Mathematical modeling of the recrystallization kinetics of Nb microalloyedsteels

The recrystallization behavior of Nb microalloyed steels was studied using hot torsion testing with the aim of modeling the recrystallization processe s taking place during hot rolling. Continuous and Interrupted torsion tests were performed in the temperature range 850 to 1050 degreesC at strain rat es of 0.05 to 5/s on selected low carbon steels containing Cr, Mo, Nb, Ni a nd Ti. The kinetics of static and metadynamic recrystallization were charac terized and appropriate expressions were formulated for the recrystallizati on kinetics. These are shown to depend on steel composition and the process ing conditions. The rate of metadynamic recrystallization increases with st rain rate and temperature and is observed to be independent of strain, in c ontrast to the observations for static recrystallization. By means of extra polations to mill strain rates,it is shown that metadynamic recrystallizati on will always be more rapid than static recrystallization, even at the lar gest possible accumulated strains. These calculations support the view that the unexpected load drops occasionally observed in industrial mills (parti cularly in the final few passes) are probably due to strain accumulation le ading to the initiation of dynamic recrystallization, followed by metadynam ic recrystallization.