Effects of aluminium and nitrogen on static recrystallisation in V-microalloyed steels

Static recrystallisation of three steels having different Al, V and N contents (one of them without V) has been studied by means of hot torsion tests. It has been found that strain-induced precipitates of AlN in the austenite have a mean size of approximately 86 nm. These particles barely inhibit the static recrystallisation, as associated pinning forces are very weak. However, mean size of VCN particles is equal to 11 nm. This fine size leads to a temporary inhibition of recrystallisation, revealed by the characteristic “plateau” of the plots of recrystallised fraction versus holding time after deformation. Besides, activation energy for recrystallisation considerably augments when VCN precipitation occurs, but it hardly increases when AlN particles precipitate. On the other hand, diffusion coefficient of Al in austenite is two orders of magnitude higher than for V. Furthermore, according to thermodynamic calculations based on Hillert and Staffanson method, precipitation of AlN particles starts at much higher temperatures than VCN. Aforesaid reasons make AlN particles to be much coarser than VCN precipitates. From the results it can be concluded that low Al contents would lead to more intense precipitation of VCN that is beneficial for V-microalloyed steels.