JP5673089B2 - Threaded joints for steel pipes - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a threaded joint for steel pipes, and in particular, oil well pipes including tubing and casings generally used for exploration and production of oil wells and gas wells, that is, OCTG (oil country tubular goods), riser pipes, line pipes, etc. The present invention relates to a threaded joint for steel pipes that is suitable for use in connecting steel pipes and has excellent sealing properties and compression resistance.

  Threaded joints are widely used to connect steel pipes used in oil industry equipment such as oil well pipes. Conventionally, standard threaded joints defined in API (American Petroleum Institute) standards have been used to connect steel pipes used in the search and production of oil and gas. However, in recent years, wells for crude oil and natural gas have been deepened, and horizontal wells and inclined wells have increased from vertical wells, and the drilling and production environment has become severe. In addition, the demand for screw joints such as compression resistance, bending resistance, and external pressure seal performance (external pressure resistance) has diversified due to the increased development of wells in poor environments such as the ocean and polar regions. ing. Therefore, the use of high-performance special threaded joints called premium joints is increasing.

  The premium joint is usually a joint in which a pin member and a box member each having a taper screw, a seal portion (specifically, a metal touch seal portion), and a shoulder portion (specifically, a torque shoulder portion) are combined. The taper screw is important for firmly fixing the pipe joint, and the seal part plays a role of ensuring the sealing performance by the metal contact between the box member and the pin member at this part, and the shoulder part is tightening the joint. It becomes the shoulder surface that plays the role of the stopper.

  3-5 is typical explanatory drawing of the premium joint for oil well pipes, These are the longitudinal cross-sectional views of the threaded joint of a circular pipe. The threaded joint includes a pin member 3 and a box member 1 corresponding thereto, and the pin member 3 (pin 3) is provided on the outer surface thereof adjacent to the male screw 7 and on the tip end side of the pin 3 adjacent to the male screw 7. And a no-thread portion called a nose portion 8 (pin nose 8). The nose portion 8 has a seal portion 11 on its outer peripheral surface and a torque shoulder portion 12 on its end surface. The opposing box member 1 has, on its inner surface, a female screw 5, a seal portion 13, and a portion that can be screwed or contacted with the male screw 7, the seal portion 11, and the shoulder portion 12 of the pin 3, respectively. The shoulder portion 14 is provided.

  Patent documents 1-6 are mentioned as conventional technology about the premium joint.

Japanese Patent No. 4535064 Japanese Patent No. 4208192 Japanese Utility Model Publication No. 61-44068 Japanese Patent No. 4300187 JP 2001-124253 A Japanese Patent No. 2705506

  3 to 5, the metal touch seal portion is located at the tip of the pin nose 8. However, in Patent Document 1, a metal touch seal portion is provided near the screw portion of the pin nose 8 in order to increase the external pressure resistance. In addition, it has been proposed to extend the nose part from the seal part to the shoulder part. In the threaded joint disclosed in Patent Document 1, the pin nose that is not in contact with the box member is configured to be elongated so as to be discontinuous with the seal portion so that the thickness of the pin nose is not reduced. In addition to the above-mentioned external pressure resistance, the axial compression resistance is also improved.

Similarly, in Patent Document 2, an appendix is formed from the seal portion to the pin nose tip, which also has a portion having a discontinuous shape with the seal portion to ensure radial rigidity and lower axial rigidity. It is described that this appendix is deformed at the time of tightening and the tensile resistance is improved by recovering the appendix when loaded.
As described in Patent Documents 1 and 2, placing the seal part near the screw part of the pin and separating it from the tip of the pin nose is stable against the screw as well as improving the external pressure resistance and tensile resistance. It is effective in giving a realistic performance, and it can be confirmed from FEM simulations and the like. Further, the pin nose that has a shape discontinuous with the seal portion is deformed itself when a strong axial compressive force is applied, and has an effect of reducing plastic deformation of the torque shoulder portion of the box member. However, on the other hand, unauthorized deformation may occur in the discontinuous portion, which is considered to depend on the tightening torque.

  Since the tightening torque is affected by lubrication conditions, surface properties, etc., a design that does not depend greatly on this is a radial seal method in which the radial contact pressure is increased. For example, Patent Document 3 discloses an example of a radial seal method having a large pin seal R shape and a small seal taper angle. However, a problem with the radial seal method in which the seal taper angle is thus reduced is that goling is likely to occur during tightening. Further, in the radial seal method, it is necessary to increase the amount of seal interference in order to ensure sealing performance and seal stability, and the ease of occurrence of goling is further increased.

  In Patent Document 4, in order to solve these problems, the radius of the toroidal (conical curved rotating surface shape) pin seal surface is specified to be large, thereby increasing the seal contact area and reducing the contact pressure. This measure is effective, and can greatly reduce the goling risk of the metal touch seal part. However, by taking a large R and lowering the contact pressure, there is a problem that the contact pressure is lowered with some slight trouble, and if a minute leak path is formed in the metal touch seal part, the leak does not stop easily. . Also, because of the large R, it is physically difficult to separate the metal touch seal part from the nose tip. When the length of the metal touch seal part and the pin nose tip is secured to a certain extent, the thickness of the pin nose tip is It leads to becoming too small.

Regarding the axial compression resistance, as described in Patent Document 5 and Patent Document 6, it is effective to reduce the gap on the stub flank side in the threaded portion. However, if this gap is too small, goling tends to occur in the threaded portion, so it is necessary to take an appropriate gap.
As described above, the conventional threaded joints still have some problems, and diversify the performance requirements for threaded joints, such as the above-mentioned compression resistance, bending resistance, and external pressure seal performance. There is room for further improvement in order to fully respond. In view of such circumstances, it is an object of the present invention to provide a threaded joint for steel pipes that has improved sealing properties, compression resistance, and galling resistance.

In order to find out the means for solving the above-described problems, the inventors have conducted intensive studies and have come to achieve the present invention having the following gist. That is, the present invention is as follows.
(1) a pin member having a male screw part, a nose part extending from the male screw part toward the tube end side, and a shoulder part provided at the tip of the nose part;
A female screw part that is screw-coupled to the male screw part, a nose part inner peripheral surface that faces the outer peripheral surface of the nose part of the pin member, and a box member that has a shoulder part that contacts the shoulder part of the pin member,
A screw for a steel pipe in which the pin member and the box member are connected by the screw connection, and the outer peripheral surface of the nose portion of the pin member and the inner peripheral surface of the nose portion of the box member are in metal-metal contact, and the contact interface forms a sealing surface. A joint,
The inner peripheral surface of the nose portion of the box member has an inwardly convex curve in the axial sectional view of the box member, and the convex curve has a plurality of inwardly convex arcs having different curvature radii R. The curved curve shape is such that the radius of curvature R of the arc increases with increasing distance from the female thread portion, and the tangent line on the connection point of the arc matches that of the connection partner arc ,
In addition, the angle formed by each arc in the composite R curve is larger as the arc closer to the female thread portion .
The screw joint for steel pipes, wherein the outer peripheral surface of the nose portion of the pin member is a tapered surface that interferes with the inner peripheral surface of the nose portion of the box member when coupled to the box member.
(2) The threaded joint for steel pipes according to (1), wherein any of the connection points in the composite R curve is a tangent point of the tapered surface.
(3) The threaded joint for steel pipes according to (1) or (2) , wherein an angle formed between the tapered surface and the axial direction of the joint is within 10 degrees.
(4) The length of the nose part of the said pin member is 20 mm or more, The threaded joint for steel pipes in any one of said (1)- (3) characterized by the above-mentioned.
(5) The threaded joint for steel pipes according to any one of (1) to (4) , wherein the male thread part and the female thread part have a stub flank angle in a range of 0 degrees to 30 degrees. .
(6) The screw for a steel pipe according to any one of (1) to (5) , wherein the male screw portion and the female screw portion have a load flank angle in a range of −5 degrees to 4 degrees. Fittings.
(7) The threaded joint for steel pipes according to any one of (1) to (6) , wherein a shoulder angle of the shoulder portion is in a range of 0 degrees to 20 degrees.
(8) The screw for a steel pipe according to any one of (1) to (7) , wherein the male screw part and the female screw part have a screw gap in a range of 0.01 to 0.1 mm. Fittings.
(9) In any one of the above (1) to (8) , instead of a composite R curve in which a plurality of arcs having different curvature radii R are sequentially connected, a plurality of arcs having different curvature radii R are directly or A threaded joint for steel pipes, characterized in that it is a composite R-curve connected sequentially through line segments.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to obtain the screw joint for steel pipes which improved the sealing performance, compression resistance, and also galling resistance.

Sectional drawing which shows the nose part of the threaded joint for steel pipes concerning embodiment of this invention Partial enlarged view of the vicinity of the seal portion in FIG. Sectional view showing a conventional threaded joint for steel pipes FIG. 3 is an enlarged sectional view showing the vicinity of the pin nose in FIG. FIG. 3 is an enlarged sectional view showing a screw portion in FIG. Sectional view showing definitions of screw clearance, load flank angle, and stub flank angle Chart showing load history in leak test simulation

  As described above, providing a seal part at a position away from the tip of the nose and extending the nose part from the seal part to the shoulder part for a long time has improved external pressure resistance and tensile resistance, as well as stable performance against screws. In particular, it is effective that the tangent point, which means the first contact point with the box member in the seal portion of the pin member, is as far as possible from the nose tip of the pin member. Therefore, the inventors further examined the shape around the seal part so that the seal part can be separated from the tip of the nose (or the shoulder).

  As a result, the inner peripheral surface of the nose portion of the box member has a surface shape that forms a convex curve inward in the axial sectional view of the box member, and the outer peripheral surface of the nose portion of the pin member is an axial sectional view of the pin member A taper surface shape that intersects the convex curve of the box member at two points above, and a metal touch seal portion is formed by the inner peripheral surface of the nose portion of the box member and the outer peripheral surface of the nose portion of the pin member, and the seal portion When each interface on the box member side and the pin member side is a sealing surface of the same member, the convex curve of the box member is a composite R curve in which a plurality of arcs having different curvature radii R are sequentially connected, The radius of curvature R of the arc increases as the distance from the female thread increases, and the tangent line at the connection point of the arc coincides with that of the arc of the connection partner. The stringent point reached on the idea that it is possible to separate from the nose tip. Furthermore, the contact surface pressure distribution of the seal portion is provided with a portion where R is large, the surface pressure is low, and the contact length is long, and a portion where R is small, the surface pressure is high, and the contact length is short. It came to the knowledge which has the effect of improving sealing performance.

  FIG. 1 is a cross-sectional view showing a nose portion of a threaded joint for steel pipes according to an embodiment of the present invention, where (a) shows a pin member 3, (b) shows a box member 1, and (c) shows a pin member 3. And the box member 1 are shown in a coupled state. The pin member 3 is provided at the end of the steel pipe, and includes a male screw portion 7, a nose portion 8 connected to the pipe end side from the male screw portion 7, and a torque shoulder portion 12 provided at the tip of the nose portion 8. Have On the other hand, the box member 1 includes a female screw portion 5 screwed to the male screw portion 7 of the pin member 3, and an outer peripheral surface (nose portion) of the nose portion 8 in a coupled state of the pin member 3 and the box member 1 by the screw coupling. It has an inner peripheral surface (nose portion inner peripheral surface) 20 of the box member 1 facing the outer peripheral surface 30) and a shoulder portion 14 that comes into contact with the shoulder portion 12.

  The inner peripheral surface 20 of the nose portion of the box member 1 has a convex curve inward in a sectional view of the box member 1 in the axial direction. On the other hand, the outer peripheral surface of the nose portion of the pin member 3 facing the inner peripheral surface 20 of the nose portion is a tapered surface 30 (conical surface) having a constant inclination angle (called a taper angle) α with respect to the axial direction of the threaded joint. . When the pin member 3 and the box member 1 are coupled, the taper surface 30 and the nose portion inner peripheral surface 20 interfere with each other to form the seal portion 40. The taper angle α is the convex curve and the taper in a cross-sectional view in the axial direction of the threaded joint in a virtual non-interference coupling state in which the pin member 3 and the box member 1 are coupled on the assumption that they do not interfere with each other. It is set so that the generatrix of the surface 30 intersects at two points A and B. Actually, the seal portion 40 is formed in a range (interference area 40a) sandwiched between the two intersections A and B. In the virtual no-interference coupling state, the box nose inner peripheral surface 20 has a virtual projecting portion inward from the tapered surface 30. The maximum projecting amount of the virtual projecting portion from the tapered surface 30 is determined by the seal portion. The joint interference amount S is 40, and the joint axial length of the virtual projecting portion is called the seal contact length l of the seal portion 40. Here, the actual contact length when the screw is actually tightened is different from the contact length l, that is, the actual contact length varies depending on the load condition applied.

The convex curve formed in the nose portion inner peripheral surface 20 is arcuate N _1, N _2, N ₃ having to be linked to a portion adjacent to the internally threaded portion 5, the different radii R _1, R _2, R ₃ Are combined so as to share the tangent line at the connection point, and the radius of curvature of the arc increases with increasing distance from the female screw portion 5, that is, R ₁ <R _2. <there is a curved shape that is R _3. It should be noted that the method of connecting the composite R curve N to the portion adjacent to the female thread portion 5 and the value of the curvature radius R ₁ <R ₂ <R ₃ are appropriately determined so that a desired seal contact length and seal interference amount can be obtained. Is set.

Thereby, as shown in FIG. 2, the distance from the tip of the nose portion of the tangent point can be made larger. FIG. 2 is an enlarged view of the vicinity of the seal interference area 40a in FIG. Here, in FIG. 2, as a comparison, the convex curve is a single R curve M (single arc of curvature radius R), and the interference area and the interference amount of the seal portion are the seal interference area 40a of the composite R curve N. A case where the amount is equal to the seal interference amount S is indicated by a broken line. When the box member and the pin member are joined, the taper surface 30 of the pin member is 30a to 30b as shown by the arrow in FIG. 2 with respect to the convex curve N or M forming the seal portion of the box member. Move to the position. The time when the tapered surface 30 is at the position 30a is the time when the convex curve N or M of the box member and the tapered surface 30 start contact. Contact points P and P ′ between the tapered surface 30 and the convex curves N and M at this time are tangent points when the convex curve is N and M, respectively. As apparent from the _figure, R _{1 <R} 2 <composite R curve obtained by sequentially connecting the circular arc _{N 1,} N 2, _{N 3} with different radii _{R 1,} R 2, _{R 3} have the relationship of _{R 3} It can be seen that the tangent point P in the case of N is located on the right side of the tangent point P ′ in the case of the single R curve M. That is, the distance from the tip of the nose located on the left side of FIG. 2 to the tangent point is larger when the composite R curve N is used as a convex curve than when the single R curve M is used. Become. As a result, it is possible to improve the external pressure resistance and the tensile resistance, and to give a stable performance to the screw.

The composite R curve N has a curved shape in which the tangent line at the connection point of the arc coincides with that of the connection partner arc. For example, the tangents of the two arcs to be connected are made coincident with each other at the connection point between the arcs N ₁ and N ₂ and the connection point between the arcs N ₂ and N ₃ . Therefore, the convex curve has a continuous curve shape with no inflection point on the curve. If this is a discontinuous shape, a partial high surface pressure may occur in the seal contact. The two arcs to be connected may be directly connected or may be connected via a line segment that overlaps a common tangent line between the arcs.

Here, the angles θ ₁ , θ ₂ , and θ ₃ formed by the arcs N ₁ , N ₂ , and N ₃ are larger as the arc closer to the female screw portion 5, that is, θ ₁ > θ ₂ > θ _3. It is preferable that Otherwise, the length of the inner peripheral surface 20 of the nose portion of the box member 1 corresponding to the length of the nose portion 8 of the limited pin member 3 (pin nose length L in FIG. 1A) or limited interference. It becomes difficult to design a composite R curve within the length of the region 40a (referred to as the seal contact length).

Further, the connecting point of the arc in the composite R curve, for example, the connecting point of the arcs N ₁ and N ₂ or the connecting point of the arcs N ₂ and N ₃ is either the tapered surface 30 of the pin member 3 or the like. It is preferably coincident with the tangent point which means the first contact point. By using one of the connection points of the arcs in the composite R curve as a tangent point, the seal portion has a portion where R is large and the contact pressure is low and the contact length is long, and R is small and the contact pressure is high and the contact length is short. Therefore, it is possible to reliably provide an effect that a leak path is hardly formed and the ultimate sealing performance is improved.

  Note that the tangent point of the pin member is preferably placed at a position where the distance from the tip of the male screw portion is 0.7 L or less (L is the pin nose length as described above) from the viewpoint of separating the seal portion from the tip of the nose. Furthermore, when the distance from the distal end of the male screw portion of the tangent point is less than 0.2L, interference between the seal portion and the screw portion is likely to occur during tightening, so 0.2L or more is preferable. Furthermore, 0.3L or more is good for safety.

The taper angle α of the taper surface 30 of the pin member 3 is preferably within 10 degrees. By making the taper angle α within 10 degrees, more preferably within 5 degrees, the radial seal method can be suitably realized, and the tightening torque dependency of the sealing performance becomes relatively low.
The pin nose length L is preferably 20 mm or more. According to this, the seal portion is sufficiently separated from the tip of the pin nose, and as a result, damage to the seal portion can be greatly reduced by elastic deformation within this separation distance range, which is effective in stabilizing the sealing performance. . Since the seal performance is stabilized, the seal interference amount S (see FIG. 1 (c)) can be relatively small as a radial seal method, and the goling risk is small.

  The two or more types of R in the composite R curve are preferably 1 inch or less for a relatively small R, 2 inches or more for a relatively large R, and 3 inches or more for a larger R. Specifically, at least one of a plurality of Rs in the composite R curve is 2 inches or more (more preferably 3 inches or more), and at least one R is less than 2 inches (more preferably 1 inch or less). Is preferred. By making at least one of a plurality of Rs of the composite R curve 2 inches or more (more preferably 3 inches or more), it becomes easy to secure the contact length of the seal portion, and at least one of the remaining Rs is 2 By making it less than 1 inch (preferably 1 inch or less), it becomes easy to achieve high surface pressure.

  Further, the number of arcs in the composite R curve (the number of arcs having different R) may be two, may be three illustrated in FIG. 1, or may be four or more. As the number of arcs increases, the degree of freedom in design increases and it is easy to achieve a shape design with improved sealing performance. The number should be designed according to the performance actually required for the threaded joint.

  In addition to the above-mentioned shape limitation around the seal portion, for the male screw portion and the female screw portion, any one or two or more of the load flank angle, the stub flank angle, and the screw gap are defined in a preferable range. It was confirmed that the overall sealing performance was further improved by the combined effect. Here, the load flank angle is the load flank angle β shown in FIG. 6, that is, the angle β formed by the load flank surface 18 with respect to the joint axis orthogonal surface (the surface orthogonal to the axial direction of the threaded joint; the same applies hereinafter). It is. The stub flank angle is the stub flank angle γ shown in FIG. 6, that is, the angle γ formed by the stub flank surface 19 with respect to the joint axis orthogonal surface. Further, the screw gap is the screw gap G shown in FIG. 5, that is, the gap G between the thread 7a of the male screw and the screw groove 5a of the female screw meshing with the screw thread 7a.

The preferred range of the load flank angle β is -5 ° to 4 °. The lower limit of the preferred range is determined from the viewpoint of galling resistance and tool life of the threaded portion, and the upper limit is determined from the viewpoint of bending resistance.
The preferred range of the stub flank angle γ is 0 to 30 degrees. The lower limit of the preferred range is determined from the viewpoint of galling resistance, tool life, and tightening performance of the threaded portion, and the upper limit is determined from the viewpoint of axial compression resistance. It was.

  The preferable range of the screw gap G is 0.01 to 0.1 mm. The lower limit of the preferable range is determined from the viewpoint of reducing the goling risk, and the upper limit is determined from the viewpoint of reducing the load on the pin tip during the axial compression load. It was. In consideration of the lead error at the time of threading, the screw gap G is preferably about 0.03 mm at least. Moreover, since it discovered that the screw gap G was about 0.045 mm and can exhibit sufficient performance effectively, it is good also as about 0.045 mm according to a condition.

By defining one or more of the load flank angle, stub flank angle, and screw gap as described above, the overall effect of improving the sealing performance is particularly the axial tension + internal pressure or This is remarkable under conditions where external pressure is applied.
Also, the shoulder angle δ of the shoulder portion (when the end surface of the shoulder portion in the joint axis direction is an angle formed with respect to the joint axis orthogonal surface, and the pin outer peripheral side of the interface protrudes outward in the joint axial direction when viewed from the pin inner peripheral side. Is a positive angle) is preferably 0 to 20 degrees. If the shoulder angle is less than 0 degrees, it is disadvantageous in terms of sealing performance and tightening characteristics, while if it exceeds 20 degrees, it is disadvantageous in that plastic deformation of the box shoulder part and local deformation of the seal part are likely to occur. Preferably it is 15 degrees or less. Furthermore, depending on the situation, 7 degrees or less is preferable.

  As an example of the invention, the threaded joint for steel pipes according to the present invention shown in FIG. 1 or in which any two of the arcs of the composite R curve in FIG. 1 are connected via line segments conforms to ISO 13679. A leak test was simulated, and the contact area pressure (ksi · inch) at the seal portion at this time was determined by FEM analysis. Note that contact area pressure = contact surface pressure × seal contact length, and is obtained by integral calculation. In this leak test, a biaxial stress corresponding to 95% of the yield condition of the material, an internal pressure, and an external pressure are applied to the threaded joint for steel pipes according to the history shown in FIG.

In addition, as an index indicating the risk of goling during screw tightening, a Goring index (defined by the product of sliding distance (inch) and contact surface pressure (psi) at each axial position of the seal part from the start to the end of tightening. (psi · inch) = contact surface pressure × sliding distance, was obtained by FEM analysis. This is also obtained by integral calculation. It can be said that the smaller the Goring index, the smaller the Goring risk.
For comparison,
Comparative Example 1: When the generatrix of the inner peripheral surface of the seal portion of the box member is a convex curve having a single R (single R curve M indicated by a broken line in FIG. 1),
-Comparative example 2: Although the generating line of the inner peripheral surface of the seal part of the box member is a composite R curve, if it does not satisfy the requirement that the R of the arc increases as it moves away from the female thread part 5,
In the same manner, the contact area pressure and Goling index were obtained.

  Table 1 shows the contact area pressure and Goling index determined by FEM calculation for the inventive example and the comparative example, together with the dimensions of each part of the threaded joint. Note that the internal pressure conditions and the contact area pressures all showed local minimum values (corresponding to a state in which leakage is most likely to occur) in the vicinity of the load step L18 (biaxial tensile stress + internal pressure) in the history of FIG. This load point is not specified in ISO 13679, but the internal pressure + tensile condition is the most severe condition and may be required. In addition, since the load performance of the load step L18 after receiving the compression history is lower than that of the load step L3 which is the same load point before receiving the compression history, the comparison at L18 is good. In Table 1, the minimum value of the contact area pressure in each example is displayed as a relative minimum value (the minimum value in all examples is 100, and the others are expressed as ratios relative thereto). In addition, the joint axis direction position showing the maximum value (corresponding to the state with the highest goling risk) of the Goling index was different for each example. In Table 1, the maximum value of the Goring index of each example is displayed as a relative maximum value (the maximum maximum value is 100 among all examples, and the others are expressed as ratios relative thereto). Several samples were made and a physical test was conducted to confirm that there was no leakage under the external pressure conditions. The problem is only in the Q1 region, and it has already been confirmed that when the amount of seal interference is reduced, there is a case where a leak occurs first in L18 after undergoing axial compression.

  From Table 1, it can be seen that, in all of the inventive examples, compared with the comparative example, although the contact area pressure is high, the galling index is small or similar, and a screw joint excellent in sealing performance and galling resistance has been realized. Recognize.

1 Box material
3 Pin (Pin material)
5 Female thread (Female thread)
5a Female thread groove
7 Male thread (Male thread)
7a Male thread thread
8 Nose (pin nose)
11, 13, 40 Seal (Details are metal touch seal)
12, 14 Shoulder part (For details, torque shoulder part)
18 Road flank surface
19 Stub flank surface
20 Inner peripheral surface of box member nose (inner peripheral surface of box nose)
30 Tapered surface forming the pin member nose outer peripheral surface (pin nose outer peripheral surface)
40 Seal part
40a Interference zone

Claims (9)

A pin member having a male screw part, a nose part extending from the male screw part to the tube end side, and a shoulder part provided at the tip of the nose part;
A female screw part that is screw-coupled to the male screw part, a nose part inner peripheral surface that faces the outer peripheral surface of the nose part of the pin member, and a box member that has a shoulder part that contacts the shoulder part of the pin member,
A screw for a steel pipe in which the pin member and the box member are connected by the screw connection, and the outer peripheral surface of the nose portion of the pin member and the inner peripheral surface of the nose portion of the box member are in metal-metal contact, and the contact interface forms a sealing surface. A joint,
The inner peripheral surface of the nose portion of the box member has an inwardly convex curve in the axial sectional view of the box member, and the convex curve has a plurality of inwardly convex arcs having different curvature radii R. The curved curve shape is such that the radius of curvature R of the arc increases with increasing distance from the female thread portion, and the tangent line on the connection point of the arc matches that of the connection partner arc ,
In addition, the angle formed by each arc in the composite R curve is larger as the arc closer to the female thread portion .
The screw joint for steel pipes, wherein the outer peripheral surface of the nose portion of the pin member is a tapered surface that interferes with the inner peripheral surface of the nose portion of the box member when coupled to the box member. 2. The threaded joint for steel pipes according to claim 1, wherein any one of the connection points in the composite R curve is a tangent point of the tapered surface. The tapered surface is a threaded joint for steel pipes according to claim 1 or 2 angle between the axial direction of the joint is equal to or is within 10 degrees. The length of the nose part of the said pin member is 20 mm or more, The threaded joint for steel pipes in any one of Claims 1-3 characterized by the above-mentioned. The threaded joint for steel pipes according to any one of claims 1 to 4 , wherein the male thread part and the female thread part have a stub flank angle within a range of 0 degrees to 30 degrees. The threaded joint for steel pipes according to any one of claims 1 to 5 , wherein the male thread part and the female thread part have a load flank angle within a range of -5 degrees to 4 degrees. The threaded joint for steel pipes according to any one of claims 1 to 6 , wherein a shoulder angle of the shoulder portion is within a range of 0 degrees to 20 degrees. The threaded joint for steel pipes according to any one of claims 1 to 7 , wherein the male screw part and the female screw part have a screw gap in a range of 0.01 to 0.1 mm. 9. In any one of Claims 1-8 , it replaces with the composite R curve which connected the some circular arc which has a different curvature radius R sequentially, and the several circular arc which has a different curvature radius R is sequentially or directly via a line segment. A threaded joint for steel pipes characterized in that it is a composite R curve connected.

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