Effect of quenching temperature on the austenite stability and mechanical properties of high-strength air-cooled TRIP steel prepared with hot-rolled C–Si–Mn sheets

This paper systematically studies the effect of quenching temperature on the microstructure evolution and the relationship between the microstructure and mechanical properties of air-cooled TRIP steel prepared with C–Si–Mn hot-rolled sheet. The results indicate that when the quenching temperature is lower than (200 °C) or close to (250 °C) the Ms temperature, the martensite transformation mainly occurs during quenching or at early stage of air cooling. When the quenching temperature reaches 300 °C, lathy bainite begins to form. As the quenching temperature increases, the morphology of bainite is transformed from lathy to granular. Bainite transformation during air cooling can promote the C enrichment in austenite, thus improving the thermal stability of the austenite. However, martensite transformation also occurs during subsequent air cooling, and the start temperature of martensite transformation first decreases and then increases with the increase of quenching temperature from 300 °C to 450 °C. The yield and tensile strength of TRIP specimens first decrease and then increase, while the retained austenite content, elongation and product of strength and elongation (PSE) first increase and then decrease. When the quenching temperature is 400 °C, the best elongation and PSE are obtained. The best elongation of 400TRIP specimen is mainly attributed to its highest content and suitable mechanical stability of retained austenite, which is favorable for the expansion of the TRIP effect range. The TRIP specimens prepared with hot-rolled sheet all exhibit ultra-high tensile strength (over 1050 MPa), which is related to the ultrafine ferrite grains, martensite transformation during air cooling and TRIP effect of retained austenite.