Galvanized steel coils sometimes have quality defects during production, and you should check the surface quality before using them.
Holes
The non-continuous defects on the surface of the steel strip that penetrate the upper and lower surfaces of the steel strip are called “holes”. They are generally located in the middle or edge of the steel strip, and are mostly distributed in a string.
Causes
The holes are caused by the steel quality of steelmaking or scarring on the surface of the plate and strip, foreign matter pressing into the surface, etc., which are torn or detached during the rolling process.
Dezincification
When the steel strip is subjected to external force, the surface zinc layer falls off, exposing the substrate, which is called “dezincification”.
Causes
The annealing furnace leaks air or water or the reduction conditions are not good, the raw materials are severely corroded, and unreduced oxides remain on the surface of the steel strip;
The raw rolling oil remains a lot and is not completely volatilized in the furnace;
The operating conditions are not good, such as high zinc liquid temperature, long zinc immersion time and improper control of aluminum content; the Fe2Al5 intermediate layer is not formed.
Air knife streaks
The wider streaks that are easily visible along the longitudinal direction of the steel strip are called “air knife streaks”. It is characterized by a thicker metal coating.
Causes
The air knife sticks zinc, causing local airflow blockage of the air knife, or local gaps or damage to the air knife, resulting in air knife streaks.
Zinc slag
Small particles or block-shaped irregular particles similar to rice grains distributed on the surface of hot-dip galvanized sheet are called “zinc slag”.
Causes
Bottom slag, suspended slag and floating slag are deposited on the steel strip to form slag particles;
Zinc liquid temperature fluctuations, unclean slag removal, overdue use of sinking rollers, excessive aluminum content, etc.
Roller marks
The groove shape of the sinking roller is printed on the surface of the steel strip, forming longitudinal (oblique) stripes along the rolling direction, which is called “sinking roller marks”.
Uneven zinc flowers
The zinc flowers on the surface of the steel plate are partially too large or too small, which does not meet the normal requirements, which is called “uneven zinc flowers”. They are distributed in blocks or strips, and appear locally or on the entire surface of the steel strip.
Causes
The speed of zinc flower formation is different.
Edge twill
During the cooling process of the steel strip, the feather-like twill formed on the edge is called “edge twill”.
Causes
The zinc layer is not cooled enough. Although the surface zinc layer is cooled, the internal zinc liquid is not fully cooled. The uncooled part of the zinc liquid has a tendency to flow and produce twill.
White rust
The gray-white powder on the metal surface is called “white rust”. White rust can cover the entire surface or appear in irregular fragments.
Causes
The steel strip is in contact with a humid environment for a long time, causing the surface zinc layer to oxidize;
The effect of squeezing or drying after finishing or passivation during the production process is not good;
Water from the outside world drips directly on the surface of the steel plate, and the plate surface is wet after rolling, causing the zinc layer to oxidize
