THE RECRYSTALLIZATION BEHAVIOUR OF Nb MICROALLOYED STEELS

Continuous and interrupted hot torsion tests were carried out to investigate the hot deformation and recrystallisation behaviour of a range of Nb microalloyed steels. This study focused on the kinetics of post deformation softening, t(50), as a function of the alloy content, initial microstructure and deformation conditions. It was found that there is a complex interaction between deformation, recrystallisation and strain induced precipitation. The post-deformation softening behaviour of austenite was influenced by carbon with different effects in different temperature regimes. At deformation temperatures above the non-recrystallisation temperature, T-nr, the post-deformation softening was slightly influenced by carbon. However, the post-deformation softening was significantly retarded with carbon at the deformation temperature lower than T-nr due to Nb(C,N) precipitates. If these precipitates formed during deformation then the post deformation recrystallisation rate was significantly reduced, even at very high strains. However, if the precipitates were formed before the deformation there was no noticeable change in the post deformation softening behaviour of steel compared with the steel without any strain induced