The main adverse effects of surface defects on subsequent processing are as follows：
1) Affect the aesthetics of the final product. Some products processed with cold-rolled strip steel, such as automobile outer plate and home appliance panel, pay great attention to the appearance. If the surface defects can not be covered up in the subsequent processing, they will directly affect the beauty of the final products and make it difficult for users to accept.
2) The coating property of reprocessing is affected. In order to enhance the corrosion resistance and aesthetics of cold-rolled strips, the surface of the cold-rolled strip is often coated with various metal or organic materials, such as zinc plating, color coating, film coating, etc. Surface defects directly affect the coating property and the quality of processed products, resulting in the failure to process or degradation of products.
3) It affects the deep drawing property of reprocessing. Surface defects in deep drawing, shrinkage, and other follow-up processing are easy to cause cracking, perforation, and so on, which directly cause scrap.
The types of surface defects in cold rolling mainly include inclusions, holes and hot scratches (as shown in the figure). The defects such as inclusions and holes are brought by hot rolling raw materials and gradually exposed in the cold rolling process.
Causes of defects in hot rolled raw materials
Inclusion is the existence of spot, block, strip, or willow leaf impurities on the surface of strip steel. It is a kind of defect that refractory material or continuous casting mold powder enter into the steel water during steel casting, and gradually expose to the surface of strip steel in the subsequent rolling process.
The hole is mainly due to the inclusion, bubble, and shrinkage cavity existing in the slab during the casting process. In the subsequent rolling process, with the thinning of the thickness, the local fracture of the strip steel occurs, or the non-metallic inclusions in the matrix crack during the rolling process due to poor ductility and finally form holes on the strip.
Strictly control the quality of incoming raw materials
The defects such as inclusions and holes have been produced in the previous process. These defects are hereditary and can not be eliminated in the cold rolling process, and will be further exposed. In serious cases, the strip will be broken. Therefore, the quality of raw materials must be controlled by the source. The quality of hot-rolled raw materials should be checked and accepted in the process of material receiving; when feeding, the defects of raw materials should be judged before the walking beam, and there should be no serious defects harmful to the surface quality of the hot-rolled strip. If an unqualified hot-rolled coil is found, it is forbidden to enter the production line. At the same time, these defects will be fed back to the previous process, communication, and coordination, take measures to reduce the relevant defects of the hot rolling coil.
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