1. Entry Section

• Equipment: Uncolier → Shear → Welder → Entry Accumulator → Tension Leveler.

• Functions:

  • Uncoiling & Splicing: Uncoils cold-rolled steel coils (from Continuous Annealing Lines or recoiling lines) and joins the trailing end of one coil to the leading end of the next using a narrow overlap welder for continuous operation.

  • Accumulator Storage: The entry accumulator stores a sufficient length of strip, allowing the process section (cleaning, plating) to continue at high speed while the entry section stops for coil changeovers and welding.

  • Strip Preparation: The tension leveler removes coil set (crossbow) and edge wave, ensuring the strip enters subsequent precision processes flat, straight, and centered.

2. Pre-Treatment (Cleaning & Activation)

• Objective: To obtain an absolutely clean, chemically activated metallic surface for optimal coating adhesion.

• Typical Process Flow:

  1. Alkaline Electro-Cleaning: The strip acts as the cathode in a hot alkaline solution. Electrical current causes gas evolution (H₂), which physically scrubs away rolling oils and greases, aided by saponification.

  2. Electrolytic Picking: Electrolysis in a dilute sulfuric or hydrochloric acid bath effectively removes the thin oxide layer and iron fines.

  3. Multi-Stage Rinsing: Deionized water is used for spray and immersion rinsing to completely remove residual acid/alkali electrolytes.

• Critical Point: Even trace contaminants can lead to poor adhesion, blistering, or bare spots in the zinc coating.

3. Electro-Galvanizing Section (Core Process)

• Principle: With electrical current applied, zinc ions at the strip surface (cathode) gain electrons and are reduced to metallic zinc, which deposits onto the steel.

• Main Process Methods:

  • Selas Method (Vertical): The strip passes vertically through the plating cell, receiving current via conductor rolls. Precise anode placement and masking allow for one-side (OS), two-side equal (EQ), or differential (DF) coating. This offers high flexibility and is the mainstream technology for producing automotive differential coated sheets.

  • Horizontal Radiant Type: The strip passes horizontally with anodes above and below, enabling simultaneous two-side plating. This design is compact and efficient.

• Process Control:

  • Plating Solution: Mainly acidic chloride or sulfate baths containing zinc ions, conducting salts, brighteners, and stabilizers.

  • Key Parameters: Current Density (determines plating rate), bath temperature, pH, zinc ion concentration.

  • Coating Weight Control: Achieved through closed-loop automatic control using Ampere-minute meters for precise charge calculation.

4. Post-Treatment (Coating Conversion & Protection)

• Objective: To enhance the short-term corrosion resistance, paint adhesion, and special functional properties of the electro-galvanized sheet.

• Main Processes:

  1. Phosphating:

     • A microcrystalline phosphate conversion coating (e.g., zinc phosphate) is formed on the zinc surface via chemical or electrochemical reaction.

     • Functions: ① Provides short-term rust prevention. ② Serves as an excellent paint base, enhancing paint adhesion. ③ Reduces "white rust" formation during storage/transport.

  2. Chromium-Free Passivation:

     • The modern, environmentally friendly standard. Uses solutions containing zirconium, titanium, silicon, etc., to form a dense oxide film.

     • Provides good, eco-friendly corrosion resistance.

  3. Anti-Fingerprint (AFP) Treatment:

     • Applies a very thin organic or organic-inorganic hybrid coating.

     • Functions: ① Prevents fingerprint corrosion from sweat/oils. ② Improves corrosion resistance. ③ Provides some lubricity, enhancing formability in subsequent stamping.

5. Exit Section

• Equipment: Exit Accumulator → Online Inspection System → Oiler → Tension Reel → Automatic Packaging Line.

• Functions:

  • Accumulator & Inspection: The exit accumulator ensures continuous process section operation during exit operations. Equipped with X-ray Fluorescence (XRF) coating gauges and surface inspection cameras for 100% online quality monitoring.

  • Finishing & Recoiling: An oiler applies a micro-thin layer of protective oil. The tension reel winds the strip under constant tension to form the finished coil.

  • Packaging: The automatic packaging line performs strapping, edge protection application, and wrapping with anti-rust paper/film to shield the product from damage and corrosion during handling and storage.