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HaiGang Yunyi
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Tianjin Haigang Steel Coil
Service first, quality assured. More dedicated and focused on crafting even better boards.
05
2021
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02
Classification and Performance Characteristics of Galvanized Strip Steel
Galvanizing processes, classified according to their methods, include the following:
Cold galvanizing
Cold galvanizing is also called Electroplated zinc , is to utilize Electrolysis equipment The fittings are first degreased and pickled, then immersed in a solution containing zinc salts. The negative electrode of an electrolytic device is connected to the fittings, while a zinc plate is placed on the opposite side of the fittings and connected to the positive electrode of the electrolytic device. When power is applied, the directed flow of current from the positive to the negative electrode causes a layer of zinc to deposit onto the fittings. Cold-galvanized fittings are processed first and then galvanized afterward.
Mechanical galvanizing
In a rotating drum equipped with plated parts, glass beads, zinc powder, water, and an accelerator, the glass beads—serving as impact media—rotate along with the drum, generating mechanical and physical energy through friction and hammering against the surface of the plated parts. Under the action of the chemical accelerator, the deposited zinc powder is... Cold welding It deposits onto the surface of the plated part, forming a smooth, uniform, and fine plating layer with a certain thickness.
Hot-dip galvanizing
The primary method used for galvanizing the surface of steel strips is hot-dip galvanizing.
The production process of hot-dip galvanized steel sheet mainly includes: preparation of the base metal → pre-galvanizing treatment → Hot-dip galvanizing →Post-plating treatment → Finished-product inspection, etc. By convention, based on the differences in pre-plating treatments, the hot-dip galvanizing process is generally categorized into two major types: off-line annealing and on-line annealing. These include the wet-process (single-sheet steel hot-dip galvanizing method), off-line annealing (single-sheet steel hot-dip galvanizing method), Wheeling hot-dip galvanizing method (continuous strip steel hot-dip galvanizing method), on-line annealing Sendzimir method (protected-gas method), modified Sendzimir method, U.S.-Japanese method (similar to the Japanese Kawasaki method), Selas method, and Sharon method.
Off-line annealing
It's hot-rolled or Cold-rolled strip steel Before entering the hot-dip galvanizing line, first pass through a bottom-draw annealing furnace or... Enclosure Annealing Furnace in the middle Recrystallization annealing Thus, the galvanizing line no longer requires an annealing process. Before hot-dip galvanizing, the steel sheet must maintain a clean, active surface of pure iron free from oxides and other contaminants. This method first uses acid pickling to remove any oxide layers formed during the annealing process. Iron oxide scale Clean the surface, then apply a protective layer consisting of a solvent made from zinc chloride or a mixture of ammonium chloride and zinc chloride to prevent the steel plate from oxidizing again.
(1) Wet-process hot-dip galvanizing
The solvent on the steel plate surface enters the zinc bath—where the surface is already covered with molten solvent—without being dried first (i.e., the surface remains wet). The drawback of this method is:
a. Galvanizing can only be performed in a lead-free environment; the alloy layer of the coating is very thick and has poor adhesion.
b. The generated zinc dross accumulates at the interface between the zinc and lead melts and cannot settle at the bottom of the pot (because the specific gravity of the zinc dross is greater than that of the zinc melt but less than that of the lead melt). As a result, the steel plates become contaminated on their surfaces as they pass through the zinc layer. Consequently, this method has largely been phased out.
(2) Single steel plate
This method typically uses hot-rolled clad plates as the raw material. First, the annealed steel plates are sent to the pickling workshop, where sulfuric acid or hydrochloric acid is used to remove surface impurities from the steel plates. Iron oxide scale After pickling, the steel plates are immediately immersed in a water tank to wait for galvanizing. This step helps prevent the steel plates from re-oxidizing. Subsequently, the plates undergo pickling, water rinsing, squeezing to remove excess moisture, and drying before being fed into the zinc bath (maintained at a constant temperature of 445–465°C) for hot-dip galvanizing. They then proceed to oiling and chromate treatment. The hot-dip galvanized sheets produced by this method exhibit significantly improved quality compared to those produced by the wet-process galvanizing method; however, this method is only economically viable for small-scale production.
(3) Huilin Fa Re
This continuous galvanizing production line includes a series of pretreatment processes such as alkali degreasing, hydrochloric acid pickling, water rinsing, solvent coating, and drying. Moreover, before the raw steel sheet enters the galvanizing line for galvanizing, it must first undergo annealing in a bell-type furnace. This annealing process is characterized by a complex production technology and high production costs. More importantly, products manufactured using this method often exhibit solvent-related defects, which adversely affect the quality of the coating. Corrosion resistance Moreover, AL in the zinc bath often reacts with the solvent on the surface of the steel plate, forming... Aluminum trichloride As a result, the adhesion of the coating deteriorates. Consequently, although this method has been around for nearly thirty years, it has not gained widespread adoption in the global hot-dip galvanizing industry.
In-line annealing
The coil stock is directly supplied by the cold-rolling or hot-rolling mill as the raw material for hot-dip galvanizing, and gas shielding is carried out within the hot-dip galvanizing production line. Recrystallization annealing The hot-dip galvanizing methods belonging to this category include: the Sendzimir process, the modified Sendzimir process, the U.S. Steel process (also known as the Kawasaki process in Japan), the Silas process, and the Sharon process.
(1) The Sengimil Method
It combines the annealing process with the hot-dip galvanizing process. The in-line annealing process mainly consists of two sections: an oxidation furnace and a reduction furnace. In the oxidation furnace, the strip steel is directly heated by a gas flame to around 450 degrees Celsius, burning off any residual rolling oil remaining on the surface and thereby cleaning and purifying the surface. The strip steel is then further heated to 700–800 degrees Celsius to complete the process. Recrystallization annealing After being cooled, the strip is brought to a temperature of around 480 degrees Celsius before entering the zinc bath. Finally, the strip is immersed in the zinc bath for galvanizing without coming into contact with air. As a result, the Sendzimir process boasts high production efficiency and excellent galvanizing quality, and it was once widely adopted.
(2) U.S. Steel Act
It is a variant of the Sengimil process, in which an alkaline electrochemical degreasing tank replaces the degreasing function of the oxidation furnace, while all other process steps remain essentially the same as those in the Sengimil method. After the raw strip enters the processing line, it first undergoes electrochemical degreasing, followed by water washing and drying. Then, it is subjected to recrystallization annealing in a reduction furnace equipped with a protective gas atmosphere. Finally, under sealed conditions, it is hot-dip galvanized in a zinc bath. Since the steel strip does not pass through the oxidation furnace for heating in this method, the surface oxide film is relatively thin, allowing the hydrogen content in the protective gas used in the reduction furnace to be appropriately reduced. This, in turn, is beneficial both for furnace safety and for lowering production costs. However, because the steel strip enters the reduction furnace without prior preheating, the thermal load on the furnace is undoubtedly increased, thereby shortening the furnace’s service life. Consequently, this method has not gained widespread adoption.
(3) Silas Method
Also known as the flame-direct heating method: First, the strip steel undergoes alkali washing to remove oil and grease. It is then treated with hydrochloric acid to remove surface scale, followed by water washing and drying before being fed into a vertical continuous annealing furnace heated directly by a gas flame. By precisely controlling the combustion ratio of gas to air within the furnace—maintaining a condition of excess gas and insufficient oxygen—the furnace operates under incomplete combustion, creating a reducing atmosphere inside. This allows for rapid heating to achieve... Recrystallization temperature The strip steel is then cooled under a low-hydrogen protective atmosphere and finally immersed in a molten zinc bath in a sealed environment for hot-dip galvanizing. This method features compact equipment, low investment costs, and high production capacity (up to 50 tons per hour at maximum). However, the production process is complex, especially when the production line is shut down. To prevent the strip steel from burning through, it is necessary to move the furnace horizontally away from the line. Steel belt The method has many operational issues, so the hot-dip galvanizing industry rarely adopts this approach.
(4) Sharon Law
In 1939, the American Sharon Company put into operation a new type of hot-dip galvanizing unit, which is therefore also known as the Sharon process. In this process, hydrogen chloride gas is sprayed onto the steel strip inside an annealing furnace, bringing the steel strip to a certain temperature. Recrystallization temperature Therefore, it is also known as the gas pickling method. Using hydrogen chloride gas for pickling not only removes the oxide scale from the surface of the strip steel but also simultaneously eliminates any oil and grease on its surface. This is because the surface of the strip steel has been oxidized. Gas corrosion This results in a rough surface, which is why coatings obtained using the Sharon process exhibit exceptionally good adhesion. However, due to severe equipment corrosion, this method incurs high costs for equipment maintenance and replacement. Consequently, this approach is rarely employed.
(5) Improved Senjimi
It is a superior hot-dip galvanizing process. In this method, the oxidation furnace and reduction furnace—previously separate units in the Sendzimir process—are connected by a narrow-gauge passageway, thereby integrating the entire annealing furnace—including the preheating furnace, reduction furnace, and cooling section—into a single, cohesive unit. Practice has demonstrated that this method offers numerous advantages: high quality, high productivity, low energy consumption, and enhanced safety—all of which have gradually gained widespread recognition. The method has been developing at an exceptionally rapid pace; since 1965, nearly all newly built production lines have adopted this approach, and in recent years, most of the older Sendzimir units have also been retrofitted according to this method.
Performance Features
A relatively thick and dense layer of pure zinc coats the surface of steel fasteners, effectively preventing any contact between the steel substrate and corrosive solutions, thereby protecting the steel fastener substrate from corrosion. In typical atmospheric conditions, a very thin yet dense layer of zinc oxide forms on the surface of the zinc coating. This layer is hardly soluble in water, thus providing a certain degree of protection to the steel fastener substrate. If the zinc oxide reacts with other atmospheric components to form insoluble zinc salts, the anti-corrosion effect becomes even more pronounced. The presence of a zinc-iron alloy layer, which is tightly bonded and highly compact, imparts exceptional corrosion resistance in marine salt-spray atmospheres as well as industrial atmospheres. Due to the strong bond and mutual solubility between zinc and iron, this alloy exhibits remarkable durability and strength. Wear resistance Because zinc has excellent ductility, its alloy layer adheres firmly to the steel substrate. Therefore, hot-dip galvanizing allows for various forming processes—such as cold stamping, rolling, wire drawing, and bending—without damaging the coating. Steel structure After hot-dip galvanizing, it is equivalent to a single annealing treatment, which can effectively improve the steel matrix. Mechanical properties It eliminates stresses generated during the forming and welding of steel components, thereby facilitating turning operations on structural steel parts. Fasteners coated with hot-dip galvanizing have a bright and aesthetically pleasing surface. The pure zinc layer is the most ductile among all hot-dip galvanized coatings; its properties are essentially similar to those of pure zinc and it exhibits excellent ductility.
Scope of application
This plating method is particularly suitable for various strong acids, alkaline fumes, and other harsh environments. Corrosive environment Middle.
Hot-dip galvanizing, galvanizing, strip steel, galvanized strip steel
We primarily produce various products including galvanized strip steel, pickled strip steel, and cold-rolled strip steel. These products are widely used in industries such as construction, mechanical manufacturing, automotive, and household appliances.
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