Abstract: Annealing and hot-dip galvanizing simulations were conducted on the cold-rolled IF steel sheet with localized surfacerust by using IWATANI hot-dip galvanizing simulator. Effects of substrate surface rust on annealed, hot-dip galvanized (GI) and galvannealed (GA) coatings were studied. The results show that after continuous annealing in an atmosphere of N₂+5% H₂ at 800 ℃, the rust is reduced to reduced iron with small grains, loose structure and numerous micropores between each other. During hot-dip galvanizing, the reduced iron rapidly diffuses with the molten zinc, causing the appearance of a zinc iron alloy phase in the coating and reducing the fluidity of the molten zinc, which further affects the control of the coating thickness when the specimen passes through the air knife, resulting in localized thickening of the GI coating at the rust location. After coating alloying, the morphology of the GA coating at the rust position is completely different from that of the normal coating, that is, the coating is flat and thick, and ultimately shows as white spot defects on the surface of the GA coating.

Key words: substrate rust, reduced iron, hot-dip galvanizing, galvannealed coating