Tailoring multiscale austenite for synergistic strengthening via gradient TRIP effect in medium-Mn steel

This study developed a dual-stage annealing process designed to construct a gradient austenite stability heterostructure in medium-Mn steel through a minimalist thermomechanical treatment process, thereby synergistically enhancing its strength and ductility. After hot rolling, the samples were subjected to either traditional austenite reversion treatment single stage annealing or intercritical stabilization dual-stage annealing (IS). IS treatment could promote the efficient distribution of Mn and C elements in the austenite, increasing the volume fraction of retained austenite (RA) and mechanical stability. The samples subjected to dual-stage annealing formed a heterogeneous microstructure consisting of lamellar, equiaxed and granular austenite, while the Cu-containing The IS treatment promoted the effective partitioning of Mn and C into austenite. Subsequently, the samples processed by IS exhibited excellent work hardening ability and uniform elongation, with a strength-ductility product reaching 40 Gpa·%. The synergistic enhancement of strength and ductility stems from the transformation induced plasticity effects (TRIP) induced by the gradient within the heterostructure, and is achieved through the accumulation of geometrically necessary dislocations to realize heterogeneous deformation-induced strengthening (HDI). The core-shell structure of equiaxed austenite acts as a transitional phase, coordinating deformation between the high and low strain area and effectively delaying localized necking. The fracture analysis indicates that the IS samples exhibited a more uniform distribution of ductile dimples and enhanced toughness. Combined with the microstructural characterization, the IS treatment enhances the deformation coordination by activating the graded TRIP and HDI strengthening mechanisms in the heterostructure.