Threaded connection

Abstract

The purpose of the present invention is to provide a threaded joint that enables reduction of time for machining a thread section and has a high torque. A threaded joint (1) is obtained by connecting a pair of pipes. The threaded joint (1) is provided with a pin (10) and a box (20). The pin (10) has a male thread part (11) around the outer circumference thereof. The box (20) has, in the inner circumference thereof, a female thread part (21) corresponding to the male thread part (11). The box (20) is faste

Description

Screw fittings The present invention relates to a threaded joint for connecting a pair of pipes. Traditionally, oil wells and natural gas wells (hereinafter collectively referred to as "oil wells") have used casings, tubing, and other oil well pipes to extract underground resources. These oil well pipes are composed of sequentially connected steel pipes. Threaded joints are used to connect the steel pipes. Threaded joints for steel pipes are broadly classified into two types: coupling type and integral type. In the coupling type, one of the pair of pipes being connected is a steel pipe, and the other is a coupling. Male threads are provided on the outer circumference of both ends of the steel pipe. Female threads are provided on the inner circumference of both ends of the coupling. The steel pipe and the coupling are fastened and connected by screwing the male threads of the steel pipe into the female threads of the coupling. In the integral type, both of the connecting pipes are made of steel, and no coupling is used. Each steel pipe has a male threaded portion on the outer circumference of one end and a female threaded portion on the inner circumference of the other end. The two steel pipes are fastened and connected by screwing the male threaded portion of one steel pipe into the female threaded portion of the other steel pipe. Generally, the portion of a pipe to be connected that has a male thread is called a pin because it is inserted into the female thread. The portion of a pipe to be connected that has a female thread is called a box because it receives the male thread. Since pins and boxes are parts of a pipe, they are both tubular in shape. A type of threaded joint for steel pipes is known that has a threaded section in which the thread width and thread root width vary at a constant rate. Threaded joints with this structure provide high torque. In this type of threaded joint, the stub lead and load lead are usually constant throughout the entire threaded section. However, the stub lead and load lead do not coincide. Japanese Patent Publication No. 4087798 discloses a screw joint in which the stub lead and / or the load lead changes in the middle of the threaded portion. That is, in the screw joint of Japanese Patent Publication No. 4087798, the rate of change of the thread width and the thread groove width is changed in the middle of the threaded portion. Thereby, with respect to a screw joint of a type in which the thread width and the thread groove width change, it is possible to improve the resistance to tensile and compressive stresses. Generally, for machining the threaded portion, a cutting tool having a size corresponding to the minimum thread groove width is used. Therefore, when cutting a thread groove having a large thread groove width, the number of cutting paths in the pipe axis direction inevitably increases. In the screw joint of Japanese Patent Publication No. 4087798, although the rate of change of the thread groove width is not constant, the thread groove width changes over the entire threaded portion. Therefore, the difference between the minimum thread groove width and the maximum thread groove width is large. For this reason, the number of cutting paths in the pipe axis direction of the thread groove having the largest thread groove width becomes particularly large, and the machining time for the entire threaded portion increases. Further, when the thread width changes over the entire threaded portion, the thread width at the outermost end inevitably becomes considerably small. When the thread width becomes equal to or smaller than the height of the thread, when a tensile load is applied in the pipe axis direction, shear fracture is likely to occur at the root of the thread having the minimum thread width. An object of the present disclosure is to provide a screw joint that can shorten the machining time of the threaded portion while having high torque. The steel pipe screw joint according to the present disclosure is for connecting a pair of pipes. The screw joint includes a pin and a box. The pin has an external thread portion on the outer periphery. The box has an internal thread portion corresponding to the external thread portion on the inner periphery. The box is fastened to the pin. The external thread portion includes a constant thread width portion and a variable thread width portion. The constant thread width portion of the external thread portion has a constant thread groove width. The variable thread width portion of the external thread portion has a thread groove width that is not less than the thread groove width of the constant thread width portion of the external thread portion and gradually increases from the constant thread width portion of the external thread portion toward the tip of the pin. The internal thread portion includes a constant thread width portion and a variable thread width portion. The constant thread width portion of the internal thread portion has a constant thread width. The variable thread width portion of the internal thread portion has a thread width that is not less than the thread width of the constant thread width portion of the internal thread portion and gradually increases from the constant thread width portion of the internal thread portion toward the center of the box. According to the threaded joint for steel pipes described herein, it is possible to shorten the processing time of the threaded portion while providing a threaded joint with high torque. Figure 1 is a longitudinal cross-sectional view showing a schematic of a screw joint according to the first embodiment. Figure 2A is an enlarged longitudinal cross-sectional view of the constant thread width portion of the screw joint shown in Figure 1. Figure 2B is an enlarged longitudinal cross-sectional view of the variable thread width portion of the screw joint shown in Figure 1. Figure 3 is an enlarged longitudinal cross-sectional view of a part of the screw joint shown in Figure 1. Figure 4A is a graph showing the stub lead and load lead of the screw joint shown in Figure 1. Figure 4B is an enlarged longitudinal cross-sectional view of the thread portion of the screw joint shown in Figure 1. Figure 5 is a longitudinal cross-sectional view showing the state in which the tips of the pins are in contact with each other in the screw joint according to the first embodiment. Figure 6 is a longitudinal cross-sectional view showing a schematic of a screw joint according to the second embodiment. Figure 7 is a longitudinal cross-sectional view showing a schematic of a screw joint according to the third embodiment. Figure 8 is a longitudinal cross-sectional view showing a schematic of a screw joint according to the fourth embodiment. Figure 9 is a longitudinal cross-sectional view showing a schematic of a screw joint according to an embodiment. Figure 10 is a longitudinal cross-sectional view showing a schematic of a screw joint according to a comparative example. Figure 11 is a graph showing the processing time of the screw joints according to the embodiment and comparative example. The threaded joint according to the embodiment connects a pair of pipes. The threaded joint comprises a pin and a box. The pin has a male threaded portion on its outer circumference. The box has a female threaded portion on its inner circumference that corresponds to the male threaded portion. The box is fastened to the pin. The male threaded portion includes a constant thread width portion and a variable thread width portion. The constant thread width portion of the male threaded portion has a constant thread root width. The variable thread width portion of the male threaded portion has a thread root width that is greater than or equal to the thread root width of the constant thread width portion of the male threaded portion and gradually increases from the constant thread width portion toward the tip of the pin. The female threaded portion includes a constant thread width portion and a variable thread width portion. The constant thread width portion of the female threaded portion has a constant thread width. The variable thread width portion of the female threaded portion has a thread width that is greater than or equal to the thread width of the constant thread width portion of the female threaded portion and gradually increases from the constant thread width portion toward the center of the box (first configuration). In the first configuration, the male and female threaded sections include a constant thread width section and a variable thread width section. In the variable thread width section, the thread root width and thread crest width change, but in the constant thread width section, the thread root width and thread crest width do not change. By providing a constant thread width section in the male and female threaded sections, the difference between the minimum and maximum thread root widths in the male and female threaded sections becomes smaller compared to the case where the thread root width gradually changes throughout the entire male and female threaded sections. Therefore, the number of cutting passes in the pipe axis direction for the thread root with the maximum thread root width in the male and female threaded sections is reduced, and the total number of cutting passes in the pipe axis direction can be reduced. As a result, the machining time for the threaded section including the male and female threaded sections can be shortened. Furthermore, by providing a constant thread width section in the male and female threaded sections, the difference between the minimum and maximum thread crest widths in the male and female threaded sections becomes smaller compared to the case where the thread crest width gradually changes throughout the entire male and female threaded sections. Therefore, when a tensile load is applied in the axial direction of the pipe in the male or female threaded portion, shear failure is less likely to occur at the base of the thread with the smallest thread width. Preferably, in both the male and female threaded portions, the smallest thread width is greater than the height of each thread. Furthermore, in the first configuration, a variable thread width section is provided in the male thread section, in which the thread root width changes, and the variable thread width section of the female thread section is configured to correspond to this variable thread width section of the male thread section. Therefore, a screw joint having the first configuration can have high torque. The maximum thread root width of the male and female thread portions may be no more than twice the minimum thread root width of the male and female thread portions (second configuration). For example, in the screw joint described in Japanese Patent Publication No. 4087798, the thread root width changes throughout the entire threaded portion, and the maximum thread root width is more than three times the minimum thread root width, requiring three or more cutting passes for the thread root. On the other hand, with the screw joint having the second configuration described above, the number of cutting passes for the thread root can be reduced to two or fewer. Therefore, the machining time for the threaded portion can be more reliably shortened. In screwing a pin into a box, the tip of the pin does not contact the tip of any other pin fastened to the box when both the insertion surface and the load surface of the variable-width screw portion are in contact with the box, but may contact the tip of any other pin after both the insertion surface and the load surface of the variable-width screw portion are in contact with the box and before the male thread portion yields (third configuration). According to the third configuration, torque can be further increased as needed. In other words, when high torque performance is not required for the threaded joint, fastening is completed when both the insertion surface and the load surface of the variable thread width portion of the male threaded section come into contact with the box during the screwing of the pin into the box. At this time, the tip of the pin screwed into the box does not come into contact with the tips of other pins fastened to that box. On the other hand, when high torque performance is required for a threaded joint, the pin is screwed in further, bringing the tip of that pin into contact with the tip of another pin before the male thread yields. The contact between the tips of the two pins applies axial force to each pin, thereby increasing the torque. The pin may further have a shoulder surface. The box may further have a shoulder surface that corresponds to the shoulder surface of the pin and is capable of contacting the shoulder surface of the pin when fastened (fourth configuration). According to the fourth configuration, when the shoulder surfaces of the pin and the box come into contact, an axial force is applied to the pin. This allows for improved torque. In screwing a pin into a box, the shoulder surface of the pin does not contact the box's shoulder surface when both the insertion surface and the load surface of the variable-width screw portion are in contact with the box, but may contact the box's shoulder surface after both the insertion surface and the load surface of the variable-width screw portion are in contact with the box and before the male thread portion yields (fifth configuration). Even in the fifth configuration, the torque can be increased as needed. In other words, when high torque performance is not required for the threaded joint, the fastening of the pin into the box is completed when both the insertion surface and the load surface of the variable thread width portion of the male threaded section come into contact with the box. At this time, the shoulder surfaces of the pin and the box are not in contact with each other. On the other hand, when high torque performance is required for a threaded joint, the pin is screwed in further, bringing the shoulder surfaces of the pin and the box into contact before the male thread yields. This applies axial force to the pin, increasing the torque. The length in the axial direction of the variable thread width portion of the female thread may be longer than the length in the axial direction of the variable thread width portion of the male thread (sixth configuration). According to the sixth configuration, the threads of the male screw portion easily fit into the thread valleys of the female screw portion, making it easy to screw the pin into the box. <Implementation> The following describes each embodiment with reference to the drawings. In the drawings, identical and corresponding components are denoted by the same reference numerals, and the same description will not be repeated. [First Embodiment] (Overall structure) Figure 1 is a longitudinal cross-sectional view showing a threaded joint 1 according to the first embodiment. The threaded joint 1 according to this embodiment is a coupling-type threaded joint. However, the configuration of the threaded joint described in this embodiment can also be applied to an integral-type threaded joint. As shown in Figure 1, the threaded joint 1 comprises two pins 10 and a box 20. The two pins 10 and the box 20 are each tubular in shape. The box 20 is fastened to each pin 10. One pin 10 is inserted into the box 20 from one end in the axial direction of the pipe, and the other pin 10 is inserted from the other end in the axial direction of the pipe. Each pin 10 has a male threaded portion 11 on its outer circumference. The box 20 has a female threaded portion 21 on its inner circumference that corresponds to each male threaded portion 11. Each male threaded portion 11 and each female threaded portion 21 are composed of tapered threads that mesh with each other. The thread shape of each male threaded portion 11 and each female threaded portion 21 is a dovetail shape. Each male threaded portion 11 of the pin 10 includes a constant thread width portion 111 and a variable thread width portion 112. Each female threaded portion 21 of the box 20 includes a constant thread width portion 211 and a variable thread width portion 212. In the constant thread width portions 111, 211, the thread width and thread root width are constant, while in the variable thread width portions 112, 212, the thread width and thread root width change. This point will be explained in more detail later. Figure 2A is an enlarged longitudinal cross-sectional view of the constant screw width sections 111 and 211. Figure 2B is an enlarged longitudinal cross-sectional view of the variable screw width sections 112 and 212. As shown in Figures 2A and 2B, when viewed from a cross-section obtained by cutting the threaded joint 1 through a plane passing through the pipe axis CL, the male threaded portion 11 has multiple thread crests 11a, thread roots 11b, insertion surfaces 11c, and load surfaces 11d. Each insertion surface 11c and each load surface 11d connects the thread crests 11a and the thread roots 11b. Each insertion surface 11c is the surface that precedes the screwing of the pin 10 into the box 20. Each load surface 11d is located on the opposite side of the corresponding insertion surface 11c. When viewed from a cross-section obtained by cutting the threaded joint 1 through a plane passing through the pipe axis CL, the female threaded portion 21 has multiple thread crests 21a, thread roots 21b, insertion surfaces 21c, and load surfaces 21d. Each thread crest 21a faces the thread root 11b of the male threaded portion 11. Each thread root 21b faces the thread crest 11a of the male threaded portion 11. Each insertion surface 21c faces the insertion surface 11c of the male threaded portion 11. Each load surface 21d faces the load surface 11d of the male threaded portion 11. The flank angles of the insertion surfaces 11c, 21c and the load surfaces 11d, 21d are all negative angles less than 0°. Here, the flank angle refers to the angle between the plane perpendicular to the pipe axis CL and the insertion surfaces 11c, 21c or the load surfaces 11d, 21d. In Figures 2A and 2B, the flank angles of the insertion surfaces 11c, 21c are considered positive when counterclockwise, and the flank angles of the load surfaces 11d, 21d are considered positive when clockwise. When viewed from a cross-section obtained by cutting the threaded joint 1 in a plane passing through the pipe axis CL, the male threaded portion 11 preferably has a plurality of chamfered surfaces 11e. Each chamfered surface 11e is an inclined surface connecting the insertion surface 11c and the thread root bottom surface 11b. The angle θ of the chamfered surface 11e with respect to the thread root bottom surface 11b is preferably 25° to 75°. If the male threaded portion 11 has chamfered surfaces 11e, the female threaded portion 21 has chamfered surfaces 21e corresponding to each chamfered surface 11e of the male threaded portion 11. Each chamfered surface 21e of the female threaded portion 21 faces each chamfered surface 11e of the male threaded portion 11. As shown in Figure 2A, in the fastened state, in the constant thread width sections 111 and 211, the load surface 11d of the male thread section 11 contacts the load surface 21d of the female thread section 21. Also, in the constant thread width sections 111 and 211, the thread crest surface 11a and thread root surface 11b of the male thread section 11 contact the thread root surface 21b and thread crest surface 21a of the female thread section 21, respectively. However, in the constant thread width sections 111 and 211, the insertion surfaces 11c and 21c do not contact each other. If the male thread section 11 and the female thread section 21 each have chamfered surfaces 11e and 21e, then in the constant thread width sections 111 and 211, the chamfered surfaces 11e and 21e also do not contact each other. In other words, in the constant thread width sections 111 and 211, a gap is created between the male thread section 11 and the female thread section 21 in the fastened state. On the other hand, as shown in Figure 2B, in the variable screw width sections 112 and 212, no gap is created between the male screw section 11 and the female screw section 21 when fastened. That is, in the fastened state, the load surfaces 11d and 21d and the insertion surfaces 11c and 21c are in contact with each other in the variable screw width sections 112 and 212. In addition, in the variable screw width sections 112 and 212, the thread crest surface 11a of the male screw section 11 and the thread root surface 21b of the female screw section 21 are in contact with each other, and the thread root surface 11b of the male screw section 11 and the thread crest surface 21a of the female screw section 21 are in contact with each other. If the male screw section 11 and the female screw section 21 each have chamfered surfaces 11e and 21e, then in the variable screw width sections 112 and 212, the chamfered surfaces 11e and 21e are also in contact with each other. Figure 3 is an enlarged longitudinal cross-sectional view of a part of the screw joint 1. In Figure 3, one of the two pins 10 and the portion of the box 20 corresponding to that pin 10 are shown. As described above, the male threaded portion 11 of the pin 10 includes a constant thread width portion 111 and a variable thread width portion 112. The constant thread width portion 111 is located on the base end side (pipe body side) of the pin 10 in the male threaded portion 11. The constant thread width portion 111 has a constant thread root width and thread width. The variable screw width section 112 is positioned on the male screw section 11 closer to the tip of the pin 10 than the constant screw width section 111. The variable screw width section 112 is provided continuously with the constant screw width section 111. The variable screw width section 112 has a thread root width greater than or equal to the thread root width of the constant screw width section 111. The thread root width of the variable screw width section 112 gradually increases from the constant screw width section 111 toward the tip of the pin 10. Therefore, the thread width of the variable screw width section 112 gradually decreases toward the tip of the pin 10. In the male threaded portion 11, it is preferable that the maximum thread root width W11max is no more than twice the minimum thread root width W11min. The thread root width of the variable thread width portion 112 changes within a range that does not fall below the thread root width of the constant thread width portion 111. For this reason, the variable thread width portion 112 has the maximum thread root width W11max in the male threaded portion 11. Also, the minimum thread root width W11min in the male threaded portion 11 is the thread root width of the constant thread width portion 111. The female thread portion 21 of the box 20 is provided with a constant thread width portion 211 and a variable thread width portion 212, respectively, corresponding to the constant thread width portion 111 and variable thread width portion 112 of the male thread portion 11. The constant thread width section 211 faces the constant thread width section 111 of the male thread section 11. The constant thread width section 211 has a constant thread width and thread root width. The thread width and thread root width of the constant thread width section 211 correspond to the thread root width and thread width of the constant thread width section 111 of the male thread section 11, respectively. The variable thread width section 212 faces the variable thread width section 112 of the male thread section 11. The thread width and thread root width of the variable thread width section 212 correspond to the thread root width and thread width of the variable thread width section 112 of the male thread section 11, respectively. That is, the thread width of the variable thread width section 212 corresponds to the thread root width of the variable thread width section 112 of the male thread section 11, and gradually increases from the constant thread width section 211 toward the center of the box 20 (opposite the front end of the box 20). The thread root width of the variable thread width section 212 corresponds to the thread width of the variable thread width section 112 of the male thread section 11, and gradually decreases from the constant thread width section 211 toward the center of the box 20. In the female thread portion 21, it is preferable that the maximum thread root width W21max is no more than twice the minimum thread root width W21min. The thread root width of the variable thread width portion 212 changes within a range that does not exceed the thread root width of the constant thread width portion 211. For this reason, the variable thread width portion 212 has the minimum thread root width W21min in the female thread portion 21. Also, the maximum thread root width W21max in the female thread portion 21 is the thread root width of the constant thread width portion 211. In the male threaded portion 11 and the female threaded portion 21, the variable thread width portions 112 and 212 primarily contribute to the improvement of torque. The ratio of the variable thread width portion 112 to the entire male threaded portion 11 can be appropriately determined considering the required torque. Similarly, the ratio of the variable thread width portion 212 to the entire female threaded portion 21 can be appropriately determined considering the required torque. For example, if the length of the entire male threaded portion 11 in the axial direction is L11 and the length of the variable thread width portion 112 in the axial direction is L112, then 0 < L112 / L11 < 1 can be set. Preferably, 0.3 < L112 / L11 < 1.0. Also, if the length of the entire female threaded portion 21 in the axial direction is L21 and the length of the variable thread width portion 212 in the axial direction is L212, then 0 < L212 / L21 < 1 can be set. Preferably, 0.2 < L212 / L21 < 0.9. Figure 4A is a graph showing the stub lead and load lead of the pin and box. In the graph shown in Figure 4A, the vertical axis represents the lead size, and the horizontal axis represents the number of threads counted from the tip of the pin or the center of the box. Here, with reference to Figure 4B, the stub leads and load leads of the pin and box will be described. The stub lead of the pin is the distance D1a in the pipe axis direction between adjacent insertion surfaces 11c in the male threaded portion 11. The load lead of the pin is the distance D1b in the pipe axis direction between adjacent load surfaces 11d in the male threaded portion 11. The stub lead of the box is the distance D2a in the pipe axis direction between adjacent insertion surfaces 21c in the female threaded portion 21. The load lead of the box is the distance D2b in the pipe axis direction between adjacent load surfaces 21d in the female threaded portion 21. Returning to Figure 4A, let's explain the transition between the stub lead and the load lead. As shown in Figure 4A, at the tip of the pin, i.e., the variable thread width portion 112 of the male thread portion 11 (Figure 3), the load lead is larger than the stub lead. Similarly, at the central portion of the box, i.e., the variable thread width portion 212 of the female thread portion 21 (Figure 3), the load lead is larger than the stub lead. At the base end of the pin, i.e., the constant thread width portion 111 of the male thread portion 11 (Figure 3), the stub lead and the load lead are equal. Similarly, at the end of the box 20, i.e., the constant thread width portion 211 of the female thread portion 21 (Figure 3), the stub lead and the load lead are equal. The load lead of each pin and box is constant throughout the entire threaded portion. However, the stub load changes in both the pin and box. That is, in both the pin and box, the stub load increases when switching from the variable thread width section to the constant thread width section. As can be seen from Figure 4A, the length L212 in the axial direction of the variable-width section of the female thread is longer than the length L112 in the axial direction of the variable-width section of the male thread. In other words, the position where the variable-width section switches to a constant-width section in the pin does not perfectly coincide with the position where the variable-width section switches to a constant-width section in the box. Specifically, L212 / L112 > 1.05. If this position were to perfectly coincide between the pin and the box, the threads of the male thread would have difficulty fitting into the thread valleys of the female thread, making it difficult to screw the pin into the box. (Method of fastening pins to boxes) Next, we will explain how to fasten each pin 10 to the box 20. As described above, the threaded joint 1 according to this embodiment is a coupling-type threaded joint. Therefore, the pins 10 are screwed into the box 20 from each end in the direction of the pipe axis. Referring to Figures 1 and 2B, when one pin 10 is screwed into one end of the box 20 in the axial direction, the male threaded portion 11 of the pin 10 and the female threaded portion 21 of the box 20 interfere with each other, and at some point the load surface 11d and insertion surface 11c of the variable thread width portion 112 of the male threaded portion 11 come into contact with the box 20. More specifically, each of the load surface 11d and insertion surface 11c of the variable thread width portion 112 comes into contact with the corresponding load surface 21d and insertion surface 21c of the female threaded portion 21. Similarly, when another pin 10 is screwed into the other end of the box 20 in the axial direction, at some point each of the load surface 11d and insertion surface 11c of the variable thread width portion 112 of the male threaded portion 11 of that other pin 10 also comes into contact with the corresponding load surface 21d and insertion surface 21c of the female threaded portion 21 (Figure 2B). At this point, as shown in Figure 1, the tips of the pins 10 do not come into contact with each other. For example, when the threaded joint 1 is used for horizontal drilling of shale wells, high torque performance is required for the threaded joint 1. In this case, each pin 10 can be further screwed into the box 20 from the state shown in Figure 1. That is, after the load surface 11d and insertion surface 11c of the variable thread width portion 112 come into contact with the box 20 (Figure 2B), when each pin 10 is further screwed into the box 20, the tips of the pins 10 come into contact with each other, as shown in Figure 5. The tips of the pins 10 come into contact with each other before each male thread portion 11 yields. As a result, axial force is applied to each pin 10, and the torque increases. In order to reliably apply axial force to each pin 10, it is preferable that the tip of each pin 10 is composed of an annular surface substantially perpendicular to the pipe axis CL. (Effects of the first embodiment) As described above, in the screw joint 1 according to this embodiment, the male screw portion 11 has a constant screw width portion 111 and a variable screw width portion 112. The screw root width of the variable screw width portion 112 increases from the constant screw width portion 111 toward the tip of the pin 10, but the screw root width of the constant screw width portion 111 does not change. By providing such a constant screw width portion 111 in the male screw portion 11, the difference in screw root width that occurs in the male screw portion 11 can be reduced compared to the case in which the screw root width is gradually changed over the entire male screw portion. Therefore, when machining the male screw portion 11 using a cutting tool sized to match the minimum screw root width, the number of cutting passes in the pipe axis direction can be reduced. The female thread portion 21 is provided in correspondence with the male thread portion 11. That is, the female thread portion 21 has a constant thread width portion 211 that corresponds to the constant thread width portion 111 of the male thread portion 11. Therefore, the difference in thread root width can be reduced in the female thread portion 21 as well, and the number of cutting passes in the pipe axis direction can be reduced. Thus, according to the threaded joint 1 of this embodiment, the number of cutting passes in the pipe axis direction can be reduced in the male threaded portion 11 and the female threaded portion 21. Therefore, the processing time of the threaded portion composed of the male threaded portion 11 and the female threaded portion 21 can be shortened. Furthermore, the male threaded portion 11 and the female threaded portion 21 have constant thread width sections 111 and 211 and variable thread width sections 112 and 212. By providing such constant thread width sections 111 and 211 in the male threaded portion 11 and the female threaded portion 21, the difference between the minimum thread width and the maximum thread width in the male threaded portion 11 or the female threaded portion becomes smaller compared to the case where the thread width gradually changes throughout the entire male threaded portion 11 or the female threaded portion 12. Therefore, when a tensile load is applied in the pipe axial direction in the male threaded portion and the female threaded portion, shear failure is less likely to occur at the base of the thread with the minimum thread width. As described above, the male threaded portion 11 is provided with a thread width variable portion 112 in which the thread root width increases towards the tip of the pin 10. The female threaded portion is provided with a thread width variable portion 212 in which the thread root width changes in correspondence with the thread width variable portion 112 of the male threaded portion 11. Therefore, the screw joint 1 according to this embodiment can also have high torque. Here, if the only goal is to reduce the difference between the maximum and minimum thread root widths in the male thread section and the maximum and minimum thread root widths in the female thread section, then the male or female thread section may have variable thread width sections along its entire length, where the rate of change in thread root width and thread width is small, and may not have constant thread width sections. However, it is advantageous for both the male and female thread sections to have variable thread width sections 111, 211 and constant thread width sections 112, 212, respectively, where the rate of change in thread root width and thread width is large, in order to achieve high torque. In the male thread portion 11, the maximum thread root width W11max is preferably twice or less the minimum thread root width W11min. Therefore, the number of cutting passes for the thread root in the male thread portion 11 can be kept to two passes or less. In the female thread portion 21, the maximum thread root width W21max is preferably twice or less the minimum thread root width W21min. Therefore, in the female thread portion 21, the number of cutting passes for the thread root can be kept to two passes or less. In order to prevent shear failure at the base of the threads when a tensile load is applied in the axial direction of the pipe, it is preferable that the minimum thread width in the male thread portion 11 and the female thread portion 21 be greater than or equal to the height of the threads. When screwing each pin 10 into the box 20, the tips of the pins 10 do not come into contact with each other at the point when both the insertion surface and the load surface of each variable screw width portion 112 come into contact with the box 20. In particular, when high torque performance is not required, the screw joint 1 is used with the tips of the pins 10 not in contact with each other. When high torque performance is required for the screw joint 1, the tips of the pins 10 can be brought into contact with each other before the male threaded portion 11 yields by further screwing each pin 10 into the box 20. This applies axial force to each pin 10, thereby increasing the torque. In this way, the screw joint 1 according to this embodiment can increase the torque as needed. In the screw joint 1 according to this embodiment, the male thread portion 11 and the female thread portion 21 each include not only constant thread width portions 111 and 211, but also variable thread width portions 112 and 212. Therefore, the required torque can be secured. In the fastened state, if the thread root surface of the male thread portion and the thread crest surface of the female thread portion are in contact, and the thread crest surface of the male thread portion and the thread root surface of the female thread portion are not in contact, the threaded portion composed of the male thread portion and the female thread portion will not have sealing performance. On the other hand, in the fastened state of the threaded joint 1 according to this embodiment, in the variable thread width portions 112, 212, the thread root surface 11b of the male thread portion 11 and the thread crest surface 21a of the female thread portion 21 are in contact, and the thread crest surface 11a of the male thread portion 11 and the thread root surface 21b of the female thread portion 21 are in contact. With such a configuration, sealing performance can be provided to the threaded portion composed of the male thread portion 11 and the female thread portion 21. [Second Embodiment] Figure 6 is a longitudinal cross-sectional view showing a threaded joint 1A according to the second embodiment. The threaded joint 1A differs from the threaded joint 1 according to the first embodiment in that the pin 10A and the box 20A each have shoulder surfaces 12, 22 and sealing surfaces 13, 23. Other configurations of the threaded joint 1A can be the same as those of the threaded joint 1 according to the first embodiment. The pin 10A includes a shoulder surface 12 and a sealing surface 13. The shoulder surface 12 is located at the base end of the pin 10A. The shoulder surface 12 can be an annular surface substantially perpendicular to the pipe axis CL. The sealing surface 13 is provided on the outer circumference of the pin 10A. The sealing surface 13 is located between the male threaded portion 11 and the shoulder surface 12 of the pin 10A. The box 20A includes a shoulder surface 22 and a sealing surface 23. The shoulder surface 22 is provided corresponding to the shoulder surface 12 of the pin 10A. The shoulder surface 22 can be an annular surface substantially perpendicular to the pipe axis CL. The sealing surface 23 is provided on the inner circumference of the box 20A corresponding to the sealing surface 13 of the pin 10A. The sealing surface 23 is located between the female thread portion 21 and the shoulder surface 22. The sealing surfaces 13 and 23 come into contact with each other as the pin 10A is screwed into the box 20A, and in the fastened state, they fit tightly together to form an interference fit. As a result, the sealing surfaces 13 and 23 form a sealing portion by metal contact. Consequently, the sealing performance of the screw joint 1A is improved. The sealing portion formed by the sealing surfaces 13 and 23 exhibits excellent sealing performance particularly against external pressure. The shoulder surfaces 12 and 22 come into contact with and press against each other as the pin 10A is screwed into the box 20A. When the shoulder surfaces 12 and 22 come into contact with each other, an axial force is applied to the pin 10A and the torque increases. Although not shown in the drawing, the male screw portion 11 of the pin 10A has a variable screw width portion as in the first embodiment. The shoulder surfaces 12 and 22 may come into contact with each other when the load surface and the insertion surface in the variable screw width portion of the male screw portion 11 contact the box 20A during the screwing of the pin 10A into the box 20A, or they may not come into contact with each other at that time. When the shoulder surfaces 12 and 22 do not come into contact with each other when the load surface and the insertion surface in the variable screw width portion of the male screw portion 11 contact the box 20A, the shoulder surfaces 12 and 22 can be brought into contact with each other by further screwing the pin 10A into the box 20A. That is, after the load surface and the insertion surface in the variable screw width portion of the male screw portion 11 contact the box 20A, the pin 10A is further screwed into the box 20A, and the shoulder surfaces 12 and 22 are brought into contact with each other before the male screw portion 11 yields. As a result, an axial force is applied to the pin 10A and the torque increases compared to the state where the shoulder surfaces 12 and 22 are not in contact with each other. By configuring it in this way, as in the first embodiment, the torque can be increased as needed. In the second embodiment, the pin 10A has the shoulder surface 12 and the sealing surface 13, but the pin 10A may have only one of the shoulder surface 12 and the sealing surface 13. In this case, the box 20A has a shoulder surface 22 corresponding to the shoulder surface 12 of the pin 10A, or a sealing surface 23 corresponding to the sealing surface 13 of the pin 10A. [Third Embodiment] FIG. 7 is a longitudinal sectional view showing a screw joint 1B according to the third embodiment. The screw joint 1B is different from the screw joint 1 according to the first embodiment in that the pin 10B and the box 20B each have seal surfaces 14 and 24. Other configurations of the screw joint 1B can be the same as those of the screw joint 1 according to the first embodiment. The pin 10B includes the seal surface 14. The seal surface 14 is provided on the outer periphery of the pin 10B. The seal surface 14 is disposed at the tip portion of the pin 10B. The box 20B includes the seal surface 24. The seal surface 24 is provided on the inner periphery of the box 20B corresponding to the seal surface 14 of the pin 10B. The seal surfaces 14 and 24 come into contact with each other as the pin 10B is screwed into the box 20B, and in the fastened state, they are fitted and adhered to each other to be in an interference fit state. As a result, the seal surfaces 14 and 24 form a seal portion by metal contact. As a result, the sealing performance of the screw joint 1B is improved. The seal portion formed by the seal surfaces 14 and 24 exhibits particularly excellent sealing performance against internal pressure. In the screw joint 1B according to the third embodiment, the pin 10B may further have a seal surface 13 (FIG. 6) similar to that of the second embodiment. In this case, the box 20B has a seal surface 23 (FIG. 6) corresponding to the seal surface 13. According to such a configuration, the sealing performance against internal pressure can be improved by the seal portion formed by the seal surfaces 14 and 24, and the sealing performance against external pressure can be improved by the seal portion formed by the seal surfaces 13 and 23. [Fourth Embodiment] FIG. 8 is a longitudinal sectional view showing a screw joint 1C according to the fourth embodiment. In the screw joint 1C, the pin 10C and the box 20C each have seal surfaces 14 and 24 similar to those of the third embodiment. Further, the pin 10C and the box 20C each have shoulder surfaces 15 and 25. The shoulder surface 15 is provided at the tip of the pin 10C. The shoulder surface 25 is provided on the box 20C corresponding to the shoulder surface 15 of the pin 10C. The shoulder surfaces 15 and 25 are preferably each formed of an annular surface substantially perpendicular to the tube axis CL. The shoulder surfaces 15 and 25 come into contact with each other and are pressed together as the pin 10C is screwed into the box 20C. The contact between the shoulder surfaces 15 and 25 imparts axial force to the pin 10C, increasing the torque. The male threaded portion 11 of the pin 10C has a variable thread width portion, similar to the first embodiment. The shoulder surfaces 15 and 25 may or may not come into contact with each other at the time the load surface and insertion surface of the variable thread width portion of the male threaded portion 11 come into contact with the box 20C during the screwing of the pin 10C into the box 20C. If the shoulder surfaces 15 and 25 do not come into contact with each other when the load surface and insertion surface of the variable-width thread portion of the male thread portion 11 come into contact with the box 20C, the shoulder surfaces 15 and 25 can be brought into contact with each other by further screwing the pin 10C into the box 20C. That is, after the load surface and insertion surface of the variable-width thread portion of the male thread portion 11 come into contact with the box 20C, the pin 10C is further screwed into the box 20C, bringing the shoulder surfaces 15 and 25 into contact with each other before the male thread portion 11 yields. As a result, an axial force is applied to the pin 10C, and the torque increases compared to the state where the shoulder surfaces 15 and 25 are not in contact with each other. With this configuration, the torque can be increased as needed, as in the first embodiment. In the fourth embodiment, the pin 10C has a shoulder surface 15 and a sealing surface 14, but the pin 10C may have only one of the shoulder surface 15 and the sealing surface 14. In this case, the box 20C has a shoulder surface 25 corresponding to the shoulder surface 15 of the pin 10C, or a sealing surface 24 corresponding to the sealing surface 14 of the pin 10C. The pin 10C may further have a shoulder surface 12 similar to that of the second embodiment. That is, the pin 10C may have shoulder surfaces 12 and 15 at its base and tip. In this case, the box 20C has shoulder surfaces 22 and 25 corresponding to the shoulder surfaces 12 and 15 of the pin 10C, respectively. [Variations] Although each embodiment has been described above, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the invention. In each of the above embodiments, the male and female threaded portions are composed of one constant-width portion and one variable-width portion. However, the male and female threaded portions may also include multiple constant-width portions and / or multiple variable-width portions. When the male and female threaded portions include multiple constant-width portions, each constant-width portion may have a different thread root width. When the male and female threaded portions include multiple variable-width portions, the rate of change of the thread root width in each variable-width portion may be the same or different from one another. In each of the embodiments described above, the male and female threaded portions are made of dovetail-shaped threads. However, the thread shape of the male and female threaded portions is not limited to this. The male and female threaded portions may be made of trapezoidal threads, for example. In the embodiments described above, the male and female threaded portions were composed of a single thread, but they can also be composed of multiple threads. Furthermore, the threaded portions composed of the male and female threaded portions may be single-start threads or double-start threads. In the second and fourth embodiments described above, the pin has shoulder surfaces at its tip and / or base. However, the pin may also have an intermediate shoulder surface provided in the middle. The box is provided with an intermediate shoulder surface corresponding to the intermediate shoulder surface of the pin. The pin and the box may each have two or more intermediate shoulder surfaces. In this case, the male and female threaded portions are composed of multiple stages of threads with intermediate shoulder surfaces in between. In the third embodiment described above (Figure 7), a male sealing surface 14 is positioned at the tip of the pin, and a female sealing surface 24 is positioned at the corresponding portion of the box. However, instead of this structure, or in combination with this structure, a male sealing surface may be positioned at the tip of one pin and a female sealing surface at the tip of the other pin. In this case, the sealing surfaces of both pins form a sealing portion through metal-to-metal contact. As a result, the sealing performance of the screw joint is improved. To confirm the effectiveness of the threaded joint described herein, the following tests were conducted. (Examples) Figure 9 is a longitudinal cross-sectional view of a screw joint according to an embodiment. The male threaded portion 11 of the screw joint shown in Figure 9 was machined, and the machining time was measured. A constant thread width section 111 with a constant thread root width and a variable thread width section 112 where the thread root width gradually increases from the constant thread width section 111 towards the tip of the pin 10 are formed. The lengths of the constant thread width section 111 and the variable thread width section 112 in the axial direction of the pipe are equal. That is, half of the male thread section 11 is the constant thread width section 111. As a result, in the male thread section 11, the maximum thread root width W11max is twice the minimum thread root width W11min. The male thread portion 11 was machined using a cutting tool sized to match the minimum thread root width W11min. As described above, in the male thread portion 11, the maximum thread root width W11max is twice the minimum thread root width W11min. Therefore, taking the number of cutting passes in the pipe axis direction for the thread root with the minimum thread root width W11min as 1, the number of cutting passes in the pipe axis direction for the thread root with the maximum thread root width W11max (maximum number of cutting passes) was 2. In addition, the total number of cutting passes in the pipe axis direction was 14. (Comparative example) Figure 10 is a longitudinal cross-sectional view of a threaded joint according to a comparative example. The male threaded portion 31 of the threaded joint shown in Figure 10 was machined, and the machining time was measured. In the comparative example of the screw joint, the thread root width was varied throughout the male thread portion 31. The thread root width of the male thread portion 31 gradually increased from the base end to the tip of the pin 30. The length of the male thread portion 31 in the pipe axis direction was approximately equal to the length of the male thread portion 11 in the pipe axis direction according to the embodiment. The rate of change of the thread root width of the male thread portion 31 was also substantially the same as the rate of change of the thread root width of the variable thread width portion 112 according to the embodiment. As a result, in the male thread portion 31, the maximum thread root width was four times the minimum thread root width. The male thread portion 31 was machined using a cutting tool sized to match the minimum thread root width. In the male thread portion 31, the maximum thread root width is four times the minimum thread root width, so the maximum number of cutting passes was 4. The total number of cutting passes in the pipe axis direction was 28. (evaluation) Figure 11 shows graphs of the machining times for the screw joints in the embodiment and comparative example. The maximum number of cutting passes and the total number of cutting passes for the male thread portion 11 in the embodiment are half the maximum number of cutting passes and the total number of cutting passes for the male thread portion 31 in the comparative example, respectively. Therefore, as shown in Figure 11, the machining time for the male thread portion 11 in the embodiment is also approximately half the machining time for the male thread portion 31 in the comparative example. As described above, it has been confirmed that by providing a constant thread width section 111 in the male thread section 11, the processing time of the male thread section 11 can be shortened compared to the male thread section 31 which does not include a constant thread width section. Similar effects can be expected for the female thread section 21 which has a configuration corresponding to the male thread section 11. This invention is applicable to threaded joints.

Claims

1. A threaded joint for connecting a pair of pipes, comprising: a pin having a male threaded portion on its outer circumference; and a box having a female threaded portion on its inner circumference corresponding to the male threaded portion and fastened with the pin, wherein the male threaded portion includes a constant thread width portion having a constant thread root width, and a variable thread width portion having a thread root width greater than or equal to the thread root width of the constant thread width portion of the male threaded portion and gradually increasing from the constant thread width portion of the male threaded portion toward the tip of the pin, and the female threaded portion includes a constant thread width portion having a constant thread width portion and a variable thread width portion having a thread width greater than or equal to the thread width of the constant thread width portion of the female threaded portion and gradually increasing from the constant thread width portion of the female threaded portion toward the center of the box, the threaded joint.

2. A threaded joint according to claim 1, wherein the maximum thread root width of the male thread portion is no more than twice the minimum thread root width of the male thread portion. 。 3. A threaded joint according to claim 1 or 2, wherein, in the screwing of the pin into the box, the tip of the pin does not contact the tip of the other pin fastened to the box when both the insertion surface and the load surface of the variable thread width portion contact the box, and contacts the tip of the other pin after both the insertion surface and the load surface of the variable thread width portion contact the box and before the male thread portion yields. hand.

4. A threaded joint according to claim 1 or 2, wherein the pin further has a shoulder surface, and the box further has a shoulder surface provided corresponding to the shoulder surface of the pin and capable of contacting the shoulder surface of the pin when fastened. Fittings.

5. The threaded joint according to claim 4, wherein, in the screwing of the pin into the box, the shoulder surface of the pin does not contact the shoulder surface of the box when both the insertion surface and the load surface of the variable thread width portion contact the box, and contacts the shoulder surface of the box after both the insertion surface and the load surface of the variable thread width portion have contacted the box and before the male thread portion yields. Joint.

6. A threaded joint according to any one of claims 1 to 5, wherein the length in the axial direction of the variable thread width portion of the female thread is longer than the length in the axial direction of the variable thread width portion of the male thread. and screw joint.

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