Microstructure evolution and enhanced mechanical properties of a novel Nb-Ti micro-alloyed medium-Mn steel

A novel Nb-Ti micro-alloyed medium-Mn steel was proposed and subjected to austenite reverted transformation annealing process. The suitable annealing process involving intercritical annealing at 690-700 degrees C for 5 h was summarized by means of thermodynamic and kinetic calculation, which was proved to provide good guidance for the experiment. The 700 degrees C annealed steel was characterized by a dual-phase microstructure consisting of ultra-fine grained tempered martensite and retained austenite. Both blocky and lath-like retained austenite with volume fraction of 37.4% and moderate stability was obtained. Moreover, a large number of spherical TiC and rod-like (Nb,Ti)C precipitates were observed, which increased the yield strength significantly. The transformation-induced plasticity (TRIP) effect of retained austenite and precipitation strengthening of nano-sized precipitates were combined in the studied steel, resulting in excellent trade-off between yield strength, tensile strength and total elongation. The yield strength, tensile strength, uniform elongation and total elongation was 785 MPa, 1015 MPa, 32.3% and 42.5%, respectively. The product of tensile strength and uniform elongation was up to 32.78 GPa%, which was superior to that of low-alloy steels, TRIP steels and Q&P steels, and was comparable with some high-Mn high-Al steels.