Crucial microstructural feature to determine the impact toughness of intercritically annealed medium-Mn steel with triplex-phase microstructure

We investigated the correlation between the impact toughness and microstructures of annealed Fe-8Mn-0.2C-3Al-1.3Si (wt.%) steel to identify the key microstructural feature determining the impact toughness of medium-Mn steel. The microstructural constituents were varied by changing the hot-rolling temperature in the range of 1000-1200 degrees C before intercritical annealing. The annealed steels exhibited a triplex-phase microstructure consisting of delta ferrite with coarse grains and an elongated structure along the rolling and transverse directions and nanolaminate alpha martensite plus gamma(R) retained austenite with ultrafine size. While the volume fraction of gamma(R) remained almost constant regardless of the hot-rolling temperature, the volume fraction of delta increased and that of alpha decreased with increase in the hot-rolling temperature. The average grain size for all phases increased with the hot-rolling temperature. The stability of gamma(R) decreased with the increase of the hot-rolling temperature owing to grain coarsening and a reduction in the Mn and C concentrations. A lower hot-rolling temperature resulted in improved impact toughness. We observed that deep parallel cracks formed and propagated along the delta interface decorated with Mn, ultimately causing a fracture. This result indicates that delta ferrite was the crucial factor determining the toughness among the existing phases, and the steels with a higher fraction of delta exhibited a lower impact toughness. The decrease of the retained austenite stability and the increase of the size of prior gamma grains with increasing hot-rolling temperature were identified as other microstructural factors determining the impact toughness. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.