Abstract: Effect of direct quenching+tempering process on the microstructure and mechanical properties of 1000 MPa grade high strength steel was studied. The microstructure was characterized by means of metallographic microscope and transmission electron microscope, and the Vickers hardness and tensile properties were tested. The results show that as the tempering temperature increases, the decreasing trend of microhardness and tensile strength is divided into three stages: slowly descending Ⅰ_σ stage (room temperature to 350 ℃), rapidly decreasing Ⅱ_σ stage (350-500 ℃) and basically stable Ⅲ_σ stage (above 500 ℃). There is a peak in yield strength after tempering at 250-400 ℃, and the yield strength ratio increases significantly. When the tempering temperature reaches above 500 ℃, the elongation can reach over 15%. After finishing rolling at 850 ℃ and tempering at 350-450 ℃ for 30-60 min, a good combination of strength and toughness can be obtained, which meets the performance indicators of the 1000 MPa grade high-strength steel. There are a lot of dislocation substructures in the martensite structure, in which the movable dislocation can alleviate the local stress concentration, reduce the possibility of microcrack formation, so that the material can obtain large uniform plastic deformation.

Key words: heat treatment, microstructure, mechanical properties, direct quenching-tempering