Experimental investigation on a novel medium Mn steel combining transformation-induced plasticity and twinning-induced plasticity effects

Abstract

The additional deep cryogenic treatment process prior to intercritical annealing was employed to tailor the mechanical stability of austenite grains in a new medium Mn steel. As a consequence, the medium Mn steel after intercritical annealing contains the austenite grains with different mechanical stability due to the different grain size and C content. The large austenite grains with low C content transform to martensite prior to the strain of 9.5% due to their low mechanical stability and provide the transformation-induced plasticity (TRIP) effect during tensile test. On the other hand, the small austenite grains with high C content have high mechanical stability and proper stacking fault energy, therefore do not transform to martensite but offer the twinning-induced plasticity (TWIP) effect from 9.5% strain up to fracture. Subsequently some of the twinned austenite grains provide the nucleation site for martensite formation from the strain of 26.2% up to fracture, providing the TRIP effect again in the large strain regime. In summary, the present novel medium Mn steel has TRIP effect firstly, followed by TWIP effect and then TWIP + TRIP effects during tensile test, therefore demonstrating enhanced work hardening behavior and excellent tensile properties.