Steel pipe for hydrogen gas accumulator, hydrogen gas accumulator, and method for manufacturing steel pipe for hydrogen gas accumulator

Abstract

The present invention provides a steel pipe for a hydrogen gas accumulator which can be machined with high quality when subjected to machining and can be suitably used as a material of a long hydrogen gas accumulator. The steel pipe for a hydrogen gas accumulator has a total length in the longitudinal direction of 5000-8000 mm, and a bending amount of 1.20 mm or less per 1000 mm in the longitudinal direction.

Description

Steel pipe for hydrogen gas pressure vessel, hydrogen gas pressure vessel, and method of manufacturing steel pipe for hydrogen gas pressure vessel

[0001] The present invention relates to a steel pipe for a hydrogen gas pressure vessel, a hydrogen gas pressure vessel, and a method for manufacturing a steel pipe for a hydrogen gas pressure vessel.

[0002] In recent years, in order to realize a low-carbon society, attention has been focused on hydrogen, a clean energy source, and fuel cells that use it, and it is expected that fuel cells will be installed and used in automobiles, buses, trucks, and industrial transport machinery. Hydrogen stations that supply hydrogen to these vehicles and machinery are equipped with pressure vessels called hydrogen gas accumulators (hereinafter sometimes simply referred to as accumulators), which store hydrogen gas at high pressure.

[0003] As fuel cell vehicles have become more widespread, the construction of hydrogen stations has been progressing, primarily in urban areas. However, due to the limited amount of land available in urban areas, it has been necessary to install a large amount of equipment on a small lot similar to that of a typical gas station today, and the accumulators used have also had to be compact. Meanwhile, in recent years, there has been an increasing demand for hydrogen stations in suburban areas or on factory grounds. When building hydrogen stations in such locations with ample land, there is a need to increase the internal volume of the accumulator in order to utilize the larger lot area and operate the hydrogen station more efficiently.

[0004] The pressure vessel can be manufactured by using a steel pipe for a hydrogen gas pressure vessel (hereinafter, sometimes simply referred to as a steel pipe) for the vessel body, and for example, the steel pipes described in Patent Documents 1 and 2 are known. Patent Document 1 describes a steel pipe that has been subjected to quenching and tempering as heat treatments. Patent Document 2 describes a steel pipe that has been subjected to straightening after heat treatment.

[0005] JP 2018-53357 A International Publication No. 2018 / 055937

[0006] Standards for Ultra-High Pressure Gas Equipment KHKS0220 (2010), March 31, 2010, High Pressure Gas Safety Institute

[0007] For example, as described in Patent Document 1, when a steel pipe is subjected to heat treatment, the entire steel pipe after the heat treatment is bent in the longitudinal direction. Therefore, as described in Patent Document 2, attempts have been made to eliminate the bend in the steel pipe by performing straightening after the heat treatment. However, it is difficult to eliminate the bend in the steel pipe by cold straightening, and even when hot straightening is performed, there is a problem that the bend in the steel pipe increases during cooling after the hot straightening.

[0008] Furthermore, in the past, pressure accumulators were generally manufactured using steel pipes of less than 5000 mm in length in order to make them compact. However, in order to improve the efficiency of hydrogen stations, it was necessary to increase the internal volume of the pressure accumulator, which required the use of steel pipes with a long overall length to increase the length of the pressure accumulator.

[0009] However, a steel pipe with a long overall length is more likely to bend than a short steel pipe. In other words, when a steel pipe with a long overall length is manufactured by a conventional method such as that described in Patent Document 1 or 2, the steel pipe will bend more than a short steel pipe.

[0010] Here, if the bending of the steel pipe becomes large, problems arise when manufacturing a pressure vessel from the steel pipe, particularly in machining the inner surface of the steel pipe.

[0011] When manufacturing a pressure vessel from a steel pipe, machining the inner surface of the steel pipe makes the inner surface smooth and prevents deterioration of the steel material due to hydrogen. In particular, if the inner surface of the steel pipe has a high surface roughness, hydrogen-induced cracking may occur when the steel pipe is used as a pressure vessel, but machining can prevent this. In the machining, the steel pipe itself is generally rotated while a tool is brought into contact with the steel pipe.

[0012] However, when a long steel pipe is manufactured by the conventional method described in Patent Document 1 or 2, there are cases where machining is impossible due to large vibrations when the steel pipe is rotated during machining. Moreover, even if machining is possible, the quality of the machining is reduced.

[0013] That is, when a steel pipe is rotated during machining, if the steel pipe is bent in the longitudinal direction, the distance from the axis of the rotating steel pipe increases. The centrifugal force caused by this bending of the steel pipe promotes vibration of the rotating body. If the steel pipe is bent significantly when rotated, the vibration of the steel pipe may prevent stable rotation, and machining itself may not be possible. In addition, even if the vibration is reduced to a level that allows machining, the vibration may reduce machining accuracy. Poor machining accuracy may prevent the pressure accumulator from performing as designed. Furthermore, there is a high possibility that insufficient machining or defects may occur, requiring partial maintenance, which reduces productivity. Furthermore, the quality degradation caused by such bending of the steel pipe becomes more pronounced as the pressure accumulator becomes longer.

[0014] The present invention has been made in view of the above-mentioned problems, and aims to provide a steel pipe for a hydrogen gas pressure tank that can be machined with high quality and that can be suitably used as a material for long hydrogen gas pressure tanks, and also to provide a method for manufacturing the steel pipe.

[0015] The gist of the present invention for solving the above problems is as follows.

[0016] 1. A steel pipe for a hydrogen gas pressure tank, having a total length in the longitudinal direction of 5,000 mm or more and 8,000 mm or less, and a bending amount of 1.20 mm or less per 1,000 mm in the longitudinal direction.

[0017] 2. The steel pipe for a hydrogen gas pressure vessel according to 1 above, having a tensile strength of 750 MPa or more.

[0018] 3. A hydrogen gas pressure vessel using the steel pipe for a hydrogen gas pressure vessel according to 1 or 2 above.

[0019] 4. A method for manufacturing a steel pipe for a hydrogen gas pressure tank, comprising hot straightening a base steel pipe and cooling the base steel pipe after hot straightening, wherein the base steel pipe has a total longitudinal length of 5,000 mm or more and 8,000 mm or less, and the cooling comprises rotating the base steel pipe in a temperature range of at least up to 200°C at an average cooling rate of 20°C / min or less.

[0020] 5. The method for producing a steel pipe for a hydrogen gas pressure vessel according to 4 above, wherein cooling the base steel pipe while rotating is initiated from a temperature equal to or higher than the temperature of the hot straightening minus 50°C.

[0021] According to the present invention, it is possible to provide a steel pipe for a hydrogen gas pressure vessel that can be machined with high quality when subjected to machining and that can be suitably used as a material for a long hydrogen gas pressure vessel.

[0022] FIG. 2 is a schematic diagram of a steel pipe and the amount of bending.

[0023] The steel pipe according to the present invention will be described below.

[0024] (Total Length) The longer the total length of a steel pipe, the lower the hydrogen storage cost per unit volume when used as a pressure accumulator. Therefore, in the present invention, the total length in the longitudinal direction of the steel pipe is set to 5000 mm or more. On the other hand, if the total length of the steel pipe is long, it becomes difficult to uniformly heat-treat the steel pipe. In addition, when the steel pipe is heat-treated, the cooling rate of the inner surface of the steel pipe becomes slow. That is, in order to quickly cool the inner surface of the steel pipe, a refrigerant such as water or air may be passed through the inner surface of the steel pipe. However, if the total length of the steel pipe is long, the temperature of the refrigerant rises in the middle of the steel pipe, reducing the effectiveness of the refrigerant in cooling the inner surface. Therefore, the total length in the longitudinal direction of the steel pipe is set to 8000 mm or less, preferably 7000 mm or less.

[0025] (Amount of bending) The amount of bending of the steel pipe needs to be suppressed, and the reason for this will be explained below.

[0026] The first problem with an increase in the amount of bending is that, as mentioned above, machining becomes impossible or the quality of the machining process deteriorates. Here, when the amount of bending increases, the vibrations during rotation of the steel pipe increase, resulting in a deterioration in quality, not only when machining the inner surface of the steel pipe but also when machining the outer surface.

[0027] The second problem is that an increase in the amount of bending increases the risk of design errors. When designing a pressure vessel, if the degree of bending is small, it is possible to assume that the center line of the steel pipe is approximately straight and that the outer and inner surfaces are parallel to the center line. However, as the length of the steel pipe increases and the amount of bending increases, it becomes difficult to manufacture the pressure vessel according to the design.

[0028] A third problem is that an increase in the amount of bending increases the stress generated on the inner surface of the steel pipe. Pressure vessels are designed using stress analysis based on material mechanics, as described in Non-Patent Document 1, for example. Generally, the analysis is performed under the assumption that no moment occurs in the longitudinal direction. However, in practice, stress analysis can be performed taking longitudinal moments into account by performing numerical calculations using finite element analysis. This stress analysis reveals that bending moments occur due to longitudinal bending, increasing the stress generated on the inner surface of the pressure vessel. Since the performance of the pressure vessel deteriorates as the stress increases, it is important to suppress the amount of bending.

[0029] For the reasons mentioned above, the bending amount of the steel pipe is set to 1.20 mm or less per 1000 mm in the longitudinal direction, and preferably 1.00 mm or less per 1000 mm in the longitudinal direction. The lower limit of the bending amount of the steel pipe is not particularly limited, and may be 0.00 mm per 1000 mm in the longitudinal direction. However, from the viewpoint of ease of manufacture, the bending amount of the steel pipe may be set to, for example, 0.50 mm or more per 1000 mm in the longitudinal direction.

[0030] Here, the bending amount of a steel pipe is the displacement (maximum displacement) at the position where the outer surface of the steel pipe is furthest outward from the solid body, when the outer peripheral surfaces of both longitudinal ends of the steel pipe are assumed to be the bottom surfaces and the two bottom surfaces are connected by the shortest path. For example, the schematic diagram in Figure 1 shows the cross section of a steel pipe 1, and the bending amount of the steel pipe 1 corresponds to the length of the arrow part.

[0031] The amount of bending can be measured, for example, by stretching a water line along the longitudinal direction of the steel pipe and measuring the distance between the water line and the steel pipe, or it can be measured using a measuring device that combines multiple laser displacement meters.

[0032] The bending of steel pipes after hot straightening is a phenomenon in which the entire steel pipe is deformed mainly due to the force of the steel pipe's own weight. Therefore, the longer the overall length, the heavier the steel pipe becomes, and the greater the amount of bending of the steel pipe. In contrast, the present invention can reduce the amount of bending even in steel pipes with a long overall length.

[0033] (Wall Thickness) The wall thickness of the steel pipe is not particularly limited, and the steel pipe may have any wall thickness. However, the greater the wall thickness of the steel pipe, the higher the design strength when used as a pressure accumulator. Therefore, the wall thickness of the steel pipe is preferably 35 mm or more, and more preferably 45 mm or more.

[0034] Conventionally, when the wall thickness is increased, it has been difficult to reduce the amount of bending, mainly for the following two reasons. First, the thicker the wall thickness, the heavier the steel pipe, which means that it is more likely to bend during cooling after hot straightening. Second, the cooling rate of the steel pipe decreases during cooling after hot straightening. The bending of steel pipes after hot straightening is creep deformation that occurs at temperatures higher than room temperature. Therefore, when the general method of leaving the steel pipe at a high temperature after hot straightening is adopted, the longer the cooling time, the more likely the bending of the steel pipe is to increase.

[0035] In contrast, in the present invention, even if the wall thickness is thick, a reduced amount of bending can be achieved. From this viewpoint, the wall thickness of the steel pipe is preferably 35 mm or more, and more preferably 45 mm or more.

[0036] The upper limit of the wall thickness of the steel pipe is not particularly limited, but is preferably 70 mm or less, and more preferably 60 mm or less.

[0037] (Outer Diameter) The outer diameter of the steel pipe is not particularly limited, and the steel pipe may have any outer diameter. However, the larger the outer diameter of the steel pipe, the lower the hydrogen storage cost per unit volume when used as a pressure accumulator. Therefore, it is preferable to increase the outer diameter of the steel pipe. Furthermore, as with increasing the wall thickness, increasing the outer diameter also increases the weight of the steel pipe and increases the cooling time of the steel pipe after hot straightening. However, according to the present invention, even when the outer diameter is large, a reduced amount of bending can be achieved. From this perspective, it is preferable to increase the outer diameter of the steel pipe; specifically, it is preferably 300 mm or more, and more preferably 350 mm or more. On the other hand, the upper limit of the steel pipe is not particularly limited, but it is preferably 500 mm or less, and more preferably 450 mm or less.

[0038] (Material) The material of the steel pipe is not particularly limited, and any material can be used. The steel pipe may be made of low-alloy steel. Examples of the low-alloy steel include chromium-molybdenum steel and nickel-chromium-molybdenum steel.

[0039] The method for manufacturing the steel pipe is not particularly limited, and any method for manufacturing the steel pipe can be used. As the steel pipe, any type of steel pipe, such as an electric resistance welded steel pipe or a seamless steel pipe, can be used, and processed steel pipes can also be used, but it is preferable to use a seamless steel pipe. Pressure vessels manufactured from seamless steel pipes have superior properties such as toughness compared to pressure vessels manufactured by boring, and also have no welded parts, making them extremely suitable for use with hydrogen gas.

[0040] (Strength) The strength of the steel pipe is not particularly limited and may be any tensile strength. However, the higher the strength of the steel pipe, the smaller the amount of bending can be, and the more improved the quality of machining can be. Therefore, the tensile strength of the steel pipe is preferably 750 MPa or more. On the other hand, if the strength is too high, the steel pipe is prone to hydrogen embrittlement, which deteriorates the material properties. Therefore, from the viewpoint of preventing hydrogen embrittlement, the tensile strength of the steel pipe is preferably 1000 MPa or less, and more preferably 950 MPa or less.

[0041] The tensile strength is a value measured in the circumferential direction of the steel pipe by the method specified in JIS Z 2241:2011.

[0042] (Machining) The steel pipe may be machined. At least one of the inner and outer surfaces of the steel pipe may be machined, but it is preferable that the inner surface of the steel pipe is machined. Furthermore, the machining is more preferably machined to smooth the inner surface. By machining the inner surface of the steel pipe to smooth the inner surface, deterioration of the steel material due to hydrogen can be prevented. In particular, if the surface roughness of the inner surface of the steel pipe is high, hydrogen-induced cracking may occur when the steel pipe is used as a pressure vessel, but this can be prevented by machining.

[0043] The outer surface of the steel pipe may be machined, or may be machined to make the outer surface smooth. In addition, the outer surface, the inner surface, or both surfaces may be painted.

[0044] Examples of the machining include cutting, grinding, polishing, and shot blasting, all of which can be used to smooth the inner and outer surfaces. The machining preferably involves one or more processes selected from the group consisting of cutting, grinding, polishing, and shot blasting. The cutting can be performed using, for example, a lathe, particularly a large lathe. More specifically, it can be performed using a BTA (Boring & Trepanning Association) deep hole drilling machine. The amount of cutting in the cutting is not limited, as long as it is greater than 0 mm, but may be, for example, 5 mm or less. The grinding can be performed partially to remove defects on the inner or outer surface of the steel pipe, and can be performed using, for example, a grinder. The amount of cutting in the grinding is not limited, as long as it is greater than 0 mm, but may be, for example, 1 mm or less. The polishing can be performed using, for example, an automatic polishing machine equipped with a polishing wheel. The amount of polishing in the polishing is not limited, as long as it is greater than 0 mm, but may be, for example, 3 mm or less.

[0045] The machining is preferably carried out by rotating the steel pipe itself while bringing a tool into contact with it.

[0046] (Surface roughness) The surface roughness of the inner surface of the steel pipe is not particularly limited and may be any surface roughness. However, reducing the surface roughness of the inner surface of the steel pipe can prevent deterioration of the steel material due to hydrogen, and in particular, can prevent the occurrence of hydrogen-induced cracking when used as a pressure vessel. Therefore, the surface roughness of the inner surface of the steel pipe is preferably 100 μm or less, and more preferably 50 μm or less. On the other hand, the lower limit of the surface roughness of the inner surface of the steel pipe is not particularly limited and may be 0 μm, but from the viewpoint of ease of manufacture, it is preferably 10 μm or more.

[0047] The surface roughness is the maximum height Rz according to JIS B0601:2001, and is measured using a roughness measuring instrument. Specifically, measurements are taken five times at measurement positions 100 mm, 200 mm, and 300 mm (total of six positions) from both ends of the inner surface of the steel pipe in the longitudinal direction, resulting in a total of 30 measurements, which are averaged to determine the surface roughness. The measurement direction is the longitudinal direction.

[0048] Hereinafter, a method for manufacturing a steel pipe for a hydrogen gas pressure vessel according to the present invention will be described. The manufacturing method is a method for manufacturing a steel pipe for a hydrogen gas pressure vessel, which comprises hot-straightening a base steel pipe and cooling the base steel pipe after hot-straightening. Here, it is important that the base steel pipe has a total longitudinal length of 5000 mm or more and 8000 mm or less, and that during the cooling, the base steel pipe is rotated and cooled at an average cooling rate of 20°C / min or less in a temperature range of at least up to 200°C. According to this method, it is possible to manufacture a steel pipe for a hydrogen gas pressure vessel having a total longitudinal length of 5000 mm or more and 8000 mm or less and a bending amount of 1.20 mm or less per 1000 mm in the longitudinal direction.

[0049] In the following description of the manufacturing method, the temperature of the raw steel pipe is measured on the surface of the raw steel pipe.

[0050] In order to make the total length of the final steel pipe 5000 mm or more, the total length of the base steel pipe is set to 5000 mm or more. On the other hand, in order to make the total length of the final steel pipe 8000 mm or less, the total length of the base steel pipe is set to 8000 mm or less. The total length is preferably set to 7000 mm or less.

[0051] Any method can be used to obtain a base steel pipe from steel. The base steel pipe according to the present invention can be any type, such as an electric resistance welded steel pipe or a seamless steel pipe, but it is preferable to use a seamless steel pipe.

[0052] The base steel pipe may be heat-treated in advance. The heat treatment can adjust the structure and material properties of the steel pipe. The heat treatment may be, for example, quenching only, or quenching and tempering.

[0053] In the method for producing a steel pipe according to the present invention, the raw steel pipe is hot-straightened. That is, in order to reduce the amount of bending in the steel pipe that is finally produced, the raw steel pipe is first mechanically straightened to eliminate bending in the raw steel pipe. Hot straightening can be performed, for example, using a straightening machine equipped with drum-shaped rolls, in other words, concave-shaped rolls. The raw steel pipe may be hot-straightened after being heated (reheated). Furthermore, if the quenching and tempering are performed, hot straightening may be performed during cooling immediately after tempering.

[0054] The hot straightening temperature may be 400°C or higher, or may be 550°C or lower. Furthermore, if quenching and tempering have been performed in advance, the hot straightening temperature may be (the maximum tempering temperature - 50°C) or lower. By setting the hot straightening temperature lower than the maximum tempering temperature, the amount of bending can be reduced without affecting the properties of the steel pipe. Furthermore, the hot straightening temperature may be (the maximum tempering temperature - 120°C) or higher, or (the maximum tempering temperature - 100°C) or higher.

[0055] Next, the raw steel pipe after hot straightening is cooled. Here, it is important to rotate the raw steel pipe in a temperature range up to at least 200°C. It is also important to set the average cooling rate to 20°C / min or less in a temperature range up to at least 200°C.

[0056] First, we will explain the rotation of the base steel pipe. In the present invention, it is necessary to reduce the amount of bending in the steel pipe. However, if a steel pipe after hot straightening is left at a high temperature, the entire steel pipe will bend, and as the temperature decreases while maintaining the bent shape, bending will occur in the steel pipe after cooling. For example, if a steel pipe after hot straightening is stored on a beam arranged in a direction transverse to the longitudinal direction of the steel pipe, the entire steel pipe will deform, with the end of the beam as a fulcrum. However, since bending of the steel pipe is creep deformation, which is a quasi-static deformation, it takes a certain amount of time for deformation to occur. Therefore, bending can be avoided by continuously rotating the steel pipe to change the surface receiving the force before deformation occurs. In other words, from the perspective of reducing the amount of bending in the final steel pipe, it is important to rotate the base steel pipe after hot straightening. Note that the base steel pipe may be moved longitudinally simultaneously with or prior to rotation of the base steel pipe. This further reduces the amount of bending in the steel pipe.

[0057] The method for rotating the material steel pipe is not particularly limited, but it is preferable to rotate it with a motor, as this makes it easy to rotate it continuously.

[0058] The material steel pipe may be rotated intermittently, but is preferably rotated continuously. Examples of intermittent rotation include automatic transport using a mechanical arm or manual transport. When rotating intermittently, there are periods when the rotation of the steel pipe stops, but shortening the period during which the rotation of the steel pipe is continuously stopped can further prevent bending of the steel pipe. Specifically, the period during which the rotation of the material steel pipe is continuously stopped is preferably 5 seconds or less per rotation. The lower limit of the period during which the rotation of the material steel pipe is continuously stopped is not limited, and may be 0 seconds or more per rotation. Furthermore, the temperature drop during the period during which the rotation of the material steel pipe is continuously stopped is preferably 1.7°C or less per rotation. The lower limit of the temperature drop is not limited, and may be 0°C or more per rotation.

[0059] The average rotation speed of the material steel pipe is not particularly limited, but a higher average rotation speed of the steel pipe makes it easier to prevent bending. From this perspective, the average rotation speed (average time per rotation) is preferably 120 seconds or less per rotation. On the other hand, the average rotation speed may be greater than 0 seconds per rotation. However, since a larger motor driving force is required to increase the rotation speed of the steel pipe, from the viewpoint of reducing electricity costs and equipment costs, the average rotation speed is preferably 1 second or more per rotation, and more preferably 5 seconds or more.

[0060] The average rotation speed refers to the average rotation speed in a temperature range described below.

[0061] The material steel pipe may be rotated in the circumferential direction. Specifically, the material steel pipe may be laid on its side, with its longitudinal direction parallel to the ground, and rotated in the circumferential direction. The rotation direction is not limited. During rotation, the lower portion of the material steel pipe may be supported entirely, partially, or in some other manner. When a portion of the lower portion of the material steel pipe is supported, the number of support locations is not particularly limited, but it is preferable that the supports support three or more locations on the outer surface of the material steel pipe. Increasing the number of support locations in this manner can further prevent bending of the intermediate portion of the steel pipe. The support may be, for example, a beam or a roller. Furthermore, when supports support the outer surface of the material steel pipe at multiple locations, it is preferable to shorten the distance between the position on the outer surface of the material steel pipe closest to the longitudinal end and the end. This can further prevent bending of the steel pipe end. Specifically, the distance is preferably 15% or less of the entire longitudinal length of the material steel pipe, and more preferably 10% or less. The lower limit of the distance is not limited and may be 0%.

[0062] Next, the average cooling rate will be explained. If the average cooling rate of a steel pipe is increased, a temperature difference will occur in the wall thickness direction. This will cause the steel pipe to bend due to the difference in expansion coefficient between the inner and outer surfaces of the steel pipe. In particular, when a thick-walled steel pipe is used as the base steel pipe, bending of the steel pipe will occur significantly. Therefore, from the viewpoint of preventing an increase in the amount of bending, in the present invention, the average cooling rate is set to 20°C / min or less, preferably 10°C / min or less. There is no particular upper limit to the average cooling rate, and it may be more than 0°C / min, but from the viewpoint of ease of manufacture, it is preferably set to 1°C / min or more.

[0063] The average cooling rate refers to the average cooling rate in the temperature range described below.

[0064] Next, the above temperature range will be explained. In the present invention, it is necessary to control the rotation and cooling rate of the raw steel pipe to a temperature range where creep deformation is negligible. Therefore, after the hot straightening, it is necessary to cool the raw steel pipe at the above average cooling rate while rotating it in a temperature range of at least 200°C. The lower limit of the above temperature range is 200°C, preferably 150°C, and more preferably 100°C. The temperature at which the process of cooling the raw steel pipe while rotating it can be started at (the hot straightening temperature - 50°C) or higher. That is, the upper limit of the above temperature range can be (the hot straightening temperature - 50°C), and the raw steel pipe can be cooled while rotating in a temperature range of at least (the hot straightening temperature - 50°C) to 200°C. Cooling while rotating can be started immediately after hot straightening, for example, within 180 seconds after hot straightening. That is, the upper limit of the temperature range in which the raw steel pipe is cooled while rotating it can be the temperature immediately after hot straightening, for example, the temperature 180 seconds after hot straightening.

[0065] The cooling method performed in the above temperature range is not particularly limited, and any method such as water cooling or air cooling can be used. However, in order to slow down the average cooling rate, cooling by air cooling or water cooling is preferred, and cooling by air cooling is more preferred. When cooling by air cooling, for example, a blower can be used. Natural cooling may also be performed as air cooling. When cooling by water cooling, for example, mist can be used.

[0066] At temperatures lower than the lower limit of the above temperature range, any cooling rate can be used. Here, from the viewpoint of being able to shorten the cooling time without affecting the amount of bending of the steel pipe, it is preferable to increase the average cooling rate.

[0067] Furthermore, at temperatures lower than the lower limit of the above temperature range, any cooling method such as water cooling or air cooling can be used. The cooling method may be natural cooling or forced cooling. In particular, water cooling is preferred from the viewpoint of being able to shorten the cooling time without affecting the amount of bending of the steel pipe. Any cooling method using water can be used as water cooling.

[0068] At temperatures lower than the lower limit of the above temperature range, the steel pipe may or may not be rotated.

[0069] The steel pipe obtained by the above-mentioned method may be further machined. The part to be machined is not particularly limited, and may be, for example, the inner or outer surface of the steel pipe, but it is preferable to machine the inner surface of the steel pipe. Specifically, the steel pipe can be machined by the above-mentioned method.

[0070] (Pressure Accumulator) The pressure accumulator according to the present invention can be manufactured using the above-mentioned steel pipe. The type of pressure accumulator is not limited. For example, it may be a "composite container pressure accumulator" (such as a Type 2 pressure accumulator) manufactured by using a steel pipe as a liner and covering the periphery with carbon fiber reinforced resin or the like, or it may be a "pressure accumulator" (such as a Type 1 pressure accumulator) that does not have a covering such as carbon fiber reinforced resin.

[0071] For the same reasons as for the steel pipe, the tensile strength of the pressure accumulator is preferably 750 MPa or more. Also, for the same reasons as for the steel pipe, the tensile strength of the pressure accumulator is preferably 1000 MPa or less, and more preferably 950 MPa or less.

[0072] The present invention will now be described in more detail with reference to examples. The following examples are intended to illustrate preferred examples of the present invention, and are not intended to limit the scope of the present invention in any way.

[0073] Steel tubes for hydrogen gas pressure tanks were manufactured using steel tubes having the total longitudinal length shown in Table 1. Each steel tube had an outer diameter of 406 mm and a wall thickness of 50 mm. In the manufacturing process, the steel tubes were first quenched and tempered in advance. The tempering was carried out at a maximum temperature of 670°C.

[0074] (Hot Straightening) Next, hot straightening was carried out at 550°C using a straightener equipped with an hourglass-shaped roll (concave roll).

[0075] (Cooling) After hot straightening, the raw steel pipe was cooled by air cooling (cooling using a blower or natural cooling) while being continuously rotated at the rotational speed shown in Table 1. First, the raw steel pipe immediately after hot straightening was placed on a beam, and the raw steel pipe was supported by multiple beams extending in a direction perpendicular to the longitudinal direction of the raw steel pipe. The beam had a belt on its upper surface, and the placed raw steel pipe could be rotated by moving the belt in the direction of the extension of the beam. The raw steel pipe was rotated at the rotational speed shown in Table 1 using the beam. Rotation of the raw steel pipe started at the rotation start temperature shown in Table 1.

[0076] The rotation of the raw steel pipe was not stopped until the air cooling was completed, so the average rotation speed of the raw steel pipe was equal to the rotation speed shown in Table 1.

[0077] In addition, in steel pipe No. 5, the base steel pipe was not rotated.

[0078] The average cooling rate in the temperature range from the rotation start temperature to the rotation and air-cooling end temperature was as shown in Table 1. Here, in the examples where the rotation and air-cooling end temperature was less than 200°C, the average cooling rate in the temperature range from the rotation start temperature to 200°C was 20°C / min or less in all cases.

[0079] When the surface temperature of the raw steel pipe reached the end temperature of rotation and air cooling shown in Table 1, the rotation and air cooling were terminated, and after completion of rotation and air cooling, the steel pipe was cooled by water to room temperature.

[0080] The amount of bending of the obtained steel pipe was measured. The amount of bending was measured using a measuring device that uses a laser meter. The tensile strength of the obtained steel pipe was also measured using the method described above. The measurement results are shown in Table 1.

[0081] (Rotation Test) First, it was investigated whether any problems would arise when using the machining device.

[0082] The machining device used was a BTA type deep hole drilling device.

[0083] The obtained steel pipes were set in the machining device and rotated, and those that generated abnormal noise during rotation were deemed to have failed, while those that did not generated abnormal noise were deemed to have passed. For steel pipes that failed, there was a large amount of vibration during rotation, and it was necessary to stop processing from a safety standpoint, so subsequent cutting and polishing were not performed. For steel pipes that passed, cutting and polishing were performed.

[0084] (Cutting) Of the obtained steel pipes, those that passed the rotation test were subjected to inner cutting using the machining device, with the goal of reducing the wall thickness on the inner surface by 5.0 mm.

[0085] The steel pipe was then cut in the center of its length, and the outer and inner surfaces of the center were visually inspected. It is very difficult to remove defects in the center by spot repair, which significantly reduces productivity, so it is important that there are no defects in the center of the length. In the visual inspection, the outer surface was visually inspected for scratches at the area where the beam supporting the outer surface had been in contact during the rotation performed during cooling in the manufacturing process. For the inner surface, a light was shone on the inner surface, and any shadows visually observed were considered to be defects. Defects on the outer surface and inner surface can be visually detected if they are approximately 0.5 mm deep or greater. A product with at least one of the inner surface and outer surface defects was deemed to have failed, while a product with neither was deemed to have passed.

[0086] Furthermore, for steel pipes that passed the visual inspection, wall thickness measurements were carried out to examine the processing accuracy. For wall thickness measurements, three different positions were selected along the longitudinal direction of the steel pipe, and measurements were taken at four positions along the circumferential direction at each of these positions. The smallest wall thickness (minimum wall thickness) among the 12 measurement results was used for evaluation.

[0087] The criterion for judgment was whether the minimum wall thickness was below 42.5 mm, with a tolerance of 5% of the wall thickness of the steel pipe. That is, if the wall thickness of the steel pipe is 50.0 mm, 42.5 mm is the value obtained by subtracting a cutting allowance of 5.0 mm and a tolerance of 2.5 mm from the wall thickness of the steel pipe. If the wall thickness was 42.5 mm or more, the wall thickness judgment was considered to be pass, and if it was less than 42.5 mm, the wall thickness judgment was considered to be fail.

[0088] (Polishing) For the steel pipes obtained that passed the rotation test, the inner surface was polished using an automatic polishing device to smooth the inner surface. The automatic polishing device is configured to perform polishing by rotating the steel pipe while rotating a bar with a grinding wheel at its tip in the opposite direction to the steel pipe, bringing the grinding wheel into contact with the inner surface of the steel pipe. Since the oxide film is removed by the polishing process, the presence or absence of an oxide film can be used to determine whether polishing was sufficient. Therefore, after two rounds of automatic polishing, the presence or absence of a remaining oxide film was visually confirmed. If no oxide film remained, the pipe was rated as passed, and if the oxide film remained, the pipe was rated as failed.

[0089] The results of the rotation test, the appearance inspection and thickness judgment after cutting, and the appearance inspection after polishing are shown in Table 2.

[0090] As can be seen from the results shown in Table 2, the steel pipes satisfying the conditions of the present invention passed all of the rotation test, the visual inspection and wall thickness judgment after cutting, and the visual inspection after polishing. In other words, the steel pipes satisfying the conditions of the present invention could be machined with high quality when subjected to machining. In contrast, the steel pipes of the comparative examples that did not satisfy the conditions of the present invention failed in all of the tests. In other words, the steel pipes of the comparative examples experienced large vibrations when rotated, making it impossible to machine them, or, even if they could be machined, the quality of the machining was reduced.

[0091]

[0092]

[0093] 1 steel pipe

Claims

1. A steel pipe for a hydrogen gas pressure tank, having a total length in the longitudinal direction of 5,000 mm or more and 8,000 mm or less, and a bending amount of 1.20 mm or less per 1,000 mm in the longitudinal direction.

2. A steel pipe for a hydrogen gas pressure vessel according to claim 1, having a tensile strength of 750 MPa or more.

3. A hydrogen gas pressure vessel using the steel pipe for hydrogen gas pressure vessels according to claim 1 or 2.

4. A method for manufacturing a steel pipe for a hydrogen gas pressure tank, which comprises hot straightening a base steel pipe and cooling the base steel pipe after hot straightening, wherein the base steel pipe has a total longitudinal length of 5,000 mm or more and 8,000 mm or less, and the cooling comprises rotating the base steel pipe and cooling it at an average cooling rate of 20°C / min or less in a temperature range of at least up to 200°C.

5. A method for producing a steel pipe for a hydrogen gas pressure vessel according to claim 4, wherein cooling the raw steel pipe while rotating is initiated from a temperature equal to or higher than the temperature of the hot straightening minus 50°C.

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