What are the causes of powdering defects during annealing of cold-rolled coils?
A: 1. Excessive rolling or coiling tension leads to poor circulation of the protective gas, causing thermal resistance. 2. Poor sheet shape, such as edge waviness or center waviness, easily leads to powdering during annealing. 3. Impurities in the emulsion, with impurities remaining between the steel sheets, can cause powdering if not thoroughly removed.
How do oxidation color defects occur in cold-rolled coils?
A: Inadequate sealing or leakage of the protective cover during annealing; high-temperature exit of the steel coil from the bell-type furnace after annealing (temperature greater than 110℃); excessively high dew point of the protective gas, excessively low hydrogen content, and insufficient pre-purging time before heating, leaving residual oxygen in the furnace. All these factors can lead to oxidation of different colors on the surface of the steel coil.
What are the manifestations and causes of emulsion spot defects?
A: These manifest as irregular or small island-like black or brown patterns on the surface of the annealed steel sheet. The causes are: the emulsion at the mill exit was not completely blown away, leading to carbonization during heating; insufficient pressure of the blower at the last stand exit; the blower not being turned on during threading or being shut off during tail-end rolling; insufficient pre-blowing time after loading into the furnace, resulting in carbonization of the emulsion during heating.
What are the types of adhesion defects in cold-rolled coils? What are their causes?
Answer: The types are spot adhesion, strip adhesion, and surface adhesion. Spot adhesion, after being smoothed out, produces dot-shaped arc-shaped creases; strip adhesion presents as rows of arc-shaped creases; and severe surface adhesion can cause tearing when uncoiling. Causes include excessive or fluctuating coiling tension, insufficient surface roughness of the strip, excessively soft steel, excessively high annealing furnace temperature, or excessively rapid cooling.
What are the characteristics of pressed-in iron oxide scale defects in cold-rolled coils?
Answer: The defects appear as dot-shaped, strip-shaped, or fish-scale-like black spots, with uneven distribution area and indentation depth. This type of iron oxide scale is difficult to remove during pickling and will form pits on the surface of the cold-rolled coil when it falls off.