Role of annealing conditions prior to cold rolling on microstructure and tensile deformation behavior of a medium-Mn steel

Abstract

A low-carbon medium-Mn steel is designed utilizing CALPHAD approach to leverage the transformation induced plasticity (TRIP) for the enhancement of strength-ductility combination. The as-cast steel produced by vacuum induction melting, was hot forged, and was subsequently hot-rolled and air-cooled to room temperature (HRAC). The hot-rolled plate was subjected to two different thermomechanical processing routes- (i) inter-critical annealing (IA) to obtain partial austenitization followed by air cooling (S1 or HRIA), and (ii) annealing above the upper critical temperature to obtain full austenitization and subsequent air cooling (S2). Both steels were cold-rolled followed by IA and subsequent water quenching to room temperature (S1-IA and S2-IA, or CRIA) to obtain different microstructures. The steels were characterized for phase fractions and recrystallization, grain boundary characteristics, and dislocation densities. The HRIA steel showed greater strain-hardening but lower ductility than the CRIA steels due to HCP-martensite mediated TRIP. The CRIA steels exhibited same UTS $\times$ TE$\approx$20.5 GPa%. S1-IA exhibited higher yield and tensile strength, but lower uniform elongation than S2-IA, despite having higher austenite volume fraction. The higher yield strength is attributed to the higher dislocation density of ferrite in S1-IA, in which TRIP activates at a higher stress due to smaller austenite grains size. The lower yield strength, and easier initiation of TRIP in S2-IA are attributed to a more recrystallized ferrite with lower dislocation density, and coarser austenite grains, respectively. However, the higher C of retained austenite in S2-IA increases its stability requiring larger tensile strain for continuation of TRIP effect leading to higher uniform elongation than S1-IA.