In order to eliminate the uneven effect of spangle on the surface, the production of small spangle hot dip galvanized steel sheet products for the production of color-coated steel sheets is a solution. After analyzing the mechanism of the formation of large spangles, it is known that the formation and growth of spangles are related to the composition of the molten zinc alloy, the solidification speed of the molten zinc on the surface of the steel plate, and the number of crystal nuclei generated during solidification.
Pure plating solution does not form spangles when solidified. Spangles can be formed only after adding a certain amount of lead, antimony, and other metals to the zinc solution. The so-called solidification rate influences the crystal growth time during the solidification of the zinc liquid. The length of cooling time determines the length of time provided for crystallization. It takes a certain amount of time for the spangles to grow, and the longer the time, the bigger the spangles are likely to grow.
The growth of a crystal nucleus forms each spangle formed by the solidification of liquid zinc. In the same area, the more the number of nuclei, the more spangles will be finally generated, and naturally, the area occupied by each spangle will be smaller.
Given the mechanism explained above, the spangles on the surface of the hot dip galvanized steel sheet are controlled from different angles in production: one is to make the hot dip galvanized steel sheet and a special steel mesh or unique surface before the solidification of the zinc liquid. The cooling rollers with uniformly distributed upper bumps are in contact with each other. The uniformly distributed crystal nuclei are formed at contact with the zinc liquid and then grow into acceptable and uniformly distributed spangles. The other is to spray atomized water on the surface of the zinc liquid before the coating zinc liquid solidifies, and a large number of cooling sections are formed on the surface of the zinc liquid. Finally, a lot of nitrogen is sprayed on the steel strip in the furnace. At this time, some nitrogen infiltrates In steel; when hot-dip galvanizing, fine spangles will be formed at the nitrides.
Starting from the composition of the bath components, it can also be controlled by the composition of the zinc bath. For example, a high-purity zinc solution is used to control lead, and the content of antimony and other metals that can produce spangles is less than 0.005%. For hot dip galvanized steel sheet, although the surface has a thicker zinc layer and has better corrosion resistance, to enhance the corrosion resistance during use, transportation, and storage, chemical passivation treatment or electrostatic coating are generally required. Oil. Passivation or oiling treatment can be exempted as appropriate if the surface quality of the substrate is guaranteed.