Grain Structure Development during Reheating and Deformation of Niobium-Microalloyed Steels

The present investigation looks into the effects of reheating and simulated rolling deformation on development of grain morphology and distribution in specimens taken from quarter thickness position of a 290mm thick cast slab of Nb-microalloyed steel. Plane strain compression in a Gleeble 3500 thermomechanical simulator has been carried out at a strain rate of 10/s up to a maximum deformation of 30%. It was observed previously that, depending upon the reheat temperature, the grain structure of reheated specimens could be either unimodal or bimodal. The variation in the reheated grain structure has been related to the segregation of microalloying element (Nb) and stability of Nb(C, N). The effect of deformation on the reheated specimens towards development of final grain structure is found to depend upon the deformation temperature through the interaction between recrystallization and precipitation phenomena. The Dutta-Sellars model has been applied to the experimental condition and indicates that the segregated microalloying elements (particularly Nb) can promote bimodality during deformation by affecting the local recrystallization kinetics over a much wider range of temperatures than for a homogeneous material.