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 (-110)α′//(0001)ɛ, [111]α′//[11-20]epsiron for the case of as-quenched sample, and (-110)α′//(-111)γ//(0001)ɛ, [111]α′//[110]γ//[11-20]ɛ for the case of intercritical-annealed sample, respectively. In addition, the presences of core-shell type microstructures were confirmed with ɛ–martensite being surrounded by γ–austenite. Mn content of phases were found dependent on the solubility limit of Mn of phases by the combination of scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The γ–austenite grow further by the shifting the interface between γ–austenite and α'–martensite, and forming of Mn depletion zone. Furthermore, the estimated diffusion distance of Mn within with γ–austenite was calculated about 25 nm.