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Description
Medium-Mn steels are promising candidates for next-generation advanced high-strength steels owing to their exceptional strength–ductility balance. The heterogeneity in retained austenite (RA), particularly in terms of chemical composition and morphology, has sometimes been reported to be beneficial. In this study, this view is critically examined to elucidate the effect of austenite heterogeneity on mechanical properties of a 0.4C–6Mn–2Al–1Si–0.05Nb (in wt.%) steel.
The processing was started by intercritical annealing (IA1) of a fully martensitic microstructure to obtain a microstructure approaching the local equilibrium with ~38% RA. 50% cold rolling of this structure led to the partial transformation of RA into strain-induced martensite (SIM). A subsequent flash annealing treatment (IA2), heating at a rate of 100 °C/s up to 780 °C, followed by cooling at a rate of 50 °C/s to room temperature, resulted in the final microstructure consisting of ~58% ferritic phases (ferrite and fresh martensite), and ~42% RA with a mixture of lamellar and blocky morphologies. Additionally, an APT analysis revealed two distinct RA populations in terms of chemical composition: high-Mn RA (deformed RA and reverted SIM) and low-Mn RA (reverted from ferrite), both of them exhibiting reduced C content compared to the RA formed during IA1.
A comparison of tensile properties between IA2- and IA1-treated specimens showed significantly higher work hardening and ultimate tensile strength but a serious 65% reduction in fracture elongation of the IA2 structure. This deterioration in mechanical performance can be attributed to the high fraction of less stable RA and the presence of fresh martensite after IA2, which led to rapid exhaustion of the austenite-to-martensite transformation at an early stage of straining. These results demonstrate that RA heterogeneity is not always advantageous; instead, RA stability plays a pivotal role in mechanical performance.