Achieving heterogeneous TWIP/TRIP steel with 1GPa strength and 72% ductility by introducing austenite + martensite lamella structure

A heterogeneous medium-Mn steel with ultra-high ductility and good strength was designed, which displayed yield strength of 752 MPa, ultimate tensile strength of 1068 MPa, total elongation of 72.4%, and strength-elongation product of 77.3 GPa·%. A unique pre-existing austenite + martensite lamella structure was introduced by warm rolling followed by cold rolling. During annealing, it transformed into a heterogeneous duplex-phase (ferrite and austenite) microstructure with multiple morphologies (lath-like, granular, and block) and a dispersed grain size distribution. The relationships between the microstructural evolution and Mn segregation, deformation mechanisms, and mechanical behavior were characterized. The heterogeneous austenite structure yielded different austenite-stability gradients, which triggered multi-stage transformation-induced plasticity (TRIP) effects during the tensile test, accompanied by the twinning-induced plasticity (TWIP) and microband-induced plasticity (MBIP) effects. Therefore, the synergy of multiple strengthening mechanisms significantly improved the steel ductility without affecting its strength, resulting in better mechanical properties than other medium-Mn steels.