A strong and ductile 7Mn steel manufactured by warm rolling and exhibiting both transformation and twinning induced plasticity

Using a new design strategy for the metastable phase in transformation induced plasticity (TRIP) steels, we have successfully manufactured a 7 wt.% Mn containing steel with an excellent combination of mechanical properties (950 MPa ultimate tensile strength and 63% total elongation) via warm rolling and intercritical annealing (IA) processes. Compared with the cold rolled microstructure that is usually completely recrystallized during annealing, the warm rolled microstructure is just partially recrystallized due to the lower driving force that is accumulated. This leads to about 50 vol.% of the retained austenite grains having both lamellar and equiaxed morphologies after IA, with the latter having a wide and almost evenly spaced size distribution. In-situ synchrotron X-ray examination revealed that about 40 vol.% of the retained austenite grains could transform to martensite in a sustainable way during deformation because they had a range of mechanical stability owing to their different morphologies and wide size distributions. Moreover, the partitioning of the solute elements during IA leads to some austenite grains having proper values of stacking fault energy using which they can twin during deformation. Finally, the retained austenite grains in the medium-Mn steel can either transform to martensite gradually or twin during deformation, and contribute to large elongation at high strengths via both TRIP-assisted and TWIP effects.