1.How does incomplete cleaning manifest itself?
Key process points: Degreasing (alkaline/electrolytic cleaning) concentration, temperature, and time; acid concentration and speed for pickling (if required).
Problem manifestations:
Residual rolling oil, iron dust, or oil stains on the substrate surface cause carbonization during annealing, forming "oil spots." This prevents the zinc from adhering during galvanizing, resulting in plating leaks.
Oxide scale is not completely removed, weakening the bond between the oxide scale and the substrate after annealing. During galvanizing, zinc slag forms as the zinc solution flows, leading to zinc particle defects.
2.Where does the problem of poor substrate shape manifest itself?
Key process points: Flatness of the cold-rolled substrate (e.g., wavy or camber).
Problem manifestations: Edge or center wavy strips result in uneven heating when entering the annealing furnace, which can easily cause localized wrinkles.
Poor strip shape leads to uneven tension distribution during strip operation in the mill, inconsistent distance from the air knife during galvanizing, and uneven zinc layer thickness.
3.What problems will be caused by improper control of annealing temperature/time?
Too low a temperature or insufficient insulation: Cold rolling stresses on the substrate are not completely eliminated, resulting in high hardness and insufficient elongation, making subsequent processing (such as stamping) prone to cracking.
Too high a temperature or excessive insulation: Coarse grains on the substrate lead to decreased mechanical properties (such as reduced tensile strength) and a thick oxide layer easily forms on the surface (if the atmosphere is poorly controlled).
4.What problems will an abnormal annealing atmosphere cause?
Key process points: Hydrogen purity (must be ≥99.9%), atmosphere dew point (must be ≤-40°C to avoid moisture-induced oxidation), and oxygen content (must be ≤10 ppm).
Problem manifestations:
Excessive dew point (excessive moisture) or excessive oxygen content: Oxidation forms a FeO/Fe₃O₄ film on the substrate surface. During zinc plating, the zinc solution cannot penetrate the oxidized area, resulting in plating skips or zinc layer shedding.
Insufficient hydrogen content: The reducing power is weak, unable to remove trace oxide scale remaining on the substrate surface, affecting the adhesion of the zinc layer.
5.What problems can strip deviation or tension fluctuations cause?
Key process points: Parallelism of the guide rollers in the annealing furnace and tension control accuracy.
Problem manifestations:
Strip deviation causes the edge of the strip to rub against the furnace wall, resulting in "edge burn" or scratches.
Excessive tension causes the strip to deform or even break. Excessive tension causes the strip to sag, resulting in uneven contact with the rollers and prone to roller marks.