How is the alloying treatment of alloyed hot-dip galvanized sheet (GA) achieved?

How is the "alloying treatment" of alloyed hot-dip galvanized sheet (GA) achieved?
GA adds an "alloying furnace" step after GI galvanizing. Specifically:
The hot-dip galvanized sheet (with a pure zinc layer) enters the alloying furnace, where the temperature is controlled at 550-560°C (the critical temperature for zinc-iron alloying).
The temperature is maintained for 10-30 seconds, during which the zinc atoms in the pure zinc layer diffuse with the iron atoms in the substrate, forming a Zn-Fe alloy layer (primarily composed of Zn₁₃Fe and ZnFe₂).
The zinc layer is rapidly cooled (to prevent the alloy layer from becoming too thick and brittle), ultimately completely converting the zinc layer to an alloy, leaving no pure zinc residue.
Critical control points: Too high a temperature leads to an excessively thick alloy layer (brittleness); too low a temperature leads to incomplete alloying (a pure zinc layer remains).
What precautions should be taken when cutting galvanized sheet? What problems should be avoided?
When cutting (shearing or sawing), pay attention to three points to avoid quality issues:
Tool Selection: Use high-speed steel or carbide tools, keeping the blades sharp to prevent the zinc layer from being "ripped" due to blunt tools.
Gap Adjustment: The gap between the upper and lower blades of the shear should match the sheet thickness (usually 5%-10% of the sheet thickness). Too much gap can easily cause burrs on the edges, while too little gap can damage the zinc layer.
Rust Prevention: After cutting, the edges, unprotected by the zinc layer, should be immediately coated with anti-rust paint or zinc powder paint to prevent rust ("cut corrosion").
Can galvanized sheet be bent? What are the key parameters for bending? Bending is possible, but two key parameters must be controlled to prevent zinc flaking:
Bending radius (R): This refers to the inner arc radius of the bend. It must be 1-2 times the sheet thickness (e.g., for a 2mm thick galvanized sheet, the bending radius must be 2mm or greater). A smaller radius can cause the zinc layer to stretch and crack.
Bending direction: Bending in the direction of the sheet's rolling will result in higher elongation and less cracking of the zinc layer. Bending against the rolling direction requires a larger radius.
Note: GA zinc coatings have slightly lower toughness, so the bending radius must be 10%-20% larger than GI.
Can galvanized sheet be painted or coated? What pre-treatment is required? It can be painted (e.g., appliance housings) or coated (e.g., architectural decorative panels). Pretreatment is crucial (to ensure adhesion):
Degreasing: Use a solvent or alkaline cleaner to remove surface oil (rust-proof oil applied during production);
Rust removal: If there are slight oxidized spots on the surface, remove them with fine sandpaper or phosphate treatment;
Passivation: If the original passivation layer is damaged, reapply passivation solution (to enhance coating adhesion);
Drying: Ensure the surface is free of moisture and impurities before painting or coating.
Principle: Pretreatment removes surface contaminants, creating an "active surface" that allows the coating/film to adhere tightly to the zinc layer and prevent detachment.
What is the purpose of adding "aluminum" in galvanized sheet production? Aluminum is typically added to hot-dip galvanizing baths at a concentration of 0.15%-0.25%. It serves two key functions:
Preventing excessive iron-zinc reaction: Aluminum forms an Al-Fe alloy layer on the substrate surface, preventing the diffusion of large amounts of iron atoms from the substrate into the zinc bath, thus preventing the formation of an excessively thick, brittle zinc-iron alloy layer (which reduces zinc adhesion);
Improving the brightness and uniformity of the zinc layer: Aluminum refines zinc crystals, resulting in more uniform zinc spangles and a brighter surface.
Note: Excessive aluminum content (>0.3%) can reduce the fluidity of the zinc bath and increase the risk of plating leaks.