Microstructure and Mechanical Properties of Cold-Rolled TRIP Steel Sheets

Transformation induced plasticity (TRIP) steels have complex multiphase microstructure composed of ferrite, bainite and retained austenite [1]. These metastable retained austenite can transforms into martensite during plastic deformation, which generates a TRIP effect resulting in excellent combination of high strength and ductility even at high strength level [2-5]. For this reason, the TRIP-aided steel sheets are suitable to fabricate automobile parts, as they can offer excellent formability without sacrifice the strength and safety requirement of the steel sheets. As a result, the development of TRIP-aided steels has been a very important issue in the automobile field. Conventional low carbon TRIP steels (C-Mn-Si) contain large amount of Si to retard cementite nucleation during the isothermal bainitic transformation(IBT) because of it insolubility in cementite, then the retained austenite are carbon enrichment and maintain at room temperature[1,3]. However, in the processing of hot rolling or annealing, complex oxides will be formed on surface in the high Si(>1.5 wt.%) TRIP steels, which causes poor wettability and deteriorates hot dip galvanizing property [2, 5]. Thus, other elements such as Al [3, 8] are full or partially substituted Si with similar function of inhibiting carbide precipitation. In this work, the microstructure and the mechanical properties of low alloyed CMnAlCu-TRIP steels is investigated. The relationship between mechanical properties and mechanical stability of retained austenite is also discussed.