The Shear Strength of Galvannealed Coatings on IF Steels

Abstract

The present contribution focusses on the characterisation of the shear strength of galvannealed on Ti and TiNb interstitial-free steels currently used in the automotive industry. A modified lap shear testing method was developed which gave the shear strength of the substrate/coating interface independently of the mechanical charateristics of the substrate. The criteria for adhesive selection and the required sample geometry are reviewed in detail. An in-depth electron microscopic study of the fracture plane structure was also carried out. It revealed, among other things, a quantitative relation between the shear strength of the coating and the area of the steel/Γ phase fracture plane. It is shown that the shear strength of galvannealed coatings is influenced by (1) the Fe content of the galvannealed layer, (2) the type of steel substrate (Ti versus TiNb interstitial-free steel, drawing IF steels versus high strength IF steels) and (3) the effect of certain alloying elements.