Abstract: A novel type of 1 500 MPa grade Si-Mn-Cr-Ni-Mo multi-component ultrahigh-strength low-alloy structural steel was designed.The effects of tempering temperature on the microstructure and mechanical properties of the directly-quenched steel were investigated.The results showed that with increasing tempering temperature the tensile strength continuously reduced while the yield strength gradually increased and then decreased.The elongation and impact energy firstly increased,then decreased and increased again with increasing tempering temperature.The physical mechanism of microstructural evolution during tempering includes two aspects:(1) recovery and recrystallization softening processes of lath martensite and dislocation substructure;(2) strengthening mechanism due to decomposition of retained austenite,dissolution of supersaturated carbon and precipitation of the second phase in martensite.ε-carbide precipitation was found in lath martensite after tempering at 250 ℃,and coarsened after tempering at 400 ℃,leading to temper embrittlement.After tempering at 600 ℃,part of the precipitate phase nucleated in austenite,grew up and coarsened in martensite,eventually showed quasi-spherical shape,the other part of the precipitate nucleated and grew up in martensite,and appeared ellipsoidal or rectangular.