JP2001021072A - Pipe joint excellent in seozure-resistance and machining thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a tapered thread joint of a pipe used as an oil well pipe and a pile, which is hardly allowed to seize during fastening thereof. SOLUTION: In a tapered thread joint composed of a pin P formed on the outer peripheral surface of its distal end part a tapered make thread part 1, and a box B formed on the inner peripheral surfaces of both end parts of a short pipe with a tapered female thread part 2 which is meshed with the tapered make thread part 1, during stamping by vertically inserting pin P into the box B so as to start screwing, one or both of the thread crests 3 in the vicinity of the inlet of the box and in the vicinity of the distal end of the pin P define(s) a gap or gaps between both stamping surfaces 4, in such a condition that stamping flank faces 4 of both thread crests 3 are overlapped with each other. By setting pitches on a final path or only the final pass during tapping to a slightly greater value, thereby it is possible to grind the stamping flank surfaces while the pitches on a load flank surface is maintained to be constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a threaded pipe joint for pipes used in oil country tubular goods and civil engineering piles, etc., which has a screw shape that is less likely to seize at the time of fastening, and a method of processing the same.

[0002]

2. Description of the Related Art In an oil well drilling operation, an oil well pipe having a tapered thread joint at a pipe end is fastened and lowered vertically one after another on a drill well. The tapered threaded joint is composed of a pin having a tapered male thread formed on the outer peripheral surface of the oil well tube tip, and a box having a tapered female thread formed on the inner peripheral surface of both ends of the short pipe to be screwed to the tapered male thread. At the time of fastening, grease is used as a lubricant, or a solid lubricating coating is applied.
In the fastening operation, as shown in FIG. 1 (a), the pin P at the tip of the oil country tubular good is inserted into the box B in the vertical direction indicated by the arrow, and then screwed.
Threadedly into the tapered female screw portion 2.

When screwing is started (hereinafter, the state of insertion before starting screwing is called stubbing), FIG.
As shown in the enlarged view of (b), the screw thread 3 of the pin P fits between the screw threads 3 of the box B, and the stubbing flank surfaces 4 of the screw thread 3 overlap and screw together. . w indicates the thread width, and 5 is the load flank surface. At the time of stubbing, the fitting of the threads 3 is shallow as shown in FIG. 1B, so that the pin P is displaced with respect to the center line C as shown in FIG. It fits while making a rotating motion.

[0004]

In the above-described operation for fastening an oil country tubular good, when stubbing is performed, the ends of the thread rows overlapping each other, and the thread Nos. 1, 2, 3, and 11, and 11 in FIG.
In the parts 12 and 13, as shown in the example shown by the solid line in FIG.
The load sharing on the stubbing flank surface 4 is larger than other portions. Further, as described above, the pin P is fitted while being shifted with respect to the center line C. For this reason, at the end of the screw row, the stubbing flank face 4 overlapping as shown in FIG. 1B and the thread crest top corner 6 adjacent thereto.
In some cases, burn-in occurs.

[0005] In recent oil well drilling work, when fastening oil well pipes, grease containing no heavy metal is applied from the viewpoint of environmental protection, and further, introduction of a solid lubricating joint omitting grease, or fastening and lowering of oil well pipes The aim is to speed up the work and further automate the fastening work. However, such an orientation causes a decrease in the lubrication function of the grease and a lack of attention in fastening work, etc.
This is a factor that promotes seizure when fastening the tapered threaded joint. Under such circumstances, the problem to be solved by the present invention is to provide a pipe joint having a thread shape that is less likely to seize at the time of fastening in a tapered threaded joint of a pipe used for an oil country tubular good or an civil engineering pile, and a method of processing the same. It is.

[0006]

According to a first aspect of the present invention, there is provided a pipe joint according to the present invention, wherein a pin having a tapered male thread formed on an outer peripheral surface of a distal end portion of a pipe is provided. And a box formed with a tapered female thread portion screwed to the tapered male thread portion on the peripheral surface.
The pins are inserted into the box in the vertical direction and screwing is started.When the stubbing flank surfaces of the threads of the tapered male screw portion and the tapered female screw portion overlap with each other, the two studs of the screw thread near the box entrance are overlapped. This is a pipe joint with excellent seizure resistance, characterized by having a gap between the bing flank surfaces.

In a first aspect of the first invention pipe joint, the tapered female thread of the box is formed with a uniform thread width, and the tapered male thread of the pin has a load flank surface with a constant pitch. Excellent in seizure resistance, characterized in that the thread width is gradually narrowed from the pin tip toward the pin root, or is gradually narrowed from the middle of the pin screw row toward the pin root. It is a pipe joint.

In a second aspect of the first invention pipe joint, the tapered male thread of the pin is formed with a uniform thread width, and the tapered female thread of the box has a load flank surface at a constant pitch. This is a pipe joint with excellent seizure resistance, characterized in that the thread width near the entrance of the box is formed narrow.

A second invention pipe joint comprises a pin having a tapered male thread formed on an outer peripheral surface of a distal end portion of a pipe, and a tapered female thread portion screwed into the tapered male thread portion on inner peripheral surfaces of both ends of a short pipe. In the tapered threaded joint composed of the box formed with the above, the pin is inserted into the box in the vertical direction and screwing is started, and the stubbing flank surfaces of the thread of the tapered male screw portion and the tapered female screw portion overlap each other. This is a pipe joint having excellent seizure resistance, characterized by having a gap between both stubbing flank surfaces in the thread near the pin tip in the state.

In a first aspect of the pipe joint according to the second aspect of the invention, the tapered male thread of the pin is formed with a uniform thread width, and the tapered female thread of the box has a load flank surface with a constant pitch. Excellent in seizure resistance, characterized in that the thread width is gradually narrowed from the box entrance to the center of the box, or gradually from the middle of the box screw row toward the center of the box It is a pipe fitting.

In a second aspect of the second invention, the tapered female thread of the box is formed with a uniform thread width, and the tapered male thread of the pin has a load flank surface at a constant pitch. A pipe joint having excellent seizure resistance, characterized in that the thread width near the tip of the pin is formed narrow.

According to a third aspect of the present invention, there is provided a pipe joint comprising: a pin having a tapered male thread formed on an outer peripheral surface of a distal end portion of a pipe; and a tapered female thread portion screwed to the tapered male thread portion on inner peripheral surfaces of both ends of the short pipe. In the tapered threaded joint composed of the box formed with the above, the pin is inserted into the box in the vertical direction and screwing is started, and the stubbing flank surfaces of the thread of the tapered male screw portion and the tapered female screw portion overlap each other. In this state, a pipe joint having excellent seizure resistance is characterized by having a gap between both stubbing flank surfaces for a screw thread near a box entrance and a screw thread near a pin tip.

[0013] A first invention method of the present invention for solving the above-mentioned problems is that, when the pin in the first aspect of the first invention pipe joint is threaded by an NC lathe, a final pass or a final threading process is performed. By slightly increasing the pitch of only a few passes, while keeping the pitch of the load flank constant, the thread width is gradually narrowed from the pin tip to the pin root, or the final pass or final pass Only by slightly increasing the pitch in the middle of the pin screw row, the pitch of the load flank face is kept constant, and the thread width is gradually narrowed from the middle of the pin screw row toward the pin root. This is a method for processing pipe joints with excellent seizure resistance.

[0014] The second invention method is characterized in that when the box in the second aspect of the second invention pipe joint is threaded by an NC lathe,
By slightly increasing the pitch only in the final pass or the last few passes of thread cutting, while keeping the pitch of the road flank constant, gradually narrow the thread width from the box entrance to the center of the box, Alternatively, by slightly increasing the pitch from the middle of the box screw row only in the final pass or the last few passes, while keeping the pitch of the load flank constant, the screw thread width is shifted from the middle of the box screw row toward the center of the box. This is a method for processing a pipe joint having excellent seizure resistance, characterized in that the pipe joint is gradually narrowed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A pipe joint according to the present invention comprises, as shown in FIG. 1 (a), a pin P having a tapered male thread 1 formed on an outer peripheral surface of a distal end portion of a pipe, and a pipe P having two ends. Box B having a peripheral surface formed with a tapered female thread 2 screwed to the tapered male thread 1
In the tapered threaded joint composed of the following, when the pin P is inserted into the box B in the vertical direction and the screwing is started, the stubbing flank surfaces 4 of the two screw threads 3 overlap each other as shown in FIG. One or both screw threads 3 near the box entrance and near the pin tip are provided with a gap between both stubbing flank surfaces 4.

Such a pipe joint according to the present invention has several threads located at the end of the screw thread to be screwed together at the time of stubbing for fastening. In the example of FIG. The stubbing flank face 4 of the thread 3 of No. 3 or / and No. 11, 12, 13 is gradually and gradually contacted in the direction of the thread No. 6, 7, 8 in the center of the screw row. be able to. For this reason, the corresponding portion of the pin P or the box B, the thread N
o, 1, 2 and 3 or / and No. 11, 12, and 13 the excessive load increase on the stubbing flank surface 4 is alleviated,
Burn-in can be avoided.

FIG. 2 shows the result of calculating the distribution of the load applied to the stub flank surface 4 of each thread 3 of the screw array by the FEM (finite element method) during stubbing. The solid line is an example of a conventional joint as described above. The API oil well pipe (outside diameter 9-5 / 8 in., Single weight 53.5 lbs / ft buttress joint) having a pin P formed at the tip is hung 9 m from above and inserted into box B, and the load distribution during stubbing is shown. From this figure, it can be seen that the load on the thread at the end of the thread row is significantly larger than that at the center of the thread row.

A broken line is an example of the pipe joint of the present invention, which is a pipe joint corresponding to the second aspect of the first invention described below. In this example, the stubbing flank surface 4 is set to 0.04 mm, 0.035 mm.
A pin P having a normal thread row, similar to the above-mentioned conventional joint, is inserted into the box B which has been cut down while reducing by m, and the load distribution at the time of stubbing is shown. In this example, the thread near the entrance of the box is provided with a gap between the stubbing flank surfaces that overlap each other. When stubbing, the gap at the moment when the thread 3 fits into each other is the thread No.13. 0.04mm, No.12 0.035mm, N
o.11 is 0.03 mm ... For this reason, it can be seen that the remarkable increase in the load at the entrance of the box during stubbing is eliminated. Although a load distribution is not shown when a gap is provided for the thread near the tip of the pin, a remarkable increase in load at the tip of the pin during stubbing is eliminated. In addition, when a gap is provided both in the vicinity of the box entrance and in the vicinity of the pin tip, a remarkable increase in the load at both ends of the screw array is eliminated.

The first invention pipe joint has a gap between the two stubbing flank surfaces 4 in the vicinity of the box entrance with the stubbing flank surfaces 4 of the two screw threads 3 overlapping each other during stubbing. It is a thing. FIG. 3 shows a first embodiment of the first invention pipe joint. In this example, the screw shape is exaggerated. The tapered female thread portion 2 of the box B is formed with a thread 3 having a uniform thread width, and the tapered male thread portion 1 of the pin P is formed such that the load flank surface 5 of the thread 3 has a constant pitch and the thread width is The w is gradually narrowed from the pin tip 8 toward the pin root. As a result, a gap 9 is provided between the overlapping stubbing flank surfaces 4 of the thread 3 near the box entrance 7.

In order to manufacture a pipe joint as shown in FIG. 3, for example, when the tapered male thread portion 1 of the pin P is threaded, only the final finish 1 to 2 passes, the origin at the beginning of threading, ie, the pin tip The processing pass line from 8 to the root is not changed, and only the pitch is slightly increased. In this way, the stubbing flank surface 4 of the pin P is gradually cut from the pin tip 8 toward the root without changing the position (pitch) of the load flank surface 5 of the pin P while gradually increasing the cutting margin. The thread width w as described above
Are gradually narrowed.

At the time of stubbing the pipe joint as shown in FIG.
Then touch each other. But b near box entrance 7
Has a gap 9, and the gap 9 gradually widens from the pin tip 8 side toward the box entrance 7, so that the stubbing flank surface 4 on the box entrance 7 side during stubbing.
Is gradually performed toward the center of the screw array from a state where the gap 9 is present to a state where the contact is surely made, and the contact of the stubbing flank surface is reduced toward the box entrance 7 side. As a result, seizure of the pins P and the box B near the box entrance 7 can be avoided.
Also, due to the displacement of the pin P at the time of screwing in, the conventional joint has a large one-side contact at the box entrance 7 side.
In the invention pipe joint, this one-sided contact is also alleviated and seizure is effectively avoided.

The first aspect of the first invention pipe joint is shown in FIG.
As shown in the exaggerated example, the thread width w of the pin P may be gradually narrowed from the middle c of the pin screw row toward the pin root. In this way, even if a gap 9 is provided between the stubbing flank surfaces 4 for the screw threads 3 near the box entrance 7, the contact of the stubbing flank is reduced toward the box entrance 7 side during stubbing. The one-side contact on the entrance 7 side is also reduced, and the sticking of the pin P and the box B near the box entrance 7 can be avoided. The pipe joint as shown in FIG.
When the tapered male screw portion 1 is threaded, it can be manufactured by slightly increasing the pitch from the middle c of the pin screw row toward the root of the pin only in the final finishing 1 to 2 passes.

In a second aspect of the first invention pipe joint, the tapered male thread portion 1 of the pin P is formed with a uniform thread width, and the tapered female thread portion 2 of the box B has a constant load flank surface 5. At the pitch, the thread width w near the box entrance 7 is formed narrow. In this way, even if the gap 3 is provided between the stubbing flank surfaces 4 for the threads 3 near the box entrance 7, the contact of the stubbing flank near the box entrance 7 during stubbing is eased, and the box entrance is also reduced. The one-side contact on the 7 side is also alleviated, and seizure of the pin P and the box B can be avoided. Such a box B
For example, in the finishing pass of the threading process, the stubbing flank surface at the end of the box entrance 7 side is shifted by shifting the phase of the threading process. Further, since the overlap margin is as small as less than 0.5 mm for any screw, a method may be used in which a normal screw row is machined and then only the stubbing flank surface is slightly cut as a cleaning pass.

In the pipe joint according to the second aspect of the present invention, a gap is provided between the stubbing flank surfaces 4 in the vicinity of the tip of the pin with the stubbing flank surfaces 4 of the two screw threads 3 overlapping each other during stubbing. It is a thing. FIG. 4 shows a first embodiment of the second invention pipe joint. In this example, the screw shape is exaggerated. The tapered male thread portion 1 of the pin P is formed with a thread 3 having a uniform thread width, and the tapered female thread portion 2 of the box B is formed such that the load flank surface 5 of the thread 3 has a constant pitch and the thread width is w is gradually narrowed from the box entrance 7 toward the center of the box. As a result, a gap 9 is provided between the overlapping stubbing flank surfaces 4 in the thread 3 near the pin tip 8.

In order to manufacture the pipe joint as shown in FIG. 4, for example, when threading the tapered female thread portion 2 of the box B, only the final finish of one or two passes, the origin at the beginning of threading, ie, the box entrance. The processing pass line from 7 to the center of the box is not changed, and only the pitch is slightly increased. In this way, the stubbing flank face 4 of the box B is gradually cut from the box entrance 7 toward the center of the box without changing the position (pitch) of the load flank face 5 while increasing the allowance. And the thread width w is gradually narrowed as described above.

At the time of stubbing the pipe joint as shown in FIG. 4, the overlapping stubbing flank surfaces 4 come into contact with each other at a point b near the box entrance 7. However, a near the pin tip 8
Has a gap 9, and the gap 9 gradually widens from the box entrance 7 side toward the pin tip 8, so that the stubbing flank surface 4 on the pin tip 8 side is in contact with the gap 9 during stubbing. The contact is made gradually toward the center of the thread row, and the contact of the stubbing flank surface is reduced toward the pin tip 8 side. As a result, seizure of the pin P and the box B near the pin tip 8 can be avoided.

The first aspect of the second invention pipe joint is shown in FIG.
As shown in the exaggerated example of FIG.
However, it may be formed gradually narrower from the middle c of the pin screw row toward the center of the box. In this manner, the stubbing flank surface 4 of the thread 3 near the pin tip 8 is formed.
4, the contact of the stubbing flank surface is softened toward the pin tip 8 side during stubbing, as in the example of FIG. 4, and the sticking of the pin P and the box B near the pin tip 8 is prevented. Can be avoided. The pipe joint as shown in FIG.
For example, when the tapered female thread portion 2 of the box B is threaded, it can be manufactured by slightly increasing the pitch from the middle c of the pin screw row toward the center of the box only in the final finishing 1 to 2 passes.

In a second aspect of the second invention pipe joint, the tapered female thread 2 of the box B is formed to have a uniform thread width, and the tapered male thread 1 of the pin P has a constant load flank surface 5. , The thread width w near the pin tip 8 is formed to be narrow. Thus, the screw thread 3 near the pin tip 8
However, even if a gap 9 is provided between the stubbing flank surfaces 4, the contact of the stubbing flank surface near the tip 8 of the pin during stubbing is alleviated, and seizure of the pin P and the box B can be avoided. Such a pin P can be obtained, for example, by shifting the phase of the screw processing on the stubbing flank surfaces at the several ends on the pin tip 8 side in the finishing pass of the screw cutting. Also, the overlap margin is 0.5 for any screw.
Since it is a little less than mm, after processing a normal screw row,
As a cleaning pass, a method of slightly shaving only the stubbing flank surface may be used.

In the third invention pipe joint, the stubbing flank surfaces 4 of the two screw threads 3 are overlapped with each other during stubbing, and the screw threads 3 near the box entrance and near the tip of the pin are formed.
In this case, a gap is provided between both stubbing flank surfaces 4. This aspect may be a combination of each aspect of the first invention pipe joint and each aspect of the second invention pipe joint.

According to the first invention method of the present invention, when the pin P in the first embodiment of the first invention pipe joint is thread-cut with an NC lathe, the pitch is slightly reduced only in the last pass or the last several passes of the thread cutting. By increasing the width, the thread width w is gradually narrowed from the pin tip 8 toward the pin root while the pitch of the load flank surface 5 is kept constant, or only the last pass or the last few passes are provided with a pin screw row. By slightly increasing the pitch in the middle, the thread width w is gradually reduced from the middle of the pin screw array toward the base of the pin while keeping the pitch of the load flank surface 5 constant. Here, the final pass or the final number of passes is a path from the pin tip 8 to the root. In these passes, only the pitch is slightly increased without changing the machining pass line, so that the position of the load flank surface 5 is not changed and the stubbing flank surface is gradually cut while increasing the margin. it can. The final number of passes may be 2-3 passes. By using the first invention method, machining can be performed with the same tool without changing the threading tool, so that it is only necessary to add a threading path having a larger pitch to the conventional threading program to the NC program.

In the second invention method, when the box B in the second embodiment of the second invention pipe joint is thread-cut with an NC lathe, the pitch is slightly increased only in the last pass or the last several passes of the thread cutting process. With the pitch of the load flank surface 5 kept constant, the screw thread width w is gradually narrowed from the box entrance 7 toward the center of the box, or the pitch from the middle of the box screw row only in the last pass or the last few passes. By slightly increasing the
This is a processing method in which the thread width w is gradually narrowed from the middle of the box screw row toward the center of the box while keeping the pitch of the load flank surface 5 constant. Here, the final pass or the final number pass is a pass from the box entrance 7 to the center of the box. In these passes, the pitch is slightly increased without changing the machining path line,
Without changing the position of the load flank surface 5, the stubbing flank surface can be cut little by little while increasing the allowance. The final number of passes may be 2-3 passes. If the second invention method is used, machining can be performed with the same tool without changing the threading tool, so that it is only necessary to add a threading path having a larger pitch to the conventional threading program to the NC program.

[0032]

EXAMPLE An API buttress joint having an outer diameter of 7 inches and a single weight of 29 lb / ft was processed by threading a pin P according to the first invention and a box B according to the second invention. The pitch of the final pass of the male screw 1 of the pin P is changed from 5.08 mm to 5.11 mm, the pitch of the load flank 5 is not changed, and the thread width w is gradually increased from the pin tip 8 toward the pin root. It was formed narrow. The female screw 2 of the box B was also formed so that the pitch of the final pass was 5.08 mm to 5.11 mm, the pitch of the load flank face 5 was not changed, and the thread width w was gradually narrowed from the box entrance 7 toward the center of the box. . Further, the pin P and the box B having a uniform thread width w were thread-cut by a conventional processing method in which the pitch was constant at 5.08 mm until the final pass.

Then, the pin P processed by the first invention method is used.
And a box joint processed by the above-mentioned conventional method, and a box joint processed by the above-mentioned second invention method.
For the second invention pipe joint obtained by combining the above and the pin P processed by the conventional method, a tightening test was repeatedly performed with a vertical power tong. In the experiment, manganese phosphate treatment was applied to the inner surface of box B, and environmentally compatible grease (multi-make white) containing no heavy metal was applied to pin P and box B for each tightening, and assuming actual on-site work A pin P having a weight of 500 kg attached to the upper part of the 2 m sample was inserted into the box B and screwed and loosened 10 times, but no seizure was observed. No abnormalities due to differences in location were observed.

On the other hand, in a similar test on a conventional joint in which the pin P and the box B processed by the above-mentioned conventional method were combined, no seizure occurred, but the area near the pin tip 8 and the box entrance 7 during stubbing was equivalent. The stubbing flank surface 4 near the crest of the thread 3 to be scratched had scratches, indicating that seizure was likely to occur.

[0035]

As described above, by using the pipe joint of the present invention, an oil well pipe can be lowered while fastening an oil well pipe in an oil well drilling operation, or a steel pipe used as a civil engineering pile at a construction site or the like can be connected. During stubbing when screwing a pin into a box, the concentrated load on the stubbing flank near the entrance of the box or at the end of the screw row near the end of the pin can be reduced, and the pin can be seen at the initial stage of screwing. The contact with the thread near the entrance of the box due to the drift can also be reduced. Therefore, from the viewpoint of environmental protection, grease containing no heavy metal is adopted as the lubricant, and furthermore, seizure can be avoided even under more severe conditions, such as when automating pipe fastening. It is also effective for joints that have been coated with a solid lubricant to completely eliminate grease.

[Brief description of the drawings]

FIG. 1A shows an example of a tapered threaded joint to which the present invention is applied, and is a cross-sectional view at the time of stubbing in which a pin P is inserted into a box B and screwing is started. (B) is an enlarged view of the vicinity of the tip of the pin surrounded by the square in (a).

FIG. 2 is a graph showing a thread load distribution at the time of stubbing in the pipe joint of the present invention and a conventional joint.

FIG. 3 is a sectional view showing an example of a first aspect of the first invention pipe joint;

FIG. 4 is a cross-sectional view showing an example of a first aspect of the second invention pipe joint.

FIG. 5 is a cross-sectional view showing another example of the first aspect of the first invention pipe joint.

FIG. 6 is a cross-sectional view showing another example of the first aspect of the second invention pipe joint.

[Description of Signs] 1 ... Tapered male screw portion 2 ... Tapered female screw portion 3 ... Thread 4 ... Stubbing flank surface 5 ... Load flank surface 6 ... Thread ridge top corner 7 ... Box entrance 8 ... Pin tip 9 ... Gap P: Pin B: Box w: Thread width

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Haruyuki Nagayoshi 1-1 Hibata-cho, Tobata-ku, Kitakyushu-shi Nippon Steel Corporation Yawata Works F-term (reference) 3H013 GA01 GA08

Claims (9)

[Claims] 1. A pin having a tapered external thread formed on an outer peripheral surface of a distal end portion of a tube, and a box having a tapered female screw portion formed on an inner peripheral surface of both ends of a short tube to be screwed with the tapered external thread portion. In the tapered threaded joint formed by inserting the pin into the box in the vertical direction and starting screwing, the stubbing flank faces of the threads of the tapered male thread portion and the tapered female thread portion overlap with each other at the box entrance. A pipe joint with excellent seizure resistance characterized by having a gap between both stubbing flank faces in the vicinity of the thread. 2. The pipe joint according to claim 1, wherein the tapered female screw portion of the box is formed with a uniform thread width, and the tapered male screw portion of the pin has a load flank surface with a constant pitch, and A pipe joint with excellent seizure resistance, characterized in that the peak width is gradually narrowed from the pin tip toward the pin root, or is gradually narrowed from the middle of the pin screw row toward the pin root. . 3. The pipe joint according to claim 1, wherein the tapered male thread portion of the pin is formed with a uniform thread width, and the tapered female thread portion of the box has a load flank surface at a constant pitch, A pipe joint with excellent seizure resistance characterized by a narrow thread width near the entrance. 4. A pin having a tapered external thread formed on an outer peripheral surface of a distal end portion of a tube, and a box having a tapered female screw portion formed on an inner peripheral surface of both ends of a short tube to be screwed with the tapered external thread portion. In the tapered threaded joint composed of, the pin is inserted into the box in the vertical direction and screwing is started, and the stubbing flank surfaces of the threads of the tapered male thread portion and the tapered female thread portion are overlapped with each other. A pipe joint with excellent seizure resistance characterized by having a gap between both stubbing flank faces in the vicinity of the thread. 5. The pipe joint according to claim 4, wherein the tapered male thread portion of the pin is formed with a uniform thread width, and the tapered female thread portion of the box is formed such that a load flank surface has a constant pitch. A pipe with excellent seizure resistance, characterized in that the peak width is gradually narrowed from the box entrance toward the center of the box, or is gradually narrowed from the middle of the box screw row toward the center of the box. Fittings. 6. The pipe joint according to claim 4, wherein the tapered female thread portion of the box is formed with a uniform thread width, and the tapered male thread portion of the pin has a load flank surface with a constant pitch, and A pipe joint with excellent seizure resistance, characterized by a narrow thread width near the tip. 7. A pin having a tapered external thread formed on an outer peripheral surface of a distal end portion of a tube, and a box having a tapered female screw portion formed on an inner peripheral surface of both ends of a short tube to be screwed with the tapered external thread portion. In the tapered threaded joint formed by inserting the pin into the box in the vertical direction and starting screwing, the stubbing flank faces of the threads of the tapered male thread portion and the tapered female thread portion overlap with each other at the box entrance. A pipe joint with excellent seizure resistance, characterized in that there is a gap between both stubbing flank surfaces for the thread near the pin and the thread near the tip of the pin. 8. When the pin of the pipe joint according to claim 2 is threaded by an NC lathe, the pitch of the load flank surface is constant by slightly increasing the pitch only in the last pass or the last several passes of the threading process. , Or gradually narrow the thread width from the pin tip to the pin root,
Alternatively, by slightly increasing the pitch from the middle of the pin screw row only in the final pass or the last few passes, the thread width is gradually increased from the middle of the pin screw row toward the pin root while the pitch of the load flank surface is kept constant. A method of processing a pipe joint having excellent seizure resistance, characterized in that it is made narrower. 9. When the box of the pipe joint according to claim 5 is threaded by an NC lathe, the pitch of the load flank surface is constant by slightly increasing the pitch only in the last pass or the last several passes of the threading process. While keeping the screw width, gradually narrow the thread width from the entrance of the box toward the center of the box, or increase the pitch slightly from the middle of the box screw row only in the last pass or the last few passes to obtain the road flank surface. Characterized in that the thread width is gradually narrowed from the middle of the box screw row toward the center of the box while keeping the pitch constant.