Abstract: Effects of deformation temperature and partitioning time on microstructure evolution of low carbon Si-Mn steel during direct quenching-partitioning process were investigated. The volume fraction of retained austenite and the carbon concentration in retained austenite were quantitatively studied. The results showed that the typical microstructure of Q&P steel consists of a small amount of retained austenite， ferrite and martensite containing lath martensite and twin martensite. With increase of the partitioning time， the microstructure shows a tempered transformation tendency. When deformation temperature is 950℃and 880℃， the volume fraction of retained austenite decreases after the first increase and reaches the maximum， 9.1%， 10.1%， respectively at the partitioning time of 150s. When deformation temperature is 820℃， the volume fraction of retained austenite increases up to 11.9% because of proeutectoid ferrite. And the carbon concentration in austenite is relatively lower.

Key words: low carbon Si-Mn steel, direct quenching, deformation temperature, partitioning time, retained austenite