Enhanced stability of retained austenite to facilitate deformation compatibility of surrounding ferrite in a medium manganese steel

Austenite stability and its effect on the co-deformation behaviors with surrounding ferrite phase are critical issues in optimizing the mechanical performance of medium manganese steel. In this study, we produced two medium manganese steels intercritically annealed at 625 °C and 645 °C with very narrow breadth of temperatures, yet showing significant discrepancy of strength and ductility. With quantitative measurements through atom probe tomography, more pronounced partitioning of C and Mn was found to occur in retained austenite of the 625 °C-annealed sample. The C concentration is 0.39 wt % and Mn concentration is 11.32 wt %, higher than that in the 645 °C-annealed sample (0.32 wt % C and 8.05 wt % Mn), endowing higher mechanical stability of austenite in the 625 °C-annealed sample. The enhanced stability decelerated the martensitic transformation of retained austenite over the whole regime of tensile deformation, yet accelerated the sufficient plastic deformation of surrounding ferrite in terms of multiple dislocation motion. Such compatible deformation between the austenite and ferrite, led to excellent combination of tensile strength (1084 ± 3 MPa) and uniform elongation (31.1 ± 1.2 %). This work expands the understanding on compatible deformation mechanisms among various phases and retained austenite with moderate mechanical stability in medium manganese steels and quenching and partitioning steels.