Title of invention: Threaded joint for steel pipe
Technical field
[0001]
　The present invention relates to a threaded joint used for connecting steel pipes.
Background technology
[0002]
　In oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), steel pipes called oil well pipes (OCTG: Oil Country Tubular Goods) are used to mine underground resources. The steel pipes are connected in sequence. Threaded joints are used to connect steel pipes.
[0003]
　The types of threaded joints for steel pipes are roughly classified into coupling type and integral type. In the case of a coupling type threaded joint, one of the pair of pipe materials to be connected is a steel pipe and the other pipe material is a coupling. In this case, male threaded portions are formed on the outer circumferences of both end portions of the steel pipe, and female threaded portions are formed on the inner circumferences of both end portions of the coupling. Then, the steel pipe and the coupling are connected. In the case of integral type threaded joints, the pair of pipe materials to be connected are both steel pipes, and separate couplings are not used. In this case, a male threaded portion is formed on the outer circumference of one end of the steel pipe, and a female threaded portion is formed on the inner circumference of the other end. Then, one steel pipe and the other steel pipe are connected.
[0004]
　The joint portion of the pipe end portion on which the male thread portion is formed is referred to as a pin because it includes an element to be inserted into the female thread portion. On the other hand, the joint portion of the pipe end portion on which the female thread portion is formed is called a box because it includes an element that accepts the male thread portion. Both these pins and boxes are tubular because they are the ends of the tube.
[0005]
　FIG. 1 is a vertical cross-sectional view showing a conventional typical threaded joint for steel pipe. The threaded joint shown in FIG. 1 is a coupling type threaded joint and is composed of a pin 10 and a box 20 (see, for example, Japanese Patent Application Laid-Open No. 10-096489 (Patent Document 1)).
[0006]
　The pin 10 includes an annular shoulder surface 12, an annular sealing surface 13, and a male screw portion 14 in this order from the tip of the pin 10 toward the tube body 11 of the pin 10. At the pin 10, the sealing surface 13 is adjacent to the shoulder surface 12. The box 20 includes an annular shoulder surface 22, an annular sealing surface 23, and a female threaded portion 24 in this order from the tube body 21 of the box 20 toward the tip of the box 20. In the box 20, the sealing surface 23 is adjacent to the shoulder surface 22.
[0007]
　When connecting the pin 10 and the box 20, the shoulder surface 12 of the pin 10 comes into contact with the shoulder surface 22 of the box 20 by screwing the pin 10 into the box 20. When the pin 10 is continuously rotated by a predetermined amount, a tightening axial force is generated in the male screw portion 14 and the female screw portion 24 that mesh with each other, and the fastening is completed. In the state where the fastening is completed (hereinafter, also referred to as “fastened state”), the sealing surface 13 of the pin 10 comes into contact with the sealing surface 23 of the box 20 while interfering with each other, and a sealing portion is formed by metal contact. This sealing portion ensures the sealing performance of the threaded joint.
[0008]
　In recent years, deepening and ultra-deep seas of oil wells have progressed, and along with this, the oil well environment has become a harsh environment with high temperature and high pressure. In such an well environment, the compressive load, tensile load, external pressure (hereinafter, also referred to as “external pressure”), and internal pressure (hereinafter, also referred to as “internal pressure”) applied to the well pipe are extremely high. Therefore, thick-walled steel pipes are used especially as oil country tubular goods used for casings and tubing. When a threaded joint as shown in FIG. 1 is used for connecting such thick-walled steel pipes, the threaded joint is required to have the same strength and high sealing performance as the thick-walled steel pipe body. In particular, sealing performance against external pressure is required.
[0009]
　Here, the compressive load is received by the contact surface (hereinafter, also referred to as “shoulder contact surface”) between the shoulder surface 12 of the pin 10 and the shoulder surface 22 of the box 20. With reference to FIG. 1, in a conventional threaded joint, the entire shoulder surface 12 of the pin 10 contacts the entire shoulder surface 22 of the box 20 in order to maximize the area of ​​the shoulder contact surface. That is, the diameter Dpi of the inner peripheral edge of the shoulder surface 12 of the pin 10 is substantially the same as the diameter Dbi of the inner peripheral edge of the shoulder surface 22 of the box 20.
Prior art literature
Patent documents
[0010]
Patent Document 1: Japanese Unexamined Patent Publication No. 10-096489
Outline of the invention
Problems to be solved by the invention
[0011]
　One object of the present invention is to provide a threaded joint for steel pipes which can sufficiently secure sealing performance against external pressure even when a thick-walled steel pipe is used.
Means to solve problems
[0012]
　The threaded joint for steel pipe according to the embodiment of the present invention comprises a tubular pin and a tubular box. The pin includes an annular shoulder surface, an annular sealing surface adjacent to the pin shoulder surface, and a male thread portion in order from the tip of the pin toward the tube body of the pin. The box includes an annular shoulder surface, an annular sealing surface adjacent to the shoulder surface of the box, and a female thread portion, in order from the tube body of the box toward the tip of the box. The shoulder surface of each of the pin and the box is tilted in the screwing traveling direction of the pin from the surface perpendicular to the pipe axis. The diameter of the inner edge of the shoulder surface of the pin is smaller than the diameter of the inner edge of the shoulder surface of the box.
The invention's effect
[0013]
　According to the threaded joint for steel pipes according to the embodiment of the present invention, sufficient sealing performance against external pressure can be ensured even when a thick-walled steel pipe is used.
A brief description of the drawing
[0014]
FIG. 1 is a vertical cross-sectional view showing a conventional typical threaded joint for steel pipe.
FIG. 2 is a vertical cross-sectional view showing a threaded joint for a steel pipe according to the present embodiment.
FIG. 3 is an enlarged vertical cross-sectional view of the vicinity of the tip of a pin of the screw joint shown in FIG.
FIG. 4 is an enlarged vertical cross-sectional view of the vicinity of the tip of a pin of the screw joint shown in FIG.
Mode for carrying out the invention
[0015]
　In order to solve the above problems, the present inventors carried out various analyzes and tests, and repeated diligent studies. As a result, the following findings were obtained.
[0016]
　When trying to improve the sealing performance with a threaded joint using a thick-walled steel pipe, the following methods can be considered. As a first method, it is conceivable to increase the wall thickness of the portion 15 (hereinafter, also referred to as “pin seal portion”) including the seal surface 13 of the pin 10 with reference to FIG.
[0017]
　The first method was derived from the following reasoning. If the wall thickness of the pin seal portion 15 is thick, the rigidity of the pin seal portion 15 in the radial direction is increased. As a result, the elastic recovery force of the pin seal portion 15 is improved, and the contact force between the seal surface 13 of the pin 10 and the seal surface 23 of the box 20 (hereinafter, also referred to as “seal contact force”) is increased in the fastened state. Improves sealing performance against internal and external pressure. Further, the diameter reduction deformation of the pin seal portion 15 when an external pressure is applied to the pin 10 is suppressed. Therefore, even when an external pressure is applied, the decrease in the seal contact force can be reduced. Therefore, it may be said that the deterioration of the sealing performance with respect to the external pressure can be suppressed.
[0018]
　In the first method, the area of ​​the shoulder surface 12 of the pin 10 is large due to the thickening of the pin seal portion 15. In a conventional threaded joint, the entire area of ​​the shoulder surface 12 of the pin 10 comes into contact with the entire area of ​​the shoulder surface 22 of the box 20. Therefore, the area of ​​the shoulder surface 22 of the box 20 is also large. That is, the area of ​​the shoulder contact surface is large.
[0019]
　However, if the area of ​​the shoulder contact surface is too large, the contact force between the shoulder surface 12 of the pin 10 and the shoulder surface 22 of the box 20 (hereinafter, also referred to as “shoulder contact force”) is non-uniform within the shoulder contact surface. become. The seal portion adjacent to the shoulder contact surface is greatly affected by this. Therefore, in reality, the contact of the seal portion becomes unstable and the seal performance deteriorates.
[0020]
　As a second method, as opposed to such a first method, it is conceivable to reduce the wall thickness of the pin seal portion 15. In this case, the area of ​​the shoulder surface 12 of the pin 10 is small and the area of ​​the shoulder surface 22 of the box 20 is also small due to the thinning of the pin seal portion 15. As a result, the area of ​​the shoulder contact surface is small. Therefore, the shoulder contact force can be made uniform.
[0021]
　However, in the second method, the rigidity of the pin seal portion 15 in the radial direction is low due to the thinning of the pin seal portion 15. As a result, when an external pressure is applied to the pin 10, the pin seal portion 15 is easily deformed in diameter. Therefore, in reality, the sealing performance against external pressure is lowered.
[0022]
　In short, neither the first method nor the second method can secure the sealing performance against the external pressure.
[0023]
　Therefore, the present inventors have focused on the pin seal portion and the shoulder contact surface in consideration of the problems of the first and second methods. Specifically, the wall thickness of the pin seal portion is increased and the area of ​​the shoulder contact surface is reduced. As a result, the rigidity of the pin seal portion in the radial direction is increased, and the shoulder contact force becomes uniform in the shoulder contact surface. Therefore, it is possible to stabilize the contact of the seal portion. As a result, the sealing performance against external pressure can be ensured.
[0024]
　The threaded joint for steel pipes of the present invention has been completed based on the above findings.
[0025]
　The threaded joint for steel pipe according to the embodiment of the present invention comprises a tubular pin and a tubular box. The pin includes an annular shoulder surface, an annular sealing surface adjacent to the pin shoulder surface, and a male thread portion in order from the tip of the pin toward the tube body of the pin. The box includes an annular shoulder surface, an annular sealing surface adjacent to the shoulder surface of the box, and a female thread portion, in order from the tube body of the box toward the tip of the box. The shoulder surface of each of the pin and the box is tilted in the screwing traveling direction of the pin from the surface perpendicular to the pipe axis. The diameter of the inner edge of the shoulder surface of the pin is smaller than the diameter of the inner edge of the shoulder surface of the box.
[0026]
　In a typical example, the threaded joint of this embodiment is used to connect thick steel pipes used for casing and tubing. The wall thickness of the thick steel pipe exceeds 1 inch (25.4 mm).
[0027]
　According to the threaded joint of the present embodiment, the area of ​​the shoulder surface of the pin is large and the area of ​​the shoulder surface of the box is small. This is because the diameter of the inner peripheral edge of the shoulder surface of the pin is smaller than the diameter of the inner peripheral edge of the shoulder surface of the box. As a result, since the thickness of the pin seal portion is thick, the rigidity of the pin seal portion in the radial direction is increased. Further, since the area of ​​the shoulder contact surface is small, the shoulder contact force becomes uniform in the shoulder contact surface.
[0028]
　Further, according to the screw joint of the present embodiment, the shoulder surface of the pin is in pressure contact with the shoulder surface of the box in a hook shape in the fastened state. This is because the shoulder surfaces of the pin and the box are tilted in the screwing traveling direction of the pin from the surface perpendicular to the pipe axis. As a result, the pin seal portion always receives a reaction force in the direction of increasing the diameter. Therefore, when an external pressure is applied to the pin, the pin seal portion is unlikely to be deformed in diameter.
[0029]
　From the above, even when a thick-walled steel pipe is used, the contact of the seal portion is stable. Therefore, sufficient sealing performance against external pressure can be ensured.
[0030]
　In the above threaded joint, it is preferable that the inclination angle of the shoulder surface of each of the pin and the box with respect to the surface perpendicular to the pipe axis is 5 ° to 20 °. When the inclination angle of the shoulder surface is 5 ° or more, the shoulder surface of the pin is effectively pressed and contacted with the shoulder surface of the box in the form of a hook in the fastened state. Preferably, the inclination angle of the shoulder surface is 10 ° or more. On the other hand, if the inclination angle of the shoulder surface is 20 ° or less, the deformation of the shoulder portion of the box is small even if a compressive load is repeatedly applied. Therefore, the pressing contact between the shoulder surfaces in the hook shape is effectively maintained.
[0031]
　In the above threaded joint, the thickness tp of the annular pin shoulder region that appears when the shoulder surface of the pin is projected onto the surface perpendicular to the pipe axis is preferably 60% or more of the wall thickness t of the pipe body of the pin. .. When the thickness tp of the pin shoulder region is 60% or more of the wall thickness t of the tube body of the pin, the wall thickness of the pin seal portion is effectively increased.
[0032]
　On the other hand, the upper limit of the thickness tp of the pin shoulder region is not particularly limited. However, if the thickness tp of the pin shoulder region is too thick, it becomes difficult to secure the length of the male threaded portion. Therefore, the thickness tp of the pin shoulder region is preferably 80% or less of the wall thickness t of the tube body of the pin.
[0033]
　In the above threaded joint, the thickness tb of the annular box shoulder region that appears when the shoulder surface of the box is projected onto the surface perpendicular to the pipe axis is 20% or more and 55% or less of the wall thickness t of the pipe body of the pin. It is preferable to have. The box shoulder region corresponds to an annular shoulder contact surface region that appears when the shoulder contact surface is projected onto a plane perpendicular to the tube axis.
[0034]
　If the thickness tb of the box shoulder region (shoulder contact surface region) is 20% or more of the wall thickness t of the tube body of the pin, if an excessive compressive load is applied to the threaded joint, the shoulder surface and adjacent to it The plastic deformation of the sealing surface can be suppressed, and the contact state of the sealing surface can be stabilized. Therefore, the seal contact force can be secured. More preferably, the thickness tb of the box shoulder region is 30% or more of the wall thickness t of the tube body of the pin. On the other hand, if the thickness tb of the box shoulder region is 55% or less of the wall thickness t of the tube body of the pin, the area of ​​the shoulder contact surface is effectively reduced. More preferably, the thickness tb of the box shoulder region is 45% or less of the wall thickness t of the tube body of the pin.
[0035]
　Strictly speaking, in the vertical cross section of the pin, there is a corner portion between the shoulder surface and the sealing surface. This corner smoothly connects the shoulder surface and the sealing surface of the pin. Similarly, in the vertical section of the box, there is a corner between the shoulder surface and the sealing surface. This corner smoothly connects the shoulder surface and the sealing surface of the box. The radius in the vertical cross section of these corners and corners is about 1.5 mm at the maximum. In this case, the thickness tp of the pin shoulder region does not include the corner region. Similarly, the thickness tb of the box shoulder region does not include the corner region.
[0036]
　Hereinafter, specific examples of the threaded joint for steel pipes of the present embodiment will be described with reference to the drawings.
[0037]
　FIG. 2 is a vertical cross-sectional view showing a threaded joint for steel pipes of the present embodiment. 3 and 4 are enlarged vertical cross-sectional views of the vicinity of the tip of the pin of the threaded joint shown in FIG. FIG. 3 shows the fastening state. FIG. 4 shows a state in which the pin 10 is separated from the box 20 for convenience of explanation. The white arrows in FIGS. 2 to 4 indicate the screwing traveling direction of the pin 10 with respect to the box 20. In the present specification, the vertical cross section means a cross section including a pipe shaft CL (see FIG. 2) of a threaded joint.
[0038]
　With reference to FIGS. 2 to 4, the threaded joint of the present embodiment is a coupling type threaded joint, and is composed of a pin 10 and a box 20. The pin 10 is a thick steel pipe.
[0039]
　The pin 10 includes an annular shoulder surface 12, an annular sealing surface 13, and a male screw portion 14 in this order from the tip of the pin 10 toward the tube body 11 of the pin 10. Hereinafter, the shoulder surface 12 of the pin 10 is also referred to as a “pin shoulder surface”. The sealing surface 13 of the pin 10 is also referred to as a “pin sealing surface”.
[0040]
　The pin shoulder surface 12 is an annular surface forming the tip surface of the pin 10, and is tilted in the screwing traveling direction of the pin 10 from a surface perpendicular to the tube axis CL. As a result, the outer peripheral edge 12b (edge ​​on the side far from the tube axis CL) of the pin shoulder surface 12 protrudes in the screwing traveling direction of the pin 10 from the inner peripheral edge 12a (edge ​​on the side closer to the tube axis CL) of the pin shoulder surface 12. To do. The pin seal surface 13 is adjacent to the pin shoulder surface 12. That is, the pin seal surface 13 is connected to the outer peripheral edge 12b of the pin shoulder surface 12. The pin seal surface 13 is a tapered annular surface. However, the pin seal surface 13 may have a shape in which a tapered annular surface and a surface corresponding to the peripheral surface of a rotating body obtained by rotating a curved line such as an arc around the tube axis CL are combined. The diameter of the pin seal surface 13 is smaller toward the tip end side of the pin 10 (pin shoulder surface 12 side).
[0041]
　The box 20 includes an annular shoulder surface 22, an annular sealing surface 23, and a female threaded portion 24 in this order from the tube body 21 of the box 20 toward the tip of the box 20. Hereinafter, the shoulder surface 22 of the box 20 is also referred to as a “box shoulder surface”. The sealing surface 23 of the box 20 is also referred to as a “box sealing surface”.
[0042]
　The box shoulder surface 22 is an annular surface corresponding to the pin shoulder surface 12, and is tilted in the screwing traveling direction of the pin 10 from the surface perpendicular to the tube axis CL. As a result, the inner peripheral edge 22a of the box shoulder surface 22 (the edge on the side closer to the tube axis CL) is in the screwing traveling direction of the pin 10 than the outer peripheral edge 22b (the edge on the side far from the tube axis CL) of the box shoulder surface 22. Protrudes in the opposite direction. The box seal surface 23 is adjacent to the box shoulder surface 22. That is, the box seal surface 23 is connected to the outer peripheral edge 22b of the box shoulder surface 22. The box seal surface 23 is a tapered annular surface corresponding to the pin seal surface 13. However, the box seal surface 23 may have a shape obtained by combining a tapered annular surface and a surface corresponding to the peripheral surface of a rotating body obtained by rotating a curved line such as an arc around the tube axis CL. ..
[0043]
　The male threaded portion 14 of the pin 10 corresponds to the female threaded portion 24 of the box 20. The male screw portion 14 and the female screw portion 24 include a screw top surface, a screw bottom surface, an insertion flank surface, and a load flank surface, respectively.
[0044]
　In the present embodiment, the diameter Dpi of the inner peripheral edge 12a of the pin shoulder surface 12 is smaller than the diameter Dbi of the inner peripheral edge 22a of the box shoulder surface 22. That is, the area of ​​the pin shoulder surface 12 is large, and the area of ​​the box shoulder surface 22 is small. Therefore, the area of ​​the shoulder contact surface 30 is small. The diameter Dpi of the inner peripheral edge 12a of the pin shoulder surface 12 is the same as the inner diameter of the tube body 11 of the pin 10. That is, the inner diameter of the pin 10 is constant.
[0045]
　Further, in the present embodiment, the inclination angles θp and θb of the pin shoulder surface 12 and the box shoulder surface 22 with respect to the surface perpendicular to the tube axis CL are 5 ° to 20 °, respectively. The thickness tp of the annular pin shoulder region that appears when the pin shoulder surface 12 is projected onto the plane perpendicular to the tube axis CL is 60% or more of the wall thickness t of the tube body 11 of the pin 10. The thickness tb of the annular box shoulder region that appears when the box shoulder surface 22 is projected onto the plane perpendicular to the tube axis CL is 20% or more and 55% or less of the wall thickness t of the tube body 11 of the pin 10.
[0046]
　When connecting the pin 10 and the box 20, the male screw portion 14 meshes with the female screw portion 24 by screwing the pin 10 into the box 20. A part of the pin shoulder surface 12 comes into contact with the entire area of ​​the box shoulder surface 22. That is, the pin shoulder surface 12 comes into contact with the box shoulder surface 22 within the range of the shoulder contact surface 30. When the pin 10 is subsequently rotated by a predetermined amount, a part of the pin shoulder surface 12 comes into pressure contact with the entire area of ​​the box shoulder surface 22 in a hook shape. As a result, a tightening axial force is generated in the male screw portion 14 and the female screw portion 24 that mesh with each other, and the fastening is completed. In the fastened state, the pin seal surface 13 contacts the box seal surface 23 while interfering with each other, and a seal portion is formed by metal contact. This sealing portion ensures the sealing performance of the threaded joint.
[0047]
　With reference to FIGS. 2 to 4, according to the threaded joint of the present embodiment, the area of ​​the pin shoulder surface 12 is large and the area of ​​the box shoulder surface 22 is small. As a result, the thickness of the pin seal portion 15 is large, so that the rigidity of the pin seal portion 15 in the radial direction is increased. Further, since the area of ​​the shoulder contact surface 30 is small, the shoulder contact force becomes uniform in the shoulder contact surface 30.
[0048]
　Further, according to the screw joint of the present embodiment, a part of the pin shoulder surface 12 is pressed and contacted with the entire area of ​​the box shoulder surface 22 in a hook shape in the fastened state. As a result, the pin seal portion 15 always receives a reaction force in the direction of increasing the diameter. Therefore, when an external pressure is applied to the pin 10, the pin seal portion 15 is unlikely to be deformed in diameter.
[0049]
　From the above, even when a thick-walled steel pipe is used, the contact of the seal portion is stable. Therefore, sufficient sealing performance against external pressure can be ensured.
Example
[0050]
　In order to confirm the effect of this embodiment, numerical simulation analysis (FEM analysis) by the elasto-plastic finite element method was performed.
[0051]
　[Example 1]
　[Test conditions] In the
　FEM analysis, a model of a coupling type threaded joint in which the diameter Dpi of the inner peripheral edge of the pin shoulder surface and the diameter Dbi of the inner peripheral edge of the box shoulder surface were changed was used. The common conditions are as follows.
・ Dimensions of steel pipe (pin body): 7-5 / 8 inch × 1.06 inch (outer diameter 193.7 mm, wall thickness 27.0 mm)
・ Steel pipe grade: API standard P110 (carbon steel with nominal yield stress of 110 ksi) )
・ Diameter of the outer peripheral edge of the shoulder surface (pin shoulder surface and box shoulder surface) Do: 179.9 mm
・ Shoulder surface inclination angle: 15 °
・ Screw pitch: 5.08 mm
・ Load flank surface flank angle: -3 °
・Frank angle of insertion flank surface: 10 °
・ Gap in insertion flank surface: 0.15mm
[0052]
　The changed dimensional conditions are shown in Table 1 below.
[0053]
[table 1]

[0054]
　In the FEM analysis, the material was an isotropically cured elasto-plastic body. The elastic modulus was 210 GPa, and the yield strength as 0.2% proof stress was 110 ksi (758.3 MPa). Tightening was performed until the pin shoulder surface came into contact with the box shoulder surface and then further rotated 1.0 / 100.
[0055]
　Test No. 1 and 3 to 6 are examples of the present invention assuming the threaded joint of the present embodiment, and the diameter Dpi of the inner peripheral edge of the pin shoulder surface is smaller than the diameter Dbi of the inner peripheral edge of the box shoulder surface. Test No. Reference numeral 2 denotes a comparative example of a standard assuming a conventional threaded joint, in which the diameter Dpi of the inner peripheral edge of the pin shoulder surface was the same as the diameter Dbi of the inner peripheral edge of the box shoulder surface. Test No. 7 to 9 are comparative examples, and the diameter Dpi of the inner peripheral edge of the pin shoulder surface was the same as the diameter Dbi of the inner peripheral edge of the box shoulder surface.
[0056]
　[Evaluation method] In the
　FEM analysis, a load step (internal pressure, external pressure, tensile load, and compressive load) simulating the ISO13679 2011 Series A test was applied to the model in the fastened state. Focusing on the external pressure and compressive load load points among the external pressure points in the external pressure cycle of the load step history, and the load points of only the external pressure, the sealing performance of the seal portion at each load point was evaluated. Here, the seal contact force [N / mm] when an external pressure and a compressive load are applied, and the seal contact force [N / mm] when an external pressure only is applied are investigated. The seal contact force referred to here is a value of "average contact surface pressure between sealing surfaces" x "contact width", and the higher this value is, the better the sealing performance is.
[0057]
　The specific evaluation of the sealing performance is described in Test No. It was done based on 2. That is, the test No. The seal contact force under load of external pressure and compressive load in No. 2 and the seal contact force under load of only external pressure were used as the reference (1.00), and the test No. Each test No. 2 for the seal contact force of 2. The ratio of the seal contact force of was compared.
[0058]
　[Test Results]
　Table 1 above shows the test results. From the results shown in Table 1, the following is shown. Test No. of the example of the present invention. In Nos. 1 and 3 to 6, the reference test Nos. Compared with 2, the sealing performance has improved. This is because the diameter Dpi of the inner peripheral edge of the pin shoulder surface was smaller than the diameter Dbi of the inner peripheral edge of the box shoulder surface. In particular, Test No. In 1 and 3 to 6, the sealing performance is further improved. This is because the thickness tp of the pin shoulder region is 60% or more of the wall thickness t of the tube body of the pin, and the wall thickness of the pin seal portion is effectively increased. Further, the thickness tb of the box shoulder region is 55% or less of the wall thickness t of the tube body of the pin, and the area of ​​the shoulder contact surface is effectively small.
[0059]
　On the other hand, the test No. of the comparative example. In 7 to 9, the reference test No. Compared with 2, the sealing performance was lowered. This is because the diameter Dpi of the inner peripheral edge of the pin shoulder surface was the same as the diameter Dbi of the inner peripheral edge of the box shoulder surface. In particular, since the thickness tp of the pin shoulder region did not reach 60% of the wall thickness t of the tube body of the pin, the wall thickness of the pin seal portion became thin.
[0060]
　[Example 2]
　[Test conditions] In
　Example 2, the same FEM analysis as in Example 1 above was performed. In particular, in Example 2, the inclination angle of the shoulder surface was set to 5 °. Other than that, the common conditions were the same as in Example 1 above. The changed dimensional conditions (diameter Dpi of the inner peripheral edge of the pin shoulder surface and diameter Dbi of the inner peripheral edge of the box shoulder surface) are as shown in Table 2 below.
[0061]
[Table 2]

[0062]
　Test No. 10 and 12 to 15 are examples of the present invention assuming the threaded joint of the present embodiment, and the diameter Dpi of the inner peripheral edge of the pin shoulder surface is smaller than the diameter Dbi of the inner peripheral edge of the box shoulder surface. Test No. Reference numeral 11 denotes a comparative example of the standard assuming a conventional threaded joint, in which the diameter Dpi of the inner peripheral edge of the pin shoulder surface was the same as the diameter Dbi of the inner peripheral edge of the box shoulder surface. Test No. 16 to 18 are comparative examples, and the diameter Dpi of the inner peripheral edge of the pin shoulder surface was the same as the diameter Dbi of the inner peripheral edge of the box shoulder surface.
[0063]
　[Evaluation method]
　The sealing performance was evaluated in the same manner as in Example 1 above. The specific evaluation of the sealing performance is described in Test No. It was done based on 11. That is, the test No. The seal contact force when the external pressure and the compressive load were applied in No. 11 and the seal contact force when the external pressure was applied were set as the reference (1.00), respectively, and the test No. 11 was used. Each test No. 11 for the seal contact force. The ratio of the seal contact force of was compared.
[0064]
　[Test Results]
　Table 2 above shows the test results. From the results shown in Table 2, the following is shown. Test No. of the example of the present invention. In Nos. 10 and 12 to 15, the reference test Nos. Compared with 11, the sealing performance was improved. This is because the diameter Dpi of the inner peripheral edge of the pin shoulder surface was smaller than the diameter Dbi of the inner peripheral edge of the box shoulder surface.
[0065]
　On the other hand, the test No. of the comparative example. In 16 to 18, the reference test No. Compared with 11, the sealing performance was lowered. This is because the diameter Dpi of the inner peripheral edge of the pin shoulder surface was the same as the diameter Dbi of the inner peripheral edge of the box shoulder surface.
[0066]
　In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the type of threaded joint may be either a coupling type or an integral type.
Industrial applicability
[0067]
　The threaded joint of the present invention can be effectively used for connecting a steel pipe used as an oil well pipe.
Description of the sign
[0068]
　10 Pin
　11 Tube body
　12 Shoulder surface
　12a Inner peripheral edge
　12b Outer peripheral edge
　13 Seal surface
　14
　Male threaded part 15 Pin seal part
　20 Box
　21 Tube body
　22 Shoulder surface
　22a Inner peripheral edge
　22b Outer peripheral edge
　23 Sealing surface
　24 Female threaded part
　30 Shoulder contact surface
　Do Shoulder surface Diameter of the outer peripheral edge of
　Dpi Diameter of the inner peripheral edge of the pin
　shoulder surface Dbi Diameter of the inner peripheral edge of the box shoulder surface
　tp Thickness of the 　pin
　shoulder area tb Thickness of the box shoulder area
t Thickness of the tube body of the pin
　CL
The scope of the claims
[Claim 1]
　A threaded joint for a steel pipe composed of a tubular pin and a tubular box,
　wherein the pin is adjacent to an annular shoulder surface and the shoulder surface of the pin in order from the tip of the pin toward the pipe body of the pin. The
　box includes an annular sealing surface and a male screw portion, and the box has an annular shoulder surface and an annular seal adjacent to the shoulder surface of the box in this order from the pipe body of the box toward the tip of the box. The
　shoulder surface of each of the pin and the box , including the surface and the female thread portion, is tilted in the screwing traveling direction of the pin from the surface perpendicular to the pipe axis, and
　the diameter of the inner peripheral edge of the shoulder surface of the pin is increased. , A threaded joint for steel pipes, which is smaller than the diameter of the inner peripheral edge of the shoulder surface of the box.
[Claim 2]
　The threaded joint for steel pipe according to claim 1,
　wherein the inclination angle of the shoulder surface of each of the pin and the box with respect to the surface perpendicular to the pipe axis is 5 ° to 20 °.
[Claim 3]
　In the threaded joint for steel pipe according to claim 1 or 2,
　the thickness of the annular pin shoulder region that appears when the shoulder surface of the pin is projected onto the surface perpendicular to the pipe axis is the thickness of the pin. A threaded joint for a steel pipe, which is 60% or more of the wall thickness of the pipe body.
[Claim 4]

　The thickness of the annular box shoulder region of the 　threaded joint for steel pipe according to any one of claims 1 to 3, which appears when the shoulder surface of the box is projected onto a surface perpendicular to the pipe axis. A threaded joint for a steel pipe having a thickness of 20% or more and 55% or less of the wall thickness of the pipe body of the pin.