CN118374731A

CN118374731A - Austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas and method for producing same

Info

Publication number: CN118374731A
Application number: CN202410072684.XA
Authority: CN
Inventors: 工藤大辅; 高桥伸幸; 安东知洋; 小柳祯彦
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 2023-01-20
Filing date: 2024-01-18
Publication date: 2024-07-23

Abstract

The invention relates to an austenitic stainless steel for high-pressure hydrogen or liquid hydrogen, having a composition consisting of the following elements: 0.20 mass% or less of C, 1.00 mass% or less of Si, 2.0 mass% or less of Mn, 0.050 mass% or less of P, 0.050 mass% or less of S, 2.0 mass% or less of Cu <4.0 mass%, 8.0 mass% or less of Ni 11.5 mass%, 17.0 mass% or less of Cr < 22.0 mass%, 0.20 mass% or less of Mo, and 0.050 mass% or less of N, the balance being Fe and unavoidable impurities; the Ni equivalent Nieq is more than 24.0; and the relative reduction of area is 0.8 or more.

Description

Austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas and method for producing same

Technical Field

The present invention relates to austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas and a method for producing the same. More specifically, the present invention relates to an austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen excellent in hydrogen embrittlement resistance, and a method for producing the same.

Background

In recent years, fuel cell automobiles using hydrogen as fuel and a hydrogen addition station for supplying hydrogen to the fuel cell automobiles have been developed. Various devices used in fuel cell automobiles, hydrogen stations, and the like include devices used in a high-pressure hydrogen environment (hereinafter, also collectively referred to as "high-pressure hydrogen devices") and devices used in a liquid hydrogen environment (hereinafter, also collectively referred to as "liquid hydrogen devices"). Materials used in these devices are required to have excellent hydrogen embrittlement resistance. Stainless steel (particularly austenitic stainless steel with higher Ni equivalent) has excellent resistance to hydrogen embrittlement and is therefore suitable for such applications.

Among various austenitic stainless steels, SUS316L is known to be a material excellent in hydrogen embrittlement resistance. Currently, SUS316L is approved as stainless steel excellent in hydrogen embrittlement resistance according to the compressed hydrogen container standard for automobiles prescribed in the japanese high-pressure gas safety act. However, since SUS316L is low in strength, in the case where SUS316L is used for a structural member of a high-pressure hydrogen gas apparatus, the structural member needs to be designed thick. As a result, there is a problem in that the size and weight of the apparatus are inevitably increased. In order to reduce the weight of the fuel cell vehicle, miniaturize the hydrogen addition station, and achieve high pressure operation in the hydrogen addition station, it is preferable that the strength and hydrogen embrittlement resistance of stainless steel used for these applications be high.

In order to solve this problem, various proposals have been made in the prior art.

For example, patent document 1 discloses an austenitic high Mn stainless steel containing predetermined amounts C, N, si, cr, mn, cu and Ni and an austenite stability index Md30 within a predetermined range.

Patent document 1 discloses (a) that strain-induced martensite is generated and embrittled in a low-temperature hydrogen environment even in SUS 316-based austenitic stainless steel, and (B) that when Md30 is designed to satisfy a specific condition in a low-temperature hydrogen environment, the generation of strain-induced martensite is suppressed, whereby hydrogen embrittlement resistance exceeding that of SUS 316-based austenitic stainless steel is obtained.

Patent document 2 discloses an austenitic stainless steel containing predetermined amounts C, si, mn, P, S, ni, cr, mo, cu, N, al, ca, O, B, ti, nb and V, wherein Cr equivalent/Ni equivalent is 1.56 or less, and P value (an index indicating the contents of S, O and Ca) is-5 or less.

Patent document 2 discloses (a) that hydrogen embrittlement occurs even in the case of SUS 316-based austenitic stainless steel in a low-temperature, high-pressure hydrogen atmosphere, (B) that when the Cr equivalent/Ni equivalent is set to 1.56 or less, variation in Ni concentration and decrease in hydrogen embrittlement resistance due to the variation can be suppressed, (C) that when the S content is reduced as much as possible by Al deoxidation and Ca addition, hot workability can be improved, and (D) that accordingly, austenitic stainless steel having both excellent hot workability and hydrogen embrittlement resistance in a low-temperature, high-pressure hydrogen atmosphere of more than 40MPa can be obtained.

Patent document 3 discloses an austenitic stainless steel for high-pressure hydrogen gas, which contains predetermined amounts C, si, mn, P, S, ni, cr and N, wherein the area ratio of Cr carbide is 23% or more.

Patent document 3 discloses that (a) when the area ratio of Cr carbide is set to 23% or more, an austenitic stainless steel having a 0.2% yield strength of 330MPa or more and a hardness of 200Hv or more in a solution heat treated state can be obtained, and (B) when the amount of Ni is set to 8 to 14 mass%, the generation of δ ferrite and the generation of deformation-induced martensite can be suppressed.

Patent document 4 discloses an austenitic stainless steel welded pipe for high-pressure hydrogen transportation, which (a) contains predetermined amounts C, si, mn, P, S, ni, cr and N, the balance being Fe and unavoidable impurities, (b) wherein the content of each element is adjusted to satisfy a predetermined relational expression, and (c) the area ratio of delta ferrite phase of the welded portion is 0.5% or less.

Patent document 4 discloses that the hydrogen embrittlement resistance of austenitic stainless steel welded pipes can be improved by restricting the delta ferrite phase of the welded portion.

Further, patent document 5 discloses an austenitic stainless steel for hydrogen gas, which is obtained by: (a) Plastic working an austenitic stainless steel containing predetermined amounts of C, si, mn, P, S, cr, ni, al and N, and the balance of Fe and impurities at room temperature to 200 ℃, at a cross-sectional shrinkage of 10% to 50%, and (b) plastic working the austenitic stainless steel at a cross-sectional shrinkage of 5% or more in a direction different from the plastic working direction.

Patent document 5 discloses (a) that strength is improved when austenitic stainless steel is cold worked, but when excessive dislocations are introduced into austenitic stainless steel, hydrogen embrittlement sensitivity is increased, and (B) that when austenitic stainless steel is cold worked while changing the working direction, hydrogen embrittlement sensitivity is significantly reduced.

In a hydrogen station as a hydrogen supply infrastructure, hydrogen gas at an extremely low temperature of-40 ℃ and an extremely high pressure of 82MPa is prepared for treatment. In addition, in order to improve the use efficiency of hydrogen in the future, it is also desired to further increase the hydrogen pressure.

However, even SUS316 may become brittle in a low-temperature, high-pressure hydrogen atmosphere as disclosed in patent document 1. In addition, SUS316 is expensive because it contains a large amount of Ni and Mo as rare metals.

On the other hand, patent document 2 discloses an austenitic stainless steel excellent in hydrogen embrittlement resistance under a hydrogen atmosphere of 40MPa at a low temperature. However, patent document 2 does not mention a high-pressure hydrogen atmosphere of about 82MPa required for a hydrogen addition station. In addition, in the austenitic stainless steel disclosed in patent document 2, the content of Mo as a rare metal exceeds 2%.

Patent document 3 discloses an austenitic stainless steel which improves hydrogen embrittlement resistance in a high-pressure hydrogen atmosphere by suppressing deformation-induced martensitic transformation at low temperatures. However, in the austenitic stainless steel disclosed in patent document 3, since the content of Ni as a rare metal is substantially the same as that in conventional SUS316, the material cost and the resource risk are high.

Patent document 1: JP2007-126688A

Patent document 2: JP2015-196842A

Patent document 3: JP2018-135592A

Patent document 4: JP2010-121190A

Patent document 5: WO2004/111285

Disclosure of Invention

The purpose of the present invention is to provide an austenitic stainless steel for high-pressure hydrogen or liquid hydrogen, which has a low content of expensive Ni and Mo and is excellent in hydrogen embrittlement resistance in a low-temperature, high-pressure hydrogen or liquid hydrogen environment.

Another object of the present invention is to provide a method for producing austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen.

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas according to the present invention for solving the above object has the following configuration:

(1) The austenitic stainless steel for high-pressure hydrogen or liquid hydrogen comprises:

C is less than or equal to 0.20 mass percent,

Si is more than or equal to 0.10 mass percent and less than or equal to 1.00 mass percent,

Mn is more than or equal to 0.10 mass percent and less than or equal to 2.0 mass percent,

P is less than or equal to 0.050 mass percent,

S is less than or equal to 0.050 mass percent,

2.0 Mass percent or less and 4.0 mass percent of Cu,

Ni is more than or equal to 8.0 mass percent and less than or equal to 11.5 mass percent,

17.0 Mass percent of < Cr is less than or equal to 22.0 mass percent,

Mo is less than or equal to 0.20 percent by mass,

N is less than or equal to 0.050 mass%, and

Optionally, at least one selected from the group consisting of:

v is less than or equal to 0.5 mass percent,

Nb is less than or equal to 0.5 mass percent,

Ca is less than or equal to 0.03 mass percent,

B is less than or equal to 0.05 mass percent,

Zr is less than or equal to 0.5 mass percent,

W is less than or equal to 2.0 mass percent,

Al is less than or equal to 0.05 mass percent,

Mg not more than 0.01 mass%, and

Co less than or equal to 1.0 mass percent,

The balance of Fe and unavoidable impurities;

(2) The austenitic stainless steel for high-pressure hydrogen or liquid hydrogen has a Ni equivalent Nieq represented by the following formula (1) of 24.0 or more:

Nieq＝[％Ni]+15.9[％C]+0.32[％Si]+0.66[％Mn]+0.47[％Cr]+0.64[％Mo]+[％Cu]+15.9[％N](1)

here, [% Z ] represents the content (mass%) of element Z, and

(3) The austenitic stainless steel for high-pressure hydrogen or liquid hydrogen has a relative reduction of area represented by the following formula (2) of 0.8 or more:

relative area reduction = a/B (2)

Here the number of the elements is the number,

A is the area reduction rate of a round bar tensile specimen when a slow strain rate test is conducted under the conditions of a test temperature of-60 ℃ and a test atmosphere of 87.5MPa hydrogen, and

B is the area reduction rate of the round bar tensile sample when the slow strain rate test is carried out under the conditions of the test temperature of-60 ℃ and the test atmosphere of 87.5MPa helium.

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to the present invention may further comprise one or more selected from the group consisting of:

0 mass% < V < 0.5 mass%,

0 Mass% < Nb less than or equal to 0.5 mass%,

0 Mass percent of Ca is less than or equal to 0.03 mass percent,

0 Mass percent of < B is less than or equal to 0.05 mass percent,

0 Mass percent < Zr is less than or equal to 0.5 mass percent,

0 Mass% < W.ltoreq.2.0 mass%,

0 Mass% < Al.ltoreq.0.05 mass%, and

0 Mass% of < Mg is less than or equal to 0.01 mass%.

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas according to the present invention may further contain 0 mass% < Co.ltoreq.1.0 mass% in place of a part of Fe.

The method for manufacturing austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to the present invention comprises:

a first step of melting and casting a raw material to obtain an ingot, the ingot comprising:

C is less than or equal to 0.20 mass percent,

P is less than or equal to 0.050 mass percent,

S is less than or equal to 0.050 mass percent,

2.0 Mass percent or less and 4.0 mass percent of Cu,

17.0 Mass percent of < Cr is less than or equal to 22.0 mass percent,

Mo is less than or equal to 0.20 mass percent

N is less than or equal to 0.050 mass%, and

Optionally, at least one selected from the group consisting of:

v is less than or equal to 0.5 mass percent,

Nb is less than or equal to 0.5 mass percent,

Ca is less than or equal to 0.03 mass percent,

B is less than or equal to 0.05 mass percent,

Zr is less than or equal to 0.5 mass percent,

W is less than or equal to 2.0 mass percent,

Al is less than or equal to 0.05 mass percent,

Mg less than or equal to 0.01 mass percent

Co less than or equal to 1.0 mass percent,

The balance being Fe and unavoidable impurities, and the Ni equivalent Nieq of the ingot represented by formula (1) is 24.0 or more:

a second step of soaking the ingot at a temperature of 1,200deg.C or higher, and

And thirdly, performing primary heat treatment on the soaked material.

When a relatively large amount of Cu is added to austenitic stainless steel, cu segregates or low-melting point Cu compounds are easily precipitated when the heat treatment conditions are inappropriate. On the other hand, when austenitic stainless steel containing a relatively large amount of Cu is subjected to soaking treatment at a temperature of 1,200 ℃ or higher, cu is solid-dissolved in the austenitic phase, and therefore austenitic stainless steel having less Cu segregation and low melting point Cu compounds can be obtained.

Cu is an austenite stabilizing element. Therefore, when a large amount of Cu is solid-dissolved in the austenite phase, nieq becomes large even when the content of Ni or Mo is small, and austenite is stabilized. As a result, the formation of deformation-induced martensite, which causes hydrogen embrittlement, is suppressed, and the hydrogen embrittlement resistance is improved.

Detailed Description

Hereinafter, embodiments of the present invention will be described in detail.

[1. Austenitic stainless Steel for high pressure Hydrogen gas or liquid Hydrogen ]

[1.1. Major constituent elements ]

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas (hereinafter, also simply referred to as "austenitic stainless steel") according to the present invention contains the following elements, the balance being Fe and unavoidable impurities. The types of the additive elements, the ranges of the components thereof, and the reasons for limitation are as follows.

(1) C is less than or equal to 0.20 mass%:

In the present invention, C is an impurity. In the case where the amount of C is excessive, toughness and ductility may be reduced. Therefore, the amount of C needs to be 0.20 mass% or less. The amount of C is preferably 0.1 mass% or less.

The smaller the amount of C, the better. However, an extremely reduced amount of C leads to an increase in manufacturing costs. The amount of C is preferably 0.01 mass% or more in view of manufacturing cost.

(2) Si is more than or equal to 0.10 mass percent and less than or equal to 1.00 mass percent:

Si has an effect of improving tensile strength by being solid-dissolved in an austenite phase. In addition, si is an austenite stabilizing element, and thus contributes to improvement in hydrogen embrittlement resistance. In order to achieve such an effect, the amount of Si needs to be 0.10 mass% or more. The amount of Si is preferably 0.50 mass% or more.

On the other hand, in the case where the amount of Si is excessive, the grain boundary strength may be lowered, and the hydrogen embrittlement resistance may be lowered. Therefore, the amount of Si needs to be 1.00 mass% or less.

(3) Mn is more than or equal to 0.10 mass percent and less than or equal to 2.0 mass percent:

Mn has the effect of forming inclusions such as MnS and improving manufacturability. Further, mn is an austenite stabilizing element, and thus contributes to improvement in hydrogen embrittlement resistance. In order to obtain such effects, the amount of Mn needs to be 0.10 mass% or more. The amount of Mn is preferably 0.50 mass% or more, and more preferably 1.50 mass% or more.

On the other hand, when the amount of Mn is too large, the solid solubility limit of Cu may be lowered, and thus a low-melting point Cu compound may be precipitated. The low-melting Cu compound melts during hot working, which leads to a decrease in hot workability. Therefore, the amount of Mn needs to be 2.0 mass% or less.

(4) P is less than or equal to 0.050 mass%:

in the present invention, P is an impurity. In the case where the amount of P is excessive, hot workability may be lowered. Therefore, the amount of P needs to be 0.050 mass% or less.

The smaller the amount of P, the better. However, an extremely reduced amount of P leads to an increase in manufacturing costs. The amount of P is preferably 0.001 mass% or more in view of manufacturing cost.

(5) S is less than or equal to 0.050 mass%:

In the present invention, S is an impurity. In the case where the amount of S is excessive, hot workability may be lowered. Therefore, the amount of S needs to be 0.050 mass% or less. The amount of S is preferably 0.030 mass% or less.

The smaller the amount of S, the better. However, an extreme reduction in the amount of S leads to an increase in manufacturing costs. The amount of S is preferably 0.001 mass% or more in view of manufacturing cost.

(6) 2.0 Mass% or less of Cu <4.0 mass%:

Cu has the effects of suppressing the formation of deformation-induced martensite and improving hydrogen embrittlement resistance. Further, cu has an effect of improving cold workability. In order to achieve such an effect, the amount of Cu needs to be 2.0 mass% or more. The amount of Cu is preferably 3.0 mass% or more.

On the other hand, when the amount of Cu is too large, segregation of components is promoted, and thus hydrogen embrittlement resistance of some portions may be unstable. Further, in the case where the amount of Cu is excessive, a Cu concentrated portion may be formed, or Cu exceeding the solid solubility limit may be precipitated as a low melting point Cu compound. The Cu concentrated portion or the low melting point Cu compound melts during hot working, which results in a decrease in hot workability. Therefore, the amount of Cu needs to be less than 4.0 mass%.

(7) Ni is more than or equal to 8.0 mass percent and less than or equal to 11.5 mass percent:

Ni has the effects of suppressing the formation of deformation-induced martensite and improving hydrogen embrittlement resistance. In addition, ni has the effect of improving the solid solubility limit of Cu and improving hot workability. In order to achieve such an effect, the amount of Ni is required to be 8.0 mass% or more. The amount of Ni is preferably 9.0 mass% or more.

On the other hand, since Ni is expensive, when the amount of Ni is excessive, the cost of raw materials increases. Therefore, the amount of Ni is required to be 11.5 mass% or less. The amount of Ni is preferably 10.5 mass% or less.

(8) 17.0 Mass% < Cr is not more than 22.0 mass%:

Cr is a ferrite stabilizing element, and in the composition range of the austenitic stainless steel according to the present invention, cr has the effects of suppressing the formation of deformation-induced martensite and improving hydrogen embrittlement resistance. In addition, cr has the effect of enhancing corrosion resistance required for austenitic stainless steel. In order to achieve such an effect, the amount of Cr needs to exceed 17.0 mass%. The amount of Cr is preferably 17.5 mass% or more, and more preferably 18.5 mass% or more.

On the other hand, cr is expensive, and therefore, when the amount of Cr is excessive, the raw material cost increases. In addition, when the amount of Cr is too large, the generation of δ ferrite is promoted, and therefore hydrogen embrittlement resistance may be reduced. Therefore, the amount of Cr is required to be 22.0 mass% or less. The amount of Cr is preferably 19.0 mass% or less.

(9) Mo is less than or equal to 0.20 mass%:

In the present invention, mo is an element that can be contained as an impurity. Mo is an element contributing to the improvement of hydrogen embrittlement resistance, but is also an element that is expensive. Therefore, when the amount of Mo is excessive, the raw material cost increases. Therefore, the amount of Mo needs to be 0.20 mass% or less.

In the present invention, the amount of Mo may be 0. However, as described above, mo contributes to improvement in hydrogen embrittlement resistance. In order to achieve such an effect, the amount of Mo is preferably 0.01 mass% or more.

(10) N is less than or equal to 0.050 mass%:

in the present invention, N is an impurity. In the case where the amount of N is excessive, the stacking fault energy may be lowered, and hydrogen embrittlement resistance may be lowered. Therefore, the amount of N needs to be 0.050 mass% or less.

The smaller the amount of N, the better. However, an extremely reduced amount of N leads to an increase in manufacturing costs. The amount of N is preferably 0.01 mass% or more in view of manufacturing cost.

(11) Unavoidable impurities:

the austenitic stainless steel according to the invention may contain unavoidable impurities.

Here, the "unavoidable impurities" means components which are mixed into the austenitic stainless steel due to various reasons such as raw materials, manufacturing processes, etc. in the industrial manufacture of the austenitic stainless steel, and the content of each component is within a range that does not adversely affect the austenitic stainless steel according to the present invention.

Examples of unavoidable impurities other than C, P, S, mo and N described above include Sn, pb, ti, ta and Hf. The content of each of these impurities is preferably 0.05 mass% or less.

Further, the total content of unavoidable impurities is preferably 1.00 mass% or less. The total content is more preferably 0.50 mass% or less.

[1.2. Minor constituent elements ]

The austenitic stainless steel according to the present invention may contain one or two or more of the following elements in addition to the above-described main constituent elements and unavoidable impurities. The types of the additive elements, the ranges of the components thereof, and the reasons for limitation are as follows.

(1) 0 Mass% < V.ltoreq.0.5 mass%:

V has an effect of increasing strength by generating carbonitrides. Thus, the austenitic stainless steel according to the invention may comprise V.

On the other hand, in the case where the amount of V is excessive, a large amount of V-based compound may be generated, and manufacturability may be lowered. Therefore, the amount of V is preferably 0.5 mass% or less.

(2) 0 Mass% of < Nb is less than or equal to 0.5 mass%:

nb has effects of refining grains and improving strength. Thus, the austenitic stainless steel according to the invention may comprise Nb.

On the other hand, in the case where the amount of Nb is excessive, a large amount of Nb-based compound may be generated, and hot workability may be lowered. Therefore, the amount of Nb is preferably 0.5 mass% or less.

(3) 0 Mass% < Ca < 0.03 mass%:

ca has the effect of improving hot workability and improving manufacturability. Thus, the austenitic stainless steel according to the invention may comprise Ca.

On the other hand, in the case where the amount of Ca is excessive, a large amount of Ca-based compound may be generated, and manufacturability and corrosion resistance may be lowered. Therefore, the amount of Ca is preferably 0.03 mass% or less.

(4) 0 Mass% of < B is less than or equal to 0.05 mass%:

b has effects of segregation in grain boundaries and improving hot workability. Thus, the austenitic stainless steel according to the invention may comprise B.

On the other hand, in the case where the amount of B is excessive, a large amount of B-based compound may be generated, and workability and corrosion resistance may be lowered. Therefore, the amount of B is preferably 0.05 mass% or less.

(5) 0 Mass% of < Zr is less than or equal to 0.5 mass%:

zr is a deoxidizing element. Thus, the austenitic stainless steel according to the invention may comprise Zr.

On the other hand, in the case where the amount of Zr is excessive, a large amount of Zr-based compound may be generated, and manufacturability may be lowered. Accordingly, the amount of Zr is preferably 0.5 mass% or less.

(6) 0 Mass% < W.ltoreq.2.0 mass%:

W has the effect of improving strength by forming carbide. Thus, the austenitic stainless steel according to the invention may comprise W.

On the other hand, in the case where the amount of W is excessive, the manufacturing cost and the raw material cost may increase. Therefore, the content of W is preferably 2.0 mass% or less.

(7) 0 Mass% of < Al is less than or equal to 0.05 mass%:

Al is a deoxidizing element, and has an effect of improving manufacturability. Thus, the austenitic stainless steel according to the invention may comprise Al.

On the other hand, in the case where the amount of Al is excessive, a large amount of Al-based compound or σ phase may be generated, and manufacturability may be lowered. Therefore, the amount of Al is preferably 0.05 mass% or less. The amount of Al is more preferably 0.01 mass% or less.

(8) 0 Mass% of < Mg is less than or equal to 0.01 mass%:

Mg has the effect of improving hot workability and improving manufacturability. Thus, the austenitic stainless steel according to the invention may comprise Mg.

On the other hand, in the case where the amount of Mg is excessive, a large amount of Mg-based compound may be generated, and manufacturability may be lowered. Therefore, the amount of Mg is preferably 0.01 mass% or less.

(9) 0 Mass% of Co is less than or equal to 1.0 mass%:

co has the effect of suppressing the formation of deformation-induced martensite and improving hydrogen embrittlement resistance. Such effects can be obtained even if Co is contained in a small amount. Therefore, the amount of Co is preferably more than 0 mass%. The amount of Co is more preferably 0.01 mass% or more.

On the other hand, when the amount of Co is excessive, the ductility slightly decreases. In addition, co is an expensive element, and thus raw material cost increases. Therefore, the amount of Co is preferably 1.0 mass% or less. The amount of Co is more preferably 0.5 mass% or less, still more preferably 0.4 mass% or less, and still more preferably 0.3 mass% or less.

[1.3 Component Balancing ]

In the present invention, "Ni equivalent Nieq" means a value represented by the following formula (1):

here, [% Z ] represents the content (mass%) of the element Z.

Nieq is an index indicating the stability of austenite. The larger Nieq is, the more deformation-induced martensitic transformation in a low-temperature high-pressure hydrogen environment can be inhibited, and therefore the better the hydrogen embrittlement resistance is. In order to obtain excellent hydrogen embrittlement resistance, nieq needs to be 24.0 or more. Nieq is preferably 25.0 or more.

[1.4. Properties ]

[1.4.1 Relative area reduction ]

The degree of hydrogen embrittlement resistance can be evaluated based on the magnitude of the relative reduction of area.

Here, "relative reduction of area" refers to a value represented by the following formula (2). The higher the relative reduction of area value represented by formula (2), the better the hydrogen embrittlement resistance.

Relative area reduction = a/B (2)

Here the number of the elements to be processed is,

B is the area reduction rate of the round bar tensile specimen when the slow strain rate test is performed under the test temperature of-60 ℃ and the test atmosphere of 87.5MPa helium.

In each of the A and B determinations, a round bar tensile specimen having a parallel portion diameter of 4mm was used, and the strain rate was 7X 10 ^-5/s.

The austenitic stainless steel according to the present invention is excellent in hydrogen embrittlement resistance due to its optimized composition, and exhibits high ductility in a low-temperature, high-pressure hydrogen atmosphere. In the austenitic stainless steel according to the present invention, the relative reduction of area may be 0.8 or more with the composition and structure optimized. The relative reduction of area may be above 0.9 with further optimization of composition and/or tissue. Such austenitic stainless steel may be obtained by subjecting an ingot having a predetermined composition to soaking treatment and then to primary heat treatment. If necessary, it may be further subjected to secondary hot working, solution treatment, cold working, etc.

[1.4.2. Tensile Strength ]

"Tensile strength" means a tensile strength obtained by a tensile test according to JIS Z2241:2011 using No.14A specimen having a parallel portion diameter of 6 mm.

The austenitic stainless steel according to the invention has a tensile strength of more than 500MPa measured at 25 c by optimizing the hot working conditions, the solution treatment conditions and/or the cold working conditions. In the case of further optimizing the composition, the tensile strength may be 600MPa or more or 650MPa or more. In order to obtain austenitic stainless steel having a high strength of 650MPa or more, cold working is preferably performed.

[1.4.3. Metal Structure ]

"Amount of martensite (volume%)" is a value calculated by a method (so-called 5-peak method) using (200) peak intensity and (211) peak intensity of ferrite phase and (200) peak intensity, (220) peak intensity and (311) peak intensity of austenite phase, which are measured by X-ray diffraction using Mo tube; and means a value obtained by subtracting the volume fraction (volume%) of the austenite phase calculated based on the integrated intensity ratio of the diffraction peaks from 100%.

When austenitic stainless steel is cold worked, deformation induced martensite may be generated. Deformation induced martensite may reduce hydrogen embrittlement resistance. On the other hand, since the austenitic stainless steel according to the present invention has high austenite stability, the amount of martensite in the metallic structure is small even after cold working.

In the austenitic stainless steel according to the present invention, the amount of martensite in the metallic structure may be 1.0 vol% or less with the hot working condition, the solution treatment condition, and/or the cold working condition being optimized. In the case of further optimizing the manufacturing conditions, the amount of martensite may be 0.5% by volume or less.

[1.5. Application ]

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas according to the present invention may be in any of a hot worked state, a solution treated state, a cold worked state, or a state in which a necessary post-treatment is performed after the solution treatment.

Further, the shape of the austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas according to the present invention is not particularly limited, and an optimal shape may be selected according to the purpose thereof.

Examples of shapes include tubes, rods, wires, plates, and the like.

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to the present invention is excellent in hydrogen embrittlement resistance, and therefore can be used not only for various parts exposed to a high-pressure hydrogen gas environment but also for various parts exposed to a liquid hydrogen environment.

In particular, since the austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to the present invention is excellent in toughness at extremely low temperatures in addition to hydrogen embrittlement resistance, the austenitic stainless steel can be used as a material of components used in a liquid hydrogen environment, such as (a) components for a pressurized hydrogen station of a liquid hydrogen pump, and (b) components used in valves and pump components for liquid hydrogen.

[2 ] Process for producing austenitic stainless Steel for high-pressure Hydrogen gas or liquid Hydrogen gas ]

The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to the present invention can be obtained by:

(a) The raw materials mixed to obtain austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to the present invention are melted and cast,

(B) Soaking the obtained cast ingot at a temperature of 1,200 ℃ or higher,

(C) Performing primary heat treatment on the material subjected to soaking treatment,

(D) Secondary heat processing is carried out on the material subjected to the primary heat processing according to the requirement,

(E) Carrying out solid solution treatment on the material subjected to the secondary heat treatment according to the requirement,

(F) Cold working the material after the secondary hot working or solution treatment as required, and

(G) The material after the primary heat treatment, the secondary heat treatment, the solution treatment or the cold treatment is post-treated as needed.

[2.1. First step ]

First, raw materials mixed to obtain austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen gas according to the present invention are melted and cast to obtain an ingot (first step). The method and conditions for melting and casting the raw materials are not particularly limited, and the optimum method and conditions may be selected according to the purpose thereof. For example, an electric furnace, an Argon Oxygen Decarburization (AOD) furnace, a Vacuum Oxygen Decarburization (VOD) furnace, or the like may be used to manufacture molten steel.

[2.2. Second step ]

Next, the resulting ingot was subjected to soaking treatment at a temperature of 1,200 ℃ or higher (second step). The soaking treatment is performed to induce the diffusion of components in the steel ingot and to remove the segregation of the components.

The temperature of the soaking treatment affects the composition segregation. In the case where soaking treatment is not performed, or in the case where the temperature of soaking treatment is too low, a Cu concentrated portion may be generated at the interface between the substrate and the oxide scale on the surface, or a low melting point Cu compound may be precipitated at the grain boundary. The Cu concentrated portion or the low melting point Cu compound causes localized melting during hot working, thereby significantly deteriorating hot workability. In addition, the solution temperature of alloy carbide also increases with the concentration of Cu. Therefore, in the case where soaking is not performed or in the case where the temperature of soaking is too low, coarse alloy carbide may remain in the steel material. Coarse alloy carbides cause a decrease in toughness and ductility.

In contrast, in the case of soaking treatment at high temperature, hot workability and toughness as well as ductility are improved. To achieve such an effect, the soaking temperature is required to be 1,200 ℃ or higher.

For the holding time of the soaking treatment temperature, an optimal time may be selected according to the purpose thereof. In general, the longer the soaking temperature is maintained, the less Cu segregates. The optimal holding time depends on the temperature of the soaking treatment, but is usually 1 minute to 24 hours. After the end of the holding time, the material is cooled by water cooling, oil cooling, air cooling or a method that enables a cooling rate comparable thereto.

In the case where the primary heat treatment is performed immediately after the soaking treatment, cooling after the soaking treatment may be omitted.

[2.3 Third step ]

Next, the soaking-treated material is subjected to primary heat treatment (third step). Primary hot working is performed to break up the coarse cast structure and refine the structure while converting the ingot into steel such as slabs, billets, and bars. The primary heat processing method is not particularly limited, and an optimal method may be selected according to the purpose thereof. Examples of the primary hot working method include hot forging and hot rolling.

[2.4. Fourth step ]

Next, the material after the primary heat treatment is subjected to secondary heat treatment as needed (fourth step). The secondary heat working is performed to process the material obtained in the primary heat working process into a product shape (e.g., steel plate, steel bar, steel wire, steel pipe, etc.) or a shape close thereto. The method of the secondary heat treatment is not particularly limited, and an optimal method may be selected according to the purpose thereof. Examples of the method of the secondary hot working include hot rolling, hot extrusion, and hot piercing rolling.

The conditions for the secondary heat treatment are not particularly limited, and the optimum conditions may be selected according to the purpose thereof. Further, according to the purpose thereof, the secondary heat processing may be performed a plurality of times. The heating temperature of the steel material before the secondary hot working is preferably 900 ℃ to 1,200 ℃ inclusive.

In the case of performing the secondary heat treatment a plurality of times, the temperature of the steel material at the time of completion of the final secondary heat treatment is preferably 800 ℃ to 1200 ℃. This is to optimize the grain.

[2.5. Fifth step ]

Next, the material after the secondary heat treatment may be subjected to solution treatment as needed (fifth step). The solution treatment may be performed only once, or may be performed a plurality of times.

The temperature of the solution treatment affects the properties of the material. Without solution treatment, or at too low a temperature of solution treatment, coarse alloy carbides may be excessively retained, and toughness and ductility may be reduced. Therefore, the temperature of the solution treatment is preferably 1,000 ℃ or higher.

On the other hand, in the case where the temperature of the solution treatment is too high, the crystal grains may become too coarse, and the strength may be lowered. Therefore, the temperature of the solution treatment is preferably 1,200 ℃ or less, and more preferably 1,150 ℃ or less.

For the holding time of the solution treatment temperature, an optimal time may be selected according to the purpose thereof. In general, the longer the hold time of the solution treatment, the smaller the number density of coarse alloy carbides. On the other hand, if the holding time is prolonged beyond the necessary time, the crystal grains are excessively coarsened. The optimal holding time depends on the temperature of the solution treatment, but is usually 1 minute to 3 hours. After the end of the holding time, the material is cooled by water cooling, oil cooling, air cooling or a method that enables a cooling rate comparable thereto.

[2.6. Sixth step ]

Next, the material after the secondary heat treatment or the solution treatment may be subjected to cold working (sixth step), as necessary. The method of cold working is not particularly limited, and an optimal method may be selected according to the purpose thereof. For example, in the case of cold working a material to obtain a steel pipe, a cold drawing method is preferably used. Or in the case of processing the material into a steel sheet, a cold rolling method is preferably used.

Cold working affects the properties of the material. For cold working, the optimum working ratio may be selected according to the purpose thereof. In general, as the working rate of cold working increases, the strength of the material increases. On the other hand, excessive cold working causes the formation of deformation-induced martensite and a decrease in hydrogen embrittlement resistance. Therefore, the reduction of area is preferably 40% or less in terms of the cold working rate.

[2.7. Seventh step ]

Next, the material after the primary heat treatment, the secondary heat treatment, the solution treatment, or the cold treatment may be subjected to a post-treatment as needed (seventh step). Examples of post-processing include cutting, welding, and cold working. The component thus obtained is used for various applications.

[3. Effect ]

When a relatively large amount of Cu is added to austenitic stainless steel, cu segregates or low-melting point Cu compounds are easily precipitated when the heat treatment conditions are inappropriate. On the other hand, when austenitic stainless steel containing a large amount of Cu is subjected to soaking treatment at a temperature of 1,200 ℃ or higher, cu is solid-dissolved in the austenitic phase, and therefore austenitic stainless steel with less Cu segregation and less low-melting point Cu compounds can be obtained.

Cu is an austenite stabilizing element. Therefore, when Cu is dissolved in a relatively large amount in the austenite phase, nieq becomes large even when the content of Ni or Mo is small, and the austenite is stabilized. As a result, the formation of deformation-induced martensite, which causes hydrogen embrittlement, is suppressed, and the hydrogen embrittlement resistance is improved.

In addition, austenitic stainless steel having a relatively large amount of Cu dissolved therein is excellent in not only hydrogen embrittlement resistance at low temperatures but also excellent workability and excellent free-cutting property, and cold working can be performed at a high working rate by virtue of the excellent workability.

In addition, in the austenitic stainless steel according to the present invention, it is difficult to generate deformation-induced martensite due to high stability of austenite. Therefore, in the case of cold working the austenitic stainless steel according to the present invention at a predetermined cold working rate, high strength can be obtained without lowering the hydrogen embrittlement resistance at low temperatures.

Example

(Examples 1 to 14 and comparative examples 1 to 7)

[1. Preparation of sample ]

In a vacuum induction furnace, 50kg of steel having the composition shown in table 1 was melted and cast into ingots. Then, the ingot (except comparative example 6) was subjected to soaking treatment at 1,200℃. Next, the soaked material is hot forged. Next, the hot-forged material was subjected to solution treatment at 1,080 ℃, and further, in examples 2 to 3 and comparative example 7, the solution-treated material was subjected to cold working. The reduction of area of these materials was 25%, 38% and 75%, respectively, in that order. Samples were cut from the resulting materials and various tests were performed. However, in comparative example 6, breakage occurred during hot forging, and thus a sample could not be prepared.

TABLE 1

Table 1 (subsequent)

[2. Test method ]

[2.1 Amount of martensite ]

The amount of martensite was measured by using the 5-peak method.

[2.2. Tensile Strength ]

Tensile test was carried out in accordance with JIS Z2241:2021.

That is, a round bar tensile specimen (No. 14A specimen) was cut out from the obtained material. The parallel portion of the round bar tensile specimen is parallel to the rolling direction of the steel bar. The diameter of the parallel portion was 6mm. Tensile strength TS (MPa) was obtained by subjecting a round bar tensile specimen to a tensile test at room temperature (25 ℃) in the atmosphere.

[2.3. Evaluation of Hydrogen embrittlement resistance ]

To evaluate hydrogen compatibility, a slow strain rate test was performed. The test temperature was-60℃and the test atmosphere was helium or hydrogen at 87.5 MPa. As a sample, a round bar tensile sample having a parallel portion diameter of 4mm was used. The strain rate was 7X 10 ^-5/s.

The reduction of area in hydrogen and the reduction of area in helium were calculated from the area of the fracture surface of the round bar tensile specimen after the slow strain rate test. Furthermore, they were used to calculate the relative area shrinkage (=a/B) at-60 ℃.

[3. Results ]

The results are shown in Table 2. The following can be found from table 2.

With respect to the amount of martensite, "a" means that the amount of martensite is 0.5% by volume or less, "B" means that the amount of martensite is more than 0.5% by volume and 1.0% by volume or less, and "C" means that the amount of martensite is more than 1.0% by volume.

With respect to the tensile strength, "a" means that the tensile strength is 650MPa or more, "B" means that the tensile strength is 500MPa or more and less than 650MPa, and "C" means that the tensile strength is less than 500MPa.

Further, regarding hydrogen embrittlement resistance, "a" means that the relative area Reduction Ratio (RRA) is 0.9 or more, "B" means that RRA is 0.8 or more and less than 0.9, and "C" means that RRA is less than 0.8.

(1) In comparative example 1, RRA was less than 0.8. The reason for the decrease in RRA is thought to be that the formation of deformation-induced martensite in the slow strain rate test causes hydrogen embrittlement due to the small amount of Cu.

(2) In comparative example 2, the tensile strength after the solution treatment was less than 500MPa, and RRA was less than 0.8. The reason for the decrease in tensile strength is considered to be that the amount of dissolved solute element is relatively small (Nieq is less than 24), and thus sufficient solid solution strengthening does not occur. Furthermore, it is believed that the cause of the RRA reduction is due to Nieq less than 24.0, and thus the formation of deformation induced martensite in the slow strain rate test causes hydrogen embrittlement.

(3) In comparative example 3, RRA was less than 0.8. This is thought to be because the amount of N is excessive.

(4) In comparative example 4, RRA was less than 0.8. This is thought to be because the amount of C is excessive.

(5) In comparative example 5, RRA was less than 0.8. This is considered to be because the amount of Cr is small and the amount of Cu is slightly small.

(6) In comparative example 6, cracking occurred during hot forging, and thus a sample could not be prepared. This is considered to be because Cu segregates because soaking treatment is not performed.

(7) In comparative example 7, the amount of martensite exceeded 1.0 vol%, and the RRA decreased. This is considered to be because the cold working rate is too high, and thus deformation-induced martensite is generated.

(8) In all of examples 1, 4 to 11 and 13 to 14, the amount of martensite was 0.5% by volume or less, the tensile strength after solution treatment was 500MPa or more, and RRA was 0.9 or more.

(9) In examples 2 to 3, the tensile strength after cold working was 650MPa or more. In examples 2 to 3, the amount of martensite was 0.5% by volume or less even after cold working.

(10) In example 12, RRA was slightly reduced. This is thought to be because the amount of Mn and the amount of Cu are slightly smaller.

TABLE 2

(Examples 15 to 30 and comparative examples 8 to 12)

[1. Preparation of sample ]

In a vacuum induction furnace, 50kg of steel having the composition shown in table 3 was melted and cast into ingots. Then, the ingot was subjected to soaking treatment at 1,200℃. Next, the soaked material is hot forged. Next, the hot-forged material was subjected to solution treatment at 1,080 ℃, and further, in examples 16 and 17, the solution-treated material was subjected to cold working. The reduction of area of these materials was 25% and 38%, respectively, in order. Samples were cut from the resulting materials and various tests were performed.

TABLE 3

Table 3 (subsequent)

[2. Test method ]

The amount of martensite, tensile strength and hydrogen embrittlement resistance were evaluated in the same manner as in example 1.

[3. Results ]

The results are shown in Table 4. The following results can be found from table 4. In table 4, the meaning of each evaluation item "a", "B", and "C" is the same as that in table 2.

(1) In comparative example 8, RRA was less than 0.8. The reason for the decrease in RRA is thought to be that the formation of deformation-induced martensite in the slow strain rate test causes hydrogen embrittlement due to the small amount of Cu.

(2) In comparative example 9, the tensile strength after the solution treatment was less than 500MPa, and RRA was less than 0.8. The reason for the decrease in tensile strength is considered to be that the amount of dissolved solute element is relatively small (Nieq is less than 24), and thus sufficient solid solution strengthening does not occur. Furthermore, it is believed that the cause of the RRA reduction is due to Nieq less than 24.0, and thus the formation of deformation induced martensite in the slow strain rate test causes hydrogen embrittlement.

(3) In comparative example 10, RRA was less than 0.8. This is thought to be because the amount of N is excessive.

(4) In comparative example 11, RRA was less than 0.8. This is thought to be because the amount of C is excessive.

(5) In comparative example 12, RRA was less than 0.8. This is considered to be because the amount of Cr is small and the amount of Cu is slightly small.

(6) In all of examples 15, 18 to 26 and 28 to 30, the amount of martensite was 0.5% by volume or less, the tensile strength after solution treatment was 500MPa or more, and RRA was 0.9 or more.

(7) In examples 16 to 17, the tensile strength after cold working was 650MPa or more. In examples 16 to 17, the amount of martensite after cold working was 0.5% by volume or less.

(8) In example 27, RRA was slightly reduced. This is thought to be because the amount of Mn and the amount of Cu are slightly smaller.

TABLE 4

The embodiments of the present invention have been described above in detail, but the present invention is not limited to the above embodiments and various changes can be made within the scope not departing from the gist of the present invention.

The present application is based on Japanese patent application No.2023-007019 filed on 1 month 20 of 2023 and Japanese patent application No.2023-134998 filed on 8 month 22 of 2023, and the contents of the above Japanese patent applications are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The austenitic stainless steel for high-pressure hydrogen or liquid hydrogen according to the present invention can be used as a structural member used in a high-pressure hydrogen device or a liquid hydrogen device.

Claims

1. An austenitic stainless steel for high pressure hydrogen or liquid hydrogen, having a composition consisting of:

C is less than or equal to 0.20 mass percent,

P is less than or equal to 0.050 mass percent,

S is less than or equal to 0.050 mass percent,

2.0 Mass percent or less and 4.0 mass percent of Cu,

17.0 Mass percent of < Cr is less than or equal to 22.0 mass percent,

Mo is less than or equal to 0.20 mass percent, and

N is less than or equal to 0.050 mass%, and

Optionally, at least one selected from the group consisting of:

v is less than or equal to 0.5 mass percent,

Nb is less than or equal to 0.5 mass percent,

Ca is less than or equal to 0.03 mass percent,

B is less than or equal to 0.05 mass percent,

Zr is less than or equal to 0.5 mass percent,

W is less than or equal to 2.0 mass percent,

Al is less than or equal to 0.05 mass percent,

Mg not more than 0.01 mass%, and

Co less than or equal to 1.0 mass percent,

The balance of Fe and unavoidable impurities;

The austenitic stainless steel for high-pressure hydrogen or liquid hydrogen has a Ni equivalent Nieq represented by the following formula (1) of 24.0 or more:

Nieq＝[％Ni]+15.9[％C]+0.32[％Si]+0.66[％Mn]+0.47[％Cr]+0.64[％Mo]+[％Cu]+15.9[％N] (1)

here, [% Z ] represents the content (mass%) of the element Z; and

The austenitic stainless steel for high-pressure hydrogen or liquid hydrogen has a relative reduction of area represented by the following formula (2) of 0.8 or more:

Relative area reduction = a/B (2)

Here the number of the elements is the number,

B is the area reduction rate of the round bar tensile specimen when the slow strain rate test is performed under the conditions of the test temperature of-60 ℃ and the test atmosphere of 87.5MPa helium.

2. The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to claim 1,

A tensile strength of 500MPa or more as measured at 25 ℃, and

The amount of martensite in the metallic structure is 1.0% by volume or less.

3. The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to claim 1, further satisfying one or more selected from the group consisting of:

0 mass% < V < 0.5 mass%,

0 Mass% < Nb less than or equal to 0.5 mass%,

0 Mass percent of Ca is less than or equal to 0.03 mass percent,

0 Mass percent of < B is less than or equal to 0.05 mass percent,

0 Mass percent < Zr is less than or equal to 0.5 mass percent,

0 Mass% < W.ltoreq.2.0 mass%,

0 Mass% < Al.ltoreq.0.05 mass%, and

0 Mass% of < Mg is less than or equal to 0.01 mass%.

4. The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to claim 2, further satisfying one or more selected from the group consisting of:

0 mass% < V < 0.5 mass%,

0 Mass% < Nb less than or equal to 0.5 mass%,

0 Mass percent of Ca is less than or equal to 0.03 mass percent,

0 Mass percent of < B is less than or equal to 0.05 mass percent,

0 Mass percent < Zr is less than or equal to 0.5 mass percent,

0 Mass% < W.ltoreq.2.0 mass%,

0 Mass% < Al.ltoreq.0.05 mass%, and

0 Mass% of < Mg is less than or equal to 0.01 mass%.

5. The austenitic stainless steel for high-pressure hydrogen gas or liquid hydrogen according to any one of claims 1 to 4, further satisfying:

0 mass% of Co is less than or equal to 1.0 mass%.

6. A method of manufacturing austenitic stainless steel for high pressure hydrogen or liquid hydrogen, comprising:

A first step of melting and casting a raw material to obtain an ingot

The ingot has a composition consisting of:

C is less than or equal to 0.20 mass percent,

P is less than or equal to 0.050 mass percent,

S is less than or equal to 0.050 mass percent,

2.0 Mass percent or less and 4.0 mass percent of Cu,

17.0 Mass percent of < Cr is less than or equal to 22.0 mass percent,

Mo is less than or equal to 0.20 mass percent, and

N is less than or equal to 0.050 mass%, and

Optionally, at least one selected from the group consisting of:

v is less than or equal to 0.5 mass percent,

Nb is less than or equal to 0.5 mass percent,

Ca is less than or equal to 0.03 mass percent,

B is less than or equal to 0.05 mass percent,

Zr is less than or equal to 0.5 mass percent,

W is less than or equal to 2.0 mass percent,

Al is less than or equal to 0.05 mass percent,

Mg not more than 0.01 mass%, and

Co less than or equal to 1.0 mass percent,

The balance being Fe and unavoidable impurities, and

The Ni equivalent Nieq of the ingot represented by the following formula (1) is 24.0 or more:

here, [% Z ] represents the content (mass%) of the element Z;

And thirdly, performing primary heat treatment on the material subjected to the soaking treatment.