Cold Rolling Mill Technology Sharing - The defect of hot rolled material brings adverse effect on cold rolling process

The surface defects of cold-rolled sheet coil have the following adverse effects on post-processing :

1)  The aesthetics of the final product will be affected.  Some products processed with cold rolled strip steel, such as automobile outer plate, home appliance panel, have very strict requirements on product appearance.  If the surface defects can not be covered up in the subsequent processing, it will directly affect the appearance of the final product, which is difficult to be accepted by customers. 

2)  Affect the coating and plating property of reprocessing.  In order to enhance the corrosion resistance and beauty of the strip, the surface of cold rolled strip is often coated with various metal or organic materials, such as zinc plating, color coating, film coating, etc.  Surface defects will directly affect the coating and the quality of processed products, resulting in unable to process or product degradation.   

3)  It will affect the deep drawing of reprocessing.  Surface defects in deep drawing, expansion and contraction and other subsequent processing procedures are easy to cause cracking, perforation and other problems, will directly cause scrap.  

The surface defects of cold rolling mainly include inclusions, holes and thermal scratches (as shown in figure).  Inclusions and holes and other defects of hot rolling raw materials themselves, will gradually expose defects in the process of cold rolling. 

Causes of defects in hot rolling raw materials  

Inclusion refers to spot, block, strip or willow - leaf impurities in local area of strip surface.  It is the defect that refractory material or continuous casting protection slag enters into molten steel during steel making and is gradually exposed to the surface of strip steel during subsequent rolling process.  The main reason for the holes is that the inclusion, bubble and shrinkage holes in the casting slab break partially with the thickness reduction in the subsequent rolling process, or the non-metallic inclusion in the matrix cracks in the rolling process due to poor extensibility, and eventually forms holes in the strip. 

Strictly control the quality of incoming raw materials  

Inclusions and holes and other defects in the previous process have been produced, these defects are genetic, can not be eliminated in the process of cold rolling, and will be further exposed, severe conditions will lead to belt breaking.  Therefore, the quality of raw materials must be controlled from the source.  The quality of hot-rolled raw materials should be checked and accepted in the receiving process.  When feeding, raw material defects should be judged before stepping beam, hot rolled strip surface should not have serious defects harmful to strip surface quality.  If unqualified hot rolled coil is found, it is forbidden to enter the production line.  At the same time, feedback these defects to the previous process, communication and coordination, take measures to reduce the hot rolled coil related defects.