Effect of the substrate properties on the surface quality of galvannealed steel sheets

Interstitial-five high strengthened steels with the tensile strength of 340-440 MPa produced by the addition of Ti, Nb, Mn and P are galvannealed for automobile panels to increase their corrosion resistance. It has been well known that the substrate characteristics such as chemical composition, roughness, grain size and texture significantly affects the visual appearance of the galvannealed coating. An understanding of the micro-structural phase evolution that takes place at the interface between zinc and substrate steel is essential to obtain good surface quality of coating. In the present study, both conventional interstitial-free scavenged by Ti and Nb and interstitial-free high Strengthened sheet steels containing Mn and P were galvannealed in a Rhesca hot-dip simulator. The effect of chemical composition and substrate properties on the surface quality of galvannealed steel sheets has been investigated by glow discharge spectrometer, scanning electron microscopy, transmission electron microscopy with Gatan image filter and focused ion beam techniques.