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The effect of non-metallic particles on as-cast austenitic structure of low carbon steel Komatsu, Nobuyuki 1993

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The Effect of Non-Metallic Particles on As-Cast Austenitic Structure of LowCarbon SteelByNobuyuki KomatsuB.Eng. Mechanical Engineering, The University of Tokyo, 1983A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE STUDIESMETALS AND MATERIALS ENGINEERINGWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAJuly 1993© Nobuyuki KomatsuIn 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 vietas 01.„(A tiater,,,Axs chs; iteeh.The University of British ColumbiaVancouver, CanadaDate^8^iqq3DE-6 (2/88)AbstractThe effect of second-phase particles and cooling rate on the as-cast austenite grain size ofperitectic plain-carbon steel has been examined. Remelting tests followed by continuous coolingand helium quenching were performed on a Gleeble 1500 Thermomechanical Simulator. Twolow alloy types of steel were used; both had basically the same basic chemical composition, butcontained different amounts of titanium and calcium. The as-cast austenite grain size wascharacterized in terms of both the cooling rate, which was obtained by the Gleeble measuringsystem through the thermocouples welded on the sample surface, and the volume fraction andsize distribution of second-phase particles which were obtained by electron metallography. Theclassical Zener equation and Gladman model were applied to explain the experimental resultsrelating to the pinning effect of second-phase particle against as-cast austenitic grain growth.It was found that at relatively high cooling rates (4.5 °C/s — 17.5 °C/s), the effect ofcooling rate was dominant in austenite grain growth behavior, while at slower cooling rates (0.4°C/s — 1.2 °C/s), the effect of second-phase particles in grain boundary pinning could beobserved. Regarding the effect of second-phase particles, it was found that the titanium nitride,which is an effective precipitate for inhibiting austenite grain growth in the conventionalcontinuous casting and reheating process, was not effective for pinning the coarse as-castaustenite grain growth. This was due to both the small volume fraction and the small size oftitanium nitrides. On the other hand, oxides, which mainly were Ca-aluminates, were effectivefor pinning the as-cast austenite grain boundaries due to the large volume fraction and therelatively large particle size of oxides. The results were explained using the Gladman model.11Table of ContentsAbstract^ iiTable of Contents ^ iiiList of Tables vList of Figures^ viList of Symbols xiAcknowledgment^ xiv1. Introduction 12. Literature Search^ 32.1. Austenite Grain Growth Behavior in the High Temperature Region 32.2. Effect of Second Phase Particles on Austenite Grain Growth^ 82.3. Effective Particle Size for Pinning Austenite Grain Boundary^ 193. Scope and Objectives^ 214. Experimental Procedure 234.1. Test Apparatus^ 234.2. Sample Preparation 244.3. Remelting and Continuous Cooling Tests^ 274.4. Metallographic Tests^ 284.4.1.^Sample Preparation 284.4.2.^Secondary Dendrite Arm Spacing^ 304.4.3.^Austenite Grain Size^ 304.4.4.^Electron Microscopy 315. Results and Discussion 335.1. Remelting and Continuous Cooling Tests^ 335.1.1.^Thermocouple Output^ 335.1.2.^Thermal History 365.2. Dendritic Structure^ 391115.3. Austenitic Structure^ 485.4. Particle Size Measurement 575.5. Reproducibility Test^ 705.6. Volume Fraction of Second Phase Particles^ 775.7. Application of Gladman's Model^ 996. Conclusions^ 109References 111Appendix (The Program for Gladman Model)^ 114A.1 Flowchart^ 114A.2 Source Code 115ivList of Tables Table 2.1 Chemical compositions of steels used (wt%) [5].^ 4Table 4.1 Chemical compositions of steels tested (wt %). 25Table 5.1 Results of thermocouple calibration.^ 34Table 5.2 Actual cooling rates for each test. 36Table 5.3 The number of austenite grains counted (N) and the standard deviation (a) ^ 48Table 5.3 Calculation for particle forming.^ 63Table 5.4 Calculation of volume fraction of second phase particles on the replicas ^ 78Table 5.5 Summary of the relationship between il f and R ^ 79Table 5.7 The values of r* (pm) for various Ro and Z 107vList of FiguresFig. 2.1 The effect of carbon content on 7 grain size [5].^ 3Fig. 2.2 Grain growth of 7 phase during continuous cooling. The specimens were remeltedat 1580 °C, cooled to a given temperature at a rate of 0.28 °C/s, and quenched in water [5].^ 3Fig. 2.3 The delta ferrite field of a carbon-iron phase diagram [6]^ 4Fig. 2.4 Effects of Mn and Ni on the C dipendence of Ty [7] 5Fig. 2.5 Relationship between Dy and Ty in various steels [7].^ 6Fig. 2.6 Austenite grain growth during continuous cooling at rates of v, 2v, 0.5v, betweenTy and 1000°C [7].^ 7Fig. 2.7 The relationship between austenite grain size and cooling rate based on the datafrom previously published literature [5, 7, 8].^ 8Fig. 2.8 Schematic presentation of interaction between TiN particle and austenite grainboundary [16] . ^ 10Fig. 2.9 Relationship between contact angle and interaction energy [16].^ 12Fig. 2.10 Relationship between contact angle and particle size of TiN [16]^ 12Fig. 2.11 Schematic diagram illustrating a model for unpinning [17]. ^ 15Fig. 2.12 Model for grain growth. Grain A, R=Ro; grain B, R>Ro [17]^ 16Fig. 2.13 Energy changes during unpinning for f=0.0005, Ro=14ium; r=0.0350[1m; Z=1.5;7=800 erg/cm2 [17].^ 18Fig. 2.14 Effect of particle size on the barrier to growth for Ro=141-tm; Z=1.5; 7=800erg/cm2 [17].^ 19Fig. 2.15 Effect of particles on the inhibition of grain growth [20].^ 20Fig. 4.1 Schematic diagram of specimen mounting.^ 24Fig. 4.2 Cylindrical steel specimen for Gleeble melting test 26Fig. 4.3 Schematic diagram for sectioning of specimen^ 29Fig. 5.1 A thermal history of a remelting and continuous cooling test^ 36viFig. 5.2 Cooling curves for air cooling^ 37Fig. 5.3 Cooling curves for cooling rate of 4.5°C/s^ 37Fig. 5.4 Cooling curves for cooling rate of 1.2°C/s 38Fig. 5.5 Cooling curves for cooling rate of 0.4°C/s^ 38Fig. 5.6 Plot of measured secondary dendrite arm spacing against cooling rate. ^ 39Fig. 5.7 The dendritic structure of Steel A cooled in air.^ 40Fig. 5.8 The dendritic structure of Steel B cooled in air 41Fig. 5.9 The dendritic structure of Steel A cooled at 4.5°C/s^ 42Fig. 5.10 The dendritic structure of Steel B cooled at 4.5°C/s. 43Fig. 5.11 The dendritic structure of Steel A cooled at 1.2°C/s^ 44Fig. 5.12 The dendritic structure of Steel B cooled at 1.2°C/s. 45Fig. 5.13 The dendritic structure of Steel A cooled at 0.4°C/s.^ 46Fig. 5.14 The dendritic structure of Steel B cooled at 0.4°C/s. 47Fig. 5.15 The austenitic structure of Steel A cooled in air.^ 49Fig. 5.16 The austenitic structure of Steel B cooled in air. 50Fig. 5.17 The austenitic structure of Steel A cooled at 4.5°C/s^ 51Fig. 5.18 The austenitic structure of Steel B cooled at 4.5°C/s. 52Fig. 5.19 The austenitic structure of Steel A cooled at 1.2°C/s^ 53Fig. 5.20 The austenitic structure of Steel B cooled at 1.2°C/s. 54Fig. 5.21 The austenitic structure of Steel A cooled at 0.4°C/s^ 55Fig. 5.22 The austenitic structure of Steel B cooled at 0.4°C/s. 56Fig. 5.23 Plot of measured austenite grain size against cooling rate.^ 57Fig. 5.24 The second phase particle size distribution for the samples air cooled^ 58Fig. 5.25 The second phase particle size distribution for the samples cooled at 4.5°C/s.^ 59viiFig. 5.26 The second phase particle size distribution for the samples cooled at 1.2°C/s.^ 60Fig. 5.27 The second phase particle size distribution for the samples cooled at 0.4°C/s.61Fig. 5.28 The log-normal probability plot of the second phase particle size distribution forthe samples cooled in air. ^ 65Fig. 5.29 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 4.5°C/s. 66Fig. 5.30 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 1.2°C/s^ 67Fig. 5.31 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 0.4°C/s 68Fig. 5.32 The cooling curve for the reproducibility test plotted with the curves for theearlier tests (cooling rate: 1.2°C/s).^ 70Fig. 5.33 The dendritic structure for the repeated test (Steel B, C.R.=1.2°C/s).^ 71Fig. 5.34 The austenitic structure for the repeated test (Steel B, C.R.=1.2°C/s).^ 72Fig. 5.35 The relationship between secondary arm spacing and cooling rate. ^ 73Fig. 5.36 The relationship between austenite grain size and cooling rate.^ 74Fig. 5.37 The second phase particle size distribution for the sample of the Steel B cooledat 1.2°C/s.^ 75Fig. 5.38 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 1.2°C/s.^ 76Fig. 5.39 The relationship between R and lif.^ 80Fig. 5.40 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-1)^ 83Fig. 5.41 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-2) 84Fig. 5.42 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-3)^ 85Fig. 5.43 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-4) 86viiiFig. 5.44 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-5)^ 87Fig. 5.45 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: A1-Ca-0-6) 88Fig. 5.46 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: A1-Ca-0-7)^ 89Fig. 5.47 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-8) 90Fig. 5.48 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-9)^ 91Fig. 5.49 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-10) 92Fig. 5.50 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-11)^ 93Fig. 5.51 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: Al-Ca-0-12) 94Fig. 5.52 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: A1-1)^ 95Fig. 5.53 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: A1-2) 96Fig. 5.54 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: A1-3)^ 97Fig. 5.55 SEM photographs of particles on boundaries in Steel B cooled at 1.2°C/s.(Particle No.: A1-4) 98Fig. 5.56 Effect of particle size on the barrier to grain growth. The verage austenite grainradius = 0.4 mm, the volume fraction of particles = 0.001, and the surface energy per unitarea of boundary of austenite = 0.8 J/m2  101Fig. 5.57 Effect of particle size on the barrier to grain growth. The average austenite grainradius = 0.8 mm, the volume fraction of particles = 0.001, and the surface energy per unitarea of boundary of austenite = 0.8 J/m2 ^ 103Fig. 5.58 Effect of particle size on the barrier to grain growth. The average austenite grainradius = 1.4 mm, the volume fraction of particles = 0.001, and the surface energy per unitarea of boundary of austenite = 0.8 J/m2 ^ 105ixFig. 5.59 Effect of austenite grain size and heterogeneity on r*^ 107xList of SymbolsThe Relationship between Cooling Rate and Slab Thicknessd^Slab thickness (mm)G^Cooling rate (°C/s)Austenite Grain Growth at High TemperatureC^A constant which describes grain growth behavior (m3/s2)D^Austenite grain diameter at 1000 °C (mm)Do^Initial austenite grain diameter during continuous cooling (mm)Dy^Austenite grain diameter at 1300 °C (mm)Q Activation energy for grain boundary movement (J/mol)91^Gas constant (J/K-mol)t^Time (s)T^Absolute temperature (K)Ty^The temperature at which steels become single phase austenite (K)1)^Cooling rate (°C/s)Okumura-Matsuda Modelf^Volume fraction of TiNAF(r,co)^Increment of the interfacial energy per TiN particle (J)F^Grain boundary energy per unit area (J/m2)8^The interaction parameter between the austenite grain and the TiN particle(dimensionless)K^A dimensionless factor, which depends on the nature of the precipitate particlesr^Mean particle radius of TiN (vim)R^Austenite grain radius (ium)s^Contact arc length between a TiN particle and an austenite grain boundary (tim)xia0)^Contact angle between a TiN particle and an austenite grain boundary (rad)Y^Axis which shows the direction of boundary movementGladman Modelia variable described by the equation, a = s 2r —[1^)2(— s]2CUM2)2 )A^Area of the distorted planar boundary (i_tm2)A,^Increase in boundary area per unit area of interface (dimensionless)A,^Area of grain boundaries which is eliminated by growth of the grain per unit areaof interface (dimensionless)An^Net change in area of grain boundary per unit area of growing interface(dimensionless)M^Change in area of the distorted boundary (mn2)Ei^Net energy change per pinning particle due to grain growth (J)En^Net energy change per unit area of interface due to grain growth (J/tun2)Ep^Pinning energy per pinning particle due to the distortion of grain boundary (J)ET^Total energy change associated with the unpinning of a single particle (J)f^Volume fraction of particle (dimensionless)(1), 0^Interfacial angles between a particle and a grain boundary (rad)I^Grain boundary energy per unit area (J/m2)H^Energy barrier against grain boundary movement exerted by pinning particles (J)k^A constant described by the equation, y(s — x) = k (tim2)Li^Junction between a grain boundary and a pinning particle (pm)L2^Upper limit of the integration for deriving the area of the distorted grain boundary(Pm)n^A dimensionless constant which gives the upper limit L2 (L2 = nr)xiiNumber of particles per unit volume (pm-3)Radius of a pinning particle (gm)Critical particle radius above which grain coarsening can occur (pm)Matrix grain radius (pm)Displacement of a boundary (gm)Axis which shows the direction of grain boundary movementAxis which is perpendicular to the direction of grain boundary movementRatio of radii of growing grains to matrix grains (dimensionless)Temperature CorrectionTu,^Liquidus temperature of steels (°C)TCOr.^Corrected temperature (°C)T meas.^Output temperature from Gleeble (°C)Particle Size Distributiona^Fraction of the number of particles extracted on a replica (dimensionless)f Volume fraction of second phase particles (dimensionless)N^Number of particles in examined area on a replica (pm-2)Ns^Total number of particles intersecting a unit area of a sample (pm-2)N's^Number of particles per unit area of a replica (pm-2)F^Arithmetical mean of a particle radius distribution (gm)ac^Geometric standard deviation of a particle diameter distribution (pm)YG^Geometric mean of a particle diameter distribution (gm)AcknowledgmentI would like to thank professor J.K. Brimacombe, who supervised the work describedherein, and professor E.B. Hawbolt, for their advice and guidance during the course of this work.The assistance of Binh Chau, who operated the Gleeble 1500, is also appreciated.The discussion with F. Reyes, Y. Haruna, M. Militzer, and K. Scholey was considerablyhelpful. The advice of A. Schmalz, W. Chen, and N. Walker concerning the experimentalprocedure was also useful.I am indebted to Nippon Steel Corporation for the financial support and for the supply ofthe steels used in this work. Lastly, I would like to thank Kayoko for her love, support, andencouragement.xivChapter 1. Introduction1. Introduction In the manufacturing process of steel products, as in the Continuous Casting - DirectRolling process or in Near Net Shape Casting, the linkage between casting and rolling (or eventhe elimination of rolling) is receiving increasing attention because of energy savings andreduction of production costs. With respect to energy savings, the consumption of fossil fuel isreduced with less emission of CO2 due to the elimination of reheating furnaces, which is of greatimportance for solving the global environmental problem. The reduction of production cost isachieved because the building and maintenance costs for reheating furnaces are not needed.In these operations, there are fewer transformations than that of conventional processesand smaller reductions in the thickness of the material between casting and the finished productsuch that the as-cast structure has a larger influence on the final microstructure, whichdetermines the mechanical properties of the final products, than that of conventional processes:Consequently, the mechanical properties of the products by Near Net Shape Casting are poorercompared to those by the conventional processes due to the coarse as-cast structure; this hindersthe commercialization of the new process. Therefore, it is essential to make the cast structure asfine as possible in order to exploit Near Net Shape Casting.Various techniques, such as single roll, twin roll, or twin belt, have been developed forNear Net Shape Casting [1]. According to Itoh [2], the cooling rate of the casting processes atthe mid-thickness of material during casting is a function of thickness of the material to be cast,regardless of the type of caster, as can be seen in the following equation:G =803d-136^ (1.1)where, G is cooling rate in °C/s and d is the casting thickness in mm.In the case of strip casting with thicknesses in the order of 1 mm, very high cooling ratescan be achieved resulting in fine cast structures. Consequently, the most serious technicalimpediment to reach this goal has been surface quality [3]. For stainless steel strip production,1Chapter 1. Introductionthe twin-roll strip casting technique enables the Near Net Shape Casting processes to becommercialized to some extent [4]. Near Net Shape Casting processes for plates, whosethicknesses are of the order of 10-100 mm, are still far from commercialization because of therelatively slow cooling rate in the interior of the section which causes too coarse a structure inthe mid-thickness region to obtain proper mechanical properties. Therefore, a method for grainrefining the as-cast austenitic structure is needed in order to commercialize the near-net-shapecasting process for plates.For a given cooling rate, in which the primary cast structure is considered to be fixed, oneof the effective ways to obtain a fine microstructure is to utilize second-phase particles forpinning the grain boundaries of austenite which grow rapidly after solidification is completed.As a first step of this project, the as-cast austenite grain growth behavior in the high temperatureregion and the effect of second-phase particles on austenite grain growth will be discussed basedon previously published papers. Following this, the effect of second-phase particles on the as-cast austenite grain size of low carbon steels will be examined in the light of both experimentaland theoretical results. The major aim of this project has been to examine the particle size whichis effective for inhibiting the grain growth of coarse austenite grains.2Chapter 2. Literature Search2. Literature Search 2.1. Austenite Grain Growth Behavior in the High Temperature RegionSumitomo Metal Industry Co. [5] has reported that the austenite grain size of as-caststeels was found to depend largely on carbon content; the maximum grain size appeared in the0.10-0.15% carbon region (See Fig. 2.1). This phenomenon was explained by the higheraustenite formation temperature in this carbon region. This explanation was supported by otherexperimental results which are shown in Fig. 2.2.C (IQFig. 2.1 The effect of carbon content on 7 grain size [5].15001400 1300 1200 1100 1000 900QUENCHING TEMPERATURE (C)Fig. 2.2 Grain growth of 7 phase during continuous cooling. The specimens were remeltedat 1580 °C, cooled to a given temperature at a rate of 0.28 °C/s, and quenched in water [5].3ATOMIC PERCENTAGE CARBON1^ 2°C1580rigor15601120F1540veer1520reoF1500272oF14802680F1460eeeoF14403600F1420neoF14002540F1380mar1360Fe30.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75WEIGHT PERCENTAGE CARBONChapter 2. Literature SearchThe chemical composition of these steels are listed in Table 2.1.Table 2.1 Chemical compositions of steels used (wt %) [5].Steel^C^Si^Mn^P^S^Al^N Cl^0.029 0.84^0.020^0.006^0.025^0.0047C5 0.16 0.68^0.022^0.006^0.010^0.0043C9^0.28^0.89^0.022^0.007^0.031^0.0066Figure 2.2 shows that the growth of austenite grains occurs rapidly below sometemperature. In the case of steel C5 (0.16%), the temperature was about 1450 °C, while in theother cases it was relatively lower. A separate thermal analysis confirmed that thesetemperatures were almost equal to the temperatures at which the steels become single austenitephase (Ty). This suggests that the existence of a second-phase, such as liquid phase and 8 ferrite,suppresses the austenite grain growth considerably. Therefore, the grain growth of the austenitephase is primarily affected by the temperature of complete transformation to austenite(Ty). Thehighest Ty for the steels with the peritectic carbon content can qualitatively be understood by thecarbon-iron binary phase diagram in Fig 2.3 [6]. The effect of Mn, which is usually added tosteel plates, on the dependence of Ty on carbon content is shown in Fig. 2.4 [7]. This figureFig. 2.3 The delta ferrite field of a carbon-iron phase diagram [6].4ij 153015t-• 1440142- 14000 0.5*Coa 1.5•Cs-'z 1440LJ 1420o 14000 0.1^0.2^0.3^0.4^0.5C (%)Chapter 2. Literature Searchshows that the carbon content which gives maximum Ty decreases slightly due to a 2 wt% Mnaddition.Fig. 2.4 Effects of Mn and Ni on the C dependence of Ty [7].This research [7] also showed the grain growth of austenite after the 8-47 transformationmore quantitatively. As was shown in the previous work, grain growth of the 7 phase occurredrapidly below the completion temperature of transformation into 7 phase (T?). As a result of thatthe strong inhibiting effect of a second phase, such as 8 ferrite and/or liquid phase, on 7 grainboundary migration disappeared. Thus, the grain size of as-cast steels can be determined mainlyby T. Figure 2.5 shows the relation between Dy and T, where D, is the 7 grain size ofspecimens which were cooled to 1300 °C at a constant cooling rate, followed by waterquenching. This figure shows that the increased cooling rate also reduces the temperature ofattaining the single phase field, T1, and thereby reduces the resulting 7 grain size.5O Fe-C-2Mn- 0 Fe-C-2NiA Fe-C-2Cr^ Fe-C-1Mo41350 1400^1450(°C)32Chapter 2. Literature SearchFig. 25 Relation between Dy and Ty in various steels [7],(Open: Cooled at 0.5 °C/s, Filled: Cooled at 1.5 °C/s).The authors proposed the following equation concerning grain growth based on theirexperimental observation that the austenite grain growth started at T,Q—Do' = C iti t exp(—^dt (2.1)TTwhereDo: initial 7 grain diameterD : 7 grain diameter at 1000°CC : a constantt : timeti : time at which temperature is 1000°CQ : activation energy for grain boundary movement• : gas constantT: absolute temperature.In the case of continuous cooling at a rate of v,T = Ty — Vt^ (2.2)6Chapter 2. Literature Search05v v 2v1400^1500Ty (T)Fig. 2.6 Austenite grain growth during continuous cooling at a rates of v, 2v, 0.5v, betweenTy and 1000°C [7].Equations (2.1) and (2.2) indicate that the relative 7 grain size depends only on thecooling rate,v and the temperature, T, as is schematically shown in Fig. 2.6, in which theordinate axis denotes the relative grain size shown in arbitrary units. If Ty decreases from1450°C to 1400 °C, at the same cooling rate, the 7 grain size is halved as can be seen at CD inFig. 2.6; that is in good agreement with the experimental results shown in Fig. 2.5.Matsuura et al. [8] studied the effects of carbon content and cooling rate on thedevelopment of columnar austenitic grains in as-cast steel. They melted steels containing 0.01 to0.49 wt% carbon, and cooled them at rates of 0.3 to 40 °C/s down to 1000 °C followed byquenching. Their results are summarized as follows;1) large columnar grains were developed in the 0.1-0.3 wt%C region,2) the region of the carbon content favorable for the development of columnar grainsextended with increasing cooling rate.From these experiments and the simulations, they concluded that the formation of columnargrains was attributable to the steep temperature gradient at the 5—>y transformation front.Some of their data on austenite grain size are plotted as a function of cooling rate inFig. 2.7 [8] and compared with results obtained by researchers at Sumitomo Metal [5, 7]. Since70Chapter 2. Literature SearchMatsuura et al. [8] studied the formation of columnar austenite, both the width (the minordimension of a columnar grain) and the length (the major dimension of a columnar grain) weredistinguished in this paper, as shown in Fig. 2.7.6.0 ^5.5-5.0-—0— Matsuura.1--•— Matsuura.2A Matsuura.3—0— Sumitom0.1—m— Sumitomo.2• Sumitomo.3EECI) 4.0'CT)•c;_71 3.0-.cp 2.5-Ea) 2.0-T/5^•1.5-0.0^0.1 1^ 10Cooling Rate (°C/s)Data description;Matsuura 1^: 0.18%C-0.25%Mn, quenched from 1000 °C, width, [8]Matsuura 2^: 0.18%C-0.25%Mn, quenched from 1000 °C, length, [8]Matsuura 3^: 0.23%C-0.25%Mn, quenched from 1000 °C, width, [8]Sumitomo 1 : 0.20%C-2%Mn, quenched from 1300 °C, [7]Sumitomo 2 : 0.14%C-2%Mn, quenched from 1300 °C, [7]Sumitomo 3 : 0.16%C-2%Mn, quenched from 1300 °C, [5]Fig. 2.7 The relationship between austenite grain size and cooling rate based on the datafrom previously published literature [5, 7, 8].From this figure, in which the cooling rate dependence of as-cast austenite grain size canbe seen, it is apparent that the as-cast austenite grain size is very coarse, in the order of mm, evenat relatively high cooling rates.2.2. Effect of Second Phase Particles on Austenite Grain GrowthAs Leslie wrote [9], it is well recognized that grain refinement can be achieved by theaddition of Al, Nb [10, 11], or Ti [12], with the reason being that these elements form carbides,8Chapter 2. Literature Searchnitrides, or carbonitrides which inhibit austenite grain growth. This effect, known as Zenerpinning, is named after C. Zener [13] who quantified the phenomenon for the first time.Although there have been many studies concerning Zener pinning [e.g. 11-13] in steels, most ofthe research was concerned with grain coarsening temperatures up to 1350°C obtained during thereheating and soaking process.There have been very few works which studied the effect of second phase particles on as-cast austenite grain growth. Zhang et al. [14] studied the effect of niobium on the continuouscasting solidification structure of HSLA (High Strength Low Alloyed) steels. They simulatedcontinuous casting conditions of several steels in the laboratory by unidirectional solidification,in which a water-cooled copper chill, an exothermic mold, and a hot top were used. Thetemperature of the ingots at the time of quenching averaged 1000°C. From the experimentalresults, they concluded that the effect of Nb on cast structures was as follows:• Small additions of Nb (<0.08 wt%) to the low alloy steels resulted in nosignificant changes in primary dendrite arm spacing.• Small additions of Nb (<0.08 wt%) distinctly refined the columnar austenite grainsize.They explained the refinement of the austenite grains in the Nb bearing steel as the resultof a reduced austenite grain boundary mobility due to the pinning force exerted by the niobiumcarbonitride particles precipitated at the boundary. Ueshima et al. [15] studied the effect of MnSon grain boundary migration in a low carbon Al-killed steel using a laboratory-scale twin-rollcaster. They reported that MnS played an important role in pinning migrating grain boundaries athigh temperatures.Matsuda and Okumura [16] studied the effects of TiN precipitate particles on theaustenite grain size of low carbon, low alloy steels. Although they studied the microstructures ofsteels which were heat treated simulating the conventional continuous casting and reheatingprocess, some of their fundamental discussion is considered to be applicable to as-castmicrostructures.9XGrain boundary- —11 TiNL_^ JYRChapter 2. Literature SearchThey considered the interaction between an austenite grain boundary and a TiN particlefrom the standpoint of a balance of forces. The driving force for grain growth would be providedthrough equilibration of two forces: the first is the force due to the increase in the grainboundary energy tending to contract the austenite grain surface, and the second is related to achange in the energy of that part of the grain boundary which contacted the TiN particles. Theyproposed that the magnitude of this second force, which Gladman [17] did not take into account,depended on the type of precipitation in terms of, for example, chemical composition.Their schematic illustration of the interaction between an austenite grain boundary andTiN particles is shown in Fig. 2.8. For simplicity, they assumed that a TiN particle was a sphereof radius r.r: TiN particle sizeR: Austenite grain size2w: Contact angleFig. 2.8 Schematic representation of interaction between TiN particle andaustenite grain boundary [16].10Chapter 2. Literature SearchThey obtained the following equation both empirically and theoretically;R= K(rl f)^ (2.3)where,R : the austenite grain sizef : the volume fraction of TiNr : the mean particle size of TiNK: a dimensionless factor, which depends on the nature of the precipitateparticles.When the grain A grows in the direction of the Y axis, it meets a resistive forceoriginating from the TiN particle until finally a steady state is attained. They defined the contactangle (2w) as follows;20) =—sr(2.4)where s is the contact length of the arc. The increase in the free energy (AF) of the system,which gives rise to a resistive force against the austenite grain growth, is given by the followingequation;AF(r,w)= I-(1+ 6)5027cr sin w • rdw— Tic(r sin (0)2^(2.5)where,AF(r,w) :^the increment of the interfacial energy per TiN particleF :^the grain boundary energy per unit area (--,-800 erg/cm2 according toKazenec and Kamenska [18])8 :^the interaction parameter between the austenite grain and the TiNparticle.It is evident that Eq. (2.5) holds for any kind of precipitate particles, with the parameter 8denoting the nature of the particular particle.From Eq. (2.5), the resistive force is obtained as follows;11o22/40., (radian)2/2Fig. 2.10 Relationship betweencontact angle and particle size ofTiN [16].Chapter 2. Literature Searcht(0),r) = d(AF) I dY = 27c1"(1+ 5 —cos(o)^(2.6)Y=r—rcosw^ (2.7)where t(co,r) is the resistive force per TiN particle in the direction of the Y axis. In this case itis sufficient to examine the balance of forces only in the direction of the Y axis, since both thedriving force for grain growth and the resistive force are symmetric with respect to the Y axis.The driving force for grain growth per unit area of grain boundary is given as2F/R^ (2.8)where R is the radius of the grain A which is assumed to be a sphere. In an equilibrium state,this driving force is in balance with the resistive force originating from the TiN particle,whereby,2F/R=N•t(r,(0)^ (2.9)where N is the number of TiN particles per unit area of grain boundary. By substitutingEq. (2.6) into Eq. (2.9), the following equation can be obtained:R = { 43(1+5—cosc))f(r I f) (2.10)where the volume fraction of TiN particles (f) is equal to 4/ 3nr3/ (r I N), and (r If) gives theF: Austenite grain boundary per unit area(3-r: Interaction energy between austenite grainand TiN particleFig. 2.9 Relationship between contactangle and interaction energy [16].12Chapter 2. Literature Searchmean distance of the particles.Equation (2.3), which initially was obtained empirically, is identical to Eq. (2.10), where thedimensionless factor K is defined as,4K= ^ (2.11)3(1+8 —cos co)It will be noted that K depends on the nature of the precipitate particles through 8 and co; it was1.5 in the case of TiN according to their experimental results. They showed the relationshipbetween 8 and co for K=1.5, as in Fig. 2.9 and also the relationship between contact angle (co)and the size of TiN particles (See Fig. 2.10). Thus they found that the term 8 was much lessthan unity, as can be seen in Fig. 2.9. From the analysis above, they concluded that the increasein the interfacial energy of the grain boundary was the main factor controlling the austenite graingrowth in the presence of TiN precipitate particles.Although Matsuda and Okumura [16] pointed out that Gladman's theory [17] did not takeinto account the kind of pinning particles, they concluded that the parameter 8 denoting thenature of a particular particle was much less than unity and did not inhibit the austenite graingrowth very much; therefore it is meaningful to refer to the Gladman theory.Gladman [17] considered the balance of the energy changes between those accompanyingthe unpinning process and those accompanying grain growth. He presented a schematic diagrammodeling the pinning of a planar boundary by a spherical particle, as shown in Fig. 2.11. Thepinning mechanism between a grain boundary and a particle is largely due to the reduction ingrain boundary area when a grain boundary intersects a particle. The lowest energy position ofthe boundary when it contacts a single spherical particle will occur when the boundary intersectsthe particle across a diametrical plane. As the planar boundary moves away from the particle, theboundary will distort locally to a position of lower energy for a given displacement of the planarboundary, as shown in Fig. 2.11. The change in surface area at the boundary will thus depend ontwo factors. First, the curvature of the boundary will increase the area, and second, because theparticle still occupies a position on the boundary, it tends to reduce the grain boundary area. The13Chapter 2. Literature Searchactual change in grain boundary area, thus, depends on the shape of the boundary in the region ofthe particle and on the circle of contact between the boundary and the particle. He assumed thatthe distortion from a planar boundary varied inversely as the distance from the particle,y(s — x)= k ,^ (2.12)and that the interfacial angles between the particle and grain A and the particle and grain B wereequal,0 = 0.^ (2.13)The equation for the grain boundary profile is derived from Eqs. (2.12) and (2.13) for a givendisplacement of the boundary, s , as follows;1y = s[r2 —(— s21 )2]2(s—x).^ (2.14)Having obtained the grain boundary profile, the grain boundary area may be obtained byevaluating the area of revolution of the profile. The surface area, A, is obtained from,iL2A= 27cf y[1+ (dy I dx) 2 1 2 dx .LiThe solution to this integral for the geometry in Fig. 2.11 is,(2.15)_i - L2Isinh-1{2(s a— x)2} {1-F(^a2(s— x)2 ) (2.16)_ Li-whereia = s r2 — (-1 s2[^)2ii(2.17)1A = --ica2This equation describes the area of the distorted boundary between the limits Li and L2. Thelower limit will occur at the junction between the grain boundary and the precipitate particle; that14Chapter 2. Literature Searchgrain Apinning particlediametral position grain Bz-pinning distortionrigid planar boundarydistorted boundaryFig. 2.11 Schematic diagram illustrating a model for unpinning [17].is, Li =ls. The upper limit of x is s , but this involves an infinite area of grain boundary;2Gladman assumed the upper limit L2 by the condition y= nr. Although he did not mention thevalue of n in his paper clearly, it is estimated at 4 by Fig. 2.11. Then the general equation for thearea of the distorted planar boundary is given byA = --1 Ira2I^1_2a 2 2 (2.18)sinh-11 a^^sinh(—s2 )-- 1 +(2n2r2 2.i. + {1 +(-T.) }2n2r2)^2a^a^s_The change in area from the fully pinned condition may be obtained by subtracting thearea of the boundary when in the diametrical plane of the particle, that is,AA = A –{n(nr)2 –nr2}.^ (2.19)As the second step, Gladman examined the energy change accompanying the grain15Chapter 2. Literature Searchgrowth. When a grain grows and absorbs neighboring grains, there are two principal sources ofenergy change. Firstly, the energy of the system is increased by the expansion of the interface ofthe growing grain, and, secondly, the energy of the system is decreased by the elimination of thegrain interfaces of the grains which are absorbed by the growing one. He showed a two-dimensional representation of his model, as in Fig. 2.12. The area of grain boundaries per unitarea of interface which is eliminated by growth of the grain from a radius R to a radius (R+ s),is,Ae ,--- 3s/2Ro,^ (2.20)where Ro is the average grain size of the matrix.The growing grain, however, also increases its own boundary area. The increase in boundaryarea per unit area of interface, Ac, is given byAc ,---  2sIR.^ (2.21)Thus the net change in area, A, of grain boundary per unit area of growing interfacewhen a grain increases its radius from R to (r + s) isAn = S(2/ R - 3/2Ro).^ (2.22)Fig. 2.12 Model for grain growth. Grain A, R= Ro; grain B, R> Ro [17].16Chapter 2. Literature SearchBy defining the ratio of the radii of the growing grain and its neighbor as Z, the energychange is written as,En= sa(-2 --31Ro Z 2/(2.23)where 7 is the grain boundary energy per unit area. This is the general equation governing theenergy changes during grain growth when the boundary displacement is very small with respectto the grain size.As the third step, Gladman combined two energy changes: that accompanying theunpinning process and that accompanying grain growth. The unpinning energy was derived for asingle particle, whereas the energy change accompanying grain growth was expressed in terms ofenergy per unit area of the advancing interface. In order to combine these energy changes, it isnecessary to determine the area of the moving interface which is occupied by a single particle.The volume fraction of spherical particles, f, is expressed by the following equation,4f = —nacr',3(2.24)where fly is the number of particles per unit volume. The condition that a particle is in contactwith an interface is that, before the start of grain growth, the center of the particle must lie within±r of the interface. Thus there will be 2rnv particles per unit area of the interface. Then energyrelease per particle due to grain growth, El, isEi = Enl2rri,orEi . 2sicr27 ( 2 3 ).3Rof Z 2 )(2.25)Thus the total energy change associated with the unpinning of a single particle, ET, is17Chapter 2. Literature Search_/pinned^IIunpinned-0^200^400^600boundary displacement, 8 (A)Fig. 2.13 Energy changes during unpinning for f=0.0005, R0=l4pm;r=0.0350pm; Z=1.5; y=800 erg/cm2 [17].ET= Ep+ Ei= 7'(M)± 2sicr2y  ( 2 3),3Rof LZ 2)+50o-50(2.26)where Ep is the pinning energy and AA is the change in area of the distorted boundary derivedbefore, as in Eq. (2.19). Gladman showed a typical sequence of energy change, occurring when aboundary is unpinned, as in Fig. 2.13, and he mentioned that unless the rate of decrease in energydue to grain growth exceeded the maximum rate of increase in energy due to unpinning, anenergy barrier (H) would occur in the total energy change accompanying grain boundarymovement. Such a barrier varies as a function of particle size and volume fraction of precipitateparticles, as he showed in Fig. 2.14.Gladman applied his model only to a relatively small austenite grain size, as shown inFig. 2.14; however, Eq. (2.23) which gives energy change during grain growth contains the termRo, the matrix austenite grain size; therefore his model is useful to study the pinning effect ofsecond phase particles in a coarse as-cast austenitic structure assuming that the model is valid toa coarse grained structure.18f=0-00057,'D 400r-o.—a3s. 200r...4)I1y=o-oolo11Chapter 2. Literature Search500^1000precipitate particle radius r (A)Fig. 2.14 Effect of particle size on the energy barrier to growth for R0=14pm;Z=1.5; 7=800 erg/cm2 [17].2.3. Effective Particle Size for Pinning Austenite Grain BoundaryAs mentioned earlier, most of the studies concerning the effect of second phase particleson austenite grain growth concerned the grain coarsening temperatures attained during thereheating and austenitic soaking processes [10-12]. Gladman and Pickering [11] suggested thatthe main factor controlling grain growth was particle coalescence, showing that there was acritical particle diameter at which the particles lost their pinning effectiveness, and was given bythe following equation;where= 6Rof  ( 3 2it t 2 Z )r . : critical particle radius above which grain coarsening can occur,Ro^: matrix grain radius,: volume fraction of particle,: the ratio of radii of growing grains to matrix grains.(2.27)19Chapter 2. Literature SearchThese researchers have reported a critical diameter of 0.05-0.1 gm for MN, which meant thatonly very small particles could inhibit the austenite grain growth.Another field of study concerning the effect of second phase particles on microstructure isthat of the weld fusion zone. Kanazawa et al. [19] studied the improved toughness of the weldfusion zone by fine TiN particles and showed experimentally that TiN particles smaller than 0.05gm gave rise to a decrease in austenite grain size.These two studies suggested that the smaller particles were more effective in pinning theaustenite grain boundaries. However, this statement may not be applicable for a coarse as-castaustenitic structure because, as can be seen in Eq. (2.27), r* is a function of the matrix grain size.Also, as Gladman [20] showed theoretically in another paper, for large grains, large particles, e.g.diameter of the order of 1 gm, possibly inhibit the grain growth (see Fig. 2.15); therefore, oxides,which usually have larger sizes than AIN, Ti(C, N), or Nb(C, N) and are more stable at highertemperatures than these precipitates, may be effective for pinning as-cast austenite grainboundaries.GA MUSI2E0.01^ OA^1.00AY VOLUME 0 PARTICLESFig. 2.15 Effect of particles on the inhibition of grain growth [20].20Chapter 3. Scope and Objectives3. Scope and ObjectivesThe mid-thickness cooling rate of a thin slab whose thickness is 50 mm is estimated at0.8 °C/s from Eq. (1.1). Judging from Fig. 2.6, this will produce a large as-cast austenite grainstructure which may be too coarse to obtain preferred mechanical properties with as-castmaterial. References [5],[7],[8], also showed that the as-cast austenite grain of carbon steel tendsto be coarsest at the peritectic carbon region (0.17wt%C), a carbon level common in steel platesfor structural purposes. Therefore, the need for structure refinement is apparent for steels in thiscarbon region.To refine as-cast austenitic structures, the pinning action of second phase particles againstaustenite grain boundary movement should be considered; however, little research has beenreported on this subject. Although the effect of niobium on continuous casting solidificationstructure has been published [14] and the effectiveness of the niobium addition for refining thecolumnar austenite grain has been experimentally observed, the mechanism of this phenomenon,in particular the size of the precipitates which were responsible for pinning the austenite grainboundary, was not clear.This thesis was undertaken to:1. Examine the effect of cooling rate on the precipitation behavior of second phaseparticles (such as TiN) and their effect on the as-cast austenite grain size of steelsin the peritectic region; and2. Examine the effect of second phase particle size on its pinning effectiveness onas-cast austenite grain boundaries in peritectic steels.The experimental work was divided into two major phases:1. Remelting and solidifying tests at four different cooling rates, followed bycharacterization of the as-cast dendritic and austenitic structure for two kinds ofsteels, both of which had the same carbon content but had slightly different21Chapter 3. Scope and Objectivesmicro-alloying elements; and2. Optical and electron microscopy characterization of the size and composition ofsecond phase particles resulting from the remelting test.22Chapter 4. Experimental Procedure4. Experimental Procedure4.1. Test ApparatusThe remelting tests, which were followed by continuous cooling and helium quenching,were performed on a Gleeble 1500 Thermomechanical Simulator. The Gleeble is equipped witha sealed chamber, inside of which, samples of various shapes and sizes can be resistively heated,gas or water quenched, and hydraulically tested in compression or tension. The machine iscontrolled by a computer which provides a wide variety of heating and cooling rates, stress andstrain rates, and also stores data from each test at a set sampling rate; these conditions areprogrammed for each test.The schematic diagram of the specimen mounting procedure for the remelting tests isshown in Fig. 4.1. To control and measure the sample temperature during heating, melting, andcooling followed by helium quenching, a Pt/Pt-10%Rh thermocouple of diameter 0.25 mm wasspot welded to the sample surface at the diametrical plane at mid length of the specimen. Thetwo thermocouple wires were separated by two wire diameters. The thermocouple was sheathedwith a two-hole alumina tube, which was supported by a glass holder made specifically for thispurpose. This holder allowed the thermocouple to move only in the vertical direction after thesteel became liquid. Without the holder, stable temperature control could not be obtainedbecause of unexpected movements of the thermocouple after melting of the steel.In order to contain the liquid steel during the melting tests, a quartz tube was used, as isrecommended in the Gleeble operational manual [21]. The tube was slipped over the initiallysolid specimen. The diametral clearance required for steel was approximately 2% [21]; this isrequired to accommodate the larger linear coefficient of thermal expansion of the steel. As canbe seen in Fig. 4.1, a slit was cut on the top of each quartz tube to provide for thermocoupleaccess and an exit for gas.During each test, the time, programmed temperature, measured temperature, and23A...„...----Rt/Pt-10%Rh ThermocoupleAlumina TubeAlumina Tube Holder/Quartz Tube End ViewSide View ------ SpecimenJawChapter 4. Experimental Proceduremeasured stroke were recorded continuously. Tests were run in an atmosphere of pre-purifiedargon admitted to the test chamber after mechanical and diffusion pumping to a vacuum ofapproximately 10-4 tom When back filling the argon into the chamber after vacuum pumping,the pressure inside the chamber was set slightly below atmospheric pressure to maintain the 0-ring seal at the glass lid of the test chamber.Top ViewFig. 4.1 Schematic diagram of specimen mounting for remelting tests.4.2. Sample PreparationThe two steels chosen to examine the effect of second phase particles on the as-castaustenitic structure of carbon steel were selected for the following reasons:24Chapter 4. Experimental Procedure1. The basic chemical compositions, aside from the particle forming elements suchas Ti and Ca, were similar, as shown in Table 4.1.2. The carbon contents were close to the peritectic point, where carbon steels havebeen reported to have the coarsest as-cast austenite grains; consequently, anyeffect of second phase particles on the structure was expected to be visible.3. The quantity of elements which form second phase particles, such as titanium, wasdifferent between the two steels.4. These were common grades, and were easily obtained.Table 4.1 Chemical compositions of steels tested (wt %).C Si Mn P S Cu Ni Cr Al Ca Ti N 0Steel A 0.16 0.45 1.46 0.013 0.006 0.02 0.01 0.02 0.023 0 0.003 0.0020 0.0010Steel B 0.16 0.44 1.43 0.012 0.001 0.01 0.01 0.02 0.028 0.0017 0.012 0.0031 0.0010These are typical plain low-carbon steels used for structural purposes whose nominaltensile strength is 500 MPa. The main differences between these steels are as follows:1. Steel A is for general use.2. Steel B is usually used for specific parts such as the box column of high-risebuildings, which requires good weldability, good toughness in the heat affectedzone, and lamella tear resistance; thus Steel B contains less sulphur, morecalcium, and more titanium than Steel A.The test material was obtained from two pieces of plate, 45mmx120mmx450mm forSteel A, and 40mmx140mmx450mm for Steel B, supplied by Nippon Steel Corporation, KimitsuWorks. Test samples, 13mmx13mmx140mm were cut from these plates and then machined intothe final cylindrical shape as shown in Fig. 4.2.25Chapter 4. Experimental ProcedureFig. 4.2 Cylindrical steel specimen dimensions for Gleeble melting test.The basic specimen diameter was taken from the recommendations of the Gleebleoperational manual [21], in which tests assessing the volume-to-surface area ratio of thespecimen were reported for three diameters, viz. 6mm, lOmm, and 12mm. In order to minimizethe influence of the quartz tube inner surface on the solidified specimen, a large volume tosurface was recommended. However, the specimen with 12mm diameter, which had the largestvolume-to-surface ratio, was not recommended because it could not be cooled as fast asspecimens with smaller diameters. For these reasons, the diameter of lOmm was chosen.The outer diameter of the specimen was determined by the inner diameter of the quartztube which was used to contain the molten steel. If the gap between the specimen and the quartztube was too narrow, the quartz tube would be broken because of thermal expansion of the steelspecimen. If the gap was too large, the molten steel would flow out of the center of the specimeninto the gap between the specimen and the quartz tube. For this reason, the diameter of thespecimen was machined 2% smaller than the 10.01mm inner diameter of the quartz tube.The test specimen length was selected to accommodate the 12.5mm thread length and the26Chapter 4. Experimental Procedure30mm length of the copper electrical contact jaws at each end of the specimen, requiring a lengthof 85mm as shown in Figs. 4.1 and 4.2. The rest of the specimen length, the so-called free span,was limited by the length of the quartz tube which held the molten steel. According to Duffers[21], a suitable melt length should be 1 to 11/2 times the diameter. The typical quartz tube lengthfor the lOmm specimen was 30mm to which 2 to 4mm had to be left as clearance on each end ofthe quartz tube. Thus a total free span of 34 to 38mm was recommended [21]. In preliminarytests, specimens of 60mm free span were attempted; however, the long melting zone, which wasabout 30mm, was difficult to control, resulting in unstable thermocouple output and molten steelflowing out of the quartz tube. Thus the free span of 40mm was used.4.3. Remelting and Continuous Cooling TestsRemelting and controlled continuous cooling tests, followed by helium quenching wereperformed as follows. (Note: During these tests, errors in thermocouple output were recognized.This detail will be discussed in the "Results and Discussion" section. The temperaturesmentioned in this section are corrected values.)Before heating the specimens, the ram of Gleeble was backed off by 2mm to allow thespecimens to thermally expand during heating. The specimens were initially heated up to1425 °C at the rate of 10 °C/s, and then at a rate of 0.5 °C/s to 1520 °C, this being about 10 °Cabove the liquidus temperature of the steels tested. A slower heating rate on approaching themelting temperature was chosen to enable the Gleeble to provide the latent heat of meltingsmoothly without a sudden change of power supply, which would cause unstable experimentalconditions.The peak temperature was held for one minute, followed by continuous cooling at one offour cooling rates down to 1260 °C at which time the steels were expected to be single-phaseaustenite. The cooling rates ranged from 0.4 °C/s, which approximately corresponds to thecooling rate of conventional continuously cast slabs (210 mm thick) at the quarter thickness [22],27Chapter 4. Experimental Procedureto 17.5 °C/s, which approximately corresponds to the cooling rate at the center of thin slabs (10mm thick) cast by a belt caster [23]. Between these two cooling rates, two other cooling rates,1.2 °C/s and 4.5 °C/s, were chosen to divide the interval between minimum and maximumcooling rates into approximately equal portions on a logarithmic scale. The maximum coolingrate was obtained by air cooling of the specimen, which meant that after holding at peaktemperature for one minute the electric current through the specimen was shut off and the heat inthe specimen was extracted by the atmosphere and copper jaws. The other three cooling rateswere obtained by computer controlling the resistive heating power to the specimen.During melting, stirring of the liquid due to the magnetic field caused by the heatingcurrent flow was expected [21]. To reduce this condition, the MODE switch of the 1531 T-SERVO of the Gleeble was set at 1 to reduce the heating frequency, as was recommended byDuffers; this resulted in the heating frequency being reduced to half of the line frequency.The control of stroke was also important to maintain the full sample cross section in themolten zone and to accommodate the shrinkage of liquid steel during solidification. Thecompression pattern was decided as follows according to the results of the preliminary tests;• compressed 0.5 mm during holding at peak temperature to ensure a full samplecross section during melting, and• compressed another 1 5 mm during cooling from peak temperature down to1350°C to accommodate sample shrinkage and minimize the thermal stressesduring solidification and cooling.After cooling down to 1260 °C, the specimens were quenched by helium to retain theevidence of the austenite grain size at this temperature.4.4. Metallographic Tests4.4.1.^Sample PreparationFirst, the specimens were cut at the cross section where the thermocouples remained28Chapter 4. Experimental Procedureattached. This location was not necessarily the exact longitudinal center of the specimensbecause the thermocouples, which had been welded at the center, sometimes moved slightly fromthe original position during melting. The microstructure at the thermocouple position wasconsidered to represent the thermal history measured during the test. The microstructure wasexpected to change with increased distance from the controlling thermocouple due to the steepthermal gradients present along the axis of the sample. A lOmm long cylindrical piece of thesample containing the cross section at the thermocouple position at one end was cut and mountedin plastic. The determination of austenite grain boundaries might be difficult at a cross sectionfor some cases; therefore, the longitudinal section was examined as well as the cross section foreach specimen. The adjacent lOmm length of each specimen, also containing the cross section atthe thermocouple position was used for longitudinal section examination. This section was cutin half along the longitudinal axis, one of the two halves being mounted for longitudinalmicrostructure examination (See Fig. 4.3). All samples were polished to l[tm diamond prior toexamination.Fig. 4.3 Schematic diagram for sectioning of specimen.29Chapter 4. Experimental Procedure^4.4.2.^Secondary Dendrite Arm SpacingThe secondary dendrite arm spacing was measured for each cooling rate. The dendriticstructure was revealed using the following picric acid etch and etching procedure [24]:1. The composition of the etchant was 5g of picric acid, 100 ml of H20, and 2 drops of liquidsoap.2. The etchant was heated up to 80 °C in a water bath.3. The sample was immersed into the etchant for 2 minutes, then polished lightly with liimalumina.4. The polished surface was examined with an optical microscope.5. If the dendritic structure was not revealed clearly enough, the sample was immersed intothe etchant for an additional 30 seconds followed by polishing with him alumina.6. Steps 4 and 5 were repeated until the dendritic structure was revealed clearly.Two or three photographs of dendritic structure were taken for each sample at amagnification of 50 times revealing at least eight secondary dendrite arm spacings for eachsample. The number of measurements was determined by the number of clearly revealedsecondary dendrite arms. The dendrite arm spacing was not clearly revealed in the slowly cooledsamples.^4.4.3.^Austenite Grain SizePrior austenite grain boundaries were revealed by etching with 2% nital, as was done inprevious studies [5, 7, 8]. Photographs of the macrostructure at a magnification of 6.5 weretaken to measure the austenite grain size. Each sample was also examined at a highermagnification, up to 400 times, to clearly identify the individual austenite grain boundaries. Theaustenite grain boundaries were revealed by the existence of ferrite plates or feather-like bainitealong the boundaries. The boundaries were drawn on transparent sheets placed on themacrophotographs to facilitate the measurement of austenite grain size.The size of each austenite grain was determined as follows:1. Each grain was assumed to have an ellipsoidal shape.30Chapter 4. Experimental Procedure2. The minor axis and the major axis of the ellipse were measured on the transparentsheet which corresponded to the macrostructure at a magnification of 6.5.3. The arithmetical mean of the minor and major axes was defined to be the size ofeach grain.4.4.4.^Electron MicroscopySecond phase particles were examined with a Transmission Electron Microscopy (TEM)and a Scanning Electron Microscopy (SEM). The TEM was used mainly to determine the sizedistribution of the second-phase particles, and the SEM was used to assess the relationshipbetween second-phase particles and austenite grain boundaries. Both the TEM and SEM wereattached to an energy dispersive X-ray spectrograph (EDX), which enabled the chemicalcompositions of particles to be determined.For the TEM examination, the conventional carbon extraction replica technique wasemployed. In this technique, non-metallic particles on a etched sample surface can be extractedby a very thin carbon layer, whose thickness is the order of 100 A, deposited on the samplesurface by a carbon evaporator. Replicas were prepared by the following procedure [25],[26]:1. A polished surface was etched with 2% nital before carbon coating.2. The etched sample was placed in a carbon evaporator with carbon rods placed100 mm above the sample, and the carbon was evaporated for about 20 seconds.3. After the evaporation, the sample was removed from the evaporating unit and thecarbon film was scored into several squares with sides of about 3 mm.4. The sample was immersed in 5% nital to strip the carbon film off. Some pieces ofthe stripped carbon film were immersed in distilled water where the surfacetension forces prevented those pieces of carbon film from rolling up.5. The pieces of carbon film designated as to location on the sample were picked upwith TEM grids, and placed on a piece of filter paper to dry.The pieces of carbon film were identified as to location in the sample (step 5) because thecenter of the sample, which was considered to be the last part to solidify, might have a different31Chapter 4. Experimental Proceduredistribution of second phase particles because of the segregation of solutes.The examination of the extraction replicas quickly revealed the second phase particleswhich were usually dark compared to the light carbon of the TEM image. However, it was verydifficult to identify austenite grain boundaries in the TEM image of the replicas. Therefore, theSEM, which made it possible to observe the surface of the sample directly, was employed toobserve the relationship between austenite grain boundaries and second phase particles. For theSEM examination, the polished surface of a sample was etched with 2% nital.The size of the second phase particles was determined in a similar way as was done indetermining the austenite grain size. Namely, the shape of a particle was assumed to be anellipsoid and the arithmetical mean of the minor and major axes of the ellipse was defined as thediameter of the particle.In order to obtain the size distribution of second phase particles, two replicas wereexamined, one from the center and the other further away from the center. For each replica, threefields were investigated; each field was a square of 110[tmx1101.1m, which corresponded to thegrids of the TEM specimen holder.32Chapter 5. Results and Discussion5. Results and Discussion5.1. Remelting and Continuous Cooling Tests5.1.1.^Thermocouple OutputIn preliminary tests, some error in the thermocouple output was noticed, namely theoutput temperature of the Gleeble was lower than the expected temperature. The liquidustemperature of the steels used in the experiments was considered to be about 1512-1513°Caccording to the Kawawa equation [27] which indicates the effect of compositions as shownbelow:TLL=1536-78[wt%C]-7.6[wt%Si]-4.9[wt%Mn]-34.4[wt%P]-38[wt%S]^(5.1)where I'LL is the liquidus temperature of steels in °C.However, the Gleeble thermocouple showed that the samples melted completely at around1430°C.This error was considered to be due to the following two factors:• the error of the converter installed on the Gleeble to convert the thermocoupleoutput into temperature, and• the error of the thermocouple output, i.e. the E.M.F. (electromotive force), itself.Concerning the first factor, the calibration of the Gleeble was carried out and therelationship between input voltage and the temperature output from the converter was examined.The results are shown in Table 5.1. From these results, it can be seen that the error is about 40°Cwhen the output from the converter is around 1400°C; however, the difference between theGleeble output and the expected temperature when the steels melted was 80°C (=1510°C-1430°C), larger than the error caused by the converter, suggesting that the output from thethermocouple, i.e. the E.M.F. itself had some error.33Chapter 5. Results and DiscussionTable 5.1 Results of thermocouple calibration.Input Voltage Temperature Output from the Error(mV) corresponding to the converter of Gleeble (°C)Input Voltage* (°C) (°C)4.26 519 509 -108.46 917 906 -1111.96 1217 1196 -2114.38 1417 1385 -3215.58 1516 1477 -3916.77 1616 1569 -47*)The relationship between the input voltage (E.M.F.) and the corresponding temperature was based on thetemperature vs. E.M.F. table for type S thermocouple (Pt/Pt-10%Rh) from ASTM [28]; however, this table was for0°C reference junction; therefore, the room temperature at the time when the calibration was done (=-16°C) wasadded to the values from the ASTM table.The error in E.M.F. of the thermocouple, the second factor, was considered to be causedby the thermocouple mounting technique; namely, the thermocouple wires were directly weldedonto the surface of the sample (so called intrinsic thermocouple). Although no quantitative dataconcerning the error of E.M.F. of an intrinsic thermocouple, applicable for this work, werefound, it is said that iron affects the E.M.F. of Pt/Pt-Rh thermocouples [29]. Also Walker et al.[30], [31] reported that a small amount of iron in ceramic protection tubes covering Pt/Pt-Rhthermocouples caused a significant error (---5%) in the E.M.F. after long time use such as 120hours at 1600°C. Although the experimental time was relatively short in the present work, theearlier study suggests that the intrinsic thermocouple which is welded directly onto the steelgives an error due to the iron contamination which is caused by the diffusion of iron from thesample to the thermocouple wires, and due to the diffusion of platinum or rhodium from thethermocouple wires to the sample [32], especially above the liquidus temperature of a steel,where the diffusion rate of solutes is very large. This supposition is supported by the fact that apreviously used thermocouple showed a lower steel melting temperature after a few tests thanwas measured using a new thermocouple, although the 5mm to lOmm tip of the thermocouplewas cut after each test.34Chapter 5. Results and DiscussionIn order to eliminate the error due to the contamination of iron and the diffusion betweenthe sample and the thermocouple wires, a thermocouple sheath was needed. Alumina tubes weretried; however, the response of the thermocouple output was not fast enough to control thesample temperature properly. One study suggested [33] a ceramic coating method to protectthermocouples from attack of liquid metal without losing a good thermal contact; however, thefeasibility of this approach is a subject left for future study, as was the quantification of theE.M.F. error with respect to diffusion.Although the errors in E.M.F. were not fully understood or minimized, tests wereperformed nonetheless because of the limited time of Gleeble availability for this project, andbecause the relative cooling rates were considered to be more important than the absolute valuesof temperature. The following two measures were taken to correct the temperature data as muchas possible and to make the comparison between the tests in this project reasonable;• new thermocouple wires were used for each test, and• the measured temperatures were corrected as follows.Tcor. Tmeas.(1+=^1520 — 1440 )1440 )(5.2)where Tcor is the corrected temperature and Tmeas is the output temperature from theGleeble. Eq. (5.2) was determined for the following reasons:1. the peak temperature programmed on the Gleeble thermocouple response was1440°C, which was 10°C higher than the melting temperature, and which wasactually considered to be 1520°C, and2. the difference between the output temperature and the actual temperature wasconsidered to increase as the output temperature increased, judging from thecalibration results shown in Table 5.1.All temperatures which appear in this work have been corrected using Eq. (5.2), unless otherwisestated.35Chapter 5. Results and Discussion5.1.2.^Thermal HistoryAn example of a thermal history for a remelting and continuous cooling test is shown in Fig. 5.1,for air cooled Steel B.Steel B, Air Cooling50^100^150^200^250^3(X)^350^400^450^500Time (s)Fig. 5.1 A thermal history of a remelting and continuous cooling test.The thermal history associated with the heating at two different rates, holding at the peaktemperature, air cooling, and helium quenching can be seen in this figure. The thermal historieswere the same for all tests up to the start of cooling, which occurred after an elapsed time ofapproximately 420 seconds.Figures 5.2 to 5.5 show a comparison of the thermal histories of the A and B steels, thecontrolled cooling rate of each steel following melting being apparent. The cooling curves for aircooling (Fig. 5.2), were not linear due to the release of the latent heat of solidification. Theprogrammed and measured value of the cooling rate for each test are listed in Table 5.2.Table 5.2 Actual cooling rates for each test. (°C/s) Air Cooling *^C.R.=4.5°C/s^C.R.=1.2°C/s^C.R.=0.4°C/sSteel A 17.5 4.45 1.20 0.37Steel B 15.9 4.46 1.18 0.36* The average cooling rate was calculated between the start of the cooling and the start of quenching.36440 450410^420^430Time (s)^ Steel A■ m " Steel BC)it, 14 0 0 —3Ea)a 1300 —Ea)i-410^420^430^440^450^460^470^480^490Time (s)500Chapter 5. Results and DiscussionFig. 5.2 Cooling curves for air cooling.Fig. 5.3 Cooling curves for cooling rate of 4.5°C/s.37Chapter 5. Results and Discussion400^450^5C0^550^600^650Time (s)Fig. 5.4 Cooling curves for cooling rate of 1.2°C/s.Fig. 5.5 Cooling curves for cooling rate of 0.4°C/s.38100.1o SteelA• SteelB^ Sumitomo- Kawasaki10^Chapter 5. Results and Discussion5.2. Dendritic StructureThe dendritic structure of each test sample is shown in Figs. 5.7-5.14. For each sample,two to three regions were observed. From these photographs secondary dendrite arm spacingswere measured. The values obtained are plotted in Fig. 5.6 as a function of cooling rate. In thisfigure, the error bars correspond to ±a, where a is the standard deviation around an arithmeticmean, and the lines are taken from studies undertaken at Kawasaki Steel [34] and SumitomoMetal [35].Cooling Rate (°C/s)Fig. 5.6 Plot of measured secondary dendrite arm spacing against cooling rate.The relationship between secondary dendrite arm spacing and cooling rate determined in thecurrent work agrees well with the previous investigations, suggesting that the temperaturecorrection described in the previous section (as well as the measured temperature) is reasonable,at least for obtaining cooling rates.39(b)^ 500pmFig. 5.7 The dendritic structure of Steel A cooled in air. (x50)Chapter 5. Results and Discussion40Chapter 5. Results and DiscussionFig. 5.8 The dendritic structure of Steel B cooled in air. (x50)41Chapter 5. Results and DiscussionFig. 5.9 The dendritic structure of Steel A cooled at 4.5°C/s. (x50)42Chapter 5. Results and Discussionmorr .4,1,^10'il^'''^.'^ - • '',Ift*F474^J.<::--?(a) 500pm (b)^50011m(C)^ 500 pmFig. 5.11 The dendritic structure of Steel A cooled at 1.2°C/s. (x50)44Chapter 5. Results and Discussion(C)^ 500pmFig. 5.12 The dendritic structure of Steel B cooled at 1.2°C/s. (x50)45Chapter 5. Results and DiscussionFig. 5.13 The dendritic structure of Steel A cooled at 0.4°C/s. (x50)46Chapter 5. Results and Discussion•,vo,,v(a)(C)^ 500pmFig. 5.14 The dendritic structure of Steel B cooled at 0.4°C/s. (x50)47Chapter 5. Results and DiscussionThe secondary dendrite arm spacings of the samples cooled at the higher cooling rateswere clearly revealed as in Figs. 5.7-5.10. However, those obtained at the slower cooling rateswere revealed less clearly, as shown in Figs. 5.11-5.14. Consequently, fewer dendrite armspacing measurements were made for samples cooled at slower cooling rates; at least 20measurements were obtained for each high cooling rate condition, whereas, only 8 was possiblefor the lower cooling rates.5.3. Austenitic StructureAustenitic macrostructures of the samples are shown in Figs. 5.15-5.22. From thesephotographs, austenite grain sizes were measured, the values of which are plotted as a function ofcooling rate in Fig. 5.23. In this figure, the circles represent the arithmetic mean of the austenitegrain size for each sample and an error bar, ±a = the standard deviation around an arithmeticmean is included. In Table 5.3, the number of grains counted and the value of o' for each test islisted. The a's are relatively large especially those for the slower cooling rate; this is becauseonly a few grains were observed in the cross sections examined and those grains had a range ofsizes, as can be seen in Figs. 5.15-5.22. The grain sizes for the samples, with the exceptions ofthose that were air cooled were measured by the procedure described in Chapter 4.Measurements for the air cooled samples were made by linear analysis; the photographs of thelongitudinal sections (Figs. 5.15 (b), 5.16 (b)) were examined and the diametric line lengthswhich were intersected by two adjacent grain boundaries at the thermocouple position weremeasured to obtain the mean grain sizes and the standard deviation. These values are consideredto be reasonable because the austenite grains in the air cooled samples were very uniform, asshown in Figs. 5.15 (a), 5.16 (a).Table 5.3 The number of austenite grains counted (N) and the standard deviation (a).Air Cooling C.R.=4.5°C/s C.R.=1.2°C/s C.R.=0.4°C/sN a (mm) N a (mm) N a (mm) N a (mm)Steel A 12 0.43 47 0.76 11 1.8 7 2.6Steel B 11 0.56 35 0.98 34 0.8 11 1.648(a) Cross section at thermocouple position.Chapter 5. Results and Discussion(b) Longitudinal section.(Thermocouple position at top)4mmFig. 5.15 The austenitic structure of Steel A cooled in air. (x6.5)49(a) Cross section at thermocouple position. imm(b) Longitudinal section.(Thermocouple position at top)Chapter 5. Results and DiscussionFig. 5.16 The austenitic structure of Steel B cooled in air. (x6.5)50(a) Cross section at thermocouple position.Chapter 5. Results and Discussion(b) Longitudinal section.(Thermocouple position at top)4mmFig. 5.17 The austenitic structure of Steel A cooled at 4.5°C/s. (x6.5)51(a) Cross section at thermocouple position.(b) Longitudinal section.(Thermocouple position at top)Chapter 5. Results and DiscussionFig. 5.18 The austenitic structure of Steel B cooled at 4.5°C/s. (x6.5)52(a) Cross section at thermocouple position.(b) Longitudinal section.(Thermocouple position at top)Chapter 5. Results and DiscussionFig. 5.19 The austenitic structure of Steel A cooled at 1.2°C/s. (x6.5)53Chapter 5. Results and Discussion(a) Cross section at thermocouple position.^imni(b) Longitudinal section.^imm(Thermocouple position at top)Fig. 5.20 The austenitic structure of Steel B cooled at 1.2°C/s. (x6.5)54(a) Cross section at thermocouple position.(b) Longitudinal section.(Thermocouple position at top)Chapter 5. Results and DiscussionFig. 5.21 The austenitic structure of Steel A cooled at 0.4°C/s. (x6.5)55(a) Cross section at thermocouple position.(b) Longitudinal section.(Thermocouple position at top)Chapter 5. Results and DiscussionFig. 5.22 The austenitic structure of Steel B cooled at 0.4°C/s. (x6.5)56Chapter 5. Results and DiscussionEa) 4.0 -N^.5 3.5 -c^.3.0 -0 -.0 2.5• 2.0 -tn.• 1.5-1.0-'0.5 -0.0^0.1 1 10Cooling Rate (°C/s)Fig. 5.23 Plot of measured austenite grain size against cooling rate.Figure 5.23 shows a similar tendency of change in austenite grain size as a function ofcooling rate as can be seen in Fig. 2.6, which summarizes the results from published works. At agiven cooling rate, the austenite grain sizes are almost the same for Steel A and Steel B, except atthe cooling rate of 1.2°C/s. The explanation for this observation will be discussed in thefollowing sections.5.4. Particle Size MeasurementThe size distribution of second phase particles, which were obtained by examining carbonextraction replicas with the TEM, are shown in Figs. 5.24-5.27. Normal and log-normal particlesize distributions are common in nature [36], [37], and the log-normal distribution seemed toreveal the tendency of these measurements better. For this reason, the particle diameter is plottedon a log scale in these figures.The maximum magnification of the TEM was 100K in the scanning mode in which thequalitative chemical analysis of particles was carried out with EDX; this resulted in a resolutionof 0.05m in particle diameter.57ao3545105o • I.^Rr^.. . Iv wwf• ■•■lIlFib, 4111 nr . rnSteel B, C.R.=15.9 °Cis- Total- - - - MnS TiChapter 5. Results and Discussion(a) Steel A0.01^ 0.1^ 1^ 10Particle Diameter (pm)(b) Steel BFig. 5.24 The second phase particle size distribution for the samples air cooled.58Steel B, C.R.=4.5 °C/sl• 11-16-n-roTk„.._45403530c 2520u_1510Chapter 5. Results and Discussion(a) Steel A0.01^ 0.1^ 1^ 10Particle Diameter (pm)(b) Steel BFig. 5.25 The second phase particle size distribution for the samples cooled at 4.5°C/s.59Chapter 5. Results and Discussion(a)Steel A(b)Steel BFig. 5.26 The second phase particle size distribution for the samples cooled at 1.2°C/s.60Chapter 5. Results and Discussion(a) Steel A(b) Steel BFig. 5.27 The second phase particle size distribution for the samples cooled at 0.4°C/s.61Chapter 5. Results and DiscussionAlthough, light elements, such as carbon, nitrogen, and oxygen, could not be detectedwith the EDX attached to the TEM, most of the particles were considered to be oxides whichcontained Al, Si, and/or Ca, judging from both the shape and the chemical composition of theparticles. Also some Ti bearing particles, some of which were identified as titanium-sulphides,and the rest of which were considered to be titanium-carbonitrides (or titanium-nitrides,titanium-carbides), were found. In Steel A, which contained more sulphur than Steel B, somemanganese sulphides were also found.In the foregoing figures, the size distribution of titanium bearing particles and manganesesulphides is plotted with different notations from that of the total (thick solid line for total, thinsolid line for titanium bearing particles, and dotted line for manganese sulphides). The resultingsize distributions for the totals are different for the two steels at a given cooling rate, and thedifference has no clear dependence on cooling rate. Similarly, for a given steel, no clearrelationship exists between the size distribution and the cooling rate. This is considered to bedue to the fact that most of the particles found in each steel were oxides which were stable evenin the liquid steel, and whose distribution did not change from that in as received steel plates.The degree of stability of particles can be understood quantitatively by comparing theactivity product of the particle forming elements with their respective solubility limits shown inTable 5.3. The calculation of the solubility in liquid iron was based on data published byElliot [38], and the activity of the compounds was assumed to be one. In addition, the first-orderinteraction coefficients among solutes were taken into account in the calculation. The solubilitylimits were calculated at 1520 °C which is the peak temperature of the melting tests in thepresent work. The calculation was performed for TiN, MnS, Al203, and CaO.62Chapter 5. Results and DiscussionTable 5.4 Calculation for particle forming.Reaction ActualActivityProduct forSteel AActualActivityProduct forSteel BSolubilityProduct at1520 °C inwt%ParticleFormingTemperaturefor Steel A(°C)ParticleFormingTemperaturefor Steel B(°C)1. 6.1x 10-6 3.7 x 10-5 6.7 x10-4 1170 2) 1410 2)[Ti] +—[N2] = TiN21[Mn]+—[S2] = MnS8.7x103 1.4 x 10-3 2.2 1485 3) 1270 4)2, A „^3 ,, ,^A i^‘,..., 9.7 x10-14 1.2 x10-13 1.4 x 10-155) 5)21±11 j + -I_ l_121 = IAl2l.J3211) 7.2x107 1.6x10' 1) 5)[Ca]+—[02] = CaO21) Steel A does not contain Ca.2) Calculated by the solubility product measured by Matsuda et al. [16].3) Estimated as the temperature at which the steel becomes single phase austenite.4) Calculated by the solubility product measured by Turkdogan eta!. [39].5) Considered to exist in the liquid steel.In the table above, a smaller activity product of particle forming elements than thesolubility limit of the compounds means that the particle forming elements are soluble in theliquid steel. This is the case for TiN and MnS; however, the activity products of Al203 and CaOexceeds the solubility limits, implying that these oxide particles exist in the liquid steel at1520°C. Although it should be emphasized that this calculation can not be directly applied to theexperiments in the current study because the activity of the compounds were assumed to be one(pure compounds) and the experiments were not considered to have attained equilibrium, thecalculation does support the stability of oxides in the molten steel. Furthermore, particle formingtemperatures for TiN and MnS were estimated with the solubility products in the literature. ForTiN, the solubility product in austenite which is expressed by the following equation [16] wasused.63Chapter 5. Results and Discussion—8000 + 0.32log[%Ti][%N]=T(5.3)where T is an absolute temperature. For MnS, the solubility product by Turkdogan et al. [39]shown below was used.Ks =[%Mn][%S]fsmnlog Ks = (-9020/T) + 2.929^ (5.4)log fsmn = r(-215/T) + 0.097][% Mn]For Steel A, the solubility of MnS in 8-ferrite was calculated as well based on the free energychange for formation of MnS in 8-phase [40]. The results showed that MnS in Steel A wascompletely soluble in 8-phase and not completely soluble in 7-phase and that the change of MnSsolubility between the two phases was discontinuous; therefore, the formation temperature wasestimated as the temperature at which the steel became single phase austenite. The calculationsare based on the equilibrium state; therefore, the results are not directly applicable to the presentstudy. However, the results are helpful to have a rough idea of the particle forming sequenceduring continuous cooling.In order to examine the size distributions more quantitatively, it is useful to plot thecumulative percentage on log-normal probability paper [41]. Those plots are shown inFigs. 5.28-5.31. In these figures, all particles are plotted. If the cumulative percentage lies abouta straight line in the plot, then the log of the geometric mean of this distribution, logiG, is givenby the 50 percent point, and the log of the geometric standard deviation, log aG, is given by thedifference between the 50 and 84 percent points [41].64Chapter 5. Results and Discussiono SteelA• SteelB^ SteelA.LINE- SteelB.LINEAir Cooling1^1 1 1 1 111aa aaaa•••0 0^t^a a^a■,^,, •o^1I I^1 11ADIII.9/■ e■ aaa^aa^Ir^1111111•I1111911111111111 111^11111111110.01^0.1^10Particle Diameter (pm)Fig. 5.28 The log-normal probability plot of the second phase particle size distribution forthe samples cooled in air.99.999(.3(` 90>.-0 70_o2 50CL• 30.1:71Ez0.10.01650.01 ■^ ■ i ■, ,4 I I,99.99910.1I^11 ••IIIe 90>.=la 70ai_o2 500_° 30>Chapter 5. Results and Discussiono SteelA• SteelB^ SteelA.LINE-SteelB.LINEC.R.=4.5 °C/s0.01 01^i^10Particle Diameter (pm)Fig. 5.29 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 4.5°C/s.66Chapter 5. Results and DiscussionC.R.=1.2 °C/so SteelA• SteelB^ SteelA.LINE- SteelB.LINE 3 , .^1^1^1^1^11 1^1^1^1^1^1^1^1^1/.^•)11.1^I^I^I^II^I^I^I)),b' ".,..9 •)1^1^1^1^1 ./•U0'^.^.,1. . .^.^. "0e0.I •■ I ,.^.I1^1^IQ1^11."1^1^1 'I I I^1^i^III 4 I 1^1^1^111 I 1 I 11110.01^01 10Particle Diameter (pm)Fig. 5.30 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 1.2°C/s.99.9975310.10.016799.90.10.01Chapter 5. Results and DiscussionC.R.=0.4 °C/so SteelA• SteelB^ SteelA.LINE- SteelB.LINE 11111^111 1)1a^a11..^a^Ai a A^a^a^la A^a^./a^•••••1•it^I^tot)^•.iI^■o^•Hei i1/1^11.1111111lir• 11';11111t/7,• 41/'.11e ■^I^1^■a,■11I111111111It 1111 11^111 111 1111 I^ 11111 111 1 /11111110.01^0.1^10Particle Diameter (pm)Fig. 5.31 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 0.4°C/s.68Chapter 5. Results and DiscussionFrom these figures, the following statements can be made:• For the samples air cooled and cooled at 0.4°C/s, no significant difference in sizedistribution of second phase particles is observed between the two steels.• For the samples cooled at 4.5°C/s, the particle size distributions are differentbetween the two steels; however, the geometric means for both steels are almostthe same.• For the samples cooled at 1.2°C/s, the geometric means, as well as the particlesize distributions, are different between the two steels.As can be seen in Figs. 5.15-5.23, the difference in the as-cast austenite grain sizebetween the two steels at a given cooling rate was the largest at 1.2°C/s, and no significantdifference was observed for the rest of the cooling rates. This result can be related to thedifference in the size distribution of second phase particles mentioned above. This reasoningwill be discussed more thoroughly in later sections.To examine the cause of the different particle size distributions, two facts should be keptin mind:1) There is no clear relationship between the size distribution of second phaseparticles and related factors such as the cooling rate and the type of steel, and2) Most of the particles found were oxides which are very stable even at hightemperatures above the liquidus temperature of the steels, meaning that the sizedistribution of the oxides would not be changed from the initial stage very muchby the heat treatments applied to the samples.Thus the difference in size distribution among the samples is considered to be due to the unevendistribution of second phase particles in the as received plate steels, rather than the cooling rateand chemical composition.In order to verify the above supposition, a reproducibility test was carried out with SteelB at the cooling rate of 1.2°C/s; previous results indicated the austenite grain size was clearlyfiner than that of Steel A at the same cooling rate.69450^509^550Time (s)600 650Chapter 5. Results and Discussion5.5. Reproducibility TestThe thermal history of the repeated test is shown in Fig. 5.32, which also presents thecooling curves of the earlier tests at the same cooling rate.Fig. 5.32 The cooling curve for the reproducibility test plotted with the curves for theearlier tests (cooling rate: 1.2°C/s).In this figure, it can be seen that the cooling rate was well controlled for the reproducibility test.The resulting dendritic structure and the as-cast austenitic macro structure are shown inFigs. 5.33 and 5.34.70Chapter 5. Results and Discussion•i="'k-4'4c.1404K-V-?!::, +7}(a) Soot"500pin (b)(C)^ 500pinFig. 5.33 The dendritic structure for the repeated test (Steel B, C.R.=1.2°C/s).71(a) Cross section at thermocouple position.Chapter 5. Results and Discussion(b) Longitudinal section.(Thermocouple position at top)Fig. 5.34 The austenitic structure for the repeated test (Steel B, C.R..1.2°C/s, x6.5).72Chapter 5. Results and DiscussionThe secondary dendrite arm spacing was measured from Fig. 5.33, and plotted inFig. 5.35 together with the secondary arm spacing of the other tests. The secondary dendrite armspacing of the repeated test is consistent with that of the other tests, and is a measure of thereproducibility of the thermal history.o SteelA• SteelBFig. 5.35 The relationship between secondary arm spacing and cooling rate.In Fig. 5.34, much coarser austenite grains are observed as compared to the original testresults shown in Fig. 5.20. The austenite grains were measured and plotted in Fig. 5.36, togetherwith the grain sizes obtained in the other tests.73Chapter 5. Results and DiscussionFig. 5.36 The relationship between austenite grain size and cooling rate.The austenite grain size of the repeat test does not agree with that of the previous test under thesame conditions. Instead, it is as coarse as that of the Steel A at the same cooling rate. The sizedistribution of second phase particles was measured and plotted in Fig. 5.37.74Chapter 5. Results and DiscussionFig. 5.37 The second phase particle size distribution for the sample of the Steel B cooled at1.2°C/s.From Fig. 5.37, it can be seen that the particles are fewer in number than those in thesamples of the previous tests obtained at the same cooling rate, as shown in Fig. 5.26.The cumulative percentage plot on log-normal probability paper for this sample is shownin Fig. 5.38 together with the previous tests obtained at the same cooling rate.75I I^I^I1^1^11•1111^1^111^1• 1 1^1^1■r1-, .^..,^..,1^1 1^1^1 e,.1 1^1^11111^11I111I1 111 1^1^11^1^199.999, 90702 50a) 30as1000.10.01Chapter 5. Results and DiscussionC.R.=1.2 °C/so SteelA• SteelB^ SteelA.LINE^ SteelB.LINE• SteelB.Rep.SteelB.Rep..LINE0.01^01^1 10Particle Diameter (pm)Fig. 5.38 The log-normal probability plot of the second phase particle size distribution forthe samples cooled at 1.2°C/s.The size distribution of the second-phase particles in the sample which was thermallytreated to reproduce the test of Steel B cooled at 1.2°C/s, is different than previously obtainedand is very close to that obtained for Steel A cooled at 1.2°C/s.The results of the reproducibility test strongly suggest that the difference in sizedistribution of second phase particles among the samples is due to the uneven distribution ofsecond phase particles in the as-received steel plates, rather than to cooling rate and chemicalcomposition.76Chapter 5. Results and Discussion5.6. Volume Fraction of Second Phase ParticlesIn discussing the effect of second phase particles on the austenite grain size, the volumefraction of second phase particles is important, as well as the mean particle size which isdetermined by the size distribution, as can be seen in the literature [16, 17].Ashby and Ebeling [41] suggested that from an examination of extraction replicas thevolume fraction of spherical second phase particles could be determined using the followingequation based on the log-normal distribution:lnf =In—ic Ns + 2 ln 7x-G + 2 ln 2 crc6(5.5)where f is the volume fraction of second phase particles, Ns is the total number of particlesintersecting a unit area of the sample, YG is the geometric mean of the particle diameterdistribution, and cyc is the geometric standard deviation of the particle diameter distributionaround the mean. Ns is related to the number of particles per unit area of the replica, N's, byaNs = N's; where a is the fraction of the number of particles extracted on the replica. Althoughthere is no data regarding the value of a, it is meaningful to calculate f by using N's, instead ofNs in Eq. (5.5) in order to compare the volume fractions of second phase particles observed inthe samples obtained in the current work. All of the replicas of the current work were preparedby the same procedure, which means that the values of a should not vary from sample to sample.The results of the calculation are shown in Table 5.5.77Chapter 5. Results and DiscussionTable 5.5 Calculation of volume fraction of second phase particles on the replicas.Cooling Rate Steel N** N's TG ogo ln f f(°C/s) (111"1-2) (Pm) (gm)17.5 A 124 1.71x10-3 0.18 0.37 -8.47 2.1x10-415.9 B 69 9.50x10-4 0.20 0.71 -10.59 2.5x10-54.5 A 76 1.05x10-3 0.28 0.31 -7.31 6.7x10-44.5 B 99 1.36x10-3 0.23 1.43 -9.93 4.9x10-51.2 A 159 2.19x10-3 0.18 0.32 -7.60 5.0x10-41.2 B 73 1.01x10-3 0.53 0.61 -8.33 2.4x10-40.4 A 77 1.06x10-3 0.19 0.36 -8.73 1.6x10-40.4 B 181 2.49x10-3 0.24 0.49 -8.48 2.1x10-41.2* B 29 4.00x10-4 0.16 0.71 -11.90 6.8x10-6*) Repeated test.**) Number of particles in examined area, which was six 110pmx110um squares.The calculated volume fractions range from 6.8x10-6 to 6.7x10-4. According toMatsumiya et al. [42], who studied the crystallization behavior of inclusions during solidificationof a steel which had a similar chemical composition to the steels used in the current study, andwhich also was considered to have the same degree of cleanliness due to the fact the steel wassupplied by the same mill as the steels used in the present work, the weight percent of oxidessuch as Al203 and CaO were of the order of 10-2, which is an approximate volume fraction of10-4- 10-3. Therefore, the calculated values of volume fraction are considered to be reasonableestimations, considering that some of the particles might not be extracted into the replicas andwould remain in the samples.According to classical theory [13], the average austenite grain size in a steel whichcontains second-phase particles is related to the mean particle radius and the volume fraction ofthe particles, as can be seen in the following equation:R = K 21f(5.6)where R is the average austenite grain radius, K is a constant, F is the mean particle radius andf is the volume fraction of the particles. Although it was not clearly stated, F is considered to78Chapter 5. Results and Discussionbe the arithmetic average of the radius distribution of second phase particles. Therefore, thearithmetic average of the second phase particles was calculated for each size distribution, andthese values are included in Table 5.6 with the other measured parameters of importance toEq. (5.6).Table 5.6 Summary of the relationship between  Fl f and R.Cooling Steel F f 11 f R RRate (/j )(°C/s) (j1m) (mm) (mm)17.5 A 0.23 2.1x10-4 1.10 0.42 0.3815.9 B 0.24 2.5x1 05 9.34 0.45 0.0484.5 A 0.20 6.7x10-4 0.29 0.75 2.64.5 B 0.37 4.9x10-5 7.49 0.85 0.111.2 A 0.24 5.0x10-4 0.48 1.5 3.01.2 B 0.45 2.4x1 04 1.87 0.90 0.480.4 A 0.21 1.6x10-4 1.27 1.7 1.30.4 B 0.24 2.1x10-4 1.15 1.5 1.31.2* B 0.24 6.8x1 06 34.7 1.4 0.040*) Repeated test.The average austenite grain radius, R, as a function of I-if is plotted in Fig. 5.39.79Chapter 5. Results and Discussion1.8 1.6 —? 1.4a) 1.2 —1 —asa 0.8= 0.6 —a)04 —ct •0.2 —•^Ar Cooling-^CR.=4.5°C/s— • —C.R..4°CA0^5^10^15^20^25^30^35r/f (mm)Fig. 539 The relationship between R and ?I f .Although various values of K in Eq. (5.6) have been reported for different alloy system,consisting of a metal matrix and second-phase particles, as Nishizawa reviewed [43], the value ofK was a constant in each system. In the present work, both Steel A and Steel B consisted of asteel matrix and oxide particles accounting for the majority of the second-phase particles: for thiscondition the value of K was expected to be constant for a given cooling rate. However, asshown in Table 5.6, the values of, ,^) , which corresponds to K are different for each steel,except for the cooling rate of 0.4°C/s. This suggests that the particle pinning phenomenonrestraining the grain growth of as-cast austenite grains at the high temperature can not bedescribed by the classical Eq. (5.6) except for the cooling rate of 0.4°C/s, in which K is equal to1.3 for both samples. For the higher cooling rates, such as air cooling and 4.5 °C/s, the effect ofsecond phase particles on austenite grain size is smaller than that of a slower cooling rate(0.4°C/s) as can be seen in the small gradients of the lines which give the relationship between R80Chapter 5. Results and Discussionand 11 f .The experimental observations can be explained as follows:1) For the low cooling rate of 0.4°C/s, the balance between the driving force foraustenite grain growth and the energy barrier of pinning could be close toequilibrium. Matsuda and Okumura [16] have reported that for this condition,when f is much less than unity, as is the case in the present study, the austenitegrain size is related to il f which gives the mean particle distance.2) For high cooling rates, such as air cooling and 4.5 °C/s, the effect of cooling rateon the austenite grain growth behavior is predominant compared to that of pinningby second phase particles. The higher the cooling rate, the lower the temperatureof complete transformation to austenite (Ty) [7] below which austenite graingrowth occurs rapidly [5]. Thus at higher cooling rates, the temperature region inwhich austenite can grow is smaller and lower compared to that obtained at alower cooling rates [7]; this results in reduced austenite grain growth at the highcooling rates.Although the measured dependence of the austenite grain size on il f is not welldescribed by Eq. (5.6) for the high cooling rates, such as air cooling and 4.5 °C/s, the qualitativetrend is consistent with the conventional theory; namely, the larger lj f , the larger the austenitegrain size. However, two of the data points for a cooling rate of 1.2 °C/s, clearly show negativeslope in Fig. 5.39. This phenomenon is considered to be due to the different way that the secondphase particles affect the austenite grain growth for the medium cooling rate, as compared to thatobtained for the slow cooling rate at which Eq. (5.6) is valid and that obtained for high coolingrates at which the cooling rate affects the austenite grain size more than does the particle size anddistribution.To pursue this possibility, the particle size distribution of the two samples cooled at 1.2°C/s has been examined more thoroughly. From Figs. 5.26 and 5.30, the following points can bedescribed:81Chapter 5. Results and Discussion• The number of particles in Steel A is larger than that in Steel B; however, most ofthe particles in Steel A are smaller than 1 gm.• The number of particles which have a diameter of approximately 1 11111, thosebetween 0 6 gm and 3 gm, is larger in Steel B than in Steel A.• The average particle diameter is larger for Steel B than for Steel A.From these observations, it is suggested that the fine particles which are effective forpinning austenite grain growth in the classical theory, as can be described by Eq. (5.6), are notcapable of pinning the coarse as-cast austenite grains. In order to test this supposition, theparticles which existed on grain boundaries were examined for the sample of Steel B cooled at1.2 °C/s; this steel showed a smaller austenite grain size than Steel A at the same cooling rate.The results are summarized in Table 5.7 and the corresponding photographs of the particles areshown in Figs. 5.40-5.55.Table 5.7 The particles found on grain boundaries in Steel  B cooled at 1.2°C/s.Particle^Elements detected^Correctednumber * Diameter **(11m) A1-Ca-0-1^Al, Ca, Mn, Si, Ti^2.7A1-Ca-0-2^Al, Ca, Mn, Si, Ti^4.4Al-Ca-0-3^Al, Ca, Mn, Si, Ti^2.5Al-Ca-0-4^Al, Ca, Mn, Si, Ti^6.0Al-Ca-0-5^Al, Ca, Mn, Si, Ti^3.9Al-Ca-0-6^Al, Ca, Mn, Si, Ti^2.2Al-Ca-0-7^Al, Ca, Mn, Si, Ti^2.8Al-Ca-0-8^Al, Ca, Mn, Si, Ti^2.7Al-Ca-0-9^Al, Ca, Mn, Si, Ti 1.8Al-Ca-0-10^Al, Ca, Mn, Si, Ti^1.8Al-Ca-0-11^Al, Ca, Mn, Si, Ti^2.2Al-Ca-0-12^Al, Ca, Mn, Si, Ti 1.0^A1-1 Al, Mn, Si, Ti^2.5A1-2^Al, Mn, Si, Ti 0.9A1-3 Al, Mn, Si, Ti^1.2A1-4^Al, Mn, Si, Ti 1.1*) Corresponds to the identification number of photographs, implying the constituents of particles.**) The particle diameter was measured on photographs at x6000 magnification. The scale on these photographswas calibrated using a standard specimen examined at the same SEM conditions. This revealed that the scale was11.3% shorter than the actual length requiring the values listed in Table 5.6, to be corrected accordingly.82Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.40 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: A1-Ca-0-1)83Chapter 5. Results and Discussion\\`'(a) Low magnification.(b) High magnification..^...^.171 2 5 R 42 20KV^X.1.00K^30u mFig. 5.41 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-Ca-0-2)84Chapter 5. Results and Discussion'k.`riP9T:(a) Low magnification.(b) High magnification.Fig. 5.42 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-Ca-0-3)85Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.43 SEM photographs of particleson boundaries in Steel B cooled at1.2°C/s.(Particle No.: Al-Ca-0-4)86Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.44 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-Ca-0-5)87Chapter 5. Results and Discussion(a) Low magnification.2 5 4 7 2 ci K(b) High magnification.Fig. 5.45 SEM photographs of particleson boundaries in Steel B cooled at1.2°C/s.(Particle No.: Al-Ca-0-6)88Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.46 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: A1-Ca-0-7)89Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.47 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-Ca-0-8)- '5944. 20KV^X6.00 I(^u. 0 u m90Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.48 SEM photographs of particleson boundaries in Steel B cooled at1.2°C/s.(Particle No.: Al-Ca-0-9)91Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.49 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-Ca-0-10)92Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.50 SEM photographs of particleson boundaries in Steel B cooled at1.2°C/s.(Particle No.: A1-Ca-0-11)93Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.51 SEM photographs of particleson boundaries in Steel B cooled at1.2°C/s.(Particle No.: Al-Ca-0-12)94Chapter 5. Results and Discussion(a) Low magnification.VI 2 5 9 7: 2 2 0(b) High magnification.Fig. 5.52 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-1)95Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.53 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-2)96Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.54 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-3)97Chapter 5. Results and Discussion(a) Low magnification.(b) High magnification.Fig. 5.55 SEM photographs of particleson boundaries in Steel B cooled at1.2° C/s.(Particle No.: Al-4)98Chapter 5. Results and DiscussionAll of the particles were considered to be oxides judging from their rounded shape and theirchemical compositions. The size of the particles was quite large, most being larger than 1 pm,which supports the supposition that the fine particles are not effective in pinning the coarse as-cast austenite grain boundaries. This can be explained by the influence of thermal activation onthe interaction between particles and migrating grain boundaries. Gore et al. [44] reported thatthe thermally activated unpinning of boundaries was found to become significant at smallparticle sizes, << 1 lum, and high temperatures such as above 870°C. In the present study, thelower limit of particle size observed was 0.05 gm.5.7. Application of Gladman's ModelTo support the previous discussion, Gladman's model [17] was applied to calculate theenergy barrier of unpinning as a function of particle diameter. The calculation conditions wereas follows:1) The maximum particle radius for which the calculation was carried out was lOpm.2) The average austenite grain radius of the matrix was assumed to be Ro = 0.4, 0.8,and 1.4 mm.3) The volume fraction of particle, f was taken to be 0.001.4) The ratio of the radii of growing grains to matrix grains, Z was examined forvalues of 1.0, 1.2, 1.4, 1.6, 2.0, 3.0, 4.0, 5.0, 6.0, and 8.0.5) The surface energy per unit area of boundary of austenite, y, was assumed to be0.8 J/m2.These conditions were chosen to simulate the sample of Steel B cooled at 1.2 °C/s. Theresults are shown in Figs. 5.56-5.58.99Chapter 5. Results and Discussion4.50E-09 ^4.00E-093.50E-093.00E-09 —2.50E-092.00E-091.50E-091.00E-095.00E-10aooE+oo ^Matrix Grain Radius=0.4mm, f=0.001Z=1.0Z=1.2Z=1.40 1^2^3^4^5^6^7^8^9^10Particle Radius (pm)(a) Ratio of radii of growing grains to matrix grains: Z=1.0, 1.2, 1.4Matrix Grain Radius=0.4mm, f=0.0012.50E-112.00E-111.50E-111.03E-11 —5.00E-12 —0.00E+00 ^Z=1.40^1^2^3^4^5^6^7^8^9^10Particle Radius (pm)(b) Ratio of radii of growing grains to matrix grains: Z=1.41001.80E-121.60E-121.40E-121.20E-121.00E-128.00E-136.00E-134.00E-132.03E-130.00E+001.20E-13 ^1.00E-13C)8.00E-14in 6.00E-142 4.00E-14Ui2.COE -140.00E+00Z=4.0Z=5.0"' Z=6.0- Z=8.0Chapter 5. Results and DiscussionMatrix Grain Radius=0.4mm, f=0.001 Z=1.6Z=2.0'" Z=3.01^ 2^3Particle Radius (pm)(c) Ratio of radii of growing grains to matrix grains: Z=1.6, 2.0, 3.0Matrix Grain Radius=0.4mm, f=0.0010^0.1^0.2^0.3^0.4^0.5^0.6^07Particle Radius (pm)(d) Ratio of radii of growing grains to matrix grains: Z=4.0, 5.0, 6.0, 8.0Fig. 5.56 Effect of particle size on the barrier to grain growth. The average austenite grainradius = 0.4 mm, the volume fraction of particles = 0.001, and the surface energy per unitarea of boundary of austenite = 0.8 J/m2.1012.50E-09tr)•E2.00E-091.50E-09COMatrix Grain Radius=0.8mm, f=0.001Z=1.0Z=1.2"11 Z=1.4-............ ............2^3^4^5^6^7^8 9Particle Radius (pm) 1 10En 1.00E-09 -C)Ui5.00E-10 -0.00E+00Chapter 5. Results and Discussion(a) Ratio of radii of growing grains to matrix grains: Z=1.0, 1.2, 1.4Matrix Grain Radius=0.8mm, f=0.0019.00E-11 ^8.00E-117.00E-11 -17').4.2 6.00E-11r5 5.00E-11tO>. 4.00E-11t 3.00E-11 -c2.00E-11 -1.09E-110.00E+00 ^0 1^2^3^4^5^6^7^8^9^10Particle Radius (pm)Z=1.4(b) Ratio of radii of growing grains to matrix grains: Z=1.41026.00E-12 --2 5.00E-12• 4.00E-12CO3 00E-12ui• 2.00E-127.00E-121.03E-120.COE +00Z=1.6Z=2.0Z=3.04.50E-134.00E-133.50E-133.COE -132.50E-132.00E-131.50E-131.00E-135.1E-140.COE +COChapter 5. Results and DiscussionMatrix Grain Radius=0.8mm, f=0.0010^ 2^3^4^5^6Particle Radius (pm)(c) Ratio of radii of growing grains to matrix grains: Z=1.6, 2.0, 3.0Matrix Grain Radius=0.8mm, f=0.0010^0.2^0.4^0.6^0.8^1.2^14Particle Radius (pm)Z=4.0Z=5.0''"• Z=6.0- Z=8.0(d) Ratio of radii of growing grains to matrix grains: Z=4.0, 5.0, 6.0, 8.0Fig. 5.57 Effect of particle size on the barrier to grain growth. The average austenite grainradius = 0.8 mm, the volume fraction of particles = 0.001, and the surface energy per unitarea of boundary of austenite = 0.8 J/m2.1031.60E-091.40E-091.20E-09ft 1.00E-09L.m 8.00E-1012 6.00E-10 —a)4.00E-102.00E-100.00E+00 ^F I^2^3^4^5^6^7^8^9Particle Radius (pm)Z=1.0Z=1.2I"' Z=1.41 10Chapter 5. Results and DiscussionMatrix Grain Radius=1.4mm, f=0.001(a) Ratio of radii of growing grains to matrix grains: Z=1.0, 1.2, 1.4Matrix Grain Radius=1.4mm, f=0.0011.80E-101.60E-101.40E-101.20E-10 —soi 1.00E-10 —CO>. 8.00E-11 —C)6.00E-114.00E-112.03E-11 —0.00E+09 ^ Z=1.40^1^2^3^4^5^6^7^8^9^10Particle Radius (pm)(b) Ratio of radii of growing grains to matrix grains: Z=1.41042.50E-112.00E-111.13•E 1.50E-11COEn 1.00E-11LU5.00E-120.00E+00Matrix Grain Radius=1.4mm, f=0.001 Z=1.6Z=2.0Z=3.0Z=4.0Z=5.0Z=6.0- Z=8.01.40E-121.20E-12-2 1.00E-12 -r)L- 8.00E-13CO• 6.00E-13a)La• 4.00E-13 -2.00E-130.00E+09Chapter 5. Results and Discussion0^1^2^3^4^5^6^7^8^9^10Particle Radius (pm)(c) Ratio of radii of growing grains to matrix grains: Z=1.6, 2.0, 3.0Matrix Grain Radius=1.4mm, f=0.0010.5^1.5^2^25Particle Radius (pm)(d) Ratio of radii of growing grains to matrix grains: Z=4.0, 5.0, 6.0, 8.0Fig. 5.58 Effect of particle size on the barrier to grain growth. The average austenite grainradius = 1.4 mm, the volume fraction of particles = 0.001, and the surface energy per unitarea of boundary of austenite = 0.8 J/m2.105Chapter 5. Results and DiscussionFor Z (the ratio of the radii of growing grains to the matrix grains) = 1.0 and 1.2, theenergy barrier increases as the particle diameter increases regardless of the average austenitegrain size of the matrix up to the particle diameter of 10 pm employed in the calculation.However, if Z is greater than 1.4, it can be seen that there is a specific particle diametercorresponding to a maximum energy barrier for each condition. Denoted this particle diameter asr*, it can be seen that r* varies depending on the average austenite grain size and Z whichrepresents the heterogeneity of the austenitic structure. One qualitative explanation is as follows:1) As reviewed in Chapter 2, the total energy change associated with theunpinning of a single particle, ET, isET= Ep+ Ei= y . (AA) + 2snr2y r 2 3) (see Eq. (2.27));3Rof Z 2)therefore, if Z <4/3=1.33, the second term of the equation above, which represents the energyrelease per particle due to grain growth, becomes positive, which means that there is an increasein boundary area and the heterogeneity is not large enough for grain growth to occur. As a result,the total energy change associated with the unpinning of a single particle, ET, increasesmonotonically as the particle diameter, r, increases as shown in Eq. (2.27). AA is also afunction of r, as can be seen in Eqs. (2.19) and (2.20), where, AA increases as r increases.Therefore, the energy barrier to grain growth increases monotonically as the particle diameterincreases. In the case of Z=1.4, which is slightly greater than 4/3=1.33, the energy barrier tograin growth increases monotonically as r increases up to r = 101.im for R 0=0.8mm and 1.4mm;however, the energy barrier is expected to decrease at some point as r increases further, as isseen in the case of Z=1.4 and Ro=0.4mm.2) If Z >4/3=1.33, there is a decrease in the boundary area, which is a requirementfor grain growth to occur, and the second term of Eq. (2.27) becomes negative. For thiscondition, the balance between the increase in the area of grain boundary due to the distortionexerted by a pinning particle and the decrease in the area due to grain growth should be106Chapter 5. Results and Discussionconsidered. For particles of small diameter, the first term, namely the increase in the area due tothe distortion is dominant, resulting in a larger energy barrier for larger particles. On the otherhand, as the particle diameter increases, the number of particles per unit area of grain boundarymay decrease, assuming the volume fraction, f, is constant. As a result, the second term, theenergy release per particle due to grain growth, becomes larger and dominant in Eq. (2.27),resulting in a decrease in the energy barrier for larger particles.Thus, the presence of a particle diameter associated with a peak energy barrier, r*, can beunderstood. The results of Ro and r* are summarized in Table 5.8 and the r* vs. Z is plotted inFig. 5.59. It can be seen that r* increases as Z decreases, and that r* increases as Ro increases.Table 5.8 The values of r* (pm) for various Ro and Z.Z 1.6 2.0 3.0 4.0 5.0 6.0 8.0Ro (mm)0.40 1.4 0.74 0.40 0.36 0.34 0.30 0.280.80 2.9 1.4 0.90 0.72 0.68 0.62 0.581.4 5.4 2.6 1.5 1.3 1.2 1.1 1.06o• Rc=0.4rrrn- -0- - Rc=0.8rrrn- - - -. R cl .4rrrn:)... ,.....s.. .....^hr ......... A- AA ........ 4,•C^ -o•1^2^3^4^5^6^7^8^9zFig. 5.59 Effect of austenite grain size and heterogeneity on r*.107Chapter 5. Results and DiscussionAlthough the particle size used in Gladman's model is the average of the size distributionof particles, which assumes that all of the particles have the same radius, r, occupying thevolume fraction, f, in the matrix, these results provide information as to the particle size that iseffective for pinning the coarse as-cast austenite grains and helps to explain the preponderance oflarger particles found on the austenite grain boundaries (Table 5.7).108Chapter 6. Conclusions6. Conclusions The results of this research can be summarized as follows:1) Ti enriched particles whose diameters were in the range of 0.1 - 0.5 gm did noteffectively inhibit the as-cast austenite growth.2) Oxides with larger diameters than TiN particles were effective pinning particlesfor the grain boundaries of as-cast austenite grain, in the temperature region of1200 to 1300°C.3) The higher cooling rates examined in this study, 4 — 20 °C/s, reduced austenitegrain growth more effectively than second phase particles.It is thought that the amount and the size of TiN particles in Steel B were not largeenough for pinning the coarse as-cast austenite grains in the experimental condition examined inthis work. However, if more titanium is added to a steel, titanium nitrides may form duringsolidification resulting in their larger diameters [45], which will be favorable in terms of pinningaustenite grain boundary and reducing grain growth.The experimental results presented in this thesis suggest that the size and composition ofsecond phase particles which are effective for the pinning of grain growth may vary depending onthe temperature where the grain growth takes place and on the grain size of the matrix.The cooling rates applied in this research, correspond to those at the mid-thickness ofslabs whose thickness is 10 — 20 mm in a thin slab casting process, as shown in Eq. (1.1). Forthicker slabs, slower cooling rates would be expected at the mid-thickness, even though thecooling rates in the surface region will remain high. For this reason, a relatively thick slab,whose thickness is e.g. 50 mm, second phase particles will be effective to producing a finemicrostructure throughout the thickness.The following recommendations are made for future work:109Chapter 6. Conclusions1) Quantify the effect of cooling rate on the amount and particle diameter of oxidesforming during melting.2) Find a proper condition for the formation of TiN particles which are effective forpinning as-cast austenite grain boundaries at the high temperatures followingsolidification.3) Relate the as-cast austenitic structure to the ferritic structure and the presence ofsecond phase particles which form at different temperatures.110ReferencesReferences[1] M.Cygler and M.Wolf: Iron and Steel Maker, Vol.13, No.8 (1986), pp. 27-33[2] Y.Itoh: Tetsu-to-Hagane, 72 (1986), pp. 1667-1673[3] B.Pollock: American Metal Market, 100, (48), (Mar. 1992), p. 1, p. 16[4] K.Shibuya and M.Ozawa: ISIJ International, 31 (1991), pp. 661-668[5] Y.Maehara, K.Yasumoto, Y.Sugitani, and K.Gunji: Tetsu-to-Hagane, 71 (1985), pp. 1534-1541[6] W.C.Leslie: The Physical Metallurgy of Steels, TechBooks (1991), p. 77[7] K.Yasumoto, T.Nagamichi, Y.Maehara, and K.Gunji: Tetsu-to-Hagand, 73 (1987), pp. 1738-1745[8] K.Matsuura, Y.Itoh, and K.Matsubara: Tetsu-to-Hagane, 76 (1990), pp. 714-721[9] W.C.Leslie: The Physical Metallurgy of Steels, TechBooks (1991), p. 169[10] K.J.Irvine, F.B.Pickering, and T.Gladman: JISI, 205 (1967), pp. 161-182[11] T.Gladman and F.B.Pickering: JISI, 205 (1967), pp. 653-664[12] T.J.George and J.J.Irani: J.Aus.Ins.Metals, 13 (1968), pp. 94-106[13] C.Zener: Private communication to C.S.Smith in Trans.AIME, 175 (1948), pp. 47-58[14] H.Zhang, N.S.Pottore, C.I.Garcia, M.G.Burke, and A.J.Deardo: HSLA Steels '85, Proc. Int.Conf. on HSLA Steels '85, Beijing, 1985, (1985), pp. 445-448[15] Y.Ueshima, T.Sawai, T.Mizoguchi, K.Miyazawa, and S.Mizoguchi: Proc. SixthInternational Iron and Steel Congress, Oct.21-26, 1990 at Nagoya, Iron and Steel Institute ofJapan, Val Fundamentals, (1990), pp. 642-648[16] S.Matsuda and N.Okumura: Transfron Steel Inst.Jpn., 18 (1978), pp. 198-205[17] T.Gladman: Proc.Roy.Soc., 294 (1966), pp. 298-309[18] K.Kazence and E.Kamenska: Fizika.Metal., 12 (1961), pp. 91-96[19] S.Kanazawa, A.Nakajima, K.Okamoto, and K.Kanaya: Tetsu-to-Hagane, 61 (1975),pp. 2589-2603[20] T.Gladman: Inclusions (edited by F.B.Pickering), The Institution of Metallurgists,Monograph No.3, (1979), pp. 172-181111References[21] Duffers Scientific, Inc.: Dynamic Thermal/Mechanical Metallurgy Using The Gleeble 1500,2nd ed., pp. 165-175[22] A Nippon Steel Corporation Internal Report, (Unpublished work)[23] J.Inagaki, Y.Takada, K.Nakaoka, and K.Yamamoto: Tetsu-to-Hagane, 71 (No.10, 1985),A233-A236[24] N.Walker: private communication (March 24, 1993)[25] C.O.I.Emenike and J.C.Billington: Materials Science and Technology, 5 (June 1989),pp. 566-574[26] G.Thomas: Transmission Electron Microscopy of Metals, John Wiley & Sons, Inc. (1962),pp. 135-139[27] T.Kawawa: Tokko-Binran (Handbook for Steel), by ISIJ, 3rd ed., Maruzen, Tokyo, 1981,Val., p. 205[28] 1991 Annual Book of ASTM Standards, Vol.14.03, Temperature Measurement, p. 158[29] R.E.Bedford, T.M.Dauphinee, and H.Preston: Tools and Techniques in Physical Metallurgy,(edited by F.Weinberg), Vol.1, pp. 26-29[30] B.E.Walker, C.T.Ewing, and R.R.Miller: The Review of Scientific Instruments, 33 (Oct.1962), pp. 1029-1040[31] B.E.Walker, C.T.Ewing, and R.R.Miller: The Review of Scientific Instruments, 36 (May1965), pp. 601-606[32] P.A.Kinzie: Thermocouple Temperature Measurement, John Wiley & Sons, Inc. (1973),pp. 36-37[33]G.N.Heintze, J.D.Gates, R.McPherson, and R.A.Jago: Mat.Sci.Tech., 2 (1986), pp. 1245-1246[34] S.Itoyama, H.Nakato, T.Nozaki, Y.Habu, and T.Emi: Fifth Intl. Iron and Steel Congress,Washington, D.C., Steelmaking Proceedings, (1986), Vol.69, pp. 833-838[35] K.Ushijima, T.Okazaki, M.Yoshihara, S.Shiode, and M.Koide: Fifth Intl. Iron and SteelCongress, Washington, D.C., Steelmaking Proceedings, (1986), Vol.69, pp. 827-833[36] G.Herdan: Small Particle Statistics, Butterworths, London, (1960)[37] F.E.Luborsky: J.AppLPhys., 33 (1962), pp. 1909-1913[38] J.F.Elliot: Electric Furnace Steelmaking (edited by C.R.Taylor), Iron and Steel Society(1985), pp. 291-319112References[39] E.T.Turkdogan, S.Ignatowicz, and J.Pearson: JISI, 180 (1955), pp. 349-354[40] H.A.Wriedt and H.Hu: Metall.Trans.A, 7(1976), pp. 711-718[41] M.F.Ashby and R.Ebeling: Trans.Met.Soc.AIME, 236 (Oct., 1966), pp. 1396-1404[42] T.Matsumiya and T.Ohashi: Tetsu-to-Hagane, 71 (1985), S1069[43] T.Nishizawa: Tetsu-to-Hagane, 70 (1984), pp. 1984-1992[44] M.J.Gore, M.Grujicic, G.B.Olson, and M.Cohen: Acta Metall., 37 (1989), pp. 2849-2854[45] S.V.Subramanian, S.Shima, G.Ocambo, T.Castillo, J.D.Embury, and G.R.Purdy: HSLASteels '85, Proc. Int. Conf on HSLA Steels '85, Beijing, 1985, (1985), pp. 151-161113Appendix (The Program for Gladman Model)A.1 Flowchart( Start )Input austenite grairadius(Ro) and Z.41 ^/ Input particleradiusalculation of pinning energy for agiven boundary displacement (S)Determine the minimum and themaximum for the pinning energy(PE_min, PE_max)Calculate the energy barrier forunpinning (PE_max-PE_min)Yeseed another calculationr another Z and Ro?No ^( Stop )YesAppendixAppendixA.2 Source Code************************************************************************C Program to calculate the energy barrier for grain growth************************************************************************C MAIN PROGRAMIMPLICIT DOUBLE PRECISION (A-H2O-Z)DIMENSION GS(10), ZZ(10)C GS:Austenite grain size; dimension:micronGS (1)=400.D0GS(2)=1000.D0GS(3)=1200.D0GS(4)=1400.D0GS(5)=1600.D0GS(6)=1800. DOGS(7)=2000.D0GS(8)=2200.D0GS(9)=2400.D0GS(10)=2600.D0ZZ(1)=1ZZ(2)=1.2D0ZZ(3)=1.4D0ZZ(4)=1.6D0ZZ(5)=2.D0ZZ(6)=3.D0ZZ(7)=4.D0ZZ(8)=5.D0ZZ(9)=6.D0ZZ(10)=8.D0C*DO 10 1=1,10DO 20 J=1,10RO=GS(I)Z=ZZ(J)OPEN (UNIT=5, FILE='PIN.TXT', STATUS='UNKNOWN')WRITE (5,501) RO, Z501 FORMAT (4H R0=,1F6.1,3H Z=,1F3.1)CALL ENERGY(RO,Z)20 CONTINUE10 CONTINUESTOPENDAppendixSUBROUTINE ENERGY (RO,Z)************************************************************************C This subroutine calculates the energy barrier for grain growth.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)COMMON /A01/ PIDIMENSION ET(20001), H(100), PR(100)CALL DATAIN (N,GAMMA,F)OPEN (UNIT=5, FILE='PIN.TXT', STATUS='UNKNOWN')WRITE (5,501) N,GAMIvIA,F501 FORMAT (3H N=,112,7H GAMIVIA=,1F3.1,3H F=,1F6.4)R=0.D0PR(1)=RH(1)=0.D0C*DO 30 J=2,101R=R+0.1D0PR(J)=RCALL INIT (S,ET,ET_MAX,ET_MIN)DO 10 1=2, 20001C*S=S+0.001D0IF (S.GT.(2.D0*R)) GO TO 20CALL,AREA_B (S,R,N,AREA)CALL ENE_GG (S,R,GAMMA,R0,F,Z,E1)CALL ENE_TO (1,AREA,GAMMA,E1,ET)IF (I.GE.3) CALL MINMAX (I,ET,ET_MAX,ET_M1N)10 CONTINUE20 H(J)=ET_MAX-ET_MINIF ((J.GT.10).AND.(H(J).LE.0.D0)) GO TO 4030 CONTINUE40 CALL OUTPUT (J,PR,H)RETURNENDSUBROUTINE DATAIN (N,GAMMA,F)116Appendix************************************************************************C This subroutine reads the input data to run the program.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)COMMON /A01/ PIDATA P113. 141592D01C Dimension of S:micron, R:micron, RO:micronC S =O.DOC R=0.0350D0C R0=1800.D0N=4C Z=8.D0F=0.001D0C Dimension of GAMMA: Joule/m**2GAMMA=0.8D0RETURNENDSUBROUTINE INIT (S,ET,ET_MAX,ET_MIN)************************************************************************C This subroutine initializes some values.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)DIMENSION ET(20001)S=0.D0ET_MAX=0.D0ET_MIN=0.D0DO 10 I=1, 1000ET(I)=0.D010 CONTINUERETURNENDFUNCTION ASH(X)************************************************************************C This subroutine calculates the inverse function of hyperbolicC sine.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)ASH=DLOG(X+DSQRT(X**2.D0+1.D0))RETURNEND117AppendixSUBROUTINE AREA _B (S,R,N,AREA)************************************************************************C This subroutine calculates the change in the area of the grainC boundary including the effect of the distorted planar boundary.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)COMMON /A01/ PIAREA-0.D0A=S*DSQRT(R**2.D0-(0.5DO*S)**2.D0)T I =ASH(A/2.DO/DBLE(FLOAT(N))**2.DO/R**2.D0)T2=ASH(S**2.D0/2.DO/A)T3=DSQRT( 1 .D0+(2.D0*(DBLE(FLOAT(N))**2.D0)*(R**2.D0)/A)**2.D0)T4=DSQRT( 1 .D0+(2.DO*A/(S**2.D0))**2.D0)AREA=-0.5DO*PI*A*(T1-T2-T3+T4)A_INIT=PI*(DBLE(FLOAT(N))*R)**2.DO-PI*(R**2.D0)AREA=AREA-A_INITC Dimension of AREA: (micron)**2RETURNENDSUBROUTINE ENE_GG (S,R,GAMMA,RO,F,Z,E1)************************************************************************C This subroutine calculates the energy release per particleC due to grain growth.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)COMMON /A01/ PIEl =2.DO*S*PI*(R**2.D0)*GAMMA/(3.DO*RO*F)E 1 =E 1 *(2.DO/Z- 1.5D0)/( 10.D0** 1 2. DO)RETURNENDSUBROUTINE ENE_TO (I,AREA,GAMMA,E1,ET)************************************************************************This subroutine calculates the total energy change assosiatedwith the unpinning of a single particle.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)DIMENSION ET(20001)118AppendixET(I)=AREA*GAMMA/(10. DO**12.D0)+ElRETURNENDSUBROUTINE MINMAX (I,ET,ET_MAX,ET_MIN)************************************************************************C This subroutine calculates the minimum and maxmum values ofC the total energy change which is a function of grain boundaryC displacement.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)DIMENSION ET(20001)D1=ET(I-1)-ET(I-2)D2=ET(I)-ET(I-1)PROD=D1*D2IF ((D1 .GT.O.D0).AND.(PRODIT.O.D0)) ET_MAX=ET(I-1)IF ((DI .LT.O.D0).AND.(PROD.LT.O.D0)) ET_MIN=ET(I-1)RETURNENDSUBROUTINE OUTPUT (JJ,PR,H)************************************************************************C This subroutine outputs the results.************************************************************************IMPLICIT DOUBLE PRECISION (A-H2O-Z)DIMENSION PR(100), H(100)OPEN (UNIT=5, FILE='PIN.TXT', STATUS='UNKNOWN)DO 10 J=1, JJ-1WRITE (5,502) PR(J), H(J)502 FORMAT (1F5.2,1H„1E14.6)10 CONTINUERETURNEND

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