1.What are the core principles and overall requirements for the surface quality of galvanized coils?
Coating Continuity: The surface must have a complete zinc coating, with no critical defects such as missing plating (bare spots) or rust spots. This is a fundamental requirement; otherwise, the corrosion protection will be lost.
Coating Uniformity: The zinc layer should be evenly distributed across the steel surface, without noticeable uneven thickness or streaking.
Surface Cleanliness: The surface must be spotless, free of contaminants such as oil, dust, fingerprints, and black spots, which could affect subsequent painting or welding.
2.What are the common surface defects and requirements of galvanized coils?
Serious defects: missing plating, bare spots; zinc slag (burrs); rust
Common appearance defects (strictly controlled): scratches, abrasions; embossing, roller marks; air knife streaks; uneven zinc spangle size
Process-related defects: waves, warping, chromate passivation spots
3.What are the surface treatment grade classifications?
Surface A (Normal Surface): A normal spangled surface. Suitable for most applications where direct exposure or aesthetics are not crucial, such as structural components and internal parts.
Surface B (Minimal Spangle Surface): Technical measures are used to suppress spangle growth, resulting in a more uniform and fine surface. It is ideal as a substrate for color-coated steel sheets, as it offers a smooth appearance after painting.
Surface C (Finished Surface): Rolling through a skin-pass mill not only eliminates spangles but also improves surface smoothness, hardness, and flatness. It is used in applications where appearance is paramount, such as high-end home appliances and automotive exterior panels.
4.What are the different focuses for different purposes?
For color-coated substrates (color-coated coils): The highest requirements apply. The surface must be free of zinc spangles or small spangles (surfaces B/C), extremely clean, and free of any defects that could affect paint film adhesion (such as zinc slag and dirt).
For automotive exterior panels: Extremely high surface quality, flatness, and formability are required. Electro-galvanized or hot-dip galvanized sheet with a smooth surface (surface C) is typically used.
For building structures/home appliance internal components: Corrosion resistance (zinc coating weight) and mechanical properties are prioritized, with less stringent requirements for surface aesthetics (such as spangle size).
For general hardware: Basic corrosion resistance and cost are emphasized, with a higher tolerance for minor cosmetic defects.
5.Summary of surface quality requirements for galvanized coils?
No fatal defects: No missing plating or red rust is permitted.
Strict control of process defects: Minimize zinc slag, severe scratches, and embossing.
Surface condition suitable for the intended use: Select the appropriate surface grade (e.g., ordinary spangle, fine spangle, or smooth finish) based on the end use.
Excellent cleanliness and flatness: Ensure smooth subsequent processing.