Shearing occurs when a longitudinal strip of base metal is torn off a bar during rolling. This strip often reattaches as rolling continues, not necessarily to the same bar. Shearing can refer either to the the discontinuity resulting from the detachment or to the subsequent reattachment. There are usually several occurrences of shearing with a single orientation along the bar.–AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
Mill shearing is usually detected visually and appears longer than scabs. also, the surface below the defect is smoother and more uniform than found below scabs.
Excessive rubbing of the steel as it rolls through the mill causes overheating, shearing material off the bar, which is later picked up from mill components on the same or another billet.
Improved guiding, pass design, and better section control can reduce incidents of mill shearing.
- Rolled in material may have come from some source other than the base material.
- If the material which is removed by shearing is not reattached, the remaining gouge in the surface may form other defects upon further rolling.
- Intergranular precipitates or segregation can contribute to mill shearing.
- Adjust mill to reduce sources of friction(al) heating
In my experience, mill shearing presents as and is easily confused with laps and slivers. Confirming that it is a piece of foreign material that has been rolled into the product is easily confirmed with a pair of pliers to remove it. Mill shearing almost always is removable by such means, and will show as two completely separate pieces of material in a micro.