Abstract: Effect of tempering temperature on microstructure and properties of 30Cr3Si2NiMoWNb ultra-high strength steel was studied by means of scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and mechanical test. The resules show that the tempering temperature can greatly affect and regulate the mechanical properties of the steel. When tempered at 200-350 ℃, the microstructure is composed of tempered martensite and fine dispersed ε-carbides. The fluctuation of strength and toughness is small, the tensile strength grade is 1700 MPa, the yield strength grade is 1300 MPa. Due to the inhomogeneous precipitation of cementite when tempered at 350-500 ℃, the strength and toughness decrease at the same time, and the brittleness is the most serious at about 500 ℃, with impact absorbed energy decreasing to the lowest point. When tempered at 500-700 ℃, stable spherical cementite is formed, and strength decreases and toughness increases greatly. The influence mechanism of tempering temperature on strength and toughness is the evolution process of precipitates such as ε-carbide and cementite. Certain amount of Si can raise the forming temperature of cementite and temper brittleness temperature of the steel.

Key words: ultra-high strength steel, tempering temperature, mechanical properties, cementite