Simultaneous enhancement of strength and ductility in ferrite-martensite steel via increasing the martensite fraction

Abstract

In the current research, the effect of intercritical annealing temperature on the microstructural evolution and mechanical properties of dual-phase (DP) steel was investigated. The results showed that a homogeneous distribution of martensite in the ferrite caused by performing the asymmetric rolling before the intercritical annealing. By increment of the temperature of intercritical annealing to 830 °C, the volume fraction of martensite increased to 0.68. The DP770 and DP830 steels had the plate and lath martensite, respectively. Interestingly, both strength and ductility remarkably increased with increasing of martensite fraction owing to the decrement of martensite carbon concentration. Increasing the yield and ultimate tensile strengths (YS and UTS) was more than 174% and 130% in comparison with the as-received steel, respectively. There was a strong linear relationship (with R-squared more than 0.95) between the tensile properties and the martensite fraction. When the temperature of annealing increased, the rate of work hardening enhanced, especially in the early stages of plastic deformation, due to the increment of martensite fraction in the microstructure. The DP830 steel has one stage of strain hardening, which was owing to the improvement of strain hardening ability of the martensite phase caused by the lower carbon content. The results of the current research exhibited that the failure mode of DP steels was a mixture of brittle and ductile fractures, however, the DP830 steel has a more ductile fracture. The decreasing the hardness difference between ferrite and martensite in DP steels decreased the severity of interface decohesion and delayed void formation and fracture during the tensile deformation.