Research on Tempering Performance of Seismic Steel with Yield Strength 550 MPa

Effects of tempering temperature on microstructure,precipitation and mechanical properties of Seismic building steel with yield strength of 550 MPa produced by TMCP,were researched by optical microscope,TEM and mechanical properties test. The results show that with the increase of tempering temperature,both yield strength and tensile strength of the test steel increase,and the ratio of R_p0.2/R_m increases obviously. The changes of mechanical properties before and after tempering are mainly related to the precipitation of fine particles below 25 nm size,the decomposition of M/A islands and the recovery of dislocation substructure after tempering. When the tempering temperature is 450 ℃,the best comprehensive mechanical properties of the tested steel are achieved with tensile strength at 703 MPa,yield strength at 588 MPa,elongation of 17.5% and impact energy at-20 ℃ reaching 240 J,as well as lower R_p0.2/R_m of 0.84.