Phase transformation under thermal fatigue of high Mn-TWIP steel: Microstructure and mechanical properties

High Mn steels present both high tensile strength and good ductility, therefore they have attracted attention as promising candidates for the next-generation of automotive steels. However, slight changes during the manufacturing process or in service conditions (i.e. chemical composition, annealing temperature, among others) can promote significant variations in their microstructure, leading to a strong modification in their mechanical response. In this regard, this paper discusses the relationship between the different microstructures generated on a high Mn-twinning induced plasticity (TWIP) steel and its mechanical properties evaluated by means of Vickers's hardness, tensile testing and also high cycle fatigue response. Different conditions, namely: as received, annealed at 500 °C and thermally cycled at 500 °C during 15, 36, 56 and 75 cycles, have been analyzed. The results exhibit the development of a heterogeneous pearlitic microstructure with a plateau on its fraction content and Vickers's hardness at ~24% and 292±5 HV, respectively, after 36 thermal fatigue cycles. Finally, pearlite colonies have been nucleated during the thermal fatigue treatment along the austenitic grain boundary producing a deleterious effect on the tensile and high cycle fatigue behavior.