JP7513867B2 - Austenitic stainless steel and method for producing same - Google Patents

Description

This disclosure relates to austenitic stainless steel and a method for producing austenitic stainless steel.

Research into the practical application of transportation equipment that uses hydrogen as an energy source instead of fossil fuels is being actively carried out. These transportation equipment, particularly fuel cell vehicles, are equipped with tanks that store high-pressure hydrogen and piping for high-pressure hydrogen. High-pressure hydrogen tanks and piping pose a problem of hydrogen embrittlement, in which hydrogen penetrates into steel material, significantly impairing its ductility and toughness. For this reason, austenitic stainless steels used in high-pressure hydrogen tanks and piping are required to have excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. Furthermore, austenitic stainless steels used in high-pressure hydrogen tanks and piping are required to have high strength in order to withstand high-pressure hydrogen.

For example, JP 2018-135592 A (Patent Document 1) proposes a technology to improve the hydrogen embrittlement resistance and yield strength of austenitic stainless steel.

The austenitic stainless steel for high-pressure hydrogen described in Patent Document 1 contains, by mass%, C: 0.40-1.00%, Si: 1.00% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.030% or less, Ni: 8.00-14.00%, Cr: 16.00-21.00%, N: 0.09% or less, with the balance being Fe and impurity elements, and further satisfies the condition 54.8C + 3.7Ni + 2.5Mn - 1.6Cr - 0.9Si + 266N - 39.6 > 0 (Equation 1), is used as is after solution heat treatment, and is characterized in that Cr carbides are present in the steel at an area ratio of 23% or more. This makes it possible to obtain a steel that is inexpensive in terms of components, such as one that does not require the addition of expensive Mo, and that has excellent yield strength and hardness even after solution heat treatment without relying on cold working to improve strength, and also has excellent resistance to hydrogen embrittlement at low temperatures, according to Patent Document 1.

JP 2018-135592 A

Stainless steels generally contain large amounts of alloying elements such as Ni, Cr, Mo, and Cu. This makes them prone to poor hot workability. In particular, austenitic stainless steels for high-pressure hydrogen contain a large amount of Ni to improve hydrogen embrittlement resistance and strength. For this reason, austenitic stainless steels for high-pressure hydrogen are particularly prone to poor hot workability, and there has been a demand for improved hot workability.

The objective of this disclosure is to provide an austenitic stainless steel that has high strength and combines excellent hydrogen embrittlement resistance with excellent hot workability, and a method for manufacturing an austenitic stainless steel.

The austenitic stainless steel of the present disclosure comprises, in mass %,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 1.50 to 6.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 4.0 to 12.0%,
Cr: 17.0 to 19.0%,
Mo: 0 to 0.100%,
N: 0.12 to 0.30%,
Cu: 0 to 0.50%,
Nb: 0.001 to 0.200%,
V: 0.001 to 0.100%, and
The balance is Fe and impurities, and has a chemical composition that satisfies formulas (1) to (4).
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧0 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formulas (1) to (4) is substituted for the corresponding element symbol.

The method for producing an austenitic stainless steel according to the present disclosure comprises, in mass %,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 1.50 to 6.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 4.0 to 12.0%,
Cr: 17.0 to 19.0%,
Mo: 0 to 0.100%,
N: 0.12 to 0.30%,
Cu: 0 to 0.50%,
Nb: 0.001 to 0.200%,
V: 0.001 to 0.100%, and
preparing a material having a chemical composition satisfying formulas (1) to (4), the balance being Fe and impurities;
A process of producing an intermediate material by hot processing the material;
and finally heat treating the intermediate material.
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧0 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formulas (1) to (4) is substituted for the corresponding element symbol.

The austenitic stainless steel disclosed herein has high strength and is equipped with excellent resistance to hydrogen embrittlement and excellent hot workability. Furthermore, the austenitic stainless steel disclosed herein can be manufactured, for example, by the manufacturing method disclosed herein.

The inventors have investigated and examined the strength, hydrogen embrittlement resistance, and hot workability of austenitic stainless steels, and have obtained the following findings.

Previous research has shown that the mechanism by which hydrogen embrittlement occurs is hydrogen embrittlement due to processing-induced martensite. When processing-induced martensite transformation occurs during processing of austenitic stainless steel, hydrogen embrittlement resistance is significantly reduced. The Ni equivalent is used as an index of the difficulty of this processing-induced martensite transformation to occur. The higher the Ni equivalent, the higher the austenite stability and the less likely processing-induced martensite transformation, which is harmful to hydrogen embrittlement resistance, to occur.

Deformation-induced martensitic transformation is the dominant factor in hydrogen embrittlement. For this reason, in previous studies, austenitic stainless steels for high-pressure hydrogen use have been made to contain large amounts of Ni in order to increase the Ni equivalent. However, austenitic stainless steels that contain large amounts of Ni tend to have poor hot workability.

In order to improve hot workability, it is conceivable to reduce the Ni content of austenitic stainless steel. The inventors investigated the hydrogen embrittlement resistance of austenitic stainless steel with reduced Ni content. As a result, it was found that simply lowering the Ni content reduces the hydrogen embrittlement resistance of austenitic stainless steel, and that excellent hydrogen embrittlement resistance and excellent hot workability cannot be achieved at the same time.

The inventors therefore investigated a method for achieving both excellent hydrogen embrittlement resistance and excellent hot workability in austenitic stainless steels for high-pressure hydrogen. As a result, they obtained the following previously unknown findings.

In addition to the previously known mechanism of deformation-induced martensite, it has been found that dislocations in austenitic stainless steel also play a role in the mechanism of hydrogen embrittlement. Dislocations introduced by plastic deformation move to adopt an arrangement that minimizes the overall elastic energy. As a result, areas with relatively low dislocation density and areas where dislocations have accumulated in a wall-like shape and have a high dislocation density are observed. These are called dislocation cells. The inventors have discovered that when the formation of these dislocation cells is suppressed and a planar dislocation structure is formed, the direction of slip deformation in the metal structure becomes fixed, making hydrogen embrittlement more likely to occur.

Deformation-induced martensite is the dominant factor in hydrogen embrittlement, and in austenitic stainless steels with a high Ni content, hydrogen embrittlement resistance can be improved without considering the dislocation structure. However, it was found that when the Ni content is reduced, hydrogen embrittlement resistance cannot be improved unless the effect of the dislocation structure, which is a subordinate factor, is also taken into consideration.

Therefore, the present inventors investigated and examined the chemical composition of austenitic stainless steel that can improve hot workability and further improve hydrogen embrittlement resistance by suppressing the formation of planar dislocation structures. As a result, it was found that if the chemical composition of austenitic stainless steel satisfies the following formulas (1) and (2), it is possible to achieve both excellent hydrogen embrittlement resistance and excellent hot workability even when the Ni content is reduced to a certain extent.
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧0 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Here, the content (mass %) of each element in formula (1) and formula (2) is substituted for the corresponding element symbol.

Furthermore, the inventors have discovered that in order to suppress strain-induced martensitic transformation, which is a dominant factor in the hydrogen embrittlement resistance of austenitic stainless steel, the chemical composition of the austenitic stainless steel must satisfy the following formula (3).
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
Here, the content (mass %) of the corresponding element is substituted for each element symbol in formula (3).

Moreover, in order to improve hot workability, the chemical composition of the austenitic stainless steel of the present application has a lower Ni content than conventional ones, and therefore the strength is increased by solid solution strengthening of C and N. If the chemical composition of the austenitic stainless steel satisfies the following formula (4), the strength of the austenitic stainless steel is increased.
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formula (4) is substituted for the corresponding element symbol.

The austenitic stainless steel [1] developed based on the above findings is, in mass%,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 1.50 to 6.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 4.0 to 12.0%,
Cr: 17.0 to 19.0%,
Mo: 0 to 0.100%,
N: 0.12 to 0.30%,
Cu: 0 to 0.50%,
Nb: 0.001 to 0.200%,
V: 0.001 to 0.100%, and
The balance is Fe and impurities, and has a chemical composition that satisfies formulas (1) to (4).
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧0 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formulas (1) to (4) is substituted for the corresponding element symbol.

The austenitic stainless steel disclosed herein satisfies formulas (1) to (4). Therefore, even though the Ni content is reduced compared to conventional methods, it has high strength and high resistance to hydrogen embrittlement. As a result, the austenitic stainless steel disclosed herein has high strength and can achieve both excellent resistance to hydrogen embrittlement and excellent hot workability.

The austenitic stainless steel of [2] is the austenitic stainless steel of [1], and its chemical composition, in mass%, is as follows:
Mo: Contains 0.001 to 0.100%.

The austenitic stainless steel of [3] is the austenitic stainless steel according to [1] or [2], and has a chemical composition, in mass%, of:
Cu: Contains 0.01 to 0.50%.

The method for producing an austenitic stainless steel according to [4] comprises, in mass%,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 1.50 to 6.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 4.0 to 12.0%,
Cr: 17.0 to 19.0%,
Mo: 0 to 0.100%,
N: 0.12 to 0.30%,
Cu: 0 to 0.50%,
Nb: 0.001 to 0.200%,
V: 0.001 to 0.100%, and
preparing a material having a chemical composition satisfying formulas (1) to (4), the balance being Fe and impurities;
A process of producing an intermediate material by hot processing the material;
and finally heat treating the intermediate material.
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧0 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formulas (1) to (4) is substituted for the corresponding element symbol.

The austenitic stainless steel disclosed herein can be manufactured, for example, by the manufacturing method described above.

The method for producing an austenitic stainless steel according to [5] is a method for producing an austenitic stainless steel according to [4], wherein the chemical composition, in mass%, is:
Mo: Contains 0.001 to 0.100%.

The method for producing an austenitic stainless steel according to [6] is a method for producing an austenitic stainless steel according to [4] or [5], wherein the chemical composition, in mass%, is:
Cu: Contains 0.01 to 0.50%.

The austenitic stainless steel disclosed herein is described in detail below. "%" for elements means mass % unless otherwise specified.

[Chemical composition]
The chemical composition of the austenitic stainless steel of the present disclosure contains the following elements:

C: 0.100% or less Carbon (C) is not an element that is actively added to the austenitic stainless steel of the present disclosure. If the C content exceeds 0.100%, carbides precipitate at the grain boundaries, adversely affecting toughness, etc. Therefore, the C content is 0.100% or less. The preferred upper limit of the C content is 0.040%, and more preferably 0.020%. It is better to have as little C content as possible, but an extreme reduction in the C content leads to an increase in refining costs, so in practice, it is preferable to set the lower limit of the C content to 0.001%.

Si: 1.00% or less Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si deoxidizes steel. However, if the Si content is too high, Si bonds with Ni and Cr to form intermetallic compounds, or promotes the formation of intermetallic compounds such as sigma phases. As a result, the hot workability and toughness of the steel material are reduced. Therefore, the Si content is 1.00% or less. The preferred upper limit of the Si content is 0.90%, more preferably 0.70%, and even more preferably 0.50%. If the Si content is reduced excessively, the manufacturing cost will increase. Therefore, in consideration of normal industrial production, the preferred lower limit of the Si content is 0.0001%, more preferably 0.001%, even more preferably 0.01%, and even more preferably 0.10%.

Mn: 1.50 to 6.00%
Manganese (Mn) stabilizes austenite and suppresses the formation of martensite, which is highly susceptible to hydrogen embrittlement. Mn also increases the amount of dissolved N and enhances the effect of solid solution strengthening of N. If the Mn content is too low, the above effects cannot be obtained. On the other hand, if the Mn content is too high, the ductility and hot workability of the steel are reduced. Therefore, the Mn content is 1.50 to 6.00%. The lower limit of the Mn content is preferably 1.60%, more preferably 1.70%, and even more preferably 1.80%. The upper limit of the Mn content is preferably 5.80%, more preferably 5.50%, and even more preferably 5.00%.

P: 0.050% or less Phosphorus (P) is an impurity that is inevitably contained. In other words, the P content is more than 0%. P reduces the hot workability and toughness of steel materials. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.030%, and more preferably 0.025%. It is preferable that the P content is as low as possible. However, excessive reduction of the P content increases the manufacturing cost. Therefore, in consideration of normal industrial production, the preferred lower limit of the P content is 0.0001%, and more preferably 0.0005%.

S: 0.030% or less Sulfur (S) is an impurity that is inevitably contained. In other words, the S content is more than 0%. S reduces the hot workability and toughness of steel materials. Therefore, the S content is 0.030% or less. The preferred upper limit of the S content is 0.025%, more preferably 0.020%. The S content is preferably as low as possible. However, excessive reduction of the S content increases the manufacturing cost. Therefore, in consideration of normal industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0005%.

Ni: 4.0 to 12.0%
Nickel (Ni) stabilizes austenite, inhibits the formation of processing-induced martensite, and enhances hydrogen embrittlement resistance. However, if the Ni content is too high, the hot workability of the austenitic stainless steel decreases. Therefore, the Ni content is 4.0 to 12.0%. The preferred lower limit of the Ni content is 4.5%, more preferably 5.0%, and even more preferably 5.5%. The preferred upper limit of the Ni content is 11.50%, more preferably 11.00%, and even more preferably 10.00%.

Cr: 17.0 to 19.0%
Chromium (Cr) enhances the corrosion resistance of steel. Cr also increases the amount of dissolved N and enhances the effect of solid solution strengthening of N. If the Cr content is too low, the above effects cannot be obtained. On the other hand, if the Cr content is too high, _{M23C6 type} carbides are formed, and the ductility and toughness of the austenitic stainless steel are reduced. Therefore, the Cr content is 17.0 to 19.0%. The lower limit of the Cr content is preferably 17.5%, more preferably 17.8%, and even more preferably 18.0%. The upper limit of the Cr content is preferably 18.8%, and even more preferably 18.5%.

N: 0.12 to 0.30%
Nitrogen (N) stabilizes austenite. N also increases the strength of steel by solid solution strengthening. If the N content is too low, the above effects cannot be obtained. On the other hand, if the N content is too high, the toughness and workability of the austenitic stainless steel decrease. Therefore, the N content is 0.12 to 0.30%. The lower limit of the N content is preferably 0.15%, more preferably 0.18%, and even more preferably 0.20%. The upper limit of the N content is preferably 0.28%, and even more preferably 0.25%.

Nb: 0.001 to 0.200%
Niobium (Nb) forms carbonitrides and increases the strength of austenitic stainless steel. If the Nb content is too low, this effect cannot be obtained. However, if the Nb content is too high, the effect saturates and the manufacturing cost increases. Therefore, the Nb content is 0.001 to 0.200%. The lower limit of the Nb content is preferably 0.005%, more preferably 0.008%. The upper limit of the Nb content is preferably 0.150%, more preferably 0.100%, and more preferably 0.050%.

V: 0.001 to 0.100%
Vanadium (V) forms carbonitrides and increases the strength of austenitic stainless steel. If the V content is too low, this effect cannot be obtained. However, if the V content is too high, the effect saturates and the manufacturing cost increases. Therefore, the V content is 0.001 to 0.100%. The lower limit of the V content is preferably 0.005%, and more preferably 0.008%. The upper limit of the V content is preferably 0.080%, and more preferably 0.050%, and even more preferably 0.030%.

The balance of the chemical composition of the austenitic stainless steel of the present disclosure consists of Fe and impurities. Here, impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during the industrial production of austenitic stainless steel, and are acceptable to the extent that they do not adversely affect the austenitic stainless steel of the present disclosure.

[Optional elements]
The austenitic stainless steel according to the present disclosure may further contain Mo in place of a portion of Fe.

Mo: 0 to 0.100%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the hydrogen embrittlement resistance and strength of austenitic stainless steel. Mo also increases the corrosion resistance of austenitic stainless steel. However, if the Mo content is too high, intermetallic compounds are likely to precipitate, and the ductility and toughness of austenitic stainless steel are reduced. Therefore, the Mo content is 0 to 0.100%. The lower limit of the Mo content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.008%. The upper limit of the Mo content is preferably 0.080%, more preferably 0.060%, and even more preferably 0.050%.

The austenitic stainless steel according to the present disclosure may further contain Cu in place of a portion of the Fe.

Cu: 0 to 0.50%
Copper (Cu) is an optional element and may not be included. When included, Cu stabilizes austenite. Cu also increases the strength of austenitic stainless steels by solid solution strengthening. On the other hand, if the Cu content is too high, the hot workability of austenitic stainless steels decreases. Therefore, the Cu content is 0 to 0.50%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Cu content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[Regarding formulas (1) to (4)]
The chemical composition of the austenitic stainless steel disclosed herein satisfies formulas (1) to (4) on the premise that the contents of the above-mentioned elements are satisfied, thereby providing an austenitic stainless steel having high strength and achieving both excellent hydrogen embrittlement resistance and excellent hot workability.

[Regarding formula (1)]
The chemical composition of the austenitic stainless steel of the present disclosure satisfies formula (1).
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧0 (1)
Here, the content (mass %) of each element in formula (1) is substituted for the corresponding element symbol.

F1 is defined as (Cr + 1.5Si + Mo + 0.5Nb) - (Ni + 0.5Mn + 30C + 30(N - 0.06)). If F1 is 0 or more, the hot workability of the austenitic stainless steel is improved. The lower limit of F1 is preferably 0.50, more preferably 1.00, and even more preferably 2.00. The upper limit of F1 is not particularly limited, but is, for example, 14.15.

[Regarding formula (2)]
The chemical composition of the austenitic stainless steel of the present disclosure satisfies formula (2).
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Here, the content (mass %) of each element in formula (2) is substituted for the corresponding element symbol.

F2 is defined as -7.1 + 2.7Ni + 0.49Cr + 2.0Mo - 2.0Si + 0.75Mn - 5.7C - 24N. If F2 is 10.00 or more, the formation of a planar dislocation structure can be suppressed. As a result, even if the Ni content is reduced, the hydrogen embrittlement resistance of the austenitic stainless steel can be improved. The lower limit of F2 is preferably 10.50, and more preferably 11.00. The upper limit of F2 is not particularly limited, but is, for example, 36.43. The upper limit of F2 may be 27.50.

[Regarding formula (3)]
The chemical composition of the austenitic stainless steel of the present disclosure satisfies formula (3).
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
Here, the content (mass %) of each element in formula (3) is substituted for the corresponding element symbol.

Formula (3) is one of the formulas for calculating the Ni equivalent in the austenitic stainless steel of the present disclosure. It is defined as F3 = Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N. If F3 is 25.00 or more, the hydrogen embrittlement resistance of the austenitic stainless steel is increased. The lower limit of F3 is preferably 25.10, and more preferably 25.50. The higher the value of F3, the better. The upper limit of F3 is not particularly limited, but is, for example, 36.25.

[Regarding formula (4)]
The chemical composition of the austenitic stainless steel of the present disclosure satisfies formula (4).
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formula (4) is substituted for the corresponding element symbol.

Formula (4) is an index of the strength of the austenitic stainless steel of the present disclosure due to solid solution strengthening. It is defined as F4 = C + N. If F4 is 0.22 or more, the strength of the austenitic stainless steel is increased. The lower limit of F4 is preferably 0.23, and more preferably 0.24. The higher the value of F4, the better. The upper limit of F4 is not particularly limited, but is, for example, 0.40.

[About tensile strength]
The tensile strength of the austenitic stainless steel of the present disclosure is preferably 650 MPa or more. The lower limit of the tensile strength is more preferably 660 MPa, and even more preferably 670 MPa. The upper limit of the tensile strength is not particularly limited, but is, for example, 900 MPa, preferably 800 MPa, and more preferably 700 MPa.

[Method of measuring tensile strength]
The tensile strength is measured in accordance with JIS Z2241 (2011). Two round bar tensile test pieces with a parallel part diameter of 6.0 mm and a gauge length of 20 mm are taken. The direction in which the round bar tensile test pieces are taken is the rolling direction. A tensile test is performed on the taken round bar tensile test pieces at room temperature using a method in accordance with JIS Z2241 (2011) to determine the tensile strength.

The shape of the austenitic stainless steel of the present disclosure is not particularly limited. The austenitic stainless steel of the present disclosure may be a steel pipe, a steel plate, a steel bar, or another shape.

The austenitic stainless steel of the present disclosure is widely applicable to applications requiring strength and/or hydrogen embrittlement resistance. The austenitic stainless steel of the present disclosure is particularly applicable to components for high-pressure hydrogen gas environment applications. Examples of high-pressure hydrogen gas environment applications include components used for high-pressure hydrogen containers mounted on fuel cell vehicles and high-pressure hydrogen containers installed at hydrogen stations that supply hydrogen to fuel cell vehicles. However, the austenitic stainless steel of the present disclosure is not limited to high-pressure hydrogen gas environment applications. As described above, the austenitic stainless steel of the present disclosure is widely applicable to applications requiring strength and/or hydrogen embrittlement resistance.

[Production method]
An example of a method for producing the austenitic stainless steel of the present disclosure will be described. The example of the method for producing the austenitic stainless steel of the present disclosure includes a preparation step, a hot working step, and a final heat treatment step. Each step will be described in detail below.

The steel material satisfying the above-mentioned chemical composition and formulas (1) to (4) may be purchased from a third party. It may also be manufactured. When manufacturing the steel material, for example, it is manufactured by the following method.

[Preparation process]
In the preparation step, a material satisfying the above chemical composition and formulas (1) to (4) is prepared. Specifically, molten steel satisfying the above chemical composition and formulas (1) to (4) is produced. The produced molten steel is subjected to a well-known degassing process as necessary. A material is produced from the molten steel that has been degassed. A continuous casting method is one example of a method for producing the material. A continuous cast material (material) is produced by the continuous casting method. The continuous cast material is, for example, a slab, bloom, billet, etc. The molten steel may be made into an ingot by an ingot casting method.

[Hot processing process]
In the hot working process, the material is hot worked to produce an intermediate material. Examples of hot working include hot forging, hot extrusion, and hot rolling. Examples of hot forging include stretch forging. Examples of hot rolling include tandem rolling using a tandem rolling mill including a plurality of rolling stands arranged in a row (each rolling stand having a pair of work rolls) to perform rolling in multiple passes, or reverse rolling using a reverse rolling mill having a pair of work rolls to perform rolling in multiple passes. Examples of hot extrusion include hot extrusion using the Eugène-Séjournet method. Hot rolling may be performed after hot forging. An intermediate material may be produced by the above manufacturing process. Examples of intermediate materials include steel pipes, steel bars, and wire rods. The heating temperature during hot working is, for example, 1100 to 1200°C.

[Final heat treatment process]
In the final heat treatment step, the intermediate material is subjected to a final heat treatment. The temperature of the final heat treatment is, for example, 1040 to 1250°C. If the final heat treatment temperature is 1000°C or higher, the alloy elements can be sufficiently dissolved. On the other hand, if the heat treatment temperature is 1250°C or lower, coarsening of austenite grains can be suppressed. The holding time is, for example, 5 to 360 minutes. The intermediate material is preferably quenched from the final heat treatment temperature, and is preferably water-cooled.

This final heat treatment does not have to be an independent process. For example, cooling may be performed after hot working. For example, hot extrusion at 1100 to 1200°C may be performed, followed by cooling.

For example, the above manufacturing method can be used to produce the austenitic stainless steel disclosed herein.

[Other steps]
The method for producing the austenitic stainless steel of the present disclosure may include other steps in addition to the steps described above. For example, a cold working step may be included before the final heat treatment step. Also, an intermediate heat treatment step may be included after the hot working step and before the final heat treatment step. The cold working step and the heat treatment step may be performed multiple times.

Austenitic stainless steel having the chemical composition shown in Table 1 was melted in a test furnace and hot forged and cold worked to a cross-sectional area reduction rate of 10% to produce steel plates 300 mm long, 200 mm wide and 20 mm thick. The resulting steel plates were then subjected to a final heat treatment in which they were held at 1100°C for 30 minutes and then cooled in water. Test specimens for each test were prepared from the steel plates that had undergone the final heat treatment.

[Tensile test]
The tensile strength of each steel plate was measured by the method described above. The results are shown in Table 2.

[Slow strain rate tensile test]
A slow strain rate tensile test was carried out to evaluate the hydrogen embrittlement resistance of the steel plate of each test number. Two round bar tensile test pieces (first and second test pieces) were taken from the center of the steel plate of each test number. The parallel parts of the round bar tensile test pieces were all parallel to the rolling direction of the steel plate. The diameter of the parallel parts was 3 mm. A tensile test (referred to as an air tensile test) was carried out on the first test piece in air at room temperature (25°C) to measure the breaking elongation BE _Air . Furthermore, a tensile test (referred to as a hydrogen tensile test) was carried out on the second test piece in a high pressure hydrogen atmosphere of 45 MPa at room temperature (25°C) to measure the breaking elongation BE _H. In both the air tensile test and the hydrogen tensile test, the strain rate was set to 3×10 ⁻⁶ /s. The obtained breaking elongations BE _Air and BE _H were substituted into formula (5) to calculate the relative breaking elongation RRA (Relationship between relative reduction of area). The results are shown in the RRA (%) column in Table 2.
Relative breaking elongation RRA (%) = BE _H / BE _Air × 100 (5)

[900°C fracture reduction measurement test]
In order to evaluate the hot workability of the steel plate of each test number, the reduction in area at break at 900°C was measured using a Gleeble tester. Gleeble test pieces were taken from the center of the steel plate of each test number. The parallel part of the round bar tensile test piece was parallel to the rolling direction of the steel plate. The round bar test piece was heated electrically to 900°C and held for 3 minutes, after which the test piece was pulled at a strain rate of 10/s to break, and the reduction in area at break (%) was measured. The results are shown in Table 2.

In Table 2, a white circle (○) in the "900°C Fracture Reduction" column indicates that the fracture reduction at 900°C was 60% or more. A cross (×) in the "900°C Fracture Reduction" column indicates that the fracture reduction at 900°C was less than 60%.

[Evaluation results]
With reference to Tables 1 and 2, the steel plates of Test Nos. 1 to 5 had appropriate contents of each element and satisfied formulas (1) to (4). Therefore, the tensile strength was 650 MPa or more, the relative reduction in area at break was 80% or more, and the reduction in area at break at 900°C was 60% or more. In other words, the austenitic stainless steels of Test Nos. 1 to 5 had high strength and further achieved both excellent hydrogen embrittlement resistance and excellent hot workability.

On the other hand, the N content of the steel plate of test number 6 was too low and did not satisfy formula (4). Therefore, the tensile strength of the steel plate of test number 6 was 611 MPa, which was low.

Although the content of each element in the steel plate of test number 7 was appropriate, it did not satisfy formula (4). Therefore, the tensile strength of the steel plate of test number 7 was 634 MPa, which was low.

Although the content of each element in the steel plate of test number 8 was appropriate, it did not satisfy formula (3). Therefore, the relative reduction in area at break of the steel plate of test number 8 was 78%, and the resistance to hydrogen embrittlement was low.

Although the contents of each element in the steel plate of test number 9 were appropriate, it did not satisfy formula (2) and formula (3). Therefore, the relative reduction in area at break of the steel plate of test number 9 was 67%, and the hydrogen embrittlement resistance was low.

The steel plate of test number 10 had too low a Ni content and did not satisfy formula (2). Therefore, the relative reduction in area at break of the steel plate of test number 10 was 53%, and the hydrogen embrittlement resistance was low.

The steel plate of test number 11 had too high Ni and Cr contents and did not satisfy formula (1). Therefore, the steel plate of test number 11 had a fracture reduction of less than 60% at 900°C and had poor hot workability.

Although the contents of each element in the steel plate of test number 12 were appropriate, it did not satisfy formula (1). Therefore, the fracture reduction at 900°C of the steel plate of test number 12 was less than 60%, and the hot workability was poor.

The steel plate of test number 13 had too high Ni content, Cr content, and N content, and did not satisfy formula (1). Therefore, the steel plate of test number 13 had a fracture reduction of less than 60% at 900°C and had poor hot workability.

Although the content of each element in the steel plate of test number 14 was appropriate, it did not satisfy formula (2). Therefore, the steel plate of test number 14 had a relative reduction in area at break of 61% and low resistance to hydrogen embrittlement.

The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit of the present disclosure.

Claims (6)

In mass percent,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 1.50 to 6.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 4.0 to 12.0%,
Cr: 17.0 to 19.0%,
Mo: 0 to 0.100%,
N: 0.12 to 0.30%,
Cu: 0 to 0.50%,
Nb: 0.001 to 0.200%,
V: 0.001 to 0.080 %, and
The balance is Fe and impurities, and the austenitic stainless steel has a chemical composition that satisfies formulas (1) to (4).
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧ 3.38 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formulas (1) to (4) is substituted for the corresponding element symbol. 2. The austenitic stainless steel according to claim 1, wherein the chemical composition is, in mass%,
An austenitic stainless steel containing Mo: 0.001 to 0.100%. 3. The austenitic stainless steel according to claim 1 or 2, wherein the chemical composition is, in mass%,
An austenitic stainless steel containing Cu: 0.01 to 0.50%. A method for producing an austenitic stainless steel, comprising the steps of:
In mass percent,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 1.50 to 6.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 4.0 to 12.0%,
Cr: 17.0 to 19.0%,
Mo: 0 to 0.100%,
N: 0.12 to 0.30%,
Cu: 0 to 0.50%,
Nb: 0.001 to 0.200%,
V: 0.001 to 0.080 %, and
preparing a material having a chemical composition satisfying formulas (1) to (4), the balance being Fe and impurities;
hot working the material to produce an intermediate material;
and subjecting the intermediate material to final heat treatment.
(Cr+1.5Si+Mo+0.5Nb)-(Ni+0.5Mn+30C+30(N-0.06))≧ 3.38 (1)
-7.1 + 2.7Ni + 0.49Cr + 2.0Mo-2.0Si + 0.75Mn-5.7C-24N ≧ 10.00 (2)
Ni + 0.72Cr + 0.88Mo + 1.11Mn - 0.27Si + 12.93C + 0.53Cu + 7.55N ≧ 25.00 (3)
C + N ≧ 0.22 (4)
Here, the content (mass %) of each element in formulas (1) to (4) is substituted for the corresponding element symbol. 5. The method for producing an austenitic stainless steel according to claim 4, wherein the chemical composition is, in mass%,
A method for producing an austenitic stainless steel containing Mo: 0.001 to 0.100%. 6. The method for producing an austenitic stainless steel according to claim 4 or 5, wherein the chemical composition is, in mass%,
A method for producing an austenitic stainless steel containing Cu: 0.01 to 0.50%.