Why Does Hot Rolling Work Roll Peel off?

The spalling form of hot rolling work roll is different from that of fracture surface and support roll. The former mainly occurs in the upstream racks of hot tandem mills, and most of them are small pieces of spalling, resulting in pits or ellipsoidal pits on the surface of the rolls, which are distributed in the range of the roll body in contact with the rolling piece; sometimes, in the case of card steel, large flaking up to several hundred millimeters along the central axis of the roller body appears.

It can be judged that the peeling is caused by contact fatigue from the peeling fracture of the work roll. It also has the process of crack generation and development. The work roll peeling during production is mostly caused by roll surface cracks.

The contact between the work roll and the support roll also produces contact pressure stress and corresponding alternating shear stress. Since the service cycle of the support roll is as long as 2 to 3 weeks, or even more than 4 weeks, while the work roll is only in service for 3 to 6 hours, that is, off the machine for grinding, it is not easy to produce alternating shear stress fatigue cracks. During rolling, the contact width between the support roll and the work roll is less than 20. There is no significant temperature difference in the support roll, but not in the work roll. When the work roll is in contact with high-temperature strip steel, the temperature of the roll surface can rise to 500-600°C; when it comes into contact with cooling water, it is rapidly cooled to 100-150°C. The periodic heating and cooling of the surface of the roll result in a changing temperature field, resulting in significant periodic stress. The surface layer of the roll is affected by thermal fatigue stress. When the thermal stress exceeds the fatigue limit of the material, the surface of the roll will produce small net-like thermal cracks, usually called cracks.

When an accident such as steel jam occurs during rolling, the local temperature of the roll will increase, which will cause thermal stress and structural stress. The significant temperature difference of the roll caused by the cold head, cold tail and cold edge of the rolled piece also produces thermal stress. Both thermal stress and organizational stress are tensile stresses. Under the combined action of the two, the tensile stress exceeds the strength limit of the material, resulting in thermal shock cracks. In the rolling process, when the strip steel is thought to be tailed and rolled, the rolled piece will scratch the roll and may also form a new source of cracks.

In addition, the replaced rolls, especially the rolls on the upstream stand of the wide strip hot strip mill, have cracks on the surface of most rolls, which should be eliminated when the rolls are ground. If the amount of roll grinding is not enough, the cracks are not removed and remain, these cracks will become the fatigue core in the next use.

Surface cracks such as cracks on the surface of the roll, under the action of working stress, residual stress and oxidation caused by cooling, the stress at the crack tip increases sharply and exceeds the allowable stress of the material and expands into the roll.

The work roll of a rolling mill is usually an active roll, and the support roll relies on the friction between the work roll to achieve rotation, so the support roll produces a reverse friction force to the work roll. Due to friction, cracks, etc., when tilted, if the direction of rotation of the roll is opposite to the direction of rotation of the roll, the possibility of expansion through contact with the backing roll is high; and the higher the residual stress, the easier the expansion. When the crack develops into a certain angle with the roller surface or even expands in a direction parallel to the roller surface, it will eventually cause peeling.