The production process of hot-dip galvanized sheet mainly includes: plate preparation, plating pretreatment, hot-dip galvanizing, plating treatment, finished product inspection, etc.. According to the habit, the hot-dip galvanizing process is divided into two categories, that is, out of line annealing and in line annealing, according to different treatment methods before plating.
Out of line annealing
Before the hot rolled or cold rolled steel sheet enters the hot galvanizing line, recrystallization annealing is first carried out in a bottom annealing furnace or a bell type annealing furnace, so that there is no annealing process for the galvanizing line. A clean pure iron active surface without oxides and other impurities must be maintained prior to hot-dip galvanizing. This method is the first method to remove surface oxide by pickling iron annealing, then coated by a layer of zinc chloride or a mixture of ammonium chloride and zinc chloride solvent protection, so as to prevent the steel plate by oxidation. Including the hot dip galvanizing method belongs to the category of wet method, hot galvanized sheet steel and Huilinrefa.
Wet process
The solvent on the surface of the steel plate does not dry (i.e., the surface is still wet) and enters the surface of the zinc bath which is covered with a molten solvent. The disadvantage of this method is that it can only be galvanized in lead free condition, and the alloy layer of coating is very thick and bad adhesion.
The zinc accumulated not deposited to the bottom of the pot at the interface of liquid zinc and lead liquid (because the zinc slag is greater than the proportion of liquid zinc is less than the lead liquid), so that the steel plate through the zinc layer surface pollution. Therefore, the method has been basically eliminated.
Sheet steel sheet
This method is generally used hot rolling laminated plate as raw material, first of all, the annealed steel plate into the pickling workshop, with sulfuric acid or hydrochloric acid to remove the surface of the steel sheet oxygen hot galvanized iron sheet. After pickling, the steel plate immediately enters the tank and is soaked for galvanizing, thus preventing the steel plate from re oxidation. After pickling, cleaning, drying, drying, into the pot (the temperature has remained at 445 - 465 degrees Celsius) hot galvanizing, and then oiled and chromium treatment. The hot-dip galvanized sheet produced by this method has significantly improved the quality of wet galvanized products, and is only of certain value for small-scale production.
Huilinrefa
The continuous galvanizing line includes a series of pretreatment processes, such as alkaline solution degreasing, hydrochloric acid pickling, water washing, solvent coating and drying, etc., and the original plate enters the galvanizing line and needs to be covered by bell type annealing before galvanizing. The production process is complex, the production cost is high, and more importantly, the products produced by this method often have solvent defects, which affect the corrosion resistance of the coating. And the zinc pot AL often the solvent and the steel surface function to generate aluminum chloride consume, deterioration of the adhesion of the coating. Although this method has been published for nearly thirty years, it has not been developed in the world hot galvanizing industry.
In line annealing
The cold rolling or hot rolling mill is directly supplied with the coil as the original plate of hot galvanizing, and the gas protection, recrystallization and annealing are carried out in the hot galvanizing line. Including the hot galvanizing method belongs to the class of the industry: the Sendzimir method, modified Sendzimer method, UEC (Japan Kawasaki Nori); Silas method; Sharon method.
sendzimir
It combines the annealing process with hot galvanizing process, and the in line annealing mainly consists of two parts: the oxidation furnace and the reduction furnace. In the oxidation furnace, the gas flame is heated directly to about 450 degrees. The remaining rolling oil on the strip is burned and the surface is purified. The strip is then heated to 700-800 degrees to complete recrystallization annealing, controlled by the cooling section into the zinc pot temperature at about 480 degrees, and finally in the case does not contact the air into the galvanized zinc pot, therefore, high yield, good quality galvanized Sendzimer method, this method has been widely applied.
United Steel method
It is a variety of Sen Jimmy Apha, it is only using an alkaline electrolytic degreasing tank instead of oxidation furnace degreasing, and the rest of the process is basically the same as Sen Jimmy Apha. After the original plate enters the operation line, the electrolytic degreasing is carried out first, then the water is washed and dried, and then the recrystallization annealing is carried out through a reducing furnace with protective gas. Finally, the zinc is dipped into the zinc pot to be hot-dip galvanized under the seal condition. Because the strip is not heated by the oxidation furnace, the surface oxide film is thinner, and the hydrogen content of the protective gas in the reducing furnace can be appropriately reduced. In this way, it is beneficial to furnace safety and production cost reduction. However, as the strip cannot be pre heated, it enters the reduction furnace, which undoubtedly improves the heat load of the reduction furnace and affects the service life of the furnace. Therefore, this method has not been widely applied.
Silas method
Also known as direct flame heating method; first strip by alkali cleaning, then use hydrochloric acid to remove surface oxide, and after washing and drying before entering the vertical lines directly heated by the gas flame annealing furnace, through strict control of the furnace gas and air proportion of the flame burning, the flame was incomplete in gas surplus and under the condition of insufficient oxygen burning, thereby reducing atmosphere in the furnace caused by. It can be heated rapidly to achieve recrystallization temperature and cool the strip steel under low hydrogen protection atmosphere. Finally, the zinc liquid is dipped into the zinc bath under closed condition to do hot galvanizing. The method has the advantages of compact equipment, low investment cost and high output (up to 50/ hours). But the complexity of the production process, especially when the unit stops running, in order to avoid burning off the strip, the stove sliding away from the strip method, so a lot of operational problems, so the galvanizing industry using this method rarely.
Sharon law
In 1939, the American Sharon company put into operation a new type of hot-dip galvanizing line, so it was also called Sharon method. The method is in the annealing furnace