Galvanized Steel Coils: Properties, Process, Standards and Specifications

Galvanized steel coils are steel sheets coated with zinc to prevent corrosion, making them durable and long-lasting. The galvanization process involves submerging steel coils in molten zinc, creating a robust and uniform coating that enhances structural strength. The coated coils can be used directly or further processed into various products, including shapes and structures, and can be painted for added protection and aesthetic appeal.

Type of Galvanization 

Hot-Dip Galvanization

• Process description

In hot-dip galvanization, steel coils are submerged in molten zinc. This process forms a corrosion-resistant zinc coating. The immersion temperature is typically around 460°C. After removal from the zinc, the steel undergoes a quenching process to enhance its mechanical properties.

• Advantages and limitations
  

The resulting coat is thick, offering substantial protection against corrosion. This method is cost-effective for large-scale production. However, it can lead to a rough surface finish, and the size of the dip tank can limit the dimensions of treatable steel products.

Electro-Galvanization

• Process description
  

Electro-galvanization involves the application of zinc onto steel through an electrochemical process. The steel is passed through a solution of zinc sulfate or cyanide and an electric current is applied to form a zinc coating on the steel surface. This method generally yields a thinner zinc layer.

• Comparison with hot-dip galvanization
  

The electro-galvanization process provides a more precisely controlled, uniform coat compared to hot-dip galvanization. Though thinner, the resulting coat is smoother and suitable for products where aesthetics are important. However, it may not be as corrosion-resistant as a thicker hot-dipped coating.

Galvanized Steel Coil Specifications

Thickness

Common thickness ranges from 0.15 to 3.4 millimeters, allowing for use in diverse industries from construction to automotive.

Width and Size Options

Widths can vary, with standard sizes starting at 600 millimeters up to 1500 millimeters. Cut-to-length services are often available to ensure the steel coil meets specific project needs.

Steel Grade

Steel grades such as CS (Commercial Steel), FS (Forming Steel), DDS (Deep Drawing Steel), and EDDS (Extra Deep Drawing Steel) indicate the steel’s ductility and strength.

Surface Treatment

Surface treatment of galvanized steel coils is an important aspect of the production process, as it enhances the corrosion resistance and appearance of the final product, as well as its suitability for painting or further processing. Here are some common surface treatments applied to galvanized steel coils:

1. Passivation (Chromate Coating): A passivation layer, often a chromate-based solution, is applied to the surface to enhance corrosion resistance. This layer helps to prevent the formation of white rust during storage and handling. Non-chromate passivation solutions are also used for environmental reasons.

2. Oiling: A thin layer of oil is applied to the surface to prevent rusting during storage and transport. The oil provides a barrier against moisture and contaminants.

3. Phosphate Treatment: This involves applying a phosphate chemical to create a layer that improves paint adhesion and increases corrosion resistance.

4. Anti-fingerprint Treatment: A nano-coating is applied to the surface to resist fingerprints and smudges, which is particularly useful for products that will be handled during installation or used in appliances.

Surface Finish

The surface finish of galvanized steel coils can significantly affect their appearance, performance, and suitability for certain applications. Here are some common types of surface finishes for galvanized steel coils:

1. Regular Spangle: The typical finish for hot-dip galvanized steel, characterized by visible crystalline patterns across the surface, known as spangles. These are the result of the normal solidification process of the zinc coating.

2. Minimized Spangle: By controlling the cooling process and using special zinc bath compositions, the spangle size can be minimized, resulting in a more uniform appearance. This finish is often used for applications where a smoother paint finish is desired.

3. Zero Spangle: Through the use of zinc alloys and cooling techniques, it’s possible to produce a galvanized surface with a very minimal or no visible spangle. This provides a uniform surface that is preferred for painting or applications requiring a high-quality appearance.

Coating Thickness

The coating thickness on galvanized steel coils, referring to the zinc layer, is crucial for durability and corrosion resistance. It is measured in grams per square meter (g/m²) and varies based on the application. Common coating weights include G30, G60, G90, indicating the zinc weight per square foot of steel surface.

ASTM Standards: The American Society for Testing and Materials (ASTM) provides standards for coating thickness. For example, ASTM A653 specifies the minimum zinc coating weights for different designations, ranging from G30 (0.30 oz/ft² total both sides) to G235 (2.35 oz/ft² total both sides), which corresponds to different average coating thicknesses.

EN  Standards: EN 10346 uses a coating designation system to specify the minimum total coating mass on both sides of the steel. The designation system is given in terms of the coating mass in grams per square meter (g/m²). For example, a coating designation of DX51D+Z275 indicates a minimum total coating mass of 275 g/m²

It’s important to specify the desired coating thickness when ordering galvanized steel coils to ensure the product meets the required performance criteria for corrosion resistance. The actual coating thickness should conform to the relevant standards or customer specifications and can be verified using appropriate testing methods after the galvanizing process.

Properties of Galvanized Steel Coils

Galvanized steel coils have a range of properties that are enhanced by the galvanizing process, which involves coating the steel with a layer of zinc to protect it from corrosion. Here are some key properties of galvanized steel coils:

1. Corrosion Resistance: The primary benefit of galvanizing steel coils is the significant increase in corrosion resistance. The zinc coating acts as a barrier to prevent environmental elements from reaching the steel surface. Moreover, zinc corrodes preferentially to steel, offering sacrificial protection even if the coating is damaged.

2. Formability: Despite the additional layer of zinc, galvanized steel coils can still be cut, bent, and formed into various shapes without losing their protective properties, making them suitable for a wide range of applications.

3. Weldability: Galvanized steel can be welded, although precautions may need to be taken to avoid inhaling zinc fumes and to ensure the integrity of the zinc coating around the welded area.

4. Mechanical Properties: The mechanical properties of steel, such as tensile strength, yield strength, and elongation, are generally not affected by the galvanizing process. However, the total thickness of the material will increase slightly due to the addition of the zinc coating.

5. Temperature Resistance: Galvanized steel can withstand high temperatures up to a certain point before the zinc coating begins to degrade. The performance of galvanized steel at high temperatures will depend on the specific conditions and duration of exposure. Generally, galvanized coatings are not recommended for continuous exposure to temperatures above 200°C (392°F), as this can lead to peeling and reduced corrosion resistance.

Production Process of Galvanized Steel Coils

1 Uncoiling and welding

Uncoil the steel coil to provide the steel strip required for continuous operation, and weld the tail of the leading strip and the head of the trailing strip to ensure the continuous production of the unit.

2 Degreasing and cleaning

The cleaning section is to clean the surface of the strip to remove rolling oil and iron powder to obtain good coating adhesion and surface quality. It generally includes spraying, brushing, electrolytic cleaning, brushing, spraying, squeezing and hot air drying. Such equipment, the cleaning section is more important for hot-dip galvanizing, and is an important step to obtain good strip surface quality.

3 Annealing

Recrystallization annealing is performed on the hardened strip to obtain good mechanical properties of the product. By controlling the atmosphere in the furnace, good strip coating properties are obtained. The annealing furnace generally consists of preheating, heating, soaking, cooling, etc. consists of segments.

4 Air knife zinc layer control

The thickness of the coating on the surface of the strip is controlled by blowing gas, usually air or pure nitrogen, onto the surface of the strip after leaving the zinc pot.

5 Alloying treatment

Alloying treatment is a process of heating the galvanized strip to obtain a zinc-iron alloy coating.

6 Leveling and straightening

Roll the strip with a small rolling force to eliminate the yield platform of low carbon steel, improve the performance of the strip, obtain the desired surface roughness of the strip, improve the strip shape, and obtain good flatness, etc. After leveling, there is tension leveling. Tension leveling is to stretch and straighten the strip. The most important thing is to obtain a good plate shape.

7 Post-processing

Passivation or other chemical surface treatment is performed on the strip steel to obtain the corrosion resistance, lubrication or other requirements desired by the user. Generally, spray squeezing or roller coating and drying are used.

8 Trimming

Trim the strip as needed to obtain the appropriate width accuracy.

9 Oiling

Coat the strip with oil to obtain certain anti-rust and lubrication properties.

Applications of Galvanized Steel Coils

Galvanized steel coils exhibit excellent corrosion resistance and durability, making them suitable for a broad range of applications across various industries.

Construction Industry

• Roofing and Wall Cladding: These coils are commonly used to create components that endure harsh weather, such as roofs and exterior walls, due to their ability to withstand environmental degradation.

• Structural Components: They are integral in constructing frameworks and support structures because their strength and resilience contribute to the longevity of buildings.

Automotive Industry

• Body Parts and Frames: Galvanized steel’s robustness make it a preferred choice for manufacturing resilient and rust-resistant car frames and body parts.
• Safety and Durability Enhancements: The material’s properties significantly bolster the vehicle’s safety features and overall structural integrity.

Other Industries

• Home Appliances: Durability and resistance to wear and tear are essential for appliances; thus, galvanized steel is used in products like refrigerators, washing machines, and microwaves.
• Agricultural Equipment: Farming tools and machinery benefit from the protective coating of galvanized steel, ensuring they remain operational over long periods, irrespective of exposure to potentially corrosive substances.

Standards

Galvanized steel coils adhere to international standards to guarantee quality, durability, and suitability for specific applications, covering aspects like coating weight, dimensions, and strength. These standards are essential for maintaining consistency and reliability for manufacturers, suppliers, and consumers.

ASTM A653/A653M: This specification covers steel sheets, zinc-coated (galvanized), or zinc-iron alloy-coated (galvannealed) by the hot-dip process in coils and cut lengths.

ASTM A924/A924M: It outlines the general requirements for the metallic-coated steel sheet sold in the form of coils or cut lengths, intended for use in structural applications or non-structural applications.

EN 10346: This standard specifies the requirements for continuously hot-dip-coated steel flat products. It is used for cold forming and for applications where high strength, accurate dimensional tolerances, and specified minimum yield strengths are necessary.

EN 10326: This European Standard specifies requirements for continuously hot-dip coated steel sheets of structural quality in the grades and qualities given in chemical composition tables and mechanical properties tables in the delivery conditions.

AS 1397: This standard specifies the requirements for continuous hot-dip coated metallic steel sheet and strip

JIS G 3302: This standard specifies the requirements for hot-dip zinc-coated steel sheet and strip

Conclusion

Galvanized steel coils are crucial in various industries due to their durability and corrosion resistance, particularly in construction, automotive, and appliance manufacturing. These materials are expected to play a pivotal role in sustainable development efforts due to their recyclability and energy-efficient production methods. Ongoing technological advancements in galvanization are enhancing the quality and efficiency of these coils, ensuring their continued use in numerous applications.