Formability of Galvanized Steels - Revisited

A series of sheet steels with corrosion-resistant coatings was tested, by punch stretching and in tension, in both the as-received condition and after the zinc was removed chemically. The results obtained indicate that the reduced formability of galvanized steels cannot be attributed to an increase in punch-sheet friction. The punch stretching data were well explained by a model which results in the generation of local tensile stresses leading to premature failure in regions of negative curvature which contact the punch. This model is compatible with the observation that the limiting dome height in punch stretching decreases with increasing intermetallic layer thickness. The model is also compatible with the observation that no such correlation is observed between alloy layer thickness and tensile ductility.