Effect of annealing time on the microstructure and properties of Nb-bearing high-strength IF steel with super-fine grains

Taking a Nb-bearing high-strength IF steel with super-fine grains as an object of study, cold rolling and simulative continuous annealing experiments were carried out in the laboratory. The results show that its microstructure can be refined and homogenized by choosing a feasible annealing time, with a number of cake-shaped grains. The steel is solid-solution strengthened by adding solid-solution elements such as Si and Mn; fine Nb(C,N) forms in its microstructure by adding micro-alloy element Nb. The tensile strength of the steel is improved by fine-grain strengthening and precipitation strengthening due to a dispersive distribution of fine niobium earbonitride and the strengthen mechanisms are solid solution strengthening, fine-grain strengthening and precipitation strengthening. It is noted that the yield strength is low but the tensile strength is high since precipitated free zones exist on one side of grain boundaries. In contrast to a conventional IF steel, the steel has super fine grains and exhibits excellent formability such as low yield strength and high r-values.