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 delta-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 (delta-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 Luders strain, which is closely associated with HDI hardening and extra TRIP/TWIP effects.

Automated summary

This study reports a tri-phase hierarchical lamellar structure to achieve a balance between ultra-high yield strength and high ductility in a Si-Al added medium Mn lightweight TRIP/TWIP steel. By subjecting the warm-rolled sample to small cold rolling and low-temperature tempering, a nano-scale twins and martensitic laths were obtained in the austenitic matrix along with the hard delta-ferrite and nano-precipitates. The tempered sample exhibited a yield strength of 1403 MPa, which was 370 MPa higher than the warm-rolled counterpart. The propagation of plastic strain was confined by the hard zone (delta-ferrite), resulting in an intrinsic hetero-deformation induced (HDI) strengthening effect. The large ductility of 30% was attributed to the elongation of the yield point and enhanced strain hardening after the Luders strain, which were closely associated with HDI hardening and additional TRIP/TWIP effects.