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

Abstract

Provided are a steel pipe for high-pressure hydrogen piping and high-pressuring hydrogen piping using the steel pipe, which are favorable as piping for high-pressure hydrogen gas used in a fuel cell powered vehicle.　This high-pressure hydrogen piping is characterized by: having, in terms of mass%, a chemical composition of 0.17-0.27% C, 0.05-0.40% Si, 0.30-2.00% Mn, 0.035% or less P, 0.035% or less S, 0-0.50% Cu, 0-1.0% Mo, and 0-0.15% V, the remainder being Fe and impurities; the metal composition of a center section in the thickness direction of the steel pipe comprising a mixed composition of bainite and ferrite; the tensile strength in a hydrogen atmosphere being 500-900 MPa; the hardness of the center section in the thickness direction being 160-280 HV1; and the maximum value of the depth of defects present in the inner surface of the piping being 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 more particularly to a steel pipe for high-pressure hydrogen piping suitable as a piping for high-pressure hydrogen gas used in fuel cell vehicles and a high-pressure hydrogen piping using the same.

Background Art

[0002] As a countermeasure against future energy depletion, campaigns to promote energy conservation and resource recycling and the development of technologies to achieve these purposes are being actively carried out. In particular, in recent years, as a global effort to prevent global warming, there has been a strong demand to reduce CO2 emissions associated with fuel combustion. As a transportation device with low CO2 emissions, a fuel cell vehicle (FCV) that mounts and generates electricity with a fuel cell (PEFC: polymer electrolyte fuel cell) using hydrogen and oxygen as fuel and drives a motor to run can be cited. Although electric vehicles and the like can also be cited as transportation devices that play the same role, since they cannot sufficiently meet the requirements for cruising range and vehicle size, the application of technologies equipped with fuel cells is particularly expected for commercial vehicles and the like. When promoting the spread of fuel cell vehicles, the uses and demands of hydrogen utilization devices are expanding year by year, such as the construction of hydrogen stations being promoted nationwide. The hydrogen fuel in fuel cell vehicles is generally mounted on the vehicle as high-pressure hydrogen gas, and its pressure was initially 35 MPa, but it has now increased to 70 MPa for the purpose of extending the cruising range. It is also conceivable that related devices are required to cope with use in a high-pressure hydrogen gas environment. However, in order to popularize the use of these high-pressure hydrogen gases on a daily basis, there are many technical problems that must be solved in addition to regulatory relaxation and cost reduction. Among them, in particular, in order to achieve both the safety and economy of high-pressure hydrogen gas utilization devices, material selection and strength design considering the influence of "hydrogen embrittlement" in which various strength characteristics of metal materials deteriorate due to the influence of hydrogen are required.

[0003] Conventionally, materials such as pipes, joints, and valves used in hydrogen stations, considering the influence of hydrogen on the materials, are represented by stainless steels for pipes such as SUS316 and SUS316L (JIS G 3459) in the exemplary standards related to the General High-Pressure Gas Safety Regulations (High-Pressure Gas Safety Association). For materials that can be used in a high-pressure hydrogen gas environment, it is stipulated that the reduction value in the tensile test or the mill sheet is 75% or more, and the Ni equivalent is 28.5 or more when its normal operating temperature is -45°C or more and less than -10°C, 27.4 or more when the normal operating temperature is -10°C or more and less than 20°C, and 26.3 or more when the normal operating temperature is 20°C or more and 250°C or less. When confirming the hydrogen environment compatibility of materials, it is common to apply this exemplary standard. When conducting an SSRT (Slow Strain Rate Tensile) test in hydrogen, the relative reduction of area RRA (Relative Reduction of Area) in air and hydrogen gas meeting the judgment criteria (RRA≧0.8 from the judgment formula) is used as an indicator. Also, the allowable stress during design uses the reference strength as the tensile strength, and by setting the safety factor (S = 4.0) sufficiently high, safety is ensured. Since the high-pressure hydrogen pipes installed in current fuel cell vehicles are not subject to the above-mentioned exemplary standards, there are no restrictions on the use of materials. However, considering safety and performance, etc., the standard material SUS316L for hydrogen stations is applied. In addition, carbon steel, which is a relatively inexpensive material, is considered out of specification because it is easily affected by hydrogen and has poor performance records. However, since SUS316L has low strength, the pipe has a thick wall and a small diameter, resulting in a small hydrogen flow rate. To ensure the flow rate, an increase in the inner diameter is required. However, to withstand high pressure, it is desirable to increase the wall thickness accordingly. For reasons such as economy, it is ideal to keep the wall thickness to a minimum, and in the future, the application of high-strength materials that can be used in a high-pressure hydrogen gas environment is expected. As another technology in a high-pressure situation, on the other hand, there are high-pressure fuel injection pipes for diesel engines that perform high-pressure fuel injection, and Patent Document 1 and Patent Document 2 shown below are disclosed.

[0004] Patent Document 1 discloses a method for manufacturing a steel pipe used for fuel injection of a diesel engine, in which the inner surface of a seamless steel pipe material that has been hot-rolled is subjected to shot blasting treatment, grinding and polishing, and then cold drawing is performed. According to this manufacturing method, it is disclosed that since the depth of defects (such as unevenness, heges, and fine cracks) on the inner surface of the steel pipe can be made 0.10 mm or less, the strength of the steel pipe used for fuel injection can be increased. Further, Patent Document 2 discloses a steel pipe for fuel injection in which the maximum diameter of non-metallic inclusions present at a depth of at least 20 μm from the inner surface of the steel pipe is 20 μm or less and the tensile strength is 500 MPa or more.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Although the stainless steel for piping of SUS316L (JIS G 3459), which is a material that can be used in the above-mentioned high-pressure hydrogen gas environment, has hydrogen environment compatibility, it lacks economic efficiency, and the use of only standard materials has become an obstacle to the popularization of high-pressure hydrogen gas utilization equipment. Further, in a hydrogen gas environment, it is necessary to grasp the influence of hydrogen on the material, but Patent Documents 1 and 2 mentioned above do not clarify at all about "hydrogen embrittlement" in which various strength characteristics of metal materials deteriorate due to the influence of hydrogen. The present invention has been proposed in view of these actual situations, and an object thereof is to verify the influence of hydrogen on the fatigue characteristics of carbon steel and to provide a high-pressure hydrogen piping steel pipe that achieves both safety and economic efficiency and a high-pressure hydrogen piping using the same.

Means for Solving the Problems

[0007] The steel pipe for high-pressure hydrogen piping according to the present invention has a chemical composition in mass%, C: 0.17 to 0.27%, Si: 0.05 to 0.40%, Mn: 0.30 to 2.00%, P: 0.035% or less, S: 0.035% or less, Cu: 0 to 0.50%, Mo: 0 to 1.0%, V: 0 to 0.15%, the balance being Fe and impurities, the metallographic structure at the center of the wall thickness of the steel pipe is composed of a mixed structure of bainite and ferrite, the tensile strength in a hydrogen atmosphere is 500 to 900 MPa, the hardness at the center of the wall thickness is 160 to 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 existing on the inner surface of the steel pipe crack is 200 μm or less, which is characterized in that.

[0008]

Equation

[0009] Further, in the steel pipe for high-pressure hydrogen piping, instead of a part of Fe, in mass%, Ti: 0.005 to 0.015% Nb: 0.015 to 0.045% Cr: 0 to 1.0% Ni: 0 to 0.50% Al: 0.005 to 0.060% O: 0.0040% or less Ca: 0.0010% or less N: 0.0020 to 0.0080% contains one or more of the above, which is characterized in that.

[0010] Further, the chemical composition of the steel pipe is, in mass%, Cr: 0.2 to 1.0% Mo: 0.03 to 1.0% Cu: 0.03 to 0.50% Ni: 0.03 to 0.50% V: 0.06 to 0.10% It is characterized by containing one or more selected from the following.

[0011] Further, the high-pressure hydrogen pipe according to the present invention is characterized by using, as a material, a steel pipe for high-pressure hydrogen pipes having any of the above chemical compositions.

Effects of the Invention

[0012] The steel pipe for high-pressure hydrogen pipes of the present invention is excellent in fatigue characteristics, and it is possible to obtain a steel pipe for high-pressure hydrogen pipes that achieves both safety and economy. Therefore, the steel pipe for high-pressure hydrogen pipes according to the present invention can be suitably used particularly as a high-pressure hydrogen pipe used in a fuel cell vehicle.

Modes for Carrying Out the Invention

[0013] Hereinafter, each requirement of the present invention will be described in detail.

[0014] 1. Chemical Composition The reasons for limiting each element are as follows. In the following description, “%” for the content means “mass %”.

[0015] C: 0.17 to 0.27% C is an element effective for increasing the strength of steel. In order to ensure the desired tensile strength, it is necessary to set the C content to 0.17% or more. However, if the C content exceeds 0.27%, the workability deteriorates, so the C content is preferably 0.17 to 0.27%.

[0016] Si: 0.05 to 0.40% Si is preferably contained for deoxidation of steel. In order to improve the strength, it is necessary to set the Si content to 0.05% or more. However, if the Si content exceeds 0.40%, it may cause a decrease in toughness.

[0017] Mn: 0.30 to 2.0% Mn not only has a deoxidizing effect but also is an element effective in enhancing the hardenability of steel, improving strength and toughness. However, if its content is less than 0.30%, sufficient strength cannot be obtained. On the other hand, if it exceeds 2.0%, coarsening of MnS occurs, which spreads during hot rolling and instead reduces toughness. Therefore, the Mn content is set to 0.30 - 2.0%.

[0018] P: 0.035% or less P is an element that inevitably exists in steel as an impurity. If its content exceeds 0.035%, it not only causes a decrease in hot workability but also significantly reduces toughness due to grain boundary segregation. Therefore, the P content is set to 0.035% or less.

[0019] S: 0.035% or less S is an element that inevitably exists in steel as an impurity, similar to P. If its content exceeds 0.035%, it segregates at grain boundaries and generates sulfide-based inclusions, easily leading to a decrease in fatigue strength. Therefore, the S content is set to 0.035% or less.

[0020] Cu: 0 - 0.50% Cu is an element that has the effect of improving strength and toughness by enhancing the hardenability of steel. However, if the Cu content exceeds 0.50%, the effect saturates and it also results in an increase in alloy cost. Therefore, the Cu content is set to 0.50% or less.

[0021] Mo: 0 - 1.0% Mo is an element that contributes to ensuring high strength by improving hardenability and increasing the resistance to temper softening. However, if the Mo content exceeds 1.0%, the effect saturates and it also results in an increase in alloy cost. Therefore, when adding Mo, the Mo content is set to 1.0% or less.

[0022] V: 0 - 0.15% V precipitates as fine carbides (VC) during tempering, increasing the tempering softening resistance, enabling high-temperature tempering, and contributing to the strengthening and toughening of the steel. However, when the V content exceeds 0.15%, it rather causes a decrease in toughness. Therefore, the V content when adding it should be 0.15% or less.

[0023] Ti: 0.005 - 0.015% Ti is an element that contributes to preventing the coarsening of crystal grains by precipitating finely in the form of TiN, etc. However, in order to obtain this effect, the Ti content needs to be 0.005% or more. On the other hand, when the Ti content exceeds 0.015%, the effect of grain refinement tends to saturate, and in some cases, large Ti - Al complex inclusions may occur. Therefore, the Ti content is set to 0.005 - 0.015%.

[0024] Nb: 0.015 - 0.045% Nb is an essential element for obtaining a desired fine-grained structure because it has the effect of finely dispersing as carbides or carbonitrides in the steel and strongly pinning the grain boundaries. Also, due to the fine dispersion of Nb carbides or carbonitrides, the strength and toughness of the steel are improved. For these purposes, the Nb content is preferably 0.015 - 0.045%.

[0025] Cr: 0 - 1.0% Cr is an element that has the effect of improving the hardenability and wear resistance of the steel. However, when the content exceeds 1.0%, the toughness and cold workability decrease. Therefore, the Cr content when adding it should be 1.0% or less.

[0026] Ni: 0 - 0.50% Ni is an element that has the effect of improving strength and toughness by increasing the hardenability of the steel, similar to Cu. However, when the Ni content exceeds 0.50%, the effect saturates and it results in an increase in alloy cost. Therefore, the Ni content when adding it should be 0.50% or less.

[0027] Al: 0.005 - 0.060% Al is an element effective for deoxidizing steel and also has the effect of enhancing the toughness and workability of steel. 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%, there is a risk of generating large-sized Ti-Al complex inclusions. Therefore, the Al content is set to 0.005 - 0.060%.

[0028] O: 0.0040% or less O forms coarse oxides and tends to cause a decrease in the internal pressure limit due to them. From such a viewpoint, the O content needs to be 0.0040% or less.

[0029] Ca: 0.0010% or less Ca has the effect of aggregating silicate-based inclusions. If the Ca content exceeds 0.0010%, the internal pressure limit decreases due to the formation of coarse C-based inclusions. Therefore, the Ca content is set to 0.0010% or less.

[0030] N: 0.0020 - 0.0080% N is an element that inevitably exists in steel as an impurity. However, in the present invention, for the purpose of preventing grain coarsening due to the pinning effect of TiN, it is necessary to leave 0.0020% or more of N. On the other hand, if the N content exceeds 0.0080%, the risk of generating large-sized Ti-Al complex inclusions increases. Therefore, the N content is set to 0.0020 - 0.0080%.

[0031] 2. Metallic structure The metallic structure of the steel pipe for high-pressure hydrogen piping according to the present invention preferably consists of a mixed structure of bainite and ferrite. Although the presence of martensite in the structure can ensure a tensile strength higher than 1000 MPa, the hydrogen environment compatibility may not be sufficient in some cases. Also, as an example of the improvement method, tempering treatment at high temperature is essential, but it leads to an increase in cost depending on the heat treatment. In the present invention, it has been found that a steel pipe for high-pressure hydrogen piping and a high-pressure hydrogen piping using the same are realized that do not require the above-mentioned treatment, satisfy the target mechanical properties in a non-quenched and tempered metallic structure form, and achieve both safety and economy.

[0032] 3. Mechanical properties As the tensile strength of the steel pipe for high-pressure hydrogen piping according to the present invention in a hydrogen atmosphere, 500 to 900 MPa is desirable.

[0033] Also, the hardness at the center of the wall thickness is desirably 160 to 280 HV1. If the above hardness is less than 160 HV1, sufficient strength in a hydrogen atmosphere cannot be obtained. On the other hand, when the above hardness exceeds 280 HV1, the influence of hydrogen on the material properties tends to become more prominent. Note that "HV1" means the "hardness symbol" when a Vickers hardness test is carried out with a test force of 9.8 N (1 kgf) (see JIS Z 2244:2009). When the hardness at the center of the wall thickness is 160 HV1 or more, a tensile strength of 500 MPa or more can be obtained.

[0034] Furthermore, the crack The maximum value of the depth existing on the inner surface of the steel pipe is desirably 200 μm or less.

[0035] 4. Dimensions Regarding the dimensions of the steel pipe for high-pressure hydrogen piping according to the present invention, the ratio of the outer diameter to the inner diameter, etc., are appropriately set according to the applicable technology, purpose of use, etc. As the steel pipe for high-pressure hydrogen piping, for example, it is desirable 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).

[0036] However, D in the above formula (1) is the outer diameter (mm) of the steel pipe for high-pressure hydrogen piping, and d is the inner diameter (mm). On the other hand, although the upper limit of D / d is not particularly provided, if the value is too large, bending processing becomes difficult, so it is desirably 3.0 or less, and more desirably 2.8 or less.

[0037] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

Examples

[0038] Industrially, assuming that defects in members that cause crack generation are inevitable, when the service life of equipment is extremely long and it is necessary to withstand a very large number of repeated stresses, design is carried out considering the fatigue limit of the material. In addition, in order to ensure high reliability, it is important to verify the influence of defect dimensions and inclusions. One of them is the √area parameter model (Non-Patent Document: Keishin Murakami, "Metal Fatigue - Influence of Micro Defects and Inclusions", 1st Edition (1993), Yoshikendo) that quantitatively evaluates the influence of micro defects on the fatigue limit. Based on this √area parameter model and the fatigue life test results, the lower limit value ΔK of the fatigue crack growth at each stress ratio th is calculated, and finally, ΔK with R = 0, which is the high-pressure hydrogen piping specification of the fuel cell vehicle th is derived. In addition, the stress intensity factor range ΔK of the crack existing on the inner surface of the pipe under internal pressure is given by the following formula (2).

[0039]

Equation

[0040] However, in the above formula (2), S is the ratio of the inner and outer diameters, Δp is the pressure range (MPa), and a is the crack depth (μm). Here, since ΔK th, R = 0 ≧ΔK means the establishment of the fatigue limit design, the ratio of the inner and outer diameters derived when ΔK th, R = 0 =ΔK becomes the theoretical lower limit value.

[0041] Using the steel type according to the present invention controlled by the chemical components shown in Table 1, fatigue life tests were carried out at R = -1 and 0.1, and the lower limit value of the ratio of the inner and outer diameters at which the fatigue limit design is established was obtained based on the √area parameter model. A steel material (billet) having the chemical components shown in Table 2 was cut into a predetermined length and subjected to test piece processing to obtain a test material for the fatigue life test. In the test material evaluation part, 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. The metallographic structure at this time was a mixed structure of bainite and ferrite. The tensile strength was 703 MPa in air and 698 MPa in hydrogen, and the hardness at the center of the billet thickness was 223 HV1 (Table 3). From the above results, the tensile strength in air can be regarded as equivalent to that in hydrogen.

[0042] The fatigue life test conditions were such that the cyclic stress was varied to take a sine wave with respect to time at stress ratios R = -1 and 0.1. The frequency in air was 10 Hz, and the frequency in hydrogen gas was 1 Hz. The fatigue life test in hydrogen gas was carried out with 95 MPa of hydrogen gas enclosed in the pressure vessel of the testing machine. The test results show the relationship between the stress amplitude of the fatigue limit and ΔK at stress ratios R = -1 and 0.1 in Table 4. At this time, the fatigue limit was evaluated as the maximum stress amplitude at which fracture did not occur even when the number of repetitions was 1×10 th times in air and 2×10 7 times in hydrogen gas. 6

[0043] Here, based on the √area parameter model and the fatigue life test results, when deriving ΔK at R = 0 which is the high-pressure hydrogen piping specification of a fuel cell vehicle, ΔK th = 4.86 MPa m th, R = 0 1 / 2 is obtained. Assuming that an internal pressure Δp = 90 MPa acts on a pipe having a crack with a depth a = 100 μm on the inner surface, the inner-to-outer diameter ratio at which the fatigue limit design holds is obtained with S = 2.0 as the lower limit value from Equation (2). Also, when an internal pressure Δp = 90 MPa acts on a pipe having a crack with a depth a = 200 μm on the inner surface, the inner-to-outer diameter ratio at which the fatigue limit design holds is obtained with S = 2.4 as the lower limit value from Equation (2), and it satisfies Equation (1).

[0044] Regarding the inner-to-outer diameter ratio at which the fatigue limit design holds, as an example, when using carbon steel pipe for mechanical structures STKM17A under the same conditions, D / d ≥ 2.2, whereas when using the high-pressure hydrogen piping steel pipe according to the present invention, D / d ≥ 2.0, and when the inner diameter d = 3.5 mm, it is possible to thin the pipe by about 24%. ​​

[0045] In addition, in the present invention, as a result of conducting a fatigue life test (R = -1, 0.1) in high-pressure hydrogen gas using a test piece with a microdefect having a depth of 100 μm, it has been confirmed that the fatigue limit does not decrease with respect to that in the atmosphere. From these facts, it has been found that even in hydrogen, it is possible to design in the same manner as in the atmosphere, and a high-pressure hydrogen pipe using the said steel type can be obtained.

[0046]

Table 1

[0047]

Table 2

[0048]

Table 3

[0049]

Table 4

Claims

1. The chemical composition is in mass %, C: 0.17 to 0.27%, Si: 0.05 to 0.40%, Mn: 0.30 to 2.00%, P: 0.035% or less, S: 0.035% or less, Cu: 0 to 0.50%, Mo: 0 to 1.0%, V: 0 to 0.15%, The balance: Fe and impurities, the metallographic structure at the center of the wall thickness of the steel pipe consists of a mixed structure of bainite and ferrite, the tensile strength in a hydrogen atmosphere is 500 to 900 MPa, the hardness at the center of the wall thickness is 160 to 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, and the ratio of the outer diameter to the inner diameter satisfies the following formula (1), a steel pipe for high-pressure hydrogen piping, characterized in that the maximum value of the crack depth existing on the inner surface of the steel pipe is 200 μm or less. 【Number 1】

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

3. The chemical composition of the steel pipe is in mass %, Cr: 0.2 to 1.0% Mo: 0.03 to 1.0% Cu: 0.03 to 0.50% Ni: 0.03 to 0.50% V: 0.06 to 0.10% The steel pipe for high-pressure hydrogen piping according to claim 1 or 2, characterized by containing one or more selected from the above.

4. A high-pressure hydrogen pipe, characterized by using the steel pipe for high-pressure hydrogen piping according to any one of claims 1 to 3 as a material.

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