1.Why does color difference occur?
The mechanism of color difference is relatively complex, but the most direct and core reason usually points to one point: the uneven micro-roughness (Ra) of the steel strip surface. When light shines on areas with different roughness, the reflection effect is different, and the human eye will see differences in color or brightness.
2.What are the main characteristics and core causes of longitudinal color difference (light and dark stripes)?
Parallel stripes distributed along the rolling direction, approximately 5-15 mm wide, are commonly found in ultra-low carbon steel.
1. Uneven surface roughness of hot-rolled sheet: The surface roughness of hot-rolled sheet is inherently uneven, leading to uneven distribution of emulsion and rolling force during cold rolling.
2. Roughness decay of cold rolling rolls: Uneven wear of upstream stand rolls during use transfers to the strip.
3. Improper leveling process: The roughness, peak count, and leveling elongation of the leveling rolls are mismatched with the steel grade.
3.What are the main characteristics and causes of color difference at the edges?
Excessive roughness difference between the edge and center of hot-rolled steel sheets. This is usually related to improper coiling temperature and laminar flow cooling mode in the hot rolling process, resulting in different microstructure and properties between the edge and center.
4.What are the typical characteristics and main causes of watermark/patchy color difference?
It appears as water stains or spots.
Oxidation marks formed after annealing if residual rolling emulsion or water stains on the surface of cold-rolled coils (unannealed cold-rolled coils) are not promptly blown away.
5.What are the characteristics of color differences related to other processes?
It has diverse forms of expression and no fixed form.
1. Uneven rolling pressure: Excessive or insufficient reduction in certain areas leads to differences in surface condition.
2. Poor lubrication: Uneven emulsion concentration or temperature, or the introduction of impurities, results in uneven oil film thickness.
3. Localized roll wear: Prolonged use leads to severe localized wear on the roll surface.