When strip steel is hot-plated, the air flow of the air knife is used to scrape off the excess liquid plating solution on the surface. In order to improve the quality of the coating, every effort is made when designing the air knife to maximize the uniformity of the air flow blown from the air knife. However, in fact, various uneven airflow phenomena inevitably exist. First, even if the air knife is very good and adjusted in place, the relatively uniform laminar flow is only within a small range from the knife lip, and cannot be uniform beyond a certain range; second, the air flow blown out by the air knife It is greatly affected by the angle and height of the air knife, the stability of gas pressure and the vibration of the strip, and it is difficult to adjust it in place. Thirdly, in addition to the gas blown out by the air knife, the air flow on the strip will also involve outside gas. , the coating is affected by the air flow within a certain area. Therefore, it is relative that the coating is stable and consistent under the action of air flow, and unstable is. Super air knife

So, what impact will uneven airflow have on the coating? Slight uneven air flow will cause the oxide film on the surface of the strip to form "cloud-shaped ripples" and "whisker ripples" defects. More serious uneven air flow will cause "zinc undulations" or even "small pits" and "small bright spots" defects on the surface of the strip. . Analyze separately below.

First, perform defect analysis of "cloud ripples" and "whisker ripples".

On the one hand, the air flow blown out by the air knife actually changes with time in a basically wavy shape. Coupled with the effect of the surrounding ambient gas being involved, this undulating shape is more obvious. Therefore, when the strip leaves the air knife area, The effect of the air flow can be said to be in groups. On the other hand, there is an oxide film on the surface of the liquid coating. This oxide film can flow relative to the coating. It will hang downward under the action of gravity, and may also flow upward under the action of air flow. It is the instability of the air flow and the fluidity of the oxide film that lead to uneven thickness of the oxide film on the entire board surface. When the air pressure is high, it becomes thinner, and when the air pressure is low, it becomes thicker and wrinkled. For galvanizing, it is mainly a dense oxide film of aluminum. Since the oxide film of aluminum is colorless, it has little impact on the appearance. For galvanized aluminum-magnesium, the composition of the oxide film is relatively complex. It is a mixture of aluminum and magnesium oxides. The composition and proportion are different. It has a certain color. It doesn’t matter where it is thin, but it is especially wrinkled when it is thick. Where the uneven refraction of light is obvious.

If the thickness of the strip is thin, the coating is thin, or the temperature of the plating bath is low, the oxygen content in the zinc pot and air knife area is low, the surface oxide film is thin and cools and solidifies quickly, and the uneven oxide film formed is lighter. It can be seen on the surface of the zinc-aluminum-magnesium plate. It doesn't have much impact or just a faint "cloud ripple".

On the contrary, when the oxide film on the surface of zinc, aluminum and magnesium is very thick, under the combined action of gravity and air flow, the uneven phenomenon is very serious, the wrinkle area is large, and the wrinkles are obvious, and "beard ripples" like foreigners' beards will appear. . Annular air knife

Regarding "whisker ripples", analysis can reveal the existence of dense fine linear protrusions on the surface, and the oxygen content at the protrusions is relatively high, which can be determined to be wrinkles in the oxide film. The problem is that it is often confused whether the oxide film is produced in the furnace nose or in the zinc pot air knife area. We believe that this may only occur in the zinc pot air knife area. The strongest reason is that after the oxide film in the furnace nose area sticks to the surface of the strip steel, it will cause severe folding, rather than such regular wrinkles. ; Moreover, extrusion damage will occur when passing through the sinking roller and stabilizing roller, and it is impossible to be so intact and continuous.