The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution
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I.
INTRODUCTION
MICROSTRUCTURAL changes that occur during hot rolling of steel strip are a result of the complex interplay between thermal, mechanical, and metallurgical phenomena. Processes have been operated for decades, with the operating practices being established by trial and error to ensure that dimensional tolerances and mechanical property specifications are met. Advances in process control driven by the development of more accurate sensors and burgeoning computer power have allowed greater control to be exercised over thermomechanical processes and the resulting microstructure and mechanical properties of the products. However, models for process control are in an evolutionary stage, moving away from the fully empirical, which are based on accumulated data bases, to the knowledge intensive, which are rooted in the fundamental principles of transport phenomena, continuum mechanics, and physical metallurgy. This transition is important to the industry, because knowledgebased models provide a more scientific framework within
C. DEVADAS, formerly Graduate Student, The Centre for Metallurgical Process Engineering, The University of British Columbia, is Research Engineer, Comalco Research Centre, Melbourne, Victoria 3074, Australia. I.V. SAMARASEKERA, Professor, The Centre for Metallurgical Process Engineering, and E.B. HAWBOLT, Professor, Department of Metals and Materials Engineering, are with The University of British Columbia, Vancouver, BC V6T 1W5 Canada. Manuscript submitted September 5, 1989. METALLURGICAL TRANSACTIONS A
which to effect process control and produce material with the desired shape, dimensions, and mechanical properties. II.
PREVIOUS W O R K
Sellars and co-workers, [~-Sj pioneers in the field of modeling microstructural evolution during hot rolling of steel strip and plate, have placed emphasis on the importance of quantifying the thermal field in the material and the associated metallurgical phenomena. Their model developed for thermomechanical processing of strip has been applied to the rolling of C-Mn steel, and the predicted austenite grain size compares favorably with the measurements. More recently, researchers at Nippon Steel, [6,7,81 Kawasaki Steel, t9,~~ and Institut de Recherches de la Sidtrurgie Francaise (IRSID) tuJ have described their respective efforts in modeling the microstructure and mechanical properties of steel during hot rolling. Each research group has been successful in predicting the austenite grain size following finish rolling and the mechanical properties of the finished steel, particularly for plain C-Mn grades. However, it is important to note that although the equations employed by each research group to characterize dynamic and static recrystallization kinetics, recrystallized grain sizes, and grain growth kinetics bear some similarities, they also exhibit significant differences. A more detailed discussion of these differences together with the approaches adopted to model temperature and strain in the roll bite follow. VOLUME 22A, FEBRUARY 1991--335
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