The invention relates to a method and a tensioning device for the continuous straightening of metal strip, in particular thin strip. Within the scope of the invention, thin metal strip is understood to be strip with a thickness of up to 1 mm. For very soft materials, such as zinc, aluminum, etc., the thickness of the strip may be as much as 3 mm, however.
Essentially two methods are known for the continuous straightening of metal strip, i.e; stretcher leveling, in which the metal strip is elongated by being pulled between two sets of tensioning rolls, and the stretching-and-bending method using two or more bending rolls between two sets of tensioning rolls.
If the stretcher leveling method is used on wavy metal strips, fibers or particles of different length are plastically elongated to a different extent and therefore strengthened to varying degrees. In this metal strips zones differing in strength cause undesirable residual waviness. In addition, there is the danger that the strip will break and this danger increases with the stretching ratio. For metal strip with strip sabre or strip dimples, stretcher leveling cannot be used because the strip breaks frequently.
In the stretching-and-bending method metal strip is straightened by combined tensile and bending stresses in the area of bending rolls with a small diameter. The tension applied to the strip in this case is considerably lower than in the stretcher leveling method. It is possible to operate with a high stretching ratio without breaking the strip; this simultaneously eliminates the strip sabre. If the stretching-and-bending method is used on thin metal strips which are stretched more than 1%, flaws in the strip middle may occur, whose size increases with the stretching ratio and the strip width. In addition, residual stresses remain in the metal strip which are generated by the bending stresses and which may cause, for example, a transverse curvature, the so-called cross bow. Both straightening methods themselves are not well suited for straightening thin metal strip with low intrinsic stiffness, particularly not for metal strip with a low modulus of elasticity, e.g. aluminum strip with thicknesses between 0.1 and 0.35 mm.