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Method of hot galvanizing

- Apr 09, 2018 -

Method of hot galvanizing.

Continuous zinc plating refers to the surface treatment technology that is coated with zinc on the surface of metal, alloy or other materials for the purpose of aesthetics, rust prevention and so on.Now the main method is hot continuous galvanization.

Hot continuous galvanization has been developed by the older method of hot plating. Since 1836, France has applied hot continuous galvanization to the industry, it has been a hundred and forty years.However, the hot continuous galvanizing industry has been developed rapidly in the past 30 years with the rapid development of cold-rolled strip steel.The production process of hot continuous galvanized sheet mainly includes: preparation of raw plate, pretreatment, hot dip, post-treatment, finished product inspection, etc.

According to the habits often according to different plating pretreatment methods of the hot continuous galvanizing process divided into two major categories of annealing in annealing and line outside the lines, namely, wet method (hot continuous galvanized sheet steel plate method), the line outside the annealing (hot continuous galvanized sheet steel plate method), continuous hot galvanizing wheeling method (wheeling) (continuous hot strip continuous galvanizing process), annealing, Jimmy's method (Sendzimir) within (method) of shielding gas, improvement, Jimmy's method, steel union method (with Japanese kawasaki method), Silas (Selas) method and Sharon (Sharon) method.

: 1. The outside line annealing is hot rolled or cold rolled steel sheet into the continuous hot galvanizing line before the first type annealing furnace in the bottom of a pump or bell type annealing furnace of recrystallization annealing, in this way, there is no annealing process in continuous galvanizing line.The steel plate must maintain a clean pure iron active surface with no oxides and other dirt before hot continuous galvanizing.This approach is to first by pickling method by annealing the surface oxidation of iron removal, then coated with a layer of zinc chloride or consisting of ammonium chloride and zinc chloride mixed solvent, thus prevent the oxidation of the steel plate to be.

(1) wet heat continuous galvanization: the solvent on the surface of the steel plate is not dried (i.e. the surface is still wet), and the molten zinc liquid is covered by the molten state and the zinc liquid is hot and continuous zinc plating.The disadvantages of this approach are:

A. It can only be galvanized continuously in lead-free state. The alloy layer of coating is very thick and the adhesion is very bad.

B. The generated zinc slag is deposited in the interface of zinc and lead liquid, which cannot deposit the bottom of the pot (because the proportion of zinc slag is greater than that of zinc), so that the steel plate is contaminated with the surface through the zinc layer.Therefore, this method has been basically eliminated.

(2) the single plate: this method is generally adopted hot pack rolling plate as raw material, the first sent after annealing of steel pickling workshop, with sulfuric acid or hydrochloric acid to remove oxygen on the surface of the steel plate hot continuous galvanizing sheet.After pickling, the steel plate immediately enters the water tank to soak and wait for continuous zinc plating, which prevents the steel plate from reoxidation.After the acid washing, water cleaning, drying, drying, into the zinc pot (the temperature has been kept at 445-465) hot continuous galvanization, then oil and chromium treatment.This method has a significant improvement in the quality of continuous galvanizing finished products with continuous zinc plating, which is only of certain value to small-scale production.

(3) wheeling hot: the continuous continuous galvanizing line including alkali degreasing, hydrochloric acid pickling solvent, water washing, coating, drying and so on a series of pretreatment process, and the original plate into continuous galvanizing line continuous galvanizing is needed before the bell type annealing furnace.The production process is complex, the production cost is high, and the main part is that the products produced by this method often have solvent defects, which affect the corrosion resistance of the coating.And the AL in the zinc pot often reacts with the surface of the steel plate to produce aluminium trichloride, and the adhesion of the coating becomes bad.Therefore, although this method has been published for nearly 30 years, it has not been developed in the world hot continuous galvanizing industry.

2. Annealing in line: it is the cold rolling or hot rolling mill that directly provides the original plate with coil as the hot continuous galvanizing, and the gas protection recrystallization annealing in the hot continuous galvanizing line.The hot continuous galvanizing methods of this industry include: senkimmel method, the modified senkimmel method, and the United States steel union method (with the Japanese kawasaki law);Silas method;Sharon method.

Senkimmel method: it is a combination of annealing process and hot continuous galvanizing process. The annealing in line is mainly composed of oxidation furnace and reduction furnace.The gas flame is heated directly to about 450 degrees in the furnace, and the residual rolling oil on the strip surface is burned and the surface is cleaned.Again after finish the strip heated to 700-700 degrees of recrystallization annealing, the cooling control before entering zinc pot temperature at 480 degrees, and finally the air without touching the case into the continuous galvanized zinc pot, as a result, Mr Jimmy, high output, continuous galvanizing quality is better, this method was widely used.

United States steel link: it is a variant of the senkimmel method, which only replaces the degreasing effect of the oxidation furnace with an alkaline electrolytic degreasing tank, and the rest of the process is basically the same as the senkimmel method.After the original plate into the line, first for electrolytic degreasing, washing, drying, and then again by a protective gas reduction furnace recrystallization annealing, the last in a sealed case into zinc pot hot continuous galvanizing.This method is not heated by oxidizing furnace, so the oxide film on the surface is thinner, which can reduce the hydrogen content of the protective gas in the reduction furnace.In this way, it is advantageous to the furnace safety and reduce the production cost.However, due to the lack of pre-heating of strip steel, it will enter the reduction furnace, which will undoubtedly improve the heat load of the reduction furnace and affect the life of the furnace.Therefore, this method is not widely used.

Silas method: also called flame direct heating;Strip steel by alkaline degreasing first, and then use hydrochloric acid to remove surface oxide skin, and after washing, drying and then enter directly by the gas flame heating stove is retreated inside the vertical line, through strict control in furnace gas and air flame burning rate, in excess gas and oxygen shortage under the situation of incomplete burning flame, which result in reducing atmosphere in the furnace.It can rapidly heat up to recrystallization temperature and cool the strip in the atmosphere of low hydrogen protection. Finally, it is immersed in the zinc liquid in the sealed condition, and the hot continuous galvanization is carried out.It has compact equipment, low investment costs and high production capacity (up to 50 per hour).But complex production process, especially when the unit stop running, in order to avoid burning out the strip, the need to adopt the method of transverse left the stove steel belt, so many operation problem, so, continuous hot galvanizing industry adopted very little.

Sharon law: in 1939, American Sharon started a new hot, continuous galvanizing unit, so it was also called Sharon law.This method is used to spray hydrogen chloride in the annealing furnace and bring the strip to the recrystallization temperature, so it is also known as the gas pickling method.Using hydrogen chloride gas pickling, not only can remove oxide coating on the surface of the steel strip, and at the same time to in addition to the strip surface of oil, gas corrosion, due to the strip surface oxidation to form pits, so use Sharon method of coating adhesion.However, due to the serious corrosion of the equipment, high maintenance and renewal costs are incurred.This method is rarely used.

Modified sen Jimmy: it is a superior hot continuous galvanizing process;It's Jimmy, independently of oxidation furnace and reduction furnace are linked by a smaller cross-sectional area of corridor, such, including preheating furnace, reducing furnace and cooling section of the annealing furnace constitute 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 is very fast, since 1965, the new construction line has adopted this method almost entirely. In recent years, the old senkimmel units have also been modified in this way.