(1) Gas corrosion is a small black spot-like pit on the surface of the strip steel. If the emulsion, oil, salt, dirt and other residues on the surface of the strip are not removed, part of the surface or the entire surface of the strip (if it stays in the furnace for a long time) will be corroded by the gas. At high temperatures, gas corrosion on the surface of strip steel is more serious.
(2) Overheating. When overheated, the surface of the steel strip will be dark brown. Although the iron oxide scale on the surface layer has peeled off, it is not easy to be pickled clean. The reason for this defect is that the metal is heated to a high temperature or stays in the furnace for a long time. Overheating may cause intergranular corrosion.
(3) Underheating. When the strip is underheated, the surface of the strip will have a light gray metallic luster. The iron oxide scale is difficult to remove during pickling, and the strip turns gray after pickling. The reason for underheating is that the heating temperature is low or the strip passes through the furnace too fast.
(4) Gouges. Gouges refer to black dot-like pits on the lower surface of the strip steel, which are easy to see after pickling. This defect is caused by small protrusions on the working surface of the support roller that damages the strip surface. For this reason, the furnace rollers must be polished and replaced regularly.