Technical field
[0001]
　The present disclosure relates to a threaded joint for use in connecting steel pipes.
Background technique
[0002]
　Oil wells, natural gas wells and the like in (hereinafter collectively referred to as "oil wells"), a casing, an oil well pipe such as tubing are used to mine underground resources. OCTG are made are sequentially connected steel pipes, threaded joints are used for the connection.
[0003]
　The format of this type of tubular threaded joint is roughly classified into a coupling-type and integral type. For coupling type, a pair of tubing of consolidated, where one tube is steel, other tubing is coupled. In this case, a male screw portion is provided on an outer periphery of both end portions of the steel pipe, the internal thread portion is provided on the inner periphery of both end portions of the coupling. Then, the male screw portion of the steel pipe is screwed into the female screw portion of the coupling, which both are connected is fastened by. For integral type, a pair of tubing of consolidated are both steel pipe, without using a separate coupling. In this case, a male screw portion is provided on an outer periphery of one end portion of the steel pipe, the internal thread portion is provided on the inner periphery of the other end. Then, the male screw portion of one of the steel pipe is screwed into the female screw portion of the other steel pipe, which both are connected is fastened by.
[0004]
　Generally, the joint portion of the male screw portion the tube end portion formed, since the containing elements to be inserted into the female screw portion, it referred to as pin. On the other hand, the joint portion of the pipe end portion a female screw portion is formed, since it contains elements for receiving the external thread portion, referred to as boxes. These pin and the box are the ends of the tube, both of which are tubular.
[0005]
　Figure 1 is a longitudinal sectional view showing an example of a conventional general tubular threaded joint. Threaded joint shown in FIG. 1 is a coupling type of threaded joint, and a pin 110 and a box 120..
[0006]
　Pin 110 includes in sequence toward the proximal side from the distal end, the shoulder surface 111, the sealing surface 113 and the male screw portion 114. On the other hand, the box 120 in order from the base side to the tip side, comprises shoulder surface 121, the sealing surface 123 and the internally threaded portion 124. Shoulder surface 121 of the box 120, the sealing surface 123 and the female screw portion 124, respectively, the shoulder surface 111 of the pin 110 is provided corresponding to the sealing surface 113 and the male screw portion 114. The female screw portion 124 of the male thread portion 114 and the box 120 of the pin 110 is intended to mesh with each other, the screw portion constituted by these is the trapezoidal thread of the tapered thread.
[0007]
　Male thread portion 114 and the internal thread portion 124 allows the mutual screwing, is closely fit to each other by a fastening state, the interference fit state. Each sealing surface 113 and 123, with the screwing of the pin 110 in contact with each other, and because of the state interference fit in close contact with the fitting in the engagement state. Thus, the sealing surfaces 113 and 123 to form a seal by metal contact. Each shoulder surface 111 and 121 is pressed in contact with each other with the threaded pin 110, responsible for the stopper for limiting the screwing of the pin 110. Each shoulder surface 111 and 121, in the engaged state, the load surface of the male screw portion 114 of the pin 110, serves to impart a tightening axial force of the so-called screw.
[0008]
　In such an arrangement of the threaded joint, in addition to the mating contact between the male thread portion 114 and the female screw portion 124, the contact fit between the seal surfaces 113 and 123, the sealing performance is secured.
[0009]
　Recently, oil well environment, high depth of and ultra deep water of the progress, along with this, have become harsh environment of high corrosion at high temperature and high pressure. Because of these corresponding to the harsh environment, steel thick is often used as an oil well pipe. The threaded joint for connecting these steel pipes, the sealing performance is required for good against pressure and external pressure.
[0010]
　The prior art with improved sealing performance for a threaded joint, there are the following.
[0011]
　Figure 2 is a longitudinal sectional view showing an example of a conventional threaded joint for steel pipes with improved sealing performance. Threaded joint shown in Figure 2, comprises two portions a sealing portion by the metal contact. Further, in the threaded joint, and shoulder surface is placed near the center of the tube axis direction (for example, see U.S. Pat. No. 4,662,659).
[0012]
　Specifically, as shown in FIG. 2, the pin 210 is in the order toward the proximal side from the distal end side, a first sealing surface 213, a first male screw portion 214, the shoulder surface 211, the second sealing surface 216 and the second provided with a male screw portion 217. On the other hand, the box 220 includes, in order toward the end side from the base portion includes a first sealing surface 223, the first female screw portion 224, the shoulder surface 221, a second sealing surface 226 and the second female threaded portion 227. The first threaded portion constituted by the first male screw portion 214 and the first female screw portion 224, and a second male threaded portion 217 second threaded portion constituted by the second female screw portion 227, trapezoidal any taper thread it is a screw.
[0013]
　Tapered surface of the first threaded portion is made to the tube axis CL nearer the tapered surface of the second threaded portion. A first threaded portion because the shoulder surface 211 and 221 is installed between the second threaded portion.
[0014]
　The first male screw portion 214 and the first internal thread portion 224 allows the mutual screwing, is closely fit to each other by a fastening state, the interference fit state. Second male threaded portion 217 and the second female threaded portion 227 also becomes fit state interference fit in the same manner. Each first sealing surface 213, 223 and the respective second sealing surfaces 216 and 226, respectively in contact with each other with the screwing of the pin 210, in close contact with the fitting in the engaged state, the interference fit state. Each shoulder surface 211 and 221 is pressed in contact with each other with the screwing of the pin 210.
[0015]
　In such arrangement of the threaded joint, the contact fitting between the first sealing surface 213 and 223, sealing performance of the main internal pressure is ensured. Further, the contact fitting between the second sealing surface 216 and 226, mainly sealing performance against external pressure is ensured.
Disclosure of the Invention
[0016]
　In the threaded joint shown in FIG. 2, in the engaged state, and the shoulder surface 221 of the shoulder surface 211 and the box 220 of the pin 210 is in contact. The tip of the pin 210 does not contact the box 220 even if the fastening is complete. However, if there is a gap between the tip and the box 220 of the pin 210 in the engaged state, the possibility of crevice corrosion becomes high.
[0017]
　The present disclosure is configured to ensure excellent sealing performance, and to provide a tubular threaded joint which can suppress the occurrence of crevice corrosion.
[0018]
　Tubular threaded joint according to the present disclosure is comprised of a pin of the tubular and the tubular box. Pin is screwed into the box and the pin and the box is fastened. Pin includes in order from the distal end side, a first shoulder surface, a first sealing surface, the first male threaded portion of the tapered threads, the second shoulder surface, the second male threaded portion of the second sealing surface and the tapered thread. Box, first shoulder surface of the pin, the first sealing surface, the first male threaded portion, the second shoulder surface, a first shoulder surface corresponding to each of the second sealing surface and a second male threaded portion, a first sealing surface, tapered comprising first female threaded portion of the screw, the second shoulder surface, the second female threaded portion of the second sealing surface and the tapered thread. Pin is provided with a nose portion continuous with the first sealing surface between the first shoulder surface and the first sealing surface. Pin is provided with a unthreaded extension connected to the second sealing surface between the second shoulder surface and the second sealing surface. Box, a recess corresponding to the nose portion of the pin. Box has a unthreaded extension corresponding to the unthreaded extension of the pin. In the engagement state, the first shoulder faces are in contact with each other, the first sealing faces are in contact with each other, the second sealing faces are in contact with each other, a gap is formed between the nose portion and the box of the recess of the pin , a gap is formed between the free pin threaded extension and an unthreaded extension of the box fits Mari a first male screw portion and the first female screw portion is fitted, fitting a second male threaded portion and the second female screw portion Mari fit. Distance L in the tube axis direction from the first shoulder surface at the pin of the disengaged state until the second shoulder surface pin , the distance in the tube axis direction from the first shoulder surface in the disengaged state box until the second shoulder surface L box as interference amount δ of the second shoulder surface and the second shoulder surface of the box pin shld when defined by the following equation (1) satisfies the following equation (2) and (3).
[0019]
[Number 1]

[0020]
　Here, P is the screw pitch of the first external thread portion, delta min is tightened rotational speed of the lower limit after a first shoulder faces or second shoulder surfaces come into contact with each other in the fastening process, delta max is tightening rotation upper limit of the number, lambda is the elongation amount at the time of engagement of the distal portion than the second shoulder surface of the pin.
[0021]
　The inner diameter of the pin ID, and the innermost diameter D of the second shoulder surface of the pin ms , the outermost diameter of the first shoulder surface of the pin D IS and the interference amount δ of the first male screw portion and the first female screw portion th and when, lambda is expressed using the following equation (4) and (5).
[0022]
[Number 2]

[0023]
　According to a threaded joint for steel pipes according to the present disclosure, it is possible to ensure excellent sealing performance, it is possible to suppress the occurrence of crevice corrosion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024]
[1] Figure 1 is a longitudinal sectional view showing an example of a conventional general tubular threaded joint.
FIG. 2 is a longitudinal sectional view showing an example of a conventional threaded joint for steel pipes with improved sealing performance.
FIG. 3 is a longitudinal sectional view schematically showing a pin and box used in the study of tubular threaded joint according to the embodiment.
[4] FIG. 4 is a diagram showing a simplified model of the pin and box shown in Figure 3.
FIG. 5 is the speed difference tightening and Δ intermediate shoulder interference amount [delta] shld is a graph showing the relationship between.
FIG. 6 is a longitudinal sectional view showing a threaded joint for steel pipes according to the embodiment.
[7] FIG. 7 is the threaded joint shown in FIG. 6 is a longitudinal sectional view enlarging a vicinity of the inner end of the tube axis direction.
[8] FIG. 8 is the threaded joint shown in FIG. 6 is a longitudinal sectional view enlarging a vicinity of the center in the tube axis direction.
[9] FIG. 9 is a longitudinal sectional view schematically showing the pin and box of a threaded joint shown in FIG.
DESCRIPTION OF THE INVENTION
[0025]
　Threaded joint having a shoulder surface in the vicinity of the center in the tube axis direction is generally configured to the pin tip and the box is not in contact during engagement. Contacting the shoulder surfaces are in engagement, and if an attempt is made contact with the pin tip and the box, because the management of the fastening completion position becomes difficult.
[0026]
　However, if there is a gap between the pin tip and the box in the engaged state, the possibility of crevice corrosion becomes high. Accordingly, the present inventors have not only near the center in the tube axis direction, to the inner end of the tube axis direction was examined threaded joint having a shoulder surface. Hereinafter, the inner shoulder surface or first shoulder surface shoulder surface of the inner end of the tube axis direction refers to the shoulder surface near the center of the tube axis direction and an intermediate shoulder surface or the second shoulder surface.
[0027]
　If each of the pin and box having an inner shoulder surface and the intermediate shoulder surface, in order to avoid excessive surface pressure on the shoulder surface is loaded, in the course of engagement, the inner shoulder surface contact with the intermediate shoulder surfaces between it is preferred that the contact with the each other are started simultaneously. However, due to manufacturing tolerance or the like, in reality, the timing between the inner shoulder surface starts to contact, it is difficult to each other intermediate the shoulder surface is to match the timing for starting the contact. Sometimes, at the time of engagement completion, a state in which only one of each other inner shoulder faces and the intermediate shoulder surfaces are in contact.
[0028]
　The following describes the contact time between the inner shoulder faces and an intermediate shoulder surface.
[0029]
　In pin, between an inner shoulder surface and the intermediate shoulder surface is provided with a first external thread portion, the second male threaded portion is provided on the base side than the middle shoulder surface. The box first and second internally threaded portion corresponding to each of the first and second male threaded portion is provided. Upon concluding the pin and the box, the tip-side portion than the middle shoulder surfaces of the pin, the first male threaded portion reduced in diameter by interference fit between the first female thread portion, extending in the axial direction of the tube.
[0030]
　Here, the pin and box of the disengaged state, respectively, the tube axis direction of the distance between the inner shoulder surface and the intermediate shoulder surfaces L pin [mm], L box and [mm], L box L from pin minus a value intermediate shoulder interference amount [delta] shld define [mm] and.
[0031]
　[delta] shld is larger than the elongation amount of the pin, in the course of engagement, after contacting each other intermediate shoulder surface, together the inner shoulder surface is in contact. [delta] shld is too large, plastically deformed by the intermediate shoulder surface and the threaded portion breakdown before the inner shoulder faces contacting, sealing performance is lowered.
[0032]
　Engagement completion position is managed based on the relationship between the rotational speed and tightening torque (torque chart). [delta] shld result too large, the plastic deformation of the intermediate shoulder surfaces and threaded portions before beginning the contact of the inside shoulder surface at the conclusion of the process occurs, not obtain a normal torque chart. In this case, it is impossible to determine the engagement completion position.
[0033]
　[delta] shld If is too large, it may be not in contact inner shoulder faces be tightened to plastic deformation to each intermediate shoulder surface and threaded portion arises. In this case, a gap is generated between the conventional threaded joint as well as the pin tip and the box, crevice corrosion is likely to occur.
[0034]
　[delta] shld If is smaller than the elongation amount of the pin, in the course of engagement, after each other the inner shoulder surface is in contact, with each other intermediate the shoulder surface contacts. In this case, since the tip of the pin is securely in contact with the box, there is no gap corrosion problems described above. However, [delta] shld is too small, and plastic deformation respective inner shoulder surfaces before each other intermediate the shoulder surface contacts and threaded portion is surrendered, sealing performance is lowered. Further, the plastic deformation of each inner shoulder surface and the threaded portion occurs, because not obtained normal torque chart, it is impossible to determine the engagement completion position.
[0035]
　The present inventors have taken into account the above problems, an intermediate shoulder interference amount [delta] shld were studied appropriate range.
[0036]
　Figure 3 is a longitudinal sectional view showing a schematic configuration of the pin and box used in the study. In Figure 3, the pin and box of the non-fastening state is shown. At pin shown in FIG. 3, ID is the inner diameter of the tube [mm], D ms is innermost intermediate shoulder surface [mm], D IS is outermost diameter of the inner shoulder surface [mm].
[0037]
　Figure 4 is a diagram showing a simplified model of the pin and box shown in Figure 3. 4, a simplified model of the pins is represented by a stepped cylinder composed of the small diameter portion and the large diameter portion. The small diameter portion, at pin located from the intermediate shoulder surface on the tip side, and the first male screw portion corresponds to the portion that is provided. The outer diameter D of the small diameter portion int [mm] is defined by the following equation (4).
[0038]
[Number 3]

[0039]
　Interference amount δ of the first male screw portion of the pin and the first female thread portion of the box th interference fit by a small diameter portion and extends only lambda [mm] in the tube axis direction at the time of fastening. At this time, the small diameter portion, undeformed volume V [mm 3 ], the volume of the deformed V '[mm 3 and, if the volume and thickness before and after the deformation is assumed to be constant, the following equation (6) to (8) is established.
[0040]
[Formula 4]

[0041]
　The equation (6) to (8), it is possible to obtain the elongation amount λ represented by the following formula (5).
[0042]
[Formula 5]

[0043]
　Amount of elongation pin λ intermediate shoulder interference amount [delta] shld equal to the, in the process of fastening the pin and the box, between the inner shoulder surface, together intermediate shoulder surface begins to contact simultaneously. λ is [delta] shld greater than in the process of fastening, after each other the inner shoulder surface is in contact, with each other intermediate the shoulder surface contacts. λ is [delta] shld smaller than in the process of fastening, after contacting each other intermediate shoulder surface, together the inner shoulder surface is in contact.
[0044]
　As described above, lambda ≠ [delta] shld case, the timing of the contact between the inner shoulder surface, the deviation in the timing of contact between the intermediate shoulder surface occurs. After one shoulder surfaces come into contact with each other, the other rotational speed tightening up shoulder surfaces contact each other, i.e., tightening the rotational speed from the fastening start to one shoulder surfaces contact each other, the other shoulder from the engagement initiation the difference between the tightening rotational speed up to the surface contact each other delta [rotation], when the thread pitch of the first male screw portion is P [mm], is represented by the following equation (9).
[0045]
[Number 6]

[0046]
　In the formula (9), delta is in contact with the intermediate shoulder surfaces each other ahead if positive, between the inner shoulder surface if delta is negative it is assumed that contact with the first.
[0047]
　The lower limit and the upper limit of the tightening can exhibit sufficient sealing performance rotational speed difference, respectively, delta min , delta max and. Then, excellent in order to ensure the sealing performance, after contact with one of the shoulder faces, delta min or more delta max is necessary to complete the engagement with the following tightening speed. That is, the rotational speed difference Δ tightening, it is necessary to satisfy the following equation (10).
[0048]
[Number 7]

[0049]
　Figure 5 is a tightening rotational speed difference Δ and the intermediate shoulder interference amount δ is represented by the formula (9) shld is a graph showing the relationship between. If Δ satisfies the equation (10), [delta] shld is a range indicated by hatching in FIG. The present inventors have conducted extensive studies results, delta min , delta max preferred values and δ of shld found an appropriate range of, to complete the tubular threaded joint according to the embodiment.
[0050]
　A threaded joint for steel pipes according to the embodiment is constituted by a pin of the tubular, the tubular box. Pin is screwed into the box and the pin and the box is fastened. Pin includes in order from the distal end side, a first shoulder surface, a first sealing surface, the first male threaded portion of the tapered threads, the second shoulder surface, the second male threaded portion of the second sealing surface and the tapered thread. Box, first shoulder surface of the pin, the first sealing surface, the first male threaded portion, the second shoulder surface, a first shoulder surface corresponding to each of the second sealing surface and a second male threaded portion, a first sealing surface, tapered comprising first female threaded portion of the screw, the second shoulder surface, the second female threaded portion of the second sealing surface and the tapered thread. Pin is provided with a nose portion continuous with the first sealing surface between the first shoulder surface and the first sealing surface. Pin is provided with a unthreaded extension connected to the second sealing surface between the second shoulder surface and the second sealing surface. Box, a recess corresponding to the nose portion of the pin. Box has a unthreaded extension corresponding to the unthreaded extension of the pin. In the engagement state, the first shoulder faces are in contact with each other, the first sealing faces are in contact with each other, the second sealing faces are in contact with each other, a gap is formed between the nose portion and the box of the recess of the pin , a gap is formed between the free pin threaded extension and an unthreaded extension of the box fits Mari a first male screw portion and the first female screw portion is fitted, fitting a second male threaded portion and the second female screw portion Mari fit. Distance L in the tube axis direction from the first shoulder surface at the pin of the disengaged state until the second shoulder surface pin , the distance in the tube axis direction from the first shoulder surface in the disengaged state box until the second shoulder surface L box as interference amount δ of the second shoulder surface and the second shoulder surface of the box pin shld when defined by the following equation (1) satisfies the following equation (2) and (3).
[0051]
[Number 8]

[0052]
　Here, P is the screw pitch of the first external thread portion, delta min is tightened rotational speed of the lower limit after a first shoulder faces or second shoulder surfaces come into contact with each other in the fastening process, delta max is tightening rotation upper limit of the number, lambda is the elongation amount at the time of engagement of the distal portion than the second shoulder surface of the pin.
[0053]
　The inner diameter of the pin ID, and the innermost diameter D of the second shoulder surface of the pin ms , the outermost diameter of the first shoulder surface of the pin D IS and the interference amount δ of the first male screw portion and the first female screw portion th and when, lambda is expressed using the following equation (4) and (5).
[0054]
[Number 9]

[0055]
　If threaded joint which satisfies the equation (2) and (3), the second shoulder faces is a case where the contact with the above, after the contact between the second shoulder surface, delta min or more delta max following tightening rotation contacting the first shoulder faces in the range of a few, it can be completed engagement. Therefore, it is possible to it is possible to ensure excellent sealing performance, without causing a gap between the pin tip and the box, to suppress the occurrence of crevice corrosion due to the gap.
[0056]
　Intermediate shoulder interference amount δ to satisfy equation (2) and (3) shld be set, in the course of engagement, shift the contact timing between the first shoulder surface, and the contact timing between the second shoulder surface is larger that there is no. Therefore, excessive plastic deformation does not easily occur in the shoulder surface which began earlier contact, it is possible to ensure excellent sealing performance.
[0057]
　The first shoulder surface contact each other during engagement completion, even if the second shoulder faces is not in contact, the intermediate shoulder interference amount δ to satisfy equation (2) and (3) shld be set, It does not occur a large gap between the second shoulder surface. Therefore, if an excessive compressive load is applied to also contact the second shoulder faces, it is possible to prevent decrease in and sealing performance of resistance to compression.
[0058]
　It will be described in more detail with reference to the drawings embodiment. Given the same reference and the corresponding configuration in the figure does not repeat the same description. For convenience of explanation, in each figure, or are simplified or schematized configuration, in some cases or are omitted a part of the configuration.
[0059]
　Configuration of a threaded joint for steel pipes]
　FIG 6 is a longitudinal sectional view showing a threaded joint for steel pipes according to the embodiment. The threaded joint is a coupling type of threaded joint, composed of the pin 10 and the box 20. Figure 7 is the threaded joint shown in FIG. 6 is a longitudinal sectional view enlarging a vicinity of the leading end portion of the pin 10. Figure 8 is the threaded joint shown in FIG. 6 is a longitudinal sectional view enlarging a vicinity of the center in the tube axis direction. Hereinafter, in the tube axis direction, the inner or front base side of the distal and box 20 of the pin 10 may be referred to a front end side of the base portion side and the box 20 of the pin 10 and the outer or after.
[0060]
　Pin 10 comprises, in order towards the base side from the distal end side, a first shoulder surface 11, the nose portion 12, the first seal surface 13, the first male threaded portion 14, the second shoulder surface 18, a first unthreaded extension 15a, the second sealing surface 16 includes a second unthreaded extension 15b, and a second male threaded portion 17. The first sealing surface 13 and the second sealing surface 16 are both tapered. Strictly speaking, the first sealing surface 13 and the second sealing surface 16, respectively, and the shape, or frustoconical circumferential surface consisting of a surface corresponding to a peripheral surface of a truncated cone as the distal end side diameter is reduced, the arc or the like They are combined to form a surface corresponding to the peripheral surface of the rotary body obtained by rotating around the tube axis CL of the curve.
[0061]
　The nose portion 12 is cylindrical, extend out in the axial direction of the tube continues to the first sealing surface 13 of the inboard. However, the outer peripheral surface of the nose portion 12, or taper and inclination of the first sealing surface 13 are the same, small (loose), or large (steep) may be a tapered surface. If the outer peripheral surface of the nose portion 12 is tapered surfaces, the outer peripheral surface, strictly speaking, the shape comprising a surface corresponding to a peripheral surface of a truncated cone as the distal end side diameter is reduced, or frustoconical circumferential surface thereof and has a shape of a combination of a surface corresponding to the peripheral surface of the rotary body obtained by rotating the curve of the circular arc or the like around the tube axis CL.
[0062]
　The first shoulder surface 11 is provided on the tip of the nose portion 12. The first shoulder surface 11 is substantially perpendicular annular surface to the tube axis CL. Strictly speaking, the outer peripheral side of the first shoulder surface 11 is slightly tilted toward the front end side of the pin 10.
[0063]
　The second shoulder surface 18 is disposed between the first external thread portion 14 of the inboard first unthreaded extension 15a. The second shoulder surface 18 is provided continuous to the first unthreaded extension 15a. In the present embodiment, the second shoulder surface 18 is perpendicular annular surface to the tube axis CL. However, the second shoulder surface 18, similar to the first shoulder surface 11 of the tip of the pin 10, the outer peripheral side may be slightly tilted toward the front end side of the pin 10.
[0064]
　First unthreaded extension 15a is out extends continuous to the front of the second sealing surface 16 of the outboard axial direction of the tube. The first male screw portion 14 of the inboard are disposed continuous with the first unthreaded extension 15a. Second unthreaded extension 15b is out extends continuous to the rear of the second sealing surface 16 of the outboard axial direction of the tube. Second male threaded portion 17 of the outboard is provided continuous with the second unthreaded extension 15b. The outer peripheral surface of the first unthreaded extension 15a may be any shape that can ensure the rigidity, for example, small (loose) than the cylindrical surface and screw taper of the first male screw portion 14 may be a tapered surface, it may be a curved surface. The outer peripheral surface of the second unthreaded extension 15b is similar.
[0065]
　Meanwhile, box 20, in order from the base side to the tip side, a first shoulder surface 21, recess 22, the first seal surface 23, the first female threaded portion 24, the second shoulder surface 28, a first unthreaded extension 25a comprises a second sealing surface 26, a second unthreaded extension 25b, and a second female threaded portion 27. The first shoulder surface 21 of the box 20, the recess 22, the first seal surface 23, the first female threaded portion 24, the second shoulder surface 28, a first unthreaded extension 25a, the second sealing surface 26, no second screw extension 25b, and a second female threaded portion 27, respectively, the first shoulder surface 11 of the pin 10, the nose portion 12, the first seal surface 13, the first male threaded portion 14, the second shoulder surface 18, without first threaded extension part 15a, the second sealing surface 16, a second unthreaded extension 15b, and provided corresponding to the second male threaded portion 17.
[0066]
　6 and 7, the first seal surface 23 of the box 20, shows the case protruding toward the first sealing surface 13 of the pin 10. However, the first sealing surface 23 of the box 20 may not protrude. In that case, the first sealing surface 13 of the pin 10 protrudes toward the first sealing surface 23 of the box 20.
[0067]
　First first female threaded portion 24 of the external thread portion 14 and the box 20 of the pin 10 is a trapezoidal screw tapered thread meshing with each other, constituting the first threaded portion of the inboard. The second female threaded portion 27 of the second male threaded portion 17 and the box 20 of the pin 10 is also a trapezoidal thread taper thread meshing with each other, it constitutes the second threaded portion of the outboard.
[0068]
　Tapered surface of the first threaded portion is a tube axis CL nearer the tapered surface of the second threaded portion. The second shoulder surface 18, 28 is disposed between the first threaded portion (first male threaded portion 14 and the first female screw portion 24) and the second threaded portion (second male threaded portion 17 and the second female threaded portion 27) it is from. Therefore, at pin 10, the outer diameter of the region of the first male screw portion 14 and the first sealing surface 13 of the inboard decreases, the thickness of the region is reduced. On the other hand, in the pin 10, the outer diameter of the region of the second sealing surface 16 and the second male threaded portion 17 of the outboard increases, the thickness of the region becomes thicker.
[0069]
　The first male screw portion 14 and the first female screw portion 24 allows the mutual screwing, is closely fit to each other by a fastening state, a state of interference fit. The second male screw portion 17 and the second female threaded portion 27 is also in a state of interference fit in the same manner.
[0070]
　Each first sealing surface 13, 23 and the second seal surface 16 and 26, respectively in contact with each other with the screwing of the pin 10, in close contact with the fitting in the engaged state, the interference fit state. Thus, the first seal surface 13 and 23 and the second seal surface 16 and 26, respectively to form the first seal portion and the second sealing portion by the metal contact.
[0071]
　Between the engaged state, a gap is formed between the recess 22 of the nose portion 12 and the box 20 of the pin 10, a first first unthreaded extension 25a of the unthreaded extension 15a and the box 20 of the pin 10 also gaps been formed, the gap also between the second second unthreaded extension 25b of the unthreaded extension 15b and the box 20 of the pin 10 is formed.
[0072]
　In engaged state, between the first shoulder surface 11, 21 is in contact pressed against each other. Pressure contact of the first shoulder surface 11, 21 imparts an axial force fastening the load surface of the first male screw portion 14 of the main pin 10. The second shoulder surface 18 and 28, there is a case that contact each other in the engaged state, not in contact with each other even if the fastening is complete, sometimes opposed with a gap. If the second shoulder surface 18, 28 is pressed into contact, axial force is applied tightening primarily load surface of the second male threaded portion 17 of the pin 10.
[0073]
　Here, by contacting the first shoulder surface 11, 21 with each other in the engaged state, and description will be given of a configuration for contacting or sufficiently close to the second shoulder surface 18, 28 with each other.
[0074]
　Figure 9 is a diagram showing a longitudinal section of the pin 10 and box 20 of the non-engaged state are simplified. As shown in FIG. 9, in the disengaged state, the first shoulder surface 11 of the pin 10 the tube axis direction of the distance between the second shoulder surface 18 L pin and [mm]. In a disengaged state, the pipe axis direction of the distance between the first shoulder surface 21 of the box 20 and the second shoulder surface 28 L box and [mm].
[0075]
　The first shoulder surface 11, 21 and the second shoulder surface 18 and 28 is an annular surface that intersects the tube axis direction. The first shoulder surface 11, 21 and the second shoulder surface 18 and 28 may or may perpendicular to the tube axis CL, sometimes are inclined with respect to a plane perpendicular to the tube axis CL. L pin , in cross-section of the pin 10 in the disengaged state, which is cut along a plane through the tube axis CL, from the innermost end of the first shoulder surface 11, the tube axis direction to the innermost end of the second shoulder surface 18 distance to. L box , in cross-section of the non-engaged state of the box 20 which is cut along a plane through the tube axis CL, from the innermost end of the first shoulder surface 21, the tube axis direction to the innermost end of the second shoulder surface 28 distance to.
[0076]
　As shown in the following equation (1), L pin , L box intermediate shoulder interference amount δ with shld define a [mm]. [delta] shld is the interference quantity of the second shoulder surface 18, 20 without considering the deformation by fastening.
[0077]
[Formula 10]

[0078]
　In actual engagement, by interference fit of the first threaded portion, the distal end side portion than the second shoulder surface 18 of the pin 10 extends only lambda [mm]. Therefore, in consideration of the amount of elongation pin 10 lambda, it is necessary to determine the first shoulder surface 11, 21 of the positional relationship between the second shoulder surface 18, 28. In the present embodiment, so as to satisfy the following equation (2) and (3), an intermediate shoulder interference amount [delta] shld is determined.
[0079]
[Number 11]

[0080]
　Here, P is a thread pitch of the first male threaded portion 14 (FIG. 6). Delta min , delta max , respectively, in the process of fastening, after one of between the first shoulder surface 11, 21 and between the second shoulder surface 18, 28 are in contact, rotational speed difference tightened until the other contacts delta it is a lower limit and an upper limit. Tightening rotational speed difference delta is the formula (3) shows delta min or more delta max if less, it is possible to exhibit sufficient sealing performance.
[0081]
　Elongation amount of the pin 10 lambda is calculated by the following equation (4) and (5).
[0082]
[Number 12]

[0083]
　In Equation (4) and (5), inner diameter ID of the pin 10 in the disengaged state, the outermost diameter of the first shoulder surface 11 D IS , the innermost diameter of the second shoulder surface 18 D ms is represented by. [delta] th is the interference quantity of the first threaded portion.
[0084]
　Here, the interference amount δ of the first threaded portion th is from the innermost end of the first shoulder surface 11 in the cross section of the pin 10 in the disengaged state, which is cut along a plane through the tube axis CL L pin / 2 only behind the from thread root diameter of the first male threaded portion 14 at the position, from the innermost end L of the first shoulder surface 21 in the cross section of the box 20 of the non-fastening state of being cut in a plane passing through the tube axis CL pin position by / 2 behind the the first value obtained by subtracting the crest diameter of the internal thread portion 24 at.
[0085]
　In the course of engagement, the first shoulder surface 11, 21 and the second shoulder surface 18 and 28 may be simultaneously contact may contact either one first. Preferably, contacting previously or simultaneously than the first shoulder surface 11, 21 with each other more large area second shoulder surfaces 18 and 28 to each other. To preferentially contacting the second shoulder surface 18 and 28 each other, intermediate shoulder interference amount [delta] shld may be set to more than the elongation amount of the pin 10 lambda. However, as described above, in the fastening state, between the second shoulder surface 18 and 28 does not contact, only the first shoulder surface 11, 21 may be in contact with each other.
[0086]
　[Effect]
　In the threaded joint according to the present embodiment, both the first shoulder surface 11, 21 is provided on the inner end of the tube axis direction, the second shoulder surface 18, 28 is provided in the vicinity of the center in the tube axis direction . In the present embodiment, the intermediate shoulder interference amount δ so as to satisfy the above equation (2) and (3) shld is set. According to this configuration, even between the second shoulder surface 18 and 28 is a case in contact first, and then, without compromising the sealing performance, i.e. delta min or more delta max tightening rotational speed of the range of the first shoulder surfaces 11 and 21 are brought into contact with each other, it is possible to complete the fastening. Therefore, excellent while securing sealing performance, it is possible to prevent a gap from occurring between the tip and the box 20 of the pin 10, it is possible to suppress the occurrence of crevice corrosion.
[0087]
　In the present embodiment, the intermediate shoulder interference amount δ so as to satisfy the above equation (2) and (3) shld been set, the first shoulder surface 11, 21 second shoulder surface 18, 28 and appropriate positional relationship in is located. Therefore, in the process of fastening, and the contact timing between the first shoulder surface 11 and 21, never the contact timing between the second shoulder surface 18, 28 is largely deviated. Therefore, since an excessive plastic deformation is less likely to occur in the shoulder surface and the threaded portion that started earlier contact, it is possible to ensure excellent sealing performance.
[0088]
　The first shoulder surface 11, 21 contact each other at the time of engagement completion, even if the second shoulder surface 18, 28 with each other are not in contact, the intermediate shoulder interference so as to satisfy the above equation (2) and (3) the amount [delta] shld be set, it does not occur a large gap between the second shoulder surface 18, 28. Therefore, when an excessive compressive load is applied, also contacts the second shoulder faces. Thus, it is possible to prevent decrease in and sealing performance of resistance to compression.
[0089]
　A threaded joint according to the present embodiment includes a first shoulder surface 11, 21 and the second shoulder surface 18, 28. According to this configuration, it is possible to shoulder surface as compared with conventional threaded joint provided on only one place, under compressive load over a large area. Therefore, it is possible to secure a high resistance to compression.
[0090]
　The threaded joint according to the present embodiment, the contact fitting between the first sealing surface 13, 23 of the inboard, sealing performance of the main internal pressure is ensured. Further, the contact fitting between the second sealing surface 16, 26 of the outboard, mainly sealing performance against external pressure is ensured.
[0091]
　In particular, the pin 10, since the first unthreaded extension 15a continues to the front of the second sealing surface 16 of the outboard are provided, the rigidity of the first unthreaded extension 15a, the second seal pin 10 diameter resistance region of the surface 16 increases. Therefore, even when the external pressure is loaded to the screw joint, shrink deformation of the pin 10 is suppressed, reduction of the second sealing surface 16, 26 to each other of the contact surface pressure is suppressed. As a result, to improve the sealing performance against external pressure. In addition, the box 20, the first unthreaded extension 25a provided corresponding to the region of the first unthreaded extension 15a of the pin 10, first unthreaded extension 15a in the fastening state, between 25a between a gap is formed. Therefore, to accommodate the excess dope applied to the time of fastening the gap. As a result, inadvertent reduction in contact surface pressure of the second sealing surface 16, 26 with each other due to the pressure rise in the dope is avoided.
[0092]
　The pin 10 and box 20, respectively, second unthreaded extension 15b, 25b are provided continuous to the rear of the second sealing surface 16 of the outboard, second unthreaded extension 15b, between the 25b each other , a gap is formed in the engaged state. Therefore, the surface pressure reduction due to the decrease in substantial interference amount of the second sealing surface 16, 26 with each other by interference fit action of the second threaded portion is suppressed. As a result, to improve the sealing performance against external pressure.
[0093]
　Further, the pin 10, the nose portion 12 is provided continuous with the first sealing surface 13 of the inboard. The box 20, the recess 22 is provided corresponding to the region of the nose portion 12 of the pin 10. A gap is formed between their nose portion 12 and the recess 22 in the fastening state. For example, when an excessive tensile load is applied to the threaded joint, a first shoulder surface 11 is spaced from the first shoulder surface 21 of the box 20, a substantial amount of interference between the first sealing surface 13, 23 of the pin 10 reduced contact tends to loosened. Even in such a case, the amplification effect of the first seal surface 13 and 23 to each other of the contact surface pressure is obtained by the elastic recovery of the nose portion 12. Therefore, since the decrease in the contact pressure as a whole is suppressed, high sealing performance against internal pressure in a state where an excessive tensile load is applied is obtained.
[0094]
　The threaded joint according to the present embodiment, the installation of the second shoulder surface 18 and 28, inboard region of the first male screw portion 14 and the first sealing surface 13 of the pin 10 is thinned. Thus, when the internal pressure in the screw joint is loaded, the thinned region is effectively expanded deformation. Therefore, the contact surface pressure between the first seal surface 13 and 23 is amplified.
[0095]
　The installation of the second shoulder surface 18 and 28, the area of ​​the second sealing surface 16 and the second male threaded portion 17 of the outer side of the pin 10 is thickened, its rigidity is relatively high. Thus, when the external pressure is loaded to the threaded joint, since the shrink deformation of the region is suppressed, it is possible to maintain a high contact surface pressure between the second sealing surface 16, 26.
[0096]
　Hereinafter, complement the preferred embodiment of the threaded joint according to the present embodiment.
[0097]
　In the pin 10, A the cross-sectional area of the steel pipe body in a cross section perpendicular to the tube axis CL 0 and, the sum of the respective projection areas first shoulder surface 11 and the second shoulder surface 18 of the plane perpendicular to the tube axis CL a 2 a. In this case, both the area ratio (hereinafter, also referred to as "total shoulder area ratio steel body") A 2 / A 0 is preferably not less than 30%. More preferably, A 2 / A 0 is 35% or more. The reason for this is as follows. A 2 / A 0 is substantially dependent on the area of the first shoulder surface 11 and the second shoulder surface 18. A 2 / A 0 When is small, since the area of the first shoulder surface 11 and the second shoulder surface 18 is small, when an excessive compressive load is applied to the threaded joint, the first shoulder surface and the second shoulder surface 18 not fully withstand the compression load. In this case, the first shoulder surface 11 and the nose portion 12 and the first seal surface 13 continuous to this plastic deformation, the contact state between the first seal surface 13 and 23 becomes unstable. Together with this, the second shoulder surface 18 and the first unthreaded extension 15a and the second sealing surface 16 continuous to this plastic deformation, the contact state between the second seal surface 16 and 26 becomes unstable. As a result, the contact surface pressure and the second seal surface 16, 26 the contact surface pressure between the between the first seal surface 13 and 23 may be reduced. Therefore, total shoulder area rate A to the steel tube body 2 / A 0 is preferably large to some extent.
[0098]
　All shoulder area rate A against the steel pipe body 2 / A 0 limit is not particularly defined. However, A 2 / A 0 When too large, the outer diameter of the substantially since the area of the first shoulder surface 11 and the second shoulder surface 18 is too large, the first shoulder surface 11 and the second shoulder surface 18 of the pin 10 so that is too large. As a result, in box 20, the inside diameter of the region of the first female threaded portion 24 and the first seal surface 23 is increased. In order to ensure the cross-sectional area of the danger section of box 20, it is necessary to increase the box outside diameter. Also, securing the first threaded portion and the engagement length of the second screw portion is difficult. Therefore, light of practicability, total shoulder area rate A to the steel tube body 2 / A 0 is preferably 60% or less.
[0099]
　In the pin 10, the sum of each of the projected area first shoulder surface 11 and the second shoulder surface 18 of the plane perpendicular to the tube axis CL A 2 and, in the first shoulder surface 11 to a plane perpendicular to the tube axis CL the projected area a 1 and. In this case, both the area ratio (hereinafter, also referred to as "first shoulder area ratio to total shoulder") A 1 / A 2 is preferably not less than 35%. More preferably, A 1 / A 2 is 40% or more. The reason for this is as follows. A 1 / A 2 is compared with the second shoulder surface 18, substantially depends on the area of the first shoulder surface 11. A 1 / A 2 when is small, since the area of the first shoulder surface 11 is small, if an excessive compressive load is applied to the threaded joint, a first shoulder surface 11 can not completely withstand the compressive load. In this case, the first shoulder surface 11 and the nose portion 12 and the first seal surface 13 continuous to this plastic deformation, the contact state between the first seal surface 13 and 23 becomes unstable. As a result, there is a risk that the contact surface pressure between the first seal surface 13 and 23 is reduced. Therefore, the first shoulder area ratio A to all shoulder 1 / A 2 is preferably large to some extent.
[0100]
　The first shoulder area ratio A to all shoulder 1 / A 2 limit is not particularly defined. However, A 1 / A 2 when is too large, in comparison with the second shoulder surface 18, since the area of the substantially first shoulder surface 11 is too large, the nose portion 12 leading to the first shoulder surface 11 of the pin 10 so that the wall thickness, and the area of the first male screw portion 14 and the first seal surface 13 is too thick. As a result, when the internal pressure is loaded in the threaded joint, the region is not effectively expanded deformation, amplification effect of the contact surface pressure between the first sealing surface 13, 23 can not be obtained. In this case, there is a fear that the contact surface pressure between the first seal surface 13 and 23 is reduced. Therefore, light of practicability, the first shoulder area ratio A to all shoulder 1 / A 2 is preferably at most 55%.
[0101]
　As shown in FIG. 8, the pin 10, the minimum diameter of the outer diameter of the first unthreaded extension 15a is preferably larger than the diameter of the reference tapered surface 19b. Criteria tapered surface 19b refers to the second thread height twice partial outer diameter smaller tapered surface 19b than the extension surface 19a of the taper of the thread root bottom surface of the male screw portion 17 and the second male threaded portion 17. The reason for this is as follows. The outer diameter of the first unthreaded extension 15a is substantially dependent on the thickness of the first unthreaded extension 15a. If the outer diameter of the first unthreaded extension 15a is small, because the thickness of the first unthreaded extension 15a is thin, if the external pressure is loaded to the threaded joint, the due stiffness of the first unthreaded extension 15a 2 diameter resistance region of the sealing surface 16 becomes insufficient. In this case, there is a fear that the contact surface pressure between the second sealing surface 16, 26 is reduced. Therefore, the outer diameter of the first unthreaded extension 15a is preferably large to some extent.
[0102]
　The upper limit of the outer diameter of the first unthreaded extension 15a is not particularly defined. However, the outer diameter of the first unthreaded extension 15a is required to not interfere with the second sealing surface 26 of the box 20 when the fastening dimensions.
[0103]
　In the pin 10 that the length along the tube axis of the first unthreaded extension 15a is the second least 1 times the thread pitch of the external thread portion 17 as measured from the front end of the second sealing surface 16 It is preferred. If the length of the first unthreaded extension 15a is short, similar to the reason when the thickness of the first unthreaded extension 15a is thin, if the external pressure is loaded to the screw joint, the second seal surface 16 and 26 contact surface pressure between there is a fear that decrease.
[0104]
　Maximum length of the first unthreaded extension 15a is not particularly defined. However, if the length of the first unthreaded extension 15a is too long, the total length of the joint is increased, the manufacturing cost is increased by an increase in working time and material costs. Moreover, the effect of improving the sealing performance becomes higher has a length length of the first unthreaded extension 15a is substantially saturated. Therefore, light of practicality, the length of the first unthreaded extension 15a is preferably set to 5 times the thread pitch of the second male threaded portion 17.
[0105]
　In the pin 10, the length along the tube axis of the second unthreaded extension 15b is a 1-fold or more thread pitch of the second male threaded portion 17 as measured from the rear end side of the end portion of the second sealing surface 16 it is preferable. If the length of the second unthreaded extension 15b is short, the decrease in the substantial amount of interference between the second sealing surface 16, 26 by fit action interference fit of the second threaded portion, the second sealing surface 16, 26 with each other contact surface pressure there is a fear that decrease.
[0106]
　Maximum length of the second unthreaded extension 15b is not particularly specified. However, if the length of the second unthreaded extension 15b is too long, the total length of the joint is increased, the manufacturing cost is increased by an increase in working time and material costs. Moreover, the effect of improving the sealing performance becomes higher length with a length of the second unthreaded extension 15b is nearly saturated. Therefore, light of practicality, the length of the second unthreaded extension 15b is preferably set to 5 times the thread pitch of the second male threaded portion 17.
[0107]
　In the pin 10, it is preferable length along the tube axis CL of the nose portion 12 is 5mm or more. The reason for this is as follows. When the length of the nose portion 12 is short, when an excessive tensile load is applied to the threaded joint, elastic restoring force of the first seal surface 13 by the nose portion 12 becomes insufficient. In this case, there is a fear that the contact surface pressure between the first seal surface 13 and 23 is reduced. Therefore, the length of the nose portion 12 is relatively long, it is preferable.
[0108]
　The upper limit of the length of the nose portion 12 is not particularly specified. However, if too long a length of the nose portion 12, the entire length of the joint is increased, the manufacturing cost is increased by an increase in working time and material costs. Moreover, the effect of improving the sealing performance becomes longer than the length that the length of the nose portion 12 is substantially saturated. Therefore, light of practicality, the length of the nose portion 12 is preferably set to 5 times the thread pitch of the first male threaded portion 14.
[0109]
　Besides, the present disclosure is not limited to the above embodiments, without departing from the scope of the present disclosure, various modifications are possible. For example, when the internal pressure in the screw joint is loaded, it may be added measures to alleviate adhesion mating of the first threaded portion of the inboard and restricted to areas close to the first sealing portion. Thus, inboard region of the first sealing surface 13 of the pin 10 is more effectively expanded deformation, the contact surface pressure between the first seal surface 13 and 23 is further amplified. As the measure, in the region close to the first seal portion of the first threaded portion, the first male screw portion 14 or the first female threaded portion 24 may adopt a configuration in which the incomplete incomplete thread of the screw-shaped. As an example of this configuration, the in the region of incomplete thread, as a first cylindrical surface parallel to the tube axis CL of the thread crest surface of the female screw portion 24 of the box 20, the thread height lower than the thread height of the normal. Thus, only in the area of ​​the incomplete thread, a gap is provided between the thread crest surface of the first female screw portion 24 and the thread root bottom surface of the first male threaded portion 14. In this case, the length of the region of the incomplete thread is the 3-9 times the thread pitch of the first female threaded portion 24 (about 15 ~ 45 mm).
[0110]
　Furthermore, a threaded joint of the above embodiments is applicable to any integral type and coupling type.
Example
[0111]
　To confirm the effects of a threaded joint for steel pipes according to the present disclosure was conducted numerical simulation analysis by elasto-plastic finite element method.
[0112]
　
　intermediate shoulder interference amount [delta] shld for the different specimens was compared the difference in performance by implementing a finite element analysis. Each specimen is coupling type of threaded joint having the basic structure shown in FIGS. 6-8. A common test conditions are shown below.
(1) the size of the steel pipe
　7-5 / 8 [inch] × 1.06 [inch] ( outer diameter 193.68 [mm], the thickness 27.0 [mm]), or 8-5 / 8 [inch] × 1.15 [inch] (outside diameter 219.1 [mm], the thickness 29.2
[mm]] (2) steel grades
　P110 (nominal yield stress of API standard 110 [ksi] carbon
steel) ( 3) the dimensions of the screws (common to all of the screw)
　screw pitch: 5.08 [mm], the flank angle of the load surface: -3 °, insert flank angle: 10 °, insertion surface gap: 0.15 [mm ]
[0113]
　In finite element analysis, the elasto-plastic body of isotropic hardening the material, the elastic modulus 210 [GPa], as the yield strength as 0.2% proof stress is 110 [ksi] (= 758.3 [MPa]) , it was used a model of each specimen.
[0114]
　
　First Analysis
　as a first analysis, was analyzed tightening screws for each specimen. In the first analysis, after which one of the shoulder surfaces come into contact with each other, between the other shoulder surface was tightened until contact. However, after one of the shoulder surfaces come into contact with each other, if each other the other shoulder surface is not contacted even tightening 15/100 turns to complete the tightening at that point.
[0115]
　In the first analysis, the contact timing between the inner shoulder surface (11, 21), the difference between the contact timing between the intermediate shoulder surfaces (18, 28) were evaluated by the following four levels.
　Yu: intermediate shoulder faces, in contact in the order of between the inner shoulder surface, the absolute value of the tightening rotational speed difference Δ is 3 [× 1 / 100turn] or less.
　- Good: inner shoulder faces, in contact in the order of between intermediate shoulder surface, the absolute value of the tightening rotational speed difference Δ is 3 [× 1 / 100turn] or less.
　· Allowed: contact with each other the inner shoulder surface is above the absolute value of the tightening rotational speed difference Δ is greater than 3 [× 1 / 100turn].
　- Call: contact between the intermediate shoulder surface above, the absolute value of the tightening rotational speed difference Δ is greater than 3 [× 1 / 100turn].
[0116]
　Second Analysis]
　In the second analysis, each specimen was loaded into the model of the fastening state a load history simulating the Series A test ISO13679. In a second analysis, after either one of between the inner shoulder faces and the intermediate shoulder surfaces are in contact, and the engagement completion at the time of tightening further 3/100 turn.

　Is composed of a pin of the tubular, the tubular box, the pin is a threaded joint for a steel pipe and the box and the pin is screwed into the box is fastened,
　said pins, in order from the distal end side, a first shoulder surface, the first male threaded portion of the first sealing surface, tapered threads, the second shoulder surface, includes a second male threaded portion of the second sealing surface and the tapered thread,
　wherein the box, the first shoulder surface of the pin, said first 1 sealing surface, the first male threaded portion, the second shoulder surface, the first internally threaded portion of the first shoulder surface corresponding to each of the second sealing surface and the second male threaded portion, a first sealing surface, a tapered thread, the second shoulder surface, includes a second female threaded portion of the second sealing surface and the tapered thread, further
　said pins, nose portion continuous to said first sealing surface between the first shoulder surface and the first sealing surface the Bei Rutotomoni, comprising the unthreaded extension connected to the second sealing surface between the second shoulder surface and the second sealing surface,
　the box is provided with a recess corresponding to the nose portion of the pin, has a unthreaded extension corresponding to the unthreaded extension of the pin,
　in engagement condition, said first shoulder faces are in contact with each other, the first sealing faces are in contact with each other, the second seal face each other in contact with each other, a gap is formed between the recess of the box and the nose portion of the pin, a clearance between the unthreaded extension of the unthreaded extension and the box of the pin There is formed, wherein the first male threaded portion fit first Mari and the internal thread portion is fitted, wherein the second male threaded portion fit second Mari and the internal thread portion is engaged,
　said first shoulder at said pin disengaged state The distance in the tube axis direction from the surface to the second shoulder surface L pin , the distance in the tube axis direction from the first shoulder surface to the second shoulder surface L in the box disengaged state box as the pin the interference amount δ of the second shoulder surface and the second shoulder surface of the box shld when defined by the following equation (1) satisfies the following equation (2) and (3), a threaded joint for steel pipes .
[Equation 1]

　where, P is the first male screw portion of the screw pitch, delta min is tightened after contact the first shoulder surface or between the second shoulder surfaces each other in the fastening process the rotational speed of the lower limit value , delta max , the tightening rotational speed upper limit value, lambda is the elongation amount at the time of engagement of the second shoulder surface leading end side portion than the one of the pin,
　the internal diameter of the pin ID, the pin the innermost diameter D of the second shoulder surface ms , the outermost diameter of the first shoulder surface of the pin D iS and the interference amount δ between said first externally threaded portion and the first female threaded portion the when a, λ when a, λ is expressed using the following equation (4) and (5).