SQUARE TUBE FORMING METHOD AND SQUARE TUBE FORMING DEVICE

Abstract

To enhance forming performance enhanced in square tube forming of forming a round tube into a square tube by passing the round tube through a plurality of roll stands arranged in a raw tube forming direction. To give a high degree of multi-usability to a square tube forming device. To suppress the entire length of a roll stand array. A first flat roll stand 20A and a second flat roll stand 20B of rolling directions orthogonal to each other are arranged alternately in a forming direction to form a roll stand array 10. Each of the roll stands 20A and 20B includes roll gap adjusting means by which a rolling amount is independently settable, and one of the first flat roll stand 20A and the second flat roll stand 20B adjacent to each other is driven to rotate by roll driving means 40. A forming rolling amount distribution is allocated individually as a rolling amount at each of the roll stands 20A and 20B. The forming rolling amount distribution is determined in advance for stands from a first-stage stand to a final-stage stand in response to the outer diameter, thickness, and material of a forming target raw tube and an intended dimension of a square tube as a product.

Claims

1. A square tube forming method of forming a round tube of any diameter as a forming target raw tube into a square tube of any dimension as a product by passing the round tube through a plurality of stages of roll stands arranged in a raw tube forming direction, comprising: using a flat roll stand array in which a first flat roll stand and a second flat roll stand of rolling directions orthogonal to each other are arranged alternately in a forming direction for a tube member and in which a rolling amount at each roll stand is independently settable; allocating a forming rolling amount distribution individually as a rolling amount at each roll stand in the flat roll stand array, the forming rolling amount distribution being determined in advance for the stands from a first-stage stand to a final-stage stand in response to the outer diameter, thickness, and material of the forming target raw tube to be used and an intended dimension of the square tube as a product; and forming into the square tube as a product having the intended dimension by rotationally driving two-direction roll in all the stages of at least one of the first flat roll stand and the second flat roll stand adjacent to each other to apply thrust to the forming target raw tube, and causing the forming target raw tube to be subjected to bending forming using two-direction rolls at each roll stand.

2. The square tube forming method according to claim 1, wherein at one or a plurality of roll stands in a part of the flat roll stand array, thrust is applied to the tube member by forming an arc-like recess at a flat roll, the recess having a larger curvature than a roll contact surface R of the tube member to pass through the one or plurality of roll stands.

3. The square tube forming method according to claim 1, wherein gap adjusting motors in a pair intended for the first flat roll stand and the second flat roll stand in a pair adjacent to each other are arranged to be movable along the flat roll stand array, and are shared between the first flat roll stands and the second flat roll stands in a plurality of pairs in the flat roll stand array.

4.-6. (canceled)

Description

BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1 is a perspective view of a square tube forming device showing an embodiment of the present invention;

[0037] FIG. 2 is a perspective view of the square tube forming device taken from a different angle from which a roll driving mechanism is omitted;

[0038] FIG. 3 is a perspective view of a horizontal roll stand in the square tube forming device;

[0039] FIG. 4 is a perspective view of a vertical roll stand in the square tube forming device;

[0040] FIG. 5 is a perspective view showing one example of a square tube forming method of the present invention and showing a square tube forming process continuously and entirely according to this example;

[0041] FIG. 6 is an explanatory view showing the square tube forming process of forming a raw round tube into a square tube as a product in stages according to the one example by illustrating cross-sectional shapes of the tube at respective stands;

[0042] FIG. 7 is a perspective view showing a different example of the square tube forming method of the present invention and showing a square tube forming process continuously and entirely according to this example; and

[0043] FIG. 8 is an explanatory view showing the square tube forming process of forming a raw round tube into a square tube as a product in stages according to the different example by illustrating cross-sectional shapes of the tube at respective stands.

EMBODIMENTS FOR CARRYING OUT INVENTION

[0044] An embodiment of the present invention will be described below.

[0045] A square tube forming device of the embodiment is a device of forming a round tube as a forming raw member continuously into a square tube by passing the round tube through a plurality of forming roll stands sequentially. The square tube forming device is arranged in a round tube manufacturing line and used for forming a part of a manufactured round tube into a square tube to allow manufacture of both the round tube and the square tube.

[0046] As shown in FIGS. 1 and 2, this square tube forming device includes a roll stand array 10 of a square tubular shape extending long in a tube member forming direction. The square tubular roll stand array 10 has a configuration in which a horizontal roll stand 20A of a square frame-like shape having a small thickness in the tube member forming direction and a vertical roll stand 20B of a square frame-like shape also having a small thickness in the tube member forming direction are arranged alternately on a stand base 30 in the tube member forming direction. A round tube as a forming raw member passes through the roll stand array 10 from a front side toward a back side of FIG. 1. Namely, the front side of FIG. 1 is an upstream side of the roll stand array 10, and the back side of FIG. 1 is a downstream side of the roll stand array 10.

[0047] As shown in FIG. 3, the horizontal roll stand 20A includes: upper and lower horizontal fixed bases 22A, 22A coupled by a total of four vertical rods 21A arranged two on the right and two on the left; upper and lower horizontal movable bases 23A, 23A supported to be vertically movable by the right and left vertical rods 21A between the upper and lower horizontal fixed bases 22A, 22A; and upper and lower horizontal rolls 24A, 24A attached to respective surfaces of the movable bases 23A, 23A facing each other.

[0048] Each of the upper and lower horizontal rolls 24A is a flat roll having an outer diameter substantially constant along its entire length in a center axis direction. A horizontal support shaft supporting the horizontal roll 24A is rotatably supported by brackets 25A, 25A provided in bearings on the opposite sides of the horizontal roll 24A. One end portion of this support shaft projects as an input shaft 24A′ toward one side of the roll stand array 10 and is coupled to a roll driving mechanism 40 as roll driving means arranged on the one side, thereby driving the horizontal roll 24A to rotate (see FIG. 1).

[0049] A roll gap between the upper and lower horizontal rolls 24A, 24A in a pair is adjusted by a mechanical upper jack 26A attached in a downward-pointing position to the upper surface of the upper horizontal fixed base 22A at the center and by a mechanical lower jack 26A attached in an upward-pointing position to the lower surface of the lower horizontal fixed base 22A at the center.

[0050] Specifically, the downward-pointing upper jack 26A has a tip portion coupled to the upper surface of the upper movable base 23A while penetrating the upper horizontal fixed base 22A, and is driven by a horizontal roll gap adjusting motor 50A (see FIGS. 1 and 2) arranged on the one side of the roll stand array 10 through a horizontal input shaft 27A. The upward-pointing lower jack 26A has a tip portion coupled to the lower surface of the lower movable base 23A while penetrating the lower horizontal fixed base 22A, and is driven symmetrically to and synchronously with the upper jack 26A as driving force of the horizontal roll gap adjusting motor 50A (see FIGS. 1 and 2) is transmitted to the lower jack 26A through a gear box 28A attached to a lateral upper surface of the upper horizontal fixed base 22A on the other side, a vertical power transmission shaft 29A, and a gear box 28A attached to a lateral lower surface of the lower horizontal fixed base 22A on the other side.

[0051] The symmetric and synchronous driving using the upper and lower jacks 26A, 26A described above drives the upper and lower movable bases 23A, 23A to move up and down symmetrically, thereby adjusting a roll gap between the upper and lower horizontal rolls 24A, 24A. Namely, a gap adjusting mechanism for the upper and lower horizontal rolls 24A, 24A is configured using the upper and lower jacks 26A, 26A, the horizontal roll gap adjusting motor 50A, and a power transmission mechanism including the input shaft 27A, the upper and lower gear boxes 28A, 28A, and the power transmission shaft 29A.

[0052] As shown in FIG. 4, the vertical roll stand 20B includes: right and left vertical fixed bases 21B, 21B in a pair; two upper and lower horizontal rods 22B, 22B coupling the right and left vertical fixed bases 21B, 21B; right and left movable bases 23B, 23B supported to be horizontally movable by the two upper and lower horizontal rods 22B, 22B between the right and left vertical fixed bases 21B, 21B; and right and left vertical rolls 24B, 24B rotatably supported by the right and left movable bases 23B, 23B respectively.

[0053] The right and left vertical fixed bases 21B, 21B are provided in standing positions symmetrically to each other on opposite end portions of the lower horizontal fixed base 22A of the horizontal roll stand 20A described above (see FIG. 3). Namely, the foregoing lower horizontal fixed base 22A of the horizontal roll stand 20A extends toward the downstream side of the roll stand array 10, and the vertical fixed bases 21B, 21B are attached to this extending portion. In this way, the horizontal roll stand 20A and the vertical roll stand 20B in a pair adjacent to each other form a horizontal and vertical stand pair 20 integrated by the common horizontal fixed base 22A.

[0054] Each of the right and left movable bases 23B includes upper and lower sliders slidably supported by the upper and lower horizontal rods 22B, 22B, and a roll support frame 25B attached between the upper and lower sides. The roll support frame 25B, which further functions as a coupling member between the upper and lower sliders, has a configuration opened inwardly formed by a combination of upper and lower horizontal members and an outer vertical member. The foregoing right and left vertical rolls 24B, 24B are rotatably supported between these upper and lower horizontal members. A roll gap between the right and left vertical rolls 24B, 24B is adjusted by right and left jacks 26B, 26B attached to outer side surfaces of the right and left vertical fixed bases 22B, 22B respectively with the inner sides thereof pointing toward the respective outer side surfaces.

[0055] Specifically, the jack 26B on the one side has a tip portion coupled to the outer side surface of the outer vertical member of the movable base 23B on the one side while penetrating the vertical fixed base 22B on the one side, and is driven by a vertical roll gap adjusting motor 50B (see FIGS. 1 and 2) arranged on the one side of the roll stand array 10 through a vertical input shaft 27B. The jack 26B on the other side has a tip portion coupled to the outer side surface of the outer vertical member of the movable base 24B on the other side while penetrating the vertical fixed base 22B on the other side, and is driven symmetrically to and synchronously with the jack 26B on the one side as driving force of the vertical roll gap adjusting motor 50B (see FIGS. 1 and 2) is transmitted to the jack 26B on the other side through a gear box 28B attached to a lateral portion of the stand base 30 on the one side, a horizontal power transmission shaft 29B, and a gear box 28B attached to a lateral portion of the stand base 30 on the other side.

[0056] The symmetric and synchronous driving using the right and left jacks 26B, 26B described above moves the right and left movable bases 23B, 23B horizontally and symmetrically to each other, thereby adjusting a gap between the right and left vertical rolls 24B, 24B. Namely, a gap adjusting mechanism for the right and left vertical rolls 24B, 24B is configured using the right and left jacks 26B, 26B, the vertical roll gap adjusting motor 50B, and a power transmission mechanism including the input shaft 27B, the right and left gear boxes 28B, 28B, and the power transmission shaft 29B.

[0057] With the horizontal roll stand 20A and the vertical roll stand 20B in a pair having the foregoing configurations defined as the horizontal and vertical stand pair 20, a plurality of such pairs is arranged on the stand base 30 in the tube member forming direction to form the roll stand array 10 of the square tube forming device. More specifically, the roll stand array 10 is composed of 11 stand pairs 20 (23 roll stands in total including the horizontal roll stand 20A in a final stage).

[0058] As described above, in this square tube forming device, a gap between the horizontal rolls 24A, 24A in the horizontal roll stand 20A is adjusted by the horizontal roll gap adjusting motor 50A, and a gap between the vertical rolls 24B, 24B in the vertical roll stand 20B is adjusted by the vertical roll gap adjusting motor 50B. The horizontal roll gap adjusting motor 50A and the vertical roll gap adjusting motor 50B are provided only in one pair for one horizontal and vertical stand pair 20, and the horizontal roll gap adjusting motor 50A and the vertical roll gap adjusting motor 50B in one pair moves along the roll stand array 10 (here, in a self-propelled manner) to adjust gaps between the horizontal rolls 24A, 24A and gaps between the vertical rolls 24B, 24B of the 12 horizontal and vertical stand pairs 20 sequentially.

[0059] Specifically, as shown in FIGS. 1 and 2, particularly in FIG. 2, a support table 51 is provided on the one side of the roll stand array 10, particularly at the top thereof while extending along the entire length of the roll stand array 10. The support table 51 is supported horizontally by a stay 52, etc. extending from the top of the stand pair 20 toward the one side. The horizontal roll gap adjusting motor 50A and the vertical roll gap adjusting motor 50B in a pair are coupled to each other and in this state, are movable on the support table 51 along the roll stand array 10. The horizontal roll gap adjusting motor 50A and the vertical roll gap adjusting motor 50B in a pair stop at positions corresponding to the 12 horizontal and vertical stand pairs 20 and are coupled to the horizontal input shaft 27A in the horizontal roll stand 20A and to the vertical input shaft 27A in the vertical roll stand 20B at each of the stopping positions, thereby adjusting a gap between the horizontal rolls 24A, 24A in the horizontal roll stand 20A and a gap between the vertical rolls 24B, 24B in the vertical roll stand 20B.

[0060] This operation is performed on each of all the horizontal and vertical stand pairs 20 to adjust the gaps between the horizontal rolls 24A, 24A and the gaps between the vertical rolls 24B, 24B in all the respective horizontal and vertical stand pairs 20 independently of each other. Here, 53 is a flexible cable for feeding electricity to the movable horizontal roll gap adjusting motor 50A and vertical roll gap adjusting motor 50B.

[0061] As described above, in this square tube forming device, the horizontal rolls 24A, 24A in the horizontal roll stand 20A are driven to rotate by the roll driving mechanism 40 arranged on the one side of the roll stand array 10. The roll driving mechanism 40 mentioned herein is divided into three driving units 41. Each of the driving units 41 is coupled to four horizontal roll stands 20A in four stand pairs 20 adjacent to each other in the roll stand array 10, thereby driving the upper and lower horizontal rolls 24A, 24A in each of the four horizontal roll stands 20A to rotate.

[0062] Specifically, the roll driving mechanism 40 includes a driving motor 42 for driving each driving unit 41, and includes upper and lower output shafts 43, 43 corresponding to each of the four horizontal roll stands 20A and coupled to the input shafts 24A′, 24A′ of the upper and lower horizontal rolls 24A, 24A in each of the four horizontal roll stands 20A. By doing so, with the four horizontal roll stands 20A defined as one set, the upper and lower horizontal rolls 24A, 24A in each of the horizontal roll stands 20A are driven to rotate.

[0063] In the square tube forming device of the embodiment, in some of the roll stands in the roll stand array 10, here, some of the roll stands on the upstream side, more specifically, two horizontal and vertical stand pairs 20, namely, two horizontal roll stands 20A and two vertical roll stands 20B viewed from the most upstream position, extremely shallow arc-like recesses having larger curvatures than an outer surface R of the tube member to pass through between rolls are formed at the outer circumferential surfaces of the horizontal rolls 24A, 24A in each horizontal roll stand 20A and at the outer circumferential surfaces of the vertical rolls 24B, 24B in each vertical roll stand 20B.

[0064] A method of forming a round tube into a square tube using the square tube forming device of the embodiment will be described next as a square tube forming method of the embodiment.

[0065] In response to the dimension (outer diameter, thickness) and material of the round tube as a forming raw member, the dimension of the square tube as a product, etc., in a plurality of (here, 12) horizontal and vertical stand pairs 20 (namely, combinations of the horizontal roll stands 20A and the vertical roll stands 20B) in the roll stand array 10, a gap between the upper and lower horizontal rolls 24A, 24A in the horizontal roll stand 20A and a gap between the right and left vertical rolls 24B, 24B in the vertical roll stand 20B are adjusted for each stand pair 20 in order from the upstream side toward the downstream side of the roll stand array 10, for example. As described above, this adjustment is made using the movable horizontal roll gap adjusting motor 50A and vertical roll gap adjusting motor 50B in combination.

[0066] After the gap between the horizontal rolls 24A, 24A in the horizontal roll stand 20A and the gap between the vertical rolls 24B, 24B in the vertical roll stand 20B are adjusted in each of all the stand pairs 20, the roll driving mechanism 40 drives only the horizontal rolls 24A, 24A in the horizontal roll stand 20A in each of all the stand pairs 20 to rotate.

[0067] In this state, the round tube as a forming raw member is passed through the roll stand array 10. This tube member is passed through the stand pairs 20 in the roll stand array 10 (namely, combinations of the horizontal roll stands 20A and the vertical roll stands 20B) sequentially to be formed from the round tube into the square tube.

[0068] The roll stand array 10 is configured using the horizontal roll stand 20A and the vertical roll stand 20B arranged alternately. Further, the horizontal rolls 24A, 24A in the horizontal roll stand 20A and the vertical rolls 24B, 24B in the vertical roll stand 20B are both flat rolls having outer diameters substantially constant along their entire lengths in the center axis direction. These realize not only reduction in a distance between adjacent stands but also ease of avoidance of interference between rolls in the adjacent stands. This makes a roll gap in each stand independently adjustable in a wide range. Moreover, even with a reduced gap between the adjacent stands, it is still possible to use a flat roll of a relatively large diameter to avoid winding of the tube member around the roll and avoid increase in invasion resistance to be cause by such winding.

[0069] As a result, it becomes possible to increase a rolling amount at a particular position in the roll stand array 10 such as a position corresponding to some of stands on the upstream side, and to reduce a rolling amount gradually in the other positions, for example. In such a way, a rolling amount distribution oriented to forming performance can be set in consideration of a springback amount to be changed by the outer diameter, material, or thickness of a forming raw member, for example.

[0070] Additionally, a tube member invasion resistance is inherently controlled low in the roll stand array 10, and the horizontal rolls 24A, 24A are driven to rotate in each of all the horizontal roll stands 20A to apply thrust to the entire length of a tube member to pass through the roll stand array 10. Thus, even if a rolling amount at a particular position in the roll stand array 10 is increased to cause large springback correspondingly, the tube member is still passed smoothly through the roll stand array 10.

[0071] In particular, in the square tube forming device of the embodiment, in some of stands on the upstream side (while these stands are two horizontal and vertical stand pairs 20, the number of such pairs may be greater), the extremely shallow arc-like recesses having larger curvatures than the roll contact surface R of the tube member to pass through between rolls are formed at the outer circumferential surfaces of the horizontal rolls 24A, 24A in each horizontal roll stand 20A and at the outer circumferential surfaces of the vertical rolls 24B, 24B in each vertical roll stand 20B. As a result, particularly large thrust is applied to the tube member while the tube member passes through the stands.

[0072] As a result of the provision of the foregoing arc-like recesses at the horizontal rolls 24A, 24A in the horizontal roll stand 20A and at the vertical rolls 24B, 24B in the vertical roll stand 20B on the upstream side of the roll stand array 10, force of pulling the tube member into the roll stand array 10 is increased to allow smoother passage of the tube member.

[0073] The tube member to pass through the roll stand array 10 is subjected to bending forming applied from two directions using the flat horizontal rolls 24A, 24A and the flat vertical rolls 24B, 24B to form a corner portion. This functions, in addition to the smooth passage of the tube member, to enhance forming performance, thereby manufacturing a high-quality square tube. More specifically, the square tube having a predetermined sectional curvature is manufactured in the absence of a thickness reduction at the corner portion. The absence of thickness reduction at the corner portion eliminates a need to assume the occurrence of thickness reduction in the dimension of a forming raw tube, thereby achieving the effect of reducing the diameter of a base tube or the effect of giving substantially equal thicknesses to a round tube and a square tube to be manufactured.

[0074] Additionally, in the square tube forming device of the embodiment, the horizontal roll stand 20A and the vertical roll stand 20B are arranged alternately to suppress interference between rolls in adjacent stands. Further, only the horizontal rolls 24A, 24A in the horizontal roll stand 20A are driven to rotate while the vertical rolls 24B, 24B in the vertical roll stand 20B are free rollers. Thus, interference between members in the adjacent stands is suppressed to a greater extent. As a result, the entire length of the roll stand array 10 is controlled to a short length to facilitate retention of space for arrangement of the device in a tube manufacturing factory. Moreover, the weight of the device is reduced to encourage reduction in manufacturing cost.

[0075] Further, a gap between rolls in the horizontal roll stand 20A and a gap between rolls in the vertical roll stand 20B are adjusted for each horizontal and vertical stand pair 20 using a pair of the movable horizontal roll gap adjusting motor 50A and vertical roll gap adjusting motor 50B in combination. Namely, the horizontal roll gap adjusting motor 50A and the vertical roll gap adjusting motor 50B are shared between a plurality of the horizontal and vertical stand pairs 20. This simplifies the configuration of the square tube forming device, resulting in reduction in the weight of the device and further reduction in cost of manufacturing the device.

EXAMPLES

[0076] Finally, by referring to FIGS. 5 to 8, analysis result about two square tube forming examples will be described in detail as examples of the present invention. Analysis software used for obtaining the analysis result is 3D elastic-plastic deformation finite analysis software developed by the present inventors having accuracy sufficient for reproducing actual forming. These drawings are schematic drawings generated on the basis of result output from this software.

[0077] According to square tube forming examples 1 and 2, the same flat roll stand array is used. FIG. 5 continuously and entirely shows a square tube forming process implemented in the flat roll stand array according to the square tube forming example 1, and FIG. 6 shows change in a tube cross-sectional shape from a round tube as a forming raw member to a square tube as a product in stages of respective stands according to the square tube forming example 1. Likewise, FIG. 7 continuously and entirely shows a square tube forming process implemented in the flat roll stand array according to the square tube forming example 2, and FIG. 8 shows change in a tube cross-sectional shape from a round tube as a forming raw member to a square tube as a product in stages of respective stands according to the square tube forming example 2. The roll outer diameters are shown at the same ratio in FIGS. 5 and 7 to facilitate comparison between the sizes of the outer diameters of the raw tubes.

[0078] The flat roll stand array used in the square tube forming examples 1 and 2 has a configuration conforming to the square tube forming device shown in FIGS. 1 to 4. With a vertical roll stand and a horizontal roll stand defined as one pair, this roll stand array includes 24 rolls with 12 pairs of vertical roll stands and horizontal roll stands in total arranged alternately. A roll diameter in each roll stand is 150 mm, and the roll stand array has an entire length of 4660 mm. While a rolling amount is individually settable for each of the 24 roll stands, the 12 pairs of roll stands each including a vertical roll stand and a horizontal roll stand in one pair were divided into three groups with one group including four pairs, and an individual rolling amount was set for each of these groups.

[0079] In the first group at the most upstream position, for ensuring thrust, a shallow arc-like recess having a larger curvature than an outer surface R of a roll contact surface of a tube member to pass through each roll stand is formed at each roll.

[0080] In the square tube forming example 1 shown in FIGS. 5 and 6, a raw round tube of a relatively small diameter made of common steel (Ys=360 Mpa) and having an outer diameter of 44.45 mm and a thickness of 3.6 mm is to be formed into a square tube as a product having a regular square cross section with one side of 38 mm and a thickness of 3.6 mm. According to a rolling amount distribution employed for this raw tube made of common steel, a ratio of 9:3:1 was set for the first group, for the second group, and for the third group respectively.

[0081] Specifically, with a rolling amount required for forming the cross-sectional shape of the raw round tube entirely into the cross-sectional shape of the square tube as a product set at 13, the rolling amount distribution was determined in such a manner as to perform forming of a rolling amount of 9/13 (about 69.3%) in the first group, to perform forming of a rolling amount of 3/13 (about 23%) in the second group, and to perform forming of a rolling amount of 1/13 (about 7.7%) in the third group. For forming from the round tube into the regular square cross section, the same rolling amount is set for a vertical roll and a horizontal roll in a pair, and as a result of planarization of side sections, four corners of the regular square cross section are formed.

[0082] As a result, about 69% forming of a total forming amount is finished at a final stand RB8 in the first group, about 92% forming is finished at a final stand RB16 in the second group, and 100% forming both in length and width is finished at a final stand RB24 in the third group.

[0083] In the square tube forming example 2 shown in FIGS. 7 and 8, a raw round tube of a relatively large diameter made of the same common steel (Ys=360 Mpa) and having an outer diameter of 119.67 mm and a thickness of 7 mm is to be formed into a square tube as a product having a flat rectangular shape with a length of 50 mm, a width of 150 mm, and a thickness of 7 mm. In this example, a rolling amount distribution was also set to provide a ratio of 9:3:1 for the first group, for the second group, and for the third group respectively. For forming from the round tube into the rectangular cross section, a position to become a corner can be set by determining a rolling amount for a vertical roll and a horizontal roll in a pair in such a manner as to be larger on the vertical side and to be smaller on the horizontal side. As a result of planarization of side sections, four corners of the rectangular cross section are formed.

[0084] As a result, about 69% forming of a total forming amount is finished at the final stand RB8 in the first group, about 92% forming is finished at the final stand RB16 in the second group, and 100% forming both in length and width is finished at the final stand RB24 in the third group.

[0085] As comprehensibly shown particularly in FIG. 8, a rolling amount is distributed in the foregoing manner for reason of employing a forming process of performing planarization of side sections readily in the first group on the upstream side for corner forming, continuously performing the planarization of the side sections gently in the second group on the midstream side for the corner forming, and obtaining intended dimensions in the third group on the downstream side. In the two cases of square tube forming, such a rolling amount distribution is employed by giving consideration to intended dimensions of a product. Allowing employment of a forming process responsive to such product specifications according to circumstances forms the characteristic of the present invention. A range of roll multi-use defined by a model for analysis is such that an outer diameter ratio is up to three times and an aspect ratio is up to three times. Result obtained from the analysis shows that manufacturing an actual machine and actually operating the machine produces the same effect.

[0086] While roll stands in a flat roll stand array are divided into three groups with one group including one pair×4 (eight stands), these roll stands may alternatively be divided into four groups with one group including one pair×3 (six stands), into six groups with one group including one pair×2 (four stands), or into 12 groups with one group including one pair×1 (two stands). By choosing a method of defining groups in various ways together with a rolling amount distribution in response to a difference in forming specifications (the material, outer diameter, and thickness of a raw round tube, and dimensions of a square tube as a product), it becomes possible to perform a wide range of square tube forming.

REFERENCE SIGNS LIST

[0087] 10 Roll stand array

[0088] 20 Horizontal and vertical stand pair

[0089] 20A Horizontal roll stand (first flat roll stand)

[0090] 21A Vertical rod

[0091] 22A Horizontal fixed base

[0092] 23A Movable base

[0093] 24A Horizontal roll

[0094] 25A Bracket

[0095] 26A Jack

[0096] 27A Input shaft

[0097] 28A Gear box

[0098] 29A Power transmission shaft

[0099] 20B Vertical roll stand (second flat roll stand)

[0100] 21B Vertical fixed base

[0101] 22B Horizontal rod

[0102] 23B Movable base

[0103] 24B Vertical roll

[0104] 25B Roll support frame

[0105] 26B Jack

[0106] 27B Input shaft

[0107] 28B Gear box

[0108] 29B Power transmission shaft

[0109] 30 Stand base

[0110] 40 Roll driving mechanism (roll driving means)

[0111] 41 Driving unit

[0112] 42 Driving motor

[0113] 50A Horizontal roll gap adjusting motor

[0114] 50B Vertical roll gap adjusting motor

[0115] 51 Support table

[0116] 52 Stay

[0117] 53 Cable