JP2009299174A - Pressure vessel for high pressure hydrogen gas and pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pressure vessel which has excellent hydrogen embrittlement resistance and stress corrosion cracking resistance, and suitable for high pressure hydrogen gas, e.g. under ≥70 MPa without depending on remarkable thickening, and to provide a pipe for piping. <P>SOLUTION: Disclosed is a pressure vessel for storing high pressure hydrogen gas made of stainless steel, which has a steel composition comprising, by mass, ≤0.08% C, 1.3 to 3.5% Si, ≤3.5% Mn, ≤0.05% P, ≤0.03% S, 8 to 17% Ni, 15 to 20% Cr and ≤0.2% N, and, if required, further comprising one or more selected from ≤3% Mo and ≤3.5% Cu, one or more selected from V and W by ≤4% in total, one or more selected from Nb, Ti and Al by ≤0.4% in total and ≤0.01% B, and in which at least the surface on the side to be exposed to hydrogen gas is provided with a passive film in which the content of Si in the metallic elements is ≥1.0 mass%. Also, disclosed is a pipe for transporting high pressure hydrogen gas. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a pressure vessel and a pipe for high-pressure hydrogen gas that have excellent hydrogen embrittlement resistance with respect to high-pressure hydrogen gas of 1 MPa or more and also have excellent stress corrosion cracking resistance.

In recent years, hydrogen has become a clean energy source that does not emit substances that cause environmental destruction such as carbon dioxide (CO ₂ ), nitrogen oxides (NO _x ), and sulfur oxides (SO _x ) due to global environmental conservation issues. Attention has been focused on research on various fuel cells using hydrogen.

The following three methods for supplying hydrogen to automobiles equipped with fuel cells have been studied.
(1) Production of hydrogen at an external plant → Storage at a hydrogen station (offsite type) → Supplying hydrogen to a fuel cell vehicle (2) Production and storage of hydrogen at a hydrogen station (onsite type) → To a fuel cell vehicle Supplying hydrogen (3) Producing hydrogen in fuel cell vehicles

  In any of these systems, a tank for storing hydrogen gas and a pipe for transporting are required. At that time, the hydrogen gas is often handled in a high-pressure gas state of 1 MPa or more. Conventionally, Cr-Mo steel and manganese steel have been used for high-pressure tanks for hydrogen storage. However, since these steels are low in strength, it is necessary to make the thickness of the tank very thick, resulting in an increase in size of the equipment and difficulty in transportation. In particular, there is a problem that the tank for vehicle mounting conflicts with space saving and vehicle weight reduction.

  In order to reduce the thickness and weight of tanks and pipes applied to high-pressure hydrogen gas, materials having excellent hydrogen embrittlement resistance and high strength are required. Thus, it has been studied to use austenitic stainless steel, which can be made stronger than the Cr-Mo steel and manganese steel, for a hydrogen storage tank or the like. Patent Document 1 discloses a technique for improving the hydrogen embrittlement resistance of austenitic stainless steel using nitriding. Patent Document 2 discloses a technique using high Mn austenitic steel. Patent Document 3 discloses a technique for controlling texture.

JP 2007-270350 A JP 2007-126688 A International Publication No. 2004/111285 Pamphlet JP-A 64-62443 JP-A-1-159351 Japanese Patent Laid-Open No. 8218151

  High pressure gas tanks and piping are often welded, and it is desirable to employ steel that sufficiently overcomes the stress corrosion cracking resistance, which is a weak point of austenitic stainless steel. However, existing austenitic stainless steel with improved hydrogen embrittlement resistance does not necessarily have satisfactory characteristics from the viewpoint of stress corrosion cracking resistance.

  On the other hand, as an austenitic stainless steel having improved stress corrosion cracking resistance in a hot water environment, steel types having an increased Si content as disclosed in Patent Documents 4, 5, and 6 are known. However, these austenitic stainless steel grades are not considered for stress corrosion cracking resistance other than in a hot water environment, and according to the study by the inventors, the stress corrosion cracking resistance in a salty and wet environment is not always satisfactory. It turns out that it is not a thing. For this reason, it is impossible to apply these steel types as they are to, for example, on-vehicle members.

  In addition, with respect to the technique for improving the hydrogen embrittlement resistance of austenitic stainless steel, the methods described in Patent Documents 1 to 3 tend to increase the cost, and provide a technique for imparting excellent hydrogen embrittlement resistance by a simple technique. Establishment of is desired.

  The present invention is intended to provide a pressure vessel and piping pipe that are excellent in hydrogen embrittlement resistance and stress corrosion cracking resistance, and can be applied to high-pressure hydrogen gas such as 70 MPa or more without relying on a significant increase in thickness. is there.

  In order to achieve the above object, in the present invention, by mass%, C: 0.08% or less, Si: 1.3 to 3.5%, Mn: 3.5% or less, P: 0.05% or less, S: 0.03% or less, Ni: 8-17%, Cr: 15-20%, N: 0.2% or less, remaining Fe and unavoidable impurities, at least exposed to hydrogen gas A stainless steel high-pressure hydrogen gas storage pressure vessel and a high-pressure hydrogen gas transport pipe having a passive film having a Si content of 1.0% by mass or more on the surface of the metal element are provided. In addition to the above-mentioned elements, this stainless steel may further comprise (i) one or more of Mo: 3% or less, Cu: 3.5% or less, and (ii) one or more of V and W: 4% in total Hereinafter, (iii) one or more of Nb, Ti, and Al: a total of 0.4% or less, and (iv) B: 0.01% or less may be selectively contained alone or in combination. . The metal structure of the stainless steel is preferably an austenite single phase structure.

  According to the study by the inventors, when the above-described passive film having a high Si content is formed, the stress corrosion cracking resistance of the austenitic stainless steel in a salt wet / dry environment can be remarkably improved. . The composition of the passive film is specified by a microscopic analysis technique such as EDX.

  According to the present invention, it has become possible to obtain a pressure vessel and a transport pipe for high-pressure hydrogen gas that are excellent in hydrogen embrittlement resistance and stress corrosion cracking resistance using austenitic stainless steel. The pressure vessel and the pipe can be provided with a strength applicable to high-pressure hydrogen gas such as 70 MPa or more without depending on the increase in thickness, and is suitable for reducing the size and weight of the hydrogen gas supply facility. Moreover, this pressure vessel and pipe are also excellent in stress corrosion cracking resistance in a salt dry and wet environment. Therefore, the present invention is expected to contribute greatly to the spread of the fuel cell system for in-vehicle use.

  For tanks and pipes that come into contact with high-pressure hydrogen gas, plain steel, ferritic stainless steel, and martensitic stainless steel with a crystal lattice of body-centered cubic structure are not suitable because of high hydrogen diffusion coefficient and low hydrogen solubility. Adopting austenitic stainless steel with face-centered cubic structure is advantageous. However, since austenitic stainless steel is prone to stress corrosion cracking, care must be taken when applying it to a high-pressure tank or high-pressure piping. Especially for in-vehicle use, stress corrosion cracking resistance when chloride adheres is important. On the other hand, hydrogen embrittlement resistance must be imparted simultaneously with the stress corrosion cracking resistance.

As a result of detailed studies, the inventors have found that by increasing the Si content in the passive film, the stress corrosion cracking resistance of the austenitic stainless steel in a salt-wet environment can be significantly improved. discovered.
On the other hand, with regard to hydrogen embrittlement resistance, it has become clear that it is extremely advantageous to increase the Si content of solid solution in the austenitic steel above a certain level.
And it discovered that Si could be concentrated by the predetermined density | concentration in a passive film by performing pickling on specific conditions with respect to the austenitic stainless steel of the composition which increased Si content. Hereinafter, matters for specifying the present invention will be described in more detail.

[Si content in the passive film]
In the present invention, in the austenitic stainless steel adjusted to the chemical composition described later, the content of Si in the metal element in the passive film needs to be 1.0% by mass or more. This makes it possible to stably and significantly improve the stress corrosion cracking resistance of the austenitic stainless steel in a salt wet / dry environment. Although the mechanism is not necessarily clear at the present time, if a large amount of Si is present in the passive film, active dissolution of the metal in the steel substrate is promoted in a salt dry / wet environment, and the failure mode of the passive film is further improved. It can be considered to be highly uniform. The stress corrosion cracking of austenitic stainless steel is attributed to local corrosion defects, but the failure mode of the passive film as described above is advantageous in preventing the formation of local corrosion defects. It is thought that this is a factor that significantly improves the stress corrosion cracking resistance.

  Elemental analysis of the passive film can be performed by EDX on a cross section parallel to the thickness direction of the film. The analysis location is in the thickness direction, (a) near the outermost surface of the passive film (position about 1 nm from the outermost surface), (b) the central part of the film thickness of the passive film, Three points (about 3 nm from the outermost surface) may be used, and the average value may be adopted. Since the passive film is mainly composed of oxide, its constituent elements contain a large amount of O (oxygen). Further, C (carbon) is usually detected as a contamination component during analysis. As a result of various studies, it has been clarified that the resistance to stress corrosion cracking can be grasped by the amount of Si in the metal elements excluding O and C among the elements constituting the passive film. The metal element to be analyzed may be a steel component element (except for the nonmetallic elements C, P, S, and N) defined in the present invention. Hereinafter, the “Si amount in the passive film” in the present specification means “the Si amount in the metal element in the passive film” unless otherwise specified.

  The amount of Si in the passive film thus identified needs to be 1.0% by mass or more, but it is not necessary to increase the Si content too much. According to the study by the inventors, the amount of Si in the passive film may be about 1.0 to 7.0% by mass, and may be controlled to about 1.0 to 5.0% by mass.

  In order to obtain the above-described passivated film in which Si is concentrated, an austenitic stainless steel in which the Si content in the steel is adjusted to a predetermined range described below is prepared, and the hydrofluoric acid concentration: 5 to 5 for the steel. It is effective to perform pickling using 25% by mass and nitric acid concentration: 95 to 75% by mass hydrofluoric acid (mixed acid). The liquid temperature may be about 40 to 80 ° C., and the immersion time may be about 30 to 90 seconds. After the pickling, skin pass rolling may be performed as necessary. However, it is not preferable to include a process (such as polishing or shot blasting) for mechanically removing the passive film in the subsequent process.

[Chemical composition of steel]
The pressure vessel and pipe of the present invention are made of austenitic stainless steel in which the content of component elements is adjusted as follows. “%” In the steel composition means “mass%” unless otherwise specified.
C: 0.08% or less C is a strong austenite forming element and is an element effective for improving the strength. However, excessive inclusion forms coarse Cr carbide by recrystallization treatment, intergranular corrosion and weldability. Since it becomes a cause of a fall, it is made to contain in the said range.

Si: 1.3-3.5%
Si is an important element in the present invention. When the Si content in the steel is low, it becomes difficult to form a passive film having a high Si concentration as described above, and it becomes difficult to sufficiently provide excellent stress corrosion cracking resistance in a salty and wet environment.

  As a result of detailed studies by the inventors, it has been found that Si in steel is extremely effective in imparting hydrogen embrittlement resistance. The hydrogen embrittlement of steel is considered to be caused mainly by strains locally concentrated by hydrogen that has penetrated into the steel, and fracture is likely to occur (so-called hydrogen-induced local plastic deformation model). Si is an element exhibiting a solid solution strengthening action. However, when the amount of the solid solution is sufficiently large, it does not act extremely effectively in suppressing local plastic deformation induced by strain caused by hydrogen penetration. It is guessed that there is not. However, excessive Si content hardens the steel and causes deterioration of workability. Therefore, Si content in steel is prescribed | regulated in said range.

Mn: 3.5% or less Although Mn is effective as an austenite-forming element, excessive content causes deterioration in workability and surface defects, so the content is within the above range.

P: 0.05% or less Since P impairs the toughness of steel, it is preferable that the P content is low. However, since dephosphorization in refining is difficult in the production of Cr-containing steel, it is difficult to reduce the P content. Careful selection is required. For this reason, excessive P reduction is not preferable because it increases costs. As a result of various studies, the present invention allows P content within the above range.

S: 0.03% or less S is limited to the above range because it forms MnS that tends to be the starting point of pitting corrosion and causes a decrease in corrosion resistance.

Ni: 8-17%
Ni is indispensable as an austenite forming element. It is desirable that the processing-induced martensite is not generated as much as possible in the processing portion of the pressure vessel or the pipe, and for that purpose, Ni content of 8% or more is necessary. It is more preferable to ensure a Ni content of 9.5% or more. However, Ni is an expensive element and excessive content is not preferable. The upper limit of the Ni content may be managed at 15% or 13%.

Cr: 15-20%
Cr is a main constituent element of the passive film. Generally, as its content increases, resistance to local corrosion such as pitting corrosion and crevice corrosion increases. On the other hand, since Cr is a ferrite-forming element, a large amount is not preferable because a large amount of δ ferrite phase is generated at a high temperature. As a result of various studies, the Cr content is specified in the above range in order to ensure sufficient corrosion resistance in the application of the pressure vessel and pipe targeted by the present invention and to appropriately suppress the formation of δ ferrite.

N: 0.2% or less N is effective as an austenite forming element and is also effective in increasing the strength of steel. However, excessive N content causes a bad effect such as causing blow holes during casting. Therefore, N content is prescribed | regulated in said range. You may manage to 0.15% or less.

Mo: 3% or less Mo is an element effective for improving the corrosion resistance level together with Cr, and is added as necessary. In that case, it is more effective to secure a Mo content of 0.1% or more. However, addition of a large amount of Mo is undesirable because it leads to the formation of a δ ferrite phase at a high temperature and increases the cost. When adding Mo, it carries out in the said content range.

Cu: 3.5% or less Cu has the effect of increasing the stacking fault energy of the austenite phase and reducing the cross-slip interval during plastic deformation. This action suppresses local destruction of the passive film on the surface during plastic deformation, and is effective in improving stress corrosion cracking resistance. For this reason, Cu is added as needed. In that case, it is more effective to secure a Cu content of 0.1% or more. However, excessive addition of Cu hinders pitting corrosion resistance and hot workability. Therefore, when Cu is added, the content is within the above range.

One or more of V and W: 4% or less in total V and W are elements advantageous for increasing the strength of steel, and can be added as necessary. However, when these elements are added excessively, the hot workability is adversely affected. As a result of the study, when one or two of V and W are added, the total content is within the above range.

One or more of Nb, Ti and Al: 0.4% or less in total Nb, Ti and Al are effective elements for precipitation strengthening, and Al is also effective for deoxidation during steelmaking. For this reason, in this invention, 1 or more types of these elements can be added as needed. However, excessive addition of these elements causes adverse effects such as a decrease in hot workability and frequent occurrence of surface defects on the product. Therefore, when adding 1 or more types of Nb, Ti, and Al, let the total content be said range.

B: 0.01% or less B is an element effective for preventing the occurrence of hot-rolled steel strip edge cracks caused by the difference in deformation resistance between the δ ferrite phase and the austenite phase at the hot rolling temperature, and is added as necessary. be able to. However, if B is added excessively, a low-melting boride is likely to be formed, which is a factor that adversely decreases hot workability. For this reason, when adding B, it carries out in the said content range.

  In addition, REM (rare earth element), Y, Ca, and Mg are allowed to be mixed within a total range of 0.1% by mass or less. Although these elements may enter from scrap materials and refractories used in the steel making process, there is no particular adverse effect within the above range.

[Manufacture of pressure vessels and pipes]
The pressure vessel and pipe of the present invention are:
(I) an austenitic stainless steel having the above chemical composition, preferably made of a material having an austenite single phase structure,
(Ii) The passive film having a high Si concentration is formed on the inner surface, that is, the surface exposed to high-pressure hydrogen gas,
The production method is not particularly limited as long as the above is satisfied. Generally, it is manufactured by manufacturing a steel plate material in a normal austenitic stainless steel plate manufacturing line and processing it.

  In the case of a pressure vessel, each member formed of a steel plate material can be welded to form a vessel, and the inner surface can be manufactured by subjecting it to a pickling treatment as described above. Or you may form a passive film with high Si concentration by performing the above-mentioned pickling process in the process of manufacturing a steel plate raw material, for example in a continuous annealing pickling line. Thereafter, various finishing treatments such as skin pass rolling are performed as necessary within a range in which the dynamic film is not removed, and the obtained steel plate material is processed into a pressure vessel. When welding is involved in the processing stage, it is preferable to form the passive film on the entire inner surface by, for example, pickling the welded portion on the inner surface of the container.

  In the case of a pipe, a general manufacturing process of a stainless steel pipe in which a steel plate material is passed through a welded pipe making line in the state of a steel strip can be used. A steel pipe cut to a predetermined length is pickled with the above-mentioned mixed acid to form a passive film having a high Si concentration on the inner surface.

Stainless steel having the chemical composition shown in Table 1 is melted, hot rolled to a thickness of 3.5 mm, subjected to annealing, cold rolling, and solution treatment of “1050 ° C. × 1 minute holding → water cooling”. A cold-rolled annealed material having a thickness of 0.8 mm was obtained. Thereafter, any one of the following finishing treatments was applied to obtain a test material.
[Mixed pickling finish]
Pickling was performed under conditions of hydrofluoric acid concentration: 10% by mass, nitric acid concentration: 90% by mass hydrofluoric acid (mixed acid), liquid temperature: 60 ° C., immersion time: about 60 sec.
[Polished finish]
After the mixed pickling, polishing with # 600 emery paper.

When the metal structure of each test material was observed with a microscope, it was confirmed that the matrix exhibited an austenite single phase structure.
About each test material, the composition analysis of the passive film, the hydrogen charge test, the stress corrosion cracking test, and the tensile test before and after the hydrogen charge were carried out by the following methods.

[Composition analysis of passive film]
About the cross section parallel to the plate | board thickness direction of a test material, it observed with FE-TEM provided with an EDX apparatus, and analyzed the composition of the position of three positions of the passive film in the depth direction by EDX. The analysis location is (a) near the outermost surface of the passive film (position about 1 nm from the outermost surface), (b) the central part of the film thickness of the passive film, (c) near the interface between the passive film and the steel substrate (from the outermost surface) 3 points). Analytical elements are all metal elements except for C, P, S, and N shown in Table 1, and the Si content in those metal elements is calculated in mass%, and the average value of the calculation results at three points is It was adopted as the amount of Si in the passive film.

[Hydrogen charge test]
Considering the use of tanks and pipes, hydrogen was occluded in stainless steel by the hydrogen charging method using high-pressure hydrogen gas, instead of the hydrogen charging method by electrolysis.
A test piece was cut out from the test material and charged with hydrogen by holding it in an atmosphere of pure hydrogen, pressure 20 MPa, 350 ° C. for 24 hours. This temperature was adopted because austenitic stainless steel absorbs the largest amount of hydrogen at around 350 ° C. Applying the “relationship between hydrogen partial pressure and hydrogen storage amount (Sieverts's law)” at various temperatures known for austenitic stainless steels, the austenitic stainless steels have a pressure of 20 MPa and a temperature of 350 ° C. The amount of hydrogen occluded in is equivalent to the amount of hydrogen occluded under conditions of pressure 70 MPa and 100 ° C.

  Immediately after the completion of hydrogen charging, the amount of diffusible hydrogen in the test piece was measured by the MPI-MS method. The measurement conditions were temperature range: room temperature to 900 ° C., temperature increase rate: 12 ° C./min, sampling interval: 0.5 minutes, and the total amount of diffusible hydrogen from room temperature to 600 ° C. was determined.

[Tensile test before and after hydrogen charging]
With respect to the specimen material sample before the hydrogen charge test and the sample after the hydrogen charge test, a strain rate of 1 in a normal temperature atmosphere is obtained by a SSRT (Slow Strain Rate Technique) method using a tensile test piece having a parallel part length of 30 mm. A tensile test at × 10 ^-6 was performed.
The rate of change in elongation at break (%) before and after hydrogen charging was determined by the following equation (1).
[Change rate of breaking elongation] = ([breaking elongation after hydrogen charging] − [breaking elongation before hydrogen charging]) / [breaking elongation before hydrogen charging] × 100 (1)

[Stress corrosion cracking test]
A large piece of 29 mm × 31 mm and a small piece of 14 mm × 16 mm were cut out from the test material, and they were overlapped and joined by spot welding to produce a spot welding test piece having the shape shown in FIG. One cycle of “salt spray (5% aqueous sodium chloride solution, 35 ° C. × 15 minutes) → drying (60 ° C., 30% RH × 1 hour) → wet (50 ° C., 90% RH × 3 hours)” on each test piece The salt dry / wet test was performed for 600 cycles, the large piece and the small piece were separated from the spot welded test piece after the test, and it was observed whether each of the large piece and the small piece had stress corrosion cracking. For each specimen, the number of tests was n = 3, and no stress corrosion cracking was found in any of the three spot welded specimens, ○ (good), and the others x (defective) ).
These results are shown in Table 2.

  As can be seen from Table 2, the example of the present invention in which a passive film having a Si concentration of 1.0% by mass or more was formed on the surface of an austenitic stainless steel having a predetermined chemical composition is resistant to stress in a salty and dry environment. The corrosion cracking property was excellent, and the rate of change in elongation at break before and after hydrogen charging was in the range of -2 to 2%, and hydrogen embrittlement was hardly observed. Therefore, a pressure vessel and piping pipe for high-pressure hydrogen gas, which is excellent in hydrogen embrittlement resistance and stress corrosion cracking resistance, can be constructed using these austenitic stainless steels.

  On the other hand, Comparative Examples No. 21 and 22 have the chemical composition targeted by the present invention, but due to the polishing finish, the Si concentration in the passive film is insufficient, and the stress corrosion resistance in a salty and wet environment It was inferior in cracking property. In Nos. 23, 24 and 25, since the Si content in the steel was insufficient, the rate of change in elongation at break before and after hydrogen charging was a very low value (a large value on the minus side), and the hydrogen embrittlement resistance was poor. In addition, the Si concentration in the passive film was insufficient, and the resistance to stress corrosion cracking in a salt dry / wet environment was inferior. In No. 26, it was considered that sensitization due to carbide precipitation occurred due to excessive C content in the steel, and intergranular corrosion was observed. Further, the hydrogen embrittlement resistance is not improved despite the sufficiently high Si concentration in the passive film. In No. 27, it was considered that the Mn concentration in the passive film increased because the Mn content in the steel was too high, and that Mn weakened the passive film, resulting in stress corrosion cracking resistance. inferior.

The figure which showed typically the shape of the spot welding test piece.

Claims (10)

  In mass%, C: 0.08% or less, Si: 1.3-3.5%, Mn: 3.5% or less, P: 0.05% or less, S: 0.03% or less, Ni: 8 Si having a composition consisting of ˜17%, Cr: 15-20%, N: 0.2% or less, the balance Fe and unavoidable impurities, at least on the surface exposed to hydrogen gas, in the metal element A pressure vessel for high-pressure hydrogen gas storage made of stainless steel having a passive film with an amount of 1.0 mass% or more.   The pressure vessel according to claim 1, wherein the stainless steel further has a composition containing at least one of Mo: 3% or less and Cu: 3.5% or less.   The pressure vessel according to claim 1 or 2, wherein the stainless steel further has a composition containing one or more of V and W in a total range of 4% or less.   The pressure vessel according to any one of claims 1 to 3, wherein the stainless steel further has a composition containing at least one of Nb, Ti, and Al in a range of 0.4% or less in total.   The pressure vessel according to any one of claims 1 to 4, wherein the stainless steel further has a composition containing B in a range of 0.01% or less.   In mass%, C: 0.08% or less, Si: 1.3-3.5%, Mn: 3.5% or less, P: 0.05% or less, S: 0.03% or less, Ni: 8 Si having a composition consisting of ˜17%, Cr: 15-20%, N: 0.2% or less, the balance Fe and unavoidable impurities, at least on the surface exposed to hydrogen gas, in the metal element A stainless steel high-pressure hydrogen gas transport pipe having a passive film with an amount of 1.0% by mass or more.   The pipe according to claim 6, wherein the stainless steel has a composition further containing one or more of Mo: 3% or less and Cu: 3.5% or less.   The pipe according to claim 6 or 7, wherein the stainless steel further has a composition containing at least one of V and W in a total range of 4% or less.   The pipe according to any one of claims 6 to 8, wherein the stainless steel further has a composition containing at least one of Nb, Ti, and Al in a range of 0.4% or less in total.   The pipe according to any one of claims 6 to 9, wherein the stainless steel further has a composition containing B in a range of 0.01% or less.

Images