A novel two-step intercritical annealing process to improve mechanical properties of medium Mn steel

In this paper, we present a novel two-step intercritical annealing process, i.e. a-few-minute interruption during heating followed by a short intercritical annealing, to improve the mechanical properties of medium Mn steel without prolonging the intercritical annealing (IA) period. Both experimental investigations and numerical simulations were performed to investigate the microstructural inheritance between the two steps and its influence on the tensile properties. The increase of interruption temperature led to a gradual change of tensile flow curve from the discontinuous yielding to the continuous yielding at lower yield strength due to more martensite and less austenite formed after the IA at 670 degrees C. It is concluded that the two-step IA process can improve the mechanical properties of medium Mn steel when it is optimized, compared to the non-interrupted IA process. The strategy for optimizing this novel process is to carefully tailor cementite precipitates before annealing as the nuclei of austenite for promoting relatively coarse austenite grains formed with Mn gradients inside. This kind of austenitization on the deliberately introduced nuclei finally led to a considerable fraction of austenite grains retained adjacent to martensite, causing strength and ductility improved simultaneously. Moreover, we demonstrate that such a deliberate design of two-step IA process can be guided by the quantitative modeling of austenitization kinetics on a cementite nucleus. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.