Threaded joint

Abstract

To provide a threaded joint that can suppress galling during a fastening operation while employing a dope-free technology.SOLUTION: In a fastened state, a distance (LP) in a pipe axis (X) direction from a tip (10a) of a pin (10) to an end (11a1) of a complete male thread portion (11a), and a distance (LB) in the pipe axis (X) direction from the tip (10a) to an end (21a1) of a complete female thread portion (21a) satisfy formula (1): α×L / (α+T / 100)+(δ+2×t) / T×100≥LB-LP. In formula (1), the symbols have the following meanings: α is tan 0.5°; L is an engagement distance [mm] between the complete male thread portion (11a) and the complete female thread portion (21a) at a time point when a root surface of the male thread comes into contact with a crest surface of the female thread during fastening; T is thread taper [%]; δ is an interference amount [mm] between the root surface of the male thread and the crest surface of the female thread; and t is a film thickness [mm] of a laminated coating covering an inner circumferential surface of a box (20).SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

[0002] In oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), a large number of steel pipes called oil country tubular goods are used to extract underground resources. These steel pipes are connected to each other by threaded joints.

[0003] Threaded joints for steel pipes are broadly divided into coupling types and integral types. In the case of the coupling type, steel pipes are connected via a coupling, which is a different pipe material. Specifically, each steel pipe has a tapered male thread at both ends, and the coupling has a tapered female thread at both ends. The tapered male thread of the steel pipe is screwed into each of the tapered female threads of the coupling to fasten the steel pipes, thereby connecting the steel pipes. In this specification, the tapered male thread is also simply referred to as the male thread, and the tapered female thread is also simply referred to as the female thread. The tapered male thread and the tapered female thread are also collectively referred to as the thread.

[0004] In the case of an integral type, steel pipes are directly connected to each other. Specifically, each steel pipe has a male threaded portion at one end and a female threaded portion at the other end. The male threaded portion of one steel pipe is screwed into the female threaded portion of the other steel pipe to fasten them together, thereby connecting the steel pipes to each other.

[0005] Generally, the end of a steel pipe with a male thread is called a pin, and the end of a steel pipe or coupling with a female thread is called a box. Using these names, the pin and the box are fastened together by inserting the pin into the box and screwing it in.

[0006] Threaded joints for steel pipes are required to prevent seizure during the make-up process and to ensure sealing performance when used after make-up. To meet this requirement, grease is generally applied to the surface of the box and / or pin before make-up work is performed at the oil well site (platform). This grease is a lubricant specifically for threaded joints and is called dope. Traditionally, dope contains heavy metal powder (compound).

[0007] In recent years, dope-free technology, which uses lubricants instead of conventional dopes, has been promoted to improve the efficiency of fastening work, reduce environmental impact, and consider the health of workers. In dope-free technology, a polymer-based lubricating coating is usually formed on the surface of the box and / or pin. This lubricating coating is a solid or semi-solid lubricant and is called a solid lubricating coating.

[0008] For example, Japanese Patent Laid-Open Publication No. 2002-349775 (Patent Document 1) describes a tapered thread joint in which a lubricating coating layer (a solid lubricating coating or a viscous liquid lubricating coating) is formed on the surface of at least one of the pin and box. The problem with this patent document 1 is to prevent seizure that occurs in the threaded portion during the make-up operation. To solve this problem, Patent Document 1 proposes a first tapered thread joint and a second tapered thread joint.

[0009] In the case of a first tapered thread joint, the female thread portion is divided into a first tapered portion and a second tapered portion. In this female thread portion, the second tapered portion has a larger taper than the first tapered portion and is composed of an incomplete thread portion. At least a portion of the second tapered portion is joined to face the incomplete thread portion of the male thread portion. On the other hand, in the case of a second tapered thread joint, the male thread portion is divided into a third tapered portion and a fourth tapered portion. In this male thread portion, the fourth tapered portion has a smaller taper than the third tapered portion and is composed of an incomplete thread portion. At least a portion of the fourth tapered portion is joined to face the complete thread portion of the female thread portion. Patent Document 1 states that in both the first and second tapered thread joints, unnecessary interference does not occur in the incomplete thread portion, so the lubricating coating layer formed on the incomplete thread portion of the male thread portion does not peel off, and the lubricating coating layer formed on the second tapered portion of the female thread portion does not peel off, and seizure does not occur. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-349775 Summary of the Invention [Problem to be solved by the invention]

[0011] Even with the adoption of dope-free technology, there is a demand for further improvements in the efficiency of fastening operations. To meet this demand, for example, a connecting pipe consisting of two or three steel pipes connected together in a stockyard is increasingly being manufactured in advance, and then the connecting pipe's pin is inserted into a box on the rig for fastening. However, the weight of a connecting pipe is greater than the weight of a single steel pipe. Therefore, if the device (compensator) used to cancel the connecting pipe's own weight during fastening operations on the rig is not properly adjusted, the connecting pipe's own weight can cause excessive contact force on the threads, leading to seizure at the threads. Seizure, which is likely to occur due to improper compensator adjustment, can also occur when fastening the pin of a single steel pipe to a box. While such seizure may be suppressed by the threaded joint described in Patent Document 1, other technologies different from the technology described in Patent Document 1 may also be used to suppress seizure at the threads.

[0012] An object of the present disclosure is to provide a threaded joint that employs dope-free technology and is capable of suppressing seizure of the threaded portion when fastening a pin to a box. [Means for solving the problem]

[0013] A threaded joint according to the present disclosure is a threaded joint for connecting steel pipes. The threaded joint includes a tubular pin and a tubular box. The pin is provided contiguous with a steel pipe body. The pin is inserted into and fastened to the box. The pin includes a tapered male thread portion and a pin-side torque stop mechanism. The tapered male thread portion is provided on the outer peripheral surface of the pin. The pin-side torque stop mechanism is provided to limit the threading of the pin into the box when fastening the pin to the box. The box includes a tapered female thread portion and a box-side torque stop mechanism. The tapered female thread portion is provided on the inner peripheral surface of the box in correspondence with the tapered male thread portion. The tapered female thread portion has a thread taper equal to the thread taper of the tapered male thread portion. The box-side torque stop mechanism is provided in correspondence with the pin-side torque stop mechanism.

[0014] The tapered male thread portion includes a male thread crest surface, a male thread root surface, a male thread stab flank surface, and a male thread load flank surface. The tapered male thread portion is divided into a complete male thread portion and an incomplete male thread portion. The incomplete male thread portion is formed contiguous to the complete male thread portion toward the tip side of the pin. The tapered female thread portion includes a female thread root surface, a female thread crest surface, a female thread stab flank surface, and a female thread load flank surface. The female thread root surface corresponds to the male thread crest surface. The female thread crest surface corresponds to the male thread root surface. The female thread stab flank surface corresponds to the male thread stab flank surface. The female thread load flank surface corresponds to the male thread load flank surface. The tapered female thread portion is divided into a complete female thread portion and an incomplete female thread portion. The incomplete female thread portion is formed contiguous to the complete female thread portion toward the innermost side of a box. The inner peripheral surface of the box is coated with a laminate coating. The laminate coating includes a metal plating layer and a solid lubricating coating. The metal plating layer has a hardness higher than that of the base material of the pin. The solid lubricating coating is laminated on the metal plating layer.

[0015] When the pin and box are fastened together, the threaded joint has the following configuration: The male thread root surface is in interference contact with the female thread crest surface. The male thread load flank surface is in pressing contact with the female thread load flank surface. The pin-side torque stop mechanism and box-side torque stop mechanism function reciprocally. Furthermore, the distance LP in the pipe axial direction of the threaded joint from the tip of the pin to the end of the complete male thread portion on the incomplete male thread portion side, and the distance LB in the pipe axial direction from the tip of the pin to the end of the complete female thread portion on the incomplete female thread portion side, satisfy formula (1). α×L / (α+T / 100)+(δ+2×t) / T×100≧LB-LP (1) The meanings of the symbols in formula (1) are as follows: α: tan 0.5°, L: The distance in the pipe axis direction between the end of the complete male thread on the incomplete male thread side and the end of the complete female thread on the pipe end side of the box at the time when the male thread bottom surface contacts the female thread crest surface when the pin is fastened to the box [mm], T: Thread taper of the male and female threads [%] δ: Interference between the male thread root surface and the female thread crest surface [mm], and t: Thickness of the laminated coating [mm]. [Effects of the Invention]

[0016] The threaded joint according to the present disclosure employs dope-free technology while making it possible to suppress seizure of the threaded portion when fastening the pin to the box. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a longitudinal sectional view of a threaded joint according to a first embodiment. [Figure 2] FIG. 2 is a longitudinal cross-sectional view of the pin and the box in the threaded joint shown in FIG. [Figure 3] 3 is a partially enlarged view of the male thread portion of the pin in the threaded joint shown in FIG. 2. FIG. [Figure 4] 4 is a partially enlarged view of the female thread portion of the box in the threaded joint shown in FIG. 2. FIG. [Figure 5] FIG. 5 is a schematic diagram showing the fastened state of the pin and the box in a vertical cross section of a threaded joint. [Figure 6] FIG. 6 is a schematic diagram showing the state of the pin and the box in a longitudinal cross section of a threaded joint at the end of the hand make-up stage. [Figure 7] FIG. 7 is a schematic diagram showing the state of the pin and box in a longitudinal cross section of a threaded joint at the time of hand tightening. [Figure 8] FIG. 8 is a longitudinal sectional view of a threaded joint according to the second embodiment. [Figure 9] FIG. 9 is a vertical cross-sectional view of the pin and the box in the threaded joint shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0018] In order to solve the above problems, the inventors first conducted a detailed investigation into the state of seizure that occurs in the threads of threaded joints that employ dope-free technology, that is, threaded joints that are provided with a solid lubricating coating, when the pin and box are tightened together, and the causes of this seizure. In this investigation, a coupling-type threaded joint was used as the threaded joint.

[0019] The tapered male thread is provided on the outer peripheral surface of the pin, where dimensional expansion is limited. Coupling-type threaded joints typically employ a design in which the threads on the side of the steel pipe body gradually disappear as they approach the steel pipe body, in order to achieve tensile performance roughly equivalent to that of the steel pipe body. In this case, the portion of the male thread on the side of the steel pipe body is an incomplete thread, with the crest of the thread cut off by a cylindrical surface with a diameter equal to or slightly smaller than the outer diameter of the steel pipe body. This incomplete male thread on the side of the steel pipe body is continuous with the complete male thread, and its region is relatively long, occupying approximately half of the total length of the male thread. Furthermore, the end of the male thread on the tip side of the pin is also an incomplete thread. This region of the incomplete male thread on the tip side of the pin is extremely short. The thread height of these incomplete male threads is lower than that of the complete male thread. In short, the tapered male thread portion is divided into a short incomplete male thread portion, a complete male thread portion, and a long incomplete male thread portion, in that order, from the pin tip side toward the steel pipe body side. In this specification, the short incomplete male thread portion located on the pin tip side is also referred to as the "inner incomplete male thread portion," and the long incomplete male thread portion located on the steel pipe body side is also referred to as the "outer incomplete male thread portion."

[0020] On the other hand, the tapered female thread portion is provided on the inner peripheral surface of the box. The tapered female thread portion is divided into an incomplete female thread portion, a complete female thread portion, and a short incomplete female thread portion, in that order from the innermost side of the box toward the pipe end side of the box. These incomplete female thread portions are each continuous with the complete female thread portion. The incomplete female thread portion has a shape in which the crest of the thread is cut off by a tapered surface or a cylindrical surface, and its thread height is lower than that of the complete female thread portion. In this specification, the incomplete female thread portion located on the innermost side of the box is also referred to as the "inner incomplete female thread portion," and the short incomplete female thread portion located on the pipe end side of the box is also referred to as the "outer incomplete female thread portion." Generally, when the pin and the box are fastened together, the region of the complete female thread portion overlaps the region of the outer incomplete male thread portion and also overlaps with most of the region of the complete male thread portion. The region of the inner incomplete female thread overlaps the region of the complete male thread portion, and may also overlap the region of the inner incomplete male thread portion. As a result of the investigation, the present inventors have made the following findings.

[0021] In the box, when viewed in vertical cross section of the threaded joint, seizure often occurred in the innermost region of the complete female thread, approximately 5 to 9 threads from the pipe end of the box. The area where this seizure occurred was the stab flank surface (female thread stab flank surface) in the area of ​​the complete female thread that corresponds to the outer incomplete male thread. In this specification, seizure that occurs on the pipe end side of the box in this way is also referred to as "external seizure."

[0022] In addition to external seizure, in the longitudinal cross section of the threaded joint, seizure sometimes occurred in the pin in a region of the complete male thread portion up to two or three threads from the end of the tip of the pin. The area where this seizure occurred was the stab flank surface (male thread stab flank surface) in the region of the complete male thread portion that corresponds to the internal incomplete female thread portion. In this specification, seizure that occurs on the tip side of the pin in this way is also referred to as "internal seizure."

[0023] Scratches were found on the female thread stab flank surface of the fully female threaded portion where external seizure had occurred. These scratches were concentrated in an area of ​​the female thread stab flank surface that was approximately halfway around the axis of the box. Scratches were found on the male thread stab flank surface of the fully male threaded portion where internal seizure had occurred. These scratches were concentrated in an area of ​​the male thread stab flank surface that was approximately halfway around the axis of the pin.

[0024] Ideally, the fastening operation should proceed with the pin axis approximately aligned with the box axis, i.e., with the pin aligned with the box. However, the fastening operation may proceed with the pin axis deviating from the box axis, i.e., with the pin misaligned with the box. When the fastening operation proceeds in a misaligned state, scratches and seizures are more likely to occur than when the fastening operation proceeds in an aligned state.

[0025] First, regarding the external seizure that mainly occurs, as described above, an external incomplete male thread portion has a shape in which the crest of the thread is cut off by a cylindrical surface. Therefore, in an external incomplete male thread portion, the outer peripheral edge of the male thread stab flank surface forms a sharp edge. This sharp edge is inevitably formed by thread cutting of the male thread portion. In this specification, the male thread stab flank surface in an external incomplete male thread portion is also referred to as the "incomplete male thread stab flank surface."

[0026] If the pin is misaligned with the box during the tightening process, the sharp outer peripheral edge of the incomplete male thread stab flank will make uneven contact and slide against the female thread stab flank of the complete female thread. This uneven contact and sliding will cause scratches on the female thread stab flank of the complete female thread, resulting in external seizure.

[0027] Here, a comparison was made between threaded joints of the same design when a solid lubricating coating was used and when a dope was used. When dope was used, scratches were sometimes observed, but these scratches rarely developed into seizure. When a solid lubricating coating was used, seizure was clearly more likely to occur.

[0028] Specifically, in the case of a threaded joint having dope on the surface of the female thread, even if the dope is expelled by the sharp outer peripheral edge of the incomplete male thread insertion flank, the surrounding dope immediately flows into the location where the dope was expelled, thereby gradually self-repairing the lubricating layer formed by the dope.

[0029] In contrast, in threaded joints with a solid lubricating coating on the surface of the female thread, the solid lubricating coating is dug up by the sharp outer circumferential edge of the imperfect male thread stab flank. Because the coating is solid, the dug-up area of ​​the solid lubricating coating cannot be repaired, resulting in a loss of the coating. In areas where the coating is lost, the outer circumferential edge of the male thread stab flank easily reaches the surface of the base material in the female thread, resulting in metal-to-metal contact between metals of the same material. This metal-to-metal contact causes scratches, which can lead to digging and adhesion to the base material, resulting in earlier seizure than when doped materials are used.

[0030] Furthermore, in the case of threaded joints with a solid lubricating coating on the surface of the male thread, the solid lubricating coating covering the outer periphery of the incomplete male thread stab flank slides in uneven contact with the female thread stab flank at very high contact pressure, causing it to wear away. Because the worn-out solid lubricating coating cannot be repaired, the base material is exposed at the outer periphery of the incomplete male thread stab flank. This exposed base material comes into metal-to-metal contact with the female thread stab flank, where the metals are of the same quality. This metal-to-metal contact can cause the base material to dig out or adhere, resulting in early onset of seizure.

[0031] In other words, in the case of threaded joints equipped with a solid lubricating coating, the solid lubricating coating rarely self-repairs during sliding. Therefore, once localized damage such as scratches or abrasion occurs in the solid lubricating coating, the damage becomes irreparable. In this case, the damage easily develops into seizure. Therefore, outer seizure occurs on the stab flank of the female thread, which corresponds to the outer incomplete male thread portion of the complete female thread.

[0032] Therefore, in order to prevent external seizure even when the outer peripheral edge of the incomplete male thread stab flank comes into metal-to-metal contact with the female thread stab flank of the complete female thread, the inventors first considered providing a metal plating layer that is harder than the base metal and less likely to adhere to the base metal as a base for the solid lubricating coating. However, if a hard metal plating layer is provided on the surface of the pin, i.e., the male thread portion, the outer peripheral edge of the incomplete male thread stab flank is strengthened. In this case, regardless of whether the surface of the box, i.e., the female thread portion, is coated with a solid lubricating coating, the strengthened outer peripheral edge of the incomplete male thread stab flank comes into contact and slides with the female thread stab flank of the complete female thread, promoting the occurrence of scratches.

[0033] Therefore, the metal plating layer may be provided on the surface of the box, i.e., the female thread, so that the female thread stab flank is stronger than the outer peripheral edge of the incomplete male thread stab flank. In this case, the metal plating layer provided on the surface of the female thread dulls the outer peripheral edge of the incomplete male thread stab flank due to sliding contact with the metal plating layer provided on the female thread stab flank, thereby reducing the contact pressure between the two and suppressing digging into the solid lubricant coating. As a result, scratches are less likely to occur on the female thread stab flank of the complete female thread. Even if the solid lubricant coating wears away and the outer peripheral edge of the incomplete male thread stab flank comes into metal-to-metal contact with the female thread stab flank, adhesion between the dissimilar metals and seizure are less likely to occur.

[0034] Even if a solid lubricating coating is provided on the surface of the male thread of an object on which a solid lubricating coating is to be applied, as described above, the solid lubricating coating covering the outer peripheral edge of the incomplete male thread stab flank will quickly wear away. Therefore, the solid lubricating coating only needs to be provided on the surface of the box, i.e., the female thread. In other words, the solid lubricating coating only needs to be formed on the metal plating layer provided on the female thread.

[0035] When a laminate coating including a solid lubricating coating and a metal plating layer is provided on the surface of the female thread portion in this manner, the frequency of external seizure between the outer peripheral edge of the incomplete male thread insertion flank surface and the female thread insertion flank surface of the complete female thread portion is reduced.

[0036] However, with regard to the secondary occurrence of internal seizure, as described above, the internal incomplete female thread has a shape in which the crest of the thread is cut off by a tapered surface or a cylindrical surface. Therefore, in the internal incomplete female thread, the inner peripheral edge of the female thread stab flank has a sharp edge. This sharp edge is inevitably formed by thread cutting of the female thread. In this specification, the female thread stab flank in the internal incomplete female thread is also referred to as the "incomplete female thread stab flank."

[0037] When a hard metal plating layer is provided on the surface of the female thread, the sharp inner peripheral edge of the incomplete female thread stab flank is strengthened. In this case, the strengthened inner peripheral edge of the incomplete female thread stab flank slides against the male thread stab flank of the complete male thread while making uneven contact. This uneven contact and sliding causes scratches on the male thread stab flank of the complete male thread, which makes it easy for internal seizure to occur.

[0038] In order to prevent such inner seizure from occurring, the inventors further analyzed the fastening operation in detail and conducted extensive research.

[0039] During the make-up operation, torque is generated as the pin is screwed into the box, and this torque must be managed. To manage the torque, threaded joints include a torque stop mechanism. The make-up operation of screwing the pin into the box is divided into an early stage and a late stage. In the early stage, the torque is small, and the pin can be rotated by hand. This early stage is called the hand make-up stage. In contrast, in the late stage, the torque is large, and the pin is rotated by a machine (power tongs). This late stage is sometimes called the power make-up stage or the machine make-up stage.

[0040] Typically, in the fastened state where the pin and box are fully fastened, the male thread root surface is in interference contact with the female thread crest surface, and a radial interference is provided between the male thread root surface and the female thread crest surface. In the fastening operation, in the hand make-up stage, the pin is screwed into the box, and the male thread root surface contacts the female thread crest surface. When the male thread root surface contacts the female thread crest surface, torque begins to increase. The point at which the male thread root surface contacts the female thread crest surface is the end point of the hand make-up stage, and the state at this point is called hand tightening. The power make-up stage begins from the point at which hand tightening is reached. In the power make-up stage, the pin is further screwed into the box, and the torque stop mechanism is activated. When the torque stop mechanism is activated, the torque increases rapidly.

[0041] The torque stop mechanism serves to limit the screwing of the pin into the box. For example, if the type of thread that constitutes the threaded portion (male thread portion and female thread portion) is a buttress thread (trapezoidal thread) or a square thread, a pin shoulder surface is provided on the pin as the pin-side torque stop mechanism, and a box shoulder surface is provided on the box as the box-side torque stop mechanism. The torque stop mechanism functions when the shoulder surfaces come into contact with each other. This torque stop mechanism is also called a shouldering mechanism.

[0042] Furthermore, when the type of thread is a wedge thread, the male thread stab flank and male thread load flank are used as the pin-side torque stop mechanism, and the female thread stab flank and female thread load flank are used as the box-side torque stop mechanism. The torque stop mechanism functions when the stab flanks contact each other and the load flanks contact each other. This torque stop mechanism is also called a self-locking mechanism of the thread.

[0043] Once the torque stop mechanism has functioned, the pin can be turned slightly to complete the make-up. Generally, the make-up of a steel pipe threaded joint is performed using torque management, and the make-up is completed when the recommended make-up torque is reached, which is determined to be within the range from when the torque stop mechanism begins to function until irreparable damage (such as excessive plastic deformation of the components of the threaded joint, including the torque stop mechanism) occurs. The point at which make-up is complete marks the end of the power make-up phase, and the state at this point is called power tight. Therefore, the power tight state can be said to be the tightened state.

[0044] The power make-up stage is the stage after the threads begin to make interference contact, i.e., after hand tightening. At this stage, the make-up operation basically proceeds with the pin aligned with the box. Therefore, during the power make-up stage, it is unlikely that the inner peripheral edge of the incomplete female thread insertion flank will come into partial contact with and slide against the male thread insertion flank of the complete male thread.

[0045] In contrast, the hand make-up stage is the stage before the threaded portions begin to come into interference contact, i.e., the stage before hand tightening. At this stage, the engagement between the male and female threaded portions is loose, and the make-up operation is likely to proceed with the pin misaligned relative to the box. If the make-up operation proceeds in a misaligned state during the hand make-up stage, a situation may occur in which the inner peripheral edge of the incomplete female thread insertion flank comes into uneven contact with and slides against the male thread insertion flank of the complete male thread portion.

[0046] Based on such considerations of the make-up operation, it is believed that scratches that cause internal seizure occur during the hand make-up stage, when the make-up operation is likely to proceed in a misaligned state. Therefore, the inventors of the present invention have conceived that scratches can be prevented and, as a result, the occurrence of internal seizure can be suppressed by preventing the inner peripheral edge of the incomplete female thread insertion flank from coming into uneven contact with the male thread insertion flank of the complete male thread portion during the hand make-up stage.

[0047] Here, while the pin is being screwed into the box, the axis of the pin and the axis of the box rarely truly coincide. In this case, the pin is screwed into the box with a slight axial runout. Therefore, a state in which the pin is aligned with the box means that a certain degree of runout of the axis of the pin relative to the axis of the box is tolerated. If the axial runout is within a range such that the inner peripheral edge of the incomplete female thread insertion flank does not come into significant offset contact with the male thread insertion flank of the complete male thread, the local contact pressure between the two will not become extremely large, and this state can be considered to be an aligned state.

[0048] The inventors conducted extensive research into the allowable axial runout and discovered the following: If the runout angle of the pin's axis relative to the box's axis, i.e., the misalignment angle, is 0.5° or less, even if the inner peripheral edge of the incomplete female thread stab flank comes into uneven contact with the male thread stab flank of the complete male thread, significant uneven contact will not occur between the two, and extremely large localized contact pressure will not be generated. Even during the hand make-up stage, the final period is just before hand tightening, and the misalignment angle is at most 0.5°. Therefore, if the inner peripheral edge of the incomplete female thread stab flank does not come into uneven contact with the male thread stab flank of the complete male thread before the final period of the hand make-up stage, internal seizure can be suppressed.

[0049] Therefore, in order to prevent internal seizure when the pin of the connecting pipe is screwed into the box and fastened, it is sufficient that the region of the complete male thread does not reach the region of the internal incomplete female thread before the final period of the hand make-up stage. In other words, even if the region of the complete male thread overlaps with the region of the internal incomplete female thread in the fastened state (power tight state), it is sufficient that the region of the complete male thread reaches the region of the internal incomplete female thread during the final period of the hand make-up stage or the power make-up stage.

[0050] Specifically, the end of the complete male thread portion on the inner incomplete male thread portion side is referred to as the "complete male thread start end," and the end of the complete female thread portion on the inner incomplete female thread portion side is referred to as the "complete female thread start end." In other words, the complete male thread start end is the end of the complete male thread portion that is closest to the tip of the pin. The complete female thread start end is the end of the complete female thread portion that is closest to the innermost part of the box. Of the two ends of the complete female thread portion, the end of the complete female thread portion that is closest to the pipe end of the box is also referred to as the "complete female thread end end."

[0051] Furthermore, in the fastened state (power tight state), the distance in the pipe axis direction of the threaded joint from the tip of the pin to the start of the complete male thread is defined as LP, and the distance in the pipe axis direction of the threaded joint from the tip of the pin to the start of the complete female thread is defined as LB. In this case, to suppress internal seizure, when the pin is threaded into the box to be fastened, the start of the complete male thread simply needs to reach the start of the complete female thread during the final period of the hand make-up phase or the power make-up phase. To achieve this condition, the difference between the distances LB and LP, "LB - LP," needs to be less than or equal to the sum of the distance in the pipe axis direction that the pin advances as it is threaded into the box during the final period of the hand make-up phase and the distance in the pipe axis direction that the pin advances as it is threaded into the box during the power make-up phase (the period from hand tight to power tight). In this specification, the former is also referred to as the "first distance" and the latter is also referred to as the "second distance." For the first distance, the misalignment angle is a maximum of 0.5° at the beginning of the period at the end of the hand make-up stage, gradually decreases during that period, and reaches 0° at the end of that period (hand tight).

[0052] The threaded joint according to the embodiment of the present disclosure has been completed based on the above findings.

[0053] The threaded joint according to this embodiment is a threaded joint for connecting steel pipes. The threaded joint includes a tubular pin and a tubular box. The pin is provided contiguous with the steel pipe body. The pin is inserted into and fastened to the box. The pin includes a tapered male thread portion and a pin-side torque stop mechanism. The tapered male thread portion is provided on the outer peripheral surface of the pin. The pin-side torque stop mechanism is provided to limit the threading of the pin into the box when fastening the pin to the box. The box includes a tapered female thread portion and a box-side torque stop mechanism. The tapered female thread portion is provided on the inner peripheral surface of the box in correspondence with the tapered male thread portion. The tapered female thread portion has a thread taper equal to the thread taper of the tapered male thread portion. The box-side torque stop mechanism is provided in correspondence with the pin-side torque stop mechanism.

[0054] The tapered male thread portion includes a male thread crest surface, a male thread root surface, a male thread stab flank surface, and a male thread load flank surface. The tapered male thread portion is divided into a complete male thread portion and an incomplete male thread portion. The incomplete male thread portion is formed contiguous to the complete male thread portion toward the tip side of the pin. The tapered female thread portion includes a female thread root surface, a female thread crest surface, a female thread stab flank surface, and a female thread load flank surface. The female thread root surface corresponds to the male thread crest surface. The female thread crest surface corresponds to the male thread root surface. The female thread stab flank surface corresponds to the male thread stab flank surface. The female thread load flank surface corresponds to the male thread load flank surface. The tapered female thread portion is divided into a complete female thread portion and an incomplete female thread portion. The incomplete female thread portion is formed contiguous to the complete female thread portion toward the innermost side of a box. The inner peripheral surface of the box is coated with a laminate coating. The laminate coating includes a metal plating layer and a solid lubricating coating. The metal plating layer has a hardness higher than that of the base material of the pin. The solid lubricating coating is laminated on the metal plating layer.

[0055] In the fastened state of the pin and box, the threaded joint has the following configuration: The male thread root surface is in interference contact with the female thread crest surface. The male thread load flank surface is in pressing contact with the female thread load flank surface. The pin-side torque stop mechanism and the box-side torque stop mechanism function reciprocally. Furthermore, the distance LP in the pipe axial direction of the threaded joint from the tip of the pin to the end of the complete male thread portion on the incomplete male thread portion side, and the distance LB in the pipe axial direction from the tip of the pin to the end of the complete female thread portion on the incomplete female thread portion side, satisfy formula (1) (first configuration). α×L / (α+T / 100)+(δ+2×t) / T×100≧LB-LP (1) The meanings of the symbols in formula (1) are as follows: α: tan 0.5°, L: The distance in the pipe axis direction between the end of the complete male thread on the incomplete male thread side and the end of the complete female thread on the pipe end side of the box at the time when the male thread bottom surface contacts the female thread crest surface when the pin is fastened to the box [mm], T: Thread taper of the male and female threads [%] δ: Interference between the male thread root surface and the female thread crest surface [mm], and t: Thickness of the laminated coating [mm].

[0056] In a threaded joint according to the first configuration, the inner peripheral surface of the box, on which the female thread is formed, is coated with a laminated coating including a metal plating layer and a solid lubricating coating laminated on the metal plating layer. The metal plating layer is harder than the base material of the pin. In this case, the female thread of the box, i.e., the female thread stab flank, is strengthened by the metal plating layer more than the male thread of the pin. Therefore, if the male thread has an outer incomplete male thread on the steel pipe body side of the complete male thread during fastening, even if the outer peripheral edge of the stab flank of the outer incomplete male thread comes into metal-to-metal contact with the female thread stab flank of the complete female thread, excessive scratches are unlikely to occur on the female thread stab flank. This reduces external seizure.

[0057] Furthermore, in a threaded joint according to the first configuration, when the pin and box are fastened together, the distance LP in the pipe axial direction from the tip of the pin to the complete male thread start end (the end of the complete male thread on the inner incomplete male thread side) and the distance LB in the pipe axial direction from the tip of the pin to the complete female thread start end (the end of the complete female thread on the inner incomplete female thread side) satisfy formula (1). Formula (1) indicates that the difference between distances LB and LP, "LB - LP," shown on the right side, is less than or equal to the sum of the first and second terms shown on the left side. The first term on the left side corresponds to the first distance described above (the distance in the pipe axial direction that the pin advances as it is threaded into the box during the final stage of the hand make-up phase). Meanwhile, the second term on the left side corresponds to the second distance described above (the distance in the pipe axial direction that the pin advances as it is threaded into the box during the power make-up phase). In other words, a threaded joint according to the first configuration is designed so that "LB - LP" is less than or equal to the sum of the first and second distances, as shown in formula (1).

[0058] According to Equation (1), since "LB - LP" is equal to or less than the sum of the first distance and the second distance, when the pin is threaded into the box to be tightened, the complete male thread start end reaches the complete female thread start end during the final period of the hand makeup phase or during the power makeup phase. Alternatively, the complete male thread start end does not reach the complete female thread start end even at the end of the power makeup phase. In either of these cases, the complete male thread region does not reach the internal incomplete female thread region before the final period of the hand makeup phase. Therefore, as the tightening operation progresses, the inner peripheral edge of the incomplete female thread stab flank does not come into significant metal-to-metal contact with the male thread stab flank of the complete male thread, and excessive scratches are unlikely to occur on the male thread stab flank. This reduces internal seizure. Therefore, the threaded joint of the first configuration makes it possible to suppress seizure when tightening the pin into the box while employing dope-free technology.

[0059] In formula (1), the lower limit of "LB - LP" is not particularly limited. "LB - LP" is typically a positive value. In this case, the distance LB is greater than the distance LP. "LB - LP" may be zero or a negative value.

[0060] If "LB - LP" is a negative value, the distance LB is smaller than the distance LP. In this case, even at the end of the power make-up phase, the complete male thread start end does not reach the complete female thread start end, and in the tightened state, the area of ​​the inner incomplete female thread does not overlap the area of ​​the complete male thread. Therefore, as the tightening operation progresses, the inner peripheral edge of the incomplete female thread stab flank does not come into contact with the male thread stab flank of the complete male thread, and excessive scratches do not occur on the male thread stab flank.

[0061] However, in reality, the outer and inner diameters of the pin and box are subject to strict constraints. Therefore, a negative "LB-LP" value leads to the following problems. To achieve a negative "LB-LP" value, the length of the female thread must be extended toward the innermost part of the box. In this case, the thickness of the lip portion formed at the tip of the pin becomes thinner as the female thread is extended toward the innermost part. For example, a pin seal surface is often provided on the outer peripheral surface of the lip portion of the pin, and a box seal surface is provided on the inner peripheral surface of the box corresponding to the pin seal surface. A thin lip reduces the rigidity of the lip and reduces the external pressure sealing performance due to the metal-to-metal contact between the seal surfaces. Furthermore, as a torque stop mechanism, a pin shoulder surface is often provided at the tip of the pin, i.e., the lip, and a box shoulder surface is provided at the innermost part of the box corresponding to the pin shoulder surface. A thin lip reduces the contact area between the shoulder surfaces, resulting in reduced torque resistance. Furthermore, to ensure a sufficient lip thickness, the diameter of the thread must be further enlarged. In this case, as the diameter of the threaded portion increases, the area of ​​the critical cross section of the box decreases, and the tensile performance decreases.

[0062] Therefore, in order to suppress seizure while ensuring the basic performance of a threaded joint, it is preferable that "LB-LP" be as large as possible within the range that satisfies formula (1).

[0063] Specifically, in the above-mentioned threaded joint, it is further preferable that the distance LP and the distance LB satisfy formula (2) (second configuration). LB-LP>0 (2)

[0064] In the second configuration, "LB - LP" is a positive value according to equation (2). In this case, "LB - LP" exceeds zero and is equal to or less than the sum of the first distance and the second distance. Therefore, when the pin is screwed into the box to be fastened, the start end of the complete male thread reaches the start end of the complete female thread either during the final period of the hand make-up phase or during the power make-up phase.

[0065] In a threaded joint according to the first configuration, it is even more preferable that the distance LP and the distance LB satisfy formula (3) (third configuration). LB-LP≧(δ+2×t) / T×100 (3)

[0066] In the third configuration, the lower limit of "LB - LP" is the second distance according to formula (3). In this case, "LB - LP" is equal to or greater than the second distance and equal to or less than the sum of the first and second distances. Therefore, when the pin is screwed into the box and fastened, the start end of the complete male thread reaches the start end of the complete female thread during the final stage of the hand make-up phase.

[0067] The above-mentioned threaded joint can employ the following configuration. The thread pitch of the male thread stab flank, the thread pitch of the female thread stab flank, the thread pitch of the male thread load flank, and the thread pitch of the female thread load flank are all equal. The pin further includes a pin shoulder surface as a pin-side torque stop mechanism, and the box further includes a box shoulder surface as a box-side torque stop mechanism. In this case, when the pin is fastened to the box, the pin shoulder surface comes into contact with the box shoulder surface, causing the pin-side torque stop mechanism and the box-side torque stop mechanism to function together (fourth configuration). In this case, the type of thread that constitutes the threaded portion is a buttress thread (trapezoidal thread) or a square thread. A threaded joint according to the fourth configuration is equipped with a shouldering mechanism as a torque stop mechanism.

[0068] A threaded joint according to any one of the first to third configurations can employ the following configurations. The thread pitch of the male stab flank is equal to the thread pitch of the female stab flank, and the thread pitch of the male load flank is equal to the thread pitch of the female load flank. The thread pitch of each of the male stab flank and female stab flank is smaller than the thread pitch of each of the male load flank and female load flank. In a tapered male thread portion, the width of the male thread defined by the male crest, male stab flank, and male load flank decreases toward the tip of the pin, and the width of the male thread groove defined by the male root, male stab flank, and male load flank increases toward the tip of the pin. In a tapered female thread portion, the width of the female thread groove defined by the female thread root surface, female thread stab flank, and female thread load flank decreases toward the innermost side of the box, corresponding to the width of the male thread, while the width of the female thread defined by the female thread crest surface, female thread stab flank, and female thread load flank increases toward the innermost side of the box, corresponding to the width of the male thread groove. The male thread stab flank and male thread load flank form the pin-side torque stop mechanism, and the female thread stab flank and female thread load flank form the box-side torque stop mechanism. In this case, when the pin is fastened to the box, the male thread load flank comes into contact with the female thread load flank and the male thread stab flank comes into contact with the female thread stab flank, thereby causing the pin-side torque stop mechanism and the box-side torque stop mechanism to function together (fifth configuration). In this case, the type of thread that constitutes the threaded portion is a so-called wedge thread. A threaded joint according to the fifth configuration is equipped with a self-locking mechanism as a torque stop mechanism.

[0069] In the above-described threaded joint, the metal plating layer is preferably an alloy plating layer (sixth configuration). As the alloy plating layer, for example, a Zn alloy plating layer can be applied.

[0070] In the above threaded joint, preferably, the pin is provided on each of the steel pipes to be connected, and the box is provided on the coupling, which is a pipe material separate from the steel pipes (seventh configuration). In this case, the threaded joint is of the coupling type.

[0071] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and redundant description will not be repeated.

[0072] First Embodiment [Threaded joint configuration] FIG. 1 is a longitudinal cross-sectional view of a threaded joint 100 according to the first embodiment. A longitudinal cross-section of the threaded joint 100 is a cross-section obtained by cutting the threaded joint 100 along a plane including the pipe axis X. In this specification, the direction in which the pipe axis X of the threaded joint 100 extends may be referred to as the pipe axis direction. The radial direction of the threaded joint 100, i.e., the direction perpendicular to the pipe axis direction, may be simply referred to as the radial direction. FIG. 1 shows the threaded joint 100 in a made-up state (power-tight state).

[0073] Referring to Figure 1, a threaded joint 100 is used to connect steel pipes 30 together. The threaded joint 100 includes a tubular pin 10 and a tubular box 20. The pin 10 has its own pipe axis X1. The box 20 has its own pipe axis X2. In design, the pipe axis X1 of the pin 10 coincides with the pipe axis X2 of the box 20, and the pipe axes X1 and X2 coincide with the pipe axis X of the threaded joint 100.

[0074] The pin 10 is provided at the end of the steel pipe 30. In this embodiment, the box 20 is provided on a coupling 40, which is a pipe material different from the steel pipe 30 to be connected. In this case, the threaded joint 100 is a coupling type. The material of the steel pipe 30 (strictly speaking, the material of the base material of the steel pipe 30) is, for example, low-chromium alloy martensitic steel, martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel, etc. The material of the coupling 40 (strictly speaking, the material of the base material of the coupling 40) is the same as the material of the steel pipe 30. The tensile strength and hardness of the coupling 40 and the steel pipe 30 may be the same or different. Furthermore, the material of the coupling 40 may be different from the material of the steel pipe 30.

[0075] The pin 10 is inserted into the box 20 and fastened to the box 20. The pin 10 is provided contiguous with the main body 31 of the steel pipe 30. The pin 10 includes a tapered male thread portion 11. In this embodiment, the pin 10 further includes a pin seal surface 12. The threaded joint 100 of this embodiment also includes a torque stop mechanism, and as this torque stop mechanism, it includes a shouldering mechanism. Specifically, the pin 10 further includes a pin shoulder surface 13 as a pin-side torque stop mechanism. The male thread portion 11, pin seal surface 12, and pin shoulder surface 13 are arranged in this order from the steel pipe main body 31 side toward the tip 10a side of the pin 10.

[0076] The tapered male thread portion 11 is provided on the outer peripheral surface of the pin 10. The male thread portion 11 is arranged on the steel pipe body 31 side relative to the pin seal surface 12. The male thread portion 11 extends from the vicinity of the pin seal surface 12 to the vicinity of the steel pipe body 31. The male thread portion 11 is composed of a tapered thread. The male thread portion 11 has a thread taper T [%]. The thread taper T of the male thread portion 11 is constant over the entire length of the male thread portion 11. The type of thread that constitutes the male thread portion 11 is, for example, an API standard buttress thread (trapezoidal thread) or a square thread.

[0077] The pin seal surface 12 is provided on the outer peripheral surface of the pin 10 between the male thread portion 11 and the pin shoulder surface 13. Specifically, the pin seal surface 12 is provided on the outer peripheral surface of a lip portion 10b formed on the tip 10a side of the pin 10. The pin seal surface 12 is the outer surface of a rotor whose axis is the pipe axis X1, i.e., the pipe axis X of the threaded joint 100. In a vertical cross-sectional view of the threaded joint 100, the pin seal surface 12 may be a curve that convexly extends toward the box 20, or a straight line that is inclined relative to the pipe axis X so as to approach the pipe axis X toward the tip 10a of the pin 10. The pin seal surface 12 may be configured by combining two or more of these curves and / or straight lines.

[0078] The pin shoulder surface 13 is provided on the tip 10a of the pin 10. Specifically, the pin shoulder surface 13 is provided on the tip of the lip portion 10b. The pin shoulder surface 13 forms the pipe end surface of the pin 10 and is an annular surface whose axis is the pipe axis X1, i.e., the pipe axis X of the threaded joint 100. The pin shoulder surface 13 may or may not be inclined so that its outer peripheral edge is located closer to the tip 10a of the pin 10 than the inner peripheral edge.

[0079] The box 20 includes a tapered female thread portion 21. In this embodiment, the box 20 further includes a box seal surface 22. The box 20 further includes a box shoulder surface 23 as a box-side torque stop mechanism. The female thread portion 21, the box seal surface 22, and the box shoulder surface 23 are provided on the box 20 in correspondence with the male thread portion 11, the pin seal surface 12, and the pin shoulder surface 13, respectively.

[0080] The tapered female thread portion 21 is provided on the inner peripheral surface of the box 20. The female thread portion 21 is arranged on the pipe end side of the box 20 with respect to the box seal surface 22. The female thread portion 21 extends from near the box seal surface 22 to near the pipe end of the box 20. The female thread portion 21 is composed of a tapered thread that meshes with the male thread portion 11 of the pin 10. The female thread portion 21 has a thread taper T that is equal to the thread taper T [%] of the male thread portion 11. The thread taper T of the female thread portion 21 is constant over the entire length of the female thread portion 21. The type of thread that constitutes the female thread portion 21 corresponds to the male thread portion 11, for example, an API standard buttress thread (trapezoidal thread) or a square thread.

[0081] The box seal surface 22 is provided on the inner peripheral surface of the box 20 between the female thread portion 21 and the box shoulder surface 23. The box seal surface 22 is the inner surface of a body of revolution whose axis is the pipe axis X2, i.e., the pipe axis X of the threaded joint 100. In a vertical cross-sectional view of the threaded joint 100, the box seal surface 22 may be a curve that convexly extends toward the pin 10, or may be a straight line that is inclined relative to the pipe axis X so as to approach the pipe axis X as it extends toward the innermost part of the box 20. The box seal surface 22 may be configured by combining two or more of these curves and / or straight lines.

[0082] The box shoulder surface 23 is provided at the deepest part of the box 20, corresponding to the pin shoulder surface 13. Like the pin shoulder surface 13, the box shoulder surface 23 is an annular surface whose axis is the pipe axis X2, i.e., the pipe axis X of the threaded joint 100. The box shoulder surface 23 may or may not be inclined so that its outer peripheral edge is located deeper inside the box 20 than the inner peripheral edge. The inclination angle (shoulder angle) of the box shoulder surface 23 is substantially equal to the inclination angle of the pin shoulder surface 13.

[0083] The configuration of a threaded joint 100 according to this embodiment will be described in more detail below with reference to Figures 1 to 7. Figure 2 is a longitudinal cross-sectional view of the pin 10 and the box 20 in the threaded joint 100 shown in Figure 1. In order to make it easier to understand the configuration of the pin 10 and the box 20, Figure 2 shows the pin 10 and the box 20 separately, with their positions in the directions of the pipe axes X, X1, and X2 aligned.

[0084] 2, the tapered male thread portion 11 includes a male thread crest surface 111, a male thread root surface 112, a male thread stab flank surface 113, and a male thread load flank surface 114. The tapered female thread portion 21 includes a female thread root surface 211, a female thread crest surface 212, a female thread stab flank surface 213, and a female thread load flank surface 214. The female thread root surface 211, the female thread crest surface 212, the female thread stab flank surface 213, and the female thread load flank surface 214 are provided in the box 20 corresponding to the male thread crest surface 111, the male thread root surface 112, the male thread stab flank surface 113, and the male thread load flank surface 114, respectively.

[0085] In the male thread portion 11, the male thread crest surface 111 is connected to the male thread root surface 112 by the male thread stab flank surface 113 and the male thread load flank surface 114. The male thread stab flank surface 113 is located in front of the male thread crest surface 111 in the direction of threading of the pin 10 into the box 20 during make-up. The male thread load flank surface 114 is located behind the male thread crest surface 111 in the direction of threading of the pin 10.

[0086] The male thread crest surface 111 and the male thread root surface 112 are straight lines inclined with respect to the pipe axis X1 so as to approach the pipe axis X1 toward the tip 10a of the pin 10 in a vertical cross-sectional view of the threaded joint 100. The male thread crest surface 111 and the male thread root surface 112 may also be straight lines parallel to the pipe axis X1 in a vertical cross-sectional view of the threaded joint 100.

[0087] The male thread insertion flank surface 113 is a straight line that is inclined so that its outer peripheral edge is positioned closer to the steel pipe body 31 than the inner peripheral edge when viewed in vertical cross section of the threaded joint 100.

[0088] Like the male thread stab flank surface 113, the male thread load flank surface 114 is a straight line that is inclined so that its outer circumferential edge is located closer to the steel pipe body 31 than the inner circumferential edge in a longitudinal cross-sectional view of the threaded joint 100. The male thread load flank surface 114 may be a straight line that is inclined so that its outer circumferential edge is located closer to the tip 10a of the pin 10 than the inner circumferential edge in a longitudinal cross-sectional view of the threaded joint 100, or may be a straight line that is perpendicular to the pipe axis X1.

[0089] In the female thread portion 21, the female thread root surface 211 and the female thread crest surface 212 are connected by a female thread stab flank surface 213 and a female thread load flank surface 214. The female thread stab flank surface 213 is located in front of the female thread root surface 211 in the direction of threading of the pin 10 during fastening. The female thread load flank surface 214 is located behind the female thread root surface 211 in the direction of threading of the pin 10.

[0090] In a vertical cross-sectional view of the threaded joint 100, the female thread root surface 211 and the female thread crest surface 212 are straight lines inclined with respect to the pipe axis X2 so as to approach the pipe axis X2 toward the innermost side of the box 20. The inclination angle of the female thread root surface 211 and the female thread crest surface 212 with respect to the pipe axis X2 is substantially equal to the inclination angle of the male thread crest surface 111 and the male thread root surface 112 with respect to the pipe axis X1. In a vertical cross-sectional view of the threaded joint 100, the male thread crest surface 111 and the male thread root surface 112 may be straight lines parallel to the pipe axis X1, in which case the female thread root surface 211 and the female thread crest surface 212 are straight lines parallel to the pipe axis X2. In short, the female thread root surface 211 and the female thread crest surface 212 are parallel to the male thread crest surface 111 and the male thread root surface 112, respectively.

[0091] The female thread stab flank surface 213 is a straight line that is inclined so that its outer peripheral edge is located closer to the pipe end of the box 20 than the inner peripheral edge in a longitudinal cross-sectional view of the threaded joint 100. The inclination angle of the female thread stab flank surface 213 is substantially equal to the inclination angle of the male thread stab flank surface 113. In other words, the female thread stab flank surface 213 is parallel to the male thread stab flank surface 113.

[0092] Like the female thread stab flank surface 213, the female thread load flank surface 214 is a straight line that is inclined so that its outer peripheral edge is located closer to the pipe end of the box 20 than the inner peripheral edge in a longitudinal cross-sectional view of the threaded joint 100. The female thread load flank surface 214 may also be a straight line that is inclined so that its outer peripheral edge is located closer to the inner depth of the box 20 than the inner peripheral edge in a longitudinal cross-sectional view of the threaded joint 100. The inclination angle of the female thread load flank surface 214 is substantially equal to the inclination angle of the male thread load flank surface 114. Like the male thread load flank surface 114, the female thread load flank surface 214 may also be a straight line that is perpendicular to the pipe axis X2 in a longitudinal cross-sectional view of the threaded joint 100. In other words, the female thread load flank surface 214 is parallel to the male thread load flank surface 114.

[0093] In this specification, it is stated that each surface constituting the threaded portion (male threaded portion 11 and female threaded portion 21) is straight when viewed in cross section of the threaded joint 100. This means that the main area of ​​each surface of the threaded portion is straight when viewed in cross section of the threaded joint 100. In other words, the threaded portion of the threaded joint 100 may also include a threaded portion in which an arc portion or a secondary chamfer is provided at the connection between adjacent surfaces.

[0094] Specifically, the male thread stab flank surface 113 may be connected to the male thread crest surface 111 and the male thread root surface 112 via arc portions, respectively, when viewed in vertical cross section of the threaded joint 100. The male thread load flank surface 114 may be connected to the male thread crest surface 111 and the male thread root surface 112 via arc portions, respectively, when viewed in vertical cross section of the threaded joint 100.

[0095] The female thread stab flank surface 213 may be connected to the female thread root surface 211 and the female thread crest surface 212 via arc portions, respectively, when viewed in vertical cross section of the threaded joint 100. The female thread load flank surface 214 may be connected to the female thread root surface 211 and the female thread crest surface 212 via arc portions, respectively, when viewed in vertical cross section of the threaded joint 100. For example, the radius of curvature of the arc portion connecting the female thread stab flank surface 213 and the female thread crest surface 212 is larger than the radius of curvature of the other arc portions. Also, for example, a chamfer that is linear in cross section may be provided between the female thread stab flank surface 213 and the female thread crest surface 212, and both ends of the chamfer may be connected to the female thread stab flank surface 213 and the female thread crest surface 212 by arc portions, respectively.

[0096] 1 and 2, when the pin 10 is screwed into the box 20 and the pin 10 and box 20 are fastened together (power-tight state), the male thread portion 11 engages with the female thread portion 21. Specifically, the male thread root surface 112 is in interference contact with the female thread crest surface 212, and a radial interference amount δ [mm] (not shown) is provided between the male thread root surface 112 and the female thread crest surface 212. In the hand make-up stage before reaching the fastened state, the male thread root surface 112 comes into contact with the female thread crest surface 212, resulting in the hand-tight state.

[0097] In the fastened state, the male thread load flank surface 114 is in pressing contact with the female thread load flank surface 214. Meanwhile, the male thread crest surface 111 is not in contact with the female thread root surface 211, and a gap is provided between the male thread crest surface 111 and the female thread root surface 211. The male thread stab flank surface 113 is also not in contact with the female thread stab flank surface 213, and a gap is also provided between the male thread stab flank surface 113 and the female thread stab flank surface 213.

[0098] In the fastened state, the pin seal surface 12 is in interference contact with the box seal surface 22, and together with the box seal surface 22, they form a seal portion through metal-to-metal contact. In the fastened state, the pin shoulder surface 13 is in pressing contact with the box shoulder surface 23, and together with the box shoulder surface 23 they form a shoulder portion. The shoulder surfaces 13, 23 are in pressing contact with each other, and this applies a tightening axial force to the mating male thread portion 11 and female thread portion 21. In the power make-up stage before the fastened state is reached, the pin shoulder surface 13 comes into contact with the box shoulder surface 23, causing the pin-side torque stop mechanism (pin shoulder surface 13) and the box-side torque stop mechanism (box shoulder surface 23) to function together, and as a result, the shouldering mechanism (torque stop mechanism) functions.

[0099] Figures 3 and 4 are partially enlarged views of the threaded portion of the threaded joint 100 shown in Figure 2. Figure 3 shows the portion of the male threaded portion 11 on the tip side of the pin 10. Figure 4 shows the portion of the female threaded portion 21 on the innermost side of the box 20.

[0100] 2 and 3, the tapered male thread portion 11 is divided into a complete male thread portion 11a, an outer incomplete male thread portion 11b, and an inner incomplete male thread portion 11c. That is, the male thread portion 11 is composed of the complete male thread portion 11a, the outer incomplete male thread portion 11b, and the inner incomplete male thread portion 11c. The inner incomplete male thread portion 11c, the complete male thread portion 11a, and the outer incomplete male thread portion 11b are arranged in this order from the tip 10a side of the pin 10 toward the steel pipe body 31 side. The inner incomplete male thread portion 11c is continuous with the end of the complete male thread portion 11a on the pin tip 10a side, and the outer incomplete male thread portion 11b is continuous with the end of the complete male thread portion 11a on the steel pipe body 31 side.

[0101] The complete male thread portion 11a includes a male thread crest surface 111a, a male thread root surface 112a, a male thread stab flank surface 113a, and a male thread load flank surface 114a. In a longitudinal cross-sectional view of the pin 10, the crest surface 111a and the root surface 112a of the complete male thread portion 11a are, as described above, straight lines inclined with respect to the pipe axis X1 so as to approach the pipe axis X1 toward the tip 10a of the pin 10. The male thread crest surface 111a and the male thread root surface 112a may also be straight lines parallel to the pipe axis X1 in a longitudinal cross-sectional view of the pin 10. The stab flank surface 113a of the complete male thread portion 11a is a straight line inclined so that its outer circumferential edge is located closer to the steel pipe body 31 than the inner circumferential edge. The load flank surface 114a of the complete male thread portion 11a is a straight line inclined so that its outer circumferential edge is located closer to the steel pipe body 31 than the inner circumferential edge.

[0102] Referring to Figure 2, the outer incomplete male thread portion 11b has substantially the same shape as the complete male thread portion 11a. However, the outer incomplete male thread portion 11b has a shape in which the crest of the thread is cut off by a cylindrical surface having a diameter equal to the outer diameter of the steel pipe body 31. The outer incomplete male thread portion 11b may also have a shape in which the crest of the thread is cut off by a cylindrical surface having a diameter slightly smaller than the outer diameter of the steel pipe body 31. The thread height of the outer incomplete male thread portion 11b is lower than the thread height of the complete male thread portion 11a. In the dimension in the pipe axis X1 direction, the region of the outer incomplete male thread portion 11b is relatively long, and its length is, for example, about half the total length of the male thread portion 11.

[0103] 2 and 3, the inner incomplete male thread portion 11c is formed at the end of the male thread portion 11 on the tip 10a side of the pin 10. The inner incomplete male thread portion 11c has substantially the same shape as the complete male thread portion 11a. However, the thread height of the inner incomplete male thread portion 11c is naturally lower than the thread height of the complete male thread portion 11a. In terms of the dimension in the direction of the pipe axis X1, the region of the inner incomplete male thread portion 11c is extremely short, and is much shorter than the region of the outer incomplete male thread portion 11b. For example, the region of the inner incomplete male thread portion 11c has approximately 1 to 2 threads in a longitudinal cross-sectional view of the threaded joint 100.

[0104] 2 and 4, the tapered female thread portion 21 is divided into a complete female thread portion 21a, an outer incomplete female thread portion 21c, and an inner incomplete female thread portion 21b. That is, the female thread portion 21 is composed of the complete female thread portion 21a, the outer incomplete female thread portion 21c, and the inner incomplete female thread portion 21b. The inner incomplete female thread portion 21b, the complete female thread portion 21a, and the outer incomplete female thread portion 21c are arranged in this order from the innermost side of the box 20 toward the pipe end. The inner incomplete female thread portion 21b is continuous with the end of the complete female thread portion 21a on the innermost side of the box, and the outer incomplete female thread portion 21c is continuous with the end of the complete female thread portion 21a on the pipe end side of the box. The inner incomplete female thread portion 21b has a shape in which the top of the thread is cut off by a tapered surface. This tapered surface is a tapered surface whose axis is the pipe axis X2, and its diameter increases toward the innermost part of the box 20. The inner incomplete female thread portion 21b may have a shape in which the top of the thread is cut off by a cylindrical surface. This cylindrical surface is the outer surface of a cylinder whose axis is the pipe axis X2.

[0105] The complete female thread portion 21a includes a female thread root surface 211a, a female thread crest surface 212a, a female thread stab flank surface 213a, and a female thread load flank surface 214a. In a longitudinal cross-sectional view of the box 20, the bottom surface 211a and the crest surface 212a of the complete female thread portion 21a are, as described above, straight lines inclined with respect to the pipe axis X2 so as to approach the pipe axis X2 toward the innermost side of the box 20. In a longitudinal cross-sectional view of the threaded joint 100, the male thread crest surface 111a and the male thread bottom surface 112a may be straight lines parallel to the pipe axis X1. In this case, the female thread root surface 211a and the female thread crest surface 212a are straight lines parallel to the pipe axis X2. The stab flank surface 213a of the complete female thread portion 21a is a straight line inclined so that its outer circumferential edge is located closer to the pipe end of the box 20 than the inner circumferential edge. The load flank surface 214a of the complete internal thread portion 21a is a straight line that is inclined so that its outer circumferential edge is positioned closer to the pipe end of the box 20 than the inner circumferential edge.

[0106] 4, the inner incomplete female thread portion 21b includes a female thread root surface 211b, a female thread crest surface 212b, a female thread stab flank surface 213b, and a female thread load flank surface 214b. The inner incomplete female thread portion 21b has substantially the same shape as the complete female thread portion 21a. Specifically, in a vertical cross-sectional view of the box 20, the bottom surface 211b of the inner incomplete female thread portion 21b is a straight line inclined with respect to the pipe axis X2 so as to approach the pipe axis X2 toward the innermost side of the box 20, as described above. In a vertical cross-sectional view of the threaded joint 100, the male thread crest surface 111a and the male thread root surface 112a may be straight lines parallel to the pipe axis X1, in which case the female thread root surface 211b is a straight line parallel to the pipe axis X2.

[0107] In a vertical cross-sectional view of the box 20, the stab flank surface 213b of the inner incomplete female thread portion 21b is a straight line that is inclined so that its outer peripheral edge is located closer to the pipe end of the box 20 than the inner peripheral edge 213be. The load flank surface 214b of the inner incomplete female thread portion 21b is a straight line that is inclined so that its outer peripheral edge is located closer to the pipe end of the box 20 than the inner peripheral edge.

[0108] In contrast, the crest surface 212b of the inner incomplete female thread portion 21b is a straight line that moves away from the pipe axis X2 as it goes deeper inside the box 20, as viewed in vertical cross section of the box 20. In this case, the inner diameter dimension of the female thread crest surface 212b increases as it goes deeper inside the box 20. The crest surface 212b of the inner incomplete female thread portion 21b may also be a straight line parallel to the pipe axis X2, as viewed in vertical cross section of the box 20. In this case, the inner diameter dimension of the female thread crest surface 212b is constant. In such an inner incomplete female thread portion 21b, the inner peripheral edge 213be of the incomplete female thread insertion flank surface 213b is a sharp edge.

[0109] 2 to 4, the thread pitch of the male thread portion 11 is constant over the entire length of the male thread portion 11, which is composed of the complete male thread portion 11a, the outer incomplete male thread portion 11b, and the inner incomplete male thread portion 11c. The thread pitch of the female thread portion 21 is constant over the entire length of the female thread portion 21, which is composed of the complete female thread portion 21a, the outer incomplete female thread portion 21c, and the inner incomplete female thread portion 21b. The thread pitch of the male thread portion 11 is equal to the thread pitch of the female thread portion 21.

[0110] Specifically, the thread pitch sp11 of the male thread insertion flank surface 113 (113a) is constant, and the thread pitch lp11 of the male thread load flank surface 114 (114a) is constant in a vertical cross-sectional view of the male thread portion 11. The thread pitch sp11 of the male thread insertion flank surface 113 is equal to the thread pitch lp11 of the male thread load flank surface 114.

[0111] In a longitudinal cross-sectional view of the female thread portion 21, the thread pitch sp21 of the female thread stab flank surfaces 213 (213a, 213b) is constant, and the thread pitch lp21 of the female thread load flank surfaces 214 (214a, 214b) is constant. The thread pitch sp21 of the female thread stab flank surfaces 213 is equal to the thread pitch lp21 of the female thread load flank surfaces 214. Furthermore, the thread pitch sp21 of the female thread stab flank surfaces 213 is equal to the thread pitch sp11 of the male thread stab flank surfaces 113. In short, the thread pitch sp11 of the male thread stab flank surfaces 113, the thread pitch sp21 of the female thread stab flank surfaces 213, the thread pitch lp11 of the male thread load flank surfaces 114, and the thread pitch lp21 of the female thread load flank surfaces 214 are all equal to one another.

[0112] Referring to FIG. 4 , the inner circumferential surface of the box 20 is coated with a laminate coating 24. The laminate coating 24 includes a metal plating layer 241 and a solid lubricating coating 242. The metal plating layer 241 coats at least the female thread portion 21. Therefore, the female thread root surface 211 (211a, 211b), the female thread crest surface 212 (212a, 212b), the female thread stab flank surface 213 (213a, 213b), and the female thread load flank surface 214 (214a, 214b) are coated with the metal plating layer 241. In this embodiment, the box seal surface 22 and the box shoulder surface 23 ( FIG. 2 ) are also coated with the metal plating layer 241. In other words, the surface of the box 20 is coated with the metal plating layer 241.

[0113] The metal plating layer 241 has a hardness higher than that of the base material of the pin 10. More precisely, the hardness of the base material of the pin 10 is the hardness of the base material on the surface of the male thread portion 11 provided on the pin 10. For example, the hardness of the base material of the pin 10 is 200 to 396 Hv in Vickers hardness. The hardness of the base material of the box 20 may be the same as or different from the hardness of the pin 10. In this case, the hardness of the metal plating layer 241 formed on the box 20 is 400 Hv or more in Vickers hardness.

[0114] An alloy plating layer can be used as the metal plating layer 241. In this case, the metal plating layer 241 is, for example, a Zn alloy plating layer. The Zn alloy plating layer is a multi-component alloy plating in which zinc (Zn) is the main element by weight, i.e., the element with the largest proportion by weight, and contains 50 wt% or more of zinc. For example, the Zn alloy plating layer is a binary alloy plating containing zinc and any one element selected from nickel, iron, magnesium, and manganese. The Zn alloy plating layer may be a Ni-Zn binary alloy plating containing 12 wt% or more and 18 wt% or less of nickel and having a single-phase γ-phase microstructure. The Zn alloy plating layer may also be a ternary alloy plating containing zinc and any two elements selected from nickel, iron, magnesium, manganese, copper, and tin.

[0115] Alternatively, the metal plating layer 241 may be a Cu alloy plating layer. The Cu alloy plating layer is a multi-element alloy plating in which copper (Cu) is the main element by weight, i.e., the element with the largest proportion by weight. For example, the Cu alloy plating layer is a binary or ternary alloy plating containing copper and one or two elements selected from nickel, iron, magnesium, manganese, zinc, and tin.

[0116] The metal plating layer 241 can be easily formed by, for example, immersion electroplating, which forms the metal plating layer 241 homogeneously and with a substantially uniform thickness on the surface of the target component, i.e., the box 20.

[0117] The thickness of the metal plating layer 241 is, for example, 4 μm or more and 20 μm or less. If the thickness of the metal plating layer 241 is 4 μm or more, sufficient corrosion resistance is obtained, and sufficient wear resistance against sliding with the pin 10 is also obtained. On the other hand, if the thickness of the metal plating layer 241 is 20 μm or less, embrittlement due to accumulation of H2 is sufficiently suppressed. The thickness of the metal plating layer 241 is more preferably 6 μm or more and 15 μm or less.

[0118] In the box 20, the metal plating layer 241 is coated with a solid lubricating coating 242. The solid lubricating coating 242 is laminated at least on the metal plating layer 241 formed on the female thread portion 21. In this embodiment, the solid lubricating coating 242 is also laminated on the metal plating layer 241 formed on the box seal surface 22 and the box shoulder surface 23 (see FIG. 2).

[0119] The solid lubricant coating 242 is a polymer-based lubricant coating. The solid lubricant coating 242 contains a resin and a solid lubricant powder. The resin is the base material of the solid lubricant coating 242. Particles of the solid lubricant powder are dispersed in the resin. The solid lubricant coating 242 may further contain additives such as a friction-enhancing material, a surfactant, and a rust inhibitor.

[0120] The resin is, for example, an epoxy resin. In addition to epoxy resin, the resin may be polyvinyl resin, acrylic resin, polyurethane resin, polyamide-imide resin, or the like. Of these resins, epoxy resin, polyvinyl resin, and acrylic resin are particularly excellent in adhesion to the metal plating layer 241. Polyurethane resin is particularly chemically stable and easy to handle. Polyamide-imide resin is particularly excellent in abrasion resistance.

[0121] The solid lubricant powder is, for example, polytetrafluoroethylene (PTFE). In addition to PTFE, the solid lubricant powder may be molybdenum dithiocarbamate (MoDTC), molybdenum disulfide (MoS), carbon black (C), graphite fluoride (CF), or a mixture thereof. Among these solid lubricant powders, PTFE exhibits a stable coefficient of friction under strong contact pressure, resulting in good tightening torque.

[0122] When PTFE is used as the solid lubricant powder, the average particle size is preferably 10 μm or less. If the average particle size of the solid lubricant powder exceeds 10 μm, the solid lubricant powder particles will be too large compared to the film thickness of the solid lubricant coating 242, resulting in uneven dispersion of the solid lubricant powder particles in the resin. If the solid lubricant powder particles are unevenly dispersed in the resin, the solid lubricant coating 242 will be more likely to fall off due to sliding, posing a risk of seizure. More preferably, the average particle size of the solid lubricant powder is 5 μm or less.

[0123] It is preferable to combine epoxy resin as the resin and PTFE as the solid lubricant powder in the solid lubricant coating 242. In this case, the PTFE content is preferably 10% by weight or more and 25% by weight or less. The solid lubricant coating 242 may also be an inorganic binder coating.

[0124] The solid lubricating coating 242 can be industrially formed by application using a spray nozzle. Specifically, a solvent is mixed with the composition of the solid lubricating coating 242 to adjust the viscosity and prepare a fluid coating composition. This coating composition is sprayed onto the target component on which the metal plating layer 241 has been formed, i.e., onto the box 20. The coating composition applied to the target component by spraying is then solidified by heating, removing heat, air drying, or the like. This forms the solid lubricating coating 242 on the surface of the target component.

[0125] The thickness of the solid lubricant coating 242 depends on its components, but for example, if the solid lubricant coating 242 is made of fluorine-based solid lubricant powder particles dispersed in epoxy resin, it can be 5 μm or more and 50 μm or less. In this case, the more preferable thickness is 10 μm or more and 40 μm or less.

[0126] The laminate coating 24 may further include a passivation coating (not shown). The passivation coating is provided between the metal plating layer 241 and the solid lubricant coating 242. In this case, the metal plating layer 241 is formed on the base material surface of the box 20, and the passivation coating is formed on this metal plating layer 241. The solid lubricant coating 242 is formed on this passivation coating. In short, in the box 20, the passivation coating is laminated between the metal plating layer 241 and the solid lubricant coating 242. The passivation coating contains, for example, trivalent chromium (Cr(III)). Trivalent chromium is preferable because it is more stable than Cr(II) and, unlike Cr(VI), is not harmful.

[0127] The passivation coating functions as a passivation layer. The passivation coating improves the corrosion resistance of the metal plating layer 241 for the box 20 on which the solid lubricant coating 242 is provided. Alternatively, a barrier layer (not shown) may be formed on the passivation coating, and the solid lubricant coating 242 may be formed on this barrier layer. The barrier layer may be, for example, an inorganic matrix layer or an organic-inorganic matrix layer containing silicon dioxide (SiO2) particles. The barrier layer further improves corrosion resistance.

[0128] The base metal surface of the box 20 on which the metal plating layer 241 is formed may be subjected to a blasting process to form irregularities as a base. The blasting process may be, for example, sandblasting or shot peening. In this case, the adhesion between the base metal surface of the box 20 and the metal plating layer 241 is improved, and the surface of the metal plating layer 241 also becomes irregular, which improves the adhesion between the solid lubricating coating 242 and the metal plating layer 241.

[0129] The laminate coating 24 may further include a chemical conversion coating (not shown). The chemical conversion coating is provided between the metal plating layer 241 and the solid lubricant coating 242. In this case, the metal plating layer 241 is formed on the base material surface of the box 20, and the chemical conversion coating is formed on this metal plating layer 241. The solid lubricant coating 242 is formed on this chemical conversion coating. In short, in the box 20, the chemical conversion coating is laminated between the metal plating layer 241 and the solid lubricant coating 242. The chemical conversion coating is formed by applying a chemical conversion treatment to the surface of the metal plating layer 241 using zinc phosphate, manganese phosphate, or the like. The chemical conversion coating improves adhesion between the metal plating layer 241 and the solid lubricant coating 242.

[0130] Referring again to Figures 2 and 3, the outer peripheral surface of the pin 10 is not coated with a metal plating layer or a solid lubricant coating. Instead, the outer peripheral surface of the pin 10 is coated with a rust-proof coating (not shown). The rust-proof coating covers at least the male thread portion 11. Therefore, the male thread crest surface 111 (111a), the male thread root surface 112 (112a), the male thread stab flank surface 113 (113a), and the male thread load flank surface 114 (114a) are coated with the rust-proof coating. In this embodiment, the pin seal surface 12 and the pin shoulder surface 13 are also coated with the rust-proof coating. In short, the surface of the pin 10 is coated with the rust-proof coating.

[0131] Specifically, the rust-preventive coating is composed of, for example, a chemical conversion coating (not shown) and a rust-preventive oil coating (not shown). In this case, the base metal surface of the pin 10 is coated with a chemical conversion coating, and the surface coated with the chemical conversion coating is coated with a rust-preventive oil coating. In other words, the base metal surface of the pin 10 is coated with a chemical conversion coating, and the rust-preventive oil coating is formed on this chemical conversion coating.

[0132] The chemical conversion coating is formed by chemically treating the base metal surface of the pin 10 with zinc phosphate, manganese phosphate, or the like. The rust-preventive oil coating is formed by applying rust-preventive oil to the entire surface of the pin 10 on which the chemical conversion coating has been formed. The rust-preventive oil can be applied by, for example, spraying, brushing, or immersion in a tank of rust-preventive oil.

[0133] In this way, the chemical conversion coating and the rust-preventive oil coating are laminated in this order on the surface of the pin 10. This provides an inexpensive and sufficient rust-preventive treatment to the pin 10. This prevents the pin 10 from rusting during the storage period from when the pin 10 is manufactured until when it is fastened to the box 20.

[0134] However, such a surface treatment coating is not necessarily required for the pin 10. For example, only a chemical conversion coating or only a 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.

[0135] 5 to 7 are schematic diagrams of the pin 10 and the box 20 in a longitudinal cross section of a threaded joint 100. FIG. 5 corresponds to FIG. 1 or 2 and shows the make-up state (power-tight state). In other words, FIG. 5 shows the state at the end of the power make-up stage. FIG. 6 shows the state at the end of the hand make-up stage. FIG. 7 shows the state at the end of the hand make-up stage. In other words, FIG. 7 shows the state at the time of hand-tightening. In FIGS. 5 to 7, the pin 10 and the box 20 are shown separately with their positions in the pipe axis X, X1, and X2 directions aligned, in order to make it easier to understand the configuration of each of the pin 10 and the box 20.

[0136] As described above, the threaded joint 100 according to this embodiment has the following configuration when the pin 10 and box 20 are fastened together. The male thread root surface 112 is in interference contact with the female thread crest surface 212. The male thread load flank surface 114 is in pressing contact with the female thread load flank surface 214. The pin-side torque stop mechanism (pin shoulder surface 13) and the box-side torque stop mechanism (box shoulder surface 23) function mutually. Furthermore, in this embodiment, the pin seal surface 12 is in interference contact with the box seal surface 22.

[0137] Furthermore, referring to FIG. 5, the distance LP in the direction of the pipe axis X from the tip 10a of the pin 10 to the complete male thread starting end 11a1 (the end 11a1 of the complete male thread portion 11a on the inner incomplete male thread portion 11c side) and the distance LB in the direction of the pipe axis X from the tip 10a of the pin 10 to the complete female thread starting end 21a1 (the end 21a1 of the complete female thread portion 21a on the inner incomplete female thread portion 21b side) satisfy the formula (1). α×L / (α+T / 100)+(δ+2×t) / T×100≧LB-LP (1)

[0138] The meanings of the symbols in Equation (1) are as follows: α is tan 0.5°. Referring to FIG. 6, 0.5° is the misalignment angle allowed during the final stage of the hand make-up phase. Referring to FIG. 7, L is the distance [mm] in the direction of the pipe axis X between the end 11a1 of the complete male thread portion 11a on the inner incomplete male thread portion 11c side (the start end of the complete male thread, i.e., the end 11a1 of the complete male thread portion 11a on the tip side of the pin 10) and the end 21a2 of the complete female thread portion 21a on the outer incomplete female thread portion 21c side (the end end of the complete female thread, i.e., the end 21a2 of the complete female thread portion 21a on the pipe end side of the box 20) at the time when the male thread root surface 112 contacts the female thread crest surface 212 when the pin 10 is tightened into the box 20, i.e., at the time of hand tightening. Hereinafter, this distance will also be referred to as the "interlocking distance between the complete male thread portion and the complete female thread portion." T is the thread taper [%] of each of the male thread portion 11 and the female thread portion 21. δ is the interference [mm] between the male thread root surface 112 and the female thread crest surface 212. This δ represents the interference amount over the diameter of the thread portion. t is the film thickness [mm] of the multilayer coating 24 ( FIG. 4 ). This t is the total film thickness of all elements that make up the multilayer coating 24. In a typical example, t is the total film thickness of the metal plating layer 241 and the solid lubricating coating 242.

[0139] Here, the starting point of the final stage of the hand makeup stage refers to the point at which the maximum possible misalignment angle between the pipe axis X1 of the pin 10 and the pipe axis X2 of the box 20 is 0.5°. The tip 10a of the pin 10 is positioned at the part of the pin 10 farthest from the steel pipe body 31. The complete male thread starting end 11a1 is the end of the complete male thread portion 11a on the inner incomplete male thread portion 11c side, and the complete male thread starting end 11a1 is the boundary where the complete thread changes from an incomplete thread to an incomplete thread. The complete female thread starting end 21a1 is the end of the complete female thread portion 21a on the inner incomplete female thread portion 21b side, and the complete female thread starting end 21a1 is the boundary where the complete thread changes from an incomplete thread to an incomplete thread.

[0140] As described above, Equation (1) indicates that the difference "LB - LP" (FIG. 5) between distances LB and LP, shown on the right side, is less than or equal to the sum of the first and second terms shown on the left side. The first term on the left side corresponds to the first distance described above (the distance in the pipe axis X direction that the pin 10 advances as it is threaded into the box 20 during the final period of the hand make-up phase). Meanwhile, the second term on the left side corresponds to the second distance described above (the distance in the pipe axis X direction that the pin 10 advances as it is threaded into the box 20 during the power make-up phase). In other words, the threaded joint 100 is designed so that "LB - LP" is less than or equal to the sum of the first distance and the second distance, as shown in Equation (1).

[0141] 5, the distance LB is greater than the distance LP, and "LB-LP" is a positive value. That is, the distances LP and LB satisfy the formula (2). LB-LP>0 (2)

[0142] In this case, "LB-LP" exceeds zero and is equal to or less than the sum of the first distance and the second distance. Therefore, when the pin 10 is screwed into the box 20 and fastened, the complete male thread start end 11a1 reaches the complete female thread start end 21a1 either during the final stage of the hand make-up stage or during the power make-up stage.

[0143] More specifically, as shown in Fig. 6, the complete male thread starting end 11a1 reaches the complete female thread starting end 21a1 during the final stage of the manual make-up stage. In this case, "LB - LP" is equal to or greater than the second distance. That is, the distance LP and the distance LB satisfy the formula (3). LB-LP≧(δ+2×t) / T×100 (3)

[0144] [effect] In the threaded joint 100 according to this embodiment, the inner peripheral surface of the box 20, on which the female thread portion 21 is provided, is coated with a laminate coating 24 including a metal plating layer 241 and a solid lubricant coating 242 laminated on the metal plating layer 241. The metal plating layer 241 is harder than the base material of the pin 10. In this case, the female thread portion 21 of the box 20, i.e., the female thread stab flank surface 213, is strengthened by the metal plating layer 241 more than the male thread portion 11 of the pin 10. Therefore, even if the outer peripheral edge of the stab flank surface of the outer incomplete male thread portion 11b comes into metal-to-metal contact with the female thread stab flank surface 213a of the complete female thread portion 21a during make-up, excessive scratches are unlikely to occur on the female thread stab flank surface 213a. This reduces external seizure.

[0145] Furthermore, in the threaded joint 100 according to this embodiment, according to formula (1), "LB - LP" is equal to or less than the sum of the first distance and the second distance. Therefore, according to formula (2), if "LB - LP" is a positive value, when the pin 10 is threaded into the box 20 and fastened, the complete male thread start end 11a1 reaches the complete female thread start end 21a1 during the final period of the hand make-up stage or the power make-up stage. In particular, according to formula (3), if "LB - LP" is equal to or greater than the second distance, the complete male thread start end 11a1 reaches the complete female thread start end 21a1 during the final period of the hand make-up stage. If "LB - LP" is a negative value, the complete male thread start end 11a1 does not reach the complete female thread start end 21a1 even at the end of the power make-up stage. In either case, the region of the complete male thread portion 11a does not reach the region of the inner incomplete female thread portion 21b before the final period of the hand make-up stage. Therefore, as the make-up operation progresses, the inner peripheral edge 213be of the incomplete female thread stab flank surface 213b does not come into significant metal-to-metal contact with the male thread stab flank surface 113a of the complete male thread portion 11a, and excessive scratches are unlikely to occur on the male thread stab flank surface 113a. This reduces internal seizure. Therefore, the threaded joint 100 according to this embodiment employs dope-free technology and makes it possible to suppress seizure when making the pin 10 into the box 20.

[0146] Second Embodiment The configuration of a threaded joint 100A according to a second embodiment will be described with reference to Figures 8 and 9. Figure 8 is a longitudinal cross-sectional view of the threaded joint 100A. Figure 9 is a longitudinal cross-sectional view of the pin 10 and the box 20 of the threaded joint 100A shown in Figure 8. The threaded joint 100A differs from the threaded joint 100 according to the first embodiment in that the type of threads that make up the male threaded portion 11 and the female threaded portion 21 are so-called wedge threads, and in that it is equipped with a self-locking mechanism as a torque stop mechanism.

[0147] 8 and 9, the thread pitch sp11 of the male thread stab flank surface 113 is equal to the thread pitch sp21 of the female thread stab flank surface 213. The thread pitch lp11 of the male thread load flank surface 114 is equal to the thread pitch lp21 of the female thread load flank surface 214. The thread pitches sp11 and sp21 of the male thread stab flank surface 113 and the female thread load flank surface 213, respectively, are smaller than the thread pitches lp11 and lp21 of the male thread load flank surface 114 and the female thread load flank surface 214, respectively.

[0148] In the male thread portion 11, the width of the male thread defined by the male thread crest surface 111, the male thread stab flank surface 113, and the male thread load flank surface 114 decreases along the helical winding of the thread toward the tip 10a of the pin 10. In the male thread portion 11, the width of the male thread groove defined by the male thread root surface 112, the male thread stab flank surface 113, and the male thread load flank surface 114 increases along the helical winding of the thread toward the tip 10a of the pin 10.

[0149] The male thread insertion flank surface 113 is a straight line that is inclined so that its outer peripheral edge is located closer to the tip 10a of the pin 10 than the inner peripheral edge in a vertical cross-sectional view of the threaded joint 100A. The male thread load flank surface 114 is a straight line that is inclined so that its outer peripheral edge is located closer to the steel pipe main body 31 than the inner peripheral edge in a vertical cross-sectional view of the threaded joint 100A.

[0150] In the female thread portion 21, the width of the female thread groove defined by the female thread root surface 211, the female thread stab flank surface 213, and the female thread load flank surface 214 decreases along the helical winding of the thread toward the innermost side of the box 20, corresponding to the width of the male thread. In the female thread portion 21, the width of the female thread defined by the female thread crest surface 212, the female thread stab flank surface 213, and the female thread load flank surface 214 increases along the helical winding of the thread toward the innermost side of the box 20, corresponding to the width of the male thread groove.

[0151] The female thread insertion flank surface 213 is a straight line that is inclined so that its outer peripheral edge is located deeper inside the box 20 than the inner peripheral edge in a vertical cross-sectional view of the threaded joint 100A. The female thread load flank surface 214 is a straight line that is inclined so that its outer peripheral edge is located closer to the pipe end of the box 20 than the inner peripheral edge in a vertical cross-sectional view of the threaded joint 100A.

[0152] In the threaded joint 100A according to this embodiment, the male thread stab flank surface 113 and the male thread load flank surface 114 are pin-side torque stop mechanisms, and the female thread stab flank surface 213 and the female thread load flank surface 214 are box-side torque stop mechanisms.

[0153] During make-up, as the pin 10 is threaded into the box 20, the male thread load flank 114 comes into contact with the female thread load flank 214, and the male thread stab flank 113 comes into contact with the female thread stab flank 213. This allows the pin-side torque stop mechanism (the male thread stab flank 113 and the male thread load flank 114) and the box-side torque stop mechanism (the female thread stab flank 213 and the female thread load flank 214) to function reciprocally, resulting in the functioning of the self-locking mechanism (torque stop mechanism). As the pin 10 is further threaded, in the make-up-completed state (power tight state), the load flanks 114 and 214 come into pressing contact with each other, and the stab flanks 113 and 213 come into pressing contact with each other. This applies a tightening axial force to the mating male thread portion 11 and female thread portion 21. In this case, the pin shoulder surface 13 and the box shoulder surface 23 may not be provided.

[0154] In this embodiment, the torque stop mechanism may begin functioning simultaneously with the end of the hand make-up stage (hand tightening) during make-up. Specifically, when the male thread root surface 112 contacts the female thread crest surface 212, the male thread load flank surface 114 may contact the female thread load flank surface 214, and the male thread stab flank surface 113 may contact the female thread stab flank surface 213. [Example]

[0155] The present disclosure will be described in more detail below with reference to examples, although the present disclosure is not limited to the following examples.

[0156] In order to confirm the effects of the present disclosure, an analysis was carried out using a quasi-three-dimensional elastic-plastic finite element method using Fourier elements. In this analysis, a steel pipe threaded joint 100 was modeled.

[0157] The main conditions for the threaded joint analysis model are as follows: Steel pipe dimensions: 7" 29# (outer diameter: 177.8mm, wall thickness: 10.36mm) Material: API L80 steel (yield stress: 551.6 MPa, work hardening coefficient: 2.0 GPa, elastic modulus: 205.0 GPa) Thread dimensions Interlocking distance L between the fully male and female threads at the time of hand tightening: 90 mm Thread taper T: 6.25% Thread interference δ: 0.3175mm (5.08 / 16) Layered coating thickness: 0.05 mm Thread pitch: 5.08mm (5TPI)

[0158] The analysis simulated the situation when make-up is initiated in a misaligned state. Specifically, in an analysis model in which the pin was inserted into the box, a compressive load in the pipe axial direction and a bending moment load perpendicular to the pipe axial direction were applied to the end of the pin facing the steel pipe body, while the pin was moved one pitch at a time relative to the box. The changes in the contact conditions between the inner periphery of the incomplete female thread stab flank and the complete male thread stab flank during the make-up process, including the hand-tightening stage, were evaluated.

[0159] The load conditions are as follows: Compression load: 15.2 kN (equivalent to the weight of the connecting pipe made up of three steel pipes) Bending moment: 14.4 kN m (moment load when a connecting pipe made up of three steel pipes is tilted at an angle of approximately 3°)

[0160] The evaluation was performed based on the maximum value of contact pressure acting on the inner peripheral edge of the stab flank of the incomplete female thread that comes into contact with the stab flank of the complete male thread. The analysis results are shown in Table 1.

[0161] [Table 1]

[0162] Referring to Table 1, Test No. 1 is an example of the present invention that satisfies the conditions defined in this embodiment. Test Nos. 2 and 3 are comparative examples in which "LB - LP" exceeds the upper limit expressed by formula (1) and does not satisfy the conditions defined in this embodiment. In Test No. 1, an example of the present invention, the start end of the complete male thread reached the start end of the complete female thread during the final period of the hand make-up stage. In Test Nos. 2 and 3, which are comparative examples, the start end of the complete male thread reached the start end of the complete female thread before the final period of the hand make-up stage. As is clear from Table 1, the contact pressure in the examples of the present invention was significantly reduced compared to the comparative examples. This result demonstrates that the risk of internal seizure is reduced.

[0163] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

[0164] In the pin 10, the position where the pin seal surface 12 is provided is not limited to between the male thread portion 11 and the pin shoulder surface 13. For example, the pin seal surface 12 may be provided midway along the male thread portion 11 in the direction of the pipe axis X1, or may be provided on the steel pipe main body 31 side of the male thread portion 11.

[0165] In the box 20, the position where the box seal surface 22 is provided may correspond to the position where the pin seal surface 12 is provided, and is not limited to between the female thread portion 21 and the box shoulder surface 23. For example, the box seal surface 22 may be provided midway in the pipe axis X2 direction of the female thread portion 21, or may be provided on the pipe end side of the box 20 with respect to the female thread portion 21.

[0166] When the pin 10 has a pin shoulder surface 13, the position of the pin shoulder surface 13 is not limited to the tip of the pin 10. For example, the pin shoulder surface 13 may be provided midway along the male thread portion 11 in the direction of the pipe axis X1, or may be provided on the steel pipe main body 31 side of the male thread portion 11.

[0167] When the box 20 has a box shoulder surface 23, the position of the box shoulder surface 23 only needs to correspond to the position of the pin shoulder surface 13, and is not limited to the deep inside of the box 20. For example, the box shoulder surface 23 may be provided midway along the female thread portion 21 in the pipe axis X2 direction, or at the pipe end of the box 20.

[0168] The threaded joints 100, 100A may be of the integral type. [Explanation of symbols]

[0169] 100, 100A: Threaded joint 10: Pin 10a: Tip 11: Male thread 11a: Fully male threaded section 11a1: Fully male thread starting end 11b: Outer incomplete male thread 11c: Internal incomplete male thread 111,111a: Male thread top surface 112, 112a: Male thread bottom 113, 113a: Male thread insertion flank surface 114, 114a: Male thread load flank surface 12: Pin seal surface 13: Pin shoulder surface 20: Box 21: Female thread 21a: Fully internally threaded section 21a1: Complete internal thread start end 21a2: Full female thread end 21b: Internal incomplete female thread 21c: Outside incomplete female thread part 211, 211a, 211b: Female thread bottom 212, 212a, 212b: Female thread top surface 213, 213a, 213b: Female thread insertion flank surface 213be: Inner edge of female thread insertion flank 214, 214a, 214b: Female thread load flank surface 22: Box seal surface 23: Box shoulder surface 24: Laminated film 241: Metal plating layer 242: Solid lubricant coating X,X1,X2: tube axis 30: Steel pipe 31: Main body 40: Coupling

Claims

1. A threaded joint for connecting steel pipes, a tubular pin provided continuously on the steel pipe body; a tubular box into which the pin is inserted and fastened to the pin; the pin includes a tapered male thread portion provided on an outer peripheral surface of the pin, and a pin-side torque stop mechanism provided to limit threading of the pin into the box when the pin is fastened to the box, the box includes a tapered female thread portion provided on an inner peripheral surface of the box in correspondence with the tapered male thread portion and having a thread taper equal to the thread taper of the tapered male thread portion, and a box-side torque stop mechanism provided in correspondence with the pin-side torque stop mechanism, The tapered male thread portion includes a male thread crest surface, a male thread root surface, a male thread insertion flank surface, and a male thread load flank surface, and is divided into a complete male thread portion and an incomplete male thread portion formed on the tip side of the pin continuous with the complete male thread portion, The tapered female thread portion includes a female thread root surface corresponding to the male thread crest surface, a female thread crest surface corresponding to the male thread root surface, a female thread insertion flank surface corresponding to the male thread insertion flank surface, and a female thread load flank surface corresponding to the male thread load flank surface, and is divided into a complete female thread portion and an incomplete female thread portion formed on the innermost side of the box, continuous with the complete female thread portion; the inner circumferential surface of the box is coated with a laminated coating including a metal plating layer having a hardness higher than that of a base material of the pin and a solid lubricating coating laminated on the metal plating layer, A threaded joint wherein, in a fastened state between the pin and the box, the male thread bottom surface is in interference contact with the female thread crest surface, the male thread load flank surface is in pressing contact with the female thread load flank surface, the pin-side torque stop mechanism and the box-side torque stop mechanism function mutually, and the distance LP in the pipe axial direction of the threaded joint from the tip of the pin to the end of the complete male thread portion on the incomplete male thread portion side, and the distance LB in the pipe axial direction from the tip of the pin to the end of the complete female thread portion on the incomplete female thread portion side satisfy formula (1). α×L / (α+T / 100)+(δ+2×t) / T×100≧LB-LP (1) The meanings of the symbols in formula (1) are as follows: α: tan 0.5°, L: The distance [mm] in the pipe axis direction between the end of the complete male thread portion on the incomplete male thread portion side and the end of the complete female thread portion on the pipe end side of the box at the time when the male thread bottom surface contacts the female thread crest surface when the pin is fastened to the box, T: the thread taper [%] of each of the male thread portion and the female thread portion, δ: interference amount [mm] between the male thread bottom surface and the female thread top surface, and t: film thickness of the laminated coating [mm].

2. 2. A threaded joint according to claim 1, further comprising: A threaded joint, wherein the distance LP and the distance LB satisfy formula (2). LB-LP>0 (2)

3. 2. A threaded joint according to claim 1, further comprising: A threaded joint, wherein the distance LP and the distance LB satisfy formula (3). LB-LP≧(δ+2×t) / T×100 (3) The meaning of each symbol in formula (3) is the same as that of each symbol in formula (1).

4. 2. A threaded joint according to claim 1, the thread pitch of the male thread stab flank, the thread pitch of the female thread stab flank, the thread pitch of the male thread load flank, and the thread pitch of the female thread load flank are equal to each other; the pin further includes a pin shoulder surface as the pin-side torque stop mechanism, the box further includes, as the box-side torque stop mechanism, a box shoulder surface corresponding to the pin shoulder surface; a threaded joint in which, when the pin is fastened to the box, the pin shoulder surface comes into contact with the box shoulder surface, thereby causing the pin side torque stop mechanism and the box side torque stop mechanism to function together.

5. 2. A threaded joint according to claim 1, the thread pitch of the male thread flank surface is equal to the thread pitch of the female thread flank surface; the thread pitch of the male thread load flank surface is equal to the thread pitch of the female thread load flank surface; the thread pitch of each of the male thread stab flank and the female thread stab flank is smaller than the thread pitch of each of the male thread load flank and the female thread load flank; In the tapered male thread portion, the width of the male thread defined by the male thread crest surface, the male thread stab flank surface, and the male thread load flank surface decreases toward the tip side of the pin, and the width of the male thread groove defined by the male thread root surface, the male thread stab flank surface, and the male thread load flank surface increases toward the tip side of the pin, In the tapered female thread portion, the width of the female thread groove defined by the female thread root surface, the female thread stab flank surface, and the female thread load flank surface decreases toward the innermost side of the box in correspondence with the width of the male thread, and the width of the female thread defined by the female thread crest surface, the female thread stab flank surface, and the female thread load flank surface increases toward the innermost side of the box in correspondence with the width of the male thread groove, the male thread stab flank surface and the male thread load flank surface are the pin-side torque stop mechanism, the female thread stab flank surface and the female thread load flank surface are the box-side torque stop mechanism; a threaded joint in which, when the pin is fastened to the box, the male thread load flank surface comes into contact with the female thread load flank surface and the male thread stab flank surface comes into contact with the female thread stab flank surface, thereby causing the pin side torque stop mechanism and the box side torque stop mechanism to function together.

6. 2. A threaded joint according to claim 1, A threaded joint, wherein the metal plating layer is an alloy plating layer.

7. A threaded joint according to any one of claims 1 to 6, the pin is provided on each of the steel pipes to be connected, A threaded joint in which the box is provided to a coupling that is a different pipe material from the steel pipe.

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