1) Raw Material and Production Route Selection
There are generally two upstream production routes for hot-dip galvanized coils:
Route A: Steel billet → Hot-rolled coil (HRC) → Pickled and cold-rolled (CRC) → Continuous hot-dip galvanizing (CGL) → Finished product
Route B: Direct purchase of cold-rolled coil (CRC) → Continuous hot-dip galvanizing (CGL) → Finished product
Ultimately, the vast majority of galvanized coils you see are made from cold-rolled substrates on a continuous hot-dip galvanizing line because they offer better surface finish and more precise thickness.
2) Steel Billet → Hot Rolled (HRC)
2.1 Continuously Cast Steel Billet
Molten steel is continuously cast to form a slab. Key concerns include:
Composition control (C, Mn, Si, Al, P, S, etc.)
Inclusions, segregation, and central porosity (affecting subsequent surface finish and forming)
2.2 Heating Furnace + Roughing/Finishing + Coiling
The slab is heated to the hot rolling temperature zone.
After roughing and finishing rolling, hot-rolled strip is formed.
Controlling the final rolling temperature and coiling temperature affects the microstructure and mechanical properties.
Output: Hot-rolled coil (HRC), which will subsequently undergo galvanizing and generally pickling/cold rolling.
3) Pickling: Removing oxide scale
Hot-rolled surfaces have iron oxide scale, which must be cleaned before galvanizing.
Uncoiling → Inlet shearing and welding → Pickling tank (multiple tanks in series) → Rinsing → Drying → Coiling
Common acid solutions: hydrochloric acid system (sulfuric acid system also exists)
Key control points:
Acid concentration, temperature, speed, iron ion content
Rinsing quality (residual acid affects subsequent rolling/galvanizing)
Output: Pickled coil (P&O)
4) Cold Rolling: Thickness setting and surface improvement
Pickled coil enters the cold rolling mill (single stand/multiple stand series):
Thickness is rolled to the target range (e.g., 0.12–3.0 mm is common) by reduction rate
Surface roughness and sheet shape (straightness) are established here
Control points:
Rolling oil, emulsion
Tension, sheet shape (wavy, edge waviness)
Thickness accuracy (AGC) and surface defects
Output: Cold rolled hard coil (Full) Hard (High hardness, requires annealing to restore plasticity)
5) Continuous Annealing: Restoring plasticity and microstructure
Hot-dip galvanizing lines (CGL) usually have their own annealing furnace (or anneal before galvanizing):
Process:
Inlet uncoiling, welding, looping
Cleaning section (alkaline washing/brushing) to remove oil stains
Continuous annealing furnace: Heating → Holding → Controlled cooling/rapid cooling
Purpose:
Recrystallization, reducing hardness, obtaining the required strength/elongation
Providing a suitable surface condition for galvanizing
Key control points:
Furnace atmosphere (usually H₂/N₂ protective gas): Preventing oxidation
Furnace temperature profile, speed
Dew point control (affects surface oxidation/reduction ability)
6) Core of hot-dip galvanizing: Zinc pot + air knife to control the zinc layer
6.1 Pre-treatment before entering the zinc pot
After exiting the furnace, the strip steel generally passes through a reduction section/cooling section before entering the zinc pot.
Ensuring good surface "activity" is beneficial for zinc wetting and bonding.
6.2 Molten Zinc Bath
Stable zinc bath temperature
Molten zinc bath composition: Primarily Zn, often with a small amount of Al added (to inhibit over-alloying and improve coating quality). Different processes may involve trace amounts of other elements.
After the strip is immersed in the molten zinc bath:
A zinc layer forms on the surface, and simultaneously, an Fe-Zn alloy layer is formed at the interface (key to coating adhesion).
6.3 Air Knife for Quantitative Zinc Coating
After the strip exits the molten zinc bath, it is purged with an air knife to control the coating thickness:
Air pressure, air knife gap, angle, and strip speed
Determine the final coating weight (e.g., Z40/Z60/Z80/Z120/Z180/Z275, etc.)
7) Post-treatment: Cooling, shaping, passivation/oiling, and slitting
After exiting the air knife, the strip enters:
Cooling section (curing the zinc layer)
Skin Pass: Improves surface, controls yield plateau, and enhances strip shape
Tension Leveler: Improves flatness
Surface Treatment (Optional):
Passivation (Chromate/Non-chrome): Enhances resistance to white rust
Oil Coating: Rust prevention and transport protection
Fingerprint Resistance (AFP): Commonly used in home appliances/construction
Phosphating/Pretreatment: Prepares for subsequent coating
Final Inspection (Surface, Zinc Layer, Mechanical Properties, Dimensions)
Slitting or Cut-to-Length
Packaging and Warehousing
8) Quality Control Indicators (Most Important to Customers)
Substrate Material: DX51D, SGCC, S220GD/S280GD/S350GD, etc.
Thickness and Width Tolerance: Thickness accuracy, plate shape Coating weight/thickness: Zxx (total for both sides)
Surface quality: Zinc spangle, scratches, pitting, exposed iron, black spots, zinc dross
Adhesion/bending performance: T-bend, stamping performance
Corrosion resistance: Salt spray, white rust risk (strongly correlated with passivation/oil film/packaging)
9) Quick reference for common defects and process causes
Exposed iron/incomplete coating: Incomplete cleaning, oxide film, abnormal furnace gas dew point, strip surface contamination
Uneven zinc layer: Fluctuations in air knife parameters, unstable speed, zinc pot flow field, sheet shape problems
Zinc dross/particles: Poor control of zinc pot dross, filtration and dross removal management, aluminum content and temperature fluctuations
White rust: Moist packaging, insufficient passivation, improper transportation and storage
Poor sheet shape: Cold-rolled sheet shape residue, tension control, insufficient straightening