Steel pipe threaded joints

Abstract

Provided is a threaded joint for steel pipes having wedge threads that can suppress seizure during make-up. [Solution] A threaded joint for steel pipes comprises a pin (10) having a male thread portion (11) composed of wedge threads, and a box (20) having a female thread portion (21) composed of wedge threads. The male thread portion (11) includes a constant pin thread width portion (111) and a variable pin thread width portion (112). The female thread portion (12) includes a constant box thread width portion (211) and a variable box thread width portion (212). The load flank lead (W112) of the variable pin thread width portion (112) is smaller than the load flank lead of the female thread portion (21), and the load flank lead (W111) of the constant pin thread width portion (111) is smaller than the load flank lead (W112) of the variable pin thread width portion (112).

Description

[Technical Field]

[0001] The present invention relates to a threaded joint for steel pipes that connects steel pipes together. [Background technology]

[0002] Conventionally, oil well pipes such as casings and tubing have been used to extract underground resources in oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"). Oil well pipes are made up of steel pipes that are connected in sequence. Threaded joints are used to connect the steel pipes.

[0003] Threaded joints for steel pipes are broadly divided into coupling and integral types. In the case of a coupling type, one of the pair of pipes to be connected is a steel pipe and the other is a coupling. Male threads are provided on the outer periphery at both ends of the steel pipe. Female threads are provided on the inner periphery at both ends of the coupling. The male threads of the steel pipe are screwed into the female threads of the coupling, thereby fastening and connecting the steel pipe and the coupling.

[0004] In the case of an integral type, both of the pipes to be connected are steel pipes, and no couplings are used. Each steel pipe has a male thread on the outer periphery of one end and a female thread on the inner periphery of the other end. The male thread of one steel pipe is screwed into the female thread of the other steel pipe, fastening and connecting the two steel pipes.

[0005] Generally, the portion of the pipe to be connected that has a male thread is called a pin because it is inserted into the female thread. The portion of the pipe to be connected that has a female thread is called a box because it receives the male thread. The pin and box are both tubular because they are part of the pipe.

[0006] Japanese Patent Laid-Open Publication No. 2001-56075 (Patent Document 1) discloses a coupling-type pipe joint in which a pin having a tapered male thread formed on the outer peripheral surface of the tip of a pipe is screwed into a box having a tapered female thread formed on the inner peripheral surface of both ends of a short pipe, which screws into the tapered male thread. The pipe joint has a mismatch between the pitch of the male thread and the pitch of the male thread. In other words, the pitch of the male thread is smaller than the pitch of the female thread by 1 to 7 microns. This prevents seizure from occurring at the threaded mating portion when the oil well pipe thread joint is screwed into the pipe. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-56075 Summary of the Invention [Problem to be solved by the invention]

[0008] Threaded joints for steel pipes having threads formed with wedge threads are generally known. Threaded joints formed with wedge threads typically have excellent torque resistance because the load flanks and stabbing flanks of the male and female thread portions are in mating contact with each other when the threaded joint for steel pipes is made up.

[0009] To ensure consistent sealing performance in threads constructed with such wedge threads, it is necessary to minimize the gap between the opposing crest and root faces of the threads. However, as the diameter of a steel pipe increases, the elliptical error of the thread increases. Even in perfectly threaded sections, the gap between the crest and root faces of the threads increases locally in the circumferential direction due to the elliptical error. To avoid this, the angles of the flanks are increased or thread interference is provided. As the diameter of a steel pipe increases, the thread interference must also increase. However, if the thread interference becomes too large, the pin may stretch significantly in the axial direction due to the Poisson effect. In this case, the male thread at the base end of the pin (the steel pipe body) and the female thread at the tip end of the box may come into strong contact with each other on their respective load surfaces when the steel pipe threaded joint is made. This can lead to seizure.

[0010] As described above, the pipe fitting of Patent Document 1 can suppress seizure to some extent even when the threads are configured with wedge threads. However, in large-diameter steel pipes with wedge threads in which the load flanks and stabbing flanks of the male and female threads are in mating contact with each other, even if the pitch of the male thread is made smaller than the pitch of the female thread, as in the pipe fitting of Patent Document 1, seizure can still occur between the load flanks of the male thread at the base end (steel pipe body) of the pin and the female thread at the tip end of the box. On the other hand, if the pitch difference between the male thread at the tip end of the pin and the female thread at the base end (steel pipe body) of the box is increased throughout, the stabbing flanks of the male thread at the tip end of the pin and the female thread at the base end (steel pipe body) of the box will come into strong contact with each other, which can cause seizure.

[0011] An object of the present disclosure is to provide a threaded joint for large-diameter steel pipes having wedge threads, which is capable of suppressing seizure during make-up. [Means for solving the problem]

[0012] A threaded joint for steel pipes according to the present disclosure is a threaded joint for connecting steel pipes, and includes a tubular pin formed at one tip end of the steel pipe, and a tubular box into which the pin is inserted and fastened. The pin includes a male thread portion formed on the outer peripheral surface of the pin and configured with wedge-shaped threads. The box includes a female thread portion formed on the inner peripheral surface of the box, corresponding to the male thread portion, and configured with wedge-shaped threads. The male thread portion includes a constant pin thread width portion having a constant thread root width, and a variable pin thread width portion having a thread root width that is equal to or greater than the thread root width of the constant pin thread width portion and gradually increases from the constant pin thread width portion toward the tip of the pin. The load flank lead of the variable pin thread width portion is smaller than the load flank lead of the female thread portion. The load flank lead of the constant pin thread width portion is smaller than the load flank lead of the variable pin thread width portion. [Effects of the Invention]

[0013] According to the present invention, in a threaded joint for large diameter steel pipes having wedge threads, seizure during make-up can be suppressed. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a longitudinal cross-sectional view showing an outline of a threaded joint for steel pipes according to an embodiment. [Figure 2] FIG. 2 is an enlarged longitudinal sectional view of a constant thread width portion of the threaded joint for steel pipes shown in FIG. [Figure 3] FIG. 3 is an enlarged longitudinal sectional view of a variable thread width portion of the threaded joint for steel pipes shown in FIG. [Figure 4] FIG. 4 is a schematic diagram of an enlarged longitudinal cross section of a constant thread width portion of the threaded joint for steel pipes shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] (Configuration 1) A threaded joint for steel pipes according to an embodiment is a threaded joint for connecting steel pipes, and includes a tubular pin formed at one tip end of the steel pipe, and a tubular box into which the pin is inserted and fastened. The pin includes a male thread portion formed on the outer peripheral surface of the pin and configured with wedge-shaped threads. The box includes a female thread portion formed on the inner peripheral surface of the box, corresponding to the male thread portion, and configured with wedge-shaped threads. The male thread portion includes a constant pin thread width portion having a constant thread root width, and a variable pin thread width portion having a thread root width that is equal to or greater than the thread root width of the constant pin thread width portion and gradually increases from the constant pin thread width portion toward the tip of the pin. The load flank lead of the variable pin thread width portion is smaller than the load flank lead of the female thread portion. The load flank lead of the constant pin thread width portion is smaller than the load flank lead of the variable pin thread width portion.

[0016] This not only prevents seizure between the load surface of the pin thread width variable section and the load surface of the box thread width variable section when connecting large diameter steel pipes, but also prevents seizure between the load surface of the pin thread width constant section and the load surface of the box thread width constant section.

[0017] (Configuration 2) In a threaded joint for steel pipes of Configuration 1, the female thread portion may include a constant box thread width portion having a constant thread root width, and a variable box thread width portion having a thread root width that is equal to or smaller than the thread root width of the constant box thread width portion and that gradually decreases from the constant box thread width portion toward the inner end of the box. The load flank lead of the variable pin thread width portion may be smaller than the load flank lead of the variable box thread width portion. The load flank lead of the variable box thread width portion may be equal to or smaller than the load flank lead of the constant box thread width portion. This more effectively suppresses seizure between the load flanks of the constant pin thread width portion and the constant box thread width portion.

[0018] (Configuration 3) In a threaded joint for steel pipes according to Configuration 1 or 2, when the pin and box are fastened, a gap may be formed between the load flanks of the pin constant thread width portion and the load flanks of the box constant thread width portion, and / or between the stabbing flanks of the pin constant thread width portion and the stabbing flanks of the box constant thread width portion. This makes it possible to more effectively suppress seizure between the load flanks of the pin constant thread width portion and the load flanks of the box constant thread width portion.

[0019] (Configuration 4) In a threaded joint for steel pipes having any one of configurations 1 to 3, the amount of mismatch between the load flank lead of the pin thread width variable portion and the load flank lead of the female thread portion may be 1 to 10 μm.

[0020] (Configuration 5) In a threaded joint for steel pipes having any one of configurations 1 to 4, the amount of mismatch between the load flank lead of the variable pin thread width portion and the load flank lead of the constant pin thread width portion may be 1 to 180 μm.

[0021] (Configuration 6) In a threaded joint for steel pipes according to any one of configurations 3 to 5, the combined value of the gap formed between the load flank of the male thread of the pin constant thread width portion and the load flank of the box constant thread width portion corresponding to the load flank of the male thread, and the gap formed between the stabbing flank of the male thread and the stabbing flank of the box constant thread width portion corresponding to the stabbing flank of the male thread, may be 245 to 450 μm. This makes it possible to make it easier for the pin constant thread width portion and the box constant thread width portion to bear a tensile load or compressive load after make-up, while suppressing seizure between the load flank of the pin constant thread width portion and the load flank of the box constant thread width portion.

[0022] (Configuration 7) A threaded joint for steel pipes according to any one of configurations 1 to 6, wherein the steel pipe may have an outer diameter exceeding 16 inches. This threaded joint for steel pipes can be suitably used to connect steel pipes of relatively large diameters. That is, as described above, steel pipes of relatively large diameters, such as those exceeding 16 inches, have relatively thin walls, which further increases the ellipticity error. The Poisson effect also increases. Therefore, the threaded joint for steel pipes of the present disclosure can particularly contribute to preventing seizure in steel pipes of relatively large diameters.

[0023] (Configuration 8) A steel pipe according to an embodiment comprises a threaded joint for steel pipes of any one of configurations 1 to 7.

[0024] First Embodiment Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding components are designated by the same reference numerals, and the same description will not be repeated.

[0025] Figure 1 is a longitudinal cross-sectional view showing a threaded joint for steel pipes 1 according to a first embodiment of the present disclosure. The threaded joint for steel pipes 1 is an integral type threaded joint. However, the configuration of the threaded joint described in this embodiment can also be applied to a coupling type threaded joint.

[0026] As shown in Figure 1, a threaded joint 1 for steel pipes comprises a pin 10 and a box 20. The pin 10 and box 20 are each tubular. The pin 10 extending from the end of one steel pipe 2 is inserted into the box 20 extending from the other steel pipe 2 and fastened together. In this way, by providing a threaded joint 1 for steel pipes, steel pipes 2 can be connected to each other. The threaded joint 1 for steel pipes can be suitably used to connect steel pipes 2 with relatively large diameters, such as those with outer diameters exceeding 16 inches.

[0027] The pin 10 has a male thread portion 11 on its outer circumferential surface. The box 20 has a female thread portion 21 corresponding to the male thread portion 11 on its inner circumferential surface. The male thread portion 11 and the female thread portion 21 are configured with tapered threads that mesh with each other. The male thread portion 11 and the female thread portion 21 are configured with wedge-shaped threads. In other words, the thread shapes of the male thread portion 11 and the female thread portion 21 are dovetail shapes.

[0028] The male thread portion 11 of the pin 10 includes a constant pin thread width portion 111 and a variable pin thread width portion 112. The constant pin thread width portion 111 is located on the base end side (pipe main body side) of the pin 10 in the male thread portion 11. The variable pin thread width portion 112 is located on the tip side of the pin 10 relative to the constant pin thread width portion 111 in the male thread portion 11, and is provided continuous with the constant pin thread width portion 111. The female thread portion 21 of the box 20 includes a constant box thread width portion 211 and a variable box thread width portion 212. The constant box thread width portion 211 is located on the tip side of the box 20 in the female thread portion 21. The variable box thread width portion 212 is located on the inner end side of the box (pipe main body side) relative to the constant box thread width portion 211 in the female thread portion 21.

[0029] The thread width and thread root width are constant in the constant pin thread width portion 111 and the constant box thread width portion 211, while the thread width and thread root width vary in the variable pin thread width portion 112 and the variable box thread width portion 212. That is, the variable pin thread width portion 112 has a thread root width that is equal to or greater than the thread root width of the constant pin thread width portion 111, and the thread root width of the variable pin thread width portion 112 gradually increases from the constant pin thread width portion 111 toward the tip of the pin 10. Therefore, the thread width of the variable pin thread width portion 112 gradually decreases toward the tip of the pin 10. Furthermore, the variable box thread width portion 212 has a thread root width that is equal to or less than the thread root width of the constant box thread width portion 211, and the thread root width of the variable box thread width portion 212 gradually decreases from the constant box thread width portion 211 toward the innermost end of the box 20. Therefore, the thread width of the variable box thread width portion 212 gradually increases toward the innermost end of the box 20.

[0030] Fig. 2 is an enlarged longitudinal sectional view of the pin constant thread width portion 111 and the box constant thread width portion 211 when the male thread portion 11 is a complete thread. Fig. 3 is an enlarged longitudinal sectional view of the pin variable thread width portion 112 and the box variable thread width portion 212 when the male thread portion 11 is a complete thread.

[0031] As shown in Figures 2 and 3, when viewed from a cross section of the threaded joint for steel pipes 1 cut along a plane including the pipe axis CL, the male thread portion 11 has a plurality of thread crests 11a, thread roots 11b, stabbing flanks 11c, and load flanks 11d. Each stabbing flank 11c and each load flank 11d connects the thread crest 11a to the thread root 11b. Each stabbing flank 11c is the leading surface when the pin 10 is threaded into the box 20. Each load flank 11d is the surface located opposite the corresponding stabbing flank 11c. The thread crests 11a and thread roots 11b are parallel to the pipe axis CL. However, the thread crests 11a and thread roots 11b may also be parallel to the thread taper.

[0032] When viewed from a cross section of the threaded joint for steel pipes 1 cut on a plane including the pipe axis CL, the female thread portion 21 has a plurality of thread crests 21a, thread roots 21b, stabbing flanks 21c, and load flanks 21d. Each thread crest 21a faces the thread root 11b of the male thread portion 11. Each thread root 21b faces the thread crest 11a of the male thread portion 11. Each stabbing flank 21c faces the stabbing flank 11c of the male thread portion 11. Each load flank 21d faces the load flank 11d of the male thread portion 11. The thread crest 21a and thread root 21b are parallel to the pipe axis CL. However, the thread crest 21a and thread root 21b may also be parallel to the thread taper.

[0033] The flank angles of the stab flanks 11c, 21c and the load flanks 11d, 21d are all negative angles less than 0°. The flank angle here refers to the angle between a plane perpendicular to the tube axis CL and the stab flanks 11c, 21c or the load flanks 11d, 21d. In Figures 2 and 3, the flank angles of the stab flanks 11c, 21c are positive in the counterclockwise direction, and the flank angles of the load flanks 11d, 21d are positive in the clockwise direction.

[0034] 2, in the fastened state, in the constant pin thread width portion 111 and the constant box thread width portion 211, the load flank 11d of the male thread portion 11 contacts the load flank 21d of the female thread portion 21 depending on the load flank lead mismatch and external force described below, or there is no contact and a gap g1 is generated between the load flank 11d of the male thread portion 11 and the load flank 21d of the female thread portion 21. Furthermore, in the constant pin thread width portion 111 and the constant box thread width portion 211, if the male thread portion 11 is a complete thread, the thread crest 11a and the thread root 11b of the male thread portion 11 contact the thread root 21b and the thread crest 21a of the female thread portion 21, respectively. However, the stabbing flanks 11c and the stabbing flanks 21c do not contact each other. That is, in the pin thread width constant portion 111 and the box thread width constant portion 211, in a fastened state, a gap g2 is generated between the stabbing flank 11c of the male thread portion 11 and the stabbing flank 21c of the female thread portion 21. In this way, in the pin thread width constant portion 111 and the box thread width constant portion 211, a gap g1 may be generated between the load flank 11d of the male thread of the male thread portion 11 and the load flank 21d of the female thread portion 21 that faces the load flank 11d, and a gap g2 is generated between the stabbing flank 11c of the male thread and the stabbing flank 21c of the female thread portion 21 that faces the stabbing flank 11c. Details of the gap g1 and the gap g2 will be described later. In the pin thread width constant portion 111 and the box thread width constant portion 211, if the male thread portion 11 is an incomplete thread, the thread crest 11a and the thread valley root 11b of the male thread portion 11 may not contact the thread valley bottom 21b and the thread crest 21a of the female thread portion 21, respectively. In the case of a complete thread, the thread crest 11a and the thread valley bottom 11b of the male thread portion 11 contact the thread valley bottom 21b and the thread crest 21a of the female thread portion 21, respectively. However, in a complete thread portion, if there is a difference in the thread height of the male thread and the female thread due to a manufacturing error, a gap of, for example, up to about 50 μm may occur between the thread crest 11a and the thread valley bottom 21b or between the thread valley bottom 11b and the thread crest 21a. In other words, "in the case of a perfect thread, the thread crest 11a and thread valley bottom 11b of the male thread portion 11 contact the thread valley bottom 21b and thread crest 21a of the female thread portion 21, respectively" allows for gaps due to such manufacturing errors.This also applies to a pin thread width variable section 112 and a box thread width variable section 212, which will be described later.

[0035] 3, in the main portions of the pin thread width variable portion 112 and the box thread width variable portion 212, no gap is generated between the male thread portion 11 and the female thread portion 21 in the fastened state. That is, in the fastened state, the load flanks 11d, 21d and the stabbing flanks 11c, 21c come into contact with each other in the pin thread width variable portion 112 and the box thread width variable portion 212. Furthermore, in the pin thread width variable portion 112 and the box thread width variable portion 212, when the male thread portion 11 is a complete thread, the thread crest surface 11a of the male thread portion 11 and the thread root surface 21b of the female thread portion 21 come into contact with each other, and the thread root surface 11b of the male thread portion 11 and the thread crest surface 21a of the female thread portion 21 come into contact with each other. When the male thread portion 11 and the female thread portion 21 each have a chamfered surface 11e, 21e, the chamfered surfaces 11e, 21e also come into contact with each other in the pin thread width variable portion 112 and the box thread width variable portion 212. However, even if the male thread portion 11 is a fully threaded portion, the male and female threads of the pin thread width variable portion 112 and the box thread width variable portion 212 may have gaps in some places between the load flanks or stabbing flanks of the fully threaded portions due to differences in the outer diameter, thickness, and load flank pitch of the pin 10 and the box 20 in the pin thread width variable portion 112 and the box thread width variable portion 212. The above-mentioned "main portion" can be defined as a region where there are no gaps due to such factors.

[0036] Next, with reference to Figures 1 to 3, the load flank leads of the male thread portion 11 and the female thread portion 21 of the pin 10 and the box 20 will be described. In the male thread portion 11, the load flank lead W111 of the constant pin thread width portion 111 (see Figure 2) and the load flank lead W112 of the variable pin thread width portion 112 (see Figure 3) are the distance in the direction of the pipe axis CL between adjacent load flanks 11d. More specifically, as shown in Figures 2 and 3, they are the distance in the direction of the pipe axis CL between the intersections of the extensions of the load flanks 11d and the extensions of the thread roots 11b on adjacent load flanks 11d. In the female thread portion 12, the load flank lead W211 of the constant box thread width portion 211 (see Figure 2) and the load flank lead W212 of the variable box thread width portion 212 (see Figure 3) are the distance in the direction of the pipe axis CL between adjacent load flanks 21d. 2 and 3, it is the distance in the pipe axis CL direction between the intersections of the extensions of the load flanks 21d and the thread roots 21b on adjacent load flanks 21d. In this embodiment, the load flank leads W211 and W212 are equal. Therefore, the load flank leads W211 and W212 may be collectively referred to as the "load flank leads of the female thread portion 21."

[0037] The load flank lead W112 of the pin thread width variable portion 112 in the male thread portion 11 is smaller than the load flank lead of the female thread portion 21. Furthermore, the load flank lead W111 of the pin thread width constant portion is smaller than the load flank lead W112 of the pin thread width variable portion 112. In other words, a mismatch is provided between the load flank lead of the male thread portion 11 (load flank lead W111 and load flank lead W112) and the load flank lead of the female thread portion 21, and further, in the load flank lead of the male thread portion 11, a mismatch is also provided between the load flank lead W111 of the pin thread width constant portion 111 and the load flank lead W112 of the pin thread width variable portion 112.

[0038] In this way, by making the load surface lead W112 of the pin thread width variable portion 112 smaller than the load surface lead W112 of the female thread portion 21 and by making the load surface lead W111 of the pin thread width constant portion 111 smaller than the load surface lead W112 of the pin thread width variable portion 112 (load surface lead of female thread portion 21 > load surface lead W112 > load surface lead W111), when the pin 10 and the box 20 are fastened together, excessive contact between the load surface 11d of the pin thread width constant portion 111 and the load surface 21d of the box thread width constant portion 211 can be prevented due to the Poisson effect. As a result, not only can seizure between the load surface 11d of the pin thread width variable portion 112 and the load surface 21d of the box thread width variable portion 212 be prevented, but seizure between the load surface 11d of the pin thread width constant portion 111 and the load surface 21d of the box thread width constant portion 211 can also be prevented. Furthermore, the load surface 11d of the pin thread width variable portion 112 and the load surface 21d of the box thread width variable portion 212 are in fitting contact with each other, so sufficient sealing performance can be obtained.

[0039] If the mismatch between the load flank lead W112 of the pin thread width variable portion 112 and the load flank lead of the female thread portion 12 is 1 μm or more, a gap will form between the load flank 11d of the pin thread width variable portion 112 and the load flank 21d of the box thread width variable portion 212, making it less likely that excessive contact pressure will occur. On the other hand, if the mismatch exceeds 10 μm, the gap between the stabbing flank 11c of the pin thread width variable portion 112 and the stabbing flank 21c of the box thread width variable portion 212 will become small, which may result in seizure. Therefore, the mismatch between the load flank lead W112 of the pin thread width variable portion 112 and the load flank lead of the female thread portion 12 can be 1 to 10 μm, and more preferably 1 to 7 μm.

[0040] If the mismatch between the load flank lead W112 of the pin thread width variable portion 112 and the load flank lead W111 of the constant pin thread width portion 111 is 1 μm or more, a gap will form between the load flank 11d of the constant pin thread width portion 111 and the load flank 21d of the constant box thread width portion 211, making it less likely that excessive contact pressure will occur. On the other hand, if the mismatch exceeds 180 μm, the gap between the stabbing flank 11c of the constant pin thread width portion 111 and the stabbing flank 21c of the constant box thread width portion 211 will become small, which may result in seizure. Therefore, the mismatch between the load flank lead W112 of the pin thread width variable portion 112 and the load flank lead of the constant pin thread width portion 111 can be set to 1 to 180 μm, preferably 1 to 150 μm, and more preferably 1 to 100 μm.

[0041] In addition, with regard to the load flank lead of the female thread portion 21, a mismatch may be provided between the load flank lead W211 of the constant box thread width portion 211 and the load flank lead W212 of the variable box thread width portion 212. That is, the load flank lead W212 of the variable box thread width portion 212 may be smaller than the load flank lead W211 of the constant box thread width portion 211. In this case, the load flank lead W212 of the variable box thread width portion 212 is smaller than the load flank lead W211 of the constant box thread width portion 211, the load flank lead W112 of the variable pin thread width portion 112 is smaller than the load flank lead W212 of the variable box thread width portion 212, and the load flank lead W111 of the constant pin thread width portion 111 is smaller than the load flank lead W112 of the variable pin thread width portion 112 (load flank lead W211 > load flank lead W212 > load flank lead W112 > load flank lead W111). This further makes it possible to avoid contact between the load surface 11d of the constant pin thread width portion 111 and the load surface 21d of the constant box thread width portion 211, and more effectively suppresses seizure between the load surface 11d of the variable pin thread width portion 112 and the load surface 21d of the variable box thread width portion 212.

[0042] Note that the position where the variable thread width portion on the pin switches to the constant thread width portion does not completely coincide with the position where the variable thread width portion on the box switches to the constant thread width portion. If the positions on the pin and the box were completely aligned, it would be difficult to screw the pin into the box. This is because, in the constant thread width portion, the male thread width on the outer diameter side of the pin 10 would be larger than the female thread groove width on the inner diameter side of the box 20, and the threads of the pin 10 would not mesh with the thread grooves on the box 20. Therefore, it is preferable to position the position where the variable thread width portion on the pin switches to the constant thread width portion between the tip of the pin 10 and the position where the variable thread width portion on the box switches to the constant thread width portion in the direction of the pipe axis CL. This allows the thread width on the outer diameter side of the pin 10 facing the constant thread width portion of the box 20 to be smaller than the thread groove width on the inner diameter side of the box 20. From this perspective, the distance in the direction of the pipe axis CL between the position where the variable thread width portion in the pin switches to the constant thread width portion and the position where the variable thread width portion in the box switches to the constant thread width portion should be 1.25 to 8 times the load surface lead W212 of the box variable thread width portion 212.

[0043] Next, the above-mentioned gaps g1 and g2 will be described with reference to Fig. 4. Note that the gaps g1 and g2 are formed when the pin 10 and the box 20 are fastened together, but in Fig. 4, in order to make the description of the gaps g1 and g2 easier to understand, the pin 10 and the box 20 are shown separated from each other, and the inclinations of the load flanks and stab flanks, chamfered surfaces, etc. are omitted.

[0044] As shown in FIG. 4, when the pin 10 and the box 20 are fastened together, a gap g1 is formed between the load flank 11d of the pin constant thread width portion 111 of the male thread portion 11 and the load flank 21d of the box constant thread width portion 211 of the female thread portion 21. As shown in the figure, the gap g1 increases in the order of g1-1, g1-2, and g1-3 from the tip end side (right side in the figure) of the pin 10. A gap g2 is formed between the stabbing flank 11c of the pin constant thread width portion 111 and the stabbing flank 21c of the box constant thread width portion 211. The gap g2 decreases in the order of g2-1, g2-2, and g2-3 as viewed from the tip end side (right side in the figure) of the pin 10. In the fastened state, at least one of the gap g1 and the gap g2 is formed. Note that, as described above, there are cases where the gap g1 is not formed between the load flank 11d and the load flank 21d depending on the amount of load flank lead mismatch and external force.

[0045] Here, for gaps g1 and g2, the sum of gaps g1-1 and g2-1, the sum of gaps g1-2 and g2-2, and the sum of gaps g1-3 and g2-3 are the same. That is, for each male thread of constant pin thread width portion 111, the sum of gap g1 formed between load flank 11d of the male thread of constant pin thread width portion 111 and load flank 21d of constant box thread width portion 211 corresponding to load flank 11d of the male thread, and gap g2 formed between stabbing flank 11c of the male thread of constant pin thread width portion 111 and stabbing flank 21c of constant box thread width portion 211 corresponding to stabbing flank 11c of the male thread, are the same.

[0046] The total value of the gap g1 and the gap g2 can be set to 245 to 450 μm, preferably 245 to 400 μm, and more preferably 245 to 380 μm.

[0047] In this way, by forming at least one of the gap g1 and the gap g2 in the pin constant thread width portion 111 and the box constant thread width portion 211, excessive contact between the load surfaces 11d and 21d can be suppressed even if the pin 10 extends axially due to the Poisson effect during make-up, and seizure during make-up can be suppressed. However, if the sum of the gaps g1 and g2 is too large, it becomes difficult for the pin constant thread width portion 111 and the box constant thread width portion 211 to bear the tensile load or compressive load after make-up is complete. Therefore, the sum of the gaps g1 and g2 is preferably 245 μm or more, and 450 μm or less, preferably 400 μm or less, and more preferably 380 μm or less.

[0048] Although the embodiments have been described above, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. [Example]

[0049] In order to confirm the effects of the threaded joint for steel pipes according to the present disclosure, threaded joints for steel pipes, specimens 1 and 2, were prepared and the following test was carried out on seizure in each of the threaded joints for steel pipes. Specimen 1 is a comparative example, and specimen 2 is an example.

[0050] <Common conditions> The common conditions for specimens 1 and 2 are as follows: Steel pipe outer diameter: 16 inches Steel pipe thickness: 12.57 mm Steel type: Carbon steel Grade: 95ksi Length of the pin thread width variable section in the pipe axial direction (L1): 61.872 mm Length of the pin thread width in the pipe axial direction (L2): 55.928 mm Thread length ratio (L1 / L2): 1.1 Thread taper: 8.333%

[0051] [Table 1]

[0052] As shown in Table 1, a mismatch was created between the load surface lead of the female thread portion (box) and the load surface lead of the male thread portion (pin) (load surface lead of the variable pin thread width portion and load surface lead of the constant pin thread width portion), and the presence or absence of seizure was observed for each.

[0053] For specimen 1, the same mismatch was applied to both the variable thread width section and the constant thread width section of the pin relative to the load surface lead of the female thread section (box), and when seizure was observed after tightening, seizure occurred on the load surface of the constant thread width section.

[0054] On the other hand, for specimen 2, the load surface lead of the pin constant thread width portion was made smaller than the load surface lead of the pin variable thread width portion to create a mismatch, and when seizure after make-up was observed, no seizure was confirmed to have occurred in either the constant thread width portion or the variable thread width portion. [Explanation of symbols]

[0055] 1: Steel pipe threaded joints 2: Steel pipe 10: Pin 11: Male thread 111: Pin thread width constant part 112: Pin thread width variable section 11d: Load surface 20: Box 21: Female thread 211: Box thread constant width part 212: Box thread width variable section 21d: Load surface W111: Load surface lead of pin thread constant width part W112: Load surface lead of pin thread width variable section W211: Load surface lead of box thread with constant width W212: Load surface lead of box thread width variable section

Claims

1. A threaded joint for steel pipes for connecting steel pipes, a tubular pin formed at one end of the steel pipe; a tubular box into which the pin is inserted and fastened to the pin; The pin is A male thread portion formed on the outer peripheral surface of the pin and configured as a wedge-shaped thread is included, The box A female thread portion corresponding to the male thread portion is formed on the inner circumferential surface of the box and is configured as a wedge-shaped thread, the male thread portion includes a pin thread width constant portion having a constant thread root width, and a pin thread width variable portion having a thread root width that is equal to or greater than the thread root width of the pin thread width constant portion and that gradually increases from the pin thread width constant portion toward the tip of the pin, a load flank lead of the pin thread width variable portion is smaller than a load flank lead of the female thread portion, A threaded joint for steel pipes, wherein the load flank lead of the constant pin thread width portion is smaller than the load flank lead of the variable pin thread width portion.

2. A threaded joint for steel pipes according to claim 1, The female thread portion includes a box thread width constant portion having a constant thread root width, and a box thread width variable portion having a thread root width that is equal to or smaller than the thread root width of the box thread width constant portion and gradually decreases from the box thread width constant portion toward the innermost end of the box, a load surface lead of the pin thread width variable portion is smaller than a load surface lead of the box thread width variable portion; A threaded joint for steel pipes, wherein the load flank lead of the variable box thread width portion is equal to or less than the load flank lead of the constant box thread width portion.

3. A threaded joint for steel pipes according to claim 1, A threaded joint for steel pipes, wherein, when the pin and box are fastened together, a gap is formed between the load surface of the pin constant thread width portion and the load surface of the box constant thread width portion, and / or between the stabbing flanks of the pin constant thread width portion and the stabbing flanks of the box constant thread width portion.

4. A threaded joint for steel pipes according to claim 1, A threaded joint for steel pipes, wherein the amount of mismatch between the load flank lead of the pin thread width variable portion and the load flank lead of the female thread portion is 1 to 10 μm.

5. A threaded joint for steel pipes according to claim 1, A threaded joint for steel pipes, wherein the amount of mismatch between the load flank lead of the variable pin thread width portion and the load flank lead of the constant pin thread width portion is 1 to 180 μm.

6. A threaded joint for steel pipes according to claim 3, A threaded joint for steel pipes, wherein the combined value of the gap formed between the load flank of the male thread of the pin constant thread width portion and the load flank of the box constant thread width portion corresponding to the load flank of the male thread, and the gap formed between the stabbing flank of the male thread and the stabbing flank of the box constant thread width portion corresponding to the stabbing flank of the male thread, is 245 to 450 μm.

7. A threaded joint for steel pipes according to any one of claims 1 to 6, A threaded joint for steel pipes, wherein the steel pipe has an outer diameter of more than 16 inches.

8. A steel pipe provided with a steel pipe threaded joint according to any one of claims 1 to 6.

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