Wet hot dip galvanizing
The solvent on the surface of the steel plate enters the zinc liquid covered with the molten solvent on the surface for hot-dip galvanizing without drying (that is, the surface is still wet). The disadvantages of this method are:
a. Galvanizing can only be done in a lead-free state. The alloy layer of the coating is very thick and has poor adhesion.
b. The generated zinc slag accumulates at the interface between the zinc liquid and the lead liquid and cannot be deposited on the bottom of the pot (because the specific gravity of the zinc slag is greater than that of the zinc liquid but less than that of the lead liquid). In this way, the steel plate passes through the zinc layer and pollutes the surface. Therefore, this method has been basically eliminated.
(1) Single steel plate
This method generally uses hot-rolled laminated plates as raw materials. First, the annealed steel plates are sent to the pickling workshop, and sulfuric acid or hydrochloric acid is used to remove the oxygen hot-dip galvanized iron scale on the surface of the steel plates. After pickling, the steel plate immediately enters the water tank to soak and wait for galvanizing, which can prevent the steel plate from being oxidized again. Afterwards, it undergoes pickling, water cleaning, squeezing, drying, hot-dip galvanizing in a zinc pot (the temperature is maintained at 445-465°C), and then oiling and chromating. The quality of hot-dip galvanized sheets produced by this method is significantly improved compared to wet galvanizing, and it is only of certain value for small-scale production.
(2) Huilin thermal method
The continuous galvanizing production line includes a series of pre-treatment processes such as alkali degreasing, hydrochloric acid pickling, water washing, solvent coating, and drying. The original plate also needs to be annealed in a bell furnace before entering the galvanizing line for galvanizing. The production process of this method is complex and the production cost is high. More importantly, the products produced by this method often contain solvent defects, which affects the corrosion resistance of the coating. Moreover, the AL in the zinc pot often interacts with the solvent on the surface of the steel plate to form aluminum trichloride and is consumed, and the adhesion of the coating deteriorates. Therefore, although this method has been around for nearly thirty years, it has not been developed in the world's hot-dip galvanizing industry.
(3) In-line annealing
That is, the cold rolling or hot rolling workshop directly provides the coil as the original plate of hot-dip galvanizing, and performs gas-protected recrystallization annealing in the hot-dip galvanizing line. Hot-dip galvanizing methods belonging to this industry include: Sendzimir method, modified Sendzimir method, US Steel Union method (same as Japan's Kawasaki method); Silas method; Sharon method.
(4) Sendzimir method
It combines the annealing process and the hot-dip galvanizing process. The in-line annealing mainly consists of an oxidation furnace and a reduction furnace. The strip steel is directly heated to about 450 degrees by the gas flame in the oxidation furnace, which burns off the remaining rolling oil on the surface of the strip and purifies the surface. Then the strip is heated to 700-800 degrees to complete the recrystallization annealing. The cooling section controls the temperature before entering the zinc pot to be around 480 degrees. Finally, it enters the zinc pot for galvanizing without contact with air. Therefore, the output of the Sendzimir method is High and good galvanizing quality, this method has been widely used.
(5) U.S. Steel Law
It is a variant of the Sendzimir method. It only uses an alkaline electrolytic degreasing tank to replace the degreasing function of the oxidation furnace. The remaining processes are basically the same as the Sendzimir method. After the original plate enters the operation line, it is first electrolytically degreased, then washed, dried, and then passed through a reduction furnace with protective gas for recrystallization and annealing, and finally enters the zinc pot for hot-dip galvanizing under a sealed condition. In this method, because the strip steel is not heated by the oxidation furnace, the oxide film on the surface is thinner, which can appropriately reduce the hydrogen content of the protective gas in the reduction furnace. This is beneficial to furnace safety and production cost reduction. However, since the strip steel enters the reduction furnace without being preheated, this undoubtedly increases the heat load of the reduction furnace and affects the life of the furnace. Therefore this method is not widely used.
(6) Silas Law
Also known as the direct flame heating method; first, the strip is degreased by alkali washing, and then the oxide scale on the surface is removed with hydrochloric acid. After being washed and dried, it enters the vertical in-line annealing furnace directly heated by the gas flame. Through strict control of the furnace The combustion ratio of gas and air enables incomplete combustion under the condition of excess gas and insufficient oxygen, thus creating a reducing atmosphere in the furnace. It is rapidly heated to the recrystallization temperature and the strip is cooled in a low hydrogen protective atmosphere. Finally, it is immersed in zinc liquid under closed conditions and hot-dip galvanized. This method has compact equipment, low investment costs, and high output (up to 50/hour). However, the production process is complex, especially when the unit is stopped. In order to avoid burning the strip, it is necessary to move the furnace laterally away from the steel strip. This has many operational problems. Therefore, this method is rarely used in the hot-dip galvanizing industry.
(7) Sharon Law
In 1939, the American Sharon Company put into production a new hot-dip galvanizing unit, so it was also called the Sharon method. This method is to inject hydrogen chloride gas into the strip in the annealing furnace and bring the strip to the recrystallization temperature, so it is also called the gas pickling method. The use of hydrogen chloride gas pickling can not only remove the oxide scale on the surface of the strip, but also remove the grease on the surface of the strip. Since the surface of the strip is corroded by the oxidizing gas and forms pitted surfaces, the coating obtained by using the Sharon method adheres Sex is particularly good. However, due to severe equipment corrosion, high equipment maintenance and update costs are incurred. Therefore this method is rarely used.
(8) Improved Sendjimi
It is a superior hot-dip galvanizing process; it connects the independent oxidation furnace and reduction furnace in the Sendzimir method by a corridor with a smaller cross-sectional area, including the preheating furnace, reduction furnace and cooling section. The entire annealing furnace inside constitutes an organic whole. Practice has proved that this method has many advantages: high quality, high yield, low consumption, safety and other advantages have gradually been recognized by people. Its development speed is very fast. Almost all newly built operating lines since 1965 have adopted this method. In recent years, most of the old Sendzimir units have also been transformed according to this method.