Abstract: Effect of tempering temperature (300, 400, 500, 600, 700 ℃) on yield ratio and residual stress of 550 MPa high strength steel was studied by means of microstructure analysis, tensile test and stress measurement. The results show that with the increase of tempering temperature in the range of 300-500 ℃, the amount of dispersed and fine precipitates increases, the M/A constituents decompose and the dislocation density decreases gradually. When the tempering temperature rises up to 600 ℃, the amount of the precipitates reaches the maximum, while the M/A constituents completely decompose and only a small amount of dislocations exist. When the tempering temperature is 700 ℃, partial recrystallization of bainite occurs with the formation of more ferrite, and the precipitates aggregate and coarsen and dislocations are almost invisible. With the tempering temperature increasing from 300 ℃ to 700 ℃, the yield ratio first increases and then decreases, while the residual stress decreases gradually. An excellent combination of yield ratio and residual stress is obtained after tempering at 500 ℃, with yield ratio of 0.85 and residual stress of 13.98 MPa.

Key words: tempering temperature, 550 MPa high strength steel, yield ratio, residual stress