| In resent years, with the top-grade hot-dip galvanizing steel and galvannealed steel extensive using in automotive and appliance field, the substrate surface quality is valued more and more. High surface quality galvannealed steel is that strictly controlling the surface roughness and surface conditions in later craftwork, like as shape formation, high size precision, small tolerance, no surface defects, etc. Study found that the coating defects of galvannealed steel mainly causes by two reasons. One is the inheritance of cold-roll steel sheet defects made the galvannealed coating can't meet the requirements. Another is due to the galvannealed production process. The surface roughness and. scratches are surface defects which common appeared in cold-rolled steel substrate.In this paper, chosing the different roughness and different types of scratches' cold-rolled steel sheet that has for studying. Using OM, SEM, XRD and GDOES equipments to studing the effects of the cold-roll steel sheets'roughness and scratches to the galvannealed coating performance and observeing the morphology and growth process of Zn-Fe alloy in the coating layers after the different roughness and different types of scratches cold-roll steel sheets galvannealed. The powdering resistance and the corrosion resistance of the galvannealed coating were evaluated by the 60°V-bend type test, NSST and electrochemical etching method.The results show that the reaction between zinc and iron accelerated by the increasing surface roughness under the same galvannealed process. Because the specific reaction area of that the substrate and the liquid zinc increased, the galvannealed degree increased too. The amount of the increased Fe in the coating improved the coating weldability and the corrosion resistance, but reduced the powdering resistance of the coating. Under the 15 seconds in the 510℃heating of the galvannealing process, the galvannealed coating which the substrate roughness between 0.6μm and 0.8μm[0.6 |
