Abstract: Effect of tempering temperature on microstructure and properties of Q1100 ultra-high strength steel was studied by mechanical properties test, metallographic observation and SEM observation. The results show that the tested steel is composed of tempered lath martensite when water quenched from 900 ℃ and then tempered at 200-300 ℃. After tempering at 400 ℃ and 500 ℃, the microstructure is tempered troostite. After tempering at 600 ℃, the microstructure is tempered sorbite. The tested steel has high tempering stability, the α ferrite still maintains the shape and orientation of lath martensite when tempered at 400-600 ℃. After tempering at 200 ℃, the content of low angle grain boundary is high, which hinders the propagation of microcracks and the steel has good toughness. With the increase of tempering temperature, the proportion of low angle grain boundary decreases gradually. After tempering at 400 ℃, the proportion of low angle grain boundary is relatively small, the precipitation of carbides deteriorates the toughness of the tested steel, the tempering brittleness occurs and results in the worst toughness. After tempering at 500 ℃ and 600 ℃, the proportion of low angle grain boundary is not significantly different from that at 400 ℃, but the recovery degree of the microstructure is large and partial recrystallization occurs after tempering at 600 ℃. The tempering softening effect is large and the toughness is high. When the tempering temperature is 200 ℃, the tested steel has the best comprehensive properties, of which the yield strength is 1164.38 MPa, the tensile strength is 1429.70 MPa, the elongation after fracture is 14.66%, the hardness is 430.27 HV3, and the impact absorbed energy of the standard specimen at -40 ℃ is 92.30 J.

Key words: Q1100 steel, tempering temperature, mechanical properties, microstructure, electron backscattered diffraction