F. Siciliano et Jj. Jonas, Mathematical modeling of the hot strip rolling of microalloyed Nb, multiply-alloyed Cr-Mo, and plain C-Mn steels, MET MAT T A, 31(2), 2000, pp. 511-530
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.