High-pressure gas container and manufacturing method thereof

Abstract

To alleviate stress applied to a thread portion to thereby prevent fatigue fracture, in a high-pressure gas container having a metallic container.SOLUTION: A high-pressure gas container having a metallic container is provided, the metallic container comprises a metal cylinder having a female thread portion at least at an inner periphery of one end part, and a lid having an outer periphery at a male thread portion which is threaded with the female thread portion. A maximum value of residual compressive stress in a position of 0.4 mm in a depth direction from a screw bottom out of a plurality of the screw bottoms of the female thread portion and the male thread portion is equal to or more than 100 Mpa, equal to or less than the tensile strength of a material of the metal cylinder, and equal to or less than the tensile strength of a material of the lid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-pressure gas container and a method for manufacturing the same. [Background technology]

[0002] Fuel cell vehicles, which can solve the CO2 emissions problem as well as energy problems, are expected to be the new vehicles of the future. Hydrogen stations that supply hydrogen to these fuel cell vehicles are equipped with high-pressure gas containers (also called pressure vessels) that store hydrogen at pressures of 80 MPa or more.

[0003] There are two main types of high-pressure gas containers: one is a cylinder-type container, such as a gas cylinder, in which the end of the tube is drawn to create a dome part, and the other is a straight container, in which both ends of a straight tube are capped.

[0004] A cylinder-type container has a shape in which the cross-sectional area of ​​the container decreases toward the gas outlet, i.e., the longitudinal end, and this end is called the "head portion." A nozzle for letting gas in and out is provided at the tip of the head portion, and the nozzle is sealed with a clasp having a thread. Because the area of ​​the clasp is sufficiently small compared to the cross-sectional area of ​​the cylindrical portion of the cylinder-type container, stress on the threaded portion of the nozzle is reduced, and therefore there are no problems with sealing the pressure. However, high-pressure gas containers such as those used at hydrogen stations require periodic internal inspections after use, and the problem with cylinder-type containers is that internal inspection of the container is difficult.

[0005] Furthermore, when a high-pressure gas container is manufactured using a metal container, the metal container is usually subjected to a heat treatment for the purpose of improving its strength. In the heat treatment, the metal container is generally quenched by heating it and then rapidly cooling it with cooling water. However, in the case of a cylinder-type container, it takes time for the cooling water to enter and drain from the inside of the container, so the cooling rate during the heat treatment is slow and the steel structure varies greatly.

[0006] Furthermore, the heat treatment generates scale and a decarburized layer on the surface of the metal container, but in the case of a cylinder-type container, it is difficult to remove the scale and decarburized layer formed on the inner surface of the container, so the inner surface is used in the state after the heat treatment, which causes deterioration of the fatigue properties of the metal container.

[0007] Therefore, in order to avoid the above-mentioned problems, it is conceivable to use a straight-type container. A structure in which a straight tube is covered with a lid facilitates cooling during heat treatment because the opening of the tube is large, allowing for precise control of the steel material structure. Furthermore, the decarburized layer and scale generated during heat treatment can be easily removed by machining. Furthermore, by removing the lid, it is easy to inspect the inner surface of the container after use. In addition, since a straight-type container does not have a drawn mirror portion, there is almost no variation due to processing, making it possible to manufacture a uniform container. Examples of such high-pressure gas containers include those described in Patent Documents 1 and 2, for example. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-158243 [Patent Document 2] Japanese Patent Application Laid-Open No. 2017-141919 Summary of the Invention [Problem to be solved by the invention]

[0009] However, when a straight-shaped container is used, the cross section of the container is constant, so the entire internal pressure is absorbed by the lid. Therefore, the lid structure of a high-pressure gas container using a straight-shaped container is required to withstand extremely high pressures.

[0010] Possible lid structures for straight-type containers include a structure in which a flange is provided at the end of a straight-shaped container and the lid is bolted using the flange, or a structure in which the lid is screwed onto the container.

[0011] However, a flange-based lid structure has the problem that the flange increases the container size and costs, so in order to meet the demand for smaller container sizes and lower costs, it is desirable to adopt a screw-type lid structure.

[0012] However, although a screw-fastened lid structure can avoid the problems that occur when a flange is used, the stress applied to the threaded portion is high, and fatigue failure may occur starting from the threaded portion.

[0013] The present invention has been made in view of the above circumstances, and has as its object to relieve stress applied to threaded portions in high-pressure gas containers having metal containers, thereby preventing fatigue fracture. [Means for solving the problem]

[0014] The present inventors have conducted studies to solve the above problems and have made the following findings.

[0015] (1) When a high-pressure gas container is filled with gas, internal pressure is applied to the metal container, and tensile stress is generated in the metal container due to the internal pressure. The tensile stress is concentrated in the threaded portion, particularly near the bottom of the thread.

[0016] (2) Therefore, by preliminarily applying a predetermined residual compressive stress near the thread root, at least a portion of the tensile stress applied to the threads when the metal container is filled with gas is offset by the residual compressive stress, and the actual stress applied to the threads can be significantly alleviated, thereby suppressing fatigue fracture originating from the threads.

[0017] (3) When a high-pressure gas container is subjected to an internal pressure higher than the normal operating pressure of the container, excessive stress is generated, resulting in localized plastic deformation at the thread root. Since plastic deformation occurs only in a limited area, and most of the remaining area is in the elastic range, compressive stress remains at the thread root after the internal pressure is released. Therefore, during the manufacturing process of a high-pressure gas container, a predetermined residual compressive stress can be imparted to the thread by applying a high internal pressure to the container under predetermined conditions.

[0018] The present invention has been made based on the above findings, and has the following gist.

[0019] 1. A high-pressure gas container with a metal container, The metal container includes a metal cylinder having a female thread portion on the inner peripheral surface of at least one end, and a lid having a male thread portion on the outer peripheral surface that screws into the female thread portion, A high-pressure gas container in which the maximum value of residual compressive stress at a position 0.4 mm in a depth direction from the multiple thread roots of the female thread portion and the male thread portion is 100 MPa or more and is not more than the tensile strength of the material of the metal cylinder and not more than the tensile strength of the material of the lid.

[0020] 2. A high-pressure gas container as described in 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 metal cylinder and equal to or less than the yield stress of the material of the lid.

[0021] 3. The male thread of the lid is screwed into the female thread of the metal cylinder, 3. A high-pressure gas container according to claim 1 or 2, wherein the maximum value of residual compressive stress at a position 0.4 mm in a depth direction from the thread roots in the multiple thread roots of the female thread portion and the male thread portion exceeds 0.

[0022] 4. A method for manufacturing a high-pressure gas container having a metal container, the metal container comprising a metal cylinder having a female thread portion on the inner peripheral surface of at least one end, and a lid having a male thread portion on the outer peripheral surface that screws into the female thread portion, A method for manufacturing a high-pressure gas container, comprising an internal pressure applying step of applying internal pressure to the metal container under conditions that satisfy at least one of the following (A) and (B) and also satisfy both of the following (C) and (D): (A) The thread root stress of the female thread portion is greater than the yield stress of the material of the metal cylinder. (B) The thread root stress of the male thread portion is greater than the yield stress of the material of the lid. (C) The axial stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. (D) The circumferential stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder.

[0023] 5. A method for manufacturing a high-pressure gas container having a metal container, the metal container comprising a metal cylinder having a female thread portion on the inner peripheral surface of at least one end, and a lid having a male thread portion on the outer peripheral surface that screws into the female thread portion, a jig mounting process for mounting a jig having a male threaded portion on an outer circumferential surface of the metal cylinder so that the male threaded portion of the jig is threadedly engaged with the female threaded portion of the metal cylinder; an internal pressure applying step of applying internal pressure to the metallic container under conditions that satisfy the following (A), (C), and (D); a jig removal step of removing the jig from the metal container; a lid fastening step of fastening the lid to the metal cylinder so that the male thread portion of the lid is threadedly engaged with the female thread portion of the metal cylinder, In the lid tightening step, the tightening torque is adjusted so that the maximum value of residual compressive stress at a position 0.4 mm in a depth direction from the multiple thread roots of the female thread portion and the male thread portion exceeds 0. (A) The thread root stress of the female thread portion is greater than the yield stress of the material of the metal cylinder. (C) The axial stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. (D) The circumferential stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. [Effects of the Invention]

[0024] According to the present invention, the stress applied to the threaded portion of a high-pressure gas container can be alleviated, and fatigue failure can be prevented. Therefore, the high-pressure gas container of the present invention can be used at higher pressures and with larger cross sections than conventional containers. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 2 is a schematic diagram showing a position that defines a compressive residual stress in the present invention. [Figure 2] 1 is a cross-sectional view showing a structure of a high-pressure gas container according to one embodiment of the present invention. [Figure 3] FIG. 4 is a cross-sectional view showing the structure of a high-pressure gas container according to another embodiment of the present invention. [Figure 4] FIG. 4 is a cross-sectional view showing the structure of a high-pressure gas container according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] Next, a method for carrying out the present invention will be specifically described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description in any way.

[0027] [High-pressure gas container] In one embodiment of the present invention, the high-pressure gas container is a high-pressure gas container comprising a metal container, the metal container comprising a metal cylinder having a female thread portion on the inner peripheral surface of at least one end, and a lid having a male thread portion on its outer peripheral surface that screws into the female thread portion.

[0028] The lids are preferably provided on both ends of the metal container. That is, the metal container preferably includes female threads provided on the inner peripheral surfaces of both ends of the metal cylinder, and lids having male threads on the outer peripheral surface that screw into the female threads.

[0029] The high-pressure gas container can be used as a high-pressure hydrogen gas container, for example, a container for a hydrogen station, a mobile hydrogen station, or a container to be mounted on a vehicle, but is not limited thereto and can be used for any purpose.

[0030] In the present invention, it is important that the maximum value of the residual compressive stress at a position 0.4 mm in depth from the multiple thread roots of the female thread portion and the male thread portion is 100 MPa or more, and is equal to or less than the tensile strength of the material of the metal cylinder and the tensile strength of the material of the lid. The reasons for this are explained below.

[0031] FIG. 1 is a schematic diagram showing the position at which compressive residual stress is determined in the present invention. A female threaded portion 11 is provided on the inner peripheral surface of at least one end of a metal cylinder 10, and multiple thread grooves 12 are provided in the female threaded portion. The bottoms of the thread grooves 12 are referred to as the thread roots 13 of the female threaded portion. A male threaded portion 21 is provided on the outer peripheral surface of a lid 20, and the male threaded portion 21 is threadedly engaged with the thread grooves 12 of the female threaded portion 11. The male threaded portion 21 has multiple threads 22. The bottoms of the spaces between adjacent threads 22, when considered as thread grooves, are referred to as the thread roots 23 of the male threaded portion. In the present invention, the maximum residual compressive stress at positions P, 0.4 mm in depth from the thread roots 13 of the female threaded portion and the thread roots 23 of the male threaded portion, are controlled to satisfy the above-mentioned conditions. Note that FIG. 1 is a schematic diagram for illustrative purposes only and does not represent the actual shape and dimensions of the threaded portion.

[0032] As described above, when a high-pressure gas container is used, internal pressure is applied to the metal container when the container is filled with gas, generating tensile stress. This tensile stress is concentrated in the threads, particularly near the thread root. Therefore, by preliminarily applying residual compressive stress near the thread root, at least a portion of the tensile stress applied to the threads when the metal container is filled with gas is offset by the residual compressive stress, thereby significantly reducing the stress actually applied to the threads. As a result, fatigue fracture originating from the threads can be suppressed.

[0033] However, the tensile stress generated during use of a high-pressure gas container spreads not only on the surface of the thread but also inside the material. Therefore, the above-mentioned effect cannot be achieved even if residual compressive stress exists only in the surface layer of the thread. Therefore, in the present invention, the maximum value of residual compressive stress at a position 0.4 mm deep from the thread root is set to 100 MPa or more.

[0034] On the other hand, if the maximum value of the residual compressive stress is excessively high, the components constituting the metallic container will buckle and the metallic container will be deformed. Therefore, in order to prevent deformation of the metallic container, the maximum value of the residual compressive stress is set to be equal to or less than the tensile strength of the material of the metallic cylinder and the tensile strength of the material of the lid.

[0035] From the viewpoint of more reliably preventing buckling of the threaded portion, it is preferable that the maximum value of the residual compressive stress is equal to or less than the yield stress of the material of the metal cylinder and equal to or less than the yield stress of the material of the lid.

[0036] Here, the maximum value of the residual compressive stress is defined as the value when no internal pressure is applied to the metallic container. The maximum value of the residual compressive stress can be determined by elastic-plastic analysis using the finite element method (FEM). However, the maximum value of the residual compressive stress can also be determined by measuring the residual stress in the cross sections of the male and female threads using X-ray stress measurement.

[0037] [Metal cylinder] The material of the metal cylinder is not particularly limited and any metal can be used. From the viewpoint of cost reduction, it is preferable to use low-alloy steel as the material. As the low-alloy steel, it is particularly preferable 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 SMn steel), ASEM SA-723, and boron-added steels N28CB, N36CB, and N46CB. Among them, from the viewpoint of compatibility with material strength, it is more preferable to use chromium-molybdenum steel, SA723 steel, or chromium-molybdenum-nickel steel, which are easy to ensure hardenability. For example, chromium-molybdenum steel (SCM435) contains 0.33 to 0.38 mass% C, 0.15 to 0.35 mass% Si, 0.60 to 0.90 mass% Mn, 0.030 mass% or less P, 0.030 mass% or less S, 0.90 to 1.20 mass% Cr, and 0.15 to 0.30 mass% Mo.

[0038] The metal cylinder may be manufactured by any method. For example, it may be a container made by hollowing out the inside of a steel material, or it may be a processed steel pipe. The steel pipe may be any type, such as an electric resistance welded steel pipe or a seamless steel pipe, but it is preferable to use a metal cylinder made of a seamless steel pipe. A metal cylinder made of a seamless steel pipe has superior properties such as toughness compared to a metal cylinder manufactured by hollowing out, and has no welded joints, making it extremely suitable as a container for a high-pressure gas.

[0039] As described above, the inner peripheral surface of at least one end of the metal cylinder is provided with a female thread for screwing a lid. In other words, the metal cylinder has an opening at at least one end for attaching a lid, and the inner peripheral surface of the opening is provided with a female thread. It is preferable that the female thread is provided on both ends of the metal cylinder.

[0040] The size of the opening is not particularly limited, but if the opening is too small, it becomes difficult to impart sufficient compressive residual stress to the threaded portion using the method described below. Therefore, the ratio of the inner diameter Di of the metal cylinder to the inner diameter Ds of the female threaded portion of the metal cylinder, Di / Ds, is preferably 3.5 or less, more preferably 2.0 or less, and even more preferably 1.5 or less. Here, the inner diameter Ds of the female threaded portion is defined as the distance between the thread roots at opposing positions of the female threaded portion formed on the inner surface of the metal cylinder. Furthermore, the inner diameter Di of the metal cylinder refers to the inner diameter of the metal cylinder in the portion where the female thread is not formed, i.e., the inner diameter of the gas storage portion.

[0041] On the other hand, if Di / Ds is less than 0.8, the threads are too deep relative to the wall thickness of the metal cylinder, increasing the stress on the thread root. Therefore, from the viewpoint of further reducing the stress on the thread root, Di / Ds is preferably 0.8 or more, and more preferably 0.9 or more.

[0042] [Carbon fiber reinforced resin layer] A carbon fiber reinforced resin layer can be provided on the surface of the metal container, which can further improve the pressure resistance and fatigue properties of the container.

[0043] Carbon fiber reinforced resin is a composite material in which carbon fibers as a reinforcing material are impregnated with resin to improve strength, and is called CFRP (carbon-fiber-reinforced plastic). The carbon fibers are not particularly limited, and any type, such as PAN-based or pitch-based, can be used. The volume content of carbon fibers in the carbon fiber reinforced resin layer can be determined in accordance with Japanese Industrial Standard JIS K 7075 (1991), and is preferably in the range of 50% to 80%.

[0044] The carbon fiber reinforced resin layer can cover the entire outer surface of the metal container or a part of it. For example, the high-pressure gas container may have a structure in which the entire outer surface of the metal container is covered with the carbon fiber reinforced resin layer (full wrap structure). However, from the viewpoint of cost reduction, it is preferable that the carbon fiber reinforced resin layer has a hoop-wrap structure, that is, a structure in which carbon fiber is wrapped only in the circumferential direction of the metal container.

[0045] When a carbon fiber reinforced resin layer is provided on the outer surface of a metal container, it is preferable to treat the outer surface of the metal container to prevent electrolytic corrosion, such as by powder coating or wrapping with glass fiber reinforced plastic (GFRP). This prevents the metal container from rusting due to potential corrosion even if cracks or the like occur in the carbon fiber reinforced resin layer, which is the surface layer, and moisture accumulates at the interface between the liner layer and the carbon fiber reinforced resin layer. For the powder coating, thermoplastic powder paints based on vinyl chloride resins or the like, or thermosetting powder paints based on polyester resins, acrylic resins, epoxy resins, or the like can be used. Considering the heat generated when filling with gases such as hydrogen, it is preferable to use thermosetting powder paints.

[0046] [lid] The lid for the metal container is not particularly limited, and any lid can be used as long as it has a male thread on its outer surface that screws into the female thread of the metal cylinder. In other words, the high-pressure gas container of the present invention is equipped with a screw-type lid that can be attached to at least one side of the metal cylinder. The lid may be of an integral structure or may be of a structure consisting of multiple parts.

[0047] The lid preferably has a sealing member on its outer circumferential surface for sealing the high-pressure gas inside the container. The sealing member is not particularly limited, and any sealing member can be used. An O-ring is preferably used as the sealing member. When the lid has an O-ring, the O-ring is preferably provided at a position inside the container relative to the male thread portion.

[0048] The material of the lid is not particularly limited, but is preferably metal, and more preferably steel. It is even more preferable to use a steel (low alloy steel) having a tensile strength (TS) of 750 MPa or more as the steel. The same materials as those listed as the materials for the metal cylinder can also be used as the material for the lid. The materials for the lid and the metal cylinder may be the same or different, but are preferably the same. Furthermore, when the lid is made up of multiple components, as in the examples shown in Figures 3 and 4, the materials for each component can be selected independently and may be the same or different.

[0049] The shape of the male thread of the lid is not particularly limited and may be, for example, a thread specified by JIS, but it is preferable to use a shape that further reduces stress concentration, such as a screw with a large radius of curvature at the bottom tip of the screw (such as a trapezoidal screw with a rounded tip).

[0050] Below, examples of the lid structure will be described using three embodiments. Note that in the following description, a schematic diagram showing the structure in which the lid is attached to the metal cylinder is referred to, but the high-pressure gas container of the present invention is not limited to a state in which the lid is attached to the metal cylinder.

[0051] 2 is a schematic diagram showing the structure of a high-pressure gas container 1 according to one embodiment of the present invention, and shows a cross section taken along a plane passing through the central axis of the high-pressure gas container 1. The high-pressure gas container 1 comprises a metal cylinder 10, the internal space of which forms a storage section 14 for storing high-pressure gas. Furthermore, female threads 11 are provided on the inner peripheral surfaces of both ends of the metal cylinder 10.

[0052] Solid cylindrical lids 20 are provided on both ends of the metal cylinder 10. The outer circumferential surface of the lid 20 is provided with a male thread 21 that screws into the female thread of the metal cylinder 10. Such an integrally structured lid has the advantage of being easy to manufacture.

[0053] The lid 20 is provided with an O-ring 24 as a sealing member on its outer circumferential surface. The male thread portion 21 is provided on the outer side of the container than the O-ring 24 (on the side opposite to the storage portion 14).

[0054] 3 is a cross-sectional view showing the structure of a high-pressure gas container 1 according to another embodiment of the present invention. Note that unless otherwise specified, the same applies to the embodiment shown in FIG.

[0055] The lid 20 in this embodiment comprises a head plate 25 and a screw nut 26. The head plate 25 is a generally plate-shaped (disk-shaped) member for sealing the gas inside the container. Instead of having threads on its peripheral surface, it has an O-ring 24 as a sealing member. Of the two opposing main surfaces of the head plate 25, one main surface contacts the storage section 14, and the other main surface contacts the screw nut 26. The screw nut 26 is a solid cylindrical member, and its outer surface is provided with a male thread 21 that screws into the female thread of the metal cylinder 10. By attaching the head plate 25 and the screw nut 26 as shown in FIG. 3, the head plate 25 can be supported by the screw nut 26. Such a separate lid structure has the advantage that if the male thread of the screw nut is damaged, only the screw nut needs to be replaced, and the head plate can be used as is.

[0056] 4 is a cross-sectional view showing the structure of a high-pressure gas container 1 according to another embodiment of the present invention. Note that unless otherwise specified, the same applies to the embodiment shown in FIG.

[0057] The lid 20 in this embodiment comprises a head plate 25 and a screw nut 26. The head plate 25 is a substantially plate-shaped (disc-shaped) member for sealing the gas inside the container, and instead of being provided with threads on its peripheral surface, it is provided with an O-ring 24 as a sealing member. Of the two opposing main surfaces of the head plate 25, one main surface is in contact with the storage section 14, and the other main surface is in contact with the screw nut 26. The screw nut 26 is a hollow cylindrical member, and its outer circumferential surface is provided with a male thread portion 21 that screws into the female thread portion of the metal cylinder 10. By attaching the head plate 25 and the screw nut 26 as shown in FIG. 4, the head plate 25 can be supported by the screw nut 26.

[0058] 4, if the lid has such a separate structure, if the male thread of the screw nut is damaged, only the screw nut needs to be replaced, and the head plate can be used as is. Furthermore, in this embodiment, since the screw nut 26 is hollow cylindrical, the weight of the lid can be significantly reduced, thereby making the high-pressure gas container lighter. Furthermore, since the screw nut 26 is more easily stressed at the thread root of the male thread than when it is a solid cylindrical member, residual compressive stress can be imparted to the thread root of the male thread without applying excessive stress to the metal cylinder.

[0059] The hollow cylindrical screw nut 26 can be manufactured by any method. For example, the hollow cylindrical shape can be formed by hollowing out the inside of a steel material, or a steel pipe can be used. Any steel pipe, such as an electric resistance welded steel pipe or a seamless steel pipe, can be used as the steel pipe, but a seamless steel pipe is preferred. In particular, if both the metal cylinder 10 and the screw nut 26 are made of seamless steel pipe, part of the manufacturing process for the metal cylinder and the screw nut can be shared, which is extremely preferable from the standpoint of productivity.

[0060] In addition, when the lid is made up of multiple components as described above, the "tensile strength of the component that is provided with the male thread among the components that make up the lid" is defined as the "tensile strength of the lid material." Similarly, when the lid is made up of multiple components, the "yield stress of the component that is provided with the male thread among the components that make up the lid" is defined as the "yield stress of the lid material." For example, in the case of the lid shown in Figure 3 or Figure 4, the tensile strength and yield stress of the material of the screw nut 26 are defined as the tensile strength and yield stress of the lid material.

[0061] [Tightening the lid] As described above, by preliminarily applying a predetermined residual compressive stress near the thread root, it is possible to alleviate the stress applied to the threaded portion when gas is filled into the metal container. However, when the high-pressure gas container of the present invention is actually used, the stress reduction effect of the residual compressive stress changes depending on the force with which the lid is fastened.

[0062] That is, when a lid is tightened onto a high-pressure gas container, tightening is usually performed with a constant torque. However, tightening the lid places stress on the threads, and if this stress exceeds the pre-applied residual compressive stress, the residual compressive stress is canceled out. Therefore, when tightening the lid onto the high-pressure gas container of the present invention so that the male thread of the lid is threadedly engaged with the female thread of the metal cylinder, it is preferable to adjust the tightening torque so that the pre-applied compressive residual stress remains. Specifically, it is preferable that the maximum value of the residual compressive stress at a position 0.4 mm deep from the thread root in the multiple thread roots of the female thread and male thread exceeds 0.

[0063] Therefore, in one embodiment of the present invention, the high-pressure gas container is in a state in which the male threaded portion of the lid is screwed into the female threaded portion of the metal cylinder, and the maximum value of the residual compressive stress at a position 0.4 mm deep from the thread root at multiple thread roots of the female threaded portion and the male threaded portion exceeds 0.

[0064] In other words, the high-pressure gas container assembly according to one embodiment of the present invention is a high-pressure gas container assembly including a metal container, The metal container includes a metal cylinder having a female thread portion on the inner peripheral surface of at least one end, and a lid having a male thread portion on the outer peripheral surface that screws into the female thread portion, The male thread portion of the lid is screwed into the female thread portion of the metal cylinder, The maximum value of residual compressive stress at a position 0.4 mm in depth from the multiple thread roots of the female thread portion and the male thread portion is greater than 0 and is equal to or less than the tensile strength of the material of the metal cylinder and equal to or less than the tensile strength of the material of the lid.

[0065] In order to retain the compressive residual stress that has been applied beforehand, the smaller the tightening torque, the better. Therefore, the lower limit of the tightening torque can be 0. However, if the tightening torque is too small, the lid may become loose due to vibrations during transportation of the high-pressure gas container. For this reason, it is preferable to tighten the lid with a torque of 10 N·m or more, and more preferably with a torque of 100 N·m or more. Furthermore, to prevent the lid from loosening and coming off, it is also preferable to attach a jig to the high-pressure gas container to prevent the lid from falling off.

[0066] [Manufacturing method] Next, a method for manufacturing a high-pressure gas container according to one embodiment of the present invention will be described. As described above, when a high-pressure gas container is subjected to an internal pressure higher than the normal operating pressure of the high-pressure gas container, excessive stress is generated, causing localized plastic deformation at the thread root. Since plastic deformation occurs only in a limited area, and most of the other areas are in the elastic range, compressive stress remains at the thread root after the internal pressure is released. Therefore, during the manufacturing process of a high-pressure gas container, a predetermined residual compressive stress can be imparted to the thread portion by applying a high internal pressure to the high-pressure gas container under predetermined conditions.

[0067] (First embodiment) Therefore, a method for manufacturing a metal container in one embodiment of the present invention includes an internal pressure applying step of applying internal pressure to the metal container under conditions that satisfy at least one of the following (A) and (B) and also satisfy both the following (C) and (D). This manufacturing method makes it possible to manufacture a high-pressure gas container having the above-mentioned predetermined residual compressive stress. (A) The thread root stress of the female thread portion is greater than the yield stress of the material of the metal cylinder. (B) The thread root stress of the male thread portion is greater than the yield stress of the material of the lid. (C) The axial stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. (D) The circumferential stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder.

[0068] The reasons for limiting the above conditions will be explained below.

[0069] Conditions (A) and (B) In order to impart residual compressive stress near the thread root, it is necessary to induce plastic deformation at the thread root. To induce plastic deformation at the thread root, it is sufficient to impart a thread root stress exceeding the yield stress of the material. Therefore, in the present invention, in the internal pressure application step, internal pressure is applied under conditions that satisfy at least one of the above (A) and (B). When condition (A) is satisfied, residual compressive stress can be imparted to the female thread portion of the metal cylinder. Furthermore, when condition (B) is satisfied, residual compressive stress can be imparted to the male thread portion of the lid.

[0070] In order to impart residual compressive stress more effectively, it is preferable that the thread root stress imparted in the internal pressure imparting step is equal to or greater than the tensile strength of the material. In other words, it is preferable that the internal pressure be imparted in the internal pressure imparting step under conditions that satisfy at least one of the following (A') and (B'): (A') The thread root stress of the female thread portion is greater than the tensile strength of the material of the metal cylinder. (B') The thread root stress of the male thread portion is greater than the tensile strength of the material of the lid.

[0071] In the internal pressure application step, there are no particular limitations on the upper limits of the thread root stress of the female thread portion and the thread root stress of the male thread portion, and they may be adjusted so as to impart a desired residual compressive stress. From the viewpoint of effectively imparting residual compressive stress, it is preferable that the stress on the outer surface (the outer peripheral surface of the cylinder) facing the female thread portion of the metal cylinder in the internal pressure application step be equal to or less than the yield stress of the material of the metal cylinder. By making the thread root stress greater than the yield stress and at the same time making the stress on the outer surface of the metal cylinder equal to or less than the yield stress, residual compressive stress can be imparted more effectively near the thread root than in the case where a stress higher than the yield stress is applied throughout the entire thickness direction of the metal cylinder.

[0072] For the same reason, when using a lid consisting of a head plate and a solid cylindrical screw nut as shown in Figure 4, it is preferable that in the internal pressure application process, the stress on the inner surface (inner surface of the cylindrical nut) facing the male thread portion of the screw nut be less than the yield stress of the material of the screw nut.

[0073] ·Condition (B), (C) As mentioned above, to impart residual compressive stress, it is necessary to apply internal pressure to cause plastic deformation, but if the internal pressure is too high, the metal cylinder will be destroyed. To prevent the metal cylinder from being destroyed, both the axial stress and circumferential stress applied to the metal cylinder must be equal to or less than the tensile strength of the material of the metal cylinder. (B) and (C) above specifically define the above conditions.

[0074] The axial stress and circumferential stress acting on a metal cylinder can be calculated using the following equations (1) and (2), respectively. Axial stress = (pressure-receiving area of ​​the lid x internal pressure) / minimum cross-sectional area of ​​the metal cylinder...(1) Circumferential stress = (inner diameter of metal cylinder × internal pressure) / (2 × thickness of metal cylinder) ... (2) Here, the "pressure-receiving area of ​​the lid" refers to the area of ​​the surface of the lid that contacts the storage section. When a head plate is used, as shown in Figures 3 and 4, this refers to the area of ​​the head plate. The "cross-sectional area" of the metal cylinder refers to the cross-sectional area of ​​the metal portion in a cross section perpendicular to the axial direction of the metal cylinder, and does not include the cross-sectional area of ​​the internal space of the metal cylinder. The cross-sectional area of ​​the metal cylinder may vary depending on the longitudinal position of the metal cylinder, and the axial stress acting on the metal cylinder is greatest at the smallest cross-sectional area. Therefore, the minimum cross-sectional area of ​​the metal cylinder is used in the above equation (1). The axial stress and circumferential stress obtained by equations (1) and (2) are average stresses per cross-sectional area of ​​the metal cylinder. Meanwhile, the thread root stress, which is a localized stress, can be calculated using the finite element method, as described below.

[0075] However, when the outer surface of the metal container has a carbon fiber reinforced resin layer, the axial stress and circumferential stress acting on the cylindrical metal part will be lower than the values ​​calculated by the above formulas (1) and (2). Therefore, when the outer surface of the metal container has a carbon fiber reinforced resin layer, the axial stress and circumferential stress acting on the cylindrical metal part will be evaluated by numerical analysis using the finite element method.

[0076] In other words, when a carbon fiber reinforced resin layer is provided on the outer surface of a metal container, a higher internal pressure can be applied compared to when no carbon fiber reinforced resin layer is provided, and therefore residual compressive stress can be applied more effectively.

[0077] If at least one of the axial stress and circumferential stress applied to the metallic cylindrical portion exceeds the yield stress of the material of the metallic cylinder, the metallic cylinder may undergo plastic deformation, resulting in a change in the inner diameter of the metallic cylinder. Therefore, from the viewpoint of suppressing deterioration of sealing performance due to plastic deformation of the metallic cylinder, it is preferable that the axial stress and circumferential stress applied to the metallic cylindrical portion be equal to or less than the yield stress of the material of the metallic cylinder, and more preferably equal to or less than 90% of the yield stress. In other words, in the internal pressure application step, it is preferable to apply internal pressure under conditions that satisfy both of the following (C') and (D'), and it is more preferable to apply internal pressure under conditions that satisfy both of the following (C'') and (D''). (C') The axial stress of the metal cylinder is equal to or less than the yield stress of the material of the metal cylinder. (D') The circumferential stress of the metal cylinder is equal to or less than the yield stress of the material of the metal cylinder. (C'') The axial stress of the metal cylinder is 90% or less of the yield stress of the material of the metal cylinder. (D'') The circumferential stress of the metal cylinder is 90% or less of the yield stress of the material of the metal cylinder.

[0078] In order to apply internal pressure to the metallic container in the internal pressure application step, the metallic container may be filled with any pressure medium. While any medium can be used as the pressure medium without any particular limitations, from the viewpoint of safety, it is preferable to use an incompressible fluid such as water or oil. Furthermore, from the viewpoint of preventing corrosion of the metallic container, it is preferable to use an incompressible fluid containing a corrosion inhibitor or an aqueous solution of an alcohol such as ethylene glycol.

[0079] (Second embodiment) In addition, a method for manufacturing a metal container according to another embodiment of the present invention includes the steps of: a jig mounting step of mounting a jig having a male threaded portion on an outer peripheral surface of the metal cylinder so that the male threaded portion of the jig is threadedly engaged with the female threaded portion of the metal cylinder; and an internal pressure applying step of applying internal pressure to the metal container under conditions that satisfy the following (A), (C), and (D): a jig removal step of removing the jig from the metal container; a lid fastening step of fastening the lid to the metal container so that the male thread portion of the lid is threadedly engaged with the female thread portion of the metal cylinder, In the lid tightening step, the tightening torque is adjusted so that the maximum value of residual compressive stress at a position 0.4 mm deep from the multiple thread roots of the female thread portion and the male thread portion exceeds 0. (A) The thread root stress of the female thread portion is greater than the yield stress of the material of the metal cylinder. (C) The axial stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. (D) The circumferential stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder.

[0080] The above-described manufacturing method also makes it possible to manufacture a high-pressure gas container having the predetermined residual compressive stress. Each step will now be described.

[0081] [Jig installation process] First, a jig having a male thread on its outer circumferential surface is attached to a metal cylinder so that the male thread of the jig is threadedly engaged with the female thread of the metal cylinder (jig attachment process). Any jig can be used as the jig as long as it has a male thread that is threadedly engaged with the female thread of the metal cylinder. It is preferable to use a jig that is the same as a member used as a lid for a metal container. It is also more preferable to reuse the lid used as the jig as a lid in the lid fastening process described below.

[0082] The material of the jig is not particularly limited, but is preferably made of metal, and more preferably made of steel. It is even more preferable to use a steel (low alloy steel) having a tensile strength (TS) of 750 MPa or more. The same materials as those listed as the materials for the metal cylinder can also be used for the jig material. The jig material and the metal cylinder material may be the same or different, but are preferably the same.

[0083] [Internal pressure application process] After the jig is attached to the metal cylinder, internal pressure is applied to the metal container under the conditions (A), (C), and (D) below (internal pressure application step). The reasons for limiting the conditions (A), (C), and (D) are as described in the explanation of the first embodiment above. In addition, other points can be the same as those of the first embodiment above unless otherwise specified. (A) The thread root stress of the female thread portion is greater than the yield stress of the material of the metal cylinder. (C) The axial stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. (D) The circumferential stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder.

[0084] [Jig removal process] After the internal pressure is applied in the internal pressure application step, the jig is removed from the metal container (jig removal step).

[0085] [Lid tightening process] Then, the lid is fastened to the metal cylinder so that the male thread portion of the lid is threadedly engaged with the female thread portion of the metal cylinder (lid fastening step). In the lid fastening step, the fastening torque is adjusted so that the maximum value of the residual compressive stress at a position 0.4 mm in depth from the multiple thread roots of the female thread portion and the male thread portion exceeds 0.

[0086] As mentioned above, it is preferable to use a lid as a jig in the jig attachment step and reuse the lid used in the jig attachment step in the lid fastening step. However, if the lid is equipped with a sealing member such as an O-ring, the high internal pressure applied in the internal pressure application step may damage the sealing member and impair the sealing performance. Therefore, it is preferable to replace the sealing member of the lid after removing the jig in the jig removal step and then fasten the lid. Furthermore, considering that high internal pressure is applied in the internal pressure application step, it is preferable that the diameter of the sealing member used in the internal pressure application step be larger than the diameter of the sealing member used in the final lid fastening step. [Example]

[0087] The effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0088] Example 1 The thread root stress was analyzed by elastic-plastic analysis using the finite element method (FEM) with a container model. Two types of container models were used: a Type 1 container consisting of a metal container made of low-alloy steel and without a carbon fiber reinforced resin layer, and a Type 2 container consisting of a metal container (liner) made of the same low-alloy steel as the Type 1 container and a carbon fiber reinforced resin layer formed by wrapping CFRP to a thickness of 5 mm on the surface of the metal container. The metal cylinder and lid constituting the metal container were made of the same low-alloy steel, and the low-alloy steel had a tensile strength (TS) of 821 MPa and a yield stress (YP) of 705 MPa. The stress-strain curve of the low-alloy steel was that of SNCM439 steel with a TS of 900 MPa.

[0089] The dimensions of the metal cylinder that constituted the metal container were fixed at 4,500 mm in length and 404 mm in outer diameter, with the inner diameter and thickness shown in Table 1. The lid, as shown in Figure 4, consisted of a disk-shaped head plate and a hollow cylindrical screw nut, with the head plate having a thickness of 75 mm and the screw nut having a thickness of 37 mm. The screw nut thickness was measured from the apex of the thread on the outer surface to the inner surface. The thread profile was a JIS trapezoidal screw with a pitch of 12 mm, a thread depth of 12 mm, and a shoulder radius of 2.2 mm.

[0090] The above FEM analysis was carried out under the following conditions. Software: ABAQUS Ver.6.12-4 (Dassault Systemes) Calculation model: axisymmetric model Mesh division: 50 μm in stress concentration areas Boundary condition: Gas pressure is applied to the inner surface of the metal cylinder and the gas storage side of the head plate Constraint conditions: Metal cylinder: Nodes on the Y-symmetric plane, Y-direction displacement constraint Head plate, screw nut: no active fixation of nodal displacement Contact conditions: Contact friction coefficient μ=0.05

[0091] (Internal pressure application process) The thread root stress when the loaded internal pressure shown in Table 1 was applied to the above-mentioned metallic container was determined by FEM. The maximum values ​​of the thread root stress in the female thread portion and the maximum values ​​of the thread root stress in the male thread portion were as shown in Table 1. The maximum value of the thread root stress refers to the maximum value of the stress in the entire thickness direction from the thread root to the surface on the opposite side of the thread root. In this example, the maximum value of the thread root stress in the male thread portion during the internal pressure loading process was lower than the maximum value of the thread root stress in the female thread portion in all examples shown in Table 1. Therefore, only the maximum values ​​of the thread root stress in the female thread portion are shown in Table 1.

[0092] The axial stress and circumferential stress of the metal cylinder when the above-mentioned internal pressure load was applied are also shown in Table 1. When the metal container is a Type 1 container, the axial stress and circumferential stress were calculated using the following formulas (1) and (2). Axial stress = (pressure-receiving area of ​​the lid x internal pressure) / minimum cross-sectional area of ​​the metal cylinder...(1) Circumferential stress = (inner diameter of metal cylinder × internal pressure) / (2 × thickness of metal cylinder) ... (2) In addition, when the metallic container is a Type 2 container, the axial stress and circumferential stress were determined by FEM analysis.

[0093] (residual compressive stress) Next, the residual compressive stress was determined by FEM analysis at a position 0.4 mm in the depth direction from the thread root when the internal pressure was released. Table 2 shows the maximum residual compressive stress in the male thread portion, the maximum compressive residual stress in the female thread portion, and their respective maximum values.

[0094] Next, in order to evaluate the performance of each metal container, the thread root stress and thread fracture life when internal pressure was applied again were determined.

[0095] (Thread root stress under internal pressure load) Assuming the conditions under which a metal container will actually be used as a high-pressure gas container, the maximum value of the thread root stress when an internal pressure of 82 MPa is applied to the metal container was determined by FEM analysis. The results are shown in Table 2.

[0096] (Thread breakage life) The fracture life of the threaded portion in a pressure cycle test was evaluated from the stress obtained through FEM analysis. The fracture life evaluation was carried out in accordance with the "Crack Growth Analysis Method for Various Parts" (KHKS 0220 (2010) Annex IX) established by the High Pressure Gas Safety Institute of Japan. The pressure application conditions were minimum pressure: 2 MPa, maximum pressure: 82 MPa, and temperature: room temperature.

[0097] As can be seen from the results shown in Table 2, compressive residual stress that satisfies the requirements of the present invention can be introduced into the thread root by applying internal pressure under appropriate conditions. Furthermore, metallic containers whose maximum value of residual compressive stress at a depth of 0.4 mm from the thread root satisfies the requirements of the present invention have reduced thread root stress when internal pressure is applied, and as a result, exhibit excellent fatigue life.

[0098] [Table 1]

[0099] [Table 2]

[0100] Example 2 Next, the influence of the conditions for fastening the lid to the metal cylinder on the fatigue life was evaluated by FEM analysis. Specifically, for each of containers No. 6, 11, and 13 in Example 1, the lid was first removed after the internal pressure application step, and then the lid was fastened again under the conditions shown in Table 3. Next, the thread root stress and thread fracture life when internal pressure was applied again were determined using the same procedures as in Example 1. The results are shown in Table 3.

[0101] The results shown in Table 3 show that the thread fracture life can be further improved by tightening the lid so that the maximum value of the residual compressive stress at a position 0.4 mm deep from the thread root exceeds 0.

[0102] [Table 3] [Explanation of symbols]

[0103] 1. High-pressure gas containers 10 Metal Cylinder 11 Female thread 12 Thread groove 13 Thread bottom of female thread 14 Storage Unit 20 Lid 21 Male thread 22 threads 23 Thread bottom of male thread 24 O-rings 25 Headboard 26 Threaded nut P 0.4mm deep from the bottom of the screw

Claims

[Claim 1] A method for manufacturing a high-pressure gas container having a metal container, the metal container comprising: a metal cylinder having a female thread portion on an inner peripheral surface of at least one end; and a lid having a male thread portion on an outer peripheral surface thereof that screws into the female thread portion, The method includes an internal pressure applying step of applying internal pressure to the metallic container under conditions that satisfy at least one of the following (A) and (B) and also satisfy both of the following (C) and (D): In the internal pressure applying step, a pressure medium is filled inside the metallic container to apply internal pressure to the metallic container. (A) The thread root stress of the female thread portion is greater than the yield stress of the material of the metal cylinder. (B) The thread root stress of the male thread portion is greater than the yield stress of the material of the lid. (C) The axial stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder. (D) The circumferential stress of the metal cylinder is equal to or less than the tensile strength of the material of the metal cylinder.

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