Static recrystallization of Nb and Nb-B steels under continuous cooling conditions

Abstract

The recrystallization behaviour of nine steels containing various combinations of Mo, Nb and B was investigated under continuous cooling conditions. Hot torsion tests were performed using a 17 pass deformation schedule at strain rates of 0.2 to 10s-1, and interpass times of 1 to 150s. After reheating at 1 250 or 1 200°C, the first deformation was applied at 1 180°C and the last one at about 700°C. By plotting mean flow stress vs. 1 000/T, the no-recrystallization temperatures (Tnr) were determined, and the effects of chemical composition and deformation conditions were assessed. Separate empirical equations for the Tnr were derived from the data for short and long interpass times. These relations describe the influence of chemical composition and of the pass strain, strain rate, and interpass time under continuous cooling conditions. Furthermore, the relationship between the residual strain εr or the residual strain ratio λ and the deformation parameters was also established by analyzing the flow curves. This made it possible to specify the recrystallization limit temperatures (RLT) and recrystallization stop temperatures (RST) that apply to each steel and to each combination of rolling conditions.