Effect of Microstructure on Fracture Mechanisms in Galvannealed Coatings.

Fracture mechanisms in galvannealed coatings have been studied by performing draw bead tests on galvannealed Ti stabilized interstitial free and Aluminum killed low carbon steel sheets and by investigating coating microstructures by scanning electron microscopy. Galvannealing treatments, on samples galvanized using an industrial hot-dip galvanizing process, were conducted at 450, 500 and 550°C for several time periods between 1 and 360 s in a laboratory induction furnace.In the coatings with low Fe content (up to 5 g/m2 ), the amount of powdering during the draw bead test was minimal. Growth of cracks nucleated within the δ1 phase was arrested at the steel-coating interfaces where only a limited amount of decohesion occurred. A steep increase in the amount of powdering was ob-servedin coatings with Fe contents between 6–9 g/m2 . In these coatings, cracks originating from the δ1 phase reached Γ–Γ1–δ1 phase boundaries, which provided preferential crack growth paths and thus facilitated fracture within the coating. A fracture mechanics model was proposed to account for the powdering resistance of galvannealed coatings.