Effect of low-temperature tempering on microstructure and mechanical behavior of ultra-high-strength medium-Mn steel

This study comprehensively examined the effects of low-temperature tempering on the microstructure and tensile properties of cold-rolled medium Mn steel. The initial microstructure contained blocky and film-like retained austenite (RA) along with banded, blocky, and lath martensite. After tempering, the microstructure was primarily composed of banded and blocky martensite, along with some granular and blocky RA. During tempering, migration of the austenite/martensite interface into the austenite phase resulted in the transformation of 18 vol.% of the blocky and film-like austenite into martensite. The tempering process relieved residual stresses from cold rolling. Carbon segregation at the martensite/austenite interfaces and the precipitation of nano-carbides increased the yield strength of the tempered samples by 554 MPa. The harder martensitic matrix enhanced RA stability, and stress–strain accumulation induced a staged martensitic transformation in the RA, reflected by more pronounced serrations in the strain hardening rate curve (S2). The improved RA stability contributed to strain hardening, raising the tensile strength by 282 MPa.