Effects of Solute Segregation on Deformation Behavior in Austenitic High-Mn Steels

Abstract

With increasing demands of advanced mechanical properties in structural materials, austenitic high-Mn steels are strong candidates for GPa-grade cold-rolled steel sheets. The combination of strength and ductility has been greatly improved by utilizing deformation mechanisms of austenite, such as TRansformation induced plasticity (TRIP) and TWinning induced plasticity (TWIP). Even though, austenitic high-Mn steels are showing excellent tensile strength with ductility, a low yield strength of austenite should be improved. Therefore, researchers are actively researching grain refinement and precipitation strengthening. Furthermore, austenitic microstructure shows additional drawbacks of solute segregation, especially Mn-segregation. Solute segregation is inevitably generated during dendritic solidification and able to cause unintended microstructures, deformation mechanisms, or anisotropic mechanical properties, and deteriorate the ductility or formability. Therefore, in this study, we investigate the effects of solute segregation on microstructural evolution and tensile properties in high-Mn steels along the different C contents. The present steels showed relatively high yield strength by grain refinement and precipitation hardening. With increasing C contents, deformation mechanisms are changed from TRIP to TWIP and critical strain for serrated flow increases in their stress-strain curves. A martensitic transformation is actively formed step by step as localized Portevin-Le Chatelier (PLC) bands propagates to the end of the specimen gage section. In microstructural aspects, the martensitic transformation occurred sequentially along solute segregated bands due to differences in austenite stability and Mn content between high- and low-Mn bands, thereby leading to discontinuous transformation and consequently the serrated flow. In conclude, deformation behavior with solute segregation is carefully investigated in high-Mn steels. After understanding the effects of solute segregation, Mn-segregated bands are utilized 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. Along the Mn-segregated bands, partially recrystallized microstructure is observed. Non-recrystallized austenite contains nano-twinned structure and recrystallized austenite shows only 0.9 μm of grain size. 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%. This results can suggest the novel idea for next generation structural materials with utilizing the solute-segregation-induced TWIP+TRIP mechanisms.