The evolution of ferrite grain size in structural steels

The refinement of the ferrite grain size is the main aim of modern thermomechanical processes for hot rolled steels. The ferrite grain size is determined by the composition, the state of the austenite at the point of transformation and the cooling rate through transformation. By adding microalloying additions of Ti for grain refinement and Nb to retard recrystallization, it is possible to reduce the ferrite grain size to less than 5 μm at moderate to high cooling rates. However, it is not possible under even the most extreme traditional controlled rolling and accelerated cooling conditions to produce an equiaxed ferrite grain size of less than 3 μm. More recent work, though, involving rolling with high undercooling and friction conditions that lead to high shear, suggests that it is possible to produce microstructures in a single rolling pass with an average grain size less than 1 μm. This appears to involve a dynamic (ie strain induced) transformation process. The current understanding of static and dynamic transformation and the resultant grain size is reviewed and areas requiring further research are highlighted.