Production Process of Galvanized Color Coated Coils: From Galvanized Substrate to Finished Color Coated Products

I. Pre-process: Preparation of Cold-Rolled Substrate

1. Substrate Material Selection

   
   

   The raw material adopts cold-rolled low-carbon steel coils (common grades such as Q235, SPCC, DX51D, etc.) with a thickness range of 0.12–2.0 mm, and the width can be customized according to customer requirements. The steel coils must undergo appearance inspection to ensure no scratches, pitting, rust spots, or other defects on the surface.
2. Steel Coil Pre-treatment
   • Uncoiling and Leveling: The steel coil is uncoiled by an uncoiler and then leveled by a leveler to ensure the flatness of the steel sheet and avoid deviation during subsequent processing.
   • Shearing and Welding: The tail of the previous coil is welded to the head of the next coil to achieve uninterrupted continuous production and improve production efficiency.

II. Core Process 1: Preparation of Galvanized Layer (Substrate Anti-corrosion Treatment)

1. Hot-Dip Galvanizing Process (Mainstream Process)
   • Degreasing and Cleaning: The steel sheet enters an alkali washing tank to remove oil stains and dust on the surface; then it is put into a pickling tank (hydrochloric acid solution) to remove scale; finally, it is rinsed with clean water to ensure the surface of the substrate is clean.
   • Annealing and Reduction: The cleaned steel sheet is sent to a continuous annealing furnace and heated at 800–900℃ in a reducing atmosphere (nitrogen + hydrogen) to eliminate cold-rolling stress. Meanwhile, the residual iron oxide on the surface is reduced to pure iron, enhancing the adhesion between the zinc layer and the substrate.
   • Hot-Dip Galvanizing: The annealed steel sheet is directly immersed in molten zinc solution at around 450℃. A metallurgical reaction occurs between the steel sheet surface and the zinc solution, forming a double-layer structure of zinc-iron alloy layer + pure zinc layer.
   • Zinc Layer Control: High-pressure air flow from an air knife is used to blow the surface of the steel sheet, precisely controlling the zinc layer thickness (30–275 g/㎡ as standard) to meet the anti-corrosion requirements of different scenarios. Subsequently, the steel sheet is cooled by cooling air to solidify the zinc layer.
2. Electro-galvanizing Process (Suitable for Scenarios Such as Home Appliance Interiors)

   
   

   Based on the principle of electrolysis, the steel sheet is used as the cathode and placed in an electrolyte containing zinc salts. When current is applied, zinc ions deposit on the steel sheet surface to form a uniform and dense zinc layer. The zinc layer is thinner (10–50 g/㎡), the surface is smoother, and the cost is relatively lower.

III. Core Process 2: Preparation of Color Coating Layer (Surface Coating and Curing)

1. Surface Pre-treatment (Critical Step)
   • Passivation Treatment: The galvanized substrate is put into a passivation tank, and a passivation film is formed on the zinc layer surface through chemical oxidation (chromium-free passivation is currently the mainstream, complying with RoHS and REACH standards). This enhances the adhesion between the zinc layer and the coating and improves the anti-corrosion performance.
   • Water Washing and Drying: The passivated substrate is rinsed with clean water to remove residual passivation solution, and then dried in a hot air drying furnace to ensure no water stains on the surface and avoid blistering and pinholes after coating.
2. Coating Process
   • Primer Coating: The roll coating method (most commonly used) is adopted to evenly apply primer on the substrate surface with coating rollers. The primer functions to enhance topcoat adhesion, block moisture, and improve overall anti-corrosion effect. The primer thickness is controlled at 5–10 μm, with epoxy primer being the common type.
   • Pre-curing: The steel sheet coated with primer is sent to a pre-curing furnace (150–200℃) to preliminarily dry the primer, preventing interlayer mixing during subsequent topcoat application.
   • Topcoat Application: The type of topcoat (PE polyester, SMP silicon-modified polyester, PVDF polyvinylidene fluoride, etc.) is selected according to customer requirements, and roll coating is also used for application. Back paint can be added as needed (to improve the anti-corrosion performance of the back surface). The topcoat thickness ranges from 10–25 μm, and customization of RAL/Pantone standard colors is supported.
   • Finishing Treatment (Optional): For special surface textures (matte, high-gloss, frosted, etc.), the texture of coating rollers can be adjusted or additives can be added to achieve the desired effect.
3. High-Temperature Curing

   
   

   The coated steel sheet is conveyed to a curing furnace and baked at 200–250℃ for 30–60 seconds. This allows the resin in the coating to undergo a cross-linking reaction, forming a hard and dense paint film. The curing temperature and time must be precisely controlled. Excessively high temperature can cause discoloration and aging of the coating, while insufficient temperature leads to incomplete curing and easy peeling of the coating.

IV. Post-process: Finished Product Handling and Packaging

1. Cooling and Inspection

   
   

   The cured color coated coil is cooled by cooling rollers and then inspected via an online detection system, which tests coating thickness, color difference, adhesion, and surface defects (such as missed coating, scratches). Non-conforming products are marked and rejected.
2. Slitting and Coiling

   
   

   According to customer order requirements, a slitting machine is used to cut large color coated coils into small coils of specified widths. The small coils are then wound into regular rolls by a recoiler, and product labels are attached (indicating grade, specifications, zinc layer/coating thickness, color, production date).
3. Packaging and Warehousing

   
   

   Export-standard packaging is adopted: the inner layer is wrapped with moisture-proof paper, the outer layer is wrapped with plastic film and fixed with steel strip bundling, and finally placed on wooden pallets to prevent moisture damage, deformation, and scratches during sea transportation. After packaging, the finished products are warehoused and ready for delivery.