On significance of initial microstructure in governing mechanical behavior and fracture of dual-phase steels

Abstract

Different initial microstructures were obtained through combination of intercritical annealing and cold-rolling. Subsequently, steels with different microstructures of ferrite–pearlite (FP), ferrite–martensite (FM) and complete martensite (M) were intercritically annealed at 780 °C for 5 min and water quenched to obtain ferrite–martensite microstructure. The significance of initial microstructures on ultimate microstructure, mechanical properties, strain-hardening ability and fracture behavior in dual-phase steels has been elucidated. Initial microstructures of FP, FM and M yielded different martensite morphologies, notably chain-like network structure, fine and fibrous martensite structure, respectively. Furthermore, with increasing martensite content in the initial microstructure, the average grain size of ferrite was significantly refined from about 12.3 to 2.1 μm, which results in that the ultimate tensile strength (UTS) and yield strength were increased, total elongation remained unaffected, and uniform elongation (UE) and strain-hardening ability were increased. A comparison of mechanical properties for different initial microstructures suggested that when the initial microstructure was complete martensite, the steel had excellent mechanical properties, with UTS × UE of 122.5 J cm−3, which was 24% greater than the conventional continuously annealed steels with ferrite–pearlite initial microstructure (98.8 J cm−3). The variation in tensile properties, strain-hardening ability and fracture mechanism of steels with different initial microstructures were discussed in relation to the ultimate microstructures.