Microstructure and properties of Mn 7 medium-Mn steel at different partitioning temperatures

Abstract

A low-cost microalloyed medium-Mn steel with a nominal composition of Fe–7.0Mn–0.34C-0.5Si–0.5Al–0.2V–0.003B was designed. After hot forging at a lower temperature and smaller forging ratio, it was treated with a quenching‒partitioning (Q&P) process. The effects of the partitioning temperature on the microstructures and mechanical properties was investigated. Detailed microstructural analysis revealed that the volume fraction and the C content of retained austenite (RA) were enhanced after Q&P, accompanied by improved tensile strength and elongation. In particular, after partitioning treatment at 300 °C, the steel had a topological structure of alternating lath martensite and interlath RA within different regions, as well as a more persistent transformation-induced plasticity (TRIP) effect during deformation. Multiple microstrengthening mechanisms achieved a better strength‒elongation combination in the steel (1659 MPa tensile strength, 11.06 % elongation, and 18.35 GPa·% as the product of tensile strength and elongation (PSE)). The residual stresses and hardness of the steel decreased with an increase in the partitioning temperature, but increased slightly at a higher partitioning temperature of 400 °C, caused by the precipitation of carbides to form a secondary hardening effect. The dislocation density of martensite and austenite decreased slowly with an increase in the partitioning temperature. The mechanical stability of RA was inversely proportional to the transformation rate of RA and the TRIP effect. RA with excessive mechanical stability significantly reduced the occurrence of stress-induced martensitic transformation and shortened the discontinuous TRIP effect during deformation, which produced a decrease in elongation.