What are the general methods of online annealing for zinc plating?

In-line annealing of zinc plating for hardware surface treatment is that steel coils are directly provided by cold-rolling or hot-rolling workshops as the original hot-dip galvanizing plates, and gas-shielded recrystallization annealing is carried out in the hot-dip galvanizing line. Wenzhou Electroplating Company said that the hot dip galvanizing methods in this kind of industry are: Sendzimir method, improved Sendzimir method, combined steel method (the same as Kawasaki method in Japan); Silas law; Sharon method. The combined process of Meigang is a variant of the Sendzimir process, except that an alkaline electrolytic degreasing tank is used to replace the degreasing effect of the oxidation furnace, and other processes are basically the same as the Sendzimir process. After the original plate enters the production line, it is degreased by electrolysis, then washed and dried, then recrystallized and annealed in a reducing furnace with protective gas, and finally enters a zinc pot for hot galvanizing in a sealed state. In this method, because the strip steel is not heated by the oxidation furnace, the surface oxide film is thin, and the hydrogen content of the protective gas in the reduction furnace can be appropriately reduced. This is beneficial to the safety of the furnace and reduces the production cost. However, because the strip steel enters the reduction furnace without preheating, it undoubtedly increases the heat load of the reduction furnace and affects the furnace life. So this method has not been widely used. Silas method is also called flame direct heating method; Firstly, the strip steel is degreased by alkali washing, and then the oxide scale on the surface is removed by hydrochloric acid. After cleaning and drying, the strip steel enters the vertical inline annealing furnace directly heated by gas flame. By strictly controlling the flame combustion ratio of gas and air in the furnace, the combustion is incomplete under the condition of too much gas and insufficient oxygen, thus creating a reducing atmosphere in the furnace. Rapidly heating to recrystallization temperature, cooling the strip steel in a low-hydrogen protective atmosphere, and finally immersing the strip steel in zinc liquid for hot galvanizing under a closed condition. This method has the advantages of compact equipment, low investment cost and high output (up to 50 pieces/hour). However, the production process is complicated, especially when the device stops running. In order to avoid burning the steel strip, the furnace needs to move laterally away from the steel strip, which has many operational problems. Therefore, this method is rarely used in hot dip galvanizing industry.