How to reduce the diameter of a metal tube

Abstract

To provide a metal pipe diameter reduction method that reduces diameters of metallic element pipes having a substantially rectangular cross section that is configured by three or more sets of curves which have different curvature radii and face one another, and can manufacture a cylindrical member having a smooth and accurate substantially rectangular cross section.SOLUTION: A diameter of a metallic element pipe is reduced by pressing an outer peripheral surface of the element pipe with a plurality of split dies which are so arranged around an axis of the element pipe having a substantially rectangular cross section as to surround the element pipe. In a metal pipe diameter reduction method, a plurality of split dies are divided into a plurality of split die groups comprising one or adjacent two split dies, and a time difference is provided between timings for bringing the plurality of split die groups into contact with the outer peripheral surface of the element pipe.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for reducing the diameter of a metal tube, and more particularly, to a method for reducing the diameter of a metal tube having a substantially rectangular cross section constituted by three or more sets of opposing curves having different radii of curvature, capable of producing a tubular member having a smooth and accurate substantially rectangular cross section. [Background technology]

[0002] For example, in exhaust purification devices (catalytic converters, etc.) mounted on automobiles, the cross section is becoming non-circular due to requirements in terms of performance and mounting space. In the process of assembling such exhaust purification devices, a buffer mat having a predetermined thickness is wound around the outer circumferential surface of a catalyst carrier having a non-circular cross section, and the combination of the catalyst carrier and the buffer mat is inserted into a metallic tubular member (outer tube) having a non-circular cross section, and then the metal tube is reduced in diameter, so that the catalyst carrier is held in place by the restoring force of the buffer mat sandwiched between the catalyst carrier and the tubular member. In other words, the cross section of the tubular member has a shape similar to that of the catalyst carrier, but larger than that of the catalyst carrier.

[0003] Specific examples of non-circular cross-sectional shapes include an ellipse and a rectangle. However, since it is difficult to industrially mass-produce metal pipes having a true elliptical cross-section, which is a set of points whose sum of distances from two fixed points (foci) on a plane is constant, pseudo-elliptical cross-sections have been widely used in the past. A pseudo-ellipse is a pseudo-ellipse consisting of two sets of large and small opposing curves (two curves C1 and two curves C2) with different curvature radii R1 and R2, as shown in FIG. 43. Therefore, strictly speaking, there are four inflection points (points where adjacent curves are connected) in a pseudo-ellipse (see the black circles in the figure). Note that the black circles in the figure are drawn for the purpose of clearly showing the positions of the points (inflection points) where the multiple curves constituting the cross-section are connected to other adjacent curves, and do not mean that there are convex parts and / or protuberances corresponding to the shapes of the black circles on the cross-section. The same applies to each drawing referred to later in this specification.

[0004] A cylindrical member having a pseudo-elliptical cross section as shown in FIG. 43 is generally produced by reducing the diameter of a blank tube, which is a metal tube having a cross section slightly larger than the target cross section, to produce a cylindrical member, which is a metal tube having a cross section of the final size and shape. Both the blank tube and the cylindrical member have a flat shape having a major axis and a minor axis. Therefore, in a conventional diameter reduction die, the blank tube is reduced in diameter by pressing the outer peripheral surface area of ​​the blank tube corresponding to the four curves (two curves C1 and two curves C2) that constitute the cross section with four pressing members. In this case, since the amount of movement of the pressing members in the major axis direction and the minor axis direction differs, independent movement control is required for each pressing member, which complicates the control system and pressing mechanism.

[0005] In the diameter reduction die according to the invention described in Patent Document 1 (JP Patent No. 4856618), a die (pressing member) for pressing regions of the outer circumferential surface of a blank tube corresponding to two sets of opposing curves having large and small radii of curvature that constitute a pseudo-elliptical cross section is divided into a plurality of split dies. A split die group for the larger radius of curvature and a split die group for the smaller radius of curvature are configured, and each split die group is configured so that the center point of the split die group for the smaller radius of curvature is located on a center line (radius) that is a line segment connecting the end of the pressing surface of the split die group for the larger radius of curvature and the center point.

[0006] According to the above invention, it is possible to move the multiple split dies uniformly (at the same speed and by the same distance) toward the center points corresponding to the respective radii of curvature, so that the multiple split dies can be brought into contact with the outer circumferential surface of the blank tube and pressed simultaneously. In addition, since no step is generated between the pressing surfaces of the adjacent split dies, it is possible to form a smooth and accurate cross-sectional shape without requiring a complex control system. A similar invention is also described in Patent Document 2 (JP Patent Publication No. 2013-500161).

[0007] However, in recent years, there has been an increasing demand for cylindrical members having a substantially rectangular cross section formed by three or more sets of opposing curved lines having different radii of curvature, instead of the above-mentioned cylindrical members having a pseudo-elliptical cross section. A specific example of a substantially rectangular shape is a pseudo-rectangle formed by three sets of opposing curved lines having different radii of curvature (four curved lines C1, two curved lines C2, and two curved lines C3), as shown in FIG.

[0008] The cross section of a mother tube as a work for obtaining a tubular member having a substantially rectangular cross section as described above has a shape similar to the cross section of the tubular member and slightly larger than the cross section of the tubular member. That is, the cross section of the mother tube also has a substantially rectangular cross section having a cross section formed by three or more sets of opposing curves having different radii of curvature. In the case of a mother tube having such a substantially rectangular cross section, it is difficult or impossible to uniformly move the split dies in the conventional technology such as those described in Patent Documents 1 and 2 to simultaneously abut against the outer circumferential surface of the mother tube and press the outer circumferential surface.

[0009] In addition, in a mother tube having a substantially rectangular cross section as described above, for example, between a portion where the curves constituting the substantially rectangular cross section are long and a portion where the curves are short, the difference in surface rigidity of the corresponding regions on the outer circumferential surface of the mother tube may become large. In such a case, even if the split dies are uniformly moved by the diameter reducing die according to the conventional technology as described above to simultaneously abut against the outer circumferential surface of the mother tube and press the outer circumferential surface, a difference in the amount of deformation will occur in each region, and buckling is likely to occur in a region with low surface rigidity (for example, a region corresponding to the longest curve among the curves constituting the cross section) or a step will occur in a portion corresponding to a point (inflection point) where adjacent curves are connected in the cross section.

[0010] As described above, it is difficult to form a smooth and accurate cross section of a cylindrical member having a substantially rectangular cross section, which is widely used these days, using a diameter reducing die according to the prior art. Such a problem is not limited to the outer tube of an exhaust gas purification device, but is common to all cylindrical members having a substantially rectangular cross section. That is, in this technical field, there is a demand for a method for reducing the diameter of a metal tube, which can produce a cylindrical member having a smooth and accurate substantially rectangular cross section by reducing the diameter of a metal blank tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 4856618 [Patent Document 2] Special table 2013-500161 publication Summary of the Invention [Problem to be solved by the invention]

[0012] As described above, there is a need in the art for a method for reducing the diameter of a metal tube that can smoothly and accurately produce a tubular member having a substantially rectangular cross-section by reducing the diameter of a metal blank tube having a substantially rectangular cross-section formed by three or more sets of opposing curves having different radii of curvature. [Means for solving the problem]

[0013] Therefore, as a result of intensive research, the inventors have found that the above-mentioned problem can be solved by providing a difference in the timing at which the split dies abut against the mother tube, rather than moving the multiple split dies uniformly when pressing the outer peripheral surface of a metal mother tube having a substantially rectangular cross-section with the multiple split dies.

[0014] Specifically, the method for reducing the diameter of a metal tube according to the present invention (hereinafter sometimes referred to as the "method of the present invention") is a method for reducing the diameter of a metal tube, in which the diameter of a metal tube is reduced by pressing the outer peripheral surface of the mother tube with a plurality of split dies arranged around the axis of the mother tube having a substantially rectangular cross section, thereby reducing the diameter of the mother tube. In this specification, "substantially rectangular" means a shape that can be regarded as roughly a rectangle composed of three or more sets of opposing curves having different radii of curvature.

[0015] In the method of the present invention, a plurality of split dies are divided into a plurality of split die groups each consisting of one or two or more adjacent split dies, and a time lag is provided in the timing for bringing the plurality of split die groups into contact with the outer circumferential surface of the mother tube. Effect of the Invention

[0016] As described above, in the method of the present invention, a time difference is provided in the timing at which the multiple split die groups are brought into contact with the outer circumferential surface of the mother tube. Therefore, even if the difference in surface rigidity is large on the outer circumferential surface of the mother tube having a substantially rectangular cross section, the possibility of buckling occurring in an area having low surface rigidity or of a step occurring at a portion corresponding to an inflection point in the cross section can be reduced by setting the timing at which the split die groups are brought into contact with the mother tube in accordance with the difference in surface rigidity. In other words, according to the method of the present invention, a tubular member having a smooth and accurate substantially rectangular cross section can be manufactured by reducing the diameter of a metal mother tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature.

[0017] Other objects, features and attendant advantages of the present invention will become readily apparent from the following description of the embodiments of the present invention which are given with reference to the accompanying drawings. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1( a ) is a schematic diagram showing an example of a cross section of a mother tube to be reduced in diameter by a method for reducing a diameter of a metal tube according to a first embodiment of the present invention (first method), and FIGS. 1( b ) and 1 ( c ) are schematic diagrams illustrating the arrangement of a plurality of split dies and a mother tube immediately before the start of the first method. [Diagram 2] FIG. 4 is a schematic diagram showing an example of a means for driving a plurality of split dies toward the outer circumferential surface of a mother tube in the first method. [Diagram 3] FIG. 1( a ) is a schematic diagram showing an example of a cross section of a mother tube to be reduced in diameter by a method for reducing the diameter of a metal tube according to a second embodiment of the present invention (second method), and FIGS. 1( b ) and 1( c ) are schematic diagrams illustrating the arrangement of a plurality of split dies and a mother tube immediately before the start of the second method. [Figure 4] FIG. 11 is a schematic diagram showing an example of a step of reducing the diameter of a blank tube in a method for reducing the diameter of a metal tube (third method) according to a third embodiment of the present invention. [Diagram 5] FIG. 13 is a schematic view showing an example of a step of reducing the diameter of a blank tube in a third method according to a preferred embodiment. [Figure 6]FIG. 13 is a schematic diagram showing an example of a step of reducing the diameter of a mother tube in a method for reducing the diameter of a metal tube according to a fourth embodiment of the present invention (fourth method). [Figure 7] FIG. 13 is a schematic view showing an example of a step of reducing the diameter of a mother tube in a method for reducing the diameter of a metal tube (fifth method) according to a fifth embodiment of the present invention. [Figure 8] FIG. 13 is a schematic view showing an example of a step of reducing the diameter of a mother tube in a method for reducing the diameter of a metal tube (sixth method) according to a sixth embodiment of the present invention. [Figure 9] FIG. 13 is a schematic diagram for explaining the moving direction (direction in which the outer circumferential surface of the mother tube is pressed) of a plurality of split dies in a method for reducing the diameter of a metal tube (seventh method) according to a seventh embodiment of the present invention.

[0019] [Figure 10] FIG. 1 is a schematic diagram showing an arrangement of a plurality of split dies and an original pipe immediately before the start of a method for reducing a diameter of a metal pipe according to Example 1 of the present invention (Example Method 1). [Figure 11] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 12] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 13] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 14] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 15] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 16] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 17] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 18]FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment. [Figure 19] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 1 of the embodiment.

[0020] [Figure 20] FIG. 11 is a schematic diagram showing an arrangement of a plurality of split dies and an original pipe immediately before the start of a method for reducing the diameter of a metal pipe according to Example 2 of the present invention (Example Method 2). [Figure 21] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Figure 22] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Diagram 23] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Figure 24] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Diagram 25] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Figure 26] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Figure 27] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Figure 28] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Figure 29] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Diagram 30]FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Diagram 31] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment. [Diagram 32] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 2 of the embodiment.

[0021] [Diagram 33] FIG. 11 is a schematic diagram showing an arrangement of a plurality of split dies and an original pipe immediately before the start of a method for reducing the diameter of a metal pipe according to Example 3 of the present invention (Example Method 3). [Diagram 34] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Diagram 35] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Diagram 36] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Figure 37] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Figure 38] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Figure 39] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Diagram 40] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Diagram 41] FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Diagram 42]FIG. 13 is a schematic diagram showing the movement of the divided mold groups in a pressing process performed by each divided mold group in the method 3 of the embodiment. [Diagram 43] FIG. 1 is a schematic diagram showing an example of a cross section of a pseudo-ellipse, which is a pseudo-ellipse made up of two sets of curves, large and small, facing each other and having different radii of curvature, which is possessed by a conventionally used cylindrical member (outer cylinder). [Diagram 44] FIG. 1 is a schematic diagram showing an example of a substantially rectangular cross section, which is a pseudo-rectangle made up of three sets of opposing curves having different radii of curvature, of a cylindrical member (outer cylinder) that is widely used these days. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] First Embodiment Hereinafter, a method for reducing the diameter of a metal tube according to a first embodiment of the present invention (hereinafter, sometimes referred to as the "first method") will be described with reference to the drawings.

[0023] The first method is a method for reducing the diameter of a metal tube, in which the diameter of a metal tube having a substantially rectangular cross section is reduced by pressing the outer circumferential surface of the tube with a plurality of split dies arranged around the axis of the tube so as to surround the tube. As described above, in this specification, "substantially rectangular" means a shape that can be regarded as roughly a rectangle composed of three or more sets of opposing curves having different radii of curvature.

[0024] In the first method, the multiple split dies are divided into multiple split die groups each consisting of one or two or more adjacent split dies. Furthermore, a time difference is provided in the timing for contacting the multiple split die groups with the outer peripheral surface of the mother tube. That is, in the first method, the multiple split dies are divided into multiple sets (split die groups) each having a different timing for contacting with the outer peripheral surface of the mother tube, and each set (split die group) consists of one or two or more adjacent split dies.

[0025] Therefore, in the first method, even when the difference in surface stiffness is large on the outer circumferential surface of a mother tube having a substantially rectangular cross section, the possibility of buckling occurring in a region having low surface stiffness or of a step occurring at a portion corresponding to an inflection point on the cross section can be reduced by setting the timing at which the multiple split die groups contact the mother tube according to the difference in surface stiffness. It is preferable to appropriately determine the timing at which each of the multiple split die groups contacts the mother tube according to the distribution of surface stiffness on the outer circumferential surface of the mother tube. The surface stiffness of each region constituting the outer circumferential surface of the mother tube is affected by various factors, such as the shape of the cross section of the mother tube to be reduced in diameter, the thickness of the tube wall and the properties of the constituent materials (e.g., ductility and malleability), the shape (e.g., radius of curvature on the cross section) and area of ​​each region on the outer circumferential surface of the mother tube, and the surface stiffness of other surrounding regions. The surface stiffness of each region constituting the outer circumferential surface of the mother tube can be obtained by various methods, such as various simulation analyses and actual measurements using the mother tube itself. Furthermore, the specific timing for bringing the plurality of split die groups into contact with the base tube can be appropriately determined by various techniques, such as various simulation analyses and preliminary experiments (described in detail later).

[0026] The timing of bringing a certain split mold group into contact with the mother tube may be before or after the completion of the process of pressing the mother tube by another split mold group that was brought into contact with the mother tube immediately before the split mold group. That is, the process of pressing the mother tube by the split mold group may be started before the process of pressing the mother tube by the other split mold group that was brought into contact with the mother tube immediately before the split mold group is completed, the process of pressing the mother tube by the split mold group may be started at the same time as the process of pressing the mother tube by the other split mold group that was brought into contact with the mother tube immediately before the split mold group is completed, or the process of pressing the mother tube by the split mold group may be started after the process of pressing the mother tube by the other split mold group that was brought into contact with the mother tube immediately before the split mold group is completed. This point can also be appropriately determined by various methods, such as various simulation analyses and preliminary experiments.

[0027] Also, for example, depending on the configuration of the mother tube, the split mold group, and the means for driving the split mold group (described in detail later), it is not always necessary to form a tubular member having a target cross section by performing the first method once. In other words, the cross section of the mother tube may be gradually brought closer to the target cross section by performing the first method multiple times.

[0028] Fig. 1(a) is a schematic diagram showing an example of a cross section of a blank tube reduced by the first method. The blank tube 11 shown in Fig. 1(a) has a pseudo-rectangular (approximately rectangular) cross section consisting of three different sets of opposing curves (two curves 12a, four curves 12b, and two curves 12c) having different radii of curvature. When the blank tube 11 having such a substantially rectangular cross section is reduced by a reducing die according to the prior art, as described above, there is a high possibility that buckling will occur in an area having low surface rigidity, or that a step will occur in a portion corresponding to an inflection point in the cross section.

[0029] Fig. 1(b) is a schematic diagram showing an example of an arrangement of a plurality of split molds and a mother tube immediately before the start of the first method. In the example shown in Fig. 1(b), eight split mold groups (two split mold groups 21a, four split mold groups 21b, and two split mold groups 21c) are arranged to surround the mother tube 11 around its axis. Each of the plurality of split mold groups 21a, 21b, and 21c is composed of one split mold. However, at least one of the plurality of split mold groups 21a, 21b, and 21c may be composed of a plurality of split molds.

[0030] In the example shown in Fig. 1(c), two split die groups 21c are each composed of three split dies. When a split die group composed of a plurality of split dies is brought into contact with the outer circumferential surface of the mother tube in this manner, all of the split dies constituting the split die group may be brought into contact with the outer circumferential surface of the mother tube at the same time. Alternatively, a time lag may be provided in the timing at which the plurality of split dies constituting the split die group are brought into contact with the outer circumferential surface of the mother tube.

[0031] The means for driving the multiple split dies toward the outer circumferential surface of the mother tube is not particularly limited as long as it is possible to press the multiple split dies against the outer circumferential surface of the mother tube to reduce the diameter of the mother tube by a desired amount. For example, the multiple split dies can be driven toward the outer circumferential surface of the mother tube using a mechanism such as a combination of jaw segments and ring portions provided in the diameter reducing device described in the above-mentioned Patent Document 2.

[0032] Fig. 2 is a schematic diagram illustrating the configuration of the drive mechanism described above. In the center of Fig. 2(a), the three split die groups 21a, 21b, and 21c illustrated in Fig. 1(b) and the mother tube 11 are illustrated. Around them, the three split die groups 21a, 21b, and 21c observed from a direction perpendicular to the axis of the mother tube 11 (radial direction) are illustrated. The three split die groups 21a, 21b, and 21c are held so that they can approach or move away from each other, and a sliding surface that is an inclined surface inclined at a predetermined angle with respect to the axial direction of the mother tube 11 is formed on the opposite side to the surface (pressing surface) that presses the outer circumferential surface in the radial direction of the mother tube 11. That is, the three split die groups 21a, 21b, and 21c correspond to the jaw segments described above.

[0033] 2(b), the three sets of "kettles" 31a, 31b, and 31c are engaged with the three sets of split die groups 21a, 21b, and 21c so that the sliding surfaces of the three sets of split die groups 21a, 21b, and 21c come into contact with the inclined surfaces formed on the three sets of "kettles" 31a, 31b, and 31c, which are members configured to be movable in the axial direction of the mother tube 11. Then, as shown by the dashed arrows, the three sets of kettles 31a, 31b, and 31c are moved in the axial direction of the mother tube 11 in a direction in which the spacing between the opposing sliding surfaces of the three sets of split die groups 21a, 21b, and 21c becomes wider. As a result, as shown by the solid arrows, the opposing split die groups of the three sets of split die groups 21a, 21b, and 21c are biased in a direction in which they approach each other. In this manner, the three split die groups 21a, 21b, and 21c can be driven toward the outer circumferential surface of the mother tube 11, as shown in FIG. 2(c).

[0034] In the first method, a time difference is provided in the timing for bringing the plurality of split die groups into contact with the outer circumferential surface of the mother tube. When the above-mentioned drive mechanism is adopted, for example, three split die groups 21a, 21b, and 21c and three kettles are used. 31a, 31b and 31c By appropriately adjusting the range and / or inclination angle of the surface where the three split die groups 21a, 21b and 21c slide while in contact with each other, a time lag can be created in the timing at which the three split die groups 21a, 21b and 21c come into contact with the outer peripheral surface of the mother tube 11.

[0035] Alternatively, in the first method, a time lag may be provided in the timing of contacting the multiple split die groups with the outer peripheral surface of the mother tube by driving each split die group or each split die individually using an actuator such as a hydraulic cylinder or a servo cylinder.

[0036] As described above, in the first method, a time difference is set for the timing at which the multiple split die groups are brought into contact with the outer circumferential surface of the mother tube. Therefore, even if the difference in surface rigidity is large on the outer circumferential surface of the mother tube having a substantially rectangular cross section, the possibility of buckling occurring in an area having low surface rigidity or of a step occurring in a portion corresponding to an inflection point in the cross section can be reduced by setting the timing at which the split die groups are brought into contact with the mother tube according to the difference in surface rigidity. In other words, according to the first method, a cylindrical member having a smooth and accurate substantially rectangular cross section can be manufactured by reducing the diameter of a metal mother tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature.

[0037] Second Embodiment Hereinafter, a method for reducing the diameter of a metal tube according to a second embodiment of the present invention (hereinafter, sometimes referred to as the "second method") will be described with reference to the drawings.

[0038] As described above, the first method is a method for reducing the diameter of a metal tube, in which a plurality of split dies are arranged around the axis of a metal tube having a substantially rectangular cross section to press the outer circumferential surface of the tube, thereby reducing the diameter of the tube. Furthermore, the plurality of split dies are divided into a plurality of split die groups, each of which is composed of one or two or more adjacent split dies.

[0039] In the first method, the number of split dies and the size of the surface of each split die that presses the outer circumferential surface of the mother tube (hereinafter, sometimes referred to as the "pressing surface") are not particularly limited, so long as the above-mentioned effects can be achieved. For example, in the first method, the existence of a split die and / or a split die group having a pressing surface that faces the outer circumferential surface of the mother tube so as to cover the boundary between two regions corresponding to two different adjacent curves (so as to straddle the boundary between the adjacent regions) is not excluded. In other words, there may be a split die and / or a split die group having a pressing surface that faces an inflection point in the cross section of the mother tube.

[0040] In addition to the above, in the first method, by providing a time difference in the timing at which multiple split mold groups are abutted against the outer peripheral surface of the mother tube, it is possible to reduce the possibility of buckling occurring in areas with low surface rigidity or of steps occurring in parts corresponding to inflection points in the cross section, even when there is a large difference in surface rigidity on the outer peripheral surface of the mother tube having an approximately rectangular cross section.

[0041] However, the distribution of surface rigidity on the outer peripheral surface of the mother tube generally corresponds to the arrangement of the multiple curved lines constituting the cross section of the mother tube. From the viewpoint of reducing the possibility of buckling occurring in an area having low surface rigidity or of steps occurring at portions corresponding to inflection points in the cross section, it is preferable to provide a time difference in the timing of bringing the multiple split die groups into contact with the outer peripheral surface of the mother tube in accordance with the distribution of surface rigidity on the outer peripheral surface of the mother tube. Therefore, it is preferable to arrange the multiple split die groups in accordance with the arrangement of the multiple curved lines constituting the cross section of the mother tube.

[0042] Therefore, the second method is a method for reducing the diameter of a metal tube, which is the above-mentioned first method, characterized in that each of the multiple split mold groups faces an area of ​​the outer circumferential surface of the mother tube that corresponds to each of the curves that constitute the cross section of the mother tube. In other words, in the second method, the boundary between adjacent split mold groups is present at a position facing a point (inflection point) where adjacent curves are connected in the cross section of the mother tube. This makes it easy to set the timing for abutting the split mold groups for each boundary area of ​​the outer circumferential surface of the mother tube that corresponds to the multiple curves that constitute the cross section of the mother tube. In other words, it becomes easy to set a time difference in the timing for abutting the multiple split mold groups on the outer circumferential surface of the mother tube according to the distribution of surface rigidity on the outer circumferential surface of the mother tube.

[0043] Fig. 3(a) is a schematic diagram showing an example of a cross section of a mother tube whose diameter is reduced by the second method, which is the same as the mother tube 11 illustrated in Fig. 1(a). Fig. 3(b) and (c) are schematic diagrams illustrating the arrangement of a plurality of split molds and a mother tube immediately before the start of the second method. In the example shown in Fig. 3, eight split mold groups (two split mold groups 21a, four split mold groups 21b, and two split mold groups 21c) are arranged to surround the mother tube 11 around its axis, and the boundaries between adjacent split mold groups are located at positions facing the inflection points in the cross section of the mother tube 11. That is, each of the multiple split mold groups 21a, 21b, and 21c faces an area of ​​the outer circumferential surface of the mother tube 11 corresponding to each of the curves 12a, 12b, and 12c constituting the cross section of the mother tube 11.

[0044] According to the second method, it is easy to set the timing for bringing the split die groups into contact with each boundary region of the outer circumferential surface of the mother tube corresponding to the plurality of curves constituting the cross section of the mother tube. That is, it is easy to set a time difference in the timing for bringing the plurality of split die groups into contact with the outer circumferential surface of the mother tube according to the distribution of surface rigidity on the outer circumferential surface of the mother tube. As a result, even when the difference in surface rigidity is large on the outer circumferential surface of the mother tube having a substantially rectangular cross section, it is possible to more reliably reduce the possibility that buckling occurs in the region having low surface rigidity or that a step occurs in the portion corresponding to an inflection point on the cross section. That is, according to the second method, it is possible to more reliably manufacture a tubular member having a smooth and accurate substantially rectangular cross section by reducing the diameter of a metal mother tube having a substantially rectangular cross section constituted by three or more sets of opposing curves having different radii of curvature.

[0045] Third Embodiment Hereinafter, a method for reducing the diameter of a metal tube according to a third embodiment of the present invention (hereinafter, sometimes referred to as the "third method") will be described with reference to the drawings.

[0046] As described above, in a mother tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature, there may be a large difference in surface rigidity on the outer circumferential surface of the mother tube. The present inventors have found that when pressing of a region having the lowest surface rigidity of such a mother tube is started first and then pressing of a region having a higher surface rigidity is started, there is a tendency that unintended deformation such as buckling and / or steps tends to occur in the region having the lowest surface rigidity.

[0047] Therefore, the third method is a method for reducing the diameter of a metal tube, which is the above-mentioned first or second method, characterized in that the split mold group that presses the region of the outer circumferential surface of the mother tube having the lowest surface rigidity among the multiple split mold groups is brought into contact with the outer circumferential surface of the mother tube last. In other words, in the third method, among the timings at which the multiple split mold groups are brought into contact with the outer circumferential surface of the mother tube, the timing at which the split mold group that presses the region of the outer circumferential surface of the mother tube having the lowest surface rigidity is brought into contact with the outer circumferential surface of the mother tube is the latest. This makes it possible to avoid the start of pressing the region with higher surface rigidity after pressing the region with the lowest surface rigidity has started, thereby reducing the possibility of problems such as unintended deformation occurring in the region with the lowest surface rigidity.

[0048] In the third method, the timing for bringing the split die groups other than the split die group pressing the region of the outer circumferential surface of the mother tube having the lowest surface rigidity into contact with the outer circumferential surface of the mother tube is not particularly limited as long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in the region having lower surface rigidity, it is preferable to bring the split die group pressing the region of the outer circumferential surface of the mother tube having the highest surface rigidity among the multiple split die groups into contact with the outer circumferential surface of the mother tube first.

[0049] Therefore, a third method according to a preferred embodiment is a method for reducing the diameter of a metal pipe, characterized in that, in addition to bringing into contact with the outer circumferential surface of the mother pipe the split mold group that presses the region of the outer circumferential surface of the mother pipe having the lowest surface rigidity among the multiple split mold groups last, as described above, the split mold group that presses the region of the outer circumferential surface of the mother pipe having the highest surface rigidity among the multiple split mold groups is brought into contact with the outer circumferential surface of the mother pipe first. In other words, in the third method according to a preferred embodiment, among the timings at which the multiple split mold groups are brought into contact with the outer circumferential surface of the mother pipe, the timing at which the split mold group that presses the region of the outer circumferential surface of the mother pipe having the lowest surface rigidity is brought into contact with the outer circumferential surface of the mother pipe is the latest, and the timing at which the split mold group that presses the region of the outer circumferential surface of the mother pipe having the highest surface rigidity is brought into contact with the outer circumferential surface of the mother pipe is the earliest.

[0050] According to the third method, which is a preferred embodiment, it is possible to reduce the possibility of unintended deformation such as buckling and / or steps occurring in the area with the lowest surface stiffness by avoiding starting compression of an area with a higher surface stiffness after starting compression of an area with the lowest surface stiffness, as well as reducing the possibility of unintended deformation such as buckling and / or steps occurring in the area with the intermediate surface stiffness by avoiding starting compression of an area with the highest surface stiffness after starting compression of an area with an intermediate surface stiffness.

[0051] In the third method according to a preferred embodiment, the timing of contacting the outer circumferential surface of the mother tube with a split mold group that is neither a split mold group that presses an area of ​​the outer circumferential surface of the mother tube having the lowest surface rigidity nor a split mold group that presses an area of ​​the outer circumferential surface of the mother tube having the highest surface rigidity (a split mold group that presses an area of ​​intermediate surface rigidity) is not particularly limited as long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in areas with lower surface rigidity, it is preferable to contact the split mold groups with the outer circumferential surface of the mother tube in order of surface rigidity of the areas of the outer circumferential surface of the mother tube that face the pressing surface of the split mold group, from the area with the highest surface rigidity to the area with the lowest surface rigidity.

[0052] Therefore, a third method according to a more preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that the timing of bringing the multiple split die groups into contact with the outer peripheral surface of the mother tube is in descending order of the surface rigidity of the region of the outer peripheral surface of the mother tube that each split die group faces. In other words, in the third method according to a more preferred embodiment, the split die groups are brought into contact with the outer peripheral surface of the mother tube in order of the surface rigidity of the regions of the outer peripheral surface of the mother tube that face each other, from the region with the highest surface rigidity to the region with the lowest surface rigidity. This makes it possible to avoid starting pressing of a region with a higher surface rigidity after starting pressing of a region with a lower surface rigidity, and more reliably reduces the possibility of unintended deformation such as buckling and / or steps occurring in all regions.

[0053] FIG. 4 is a schematic diagram showing an example of the diameter reduction step of the blank tube in the third method. FIG. 4(a) shows the arrangement of a plurality of split dies and the blank tube immediately before the start of the diameter reduction step. The blank tube 12 shown in FIG. 4 has a cross section of a pseudo-rectangle (approximately rectangular) consisting of three or more different sets of curves facing each other with different radii of curvature. In the example shown in FIG. 4, the blank tube 12 is reduced in diameter by pressing the outer circumferential surface of the blank tube 12 with eight split die groups (two split die groups 22a, four split die groups 22b, and two split die groups 22c). In addition, in the outer circumferential surface of the blank tube 12, the surface rigidity of the region facing the pressing surface of the split die group 22a is the highest, the surface rigidity of the region facing the pressing surface of the split die group 22b is the second highest, and the surface rigidity of the region facing the pressing surface of the split die group 22c is the lowest. In addition, in order to simplify the drawings, reference numerals are omitted in FIG. 4(b) to (d).

[0054] In the process of reducing the diameter of the mother tube 12 illustrated in FIG. 4, pressing processes of the outer circumferential surface of the mother tube 12 are performed in the order of (b), (c), and (d). In FIG. 4(b), which shows the pressing process performed first, the split die group 22a of the three split die groups 22a, 22b, and 22c presses the region of the outer circumferential surface of the mother tube 12 facing the pressing surface of the split die group 22a. As described above, the region facing the pressing surface of the split die group 22a is the region of the outer circumferential surface of the mother tube 12 that has the highest surface rigidity. That is, in the process of reducing the diameter of the mother tube 12 illustrated in FIG. 4, the split die group 22a first comes into contact with the region of the outer circumferential surface of the mother tube 12 that has the highest surface rigidity, and presses that region. Therefore, the process of reducing the diameter of the mother tube 12 illustrated in FIG. 4 corresponds to the above-mentioned "third method according to a preferred embodiment".

[0055] 4(c), which shows the pressing process to be performed next, the split die group 22b of the three split die groups 22a, 22b, and 22c presses the region of the outer circumferential surface of the mother tube 12 that faces the pressing surface of the split die group 22b. As described above, the region that faces the pressing surface of the split die group 22b is a region of the outer circumferential surface of the mother tube 12 that has intermediate surface rigidity.

[0056] In (d) of Fig. 4, which shows the pressing process performed last, the split die group 22c out of the three split die groups 22a, 22b, and 22c presses the region facing the pressing surface of the split die group 22c. As described above, the region facing the pressing surface of the split die group 22c is the region on the outer circumferential surface of the mother tube 12 that has the lowest surface rigidity. That is, in the diameter reduction process of the mother tube 12 illustrated in Fig. 4, the split die group 22c finally comes into contact with the region on the outer circumferential surface of the mother tube 12 that has the lowest surface rigidity, and presses that region. Therefore, the diameter reduction process of the mother tube 12 illustrated in Fig. 4 also corresponds to the above-mentioned "third method".

[0057] As described above, as the diameter reduction process of the mother tube 12 progresses from (b), (c), to (d) in Fig. 4, the pressing process is performed in the order of the region on the outer circumferential surface of the mother tube 12 facing the pressing surface of the split die group 22a, the region facing the pressing surface of the split die group 22b, and the region facing the pressing surface of the split die group 22c. That is, as the diameter reduction process of the mother tube 12 progresses from (b), (c), to (d) in Fig. 4, the split die group abuts against the outer circumferential surface of the mother tube and presses each region in descending order of surface rigidity of the region on the outer circumferential surface of the mother tube, from the region with the highest surface rigidity to the region with the lowest surface rigidity. Therefore, the diameter reduction process of the mother tube 12 illustrated in Fig. 4 also corresponds to the above-mentioned "third method according to a more preferred embodiment".

[0058] As illustrated in FIG. 4, the third method makes it possible to avoid starting compression of an area with a higher surface stiffness after starting compression of an area with a lower surface stiffness, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the area with the lower surface stiffness.

[0059] Incidentally, as described above, when at least one of the split mold groups is composed of a plurality of split molds, all of the split molds constituting the split mold group may be brought into contact with the outer circumferential surface of the mother tube at the same time. Alternatively, a time difference may be provided for the timing at which the plurality of split molds constituting the split mold group are brought into contact with the outer circumferential surface of the mother tube. In the latter case, a time difference may be provided for the timing at which the split molds are brought into contact with the outer circumferential surface of the mother tube depending on the surface rigidity of the region of the outer circumferential surface of the mother tube that faces the pressing surface of each of the split molds constituting the split mold group. In this case, from the viewpoint of reducing the possibility that pressing of a region having a lower surface rigidity is started after pressing of a region having a higher surface rigidity is started, and thus unintended deformation such as buckling and / or steps occurs in the region having a lower surface rigidity, it is preferable to bring the split molds constituting the split mold group into contact with the outer circumferential surface of the mother tube in the order of the surface rigidity of the region of the outer circumferential surface of the mother tube that faces the pressing surface of each of the split molds.

[0060] Therefore, a third method according to another preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that when a split die group consisting of a plurality of split dies is abutted against the outer peripheral surface of the mother tube, the timing of abutting the split dies against the outer peripheral surface of the mother tube is in descending order of the surface rigidity of the area of ​​the outer peripheral surface of the mother tube that each split die faces.

[0061] FIG. 5 is a schematic diagram showing an example of the step of reducing the diameter of a blank tube in the third method according to another preferred embodiment. As shown in FIG. 5(a), the split mold group 22c shown in this example is composed of two split molds 22c1 located at both ends and a split mold 22c2 located in the center. Except for this point, the split mold group 22c shown in FIG. 5(a) has the same configuration as the split mold group 22c shown in FIG. 4. In the following description, in the region facing the pressing surface of the split mold group 22c, the surface rigidity of the region facing the pressing surface of the split mold 22c1 is higher than the surface rigidity of the region facing the pressing surface of the split mold group 22c2. In order to simplify the drawings, reference numerals are omitted in FIG. 5(d1) to (d2).

[0062] When the split mold group 22c is composed of multiple split molds 22c1 and 22c2 as described above, in order to reduce the possibility of unintended deformation such as buckling and / or steps occurring in areas having low surface rigidity as described above, it is preferable to abut the split molds 22c1 and 22c2 against the outer peripheral surface of the mother tube 12 in order of highest surface rigidity in the areas of the outer peripheral surface of the mother tube that the pressing surfaces of each split mold 22c1 and 22c2 face.

[0063] Therefore, in the example shown in Fig. 5, the pressing process by the split mold set 22c illustrated in Fig. 4(d) is performed in two stages, as illustrated in Fig. 5(d1) and (d2). First, as shown in Fig. 5(d1), the split mold 22c1 facing the region having higher surface rigidity comes into contact with the outer circumferential surface of the mother tube 12 to start the pressing process. Next, as shown in Fig. 5(d2), the split mold 22c2 facing the region having lower surface rigidity comes into contact with the outer circumferential surface of the mother tube 12 to start the pressing process.

[0064] As described above, according to the third method according to another preferred embodiment, even in the region where the pressing surfaces of one split mold group face each other, it is possible to avoid starting pressing of a region having a higher surface rigidity after starting pressing of a region having a lower surface rigidity, thereby more reliably reducing the possibility of unintended deformation such as buckling and / or steps occurring in the region having the lower surface rigidity.

[0065] Fourth Embodiment A method for reducing the diameter of a metal tube according to a fourth embodiment of the present invention (hereinafter, may be referred to as the "fourth method") will be described below with reference to the drawings.

[0066] As described above, in a mother tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature, there may be a large difference in surface rigidity on the outer circumferential surface of the mother tube. The present inventors have found that if pressing of a region closest to the end of the minor axis of such a mother tube's cross section is started first, and then pressing of a region farther from the end of the minor axis is started, there is a tendency for unintended deformation such as buckling and / or a step to be more likely to occur in the region closest to the end of the minor axis.

[0067] Therefore, the fourth method is a method for reducing the diameter of a metal tube, which is the above-mentioned first or second method, characterized in that among the multiple split mold groups, the split mold group closest to the end of the short axis of the cross section of the mother tube is brought into contact with the outer circumferential surface of the mother tube last. In other words, in the fourth method, among the timings at which the multiple split mold groups are brought into contact with the outer circumferential surface of the mother tube, the timing at which the split mold group closest to the end of the short axis of the cross section of the mother tube is brought into contact with the outer circumferential surface of the mother tube is the latest. This makes it possible to avoid starting pressing of the region closest to the end of the short axis of the cross section of the mother tube after starting pressing of the region farther from the end of the short axis, thereby reducing the possibility of problems such as unintended deformation occurring in the region closest to the end of the short axis.

[0068] In the fourth method, the timing for bringing the split die groups other than the split die group closest to the end of the minor axis of the cross section of the mother tube into contact with the outer circumferential surface of the mother tube is not particularly limited as long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in the region closest to the end of the minor axis, it is preferable to bring the split die group closest to the end of the major axis of the cross section of the mother tube into contact with the outer circumferential surface of the mother tube first, among the multiple split die groups.

[0069] Therefore, a fourth method according to a preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that, in addition to bringing the split mold group closest to the end of the minor axis of the cross section of the mother tube into contact with the outer circumferential surface of the mother tube last among the multiple split mold groups as described above, the split mold group closest to the end of the major axis of the cross section of the mother tube into contact with the outer circumferential surface of the mother tube first among the multiple split mold groups. In other words, in the fourth method according to a preferred embodiment, among the timings at which the multiple split mold groups are brought into contact with the outer circumferential surface of the mother tube, the timing at which the split mold group closest to the end of the minor axis of the cross section of the mother tube is brought into contact with the outer circumferential surface of the mother tube is the latest, and the timing at which the split mold group closest to the end of the major axis of the cross section of the mother tube is brought into contact with the outer circumferential surface of the mother tube is the earliest.

[0070] According to the fourth method, which is a preferred embodiment, it is possible to reduce not only the possibility of unintended deformation such as buckling and / or steps occurring in the area pressed by the split die group closest to the end of the minor axis of the cross section of the mother tube, by avoiding the start of pressing by another split die group after pressing by the split die group closest to the end of the major axis of the cross section of the mother tube has begun, but also the possibility of unintended deformation such as buckling and / or steps occurring in the area pressed by the other split die group, by avoiding the start of pressing by the split die group closest to the end of the major axis of the cross section of the mother tube after pressing by the other split die group has begun.

[0071] In the fourth method according to a preferred embodiment, the timing for bringing into contact with the outer circumferential surface of the mother tube a split mold group other than the split mold group closest to the end of the minor axis of the cross section of the mother tube or the split mold group closest to the end of the major axis of the cross section of the mother tube (a group located between them) is not particularly limited as long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in all regions, it is preferable to bring the split mold groups into contact with the outer circumferential surface of the mother tube in the order of proximity to the end of the major axis of the cross section of the mother tube, starting from the split mold group closest to the end of the major axis of the cross section of the mother tube, and then proceeding to the split mold group closest to the end of the minor axis of the cross section of the mother tube.

[0072] Therefore, a fourth method according to a more preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that the timings for bringing a plurality of split die groups into contact with the outer peripheral surface of the mother tube are in ascending order of the distance between each split die group and the end of the major axis. In other words, in the fourth method according to a more preferred embodiment, the split die groups are brought into contact with the outer peripheral surface of the mother tube in order of proximity to the end of the major axis of the cross section of the mother tube, starting from the split die group closest to the end of the major axis of the cross section of the mother tube to the split die group closest to the end of the minor axis of the cross section of the mother tube. This makes it possible to avoid a situation in which pressing by a split die group closer to the end of the major axis of the cross section of the mother tube is started after pressing by a split die group closer to the end of the minor axis of the cross section of the mother tube is started, and thus it is possible to more reliably reduce the possibility of unintended deformation such as buckling and / or steps occurring in all regions.

[0073] FIG. 6 is a schematic diagram showing an example of a diameter reduction step of a mother tube in the fourth method. FIG. 6(a) shows the arrangement of the multiple split dies and the mother tube just before the start of the diameter reduction step, and FIG. 6(b) shows the arrangement of the multiple split dies and the mother tube at the end of the diameter reduction step. The mother tube 13 shown in FIG. 6 has a short axis DS and a long axis DL of a substantially rectangular cross section indicated by thick double-headed arrows. In the example shown in FIG. 6, the outer peripheral surface of the mother tube 13 is pressed by 16 split dies (two split dies 23a, four split dies 23b, four split dies 23c, four split dies 23d, and two split dies 23e) to reduce the diameter of the mother tube 13. In order to simplify the drawing, reference numerals are omitted in FIG. 6(b).

[0074] In the diameter reduction process of the mother tube 13 illustrated in FIG. 6, the split die group 23a, which is the closest to the end (tip of the double arrow) of the long axis DL of the cross section of the mother tube 13 among the 16 split dies 23a to 23e, comes into contact with the outer circumferential surface of the mother tube 13 first, and starts pressing the area facing the pressing surface. Therefore, the diameter reduction process of the mother tube 13 illustrated in FIG. 6 corresponds to the above-mentioned "fourth method according to a preferred embodiment". Thereafter, the split die 23b, the split die 23c, and the split die 23d come into contact with the outer circumferential surface of the mother tube 13 in this order, and start pressing the area facing the pressing surface of each split die. Finally, the split die group 23e, which is the closest to the end (tip of the double arrow) of the short axis DS of the cross section of the mother tube 13 among the 16 split dies 23a to 23e, comes into contact with the outer circumferential surface of the mother tube 13 first, and starts pressing the area facing the pressing surface. Therefore, the diameter reduction process of the mother tube 13 illustrated in FIG. 6 corresponds to the above-mentioned "fourth method".

[0075] 6, the split dies come into contact with the outer circumferential surface of the mother tube 13 in ascending order of the distance between each split die and the end of the long axis DL, starting from the split die group 23a closest to the end of the long axis DL of the cross section of the mother tube 13 among the 16 split dies 23a to 23e, to the split die group 23e closest to the end of the short axis DS of the cross section of the mother tube 13, and start pressing the area facing the pressing surface of each split die (see the dashed arrow in the figure). Therefore, the diameter reducing process of the mother tube 13 illustrated in FIG. 4 also corresponds to the above-mentioned "fourth method according to a more preferred embodiment".

[0076] As illustrated in Figure 6, the fourth method avoids starting pressing by a split die group closer to the end of the long axis of the cross section of the mother tube after pressing by a split die group closer to the end of the short axis of the cross section of the mother tube has started, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the area pressed by the split die group closer to the end of the short axis of the cross section of the mother tube.

[0077] In the above description, the 16 split dies (two split dies 23a, four split dies 23b, four split dies 23c, four split dies 23d, and two split dies 23e) are each in contact with the outer circumferential surface of the mother tube 13 as a split die group, and press the region opposite the pressing surface. However, as described above, at least one of the multiple split die groups may be composed of multiple split dies. In this case, all of the split dies constituting the split die group may be brought into contact with the outer circumferential surface of the mother tube at the same time, or a time lag may be provided in the timing at which the multiple split dies constituting the split die group are brought into contact with the outer circumferential surface of the mother tube.

[0078] In the latter case, a time lag may be set in the timing of bringing the split dies into contact with the outer circumferential surface of the mother tube depending on the distance between each of the split dies constituting the split die group and the end of the long axis of the cross section of the mother tube. In this case, from the viewpoint of reducing the possibility of unintended deformation such as buckling and / or steps occurring in the region pressed by the split die group closer to the end of the short axis of the cross section of the mother tube, which is pressed by the split die group closer to the end of the long axis of the cross section of the mother tube, after pressing by the split die closer to the end of the long axis of the cross section of the mother tube is started, it is preferable to bring the split dies constituting the split die group into contact with the outer circumferential surface of the mother tube in the order of proximity to the end of the long axis of the cross section of the mother tube.

[0079] Therefore, a fourth method according to another preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that when a split mold group consisting of a plurality of split dies is abutted against the outer peripheral surface of the mother tube, the timing at which the split dies are abutted against the outer peripheral surface of the mother tube is in ascending order of the distance between each split dies and the end of the long axis.

[0080] For example, if, among the 16 split dies illustrated in (a) of Figure 6, the split die 23e and the split dies 23d located on either side of them form one split die group, when the split die group is brought into contact with the outer peripheral surface of the mother tube 13, the split die 23d, which is the split die closer to the end of the long axis DL of the cross-section of the mother tube 13, is brought into contact with the outer peripheral surface of the mother tube 13 first, and the split die 23e, which is the split die farther from the end of the long axis DL of the cross-section of the mother tube 13 (i.e., closer to the end of the short axis DS), is brought into contact with the outer peripheral surface of the mother tube 13 last.

[0081] As described above, according to the fourth method according to another preferred embodiment, even in a region where the pressing surfaces of one split die group face each other, it is possible to avoid a situation where pressing by a split die closer to the end of the long axis of the cross section of the mother tube is started after pressing by a split die closer to the end of the short axis of the cross section of the mother tube is started. As a result, it is possible to more reliably reduce the possibility of unintended deformation such as buckling and / or steps occurring in the region pressed by the split die farther from the end of the long axis of the cross section of the mother tube (i.e., closer to the end of the short axis).

[0082] Fifth Embodiment Hereinafter, a method for reducing the diameter of a metal tube according to a fifth embodiment of the present invention (hereinafter, sometimes referred to as the "fifth method") will be described with reference to the drawings.

[0083] As described above, in the second method, the boundary between adjacent split mold groups exists at a position facing the point (inflection point) where adjacent curves are connected in the cross section of the mother tube. That is, each of the multiple split mold groups faces an area of ​​the outer circumferential surface of the mother tube corresponding to each of the curves constituting the cross section of the mother tube. This makes it easy to set the timing for abutting the split mold groups for each boundary area of ​​the outer circumferential surface of the mother tube corresponding to the multiple curves constituting the cross section of the mother tube. That is, it makes it easy to set a time difference in the timing for abutting the multiple split mold groups on the outer circumferential surface of the mother tube depending on the distribution of surface rigidity on the outer circumferential surface of the mother tube.

[0084] The inventor has discovered that when, as described above, each of the multiple split die groups faces an area of ​​the outer peripheral surface of the mother tube that corresponds to each of the curves that make up the cross section of the mother tube, the possibility of unintended deformation such as buckling and / or steps occurring in that area can be reduced by lastly abutting the split die group that presses the area corresponding to the longest curve among the curves that make up the cross section of the mother tube against the outer peripheral surface of the mother tube.

[0085] Therefore, the fifth method is the above-mentioned second method, characterized in that the split mold group, which presses the region of the outer circumferential surface of the mother tube corresponding to the longest curve among the curves constituting the cross section of the mother tube, among the multiple split mold groups, is brought into contact with the outer circumferential surface of the mother tube last. In other words, in the fifth method, among the timings at which the multiple split mold groups are brought into contact with the outer circumferential surface of the mother tube, the timing at which the split mold group, which presses the region of the outer circumferential surface of the mother tube corresponding to the longest curve among the curves constituting the cross section of the mother tube, is brought into contact with the outer circumferential surface of the mother tube last. This makes it possible to avoid starting pressing the region corresponding to a shorter curve after starting pressing the region corresponding to the longest curve, thereby reducing the possibility of problems such as unintended deformation occurring in the region of the outer circumferential surface of the mother tube corresponding to the longest curve. The "length of the curve" referred to here may be the actual length of the curve constituting the cross section of the mother tube, or may be the length of an arc or a chord when the curve constituting the cross section of the mother tube is regarded as a circular arc.

[0086] In the fifth method, the timing for bringing the split die groups other than the split die group that presses the region of the outer circumferential surface of the mother tube corresponding to the longest curve among the curves that make up the cross section of the mother tube into contact with the outer circumferential surface of the mother tube is not particularly limited, as long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in the region of the outer circumferential surface of the mother tube that corresponds to the longest curve among the curves that make up the cross section of the mother tube, it is preferable to bring the split die group that presses the region of the outer circumferential surface of the mother tube that corresponds to the shortest curve among the curves that make up the cross section of the mother tube into contact with the outer circumferential surface of the mother tube first.

[0087] Therefore, a fifth method according to a preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that, in addition to bringing into contact with the outer circumferential surface of the mother tube last, the split mold group among the plurality of split mold groups that presses a region of the outer circumferential surface of the mother tube that corresponds to the longest curve among the curves that constitute the cross section of the mother tube, as described above, the split mold group among the plurality of split mold groups that presses a region of the outer circumferential surface of the mother tube that corresponds to the shortest curve among the curves that constitute the cross section of the mother tube is brought into contact with the outer circumferential surface of the mother tube first. In other words, in the fifth method according to a preferred embodiment, among the timings at which the plurality of split mold groups are brought into contact with the outer circumferential surface of the mother tube, the timing at which the split mold group is brought into contact with the region of the outer circumferential surface of the mother tube that corresponds to the longest curve among the curves that constitute the cross section of the mother tube is the latest, and the timing at which the split mold group is brought into contact with the region of the outer circumferential surface of the mother tube that corresponds to the shortest curve among the curves that constitute the cross section of the mother tube is the earliest.

[0088] According to a fifth method according to a preferred embodiment, the possibility of unintended deformation such as buckling and / or steps occurring in the region on the outer circumferential surface of the mother tube corresponding to the longest curve among the curves constituting the cross section of the mother tube can be reduced by avoiding the start of pressing of other regions after pressing of the region corresponding to the longest curve among the curves constituting the cross section of the mother tube has started. In addition, the possibility of unintended deformation such as buckling and / or steps occurring in the other regions can be reduced by avoiding the start of pressing of the region on the outer circumferential surface of the mother tube corresponding to the shortest curve among the curves constituting the cross section of the mother tube after pressing of the other regions has started.

[0089] In the fifth method according to a preferred embodiment, the timing for bringing the split die set into contact with a region of the outer circumferential surface of the mother tube that is neither the region corresponding to the longest curve nor the region corresponding to the shortest curve among the curves constituting the cross section of the mother tube is not particularly limited, so long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in all regions, it is preferable to bring the split die set into contact with the region of the outer circumferential surface of the mother tube that is corresponding to the shortest curve among the curves constituting the cross section of the mother tube, in ascending order of the length of the curves constituting the cross section of the mother tube, from the region of the outer circumferential surface of the mother tube that is corresponding to the shortest curve among the curves constituting the cross section of the mother tube to the region of the outer circumferential surface of the mother tube that is corresponding to the longest curve.

[0090] Therefore, a fifth method according to a more preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that the timings for bringing the multiple split die groups into contact with the outer peripheral surface of the mother tube are in ascending order of the length of the curve constituting the cross section of the mother tube corresponding to the region of the outer peripheral surface of the mother tube that each split die group faces. In other words, in the fifth method according to a more preferred embodiment, the split die groups are brought into contact with the outer peripheral surface of the mother tube in ascending order of the length of the curve constituting the cross section of the mother tube, from the region of the outer peripheral surface of the mother tube corresponding to the shortest curve among the curves constituting the cross section of the mother tube to the region of the outer peripheral surface of the mother tube corresponding to the longest curve. This makes it possible to avoid starting pressing of the region of the outer peripheral surface of the mother tube with a longer curve before starting pressing of the region of the outer peripheral surface of the mother tube with a shorter curve, and more reliably reduces the possibility of unintended deformation such as buckling and / or steps occurring in all regions.

[0091] FIG. 7 is a schematic diagram showing an example of the diameter reduction step of the blank tube in the fifth method. FIG. 7(a) shows the arrangement of the multiple split dies and blank tube just before the start of the diameter reduction step. The blank tube 14 shown in FIG. 7 has a substantially rectangular cross section consisting of three or more sets of different opposing curves having different radii of curvature and lengths. In the example shown in FIG. 7, the blank tube 14 is reduced in diameter by pressing the outer circumferential surface of the blank tube 14 with eight split die groups (two split die groups 24a, four split die groups 24b, and two split die groups 24c). Of the three sets of curves constituting the cross section of the blank tube 14, the curve corresponding to the region facing the pressing surface of the split die group 24a is the shortest, the curve corresponding to the region facing the pressing surface of the split die group 24b is the next shortest, and the curve corresponding to the region facing the pressing surface of the split die group 24c is the longest. In order to simplify the drawings, reference numerals are omitted in FIG. 7(b) to (d).

[0092] In the process of reducing the diameter of the mother tube 14 illustrated in FIG. 7, pressing processes of the outer circumferential surface of the mother tube 14 are performed in the order of (b), (c), and (d). In FIG. 7(b), which shows the pressing process performed first, the split die group 24a of the three split die groups 24a, 24b, and 24c presses the region of the outer circumferential surface of the mother tube 14 facing the pressing surface of the split die group 24a. As described above, the region facing the pressing surface of the split die group 24a is the region corresponding to the shortest curve among the three curves constituting the cross section of the mother tube 14. That is, in the process of reducing the diameter of the mother tube 14 illustrated in FIG. 7, the split die group 24a first comes into contact with the region of the outer circumferential surface of the mother tube 14 corresponding to the shortest curve among the curves constituting the cross section of the mother tube 14, and presses the region. Therefore, the process of reducing the diameter of the mother tube 14 illustrated in FIG. 7 corresponds to the above-mentioned "fifth method according to a preferred embodiment".

[0093] 7(c), which shows the pressing process to be performed next, the split die group 24b of the three split die groups 24a, 24b, and 24c presses the region of the outer circumferential surface of the mother tube 14 that faces the pressing surface of the split die group 24b. As described above, the region that faces the pressing surface of the split die group 24b is a region that corresponds to a curve having an intermediate length among the three sets of curves that constitute the cross section of the mother tube 14.

[0094] In (d) of Fig. 7, which shows the pressing process to be performed last, the split die group 24c out of the three split die groups 24a, 24b, and 24c presses the region facing the pressing surface of the split die group 24c. As described above, the region facing the pressing surface of the split die group 24c is the region corresponding to the longest curve among the three curves constituting the cross section of the mother tube 14. That is, in the diameter reduction process of the mother tube 14 illustrated in Fig. 7, the split die group 24c finally comes into contact with the region of the outer circumferential surface of the mother tube 14 corresponding to the longest curve among the curves constituting the cross section of the mother tube 14, and presses that region. Therefore, the diameter reduction process of the mother tube 14 illustrated in Fig. 7 also corresponds to the above-mentioned "fifth method".

[0095] 7(b), (c), and (d), the pressing process is performed on the outer circumferential surface of the mother tube 14 in the order of the region facing the pressing surface of the split die group 24a, the region facing the pressing surface of the split die group 24b, and the region facing the pressing surface of the split die group 24c. That is, as the diameter reduction process of the mother tube 14 progresses from the region of the outer circumferential surface of the mother tube 14 corresponding to the shortest curve among the curves constituting the cross section of the mother tube 14 to the region of the outer circumferential surface of the mother tube 14 corresponding to the longest curve among the curves constituting the cross section of the mother tube 14, the split die groups 24a, 24b, and 24c come into contact with the outer circumferential surface of the mother tube 14 and press each region in order of the length of the curve constituting the cross section of the mother tube 14. Therefore, the process of reducing the diameter of the raw tube 14 illustrated in FIG. 7 also corresponds to the above-mentioned "fifth method according to a more preferred embodiment."

[0096] As illustrated in Figure 7, the fifth method avoids starting pressing of the region in the cross section of the mother tube where the curve is longer after starting pressing of the region in the cross section of the mother tube where the curve is shorter, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the region in the cross section of the mother tube where the curve is longer.

[0097] Sixth Embodiment A method for reducing the diameter of a metal tube according to a sixth embodiment of the present invention (hereinafter, may be referred to as the "sixth method") will be described below with reference to the drawings.

[0098] The inventor has discovered that when, as in the second method described above, each of the multiple split die groups faces a region of the outer circumferential surface of the mother tube corresponding to each of the curves that constitute the cross section of the mother tube, the possibility of unintended deformation such as buckling and / or steps occurring in the region of the outer circumferential surface of the mother tube that corresponds to the curve with the largest radius of curvature among the curves that constitute the cross section of the mother tube can be reduced by abutting the outer circumferential surface of the mother tube last, among the multiple split die groups, which presses the region of the outer circumferential surface of the mother tube that corresponds to the curve with the largest radius of curvature among the curves that constitute the cross section of the mother tube.

[0099] Therefore, the sixth method is a method for reducing the diameter of a metal tube, which is the second method described above, characterized in that the split mold group, which presses the region of the outer circumferential surface of the mother tube corresponding to the curve having the largest radius of curvature among the curves constituting the cross section of the mother tube, among the multiple split mold groups is brought into contact with the outer circumferential surface of the mother tube last. In other words, in the sixth method, among the timings at which the multiple split mold groups are brought into contact with the outer circumferential surface of the mother tube, the timing at which the split mold group, which presses the region of the outer circumferential surface of the mother tube corresponding to the curve having the largest radius of curvature among the curves constituting the cross section of the mother tube, is brought into contact with the outer circumferential surface of the mother tube last. This makes it possible to avoid the start of pressing the region corresponding to the curve having a smaller radius of curvature after pressing the region corresponding to the curve having the largest radius of curvature has started, thereby reducing the possibility of problems such as unintended deformation occurring in the region of the outer circumferential surface of the mother tube corresponding to the curve having the largest radius of curvature. The "radius of curvature" referred to here does not necessarily have to be the radius of curvature of the curve constituting the cross section of the mother tube in the strict sense, but may be, for example, the radius of curvature when the curve constituting the cross section of the mother tube is regarded as a circular arc.

[0100] In the sixth method, the timing for bringing the segment die groups other than the segment die group that presses the region of the outer circumferential surface of the mother tube corresponding to the curve with the largest radius of curvature among the curves that make up the cross section of the mother tube into contact with the outer circumferential surface of the mother tube is not particularly limited, as long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in the region of the outer circumferential surface of the mother tube that corresponds to the curve with the largest radius of curvature among the curves that make up the cross section of the mother tube, it is preferable to bring the segment die group that presses the region of the outer circumferential surface of the mother tube that corresponds to the curve with the smallest radius of curvature among the curves that make up the cross section of the mother tube into contact with the outer circumferential surface of the mother tube first.

[0101] Therefore, a sixth method according to a preferred embodiment is a method for reducing the diameter of a metal pipe, characterized in that, in addition to bringing into contact with the outer circumferential surface of the mother pipe last the split mold group that presses a region of the outer circumferential surface of the mother pipe corresponding to the curve having the largest radius of curvature among the curves that constitute the cross section of the mother pipe, among the plurality of split mold groups, as described above, into contact with the outer circumferential surface of the mother pipe first the split mold group that presses a region of the outer circumferential surface of the mother pipe corresponding to the curve having the smallest radius of curvature among the curves that constitute the cross section of the mother pipe. In other words, in the sixth method according to a preferred embodiment, among the timings at which the plurality of split mold groups are brought into contact with the outer circumferential surface of the mother pipe, the timing at which the split mold group is brought into contact with the region of the outer circumferential surface of the mother pipe corresponding to the curve having the largest radius of curvature among the curves that constitute the cross section of the mother pipe is the latest, and the timing at which the split mold group is brought into contact with the region of the outer circumferential surface of the mother pipe corresponding to the curve having the smallest radius of curvature among the curves that constitute the cross section of the mother pipe is the earliest.

[0102] According to a sixth method according to a preferred embodiment, the possibility of unintended deformation such as buckling and / or steps occurring in the region on the outer circumferential surface of the mother tube corresponding to the curve with the largest radius of curvature among the curves constituting the cross section of the mother tube can be reduced by avoiding the start of pressing of other regions after pressing of the region corresponding to the curve with the largest radius of curvature among the curves constituting the cross section of the mother tube has started. In addition, the possibility of unintended deformation such as buckling and / or steps occurring in the other regions can be reduced by avoiding the start of pressing of the region on the outer circumferential surface of the mother tube corresponding to the curve with the smallest radius of curvature among the curves constituting the cross section of the mother tube after pressing of the other regions has started.

[0103] In the sixth method according to a preferred embodiment, the timing for bringing the split die set into contact with a region of the outer circumferential surface of the mother tube that is neither the region of the outer circumferential surface of the mother tube that corresponds to the curve with the largest radius of curvature among the curves constituting the cross section of the mother tube nor the region of the outer circumferential surface of the mother tube that corresponds to the curve with the smallest radius of curvature among the curves constituting the cross section of the mother tube is not particularly limited, so long as the above-mentioned effects can be achieved. However, from the viewpoint of reducing the possibility of problems such as unintended deformation occurring in all regions, it is preferable to bring the split die set into contact with the region of the outer circumferential surface of the mother tube that corresponds to the curve with the smallest radius of curvature among the curves constituting the cross section of the mother tube in ascending order of the radius of curvature of the curves constituting the cross section of the mother tube, from the region of the outer circumferential surface of the mother tube that corresponds to the curve with the smallest radius of curvature to the region of the outer circumferential surface of the mother tube that corresponds to the curve with the largest radius of curvature among the curves constituting the cross section of the mother tube.

[0104] Therefore, a sixth method according to a more preferred embodiment is a method for reducing the diameter of a metal tube, characterized in that the timings for bringing a plurality of split die groups into contact with the outer peripheral surface of the mother tube are in ascending order of the magnitude of the radius of curvature of the curve constituting the cross section of the mother tube corresponding to the region of the outer peripheral surface of the mother tube that each split die group faces. In other words, in the sixth method according to a more preferred embodiment, the split die groups are brought into contact with the outer peripheral surface of the mother tube in ascending order of the radius of curvature of the curve constituting the cross section of the mother tube, from the region of the outer peripheral surface of the mother tube corresponding to the curve with the smallest radius of curvature among the curves constituting the cross section of the mother tube to the region of the outer peripheral surface of the mother tube corresponding to the longest curve. This makes it possible to avoid starting pressing of the region of the curve constituting the cross section of the mother tube with a smaller radius of curvature after starting pressing of the region of the curve constituting the cross section of the mother tube with a larger radius of curvature, and more reliably reduces the possibility of unintended deformation such as buckling and / or steps occurring in all regions.

[0105] FIG. 8 is a schematic diagram showing an example of a diameter reduction step of a mother tube in the sixth method. FIG. 8(a) shows an arrangement of a plurality of split dies and a mother tube immediately before the start of the diameter reduction step. The mother tube 15 shown in FIG. 8 has a substantially rectangular cross section consisting of three different sets of opposing curves having different radii of curvature. In the example shown in FIG. 8, the outer circumferential surface of the mother tube 15 is pressed by 16 split die groups (four split die groups 25a, eight split die groups 25b, and four split die groups 25c) to reduce the diameter of the mother tube 15. Among the three sets of curves constituting the cross section of the mother tube 15, the curve corresponding to the region facing the pressing surface of the split die group 25a has the smallest curvature radius, the curve corresponding to the region facing the pressing surface of the split die group 25b has the next smallest curvature radius, and the curve corresponding to the region facing the pressing surface of the split die group 25c has the largest curvature radius. In order to simplify the drawings, reference numerals are omitted in (b) to (d) of FIG.

[0106] In the diameter reduction process of the mother tube 15 illustrated in FIG. 8, pressing processes of the outer circumferential surface of the mother tube 15 are performed in the order of (b), (c), and (d). In FIG. 8(b), which shows the pressing process performed first, the split die group 25a of the three split die groups 25a, 25b, and 25c presses the region of the outer circumferential surface of the mother tube 15 facing the pressing surface of the split die group 25a. As described above, the region facing the pressing surface of the split die group 25a is the region corresponding to the curve having the smallest radius of curvature among the three curves constituting the cross section of the mother tube 15. That is, in the diameter reduction process of the mother tube 15 illustrated in FIG. 8, the split die group 25a first comes into contact with the region of the outer circumferential surface of the mother tube 15 corresponding to the curve having the smallest radius of curvature among the curves constituting the cross section of the mother tube 15, and presses the region. Therefore, the diameter reduction process of the mother tube 15 illustrated in FIG. 8 corresponds to the above-mentioned "sixth method according to a preferred embodiment".

[0107] 8(c), which shows the pressing process to be performed next, the split die group 25b of the three split die groups 25a, 25b, and 25c presses the region of the outer circumferential surface of the mother tube 15 that faces the pressing surface of the split die group 25b. As described above, the region that faces the pressing surface of the split die group 25b corresponds to the curve having an intermediate radius of curvature among the three curves that constitute the cross section of the mother tube 15.

[0108] In (d) of Fig. 8, which shows the pressing process to be performed last, the split die group 25c out of the three split die groups 25a, 25b, and 25c presses the region facing the pressing surface of the split die group 25c. As described above, the region facing the pressing surface of the split die group 25c is the region corresponding to the curve having the largest radius of curvature among the three curves constituting the cross section of the mother tube 15. That is, in the diameter reduction process of the mother tube 15 illustrated in Fig. 8, the split die group 25c finally comes into contact with the region of the outer circumferential surface of the mother tube 15 corresponding to the curve having the largest common radius among the curves constituting the cross section of the mother tube 15, and presses the region. Therefore, the diameter reduction process of the mother tube 15 illustrated in Fig. 8 also corresponds to the above-mentioned "sixth method".

[0109] As described above, as the diameter reducing process of the mother tube 15 progresses from (b), (c), to (d) in Fig. 8, the pressing process is performed in the order of the region on the outer circumferential surface of the mother tube 15 facing the pressing surface of the split die group 25a, the region facing the pressing surface of the split die group 25b, and the region facing the pressing surface of the split die group 25c. That is, as the diameter reducing process of the mother tube 15 progresses from (b), (c), to (d) in Fig. 8, the split die groups 25a, 25b, and 25c come into contact with the outer circumferential surface of the mother tube 15 and press each region in the order of the smallest radius of curvature of the curves constituting the cross section of the mother tube 15 to the region corresponding to the largest radius of curvature of the curves constituting the cross section of the mother tube 15. Therefore, the diameter reducing process of the mother tube 15 illustrated in Fig. 8 also corresponds to the above-mentioned "sixth method according to a more preferred embodiment".

[0110] As illustrated in Figure 8, the sixth method avoids starting pressing of a region in which the curve constituting the cross section of the mother tube has a larger radius of curvature after starting pressing of a region in which the curve constituting the cross section of the mother tube has a smaller radius of curvature, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the region in which the curve constituting the cross section of the mother tube has a larger radius of curvature.

[0111] Seventh Embodiment A method for reducing the diameter of a metal tube according to a seventh embodiment of the present invention (hereinafter, may be referred to as the "seventh method") will be described below.

[0112] As described above, the methods for reducing the diameter of a metal tube according to the present invention, including the first method to the sixth method (methods of the present invention), are methods for reducing the diameter of a metal tube, in which the diameter of the mother tube is reduced by pressing the outer peripheral surface of the mother tube with a plurality of split dies that are arranged around the axis of the mother tube and has a substantially rectangular cross section, thereby reducing the diameter of the mother tube.

[0113] In the method of the present invention, the direction in which the multiple split dies press the outer circumferential surface of the mother tube is not particularly limited, so long as the effects described above can be achieved. For example, in each split die group, the outer circumferential surface of the mother tube may be pressed toward the pressing surface of the split die constituting the split die group or toward the center corresponding to the radius of curvature of the region of the mother tube pressed by the pressing surface. However, when the pressing direction of the split dies is set for each split die group in this way, independent movement control is required for each split die group, which may complicate the control system and pressing mechanism.

[0114] Therefore, the seventh method is a method for reducing the diameter of a metal tube according to the present invention (method of the present invention) including the above-mentioned first to sixth methods, and is characterized in that a plurality of split dies press the outer peripheral surface of the mother tube toward the central axis of the mother tube.

[0115] Fig. 9 is a schematic diagram for explaining the moving direction of the multiple split dies (the direction in which the outer circumferential surface of the mother tube is pressed) in the seventh method. Fig. 9(a) is a schematic diagram showing an example of a cross section of a mother tube whose diameter is reduced by the seventh method, and Fig. 9(b) is a schematic diagram showing an example of an arrangement of the multiple split dies and the mother tube just before the start of the seventh method. Fig. 9(a) and (b) respectively correspond to Fig. 1(a) and (b) referred to in the explanation of the first method described above.

[0116] 9, each of the split dies 21a, 21b, and 21c has a generally linearly symmetrical shape in a cross section taken along a plane perpendicular to the central axis of the mother tube 11, and when considered as linearly symmetric, the axis of symmetry (dash-dotted line) of each of the split dies is parallel to the moving direction of the split dies (thick solid arrow). Furthermore, the extensions of the axes of symmetry of the multiple split dies 21a, 21b, and 21c intersect at the central axis AX (open circle) of the mother tube 11. That is, in the seventh method, the multiple split dies are configured to press the outer circumferential surface of the mother tube toward the central axis of the mother tube.

[0117] According to the seventh method, it is possible to simplify the control system and pressing mechanism without the need for independent movement control for each split mold group, as in the case of setting the pressing direction of the split mold for each of the above-mentioned split mold groups. Also, from the viewpoint of smoothly operating the multiple split molds, the seventh method in which the multiple split molds press the outer circumferential surface of the mother tube toward the central axis of the mother tube is preferable.

[0118] However, the seventh method is merely one example among a wide variety of aspects included in the method for reducing the diameter of a metal tube according to the present invention (the method of the present invention). That is, in the method of the present invention, it is not necessary that all of the constituent elements of the seventh method are satisfied. For example, a split mold may exist whose shape in a cross section taken along a plane perpendicular to the central axis of the mother tube is not line-symmetric. Also, among the split molds having a shape that is approximately line-symmetric in a cross section taken along a plane perpendicular to the central axis of the mother tube, a split mold whose axis of symmetry is not parallel to the moving direction of the split mold may exist. Furthermore, among the split molds having a shape that is approximately line-symmetric in a cross section taken along a plane perpendicular to the central axis of the mother tube, a split mold whose extension line of the axis of symmetry does not intersect with the central axis of the mother tube may exist. In addition, among the split molds having a shape that is approximately line-symmetric in a cross section taken along a plane perpendicular to the central axis of the mother tube, a split mold whose axis of symmetry is parallel to the moving direction of the split mold and whose extension line of the axis of symmetry does not intersect with the central axis of the mother tube may exist. EXAMPLES

[0119] An embodiment of the method for reducing the diameter of a metal tube according to the present invention (method of the present invention), including the first to seventh methods described above, will be described below with reference to the drawings. However, the various embodiments described below are merely illustrative.

[0120] FIG. 10 is a schematic diagram showing the arrangement of a plurality of split molds and a blank tube immediately before the start of a method for reducing the diameter of a metal tube according to a first embodiment of the present invention (hereinafter, sometimes referred to as "method 1 of the embodiment"). As shown in FIG. 10, eight split mold groups (two split mold groups 26a, four split mold groups 26b, and two split mold groups 26c) are arranged to surround the blank tube 16 around the axis, and each of the split mold groups 26a, 26b, and 26c is composed of one split mold. In the method 1 of the embodiment, the outer circumferential surface of the blank tube 16 is pressed by the eight split mold groups shown in FIG. 10, and the blank tube 16 is reduced in diameter. Note that, on the outer circumferential surface of the blank tube 16, the surface rigidity of the region facing the pressing surface of the split mold group 26a is the highest, the surface rigidity of the region facing the pressing surface of the split mold group 26b is the second highest, and the surface rigidity of the region facing the pressing surface of the split mold group 26c is the lowest.

[0121] 11 to 19 are schematic diagrams showing the movement of the split mold groups in the pressing process performed by each of the split mold groups in Example Method 1. In addition, in Fig. 11 to 19, the reference numerals of the split molds and the blank tube are omitted in order to simplify the drawings.

[0122] In the embodiment method 1, first, as shown in Fig. 11, among the split die groups 26a to 26c, the split die group 26a facing the region having the highest surface rigidity on the outer circumferential surface of the mother tube 16 first comes into contact with the outer circumferential surface of the mother tube 16 and starts pressing the region facing the pressing surface. Next, as shown in Fig. 12, among the remaining split die groups 26b and 26c, the split die group 26b facing the region having an intermediate surface rigidity on the outer circumferential surface of the mother tube 16 comes into contact with the outer circumferential surface of the mother tube 16 and starts pressing the region facing the pressing surface. Next, as shown in Fig. 13, among the split die groups 26a to 26c, the split die group 26c facing the region having the lowest surface rigidity on the outer circumferential surface of the mother tube 16 comes into contact with the outer circumferential surface of the mother tube 16 and starts pressing the region facing the pressing surface. 11, 12, and 13, the split die group comes into contact with and presses each region on the outer circumferential surface of the mother tube 16 in descending order of surface rigidity, from the region with the highest surface rigidity to the region with the lowest surface rigidity. Therefore, the diameter reducing process of the mother tube 16 in embodiment method 1 illustrated in Figures 11 to 13 corresponds to the above-mentioned "third method according to a more preferred embodiment."

[0123] In addition, in embodiment method 1, as shown in Figures 14 to 19, a series of pressing processes similar to those illustrated in Figures 11 to 13 described above are repeated two more times to gradually bring the cross section of the blank tube 16 closer to the target cross section.

[0124] 10 to 19, in the embodiment method 1, it is possible to avoid starting pressing of an area having a higher surface rigidity after starting pressing of an area having a lower surface rigidity, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the area having a lower surface rigidity. As a result, according to the embodiment method 1, it is possible to manufacture a tubular member having a smooth and accurate substantially rectangular cross section by reducing the diameter of a metal blank tube having a substantially rectangular cross section constituted by three or more sets of opposing curves having different radii of curvature. EXAMPLES

[0125] Another embodiment of the method of the present invention will be described below with reference to the drawings. Fig. 20 is a schematic diagram showing the arrangement of multiple split dies and a blank tube immediately before the start of a method of reducing the diameter of a metal tube according to embodiment 2 of the present invention (hereinafter, may be referred to as "embodiment method 2"), in which the minor axis DS and major axis DL of the approximately rectangular cross section are indicated by thick double-headed arrows.

[0126] In the example shown in FIG. 20, 12 split mold groups (two split mold groups 27a, four split mold groups 27b, four split mold groups 27c, and two split mold groups 27d) are arranged to surround the mother tube 17 around the axis. That is, each of the multiple split mold groups 27a, 27b, 27c, and 27d is composed of one split mold. In the embodiment method 2, the outer circumferential surface of the mother tube 17 is pressed by the 12 split mold groups shown in FIG. 20, and the mother tube 17 is reduced in diameter. FIGS. 21 to 32 are schematic diagrams showing the movement of the split mold groups in the pressing process performed by each split mold group in the embodiment method 2. In addition, in order to simplify the drawings, the reference symbols of the split molds and the mother tube are omitted in FIGS. 21 to 32.

[0127] In the second embodiment method, first, as shown in Fig. 21, among the split dies 27a to 27d, the split die group 27a closest to the end (tip of the double arrow) of the long axis DL of the cross section of the mother tube 17 comes into contact with the outer circumferential surface of the mother tube 17 and starts pressing the area facing the pressing surface. Next, as shown in Fig. 22, among the remaining split dies 27b to 27d, the split die 27b closest to the end of the long axis DL of the cross section of the mother tube 17 comes into contact with the outer circumferential surface of the mother tube 17 and starts pressing the area facing the pressing surface. Next, as shown in Fig. 23, among the remaining split dies 27c and 27d, the split die 27c closest to the end of the long axis DL of the cross section of the mother tube 17 comes into contact with the outer circumferential surface of the mother tube 17 and starts pressing the area facing the pressing surface. Next, as shown in Fig. 24, among the split dies 27a to 27d, the split die 27d that is furthest from the end of the long axis DL of the cross section of the mother tube 17, i.e., that is closest to the end of the short axis DS of the cross section of the mother tube 17 (the tip of the double arrow), comes into contact with the outer circumferential surface of the mother tube 17 and starts pressing the area opposite to the pressing surface. Thus, in the embodiment method 2, the timings at which the four types of split dies 27a to 27d are brought into contact with the outer circumferential surface of the mother tube 17 are in ascending order of the distance between each of the split die groups and the end of the long axis DL. Therefore, the diameter reduction process of the mother tube 17 illustrated in Figs. 20 to 24 corresponds to the above-mentioned "fourth method according to a more preferred embodiment".

[0128] In addition, in embodiment method 1, as shown in Figures 25 to 32, a series of pressing processes similar to those illustrated in Figures 21 to 24 described above are repeated two more times to gradually bring the cross-section of the blank tube 17 closer to the target cross-section.

[0129] 20 to 32, in the embodiment method 2, it is possible to avoid starting pressing by the split die group closer to the end of the long axis of the cross section of the mother tube after starting pressing by the split die group closer to the end of the short axis of the cross section of the mother tube, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the region pressed by the split die group closer to the end of the short axis of the cross section of the mother tube. As a result, according to the embodiment method 2, it is possible to manufacture a tubular member having a smooth and accurate substantially rectangular cross section by reducing the diameter of a metal mother tube having a substantially rectangular cross section constituted by three or more sets of opposing curves having different radii of curvature. EXAMPLES

[0130] Another embodiment of the method of the present invention will be described below with reference to the drawings. Fig. 33 is a schematic diagram showing the arrangement of a plurality of split dies and a mother tube immediately before the start of a method for reducing the diameter of a metal tube according to embodiment 3 of the present invention (hereinafter, sometimes referred to as "embodiment method 3"). The mother tube 18 illustrated in Fig. 33 has a substantially rectangular cross section consisting of three different sets of opposing curves having different radii of curvature. In the example shown in Fig. 33, the outer circumferential surface of the mother tube 18 is pressed by 16 split die groups (four split die groups 28a, eight split die groups 28b, and four split die groups 28c) to reduce the diameter of the mother tube 18. Among the three sets of curves constituting the cross section of the mother tube 18, the curve corresponding to the region facing the pressing surface of the split die group 28a has the smallest curvature radius, the curve corresponding to the region facing the pressing surface of the split die group 28b has the next smallest curvature radius, and the curve corresponding to the region facing the pressing surface of the split die group 28c has the largest curvature radius. 34 to 42 are schematic diagrams showing the movement of the divided mold groups in the pressing process performed by each divided mold group in Example Method 3. In order to simplify the drawings, reference numerals are omitted in Figs. 34 to 42.

[0131] In the embodiment method 3, first, as shown in Fig. 34, the split die group 28a comes into contact with an area corresponding to a curve having the smallest radius of curvature among the curves constituting the cross section of the mother tube 18, and starts pressing the area facing the pressing surface. Next, as shown in Fig. 35, the split die group 28b comes into contact with an area corresponding to a curve having an intermediate radius of curvature among the curves constituting the cross section of the mother tube 18, and starts pressing the area facing the pressing surface. Next, as shown in Fig. 36, the split die group 28c comes into contact with an area corresponding to a curve having the largest radius of curvature among the curves constituting the cross section of the mother tube 18, and starts pressing the area facing the pressing surface. Thus, in the embodiment method 3, the timings at which the three types of split dies 28a to 28c are brought into contact with the outer circumferential surface of the mother tube 18 are in ascending order of the magnitude of the radius of curvature of the curve constituting the cross section of the mother tube 18 corresponding to the area of ​​the outer circumferential surface of the mother tube 18 that each split die group faces. Therefore, the process of reducing the diameter of the blank tube 18 illustrated in Figs. 33 to 36 corresponds to the above-mentioned "sixth method according to a more preferred embodiment."

[0132] In addition, in embodiment method 2, as shown in Figures 37 to 42, a series of pressing processes similar to those illustrated in Figures 34 to 36 described above are repeated two more times to gradually bring the cross-section of the blank tube 18 closer to the target cross-section.

[0133] 33 to 42, in the third embodiment method, it is possible to avoid starting pressing of a region in which the curve constituting the cross section of the mother tube has a smaller radius of curvature after starting pressing of a region in which the curve constituting the cross section of the mother tube has a larger radius of curvature, thereby reducing the possibility of unintended deformation such as buckling and / or steps occurring in the region in which the curve constituting the cross section of the mother tube has a larger radius of curvature. As a result, the third embodiment method makes it possible to manufacture a tubular member having a smooth and accurate substantially rectangular cross section by reducing the diameter of a metal mother tube having a substantially rectangular cross section constituted by three or more sets of opposing curves having different radii of curvature.

[0134] For the purpose of explaining the present invention, several embodiments and examples having specific configurations have been described above, sometimes with reference to the attached drawings. However, the scope of the present invention should not be construed as being limited to these exemplary embodiments and examples, and it goes without saying that appropriate modifications can be made within the scope of the claims and the matters described in the specification. [Explanation of symbols]

[0135] 11, 12, 13, 14, 15, 16, 17, 18...Plain tube 21a,21b,21c,22a,22b,22c,23a,23b,23c,23d,23e,24a,24b,24c,25a,25b,25c,26a,26b,26c,27a,27b,27c,27d,28a,28b,28c...Divided type group (divided type) 22c1,22c2…split type 31a, 31b, 31c…Kama

Claims

1. 1. A method for reducing a diameter of a metal tube, comprising the steps of: pressing an outer circumferential surface of a metal mother tube having a substantially rectangular cross section constituted by three or more sets of opposing curves having different radii of curvature with a plurality of split dies arranged around an axis of the mother tube so as to surround the mother tube, thereby reducing a diameter of the mother tube, The plurality of division types are divided into a plurality of division type groups each consisting of one or two or more adjacent division types, a split die group that presses an area of ​​the outer circumferential surface of the mother tube having the lowest surface rigidity among the plurality of split die groups, and finally abuts the outer circumferential surface of the mother tube.

2. 2. The method for reducing a diameter of a metal tube according to claim 1, a split die group that presses an area of ​​the outer circumferential surface of the mother tube having the highest surface rigidity among the plurality of split die groups, the split die group being brought into contact with the outer circumferential surface of the mother tube first.

3. 3. A method for reducing a diameter of a metal tube according to claim 2, comprising the steps of: a timing at which the plurality of split die groups are brought into contact with the outer peripheral surface of the mother tube is determined in descending order of surface rigidity of an area of ​​the outer peripheral surface of the mother tube that each of the split die groups faces.

4. A method for reducing a diameter of a metal tube according to any one of claims 1 to 3, comprising the steps of: a first split die group including a first die and a second split die, the second split die group being configured to contact the outer peripheral surface of the mother tube with each other at different timings in order of surface rigidity of an area of ​​the outer peripheral surface of the mother tube that each split die faces.

5. A method for reducing the diameter of a metal tube, comprising the steps of: compressing an outer peripheral surface of a metal tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature using a plurality of split dies arranged around the axis of the metal tube so as to surround the tube, thereby reducing the diameter of the tube; The plurality of division types are divided into a plurality of division type groups each consisting of one or two or more adjacent division types, a split die group that is closest to an end of a minor axis of the cross section of the mother tube is brought into contact with the outer peripheral surface of the mother tube last, among the plurality of split die groups.

6. 6. A method for reducing a diameter of a metal tube according to claim 5, comprising the steps of: a split die group that is closest to an end of a major axis of the mother tube in the cross section of the mother tube is first brought into contact with the outer circumferential surface of the mother tube.

7. 7. A method for reducing a diameter of a metal tube according to claim 6, comprising the steps of: a timing at which the plurality of split die groups are brought into contact with the outer peripheral surface of the mother tube is in ascending order of the distance between each of the split die groups and an end of the major axis.

8. A method for reducing a diameter of a metal tube according to any one of claims 5 to 7, comprising the steps of: abutting the split dies against the outer peripheral surface of the mother tube at timings in which the split dies are brought into contact with the outer peripheral surface of the mother tube in ascending order of distance between each of the split dies and an end of a major axis in the cross section of the mother tube.

9. A method for reducing the diameter of a metal tube according to any one of claims 1 to 8, comprising: a plurality of said split die groups each facing an area of ​​the outer circumferential surface of the mother tube corresponding to each of said curves constituting the cross section of the mother tube.

10. A method for reducing the diameter of a metal tube, comprising the steps of: compressing an outer peripheral surface of a metal tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature using a plurality of split dies arranged around the axis of the metal tube so as to surround the tube, thereby reducing the diameter of the tube; The plurality of division types are divided into a plurality of division type groups each consisting of one or two or more adjacent division types, each of the plurality of split mold groups faces an area of ​​an outer circumferential surface of the mother tube corresponding to each of the curves constituting the cross section of the mother tube, a split die group that presses an area of ​​the outer circumferential surface of the mother tube that corresponds to the longest curve among the curves that constitute the cross section of the mother tube, is finally brought into contact with the outer circumferential surface of the mother tube.

11. A method for reducing a diameter of a metal tube according to claim 10, comprising the steps of: a split die group that presses an area of ​​the outer circumferential surface of the mother tube that corresponds to the shortest curve among the curves that constitute the cross section of the mother tube, and a split die group that presses an area of ​​the outer circumferential surface of the mother tube that corresponds to the shortest curve among the curves that constitute the cross section of the mother tube is first brought into contact with the outer circumferential surface of the mother tube.

12. A method for reducing a diameter of a metal tube according to claim 11, comprising the steps of: a plurality of split die groups are brought into contact with the outer peripheral surface of the mother tube at timings in ascending order of lengths of the curves constituting the cross section of the mother tube that correspond to regions of the outer peripheral surface of the mother tube that each of the split die groups faces.

13. A method for reducing the diameter of a metal tube, comprising the steps of: compressing an outer peripheral surface of a metal tube having a substantially rectangular cross section formed by three or more sets of opposing curves having different radii of curvature using a plurality of split dies arranged around an axis of the metal tube so as to surround the tube, thereby reducing the diameter of the tube; The plurality of division types are divided into a plurality of division type groups each consisting of one or two or more adjacent division types, each of the plurality of split mold groups faces an area of ​​an outer circumferential surface of the mother tube corresponding to each of the curves constituting the cross section of the mother tube, a split die group that presses a region of the outer circumferential surface of the mother tube that corresponds to the curve that has the largest radius of curvature among the curves that constitute the cross section of the mother tube, among the plurality of split die groups, is finally brought into contact with the outer circumferential surface of the mother tube.

14. A method for reducing a diameter of a metal tube according to claim 13, comprising the steps of: a split die group for pressing a region of the outer circumferential surface of the mother tube that corresponds to the curve that has the smallest radius of curvature among the curves that constitute the cross section of the mother tube, the split die group being one of the plurality of split die groups first brought into contact with the outer circumferential surface of the mother tube.

15. 15. A method for reducing a diameter of a metal tube according to claim 14, comprising the steps of: a plurality of split die groups are brought into contact with the outer peripheral surface of the mother tube at timings in ascending order of magnitude of the radii of curvature of the curves constituting the cross section of the mother tube corresponding to regions of the outer peripheral surface of the mother tube that each of the split die groups faces.

16. A method for reducing a diameter of a metal tube according to any one of claims 1 to 15, comprising the steps of: a plurality of split dies pressing the outer circumferential surface of the mother tube toward a central axis of the mother tube.

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