Achieving high ductility in a 1.4 GPa grade medium Mn lightweight TRIP/TWIP steel with hierarchical lamellar structure

The present work reports a tri–phase hierarchical lamellar structure to tailor the ultra–high yield strength and high ductility balance in a novel Si–Al added medium Mn lightweight TRIP/TWIP steel. The warm–rolled sample was subjected to small cold rolling and low–temperature tempering to obtain the nano–scale twins and martensitic laths in the austenitic matrix, together with the hard δ–ferrite with nano–precipitates. The yield strength of 1403 MPa was obtained in the tempered sample, which is 370 MPa higher than the warm–rolled counterpart. Interestingly, the hard zone (δ–ferrite) confined the propagation of the plastic strain, leading to an intrinsic hetero–deformation induced (HDI) strengthening effect. The large ductility of 30% was ascribed to the large yield point elongation and the enhanced strain hardening after the Lüders strain, which is closely associated with HDI hardening and extra TRIP/TWIP effects.