Retained Austenite Transformation and Portevin–Le Chatelier Effect in 44CrMn2Si2Mo Steel under Tension

The 44CrMn2Si2Mo steel heat treated by quenching and partitioning demonstrates a unique combination of strength characteristics: the yield stress σ0.2 = 1140 MPa, ultimate strength σВ = 1690 MPa, and elongation δ = 20.7%. Quenching and partitioning leads to the formation of a multiphase structure consisting of primary martensite, retained austenite, bainite, and secondary martensite. Primary martensite and bainite contain transition-metal carbides Fe2C. The high ductility of the steel is due to the transformation of retained austenite into strain-induced martensite during tension, which ensures high strain hardening. Stable plastic flow is observed at low strain, when a significant fraction of retained austenite is transformed into strain-induced martensite. The plastic flow instability, which appears as the Portevin–Le Chatelier effect on deformation curves and plastic flow localization in deformation bands, occurs at higher strains and is associated with the transformation of film-like retained austenite. The velocity of deformation bands decreases with a decrease in the volume fraction of retained austenite. Localization of plastic flow in the neck and fracture occur when the transformation of retained austenite into strain-induced martensite cannot provide strain hardening, and deformation bands lose their mobility.