Exceptional combination of ultra-high strength and excellent ductility by inevitably generated Mn-segregation in austenitic steel

Austenitic high-Mn steels have been nominated as potential alloys for ultra-high strength and excellent ductility, but the high-Mn content inevitably generates band-shaped Mn-segregation due to dendritic solidification from the steel melt. This segregation band often causes unintended deformation mechanisms and anisotropic mechanical properties, and deteriorates formability. Here, we effectively utilize the Mn-segregated bands for the improvement of tensile properties by actively producing TWinning- and TRansformation-induced plasticity (TWIP and TRIP) mechanisms in high- and low-Mn-segregated bands. We also adopt a mixed microstructure of non-recrystallized and recrystallized austenite mainly formed in high- and low-Mn-segregated bands, respectively. The TWIP+TRIP mechanisms generating highly-sustained strain hardening and high strain hardening, coupled with partial recrystallization and precipitation hardening, are working successfully for overcoming low-yield-strength characteristics of austenite to reach 1 GPa and for achieving the excellent tensile strength of 1.5 GPa and ductility of 44%. Our results demonstrate how the Mn-segregation-induced TWIP+TRIP mechanisms can be a novel idea in ultra-high-strength steel design.