STRETCH-REDUCING MILL HAVING IMPROVED DIAMETER TOLERANCE AND WALL THICKNESS TOLERANCE

Abstract

A stretch-reducing mill (1) for producing seamless pipes (R), which comprises a plurality of roll stands (10), which are arranged one after the other in a conveying direction (F) of the pipes (R) and each have three rolls (11) arranged at an angular distance of 120, wherein the roll stands (10) are divided into at least two groups (A, B) each having at least two roll stands (10); the rolls (11) of adjacent roll stands (10) within a group (A, B) are inclined relative to each other by an intra-group angle I; and the rolls (11) of the roll stands (10) of adjacent groups (A, B) are inclined relative to each other by a group angle G that is less than the intra-group angle I.

Claims

1. Stretch-reducing mill (1) for the production of seamless tubes (R), comprising: plural roll stands (10) arranged one behind the other in a conveying direction (F) of the tubes (R), each of said roll stands having three rolls (11) arranged at an angular distance of 120, wherein the roll stands (10) are divided into at least two groups (A, B) each including at least two roll stands (10), the rolls (11) of adjacent ones of the roll stands (10) within a group (A, B) are inclined relative to one another by an angle I within the group, and wherein the rolls (11) of the roll stands (10) of adjacent groups (A, B) are inclined relative to one another by a group angle G which is smaller than the group-internal angle I.

2. Stretch-reducing mill (1) according to claim 1, characterized in that the group-internal angle I is approximately 60.

3. Stretch reducing mill (1) according to claim 1, characterized in that the group angle G is 60/n, where n is an integer greater than 1.

4. Stretch reducing mill (1) according to claim 3, characterized in that the group angle G is approximately 30.

5. Stretch reducing mill (1) according to claim 1, characterized in that the rolls (11) of the roll stands (10) of the groups (A, B) have a caliber shape deviating from the circular shape.

6. Stretch-reducing mill (1) according to claim 1, characterized in that between two groups (A, B) at least one neutral roll stand (10) is provided, each with three rollers (11) arranged at an angular distance of 120, their caliber shape counteracts a torsional moment acting on the tube (R).

7. Stretch reducing mill (1) according to claim 6, characterized in that the caliber shape of the rollers (11) of the neutral roll stand (10) is in the shape of a segment of a circle.

8. Stretch reducing mill (1) according to claim 1, characterized in that 35% to 70% of the total diameter reduction of the tube (R) takes place in the first group (A) and the remaining diameter reduction in the second group (B).

9. Stretch reducing mill (1) according to claim 1, characterized in that this is an extraction mill.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0019] FIG. 1 is a schematic view of a stretch-reducing mill with grouped rolling stands.

[0020] FIG. 2 shows a rolling stand in three-roll design.

[0021] FIG. 3 shows schematically a group-wise inclination of roll stands.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

[0022] Preferred exemplary embodiments are described below with reference to the Figures. Identical, similar or identically functioning elements are provided with identical reference symbols in the Figures, and a repetitive description of these elements is partially dispensed with in order to avoid redundancies.

[0023] FIG. 1 is a schematic view of a stretch-reducing mill 1. The stretch-reducing mill 1 has a plurality, here fifteen, roll stands 10. The roll stands 10 can preferably be controlled individually. In particular, the speeds of the rolls 11 (see FIG. 2) of the roll stands 10 can be set individually.

[0024] The rolling stands 10 are controlled via a control device 2, preferably computer-based. The control device 2 takes over the control of further components of the stretch-reducing mill 1 if necessary. It should be pointed out that the term control device includes both centralized and decentralized structures for controlling the stretch-reducing mill 1. The control device 2 therefore does not have to be located at the location of the stretch-reducing mill 1 or be part of it. In addition, control tasks, data processing steps, etc. can be distributed to different computing devices, which then fall under the term control device in their entirety. Furthermore, the communication of the control device 2 with the components to be controlled can take place both physically via cable as well as wirelessly.

[0025] To roll a pipe R, the pipe runs in a conveying direction F through the stretch-reducing mill 1. Before entering the stretch-reducing mill 1, the pipe R has an inlet-side wall thickness d1. When exiting the last roll stand 10, the pipe R has a wall thickness d2 and a reduced diameter. The wall thickness d2 is not necessarily reduced compared to the wall thickness d1; rather, depending on the roll speed, it can be smaller, the same, but also larger than the initial wall thickness d1.

[0026] The inlet-side and/or outlet-side wall thickness d1 or d2 can be measured by means of one or more wall thickness measuring devices (not shown). In addition, further process parameters can be measured or otherwise determined, for example the inlet-side and/or outlet-side speed of the pipe R, the inlet-side and/or outlet-side weight of the pipe R, etc. The determined process parameters can be transmitted to the control device 2 to control the rolling process.

[0027] FIG. 2 shows a roll stand 10, 10 with three rolls 11 arranged symmetrically about the pipe R at an angular distance of 120. According to this exemplary embodiment, the rolls 11 each have an arcuate cross section of the roll surface, i.e., a round caliber shape. The caliber base 13 refers to the center of the roll surface, viewed in the cross section of FIG. 2 and in the axial direction of the corresponding roll 11. The two outer ends of the roll surfacealso seen in the cross section of FIG. 2 and in the axial direction of the corresponding roller 11are referred to as the caliber jump 14. The caliber base 13 and the caliber jump 14 are characteristic positions of the roll surface.

[0028] The shape of the caliber preferably deviates from a perfect circular arc. The reason for the deviation from the circular shape is that in this way material can be prevented from entering the gap between adjacent rolls 11more precisely, between the caliber jumps 14 of adjacent rolls 11. Local caliber size reduction and local caliber size increase allows compensation of deviations in the pipe diameter.

[0029] Referring again to FIG. 1, the roll stands 10 are divided into two groups A and B, each of which has seven roll stands 10. The groups A, B are disjoint, i.e., they are arranged sequentially one behind the other and do not overlap or interpenetrate. Within a group A, B, the rolls 11 of the roll stands 10 according to the present exemplary embodiment are inclined relative to one another by an internal group angle ai of 60 or approximately 60, so that the tube sections are alternately rolled in the caliber base 13 or in the caliber jump 14. The number of roll stands 10 per group A, B is selected so as to minimize the formation of the inner polygon. The number of roll stands 10 per group A, B is at least two; preferably in the range from 2 to 8. The number of roll stands 10 can vary from group A to group B.

[0030] The groups A, Bmore precisely, the rolls 11 of the roll stands 10 of two, preferably adjacent groups A, Bare inclined relative to each other by an angle which is referred to herein as the group angle OG. Preferably, OG is equal to or about 30. As a result of the inclination, the inner polygon formation of a group A is superimposed with an inner polygon formation inclined by OG of the following group B. To a certain extent, an inner dodecahedron is generated, which deviates between a maximum wall thickness and a minimum wall thickness significantly less compared to an inner hexagon. The internal geometry of the pipe R is thus approximated to an ideal round.

[0031] The group-related rotation or inclination described above is shown schematically in FIG. 3. The roll stands 10 of groups A, B are shown schematically in FIG. 3. Within a group A, B, the roll stands 10 or their rolls 11 (without reference numerals in FIG. 3) are alternately rotated or tilted by 180, whereby the above-described inclination of 60 is achieved. A rotation of 90 was made between groups A and B, resulting in an inclination of 30. Of course, the two types of inclination can also be achieved directly by rotating about the group angle OG and the group-internal angle ai. The variant shown in FIG. 3, however, has structural advantages, in particular when coupling the rolls 11 to the drives (not shown). More precisely, when using roll stands 10 each with three rolls 11, the offset of 60 results, for example, by tilting the roll stands 10 by 180. As a result, the drives can be arranged on the usual feed-in side and opposite the feed-in side with internal power distribution. This simplifies the installation and maintenance of the drives, since the coupling can take place directly when the scaffolding is pushed in.

[0032] The size of the groups A, B, i.e., the number of roll stands 10 in the respective group A, B, can be made dependent on the dimension to be rolled. Thus, preferably about 35% to 70% of the total diameter reduction takes place in the first group A and the remaining diameter reduction in the second group B. The reason for this distribution is that the inner polygon formation gradually increases and thus there is a risk of overcompensation. In other words, in the case of an unfavorable distribution, the optimal compensation is not downstream of the last roll stand 10, but on an inner roll stand 10.

[0033] FIGS. 1 and 4 also show a roll stand 10, which is referred to herein as a transition pass or neutral roll stand. The neutral roll stand 10 serves to avoid twisting of the pipe R between groups A and B. The cause of any twisting of the pipe R is that in the case of non-circular deformation, a torsional moment can act on the pipe R about its own axis if a group-related inclination of OG is applied. In order to counteract such a tendency to torsion, at least one neutral roll stand 10 is preferably connected between groups A, B. The caliber shape of the rolls 11 of the neutral roll stand 10 is thus determined so as to counteract the tendency to torsion. The rolls 11 of the neutral roll stand 10 preferably have a circular or approximately circular caliber shape, as shown in FIG. 2.

[0034] The group-related inclination about the group angle OG, as described above, results in an improved approximation of the the internal geometry of the tube R to an ideal circular cross-section.

[0035] Another technical effect is that the caliber base 13, which is offset in groups, improves temperature compensation in the event of a temperature gradient in the pipe R. This effect particularly comes to bear in the case of inhomogeneous temperature distribution along the radial direction of the tube R, as is the case in particular in extraction mills. Extracting mills, which are normally located immediately downstream of a stretching mill with a mandrel, serve to separate the tube R from the mandrel. In this process stage, the tube R has a comparatively strong temperature gradient from the outside (cold) to the inside (warm). If the extraction mill is designed as a stretch reducing mill 1, i.e., in addition to the separation of mandrel and tube R, it is also designed for strong deformation of the tube R, the inhomogeneous temperature distribution in the tube R can have a detrimental effect on the rolling result. For this reason, a group-related inclination according to the disclosed exemplary embodiments, is particularly suitable for an extraction mill, in particular an extraction rolling mill designed as a stretch-reducing mill 1.

[0036] As far as applicable, all of the individual features set out in the exemplary embodiments can be combined with one another and/or exchanged without departing from the scope of the invention.

LIST OF REFERENCE NUMERALS

[0037] 1 stretch-reducing mill [0038] 2 control device [0039] 10 roll stand [0040] 10 neutral mill stand [0041] 11 roller [0042] 13 caliber base [0043] 14 caliber jump [0044] A group of roll stands [0045] B group of rolling stands [0046] R pipe [0047] F direction of travel [0048] d1 wall thickness on the inlet side of the pipe [0049] d2 Wall thickness on the outlet side of the pipe Qi Group-internal angle [0050] OG group angle