The main reason is that the wettability of the Al-Zn-Si melt and the steel plate is much worse than that of the pure Zn melt. At the same time, the temperature of the plating bath and the aluminum content have been greatly increased, which has caused a fundamental change in the production process of GL products. The melting point of Zn is 419.5°C, and it is sufficient to control the hot-dip galvanizing temperature to 455～475°C. It can be seen from the figure that the liquidus temperature of the GL product alloy in equilibrium is about 590 °C, so the temperature of the plating bath must be controlled above 600 °C, and the units of Baosteel and Pangang are controlled at 620 ~ 650 °C.
A1-Zn alloy equilibrium state pattern leaking defects.
At such a high temperature, the zinc liquid without an oxidation protection layer in the furnace nose is very easy to evaporate. The zinc vapor directly adheres to the steel plate or to the parts in the furnace nose. After forming zinc powder, it will fall on the steel plate, which will Affect the direct contact between the steel plate and the plating bath solution, thereby forming pinholes. On the other hand, the Al content in the plating bath melt is high, such as poor reduction of the surface of the steel strip, there are certain oxidation components or oxygen content in the protective gas Higher steam content will cause oxidation of the sack, and the density of Al oxide is similar to the density of the plating bath melt, which will disperse inside the plating bath in the furnace nose. If it sticks to the steel strip, it will also weaken the adhesion of the coating Sex.