12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

In this paper we present spangle nucleation and refinement theory in 55%Al-Zn coated steel. We propose that the precipitation of 5c intermetallic phase, (Fe5Si2Al20+Zn) in the coating overlay is the dominant mechanism for spangle refinement. Under normal process conditions, when the strip emerges from the bath, the Fe in solution in the coating overlay diffuses to the alloy layer and contributes to a third of the alloy layer thickness. At higher cooling rates, as the liquid becomes supersaturated with Fe because of the decrease in the solubility of Fe with a decrease in temperature, IMCs will precipitate in the coating overlay. The precipitation of 5c in the coating overlay is promoted by process parameters such as cooling rate which inhibits the diffusion Fe to the alloy layer which results in the precipitation of fine dispersions of IMCs in the overlay. The 5c intermetallic phase provides a potent heterogeneous nucleation site for nucleation and growth of -Al. Using an analytical solution of the diffusion equation for transient conditions, a model is developed to estimate the level of Fe supersaturation in the coating overlay as a function of cooling rate. The effect of cooling rate on spangle refinement is discussed in terms of the Fe supersaturation predicted from the model. The concentrations of Fe supersaturation predicted by the model are in good agreement with measured concentrations of Fe in the overlay of commercial products with varying spangle sizes. The role of fine dispersions of suspended IMCs present in the bath and the effects of strip surface conditions (that result in a discontinuous alloy layer) on spangle refinement are also discussed. Calculations of spangle size based on model predictions indicate that precipitation of IMCs from 10 to 20% of the available Fe would be sufficient to result in spangle refinement. The model predictions were consistent with the results of Fe analysis in the coating overlay of a product with minimized spangle: the product with spangle size of 354m show Fe content of 0.11wt% compared with 0.02wt% for a product with a spangle size of 2 mm.