high pressure tank

Abstract

A high-pressure tank which suppresses the effect of hydrogen embrittlement of metal, ensures the storage amount of gas, and reduces the weight, stores a mixed gas containing natural gas and hydrogen, which contains more than 2% of hydrogen in terms of volume ratio, in the inside of a tank main body, the tank main body has a cylindrical portion extending in a first direction, and a dome portion connected to both ends of the first direction of the cylindrical portion, the tensile strength TS of a steel material constituting the tank main body is 850 MPa or more, has a structure of 95% or more of tempered martensite in terms of area ratio, carbides of 100 nm or less are dispersed in the metal structure in the form of precipitates, and has a strength of 11250 cycles or more in a crack propagation analysis.

Description

Technical Field

[0001] This disclosure relates to a high-pressure tank for storing high-pressure hydrogen. Background Technology

[0002] As a countermeasure to address global warming, the SDGs (Social, Sustainable, and Governing Energy) have been proposed. As part of these measures, hydrogen and e-methane (synthetic methane, methane synthesized from recycled CO2 and hydrogen produced using renewable energy sources) are highly anticipated as alternative energy sources to fossil fuels.

[0003] Hydrogen engines, which use hydrogen as fuel, are environmentally friendly because they do not emit carbon dioxide (CO2), and their structure can be modified from traditional gasoline engines. However, while hydrogen is highly flammable and burns quickly, it can ignite at unpredictable times. Therefore, hydrogen engines are prone to damage and have durability issues. The dominant factors contributing to this unstable ignition of hydrogen are currently unknown.

[0004] Methane gas (CH4) is also used as an environmentally friendly fuel, and CNG (natural gas) vehicles are known to exist. However, because methane gas ignites slowly and easily produces unburned methane, NOx generation has become a problem. As a NOx countermeasure, although lean-burn technology for methane has been developed, there is a problem that the lean-burn range of methane is relatively narrow.

[0005] To address the aforementioned issues related to hydrogen and methane gases, the focus has shifted to lean combustion technology for hydrogen-hydrogen mixed gases (HCNGs). Development and validation trials of HCNG lean combustion technology were conducted in the 2000s, but development stalled due to insufficient hydrogen supply infrastructure, high hydrogen prices, and inadequate NOx reduction. However, in recent years, work on developing technologies for the utilization of hydrogen and hydrogen compounds has been restarted.

[0006] Recent studies have shown that adding hydrogen to methane gas expands the lean-burn range of methane, improves fuel efficiency, and reduces NOx due to lower combustion temperatures. Furthermore, insights have emerged regarding the potential application of lean-burn technology in dual-fuel engines using gasoline and natural gas.

[0007] When a car is equipped with hydrogen or natural gas as fuel, a tank is needed to store the gas under high pressure. As such a tank for storing high-pressure gas, there are known types 1, which are made of metal containers; types 2, which have annular layers made of fiber-reinforced resin (FRP) formed only in the cylindrical portion of the metal container; types 3, which have FRP spiral layers formed throughout the entire metal container and FRP annular layers formed in the cylindrical portion; and types 4, which have FRP spiral layers formed throughout the entire resin container and FRP annular layers formed in the cylindrical portion (for example, see Patent Document 1).

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2017-048912 Summary of the Invention

[0011] In the high-pressure tanks disclosed in Patent Document 1, except for Type 1 which is a metal container, the manufacturing process is complex and costly because it is constructed by winding fiber-reinforced resin (FRP) around a metal container. Especially when the gas stored in the high-pressure tank contains hydrogen, the high-pressure tank is manufactured by winding resin-impregnated CFRP (carbon fiber reinforced composite material) around an aluminum alloy liner that does not experience hydrogen embrittlement. The liner of the high-pressure tank has a cylindrical portion and dome-shaped openings at both ends of the cylindrical portion. Generally, considering the internal pressure acting on the cylindrical portion in such a thin-walled tank, it is known that the circumferential stress in the cylindrical portion is twice that in the axial direction. High-pressure tanks typically have a large wall thickness, resulting in a stress distribution along the wall thickness direction; however, the stresses loaded in the circumferential and axial directions of the liner of the high-pressure tank will differ as described above. Therefore, in order to achieve strength matching the aforementioned stress ratio, high-pressure tanks of types 2 to 4 require adjustment of the fiber orientation of the FRP wound around the dome and cylindrical portion. Therefore, in high-pressure tanks of types 2 to 4, there is a problem of costly FRP winding process.

[0012] On the other hand, in the high-pressure tank disclosed in Patent Document 1, if Type 1, which is a metal container, is used for a natural gas single-fuel engine, the hydrogen content in the stored gas is less than 2%, thus minimizing the impact of hydrogen embrittlement and enabling lightweight design and cost control. However, as mentioned above, since the Type 1 high-pressure tank used for storing hydrogen-added gas (HCNG) is made of steel, there is a problem of reduced durability due to hydrogen embrittlement. Furthermore, in the Type 1 high-pressure tank, while the gas storage pressure can be reduced to suppress the effects of hydrogen embrittlement, this results in the inability to store the required amount of gas for a vehicle. Conversely, when the pressure of the stored gas is increased to increase the storage capacity, the high-pressure tank needs to reduce the resulting stress to suppress hydrogen embrittlement, leading to a thicker wall. In this case, the high-pressure tank becomes unsuitable for vehicle use due to increased weight and volume.

[0013] This disclosure is intended to solve the above-mentioned problems, and its purpose is to provide a high-pressure vessel that ensures the storage capacity of hydrogen-containing gas while reducing weight.

[0014] The high-pressure tank disclosed herein is a high-pressure tank that stores a mixture of natural gas and hydrogen inside the tank body, wherein the mixture contains more than 2% hydrogen by volume. The tank body has a cylindrical portion extending in a first direction and dome-shaped portions connected to both ends of the cylindrical portion in the first direction. The steel constituting the tank body has a tensile strength TS of 850 MPa or more, has a structure with tempered martensite of more than 95% by area, has carbides smaller than 100 nm dispersed in the metal structure as precipitates, and has a strength of more than 11,250 cycles in crack propagation analysis.

[0015] According to the aforementioned high-pressure tank, hydrogen penetrates into the microstructure of the steel constituting the tank body. As the hydrogen moves within the grains of the steel microstructure, it is captured by precipitates smaller than 100 nm, thus suppressing hydrogen accumulation at the grain boundaries of the steel microstructure. Therefore, because the steel constituting the tank body can suppress grain boundary fracture, sufficient fatigue strength can be ensured even when storing fuels mixed with hydrogen and natural gas. Attached Figure Description

[0016] Figure 1 This is a cross-sectional view of the high-pressure tank 100 in embodiment 1.

[0017] Figure 2 This is a schematic diagram illustrating the effect of hydrogen embrittlement on steel.

[0018] Figure 3 This is an illustrative diagram that schematically illustrates the effects of hydrogen in traditional and new materials.

[0019] Figure 4 This is an illustration diagram schematically showing the relationship between stress and crack propagation rate in the steel that constitutes the tank body 10 of Embodiment 1 and conventional steel. Detailed Implementation

[0020] The preferred embodiments of the high-pressure tank of this disclosure will now be described in detail with reference to the accompanying drawings. It should be noted that the embodiments described below are preferred examples of this disclosure; therefore, although various technically preferred limitations are included, the scope of this disclosure is not limited to these methods unless specifically stated in the following description.

[0021] Implementation method 1.

[0022] <High-Pressure Tank 100>

[0023] Figure 1 This is a cross-sectional view of the high-pressure tank 100 according to Embodiment 1. Figure 1The high-pressure tank 100 shown schematically illustrates its structure. The high-pressure tank 100, for example, is mounted in vehicles and is used to store fuel for driving engines. The fuel is a hydrogen-containing gas mixture, particularly a mixture of hydrogen and natural gas (HCNG). HCNG, for example, contains 20% hydrogen, with the remainder consisting of natural gas. To supply the stored gas mixture to the engine, the high-pressure tank has a supply device 14, such as a valve, installed at the end interface 13. The supply device 14 delivers the gas mixture to the engine at a specified pressure. The high-pressure tank 100 can store a gas mixture sufficient to ensure the driving range required for practical use in automobiles. The high-pressure tank 100 is not limited to automobiles; it can also be used in various drive systems (industrial machinery, generators, etc.) that use hydrogen-containing gas mixtures as fuel. Therefore, the high-pressure tank 100 is a structure that can be used not only in vehicles but also in a wide range of applications. It should be noted that the mixed gas stored in the high-pressure tank 100 of Embodiment 1 is at least a gas with a hydrogen ratio greater than that of conventional CNG, containing at least 2% hydrogen by volume. As other examples, it may also contain hydrogen values ​​greater than 4%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% by volume, or hydrogen values ​​greater than those in this example.

[0024] <Structure of the main body 10 of the tank>

[0025] The tank body 10 includes a cylindrical portion 11 and domed tops 12 formed at both ends of the cylindrical portion 11. The cylindrical portion 11 is a cylindrical body with an inner diameter of Di, formed of steel with a wall thickness of t. The domed tops 12 are hollow, hemispherical sections connected to both ends of the cylindrical portion 11, formed of steel with the same wall thickness t as the cylindrical portion 11. Figure 1 In this design, the dome 12 is hemispherical, but not limited to this; it can also be a dome shape formed by a combination of curved surfaces such as spheres and planes on its outer surface. Furthermore, the wall thickness t of the dome 12 can be non-uniform. For example, the wall thickness t can be thicker in areas where stress is prone to concentration, such as around the interface 13.

[0026] An interface 13 is provided on at least one of the domes 12 at both ends, enabling the internal mixed gas to be discharged to the outside of the tank body 10. Figure 1 In the dome 12a on the X1 side, an interface 13 is provided. The interface 13 can be provided not only on the dome 12a, but also on the dome 12b located at the other end.

[0027] Interface 13 is the part for installing the supply device 14. It may have internal threads or a sealing structure to prevent the leakage of the internal mixed gas.

[0028] Preferably, the steel used for the tank body 10 has an initial defect size of less than 0.5 mm in depth and less than 1.5 mm in length, as measured by ultrasonic testing in its finished state. Furthermore, the tank body 10 preferably uses steel whose inner surface, which comes into contact with a hydrogen-containing gas mixture, has been ground.

[0029] <Strength required for pressure vessel 100>

[0030] The high-pressure tank 100 is sealed with a hydrogen-containing gas mixture within its internal space 90. The rated maximum pressure of the gas mixture sealed in the high-pressure tank 100 is at least 20 MPa, but can sometimes be 25 MPa, 35 MPa, or 70 MPa, or any pressure between 20 MPa and 70 MPa. The gas mixture in the high-pressure tank 100 decreases with each trip of the vehicle, and the internal pressure gradually decreases, for example, to approximately 0.2 MPa. When the pressure of the gas mixture decreases, it is refilled into the high-pressure tank 100, restoring the internal pressure to its high state. The internal pressure of the tank body 10 is repeatedly subjected to high-pressure and low-pressure states (e.g., 0.2 MPa to 20 MPa), generating high stress under high pressure and low stress under low pressure. In other words, the tank body 10 is repeatedly subjected to stress and must be able to withstand this stress amplitude.

[0031] Generally, if the stress amplitude is within the fatigue limit σ w The following can essentially withstand an infinite number of repetitions (10). 7 Approximately 2000 times. (Depending on the material, there is sometimes no clearly defined fatigue limit). In other words, if the high-pressure tank 100 described above is subjected to a pressure of at least 20 MPa and the stress σ generated in the tank body 10 at this time is at least the fatigue limit σ. w Therefore, the tank body 10 will not fracture due to fatigue. However, the fatigue limit σ w It is about half of the tensile strength of the material. If the fatigue limit σ is taken as... w If the high-pressure tank 100 is designed with safety factors in mind, then the wall thickness t of the tank body 10 needs to be increased, or the maximum internal pressure needs to be reduced.

[0032] Figure 2 This is a schematic diagram illustrating the effect of hydrogen embrittlement on steel. Figure 2 This graph shows the relationship between the stress applied to steel and the number of stress cycles. The steel's strength decreases relative to the repeated stress amplitude when exposed to a hydrogen environment. Although the steel fractures under repeated stress, reducing the stress will prevent it from fracturing even after 10 cycles. 6 ~10 7The steel will not fracture even after more than one cycle of repeated stress. This stress is called the fatigue limit. When steel is exposed to hydrogen, in the low-cycle region with fewer repeated stress cycles, the stress at which the steel fractures is lower compared to the case without hydrogen influence.

[0033] Because the high-pressure tank 100 contains a hydrogen-containing gas mixture, it is susceptible to hydrogen embrittlement. The high-pressure tank 100 of Embodiment 1 incorporates steel modified to accommodate the internal hydrogen embrittlement, thus enabling the storage of high-pressure gas mixtures while suppressing the effects of hydrogen embrittlement. Details of the characteristics of the steel used in the high-pressure tank 100 of Embodiment 1 will be described later. In contrast, conventional high-pressure tanks, using ordinary steel, are susceptible to hydrogen embrittlement when containing a hydrogen-containing gas mixture, resulting in reduced fatigue strength and significantly decreased resistance to repeated stress. Therefore, in high-pressure tanks using ordinary steel, it is necessary to increase the wall thickness t or reduce the internal pressure to reduce the resulting stress. However, conventional high-pressure tanks used in CNG vehicles contain less than 2% hydrogen in the internal gas, thus eliminating the need to consider the effects of hydrogen embrittlement.

[0034]

[0035] Table 1 summarizes the circumferential stress σ and lifespan of high-pressure tanks in existing examples, comparative examples, and inventive examples. In the examples shown in Table 1, the structure of the tank body is similar to... Figure 1 The tank body 10 of Embodiment 1 shown has the same structure (although the detailed dimensions are set differently), the pressure of the gas sealed inside is the same, and the wall thickness t and the material used for the tank body 10 are different. Therefore, the existing examples, comparative examples and inventive examples of high-pressure tanks in Table 1 generate different circumferential stresses σ, and their service life is also different.

[0036] <Existing Examples in Table 1>

[0037] In the existing example shown in the leftmost column of Table 1, the stored fuel is CNG (natural gas), and the hydrogen content in the gas composition is suppressed to below 2%. The high-pressure tank constituting the existing example is made of conventional materials, such as SCM435 (chromium-molybdenum steel) that has been quenched and tempered, and for example, a material with a tensile strength (TS) of 953 MPa, but it does not have the characteristics of the steel used in the high-pressure tank 100 of Embodiment 1. It should be noted that SCM435, as a conventional material, contains 0.15% to 0.30% molybdenum (Mo) by mass%. In the high-pressure tank of the existing example, the hydrogen content in the fuel stored inside is minimal and is not affected by hydrogen embrittlement. Therefore, as shown in Table 1 above, even if the circumferential stress is calculated to be σ = 592 MPa, it has sufficient resistance, thus fully ensuring the service life of the vehicle-mounted CNG high-pressure tank.

[0038] It should be noted that the circumferential stress σ generated in the high-pressure tank is calculated using the following Equation 1.

[0039] σ = Di·p / 2t (Equation 1)

[0040] Di: Inner diameter of the container [mm]

[0041] p: Internal gas pressure [MPa]

[0042] t: The wall thickness of the cylindrical part 11 of the tank body 10.

[0043] Furthermore, the lifespan of each high-pressure tank in Table 1 was determined using fatigue crack propagation analysis. The fatigue crack propagation analysis was performed according to KHK S 0220 (2020). For each high-pressure tank, the lifespan was determined by whether fracture occurred when subjected to a stress amplitude of 11,250 cycles, indicating whether the lifespan had been achieved (represented as "OK" in Table 1) or not (represented as "NG" in Table 1). The stress amplitude of 11,250 cycles was set considering the number of times a high-pressure tank, typically used as a vehicle fuel container, would be refueled before reaching the vehicle's mileage limit. It should be noted that the lifespan of the high-pressure tank 100 in Embodiment 1 can be a strength of 11,250 cycles or more in the crack propagation analysis. For example, strengths of 12,000 cycles, 15,000 cycles, 20,000 cycles, 25,000 cycles, or values ​​between these exemplified values ​​are also permissible. However, when the lifespan of the high-pressure tank 100 is long, the wall thickness of the tank body 10 tends to increase, resulting in increased weight. In this case, when the high-pressure tank 100 is mounted on a vehicle, fuel efficiency may decrease. Therefore, it is preferable to set the lifespan of the high-pressure tank 100 to, for example, less than 50,000 cycles.

[0044] As described above, although the existing high-pressure tank is made of conventional materials, it is not affected by hydrogen embrittlement because its purpose is to store CNG and the hydrogen content in the filled gas is suppressed to below 2%, thus ensuring the required lifespan for vehicle fuel containers. Specifically, the lifespan of the existing example is 15,784 cycles, relative to the stress amplitude number of 11,250 cycles used as a criterion.

[0045] <Comparative Example 1 and Comparative Example 3 in Table 1>

[0046] In Comparative Example 1 (second column from the left) and Comparative Example 3 (fourth column) of Table 1, the stored fuel is HCNG (a mixture of natural gas and hydrogen), with a hydrogen content of 20%. The high-pressure tanks constituting Comparative Examples 1 and 3 are made of the same conventional material as existing examples, such as SCM435 (chromium-molybdenum steel) that has been quenched and tempered, with a tensile strength (TS) of 953 MPa. However, they do not possess the characteristics of the steel used in the high-pressure tank 100 of Embodiment 1. In this comparative example, the high-pressure tank is affected by the hydrogen contained in the internally stored fuel, causing hydrogen embrittlement of the steel and a reduced lifespan. The high-pressure tank of Comparative Example 1 is the same as the high-pressure tank of the existing examples described above, with a wall thickness t = 6.4 mm and a calculated circumferential stress σ = 592 MPa. However, the lifespan of the high-pressure tank of Comparative Example 1 is only about 1200 to 1300 cycles, which cannot guarantee its lifespan as a vehicle-mounted high-pressure tank. Furthermore, as shown in Comparative Example 3, even with a wall thickness t = 10 mm, the lifespan cannot be guaranteed using conventional materials.

[0047] <Comparative Example 2 in Table 1>

[0048] In Comparative Example 2, shown in the third column from the left in Table 1, the fuel stored is HCNG (a mixture of natural gas and hydrogen), the same as in Comparative Example 1. The high-pressure tank of Comparative Example 2 is made of a conventional material, similar to the existing examples, Comparative Example 1, and Comparative Example 3, such as SCM435 (chromium-molybdenum steel) that has been quenched and tempered, with a tensile strength (TS) of 953 MPa. However, it does not possess the characteristics of the steel used in the high-pressure tank 100 of Embodiment 1. In the high-pressure tank of Comparative Example 2, the steel undergoes hydrogen embrittlement due to the hydrogen contained in the fuel stored inside, resulting in a reduced lifespan. However, the wall thickness of the high-pressure tank of Comparative Example 2 is set to t = 13 mm, and the circumferential stress is calculated to be as low as σ = 279 MPa. Therefore, despite the effects of hydrogen embrittlement, the high-pressure tank of Comparative Example 2 has a lifespan exceeding the 11,250 cycles used as a criterion, ensuring its lifespan as a vehicle-mounted HCNG high-pressure tank.

[0049] However, while the high-pressure tank of Comparative Example 2 achieved the benchmark lifespan, its wall thickness t was thicker, and its weight of 113 kgf was twice that of the existing examples. Furthermore, it could store less fuel than the high-pressure tanks of the existing examples, Comparative Example 1, and Comparative Example 3. When using a high-pressure tank as a vehicle fuel tank, the increased weight of the tank body 10 would lead to an increase in vehicle weight, which needs to be minimized as much as possible. For example, when mounting a heavier high-pressure tank 100 like Comparative Example 2 on a vehicle, measures such as reducing the number of passengers are usually necessary. Therefore, currently used high-pressure tanks for storing CNG for vehicles do not have thick walls. Additionally, if conventional materials were to be used directly in high-pressure tanks for storing HCNG for vehicles, a wall thickness t at least the same as that of Comparative Example 2 would be required.

[0050] <Comparative Example 4 in Table 1>

[0051] Comparative Example 4, shown in the fifth column from the left in Table 1, can reduce the stress generated by decreasing the outer diameter Do of the tank, thus reducing the wall thickness. Compared to Comparative Example 2, the wall thickness can be reduced by approximately 3 mm in Comparative Example 4. On the other hand, if only the outer diameter Do is reduced, the tank capacity will decrease, so the tank length needs to be extended to ensure the tank capacity. In Comparative Example 4, to ensure a capacity of 57 liters equivalent to existing tanks (existing examples and Comparative Example 1 in Table 1), the estimated length is 1595 mm, and the weight reaches 113 kg. Therefore, as a tank for storing HCNG, while reducing the outer diameter Do of the high-pressure tank can meet the lifespan requirement, it cannot achieve weight reduction.

[0052] <Examples of Inventions in Table 1>

[0053] The sixth column from the left in Table 1 shows Invention Example 1, which is the same as Comparative Examples 1, 2, 3, and 4, storing HCNG (a mixture of natural gas and hydrogen) as fuel. The material of the high-pressure tank constituting Invention Example 1 differs from that of the existing examples and Comparative Examples 1-4; it uses a new type of steel used in the tank body 10 of the high-pressure tank 100 of Embodiment 1. This new type of steel is equivalent to the steel used in the tank body 10 of Embodiment 1, as described later, such as the steel Nos. 1-18 listed in Table 3 as examples of the invention. In the high-pressure tank of Invention Example 1, because a new material is used for the steel, the influence of hydrogen contained in the internally stored fuel is suppressed, hydrogen embrittlement of the steel is suppressed, and the lifespan is extended compared to high-pressure tanks using conventional materials. In conventional materials, as shown in Comparative Example 3, even a wall thickness of t=10mm cannot guarantee the service life. However, as shown in Comparative Example 2, the wall thickness is increased to t=13mm, or as shown in Comparative Example 4, the wall thickness is increased to t=10.1mm and the outer diameter Do of the tank is reduced to 242mm, thus ensuring a sufficient service life. In contrast, in the high-pressure tank of Invention Example 1, with a wall thickness of t=8mm and a calculated circumferential stress of σ=445MPa, the service life exceeds the 11250 cycles used as the criterion. Even with a thinner wall thickness than conventional materials, the service life as a vehicle-mounted HCNG high-pressure tank is guaranteed. Furthermore, Invention Examples 2, 3, and 4 also utilize new materials and, like Invention Example 1, can ensure a sufficient service life as a vehicle-mounted HCNG high-pressure tank. Additionally, although Invention Example 4 increases the tank size, the outer diameter Do can be reduced, allowing the orientation of the tank on the vehicle to change from lateral to longitudinal, thus expanding the design freedom of the mounting configuration.

[0054] In Invention Example 1, strength and durability are ensured even when storing HCNG as described above, and although the weight is increased compared to existing examples, it is about 40 kgf lighter than that of Comparative Example 2 which uses conventional materials. In other words, the high-pressure tank 100 using the new materials as in Invention Example 1 can meet the practical requirements of being a vehicle-mounted HCNG high-pressure tank.

[0055] <The steel used in the tank body 10 of Embodiment 1>

[0056] As described above, by using the high-pressure tank 100 of Embodiment 1 as an on-board high-pressure tank for storing HCNG, sufficient lifespan can be ensured while storing hydrogen. The reason why sufficient lifespan can be ensured as an on-board HCNG high-pressure tank is mainly due to the characteristics of the steel used in the tank body 10 constituting the high-pressure tank 100, which will be described below. The characteristics of the steel used in the tank body 10 and the mechanism for suppressing its hydrogen embrittlement effect will be explained below.

[0057] The can body 10 of Embodiment 1 suppresses the effects of hydrogen embrittlement by dispersing carbides within the metal structure, thereby retaining hydrogen that has penetrated into the metal structure within the grains of the metal structure. Specifically, by containing a predetermined amount of molybdenum in the steel composition, the carbides present in the metal structure are ideally dispersed as precipitates at an ideal size. Therefore, hydrogen that has penetrated into the metal structure is captured by the molybdenum precipitates within the grains of the metal structure, and the effects of hydrogen embrittlement are suppressed. Furthermore, by containing a predetermined amount of molybdenum in the steel composition, molybdenum precipitates are dispersed in the metal structure and capture hydrogen. As in the aforementioned conventional and comparative examples, if steel with inappropriately distributed carbides within the metal structure is used in the can body 10, as in the prior art and comparative examples described above, hydrogen will accumulate at the grain boundaries of the metal structure, and the steel will fracture at the grain boundaries. It should be noted that, in addition to Mo2C (molybdenum carbide), other effective hydrogen trapping carbides within the metal structure include TiC (titanium carbide), VC (vanadium carbide), and NbC (niobium carbide). Carbides precipitated in the metal structure are effective as hydrogen traps if they are less than 100 nm in size, but less than 50 nm is preferred.

[0058] Figure 3 This is a schematic diagram illustrating the effects of hydrogen on traditional and new materials. Hydrogen atoms infiltrating the metal structure of the steel used in the tank body 10 move within the metal structure along with dislocations in the crystal. The metal structure of the steel contains tiny grains, and under high stress, fracture easily occurs at the boundaries between these grains. Furthermore, when hydrogen atoms accumulate at these grain boundaries, grain boundary fracture occurs due to the reduced cohesive force between hydrogen atoms, leading to a decrease in the fatigue strength of the steel.

[0059] Figure 3Figure (a) illustrates the movement of hydrogen within grains in conventional materials where carbides are not properly distributed within the grains. In conventional materials, hydrogen atoms move within the grains along with dislocations and accumulate at grain boundaries. Therefore, grain boundaries are prone to fracture due to the influence of hydrogen atoms.

[0060] Figure 3 Figure (b) illustrates the movement of hydrogen within grains in a novel material with appropriately placed carbides. In this novel material, although hydrogen atoms move within grains along with dislocations, they are captured by the carbides within the grains, suppressing hydrogen atom accumulation at grain boundaries. Therefore, in this novel material, grain boundaries are less affected by hydrogen atoms, and grain boundary fracture is suppressed compared to conventional materials.

[0061] Figure 4 This is an explanatory diagram schematically illustrating the relationship between stress and crack propagation rate in the steel constituting the tank body 10 of Embodiment 1 and conventional steel. In the case of a high-pressure tank using conventional materials, such as... Figure 4 As shown by solid line A, within a certain stress range, quasi-cleavage (QC) fracture mainly occurs within the grains (the horizontal portion of solid line A), while above a certain stress, grain boundary (IG) fracture mainly occurs (the inclined portion of solid line A). This also applies to the steel constituting the tank body 10 of Embodiment 1, but as... Figure 4 As shown by the dashed line B, since carbides are disposed within the grains of the steel constituting the tank body 10 of Embodiment 1, the overall crack propagation rate is reduced. Furthermore, in the steel constituting the tank body 10 of Embodiment 1, since hydrogen atoms are trapped within the grains and grain boundary fracture is less likely to occur, the region where quasi-cleavage (QC) fracture is the main fracture mode expands, and the stress dominated by grain boundary fracture increases.

[0062] [Steel Structure]

[0063] The steel used for the tank body 10 has a microstructure dominated by tempered martensite, in which molybdenum precipitates are present. These molybdenum precipitates include those with a diameter of 50 nm or less. The steel used for the tank body 10 is, for example, molybdenum steel or martensitic stainless steel as described in the above-described example. Conventional high-pressure tanks for storing hydrogen-containing mixed gases include those that use austenitic stainless steel to suppress the effects of hydrogen embrittlement; however, in the tank body 10 of Embodiment 1, hydrogen embrittlement is suppressed while primarily using low-alloy steel.

[0064] Main phase: Tempered martensitic phase

[0065] In the steel used for the tank body 10, a martensitic phase-dominant microstructure is adopted to ensure a high tensile strength (TS: 850 MPa or higher). However, to maintain the ductility and toughness required for the structure, tempered martensite obtained by tempering the martensitic phase is used as the main phase. Here, "main phase" refers to a single phase comprising 100% of the phase by area, or a second phase comprising 95% or more of the phase without affecting the properties, with an area percentage of less than 5%. It should be noted that examples of the second phase include bainite, retained austenite, pearlite, or mixtures thereof.

[0066] The aforementioned microstructure in the tank body 10 can be adjusted by appropriately selecting the heating temperature during quenching and the cooling rate during cooling, which correspond to the steel composition. The tensile strength of the material is adjusted to 850 MPa or more, particularly to 850 MPa or more and less than 1150 MPa, by adjusting the quenching and tempering conditions. It is further preferred that the tensile strength TS is adjusted to 850 MPa to 1000 MPa. Since the tank body 10 is filled with a hydrogen-containing mixed gas under high pressure, it is preferable that the steel used has a high tensile strength TS of 850 MPa or more. However, on the other hand, if the tensile strength TS is too high, the brittleness increases and the fatigue strength decreases. Therefore, it is necessary to suppress the tensile strength TS to a certain range. Therefore, in the case of the steel used in the tank body 10 of Embodiment 1, the tensile strength TS is at most less than 1150 MPa, more preferably 850 MPa to 1000 MPa, and it is preferable to treat it to meet the fatigue strength, including the effect of hydrogen.

[0067] When the grain size of the original austenite grains is less than 8.5, the substructure of the resulting martensite phase becomes coarser, leading to a decrease in toughness. Therefore, the grain size of the original austenite grains is preferably 8.5 or higher. It should be noted that the grain size should be the value determined according to JIS G0551.

[0068] The grain size of the original austenite grains can be adjusted by changing the heating rate, heating temperature, holding temperature, and the number of quenching processes.

[0069] Furthermore, in order to improve the resistance to hydrogen embrittlement, the concentration of molybdenum precipitates in the steel used for the tank body 10 is adjusted to an appropriate range according to their size. It should be noted that the molybdenum precipitates are identified by membrane filtration, for example, by extraction, using the technique described in Japanese Patent Application Publication No. 2010-127791 and the technique described in "Ishida et al., Analysis of the Formation State of Fine Precipitates in Steel, Iron and Steel Vol. 107 No. 08". To measure the Mo concentration of the precipitates, a 10 mm square sample taken from a section perpendicular to the rolling direction of the steel pipe (section perpendicular to the pipe axis: section C) is electrolyzed in an electrolyte solution. The precipitates, adhering to the surface of the steel sheet, are then placed in a dispersible aqueous solution and irradiated with ultrasound. The precipitates are then extracted in the aqueous solution. The aqueous solution containing the extracted precipitates is filtered through a filter membrane to separate the precipitates by size. The precipitates of different sizes are then dissolved in a solution, and the Mo concentration is analyzed using ICP (Inductively Coupled Plasma) to calculate the Mo content in each size of precipitate. In ICP-based concentration analysis, the solution can be introduced into plasma to excite the characteristic spectrum of the element. The elemental concentration in the solution is determined based on the intensity of the light emitted, thus allowing the calculation of the Mo concentration (mass%) in the precipitates. This method allows the calculation of the overall Mo content in the precipitates, and the proportion (mass%) of Mo in the precipitates can be determined from this value and the Mo content in the steel. Furthermore, according to the technique described in Japanese Patent Application Publication No. 2009-031269, the Mo solid solution concentration in the steel is obtained by analyzing the electrolyzed solution using ICP. Additionally, the Mo content in the precipitates remaining on the filter membrane is analyzed using ICP, focusing on the Mo content in precipitates larger than 50 nm. The proportion (mass%) of Mo in precipitates with a diameter of less than 50 nm can be determined by subtracting the Mo content in precipitates larger than 50 nm from the total Mo content in the precipitates.

[0070] More than 50% of the Mo contained in steel is contained in precipitates.

[0071] Including Mo in the composition of steel as precipitates improves its properties in a hydrogen environment. However, even increasing the amount of Mo added to the steel does not yield the desired effect under solid solution conditions. On the other hand, the greater the amount of molybdenum precipitates, the higher the hydrogen trapping capacity, which is significantly improved by including more than 50% of the Mo contained in the steel in the precipitates. It should be noted that preferably, more than 60% of the Mo contained in the steel is present in the precipitates. However, the Mo contained in the precipitates can be less than 50% of the Mo contained in the steel.

[0072] More than 50% of the Mo contained in the precipitate is contained in precipitates with a diameter of 50 nm or less. Molybdenum precipitates prevent hydrogen accumulation at grain boundaries by capturing hydrogen in the steel, thereby increasing grain boundary strength in a hydrogen environment. However, when their size is greater than 50 nm, the hydrogen capture capacity decreases, and the effect on increasing grain boundary strength diminishes. Therefore, it is necessary to contain a significant amount of Mo in precipitates with a diameter of 50 nm or less. Here, the higher the amount of Mo in the precipitate, the higher the hydrogen capture capacity, which is significantly improved by ensuring that the Mo contained in the fine precipitates with a diameter of 50 nm or less accounts for more than 50% of the total Mo in the precipitate. Therefore, in the steel used for the tank body 10, it is preferable that the Mo contained in the fine precipitates with a diameter of 50 nm or less is more than 50% of the total Mo in the precipitate. More preferably, the Mo contained in the fine precipitates with a diameter of 50 nm or less accounts for more than 60% of the total Mo in the precipitate. Furthermore, since the smaller the size of the molybdenum precipitate, the better the hydrogen capture capacity, it is more preferable that the diameter of the precipitate is 20 nm or less. It should be noted that since the coarsening of molybdenum precipitates is due to the aggregation and coalescence of fine molybdenum precipitates, the coarsening of molybdenum precipitates leads to a reduction in the number of fine molybdenum precipitates.

[0073] For the steel used in the tank body 10, in addition to the precipitation of molybdenum precipitates, reducing nitride and oxide inclusions, which may become fracture initiation points, is also important for improving resistance to hydrogen embrittlement. Management of the steel refining process is particularly crucial. After desulfurization and dephosphorization in the hot metal pretreatment, and decarburization and dephosphorization in the converter, the ladle undergoes heated stirring refining (LF) and RH vacuum degassing. Furthermore, the processing time of heated stirring refining (LF) and RH vacuum degassing must be adequately ensured, and the RH reflux rate must be managed. Additionally, when producing the castings (steel billets) using continuous casting, to reduce nitride and oxide inclusions, inactive gas is used for sealing during the pouring from the ladle to the tundish. Furthermore, electromagnetic stirring is implemented in the crystallizer to achieve the flotation and separation of inclusions. It should be noted that the steel refining process for the steel used in the tank body 10 is not limited to the above, but management of the refining process is still important in this case.

[0074] [composition]

[0075] First, the composition of the steel used in the tank body 10 will be explained. Hereinafter, the mass percentage in the composition will be abbreviated as "%".

[0076] C: 0.20~0.50%

[0077] Carbon (C) solution helps increase the strength of steel and improves its hardenability, facilitating the formation of a martensitic microstructure during quenching. To achieve this effect, the C content needs to be 0.20% or higher. On the other hand, if the C content exceeds 0.50%, cracking will occur during quenching, significantly reducing manufacturability. Therefore, the C content is limited to the range of 0.20% to 0.50%. It should be noted that the C content is preferably 0.20% to 0.40%, and more preferably 0.22% to 0.35%.

[0078] Si: 0.05~2.00%

[0079] Si is added for deoxidation, but the deoxidation effect is insufficient when the Si content is less than 0.05%. Therefore, the Si content is limited to 0.05% or more. On the other hand, the effect saturates when the Si content is greater than 2.00%, so the Si content is limited to 2.00% or less.

[0080] Mn: 0.30~1.5%

[0081] Like carbon (C), manganese (Mn) is an element that improves the hardenability of steel and helps increase its strength. To achieve this effect, the Mn content needs to be 0.30% or higher. On the other hand, Mn is an element that segregates in steel and locally hardens it. In cases with a high Mn content, localized hardening zones can form, leading to a decrease in resistance to hydrogen embrittlement. Therefore, in the steel used for the tank body 10, the Mn content is limited to the range of 0.30% to 1.5%. It should be noted that the Mn content is preferably 0.4% to 0.8%, and more preferably 0.5% to 0.8%.

[0082] P: below 0.015%

[0083] Phosphorus (P) is an element that not only segregates at grain boundaries in steel, causing grain boundary embrittlement, but also locally hardens the steel due to segregation. In the steel used for the tank body 10, P is an unavoidable impurity and should preferably be minimized, ideally to below 0.015%. Therefore, the P content is limited to 0.015% or less. It should be noted that the P content is preferably 0.008% or less. Lower P content is better, but from the viewpoint of refining cost, the P content is preferably 0.0001% or more.

[0084] S: below 0.005%

[0085] Sulfur (S), as an unavoidable impurity, exists primarily in steel as sulfide inclusions, reducing ductility, toughness, and resistance to sulfur oxides (SSCs). Therefore, it is preferable to minimize its content, which is permissible to be below 0.005%. Thus, the S content is limited to below 0.005%. It should be noted that the S content is preferably below 0.003%. While lower S content is better, from the viewpoint of refining cost, the S content is preferably above 0.0002%.

[0086] Al: 0.005~0.15%

[0087] Al is added as a deoxidizer, but it has no effect when the content is less than 0.005%. Therefore, the Al content is limited to 0.005% or more. On the other hand, if the Al content is greater than 0.15%, the cleanliness of the steel decreases and the toughness deteriorates. Therefore, the Al content is limited to 0.15% or less.

[0088] N: below 0.006%

[0089] Nitrogen (N) is an unavoidable impurity in steel. It combines with Al to form AlN, and in the presence of Ti, it forms TiN, which helps to refine the grain size and improve toughness. However, when the N content is greater than 0.006%, the formed nitrides become coarser, significantly reducing toughness. Therefore, the N content is limited to 0.006% or less.

[0090] Cr: greater than 0.2% and less than 1.7%

[0091] Cr is an element that increases the strength of steel by improving hardenability and also enhances its corrosion resistance. Additionally, Cr combines with C during tempering to form M3C, M7C3, and M... 23 C6 (where M is a metallic element) and other precipitates are essential elements for improving resistance to tempering softening, especially when increasing the strength of steel pipes. M3C-type precipitates, in particular, have a strong effect on improving resistance to tempering softening. To achieve this effect, the Cr content needs to exceed 0.2%. On the other hand, if the Cr content exceeds 1.7%, a large amount of M7C3 and M... 23 C6 acts as a hydrogen trapping site, thus reducing resistance to hydrogen embrittlement. Furthermore, a higher Cr content leads to coarsening of molybdenum precipitates. Since the fine molybdenum precipitates coarsen due to aggregation and coalescence, reducing the number density of these fine precipitates results in decreased resistance to hydrogen embrittlement. Therefore, the Cr content is limited to a range of greater than 0.2% and less than 1.7%. The Cr content is preferably greater than 0.2% and less than 1.0%. More preferably, the Cr content is greater than 0.2% and less than 0.4%.

[0092] Mo: greater than 1.0% and less than 3.0%

[0093] Mo is an element that forms precipitates and strengthens steel through precipitation strengthening, effectively ensuring the desired high strength by reducing dislocation density through tempering. Furthermore, Mo dissolved in steel and segregated at the original austenite grain boundaries helps improve resistance to hydrogen embrittlement. In addition, Mo densifies corrosion products, thereby inhibiting the formation and growth of pitting corrosion, which becomes the initiation point for cracking. To achieve this effect, the Mo content needs to exceed 1.0%. On the other hand, when the Mo content is greater than 3.0%, it promotes the formation of acicular M2C precipitates and, depending on the situation, the Laves phase (Fe2Mo), thereby reducing resistance to hydrogen embrittlement. Therefore, the Mo content is limited to a range greater than 1.0% and less than 3.0%. It should be noted that the Mo content is preferably greater than 1.1% and less than 3.0%, more preferably greater than 1.2% and less than 2.8%, and even more preferably 1.45% to 2.5%. Even more preferably 1.45% to 1.80%. However, even if the Mo content is in any range between 1.0% and 3.0%, it has the effect of dispersing fine hydrogen-capturing precipitates throughout the metal structure.

[0094] Nb: 0.001~0.02%

[0095] Nitrogen b (Nb) forms precipitates or carbonitrides, which help increase the strength of steel through precipitation strengthening and also contribute to the refinement of austenite grains. To achieve this effect, the Nb content needs to be 0.001% or more. On the other hand, since large Nb precipitates easily become crack initiation sites for hydrogen-induced cracking, the presence of a large amount of Nb precipitates (greater than 0.02%) in high-strength steels leads to a significant reduction in hydrogen embrittlement resistance. Therefore, from the viewpoint of achieving both the desired high strength and excellent hydrogen embrittlement resistance, the Nb content is limited to 0.001% to 0.02% in this disclosure. It should be noted that the Nb content is preferably 0.001% or more and less than 0.01%.

[0096] B: 0.0003~0.0030%

[0097] Boron (B) improves the hardenability of steel even in trace amounts by segregating at austenite grain boundaries and suppressing ferrite phase transformation from the grain boundaries. To achieve this effect, the B content needs to be 0.0003% or higher. On the other hand, when the B content exceeds 0.0030%, it precipitates as carbonitrides, reducing hardenability and consequently toughness. Therefore, the B content is limited to the range of 0.0003% to 0.0030%. It should be noted that the preferred B content is 0.0007% to 0.0025%.

[0098] O (Oxygen): below 0.0030%

[0099] Oxygen (O) is an unavoidable impurity in steel, existing as oxide inclusions. These inclusions can become crack initiation sites in a hydrogen environment, reducing resistance to hydrogen embrittlement. Therefore, in this disclosure, it is preferable to minimize O content as much as possible. However, excessive reduction leads to a sharp increase in refining costs, so an O content of 0.0030% or less is permissible. Therefore, the O content is limited to 0.0030% or less. It should be noted that the O content is preferably 0.0020% or less.

[0100] Ti: 0.003~0.025%

[0101] During the solidification of molten steel, Ti combines with N to precipitate as fine TiN, which contributes to the refinement of austenite grains through its pinning effect. To achieve this effect, a Ti content of 0.003% or more is required. The effect is minimal when the Ti content is less than 0.003%. On the other hand, when the Ti content is greater than 0.025%, the TiN becomes coarse, failing to exert the aforementioned pinning effect and instead leading to reduced toughness. Furthermore, the coarse TiN also reduces resistance to hydrogen embrittlement. Therefore, the Ti content is limited to the range of 0.003% to 0.025%.

[0102] Mo / C: 2.0–12.0

[0103] When the Mo / C ratio is less than 2.0, insufficient Mo leads to a reduced amount of molybdenum precipitates, thus preventing the formation of molybdenum precipitates sufficient to improve hydrogen embrittlement resistance. On the other hand, when the Mo / C ratio is greater than 12.0, the molybdenum precipitates tend to coarsen significantly, resulting in decreased toughness and hydrogen embrittlement resistance. Furthermore, the coarsening of molybdenum precipitates is also caused by the aggregation and coalescence of fine molybdenum precipitates, thus reducing the number density of fine molybdenum precipitates. Therefore, the Mo / C ratio is limited to the range of 2.0 to 12.0. It should be noted that the preferred range for Mo / C is 2.5 to 6.0.

[0104] The above-mentioned components are the basic components. In addition to the basic components, optional elements may be included, selected from one or more of the following: V: less than 0.3%, Cu: less than 1.0%, Ni: less than 2.0%, W: less than 3.0%, H: less than 0.0010%, or Ca: 0.0005% to 0.005%, and these elements may be combined arbitrarily.

[0105] Selected from one or more of the following: V: less than 0.3%, Cu: less than 1.0%, Ni: less than 2.0%, and W: less than 3.0%.

[0106] V, Cu, Ni, and W are all elements that help increase the strength of steel. You can choose to include one or more of them as needed.

[0107] V: Below 0.3%

[0108] V is an element that contributes to the strengthening of steel by forming precipitates and carbonitrides. The V content can be 0% or more, but to achieve the aforementioned effect, the V content is preferably 0.02% or more, more preferably 0.03% or more. On the other hand, even if the V content exceeds 0.3%, the effect will saturate, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when V is present, the V content is limited to 0.3% or less. It should be noted that the V content is preferably 0.20% or less, more preferably 0.15% or less.

[0109] Cu: below 1.0%

[0110] Cu is an effective element for improving toughness and strength, but excessive content can lead to deterioration of weldability. Therefore, when Cu is present, its content is limited to 1.0% or less. The Cu content can be 0% or more, but to achieve the desired improvement in toughness and strength, a content of 0.01% or more is preferred.

[0111] Ni: below 2.0%

[0112] Ni is an element that helps increase the strength, toughness, and corrosion resistance of steel. To achieve these effects, a Ni content of 0.03% or more is preferred. On the other hand, even if the Ni content exceeds 2.0%, the effect will saturate, and a result commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when Ni is present, the Ni content is limited to 2.0% or less.

[0113] W: Below 3.0%

[0114] W is an element that helps increase the strength of steel by forming precipitates and strengthening it through precipitation, while also contributing to improved resistance to hydrogen embrittlement through solid solution and segregation at the original austenite grain boundaries. To achieve this effect, a W content of 0.03% or more is preferred. On the other hand, when the W content exceeds 3.0%, the effects of increased steel strength and improved resistance to hydrogen embrittlement become saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when W is present, the W content is limited to 3.0% or less.

[0115] H: Below 0.0010%

[0116] Hydrogen (H) can sometimes be introduced into steel during various manufacturing processes. Higher concentrations increase the risk of post-solidification cracking and worsen resistance to hydrogen embrittlement. Therefore, it is important to reduce the hydrogen content in steel. These effects are not problematic if the H content is below 0.0010%, so the H content is preferably below 0.0010%. More preferably, it is below 0.0001%. A H content below 0.00001% increases costs, so the H content is preferably above 0.00001%. It should be noted that the hydrogen content refers to the residual hydrogen content after the plate, pipe, etc., are formed.

[0117] Ca: 0.0005%~0.005%

[0118] Ca (Ca) is an element that effectively controls the morphology of sulfide inclusions by combining with sulfur (S) to form CaS. This morphological control helps improve toughness and resistance to hydrogen embrittlement. To achieve this effect, the Ca content needs to be 0.0005% or higher. On the other hand, when the Ca content exceeds 0.005%, its effect saturates, and a commensurate effect cannot be expected, which is economically disadvantageous. Therefore, when Ca is present, the Ca content is limited to the range of 0.0005% to 0.005%.

[0119] The remaining portion, besides the components mentioned above, consists of Fe and unavoidable impurities. As unavoidable impurities, for example, Mg content of 0.0008% or less and Co content of 0.0008% or less are permissible.

[0120] [Manufacturing Method]

[0121] Next, the manufacturing method of the tank body 10 of the high-pressure tank 100 according to Embodiment 1 will be described.

[0122] The tank body 10 is formed by heating and hot-working the steel pipe billet as described above to process a seamless steel pipe into a specified shape. It should be noted that the seamless steel pipe for high-pressure hydrogen containers is preferably used in hydrogen containers with a hydrogen pressure of 1 MPa or higher, and more preferably 20 MPa or higher.

[0123] The steel billet used to manufacture the tank body 10 (hereinafter also referred to as steel billet) is preferably made by melting molten steel with the above composition using a commonly used smelting method such as a converter, and then producing a casting (circular casting) using a commonly used casting method such as continuous casting. Alternatively, the casting can be further hot-rolled into a round steel sheet of a specified shape, or produced into a round steel sheet through ingot-bill rolling.

[0124] In the following description, the manufacturing method is illustrated using the case where the tank body 10 is a seamless tank as an example. However, it is obvious that the tank body 10 can also be manufactured by welding after processing in the same way to form a thermal process. For example, a resistance welded pipe can be manufactured by rolling a steel plate at a temperature range of Ac3 phase transformation point to 1000°C, followed by quenching at least once to a surface temperature of 200°C or less, and then tempering the steel plate at a temperature range of 600 to 740°C after the quenching process. After tempering the steel plate at an average heating rate of 0.5°C / min or more until the tempering temperature is reached, and holding the tempering temperature at the reached tempering temperature for 10 minutes or more and less than 60 minutes, the same characteristics can be obtained.

[0125] The can body 10 can be manufactured by performing the following processes (1) to (3) in sequence.

[0126] (1) The process of casting after adjusting the composition of steel billet.

[0127] (2) The rolling process of heating and rolling casting materials to obtain steel pipes, and

[0128] (3) The process of cooling and tempering the steel pipe obtained in the rolling process.

[0129] The following describes each process. It should be noted that, unless otherwise specified, the temperatures mentioned below refer to the surface temperature of the steel billet.

[0130] [Casting process]

[0131] Casting speed: below 2.0 m / min

[0132] The slower the casting speed, the more the hydrogen concentration and inclusions in the steel are reduced. The effect is significant at speeds below 2.0 m / min, therefore, a casting speed of 1.5 m / min or less is preferred. A casting speed of 1.0 m / min or less is more preferred, and 0.5 m / min or less is even more preferred.

[0133] [Heating process]

[0134] For hot rolling, a steel billet having the above composition is heated. There are no particular limitations on the steel billet; for example, a square billet obtained by a conventional continuous casting method can be used.

[0135] Heating temperature: 1050~1350℃

[0136] When the heating temperature is below 1050°C, the dissolution of precipitates in the steel billet becomes insufficient. On the other hand, when the heating temperature exceeds 1350°C, the grains coarsen, and the precipitates such as TiN that form during solidification also coarsen, as does the cementite, thus reducing the toughness of the steel pipe. Furthermore, when heated to temperatures exceeding 1350°C, a thick oxide layer forms on the surface of the steel billet, leading to surface defects during rolling and increased energy loss, which is not preferable from an energy-saving perspective. Therefore, the heating temperature is limited to the range of 1050–1350°C. It should be noted that the preferred heating temperature is 1100–1300°C.

[0137] [Rolling process]

[0138] Next, the steel billet heated by the above heating process is rolled into a steel tube shape. During rolling, hot rolling including piercing rolling can be used, either the conventional Mannesmann-plug mill or the Mannesmann-mandrel mill.

[0139] After hot rolling, the resulting seamless steel pipe (tank body 10) is cooled to a surface temperature of 200°C or lower at a cooling rate of air cooling or higher.

[0140] Cooling treatment after hot rolling: Cooling rate: above air cooling; Cooling stop temperature: below 200℃

[0141] Within the range of steel composition constituting the tank body 10 described above, if cooling is performed at a rate of air cooling or higher after hot rolling, a microstructure dominated by the martensitic phase can be obtained. If air cooling is stopped when the surface temperature exceeds 200°C, the phase transformation may not be completely completed. Therefore, the cooling treatment after hot rolling is to cool to a surface temperature below 200°C at a rate of air cooling or higher. Here, "a cooling rate of air cooling or higher" means 0.1°C / s or higher. When the cooling rate is less than 0.1°C / s, the metal microstructure after cooling becomes inhomogeneous, and the metal microstructure after subsequent heat treatment will also become inhomogeneous.

[0142] [Forming of the main body 10 of the can]

[0143] It should be noted that the shape of the can body 10 can be formed during or after hot rolling. When the can body 10 is a seamless, one-piece molded product, it is formed by narrowing both ends of a tubular, rolled steel section. Furthermore, the necessary removal processes for the can body 10 can be performed after hot rolling.

[0144] [Heat treatment process]

[0145] After cooling at a rate higher than air cooling, tempering is performed. The tempering process involves heating to a temperature in the range of 600–740°C.

[0146] Tempering temperature: 600~740℃

[0147] Tempering is performed to reduce dislocation density, induce molybdenum precipitation, and improve toughness and resistance to hydrogen embrittlement. When the tempering temperature is below 600°C, the reduction of dislocations and the precipitation of molybdenum are insufficient, thus failing to ensure excellent resistance to hydrogen embrittlement. On the other hand, at temperatures exceeding 740°C, the microstructure softens significantly, making it impossible to ensure the desired high strength. Therefore, the tempering temperature is limited to the range of 600–740°C. It should be noted that the preferred tempering temperature is 640–710°C.

[0148] The average heating rate is above 0.5℃ / min until the tempering temperature is reached.

[0149] Molybdenum precipitates form during the tempering process, and their size gradually increases. Therefore, if the heating rate is too slow until the specified tempering temperature is reached, the precipitates become too large, failing to achieve the desired resistance to hydrogen embrittlement. Therefore, the average heating rate until the tempering temperature is reached should be 0.5°C / min or more, preferably 2.0°C / min or more. While no specific upper limit is specified, excessively rapid heating can lead to uneven temperature distribution and inhomogeneous material microstructure; therefore, a rate of 50°C / min or less is preferred.

[0150] The holding time at the tempering temperature is more than 10 minutes and less than 120 minutes.

[0151] Molybdenum precipitates most readily during the tempering hold period. Shorter holding times prevent complete analysis and fail to achieve the desired resistance to hydrogen embrittlement. The holding time at the tempering temperature should be 10 minutes or more. However, excessively long holding times at the tempering temperature result in excessively large precipitates; therefore, a holding time of less than 120 minutes is preferable. It should be noted that holding time increases energy costs, so a holding time of less than 60 minutes is preferable.

[0152] It should be noted that, in order to reliably ensure the desired properties, a cooling treatment with a cooling rate of air cooling or higher can be performed after hot rolling, followed by a quenching treatment of reheating or rapid cooling using water cooling or the like at least once, and then the above-mentioned tempering treatment can be performed.

[0153] Reheating temperature for quenching treatment: Ac3 phase transformation point ~1000℃

[0154] When quenching is performed, if the reheating temperature is below the Ac3 phase transformation point, the austenitic single-phase region will not be heated, thus failing to obtain a microstructure dominated by the martensitic phase. On the other hand, temperatures exceeding 1000℃ result in grain coarsening, reduced toughness, thicker and more easily peeled oxide scale, leading to surface defects in the steel plate. Furthermore, the excessive load on the heat treatment furnace poses an energy-saving problem. For these reasons, and also from an energy-saving perspective, the reheating temperature for quenching is limited to between the Ac3 phase transformation point and 1000℃. It should be noted that the reheating temperature is preferably below 950℃.

[0155] Furthermore, after reheating, a quenching process is performed. The quenching process is preferably carried out by water cooling at an average cooling rate of 2°C / s or higher, until the temperature at the center of the plate thickness reaches 400°C or lower. Additionally, the surface temperature is rapidly cooled to below 200°C through the quenching process. Preferably, the surface temperature is cooled to below 100°C. It should be noted that the quenching process can be repeated more than twice.

[0156] It should be noted that the Ac3 phase transition point is the value calculated using the following formula.

[0157] Ac3 phase transition point (°C) = 937 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 4.9Cr - 26.6Ni + 38.1Mo + 124.8V + 136.3Ti + 198Al + 3315B

[0158] (Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Al, and B are the contents (mass%) of each element.)

[0159] When calculating the Ac3 phase transition point, if the element described in the above formula is not present, its content is counted as zero%.

[0160] It should be noted that after quenching and tempering, correction of shape defects in the tank body 10 can be performed as needed, either while hot or cold. Alternatively, the tank body 10 can be formed after quenching and tempering.

[0161] Example

[0162] The following describes an embodiment of the steel constituting the tank body 10. The following description represents a preferred example of the steel constituting the tank body 10, and this disclosure is not limited to these embodiments. It should be noted that the following embodiments are not evaluated in a storage tank actually formed into the tank body 10, but rather by evaluating the characteristics of a seamless steel tube used to manufacture an actual steel structure. Regardless of the shape of the tank body 10 or the seamless steel tube, only the shape of the final product differs. Even with the shape of the tank body 10, the evaluation results of the steel grades illustrated in the remarks column of Table 3 below can be applied. Table 2 shows the composition of steels No. 1 to 14. Furthermore, Table 3 shows the respective tempering conditions, the size and amount of molybdenum precipitates, and the relative reduction of area (RRA) for Nos. 1 to 14.

[0163]

[0164]

[0165] Seamless steel pipes are obtained by casting billets with the compositions shown in steel grades No. 1 to 14 of Table 2 at a casting speed of 0.6 m / min, heating the billets to 1250°C, and expanding them. These seamless steel pipes correspond to the examples shown in steel grades No. 1 to 14 of Table 3. Alternatively, seamless steel pipes are obtained by casting billets with the composition shown in steel grade No. 5 of Table 2 at a casting speed of 1.8 m / min, heating the billets to 1250°C, and expanding them. These seamless steel pipes correspond to the examples shown in steel grades No. 15 to 17 of Table 3. The seamless steel pipes shown in steel grades No. 1 to 18 of Table 3 are manufactured by completing the expansion process at a temperature of 820°C or higher, and then cooling them at a cooling rate of air cooling or higher until the surface temperature reaches 200°C or lower. For the obtained seamless steel pipes, those with an Ac3 point below 950°C were heated and held at 950°C, while those with an Ac3 point above 950°C were heated and held at 1000°C. These were then water-cooled to below 200°C at a rate of 0.5°C / min, followed by tempering. The tempering process was carried out according to the heating rate and holding time listed in Table 3. It should be noted that the tempering temperature was adjusted to achieve a tensile strength in the range of 850–1150 MPa. The microstructure and mechanical properties of the obtained seamless steel pipes were evaluated. The evaluation method is as follows.

[0166] The evaluation of hydrogen embrittlement resistance is based on the relative reduction of area (RRA) of the specimen after a low strain rate tensile test in hydrogen. In the atmosphere, steel undergoes plastic deformation, resulting in a smaller cross-sectional area and thus a larger reduction of area φair. On the other hand, in hydrogen, due to the reduced elongation of the steel, the material fractures before shrinkage, maintaining a larger cross-sectional area. Therefore, the reduction of area φH of the cross-section after the hydrogen test differs from that in the atmosphere, becoming smaller. This decrease in reduction of area is used to evaluate hydrogen embrittlement resistance. It should be noted that the relative reduction of area (RRA) is obtained by the following formula.

[0167] Relative reduction of area (RRA) = φH / φair × 100

[0168] The relative reduction of area (RRA) obtained from low strain rate tensile tests (tensile speed 0.002 mm / s) at room temperature under hydrogen pressure of 105 MPa is shown in Table 3. The larger the RRA, the better the resistance to hydrogen embrittlement. In this evaluation, RRA above 60% is considered good.

[0169] Furthermore, the method for measuring molybdenum precipitates in steel is described below. The identification of molybdenum precipitates is performed by an extraction method involving electrolysis of the steel and filtration of the resulting precipitates through a filter membrane. A 10 mm square sample is taken from a section perpendicular to the rolling direction of the seamless steel pipe (section perpendicular to the pipe axis: section C). The steel sheet is dissolved using a 10% AA-based electrolyte via constant current electrolysis, immersed in a 0.05 wt% sodium hexametaphosphate aqueous solution, and irradiated with ultrasound. The precipitate is then removed. Precipitates smaller than 50 nm are obtained by filtering the solution through a 50 nm filter membrane. Precipitates smaller than 50 nm that pass through the filter membrane and those larger than 50 nm that remain on the filter membrane are dissolved in hydrochloric acid after heating to produce white fumes using sulfuric acid, perchloric acid, and nitric acid. Then, the concentration of Mo, the amount of Mo, and the concentration of dissolved Mo in each size of precipitate are calculated by ICP analysis of the concentration of the precipitate solution and the electrolyte containing the solid solution.

[0170] All the steel grades shown as examples of the invention in Table 3 meet the condition that the RRA in the low strain rate tensile test in hydrogen is 60% or more.

[0171] The configuration shown in the above embodiments is an example, and some parts of the configuration may be omitted or changed without departing from the main idea.

[0172] Alternatively, the high-pressure tank 100 described above may also include combinations of the features shown in Notes 1 to 13 below. Such combinations are shown below.

[0173] [Note 1]

[0174] A high-pressure tank stores a mixture of natural gas and hydrogen, with the mixture containing more than 2% hydrogen by volume, inside the tank body.

[0175] The tank body comprises:

[0176] The cylindrical portion extending in the first direction, and

[0177] The dome-shaped tops connecting the two ends of the cylindrical portion in the first direction.

[0178] The steel constituting the main body of the tank has a tensile strength TS of 850 MPa or more, has a structure with tempered martensite of 95% or more in terms of area, and has carbides smaller than 100 nm dispersed in the metal structure as precipitates. It also has a strength of more than 11,250 cycles in crack propagation analysis.

[0179] [Note 2]

[0180] According to the high-pressure tank described in Note 1, when the maximum tangential stress generated in the cylindrical portion of the tank body when storing a hydrogen-containing mixed gas with p = 25 MPa inside is set to σ = Di·p / (2t), the condition 255 MPa < σ ≤ 445 MPa is satisfied.

[0181] [Note 3]

[0182] According to the high-pressure tank described in Note 1 or 2, the wall thickness t of the cylindrical portion satisfies: 8mm≤t.

[0183] [Note 4]

[0184] The high-pressure tank according to any one of notes 1 to 3, wherein the cylindrical portion and the dome are integrally formed.

[0185] [Note 5]

[0186] According to any one of Notes 1 to 4, the high-pressure vessel contains molybdenum precipitates, wherein the precipitates comprise at least 50% of the Mo contained in the steel.

[0187] [Note 6]

[0188] According to the high-pressure vessel described in Note 5, more than 50% of the Mo contained in the molybdenum precipitate is contained in precipitates with a diameter of less than 50 nm.

[0189] [Note 7]

[0190] According to the high-pressure tank described in Note 5 or 6, the steel constituting the main body of the tank contains Mo: greater than 1.0% and less than 3.0% by mass.

[0191] [Note 8]

[0192] According to any one of Notes 5 to 7, the high-pressure tank, wherein the steel constituting the main body of the tank is contained, by mass%, in a Mo / C ratio ranging from 2.0 to 12.0.

[0193] [Note 9]

[0194] According to the high-pressure tank described in Note 8, the steel constituting the main body of the tank contains C: 0.20 to 0.50% by mass.

[0195] [Note 10]

[0196] According to the high-pressure tank described in Note 9, the steel constituting the main body of the tank has the following composition by mass%: Si: 0.05-2.00%, Mn: 0.30-1.5%, P: less than 0.015%, S: less than 0.005%, Al: 0.005-0.15%, N: less than 0.006%, Cr: greater than 0.2% and less than 1.7%, Nb: 0.001-0.02%, B: 0.0003-0.0030%, O: less than 0.0030%, Ti: 0.003-0.025%, with the remainder consisting of Fe and unavoidable impurities.

[0197] [Note 11]

[0198] According to the high-pressure tank described in Note 10, the steel constituting the main body of the tank, in addition to the aforementioned composition, further contains, by mass percent, one or more of the following: V: less than 0.3%, Cu: less than 1.0%, Ni: less than 2.0%, and W: less than 3.0%.

[0199] [Note 12]

[0200] According to the high-pressure tank described in Note 10 or 11, the steel constituting the tank body, in addition to the aforementioned composition, further contains less than 0.0010% H by mass.

[0201] [Note 13]

[0202] According to any one of Notes 10 to 12, the high-pressure tank, in addition to the aforementioned composition, further contains, by mass %, 0.0005 to 0.005% Ca of the steel constituting the tank body.

[0203] Symbol Explanation

[0204] 10: Tank body

[0205] 11: Cylindrical section

[0206] 12, 12a, 12b: Dome

[0207] 13: Interface

[0208] 14: Supply device

[0209] 90: Space

[0210] 100: High-pressure tank

Claims

1. A high-pressure tank for storing a mixture of natural gas and hydrogen, wherein the mixture contains more than 2% hydrogen by volume, inside the tank body. The tank body comprises: The cylindrical portion extending in the first direction, and The dome-shaped tops connected to both ends of the cylindrical portion in the first direction. The steel constituting the main body of the tank has a tensile strength TS of 850 MPa or more, has a structure with tempered martensite of 95% or more in terms of area, and has carbides smaller than 100 nm dispersed in the metal structure as precipitates. It also has a strength of more than 11,250 cycles in crack propagation analysis.

2. The high-pressure tank according to claim 1, wherein, When the maximum tangential stress generated in the cylindrical part of the tank body when storing a hydrogen-containing mixed gas with p = 25 MPa inside is set to σ = Di·p / (2t), the condition 255 MPa < σ ≤ 445 MPa is satisfied.

3. The high-pressure tank according to claim 1 or 2, wherein, The wall thickness t of the cylindrical section satisfies 8mm≤t.

4. The high-pressure tank according to any one of claims 1 to 3, wherein, The cylindrical section is integrally formed with the dome.

5. The high-pressure tank according to any one of claims 1 to 4, wherein, The precipitated carbides include molybdenum precipitates, which are present in the steel as more than 50% of the Mo contained in the steel.

6. The high-pressure tank according to claim 5, wherein, More than 50% of the Mo contained in the molybdenum precipitate is contained in precipitates with a diameter of less than 50 nm.

7. The high-pressure tank according to claim 5 or 6, wherein, The steel constituting the main body of the tank contains Mo at a concentration greater than 1.0% and less than 3.0% by mass.

8. The high-pressure tank according to any one of claims 5 to 7, wherein, The steel constituting the main body of the tank contains, by mass%, a Mo / C ratio in the range of 2.0 to 12.

0.

9. The high-pressure tank according to claim 8, wherein, The steel constituting the main body of the tank contains 0.20 to 0.50% C by mass.

10. The high-pressure tank according to claim 9, wherein, The steel constituting the main body of the tank has the following composition by mass%: Si: 0.05-2.00%, Mn: 0.30-1.5%, P: less than 0.015%, S: less than 0.005%, Al: 0.005-0.15%, N: less than 0.006%, Cr: greater than 0.2% and less than 1.7%, Nb: 0.001-0.02%, B: 0.0003-0.0030%, O: less than 0.0030%, Ti: 0.003-0.025%, with the remainder consisting of Fe and unavoidable impurities.

11. The high-pressure tank according to claim 10, wherein, In addition to the aforementioned components, the steel constituting the main body of the tank further contains, by mass percent, a component selected from V: One or more of the following: less than 0.3%, Cu: less than 1.0%, Ni: less than 2.0%, and W: less than 3.0%.

12. The high-pressure tank according to claim 10 or 11, wherein, In addition to the aforementioned composition, the steel constituting the main body of the tank further contains less than 0.0010% H by mass.

13. The high-pressure tank according to any one of claims 10 to 12, wherein, In addition to the aforementioned composition, the steel constituting the main body of the tank further contains Ca by weight of 0.0005 to 0.005%.

Images