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Evolution of microstructure and texture during continuous annealing of cold rolled, Ti-stablized interstitial-free steel Mukunthan, Kannappar


Interstitial-Free (I-F) steels are increasingly being used for press forming operations due to their markedly improved deep-drawability. Additional interest is due to the fact that the cold rolled I-F steels can be effectively heat treated in a continuous annealing line without the need for any accompanying overaging process. The objective of this study was to characterize the evolution of microstructure and crystallographic texture of a 80 % cold rolled, Ti-stabilized I-F steel during heating rates applicable to batch and continuous annealing processes. Isothermal recovery kinetics, as monitored by {220} x-ray peak resolution measurements, were described using a semi-empirical logarithmic equation. Isothermal recrystallization kinetics were determined by quantitative metallographic measurements and were characterized by both Johnson-Mehl-Avrami-Kolmogorov and Speich-Fisher relationships. The isothermal recrystallization kinetics were also described in terms of the experimentally determined microstructural path function and an empirical kinetic function relating the interfaceaveraged growth rate with recrystallization time. The additivity procedure was success fully employed in conjunction with the isothermal kinetic parameters to predict continuous heating recovery and recrystallization kinetics at heating rates simulating batch and continuous annealing processing. Microstructural examination showed that the recrystallization event was heterogeneous and related to the cold rolled cell structure. The recrystallized nuclei, developed primarily by subgrain coalescence occurring during the later stages of recovery, grew into the cold rolled matrix by the migration of high misorientation boundaries. Large precipitates of TiN and TiS acting as preferred nucleation sites and fine precipitates impeding the boundary mobility were also observed. The hot band texture with the moderate presence of (112)[1ï0] yielded a strongly developed cold rolled texture extending from (001)[1ï0] to (112)[1ï0]. The recrystallization texture was characterized by the strong presence of (554)[225] and (111) [1ï0]. Grain growth following recrystallization strengthened the existing texture with an accompanying improvement in average strain ratio values. The effect of heating rate on the final recrystallization texture was found to be insignificant.

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