Influence of Tempering Temperature on Microstructure, Strength and Toughness of Air-Cooled Carbide-Free Bainite/Martensite Multiphase Steel

In this study, the influence of tempering temperatures (at 240 − 360 °C) on the microstructure evolution and mechanical properties in a novel air-cooled carbide-free bainite/martensite (CFB/M) multiphase steel was discussed. Results demonstrate that with an increase in tempering temperature, both yield strength and post-uniform elongation increase, while the impact toughness firstly increases and then decreases. The increased yield strength is attributed to the decomposition of unstable soft retained austenite (RA) during the tempering, the decrease in the mobile dislocation density, and the precipitation of transition carbides. The enhanced impact toughness is mainly attributed to the increased stability of RA during tempering. The improved mechanical stability of RA ensures excellent deformability and high fracture resistance and the resulting significant post-uniform elongation. However, a higher temperature at 360 °C leads to the precipitation of high-density long strip-shaped transition carbide in the bainite/martensite laths, as well as the transformation of RA into twin martensite during cooling step after tempering. The formation of coarse transition carbides and twin martensite can further increase the yield strength but is unfavorable to toughness. The optimum combination of strength and toughness with yield strength of 1128 MPa and Charpy V-notch impact toughness of 123 J is achieved in the air-cooled CFB/M steel after tempering at 320 °C.