The present invention concerns a method for levelling strips, the thickness of which is maximum 4 mm, by simultaneous stretching and bending and by help of two levelling rolls only.

Known levelling devices, fig 1, for levelling strips with a small thickness and widths, which can be considerable, is constituted in its central part of a number of zigzag-placed rolls 1 between which the strip 2 is passing and where the surface of the strip 2 is bended in two opposite directions. On the same time the strip 2 is subjected to a stretching in order to move the elastic zone from the centre of the strip at the bending around the levelling rolls 1. The main principle for the levelling of the strip 2 is to bring the material to exceed the yield point 8_S across the whole of its cross section in such a way that partly eventual buckles and others are removed, partly the strip leaving the last levelling roll without being bent. Thus the strip is passing the levelling device, fig 1, under a pull effected by a tractive force of the coiling reel 3 and the braking force of the decoiling reel 4. Breaking rolls 5 are arranged on both sides of the levelling roll assembly to compensate changes in the pulling force in the strip 2 at changing outer diameters of the coils at coiling and decoiling. To get a good result a constant pull is required in the strip 2. The stretching force in the strip usually is adjusted in that way that a specific pulling force of 20-40 % of the yield point is reached in the strip. The diameter of the levelling rolls usually is dimensioned according to the unlevelty of the strip before levelling and the thickness of the strip. Normally the roller diameter is about 100 times the thickness t of the strip, but can of course vary within wide limits. A varying roller diameter in the assembly of levelling rolls also can be found. The drawbacks of conventional roller levelling assemblies is among others that these, because of a great number of levelling rolls, are complicated to adjust to get level strips and rather expensive in manufacturing. Also the plastic area of the strip is utilised into a too high degree because of a great number of bendings, which reduces the area of finishing of the material before reaching the breaking point and also hardens considerably because of the cold working.

Surprisingly it has been found that strips can be levelled by only two levelling rolls if, beside: of an appropiate pull in the strip and an appropiate dimension of the diameter of the breaking rolls, the mutual relation between the roller diameters is adjusted. The invention will be described in the claim enclosed, in the closer description below and by help of the enclosed drawings on which figure 1 schematically shows an ordinary straightening machine and figure 2 a tensile test diagram of the actual strip material.

Initially pieces for tensile test are taken from the strip 2, which is to be levelled, tensile test is made and a diagram for stress and elongation is drawn up in a conventional way with the elongation s as abscissa and the stress a as ordinate, fig 2, (see the Swedish general specification SS 11 21 10). A mainly straight upward slooping line extends from the origin of the coordinates, the elasticity line 7, up to the yield point 8, which after that turns into a more level line, the plasticity line 9. The later line 9 whithout disadvantages can be approximated by a straight line. A radius R1 of the first levelling roll 1 is choosen, which because of earlier experiances from roller levelling devices shall be around or somewhat below 50 times the thickness t of the strip 2. A distance a is pointed out from the origin of coordinates along the abscissa, which distance is equal to the thickness t of the strip divided by the radius R1 of the first roll, and from that point a first parallel line 10 is drawn up against the plasticity line 9, parallel to the elasticity line 7. By that a first surface 11 is constituted in the diagram, fig 2, between the abscissa, the elasticity line 7, the first parallel line 10 and the plasticity line 9. A new second parallel line 12 is drawn to the right of the first surface 11 in that way that a second surface 13 between the abscissa, the first and the second parallel line 10, 12 and the plastcity line 9 becomes equal to the first surface 11. The distance b along the abscissa between the first and the second parallel line 10, 12 is measured. The thickness t of the strip 2 is devided with the later distance b which constitutes the radius R2 of the second levelling roll 1 in the roller levelling device, fig 1. These two rolls are used in a roller levelling device, fig 1, mentioned in the preamble, and the strip 2 is levelled in a usual manner by help of an appropriate pull. By that a level strip 2 is reached.