1.What are the manifestations of abnormal zinc liquid composition and temperature?
Key process parameters include: zinc melt temperature (usually 450-460°C), aluminum content (0.15-0.25% to ensure bonding between the zinc layer and the substrate), and iron content (≤0.03% to avoid excessive zinc slag).
Problem manifestations:
Insufficient aluminum content: A continuous Fe-Al alloy transition layer cannot be formed, resulting in poor bonding between the zinc layer and the substrate and easy detachment during processing.
Excessive iron content: A large amount of Fe-Zn alloy slag (such as δ phase and ζ phase) is generated, adhering to the strip surface as zinc particles.
Excessive zinc temperature (>470°C): The zinc melt is too fluid, making air knife control difficult and prone to flow marks.
Excessive zinc temperature (<440°C): The zinc melt is poorly fluid, and the zinc layer is prone to "cold shuts" (local discontinuities).
2.What are the symptoms of out-of-control air knife parameters?
Key process points: air knife pressure (usually 0.1-0.3 MPa), air knife-strip distance (10-30 mm), and air knife angle (perpendicular or slightly inclined to the strip).
Problem manifestations:
Uneven pressure (e.g., pressure difference >5% across the strip): Large variations in zinc coating thickness across the strip width (thicker or thinner at the edges);
Excessive distance or insufficient pressure: The zinc solution is not blown flat, resulting in sagging or an excessively thick zinc coating;
Excessive distance or excessive pressure: Partial zinc coating is blown through, causing plating defects;
Incorrect air knife angle: The zinc solution is blown in the opposite direction of the strip, causing zinc buildup or scratches.
3.What are the symptoms of sinking roller/stabilizing roller problems?
Key process points: Roller surface finish, rotational flexibility, and slagging.
Problem manifestations:
Roller surface slagging (zinc slag adhesion): When the strip contacts the rollers, slag particles are pressed into the zinc layer, forming indentations or zinc particles.
Roller table stalling or jamming: Friction between the strip and the roller surface causes scratches, forming continuous scratches along the length of the strip.
4.What are the symptoms of strip steel running speed fluctuation problems?
Key process point: Speed stability (usually 100-200 m/min).
Problem manifestations:
Sudden speed increase: Short zinc deposition time, resulting in a thin zinc layer; sudden speed decrease: Thick zinc layer, prone to flow marks;
Mismatch between speed and air knife pressure: Disrupted zinc layer thickness uniformity.
5.What are the symptoms of improper passivation treatment?
Key process points: passivation solution concentration (e.g., chromate concentration), pH value, treatment time, and drying temperature.
Problem manifestations:
Too low a concentration or insufficient treatment time: The passivation film is too thin, resulting in poor corrosion resistance and prone to white rust.
Too high a concentration or excessive drying temperature: The passivation film cracks and yellows, affecting its appearance.
Abnormal pH value: The passivation film has poor adhesion and is prone to detachment.