Mathematical modeling of the hot strip rolling of microalloyed Nb, multiply-alloyed Cr-Mo, and plain C-Mn steels

Industrial mill logs from seven different hot strip mills (HSMs) were analy zed in order to calculate the mean flow stresses (MFSs) developed in each s tand. The schedules were typical of the processing of microalloyed Nb, mult iply-alloyed Cr-Mo, and plain C-Mn steels. The calculations, based on the S ims analysis, take into account work roll flattening, redundant strain, and the forward slip ratio. The measured stresses are then compared to the pre dictions of a model based on an improved Misaka MFS equation, in which solu te effects, strain accumulation, and the kinetics of static recrystallizati on (SRX) and metadynamic recrystallization (MDRX) are fully accounted for. Good agreement between the measured and predicted MFSs is obtained over the whole range of rolling temperatures. The evolution of grain size and the f ractional softening are also predicted by the model during all stages of st rip rolling. Special attention was paid to the Nb steels, in which the occu rrence of Nb(C, N) precipitation strongly influences the rolling behavior, preventing softening between passes. The present study leads to the conclus ion that Mn addition retards the strain-induced precipitation of Nb; by con trast, Si addition has an accelerating effect. The critical strain for the onset of dynamic recrystallization (DRX) in Nb steels is derived, and it is shown that the critical strain/peak strain ratio decreases with increasing Nb content; furthermore, Mn and Si have marginal but opposite effects. Tt is demonstrated that DRX followed by MDRX occurs under most conditions of h ot strip rolling; during the initial passes, it is due to high strains, low strain rates, and high temperatures, and, in the final passes, it is a con sequence of strain accumulation.