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 their high strength, high ductility and hence excellent energy absorption capacity. These characteristics of TRIP steels are due to the existence of retained austenite in their microstructures in the ambient temperature, which transforms to the martensite phase during deformation. The microstructure of TRIP steel contains various phases and in the past-published studies mainly the volume fraction of retained austenite was investigated and there is not a quantitative comprehensive investigation about all phases in the microstructure of this steel. The main goal in this study is a comprehensive qualitative and quantitative investigation in various phases of TRIP steel microstructure. Therefore, a TRIP steel with chemical composition of 0.2C+ 1.43Si+ 1.58Mn was produced and its complicated microstructure which contained ferrite, bainite, martensite and retained austenite was investigated using 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 studies and identification of the morphologies of the phases but the EBSD results and functions were used to qualitative and quantitative studies. The volume fractions of retained austenite, ferrite+bainite and martensite phases were calculated and the amounts of 11%, 82% and 7% were obtained, respectively. The volume fraction of retained austenite was also measured with XRD and the amount of 14.3% was obtained.