The recrystallization behavior of three Nb-bearing 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%. The specimens were tested on a computerized torsion machine using a simulation schedule of 17 passes. Deformation temperatures of 1180$ sp circ$C to 700$ sp circ$C were employed, together with pass strains of 0.1 to 0.7, strain rates of 0.2 to 10 s$ sp{-1}$, and interpass times of 2 to 200 seconds. By means of mean flow stress vs. 1000/T diagrams, the effect of reheating temperature, initial grain size, and chemical composition on the T$ sb{ rm nr}$ (temperature at which recrystallization is no longer complete) was determined. Reheating temperature and microalloying elements have significant influence on the T$ sb{ rm nr}$, while initial grain size has no effect on the T$ sb{ rm nr}$ during multipass deformation. Furthermore, the effect of deformation parameters such as the pass strain, strain rate, and interpass time on the T$ sb{ rm nr}$ during multipass deformation was investigated in this way. The T$ sb{ rm nr}$ decreases with increasing strain and also decreases slightly with increasing strain rate. There is a T$ sb{ rm nr}$ minimum at times of about 12 $ sim$ 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 as the time is extended instead. The evolution of Nb(C,N) precipitation during simulated rolling was studied with the aid of carbon extraction replication and electron microscopy. Finally, by applying the additivity rule to the isothermal model of Dutta and Sellars, continuous cooling T$ sb{ rm nr}$'s were predicted from recrystallization-precipitation-temperature-time (RPTT) diagrams; these are shown to be in good agreement with the experimental observations.

Jonas, J. J. (Supervisor)

Yue, S. (Supervisor)