JP2001062519A - Metallic pipe bending method and bent metallic pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of bending a metallic pipe with relatively high accuracy of bending angle in a short time and at a low processing cost. SOLUTION: This metallic pipe bending method consists of a rough-bending process to form a bent portion 30 on a bent metallic pipe 3 with a pipe bender and a precision bending process to press-form the bent portion 30 of the metallic pipe 3 with mutually opposing an outer die 2 and an inner die 1. In the precision bending process, a pair of concave portions 33 are formed on the outside of both end portions 32 of the bent portion 30 by a pair of protruded portions 12 of the female die 1. As a result, when the bent metallic pipe 3 is removed from the male die 2 and the female die 1, the spring back of the center portion 31 of the bent portion 30 and the spring back due to the concave portion 33 of both end portions 32 are mutually offset, and variance in bending angle is also offset. Therefore, the accuracy of the bending angle of the bent portion 30 becomes relatively high while the processing can be done in a short time and at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of a metal pipe bending technique or a metal bent pipe manufacturing technique. It also belongs to the technical field of metal bent pipes as its products. As a material for the metal pipe, an iron-based metal is particularly suitable, but is not limited thereto.

[0002]

2. Description of the Related Art As a conventional method of bending a metal pipe, a rough and inexpensive method of bending a metal pipe by a pipe bender or a press die (the former) and a precise and expensive method called bulging or hydroforming. (The latter).

In the former method, for example, Japanese Patent Application Laid-Open No. 9-12
A pipe bender having a configuration very similar to the bending device disclosed in Japanese Patent No. 2763 is used. Alternatively, a metal pipe is sandwiched between a male mold and a female mold that are formed with proper consideration of springback, and press-formed by the male mold and the female mold.

The former method is excellent in that it does not take much time for processing and the processing cost is inexpensive, but has a disadvantage in that the bending accuracy varies greatly and the finishing accuracy is not high. In other words, although springback is anticipated for forming the male and female molds, the angle of the springback, including the change with time, due to variations in the material and dimensions of the metal pipe and subtle differences in processing conditions, etc. It is not always constant, and the variation is large. For example,
When a steel metal pipe is bent at 90 °, usually 2-3
Although springback of about ° occurs, the range of variation of the bending angle at that time extends to about ± 3 °.

On the other hand, the latter method is a method in which a metal pipe is expanded and plastically deformed in a precision mold while applying a liquid pressure from the inside of the metal pipe to form a bent portion into a desired bent shape. This method has a small bending angle and can be finished precisely, but it is not suitable for mass production that requires efficient production of inexpensive products because the processing takes a considerable amount of time and the processing cost is much higher. By the way, the range of the variation of the bending angle which occurs when the steel metal pipe is bent by 90 ° by this method is only ± 0.2.
° degree.

In the latter method, if the bending angle becomes large or the radius of curvature becomes small, the pipe expansion ratio and the internal pressure must be increased, which is not suitable for molding at a large bending angle or a high curvature. With the constraint that

[0007]

SUMMARY OF THE INVENTION Therefore, the present invention is to solve the problem of providing a method for bending a metal pipe, which can be processed in a short time and has a low bending cost, and has a relatively high bending angle accuracy. Make it an issue. It is another object of the present invention to provide a metal bent pipe which is inexpensive and has a high bending angle accuracy.

[0008]

Means for Solving the Problems A metal pipe bending method according to the present invention includes a rough bending step of bending a metal pipe at an approximate angle to form a bent portion in the metal pipe. And a precision bending step performed after the step. In the precision bending step, a female mold having a concave portion having a center that is tighter than the curvature of the bent portion is circumscribed around the outer peripheral surface of the bent portion, and a convex portion having a tighter curvature than the bent portion is provided at the center. A male mold having a pair of pulling portions whose curvature is looser than the bent portion on both sides in the axial direction of the convex portion is inscribed in the outer peripheral surface of the bent portion, and the outer mold and the inner mold are opposed to each other. Press-forming the bent portion of the metal pipe.

First, in the rough bending step, the metal pipe is bent at an approximate angle by a processing technique such as bending by a pipe bender or bending by press forming, similarly to the above-described conventional technique, and the bent portion is formed in the metal pipe. Is formed.
As a result, a bent metal pipe is manufactured as an intermediate product.

Next, in the precision bending step, a bent portion of a metal bent pipe bent at an approximate angle by the above-mentioned male mold and female mold is clamped, and a press is performed so that the bent portion has a precise bending angle. Processing is performed. The male and female shapes used at this time have the following features.

That is, since the female mold which is circumscribed to the outer peripheral surface of the bent portion of the metal bent pipe has a concave portion having a curvature that is steeper than the curvature of the bent portion at the center, it is natural that the concave portion is provided on both sides along the axial length direction. A protruding portion protruding toward the metal bent pipe is formed. Here, the curvature of the female concave portion is
It is desirable that the curvature be appropriately tighter than the finished curvature outside the outer peripheral surface of the bent portion, by incorporating a springback generated after forming the bent portion of the metal bent pipe. Further, as described above, a pair of protrusions protruding toward the metal bent pipe are formed on both sides along the axial length direction from the center of the female mold. Therefore, a pair of concave portions that are pressed toward these centrifugal directions are formed outside the outer peripheral surface at both ends in the axial length direction of the bent portion of the metal bent pipe that is pressed by the pair of projecting portions. Is done.

[0012] On the other hand, the male mold which is inscribed in the outer peripheral surface of the bent portion of the metal bent pipe has a convex portion having a curvature that is steeper than the finished curvature of the bent portion at the center, and is provided on both sides in the axial direction of the convex portion. It has a pair of pulling portions whose curvature is looser than the finished curvature of the bent portion. It is desirable that the curvature of the male convex portion be appropriately tighter than the finished curvature inside the outer peripheral surface of the bent portion, taking into account the springback generated after forming the bent portion of the metal bent pipe. Also,
The curvature of the pair of pulling portions on both sides of the convex portion is smaller than that of the inner portion of the outer peripheral surface of the bent portion that comes into contact with the pulling portion. It is desirable that the tensile deformation is generated to near the limit. Here, since the curvature of the tension portion is determined by the curvature of the corresponding portion of the bent portion and the elastic limit of the same portion, the curvature may or may not be reversed with respect to the curvature of the convex portion.

The cross sections of the male and female molds each having a normal to the axis of the metal bent pipe are approximately semicircular, and when the male and female molds are pressed, the metal bent pipe is formed. It is desirable that the shape be such that it encloses the entire bent portion. When the male mold and the female mold have such a shape, the finishing accuracy of the metal bent pipe is further improved. In addition, in such a shape, the male convex portion has a saddle-like shape when three-dimensionally described.

Since the metal bent pipe is press-formed by the male and female molds having such a shape, springback in the precision bending process is suppressed, and
Variations in the springback angle are reduced. Therefore, in the bent portion of the finished metal bent pipe, the variation in the bending angle and the bending dimension is small, and the molding accuracy is high. For example, when a 90 ° bend is given to a bent portion, the range of the variation is only about ± 0.5 °.

Here, the inventors interpret the reason why the precision bending process improves the finishing accuracy of the metal bent pipe as follows.

That is, in the finished metal bent pipe, a pair of dents are formed outside both ends of the bent portion as described above. Therefore, removing the metal bent pipe from the male and female molds after the precision bending process,
The residual stress remaining in the dent portion and its vicinity acts in a direction to return the dent portion to its original position, and acts in a direction to increase the bending angle of the bent portion. On the other hand, the residual stress remaining in the center of the bent portion bent deep in anticipation of springback by the female concave portion acts in a direction to reduce the bending angle of the bent portion. Then, the action of the residual stress remaining at the center or intermediate portion of the bent portion (the former) and the action of the residual stress remaining at both ends of the bent portion (the latter) cancel each other. In addition, if the metal pipe as the material is homogeneous in the axial direction, it is considered that if the springback by the former is large, the springback by the latter is also large. You. As a result, after the precision bending step, not only the springback is offset, but also the variation of the springback is offset, so that the bending accuracy of the bent portion of the finished metal bent pipe is improved.

On the other hand, in the bent portion of the finished metal bent pipe, the central portion on the inner side in the bending direction is deeply bent in consideration of an appropriate springback, and on both sides thereof, the residual stress is reduced by the tensile portions. . Therefore, the central portion of the bent portion springs back to an appropriate finished shape due to the residual stress, and the residual stress is reduced on both sides thereof, so that almost no springback occurs. If it is considered that the springback generated at the center of the bent portion is offset by the springback generated at the outer concave portion, the variation of the springback is also offset.

[0018] Therefore, only by offsetting the size of the springback that occurs at the center of the bent portion and reduces the curvature,
It is important that a pair of indentations be formed in the metal bend pipe so as to generate springback at both ends of the bend that increases the curvature. Therefore, it is clear that setting the shapes of the male mold and the female mold in the precision bending process is an important point in improving the finishing accuracy, and that the rough bending process does not require much accuracy.

As described in detail above, according to the metal pipe bending method of the present invention, the rough bending step is inexpensive and requires a small number of steps, as in the former technique, and the one-action method is also used in the precision bending step. Since press molding is sufficient, the cost is low and the number of steps is small. As a result, as described above, there is an effect that a metal bent pipe having high finishing accuracy of a bending angle can be produced at low cost and speed.

In addition, in the precision bending process,
The metal bent pipe can be pressed and fixed to the male and female molds only at the bent portion. Therefore, in this state, the portion deviating from the bent portion can be exposed, and the exposed portion can be easily subjected to incidental processing such as drilling. Also, unlike the latter of the prior art, it is possible to work to the limit of the plastic deformation (elongation) of the metal material forming the metal bent pipe, so that a bent portion is formed with a larger bending angle and a tighter curvature. There is also an effect that can be done.

(Metal bent pipe) The metal bent pipe of the present invention is a metal bent pipe having a bent portion bent at a predetermined bending angle, wherein the bent portion is formed on an outer peripheral surface at both ends in the axial direction. And having a pair of recessed portions that are recessed toward the centripetal direction.

That is, in the metal bent pipe of the present invention, unlike the metal bent pipe bent by ordinary press forming, both ends of the bent portion in the axial length direction are provided on the outer side of the outer peripheral surface in the centripetal direction. A pair of concave portions are formed. Therefore, while the central portion or the intermediate portion of the bent portion springs back in the direction to ease the bending, the springback occurs in the direction to deepen the bending by the action of the pair of dents trying to return outward.

As a result, the springback that occurs at the center or the middle of the bent portion and the springback that occurs at both ends of the bent portion cancel each other. In addition, if the magnitudes of the springbacks are substantially equal, the magnitudes of the variations of the springbacks are also substantially equal, and the directions thereof are opposite to each other, so that the variations of the springbacks are also offset.

Therefore, the average value of the bend angles in the bend portion of the metal bend pipe of the present invention approaches a constant value, and the dispersion of the bend angles is suppressed to a small value, thereby improving the bend angle accuracy. For example, 90 ° at the bend
, The range of variation is only about ± 0.5 °.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a metal pipe bending method and a metal bent pipe according to the present invention will be clearly and fully described in the following examples so that those skilled in the art can understand the present invention. .

Embodiment 1 (Structure of Metal Pipe Bending Method) A metal pipe bending method according to Embodiment 1 of the present invention is described by bending a straight steel metal pipe at an angle of approximately 90 °. As shown in FIG. 1, the method includes a rough bending step of forming a bent portion 30 in a metal pipe and a precision bending step performed after the step. In this precision bending step, a female mold having a concave portion in the center of which the curvature is tighter than the curvature of the bent portion 30 is circumscribed to the outer peripheral surface of the bent portion. At the same time, a male mold having a pair of pulling portions whose curvature is steeper than the bent portion 30 at the center and which has a curvature smaller than the bent portion on both sides in the axial direction of the convex portion is provided on the outer peripheral surface of the bent portion. Be inscribed. Then, press forming of the bent portion 30 of the metal bent pipe 3 is performed with the outer die and the inner die facing each other.

First, in the rough bending step, similarly to the above-mentioned prior art, as shown in FIG. 1, a metal pipe made of a material is bent at an angle of approximately 90 ° by bending by a pipe bender, and a bent portion 30 is formed. Is formed. As a result, the metal bent pipe 3 as an intermediate product is manufactured. The metal bent pipe 3 is 90 ° ± 1 at a predetermined curvature.
And a pair of straight portions 34 continuous with both ends in the axial direction.

Next, in the precision bending step, first, as shown in FIG. 2, the bent portion 30 of the metal bent pipe 3 bent at approximately 90 ° is sandwiched between the male mold 2 and the female mold 1. . At this time, the female mold 1 which can move up and down is directly above the male mold 2 which is fixed to the base below, and the two 1 and 2 are erected upright, and the metal bent pipe 3 is like a yajirobe. Stable on male form 2. Then, the female mold 1 descends with a pressing force against the male mold 2, and press processing is performed so that the bent portion 30 of the metal bent pipe 3 has a precise 90 ° bending angle.

(Male and Female Die) As described above, in the metal pipe bending method of the present embodiment, the male die 2 and the female die 1 are used in the precision bending step. As shown in FIG. The male mold 2 and the female mold 1 are bilaterally symmetrical and have the following characteristics in their shapes.

That is, the female mold 1 to be circumscribed on the outer peripheral surface of the bent portion 30 of the metal bent pipe 3 (see FIG. 1).
Has a concave portion 11 at the center where the curvature is steeper than the curvature of the bent portion 30. Therefore, the bent portion 30 along the axial length direction
A pair of projecting portions 12 projecting toward the metal bent pipe 3 are formed on both sides of the metal bent pipe 3. Here, the curvature of the concave portion 11 of the female mold 1 is appropriately tighter than the finished curvature outside the outer peripheral surface of the bent portion 30 by taking into account the springback generated after forming the bent portion 30 of the metal bent pipe 3. is there. Further, as described above, a pair of protruding portions 12 protruding toward the metal bent pipe are formed on both sides along the axial length direction from the central portion of the female mold 1.

On the other hand, the male mold 2 is inscribed in the outer peripheral surface of the bent portion 30 of the metal bent pipe 3 as described above. The male mold 2 has a convex portion 21 having a greater curvature than the finished curvature of the bent portion 30 at the center, and a pair of tension portions 22 having a curvature smaller than the finished curvature of the bent portion on both sides in the axial direction of the convex portion 21. Have. The curvature of the male projection 21 is
With the springback generated in the bent portion 30 of the metal bent pipe 3, the curvature is appropriately tighter than the finished curvature inside the outer peripheral surface of the bent portion 30. Further, the curvature of the pair of pulling portions 22 on both sides of the convex portion 21 is smaller than that of the inner portion of the outer peripheral surface of the bent portion 30 abutting on the pulling portion 22, so that the bending deformation of the bent portion 30 becomes close to the elastic limit. To the extent that it occurs. Here, the tension portion 22
Is inverted with respect to the curvature of the convex portion 21, and the tensile portion 22 has a shallow dent.

The cross sections of the male mold 2 and the female mold 1 with the normal to the axis of the metal bent pipe 30 are approximately semicircular, respectively. The shape is such that the bent portion 30 of the metal bent pipe 3 is entirely wrapped. At the time of press working, the male mold 2 and the female mold 1 enclose the bent portion 30, so that the finishing accuracy of the metal bent pipe 3 is further improved. Incidentally, the convex portion 21 of the male mold 2 has a saddle-like shape when described three-dimensionally.

(Operation of the Precision Bending Step) First, in the precision bending step, the metal bent pipe 3 bent at approximately 90 ° in the coarse bending step is set so that the center of the bent portion 30 is symmetric with the center of the male mold 2. After being set in the male mold 2 so that the female mold 1 is positioned, the female mold 1 is lowered from above. Then, as shown in FIG.
Of the central portion 31 of the bent portion 30 abuts against the inner surface of the central portion 31 of the bent portion 30, and the pair of protrusions 12 on both sides in the axial direction of the female mold 1 abut on the outer surfaces of both end portions 32 of the bent portion 30. Reach.

Subsequently, when the female mold 1 is further lowered, as shown in FIG. 5, the curvature of the bent portion 30 is increased by being pressed by the pair of projecting portions 12 of the female mold 1, and both ends 32 of the bent portion 30 are formed. The side surface also contacts the both end portions 32 of the male mold 2 and the bent portion 30
The increase in curvature generally stops. When the female mold 1 further descends, the two protruding portions 12 of the male mold 2 bite into the outer surfaces of both ends 32 of the bent portion 30 and are crushed to form a pair of dents 33. At this time, both ends 32 of the bent portion 30 are supported by both ends 23 of the male mold 2 from behind (below) the dent portion 33, and both ends 2 of the male mold 2 are formed.
3 is provided with an appropriate radius so as not to damage the bent metal pipe 3.

Subsequently, when the female die 1 further descends and comes into contact with the male die 2 at both ends in the circumferential direction (see FIG. 3), as shown in FIG. 6, the bent portion 30 of the metal bent pipe 3 is formed. Is
The male mold 2 and the female mold 1 which are opposed to each other and abut each other are tightly wrapped. In this state, the central portion 31 of the bent portion 30 is bent deeper by about 1 to 2 degrees than the finished shape in consideration of springback. On both sides of the central portion 31 of the bent portion 30 in the axial length direction, the inner side surface abuts against the concave pulling portion 22 of the male mold 2 and is extended by elastic deformation to near the elastic limit of the metal bent pipe 3. As a result, the residual stress in this portion is reduced.

In this state, the bent metal pipe 3 is firmly held by the male mold 2 and the female mold 1 at the bent portion 30, and the straight portions 34 on both sides of the bent portion 30 are exposed. Therefore, in this state, it is easy to perform ancillary processing such as drilling and attaching parts to both straight portions 34.

The bent portion 30 of the metal bent pipe 3 pressed in the state shown in FIG.
As shown in FIG. 7, the central portion 31 is bent slightly deeper, and a pair of dent portions 33 are formed outside both end portions. Also, on the inner surface portion facing the dent portion 33,
A gentle bulge is formed by being pulled within the range of elastic deformation.

Finally, the female mold 1 is lifted up and retracted, and the metal bent pipe 3 is removed from the male mold 2 and the female mold 1 to complete the precision bending process. The method ends. Then, in the bent portion 30 of the metal bent pipe 3, the central portion 31 causes springback and the curvature of the central portion 31 decreases, but conversely, both ends 32 where the concave portion 33 is formed also cause springback. Thus, the curvature increases at both ends 32.

That is, as shown in FIG.
When it is removed from the female mold 1, the dent portion 33 springs back in the direction of the arrow. Then, since the outer peripheral surface of the bent portion 30 extends in the axial direction in the direction of the arrow, both ends of the bent portion 30 bend in the direction of the arrow, and the curvature increases at both ends 32.

As a result, the springback in the precision bending process is offset and suppressed, and the variation in the springback angle is also offset and reduced. Therefore, in the bent portion 30 of the metal bent pipe 3 that has been finished after the precision bending process, variations in the bending angle and the bending dimension are small, and the molding accuracy is high. When a 90 ° bend is given to the bent portion 30 as in the present embodiment, the range of the variation is only about ± 0.5 °.

If the bending angle in the rough bending step is too deep, as shown in FIG. 8, at the beginning of the precision bending step, the opposite ends 23 of the male mold 2 are reversed. And the central part 11 of the female mold 1 comes into contact with the bent part 30 of the metal bent pipe 3. Even in such a case, the operation and effect of the precision bending step can be obtained in substantially the same manner as described above. That is, in the metal pipe bending method of the present embodiment, it is a point of improving the finishing accuracy that the shape of the male die 2 and the female die 1 in the precision bending process is appropriately set, and the accuracy of the rough bending process is not much required. Not done.

(Effects of Metal Pipe Bending Method) As described above in detail, according to the metal pipe bending method of the present embodiment, in the rough bending step, as in the prior art, the cost and man-hour are the same as the former. In addition, it is inexpensive and requires less man-hours because it requires only one-action press forming even in the precision bending process. As a result, there is an effect that the metal bent pipe 3 having high finishing accuracy of the bending angle as described above can be produced at low cost and speed.

In addition, as described above, in the precision bending step, the metal bent pipe 3 can be pressed and fixed to the male mold 2 and the female mold 1 only by the bent portion 30. Therefore, the bent portion 3 in that state
Since the straight portion 34 deviating from zero can be exposed, there is an effect that the straight portion 34 can be easily subjected to incidental processing such as drilling. Also, unlike the latter of the prior art, since the metal can be processed to the limit of the plastic deformation (extension) of the metal material forming the metal bent pipe 3, the bent portion 30 has a larger bending angle and a tighter curvature. Can also be formed.

(Configuration and Effect of Metal Bent Pipe)
The metal bent pipe 3 as the present embodiment is a product of the above-described metal pipe bending method, and has a bent portion 30 bent at a bending angle of 90 ° as shown in FIG. A feature of the metal bent pipe 3 of the present embodiment is that a pair of dents 33 are formed at both ends 32 of the bent part 30 on the outer side of the outer peripheral surface in the centripetal direction.

That is, in the metal bent pipe 3 of the present embodiment, unlike the metal bent pipe bent by ordinary press forming, both ends in the axial direction of the bent portion 30 are provided with the centrifugal direction outside the outer peripheral surface. A pair of dents 33 are formed which are depressed toward. Therefore, while the central portion 31 of the bent portion 30 springs back in the direction to ease the bending, the springback occurs in the direction to deepen the bending by the action of the pair of dents 33 returning outward.

As a result, the springback generated at the central portion 31 of the bent portion 30 and the springback generated at both ends 32 of the bent portion 30 cancel each other. In addition, if the magnitudes of the springbacks are substantially equal, the magnitudes of the variations of the springbacks are also substantially equal, and the directions thereof are opposite to each other, so that the variations of the springbacks are also offset.

Therefore, according to the metal bending pipe 3 of the present embodiment, the average value of the bending angle in the bending portion 30 approaches a constant value, and the variation in the bending angle is suppressed to be small, so that the bending angle accuracy is improved. effective. More specifically, in the present embodiment, the bent portion is provided with a 90 ° bend, but the range of the variation is only about ± 0.5 °.

In addition, since the metal bent pipe 3 of this embodiment can be manufactured by the above-described metal pipe bending method, the number of processing steps is small, it is inexpensive, and it is suitable for mass production. There is also an effect.

[Brief description of the drawings]

FIG. 1 is a front view showing the shape of a metal bent pipe after a rough bending step in Example 1.

FIG. 2 is a schematic view showing an outline of a precision bending process in Example 1.

FIG. 3 is a front sectional view showing a main part shape of a male mold and a female mold used in a precision bending process in Example 1.

FIG. 4 is a schematic view showing an initial state of a precision bending step in Example 1.

FIG. 5 is a schematic view showing a middle stage of a precision bending process in the first embodiment.

FIG. 6 is a schematic view showing a later stage of the precision bending process in the first embodiment.

FIG. 7 is a front sectional view showing the shape of a metal bent pipe in a later stage of the precision bending process in the first embodiment.

FIG. 8 is a schematic view showing another initial state of the precision bending step in the first embodiment.

FIG. 9 is a front view showing the outer shape of a metal bent pipe as the first embodiment;

[Explanation of symbols]

1: female type 11: concave portion 12: projecting portion 2: male type 21: convex portion 22: pulling portion 23: both ends 3: metal bent pipe 30: bent portion 31: central portion 32: both ends
33: Indented part 34: Straight part

Continued on the front page (72) Inventor Masato Asai 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Hiroshi Naoi 42 Minami lot, Higashibata Town, Anjo City, Aichi Prefecture Inside MEG Co., Ltd. (72) Inventor Kozo Onoda 42nd Minami lot, East end town, Anjo-shi, Aichi F-term in MEG Co., Ltd. (reference) 4E063 AA04 BA01 KA07 LA08 LA10

Claims (2)

[Claims] 1. A rough bending step of bending a metal pipe to an approximate angle to form a bent portion in the metal pipe; and forming a female mold having a concave portion whose curvature is tighter than the curvature of the bent portion at the center thereof. Circumscribes the outer peripheral surface of the male part, and has a convex part having a greater curvature than the curved part at the center, and a male mold having a pair of pulling parts with a smaller curvature than the curved part on both sides in the axial direction of the convex part. A precision bending step of press forming the bent portion of the metal pipe with the outer mold and the inner mold facing each other with the outer peripheral surface of the portion being in contact with the outer mold. Method. 2. A metal bent pipe having a bent portion bent at a predetermined bending angle, wherein the bent portion has a pair of concave portions bent in the centripetal direction outside the outer peripheral surfaces at both ends in the axial length direction. A bent metal pipe, comprising: