It is seems obvious that the application of kinetic theory requires an understanding of the mechanisms of transformations that occur in steels. Grain size refinement using thermomechanical processing is an important method for improving both the strength and toughness of steels. When considering austenite grain growth in large components, the time taken to reach the heat-treatment temperature can be taken into account. Thermomechanical processing is routine in the production of structural steels, with the aim of refining the austenite grain size and hence the ferrite grain size following transformation. Isothermal transformation diagrams contain curves, each of which represents a specific degree of transformation for a specific product, as a function of temperature, time and the austenite grain structure. Large particles are more effective since there are greater deformation gradients around them and hence are expected to be more effective in inducing recrystallisation.