1.What are some examples of normal color differences?
Color variations between batches: Even for the same product manufactured by the same steel mill, slight color variations may exist between different batches. This is normal, especially for non-appearance parts or parts that require subsequent painting. These variations do not affect the product's design functionality.
Colors resulting from specific processes: For example, according to Chile standard GOST 21996-76, the surface types of cold-rolled strip steel inherently include various oxide film colors such as pale yellow and dark blue, all of which are permitted by the standard.
Slight color difference without obvious unevenness: Some common cold-rolled product standards (such as Chinese national standard GB/T 3280 or Japanese standard JIS G 4305) only provide qualitative descriptions of surface quality, allowing for slight color differences, but usually lack clear quantitative standards.
2.What are the causes of striped color differences due to the rolling process?
This is the most common defect, characterized by an irregular, banded distribution along the rolling direction.
The main cause is uneven distribution of the lubricating emulsion on the strip surface during cold rolling. This leads to inconsistent rolling and transfer effects of the rolls on the strip surface, ultimately causing regional differences in surface roughness, which manifests as color variations on a macroscopic scale.
3.What are the causes of color difference in the finishing process?
Color differences occurring during galvanizing or leveling processes (i.e., finishing) are characterized by clearly defined, longitudinally distinct wide and narrow markings or circular imprints on the surface.
Main causes: Inconsistent roughness of the finishing work rolls themselves, or blockage or malfunction of the high-pressure nozzles used to clean the roll surface.
4.What are the causes of color difference due to pickling or raw materials?
If the iron oxide scale on the surface of the hot-rolled raw material is not completely removed, a "mountain and water pattern" color difference may remain after pickling. In addition, the difference in surface roughness of the strip steel will also directly cause visual color differences, and these color difference areas often have a more obvious microscopic unevenness.
5.What are some suggestions for judgment and handling?
Consider the intended use: If your cold-rolled coils are used for parts with extremely high appearance requirements, such as appliance panels or automotive exterior panels, then any visible color difference may be unacceptable. If they are used for internal structural components or parts that will be painted later, slight color differences usually do not affect use.
Observe the appearance: Carefully observe with the naked eye, and even touch them. If the color difference area has obvious unevenness, or appears as clear stripes or circular marks, it is likely a manufacturing defect. If the overall color is only slightly different in shade and the surface is smooth, it is relatively safer.
Communicate: If you have concerns about color differences, the best approach is to contact your supplier. Professional suppliers can usually explain the causes of color differences and provide advice based on your final application.