JP2016068088A - Method for manufacturing metal tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a metal tube by which generation of underfill in the metal tube can be suppressed.SOLUTION: A method for manufacturing a metal tube according to an embodiment includes one or more heating processes and plural upsetting processes. In the heating process, a tube end part of a metal tube is heated. In the upsetting process, upsetting of the heated tube end part is performed by use of a mandrel bar and a die. A taper angle of an outer taper part of a mandrel bar, which is used in the second or later upsetting processes is larger than a taper angle of an outer taper part of a mandrel bar which is used in the previous upsetting process.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a method for manufacturing a metal tube. More specifically, the present invention relates to a method of manufacturing a metal tube in which a pipe end portion of the metal tube is upset processed.

  When connecting metal pipes represented by oil well pipes, for example, a threaded portion is formed at the end of the metal pipe. In this case, the metal tubes are connected to each other through a joint having a threaded portion, or the tube ends of the metal tube are connected to each other by the threaded portion. When forming a thread part in the pipe end part of a metal tube, in order to raise the intensity of a thread part, the thickness of a pipe end part is made thicker than the thickness of other metal pipe parts.

  The upset process increases the thickness of the end of the metal tube. In the upset process, a metal tube having a heated tube end is inserted into a die, and then the tube end is upset and forged using a mandrel bar.

  In the upset process, the pipe end is deformed by upsetting forging to increase the thickness. At this time, the tube end portion may not be uniformly deformed, and a dent may occur on the inner surface of the tube end portion. Such a dent is called underfill. When the deformation resistance of the metal tube is high (for example, when the metal tube is a high alloy or when the heating temperature of the metal tube during upset processing is limited), underfill is likely to occur.

  A method for suppressing the occurrence of underfill in upset processing is disclosed in Japanese Patent Laid-Open No. 8-10889 (Patent Document 1). In the manufacturing method disclosed in Patent Document 1, the tube end is heated at a temperature distribution such that the center of the tube end is at the maximum temperature, and the temperature decreases toward the both sides from the center, and then upset processing is performed. In this case, the deformation resistance is large at the tube end, and the deformation resistance is small at the center. Therefore, Patent Document 1 describes that the thickening occurs uniformly in the axial direction.

JP-A-8-10889

  However, when upset processing is performed on a metal tube having high deformation resistance, underfill may still occur even when the method of Patent Document 1 is applied.

  The objective of this invention is providing the manufacturing method of a metal pipe | tube which can suppress that an underfill generate | occur | produces in a metal pipe | tube.

  The manufacturing method of the metal tube by embodiment of this invention is equipped with a heating process once or more and an upset process process in multiple times. In the heating step, the tube end of the metal tube is heated. In the upset process, the heated tube end is upset using a mandrel bar and a die. The taper angle of the outer taper portion of the mandrel bar used in the second and subsequent upset processing steps is larger than the taper angle of the outer taper portion of the mandrel bar used in the preceding upset processing step.

  The metal pipe manufacturing method according to the present invention can suppress underfill of the metal pipe.

FIG. 1 is a schematic view of a hot upset forging apparatus used in the method for manufacturing a metal tube according to the first embodiment. FIG. 2 is a cross-sectional view of the die shown in FIG. FIG. 3 is a side view of the mandrel bar in FIG. FIG. 4 is a flowchart of the method for manufacturing a metal tube according to this embodiment. FIG. 5 is an explanatory diagram of the process of step S2 in FIG. FIG. 6 is an explanatory diagram of the step S4 in FIG. FIG. 7 is another explanatory diagram of the step S4 in FIG. FIG. 8 is an explanatory diagram of the step S4 of the manufacturing method according to the second embodiment.

  The manufacturing method of the metal tube by this embodiment is provided with a heating process once or more and an upset process process in multiple times. In the heating step, the tube end of the metal tube is heated. In the upset process, the heated tube end is upset using a mandrel bar and a die. The taper angle of the outer taper portion of the mandrel bar used in the second and subsequent upset processing steps is larger than the taper angle of the outer taper portion of the mandrel bar used in the preceding upset processing step.

  As described above, the mandrel bar used in the nth and subsequent upset processing steps (n is a natural number of 2 or more. Hereinafter, the mandrel bar used in the nth upset processing step is referred to as an “nth mandrel bar”). The taper angle of the outer taper portion is larger than the outer taper portion of the [n-1] th mandrel bar used in the previous [n-1] -th upset machining step. Therefore, in the n-th upset processing step, a gap is formed between the outer surface of the outer tapered portion of the nth mandrel bar and the inner surface of the tube end portion of the metal tube. This gap becomes a space (for generating a metal flow) for the tube end portion to be deformed and moved again during the setup process. Therefore, a metal flow is likely to occur during the n-th upset process, and the nth mandrel bar is easily pushed. As a result, the occurrence of underfill is suppressed.

  Preferably, the heating step and the upset processing step are alternately performed twice. The taper angle of the outer taper portion of the mandrel bar (first mandrel bar) used in the first upset processing step is 0.3 to 0.5 degrees. The taper angle of the outer taper portion of the mandrel bar (second mandrel bar) used in the second upset processing step is 0.5 to 0.7 degrees.

  Preferably, the die includes a rear end bearing portion, a first inner tapered portion, a second inner tapered portion, and a front end bearing portion in order from the rear end toward the front end. The second inner taper portion has a smaller taper angle than the first inner taper portion and an axial length of 63.5 to 127 mm.

  Preferably, the axial length of the second inner tapered portion of the die used in the second and subsequent upset machining steps is longer than the second inner tapered portion of the die used in the preceding upset machining step. .

  A die used in the n-th upset processing step is referred to as an “nth die”. As described above, the axial length of the second inner tapered portion of the n-th die is the second inside of the [n−1] -th die used in the previous [n−1] -th upset machining step. Longer than the taper. In this case, a gap is formed between the inner surface of the second inner tapered portion of the nth die and the outer surface of the tube end portion of the metal tube during the n-th upset processing. Due to this gap, metal flow is likely to occur during upset processing again. Therefore, it becomes easier to push in the nth mandrel bar, and the occurrence of underfill is further suppressed.

  The method for producing a metal tube according to the present embodiment is particularly effective when the metal tube is a martensitic stainless steel tube containing 11 to 19% by mass and Cr. Generally, the higher the heating temperature, the smaller the deformation resistance of the alloy steel and the easier it is to process. However, even though such a stainless steel pipe is a high alloy having a large deformation resistance, the heating temperature at the time of upset processing cannot be increased because it is necessary to avoid the generation of the δ ferrite phase. The manufacturing method of this embodiment is particularly useful for such steel pipes that are difficult to process.

  Hereinafter, with reference to drawings, the manufacturing method of the metal tube of this embodiment is explained in full detail. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

[First Embodiment]
[manufacturing device]
FIG. 1 is a schematic view of a hot upset forging device (upsetter) 1 used in the method of manufacturing a metal tube according to the present embodiment. With reference to FIG. 1, the upsetter 1 includes a mandrel bar 2, a die 3, and a grip 5.

  The metal tube 4 heated in a heating furnace (not shown) is inserted into the die 3. Of the tube ends of the metal tube 4, the tube end at the front end is gripped by the grip 5 and fixed. The mandrel bar 2 is inserted into the tube end 41 at the rear end of the metal tube 4 and upsetting forging is performed.

[dice]
FIG. 2 is a sectional view of the die 3. Referring to FIG. 2, the die 3 includes a rear end bearing portion 31, a first inner taper portion 32, a second inner taper portion 33, and a front end in order from the rear end into which the metal tube 4 is inserted toward the front end. A bearing portion 34 is provided.

  The rear end bearing portion 31 is disposed on the rear end side of the die 3 (that is, the side on which the metal tube 4 is inserted). The inner diameter of the rear end bearing portion 31 is constant. That is, the rear end bearing portion 31 has a cylindrical inner surface.

  The first inner tapered portion 32 is disposed adjacent to the front end of the rear end bearing portion 31 and smoothly connects to the rear end bearing portion 31. The first inner tapered portion 32 has a tapered inner surface. The inner diameter of the first inner tapered portion 32 decreases from the rear end to the front end of the die 3.

  The second inner taper portion 33 is disposed adjacent to the front end of the first inner taper portion 32 and smoothly connects to the first inner taper portion. The second inner tapered portion 33 has a tapered inner surface, and the inner diameter thereof decreases from the rear end to the front end of the die 3. The taper angle A33 (°) of the second inner taper portion 33 is smaller than the taper angle A32 of the first inner taper portion 32.

  The front end bearing portion 34 is disposed adjacent to the front end of the second inner tapered portion and is smoothly connected to the second inner tapered portion 33. The inner diameter of the front end bearing portion 34 is constant and smaller than the inner diameter of the rear end bearing. Similarly to the rear end bearing portion 31, the front end bearing portion 34 also has a cylindrical inner surface.

[Mandrel bar]
FIG. 3 is a side view of the mandrel bar 2 in FIG. With reference to FIG. 3, the mandrel bar 2 includes a shoulder portion 21, an outer taper portion 22, and a tip portion 23 in order from the rear end toward the tip.

  The shoulder portion 21 includes a cylindrical outer surface 211 and a shoulder 212 which is the front surface of the shoulder portion 21. The shoulder portion 21 brings the shoulder 212 into contact with the rear end surface of the tube end portion 41 of the metal tube 4 and pushes the tube end portion 41 forward.

  The outer tapered portion 22 is disposed on the front surface of the shoulder portion 21. The outer taper portion 22 has a taper shape with a taper angle A22 (°). The outer diameter of the outer tapered portion 22 decreases from the rear end to the front end of the mandrel bar 2. The outer diameter of the rear end of the outer tapered portion 22 is smaller than the outer diameter of the shoulder portion 21. Therefore, a step (shoulder 212) is formed at the connection portion between the front surface of the shoulder portion 21 and the outer tapered portion.

  The front end portion 23 is adjacent to the front end of the outer taper portion 22 and is smoothly connected to the outer taper portion 22. The distal end portion 23 has a taper angle equal to or greater than the taper angle A22 of the outer taper portion 22. The distal end surface of the distal end portion 23 is rounded. The tip portion 23 may not be provided.

[Production method]
The manufacturing method of the metal tube of this embodiment using the above-mentioned upsetter 1 is as follows. In the following, the case where the heating process and the upset process shown in FIG. 4 are alternately performed twice (two-heat two-shot process) will be described in detail. Other embodiments will be supplementarily described as appropriate.

  FIG. 4 is a flowchart of the manufacturing method of the present embodiment. With reference to FIG. 4, the manufacturing method of the metal tube of this embodiment WHEREIN: The process (S1) of heating the pipe end part 41 of the metal pipe 4, and the process of implementing an upset process with respect to the pipe end part 41 ( S2), a step (S3) of reheating the tube end portion 41 after the upset processing, and a step (S4) of performing the upset processing again on the reheated tube end portion 41. . Hereinafter, each process is explained in full detail.

[Heating step: S1]
First, the tube end 41 at the rear end of the metal tube 4 is heated (S1). For example, the tube end 41 is heated using a high-frequency induction heating furnace (not shown).

  The heating temperature is appropriately set according to the steel type and size (wall thickness, etc.) of the metal tube 4. For example, when the metal tube 4 is a martensitic stainless steel containing 11% to 19% Cr by mass%, it is also a high alloy, so it is desirable to raise the heating temperature as much as possible from the viewpoint of deformation resistance. However, if the heating temperature is too high, a δ ferrite phase is likely to be generated.

  For example, in mass%, C: 0.03% or less, Si: 0.50% or less, Mn: 1.00% or less, Ni: 4.0-6.0%, Cr: 11.0-14.0 %, Mo: Martensitic stainless steel containing 0 to 2.0% (in addition to the above-mentioned components, it may contain additional elements or impurities that are acceptable in terms of product performance), heating When the temperature exceeds about 1250 ° C., a δ ferrite phase is rapidly formed. Therefore, in the steel having such a chemical composition, the heating temperature is set to 1150 ° C to 1250 ° C, more preferably 1200 ° C to 1250 ° C. However, the metal tube of this embodiment is not limited to steel having the above chemical composition.

[Upset machining process: S2]
After heating, an upset process is performed on the tube end 41 of the metal tube 4 using the mandrel bar 2 and the die 3. Specifically, the metal tube 4 is transported to the upsetter 1 by a well-known transport device (not shown). As shown in FIG. 1, the metal tube 4 is passed through a die 3, and the tube end portion at the tip of the metal tube 4 is held and fixed by a grip 5.

  After fixing, the axis of the mandrel bar 2 is aligned with the axis of the metal tube 4. Thereafter, the mandrel bar 2 is pushed in the axial direction from the rear end of the tube end portion 41 toward the front end of the metal tube 4. At this time, the outer tapered portion 22 of the mandrel bar 2 is inserted into the tube end portion 41. Then, the shoulder 212 of the shoulder portion 21 of the mandrel bar 2 comes into contact with the rear end surface of the tube end portion 41 and pushes the tube end portion 41 forward.

  FIG. 5 is a cross-sectional view of the metal tube 4 and the die 3 during upset processing. Referring to FIG. 5, the tapered portion 22 of the mandrel bar 2 is inserted into the metal tube 4, and the shoulder portion 21 pushes the tube end portion 4 forward. Thereby, the wall thickness of the pipe end part 41 increases toward the radial direction outer side of a metal pipe. At this time, the outer surface of the tapered portion 22 further contacts the inner surface of the tube end portion 41. As a result, the inner surface of the tube end portion 41 has a tapered shape in which the inner diameter decreases from the rear end toward the front end.

  In the present embodiment, the mandrel bar 2 has an outer tapered portion 22 having a tapered shape. Therefore, it is easy to push the mandrel bar 2 into the pipe end 41.

  Note that the outer surface of the tube end portion 41 has a two-step tapered shape due to the first inner tapered portion 32 and the second inner tapered portion 33 of the die 3.

  In the upset process of step S2, it is preferable to increase the thickness by more than half of the final thickness increase of the thickness of the pipe end 41. In this case, since the amount of increase in thickness during the second upset process in step S4 is reduced, underfill is less likely to occur.

[Second heating step: S3]
After the upset processing in step S2, the tube end 41 of the metal tube 4 is heated. Due to the upset process, the temperature of the pipe end 41 is lowered. In order to perform the upsetting process again in step S4, the tube end portion 41 is heated again. For example, the heating temperature is set to be the same as that in step S1.

[Second upset processing step: S4]
After the heating in step S3, upsetting is performed again on the tube end 41 of the metal tube 4 (S4).

  As shown in FIG. 6, in the upset process of step S <b> 4, the mandrel bar 20 is used instead of the mandrel bar 2 and the die 30 is used instead of the die 3, as compared with the upset process of step S <b> 2.

  Referring to FIG. 2, as in the case of the die 3, the die 30 is arranged in order from the rear end to the front end of the die 30, the rear end bearing portion 31, the first inner tapered portion 32, the second inner tapered portion 33, and The front end bearing portion 34 is provided. Referring to FIG. 6, the inner diameter of the rear end bearing portion 31 of the die 30 is larger than the inner diameter of the rear end bearing portion of the die 3. The inner diameters of the first and second inner tapered portions 32 and 33 of the die 30 are also larger than the inner diameters of the first and second inner tapered portions 32 and 33 of the die 3. By using the die 30 instead of the die 3, the tube end portion 41 of the metal tube 4 can be further increased in thickness.

  3 and 6, the mandrel bar 20 also has a shoulder portion 21, an outer taper portion 22, and a tip portion 23 in order from the rear end to the front end of the mandrel bar 20 in the same manner as the mandrel bar 2. Prepare.

  However, the taper angle A22 (°) of the outer taper portion 22 of the mandrel bar 20 is larger than the taper angle A22 of the outer taper portion 22 of the mandrel bar 2. Furthermore, the outer diameter (hereinafter referred to as the front end outer diameter) D22 of the outer tapered portion 22 of the mandrel bar 20 is smaller than the front end outer diameter D22 of the outer tapered portion 22 of the mandrel bar 2. By using the mandrel bar 20 instead of the mandrel bar 2 in the second upset processing step (S4), the occurrence of underfill is suppressed. Hereinafter, this point will be described in detail.

[Underfill suppression mechanism]
As shown in FIG. 6, the outer tapered portion 22 of the mandrel bar 20 is inserted into the tube end portion 41, and the shoulder portion 21 is pressed against the rear end of the tube end portion 41. At this time, the pipe end 41 is upset and the thickness of the pipe end 41 is increased.

  As described above, the taper angle A22 of the outer taper portion 22 of the mandrel bar 20 is larger than that of the mandrel bar 2, and the front end outer diameter D22 of the outer taper portion 22 of the mandrel bar 20 is smaller than that of the mandrel bar 2.

  By the upset process in step S <b> 2, the inner diameter of the tube end portion 41 of the metal tube 4 is the same as the outer diameter of the outer tapered portion 22 of the mandrel bar 2. Therefore, as shown in FIG. 7, in the upsetting process again, a gap SP <b> 1 is formed between the inner surface of the tube end portion 41 and the outer tapered portion 22 of the mandrel bar 20.

  If the gap SP1 is not formed, that is, if the outer surface of the outer tapered portion 22 of the mandrel bar 20 is in contact with the inner surface of the tube end portion 41, even if the mandrel bar 20 is further pushed, Since the contact resistance with the pipe end portion 41 is large, the mandrel bar 20 does not advance. In this case, pushing by the mandrel bar 20 is insufficient. Therefore, an underfill is formed at the tube end portion 41.

  However, in this embodiment, the gap SP1 is formed as described above. Therefore, the outer surface of the outer tapered portion 22 of the mandrel bar 20 is unlikely to come into contact with the inner surface of the tube end portion 41. Therefore, it is easy to push the mandrel bar 20. If the mandrel bar 20 is further advanced by the clearance SP1, the tube end portion 41 is further thickened by the shoulder portion 21. In this case, the metal portion (meat) flows into the gap SP1 due to the metal flow. As a result, the desired increase in thickness can be achieved while the occurrence of underfill is suppressed.

  As described above, in the present embodiment, the mandrel bar 20 is used in place of the mandrel bar 2 in the second upset process (S4). The taper angle of the outer tapered portion 22 of the mandrel bar 20 is larger than that of the mandrel bar 2, and the front end outer diameter D22 of the outer tapered portion 22 of the mandrel bar 20 is smaller than that of the mandrel bar 2. Therefore, a gap SP <b> 1 is formed between the outer surface of the outer tapered portion 22 of the mandrel bar 20 and the inner surface of the pipe end portion 41 during the upset process (S <b> 4) again. Therefore, the insufficiency of pressing of the mandrel bar 20 is solved, and the occurrence of underfill is suppressed.

  Preferably, the taper angle of the outer tapered portion 22 of the mandrel bar 2 is 0.3 to 0.5 °, and the taper angle of the outer tapered portion 22 of the mandrel bar 20 is 0.5 to 0.7 °. More preferably, it is 0.55 to 0.65 °.

  Furthermore, preferably, the axial distance L33 (see FIG. 2) of the second inner tapered portion 33 of the die 3 and the die 30 is 63.5 to 127 mm. By using a die having such a structure, the occurrence of underfill can be further suppressed.

[Second Embodiment]
In the above-described embodiment, the mandrel bar 20 is used in place of the mandrel bar 2 in the upset processing in step S4. In step S4, if a die 30 having the following shape is used, the occurrence of underfill can be further reduced. Hereinafter, the second embodiment will be described in detail.

  In the second embodiment, as compared with the first embodiment, the die 30 having the shape described below is used in the upsetting process in step S4 again.

  Referring to FIG. 2, the die 30 in the second embodiment is similar to the die 3 in that the rear end bearing portion 31, the first inner taper portion 32, 2 The inner taper part 33 and the front-end bearing part 34 are provided.

  The axial distance L33 of the second inner tapered portion 33 of the die 30 is longer than the axial distance L33 of the second inner tapered portion 33 of the die 3. The inner diameters of the front end bearing 34 of the die 30 and the die 3 are the same. Therefore, the taper angle A33 of the second inner taper portion 33 of the die 30 is smaller than the taper angle A33 of the die 3.

  In 2nd Embodiment, the upset process of step S4 is implemented using the die | dye 30 of the above-mentioned shape. Other steps (S1 to S3) are the same as those in the first embodiment. By using the die 30 having the shape described above, it becomes easier to further suppress the occurrence of underfill in the upset process in step S4. Hereinafter, this point will be described in detail.

[Suppression mechanism of underfill]
As described above, in the present embodiment, the axial distance of the second inner tapered portion 33 of the die 30 is longer than that of the die 3. More specifically, as shown in FIG. 8, axial distance L33 ₃₀ in the second tapered portion 33 of the die 30 is greater than the axial distance L33 ₃ of the second inner taper portion 33 of the die 3. Therefore, a gap SP <b> 2 is formed between the die 30 and the pipe end 41 at the time of the upsetting process in step S <b> 4 again. Therefore, at the time of upsetting forging of the tube end 41 by the mandrel bar 20, the metal flow of the tube end 41 is easily performed. Therefore, even when the metal tube 4 made of a metal having high deformation resistance is upset, the tube end portion 41 is easily deformed and the mandrel bar 20 is easily pushed. Therefore, it becomes easy to suppress the occurrence of underfill.

  As described above, in the second embodiment, the axial length L33 of the second inner tapered portion 33 of the die 30 used in step S4 is made longer than that of the die 3. In this case, the taper angle of the second inner tapered portion 33 of the die 30 is smaller than that of the die 3. Therefore, a gap SP2 is formed during upset processing. The gap SP2 makes it easier to push the mandrel bar 20 more easily, and as a result, it becomes easier to suppress the occurrence of underfill.

  Another embodiment of the present invention will be described.

  In the above-described embodiment (2-heat 2-shot process), the heating step and the upset processing step are alternately repeated twice, but these may be repeated three times or more (for example, a 3-heat 3-shot process).

  In the above-described two-heat two-shot process, one upset process is performed for one heating process. On the other hand, upset processing may be performed a plurality of times with mandrel bars having different shapes while the temperature of the tube end heated in one heating step does not drop (for example, one heat two shot process). In this case, for example, using a device in which a plurality of upsetters with mandrel bars of different shapes are arranged close to each other, the metal tube is moved while the temperature at the end of the tube heated in the heating process does not fall sufficiently. While upset processing.

  Based on the manufacturing conditions shown in Table 1, a metal tube whose tube end was upset was manufactured. The manufactured metal tube was examined for underfill.

  Referring to Table 1, the axial length of the second inner tapered portion of the die used in steps S2 and S4 was 50 mm in the comparative example and 100 mm in the present invention example. In addition, the axial direction length of the 1st inner taper part of the die | dye used by step S2 and S4 was 50 mm in both this invention example and the comparative example.

  Furthermore, in the comparative example, the taper angle of the outer tapered portion of the mandrel bar used in step S2 is the same as the taper angle of the outer tapered portion of the mandrel bar used in step S4. On the other hand, in the present invention example, the taper angle of the outer taper portion of the mandrel bar used in step S4 is made larger than the taper angle of the outer taper portion of the mandrel bar used in step S2.

  In the present invention example, the front end outer diameter of the outer tapered portion of the mandrel bar used in step S4 was smaller than the front end outer diameter of the outer tapered portion of the mandrel bar used in step S2.

  The chemical composition of the metal tube used in this example is mass%, C: 0.009%, Si: 0.25%, Mn: 0.43%, P: 0.013%, S: 0.001. %, Cu: 0.18%, Cr: 11.8%, Ni: 5.4%, Mo: 1.9%, Ti: 0.10%, Sol. Al: 0.035%, N: 0.007% was contained, and the balance was Fe and impurities. The outer diameter of the metal tube was 114.3 mm. The wall thickness was 14.22 mm. In step S1 and step S3, a region having a length of 150 mm from the end of the metal tube was heated to 1250 ° C.

  The total thickness increase of the pipe end of the metal pipe to be upset in steps S2 and S4 was 6.75 mm. The final shape upset length was 60 mm.

  At the pipe end of the metal pipe for which Step S4 was completed, the upset processed part was cut so that the longitudinal section could be seen, and the depth and width of the underfill were measured. When the maximum depth was less than 1 mm, the underfill was “not generated”, and when it was 1 mm or more, the underfill was “occurred”.

[Test results]
In the example of the present invention, no underfill was observed. As described above, the taper angle of the outer tapered portion of the mandrel bar in step S4 is larger than that of the mandrel bar in step S2, and the front end outer diameter of the outer tapered portion in step S4 is smaller than that of step S2. Also, the length of the inner taper portion of the die was appropriate. Therefore, in the second upset processing step of Step S4, it is considered that an appropriate gap is formed between the tube end portion, the mandrel bar and the die, and the occurrence of underfill is suppressed. On the other hand, underfill was observed in the comparative example.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

DESCRIPTION OF SYMBOLS 1 Upsetter 2,20 Mandrel bar 21 Shoulder 22 Outer taper part 3,30 Dies 31 Rear end bearing part 32 First inner taper part 33 Second inner taper part 34 Front end bearing part

Claims (5)

One or more heating steps for heating the tube end of the metal tube,
A plurality of upset processing steps for upsetting the heated tube end using a mandrel bar and a die,
The taper angle of the outer taper portion of the mandrel bar used in the second and subsequent upset processing steps is larger than the taper angle of the outer taper portion of the mandrel bar used in the previous upset processing step, A method of manufacturing a metal tube. The manufacturing method according to claim 1,
The heating step and the upset processing step,
Do it twice,
The taper angle of the outer taper portion of the mandrel bar used in the first upset processing step is 0.3 to 0.5 degrees,
The method of manufacturing a metal tube, wherein a taper angle of the outer tapered portion of the mandrel bar used in the second upset processing step is 0.5 to 0.7 degrees. It is a manufacturing method of Claim 1 or Claim 2, Comprising:
The die includes a rear end bearing portion, a first inner taper portion, a second inner taper portion, and a front end bearing portion in order from the rear end to the front end.
The second inner taper portion has a taper angle smaller than that of the first inner taper portion and an axial length of 63.5 to 127 mm. It is a manufacturing method of Claim 3, Comprising:
The axial length of the second inner tapered portion of the die used in the second and subsequent upset machining steps is the same as that of the second inner tapered portion of the die used in the preceding upset machining step. A method of manufacturing a metal tube that is longer than the axial length. It is a manufacturing method of any one of Claims 1-4, Comprising:
The metal tube is
A manufacturing method of a metal pipe which is a martensitic stainless steel pipe containing Cr: 11 to 19% by mass%.

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