On the significance of intercritical annealing time in governing mechanical properties of lean composition dual-phase steel

In this study, the significance of intercritical annealing time in governing mechanical properties of lean composition dual-phase steel was investigated. The dual-phase steel was produced by 40% asymmetric cold-rolling followed by intercritical annealing at 860 degrees C for 1, 5, 10, and 20 min and water-quenching. It was found that in the 860-5 and 860-10 samples, island martensite forms at grain boundaries in the ferrite matrix and produced a chain-like structure. The martensite fraction increased by increasing the holding time of intercritical annealing from 1 to 10 min. Further increasing the holding time to 20 min, decreased the fraction of martensite. Increasing the martensite fraction reduced the carbon content of martensite, resulting in decreasing the hardness and strength but improving the ductility and formability of the martensite. The yield point phenomenon (YPP) was not visible in the dual-phase samples. The 860-1 and 860-20 DP steels revealed two-stage hardening behavior, while the 860-5 and 860-10 DP samples exhibited a three-stage behavior. The fracture surface of all samples consisted of many small and large dimples, implying ductile fracture. It was concluded that the alpha/alpha ' decohesion mode occurs when the martensite fraction in the microstructure was high enough.