Grain refinement of a Nb–Ti microalloyed steel through heavy deformation controlled cooling

Heavy deformation and controlled cooling is one of the most effective ways of grain refinement in HSLA steels. In this paper, experimental specimens were prepared with a commercial line-pipe steel grade X65, the chemical compositions of which is Fe–0.094 wt.% C–1.4 wt.% Mn–0.29 wt.% Si–0.045 wt.% Nb–0.008 wt.% Ti. After heavy deformation to 80% reduction at a non-recrystallization range of austenite and controlled cooling rates of 2–64 K/s, the grain sizes varied from 9.55 to 1.06 μm. It is shown that the strength of the steel increases almost linearly with grain refinement. The ultra-fine grains might be formed dynamically through deformation-induced transformation if the reduction is large enough. The grain sizes decrease if the cooling rates increase, but when the cooling rates increase to over 10 K/s, the grain sizes change little further.