Title of invention: Threaded joint for steel pipe
Technical field
[0001]
　The present invention relates to a threaded joint used for connecting steel pipes.
Background technology
[0002]
　In oil wells, natural gas wells, etc. (hereinafter collectively referred to as "oil wells"), steel pipes called oil well pipes (OCTG: Oil Country Tubular Goods) are used to mine underground resources. The steel pipes are connected in sequence. Threaded joints are used to connect steel pipes.
[0003]
　The types of threaded joints for steel pipes are roughly classified into coupling type and integral type. In the case of a coupling type threaded joint, one of the pair of pipe materials to be connected is a steel pipe and the other pipe material is a coupling. In this case, male threaded portions are formed on the outer circumferences of both end portions of the steel pipe, and female threaded portions are formed on the inner circumferences of both end portions of the coupling. Then, the steel pipe and the coupling are connected. In the case of integral type threaded joints, the pair of pipe materials to be connected are both steel pipes, and separate couplings are not used. In this case, a male threaded portion is formed on the outer circumference of one end of the steel pipe, and a female threaded portion is formed on the inner circumference of the other end. Then, one steel pipe and the other steel pipe are connected.
[0004]
　The joint portion of the pipe end portion on which the male thread portion is formed is referred to as a pin because it includes an element to be inserted into the female thread portion. On the other hand, the joint portion of the pipe end portion on which the female thread portion is formed is called a box because it includes an element that accepts the male thread portion. Both these pins and boxes are tubular because they are the ends of the tube.
[0005]
　The threads of threaded joints for steel pipes (hereinafter, also simply referred to as "threaded joints") are tapered threads. Therefore, the pin includes a tapered male threaded portion as the male threaded portion. The box includes, as a female threaded portion, a tapered female threaded portion that meshes with the tapered male threaded portion. Further, the screw of the screw joint is a trapezoidal screw typified by an API standard buttress screw. In trapezoidal threads, the tapered male threaded portion (hereinafter, also simply referred to as “male threaded portion”) and the tapered female threaded portion (hereinafter, also simply referred to as “female threaded portion”) are the screw top surface, thread bottom surface, load flank surface, and insertion flank, respectively. It includes four faces, and further includes corners or corners such as arcs that connect the faces. The screw top surface, screw bottom surface, load flank surface and insertion flank surface are clearly distinguished by shape and function.
[0006]
　Usually, the pin and the box each include a shoulder part. By screwing the pin into the box, the shoulder part of the pin comes into contact with the shoulder part of the box. When the pin is continuously rotated by a predetermined amount, the fastening of the pin to the box is completed. As a result, a tightening axial force is generated, and the load flank surface of the pin is strongly pressed against the load flank surface of the box. In the state where the fastening is completed (hereinafter, also referred to as "fastened state"), the screw bottom surface of the male screw portion (hereinafter, also referred to as "male screw bottom surface") is the screw top surface of the female screw portion (hereinafter, also referred to as "female screw top surface"). Contact while interfering with. On the other hand, a gap is formed between the screw top surface of the male screw portion (hereinafter, also referred to as “male screw top surface”) and the screw bottom surface of the female screw portion (hereinafter, also referred to as “female screw bottom surface”).
[0007]
　Pins and boxes may each include a sealing surface. In this case, in the fastened state, the sealing surface of the pin comes into contact with the sealing surface of the box while interfering with each other, and a sealing portion is formed by metal contact.
[0008]
　Conventionally, when a pin is screwed into a box and fastened, a grease compound is applied to a threaded portion (male threaded portion and female threaded portion) as a lubricant. In recent years, in order to improve the efficiency of environmental regulations and fastening work, a solid lubricating film is formed in advance on the surface of the threaded portion instead of the grease compound (for example, International Publication WO2007 / 042231 (Patent Document 1), and International Published WO2009 / 072486 (Patent Document 2)).
[0009]
　The solid lubricating film is originally a fluid semi-solid lubricant, and is applied to the surface of the threaded portion by using a brush, a spray, or the like. The applied semi-solid lubricant is cured (eg, cooling, ultraviolet irradiation, etc.) and solidified to form a solid lubricating film.
[0010]
　However, the semi-solid lubricant applied to the threaded portion flows during the period from application to solidification. As a result, the film thickness of the lubricant before solidification becomes non-uniform. Specifically, for example, as disclosed in International Publication WO2015 / 182128 (Patent Document 3), a corner portion connecting the screw top surface and the load flank surface, and a corner portion connecting the screw top surface and the insertion flank surface. Then the film thickness becomes thin. On the screw top surface, the film thickness becomes particularly thick at the central portion thereof. Such non-uniformity of film thickness is maintained even after solidification. Therefore, the film thickness of the solid lubricating film becomes non-uniform.
[0011]
　As described above, a gap is formed between the top surface of the male screw and the bottom surface of the female screw in the fastened state. That is, a gap is formed between the top surface of the male screw and the bottom surface of the female screw even during fastening. In this case, if the solid lubricating film is thick on the top surface of the male screw, the solid lubricating film is peeled off during fastening. The peeled solid lubricant rolls randomly in the gap between the top surface of the male screw and the bottom surface of the female screw. A large amount of solid lubricant rolling through the gap impedes the smooth screwing of the pin into the box. As a result, a fastening defect occurs.
[0012]
　For example, torque chart humping may occur or high shouldering may occur. Threaded joints are usually fastened with torque control. When humping or high shouldering occurs, high torque is generated even though the fastening is in progress. In this case, the fastening is completed in the middle of the fastening. That is, the fastening is completed in a state of insufficient tightening. As a result, a predetermined amount of interference is not introduced into the seal portion and a leak is likely to occur, or a predetermined tightening axial force is not introduced into the screw and the screw is easily loosened. Therefore, the desired sealing performance and joint strength cannot be obtained.
[0013]
　In response to such a problem, in the screw joint of Patent Document 3, a shallow groove is provided in the central portion of the top surface of the male screw. The semi-solid lubricant applied to the top surface of the male screw spreads in the direction of thinning due to the groove. Therefore, on the top surface of the male screw, the film thickness of the semi-solid lubricant becomes uniform without being thickened, and the solid lubricating film also becomes uniform. Therefore, according to the threaded joint of Patent Document 3, it is possible to suppress a fastening defect.
Prior art literature
Patent documents
[0014]
Patent Document 1: International Publication WO2007 / 042231
Patent Document 2: International Publication WO2009 / 072486
Patent Document 3: International Publication WO2015 / 182128
Outline of the invention
Problems to be solved by the invention
[0015]
　In the threaded joint of Patent Document 3, in order to provide a groove on the top surface of the male thread, a special thread cutting tool having a convex shape corresponding to the groove is used. In this case, there is a disadvantage that the life of the tool is short and the tool must be replaced frequently. Further, after machining an ordinary male thread portion without a groove, a tool dedicated to grooving may be used. In this case, there is a disadvantage that the total processing time becomes long. In any case, it is undeniable that the productivity of threaded joints will decline.
[0016]
　An object of the present invention is to provide a threaded joint for steel pipes having a solid lubricating film on the surface of a male threaded portion, which can suppress a fastening defect without impairing the productivity of the threaded joint. It is to be.
Means to solve problems
[0017]
　The threaded joint for steel pipe according to the embodiment of the present invention comprises a tubular pin and a tubular box. The pin includes a tapered male thread portion and a shoulder portion. The tapered male thread portion includes a thread top surface, a thread bottom surface, a load flank surface and an insertion flank surface. The box includes a tapered female thread that meshes with the tapered male thread and a shoulder that corresponds to the shoulder of the pin. The tapered female thread portion includes a thread top surface, a thread bottom surface, a load flank surface and an insertion flank surface. With the shoulder part of the pin in contact with the shoulder part of the box and the fastening is completed, the bottom surface of the thread of the tapered male thread part comes into contact with the thread top surface of the tapered female thread part while interfering with the thread top surface of the tapered male thread part. A gap is formed between the female thread and the bottom of the thread.
[0018]
　In the vertical cross section including the pipe shaft of the threaded joint, the screw top surface of the tapered male threaded portion is connected to the load flank surface of the tapered male threaded portion via the first corner portion formed of an arc. Further, the screw top surface of the tapered male threaded portion is connected to the insertion flank surface of the tapered male threaded portion via the second corner portion formed of an arc. Further, the load flank surface and the insertion flank surface of the tapered male thread portion are each composed of straight lines. Further, the flank angle of the load flank surface of the tapered male thread portion is a negative angle. Further, the screw top surface of the tapered male thread portion is formed of a convex curve that is tangent to both the first corner portion and the second corner portion. The threaded joint is provided with a solid lubricating film on the surface of the tapered male threaded portion.
Effect of the invention
[0019]
　According to the threaded joint for steel pipes according to the embodiment of the present invention, even when the surface of the male threaded portion is provided with a solid lubricating film, it is possible to suppress a fastening defect without impairing the productivity of the threaded joint.
A brief description of the drawing
[0020]
FIG. 1 is a vertical cross-sectional view showing a typical example of a threaded joint for a steel pipe according to the first embodiment.
FIG. 2 is an enlarged vertical cross-sectional view of a threaded portion of the threaded joint for steel pipe of the first embodiment.
FIG. 3 is a vertical cross-sectional view of the male screw portion shown in FIG. 2 before the solid lubricating film is formed.
FIG. 4 is a vertical cross-sectional view of the male screw portion shown in FIG.
FIG. 5 is an enlarged vertical cross-sectional view of a threaded portion in the threaded joint for steel pipe of the second embodiment.
FIG. 6 is an enlarged vertical cross-sectional view of a threaded portion of the threaded joint for steel pipe according to the third embodiment.
FIG. 7 is a vertical cross-sectional view of the male screw portion shown in FIG. 6 before the solid lubricating film is formed.
Mode for carrying out the invention
[0021]
　The threaded joint for steel pipe according to the embodiment of the present invention comprises a tubular pin and a tubular box. The pin includes a tapered male thread portion and a shoulder portion. The tapered male thread portion includes a thread top surface, a thread bottom surface, a load flank surface and an insertion flank surface. The box includes a tapered female thread that meshes with the tapered male thread and a shoulder that corresponds to the shoulder of the pin. The tapered female thread portion includes a thread top surface, a thread bottom surface, a load flank surface and an insertion flank surface. With the shoulder part of the pin in contact with the shoulder part of the box and the fastening is completed, the bottom surface of the thread of the tapered male thread part comes into contact with the thread top surface of the tapered female thread part while interfering with the thread top surface of the tapered male thread part. A gap is formed between the female thread and the bottom of the thread.
[0022]
　In a longitudinal section including the pipe shaft of the threaded joint, the threaded joint comprises the following configuration. The screw top surface of the tapered male threaded portion is connected to the load flank surface of the tapered male threaded portion via the first corner portion formed of an arc. The screw top surface of the tapered male threaded portion is connected to the insertion flank surface of the tapered male threaded portion via a second corner portion formed of an arc. The load flank surface and the insertion flank surface of the tapered male thread portion are each composed of straight lines. The flank angle of the load flank surface of the tapered male thread portion is a negative angle. The screw top surface of the tapered male thread portion consists of a convex curve that is tangent to both the first corner portion and the second corner portion. The threaded joint is provided with a solid lubricating film on the surface of the tapered male threaded portion.
[0023]
　According to the threaded joint of the present embodiment, the entire top surface of the male screw is gently raised. That is, the top surface of the male screw is not flat, and no groove is provided on the top surface of the male screw. Surface tension acts on the semi-solid lubricant applied to the top surface of the male screw. Surface tension usually acts in the direction of minimizing the surface energy of the object, and at the gas-liquid interface, provides a driving force that minimizes the area of ​​the free surface of the liquid. Therefore, the semi-solid lubricant applied to the portion having a small radius of curvature flows to the portion having a larger radius of curvature. The greater the difference in radius of curvature between adjacent surfaces, the more pronounced this flow.
[0024]
　In a conventional trapezoidal screw, the radius of curvature of the flat screw top surface is infinite, so the difference in radius of curvature between the screw top surface and the corner portion is extremely large. Therefore, the non-uniformity of the film thickness has increased due to the remarkable flow of the semi-solid lubricant. In the screw joint of the present embodiment, since the entire male screw top surface is gently raised, the difference in radius of curvature between the screw top surface and the corner portion is considerably smaller than that of the conventional trapezoidal screw. Therefore, careless flow of the semi-solid lubricant can be suppressed. As a result, the film thickness of the semi-solid lubricant becomes uniform on the top surface of the male screw without thickening, and the film thickness of the solid lubricating film also becomes uniform. Therefore, it is possible to suppress a defect in fastening. As a result, desired sealing performance and joint strength can be obtained.
[0025]
　The thread cutting tool for forming the male thread portion has a concave shape corresponding to a gently raised thread top surface. During thread cutting, the load received by the concave shaped portion is much smaller than the load received by the convex shaped portion. Therefore, the life of the tool is not particularly shortened. Therefore, the productivity of the threaded joint is not impaired.
[0026]
　In the threaded joint of the present embodiment, there are a flat load flank surface and a flat insertion flank surface. Furthermore, the flank angle of the load flank surface is a negative angle. That is, the load flank surface is inclined like a hook. Therefore, the tightening axial force effectively acts on the load flank surface. As a result, jumpout is unlikely to occur even if a tensile load is applied. Therefore, the joint strength is high.
[0027]
　The solid lubricating film used in the threaded joint of the present embodiment is not particularly limited as long as it has fluidity at the time of coating and is hardened and solidified after coating. That is, a solid lubricating film that does not flow until solidification, such as an electrodeposition film and a pressure-bonded film, is not included.
[0028]
　In the threaded joint of the present embodiment, the shape of the female thread bottom surface is not limited as long as a gap is formed between the male thread top surface and the female thread bottom surface in the fastened state. For example, in the longitudinal section of a threaded joint, the bottom surface of the female thread is a straight line. In this case, the entire bottom surface of the female screw is flat. Further, in the vertical cross section of the threaded joint, the bottom surface of the female thread may be a concave curve so as to correspond to the top surface of the male thread. In this case, the entire bottom surface of the female screw is gently recessed.
[0029]
　In a typical example, the pin and the box each include a sealing surface. In this case, in the fastened state, the sealing surface of the pin comes into contact with the sealing surface of the box while interfering with each other, and a sealing portion is formed by metal contact. However, it is not necessary to provide a sealing surface.
[0030]
　The above threaded joint preferably includes the following configuration. In the vertical cross section of the threaded joint, it is adjacent to the virtual first circle including the arc forming the first corner, the virtual second circle including the arc forming the second corner, and the thread top surface of the tapered male thread. When a virtual straight line tangent to both the virtual first circle and the virtual second circle is drawn, the virtual straight line and the curve forming the screw top surface of the tapered male thread portion are perpendicular to the tube axis. The maximum distance b of the distances in the direction is 0.1 mm to 0.3 mm. When the maximum distance b is 0.1 mm or more, the film thickness of the solid lubricating film becomes effectively uniform. Preferably, the maximum distance b is 0.2 mm or more. On the other hand, when the maximum distance b is 0.3 mm or less, the height of the load flank surface in the direction perpendicular to the pipe axis is effectively secured, and there is no adverse effect on the joint strength.
[0031]
　In a typical example, in the longitudinal section of a threaded joint, the curve forming the thread top surface of the tapered male threaded portion is an arc, an elliptical arc or a parabola.
[0032]
　The above threaded joint preferably includes the following configuration. In the vertical cross section of the threaded joint, the height LSH in the direction perpendicular to the pipe axis of the load flank surface of the tapered male thread portion (hereinafter, also referred to as “load flank surface height”) is 30% of the thread height H of the tapered male thread portion. More than 70% or less. Further, the height SSH in the direction perpendicular to the pipe axis of the insertion flank surface of the tapered male thread portion (hereinafter, also referred to as “insertion flank surface height”) is 30% or more and 70% or less of the thread height H of the taper male thread portion. is there.
[0033]
　If the load flank surface height LSH is 30% or more of the screw height H, there is no adverse effect on the joint strength. Preferably, the load flank surface height LSH is 40% or more of the screw height H. On the other hand, if the load flank surface height LSH is 70% or less of the screw height H, an arc having a sufficient size can be used for the corners and corners described later. In this case, the corners do not damage the surface of the mating member at the time of fastening, and there is no effect on seizure resistance. Further, there is no problem in securing the maximum distance b. Further, in this case, extreme stress concentration at the corners can be avoided, and there is no adverse effect on fatigue strength and the like.
[0034]
　Further, if the insertion flank surface height SSH is 30% or more of the screw height H, there is no adverse effect on the compressive strength of the joint. Preferably, the insertion flank surface height SSH is 40% or more of the screw height H. On the other hand, if the insertion flank surface height SSH is 70% or less of the screw height H, an arc having a sufficient size can be used for the corners and corners described later. In this case, the corners do not damage the surface of the mating member at the time of fastening, and there is no effect on seizure resistance. Further, there is no problem in securing the maximum distance b. Further, in this case, extreme stress concentration at the corners can be avoided, and there is no adverse effect on fatigue strength and the like.
[0035]
　In the case of this threaded joint, it is preferable to include the following configuration. In the longitudinal section of the threaded joint, the threaded bottom surface of the tapered male threaded portion is connected to the load flank surface of the tapered male threaded portion via the first corner formed by an arc. Further, the bottom surface of the tapered male threaded portion is connected to the insertion flank surface of the tapered male threaded portion via the second corner formed by an arc. Further, the radius R1 of the first corner portion is 5% or more and 35% or less of the screw height H. Further, the radius R2 of the second corner portion is 15% or more and 50% or less of the screw height H. Further, the radius R3 of the first corner is 15% or more and 50% or less of the screw height H. Further, the radius R4 of the second corner is 5% or more and 35% or less of the screw height H.
[0036]
　The radii R1, R2, R3 and R4 of the first corner portion, the second corner portion, the first corner portion and the second corner portion are set to appropriate values ​​in design. In particular, the radius R2 of the second corner portion and the radius R3 of the first corner portion are set based on the following design concept. If the radius R2 of the second corner is too small, the stubing performance is impaired. Here, the stubing performance refers to the performance of how quickly and surely the male threaded portion can be fitted into the female threaded portion and the pin can start rotating when the pin is inserted into the box. Further, in this case, the surface of the female screw portion is scratched when the pin is inserted into the box, and the seizure resistance is deteriorated. Therefore, the radius R2 is set to a large value within the range allowed by the design circumstances. If the radius R3 of the first corner is too small, excessive stress concentration occurs during fastening and tensile load, and the fatigue resistance performance deteriorates. Therefore, the radius R3 is set to a large value within the range allowed by the design circumstances. The radius R1 of the first corner portion and the radius R4 of the second corner portion do not have the same circumstances as the radius R2 and the radius R3. Therefore, the radius R1 and the radius R4 are set so as to secure the above-mentioned load flank surface height LSH and the insertion flank surface height SSH.
[0037]
　A specific example of the threaded joint for steel pipes of the present embodiment will be described below with reference to the drawings.
[0038]
　[First Embodiment]
　FIG. 1 is a vertical cross-sectional view showing a typical example of a threaded joint for a steel pipe of the first embodiment. FIG. 2 is an enlarged vertical cross-sectional view of the threaded portion of the threaded joint for steel pipe of the first embodiment. 3 and 4 are vertical cross-sectional views of the male screw portion shown in FIG. FIG. 3 shows a state before the solid lubricating film is formed. FIG. 4 shows a state in which a solid lubricating film is formed. In FIGS. 2 to 4, the screwing traveling direction of the pin 10 with respect to the box 20 is indicated by a white arrow. In the present specification, the vertical cross section means a cross section including a pipe shaft CL of a threaded joint. Note that the solid lubricating film is not shown in FIGS. 1 and 2.
[0039]
　The threaded joint shown in FIG. 1 is a coupling type threaded joint. With reference to FIG. 1, the threaded joint is composed of a pin 10 and a box 20. The thread of the threaded joint is a tapered thread. The pin 10 includes a male screw portion 11 and a shoulder portion 16. The shoulder portion 16 is provided at the tip of the pin 10. The box 20 includes a female screw portion 21 corresponding to the male screw portion 11 of the pin 10 and a shoulder portion 26 corresponding to the shoulder portion 16 of the pin 10. Further, the pin 10 includes a sealing surface 17. The sealing surface 17 is provided between the male screw portion 11 and the shoulder portion 16. The box 20 includes a sealing surface 27 corresponding to the sealing surface 17 of the pin 10.
[0040]
　With reference to FIGS. 1 and 2, the male threaded portion 11 of the pin 10 includes a screw top surface 12, a thread bottom surface 13, a load flank surface 15 and an insertion flank surface 14. On the other hand, the female screw portion 21 of the box 20 includes a screw top surface 22, a screw bottom surface 23, a load flank surface 25, and an insertion flank surface 24.
[0041]
　The male screw top surface 12 faces the female screw bottom surface 23. Although details will be described later, in the vertical cross section of the threaded joint, the male screw top surface 12 has a convex curve. That is, the male screw top surface 12 is gently raised. In the vertical cross section of the threaded joint, the female thread bottom surface 23 is a straight line. That is, the female screw bottom surface 23 is flat.
[0042]
　The male screw bottom surface 13 faces the female screw top surface 22. In the vertical cross section of the threaded joint, the male thread bottom surface 13 is a straight line. That is, the male screw bottom surface 13 is flat. In the vertical cross section of the threaded joint, the female thread top surface 22 is a straight line. That is, the female screw top surface 22 is flat.
[0043]
　In the first embodiment, in the vertical cross section of the threaded joint, the male thread bottom surface 13 is on the same straight line over the entire longitudinal direction (tube axis CL direction) of the male threaded portion 11. The female screw top surface 22 is also on the same straight line over the entire longitudinal direction of the female screw portion 21. The female thread bottom surface 23 is also on the same straight line over the entire longitudinal direction of the female thread portion 21. These straight lines are inclined from the tube axis CL by a predetermined taper angle.
[0044]
　The insertion flank surface 14 of the male screw portion 11 faces the insertion flank surface 24 of the female screw portion 21. In the vertical cross section of the threaded joint, the insertion flank surface 14 of the male threaded portion 11 is formed of a straight line. That is, the insertion flank surface 14 of the male screw portion 11 is flat. In the vertical cross section of the threaded joint, the insertion flank surface 24 of the female threaded portion 21 is a straight line. That is, the insertion flank surface 24 of the female screw portion 21 is flat.
[0045]
　The load flank surface 15 of the male screw portion 11 faces the load flank surface 25 of the female screw portion 21. In the vertical cross section of the threaded joint, the load flank surface 15 of the male threaded portion 11 is formed of a straight line. That is, the load flank surface 15 of the male screw portion 11 is flat. In the vertical cross section of the threaded joint, the load flank surface 25 of the female threaded portion 21 is formed of a straight line. That is, the load flank surface 25 of the female screw portion 21 is flat. Here, the flank angles θ of the load flank surfaces 15 and 25 are negative angles. That is, the load flank surfaces 15 and 25 are inclined like hooks.
[0046]
　By screwing the pin 10 into the box 20, the male screw portion 11 meshes with the female screw portion 21. The shoulder portion 16 of the pin 10 comes into contact with the shoulder portion 26 of the box 20 (see FIG. 1). In the fastened state, the shoulder portions 16 and 26 generate a tightening axial force, and the load flank surface 15 of the pin 10 is strongly pressed against the load flank surface 25 of the box 20. Further, the bottom surface 13 of the male screw comes into contact with the top surface 22 of the female screw while interfering with each other. On the other hand, a gap is formed between the male screw top surface 12 and the female screw bottom surface 23. A gap is formed between the insertion flank surface 14 of the male screw portion 11 and the insertion flank surface 24 of the female screw portion 21. Further, the sealing surface 17 of the pin 10 comes into contact with the sealing surface 27 of the box 20 while interfering with each other, and a sealing portion is formed by metal contact (see FIG. 1).
[0047]
　With reference to FIGS. 3 and 4, in the vertical cross section of the threaded joint, the male thread top surface 12 is connected to the load flank surface 15 of the male thread portion 11 via the first corner portion 12a. The first corner portion 12a is formed of an arc having a radius of R1. The male screw top surface 12 is connected to the insertion flank surface 14 of the male screw portion 11 via the second corner portion 12b. The second corner portion 12b is formed of an arc having a radius of R2. The male screw bottom surface 13 is connected to the load flank surface 15 of the male screw portion 11 via the first corner portion 13a. The first corner portion 13a is formed of an arc having a radius of R3. The male screw bottom surface 13 is connected to the insertion flank surface 14 of the male screw portion 11 via the second corner portion 13b. The second corner portion 13b is formed of an arc having a radius of R4.
[0048]
　The radius R1 of the first corner portion 12a is 5% or more and 35% or less of the screw height H. The radius R2 of the second corner portion 12b is 15% or more and 50% or less of the screw height H. The radius R3 of the first corner portion 13a is 15% or more and 50% or less of the screw height H. The radius R4 of the second corner portion 13b is 5% or more and 35% or less of the screw height H.
[0049]
　Further, the height LSH of the load flank surface 15 of the male screw portion 11 in the direction perpendicular to the pipe axis CL is 30% or more and 70% or less of the screw height H of the male screw portion 11. The height SSH of the insertion flank surface 14 of the male threaded portion 11 in the direction perpendicular to the tube axis CL is 30% or more and 70% or less of the threaded height H of the male threaded portion 11. Here, the load flank surface height LSH can be read as the radial height (distance) when the load flank surface 15 is projected onto a surface perpendicular to the pipe axis CL. The height of the inserted flank surface SSH can be read as the height (distance) in the radial direction when the inserted flank surface 14 is projected on the surface perpendicular to the tube axis CL.
[0050]
　In the longitudinal section of the threaded joint, the male thread top surface 12 has a convex curve. This curve is both a virtual first circle C1 containing an arc of radius R1 forming the first corner 12a and a virtual second circle C2 containing an arc of radius R2 forming the second corner 12b. In direct contact with. 3 and 4 show an example in which the curve forming the screw top surface 12 is an arc. Specifically, with reference to FIG. 3, a virtual first circle C1 including an arc forming the first corner portion 12a is drawn. A virtual second circle C2 including an arc forming the second corner portion 12b is drawn. Then, a virtual straight line C is drawn which is adjacent to the male screw top surface 12 and is in contact with both the virtual first circle C1 and the virtual second circle C2. At this time, the maximum distance b of the distance between the virtual straight line C and the curve forming the male screw top surface 12 in the direction perpendicular to the tube axis CL is 0.1 mm to 0.3 mm.
[0051]
　For example, in design, the screw height H is determined at the male screw portion 11. In the case of the first embodiment, the male screw bottom surface 13 is on the line A inclined at a predetermined taper angle from the pipe axis CL. This line A is translated by the screw height H in the direction away from the pipe axis CL. The convex male screw top surface 12 is in contact with the parallel-moved line B. This line B is translated by a predetermined distance (maximum distance b) in the direction approaching the pipe axis CL. The translated line C is the above-mentioned virtual straight line. In the case of the first embodiment, the virtual straight line C is inclined from the pipe axis CL by a predetermined taper angle. The circle C1 in contact with both the straight line C and the load flank surface 15 is the above-mentioned virtual first circle. The circle C2 in contact with both the straight line C and the insertion flank surface 14 is the above-mentioned virtual second circle. Then, the screw top surface 12 is defined so as to be in contact with the line B and in direct contact with both the first circle C1 and the second circle C2. Further, the arc of the first circle C1 connecting the load flank surface 15 and the screw top surface 12 is the first corner portion 12a. The arc of the second circle C2 connecting the insertion flank surface 14 and the screw top surface 12 is the second corner portion 12b.
[0052]
　With reference to FIG. 4, a solid lubricating film 30 is formed on the surface of the male threaded portion 11 of the pin 10. The solid lubricating film 30 has fluidity at the time of coating, and is cured after coating to solidify. Prior to the solidification of the solid lubricating film 30, the careless flow of the semi-solid lubricant applied to the male screw top surface 12 is suppressed. This is due to the fact that the entire top surface of the male screw is not flat and is gently raised, and that surface tension acts on the semi-solid lubricant. Therefore, on the male screw top surface 12, the film thickness of the semi-solid lubricant becomes uniform without being thickened, and the film thickness of the solid lubricating film 30 also becomes uniform.
[0053]
　[Second Embodiment]
　FIG. 5 is an enlarged vertical cross-sectional view of a threaded portion in the threaded joint for steel pipe of the second embodiment. The threaded joint of the second embodiment is a modification of the threaded joint of the first embodiment described above. Hereinafter, the description of the configuration overlapping with the threaded joint of the first embodiment will be omitted. The same applies to the third embodiment described later.
[0054]
　With reference to FIG. 5, in the vertical cross section of the threaded joint, the female thread bottom surface 23 is formed of a concave curve so as to correspond to the male thread top surface 12. That is, the female screw bottom surface 23 is gently recessed. Even with such a configuration, the same effect as that of the first embodiment is obtained.
[0055]
　[Third Embodiment]
　FIG. 6 is an enlarged vertical cross-sectional view of a threaded portion in the threaded joint for steel pipe of the third embodiment. FIG. 7 is a vertical cross-sectional view of the male threaded portion shown in FIG. 6 before the solid lubricating film is formed.
[0056]
　With reference to FIGS. 6 and 7, in the vertical cross section of the threaded joint, the male thread top surface 12 has a convex curve. The male screw bottom surface 13, the female screw top surface 22, and the female screw bottom surface 23 are each composed of straight lines. However, in the third embodiment, the male screw bottom surface 13 is parallel to the pipe shaft CL in the vertical cross section of the threaded joint. The female screw top surface 22 is also parallel to the tube axis CL. The female screw bottom surface 23 is also parallel to the tube axis CL.
[0057]
　For example, in the design, in the male screw portion 11, the screw height HS on the insertion flank surface 14 side and the screw height HL on the load flank surface 15 side are determined. In the case of the third embodiment, the male screw portion 11 is designed based on the screw height HS on the insertion flank surface 14 side. The male screw bottom surface 13 is on the line A'parallel to the pipe axis CL. This line A'is translated by the screw height HS on the insertion flank surface 14 side in the direction away from the tube axis CL. The convex male screw top surface 12 touches the parallel-moved line B'. This line B'is translated by a predetermined distance (maximum distance b) in the direction approaching the pipe axis CL. The translated line C'is the virtual straight line described above. In the case of the third embodiment, the virtual straight line C'is parallel to the tube axis CL. The circle C1 in contact with both the straight line C'and the load flank surface 15 is the above-mentioned virtual first circle. The circle C2 in contact with both the straight line C'and the insertion flank surface 14 is the above-mentioned virtual second circle. Then, the screw top surface 12 is defined so as to be in contact with the line B'and in direct contact with both the first circle C1 and the second circle C2. Further, the arc of the first circle C1 connecting the load flank surface 15 and the screw top surface 12 is the first corner portion 12a. The arc of the second circle C2 connecting the insertion flank surface 14 and the screw top surface 12 is the second corner portion 12b.
[0058]
　Even with such a configuration, the same effect as that of the first embodiment is obtained. In the vertical cross section of the threaded joint as in the second embodiment, the female thread bottom surface 23 may have a concave curve so as to correspond to the male thread top surface 12.
Example
[0059]
　In order to confirm the effect of this embodiment, numerical simulation analysis by the finite element method was carried out, and the film thickness of the semi-solid lubricant applied to the male thread portion was investigated.
[0060]
　[Test conditions] As
　a model for FEM analysis, the male screw portion of the first embodiment shown in FIG. 3 was used, and the screw height H was variously changed. Test No. Model 1 was a comparative example, and the vertical cross-sectional shape of the top surface of the male screw was straight. Test No. 2 and 3 are Examples 1 and 2, respectively, and the vertical cross-sectional shape of the top surface of the male screw was a single arc. The common conditions are as follows.
[0061]
　- thread pitch: 5 TPI (1 the number of threads per inch five)
　, the thread width: pitch line on a 2.48 mm
　screw taper: 6.25% (taper angle: about 1.8 °)
　· Load Frank angle of flank surface: -3 °
　・Frank angle of inserted flank surface: 10 °
　・ Load flank surface height LSH: 0.82 mm
　・ Inserted flank surface height SSH: 0.86 mm
　・ First corner radius R1: 0.35 mm
　・ Second corner radius R2: 0.76 mm
　・ First corner radius R3: 0.35 mm
　・ Second corner radius R4: 0.15 mm
[0062]
　In the FEM analysis, a model of the male threaded portion and the lubricant before solidification with a plane strain element was used. The male threaded portion was an elastic body, and its Young's modulus was 210 GPa. The lubricant before solidification was a fluid viscous plastic fluid. Specifically, regarding the lubricant before solidification, the viscosity coefficient was 200 cm Stokes, the mass density was 1.0 × 10 -6 kg / mm 3 , and the surface tension was 22 m (mm) N / m. Test No. In any of 1 to 3, the same amount of viscoplastic fluid was spray-coated to give a uniform initial film thickness. The initial film thickness was 0.1 mm. From this state, the flow due to surface tension and viscosity was analyzed, and the film thickness after 100 seconds when the flow actually stopped was investigated. The lower limit of the film thickness at which seizure does not occur at the first and second corners connected to the top surface of the male screw is 0.012 mm.
[0063]
　[Evaluation method]
　The maximum film thickness on the top surface of the male screw was extracted. Further, the minimum film thickness at the first corner portion and the second corner portion was extracted. Then, the ratio of the maximum film thickness at the top surface of the male screw to the minimum film thickness at both corners (hereinafter, also referred to as “film thickness ratio”) was calculated, and the uniformity of the film thickness was evaluated. The smaller the film thickness ratio, the more uniform the film thickness. The results are shown in Table 1 below.
[0064]
[table 1]

[0065]
　[Test Results] From the results
　shown in Table 1, the following is shown. The film thickness ratios of Examples 1 and 2 were smaller than the film thickness ratios of Comparative Examples. Therefore, the film thickness of the solid lubricant could be made uniform by the screw joints of Examples 1 and 2. Further, the minimum film thickness of the corner portion of the comparative example was equivalent to the lower limit of the film thickness that does not cause seizure. On the other hand, the minimum film thickness of the corners of Examples 1 and 2 had a margin with respect to the lower limit of the film thickness. This means that the amount of the lubricant applied can be reduced in Examples 1 and 2.
[0066]
　In addition, the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, the type of threaded joint may be either a coupling type or an integral type. The installation location, number of installations, etc. of the shoulder section are not particularly limited. When a seal portion is provided by metal contact, the installation location, the number of installations, etc. are not limited.
Industrial applicability
[0067]
　The threaded joint of the present invention can be effectively used for connecting a steel pipe used as an oil well pipe.
Description of the sign
[0068]
　10 Pin
　11 Male thread
　12 Male top surface
　12a 1st corner
　12b 2nd corner
　13 Thread bottom
　13a 1st corner
　14b 2nd corner
　14 Insertion flank surface
　15 Load flank surface
　16 Shoulder
　17 Seal surface
　20 Box
　21 Female screw Part
　22 Thread top surface
　23 Thread bottom surface
　24 Insertion flank surface
　25 Load flank surface
　26 Shoulder part
　27 Seal surface
　30 Solid lubrication film
　CL pipe shaft
The scope of the claims
[Claim 1]
　A threaded joint for steel pipes consisting of a tubular pin and a tubular box,
　wherein the pin includes a tapered male threaded portion and a shoulder portion, and the
　tapered male threaded portion includes a thread top surface, a thread bottom surface, a load flank surface and an insertion flank surface. The
　box includes a tapered female threaded portion that meshes with the tapered male threaded portion and a shoulder portion corresponding to the shouldered portion of the pin,
　wherein the tapered female threaded portion includes a screw top surface, a thread bottom surface, a load flank surface and an insertion. In
　a state where the shoulder portion of the pin includes the flank surface and the shoulder portion of the pin is in contact with the shoulder portion of the box to complete the fastening, the screw bottom surface of the tapered male screw portion interferes with the screw top surface of the tapered female screw portion. while contacting said gap is formed between the thread top face of the tapered male thread portion and the thread bottom face of the tapered female screw portion,
　in longitudinal section including the tube axis of the threaded joint,
　the screw of the tapered male thread portion The top surface is connected to the load flank surface of the tapered male thread portion via a first corner portion formed
　of an arc, and the screw top surface of the tapered male thread portion is connected to the tapered male thread via a second corner portion formed of an arc. leads to the insertion flank parts,
　wherein the load flanks and the insertion flanks of the tapered male thread portion respectively made of a straight line,
　the flank angle of the load flanks of the tapered male thread portion is negative angle,
　the tapered male thread The screw top surface of the portion is composed of a convex curve that is in direct contact with both the first corner portion and the second corner portion.
　A threaded joint for steel pipes having a solid lubricating film on the surface of the tapered male threaded portion.
[Claim 2]
　The threaded joint for steel pipes according to claim 1,
　wherein in the vertical cross section, a virtual first circle including an arc forming the first corner portion and a virtual first circle including an arc forming the second corner portion. When a virtual straight line is drawn that is adjacent to the screw top surface of the tapered male thread portion and is in direct contact with both the virtual first circle and the virtual second circle, the virtual straight line and the virtual straight line are drawn. A threaded joint for steel pipes, wherein the maximum distance between the curve forming the screw top surface of the tapered male thread portion and the distance in the direction perpendicular to the pipe axis is 0.1 mm to 0.3 mm.
[Claim 3]
　The threaded joint for steel pipe according to claim 1 or 2,
　wherein the curve forming the thread top surface of the tapered male thread portion is an arc, an elliptical arc, or a parabola in the vertical cross section. ..
[Claim 4]
　The threaded joint for a steel pipe according to any one of claims 1 to 3,
　wherein in the vertical cross section,
　the height of the load flank surface of the tapered male threaded portion in a direction perpendicular to the pipe axis is said. The height of the tapered male threaded portion is 30% or more and 70% or less, and
　the height of the insertion flank surface of the tapered male threaded portion in the direction perpendicular to the pipe axis is 30% of the threaded height of the tapered male threaded portion. A threaded joint for steel pipes that is 70% or more and 70% or less.
[Claim 5]
　The threaded joint for a steel pipe according to claim 4,
　wherein in the vertical cross section, the
　thread bottom surface of the tapered male threaded portion is connected to the load flank surface of the tapered male threaded portion via a first corner formed of an arc. The
　bottom surface of the thread of the tapered male threaded portion is connected to the insertion flank surface of the tapered male threaded portion via a second corner formed of an arc, and
　the radius of the first corner portion is 5% or more of the thread height 35. % or less,
　the radius of the second corner is less than 50% more than 15% of the thread height,
　and the radius of the first corner portion is less than 50% more than 15% of the thread height,
　the A threaded joint for steel pipes, wherein the radius of the second corner is 5% or more and 35% or less of the thread height.