The newly designed high performance tested steel was prepared by the process of vacuum induction furnace smelting and forging.According to the ideas of online relaxation and quenching-intercritical quenching-tempering(QL-T)process,effects of offline relaxation process on microstructure and mechanical properties of the tested steel were investigated.Offline relaxation process was simulated using the MMS-300 thermal simulation testing apparatus and heat treating furnace.The results show that the microstructures are composed of bainite and martensite when the specimens are quenched in the austenite region.Once the specimens are air-cooled into the dual phase region,ferrite and granular bainite start to form.In the relaxation process,fine Nb(C,N)carbonitrides are precipitated first and then grow and coarsen.The microstructure is affected by process and determines mechanical properties.The Vickers hardness and yield strength(YS)first rise then fall when the relaxation temperature drops in austenite region,and then decline dramatically in dual phase region.When the relaxation time is 20s,the tensile strength reaches the peak(1 034MPa),and at that time YS is 872MPa,elongation is 17.7% and Charpy V-notch impact energy is 171 J at -20℃.When the relaxation time increases to 100s,the yield and tensile strengths are 750 and 934MPa respectively,elongation is 18.6% and Charpy V-notch impact energy is 165 J at -20℃. The newly designed high performance tested steel was prepared by the process of vacuum induction furnace smelting and forging. According to the ideas of online relaxation and quenching-intercritical quenching-tempering (QL-T) process, effects of offline relaxation process on microstructure and mechanical properties of the tested steel were investigated. Offline relaxation process was simulated using the MMS-300 thermal simulation testing apparatus and heat treating furnace. The results show that the microstructures are composed of bainite and martensite when the specimens are quenched in the austenite region. the specimens are air-cooled into the dual phase region, ferrite and granular bainite start to form. In the relaxation process, fine Nb (C, N) carbonitrides are precipitated first and then grow and coarsen.The microstructure is affected by process and determines mechanical properties. Vickers hardness and yield strength (YS) first rise then fall when the relaxation temperature drops in a The tensile moment is 20s, the tensile strength reaches the peak (1034 MPa), and at that time YS is 872 MPa, elongation is 17.7% and Charpy V-notch impact energy is 171 J at -20 ° C.When the relaxation time increases to 100 s, the yield and tensile strengths are 750 and 934 MPa respectively, elongation is 18.6% and Charpy V-notch impact energy is 165 J at -20 ° C.