On the microstructural characteristics influencing the yielding behavior of ultra-fine grained medium-Mn steels

This paper systematically investigated the influence of microstructural characteristics such as grain size and morphology on the yielding behavior of a cold rolled medium-Mn Fe-6.4Mn-0.1C (wt%) steel. By intercritical annealing of heavily cold-worked and fully martensitic initial state, ultra-fine grained (UFG) microstructures with different morphologies could be obtained, namely globular in the former case and predominantly lath-like in the latter case. The influence of these initial microstructures and intercritical annealing temperature (T-IA) on the final microstructure and resulting mechanical properties was presented in detail. Medium-Mn steels commonly exhibit large yield point elongations (YPE), easily exceeding 10%. Both low TIA and a globular microstructure remarkably supported the formation of large YPE. These YPE formed by localized deformation, which was analyzed by infrared (IR) thermography. Using interrupted tensile testing a vivid linear correlation between decreasing retained austenite (RA) stability and decreasing YPE could be manifested, while this dependency proved to be valid for several medium-Mn steel compositions. Besides the effect of the RA stability on YPE, STEM investigations on deformed tensile samples revealed an entirely different dislocation structure between the UFG globular and lath-like microstructure, suggesting different active deformation mechanisms depending on the overall grain size and morphology. Based on this investigation, it was recommended to provide a martensitic microstructure prior to intercritical annealing in order to limit YPE. Furthermore, special attention has to be paid to a careful design of the RA stability in order to adjust an appropriate balance between excellent mechanical properties and reduced YPE. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.