Mathematical modeling of the hot strip rolling of niobium microalloyed steels

Industrial mill logs from 7 different hot strip mills were analyzed in order to calculate the mean flow stresses developed in each stand. The schedules were typical of the processing of Nb microalloyed, multiply-alloyed and plain C-Mn steels. The calculations, based on the Sims analysis, take into account work roll flattening, redundant strain and the forward slip ratio. The measured stresses are then compared with the predictions of a model based on an improved Misaka mean flow stress equation, in which solute effects, strain accumulation, as well as the kinetics of static and metadynamic recrystallization are fully accounted for. Good agreement between the measured and predicted mean flow stresses is obtained over the whole range of rolling temperatures. The evolution of grain size and the fractional softening are also predicted by the model during all stages of strip rolling. Special attention was paid to the Nb steels, in which the occurrence of Nb(C,N) precipitation strongly influences the rolling behaviour, preventing softening between passes. The present study leads to the conclusion that Mn addition retards the strain-induced precipitation of Nb. By contrast, Si addition has an accelerating effect. The model calculates the critical strain for the onset of dynamic recrystallization in Nb steels. It shows that the critical strain/peak strain ratio decreases with increasing Nb content, and that Mn and Si have marginal but opposite effects. It is demonstrated that dynamic recrystallization followed by metadynamic recrystallization occurs under most conditions of hot strip rolling, during the initial passes due to the high strains, low strain rates and high temperatures, or in the final passes as a consequence of strain accumulation.