Abstract: Effect of austenitizing temperature on microstructure, retained austenite content and mechanical properties of 1500 MPa grade Q&P steel was studied by means of optical microscope, scanning electron microscope, X-ray diffractometer and tensile testing machine. The results indicate that the tested steel is composed of ferrite, martensite and retained austenite austenitized at different temperatures, slow cooled, quenched and partitioned, but there are significant differences in the content of each phase and grain size. After the same slow cooling, quenching and partitioning process, the content of martensite and retained austenite significantly increases during intercritical austenitizing (750, 800 ℃), while there is no significant difference in the proportion of phases between high-temperature intercritical austenitizing at 800 ℃ and complete austenitizing at 850 ℃. The average grain size of the tested steel decreases with the increase of austenitizing temperature. The volume fraction of retained austenite in the tested steel austenitized at 750, 800 and 850 ℃ is 8.7%, 13.3% and 12.0%, respectively. The comprehensive mechanical properties of the tested steel are optimal when austenitized at 800 ℃, with a yield strength of 1169 MPa, tensile strength of 1503 MPa and elongation of 13.3%. The mechanical properties are mainly affected by the phase proportion, fine grain strengthening effect and retained austenite distribution.

Key words: austenitizing temperature, Q&P steel, microstructure and properties