Formation of core-shell type structure in Duplex Martensitic Steel

Microstructures of both as-quenched and intercritical-annealed medium-Mn steel were investigated by transmission electron microscopy to acquire a better understanding of γ–ɛ and γ–α′ martensitic transformations. The orientation relationships among each phases were determined as (1¯10)_α^′//(0001)_ε,[111]_α^′//[112¯0]_ε for the case of as-quenched sample, and (1¯10)_α^′//(1¯11)_γ//(0001)_ε,[111]_α^′//[110]_γ//[112¯0]_ε for the case of intercritical-annealed sample, respectively. In addition, the presences of core-shell type microstructures with the gradient of Mn concentration were confirmed with ɛ–martensite being surrounded by retained γ. Mn concentration of each phase was found dependent on the growth of reversed γ controlled by Mn diffusion during intercritical annealing. It was strongly suggested that the gradient of Mn concentration occurred during intercritical annealing affected the phase stability, which resulted in the formation of ε/γ core-shell microstructures.