Screw joint for steel pipe

Abstract

To provide a screw joint for a steel pipe capable of improving seizure resistance and manufacturability.SOLUTION: A screw joint 1 for a steel pipe has a pin 10 and a box 20. The pin 10 has a male screw 11, a pin inner seal surface 12, and a pin outer seal surface 13. The box 20 has a female screw 12, a box inner seal surface 12, and a box outer seal surface 23. The male screw 11 includes a tapered region 111 located on the pin outer seal surface 13 side, and a tapered region 112 located on the pin inner seal surface 12 side. A gradient β1 of the tapered region 112 is smaller than a gradient α1 of the tapered region 111. An insertion surface pitch of threads included in the male screw 11 is constant. The threads included in the tapered region 112 have a thread width greater than a thread width of the threads included in the tapered region 111.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a threaded joint used for connecting steel pipes.

Background Art

[0002] In oil wells, natural gas wells, etc. (hereinafter, also collectively referred to as "oil wells"), oil well pipes such as casings and tubing are used to extract underground resources. The oil well pipes are formed by sequentially connecting steel pipes, and threaded joints are used for the connection.

[0003] The types of threaded joints for this type of steel pipe are roughly classified into a coupling type and an integral type. In the case of the coupling type, among a pair of pipe materials to be connected, one pipe material is a steel pipe and the other pipe material is a coupling. In this case, male threads are provided on the outer circumferences of both ends of the steel pipe, and female threads are provided on the inner circumferences of both ends of the coupling. Then, the male threads of the steel pipe are screwed into the female threads of the coupling, and thereby the two are fastened and connected. In the case of the integral type, a pair of pipe materials to be connected are both steel pipes, and no separate coupling is used. In this case, male threads are provided on the outer circumference of one end of the steel pipe, and female threads are provided on the inner circumference of the other end. Then, the male thread portion of one steel pipe is screwed into the female thread portion of the other steel pipe, and thereby the two are fastened and connected.

[0004] Generally, the joint portion of the pipe end where the male thread portion is formed includes an element to be inserted into the female thread, and thus is called a pin. On the other hand, the joint portion of the pipe end where the female thread is formed includes an element to receive the male thread, and thus is called a box. Since these pins and boxes are the ends of the pipe materials, they are both tubular.

[0005] In recent years, the depth of oil wells has been increasing. In deep wells, generally, oil well pipes having high pressure resistance are used. Threaded joints for connecting oil well pipes are required not only to have high strength and sealing performance, but also to be subject to strict outer diameter dimension constraints for arranging oil well pipes in multiple layers.

[0006] The threaded joint for oil well pipes exhibits high sealing performance at the seal part. Generally, the diameter of the seal part of the pin is larger than that of the seal part of the box. Therefore, in the tightened state, the two seal parts fit and adhere to each other to form an interference fit state, forming a seal part by metal contact. The difference between the diameter of the seal part of the pin and the diameter of the seal part of the box is called the "seal interference amount". The larger the seal interference amount, the higher the seal contact force, and better sealing performance can be obtained.

[0007] In order to improve the sealing performance against external pressure, it is effective to increase the wall thickness of the seal part of the pin where the external pressure acts. This increases the shrinkage resistance of the seal part of the pin when the threaded joint is loaded with external pressure, so the substantial decrease in the seal interference amount is reduced, and the decrease in the seal contact force is reduced. In addition, in order to exhibit stable sealing performance even under high tensile / compressive loads, a certain length of the threaded part and the area of the shoulder surface are also required.

[0008] On the other hand, in order to reduce the outer diameter dimension of the joint part, at least one of the pin and the box can be thinned. As a threaded joint with a thinned pin and / or box, for example, one called a slim type can be mentioned. The slim type threaded joint has an outer diameter similar to that of the oil well pipe body. The slim type threaded joint has difficulty ensuring the wall thickness of the seal part and the area of the shoulder surface due to thinning.

[0009] International Publication No. 2019 / 224345 (Patent Document 1) discloses a tubular screw connection. The tubular screw connection includes a tubular male end (pin) and a tubular female end (box). The tubular male end includes an outer male seal surface, an inner male seal surface, an outer male thread, an inner male thread, and a male shoulder located between the outer male thread and the inner male thread. The tubular female end includes an outer female seal surface corresponding to the outer male seal surface, an inner female seal surface corresponding to the inner male seal surface, an outer female thread meshing with the outer male thread, an inner female thread meshing with the inner male thread, and a female shoulder contacting the male shoulder. Further, Patent Document 1 discloses that the outer female thread and the inner female thread meshing with the outer male thread and the inner male thread respectively are tapered along the same taper angle, and the pitches and leads of the load surface and the insertion surface are exactly the same.

[0010] Japanese Patent No. 6916277 (Patent Document 2) discloses a screw joint for steel pipes. The screw joint for steel pipes consists of a tubular pin and a tubular box. The pin includes, in order from the tip side of the pin toward the pipe body side, an inner seal surface, an inner male thread portion, a shoulder portion, an outer male thread portion, and an outer seal surface. The box includes, in order from the pipe body side of the box toward the tip side, an inner seal surface, an inner female thread portion, a shoulder portion, an outer female thread portion, and an outer seal surface. In the screw joint for steel pipes, some teeth of the inner male thread portion of the pin are accommodated in an inner groove portion provided between the inner seal surface and the inner female thread portion of the box, or some teeth of the outer female thread portion are accommodated in an outer groove portion provided between the outer seal surface and the outer male thread portion of the pin. Further, Patent Document 2 discloses that the inner seal surface of the pin, the inner seal surface of the box, the outer seal surface of the pin, and the outer seal surface of the box are all tapered.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] An object of the present disclosure is to provide a threaded joint for steel pipes that can improve seizure resistance performance and manufacturing cost.

Means for Solving the Problems

[0013] The threaded joint for steel pipes according to the present disclosure may include a tubular pin and a tubular box into which the pin is screwed and fastened to the pin. The pin may include a pin inner seal surface formed at the tip of the pin, a male thread formed on the outer peripheral surface of the pin, which is located between the pin inner seal surface and the steel pipe body, and a pin outer seal surface formed between the steel pipe body and the male thread. The box may include a box inner seal surface facing the pin inner seal surface and contacting the pin inner seal surface when the pin and the box are fastened, a female thread corresponding to the male thread and formed on the inner peripheral surface of the box, and a box outer seal surface facing the pin outer seal surface and contacting the pin outer seal surface when the pin and the box are fastened. The male thread may include a first taper region located between the pin inner seal surface and the pin outer seal surface, and a second taper region located between the pin inner seal surface and the first taper region and having a gradient smaller than that of the first taper region. The female thread may include a third taper region located between the box inner seal surface and the box outer seal surface, and a fourth taper region located between the box outer seal surface and the third taper region and having a gradient smaller than that of the third taper region. The thread of the second taper region may have a wider width than the thread of the first taper region. The thread of the fourth taper region may have a wider width than the thread width of the third taper region. The insertion surface pitch of the male thread and the female thread may be constant.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0015] The above-described tubular screw connection portion is provided with an outer male seal surface, an inner male seal surface, an outer female seal surface corresponding to the outer male seal surface, and an inner female seal surface corresponding to the inner male seal surface. Similarly, the above-described steel pipe screw joint is provided with an inner seal surface and an outer seal surface of the pin, and an inner seal surface and an outer seal surface of the box, so as to improve the sealing performance against external pressure and internal pressure. As described above, in order to obtain excellent sealing performance under strict outer diameter constraints, the slim-type steel pipe screw joint is required to ensure the wall thickness of the seal portion. There is still room for further consideration regarding ensuring the wall thickness of the seal portion. The inventors considered making the screw bottom surface at the tip of the male screw located on the tip side of the pin and the screw bottom surface at the tip of the female screw located on the tip side of the box parallel to the pipe axis of the steel pipe body in a screw joint having a tapered screw portion, or making the taper gradient of the tip of the male screw and the tip of the female screw smaller than the taper gradient of the male screw and the female screw excluding these tips. Thereby, the wall thickness of the pin inner seal surface and the box outer seal surface can be increased to improve rigidity and sealing performance.

[0016] Here, the joint efficiency of the slim screw joint is less than 1. The joint efficiency is the ratio of the tensile strength of the joint part to the tensile strength of the steel pipe body, and is defined as the area of the cross-section of the joint part (generally referred to as the "critical section") where the area that withstands the tensile load is the smallest / the area of the cross-section of the oil well pipe body. In order to obtain an appropriate joint efficiency, it is important to ensure that the male screw and the female screw mesh properly and that both the pin and the box bear the tensile load when the tensile load is applied to the screw joint. In order to ensure that the male screw and the female screw mesh properly, it is preferable to make the load surface pitch and the insertion surface pitch the same.

[0017] However, as described above, if the screw bottom surface of the tip of the male screw and the screw bottom surface of the tip of the female screw are parallel, or if the taper gradient of the tip of the male screw and the taper gradient of the tip of the female screw are made smaller, the load surface pitch and the insertion surface pitch cannot be made the same in a single cutting process. That is, it is necessary to cut the load surface and the insertion surface separately, resulting in a decrease in manufacturability. Also, from the viewpoint of ensuring the tensile strength after fastening, only the load surface pitch can be made the same in a single cutting process, but the insertion surface pitch will not be the same.

[0018] More specifically, with reference to FIG. 1, a method of cutting a male thread 11 using a single chaser 100 will be described. From the above-described viewpoint, cutting is performed such that the load surface pitch is constant, that is, the load surface pitch LPα and the load surface pitch LPβ are the same. When cutting with a taper gradient β smaller than the taper gradient α with respect to the pipe axis CL, as shown in the figure, the cutting amount by the chaser 100 (the moving amount of the chaser 100 downward in the figure) becomes smaller. Further, the chaser 100 for cutting a trapezoidal thread is tapered toward the cutting direction (downward in the figure) in a longitudinal cross-sectional view. Therefore, when cutting so that the load surface pitches LPα and LPβ are constant, the insertion surface pitch changes between the taper region with the gradient α and the taper region with the gradient β. That is, compared with the case where the gradients α and β are constant, in the taper region with the gradient β, the insertion surface SPβ moves by the difference Δ in the figure so that the thread width becomes wider. When the thread width in the taper region with the gradient β becomes wider in this way, the insertion surfaces come into excessive contact with each other during the fastening process of the pin and the box, and seizure is likely to occur.

[0019] As a result of intensive studies, the present inventors newly found that when the gradients α and β are different, if the insertion surface pitch is made constant, the seizure resistance performance and the manufacturability can be improved while ensuring the sealing performance. Based on this finding, the present inventors invented the following steel pipe screw joint.

[0020] The threaded joint for steel pipes according to this embodiment may include a tubular pin and a tubular box into which the pin is screwed and fastened to the pin. The pin may include a pin inner seal surface formed at the tip of the pin, a male thread formed on the outer peripheral surface of the pin located between the pin inner seal surface and the steel pipe body, and a pin outer seal surface formed between the steel pipe body and the male thread. The box may include a box inner seal surface facing the pin inner seal surface and contacting the pin inner seal surface when the pin and the box are fastened, a female thread formed on the inner peripheral surface of the box corresponding to the male thread, and a box outer seal surface facing the pin outer seal surface and contacting the pin outer seal surface when the pin and the box are fastened. The male thread may include a first taper region located between the pin inner seal surface and the pin outer seal surface, and a second taper region located between the pin inner seal surface and the first taper region and having a gradient smaller than that of the first taper region. The female thread may include a third taper region located between the box inner seal surface and the box outer seal surface, and a fourth taper region located between the box outer seal surface and the third taper region and having a gradient smaller than that of the third taper region. The thread crest included in the second taper region may have a width wider than the thread crest of the thread included in the first taper region. The thread crest included in the fourth taper region may have a width wider than the thread width of the thread included in the third taper region. The insertion surface pitch of the male thread and the female thread may be constant.

[0021] The thread crest included in the second taper region has a width wider than the thread crest width of the thread included in the first taper region, the thread crest included in the fourth taper region has a width wider than the thread width of the thread included in the third taper region, and the insertion surface pitch of the male thread and the female thread is constant. That is, the load surface pitch of the male thread and the female thread is changing. The load surface pitch of the thread crest included in the second and fourth taper regions is smaller than the load surface pitch of the thread crest included in the first and third regions. In the present disclosure, the gradients of the second and fourth taper regions include the case where they are parallel to the pipe axis CL (or a straight line parallel thereto).

[0022] Thereby, it is possible to improve seizure resistance performance and manufacturability in the fastening process of the pin and the box.

[0023] The steel pipe screw joint may include a tubular pin and a tubular box into which the pin is screwed and fastened to the pin. The pin may include a pin inner seal surface formed at the tip of the pin, an internal male thread formed on the outer peripheral surface of the pin and located between the pin inner seal surface and the steel pipe body, an external male thread formed on the outer peripheral surface of the pin and located between the steel pipe body and the internal male thread, and a pin intermediate seal surface formed between the internal male thread and the external male thread. The box may include a box inner seal surface facing the pin inner seal surface and contacting the pin inner seal surface when the pin and the box are fastened, an internal female thread corresponding to the internal male thread and formed on the inner peripheral surface of the box, an external female thread corresponding to the external male thread and formed on the inner peripheral surface of the box, and a box intermediate seal surface facing the pin intermediate seal surface and contacting the pin intermediate seal surface when the pin and the box are fastened. The internal male thread may include a fifth taper region located between the pin inner seal surface and the pin intermediate seal surface, and a sixth taper region located between the pin inner seal surface and the fifth taper region and having a gradient smaller than that of the fifth taper region. The external male thread may include a seventh taper region located between the pin intermediate seal surface and the steel pipe body, and an eighth taper region located between the pin intermediate seal surface and the seventh taper region and having a gradient smaller than that of the seventh taper region. The internal female thread may include a ninth taper region located between the box inner seal surface and the box intermediate seal surface, and a tenth taper region located between the box intermediate seal surface and the ninth taper region and having a gradient smaller than that of the ninth taper region. The external female thread may include an eleventh taper region located between the box intermediate seal surface and the tip of the box, and a twelfth taper region located between the tip of the box and the eleventh taper region and having a gradient smaller than that of the eleventh taper region. The thread pitches of the threads included in the sixth and eighth taper regions may each have a wider width than the threads included in the fifth and seventh taper regions. The thread pitches of the threads included in the tenth and twelfth taper regions may have a wider width than the threads included in the ninth and eleventh taper regions. The insertion surface pitches of the internal male thread and the internal female thread may be constant. The insertion surface pitches of the external male thread and the external female thread may be constant. Thereby, the seizure resistance performance and manufacturability in the fastening process of the pin and the box can be improved.

[0024] The steel pipe screw joint may include a tubular pin and a tubular box into which the pin is screwed and fastened to the pin. The pin may include a pin inner seal surface formed at the tip of the pin, a pin outer seal surface formed between the steel pipe body and the pin inner seal surface, an internal male thread formed on the outer peripheral surface of the pin and located between the pin inner seal surface and the pin outer seal surface, an external male thread formed on the outer peripheral surface of the pin and located between the pin outer seal surface and the internal male thread, and a pin intermediate shoulder surface formed on the outer peripheral surface of the pin and located between the internal male thread and the external male thread. The box may include a box inner seal surface facing the pin inner seal surface and contacting the pin inner seal surface when the pin and the box are fastened, a box outer seal surface facing the pin outer seal surface and contacting the pin outer seal surface when the pin and the box are fastened, an internal female thread formed on the inner peripheral surface of the box corresponding to the internal male thread, an external female thread formed on the inner peripheral surface of the box corresponding to the external male thread, and a box intermediate shoulder surface facing the pin intermediate shoulder surface and contacting the pin intermediate shoulder surface when the pin and the box are fastened. The internal male thread may include a fifth taper region located between the pin inner seal surface and the pin intermediate shoulder surface, and a sixth taper region located between the pin inner seal surface and the fifth taper region and having a gradient smaller than that of the fifth taper region. The external male thread may include a seventh taper region located between the pin intermediate shoulder surface and the pin outer seal surface, and an eighth taper region located between the pin intermediate shoulder surface and the seventh taper region and having a gradient smaller than that of the seventh taper region. The internal female thread may include a ninth taper region located between the box inner seal surface and the box intermediate shoulder surface, and a tenth taper region located between the box intermediate shoulder surface and the ninth taper region and having a gradient smaller than that of the ninth taper region. The external female thread may include an eleventh taper region located between the box intermediate shoulder surface and the box outer seal surface, and a twelfth taper region located between the box outer seal surface and the eleventh taper region and having a gradient smaller than that of the eleventh taper region. The threads included in the sixth and eighth taper regions may each have a wider width than the threads included in the fifth and seventh taper regions. The threads included in the tenth and twelfth taper regions may have a wider width than the threads included in the ninth and eleventh taper regions.The insertion surface pitch of the internal male thread and the internal female thread is constant, and the insertion surface pitch of the external male thread and the external female thread is constant. Thereby, the seizure resistance performance and manufacturability in the fastening process of the pin and the box can be improved.

[0025] The pin may further include an annular pin groove portion that faces the 12th tapered region of the external female thread and is spaced apart from the 12th tapered region when the pin and the box are fastened. The box may further include an annular box groove portion that faces the 6th tapered region of the internal male thread and is spaced apart from the 6th tapered region when the pin and the box are fastened. Thereby, excessive interference between the load surfaces of the threads included in the 6th tapered region and the 12th tapered region can be suppressed, and more excellent sealing performance can be ensured.

[0026] The male thread and the female thread may include a complete thread portion having a thread top surface and a thread bottom surface. The thread top surface and the thread bottom surface may be parallel to the tube axis of the steel pipe body.

[0027] The screw joint for steel pipes may be an integral type screw joint.

[0028] The screw joint for steel pipes may have an outer diameter of 105% or less with respect to the steel pipe body.

[0029] Either a solid lubricating film or a semi-solid lubricating film may be formed on at least one of the outer peripheral surface of the pin and the inner peripheral surface of the box.

[0030] Hereinafter, with reference to the drawings, the present embodiment will be described in detail. The same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated.

[0031] [Configuration of Screw Joint for Steel Pipes] [First Embodiment] Referring to FIG. 2, the steel pipe screw joint 1 according to the first embodiment is a slim - type screw joint, and includes a tubular pin 10 and a tubular box 20 into which the pin 10 is screwed and fastened to the pin 10. The steel pipe screw joint 1 according to the present disclosure can be more preferably used in the slim - type steel pipe screw joint 1. The slim - type steel pipe screw joint 1 has an outer diameter of 105% or less with respect to the steel pipe body 2, for example.

[0032] The pin 10 is provided at the pipe end of one steel pipe body 2. The pin 10 includes a tapered male thread 11, an inner - pin sealing surface 12, an outer - pin sealing surface 13, and an inner - pin shoulder surface 14.

[0033] The male thread 11 is formed on the outer peripheral surface of the pin 10. The inner - pin shoulder surface 14 is formed at the tip of the pin 10. The inner - pin shoulder surface 14 is an annular surface substantially perpendicular to the pipe axis CL. The inner - pin sealing surface 12 is formed on the outer peripheral surface of the pin 10 between the male thread 11 and the inner - pin shoulder surface 14. The outer - pin sealing surface 13 is formed on the outer peripheral surface of the pin 10 between the male thread 11 and one steel pipe body 2.

[0034] The male thread 11 includes a taper region 111 and a taper region 112. The taper region 112 is formed on the tip side of the pin 10. The taper region 111 is formed between the region 112 and the outer - pin sealing surface 13. The taper region 111 has a taper gradient α1. The taper region 112 has a gradient β1 smaller than the gradient α1 of the taper region 111. The gradient β1 of the taper region 112 is smaller than the taper gradient α1. Thereby, compared with the case where the taper of the male thread 11 is formed with a constant gradient α1, the wall thickness at the inner - pin sealing surface 12 can be more ensured, which can contribute to the internal pressure sealing performance. In the present disclosure, the gradient β1 includes the case where it is parallel to the pipe axis CL (or a straight line parallel thereto).

[0035] When the thread bottom surface of the male thread 11 is parallel to the thread taper, the gradients α1 and β1 are the gradients between the straight line connecting the thread bottom surface of the male thread 11 and the pipe axis CL (or a straight line parallel thereto) in the longitudinal section of the pin 10 including the pipe axis CL. On the other hand, when the thread bottom surface of the male thread 11 is parallel to the pipe axis CL, the gradients α1 and β1 of the taper are the gradients between the straight line connecting the intersection point of the extension line of the bottom surface of the male thread 11 and the extension line of the load surface and the pipe axis CL (or a straight line parallel thereto) in the longitudinal section of the pin 10 including the pipe axis CL.

[0036] As shown in FIG. 3, the male thread 11 is a trapezoidal thread. The male thread 11 has a thread bottom surface 1111, a thread top surface 1112, an insertion surface 1113, and a load surface 1114. The insertion surface 1113 has a positive flank angle. The load surface 1114 has a negative flank angle. The absolute value of the flank angle of the insertion surface 1113 is larger than the absolute value of the flank angle of the load surface 1114. The flank angle refers to the angle formed by a straight line perpendicular to the pipe axis CL and the insertion surface or the load surface in the longitudinal section of the steel pipe threaded joint 1. For the flank angle of the insertion surface 1113, the counterclockwise direction in the figure is defined as the positive direction. Therefore, the insertion surface 1113 is inclined such that the outer peripheral portion is located rearward in the pipe axis direction than the inner peripheral portion. For the flank angle of the load surface 1114, the counterclockwise direction in the figure is defined as the negative direction. Therefore, the load surface 1114 is inclined such that the outer peripheral portion is located rearward in the pipe axis direction than the inner peripheral portion. In the full thread portion of the male thread 11, the thread bottom surface 1111 and the thread top surface 1112 are parallel to the thread taper in a longitudinal section view. However, in the full thread portion of the male thread 11, the thread bottom surface 1111 and the thread top surface 1112 may be parallel to the pipe axis CL in a longitudinal section view.

[0037] The box 20 is provided at the pipe end portion of the other steel pipe body 2. The box 20 includes a tapered female thread 21, an inner box seal surface 22, an outer box seal surface 23, and an inner box shoulder surface 24.

[0038] The female thread 21 corresponds to the male thread 11 and is formed on the inner peripheral surface of the box 20. The inner shoulder surface 24 of the box corresponds to the inner shoulder surface 14 of the pin and is formed at the rear end side of the box 20. The inner shoulder surface 24 of the box is an annular surface substantially perpendicular to the pipe axis CL. The inner shoulder surface 24 of the box contacts the inner shoulder surface 14 of the pin when the pin 10 and the box 20 are fastened. The inner seal surface 22 of the box corresponds to the inner seal surface 12 of the pin and is formed on the inner peripheral surface of the box 20. The inner seal surface 22 of the box contacts the inner seal surface 12 of the pin when the pin 10 and the box 20 are fastened. The outer seal surface 23 of the box corresponds to the outer seal surface 13 of the pin and is formed on the inner peripheral surface of the box. The outer seal surface 23 of the box contacts the outer seal surface 13 of the pin when the pin 10 and the box 20 are fastened.

[0039] The female thread 21 includes a taper region 211 and a taper region 212. The taper region 212 is formed at the front end side of the box 20. The taper region 211 is formed between the taper region 212 and the inner seal surface 22 of the box. The taper region 211 has a taper gradient α2. The taper region 212 has a gradient β2 smaller than the gradient α2. The gradient β2 is smaller than the gradient α2. Thereby, compared with the case where the taper of the female thread 21 is formed with a constant gradient α1, the wall thickness at the outer seal surface 23 of the box can be sufficiently ensured, and it can contribute to the external pressure sealing performance. In the present disclosure, the taper gradient β2 includes the case where it is parallel to the pipe axis CL (or a straight line parallel thereto).

[0040] When the bottom surface of the female thread 21 is parallel to the thread taper, the taper gradients α2 and β2 are the gradients between the straight line connecting the bottom surface of the female thread 21 and the pipe axis CL (or a straight line parallel thereto) in the longitudinal section of the pin 10 including the pipe axis CL. On the other hand, when the bottom surface of the female thread 21 is parallel to the pipe axis CL, the taper gradients α2 and β2 are the gradients between the straight line connecting the intersection of the extension line of the bottom surface of the female thread 21 and the extension line of the load surface and the pipe axis CL (or a straight line parallel thereto) in the longitudinal section of the pin 10 including the pipe axis CL.

[0041] As shown in Fig. 3, the female screw 21 is a trapezoidal screw. The female screw 21 has a screw bottom surface 2111, a screw top surface 2112, an insertion surface 2113, and a load surface 2114. The insertion surface 2113 has a positive flank angle. The load surface 2114 has a negative flank angle. The absolute value of the flank angle of the insertion surface 2113 is larger than the absolute value of the flank angle of the load surface 2114. For the flank angle of the insertion surface 2113, the counterclockwise direction in the figure is defined as the positive direction. Therefore, the insertion surface 2113 is inclined such that the outer peripheral portion is located forward in the tube axis direction than the inner peripheral portion. For the flank angle of the load surface 2114, the counterclockwise direction in the figure is defined as the negative direction. Therefore, the load surface 2114 is inclined such that the outer peripheral portion is located forward in the tube axis direction than the inner peripheral portion. In the complete thread portion of the female screw 21, the screw bottom surface 2111 and the screw top surface 2112 are parallel to the screw taper in a longitudinal cross-sectional view. However, in the complete thread portion of the female screw 21, the screw bottom surface 2111 and the screw top surface 2112 may be parallel to the tube axis CL in a longitudinal cross-sectional view.

[0042] When the fastening of the pin 10 and the box 20 is completed, in the complete thread portions of the male screw 11 and the female screw 21, the screw bottom surface 1111 of the male screw 11 contacts the screw top surface 2112 of the female screw 21, and the load surface 1114 of the male screw 11 contacts the load surface 2114 of the female screw 21. Also, the insertion surface 1113 of the male screw 11 and the insertion surface 2113 of the female screw 21 face each other with a gap therebetween, and the screw top surface 1112 of the male screw 11 and the bottom surface 2111 of the female screw 21 face each other with a gap therebetween.

[0043] The insertion surface pitch of the male thread 11 and the insertion surface pitch of the female thread 21 are constant. As shown in FIG. 4, by cutting with a single chaser 100 so that the insertion surface pitch SPα and the insertion surface pitch SPβ are constant, the insertion surface pitch becomes constant. As a result, in the process of fastening the pin 10 and the box 20, the load is dispersed to each thread crest without concentrating on the taper region 112 at the tip of the male thread 11. As a result, the seizure resistance performance when fastening the pin 10 and the box 20 can be improved. In addition, the thread crest can be formed by one cutting process, and the productivity can be improved. Further, as shown in FIG. 4, the load surface of the male thread 11 moves by the amount of the difference Δ in the figure so that the thread crest width of the male thread 11 becomes wider. That is, the load surface pitch LPα and the load surface pitch LPβ change between the gradients α1 and β1. In this way, since the gradient β1 is smaller than the gradient α1, the thread crest width of the thread crest included in the taper region 112 becomes wider than the thread crest width of the taper region 111. That is, since the insertion surface pitch is constant, the thickness increases on the load surface 1114 of the male thread 11. As a result, an interference amount in the pipe axis direction is introduced between the load surface 1114 of the male thread 11 and the load surface 2114 of the female thread 21 in the taper region 112 and the taper region 212. This interference amount becomes larger toward the tip side of the pin in the taper region 112 and larger toward the tip side of the box (the pipe body side of the pin) in the taper region 212. As a result, at the end of fastening, compared with the region where the taper region 111 and the taper region 211 face each other, that is, the region where the load surface pitches of the male thread 11 and the female thread 21 are constant, in the taper region 112 and the taper region 212, the load surfaces of each other come into strong contact. Further, this contact becomes stronger toward the tip side of the pin in the taper region 112 and toward the tip side of the box (the pipe body side of the pin) in the taper region 212. By the way, in the steel pipe screw joint 1 shown in FIG. 2, the dangerous cross-section of the pin 10 is the screwed end on the side of one steel pipe body 2, and the dangerous cross-section of the box 20 is the screwed end on the tip side of the pin 10.In these critical cross - sectional areas, if sufficient engagement cannot be obtained between the male thread 11 and the female thread 21, the applied tensile load may not be reliably transmitted to the pin 10 and the box 20, and an excessive load may be applied to either the pin 10 or the box 20, leading to a decrease in tensile strength. As described above, in the tapered regions 112 and 212, the load surfaces of each other come into strong contact. That is, the contact between the load surfaces of the male thread 11 and the female thread 12 in these critical cross - sectional areas becomes stronger. From this, it is possible to surely engage the load surfaces with each other in these critical cross - sectional areas, and reliable transmission of the tensile load to the pin 10 and the box 20 becomes possible. That is, it is possible to improve the tensile strength.

[0044] [Second Embodiment] Next, with reference to FIG. 5, the steel pipe screw joint 1 according to the second embodiment will be described. The description of the same configuration as that of the first embodiment will be omitted, and the configuration different from the first embodiment will be described in detail.

[0045] The steel pipe screw joint 1 according to the second embodiment is a so - called two - stage screw. The steel pipe screw joint 1 has a pin 10 and a box 20. The pin 10 has an internal male thread 11a, an external male thread 11b, and a pin intermediate seal surface 15. The pin intermediate seal surface 15 is formed on the outer peripheral surface of the pin 10 between the internal male thread 11a and the external male thread 11b. The internal male thread 11a is formed on the outer peripheral surface of the pin 10 between the pin internal seal surface 12 and the pin intermediate seal surface 15. The external male thread 11b is formed on the outer peripheral surface of the pin 10 between the pin intermediate seal surface 15 and the steel pipe body 2. In the steel pipe screw joint 1 according to the second embodiment, although the pin outer seal surface 13 is not formed, the pin outer seal surface 13 may be formed on the outer peripheral surface of the pin 10 between the external male thread 11b and the steel pipe body 2.

[0046] The box 20 has an internal female thread 21a, an external female thread 21b, and a box intermediate seal surface 25. The internal female thread 21a corresponds to the internal male thread 11a and is formed on the inner peripheral surface of the box 20. The external female thread 21b corresponds to the external male thread 11b and is formed on the inner peripheral surface of the box 20. The box intermediate seal surface 25 corresponds to the pin intermediate seal surface 15 and is formed on the inner peripheral surface of the box 20. When the pin 10 and the box 20 are fastened, the box intermediate seal surface 25 contacts the pin intermediate seal surface 15. In addition, when the above-mentioned pin outer seal surface 13 is formed, a box outer seal surface 23 corresponding to the pin outer seal surface 13 may be formed.

[0047] The internal male thread 11a includes a tapered region 111a and a tapered region 112a. The tapered region 112a is formed on the tip side of the pin 10. The tapered region 111a is formed between the tapered region 112a and the pin intermediate seal surface 15. The tapered region 111a has a taper gradient α1. The tapered region 112a has a gradient β1 that is smaller than the gradient α1. The gradient β1 is smaller than the taper gradient α1. Thereby, compared with the case where the taper of the male thread 11a is formed with a constant gradient α1, the wall thickness at the pin inner seal surface 12 can be more ensured, and it can contribute to the internal pressure sealing performance. In the present disclosure, the gradient β1 includes the case where it is parallel to the tube axis CL (or a straight line parallel thereto).

[0048] The internal female thread 21a includes a tapered region 211a and a tapered region 212a. The tapered region 212a is formed on the tip side of the box 20. The tapered region 211a is formed between the tapered region 212a and the box inner seal surface 22. The tapered region 211a has a taper gradient α2. The tapered region 212a has a gradient β2 that is smaller than the gradient α2. The gradient β2 is smaller than the gradient α2. Thereby, compared with the case where the taper of the female thread 21a is formed with a constant gradient α2, the wall thickness at the box intermediate seal surface 25 can be sufficiently ensured, and it can contribute to the external pressure sealing performance. In the present disclosure, the gradient β2 includes the case where it is parallel to the tube axis CL (or a straight line parallel thereto).

[0049] The external male thread 11b includes a taper region 111b and a taper region 112b. The taper region 112b is formed on the side of the pin intermediate seal surface 15. The taper region 111b is formed between the taper region 112b and the steel pipe body 2. The taper region 111b has a taper gradient α3. The taper region 112b has a gradient β3 smaller than the gradient α3. The gradient β3 is smaller than the gradient α3. Thereby, compared with the case where the taper of the male thread 11b is formed with a constant gradient α1, the wall thickness at the pin inner seal surface 12 and the pin intermediate seal surface 15 can be more ensured, and it can contribute to the internal pressure sealing performance. In the present disclosure, the gradient β3 includes the case where it is parallel to the pipe axis CL (or a straight line parallel thereto).

[0050] The external female thread 21b includes a taper region 211b and a taper region 212b. The taper region 212b is formed on the tip side of the box 20. The taper region 211b is formed between the taper region 212b and the box intermediate seal surface 25. The taper region 211b has a taper gradient α4. The taper region 212b has a gradient β4 smaller than the gradient α4. Thereby, compared with the case where the taper of the female thread 21b is formed with a constant gradient α4, when the box outer seal surface 23 is formed, the wall thickness at the box outer seal surface 23 can be sufficiently ensured, and it can contribute to the external pressure sealing performance. In the present disclosure, the taper gradient β4 includes the case where it is parallel to the pipe axis CL (or a straight line parallel thereto).

[0051] Similar to the male thread 11 and the female thread 21 according to the first embodiment, the insertion surface pitch of the internal male thread 11a and the internal female thread 21a is constant, and the insertion surface pitch of the external male thread 11b and the external female thread 21b is also constant. Thereby, also in the steel pipe screw joint 1 according to the second embodiment, seizure can be suppressed in the fastening process of the pin 10 and the box 20. Further, in the taper region 112a of the internal male thread 11a and the taper region 112b of the male thread 11b, the wall thickness at the load surface of the thread increases. Thereby, the tensile strength can be improved.

[0052] [Third Embodiment] Next, with reference to FIG. 6, the steel pipe screw joint 1 of the third embodiment will be described. The description of the same configuration as that of the second embodiment will be omitted, and the configuration different from that of the second embodiment will be described in detail.

[0053] The steel pipe screw joint 1 according to the third embodiment includes a pin 10 and a box 20. The pin 10 has a pin intermediate shoulder surface 16, a pin outer seal surface 13, and a pin groove portion 17. The pin intermediate shoulder surface 16 is formed on the outer peripheral surface of the pin 10 between the internal male thread 11a and the external male thread 11b. The pin intermediate shoulder surface 16 is an annular surface substantially perpendicular to the pipe axis CL. The pin outer seal surface 13 is formed on the outer peripheral surface of the pin 10 between the external male thread 11b and one steel pipe body 2.

[0054] The pin groove portion 17 is an annular groove portion formed on the outer peripheral surface of the pin 10 and facing the taper region 212b of the female thread 21b. The pin groove portion 17 is separated from the taper region 212b of the female thread 21b when the pin 10 and the box 20 are fastened and does not contact the taper region 212b. That is, a gap is provided between the pin groove portion 17 and the taper region 212b of the female thread 21b. As described above, since the gradient β4 of the taper region 212b is smaller than the taper gradient α4 of the taper region 211b, the thread width of the thread included in the taper region 212b is wider than the thread width of the thread included in the taper region 211b. If the load surfaces of the threads included in the taper region 212b and the load surfaces of the threads included in the taper region 111b interfere excessively, it may affect the contact between the pin outer seal surface 13 and the box outer seal surface 23. Therefore, by providing the pin groove portion 17, excessive interference between the load surfaces can be suppressed, and the external pressure sealing performance can be further improved.

[0055] The box 20 has a box intermediate shoulder surface 26, a box outer seal surface 23, and a box groove portion 27. The box intermediate shoulder surface 26 corresponds to the pin intermediate shoulder surface 16 and is formed on the inner peripheral surface of the box 20. The box intermediate shoulder surface 26 is an annular surface substantially perpendicular to the tube axis CL. The box intermediate shoulder surface 26 contacts the pin intermediate shoulder surface 16 when the pin 10 and the box 20 are fastened. The box outer seal surface 23 corresponds to the pin outer seal surface 13 and is formed on the inner peripheral surface of the box 20. The box outer seal surface 23 contacts the pin outer seal surface 13 when the pin 10 and the box 20 are fastened.

[0056] The box groove portion 27 is an annular groove portion formed on the outer peripheral surface of the box 20 and facing the taper region 112a of the internal male thread 11a. The box groove portion 27 is separated from the taper region 112a of the internal male thread 11a and does not contact the taper region 112a when the pin 10 and the box 20 are fastened. That is, a gap is provided between the box groove portion 27 and the taper region 112a of the male thread 11a. As described above, since the gradient β1 of the taper region 112a is smaller than the gradient α1 of the taper region 111a, the thread width of the thread included in the taper region 112a is wider than the thread width of the thread included in the taper region 111a. If the load surfaces of the threads included in the taper region 112a and the load surfaces of the threads included in the taper region 221a interfere excessively in the tube axis direction, it may affect the contact between the pin inner seal surface 12 and the box inner seal surface 22. Therefore, by providing the box groove portion 27, excessive interference between the load surfaces can be suppressed, and the external pressure sealing performance can be further improved.

[0057] Since the gradient β2 of the tapered region 212a of the internal female thread 21a is smaller than the gradient α1 of the tapered region 111a, the thread width of the thread included in the tapered region 212a becomes wider than the thread width of the thread included in the tapered region 111a. Therefore, an interference amount in the pipe axis direction is introduced between the load surfaces of the threads included in the tapered region 212a and the load surfaces of the threads included in the tapered region 111a. Also, since the gradient β3 of the tapered region 112b of the external male thread 11b is smaller than the gradient α4 of the tapered region 211b, the thread width of the thread included in the tapered region 112b becomes wider than the thread width of the thread included in the tapered region 211b. Therefore, an interference amount in the pipe axis direction is introduced between the load surfaces of the threads included in the tapered region 112b and the load surfaces of the threads included in the tapered region 211b. These interference amounts increase as they approach the pin intermediate shoulder surface 16 and the box intermediate shoulder surface 26. As a result, at the positions of the pin intermediate critical section PCCS1 and the box intermediate critical section BCCS1, the internal male thread 11a and the internal female thread 21a come into contact reliably, and also, the external male thread 11b and the external female thread 21b come into contact reliably, so that the tensile load is transmitted to the pin intermediate shoulder surface 16 and the box intermediate shoulder surface 26. Consequently, the tensile strength can be improved.

[0058] Note that the pin 10 includes a pin intermediate critical section PSSC1 and a pin critical section PSSC2. The box 20 includes a box intermediate critical section BCCS1 and a box critical section BCCS2. As described above, the critical section is the cross section of the joint portion where the area that withstands the tensile load is the smallest in the fastened state. Usually, the pin intermediate critical section PSSC1 is located near the end portion on the pin intermediate shoulder 16 side of the internal male thread 11a. The pin critical section PSSC2 is located near the end portion on the pin outer seal surface 13 side of the external male thread 11b. The box intermediate critical section BSSC1 is located near the end portion on the box intermediate shoulder surface 26 side of the external female thread 21b. The box critical section BSSC2 is located near the end portion on the box inner seal surface 22 side.

[0059] The steel pipe screw joint 1 according to the present disclosure can also be applied to a coupling type or integral type steel pipe screw joint 1. However, the steel pipe screw joint 1 of the present disclosure can obtain particularly great effects in a slim type steel pipe screw joint 1 that is subject to strict restrictions on the outer diameter dimension.

[0060] In addition, the steel pipe screw joint 1 according to the present disclosure can be suitably applied to a so-called dope-free screw joint. A dope-free screw joint is a screw joint in which a solid lubricating film or a semi-solid lubricating film is pre-formed on either the pin 10 or the box 20 in advance before fastening, without applying dope when fastening the pin 10 and the box 20. The dope-free screw joint is repeatedly fastened and disassembled without re-applying dope. Therefore, excellent seizure resistance performance is required. Thus, the steel pipe screw joint 1 of the present disclosure having excellent seizure resistance performance can be suitably applied to the dope-free screw joint.

[0061] As described above, the embodiments have been described, but the present disclosure is not limited to the above embodiments, and various modifications are possible without departing from the spirit thereof.

Example

[0062] [First Analysis (Stabbing Property Evaluation)] Using a two-dimensional axisymmetric model for finite element method analysis, an elastoplastic analysis was performed to simulate the state when the screw joint was stabbed. Specifically, from the state where the pin and the box shown in FIG. 6 were separated in the pipe axis direction, an axial force load corresponding to the self-weight of the steel pipe body was applied, and the contact state of the screw portion during stabbing was evaluated.

[0063] [Analysis Conditions] 14” 116# Slim type screw joint for steel pipe Length of steel pipe body 12m Q125 steel specified by API standard (nominal yield stress 125 ksi = 861.8 MPa) Young's modulus 210 GPa, Poisson's ratio 0.3 Screw shape The taper gradients α1 to 4 shown in Fig. 6 are 1 / 18 [-] The taper gradients β1 to 4 shown in Fig. 6 are 0 [-] Flank angle of the load surface: -3 [deg] (the tip side of the pin is positive with respect to the plane perpendicular to the pipe axis CL). Flank angle of the insertion surface: 10 [deg] (the pipe body side is positive with respect to the plane perpendicular to the pipe axis CL). Load surface pitch: 5.08 [mm] Insertion surface pitch of the taper regions 111a, 111b, 211a, and 211b shown in Fig. 6: 5.08 [mm]

[0064] [Second analysis (sealing performance evaluation)] Regarding the model used in the first analysis, after performing an analysis simulating the fastening of the screw joint, a repeated combined load corresponding to the Series A test of ISO 13679 was applied, and the sealing performance of the inner seal (the inner seal surface of the pin and the inner seal surface of the box) and the outer seal (the outer seal surface of the pin and the outer seal surface of the box) was evaluated. The sealing performance was evaluated by the seal contact force [N / mm] per unit length in the circumferential direction of the inner seal and the outer seal, and it was judged that the better the sealing performance, the larger the minimum value of the seal contact force during the load cycle application.

[0065] [Analysis results] The analysis results in the first and second analyses are shown in Table 1. Specimens 1 and 2 are comparative examples, and specimen 3 is an example. Also, in Table 1, the contact pressure refers to the maximum contact pressure applied to the threaded part during stabbing.

[0066]

Table 1

[0067] In the first analysis, since the insertion surface pitch of Specimens 1 and 3 was constant, the load applied to the thread was dispersed, and the contact pressure in the threaded portion was suppressed. On the other hand, in Specimen 2 where the insertion surface pitch changes (see Fig. 1), only the threads at the ends of the internal male thread, internal female thread, external male thread, and external female thread came into contact, and the load concentrated on these threads, resulting in an extremely high contact pressure in the threaded portion.

[0068] As shown in Table 1 presenting the evaluation results of the sealing performance obtained in the second analysis, it can be said that the sealing performances of Specimens 1 to 3 are substantially the same.

[0069] In the above-described screw joint where the taper gradient changes, when both the load surface pitch and the insertion surface pitch are made constant (Specimen 1), it is necessary to cut the load surface and the insertion surface separately, increasing the cutting process. Therefore, Specimen 1 was evaluated as "B" due to inferior manufacturability, while Specimens 2 and 3, which only need to be cut so that either the load surface pitch or the insertion surface pitch is aligned, were evaluated as "A" due to excellent manufacturability.

[0070] From the above, it can be said that Specimen 3 has excellent seizure resistance performance and manufacturability, and its sealing performance is also equivalent to that of Specimens 1 and 2.

Explanation of Symbols

[0071] 1: Steel pipe screw joint 2: Steel pipe body 10: Pin 11: Male thread 11a: Internal male thread 11b: External male thread 12: Inner seal surface of pin 13: Outer seal surface of pin 14: Inner shoulder surface of pin 15: Intermediate seal surface of pin 16: Intermediate shoulder surface of pin 17: Groove portion of pin 20: Box 21: Female thread 21a: Internal female thread 21b: External female thread 22: Inner seal surface of box 23: Outer seal surface of the box 24: Inner shoulder surface of the box 25: Intermediate seal surface of the box 26: Intermediate shoulder surface of the box 27: Groove portion of the box α1 to α4, β1 to β4: Gradients of the tapered regions

Claims

1. A screw joint for steel pipes for connecting steel pipe bodies, comprising: a tubular pin; a tubular box into which the pin is screwed and fastened to the pin, wherein the pin includes: a pin inner seal surface formed at the tip of the pin; a male thread formed on the outer peripheral surface of the pin, located between the pin inner seal surface and the steel pipe body; a pin outer seal surface formed between the steel pipe body and the male thread; wherein the box includes: a box inner seal surface facing the pin inner seal surface and contacting the pin inner seal surface when the pin and the box are fastened; a female thread formed on the inner peripheral surface of the box, corresponding to the male thread; a box outer seal surface facing the pin outer seal surface and contacting the pin outer seal surface when the pin and the box are fastened; wherein the male thread includes a first taper region located between the pin inner seal surface and the pin outer seal surface, and a second taper region located between the pin inner seal surface and the first taper region and having a smaller gradient than the first taper region; wherein the female thread includes a third taper region located between the box inner seal surface and the box outer seal surface, and a fourth taper region located between the box outer seal surface and the third taper region and having a smaller gradient than the third taper region; the thread in the second taper region has a wider width than the thread in the first taper region; the thread in the fourth taper region has a wider width than the thread width in the third taper region; A screw joint for steel pipes, wherein the insertion surface pitch of the male thread and the female thread is constant.

2. A screw joint for steel pipes for connecting steel pipe bodies, comprising: a tubular pin; a tubular box into which the pin is screwed and fastened to the pin, wherein the pin includes: a pin inner seal surface formed at the tip of the pin; an inner male thread formed on the outer peripheral surface of the pin, located between the pin inner seal surface and the steel pipe body; an outer male thread formed on the outer peripheral surface of the pin, located between the steel pipe body and the inner male thread; a pin intermediate seal surface formed between the inner male thread and the outer male thread; wherein the box includes: a box inner seal surface facing the pin inner seal surface and contacting the pin inner seal surface when the pin and the box are fastened; An internal female thread formed on the inner peripheral surface of the box corresponding to the internal male thread, An external female thread formed on the inner peripheral surface of the box corresponding to the external male thread, Including a box intermediate seal surface that faces the pin intermediate seal surface and contacts the pin intermediate seal surface when the pin and the box are fastened, The internal male thread includes a fifth taper region located between the pin inner seal surface and the pin intermediate seal surface, and a sixth taper region located between the pin inner seal surface and the fifth taper region and having a gradient smaller than that of the fifth taper region, The external male thread includes a seventh taper region located between the pin intermediate seal surface and the steel pipe body, and an eighth taper region located between the pin intermediate seal surface and the seventh taper region and having a gradient smaller than that of the seventh taper region, The internal female thread includes a ninth taper region located between the box inner seal surface and the box intermediate seal surface, and a tenth taper region located between the box intermediate seal surface and the ninth taper region and having a gradient smaller than that of the ninth taper region, The external female thread includes an eleventh taper region located between the box intermediate seal surface and the tip of the box, and a twelfth taper region located between the tip of the box and the eleventh taper region and having a gradient smaller than that of the eleventh taper region, The threads included in the sixth and eighth taper regions each have a wider width than the threads included in the fifth and seventh taper regions, The threads included in the tenth and twelfth taper regions have a wider width than the threads included in the ninth and eleventh taper regions, A screw joint for a steel pipe, wherein the insertion surface pitches of the internal male thread and the internal female thread are constant, and the insertion surface pitches of the external male thread and the external female thread are constant.

3. A screw joint for a steel pipe for connecting a steel pipe body, A tubular pin, A tubular box into which the pin is screwed and fastened to the pin, The pin is, A pin inner seal surface formed at the tip of the pin, A pin outer seal surface formed between the steel pipe body and the pin inner seal surface, An internal male thread located between the pin inner seal surface and the pin outer seal surface and formed on the outer peripheral surface of the pin, An external male thread located between the pin outer seal surface and the internal male thread and formed on the outer peripheral surface of the pin, It is located between the internal male thread and the external male thread and includes a pin intermediate shoulder surface formed on the outer peripheral surface of the pin. The box is A box inner seal surface that faces the inner seal surface of the pin and contacts the inner seal surface of the pin when the pin and the box are fastened, A box outer seal surface that faces the outer seal surface of the pin and contacts the outer seal surface of the pin when the pin and the box are fastened, An internal female thread corresponding to the internal male thread and formed on the inner peripheral surface of the box, An external female thread corresponding to the external male thread and formed on the inner peripheral surface of the box, It includes a box intermediate shoulder surface that faces the pin intermediate shoulder surface and contacts the pin intermediate shoulder surface when the pin and the box are fastened. The internal male thread includes a fifth taper region located between the inner seal surface of the pin and the pin intermediate shoulder surface, and a sixth taper region located between the inner seal surface of the pin and the fifth taper region and having a gradient smaller than that of the fifth taper region. The external male thread includes a seventh taper region located between the pin intermediate shoulder surface and the outer seal surface of the pin, and an eighth taper region located between the pin intermediate shoulder surface and the seventh taper region and having a gradient smaller than that of the seventh taper region. The internal female thread includes a ninth taper region located between the box inner seal surface and the box intermediate shoulder surface, and a tenth taper region located between the box intermediate shoulder surface and the ninth taper region and having a gradient smaller than that of the ninth taper region. The external female thread includes an eleventh taper region located between the box intermediate shoulder surface and the box outer seal surface, and a twelfth taper region located between the box outer seal surface and the eleventh taper region and having a gradient smaller than that of the eleventh taper region. The threads included in the sixth and eighth taper regions each have a wider width than the threads included in the fifth and seventh taper regions. The threads included in the tenth and twelfth taper regions have a wider width than the threads included in the ninth and eleventh taper regions. A screw joint for steel pipes, wherein the insertion surface pitch of the internal male thread and the internal female thread is constant, and the insertion surface pitch of the external male thread and the external female thread is constant.

4. The screw joint for steel pipes according to claim 3, The pin faces the 12th tapered region of the external female thread and further includes an annular pin groove portion that is spaced apart from the 12th tapered region when the pin and the box are fastened. The box faces the 6th tapered region of the internal male thread and further includes an annular box groove portion that is spaced apart from the 6th tapered region when the pin and the box are fastened, the steel pipe screw joint.

5. The steel pipe screw joint according to any one of Claims 1 to 4, wherein the male thread and the female thread include a complete thread portion having a thread top surface and a thread bottom surface, and the thread top surface and the thread bottom surface are parallel to the pipe axis of the steel pipe body, the steel pipe screw joint.

6. The steel pipe screw joint according to any one of Claims 1 to 5, wherein the steel pipe screw joint is an integral type screw joint, the steel pipe screw joint.

7. The steel pipe screw joint according to any one of Claims 1 to 6, wherein the steel pipe screw joint has an outer diameter that is 105% or less with respect to the steel pipe body, the steel pipe screw joint.

8. The steel pipe screw joint according to any one of Claims 1 to 7, wherein either a solid lubricating film or a semi-solid lubricating film is formed on at least one of the outer peripheral surface of the pin and the inner peripheral surface of the box, the steel pipe screw joint.

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