JP2024136102A - Steel pipe threaded joint and manufacturing method thereof - Google Patents

Abstract

To achieve weight reduction in a body part by reducing wall thickness and increasing strength, and to increase tensile strength of a steel pipe.SOLUTION: A steel pipe screw joint has: a pin 2 in which a male screw 2A is formed on an outer surface of a first steel pipe 10A; and a box 3 in which a female screw 3A which can be screwed with the male screw 2A of the pin 2 is formed on an inner surface of a second steel pipe 10B. In a screw row of the pin 2 and the box 3, screw row taper parts 21, 31 are formed which are inclined with respect to a pipe axis O of the steel pipe 10 at a screw row taper angle θ1. A minimum inner diameter D1 of the pin 2 is smaller than a steel pipe inner diameter D2 of body parts 11A, 11B excluding the pin 2 and the box 3 out of the steel pipe 10. The pin 2 has a pin continuously-provided part 22 which is inclined by a pin taper angle θ2 from a male screw terminal 21a of the male screw 2A to the pin body part 11A. The pin taper angle θ2 is equal to or less than two times the screw row taper angle θ1.SELECTED DRAWING: Figure 2

Description

The present invention relates to a steel pipe threaded joint and a method for manufacturing a steel pipe threaded joint.

Traditionally, when excavating tunnels in soft ground, the AGF method (All Ground Fasten, a long steel pipe fore-supporting method) has been used to prevent collapse. With this type of AGF method, multiple steel pipes, for example with outer diameters of 76.3 mm to 114.3 mm, are often driven into the cross section of the tunnel face to a depth of 12.5 m. In this case, the steel pipes are about 3 m long each, and threaded joints machined into the ends of the pipes on an excavator commonly known as a drill jumbo are tightened manually at three points per pile, and the total length is completed to 12.5 m while excavating.

In conventional joining of steel pipes for piles as described above, cut threaded joints have been used from the viewpoints of cost and ease of construction (see, for example, Patent Documents 1 and 2).
Known examples of cut threaded joints include threaded joints 100A, 100B, 100C and the like of the types shown in Figures 9(a) to 9(c).

The first threaded joint 100A shown in Fig. 9(a) is a type in which a pin 104 with a male thread 101 and a box 105 with a female thread 102 are previously formed in a thick-walled steel pipe, and are integrated and screwed to a pile body 103 by welding, friction welding, diffusion welding, etc. The second threaded joint 100B shown in Fig. 9(b) is a type in which a box 106 with a female thread 102 is screwed to the pile body 103 with nipples 107 with male threads on both ends. The third threaded joint 100C shown in Fig. 9(c) is a type in which a pin 104 with a male thread 101 and a box 105 with a female thread 102 are screwed directly to the pipe end of the pile body 103 without preliminary processing.
Another type of threaded joint is the coupling type, which protrudes outward toward the outer diameter of the steel pipe, but this is not preferred because it creates resistance when driving the joint into the ground.

Japanese Patent Application Publication No. 11-107272 JP 2015-110994 A

However, conventional steel pipe threaded joints have the following problems.
That is, in the first threaded joint 100A shown in Fig. 9(a) among conventional steel pipe threaded joints, from the viewpoint of manufacturing cost, a joint blank pipe must be supplied to integrate the pre-machined threaded portion by welding or the like, and the cost of welding must be increased. In the case of the second threaded joint 100B shown in Fig. 9(b), a joint blank pipe must be supplied to use a joining part such as a nipple 107, and two sets of male and female threads must be machined for one joint of the pile, which increases the cost. In addition, the third threaded joint 100C shown in Fig. 9(c) is more desirable than the first threaded joint 100A and the second threaded joint 100B described above from the viewpoint of manufacturing cost, but has a problem that it becomes difficult to ensure the tensile strength of the joint when the steel pipe is thin-walled.

Furthermore, in tunnel construction methods, steel pipes made of STK400 material have been used, but with a wall thickness of about 5mm to 6mm, they weigh up to about 50kg per 3m length, and because they are handled as a 3m component including the weight of the drill bit and the shaft that transmits power to the bit, rather than as a single steel pipe, the weight of each component can exceed 50kg. Since it requires a considerable amount of effort for one person to insert and fasten a component weighing more than 50kg on the drill jumbo excavator mentioned above, there is a demand for lighter steel pipes and easier fastening of threaded joints from the standpoints of safety and efficiency.

The present invention was made in consideration of the above-mentioned problems, and aims to provide a steel pipe threaded joint and a method for manufacturing a steel pipe threaded joint that can reduce the weight of the main body by making it thinner and stronger, and can increase the tensile strength of the steel pipe.

(1) Aspect 1 of the steel pipe threaded joint according to the present invention is a steel pipe threaded joint for fastening steel pipes to be driven into the ground in construction work by screwing them together, comprising a pin having a male thread formed on the outer surface of one of the steel pipes, and a box having a female thread formed on the inner surface of the other steel pipe that can be screwed into the male thread of the pin, the thread row of the pin and the box is formed with a thread row taper portion inclined at a first taper angle with respect to the pipe axis of the steel pipe, the minimum inner diameter of the pin is smaller than the steel pipe inner diameter of the main body portion of the steel pipe excluding the pin and the box, the pin has a pin connection portion inclined by a second taper angle from the end of the complete thread portion of the male thread to the main body portion, and the second taper angle is less than twice the first taper angle.

According to the steel pipe threaded joint of the present invention, in a thread structure in which a pin having a male thread machined directly on the end of one steel pipe is screwed into a box having a female thread machined on the end of the other steel pipe, the outer diameters of both steel pipes are matched, and the minimum inner diameter of the pin is made smaller than the inner diameter of the steel pipe of the main body, and the second taper angle of the pin connecting portion is set to be equal to or less than twice the first taper angle, so that the inner hollow cross-sectional area of the steel pipe can be secured without hindrance to construction, and the steel pipe can be thinned while maintaining the same outer diameter while suppressing a decrease in tensile strength. Therefore, with the steel pipe threaded joint of the present invention, the tensile strength of the steel pipe can be increased compared to conventional steel pipe joints, and the steel pipe can be thinned even with the same tensile strength as conventional steel pipes, thereby reducing the weight of the steel pipe.
In this way, the present invention reduces the work of manually tightening the steel pipes because the steel pipes are thin-walled and lightweight, and improves the efficiency of the work. In this case, the tensile strength of the steel pipes can be ensured, so that the steel pipes can be driven into the ground without changing the driving intervals, etc.

(2) In aspect 2 of the present invention, in the steel pipe threaded joint of aspect 1, it is preferable that the minimum inner diameter of the pin is 80% or more of the inner diameter of the steel pipe of the main body when the steel pipes are fastened together.

In this case, by limiting the minimum inner diameter of the pin to 80% or more of the inner diameter of the main steel pipe when the steel pipes are fastened together, it is possible to generally allow the passage of excavation drills, mortar injection pipes, etc., inside the steel pipe. Therefore, even if the pin protrudes into the inner diameter of the steel pipe within the above range, it is possible to ensure a cross-sectional area inside the steel pipe that allows conventional work inside the steel pipe to be carried out without hindrance.

(3) Aspect 3 of the method for manufacturing a steel pipe threaded joint according to the present invention is a method for manufacturing a steel pipe threaded joint for manufacturing a steel pipe threaded joint according to aspect 1 or aspect 2, characterized in that it comprises the steps of machining the box including the female thread at one end of the steel pipe, performing a diameter reduction process to reduce the diameter of one end of the steel pipe, and machining the pin including the male thread in the reduced diameter portion after the diameter reduction process.

According to the manufacturing method of the steel pipe threaded joint of the present invention, the precision of the male and female threads can be ensured in the steel pipe threaded joint having the above-mentioned configuration, and the thread row taper portion and the pin connecting portion of the pin can be efficiently machined.

The steel pipe threaded joint and manufacturing method for the steel pipe threaded joint of the present invention can reduce the weight of the main body by making it thinner and increasing its strength, and can increase the tensile strength of the steel pipe.

FIG. 1 is a longitudinal cross-sectional view showing a steel pipe threaded joint according to an embodiment of the present invention, in which (a) is a view of steel pipes fastened to each other, and (b) is a view before fastening. FIG. 2 is an enlarged view of a main portion of the steel pipe threaded joint shown in FIG. FIG. 3 is an enlarged view of a main portion showing a pin of the steel pipe threaded joint shown in FIG. 2 . FIG. 2 is a diagram showing an example of a method for manufacturing a steel pipe threaded joint according to an embodiment. FIG. 5 is a detailed view of a die used in the method for manufacturing the steel pipe threaded joint shown in FIG. FIG. 11 is a diagram showing another example of the method for manufacturing a steel pipe threaded joint. FIG. 11 is a diagram showing the stress distribution in a steel pipe threaded joint based on the analysis results of an example. FIG. 13 is a diagram showing an analysis result according to an embodiment, and is a diagram showing the relationship between the taper ratio and the joint tensile strength. 1A to 1C are partial cross-sectional views showing a conventional steel pipe threaded joint.

Below, a steel pipe threaded joint and a method for manufacturing a steel pipe threaded joint according to an embodiment of the present invention will be described with reference to the drawings.

As shown in FIG. 1, the steel pipe threaded joint 1 according to this embodiment is a taper threaded joint in which threads are machined directly into the pipe end in construction work, and is intended to fasten two steel pipes 10 (10A, 10B) together in the pipe axial direction by screwing them together.

The steel pipes 10A and 10B are straight pipes with no irregularities on the surface of the pipe body, and are applied to short steel pipes (e.g., about 3 m in length) with an outer diameter of 76.3 mm and a wall thickness of 2.6 mm, for example. Civil engineering steel pipes in which the steel pipes 10A and 10B are fastened in the axial direction of the pipe by the steel pipe threaded joint 1 of this embodiment are used as ground reinforcement steel pipes (e.g., tunnel head bolts or long forepiles, etc.) to reinforce the ground of tunnels or slopes, for example, in the AGF method, etc.

The steel pipes 10A and 10B connected using the steel pipe threaded joint 1 of this embodiment may be used as steel pipe piles for other purposes, such as foundation piles for buildings. For example, carbon steel pipes for general construction, carbon steel pipes for building construction, etc. can be used as the steel pipes 10A and 10B.

The steel pipe threaded joint 1 has a pin 2 with a male thread 2A machined on the outer surface of one steel pipe (first steel pipe 10A) on the right side of the paper in Figures 1(a) and (b), and a box 3 with a female thread 3A machined on the inner surface of the other steel pipe (second steel pipe 10B) on the left side of the paper, which can be screwed into the male thread 2A of the pin 2. When the male thread 2A of the pin 2 and the female thread 3A of the box 3 are completely connected, the male thread 2A and the female thread 3A are screwed together in close contact, and the pin 2 and the box 3 are stably fastened. The male thread 2A and the female thread 3A have a symmetrical concave-convex shape so that they fit together without any gaps. In the following, the parts of the steel pipe 10 other than the pin 2 and the box 3 are referred to as the main body parts 11A and 11B.

In the following description, the direction along the central axis (pipe axis O) of the steel pipes 10A and 10B formed by the pin 2 and box 3 is defined as the pipe axis direction. The end side of the pin 2 and box 3 in the pipe axis direction X is defined as the pipe end side, and the main body 11A, 11B side opposite the pipe end side is defined as the base end side. The direction perpendicular to the pipe axis O is defined as the radial direction, and the direction going around the pipe axis O as seen from the pipe axis direction X is defined as the circumferential direction. The side facing the pipe axis O in the radial direction is defined as the inner diameter side, and the direction away from the pipe axis O on the opposite side to the inner diameter side is defined as the outer diameter side. The imaginary axis parallel to the pipe axis O is indicated by the symbol O1.

The thread rows of the pin 2 and the box 3 are formed with thread row taper sections 21, 31 inclined at a thread row taper angle θ1 (first taper angle) relative to the pipe axis O of the steel pipes 10A, 10B, respectively. In the drawings, the thread row taper angle θ1 is the angle between a virtual axis O1 parallel to the pipe axis O and the pin thread row taper section 21, and the angle between the virtual axis O1 and the box thread row reverse taper section 31.

The pin thread row taper portion 21 of the pin 2 forms a male thread 2A on the outer surface of one end side of the first steel pipe 10A, with a taper that gradually reduces in diameter as it approaches the pipe axis O from the base end side to the pipe end side in the pipe axis direction X.
The box thread row reverse taper section 31 of the box 3 forms a female thread 3A on the inner surface on one end side of the second steel pipe 10B, with a taper that gradually expands in diameter away from the pipe axis O as it moves from the base end side to the pipe end side in the pipe axis direction X.

As shown in FIG. 2, the pin 2 is provided with, in order from the pin tip 2a to the pin base end 2b (the pin body 11A side of the first steel pipe 10A), a pin thread row taper portion 21 and a pin connection portion 22 that connects the male thread terminal end 21a of the male thread 2A to the outer circumferential surface 11a of the pin body 11A.

The box 3 has a box thread row reverse taper section 31 and a shoulder section 32 arranged on the base end side of the box thread row reverse taper section 31. The box 3 is connected to the inner peripheral surface 11d of the box body section 11B via the shoulder section 32.

When the steel pipes 10A and 10B are fastened by screwing the male thread 2A of the pin 2 into the female thread 3A of the box 3, the pin tip 2a and the female thread end 31a of the box 3 face each other radially, and the box tip 3a and the male thread end 21a of the pin 2 face each other radially.

The shoulder portion 32 of the box 3 is composed of an annular surface 32a that is nearly perpendicular to the pipe axis O, and an inner peripheral surface 11d of the box body 11B on the box 3 side. A small gap is formed between the annular surface 32a of the shoulder portion 32 and the pin tip 2a when the steel pipes 10A, 10B are fastened to each other. The shoulder portion 32 functions as a stopper that prevents the pin 2 from climbing up against the box 3. In addition, by providing a gap between the shoulder portion 32 and the pin 2, an allowable region is created in which the steel pipes 10 can be further tightened together due to vibrations during drilling of the steel pipes 10, and the steel pipes 10A, 10B can be fastened together more firmly.

The male thread 2A of the pin 2 and the female thread 3A of the box 3 are fitted (screwed) with each other at the pin thread row taper section 21 and the box thread row reverse taper section 31 with the same thread pitch, thread height, and thread row taper angle θ1. In the steel pipe thread joint 1, the pin 2 is inserted into the box 3 while rotating, and the male thread 2A of the pin 2 and the female thread 3A of the box 3 are sufficiently screwed together, so that the pin 2 and the box 3 are fastened together as shown in FIG. 1(a), and the two steel pipes 10A and 10B can be connected in the pipe axis direction X. The thickness dimension of the joint part where the male thread 2A and the female thread 3A are screwed together is greater than the thickness of each main body part 11A and 11B. Therefore, when the steel pipes 10A and 10B are fastened together, the pin 2 of the joint part protrudes toward the inner diameter side of the pin main body part 11A as described above.

As shown in Figures 1(b), 2 and 3, the minimum inner diameter D1 of the pin 2 is smaller than the steel pipe inner diameter D2 of the pin body 11A. In other words, the pin 2 protrudes toward the inner diameter side from the inner peripheral surface 11b of the pin body 11A. The minimum inner diameter D1 of the pin 2 is set to 80% or more, and more preferably 90% or more, of the steel pipe inner diameter D2 of the pin body 11A when the steel pipes 10A and 10B are fastened together.

Since various objects such as drills and mortar injection pipes pass through the inside of piles and steel pipes used in civil engineering works, it is necessary to limit the amount by which the joint part (pin 2) protrudes from the inner circumferential surface 11b of the pin body 11A toward the inner diameter. The upper limit of the protrusion amount varies depending on the type of work, but by setting the minimum inner diameter D1 of the pin 2 to 80% or more of the steel pipe inner diameter D2 of the pin body 11A, it is possible to allow the passage of drills, mortar injection pipes, etc.

In particular, assuming that thin-walled steel pipes with an outer diameter of 76.3 mm and a thickness of approximately 2.0 mm (inner diameter of approximately 72.3 mm) will be used in tunnel reinforcement work, if the minimum inner diameter D1 of pin 2 is made larger than the inner diameter 65.9 mm of the current steel pipe (outer diameter 76.3 mm, thickness 5.2 mm), currently used tools and equipment can be used as is without the need to prepare new tools and equipment. Therefore, it is more preferable to set the minimum inner diameter D1 of pin 2 to 90% or more of the steel pipe inner diameter D2 of pin body 11A (the ratio of the inner diameter 65.9 mm of the current steel pipe to the inner diameter 72.3 mm of the thin-walled steel pipe is approximately 91%).

3, the pin 2 has a pin connecting portion 22 extending from the male thread end 21a of the male thread 2A to the pin body 11A, which is inclined by a pin taper angle θ2 (second taper angle). In FIG. 3, the pin taper angle θ2 is the angle between the imaginary axis O1 parallel to the tube axis O and the pin connecting portion 22.
At this time, the pin taper angle θ2 is set to be equal to or less than twice the thread row taper angle θ1 (taper ratio θ2/θ1≦2). The pin connecting portion 22 is formed on the outer circumferential surface on the male thread end 21a side with a joint relief portion 23 for allowing a cutting tool to escape to the outer diameter side of the steel pipe during thread machining.

As shown in FIG. 3, by setting the pin taper angle θ2 in the pin-connected portion 22 to be no more than twice the thread row taper angle θ1, it is possible to set the minimum thickness t of the joint relief portion 23 to 90% or more relative to the dangerous cross section S. In other words, in order to suppress a decrease in the tensile strength of the joint due to the joint relief portion 23 being thinner relative to the dangerous cross section S of the joint, it is preferable to set the pin taper angle θ2 to be no more than twice the thread row taper angle θ1, taking into account an allowance for a thin wall of about 10% relative to the dangerous cross section S.

In addition, as the pin taper angle θ2 becomes smaller, the thickness of the pin connection portion 22 becomes thicker, so there is no lower limit for θ2 from the perspective of joint tensile strength. On the other hand, if the pin connection portion 22 protrudes inward from the inner diameter of the pin 2, there is a possibility that a tool such as a shaft passing through the inner diameter of the steel pipe may interfere, so it is preferable that θ2 is greater than 0. In other words, a more desirable range for the taper ratio θ2/θ1 is 0<θ2/θ1≦2.

Let me explain in more detail. General structural steel pipes (JIS G3444) that are often used for base pipes have a wall thickness tolerance of -12.5% to +15%, and the wall thickness of the base pipe is inevitably thinned to a maximum of -12.5%. In other words, considering that the tensile strength may decrease by that amount, it is acceptable for the minimum wall thickness t of the joint relief portion 23 to be approximately 10% thinner than the critical cross section S.

The length of the joint relief portion 23 in the pipe axial direction X is generally about 1 time the thread pitch, and the maximum thread pitch is assumed to be about 7 mm. In addition, the taper angle θ of a taper thread joint is generally about 1/12 to 1/16 based on the outer diameter. When converted to a taper angle relative to the wall thickness, it is about 1/24 to 1/32. Here, 1/32 relative to the wall thickness is an angle inclined at a ratio of 32 mm in the pipe axial direction X to 1 mm in the wall thickness direction, which is about 1.8 degrees. On the other hand, the maximum wall thickness of a general structural steel pipe with an outer diameter of about 200 mm is about 10 mm. Considering the thinning (weight reduction) of the wall thickness of the steel pipe to about half, the wall thickness of the dangerous section S is assumed to be about 4.5 mm. In order to suppress the amount of reduction in thickness of the threaded joint to 10% of the critical cross section S, that is, to 0.45 mm or less for a thread pitch of 7 mm, from the relationship in FIG. 3, the taper angle θ2 of the pin-connected portion 22 is tan-1 (0.45 mm/7.0 mm) = approximately 3.7 degrees.
As a result, by restricting the taper angle θ2 in the pin-connected portion 22 to less than twice the thread row taper angle θ1, the minimum thickness t of the joint relief portion 23 can be made 90% or more of the critical cross section S.

As shown in Figures 4 to 6, the manufacturing method for manufacturing the steel pipe threaded joint 1 of this embodiment includes the steps of machining a box 3 including a female thread 3A at one end of the second steel pipe 10B, performing a diameter reduction process to reduce the diameter of one end of the first steel pipe 10A, and machining a pin 2 including a male thread 2A in the reduced diameter portion after the diameter reduction process.

4 and 5 show an example of a method for reducing the diameter of the first steel tube 10A, and show a method in which the first steel tube 10A is moved in the tube axis direction X and pressed into a first die 4A of a predetermined shape.
As shown in FIG. 5, the first die 4A has an opening inclined surface 41 that expands in diameter toward the opening end surface 4b at the opening 4a, and a pin diameter-reducing tapered surface 42 formed to reduce the diameter from the base end 41a of the opening inclined surface 41 toward the back side in the insertion direction (pipe axis direction X) of the die 4. The pin diameter-reducing tapered surface 42 has the same angle as the angle of the pin thread row tapered portion 21 of the pin 2 (thread row taper angle θ1). The inclination angle θ3 of the opening inclined surface 41 is set to an angle approximately equal to the pin taper angle θ2 of the pin connection portion 22. The first steel pipe 10A can be reduced in diameter by moving it so as to be pushed into the first die 4A in the pipe axis direction X.

Figure 6 shows another example of a method for reducing the diameter of the first steel pipe 10A, and shows a method of reducing the diameter by pressing the pipe between a second mold 4B of a predetermined shape from above and below. The second mold 4B also has an opening inclined surface 41 and a pin diameter reduction tapered surface 42 similar to the first mold 4A shown in Figure 5 described above.

Next, the operation of the above-mentioned steel pipe threaded joint and manufacturing method of the steel pipe threaded joint will be described in detail with reference to Figures 1 to 6.

According to the steel pipe threaded joint 1 of this embodiment, the steel pipes 10A, 10B to be driven into the ground are fastened by being screwed together. The steel pipe threaded joint 1 has a pin 2 having a male thread 2A formed on the outer surface of a first steel pipe 10A, and a box 3 having a female thread 3A formed on the inner surface of a second steel pipe 10B that can be screwed into the male thread 2A of the pin 2.
The thread rows of the pin 2 and the box 3 are formed with thread row taper portions 21, 31 inclined at a thread row taper angle θ1 with respect to the pipe axis O of the steel pipe 10, and the minimum inner diameter D1 of the pin 2 is smaller than the steel pipe inner diameter D2 of the main body portions 11A, 11B of the first steel pipe 10A excluding the pin 2 and the box 3. The pin 2 has a pin connecting portion 22 inclined by a pin taper angle θ2 from the male thread end 21a of the male thread 2A to the pin main body portion 11A. The pin taper angle θ2 is equal to or less than twice the thread row taper angle θ1 (θ2/θ1≦2).

According to the steel pipe threaded joint 1 of this embodiment, the pipe end of the first steel pipe 10A is reduced in diameter, and a pin 2 with a male thread 2A directly machined is threaded into a box 3 with a female thread 3A engraved into the pipe end. In this screw structure, the outer diameters of the first steel pipe 10A and the second steel pipe 10B are matched, and the minimum inner diameter D1 of the pin 2 is made smaller than the steel pipe inner diameter D2 of the pin main body 11A, and the pin taper angle θ2 of the pin connecting portion 22 is set to be equal to or less than twice the thread row taper angle θ1. This allows the internal cross-sectional area of the steel pipe 10 to be secured without hindrance to construction, and the steel pipe can be thinned while maintaining the same outer diameter while suppressing a decrease in tensile strength. Therefore, with the steel pipe threaded joint 1 of this embodiment, the tensile strength of the steel pipe 10 can be increased compared to conventional steel pipe joints, and the steel pipe 10 can be thinned even with the same tensile strength as conventional steel pipes, thereby reducing the weight of the steel pipe 10.

Because the steel pipe 10 is thus made thin and lightweight, the work of manually holding and tightening the steel pipe 10 can be reduced, improving work efficiency. In this case, the tensile strength of the steel pipe 10 can be ensured, so the steel pipe 10 can be cast into the ground without changing the casting interval, etc.

Specifically, by forming the pin connected portion 22 and the pin thread row tapered portion 21 by subjecting the pin 2 to diameter reduction processing as shown in FIG. 2, it becomes possible to machine the male thread 2A without difficulty.
That is, in the steel pipe threaded joint 1 according to the present embodiment, the minimum inner diameter D1 of the pin 2 is made smaller than the steel pipe inner diameter D2 of the pin main body 11A, and the pin taper angle θ2 is made less than twice the thread row taper angle θ1 (θ2/θ1≦2), thereby solving the conventional problem that, as in the case of a threaded joint where the steel pipe inner diameter of the pin main body and the inner diameter of the pin are the same (the inner diameter does not change over the entire length of the steel pipe including the pin), although the male and female threads themselves can be machined, there are fewer threads that can be engaged even if some interference is tolerated, and the threads easily slip out (jump out) when a tensile force is applied.

That is, as in the case of the conventional threaded joint in which the inner diameter of the pin body and the inner diameter of the pin are the same, when a tensile force acts on the threaded joint, in the case of many joints, if there is no jump-out, the joint will break from the end of the male thread of the box or pin, that is, the so-called dangerous cross section. Therefore, when machining a threaded joint on a steel pipe after diameter reduction, it is necessary to perform machining so that the thickness of the dangerous cross section is as designed. In machining, the process starts from the pipe end side of the steel pipe, and after machining the complete thread portion, a joint relief portion is required to allow the cutting tool to escape to the outer diameter side of the steel pipe. In this embodiment in which the pin connecting portion 22 is provided, when the dangerous cross section S is provided near the boundary between the pin thread row tapered portion 21 and the pin connecting portion 22, an escape allowance is provided in the pin connecting portion 22.
In this case, if the pin taper angle θ2 from the pin connected portion 22 to the pin main body 11A is too steep, the thickness of the relief portion will be thinner than the critical cross section S, and as a result, it may break at that point when a tensile load is applied, and sufficient tensile strength may not be obtained. Therefore, by setting the pin taper angle θ2 to no more than twice the thread row taper angle θ1 (θ2/θ1≦2), it is possible to ensure that the cross-sectional area of the minimum thickness t of the joint relief portion 23 is 90% or more of the critical cross section S.

It is possible to move the position of the joint relief section 23 from the pin connection section 22 to the pin thread row taper section 21 by moving the dangerous section S toward the pipe end (toward the pin tip 2a); however, in this case, it is necessary to move the radial position of the male thread 2A further toward the inner diameter side, which increases the amount of protrusion of the pin 2 toward the inner diameter side, so this must be taken into consideration.

In addition, in this embodiment, by limiting the minimum inner diameter D1 of the pin 2 to 80% or more of the steel pipe inner diameter D2 of the pin body 11A when the steel pipes 10A and 10B are fastened together, it is possible to generally allow the passage of an excavation drill, a mortar injection pipe, etc., inside the steel pipe 10. Therefore, even if the pin 2 protrudes into the inner diameter side of the steel pipe 10 within the above range, it is possible to ensure a cross-sectional area inside the steel pipe that allows work to be performed without hindrance inside conventional steel pipes.

In addition, the manufacturing method for the steel pipe threaded joint 1 of this embodiment includes the steps of machining a box 3 including a female thread 3A at one end of the second steel pipe 10B, performing diameter reduction processing to reduce the diameter of one end of the first steel pipe 10A, and machining a pin 2 including a male thread 2A in the reduced diameter portion after the diameter reduction processing.
As a result, in the steel pipe threaded joint 1 configured as described above, the precision of the male thread 2A and female thread 3A can be ensured, and the pin thread row tapered portion 21 and pin connected portion 22 of the pin 2 can be efficiently machined.

The steel pipe threaded joint 1 and the manufacturing method for the steel pipe threaded joint 1 according to the present embodiment described above can reduce the weight of the main body portions 11A and 11B by reducing the thickness and increasing the strength, thereby increasing the tensile strength of the steel pipe 10.

Next, examples conducted to verify the effects of the steel pipe threaded joint 1 and the manufacturing method of the steel pipe threaded joint 1 according to the above-described embodiment will be described below.

(Example)
In the examples, the tensile strength was verified by structural analysis for four types of steel pipes (cases 1 to 4) shown in Table 1, which used the steel pipe threaded joint of the above-mentioned embodiment (see Figure 2). That is, the steel pipe threaded joints of the steel pipes of each of cases 1 to 4 have a configuration in which the pin 2 and box 3 shown in the above-mentioned embodiment are fastened, and in order to confirm the tensile strength, deformation, and equivalent stress state of these steel pipe threaded joints of cases 1 to 4, an FEA model was created using the elastic-plastic finite element method (FEA), and a numerical simulation analysis was performed at maximum load to confirm the effects.

Table 1 shows the conditions of the steel pipe threaded joints for each of Cases 1 to 4 used in the analysis of the examples and the tensile strengths of the analysis results.
As shown in Table 1, the steel pipes used in cases 1 to 4 are all blank pipes with an outer diameter of 76.3 mm and a wall thickness of 2.6 mm. In addition, the thread shapes of the steel pipe threaded joints in cases 1 to 4 are all the same, and the taper angle of the pin-connected portion (pin taper angle θ2) was changed. Table 1 shows the taper angle (°) and wall thickness (mm) conditions in each of cases 1 to 4. The taper angle indicates the thread row taper angle θ1 of the thread (male thread of the pin and female thread of the box), the pin taper angle θ2 of the pin-connected portion, and the taper ratio (θ2/θ1) of the thread row taper angle θ1 and the pin taper angle θ2. The wall thickness indicates the thickness of the pin, and indicates the wall thickness t1 of the dangerous cross section, the minimum wall thickness t2, and the wall thickness ratio (t2/t1) of the minimum wall thickness t2 to the wall thickness t1 of the dangerous cross section.

As an example, Figure 7 shows the equivalent stress distribution in a steel pipe threaded joint in tensile analysis using a bending FEA model for case 1. As a result of the structural analysis, as shown in Figure 7, it was confirmed that in all cases 1 to 4, stress was concentrated near the critical cross section of pin 2, not box 3, and the pin of the steel pipe threaded joint broke (pin fracture). The symbol T in Figure 7 indicates the pin fracture point.

The standard for tensile strength is set at 77.5% or more of the tensile strength of the blank pipe. This is because, as mentioned above, if the thickness of the blank pipe is -12.5% smaller than the standard, the tensile strength of the blank pipe is proportional to the cross-sectional area, so it is estimated that it will decrease by 12.5% squared, or 22.5% (12.5 x 12.5), relative to the standard dimensions.

Fig. 8 is a graph showing the analysis results according to the embodiment, showing the relationship between the taper ratio and the joint tensile strength, in which the horizontal axis represents the taper ratio (θ2/θ1) and the vertical axis represents the joint tensile strength (%), with the data in Table 1 plotted on the graph and the plots connected by straight lines.
As shown in Table 1 and Figure 8, it can be seen that as the taper ratio (θ2/θ1) increases, the joint tensile strength (%) decreases. For example, it can be seen that when the taper ratio (θ2/θ1) exceeds 2, the joint tensile strength decreases to 20% or less. From this, it can be seen that if the pin taper angle θ2 of the pin-connected portion is equal to or less than twice the thread row taper angle θ1, the joint tensile strength of a steel pipe threaded joint can be ensured to be 77.5% or more of that of the bare pipe portion (corresponding to the main body portion 11 in the above embodiment).

In addition, assuming that STK400, which is a general structural steel pipe, is strengthened to STK540, the standard yield strength of STK540 is 390 N/ ^mm2 , the standard tensile strength is 540 N/ ^mm2, and the yield ratio (yield strength/tensile strength) is about 72%. If the wall thickness of the STK540 mother pipe is the standard thickness and the tensile strength of the joint is 77.5% of the tensile strength of the mother pipe, the strength will be close to the yield strength of the mother pipe, and the joint may break before the mother pipe yields. Therefore, it is more preferable that the tensile strength of the joint is 10% or more higher than the yield ratio of STK540, that is, about 85%, and the pin taper angle θ2 of the pin-connected portion required in this case is 1.6 times or less the thread row taper angle θ1 as shown in FIG. 8.

The above describes the embodiments of the steel pipe threaded joint and the manufacturing method of the steel pipe threaded joint according to the present invention, but the present invention is not limited to the above-mentioned embodiments and can be modified as appropriate without departing from the spirit of the invention.

For example, in this embodiment, the minimum inner diameter D1 of the pin 2 is preferably 80% or more of the steel pipe inner diameter D2 of the pin body 11A when the steel pipes 10A, 10B are fastened together. However, if the steel pipe is not used for construction that requires the passage of an excavation drill or mortar injection pipe inside the steel pipe, it is also possible to set the amount of protrusion from the inner circumferential surface 11b of the body 11 of the pin 2 to be less than the above 80% in terms of inner diameter ratio.

In addition, this embodiment shows an example of application to civil engineering steel pipes for reinforcing the ground of a tunnel, and is mainly intended for steel pipes that are driven horizontally or diagonally forward into the ground, but it may also be a steel pipe threaded joint for a steel pipe pile that is driven vertically into the ground. Furthermore, it is not limited to tunnel construction.

In addition, the components in the above-described embodiments may be replaced with well-known components as appropriate without departing from the spirit of the present invention.

1 Steel pipe threaded joint 10, 10A, 10B Steel pipe 2 Pin 2A Male thread 3 Box 3A Female thread 11A Pin body 11B Box body 11a, 11c Outer circumferential surface 11b, 11d Inner circumferential surface 21 Pin thread row taper portion 21a Male thread end 22 Pin connecting portion 31 Box thread row reverse taper portion 32 Shoulder portion O Pipe axis X Pipe axis direction θ1 Thread row taper angle (first taper angle)
θ2 Pin taper angle (second taper angle)

Claims (3)

A steel pipe threaded joint for fastening steel pipes to be driven into the ground in construction work by screwing them together,
A pin having a male thread formed on the outer surface of one of the steel pipes;
A box having an internal thread formed on the inner surface of the other steel pipe that can be screwed onto the external thread of the pin,
The thread row of the pin and the box has a thread row taper portion inclined at a first taper angle with respect to the pipe axis of the steel pipe,
The minimum inner diameter of the pin is smaller than the inner diameter of the steel pipe of the main body portion excluding the pin and the box,
The pin has a pin connecting portion inclined by a second taper angle from a terminal end of a fully threaded portion of the male screw to the main body portion,
A steel pipe threaded joint, wherein the second taper angle is not more than twice the first taper angle. The steel pipe threaded joint according to claim 1, wherein the minimum inner diameter of the pin is 80% or more of the inner diameter of the steel pipe of the main body when the steel pipes are fastened together. A method for producing a steel pipe threaded joint according to claim 1 or 2,
machining the box including the female thread on one end of the steel pipe;
A step of performing a diameter reduction process to reduce the diameter of one end of the steel pipe;
machining the pin including the male thread in a reduced diameter portion after the diameter reduction process;
A method for manufacturing a steel pipe threaded joint having the above structure.

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