Classification and Quality Inspection Standards for Galvanized Steel Sheets

Classification
Galvanized steel sheets refer to welded steel plates with a hot-dipped or electro-galvanized coating on the surface. They are widely used in construction, home appliances, automotive, shipbuilding, container manufacturing, electromechanical industries, and more.
1. Hot-Dip Galvanized Steel Sheets (HDG)
• Produced by immersing thin steel plates into a bath of molten zinc, allowing a layer of zinc to adhere to the surface.
• Commonly produced using continuous galvanizing processes where coiled steel is continuously immersed in a molten zinc bath.
2. Alloyed Galvanized Steel Sheets
• Manufactured using the hot-dip method but heated to around 500°C after exiting the zinc bath to form an alloy film of zinc and iron.
• Offers excellent paint adhesion and weldability.
3. Electro-Galvanized Steel Sheets (EG)
• Produced via electroplating, providing good formability.
• Has thinner coatings compared to HDG, offering slightly less corrosion resistance.
4. Single-Sided and Differential Galvanized Steel Sheets
• Single-sided galvanized sheets have zinc coating only on one side, suitable for welding, painting, rust prevention, and processing.
• To address the lack of zinc on the other side, differential galvanized sheets apply a thin layer of zinc on the non-coated side.
5. Alloy and Composite Galvanized Steel Sheets
• Made from alloys of zinc with other metals like aluminum, lead, etc., or composite coatings.
• Provides superior coating performance.
6. Other Types
• Includes color-coated galvanized steel, printed coated galvanized steel, PVC laminated galvanized steel, etc.
• The most commonly used type remains hot-dip galvanized steel.
Quality Inspection Standards
High-quality galvanized steel sheets must meet several criteria regarding dimensions, surface quality, zinc coating weight, chemical composition, mechanical properties, and packaging.
1. Packaging
• Galvanized sheets can be packaged either as cut-to-length sheets or coil-packaged.
• Typically wrapped in iron sheets with moisture-proof paper inside and secured with iron straps to prevent internal friction.
2. Dimensions and Tolerances
• Standards specify recommended thicknesses, lengths, and widths along with allowable tolerances.
• Custom sizes based on user requirements are also available.
3. Surface Condition
• Varies depending on the coating process: regular zinc pattern, fine zinc pattern, smooth zinc pattern, no visible zinc pattern, phosphated finish, etc.
• Cut-to-length sheets and coils should not have defects that affect usage, though minor imperfections near welds are acceptable.
4. Zinc Coating Weight
• Determined by measuring the zinc coating per unit area (g/m²).
• Can be equal on both sides (equal-thickness galvanizing) or different (differential-thickness galvanizing).
5. Mechanical Properties
• Tensile Testing: Generally required for structural, drawing, and deep-drawing applications.
• Bend Testing: A critical measure of sheet metal formability. After bending 180°, the outer zinc layer should not peel off, and the base material should not crack.
6. Chemical Composition
• Requirements vary by country. For instance, Japan specifies chemical compositions, while the U.S. does not.
• Typically not checked for finished products.
7. Flatness
• Measured by flatness deviation and edge wave (scythe bend).
• Maximum allowable values are specified in standards.
In summary, understanding these classifications and adhering to strict quality inspection standards ensures the durability, reliability, and safety of galvanized steel sheets in various applications.