Hot dip galvanizing production process
Hot-dip galvanizing (galvanizing/HDG) is also called hot-dip galvanizing and hot-dip galvanizing: it is an effective method of metal anti-corrosion and is mainly used for metal structural facilities in various industries. It is to immerse the rust-removed steel parts into molten zinc liquid at about 500°C, so that the zinc layer is attached to the surface of the steel components, thereby achieving the purpose of anti-corrosion. Hot-dip galvanizing process flow: finished product pickling - water washing - adding plating solution - drying - rack plating - cooling - chemical treatment - cleaning - polishing - hot-dip galvanizing is completed. Hot-dip galvanizing was developed from the older hot-dip method. It has a history of 140 years since France applied hot-dip galvanizing to industry in 1836. However, the hot-dip galvanizing industry has developed on a large scale in the past thirty years with the rapid development of cold-rolled strip steel.
Because the resulting coating is thicker, hot-dip galvanizing has better protective properties than electro-galvanizing, so it is an important protective coating for steel parts used in harsh working environments. Hot-dip galvanized products are widely used in chemical equipment, petroleum processing, ocean exploration, metal structures, power transmission, shipbuilding and other industries. They are also used in agricultural fields such as pesticide sprinkler irrigation, greenhouses, and construction industries such as water and gas transmission, wire casings, scaffolding, and bridges. , highway guardrails, etc., they have been widely used in recent years.
1. Production process
The production process of hot-dip galvanized sheets mainly includes: original sheet preparation → pre-plating treatment → hot-dip plating → post-plating treatment → finished product inspection, etc. According to custom, the hot-dip galvanizing process is divided into two categories: off-line annealing and in-line annealing based on different pre-plating treatment methods, namely:
Wet method (single steel plate hot-dip galvanizing method)
Off-line annealing single steel plate hot dip galvanizing method
Hot-dip galvanizing Wheeling method (continuous hot-dip galvanizing method for strip steel)
In-line annealing Sendzimir method (protective gas method) modified Sendzimir method
U.S. Steel Federation method (same as Japan's Kawasaki method) Selas method
Sharon Law
1) Off-line annealing: Before the hot-rolled or cold-rolled steel plate enters the hot-dip galvanizing line, it is first recrystallized and annealed in a bottom-type annealing furnace or a bell-type annealing furnace. In this way, there is no annealing process in the galvanizing line. . Before hot-dip galvanizing, the steel plate must maintain a clean, pure iron active surface free of oxides and other dirt. This method is to first remove the annealed surface iron scale by pickling, and then apply a layer of zinc chloride or a solvent composed of a mixture of ammonium chloride and zinc chloride for protection, thereby preventing the steel plate from being oxidized again. .
(1) Wet hot-dip galvanizing: The solvent on the surface of the steel plate enters the zinc liquid covered with molten solvent on the surface without drying (that is, the surface is still wet) for hot-dip galvanizing. 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.
(2) Single steel plate: This method generally uses hot-rolled stacked 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.
(3) Huilin Fare: The continuous galvanizing production line includes a series of pre-treatment processes such as alkali degreasing, hydrochloric acid pickling, water washing, solvent coating, drying, etc., and the original plate needs to be covered before entering the galvanizing line for galvanizing. furnace annealing. 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.
2) In-line annealing: The cold rolling or hot rolling workshop directly provides the coil as the original plate of hot-dip galvanizing, and performs gas protection 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.
