[0001]The present invention relates to a method of manufacturing a hollow tube member. 　This application, on February 5, 2016, claiming priority on Japanese Patent Application No. 2016-021153 filed in Japanese, which is incorporated herein by reference. Background technique [0002]Currently, from the point of view of protection of the global environment, the weight of the vehicle is being sought. For example, thinning and the plate thickness by the high tensile of a steel sheet that constitutes the vehicle body, weight of various automobile mounting parts have been promoted strongly. Therefore, the manufacturing cost of the motor vehicle tend to increase, and also required more of the cost of the various automobile mounted parts. [0003] 　For example, a propeller shaft for transmitting to the rear wheel and a drive shaft for transmitting the driving force output via the transmission from the engine to the tires, a driving wheel output of an engine mounted in the front part of the vehicle body, Furthermore, in the power transmission system shaft for a motor vehicle is connected to the left and right drive shafts to prevent torque steer, already weight by hollowing from the real parts in the old have been put to practical use. [0004] 　Such hollow drive train shaft, often a change in outer diameter and an inner diameter at each position along its axial direction, so far have been manufactured by the method listed below. [0005] 　(A) manufacturing method using the friction welding 　in this way, making the power transmission system shaft having a shape of varying outer diameter and an inner diameter in the axial direction, separately and axially central portion and the axial ends produced in, joining the by friction welding. In this way, the axial central portion is prepared by cutting the steel pipe, the axial ends produced shaving forgings. [0006] 　(B) manufacturing method using a rotary swaging 　In this way, in the axial direction to prepare a steel pipe is a wall thickness constant and thinner and reduced in diameter to thickness increasing it by a rotary swaging both ends thereof produced by. The following Patent Document 1, this method invention for producing a hollow propeller shaft and a drive shaft is disclosed. CITATION Patent Document [0007] Patent Document 1: Japanese Patent 2011-121068 JP Summary of the Invention Problems that the Invention is to Solve [0008] 　According to the manufacturing method using the friction welding, it is possible to thin the axially central portion of the product, it is certainly possible to reduce the weight of the power transmission system shaft. However, since the axially central portion and the axially opposite end it is necessary step of bonding between the portions, increase in production costs is inevitable. There are also necessary to perform strict quality control of the joint, also increase the manufacturing cost from this plane. [0009] 　Further, the rotary in the sway manufacturing method using the managing, along with equipment to do this is very expensive, since the processing time by this manufacturing method is inevitably long, also increase the production cost. [0010] 　Thus, in the prior art, for example, a drive shaft or a propeller shaft, further a hollow power transmission system shaft which is connected to the left and right drive shafts (hollow tube member), it is difficult to inexpensively manufacture. [0011] 　An object of the present invention is to provide a method for inexpensively producing a different hollow tubing cross-sectional shape along the axial direction. Means for Solving the Problems [0012] 　The present inventors have made intensive studies in order to solve the above problem, by employing the following steps (A) and (B), manufactured inexpensively with different hollow tubing cross-sectional shape along the axial direction it was found that, and completed the present invention by overlapping a further study. The steel pipe is (A) material disposed within the outer die having an inner surface shape of the same shape as the outer shape of the product to be produced, (B) axial opposing axial ends each steel pipe of the steel pipe and capable of pressing the base portion toward the center, this is provided in the base portion and a metal core portion having an outer surface shape of the same shape as the inner surface shape of the axially opposite end portions of the product, between the pair of molds to compress the steel pipe in the axial direction. [0013] 　That is, the present invention adopts the aspects of the listed below. (1) One aspect of the present invention, toward the end portions in the axial direction in the axial direction of the center position, and the thickened portion, a bulged portion having a larger outer diameter than the thickening unit, from the bulging portion a method is also producing a hollow tube member having a central portion smaller in outer diameter, and the steel pipe which is a material, inside of the outer mold having an inner surface of the same shape as the outer shape of the hollow tube member, at least from the inner surface placing by separating the part; between a pair of pressure圧金type on both end faces in the axial direction of the steel pipe abuts the outer surface shape of the same shape as the inner surface shape of the both end portions in the axial direction of said hollow tubular member in a state where the cored bar was inserted by separating the at least a portion against the inner surface of the steel pipe having, compressing the steel pipe in the axial direction is shortened relative distance between the pair of pressing圧金type having; a step. (2) In the above (1), wherein the core metal portion may be an outer diameter becomes small toward its tip. (3) When the above (2), the outer diameter of the core metal portion may be changed stepwise or continuously in accordance with the thickness of the thickening unit. (4) In any one of the above (1) to (3), the contact portion between the axial end faces of the steel pipe in the pair of pressing圧金type, inclined toward the outside of the steel pipe it may be. (5) In any one of the above (1) to (4), said hollow tube member may be a drive train shaft for an automobile. (6) above (5), the power transmission system shaft, the drive shaft may be a drive train shaft connected to the propeller shaft or the left and right drive shafts. Effect of the invention [0014] 　According to the above aspect, according to the manufacturing method of hollow tubing, providing for example a drive shaft or a propeller shaft, further a hollow tubing which is suitably used as a hollow power transmission system shaft which is connected to the left and right drive shafts, low cost it can. BRIEF DESCRIPTION OF THE DRAWINGS [0015] [Figure 1] A sectional view showing a manufacturing method of hollow tubing according to an embodiment of the present invention, (a) represents a front forging shows after forging is (b). [2] a cross-sectional view for explaining the dimensions of the hollow pipe material to be processed by the production method of the hollow tube member according to the embodiment, shows the dimensions of the steel pipe to be the processing material (a), (b ) indicates the size of the power transmission system shaft after forging. [Figure 3] a cross-sectional view showing a modification of the embodiment, (a) corresponds to FIG. 1 (a), (b) corresponds to FIG. 1 (b). A diagram for illustrating a taper angle α of the core portion used [4] In the modification, a cross-sectional view when viewed in cross-section including the axis. [Figure 5] The hollow tube member and the core metal portion, a cross-sectional view when viewed in cross-section containing their axes, (a) shows the, if the taper angle is used a certain core metal portion along the axis indicates the time of molding completion, (b) shows the time of molding completion when the taper angle using cored bar which changes stepwise along the axis, further, (c) is the taper angle axis It shows the time of molding the completion of the case of using the cored bar which continuously varies along. [6] A sectional view showing another modification of the embodiment shown in FIG. 1, (a) corresponds to FIG. 1 (a), (b) corresponds to FIG. 1 (b). [7] a diagram showing the process of forging in the modification, (a) is an enlarged sectional view of a portion corresponding to portion A1 of FIG. 6 (a), (b) in FIG. 6 (b ) is an enlarged sectional view of a portion corresponding to A2 parts. A cross-sectional view showing still another modification of the embodiment shown in FIG. 8 Fig. 1, (a) corresponds to FIG. 1 (a), (b) corresponds to FIG. 1 (b) . [9] a cross-sectional view showing still another modification of the embodiment shown in FIG. 1 is an enlarged cross-sectional view of a portion corresponding to A3 portion in Fig. 1 (a). DESCRIPTION OF THE INVENTION [0016] 　With reference to the accompanying drawings, illustrating a manufacturing method of hollow tubing according to an embodiment of the present invention are described below. In the following description, the hollow tube material will be exemplified a case where the hollow drive train shaft for an automobile, the present invention is equally applicable to a hollow tube member other than the power transmission system shaft . [0017] 　1. Hollow power transmission system shaft 1 for an automobile 　1 is a cross-sectional view showing a manufacturing method of hollow tubing according to an embodiment of the present invention, it shows a prior forging is (a), (b) forging It shows the after processing. [0018] 　As shown in FIG. 1 (b), in this embodiment, a hollow power transmission system shaft 1 for a motor vehicle, to produce in one step by cold forging. [0019] 　Drive train shaft 1, the axial direction along the CL (hereinafter, referred to as axial direction) toward the both end positions 2-1 and 2-2 in the axial direction of the center position 2-3, thickened portions 3 having a 1,3-2, a bulging portion 4-1 and 4-2, and a central portion 5. That is, as shown in FIG. 1 (b), the power transmission system shaft 1, thickened portion 3-1, the bulging portion 4-1, the central portion 5, the bulging portion 4-2 thickened portion 3-2 There are formed continuously in this order. [0020] 　Thickened portions 3-1 and 3-2, thickened portions 31 and 32, among the bulging portions 4-1 and 4-2 and the central portion 5, a wall thickness of the thickest portion. In this embodiment, the thickness of the thickened portion 31 and 32, is substantially constant at each position in the axial direction of the power transmission system shaft 1. The outside diameter of the thickened portion 3-1 and 3-2, and a substantially constant at each position in the axial direction of the drive train shaft 1 is substantially the same as the outer diameter of the steel pipe 6 which is a material. The inner diameter of the thickened portion 3-1 and 3-2, and a substantially constant at each position in the axial direction of the drive train shaft 1, smaller than the inner diameter of the steel pipe 6. 　When the power transmission system shaft 1 is hollow driveline shaft is connected to the left and right drive shafts, splines are engraved on the outer surface of the thickened portion 31 and 32. [0021] 　Bulging portions 4-1 and 4-2, and an outer diameter greater than the thickened portions 31 and 32, thicker than thickened portion 31 and 32 is a thin portion. The outer diameter of the bulging portion 4-1 and 4-2, gradually increasing the outside diameter of the thickened portion 31 and 32 as a starting point, in the axial direction of the bulging portion 4-1 and 4-2 maximum value becomes substantially the center position, then gradually decreases toward the central portion 5. Also, the inner diameter of the bulging portion 4-1 and 4-2, gradually increasing starting from the inner diameter of the thickened portion 31 and 32, in the axial direction of the bulging portion 4-1 and 4-2 maximum value becomes substantially the center position, then gradually decreases toward the central portion 5. [0022] 　Central portion 5, the outer diameter is smaller than the bulged portion 41 and 42 of both sides thereof. Further, the outer diameter of the central portion 5, and a substantially constant at each position in the axial direction of the drive train shaft 1, substantially coincides outer diameter of the steel pipe 6 and a material. 　The inner diameter of the central portion 5, and a substantially constant at each position in the axial direction of the drive train shaft 1, substantially coincides with the inner diameter of the steel tube 6. Therefore, the inner diameter of the central portion 5 is larger than the inner diameter of the thickened portion 31 and 32. 　Accordingly, since the central portion 5 is not also increased even diameter wall, the wall thickness of the central portion 5, at each position in the axial direction of the drive train shaft 1, and approximately the same as the thickness of the steel pipe 6, substantially It is constant. [0023] 　Hardness of the central part 5, has a hardness substantially coincides steel pipe 6 which is a material, changes before and after forging, almost no. 　Meanwhile, since the thickened portion 31 and 32 is thickened by forging, it has work hardening, which is harder than the hardness of the case of the steel pipe 6. Further, bulging portions 4-1 and 4-2 is also because it is expanded by forging, has work hardening, which is harder than the hardness of the case of the steel pipe 6. [0024] 　By respective thickened portions 3-1, 3-2 and bulging portions 4-1 and 4-2 are work hardened in this manner, the power transmission system shaft 1, the power transmission system shafts for motor vehicles characteristic improvement of the torsional strength and torsional fatigue is achieved fully required as the basic performance. [0025] 　The material of the power transmission system shaft 1, but S45CB softening material (tensile strength TS = 550 MPa grade) is exemplified, but the invention is not limited to this material. Drive train shaft 1, for performing the thickening and tube expansion by axial pressing by forging, the deformation caused by machining, compression deformation is mainly a tensile deformation amount is small. Therefore, the risk of rupture due to the high strength of the material is very small. Therefore, as the material of the power transmission system shaft 1, it is also applicable to low strength material than S45CB softening material. Furthermore, it is possible to mold the drive train shaft 1 without breaking even when applied to a high-strength material than S45CB softening material. [0026] 　The high-strength materials than S45CB softening material, load press axis is increased in proportion to the strength of the material. However, the load press axis, because it is about 350 tons in S35CB softening material, requires only about 700 tons in 1000MPa class high strength material, is sufficiently possible manufactured in mass production press. [0027] 　2. The method of manufacturing a power transmission system shaft 1 　First, as shown in FIG. 1 (a), which is a material, a constant thickness of the steel pipe 6, the outer gold having an inner surface shape 7a of the same shape as the outer shape of the power transmission system shaft 1 inside the mold 7 is arranged from the inner surface of the outer die 7 is slightly spaced apart. That is, the inner surface and the axis the axis of the steel pipe 6 (inner shape 7a) arranged so as to be coaxial (axis CL). Although a clearance is provided between the outer surface and the inner surface of the outer die 7 of the steel pipe 6, the size of the gap is small. Yet, the base portion 81 and 82 will be described later, has a substantially cylindrical shape of the outer diameter of the end portion inner diameter substantially matching the inner surface shape 7a of the outer mold 7. Therefore, the steel pipe 6 and the base portion 81 and 82 only normally placed in an outer mold 7 may be substantially coaxial arrangement. If the need to coaxially place the steel pipe 6 with higher accuracy there is structure which will be described below with reference to FIG. 9 may be adopted. [0028] 　Details of the inner shape 7a. As shown in FIG. 1 (a), when viewed along the axial direction of the outer die 7, portions corresponding to the thickened portion 3-1, the distance from the axis line CL is constant r1. 　As in the portion corresponding to the bulging portion 4-1 followed, distance r2 from the axis CL is is the same as the distance r1 thickened portion 3-1 at one end thereof, closer toward the center 5 will decrease gradually toward the other end from through will increase gradually, and the maximum value. [0029] 　Subsequently, the distance r3 of region corresponding to the central portion 5, the and the same as the distance r2 at the other end, it is constant up to the bulging portion 4-2 along the axis line CL. 　Gradually as Subsequently, in the portion corresponding to the bulging portion 4-2, the distance r4 from the axis line CL is is the same as the distance r3 at one end thereof, closer towards the thickened portion 3-2 increased and go, and will decrease gradually toward the other end from through the maximum value. 　Then, portions corresponding to the thickened portion 3-2, the distance from the axis line CL and a constant r5, which is the same as the distance r4 in the other end. 　The distance r1 and the distance r5 are equal to each other. [0030] 　Between the inner face 7a of the outer surface and the outer die 7 of the steel pipe 6, a gap is provided. The dimensions of the gap is not constant when viewed along the direction of the axis CL, the difference is provided according to the purpose. 　Specifically, the A portion of Fig. 1 (b) (thickened portion 31 and 32), to pass smoothly when putting the steel pipe 6 into the outer mold 7 (the object 1), since the steel tube 6 which is deformed by the die forging in the outer die 7 is smoothly flow friction restrained by material occurring between the outer die 7 and (object 2), the dual purpose of, the gap dimension It is defined. Incidentally, if only to achieve these two objectives 1,2, simply it is conceivable to increase the gap, So the deformation to be Futoro excessive steel 6 radially outward constraint than its surroundings can not, there is a possibility of causing buckling deformation of the steel pipe 6. Therefore, although is a gap positively, its dimensions are determined so as not excessive Moto. 　Also, B part shown in FIG. 1 (b) in (bulging portion 4-1 and 4-2), in order to fatten the outer diameter of the steel pipe 6, it employs a large gap size. 　Then, in the C section of FIG. 1 (b) (central 5), since diameter larger not performed thickening, for the sole purpose of passing smoothly when putting the steel pipe 6 into the outer mold 7, the minimum It has adopted the gap dimension. In this portion C, as much as possible, it is preferable not a gap. [0031] 　The gap dimension in each part of the part A ~ C portion in FIG. 1 (b), for the reasons described above, it is preferable to set the upper and lower limit values, respectively. 　First, they say the lower limit value of the gap in the A section, in the radial direction of the steel tube 6, the gap between the outer surface of the inner surface and the steel pipe 6 of the outer die 7 W1 (mm) is a mother pipe prior to machining steel 6 the outer diameter in the axial direction of the arbitrary position when the d1 (mm), W1 is be 0.01 × d1 more desirable from the viewpoint of the object 1. Then, say the upper limit of the gap in the A section, W1 is be 0.05 × d1 less desirable from the viewpoint of the object 2. As described above, in the A section, it is preferable to employ a gap W1 (mm) from the range defined by the formula of 0.01 × d1 ≦ W1 ≦ 0.05 × d1. [0032] 　Subsequently, the gap in the B portion, it is preferable to employ a gap W1 (mm) from the range defined by the formula of 0.10 × d1 ≦ W1 ≦ 0.25 × d1. 　On the other hand, in the central portion 5, may be Tosere the steel pipe 6 into the outer mold 7, the gap W1 (mm) is preferably substantially 0 (zero) mm. [0033] 　Although not shown, the outer mold 7, as well as the mold used in general die forging are left and right bisected structure, be taken out product after molding by halving It is configured to be able. That is, the outer mold 7, FIG. 1 because it is composed of bisected pair of molds by a dividing surface having a shape (b), the bulging portion 4-1 powertrain shaft 1 after the molding , even if there is 4-2, by bisecting the pair of molds, it is possible to take out the power transmission system shaft 1 than within the outer die 7. [0034] 　Next, as shown in FIG. 1 (a), the respective axial end faces 6-1 and 6-2 of the steel pipe 6, a pair of upper and lower molds (metal core with a punch) 10-1 and 10-2 among performs die forging to axial compression. Mold 10-1 includes a base portion 8-1, and a metal core portion 9-1. Mold 10-2 includes a base portion 8-2, and a metal core portion 9-2. 　Base unit 81 and 82 is pressed toward the steel pipe 6 in the center of the axial direction from both ends. Base unit 81 and 82 has a substantially cylindrical shape having an outer diameter that substantially coincides with the end portion inner diameter of the inner surface shape 7a of the outer die 7, it can be inserted and removed to the edge portion of the outer die 7 it is. [0035] 　Core metal 9-1 and 9-2 are provided coaxially and integrally with the base portion 81 and 82, the inner surface shape 1-1 at both ends of the axial direction of the power transmission system shaft 1 has an outer surface shape 9-1a, 9-2a of the same shape as 1-2 (i.e., the same shape as the inner surface of the thickened portion 31 and 32). Clearance between the outer surface and the forging prior to the inner surface of the steel pipe 6 of the core portion 9-1 and 9-2 W2 (mm) is the inside diameter of the steel tube 6 d2 (mm), the thickness and t (mm) when, from the range of 0.10 × (d2-2 × t) ≦ W2 ≦ 0.25 × (d2-2 × t), it can be set as appropriate. [0036] 　During die forging, the part A in FIG. 1 (b), the by axial pressing with a die 10-1, thickened portions 31 and 32 are formed. To put in detail, as shown in FIG. 1 (a), prior to pushing the shaft by the base unit 8-1 and 8-2, between the outer surface and the inner surface of the steel pipe 6 of the core portion 9-1, 9-2 gap is provided. Then, by receiving compressed by the base portions 81 and 82, gradually the thickness is increased while decreases the inner diameter of the portion to be thickened, finally the core metal 9-1 and 9-2 the outer surface the thickened portion 31 and 32 having the internal shape of the final product that meet. 　Also, B portion in FIG. 1 (b), in the state before pressing the shaft shown in FIG. 1 (a), the periphery of the steel pipe 6 is not restrained. Therefore, the B unit is thickened by axial pressing the bulged portion 4-1 and 4-2 are formed by tube expansion with little. Maximum outer diameter of the bulging portion 4-1 and 4-2 are enlarged to approximately 1.2-1.5 times the outer dimensions of the steel pipe 6 before forging. 　The C portion in FIG. 1 (b), the tube expansion is also not been made thickened. [0037] 　Figure 2 is a cross-sectional view for explaining the dimensions of the hollow pipe material to be processed by the manufacturing method of hollow tubing according to the present embodiment, shows the various dimensions of the steel pipe 6 serving as the processing material (a), (b) shows the various dimensions of the power transmission system shaft 1 after the processing. 　When using S45CB softening material (tensile strength TS = 550 MPa grade) as a material of the steel pipe 6, the overall length L is 100 mm ~ 2000 mm, an outer diameter d is 20 mm ~ 100 mm, the plate thickness t is 2 mm ~ 20 mm at and t ≦ d / 2, it can be suitably adopted to satisfy the. [0038] 　Power transmission system shaft 1 illustrated in FIG. 2 (b), satisfies the following condition 1 regarding the thickness of each part, both the following conditions 2 about the outer diameter of each part. Condition 1: t 1 > t 2 and t 1 > t 3 and a, t 1 , t 2 , t 3 each is 4mm or more and 15mm or less. Condition 2: D 2 > D 1 and D 2 > D 3 and a, 20 mm