What is it?
This cold-roll defect usually appears as a dark, brown/black stain on cold-rolled steel. It is also known as a black stain or a carbon stain. This defect happens as a result of various mill solutions that get baked into the surface by the batch anneal. The leftover solution is a result of a mechanical failure of the wiping system. Picture 1: Leftover cold reduction mill solution; Picture 2: leftover temper mill solution; Picture 3: an uneven application of rust prevention oil at temper mill.