Transformation behavior of TRIP steels during tensile deformation

Abstract

The topic of the presented diploma thesis is the transformation behavior of retained austenite (RA) in Fe-2Mn-1.5Si-0.4C TRIP steel during tensile deformation with focus on a mechanical stability of individual grains of RA. The performed heat treatment consisted of austenitization followed by an isothermal holding at a temperature 400 °C for 1.8 ks. Two main morphologies of RA were observed, lath-shaped RA located between bainitic ferrite and blocky RA distributed mainly at prior austenite grain, packet or block boundaries. The prepared tensile specimens were step-wise strained up to the different levels of strain using in situ micro-tensile stage. At each level of strain, the tensile test was interrupted and the transformation behavior of individual RA grains was observed by electron backscattered diffraction (EBSD) method. The transformation of RA into deformation induced martensite with increasing strain was inhomogeneous, depending on packets of bainite structure. Deformation induced martensite seemed to be formed mainly with specific orientation to adjacent bainitic ferrite. Formation of specific variant pairs between deformation-induced martensite and surrounding bainitic ferrite were frequently observed, such as V1/V4 pair misorientated by a low angle (approximately 10°) or twin-related pair V1/V2.