Effect of martensite morphology and volume fraction on strain hardening and fracture behavior of martensite–ferrite dual phase steel

Two different morphologies of martensite in dual phase (DP) steel were obtained using two different processing routes. In one case, intermediate quenching (IQ) was adapted, where DP steel was water-quenched to obtain martensite phase, followed by inter-critical annealing. In the second case, the steel was cold rolled, followed by inter-critical annealing (CR-IA). For IQ and CR-IA steels, the inter-critical temperatures varied from 750 °C to 850 °C to obtain different volume fractions of martensite. An understanding of structure–property was obtained using a combination of scanning electron microscope (SEM), transmission electron microscope (TEM), and tensile tests. It was observed that fibrous martensite presented in IQ samples, gradually transformed to blocky martensite with increase in inter-critical temperature, resembling the CR-IA steels. The fibrous martensite encouraged martensite cracking, however, the martensite cracking was dramatically decreased in the IQ samples with increase in martensite fraction. The strain hardening behavior studied using the differential C–J model indicated multistage depending on the fraction of martensite. The low volume fraction of martensite in the DP steel provided high ductility–toughness combination and improved strain hardening ability due to the presence of soft ferrite phase in DP steel. Fibrous martensite in DP steel resulted in less strain hardening than blocky martensite, prior to exceeding a threshold volume fraction. The threshold value was significantly smaller for DP steel with blocky martensite.