Threaded joint

Abstract

A threaded joint (100) comprises a pin (10) and a box (20), and in a fastened state, seal surfaces (12, 22) interfere and come into contact with each other, a male thread bottom surface (112) interferes and comes into contact with a female thread top surface (212), and load flank faces (114, 214) come into press-contact with each other. When the surface of the box (20) is covered with a base coating film (25) including an alloy plating layer (251) and with a single solid lubricating coating film (26) in a single body state, the film thickness (tS214) of the solid lubricating coating film (26) on the female thread load flank face (214) is smaller than the film thickness (tS212) of the solid lubricating coating film (26) on the female thread top surface (212), and the film thickness (tS22) of the solid lubricating coating film (26) on the box seal surface (22) is greater than the film thickness (tS214) of the solid lubricating coating film (26) on the female thread load flank face (214).

Description

Threaded joints

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

[0002] In oil wells, natural gas wells, and the like (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 coupling types, steel pipes are connected to each other via a coupling, which is another tubular member. Specifically, each steel pipe has male threads at both ends, and the coupling has female threads at both ends. The male threads of the steel pipe are screwed into each of the female threads of the coupling to fasten the steel pipes together. On the other hand, in the case of integral types, steel pipes are directly connected to each other. Specifically, each steel pipe has a male thread at one end and a female thread at the other end. The male threads of one steel pipe are screwed into the female threads of the other steel pipe to fasten the steel pipes together. Generally, the end of the steel pipe with the male threads is called a pin, and the end of the steel pipe or coupling with the female threads is called a box. Using these names, the pin and box are fastened by inserting and screwing the pin into the box.

[0004] Threaded joints for steel pipes are required to prevent seizure during make-up and ensure sealing performance during use after make-up. To meet these requirements, 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, grease contains heavy metal powder (compound). For example, a standard compound grease is specified in Recommended Practice 5A3 of the American Petroleum Institute (API) standard.

[0005] During tightening, torque is generated as the pin is screwed into the box, and this torque is managed. In recent years, a method of monitoring a torque chart and judging the soundness of the tightening from the chart shape has been widely adopted. A torque chart is a graph showing the number of screw-in turns on the horizontal axis and the torque on the vertical axis. In short, the torque chart shows the behavior of torque relative to the number of screw-in turns.

[0006] Typically, threaded joints for steel pipes have a torque stop mechanism. For example, when the type of thread that constitutes the threaded portions (male and female threaded portions) is a buttress thread (trapezoidal thread) or a square thread, the pin and box each include a shoulder surface as a torque stop mechanism. The torque stop mechanism functions when the shoulder surfaces come into contact with each other. In this specification, this torque stop mechanism is also referred to as a shouldering mechanism. When the type of thread is a wedge thread, the load flank surface and the stab flank surface of the male and female threaded portions, respectively, are used as the torque stop mechanism. The torque stop mechanism functions when the load flank surfaces come into contact with each other and when the stab flank surfaces come into contact with each other. In this specification, this torque stop mechanism is also referred to as a self-locking mechanism.

[0007] FIG. 1 shows an example of a torque chart. Referring to FIG. 1 , the torque increases with the number of turns of threading. Specifically, when threading begins, the radial contact surfaces of the pin and box (the female thread crest surface, male thread root surface, and seal surface) first come into contact with each other and begin to interfere. As a result, the torque chart has a gentle slope at the beginning of threading, and the torque gradually increases. In the case of a threaded joint having a shouldering mechanism, the torque chart changes significantly when the shouldering mechanism begins to function as the threading progresses. This point of change is called shouldering, and the torque at this point of change is called shouldering torque. The torque chart following shouldering becomes steep, and the torque rises rapidly. As the threading progresses further, the torque chart changes from a steep slope to a gentle slope. The torque at this point of change is called yield torque, and the torque after this point of change is called over-torque. The over-torque state is a state in which excessive threading is occurring. In most cases, when an over-torque condition occurs, the threads and shoulder surfaces undergo significant plastic deformation to the point that they cannot be reused.

[0008] In the case of a threaded joint with a self-locking mechanism, the torque chart will be similar to that of a threaded joint with a shouldering mechanism. In this case, the torque chart changes significantly when the self-locking mechanism begins to function. This point of change is called self-locking, and the torque at this point of change is called locking torque. Self-locking corresponds to shouldering, and locking torque corresponds to shouldering torque.

[0009] When making up a threaded joint, if the threading is completed at a torque between the shouldering torque (locking torque) and the yield torque, the desired joint strength and sealing performance can be obtained. For this reason, a recommended torque for completing the threading is set. The recommended torque is set so that the torque at the completion of the threading falls between the shouldering torque and the yield torque, even if various dimensions change within the range of manufacturing tolerances.

[0010] FIG. 2 is a diagram for explaining the recommended torque. FIG. 2 shows a torque chart (hereinafter also referred to as a "high-side torque chart") in which it is assumed that shouldering torque will be highest within the range of manufacturing tolerances, and a torque chart (hereinafter also referred to as a "low-side torque chart") in which it is assumed that yield torque will be lowest within the range of manufacturing tolerances. In FIG. 2, the torque chart on the left side is the high-side torque chart, and the torque chart on the right side is the low-side torque chart. The range from shouldering torque (Max. shouldering torque) on the high-side torque chart to yield torque (Min. yield torque) on the low-side torque chart is called the torque window. The recommended torque is set within this torque window.

[0011] However, depending on the design of the threaded joint, the type of lubricant, and other factors, the shouldering torque on the high-end torque chart may be higher than the yield torque on the low-end torque chart. In this case, there is no valid torque window, making it difficult to set a recommended torque. Therefore, in the design of threaded joints, efforts are made to widen the torque window as much as possible by keeping the shouldering torque (locking torque) as low as possible and the yield torque as high as possible.

[0012] Recently, in order to improve the natural environment, workability at oil well sites, and worker health, technologies that use lubricants instead of conventional dopes, i.e., dope-free technologies, have been promoted. In dope-free technologies, a polymer-based lubricating coating is formed on the surface of the box and / or pin. This lubricating coating is a solid or semi-solid lubricant that covers the box and / or pin, and is called a solid lubricating coating. A solid lubricating coating is prepared by mixing, for example, various lubricants, friction-enhancing agents, rust inhibitors, etc., and exhibits various properties such as favorable friction characteristics, adhesion, and durability.

[0013] In a threaded joint equipped with a solid lubricating coating, if the solid lubricating coating is prepared so that the coefficient of friction is small in order to reduce shouldering torque, the yield torque will naturally also decrease. Conversely, if the solid lubricating coating is prepared so that the coefficient of friction is large in order to increase yield torque, the shouldering torque will naturally increase as well. In short, adjusting only the friction coefficient of the solid lubricating coating will not widen the torque window.

[0014] Furthermore, if the coefficient of friction of a solid lubricant coating is too low, the tightened threaded joint will be prone to loosening, resulting in the so-called low breakout torque problem. Conversely, if the coefficient of friction of a solid lubricant coating is too high, the torque required for threading will exceed the rated torque of the screw tightening device, making the tightening process difficult. Furthermore, there is an increased risk of seizure during tightening.

[0015] Techniques for overcoming such problems and widening the torque window have been proposed, for example, in Japanese Patent Publication No. 2009-517614 (Patent Document 1) and Japanese Patent Publication No. 2015-506445 (Patent Document 2).

[0016] The threaded joint described in Patent Document 1 comprises a first layer with high-friction anti-seizure properties and a second layer with low-friction properties. The first layer is provided on the surface of either the pin or the box. The second layer is provided on a portion of the surface of either the pin or the box that is adapted to bring them into radial contact with each other. The portions on which the second layer is provided are the crest surface of the female thread of the box, the root surface of the male thread of the pin, and the seal surface. The first and second layers are different types of solid lubricant coatings. The technology of Patent Document 1 aims to achieve low shouldering torque and high yield torque by providing the first and second layers.

[0017] The threaded joint described in Patent Document 2 includes a first solid lubricating coating and a second solid lubricating coating. The first solid lubricating coating has a higher Knoop hardness than the second solid lubricating coating. The first solid lubricating coating is provided on a portion of the contact surface of at least one of the pin and the box. The portion of the contact surface on which the first solid lubricating coating is provided includes the shoulder portion and also the seal portion. The second solid lubricating coating is provided on at least a portion of the contact surface of at least one of the pin and the box that is not provided with the first solid lubricating coating. The portion of the contact surface on which the second solid lubricating coating is provided includes at least the thread portion. The technology of Patent Document 2 aims to achieve low shouldering torque and high yield torque by providing the first solid lubricating coating and the second solid lubricating coating.

[0018] Special table 2009-517614 Publication Special table 2015-506445

[0019] In both of the threaded joints described in Patent Documents 1 and 2, two types of solid lubricant coatings are essential. Therefore, the two types of solid lubricant coatings must be procured separately and strictly managed. Furthermore, the two types of solid lubricant coatings are applied to different parts of the pin and box. Therefore, the two types of solid lubricant coatings must be applied to the pin and box depending on the parts where they are needed. Therefore, the configuration of the solid lubricant coatings required to expand the torque window is complex, and the cost of achieving this configuration tends to be high.

[0020] An object of the present disclosure is to provide a threaded joint that is capable of widening the torque window with a simple and inexpensive configuration.

[0021] 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 male thread portion and a pin seal surface. The male thread portion is provided on the outer peripheral surface of the pin. The pin seal surface is provided on the outer peripheral surface of the pin. The box includes a female thread portion and a box seal surface. The female thread portion is provided on the inner peripheral surface of the box in correspondence with the male thread portion, and meshes with the male thread portion when the pin and box are fastened together. The box seal surface is provided on the inner peripheral surface of the box in correspondence with the pin seal surface. The 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 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.

[0022] In the individual state before the pin and box are fastened together, the surface of at least one of the box and the pin is coated with a base coating, and the surface coated with the base coating is coated with a single solid lubricating coating. In the individual state, the base coating includes an alloy plating layer, and the solid lubricating coating is formed on the alloy plating layer. In the fastened state, the pin seal surface is in interference contact with the box seal surface, the male thread root surface is in interference contact with the female thread crest surface, and the male thread load flank surface is in pressing contact with the female thread load flank surface.

[0023] The threaded joint in its standalone state has the following features: The film thickness of the solid lubricant coating on the female thread load flank surface in the pipe axial direction of the threaded joint is smaller than the film thickness of the solid lubricant coating on the female thread crest surface in the radial direction perpendicular to the pipe axial direction. The radial film thickness of the solid lubricant coating on the box seal surface is larger than the above-mentioned film thickness of the solid lubricant coating on the female thread load flank surface. The film thickness of the solid lubricant coating on the male thread load flank surface in the pipe axial direction is smaller than the radial film thickness of the solid lubricant coating on the male thread root surface. The radial film thickness of the solid lubricant coating on the pin seal surface is larger than the above-mentioned film thickness of the solid lubricant coating on the male thread load flank surface. Furthermore, with regard to the first film thickness which is the combination of the solid lubricating coating and the base coating at the root surface of the female thread, the second film thickness which is the combination of the solid lubricating coating and the base coating at the crest surface of the male thread, the third film thickness which is the combination of the solid lubricating coating and the base coating at the crest surface of the female thread, and the fourth film thickness which is the combination of the solid lubricating coating and the base coating at the root surface of the male thread, the first index, which is expressed by the value obtained by subtracting the sum of the third film thickness and the fourth film thickness from the sum of the first film thickness and the second film thickness, is smaller than the second index, which is expressed by the distance between the male thread crest surface and the female thread root surface in the make-up state when it is assumed that the box and the pin do not have a base coating or a solid lubricating coating.

[0024] The threaded joint according to the present disclosure makes it possible to widen the torque window with a simple and inexpensive configuration.

[0025] FIG. 1 is a diagram showing an example of a torque chart. FIG. 2 is a diagram for explaining recommended torque. FIG. 3 is a diagram showing an example of a torque chart that appears in a threaded joint provided with a solid lubricating coating. FIG. 4 is a longitudinal sectional view of a threaded joint according to the first embodiment. FIG. 5 is a partially enlarged view of the threaded joint shown in FIG. 4. FIG. 6A is a longitudinal sectional view showing a pin in the threaded joint shown in FIG. 5 in an individual state before make-up. FIG. 6B is a longitudinal sectional view showing a box in the threaded joint shown in FIG. 5 in an individual state before make-up. FIG. 7 is a schematic diagram showing surface layer portions of the pin and box in the threaded joint according to the first embodiment, in an individual state. FIG. 8 is a longitudinal sectional view showing a make-up state of the threaded joint shown in FIG. 5, assuming that the pin and box do not have a base coating or a solid lubricating coating. FIG. 9 is a schematic diagram showing surface layer portions of the pin and box in an individual state, for explaining Variation 1 of the threaded joint according to the first embodiment. FIG. 10 is a schematic diagram showing surface layer portions of the pin and box in an individual state, for explaining Variation 3 of the threaded joint according to the first embodiment. FIG. 11 is a schematic diagram showing surface layer portions of a pin and a box in a separate state, for explaining Modification 4 of the threaded joint according to the first embodiment. FIG. 12A is a longitudinal sectional view showing a portion of a pin in a threaded joint according to the second embodiment in a separate state. FIG. 12B is a longitudinal sectional view showing a portion of a box in a threaded joint according to the second embodiment in a separate state. FIG. 13 is a schematic diagram showing surface layer portions of a pin and a box in a threaded joint according to the second embodiment in a separate state. FIG. 14A is a longitudinal sectional view showing a portion of a pin in a threaded joint according to the third embodiment in a separate state. FIG. 14B is a longitudinal sectional view showing a portion of a box in a threaded joint according to the third embodiment in a separate state. FIG. 15 is a schematic diagram showing surface layer portions of a pin and a box in a threaded joint according to the third embodiment in a separate state. FIG. 16 is a longitudinal sectional view showing a portion of a threaded joint according to the fourth embodiment in a fastened state. FIG. 17A is a longitudinal sectional view showing a portion of a pin in the threaded joint shown in FIG. 16 in a separate state. FIG. 17B is a longitudinal sectional view showing a portion of a box in the threaded joint shown in FIG. 16 in a separate state.

[0026] In order to solve the above problems, the present inventors conducted extensive research and intensive investigation into the effect of a solid lubricating coating on a torque chart, and as a result, they discovered the following.

[0027] The coefficient of friction of a solid lubricating coating is comparable to that of a dope. Nevertheless, the yield torque of a threaded joint equipped with a solid lubricating coating generally tends to be lower than the yield torque of a threaded joint using a dope.

[0028] However, it has been found that when a threaded joint equipped with a solid lubricating coating is made up, even if the thread is tightened beyond the torque determined to be the yield torque, the thread portion and shoulder surface have not yet actually yielded (undergone large-scale plastic deformation). Based on this, the inventors have determined that the torque determined to be the yield torque merely indicates a change in the slope of the torque chart, and is not the true yield torque.

[0029] Figure 3 shows an example of a torque chart that appears in a threaded joint equipped with a solid lubricating coating. Referring to Figure 3, the torque chart with a steep slope following shouldering (self-locking) breaks down at a torque lower than the true yield torque. In other words, the torque chart loses linearity at the break point and the slope changes. In short, in the torque chart with a steep slope following shouldering, an inflection point torque (torque at loss of linearity) appears before the true yield torque appears. The range from the inflection point torque to the true yield torque is not an over-torque state, but is within the torque window.

[0030] At oil well sites, workers judge the integrity of a joint solely by the change in the slope of the torque chart. Therefore, if an inflection point torque is identified in the torque chart following shouldering, that inflection point torque is considered the yield torque for tightening control, even if the threaded joint has not actually yielded. This inflection point torque is an apparent yield torque, and once apparent yield torque is identified, the threaded joint tightening operation is halted. The threaded joint is then disassembled, inspected, and re-tightened. This results in unnecessary work.

[0031] Therefore, the inventors investigated the reason why apparent yield torque appears on a torque chart in a threaded joint equipped with a solid lubricating coating, even though the threads and shoulder surface have not yielded. As a result, they identified the following cause. After shouldering, the load flanks slide against each other while coming into strong contact, generating torque. The load flanks are contact surfaces in the axial direction of the pipe. In this case, the greater the thickness of the solid lubricating coating on the load flanks, the lower the apparent yield torque, which is clearly shown on the torque chart. Furthermore, the shoulder surface is also a contact surface in the axial direction of the pipe, and after shouldering, the two surfaces slide against each other while coming into strong contact, generating torque. In this case, just like the load flanks, the greater the thickness of the solid lubricating coating on the shoulder surface, the lower the apparent yield torque, which is clearly shown on the torque chart.

[0032] Specifically, when a surface with a solid lubricant coating slides against a mating metal surface while pressing it against the contact surface (load flank surface, shoulder surface) in the axial direction of the pipe, if the pressing force is small, the torque (force) required for sliding depends on the frictional resistance between the free surface of the solid lubricant coating and the mating metal surface. If the pressing force increases, shear fracture occurs inside the solid lubricant coating, reducing its rigidity (shear resistance). This reduces the torque required for sliding by the amount of the reduction in rigidity of the solid lubricant coating. This is reflected in a change in the gradient of the torque chart. Furthermore, the thicker the solid lubricant coating, the more likely it is that shear fracture will occur inside the solid lubricant coating, resulting in a significant reduction in the rigidity of the solid lubricant coating. This significant reduction in the rigidity of the solid lubricant coating increases the amount of discounting of the torque required for sliding, resulting in a clear change in the gradient of the torque chart.

[0033] In other words, in a threaded joint equipped with a solid lubricating coating, the areas that contribute to the apparent yield torque are the contact surfaces in the axial direction of the pipe (load flank surfaces and shoulder surfaces), and the thickness of the solid lubricating coating on these contact surfaces in the axial direction contributes to the apparent yield torque.

[0034] On the other hand, before shouldering, the radial contact surfaces come into contact with each other and slide while interfering with each other. The radial contact surfaces are the top surface of the female thread of the box, the bottom surface of the male thread of the pin, and the seal surface. The sliding between these radial contact surfaces generates torque before shouldering. Therefore, if the thickness of the solid lubricating coating on the radial contact surfaces is large, lubrication is enhanced and frictional resistance is reduced. As a result, shouldering torque is reduced.

[0035] In short, in a threaded joint equipped with a solid lubricating coating, the areas that contribute to shouldering torque are the radial contact surfaces (the female thread crest surface, male thread root surface, and seal surface), and the film thickness of the solid lubricating coating on these radial contact surfaces contributes to shouldering torque.

[0036] From the above, the inventors have arrived at the following technical idea. If the thickness of the solid lubricating coating is relatively large on the radial contact surfaces that are involved in shouldering torque (the female thread crest surface, male thread root surface, and seal surface), good lubrication can be ensured. On the other hand, if the thickness of the solid lubricating coating is relatively small on the axial contact surfaces that are involved in apparent yield torque (the load flank surface and shoulder surface), the effect of reduced rigidity of the solid lubricating coating can be reduced. In this way, if the thickness of the solid lubricating coating is appropriate at each location in a threaded joint, it is possible to achieve low shouldering torque and high yield torque even when a single solid lubricating coating is used.

[0037] The threaded joint according to the embodiment of the present disclosure has been completed based on the above findings.

[0038] 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 box is fastened to the pin by inserting the pin. The pin includes a male thread portion and a pin seal surface. The male thread portion is provided on the outer peripheral surface of the pin. The pin seal surface is provided on the outer peripheral surface of the pin. The box includes a female thread portion and a box seal surface. The female thread portion is provided on the inner peripheral surface of the box in correspondence with the male thread portion, and meshes with the male thread portion when the pin and box are fastened together. The box seal surface is provided on the inner peripheral surface of the box in correspondence with the pin seal surface. The 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 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, and the female thread load flank surface corresponds to the male thread load flank surface.

[0039] In the individual state before the pin and box are fastened together, the surface of at least one of the box and the pin is coated with a base coating, and the surface coated with the base coating is coated with a single solid lubricating coating. In the individual state, the base coating includes an alloy plating layer, and the solid lubricating coating is formed on the alloy plating layer. In the fastened state, the pin seal surface is in interference contact with the box seal surface, the male thread root surface is in interference contact with the female thread crest surface, and the male thread load flank surface is in pressing contact with the female thread load flank surface.

[0040] The threaded joint in its standalone state has the following features: The film thickness of the solid lubricant coating on the female thread load flank surface in the pipe axial direction of the threaded joint is smaller than the film thickness of the solid lubricant coating on the female thread crest surface in the radial direction perpendicular to the pipe axial direction. The radial film thickness of the solid lubricant coating on the box seal surface is larger than the above-mentioned film thickness of the solid lubricant coating on the female thread load flank surface. The film thickness of the solid lubricant coating on the male thread load flank surface in the pipe axial direction is smaller than the radial film thickness of the solid lubricant coating on the male thread root surface. The radial film thickness of the solid lubricant coating on the pin seal surface is larger than the above-mentioned film thickness of the solid lubricant coating on the male thread load flank surface. Furthermore, with regard to a first film thickness which is the combination of the solid lubricating coating and the base coating at the root surface of the female thread, a second film thickness which is the combination of the solid lubricating coating and the base coating at the crest surface of the male thread, a third film thickness which is the combination of the solid lubricating coating and the base coating at the crest surface of the female thread, and a fourth film thickness which is the combination of the solid lubricating coating and the base coating at the root surface of the male thread, a first index which is expressed by the value obtained by subtracting the sum of the third film thickness and the fourth film thickness from the sum of the first film thickness and the second film thickness is smaller than a second index which is expressed by the distance between the male thread crest surface and the female thread root surface in the make-up state when it is assumed that the box and the pin do not have a base coating or a solid lubricating coating (first configuration).

[0041] In a threaded joint according to the first configuration, the surface of at least one of the box and the pin is coated with a base coating including an alloy plating layer, and the surface coated with the base coating is then coated with a single solid lubricating coating. That is, the base coating and the single solid lubricating coating are laminated in this order on the surface of at least one of the box and the pin. The base coating including the alloy plating layer improves adhesion between the solid lubricating coating and the surface of the member on which it is provided. In this specification, the base coating and the solid lubricating coating may be collectively referred to as a "laminated coating."

[0042] In a threaded joint according to the first configuration, when the pin and box are fastened together, that is, when the pin has been completely fastened to the box, the pin seal surface is in interference contact with the box seal surface, the male thread root surface is in interference contact with the female thread crest surface, and the male thread load flank surface is in pressing contact with the female thread load flank surface. A laminate coating is interposed between the pin seal surface and the box seal surface, which are in interference contact with each other. A laminate coating is interposed between the male thread root surface and the female thread crest surface, which are in interference contact with each other. A laminate coating is interposed between the male thread load flank surface and the female thread load flank surface, which are in pressing contact with each other.

[0043] Furthermore, when the pin and box are in their individual state before being fastened together and a laminate coating is provided on both the box and pin surfaces, for example, the thickness of the solid lubricating coating on the female thread load flank is smaller than the thickness of the solid lubricating coating on the female thread crest surface and the thickness of the solid lubricating coating on the male thread load flank is smaller than the thickness of the solid lubricating coating on the male thread root surface. Furthermore, the thickness of the solid lubricating coating on the box seal surface is greater than the thickness of the solid lubricating coating on the female thread load flank, and the thickness of the solid lubricating coating on the pin seal surface is greater than the thickness of the solid lubricating coating on the male thread load flank. In this case, of the female thread load flank, female thread crest surface, and box seal surface, the thickness of the solid lubricating coating on the box is smallest on the female thread load flank and is greater on the female thread crest and box seal surface than on the female thread load flank. On the pin, the thickness of the solid lubricating coating is smallest on the male thread load flank, male thread root, and pin seal surface, and is greater on the male thread root and pin seal surfaces than on the male thread load flank. Therefore, the thickness of the solid lubricating coating is relatively large on the radial contact surfaces involved in shouldering torque (the female thread crest and male thread root surfaces, the box seal surface, and the pin seal surface), and is relatively small on the axial contact surfaces involved in apparent yield torque (the female thread load flank and male thread load flank surfaces).

[0044] Furthermore, for example, when a laminate coating is provided only on the surface of the box, the thickness of the solid lubricating coating on the female thread load flank is smaller than that on the female thread crest. Furthermore, the thickness of the solid lubricating coating on the box seal surface is larger than that on the female thread load flank. In this case, the thickness of the solid lubricating coating is smallest on the female thread load flank among the female thread load flank, female thread crest, and box seal surface, and is larger on the female thread crest and box seal surface than on the female thread load flank. Therefore, the thickness of the solid lubricating coating is relatively large on the radial contact surfaces (female thread crest and box seal surface) involved in shouldering torque, and is relatively small on the axial contact surface (female thread load flank) involved in apparent yield torque.

[0045] Furthermore, for example, when a laminate coating is provided only on the surface of the pin, the thickness of the solid lubricating coating on the male thread load flank surface is smaller than that on the male thread root surface. Furthermore, the thickness of the solid lubricating coating on the pin seal surface is larger than that on the male thread load flank surface. In this case, the thickness of the solid lubricating coating is smallest on the male thread load flank surface among the male thread load flank surface, male thread root surface, and pin seal surface, and is larger on the male thread root surface and pin seal surface than on the male thread load flank surface. Therefore, the thickness of the solid lubricating coating is relatively large on the radial contact surfaces (male thread root surface and pin seal surface) involved in shouldering torque, and is relatively small on the axial contact surface (male thread load flank surface) involved in apparent yield torque.

[0046] In either case, the thickness of the solid lubricating coating is relatively large on the radial contact surfaces that contribute to shouldering torque, ensuring good lubrication. Furthermore, the thickness of the solid lubricating coating is relatively small on the axial contact surfaces that contribute to apparent yield torque, reducing the effect of reduced rigidity of the solid lubricating coating. In this way, the thickness of the solid lubricating coating is appropriate at each location in the threaded joint, making it possible to achieve low shouldering torque and high yield torque. Moreover, since a single solid lubricating coating is used, procurement and management of the solid lubricating coating are easier than when two types of solid lubricating coatings are used, and the solid lubricating coating configuration is simpler. Therefore, the threaded joint of the first configuration makes it possible to expand the torque window with a simple and inexpensive configuration.

[0047] As described above, in the fastened state, the male thread root surface comes into interference contact with the female thread crest surface. In this case, the male thread crest surface does not contact the female thread root surface, and a gap is provided between the male thread crest surface and the female thread root surface. During the fastening operation, the male thread crest surface and the female thread root surface do not generally slide from the start of threading to the completion of threading (fastening completion), except immediately after the pin is inserted into the box. However, if the laminate coating provided on the male thread crest surface and / or the female thread root surface is thick enough to fill the gap, the surface of the solid lubricating coating may come into contact with the mating surface. When this occurs, the solid lubricating coating is not pressed against the base metal surface, and only shear forces act on the solid lubricating coating. This makes the solid lubricating coating prone to peeling from the base metal surface. Furthermore, because a pressing force acts between the male thread root surface and the female thread crest surface, which are in interference contact, the solid lubricating coating is less likely to peel off even when a shear force acts on it. When large separations of the solid lubricant coating occur, the separated solid lubricant coating accumulates unevenly in the gap between the crest surface of the male thread and the root surface of the female thread, hindering smooth screwing of the pin. This causes fastening problems such as high shouldering, resulting in a narrowing of the torque window.

[0048] In contrast, a threaded joint according to the first configuration has the following configurations regarding a first film thickness which is the combined thickness of the solid lubricating coating and the base coating at the root surface of the female thread, a second film thickness which is the combined thickness of the solid lubricating coating and the base coating at the crest surface of the male thread, a third film thickness which is the combined thickness of the solid lubricating coating and the base coating at the crest surface of the female thread, and a fourth film thickness which is the combined thickness of the solid lubricating coating and the base coating at the root surface of the male thread: The first index, which is expressed as the sum of the first film thickness and the second film thickness minus the sum of the third film thickness and the fourth film thickness, is smaller than the second index, which is expressed as the distance between the male thread crest surface and the female thread root surface in the make-up state when it is assumed that the box and pin do not have a coating stack.

[0049] Specifically, for example, when the laminate coating is provided on both the box and pin surfaces, the first index is expressed as the sum of the first thickness (the laminate coating thickness at the female thread root surface) and the second thickness (the laminate coating thickness at the male thread crest surface) minus the sum of the third thickness (the laminate coating thickness at the female thread crest surface) and the fourth thickness (the laminate coating thickness at the male thread root surface). For example, when the laminate coating is provided only on the box surface, the first index is expressed as the first thickness (the laminate coating thickness at the female thread root surface) minus the third thickness (the laminate coating thickness at the female thread crest surface). For example, when the laminate coating is provided only on the pin surface, the first index is expressed as the second thickness (the laminate coating thickness at the male thread crest surface) minus the fourth thickness (the laminate coating thickness at the male thread root surface). The second index corresponds to the radial distance (gap) between the male thread crest surface and the female thread root surface when the box and pin without the laminate coating have completed make-up and the male thread root surface and the female thread crest surface are in interference contact.

[0050] In either of these cases, if the first index is smaller than the second index, the surface of the solid lubricating coating will not come into contact with the surface of the mating member between the male thread crest surface and the female thread root surface during make-up. With a threaded joint of the first configuration, because the first index is smaller than the second index, make-up problems can be prevented, and in particular, shouldering torque can be stabilized at a low level, preventing the torque window from narrowing.

[0051] In the threaded joint according to the first configuration, preferably the base coating further includes a passivation coating, which is formed between the alloy plating layer and the solid lubricating coating (second configuration). In this case, the passivation coating can improve the corrosion resistance of the alloy plating layer on the box and / or pin on which the solid lubricating coating is provided.

[0052] In a threaded joint according to the first or second configuration, preferably the surface on which the base coating is formed is roughened by blasting (third configuration). In this case, the roughness improves adhesion between the base coating and the surface of the box and / or pin on which the solid lubricating coating is formed, and the roughness of the base coating surface also improves adhesion between the solid lubricating coating and the base coating.

[0053] A threaded joint according to any one of the first to third configurations preferably has the following configuration: The pin further includes a pin shoulder surface. The pin shoulder surface is provided at the tip of the pin. The box further includes a box shoulder surface. The box shoulder surface is provided at the deepest part of the box corresponding to the pin shoulder surface. In the made-up state, the pin shoulder surface is in pressing contact with the box shoulder surface. A threaded joint in a standalone state has the following configuration: The film thickness in the pipe axial direction of the solid lubricating coating on the box shoulder surface is smaller than the above-mentioned film thickness of the solid lubricating coating on the female thread crest surface. The film thickness in the pipe axial direction of the solid lubricating coating on the pin shoulder surface is smaller than the above-mentioned film thickness of the solid lubricating coating on the male thread root surface (fourth configuration).

[0054] In a threaded joint according to the fourth configuration, the pin and the box each have shoulder surfaces (pin shoulder surface and box shoulder surface) that are in pressing contact with each other in the made-up state. In other words, this threaded joint has a shouldering mechanism. In this case, the type of thread is typically a buttress thread (trapezoidal thread). A laminate coating is interposed between the pin shoulder surface and the box shoulder surface that are in pressing contact with each other in the made-up state.

[0055] In a threaded joint according to the fourth configuration, when a laminate coating is provided on both the box and pin surfaces in a single state, for example, the thickness of the solid lubricating coating on the box shoulder surface is smaller than the thickness of the solid lubricating coating on the female thread crest surface, and the thickness of the solid lubricating coating on the pin shoulder surface is smaller than the thickness of the solid lubricating coating on the male thread root surface. In this case, in the box, the thickness of the solid lubricating coating is greater on the female thread crest surface and the box seal surface than on the female thread load flank surface, as described above, and is further smaller on the box shoulder surface than on the female thread crest surface. In the pin, the thickness of the solid lubricating coating is greater on the male thread root surface and the pin seal surface than on the male thread load flank surface, as described above, and is further smaller on the pin shoulder surface than on the male thread root surface. Therefore, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (the female thread load flank surface, the male thread load flank surface, the box shoulder surface, and the pin shoulder surface).

[0056] Furthermore, for example, if the laminate coating is provided only on the surface of the box, the thickness of the solid lubricating coating on the box shoulder surface is smaller than that on the female thread crest surface. In this case, as described above, the thickness of the solid lubricating coating is greater on the female thread crest surface and box seal surface than on the female thread load flank surface, and is also smaller on the box shoulder surface than on the female thread crest surface. Therefore, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the axial direction of the pipe that are involved in the apparent yield torque (the female thread load flank surface and the box shoulder surface).

[0057] Furthermore, for example, when a laminate coating is provided only on the surface of the pin, the thickness of the solid lubricating coating on the pin shoulder surface is smaller than that on the male thread root surface. In this case, as described above, the thickness of the solid lubricating coating is greater on the male thread root surface and pin seal surface than on the male thread load flank surface, and is further smaller on the pin shoulder surface than on the male thread root surface. Therefore, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (male thread load flank surface and pin shoulder surface).

[0058] In any case of a threaded joint according to the fourth configuration having a shouldering mechanism, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the pipe axial direction that contribute to the apparent yield torque. Therefore, a threaded joint according to the fourth configuration achieves the same effects as the threaded joint according to the first configuration described above.

[0059] A threaded joint according to the fourth configuration preferably has the following configuration. A threaded joint in a standalone state has the following configuration. The film thickness of the solid lubricating coating on the box shoulder surface is smaller than the film thickness of the solid lubricating coating on the box seal surface. The film thickness of the solid lubricating coating on the pin shoulder surface is smaller than the film thickness of the solid lubricating coating on the pin seal surface (fifth configuration).

[0060] A threaded joint according to the fifth configuration is based on the threaded joint according to the fourth configuration, which has a shouldering mechanism. In a threaded joint according to the fifth configuration, when a laminate coating is provided on both the box and pin surfaces in a standalone state, for example, the thickness of the solid lubricating coating on the box shoulder surface is smaller than the thickness of the solid lubricating coating on the box seal surface, and the thickness of the solid lubricating coating on the pin shoulder surface is smaller than the thickness of the solid lubricating coating on the pin seal surface. In this case, on the box, the thickness of the solid lubricating coating is greater on the female thread crest surface and the box seal surface than on the female thread load flank surface, as described above, and is also smaller on the box shoulder surface than on the box seal surface. On the pin, the thickness of the solid lubricating coating is greater on the male thread root surface and the pin seal surface than on the male thread load flank surface, as described above, and is also smaller on the pin shoulder surface than on the pin seal surface. For this reason, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the axial direction of the pipe that are involved in the apparent yield torque (female thread load flank surface, male thread load flank surface, box shoulder surface, and pin shoulder surface).

[0061] Furthermore, for example, if the laminate coating is provided only on the surface of the box, the thickness of the solid lubricating coating on the box shoulder surface is smaller than that on the box seal surface. In this case, as described above, the thickness of the solid lubricating coating is greater on the female thread crest surface and box seal surface than on the female thread load flank surface, and is also smaller on the box shoulder surface than on the box seal surface. Therefore, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the axial direction of the pipe that are involved in the apparent yield torque (female thread load flank surface and box shoulder surface).

[0062] Furthermore, for example, when a laminate coating is provided only on the surface of the pin, the thickness of the solid lubricating coating on the pin shoulder surface is smaller than that on the pin seal surface. In this case, as described above, the thickness of the solid lubricating coating is greater on the male thread root surface and pin seal surface than on the male thread load flank surface, and is also smaller on the pin shoulder surface than on the pin seal surface. Therefore, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the axial direction of the pipe that are involved in the apparent yield torque (the male thread load flank surface and the pin shoulder surface).

[0063] In this way, in all cases of threaded joints according to the fifth configuration, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the pipe axial direction that are involved in the apparent yield torque. Therefore, threaded joints according to the fifth configuration achieve the same effects as the threaded joint according to the first configuration described above.

[0064] A threaded joint according to the fourth or fifth configuration preferably has the following configuration: With regard to a fifth film thickness of the solid lubricating coating and the base coating on the load flank of the female thread, a sixth film thickness of the solid lubricating coating and the base coating on the load flank of the male thread, a seventh film thickness of the solid lubricating coating and the base coating on the load flank of the female thread, and an eighth film thickness of the solid lubricating coating and the base coating on the stab flank of the male thread, the third index, which is expressed as the sum of the fifth film thickness, the sixth film thickness, the seventh film thickness, and the eighth film thickness, is smaller than a fourth index, which is expressed as the distance between the stab flank of the male thread and the stab flank of the female thread in the fastened state when it is assumed that the box and the pin do not have a base coating or a solid lubricating coating (sixth configuration).

[0065] As described above, in the fastened state, the male thread load flank is in pressing contact with the female thread load flank. In this case, in a threaded joint having a shouldering mechanism, the male thread stab flank is not in contact with the female thread stab flank, and a gap is provided between the male thread stab flank and the female thread stab flank. If the total thickness of the laminate coating provided on each flank is greater than the gap between the stab flanks in the fastened state assuming that the box and pin do not have a laminate coating, contact sliding occurs between both the load flanks and the stab flanks when the male thread root surface and the female thread crest surface slide in interference contact. In this case, not only does the sliding area increase, but the load flanks and the stab flanks press against each other simultaneously. Therefore, even if the solid lubricating coating is functioning, torque resistance increases, which in turn increases shouldering torque, resulting in a narrowing of the torque window.

[0066] In contrast, a threaded joint according to a sixth configuration is based on the threaded joint according to the fourth configuration, which has a shouldering mechanism. This threaded joint is configured as follows with respect to a fifth film thickness which is the sum of the solid lubricating coating and the base coating on the load flank of the female thread, a sixth film thickness which is the sum of the solid lubricating coating and the base coating on the load flank of the male thread, a seventh film thickness which is the sum of the solid lubricating coating and the base coating on the stab flank of the female thread, and an eighth film thickness which is the sum of the solid lubricating coating and the base coating on the stab flank of the male thread. The third index, which is represented by the sum of the fifth, sixth, seventh, and eighth film thicknesses, is smaller than the fourth index, which is represented by the distance between the male and female stab flanks in the fastened state when it is assumed that the box and pin do not have a coating stack.

[0067] Specifically, for example, when the laminate coating is provided on the surfaces of both the box and the pin, the third index is expressed as the sum of the fifth thickness (the thickness of the laminate coating on the load flank of the female thread), the sixth thickness (the thickness of the laminate coating on the load flank of the male thread), the seventh thickness (the thickness of the laminate coating on the stab flank of the female thread), and the eighth thickness (the thickness of the laminate coating on the stab flank of the male thread). Also, for example, when the laminate coating is provided on the surface of only the box, the third index is expressed as the sum of the fifth thickness (the thickness of the laminate coating on the load flank of the female thread) and the seventh thickness (the thickness of the laminate coating on the stab flank of the female thread). Also, for example, when the laminate coating is provided on the surface of only the pin, the third index is expressed as the sum of the sixth thickness (the thickness of the laminate coating on the load flank of the male thread) and the eighth thickness (the thickness of the laminate coating on the stab flank of the male thread). The fourth index corresponds to the distance (gap) in the pipe axis direction between the male thread insertion flank surface and the female thread insertion flank surface when the box and pin without the laminate coating have completed fastening and the male thread load flank surface and the female thread load flank surface are in interference contact.

[0068] In either of these cases, if the third index is smaller than the fourth index, at the stage where the male thread root surface and the female thread crest surface slide while in interference contact, contact sliding does not occur between both the load flank surfaces and the stabbing flank surfaces. With a threaded joint according to the sixth configuration, because the third index is smaller than the fourth index, an increase in torque resistance can be suppressed, and therefore an increase in shouldering torque can be suppressed, and as a result, a narrowing of the torque window can be prevented.

[0069] In the threaded joint according to any one of the first to sixth configurations, 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 tubular member separate from the steel pipes (seventh configuration). In this case, the threaded joint is of the coupling type.

[0070] In a threaded joint according to any one of the first to seventh configurations, the solid lubricating coating preferably contains a resin and a solid lubricating powder (eighth configuration).

[0071] In a threaded joint according to the eighth configuration, preferably, the resin is an epoxy resin and the solid lubricating powder is polytetrafluoroethylene (ninth configuration).

[0072] In the threaded joint according to the first configuration, preferably, in the stand-alone state, the box has a base coating and a solid lubricating coating, and the pin has no base coating or solid lubricating coating (tenth configuration).

[0073] Generally, a solid lubricating coating is formed by spraying a fluid coating composition onto the target component (box and / or pin) and allowing it to solidify. For example, in a threaded joint according to the first configuration, when a solid lubricating coating is applied to a pin, the coating thickness of the male thread must be large at the male thread root surface and small at the male thread load flank surface. When the coating composition is sprayed onto the male thread, the coating composition sprayed onto the male thread crest region adheres directly to the male thread crest surface, while the coating composition sprayed onto the male thread root region is distributed to the male thread root surface, male thread load flank surface, and male thread stab flank surface. For this reason, the thickness of the solid lubricating coating on the male thread root surface is unlikely to be large.

[0074] On the other hand, when a solid lubricating coating is applied to a box, the film thickness of the female thread needs to be large on the female thread crest and small on the female thread load flank. When a coating composition is sprayed on a female thread, the coating composition sprayed on the female thread crest region adheres directly to the female thread crest, while the coating composition sprayed on the female thread root region is distributed to the female thread root, female thread load flank, and female thread stab flank. As a result, the film thickness of the solid lubricating coating on the female thread crest tends to be large, while the film thickness of the solid lubricating coating on the female thread load flank tends to be small. In a threaded joint according to the tenth configuration, a solid lubricating coating is applied only to the box. As a result, a solid lubricating coating of an appropriate thickness can be easily formed in the required area.

[0075] In a threaded joint according to the tenth configuration, as with the threaded joint according to the first configuration in which a laminate coating is provided only on the surface of the box, the thickness of the solid lubricant coating on the female thread load flank is smaller than the thickness of the solid lubricant coating on the female thread crest. Furthermore, the thickness of the solid lubricant coating on the box seal surface is greater than the thickness of the solid lubricant coating on the female thread load flank. In this case, the thickness of the solid lubricant coating is smallest on the female thread load flank among the female thread load flank, the female thread crest, and the box seal surface, and is greater on the female thread crest and box seal surface than on the female thread load flank. Therefore, the thickness of the solid lubricant coating is relatively large on the radial contact surfaces (female thread crest and box seal surfaces) that are involved in shouldering torque, and is relatively small on the axial contact surfaces (female thread load flank) that are involved in apparent yield torque. Therefore, with the threaded joint according to the tenth configuration, as with the threaded joint according to the first configuration, it is possible to expand the torque window with a simple and inexpensive configuration.

[0076] Furthermore, in a threaded joint according to the tenth configuration, as in the threaded joint according to the first configuration in which the laminate coating is provided only on the surface of the box, the first index is expressed as the first film thickness (the film thickness of the laminate coating at the root surface of the female thread) minus the third film thickness (the film thickness of the laminate coating at the crest surface of the female thread). The second index corresponds to the radial distance (gap) between the male thread crest surface and the female thread root surface when the box and pin without the laminate coating are made up and the male thread root surface and the female thread crest surface are in interference contact. Furthermore, because the first index is smaller than the second index, the surface of the solid lubricating coating does not come into contact with the surface of the mating member between the male thread crest surface and the female thread root surface during make-up. Therefore, as in the threaded joint according to the first configuration, the threaded joint according to the tenth configuration can prevent make-up problems, and in particular, can stabilize shouldering torque at a low level and prevent the torque window from narrowing.

[0077] A threaded joint according to the tenth configuration preferably has the following configuration. The pin further includes a pin shoulder surface. The pin shoulder surface is provided at the tip of the pin. The box further includes a box shoulder surface. The box shoulder surface is provided at the innermost part of the box corresponding to the pin shoulder surface. In a fastened state, the pin shoulder surface is in pressing contact with the box shoulder surface. A pin is provided on each of the steel pipes to be connected. The box is provided on a coupling, which is a tubular member separate from the steel pipes. The solid lubricant coating contains epoxy resin and solid lubricant powder. The solid lubricant powder is made of polytetrafluoroethylene. The undercoat includes a Zn alloy plating layer as an alloy plating layer, and a passivation coating. The passivation coating is formed between the Zn alloy plating layer and the solid lubricant coating. The surface of the box on which the undercoat is formed has irregularities formed by blasting. In the stand-alone state, the thickness of the solid lubricating coating on the box shoulder surface in the axial direction of the pipe is smaller than the thickness of the solid lubricating coating on the crest surface of the female thread (eleventh configuration).

[0078] A threaded joint according to an eleventh configuration is based on the threaded joint according to the tenth configuration, in which a laminate coating is provided only on the box. The threaded joint according to the eleventh configuration has a shouldering mechanism, similar to the threaded joint according to the fourth configuration, and the thickness of the solid lubricating coating on the box shoulder surface is smaller than the thickness of the solid lubricating coating on the female thread crest surface. In this case, the thickness of the solid lubricating coating is greater on the female thread crest surface and the box seal surface than on the female thread load flank surface, as described above, and is also smaller on the box shoulder surface than on the female thread crest surface. The threaded joint according to the eleventh configuration is also a coupling type, similar to the threaded joint according to the seventh configuration. In the threaded joint according to the eleventh configuration, the basecoat including the Zn alloy plating layer improves adhesion between the solid lubricating coating and the surface of the box on which the solid lubricating coating is provided. Furthermore, similar to the threaded joint according to the second configuration, the corrosion resistance of the Zn alloy plating layer can be improved on the box on which the solid lubricating coating is provided by a passivation coating. Furthermore, as with the threaded joint of the third configuration, the unevenness improves the adhesion between the surface of the box on which the solid lubricating coating is provided and the base coating, and since the surface of the base coating also becomes uneven, the adhesion between the solid lubricating coating and the base coating is also improved.

[0079] In a threaded joint according to the eleventh configuration, the film thickness of the solid lubricating coating on the box shoulder surface is preferably smaller than the film thickness of the solid lubricating coating on the box seal surface in the stand-alone state (twelfth configuration).

[0080] A threaded joint according to the twelfth configuration is based on the threaded joint according to the eleventh configuration. That is, the threaded joint according to the twelfth configuration is based on the threaded joint according to the tenth configuration, in which a laminate coating is provided only on the box. In the threaded joint according to the twelfth configuration, the thickness of the solid lubricating coating on the box shoulder surface is smaller than that on the box seal surface. Therefore, as with the threaded joint according to the fifth configuration, the thickness of the solid lubricating coating is greater on the female thread crest surface and the box seal surface than on the female thread load flank surface, and is smaller on the box shoulder surface than on the box seal surface, as described above. Therefore, the thickness of the solid lubricating coating is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (female thread load flank surface and box shoulder surface). Therefore, the threaded joint according to the twelfth configuration achieves the same effects as the threaded joint according to the fifth configuration described above.

[0081] A threaded joint according to the eleventh or twelfth configuration preferably has the following configuration: With regard to a fifth film thickness which is the sum of the solid lubricating coating and the base coating on the load flank of the female thread, and a seventh film thickness which is the sum of the solid lubricating coating and the base coating on the stab flank of the female thread, the third index represented by the sum of the fifth film thickness and the seventh film thickness is smaller than a fourth index represented by the distance between the stab flank of the male thread and the stab flank of the female thread in the fastened state when it is assumed that the box and the pin do not have a base coating or a solid lubricating coating (thirteenth configuration).

[0082] A threaded joint according to the thirteenth configuration is based on the threaded joint according to the eleventh configuration. That is, the threaded joint according to the thirteenth configuration is based on the threaded joint according to the tenth configuration, in which the laminate coating is provided only on the box. In the threaded joint according to the thirteenth configuration, as in the threaded joint according to the sixth configuration in which the laminate coating is provided only on the surface of the box, the third index is expressed as the sum of the fifth film thickness (the film thickness of the laminate coating on the load flank of the female thread) and the seventh film thickness (the film thickness of the laminate coating on the stab flank of the female thread). The fourth index corresponds to the axial distance (gap) between the male thread stab flank and the female thread stab flank when the box and pin without the laminate coating are tightened and the male thread stab flank and the female thread stab flank are in interference contact. Furthermore, because the third index is smaller than the fourth index, when the male thread root surface and the female thread crest surface slide while in interference contact, no contact sliding occurs between both the load flank and the stab flank. Therefore, with the threaded joint of the thirteenth configuration, similar to the threaded joint of the sixth configuration, an increase in torque resistance can be suppressed, and in turn an increase in shouldering torque can be suppressed, and as a result, narrowing of the torque window can be prevented.

[0083] In a threaded joint according to any one of the tenth to thirteenth configurations, preferably, in the state of being a single piece, the surface of the pin is coated with a chemical conversion coating, and the surface coated with the chemical conversion coating is coated with a rust-preventive oil coating (fourteenth configuration).

[0084] The threaded joint according to the fourteenth configuration is based on the threaded joint according to the tenth configuration, in which a laminate coating is provided only on the box. In the threaded joint according to the fourteenth configuration, a laminate coating is not provided on the pin. Instead, a chemical conversion coating and a rust-preventive oil coating are laminated in this order on the surface of the pin. This provides the pin with an inexpensive and sufficient rust-preventive treatment.

[0085] 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.

[0086] <First embodiment> [Configuration of threaded joint] Figure 4 is a longitudinal cross-sectional view of a threaded joint 100 according to the first embodiment. The longitudinal cross-section of the threaded joint 100 is a cross-section obtained by cutting the threaded joint 100 along a plane that includes the pipe axis X. In this specification, the direction in which the pipe axis X 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.

[0087] Referring to FIG. 4 , 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 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 tubular member separate from the steel pipes 30 to be connected. In other words, the threaded joint 100 is a coupling type. The material of the steel pipes 30 (strictly speaking, the material of the base metal of the steel pipes 30) is, for example, low-chromium alloy martensitic steel, martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel, or the like. The material of the coupling 40 (strictly speaking, the material of the base metal of the coupling 40) may be the same as or different from the material of the steel pipes 30.

[0088] 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 male thread portion 11 and a pin seal surface 12. In this embodiment, the pin 10 further includes a pin shoulder surface 13. The male thread portion 11, the pin seal surface 12, and the pin shoulder surface 13 are arranged in this order from the steel pipe main body 31 side toward the tip side of the pin 10.

[0089] The 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 main body 31 side with respect 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 main body 31. The male thread portion 11 is composed of a tapered thread. 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.

[0090] 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. The pin seal surface 12 is the outer surface of the side of a rotating body whose axis is the pipe axis X. 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 of the pin 10. The pin seal surface 12 may be configured by combining two or more of these curves and / or straight lines.

[0091] The pin shoulder surface 13 is provided at the tip of the pin 10. The pin shoulder surface 13 forms the tube end surface of the pin 10 and is an annular surface whose axis is the tube axis X. The pin shoulder surface 13 may be inclined so that its outer peripheral edge is located closer to the tip of the pin 10 than the inner peripheral edge, or it may not be inclined.

[0092] The box 20 includes a female thread portion 21 and a box seal surface 22. In this embodiment, the box 20 further includes a box shoulder surface 23. 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.

[0093] The female thread portion 21 is provided on the inner peripheral surface of the box 20. The female thread portion 21 is disposed on the pipe end side of the box 20 with respect to the box seal surface 22. The female thread portion 21 extends from the vicinity of the box seal surface 22 to the vicinity of the pipe end of the box 20. The female thread portion 21 is configured with a tapered thread that meshes with the male thread portion 11 of the pin 10. The type of thread that constitutes the female thread portion 21 corresponds to the male thread portion 11, and is, for example, an API standard buttress thread (trapezoidal thread) or a square thread.

[0094] 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 the side of a rotating body whose axis is the pipe axis X. 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 with respect 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 such curves and / or straight lines.

[0095] 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 X. 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.

[0096] The configuration of the threaded joint 100 according to this embodiment will be described in more detail below with reference to Figures 5 to 11. Figure 5 is a partially enlarged view of the threaded joint 100 shown in Figure 4. Figures 6A and 6B are vertical cross-sectional views showing the pin 10 and box 20 in the threaded joint 100 shown in Figure 5 in an individual state before make-up. Figure 6A shows the pin 10. Figure 6B shows the box 20. Figure 7 is a schematic view showing the surface layer portions of the pin 10 and box 20 in the threaded joint 100 in an individual state.

[0097] 5, 6A, and 6B, the 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 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.

[0098] 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 113 and the male thread load flank 114. The male thread stab flank 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 114 is located behind the male thread crest surface 111 in the direction of threading of the pin 10.

[0099] The male thread crest surface 111 and the male thread root surface 112 are straight lines inclined with respect to the pipe axis X so as to approach the pipe axis X toward the tip side of the pin 10 in a longitudinal 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 X in a longitudinal cross-sectional view of the threaded joint 100.

[0100] 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.

[0101] 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 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 X.

[0102] 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.

[0103] The female thread root surface 211 and the female thread crest surface 212 are straight lines inclined with respect to the pipe axis X so as to approach the pipe axis X toward the innermost side of the box 20 in a longitudinal cross-sectional view of the threaded joint 100. The inclination angle of the female thread root surface 211 and the female thread crest surface 212 is substantially equal to the inclination angle of the male thread crest surface 111 and the male thread root surface 112. Like the male thread crest surface 111 and the male thread root surface 112, the female thread root surface 211 and the female thread crest surface 212 may be straight lines parallel to the pipe axis X in a longitudinal cross-sectional view of the threaded joint 100. In other words, 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.

[0104] 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.

[0105] 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 be a straight line that is perpendicular to the pipe axis X 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.

[0106] In this specification, in order to briefly explain the essence of the disclosed technical idea, it is stated that each surface constituting the threaded portion (the male threaded portion 11 and the female threaded portion 21) is straight in a cross-sectional view of the threaded joint 100. This means that the main area of ​​each surface of the threaded portion is straight in a cross-sectional view 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 portion between adjacent surfaces.

[0107] 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, in a vertical cross-sectional view 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, in a vertical cross-sectional view of the threaded joint 100.

[0108] 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, in a longitudinal cross-sectional view 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, in a longitudinal cross-sectional view 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 a cross-sectional view 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.

[0109] 5 , when the pin 10 is screwed into the box 20 and the pin 10 and box 20 are fastened together, 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 the male thread load flank surface 114 is in pressing contact with the female thread load flank surface 214. Meanwhile, in the fastened state, 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. Furthermore, in the fastened state, the male thread stab flank surface 113 is 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.

[0110] In a fastened state, the laminate coatings 14, 24 are interposed between the male thread root surface 112 and the female thread crest surface 212, which are in interference contact with each other. The laminate coatings 14, 24 are also interposed between the male thread load flank surface 114 and the female thread load flank surface 214, which are in pressing contact with each other. The laminate coatings 14, 24 are also present between the male thread crest surface 111 and the female thread root surface 211. The laminate coatings 14, 24 are also present between the male thread stab flank surface 113 and the female thread stab flank surface 213.

[0111] In a fastened state, the pin seal surface 12 comes into interference contact with the box seal surface 22, forming a seal portion by metal-to-metal contact together with the box seal surface 22. The laminate coatings 14, 24 are interposed between the pin seal surface 12 and the box seal surface 22, which come into interference contact with each other.

[0112] In the fastened state, the pin shoulder surface 13 comes into pressing contact with the box shoulder surface 23, forming a shoulder portion together with the box shoulder surface 23. During make-up, the shoulder surfaces 13, 23 come into contact with each other as the pin 10 is threaded into the box 20. The point at which this contact occurs is shouldering. Therefore, the threaded joint 100 has a shouldering mechanism as a torque stop mechanism. In the fastened state, the laminate coatings 14, 24 are interposed between the pin shoulder surface 13 and the box shoulder surface 23, which are in pressing contact with each other.

[0113] The female thread crest surface 212 and the male thread root surface 112 are in interference contact with each other substantially in the radial direction, and are therefore radial contact surfaces. The box seal surface 22 and the pin seal surface 12 are in interference contact with each other substantially in the radial direction, and are therefore radial contact surfaces. The female thread load flank surface 214 and the male thread load flank surface 114 are in pressing contact with each other substantially in the pipe axial direction, and are therefore axial contact surfaces. The box shoulder surface 23 and the pin shoulder surface 13 are in pressing contact with each other substantially in the pipe axial direction, and are therefore axial contact surfaces.

[0114] 6A, 6B, and 7, in the individual state before the pin 10 and box 20 are fastened together, the surface of the pin 10 is covered with a single coating laminate 14. In the individual state, the surface of the box 20 is covered with a single coating laminate 24. In this specification, a single solid lubricating coating 16, 26 means that it is composed of one type of solid lubricating coating. In other words, the solid lubricating coatings 16, 26 are not composed of multiple types of solid lubricating coatings laminated together.

[0115] In the pin 10, the surfaces coated with the laminate coating 14 are at least the male thread portion 11, the pin seal surface 12, and the pin shoulder surface 13. Therefore, the male thread crest surface 111, the male thread root surface 112, the male thread stab flank surface 113, the male thread load flank surface 114, the pin seal surface 12, and the pin shoulder surface 13 are coated with the laminate coating 14.

[0116] In the box 20, the surfaces coated with the laminate coating 24 are at least the female thread portion 21, the box seal surface 22, and the box shoulder surface 23. Therefore, the female thread root surface 211, the female thread crest surface 212, the female thread stab flank surface 213, the female thread load flank surface 214, the box seal surface 22, and the box shoulder surface 23 are coated with the laminate coating 24.

[0117] The laminate coating 14 that covers the surface of the pin 10 is composed of a base coating 15 and a solid lubricating coating 16. Specifically, the surface of the pin 10 is coated with the base coating 15, and the surface of the pin 10 that is coated with the base coating 15 is further coated with the solid lubricating coating 16. In other words, the base coating 15 and the solid lubricating coating 16 are laminated in this order on the surface of the pin 10. In a standalone state, the base coating 15 includes an alloy plating layer 151, and the solid lubricating coating 16 is formed on the alloy plating layer 151. In this embodiment, the base coating 15 is composed of the alloy plating layer 151.

[0118] The laminate coating 24 that covers the surface of the box 20 is composed of a base coating 25 and a solid lubricating coating 26. Specifically, the surface of the box 20 is coated with the base coating 25, and the surface of the box 20 that is coated with the base coating 25 is further coated with the solid lubricating coating 26. In other words, the base coating 25 and the solid lubricating coating 26 are layered in this order on the surface of the box 20. In a standalone state, the base coating 25 includes an alloy plating layer 251, and the solid lubricating coating 26 is formed on the alloy plating layer 251. In this embodiment, the base coating 25 is composed of the alloy plating layer 251.

[0119] The solid lubricating coatings 16, 26 are polymer-based lubricating coatings. The solid lubricating coatings 16, 26 contain a resin and a solid lubricating powder. The resin is the base material of the solid lubricating coatings 16, 26. Particles of the solid lubricating powder are dispersed in the resin. The solid lubricating coatings 16, 26 may further contain additives such as friction-enhancing materials, surfactants, and rust inhibitors.

[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 alloy plating layers 151, 251 contained in the undercoatings 15, 25. Polyurethane resin is particularly chemically stable and easy to handle. Polyamide-imide resin is particularly excellent in wear resistance.

[0121] The solid lubricant powder is, for example, polytetrafluoroethylene (PTFE). In addition to PTFE, the solid lubricant powder may also be molybdenum dithiocarbamate (MoDTC), molybdenum disulfide (MoS ２ ), carbon black (C), graphite fluoride (CFx), or a mixture thereof. Of these solid lubricant powders, PTFE exhibits a stable coefficient of friction under strong contact pressure and provides 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 coatings 16, 26, 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 coatings 16, 26 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] The solid lubricating coatings 16, 26 preferably combine an epoxy resin as the resin and PTFE as the solid lubricating powder. In this case, the PTFE content is preferably 10% by weight or more and 25% by weight or less. The solid lubricating coatings 16, 26 may also be an inorganic binder coating.

[0124] The base coating 15, 25 on which the solid lubricating coating 16, 26 is laminated includes an alloy plating layer 151, 251. The alloy plating layer 151, 251 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 predominant 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.

[0125] The alloy plating layers 151, 251 may also be Cu alloy plating layers. The Cu alloy plating layers are multi-element alloy plating layers 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 layers are binary or ternary alloy plating layers containing copper and one or two elements selected from nickel, iron, magnesium, manganese, zinc, and tin.

[0126] The alloy plating layers 151, 251 can be easily formed by, for example, immersion electroplating, which forms the alloy plating layers 151, 251 homogeneously and with a substantially uniform thickness on the surfaces of the target components, i.e., the pin 10 and the box 20.

[0127] The thickness of the alloy plating layer 151, 251 is, for example, 4 μm or more and 20 μm or less. If the thickness of the alloy plating layer 151, 251 is 4 μm or more, sufficient corrosion resistance is obtained. On the other hand, if the thickness of the alloy plating layer 151, 251 is 20 μm or less, a decrease in adhesion with the pin 10 or box 20 due to an increase in internal stress is sufficiently suppressed. The thickness of the alloy plating layer 151, 251 is more preferably 6 μm or more and 15 μm or less.

[0128] (Film Thickness of the Laminate Coatings 14, 24 and the Solid Lubricant Coatings 16, 26) First, the film thicknesses of the solid lubricant coatings 16, 26 will be described. Referring to Figure 6B, in the box 20 in its standalone state, the film thickness tS214 of the solid lubricant coating 26 on the female thread load flank surface 214 is smaller than the film thickness tS212 of the solid lubricant coating 26 on the female thread crest surface 212. The film thickness tS214 of the solid lubricant coating 26 on the female thread load flank surface 214 refers to the thickness in the tube axis direction of the solid lubricant coating 26 formed on the female thread load flank surface 214. The film thickness tS212 of the solid lubricant coating 26 on the female thread crest surface 212 refers to the thickness in the radial direction of the solid lubricant coating 26 formed on the female thread crest surface 212.

[0129] In the box 20 in its standalone state, the film thickness tS22 of the solid lubricant coating 26 on the box seal surface 22 is greater than the film thickness tS214 of the solid lubricant coating 26 on the female thread load flank surface 214. The film thickness tS22 of the solid lubricant coating 26 on the box seal surface 22 refers to the radial thickness of the solid lubricant coating 26 formed on the box seal surface 22.

[0130] In this case, the film thickness of the solid lubricating coating 26 is smallest on the female thread load flank 214 (film thickness tS214) among the female thread load flank 214, the female thread crest 212, and the box seal surface 22, and is greater on the female thread crest 212 (film thickness tS212) and the box seal surface 22 (film thickness tS22) than on the female thread load flank 214 (film thickness tS214). Therefore, the film thickness of the solid lubricating coating 26 is relatively large on the radial contact surfaces (female thread crest 212, box seal surface 22) involved in shouldering torque, and the film thickness of the solid lubricating coating 26 is relatively small on the axial contact surface (female thread load flank 214) involved in apparent yield torque.

[0131] In addition, in the box 20 in its standalone state, the film thickness tS23 of the solid lubricating coating 26 on the box shoulder surface 23 is smaller than the film thickness tS212 of the solid lubricating coating 26 on the female thread crest surface 212. The film thickness tS23 of the solid lubricating coating 26 on the box shoulder surface 23 refers to the thickness dimension in the pipe axis direction of the solid lubricating coating 26 formed on the box shoulder surface 23. Furthermore, the film thickness tS23 of the solid lubricating coating 26 on the box shoulder surface 23 is smaller than the film thickness tS22 of the solid lubricating coating 26 on the box seal surface 22.

[0132] In this case, as described above, the thickness of the solid lubricating coating 26 is greater on the female thread crest 212 (film thickness tS212) and the box seal surface 22 (film thickness tS22) than on the female thread load flank 214 (film thickness tS214), and is furthermore smaller on the box shoulder surface 23 (film thickness tS23) than on the female thread crest 212 (film thickness tS212). Furthermore, the thickness of the solid lubricating coating 26 is smaller on the box shoulder surface 23 (film thickness tS23) than on the box seal surface 22 (film thickness tS22). Therefore, the thickness of the solid lubricating coating 26 is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (the female thread load flank surface 214 and the box shoulder surface 23).

[0133] 6A , in the pin 10 in its standalone state, the film thickness tS114 of the solid lubricant coating 16 on the male thread load flank surface 114 is smaller than the film thickness tS112 of the solid lubricant coating 16 on the male thread root surface 112. The film thickness tS114 of the solid lubricant coating 16 on the male thread load flank surface 114 refers to the thickness dimension in the axial direction of the solid lubricant coating 16 formed on the male thread load flank surface 114. The film thickness tS112 of the solid lubricant coating 16 on the male thread root surface 112 refers to the thickness dimension in the radial direction of the solid lubricant coating 16 formed on the male thread root surface 112.

[0134] In the pin 10 in its standalone state, the film thickness tS12 of the solid lubricant coating 16 on the pin seal surface 12 is greater than the film thickness tS114 of the solid lubricant coating 16 on the male thread load flank surface 114. The film thickness tS12 of the solid lubricant coating 16 on the pin seal surface 12 refers to the radial thickness of the solid lubricant coating 16 formed on the pin seal surface 12.

[0135] In this case, the film thickness of the solid lubricating coating 16 is smallest on the male thread load flank surface 114 (film thickness tS114) among the male thread load flank surface 114, the male thread root surface 112, and the pin seal surface 12, and is greater on the male thread root surface 112 (film thickness tS112) and the pin seal surface 12 (film thickness tS12) than on the male thread load flank surface 114 (film thickness tS114). Therefore, the film thickness of the solid lubricating coating 16 is relatively large on the radial contact surfaces (male thread root surface 112, pin seal surface 12) involved in shouldering torque, and the film thickness of the solid lubricating coating 16 is relatively small on the axial contact surface (male thread load flank surface 114) involved in apparent yield torque.

[0136] In addition, in the pin 10 in its standalone state, the film thickness tS13 of the solid lubricant coating 16 on the pin shoulder surface 13 is smaller than the film thickness tS112 of the solid lubricant coating 16 on the male thread root surface 112. The film thickness tS13 of the solid lubricant coating 16 on the pin shoulder surface 13 refers to the thickness dimension in the pipe axis direction of the solid lubricant coating 16 formed on the pin shoulder surface 13. Furthermore, the film thickness tS13 of the solid lubricant coating 16 on the pin shoulder surface 13 is smaller than the film thickness tS12 of the solid lubricant coating 16 on the pin seal surface 12.

[0137] In this case, as described above, the thickness of the solid lubricating coating 16 is greater on the male thread root surface 112 (film thickness tS112) and the pin seal surface 12 (film thickness tS12) than on the male thread load flank surface 114 (film thickness tS114), and is furthermore smaller on the pin shoulder surface 13 (film thickness tS13) than on the male thread root surface 112 (film thickness tS112). Furthermore, the thickness of the solid lubricating coating 16 is smaller on the pin shoulder surface 13 (film thickness tS13) than on the pin seal surface 12 (film thickness tS12). Therefore, the thickness of the solid lubricating coating 16 is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (male thread load flank surface 114 and pin shoulder surface 13).

[0138] Next, the film thicknesses of the laminate coatings 14, 24 (the base coatings 15, 25 and the solid lubricant coatings 16, 26) will be described. Referring to Figures 6A and 6B, the film thickness tL211 of the laminate coating 24 at the female thread root surface 211 is designated as the first film thickness tL211. The film thickness tL111 of the laminate coating 14 at the male thread crest surface 111 is designated as the second film thickness tL111. The film thickness tL212 of the laminate coating 24 at the female thread crest surface 212 is designated as the third film thickness tL212. The film thickness tL112 of the laminate coating 14 at the male thread root surface 112 is designated as the fourth film thickness tL112. The first film thickness tL211 is the combined film thickness of the solid lubricant coating 26 and the base coating 25 formed on the female thread root surface 211. The second film thickness tL111 is the combined film thickness of the solid lubricant coating 16 and the base coating 15 formed on the male thread crest surface 111. The third film thickness tL212 is the combined film thickness of the solid lubricant coating 26 and the base coating 25 formed on the female thread crest surface 212. The fourth film thickness tL112 is the combined film thickness of the solid lubricant coating 16 and the base coating 15 formed on the male thread root surface 112. The first film thickness tL211, the second film thickness tL111, the third film thickness tL212, and the fourth film thickness tL112 are all thickness dimensions in the radial direction of the coating stack 14, 24.

[0139] With respect to the first thickness tL211, the second thickness tL111, the third thickness tL212, and the fourth thickness tL112, a first index is defined which is expressed by the sum of the first thickness tL211 and the second thickness tL111 minus the sum of the third thickness tL212 and the fourth thickness tL112. In this first index, the sum of the first thickness tL211 and the second thickness tL111 is the thickness of the entire laminate coating 14, 24 present between the female thread root surface 211 and the male thread crest surface 111 in the made-up state, and the sum of the third thickness tL212 and the fourth thickness tL112 is the thickness of the entire laminate coating 14, 24 interposed between the female thread crest surface 212 and the male thread root surface 112 in the made-up state.

[0140] Figure 8 is a vertical cross-sectional view showing the make-up state of the threaded joint 100 shown in Figure 5 when it is assumed that the pin 10 and box 20 do not have the laminate coatings 14, 24 (primary coatings 15, 25 and solid lubricating coatings 16, 26). In other words, Figure 8 shows the state in which the box 20 and pin 10 without the laminate coatings 14, 24 have completed make-up, with the male thread root surface 112 and the female thread crest surface 212 in direct interference contact and the male thread load flank surface 114 and the female thread load flank surface 214 in direct pressing contact.

[0141] 8, a second index represented by the distance d2 between the male thread crest surface 111 and the female thread root surface 211 in a fastened state is defined. The second index represented by this distance d2 corresponds to the radial interval (gap) between the male thread crest surface 111 and the female thread root surface 211 in a fastened state when it is assumed that the box 20 and the pin 10 do not have the laminate coatings 14, 24.

[0142] 6A and 6B , the first film thickness tL211, the second film thickness tL111, the third film thickness tL212, and the fourth film thickness tL112 are formed so that the first index is smaller than the second index (distance d2). In other words, in the radial dimensions of the threaded joint 100, the film thicknesses tL211 and tL212 of the laminate coating 24 formed on the female thread portion 21 and the film thicknesses tL111 and tL112 of the laminate coating 14 formed on the male thread portion 11 satisfy the condition that the first index is smaller than the second index.

[0143] The thickness tL214 of the laminate coating 24 on the female thread load flank surface 214 is designated as the fifth thickness tL214. The thickness tL114 of the laminate coating 14 on the male thread load flank surface 114 is designated as the sixth thickness tL114. The thickness tL213 of the laminate coating 24 on the female thread stab flank surface 213 is designated as the seventh thickness tL213. The thickness tL113 of the laminate coating 14 on the male thread stab flank surface 113 is designated as the eighth thickness tL113. The fifth thickness tL214 is the combined thickness of the solid lubricant coating 26 and the base coating 25 formed on the female thread load flank surface 214. The sixth thickness tL114 is the combined thickness of the solid lubricant coating 16 and the base coating 15 formed on the male thread load flank surface 114. The seventh film thickness tL213 is the combined film thickness of the solid lubricant coating 26 and the base coating 25 formed on the female thread stab flank 213. The eighth film thickness tL113 is the combined film thickness of the solid lubricant coating 16 and the base coating 15 formed on the male thread stab flank 113. The fifth film thickness tL214, the sixth film thickness tL114, the seventh film thickness tL213, and the eighth film thickness tL113 are all thickness dimensions of the stacked film 14, 24 in the tube axis direction.

[0144] With respect to the fifth thickness tL214, the sixth thickness tL114, the seventh thickness tL213, and the eighth thickness tL113, a third index is defined which is expressed as the sum of the fifth thickness tL214, the sixth thickness tL114, the seventh thickness tL213, and the eighth thickness tL113. In this third index, the sum of the fifth thickness tL214 and the sixth thickness tL114 is the thickness of the entire laminate coating 14, 24 interposed between the female thread load flank surface 214 and the male thread load flank surface 114, and the sum of the seventh thickness tL213 and the eighth thickness tL113 is the thickness of the entire laminate coating 14, 24 present between the female thread stab flank surface 213 and the male thread stab flank surface 113.

[0145] 8, a fourth index represented by a distance d4 between the male thread stab flank 113 and the female thread stab flank 213 in the fastened state is defined. The fourth index represented by the distance d4 corresponds to the axial distance (gap) between the male thread stab flank 113 and the female thread stab flank 213 in the fastened state when it is assumed that the box 20 and the pin 10 do not have the laminate coatings 14, 24.

[0146] 6A and 6B , the fifth film thickness tL214, the sixth film thickness tL114, the seventh film thickness tL213, and the eighth film thickness tL113 are formed so that the third index is smaller than the fourth index (distance d4). In other words, in the axial dimension of the threaded joint 100, the film thicknesses tL214 and tL213 of the laminate coating 24 formed on the female thread portion 21 and the film thicknesses tL114 and tL113 of the laminate coating 14 formed on the male thread portion 11 satisfy the condition that the third index is smaller than the fourth index.

[0147] The thickness of the solid lubricating coatings 16, 26 will depend on their components, but for example, if the solid lubricating coatings 16, 26 are made of fluorine-based solid lubricating powder particles dispersed in epoxy resin, it can be 5 μm or more and 50 μm or less, and in this case, a more preferable thickness is 10 μm or more and 40 μm or less.

[0148] In the female thread portion 21 of the box 20, the film thickness tS212 of the solid lubricant coating 26 at the female thread crest 212 may be, for example, 25 μm or more and 50 μm or less. A more preferable film thickness tS212 is 25 μm or more and 40 μm or less. The film thickness tS214 of the solid lubricant coating 26 at the female thread load flank 214 may be, for example, 5 μm or more and 25 μm or less, and may be smaller than the film thickness tS212 at the female thread crest 212. A more preferable film thickness tS214 is 10 μm or more and 25 μm or less. The film thickness tS212 at the female thread crest 212 depends on the method for forming the solid lubricant coating 26, but in the case of application by spraying, for example, it is usually at least 1.5 times the film thickness tS214 at the female thread load flank 214.

[0149] In the male thread portion 11 of the pin 10, the film thickness tS112 of the solid lubricant coating 16 at the male thread root surface 112 may be, for example, 25 μm or more and 50 μm or less. A more preferable film thickness tS112 is 25 μm or more and 40 μm or less. The film thickness tS114 of the solid lubricant coating 16 at the male thread load flank surface 114 may be, for example, 5 μm or more and 25 μm or less, and may be smaller than the film thickness tS112 at the male thread root surface 112. A more preferable film thickness tS114 is 10 μm or more and 25 μm or less. The film thickness tS112 at the male thread root surface 112 depends on the method for forming the solid lubricant coating 16, but in the case of application by spraying, for example, it is usually at least 1.5 times the film thickness tS114 at the male thread load flank surface 114.

[0150] In this embodiment, solid lubricant coatings 16, 26 are provided on both the pin 10 and the box 20. In this case, the film thickness tS212 at the female thread crest surface 212 and the film thickness tS112 at the male thread root surface 112 are preferably limited to 35 μm or less. The reason for this is as follows: Because the male thread root surface 112 is in interference contact with the female thread crest surface 212, the solid lubricant coatings 16, 26 slide against each other between the male thread root surface 112 and the female thread crest surface 212 during make-up. This sliding causes the solid lubricant coatings 16, 26 to adhere to each other, and the adhered portions are likely to fall off. This detached adhered debris may get between the male thread load flank surface 114 and the female thread load flank surface 214, or between the male thread stab flank surface 113 and the female thread stab flank surface 213, causing abnormal disturbances in the torque chart.

[0151] As described above, in the box 20, the film thickness of the solid lubricant coating 26 is greater on the female thread crest 212 (film thickness tS212) and the box seal surface 22 (film thickness tS22) than on the female thread load flank 214 (film thickness tS214). That is, the film thickness tS22 of the solid lubricant coating 26 on the box seal surface 22 is greater than the film thickness tS214 of the solid lubricant coating 26 on the female thread load flank 214. Because the box seal surface 22, together with the female thread crest 212, is a radial contact surface, the film thickness tS22 thereof is, for example, equivalent to the film thickness tS212 of the solid lubricant coating 26 on the female thread crest 212. The film thickness tS22 on the box seal surface 22 may be, for example, 25 μm or more and 50 μm or less. A film thickness tS22 of 25 μm or more and 40 μm or less is more preferable.

[0152] As described above, in the pin 10, the film thickness of the solid lubricant coating 16 is greater on the male thread root surface 112 (film thickness tS112) and the pin seal surface 12 (film thickness tS12) than on the male thread load flank surface 114 (film thickness tS114). That is, the film thickness tS12 of the solid lubricant coating 16 on the pin seal surface 12 is greater than the film thickness tS114 of the solid lubricant coating 16 on the male thread load flank surface 114. Because the pin seal surface 12, together with the male thread root surface 112, is a radial contact surface, the film thickness tS12 thereof is, for example, equal to the film thickness tS112 of the solid lubricant coating 16 on the male thread root surface 112. The film thickness tS12 on the pin seal surface 12 may be, for example, 25 μm or more and 50 μm or less. A more preferable film thickness tS12 is 25 μm or more and 40 μm or less.

[0153] In the box 20, the film thickness tS23 of the solid lubricating coating 26 on the box shoulder surface 23 may be, for example, 5 μm or more and 50 μm or less. A more preferable film thickness tS23 is 5 μm or more and 40 μm or less. Because the box shoulder surface 23, together with the female thread load flank surface 214, is a contact surface in the pipe axial direction, the film thickness tS23 thereof is preferably approximately the same as the film thickness tS214 on the female thread load flank surface 214. For example, the film thickness tS23 on the box shoulder surface 23 is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 30 μm or less.

[0154] In the pin 10, the film thickness tS13 of the solid lubricating coating 16 on the pin shoulder surface 13 may be, for example, 5 μm or more and 50 μm or less. A more preferable film thickness tS13 is 5 μm or more and 40 μm or less. Because the pin shoulder surface 13, together with the male thread load flank surface 114, is a contact surface in the pipe axial direction, the film thickness tS13 thereof is preferably approximately the same as the film thickness tS114 on the male thread load flank surface 114. For example, the film thickness tS13 on the pin shoulder surface 13 is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 30 μm or less.

[0155] (Method of Forming Solid Lubricating Coatings 16, 26) The threaded joint 100 is provided with solid lubricating coatings 16, 26 having appropriate film thicknesses for each portion. Such solid lubricating coatings 16, 26 can be industrially formed by application using a spray nozzle. Specifically, a solvent is mixed with the composition of the solid lubricating coatings 16, 26 to adjust the viscosity and prepare a fluid coating composition. This coating composition is sprayed onto the target components on which the base coatings 15, 25 have been formed, i.e., the pin 10 and the box 20. The coating composition applied to the target components by spraying is then solidified by heating, removing heat, air drying, or the like. This forms the solid lubricating coatings 16, 26 on the surfaces of the target components.

[0156] When a solid lubricating coating 16 is applied to the pin 10, it is necessary to increase the film thickness tS112 at the male thread root surface 112 and decrease the film thickness tS114 at the male thread load flank surface 114 in the male thread portion 11. In this case, for example, the spray angle of the spray nozzle is narrowed and the coating composition is applied with the spray nozzle positioned close to the male thread root surface 112. At this time, the feed of the spray nozzle relative to the pin 10 is adjusted in the circumferential and axial directions so that the spray nozzle moves along the lead (helix) of the thread. This allows the coating composition to be applied preferentially to the male thread root surface 112. As a result, a solid lubricating coating 16 with a large film thickness tS112 is formed at the male thread root surface 112, and a solid lubricating coating 16 with a small film thickness tS114 is formed at the male thread load flank surface 114.

[0157] When a solid lubricating coating 26 is applied to the box 20, it is necessary to increase the film thickness tS212 at the female thread crest 212 of the female thread portion 21 and decrease the film thickness tS214 at the female thread load flank 214. In this case, for example, if the coating composition is sprayed onto the female thread portion 21, the coating composition sprayed onto the female thread crest 212 region will adhere directly to the female thread crest 212, while the coating composition sprayed onto the female thread root 211 region will be distributed to the female thread root 211, the female thread load flank 214, and the female thread stab flank 213. For this reason, the film thickness tS212 of the solid lubricating coating 26 at the female thread crest 212 tends to be large, and the film thickness tS214 of the solid lubricating coating 26 at the female thread load flank 214 tends to be small. As a result, a solid lubricating coating 26 with a large thickness tS 212 is formed on the female thread crest surface 212, and a solid lubricating coating 26 with a small thickness tS 214 is formed on the female thread load flank surface 214. When the solid lubricating coating 26 is provided on the box 20 in this way, it is easier to form the solid lubricating coating 26 with an appropriate thickness where it is required, compared to when the solid lubricating coating 16 is provided on the pin 10.

[0158] Furthermore, when a solid lubricating coating 16 is applied to the pin 10, the film thickness tS12 on the pin seal surface 12 needs to be large, similar to the film thickness tS112 on the male thread root surface 112. On the other hand, the film thickness tS13 on the pin shoulder surface 13 needs to be small, similar to the film thickness tS114 on the male thread load flank surface 114. Although the pin seal surface 12 is close to the pin shoulder surface 13, there is an angular difference of about 90° between the pin seal surface 12 and the pin shoulder surface 13. Therefore, if the coating composition is sprayed primarily toward the pin seal surface 12, a solid lubricating coating 16 with a large film thickness tS12 will naturally form on the pin seal surface 12, and a solid lubricating coating 16 with a small film thickness tS13 will naturally form on the pin shoulder surface 13.

[0159] Furthermore, when a solid lubricating coating 26 is applied to the box 20, the film thickness tS22 on the box seal surface 22 needs to be large, similar to the film thickness tS212 on the female thread crest surface 212. On the other hand, the film thickness tS23 on the box shoulder surface 23 needs to be small, similar to the film thickness tS214 on the female thread load flank surface 214. Although the box seal surface 22 is close to the box shoulder surface 23, there is an angular difference of about 90° between the box seal surface 22 and the box shoulder surface 23. Therefore, if the coating composition is sprayed primarily toward the box seal surface 22, a solid lubricating coating 26 with a large film thickness tS22 will naturally form on the box seal surface 22, and a solid lubricating coating 26 with a small film thickness tS23 will naturally form on the box shoulder surface 23.

[0160] [Effect] In the threaded joint 100 according to this embodiment, the surfaces of both the box 20 and the pin 10 are coated with base coatings 15, 25 including alloy plating layers 151, 251, and the surfaces coated with the base coatings 15, 25 are coated with single solid lubricating coatings 16, 26. In other words, laminate coatings 14, 24 are formed on the surfaces of both the box 20 and the pin 10, in which the base coatings 15, 25 and the single solid lubricating coatings 16, 26 are laminated in this order. The base coatings 15, 25 including the alloy plating layers 151, 251 improve the adhesion between the solid lubricating coatings 16, 26 and the surfaces of the box 20 and the pin 10 on which the solid lubricating coatings 16, 26 are provided.

[0161] In the threaded joint 100 according to this embodiment, in the made-up state, the pin seal surface 12 is in interference contact with the box seal surface 22, and the laminate coatings 14, 24 are interposed between the pin seal surface 12 and the box seal surface 22, which are in interference contact with each other. In the made-up state, the male thread root surface 112 is in interference contact with the female thread crest surface 212, and the laminate coatings 14, 24 are also interposed between the male thread root surface 112 and the female thread crest surface 212, which are in interference contact with each other. In the made-up state, the male thread load flank surface 114 is in pressing contact with the female thread load flank surface 214, and the laminate coatings 14, 24 are also interposed between the male thread load flank surface 114 and the female thread load flank surface 214, which are in pressing contact with each other.

[0162] Furthermore, in the box 20, in a standalone state, the film thickness of the solid lubricant coating 26 is smallest on the female thread load flank surface 214 among the female thread load flank surface 214, the female thread crest surface 212, and the box seal surface 22, and is greater on the female thread crest surface 212 and the box seal surface 22 than on the female thread load flank surface 214. On the pin 10, the film thickness of the solid lubricant coating 16 is smallest on the male thread load flank surface 114 among the male thread load flank surface 114, the male thread root surface 112, and the pin seal surface 12, and is greater on the male thread root surface 112 and the pin seal surface 12 than on the male thread load flank surface 114. For this reason, the thickness of the solid lubricating coatings 16, 26 is relatively large on the radial contact surfaces involved in shouldering torque (the female thread crest surface 212 and the male thread root surface 112, the box seal surface 22, and the pin seal surface 12), while the thickness of the solid lubricating coatings 16, 26 is relatively small on the axial contact surfaces involved in apparent yield torque (the female thread load flank surface 214 and the male thread load flank surface 114).

[0163] Because the solid lubricating coatings 16, 26 have a relatively large thickness at the radial contact surfaces that contribute to shouldering torque, good lubrication is ensured. As a result, a low shouldering torque is displayed on the torque chart. Furthermore, because the solid lubricating coatings 16, 26 have a relatively small thickness at the axial contact surfaces that contribute to apparent yield torque, the effect of reduced rigidity of the solid lubricating coatings 16, 26 is reduced. As a result, the torque chart shows a high yield torque, suppressing the occurrence of apparent yield torque. In this way, the thickness of the solid lubricating coatings 16, 26 is appropriate at each location of the threaded joint 100, making it possible to achieve low shouldering torque and high yield torque. Low shouldering torque and high yield torque result in a wide torque window. Furthermore, because a single solid lubricating coating 16, 26 is used, procurement and management of the solid lubricating coatings 16, 26 are easier than when two types of solid lubricating coatings are used, and the structure of the solid lubricating coatings 16, 26 is simpler. Therefore, with the threaded joint 100 according to this embodiment, it is possible to widen the torque window with a simple and inexpensive configuration.

[0164] Furthermore, in the threaded joint 100 according to this embodiment, the first index is smaller than the second index for the first thickness tL211 (thickness of the laminate coating 24 at the female thread root surface 211), the second thickness tL111 (thickness of the laminate coating 14 at the male thread crest surface 111), the third thickness tL212 (thickness of the laminate coating 24 at the female thread crest surface 212), and the fourth thickness tL112 (thickness of the laminate coating 14 at the male thread root surface 112). The first index is expressed as the sum of the first thickness tL211 and the second thickness tL111 minus the sum of the third thickness tL212 and the fourth thickness tL112. The second index is expressed as the distance d2 between the male thread crest surface 111 and the female thread root surface 211 in the make-up state, assuming that the box 20 and the pin 10 do not have the laminate coatings 14, 24.

[0165] Because the first index is smaller than the second index, during make-up, the surface of one solid lubricating coating 16 does not come into contact with the surface of the other solid lubricating coating 26 between the male thread crest surface 111 and the female thread root surface 211. This makes it possible to prevent make-up problems, and in particular, it is possible to stabilize the shouldering torque at a low level and prevent the torque window from narrowing.

[0166] In this embodiment, in the fastened state, the pin shoulder surface 13 is in pressing contact with the box shoulder surface 23, and the laminate coatings 14, 24 are interposed between the pin shoulder surface 13 and the box shoulder surface 23, which are in pressing contact with each other. In the standalone state, the film thickness of the solid lubricating coating 26 in the box 20 is greater on the female thread crest surface 212 and the box seal surface 22 than on the female thread load flank surface 214, and is further smaller on the box shoulder surface 23 than on the female thread crest surface 212. Furthermore, the film thickness of the solid lubricating coating 26 is smaller on the box shoulder surface 23 than on the box seal surface 22. In the pin 10, the film thickness of the solid lubricating coating 16 is greater on the male thread root surface 112 and the pin seal surface 12 than on the male thread load flank surface 114, and is further smaller on the pin shoulder surface 13 than on the male thread root surface 112. Furthermore, the film thickness of the solid lubricating coating 16 is smaller on the pin shoulder surface 13 than on the pin seal surface 12. Therefore, the film thickness of the solid lubricating coatings 16, 26 is relatively small on the contact surfaces in the axial direction that are involved in the apparent yield torque (the female thread load flank surface 214 and the male thread load flank surface 114, the box shoulder surface 23, and the pin shoulder surface 13). Because the film thickness of the solid lubricating coatings 16, 26 is relatively small on the contact surfaces in the axial direction that are involved in the apparent yield torque, a high yield torque appears on the torque chart as described above.

[0167] Furthermore, in the threaded joint 100 according to this embodiment, the third index is smaller than the fourth index for the fifth thickness tL214 (thickness of the laminate coating 24 on the female thread load flank 214), the sixth thickness tL114 (thickness of the laminate coating 14 on the male thread load flank 114), the seventh thickness tL213 (thickness of the laminate coating 24 on the female thread stab flank 213), and the eighth thickness tL113 (thickness of the laminate coating 14 on the male thread stab flank 113). The third index is expressed as the sum of the fifth thickness tL214, the sixth thickness tL114, the seventh thickness tL213, and the eighth thickness tL113. The fourth index is expressed as the distance d4 between the male thread stab flank 113 and the female thread stab flank 213 in the fastened state, assuming that the box 20 and the pin 10 do not have the laminate coatings 14, 24.

[0168] Because the third index is smaller than the fourth index, at the stage where the male thread root surface 112 and the female thread crest surface 212 slide while in interference contact, contact sliding does not occur between both the load flank surfaces 114, 214 and the stab flank surfaces 113, 213. Therefore, an increase in torque resistance can be suppressed, and in turn an increase in shouldering torque can be suppressed, and as a result, narrowing of the torque window can be prevented.

[0169] In the threaded joint 100 according to this embodiment, the alloy plating layers 151, 251 included in the base coatings 15, 25 contribute to seizure resistance and rust prevention. It is preferable to use a Zn alloy plating layer as the alloy plating layer 151, 251. If a pure zinc plating layer were used, zinc would be consumed at a high rate due to sacrificial corrosion protection, necessitating a thick plating thickness. A thick plating thickness reduces the necessary gap between the male thread portion 11 and the female thread portion 21, potentially resulting in undue interference. This can result in a high torque being required during make-up or seizure. By using a Zn alloy plating layer as the alloy plating layer 151, 251, such problems can be prevented.

[0170] (Variation 1) Variation 1 of the threaded joint 100 according to the first embodiment will be described with reference to Figure 9. Figure 9 is a schematic diagram showing the surface layers of the pin 10 and the box 20 in a separate state, illustrating Variation 1 of the threaded joint 100. In Variation 1, the base coating 15 that covers the surface of the pin 10 further includes a passivation coating 152. That is, the base coating 15 is composed of an alloy plating layer 151 and a passivation coating 152. Specifically, the alloy plating layer 151 is formed on the base metal surface of the pin 10, and the passivation coating 152 is formed on this alloy plating layer 151. The solid lubricating coating 16 is formed on this passivation coating 152. In other words, in the pin 10, the passivation coating 152 is formed between the alloy plating layer 151 and the solid lubricating coating 16.

[0171] Furthermore, in Modification 1, the base coating 25 covering the surface of the box 20 further includes a passivation coating 252. That is, the base coating 25 is composed of an alloy plating layer 251 and a passivation coating 252. Specifically, the alloy plating layer 251 is formed on the base metal surface of the box 20, and the passivation coating 252 is formed on this alloy plating layer 251. The solid lubricant coating 26 is formed on this passivation coating 252. In short, in the box 20, the passivation coating 252 is formed between the alloy plating layer 251 and the solid lubricant coating 26.

[0172] The passivation coatings 152 and 252 contain, for example, trivalent chromium (Cr(III)), which is preferable because it is more stable than Cr(II) and, unlike Cr(VI), is not harmful.

[0173] In Modification 1, the passivation coatings 152, 252 function as passivation layers. The passivation coating 152 improves the corrosion resistance of the alloy plating layer 151 for the pin 10 on which the solid lubricant coating 16 is provided. The passivation coating 252 improves the corrosion resistance of the alloy plating layer 251 for the box 20 on which the solid lubricant coating 26 is provided.

[0174] In the first modification, a barrier layer (not shown) may be formed on the passivation coating 152, 252, and the solid lubricant coating 16, 26 may be formed on this barrier layer. The barrier layer may be made of, for example, silicon dioxide (SiO ２ The barrier layer is an inorganic matrix layer or an organic-inorganic matrix layer containing particles of the metal ion. The barrier layer further improves corrosion resistance.

[0175] (Variation 2) Variation 2 of the threaded joint 100 according to the first embodiment will be described using Figures 7 and 9 . In Variation 2, unevenness (not shown) is formed by blasting on the base metal surface of the pin 10 on which the base coating 15 is formed. Furthermore, unevenness (not shown) is formed by blasting on the base metal surface of the box 20 on which the base coating 25 is formed. As the blasting process, for example, sandblasting or shot peening can be used. In this case, the unevenness of the pin 10 improves adhesion between the surface of the pin 10 and the base coating 15 (alloy plating layer 151), and because the surface of the base coating 15 also becomes uneven, adhesion between the solid lubricating coating 16 and the base coating 15 is also improved. Furthermore, the unevenness of the box 20 improves the adhesion between the surface of the box 20 and the undercoating 25 (alloy plating layer 251), and since the surface of the undercoating 25 also becomes uneven, the adhesion between the solid lubricating coating 26 and the undercoating 25 is also improved.

[0176] (Variation 3) Variation 3 of the threaded joint 100 according to the first embodiment will be described with reference to Figure 10 . Figure 10 is a schematic diagram showing the surface layer portions of the pin 10 and the box 20 in a separate state, for illustrating Variation 3 of the threaded joint 100. In Variation 3, the base coating 15 that covers the surface of the pin 10 further includes a chemical conversion coating 153. That is, the base coating 15 is composed of the chemical conversion coating 153 and the alloy plating layer 151. Specifically, the base metal surface of the pin 10 is coated with the chemical conversion coating 153, and the alloy plating layer 151 is formed on this chemical conversion coating 153. The solid lubricating coating 16 is formed on this alloy plating layer 151. In other words, the chemical conversion coating 153 is formed between the base metal surface of the pin 10 and the alloy plating layer 151. The chemical conversion coating 153 is formed by subjecting the surface of the base material of the pin 10 to a chemical conversion treatment using zinc phosphate, manganese phosphate, or the like.

[0177] In addition, in Modification 3, the base coating 25 covering the surface of the box 20 further includes a chemical conversion coating 253. That is, the base coating 25 is composed of the chemical conversion coating 253 and the alloy plating layer 251. Specifically, the base metal surface of the box 20 is covered with the chemical conversion coating 253, and the alloy plating layer 251 is formed on this chemical conversion coating 253. The solid lubricant coating 26 is formed on this alloy plating layer 251. In other words, the chemical conversion coating 253 is formed between the base metal surface of the box 20 and the alloy plating layer 251. The chemical conversion coating 253 is formed by performing a chemical conversion treatment on the base metal surface of the box 20 using zinc phosphate, manganese phosphate, or the like, similar to the treatment used to form the chemical conversion coating 153 on the pin 10.

[0178] In the third modification, the chemical conversion coating 153 improves the adhesion between the surface of the pin 10 and the base coating 15 (alloy plating layer 151). The chemical conversion coating 253 improves the adhesion between the surface of the box 20 and the base coating 25 (alloy plating layer 251).

[0179] (Variation 4) Variation 4 of the threaded joint 100 according to the first embodiment will be described with reference to Figure 11 . Figure 11 is a schematic diagram showing the surface layer portions of the pin 10 and the box 20 in a separate state, for illustrating Variation 4 of the threaded joint 100. In Variation 4, the base coating 15 that covers the surface of the pin 10 further includes a chemical conversion coating 154. That is, the base coating 15 is composed of an alloy plating layer 151 and a chemical conversion coating 154. Specifically, the base metal surface of the pin 10 is covered with an alloy plating layer 151, and the chemical conversion coating 154 is formed on this alloy plating layer 151. The solid lubricating coating 16 is formed on this chemical conversion coating 154. In other words, in the pin 10, the chemical conversion coating 154 is formed between the alloy plating layer 151 and the solid lubricating coating 16. The chemical conversion coating 154 is formed by subjecting the surface of the alloy plating layer 151 to a chemical conversion treatment using zinc phosphate, manganese phosphate, or the like.

[0180] Furthermore, in Modification 4, the base coating 25 covering the surface of the box 20 further includes a chemical conversion coating 254. That is, the base coating 25 is composed of an alloy plating layer 251 and a chemical conversion coating 254. Specifically, the base metal surface of the box 20 is covered with an alloy plating layer 251, and the chemical conversion coating 254 is formed on this alloy plating layer 251. The solid lubricant coating 26 is formed on this chemical conversion coating 254. In other words, in the box 20, the chemical conversion coating 254 is formed between the alloy plating layer 251 and the solid lubricant coating 26. The chemical conversion coating 254 is formed by performing a chemical conversion treatment on the surface of the alloy plating layer 251 using zinc phosphate, manganese phosphate, or the like, similar to the treatment used to form the chemical conversion coating 154 on the pin 10.

[0181] In Modification 4, chemical conversion coating 154 improves the adhesion between base coating 15 (alloy plating layer 151) and solid lubricating coating 16 in pin 10. Chemical conversion coating 254 improves the adhesion between base coating 25 (alloy plating layer 251) and solid lubricating coating 26 in box 20.

[0182] In the threaded joint 100 according to the first embodiment, it is also possible to combine Modifications 3 and 4. That is, in the pin 10, the base coating 15 may be composed of a chemical conversion coating 153, an alloy plating layer 151, and a chemical conversion coating 154. In the box 20, the base coating 25 may be composed of a chemical conversion coating 253, an alloy plating layer 251, and a chemical conversion coating 254.

[0183] Second Embodiment The configuration of a threaded joint 100A according to a second embodiment will be described with reference to Figures 12A, 12B, and 13. Figures 12A and 12B are longitudinal cross-sectional views showing the pin 10 and a portion of the box 20 of the threaded joint 100A in a separate state. Figure 12A shows the pin 10. Figure 12B shows the box 20. Figure 13 is a schematic diagram showing the surface layer portions of the pin 10 and the box 20 of the threaded joint 100A in a separate state. The threaded joint 100A differs from the threaded joint 100 according to the first embodiment in that the pin 10 does not have a base coating or solid lubricating coating. That is, in the threaded joint 100A, a base coating 25 and a solid lubricating coating 26 (laminated coating 24) are provided only on the surface of the box 20.

[0184] 12A and 12B , in the threaded joint 100A, when the components are separated, the box 20 has a base coating 25 and a solid lubricant coating 26, while the pin 10 has neither a base coating nor a solid lubricant coating. In the threaded joint 100A, as in the threaded joint 100, the film thickness tS214 of the solid lubricant coating 26 on the female thread load flank surface 214 is smaller than the film thickness tS212 of the solid lubricant coating 26 on the female thread crest surface 212. Furthermore, the film thickness tS22 of the solid lubricant coating 26 on the box seal surface 22 is greater than the film thickness tS214 of the solid lubricant coating 26 on the female thread load flank surface 214.

[0185] In this case, the film thickness of the solid lubricant coating 26 is smallest on the female thread load flank 214 (film thickness tS214) among the female thread load flank 214, the female thread crest 212, and the box seal surface 22, and is greater on the female thread crest 212 (film thickness tS212) and the box seal surface 22 (film thickness tS22) than on the female thread load flank 214 (film thickness tS214). Therefore, the film thickness of the solid lubricant coating 26 is relatively large on the radial contact surfaces (female thread crest 212, box seal surface 22) that are involved in shouldering torque, and the film thickness of the solid lubricant coating 26 is relatively small on the axial contact surface (female thread load flank 214) that is involved in apparent yield torque. Therefore, with the threaded joint 100A according to the second embodiment, it is possible to expand the torque window with a simple and inexpensive configuration, as with the threaded joint 100 according to the first embodiment.

[0186] Furthermore, in the threaded joint 100A, the laminate coating 24 is provided only on the surface of the box 20, so the second film thickness (film thickness of the laminate coating at the male thread crest surface 111) and the fourth film thickness (film thickness of the laminate coating at the male thread root surface 112) are zero. Therefore, the first index is expressed as the first film thickness tL211 (film thickness of the laminate coating 24 at the female thread root surface 211) minus the third film thickness tL212 (film thickness of the laminate coating 24 at the female thread crest surface 212). The second index is expressed as the distance d2 between the male thread crest surface 111 and the female thread root surface 211 in the make-up state, assuming that the box 20 and pin 10 do not have the laminate coatings 14, 24, as in the threaded joint 100 (see FIG. 8 ). The first index is smaller than the second index.

[0187] Because the first index is smaller than the second index, during make-up, the surface of the solid lubricating coating 26 will not come into contact with the surface of the mating part between the male thread crest surface 111 and the female thread root surface 211. Therefore, with the threaded joint 100A according to the second embodiment, similar to the threaded joint 100 according to the first embodiment, it is possible to prevent make-up problems before they occur, and in particular, it is possible to stabilize shouldering torque at a low level and prevent the torque window from narrowing.

[0188] In the threaded joint 100A, as in the threaded joint 100, the film thickness tS23 of the solid lubricating coating 26 on the box shoulder surface 23 is smaller than the film thickness tS212 of the solid lubricating coating 26 on the female thread crest surface 212. Furthermore, the film thickness tS23 of the solid lubricating coating 26 on the box shoulder surface 23 is smaller than the film thickness tS22 of the solid lubricating coating 26 on the box seal surface 22. In this case, in the box 20, as in the threaded joint 100, the film thickness of the solid lubricating coating 26 is greater on the female thread crest surface 212 and the box seal surface 22 than on the female thread load flank surface 214. Furthermore, the film thickness of the solid lubricating coating 26 is smaller on the box shoulder surface 23 than on the female thread crest surface 212. Furthermore, the film thickness of the solid lubricating coating 26 is smaller on the box shoulder surface 23 than on the box seal surface 22.

[0189] For this reason, the film thickness of the solid lubricant coating 26 is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (the female thread load flank surface 214 and the box shoulder surface 23). Therefore, with the threaded joint 100A according to the second embodiment, a high yield torque appears on the torque chart, just like the threaded joint 100 according to the first embodiment.

[0190] Furthermore, in the threaded joint 100A, since the laminate coating 24 is provided only on the surface of the box 20, the sixth film thickness (film thickness of the laminate coating on the male thread load flank 114) and the eighth film thickness (film thickness of the laminate coating on the male thread stab flank 113) are zero. Therefore, the third index is expressed as the sum of the fifth film thickness tL214 (film thickness of the laminate coating 24 on the female thread load flank 214) and the seventh film thickness tL213 (film thickness of the laminate coating 24 on the female thread stab flank 213). The fourth index is expressed as the distance d4 between the male thread stab flank 113 and the female thread stab flank 213 in the fastened state, assuming that the box 20 and pin 10 do not have the laminate coatings 14, 24 (see FIG. 8 ). The third index is smaller than the fourth index.

[0191] Because the third index is smaller than the fourth index, at the stage where the male thread root surface 112 and the female thread crest surface 212 slide while in interference contact, contact sliding does not occur between both the load flank surfaces 114, 214 and the stabbing flank surfaces 113, 213. Therefore, according to the threaded joint 100A according to the second embodiment, similar to the threaded joint 100 according to the first embodiment, an increase in torque resistance can be suppressed, and therefore an increase in shouldering torque can be suppressed, and as a result, narrowing of the torque window can be prevented.

[0192] 13 , in a threaded joint 100A, when the pin 10 is in a standalone state, the surface of the pin 10 is not coated with a primer coating or a solid lubricant coating. Instead, the surface of the pin 10 is coated with a chemical conversion coating 155, and the surface coated with the chemical conversion coating 155 is coated with a rust-preventive oil coating 17. In other words, the base metal surface of the pin 10 is coated with the chemical conversion coating 155, and the rust-preventive oil coating 17 is formed on this chemical conversion coating 155.

[0193] The chemical conversion coating 155 is formed by performing a chemical conversion treatment using zinc phosphate, manganese phosphate, or the like on the base metal surface of the pin 10, similar to the treatment for forming the chemical conversion coating 153 shown in Modification Example 3. The rust-preventive oil coating 17 is formed by applying rust-preventive oil to the entire surface of the pin 10 on which the chemical conversion coating 155 has been formed. The rust-preventive oil can be applied by, for example, spraying, brushing, or immersion in a tank of rust-preventive oil.

[0194] In this way, the chemical conversion coating 155 and the rust-preventive oil coating 17 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 makes it possible to prevent rust from occurring on the pin 10 during the storage period from when the pin 10 is manufactured until when it is fastened to the box 20.

[0195] However, such a surface treatment coating is not necessarily required for the pin 10. For example, only the chemical conversion coating 155 may be formed on the base metal surface of the pin 10, or only the rust-preventive oil coating 17 may be formed. The base metal surface of the pin 10 may be exposed.

[0196] As described in the first embodiment, when the solid lubricating coating 26 is provided on the box 20, it is easier to form the solid lubricating coating 26 of an appropriate thickness where it is needed, compared to when the solid lubricating coating 16 is provided on the pin 10. In this regard, in the threaded joint 100A, the solid lubricating coating 26 is provided only on the box 20. Therefore, the threaded joint 100A according to the second embodiment is useful in that it can be easily manufactured.

[0197] In the threaded joint 100A according to the second embodiment, the configurations shown in the above-described modified examples 1 to 4 may be applied to the box 20 on which the laminate coating 24 (primary coating 25 and solid lubricating coating 26) is provided.

[0198] Third Embodiment The configuration of a threaded joint 100B according to the third embodiment will be described with reference to Figures 14A, 14B, and 15. Figures 14A and 14B are longitudinal cross-sectional views showing the pin 10 and a portion of the box 20 of the threaded joint 100B in a separate state. Figure 14A shows the pin 10. Figure 14B shows the box 20. Figure 15 is a schematic diagram showing the surface layer portions of the pin 10 and the box 20 of the threaded joint 100B in a separate state. The threaded joint 100B differs from the threaded joint 100 according to the first embodiment in that the box 20 does not have a base coating or solid lubricating coating. That is, in the threaded joint 100B, the base coating 15 and solid lubricating coating 16 (laminated coating 14) are provided on the surface of the pin 10 only.

[0199] 14A and 14B , in a threaded joint 100B, when the pin 10 is in a standalone state, the pin 10 has a base coating 15 and a solid lubricant coating 16, while the box 20 has neither a base coating nor a solid lubricant coating. In the threaded joint 100B, as in the threaded joint 100, the film thickness tS114 of the solid lubricant coating 16 on the male thread load flank surface 114 is smaller than the film thickness tS112 of the solid lubricant coating 16 on the male thread root surface 112. Furthermore, the film thickness tS12 of the solid lubricant coating 16 on the pin seal surface 12 is greater than the film thickness tS114 of the solid lubricant coating 16 on the male thread load flank surface 114.

[0200] In this case, the film thickness of the solid lubricant coating 16 is smallest on the male thread load flank surface 114 (film thickness tS114) among the male thread load flank surface 114, the male thread root surface 112, and the pin seal surface 12, and is greater on the male thread root surface 112 (film thickness tS112) and the pin seal surface 12 (film thickness tS12) than on the male thread load flank surface 114 (film thickness tS114). Therefore, the film thickness of the solid lubricant coating 16 is relatively large on the radial contact surfaces (male thread root surface 112, pin seal surface 12) that are involved in shouldering torque, and the film thickness of the solid lubricant coating 16 is relatively small on the axial contact surface (male thread load flank surface 114) that is involved in apparent yield torque. Therefore, with the threaded joint 100B according to the third embodiment, it is possible to expand the torque window with a simple and inexpensive configuration, as with the threaded joint 100 according to the first embodiment.

[0201] In the threaded joint 100B, the laminate coating 14 is provided only on the surface of the pin 10, and therefore the first film thickness (film thickness of the laminate coating at the female thread root surface 211) and the third film thickness (film thickness of the laminate coating at the female thread crest surface 212) are zero. Therefore, the first index is expressed as the second film thickness tL111 (film thickness of the laminate coating 14 at the male thread crest surface 111) minus the fourth film thickness tL112 (film thickness of the laminate coating 14 at the male thread root surface 112). The second index is expressed as the distance d2 between the male thread crest surface 111 and the female thread root surface 211 in the make-up state, assuming that the box 20 and the pin 10 do not have the laminate coatings 14, 24, as in the threaded joint 100 (see FIG. 8 ). The first index is smaller than the second index.

[0202] Because the first index is smaller than the second index, during make-up, the surface of the solid lubricating coating 16 does not come into contact with the surface of the mating member between the male thread crest surface 111 and the female thread root surface 211. Therefore, with the threaded joint 100B according to the third embodiment, similar to the threaded joint 100 according to the first embodiment, it is possible to prevent make-up problems, and in particular, it is possible to stabilize shouldering torque at a low level and prevent the torque window from narrowing.

[0203] In the threaded joint 100B, as in the threaded joint 100, the film thickness tS13 of the solid lubricating coating 16 on the pin shoulder surface 13 is smaller than the film thickness tS112 of the solid lubricating coating 16 on the male thread root surface 112. Furthermore, the film thickness tS13 of the solid lubricating coating 16 on the pin shoulder surface 13 is smaller than the film thickness tS12 of the solid lubricating coating 16 on the pin seal surface 12. In this case, as in the threaded joint 100, the film thickness of the solid lubricating coating 16 in the pin 10 is greater on the male thread root surface 112 and the pin seal surface 12 than on the male thread load flank surface 114. Furthermore, the film thickness of the solid lubricating coating 16 is smaller on the pin shoulder surface 13 than on the male thread root surface 112. Furthermore, the film thickness of the solid lubricating coating 16 is smaller on the pin shoulder surface 13 than on the pin seal surface 12.

[0204] For this reason, the thickness of the solid lubricant coating 16 is relatively small on the contact surfaces in the pipe axis direction that are involved in the apparent yield torque (male thread load flank surface 114 and pin shoulder surface 13). Therefore, with the threaded joint 100B according to the third embodiment, a high yield torque appears on the torque chart, just like the threaded joint 100 according to the first embodiment.

[0205] In addition, in the threaded joint 100B, the laminate coating 14 is provided only on the surface of the pin 10, so the fifth thickness (thickness of the laminate coating on the female thread load flank 214) and the seventh thickness (thickness of the laminate coating on the female thread stab flank 213) are zero. Therefore, the third index is expressed as the sum of the sixth thickness tL114 (thickness of the laminate coating 14 on the male thread load flank 114) and the eighth thickness tL113 (thickness of the laminate coating 14 on the male thread stab flank 113). The fourth index is expressed as the distance d4 between the male thread stab flank 113 and the female thread stab flank 213 in the fastened state, assuming that the box 20 and pin 10 do not have the laminate coatings 14, 24 (see FIG. 8 ). The third index is smaller than the fourth index.

[0206] Because the third index is smaller than the fourth index, at the stage where the male thread root surface 112 and the female thread crest surface 212 slide while in interference contact, contact sliding does not occur between both the load flank surfaces 114, 214 and the stabbing flank surfaces 113, 213. Therefore, according to the threaded joint 100B according to the third embodiment, similar to the threaded joint 100 according to the first embodiment, an increase in torque resistance can be suppressed, and therefore an increase in shouldering torque can be suppressed, and as a result, a narrowing of the torque window can be prevented.

[0207] 15 , in a threaded joint 100B, when used alone, the surface of the box 20 is not coated with a primer coating or a solid lubricant coating. Instead, the surface of the box 20 is coated with a chemical conversion coating 255, and the surface coated with the chemical conversion coating 255 is coated with a rust-preventive oil coating 27. In other words, the base metal surface of the box 20 is coated with the chemical conversion coating 255, and the rust-preventive oil coating 27 is formed on top of this chemical conversion coating 255.

[0208] The chemical conversion coating 255 is formed by performing a chemical conversion treatment using zinc phosphate, manganese phosphate, or the like on the base material surface of the box 20, similar to the treatment for forming the chemical conversion coating 253 shown in Modification Example 3. The rust-preventive oil coating 27 is formed by applying rust-preventive oil to the entire surface of the box 20 on which the chemical conversion coating 255 has been formed. The rust-preventive oil can be applied by, for example, spraying, brushing, or immersion in a tank of rust-preventive oil.

[0209] In this way, the chemical conversion coating 255 and the rust-preventive oil coating 27 are laminated in this order on the surface of the box 20. This provides an inexpensive and sufficient rust-preventive treatment to the box 20. This makes it possible to prevent rust from occurring in the box 20 during the storage period from when the box 20 is manufactured until when it is fastened to the pin 10.

[0210] However, such a surface treatment coating is not necessarily required for the box 20. For example, only the chemical conversion coating 255 may be formed on the base metal surface of the box 20, or only the rust-preventive oil coating 27 may be formed. The base metal surface of the box 20 may be exposed.

[0211] In the threaded joint 100B according to the third embodiment, the configurations shown in Modifications 1 to 4 above may be applied to the pin 10 on which the laminate coating 14 (primary coating 15 and solid lubricant coating 16) is provided.

[0212] <Fourth embodiment> The configuration of a threaded joint 100C according to the fourth embodiment will be described with reference to Figures 16, 17A, and 17B. Figure 16 is a longitudinal cross-sectional view showing a portion of the threaded joint 100C in a fastened state. Figures 17A and 17B are longitudinal cross-sectional views showing the pin 10 and box 20 in the threaded joint shown in Figure 16 in a separate state. Figure 17A shows the pin 10. Figure 17B shows the box 20. The threaded joint 100C differs from the threaded joint 100 according to the first embodiment in that it has a configuration that includes a self-locking mechanism. The threaded joint 100C does not have to include a pin shoulder surface and a box shoulder surface.

[0213] In the threaded joint 100C, the male thread portion 11 of the pin 10, together with the female thread portion 21 of the box 20, constitutes a self-locking mechanism. Specifically, in the male thread portion 11, the width of the male thread ridge 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 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 of the pin 10. In other words, the type of thread that constitutes the male thread portion 11 is a wedge thread.

[0214] The male thread stab flank surface 113 is a straight line that is inclined so that its outer peripheral edge is located closer to the tip of the pin 10 than the inner peripheral edge in a vertical cross-sectional view of the threaded joint 100C. 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 body 31 than the inner peripheral edge in a vertical cross-sectional view of the threaded joint 100C.

[0215] 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 as it moves 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 as it moves toward the innermost side of the box 20, corresponding to the width of the male thread groove. In other words, the type of thread that constitutes the female thread portion 21 is a wedge thread, corresponding to the male thread portion 11.

[0216] The female thread stab 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 100C. 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 100C.

[0217] 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. The point at which these contacts occur is self-locking. Self-locking corresponds to shouldering in the threaded joint 100 according to the first embodiment. As the threading progresses further, when make-up is complete, the load flanks 114, 214 come into pressing contact with each other, and the stab flanks 113, 213 come into pressing contact with each other. In this made-up state, the laminate coatings 14, 24 are interposed between the male thread load flank 114 and the female thread load flank 214, which are in pressing contact with each other. The laminate coatings 14, 24 are also interposed between the male thread stab flank 113 and the female thread stab flank 213, which are in pressing contact with each other.

[0218] The threaded joint 100C according to the fourth embodiment achieves the same effects as the first embodiment. However, in the fourth embodiment, the stab flank surfaces 113, 213 are in pressing contact with each other in a fastened state assuming that the box 20 and the pin 10 do not have the laminate coatings 14, 24. Therefore, the fourth index according to the first embodiment becomes zero. Therefore, the condition in the first embodiment that the third index be smaller than the fourth index does not apply in the fourth embodiment.

[0219] The configuration of the fourth embodiment may be applied to the threaded joints 100A, 100B according to the second and third embodiments.

[0220] 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.

[0221] In the pin 10, the position at which 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 in the pipe axial direction of the male thread portion 11, or may be provided on the steel pipe body 31 side of the male thread portion 11.

[0222] 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 axial 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.

[0223] 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 pipe axis direction, or may be provided on the steel pipe body 31 side of the male thread portion 11.

[0224] 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 in the pipe axial direction of the female thread portion 21, or may be provided at the pipe end of the box 20.

[0225] The threaded joints 100, 100A, 100B, and 100C may be integral types. Most preferred is the coupling-type threaded joint 100A, in which the solid lubricant coating 26 is provided only on the surface of the box 20. This is because the coupling 40 is a tubular member that is significantly shorter than the steel pipe 30, and can be easily subjected to various treatments.

[0226] 100, 100A, 100B, 100C: Threaded joint 10: Pin 11: Male thread portion 111: Male thread crest surface 112: Male thread root surface 113: Male thread stab flank surface 114: Male thread load flank surface 12: Pin seal surface 13: Pin shoulder surface 14: Laminate coating 15: Undercoat 151: Alloy plating layer 16: Solid lubricant coating 20: Box 21: Female thread portion 211: Female thread root surface 212: Female thread crest surface 213: Female thread stab flank surface 214: Female thread load flank surface 22: Box seal surface 23: Box shoulder surface 24: Laminate coating 25: Undercoat 251: Alloy plating layer 26: Solid lubricant coating tS112: Film thickness tS114: Film thickness tS12: Film thickness tS13: Film thickness tS212: Film thickness tS214: Film thickness tS22: Film thickness tS23: Film thickness tL111: Second film thickness tL112: Fourth film thickness tL113: Eighth film thickness tL114: Sixth film thickness tL211: First film thickness tL212: Third film thickness tL213: Seventh film thickness tL214: Fifth film thickness d2: Distance (Second index) d4: Distance (Fourth index) X: Tube axis 30: Steel tube 31: Body 40: Coupling

Claims

1. A threaded joint for connecting steel pipes, comprising: a tubular pin provided contiguous with a steel pipe body; and a tubular box into which the pin is inserted and fastened, wherein the pin includes a male thread portion provided on an outer circumferential surface of the pin, and a pin seal surface provided on the outer circumferential surface, wherein the box includes a female thread portion provided on an inner circumferential surface of the box corresponding to the male thread portion, and which engages with the male thread portion when the pin and the box are fastened together, and a box seal surface provided on the inner circumferential surface corresponding to the pin seal surface, wherein the male thread portion includes a male thread crest surface, a male thread bottom surface, a male thread insertion flank surface, and a male thread load flank surface, and wherein the female thread portion includes a female thread bottom surface corresponding to the male thread crest surface, a female thread crest surface corresponding to the male thread bottom 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, In an individual state before the pin and the box are fastened together, at least one surface of the box and the pin is coated with a base coating, and the surface coated with the base coating is coated with a single solid lubricating coating, the base coating includes an alloy plating layer, and the solid lubricating coating is formed on the alloy plating layer, and in the fastened state, the pin seal surface is in interference contact with the box seal surface, the male thread root surface is in interference contact with the female thread crest surface, and the male thread load flank surface is in pressing contact with the female thread load flank surface, and the threaded joint in the individual state has the following configuration: a film thickness of the solid lubricating coating on the female thread load flank surface in the pipe axial direction of the threaded joint is smaller than a film thickness of the solid lubricating coating on the female thread crest surface in a radial direction perpendicular to the pipe axial direction, the film thickness of the solid lubricating coating on the box seal surface is greater than the film thickness of the solid lubricating coating on the female thread load flank surface, a thickness of the solid lubricant coating in the axial direction on the male thread load flank surface is smaller than a thickness of the solid lubricant coating in the radial direction on the male thread root surface, a thickness of the solid lubricant coating in the radial direction on the pin seal surface is larger than the thickness of the solid lubricant coating on the male thread load flank surface, anda first index expressed by subtracting the sum of the third film thicknesses and the fourth film thicknesses from the sum of the first film thicknesses and the second film thicknesses, regarding a first film thickness which is the combination of the solid lubricating coating and the base coating at the bottom surface of the female thread, a second film thickness which is the combination of the solid lubricating coating and the base coating at the crest surface of the male thread, a third film thickness which is the combination of the solid lubricating coating and the base coating at the crest surface of the female thread, and a fourth film thickness which is the combination of the solid lubricating coating and the base coating at the bottom surface of the male thread, is smaller than a second index expressed by the distance between the male thread crest surface and the female thread root surface in a made-up state when it is assumed that the box and the pin do not have the base coating and the solid lubricating coating.

2. A threaded joint as claimed in claim 1, wherein the undercoat further includes a passivation coating, the passivation coating being formed between the alloy plating layer and the solid lubricant coating.

3. A threaded joint as claimed in claim 1, wherein the surface on which the undercoat is formed is provided with irregularities by blasting.

4. A threaded joint as claimed in claim 1, wherein the pin further includes a pin shoulder surface provided at the tip of the pin, and the box further includes a box shoulder surface provided at the inner back of the box corresponding to the pin shoulder surface, and in the fastened state, the pin shoulder surface is in pressing contact with the box shoulder surface, and the threaded joint in the standalone state has the following configuration: the film thickness in the tube axial direction of the solid lubricant coating on the box shoulder surface is smaller than the film thickness of the solid lubricant coating on the female thread crest surface, and the film thickness in the tube axial direction of the solid lubricant coating on the pin shoulder surface is smaller than the film thickness of the solid lubricant coating on the male thread root surface.

5. A threaded joint as claimed in claim 4, wherein the threaded joint in the standalone state has the following configuration: the film thickness of the solid lubricant coating on the box shoulder surface is smaller than the film thickness of the solid lubricant coating on the box seal surface, and the film thickness of the solid lubricant coating on the pin shoulder surface is smaller than the film thickness of the solid lubricant coating on the pin seal surface.

6. A threaded joint as claimed in claim 4, wherein a third index represented by the sum of a fifth film thickness of the solid lubricant coating and the base coating on the load flank of the female thread, a sixth film thickness of the solid lubricant coating and the base coating on the load flank of the male thread, a seventh film thickness of the solid lubricant coating and the base coating on the insertion flank of the female thread, and an eighth film thickness of the solid lubricant coating and the base coating on the insertion flank of the male thread, is smaller than a fourth index represented by the distance between the male thread insertion flank and the female thread insertion flank in a fastened state when it is assumed that the box and the pin do not have the base coating and the solid lubricant coating.

7. A threaded joint as claimed in claim 1, wherein the pin is provided on each of the steel pipes to be connected, and the box is provided on a coupling which is a tubular member separate from the steel pipes.

8. A threaded joint as claimed in claim 1, wherein the solid lubricant coating contains a resin and a solid lubricant powder.

9. A threaded joint according to claim 8, wherein the resin is an epoxy resin and the solid lubricant powder is polytetrafluoroethylene.

10. A threaded joint as claimed in claim 1, wherein, in the individual state, the box has the base coating and the solid lubricating coating, and the pin does not have the base coating or the solid lubricating coating.

11. A threaded joint as claimed in claim 10, wherein the pin further includes a pin shoulder surface provided at the tip of the pin, the box further includes a box shoulder surface provided at the inner back of the box corresponding to the pin shoulder surface, and in the fastened state, the pin shoulder surface is in pressing contact with the box shoulder surface, the pin is provided on each of the steel pipes to be connected, the box is provided on a coupling which is a tubular member separate from the steel pipes, the solid lubricant coating contains epoxy resin and solid lubricant powder formed of polytetrafluoroethylene, the base coating includes a Zn alloy plating layer as the alloy plating layer and a passivation coating, and the passivation coating is formed between the Zn alloy plating layer and the solid lubricant coating, and unevenness is formed on the surface of the box on which the base coating is formed by blasting, a thickness of the solid lubricating coating on the box shoulder surface in the axial direction of the tube, in the state of the single member, that is smaller than the thickness of the solid lubricating coating on the female thread crest surface.

12. A threaded joint as claimed in claim 11, wherein, in the standalone state, the film thickness of the solid lubricant coating on the box shoulder surface is smaller than the film thickness of the solid lubricant coating on the box seal surface.

13. A threaded joint as claimed in claim 11, wherein a third index represented by the sum of a fifth film thickness which is the sum of the solid lubricant coating and the base coating on the female thread load flank and a seventh film thickness which is the sum of the solid lubricant coating and the base coating on the female thread insertion flank is smaller than a fourth index represented by the distance between the male thread insertion flank and the female thread insertion flank in a fastened state when it is assumed that the box and the pin do not have the base coating and the solid lubricant coating.

14. A threaded joint as claimed in any one of claims 10 to 13, wherein, in the individual state, the surface of the pin is coated with a chemical conversion coating, and the surface coated with the chemical conversion coating is coated with an anti-rust oil coating.