Effects of Tempering Condition on the Microstructure and Mechanical Properties of 30MnB5 Hot-Stamping steel

Abstract

The effects of tempering condition on the microstructure and mechanical properties of 30MnB5 hot stamping steel were investigated in this study. Before the tempering, hot-stamped 30MnB5 steel was composed of only α′-martensite microstructure without precipitates. After the tempering at 180 ℃ for 120 min, nano-sized ε-carbides were precipitated in the α′-martensite laths. After tempering at 250 ℃ for 60 min, cementite was precipitated along the α′-martensite lath boundaries. The cementite was also observed in the specimens tempered at 350 ℃ for 30 min and 450 ℃ for 6 min, respectively. The globular α-ferrite appeared at 350 ℃-30 min tempering, and the volume fraction of α-ferrite increased when the tempering temperature was increased. The yield strength increased after tempering, and it reached a peak with the tempering condition of 180 ℃-120 min, due to the nano-sized precipitates in the α′-martensite lath. After the tempering, the steel’s ultimate tensile strength (UTS) was decreased due to the reduction in dislocation density and C segregation to lath boundaries. The highest elongation was observed at the 180 ℃-120 min tempering condition, due to the reduction of residual stress, and the lack of precipitates along the lath boundaries. The 180 ℃-120 min tempering condition was considered to have outstanding crash performance, according to toughness and anti-intrusion calculation results. In drop tower crash tests, the 30MnB5 door impact beam tempered at 180 ℃ for 120 min showed better crash performance compared to a 22MnB5 door impact beam. Key words: hot stamping, 30MnB5 steel, tempering, martensite, cementite, carbide