Martensite Transformation and Its Control in DP, TRIP and TWIP Steels

Designing of alloy concept and process for DP, TRIP and TWIP steels stressing at martensite transformation are analyzed. For DP steel, austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel, which meet well with the experimental result. The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation. Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements. Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie. In order to maintain the austenite stability or to prevent precipitation of cementite, minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation. Bainite transformation is estimated, which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful. Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced. Relationship between transformation and mechanical properties in the steel is also mentioned.