Influence of carbon content on the microstructure, martensitic transformation and mechanical properties in austenite/epsilon-martensite dual-phase Fe-Mn-C steels

We report on the effects of carbon content on the martensitic transformation and its contribution to the work-hardening behavior of Fe-Mn-C steels during tensile deformation based on analysis by X-ray diffraction, electron backscatter diffraction and transmission electron microscopy. Austenite/epsilon-martensite dual-phase Fe-17Mn-C (wt.%) steels containing different carbon contents (0.01, 0.10, 0.20 wt.%) were investigated before, during and after tensile deformation. Before deformation, the transformation of austenite to thermally induced epsilon-martensite on cooling was suppressed as the carbon content increases. To precisely monitor microstructural changes during deformation, stepwise loading experiments were carried out in combination with electron backscatter diffraction analysis. This approach revealed that with increasing carbon content, the kinetics of transformation of gamma phase to deformation stimulated epsilon-martensite became faster, while that of epsilon-martensite to alpha'-martensite was sluggish. We attribute this controversial effect to an increased gamma grain size by the microstructural refinement of thermally induced epsilon-martensite and the reduction of solid solution strengthening effects by the redistribution of solute carbon. In addition, the dependence of deformation-induced epsilon-martensite on the loading direction differed from that of alpha'-martensite, and the evolution of alpha' morphology was controlled by achieving appropriate levels of strain during stepwise loading. Based on the observations at the surface and inside the bulk after deformation, insights into various deformation-driven displacive phenomena, such as the formation of alpha'-martensite at the nonintersecting parts of two epsilon(initial) bands, the presence of nanotwinned bundles inside austenite, cementite precipitation inside alpha'-martensite, and the origin of the serrated flow in strain stress curves, were obtained. Therefore, the present study is able assist in identifying whether the deformation-induced martensitic transformation varied as a function of carbon content and the resulting fracture behavior, thereby enabling us to understand the work-hardening behavior of these steels. Crown Copyright (c) 2012 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.