Screw joint

Abstract

Provided is a screw joint capable of suppressing scratches and seizure caused by misalignment during fastening work while employing a doping-free technique. A screw joint (100) comprises a pin (10) including a tapered male screw part (11), and a box (20) including a tapered female screw part (21). The female screw part (21) has a screw taper that is constant over the entire length thereof. The male screw part (11) is divided into a complete screw part (11a) and an incomplete screw part (11b). The complete screw part (11a) has a screw taper equivalent to the screw taper of the female screw part (21). The incomplete screw part (11b) has a screw taper smaller than the screw taper of the complete screw part (11a). The inner peripheral surface of the box (20) is covered with a metal plating layer. The metal plating layer has a hardness higher than the hardness of the base material of the pin (10). The metal plating layer is covered with a solid lubrication film.

Description

TITLE OF INVENTION: THREADED CONNECTION TECHNICAL FIELD

[0001] The present disclosure relates to a threaded connection that connects steel pipes with each other. BACKGROUND ART

[0002] In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as "oil well"), a large number of steel pipes referred to as oil country tubular goods are used to mine underground resources. These steel pipes are connected with each other with threaded connections.

[0003] The types of threaded connections for steel pipes are broadly classified into a coupling type and an integral type. In the case of the coupling type, steel pipes are connected with each other via couplings, which are parts having a pipe shape separately from the steel pipes. Specifically, each steel pipe includes tapered male thread parts at both end portions thereof, and each coupling includes tapered female thread parts at both end portions thereof. The tapered male thread part of the steel pipe is screwed into and made up to each tapered female thread part of the coupling, so that steel pipes are connected with each other. In this specification, the tapered male thread part is also simply referred to as "male thread part", and the tapered female thread part is also simply referred to as "female thread part". The tapered male thread part and the tapered female thread part are also collectively referred to as "thread part".

[0004] In the case of the integral type, steel pipes are directly connected with each other. Specifically, each steel pipe includes a male thread part at one end portion thereof, and includes a female thread part at the other end portion thereof. The male thread part of one steel pipe is screwed into and made up to the female thread part of the other steel pipe, so that the steel pipes are connected with each other.

[0005] In general, the end portion of a steel pipe that includes a male thread part is referred to as "pin", and the end portion of a steel pipe or a coupling that includes a female thread part is referred to as "box". Making a description with these terms, a pin is made up to a box by inserting and screwing the pin into the box.

[0006] A threaded connection for a steel pipe is required to prevent galling in the course of a making up operation and to ensure sealing performance after the made up and during use. Due to such requirements, in general, a grease is applied to the surface of a box and / or pin on an oil well site (platform) before the making up operation is executed. This grease is a lubricant specialized for threaded connections, and is referred to as "dope". Conventionally, the dope contains a heavy metal powder (compound).

[0007] In recent years, in considerations of improvements in the efficiency of the making up operation, reduction of environmental impact, and the health of operators, a technique that uses a lubricant instead of a conventionally-used dope, that is, a dope-free technique, has been promoted. In the dope-free technique, a polymerbased lubricating coating is usually formed on the surface of a box and / or pin. This lubricating coating is a solid or semi-solid lubricant, and is referred to as "solid lubricating coating".

[0008] For example, Japanese Patent Application Publication No. 2002-349775 (Patent Literature 1) describes a tapered threaded connection in which a lubricating coating layer (solid lubricating coating or viscous liquid lubricating coating) is formed on the surface of at least one of a pin and a box. The problem of this Patent Literature 1 is to prevent galling that occurs at a thread part in the course of a making up operation. In order to solve this problem, Patent Literature 1 proposes a first tapered threaded connection and a second tapered threaded connection.

[0009] In the case of the first tapered threaded connection, a female thread part is divided into a first tapered portion and a second tapered portion. In this female thread part, the second tapered portion has a taper larger than the first tapered portion, and is formed by an imperfect thread part. At least a part of the second tapered portion faces and is joined to an imperfect thread part of a male thread part. In contrast, in the case of the second tapered threaded connection, the male thread part is divided into a third tapered portion and a fourth tapered portion. In this male thread part, the fourth tapered portion has a taper smaller than the third tapered portion, and is formed by an imperfect thread part. At least a part of the fourth tapered portion faces and is joined to a perfect thread part of a female thread part. Patent Literature 1 describes that, since unnecessary interference does not occur in both the first and second tapered threaded connections, a lubricating coating layer formed on the imperfect thread part of the male thread part does not peel off, or a lubricating coating layer formed on the second tapered portion of the female thread part does not peel off, and thus galling does not occur. CITATION LIST PATENT LITERATURE

[0010] Patent Literature 1: Japanese Patent Application Publication No. 2002-349775 SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0011] With the spread of the dope-free technique, in an automation rig, a making up operation may be performed only by machine without any human intervention. Additionally, in an unequipped rig, a making up operation may proceed with the alignment (alignment between the axial centerline of a pin and the axial centerline of a box) of the pin to the box being insufficiently adjusted. These making up operations are not envisioned by manufacturers of threaded connections, and the following problems may occur.

[0012] In an initial stage of a making up operation, a pin may be inserted at an angle to a box, and the pin may not be aligned to the box. When the making up operation advances in such a misalignment state, a solid lubricating coating of a thread part is locally destroyed, and scratches occur in the thread part. When the making up operation is continued with the scratches, the scratches may cause galling. Especially in recent years, in many cases, a jointed pipe formed by connecting two or three steel pipes with each other in advance, and a pin of this jointed pipe is inserted into a box. In this case, when an apparatus (compensator) for canceling the selfweight of the jointed pipe is not well adjusted, the contact force to the thread part becomes excessive due to the self-weight of the jointed pipe, and even slight misalignment can easily cause scratches.

[0013] Upon investigation by inventors of the present disclosure, the following actual conditions have been confirmed. Scratches and galling due to misalignment at the time of a making up operation often occur in an imperfect thread part of a male thread part, and in a portion of a female thread part corresponding to the imperfect thread part of the male thread part. Particularly, a portion where scratches and galling occur in the female thread part is a stabbing flank. Further, if it is a threaded connection of the same design, galling is more likely to occur when a solid lubricating coating is used than when a dope is used. However, Patent Literature 1 does not focus on scratches and galling due to misalignment.

[0014] It is an objective of the present disclosure to provide a threaded connection that can suppress scratches and galling due to misalignment at the time of a making up operation, while adopting a dope-free technique. SOLUTION TO PROBLEM

[0015] A threaded connection according to the present disclosure is a threaded connection that connects steel pipes with each other. The threaded connection includes a pin having a pipe shape and a box having a pipe shape. The pin is provided continuously to a steel pipe body. The box is made up to the pin with insertion of the pin into the box. The pin includes a tapered male thread part. The tapered male thread part is provided to an outer peripheral surface of the pin. The box includes a tapered female thread part. The tapered female thread part is provided to an inner peripheral surface of the box in such a way as to correspond to the tapered male thread part. The tapered female thread part has a constant thread taper over the entire length of the tapered female thread part. The tapered male thread part is divided into a perfect thread part and an imperfect thread part. The perfect thread part has a thread taper equal to a thread taper of the tapered female thread part. The imperfect thread part has a thread taper smaller than the thread taper of the perfect thread part. The inner peripheral surface of the box is covered by a metal plating layer. The metal plating layer has a hardness higher than the hardness of the base metal of the pin. The metal plating layer is covered by a solid lubricating coating. ADVANTAGEOUS EFFECTS OF INVENTION

[0016] According to the threaded connection according to the present disclosure, it is possible to suppress scratches and galling due to misalignment at the time of a making up operation, while adopting a dope-free technique. BRIEF DESCRIPTION OF DRAWINGS

[0017] [FIG. 1] FIG. 1 is a longitudinal cross-sectional view of a threaded connection according to a first embodiment. [FIG. 2] FIG. 2 is a longitudinal cross-sectional view of a pin and a box in the threaded connection shown in FIG. 1. [FIG. 3] FIG. 3 is a partial enlarged view of a male thread part of the pin in the threaded connection shown in FIG. 2. [FIG. 4] FIG. 4 is a partial enlarged view of a female thread part of the box in the threaded connection shown in FIG. 2. [FIG. 5] FIG. 5 is a schematic diagram showing a state in which a making up operation advances in a misalignment state. [FIG. 6] FIG. 6 is a partial enlarged view of a female thread part of a box in a threaded connection according to a second embodiment. [FIG. 7] FIG. 7 is a partial enlarged view of a male thread part of a pin in a threaded connection according to the third embodiment. [FIG. 8] FIG. 8 is a partial enlarged view of a male thread part of a pin in a threaded connection according to a fourth embodiment. [FIG. 9] FIG. 9 is a partial enlarged view of a female thread part of a box in a threaded connection according to a fourth embodiment. DESCRIPTION OF EMBODIMENTS

[0018] To solve the above-mentioned problems, the inventors of the present disclosure have investigated the condition of galling that occurs in a thread part at the time of making up of a pin to a box in a misalignment state and the cause of occurrence of galling for a threaded connection that adopts a dope-free technique, that is, a threaded connection that includes a solid lubricating coating. In this investigation, a threaded connection of a coupling type was used as a threaded connection. The threaded connection of the coupling type usually adopts a design in which a thread ridge on a steel pipe body side of a tapered male thread part gradually disappears as it approaches a steel pipe body, so as to exhibit tensile performance substantially equivalent to the steel pipe body. In this case, in a portion closer to a steel pipe body of the tapered male thread part, the top of the thread ridge is an imperfect thread part that has a shape cut out by a cylindrical surface having a diameter that is equal to the outer diameter of the steel pipe body, or that is slightly smaller than the outer diameter of the steel pipe body. This imperfect thread part is continuous with a perfect thread part, and its area is relatively long. The thread height of the imperfect thread part is lower than the thread height of the perfect thread part. In this specification, a perfect thread part in a male thread part is also referred to as "perfect male thread part", and an imperfect thread part in a male thread part is also referred to as "imperfect male thread part". As a result of the investigation, the inventors of the present disclosure have obtained the following findings.

[0019] As for the occurrence position of galling, in the pin, galling occurred in the area of the imperfect male thread part. In the box, when a threaded connection is viewed in a longitudinal cross-sectional view, galling occurred in an area about 5 to 9 threads inner deep from a pipe end of the box of the female thread part. Particularly, in the female thread part, the portion where galling occurred was a stabbing flank (female thread stabbing flank) of the area corresponding to the imperfect male thread part.

[0020] In a sample of the threaded connection in which relatively minor galling occurred, scratches were confirmed in the female thread stabbing flank of the box. These scratches were concentrated in an area around half the circumference of the axial centerline of the box of the female thread stabbing flank. Based on the conditions, the cause of occurrence of galling is presumed to be as follows.

[0021] In the imperfect male thread part, as described above, the top of the thread ridge has a shape cut out by a cylindrical surface having a diameter that is equal to the outer diameter of the steel pipe body, or that is slightly smaller than the outer diameter of the steel pipe body. Therefore, in the imperfect male thread part, an outer peripheral edge of a stabbing flank (male thread stabbing flank) is a sharp edge. This sharp edge is inevitably formed by a threading process of the male thread part. In this specification, the male thread stabbing flank in the imperfect male thread part is also referred to as "imperfect male thread stabbing flank".

[0022] When a making up operation advances in a state in which the pin is in the misalignment state with respect to the box, the outer peripheral edge of the imperfect male thread stabbing flank slides in uneven contact with the female thread stabbing flank. Due to such uneven contact and sliding, scratches occur in the female thread stabbing flank corresponding to the imperfect male thread part and as a result, galling occurs.

[0023] Here, a comparison was made between a case in which a solid lubricating coating was used and a case in which a dope was used in threaded connections of the same design. In the case where the dope is used, although scratches may occur, the scratches hardly develop into galling. Galling clearly more likely occur in the case where the solid lubricating coating is used.

[0024] Specifically, in a case of a threaded connection that includes a dope on a surface of a female thread part, even when the dope is discharged by a sharp outer peripheral edge of an imperfect male thread stabbing flank, the dope around a location from which the dope has been discharged immediately flows into the location. Accordingly, self-repairing of a lubricating layer formed by the dope is successively performed.

[0025] On the other hand, in a case of a threaded connection that includes a solid lubricating coating on a surface of a female thread part, the solid lubricating coating is dug up by a sharp outer peripheral edge of an imperfect male thread stabbing flank. In this case, since the coating is a solid, the location where the solid lubricating coating has been dug up is not repaired, resulting in a state in which the coating is lost. At the location where the coating is lost, the outer peripheral edge of the imperfect male thread stabbing flank easily reaches a surface of a base in the female thread part, scratches occur, and homogeneous metals are in the state of metal-to-metal contact with each other. Due to this metal-to-metal contact, digging and adhesion occur even in the base, and galling occurs at an earlier stage than in the case where the dope is used.

[0026] Additionally, in a case of a threaded connection that includes a solid lubricating coating on a surface of a male thread part, the solid lubricating coating covering an outer peripheral edge of an imperfect male thread stabbing flank slides in uneven contact with a female thread stabbing flank at a very high contact pressure, and is worn away. Since the solid lubricating coating that has been worn away is not repaired, a base metal is exposed at the outer peripheral edge of the imperfect male thread stabbing flank. As a result, the exposed base metal and the female thread stabbing flank are in a state of metal-to-metal contact of homogeneous metals. Due to this metal-to-metal contact, digging and adhesion occur in the base metal, and galling occurs at an early stage.

[0027] In short, in the case of the threaded connection that includes the solid lubricating coating, the solid lubricating coating hardly performs self-repairing during sliding. Therefore, in the solid lubricating coating, once local damages, for example, digging such as scratches or wearing, occur, it will be impossible to recover the damages. In this case, the damages easily develop into galling. Accordingly, galling occurs in the imperfect male thread part, or in the female thread stabbing flank of the female thread part corresponding to the imperfect male thread part.

[0028] Based on such an occurrence mechanism of scratches and galling due to misalignment at the time of a making up operation, the inventors of the present disclosure have further conducted extensive studies.

[0029] First, the inventors of the present disclosure have conducted studies on providing, as a base of the solid lubricating coating, a metal plating layer that is harder than the base metal, and that is less likely to adhere to the base metal, so that when the making up operation advances in the misalignment state, the occurrence of galling can be suppressed even if the outer peripheral edge of the imperfect male thread stabbing flank and the female thread stabbing flank happen to be in metal-to-metal contact. However, when a hard metal plating layer is provided on the pin, that is, a surface of the male thread part, the outer peripheral edge of the imperfect male thread stabbing flank is reinforced. In this case, irrespective of whether or not the box, that is, a surface of the female thread part, is covered by the solid lubricating coating, the reinforced outer peripheral edge of the imperfect male thread stabbing flank slides in contact with the female thread stabbing flank, which encourages the occurrence of scratches.

[0030] Accordingly, it is sufficient that a metal plating layer be provided on the box, that is, the surface of the female thread part, so that the female thread stabbing flank is reinforced than the outer peripheral edge of the imperfect male thread stabbing flank. In this case, due to the metal plating layer provided on the surface of the female thread part, the outer peripheral edge of the imperfect male thread stabbing flank is blunted by sliding contact with the metal plating layer provided on the female thread stabbing flank, the contact pressure between the both is relieved, and further, digging of the solid lubricating coating is also suppressed. Therefore, scratches are less likely to occur in the female thread stabbing flank. Even if the solid lubricating coating is worn away, and the outer peripheral edge of the imperfect male thread stabbing flank and the female thread stabbing flank are in metal-to-metal contact, since they are dissimilar metals, adhesion is less likely to occur, and galling is less likely to occur.

[0031] As for an installation target of the solid lubricating coating, even if the solid lubricating coating is provided on the surface of the male thread part, as described above, the solid lubricating coating covering the outer peripheral edge of the imperfect male thread stabbing flank is worn away at an early stage. Therefore, it is sufficient that the solid lubricating coating be provided on the box, that is, the surface of the female thread part. That is, it is sufficient that the solid lubricating coating be provided on the metal plating layer provided on the female thread part.

[0032] In this manner, when the solid lubricating coating and the metal plating layer were provided on the surface of the female thread part, the occurrence frequency of galling between the outer peripheral edge of the imperfect male thread stabbing flank and the female thread stabbing flank was reduced.

[0033] The inventors of the present disclosure have conducted studies on further improvement. Specifically, the inventors of the present disclosure considered that, when the making up operation advanced in the misalignment state, if it is possible to prevent the outer peripheral edge of the imperfect male thread stabbing flank from contacting the female thread stabbing flank when the pin is rotated about the axial centerline with respect to the box, or to sufficiently reduce the contact pressure even if the outer peripheral edge contacted the female thread stabbing flank, the occurrence of scratches could be suppressed and, as a result, the occurrence of galling could be further suppressed. Therefore, assuming that the thread taper of the female thread part is constant over the entire length of the female thread part, it is sufficient that the thread taper of the perfect male thread part of the thread taper of the male thread part be equal to the thread taper of the female thread part, and the thread taper of the imperfect male thread part be smaller than the thread taper of the female thread part, that is, the thread taper of the perfect male thread part. That is, it is sufficient that the male thread part has a so-called two-stage taper.

[0034] When the male thread part has a two-stage taper, even if the axial centerline of the pin is inclined with respect to the axial centerline of the box in the misalignment state at the time of the making up operation, the outer peripheral edge of the imperfect male thread stabbing flank makes uneven contact with the female thread stabbing flank in a wide range in a pipe axis direction of the threaded connection. That is, in a longitudinal cross section of the threaded connection, stabbing flanks of a plurality of thread ridges of the imperfect male thread part make uneven contact with corresponding female thread stabbing flanks of a plurality of thread ridges of the female thread part. Accordingly, in a longitudinal cross section of the threaded connection, the contact force due to the uneven contact is borne by the plurality of thread ridges and, as a result, the contact pressure is relieved. Since the contact pressure is relieved, the occurrence of scratches is suppressed.

[0035] A threaded connection according to the embodiments of the present disclosure is completed based on the above-mentioned findings.

[0036] A threaded connection according to the present embodiment is a threaded connection that connects steel pipes with each other. The threaded connection includes a pin having a pipe shape and a box having a pipe shape. The pin is provided continuously to a steel pipe body. The box is made up to the pin with insertion of the pin into the box. The pin includes a tapered male thread part. The tapered male thread part is provided to an outer peripheral surface of the pin. The box includes a tapered female thread part. The tapered female thread part is provided to an inner peripheral surface of the box in such a way as to correspond to the tapered male thread part. The tapered female thread part has a constant thread taper over the entire length of the tapered female thread part. The tapered male thread part is divided into a perfect thread part and an imperfect thread part. The perfect thread part has a thread taper equal to a thread taper of the tapered female thread part. The imperfect thread part has a thread taper smaller than the thread taper of the perfect thread part. The inner peripheral surface of the box is covered by a metal plating layer. The metal plating layer has a hardness higher than the hardness of the base metal of the pin. The metal plating layer is covered by a solid lubricating coating (a first configuration).

[0037] In the threaded connection according to the first configuration, the inner peripheral surface of the box to which the female thread part is provided is covered by the metal plating layer, and this metal plating layer is covered by the solid lubricating coating. The metal plating layer is harder than the base metal of the pin. In this case, the female thread part of the box, that is, the female thread stabbing flank, is reinforced by the metal plating layer more than the male thread part of the pin. Therefore, when the making up operation advances in the misalignment state, even if the outer peripheral edge of the imperfect male thread stabbing flank and the female thread stabbing flank happen to be in metal-to-metal contact, excessive scratches are unlikely to occur in the female thread stabbing flank. Further, in the threaded connection according to the first configuration, the thread taper of the imperfect male thread part is smaller than the thread taper of the perfect male thread part, in other words, the thread taper of the female thread part. In this case, when the making up operation advances in the misalignment state, the outer peripheral edge of the imperfect male thread stabbing flank unevenly contacts the female thread stabbing flank in a wide range in the pipe axis direction of the box. Therefore, even if the outer peripheral edge of the imperfect male thread part contacts the female thread stabbing flank, the contact pressure can be relieved, and the occurrence of scratches is further suppressed. Therefore, according to the threaded connection according to the first configuration, it is possible to suppress scratches and galling due to misalignment at the time of the making up operation, while adopting a dope-free technique.

[0038] In the above-described threaded connection, the tapered female thread part includes a female thread crest, a female thread root, a female thread stabbing flank, and a female thread load flank. In this case, it is preferable that a chamfered surface for coupling the female thread stabbing flank to the female thread crest is provided to the tapered female thread part (a second configuration).

[0039] In the threaded connection according to the second configuration, when the making up operation advances in the misalignment state, the outer peripheral edge of the stabbing flank in the imperfect male thread part is more likely to contact the chamfered surface than the female thread stabbing flank in the female thread part. When the outer peripheral edge of the imperfect male thread stabbing flank contacts the chamfered surface of the female thread part, the contact angle between the outer peripheral edge and chamfered surface becomes large. This contact angle is the angle formed by the direction of the pressing force that acts on the outer peripheral edge of the imperfect male thread stabbing flank (that is, the self-weight of the steel pipe that has the pin), and the normal direction of the chamfered surface. When the contact angle is increased, the contact force that occurs in the chamfered surface of the female thread part due to the self-weight of the steel pipe that has the pin is decreased. The contact force that occurs in the chamfered surface corresponds to a component force of the self-weight of the steel pipe, and the direction of this component force is the normal direction of the chamfered surface. Therefore, also in an initial stage of sliding in which blunting of the outer peripheral edge of the imperfect male thread stabbing flank has not occurred, it is possible to further relieve the contact pressure between the outer peripheral edge of the imperfect male thread stabbing flank and the female thread stabbing flank.

[0040] In the above-described threaded connection, the tapered female thread part has a constant thread pitch over the entire length of the tapered female thread part. In this case, in the tapered male thread part, it is preferable that the thread pitch of the perfect thread part is equal to the thread pitch of the tapered female thread part, and the thread pitch of the imperfect thread part is larger than the thread pitch of the perfect thread part (a third configuration).

[0041] In the threaded connection according to the third configuration, the thread pitch of the male thread stabbing flank of the imperfect male thread part is larger than the thread pitch of the perfect male thread part, in other words, the thread pitch of the female thread part. In this case, the interval between the male thread stabbing flank and the female thread stabbing flank in the pipe axis direction of the threaded connection is larger in the area of the imperfect male thread part than in the area of the perfect male thread part. Therefore, it is possible to relieve the contact pressure between the outer peripheral edge of the imperfect male thread stabbing flank and the female thread stabbing flank.

[0042] In the above-described threaded connection, it is preferable that the metal plating layer is an alloy plating layer (a fourth configuration). For example, a Zn alloy plating layer can be applied as the alloy plating layer.

[0043] In the above-described threaded connection, it is preferable that the pin is provided to each of the steel pipes that are to be connected with each other, and the box is provided to a coupling that is a part having a pipe shape separately from the steel pipes (a fifth configuration). In this case, the threaded connection is of a coupling type.

[0044] Hereinafter, embodiments of the present disclosure will be described with reference to drawings. In the respective drawings, the same or identical components are given the same reference symbols, and the repeated description will be omitted.

[0045] <First embodiment> [Configuration of threaded connection] FIG. 1 is a longitudinal cross-sectional view of a threaded connection 100 according to a first embodiment. The longitudinal cross section of the threaded connection 100 is a cross section of the threaded connection 100 taken along a plane that includes a pipe axis X. In this specification, a direction in which the pipe axis X of the threaded connection 100 extends may be referred to as "pipe axis direction". The radial direction of the threaded connection 100, that is, a direction perpendicular to the pipe axis direction may be simply referred to as "radial direction".

[0046] Referring to FIG. 1, the threaded connection 100 is used to connect steel pipes 30 with each other. The threaded connection 100 includes a pin 10 having a pipe shape and a box 20 having a pipe shape. The pin 10 has a pipe axis X1 for itself. The box 20 has a pipe axis X2 for itself. By design, the pipe axis X1 of the pin 10 is aligned with the pipe axis X2 of the box 20, and the pipe axes X1 and X2 are aligned with the pipe axis X of the threaded connection 100.

[0047] The pin 10 is provided at an end portion of the steel pipe 30. In the example of the present embodiment, the box 20 is provided to a coupling 40 that is a part having a pipe shape separately from the steel pipe 30, which is a connection target. In this case, the threaded connection 100 is of a coupling type. Examples of the material of the steel pipe 30 (strictly speaking, the material of the base metal of the steel pipe 30) include low chromium alloy martensitic steel, martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, and duplex stainless steel. The material of the coupling 40 (strictly speaking, the material of the base metal of the coupling 40) may be the same as the material of the steel pipe 30. The tensile strength and the hardness may be equivalent or different between the coupling 40 and the steel pipe 30. Further, the material of the coupling 40 may be different from the material of the steel pipe 30.

[0048] The pin 10 is inserted into and made up to the box 20. The pin 10 is provided continuously to a body 31 of the steel pipe 30. The pin 10 includes a tapered male thread part 11. In the example of the present embodiment, the pin 10 further includes a pin sealing surface 12 and a pin shoulder surface 13. The male thread part 11, the pin sealing surface 12, and the pin shoulder surface 13 are arranged in this order from the steel pipe body 31 side toward the front end side of the pin 10.

[0049] The tapered male thread part 11 is provided to the outer peripheral surface of the pin 10. The male thread part 11 is disposed at a position closer to the steel pipe body 31 side than the pin sealing surface 12. The male thread part 11 extends from the vicinity of the pin sealing surface 12 to the vicinity of the steel pipe body 31. The male thread part 11 is formed of a tapered thread. Examples of the kind of thread forming the male thread part 11 include an API-standard buttress thread (trapezoidal-shaped thread) and a square thread.

[0050] The pin sealing surface 12 is provided to the outer peripheral surface of the pin 10 at a position between the male thread part 11 and the pin shoulder surface 13. The pin sealing surface 12 is the outside surface of the side of a rotary body that uses the pipe axis X1, that is, the pipe axis X of the threaded connection 100, as the axial centerline. When the threaded connection 100 is viewed in a longitudinal crosssectional view, the pin sealing surface 12 may be a curved line that protrudes toward the box 20, or may be a straight line that is inclined with respect to the pipe axis X in such a way as to approach the pipe axis X as the pin sealing surface 12 extends toward the front end side of the pin 10. The pin sealing surface 12 may be formed by combining two or more kinds of such a curved line and / or straight line.

[0051] The pin shoulder surface 13 is provided at the front end of the pin 10. The pin shoulder surface 13 is an annular face that forms the pipe end face of the pin 10 and that uses the pipe axis X1, that is, the pipe axis X of the threaded connection 100, as the axial centerline. The pin shoulder surface 13 may be or may not be inclined such that the outer circumferential edge of the pin shoulder surface 13 is located at a position closer to the front end side of the pin 10 than the inner circumferential edge of the pin shoulder surface 13.

[0052] The box 20 includes a tapered female thread part 21. In the example of the present embodiment, the box 20 further includes a box sealing surface 22 and a box shoulder surface 23. The female thread part 21, the box sealing surface 22, and the box shoulder surface 23 are provided to the box 20 in such a way as to respectively correspond to the male thread part 11, the pin sealing surface 12, and the pin shoulder surface 13.

[0053] The tapered female thread part 21 is provided to the inner peripheral surface of the box 20. The female thread part 21 is disposed at a position closer to the pipe end side of the box 20 than the box sealing surface 22. The female thread part 21 extends from the vicinity of the box sealing surface 22 to the vicinity of the pipe end of the box 20. The female thread part 21 is formed of a tapered thread that engages with the male thread part 11 of the pin 10. To allow the female thread part 21 to correspond to the male thread part 11, examples of the kind of thread forming the female thread part 21 include an API-standard buttress thread (trapezoidal-shaped thread) and a square thread.

[0054] The box sealing surface 22 is provided to the inner peripheral surface of the box 20 at a position between the female thread part 21 and the box shoulder surface 23. The box sealing surface 22 is an inside surface of the side of a rotary body that uses the pipe axis X2, that is, the pipe axis X of the threaded connection 100, as the axial centerline. When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the box sealing surface 22 may be a curved line that protrudes toward the pin 10, or may be a straight line that is inclined with respect to the pipe axis X in such a way as to approach the pipe axis X as the box sealing surface 22 extends toward the inner depth side of the box 20. The box sealing surface 22 may be formed by combining two or more kinds of such a curved line and / or straight line.

[0055] The box shoulder surface 23 is provided at the inner depth portion of the box 20 in such a way as to correspond to the pin shoulder surface 13. In the same manner as the pin shoulder surface 13, the box shoulder surface 23 is an annular face that uses the pipe axis X2, that is, the pipe axis X of the threaded connection 100, as the axial centerline. The box shoulder surface 23 may be or may not be inclined such that the outer circumferential edge of the box shoulder surface 23 is located at a position closer to the inner depth side of the box 20 than the inner circumferential edge of the box shoulder surface 23. The inclination angle (shoulder angle) of the box shoulder surface 23 is substantially equal to the inclination angle of the pin shoulder surface 13.

[0056] Hereinafter, the configuration of the threaded connection 100 according to the present embodiment will be described in more detail with reference to FIG. 1 to FIG. 4. FIG. 2 is a longitudinal cross-sectional view of the pin 10 and the box 20 in the threaded connection 100 shown in FIG. 1. FIG. 2 separately shows the pin 10 and the box 20 in a state in which the positions of the pipe axes X, X1, and X2 directions are aligned, so that the respective configurations of the pin 10 and the box 20 can be easily understood.

[0057] Referring to FIG. 2, the tapered male thread part 11 includes a male thread crest 111, a male thread root 112, a male thread stabbing flank 113, and a male thread load flank 114. The tapered female thread part 21 includes a female thread root 211, a female thread crest 212, a female thread stabbing flank 213, and a female thread load flank 214. The female thread root 211, the female thread crest 212, the female thread stabbing flank 213, and the female thread load flank 214 are provided to the box 20 in such a way as to respectively correspond to the male thread crest 111, the male thread root 112, the male thread stabbing flank 113, and the male thread load flank 114.

[0058] In the male thread part 11, the male thread crest 111 is coupled to the male thread root 112 by the male thread stabbing flank 113 and the male thread load flank 114. The male thread stabbing flank 113 is located at a position frontward of the male thread crest 111 in the direction in which screwing of the pin 10 into the box 20 advances during making up. The male thread load flank 114 is located at a position rearward of the male thread crest 111 in the direction in which screwing of the pin 10 advances.

[0059] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread crest 111 and the male thread root 112 are straight lines inclined with respect to the pipe axis X1 in such a way as to approach the pipe axis X1 as the male thread crest 111 and the male thread root 112 extend toward the front end side of the pin 10.

[0060] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread stabbing flank 113 is a straight line that is inclined such that the outer circumferential edge of the male thread stabbing flank 113 is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the male thread stabbing flank 113.

[0061] In the same manner as the male thread stabbing flank 113, when the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread load flank 114 is a straight line that is inclined such that the outer circumferential edge of the male thread load flank 114 is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the male thread load flank 114. When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread load flank 114 may be a straight line that is inclined such that the outer circumferential edge of the male thread load flank 114 is located at a position closer to the front end side of the pin 10 than the inner circumferential edge of the male thread load flank 114, or may be a straight line perpendicular to the pipe axis X1.

[0062] In the female thread part 21, the female thread root 211 and the female thread crest 212 are coupled to each other by the female thread stabbing flank 213 and the female thread load flank 214. The female thread stabbing flank 213 is located at a position frontward of the female thread root 211 in the direction in which screwing of the pin 10 advances during making up. The female thread load flank 214 is located at a position rearward of the female thread root 211 in the direction in which screwing of the pin 10 advances.

[0063] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread root 211 and the female thread crest 212 are straight lines inclined with respect to the pipe axis X2 in such a way as to approach the pipe axis X2 as the female thread root 211 and the female thread crest 212 extend toward the inner depth side of the box 20. The inclination angles of the female thread root 211 and the female thread crest 212 with respect to the pipe axis X2 are substantially equal to the inclination angles of the male thread crest 111 and the male thread root 112 with respect to the pipe axis X1. In short, the female thread root 211 and the female thread crest 212 are parallel to the male thread crest 111 and the male thread root 112, respectively.

[0064] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread stabbing flank 213 is a straight line that is inclined such that the outer circumferential edge of the female thread stabbing flank 213 is located at a position closer to the pipe end side of the box 20 than the inner circumferential edge of the female thread stabbing flank 213. The inclination angle of the female thread stabbing flank 213 is substantially equal to the inclination angle of the male thread stabbing flank 113. In short, the female thread stabbing flank 213 is parallel to the male thread stabbing flank 113.

[0065] In the same manner as the female thread stabbing flank 213, when the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread load flank 214 is a straight line that is inclined such that the outer circumferential edge of the female thread load flank 214 is located at a position closer to the pipe end side of the box 20 than the inner circumferential edge of the female thread load flank 214. When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread load flank 214 may be a straight line that is inclined such that the outer circumferential edge of the female thread load flank 214 is located at a position closer to the inner depth side of the box 20 than the inner circumferential edge of the female thread load flank 214. The inclination angle of the female thread load flank 214 is substantially equal to the inclination angle of the male thread load flank 114. In the same manner as the male thread load flank 114, when the threaded connection 100 is viewed in a longitudinal crosssectional view, the female thread load flank 214 may be a straight line perpendicular to the pipe axis X2. In short, the female thread load flank 214 is parallel to the male thread load flank 114.

[0066] In this specification, it is described that each face that forms the thread part (the male thread part 11 or the female thread part 21) is a straight line when the threaded connection 100 is viewed in a cross-sectional view. This means that the main area of each face of the thread part is a straight line when the threaded connection 100 is viewed in a cross-sectional view. That is, a thread part in which an arc portion or a secondary chamfered portion is provided to a coupling portion between adjacent faces is also allowed as the thread part of the threaded connection 100.

[0067] Specifically, when the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread stabbing flank 113 may be coupled to the male thread crest 111 via an arc portion, and may be coupled to the male thread root 112 via an arc portion. When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread load flank 114 may be coupled to the male thread crest 111 via an arc portion, and may be coupled to the male thread root 112 via an arc portion.

[0068] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread stabbing flank 213 may be coupled to the female thread root 211 via an arc portion, and may be coupled to the female thread crest 212 via an arc portion. When the threaded connection 100 is viewed in a longitudinal crosssectional view, the female thread load flank 214 may be coupled to the female thread root 211 via an arc portion, and may be coupled to the female thread crest 212 via an arc portion. For example, the radius of curvature of the arc portion that couples the female thread stabbing flank 213 to the female thread crest 212 is large compared with the radius of curvatures of other arc portions. Further, for example, a chamfered portion found as a straight line when viewed in a cross-sectional view may be provided between the female thread stabbing flank 213 and the female thread crest 212, and both ends of the chamfered portion may be coupled to the female thread stabbing flank 213 and the female thread crest 212 via arc portions.

[0069] Referring to FIG. 1 and FIG. 2, in a state in which making up between the pin 10 and the box 20 is completed due to screwing of the pin 10 into the box 20, the male thread part 11 is engaged with the female thread part 21. Specifically, the male thread root 112 comes into interference contact with the female thread crest 212. The male thread load flank 114 comes into pressure contact with the female thread load flank 214. In contrast, the male thread crest 111 is in no contact with the female thread root 211, so that a gap is formed between the male thread crest 111 and the female thread root 211. The male thread stabbing flank 113 is also in no contact with the female thread stabbing flank 213, so that a gap is also formed between the male thread stabbing flank 113 and the female thread stabbing flank 213.

[0070] In the made up state, the pin sealing surface 12 comes into interference contact with the box sealing surface 22, thus forming a sealing part by metal-to-metal contact in coordination with the box sealing surface 22. In the made up state, the pin shoulder surface 13 comes into pressure contact with the box shoulder surface 23, thus forming a shoulder part in coordination with the box shoulder surface 23. When the shoulder surfaces 13 and 23 come into pressure contact with each other, a tightening axial force is given to the male thread part 11 and the female thread part 21 that are engaged with each other.

[0071] FIG. 3 and FIG. 4 are partial enlarged views of the thread part in the threaded connection 100 shown in FIG. 2. FIG. 3 shows the male thread part 11 of the pin 10. FIG. 4 shows the female thread part 21 of the box 20 corresponding to the male thread part 11 shown in FIG. 3.

[0072] Referring to FIG. 2 and FIG. 3, the tapered male thread part 11 is divided into a perfect male thread part 11a and an imperfect male thread part 11b. That is, the male thread part 11 is formed of the perfect male thread part 11a and the imperfect male thread part 11b. The perfect male thread part 11a and the imperfect male thread part 11b are arranged in this order from the front end side of the pin 10 toward the steel pipe body 31 side. The imperfect male thread part 11b is continuous with an end of the perfect male thread part 11a on the steel pipe body 31 side. In the imperfect male thread part 11b, the top of the thread ridge has a shape cut out by a cylindrical surface having a diameter that is equal to the outer diameter of the steel pipe body 31. In the imperfect male thread part 11b, the top of the thread ridge may also have a shape cut out by a cylindrical surface having a diameter slightly smaller than the outer diameter of the steel pipe body 31. The thread height of the imperfect male thread part 11b is lower than the thread height of the perfect male thread part 11a. In the dimension in the pipe axis X1 direction, the area of the imperfect male thread part 11b is relatively long, and the length is, for example, approximately a half of the total length of the male thread part 11.

[0073] The perfect male thread part 11a includes a male thread crest 111a, a male thread root 112a, a male thread stabbing flank 113a, and a male thread load flank 114a. When the pin 10 is viewed in a longitudinal cross-sectional view, as described above, the crest 111a and the root 112a of the perfect male thread part 11a are straight lines inclined with respect to the pipe axis X1 in such a way as to approach the pipe axis X1 as the crest 111a and the root 112a extend toward the front end side of the pin 10. The stabbing flank 113a of the perfect male thread part 11a is a straight line that is inclined such that the outer circumferential edge of the stabbing flank 113a is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the stabbing flank 113a. The load flank 114a of the perfect male thread part 11a is a straight line that is inclined such that the outer circumferential edge of the load flank 114a is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the load flank 114a.

[0074] The imperfect male thread part 11b includes a male thread crest 111b, a male thread root 112b, a male thread stabbing flank 113b, and a male thread load flank 114b. When the pin 10 is viewed in a longitudinal cross-sectional view, as described above, the root 112b of the imperfect male thread part 11b is a straight line inclined with respect to the pipe axis X1 in such a way as to approach the pipe axis X1 as the root 112b extends toward the front end side of the pin 10. The stabbing flank 113b of the imperfect male thread part 11b is a straight line that is inclined such that an outer circumferential edge 113be of the stabbing flank 113b is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the stabbing flank 113b. The load flank 114b of the imperfect male thread part 11b is a straight line that is inclined such that the outer circumferential edge of the load flank 114b is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the load flank 114b. On the other hand, the crest 111b of the imperfect male thread part 11b is a straight line parallel to the pipe axis X1. The outer diameter dimension of the male thread crest 111b is equal to or slightly smaller than the outer diameter of the steel pipe body 31. In such imperfect male thread part 11b, the outer peripheral edge 113be of the male thread stabbing flank 113b is a sharp edge.

[0075] Referring to FIG. 2 and FIG. 4, the tapered female thread part 21 is formed of a perfect thread part over the entire length. The female thread part 21 includes the female thread root 211, the female thread crest 212, the female thread stabbing flank 213, and the female thread load flank 214. When the box 20 is viewed in a longitudinal cross-sectional view, as described above, the female thread root 211 and the female thread crest 212 are straight lines inclined with respect to the pipe axis X2 in such a way as to approach the pipe axis X2 as the female thread root 211 and the female thread crest 212 extend toward the inner depth side of the box 20. The female thread stabbing flank 213 is a straight line that is inclined such that the outer circumferential edge of the female thread stabbing flank 213 is located at a position closer to the pipe end side of the box 20 than the inner circumferential edge of the female thread stabbing flank 213. The female thread load flank 214 is a straight line that is inclined such that the outer circumferential edge of the female thread load flank 214 is located at a position closer to the pipe end side of the box 20 than the inner circumferential edge of the female thread load flank 214.

[0076] The female thread part 21 has a thread taper tp21. The thread taper tp21 of the female thread part 21 is constant over the entire length of the female thread part 21. When the box 20 is viewed in a longitudinal cross-sectional view, the thread taper tp21 of the female thread part 21 is specified by the following straight line that is inclined with respect to the pipe axis X2. In a certain thread ridge in the female thread part 21, an intersection point i21p1 of an extension line of the female thread crest 212 and an extension line of the female thread stabbing flank 213 is defined. In a thread ridge that is adjacent to the above-mentioned thread ridge in the female thread part 21, an intersection point i21p2 of an extension line of the female thread crest 212 and an extension line of the female thread stabbing flank 213 is defined. A straight line that connects these two intersection points i21p1 and i21p2 is a straight line that specifies the thread taper tp21 of the female thread part 21.

[0077] In the female thread part 21, the thread pitch is constant over the entire length of the female thread part 21. Specifically, when the female thread part 21 is viewed in a longitudinal cross-sectional view, a thread pitch sp21 of the female thread stabbing flank 213 is constant, and a thread pitch lp21 of the female thread load flank 214 is constant. The thread pitch sp21 of the female thread stabbing flank 213 is equal to thread pitch lp21 of the female thread load flank 214.

[0078] Returning to FIG. 3, the perfect male thread part 11a has a thread taper tp11a, and the imperfect male thread part 11b has a thread taper tp11b. When the pin 10 is viewed in a longitudinal cross-sectional view, the thread taper tp11a of the perfect male thread part 11a is specified by the following straight line that is inclined with respect to the pipe axis X1. In a certain thread ridge in the perfect male thread part 11a, an intersection point i11ap1 of an extension line of the male thread root 112a and an extension line of the male thread stabbing flank 113a is defined. In a thread ridge that is adjacent to the thread ridge in the perfect male thread part 11a, an intersection point i11ap2 of an extension line of the male thread root 112a and an extension line of the male thread stabbing flank 113a is defined. A straight line that connects these two intersection points i11ap1 and i11ap2 is a straight line that specifies the thread taper tp11a of the perfect male thread part 11a.

[0079] Similar to this, when the pin 10 is viewed in a longitudinal cross-sectional view, the thread taper tp11b of the imperfect male thread part 11b is specified by the following straight line that is inclined with respect to the pipe axis X1. In a certain thread ridge in the imperfect male thread part 11b, an intersection point i11bp1 of an extension line of the male thread root 112b and an extension line of the male thread stabbing flank 113b is defined. In a thread ridge that is adjacent to the thread ridge in the imperfect male thread part 11b, an intersection point i11bp2 of an extension line of the male thread root 112b and an extension line of the male thread stabbing flank 113b is defined. A straight line that connects these two intersection points i11bp1 and i11bp2 is a straight line that specifies the thread taper tp11b of the imperfect male thread part 11b.

[0080] The thread taper tp11a of the perfect male thread part 11a is equal to the thread taper tp21 (FIG. 4) of the female thread part 21. In contrast, the thread taper tp11b of the imperfect male thread part 11b is smaller than thread taper tp11a of the perfect male thread part 11a. That is, the thread taper tp11b of the imperfect male thread part 11b is smaller than the thread taper tp21 (FIG. 4) of the female thread part 21. In short, the male thread part 11 has the so-called two-stage taper, and the area of the imperfect male thread part 11b has a looser taper than the area of the perfect male thread part 11a.

[0081] It is sufficient that the thread taper tp11b of the imperfect male thread part 11b be smaller than the thread taper tp11a of the perfect male thread part 11a (the thread taper tp21 of the female thread part 21), and the upper limit and lower limit of the thread taper tp11b are not particularly limited. However, when the thread taper tp11b of the imperfect male thread part 11b is 98.5% or less of the thread taper tp11a of the perfect male thread part 11a, the effect described later will be effectively exhibited. Accordingly, it is preferable that the thread taper tp11b of the imperfect male thread part 11b is 98.5% or less of the thread taper tp11a of the perfect male thread part 11a.

[0082] In contrast, when the thread taper tp11b of the imperfect male thread part 11b is too small compared to the thread taper tp11a of the perfect male thread part 11a, the thread root diameter of the imperfect male thread part 11b becomes too small in an engaging end of the imperfect male thread part 11b and the female thread part 21. In this case, the critical cross-sectional area of the pin 10 becomes too small, and the tensile strength (connection strength) of the threaded connection 100 is decreased. Accordingly, it is preferable that the thread taper tp11b of the imperfect male thread part 11b is 50.0% or more of the thread taper tp11a of the perfect male thread part 11a.

[0083] In the male thread part 11, the thread pitch is constant over the entire length of the male thread part 11 that is formed of the perfect male thread part 11a and the imperfect male thread part 11b. Then, the thread pitch of the male thread part 11 is equal to the thread pitch of the female thread part 21.

[0084] Specifically, when the male thread part 11 is viewed in a longitudinal crosssection view, a thread pitch lp11 of the male thread load flank 114 (114a, 114b) is constant. The thread pitch lp11 of the male thread load flank 114 is equal to a thread pitch sp11 of the male thread stabbing flank 113a of the perfect male thread part 11a. Further, the thread pitch lp11 of the male thread load flank 114 is equal to the thread pitch sp21 of the female thread stabbing flank 213, and to the thread pitch lp21 of the female thread load flank 214.

[0085] In a case of the male thread part 11 that has such a two-stage taper, in the made up state in which making up between the pin 10 and the box 20 is completed, in the perfect male thread part 11a, the male thread root 112a comes into interference contact with the female thread crest 212, and the male thread load flank 114a comes into pressure contact with the female thread load flank 214. In contrast, in the imperfect male thread part 11b, in the same manner as the perfect male thread part 11a, the male thread load flank 114b comes into pressure contact with the female thread load flank 214. Additionally, in the imperfect male thread part 11b, the contact force due to the interference contact between the male thread root 112b and the female thread crest 212 is gradually decreased as it is distant from the perfect male thread part 11a, and may also finally become zero. Therefore, the male thread root 112b may be in no contact with the female thread crest 212 at a position distant from the perfect male thread part 11a, so that a gap may be provided between the male thread root 112b and the female thread crest 212.

[0086] Referring to FIG. 4, the inner peripheral surface of the box 20 is covered by a metal plating layer 24. The metal plating layer 24 covers at least the female thread part 21. Therefore, the female thread root 211, the female thread crest 212, the female thread stabbing flank 213, and the female thread load flank 214 are covered by the metal plating layer 24. In the example of the present embodiment, further, the box sealing surface 22 and the box shoulder surface 23 (FIG. 2) are covered by the metal plating layer 24. In short, the surface of the box 20 is covered by the metal plating layer 24.

[0087] The metal plating layer 24 has a hardness higher than the hardness of the base metal of the pin 10. More correctly, the hardness of the base metal of the pin 10 is the hardness of the base metal in the surface of the male thread part 11 provided in the pin 10. For example, the hardness of the base metal of the pin 10 is 200 to 396 Hv in the Vickers hardness. The hardness of the base metal of the box 20 may be substantially equal to or different from the hardness of the pin 10. In this case, the hardness of the metal plating layer 24 formed in the box 20 is 400 Hv or more in the Vickers hardness.

[0088] An alloy plating layer can be applied as the metal plating layer 24. In this case, the metal plating layer 24 is, for example, a Zn alloy plating layer. The Zn alloy plating layer is a multi-component alloy plating that contains zinc (Zn) as the main element by weight, that is, the element with the largest percentage by weight, and contains 50 weight% or more of zinc. For example, the Zn alloy plating layer is a binary alloy plating that contains zinc and any one element selected from nickel, iron, magnesium, and manganese. The Zn alloy plating layer may also be a Ni-Zn binary alloy plating that contains 12 weight% or more and 18 weight% or less of nickel, and in which the microstructure is a single phase and is present in the y phase. The Zn alloy plating layer may also be a ternary alloy plating that contains zinc and any two elements selected from nickel, iron, magnesium, manganese, copper, and tin.

[0089] The metal plating layer 24 may also be a Cu alloy plating layer. The Cu alloy plating layer is a multi-component alloy plating that contains copper (Cu) as the main element by weight, that is, the element with the largest percentage by weight. The Cu alloy plating layer is, for example, a binary or ternary alloy plating that contains copper and any one or two elements selected from nickel, iron, magnesium, manganese, zinc, and tin.

[0090] The metal plating layer 24 can be easily formed by an immersion electroplating method, for example. With the immersion electroplating method, the metal plating layer 24 is homogeneously formed on the surface of a target member, that is, the box 20, with a substantially uniform coating thickness.

[0091] The coating thickness of the metal plating layer 24 is 4 pm or more and 20 pm or less, for example. When the coating thickness of the metal plating layer 24 is 4 pm or more, it is possible to obtain sufficient anticorrosion property, and it is also possible to obtain sufficient wear resistance against sliding on the pin 10. In contrast, when the coating thickness of the metal plating layer 24 is 20 pm or less, it is possible to sufficiently suppress a decrease in adhesiveness with the surface of the pin 10 or the box 20 caused by increasing internal stress. The coating thickness of the metal plating layer 24 is more preferably 6 pm or more and 15 pm or less.

[0092] In the box 20, the metal plating layer 24 is covered by a solid lubricating coating 25. The solid lubricating coating 25 covers at least the metal plating layer 24 formed on the female thread part 21. In the example of the present embodiment, the solid lubricating coating 25 further covers the metal plating layer 24 formed on the box sealing surface 22 and the box shoulder surface 23 (see FIG. 2).

[0093] The solid lubricating coating 25 is a polymer-based lubricating coating. The solid lubricating coating 25 contains a resin and a solid lubricating powder. The resin is a substance that forms the base of the solid lubricating coating 25. Solid lubricating powder particles are dispersed in the resin. The solid lubricating coating 25 may further contain additives, such as an antislipping agent, a surfactant, or a rust inhibitor.

[0094] An example of the resin includes an epoxy resin. Instead of an epoxy resin, the resin may be a polyvinyl resin, an acrylic resin, a polyurethane resin, or a polyamide-imide resin, for example. Of these resins, particularly, epoxy resin, polyvinyl resin, and acrylic resin have excellent adhesiveness with the metal plating layer 24. Particularly, polyurethane resin is chemically stable, thus can be easily handled. Particularly, polyamide-imide resin has excellent wear resistance.

[0095] An example of the solid lubricating powder is polytetrafluoroethylene (PTFE). Instead of PTFE, the solid lubricating powder may be molybdenum dithiocarbamate (MoDTC), molybdenum disulfide (MoS2), carbon black (C), or graphite fluoride (CFx), for example, or may be a mixture of the above. Of these solid lubricating powders, PTFE exhibits stable friction coefficient under a high contact pressure, thus bringing about a good fastening torque.

[0096] In the case in which PTFE is used as the solid lubricating powder, it is preferable that the average particle size of the solid lubricating powder be 10 pm or less. When the average particle size of the solid lubricating powder is more than 10 pm, solid lubricating powder particles are excessively large relative to the coating thickness of the solid lubricating coating 25, so that the solid lubricating powder particles are ununiformly dispersed in the resin. When the solid lubricating powder particles are ununiformly dispersed in the resin, there is a risk that the solid lubricating coating 25 is likely to fall off due to sliding, thus causing galling. It is more preferable that the average particle size of the solid lubricating powder be 5 pm or less.

[0097] In the solid lubricating coating 25, it is preferable to use, in combination, an epoxy resin as the resin and PTFE as the solid lubricating powder. In this case, it is preferable that a content of PTFE be 10 weight% or more and 25 weight% or less. The solid lubricating coating 25 may also be an inorganic binder coating.

[0098] The solid lubricating coating 25 can be industrially formed by performing application by using a spray nozzle. Specifically, a solvent is mixed into the composition of the solid lubricating coating 25 to prepare a coating composition having flowability and adjusted viscosity. This coating composition is sprayed on a target member on which the metal plating layer 24 is formed, that is, on the box 20. The coating composition applied to the target member by spraying is solidified by heating, heat removal, or air drying, for example. With such operations, the solid lubricating coating 25 is formed on the surface of the target member.

[0099] In the case in which the solid lubricating coating 25 is, for example, a coating in which fluorine-based solid lubricating powder particles are dispersed in an epoxy resin, the coating thickness of the solid lubricating coating 25 may be set to 5 um or more and 50 um or less, although this depends on the components of the coating. In this case, it is more preferable that the coating thickness be 10 um or more and 40 um or less.

[0100] Referring to FIG. 2 and FIG. 3 again, the outer peripheral surface of the pin 10 is not covered by a metal plating layer and a solid lubricating coating. Instead, the outer peripheral surface of the pin 10 is covered by a rust preventive coating (not shown in the drawing). The rust preventive coating covers at least the male thread part 11. Therefore, the male thread crest 111, the male thread root 112, the male thread stabbing flank 113, and the male thread load flank 114 are covered by the rust preventive coating. In the example of the present embodiment, the pin sealing surface 12 and the pin shoulder surface 13 are further covered by the rust preventive coating. In short, the surface of the pin 10 is covered by the rust preventive coating.

[0101] Specifically, the rust preventive coating is formed by a chemical treatment coating (not shown in the drawing) and a rust preventive oil coating (not shown in the drawing), for example. In this case, the base metal surface of the pin 10 is covered by the chemical treatment coating, and the surface covered by the chemical treatment coating is covered by the rust preventive oil coating. That is, the base metal surface of the pin 10 is covered by the chemical treatment coating, and the rust preventive oil coating is formed on the chemical treatment coating.

[0102] The chemical treatment coating is formed by performing chemical treatment with zinc phosphate or manganese phosphate, for example, on the base metal surface of the pin 10. The rust preventive oil coating is formed by applying a rust preventive oil to the entire surface of the pin 10 on which the chemical treatment coating is formed. Examples of a method for applying a rust preventive oil include spraying, brushing, and immersion into a rust preventive oil tank.

[0103] The chemical treatment coating and the rust preventive oil coating are laminated on the surface of the pin 10 in this order as described above. Consequently, rust-proof treatment is sufficiently applied to the pin 10 at a low cost. Therefore, it is possible to prevent generation of rust on the pin 10 during a storage period from production of the pin 10 to the point in time at which the pin 10 is made up to the box 20.

[0104] However, in the pin 10, such a surface treatment coating is not always necessary. For example, only the chemical treatment coating may be formed on the base metal surface of the pin 10, or only the rust preventive oil coating may be formed on the base metal surface of the pin 10. The base metal surface of the pin 10 may be exposed.

[0105] [Advantageous effect] FIG. 5 is a schematic diagram showing a state in which a making up operation advances in a misalignment state. FIG. 5 shows the state in which, in an initial stage of a making up operation, for example, since the pin 10 is inserted at an angle to the box 20, the making up operation advances in the misalignment state in which the pipe axis X1 (axial centerline) of the pin 10 is inclined with respect to the pipe axis X2 (axial centerline) of the box 20. Referring to FIG. 5, when the making up operation advances in the misalignment state, the imperfect male thread part 11b of the male thread part 11 of the pin 10 will be in a status in which the imperfect male thread part 11b slides while contacting the female thread part 21 of the box 20 in an area CA about half way around of the pipe axis X2 of the box 20.

[0106] Specifically, referring to FIG. 3 to FIG. 5, the outer peripheral edge 113be of the stabbing flank 113b in the imperfect male thread part 11b is the sharp edge. Such outer peripheral edge 113be of the imperfect male thread stabbing flank 113b will be in a status in which the outer peripheral edge 113be slides while contacting the portion of the area CA of the female thread stabbing flank 213. When such uneven contact and sliding become significant, scratches may occur in the female thread stabbing flank 213.

[0107] However, in the threaded connection 100 according to the present embodiment, the inner peripheral surface of the box 20 to which the female thread part 21 is provided is covered by the metal plating layer 24, and this metal plating layer 24 is covered by the solid lubricating coating 25. The metal plating layer 24 is harder than the base metal of the pin 10. In this case, the female thread part 21 of the box 20, that is, the female thread stabbing flank 213, is reinforced by the metal plating layer 24 more than the male thread part 11 of the pin 10. Therefore, when the making up operation advances in the misalignment state, even if the outer peripheral edge 113be of the imperfect male thread stabbing flank 113b digs up the solid lubricating coating 25 on the female thread stabbing flank 213 and happens to be in metal-to-metal contact with the female thread stabbing flank 213, the outer peripheral edge 113be performs contact sliding with the female thread stabbing flank 213 reinforced by the metal plating layer 24. Therefore, excessive scratches are unlikely to occur in the female thread stabbing flank 213.

[0108] Further, in the threaded connection 100 according to the present embodiment, the thread taper tp11b of the imperfect male thread part 11b is smaller than the thread taper tp11a of the perfect male thread part 11a. That is, the thread taper tp11b of the imperfect male thread part 11b is smaller than the thread taper tp21 of the female thread part 21. In this case, when the making up operation advances in the misalignment state, the outer peripheral edge 113be of the imperfect male thread stabbing flank 113b comes into contact with the portion of the area CA of the female thread stabbing flank 213 in a wide area in the pipe axis X2 direction of the box 20. Therefore, even if the outer peripheral edge 113be of the imperfect male thread part 11b comes into contact with the female thread stabbing flank 213, the contact force is distributed, the contact pressure can be relieved and, as a result, the occurrence of scratches is further suppressed. Therefore, according to the threaded connection 100 according to the present embodiment, it is possible to suppress scratches and galling due to misalignment at the time of the making up operation, while adopting a dope-free technique.

[0109] (Modification 1) The box 20 may include a passivation treatment film (not shown in the drawing) between the metal plating layer 24 and the solid lubricating coating 25. In this case, the metal plating layer 24 is formed on the base metal surface of the box 20, and the passivation treatment film is formed on the metal plating layer 24. The solid lubricating coating 25 is formed on the passivation treatment film. In short, in the box 20, the passivation treatment film is laminated between the metal plating layer 24 and the solid lubricating coating 25. The passivation treatment film contains trivalent chromium (Cr (III)), for example. Trivalent chromium is more stable than Cr (II), and is preferable in that it is unharmful unlike Cr (VI).

[0110] The passivation treatment film serves as a passivation layer. The passivation treatment film improves corrosion resistance of the metal plating layer 24 in the box 20 to which the solid lubricating coating 25 is provided. A barrier layer (not shown in the drawing) may also be formed on the passivation treatment film, and the solid lubricating coating 25 may be formed on the barrier layer. The barrier layer is, for example, an inorganic matrix layer or an organic-inorganic matrix layer that contains silicon dioxide (SiO2) particles. Corrosion resistance is further improved by the barrier layer.

[0111] (Modification 2) Unevenness may be formed as a base on the base metal surface of the box 20, on which the metal plating layer 24 is formed, by performing blasting treatment. The blasting treatment is sandblasting or shot peening, for example. In this case, since the adhesiveness between the base metal surface of the box 20 and the metal plating layer 24 is improved, and the surface of the metal plating layer 24 also has unevenness, adhesiveness between the solid lubricating coating 25 and the metal plating layer 24 also increases.

[0112] (Modification 3) The box 20 may include a chemical treatment coating (not shown in the drawing) between the metal plating layer 24 and the solid lubricating coating 25. In this case, the metal plating layer 24 is formed on the base metal surface of the box 20, and the chemical treatment coating is formed on the metal plating layer 24. The solid lubricating coating 25 is formed on the chemical treatment coating. In short, in the box 20, the chemical treatment coating is laminated between the metal plating layer 24 and the solid lubricating coating 25. The chemical treatment coating is formed by performing chemical treatment with zinc phosphate or manganese phosphate, for example, on the surface of the metal plating layer 24. The chemical treatment coating improves the adhesiveness between the metal plating layer 24 and the solid lubricating coating 25.

[0113] (Modification 4) The kind of thread forming the male thread part 11 and the female thread part 21 may be a so-called wedge thread. Specifically, referring to FIG. 2, in the male thread part 11, the width of a male thread ridge defined by the male thread crest 111, the male thread stabbing flank 113, and the male thread load flank 114 reduces progressively along the thread helix towards the front end side of the pin 10. In the male thread part 11, the width of a male thread groove defined by the male thread root 112, the male thread stabbing flank 113, and the male thread load flank 114 increases progressively along the thread helix towards the front end side of the pin 10.

[0114] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread stabbing flank 113 is a straight line that is inclined such that the outer circumferential edge of the male thread stabbing flank 113 is located at a position closer to the front end side of the pin 10 than the inner circumferential edge of the male thread stabbing flank 113. When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the male thread load flank 114 is a straight line that is inclined such that the outer circumferential edge of the male thread load flank 114 is located at a position closer to the steel pipe body 31 than the inner circumferential edge of the male thread load flank 114.

[0115] In the female thread part 21, the width of a female thread groove defined by the female thread root 211, the female thread stabbing flank 213, and the female thread load flank 214 reduces progressively along the thread helix towards the inner depth side of the box 20 as corresponding to the width of the male thread ridge. In the female thread part 21, the width of a female thread ridge defined by the female thread crest 212, the female thread stabbing flank 213, and the female thread load flank 214 increases progressively along the thread helix towards the inner depth side of the box 20 as corresponding to the width of the male thread groove.

[0116] When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread stabbing flank 213 is a straight line that is inclined such that the outer circumferential edge of the female thread stabbing flank 213 is located at a position closer to the inner depth side of the box 20 than the inner circumferential edge of the female thread stabbing flank 213. When the threaded connection 100 is viewed in a longitudinal cross-sectional view, the female thread load flank 214 is a straight line that is inclined such that the outer circumferential edge of the female thread load flank 214 is located at a position closer to the pipe end side of the box 20 than the inner circumferential edge of the female thread load flank 214.

[0117] During making up, due to screwing of the pin 10 into the box 20, the male thread load flank 114 comes into contact with the female thread load flank 214, and the male thread stabbing flank 113 comes into contact with the female thread stabbing flank 213. In a state in which making up is completed with further advancing of screwing, the load flanks 114, 214 come into pressure contact with each other, and the stabbing flanks 113, 213 come into pressure contact with each other. Accordingly, a tightening axial force is given to the male thread part 11 and the female thread part 21 that are engaged with each other. In this case, the pin shoulder surface 13 and the box shoulder surface 23 need not be provided.

[0118] <Second embodiment> Referring to FIG. 3 and FIG. 6, the configuration of a threaded connection 100A according to a second embodiment will be described. FIG. 6 is a partial enlarged view of a female thread part 21A of a box 20A in the threaded connection 100A. The threaded connection 100A differs from the threaded connection 100 according to the first embodiment in that the female thread part 21A includes a chamfered surface 215.

[0119] As shown in FIG. 6, the chamfered surface 215 is provided to the tapered female thread part 21A. The chamfered surface 215 couples the female thread stabbing flank 213 to the female thread crest 212. That is, the outer peripheral edge of the chamfered surface 215 is connected to the female thread stabbing flank 213, and the inner periphery edge of the chamfered surface 215 is connected to the female thread crest 212. When the box 20 is viewed in a longitudinal cross-sectional view, the chamfered surface 215 is a straight line that is inclined such that the outer circumferential edge of the chamfered surface 215 is located at a position closer to the pipe end side of the box 20 than the inner circumferential edge of the chamfered surface 215.

[0120] Referring to FIG. 3 and FIG. 6, in the threaded connection 100A, when a making up operation advances in a misalignment state, the outer peripheral edge 113be of the stabbing flank 113b in the imperfect male thread part 11b is more likely to contact the chamfered surface 215 than the female thread stabbing flank 213 in the female thread part 21. When the outer peripheral edge 113be of the imperfect male thread stabbing flank 113b comes into contact with the chamfered surface 215 of the female thread part 21, the contact angle between the outer peripheral edge 113be and the chamfered surface 215 becomes large. When the contact angle is increased, the contact force that occurs in the chamfered surface 215 of the female thread part 21 due to the self-weight of the steel pipe 30 that has the pin 10 is decreased compared to a case without the chamfered surface 215. Therefore, also in an initial stage of sliding in which blunting of the outer peripheral edge 113be of the imperfect male thread stabbing flank 113b has not occurred, it is possible to further relieve the contact pressure between the outer peripheral edge 113be of the imperfect male thread stabbing flank 113b and the female thread stabbing flank 213.

[0121] Referring to FIG. 6, a height hc of the chamfered surface 215 is not particularly limited. The height hc of the chamfered surface 215 means the distance in the radial direction (the direction perpendicular to the pipe axis X2 direction) from the outer peripheral edge to the inner periphery edge of the chamfered surface 215 when the box 20A is viewed in a longitudinal cross-sectional view. However, when the height hc of the chamfered surface 215 is excessively small, it is difficult to obtain the above-described advantageous effects by the chamfered surface 215. In contrast, when the height hc of the chamfered surface 215 is excessively large, in a case in which the compressive load in the pipe axis X direction is applied to the threaded connection 100A during use after making up, a situation occurs in which the engagement between the male thread stabbing flank 113 and the female thread stabbing flank 213 may be disengaged. This situation is referred to as jump-in (overcoming of a thread ridge). When jump-in occurs, the threaded connection 100A is destroyed. Accordingly, it is preferable that the height hc of the chamfered surface 215 is 25% or more and 50% or less of a thread height ht of the female thread part 21A.

[0122] In the same manner as the height hc of the chamfered surface 215, an angle 0c of the chamfered surface 215 is not particularly limited. The angle 0c of the chamfered surface 215 means the inclination angle of the chamfered surface 215 with respect to a surface perpendicular to the pipe axis X2 when the box 20 is viewed in a longitudinal cross-sectional view. However, for the same reason as the height hc of the chamfered surface 215, it is preferable that the angle 0c of the chamfered surface 215 is 25° or more and 65° or less.

[0123] <Third embodiment> Referring to FIG. 4 and FIG. 7, the configuration of a threaded connection 100B according to a third embodiment will be described. FIG. 7 is a partial enlarged view of a male thread part 11B of a pin 10B in the threaded connection 100B. The threaded connection 100B differs from the threaded connection 100 according to the first embodiment in the configuration of the imperfect male thread part 11b of the male thread part 11B.

[0124] As shown in FIG. 7, in the male thread part 11B, the thread pitch of the perfect male thread part 11a is constant over the entire length of the perfect male thread part 11a, and the thread pitch of the imperfect male thread part 11b is constant over the entire length of the imperfect male thread part 11b. Then, the thread pitch of the imperfect male thread part 11b is larger than the thread pitch of the perfect male thread part 11a, in other words, the thread pitch of the female thread part 21.

[0125] Specifically, when the male thread part 11B is viewed in a longitudinal crosssectional view, a thread pitch sp11a of the stabbing flank 113 of the perfect male thread part 11a is constant, and a thread pitch lp11a of the load flank 114 of the perfect male thread part 11a is constant. Additionally, when the male thread part 11B is viewed in a longitudinal cross-sectional view, a thread pitch sp11b of the stabbing flank 113 of the imperfect male thread part 11b is constant, and a thread pitch lp11b of the load flank 114 of the imperfect male thread part 11b is constant.

[0126] The thread pitch sp11a of the stabbing flank 113 of the perfect male thread part 11a is equal to the thread pitch lp11a of the load flank 114 of the perfect male thread part 11a, and is equal to the thread pitch lp11b of the load flank 114 of the imperfect male thread part 11b. Further, the thread pitch sp11a of the stabbing flank 113 of the perfect male thread part 11a is equal to the thread pitch sp21 (FIG. 4) of the female thread stabbing flank 213, and the thread pitch lp21 (FIG. 4) of the female thread load flank 214.

[0127] However, the thread pitch sp11b of the stabbing flank 113b of the imperfect male thread part 11b is larger than the thread pitch sp11a of the stabbing flank 113a of the perfect male thread part 11a. That is, the thread pitch sp11b of the stabbing flank 113b of the imperfect male thread part 11b is larger than the thread pitch sp21 (FIG. 4) of the female thread stabbing flank 213.

[0128] Referring to FIG. 4 and FIG. 7, when the thread pitch sp11b of the stabbing flank 113b of the imperfect male thread part 11b is larger than the thread pitch sp11a of the stabbing flank 113a of the perfect male thread part 11a, in other words, the thread pitch sp21 of the female thread stabbing flank 213, the interval between the male thread stabbing flank 113 and the female thread stabbing flank 213 in the pipe axis X direction of the threaded connection 100B is larger in the area of the imperfect male thread part 11b than in the area of the perfect male thread part 11a. Therefore, it is possible to relieve the contact pressure between the outer peripheral edge 113be of the imperfect male thread stabbing flank 113b and the female thread stabbing flank 213.

[0129] Referring to FIG. 7, when the thread pitch sp11b of the stabbing flank 113b of the imperfect male thread part 11b is slightly larger than the thread pitch sp11a of the stabbing flank 113a of the perfect male thread part 11a, the above-described advantageous effects can be obtained. It is preferable that the thread pitch sp11b of the imperfect male thread part 11b is 100.4% or more of the thread pitch sp11a of the perfect male thread part 11a. In contrast, when the thread pitch sp11b of the imperfect male thread part 11b is excessively large with respect to the thread pitch sp11a of the perfect male thread part 11a, the above-described advantageous effects are saturated. Accordingly, it is preferable that the thread pitch sp11b of the imperfect male thread part 11b is 101.0% or less of the thread pitch sp11a of the perfect male thread part 11a.

[0130] The configuration of the third embodiment may be applied to the threaded connection 100A according to the second embodiment.

[0131] <Fourth embodiment> Referring to FIG. 8 and FIG. 9, the configuration of a threaded connection 100C according to a fourth embodiment will be described. FIG. 8 and FIG. 9 are partial enlarged views of a thread part in the threaded connection 100C. FIG. 8 shows a male thread part 11C of a pin 10C. FIG. 9 shows a female thread part 21C of a box 20C corresponding to the male thread part 11C shown in FIG. 8. The threaded connection 100C differs from the threaded connection 100 according to the first embodiment in the configurations of the male thread part 11C and the female thread part 21C.

[0132] As shown in FIG. 8, when the pin 10C is viewed in a longitudinal crosssectional view, the crest 111a of the perfect male thread part 11a is a straight line parallel to the pipe axis X1. In the same manner as this, the root 112a of the perfect male thread part 11a is a straight line parallel to the pipe axis X1. Further, when the pin 10C is viewed in a longitudinal cross-sectional view, in the same manner as the root 112a of the perfect male thread part 11a, the root 112b of the imperfect male thread part 11b is also a straight line parallel to the pipe axis X1.

[0133] As shown in FIG. 9, when the box 20C is viewed in a longitudinal crosssectional view, the female thread root 211 is a straight line parallel to the pipe axis X2. In the same manner as this, the female thread crest 212 is a straight line parallel to the pipe axis X2.

[0134] The threaded connection 100C according to the fourth embodiment can obtain advantageous effects substantially equal to the advantageous effects obtained by the first embodiment. The configuration of the fourth embodiment may be applied to the threaded connection 100A according to the second embodiment and the threaded connection 100B according to the third embodiment. EXAMPLE

[0135] Hereinafter, the present disclosure will be described in more detail by using an example. However, the present disclosure is not limited to the following example.

[0136] In order to observe the advantageous effects by the present disclosure, analysis according to a quasi three-dimensional elastoplastic finite element method that uses a Fourier element was performed. In the analysis, the threaded connections 100, 100A, 100B, 100C for steel pipes were modeled.

[0137] The main conditions for a threaded connection analysis model were as follows. - dimensions of steel pipe: 9-5 / 8" 53.5# (outer diameter: 244.5 mm, wall thickness: 13.8 mm) - material: API-standard L80 steel (yield stress: 552 N / mm2, elastic coefficient: 210 kN / mm2) - dimensions of thread part thread height of female thread part: 2.4 mm length of perfect male thread part: 55 mm length of imperfect male thread part: 55 mm height of chamfered surface of female thread part: 1.0 mm angle of the chamfered surface: 30°

[0138] For a model of a box, a hard metal plating layer of 400 Hv or more in the Vickers hardness was simulated by setting only the yield stress to 1200 N / mm2. Since the coating thickness of a solid lubricating coating is very small compared to the size of a thread, and has little influence on the relative evaluation of the contact pressure, the solid lubricating coating is omitted.

[0139] The analysis simulated making up in a misalignment state. Specifically, in an analysis model in a state in which a pin was inserted into a box, a compressive load in a pipe axis direction and a bending moment load in a direction perpendicular to the pipe axis direction were applied to the pin, and the contact status between an outer peripheral edge of a stabbing flank of an imperfect male thread part and a stabbing flank of a female thread part was evaluated. In performing such an operation, the relative position of the pin with respect to the box was set to a position at which three turns remained until shoulders come into contact with each other.

[0140] The conditions for loading were as follows. - compressive load: 28.1 kN (equivalent to the self-weight of a jointed pipe formed by joining three steel pipes) - bending moment: 25.5 kN-m (the moment load in a case where it was assumed that the jointed pipe formed by joining three steel pipes was tilted by approximately 3°)

[0141] The evaluation was made at the maximum value of the contact pressure that acted on the stabbing flank of the female thread part due to contact with the outer peripheral edge of the imperfect male thread part. The analysis result is shown in Table 1.

[0142] [Table 1] TABLE 1 Test No. Male thread part Female thread part Maximum contact pressure (relative value) Classification Perfect thread part Imperfect thread part Stabbing flank pitch [mm] Thread taper [%] Stabbing flank pitch [mm] Thread taper [%] Stabbing flank pitch [mm] Thread taper [%] Chamfered surface 1 6.35 6.25 6.45 6.00 6.35 6.25 Yes 0.89 Inventive Example of present invention 2 6.35 6.25 6.35 6.00 6.35 6.25 Yes 0.68 Inventive Example of present invention 3 6.35 6.25 6.35 5.75 6.35 6.25 No 0.49 Inventive Example of present invention 4 6.35 6.25 6.35 6.00 6.35 6.25 No 0.75 Inventive Example of present invention 5 6.35 6.25 6.35 6.15 6.35 6.25 No 0.91 Inventive Example of present invention 6 6.35 6.25 6.45 6.00 6.35 6.25 No 0.77 Inventive Example of present invention 7 6.35 6.25 6.35 6.25 * 6.35 6.25 No 1.00 Comparative Example Note: * mark means that the requirements specified for the threaded connection of the present embodiment are not met.

[0143] Referring to Table 1, tests No. 1 to No. 6 are Inventive Examples of the present invention that satisfy the conditions specified in the present embodiment. Test No. 7 is Comparative Example in which the imperfect male thread part and the perfect male thread part have the same thread taper, and the conditions specified in the present embodiment are not satisfied. As is clear from Table 1, in Inventive Examples of the present invention, the contact pressure was decreased by 5% or more compared to Comparative Example. From this result, it can be seen that the risk of scratches and galling is reduced in the stabbing flank of the female thread part.

[0144] The embodiments of the present disclosure have been described heretofore. However, the above-described embodiments merely form examples for executing the present disclosure. Accordingly, the present disclosure is not limited to the above described embodiments, and the above-described embodiments may be suitably modified without departing from the gist of the present disclosure.

[0145] In the pin 10, the position at which the pin sealing surface 12 is provided is not limited to between the male thread part 11 and the pin shoulder surface 13. For example, the pin sealing surface 12 may be provided in the middle of the male thread part 11 in the pipe axis X1 direction, or may be provided to the steel pipe body 31 side in the male thread part 11.

[0146] In the box 20, it is sufficient that the position at which the box sealing surface 22 is provided correspond to the position at which the pin sealing surface 12 is provided, and is not limited to between the female thread part 21 and the box shoulder surface 23. For example, the box sealing surface 22 may be provided in the middle of the female thread part 21 in the pipe axis X2 direction, or may be provided to the pipe end side of the box 20 in the female thread part 21.

[0147] In the pin 10, when the pin shoulder surface 13 is provided, the position of the pin shoulder surface 13 is not limited to the front end of the pin 10. For example, the pin shoulder surface 13 may be provided in the middle of the male thread part 11 in the pipe axis X1 direction, or may be provided to the steel pipe body 31 side in the male thread part 11.

[0148] In the box 20, when the box shoulder surface 23 is provided, it is sufficient that the position of the box shoulder surface 23 correspond to the position of the pin shoulder surface 13, and is not limited to the inner depth of the box 20. For example, the box shoulder surface 23 may be provided in the middle of the female thread part 21 in the pipe axis X2 direction, or may be provided to the pipe end of the box 20.

[0149] The threaded connections 100, 100A, 100B, 100C may be of an integral type. REFERENCE SIGNS LIST

[0150] 100, 100A, 100B, 100C: threaded connection 10, 10B, 10C: pin 11, 11B, 11C: male thread part 11a: perfect male thread part 11b: imperfect male thread part 111, 111a, 111b: male thread crest 112, 112a, 112b: male thread root 113, 113a, 113b: male thread stabbing flank 113be: outer peripheral edge of male thread stabbing flank 114, 114a, 114b: male thread load flank 12: pin sealing surface 13: pin shoulder surface 20, 20A, 20C: box 21, 21A, 21C: female thread part 211: female thread root 212: female thread crest 213: female thread stabbing flank 214: female thread load flank 215: chamfered surface 22: box sealing surface 23: box shoulder surface 24: metal plating layer 25: solid lubricating coating X, X1, X2: pipe axis tp11a: thread taper of perfect male thread part tp11b: thread taper of imperfect male thread part tp21: thread taper of female thread part 30: steel pipe 31: body 40: coupling

Claims

1. A threaded connection that connects steel pipes with each other, the threaded connection comprising:a pin having a pipe shape and provided continuously to a steel pipe body; anda box having a pipe shape and made up to the pin with insertion of the pin into the box, whereinthe pin includes a tapered male thread part provided to an outer peripheral surface of the pin,the box includes a tapered female thread part provided to an inner peripheral surface of the box in such a way as to correspond to the tapered male thread part,the tapered female thread part has a constant thread taper over an entire length of the tapered female thread part,the tapered male thread part is divided into a perfect thread part that has a thread taper equal to the thread taper of the tapered female thread part, and an imperfect thread part that has a thread taper smaller than the thread taper of the perfect thread part,the inner peripheral surface of the box is covered by a metal plating layer that has a hardness higher than a hardness of a base metal of the pin, andthe metal plating layer is covered by a solid lubricating coating.

2. The threaded connection according to claim 1, whereinthe tapered female thread part includes a female thread crest, a female thread root, a female thread stabbing flank, and a female thread load flank, anda chamfered surface for coupling the female thread stabbing flank to the female thread crest is provided to the tapered female thread part.

3. The threaded connection according to claim 1, whereinthe tapered female thread part has a constant thread pitch over an entire length of the tapered female thread part, andin the tapered male thread part, a thread pitch of the perfect thread part is equal to the thread pitch of the tapered female thread part, and a thread pitch of the imperfect thread part is larger than the thread pitch of the perfect thread part.

4. The threaded connection according to claim 1, whereinthe metal plating layer is an alloy plating layer.

5. The threaded connection according to any one of claims 1 to 4, whereinthe pin is provided to each of the steel pipes that are to be connected with each other, andthe box is provided to a coupling that is a part having a pipe shape separately from the steel pipes.

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