Effect of deformation parameters on the no-recrystallization temperature in Nb-bearing steels

The recrystallization behavior of three Nb-bearing, high-strength low-alloy (HSLA) steels was investigated during multipass deformation under continuous cooling conditions. The niobium concentrations of these steels varied from 0.05 to 0.09 wt pct. The specimens were tested on a computerized torsion machine using a simulation schedule of 17 passes. Deformation temperatures of 1,180 C to 700 C were employed, together with pass strains of 0.1 to 0.7, strain rates of 0.2 to 10 s-1. and interpass times of 5 to 200 seconds. By means of mean flow stress vs 1,000/T diagrams, the effect of the deformation conditions on the no-recrystallization temperature (Tnr ), the temperature at which recrystallization is no longer complete, was determined. It decreases with increasing strain and also decreases slightly with increasing strain rate. There is a Tnr minimum at times of about 12 ∼ 15 seconds, and both increases and decreases from this value raise this characteristic temperature. When the interpass times are short, solute atoms control the rate of recrystallization, the extent of which decreases as the time is decreased. When the interpass times are long, precipitation takes place and retards recrystallization, so that the extent of softening decreases