The influence of microstructure and composition on the plastic behaviour of dual-phase steels

This paper presents a systematic experimental study of the relative contributions of the martensite volume fraction, morphology and carbon content to the overall strength and ductility of dual-phase steels. To this end, model dual-phase steels combining three volume fractions of martensite, three martensitic carbon contents, and three arrangements of the phases were generated by suitable control of the heat treatments. The microstructures include long and short elongated, as well as equiaxed martensitic islands. The yield and tensile strengths increase with both the amount of martensite and its carbon content. The ductility in general increases with the volume fraction of martensite at constant martensitic carbon content, and conversely increases with the martensitic carbon content for a fixed volume fraction of martensite. Departure from the general trend is observed for combinations of a large volume fraction of martensite with a large martensitic carbon content, and is attributed to damage-induced softening or fracture. Specimens with long elongated martensite demonstrate an improved ductility compared to other microstructures. The plastic flow properties were further investigated by relying on a non-linear homogenization model which explicitly accounts for the microscopic parameters. The micromechanical model captures reasonably well the tensile behaviour for the complete range of martensite carbon contents and volume fractions.