A new 1.2 GPa-strength plain low carbon steel with high ductility obtained by SRDR of martensite and intercritical annealing

Abstract

In the current research, a new technique to produce ultra-high strength ferrite-martensite dual phase (DP) steels with high ductility was developed for a low-cost plain low carbon steel with 0.16 wt% C. A combination of austenitization, water quenching, single-roll drive rolling (SRDR), and intercritical annealing at 830 °C for three different holding times was used. The microstructure and mechanical properties of DP samples were investigated. It was found that the martensite islands were very uniformly distributed in both RD–TD and RD–ND planes of DP steels due to performing the SRDR process. The average sizes of the martensite islands for the holding times of 1 (DP1), 5 (DP5), and 15 (DP15) min were 7.1, 11.6, and 15.8 μm, respectively. Also, the volume fraction of martensite was about 0.29, 0.47, and 0.68 for the DP1, DP5, and DP15 steels, respectively. With increasing the annealing time, the hardness, strength, and ductility increased. The DP15 showed a superior hardness (354.1 H V) strength (1172.5 MPa) and high ductility (13.5%). The martensite phase in the DP15 steel exhibited remarkable plastic deformation because its hardness decreased by reducing the carbon content and therefore the strain hardening ability of the martensite phase increased. Achieving superior strength with high ductility in the DP15 was attributed to the uniform distribution of martensite caused by the SRDR process. As the holding time increased to 15 min, two stages of strain hardening transformed to one stage owing to increasing the ability of strain hardening of martensite with lower carbon content. All DP steels mainly consisted of dimples indicating the ductile fracture, however, some cleavage facets were observed on the fracture surfaces. In DP1 and DP5 steels voids formed mainly by decohesion at the ferrite-martensite interface. With increasing the holding time to 15 min, the severity of decohesion decreased due to increasing the critical strain required for the formation of voids through interface decohesion.