The effect of Al content in galvanizing bath and chemical composition of steel sheet on the coating adhesion of galvannealed ultra-low carbon steel sheet was investigated using the lap-shear method and the gravel method.The exposed steel surfaces after both the lap-shear test and gravel test were examined so as to discuss the relationship between the geometrical shape of the coating/steel interface and the coating adhesion strength.In the case of ultra-low carbon Ti added steel, high Al addition to the galvanizing bath increased the adhesion strength of galvanealed coating, being previously announced.
In the case of phosphorus added steel, high Al addition to the galvanizing bath didn't alter the morphology of coating/steel interface which is even and smooth, therefore the adhesion strength didn't increase.Si additions in the range of 0.05-0.10% to steel increasaed the adhesion strength of Galvanealed coating of phosphorus added steel, regardless of Al content in galvanizing bath.Zn invasion into the steel grain boundary was promoted by Si additions in the range of 0.05-0.10% to steel, and as a result, αFe grain boundary became brittle.Because the propagation course of crack in coating detoured round or branched, it was assumed that the peeling of coating/steel is relieved.Moreover, the addition of Si to steel has an excellent powdering resistance of galvannealed steel.
