Modeling recovery and recrystallization kinetics in cold-rolled Ti-Nb stabilized interstitial-free steel

The recovery and recrystallization kinetics in an 80 pct cold-rolled Ti-Nb stabilized interstitial-free (IF) steel have been characterized for isothermal (500 to 760 degrees C) and continuous heating (0.025 degrees C s(-1) to 20.2 degrees C s(-1)) annealing. Isothermal recovery kinetics, as monitored by {220} X-ray peak resolution measurements, were described using a Semiempirical logarithmic equation. The IF steel recovered relatively easily, with approximately 45 to 60 pct of the total 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 processes. Microstructural observations indicated that the recrystallization event was heterogeneous, with preferential nucleation and early site saturation 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 Speich-Fisher (SF) relationships. An alternative description of the isothermal recrystallization kinetics was provided by the experimentally determined microstructural path function, independent of the thermal path, and an empirical kinetic function describing the interface averaged growth rate. The kinetic analysis yielded an apparent recrystallization activation energy of 501.7 kJ/mole, indicating severe retardation of recrystallization in IF steels. Recovery and recrystallization kinetics during continuous heating have been modeled using the isothermal kinetic parameters, assuming the validity of the principle of additivity. The results were validated by experimental measurements obtained at heating rates simulating both batch and continuous annealing. Although the Scheil additivity equation overestimated the recrystallization start time for continuous heating conditions, the associated higher temperature and more rapid initial recrystallization resulted in similar overall kinetics.