MODELING RECOVERY AND RECRYSTALLIZATION KINETICS IN COLD-ROLLED TI-NBSTABILIZED INTERSTITIAL-FREE STEEL

The recovery and recrystallization kinetics in an 80 pct cold-rolled T i-Nb stabilized interstitial-free (IF) steel have been characterized f or isothermal (500 to 760 degrees C) and continuous heating (0.025 deg rees C s(-1) to 20.2 degrees C s(-1)) annealing. Isothermal recovery k inetics, as monitored by {220} X-ray peak resolution measurements, wer e described using a Semiempirical logarithmic equation. The IF steel r ecovered relatively easily, with approximately 45 to 60 pct of the tot al peak resolution occurring prior to the onset of recrystallization. An iterative procedure was adopted to separate the diffraction effects associated with the concurrent recovery and recrystallization process es. Microstructural observations indicated that the recrystallization event was heterogeneous, with preferential nucleation and early site s aturation at grain boundaries in the cold-rolled material. Isothermal recrystallization kinetics, determined by quantitative metallography, were described using the Johnson-Mehl-Avrami-Kolmogorav (JMAK) and Spe ich-Fisher (SF) relationships. An alternative description of the isoth ermal recrystallization kinetics was provided by the experimentally de termined microstructural path function, independent of the thermal pat h, and an empirical kinetic function describing the interface averaged growth rate. The kinetic analysis yielded an apparent recrystallizati on activation energy of 501.7 kJ/mole, indicating severe retardation o f recrystallization in IF steels. Recovery and recrystallization kinet ics during continuous heating have been modeled using the isothermal k inetic parameters, assuming the validity of the principle of additivit y. The results were validated by experimental measurements obtained at heating rates simulating both batch and continuous annealing. Althoug h the Scheil additivity equation overestimated the recrystallization s tart time for continuous heating conditions, the associated higher tem perature and more rapid initial recrystallization resulted in similar overall kinetics.