Diameter-reducing apparatus for metal pipe

Abstract

To provide a technique capable of reducing the occurrence of buckling originating from a bulging portion in diameter-reducing processing of a metal pipe having a non-circular cross section.SOLUTION: In a diameter-reducing apparatus of a metal pipe which comprises a plurality of split dies arranged so as to surround a metal element pipe around a pipe axis of the element pipe having a non-circular cross-sectional shape that is a cross section by a plane perpendicular to the pipe axis, and a drive mechanism that drives the plurality of split dies toward an outer peripheral surface of the element pipe, and which is configured to reduce a diameter of the element pipe by pressing the outer peripheral surface of the element pipe by the plurality of split dies, a recess, which is a concave portion having a shape capable of accommodating the bulging portion, is formed in a region facing the bulging portion of a pressing surface being the surface that presses the element pipe of the split die coming into contact during the diameter-reducing processing with at least one row of bulging portions that bulge outward in a radial direction on the outer peripheral surface of the element pipe and extend in a pipe axial direction.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a device for reducing the diameter of a metal tube. [Background technology]

[0002] In the case of reducing the diameter of a metal tube having a non-circular (particularly rotationally symmetric) cross section, methods for obtaining a metal tube having a desired cross section by individually controlling the stroke of radially arranged split dies have been proposed, for example, in Patent Document 1 (JP 2023-16090 A) and Patent Document 2 (JP 4856618 A). These methods make it possible to form a metal tube as a container having a non-circular and rotationally symmetric cross section (for example, an ellipse, pseudo-ellipse, oval, or pseudo-rectangle) that is often used, for example, in catalytic converters and / or silencers (mufflers) in automobile exhaust systems.

[0003] In so-called "plate-wrapped pipes" formed by plate-wrapped pipe manufacturing, a bulge (hereinafter sometimes referred to as a "bead") extending in the pipe axial direction exists at the location where the raw metal sheet is wound, butt-jointed, and welded. Because this bead bulges outward and inward in the radial direction, when a plate-wrapped pipe is used as a tubular member (hereinafter sometimes referred to as a "blank pipe") to form the above-mentioned metal pipe, the bead may cause the raw material to buckle (bend inward in the radial direction) during diameter reduction, as described above. This problem is unlikely to occur in metal pipes with a perfectly circular cross-section because a force acts evenly around the entire circumference of the metal pipe wall during diameter reduction. However, it tends to occur more easily in metal pipes with a noncircular cross-section as described above because a force acts unevenly on the metal pipe wall during diameter reduction. This problem is believed to occur due to the mechanism described below.

[0004] Figure 5 is a schematic diagram illustrating the process of buckling starting from the bead portion during diameter reduction of a sheet-wound tube as described above. Figure 5(a) is a schematic cross-sectional view illustrating the state in which the outer side of the bead portion is in contact with the split die at the start of diameter reduction, and corresponds to the area surrounded by the thick dashed line in Figure 5(c). As illustrated in Figure 5(a), the bead portion protrudes outward from the outer circumferential surface of the mother tube. Therefore, the tip of the bead portion abuts against the split die located outside the mother tube, forming gaps between the split die and the mother tube on both sides of the bead portion in the circumferential direction (see the area surrounded by the thick dashed line). Therefore, due to the combined force of the split die pressing the bead portion in the centripetal direction (centripetal direction) and the force acting on the metal tube in the direction shortening the circumferential length as the diameter is reduced, a force acting in a direction inclined relative to the centripetal direction (see the open arrow) acts on both sides of the bead portion. This force causes buckling to occur starting from the bead portion, as shown in FIG. 5(b).

[0005] As a result of the above, as shown in Figure 5(c), the cross-sectional outline of the metal pipe (see the thick solid line), which should have been uniformly reduced in diameter all around, has a distorted shape due to buckling starting from the bead (see the thick chain line). In view of this mechanism, it is thought that the occurrence of buckling can be reduced by reducing the occurrence of gaps between the split dies and the mother pipe on both sides of the bead.

[0006] Meanwhile, in this technical field, a technique for providing a recess (concave portion) in a die against which a bead portion abuts has been conventionally known in order to prevent deformation of the bead portion itself during expansion and / or straightening of a pipe material. Specific examples of such techniques include Patent Document 3 (JP 2002-35869 A) and Patent Document 4 (JP 6-34825 A) for expanding a pipe material, and Patent Document 5 (JP 2021-87970 A), Patent Document 6 (JP 2016-168620 A), and Patent Document 7 (JP 2009-183983 A) for straightening a pipe material.

[0007] As described above, the provision of a recess in the part of the die where the bead contacts is known, and it is expected that the bead will fit into the recess, eliminating gaps on both sides of the bead between the die and the pipe material. However, as mentioned at the beginning of this specification, there are no examples in the art of applying a recess to the diameter reduction of a metal pipe having a non-circular cross section. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2023-16090 [Patent Document 2] Patent No. 4856618 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-35869 [Patent Document 4] Japanese Utility Model Application Publication No. 6-34825 [Patent Document 5] Patent Publication No. 2021-87970 [Patent Document 6] Japanese Patent Application Laid-Open No. 2016-168620 [Patent Document 7] Japanese Patent Application Laid-Open No. 2009-183983 Summary of the Invention [Problem to be solved by the invention]

[0009] As described above, there is a need in the art for a technique that can reduce the occurrence of buckling originating from a bulge portion during diameter reduction of a metal tube having a non-circular cross section. [Means for solving the problem]

[0010] Therefore, as a result of intensive research, the inventors discovered that the above problem can be solved by forming a recess corresponding to the bulge portion on the surface of one of the multiple split dies that presses the outer peripheral surface of a metal blank tube having a non-circular cross section, the split die that abuts the bulge portion and presses the blank tube.

[0011] Specifically, the metal tube diameter reduction apparatus according to the present invention (hereinafter sometimes referred to as the "apparatus of the present invention") comprises a plurality of segmented dies arranged around the axis of a metal mother tube having a non-circular cross section, which is a cross section taken along a plane perpendicular to the tube axis, so as to surround the mother tube, and a drive mechanism that drives the plurality of segmented dies toward the outer peripheral surface of the mother tube. The apparatus of the present invention is configured to reduce the diameter of the mother tube by pressing the outer peripheral surface of the mother tube with the plurality of segmented dies.

[0012] Furthermore, the outer peripheral surface of the mother tube whose diameter is reduced by the device of the present invention has at least one bulging portion that bulges outward in the radial direction of the mother tube and extends in the axial direction of the tube.

[0013] In addition, in the device of the present invention, a recess, which is a concave portion having a shape that can accommodate the bulge portion, is formed in the area facing the bulge portion of the pressing surface of the abutting split mold, which is the split mold that abuts against the bulge portion during diameter reduction processing and is the surface that presses the base tube. [Effects of the Invention]

[0014] As described above, in the apparatus of the present invention, a recess having a shape capable of accommodating the bulge is formed in a region facing the bulge on a pressing surface of a contact split die, which is a split die that abuts against the bulge and is one of a plurality of split dies that press the outer circumferential surface of a metal mother tube having a noncircular cross section. Therefore, in the apparatus of the present invention, when the outer circumferential surface of the mother tube is pressed by the plurality of split dies to reduce the diameter of the mother tube, gaps formed between the contact split die and the mother tube on both sides of the bulge in the circumferential direction are eliminated or reduced. As a result, the apparatus of the present invention can reduce the occurrence of buckling originating from the bulge during diameter reduction of a mother tube having a noncircular cross section.

[0015] Other objects, other features and attendant advantages of the present invention will be readily apparent from the following description of the embodiments of the present invention which will be given with reference to the drawings. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram showing an example of the configuration of a diameter reduction device (first device) for a metal tube according to a first embodiment of the present invention. [Figure 2] 4A and 4B are schematic diagrams illustrating how the first device reduces the occurrence of buckling originating from a bulging portion when the diameter of the mother tube is reduced. [Figure 3] 10A and 10B are schematic diagrams illustrating the configuration of a contact split mold in which a recess is formed on a pressing surface. [Figure 4] FIG. 1 is a schematic diagram illustrating a cross section of a mother tube to be reduced in diameter by a metal tube reduction device (second device) according to a first embodiment of the present invention, and an arrangement of a plurality of split dies and the mother tube immediately before the start of diameter reduction processing by the second device. [Figure 5] 1 is a schematic diagram illustrating the process of buckling occurring starting from a bead portion when the diameter of a plate-wound tube is reduced. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0017] First Embodiment Hereinafter, an apparatus for reducing the diameter of a metal tube according to a first embodiment of the present invention (hereinafter, sometimes referred to as "first apparatus") will be described with reference to the drawings.

[0018] <composition> Fig. 1 is a schematic diagram showing an example of the configuration of a first apparatus. As shown in Fig. 1, the first apparatus 101 includes a plurality of split dies 21 arranged around the tube axis AX of a metal mother tube 11 having a non-circular cross section, which is a cross section taken along a plane perpendicular to the tube axis AX, so as to surround the mother tube 11, and a drive mechanism (not shown) that drives the plurality of split dies 21 toward the outer peripheral surface of the mother tube 11. The first apparatus 101 is configured to reduce the diameter of the mother tube 11 by pressing the outer peripheral surface of the mother tube 11 with the plurality of split dies 21.

[0019] As described above, the cross-sectional shape of the mother tube reduced by the first device is noncircular. In this specification, "noncircular" includes various shapes that are not perfect circles, such as elliptical, pseudo-elliptical, oval, or pseudo-rectangular. Details of the metal tube reduction device, such as the configuration and arrangement of the multiple split dies 21 and the configuration and operation of the drive mechanism, are well known to those skilled in the art, as disclosed in, for example, Patent Documents 1 and 2, and therefore will not be described in detail here. However, for example, a mechanism such as a combination of jaw segments and a ring portion can be used as the drive mechanism.

[0020] Furthermore, the outer peripheral surface of the mother tube whose diameter is reduced by the first device has at least one bulging portion that bulges outward in the radial direction of the mother tube and extends in the axial direction of the tube.

[0021] The bulging portion, which is the starting point for buckling during diameter reduction of a metal tube having a noncircular cross section to be reduced by the first device, is a portion that bulges outward at least in the radial direction of the mother tube, but does not necessarily have to bulge only outward in the radial direction. That is, the bulging portion may be a portion that bulges only outward in the radial direction, or a portion that bulges both outward and inward in the radial direction.

[0022] The mother pipe reduced by the first device is typically a so-called "plate-wrapped pipe" formed by a plate-wrapped pipe manufacturing process in which a metal pipe is formed by winding a metal plate and butting and welding the ends of the metal plate together. The welded portion generally bulges not only toward the outside but also toward the inside of the mother pipe, but it does not necessarily have to bulge toward the inside of the mother pipe. The mother pipe reduced by the first device is not particularly limited as long as it is a metal mother pipe with a noncircular cross-section and a bulged portion on the outer circumferential surface of the mother pipe that bulges outward and extends in the axial direction.

[0023] In addition, in the device of the present invention, a recess, which is a concave portion having a shape that can accommodate the bulge portion, is formed in the area facing the bulge portion of the pressing surface of the abutting split mold, which is the split mold that abuts against the bulge portion during diameter reduction processing and is the surface that presses the base tube.

[0024] Figure 2 is a schematic diagram illustrating how the first device reduces the occurrence of buckling originating from the bulging portion during diameter reduction of a mother tube. Figure 2(a) is a schematic cross-sectional view illustrating a state in which the outside of the bulging portion is in contact with a split mold at the start of diameter reduction, and corresponds to the area surrounded by a thick dashed line in Figure 2(c). That is, the split mold 21 designated by the reference numeral "21" in Figures 1 and 2(c) is a "contact split mold," which is one of the multiple split molds that abuts against the bulging portion during diameter reduction.

[0025] 2(a), a bulge 11b protrudes outward (and inward) from the outer peripheral surface of the mother tube 11. However, in the first device, a recess 21r, which is a recess having a shape capable of accommodating the bulge 11b, is formed in an area facing the bulge 11b on the pressing surface, which is the surface that presses the mother tube 11, of the abutting split die 21, which is one of the multiple split dies that abuts against the bulge 11b during diameter reduction processing.

[0026] FIG. 3 is a schematic diagram illustrating the configuration of the split contact die 21 having the recess 21r formed on the pressing surface as described above. FIG. 3(a) is a plan view of the entire split contact die 21 as viewed from the axial direction of the mother tube 11 (not shown), and FIG. 3(b) is an enlarged view of the portion surrounded by the thick dashed line in FIG. 3(a). As illustrated in FIG. 3(b), the pressing surface (the lower surface as viewed in the drawing) of the split contact die 21 has the recess 21r as a concave portion having a width Dw and a depth Dd. The width Dw and depth Dd of the recess 21r are appropriately determined according to the size and shape of the bulge 11b present on the outer peripheral surface of the mother tube 11 to be reduced, so that the bulge 11b can be accommodated inside the recess 21r during the diameter reduction process.

[0027] Therefore, when the pressing surface of the split abutment die 21 presses the outer peripheral surface of the mother tube 11 during diameter reduction, the outwardly protruding portion of the bulge 11b is accommodated in the recess 21r, as shown in Fig. 2(a) . As a result, the gap formed between the split abutment die 21 and the mother tube 11 on both sides of the bulge 11b in the circumferential direction is smaller than in Fig. 5(a) (see the area surrounded by the thick dashed line). This prevents the split abutment die 21 from locally pressing the bulge 11b in the centripetal direction (centripetal direction), and therefore prevents a force acting in a direction inclined relative to the centripetal direction (centripetal direction) from acting on the portions on both sides of the bulge. As a result, as illustrated in (c) of Figure 2, the cross-sectional contour of the mother tube 11 is uniformly reduced in diameter without buckling starting from the bulge portion 11b, and a metal tube having the desired shape can be formed (see the thick solid line).

[0028] <effect> As described above, in the first device, a recess having a shape capable of accommodating the bulge is formed in a region facing the bulge on a pressing surface of a contact split die, which is a split die that abuts against the bulge and is one of a plurality of split dies that press the outer circumferential surface of a metal mother tube having a noncircular cross section. Therefore, in the first device, gaps formed between the contact split die and the mother tube on both sides of the bulge in the circumferential direction are eliminated or reduced. As a result, the first device can reduce the occurrence of buckling originating from the bulge when the outer circumferential surface of the mother tube is reduced by pressing the outer circumferential surface of the mother tube with a plurality of split dies.

[0029] Second Embodiment Hereinafter, a metal tube diameter reducing apparatus according to a second embodiment of the present invention (hereinafter, sometimes referred to as "second apparatus") will be described with reference to the drawings.

[0030] As described above, the cross-sectional shape of the raw tube that is reduced by the metal tube reduction device (device of the present invention) of the present invention, including the first device, is non-circular, and non-circular includes various shapes that are not perfect circles, such as ellipse, pseudo-ellipse, oval, or pseudo-rectangle.

[0031] In a metal pipe having a noncircular cross section as described above, a cross section in which the curvature changes continuously in the circumferential direction (without inflection points) is ideal, but it is difficult to industrially mass-produce metal pipes having such a cross section. Therefore, in this technical field, it is common practice to substitute a pseudo cross section (with inflection points) by combining multiple sets of curves (with curvature radii R). The multiple sets of curves have different lengths (arc lengths) and curvature radii R, so each set has different rigidity against deformation (difficulty in diameter reduction deformation).

[0032] As a result of intensive research by the inventors, it was found that buckling originating from a bulge portion during diameter reduction processing of a mother tube having a non-circular cross section is likely to occur when the shape of the mother tube's cross section is non-circular (e.g., approximately rectangular) composed of multiple sets of opposing curves having different radii of curvature, and when the bulge portion is present in a region of the outer surface corresponding to the longest curve, which is the longest curve among the multiple sets of curves that make up the non-circular shape.

[0033] On the other hand, in plate-wound tubes, which are frequently used as metal tubes having noncircular cross sections, in order to facilitate the process of forming the plate-wound tube, the portion where the raw metal sheet is wound and the ends are butted together to be welded (i.e., the bead portion that becomes the bulge portion) is often formed in a region of the outer circumferential surface corresponding to the above-mentioned longest curve. Therefore, the device of the present invention, which can reduce the occurrence of buckling originating from the bulge portion during diameter reduction of a mother tube having a noncircular cross section, is preferably applied to diameter reduction of a mother tube whose cross section has a noncircular shape composed of multiple sets of opposing curves with different radii of curvature and in which the bulge portion exists in a region of the outer circumferential surface corresponding to the longest curve, which is the longest curve among the multiple sets of curves that make up the noncircular cross section.

[0034] <composition> That is, the second device is the above-mentioned first device, which is a device for reducing the diameter of a metal tube, characterized in that the cross-sectional shape of the mother tube is formed by multiple sets of opposing curves having different radii of curvature, a bulge portion is present in a region of the outer surface corresponding to the longest curve, which is the longest of the multiple sets of curves that form the cross-sectional shape of the mother tube, and a recess is formed in a region opposite the bulge portion of the abutting split-type pressing surface.

[0035] Fig. 4(a) is a schematic diagram showing an example of a cross section of a mother tube to be reduced by the second device. The mother tube 12 shown in Fig. 4(a) has a pseudo-rectangular (approximately rectangular) cross section consisting of three sets of opposing curves (two curves 12a, four curves 12b, and two curves 12c) with different radii of curvature. When a mother tube 12 having such an approximately rectangular cross section is reduced by a conventional reduction device, there is a high possibility that buckling will occur in a region with low surface rigidity (i.e., the region corresponding to the longest curve 12c) or that a step will occur in the cross section at a portion corresponding to an inflection point.

[0036] 4(a) has a quasi-rectangular (approximately rectangular) cross section consisting of three different sets of opposing curves (two curves 12a, four curves 12b, and two curves 12c) with different radii of curvature, as described above. However, the shape of the cross section of the mother tube reduced by the second device is not particularly limited as long as it is non-circular, and the number of sets of curves constituting such a non-circular cross section is not limited to three, but may be two, four, or more.

[0037] Fig. 4(b) is a schematic diagram showing an example of the arrangement of the plurality of segmented dies and the mother tube immediately before the start of diameter reduction processing by the second device. In the example shown in Fig. 4(b), eight segmented die groups (two segmented die groups 22a, four segmented die groups 22b, and two segmented die groups 22c) corresponding to the eight curved line groups (two curved lines 12a, four curved lines 12b, and two curved lines 12c) shown in Fig. 4(a) are arranged to surround the mother tube 12 around the tube axis. Each of the plurality of segmented die groups 22a, 22b, and 22c is composed of one segmented die. However, at least one of the plurality of segmented die groups 22a, 22b, and 22c may be composed of multiple segmented dies.

[0038] In the example shown in Fig. 4(c), two split die groups 22c 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 peripheral 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 peripheral surface of the mother tube at the same time. Alternatively, a time lag may be provided between the timings at which the split dies constituting the split die group are brought into contact with the outer peripheral surface of the mother tube.

[0039] In addition to the above, although not shown, in the mother tube reduced by the second device, as described above, a bulge exists in the outer peripheral surface region corresponding to the longest curve, which is the longest curve among the plurality of sets of curves that constitute the non-circular cross-sectional shape of the mother tube. In the mother tube 12 illustrated in Fig. 4, the longest curve among the plurality of sets of curves 12a, 12b, and 12c that constitute the approximately rectangular cross-sectional shape of the mother tube 12 is curve 12c. That is, in the example shown in Fig. 4, the longest curve in the mother tube 12 is curve 12c, and a bulge exists in the outer peripheral surface region corresponding to curve 12c.

[0040] 4, among the eight split die groups (two split die groups 22a, four split die groups 22b, and two split die groups 22c) provided in the second apparatus, the split die 22c is the abutting split die that faces the region of the outer circumferential surface of the mother tube 12 that corresponds to curve 12c, the longest curve in the diameter reduction process. Therefore, although not shown, in the second apparatus provided with the multiple split die groups 22a, 22b, and 22c illustrated in FIG. 4, a recess that is a depression having a shape that can accommodate the bulge, as illustrated in FIG. 3 referred to in the description of the first apparatus above, is formed in the region facing the bulge on the pressing surface that presses the mother tube 12 of the split die 22c that is the abutting split die.

[0041] <effect> As described above, in the second device, the cross-sectional shape of the mother tube is non-circular, consisting of multiple sets of opposing curves with different radii of curvature. Furthermore, a bulge is present in the region of the outer circumferential surface corresponding to the longest curve, which is the longest of the multiple sets of curves that make up the cross-sectional shape of the mother tube. Therefore, among the multiple split dies provided in the second device, a recess having a shape capable of accommodating the bulge is formed in the region facing the bulge of the abutting split die, which is the split die that faces the region of the outer circumferential surface of the mother tube that corresponds to the longest curve during diameter reduction.

[0042] That is, in the second device, recesses are formed at positions suitable for reducing the occurrence of buckling in a mother tube having a bulge in a region of the outer circumferential surface corresponding to the longest curve, which is a location where buckling originating from a bulge is likely to occur during diameter reduction of a mother tube having a noncircular cross section composed of multiple sets of opposing curves. Therefore, even in plate-wound tubes, which are commonly used as metal tubes having noncircular cross sections, where the location where the raw metal sheet is wound and the ends are butted together to be welded (i.e., the bead portion that becomes the bulge) is formed in the region of the outer circumferential surface corresponding to the longest curve, the second device can reduce the occurrence of buckling originating from a bulge during diameter reduction of a mother tube having a noncircular cross section.

[0043] Third Embodiment Hereinafter, a metal tube diameter reducing apparatus according to a third embodiment of the present invention (hereinafter, sometimes referred to as "third apparatus") will be described with reference to the drawings.

[0044] As described above, in a mother tube that is reduced by a diameter reducing apparatus for a metal tube according to the present invention (the apparatus of the present invention), including the first and second apparatuses, the cross-sectional shape is noncircular, and at least one bulging portion that bulges outward in the radial direction and extends in the axial direction of the tube is present on the outer circumferential surface of the mother tube. Furthermore, in the apparatus of the present invention, a recess, which is a concave portion having a shape capable of accommodating the bulging portion, is formed in a region facing the bulging portion on the pressing surface of the abutting split mold, which is a split mold that abuts against the bulging portion during the diameter reduction process, among the multiple split dies. Therefore, the apparatus of the present invention can reduce the gap formed between the abutting split mold and the mother tube on both sides of the bulging portion in the circumferential direction, thereby reducing the occurrence of buckling originating from the bulging portion during the diameter reduction of the mother tube.

[0045] As described above, the apparatus of the present invention includes a plurality of segmented dies arranged around the axis of the mother tube to surround the mother tube. Furthermore, as illustrated in FIG. 4 , which was referred to in the description of the second apparatus above, before the diameter reduction process starts, there are small gaps between adjacent segmented dies included in the plurality of segmented dies arranged to surround the mother tube.

[0046] The region of the outer surface of the mother tube facing the gap is less susceptible to the pressing force of the split mold than other regions. Therefore, this region is more likely to undergo deformation such as buckling due to the difference in the pressing force from the split mold between this region and other regions. Furthermore, if a bulge is present in this region, buckling originating from the bulge becomes even more likely to occur during the diameter reduction process of the mother tube.

[0047] Therefore, the third device is a device for reducing the diameter of a metal tube, which is the above-mentioned first or second device, characterized in that a bulge portion is present at a position on the outer surface of the mother tube opposite the circumferential center of the abutting split mold, and a recess is formed in the circumferential center of the pressing surface of the abutting split mold.

[0048] Here, reference is made again to Figure 4. As described above in the explanation of the second apparatus, the mother tube 12 illustrated in Figure 4 has a bulge in the region of the outer peripheral surface corresponding to the longest curve, curve 12c, and a corresponding recess is formed in the region facing the bulge on the pressing surface of the split die 22c, which is a contact split die, facing curve 12c. The eight split die groups (two split die groups 22a, four split die groups 22b, and two split die groups 22c) illustrated in Figure 4(a) correspond to the eight curved lines (two curved lines 12a, four curved lines 12b, and two curved lines 12c) that make up the cross-sectional shape of the mother tube 12. That is, both ends of curve 12c face both ends in the circumferential direction of the split die 22c, which is a contact split die.

[0049] As described above, before the diameter reduction process begins, there are gaps between the ends of the split die 22c, which is a contact split die, and the adjacent split die 22b. Therefore, the ends of the curved line 12c facing the ends of the split die 22c are less likely to receive the pressing force from the split die than other regions, and deformation such as buckling is more likely to occur due to the difference in the pressing force received from the split die compared to other regions. Furthermore, if a bulge exists at the end of the curved line 12c, buckling originating from the bulge is even more likely to occur during the diameter reduction process of the mother tube 12. According to the findings of the present inventors, this tendency becomes smaller as the bulge is farther away from the circumferential end of the contact split die.

[0050] However, the position of the bulged portion on the outer circumferential surface of the mother tube reduced by the third device does not need to strictly correspond to the position facing the circumferential center of the split mold on the outer circumferential surface of the mother tube, as long as the likelihood of buckling occurring due to the presence of the bulged portion at a position facing the circumferential end of the split mold is reduced. In other words, the position of the bulged portion on the outer circumferential surface of the mother tube reduced by the third device may be somewhat offset from the position facing the circumferential center of the split mold on the outer circumferential surface of the mother tube, as long as the likelihood of buckling occurring due to the presence of the bulged portion at a position facing the circumferential end of the split mold is reduced. In this specification, the term "center" is defined to include this concept.

[0051] Furthermore, when two split mold groups 22c are each composed of three split molds as illustrated in (c) of Figure 4, a bulge portion is present at a position opposite to the circumferential center of the split mold that abuts the bulge portion among the three split molds that make up the split mold group 22c, and a recess is formed in the circumferential center of the pressing surface of the abutting split mold.

[0052] <effect> As described above, in the third device, a bulge is present on the outer peripheral surface of the mother tube at a position facing the circumferential center of the abutment split mold, and a recess is formed in the circumferential center of the pressing surface of the abutment split mold. That is, in the third device, the bulge is present at a position farthest from the circumferential end of the abutment split mold. Therefore, in the third device, deformation such as buckling due to differences in pressing force caused by gaps between the abutment split mold and adjacent split molds is less likely to occur. Therefore, the third device can more reliably reduce the occurrence of buckling originating from the bulge during diameter reduction of the mother tube. [Example]

[0053] An embodiment of the metal tube diameter reduction device (the device of the present invention), including the first to third devices described above, will be described below with reference to the drawings. However, the embodiment described below is merely illustrative and should not be construed as limiting the scope of the present invention.

[0054] In this example, a plate-wound tube formed by plate-wound pipe manufacturing was used as the mother tube. Specifically, as shown in Fig. 4(a), the mother tube had a substantially rectangular cross section formed by three sets of opposing curves 12a, 12b, and 12c having different radii of curvature, and a bead portion as a bulge was present in a region of the outer circumferential surface of the mother tube corresponding to the center of longest curve 12c, which was the longest of the three sets of curves. The mother tube was subjected to diameter reduction.

[0055] On the other hand, the diameter reducing device used in this example was one including 12 segmented dies arranged to surround the mother tube around the tube axis and a drive mechanism for driving these 12 segmented dies toward the outer circumferential surface of the mother tube, as shown in Fig. 1. In this diameter reducing device, three segmented dies face the region of the outer circumferential surface of the mother tube that corresponds to the longest curve 12c of the three sets of curves that make up the cross section of the mother tube, similar to the segmented dies 21 designated by the reference numeral "21" in Figs. 1 and 2(c).

[0056] As described above, in the above-mentioned mother tube, a bulge exists in the region of the outer circumferential surface of the mother tube that corresponds to the center of the longest curve 12c, which is the longest of the three sets of curves that make up the cross section. Therefore, of the three split dies that face the region of the outer circumferential surface of the mother tube that corresponds to the longest curve 12c, the central split dies are made into a contact split dies, and a recess is formed in the pressing surface of this contact split dies. In other words, the diameter reducing device for a metal tube according to this embodiment has a configuration corresponding to the third device described above.

[0057] In this embodiment, the upper limits of the width and height of the bead portion were set to 6 mm and 0.3 mm, respectively, and a plate-wound tube formed by plate-wound tube manufacturing was used as the base tube. Therefore, the width Dw and depth Dd (see Figure 3(b)) of the recess formed on the pressing surface of the abutting split mold were set to 6 mm and 0.3 mm, respectively.

[0058] As a comparative example, a diameter reducing device according to the prior art, which had the same configuration as the diameter reducing device of this embodiment except that no recess was formed on the pressing surface of the abutting split type, was used to reduce the diameter of a blank tube, which was the same plate tube as the above.

[0059] As a result of performing diameter reduction processing on the above-mentioned mother tube using the diameter reduction devices according to the present example and the comparative example, buckling occurred near the bulged portion with a frequency of several percent to approximately 30% in diameter reduction processing using the diameter reduction device according to the comparative example, whereas buckling did not occur near the bulged portion at all in diameter reduction processing using the diameter reduction device according to the present example.

[0060] As described above, it has been confirmed that the diameter reduction device of this embodiment, in which a recess having a shape capable of accommodating the bulge portion is formed in the area facing the bulge portion on the pressing surface of the abutting split mold, can reduce the occurrence of buckling originating from the bulge portion when reducing the diameter of the mother tube by pressing the outer surface of the mother tube with multiple split molds.

[0061] For the purpose of explaining the present invention, several embodiments and examples having specific configurations have been described above with reference to the accompanying 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]

[0062] 101...Metal pipe diameter reduction device 11, 12...Element tube 11b...bulge 12a,12b,12c...Curve 21,22a,22b,22c...Divided type (divided type group) 21r…Recess

Claims

1. A metal tube diameter reducing device comprising: a plurality of split dies arranged around a tube axis of a metal mother tube having a non-circular cross section, which is a cross section taken along a plane perpendicular to a tube axis, so as to surround the mother tube; and a drive mechanism that drives the plurality of split dies toward an outer peripheral surface of the mother tube, wherein the device is configured to reduce a diameter of the mother tube by pressing the outer peripheral surface of the mother tube with the plurality of split dies, at least one bulging portion is present on the outer peripheral surface of the blank tube, the bulging portion bulging outward in the radial direction and extending in the tube axial direction; a contact split mold, which is a split mold that comes into contact with the bulging portion during the diameter reduction process, among the plurality of split molds; a pressing surface that presses the mother tube, the pressing surface having a recess that is a recess shape capable of accommodating the bulging portion, in an area facing the bulging portion; A metal tube diameter reduction device characterized by:

2. 2. The metal tube diameter reducing device according to claim 1, the cross-sectional shape of the blank tube is formed by a plurality of pairs of opposing curved lines having different radii of curvature, the bulge portion is present in a region of the outer circumferential surface corresponding to a longest curve that is the longest curve among the plurality of sets of curves that form the shape of the cross section, the recess is formed in a region of the pressing surface of the abutment split mold that faces the bulging portion, A metal tube diameter reduction device characterized by:

3. The metal tube diameter reduction device according to claim 1 or 2, the bulge portion is present on the outer peripheral surface of the mother tube at a position facing a central portion of the abutment split die in a circumferential direction, the recess is formed in the central portion in the circumferential direction of the pressing surface of the abutment split mold; A metal tube diameter reduction device characterized by:

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