Steel pipe for high-pressure hydrogen piping and high-pressure hydrogen piping using the same

Abstract

The present invention provides a steel pipe for high-pressure hydrogen piping suitable for piping of high-pressure hydrogen gas used in fuel cell vehicles and a high-pressure hydrogen piping using the steel pipe. The steel pipe for high-pressure hydrogen piping is characterized in that the chemical composition is C: 0.17-0.27%, Si: 0.05-0.40%, Mn: 0.30-2.00%, P: 0.035% or less, S: 0.035% or less, Cu: 0-0.50%, Mo: 0-1.0%, V: 0-0.15%, and the balance is Fe and impurities, the metallographic structure of the central part of the wall thickness of the steel pipe includes a mixed structure of bainite and ferrite, the tensile strength in a hydrogen atmosphere is 500-900 MPa, the hardness of the central part of the wall thickness is 160-280 HV1, and the maximum value of the depth of defects existing on the inner surface of the steel pipe is 200 μm or less.

Description

Technical Field

[0001] The present invention relates to a steel pipe for high-pressure hydrogen piping and a high-pressure hydrogen piping using the same, and in particular to a steel pipe for high-pressure hydrogen piping suitable as piping for high-pressure hydrogen used in a fuel cell vehicle and a high-pressure hydrogen piping using the same. Background Art

[0002] As a countermeasure to future energy depletion, the movement to promote energy conservation and resource recycling and the development of technologies to achieve these goals are prevalent. In recent years, in particular, as a global measure, in order to prevent global warming, there is a strong need to reduce the CO2 emissions associated with fuel combustion. As a transportation device with low CO2 emissions, a fuel cell (PEFC: solid polymer fuel cell) using hydrogen and oxygen as fuel is installed on the vehicle to generate electricity and drive the motor to travel. As a transportation device that plays the same role, electric vehicles can also be cited, but because they cannot fully meet the requirements of cruising range and vehicle size, the application of fuel cell technology is particularly expected in commercial vehicles. In promoting the popularization of fuel cell vehicles, the construction of hydrogen stations is being promoted throughout Japan, and the use and demand of hydrogen utilization equipment are expanding year by year. The fuel hydrogen in fuel cell vehicles is usually installed in the vehicle as high-pressure hydrogen gas, and its pressure was originally 35MPa, but in order to extend the cruising range, it has now risen to 70MPa, etc., and it is believed that the relevant equipment also needs to cope with the use in a high-pressure hydrogen environment.

[0003] However, in order to popularize the use of high-pressure hydrogen in daily life, there are many technical issues that need to be solved in addition to relaxing restrictions and reducing costs. In particular, in order to achieve a balance between safety and economy of high-pressure hydrogen utilization equipment, it is necessary to consider the influence of "hydrogen embrittlement" that deteriorates the various strength characteristics of metal materials due to the influence of hydrogen and select materials and perform strength design.

[0004] In the past, with regard to the materials for piping, joints, valves, etc. used in hydrogen stations, taking into account the impact of hydrogen on the materials, the illustrative standard for general high-pressure gas safety rules (Japan High Pressure Gas Safety Association) stipulates that the following materials are representative examples of materials that can be used in high-pressure hydrogen environments: SUS316·SUS316L (JIS G 3459) stainless steel for piping, in which the cross-sectional shrinkage rate in the tensile test or the steel material certificate is 75% or more, and the Ni equivalent is 28.5 or more when the normal temperature is above -45°C and less than -10°C, the Ni equivalent is 27.4 or more when the normal temperature is above -10°C and less than 20°C, and the Ni equivalent is 26.3 or more when the normal temperature is above 20°C and below 250°C. When confirming the hydrogen environment adaptability of materials, the illustrative benchmark is usually applied to implement the SSRT (Slow Strain Rate Tensile) test in hydrogen, and the relative cross-sectional shrinkage RRA (Relative Reduction of Area) in the atmosphere and in hydrogen meets the judgment standard (according to the judgment formula RRA ≥ 0.8) as an indicator. In addition, the allowable stress during design is based on the benchmark strength as the tensile strength, and the safety factor is fully increased (S = 4.0) to ensure safety. The high-pressure hydrogen piping currently installed in fuel cell vehicles is not subject to the above-mentioned illustrative benchmark, so there is no restriction on the use of materials, but considering safety, actual results, etc., the standard material SUS316L of hydrogen stations is used.

[0005] In addition, carbon steel, which is a relatively cheap material, is easily affected by hydrogen and lacks practical results, so it does not meet the standards. However, since SUS316L is low in strength, the piping is thick-walled and small-diameter, and the hydrogen flow rate becomes smaller. In order to ensure the flow rate, the inner diameter needs to be increased, but in order to withstand high pressure, it is hoped that the wall thickness will be increased accordingly. Due to reasons such as economy, it is ideal to control the wall thickness to a minimum, and it is expected to be applied to high-strength materials that can be used in high-pressure hydrogen environments in the future. As other technologies in the case of applying high pressure, on the other hand, there are high-pressure fuel injection pipes for diesel engines that perform high-pressure fuel injection, and the following patent documents 1 and 2 are disclosed.

[0006] Patent document 1 discloses a method for manufacturing a steel pipe for fuel injection of a diesel engine, wherein the inner surface of a hot-rolled seamless steel pipe blank is ground and polished by shot blasting and then cold drawn. According to this manufacturing method, it is disclosed that the depth of defects (concavities, flaking, microcracks, etc.) on the inner surface of the steel pipe can be made less than 0.10 mm, thereby achieving high strength of the steel pipe for fuel injection. In addition, Patent document 2 discloses a steel pipe for fuel injection pipe having a maximum diameter of less than 20 μm of non-metallic inclusions present at least from the inner surface of the steel pipe to a depth of 20 μm and a tensile strength of more than 500 MPa.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 9-57329

[0010] Patent Document 2: Japanese Patent Application Publication No. 2007-284711 Summary of the invention

[0011] Problems to be solved by the invention

[0012] The above-mentioned material that can be used in a high-pressure hydrogen environment, namely, SUS316L (JIS G 3459) stainless steel for piping, has hydrogen environment adaptability, but lacks economic efficiency. In addition, the use of only standard materials has become a barrier to the popularization of high-pressure hydrogen utilization equipment. Furthermore, in a hydrogen environment, it is necessary to understand the influence of hydrogen on materials. In the above-mentioned Patent Documents 1 and 2, it is completely unclear about "hydrogen embrittlement" that deteriorates various strength characteristics of metal materials due to the influence of hydrogen.

[0013] The present invention has been made in view of these actual conditions, and an object of the present invention is to provide a steel pipe for high-pressure hydrogen piping and a high-pressure hydrogen piping using the same, which verifies the influence of hydrogen on the fatigue properties of carbon steel and has both safety and economy.

[0014] Means for solving problems

[0015] The present invention relates to a steel pipe for high-pressure hydrogen piping, characterized in that the chemical composition is, in mass%, C: 0.17-0.27%, Si: 0.05-0.40%, Mn: 0.30-2.00%, P: 0.035% or less, S: 0.035% or less, Cu: 0-0.50%, Mo: 0-1.0%, V: 0-0.15%, and the balance is Fe and impurities, the metallographic structure at the center of the wall thickness of the steel pipe comprises a mixed structure of bainite and ferrite, the tensile strength in a hydrogen atmosphere is 500-900 MPa, the hardness at the center of the wall thickness is 160-280 HV1, the inner diameter d is 3 mm or more, the outer diameter D is 12 mm or less, the wall thickness is 1 mm or more, the ratio of the outer diameter to the inner diameter satisfies the following formula (1), and the maximum value of the depth of defects present on the inner surface of the steel pipe is 200 μm or less.

[0016] [Number 1]

[0017] D / d≥2.0···(1)

[0018] In addition, the steel pipe for high-pressure hydrogen piping is characterized in that, in place of a part of Fe, it contains, in mass%, one or more of Ti: 0.005-0.015%, Nb: 0.015-0.045%, Cr: 0-1.0%, Ni: 0-0.50%, Al: 0.005-0.060%, O: 0.0040% or less, Ca: 0.0010% or less, and N: 0.0020-0.0080%.

[0019] The steel pipe is also characterized in that the chemical composition thereof contains, in mass%, one or more selected from the group consisting of Cr: 0.2-1.0%, Mo: 0.03-1.0%, Cu: 0.03-0.50%, Ni: 0.03-0.50%, and V: 0.06-0.10%.

[0020] Furthermore, the high-pressure hydrogen piping according to the present invention is characterized in that a steel pipe for high-pressure hydrogen piping having any of the above-mentioned chemical compositions is used as a material.

[0021] Effects of the Invention

[0022] The steel pipe for high-pressure hydrogen piping of the present invention can provide a steel pipe for high-pressure hydrogen piping having excellent fatigue properties, safety and economy. Therefore, the steel pipe for high-pressure hydrogen piping of the present invention can be particularly suitably used as a high-pressure hydrogen piping used in fuel cell vehicles. DETAILED DESCRIPTION

[0023] Each of the requirements of the present invention will be described in detail below.

[0024] 1. Chemical composition

[0025] The reasons for limiting the content of each element are as follows. In the following description, "%" for content means "mass %".

[0026] C: 0.17~0.27%

[0027] C is an element effective in improving the strength of steel. In order to ensure the required tensile strength, the C content needs to be 0.17% or more. However, if the C content exceeds 0.27%, the workability decreases, so the C content is preferably 0.17 to 0.27%.

[0028] Si: 0.05-0.40%

[0029] It is preferable to contain Si for deoxidation of the steel, and it is necessary to make it 0.05% or more in order to improve the strength. If the Si content exceeds 0.40%, the toughness may be reduced.

[0030] Mn: 0.30~2.0%

[0031] Mn is an element that not only has a deoxidizing effect, but is also effective in improving the hardenability, strength and toughness of steel. However, if its content is less than 0.30%, sufficient strength cannot be obtained. On the other hand, if it exceeds 2.0%, MnS coarsening occurs, stretching during hot rolling, and toughness decreases instead. Therefore, the Mn content is set to 0.30-2.0%.

[0032] P: 0.035% or less

[0033] P is an element that inevitably exists in steel as an impurity. If its content exceeds 0.035%, not only will the hot workability be reduced, but also the toughness will be significantly reduced due to grain boundary segregation. Therefore, the P content is set to 0.035% or less.

[0034] S: 0.035% or less

[0035] S is an element that inevitably exists in steel as an impurity, similar to P. If its content exceeds 0.035%, it will segregate at the grain boundaries and easily form sulfide-based inclusions, resulting in a decrease in fatigue strength. Therefore, the S content is set to 0.035% or less.

[0036] Cu: 0~0.50%

[0037] Cu is an element that has the effect of improving the hardenability of steel and thus improving strength and toughness. However, even if the Cu content exceeds 0.50%, the effect is saturated and the alloy cost increases as a result, so the Cu content is set to 0.50% or less.

[0038] Mo: 0~1.0%

[0039] Mo is an element that helps ensure high strength by improving hardenability and temper softening resistance. However, even if the Mo content exceeds 1.0%, its effect is saturated, and the alloy cost increases as a result. Therefore, the Mo content, when contained, is set to 1.0% or less.

[0040] V: 0~0.15%

[0041] V is an element that precipitates as fine carbides (VC) during tempering, increases tempering softening resistance, enables high-temperature tempering, and contributes to high strength and high toughness of steel. However, if the V content exceeds 0.15%, toughness is reduced, so the V content, when contained, is set to 0.15% or less.

[0042] Ti: 0.005~0.015%

[0043] Ti is an element that contributes to preventing the coarsening of crystal grains by finely precipitating in the form of TiN, etc. To achieve this effect, the Ti content needs to be set to 0.005% or more. On the other hand, if the Ti content exceeds 0.015%, the grain refinement effect tends to be saturated, and large Ti-Al composite inclusions may sometimes be generated. Therefore, the Ti content is set to 0.005-0.015%.

[0044] Nb: 0.015~0.045%

[0045] Nb is finely dispersed in steel as carbides or carbonitrides and has a strong effect of pinning grain boundaries, so it is an element necessary to obtain the desired fine-grained structure. In addition, the fine dispersion of Nb carbides or carbonitrides improves the strength and toughness of steel. For these purposes, the Nb content is preferably 0.015 to 0.045%.

[0046] Cr: 0~1.0%

[0047] Cr is an element having the effect of improving the hardenability and wear resistance of steel. If the content exceeds 1.0%, the toughness and cold workability are reduced. Therefore, when Cr is contained, the content is made 1.0% or less.

[0048] Ni: 0~0.50%

[0049] Ni is an element that has the effect of improving the hardenability of steel and thus improving strength and toughness, similar to Cu. However, even if the Ni content exceeds 0.50%, the effect is saturated, and the alloy cost is increased as a result, so the Ni content, when contained, is set to 0.50% or less.

[0050] Al: 0.005~0.060%

[0051] Al is an element effective in deoxidizing steel and has the function of improving the toughness and workability of steel. In order to obtain these effects, it is necessary to contain 0.005% or more of Al. On the other hand, if the Al content exceeds 0.060%, large Ti-Al composite inclusions may be generated. Therefore, the Al content is set to 0.005-0.060%.

[0052] O: 0.0040% or less

[0053] O forms coarse oxides, which easily cause a decrease in the ultimate internal pressure. From this viewpoint, the O content needs to be 0.0040% or less.

[0054] Ca: 0.0010% or less

[0055] Ca has the function of agglomerating silicate inclusions. If the Ca content exceeds 0.0010%, the ultimate internal pressure decreases due to the formation of coarse C-based inclusions. Therefore, the Ca content is set to 0.0010% or less.

[0056] N: 0.0020~0.0080%

[0057] N is an element that inevitably exists in steel as an impurity. However, in the present invention, in order to prevent the coarsening of grains caused by the pinning effect of TiN, it is necessary to retain 0.0020% or more of N. On the other hand, if the N content exceeds 0.0080%, the possibility of generating large Ti-Al composite inclusions increases. Therefore, the N content is set to 0.0020-0.0080%.

[0058] 2. Metallographic structure

[0059] The metallographic structure of the steel pipe for high-pressure hydrogen piping according to the present invention is preferably composed of a mixed structure of bainite and ferrite. If martensite is present in the structure, although a tensile strength higher than 1000MPa can be ensured, the adaptability to hydrogen environment is sometimes insufficient. In addition, as an example of an improvement method, tempering treatment at high temperature is required, which leads to an increase in cost due to heat treatment. In the present invention, it is found that the above-mentioned treatment is not required, and a steel pipe for high-pressure hydrogen piping that meets the target mechanical properties, has both safety and economy with a non-quenched and tempered metallographic structure morphology and a high-pressure hydrogen piping using the same are achieved.

[0060] 3. Mechanical properties

[0061] The tensile strength in a hydrogen atmosphere of the steel pipe for high-pressure hydrogen piping according to the present invention is preferably 500 to 900 MPa.

[0062] In addition, it is preferred that the hardness of the central portion of the wall thickness is 160 to 280 HV1. If the hardness is less than 160 HV1, sufficient strength in a hydrogen atmosphere cannot be obtained. On the other hand, if the hardness exceeds 280 HV1, there is a tendency that the influence of hydrogen on material properties becomes more significant. It should be noted that "HV1" refers to the "hardness symbol" when the test force is set to 9.8 N (1 kgf) and the Vickers hardness test is performed (refer to JIS Z 2244: 2009).

[0063] When the hardness of the center portion of the wall thickness is 160 HV1 or more, a tensile strength of 500 MPa or more can be obtained.

[0064] Furthermore, the maximum value of the depth of defects existing on the inner surface of the steel pipe is preferably 200 μm or less.

[0065] 4. Size

[0066] The dimensions and the ratio of the outer diameter to the inner diameter of the steel pipe for high-pressure hydrogen piping according to the present invention are appropriately set according to the applied technology, the purpose of use, etc. As the steel pipe for high-pressure hydrogen piping, for example, it is preferred that the inner diameter d is 3 mm or more, the outer diameter D is 12 mm or less, the wall thickness is 1 mm or more, and the ratio of the outer diameter to the inner diameter satisfies the above formula (1).

[0067] In the formula (1), D is the outer diameter (mm) of the steel pipe for high-pressure hydrogen piping, and d is the inner diameter (mm).

[0068] On the other hand, the upper limit of D / d is not particularly set, but if the value is too large, bending becomes difficult, so it is preferably 3.0 or less, and more preferably 2.8 or less.

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0070] Example

[0071] In industry, defects in components that cause cracks are unavoidable, and when the equipment has an extremely long service life and needs to withstand a lot of repeated stress, the fatigue limit of the material is considered for design. In addition, in order to ensure high reliability, it is important to verify the influence of defect size and inclusions. One of them is the √area parameter model that quantitatively evaluates the influence of small defects on fatigue limit (non-patent literature: "Metal Fatigue - The Effect of Small Defects and Inclusions", 1st edition (1993), Yangxiantang, written by Takanobu Murakami). Based on this √area parameter model and fatigue life test results, the limit value ΔK under the progression of fatigue cracking at each stress ratio is calculated. th Finally, the ΔK of R = 0 is derived as the high-pressure hydrogen piping specification for fuel cell vehicles. thIn addition, the stress expansion coefficient width ΔK of cracks existing on the inner surface of the pipe subjected to internal pressure is given by the following formula (2).

[0072] [Number 2]

[0073] ΔK=(2.24S 2 Δp / S 2 -1)√πa······(2)

[0074] Where, S in the above formula (2) is the ratio of the inner diameter to the outer diameter, Δp is the pressure amplitude (MPa), and a is the crack depth (μm). th,R＝0 ≥ΔK means that the fatigue limit design is established, ΔK th,R＝0 =ΔK, the inner-outer diameter ratio derived becomes the theoretical lower limit value.

[0075] Using the steel type according to the present invention managed by the chemical composition shown in Table 1, fatigue life tests were carried out at R=-1 and 0.1, and the lower limit value of the inner and outer diameter ratio at which fatigue limit design was established was found based on the √area parameter model.

[0076] Steel raw materials (billets) having the chemical composition shown in Table 2 were cut into specified lengths and processed into test pieces to prepare test materials for fatigue life tests. In the evaluation of the test materials, a micro defect with a depth of 100 μm (√area = 125 μm) was introduced as a defect assumed to exist in the piping material based on the √area parameter model.

[0077] The metallographic structure at this time is a mixed structure of bainite and ferrite, the tensile strength is 703MPa in air and 698MPa in hydrogen, and the hardness of the central part of the billet wall thickness is 223HV1 (Table 3). Based on the above results, it can be considered that the tensile strength in air is equivalent to that in hydrogen.

[0078] The fatigue life test conditions are: at a stress ratio of R = -1, 0.1, the stress is repeatedly varied in a sinusoidal manner relative to time, the frequency in the atmosphere is set to 10 Hz, and the frequency in hydrogen is set to 1 Hz. The fatigue life test in hydrogen is carried out with 95 MPa of hydrogen sealed in the pressure vessel of the test machine. As for the test results, the stress amplitude of the fatigue limit at the stress ratio of R = -1, 0.1 and ΔK th The relationship between is shown in Table 4. At this time, even if the number of repetitions in the atmosphere is 1×10 7 times, in hydrogen: 2×10 6 The maximum stress amplitude without fracture was evaluated as the fatigue limit.

[0079] Here, based on the √area parameter model and fatigue life test results, the ΔK of R = 0, which is the high-pressure hydrogen piping specification for fuel cell vehicles, is used.th Substituting this into ΔK th,R＝0 =4.86MPa m 1 / 2 Assuming that an internal pressure Δp = 90 MPa acts on a pipe having a crack with a depth of a = 100 μm on the inner surface, the inner and outer diameter ratio for which fatigue limit design is established is S = 2.0 as the lower limit value obtained from equation (2).

[0080] When an internal pressure Δp=90 MPa acts on a pipe having a crack with a depth of a=200 μm on the inner surface, the inner and outer diameter ratio for which fatigue limit design is established is S=2.4 as the lower limit value from equation (2), satisfying equation (1).

[0081] As an example of the inner and outer diameter ratio for which fatigue limit design is established, under the same conditions, when using the carbon steel pipe STKM17A for mechanical structures, D / d ≥ 2.2 is obtained. However, if the steel pipe for high-pressure hydrogen piping according to the present invention is used, when D / d ≥ 2.0 and the inner diameter d = 3.5 mm, the pipe wall can be thinned by about 24%.

[0082] In the present invention, fatigue life tests in high-pressure hydrogen (R=-1, 0.1) were carried out using test pieces into which micro defects with a depth of 100 μm were introduced. As a result, it was confirmed that the fatigue limit did not decrease compared to that in the atmosphere.

[0083] This revealed that a high-pressure hydrogen pipe can be designed in hydrogen in the same manner as in the air, and that the steel grade is used to obtain a high-pressure hydrogen pipe.

[0084] [Table 1]

[0085]

[0086] [Table 2]

[0087]

[0088] [Table 3]

[0089]

[0090] [Table 4]

[0091]

Claims

1. A steel pipe for high-pressure hydrogen piping, characterized in that: The chemical composition is, in mass%, C: 0.17-0.27%, Si: 0.05-0.40%, Mn: 0.30-2.00%, P: 0.035% or less, S: 0.035% or less, Cu: 0-0.50%, Mo: 0-1.0%, V: 0-0.15%, and the balance is Fe and impurities, wherein the metallographic structure of the central portion of the wall thickness of the steel pipe comprises a mixed structure of bainite and ferrite, the tensile strength in a hydrogen atmosphere is 500-900 MPa, the hardness of the central portion of the wall thickness is 160-280 HV1, the inner diameter d is 3 mm or more, the outer diameter D is 12 mm or less, the wall thickness is 1 mm or more, the ratio of the outer diameter to the inner diameter satisfies the following formula (1), and the maximum value of the depth of defects present on the inner surface of the steel pipe is 200 μm or less, D / d≥2.0···(1).

2. The steel pipe for high-pressure hydrogen piping according to claim 1, characterized in that: Substituting for a part of Fe, the alloy contains, by mass%, at least one of Ti: 0.005-0.015%, Nb: 0.015-0.045%, Cr: 0-1.0%, Ni: 0-0.50%, Al: 0.005-0.060%, O: 0.0040% or less, Ca: 0.0010% or less, and N: 0.0020-0.0080%.

3. The steel pipe for high-pressure hydrogen piping according to claim 2, characterized in that: The chemical composition of the steel pipe contains, in mass%, one or more selected from the group consisting of 0.2% to 1.0% Cr, 0.03% to 1.0% Mo, 0.03% to 0.50% Cu, 0.03% to 0.50% Ni, and 0.06% to 0.10% V.

4. High-pressure hydrogen piping, characterized in that: The steel pipe for high-pressure hydrogen piping according to any one of claims 1 to 3 is used as a material.

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