Deformation microstructures and strengthening mechanisms of an ultrafine grained duplex medium-Mn steel

The medium-Mn steel with an ultrafine grained duplex microstructure was achieved by intercritical annealing treatment. The deformation microstructures of the UFG duplex steel during tension were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). With increasing tensile strain the volume fraction of austenite phase decreased gradually. TEM observations showed that during tensile straining a lamellar-type structure was gradually formed in austenite grains and the lath thickness decreased due to phase transformation from austenite to martensite, while little change was found in ferrite grains except for a slight increase in dislocation density. The strain-induced martensite transformation obeyed the Kurdjumov–Sachs (K–S) orientation relationship. The different deformation behaviors of austenite and ferrite grains did different contributions to the tensile flow stress and the strengthening mechanisms were discussed based on experimental results.