Influence of Strain History and Cooling Rate on the Austenite Decomposition Behavior and Phase Transformation Products i
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TRODUCTION
MODERN industrial thermomechanical-controlled processing (TMCP) relies on sophisticated computer models for process optimization and control and subsequent prediction of product properties.[1] These models,[2–4] developed and improved by modeling based on understanding the fundamental physics behind microstructural changes, are now standard tools for both offline optimization and online control for TMCP. However, comparing to most industrial processes where the stock material is subjected to complex deformation histories with non-linear strain paths, most of the current microstructural models have been developed by studying microstructure evolution under simplified laboratory conditions using monotonic deformation tests, i.e., linear strain path, and equivalent von Mises strain evm as a surrogate for the microstructure state variables without any consideration of strain paths.[3,4] While this type of model is adequate for the prediction of microstructure after monotonic compression in rolling, i.e., at the center of the plate, it is totally inadequate to predict the variation of microstructures through the thickness of rolled slab, i.e., where there are significant changes in strain path as well as thermal history.[1,5,6] L. SUN, formerly Ph.D. Student with the Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, U.K., is now Research Associate with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge, CB3 0FS, U.K. Contact e-mail: [email protected] K. MUSZKA, Assistant Professor, is with the Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Mickiewicza 30, 30-059 Krako´w, Poland. B.P. WYNNE, Senior Lecturer, and E.J. PALMIERE, Reader, are with the Department of Materials Science and Engineering, The University of Sheffield. Manuscript submitted October 24, 2013. Article published online April 4, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
Therefore, it is important to understand the effects of non-linear strain paths and thermal history on mechanical behavior and microstructure development during TMCP, which is still an area of few publications, and hence very limited knowledge. The magnitude of the strain path change can be characterized by the parameter proposed by Schmitt et al.[7,8] It is defined as the cosine of the strain path angle, i.e., the angle (Hs) between strain tensors of the pre-strain (dep ) and the subsequent strain (des ) as follows: cos Hs ¼
dep des : kdep kkdes k
½1
For most published research work on strain path effects under hot working conditions, the experimental conditions are simplified to a single strain path reversal (cos Hs = 1), e.g., forward torsion-forward torsion compared to forward torsion-reverse torsion[9–11] or tension–tension compared to tension–compression.[12,13] Although this is in contrast to the more complex strain path history experienced by stock materials during in
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