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An expert system to diagnose quality problems in the continuous casting of steel billets Kumar, Sunil 1991

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AN EXPERT SYSTEM TO DIAGNOSE QUALITY PROBLEMS IN THECONTINUOUS CASTING OF STEEL BILLETSbySUNIL KUMARB.Tech. ( Metallurgical Engineering ) Banaras Hindu University, India, 1987A THESIS SUBMITTED IN PARTIAL FULFILMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE STUDIESDepartment of Metals and Materials EngineeringWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAOctober 1991© Sunil Kumar, 1991In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature) Department of  titE TAG S & 1.0 A 7-CIQ. is^(^2 N 6)The University of British ColumbiaVancouver, CanadaDate^ / 9 c?/DE-6 (2/88)ABSTRACTQuality problems, such as cracks, rhomboidity and breakouts are often observed incontinuously cast steel billets. Although information is available in the literature on the causesof, and solutions to, these problems, the transfer of this knowledge to operating personnel isnot always an easy task. To address this issue, an expert system has been developed.The important features of knowledge engineering an expert system to diagnose qualityproblems in the continuous casting of steel billets are described in this thesis. The process usedto extract knowledge from the Experts, the experiments employed to represent knowledge indifferent ways, the strategies adopted to accumulate evidence and the methodologies to presentcoherent recommendations to both experienced and inexperienced operating personnel arediscussed.Expertise was derived from diverse sources. On the one hand fundamental knowledgeabout heat transfer, solidification and mechanical behaviour of steel was applied to identify theorigin of a quality problem in the process; on the other, heuristic knowledge associated withbillet characteristics, machine design and operating factors was required to focus on the problemcauses. Integration of these two different types of knowledge was essential in developing a usefulsystem.From the Experts' viewpoint, this exercise provided them with a formal representation oftheir knowledge to solve quality problems and identified new areas for future research. Moreover,the Experts actually generated new ideas about the domain which previously were unrecognized.For the users, this system serves two important functions - as a diagnostic tool for analysingquality problems and as a teaching tool for new operating personnel.iiTABLE OF CONTENTSABSTRACT ^  iiTABLE OF CONTENTS ^  iiiLIST OF TABLES  viiLIST OF FIGURES ^  viiiLIST OF SYMBOLS xACKNOWLEDGEMENTS^  xiCHAPTER 1 INTRODUCTION^  1CHAPTER 2 SCOPE AND OBJECTIVES ^  3CHAPTER 3 EXPERT SYSTEMS TECHNOLOGY^ 53.1 Expert Systems - An overview ^  53.2 The Development Process  6CHAPTER 4 KNOWLEDGE DOMAIN - Part I ^  124.1 Billet Casting Machine ^  124.2 Fundamental Knowledge on Billet Casting ^  154.2.1 Heat Transfer^ Mould Heat Transfer ^ Spray Heat Transfer Heat-transfer Mathematical Model^  214.2.2 Mechanical Properties of Steel^  214.2.3 Stress Generation during the Casting Process^  25iii4.2.4 Solidification of Steel^  274.2.5 Operating Parameters  304.2.5.1 Mould^  304. Mould Distortion^  304. Mould Taper  394. Mould Oscillation ^  404. Mould Lubrication  424.2.5.2 Sprays ^  444.2.5.3 Liquid steel  474.2.5.4 Pinch-rolls ^  48CHAPTER 5 KNOWLEDGE DOMAIN - Part II ^  495.1 Specificity Issues ^  495.2 Quality Problems  515.2.1 Cracks^  515.2.1.1 Internal Cracks^  535. Off-Corner Cracks ^  555. Midway Cracks  615. Diagonal Cracks^  645. Centreline Cracks  665. Pinch-roll Cracks ^  685.2.1.2 Surface Cracks^  695. Transverse Cracks (and depressions) ^  695. Longitudinal Midface Cracks ^  755. Longitudinal Corner Cracks  76iv5. Craze Cracks ^  795.2.2 Rhomboidity ^  815.2.3 Breakouts  855.3 Other Observations ^  88CHAPTER 6 KNOWLEDGE ENGINEERING ^  916.1 Knowledge Acquisition ^  916.1.1 Identification/Definition of Problem Domain^  916.1.2 Background Knowledge on Continuous Casting  916.1.3 Expert System Development^  926.2 Knowledge Representation  1006.2.1 Knowledge units^  1006.2.1.1 Keyword Triplets  1026.2.1.2 Rules ^  1046.2.1.3 Procedures  1056.2.1.4 Meta-knowledge ^  1056.2.2 Search Techniques and Conflict Resolution ^  1076.2.3 Information used in the analysis^  1096.2.4 A novel inferencing strategy for the analysis of midway cracks ^ 1116.3 Structure Of The Expert System ^  1156.4 Justification of Knowledge Engineering Decisions ^  117CHAPTER 7 TESTING AND EVALUATION OF THE EXPERT SYSTEM^ 1237.1 Evaluation Procedure ^  1237.2 System Operation - Results of the feedback^  124v7.3 Analysis of User Feedback ^  1277.3.1 System's Successes  1277.3.2 System's Drawbacks ^  1297.3.3 Areas of Future Expansion  1307.4 Implementation Of Feedback^  131CHAPTER 8 CONSULTATION SESSIONS - CASE STUDIES^ 133CHAPTER 9 SUMMARY AND CONCLUSIONS^  146REFERENCES ^  148APPENDIX A^  156APPENDIX B^  157APPENDIX C^  158APPENDIX D^  168APPENDIX E^  174GLOSSARY^  178viLIST OF TABLESTable 5.1^Internal Cracks in Billet Casting. ^54Table 5.2^Midway Cracks in Billet Casting. ^55Table 5.3^Surface Cracks in Billet Casting. ^70Table 5.4^Rhomboidity Problem in Billet Casting. ^81Table 5.5^Breakout Problem in Billet Casting. ^86Table 6.1^Initial scheme proposed by the Experts to analyse midwaycracks originating in the radiation zone. ^93Table 6.2^Procedural knowledge units : Library functions inCOMDALE/X used for system development. ^106Table 7.1^Feedback implementation strategy. ^132Table 8.1^Final report. ^138viiLIST OF FIGURESFig. 3.1^Stages in expert system development. ^9Fig. 4.1^A schematic diagram of a billet casting machine.  ^12Fig. 4.2^Billet casting mould [67] ^14Fig. 4.3^Schematic representation of thermal resistance encountered in themould [15]. ^16Fig. 4.4^Mould heat-fluxes for various steel grades [19].  ^18Fig. 4.5^Variation of heat transfer coefficient versus billet surfacetemperature [23]. ^20Fig. 4.6^Schematic representation of temperature zones of reduced hotductility of steel related to embrittling mechanisms [42]. ^22Fig. 4.7^Mechanism of oscillation mark formation [19]. ^41Fig. 5.1^Schematic diagram of a rhomboid billet showing various types ofcracks. ^52Fig. 5.2^Transverse section of a billet showing off-corner internal cracks.^56Fig. 5.3^Schematic diagram showing generation of an internal crack due tobulging of the billet shell and a hinging action in the off-cornerregion [73]. ^58Fig. 5.4^Schematic diagram illustrating the mechanism generatingoff-corner internal cracks due to bulging and hinging of the shell^59[68].Fig. 5.5^Transverse section of a billet showing midway cracks. ^62Fig. 5.6^Transverse section of a billet showing centreline cracks. ^67viiiFig. 5.7^Photograph of a billet showing transverse depressions.  ^71Fig. 5.8^Longitudinal section of a billet showing an internal crack below adepression ^73Fig. 5.9^Schematic diagram showing the formation of transversedepressions and cracks due to sticking or binding in the mould [20].^74Fig. 5.10^Schematic diagram showing the formation of subsurface,longitudinal crack on diagonal at obtuse-angle corners of rhomboidbillet [22]. ^78Fig. 5.11^Photograph of craze cracks. ^80Fig. 5.12^Schematic diagram showing a billet with non-uniform shellthickness being distorted into rhomboid shape by spray cooling[64• ^83Fig. 6.1^A typical output of the mathematical model showing predicted solidshell thickness and billet surface temperature profiles. ^96Fig. 6.2^Scheme for diagnosing quality problems requiring detailed mouldand/or spray analysis. ^98Fig. 6.3^A general outline of mould-related factors contributing to qualityproblems in billet casting. ^99Fig. 6.4^Knowledge units available in COMDALE/X Development Tool.^101Fig. 6.5^A schematic diagram illustrating the calculation of degree of beliefin a spray-related tensile strain problem from belief in high reheatin the radiation zone, high temperature and composition problems.^113Fig. 6.6^A schematic diagram illustrating the calculation of final belief in aspray-related tensile strain problem following the detection of amould disorder. ^114Fig. 6.7^A flow-sheet representing the structure of the expert system  ^116ixLIST OF SYMBOLStN^Negative strip time ( s )n^3.142f^Oscillation frequency ( Hz )v,^Casting speed ( m/s )S^Oscillation stroke length ( m )ACKNOWLEDGEMENTSI would like to express my sincere gratitude to my supervisors Dr. Keith Brimacombe, Dr.Indira Samarasekera and Dr. John Meech, for providing excellent guidance, valuable help andencouragement throughout this work. It was indeed an intellectually stimulating experience forme.This work would not have been possible without the knowledge acquired by researchers inthe field of continuous casting. I take this opportunity to thank all the individuals who contributedto this knowledge base. During the development, I had useful discussions with Ian Bakshi, SanjayChandra, Neil Walker and Bob Hapke. I thank them all. I am also grateful to Harold Ng forpreparing the photographs used in this thesis.I am greatly indebted to Stelco Steel, Ivaco Rolling Mills, Slater Steel, Sidbec-Dosco Inc.,Courtice Steel, Western Steel, Hatch Associates and the Natural Sciences and EngineeringResearch Council of Canada for support of this study. Gratitude also is expressed to ComdaleTechnologies Inc. for their assistance with the software.I am grateful to Dr. Amit Chatterjee, Assistant General Manager (Research), Tata Steel,for his support and encouragement. Sanjay Chandra and his family need special thanks for hostingme and helping me settle down comfortably in Vancouver. The two years were exciting thanksto the company of fellow graduate students at UBC and excellent room-mates at home. Lastly,I would like to thank my parents, for supporting my decision to pursue higher studies in Canada.xiCHAPTER 1 INTRODUCTIONThe continuous casting of billets, slabs and blooms has been accepted worldwide bysteelmakers in favour of the traditional ingot (casting and rolling) route. Today, emphasis onquality control is a prerequisite in the competitive environment to meet customer specificationsand to reduce operating costs. The goal of high quality cannot be realized, however, withoutknowledgeable operating and maintenance personnel who understand their casting system.Similarly, the expertise these individuals possess should be considered invaluable and a methodto retain and transfer this expertise within an organization can be of great advantage.There has been significant research into the process control of billet casting which hasrelated operating parameters to quality problems. Although well-documented in the literature,transfer of this fundamental knowledge to operating personnel has been difficult despite the factthat numerous short courses on the subject have been given. There is a need then, for an expertsystem to guide operators in analysing quality-related problems and to provide them with a readysource of fundamental knowledge related to the operation of the casting machineThis thesis describes such a system that has been developed with this purpose in mind. Itis a diagnostic tool for analysing quality problems in continuously cast steel billets, specificallydesigned for use by operating personnel. The interface provided by the development tool,COMDALE/X, helps a user with explanations of terminology in the system, justification ofquestions asked and mapping of the strategy followed to arrive at final conclusions andrecommendations. In this way, the system is able to train inexperienced operators.1The thesis consists of nine chapters. Chapter 2 outlines the scope and objectives of theproject. Chapter 3 presents a brief overview of expert systems technology. Chapter 4 reviewsthe fundamental knowledge required to analyse quality problems in billet casting. The types ofproblems (cracks, rhomboidity and breakouts) are described in Chapter 5 with emphasis onmechanisms of formation and influencing factors. Chapter 6 deals with the major stages ofknowledge engineering involved in the development of this system - knowledge acquisition andrepresentation; the structure of the expert system is described and benefits of the knowledgeengineering methodology are outlined. This chapter also justifies various decisions made duringstructuring and coding of the system. Chapter 7 describes the procedure used to evaluate theexpert system and the results of this exercise. Chapter 8 presents several case studies and discussesthe consultation sessions. Chapter 9 presents the final conclusions and recommendations forfuture work.2CHAPTER 2 SCOPE AND OBJECTIVESThis research work was undertaken to develop an expert system to meet the followingobjectives:[1] diagnosing quality problems in billet casting - cracks, shape defects and breakouts; and[2] training less experienced operating personnelThe knowledge domain used to build the expert system has been generated over two decadesof research work and industrial experience in this field.Domain Experts^J.K.BrimacombeI.V.SamarasekeraKnowledge Engineers^S.KumarJ.A.MeechSystem Development Tool^COMDALE/X - version 3.0Comdale Technologies Inc.833, The Queensway,Toronto, Ontario,Canada M8Z 5Z1The steps followed to achieve the above objectives are outlined below:(a) The knowledge base was reviewed and a practical understanding of the problem domainwas established. Numerous consultation sessions with the Experts were held during thedevelopment of the system.3(b) The knowledge base was structured. Flow-charts of the analysis schemes were constructedat various stages and modified as recommended by the Experts.(c) Coding of the knowledge was done using COMDALE/X as the development tool.(d) A two-dimensional heat-transfer mathematical model for billet solidification wasdeveloped to predict billet solid shell thickness and surface reheat temperature profiles.The alternating direction implicit fmite-difference method was used to solve the unsteadystate heat-conduction equation.(e) The expert system was evaluated at five Canadian steel companies.The main driving force for this project was the need to ease the transfer of Experts'knowledge to the industries. A system was desired that could be used as a consultant by the usersfor solving quality problems in billet casting. The initial development focussed on the majorquality problems in billet casting- cracks, rhomboidity and breakouts.4CHAPTER 3 EXPERT SYSTEMS TECHNOLOGY3.1 Expert Systems - An overviewExpert Systems evolved from the field of Artificial Intelligence (A.I.) when researchersrealized the power of "expert" or "knowledge-based" systems in solving problems. An ExpertSystem is a computer program that embodies human knowledge and understanding about aparticular domain and uses this information to simulate human thought processes in the analysisof a problem for end-users [1]. It consists of equations, models, rules of thumb, do's anddon'ts, etc., all based on expert's knowledge required for solving a particular problem.Expert Systems are preferred over conventional computer programs for three mainreasons:* Knowledge is separated from the inference mechanism whereas in conventional programsdata and reasoning are integrated.* Both quantitative and qualitative information can be handled; conventional programs canhandle only quantitative information.* Incomplete and uncertain data can be accommodated in the operation whereas conventionalprograms simply cannot function without all the facts.Expert Systems are classified on the basis of the performed task. The major classes areinterpretation, diagnosis, monitoring, prediction, design, planning, and control [2].53.2 The Development ProcessThe power of Expert Systems derives from the knowledge they possess, not from theparticular formalisms and inference schemes they employ [3]. Three main reasons can beidentified for the need to focus on knowledge as the primary component in the developmentprocess [2]:• Many problems do not have well-defined algorithmic solutions. Planning, reasoning anddiagnosis are examples where the solutions can be extremely complex making it difficultto define precisely a rigorous scheme for analysis.• Human experts have achieved tremendous success with their knowledge in solvingproblems. If a computer program can be designed that utilizes the problem-solving abilitiesof the experts, then it is possible to attain high performance levels in making decisionscomparable to the experts, with the help of computers. A number of systems based onexpert knowledge have proven to be very successful [2]. These areas include mineralprospecting [4], medical diagnosis [3, 5 - 8] and computer configuration [9].• Expert knowledge is often a scarce resource and its capture can be of great benefit to anorganization or a group. Once this knowledge has been extracted from the experts and theinformation stored in a computer, lesser experts or novices can have easy access to it.This enhances transfer of high-level knowledge to users who require it.Researchers in the field of expert system development have coined the term "knowledgeengineering" to describe their activities and the title "knowledge engineer" for a worker involvedin this area. Knowledge engineering investigates methodologies and techniques necessary tocode expert knowledge (or expertise) into computer programs for solving problems. Theexpertise generally consists of knowledge and skills needed for solving problems related to a6particular domain. Knowledge is made up of two kinds: fundamental and experiential [2].Fundamental knowledge consists of published information, facts and theories in textbooks andjournals. Experiential knowledge is possessed by the human experts and involves "heuristics"- largely comprised of "rules-of-thumb" or "do's" and "don'ts". This knowledge enables humanexperts to make intelligent guesses, wherever necessary, and to deal effectively withincomplete, incorrect or conflicting information during problem solving. An important taskin knowledge engineering is to integrate these two very diverse sources of expertise.In addition to expert knowledge, these systems have two other important components:an inference engine and a user interface. The inference engine is the thinking part of the expertsystem. It consists of an interpreter to apply the rules in the knowledge base, a scheduler tocontrol the order of rule processing, a knowledge accumulator to adjust belief in previousconclusions when new data become known, and a justifier to rationalize and explain the system'sbehaviour [2].The user interface allows communication between the user and the computer. The usercan ask "why" a question is being asked or may need to know more about some fact in theknowledge base. In addition, when the final conclusions are presented, there may be a needto obtain justification for each piece of advice. With the help of this facility, expert systemscan help in the training of an inexperienced user. There are numerous ways in which data canbe requested: simple Boolean Yes/No, multiple choice selections, numerical inputs, stringinputs, mutually exclusive concepts or form-field inputs. The different techniques must becarefully chosen to ease the process of data input for different types of users. Similarly, thestyle of data-output presentation must be flexible. Some data must be presented as text messagesand others as icons or graphs. The ability to customize output reports is an important featureof a truly intelligent system.7Stages in Expert System DevelopmentThe development of expert systems can be broken down into the following principalstages: definition of problem domain, acquisition of background knowldege about the domain,development of the expert system, testing and evaluation, delivery and acceptance, and systemmaintenance. Figure 3.1 shows the different stages involved and the various linkages.Definition of Problem Domain: During this stage a problem is identified and its scope is defined.In addition, participants (experts, knowledge engineer and end-users) are also identified.Acquisition of Domain Knowledge: This involves the extraction of experts' knowledge. Theexperts and the knowledge engineers identify key concepts, relationships and information-flowpatterns needed for problem solving. Subtasks, strategies and constraints involved in theanalysis are also outlined.Development of the Expert System: Important decisions related to knowledge representationare made during this stage. A programming language or development tool/shell is selected forthe project and evaluated. The knowledge acquired is programmed into the computer and aprototype system is developed. The knowledge base is refined and expanded.Testing and Evaluation : Expert Systems, like any other conventional software, has to betested for accuracy and user-friendliness. In fact, the need for validation is much greater inthe case of expert systems since they are based on far less certain theories such as heuristicsand rules of thumb, all of which have been utilized in problem solving but were never subjectedto any detailed examination. Expert systems are evaluated by the Knowledge Engineers, theDomain Experts and the Users. During development, the system is constantly reviewed andtested by the Knowledge Engineers. Evaluation by the Domain Experts determine the accuracy8YSYSTEM EVALUATIONSYSTEM DELIVERYAND ACCEPTANCESYSTEM MAINTENANCEFeedbackFeedbackPROBLEM DEFINITIONyACQUISITION OF DOMAINKNOWLEDGERedefinitionReformulation^EXPERT SYSTEM^Refinement^DEVELOPMENT RedesignFig. 3.1^Stages in expert system development.9of the knowledge used and the advice/conclusions provided by the system whereas those bythe users helps to assess the system with respect to its user-friendliness, efficiency, speed andreliability.Delivery and Acceptance  : The system is delivered to the users and installed for actual use.User groups examine and test the system, often discovering "failures" or "bugs" or "areas ofimprovement" to be modified by the developers.System Maintenance  : The system is updated and modified according to the future requirements.New knowledge is added as required.There are no clear-cut boundaries between the above stages, which are closely linkedand heavily dependent on each other. The term, "knowledge acquisition", is used by workersin this area to characterize the complex processes involved in the development process.Knowledge acquisition has been defined as the transfer and transformation of problem-solvingexpertise from some knowledge source to a computer program [2]. It is an iterative processwhere each of the above stages interact with each other. The knowledge engineer redesigns,reformats, reformulates and refines the system in an on-going, never-ending process. Figure3.1 shows the different stages and the various interactions involved. The last two stages areconcerned with installation of the system at the place of the user and its maintenance. It mustbe noted that once the system is put into practice by the users, the process may or may notcontinue in the maintenance mode.Knowledge representation methodsKnowledge representation involves the development of structures that assist in codingknowledge into the system so that intelligent behaviour is exhibited. The main knowledge10items used in this expert system can be grouped into "structural", "procedural", "external" and"meta-knowledge" [10]. Structural knowledge consists of rules (statements and procedures),and facts (classes, objects and their description). These units provide the various links amongdifferent knowledge elements as the system searches for final conclusions. Proceduralknowledge units are employed to direct the inferencing process for a number ofknowledge-specific tasks. External knowledge provides an interface for the system to interactwith external programs. Meta-knowledge or "knowledge about knowledge" gives useful andappropriate explanations and justifications to the users. Some workers in this field, for exampleTurban [11], use the term "meta-knowledge" to distinguish rule structures that help direct thethought process from rules that deal specifically about the domain. In this work, the term"procedural knowledge" is used for this while "meta-knowledge" is reserved for describinguser-support information supplied by the system to assist users.Future trendsExpert systems technology is expanding rapidly and numerous applications have beendeveloped in the last decade. Commercialization of expert system development tools has furtherenhanced the growth of this technology. Future advancements include development ofautomated methods for knowledge acquisition and of learning systems (neural networks). Theseimprovements will considerably ease the knowledge engineering process.11TUNDISHMOULD4 SPRAYSL^W 7^PINCH-ROLLS^SHEAR/TORCHILCHAPTER 4 KNOWLEDGE DOMAIN - Part I4.1 Billet Casting MachineFigure 4.1 is a schematic diagram of a billet casting machine. The important units of abillet casting machine are tundish, copper mould, water sprays, pinch rolls and shear/torch.Fig 4.1 A schematic diagram of a billet casting machine.12Tundish - Molten steel from the steelmaking shop is brought to the casting shop in a refractorylined ladle. Steel from the ladle is transferred to another vessel called a "tundish" which is animportant unit in the casting machine. Apart from acting as a reservoir of liquid steel, it helpsin controlling metal flow and also in floating out inclusions. Details have been published earlier[12 - 14].Mould - Figure 4.2 shows a typical billet casting mould [15]. The mould generally consistsof two types- vertical and curved in which a copper tube is held concentrically inside a steeljacket. Cooling water flows in an upward direction through the gap between the mould tubeand the steel liner, steel spacers or set screws are used to maintain a uniform gap. The annulusis always full of water. The mould is oscillated either mechanically or hydraulically, with thehelp of a cam arrangement to provide the necessary stripping action for the newly formed solidshell. The mould is lubricated with the help of oil (or more recently, powders). Oil is pumpedand distributed through a splitter to channels in an oiling plate from which it is transmitteduniformly around the mould wall. In case of powders, the lubricant is fed manually by operatorsand a submerged entry nozzle to deliver the steel to the mould.Sprays - Below the mould the strand is cooled by banks of pressure-atomized water sprays.Water from a common header is sprayed onto the strand via nozzles.Pinch rolls - Pinch rolls are used to provide the pulling action necessary for the withdrawal ofthe strand from the mould.,Shear/Torch - A torch (or shears) is used to cut the strand into the desired lengths.13 i Mould2 Steel jacket3 Housing4 Support plate5 Lubricator plate6 Cover plate7 Water channelFig 4.2 Billet casting mould [67].144.2 Fundamental Knowledge of Billet CastingIn billet casting, the generation of quality problems stems from the nature of the processwhere rapid cooling of the steel results in steep temperature gradients in the solid shell thatchange rapidly and generate thermal strains as the shell differentially expands or contracts.These problems have been linked unequivocally to the operation and design of the castingmachine and also to the mechanical behaviour of steel at continuous casting temperatures.Therefore, at the heart of most quality problems in this process is the nature of cooling (intensityand uniformity) and at the same time, the mould/shell interaction.The analysis of quality problems in billet casting, like in any other process, requires athorough understanding of the fundamental principles governing it. In this case, it is essentialto study the basics of heat transfer, mechanical properties of steel at high temperatures, sourcesof stress generation in the machine (thermal and mechanical), and solidification of the steel[121 Once possible mechanisms are identified, links between quality problems and relatedoperating and/or design variables can be established. Then, these quality problems can beeliminated by altering related process parameters or design conditions, identified as importantin the analysis.4.2.1 Heat Transfer4.2.1.1 Mould Heat TransferIn the mould, heat is transferred from the liquid steel to the cooling water throughfour media namely, the solidfying shell, the air gap between the mould and the strand,15the mould wall, and the mould / cooling water interface.Figure 4.3 is a schematic representation of the thermal resistance encountered in themould [15]. The air gap constitutes the largest resistance to heat flow [16]. In the upperregion of the mould, conduction through the air gap constitutes nearly 84 percent of thetotal and is the dominant mode of heat transfer whereas in the lower part of the mould,conduction through the solid shell is the major component [15]. The amount of heatextracted is inversely proportional to the width in each case, in accordance with Fourier'sLaw.I^1^1I^1^I1^I^II^I^1I waterI^I^II^I^I1I^11^i^II IgapFig 4.3 Schematic representation of thermal resistance encountered in the mould[l 5].16The mould/shell air gap is a complex function of design and operating variablesnamely, shell shrinkage, mould distortion, oscillation marks and ferro static pressure.The gap also varies in width in both the longitudinal and transverse directions [17]. Thus,it is difficult to predict the width of the air gap accurately as a function of distance belowthe meniscus. For these reasons, measurement of heat-transfer boundary conditions atthe mould/billet interface is necessary. The heat fluxes at the billet/mould interface havebeen calculated from time-averaged responses of thermocouples embedded in the mouldwall a set distance away from the hot face during normal casting [18]. Details about thescheme of calculation of the heat-extraction profiles for single- and double-tapered mouldtubes have been published earlier [19 - 22]. In addition, heat transfer in the mould isextremely sensitive to the shape of the copper tube which during operation changes dueto differential heating and expansion. Samarasekera et al.[22], with the aid of a finiteelement stress-strain model, have studied the thermo-mechanical behaviour of the mouldduring operation.Heat transfer in the mould is influenced by operating parameters such as steelcomposition, superheat and cooling water velocity [19]. The effect of steel carbon contenton mould heat transfer is shown in Figure 4.4. In the upper regions of the mould, themagnitude of heat flux decreases with decreasing carbon level in the 0.14-0.36 percentrange. The peak and the average mould heat fluxes are a minimum for 0.1 percent carbonsteels but increases with increasing carbon content up to 0.20 percent and are fairly constantfor higher carbon grades. This behaviour has been attributed to the peritectic 8 — yreaction.In the range of 0.016 to 0.028 percent phosphorus, for medium carbon steels, higherphosphorus levels lead to a reduction in heat transfer especially at the meniscus. A greaterreduction in the upper mould heat-flux is seen with increasing sulphur levels. This17150050^00^50^100Time ( s)5000se., No C(VP(%)L44LK)SItoSoMIATraK.(m/s1VImAr41— 24268 014 0020 0 55 0039 014 19 9 7 2 4- - -^24259 046 0024 O51 0033 042 (28) 92 49— - 242481 028 0022 100 0036 0 48 (27) 93 23— 24254 036 0026 086 0028 012 (32) 9 7 21300041-Z-r 200010004200behaviour is attributed to the deleterious effect of sulphur and phosphorus on themechanical properties of steel which influences the resistance of the steel to deformationclose to the meniscus during mould oscillation.Fig 4.4 Axial profiles of mould heat flux for various steel grades [19].18The effect of mould cooling water velocity is quite significant [19]. For low carbongrades, the magnitude of peak heat flux is maximum at a water velocity of 4.0 m/s whereasfor medium carbon ranges, the peak is observed to be a maximum at a velocity of 5.0 m/s.Heat transfer in the mould is also a strong function of oscillation parameters [19].In the upper part of the mould, the presence of oscillation marks on the billet surfaceinfluences the width of the mould/shell air gap. Hence, deep oscillation marks reduceheat extraction considerably in the mould. Also, varying depths of oscillation marksacross the billet surface leads to non-uniform heat transfer conditions in the mould. Spray Heat TransferCooling of the strand beneath the mould is achieved by banks of pressure-atomizedwater sprays. Figure 4.5 shows the variation in the heat-transfer coefficient with surfacetemperature during spray cooling for a 1/4 GG10 nozzle operating at 90 psi and set backapproximately 200 nun from the billet surface [23]. As shown in the figure, above 550°C,the heat-transfer coefficient changes very little with temperature, which is characteristicof film boiling [24]. At lower temperatures, the heat-transfer coefficient increases sharplyas the heat transfer mechanism changes from film boiling to transition boiling (breakdownof the steam barrier). The critical temperature at which this occurs is called the Liedenfrosttemperature which increases with increasing water flux [25]. Numerous laboratoryexperiments and in-plant trials have established relationships between the sprayheat-transfer coefficient and spray water flux [23, 25 - 39].19j„,..........,**N...2.•1.0Critical Temperature..""ll'4'Nfir•r•fririr*"4re"..411...w**1....*•••PNozzle - 1,4 GG 10Spray Distance - 8 In.Spray Pressure - 90 psigoo1^I^J200^400^600^800^1000 1200 1400 1600 1800 2000SURFACE TEMPERATURE, uFFig 4.5 Variation of heat transfer coefficient with billet surface temperature [23]. Heat-Transfer Mathematical ModelIn the continuous casting of steel billets, the mathematical model employed forpredicting temperature distribution, the solid shell profile and the liquid pool depth isbased on the unsteady-state, two-dimensional heat conduction equation:a ( aT) a (al aTk ax + =pCpaT (1)Details of the mathematical model i.e. assumptions, formulation scheme, boundaryconditions and the predictions have been adequately described in earlier works [41,42].Correlations which relate the working mould length, casting speed and shell thickness atthe mould exit have been established by Brimacombe [40]. In the same publication, designcurves of spray length, spray heat-transfer coefficient and water flux distribution werepresented. The boundary conditions for the model are derived from the heat-transfermeasurements made in the mould and sprays.4.2.2 Mechanical Properties of SteelThere are three distinct high-temperature zones where steel exhibits low strength andpoor ductility and, therefore, is highly susceptible to cracking. Figure 4.6 is a schematicrepresentation of the reduced hot ductility of steel based on a review of the literature byThomas et al. [42]. As shown in the diagram, a high-temperature zone of low ductility occurswithin 30-50 °C of the solidus temperature, the intermediate temperature zone of low ductilitylies between the Ar3 temperature and about 1200 °C while a low temperature zone extendsbelow the Ar3 temperature. In the continuous casting of steel billets, only thehigh-temperature zone is critical from the viewpoint of crack formation.21Crock600^900^1200^1500Temperature (°C)Fig 4.6 Schematic representation of temperature zones of reduced hot ductility of steelrelated to embrittling mechanisms [42].22High Temperature Zone : 1340 °C to SolidusAt temperatures well above 1340 °C, there is significant deterioration of the strengthand ductility of steel. In a review conducted by Brimacombe and Sorimachi [43], strengthdata measured at various strain rates in four different studies were extrapolated to a commonstrain rate of 10-3 s"' (commonly obtained in the continuous casting process) and werecompared. The strain-to-fracture of steel in the high temperature zone of low ductility appearsto be of the order of 0.20 to 0.30 percent according to Vom Ende and Vogt [44].The low strength and poor ductility of steel close to the solidification front is due tothe presence of liquid films in the interdendritic regions which remain molten untiltemperatures well below the solidus are reached [45]. These liquid films are rich in solutessuch as sulphur and phosphorus which have segregation coefficients (CJC 1) less than unity.Scanning electron micrographs of the interior surface of a crack that originated close to thesolidus temperature have shown a smooth surface, with no signs of solid fracture, indicatingthe presence of a liquid film at the time of crack formation [46].Effect of Phosphorus  : Sopher [47] studied the influence of phosphorus on fracture strengthand ductility of SAE 4340 steel at different temperatures. It was found that steel with aphosphorus concentration of 0.039 percent has drastically reduced strength and ductility ascompared to steel containing 0.017 percent phosphorus.Effect of sulphur  : The effect of sulphur on the strain-to-fracture and the UTS of steel nearthe solidus temperature was studied by Morozenskii et al.[48]. Two points can be noted fromtheir study. Firstly, the presence of sulphur in steel adversely affects both strain-to-fractureand the UTS; and secondly, an increase in Mn/S ratio in steel improves the strain to fracturebut has no significant effect on the UTS. Similar results were obtained by Kinoshita and23Kuroki [49] in another study related to steel at 1350 °C. This study also investigated theeffect of Mn concentration on the proportion of MnS and the FeS in sulphide inclusions. Itwas shown that FeS content decreased with increasing Mn concentration. The authors alsofound that in 0.2 percent C steel, for Mn concentrations less than 0.7 percent, sulphideshave a tendency to exist in liquid form below the solidification temperature of 1480 °C. AMn/S ratio greater than 20 is effective in minimizing the cracking tendency by preventingthe liquid film formation [48].Effect of carbon  : The effect of carbon on mechanical properties was studied by Morozenskiiet al.[48]. The strain-to-fracture, S E , and its plastic component, 5,1„stic , were found to beminimum for steel containing 0.17 to 0.20 percent C. Similar results were reported byGuessier and Castro [50].Effect of tin and copper : Increased contents of Sn[51] and Cu[52,53] deleteriously affectthe ductility of steel. This behaviour is attributed to the presence of the above mentionedlow-melting impurities at the grain boundaries. Failure occurs due to liquid metalembrittlement.The high temperature zone of low ductility has also been examined by numerous otherauthors [55 - 60]. It has been established that this zone of low ductility is operative attemperatures within 30 to 70°C of the solidus temperature, and the associatedstrain-to-fracture is virtually zero [61]. Thus, the presence of even a small tensile strain inthe solidfying shell in this temperature region will initiate a crack which will propagateoutward from the solidification front between dendrites.244.2.3 Stress Generation during the Casting ProcessDuring the casting process, the strand is subjected to varying thermal conditions andchanging mechanical loading which generate stresses and strain in the strand. In order fora crack to originate at a given location, two conditions must be present [12]:(i) the stress must be tensile in nature(ii) the fracture strain must be exceededThermal Stresses/Strains  - The rapid rate of cooling in continuous casting, results in steeptemperature gradients in the solid shell that can change rapidly and generate thermal strainsas the shell expands or contracts differentially. Thermal stresses can be generated eitherwhen free expansion or contraction of the material is constrained or when the thermal gradientin the material is non-linear and changing [12]. In the continuous casting of steel, the thermalstress condition in the strand can be approximated to that of a generalized plane-straincondition [12]. In the process, there is some allowance for expansion in the casting directionwhich helps minimize generation of longitudinal stresses and strains. Further, thermalgradients in the longitudinal direction are quite small and unlikely to cause any cracking.Thus, transverse cracks, which require a longitudinal stress component, simply cannotoriginate due to adverse thermal gradients in the billet but instead are generated mechanically.In the transverse plane however, conditions are present which generate high thermalstresses. The free thermal expansion is restrained and also the temperature gradients aresteep and frequently non-linear [12]. In addition, sudden changes in heat extraction rates,which could occur at the boundary between any two consecutive cooling zones, may causethe thermal gradients to shift, particularly at the surface; the resulting expansion or25contraction of this region also generates stresses in the transverse plane. If this strain istensile in nature and its value exceeds the strain-to-fracture in a zone of low ductility,particularly close to the solidification front, then longitudinal cracks may form.Reheating of the surface of the billet below the mould or the sprays causes expansionof the surface layers which imposes tensile stresses at the solidification front where the steelhas the lowest ductility [62]. Non-uniform cooling in the mould and the sprays imposestensile stresses at the obtuse angle corners or off-corners regions of billets [22,61]. Also,during binding at the corners of the billet in the mould, excessive cooling at some locationsof a face may cause localized tension [12].Van Drunen et al.[46] have suggested another source of thermal stress generation. Atthe point of complete solidification, when the final trace of latent heat has been extractedfrom the centre region, the drop in centreline temperature is considerably more rapid thanthe decrease in surface temperature with the result that the central region contracts more thanthe surface. The centre, however, is constrained from contraction by the surrounding coldersteel and is therefore, put into tension. Depending on the magnitude of the stress and thestrength of the steel, centreline cracks may form. Segregation of elements in the centralregion of the billet adversely affects the strength and ductility of steel and increases thesusceptibility to cracking.Mechanical Stresses - Sticking in the mould due to inadequate lubrication and/or oscillationconditions or binding of the strand in the mould due to excessive taper, imposes resistanceto smooth withdrawal of the strand. This generates axial tensile stresses and strains thatconcentrate on locally thin regions of the shell and may cause transverse depressions withtransverse cracks [20]. The nature of this transverse crack, whether it appears as an internal26crack or a surface crack, depends on the magnitude of the stress and the shell thickness atthat location. If the taper is insufficient in the lower portion of the mould, bulging of thesolid shell can occur and may lead to hinging of the strand at the off-corner sites whichgenerates a tensile strain at the solidification front and internal cracks [63]. Straighteningor bending operations, when carried out on a section with a liquid centre or when the centreis solid but within 50 to 70 °C of the solidus temperature, may also lead to the generationof a tensile stress at the solidification front [43]. Tensile stresses are also generated whenexcessive pinch-roll pressures are applied on the strand with a liquid core [43].4.2.4 Solidification of SteelIn continuous casting, two aspects of solidification are important from the viewpointof quality: cast structure (columnar / equiaxed) and the nature of shell growth around theliquid pool.Cast Structure - Cast structure influences internal crack formation and macrosegregation.Dahl and Hengstenberg have proposed a mechanism of crack formation based on dendriticseparation in the columnar zone [63]. The columnar structure is more susceptible to crackformation as compared to the equiaxed structure because it provides an easy path for crackpropagation. There are a number of operating parameters that influence the size of the centralequiaxed zone relative to that of the surrounding columnar zone in a continuously cast billets.These factors are superheat, steel composition, section size, casting speed and machinedesign (straight or curved mould-tubes).Effect of superheat: The effect of superheat (usually measured in the tundish) on the caststructure was investigated by Van Drunen et. al. [46]. It was found that the columnar zone27is favoured at the expense of the equiaxed zone with increasing steel temperature (liquidus+ superheat). A superheat level of 30 °C or below is desired to maximize the equiaxedstructure which makes the strand more resistant to cracking and minimizes macrosegregation.Effect of carbon conteni  : The effect of carbon content on the length of the columnar zonein a continuously cast billet has been examined by Bommaraju et al.[64]. Equiaxed structureis favoured in the medium-carbon range of 0.17 to 0.38 percent carbon. Increasing thephosphorus content from 0.008 to 0.02 percent in 0.13 to 0.20 percent C steel also causesthe columnar zone to shrink [64].effect of section size  : A large section size favours the growth of an equiaxed zone [12].Hence, crack formation by dendritic separation in the columnar zone is less likely to beserious as compared to smaller sizes.Effect of machine design  : The design of the casting machine (i.e. straight or curved) alsoinfluences the cast structure. Van Drunen et al.[46] observed that the length of the columnarzone adjacent to the inner-radius face was greater than that adjacent to the outside-radiusface. Lait et al.[65] postulated that this non-symmetrical structure results from the settlingof crystallites (dendritic debris) against the solidification front advancing from the outsideradius face. In this way, the crystallites interfere and prevent further growth of the columnargrains. On the other hand, the columnar structure adjacent to the inside radius experiencesunimpeded growth.Solid Shell Formation - The initial shell growth in the mould is governed by the mouldheat-flux distribution. Watanabe et al.[16] observed that in the upper part of the mould, themould/shell air gap constitutes a large fraction of the total thermal resistance encountered.Thus, the nature of shell growth in the upper part of the mould is largely dependent on the28dynamics of air gap formation. Hence, variables influencing air-gap formation, and thereforethe heat-transfer in the mould, directly impact on the nature of shell growth. In the lowerpart of the mould and further down the casting machine, after a reasonable amount ofsolidification has occurred, conduction of heat through the solid steel shell is critical incontrolling heat transfer and solid shell formation [15].Two aspects of solid shell formation that are important from the viewpoint of billetquality, are the magnitude of the shell thickness and the uniformity of the shell around theliquid core [12]. The solid shell at the mould exit must be strong enough to counteract theferrostatic pressure exerted by the liquid core. If the shell thickness is inadequate and themould taper insufficient, bulging of the solid shell in the mould may occur [64]. Bulgingcan also occur near the mould exit, before the spray zone, if the shell is thin. In an adversesituation, a thin shell at the mould exit may be incapable of withstanding the ferrostaticpressure and could rupture. Breakouts can also occur if there is a local reduction in shellthickness due to surface depressions, cracks or deep oscillation marks. Therefore, uniformityof the shell around the liquid core is also a critical parameter for quality and operations.Non-uniform shell thickness may result from deep and non-uniform oscillation marks,wrinkling of the surface especially in low-carbon grades, or the presence of surfacedepressions. Singh and Blazek [66] showed the influence of the carbon content of steel onthe uniformity of shell growth. Non-uniformly deep oscillation marks on the billet surfacelocally reduce the heat-extraction rate in the mould, relative to other areas around theperiphery of the billet, as a result of an increased air gap width [64]. Thus, the billet leavesthe mould with a non-uniform shell. The severity of this non-uniformity depends on theextent of variation in the heat-extraction rate around the billet periphery in the mould. Linkswill be established later between this event and rhomboidity.294.2.5 Operating Parameters4.2.5.1 MouldMould/shell interaction at the meniscus is by far the most important phenomenonoccurring in the mould as it directly impacts on the formation of oscillation marks whichaffect the width of the mould/shell air-gap and, hence, influence the mould heat-extraction[19]. Mould/shell interaction away from the meniscus is also important because itinfluences binding in the mould which occurs whenever the size of the billet exceeds themould tube dimensions. A mould/shell gap that is too large, is also not desirable. In theupper part of the mould, a large mould/shell gap leads to significant reduction in the heatextraction capability of the mould while in the lower part of the mould, coupled with athin solid shell, it causes bulging and cracking at off-corner locations [64]. Mould DistortionMould-tube distortion affects the shape of the mould during operation and isdirectly linked to the thermo-mechanical behaviour of the mould [21]. As shown inFigure 4.4, the heat flux is a maximum very close to the meniscus due to the small airgap across which heat flows essentially by conduction. Below the meniscus, the heatflux falls as the gap increases in width due to steel shrinkage. The second maximumseen in the mould heat-flux profile is due to mould distortion [19]. Above the meniscuswhere the steel does not contact the mould, the heat flux declines rapidly and causesrelatively steep temperature gradients to be set up in the axial as well as thethrough-thickness direction in the mould wall.30In the study by Samarasekera and Brimacombe [22], it was found that althoughthe maximum heat flux is at the meniscus, the peak temperature is not. The maximumtemperature is observed at a location slightly below the meniscus. This behaviour isattributed to significant heat conduction vertically up the mould wall. The axialtemperature distribution observed in the mould determines the expansion of themould-tube. The hottest region of the mould wall expands the most and causes bulgingof the mould tube. The peak bulge in the mould appears about 75 mm below the meniscus.As a result, the mould has a negative taper above the peak bulge and a positive taperbelow the maximum distortion point. The second peak in the mould heat-flux profile,according to S amarasekera and Brimacombe, is due to twisting of the mould as a resultof non-uniform mould tube support [22].Classification of Mould Distortion - Profiles of distance between opposite faces for alarge number of used moulds were examined by Brimacombe et al.[67]. Used mouldswere classified into three main categories based on the extent of permanent deformationobserved:Minimal distortion  : In this category, moulds exhibited relatively little distortion in themeniscus area. Small permanent deformation, less than 0.50 mm or less between theopposite faces, was observed. It was found that these moulds were operated with deepmeniscus levels, high water velocities and good water quality.Outward bulging near the meniscus  : Moulds in this group exhibited an outward bulgetoward the water channel, very close to the meniscus region. Permanent distortion waswithin 0.50 mm and 2.00 mm between the opposite faces; the constrained sides weredeformed more than the unconstrained sides because the former are constrained against31expansion and undergo greater plastic strains, and therefore have larger permanentdistortion. These moulds had been operated with shallow meniscus levels, poor waterquality or low water velocity.Abnormal / severe distortion  : In this case, moulds showed permanent deformationwhich exceeded a value of 2.0 mm between opposite faces. This strongly indicatedproblems such as poor water quality, low water velocity, shallow meniscus and poorcentering of the mould tube in the liner.Regular distortion measurements of both new and used mould tubes is importantin the analysis of mould disorders. Measurements made on used moulds reflect thenature of operating parameters employed in the process whereas, checks on thedimensions of new moulds are necessary to ensure that mould-tube specifications arebeing met.Factors Influencing Mould Distortion(i) Cooling Water Quality : Water quality strongly influences the thermal field and thedistortion of the mould-tube. This is linked to the deposition of scale from the coolingwater on to the cold face of the mould-tube. The presence of a layer of scale introducesa large resistance to heat flow which raises the mould temperature [21]. It is clear thateven with a relatively thin layer of deposit, the local cold-face temperature is hotterthan the hot-face temperature in the absence of scale [21]. Fouling nearly doubles thebulging of the mould-tube below the meniscus from 0.20 mm to 0.35 mm [21].The presence of scale deposits on the cold face of the mould is a clear indicationof poor water quality. The severity of fouling was quantified by Brimacombe et al.[67]32with an index, F, which was defined on the basis of the ratio of the deposit length, 1,to the effective mould length, L. The higher the value of F, the poorer is the waterquality.Poor water quality may be due to a high hardness level in the cooling water ordue to the presence of corrosion products, oil, grease or as a result of biological foulingstemming from algae and moss. Inferences on the source of the contamination may bedrawn from the colour and properties of the scale deposit. A red deposit, similar torust, suggests oxide of iron and corrosion in the water delivery system. A black depositmay be due to magnetite (Fe304) or carbonaceous matter such as oil, grease or biologicalmatter. The presence of iron oxide suggests corrosion in the water delivery system whilecarbon-rich matter indicates petroleum products or biological fouling. With the help ofchemical analysis, it is possible to distinguish between the two contaminants. Inaddition, if the loss on ignition of the scale deposit is large, the scale is expected to berich in carbon. The black deposit can be tested with an ordinary magnet to check forFe304. A light-coloured scale deposit suggests excessive hardness in the cooling water.(ii)  Cooling Water Velocity : Cooling water velocity is directly linked to the thermaldistortion of the mould [21]. A high water velocity is recommended to preventuncontrolled nucleate boiling in the cooling channel. It has been shown that nucleateboiling, which occurs on the hottest region of the mould very close to the meniscus,could be both asynchronous and intermittent. Water velocity in excess of 12 m/s isrequired to prevent boiling [68]. In addition, it is also important that the mould coolingwater velocity is uniform around the periphery of the mould.33(ii)  Mould-Tube Alignment and Tolerances  : It is not only important to maintain a highcooling water velocity but also a uniform water flow around the periphery of the mould.If the flow is non-uniform then one or more walls of the mould may operate hotter thanthe others and cause non-uniform distortion of the mould tube which in turn may leadto uneven shell growth in the billet. Hence, it is essential to ensure that the width ofthe annulus between the steel jacket and the mould tube, through which the coolingwater flows, is constant around the mould periphery.The water gap is typically 4.8 mm (3/16 inch), although some plants employ gapsof 3.2 mm (1/8 inch). It is important to maintain a close tolerance on the outsidedimensions of the water jacket due to the small width of the water gap [68]. For example,in the case of 4.8 mm wide gaps, a loss of dimension of 1 mm can change the watervelocity by up to 20 percent. In addition, the tolerances on the outside dimension ofthe mould wall and the inside dimension of the water jacket are also important. Typicallyvalues for each is about 0.5 mm. On its own, this may give rise to a variation of 1.0mm on different faces. In addition, the mould tube could translate a further 1 mm fromthe concentric position. because of the tolerances on the support plates and flanges. Inthe worst case, the water gap may differ by as much as 2 mm (roughly 40 percent ofgap width) between the opposite faces of the mould. Local increase in the gap widthmay also occur due to non-uniform distortion or bulging or misalignment or evenoff-centring of the mould tube. An increase in the gap width leads to local reduction inwater velocity which causes a marked increase in the mould temperature [21].Frequently, at the corners of the water jacket, the gap is not equal to the gap atthe midface [68]. This occurs when the jacket has square corners which is typical ofjackets fabricated from steel plates which are welded at the corners. When the two are34assembled, the water gap in the corners is larger than that in the midface [68]. A largergap at the corners leads to a reduction in the resistance to water flow through it. Thiscauses the water to flow preferentially up the corners thereby reducing the water velocityon the faces. A large gap at the corners may also isolate one face from another leadingto severe non-uniformity in the heat transfer conditions around the mould periphery[68].The material of the water jacket is also important [68]. A mild steel jacket corrodeseasily and affects the tolerances. A material, such as stainless steel, that is resistant tocorrosion is desired to maintain the dimenional integrity of the water jacket during theoperation.(iii)  Mould-wall Thickness  : Thick mould walls give rise to a smaller negative taper atthe meniscus [68]. This is because thicker walls have larger thermal gradients and hencebending of the wall is a significant mode of deformation. Thus, the midface bowsinwards relative to the corners at the meniscus. The effect of thick wall mould tubeshave been observed on the depth of oscillation marks [68]. It has been found that thickerwalled tubes generate shallower oscillation marks as compared to thinner ones. This isexpected because smaller negative taper of the thick wall tube at the meniscus resultsin reduced mould/shell interaction during negative strip. A thick wall mould tube alsoenhances heat transfer in the meniscus region [68]. It was found that the heat flux tothe thick wall tube is greater and remains at a higher level close to the meniscus ascompared to thin wall tube. The rapid drop in the heat extraction rate for the thin walltube is associated with greater negative taper which leads to quicker formation of a widermould/shell gap below the meniscus. A thicker mould wall is desired even for small35billets. From the viewpoint of mould distortion, wall thickness must be greater thanone-tenth of the largest dimension of the billet cross-section and should not be less than12.5 mm.(iv) Mould-Copper  : The selection of a copper grade for the mould-tube depends onthree critical parameters, namely, thermal conductivity, yield stress and softeningresistance [68]. A high thermal conductivity is required to minimize mould temperaturesand resulting thermal distortion. A high yield stress and an adequate softening resistanceare necessary to minimize permanent distortion of the mould tube.Pure copper is better than other metals for the mould owing to its high thermalconductivity. Unfortunately, it lacks the softening resistance and cannot maintain itsshape at peak operating temperatures for more than one or two heats. Therefore, it isnecessary to use an alloy of copper having high yield strength and good softeningresistance.DHP-122, a phosphorus deoxidized, high residual phosphorus copper with onlya small residual silver content, is commonly used as a mould material for billet casting.It has adequate softening resistance and does not undergo plastic deformation early inthe campaign. The phosphorus content should be greater than 0.03 percent to ensuregood softening resistance.DLP-120 is a phosphorus-deoxidized, low residual phosphorus grade of copper.The phosphorus content is below 0.005 percent and hence its softening resistance islower than that of DHP-122 grade. However, a silver content greater than 0.1 percentenhances the softening resistance of the alloy.36The STP grade is a tough-pitch alloy containing some silver. It lacks adequatesoftening resistance required to maintan its shape at peak operating temperatures formore than one or two heats, a behaviour very similar to that of pure copper.Copper-Silver alloys with a silver content exceeding 0.10 percent is frequentlyused as a mould material. Adequate silver content is necessary to ensure a good softeningresistance.New alloys of copper containing elements such as chromium, zirconium, nickeland aluminum are also being introduced as mould materials [68]. The alloying elementssuch as chromium, nickel and aluminium strengthen the copper by solid solutionhardening, whereas zirconium is a precipitation hardening agent. These elementsimprove the softening resistance as well as the yield strength of the copper beyond thatof phosphorus and silver. These alloys substantially improve the mould life. However,it is important to ensure that the thermal conductivity of the alloy is greater than 80percent of pure copper. Three conditions are essential in a mould material:* Thermal conductivity > 300 W/m°C(W/m°C) (80 percent of pure copper or more)* Half-softening point (°C) > 350 °C* Yield stress ( MPa ) > 250 MPa at 20 °CAn alloy satisfying the above conditions is unlikely to contribute to moulddistortion provided that the cooling water velocity exceeds 12 m/s, cooling water qualityis controlled carefully and the metal level is at least 100 mm below the top of the mould.37(v) Mould-tube support system : The mould tube is held inside a steel jacket or baffleby steel plates that fit into slots in the upper part of the tube. The slots may appear onlyon two opposite faces (usually the straight sides in a curved mould machine) or on allthe four faces of the mould tube. There is yet another design in which the mould tubeis held on the top and bottom by expansion plates with gaskets for sealing the matingsurfaces.The two-sided constraint system is undesirable because it gives rise to non-uniformmould distortion and greater negative taper at the off-corners relative to the midface[68]. The constrained sides deform more than the unconstrained sides. Non-uniformdistortion results in greater mould/shell interaction at the off-corners during negativestrip, causing deeper and more non-uniform oscillation marks at these locations.A four-sided constraint system results in more uniform mould distortion [68]. Thenegative taper at the off-corner and the midface is comparable. Mould/shell interactionand hence, the depth of oscillation marks is uniform around the mould periphery.However, these advantages may be lost if the keeper plates do not fit snugly into thebottom of the mould slots and also if mould tube is not positioned concentrically withinthe steel liner.A top and bottom support system for the mould-tube has been found to be a safedesign since it does not lead to non-uniform mould distortion, provided the supportsare parallel with the top and bottom of the mould [67].(vi) Position of the meniscus  : Metal level has a significant effect on the permanentdistortion of side-supported mould tubes. Permanent bulging was observed in mouldsoperated with the meniscus very close to the top of the mould [67]. This happens because38the point of maximum expansion of the mould tube which is very near the meniscuscomes closer to the support plates which oppose the outward movement. Hence, thestresses in the mould wall are large.The metal level and the type of mould support system are closely related especiallyin the generation of mould distortion. A metal level which is closer than 100 mm to thetop of the mould is likely to create adverse conditions for mould distortion in the caseof both two-sided and four-sided mould constraint systems. Hence, in moulds havinga side-supported system, the distance of the meniscus from the top of the mould iscritical and must not be less than 100 mm. However, in the case of moulds with a topand bottom support system this is not a criterion and therefore operating with shallowmeniscus levels is less likely to be a serious concern. Mould TaperDippenaar et al.[69] have found that a single taper of 0.6 to 0.7 % per metre isinadequate to compensate for shrinkage of the cooling steel and gives rise to a largemould/shell air gap. Also, it is insufficient to counter the development of negative taperand resulting mould/shell interaction. A double tapered mould, with an upper taper of3.5 to 4.0 % per m and a lower taper of 0.5 % per m, is considered to be better than asingle tapered mould. The advantages of a double tapered mould has been discussedin earlier publications [68,69]. Recent work at UBC has examined the use of multipletapered moulds. Taper calculations have been made for different carbon grades [70].Caution must be exercised in employing multiple tapered mould tubes since themould heat-flux profile and billet shrinkage profile are strongly dependent on casting39speed, mould-tube distortion and metal level. Since the upper taper is based on aconstant residence time between the meniscus and the break-point to the next taper, itis mandatory to control the above mentioned parameters when using multiple taperedmould tubes. Mould OscillationMechanism of Oscillation-Mark Formation  - Oscillation of the mould is essential toprovide a stripping action which aids withdrawal of the strand; but it also generatesoscillation marks. The mechanism of oscillation mark formation has been discussed bySamarasekera et al.[71] and is shown schematically in Figure 4.7. During negative strip,(the period when the mould moves downward faster than the strand), due to the negativetaper at the meniscus, the mould squeezes inward and downward and jams on the newlyformed solid shell. Due to this action, a downward compressive force is generated, theshell deforms and buckles to generate an oscillation mark on the billet surface.Factors Influencing Mould Oscillation(i) Negative strip time  : Negative strip time is expressed in terms of stroke length,oscillation frequency and the casting speed.k=-- cos —I.itfS(2)The effect of negative strip time on the depth and uniformity of oscillation markson the billet surface was studied by Samarasekera et al. [71]. A reduction in the negativestrip time from 0.21 to 0.12 s, reduces the depth and improves the uniformity of40oscillation marks on the billet surface. The average depth of oscillation marks and thedifference in their depth between the midface and the off-corner has been shown toincrease with increasing negative strip time. By reducing the time of this mechanicalinteraction, it is possible to decrease the depth and improve the uniformity of oscillationmarks. On the other hand, operating with extremely low negative strip time causessticking of the strand in the mould. An optimum range for negative strip time to minimizesticking as well as problems related to deep and non-uniform oscillation marks is 0.12to 0.15 seconds.Fig 4.7 Mechanism of oscillation mark formation [19].41(ii) Mould Lead  : Mould lead is the downward displacement of the mould relative tothat of the strand during negative strip. It is related to negative strip time, oscillationfrequency, stroke length and the casting speed by the following mathematical formula:MouldLead = S sin(nftN) — VCtN (3)The stripping action appears to be related to the mould lead generated during theoscillation of the mould. Operating with small mould leads increases the tendency ofsticking in the mould. The stripping action improves as the displacement of the mouldduring negative strip time is increased.(iii) Other variables  : In addition to negative strip time and mould lead, stroke lengthand oscillation frequency must be within specified limits. The desired range for thestroke is 9 to 16 mm. The upper limit imposed on oscillation frequency is around 4 Hz.and is based on vibration problems (of the structures) encountered at higher frequencies.Also, the oscillation frequency should not be linked to casting speed. Regularmaintenance of the oscillation system is necessary to avoid wobbling and vibration ofthe oscillation system. Mould LubricationLubrication in the mould is necessary to prevent sticking of the steel to the mouldwall at the meniscus. Sticking hinders smooth withdrawal of the strand from the mouldand leads to the generation of quality problems in billet casting.42Factors Influencing Mould Lubrication(i) Oil - flow and distribution  : Insufficient lubrication increases the tendency forsticking. On the other hand excessive supply of oil for lubrication leads to entrapmentof hydrogen and pinhole formation in the billet. Thus, it is extremely important to havethe proper amount of oil in the mould. Flow rates within 0.10 to 0.20 ml of oil per minper mm of mould tube inside perimeter are considered to be an optimum range.In addition, the oil must be uniformly distributed around the mould periphery.Regular checks on the distribution of oil are necessary. A simple experiment can bedone on a cold mould when casting is not in progress to assess the adequacy of the oildistribution system. Oil is allowed to flow down the inside mould wall and is collectedin containers placed below each face at the bottom of the mould tube. The volume ofoil collected in each container is compared. A difference exceeding 10 percent isconsidered to be unsatisfactory.(ii) Oil - Physical Properties  : Viscosity, boiling point and flash point of the lubricantare important parameters. Oil viscosity determines the resistance to oil flow down themould wall. A high flash point of oil is desired to prevent the oil from burning off beforereaching the metal level. Similarly, the boiling point of oil must also be high such thatthe oil does not vapourize before reaching the metal level. Flash point and boiling pointgreater than 300 °C are considered to be adequate provided there are no scale depositson the cold face of the mould. If scale deposits are present on the cold face of the mould,the mould operates hotter than normal and therefore, the lubricating oil vaporizes orburns before reaching the metal level. In this situation, the mould lubrication is likelyto be poor.43(iii) Cleanliness of oil distribution system  : Regular cleaning is essential to preventblockage of oil-distribution slots. Blockage of these slots disturbs the flow anddistribution of oil.(iv) Removal of oil-pyrolysis products from the meniscus  : The meniscus is the mostimportant region of the mould from the viewpoint of heat extraction and initialsolidification. If there is a build up of oil pyrolysis products at the meniscus during theprocess, the debris interferes with heat transfer and introduces non-uniformity in therates of heat extraction. Cleaning of this area is therefore essential and can be done intwo stages. Firstly, the solid loose oil particles and residual oil are removed with a ragsoaked in solvent. Secondly, the mould surface is rubbed with emery paper to removeadherent materials and uncover the chromium layer on the mould surface. SpraysFrom the viewpoint of quality products, an ideal spray cooling operation is one thatgenerates minimum surface reheat, provides uniform cooling conditions around the billetperiphery and ensures a high solidification rate.(i)  Spray Design and Operation  : The important parameters are spray length, specificwater flow, water pressure, nozzle configuration, nozzle positions and stand-off distancesfrom the strand.Reheating is minimized by preventing sharp reduction in heat extraction rates as thebillet moves from one cooling zone to another. Therefore, the cooling rate in the upperportions of the sprays must match the heat flux at the mould exit, and the gap betweenthe mould and the sprays must be as small as possible. The presence of a large gap may44cause a significant rebound in the billet surface temperature. In addition, the presence ofsupport rolls at the mould exit hinders cooling of the billet and may cause the surface toreheat. At the exit, the spray cooling rate must be comparable to the radiative heat flux.The spray water flux must be reduced with axial distance to minimize drastic transitionsin the cooling conditions.Brimacombe et al.[62] proposed a methodology for spray design by combining theunderstanding of the origin of thermal stresses with a mathematical model capable ofsimulating the thermal history of steel (profiles of solid shell thickness and surfacetemperature profiles along with the prediction of liquid pool depth). Based on experience,it is felt that surface reheating of the billet can be minimized by operating with a specificwater flow less than 1.0 litre per kg of steel cast and a spray length greater than 3 m. Theoptimum operating conditions can be obtained with the help of the mathematical modelwhich takes into account cooling in the mould, casting speed, section size and grade ofsteel being cast.The significance of nozzle configuration has been discussed by many authors.Brimacombe et al.[62] have outlined the methodology of translating the distribution ofspray heat-transfer coefficients into a spray water flux distribution which can be linked tothe operating spray-nozzle variables. In this study, the authors maintained the surfacetemperature at 1100 °C and predicted the heat transfer coefficients required to achieve thiscondition. Higher values of heat transfer coefficients are required for the central regionsof the face in the upper spray zone than towards the bottom. Cooling requirements decreasetowards the corners because of the effect of two-dimensional heat flow. The coefficients45are smaller for the lower regions of the sprays because the rate of conduction of heat tothe surface is lower due to increased thickness of the solid shell. The effect of the spraylength on the magnitude of reheating of the billet surface was also investigated.The stand-off distance, which is the distance of the spray nozzles from the billetsurface, is important. The distance must be such that the edge of the water sprays coincidewith the billet corners. In addition, the alignment of the nozzles must ensure uniformcooling around the billet. The distance between the nozzles in each spray zone is dictatedby the need to maintain a uniform water-flux over the entire spray length. Mizikar [23]has shown that the water flux between two sprays is about 20 to 30 percent greater thanthe algebraic sum of the individual contributions of each nozzle. The calculation wasbased on the knowledge about water flux resulting from overlapping spray patterns. Spraypressure and spray angle are other variables that influence the spray water flux. Duringthe operation the pressure drop over each header must be small as compared to that througheach nozzle. Therefore, careful manipulation of the spray variables is necessary to optimizethe spray heat flux profile for each zone. Brimacombe et al. [62] have evaluated numerousempirical correlations for different spray configurations proposed by various workers.Van Drunen et al. [46] proposed that a sudden decrease in the centreline temperatureat the point of complete solidification generates stresses and strains that can give rise tocentreline cracks. The mathematical model is used to predict the location of the point ofcomplete solidification and the amount of cooling required to suppress the stressgeneration.(ii)  Spray Maintenance  : Regular maintenance of the spray cooling system is necessary toeliminate problems such as blocked or bent spray nozzles and also issues related to the46alignment of the spray nozzles. The blockage of spray nozzles due to poor spray waterquality not only introduces non-uniformity in the cooling conditions but also can causesurface reheating of the billet. Bent or damaged spray nozzles also disturb the uniformityin the cooling conditions around the billet and thus generates rhomboidity and diagonalcracks. The only remedy in this situation is the immediate replacement of the damagedspray nozzles. However, while replacing, care must be taken to ensure that the newnozzle selected is appropriate for that location to avoid non-uniformity in the coolingconditions.4.2.53 Liquid steelThe important parameters from the viewpoint of billet quality are the hightemperature strength of the steel and its cast structure. The high temperature strength ofsteel is related to Mn/S ratio in the steel. The severity of cracking problem reduces as theMn/S ratio is increased above 25 [64].The cast structure is linked to the steel superheat. The formation of columnarstructure is favoured at high superheat levels. A columnar structure is undesirable sinceit provides easy paths for crack propagation. Hence, operation with low superheat levels,preferably below 25 °C, is recommended provided the flowability of steel is not adverselyaffected. The influence of higher superheat levels is more significant in low and highcarbon steels as compared to the medium carbon grades [64].The presence of low-melting impurities such as copper, tin, antimony, etc. whichcause liquid-metal embrittlement is not desired. Copper is by far the most commonly47found contaminant in liquid steel. Copper levels greater than 0.2 percent are consideredto be undesirable. Nickel in steel is beneficial and a Ni/Cu ratio of 1.0 minimizes theseverity of this problem [72]. Pinch-rollsThe pinch rolls provide the necessary withdrawal action during casting. Theimportant parameters for the pinch rolls are the magnitude of pressure exerted on the strandand also the point of application. The pressure exerted on the strand is based on the forceneeded to pull the strand. However, if this pressure is excessive, it can generate tensilestrains close to the solidification front, and cause internal cracks.48CHAPTER 5 KNOWLEDGE DOMAIN - Part IIAnalysis of Cracks, Rhomboidity and Breakouts in Billet Casting5.1 Specific IssuesA particular quality problem may be strand- or grade-specific, may occur at specifictimes during a heat and may also have a specific orientation. These trends provide importantinformation about the causes of the problem that help identify related operating parameters.(a) Quality problem is predominant on certain specific strands  : If a quality problem ispredominant on certain specific strands as compared to the others, then the problem is likelyto be influenced by factors that differ from one strand to another. Assuming the same designand operating parameters for all the strands, factors that could differ are those related tomaintenance of the casting machine - mould, sprays and other accessories. Improper tolerances,misaligned mould tube, presence of scratch or gouge marks on the mould wall, wobblingoscillation system and plugged oil distribution slots are some of the major mould-relatedmaintenence issues. For the sprays, the condition of the nozzles (blockage or any physicaldamage) as well as the overall alignment with respect to the strand are important. When thequality problem(s) are more frequent on the inboard strands as compared to the outer ones, theproblem is likely related to the condition of the metal stream flowing into the mould. Thus,design and operation of the tundish are critical.On the other hand, if the quality problem appears on all the strands, then operatingvariables which are common to all the strands such as liquid steel quality, cooling water qualityand design parameters are likely to be the cause of the problem.49(b) Quality problem is predominant at certain specific locations on the strand:  When a qualityproblem appears on some specific region (corners or midface) of the billet, strongnon-uniformity in the operating conditions in the mould around the billet periphery, issuggested. Non-uniformity may exist in the heat-extraction rate, lubrication and moulddistortion. The presence of dark and bright patches on the billet surface is a clear indicationof non-uniform cooling.(c) Quality problem is predominant at a specific orientation:  A quality problem may appearat a specific orientation more frequently as compared to the others. This is related to non-uniformcooling conditions arising from some abnormal operating conditions of a permanent naturesuch as misalignment or mould distortion that does not vary with time. On the other hand,when its orientation changes with time, the governing phenomena is transient in nature.Intermittent boiling in the cooling water jacket which varies with time, is likely to be the causeof the problem.(d)  Quality problem is predominant at a specific time in the heat:  Depending on its cause, aquality problem may occur more frequently at some specific time in the heat. The higherfrequency of the problem in the initial stage of a heat could be related to excessive steel superheator to poor start-up practice - improper linkage between the dummy bar and the steel strand.Faulty engagement drastically reduces the negative strip time (sometime the stripping actionmay be totally be absent) and can lead to sticking of the solid shell in the mould.Higher superheat favours the formation of a columnar structure in the billet. If thefrequency of the problem, especially internal cracks, is higher in the initial stage of heats, itpossible that high superheat may be responsible. In addition, with thermal stratification of thesteel in the ladle, it is also possible to have higher superheat levels at the end of heats and50therefore, problems could be more severe in the later stage of a heat as well.(e) Ouality problem is predominant in certain specific grades of steel: The columnar-zonelength is a minimum for steels having carbon content in the range 0.20 to 0.38 percent andincreases sharply at carbon levels greater than 0.38 percent [64]. Also, the columnar-zonelength increases with decreasing carbon content below about 0.15 percent. Thus, qualityproblems influenced by a large columnar zone are less frequent in steel grades with carboncontent in range of 0.20 to 0.38 percent. In low carbon steels having carbon content in range0.10 to 0.14 percent, billet surface is rougher and the oscillation marks, deeper than in othergrades [61]. Thus, binding is likely to be worse in this grade because less heat is transferredto the mould because of a larger mould/shell air gap as a result of which cooling and shrinkageof the shell are less than in the case of other grades. In addition, steels having 0.17 to 0.24percent carbon exhibits a higher frequency of defects due to reduced ductility at elevatedtemperatures [61].5.2 Quality Problems5.2.1 CracksCrack formation is a major problem in continuously cast steel billets. Cracks havebeen observed at almost every conceivable location in cast steel as shown in Figure 5.1. Inthe interior, cracks may be seen near the corners, midway between the centre and the surface,at the centreline or diagonally between the obtuse corners. On the surface, transverse andlongitudinal cracks may appear in both the midface and the corner regions.51TRANSVERSE MIDFACE CRACKRHOMBOIDBILLETLONGITUDINAL MIDFACE CRACKCRAZE CRACKSTRANSVERSE CORNERCRACKLONGITUDINALCORNER CRACKDIAGONALCRACKCENTRELINE CRACKMIDWAY CRACKSPINCH-ROLL CRACKSOFF-CORNER INTERNAL CRACKSFig. 5.1^Schematic diagram of a rhomboid billet showing various types of cracks.52Another kind of surface cracking problem is craze cracks which are linked to highcopper levels in steel. The surface cracks pose a more serious problem than the internalcracks because, they do not reweld during rolling and the crack surfaces oxidize giving riseto oxide-rich seams in the rolled products. The internal cracks can also be a problemparticularly if during rolling, they do not close leaving voids in the steel products. Themechanism for the formation of different cracks has been established in numerous studies,most of which were reviewed by Brimacombe and Sorimachi [43]. Off-corner cracks werestudied by Brimacombe et al.[73] and Bommaraju et al.[64]. Internal CracksThe internal cracks result from high tensile strains and stresses acting on regions ofthe solid shell that are in the high temperature zone of low strength and ductility. Theyare influenced by sulphur and phosphorus content of steel and the superheat level. VanDrunen et al.[46] showed that the internal cracks are hot tears that initiate close to thesolidification front. Hence, the depth of the outer tip of an internal crack from the billetsurface directly reflects the local shell thickness at the time of its formation. On comparingthis with the solid shell thickness profile predicted by the heat-transfer mathematical modelfor billet solidification, it is possible to ascertain where in the machine the crack originated.The orientation of the crack gives the direction of tensile stress causing it as this stress isnormal to the direction of crack orientation.The internal cracking problems in billet casting have been summarized in Tables5.1 and 5.2.53Table 5.1 Internalaacksasug'nQUALITYPROBLEMSLOCATION CAUSES INFLUENCINGFACTORSOff-cornerCrackLower part ofmould or veryclose to mouldexitBulging of solidshell andhinging atoff-cornersThermo-mechanical behaviourof the mould; Adversemould/shell interaction; Deepand non-uniform oscillationmarks; Steel composition andsuperheatDiagonal Crack Spray zone Non-uniformshell generatedby the mouldThermo-mechanical behaviourof the mould; Adversemould/shell interaction;Asynchronous intermittentboiling in the mould; Deepand non-uniform oscillationmarks; steel composition andsuperheatAsymmetricspray coolingPoor spray design andmaintenence; Steelcomposition and superheatCentreline Crack Near the point ofcompletesolidificationSudden decreasein the centrelinetemperature atthe point ofcompletesolidificationInadequate spray cooling nearthe point of completesolidification; Steelcomposition and superheatPinch-roll Crack Close to thepinch rollsSqueezing on astrand withliquid coreExcessive pinch roll pressure;Steel composition andsuperheatUnbendingCracksClose to the pointof unbendingUnbending on astrand withliquid centreExcessive bending strains;Steel composition andsuperheat; High casting speed54Table 5.2^Midway Cracks in Billet CastingQUALITYPROBLEMLOCATIONS CAUSES INFLUENCINGFACTORSMidway Crack Mould exit or inthe gap betweenthe mould and thespraysReheating ofthe billetsurfaceMismatch between the mouldand the sprays: due to designor maintenance problem;Poor design of cooling jacketnear the mould exit; Steelcomposition and superheatUpper portion ofthe spraysReheating ofthe billetsurfacePoor spray maintenance :bent or plugged spraynozzles; Steel compositionand superheatLower portion ofthe sprays or theradiation coolingzoneReheating ofthe billetsurface due tothe spraysSprays : design andmaintenance issues; Steelcomposition and superheatReheating ofdark overcooledpatchesgenerated bythe mouldThermo-mechanicalbehaviour of the mould;Adverse mould/shellinteraction; Deep andnon-uniform oscillationmarks; Steel compositionand superheat5. Off-Corner CracksLocation - Off-corner internal cracks can be observed in the transverse section of thebillet shown in Figure 5.2. They occur roughly at a distance of about 15 mm from a55Fig. 5.2^Transverse section of a billet showing off-corner internal cracks.56given corner and 4 to 6 mm from the surface. These cracks appear randomly on anyone, or several, of the eight off-corner sites. The cracks may be seen together withsurface depressions and deep oscillation marks.Mechanism of off-corner crack formation - The position of the outer tip of anoff-corner crack indicates whether the crack formed in the lower part of the mould ornear the mould exit. Brimacombe et al. [67] postulated that off-corner internal cracksare generated by bulging of a given face of the billet in the lower part of the mould. Itwas also proposed that bulging of the solid shell may not extend to the corner but canhinge about the off-corner region. This hinging action generates a tensile strain at thesolidification front and leads to off-corner cracks. A schematic diagram illustrating thismechanism is shown in Figure 5.3. Very often, the outer tips of the off-corner cracksoverlap with white bands. This suggests that both phenomena originate at the same timeas bulging of the shell. The bulging of the shell disturbs the interdendritic liquid andcreates these white bands [64].Bommaraju et al.[64] linked the generation of off-corner cracks to the depth andthe uniformity of oscillation marks on the billet surface. In another study, Samarasekeraet al.[71] observed that the oscillation marks at the off-corner locations were deeperthan at the midface. Deep oscillation marks in the off-corner regions locally reduce theheat extraction rate and the shell growth as the billet passes through the mould. In thelower part of the mould where the mould/shell gap is large and the billet contractionhas virtually stopped, bulging can occur, particularly on faces having thin and weakshells in the off-corner regions. The resultant hinging action then generates tensilestrains57Coldcorner^Fe rrostati c preSsureLiquid poolOff - cornercrack (IISolid shel lMould wallFig. 5.3^Schematic diagram showing generation of an internal crack due to bulgingof the billet shell and a hinging action in the off-corner region [73].58Mould wallHingingandCrackFormation.111.■••••tFerrostaticPressurestrains at the solidification front near the off-corners. A schematic diagram illustratingthe significance of deep oscillation marks in the generation of off-corner cracks is shownin Figure 5.4 [681Deep oscillationmarkFig. 5.4 Schematic diagram illustrating the mechanism generating off-cornerinternal cracks due to bulging and hinging of the shell. Significance ofdeep oscillation marks in the generation of this problem is also evident[68].59Factors influencing ner cracking problem - Off-corner cracks result frombulging of the solid shell. Hence, factors that increase the mould/shell gap will influencebulging and affect the cracking problem. Off-corner cracking is also linked to theformation of oscillation marks. Therefore, any variable that influences the moulcYshellinteraction near the meniscus will directly impact on the nature of oscillation marks,and the cracks.Mould taper that does not follow the billet shrinkage profile leads to a largemould/shell gap which may cause a thin shell to bulge. Bulging is exacerbated bythermally generated reverse taper and wear in the lower part of the mould. At the mouldexit, if the solid shell is thin and weak, bulging can occur. This can be prevented byincreasing the water flux in the upper sprays which will cool the strand and increase thesolid shell thickness. The formation of deep oscillation marks is linked in a complexway to the thermo-mechanical behaviour of the mould tube during the process [71]. Thedistortion of the mould (both elastic and plastic) together with the lubrication conditionsand the oscillation characteristics, govern the mould/shell interaction near the meniscusand influence the nature of oscillation marks - depth and uniformity across the billetsurface. Off-corner cracks were randomly seen on the eight off-corner sites [64]. Thisobservation suggests that bulging of the shell is not localized to a single face. Thebehaviour may be due to intermittent boiling in the mould cooling jacket, wobbling ofthe mould system, poor setting of the foot-rolls or misalignment of the casting machine.Casting speed is important as it influences the solid shell thickness. Increasing thecasting speed reduces the shell thickness in the mould, thereby exacerbating the tendencyfor bulging of the shell. Since, the cracking phenomena is linked to dendritic separation,a low Mn/S ratio will increases the severity of this problem [64].605. Midway CracksLocation - Midway cracks are also called radial streaks, halfway cracks and ghost lines.They can be seen in the transverse section of the billet shown in Figure 5.5. They appearas dark lines running normal to a given face in a region midway between the surfaceand the centreline. As described earlier, the distance of the outer tip of the crack fromthe billet surface can be utilized to determine where in the machine the crack formed.It can occur close to the mould exit, in the gap between the mould and the sprays, inthe spray zone or in the radiation cooling zone.Mechanism of midway crack formation  - Midway cracks are caused by reheating ofthe surface of the strand which causes the billet surface to expand [47]. This expansionimposes a tensile strain on the interior, hotter regions of the solid shell which beingweaker, may crack. The stress distribution that results from reheating was calculatedusing finite-element analysis by Grill et al.[74]. The stress distribution in the billetclearly indicated relatively large tensile stresses parallel to the wide face near thesolidification front. In an another study, Brimacombe and Sorimachi [43] found thatthe location of the high stress region corresponds closely to that of midway cracks seenin the billet. Van Drunen et al.[46] postulated that midway cracks are hot tears whichform in the high temperature zone of low ductility. An examination of the inside surfaceof an open midway crack viewed with a scanning electron microscope showed smoothdendrite branches with no signs of deformation. This suggests the presence of a liquidduring the dendritic separation. This liquid is rich in sulphur and other elements witha positive segregation coefficient.61Fig. 5.5^Transverse section of a billet showing midway cracks.62E Actors influencing midway cracks - Three conditions appear necessary for midwaycrack formation: excessive reheating of the billet surface, the presence of a longcolumnar zone and a low Mn/S ratio in the steel. Reheating of the billet surface generatestensile stresses acting on the solid steel close to the solidification front. A columnarstructure provides easy paths for crack propagation. The Mn/S ratio directly impactson the high temperature strength and ductility of steel and the formation of liquid filmbetween the dendrites.Surface reheating occurs when the billet experiences a sudden reduction in theheat extraction rate after a period of excessive cooling. This may happen at the mouldexit, in the spray zone or at the end of the sprays. Reheating in the radiation coolingzone occurs due to spray- and/or mould- related issues. If spray cooling is excessiveover a short length, the billet experiences a sudden reduction in the heat extraction rateas it exits the sprays and enters the radiant cooling zone.A reheating problem in the radiation zone has its origin in the mould if the billetleaving the mould has dark and bright patches. The dark patches cool very rapidly inthe spray zone due to nucleate boiling. When the billet enters the radiation cooling zone,these over-cooled regions on the billet surface begin to reheat excessively leading tomidway cracks. These patches indicate non-uniform cooling in the mould which isrelated to the thermo-mechanical behaviour of the mould during operation. Hence,factors related to mould distortion, oscillation mark formation, taper and lubricationare important.Reheating can occur near the mould exit if the distance of the first spray nozzlefrom the bottom of the mould is large or if there is a mismatch between the heat fluxes63at the bottom of the mould and the first spray zone. Therefore, spray cooling at themould exit and the position of the first spray nozzle are critical parameters. When themould cooling jacket does not extend to the bottom of the mould, a drastic reductionin the heat extraction rate is likely to occur.Maintenance issues contribute to reheating if the spray nozzles are bent or blockedas a result of which there is a sudden reduction in the heat extraction rate. Midwaycracks in the upper spray region occur due to this problem. This conclusion followsfrom the logic that the mathematical model assumes ideal spray conditions and thereforecannot predict a high reheat for the upper spray zone. The only cause, for the midwaycracks in this region are issues related to maintenance. Similarly, if midway cracks areseen in the region between the mould and the sprays and if the reheat predicted by themodel for this zone is not high, it is very likely that issues related to spray maintenanceare a problem.Factors such as superheat and carbon content of steel, machine design (straightvs curved mould design) and section size are critical parameters influencing the columnarstructure in the billet. The presence of a sulphur-rich liquid film between the dendritesleads to easy separation of the dendritic arms. A Mn/S ratio greater than 25 minimizesthe cracking problem by preventing the formation of a liquid film. Diagonal CracksLocation - Diagonal cracks run between the obtuse corners of a rhomboid section [43].Sometimes these cracks may grow outwards towards the corners of the billet to formlongitudinal cracks depending on the magnitude of the strain and the thickness of the64solid shell.Mechanism of diagonal crack formation  - When two adjacent faces of a billet coolmore rapidly than the others, the billet contracts and generates a diagonal strain betweenthe colder faces. If this strain is large, the billet distorts and take on a rhomboid shapewith an acute angle between the colder faces and an obtuse angle between the hotterones. A crack may then initiate near the solidification front at right angles to the strainaxis which runs along the diagonal joining the obtuse angled corners.Factors influencing diagonal cracks  - Diagonal cracks result from the distortion ofthe billet due to unsymmetrical cooling. This is an extreme case of rhomboidity and itspresence clearly indicates severe asymmetric cooling in the sprays and/or the mould.Diagonal cracks are linked to mould cooling but the sprays are also definitely involved.Non-uniform shell growth in the mould generates cracking strains along the diagonaljoining the hotter corners, even with a perfectly aligned and a uniform spray coolingsystem.Cooling conditions in the mould are very important from the viewpoint of shellgrowth. The thermo-mechanical behaviour of the mould which influences the mouldshell interaction, directly impacts on the uniformity of heat extraction rates around thebillet periphery. Asynchronous intermittent boiling in the mould is also an importantphenomena leads to varying shell thickness around the billet periphery. Thus,mould-distortion, oscillation, lubrication and taper are critical. The design tolerancesof the mould tube, liners (baffle-tubes) and support system are important since theyinfluence the augment of the mould-tube.65The spray cooling may also be asymmetrical. If this is the case, maintenanceissues such as bent or plugged nozzles and their physical alignment with respect to thestrand are certain causes of diagonal cracks.According to Mori [75,76], the rhomboidity problem is generally greater insmaller size billets and also with higher superheat levels. Based on the same logic,diagonal cracks will also tend to be more frequent under these conditions. Centreline Crackslocation - Centreline cracks or core cracks appear in the central region of a cast sectionand form towards the end of solidification. Figure 5.6 shows a photograph of a transversesection of a billet showing centreline cracks.Mechanism of centreline crack formation  - A sudden decrease in the centrelinetemperature at the point of complete solidification generates strains which could causecracking [46]. An abrupt decrease in the centreline temperature occurs when the latentheat of fusion has been completely removed from the central region of the billet. Atthis point, the centreline cools faster than the surface as a result of which, the centreregion contracts. The centre is constrained from contraction by the surrounding coldersteel and is therefore put into tension. If reheating of the billet surface coincides withthe bottom of the pool, then the resulting tensile strains acting on the centreline will bethe sum of the two stresses and then the effect is likely to be large.factors influencing centreline cracks - The abrupt decrease in the centrelinetemperature, occurring as a result of complete removal of latent heat, is a naturalphenonema and will happen in any casting process.66Fig. 5.6^Transverse section of a billet showing centreline cracks.67The severity of this problem, however, can be reduced by minimizing thedifference in the cooling rates between the surface and the centre. Brovman et al.r77]suggested that centreline cracks could be suppressed by applying spray cooling at thepoint of complete solidification.When surface reheating coincides with the point of complete solidification, it isfirst necessary to isolate the two phenonema and then take appropriate steps to eliminatethem. Reheating can be suppressed, as was suggested for midway cracking, by adequatespray design and also by eliminating the dark and bright patches on the billet surface atthe mould exit. Spray design to minimize the stresses below the liquid pool has beenstudied by Grill et al.f78J. Pinch-roll CracksLocation - Pinch roll cracks can be seen on a sulphurprint of a longitudinal or a transversesection of a billet. They are very similar to midway cracks in their appearance exceptthat they always appear normal to the faces parallel to the axis of the pinch-rolls.Mechanism of pinch-roll crack formation  - The application of excessive pinch-rollpressure on a strand having a liquid core or with its centreline above 1340 °C, causescracking by dendritic separation in the high temperature zone of low ductility [43].factors influencing pinch-rolls cracks - The positioning of the pinch-rolls is a criticalfactor from the viewpoint of crack formation as it is necessary to avoid squeezing on aliquid centre. A safe location for the pinch-rolls can be predicted based on the modelcalculation of liquid pool depth. The cracking severity depends on the magnitude ofthe pressure exerted by the pinch-rolls on the liquid core. Therefore, reduction of the68roll pressure decreases the tensile stresses and hence minimizes cracking. Like anyinternal cracks, pinch-roll cracks are hot tears and therefore are influenced by the extentof the columnar structure and also by the high temperature mechanical properties ofsteel. Hence, superheat and steel composition are important. Surface CracksSurface cracks discussed in this section include transverse cracks (and depressions),longitudinal midface and corner cracks and craze cracks. A summary of these cracks areprovided in Table Transverse Cracks (and depressions)mcation - Transverse cracks are usually seen at the midface or near the corners on thebillet surface. Figure 5.7 is a photograph of a billet showing transverse depressions.Transverse depressions may or may not have a crack at their base. In the case of adepression without a surface crack, if a longitudinal section of the billet across thedepression is observed, an internal crack will be seen below the depression. Figure 5.8is a photograph of a longitudinal section of a billet across a depression showing aninternal crack below it.Mechanism of transverse crack (and depression) formation  - Samarasekera andBrimacombe [20] postulated that transverse cracks and depressions originate in themould. Due to sticking or binding of the billet in the mould, the withdrawal systemmechanically pulls the strand. The pulling action imposes a longitudinal tensile stresson the shell as a result of which a transverse crack can form close to the solidification69Table 5.3^Surface Cracks in Billet CastingQUALITYPROBLEMSORIGINS CAUSES INFLUENCINGFACTORSTransverse Crack(and depression)In the mould Pulling action on thestrand as a result ofbinding or stickingin the mouldThermo-mechanicalbehaviour of the mould;Adverse mould/shellinteraction; Deep andnon-uniform oscillationmarks; Steelcomposition andsuperheatLongitudinalCorner CrackIn the mould Reheating of thebillet corner due to alarge mould/shellgapand / orPresence of thin andweak shell at thehotter cornersLarge corner radius;Presence of corner "keyholes"; Mould tubealignment;Asynchronousintermittent boiling in themould; Deep andnon-uniform oscillationmarks;Thermo-mechanicalbehaviour of the mould;Adverse mould/shellinteraction; SteelcompositionLongitudinalMidface CrackIn the mould Excessive reheatingof a localizedportion of the billetsurface; Streamimpingement on afacePresence of scratch orgouge marks on the innersurface of the mouldwall; Misalignment ofmetal streamCraze Crack In the sprays Grain boundaryembrittlement due tothe presence of Cu,Sn or low meltingimpurities in steel.High level of Cu, Sn orlow melting impurity insteel70Fig. 5.7^Photograph of a billet showing transverse depressions.71front where the ductility is extremely small. The surface of the billet is in the temperaturerange 1150 to 1430°C and therefore has a high ductility [42,43]. Depending on themagnitude of the stress, the solid shell may flow plastically and form a depression onthe billet surface. This mechanism is similar to necking in tensile testing. Figure 5.9is a schematic diagram illustrating this mechanism. The internal crack, depending onthe solid shell thickness at the time of its formation, as well as the extent of necking,may penetrate to the surface.Transverse crack formation is linked to the nature of oscillation marks. In anearlier study [64], it was found that the obtuse angle corners of rhomboid billets havedeeper oscillation marks than the acute angled corners. This suggests that theobtuse-angled corners are hotter than the acute angled corners due to lower heat transferarising from a larger mould/shell gap. The colder corners having shallower oscillationmarks tend to be in closest contact with the mould. The billet can bind in the mould atthese corners and develop large axial stresses during the withdrawal of the strand.Depending on the magnitude of the force, transverse cracks may form at these corners.Factors influencing transverse cracks (and depressions)  - Transverse defects arelinked to sticking and/or binding of the billet in the mould. They are also related to thenature of the oscillation marks. Hence, the thermo-mechanical behaviour of the mouldand the mould/shell interaction at the meniscus as well as away from it are critical.Hence, design and operating variables influencing mould distortion, lubrication,oscillation characteristics and taper are important parameters.72Fig. 5.8^Longitudinal section of a billet showing an internal crack below adepression.73Binding(High Friction)HighDuctility LiquidSteel— CrackNecking ofDuctile Shellto FormDepressionZone ofLow DuctilityN./WithdrawalForceFig. 5.9^Schematic diagram showing the formation of transverse depressions andcracks in billet due to sticking or binding in the mould [20].74Mould taper is an important design variable that significantly affects heatextraction and binding in the mould. It is calculated based on a designed dwell time inthe mould. The successful implementation of a mould taper requires a strict control ofmetal level and casting speed. If the casting speed is reduced, the dwell time increasesand the influence of the thermal resistance of the shell increases with a decrease in theheat flux and the cooling rate. This reduces billet shrinkage and may influence binding.By the same logic, increasing the casting speed will reduce the tendency of the billetto bind. If the upper taper is too steep, then the metal level can be lowered to decreasethe dwell time and minimize the binding problem. The dwell time in the mould mustbe sufficiently long to prevent problems related to thin and weak solid shell- bulgingof the shell and generation of off-corner internal crack and perhaps, breakouts.In low carbon steels containing 0.10 to 0.14 percent carbon, the surface of thebillet is rougher due to shrinkage associated with the 8 — ?phase transformation and lessheat is transferred to the mould as a result of which cooling and shrinkage of the shellis small. Therefore, binding is expected to be worse in this grade as compared to theothers [61]. The frequency of transverse cracking was also found to be higher in steelshaving carbon in the range 0.17 to 0.24 percent due to poor ductility at high temperatures[61]. Longitudinal Midface CracksLocation - A longitudinal midface crack is located on the midface of a billet surfaceand is parallel to the withdrawal direction.75Mechanism of longitudinal midface crack formation  - The mechanism of longitudinalmidface cracking problem in billet casting is related to the presence of some scratch orgouge marks on the mould wall which locally increases the mould/shell gap adjacentto it. This leads to a drastic drop in the heat extraction rate over a small localized regionon the billet surface. This area reheats and expands as the surface temperature approachesthe high temperature zone of low ductility. The colder steel surrounding it prevents thisexpansion and results in the generation of tensile stresses at the solidification front whichultimately leads to cracking. Longitudinal midface cracks tends to form more easilywhere the air-gap resistance dominates the heat extraction rate (i.e. near the meniscus).Longitudinal midface cracks can also form due to stream impingement on a face due tococked or misaligned stream.Factors influencing longitudinal midface cracks - Scratches, gouge marks or anydefect present on the inside wall of the mould that increase the air gap resistance overa localized area on the billet surface will influence the formation of longitudinal midfacecrack. Misalignment of the metal stream is also an important issue. Longitudinal Corner CracksLocation - These cracks directly appear on the hotter, obtuse corners of the billet.Mechanism of longitudinal corner crack formation  - Longitudinal corner crackingoriginates in the mould. Two mechanisms have been proposed for this problem. Thefirst mechanism is based on the formation of a large mould/shell gap near the cornerswhich forces the billet surface to reheat to temperatures approaching the hightemperature zone of low ductility. The tensile stresses generated due to thermal76shrinkage and ferrostatic pressure lead to cracking if the ductility of steel above 1340°Cis exceeded [79 - 81]. Figure 5.10 is a schematic diagram illustrating this mechanismof formation of longitudinal corner cracks. The second mechanism is linked torhomboidity and thus, to the formation of oscillation marks on the billet surface [61].It was observed that corners with deeper oscillation marks are hotter at the mould exit.The presence of deep oscillation marks lead to the formation of a large mould/shell airgap which reduced the rates of heat extraction and solidification.Therefore, at the hotter corners, the shell tends to be thin and weak. Further,contraction of the cooling faces generates tensile strains in the transverse direction.Depending on the magnitude of this strain, cracks may initiate near the solidificationfront and penetrate the solid shell as a longitudinal corner crack.Factors influencing longitudinal corner cracks - Longitudinal corner cracking hasbeen observed in moulds with large corner radius, usually more than 4 mm. Sometimeskey-hole effects at the mould corners can also generate conditions for the formation oflongitudinal corner cracks. Asynchronous intermittent boiling in the cooling waterchannel can also generate longitudinal corner cracks on the billet surface as it leads torhomboid mould distortion, which varies with time as boiling events changeasynchronously on different faces and causes non-symmetrical cooling of the billet dueto the changing air-gap width [22]. In addition, since the longitudinal corner crack isalso linked to the nature of oscillation marks on the billet surface, its formation is relatedto the thermo-mechanical behaviour of the mould and to the nature of mould/shellinteraction. Factors related to the mould (distortion, oscillation, lubrication and taper)are critical.77strainstrainFig. 5.10 Schematic diagram showing the formation of subsurface, longitudinalcrack on diagonal at obtuse-angle corners of rhomboid billet [22].785. Craze CracksCraze cracks appear as a fine interwoven network of cracks as shown in Figure5.11. These are caused by grain boundary embrittlement. It is difficult to spot crazecracks on the billet surface during normal inspection. However, etching of the surfacewith a hot acid can reveal the presence of craze cracks. Craze cracks appear as finestriations on final rolled products such as wire-rods and ribbed-bars.It is believed that craze cracks are related to the presence of impurities with lowmelting point such as copper, tin and antimony in the steel. The mechanism of crazecracking, though not well established, appears to be linked in some way to theenrichment of the low melting point elements as a result of surface oxidation of the billet[72]. The high temperature oxidation of steel results in the formation of iron oxidescales. Copper being nobler than iron remains unoxidized and is concentrated at thesteel/oxide interface [82]. Since the oxidation rate of iron is high and the time for backdiffusion of copper is insufficient, the solubility of copper in austenite is exceeded. Amolten copper-rich phase forms at the scale/steel interface and penetrates into cracks atthe surface and along the grain boundaries [82]. In the sprays, cooling of the billetresults in thermal strains which generate cracks along the grain boundaries embrittledby the molten phase.It has been observed that the presence of nickel in the steel reduces the frequencyof craze cracking. Copper levels below 0.2 percent [72] or Ni/Cu ratio greater than 1.0are helpful in reducing the severity of this problem [82]. Two theories have beenproposed for the beneficial role of nickel in steels containing copper [82]. The first isrelated to the fact that nickel increases the solubility of copper in austenite. The second791states that nickel alloys with the copper-rich phase and reduces its melting point. Inboth, the formation of the low melting copper-rich phase is prevented and thus, crazecrack formation is avoided.Fig. 5.11^Photograph of craze cracks.805.2.2 RhomboiditySeverely rhomboid billets pose problems in pusher-type reheat furnaces and also duringrolling where the corners may fold over and generate seams in the final product. In squarebillets, the term "off-squareness" is often used whereas in rounds, this problem is called"ovality". The difference between the lengths of the two diagonals is a measure of the severityof this problem. In rounds, the difference between the longest and the shortest diameters isused to indicate ovality. An upper limit imposed by the rolling mills for this difference is inthe region of 6 mm. Table 5.4 is a summary of rhomboidity- mechanisms and influencingfactors.Table 5.4^Rhomboidity Problem in Billet Casting.QUALITYPROBLEMORIGINS CAUSES INFLUENCINGFACTORSRhomboidity In the mould Non-uniform shell Thermo-mechanicaland/or the generated in the behaviour of mould;sprays mould..Adverse mould/shellinteraction; Asynchronousintermittent boiling in themould; Deep andnon-uniform oscillationmarks; Mould-tubealignment; Steel superheatIn the sprays Asymmetric spraycoolingPoor spray design andmaintenence; SteelsuperheatMechanism of rhomboidity - The mechanism for the generation of rhomboidity, likeoff-corner cracking involves oscillation mark formation and non-uniform heat extraction in81the mould and the sprays. The problem usually begins with the formation of deep andnon-uniform oscillation marks around the billet periphery. In the vicinity of deep oscillationmarks, due to a wide billet/mould air gap, the rate of heat removal is low. On the otherhand, regions of the billet having shallow oscillation marks experience higher rates of heatextraction. Thus, the presence of non-uniform oscillation marks on the billet surface givesrise to widely differing heat extraction rates around the billet periphery which ultimatelyleads to a solid shell having non-uniform thickness. The situation of non-uniform heatextraction is further exacerbated by rapid cooling of some portions of the solid shell whichleads to shrinkage as a result of which the hotter corners of the billet get pulled farther awayfrom the mould wall. The billet exiting the mould, although reasonably square, has anon-uniform solid shell as shown in Figure 5.12 (a schematic representation of this idea)[64]. In the sprays, the colder portions of the billet, having a thicker solid shell, tend tocool faster than the hotter regions because of the greater thermal path and effects of unstableboiling which leads to non-uniform shrinkage of the billet and generates rhomboidity.The above mechanism is supported by the following observations made on squarebillets during the casting process :(i) the obtuse angle corners of rhomboid billets usually have the deepest oscillation marks.(ii) the billets emerging from the mould, when observed through the peep-hole, showedthat, of the two corners in view, one was cold (dark) and the other was hot (bright).Subsequent billet inspection on the cooling bed indicated that the acute-angle corners in thebillet corresponded to the colder corners whereas the hot corners formed the obtuse angle ofthe billet.82.■•^■.." ■..0"^ ■I' ■5%,,,^ ."-.....■...."■'NeiUpperSpraysOff-squarebillet containingoff-cornerinternal cracksFig. 5.12 Schematic diagram showing a billet with non-uniform shell thickness beingdistorted into rhomoid shape by spray cooling [64].83Asynchronous intermittent boiling in the cooling water channel has also been linkedto the generation of rhomboidity in the billet [22]. It is proposed that mould distortion varieswith time as the boiling events change asynchronously on different faces and causesnon-symmetrical cooling of the billet due to the changing air-gap width. The orientation andseverity of rhomboidity varies during the heat due to this phenomenon and so, supports theabove mechanism. Further, this observation is useful as it can be used to distinguishrhomboidity due to asynchronous intermittent boiling from that caused by other factors suchas machine alignment, wobbly mould oscillation, or poor spray cooling. In the absence ofany mould effects, rhomboidity must be caused by asymmetric spray cooling.Factors influencing rhomboidity - Since rhomboidity is linked to deep and non-uniformoscillation marks on the billet surface, factors related to the thermo-mechanical behaviourof the mould and the nature of mould/shell interaction are critical. Hence, mould distortion,taper, oscillation characteristics and lubrication parameters are important. Factors such astolerances with respect to the water gap, mould tube alignment and the nature of the mouldconstraint system also need special attention.Even though the mould generates rhomboidity, the difference between the lengths ofbillet diagonals at the mould exit is small. This is expected since the rhomboidity in the billetat the mould exit cannot exceed the rhomboidity of the mould. The degree of rhomboiditycan then increase significantly as the billet moves through the spray cooling zone. However,the contribution of the mould to generate a non-uniform shell and therefore rhomboidity, isquite significant. Hence, mould parameters (operating, design and maintenance) areextremely critical. When rhomboidity is caused by asynchronous intermittent boiling andmould-distortion, simple suppression of the boiling phenomena can help to minimize the84problem. In this case, factors influencing asynchronous boiling such as cooling watervelocity, non-uniform scale deposition on the cold wall of the mould, surface roughnessand mould design parameters such as wall thickness and support system are critical.Spray cooling influences rhomboidity in two ways. First, it acts on the non-uniformshell generated by the mould and further aggravates non-uniformg heat extraction, unevenshell growth and the rhomboidity problem. Thus, when the mould effects are present, thesprays influence rhomboidity under all circumstances- whether symmetric or asymmetric.However, when the spray cooling is symmetric, the mould is the only cause of rhomboidity.Second, in the absence of any adverse mould effects, asymmetric spray cooling gives riseto rhomboidity. Non-uniform spray cooling is usually a maintenance problem and occursdue to plugged or bent nozzles and poorly positioned spray risers.5.2.3 BreakoutsBreakouts occur when there is a rupture of the solid shell as a result of which moltenmetal flows out. This is a serious problem as it not only hampers productivity but is also ahazard. Table 5.5 is a summary of factors influencing breakouts.Mechanism of breakouts - Breakouts are linked to the presence of a thin and weak solidsteel shell at the mould exit. The shell may be thin at localized regions or all around thebillet periphery. The presence of cracks may also lead to breakouts, particularly if the cracksact as stress concentration sites and aid the rupture of the shell.The mechanism by which thin shells are generated over localized regions in the billetis based on the formation of a large mould/billet air gap that reduces the rate of heat extractionand retards the shell growth. These thin regions of the shell tend to be hotter as compared85to the areas with thicker shell. The presence of depressions or deep oscillation marks on thebillet surface increases the mould/billet air gap which in turn influences the generation ofthin shells especially in those regions of the mould where thermal resistance of the air gapdominates the heat transfer.Table 5.5^$reakout Problem in Billet Casting. QUALITYPROBLEMORIGINS CAUSES INFLUENCINGFACTORSBreakouts Close toobtuse-anglecorners of thestrandThin shells generatedat the obtuse anglecorners due toasymmetricalcooling in the mouldThermo-mechanicalbehaviour of the mould;Adverse mould/shellinteraction; Deep andnon-uniform oscillationmarks; Mould-tubealignment; Steel superheatClose totransversedepressions ordeep oscillationmarks on thebillet surfaceLocal reduction inthe shell thicknessdue to the presenceof depressions ordeep oscillationmarksAdverse mould/shellinteraction; Deep andnon-uniform oscillationmarks; Thermo-mechanicalbehaviour of the mouldClose to a weakspot in the shellInadequate shellthickness at themould exitInsufficient dwell time inthe mould; Entrapment ofslag or scum between mouldand billetMould overflow Operator error Poor metal control86Breakouts in billet casting have been evaluated by Samarasekera and Brimacombe[83]. The results of this study indicate that breakouts occur close to transverse depressions(with cracks) and also very close to the obtuse corners which are hotter and have a thinnersolid shell. This finding supports the above mechanism quite adequately.Breakouts have also been related to the entrapment of a scum between the steel andthe mould-wall. This leads to the formation of an extremely thin shell adjacent to it. Onceoutside the mould system, it is possible that this region of the shell may rupture and causebreakouts. Mould-overflow which is mainly an operator error may also cause breakouts.Factors influencing breakouts  - Breakouts are directly linked to the nature of shell growthwhich are further related to the characteristics of the oscillation marks. The formation ofoscillation marks is related to the thermo-mechanical behaviour of the mould and to thenature of mould/shell interaction. In the above study, Samarasekera and Brimacombe [83]related breakouts to sticking in the mould, caused by poor mould lubrication, which resultsin the formation of transverse depressions (and cracks) and also to non-uniform coolingwhich cause the development of hot and thin corners. Therefore, factors influencing moulddistortion, taper, lubrication parameters and oscillation characteristics are important issues.Factors causing breakouts can be identified by examining the breakout shells. Whenbreakouts occurs close to transverse depressions (with cracks), it suggests strong linkageswith the mechanisms involving transverse depressions. Hence, sticking in the mould dueto poor lubrication or due to poor stripping action are important. In other situations whenbreakouts occurs close to the obtuse-angled corners, non-uniform cooling becomes a keyissue. Non-uniform cooling arises from mould-tube misalignment, poor tolerances or design87parameters. It can also be related to deep and non-uniform oscillation marks. Higaki etal.[84] suggested that in billet casting, breakouts can be minimized by effective lubricationand by maintaining proper mould alignment.The dwell time in the mould is an important parameter which must be selected so thatthe shell thickness at the bottom of the mould is sufficient to counter the ferrostatic pressureexerted by the liquid core. Dean [85] has emphasized the importance of using an optimummould length. This is because a mould that is too short will result in a thin shell, whereasexcessively long moulds give rise to a higher frequency of breakouts from corners, throughcracks which occur during resetting by the roller apron. The dwell time in the mould can bealso be controlled by changing the position of the metal level or by changing the castingspeed. Metal level fluctuations, by influencing mould lubrication, dwell time and castingspeed, indirectly affect the nature of oscillation marks and therefore, breakout problems.Proper metal level control has been proposed to minimize breakouts [84,86].5.3 Other ObservationsObservations made during casting can be used to focus the diagnosis, verify predictionsfrom the mathematical model and also keep a check on conflicting inputs provided for analysis.Thus, they provide a means to integrate two kinds of, very diverse, expertise: fundamentalknowledge about the casting process and operating experience in the plant.(a) Dark and bright patches on the billet surface at the mould exit :  In the case of rhomboidity,the billet emerging from the mould has bright and dark corners. The bright corners which arethin and hot, become the obtuse-angled corners of the rhomboid billet. On the other hand, thedark corners are colder and form the acute-angled corners. This indicates varying amounts of88solidification, due to non-uniform cooling around the billet periphery. Non-uniform coolingconditions arise due to misaligned mould tube and/or non-uniform mould distortion. It couldalso be related to non-uniform oscillation marks- deeper oscillation marks at the hotter corners.In addition, the presence of dark and bright patches on the billet surface at mould exit can leadto the formation of midway cracks. The dark patches which are overcooled regions on the billetsurface, tend to cool very rapidly in the spray zone due to nucleate boiling and later, reheatexcessively in the radiation zone.(b) Scale deposits on the cold face of the mould : The presence of scale deposits on the coldface of the mould wall indicates that the cooling water is contaminated. The colour of thesedeposits is indicative of the nature of contamination. The uniformity of scale deposits aroundthe mould periphery is a check for misalignment and can be employed to assess the nature ofthe cooling.(c) Change in billet colour in the radiation zone :  A change in the billet colour from dark redin the spray zone to bright red in the radiation zone, is indicative of reheating of the billetsurface and is very likely to cause midway cracks in the billet. This information can be usedto verify the magnitude of reheat predicted by the mathematical model.(d) Breakout shell  : Breakouts can occur close to transverse depressions, cracks, deeposcillation marks, laps, slag patches and even thin and hot billet corners. Examination ofbreakout shells can be used to infer about the possible cause(s) of breakout.(e) Condition of metal stream flowing into the mould :  A ropey metal stream leads to excessivegas entrainment in the liquid steel. In the mould, these gases escape from the metal surface89as bubbles creating turbulence and therefore, metal level variation. This is also indicative ofsome deficiency in the tundish design with regards to controlling flow of metal to the inboardand the outboard strands.(f) Distribution of pinholes on the billet surface  : The presence of more pinholes in some regionsof the billet as compared to others indicates hydrogen pick-up due to excessive lubricating oilon that face. If this problem was related to liquid steel, it would have resulted in a uniformdistribution of pinholes on the billet surface. A deficiency in the lubrication system is indicative.(g) Position of chromium layer discolouration in a used mould  : This indicates the position ofthe meniscus during the operation. This information can be used to cross check the metal levelduring the operation and can be used to assess the efficiency of the metal level control systemin use.90CHAPTER 6 KNOWLEDGE ENGINEERING6.1 Knowledge AcquisitionKnowledge acquisition consisted of four principal stages: identification/definition of theproblem, acquisition of background knowledge on continuous casting, development of theexpert system and testing and validation in the field. The system has not been put into actualuse and therefore the last two stages described in Chapter 3 are not applicable at the presentmoment.6.1.1 Identification/Definition of Problem DomainThe problem identification stage was driven by the experts who wanted a system tohelp transfer their knowledge to industry. The experts defined the problem domain and theneed for an expert system to diagnose quality problems in continuously cast steel billets andto train less experienced operating personnel. It was envisaged that the system could be usedas a consultant by users and so provide the experts with more time for other activities. Thus,the problem modality was well-defined and the experts were ready and willing to participatein the development process.6.1.2 Background Knowledge on Continuous CastingThe experts provided the knowledge engineers with literature on continuous castingof steel billets - operations, design and product quality. One of the knowledge engineersattended a short course provided by the experts and their colleagues in Vancouver. Thisestablished a practical understanding of the problem domain and helped considerably in91subsequent discussions with the experts during development. Buchanan et al. [2] have pointedout that communication problems during knowledge acquisition can be severe when theknowledge engineer has far less knowledge about the domain than the experts. In this work,adequate steps were taken to ensure that this was not an issue.An outline of quality problems, comprising the origin of defects, their causes offormation and suggested remedies for prevention was prepared by the Knowledge Engineers.This preliminary categorization have been summarized in Tables 5.1 to 5.5. Although thesetables contain the major elements of the knowledge domain, the methodology for diagnosisis much more complex than a simple "look-up" table. With a desire for a system possessinghuman-like thought and communication processes, considerable efforts was expended tostructure the actual approach used by the Experts in the analysis of quality problems.6.1.3 Expert System DevelopmentThe problem domain consisting of cracks (internal and surface), rhomboidity andbreakouts was initially broken down into individual modules- each of which could be workedon independently. Accordingly, the development process began with an examination ofmidway cracking as this problem was considered to be simple and well-understood. TheExperts initiated the knowledge acquisition process by identifying that midway cracks werecaused by reheating of the billet surface. The focus then, was aimed at analysing spray-relatedissues that are less complex than mould disorders. It quickly became apparent, however,that poor mould operation could also contribute to midway cracks. In the end, midwaycracking turned out to be the most complicated problem from the viewpoint of diagnosis andknowledge representation.92The experts outlined the steps followed in dealing with midway cracks and explainedthe significance of operating parameters involved at each stage. The initial scheme to analysemidway cracks originating in the radiation zone is summarized in Table 6.1. Factors such asspray length, water flow rate, steel temperature and composition were considered to beimportant.Table 6.1 h in the Radiation Zone. STAGES DETAILS1 Input Distance of the outer tip of the crack from the billet surface2 Compare Measured distance with solid shell thickness profile3 Establish Location of crack origin in the machine4 Input Model-predicted billet surface reheat for radiation zoneSpray parameters - zone length and water flow rateHigh temperature strength of steel - Mn/S ratioCast structure - Superheat level5 Analyse Compare input parameters with values specified by the experts6 Correlate Generation of midway crack with the operating parameters7 Conclude Conclusions and justificationBased on this information, a small prototype was developed. Almost immediately,questions were raised about the solid-shell thickness profile and the surface temperaturedistribution. In the expert system, it was assumed that the users would have prior informationabout these predictions. At this point, the system simply informed the users when operating93parameters were high, low, or acceptable, and therefore, its usefulness was nothing morethan a check-list with input based on intuition rather than measurements or analysis. Thus,the first stage of implementation was not very successful.The knowledge base was reviewed and emphasis was re-focused on the consultationprocess used to deal with a client rather than the actual details of the data. The system wasmodified to provide conclusions and suggest remedial actions in the same way as the experts.The knowledge base was re-structured to facilitate examination of all parameters in theanalysis. In addition, information for the users was incorporated into the system in the formof explanations, screen-messages and rule-descriptions, using the user-interface facilitiesof COMDALE/X. This approach was acceptable to the experts.At this stage, the expert system was capable of handling only typical situations whichcause midway cracks to form - high reheat, high superheat and poor strength at hightemperature. The Experts were consulted on other situations where midway cracks mightbe seen such as; when both spray and steel parameters were satisfactory, when only steelparameters were satisfactory and when only spray parameters were satisfactory. The Expertsfelt that it was impossible for midway cracks to form when both spray and steel parameterswere satisfactory. However, when a problem related to steel quality was evident but sprayparameters were satisfactory, the Experts believed that reheating must be related to a moulddisorder. Under these circumstances, the presence of dark and bright patches on the billetsurface at the mould exit would confirm a mould-related problem. These overcooled regionscan reheat excessively in the radiation zone, leading to midway cracks. Therefore, a majorexpansion in the system was necessary to include knowledge dealing with mouldparameters (distortion, oscillation, taper and lubrication).94It was pointed out that the Users would not be in a position to predict surface reheatlevels and the solid shell thickness profile. A heat-transfer mathematical model for billetsolidification was developed to predict these profiles. The model uses operating parametersas inputs and predicts solid shell thickness and maximum reheat temperatures at key locationsin the casting machine. The details about the mathematical model have been discussed earlier.A typical output profile for a test situation is shown in Figure 6.1. Here, one-quarter of thebillet section was modelled using a 30 x 30 mesh and a time increment of 0.10 second.Until this stage, the focus was on midway cracks originating in the radiation zone.But these cracks can also originate in the spray zone and in the region between the mouldand the sprays (submould). It is difficult to visualize the origin of midway cracks in the sprayzone because cooling rates in this region are high and therefore, the billet surface is unlikelyto reheat. If midway cracks do originate in the spray zone, the Experts surmised that theonly factor which could influence surface reheating is poor spray maintenance- damaged orblocked spray nozzles. The system was expanded to include the above situations. Thiscompleted the prototype for midway cracks and the next problem was selected fordevelopment.It was pointed out by the Knowledge Engineer that more than one quality problemmight be observed at the same time. This meant that the plan to have a separate module foreach problem was inappropriate. There was need for a general strategy to handle anycombination of quality problems.958001-/1550132511000^5^10^15^20Distance from the meniscus ( m )25Fig 6.1^A typical output of the mathematical model showing predicted solid shellthickness and billet surface temperature profiles. (Casting speed- 25 mm persecond; Section size- 150 mm square)Development of Strategy for Handling Combinations of Quality ProblemsBased on knowledge about factors that influence the formation of quality problems inbillets, the domain was divided into two broad groups. The first group consisted of thoseproblems that require a detailed analysis of the mould and/or the spray cooling system.Off-corner cracks, transverse cracks and depressions, midway cracks, rhomboidity, diagonalcracks and breakouts are included in this category. The second group deals with very specific96issues; a detailed analysis of the mould and/or the sprays is not needed. Longitudinal cornercracks, longitudinal midface cracks, pinch-roll cracks, centreline cracks and craze cracksare in this group.The knowledge base was organized into three modules as shown in Figure 6.2 to handlecombinations of quality problems and to accommodate the memory limitations of the DOSoperating system. This classification is based on three different approaches identified forthe diagnosis of quality problems that require detailed mould and/or spray analysis. Module-1is designed to deal with situations where no midway cracks are present but any combinationof off-corner cracks, transverse cracks and depressions, breakouts, rhomboidity and diagonalcracks are observed. In this situation, only a detailed analysis of the mould is necessary,although the system can provide warnings about possible spray maintenance problems.Module-2 examines cases that include midway cracks with any other type of quality problem.Mould issues are examined first followed by a detailed spray analysis to address midwaycracking problem. Module-3 deals with midway cracks as the sole quality problem. In thiscase, the sprays are examined first. If a spray problem is uncertain, then mould issues areexamined. As well, the mould may be examined if required by inference of the system orby request of the user. If such a request is not desired but the system detects certain moulddisorders, a severe warning message is issued. In all three modules, there are rules dealingwith quality problems that do not require detailed analysis of the mould or the sprays.Adverse mould-shell interaction is the root of all mould related problems. A thorough"breadth-search" of factors related to adverse mould-shell interaction, both at the meniscusand away from it, is conducted. Figure 6.3 shows the factors examined.97Midway CracksNO YESilleb BeliefHigh DisbeliefVacertaleDOES SPRAY PROBLEMEXIST ?QUALITY PROBLEMS REQUIRINGDETAILED ANALYSISASCERTAIN CRACKORIGIN DETAILEDSPRAY ANALYSISOff-corner CrackTransverse DefectsRhomboidityBreakoutsMidway CracksDETAILED DETAILED DETAILED MOULD ANALYSISMOULD ANALYSIS MOULD ANALYSIS MOULD ANALYSIS NEEDEDDETAILED MODIFICATION OF BELIEFSPRAY ANALYSIS IN A SPRAY PROBLEMI CONCLUSIONFig 6.2^Scheme for diagnosing quality problems requiring detailed mould and/or sprayanalysis.98AWAY FROMMENISCUSMOULD TAPERADVERSE MOULD-SHELLINTERACTIONAT MENISCUSOSCILLATIONCHARACTERISTICSMOULD DISTORTION1: Cooling Water Velocity2: Cooling Water Quality3: Mould Tube Alignment4: Type of Mould Constraint5: Mould Wall Thickness6: Position of Metal Level7: Mould Copper8: Mould Design Tolerances1: 011 Flowrate2: 011 Distribution3: OU-Phydcal Properties4: OU System Cleanliness5: Cleanliness ((MeniscusRegion6: Mould Cooling WaterTemperature1: Negative Strip Time2: Mould Lead3: Nature of Oscillation Marks4: Metal Level VariationMOULD LUBRICATIONFig 6.3^A general outline of mould-related factors contributing to quality problems inbillet casting.The important issues are mould-tube distortion, oscillation characteristics, mouldlubrication and taper. Mould-tube distortion is related to cooling water quality and velocity,the nature of the mould constraint system, copper composition, wall thickness, metal level,tube alignment and design tolerances [68]. The analysis of oscillation characteristics involvesevaluation of negative strip time, mould lead and the nature of oscillation marks - depthand uniformity across the billet surface. Important parameters dealing with lubrication in99the mould are oil flow conditions (flowrate and distribution) , physical properties of oil(boiling point, flash point and viscosity), and the maintenance of the oil-distribution system.Finally, an analysis of the type of taper and its magnitude is conducted. Taper can influencethe formation of both off-corner cracks and transverse depressions and cracks [68]. A detailedspray analysis is done to analyse the cause of midway cracks. In this methodology, positionsof the outer tip of the midway cracks is compared with the predicted solid shell thicknessprofile and the crack origin sites in the machine are determined.This development had a major impact on the project as it presented to the Experts astructure that was very similar to their own. The Experts were able to observe links betweenvarious quality problems that had not been considered previously in the analysis. This strategywas also beneficial as it significantly reduced the amount of coding necessary as per theearlier plan since only three modules were needed.6.2 Knowledge Representation6.2.1 Knowledge unitsThe development tool used for this expert system is COMDALE/X version 3.0. Themain knowledge units available in this tool are shown in Figure 6.4. Here, facts are storedas keyword triplets. Procedures are used to control the inference process - breadth searchversus depth search, forward chaining versus backward chaining and they are an importantfeature of this tool. The tool provides considerable flexibility in knowledge accumulationwhich are domain-specific [87]. It also allows specific facts to be represented as exclusivesets, fuzzy sets, restrictions, default values, etc.100KNOWLEDGEClassesI--- Objects — Attributes — Values — Degree of BeliefKeyword Triplets^ LogicalStringE Exclusive sets^ Multichoice setsFuzzy setsNumericRules— Condition Statements— Else Statements— Conclusion StatementsE Single  DoubleIntegerKeyword TripletsLogical ConnectivesPredicates / OperatorsFunctionsKeyword TripletsCertainty FactorsAssignmentsFunctionsProceduresMeta-KnowledgeSearch StrategiesInference StrategiesInput/Output StrategiesCommunication ControlTriplet RepresentationsCustomized QuestionsCustomized Rule DescriptionsCustomized ExplanationsFig. 6.4^Knowledge units available in COMDALE/X Developent Tool.1016.2.1.1 Keyword TripletsA keyword triplet combines an object together with a particular attribute and value.Objects are physical entities in the real world such as sprays for cooling the billet. Objectshave attributes which describe properties or characteristics such as "length" and "flow"for sprays. A value is attached to the object-attribute pair such as "short" and "satisfactory"for "spray length". Classes represent hierarchical relationships between objects. Allobjects in a class automatically inherit the attributes of that class. Examples of Keywordtriplets are as follows:OBJECT ATTRIBUTE VALUEspray^length^shortspray^length^satisfactoryspray^flow^excessivespray^flow^satisfactoryA degree of certainty, which is a number ranging from 0 to 100, is attached to eachkeyword-triplet and this number determines whether the keyword triplet is significant ornot at this stage of instantiation. Initially, the degree of certainty of the keyword tripletis "not known". When information about a keyword-triplet is needed but the degree ofcertainty is "not known", the system is forced to search through rules in the knowledgebase. Keyword triplets together with degrees of certainty fonn the fundamental basis forrepresenting knowledge as well as providing a driving force for the system in its searchfor knowledge during system-execution.Mutually Exclusive Sets:  In the knowledge base, there are numerous keyword-tripletswhich have common states of an Object's Attribute.102For example, the following keyword triplets describe different materials used asmould material:mould^copper^Pure Coppermould^copper^DHP_122mould^copper^DLP 120mould^copper^Copper Silver Alloymould^copper^STP_Grademould^copper^Cr Zr Coppermould^copper^Special Alloysmould^copper^Other GradesHere, when one value has been instantiated with definite certainty, there is no needto search for the degree of certainty of the other possibilities. This is prevented in thesystem by defining these keyword-triplets as mutually exclusive sets. By doing this, theremaining keyword-triplets in the set are automatically assigned a certainty of 0.Fuzzy sets: There are occasions, when keyword-triplets with common object-attributepairs, but different values, are not mutually exclusive. Also, there can be a need to allowflexibility in defining multiple sets with varying degree of certainty depending upon anumerical value assigned to the keyword triplet. For example, three keyword-triplets areset up to characterize the negative strip time based on the operating parameters :calculated negative strip time^highcalculated negative strip_time^okaycalculated negative strip_time^lowEach numerical negative strip time value is assigned a membership percentage ineach of these sets which is further interpreted as the degree of certainty of the respective103keyword-triplet. Fuzzy sets have been created to represent extremely subjective conceptsin the knowledge base. The fuzzy sets in the knowledge base have been defined based onthe experts' definition of the subjective concepts. The advantage of fuzzy sets is that itallows the knowledge engineer to program the entire set of conditions, associated withthe subjective concept, using a much reduced rule set. RulesIn COMDALE/X, keyword triplets are linked through rules. A rule is a statementcomposed of a premise and a conclusion. A rule may contain more than one premise andconclusion statement. Each condition of a premise or conclusion consists of akeyword-triplet in which the attribute and value are separated by a predicate function. Forexample:IF^midway_crack origin^IS^radiant zoneAND^radiant model reheat^IS^high_AND^billet colour^MIGHT BE bright redTHEN radiant zone model reheat^IS^high CF = 100 #When a rule is fired, the inference engine examines each condition in the premiseto establish the "degree of truth" about each statement. The degree of truth is calculatedas a function of the degree of certainty of the keyword-triplet and the predicate functionbeing used. After examining all the conditions in the premise, a net degree of truth isdetermined. The product of net degree of truth and the certainty factor associated witheach statement of the rule conclusion is assigned to the degree of certainty of thekeyword-triplet in the conclusion of the rule. COMDALE/X also has the facility to104establish complex mathematical relationships between the degrees of truth of each premisestatement and the degree of belief in a conclusion fact. This unit is known as an "inference"knowledge unit. ProceduresProcedures refer to instructions which perform special functions during the executionof the expert system. These are similar to conventional programming where the sequenceof tasks to be performed, as is the case with batch-processing, can be controlled througha series of statements. Table 6.2 is a list of library functions of COMDALE/X that wereused in developing this system and the tasks each perform. Meta-knowledgeMeta-knowledge or knowledge about facts have been extensively used in theknowledge base with the purpose of providing technical assistance as well as training tothe users. The user interface feature of COMDALE/X, which provides features likecustomized questions, rule descriptions and explanations, have been extensively employedin the knowledge base. An example of each related to the presence of dark and brightpatches on the billet surface at the mould exit is given in the following paragraphs.Customized Question : In the analysis of midway cracks, before the examination ofmould-related factors, the system asks the user whether any dark patches are seen on thebillet surface at the mould exit. The customized question posed by the system is:Do you see any dark patches on the billet surface as it comes out of the mould?105Table 6.2 procedural Knowledge Units: Library Functions in COMDALE/X used forSystem DevelopmentACTIVATE Executes external programs or MS-DOS commands.APPLYRULE Conduct a breadth-search in which every rule containing a specificfact in its premise is fired.ASNCERTAINTY Assigns the degree of certainty of a keyword triplet to a numericalkeyword triplet.AS NVALUE Assigns the value held by a numerical keyword triplet to anothernumerical keyword triplet.DISPLAY Displays the content of a document file of COMDALE/X.EXPORT Creates a file and writes data to it.FIND Conduct backward search for a particular fact. Here, every rulecontaining this fact in its conclusion is fired.FORGET Forgets a fact (key-word triplet) that was instantiated during theconsultation.FREERULE Makes already evaluated rules available for future examination.GOTO Ditcates the next rule to be examined.HALT Terminates the consultation.IGNORE Ignores rules or rulesets in the knowledge base.IMPORT Reads data from a file.LOAD Loads a new knowledge module.MACRO Dictates the next rule to be examined and then returns control to thepresent rule.TEXT To present textual material.VALUECERTAINTY Assigns the degree of certainty of a keyword triplet to the value of anumeric keyword triplet.106Customized Explanation : The user may request information on dark patches beforeanswering the above question. The customized explanation facility of the system providesthe user with a text message containing knowledge about this subject:These are dark regions present on the billet surface as the billet exits the mould. They aredark because of excessive localized cooling in the mould as a result of a mould disorder.Their presence suggests that there are problems in the mould cooling system. These darkpatches cool very rapidly in the spray zone due to nucleate boiling. When the billet entersthe radiation cooling zone, these over-cooled regions on the billet surface begin to reheatexcessively leading to midway cracks. It should be noted then, that poorly designed spraysare not the only source of midway cracks.Customized Rule description : The customized rule description has been used to providethe user an explanation of "why" a particular question is being asked. This is the englishversion of the rule currently being examined by the system. In response to a "why" fromthe user for the above question related to the presence of dark patches, the following textmessage is presented:If dark patches are seen on the billet surface as it leaves the mould, then an adversemould-shell interaction is confirmed.6.2.2 Search Techniques and Conflict ResolutionBy default, COMDALE/X uses a forward chaining depth-first strategy that allowsbackward-chaining to interrupt the search for a fmal conclusion whenever sub-goalinformation is required. However, the tool allows backward-chaining and breadth-searchcontrol strategies to occur from rule conclusions through the use of library functions such as107"FIND" and "APPLYRULE". A breadth search has been used for the analysis ofmould-related parameters, where all possible factors such as distortion, taper, oscillationand lubrication need to be examined to arrive at the correct diagnosis. A backward-chainingapproach has been selected to modify belief in a spray-related problem for midway cracksbased on reheat temperature, superheat and high temperature strength of steel.The accuracy of the heat-transfer model is limited only by certain assumptions madeabout the physical characteristics of the casting machine and the associated coolingconditions. For example, significant blockage of spray nozzles could lead to localizedreheating when the model is suggesting no reheating. Mould design and operating factorscould also contribute to reheating in the radiation zone by producing a non-uniform solidshell in the billet. These non-uniformly cooled regions appear as dark and bright patches onthe billet surface at the mould exit. As pointed out earlier, these dark patches reheatsignificantly in the radiation cooling zone. The model cannot predict reheating in thesesituations.Measurement estimates and the large subjectivity associated with visual observationsputs increased doubt on the reliability of these sources of information. Therefore, factsobtained in these ways are cross-checked by the system wherever possible. Whencontradictions do occur, the system resolves them in a manner comparable to the experts.An example is the conclusion reached about the severity of a rhomboidity problem. At thestart of the consultation, the expert system asks the user to select his choice ("yes" or "no")from a list of quality problem(s) present in the billet. Let's say, the user said "no" torhomboidity as well as "no" to diagonal cracks. The system verifies these inputs by askingthe user to enter the difference in the lengths of billet diagonals. If the user enters a valuewhich is large as per the experts' definition, the system concludes that rhomboidity is severe.108entered a small value for the difference in the lengths of billet diagonals, the system informsthe user that rhomboidity is severe and that the measurements is incorrect. This is donebecause the presence of diagonal cracks in the billet is directly indicative of a severerhomboidity problem.The following example illustrates a conflicting situation encountered during knowledgeprocessing and the approach adopted for its resolution. Let's assume that the midway crackorigin corresponds to the upper part of the spray cooling zone. This clearly suggests reheatingof the billet surface in this zone. However, the model does not predict excessive reheatingfor this region because it is assumed that all parameters are satisfactory. This predictiontherefore does not conform to the conditions prevalent in the operation. This situation isresolved by concluding that spray nozzles, particularly those lying on the same side of thebillet as the crack, are plugged or damaged and that the cooling is insufficient.6.2.3 Information used in the analysisThere are three different sources of information used by the system namely; user-input,calculations from external programs and facts inferred from rules in the knowledge base.Information from users  : Users enter the majority of data through an external program. Thisinformation relates to the operating parameters such as steel composition, castingtemperature, casting speed, oscillation frequency, oscillation stroke, billet dimensions,mould cooling water flow rate, spray cooling water flow rate and lubrication oil flow rate.Other parameters such as machine radius, position of spray nozzles and pinch-roll/shearfrom the mould exit, mould parameters - length, metal level, corner radius, wall thickness,water channel gap, type of copper and nature of constraints, spray length and physical109properties of lubrication oil are also included. In addition, during the consultation, thesystem requests observations needed for the analysis. The user is asked about the presenceof dark and bright patches on the billet surface at the mould exit, colour change in the billetin the radiation cooling zone and nature of scale deposits - severity and colour. Questionsare asked about measurements such as mould tube distortion, mould cooling water velocityand oil distribution. This data provide the main source of information for the expert system.Also, users are asked to examine a macro-etched transverse section or a sulphur print of abillet and enter information about the outer tip position of any internal cracks seen.Calculated information  : The heat-transfer finite-difference model calculates solid shellthickness profile and billet surface reheat temperature. It is impossible for the users to providean estimate of these prediction. The mathematical model, with the help of additionalsubroutines, also calculates mould cooling water velocity (linear) based on mould-designand cooling water flow rate (volumetric), negative strip time, mould lead and specific spraywater flow rate. This program creates a data-file which interfaces with the expert system.Inferred information  : During consultation, the system concludes about facts based on rulesin the knowledge base. For example, deep and non-uniform oscillation marks are inferredfrom knowledge about the uniformity of the solid shell around the billet periphery. Thepresence of off-corner cracks and also dark and bright patches on the billet at the mould exitalso suggests that the oscillation marks are deep and non-uniform. Frequently, off-cornercracks have been found together with deep oscillation marks. In case, the system is unableto infer about "deep and non-uniform oscillation marks", it asks the user for measurementsof the oscillation marks and then tries to conclude about "deep and non-uniform oscillationmarks" based on the Experts' definition of "deep" and "non-uniform".1106.2.4 A novel inferencing strategy for the analysis of midway cracksThe appearance of midway cracks in a billet suggests that solid steel close to thesolid-liquid interface has been subjected to stresses and strains during processing. Thegeneration of such stress is frequently due to reheating of the strand surface due to a suddenreduction in heat extraction. Reheating causes surface expansion and imposes a tensile strainin the interior region of the solid shell, which is weak and brittle at temperatures within 50°Cof the solidus temperature [42]. The level of reheat determines the amount of stress generatedand therefore the intensity of a cracking problem. It is assumed therefore that belief in a highstress level is equivalent to the degree of belief in a "high" reheat. Belief in a "high" reheatlevel is determined by defining a fuzzy set for the adjective "high":Radiation Zone Reheat Temperature (°C) 0 50 75 100 150Degree of belief in a high reheat (%) 0 25 50 75 100It is known that a low Mn/S ratio and a high superheat are two operating factors thatcan aggravate the cracking susceptibility of steel when the reheat is high [40]. Although thepresence of a high stress level is mandatory for crack formation, problems with compositionand superheat can lead to cracking at lower stress levels and at lower reheat levels. On theirown, superheat and composition problems are not of concern since cracks cannot initiate inthe absence of a stress.Figure 6.5 depicts how belief in a high stress level is inferred. In the methodologyadopted, belief in a composition and/or superheat problem must be higher than belief in ahigh reheat in order to increase belief in a stress problem when a high reheat "might" exist.If a high reheat "might not" be present, then belief in a stress problem can only be increased111if the composition or superheat problems are known with a certainty greater than 50 percent.Thus, when the fact "reheat temperature is high"  is true or false with equal certainty (degreeof belief = 50 percent) the absolute presence of one or both of these problems will increasebelief in a stress problem to 70 or 90 percent respectively.Belief in a spray-related problem may be affected when a mould problem is alsoidentified. If the system discovers a mould problem, belief in a spray-related problem willbe reduced, provided belief in the mould-related problem is greater than belief in thespray-related problem. Thus, when the fact "midway cracking problem is spray-related"  istrue or false with equal certainty (degree of belief = 50 percent) the absolute presence of amould-related problem will reduce belief in a spray related problem to 30 percent. Figure6.6 illustrates this reduction in belief in a spray problem when a mould problem is identifiedwith 100 percent certainty.When there is uncertainty associated with the fact "reheat temperature is high",  theabsolute presence of problems associated with both superheat and high temperature strength,cannot increase belief in a spray related stress problem to 100 percent. This is logical since"a high reheat temperature"  is known with less than full certainty and the modified belief ina spray-related stress problem must reflect this uncertainty. Thus, when there is equaluncertainty about the fact "reheat temperature is high"  being true or false (degree of belief= 50 percent), the maximum possible belief is 90 percent. Using the same logic, the absolutepresence of either problem ( high superheat or poor high temperature strength of steel ) willincrease belief in a spray-related stress problem to 70 percent. Similarly, when the fact"midway cracking problem is mould-related" is true with a 100 percent certainty, belief inspray related problem is reduced to 30 percent.112100////BA /---___- — —,...__..^--- ..--...- -7-----_----^..------ --,.- --...----/ I/ // I-/ /// /^,t../ /1// // // 1/ // // / OCD - Only Reheat/ // / "OR"OBD - Composition^Superheat Problem/ / OAD - Composition "AND" Superheat Problem//////.0^ 50^ 100BELIEF IN HIGH REHEAT DUE TO THE SPRAYS ( % )Fig 6.5^A schematic diagram illustrating the calculation of degree of belief in aspray-related tensile strain problem from belief in high reheat in the radiationzone, high superheat and composition problems.D113//////////,///,/t ./. .B . ./.A...-"■ OBC-NohloWdProbkm■ .."'... OAC-NloWdProblem■■C0^ 50^ 100INITIAL BELIEF IN SPRAY-RELATED STRAIN PROBLEM ( % )Fig 6.6^A schematic diagram illustrating the calculation of fmal belief in a spray-relatedtensile strain problem following the detection of a mould disorder.114Thus, the contribution of belief in these effects on increasing or decreasing belief ina spray-related stress problem is considered to be similar. The influence of a mould problemon belief in a spray problem is only taken into account after the effects of superheat and hightemperature strength of steel have been considered.6.3 Structure Of The Expert SystemFigure 6.7 outlines the methodology adopted in this system for the analysis of qualityproblems. At the start of a consultation, the system acquires operating parameters with the helpof an external program. The heat transfer model is executed to calculate the solid shell thicknessand the surface temperature profiles. An output file is created to interface with the expert systemshell, COMDALE/X. The user is then prompted to describe the quality problems and to selectthe most important one. The outputs from the mathematical model as well as the quality problemsentered by the user are "imported" into the "main" module of the system. In the "main" module,the system verifies the quality problem(s) entered by the user. In the case of midway cracksand pinch-roll cracks for example, the user is asked questions on the distances of the outer tipof the cracks from the billet surface. This establishes initiation sites for midway cracks andconfirms or denies the observation about pinch roll cracks.For the most important quality problem, the system addresses issues related to thefrequency of the problem with regard to particular strands, grade of steel, time in the heat,location and orientation. One of the three analysis modes is then used depending on thecombination of quality problems observed. Prior to presenting a summary of the analysis inthe "report" module, the system provides the user with an opportunity to examine all finalconclusions and to probe into the knowledge base to obtain justification for each piece of advice.115OPERATING PARAMETERSINPUT HEAT TRANSFERMODEL QUALITY PROBLEMSINPUTMAIN MODULEAnalysis of specificity associatedwith the most important problemI1 MODULE 1 I 1 MODULE 2 I I MODULE 3I REPORT IFig 6.7 A flow-sheet representing the structure of the expert system.116The following are some of the highlights of this system:* RULESMain Module 310Module - 1 300Module - 2 384Module - 3 362Report Module 94* KEYWORD TRIPLETS 552* FUZZY-SETS 68* MUTUAL-SETS 9* EXPLANATIONS 73* QUESTIONS 73* INPUT DATA ELEMENTS 44* DISPLAY FILE PAGES 271 pages in 10 files* FINAL CONCLUSIONS 6 ( 30 )* EXTERNAL PROGRAMS 3- Data Input- Quality Problem Input- Heat Transfer Model for Billet Solidification6.4 Justification of Knowledge Engineering DecisionsDuring the development of the expert system, various decisions concerning knowledgeengineering were made to accommodate constraints related to computer software(COMDALE/X version-3.0 and the operating system, MS-DOS version-4.01) and thecomplexities involved in the analysis of quality problems. Wherever possible, the input of the117user is tested for consistency and correctness. Sometimes, this is irritating for the users. Forexample, one user pointed out that the system always asked for the difference in the lengthsof the diagonals, irrespective of whether a rhomboidity problem was selected or not.(a) Due to the large size of this knowledge base and also the memory limitations imposed bythe operating system, it was decided to break the knowledge base into five modules, four foranalysis and one for report-generation. The details about these modules have been discussedearlier. The modules are linked to each other through data-files. There exists a potential problemduring justification of the final conclusions - it is difficult to chain back for a particular factpresent in a different module. At this point, the system ends the justification process by issuinga statement saying "information was imported from data file". Thus, a complete justificationis not possible with the approach taken in constructing this system.(b) A batch-file has been used to provide connections between the various programs used inthis expert system because of memory restrictions. Due to memory limitations, it was notpossible to "ACTIVATE" external programs from within the knowledge base and also it wasdifficult to "LOAD" one module from another. Batch-processing also allowed the users to"REDO" a consultation with a new set of input data and quality problems.(c) According to the Experts, mould parameters are linked to each other in a complex mannerand therefore, when a quality problem is mould-related, all parameters need to be analysed.Therefore, decision was made to adopt a "breadth-search" strategy for mould analysis using aCOMDALE/X function, "APPLYRULE". This slows down the consultation considerablysince this function examines all rules dealing with a particular keyword triplet, whethersuccessful or not. However, with the help of "IGNORE-RULE and RANGE" functions, it118was possible to enhance speed of the consultation to a great extent. Here, once a rule in arule-set is successful, all remaining rules are ignored. However, if no rule is fired, then allpossible members of the set must be exhausted during the search.(d)In the case of a midway crack analysis, the system examines the possibility of a moulddisorders even when there exists a high belief in a spray problem. The user is asked about thepresence of dark and bright patches on the billet surface at the mould exit. If the response is"no", the system asks if the user wishes to go through a detailed mould analysis. There is alarge emphasis on the mould because the experts strongly believe that mould issues must alsobe considered. In fact, when the user does not want to go through the mould analysis, thesystem issues a warning message- "be forewarned - a mould analysis may be essential forcomplete diagnosis of the problem cause".(e)During the development, it was decided to transfer all the "TEXT-MESSAGES" in theknowledge base into external "DISPLAY-FILES". This was beneficial because it enhancedthe speed of the compilation process. Maintenance of the knowledge base was easier sincecorrections to the text could be made independently of the knowledge base. However, thejustification feature (how the system found out about certain facts presented in the text-message)associated with "TEXT-MESSAGES" was lost. The "DISPLAY-FILE" does not allow theusers to have immediate justification.(f) The system summarizes its final conclusions under one or more of the following:* Quality problem is mould-related* Quality problem is spray-related* Quality problem is related to steel119* Quality problem is related to pinch-rolls* Abnormality in certain operating parameters is present* Additional information is neededThis approach was taken to prevent cluttering of the conclusions. A user can justify eachof them as usual.The following is the rule structure used for the conclusion "quality problem ismould-related":RULE 289IF^mould problem is due to distortionOR^mould taper is a potential problemOR^mould problem is related to oscillation systemOR^mould problem is related to lubrication systemOR^metal level control in the mould is a problemOR^longitudinal midface crack is mould-relatedOR^longitudinal corner crack is mould-relatedTHEN quality problem is mould-related cf=100(g) In the above approach, a fmal conclusion is made up of different facts, some of which mayor may not be instantiated during the consultation. In case, a particular fact is not instantiatedby the system, the fact is "considered to be "unknown". During justification, the systemindicates that the "degree of certainty of the fact is "unknown". This information is not requiredduring justification and therefore required elimination. In order to do this, additional rules120were created to "FORGET" these facts before the final conclusions were presented. Since, thenumber of such facts was large, the knowledge base increased in size. This also slowed downthe consultation process.(h) In addition, in this approach of summarizing final conclusions, all facts associated withthe conclusion have been given equal weightage. This is again based on the complexitiesassociated with mould analysis and also on the Experts' view that all mould parameters mustbe within limits for good operation.(i) In the inference strategy for midway crack analysis, upper limits of 70 and 90 percent andlower limit of 30 percent were arbitrarily selected. This may need some refining based onusers' feedback. In addition, the variation is assumed to be linear - an asymptotic variationmay be better, although the author believes this to be only of academic interest.(j) Reheat predicted by the mathematical model for the radiation zone is verified based on users'observation of billet colour. There are two conflicting situations encountered and they areresolved as follows:* When the model predicts a high reheat and no colour change is observed, the degreeof belief in high reheat is reduced by a factor of 0.90.When the model does not predict a high reheat and a colour change is observed, thedegree of belief in high reheat is increased from high disbelief (below 50 percent) to 80percent.Again, these limits are arbitrary and only reflect the decision of the Experts and their approach.121(k) At the end of the consultation, important points in the analysis are summarized dynamicallyin a report where values of concerned operating parameter are listed and compared with thoserecommended by the Experts. Against each parameter, comments of the Experts are provided.Comments have been created with the help of embedded keyword triplets. These keywordtriplets are strings which are instantiated during the consultation process. In the end, thesetriplets are "EXPORTED" to a data-file which is subsequently "IMPORTED" by the reportmodule.For creating the report, a "WRITE" function was used. This function was developed atUBC and linked to the inference engine of COMDALE/X. It creates an ASCII file into whichconcerned information is written. With the help of another external program, this ASCII fileis displayed. This program also provides user-friendly features such as, "quit", "page-up" and"page-down".122CHAPTER 7 TESTING AND EVALUATION OF THE EXPERT SYSTEM7.1 Evaluation ProcedureIn this work, the system was evaluated in two stages: informal evaluation by theKnowledge Engineers and the Domain Experts and a formal evaluation by the End-users.(a) Informal Evaluation: This stage formed an integral part of the knowledge acquisition andrepresentation where the system's performance was continuously assessed by the Experts andthe Knowledge Engineers. Recommendations and feed-back were incorporated immediatelyinto the system.(b)Formal Evaluation: At a later stage in the development, the system was installed at severalCanadian steel companies for formal evaluation. The procedure followed was:(i) Five Canadian steel companies were selected. The operating personnel in these plantswere willing to participate in the evaluation process.(ii) A list of operating parameters for input needed in the analysis (as shown in Appendix-A)was prepared and despatched to the users before evaluation. This provided the operatorswith sufficient time for collecting data. A list of basic computer (hardware/software)requirements necessary for the implementation of the system was also sent to them.The list is provided in Appendix-B.(iii)^One of the Knowledge Engineers installed the system in each of the companies. Abrief demonstration of the system was presented to the operating personnel.123(iv) One person was identified in each company for co-ordinating the evaluation process.The Knowledge Engineer explained the features of the system in detail. The systemwas consulted for quality problems using operating plant data.(v) Preliminary feedback was obtained at this stage. Errors in the computer code werediscovered and corrected immediately. A summary report (shown in Appendix-C) wasprepared by the Knowledge Engineer.(vi) A questionaire was distributed to the Users for providing detailed feedback after usingthe system for a longer period. Completed questionaire were obtained from threecompanies. The responses/comments obtained from them have been summarized inAppendix-D.(vii) The feedback obtained was analysed and decisions for implementation were made bythe Experts and the Knowledge Engineers.7.2 System Operation - Results of the feedbackThis section summarizes feedback associated with various aspects of the expert systembased on the preliminary feedback and those provided in the questionaire.(a) Operating Parameter - Input Module * This module serves its purpose of providing a way to enter data quite adequately. However,some Users found it a little slow to operate as only one parameter can be changed at atime. In addition, the units assigned to these parameters do not match with those used inthe plant.124• The terminology used is easy to follow for the "educated" Users such as metallurgists orprocess-control engineers. But there may be a problem with operators on the shop-floor.The Users feel that they are aware of the role of each input in the analysis and that the datais available easily.• Even though additional information on measurement techniques is adequate, some Usersfeel the need to include a glossary of terms used in the casting business.(b) Quality Problem - Input Module* The terms used in this module are easy to understand. However, Users also feel the needto have some background information on quality problems in the cast product.* There is a need to include more quality problems in the list such as laps, bleeds, centrelinesegregation.* There is no problem in handling the issue of selecting the most important problem.(c) Mathematical Model* Additional information is needed on the mathematical model- formulation of governingequations, underlying assumptions and its role in the analysis of quality problems in billetcasting.• One User felt that the time-delay associated with the model was significant. However,other did not find this issue to be a problem.* The output format is satisfactory. However, Users feel that graphical outputs of the modelpredictions will be more useful.* One user pointed out that the shell thickness predicted by the model did not match withthose measured in the billet (based on his judgement). There exists doubts about the125boundary conditions used in the model. The Users pointed out that the spray configurationas well as the mould heat-flux differs from plant to plant and this must be accounted forin the model.(d) Customized Questions. Explanations and Rule Description* Questions asked by the system and associated rule-descriptions and explanations are clearand easy to follow.* The technical level of information provided through the customized explanations andrule-descriptions is adequate. However, there still exists a need to account for differentuser expertise-levels in these presentations.* The customized features of COMDALE/X have a great potential in training novices.* No discrepancy was noted in the information provided through the customized explanationsand rule-descriptions. The Users agreed with the contents.(e) System's Recommendations * The recommendations are accurate, complete, clear and easy to understand. They areeasy to implement. (one User was neutral).* No discrepancy was encountered in the recommendations provided. The Users agreedwith these recommendations. (one User did not answer questions related to discrepancyand disagreement)* In addition, the Users feel the need to know about other potential problems which maystart coming up based on the existing operating conditions.(f) System's Final Conclusions and Justification* Final conclusions are complete, clear and easy to understand (one User was neutral).* The justification scheme is useful, clear and easy to understand (one User was neutral).126* The justification feature is a potential training tool for novices.* The logic adopted by the system for analysis was mostly satisfactory. Only one deficiencywas pointed out. The explanations provided by the system for transverse depression inAISI 1018 grade steel was found to be inadequate by one User.(g) Consultation Session* The system is adequate at diagnosing and for training. However, some Users feel that itsoperation may be a little complicated for personnel on shop-floor. This may hamper theissue of training these operators.* The consultation sessions were found to be long for some cases.* The system informs the Users about its course of action at each stage in the analysis.Changes in the system's thought process are smooth. Thus, the User is aware of theanalysis procedure and this feature of the system makes it very easy to understand.* Report is useful, satisfactory and complete.* Problems related to insufficient computer memory were encountered.* One of the Users disagreed with the approach adopted for concluding about a severerhomboidity problem. His comment was, "rhomboidity is always a problem". The expertspointed out that their definition of rhomboidity is satisfactory.73 Analysis of User Feedback7.3.1 System's Successes(a) The system proved to be successful in linking dominant problems at several sites withexpected causes. Some examples are as follows:127Company A The system related a problem with rhomboidity to plugged spray nozzles.The operators solved this problem by improving the spray cooling waterquality.Company B Off-corner cracks were found to originate near the mould exit. The systemsuggested a mismatch between spray and mould heat extraction rates dueto a large gap between the mould exit and the first spray nozzle.Company The heat transfer model calculated a high reheat in the radiation zone.Midway cracks were seen in the radiation zone. The cause of this highreheat was a short spray zone.(b) The following features of the system were useful:* The User is informed about what the system is doing at each stage during theconsultation.* The mechanisms involved in the generation of the quality problems are explained atrelevant stages.* Text-messages, explanations and rule descriptions are important tools for training.* The system not only points out flaws in design and operation but also informs theUsers about those parameters that are satisfactory.(c) The knowledge structure provides the Users with a consistent interpretation of thedomain knowledge.(d) The system serves as a valuable consulting tool for operators during brain-stormingsessions for solving quality problems.1287.3.2 System's Drawbacks(a) During an analysis on transverse depressions (and cracks), the system reached thelimit of its knowledge. When consulted, the Experts explained that the problem wasrelated to the effects of oscillation frequency on heat-transfer in the mould (a recentfinding). Expansion of the knowledge base is necessary to include this knowledge.(b) It was pointed out that the system cannot be used for day-to-day quality-problembecause of the approach adopted. Some of the recommendations require large scalemodification to the operation. However, inexperienced operators can use it forlearning the subject.(c) There is a need to recognize different levels of expertise among the possible Userssince the system considers all the Users to be the same.(d) There are severe computer memory limits imposed by COMDALE/X version-3.0 andMS-DOS version-4.01. Hence, expansion of the knowledge base under the presentconstraints is difficult.* RAM (computer memory) available for application is around 580 Kb. The size ofthe inference engine "COMDX.EXE" is about 385 Kb. The knowledge base can onlybe expanded to 160 to 170 Kb. Problems of memory are encountered when MS-DOScommands are activated from within the COMDX program.* External programs such as "DATA-INPUT' program or the "HEAT-TRANSFERMODEL" are difficult to "LOAD" from within the COMDX program because of theirlarge size.(e) Errors in the code- omission, misprints, incorrect units, etc. were present. Thesewere corrected.129(f) At the present moment, the system is incapable of handling different sprayconfigurations. The inputs to the mathematical model are number of spray zones,zone length and water flow rate. As well, the data-base of mould heat-flux for differentcasting conditions is not complete.(g) The Users found the data input stage to be very cumbersome.(h) The units assigned to operating parameters in the data-input module does not matchwith those followed in the plant. The Users are uncomfortable with this. The majorproblem in most plant is the inconsistency in the use of unit-systems, FPS, METRICor SI.(i) The system does not attract the attention of the Users. As a starting point, the focusof this project was the knowledge base. Efforts will have to be made to enhance theuser-interface.733 Areas of Future Expansion(a) Expansion of the present knowledge base is needed to include centreline segregation,laps, bleeds, blowholes and pinholes.(b) There is a need to enhance the user-interface such as graphical output profiles of billetsurface temperature and solid shell thickness.(c) Mould heat-transfer boundary conditions are not available for different companies.The system uses a general heat-flux profile. Also, the system does not take into130account the effects of oscillation frequency on mould-heat transfer (recent findings).Data base of heat-fluxes for different grades, casting speeds, oscillation frequenciesand mould-taper has to be developed.(d) There is a need to recognize different expertise levels for the Users while presentingexplanations, rule-descriptions or text-messages.7.4 Implementation Of FeedbackDue to the constraints, a decision has been made to change to a newer version ofCOMDALE/X-version-5.00. Important features of this new tool are as follows:COMDALE/X version 5.0* Runs under WINDOWS/3 environment which is easy-to-use and does not have memorylimitations.* Handles different expertise levels for the Users.* Has a form-generator which can considerably ease the data-input process.* Hypertext is an inherent feature of this environment which can enhance the presentationof "help", "text-messages" and "display-files".* Has enhanced graphics features, much better than the previous version.* The multi-tasking feature of WINDOWS/3 can be utilized to reduce waiting time duringthe execution of the mathematical model.131* There are also new features for the developer:- Incremental Compiler- Rule and Class Editors- Rule and Class Browsers- Dynamic Objectsimplementation SchemeThe decisions made to implement the feedback are summarized in Table 7.1.Table 7.1^Feedback Implementation StrategyFEEDBACK COMMENTS* Include other quality problems The knowledge base will be expanded andmodified to include these problems.* Enhance user-friendliness andgraphics.Will be implemented in version 5.0 ofCOMDALE/X.* Need to recognize different userlevelsWill be implemented in version 5.0 ofCOMDALE/X.* Ease data input process Use of form generator provided in version 5.0 ofCOMDALE/X will be useful. Also,explanations will be provided for each inputparameter.* Add spray configurations in themathematical modelMathematical model will be expanded to includea subroutine dealing with spray configuration.* Establish boundary conditions forthe mathematical modelData bases will be created for a variety ofconditions and for each company throughindustrial trials.132CHAPTER 8 CONSULTATION SESSIONS - CASE STUDIESIn this chapter, three cases have been presented to highlight the consultation process. Theoperating parameters for the three cases have been provided in Appendix E. After data input, theheat transfer model for billet solidification is activated to calculate shell thickness and billet surfacetemperature profiles. With the help of an additional subroutine in this program, mould coolingwater velocity, negative strip time and mould lead are calculated. The outputs are written to adata file which is subsequently acquired by the expert system module. At this stage, qualityproblems are selected by the user from a list.CASE 1 Midway CracksLet's assume midway cracks were seen in the billet (at depths between 22 to 25 mm fromthe surface of a 150 mm square billet). No other problem was indicated by the user. The systembegins the consultation by presenting the following predictions of the mathematical model:Solid Shell Thicknesses Maximum Reheat TemperaturesMould exit = 7 mm Submould = 25 °CSpray entry = 9 mm Radiation zone = 158 °CSpray midpoint = 12 mmSpray exit = 18 mmPinch roll = 31 mmThe user did not select rhomboidity. The system verifies this information by requestingthe difference in the lengths of the two diagonals. The following is the dialogue that takes placebetween the system and the user.133System The difference in the length of the diagonals of the billet is a measure of rhomboidity.What is the difference in the length of the two diagonals in mm?User Why are you asking me this question?System You have indicated that the billet is not rhomboid. This suggests that rhomboidity isnot a problem. However, if the difference between the lengths of the two diagonals islarge, then a rhomboidity problem is confirmed.User Please explain the meaning of this question.System The presence of rhomboidity indicates that the heat extraction rates are not uniformon all foursides of the billet. Non-uniform heat removal suggests that oscillation marksare probably unevenly deep. We can confirm rhomboidity from the difference in thelength of the two diagonals. A difference greater than 6 mm is unacceptable to therolling mills. A difference greater than 4 mm strongly suggests problems related tonon uniform cooling.User The difference between the lengths of the diagonals is 2 mm.Thus, rhomboidity is not a problem. The session continues with a series of questions toestablish the location of cracks in the casting machine. Since the outer tips of the midway cracksare located between 20 to 25 mm from the surface of the billet, the system concludes that thesecracks originated in the radiation zone. The system also cross-checks the observation aboutpinch-roll cracks. It confirms that pinch-roll cracks are truly absent since no cracks originated atdepths between 28 mm to 34 mm from the billet surface. Since midway cracking is the only134quality problem, specific issues related to midway crack(s) are analysed. These includeobservations about the frequency of the problem with respect to strand location, time in the heatand steel grade.To confirm belief in high reheat as predicted by the model, the system looks for informationon the colour of the billet.System A change in the colour of the billet from red ( in the sprays ) to bright red ( in theradiation cooling zone ), if observed, will help confirm or deny the belief in a highradiation zone reheat as per the model prediction. Colour change should be observedif reheating is severe. Do you see any such colour change?User NoSince a colour change was not observed, the system reduces its degree of belief in themodel-predicted high reheat from 100 percent to 90 percent. The following message warningthe user of this contradiction is generated to suggest that the input data may be faulty.System Our belief that the radiation zone reheat is high based on the model prediction is 100percent. Since you did not observe a colour change in the billet in the radiation zone,the model prediction is not confirmed. Belief in a high radiation zone reheat must bereduced to 90 percent.The belief in a strain problem is then modified based on the presence of a compositionproblem or a superheat problem as shown in Figure 6.5. The system examines the sprays firstsince belief in a spray-related strain problem is high. Spray parameters include spray length andspecific spray water flow. The levels of these are assessed with comments and recommendationspresented by the system as follows:135Radiant Zone Reheat is 160 °CSpray Water flow is 127 litre water / kg steel.Spray Length^is 2.07 m.We are 91 percent sure that faulty sprays have generated a high stress level. This is due toproblems with the spray water parameters. We are 100 percent sure that the spray flow is excessiveand 69 percent certain that the spray zone is short.RECOMMENDATIONS:1. Reduce the specific spray water flow to 1.0 liters/kg of steel produced.2. The length of the spray zone should be increased to at least 5 m.Even though a problem related to the sprays was present, the system still examines thepossibility of a mould disorder. The system inquires about the presence of dark and bright patcheson the billet at the mould exit. Let's say that no such patches were seen. The system then asksif the user wishes to go through a detailed mould analysis. Let's say the answer is "no". A detailedmould-analysis is not done. However, the following warning is presented.Dark patches were not seen on the billet surface at the exit of the mould. Since you do not wantto conduct a detailed mould analysis, we will not examine the mould parameters. However, BEFOREWARNED - a mould analysis may be essential for complete diagnosis of the problem cause.The analysis is then complete and the system presents the following final conclusions forjustification:* Quality Problem is Spray-related.* Quality Problem is Related to Steel Parameters.Let's evaluate the conclusion related to steel parameters.136Quality problem is related to steel parameters because of a rule that states..Quality problem related to steel parameters can occur if problems exist with superheat and/orsteel composition.And it is known that..1.It appears that high temperature strength of steel is poor2. I am certain that a large columnar zone is presentLet's examine reasons for a large columnar zone.I am certain that a large columnar zone is present because of a rule that states..A high superheat leads to an enlarged columnar zone which provides an easy path for crackpropagation. The effect of superheat is more pronounced in high and low carbon grade steelsas compared with medium carbon grades.And it is known that..1.I am certain that cast superheat is high2. Cast grade is high carbon.Let's see why cast superheat is high.I am certain that cast superheat is high because cast superheat = 33 r and a high superheatlevel is described by the following fuzzy set:Superheat (°C) 5 10 15 20 25 30 35 40Degree of belief (%) 0 5 10 45 80 90 95 100137In the same way the user can justify each of the remaining conclusions. At the end of theconsultation, the system summarizes the important points of the analysis and compares existingoperating parameters with those recommended by the experts. Table 8.1 shows the final report.Table 8.1^Final ReportSUMMARIZED REPORT ON BILLET QUALITY PROBLEMSQUALITY PROBLEM - Midway CracksLIQUID STEELPARAMETERSYOURVALUERECOMMENDEDVALUECOMMENTSMn/S Ratio 27 above 25 satisfactorySuperheat (°C) 58 below 25 too highCopper Level 0.15 below 0.20 satisfactorySPRAYPARAMETERSLength (m) 2.1 above 5 should be increasedSpecific Flow (1/kg) 1.3 below 1.0 should be decreasedNozzle Blockage NO NO no problemColour change observed NO NO check observation138CASE 2 Off-corner cracks and Transverse defects (cracks and depressions).Most Important Problem : Transverse DepressionsThe system begins the analysis by presenting the model predictions. The system then verifiesthe information about rhomboidity. Let's say the difference in the lengths of the diagonals is 5mm. The following warning message informing the user about a severe rhomboidity problem ispresented.You did not observe rhomboidity in the billet. However, the difference in the lengths of thediagonals is 5 nun. We are 75 percent sure that this is large. This strongly suggests that the billetis rhomboid and the problem is severe.Please note : RHOMBOIDITY PROBLEM IS SEVERE !!The system then includes rhomboidity in the list of quality problems to be analysed. Sincetransverse depression is the most important quality problem, specific issues related to it areanalysed. The following message is presented before the start of the consultation.You have selected TRANSVERSE DEFECTS as the most important important quality problem.Pertinent observations ( i.e. if the transverse cracks andl or depressions occur more on somespecific occasions ) provide important clues related to its formation. Some of these are as follows:If the transverse cracks andl or depressions are predominant :* on some specific strand(s)* on some specific region(s) of the billet* in some specific grades* at some specific time in the heatsSystem Do you see transverse defects on all the strands?139User^NoSystem Are the transverse defects more on the inboard strands as compared to the outboardstrands?User^YesSystem Are the transverse defects more frequent in certain specific regions on the billetsurface?User^NoSystem You indicated that the transverse defect is present on the inboard strands. You alsoindicated that it does not appear on some specific portions of the billet surface. Wefeel that this problem is affected strongly by the following factors:1. Metal level variation2. Mould-tube alignment3. Mould lubricationThese factors will be analysed now.System Would you describe the surface of the metal stream flowing into the mould as being"ropey" or "rough"?User^YesSystem WARNING : Metal level variation is significant !!You have observed that the condition of the metal stream coming out of the tundish isrough. It is certain that this could be leading to excessive gas entrainment in theliquid steel. In the mould, these gases escape from the liquid pool surface as bubbles140creating turbulence and metal level variation. The situation is worse in the absenceof an adequate tundish design for flow control.We advise you to localize turbulence from the ladle stream by the use of dams andweirs in the tundish. We strongly re commend water modelling of the tundish to establishoptimum tundish design.System Let's examine issues related to mould-tube misalignment. Misalignment of the mouldtube can be deduced from the following:1. Uniformity of scale deposits on the cold face of the mould2. Cooling water velocity measurementsSystem Is the scale deposits seen on the cold face of the mould?User^YesSystem Would you describe the scale deposits as extensive?User^YesSystem Is the scale uniformly deposited on all faces of the mould?User^NoSystem Non-uniform scale deposits indicate that the water gap around the mould peripheryis not constant and this leads to non-uniform cooling water velocity. The water gapmay differ by 1 mm or more around the mould periphery due to poor tolerances on theoutside dimensions of the mould wall and the inside dimensions of the water jacket or141due to poor tolerances on the mould assembly, especially support plates, which resultin non-concentricity of the mould tube in the baffle. We are certain that the tolerancesof the mould assembly are inadequate.The system continues the consultation and examines the following aspects of mouldlubrication system:* Lubrication oil flow and distribution in the mould* Lubrication oil physical properties* Cleanliness of the oil slot system* Cleanliness of the meniscus region in the mouldSystem Are the transverse defects more frequent in the initial stage of the heats?User^YesSystem Does the frequency of transverse defects reduce after the dummy bar has crossed thepinch-rolls?User^YesSystem You have indicated that this problem appeared more in the initial stage of the heat ascompared to the other parts of the casting cycle. It could be related to poor start-uppractice. We feel that the higher frequency of defects in the initial stage of the heat,might be due to the dummy bar practice. Since the frequency of transverse defectsreduces considerably after the dummy bar has crossed the pinch-rolls, it is certainthat this problem is related to the dummy bar practice. Our experience has shown thatthe transverse defects may also be related to poor casting speed control at the beginningof the heats.142The system conducts a detailed analysis of the mould system to ascertain the causes of thequality problems observed. Other issues related to the mould (distortion, oscillation and taper)are analysed in detail. Spray maintenance issues as well as steel quality parameters are alsoexamined. In addition, for off-corner cracks, the following analysis is also done:System Please let me know whether the outer tip of any of the off-corner cracks is close to 7.0mm from the billet surface so that we can establish whether there exists a problemnear the mould exit.User^YesSystem Off-corner crack depth, when compared with the model predictions, suggests thatbulging of the solid shell occurred in the last stages of mould cooling or just at thestart of the sprays. It is very likely that one or more of the following factors may alsobe contributing to off-corner cracks:* Wear in the lower part of the mould* Distortion in the lower part of the mould* Mould cooling jacket arrangement* Insufficient spray cooling at the mould exitThe above issues are analysed. The analysis ends with the following final conclusions:* Quality Problem is Mould-Related* Quality Problem is Spray-Related* Quality Problem is Related to Steel* Abnormality in Certain Operating Parameters is Present* Additional Information is Needed143Justification can be obtained for each of the above conclusions. The system ends theconsultation by presenting the summarized report.CASE 3 Craze CracksOnly craze cracks were seen. The system considers this problem to be the most importantproblem. The following is the consultation:System Do you see craze cracks on all the strands?User^NoSystem You indicated that craze cracking problem occurs on certain specific strands. Sincethis is a steel related issue, it is very unlikely that your observation is correct. Westrongly feel that the problem is present on the other strands as well. Please examinethe billets from other strands once again.We will now examine steel quality parameters.System The copper levels in the steel is 030 percent. This is high.The ratio of Ni/Cu is 033. This is low.You may reduce the severity of this problem by reducing the level of copper in the steelto below 020 percent or by increasing the Ni/Cu ratio in steel to more than 1.0.The consultation is over. The following final conclusion is presented by the system:* Quality Problem is Related to SteelJustification can be obtained for this conclusion in the usual manner. The system thengenerates the final report.144The system is capable of analysing other problems occuring in any combinations. Theabove examples exhibit that the analysis procedure can be very straightforward and short orextremely detailed and long. The nature of the analysis solely depends on the kinds of problemsbeing analysed.145CHAPTER 9 SUMMARY AND CONCLUSIONSAn expert system has been developed to guide operators in analysing quality-relatedproblems and in providing them with a ready source of knowledge related to the operation of thecasting machine.Outcomes of Knowledge EngineeringKnowledge engineering this expert system was beneficial in many ways. Fundamental andheuristic knowledge required for problem solving were integrated during system development.This exercise lead to a formal representation of the Experts' knowledge which had a major impacton the problem domain. A structure evolved that was very similar to that of the Experts and waseasy to understand and explain to others. It provided the users with a consistent interpretation ofthe domain knowledge.A new classification scheme for quality problems in billet casting was developed. Theproblems are divided into groups; those that have complex origins and those where very specificissues need to be examined. With this strategy, all combinations have been handled.A new knowledge link was established between mould cooling water quality and mouldlubrication. When scale deposits are present on the cold face of the mould, the mould operateshotter than normal and therefore, the lubricating oil vaporizes or burns before reaching the metallevel. In this situation, mould lubrication is likely to be poor.146A novel inferencing strategy was developed in the analysis of midway cracks to integratethe influence of important parameters such as surface reheat temperature, steel superheat andcomposition. The effect of mould disorders on belief in a spray-related strain problem was alsoincluded in this scheme.Future WorkThe knowledge base will be modified to a WINDOWS/3 environment. User-feedbackrelated to expansion of the problem domain, enhancement of user-interface and refining of theknowledge base will be incorporated into the system. Future efforts will also examine theapplication of this knowledge base to a real-time on-line monitoring and process-control system.By providing this assistance to the quality-control process, significant improvements inproductivity and product quality are likely to occur.147REFERENCES1^COMDALE/X version 3.0- Users' Manual, Comdale Technologies Inc., Toronto,Canada.2^F.Hayes-Roth et al. : Building Expert Systems. 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Meta11k, 1969, Vol.60, pp.340-350.64^R. Bommaraju et al.: ISS Transactions, 1984, Vol.5, pp.95-105.65^J.E.Lait and J.K.Brimacombe: ISS Transactions, 1982, Vol.1, pp.1-13.66^S.N.Singh and K.E.Blazek: Open Hearth Proceedings, AIME, 1976, Vol.59,pp•264-283.67^J.K.Brimacombe et al.: ISS Transactions, Vol.1, 1982, pp.29-40.68^J.K.Brimacombe et al.: Proceedings of 69th Steelmaking Conference, 5thInemational Iron & Steel Congress, April, 1986, Washington D.C., USA.69^R.J.Dippenaar et al.: Electric Furnace Conference Proceedings, Atlanta, Vol.43,1985.70^The Centre for Metallurgical Process Engineering, UBC, Vancouver, B.C.,Canada. Unpublished work.71^I.V.Samarasekera et al.: Proceedings of International Symposium on ContinuousCasting of Billets, Vancouver, Canada, CIM, 1985, pp.33-58.72^R.Hapke: Private Communication.73^J.K.Brimacombe et al.: Canadian Metallurgical Quarterly., 1980, Vol.19,pp•215-227.15374^A.Grill et al.: Ironmaking and Steelmaking, 1976, Vol.3, pp.38-47.75^H.Mori: Tetsu-to-Hagane, 1972, Vol.58, pp.1511-1534 [HB Transl. No.9000-76^H.Mori: Tetsu-to-Hagane, 1974, Vol.60, pp.784-806 [HB Transl. No. 9355 -I&II].77^M.Ya.Brovman: Stal Eng., 1967, Vol.1, p.26.78^A.Grill et al.: Ironmaking and Steelmaking, 1976, No.1, pp.38-47.79^I.V.Samarasekera and J.K.Brimacombe: Proceedings of 3rd PTD Conference onApplications of Mathematical and Physical Models in Iron and Steel Industries,ISS -AIME, 1982, pp.76-91.80^I.V.Samarasekera and J.K.Brimacombe: Metallurgical Transactions B, Vol.13B,1982, pp.105-116.81^I.V.Samarasekera and J.K.Brimacombe: Proceedings of 2nd PTD Conference onContinuous Casting of Steel, ISS-AIME, 1981, pp.2-21.82^G.L.Fisher: Journal of Iron and Steel Institute, July 1969, pp.1010-1016.83^I.V.S amarasekera and J.K.Brimacombe: Report to Stelco Edmonton Steel Works,unpublished work, 1987.84^T.Higaki et al.: Continuous Casting of Steel, 1977, London, The Metals Society,pp.177-181.15485^C.W.Dean: Open Hearth Proceedings, AIME, 1974, Vol.57, pp.253-257.86^H.Ichikawa et al.: Continuous Casting of Steel, 1977, London, The MetalsSociety, pp.304-308.87^J.A.Meech: Processing Complex Ores, R.Rao and G.Dobby (Eds.), London,Pergamon Press, 1989, pp.575-585.155APPENDIX A - OPERATING PARAMETERS REQUIRED IN THE ANALYSIS* Steel Composition* Pouring Temperature (three times in the heat)* Section Size* Casting Speed (minimum, maximum and average values)* Mould Design (length, wall thickness, corner radius, type of constraint, taper, width ofcooling channel)* Mould Copper (type, chemical composition, thermal conductivity, half-softening point,yield strength)* Mould Operation (metal level, cooling water flow rate, cooling water quality- hardness)* Mould Oscillation (stroke length and frequency- minimum, maximum and average values)* Mould Lubrication (oil flow rate, physical properties of oil- flash point, boiling point andviscosity)* Spray Parameters (number of spray zones, zone length, water flow rate in each zone andwater temperature)156APPENDIX B - EXPERT SYSTEM REQUIREMENTS1. HARDWARE AND SOFTWARE REQUIREMENTS- IBM PC 386 with MS-DOS ver 3.3- 640 Kb RAM- Free hard disk space - not less than 5.0 Mb2. USERS REQUIREMENTS- Familiarity with computers ( IBM-PC )- Little knowledge about metallurgical terminologies employed in casting- Some knowledge about the continuous casting process3. RECOMMENDED USERS- At least three different levels of operating personnel- An inexperienced operator - to assess training capabilitiesAPPENDIX CPRELIMINARY REPORT ON USERS' FEEDBACK1581. COMPANY A1. Hardware / Software :- 1st computer^(a)^286 COMPAQ machine(b) Operating system : MS-DOS version 3.30(c) High density disk drive (51/4") was not available(d) High density disk drive (31/2") was not available(e) Expert system program could not be loaded2nd computer^(a)^High density disk drive (51/4") was available(b) High density disk drive (31/2") was not available(c) Operating system : MS-DOS version 3.20(d) Expert system program was loaded and started(e) Math co-processor was not installed in the computer and thereforeit was not possible to run FORTRAN programs because of floatingpoint error arising due to the absence of a math co-processor -programs were compiled for machines having math co-processor.(0^FORTRAN programs were re-compiled for a machine with no mathco-processor. The system was restarted. The mathematical modeltook about 20 -25 minutes to run compared to 5 to 50 seconds forconventional machines with a co-processor.(g) In addition, version 3.20 of DOS did not support the "CALL"function used in the batch file.(h) Manuals for the computer were not available and therefore it wasnot possible to ascertain the type of the machine being used.Based on the above experience, the following requirements (hardware and software) for runningthe EXPERT SYSTEM using COMDALE/X version 3.0, have been identified as mandatory:(i) IBM-PC - 386 or 286 with math co-processor(ii) MS-DOS version 3.30 or above(iii) 640 kb of RAM(iv) Hard-disk space of 5.0 Mb or more2. Error(s) in the code : (a)^Unit of metal level in the mould was being incorrectly reported. This was pointed out bythe operating personnel and was later corrected.3. Quality problem analysis : (a) The expert system was demonstrated to the operating personnel using the TOSHIBA 286machine.(b) A "TEST" file was created and used as an example for the demonstration. The programwas run with quality problems : transverse depression, rhomboidity and centreline cracks.159(c) Transverse section of the billet was not available and therefore it was impossible to identifyinternal cracks in the billet.(d) It is impossible to observe dark and bright patches on the billet surface as it comes out ofthe mould because of design restrictions.(e) The expert system suggested that rhomboidity problem could be due to blocked nozzlesas a result of poor spray water quality. It was immediately pointed out by the operatorsthat they had problems of blocked spray nozzles and they minimized rhomboidity problemby incorporating chemical treatment for their spray cooling water.(f) The expert system highlighted the mechanism of formation of transverse depressions andcentreline cracks. This information was useful to the operating personnel in learning aboutthe source of the problem.(g) The operating personnel felt that centreline segregation problems must be included in thelist.(h) In general, measurements of mould tube distortion, oscillation mark depths, coolingwater velocity, oil distribution and the chemical analysis of the scale deposits were notavailable. In some cases, lack of the above information could be a potential threat tosuccessful analysis.1602. COMPANY B1. Hardware / Software :-(a) A 386-computer with a math co-processor and MS-DOS version 3.30 was available. Therewas no problem during the installation of the expert system program.(b) Option for exiting in the middle of a consultation session was not available in the expertsystem. Thus, the user could exit only at the end of the session by selecting the option"STOP". The operator requested this option. It was incorporated through the batch filebut there was a memory problem (due to insufficient memory) while "ACTIVATING" the"COPY" command in COMDALE/X. This problem was later solved by modifying the"CONFIG.SYS" file (to increase available memory).2. Error(s) in the code : (a)^Missing Fuzzy sets:^• position_of^chromium_layer_discolouration^closeto_the_mould_top* distortion in_the_lower_part_of the_mould large3. Quality_problem analysis : (a) The term "distance of the first spray nozzle from the mould exit" was not very clear to theoperating personnel. Although this distance was 7 cm, the spray water reached pointsvery close to the mould exit and therefore the actual gap between the mould and the spraywas negligible. This term needs to be modified in the data input program. This point willbe taken care of in the mathematical model by including a spray-configuration subroutine.(b) The mathematical model does not take into account the spray configuration - gap betweenthe spray zones, nozzle types and the stand-off distance. It is necessary to incorporatethis in the model to be more accurate. The operators were doubtful about the calculationof heat transfer coefficient for the spray.(c)^Units employed in the expert system:• preferred units for shell thickness- mm• conversion for SUS (unit for oil viscosity) was not available• in general, users felt that they must be comfortable with the units employed in theexpert system- English, SI or even mixed. It is very inconvenient to switch fromthe units commonly used to SI or to any others.(d)^Nb needs to be included in the list of elements in the steel composition. Also, the effectof Nb on the high temperature ductility of steel is to be included in the knowledge base,particularly with an emphasis on the generation of transverse corner cracks.(e) It is assumed in the expert system that mechanisms involved in the generation of transversecracks (midface and corner) are the same and therefore are treated in the same way.However, the operators felt that transverse midface and corner cracks arise due to differentreasons. Expert's view on this issue is required. Are the operators correct ?(f) In the expert system, the recommended value for superheat is below 25°C. The operatorsfelt that this is "too low" and therefore, not a practical recommendation. According tothem, it is very difficult to cast steel with a superheat of 25-30°C. In company B, superheatlevels were generally in the range 50-80°C. Therefore, a need exists to re-examine thedefinition of "high" superheat. The Experts' resolved this issue by saying that theirdefinition of "high" superheat is correct and a redefinition is not necessary.161(g) Here again, it is difficult to observe dark and bright patches on the billet surface as itcomes out of the mould. It is possible that in some situations the subsequent analysisstrategy may critically depend on this information. Therefore, ways must be devised toget around this problem.(h) Some of the measurements are available in the database and very often these are used asanswers to questions in the analysis. In some cases, it is possible that these may notcorrelate with the quality problems in question. Thus, it is necessary to ensure that thedata corresponds to the actual situation. The Experts pointed out that they also face thisproblem.(i) In the expert system, metal level variation in the mould has been related only to thecondition of the metal stream. It was pointed out by the operating personnel that anotherimportant factor that could be added to this is metal level control system - bad signal. Thispoint will be emphasized through a text message.(j) Water velocity measurement data was not available. Operators felt that most companieswill not have this facility for regular process control. The Experts felt that this option muststay in the knowledge base.1623. COMPANY C1. Hardware / Software :-(a) 386-computer with math co-processor and MS-DOS version 3.30. There was no problemin the installation of the expert system program.(b) Operator printed the report file. There were some errors in the report generator. Thesewere corrected immediately.1 Error(s) in the code : (a) IGNORE RANGE or RULE statements were missing. The system became unstable at thecross-over point (fuzzy set for large and small distortion cross over at 0.2 mm distortion).This was corrected by incorporating IGNORE RANGE/RULE statements in the concernedrules.(b) A misprint was discovered in one keyword triplet - "boiling problempresent_in_the_mould" was printed as "boiling problem persent_in_the_mould". This wasnoticed during the justification session.3. Quality problem analysis : (a) The model predicted a high reheat in the radiation zone. Midway cracks were seen in theradiation zone. A direct correlation was observed. Mn:S ratio was also low and thesuperheat was high. This was pointed out by the expert system. A sulphur print havingmidway cracks were observed. The above prediction was verified - midway crack in theradiation zone.(b) Frequency of midway cracks were usually higher at the end of heats as compared to therest. According to the operators, this happened because the withdrawal rate increasedsubstantially at the end of heats - purpose to finish off fast. The superheat at the end ofthe heat was the same as the start. No correlation could be established between thefrequency of the problem and the superheat level. Does this mean that the problem ismould related - reheating associated with dark patches ?(c) Superheat at the start and the end of heats were lower than that in the middle. This was ageneral observation in all the plants. The effect of thermal stratification in the ladle wasnot very obvious. In addition, the superheat at the start and the end were valid for smallertime as compared to the superheat in the middle of the heats which covered a larger period.(d) The problem of transverse depression (and cracks) was analysed. The permanent moulddistortion was 0.2 mm (50 percent sure that this is small based on the fuzzy set for "small").Dark patches were not seen. According to the expert system, distortion was not a problemand therefore analysis of parameters related to mould distortion was not necessary.Oscillation parameters- mould lead (5-7 mm) and negative strip time (0.16-0.17 s) weresatisfactory. Oil flow was satisfactory. No jerking was observed. Oil distribution wasmeasured and was uniform. Meniscus area and the oil distribution slot were cleanedregularly. Double taper was used. Oscillation marks were not deep (visual). Everythingseemed to be satisfactory. The only other reason could be improper taper. The expertsystem reached its limit.163(e) The definition of severe rhomboidity - a difference of 4 mm or more between the lengthof the two diagonals is considered to be a problem. Is this range valid for larger sectionsizes such as 190 mm 9 Is there a need of a factor say (DIFFERNCE/SECTIONSIZE)that takes into account section size as well ?The Experts said that the present definition of "large" rhomboidity is satisfctory.(f) The heat-flux profile used for the mould was general and not specific to the steel company"C". There were some doubts in the minds of the operating personnel about the heat fluxprofile being used. It was later pointed out to them that this was an assumption in themodel. It was also clarified that mathematical model was a heat transfer solidificationmodel using finite difference formulations and that its prediction was much better than akt" model for shell thickness profile. This was appreciated by them. It is necessary toinform the users about the formulation scheme.It was again pointed out by the users that the mathematical model does not take into accountthe spray configuration - gap between the spray zones, nozzle types and the stand-offdistance. It is therefore necessary to incorporate this in the model to be more accurate.Knowledge about the width of oil slot has not been incorporated in the expert system. Theusers wanted to about its significance.There is a need to refine the definition of "high" and "okay" superheat level based onpractical limitations.There is a need to incorporate the effects of Nb and B on the severity of cracking - transversecracks.Craze cracks are not a problem. No complaints from the company "C". Is it because theyare not aware of the problem and also how to locate them in the billet - etching techniqueused by R. Hapke at UBC ?The knowledge related to taper has to be incorporated in the system and also the strategyin a situation where large mould distortion has been measured or inferred. If moulddistortion is a problem, how would one handle the taper issue ?1644. COMPANY D1. Hardware / Software : (a)^286-computer with math co-processor and MS-DOS version 3.30 was available. Therewas no problem in the installation of the expert system program.2. Errors) in the code :-(a)^The system was demonstrated to the operating staff. No errors were noticed.3. Quality problem analysis : (a) Operating parameters were not easily available. Completed data sheet was not available.The operators did not have sufficient time to spare.(b) The spray parameters (zone length and flow) were not clearly known to the operators. Itmay be necessary to provide the users with some help on converting spray pressures toflow based on some data base on spray nozzles -type and configuration.(c) A consultation session involving transverse depressions and cracks was demonstratedusing the limited data available from company "D". Problem related to dummy bar practicewas identified by the expert system - analysis of specificity issues.(d) The users agreed that there was a lot of information in the expert system. They wereinterested in the information.165$, COMPANY E1. Hardware / Software : (a)^286-computer with math co-processor and MS-DOS version 3.30 was available. Therewas no problem in the installation of the expert system program.2. Error(s) in the code :-(a)^The system was demonstrated to the operating staff. No errors were noticed.3. Quality problem analysis : (a) Eventhough there are three strands, only two may be used at a given time. There is a needto include a question on "number of strands in operation".(b) There was some problems in the model predicted shell thickness for high carbon grades(carbon > 1.0 percent) - model predicted a small shell. It was later discovered that the database did not have the heat-flux for this carbon range. The heat-flux being used may notbe valid. Also, large mushy-zone associated with these grades was influencing theprediction. Data-base of heat-fluxes for different grades will be developed for each plant.(c) Distortion measurements involves checking the distance between two opposite faces. Thisobviously does not take into account twist. The operators did not observe any correlationbetween distortion measurements and mould-related quality problems.(d) Problems such as laps and bleeds needs to be included.166GENERAL COMMENTS1^The system will be useful during "brain-storming" sessions for trouble-shooting qualityproblems. Because of the "general" nature of the knowledge base, the system is unlikelyto be successful in day-to-day problem solving.2^English language will be a problem in non-english speaking areas.3^Computer literacy is very low in the steel companies. This may be a potential problem.One way to reduce this threat is to increase the user-friendliness of the system.4^The system should attract the users - like computer games. Version 5.0 of Comdale/Xwill handle part of this problem.5^Graphical images must be introduced - pictures of shell thickness and temperature profilesshould appear on the screen dynamically while the mathematical model is running.6^According to the officials, users particularly on the shop floor will use the system duringcoffee breaks and some other free time. This may be a potential problem from the viewpointof teaching/training. The company must incorporate training schemes or encourage theusers on the shop floor.7^The text message provided at various stages during the consultation must take into accountthe expertise/level of the users. This is important because the system appears to be a"TEACHER" all the time and a lot of information is supplied at inappropriate places/time.The system treats all the users in the same way. This aspect of different levels of userswill be dealt with in the version 5.0 of Comdale/X.8^The "JUSTIFICATION" feature of the expert system, though useful, did not interest theusers very much. The main concern of users was to solve the problem quickly. However,the elimination of this feature is not a consideration.9^It is not easy to establish the limit for "high pinch-roll pressure". Usually, it is limited bythe force required to pull the billet out of the mould. Operators do not control this parameter.Hence, it is very difficult to judge this parameter. One of the operators suggested that byexamining the deformation due to the pinch rolls - comparing the billet size at the mouldexit with the final cold billet dimension, it is possible to conclude about the pinch rollpressure. In case of pinch-roll cracks, the expert system recommends a decrease inpinch-roll Pressure. What happens if the pinch pressure is at the lowest set-point ?10^Some operators suggested that rhomboidity can also be minimized by increasing the spraywater flow. This point has not been included in the recommendation.11^One user felt that the concepts highlighted in the expert system made sense to him becausehe had attended the Casting Course in Vancouver. It may be necessary to give a shortcourse in the concerned company not only on the expert system but also on continuouscasting during the installation. An overview on continuous casting will get them started.The concepts in the expert system will be easy to follow after this "start-up".167APPENDIX D : RESULTS OF FORMAL EVALUATIONThe following is the key to the rating:-Strongly Disagree 1Mildly Disagree 2Neutral 3Mildly Agree 4Strongly Agree 5The Companies are indicated by the letters "A" to "E". Completed questionaire were receivedonly from three companies namely: "B", "C" and "E".1. OPERATING PARAMETERS - INPUT MODULESTATEMENTS RATING COMMENTSB C E1 Easy and convenient to operate 1* 5 4 *Too slow and cumbersome tooperate. Also, units aredifferent from those followed inthe plant.2 Terminology used is easy tounderstand.5* 5 4 *Difficult for operators on theshop-floor.3 Need to find out why certainoperating parameters are requested.3 1 3 Not necessary to provideexplanation on "why" certainparameters are requested.4 Requested operating parameterdata are readily available.5 5 4 Input data for the system is not aproblem5 Need for more information onmeasurement techniques foroperating parameters.3 1 1 Not necessary to provideexplanation on the measurementtechniques.6 Need a glossary of basic terms incontinuous casting.4* 1 1 *Hyper-Text feature ofCOMDALE/X version 5.0 willbe beneficial.1682. QUALITY PROBLEMS - INPUT MODULESTATEMENTS RATING COMMENTSB C E1 Terms used are easy to understand. 5* 5 4 *Not sure about operatorson the shop-floor.2 Need for information on where and howto look for the quality problems in thecast product is necessary.5 1 2 Some operators mayrequire additionalinformation3 Background information on qualityproblems in billet casting is necessary.5 1 2 Some operators mayrequire additionalinformation4 Other quality problems need to beincluded in the list of options.3 1 4 Need to include laps,bleeds, centrelinesegregation.5 It is difficult to select "the mostimportant problem".3 1 1 No problem with the mostimportant problem6 The "most important problem" needs tobe defined better.3 1 1 No problem with the mostimportant problem3. MATHEMATICAL MODELSTATEMENTS RATING COMMENTSB C E1 Additional information on theheat-transfer mathematical model wouldbe useful.5 5 2 Not sure about operatorson the shop-floor.2 Access to information on the formulationof equations in the mathematical modelwould be useful.5 5 2 Definitely required3 It would be useful if the underlyingassumptions in the mathematical modelwere available for the users to examine.5* 5 2 *Need to incorporate spraydesign into the model4 The time delay associated with runningthe mathematical model is too long.3 1 45 The output format from the mathematicalmodel is satisfactory.3 5 -1694. QUESTIONS ASKED BY THE SYSTEMSTATEMENTS RATING COMMENTSB C E1 Questions asked are clear and easy toanswer.5 5 52a The "EXPLANATIONS" provides usefuland additional information about facts4 1 * 5 *Explanations needed forthe "data-input" module2b The"EXPLANATIONS" are clear andeasy to understand.4 5 42c It would be better to have an explanationsuch that excess information forexperienced personnel is avoided and atthe same time, more detail for novicesare provided.3 5 4 Definitely need to addressdifferent user-levels3a The "WHY" feature was useful inexplaining the purpose behind questionsasked.4 4 43b "WHY" messages are clear and easy tounderstand.4 5 54 The "EXPLANATION" and "WHY"features provided in the system would beuseful in the training of a novice.5 5 55 The technical level of informationprovided through "EXPLANATIONS"and "WHY" features is :(a) very high 3 1 4*The present level isadequate.*High for operators on theshop-floor(b) too elementary 3 3 26 The guidelines provided by the expertsystem for collection of data and formaintenance of the caster are easy tofollow.4 4 57 Discrepancies were encountered in theinformation provided by the"EXPLANATION" or the "WHY"feature.3 1 28 There exists some disagreement withinformation provided through the"EXPLANATION" and/or the "WHY"feature of the expert system.3 1 11705. RECOMMENDATIONS PROVIDED BY THE SYSTEMSTATEMENTS RATING COMMENTSE1 System's recommendations are clear and easyto understand.4 5 52 System's recommendations are easy toimplement (they are feasible).3 5 43 Discrepancies were encountered in therecommendations provided by the expertsystem.- 1 14 There exists some disagreement with some ofthe recommendation(s) made by the system.- 1 15 It would be useful to know about otherpossible problem areas during theconsultation.5 5 36 System's recommendations are accurate andcomplete.3 5 46. FINAL CONCLUSIONS AND JUSTIFICATIONSTATEMENTS RATING COMMENTSB C E1 The conclusions reached by the system arecomplete.3 4 2* *Explanation fortransverse depressionsin AISI- 1018 gradewas found to beinadequate2 The conclusions are clear and easy tounderstand.4 5 53 The justification scheme provided at the endof the consultation is useful.5 3 44 The justification scheme is clear and easy tounderstand.4 5 45 The justification of the conclusions at the endof the consultation will be extremely useful inthe training of new operators.5 5 56 Discrepancies were encountered during thejustification process.- 1 4* *same as 17 There exists some disagreement with the logicused by the system in analysing data.- 1 4* *same as 11717. CONSULTATION SESSIONSTATEMENTS RATING COMMENTSB C E1 The consultation session is too long. 4 1 4 Session is too long forsome cases2 The system is adequate at diagnosingquality problems.4 4 4* *Updating of knowledge isneeded3 The system is adequate for trainingoperating personnel.5* 5 4 *May be complicated foroperators on shop-floor4 System instabilities were encounteredduring the consultation.51 1 52 'Problems related toinsufficient memory2Problem related tocorrupted files5 The ability of the system to handleconflicting information or input issatisfactory.- 2* 4 *Rhomboidity is always aproblem. For a largesection size (190 mm), 4-5mm is large as per thepresent definition6 The system informs the user about whatit is doing at different stages of theanalysis.5 5 47 Changes in the system's thought process,during the analysis, are smooth andcomfortable.4 5 2* *Sometimes difficult tounderstand why certainmodules are loaded.8 The final report generated by the systemis satisfactory and complete4 5 59 The information contained in the report isadequately summarized.3 5 510 There is a need to include some moreinformation in the report.- 1 211 A computer print-out of the report forrecord purposes would be useful.5 5 4 Report can be printed out -feature already exists1728. SYSTEM OPERATIONSTATEMENTS RATING COMMENTSB C E1 The system is easy and convenient tolearn and operate.21,2 5 22 'Need more places to exitfrom the program;2Not sure about operators;easy for metallurgists2 The user interface provided byCOMDALE/X is sufficiently "friendly".4 5 1 * *Not very easy foroperators with nocomputer background3 System problems were encounteredduring the consultation session(s).1 52 'Problems related toinsufficient memory;Need explanations forerror messages2Problems related toboundary conditions usedin the model173APPENDIX E : OPERATING PARAMETERS FOR CASE STUDIES1. Steel CompositionCASE "A" CASE "B" CASE "C"Carbon 0.40 0.17 0.51Manganese 0.80 0.80 0.88Sulphur 0.03 0.030 0.035Phosphorus 0.02 0.010 0.010Copper 0.15 0.14 0.30Nickel 0.12 0.07 0.10Silicon 0.30 0.22 0.25Molybdenum 0.04 0.02 0.02Chromium 0.12 0.12 0.12Aluminium 0.001 0.001 0.001Titanium 0.001 0.001 0.001Vanadium 0.001 0.001 0.001174Unit CASE "A" CASE "B" CASE "C"Section size mm 150 x 150 150 x 150 150 x 150Pouring Temperature(start of heat)°C 1560 1580 1548Pouring Temperature(middle of heat)°C 1550 1570 1540Pouring Temperature(end of heat)°C 1540 1560 1532Casting Speed (min) mm/s 25.4 25.4 25.4Oscillation Frequency Hz 2.33 2.33 2.33Casting Speed (max) mm/s 42.3 42.3 42.3Oscillation Frequency Hz 2.67 2.67 2.67Casting Speed (avg) mm/s 31.8 31.8 31.8Oscillation Frequency Hz 2.50 2.50 2.50Oscillation Stroke mm 12.7 12.7 12.71753. Mould ParametersUnits CASE "A" CASE "B" CASE "C"Mould length mm 740 740 740Metal level mm 125 125 125Corner radius mm 3.175 3.175 3.175Type of constraint 4-sided 2-sided 4-sidedGap of cooling waterchannelmm 3.175 3.175 3.175Cooling water flow rate m3/s 0.0395 0.0300 0.0395Oil flow rate mVs 1.33 1.25 1.33Oil- boiling point °C 240 240 240Oil- flash point °C 320 300 320Oil- viscosity Poiseat25°C0.50 0.60 0.501764. Machine ParametersUnits CASE "A" CASE "B" CASE "C"Number of strands 6 6 6Machine radius m 6.0 6.0 6.0Distance of the firstspray nozzle from thebottom of the mouldmm 10.0 10.0 10.0Distance of pinch-rollsfrom the meniscusm 9.0 9.0 9.0Distance of shear fromthe meniscusm 26.5 26.5 26.55. Spray ParametersUnits CASE "A" CASE "B" CASE "C"Number of spray zones 3 3 3ZONE 1Length m 12 12 12Water Flow-rate m3/s 16.2 16.2 16.2ZONE 2Length m 44 44 44Water Flow-rate m3/s 131.7 131.7 131.7ZONE 3Length m 151 151 151Water Flow-rate m3/s 263.4 263.4 263.4Water Temperature °C 23 23 23177GLOSSARYAlgorithm A step-by-step approach for solving a problem with the help of a precisely definedgroup of rules or processes that leads to a desired output from a given set of inputs.Arc The lines interconnecting nodes in a search tree.Artificial Intelligence (AI) The branch of computer science that is devoted to the study of howcomputers can be used to simulate or duplicate functions of the human brain. It deals withhardware and software techniques that make it appear as though a computer is thinking,reasoning, making decisions, storing or retrieving knowledge, solving problems, andlearning.Backtracking A technique used in tree searches by which the system works backwards from afailed objective or an incorrect result in order to examine unexplored alternatives. It issimply the process of backing up to a point where a previous choice point was made in acomputation and then trying again.Backward Chaining A "goal-driven" method of reasoning technique used in tree searches. Itstarts with a given goal and works backwards, examining facts and rules that support thedesired outcome. It is also called "expectation driven", top-down reasoning" and "backwardreasoning".Belief A hypothesis about some unobservable situation; a measure of the believer's confidencein an uncertain proposition.Blind Search A general search technique that does not utilize any knowledge or heuristics tohelp accelerate or simplify the search process. Thus, it is a time-consuming process thatattempts to exhaust all possibilities in searching instead of relying upon information thatcan help narrow down the search.178Breadth-First Search A search strategy where all the nodes on one level of the search tree areexamined before any of the nodes of the next lower level.Certainty Factor A number ranging from 0 to 100 percent that indicates the possibility that aconclusion reached by the system is true. It is also the degree of belief of a domain expertabout a conclusion being true when the premise of a rule is evaluated with a net degree oftruth of 100 percent.Class A grouping of similar objects in a hierarchical and schematic manner.Cognition The process of knowing.Confidence Level A number lying in the range of 0 to 100 percent which represents a thresholdvalue that must be overcome in order for a rule to be successful. If a keyword triplet is tobe utilized in forward chaining or to be displayed as a final conclusion, its degree of beliefmust exceed the confidence level.Conflict Resolution (of rules) The priority-ordering process which is employed by an inferencingsystem to decide which rule to recognize or "fire" when more than one rule's IF statement(s)matches the current focus of the system.Consultation System A general term used for expert and knowledge-based systems that provideadvice or diagnose problems.Control (of expert systems) Any procedure, explicit or implicit, that determines the overallorder of problem-solving activities; the temporal organization of subprocesses.Control Strategy A method of reasoning in a search space such as forward- or backward-chaining,depth- or breadth-first search etc..Data Base Information stored in a computer.179Data Driven Data-directed inferencing strategy used in tree searches.Decisions Tree A graphical structure of nodes and arcs that presents alternative paths for variousdecisions or outcomes.Declarative Knowledge Information about objects, events or situations in the form of facts.Default Values A value given to a symbol or variable automatically when no other value isdefined by the programmer or user.Degree of Belief A measure of the believer's confidence in an uncertain proposition.Degree of Certainty A number in the range of 0 to 100 percent that represents the truth aboutthe value of a keyword triplet. True equals 100 percent while False is 0 percent.Depth-First Search A search strategy that explores each branch of a search tree to its full verticallength. It is a general search technique that is used when no guiding information is available.Here, each branch is searched for a solution and if none is found, a new branch is searchedto its depth. This procedure is carried on until a solution is found.Domain A field of knowledge or expertise; a problem area of interest in expert systemsapplication.Domain Expert A person with expertise in the domain in which the expert system is beingdeveloped.DOS Disk Operating System for IBM-compatible machines.Expertise The set of capabilities that underlies the high performance of human experts, includingextensive domain knowledge, heuristic rules that simplify and improve approaches toproblem solving, metaknowledge and metacognition, and compiled forms of behaviourthat afford economy in skilled performance.180Expert System A computer program that embodies facts, knowledge, rules of thumb, and otherelements of heuristic expertise in a particular domain and then uses this information toimitate the human thought process to solve a problem.Explanation Facility That part of an expert system which explains why a question is being askedor how the system reached a particular conclusion.Fact A proposition or datum whose validity is accepted.Firing a Rule Initiating the action specified by a rule if the conditions are met.Focus The keyword triplet being used by the system in its forward chaining search strategy orthe keyword triplet being looked for by the system in its backward chaining search strategy.Forward Chaining A problem-solving technique used in production and rule-based systems inwhich conclusions are drawn or decisions are made by starting with known facts. A searchprocedure or reasoning process that uses known facts to produce new facts and to reach afinal conclusion. It is also known as "data driven" and "inductive " or "antecedent reasoning".Frame A knowledge representation scheme that associates one or more features with an objectin terms of various slots and particular slot values (similar to classes).Fuzzy Logic A well defined reasoning system that is based on the use of fuzzy sets rather thanbinary values associated with the traditional on/off logic. The development of fuzzy logicwas pioneered by L.Zadeh at The University of California at Berkeley in the 1960's.Fuzzy Reasoning A method of dealing with inexact or imprecise information. It involvestechniques for avoiding complexities when dealing with subjective information or poorlyunderstood processes. It is a method of determining an adequate solution from impreciseinformation.181Fuzzy Set A mapping of discrete numerical concepts into a set of ranking numbers that describemembership in a particular concept or the degree of belief of a particular linguisticexpression.Goal Driven A method of reasoning that begins with the goal or a conclusion and works backwardsthrough the rules and facts of a knowledge base searching for the path that will achieve thedesired goal.Heuristic Consists of empirical information, rules of thumb, tricks, procedural tips and otherexperiential knowledge that help to guide, limit, and speed up the search process. Anythingthat helps a human or a computer to discover or learn.Hierarchy A ranked or graded series of persons or things; a body of knowledge or informationorganized into successive ranks or grades. Most hierarchies can be illustrated with the useof tree graphs.High-Level Language A programming language in which the instructions are closer to naturallanguage or algebraic notations than to machine language.IF-THEN The form of rules used in many AI systems, programs and expert systems; a conditionalrule in which a certain action is taken only if some condition is satisfied; decision-makingtests that initiate an action if a specific condition is met.Inference The process of drawing a conclusion from given evidence; to reach a decision byreasoning. Different types of evidence associations may produce different degrees of beliefin a particular conclusion.Inference Engine The "thinking part" or the "brain" of an expert system that actually performsthe reasoning function; that part of an AI program which analyses the information orknowledge base using rules to make decisions or reach conclusions.182Instantiation Applied to a keyword triplet when the degree of certainty of a keyword triplet isknown.Intelligence The ability to acquire and apply knowledge through thought and reason.Interface That portion of a computer system or program that links two other portions of thesystem and then allows them to communicate; a portion of a computer program thatinterfaces with both the computer user and the remainder of the program or system.Keyword Triplet Consists of an object-attribute-value combination. An object is an actual orconceptual entity. Attributes are properties associated with objects. Each attribute can takeon different values.Knowledge Understanding, awareness, or familiarity acquired through education or experience;anything that has been learned, perceived, discovered, inferred, and understood.Knowledge Acquisition The term used by workers in the area of expert systems to characterizethe complex activities involved in the development process. It is defined as the transferand transformation of problem-solving expertise from some knowledge source to a computerprogram.Knowledge Base That part of an expert system that is made up of data, rules, inferences, andprocedures organized into frames, blackboard, semantic networks, scripts, rules, andother formats; the assembly of all the information and knowledge of a specific domain thatforms the basis of an intelligent computer system.Knowledge-Based Systems Al programs that use a knowledge base as their source for problemsolving in a particular field of interest.183Knowledge Engineer A person who designs and builds expert systems; computer science orAI specialist who acquire knowledge from all sources including human experts and organizeit into a knowledge base.Knowledge Engineering The process of acquiring and formatting knowledge to form aknowledge base.Knowledge Representation A vocabulary and syntax of symbols and conventions used todescribe and present knowledge and information; the structure and organization ofinformation (or knowledge) used to solve a problem.Learning The process of improving performance by changing or control.LISP Acronym for List Processor. The most widely used AI programming language developedby John McCarthy at MIT in the 1950's.Logic A system of reasoning based on the study of propositions and their analysis in makingdeductions; a system developed by philosophers and mathematicians for the process ofmaking inferences and facts.Logical Connectives The words "AND" and "OR" that are used to link conditions in rules.Meta A prefix of a term that designates a self-reference to the given term.Meta-Knowledge Knowledge about knowledge.Net Degree of Truth A number in the range of 0 to 100 percent that represents the degree ofbelief that the premise of any rule is true. This number is ascertained by combining thedegrees of truth of each condition statement, and depends on the logical connectives (AND,OR) used to combine condition statements. "ANDing" will assign the minimum degreeof truth while "ORing" will assign the maximum.184Path A route through a search tree.Predicate A statement about the subject of a proposition. An assertion that denotes therelationship among two or more objects or elements.Premise The condition or the "IF" part of an "IF-THEN" rule.Problem Solving The purpose of most AI programs. This includes the process of answering aquestion, seeking a solution to an issue, resolving a conflict, and making a decision.Procedural Knowledge Information about the course of action.Production Rule An IF-THEN rule.PROLOG Acronym for Programming in Logic; an AI language that was developed in Franceusing the concepts of predicate calculus.Reasoning The mental process of drawing conclusions from facts, observations, or hypotheses;making inferences from arguments.Rule A regulation or statement defining a particular conduct, habit, or behaviour; in AI, atwo-part direction consisting of a condition and a consequent action. An IF-THEN rulestates that if a given condition is true, then a specific action should be taken.Rule-Based Any program or system that uses a set of rules to draw conclusions, make decisions,and solve problems.Rule of Thumb A heuristic, principle, technique, trick, or method often used to simplify,accelerate, or facilitate a process; a technique that does not always work or that is notalways accurate.Ruleset A collection of rules that constitute a module of heuristic knowledge.185Search The basic process involved in AI applications; to explore or examine in order to discoveror learn something; to seek a particular object or goal.Search Tree A graph that looks like an inverted tree and is used to illustrate the search of all thevarious alternatives in a search space; a hierarchical structure showing all the goals andsubgoals (nodes) interconnected by arcs discovered during the search process.Shell An expert system generator; a software package that allows the users to create an expertsystem without programmingSoftware A collection of programs and routines that support the operation of the computer system.Thinking The process of thought or reasoning; to formulate mentally, decide, judge, consider,and otherwise reflect upon a subject.Tool A software package, such as an expert system shell, that makes it easier to create othersoftwares; includes high level languages such as LISP and PROLOG.Top-Down Reasoning A structured method of working backward from a desired goal todetermine subgoals and a path through the search tree that will yield a suitable solution toa problem.Uncertainty In the context of expert systems, it refers to an intermediate truth value. Manyexpert systems can accommodate uncertainty, i.e. they allow the users to indicate theirdegree of belief in a concept.User A computer operator.User-Friendly The term used to describe a facility designed to make interaction with a computersystem easy and comfortable for the user.186User-Interface That portion of a computer program which communicates with the operator; aportion of the program that accepts inputs and generates outputs with such techniques asnatural language and menus.REFERENCES Course notes on "Expert System for Metallurgists", 1989, The University of British Columbia,Vancouver, B.C, Canada.E.Turban, Decision Support and Expert Systems, 1990, Macmillan Publishing Company, NewYork, NY, USA.Building Expert Systems, Eds.F.A.Hayes-Roth, D.A.Waterman, D.B.Lenat, 1983,Addison-Wesley Publishing Company Inc., Reading, MA, USA.187


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