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

Abstract

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