Ductile fracture mechanism for dual phase steel with high strength second phase

The purpose of this study is to discuss the effect of high strength second phase in Ferrite-Martensite dual-phase steel on ductile fracture mechanism. Ferrite and Martensite single-phase steels with the same properties as Ferrite and Martensite phases in the dual-phase steel were made. The smooth and circumferentially notched round-bar tensile tests for the steels were conducted. The Martensite single-phase steel has low ductility and toughness. However, no cleavage surface was observed in the fracture surface of round-bar tensile specimen of the dual-phase steel. The micro-voids were nucleated mainly in the Ferrite phase near Ferrite/Martensite boundary. The strain of Martensite phase in dualphase steel below fracture surface was greater than critical strain obtained by round-bar tensile test for Martensite single-phase steel. Elasticplastic analysis using 3D micro-structural model was conducted to obtain localization of stress/strain due to micro-structural heterogeneity. The plastic strain and the stress triaxiality in the Ferrite phase near Ferrite/Martensite boundary where micro-voids were nucleated in the experiments were higher than those in the other region. The Martensite phase was compressed in the perpendicular direction to loading direction due to deformation of Ferrite phase. Therefore, stress triaxiality in the Martensite phase was lower than that in the Ferrite phase.