Carbide-Mediated Austenite Formation and TRIP Activation in Medium-Mn Steel: An In-Situ Synchrotron X-Ray Study

Microstructure evolution of a medium-Mn steel (Fe–0.4C–1Si–6Mn–2Al–0.05Nb, wt%) during two intercritical annealing (IA) routes was investigated. In the first route (IA1), the initial condition prior to annealing was hot-rolled steel consisting of an auto-tempered martensitic matrix with fine, closely spaced carbides and about 10% retained austenite (RA). Annealing at 680 °C for 600 s increased the RA fraction to ~25%, primarily as lamellar films with relatively low solute enrichment. In the second route (IA2), the IA1 microstructure was cold-rolled and reheated to 680 °C for 600 s. Heating caused partial decomposition of deformed RA (γ–pre) into coarser, more widely spaced carbides, leaving ~10% γ–pre at the onset of annealing. During holding of 600 s, only about 5% submicron blocky austenite enriched in C and Mn was formed. In-situ high-energy X-ray diffraction and electron microscopy revealed that austenite formation was faster in IA1, although carbide dissolution proceeded at nearly the same rate in both routes. The faster kinetics in IA1 are attributed to a greater number of nucleation sites. Tensile testing showed much slower initial work hardening in IA1, indicating higher stability of RA, whereas IA2 displayed multi-stage hardening with early transformation of γ–pre and delayed transformation-induced plasticity (TRIP) from enriched blocky RA. The total elongation was ~25% in IA1 and ~40% in IA2. These results highlight that mechanical performance is controlled not simply by the RA fraction, but by the sequence of TRIP activation among austenite populations with different stabilities.