Enhancing the TRIP Effect by Pre-Ferrite in the Hot Rolled Fe-8Mn-4Ni-3Al-0.1C Steel

As is well known, reconstruction of the initial microstructure before an inter-critical annealing (IA) can optimize the final microstructure as well as improve mechanical performance. In this study, the initial microstructure with “pre-ferrite + martensite” was designed via two-stage cooling in the rolling process. The final microstructure observation showed that bimodal size distributed austenite and double morphologies of austenite. Moreover, lath-shaped austenite existed in the martensitic matrix; blocky austenite occurred at the boundaries of pre-ferrite. According to the numerical simulations of the IA process, blocky austenite tended to be more stable with smaller size and higher Mn concentrations than lath-shaped austenite. The characteristics of austenite were similar at different IA temperatures, but the lower IA temperatures, the smaller size of austenite. An excellent combination of strength (1308 MPa) and ductility (34%) was obtained at 928 K, which was ascribed to a positive TRIP effect induced by the austenite with a large stability gradient.