What is it?

This defect appears on hot-rolled steel. It looks like a wavy surface under the scale layer, and is usually found by using a polish or scrubber to remove the scale layer and highlight the surface. It is also called roll pattern and wavy line texture. The cause of this defect is as a result of a failure point on a finishing mill-stand work-roll, usually due to excessive heat (cobble in mill stands stays in contact with the roll during clean up) or excessive wear (roll near the end of its cycle and needs reground in the roll shop). The pattern that develops in the failure point is imprinted on the hot coil surface during rolling and repeats in a pattern like a roll mark.

Firecrack