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Mould thermal response and formation of defects in the continuous casting of steel billets

University of British Columbia

With the drive to optimize the design of billet moulds and establish standard operating practices, significant improvements in billet quality have been achieved in the last two decades. As this trend continues, the role of process upsets (or unsteady-state events) in the mould, such as metal level fluctuations and/or poor maintenance, are assuming increasing importance, relative to caster design and operation, in the generation of quality problems. Of the common process upsets encountered in billet casting, random metal level fluctuations due to open-stream pouring, are by far the most significant. The metal level fluctuations originate from the roughness of the metal stream plunging from the tundish into the mould pool. Rougher streams entrain more gas in the mould pool creating more bubbles which upon their eruption from the surface generate turbulence and waves. The resulting continuously shifting meniscus interacts with the oil weeping down the mould wall to create variable lubrication and heat-flow conditions, both on a given face and one face relative to another; this generates a number of quality problems in billets. The main focus of this study was to understand the nature of metal level fluctuations and their impact on the formation of billet defects such as laps, bleeds and rhomboidity. A series of six instrumented-mould trials were conducted at four Canadian mini-mills and data on mould wall temperature, cooling water temperature, metal level fluctuations, casting speed, oscillation characteristics and mould/strand friction, were acquired using a computer-based data acquisition system. In addition, the tundish pouring stream and the meniscus in the mould were filmed using a video camera; and 35mm photographs were also taken. A number of billet samples corresponding to the data gathered, were collected for quality evaluation. In the more recent trials, a Supervisory Control And Data Acquisition (SCADA) system was installed and the mould temperatures were monitored on-line as well. This study has uncovered the mechanism of defect formation in the meniscus region due to random metal level fluctuations. The importance of peak hot face temperature relative to the boiling range of oil has been highlighted with respect to mould heat transfer and defect formation. The analysis of mould heat transfer suggests that the greatest enhancement in heat transfer due to oil boiling and/or pyrolysis, occurs when the peak hot face temperature falls i n the boiling range of oil. A new mechanism rooted in metal level luctuations, peak hot face temperature and oil distribution has been proposed to explain the formation of laps and bleeds in billet casting. With respect to rhomboidity, it was shown that a combination of high mould heat transfer and a short freezing range is important from the standpoint of severity of the problem, in addition to the non-uniform heat transfer conditions present in the meniscus region. This mechanism explains the higher severity of rhomboidity in medium carbon grades observed in this study and also reported in the literature. With respect to the development of the "intelligent mould", an expert-system-based monitoring and control system for billet casting, strategies for the on-line, hot inspection of billet defects were outlined based on a detailed analysis of measured mould thermal response and billet quality evaluation. Further, a number of interesting simulations were undertaken to evaluate the effect of various mould events on the thermal response of the mould. In the case of transverse depressions, the mould thermal response was computed using the characteristics of depressions measured on an entire billet; a good agreement was obtained between the measured and the computed mould thermal response. Thus, the findings of this study have a significant role to play in the creation of the "intelligent mould" and also, in the development of a knowledge base for the on-line, hot surface inspection of billets.

Thesis/Dissertation

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2009-02-19

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http://hdl.handle.net/2429/4821

Materials Engineering

University of British Columbia

UBCV

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