Abstract: The heat treatment processes of a lowcarbon SiMn quenched and partitioned (Q&P) steel were designed and investigated. The microstructures and phase components were characterized and analyzed using SEM， TEM and XRD. Theoretical calculation results showed that the optimum quenching temperature for a Fe021C binary alloy is 290℃ and the maximum amount of retained austenite is 179% (mole basis)， respectively. Process simulation results indicate that the volume fraction and the carbon concentration of the retained austenite in the tested steel are 67%~172% and 102%~148%， respectively. The orientation relationship between the retained austenite and adjacent martensite laths is identified as KS relationship or NW relationship. Fresh martensite forms in all the involved experimental processes. The amount of the retained austenite increases first and decreases after 50s and the carbon concentration of retained austenite rises monotonously with the increase of partitioning time. The optimum partitioning time is 50s. The carbides appear in the martensite laths during the later stage of the partitioning process and result in the decrease of the retained austenite.

Key words: Q&P steel, martensitic transformation, CCE model, microstructure, retained austenite