Low-carbon(0.08 wt% C) steel has been subjected to three different heat treatments to obtain dual-phase steels with different microstructures. An understanding of structure–property was established through tensile tests, in conjunction with scanning electron microscope and transmission electron microscope. The results show that the steel after intermediate quenching(IQ) consisting of fine and fibrous martensite exhibited the intermediate strength, highest elongation and the best comprehensive performance of mechanical properties, whereas the steel subjected to intercritical annealing(IA) produced a network martensite along ferrite grain boundaries, having the lowest strength and intermediate elongation. Besides, step quenching(SQ) resulted in a coarse and blocky ferrite–martensite microstructure showing the worst mechanical properties of the three different heat-treatment conditions. The strain-hardening behavior was studied through the modified Crussard–Jaoul model, indicating two stages of strain-hardening behavior for all three samples. The highest magnitude of strainhardening ability was obtained by IQ annealing routes. The analysis of the fractured surface revealed that ferrite/martensite interfaces are the most susceptible for microvoid nucleation. However, martensite microcracks were also observed in SQ sample, and the microvoids are nucleated within the ferrite grain in IA sample as well. The variations in strength,elongation, strain-hardening behavior and fracture mechanism of the steel with different heat-treatment schedules were further discussed in relation to the microstructural features.
Due to the largely inhomogeneous deformation among constituent phases, the advanced high-strength multi-phase steels are always facing challenges when applied to automotive parts where local formability is critically required. In this work, two characteristic microstructures were produced from a low carbon Ti-V microalloyed steel by varying the cooling path. In the ferrite single-phase microstructure resulted from "ultra-fast cooling(UFC) + furnace-cooling(FC)", the hole-expanding ratio of 200% and tensile strength of 647 MPa were achieved. In the ferrite-bainite-martensite(F+B+M) multi-phase microstructure produced by "UFC + air-cooling(AC) + UFC", the ferrite has been strengthened by Ti-V carbides to promote the strain partitioning, which resulted in the tensile strength of ≥780 MPa, a moderate elongation and hole-expanding ratio of 93%. The strengthening contributions of Ti-V carbides were calculated to be 126 MPa and 149 MPa in the ferrite single-phase and F+B+M multi-phase microstructure, respectively.
