Ultra-high strength and high ductility 60Si2CrVNb spring steel with multiphase microstructure controlled by austempering

In this study, a series of austempering treatments were carried out on the designed novel 60Si2CrVNb spring steel and the effects of different austempering temperatures on the microstructure and mechanical properties of the steel were studied. The results show that after austempering treatments, the microstructure of 60Si2CrVNb spring steel exhibited a multiphase microstructure composed of martensite, bainite and retained austenite (RA). As the austempering temperature increased, the proportion of martensite gradually decreased, while the proportion of bainite significantly increased, and the proportion of RA first increased and then slightly decreased. The carbon content and stability of RA gradually increased. The toughness and ductility of steel gradually increased, while the strength decreased gradually. At the austempering temperature of 240 °C for 90 min, due to the presence thin and long lath bundles of martensite and bainite, as well as the effective synergy between multiphase microstructure, the spring steel exhibited excellent comprehensive properties with an ultimate tensile strength of 2578 MPa, a total elongation of 11.2 %, an impact toughness of 11.8 J, and PSE of 28.6 GPa%. After the austempering treatment at 350 °C for 90 min, the highly stable RA in the spring steel exhibited a sustained TRIP effect during the tensile process. The RA gradually transformed into twinned martensite, enabling the spring steel to exhibit a long-lasting work hardening capacity and achieve excellent toughness and ductility, resulting in a total elongation of 24.1 %, an impact toughness of 29.2 J, and a PSE of 36.4 GPa%.