The rolled parts are more thicker than the required thickness of metal because of the rolls get deflected by high rolling forces apply. The Elastic deformation of mill takes place on the metal.So some of rolling defects are provided in the metal forming.
The rolling defects are mainly two types
- Surface rolling defects
- Internal structural rolling defects.
They are main category of rolling defects.
Surface defects are provided from impurities and inclusion in the material surface, roll marks, dirt, rust and other cause related to prior treatment and working of metal.
Internal structural defects:
The internal structural defects are
- Wavy edges
- Zipper cracks in center of strip
- Edge cracks
The roll part consists of middle position bend or deflection due to compressive load on work rolls resulting more thicker than end of work piece. The thicker of center implies the edges are plastically elongated than center. This reason to induce the residual stress of compression at the edge of part and tension along with the center line of work piece. The uneven distribution of stress in the work piece to provide center line cracking (zipper cracks) warping or wavy edge of final work metal sheet.
In the case the roll or cover- cambered together the work piece edge thicker than center and residual stress is provided opposite to the insufficient cambering. The tension at edge of work piece. For the resulting work piece produced in such a manner are splitting, edge cracking.
The cracks occurred on edge of hot roll coil due to excess amount of quenching effect. Origion defects are
- Strip edge are cooled with excess water
- Un- flatness leading to water carryover.
The formed sheet metal will adhere to rolled surface and to follow the path of their respective rolls, sheet will shear in the plane and defect is called as alligatoring.