Crystallographic Reconstruction Study of the Effects of Finish Rolling Temperature on the Variant Selection During Baini

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HIGH-STRENGTH bainitic steels in hot worked conditions have been developed for a number of applications in a variety of industries, such as the automotive,[1] railway,[2] heavy machinery, and general construction industries, because they combine superior tensile strength levels with improved toughness, ductility, formability, and weldability using a low amount of carbon or other alloying elements.[3] Thermomechanical controlled processing (TMCP) is a common way to obtain this combination of mechanical properties.[4] It consists of slab reheating at well-defined temperatures followed by controlled rolling in the unrecrystallized c temperature range, i.e., below the non-recrystallization temperature (Tnr). Under these circumstances, sufficient strain is accumulated in c, resulting in a fairly sharp rolling texture and the formation of structural defects (in-grain shear bands, deformation bands, mechanical NICOLAS BERNIER, Research Engineer, formerly with the OCAS N.V., ArcelorMittal Global R&D Ghent, Pres. J.F. Kennedylaan 3, 9060 Zelzate, Belgium, is now with the University of Grenoble Alpes/CEA, LETI, MINATEC Campus, Grenoble, France. Contact e-mails: [email protected], [email protected] LIEVEN BRACKE, Research Engineer, is with the OCAS N.V., ArcelorMittal Global R&D Ghent. LOI¨C MALET, Research Engineer, and STE´PHANE GODET, Professor, are with the 4 MAT (Materials Engineering, Characterisation, Synthesis and Recycling) Department, Universite´ Libre de Bruxelles, 50 Avenue FD Roosevelt, CP 194/03, 1050 Brussels, Belgium. Manuscript submitted January 10, 2014. Article published online September 16, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

twins, etc.) acting as preferential nucleation sites for the transformation products. Combined with an accelerated cooling, this process can produce low-C steels exhibiting substantial grain refinement,[5,6] which is one of the most effective ways to improve both strength and toughness. The presence of carbides and secondary phases, the density of dislocations, and the transformation texture also affect the final mechanical properties of bainitic steels, e.g., the fracture behavior.[7–9] The inheritance of these microstructure details depends on both the c conditioning during hot forming and the c-to-a phase transformation. It is, therefore, crucial to investigate the effects of various TMCP parameters on the c conditioning and phase transformation to fine-tune the current processing route for plates and coils, improving upon the current strength and toughness limits. The finish rolling temperature (FRT) is one of the main parameters that affects the c condition prior to transformation, as it determines the lowest temperature at which the deformation is applied. Ray et al.[10] reported a sharp transformation texture in plain C and Nb microalloyed steels for low FRTs, typically inthe c non-recrystallisation range. It promotes the {332}  1 13 component in particular, which is believed to be the most beneficial component for mechanical properties. As this component   is mo