Effect of deformation paramenters and reheating temperature on the non-recrystallisation temperature in Nb and Nb-Ti microalloyed steels

The main objective of the present work has been to determinate the no-recrystallisation temperature (T(nr)) of two Nb-bearing steels and two Nb-Ti microalloyed steels. The T(nr) has been obtained performing multipass torsion tests under continuous cooling conditions. The influence of deformation parameters: interpass time, t(cp), pass strain epsilon, and strain rate, (epsilon) over dot and reheating temperature, T, on the non-recrystallisation temperature has been investigated. In the range of short interpass time (t(ep) < 20 s), recrystallisation is retarded by solute drag so the T(nr) decreases as interpass time increases. For intermediate interpass time, precipitation takes place leading to an increase in the T(nr). However, in the range of long interpass time (t(ep) > 60 s), precipitated particles coarse leasing its effectiveness as recrystallisation retarding agents, which leads again to a decrease in the no-recrystallisation temperature. The T(nr) decreases with increasing pass strain following this type of relationship: T(nr) = beta . epsilon (alpha). Moreover, an increase in the strain rate leads to a decrease in the non-recrystallisation temperature. By increasing the reheating temperature, the amount of dissolved elements that can precipitate at lower temperatures rises, which results in an increase of the T(nr).