Abstract: Effect of intercritical tempering temperature on microstructure and mechanical properties of 0.02C-7Mn steel was investigated by means of scanning electron microscope (SEM),X-ray diffraction (XRD),transmission electron microscope (TEM) and Thermo-Calc software calculation.The results indicate that the microstructure is basically quenched martensite accompanied by little amount of retained austenite after direct quenching,while the microstructure is mainly tempered martensite and the reverted austenite (retained austenite) after intercritical tempering.With the increase of tempering temperature,the content of retained austenite gradually increases and the maximum content of retained austenite is 18.78% at 650 ℃.Meanwhile,ε-martensite is appeared and its content is 6.57%.It is the decline of dislocation density and dispersed precipitation during the tempering that has resulted in the decrease of tensile strength and the increase of yield strength in different effects.Furthermore,the existence of ε-martensite not only reduces yield strength but also damages low-temperature toughness.After tempered at 600 ℃,the tested steel has excellent comprehensive mechanical properties (transverse direction:tensile strength of 984 MPa,yield strength of 973 MPa and the -40 ℃ impact absorbed energy of 163 J;rolling direction:tensile strength of 947 MPa,yield strength of 919 MPa and the -40 ℃ impact absorbed energy of 186 J) that can meet the properties requirement of Q690 steel.

Key words: Q690 steel, intercritical tempering, retained austenite, ε-martensite, mechanical properties