Tailoring hetero-deformation induced strengthening to improve strength-ductility balance in warm rolled medium Mn steel

The heterogenous microstructures are designed through controlling warm rolling (WR) reduction in two-stage rolling process to control geometrically necessary dislocations (GND) density, greatly improving the strength and ductility balance of medium Mn steel in this contribution. With an elevated WR reduction in two-stage rolling process, the austenite grain size is refined and the grains change from equiaxed to lath-like in morphology, which can be attributed to the accumulation of stored strain energy during the rolling process. In addition, the volume fraction of partially recrystallized austenite also increases, which results in a higher GND density of austenite, providing a stronger hetero-deformation induced (HDI) strengthening. The stronger HDI hardening at low strain as well as the transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effect results in a slower decrease in work hardening rate. These results lead to an enhanced yield strength (YS) without damaging ductility. The best strength and ductility combination with YS of 1088 MPa, ultimate tensile strength (UTS) of 1221 MPa and total elongation (TE) of 36.4 % has been achieved in the annealed sample with 82 % WR reduction.