Study of retained austenite and nano-scale precipitation and their effects on properties of a low alloyed multi-phase steel by the two-step intercritical treatment

Microstructure evolution and properties were studied in a low carbon low alloyed hot-rolled bainitic steel by annealing and annealing plus tempering. Microstructure of the hot-rolled steel consists of lath bainite and martensite. By annealing at 720°C for 30min and water quenching, multi-phase microstructure consisting of intercritical ferrite, tempered bainite/martensite, retained austenite and fresh martensite was obtained. With increasing annealing temperature to 760°C, microstructure of the steel consisted of intercritical ferrite, fresh martensite without retained austenite. After the second step of tempering at 680°C for samples annealed both at 720°C and 760°C, ~8–9% volume fraction of retained austenite was obtained in the multi-phase microstructure. Moreover, fine precipitates of VC with size smaller than 10nm and copper precipitates with size of ~10–50nm were obtained after tempering. Results from scanning transmission electron microscopy (STEM) give evidence to support that the partitioning of Mn, Ni and Cu is of significance for retained austenite stabilization. Due to the combined contribution of multiphase microstructure, the transformation-induced-plasticity effect of retained austenite and strengthening effect of nanometer-sized precipitates, yield strength greater than 800MPa, yield to tensile ratio of 0.9, uniform elongation of ~9% and good low temperature impact toughness of 147J at −40°C were achieved.