Coating Adhesion and Interface Structure of Galvannealed Steel

Abstract

The effect of Fe content of coating and Al content in galvanizing bath on the coating adhesion of galvannealed ultra-low carbon IF (Interstitial Free) steel sheet was investigated using a lap-shear method. The exposed steel surface after removal of the galvannealed coating with hydrochloric acid solution was examined so as to discuss the relation between the geometrical shape of the coating/steel interface and the coating adhesion. Galvannealed coating exhibits the minimum adhesion strength in the region of 8–10 mass% Fe, which was thought to relate to the extinction of η-phase. Further increment of Fe content raised the coating adhesion, and high Al addition to galvanizing bath also raised the minimum adhesion strength of the coating around 9 mass% Fe. The steel surface exposed with the acid was even and smooth for the low Al content coatings with 8–9 mass% Fe, while that was uneven or rugged for the coatings with relatively high coating adhesion. The further increment of Fe content in the coating developed Γ-phase to form ledges with the pitch of about 100 nm at the coating/steel interface, and the increase of Al in galvanizing bath resulted in uneven alloy formation, particularly discontinuous formation of Γ-phase to cause uneven erosion of the steel surface and form the ruggedness with the pitch comparable with α-Fe grain size at the coating/steel interface. The ruggedness with the pitch comparable with α-Fe grain size was thought to result from the dependency of Fe-Zn interaction on the microstructure of the base steel. It was assumed that the coating adhesion is governed by the ruggedness of the coating/steel interface and formation of Γ-phase contributes to increasing the adhesion strength. © 1995, The Iron and Steel Institute of Japan. All rights reserved.