Effect of deformation and Nb segregation on grain size bimodality in HSLA steel

Bimodal ferrite grain sizes ( mixed coarse and fine grain bands) have been observed in Nb microalloyed thermomechanically controlled rolled (TMCR) steel plates and are undesirable as they can reduce toughness. This paper examines the role of rolling deformation on the formation of bimodal grain structures in reheated continuously cast slab material with initial uniform or bimodal austenitic grain structures. The slab material contains solute rich and solute poor regions, due to interdendritic segregation, which have been shown to cause bimodal austenite grain structures during reheating within a certain temperature range. It is known that deformation in the partial recrystallisation region can result in a mixed coarse and fine grained structure. Theoretical calculations (based on the Dutta-Sellars model) and deformation experiments indicated that the segregated microalloying elements ( particularly Nb) can promote bimodality during deformation by affecting the local recrystallisation kinetics over a much wider range of temperatures than for a homogeneous material.