Abstract: The multiphase microstructures comprising tempered matensite, fresh martensite and retained austenite with the low carbon medium manganese steel containing silicon was obtained by full austenitizing Q&P process. The microstructure and mechanical properties of the steel under different heat treatment processes were studied by means of SEM, TEM, XRD and tensile testing.The results show that with the increase of quenching temperature, the tensile strength decreases at first and then increases, the yield strength decreases,and the total elongation first increases and then decreases. When quenching temperature is 250 ℃, the tested steel gain excellent comprehensive mechanical properties with tensile strength of 1331 MPa, the total elongation of 17.3%, and the product of strength-plasticity up to 23 GPa·%. This is mainly due to the multiphase microstructures with a certain proportion of film-like retained austenite which give play to TRIP effect. The continuous work-hardening ability during the tensile deformation stagewhich is helpful for the improvement of strength and plasticity. Increasing the quenching temperature to 270 ℃, the stability of retained austenite decreases, a large number of fresh martensite exists, which both makes the plasticity decrease.

Key words: Q&P treatment, quenching temperature, retained austenite, mechanical properties