0001]The present disclosure relates to a threaded joint for steel pipes, and more particularly to a threaded joint for connecting a large diameter and a steel pipe of thick each other.
BACKGROUND
[0002]For example, oil wells and natural gas wells and the like exploration or production, the development of unconventional resources such as oil sand and shale gas, the carbon dioxide recovery and storage (CCS (Carbon (hereinafter, also collectively referred to as "oil wells") dioxide Capture and Storage)), in geothermal, or springs or the like, the steel pipe called oil well pipe is used. The connection of steel pipes, threaded joints are used.
[0003]
　The format of this type of tubular threaded joint is roughly classified into a coupling-type and integral type. For coupling type, a pair of tubing of consolidated, where one tube is steel, other tubing is coupled. In this case, the external thread is formed on an outer periphery of both end portions of the steel pipe, the internal thread is formed on the inner periphery of both end portions of the coupling. The external thread of the steel pipe is screwed into the female screw of the coupling, which both are connected is fastened by. For integral type, a pair of tubing of consolidated are both steel pipe, without using a separate coupling. In this case, the external thread is formed on an outer periphery of one end portion of the steel pipe, the internal thread is formed on the inner periphery of the other end. The external thread of one of the steel pipe is screwed into the female screw of the other steel pipe, which both are connected is fastened by.
[0004]
　Generally, the joint portion of the tube end portion formed with a male, since the containing elements to be inserted into the female screw, referred to as "pin". On the other hand, the joint portion of the female thread is formed tube end, since it contains an element for receiving an external thread, referred to as the "box". These pin and box are the ends of the tube, both of which are tubular.
[0005]
　Oil wells, as Anakabe during drilling does not collapse, since the burrows with reinforced anti wall OCTG will result in oil well pipes are arranged in multiple structure. Recently, high-depth reduction and ultra deep water of wells are increasingly progress, in such environments, for developing efficient oil well, the connection of the oil well pipe, the joint inner and outer diameters of the steel pipe inner and outer diameters and screw joints are comparable is frequently used. Such threaded joint by using, it is possible to reduce the gap between the oil well pipe with each other are arranged in multiple utmost, deeper in the well of the diameter can be developed efficiently oil well not so large. Under such an inner diameter and an outer diameter restriction, the threaded joint, the fluid pressure from the interior to (hereinafter also referred to as "internal pressure") and fluid pressure from the outside (hereinafter, also referred to as "external pressure"), excellent sealing performance is required. Generally, the sealing performance for the internal pressure is referred to as "inner pressure sealing performance", sealing performance against external pressure is referred to as "external pressure sealing performance."
[0006]
　As threaded joints for securing the sealing performance, a metal - sealing by metal contact (. Hereinafter, "metal seal" hereinafter) those having known. The metal seal, (referred to as the difference between the diameter "interference amount") slightly larger than the diameter of the sealing surface of the diameter of the sealing surface of the pin box, the seal faces are fitted with fastening the threaded joint If, the amount of interference sealing surface of the pin is reduced in diameter, the larger diameter the sealing surface of the box, the entire circumference the contact pressure on the sealing surfaces occurs respective sealing surfaces by the elastic restoring force of returning to the original diameter coherent, a structure to exhibit sealing performance.
[0007]
　Kohyo 2006-526747 (Patent Document 1), in a threaded joint for steel pipes comprising a pin and box, by a pin provided nose, discloses a technique for improving the external pressure sealing performance while maintaining the internal pressure sealing performance .
[0008]
　JP 2015-132285 (Patent Document 2), in a threaded joint for steel pipes comprising a pin and box, discloses a technique for preventing sticking while maintaining the internal pressure and external pressure sealing performance.
[0009]
　Patent No. 4535064 (Patent Document 3), a threaded joint comprising one metal seal and nose, discloses that it has a high external pressure sealing performance than threaded joint without a nose.
[0010]
　Herein are incorporated by reference prior art documents described below.
Patent Document 1: JP-T 2006-526747 Patent Publication
Patent Document 2: JP 2015-132285 Patent Publication
Patent Document 3: Japanese Patent No. 4535064
Summary of the Invention
[0011]
　The purpose of the present disclosure is to provide a threaded joint capable of improving the internal pressure sealing performance while maintaining a high external pressure sealing performance of the nose with a threaded joint for use in steel tube of larger diameter and thick-walled.
[0012]
　Tubular threaded joint according to the present disclosure is a threaded joint for connecting steel pipes body together with an outer diameter of more than 7 inches and 0.7 inches or more wall thickness. The threaded joint includes a pin tubular and a tubular box. Box, the pin is fastened to the inserted into the pin. Pin includes a nose, and Pinshoruda surface, and Pintepa surface, and the pin round face, and the male screw, and the pin seal surface. Nose is formed at the tip portion of the pin has a smaller outer diameter than the inner diameter of the opposing portion of the box with clamping state. Pinshoruda surface is formed at the tip of the nose. Pintepa surface is formed on the outer peripheral surface of the nose. Pin round surface is formed at the corner between the Pinshoruda surface and Pintepa surface. External thread is formed on the outer circumference of the pin. Pin seal surface is formed on the outer peripheral surface of the pin A between the nose and the male screw. Box includes a box shoulder surface, and the female screw, and a box seal surface. Box shoulder surface is opposed to Pinshoruda surface, contacts the Pinshoruda surface in the engaged state. Internal thread corresponds to the external thread is formed on the inner periphery of the box. Box sealing surface facing the pin seal surface, contacts the pin seal surface during fastening.
[0013]
　The gap amount of time of fastening between the outer surface and the box of the inner surface of the pin on the boundary line between the Pintepa surface and the pin round surface [delta] [mm], the outer diameter of the steel tube body D [inch], steel body of the wall thickness t [inch], the length of time of fastening in the tube axis direction between the pivot point and the end of the pin is a position closest to the tip of the pin out of the region where the pin seal surface is in contact with the box sealing surface L [mm], the thickness of the pin at the pivot point t L [mm], the thickness of the box at the pivot point t B when the [mm], satisfy the following equation (1), t B / t L > 1.4.
[0014]
　0.30＋min(a nt L＋b nD, n=1,2,3)＞δ＞min(a nt L＋b nD, n=1,2,3)　（１）
[0015]
　Here, min is a function that takes a minimum value of the three arguments in parentheses,
　a1 = (L / t) × 0.0115 × (t L / t) -1.68778 -0.00247 × (t L / t) -2.02052 ,
　b1 = (L / t) × 0.0076 × (t L / t) -0.65672 -0.00120 × (t L / t) -1.06817 ,
　a2 = (L / t) × .00725 × (t L / t) -1.67341 - × 0.00157 (t L / t) -1.93212 ,
　b2 = 0,
　a3 = - (L / t) × 0.00256 × (t L / t) -2.40620 + 0.00372 × (t L / t) -2.19899 ,
　b3 = (L / t) × 0.01986 × (t L / t) -0.88615 -0.01048 × (t L / t) -1.00532 is.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[1] Figure 1 is a longitudinal sectional view showing a schematic configuration of a coupling-type threaded joint according to the embodiment.
FIG. 2 is a longitudinal sectional view showing a schematic configuration of the different integral type threaded joint Fig.
FIG. 3 is a longitudinal sectional view of the tube axis direction showing the nose and around the threaded joint shown in Figure 1 or Figure 2.
[4] FIG. 4 is a longitudinal sectional view of the tube axis direction showing the nose and around the different threaded joint and FIG.
FIG. 5 is a graph showing the correlation between the gap amount and the external pressure seal of the nose.
DESCRIPTION OF THE INVENTION
[0017]
　[External pressure sealing performance]
　In the thick tubular threaded joint, closer to the external pressure value of API5C3 standards VME100% of external pressure values (i.e., not considering the collapse strength, yield load value of the material itself). When such a very high external pressure is loaded, the external pressure leaks in tubular threaded joint thick occurs.
[0018]
　The present inventors have intensively studied and, as a result, have revealed the cause of the external pressure leaks. That is, the screw having to be reduced in diameter across pin high external pressure is behind the metal seal penetrated, ahead metal seal is not reduced in diameter because external pressure to prevent entry of the result as a metal seal, than the pin of the male thread including a nose tip (hereinafter, referred to as "pin lip.") so that the warped outward in the radial direction. Therefore, the seal point is sealed suitably surface (surface roughness and surface treatment for metal seals formed are properly prepared, and surface familiar with moderate sliding during fitting engagement) to move distally of the pin beyond the and thus whether or because pin lip is strong contact with the inner peripheral surface of the box seal contact force decreases significantly, external pressure sealing performance is significantly reduced.
[0019]
　The present inventors Based on such a new finding, as the tip of the nose be load is high external pressure is not strong contact with the inner peripheral surface of the box, if defines the lower limit of the gap amount [delta], of the thick in tubular threaded joint was considered possible to prevent a significant reduction in external pressure sealing performance. The estimated amount of warping deformation of the nose during external pressure load (diameter of half) by an elastic cylindrical shell theory has identified preferred lower limit of the gap amount [delta].
[0020]
　[Pressure sealing performance]
　If the internal pressure is loaded, pin lip is enlarged, the effect of sealing contact force is amplified. However, the steel pipes of a large diameter and thick-walled, the wall thickness of the box is relatively thin relative to the pin, since the greater diameter of the box due to internal pressure load, the amplification effect of the seal contact force decreases. Accordingly, the present inventors have found that in order to improve the internal pressure sealing performance by increasing the amplification effect of the seal contact force, considered to be preferred value for seal thickness ratio of the pin and the box are present.
[0021]
　Based on the above findings, the present inventors have found that a threaded joint for steel pipes which can improve the internal pressure sealing performance while maintaining a high external pressure sealing performance of the nose with a threaded joint for use in steel tube of larger diameter and thicker It was invention.
[0022]
　Tubular threaded joint according to the embodiment is a threaded joint for connecting steel pipes body together with an outer diameter of more than 7 inches and 0.7 inches or more wall thickness. The threaded joint includes a pin tubular and a tubular box. Box, the pin is fastened to the inserted into the pin. Pin includes a nose, and Pinshoruda surface, and Pintepa surface, and the pin round face, and the male screw, and the pin seal surface. Nose is formed at the tip portion of the pin has a smaller outer diameter than the inner diameter of the opposing portion of the box with clamping state. Pinshoruda surface is formed at the tip of the nose. Pintepa surface is formed on the outer peripheral surface of the nose. Pin round surface is formed at the corner between the Pinshoruda surface and Pintepa surface. External thread is formed on the outer circumference of the pin. Pin seal surface is formed on the outer peripheral surface of the pin A between the nose and the male screw. Box includes a box shoulder surface, and the female screw, and a box seal surface. Box shoulder surface is opposed to Pinshoruda surface, contacts the Pinshoruda surface in the engaged state. Internal thread corresponds to the external thread is formed on the inner periphery of the box. Box sealing surface facing the pin seal surface, contacts the pin seal surface during fastening.
[0023]
　The gap amount of time of fastening between the outer surface and the box of the inner surface of the pin on the boundary line between the Pintepa surface and the pin round surface [delta] [mm], the outer diameter of the steel tube body D [inch], steel body of the wall thickness t [inch], the length of time of fastening in the tube axis direction between the pivot point and the end of the pin is a position closest to the tip of the pin out of the region where the pin seal surface is in contact with the box sealing surface L [mm], the thickness of the pin at the pivot point t L [mm], the thickness of the box at the pivot point t B when the [mm], satisfy the following equation (1), t B / t L > 1.4.
[0024]
　0.30＋min(a nt L＋b nD, n=1,2,3)＞δ＞min(a nt L＋b nD, n=1,2,3)　（１）
[0025]
　Here, min is a function that takes a minimum value of the three arguments in parentheses.
　a1, a2, a3, b1, b2, b3 are defined as follows.
　= A1 (L / T) × 0.0115 × (T L / T) -1.68778 -0.00247 × (T L / T) -2.02052
　B1 = (L / T) × 0.0076 × (T L / T) -0.65672 -0.00120 × (T L / T) -1.06817
　A2 = (L / T) × 0.00725 × (T L / T) -1.67341 -0.00157 × (T L / T) -1.93212
　B2 = 0
　A3 = - (L / T) × 0.00256 × (T L / T) -2.40620 Tasu 0.00372 × (T L / T) -2.19899
　b3=(L/t)×0.01986×(t L/t) -0.88615－0.01048×(t L/t) -1.00532
[0026]
　A threaded joint for steel pipes according to the present embodiment, since the lower limit of the gap amount δ is limited, high external pressure that has entered the screw is reduced in diameter across pins behind the pin seal surface, resulting reaction, nose pivot point the even if the warp deformation as a boundary, it is possible to maintain the external pressure sealing performance by the nose. The upper limit of the gap amount δ is the depth of cut or the number of passes during cutting so as not to increase unnecessarily, is set to "gap amount of the lower limit value + 0.3 mm".
[0027]
　On the other hand, the thickness ratio t B / t L for the lower limit of which is limited to 1.4, even steel pipe large-diameter and thick-walled, expanded box due to internal pressure load is suppressed. Therefore, the amplification effect of the seal contact force is increased, thereby improving the internal pressure sealing performance.
[0028]
　As described above, a threaded joint according to the present embodiment, it is possible to improve the internal pressure sealing performance while maintaining a high external pressure sealing performance of the nose with a threaded joint for use in steel tube of larger diameter and thick-walled.
[0029]
Preferably, t B / t L a> 2.0, more preferably, t B / t L is> 2.2.
[0030]
　Threaded joint may be a coupling type in integral type.
[0031]
　Nose, the fastening front and circumferentially, may have a 0.2% offset yield strength of 1.05 times or more the 0.2% offset yield strength of the steel pipe pile body.
[0032]
　As a specific method of increasing the 0.2% offset yield strength, or subjected to cold drawing the tube end prior to thread cutting, there is a method or perform reinforced hardened by high-frequency heating or the like.
[0033]
　At least one of the external thread and the internal thread, at least one of the pin seal surface and the box sealing surface, and on at least one of Pinshoruda surface and the box shoulder surface, chemical treatment, sandblasting, at least one may be subjected in the metal plating.
[0034]
　[Embodiment]
　Hereinafter will be described with reference to the accompanying drawings, embodiments of the tubular threaded joint. Given the same reference and the corresponding configuration in the figure does not repeat the same description.
[0035]
　Figure 1 is a longitudinal sectional view showing a schematic configuration of a tubular threaded joint 1 according to the embodiment. As shown in FIG. 1, a threaded joint 1 is a threaded joint for connecting the steel pipe pile body 2 to each other. Threaded joint 1 is provided with a pin 10 of the tubular, a tubular box 20. Box 20, the pin 10 is fastened to the pin 10 is inserted. Steel pipe body 2 of the pin 10 is a portion which is not inserted in the steel pipe includes a pin 10 in the box 20. Steel body 2 has an outside diameter of more than 7 inches, more than 0.7 inches of wall thickness. That is, the threaded joint 1 is used to steel pipe diameter and thick-walled.
[0036]
　Threaded joint 1 shown in FIG. 1 is a coupling type, comprising a coupling 3. Coupling 3 connects the two steel pipes. Coupling 3 comprises two boxes 20.
[0037]
　However, a threaded joint 1 may be a integral type. As shown in FIG. 2, comprises a threaded joint 1 integral type is also a threaded joint for connecting the steel pipe pile body 2 to each other, the pin 10, and a box 20. In integral type threaded joint 1, comprising one of the steel pipe is a pin 10, the other steel pipe comprises a box 20.
[0038]
　As shown in FIG. 3, the pin 10 includes a nose 11, a Pinshoruda surface 12, and Pintepa surface 15, the pin round surface 16, a male screw 13, and a pin seal surface 14. Nose 11 is formed on the tip portion of the pin 10 has a smaller outer diameter than the inner diameter of the opposing portion of the box 20 in the engaged state. Pinshoruda surface 12 is formed at the tip of the nose 11. Pintepa surface 15 is formed on the outer peripheral surface of the nose 11. Pin round surface 16 is formed at the corner between the Pinshoruda surface 12 and Pintepa surface 15. External thread 13 is formed on the outer periphery of the pin 10. Pin seal surface 14 is formed on the outer peripheral surface of the pin 10 a between the nose 11 and the external thread 13.
[0039]
　Box 20 includes a box shoulder surface 21, a female screw 22, the box sealing surface 23, a box taper surface 24, and a box round surface 25. Box shoulder surface 21 is opposed to Pinshoruda surface 12 contacts the Pinshoruda surface 12 in the engaged state. Female thread 22 corresponds to the external thread 13, formed on the inner periphery of the box 20. Box sealing surface 23 is opposed to the pin seal surface 14 in contact with the pin seal surface 14 during tightening. Box tapered surface 24 is opposed to Pintepa surface 15. Box round surface 25 is opposed to the pin round surface 16.
[0040]
　The gap amount of time of fastening between the outer and inner surfaces of the box 20 of the pin 10 on the boundary line between the Pintepa surface 15 and the pin round surface 16 δ [mm], the outer diameter of the steel pipe pile body 2 D [ inch], the thickness of the steel pipe body 2 t [inch], the length of time of fastening in the tube axis direction CL L [mm], pin seal surface 14 is box sealing surface between the tip of the pivot point PP and the pin 10 23 position closest is the thickness t of the pin 10 in the pivot point PP to the tip of the pin 10 in the region in contact with the L [mm], the thickness of the box at the pivot point PP t B when the [mm] satisfies the following equation (1), t B / t L is> 1.4.
[0041]
　0.30＋min(a nt L＋b nD, n=1,2,3)＞δ＞min(a nt L＋b nD, n=1,2,3)　（１）
[0042]
　Here, min is a function that takes a minimum value of the three arguments in parentheses.
　a1, a2, a3, b1, b2, b3 are defined as follows.
　= A1 (L / T) × 0.0115 × (T L / T) -1.68778 -0.00247 × (T L / T) -2.02052
　B1 = (L / T) × 0.0076 × (T L / T) -0.65672 -0.00120 × (T L / T) -1.06817
　A2 = (L / T) × 0.00725 × (T L / T) -1.67341 -0.00157 × (T L / T) -1.93212
　B2 = 0
　A3 = - (L / T) × 0.00256 × (T L / T) -2.40620 Tasu 0.00372 × (T L / T) -2.19899
　b3=(L/t)×0.01986×(t L/t) -0.88615－0.01048×(t L/t) -1.00532
[0043]
　Since the steel pipe outer diameter D and thickness t of the main body 2 is not constant exactly throughout the steel body 2, using the average outer diameter as the outer diameter D, it may be used an average thickness as the wall thickness t. The average outer diameter are those at positions 1 inch from the box end face of the fastened joint, measured in two directions perpendicular to the outer diameter of the steel pipe body 2 before fastening on average. The average wall thickness in the same was concluded one inch position from the box end face of the joint, in which the thickness of the circumference eight fastening preceding steel body 2 (45 ° pitch) were measured and averaged.
[0044]
　In the nose 11 shown in FIG. 3, Pintepa surface 15 is formed substantially parallel to the box tapered surface 24. That is, the interval between Pintepa surface 15 and the box tapered surface 24 are substantially equally spaced. However, as shown in FIG. 4, the interval between Pintepa surface 15 and the box tapered surface 24 may be spread gradually toward the distal end of the nose 11. In this case, the region where pin seal surface 14 is in contact with the box sealing surface 23 when fastening the wider than the area of ​​the case shown in FIG. 3, it may be close to (line contact on longitudinal section) face contact. Therefore, the position in the tube axis direction CL of the pivot point PP may be different depending on the amount of warping deformation of the fastening force and the nose 11.
[0045]
　Having described embodiments, the present invention is not limited to the above embodiments and can be variously modified without departing from the spirit.
Example
[0046]
　To confirm the effect of the embodiment was conducted numerical simulation analysis by elasto-plastic finite element method.
[0047]
　
　The elastic-plastic finite element analysis, using a model of a threaded joint having the basic configuration shown in FIG. Common test conditions are as follows.
[0048]
　- steel dimensions: 14 inches, 100 # (outer diameter 355.6 mm, wall thickness 17.78 mm)
　, material: API (American Petroleum Institute) P110 steel standard (yield strength 862N / mm 2 , modulus 205KN / mm 2 , Poisson's ratio 0.3)
　in screw joints type: coupling type (outer diameter 386Mm)
　the shape and dimensions of the screw: a buttress-type trapezoidal thread, the load surface angle -3 °, the insertion surface angle 10 degrees, male thread height 1.978mm, screw pitch 6.35mm (4TPI), one of the screws tapered 1/16
　the size of the nose: L = 17Mm, T L = 10.3Mm
　-closing conditions: further 1/100 turn from the shoulder ring tightening
　· load
　condition: (1) pressure: steel pipe body yield strength of 100% for simple pressure load
　(2) external pressure: 100% yield strength of the steel pipe body (API5C3 standard 100%) Net external pressure load
[0049]
　Pressure is loaded on the inner peripheral surface of the pin and box, as well as the entire inner surface side from the contact portion of the metal seal (outer peripheral surface of the pin nose, the inner peripheral surface of the box corresponding to the pin nose, shoulder surfaces of the pin and box, etc.) .
[0050]
　External pressure, the outer circumferential surface of the pin and box, loaded on the outer surface side entire surface (such as a pin and box threaded surface) from the contact portion of the metal seal.
[0051]
　
　As shown in the following Table 1, after an analysis that simulates the fastening at the test conditions for the model 1-6, was calculated internal pressure tightness and external pressure sealability. Pressure or external pressure sealability is divided by the sealing contact force at the time of fastening the sealing contact force (seal contact force per unit circumferential length exactly) when the internal pressure or external pressure and loaded loading conditions described above (i.e. a reduction rate from the time of fastening of the seal contact force). Sealability is the ratio of the seal contact force under pressure applied load relative to the seal contact force at the time of fastening, good sealability larger value.
[0052]
[Table 1]

[0053]
　In this example, a 0.43mm> δ> 0.13mm. Therefore, the model 1 and 5 are examples, model 2-4 and 6 are comparative examples. Model 2 is the gap amount δ is an upper limit value (0.43 mm) or more. Model 3 and 6 is equal to or less than the gap amount δ is a lower limit value (0.13 mm). Model 4 is the thickness ratio t B / t L is the lower limit (1.4) or less.
[0054]
　The external pressure seal which is other than the model 4 shown in FIG. As it is apparent from this graph, external pressure sealability is remarkably changes the boundary of the lower limit value of the gap amount [delta]. That is, when the gap amount δ is less than or equal to the lower value, the external pressure sealability is greatly reduced. This nose 11 is a warped, its distal end is considered to hit the inner peripheral surface of the box 20.
[0055]
　For smaller model 4 than the lower limit wall thickness ratio, it is significantly lower pressure sealability. Model 2 has a model 1 and 5 equivalent pressure and external pressure sealability, the gap amount δ is not less than the upper limit, since the cutting amount increases, manufacturing cost increases.
[0056]
　These results threaded joint according to the example was found to be excellent in comparison with the threaded joint, the internal pressure tightness and external pressure tightness both according to the comparative example.
DESCRIPTION OF SYMBOLS
[0057]
1: a threaded joint for steel pipes
2: steel pipe body
10: Pin
11: Nose
12: Pinshoruda surface
13: external thread
14: pin seal surface
15: Pintepa surface
16: Pin Round surface
20: Box
21: Box shoulder surface
22: female thread
23: Box sealing surface
24: box tapered surface
25: box round surface

WE CLAIM

A threaded joint for connecting steel pipes body together with an outer diameter of more than 7 inches and 0.7 inches or more wall thickness,
　and the pin of the tubular,
　the tubular the pin is fastened to the pin is inserted box with the door,
　the pin is
　formed at the tip portion of the pin, and a nose having an outer diameter smaller than the inner diameter of the opposing portion of the box fastening state,
　Pinshoruda surface formed at the tip of the nose ,
　a Pintepa surfaces formed on the outer peripheral surface of the nose,
　and a pin round surface formed at the corner between the Pinshoruda surface and the Pintepa surface,
　a male thread formed on an outer periphery of the pin,
　and the nose the and a pin seal surface formed on the outer peripheral surface of the pin a between the male screw,
　the box,
　facing the Pinshoruda surface, contacting the Pinshoruda surface in engagement state And the box shoulder surface that,
　corresponding to the external thread, and the female screw formed on the inner periphery of the box,
　facing the pin seal surface, and a box seal surface that contacts the pin seal surface during fastening,
　[Delta] [mm] amount of clearance when the fastening between the outer surface and the inner surface of the box of the pin on the boundary line between the Pintepa surface and the pin round face, the outer diameter of the steel tube body D [ inch], the thickness of the steel pipe body t [inch], wherein the pin seal surface between the tip of the pin and the pivot point is a position closest to the tip of the pin in the region in contact with the box sealing surface L [mm] of the length of time of fastening in the tube axis direction, the wall thickness of the pin at the pivot point t L [mm], the thickness of the box at the pivot point t B when the (1),[mm], t B / t L is> 1.4, the threaded joint.
　+ Min 0.30 (a n t L + b n D, n = 1, 2, 3)> [delta]> min (a n t L + b n D, n = 1, 2, 3) (1)
　where, min in parentheses of a function that takes a minimum value of the three arguments,
　a1 = (L / t) × 0.0115 × (t L / t) -1.68778－0.00247×(t L/t) -2.02052、
　b1=(L/t)×0.0076×(t L/t) -0.65672－0.00120×(t L/t) -1.06817、
　a2=(L/t)×0.00725×(t L/t) -1.67341－0.00157×(t L/t) -1.93212、
　b2=0、
　a3=-(L/t)×0.00256×(t L/t) -2.40620＋0.00372×(t L/t) -2.19899、
　b3=(L/t)×0.01986×(t L/t) -0.88615× -0.01048 (T L / T) -1.00532 is.
[Requested item 2]
　A threaded joint according to claim
　1, t B / t L is> 2.0, the threaded joint.
[Requested item 3]
　A threaded joint according to claim 2,
　a coupling type, threaded fittings.
[Requested item 4]
　A threaded joint according to any one of claims 1 to 3,
　wherein the nose is in engagement before and circumferentially, of more than 1.05 times the 0.2% offset yield strength of the steel pipe body 0.2 % with offset yield strength, a threaded joint.
[Requested item 5]
　A threaded joint according to claim 1, any one of 4,
　at least one of said external thread and said internal thread, at least one of the pin seal surface and the box sealing surface, as well as of the Pinshoruda surface and the box shoulder surface at least one chemical conversion treatment, sandblasting, at least one of the metal plating process has been performed, a threaded joint.