Evolution of microstructure and deformation behavior in Al–Ni added medium-Mn steel processed through intercritical / cold rolling and annealing

In the present work, the effect of intercritical annealing on Fe–8Mn–4Al-3.5Ni-0.8Si-0.25C (wt.%) medium-Mn steel, processed through two different processing routes viz. intercritical rolling (IR) and cold rolling (CR), has been studied. The microstructure, mechanical properties, austenite stability and deformation behavior of the steel, after intercritical annealing (IA) at different temperatures (760, 800 and 840 °C), has been investigated. It has been observed that different rolling regimes (intercritical rolling and cold rolling) give rise to distinct variation in the grain structure owing to different recrystallization behavior during intercritical annealing. The IR-annealed samples showed a mixture of lath and equiaxed grains while the CR-annealed samples showed mostly equiaxed grain morphology. Due to extensive recrystallization in CR-annealed samples, a higher fraction of retained austenite was formed, as compared to IR-annealed samples. The yield strength (YS) and ultimate tensile strength (UTS) was found to be higher for IR-annealed samples, as compared to CR-annealed samples, at all annealing temperatures. However, a high elongation (EL) of 57 % with (UTS x %El) product of 47.7 GPa% was obtained in CR sample, annealed at 800 °C. The mechanical stability of retained austenite was found to influence the work hardening behavior of the annealed samples. The CR-annealed samples exhibited strain-induced transformation to α′-martensite, as compared to ε-martensite transformation in IR-annealed samples. A sustained three-stage strain hardening behavior during deformation, comprising combined TWIP and discontinuous TRIP effect, led to excellent strength-ductility combination in CR sample annealed at 800 °C.