Abstract
Anti-exfoliation properties of galvannealed coatings on extra low carbon Ti-added steel galvannealed under various heating conditions were investigated by using a bead-drawing test, and the result was compared with that of a cylindrical cupping test and an adhesive-bonding tensile test.
Exfoliation amounts in the bead-drawing test are not necessarily correlated with those in the cupping test: regarding ζ-rich coatings, significant discrepancy between exfoliation amounts in the bead-drawing test and those of the cupping test was observed, and exfoliation amounts in the bead drawing test were larger than expected values deduced from the result in the cupping test. ζ-rich coatings are subject to coating exfoliation at coating/steel boundary in the bead drawing test as well as in the adhesive-bonding tensile test. It was thought that the deterioration of exfoliation resistance of ζ-rich coatings is attributable to high frictional coefficient and relatively poor coating adhesion at coating/steel boundary. Coating adhesion of ζ-rich coatings to steel substrate is improved by eliminating ζ-phase by means of galvannealing at temperatures below 500°C.
Coating adhesion of δ1-rich ones galvannealed above 550°C was proved very poor in both the bead-drawing test and the cupping test; in both cases, exfoliation of this type coatings seems to be caused by bending-unbending deformation.
