HDG is a relatively old hot-dip method evolved, since 1836, France applied to the galvanizing industry, we have 170 years of history. However, the hot-dip galvanizing industry is accompanied by the rapid development of the last three decades of cold-rolled strip and get a large-scale development.Hot galvanized sheet production processes include: original plate ready → → → hot-dip plating process → finished product inspection post before plating process. By convention often based on pre-plating processing methodThe process is divided into different China Color Coated Steel sheets outside the inner wire annealing and annealing two categories, namely wet (single galvanized steel sheet method), the line outside annealing (leaflets galvanized steel sheet method), hot galvanized Wheeling (Wheeling) method (continuous hot-dip galvanizing method), in-line annealing Sendzimir (Sendzimir) Act (Protection Gas Act), modified Sendzimir US Steel Union Act (Act with Kawasaki, Japan), Silas (Selas ) Act and Sharon (Sharon) method.
1. Line outside the annealing: Before that hot-rolled or cold-rolled steel into the hot-dip galvanizing line operation, first pumping bottom type annealing furnace or annealing furnace in the recrystallization annealing, so that does not exist galvanized wire annealing step up. Before hot-dip galvanized steel sheets must remain a non-oxide and other dirt present in the active surface of pure iron clean. This method is for the pickling method to annealed surface oxide skin clear, and then coated with a layer protected by a zinc chloride or solvent consisting of a mixture of ammonium chloride and zinc chloride, thereby preventing oxidation of the steel sheet again .(1) Wet Hot dip galvanizing: hot-dip galvanizing proceeds from the surface is covered with molten zinc solvent liquid solvent steel surface without drying (ie, the surface is still wet). The disadvantage of this approach are:a. only in Free Status galvanized coating alloy layer is very thick and very bad adhesion.b. generated dross are accumulated at the interface between the liquid zinc and lead deposits pot liquid and can not (because the proportion of liquid zinc dross is greater than and less than lead solution), so steel through the zinc layer due to surface contamination. Therefore, this method has basically been eliminated.(2) a single plate: This method is generally used as a raw material of hot-rolled plate Roll, first annealed steel sheets into the pickling plant, sulfuric acid or hydrochloric acid scavenging oxygen hot galvanized steel surface of the iron law. Steel pickling immediately after immersion into the tank waiting for zinc, which can prevent steel re-oxidation. After pickling, water cleaning, squeezed, dried, into the zinc pot (temperature kept at 445-465 ℃) hot dip galvanized and then coated with oil and chromium treatment. This method of production of hot dip galvanized sheet has improved significantly over the wet galvanized product quality, only a certain value to small-scale production.(3) Wheeling law Heat: The continuous galvanizing line, including alkali degreasing, hydrochloric acid, water washing, solvent coating, drying and a series of pre-treatment process, and the original panel in the top galvanized wire galvanizing also need to cover annealing furnace. This method of production of complex, high production costs, more important is this method often with solvent products defects affect the corrosion resistance of the coating. And zinc pot AL and solvents often play a role in generating the steel surface aluminum trichloride and consume, coating adhesion deteriorates. Although this method has been available thus nearly three decades, but in the world of hot-dip galvanizing industry has not been developed.2. In-line annealing: is provided by cold rolling or hot rolling plant directly with the original volume as hot galvanized plate, the gas protection in the hot-dip galvanizing line recrystallization annealing operation. Hot-dip galvanizing industry belonging to this class include: Sendzimir, modified Sendzimir US Steel Union Act (Act with Kawasaki, Japan); Silas law; Sharon method.Sendzimir law: It is the annealing process and hot dip galvanizing process together, including its line annealing furnace oxidation, reduction furnace composed of two parts. Strip in the oxidation furnace gas flame directly heated to about 450 degrees, the strip surface residual rolling oil burned, clean surface. Then the strip is heated to 700-800 degrees complete recrystallization annealing, case control passes through the cooling section before zinc pot temperature of about 480 degrees, and finally without contacting air into the zinc pot galvanizing, therefore, yield Sendzimir method high, galvanized better quality, this method has been widely used.
US Steel Union law: it is a variant of Sendzimir law, it is only the use of an alkaline electrolytic degreasing tank replaces the skimmed oxidation furnace, basically the same as the rest of the process and Sendzimir. After the original plate into the operating line, the first electrolytic degreasing, then washed, dried, and then by a protective gas reduction furnace recrystallization annealing, and finally into the hot dip galvanized zinc pot in a sealed case. This method is not oxidized because of the strip heating furnace, so the surface of the thin oxide film, the reduction furnace can be appropriately reduced hydrogen content in the protective gas. Thus, for furnace favorable safety and reduce production costs. However, due to the lack of pre-heating the strip to enter the reduction furnace, this will undoubtedly increase the heat load reduction furnace, affect the life of the furnace. Therefore, this method does not have a wide range of applications.Silas method: also known as the direct flame heating method; first strip by caustic degreasing, then with hydrochloric acid to remove surface oxide scale, and by washing, drying and then into the directly heated by the gas flame vertical line annealing furnace, By strictly controlling the flame burning furnace gas and air ratio, so that in the case of the gas surplus and shortage of oxygen flame burn incompletely, thereby resulting in a reducing atmosphere in the furnace. It reached the recrystallization temperature rapid heating and cooling the strip at low hydrogen atmosphere, and finally immersed in liquid zinc in a closed case, hot-dip galvanizing. The Act apparatus compact, low investment costs, high yield (up to 50 / hour). But the production process is complicated, especially when the unit is stopped, in order to avoid blowing the strip, we need to leave the stove traverse the strip method, this operation a lot of problems, so the hot-dip galvanizing industry rarely use this method.Sharon Laws: 1939 American company Sharon commissioned a new hot dip galvanizing line, also called Sharon method. The law is within the strip annealing furnace injection of hydrogen chloride gas and strip reach the recrystallization temperature, also known as gas pickling. Using hydrogen chloride gas pickled, not only to descaling the strip surface, and at the same time removes grease strip surface, since the strip surface oxidized gas corrosion, forming pockmarked face, so Sharon method using the resulting coating adhesion particularly good. However, due to serious corrosion of equipment, and the resulting high cost of equipment maintenance and updates. Thus this method is rarely used.Improved Sen Jimi: It is a superior method for hot dip galvanizing process; it Sendzimir method to separate the oxidation furnace and the reduction furnace consists of a cross-sectional area smaller aisle connected, so including preheating furnace, reduction furnace and the cooling section, including the entire annealing furnace to form an organic whole. Practice has proved that the method has many advantages: high quality, high yield, low cost, safe, etc. has gradually been recognized. Its development is very fast, since 1965, almost all the new jobs line adopted this approach in recent years, the old Sendzimir unit also mostly been transformed in accordance with this method.Equipped with a plating process principle, the rotary drum glass ball, zinc powder, water and accelerator, as the impact of media glass ball with the drum rotates, the plated surface friction and hammered produce mechanical and physical energy, in the role of chemical accelerators, zinc plated coated "cold welding" to plating on the surface to form a smooth, uniform and fine coating having a certain thickness.ASTM B695-2000 technical standards used by the US military C-81562 and mechanical galvanizing technology standards.
Source : articlesbase.com
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