The role of recrystallization and coarsening in the formation of ultrafine grained steels through thermomechanical processing

There is now considerable interest in the development of ultrafine grained steels with an average grain size of the order of 1 mum. One of the methods with currently the greatest industrial interest is by dynamic strain induced transformation from austenite to ferrite. This involves deformation below the equilibrium transformation temperature so that transformation occurs during the deformation. However, large strains are required to completely transform the microstructure during deformation. It is potentially possible to activate transformation during deformation then continue transformation during subsequent cooling. It is shown that there are two critical strains: the first is where dynamic transformation commences and the second is the minimum strain for a fully ultrafine final microstructure after cooling to room temperature. The deformation and potential role of dynamic recrystallization of the dynamically formed ferrite is also considered. Overall it is clear that for full industrial exploitation there is a need to understand and exploit the competing issues of nucleation, growth and recrystallization of the ferrite by both dynamic and static processes.