Plasticity enhancement mechanism: Effect of the annealing temperature on strain-induced segmented martensitic transformations and Portevin–Le Chatelier bands in 7Mn steel after deep cryogenic treatment

Abstract

This study investigated the microstructural evolution and strain localization of warm deformed Fe-0.15C-7Mn steel after deep cryogenic treatment (DCT) and intercritical annealing (IA) at 600, 630, and 660 °C. After DCT and IA, the steel contained nanoscale austenite grains with inhomogeneous thermal stability and grain size. Lüders bands appeared in all the samples, whereas the Portevin–Le Chatelier (PLC) effect only occurred in the samples annealed at 630 and 660 °C. The occurrence and initiation position of the PLC bands depended on the IA temperature, which affected the final grain size, retained austenite (RA) stability, and interactions between the solute atoms/C-Mn pairs and mobile dislocations. This indicates that segmented martensitic transformations occurred in the samples annealed at 600 and 630 °C, and the transformation sequence γ → ε → α’ occurred in the samples annealed at 630 and 660 °C. The segmented martensitic transformation depended on the IA temperature, which directly affected the amplitude of fluctuations in the Mn concentration in the RA grains and resulted in the heterogeneous thermal stability of the RA. The excellent uniaxial tensile properties of the samples were ascribed to the transformation of RA with heterogeneous thermal stability to segmented martensite during deformation. The segmented martensite acted as a protective shell that shielded the encapsulated austenite in the core. • Microstructural evolution and strain localization of a medium Mn steel were studied. • The annealing temperature affects the PLC bands and microstructural evolution. • Relationship between heterogeneous stability of RA and martensitic transformations. • Mechanisms underlying the observed physical properties were investigated. • Segmented martensite transformation behavior occurs and plays a protective role in the encapsulated austenite.