Black spot is a defect which is not easy or unable to find during the production of cold-rolled plate. It will impact the surface quality of follow-up processing. To study black spot has positive effects on improving product quality of cold-rolled galvanized steel roofing sheet. Three common black spots are introduced below by CAMELSTEEL.
1. This kind of black spots mostly distributes in the 1/4 or 3/4 part of steel belt in width direction and presents a strip following rolling direction. During uncoiling, black spots appear in cycles. Yet, cycles gradually decrease from outer ring to inner ring and correspond with the diameter of uncoiled steel coil. Black spot disappears until the galvanized steel coil opens to a certain level.
2. This kind of black spots can not be seen after uncoiling and will appear after deep processing. Most of black spots distribute in the surface of parts after processing.
3. This kind of black spots mostly distribute in the edge of steel belt in width direction, presenting the shape of water drop following the direction of rolling of steel belt. Also, abrasion mark can be seen in the plate surface.