EP2585236B1 - Method for straightening by stretch-bending - Google Patents

Description

The invention relates to a method for stretch-bend straightening metal strips, with several straightening rollers arranged one behind the other in the (horizontal or essentially horizontal) direction of passage of the strip and spaced apart in the direction of passage of the strip, wherein the strip, which is under a tensile stress below the elastic limit, is bent alternately around the straightening rollers and thereby undergoes plastic stretching is experienced, wherein one or more straightening rollers that can be adjusted relative to the belt are each preceded by a deflection roller which has a larger diameter than the straightening roller arranged downstream in each case.

During stretch bending straightening in a stretch bending straightening system or a stretch bending straightening stand, the strip is usually under a tensile stress below the elastic limit or yield point and the strip is alternately bent around the straightening rollers in the plastic or elastic-plastic range. The plastic or elastic-plastic straightening rollers are also referred to as stretching rollers. The amount by which the tape (overall) is plastically stretched and consequently lengthened is referred to as the degree of stretching.

With such a stretch bending straightening system, non-flat metal strips can be straightened and consequently non-flatness can be eliminated. Non-flatness means, for example, sliver waviness and/or sliver sagging, which result from differences in the length of the sliver fibers in the sliver plane. However, unevenness also means strip curvatures in the longitudinal and/or transverse direction, which are caused by bending moments in the strip, for example if the strip was bent elastically-plastically around deflection rollers or due to elastic-plastic deformations when the strip is wound up. Longitudinal curvatures will also referred to as a coilset, transverse curvature as a crossbow. In the course of stretch-bend straightening, the unplanar strip is bent around rollers with a sufficiently small diameter (alternately) under a tensile stress that is below the elastic limit R _E or the technical elastic limit R _p0.01 of the strip material, so that the superimposition of the tensile stress with the Bending an elastic / plastic deformation is generated in the band. The tape is stretched plastically, with the amount of plastic stretching being referred to as the degree of stretching. During plastic elongation, the originally short ligament fibers are elongated to a relatively greater extent. Ideally, all ribbon fibers have the same length after straightening, so that an ideally straightened ribbon should result, free from waviness or ribbon sagging. In practice, this is not always completely successful, so that small residual imperfections (e.g. center waves or edge waves) can remain in the strip. In addition, residual bending moments are induced in the strip due to the alternating bending, which can lead to undesirable plastic residual curvatures after straightening in the longitudinal direction (coilset) or transverse direction (crossbow). The residual bending moments can generally be minimized by suitably matching the bending intensities on the individual rollers. To this end, it has already been proposed to adjust the geometry of the stretch bending straightening stand in a wide variety of ways in order to achieve the best possible straightening results.

In addition, it is basically known from practice to provide deflection rollers before, between and/or after the straightening rollers, which have a significantly larger diameter than the straightening rollers and usually only have an elastic effect.

So you know, for example, from the EP 0 298 852 B1 (or. DE 38 85 019 T2 ) a device for straightening a metallic strip, in which between an inlet idler set and an outlet idler set several Stretch bending straightening rollers are arranged. Deflection rollers can be provided at a wide variety of points in the system, which primarily serve to guide the strip along a specific path as it runs through the system.

Similarly, the technical article " Benefits of a new leveler technology for packaging steels: Multi-Roller Tension Leveller" (Emmanuel Dechassey, Irsid, Arcelor group, METEC Congress June 2003 ) a stretch leveler with four straightening rolls. The overlap of the first straightening unit (first and second straightening roll) and the overlap of the second straightening unit (third and fourth roll) can be adjusted. In turn, several deflection rollers with a larger diameter are integrated into the system.

The stretch-bend straightening methods known from practice, in which elastically acting deflection rollers may also be provided, have basically proven their worth, but they can be further developed.

A method for stretch bending is otherwise from the JP 59-137121 , on which the preamble of claim 1 is based, known.

A sensor is provided with which vibrations in the strip can be detected, so that the roller positions can be adjusted as a function of the measurement signal.

Finally, in the DE 197 08 488 A1 describes a special method for bend straightening metal strip, in which a straightening roller is arranged in the gusset between two guide rollers and the strip wraps around the straightening roller in a form-fitting manner between two contact lines, along which the guide rollers are in indirect contact with the straightening roller via the strip.

The invention is therefore based on the technical problem of creating a method for stretch-bend straightening of the type described at the outset, with which unflatness can be eliminated in a particularly reliable and efficient manner.

To solve this problem, the invention teaches a method for stretch-bend straightening of metal strips with the features of claim 1. It is provided that the straightening roller for varying the immersion depth is adjusted relative to the associated deflection roller in such a way that the free strip length between the point where the strip runs off from the Deflection roller up to the run-on point of the belt on the straightening roller does not exceed a predetermined maximum value, above which longitudinal waviness of the belt develops. It is provided that the straightening roller for varying the immersion depth is adjusted relative to the deflection roller in such a way that the free strip length does not exceed a maximum value of 8% of the (maximum) strip width, preferably 4% of the (maximum) strip width. Thus, taking into account usual strip widths, it can be expedient for the free strip length not to exceed a maximum value of 150 mm, preferably 100 mm, particularly preferably 70 mm.

The invention is initially based on the finding that in a conventional stretch-bend straightening stand in a first plant section with several straightening rollers primarily the degree of stretching is generated and that in a second plant section with further straightening rollers the residual bending moments are essentially eliminated. Furthermore, the invention is based on the finding that it is expedient, at least in the first plant section with the straightening rollers, with which the degree of stretching is primarily generated, to assign deflection rollers to the straightening rollers. Practical investigations and theoretical calculations using the finite element method (FEM) have shown that the straightening result for a given strip depends on the Number and geometric arrangement of straightening rollers among themselves and between straightening rollers and deflection rollers. Larger distances between the straightening rollers, viewed in the direction of strip travel, result in less or completely eliminated residual waviness. As a result, efforts are made in practice to provide the largest possible distances between the individual straightening rollers. In practice, however, these large distances between the individual straightening rollers can lead to the belts running through tending towards (elastic) longitudinal waviness. This longitudinal waviness is also referred to as the "towel effect". This effect occurs in particular with thin strips that are under high tensile stress over a longer distance. If such a longitudinally corrugated strip now runs over a straightening roller, this longitudinal waviness is plastically embossed in the strip, so that the straightening result is negatively influenced. Such an effect occurs mainly on straightening rollers, where the strip is still noticeably stretched plastically. For the last straightening rolls, with which essentially only a partially plastic residual curvature correction is operated, this elastic longitudinal waviness plays no or only a minor role.

The measures according to the invention now make it possible to work with relatively large distances between the straightening rollers without the problems described arising. Because according to the invention, the elastic longitudinal waviness is first eliminated by first guiding the strip over a deflection roller in front of the straightening roller. After the strip has run off from the deflection roller, after a certain free strip length, the strip runs onto the straightening roller downstream of the deflection roller. Investigations have now led to the surprising result that longitudinal waviness in front of the straightening roller can be avoided in a particularly reliable manner if the free strip length between the point at which the belt runs off from the deflection roller to the point at which the belt runs onto the subsequent straightening roller is minimized. Within the scope of the invention, the advantageous larger distances between the straightening rollers can consequently be set up, which lead to low or completely eliminated residual waviness, without the problems with elastic residual waviness that are then to be expected occurring.

As a rule, it is necessary to process strips of different thicknesses, widths and yield points in a stretch leveling system. For this it is necessary to be able to vary the bending intensity of the individual straightening rollers. To do this, the immersion depth or the angle of wrap of the strip around the straightening roller is varied. In the course of this variation, the straightening roller is preferably adjusted at different immersion depths or angles of wrap in such a way that the free belt length does not exceed the maximum value over the entire adjustment range. The stretch leveling system is consequently designed in such a way that the formation of longitudinal waviness is avoided over the entire adjustment area. With a conventional adjustment of the straightening roller vertically or approximately vertically to the strip throughput direction, this is possible when the distance between the straightening roller and the deflection roller is small, namely so small that the free strip length over the entire adjustment area always remains below the maximum value or the maximum value does not fall below. Consequently, it can be expedient to arrange the straightening roller relatively close behind the deflection roller or to arrange the deflection roller relatively close in front of the straightening roller, so that the free belt lengths are minimized even with conventional adjustment. The available setting range for the angle of wrap or the immersion depth is relatively small with this procedure.

In a preferred development, the invention therefore proposes that the straightening roller, in the course of the variations in the immersion depth, essentially along the circumferential direction of the deflection rollers or tangentially to the deflection roller is movable. This ensures that the free strip length is kept to a minimum as the immersion depth varies, even when there is a relatively large distance between the deflection roller and the straightening roller. Such a large distance ensures that the immersion depth or the angle of wrap can be varied over a wide range. Nevertheless, the free strip length is always minimized due to the particularly preferred type of pitch, so that longitudinal waviness can be avoided.

This can be realized, for example, in that the straightening roller is moved linearly in the sense of a linear adjustment on an adjustment path arranged obliquely to the strip throughput direction, specifically preferably relative to a stationary arranged deflection roller. The adjustment on an inclined adjustment path has the advantage over the conventional "vertical" adjustment that the straightening roller moves tangentially along the surface of the deflection roller during the adjustment, so that the free strip length remains relatively small, even with relatively large variations the immersion depth.

Alternatively, there is the possibility of moving the straightening roller in the sense of a pivoting adjustment on an arcuate, for example circular adjustment path, with this arcuate or circular adjustment path surrounding the outer circumference of the deflection roller. In this case, too, the deflection roller can be stationary. Such a pivoting position on an arcuate path of movement surrounding the deflection roller also means that there are only slight differences in the free belt length at different angles of wrap and the free belt length always remains below the desired limit value, above which longitudinal waviness between the deflection roller and straightening roller would occur.

Finally, in an alternative embodiment, the free belt lengths can also be minimized by not only moving the straightening roller relative to the deflection roller, but also moving both the straightening roller and the associated deflection roller in a suitable manner. For example, there is the possibility of moving the straightening roller vertically to the direction of passage of the strip and (at the same time) of moving the deflection roller in the horizontal direction or along the direction of passage of the strip. This combined movement also causes the straightening roller to move relative to the deflection roller on a kind of circumferential path or a tangential path.

Within the scope of the invention, relatively large distances can always be set up between two straightening rollers arranged one after the other, the distance preferably being at least 30%, particularly preferably at least 50% of the (maximum) strip width. Maximum strip width means the maximum strip width of the metal strip that can be processed in a specifically designed stretch bending leveling system.

In the context of the invention, it is also important that the deflection rollers have a significantly larger diameter than the straightening rollers, because the deflection rollers should only have an elastic effect. It is therefore expedient if the diameter of the deflection rollers is at least a factor of 5, preferably at least a factor of 8, larger than the diameter of the respectively associated straightening roller. It can be expedient if the diameter of the deflection roller is about a factor of 10 larger than the diameter of the respectively associated straightening roller.

The diameter of the straightening roller(s) is preferably up to 70 mm, for example up to 50 mm. So the diameter of the straightening roller or rollers for example 15 mm to 70 mm, preferably 25 mm to 50 mm.

The diameter of the deflection roller or deflection rollers is preferably at least 150 mm, particularly preferably at least 250 mm. For example, the diameter of a deflection roller or a plurality of deflection rollers or of all deflection rollers can be 150 mm to 700 mm, preferably 250 mm to 600 mm.

The angle of wrap of the belt around one or more straightening rollers or around the straightening rollers is generally 2° to 45°, preferably 3° to 30°.

In the context of the invention, it is therefore particularly important to have one or more roller combinations, each of which has a deflection roller and an associated straightening roller. Two such roller combinations are preferably provided, particularly preferably three roller combinations. It is basically within the scope of the invention if each straightening roller is actually also assigned an upstream deflection roller. However, it is preferably provided that the roller combinations described, each consisting of a deflection roller and a straightening roller, are provided in a first system section in which at least 75%, preferably at least 95%, of the total plastic elongation of the strip is generated. Because in stretch bending straightening systems, it is common for the main part of the degree of stretching to be generated on the first straightening rollers and consequently in the first plant section, while the remaining, rear straightening rollers mainly eliminate the residual bending moments. According to the invention, it is now provided that the roller combinations described are preferably arranged in the (front) first system section in which at least 75%, preferably at least 95%, of the plastic elongation of the strip is generated will. Accordingly, it can be expedient or sufficient if, in a (rear) second plant section, in which essentially the residual curvatures in the strip after straightening are minimized or set to desired values and usually only up to 25%, preferably only up to up to 5% of the plastic elongation are generated, only one or more straightening rollers are provided, in which case associated deflection rollers can be dispensed with in this area. It is expedient here if the first plant section has two or three roller combinations, each consisting of a straightening roller and an upstream deflection roller with the adjustment options described. The second plant section for the residual curvature correction then has at least two individually position-controlled straightening rollers, it being possible to dispense with deflection rollers in this area. However, it is within the scope of the invention that deflection rollers are also provided in the area of this second plant section.

Alternatively, a multi-roller straightening set can also be arranged downstream of the described roller combinations with a short free belt length.

Fig. 1

Schematisch eine Streckbiegerichtanlage in einer ersten Ausführungsform zur Durchführung des erfindungsgemäßen Verfahrens,

Fig. 2

eine Streckbiegerichtanlage in einer zweiten Ausführungsform,

Fig. 3

eine dritte Ausführungsform der Streckbiegerichtanlage und

Fig. 4

eine vierte Ausführungsform der Streckbiegerichtanlage.

The invention is explained in more detail below with reference to drawings that merely show exemplary embodiments. Show it:

1

Schematic of a stretch leveling system in a first embodiment for carrying out the method according to the invention,

2

a stretch bending straightening system in a second embodiment,

3

a third embodiment of the stretch bending straightening system and

4

a fourth embodiment of the stretch leveling system.

A stretch bending straightening system for stretch bending straightening of metal strips 1 is shown in each of the figures. In its basic structure, such a stretch bending straightening system has an inlet tensioning roller set 2 and an outlet tensioning roller set 3 as well as straightening rollers 5a, 6a, 7a, 8a arranged between these tensioning roller sets 2, 3. The inlet tension roller set 2 with the tension rollers 4 can be designed as a brake roller set, while the outlet tension roller set 3 with the tension rollers 4 can be designed as a traction roller set. The belt running direction D runs horizontally or essentially horizontally.

With these clamping sets 2, 3, a tensile stress is generated in the metal strip 1, but this tensile stress is below the elastic limit of the strip material. The strip is then alternately bent around the straightening rollers 5a to 8a in the plastic or elastic-plastic range and is thereby plastically stretched and consequently lengthened. Each straightening roller 5a, 6a, 7a, 8a can in principle be supported by at least two support rollers in a manner known per se. These are not shown in the figures.

The individual straightening rollers 5a, 6a, 7a and 8a can be adjusted relative to the strip to adapt the process to a wide variety of circumstances, so that the immersion depth of the straightening roller 5a, 6a, 7a, 8a and thus also the angle of wrap of the strip around the straightening roller can be varied can. The figures also show that some of the straightening rollers are assigned deflection rollers 5b, 6b, 7b.

For this purpose, first on 1 Reference is made, which shows, in a schematically simplified manner, a stretch leveling system in a first embodiment. First of all, it can be seen that stretch bending straightening systems with several straightening rollers 5a - 8a usually consist of a first system section A1 and a second system section A2, with several straightening rollers 5a, 6a being provided in the first system section A1, with which the degree of stretching is essentially generated becomes. Several straightening rollers 7a, 8a are also provided in the second plant section A2, with which the residual bending moments are eliminated. In the first system section A1, the deflection rollers 5b and 6b are assigned to the straightening rollers 5a and 6a, with each straightening roller 5a, 6a being preceded by a deflection roller 5b, 6b. The deflection rollers 5b, 6b have a much larger diameter than the straightening rollers 5a, 6a, so that the deflection rollers 5b, 6b essentially only act elastically.

It is fundamentally advantageous if - as also in 1 indicated - relatively large distances are provided between the individual straightening rollers 5a, 6a, 7a, 8a, since these larger distances between the straightening rollers result in lower or completely eliminated residual ripples. However, the problem can arise that the tapes tend to elastic longitudinal waviness at such large distances, the so-called towel effect. This problem occurs in particular with thin strips which are under high tensile stress over a relatively long distance. If a strip that is longitudinally corrugated in this way runs over a straightening roller, this longitudinal waviness is plastically embossed in the strip and has a negative effect on the straightening result. Within the scope of the invention, such longitudinal waviness should now be prevented in front of the straightening roller will. This effect mainly has an effect on the straightening rollers, where the strip is still plastically lengthened noticeably. For the last straightening rolls, with which essentially only a partially plastic residual curvature correction is operated, this elastic longitudinal waviness plays no or only a minor role.

In Fig.1 it can now be seen that the straightening rollers 5a, 6a, which are preceded by a deflection roller, can be adjusted against the strip so that the immersion depth can be varied. In principle, a pivoting position is known from the prior art, which is indicated at the straightening roller 5a, and a vertical position, which is indicated at the straightening roller 6a. It is in 1 recognizable that in the course of the variation of the immersion depth the free belt length F varies between the run-off point 9 of the belt from the deflection roller to the run-on point 10 of the belt onto the straightening roller. The free belt length F can now also be minimized with this conventional adjustment if the straightening roller 5a or 6a is arranged relatively close behind the associated deflection roller 5b or 6b and if the distance A in relation to the belt passage direction D is kept relatively small. Then, even with conventional adjustment of the straightening rollers 5a and 6a, respectively 1 the free path length F is very small, so that the formation of longitudinal waviness is avoided.

the 2 , 3 and 4 show, in contrast, a preferred embodiment in which the free belt length can also be varied over a large adjustment range of the angle of wrap.

For this purpose, the straightening rollers 5a, 6a and 7a to vary the immersion depth are now employed in such a way relative to the associated deflection roller that the free belt length F between the run-off point 9 of the belt from the deflection roller to the Run-on point 10 of the band on the associated straightening roller remains as small as possible and does not exceed a predetermined maximum value. For this purpose, the straightening rollers or some of the straightening rollers can be moved essentially along the circumferential direction of the deflection rollers or tangentially to the deflection rollers in the course of varying the immersion depth.

This can be realized structurally in various ways. In the 2 , 3 , 4 three different options are shown. Drain point 9, run-on point 10 and free tape length F are in the 2 , 3 and 4 not shown explicitly, but the definition follows from 1 and the corresponding explanations.

2 shows an example of the three different options for adjusting the individual straightening rollers.

According to the system in the area of the first straightening roller 5a 2 the possibility is shown of moving the straightening roller 5a in the sense of a pivoting position on an arcuate and, in the exemplary embodiment, circular position path 12. In contrast to the in 1 shown known approach track surrounds the approach track 12 in the embodiment according to 2 the outer circumference of the deflection roller 5b, which can be stationary in the exemplary embodiment. Due to this special configuration of the swivel adjustment, the straightening roller 5a is moved over a relatively wide adjustment path in the course of the variation of the immersion depth in such a way that a minimum free belt length F is always maintained.

At the in 2 illustrated second straightening roller 6a, this straightening roller 6a can be moved linearly in the sense of a linear adjustment. In contrast to the in 1 however, the linear engagement shown for the second roll is no vertical adjustment is realized, but the straightening roller 6a can be moved linearly on an adjustment track 11 running obliquely to the strip throughput direction D. The deflection roller 6b can in turn be stationary. 2 shows that in this way a substantially tangential approach track 11 is realized, so that a minimization of the free strip length F is also maintained over relatively long approach paths.

Based on the third straightening roller 7a according to 2 another possibility can be seen of how a minimum free strip length can be obtained in the course of varying the immersion depth. For this purpose, both the straightening roller 7a and the deflection roller 7b can be moved, specifically in different directions. While the straightening roller 7a can be moved in the sense of a vertical adjustment transversely to the direction D of the strip throughput, the associated deflection roller 7b can be moved in the sense of a horizontal position along the direction of the strip throughput D and consequently horizontally. The free belt lengths F can also be minimized by combined movement.

with the inside 2 Using the different options shown, the free strip lengths can always be kept below a limit value above which longitudinal waviness would occur between the deflection roller and the straightening roller. The towel effect described can consequently be avoided, so that overall optimal flatness results are achieved. This is particularly successful when the bending intensities of the individual straightening rollers are varied and the immersion depth or the angle of wrap of the strip around the straightening roller is varied. Strips of different thicknesses, widths and yield points can therefore be straightened with the stretch-bend straightening systems listed above. Fig.2 is intended to illustrate in particular the various adjustment options, so that the adjustment according to the invention for all straightening rollers 5a, 6a and 7a is shown there.

In practice, however, it is usually sufficient if such an adjustment of the straightening rollers in relation to the deflection rollers in the first plant section A1 is provided according to the invention. For this purpose, the exemplary embodiments according to the Figures 3 and 4 referred.

3 again shows a stretch bending straightening system with four straightening rollers 5a, 6a, 7a and 8a. At least the first two straightening rollers 5a, 6a are each assigned a deflection roller 5b or 6b, which enables adjustment according to the invention.

In the embodiment after 3 the described oblique position is realized. In contrast, a conventional vertical adjustment is provided for the two straightening rollers 7a, 8a in the second plant section A2. Because, as already explained, the problem of longitudinal waviness in front of the straightening roll primarily occurs on the straightening rolls, where the strip is still noticeably lengthened plastically. For the last straightening rollers, with which essentially only a partially plastic residual curvature correction is operated, the elastic longitudinal waviness only plays a subordinate role. In this respect, in the region of the straightening rollers 7a, 8a, the adjustment of the straightening rollers in relation to the deflection rollers according to the invention can be dispensed with. The residual curvature is consequently corrected by at least the two straightening rollers 7a and 8a, which are individually position-controlled. However, it is also within the scope of the invention for deflection rollers to be arranged upstream or downstream of these straightening rollers 7a, 8a.

4 shows an alternative embodiment of a stretch bending straightening system, in which the two straightening rollers 5a and 6a, to which corresponding deflection rollers 5b and 6b are assigned, a multi-roller straightening unit 15 is arranged overall. The residual curvature can be corrected with this multi-roller straightening unit. This multi-roller straightening unit 15 can have stationary lower rollers and adjustable upper rollers. This is in 4 only hinted at.

the 3 and 4 12 also show embodiments in which only the first two straightening rollers 5a, 6a are preceded by corresponding deflection rollers 5b, 6b. However, it can also be expedient to arrange a corresponding deflection roller 7b in front of a third straightening roller 7a. This embodiment is in the 3 and 4 not shown. However, it must be taken into account that with several straightening rolls arranged one behind the other, the degree of stretching is gradually reduced from straightening roll to straightening roll, so that the problem of longitudinal waviness becomes less critical towards the end of the system.

Claims (12)

1. Method for stretch-bend-levelling metal strips (1), having a stretch-bending stretch-bend-levelling installation with a plurality of levelling rollers (5a, 6a, 7a, 8a) arranged one behind the other in the strip running direction (D) and spaced apart in the strip running direction (D),

   wherein the strip (1), which is under a tensile stress below the elasticity limit, is alternately bent around the levelling rollers (5a, 6a, 7a, 8a) and thereby undergoes plastic stretching,

   wherein one or more levelling rollers (5a, 6a, 7a), which can be adjusted relative to the strip (1), are each preceded by a deflection roller (5b, 6b, 7b) which has a larger diameter than the levelling roller arranged downstream in each case, characterised in that

   the levelling roller (5a, 6a, 7a) is adjusted relative to the associated deflection roller (5b, 6b, 7b) for varying the immersion depth in such a way that the free strip length (F) between the run-off point (9) of the strip (1) from the deflection roller up to the run-up point (10) of the strip (1) onto the associated levelling roller always does not exceed a maximum value of 8 % of the strip width over the entire adjustment range in the case of different immersion depths or wrap angles.
2. Method according to claim 1, characterised in that the levelling roller
   (5a, 6a, 7a) for varying the immersion depth relative to the deflection roller (5b, 6b, 7b) is adjusted in such a way that the free strip length (F) does not exceed a maximum value of 4% of the strip width.
3. Method according to one of claims 1 to 2, characterised in that the free strip length (F) does not exceed a maximum value of 150 mm, preferably 100 mm, particularly preferably 70 mm, and indeed preferably over the entire adjustment range of the levelling roller.
4. Method according to one of claims 1 to 3, characterised in that the levelling roller (5a, 6a, 7a) is adjusted vertically or approximately vertically to the strip running direction (D) in order to vary the immersion depth, wherein the distance (A) between the levelling roller (5a, 6a, 7a) and the deflection roller (5b, 6b, 7b) is so small that the free strip length (F) is always below the maximum value or does not exceed it over the entire adjustment range.
5. Method according to any one of claims 1 to 3, characterised in that the levelling roller (5a, 6a, 7a) is moved substantially along the circumferential direction of the deflection roller (5b, 6b, 7b) or tangentially to the deflection roller (5b, 6b, 7b) in the course of varying the immersion depth.
6. Method according to claim 5, characterised in that the levelling roller (5a, 6a, 7a) is moved linearly in the sense of a linear adjustment on an adjustment track (11) arranged obliquely to the strip running direction (D), preferably relative to a stationary deflection roller (5b, 6b, 7b).
7. Method according to claim 5, characterised in that the levelling roller (5a, 6a, 7a) is moved in the sense of a pivoted position on an arcuate, for example circular, adjustment track (12) that surrounds the outer circumference of the preferably stationarily arranged deflection roller (5b, 6b, 7b).
8. Method according to claim 5, characterised in that in the course of varying the immersion depth, both the levelling roller (5a, 6a, 7a) and the associated deflection roller (5b, 6b, 7b) are moved.
9. Method according to claim 8, characterised in that the levelling roller (5a, 6a, 7a) is moved linearly, vertically to the strip running direction and the deflection roller (5b, 6b, 7b) is moved linearly, horizontally to the strip running direction.
10. Method according to one of claims 1 to 9, characterised in that the wrap angle of the strip around the levelling roller is 2° to 45°, preferably 3° to 30°.
11. Method according to one of claims 1 to 10, with one or more roller combinations each consisting of a deflection roller (5b, 6b, 7b) and a levelling roller (5a, 6a, 7a), characterised in that these roller combinations are arranged in a first plant section (Al), for example on the inlet side, in which at least 75% of the total plastic stretching, preferably at least 95% of the total plastic stretching, is produced.
12. Method according to claim 11, characterised in that one or more levelling roller (7a, 8a) are arranged downstream from the roller combinations of deflection roller (5b, 6b, 7b) and levelling roller (5a, 6a, 7a) in each case, wherein these downstream straightening rollers (7a, 8a) are arranged in a second plant section (A2) in which only up to 25%, preferably only up to 5%, of the total plastic stretching is produced in order to correct residual curvatures.

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