Microstructural evolution and mechanical behavior of a novel heterogeneous medium Mn cold-rolled steel

A novel heterogeneous medium Mn steel with a nominal composition of Fe-0.18C-7.8Mn-1.65Al-0.04Ce wt.% is successfully fabricated by introducing various nucleation energy barriers for reverse austenite in a cold-rolled specimen prior to final annealing. The obtained steel demonstrates a ferrite and austenite duplex phase with multiple morphologies (i.e., granular, block-shaped, and lath-like), dispersed grain size, and heterogeneous chemical components. The heterogeneous microstructure yields superior synergy of 1060 MPa yield strength, 1270 MPa tensile strength, and 54.5% total elongation. The tensile behavior and microstructural evolution of the fabricated heterogeneous steel are investigated in detail. The stacking fault and dislocation accumulations are found to dominate during the initial deformation stage, and granular austenite transforms into martensite in the intermediate strain stage. Finally, block-shaped and lath-like austenite undergoes a phase transformation. Moreover, the back stress caused by the heterogeneous microstructures significantly improves the yield strength and work hardening ability of the heterogeneous specimen. Therefore, the combination of successive transformation-induced plasticity (TRIP) effect, dislocation strengthening, and back stress hardening contributes to the outstanding mechanical properties of this steel.