The Effect of Texture on the Stability of Retained Austenite in Al-Alloyed TRIP Steels
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The Effect of Texture on the Stability of Retained Austenite in Al-Alloyed TRIP Steels Kemal Davut1 and Stefan Zaefferer1 1 Max-Planck-Institute for Iron Research, Max-Planck-Street 1, Duesseldorf, D-40237, Germany. ABSTRACT Steels with transformation induced plasticity (TRIP) offer an excellent combination of high strength and ductility. The transformation of meta-stable austenite into martensite during straining leads to strong local hardening and prevents early localization of strain. Therefore, the mechanical properties of TRIP steels, including the damage resistance depend to a significant extent on the stability of retained austenite. The aim of this study was to evaluate the effect of texture on the stability of retained austenite. In order to compare the changes in both tension and compression the steel was deformed by a micro 3-point-bending device. The texture development upon bending was followed by electron backscatter diffraction (EBSD) technique. Based on a simple analysis using the relation between face centered cube (FCC) and body centered cube (BCC) shear geometries theoretically expected changes of texture components due to deformation are proposed. Using the results of this analysis the observed changes of the austenite texture due to deformation could be distinguished from those due to transformation, by comparing the experimental results with the theoretically expected behavior. From this comparison, austenite grains with “Brass (B) {011} ” and “Goss (G) {110} ” texture components were found to transform into martensite much easier than differently oriented grains. INTRODUCTION Conventional steel types such as interstitial free steels, low carbon steels and high strength steels can have only good ductility at the expense of strength or vice versa. However, there is an increasing demand for steels with better combination of mechanical properties [1-3]. The TRIP steels offer a good balance of ductility and strength, which is a result of transformation of austenite into martensite during straining. Thus, the mechanical properties of TRIP steels are determined mainly by the stability of retained austenite. The aim of is this study is to evaluate the effect of texture on the stability of retained austenite. The texture developed due to transformation of austenite in TRIP-assisted steels has been subject of research over the last 15 years. Park et. al. [4] showed that the measured BCC texture is quite similar to the transformation texture predicted without any variant selection mechanism and using Kurdjumov-Sachs (KS) orientation relationship. Verlinden et al [5] found similar results that no variant selection occurs during transformation of austenite into bainitic ferrite. In contrast, Hutchinson et al [6] found indications of variant selection in hot rolled TRIP steels. He et al [7] observed variant selection only in undeformed austenite grains; already deformed γ-grains show no variant selection during transformation into bainitic ferrite. Raabe [8] reported selective transformation of the “Brass (
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