How is a galvanized steel coil made?

Aug. 24, 2023

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Understanding Galvanized Steel Coil

Galvanized steel coil is a product of steel that has undergone a protective coating process to prevent corrosion. The coating primarily consists of zinc, which forms a barrier between the steel substrate and the corrosive environment. This process not only enhances the longevity of the steel but also imparts an attractive appearance to it.

Composition of Galvanized Steel Coil

The primary component of galvanized steel coil is, of course, steel. However, the defining characteristic lies in the zinc coating that encapsulates the steel substrate. This zinc layer serves as a sacrificial barrier, corroding in place of the steel when exposed to corrosive elements. Additionally, the thickness of the zinc coating can vary, influencing the level of corrosion protection provided.

Manufacturing Process

Galvanized steel coil, a versatile material cherished for its corrosion resistance and durability, undergoes a meticulous manufacturing process that involves several distinct stages. This article delves into the intricate steps that transform raw steel into galvanized steel coil, ready to serve diverse industries and applications.

 1. Surface Preparation:

The journey begins with the preparation of the steel substrate. Raw steel coils are subjected to a thorough cleaning process to remove any contaminants, such as rust, oil, or dirt, from the surface. This preparation ensures optimal adhesion of the zinc coating that will be applied in subsequent steps.

 2. Galvanizing Bath:

Once cleaned, the steel coils are immersed in a bath of molten zinc. This step, known as hot-dip galvanizing, is pivotal to the process. The zinc bath is maintained at a high temperature to ensure proper metallurgical bonding between the zinc and the steel. This metallurgical bond enhances the durability and longevity of the resulting galvanized steel.

 3. Alloying and Solidification:

As the steel coils are withdrawn from the galvanizing bath, excess molten zinc drips off, leaving behind a layer of zinc that adheres to the steel surface. During this phase, alloying may occur between the zinc and the steel, further enhancing the bond between the two materials. The zinc layer solidifies on the steel substrate, forming the protective coating.

 4. Cooling and Inspection:

The galvanized steel coils are then cooled, allowing the zinc coating to fully solidify and bond to the steel. After cooling, the coils undergo thorough inspection to ensure uniformity of the coating thickness and its adherence to the steel. This inspection is crucial to ensuring that the final product meets the required quality standards.

 5. Post-Treatment (Optional):

Depending on the intended application and desired characteristics, galvanized steel coils may undergo additional treatments. One common post-treatment is passivation, which involves applying a chemical solution to the zinc coating to enhance its corrosion resistance. Additionally, a topcoat may be applied for aesthetic purposes or to provide additional protection against specific environmental factors.

 The Role of Zinc in Galvanized Steel Coil:

The primary protective agent in galvanized steel coil is the zinc coating. Zinc's unique properties play a vital role in preserving the underlying steel:

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1. Sacrificial Protection: Zinc is more reactive than steel, making it a sacrificial anode. In the presence of corrosive agents, zinc corrodes before the steel, thus preventing the steel from being exposed to the corrosive elements.

2. Barrier Effect: The zinc coating acts as a physical barrier, preventing moisture and corrosive agents from reaching the steel surface. This barrier effect further enhances the longevity of the steel.

3. Self-Healing: In the event that the zinc coating is scratched or damaged, zinc's self-healing properties allow it to mend minor breaches in the coating, preventing corrosion from spreading.

4. Cathodic Protection: Zinc provides cathodic protection to the steel, meaning it actively prevents the electrochemical reactions that lead to corrosion.

 Conclusion:

The manufacturing process of galvanized steel coil is a testament to the intricate fusion of science and industry. Each step, from surface preparation to the application of the zinc coating, contributes to the creation of a material renowned for its corrosion resistance, durability, and versatility. As galvanized steel coil continues to serve diverse industries such as construction, automotive, and manufacturing, its manufacturing process remains a cornerstone of modern materials engineering.

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Why use Zinc to produce galvanized steel coil

Zinc is a blue-white metal having a melting point of 419.4 ° C and a density of 7.14 g/cm 3 . Zinc has moderate ductility. Zinc does not change in dry air at room temperature. In the moist air, the zinc surface forms a dense film of zinc carbonate, which protects the interior of the zinc from corrosion. Since zinc has such an excellent property, zinc is plated on the surface of the steel sheet to prevent corrosion, and this steel sheet is called a galvanized sheet.

Advantages of Galvanized Steel Coil

After the steel coil or plate is galvanized, the service life can be greatly extended. If the zinc layer on the steel sheet is not destroyed, the zinc can prevent the corrosive medium (water, oxygen, carbon dioxide, etc.) from contacting the surface of the steel sheet, which is exactly the same as the anti-corrosion effect of the tin plating layer. However, if the galvanized layer is destroyed and the iron of the individual parts is exposed to the surface, since the chemical properties of zinc are more active than iron, zinc and iron form micro-batteries during the etching process, and zinc is the anode of the micro-battery. It is dissolved, and iron is protected by the cathode.

Therefore, the galvanized vessel will not rust even if it is partially exposed to iron. If the iron surface is exposed too much, the iron is corroded like ungalvanized. It can be seen that the galvanized sheet has good anti-rust properties and is widely used in the construction, vehicle, furniture, mechanical, electrical and packaging industries.

Steel Strip Galvanizing

Steel strip galvanizing is available in both electrogalvanized and hot-dip galvanized. The strip produced by the electroplating method has a thin coating but a complicated process.

What is the production process of hot-dip galvanized sheet?

The hot-dip galvanizing sheet production method is developed from the hot tin plating production method and can be roughly classified into the following types:

The term "out-of-line annealing" means that the raw material (hot and cold-rolled sheet) is first subjected to recrystallization annealing before entering the hot-dip galvanizing line; the so-called in-line annealing means that the steel sheet is first subjected to recrystallization annealing on the hot-dip galvanizing line. Then hot galvanized.

At present, the Sendzimir method, also known as the oxidation-reduction method is widely used. After cold rolling, the steel is directly sent to the hot-dip galvanizing unit. The steel strip is first burned in the continuous annealing process, and a thin oxide film is formed on the surface of the strip, and then passed through the reduction section of the annealing furnace. The oxide film on the surface is reduced to a pure iron body, so that the plating layer is well bonded to the iron base when entering the zinc pot.

For a 1700mm continuous hot dip galvanizing unit. The equipment layout and operation of the feed section and the discharge section of the unit are basically the same as those of the continuous electroplating tin unit. The continuous hot-dip galvanizing unit process section includes four main processes: pre-plating (degreasing and heat treatment), hot-dip galvanizing, and correction and passivation. The speed of the unit is 10-180m/min.

The 1700mm continuous hot-dip galvanizing unit adopts the method of degreasing annealing of the rapid heating furnace. The maximum temperature of the annealing furnace is up to 980 °C, and the annealed strip enters the zinc tank at a temperature of 450-470 °C to keep the temperature of the zinc liquid constant. Control the thickness of the coating by “air knife method”, that is, using a controllable nozzle at the outlet of the zinc tank to blow compressed air or superheated steam to the strip at a certain angle to remove excess zinc liquid, and in this way, positive and negative can be produced. A thick galvanized steel sheet with different thicknesses on both sides. In order to form a layer of zinc-iron alloy on the surface of the original plate to make it have good elongation, the plate after galvanizing should be reheated (ie, through a coating annealing furnace), and the heating temperature is about 550 ° C.

Tip: In order to improve the corrosion resistance of the galvanized steel sheet, the strip steel should be passivated in chromic acid or phosphoric acid solution after cooling.

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