Effects of Heat Treatment Temperature and Cooling Rate on Bake-Hardening Properties of DP980 Steel

With the help of Gleeble-1500D thermomechanical simulator, the effects of heat treatment temperature and cooling rate on bake-hardening properties of DP980 steel were studied. The steel was treated at different temperature (750℃, 850℃ and 950℃) for 2 min, and followed by cooling with cooling rates of 10, 20, 50℃·s^-1; after that, pre-strains were carried out at deformation amounts of 0.2% and 2%, at last the bake-hardening aging process was completed at 170℃ for 20 min. The results show that, DP980 steel has higher bake-hardening property because there are a lot of precipitates with size of 10-20 nm in ferrite after bake-hardening treatment; when heat treatment temperature was 750℃ and 850℃, the bake-hardening value firstly decreased and then increased as the cooling rate increased. But When the heat treatment temperature was 950℃, the bake-hardening value exhibited the contrary trend. At the same cooling rate, the bake-hardening value is greatly affected by the heat treatment temperature, and the influence on the bake-hardening value would be greater when the cooling rate more faster. When the heat treatment temperature was 850℃ and the cooling rate was 50℃·s^-1, the highest bake-hardening could be obtained, and the microstructure of the steel was fine martensite laths and ferrite.