Abstract
Galvannealed (GA) steel sheets have been widely used in the automotive industry because of their good weldability, paintability, and corrosion resistance. However, coating failure and exfoliation originating from the brittle and hard FeZn phases during press forming is a serious problem. This study investigates the mechanism of flaking deterioration in GA coatings with high Fe content. For this, specimens with various Fe contents were fabricated with different galvannealing temperatures. As the temperature increased, the η and ζ phases became extinct and both δ and Γ phases grew simultaneously. Accordingly, surface became homogeneously flattened with crater degeneration. Simultaneously, the interface was flattened by lateral Γ phase growth. The flat region where the Γ existed was widened by broadening the Γ phase area, by which the etch-pit shape rough knoll found under craters was gradually eliminated. As a result, the Γ phase growth resulted in interface flattening, which deteriorated the flaking resistance.
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