Effect of Austenite Deformation on the Microstructure Evolution and Grain Refinement Under Accelerated Cooling Condition
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E essence of thermomechanical processing is austenite deformation below the recrystallization-stop temperature (T5pct).[1] After austenite deformation below the T5pct, several changes in austenite state are made, including grain shape, texture, density of substructures, annealing twin boundaries (changed into normal high-angle grain boundaries (HAGBs) during deformation), and bulging of austenite grain boundaries.[2] This austenite conditioning strongly affects the transformed microstructure and, hence, the final mechanical properties.[3,4] For thermomechanically processed microalloyed steels, air cooling often leads to a ferrite-pearlite microstructure with mechanical properties commonly below the X-70 grade. To further improve the properties, accelerated cooling (ACC) is necessary. Based on the understanding of the transformation behaviors of deformed austenite during continuous cooling, the
H. ZHAO and E.J. PALMIERE are with the Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, U.K. Contact e-mail: e.j.palmiere@sheffield.ac.uk Manuscript submitted August 12, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
advantage of ACC on the improvement of both strength and toughness was studied during the 1970s.[5] The wide application of online ACC equipment not only increases the productivity significantly in steel plate mills,[6] but also brings good control of the transformed microstructures.[7] For low-carbon microalloyed steels, ACC after finish rolling exploits the enhanced hardenability of these steels to produce low-temperature transformation products, mainly bainitic ferrite (BF) and acicular ferrite (AF), to improve strength. As classic bainite, BF grows in the form of clusters of parallel thin lenticular plates or laths, known as packets, but owing to the low-carbon concentration in the microalloyed steels, cementite is usually absent, yielding a well-organized microstructure consisting of BF and microconstituents, such as martensite and retained austenite (M/A). The formation of M/ A constituents can be attributed to the partitioning of carbon from these BF laths to the surrounding austenite during cooling. Differently, AF was first introduced by Smith et al.[8] in 1972. It was defined as being comprised of nonequiaxed ferrite laths with high-density substructures formed at relatively higher temperatures than bainite by a mixture of diffusion and displacive transformation mechanisms. Despite the unique morphology of AF, many investigations have shown that the
transformation mechanism of AF is similar to that of BF.[8–13] As for the grain refinement of low-temperature transformation products, although the effect of austenite deformation has been studied by many researchers, the results are still controversial. In a prior investigation,[14] it was shown that the effective grain size of bainite increases from 3.2 to 3.8 lm when austenite was deformed by 30 pct. Similarly, the block size of bainite was found to increase after austenite defo
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