High-performance medium Mn steels with expanded processing windows enabled by rapid austenite reversion

Abstract

Medium Mn steels (MMnS) with high strength and large ductility are desirable for constructing structural components, and multiple plasticity carriers are indispensable for high-performance MMnS. The key to invoking collective deformation mechanisms is the proper tuning of austenite characteristics through the intercritical annealing process. However, since other metallurgical features are also impacted by intercritical annealing both kinetically and thermodynamically, MMnS are sensitive to minor variations of annealing conditions and plagued by narrow thermo-mechanical processing windows. To overcome this limitation, we propose a high-temperature pre-annealing (HA) strategy that enables strong and ductile MMnS across broad ranges of annealing temperature and duration. The HA process facilitates austenite reversion by enhancing nucleation sites and elemental pipe diffusion through dislocations, enabling austenite fractions exceeding the values predicted by thermodynamic equilibrium. Consequently, superior and stable mechanical properties with yield strength of ∼1200 MPa and ductility up to 50% are achieved. Multi-scale characterizations reveal that this exceptional ductility stems from hierarchical austenite with graded mechanical stability and synergistic activation of plasticity mechanisms—martensitic transformation, dislocation glide, deformation twinning, and stacking faults—to mitigate strain localization. In contrast, the same MMnS without HA process exhibits inferior tensile properties and a strong dependence on annealing conditions. Our findings underscore the capability of the HA strategy to decouple mechanical performance from precise processing control, offering a scalable pathway for industrial-scale production of high-performance MMnS.