Evaluation of Zinc Coating Adhesion in Stamping Advanced High Strength Steel

Abstract

Advanced high strength steels (AHSS) have been widely used in vehicle structural components due to their high strength properties balanced with good formability. In many applications, a zinc coating is often applied to AHSS for the corrosion protection. Due to the high strength properties of AHSS, higher forming forces and binder pressures are also required in stamping AHSS, which often results in coating powdering/flaking or galling around the draw bead and die radius areas. The degree of coating powdering/flaking depends upon forming process variables such as draw bead force, forming speed, lubrication, and die temperatures. This study was designed to develop a lab-based coating adhesion test to quickly characterize the coating adhesion behavior of AHSS. In this study, both the bending under tension test and the modified draw bead test were used. A typical substrate of AHSS is coated with either a hot-dip Galvannealed (GA) or a Galvanized (GI) coating. In this study, dual phase (DP) steels with tensile strength levels of 600, 780 and 980 MPa were investigated at two different thicknesses and with two different coating types, GI and GA. The results reveal that GA tends to have worse coating adhesion than GI coatings under normal forming conditions. However, GI coatings are more sensitive to the lubrication condition and have worse coating adhesion than GA coatings under a larger draw bead penetration and smaller draw bead corner radius condition.