Volvo Laboratory Study of Zinc-Coated Steel Sheet - Formability Properties

An investigation of 14 different coatings has been carried out. The tendency towards crack formation in the coating during deep drawing and stretch forming was evaluated at different deformation depths. Coatings containing Ni and Fe were very brittle and cracked at small deformations, while pure Zn- and ZnAl-coatings were fairly ductile.

The maximum depth before failure (cracks in the steel) during deep drawing was also measured. It was found that steel sheets with coatings containing Ni or Fe failed at much smaller depths than uncoated steel sheet. Pure Zn, ZnAl, as well as the two-layer coatings of ZnFe and ZnNi with a thin Fe layer on top, obtained deep drawing depths which were almost equal to the uncoated steel. Three coating types showed still better results: a pre-phosphatized zinc coating, a zinc-rich paint and finally a ZnNi coating with a zinc-rich paint on top.

In a draw bead test it was found that the surface topography and the occurrence of an Al-oxide, Cr-oxide or a phosphate layer had a powerful effect on the friction values.