热镀锌表面缺陷

Surface Defects on Hot-Dip Metal Coated

Steel Sheet

1. Shell

Definition and appearance

Shell is irregular, flaky overlapping material permeated with nonmetallic inclusions. These laps appear in various forms and vary in size. They have an irregular perimeter and are separated from the base material by nonmetallic or oxidic inclusions, or scale.

The shell is covered by the metallic coating. The structure of the metallic coating is marred by shell and is characterized by an uneven appearance. Occurrence

Shell is caused by the formation of relatively large clusters of nonmetallic inclusions during the continuous casting process, which are located just under the surface and become exposed during deformation. The occurrence of shell may also be attributable to mould and scarfing

defects.

Identification

Shell can be easily identified with the naked eye.

Possibility of mistaken identity

There is little possibility of this defect being mistakenly identified or confused with some other flaw.

2. Rokes/Seams

Definition and appearance

Rokes/seams in the cold rolled strip are narrow longitudinal marks which run in the rolling direction and may, in many cases, be an extension of shell. They take the form of long, narrow bands or lines running parallel to the rolling direction which may be lighter or darker than the steel surface. Rokes/seams contain nonmetallic and/or oxidic inclusions which may penetrate to the surface of be located just underneath the surface. The structure of the metallic coating is marred by rokes/seams and is characterized by an uneven appearance.

Occurrence

The occurrence of rokes/seams may be attributable to the following phenomena:

--Nonmetallic inclusions located close to the surface which may be broken or exposed and become visible as a result of the coating process or of the deformation loading.

--Small surface defects contained originally in the starting material and lying parallel to the rolling direction.

--The formation of Al segregations in steels which are aluminium-killed as part of the ladle metallurgy.

Identification

Rokes/seams can usually be identified with the naked eye.

Covered rokes/seams only become visible in many cases after the material has been processed.

Lightly ground or polished surfaces reveal nonmetallic inclusions in the region of the defect. Since these inclusions are much more brittle than the surrounding metal, they become fractured during deformation and are therefore only found at certain places within the rokes/seams.

Possibility of mistaken identity

Many forms of rokes/seams can be confused with scale, scale pits/scabs, hot strip, pickling and cold strip scratches running parallel to the rolling direction.