Temperature-dependent mechanical properties governed by austenite stability in co-precipitation strengthened medium Mn steel with heterogeneous microstructure

Abstract

Using the partially recrystallized medium Mn steel containing B2 and MC carbide as a model system, this study reported its tensile properties at temperatures ranging from −150 to 150 °C. The heterogenous and partially recrystallized microstructure after intercritical annealing at 620 °C consisted of α, γ, and tempered α′ in the equiaxed zone and γ and tempered α′ in the lath zone. Furthermore, tempered α′/α and γ grains contained mainly MC carbide and B2 precipitates, respectively. The temperature-dependent tensile properties were investigated for the material annealed at 620 °C for 16 h. The ultimate tensile strength and strain hardening rate increased continuously with decreasing deformation temperatures. The total elongation was at maximum at 25 °C and decreased with increasing or decreasing deformation temperatures from 25 °C. The high strain hardening of the material deformed at −150 °C originated from its high dislocation density, a large amount of deformation-induced α′ contributing to HDI strengthening, and strain hardening by the TRIP effect. Therefore, the temperature-dependent mechanical properties of the investigated material are mainly governed by the mechanical stability of γ, affecting DIMT behavior and HDI strengthening. The fractography analysis showed an increase in the degree of brittle fracture with decreasing deformation temperatures. Faster DIMT kinetics with decreasing deformation temperatures can lead to higher strain localization and a predominantly brittle fracture surface. This study provides insights into the temperature-dependent phase-specific deformation mechanisms, DIMT behavior, HDI strengthening, and damage behavior of medium Mn steel.