Study On Coating Thickness Inhomogeneity Of Cold Rolling Hot-dip Galvanizing Sheet

Galvanized sheet is coated with a layer of zinc on its surface, which has many advantages including excellent corrosion resistance, good forming property, low cost as well as good appearance. During galvanizing edge over coating (EOC) may occur near the edge of the strip, which not only causes the waste of zinc but also brings about serious effect on following processes. In hot-dip galvanizing process, liquid zinc was wiped off by the high speed jet flow generated by air knife, so air jet wiping control is so crucial part to determine the coating thickness and uniformity of the zinc layer on the strip.The main purpose of this thesis is to study the influence factors on the coating thickness inhomogeneity of cold rolling hot-dip galvanizing sheet with numerical simulation combined with laboratory experiments. The distributions of static pressure and shear pressure on the strip were calculated by Fluent numerical simulation software, and the influences of air knife at the different conditions were discussed. Meanwhile the actual pressure on the strip and coating thickness were measured by laboratory method,the distribution principles were investigated and the results between simulations and experiments were compared.The main content of this thesis includes three parts:The distribution of coating thickness on the steel strip is measured and observed by optical microscope, SEM and AFM to find the principle of distribution along the width of strip in the first part.The influences of the jet nozzle pressure, the nozzle to strip distance, the nozzle slit opening, the edge baffle plate of air knife, as well as the tilting angle of air knife under the different conditions were discussed. Furthermore, the distribution of zinc on the surface of steel strip in jet wiping process is simulated by Fluent software in the second part.The distribution of pressure along the width of the strip is measured by pressure sensors in the laboratory. A simulated device that refers to the real equipment has been made for studying the distribution of zinc by the function of air-knife jet wiping. And the results from simulations and experiments are compared in the third part.