Threaded steel pipe and its manufacturing method

Abstract

A threaded steel pipe is provided that reduces stress applied to the threaded portion during use and prevents fatigue failure. The threaded steel pipe includes an internal threaded portion on the inner circumferential surface of at least one end portion. The maximum value of the residual compressive stress at the thread bottom of the internal threaded portion, measured 0.4 mm in the depth direction from the thread bottom, is 100 MPa or greater and is less than the tensile strength of the material of the threaded steel pipe.

Description

Technical Field

[0001] The present invention relates to a threaded steel pipe, and more particularly to a threaded steel pipe having excellent fatigue strength at the threaded portion and suitable for use in high-pressure gas containers, etc. The present invention also relates to a method for producing the threaded steel pipe. Background Art

[0002] Fuel cell vehicles are expected to be the new generation of vehicles of the future, addressing both CO2 emissions and energy needs. Hydrogen refueling stations that supply hydrogen to such fuel cell vehicles are equipped with high-pressure gas containers (also called accumulators) that store hydrogen at a pressure of 80 MPa or higher.

[0003] There are two main shapes of high-pressure gas containers: cylindrical containers, typically gas cylinders, where the ends of a tube are drawn to create a dome portion; and straight containers, where caps are attached to both ends of a straight tube.

[0004] The cylindrical container has a shape in which the cross-sectional area inside the container decreases toward the gas outlet, i.e., the end in the longitudinal direction, and this end is called a "mirror portion." A nozzle for gas inlet and outlet is provided at the front end of the mirror portion, and the nozzle is sealed with a stopper having a thread. The area of the stopper is sufficiently small compared to the cross-sectional area of the cylindrical portion of the cylindrical container, so the stress applied to the threaded portion of the nozzle is reduced, and therefore, there is no problem with pressure sealing. However, high-pressure gas containers such as those used in hydrogen refueling stations require regular internal surface inspections after they are put into use, and cylindrical containers have the problem of being difficult to inspect the inner surface of the container.

[0005] Furthermore, when using metal containers for high-pressure gas, they are typically heat-treated to improve strength. This heat treatment typically involves quenching the metal container by rapidly cooling it with cooling water after heating it. However, in the case of cylindrical containers, the cooling water takes time to flow in and out of the container, slowing the cooling rate during heat treatment and increasing variations in steel structure.

[0006] Furthermore, while the aforementioned heat treatment produces oxide scale and decarburized layers on the surface of metal containers, it is difficult to remove the oxide scale and decarburized layers formed on the inner surface of cylindrical containers. Consequently, direct use of the metal container after heat treatment of the inner surface deteriorates its fatigue properties.

[0007] Therefore, to avoid the aforementioned problems, the use of straight containers is considered. A straight tube with a cap provides a large opening, facilitating cooling during heat treatment and enabling precise control of the steel's structure. Furthermore, the decarburized layer and scale formed during heat treatment can be easily removed through machining. Furthermore, removing the cap facilitates inspection of the container's interior surface after use. Furthermore, straight containers lack the mirrored portion that undergoes deep drawing, resulting in virtually no variations caused by machining, enabling the manufacture of uniform containers. Examples of such high-pressure gas containers include those described in Patent Documents 1 and 2.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2015-158243

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2017-141919 Summary of the Invention

[0012] However, when using a straight container, the container cross-section is fixed, so the entire internal pressure is borne by the lid. Therefore, for high-pressure gas containers using a straight container, the lid structure must be able to withstand extremely high pressures.

[0013] As the lid structure of a straight container, there are a structure in which a flange is provided at the end of a straight-shaped container and the lid is fixed with bolts using the flange; a structure in which an internal thread portion is provided at the end of a steel pipe and a lid having an external thread portion screwed to the above-mentioned internal thread portion is threadedly fastened to the steel pipe, etc.

[0014] However, the lid structure using the flange has the problem that the container size increases and the cost increases in order to provide the flange. Therefore, in order to meet the requirements of miniaturization and cost reduction of the container size, the lid structure using screw fastening is preferably adopted.

[0015] However, while threaded cap structures can avoid the problems associated with flanges, the stress on the threads is high, and fatigue failure may occur starting from the threads. Therefore, in order to extend the fatigue life of high-pressure gas containers, it is necessary to prevent fatigue failure starting from the threads of the threaded steel pipe used as the container body.

[0016] Furthermore, when the threaded steel pipe is used for applications other than high-pressure gas containers, it is also preferable to prevent fatigue failure starting from the threaded portion in order to extend the product life.

[0017] The present invention has been made in view of the above circumstances, and an object of the present invention is to alleviate the stress applied to the thread portion of a threaded steel pipe during use and to prevent fatigue failure.

[0018] The present inventors conducted research to solve the above-mentioned problems and, as a result, obtained the following findings.

[0019] (1) When a metal container for a high-pressure gas is filled with gas, the metal container is subjected to internal pressure, which generates tensile stress in the metal container. The tensile stress is concentrated in the threaded portion, particularly near the thread bottom.

[0020] (2) Therefore, by pre-applying a predetermined residual compressive stress near the bottom of the thread of a threaded steel pipe used as a metal container, at least a portion of the tensile stress applied to the thread when the metal container is filled with gas is offset by the residual compressive stress, thereby significantly alleviating the stress actually applied to the thread. Furthermore, as a result, fatigue failure originating from the thread can be suppressed.

[0021] (3) When the threaded steel pipe is used for purposes other than metal containers, by pre-applying a predetermined residual compressive stress near the bottom of the thread, the stress applied to the thread portion during use can be relaxed, thereby suppressing fatigue failure.

[0022] (4) When internal pressure is applied to the internal thread of a threaded steel pipe while a cap having external threads screwed to the internal thread is installed, stress is generated, causing local plastic deformation at the bottom of the thread. Plastic deformation occurs only in a certain area, while the remaining area is mostly elastic. Therefore, after the internal pressure is released, compressive stress remains at the bottom of the thread. Therefore, by applying an appropriate load to the threaded steel pipe while the cap is installed, a predetermined residual compressive stress can be imparted to the internal thread of the threaded steel pipe.

[0023] (5) Similarly, by applying a load to the threaded steel pipe using a jig having an external thread that is screwed to the internal thread of the threaded steel pipe, a predetermined residual compressive stress can be applied to the internal thread of the threaded steel pipe.

[0024] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0025] 1. A threaded steel pipe having an internal threaded portion on the inner circumference of at least one end.

[0026] The maximum value of the residual compressive stress of the thread bottom of the female thread portion at a position 0.4 mm in a depth direction from the thread bottom is 100 MPa or more and is equal to or less than the tensile strength of the material of the threaded steel pipe.

[0027] 2. The threaded steel pipe according to item 1 above, wherein the maximum value of the residual compressive stress is equal to or less than the yield stress of the material of the threaded steel pipe.

[0028] 3. The threaded steel pipe according to item 1 or 2 above, wherein the threaded steel pipe is made of a seamless steel pipe.

[0029] 4. A method for manufacturing a threaded steel pipe, comprising the following steps:

[0030] a cover mounting step of mounting a cover having an external threaded portion on an outer circumferential surface of the threaded steel pipe having an internal threaded portion on an inner circumferential surface of at least one end portion, so that the external threaded portion is screwed into the internal threaded portion;

[0031] In the internal pressure application step, internal pressure is applied to the threaded steel pipe with the cap installed so as to meet the following conditions (A), (B) and (C):

[0032] (A) The thread bottom stress of the internal thread portion is greater than the yield stress of the material of the threaded steel pipe;

[0033] (B) the axial stress of the threaded steel pipe is less than the tensile strength of the material of the threaded steel pipe;

[0034] (C) The hoop stress of the threaded steel pipe is equal to or less than the tensile strength of the material of the threaded steel pipe.

[0035] 5. A method for manufacturing a threaded steel pipe, comprising the following steps:

[0036] a fixture installation step, wherein the threaded steel pipe having an internal thread portion on the inner circumferential surface of at least one end is installed in a fixture having an external thread portion on the outer circumferential surface so that the external thread portion is screwed into the internal thread portion; and

[0037] The load application step is to apply a load to the threaded steel pipe using the jig so as to satisfy the following two conditions (A) and (B):

[0038] (A) The thread bottom stress of the internal thread portion is greater than the yield stress of the material of the threaded steel pipe;

[0039] (B) The axial stress of the threaded steel pipe is equal to or less than the tensile strength of the material of the threaded steel pipe.

[0040] 6. The method for producing a threaded steel pipe according to 4 or 5 above, wherein the threaded steel pipe is made of a seamless steel pipe.

[0041] According to the present invention, the stress applied to the threaded portion during use of the threaded steel pipe can be alleviated, thereby preventing fatigue failure. Therefore, the threaded steel pipe of the present invention can be used under higher pressures and with larger cross-sections than conventional pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 Schematic diagram showing the position where the residual compressive stress is specified in the present invention.

[0043] Figure 2 This is a schematic diagram showing an example of a method for producing a threaded steel pipe according to the first embodiment.

[0044] Figure 3 This is a schematic diagram showing an example of a method for manufacturing a threaded steel pipe according to the second embodiment. DETAILED DESCRIPTION

[0045] Next, a method for implementing the present invention will be described in detail. However, the following description represents preferred embodiments of the present invention, and the present invention is not limited to the following description.

[0046] [Threaded steel pipe]

[0047] A threaded steel pipe according to one embodiment of the present invention has an internal threaded portion on the inner circumferential surface of at least one end. When used with this threaded steel pipe, a component having an external threaded portion corresponding to the internal threaded portion can be screwed onto the pipe. For example, when using this threaded steel pipe as a container for high-pressure gas, a screw-on cap with an external thread can be attached to the internal threaded portion.

[0048] The threaded steel pipe may have an internal thread portion on the inner circumference of only one end, or may have an internal thread portion on the inner circumference of both ends. In addition, when the threaded steel pipe has an internal thread portion on the inner circumference of only one end, the inner circumference of the other end may have an external thread portion.

[0049] In the present invention, it is important that the maximum value of the residual compressive stress at the thread bottom of the female thread portion at a depth of 0.4 mm from the thread bottom is 100 MPa or greater and equal to or less than the tensile strength of the material of the threaded steel pipe. The reasons for this are explained below.

[0050] Figure 1 This is a schematic diagram showing the position where the residual compressive stress is specified in the present invention. A plurality of thread grooves 12 are provided in the internal thread portion 11 provided on the inner circumferential surface of at least one end portion of the steel pipe 10, and the bottom of the thread groove 12 is used as the thread bottom 13 of the internal thread portion. In the present invention, the maximum value of the residual compressive stress at a position P of 0.4 mm in the depth direction from the thread bottom 13 of each internal thread portion is set to be 100 MPa or more and less than the tensile strength of the material of the threaded steel pipe. It should be noted that Figure 1 These are schematic diagrams for explanation only and do not represent the actual shape and size of the threaded portion.

[0051] As mentioned above, when using a high-pressure gas container, the metal container is subjected to internal pressure when the gas is filled into the metal container, which generates tensile stress due to this internal pressure. This tensile stress is concentrated in the threaded portion, particularly near the thread base. Therefore, by pre-applying residual compressive stress near the thread base of the threaded steel pipe, at least a portion of the tensile stress applied to the threaded portion when the gas is filled into the metal container is offset by this residual compressive stress, significantly reducing the stress actually applied to the threaded portion. Furthermore, fatigue failure originating from the threaded portion can be suppressed.

[0052] However, the tensile stress generated during use of high-pressure gas containers extends not only on the surface of the thread but also into the material. Therefore, even if residual compressive stress exists only in the surface layer of the thread, the aforementioned effects cannot be achieved. Therefore, in the present invention, the maximum residual compressive stress at a depth of 0.4 mm from the thread bottom is set to 100 MPa or greater.

[0053] On the other hand, if the maximum value of the residual compressive stress is too high, the threaded steel pipe will bend and deform. Therefore, in order to prevent deformation of the threaded steel pipe, the maximum value of the residual compressive stress is set to be less than the tensile strength of the material of the threaded steel pipe.

[0054] The maximum value of the residual compressive stress can be determined by elastic-plastic analysis using the finite element method (FEM), but can also be determined by measuring the residual stress in the cross section of the internal thread portion using X-ray stress measurement.

[0055] From the viewpoint of more reliably preventing buckling of the threaded portion, the maximum value of the residual compressive stress is preferably equal to or less than the yield stress of the material of the threaded steel pipe.

[0056] As the material of the above-mentioned threaded steel pipe, any metal can be used without particular limitation. From the viewpoint of low cost, low alloy steel is preferably used as the above-mentioned material. As the above-mentioned low alloy steel, it is particularly preferred to use any one of chromium-molybdenum steel (JIS SCM steel), nickel-chromium-molybdenum steel (JIS SNCM steel), manganese-chromium steel (JIS SMnC steel), manganese steel (JIS SMnsteel), ASEM SA-723 and boron-added steel N28CB, N36CB, and N46CB. Among them, from the viewpoint of achieving both material strength, it is more preferred to use chromium-molybdenum steel, SA-723 steel, or nickel-chromium-molybdenum steel that is easy to ensure hardenability. For example, chromium-molybdenum steel (SCM435) comprises 0.33-0.38 mass% C, 0.15-0.35 mass% Si, 0.60-0.90 mass% Mn, 0.030 mass% or less P, 0.030 mass% or less S, 0.90-1.20 mass% Cr, and 0.15-0.30 mass% Mo.

[0057] As the steel pipe, any type of steel pipe, such as an electric-welded steel pipe or a seamless steel pipe, can be used, but seamless steel pipe is preferred. Seamless steel pipe has excellent properties such as toughness and has no welded parts, making it particularly suitable for use in high-pressure gas containers.

[0058] In the present invention, the depth of the thread groove is not particularly limited. However, if the ratio Di / Ds of the inner diameter Di of the base pipe to the inner diameter Ds of the internal thread portion is less than 0.8, the stress applied to the thread bottom increases if the thread is too deep relative to the wall thickness of the steel pipe (base pipe). Therefore, from the perspective of further reducing the stress applied to the thread bottom, Di / Ds is preferably 0.8 or greater. It should be noted that the inner diameter Ds of the internal thread portion is defined as the distance between the thread bottoms at opposite positions of the internal thread portion formed on the inner circumferential surface of the steel pipe. In addition, the inner diameter Di of the base pipe refers to the inner diameter of the portion of the steel pipe where the internal thread is not formed.

[0059] The threaded steel pipe of the present invention is not particularly limited in its use and can be used in any application. However, since the threaded portion has excellent fatigue strength as described above, it is particularly suitable for use in high-pressure gas containers, joints (e.g., joints for steel pipe piles), and the like.

[0060] [Manufacturing method]

[0061] Next, a method for manufacturing a threaded steel pipe according to one embodiment of the present invention will be described. As described above, when internal pressure or load is applied to a threaded steel pipe, stress is generated, causing localized plastic deformation at the base of the thread. Plastic deformation occurs only in a certain area, while the remaining area remains elastic. After the internal pressure or load is released, compressive stress remains at the base of the thread. Therefore, by applying an appropriate internal pressure or load to the threaded steel pipe, a predetermined residual compressive stress can be applied to the internal thread of the threaded steel pipe. The following describes in detail the manufacturing methods for applying internal pressure and the manufacturing methods for applying load, respectively.

[0062] (First embodiment)

[0063] In one embodiment of the present invention, a threaded steel pipe having an internal threaded portion on the inner circumferential surface of at least one end is attached to a cap having an external threaded portion on the outer circumferential surface (cap attachment step). Subsequently, internal pressure is applied to the threaded steel pipe with the cap attached so as to satisfy predetermined conditions (internal pressure application step), thereby manufacturing a threaded steel pipe having the aforementioned predetermined residual compressive stress.

[0064] [Lid installation process]

[0065] First, a cap having an external threaded portion on its outer circumferential surface is mounted on a threaded steel pipe having an internal threaded portion on its inner circumferential surface at least one end portion so that the external threaded portion is screwed into the internal threaded portion (cap mounting step).

[0066] Figure 2 Schematic diagram showing an example of a method for manufacturing a threaded steel pipe 1 according to the present embodiment. The threaded steel pipe 1 is composed of a cylindrical steel pipe 10 , and an internal thread portion 11 is provided at an end of the steel pipe 10 . Figure 2 In the example shown, internal thread portions 11 are provided at both ends of the steel pipe 10 .

[0067] Furthermore, a cover 40 having an external threaded portion 41 on its outer peripheral surface is mounted on the threaded steel pipe 1 so that the external threaded portion 41 is screwed together with the internal threaded portion 11 of the threaded steel pipe 1. Figure 2 As shown, the cap 40 preferably includes an O-ring 42 as a sealing member on its outer peripheral surface. The external thread portion 41 is provided on the outside of the container (on the side opposite to the internal space 14) with respect to the O-ring 42.

[0068] As the cap, any cap can be used as long as it can be screwed onto the internal thread of the threaded steel pipe. When the threaded steel pipe has internal threads at both ends, caps can be installed at both ends of the threaded steel pipe to apply internal pressure.

[0069] The cover may include a through hole for inputting and withdrawing a pressure medium used in the internal pressure application step described below. The through hole may be connected to a pipe or a valve as desired.

[0070] The material of the lid is not particularly limited, but is preferably made of metal, more preferably steel. As the steel, it is further preferred to use steel having a tensile strength (TS) of 750 MPa or greater. For example, the steel may be low-alloy steel. The lid may be made of the same materials as those listed for the steel pipe. The lid and steel pipe materials may be the same or different, but are preferably the same.

[0071] [Internal pressure application process]

[0072] Next, internal pressure is applied to the threaded steel pipe with the cap attached (internal pressure application step). In the internal pressure application step, internal pressure must be applied under the following conditions (A), (B), and (C). The reasons for this are explained below.

[0073] (A) The thread bottom stress of the internal thread portion is greater than the yield stress of the material of the threaded steel pipe

[0074] (B) The axial stress of the threaded steel pipe is less than the tensile strength of the material of the threaded steel pipe.

[0075] (C) The hoop stress of the threaded steel pipe is less than the tensile strength of the material of the threaded steel pipe.

[0076] ·Condition(A)

[0077] To apply residual compressive stress near the thread base, plastic deformation must occur at the thread base. Furthermore, to induce plastic deformation at the thread base, a thread base stress greater than the material's yield stress can be applied. Therefore, in the present invention, the internal pressure application step is performed under the condition (A) above. By satisfying condition (A), residual compressive stress can be applied to the internal thread.

[0078] In order to more effectively apply residual compressive stress, it is preferable that the thread bottom stress applied in the internal pressure application step is greater than the tensile strength of the material. In other words, in the internal pressure application step, it is preferable to apply internal pressure under the following condition (A').

[0079] (A') The thread bottom stress of the internal thread portion is greater than the tensile strength of the material of the threaded steel pipe

[0080] It should be noted that the upper limit of the stress at the bottom of the thread of the internal thread portion during the internal pressure application process is not particularly limited and can be adjusted to apply the desired residual compressive stress. It should be noted that from the perspective of effectively applying residual compressive stress, it is preferable to set the stress on the outer surface of the threaded steel pipe (the outer peripheral surface of the steel pipe) opposite the internal thread portion during the internal pressure application process to be below the yield stress of the material of the threaded steel pipe. By setting the stress at the bottom of the thread to be greater than the yield stress and setting the stress on the outer surface of the threaded steel pipe to be below the yield stress, residual compressive stress can be effectively applied near the bottom of the thread compared to the case where a stress higher than the yield stress is applied throughout the entire wall thickness of the threaded steel pipe.

[0081] Conditions (B), (C)

[0082] As mentioned above, applying residual compressive stress requires internal pressure to induce plastic deformation. Excessive internal pressure can damage the steel pipe. To prevent damage, both the axial and hoop stresses applied to the threaded steel pipe must be below the tensile strength of the material used. (B) and (C) above specifically define these conditions.

[0083] It should be noted that the axial stress and the hoop stress applied to the threaded steel pipe can be obtained by the following equations (1) and (2), respectively.

[0084] Axial stress = (pressure area of the cover × internal pressure) / minimum cross-sectional area of the threaded steel pipe... (1)

[0085] Circumferential stress = (inner diameter of threaded steel pipe × internal pressure) / (2 × thickness of threaded steel pipe)…(2)

[0086] Here, the "pressure-bearing area of the lid" refers to the area of the inner surface of the lid (the surface subjected to internal pressure). Furthermore, the "cross-sectional area" of a threaded steel pipe refers to the cross-sectional area of the steel portion of the cross section perpendicular to the axial direction of the threaded steel pipe, and does not include the cross-sectional area of the internal space of the steel pipe. It should be noted that the cross-sectional area of a threaded steel pipe sometimes varies depending on the position along the longitudinal direction of the threaded steel pipe, and the axial stress applied to the threaded steel pipe is greatest in the portion with the smallest cross-sectional area. Therefore, the minimum cross-sectional area of the threaded steel pipe is used in the above formula (1).

[0087] It should be noted that when at least one of the axial stress and the circumferential stress applied to the threaded steel pipe is greater than the yield stress of the material of the threaded steel pipe, the steel pipe undergoes plastic deformation and the inner diameter of the steel pipe may change. If the inner diameter changes, there is a problem of reduced sealing when the threaded steel pipe is used in a container for high-pressure gas, etc. Therefore, from the viewpoint of suppressing the reduction in sealing caused by the plastic deformation of the steel pipe, it is preferred that the axial stress and the circumferential stress applied to the threaded steel pipe be less than the yield stress of the material of the threaded steel pipe, and more preferably less than 90% of the yield stress. In other words, in the above-mentioned internal pressure application process, it is preferred to apply the internal pressure under the conditions of satisfying the following (B') and (C'), and it is more preferred to apply the internal pressure under the conditions of satisfying the following (B") and (C").

[0088] (B') The axial stress of the threaded steel pipe is less than the yield stress of the material of the threaded steel pipe.

[0089] (C') The circumferential stress of the threaded steel pipe is less than the yield stress of the material of the threaded steel pipe.

[0090] (B") The axial stress of the threaded steel pipe is less than 90% of the yield stress of the material of the threaded steel pipe

[0091] (C") The hoop stress of the threaded steel pipe is less than 90% of the yield stress of the material of the threaded steel pipe

[0092] In order to apply internal pressure to the threaded steel pipe during the internal pressure application step, any pressure medium may be filled into the interior of the threaded steel pipe. While any medium may be used as the pressure medium without particular limitation, from a safety perspective, incompressible fluids such as water and oil are preferred. Furthermore, from a corrosion prevention perspective, incompressible fluids containing a corrosion inhibitor or aqueous solutions of alcohols such as ethylene glycol are preferred.

[0093] (Second embodiment)

[0094] In another embodiment of the present invention, a threaded steel pipe having a female thread on its inner circumference is mounted on a jig having a male thread on its outer circumference (a jig mounting step). Subsequently, a load is applied to the threaded steel pipe using the jig so as to satisfy predetermined conditions (a load application step). This allows the manufacture of a threaded steel pipe having the predetermined residual compressive stress. This will be described in detail below with reference to the accompanying drawings.

[0095] [Jig installation process]

[0096] Figure 3Schematic diagram showing a method for manufacturing a threaded steel pipe 1 according to this embodiment, that is, a method of applying a load using a jig. The threaded steel pipe 1 is composed of a cylindrical steel pipe 10, and an internal thread portion 11 is provided at the end of the steel pipe 10. Figure 3 In the illustrated example, internal thread portions 11 are provided at both ends of the steel pipe 10 .

[0097] First, a clamp 20 having an external threaded portion 21 on its outer circumferential surface is mounted on the threaded steel pipe 1 so that the external threaded portion 21 is screwed together with the internal threaded portion 11 of the threaded steel pipe 1 (a clamp mounting step). Any clamp 20 may be used as long as it has an external threaded portion 21 that can be screwed together with the internal threaded portion 11.

[0098] The material of the clamp is not particularly limited, but is preferably made of metal, more preferably steel. As the steel, it is more preferably steel having a tensile strength (TS) of 750 MPa or greater. For example, the steel may be low-alloy steel. The clamp can be made of the same materials as those listed for the steel pipe. The clamp material and the steel pipe material may be the same or different, but are preferably the same.

[0099] [Load application process]

[0100] Next, a load is applied to the threaded steel pipe 1 using the mounted jig 20 (load application step). The method of applying the load is not particularly limited, and for example, the load may be applied in a direction parallel to the axis of the threaded steel pipe 1 and in the outer direction of the pipe ( Figure 3 The load is applied by pulling the clamp 20 in the direction of the arrow A in FIG. Specifically, for example, the rod 22 mounted on the clamp 20 can be pulled by the load applier 30.

[0101] It should be explained that Figure 3 In the example shown, a load is applied to the internal thread portion 11 provided at one end of the threaded steel pipe 1. However, when internal thread portions 11 are provided at both ends of the threaded steel pipe 1, it is preferable to apply loads to the internal thread portions 11 at both ends. In this case, the application of a load to one internal thread portion 11 and the application of a load to the other internal thread portion 11 may be performed simultaneously or separately.

[0102] In the load application step, the load must be applied under the following conditions (A) and (B). The reasons for this are described below.

[0103] (A) The thread bottom stress of the internal thread portion is greater than the yield stress of the material of the threaded steel pipe

[0104] (B) The axial stress of the threaded steel pipe is less than the tensile strength of the material of the threaded steel pipe.

[0105] ·Condition(A)

[0106] To apply residual compressive stress near the thread base, it is necessary to induce plastic deformation at the thread base. Furthermore, to induce plastic deformation in the threaded portion, a thread base stress greater than the material's yield stress can be applied. Therefore, in the present invention, the load is applied during the load application step while satisfying condition (A). By satisfying condition (A), residual compressive stress can be applied to the internal threaded portion.

[0107] In order to more effectively impart residual compressive stress, it is preferred that the thread bottom stress applied in the load application step be greater than the tensile strength of the material. In other words, in the load application step, it is preferred that the load be applied under the following condition (A').

[0108] (A') The thread bottom stress of the internal thread portion is greater than the tensile strength of the material of the threaded steel pipe

[0109] It should be noted that the upper limit of the stress at the bottom of the thread of the internal thread portion during the load application process is not particularly limited and can be adjusted to apply the desired residual compressive stress. It should be noted that from the perspective of effectively applying residual compressive stress, it is preferable to set the stress on the outer surface of the threaded steel pipe (the outer peripheral surface of the steel pipe) opposite the internal thread portion during the load application process to be below the yield stress of the material of the threaded steel pipe. By setting the stress at the bottom of the thread to be greater than the yield stress and setting the stress on the outer surface of the threaded steel pipe to be below the yield stress, it is possible to effectively apply residual compressive stress near the bottom of the thread compared to applying a stress higher than the yield stress throughout the entire wall thickness of the threaded steel pipe.

[0110] Condition (B)

[0111] As mentioned above, applying a load to induce plastic deformation is necessary to impart residual compressive stress. However, excessive load can lead to pipe failure. To prevent pipe failure, the axial stress applied to the threaded pipe must be below the tensile strength of the material used. (B) above specifically defines these conditions.

[0112] It should be noted that the axial stress applied to the threaded steel pipe can be obtained by the following formula (1).

[0113] Axial stress = load / minimum cross-sectional area of threaded steel pipe...(1)

[0114] Here, the "cross-sectional area" of a threaded steel pipe refers to the cross-sectional area of the portion of the threaded steel pipe perpendicular to the axial direction where metal exists. It should be noted that the cross-sectional area of a threaded steel pipe may vary depending on the position along the longitudinal direction of the threaded steel pipe, and the axial stress applied to the threaded steel pipe is greatest in the portion with the smallest cross-sectional area. Therefore, the minimum cross-sectional area of the threaded steel pipe is used in the above formula (1).

[0115] It should be noted that when the axial stress applied to the threaded steel pipe is greater than the yield stress of the material of the threaded steel pipe, the steel pipe undergoes plastic deformation and the inner diameter of the steel pipe may change. If the inner diameter changes, there is a problem of reduced sealing when the threaded steel pipe is used for a high-pressure gas container, etc. Therefore, from the viewpoint of suppressing the reduction in sealing caused by the plastic deformation of the steel pipe, the axial stress applied to the threaded steel pipe is preferably less than the yield stress of the material of the threaded steel pipe, and more preferably less than 90% of the yield stress. In other words, in the above-mentioned load application process, it is preferred to apply internal pressure under the condition of satisfying the following (B'), and it is more preferred to apply internal pressure under the condition of satisfying the following (B").

[0116] (B') The axial stress of the threaded steel pipe is less than the yield stress of the material of the threaded steel pipe.

[0117] (B") The axial stress of the threaded steel pipe is less than 90% of the yield stress of the material of the threaded steel pipe

[0118] Example

[0119] Hereinafter, the effects of the present invention will be described using examples. However, the present invention is not limited to the following examples.

[0120] (Example 1)

[0121] The stress at the thread base was analyzed using the finite element method (FEM) using a threaded steel pipe model using elastic-plastic analysis. Assuming the threaded steel pipe is used as a high-pressure gas container, the models used were: a Type 1 container consisting of a low-alloy steel threaded steel pipe as a metal container without a carbon fiber-reinforced resin layer; and a Type 2 container consisting of the same low-alloy steel threaded steel pipe (liner) as the Type 1 container and a carbon fiber-reinforced resin layer formed by wrapping CFRP around the surface of the threaded steel pipe to a thickness of 5 mm. The threaded steel pipe constituting the metal container and the lid used in the internal pressure application process and performance evaluation described below were made of the same low-alloy steel, with a tensile strength (TS) of 821 MPa and a yield stress (YP) of 705 MPa. The stress-strain curve of the low-alloy steel used was that of SNCM439 steel with a TS: 900 MPa grade.

[0122] The dimensions of the threaded steel pipe constituting the metal container are fixed at a length of 4500 mm in the longitudinal direction and an outer diameter of 404 mm. The inner diameter and wall thickness are shown in Table 1. The lid is constructed of a disc-shaped end plate and a hollow cylindrical screw-in nut. The end plate has a wall thickness of 75 mm, and the screw-in nut has a wall thickness of 37 mm. The wall thickness of the screw-in nut is measured from the apex of the threaded portion provided on the outer circumference to the inner surface. The thread shape is a JIS trapezoidal screw with a pitch of 12 mm, a thread depth of 12 mm, and a thread shoulder radius of 2.2 mm.

[0123] The above-mentioned FEM analysis was performed under the following conditions.

[0124] Software: ABAQUS Ver.6.12-4 (Dassault Systèmes)

[0125] Calculation model: axisymmetric model

[0126] Mesh division: 50μm for stress concentration areas

[0127] Boundary conditions: Gas pressure is applied to the inner surface of the threaded steel pipe and the gas storage part of the end plate.

[0128] Constraints: Metal cylinder: Nodes on the Y symmetry plane, Y direction displacement constraint

[0129] End plates, threaded into nuts: fixing without active node displacement

[0130] Contact conditions: contact friction coefficient μ = 0.05

[0131] (Internal pressure application process)

[0132] The thread bottom stress was calculated using FEM when the internal pressure shown in Table 1 was applied to the metal container. The maximum value of the thread bottom stress in the internal thread portion is shown in Table 1. The maximum value of the thread bottom stress is the maximum value of the stress in the entire wall thickness direction from the thread bottom to the surface opposite the thread bottom.

[0133] The axial stress and hoop stress of the threaded steel pipe when the internal pressure is applied are also shown in Table 1. When the metal container is a type 1 container, the axial stress and hoop stress are calculated by the following formulas (1) and (2).

[0134] Axial stress = (pressure area of the cover × internal pressure) / minimum cross-sectional area of the threaded steel pipe... (1)

[0135] Circumferential stress = (inner diameter of threaded steel pipe × internal pressure) / (2 × thickness of threaded steel pipe)…(2)

[0136] When the metal container is a type 2 container, the axial stress and the hoop stress are obtained by FEM analysis.

[0137] (Residual compressive stress)

[0138] Next, FEM analysis was performed to determine the residual compressive stress at a position 0.4 mm in the depth direction of the thread bottom of the female thread portion in a state where the internal pressure was released. Table 2 shows the maximum value of the residual compressive stress.

[0139] Next, in order to evaluate the performance of each threaded steel pipe, the thread bottom stress and thread fracture life under internal pressure load were again determined.

[0140] (Thread bottom stress under internal pressure load)

[0141] Assuming the conditions of a threaded steel pipe actually used as a high-pressure gas container, FEM analysis was performed to determine the maximum stress at the thread base when a cap was attached to the threaded steel pipe and an internal pressure of 82 MPa was applied. The results are shown in Table 2.

[0142] (Thread fracture life)

[0143] The fracture life of the threaded portion during the pressure cycle test was evaluated using stresses calculated using FEM analysis. The fracture life evaluation was conducted according to the "Crack Growth Analysis Method for Various Locations" (Appendix IX to KHKS 0220 (2010)) established by the High-Pressure Gas Safety Research Institute. The pressure application conditions were a minimum pressure of 2 MPa, a maximum pressure of 82 MPa, and a temperature of room temperature.

[0144] The results shown in Table 2 demonstrate that applying internal pressure under appropriate conditions can induce residual compressive stress that satisfies the conditions of the present invention into the thread bottom. Furthermore, threaded steel pipes whose maximum residual compressive stress at a depth of 0.4 mm from the thread bottom satisfies the conditions of the present invention exhibit reduced stress at the thread bottom when loaded with internal pressure, resulting in excellent fatigue life.

[0145]

[0146]

[0147] (Example 2)

[0148] The stress analysis at the thread base was conducted using an elastic-plastic analysis in the same manner as in Example 1, except that a jig was used to apply the load instead of applying internal pressure with the lid attached. In this example, a jig of the same shape as the lid used in Example 1 was used as the jig.

[0149] (Load application process)

[0150] Using FEM, the thread base stress was calculated when a clamp was used to apply a load to the metal container. The maximum thread base stress values for the internal thread are shown in Table 3. Note that, for computational purposes, the load was applied by moving (displacing) the clamp parallel to the pipe axis and toward the outside of the pipe. The clamp displacement was controlled to achieve the maximum thread base stress values shown in Table 3. The maximum thread base stress value refers to the maximum stress value across the entire wall thickness from the thread base to the surface opposite the thread base.

[0151] Next, the following values were evaluated in the same procedure as in Example 1. The evaluation results are shown in Table 4.

[0152] Residual compressive stress at a position 0.4 mm from the bottom of the thread in the internal thread part in the state where the above load is released

[0153] Maximum stress at the bottom of the thread when the internal pressure is released at 82 MPa

[0154] ·Fracture life of threaded parts in force cycle test

[0155] The results shown in Table 4 demonstrate that by applying a load using a jig under appropriate conditions, residual compressive stress that satisfies the conditions of the present invention can be introduced into the thread bottom. Furthermore, threaded steel pipes whose maximum residual compressive stress at a depth of 0.4 mm from the thread bottom satisfies the conditions of the present invention exhibit reduced stress at the thread bottom when loaded with internal pressure, resulting in excellent fatigue life.

[0156]

[0157]

[0158] Explanation of symbols

[0159] 1 threaded steel pipe

[0160] 10 steel pipes

[0161] 11 Internal thread

[0162] 12 thread grooves

[0163] 13 Thread bottom of internal thread

[0164] 14 Interior Space

[0165] 20 fixtures

[0166] 21 External thread

[0167] 22 strokes

[0168] 30 Load applicators

[0169] 40 lids

[0170] 41 external thread

[0171] 42O-ring

[0172] P is 0.4mm from the bottom of the thread in the depth direction

Claims

1. A threaded steel pipe having an internal threaded portion on the inner circumference of at least one end. The maximum value of the residual compressive stress of the thread bottom of the internal thread portion at a position 0.4 mm in the depth direction from the thread bottom is 100 MPa or more and is equal to or less than the tensile strength of the material of the threaded steel pipe.

2. The threaded steel pipe according to claim 1, wherein The maximum value of the residual compressive stress is equal to or less than the yield stress of the material of the threaded steel pipe.

3. The threaded steel pipe according to claim 1 or 2, wherein: The threaded steel pipe is composed of a seamless steel pipe.

4. A method for manufacturing a threaded steel pipe, comprising the following steps: a cover mounting step of mounting a cover having an external threaded portion on the outer circumference of a threaded steel pipe having an internal threaded portion on the inner circumference of at least one end thereof, so that the external threaded portion is screwed into the internal threaded portion; and The internal pressure application step is to apply internal pressure to the threaded steel pipe with the cover installed so as to meet the following conditions (A), (B) and (C): (A) The thread bottom stress of the internal thread portion is greater than the yield stress of the material of the threaded steel pipe, (B) the axial stress of the threaded steel pipe is less than the tensile strength of the material of the threaded steel pipe, (C) The hoop stress of the threaded steel pipe is equal to or less than the tensile strength of the material of the threaded steel pipe.

5. A method for manufacturing a threaded steel pipe, comprising the following steps: a fixture installation step of installing a fixture having an external threaded portion on the outer circumference of a threaded steel pipe having an internal threaded portion on the inner circumference of at least one end thereof, so that the external threaded portion is screwed into the internal threaded portion; and The load applying step is to apply a load to the threaded steel pipe using the jig so as to satisfy both the following conditions (A) and (B): (A) The thread bottom stress of the internal thread portion is greater than the yield stress of the material of the threaded steel pipe, (B) The axial stress of the threaded steel pipe is equal to or less than the tensile strength of the material of the threaded steel pipe.

6. The method for manufacturing a threaded steel pipe according to claim 4 or 5, wherein: The threaded steel pipe is composed of a seamless steel pipe.

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