Steel pipe processing methods

Abstract

The purpose of the invention is to provide a steel pipe joint structure with which a high tightening torque can be obtained without enlarging the outer diameter and without increasing the wall thickness. The steel pipe joint structure according to the present invention connects a first steel pipe having a male thread formed on at least one end and a second steel pipe having a female thread formed in at least one end, a male thread being formed on the first steel pipe and a female thread being formed in the second steel pipe. The male threaded part includes a pin front parallel threaded part that is a parallel and incomplete thread at a pin-front-side end, and a pin complete threaded part that is a tapering and complete thread in the center of the male threaded part. The female threaded part includes a box rear incomplete threaded part that is a tapering and incomplete thread at a box-rear-side end, and a box complete threaded part that is a tapering and complete thread in the center of the female threaded part. The box rear incomplete threaded part and the pin front parallel threaded part are screwed together, and in a cross section including the pipe shaft, the width of the root of the pin front parallel threaded part is greater than the width of the root of the pin complete threaded part.

Description

Technical Field

[0001] The present invention Steel pipe processing method relates to, in particular, a Canada working method for a threaded portion formed on a steel pipe.

Background Art

[0002] As a technique for connecting oil well pipes used in exploration and production of oil fields and the like, a steel pipe joint structure that uses male and female tapered threads for connection is widely used. In the steel pipe joint structure, a male threaded portion and a female threaded portion are fitted and adhered to each other. Hereinafter, a configuration in which a pair of steel pipes are connected via a joint by screwing a female thread and a male thread together is referred to as a "steel pipe joint structure".

[0003] There are two types of steel pipe joint structures: an integral type and a coupling type. In the integral type, a male thread is provided at one end of an oil well pipe to form a pin, and a female thread is provided at the other end to form a box, and the female thread of the box and the male thread of the pin are screwed together. In the coupling type, male threads are provided at both ends of an oil well pipe to form pins, and female threads are provided at both ends of another pipe to form boxes, and the female thread on one end side of the box and the male thread of the pin portion of the oil well pipe on one end side are screwed together, and the female thread on the other end side of the same box and the male thread of the pin of the other oil well pipe are screwed together. For example, in the case of the integral type, the threaded installation portions of the pin and the box are called threaded portions. Then, the non-threaded portion from the tip end of the pin to the first thread crest of the threaded portion, and the non-threaded portion from the innermost end of the box (the portion where the tip end of the pin abuts) to the first thread valley of the threaded portion are respectively called the inner surface shoulder portion, and the innermost end of the pin and the tip end of the box are respectively called the outer surface shoulder portion. When the threaded joint is tightened, the inner surface shoulder portions of the pin and the box and the outer surface shoulder portions contact each other locally in a metal-to-metal manner, and the contacted portions serve as seals.

[0004] In steel pipe joint structures that do not have a sealing surface (non-threaded surface), the sealing and pressure resistance of the steel pipe joint structure are ensured by the fitting and tight contact between the male threaded portion and the female threaded portion. Alternatively, the sealing and pressure resistance of the steel pipe joint structure are ensured by a metal seal formed by the contact between the female thread and the male thread, as well as the contact between the sealing portions (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2015 / 033997 [Overview of the project] [Problems that the invention aims to solve]

[0006] In the steel pipe joint structure for oil well pipes disclosed in Patent Document 1, the abutment surfaces of the pin and the box come into contact and tightly close together as the pin is screwed in. The abutment surfaces act as stoppers to limit the screwing in of the pin and also apply a load to the male threaded portion in the opposite direction to the direction of screwing, so-called screw tightening axial force. This tightening axial force causes the threaded portion or metal seal portion of the steel pipe joint structure to tightly close together, ensuring the sealing and pressure resistance of the steel pipe joint structure. The root of the thread is the part where the cross-sectional area of ​​the pin and the box is smallest, and the ends of the thread engagement region, i.e., the end of the pin with a larger threaded portion and the end of the box with a smaller threaded portion, bear the tensile load applied after tightening. The cross-sections of the roots of the thread at the ends of the thread engagement region where the cross-sectional area is smallest are referred to as the critical cross-section of the pin's thread and the critical cross-section of the box's thread, respectively. The critical cross-sectional area of ​​the joint is determined by comparing the root cross-sectional area of ​​the larger diameter end of the pin thread with the root cross-sectional area of ​​the smaller diameter end of the box thread.

[0007] In the steel pipe joint structure for oil well pipes described in Patent Document 1, the critical cross-sectional area of ​​the joint exists at the end of the threaded portion's engagement area, either on the side with a larger diameter for the pin's threaded portion or on the side with a smaller diameter for the box's threaded portion. However, to increase the tensile load-bearing capacity, it is necessary to increase the respective critical cross-sectional areas. Increasing the critical cross-sectional area of ​​the pin requires increasing the wall thickness of the steel pipe, and increasing the critical cross-sectional area of ​​the box requires increasing the outer diameter of the steel pipe. In this case, the contact area between the inner shoulder portion 96 and the outer shoulder portion 95 decreases, reducing rigidity and making it unable to withstand high tightening torque. Therefore, the steel pipe joint structure had the problem of not being able to secure sufficient tightening torque.

[0008] This invention allows for high tightening torque without increasing the outer diameter or wall thickness. steel The objective is to provide a pipe processing method. [Means for solving the problem]

[0011] The steel pipe processing method according to the present invention is a steel pipe processing method for processing a male thread portion or a female thread portion at the end of a steel pipe, wherein the male thread portion or female thread portion has a parallel thread portion and a tapered thread portion formed in order from the tip of the steel pipe, and comprises a plurality of normal processing passes that feed a thread cutting insert at a first feed rate from an initial position to process the parallel thread portion and the tapered thread portion in succession, a first thread thinning processing pass that processes the stubbling flank of the parallel thread portion at a second feed rate, and the stubbling flank of the parallel thread portion at the second feed rate The machine includes a second thread thinning pass that processes the stubing flank of the tapered thread portion at the first feed rate, and a third thread thinning pass that processes the load flank of the parallel thread portion at the third feed rate and processes the tapered thread portion at the first feed rate, wherein the first and second thread thinning passes start processing from a position closer to the steel pipe than the initial position, and the third thread thinning pass starts processing from a position further from the steel pipe than the initial position. Furthermore, the steel pipe processing method according to the present invention is a steel pipe processing method for processing a male thread portion or a female thread portion at the end of a steel pipe, wherein the male thread portion or the female thread portion has a parallel thread portion and a tapered thread portion formed in order from the tip of the steel pipe, and comprises a plurality of normal processing passes that feed a thread cutting insert at a first feed rate from an initial position to process the parallel thread portion and the tapered thread portion continuously, a first thread thinning processing pass that processes the stubbling flank of a part of the parallel thread portion and the tapered thread portion at a second feed rate, and the stubbling flank of a part of the parallel thread portion and the tapered thread portion at the second feed rate The machine includes a second thread thinning pass that processes the loading flank and processes the stubbling flank of the tapered thread portion other than the aforementioned portion at the first feed rate, and a third thread thinning pass that processes the load flank of the parallel thread portion and the aforementioned portion of the tapered thread portion at the third feed rate and processes the tapered thread portion other than the aforementioned portion at the first feed rate, wherein the first thread thinning pass and the second thread thinning pass start processing from a position closer to the steel pipe than the initial position, and the third thread thinning pass starts processing from a position further from the steel pipe than the initial position. [Effects of the Invention]

[0012] According to the present invention, the incomplete threads at the tip of the pin or box of the steel pipe joint (the parallel thread portion at the front of the pin or the parallel thread portion at the front of the box) are formed with a small width, so that they can be screwed together without interfering with the threads of the incomplete thread portion at the rear of the mating pipe. Therefore, the steel pipe joint structure can increase the area of ​​the critical cross-section of the thread portion of the first steel pipe, which is the pin, and can secure the cross-sectional area near the front shoulder surface of the box of the second steel pipe without increasing the outer diameter of the second steel pipe, which is the box. Furthermore, the incomplete thread portion at the rear of the pin is formed with a small width of threads in the parallel thread portion of the mating box, so that the threads do not interfere with each other, so that it can be screwed together with the parallel thread portion on the box without reducing the root diameter. Thus, the steel pipe joint structure can secure the cross-sectional area near the rear shoulder surface of the pin provided on the first steel pipe without increasing the wall thickness of the first steel pipe. In other words, there is no need to reduce the critical cross-sectional area of ​​the pin. Therefore, the steel pipe joint structure has improved strength and rigidity against tightening axial force, making it possible to increase the tightening torque, and improving sealing performance and pressure resistance. [Brief explanation of the drawing]

[0013] [Figure 1] This is an explanatory diagram of a steel pipe joint structure 100 according to Embodiment 1. [Figure 2] This is a close-up view of a pin 11 of the first steel pipe 10 according to Embodiment 1. [Figure 3] This is an enlarged view of the shoulder portion of the steel pipe joint structure 100 according to Embodiment 1. [Figure 4] This is an enlarged view of the shoulder portion of the steel pipe joint structure 1100 related to the comparative example. [Figure 5] This is a close-up view of the box 31 of the second steel pipe 30 according to Embodiment 1. [Figure 6] This is an explanatory diagram of a steel pipe joint structure 1200 related to a comparative example. [Figure 7] This is a cross-sectional view showing an example of perfect thread engagement. [Figure 8] Figures 2 and 5 show cross-sectional views illustrating the state of complete thread engagement in section A. [Figure 9]It is a cross-sectional view showing an example of the meshing state of incomplete threads in part B of FIGS. 2 and 5. [Figure 10] It is an explanatory view of the meshing state near the connection part between the complete thread part and the incomplete thread part of the steel pipe joint structure 1000 according to the comparative example. [Figure 11] It is an explanatory view of the thread machining around the connection part 21 of the pin 11 according to Embodiment 1 and the comparative example. [Figure 12] It is an explanatory view of an example of machining using the thread insert 90 according to Embodiment 1 and the comparative example. [Figure 13] It is an explanatory view of the relationship between the thread sizes of the pin front parallel thread part 14 and the box rear incomplete thread part 37 of the steel pipe joint structure 1000 according to the comparative example of FIG. 10. [Figure 14] It is an explanatory view of an example of machining the male thread part 13 according to Embodiment 1. [Figure 15] It shows the machining process by all machining paths focusing on one valley of the pin front parallel thread part 14 in FIG. 14. [Figure 16] It is an explanatory view of the rough machining performed before the thread machining according to Embodiment 1. [Figure 17] It is a cross-sectional view showing an example of the meshing state of incomplete threads in part B of FIGS. 2 and 5.

Mode for Carrying Out the Invention

[0014] Embodiment 1. FIG. 1 is an explanatory view of the steel pipe joint structure 100 according to Embodiment 1. The upper half of FIG. 1 is a cross-sectional view along the central axis of the steel pipe joint structure 100, and the lower half of FIG. 1 is a side view. Note that each part of FIG. 1 is schematically shown, and the present invention is not limited to the illustrated form.

[0015] As shown in Figure 1, the steel pipe joint structure 100 connects the first steel pipe 10 and the second steel pipe 30 by screwing threads formed on the pin 11 and the box 31, respectively. The first steel pipe 10 has a pin 11 at at least one end. The second steel pipe 30 has a box 31 at at least one end. In Embodiment 1, the first steel pipe 10 and the second steel pipe 30 are steel pipes each with a pin 11 at one end and a box 31 at the other end, and are connected in an integral manner. In other words, as shown in Figure 1, the pin 11 and the box 31 in Embodiment 1 are one end and the other end of two steel pipes of the same structure. The tubular portion connecting the pin 11 and the box 31 at both ends of a single steel pipe is called the pipe section 50. However, the steel pipe joint structure 100 is not limited to those that connect steel pipes in an integral manner, but can also be applied to those that connect them in a coupling manner. In the case of a coupling manner, the first steel pipe 10 will have pins 11 formed at both ends, and the second steel pipe 30 will have boxes 31 formed at both ends.

[0016] The pin 11 of the first steel pipe 10 is formed to become thicker from the pin front shoulder surface 23, which is one end face, toward the pipe section 50. The pin 11 has a male threaded portion 13 (see Figure 2) which has a tapered surface in which the outer diameter increases toward the pipe section 50 from the pin front shoulder surface 23 in at least a portion of it. The male threaded portion 13 has male threads formed on its outer circumference.

[0017] The box 31 of the second steel pipe 30 is formed to become thicker from one end face, the front shoulder surface 42 of the box, toward the pipe section 50. The box 31 is provided with a female threaded section 33 (see Figure 5) which has a tapered surface in which the inner diameter decreases from the front shoulder surface 42 of the box toward the pipe section 50, at least in part. The female threaded section 33 has female threads formed on its inner circumferential surface.

[0018] The pin 11 of the first steel pipe 10 and the box 31 of the second steel pipe 30 each have tapered threads formed on them and are screwed together. The front shoulder surface 42 of the box, which is the tip surface of the second steel pipe 30, abuts against the rear shoulder surface 22 of the pin. The rear shoulder surface 22 of the pin and the front shoulder surface 42 of the box are located on the outer circumference side of the pipe portion 50 side of the pin 11 formed on the first steel pipe 10, and are surfaces that intersect the pipe axis C of the steel pipe, i.e., the central axis of the steel pipe joint structure 100, at an angle perpendicular or nearly perpendicular. In Embodiment 1, the rear shoulder surface 22 of the pin is a surface that is inclined toward the second steel pipe 30 side toward the outer diameter side than toward the inner diameter side. The front shoulder surface 42 of the box is also a surface that is inclined toward the pipe portion 50 of the second steel pipe 30 side toward the outer diameter side than toward the inner diameter side, corresponding to the rear shoulder surface 22 of the pin. The inclination of the front shoulder surface 42 of the box and the rear shoulder surface 22 of the pin improves buckling performance because, when an axial load is applied to the front shoulder surface 42 of the box, the front of the box 32 is displaced inward. In addition, the front shoulder surface 23 of the pin and the rear shoulder surface 43 of the box also come into contact and function as a metal seal.

[0019] The front shoulder surface 23 of the pin, which is the tip surface of the first steel pipe 10, is in contact with the rear shoulder surface 43 of the box. The front shoulder surface 23 of the pin is the tip surface of the first steel pipe 10 and is a surface that intersects the pipe axis C of the steel pipe, i.e., the central axis of the steel pipe joint structure 100, at an angle close to or perpendicular to it. In Embodiment 1, the front shoulder surface 23 of the pin and the rear shoulder surface 43 of the box are perpendicular to the pipe axis C. The front shoulder surface 23 of the pin and the rear shoulder surface 43 of the box may also be surfaces that are inclined toward the second steel pipe 30 on the outer diameter side rather than on the inner diameter side.

[0020] (1st steel pipe 10) Figure 2 is a close-up view of a pin 11 of the first steel pipe 10 according to Embodiment 1. The pin 11 of the first steel pipe 10 has the front pin portion 12, the male thread portion 13, and the rear pin portion 18 positioned in the direction of the pipe axis C, in that order from the tip.

[0021] The front pin portion 12 is located at the very tip of the pin 11 and includes a front pin shoulder surface 23, which is the tip surface of the first steel pipe 10, and a non-threaded portion 24 whose outer circumference is cylindrical and does not have a male thread. The front pin portion 12 is the part indicated by section P6 in Figure 2. Note that the non-threaded portion 24 is not required, in which case a male thread is formed on the front pin portion 12.

[0022] The male threaded portion 13 comprises, in order from the tip side, a pin front parallel threaded portion 14 (pin side run-in portion), a pin front incomplete tapered threaded portion 15, a pin complete threaded portion 16, and a pin rear incomplete threaded portion 17 (pin side run-out portion). The pin front parallel threaded portion 14 is located at the end on the pin front 12 side and is the portion shown as section P1 in Figure 2. Its outer circumference is a tapered surface that widens as it moves from the tip surface toward the pipe portion 50 side, and it is formed so that the effective diameter of the male thread is a predetermined value. The root diameter of the male thread in the pin front parallel threaded portion 14 is greater than or equal to the outer diameter dimension of the non-male threaded portion 24 of the pin front 12. Furthermore, because the outer circumference of the pin front parallel threaded portion 14 is a tapered surface, the male thread in the pin front parallel threaded portion 14 is an incomplete thread.

[0023] The incomplete tapered thread portion 15 at the front of the pin is the portion shown in section P2 in Figure 2. The portion shown in section P2 has a tapered outer surface that continues from the parallel thread portion 14 at the front of the pin. The effective diameter of the male thread formed in the portion shown in section P2 is a tapered thread that widens from the tip side toward the pipe portion 50 side. However, the outer circumference (thread crest) of the incomplete tapered thread portion 15 at the front of the pin in section P2 is smaller than the outer circumference (thread crest) of the complete thread portion 16 at the pin shown in section P3, and the male thread formed therein is an incomplete thread. There is a step 15a between the outer circumference (thread crest) of the incomplete tapered thread portion 15 at the front of the pin in section P2 and the outer circumference (thread crest) of the complete thread portion 16 at the pin shown in section P3, resulting in a discontinuous surface. Furthermore, the outer circumference (thread peaks) of the pin front incomplete tapered thread section 15 in section P2 is smaller than the outer circumference (thread peaks) of the pin front complete thread section 16 shown in section P3. The tip end of the pin front incomplete tapered thread section 15, i.e., the boundary between section P2 and section P1 shown in Figure 5, is a connection section 21 where the thread changes from parallel to tapered. In the pin front incomplete tapered thread section 15, the diameter of the male thread roots also widens from the tip towards the pipe section 50, so the wall thickness of the steel pipe also gradually increases towards the pipe section 50. In addition, a step 15a is formed at the pipe section 50 side end of the pin front incomplete tapered thread section 15, and it connects to the pin front complete thread section 16. Note that the pin front parallel thread section 14 and the pin front incomplete tapered thread section 15, which are incomplete thread sections, are sometimes collectively referred to as the pin front incomplete thread section. Note that the P2 section in the male threaded portion 13 may be omitted. In other words, the male threaded portion 13 may be in a form in which the pin front parallel threaded portion 14 and the pin complete threaded portion 16 are directly connected.

[0024] The fully threaded portion 16 of the pin is the part shown in section P3 in Figure 2. The fully threaded portion 16 of the pin has a tapered surface at the crest of the thread so that the male thread is a fully threaded thread. The tapered surface and the effective diameter E1 of the male thread are parallel, and the male thread is a fully threaded thread. The tapered surface on the outer circumference (crest of the thread) of the fully threaded portion 16 of the pin switches to a cylindrical surface at point 25, which has approximately the same outer diameter as the large diameter portion 19, which is the outer circumference of the shoulder. In Embodiment 1, point 25 may have an outer diameter slightly smaller than the large diameter portion 19, or it may have the same outer diameter as the large diameter portion 19. As described above, the fully threaded portion 16 of the pin is formed in section P3 up to point 25 of the male threaded portion 13.

[0025] The pin rear incomplete thread portion 17 is the end portion on the pin rear 18 side and is shown as section P4 in Figure 2. Since section P4 is a cylindrical surface with a constant outer diameter, the tapered thread formed therein is an incomplete thread where the height gradually decreases. The pin rear incomplete thread portion 17 is formed up to point 26 where the root diameter of the male thread matches the outer diameter of the outer surface.

[0026] The rear portion 18 of the pin may have a small-diameter portion 20 with a smaller outer diameter at the tip end and a large-diameter portion 19 with a larger outer diameter at the tube portion 50 end. The small-diameter portion 20 has the same outer diameter as the incomplete thread portion 17 of the rear pin and is the portion where the tapered thread is cut off. Note that there does not have to be a step between the small-diameter portion 20 and the large-diameter portion 19. In other words, the small-diameter portion 20 may be directly connected to the shoulder surface 22 of the rear pin.

[0027] The section P1 to P4 shown in Figure 2 is collectively referred to as the male threaded section 13. In other words, section P5 is the male threaded section 13. Sections P2 to P4 are referred to as the tapered threaded section 28. The male threaded section 13 is formed by combining the pin front parallel threaded section 14 and the tapered threaded section 28. With this configuration, the pin 11 can ensure a large angle between the tapered threaded section 28 and the pipe axis C, while also ensuring sufficient length of the male threaded section 13, wall thickness of the pin front 12, and wall thickness of the section where the pin rear incomplete threaded section 17 is provided. In other words, since the pin 11 has sufficient wall thickness at both the tip and the rear, its strength and rigidity against tightening torque are improved, and the tightening torque of the steel pipe joint structure 100 can be increased when the pin front shoulder surface 23 and the box rear shoulder surface 43 come into contact. As a result, the airtightness and pressure resistance of the steel pipe joint structure 100 are improved.

[0028] Figure 3 is an enlarged view of the shoulder portion of the steel pipe joint structure 100 according to Embodiment 1. Figure 4 is an enlarged view of the shoulder portion of the steel pipe joint structure 1100 according to the comparative example. In the steel pipe joint structure 1100 according to the comparative example shown in Figure 4, the complete threaded portion 1017 is formed up to the vicinity of the rear shoulder surface 22 of the pin, so the wall thickness of the critical cross section becomes thinner. In other words, at the end of the engagement of the male threaded portion 1013 of the pin 1011, the wall thickness of the first steel pipe 1010, that is, the thickness from the root of the male thread to the inner circumferential surface, becomes thinner. Consequently, the steel pipe joint structure 1100 according to the comparative example has low strength against tensile load after tightening.

[0029] On the other hand, because the pin rear incomplete thread portion 17 is formed at the end of the male thread portion 13 on the pipe portion 50 side, the wall thickness of the first steel pipe 10, that is, the thickness from the root of the male thread to the inner circumferential surface 51, can be increased at the end of the engagement of the male thread portion 13 (W1 shown in Figure 3 can be thicker than W2 shown in Figure 4). As a result, the steel pipe joint structure 100 can increase its critical cross-sectional area and its strength against tensile load after tightening. In addition, the steel pipe joint structure 100 can secure sufficient abutting surfaces between the inner shoulder portion 96 and the outer shoulder portion 95, so the tightening torque can be increased. Consequently, the airtightness and pressure resistance of the steel pipe joint structure 100 are improved.

[0030] (Second steel pipe 30) Figure 5 is a close-up view of the box 31 of the second steel pipe 30 according to Embodiment 1. The box 31 of the second steel pipe 30 has the front box portion 32, the female thread portion 33, and the rear box portion 38 positioned in the direction of the pipe axis C, in that order from the tip.

[0031] The front part 32 of the box is located at the very tip of the box 31 and includes the front shoulder surface 42 of the second steel pipe 30 and a non-threaded portion 44 whose inner circumference is cylindrical and does not have a female thread. The front part 32 of the box is the portion indicated by section B6 in Figure 5. Note that the non-threaded portion 44 is not required, in which case a female thread is formed on the front part 32 of the box.

[0032] The female thread portion 33 comprises, in order from the tip side, a box front parallel thread portion 34 (box side run-in portion), a box front incomplete tapered thread portion 35, a box complete thread portion 36, and a box rear incomplete thread portion 37 (box side run-out portion). The box front parallel thread portion 34 is the end portion on the box rear 38 side and is shown as section B1 in Figure 5. It is a tapered surface whose inner circumference decreases in diameter as it moves from the tip surface toward the pipe portion 50 side, and is formed so that the effective diameter of the female thread is a predetermined value. The root diameter of the female thread in the box front parallel thread portion 34 is less than or equal to the inner diameter dimension of the non-female thread portion 44 of the box front 32. Furthermore, because the inner circumference of the box front parallel thread portion 34 (the surface formed by the crests of the threads) is a tapered surface, the female thread in the box front parallel thread portion 34 is an incomplete thread.

[0033] The box-front incomplete tapered thread section 35 is the portion shown in section B2 of Figure 5. The portion shown in section B2 has a tapered inner circumference, continuing from the box-front parallel thread section 34. The effective diameter of the female thread formed in the portion shown in section B2 is a tapered thread that decreases in diameter from the tip side toward the pipe section 50 side. However, the inner circumference (thread crest) of the box-front incomplete tapered thread section 35 in section B2 is larger than the inner circumference (thread crest) of the box-front complete thread section 36 shown in section B3, and the formed female thread is an incomplete thread. There is a step 35a between the inner circumference (thread crest) of the box-front incomplete tapered thread section 35 in section B2 and the inner circumference (thread crest) of the box-front complete thread section 36 shown in section B3, resulting in a discontinuous surface. Furthermore, the inner circumference (thread peaks) of the box front incomplete tapered thread section 35 in section B2 is larger than the inner circumference (thread peaks) of the box front complete thread section 36 shown in section B3. Also, the tip end of the box front incomplete tapered thread section 35, i.e., the boundary between section B2 and section B1 shown in Figure 5, is a connection section 41 where the thread changes from parallel to tapered. In the box front incomplete tapered thread section 35, the diameter of the female thread roots also decreases from the tip side toward the pipe section 50 side, so the wall thickness of the steel pipe also gradually increases toward the pipe section 50 side. In addition, a step is formed at the pipe section 50 side end of the box front incomplete tapered thread section 35, and it connects to the box front complete thread section 36. The box front parallel thread section 34 and the box front incomplete tapered thread section 35, which are incomplete thread sections, are collectively referred to as the shoulder side second incomplete thread section.

[0034] The box-type complete thread portion 36 is the portion shown in section B3 of Figure 5. The box-type complete thread portion 36 has tapered surfaces at the crests of the threads so that the female thread is a complete thread. The tapered surface and the effective diameter of the female thread are parallel, and the female thread is a complete thread. The tapered surface on the inner circumference (crest of the threads) of the box-type complete thread portion 36 switches to a cylindrical surface at point 45, which has the same inner diameter dimension as the small diameter portion 39, which is the inner circumference of the rear box portion 38. In Embodiment 1, point 45 has the same inner diameter dimension as the small diameter portion 39, which is the inner circumference of the rear box portion 38. As described above, the box-type complete thread portion 36 is formed in section B3 up to point 45 of the female thread portion 33.

[0035] The box rear incomplete thread portion 37 is the end on the box rear side 38, and is the portion shown as section B4 in Figure 5. Since section B4 is a cylindrical surface with a constant inner diameter, the tapered thread formed therein is an incomplete thread with a gradually decreasing height. The box rear incomplete thread portion 37 is formed up to point 46 where the root diameter of the female thread matches the inner diameter of the inner surface of the box rear 38.

[0036] The rear part of the box 38 may have a large-diameter section 40 with a larger inner diameter at the tip end and a small-diameter section 39 with a smaller inner diameter at the pipe section 50 end. The large-diameter section 40 is formed to have the same inner diameter as the incomplete threaded section 37 of the rear part of the box and is the part where the tapered thread is cut off.

[0037] Sections B1 to B4 shown in Figure 5 are collectively referred to as the female thread section 33. Sections B2 to B4 are referred to as the tapered thread section 48. The female thread section 33 is formed by combining the box front parallel thread section 34 and the tapered thread section 48. With this configuration, the box 31 can ensure a large angle between the tapered thread section 48 and the pipe axis C, while also ensuring the length of the female thread section 33 and the wall thickness of the box front section 32 and box rear section 38. In other words, since the box 31 can simultaneously ensure sufficient wall thickness at the tip and the wall thickness of the box rear section 38, the strength and rigidity against tightening torque are improved, and the tightening torque of the steel pipe joint structure 100 can be increased when the pin rear shoulder surface 22 and the box front shoulder surface 42 come into contact, and when the pin front shoulder surface 23 and the box rear shoulder surface 43 come into contact. As a result, the airtightness and pressure resistance of the steel pipe joint structure 100 are improved.

[0038] The female thread portion 33 has an incomplete thread portion 37 at the rear of the box, which is formed at the end of the female thread portion 33 on the pipe portion 50 side. This allows the wall thickness of the second steel pipe 30, i.e., the thickness from the root of the female thread to the outer surface 52, to be increased at the engagement end of the female thread portion 33. As a result, the steel pipe joint structure 100 can increase the critical cross-sectional area and thus increase its strength against tensile load after tightening. Furthermore, the steel pipe joint structure 100 can ensure sufficient abutting surfaces between the inner shoulder portion 96 and the outer shoulder portion 95, allowing for a higher tightening torque. Consequently, the airtightness and pressure resistance of the steel pipe joint structure 100 are improved.

[0039] Figure 6 is an explanatory diagram of a steel pipe joint structure 1200 according to a comparative example. In order to avoid the condition in which the wall thickness of the critical cross section shown in Figure 4 becomes thin, it is also possible to use a box 1031 with an enlarged tip 1030 and a pin 1011 with a reduced tip 1011a, as shown in Figure 6. In the box 1031 and pin 1011 according to the comparative example, tapered threads are provided from the tip 1031a and 1011a to the pipe section 50 by changing the diameters of the tip 1031a and 1011a. As a result, in the steel pipe joint structure 1200 according to the comparative example, the engagement between the female thread and the male thread can be ensured from the tip to the pipe section. Furthermore, the tip of the female thread is pre-enlarged and the tip of the male thread is pre-reduced to form a tapered thread. However, the steel pipe joint structure 1200 in the comparative example has enlarged or reduced pipe ends 1010a and 1030, so it is not possible to sufficiently provide a structure in which the pin rear shoulder surface 22 and the box front shoulder surface 42 come into contact, and a structure in which the pin front shoulder surface 23 and the box rear shoulder surface 43 come into contact, as in the steel pipe joint structure 100 in Embodiment 1, and therefore a sufficient metal seal cannot be achieved. Accordingly, the steel pipe joint structure 1200 in the comparative example has problems with sealing performance.

[0040] (Regarding the screw engagement mechanism) Figure 7 is a cross-sectional view showing an example of complete thread engagement. Figure 7(a) shows the thread engagement state in the Root to Crest Contact method, where the root and crest of the thread are in contact, and Figure 7(b) shows the thread engagement state in the Flank to Flank Contact method, where the stubing flank and load flank are in contact. As shown in Figure 7(a), there are two types of thread engagement: the Root to Crest Contact method, where the crest of the box-side thread is in contact with the root of the pin-side thread, and the Flank to Flank Contact method, where the slopes of the box and pin threads are in contact with each other. In Embodiment 1, a Flank to Flank Contact method of engagement is adopted, which is advantageous in terms of the sealing performance and compression performance of the steel pipe joint structure 100.

[0041] In the steel pipe joint structure 100 according to Embodiment 1, since a Flank-to-Flank Contact method is employed, the cross-sectional shape of the thread is a trapezoidal or triangular thread in which the stubing flank and load flank are inclined with respect to a direction perpendicular to the pipe axis C of the thread. Note that a square thread in which the flank angle is perpendicular to the pipe axis C cannot be used for Flank-to-Flank Contact.

[0042] (Fully threaded section) Figure 8 is a cross-sectional view showing the state of engagement of the complete threads in section A of Figures 2 and 5. In the engagement of complete threads with the same effective diameter, such as the pin complete thread section 16 and the box complete thread section 36, the male thread side load surface 84 and the female thread side load surface 86, and the male thread side stubbling surface 85 and the female thread side stubbling surface 87 engage properly. In the steel pipe joint structure 100 according to Embodiment 1, a Flank to Flank Contact method is adopted, and the thread shape in the cross section including the pipe axis C is such that the load flank angle θ1 and stubbling flank angle θ2 of the male and female threads are inclined from a direction perpendicular to the pipe axis C.

[0043] (Incomplete thread section) Figure 9 is a cross-sectional view showing an example of the meshing state of incomplete threads in section B of Figures 2 and 5. Note that in Figure 9, the pin front incomplete tapered thread section 15, which is a tapered thread with a lowered thread due to crest milling, is omitted from the display. Section B of Figures 2 and 5 consists of incomplete thread sections for both the male and female threads. On the pin 11 side, it is the pin front parallel thread section 14, and on the box 31 side, it is the box rear incomplete thread section 37. The pin front parallel thread section 14, located on the tip side of the pin 11, is both a parallel and incomplete thread. In contrast, the box rear incomplete thread section 37, located on the pipe section 50 side of the box 31, is both a tapered and incomplete thread. The female thread on the box 31 side has an effective diameter E4 inclined with respect to the pipe axis C, while the male thread on the pin side has an effective diameter E2 parallel to the pipe axis C. Due to this misalignment between the effective diameters E4 and E2, the male and female threads are misaligned relative to each other on the stubing surface and the load surface. Therefore, in Embodiment 1, as shown in Figure 9, the pin front parallel thread portion 14 of the male thread portion 13 is machined so that the load surface 54 on the male thread side aligns with the load surface 56 on the female thread side. This machining will be described later. Also, the stubbling surface 55 on the male thread side is machined so that it aligns with the stubbling surface 57 on the female thread side. In other words, the threads 71 ​​of the pin front parallel thread portion 14 of the pin 11 in Embodiment 1 are machined to be narrower in width than the complete threads 76 of the complete thread portion 16 of the pin formed in the center of the male thread portion 13 formed on the pin 11. Specifically, the width of the threads 71 ​​of the pin front parallel thread portion 14 on the effective diameter E2 is smaller than the width of the complete threads 76 of the complete thread portion 16 of the pin on the effective diameter E1 (see Figures 5 and 8). Note that in Figure 9, the load surface 54 on the male thread side coincides with the load surface 56 on the female thread side. Similarly, the stubbling surface 55 on the male thread side is machined to coincide with the stubbling surface 57 on the female thread side.

[0044] In other words, the bottom of the valley 74 of the pin front parallel thread portion 14 of the pin 11 in Embodiment 1 is machined to be wider than the bottom of the valley 83 of the pin full thread portion 16 formed at the center of the male thread formed on the pin 11. For example, the width r1 of the bottom of the valley 74 of the pin front parallel thread portion 14 is wider than the width r0 of the bottom of the valley 83 of the pin full thread portion 16. Also, the bottoms r2 to r4 of the other pin front parallel thread portions 14 are similarly wider than the width r0 of the bottom of the valley 83 of the pin full thread portion 16. Furthermore, the widths r1 to r4 of the bottoms of the valleys 74 of the pin front parallel thread portion 14 become wider toward the tip side of the pin 11. That is, the relationship of the widths of the bottoms of the valleys 74 is r1 < r2 < r3 < r4. By being formed in this way, the thread 61 of the box rear incomplete thread portion 37 can be screwed into the valley of the pin front parallel thread portion 14.

[0045] In addition, in FIG. 9, the box rear incomplete thread portion 37 is a portion where a tapered thread is cut in the cylindrical portion of the box 31. Therefore, the width of the bottom of the valley 64 of the box rear incomplete thread portion 37 is the same as the width of the bottom of the valley 89 of the box full thread portion 36.

[0046] By being formed in this way, the pin front parallel thread portion 14 of the pin 11 and the box rear incomplete thread portion 37 of the box 31 are screwed together up to the vicinity of the pin front shoulder surfaces 23 and 43. Therefore, not only can a sufficient meshing length of the threads be ensured, but also the thickness of the dangerous cross-section of the pin 11 and the box 31 can be ensured sufficiently. As a result, the steel pipe joint structure 100 can sufficiently withstand the tensile load applied after tightening. Also, the steel pipe joint structure 100 can increase the tightening torque. Consequently, the steel pipe joint structure 100 has improved sealing performance and pressure resistance.

[0047] In Embodiment 1, as shown in Figure 9, the region from the tip of the pin 11 to the connecting portion 21 is a thread-thinning region where the width of the thread 71 on the effective diameter E2 is smaller than the width of the complete thread 76 of the complete thread portion 16 of the pin. However, the thread-thinning region may extend beyond the connecting portion 21 to the rear portion 18 of the pin. In other words, thread thinning may be applied to at least a part of the pin front incomplete tapered thread portion 15, which is the tapered thread portion shown in Figure 2, or to a part of the pin complete thread portion 16. In this case, at least a part of the pin front incomplete tapered thread portion 15, shown as section P2 in Figure 2, or a part of the pin complete thread portion 16, shown as section P3 in Figure 2, is machined so that the width of the thread 71 on the effective diameter E1 is smaller than the width of the complete thread 76 of the pin complete thread portion 16. In other words, in Embodiment 1, if all or part of section P2 in Figure 2 is subjected to thread thinning and becomes an incomplete thread, then part of section P3 in Figure 2 may also be subjected to thread thinning and become an incomplete thread. With this configuration, the thread thinning area of ​​the male thread portion 13 can be adjusted to correspond to the length of the box rear incomplete thread portion 37 (section B4) of the box 31 that engages with the tip-side area of ​​the male thread portion 13 of the pin 11. Therefore, the threads 71 ​​of the tip-side area of ​​the male thread portion 13 of the pin 11 can avoid interference with the threads 61 of the box rear incomplete thread portion 37 of the box 31.

[0048] Furthermore, in Figure 9, the front parallel thread portion 14 of the pin is top-milled so that the crest portion 73 of the thread 71 does not come into contact with the root 64 of the rear incomplete thread portion 37 of the box. The top-milling process is performed on the area in the rough material shape where the front parallel thread portion 14 of the pin is provided, before threading the pin 11. The top-milling process will be described later.

[0049] The box rear incomplete thread portion 37, which engages with the pin front parallel thread portion 14, is a tapered thread formed continuously with the box complete thread portion 36, and therefore the root diameter decreases as it approaches the box rear 38 side, i.e., the pipe portion 50 side. The root 64 of the box rear incomplete thread portion 37 lies on the straight line L1 shown in Figure 9, and is on the same straight line L1 as the root 89 of the box complete thread portion 36. The crest portion 73 of the pin front parallel thread portion 14 obtained by crest milling is positioned so as not to interfere with the root 64 of the tapered thread box rear incomplete thread portion 37. The roots 74 of the pin front parallel thread portion 14 are each located on a cylindrical surface (formed on the same outer diameter) and are configured so as not to interfere with the crest portion 63 of the threads 61 of the box rear incomplete thread portion 37, which are also located on a cylindrical surface. The parallel threaded portion 14 at the front of the pin is formed by cylindrical machining, so that the root of the thread 74 has a single outer diameter (see the portion to the right of the connection portion 21 of L4 shown in Figure 9).

[0050] As described above, by adjusting the width of the threads 71 ​​of the parallel thread portion 14 at the front of the pin and the incomplete thread portion 37 at the rear of the box, interference between the threads 61 of the female thread and the threads 71 ​​of the male thread can be avoided. Furthermore, interference between the peaks 73 of the male thread and the roots 64 of the female thread can also be avoided. In Embodiment 1, the thread shape of the parallel thread portion 14 at the front of the pin and the incomplete thread portion 37 at the rear of the box is, for example, set to a stubing flank angle θ2 of 45° and a load flank angle θ1 of 2°.

[0051] Furthermore, the relationship between the box rear incomplete thread portion 37 and the pin front parallel thread portion 14 described above also applies to the engagement of the incomplete thread portions of the box front parallel thread portion 34 and the pin rear incomplete thread portion 17. The engagement of the box front parallel thread portion 34 and the pin rear incomplete thread portion 17 is the engagement of the incomplete threads in section C of Figures 2 and 5. Therefore, the parallel thread shown on the lower side of Figure 9 corresponds to the box front parallel thread portion 34, and the tapered thread shown on the upper side corresponds to the pin rear incomplete thread portion 17. In this case, the box front parallel thread portion 34, which is a female thread, is machined so that the load surface 56 on the female thread side moves to match the load surface 54 on the male thread side. Also, the stubbling surface 57 on the female thread side is machined to move to match the stubbling surface 55 on the male thread side. In other words, the parallel threaded portion 34 at the front of the box 31 in Embodiment 1 is machined to have a width smaller than the complete threads 66 of the complete threaded portion 36 of the box, which is formed in the center of the female thread formed in the direction of the pipe axis C of the box 31. Specifically, the width of the threads 71 ​​of the parallel threaded portion 34 at the front of the box on the effective diameter E3 (see Figure 5) is smaller than the width of the complete threads 66 of the box on the effective diameter E4 (see Figure 5) of the complete threaded portion 36 of the box.

[0052] Furthermore, the thread-reducing region of the female thread portion 33 of the box 31 may extend beyond the connecting portion 41 (see Figure 5) to the rear portion 38 of the box. In other words, thread reduction may be applied to at least a portion of the incomplete tapered thread portion 35 at the front of the box, which is the tapered thread portion shown in Figure 5, or to a portion of the complete thread portion 36 of the box. In this case, at least a portion of the incomplete tapered thread portion 35 at the front of the box, shown as section B2 in Figure 5, or a portion of the complete thread portion 36 at the box, shown as section B3 in Figure 5, is machined such that the width of the thread 71 on the effective diameter E4 is smaller than the width of the complete thread 76 of the complete thread portion 46 of the box. In other words, in Embodiment 1, it is possible that all or part of section B2 in Figure 5 becomes an incomplete thread with reduced threads, and that part of section B3 in Figure 5 becomes an incomplete thread with reduced threads. With this configuration, the area of ​​thread reduction in the female thread portion 33 can be adjusted to correspond to the length of the incomplete thread portion 17 at the rear of the pin 11 that engages with the tip-side area of ​​the female thread portion 33 of the box 31. Therefore, the threads 71 ​​in the tip-side area of ​​the female thread portion 33 of the box 31 can avoid interference with the threads 61 of the incomplete thread portion 17 at the rear of the pin 11.

[0053] Figure 10 is an explanatory diagram of the meshing state near the connection portion 21 between the complete thread portion and the incomplete thread portion of the steel pipe joint structure 1000 according to the comparative example. Figure 10 is an enlarged view of the area corresponding to the vicinity of the connection portion 21 in Figure 2 and the connection portion 45 in Figure 4. In other words, it shows the vicinity of the boundary between the complete thread portion and the incomplete thread portion at the front of the pin 11 and the vicinity of the boundary between the complete thread portion and the incomplete thread portion at the rear of the box 31. In the steel pipe joint structure 1000 according to the comparative example, thread thinning is not applied to the parallel thread portion 14 at the front of the pin. Therefore, the thread relationship between the parallel thread portion 14 at the front of the pin and the incomplete thread portion 37 at the rear of the box, as shown in Figure 10, is such that they do not mesh.

[0054] Figure 11 is an explanatory diagram of the machining of the threads around the connection portion 21 of the pin 11 according to Embodiment 1 and the comparative example. The male thread portion 13 of the pin 11 according to Embodiment 1 is formed by thread turning using a threading insert 90. The arrows K1 and K2 shown in Figure 11 indicate the trajectory of the threading insert 90 during thread turning. The threading insert 90 has a cutting edge 91 that has the same shape as the cross-sectional shape of the root of the thread, and moves along arrows K1 and K2 to form threads on the pin 11. In Figure 11, threading insert 90a shows the threading insert 90 moving along arrow K1, and threading insert 90b shows the threading insert 90 moving along arrow K2.

[0055] As shown in Figure 11, the threading insert 90 moves parallel to the pipe axis C, as indicated by arrow K1, up to the connection portion 21, which is the boundary between the pin-front parallel thread portion 14 and the tapered thread portion 28 of the male thread portion 13. This causes the threading insert 90 to form a parallel thread. From the connection portion 21 onward, the threading insert 90 moves at an angle to the pipe axis C, as indicated by arrow K2. This causes the threading insert 90 to form a tapered thread. The pin-front parallel thread portion 14 corresponds to section P1 shown in Figure 2. The tapered thread portion 28 corresponds to sections P2 to P4 shown in Figure 2. By forming the pin-front parallel thread portion 14 at the tip of the pin 11, the male thread portion 13 can be made longer without reducing the wall thickness.

[0056] In Figure 11, the width of the thread root 74 of the pin front parallel thread portion 14 (the portion to the right of the connection portion 21 in Figure 11) is the same as the width of the thread root 83 of the pin complete thread portion 16 (the portion to the left of the connection portion 21 in Figure 11). By performing thread thinning on the pin 11 in Figure 11, a thread shape can be obtained in which the width of the thread root 74 is wider than the width of the thread root 83 of the pin complete thread portion 16, as shown in Figure 9 and other figures.

[0057] Figure 12 is an explanatory diagram of an example of machining using the threading insert 90 according to Embodiment 1 and the Comparative Example. Machining with the threading insert 90 shown in Figure 11 is performed in multiple passes, taking into consideration the lifespan of the threading insert 90 and the accuracy (appearance and dimensions) of the thread after machining. For example, the thread shown in Figure 12 shows a case where the final thread shape is formed in 5 passes. In Figure 12, the final pass has a smaller material removal allowance compared to the other passes to improve accuracy.

[0058] As shown in Figure 10, the male threaded portion 13 and female threaded portion 33 of the comparative example steel pipe joint structure 1000 have an inclined loading surface and a stubing surface. Therefore, as shown in W1 to W5 in Figure 10, when a normal thread cutting process is performed on the incomplete threaded portion of the male threaded portion 13, the width of the thread crests increases as the height of the thread decreases. This is because, in the explanatory diagram of the processing shown in Figure 11, by performing the processing with the initial position of the thread cutting insert 90 set to S1, the shape of the cutting edge 91 of the thread cutting insert 90 is transferred to the male threaded portion 13. If the processing is performed with the initial position of the thread cutting insert 90 remaining at S1, the widths W1 to W5 of the thread crests 73 of the incomplete threaded portion of the pin 11 become larger than the width X of the root of the incomplete threaded portion 64 of the box rear incomplete threaded portion 37. Therefore, the pin front parallel threaded portion 14 of the comparative example steel pipe joint structure 1000 cannot be screwed into the box rear incomplete threaded portion 37.

[0059] Figure 13 is an explanatory diagram illustrating the relationship between the thread sizes of the pin front parallel thread portion 14 and the box rear incomplete thread portion 37 of the steel pipe joint structure 1000 according to the comparative example in Figure 10. In the case of the male thread portion 13 of the steel pipe joint structure 1000 according to the comparative example, as shown in Figure 13, the width of the thread peaks W1 to W5 of the pin front parallel thread portion 14 is larger than the width of the root 64 of the box rear incomplete thread portion 37. As a result, the load flanks of the male and female threads interfere with each other by the width indicated by R in Figure 13, and the stubbling flanks interfere with each other by the width indicated by Q in Figure 13. The pin front parallel thread portion 14 of the steel pipe joint structure 100 according to Embodiment 1 undergoes the thread thinning process described below in order to avoid interference between the load flank and stubbling flank shown in Figure 13.

[0060] Figure 14 is an explanatory diagram of an example of the machining of the male thread portion 13 according to Embodiment 1. The parallel thread portion 14 at the front of the pin of the steel pipe joint structure 100 according to Embodiment 1 is subjected to thread thinning machining so that the load surface 54 and stubing surface 55 match the load surface 56 and stubing surface 57 of the incomplete thread portion 37 at the rear of the box. For example, thread thinning machining is performed by making three passes to thin the stubing surface 55 and one pass to thin the load surface 54 to thin the threads.

[0061] The topmost path shown in Figure 14 represents the normal path, which is the path used to form the threads before thread thinning. In the normal path, the threading insert 90 starts moving from the reference position S1 and moves so that when the tool feed is expressed in pitch, f2 = 0.2″ (for a thread with 5 threads per inch).

[0062] The second pass from the top in Figure 14 is the first thread thinning pass on the stubbling surface 55 side. In the first thread thinning pass, the thread cutting insert 90 starts moving from S2, which is closer to the pin 11 than S1, and moves so that the tool feed pitch is f3 = 0.1975″. In the first thread thinning pass, the starting position S2 of the thread cutting insert 90 has moved towards the pin 11, so the thread cutting insert 90 cuts in a direction that thins the stubbling surface 55 of the thread.

[0063] The third and fourth passes from the top in Figure 14 are the second and third thread thinning passes, respectively. In the second thread thinning pass, the starting position of the thread cutting insert 90 is further towards the pin 11 at S3 than in the first thread thinning pass, and in the third thread thinning pass, it is further towards the pin 11 at S4 than in the second thread thinning pass. In the second thread thinning pass, the tool feed pitch is f4 = 0.195″, and in the third thread thinning pass, the tool feed pitch is f5 = 0.1925″. In other words, in the thread thinning passes, the stubbling surface 55 is cut by gradually moving the starting position of the thread cutting insert 90 closer to the pin 11. In addition, the tool feed pitch is reduced (feed speed is slowed) by the amount that the starting position of the thread cutting insert 90 is closer to the pin 11, so that the amount of material removed decreases as the thread approaches the connection part 21.

[0064] The pass shown at the bottom of Figure 14 is the thread-reducing pass on the load surface 54 side. In the thread-reducing pass on the load surface 54 side, the starting position of the threading insert 90 is moved away from the pin 11, and the feed pitch is set to f6 = 0.2015″, which is a slightly faster feed rate. This causes the load surface 54 to be cut in a direction that reduces its thickness.

[0065] In Figure 14, the normal pass, the third thread-reducing pass on the stubing surface 55 side (the final thread-reducing pass on the stubing surface 55 side), and the thread-reducing pass on the loading surface 54 side (the final thread-reducing pass on the loading surface 54 side) are performed by feeding the tool at the connection portion 21 with a normal feed pitch f=0.2″, and the complete thread portion is machined continuously. In this way, by continuously machining the complete thread portion in the thread-reducing processes on both the loading surface 54 side and the stubing surface 55 side, the incomplete thread portions, namely the pin front parallel thread portion 14, the pin front incomplete tapered thread portion 15, and the pin complete thread portion 16, are machined to connect smoothly. Note that the threading insert 90 is complete. When the thread reaches the connection portion 21, which is the boundary with the threaded portion, the feed rate is returned to normal, so the shape of the threads in the complete threaded portion is not damaged. The connection portion 21 is the position where the incomplete parallel thread and the complete tapered thread meet, and is shown by line C1 in Figure 14. However, the thread thinning process applied to the parallel threaded portion 14 at the front of the pin may extend beyond the connection portion 21 to the region of the complete thread (tapered threaded portions 15, 16) shown in Figure 14, corresponding to the length of the incomplete threaded portion 37 at the rear of the box to which it fits. In other words, in the male threaded portion 13 according to Embodiment 1, the thread thinning process may extend to the region of section P2 or section P3 in Figure 2.

[0066] As described above, the male threaded portion 13 formed at the end of the steel pipe is machined by the threading insert 90. First, the male threaded portion 13 is machined by moving the threading insert 90 from the initial position S1 at a first feed rate, which is a feed pitch f2 = 0.2, and the pin front parallel threaded portion 14 and the tapered threaded portions 15, 16, and 17 are machined in succession. This is called a normal machining pass. A normal machining pass may consist of multiple normal machining passes. In Embodiment 1, the 1st pass to the final pass in Figure 12 and the 1st pass to the 5th pass in Figure 15 correspond to multiple normal machining passes.

[0067] Next, the thread cutting insert 90 starts moving from a machining start position closer to the steel pipe than the initial position on the male thread portion 13, and at least the pin front parallel thread portion 14 is machined at a second feed rate slower than the first feed rate. This is called the first thread thinning machining pass. The first thread thinning machining pass may consist of multiple first thread thinning machining passes starting from multiple positions closer to the steel pipe than the initial position. In this case, the second feed rate includes multiple second feed rates slower than the first feed rate. In Embodiment 1, thread thinning pass 1 starting from S2 and thread thinning pass 2 starting from S3 in Figure 14 correspond to multiple first thread thinning machining passes. The first thread thinning machining pass is a machining pass for cutting the stubbling flank of the pin front parallel thread portion 14. However, the first thread reduction machining pass applied to the parallel thread portion 14 at the front of the pin may extend beyond the connection portion 21 to the complete thread region (tapered thread portions 15, 16) shown in Figure 14, corresponding to the length of the incomplete thread portion 37 at the rear of the box to which it fits. In other words, the region machined at the second feed rate in the first thread reduction machining pass may extend beyond the connection portion 21.

[0068] Next, the male thread portion 13 starts moving from a machining start position closer to the steel pipe than its initial position, and the pin front parallel thread portion 14 is machined at a second feed rate slower than the first feed rate, and the tapered thread portions 15, 16 and 17 are machined continuously at the first feed rate. This is called the second thread thinning machining pass. In Embodiment 1, the thread thinning machining pass 3 starting from S4 in Figure 14 corresponds to the second thread thinning machining pass. The second thread thinning machining pass is intended to connect the threads at the connection portion 21 with a smooth surface by continuously machining the pin front parallel thread portion 14 and the tapered thread portions 15, 16 and 17 that are subjected to thread thinning. The second thread thinning machining pass is a machining pass for cutting the stubbling flank of the pin front parallel thread portion 14 and the tapered thread portions 15, 16 and 17. However, the second thread reduction machining pass applied to the parallel thread portion 14 at the front of the pin may extend beyond the connection portion 21 to the complete thread region (tapered thread portions 15, 16) shown in Figure 14, corresponding to the length of the incomplete thread portion 37 at the rear of the box to which it fits. In other words, the region machined at the second feed rate in the second thread reduction machining pass may extend beyond the connection portion 21.

[0069] Next, the male thread portion 13 starts moving from a machining start position further from the steel pipe than the initial position, and the pin front parallel thread portion 14 is machined at a third feed rate faster than the first feed rate, and the tapered thread portions 15, 16 and 17 are machined continuously at the first feed rate. This is called the third thread thinning machining pass. In Embodiment 1, the third thread thinning machining pass is the thread thinning pass 4 starting from S5 in Figure 14, and is for machining the load flank. The third thread thinning machining pass is also used to connect the threads at the connection portion 21 with a smooth surface by continuously machining the pin front parallel thread portion 14 and the tapered thread portions 15, 16 and 17 that are subjected to thread thinning. Furthermore, depending on the amount of material removed from the load flank, the third thread reduction machining pass may be configured to machine only the parallel thread portion 14 at the front of the pin multiple times, and finally to machine the parallel thread portion 14 at the front of the pin and the tapered thread portions 15, 16, and 17 continuously. However, the third thread reduction machining pass applied to the parallel thread portion 14 at the front of the pin may extend beyond the connection portion 21 to the complete thread region (tapered thread portions 15, 16) shown in Figure 14, corresponding to the length of the incomplete thread portion 37 at the rear of the box to which it fits. In other words, in the third thread reduction machining pass, the region machined at the third feed rate may extend beyond the connection portion 21.

[0070] The above describes the machining of the male thread portion 13, but the female thread portion 33 is machined in the same way. In machining the female thread portion 33, the box front parallel thread portion 34 and the tapered thread portions 35, 36, and 37 are machined using the above-mentioned normal machining pass, first thread thinning machining pass, second thread thinning machining pass, and third thread thinning machining pass.

[0071] Figure 15 shows the machining process using all machining passes, focusing on one of the valleys in the parallel thread section 14 at the front of the pin shown in Figure 14. In Figure 15, we focus on the valley of the parallel thread section 14 at the front of the pin where the width of the valley bottom 74 is r2 as a representative example, but other valleys are basically machined using similar machining passes, only the amount of material removed changes. The 1st to 5th passes shown in Figure 15 correspond to the normal passes in Figure 14. The normal passes shown in Figure 14 cut the rough material in five passes, for example, as shown in Figures 12 and 15. Thread thinning is performed after the normal threads have been formed. In the normal passes, the 1st to 5th passes, the valley bottom 74 is machined to the same width r0 as the normal valley bottom 83 (the valley bottom 83 in the complete thread section). Subsequently, through the thread thinning passes, the 6th to 9th passes, the valley bottom 74 is machined to a width r2, which is larger than the width r0. Similarly, valleys other than those shown in Figure 15 are also processed by mountain thinning to widths r1 to r4, which are larger than width r0.

[0072] Figure 16 is an explanatory diagram of the rough material processing performed before threading according to Embodiment 1. In Embodiment 1, if the parallel thread portion 14 at the front of the pin 11 is formed by normal threading, the crest portion 73 of the thread is at position L3 in Figure 9, and the crest portion 73a of the thread at the front of the pin parallel thread portion 14 interferes with the root of the thread 64 of the incomplete thread portion 37 at the rear of the box. For this reason, for example, the corner portion 76a of the crest portion 73a of the parallel thread portion 14 at the front of the pin may be set to have a larger corner radius than the corner radius of the root of the thread so as not to interfere with the root of the thread 64 of the incomplete thread portion 37 at the rear of the box. However, in order to enlarge the corner radius of the crest portion 73a in this way, a special tool is required to process the crest portion 73a of the thread. Therefore, in Embodiment 1, the rough material processing shown in Figure 16 is performed before threading so that the crest portion 73a of the thread of the parallel thread portion 14 at the front of the pin shown in Figure 16(a) is in the position shown in Figure 16(b), thereby preventing interference between the crest portion 73 and the root portion 64 of the thread.

[0073] The rough material of the pin 11 is machined so that its outer diameter is smaller than the tapered surface L3 (see Figures 9 and 10) formed by the crest of the threads of the complete thread portion 16 of the pin, at least in the area where the parallel thread portion 14 at the front of the pin is provided. The trajectory m shown in Figure 16 moves outward at position M, which is the end of the incomplete thread portion of the pin 11, leaving a finishing allowance for thread machining on the threads of the complete thread portion. In other words, the trajectory m shown in Figures 16(a) and (b) represents the outer shape of the rough material of the pin 11 before the threads are machined.

[0074] In other words, in a cross-section including the pipe axis C, the outer diameter of the crest 73 of the front parallel thread portion 14 of the pin is smaller than the imaginary line L3 (see Figures 10 and 11) where the crest 82 of the thread of the complete thread portion 16 of the pin is located. As shown in Figure 11, the crest 73 of the thread 71 is lower than the imaginary line L3a, and the crest 82 of the complete thread 76 coincides with the imaginary line L3b. Also, as shown in Figure 2, the pin 11 has a step 15a formed on the outer circumferential surface of the male thread portion 13 on the pin rear 18 side than the front parallel thread portion 14 of the pin.

[0075] Furthermore, the position M shown in Figure 16 can be moved to the rear 18 side of the pin beyond the front parallel thread portion 14. For example, as shown in Figure 2, if the front incomplete tapered thread portion 15 is formed following the front parallel thread portion 14, the position M can be moved to the position where the tapered thread portion of the pin 11 is formed.

[0076] Furthermore, while the above description focuses on the threads of pin 11, the parallel threaded portion 34 at the front of the box is also subjected to similar thread thinning and roughening processes. In other words, the thread engagement in section C of Figures 2 and 5 is the same as in section B, as the female thread has been subjected to thread thinning and roughening processes. To put it another way, the width of the root of the thread 74 in the parallel threaded portion 34 at the front of the box is greater than the width of the root of the thread 89 in the complete threaded portion 36 of the box. Also, the outer diameter of the crest 73 in the parallel threaded portion 34 at the front of the box is greater than the outer diameter of the crest 82 in the complete threaded portion 36 of the box. In addition, a step 35a is formed on the inner surface of the female threaded portion 33 on the rear 38 side of the box 31, relative to the parallel threaded portion 34 at the front of the box.

[0077] (modified version) Figure 17 is a cross-sectional view showing an example of the meshing state of incomplete threads in section B of Figures 2 and 5. In the above description, when meshing the pin front parallel thread portion 14 with the box rear incomplete thread portion 37 or the box front parallel thread portion 34 with the pin rear incomplete thread portion 17, the width of the threads of the pin front parallel thread portion 14 or the box front parallel thread portion 34 is machined to be smaller. In contrast, Figure 17 shows an example where the width of the threads of the box rear incomplete thread portion 37 or the pin rear incomplete thread portion 17 is machined to be smaller. In this case, the female thread of the box rear incomplete thread portion 37 is machined so that the load surface 56a on the female thread side matches the load surface 54a on the male thread side, becoming load surface 56b. Also, the stubbling surface 57a on the female thread side is machined so that it matches the stubbling surface 55a on the male thread side, becoming stubbling surface 57b. In other words, the incomplete threaded portion 37 at the rear of the box 31 in Embodiment 1 is machined to have a smaller width than the threads of the complete threaded portion 36 formed in the center of the female thread formed in the box 31. Specifically, the width of the threads 61 of the incomplete threaded portion 37 at the effective diameter E4 (see Figure 5) is smaller than the width of the complete threads 66 on the effective diameter E4 (see Figure 5) of the complete threaded portion 36. Furthermore, the crest portion 73 of the threads 71 ​​of the pin 11 should be machined as shown in Figure 16 above to avoid interference with the root of the female thread 64 of the box 31.

[0078] Furthermore, the relationship between the box rear incomplete thread portion 37 and the pin front parallel thread portion 14 shown in Figure 17 also holds true for the interlocking of the incomplete threads between the box front parallel thread portion 34 and the pin rear incomplete thread portion 17. [Industrial applicability]

[0079] According to the present invention, a sufficient critical cross-sectional area can be secured without increasing the outer diameter or wall thickness of the steel pipe joint structure, thereby improving its strength against tensile loads. Furthermore, the first and second steel pipes constituting the steel pipe joint structure do not require processes such as narrowing or expanding the opening before threading. In addition, the steel pipe joint structure can obtain a high tightening torque, improving its sealing and pressure resistance, and making it widely applicable as a joint structure. [Explanation of symbols]

[0080] 10 1st steel pipe 11 pins 12-pin front 13 Male threaded section 14 Pin front parallel thread section 15. Pin front incomplete tapered thread section (tapered thread section) 15a Step 16 Pins, fully threaded section (tapered thread section) 17. Incomplete threaded portion at the rear of the pin 18-pin rear 19 Large diameter section 20 Small diameter section 21 Connection part 22 Pin rear shoulder surface 23 Pin front shoulder surface 24 Non-male threaded portion 28 Tapered thread section 30 Second steel pipe 31 boxes 32 Front of the box 33 Female thread section 34 Box front parallel thread section 35 Box front incomplete tapered thread section (tapered thread section) 35a Step 36 Box-end fully threaded section (tapered thread section) 37 Box rear incomplete thread section 38 Rear of the box 39 Small diameter section 40 Large diameter section 41 Connection part 42 Box front shoulder surface 43 Rear shoulder surface of the box 44 Non-female threaded section 45 Connection part 46 Box Threaded Section 48 Tapered thread section 50 Pipe section 51 Inner surface 52 Outer surface 54 Road surface 54a Road surface 55 stubbling surface 55a Stubbling surface 56 Road surface 56a Load surface 56b Load surface 57 Stabbing surface 57a Stubbling surface 57b Stubbling surface 61 threads 63 Mountaintop 64 Valley bottom 71 threads 73 Mountaintop 73a Summit 74 Valley Bottom 76 Fully threaded 76a Corner 82 Mountaintop 83 Valley Bottom 84 Male thread side load surface 85 Male thread side stubbling surface 86 Female thread side load surface 87 Female thread side stubbling surface 89 Valley Bottom 90 Threading Inserts 90a threading insert 90b threading insert 91 blades 100 Steel pipe joint structure 1000 Steel pipe joint structure 1010 No. 1 steel pipe 1010a tube end 1011 pins 1011a Tip 1013 Male threaded section 1017 Complete threaded section 1030 Tip 1031 Box 1031a Tip 1100 Steel pipe joint structure 1200 Steel pipe joint structure

Claims

1. A method for processing steel pipes, which involves machining a male threaded portion or a female threaded portion at the end of a steel pipe, The male threaded portion or the female threaded portion is The steel pipe has parallel threads and tapered threads formed in order from the tip, Multiple normal machining passes are provided, in which the threading insert is fed from an initial position at a first feed rate to continuously machine the parallel thread portion and the tapered thread portion. A first thread reduction machining pass that processes the stubbling flank of the parallel thread portion at a second feed rate, A second thread reduction machining pass is used to machine the stubbling flank of the parallel thread portion at the second feed rate and the stubbling flank of the tapered thread portion at the first feed rate, The system includes a third thread reduction machining pass that machines the load flank of the parallel thread portion at a third feed rate and the tapered thread portion at a first feed rate, The first and second mountain thinning machining passes are: Processing is started from a position closer to the steel pipe than the initial position. The third mountain thinning processing path is A method for processing steel pipes, wherein the processing is started from a position further away from the steel pipe than the initial position.

2. A method for processing steel pipes, which involves machining a male threaded portion or a female threaded portion at the end of a steel pipe, The male threaded portion or the female threaded portion is The steel pipe has parallel threads and tapered threads formed in order from the tip, Multiple normal machining passes are provided, in which the threading insert is fed from an initial position at a first feed rate to continuously machine the parallel thread portion and the tapered thread portion. A first thread reduction machining pass that processes the stubbling flank of the parallel thread portion and a portion of the tapered thread portion at a second feed rate, A second thread thinning machining pass is used to machine the stubbling flank of the parallel thread portion and a portion of the tapered thread portion at the second feed rate, and to machine the stubbling flank of the tapered thread portion other than the portion mentioned above at the first feed rate, The system includes a third thread reduction machining pass which processes the load flank of the parallel thread portion and the tapered thread portion at a third feed rate, and processes the portion of the tapered thread portion other than the aforementioned part at a first feed rate, The first and second mountain thinning machining passes are: Processing is started from a position closer to the steel pipe than the initial position. The third mountain thinning processing path is A method for processing steel pipes, wherein the processing is started from a position further away from the steel pipe than the initial position.

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