A. Reasons
1. The wettability of Al-Zn-Si melt and steel is much worse than pure Zn melt. At the same time, as the drastically increase of plating bath temperature and the aluminum content, the manufacturing technique of hot dip galvalume steel sheet has undergone fundamental changes. The melting point of Zn is 419.5 ℃. And the temperature control of hot dip galvanizing at 455~ 475 ℃ is sufficient. In the equilibrium state, the liquidus temperature of hot dip galvalume steel sheet is about 590 ℃. And thus the bath temperature must be controlled above 600 ℃.
At such a high temperature, the galvanizing zinc without protective layer will easily evaporate in the furnace nose. Zinc vapor directly attached to the steel sheet or zinc vapor attached to the parts in the furnace nose to form zinc powder and fall on the steel. These will all affect the direct contact between steel plate and bath solution. And thus to the form the pinhole-like skip plating defects.
2. Al content in the plating bath melt is high. In the production of hot dip galvalume steel sheet, CAMELSTEEL found that, such as the strip surface reduction is not good, and exist a certain oxidation element or the oxygen content and steam content of shielding gas is high. These will cause the oxidation of Al. And Al oxide will be dispersed in the plating bath interior of furnace nose. If it sticks to the strip, the adhesion of the coating will also be deteriorated.
B. Measures
1. Strictly control the purity of the protective gas introduced into the furnace nose to prevent the impurities in the protective gas from reacting with the plating bath.
2. Increase the hydrogen concentration of the reducing gas in the furnace. The reducibility of steel in the annealing furnace of aluminized zinc line are high, and the oxide can not be brought into the plating bath. Staff must reduce the oxygen content of reducing gas and dew point in the furnace while increasing the hydrogen content.
3. Select the raw materials with the least oil, oxidation and iron filings, and strengthen the degreasing effect of pre-treatment. The production line of galvalume steel sheet must to remove the oil, rust, iron and dust and other impurities on the steel surface to the hilt.