1, out of line annealing
Out-of-line annealing is the recrystallization annealing of hot-rolled or cold-rolled steel before entering the hot-dip galvanizing line. Because the steel must remain clean and pure iron active surface before hot-dip galvanizing. This method is first removed from the annealed surface of the oxide iron, and then coated with a layer of zinc chloride, or a mixture of ammonium chloride and zinc chloride, to protect the steel, so as to prevent the steel from being oxidized.
Wet hot dip galvanizing
The solvent on the surface of the steel enters into the zinc bath covered with molten solvent without drying. The shortcoming of this method is:
A. can only be galvanized in lead-free condition. The alloy coating is thick and has poor adhesion.
B. The resulting zinc slag is deposited at the interface between zinc and lead and can not be deposited on the bottom of the pot. Thus, the surface of the steel is polluted by passing through the zinc layer.
Galvanized steel pipe
Therefore, wet hot galvanizing has been basically eliminated.
Single branch steel
In this method, the hot rolled strip is used as the raw material. First, the annealed steel is sent to the pickling workshop, and the steel surface of the steel surface is removed by sulfuric acid or hydrochloric acid. After pickling, the steel will immediately enter the water tank and soak and wait for zinc plating to prevent the steel from reoxidation. After pickling, water cleaning, extrusion drying, drying, zinc hot-dip galvanization, then oiling and chromium treatment. The quality of hot-dip galvanized sheet produced by this method is significantly higher than that of wet galvanized product, and it has only a certain value for small-scale production.
2, intra line annealing
In-line annealing is that strip coils are directly provided by cold rolling or hot rolling mills as hot-dip galvanized boards, and gas-shielded recrystallization annealing is carried out in the hot-dip galvanizing line. The hot-dip galvanizing methods that belong to this category include: the son gem method, the improved zameli method, the United States Steel Union (and the Japanese Kawasaki Nori), the Silas law, and the Sharon method.
(1) Sendzimir process combines annealing process with hot-dip galvanizing process. In-line annealing mainly consists of oxidation furnace and reduction furnace. The strip is heated directly to 450 degrees by gas flame in the oxidation furnace, which burns the remaining rolling oil on the strip surface and purifies the surface. Then the strip is heated to 700-800 degrees to finish the recrystallization annealing, and the temperature is about 480 degrees before the cooling section is controlled by the cooling section. Finally, the zinc pan is galvanized in the case of no contact with the air. Therefore, the high yield and the better quality of the galvanizing are obtained. This method has been widely used.
(2) United States steel combined method
It is a variant of Sen Jimmy Apha, which only uses an alkaline electrolytic degreasing tank to replace the degreasing effect of the oxidizer, and the rest of the process is basically the same as Sen Jimmy Apha. After the original plate entered the operating line, the first electrolytic degreasing was first carried out, then washed and dried, then the recrystallized annealing was carried out by the reducing furnace with protective gas. Finally, the zinc pot was hot-dip galvanized under the sealing condition. Because the strip is not heated in an oxidizing furnace, the oxide film on the surface of the strip is thin and the hydrogen content of the protective gas in the reducing furnace can be reduced appropriately. In this way, the furnace is safe and the production cost is reduced. However, because the strip can not be preheated into the reduction furnace, this undoubtedly increases the heat load of the reduction furnace, affecting the life of the furnace. Therefore, this method has not been widely used.
(3) Silas law
It is also called direct flame heating method; first, the strip is degreased by alkali washing and then cleaned by hydrochloric acid and then washed and dried into a vertical line annealing furnace which is directly heated by the gas flame. By strictly controlling the burning ratio of gas and air in the furnace, it is made in the case of excess gas and oxygen shortage. The furnace is not completely flame burning, resulting in a reduction atmosphere in the furnace. The strip is heated rapidly to recrystallization temperature and cooled in low hydrogen protective atmosphere. Finally, the strip is dipped in zinc solution under sealed condition for hot galvanizing. The method is compact in equipment, low in investment cost and high in output (up to 50/ hours). But the production process is complex, especially when the unit stops running. In order to avoid burning the strip, it is necessary to use the furnace to move away from the steel strip. So there are many problems, so the hot galvanizing industry uses this method very little.
(4) Sharon method
In 1939, Sharon company of the United States put into operation a new type of galvanizing line, so it was also called the Sharon method. This method is also called gas pickling method because it sprays hydrogen chloride gas into the strip in the annealing furnace and makes the strip reach the recrystallization temperature. The use of hydrogen chloride gas pickling can not only remove the oxide skin on the strip surface, but also remove the oil on the strip surface at the same time. Because the surface of the strip is corroded by oxidizing gas, the surface of the strip is formed, so the adhesion of the coating obtained by the Sharon method is especially good. However, due to the serious corrosion of equipment, the cost of equipment maintenance and renovation is very high. Therefore, this method is rarely used.
(5) amelioration of Jimmy
It is a better process for hot-dip galvanizing; it connects the independent oxidizing furnace and reducing furnace in the sun's method to a small cross section, which consists of an entire annealing furnace, including a preheating furnace, a reducing furnace and a cooling section. Practice has proved that this method has many advantages: high quality, high yield, low consumption and safety have been gradually recognized by people. The speed of its development is not fast. This method has been adopted almost all of the newly built operation lines since 1965. In recent years, most of the old zir machine units have been reformed according to this method.