Identification of Retained Austenite, Ferrite, Bainite and Martensite in the Microstructure of TRIP Steel

Transformation induced plasticity (TRIP) steels have a vast application in automotive industry because of theirhigh strength, high ductility and hence excellent energy absorption capacity. These characteristics of TRIPsteels are due to the existence of retained austenite in their microstructures in the ambient temperature, whichtransforms to the martensite phase during deformation. The microstructure of TRIP steel contains various phasesand in the past-published studies mainly the volume fraction of retained austenite was investigated and thereis not a quantitative comprehensive investigation about all phases in the microstructure of this steel. The maingoal in this study is a comprehensive qualitative and quantitative investigation in various phases of TRIP steelmicrostructure. Therefore, a TRIP steel with chemical composition of 0.2C+ 1.43Si+ 1.58Mn was produced andits complicated microstructure which contained ferrite, bainite, martensite and retained austenite was investigatedusing X-Ray Diffractometry (XRD), optical microscopy (OM), field emission scanning microscopy (FE-SEM)and Electron backscatter diffraction (EBSD). The OM and FE-SEM results were used only to qualitative studiesand identification of the morphologies of the phases but the EBSD results and functions were used to qualitativeand quantitative studies. The volume fractions of retained austenite, ferrite+bainite and martensite phases werecalculated and the amounts of 11%, 82% and 7% were obtained, respectively. The volume fraction of retainedaustenite was also measured with XRD and the amount of 14.3% was obtained.