High-pressure tank

Abstract

This high-pressure tank stores, inside a tank body, a mixed gas that contains natural gas and hydrogen, and that contains more than 2% hydrogen by volume. The tank body includes: a cylindrical part extending in a first direction; and a dome part connected to both ends of the cylindrical part in the first direction. The steel material constituting the tank body has a tensile strength TS of 850 MPa or higher, has a structure in which 95% or more by area ratio is tempered martensite, and includes carbides of 100 nm or less scattered in the metal structure as precipitates. The high-pressure tank has a strength of 11250 cycles or more in crack progress analysis, suppresses the influence of hydrogen embrittlement in metal, ensures gas storage capacity, and has a reduced weight.

Description

High-pressure tank

[0001] The present disclosure relates to a high-pressure tank for storing high-pressure hydrogen.

[0002] The SDGs have been proposed as a measure against global warming. As part of this, hydrogen and e-methane (synthetic methane, recovered CO ２ Methane synthesized from hydrogen produced from renewable energy sources and other sources is expected to be an alternative energy source to fossil fuels.

[0003] Hydrogen engines, which burn hydrogen as fuel, emit carbon dioxide (CO ２ It is environmentally friendly in that it does not emit CO₂, and its structure can be achieved by improving on a conventional gasoline engine. However, while hydrogen gas used as fuel has the advantage of being highly flammable and having a fast combustion speed, it also ignites at unexpected times. As a result, hydrogen engines are prone to damage and have durability issues. The dominant factors behind this unstable ignition phenomenon of hydrogen gas have yet to be clarified.

[0004] Methane gas (CH ４ Methane gas is also used as an environmentally friendly fuel, and CNG (natural gas) vehicles are well known. However, methane gas ignites slowly and is prone to producing unburned methane, which has led to the generation of NOx, which has been an issue. Lean combustion technology for methane has been developed to combat NOx, but the narrow range of lean combustion for methane remains an issue.

[0005] To address the issues with hydrogen and methane gases mentioned above, attention has been focused on lean combustion technology for mixed gas (HCNG), which is a mixture of methane and hydrogen. While HCNG lean combustion technology was developed and demonstrated in the 2000s, development stalled due to insufficient hydrogen supply infrastructure, the high price of hydrogen gas, and insufficient NOx reduction effects. However, in recent years, efforts have been relaunched to develop operational technologies for hydrogen and hydrogen compounds.

[0006] Recent research has shown that adding hydrogen gas to methane gas broadens the lean combustion range of methane, improving fuel efficiency and lowering the combustion temperature, thereby reducing NOx emissions.Furthermore, it has been discovered that lean combustion technology can also be applied to bi-fuel engines that use gasoline and natural gas.

[0007] When hydrogen gas or natural gas is used as fuel in an automobile, a tank is required to store the gas at high pressure. There are three types of tanks for storing such high-pressure gas: Type 1, which consists of a metal container; Type 2, which has a hoop layer made of fiber-reinforced plastic (FRP) formed only on the cylindrical portion of a metal container; Type 3, which has an FRP helical layer formed on the entire metal container and an FRP hoop layer formed on the cylindrical portion; and Type 4, which has an FRP helical layer formed on the entire resin container and an FRP hoop layer formed on the cylindrical portion (see, for example, Patent Document 1).

[0008] Japanese Patent Application Laid-Open No. 2017-048912

[0009] Among the high-pressure tanks disclosed in Patent Document 1, except for Type 1, which is a metal container, all are constructed by wrapping fiber-reinforced plastic (FRP) around a metal container, resulting in complex manufacturing processes and high manufacturing costs. In particular, when the gas stored in the high-pressure tank contains hydrogen, the high-pressure tank is manufactured by wrapping resin-impregnated carbon-reinforced fiber (CFRP) around an aluminum alloy liner, which is not susceptible to hydrogen embrittlement. The high-pressure tank liner comprises a cylindrical portion and dome portions at both ends of the cylindrical portion. It is generally known that in a thin-walled tank with such a structure, when internal pressure acts on the cylindrical portion, the stress applied in the circumferential direction in the cylindrical portion is twice as large as that in the axial direction. Although high-pressure tanks typically have a large wall thickness, which results in a stress distribution in the wall thickness direction, the above-mentioned difference in stress applied between the circumferential and axial directions of the high-pressure tank liner occurs. Therefore, in order to provide strength that matches the above-mentioned stress ratio, it is necessary to adjust the fiber direction of the FRP wrapped around the dome portion and cylindrical portion in Types 2 to 4 high-pressure tanks. As a result, there was a problem with types 2 to 4 of high-pressure tanks in that the FRP wrapping process was costly.

[0010] On the other hand, the Type 1 high-pressure tank disclosed in Patent Document 1, which is a metal container, is less susceptible to hydrogen embrittlement when used in a mono-fuel natural gas engine because the hydrogen content of the stored gas is 2% or less, making it lightweight and cost-effective. However, as mentioned above, Type 1 high-pressure tanks for storing gas (HCNG) obtained by adding hydrogen gas to natural gas (CNG) use steel containers, which pose a problem of reduced durability due to hydrogen embrittlement. Furthermore, while it is possible to reduce the pressure during gas storage in Type 1 high-pressure tanks to reduce the impact of hydrogen embrittlement, it is unable to store the amount of gas required for a vehicle. On the other hand, if the pressure of the stored gas is increased to increase the gas storage capacity, the high-pressure tank must reduce the stress generated to reduce the impact of hydrogen embrittlement, which increases the wall thickness. In this case, the high-pressure tank increases in weight and volume, making it unsuitable for vehicle installation.

[0011] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a high-pressure tank that ensures a storage capacity for gases including hydrogen while reducing its weight.

[0012] The high-pressure tank according to the present disclosure is a high-pressure tank that stores a mixed gas containing natural gas and hydrogen, the mixed gas containing more than 2% hydrogen by volume, inside a tank body, wherein the tank body comprises a cylindrical portion extending in a first direction and a dome portion connected to both ends of the cylindrical portion in the first direction, and the steel material constituting the tank body has a tensile strength TS of 850 MPa or more, has a structure in which tempered martensite accounts for 95% or more by area ratio, has carbides of 100 nm or less scattered throughout the metal structure as precipitates, and has a strength of 11,250 cycles or more in a crack propagation analysis.

[0013] In the above-described high-pressure tank, when hydrogen penetrates the structure of the steel material constituting the tank body and moves within the crystal grains of the steel structure, it is trapped within the crystal grains by precipitates of 100 nm or less, preventing hydrogen from accumulating at the crystal grain boundaries of the steel structure. As a result, the steel material constituting the tank body is prevented from undergoing crystal grain boundary fracture of the steel structure, ensuring sufficient fatigue strength even when storing a fuel mixture of hydrogen and natural gas.

[0014] Fig. 1 is a cross-sectional view showing a high-pressure tank 100 according to embodiment 1. Fig. 2 is an explanatory diagram schematically showing the effect of hydrogen embrittlement on steel. Fig. 3 is an explanatory diagram schematically showing the effect of hydrogen on conventional materials and new materials. Fig. 4 is an explanatory diagram schematically showing the relationship between stress and crack growth rate generated in the steel constituting the tank body 10 according to embodiment 1 and in conventional steel materials.

[0015] Preferred embodiments of the high-pressure tank of the present disclosure will be described in detail below with reference to the drawings. Note that the embodiments described below are preferred specific examples of the present disclosure, and therefore various technically preferable limitations are applied, but the scope of the present disclosure is not limited to these embodiments unless otherwise specified in the following description to the effect that the present disclosure is limited.

[0016] Embodiment 1. <High-Pressure Tank 100> FIG. 1 is a cross-sectional view of a high-pressure tank 100 according to Embodiment 1. The high-pressure tank 100 shown in FIG. 1 is a schematic representation of its structure. The high-pressure tank 100 is mounted, for example, on a vehicle to store fuel for driving the engine. The fuel is a hydrogen-containing mixed gas, particularly a mixture of hydrogen and natural gas (HCNG). HCNG contains, for example, 20% hydrogen gas and the remainder natural gas. The high-pressure tank has a supply device 14, such as a valve, attached to a nozzle 13 at its end so that the stored mixed gas can be supplied to the engine. The supply device 14 delivers the mixed gas to the engine at a predetermined pressure. The high-pressure tank 100 can store a sufficient amount of mixed gas to ensure the practical driving range of the vehicle. The high-pressure tank 100 is not limited to being mounted on a vehicle but can also be used in various driving devices (such as industrial machinery and generators) that use hydrogen-containing mixed gas as fuel. This allows the high-pressure tank 100 to be used in a wide range of applications, not just in vehicles. The mixed gas stored in the high-pressure tank 100 according to the first embodiment has a higher hydrogen gas ratio than conventional CNG, and contains at least 2% hydrogen gas by volume, and may contain hydrogen gas at a volume ratio exceeding 4%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% by volume, or may contain hydrogen gas at a value between the values ​​given here.

[0017] <Structure of Tank Body 10> The tank body 10 includes a cylindrical portion 11 and dome portions 12 formed on both ends of the cylindrical portion 11. The cylindrical portion 11 is a tube with an inner diameter Di and is made of steel with a wall thickness t. The dome portions 12 are hollow, hemispherical portions connected to both ends of the cylindrical portion 11 and are made of steel with the same wall thickness t as the cylindrical portion 11. In FIG. 1, the dome portion 12 is hemispherical in shape, but this is not limited thereto. The dome portion 12 may have a dome shape whose outer surface is formed by combining a curved surface, such as a spherical surface, with a flat surface. Furthermore, the wall thickness t of the dome portion 12 does not have to be uniform. For example, the wall thickness t may be thicker in areas where stress is likely to concentrate, such as around the nozzle 13.

[0018] A nozzle 13 is provided on at least one of the dome portions 12 at both ends, allowing the mixed gas inside to be delivered to the outside of the tank body 10. In Fig. 1, the nozzle 13 is provided on the dome portion 12a on the X1 side. The nozzle 13 may be provided not only on the dome portion 12a but also on the dome portion 12b at the other end.

[0019] The nozzle 13 is a part to which the supply device 14 is attached, and may be provided with a female screw or with a sealing structure to prevent leakage of the mixed gas inside.

[0020] The steel material used for the tank body 10 is preferably one that has initial defects in the completed state that are determined by ultrasonic flaw detection to be 0.5 mm or less in depth and 1.5 mm or less in length. In addition, the tank body 10 is preferably made of steel material with polished inner surfaces that come into contact with the hydrogen-containing mixed gas.

[0021] <Required Strength of the High-Pressure Tank 100> The high-pressure tank 100 contains a mixed gas containing hydrogen sealed in its internal space 90. The rated maximum pressure of the mixed gas sealed in the high-pressure tank 100 is at least 20 MPa or higher, and may be, for example, 25 MPa, 35 MPa, or 70 MPa, or any pressure between 20 MPa and 70 MPa. The mixed gas in the high-pressure tank 100 decreases as the vehicle travels, and the internal pressure gradually decreases, for example, to a pressure of approximately 0.2 MPa. When the mixed gas pressure decreases, the high-pressure tank 100 is refilled with the mixed gas, and the internal pressure returns to a high state. The tank body 10 is repeatedly subjected to high and low internal pressure states (e.g., 0.2 MPa to 20 MPa), and high stress occurs in the high-pressure state and low stress occurs in the low-pressure state. In other words, the tank body 10 must be able to withstand the repeated stresses and their amplitudes.

[0022] Generally, the stress amplitude is the fatigue limit σ w If it is less than this, it will be effectively infinite repetition (10 ７times. Depending on the material, there may be no clear fatigue limit. In other words, the high-pressure tank 100 described above is subjected to a high pressure of at least 20 MPa, and the stress σ generated in the tank body 10 at that time is at least equal to the fatigue limit σ w If the fatigue limit σ is less than the fatigue limit σ, the tank body 10 will not be damaged due to fatigue. w is about half the tensile strength of the material, and this fatigue limit σ w If the high-pressure tank 100 is designed based on this and taking into consideration factors such as safety factors, it is necessary to increase the wall thickness t of the tank body 10 or to lower the maximum internal pressure.

[0023] Figure 2 is an explanatory diagram that shows a schematic representation of the effect of hydrogen embrittlement on steel. Figure 2 shows the relationship between the stress applied to the steel and the number of cycles to which the stress is applied. When the steel is placed in a hydrogen environment, its strength against repeated stress amplitudes decreases. When repeated stress is applied to the steel, it breaks, but when the stress is reduced, it breaks down by 10 ６ ~10 ７ The steel will not break even if repeated stress of more than 100 cycles is applied. The stress at this point is called the fatigue limit. When steel is under the influence of hydrogen, in the low cycle region where the number of repeated stresses is low, the stress at which the steel breaks will be lower than when there is no influence of hydrogen.

[0024] The high-pressure tank 100 is susceptible to hydrogen embrittlement because it contains a mixed gas containing hydrogen gas. The high-pressure tank 100 according to the first embodiment is designed with steel materials adjusted to accommodate the hydrogen gas contained therein, thereby enabling storage of a high-pressure mixed gas while suppressing the effects of hydrogen embrittlement. The characteristics of the steel materials used in the high-pressure tank 100 according to the first embodiment will be described in detail later. In contrast, conventional high-pressure tanks use general steel materials, and therefore, when a mixed gas containing hydrogen is contained therein, the tank is susceptible to hydrogen embrittlement, resulting in a decrease in the fatigue strength of the steel material and a significant decrease in its resistance to repeated stress. Therefore, in high-pressure tanks using general steel materials, it is necessary to reduce the generated stress by increasing the wall thickness t or by reducing the internal pressure. However, conventional high-pressure tanks are used in CNG vehicles, and the amount of hydrogen gas contained in the gas contained therein is kept to 2% or less, so there is no need to consider the effects of hydrogen embrittlement.

[0025]

[0026] Table 1 summarizes the hoop stress σ generated in the high-pressure tanks of the conventional example, comparative example, and example of the invention, and the lifespan of the high-pressure tank. In each example shown in Table 1, the structure of the tank body is the same as the structure of the tank body 10 according to embodiment 1 shown in Figure 1 (however, the detailed dimensions are different for each), the pressure of the gas sealed inside is the same, but the wall thickness t and the material used for the tank body 10 are different. Therefore, the conventional example, comparative example, and example of the invention of the high-pressure tank in Table 1 each generate different hoop stress σ, and the lifespan, whether or not they are sufficient for use, also differs.

[0027] <Regarding the Conventional Examples in Table 1> The conventional examples shown in the leftmost column of Table 1 store CNG (natural gas) as fuel, and the amount of hydrogen in the gas components is kept below 2%. The high-pressure tanks of the conventional examples are made of a conventional material, such as SCM435 (chromium-molybdenum steel), which has been quenched and tempered. For example, the material has a tensile strength (TS) of 953 MPa, but does not have the characteristics of the steel material used in the high-pressure tank 100 of the first embodiment. The conventional SCM435 material contains 0.15 to 0.30% molybdenum (Mo) by mass. The high-pressure tanks of the conventional examples contain only a small amount of hydrogen in the fuel stored therein, and are therefore not affected by hydrogen embrittlement. Therefore, even with a calculated hoop stress of σ = 592 MPa, as shown in Table 1 above, the tanks have sufficient yield strength and a sufficient lifespan as on-board CNG high-pressure tanks.

[0028] The hoop stress σ generated in the high-pressure tank is calculated by the following formula: σ = Di·p / 2t (Equation 1) where Di is the inner diameter of the container [mm], p is the pressure of the gas inside [MPa], and t is the thickness of the cylindrical portion 11 of the tank body 10.

[0029] The lifespan of each high-pressure tank in Table 1 was determined by fatigue crack propagation analysis. The fatigue crack propagation analysis was performed in accordance with KHKS0220 (2020). Each high-pressure tank was determined to have reached its lifespan (indicated as "OK" in Table 1) or not (indicated as "NG" in Table 1) based on whether or not it broke when subjected to a stress amplitude of 11,250 cycles. The stress amplitude of 11,250 cycles was set taking into account the number of times a high-pressure tank typically used as an on-board fuel tank must be filled with fuel before the vehicle's mileage limit is reached. The lifespan of the high-pressure tank 100 according to embodiment 1 may be determined by the strength at or above 11,250 cycles in the crack propagation analysis. Strengths at or above any of the values, such as 12,000 cycles, 15,000 cycles, 20,000 cycles, and 25,000 cycles, are also acceptable, and the lifespan may be greater than or equal to any of the values ​​between these examples. However, if the life of the high-pressure tank 100 is long, the thickness of the tank body 10 tends to increase, resulting in an increase in weight. In this case, if the high-pressure tank 100 is mounted on a vehicle, fuel efficiency may decrease, so the life of the high-pressure tank 100 should be set to, for example, 50,000 cycles or less.

[0030] As described above, the high-pressure tank according to the conventional example was constructed from conventional materials, but because its use was to store CNG and the amount of hydrogen in the gas filled was kept to 2% or less, it was not affected by hydrogen embrittlement and was able to ensure the lifespan required for an on-board fuel container. Specifically, the lifespan of the conventional example was 15,784 cycles, compared to the criterion of 11,250 stress amplitude cycles.

[0031] <Regarding Comparative Examples 1 and 3 in Table 1> Comparative Example 1, shown in the second column from the left, and Comparative Example 3, shown in the fourth column from the left, store HCNG (a mixture of natural gas and hydrogen) as fuel, with 20% hydrogen in the gas. The high-pressure tanks of Comparative Examples 1 and 3 are made of the same conventional material as the conventional example, e.g., SCM435 (chromium-molybdenum steel), which has been quenched and tempered to have a tensile strength (TS) of 953 MPa. However, these tanks do not have the characteristics of the steel used in the high-pressure tank 100 of the first embodiment. The high-pressure tank of this comparative example is affected by the hydrogen contained in the fuel stored therein, causing hydrogen embrittlement of the steel, resulting in a shortened lifespan. The high-pressure tank of Comparative Example 1 has a wall thickness t of 6.4 mm, the same as the conventional high-pressure tank described above, and a calculated hoop stress of σ of 592 MPa. However, the high-pressure tank of Comparative Example 1 has a lifespan of approximately 1,200 to 1,300 cycles, failing to meet the lifespan required for an on-board high-pressure tank. Furthermore, as shown in Comparative Example 3, the conventional material was unable to ensure a sufficient life even when the thickness t was 10 mm.

[0032] <Regarding Comparative Example 2 in Table 1> Comparative Example 2, shown in the third column from the left in Table 1, stores HCNG (a mixture of natural gas and hydrogen) as fuel, just like Comparative Example 1. The high-pressure tank according to Comparative Example 2 is made of a conventional material, such as SCM435 (chromium-molybdenum steel), which has been quenched and tempered, just like the conventional example, Comparative Example 1, and Comparative Example 3. The material has a tensile strength (TS) of 953 MPa, but does not have the characteristics of the steel used in the high-pressure tank 100 according to the first embodiment. The high-pressure tank according to Comparative Example 2 is subject to hydrogen embrittlement of the steel due to the effects of hydrogen contained in the fuel stored therein, resulting in a shortened lifespan. However, the high-pressure tank according to Comparative Example 2 has a wall thickness t of 13 mm, and the calculated hoop stress is kept low at σ = 279 MPa. As a result, despite being subject to hydrogen embrittlement, the high-pressure tank according to Comparative Example 2 exceeded the criterion of 11,250 cycles, ensuring a sufficient lifespan for use as an on-board HCNG high-pressure tank.

[0033] However, although the high-pressure tank according to Comparative Example 2 meets the standard lifespan, its wall thickness t is thick, its weight is 113 kgf, twice that of the conventional tank, and the fuel it can store is less than the high-pressure tanks according to Comparative Examples 1 and 3. When a high-pressure tank is used as an on-vehicle fuel tank, an increase in the weight of the tank body 10 must be minimized as much as possible because it leads to an increase in the vehicle weight. For example, if a heavy high-pressure tank 100 such as that of Comparative Example 2 is installed in an automobile, measures such as reducing the passenger capacity are usually necessary. For this reason, there are no high-pressure tanks currently in use for storing on-vehicle CNG with large wall thicknesses. Furthermore, if conventional materials were to be used directly as is for a high-pressure tank for storing on-vehicle HCNG, a wall thickness t at least equivalent to that of Comparative Example 2 would be required.

[0034] <Regarding Comparative Example 4 in Table 1> In Comparative Example 4, shown in the fifth column from the left in Table 1, the wall thickness can be reduced by reducing the outer diameter Do of the tank, thereby reducing the stress generated. Compared to Comparative Example 2, the wall thickness of Comparative Example 4 can be reduced by approximately 3 mm. However, since the tank capacity decreases when only the outer diameter Do is reduced, the tank length must be extended to ensure sufficient tank capacity. In Comparative Example 4, a length of 1,595 mm is required to ensure the same 57 liters as the conventional example (the conventional example and Comparative Example 1 in Table 1), and the weight is estimated to be 113 kg. Therefore, simply reducing the outer diameter Do of a high-pressure tank for storing HCNG can provide a satisfactory lifespan but not a reduction in weight.

[0035] <Regarding the Examples of the Invention in Table 1> Example 1, shown in the sixth column from the left in Table 1, stores HCNG (a mixture of natural gas and hydrogen) as fuel, just like Comparative Examples 1, 2, 3, and 4. The material constituting the high-pressure tank of Example 1 differs from that of the conventional examples and Comparative Examples 1 to 4 in that it uses a new steel material that is also used for the tank body 10 of the high-pressure tank 100 according to the first embodiment. This new steel material corresponds to the steel material used in the tank body 10 according to the first embodiment, which will be described later, and is, for example, the steel material shown as an example of the invention among steel materials Nos. 1 to 18 listed in Table 3 below. In the high-pressure tank of Example 1, the use of a new steel material suppresses the effects of hydrogen contained in the fuel stored therein, suppresses hydrogen embrittlement of the steel material, and extends the lifespan compared to high-pressure tanks using conventional materials. With the conventional material, even with a wall thickness of 10 mm (as shown in Comparative Example 3), a sufficient lifespan could not be achieved. However, by increasing the wall thickness to 13 mm (as shown in Comparative Example 2) or by reducing the wall thickness to 10.1 mm and the tank outer diameter Do to 242 mm (as shown in Comparative Example 4), a sufficient lifespan could be achieved. In contrast, the high-pressure tank of Example 1, which has a wall thickness of 8 mm and a calculated hoop stress of σ = 445 MPa, achieved a lifespan exceeding the criterion of 11,250 cycles. This demonstrates that a sufficient lifespan for an on-board HCNG high-pressure tank can be achieved even with a thinner wall thickness than the conventional material. Examples 2, 3, and 4 also use the new material, and like Example 1, they were able to ensure a sufficient lifespan for an on-board HCNG high-pressure tank. Furthermore, although Example 4 has a longer tank, the tank outer diameter Do can be reduced, allowing the vehicle tank to be reoriented from horizontal to vertical, thereby expanding the design flexibility of the mounting configuration.

[0036] In Example 1, strength and durability are maintained even when storing HCNG as described above, and although the weight is increased compared to the conventional example, the weight is reduced by about 40 kgf compared to Comparative Example 2, which uses conventional materials. In other words, the high-pressure tank 100 using the new material as in Example 1 of the invention is satisfactory for practical use as an on-board HCNG high-pressure tank.

[0037] <Regarding the steel material used in the tank body 10 according to the first embodiment> As described above, by applying the high-pressure tank 100 according to the first embodiment as an on-board high-pressure tank for storing HCNG, it is possible to ensure a sufficient lifespan while storing hydrogen. The reason why a sufficient lifespan can be ensured as an on-board HCNG high-pressure tank is mainly due to the characteristics of the steel material that constitutes the tank body 10 of the high-pressure tank 100, and the characteristics of this steel material will be described below. The characteristics of the steel material used in the tank body 10 and the mechanism by which it suppresses the effects of hydrogen embrittlement will be described below.

[0038] The tank body 10 according to the first embodiment has carbides dispersed within the metal structure, which traps hydrogen that has penetrated the metal structure within the grains of the metal structure, thereby reducing the effects of hydrogen embrittlement. In particular, by including a predetermined amount of molybdenum in the steel composition, the carbides present within the metal structure are dispersed in a desirable size as precipitates. Therefore, hydrogen that has penetrated the metal structure is trapped by molybdenum precipitates within the grains of the metal structure, reducing the effects of hydrogen embrittlement. Furthermore, by including a predetermined amount of molybdenum in the steel composition, molybdenum precipitates are dispersed throughout the metal structure, trapping hydrogen. If a steel material in which carbides are not properly arranged within the metal structure, as in the conventional example and comparative example, were used for the tank body 10, hydrogen would accumulate at the grain boundaries of the metal structure, causing the steel material to fracture at the grain boundaries. Carbides within the metal structure that are effective as hydrogen traps include Mo. ２ In addition to molybdenum carbide (C), titanium carbide (TiC), vanadium carbide (VC), and niobium carbide (NbC) are also used. Carbides precipitated in the metal structure are effective as hydrogen traps if they are 100 nm or less, but preferably 50 nm or less.

[0039] Figure 3 is an explanatory diagram that schematically shows the effects of hydrogen on conventional and new materials. Hydrogen atoms that penetrate the metal structure of the steel used in the tank body 10 move within the metal structure along with dislocations in the crystals. The metal structure of steel contains minute crystal grains, and when high stress is applied, fractures tend to occur at the boundaries between the crystal grains. Furthermore, when hydrogen atoms accumulate at the grain boundaries, the hydrogen reduces the interatomic cohesive force, causing grain boundary fracture and reducing the fatigue strength of the steel.

[0040] 3(a) is a diagram illustrating the movement of hydrogen within crystal grains in a conventional material in which carbides are not properly arranged within the crystal grains. In the conventional material, hydrogen atoms move within the crystal grains along with dislocations and accumulate at the crystal grain boundaries. This makes the crystal grain boundaries susceptible to destruction due to the action of hydrogen atoms.

[0041] Figure 3(b) is a diagram illustrating the movement of hydrogen within crystal grains in the new material, in which carbides are properly arranged within the crystal grains. In the new material, hydrogen atoms move within the crystal grains along with dislocations, but the hydrogen atoms are trapped by the carbides arranged within the crystal grains, preventing them from accumulating at the crystal grain boundaries. Therefore, in the new material, the crystal grain boundaries are less susceptible to the action of hydrogen atoms, and grain boundary fracture is suppressed compared to conventional materials.

[0042] FIG. 4 is an explanatory diagram schematically illustrating the relationship between stress and crack propagation rate in the steel material constituting the tank body 10 according to the first embodiment and in a conventional steel material. In the case of a high-pressure tank using a conventional steel material, as shown by solid line A in FIG. 4 , intragranular quasi-cleavage (QC) fracture occurs primarily within the crystal grains up to a certain stress range (the horizontal portion of solid line A), and above a certain stress, intergranular (IG) fracture occurs primarily (the inclined portion of solid line A). This is also true for the steel material constituting the tank body 10 according to the first embodiment. However, as shown by dashed line B in FIG. 4 , the steel material constituting the tank body 10 according to the first embodiment has carbides arranged within the crystal grains, resulting in a lower overall crack propagation rate. Furthermore, in the steel material constituting the tank body 10 according to the first embodiment, hydrogen atoms are trapped within the crystal grains, making intergranular fracture less likely to occur. This expands the region in which quasi-cleavage (QC) fracture is the primary fracture mode, and increases the stress at which intergranular fracture is the primary fracture mode.

[0043] [Steel Structure] The steel used in the tank body 10 has a structure with tempered martensite as the main phase, and molybdenum precipitates are present in the structure. The molybdenum precipitates include precipitates with a diameter of 50 nm or less. The steel used in the tank body 10 is, for example, the molybdenum steel or martensitic stainless steel shown in the above-mentioned invention example. Some conventional high-pressure tanks for storing mixed gases containing hydrogen use austenitic stainless steel to suppress the effects of hydrogen embrittlement, but the tank body 10 according to the first embodiment suppresses hydrogen embrittlement while mainly using low-alloy steel.

[0044] Main Phase: Tempered Martensite Phase The steel used for the tank body 10 has a martensite-based structure to ensure high strength (TS: 850 MPa or more). However, to maintain the ductility and toughness required for a structure, the tempered martensite phase is used as the main phase. The term "main phase" as used herein refers to a single phase with an area ratio of 100%, or a phase with an area ratio of 5% or less of a secondary phase, which does not affect the properties, and is 95% or more of the secondary phase. Examples of the secondary phase include bainite, retained austenite, pearlite, or a mixture thereof.

[0045] The above-described structure of the tank body 10 can be adjusted by appropriately selecting the heating temperature during quenching and the cooling rate during cooling according to the steel's composition. The tensile strength of the material is adjusted to 850 MPa or more by adjusting the quenching and tempering conditions, particularly to 850 MPa or more but less than 1150 MPa. More preferably, the tensile strength TS is adjusted to 850 MPa or more but less than 1000 MPa. Because the tank body 10 is filled with a hydrogen-containing mixed gas at high pressure, it is desirable for the tensile strength TS of the steel used to be high, at 850 MPa or more. However, if the tensile strength TS is too high, brittleness increases and fatigue strength decreases, so the tensile strength TS must be kept within a certain range. Therefore, in the case of the steel used in the tank body 10 according to the first embodiment, the tensile strength TS should be at most less than 1150 MPa, and more preferably, 850 MPa or more but less than 1000 MPa, so as to satisfy fatigue strength requirements that take into account the effects of hydrogen.

[0046] If the grain size number of the prior austenite grains is less than 8.5, the substructure of the martensite phase that is generated becomes coarse, and toughness decreases. Therefore, it is preferable that the grain size number of the prior austenite grains is 8.5 or more. The grain size number is a value measured in accordance with the provisions of JIS G 0551.

[0047] The grain size number of the prior austenite grains can be adjusted by changing the heating rate, heating temperature, and holding temperature during quenching, as well as the number of times quenching is performed.

[0048] Furthermore, in order to improve hydrogen embrittlement resistance, the concentration of molybdenum precipitates in the steel material used for the tank body 10 is adjusted within an appropriate range depending on their size. Molybdenum precipitates are identified by filtration, for example, using the extraction method described in JP 2010-127791 A and "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, 10 mm square samples taken from a cross section perpendicular to the rolling direction of the steel pipe (a cross section perpendicular to the pipe axis: C-section) were electrolyzed in an electrolyte. The precipitates attached to the steel billet surface were then placed in a dispersible aqueous solution and irradiated with ultrasound to extract the precipitates into the aqueous solution. The aqueous solution from which the precipitates were extracted was filtered, the precipitates separated by size, and the precipitates classified by size were dissolved in respective solutions. The Mo concentration was analyzed by ICP to calculate the Mo content of the precipitates of each size. In concentration analysis using ICP, the solution is introduced into plasma to emit an element-specific spectrum, and the concentration of the element in the solution can be determined from the light emission intensity, allowing the Mo concentration (mass%) in the precipitate to be calculated. This technique allows the Mo content in the entire precipitate to be calculated, and from this value and the Mo content in the steel, the proportion (mass%) of Mo contained in the precipitates to the total Mo contained in the steel can be determined. Furthermore, the solid solution concentration of Mo in the steel was obtained by analyzing the concentration of the solution after electrolysis using ICP in accordance with the technology described in JP 2009-031269 A. Furthermore, the Mo content contained in the precipitates remaining on the filter was analyzed using ICP, and the Mo content contained in precipitates larger than 50 nm was analyzed. The proportion (mass%) of Mo contained in precipitates with a diameter of 50 nm or less to the total Mo contained in the precipitates can be determined by subtracting the Mo content contained in precipitates larger than 50 nm from the Mo content contained in all precipitates.

[0049] At least 50% of the Mo contained in the steel is contained in precipitates. The presence of Mo in the steel composition as precipitates improves the properties in a hydrogen environment. Increasing the amount of Mo added to the steel does not have the same effect if it is dissolved. On the other hand, the greater the amount of molybdenum precipitates, the better the hydrogen trapping ability, and this is significantly improved when at least 50% of the Mo contained in the steel is contained in precipitates. It is preferable that at least 60% of the Mo contained in the steel is present in precipitates. However, the Mo contained in precipitates may be 50% or less of the Mo contained in the steel.

[0050] At least 50% of the Mo contained in the precipitates is contained in precipitates with a diameter of 50 nm or less. Molybdenum precipitates trap hydrogen in steel, inhibiting hydrogen accumulation at grain boundaries and improving grain boundary strength in a hydrogen environment. However, when their size exceeds 50 nm, their hydrogen trapping ability decreases, reducing their impact on improving grain boundary strength. Therefore, it is necessary for a large amount of Mo to be contained in precipitates with a diameter of 50 nm or less. Here, the greater the amount of Mo in the precipitates, the better the hydrogen trapping ability, and this is significantly improved when the Mo contained in fine precipitates with a diameter of 50 nm or less accounts for 50% or more of the Mo in the precipitates. Therefore, in the steel used for the tank body 10, it is preferable that the Mo contained in fine precipitates with a diameter of 50 nm or less accounts for 50% or more of the Mo contained in the precipitates. Furthermore, it is preferable that the Mo contained in fine precipitates with a diameter of 50 nm or less accounts for 60% or more of the Mo contained in the precipitates. Furthermore, since smaller molybdenum precipitates have better hydrogen trapping ability, it is more preferable that the diameter of the precipitates is 20 nm or less. Note that coarsening of molybdenum precipitates occurs due to aggregation and coalescence of fine molybdenum precipitates, and therefore coarsening of molybdenum precipitates leads to a decrease in the number of fine molybdenum precipitates.

[0051] In addition to the precipitation of molybdenum precipitates, reducing nitride and oxide inclusions, which can serve as fracture initiation sites, is also important for improving hydrogen embrittlement resistance in the steel used for the tank body 10. Management of the molten steel refining process is particularly important. Desulfurization and dephosphorization are performed in the hot metal pretreatment process, followed by decarburization and dephosphorization in a converter. After that, the hot metal undergoes a hot-stirring refining process (LF) and a hot-stirring vacuum degassing process in the ladle. Sufficient processing time is then ensured for the hot-stirring refining process (LF), as well as the hot-stirring vacuum degassing process, and the RH reflux rate is controlled. Furthermore, when producing slabs (steel pipe material) using a continuous casting process, in order to reduce nitride and oxide inclusions, an inert gas seal is used when the slab is poured from the ladle into the tundish, and electromagnetic stirring is used in the mold to float and separate the inclusions. Although the refining process of the molten steel used for the tank body 10 is not limited to the above, even in that case, management of the refining process is important.

[0052] [Composition] First, the composition of the steel material used in the tank body 10 will be described. Hereinafter, mass percentages in the composition will be simply expressed as percentages. C: 0.20-0.50% C contributes to increasing the strength of the steel through solid solution, improves the hardenability of the steel, and contributes to the formation of a structure in which martensite is the main phase during quenching. To achieve this effect, the C content must be 0.20% or more. On the other hand, if the C content exceeds 0.50%, cracks will occur during quenching, significantly reducing manufacturability. For this reason, the C content is limited to the range of 0.20-0.50%. The C content is preferably 0.20-0.40%. More preferably, it is 0.22-0.35%.

[0053] Si: 0.05 to 2.00% Si is added for deoxidation, but if the Si content is less than 0.051%, the deoxidation effect is insufficient. For this reason, the Si content is limited to 0.05% or more. On the other hand, if the Si content exceeds 2.00%, the effect saturates, so the Si content is limited to 2.00% or less.

[0054] Mn: 0.30 to 1.5% Like C, Mn is an element that improves the hardenability of steel and contributes to increasing its strength. To achieve this effect, a Mn content of 0.30% or more is required. However, Mn is an element that segregates in steel and locally hardens it. Containing a large amount of Mn can form localized hardened regions, adversely affecting hydrogen embrittlement resistance. For this reason, the Mn content of the steel used for the tank body 10 is limited to the range of 0.30 to 1.5%. The Mn content is preferably 0.4 to 0.8%. The Mn content is more preferably 0.5 to 0.8%.

[0055] P: 0.015% or less P is an element that not only segregates at grain boundaries in the steel structure to cause grain boundary embrittlement, but also segregates to locally harden the steel. In the steel used for the tank body 10, P is an unavoidable impurity and is preferably reduced as much as possible, but up to 0.015% is acceptable. For this reason, the P content is limited to 0.015% or less. The P content is preferably 0.008% or less. The lower the P content, the better, but from the perspective of refining costs, the P content is preferably 0.0001% or more.

[0056] S: 0.005% or less S is an unavoidable impurity, and most of it exists in steel as sulfide-based inclusions, which reduce ductility, toughness, and SSC resistance. Therefore, it is preferable to reduce the S content as much as possible, but up to 0.005% is acceptable. For this reason, the S content is limited to 0.005% or less. The S content is preferably 0.003% or less. The lower the S content, the better, but from the viewpoint of refining costs, the S content is preferably 0.0002% or more.

[0057] Al: 0.005 to 0.15% Al is added as a deoxidizer, but if its content is less than 0.005%, it has no added effect. For this reason, the Al content is limited to 0.005% or more. On the other hand, if the Al content exceeds 0.15%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is limited to 0.15% or less.

[0058] N: 0.006% or less N is present in steel as an unavoidable impurity, but it combines with Al to form AlN, and also forms TiN when Ti is contained, which has the effect of refining crystal grains and improving toughness. However, if the N content exceeds 0.006%, the nitrides that are formed become coarse, significantly reducing toughness. For this reason, the N content is limited to 0.006% or less.

[0059] Cr: more than 0.2% and not more than 1.7% Cr is an element that increases the strength of steel by improving hardenability and also improves corrosion resistance. Cr also combines with C during tempering treatment to form M. ３ C, M ７ C ３ , M ２３ C ６ (M is a metallic element) and improves temper softening resistance, and is an element that is particularly necessary for increasing the strength of steel pipes. ３ C-type precipitates have a strong effect of improving temper softening resistance. To obtain this effect, the Cr content must be more than 0.2%. On the other hand, if the Cr content is more than 1.7%, a large amount of M ７ C ３ , M ２３ C ６ These form molybdenum precipitates, which act as hydrogen trap sites and reduce hydrogen corrosion resistance. Furthermore, a high Cr content causes coarsening of molybdenum precipitates. Fine molybdenum precipitates coarsen due to aggregation and coalescence, resulting in a decrease in the number density of the fine molybdenum precipitates and a decrease in hydrogen embrittlement resistance. For these reasons, the Cr content is limited to a range of more than 0.2% and not more than 1.7%. The Cr content is preferably more than 0.2% and not more than 1.0%. The Cr content is more preferably more than 0.2% and not more than 0.4%.

[0060] Mo: more than 1.0% and not more than 3.0% Mo is an element that forms precipitates and contributes to strengthening steel through precipitation strengthening, and effectively contributes to ensuring the desired high strength after reducing dislocation density by tempering. Mo also dissolves in steel and segregates at prior austenite grain boundaries, contributing to improving hydrogen embrittlement resistance. Furthermore, Mo has the effect of densifying corrosion products and suppressing the formation and growth of pits that serve as crack initiation points. To achieve these effects, the Mo content must be more than 1.0%. On the other hand, if the Mo content exceeds 3.0%, acicular M ２ C precipitates and, in some cases, Laves phases (Fe ２ This promotes the formation of Mo (Mo), which reduces hydrogen embrittlement resistance. For this reason, the Mo content is limited to a range of more than 1.0% and not more than 3.0%. The Mo content is preferably more than 1.1% and not more than 3.0%, more preferably more than 1.2% and not more than 2.8%, and even more preferably 1.45 to 2.5%. Still more preferably 1.45 to 1.80%. However, even if the Mo content is in any range of more than 1.0% and not more than 3.0%, it has the effect of scattering fine precipitates that trap hydrogen in the metal structure.

[0061] Nb: 0.001 to 0.02% Nb forms precipitates or carbonitrides, contributing to increased strength of steel through precipitation strengthening and also contributing to the refinement of austenite grains. To achieve this effect, the Nb content must be 0.001% or more. On the other hand, coarse Nb precipitates are likely to become crack initiation points for hydrogen-induced cracking. Therefore, the presence of a large amount of Nb precipitates due to a Nb content exceeding 0.02% leads to a significant decrease in hydrogen embrittlement resistance in high-strength steel. For this reason, 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. The Nb content is preferably 0.001% or more and less than 0.01%.

[0062] B: 0.0003 to 0.0030% B segregates at austenite grain boundaries and inhibits ferrite transformation from the grain boundaries, thereby enhancing the hardenability of steel even with trace amounts. To achieve this effect, the B content must be 0.0003% or more. On the other hand, if the B content exceeds 0.0030%, it precipitates as carbonitrides, etc., reducing hardenability and therefore toughness. For this reason, the B content is limited to the range of 0.0003 to 0.0030%. The B content is preferably 0.0007 to 0.0025%.

[0063] O (oxygen): 0.0030% or less O (oxygen) is an unavoidable impurity and exists in the steel as oxide-based inclusions. These inclusions become the starting point for crack initiation in a hydrogen gas environment and reduce hydrogen embrittlement resistance, so in the present disclosure, it is preferable to reduce O (oxygen) as much as possible. However, excessive reduction leads to an increase in refining costs, so the O (oxygen) content is permissible up to 0.0030%. For this reason, the O (oxygen) content is limited to 0.0030% or less. The O content is preferably 0.0020% or less.

[0064] Ti: 0.003 to 0.025% Ti combines with N during solidification of molten steel and precipitates as fine TiN, contributing to the refinement of austenite grains through its pinning effect. To achieve this effect, a Ti content of 0.003% or more is required. If the Ti content is less than 0.003%, the effect is small. On the other hand, if the Ti content exceeds 0.025%, the TiN becomes coarse, preventing the above-mentioned pinning effect from being exerted and instead reducing toughness. Furthermore, the coarse TiN further reduces hydrogen embrittlement resistance. For these reasons, the Ti content is limited to the range of 0.003 to 0.025%.

[0065] Mo / C: 2.0 to 12.0 If the Mo / C ratio (the ratio of Mo to C) is less than 2.0, the Mo content is insufficient, resulting in a reduced amount of molybdenum precipitates, and therefore insufficient molybdenum precipitates to improve hydrogen embrittlement resistance. On the other hand, if the Mo / C ratio exceeds 12.0, the molybdenum precipitates tend to become coarse, resulting in a decrease in toughness and hydrogen embrittlement resistance. Furthermore, coarsening of molybdenum precipitates also occurs due to aggregation and coalescence of fine molybdenum precipitates, resulting in a decrease in the number density of fine molybdenum precipitates. For these reasons, the Mo / C ratio is limited to a range of 2.0 to 12.0. The preferred range of Mo / C is 2.5 to 6.0.

[0066] The above-mentioned components are the basic components, but in addition to the basic composition, optional elements may be contained, such as one or more selected from V: 0.3% or less, Cu: 1.0% or less, Ni: 2.0% or less, and W: 3.0% or less, H: 0.0010% or less, or Ca: 0.0005 to 0.005%, or any combination of these.

[0067] One or more selected from V: 0.3% or less, Cu: 1.0% or less, Ni: 2.0% or less, and W: 3.0% or less. V, Cu, Ni, and W are all elements that contribute to increasing the strength of steel, and one or more of them can be selected and contained as necessary.

[0068] V: 0.3% or less V is an element that forms precipitates and carbonitrides and contributes to strengthening the steel. The V content may be 0% or more, but in order to obtain the above-mentioned effects, the V content is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, even if V is contained in an amount exceeding 0.3%, the effect saturates and no effect commensurate with the content can be expected, which is economically disadvantageous. For this reason, when V is contained, the V content is limited to 0.3% or less. The V content is preferably 0.20% or less, and more preferably 0.15% or less.

[0069] Cu: 1.0% or less Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates. Therefore, when Cu is contained, the Cu content is limited to 1.0% or less. The Cu content may be 0% or more, but in order to obtain the effects of improving toughness and increasing strength, it is preferable to contain 0.01% or more.

[0070] Ni: 2.0% or less Ni is an element that contributes to increasing the strength of steel and also improves toughness and corrosion resistance. To achieve this effect, the Ni content is preferably 0.03% or more. On the other hand, even if Ni is contained in an amount exceeding 2.0%, the effect saturates, and no effect commensurate with the content can be expected, which is disadvantageous in terms of economy. For this reason, when Ni is contained, the Ni content is limited to 2.0% or less.

[0071] W: 3.0% or less W is an element that forms precipitates and contributes to increasing the strength of steel through precipitation strengthening, and also dissolves and segregates at prior austenite grain boundaries, contributing to improving hydrogen embrittlement resistance. To achieve these effects, the W content is preferably 0.03% or more. On the other hand, if the W content exceeds 3.0%, the effects of increasing the strength of the steel and improving hydrogen embrittlement resistance saturate, and the expected effects are not commensurate with the content, which is disadvantageous to economics. For this reason, when W is contained, the W content is limited to 3.0% or less.

[0072] H: 0.0010% or less H may be introduced into steel during various manufacturing processes. A high introduction amount increases the risk of cracking after solidification and degrades hydrogen embrittlement resistance, so it is important to reduce the amount of hydrogen in the steel. These effects are not a problem if the H content is 0.0010% or less. Therefore, if H is contained, the H content is preferably 0.0010% or less. The H content is more preferably 0.0001% or less. Since a H content of less than 0.00001% increases costs, the H content is preferably 0.00001% or more. The hydrogen content is the amount of hydrogen remaining after forming into plates, steel pipes, etc.

[0073] Ca: 0.0005 to 0.005% Ca is an element that bonds with S to form CaS and effectively controls the morphology of sulfide-based inclusions. Through controlling the morphology of sulfide-based inclusions, it contributes to improving toughness and hydrogen embrittlement resistance. To achieve this effect, the Ca content must be 0.0005% or more. On the other hand, if the Ca content exceeds 0.005%, the effect saturates, and no effect commensurate with the content can be expected, which is economically disadvantageous. For this reason, if Ca is contained, the Ca content is limited to the range of 0.0005 to 0.005%.

[0074] The balance other than the above components is composed of Fe and unavoidable impurities. As the unavoidable impurities, for example, Mg: 0.0008% or less and Co: 0.0008% or less are allowable.

[0075] [Manufacturing Method] Next, a manufacturing method for the tank body 10 of the high-pressure tank 100 according to the first embodiment will be described.

[0076] The tank body 10 is made by heating a steel pipe material having the above-mentioned composition, and hot working the steel pipe to form a seamless steel pipe (seamless steel pipe) into a predetermined shape. Note that seamless steel pipes for high-pressure hydrogen containers are preferably used for hydrogen containers with a hydrogen pressure of 1 MPa or more, and more preferably 20 MPa or more.

[0077] The steel pipe material (hereinafter also simply referred to as steel material) for manufacturing the tank body 10 is preferably produced by melting molten steel having the above-described composition using a conventional melting method such as a converter, and then forming a slab (round slab) using a conventional casting method such as continuous casting. The slab may be further hot-rolled to form a round slab of a predetermined shape, or may be formed into a round slab by ingot-making and blooming.

[0078] In the following description, a manufacturing method will be described using an example in which the tank body 10 is a seamless tank, but it goes without saying that it is possible to manufacture the tank body 10 by welding it by performing processes that result in a similar thermal history. For example, similar properties can be obtained by manufacturing an electric resistance welded pipe by rolling a steel plate at a temperature in the range from the Ac3 transformation point to 1000°C, subsequently performing a quenching treatment at least once in which the steel plate is rapidly cooled to a surface temperature of 200°C or less, performing a tempering treatment after the quenching treatment in which the steel plate is heated to a temperature in the range of 600 to 740°C, performing a tempering treatment at an average heating rate of 0.5°C / min or more until the tempering temperature is reached and maintaining the tempering temperature for 10 minutes or more but less than 60 minutes, and then welding the steel plate.

[0079] The tank body 10 can be manufactured by sequentially performing the following steps (1) to (3): (1) a step of casting a steel material after adjusting its composition, (2) a rolling step of heating and rolling the cast material to obtain a steel pipe, and (3) a step of cooling and tempering the steel pipe obtained in the rolling step. Each step will be explained below. Note that, unless otherwise specified, temperatures in the following explanation refer to the temperature at the surface of the steel material.

[0080] [Casting process] Casting speed: 2.0 m / min or less The slower the casting speed, the more the hydrogen concentration and inclusions in the steel can be reduced, and this effect becomes more pronounced at a casting speed of 2.0 m / min or less, so the casting speed is preferably 1.5 m / min or less, more preferably 1.0 m / min or less, and even more preferably 0.5 m / min or less.

[0081] [Heating Step] In order to perform hot rolling, the steel material having the above-described composition is heated. The steel material is not particularly limited, but for example, a billet obtained by a conventional continuous casting method can be used.

[0082] Heating temperature: 1050 to 1350°C. If the heating temperature is less than 1050°C, the precipitates in the steel material will not dissolve sufficiently. On the other hand, if the heating temperature exceeds 1350°C, the crystal grains will become coarse, and precipitates such as TiN that precipitated during solidification will also become coarse. Furthermore, if the steel material is heated to a high temperature exceeding 1350°C, a thick scale layer will form on the surface of the steel material, which will cause surface defects during rolling and increase energy loss, which is undesirable from the viewpoint of energy conservation. For these reasons, the heating temperature is limited to a range of 1050 to 1350°C. The heating temperature is preferably 1100 to 1300°C.

[0083] [Rolling Step] Next, the steel material heated in the heating step is rolled into a steel pipe shape. For rolling, hot rolling including piercing rolling using a conventional Mannesmann plug mill method or a Mannesmann mandrel mill method can be used.

[0084] After hot rolling, the resulting seamless steel pipe (tank body 10) is subjected to a cooling treatment in which it is cooled at a cooling rate equal to or faster than air cooling until the surface temperature reaches 200°C or lower.

[0085] Cooling treatment after hot rolling: Cooling rate: air cooling or faster, cooling stop temperature: 200°C or lower. Within the composition range of the steel material constituting the tank body 10 described above, a structure with martensite as the main phase can be obtained by cooling at a cooling rate faster than air cooling after hot rolling. If air cooling (cooling) is stopped when the surface temperature exceeds 200°C, the transformation may not be fully completed. Therefore, the cooling treatment after hot rolling is performed at a cooling rate faster than air cooling until the surface temperature reaches 200°C or lower. Here, a "cooling rate faster than air cooling" refers to 0.1°C / s or higher. A cooling rate less than 0.1°C / s results in a non-uniform metal structure after cooling, which in turn results in a non-uniform metal structure after subsequent heat treatment.

[0086] [Forming of Tank Body 10] The shape of the tank body 10 may be formed during or after hot rolling. When the tank body 10 is a seamless, integrally formed product, the tank body 10 is formed by squeezing both ends of a tubular rolled steel material. Furthermore, removal processing required for the tank body 10 may be performed after hot rolling.

[0087] [Heat Treatment Step] After cooling at a rate faster than air cooling, the steel sheet is subjected to tempering, which involves heating to a temperature in the range of 600 to 740°C.

[0088] Tempering temperature: 600 to 740°C Tempering is performed to reduce dislocation density, precipitate molybdenum precipitates, and improve toughness and hydrogen embrittlement resistance. If the tempering temperature is less than 600°C, the reduction in dislocations and the precipitation of molybdenum precipitates are insufficient, making it impossible to ensure excellent hydrogen embrittlement resistance. On the other hand, if the temperature exceeds 740°C, the structure will soften significantly and the desired high strength will not be achieved. For this reason, the tempering temperature is limited to a range of 600 to 740°C. The tempering temperature is preferably 640 to 710°C.

[0089] Average heating rate to reach tempering temperature is 0.5°C / min or more Molybdenum precipitates precipitate and increase in size during the tempering heating process. Therefore, if the heating rate to reach the specified temperature in the tempering treatment is slow, the precipitates will become too large in size and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the average heating rate to reach the tempering temperature is set to 0.5°C / min or more, preferably 2.0°C / min or more. There is no particular upper limit, but if the heating rate is too fast, uneven temperature distribution will occur, resulting in inhomogeneity in the material structure, so a rate of 50°C / min or less is preferred.

[0090] Holding time at tempering temperature: 10 minutes or more but less than 120 minutes Molybdenum precipitates are most likely to precipitate during tempering. If this time is too short, they will not precipitate sufficiently and the desired hydrogen embrittlement resistance will not be obtained. The holding time at tempering temperature should be 10 minutes or more. If the holding time at tempering temperature is too long, the size of the precipitates will become too large, so the holding time should be less than 120 minutes. Note that the holding time is a factor in increasing costs in terms of energy, so it is preferably less than 60 minutes.

[0091] In order to stably ensure the desired properties, it is advisable to carry out a cooling treatment after hot rolling in which the material is cooled at a cooling rate equal to or faster than air cooling, and then to carry out a quenching treatment in which the material is reheated and rapidly cooled by water cooling or the like, one or more times, and then to carry out the above-mentioned tempering treatment.

[0092] Reheating temperature for quenching: Ac3 transformation point or higher and 1000°C or lower. When quenching is performed, if the reheating temperature is lower than the Ac3 transformation point, the steel sheet will not be heated to the austenite single-phase region, and therefore a structure with martensite as the main phase will not be obtained. On the other hand, if the reheating temperature exceeds 1000°C, the grains will become coarse and the toughness will decrease. In addition, the surface oxide scale will become thicker and more likely to peel off, which will cause scratches on the steel sheet surface. Furthermore, excessive load on the heat treatment furnace will be a problem from the viewpoint of energy conservation. For these reasons, and from the viewpoint of energy conservation, the reheating temperature for quenching is limited to the Ac3 transformation point or higher and 1000°C or lower. The reheating temperature is preferably 950°C or lower.

[0093] After reheating, the plate is subjected to quenching treatment. Cooling in the quenching treatment is preferably performed by water cooling at an average cooling rate of 2°C / s or more until the temperature at the center of the plate thickness reaches 400°C or less. The surface temperature is rapidly cooled to 200°C or less by the cooling in the quenching treatment. Preferably, the surface temperature is cooled to 100°C or less. The quenching treatment may be repeated two or more times.

[0094] The Ac3 transformation point is calculated using the following formula:

[0095] Ac3 transformation point (°C) = 937 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 4.9Cr - 26.6Ni + 38.1Mo + 124.8V + 136.3Ti + 198Al + 3315B (where C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Al, B: content (mass%) of each element) When calculating the Ac3 transformation point, if an element described in the above formula is not contained, the content of that element is considered to be 0%.

[0096] After the quenching and tempering treatments are performed, a warm or cold correction treatment may be performed as necessary to correct any shape defects of the tank body 10. Alternatively, the tank body 10 may be formed after the quenching and tempering treatments are performed.

[0097] Below, examples examining the steel material constituting the tank body 10 are described. The following description illustrates a preferred example of the steel material constituting the tank body 10, and the present disclosure is not limited by these examples. Note that the following examples were not evaluated using an actual tank body 10, but rather evaluated using seamless steel pipes manufactured for actual steel structures. The evaluation results for the steel types indicated as "inventive examples" in the remarks column of Table 3 below are applicable regardless of the shape of the tank body 10, whether it is a seamless steel pipe or a finished product, which differs only in shape. Table 2 shows the compositions of steels Nos. 1 to 14. Table 3 also shows the tempering conditions, size and number of molybdenum precipitates, and relative reduction of area (RRA) for each of Nos. 1 to 14.

[0098]

[0099]

[0100] Billets having the compositions shown in steel grades No. 1 to 14 in Table 2 were produced at a casting speed of 0.6 m / min, and the billets were heated to 1250°C and expanded to obtain seamless steel pipes. These seamless steel pipes correspond to the examples shown in steel grades No. 1 to 14 in Table 3. Furthermore, billets having the composition of steel grade No. 5 in Table 2 were produced at a casting speed of 1.8 m / min, and the billets were heated to 1250°C and expanded to obtain seamless steel pipes. These seamless steel pipes correspond to the examples shown in steel grades No. 15 to 17 in Table 3. The seamless steel pipes shown in steel grades No. 1 to 18 in Table 3 were produced under conditions in which expansion was completed at 820°C or higher, and after hot rolling, the pipes were cooled at a cooling rate faster than air cooling to a temperature at which the surface temperature was 200°C or lower. The obtained seamless steel pipes were heated and held at 950°C for seamless steel pipes with an Ac3 point of 950°C or less, or at 1000°C for seamless steel pipes with an Ac3 point of more than 950°C, and then water-cooled to 200°C or less at a rate of 0.5°C / min, followed by tempering. The tempering process was carried out at the heating rate and holding time shown in Table 3. The tempering temperature was adjusted so that the tensile strength was in the range of 850 to 1150 MPa. The metallographic structure and mechanical properties of the obtained seamless steel pipes were evaluated. The evaluation methods are as follows.

[0101] Hydrogen embrittlement resistance was evaluated based on the relative reduction of area (RRA) of the test specimens after slow strain rate tensile testing in hydrogen gas. In air, the steel undergoes plastic deformation, reducing the area of ​​the fracture surface, resulting in a larger reduction of area φair. On the other hand, in hydrogen, the elongation of the steel decreases, causing the material to fracture before it can be reduced, leaving the area of ​​the fracture surface large. Therefore, the reduction of area φH of the fracture surface after testing in hydrogen is smaller than in air. Hydrogen embrittlement resistance was evaluated based on this reduction in reduction of area. The relative reduction of area (RRA) is calculated as follows: RRA = φH / φair × 100. Table 3 shows the relative reduction of area obtained from a slow strain rate tensile test (tensile speed 0.002 mm / s) under 105 MPa hydrogen gas at room temperature. The larger the RRA, the better the hydrogen embrittlement resistance. In this evaluation, an RRA of 60% or more was considered good.

[0102] The method for measuring molybdenum precipitates in steel is as follows. Molybdenum precipitates were identified by an extraction method in which the steel was electrolyzed and the resulting precipitates were filtered. A 10 mm square sample taken from a cross section perpendicular to the rolling direction of a seamless steel pipe (a cross section perpendicular to the tube axis: C-section) was dissolved using constant-current electrolysis with a 10% AA-based electrolyte. The steel was then placed in a 0.05 wt % aqueous solution of sodium hexametaphosphate and subjected to ultrasonic irradiation to extract precipitates. The solution was filtered through a 50 nm filter to obtain precipitates of 50 nm or less. Precipitates of 50 nm or less that passed through the filter and precipitates exceeding 50 nm on the filter were subjected to white smoke heating treatment with sulfuric acid, perchloric acid, and nitric acid, followed by hydrochloric acid dissolution. The precipitate solution and the electrolyte containing the dissolved molybdenum were then analyzed for concentration using ICP to calculate the Mo concentration, Mo amount, and dissolved Mo concentration in precipitates of each size.

[0103] All of the steel types shown as examples of the invention in Table 3 satisfied the condition of an RRA of 60% or more in a slow strain rate tensile test in hydrogen gas.

[0104] The configurations shown in the above embodiments are merely examples, and it is possible to omit or change part of the configurations without departing from the gist of the invention.

[0105] The high-pressure tank 100 described above may also include combinations of the features shown in Supplementary Notes 1 to 13 below. These combinations are described below.

[0106] [Supplementary Note 1] A high-pressure tank for storing a mixed gas containing natural gas and hydrogen, the mixed gas containing more than 2% hydrogen by volume, wherein the tank body comprises: a cylindrical portion extending in a first direction; and dome portions connected to both ends of the cylindrical portion in the first direction, wherein a steel material constituting the tank body has a tensile strength TS of 850 MPa or more, a structure in which tempered martensite accounts for 95% or more by area, and carbides of 100 nm or less are scattered throughout the metal structure as precipitates, and a strength of 11,250 cycles or more in a crack propagation analysis. [Supplementary Note 2] The high-pressure tank according to Supplementary Note 1, wherein the tank body satisfies the condition 255 MPa < σ ≦ 445 MPa when the maximum tangential stress generated in the cylindrical portion when a mixed gas containing hydrogen of p = 25 MPa is stored therein, where σ = Di·p / (2t). [Appendix 3] The high-pressure tank according to appendix 1 or 2, wherein the wall thickness t of the cylindrical portion satisfies 8 mm≦t. [Appendix 4] The high-pressure tank according to any one of appendices 1 to 3, wherein the cylindrical portion and the dome portion are integrally molded. [Appendix 5] The high-pressure tank according to any one of appendices 1 to 4, wherein the precipitated carbides include molybdenum precipitates, and the molybdenum precipitates contain 50% or more of the Mo contained in the steel as precipitates. [Appendix 6] The high-pressure tank according to appendix 5, wherein 50% or more of the Mo contained in the molybdenum precipitates is contained in precipitates having a diameter of 50 nm or less. [Appendix 7] The high-pressure tank according to appendix 5 or 6, wherein the steel material constituting the tank body contains, by mass%, Mo: more than 1.0% and not more than 3.0%. [Appendix 8] The high-pressure tank according to any one of Appendices 5 to 7, wherein the steel material constituting the tank body contains, in mass%, Mo / C, which is the ratio of the Mo content to the C content, in the range of 2.0 to 12.0.[Supplementary Note 9] The high-pressure tank according to Supplementary Note 8, wherein the steel material constituting the tank body contains, by mass%, C: 0.20 to 0.50%. [Supplementary Note 10] The high-pressure tank according to Supplementary Note 9, wherein the steel material constituting the tank body has a composition containing, in mass%, Si: 0.05 to 2.00%, Mn: 0.30 to 1.5%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.15%, N: 0.006% or less, Cr: more than 0.2% and 1.7% or less, Nb: 0.001 to 0.02%, B: 0.0003 to 0.0030%, O: 0.0030% or less, Ti: 0.003 to 0.025%, with the balance being Fe and unavoidable impurities. [Supplementary Note 11] The high-pressure tank according to Supplementary Note 10, wherein the steel material constituting the tank body, in addition to the above composition, further contains, by mass%, one or more elements selected from: V: 0.3% or less, Cu: 1.0% or less, Ni: 2.0% or less, and W: 3.0% or less. [Supplementary Note 12] The high-pressure tank according to Supplementary Note 10 or 11, wherein the steel material constituting the tank body, in addition to the above composition, further contains, by mass%, H: 0.0010% or less. [Supplementary Note 13] The high-pressure tank according to any one of Supplements 10 to 12, wherein the steel material constituting the tank body, in addition to the above composition, further contains, by mass%, Ca: 0.0005 to 0.005%.

[0107] 10: Tank body 11: Cylindrical portion 12: Dome portion 12a: Dome portion 12b: Dome portion 13: Cap 14: Supply device 90: Space 100: High-pressure tank

Claims

1. A high-pressure tank for storing a mixed gas containing natural gas and hydrogen, wherein the hydrogen content in the mixed gas is more than 2% by volume, and the tank body includes: A cylindrical portion extending in a first direction; and Dome portions connected to both ends of the cylindrical portion in the first direction. The steel material constituting the tank body has: A tensile strength TS of 850 MPa or more; A structure with a tempered martensite area ratio of 95% or more, and carbides with a size of 100 nm or less are dispersed in the metal structure as precipitates; A strength of 11250 cycles or more in crack propagation analysis. High-pressure tank.

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

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

4. The high-pressure tank according to any one of claims 1 to 3, wherein the cylindrical portion and the dome portion are integrally formed. High-pressure tank.

5. The high-pressure tank according to any one of claims 1 to 4, wherein the precipitated carbides include molybdenum precipitates, and among the Mo contained in the steel material, 50% or more is contained as precipitates. High-pressure tank.

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

7. The high-pressure tank according to claim 5 or 6, wherein the steel material constituting the tank body contains, by mass%, Mo: more than 1.0% and 3.0% or less. High-pressure tank.

8. The high-pressure tank according to any one of claims 5 to 7, wherein the steel material constituting the tank body contains, by mass%, such that the ratio of Mo to C, which is the ratio of the content of Mo to the content of C, is in the range of 2.0 to 12.

0. High-pressure tank.

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

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

11. The high-pressure tank according to claim 10, wherein the steel material constituting the tank body further contains, in addition to the above composition, by mass%, one or more selected from V: 0.3% or less, Cu: 1.0% or less, Ni: 2.0% or less, W: 3.0% or less, a high-pressure tank.

12. The high-pressure tank according to claim 10 or 11, wherein the steel material constituting the tank body further contains, in addition to the above composition, by mass%, H: 0.0010% or less, a high-pressure tank.

13. The high-pressure tank according to any one of claims 10 to 12, wherein the steel material constituting the tank body further contains, in addition to the above composition, by mass%, Ca: 0.0005 to 0.005%, a high-pressure tank.

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