Cracking Mechanisms of Hot-Dip Galvannealed Coatings on CP1000 Steel

In recent years, ultra-high-strength steels are used as structural components more and more. For automobile industry and for different carmakers, different coating layers are chosen for corrosion resistance, such as galvanized (GI) and galvannealed (GA) layers. In this study, a hot-dip galvannealed complex phase 1000 MPa grade (CP1000) steel was chosen as the model material. Different GA parameters were adopted to get different Fe–Zn coating microstructures on CP1000, and bending and tensile loading were loaded on CP1000. Microstructure of the coating layers was investigated using SEM, and the cracking mechanisms of the coating layers were also observed using SEM. It was found that, with increasing GA time, ζ phase decreased on the surface of the coating layers, and more and more δ phase was formed from ζ phase on the surface. Furthermore, with increasing GA temperature, the formation velocity of δ phase got faster and faster. Bending tests showed that cracks could be found at the interface between the coating layers and CP1000 steel, especially when GA temperature was 550 °C. Cracks in δ phase, which were perpendicular to the interface, could also be found. Tensile tests showed that when the tensile strain was low enough, interface delamination could not be found. At the interface, when the GA temperature was 510 °C, cracks perpendicular to the interface could also be found.