AUSTENITIC ALLOY MATERIAL

Description

TECHNICAL FIELD

The present disclosure relates to an alloy material, and more particularly relates to an austenitic alloy material.

BACKGROUND ART

Recently, hydrogen has been attracting attention as a clean energy that does not generate carbon dioxide. Hydrogen is a gas at normal temperature and normal pressure, and when liquefying hydrogen for transportation, it is necessary to place hydrogen in a cryogenic state of −253° C. or less. For this reason, it is difficult to transport hydrogen alone.

Therefore, the use of ammonia as a carrier for hydrogen is being studied. Ammonia contains about 18% hydrogen by mass and liquefies at −33° C., which is higher than the boiling point of hydrogen. Therefore, studies are underway which are directed at utilizing hydrogen as energy by transporting ammonia, which is a hydrogen carrier, and then desorbing hydrogen from the ammonia at the transport destination.

In order to desorb hydrogen from ammonia, the ammonia is decomposed in a high temperature environment of about 600° C. at normal pressure using a catalyst. In the following description, an environment with an ammonia atmosphere at a high temperature of about 600° C. and normal pressure is referred to as a “high temperature ammonia environment”. Therefore, there is a need for an alloy material capable of withstanding use in a high temperature ammonia environment.

Austenitic alloy materials used in chemical plants can be considered as alloy materials that can be applied to high temperature ammonia environments for hydrogen production. This is because a chemical plant is a high temperature environment equivalent to a high temperature ammonia environment. For example, Japanese Patent Application Publication No. 2017-088957 (Patent Literature 1) discloses an austenitic alloy material for a chemical plant. The alloy material disclosed in Patent Literature 1 has a chemical composition that contains, in mass %, C: 0.02 to 0.12%, Si: 0.1 to 2%, Mn: 0.1 to 3%, P: 0.04% or less, S: 0.02% or less, Cr: 20 to 26%, Ni: more than 26% to 35% or less, W: 1 to 5.5%, V: 0.01 to 1%, Nb: 0.01 to 1%, B: 0.0005 to 0.008%, Mo: 0.3% or less, Al: 0.001 to 0.3%, Cu: 0.3% or less, Ti: 0.01% or less, N: more than 0.13% to 0.35% or less, REM: 0.003 to 0.10%, and the like. It is described in Patent Literature 1 that when the aforementioned austenitic alloy material is used in a high temperature environment, excellent creep strength is obtained.

CITATION LIST

Patent Literature

• Patent Literature 1: Japanese Patent Application Publication No. 2017-088957

SUMMARY OF INVENTION

Technical Problem

In this connection, it has been newly found that when an austenitic alloy material is used in the aforementioned high temperature ammonia environment, nitriding is promoted in the outer layer of the alloy material. If the formed nitrided layer becomes thick, an alloy portion other than the nitrided layer will become thin. In such a case, the creep strength at a high temperature will decrease. Therefore, an austenitic alloy material to be used in a high temperature ammonia environment needs to have excellent nitridation resistance.

An objective of the present disclosure is to provide an austenitic alloy material that has excellent nitridation resistance in a high temperature ammonia environment.

Solution to Problem

An austenitic alloy material of the present disclosure has a chemical composition that contains, in mass %,

• • C: more than 0 to 0.200%,
  • Si: more than 0 to 3.00%,
  • Mn: more than 0 to 3.00%,
  • P: more than 0 to 0.050%,
  • S: more than 0 to 0.050%,
  • Ni: 40.00 to 80.00%, and
  • Cr: 10.00 to 35.00%,
  • and also contains one or more kinds of element selected from a group consisting of:
  • Sn: more than 0 to 0.1000%,
  • Zn: more than 0 to 0.0100%,
  • Pb: more than 0 to 0.0100%,
  • Sb: more than 0 to 0.0100%,
  • As: more than 0 to 0.0010%, and
  • Bi: more than 0 to 0.0010%,
  • and further contains one or more kinds of element selected from a group consisting of:
  • Cu: more than 0 to 5.00%,
  • Mo: more than 0 to 20.00%,
  • Co: more than 0 to 3.00%,
  • W: more than 0 to 7.00%,
  • Ti: more than 0 to 1.00%,
  • Nb: more than 0 to 0.10%,
  • V: more than 0 to 0.50%,
  • B: more than 0 to 0.0050%,
  • N: more than 0 to 0.200%,
  • rare earth metal: more than 0 to 0.100%,
  • Al: more than 0 to 0.500%,
  • Ca: more than 0 to 0.0100%, and
  • Mg: more than 0 to 0.0150%,
  • with the balance being Fe and impurities,
  • wherein:
  • Fn1 defined by Formula (1) is less than 20, and
  • Fn2 defined by Formula (2) is more than 21 and less than 50:

Fn ⁢ 1 = 177.84 + 11.12 Si - 24.36 Mn - 8.11 Cu - 1.61 Cr - 1.78 Ni - 
 2.68 Mo ( 1 ) Fn ⁢ 2 = ( Sn + Zn + Pb + Sb + As + Bi ) × 10 3 ( 2 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formulae (1) and (2), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

Advantageous Effect of Invention

The austenitic alloy material of the present disclosure has excellent nitridation resistance in a high temperature ammonia environment.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph illustrating the relation between Fn1 and nitrided layer depth (μm) in alloy materials in which the content of each element in the chemical composition is within the range of the present embodiment, in a case where the alloy materials were held at 600° C. in a 100% ammonia atmosphere for 25 hours.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted studies regarding an alloy material in which excellent nitridation resistance is obtained when used in a high temperature ammonia environment. First, the present inventors conducted studies regarding elements that increase nitridation resistance in a high temperature ammonia environment. As a result, the present inventors have discovered that Mn, Cu, Cr, Ni, and Mo increase the nitridation resistance of an alloy material in a high temperature ammonia environment, while on the other hand, Si decreases the nitridation resistance of an alloy material in a high temperature ammonia environment.

Therefore, the present inventors conducted studies regarding alloy materials from the viewpoint of the chemical composition taking into consideration the aforementioned elements that increase nitridation resistance (Mn, Cu, Cr, Ni and Mo) and element that decreases nitridation resistance (Si). As a result, the present inventors have come to consider that if an austenitic alloy material has a chemical composition that contains, in mass %, C: more than 0 to 0.200%, Si: more than 0 to 3.00%, Mn: more than 0 to 3.00%, P: more than 0 to 0.050%, S: more than 0 to 0.050%, Ni: 40.00 to 80.00%, and Cr: 10.00 to 35.00%, and also contains one or more kinds of element selected from a group consisting of Sn: more than 0 to 0.1000%, Zn: more than 0 to 0.0100%, Pb: more than 0 to 0.0100%, Sb: more than 0 to 0.0100%, As: more than 0 to 0.0010%, and Bi: more than 0 to 0.0010%, and further contains one or more kinds of element selected from a group consisting of Cu: more than 0 to 5.00%, Mo: more than 0 to 20.00%, Co: more than 0 to 3.00%, W: more than 0 to 7.00%, Ti: more than 0 to 1.00%, Nb: more than 0 to 0.10%, V: more than 0 to 0.50%, B: more than 0 to 0.0050%, N: more than 0 to 0.200%, rare earth metal: more than 0 to 0.100%, Al: more than 0 to 0.500%, Ca: more than 0 to 0.0100%, and Mg: more than 0 to 0.0150%, with the balance being Fe and impurities, there is a possibility that excellent nitridation resistance will be obtained in a high temperature ammonia environment.

However, even when austenitic alloy materials had the chemical composition described above, there were still cases where sufficient nitridation resistance was not obtained in a high temperature ammonia environment. Therefore, the present inventors carried out further studies.

Here, the present inventors have come to consider that if the contents of the aforementioned elements that increase nitridation resistance (Mn, Cu, Cr, Ni, and Mo) and the content of the aforementioned element that decreases nitridation resistance (Si) satisfy a predetermined relation, it is likely that the nitridation resistance in a high temperature ammonia environment will increase. Therefore, with respect to an austenitic alloy material that satisfies the chemical composition described above, the present inventors investigated the relation between the contents of the elements that increase nitridation resistance (Mn, Cu, Cr, Ni, and Mo) and the element that decreases nitridation resistance (Si), and a nitrided layer depth in a high temperature ammonia environment. As a result, the present inventors have discovered that if Fn1 defined by Formula (1) that is based on the respective contents of the elements that increase nitridation resistance (Mn, Cu, Cr, Ni and Mo) and the content of the element that decreases nitridation resistance (Si) is less than 20, excellent nitridation resistance is obtained in a high temperature ammonia environment:

Fn ⁢ 1 = 177.84 + 11.12 Si - 24.36 Mn - 8.11 Cu - 1.61 Cr - 1.78 Ni - 
 2.68 Mo ( 1 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula, and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

FIG. 1 is a graph illustrating the relation between Fn1 and a nitrided layer depth (μm) in alloy materials in which the content of each element in the chemical composition is within the range of the present embodiment, in a case where the alloy materials were held at 600° C. in a 100% ammonia atmosphere for 25 hours. FIG. 1 was prepared based on data obtained by examples that are described later. Referring to FIG. 1, in alloy materials in which the content of each element in the chemical composition was within the range of the present embodiment, when Fn1 was 20 or more, a nitrided layer depth when the alloy materials were held at 600° C. in a 100% ammonia atmosphere for 25 hours was more than 30.0 μm. On the other hand, when Fn1 was less than 20, a nitrided layer depth when the alloy materials were held at 600° C. in a 100% ammonia atmosphere for 25 hours was 15.0 μm or less, and thus the nitridation resistance markedly increased.

As described above, the present inventors discovered if an austenitic alloy material has the chemical composition described above and Fn1 is less than 20, excellent nitridation resistance is obtained in a high temperature ammonia environment. However, as a new problem it was revealed that in an austenitic alloy material that has the chemical composition described above and for which Fn1 is less than 20, intergranular cracking may sometimes occur in an outer layer of the alloy material in a high temperature ammonia environment.

Therefore, the present inventors carried out further studies regarding means for suppressing the occurrence of intergranular cracking in a high temperature ammonia environment in an austenitic alloy material that has the chemical composition described above and for which Fn1 is less than 20. As a result, the present inventors have discovered that in an austenitic alloy material that has the chemical composition described above and for which Fn1 is less than 20, if, in addition, Fn2 defined by Formula (2) is more than 21 and less than 50, in a high temperature ammonia environment, excellent nitridation resistance is obtained and, furthermore, the occurrence of intergranular cracking in an outer layer can also be sufficiently suppressed:

Fn ⁢ 2 = ( Sn + Zn + Pb + Sb + As + Bi ) × 10 3 ( 2 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in the formula, and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

Although the reason why intergranular cracking in the outer layer can be suppressed in a high temperature ammonia environment if Fn2 is more than 21 and less than 50 is not certain, the present inventors consider that the reason is as follows. In an austenitic alloy material that has the chemical composition described above, containing each of the elements Sn, Zn, Pb, Sb, As, and Bi in a very small amount allows these elements to segregate at grain boundaries of the alloy material in a high temperature ammonia environment. Therefore, these elements suppress the formation of precipitates at the grain boundaries, and suppress the segregation of P and S to the grain boundaries. As a result, the grain boundaries are strengthened, and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If Fn2 is 21 or less, the aforementioned advantageous effect cannot be sufficiently obtained.

On the other hand, if Fn2 is 50 or more, Sn, Zn, Pb, Sb, As, and Bi will excessively segregate to grain boundaries, and the grain boundary strength will, on the contrary, decrease. Consequently, in a high temperature ammonia environment, intergranular cracking will easily occur in the outer layer of the alloy material.

If Fn2 is more than 21 and less than 50, on the precondition that Fn1 is less than 20, the occurrence of intergranular cracking can be sufficiently suppressed in the outer layer of the austenitic alloy material in a high temperature ammonia environment.

There is also a possibility that intergranular cracking in the outer layer is suppressed by a different mechanism to the mechanism described above. However, the fact that, on the precondition that Fn1 is less than 20, when Fn2 is more than 21 and less than 50, intergranular cracking in the outer layer is sufficiently suppressed in a high temperature ammonia environment has been proven by examples that are described later.

The austenitic alloy material of the present embodiment was completed based on the technical idea described above. The austenitic alloy material of the present embodiment is as follows.

An austenitic alloy material according to a first configuration has a chemical composition that contains, in mass %,

• • C: more than 0 to 0.200%,
  • Si: more than 0 to 3.00%,
  • Mn: more than 0 to 3.00%,
  • P: more than 0 to 0.050%,
  • S: more than 0 to 0.050%,
  • Ni: 40.00 to 80.00%, and
  • Cr: 10.00 to 35.00%,
  • and also contains one or more kinds of element selected from a group consisting of:
  • Sn: more than 0 to 0.1000%,
  • Zn: more than 0 to 0.0100%,
  • Pb: more than 0 to 0.0100%,
  • Sb: more than 0 to 0.0100%,
  • As: more than 0 to 0.0010%, and
  • Bi: more than 0 to 0.0010%,
  • and further contains one or more kinds of element selected from a group consisting of:
  • Cu: more than 0 to 5.00%,
  • Mo: more than 0 to 20.00%,
  • Co: more than 0 to 3.00%,
  • W: more than 0 to 7.00%,
  • Ti: more than 0 to 1.00%,
  • Nb: more than 0 to 0.10%,
  • V: more than 0 to 0.50%,
  • B: more than 0 to 0.0050%,
  • N: more than 0 to 0.200%,
  • rare earth metal: more than 0 to 0.100%,
  • Al: more than 0 to 0.500%,
  • Ca: more than 0 to 0.0100%, and
  • Mg: more than 0 to 0.0150%,
  • with the balance being Fe and impurities,
  • wherein:
  • Fn1 defined by Formula (1) is less than 20, and
  • Fn2 defined by Formula (2) is more than 21 and less than 50:

Fn ⁢ 1 = 177.84 + 11.12 Si - 24.36 Mn - 8.11 Cu - 1.61 Cr - 1.78 Ni - 
 2.68 Mo ( 1 ) Fn ⁢ 2 = ( Sn + Zn + Pb + Sb + As + Bi ) × 10 3 ( 2 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formulae (1) and (2), and if an element is not contained, “0” is substituted for the corresponding symbol of an element.

An austenitic alloy material according to a second configuration is in accordance with the austenitic alloy material of the first configuration, wherein:

• • when an average grain diameter in units of μm in an outer layer of the austenitic alloy material is defined as D_ave,
  • Fn3 defined by Formula (3) is more than 0.20, and
  • Fn4 defined by Formula (4) is 1000 to 5000.

Fn ⁢ 3 = Fn ⁢ 2 / D a ⁢ v ⁢ e ( 3 ) Fn ⁢ 4 = Fn ⁢ 2 × D a ⁢ v ⁢ e ( 4 )

An austenitic alloy material according to a third configuration is in accordance with the austenitic alloy material of the first configuration or second configuration, wherein the chemical composition contains, in mass %:

• • C: more than 0 to 0.050%,
  • Si: more than 0 to 0.50%,
  • Mn: more than 0 to 0.50%,
  • P: more than 0 to 0.025%,
  • S: more than 0 to 0.010%,
  • Cu: 2.00 to 4.00%,
  • Ni: 44.00 to 50.00%,
  • Cr: 20.00 to 25.00%,
  • Mo: 5.00 to 7.00%,
  • W: 2.00 to 5.00%, and
  • Fe: 12.00 to 20.00%.

An austenitic alloy material according to a fourth configuration is in accordance with the austenitic alloy material of the first configuration or second configuration, wherein the chemical composition contains, in mass %:

• • C: more than 0 to 0.150%,
  • Si: 1.00 to 2.50%,
  • Mn: more than 0 to 1.00%,
  • P: more than 0 to 0.010%,
  • S: more than 0 to 0.010%,
  • Cu: 1.50 to 3.00%,
  • Cr: 28.00 to 32.00%,
  • Mo: 1.00 to 3.00%,
  • Ti: more than 0 to 1.00%, and
  • Fe: 2.00 to 6.00%.

An austenitic alloy material according to a fifth configuration is in accordance with the austenitic alloy material of the first configuration or second configuration, wherein the chemical composition contains, in mass %:

• • C: more than 0 to 0.050%,
  • Si: more than 0 to 0.50%,
  • Mn: more than 0 to 0.50%,
  • P: more than 0 to 0.030%,
  • S: more than 0 to 0.015%,
  • Cu: more than 0 to 0.50%,
  • Cr: 27.00 to 31.00%,
  • Fe: 7.00 to 15.00%, and
  • Ni: 58.00 to 80.00%.

The austenitic alloy material of the present embodiment is described in detail hereunder. Note that, the symbol “%” in relation to elements means “mass percent” unless otherwise noted. In the following description, the austenitic alloy material is also referred to simply as “alloy material”.

[Features of Austenitic Alloy Material of Present Embodiment]

The austenitic alloy material of the present embodiment includes the following features.

(Feature 1)

The chemical composition contains, in mass %, C: more than 0 to 0.200%, Si: more than 0 to 3.00%, Mn: more than 0 to 3.00%, P: more than 0 to 0.050%, S: more than 0 to 0.050%, Ni: 40.00 to 80.00%, and Cr: 10.00 to 35.00%, and also contains one or more kinds of element selected from a group consisting of Sn: more than 0 to 0.1000%, Zn: more than 0 to 0.0100%, Pb: more than 0 to 0.0100%, Sb: more than 0 to 0.0100%, As: more than 0 to 0.0010%, and Bi: more than 0 to 0.0010%, and further contains one or more kinds of element selected from a group consisting of Cu: more than 0 to 5.00%, Mo: more than 0 to 20.00%, Co: more than 0 to 3.00%, W: more than 0 to 7.00%, Ti: more than 0 to 1.00%, Nb: more than 0 to 0.10%, V: more than 0 to 0.50%, B: more than 0 to 0.0050%, N: more than 0 to 0.200%, rare earth metal: more than 0 to 0.100%, Al: more than 0 to 0.500%, Ca: more than 0 to 0.0100%, and Mg: more than 0 to 0.0150%, with the balance being Fe and impurities.

(Feature 2)

Fn1 defined by Formula (1) is less than 20:

Fn ⁢ 1 = 177.84 + 11.12 Si - 24.36 Mn - 8.11 Cu - 1.61 Cr - 1.78 Ni - 
 2.68 Mo ( 1 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (1). If an element is not contained, “0” is substituted for the corresponding symbol of an element.

(Feature 3)

Fn2 defined by Formula (2) is more than 21 and less than 50:

Fn ⁢ 2 = ( Sn + Zn + Pb + Sb + As + Bi ) × 10 3 ( 2 )

• • where, a content in percent by mass of a corresponding element is substituted for each symbol of an element in Formula (2). If an element is not contained, “0” is substituted for the corresponding symbol of an element.

Hereunder, each feature is described.

[(Feature 1) Regarding Chemical Composition]

The chemical composition of the austenitic alloy material according to the present embodiment contains the following elements.

• • C: more than 0 to 0.200%

During use of the alloy material in a high temperature ammonia environment, carbon (C) forms fine carbides and thereby increases the creep strength. However, if the content of C is more than 0.200%, in a high temperature ammonia environment, carbides will excessively form at grain boundaries. In such case, even if the contents of other elements are within the range of the present embodiment, intergranular cracking will easily occur at the surface of the alloy material.

Therefore, the content of C is more than 0 to 0.200%.

A preferable lower limit of the content of C is 0.001%, more preferably is 0.005%, further preferably is 0.010%, and further preferably is 0.020%.

A preferable upper limit of the content of C is 0.180%, more preferably is 0.160%, and further preferably is 0.150%.

A preferable range of the content of C is, for example, 0.001 to 0.180%, more preferably is 0.005 to 0.160%, further preferably is 0.010 to 0.150%, and further preferably is 0.020 to 0.150%.

• • Si: more than 0 to 3.00%

Silicon (Si) deoxidizes the alloy. Si also increases the oxidation resistance of the alloy material in a high temperature ammonia environment. However, if the content of Si is more than 3.00%, even if the contents of other elements are within the range of the present embodiment, intergranular cracking may occur in a high temperature ammonia environment.

Therefore, the content of Si is more than 0 to 3.00%.

A preferable lower limit of the content of Si is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.30%.

A preferable upper limit of the content of Si is 2.80%, more preferably is 2.70%, and further preferably is 2.60%.

A preferable range of the content of Si is, for example, 0.01 to 2.80%, more preferably is 0.05 to 2.70%, further preferably is 0.10 to 2.60%, and further preferably is 0.30 to 2.60%.

• • Mn: more than 0 to 3.00%

Manganese (Mn) increases the nitridation resistance of the alloy material in a high temperature ammonia environment. However, if the content of Mn is more than 3.00%, the creep ductility of the alloy material will decrease in a high temperature ammonia environment even if the contents of other elements are within the range of the present embodiment. In addition, the toughness of the alloy material will decrease.

Therefore, the content of Mn is more than 0 to 3.00%.

A preferable lower limit of the content of Mn is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.30%.

A preferable upper limit of the content of Mn is 2.80%, more preferably is 2.50%, further preferably is 2.00%, and further preferably is 1.50%.

A preferable range of the content of Mn is, for example, 0.01 to 2.80%, more preferably is 0.05 to 2.50%, further preferably is 0.10 to 2.00%, and further preferably is 0.30 to 1.50%.

• • P: more than 0 to 0.050%

Phosphorus (P) is an impurity. If the content of P is more than 0.050%, in a high temperature ammonia environment, P will segregate to grain boundaries. Therefore, even if the contents of other elements are within the range of the present embodiment, intergranular cracking may occur in the alloy material in a high temperature ammonia environment.

Therefore, the content of P is more than 0 to 0.050%.

The content of P is preferably as low as possible. However, excessively reducing the content of P will significantly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of P is 0.001%, more preferably is 0.002%, and further preferably is 0.005%.

A preferable upper limit of the content of P is 0.045%, more preferably is 0.042%, and further preferably is 0.040%.

A preferable range of the content of P is, for example, 0.001 to 0.045%, more preferably is 0.002 to 0.042%, and further preferably is 0.005 to 0.040%.

• • S: more than 0 to 0.050%

Sulfur (S) is an impurity. If the content of S is more than 0.050%, in a high temperature ammonia environment, S will segregate to grain boundaries. Therefore, even if the contents of other elements are within the range of the present embodiment, intergranular cracking may occur in the alloy material in a high temperature ammonia environment.

Therefore, the content of S is more than 0 to 0.050%.

The content of S is preferably as low as possible. However, excessively reducing the content of S will significantly increase the production cost. Therefore, when taking industrial production into consideration, a preferable lower limit of the content of S is 0.001%, more preferably is 0.002%, and further preferably is 0.005%.

A preferable upper limit of the content of S is 0.045%, more preferably is 0.040%, and further preferably is 0.035%.

A preferable range of the content of S is, for example, 0.001 to 0.045%, more preferably is 0.002 to 0.040%, and further preferably is 0.005 to 0.035%.

• • Ni: 40.00 to 80.00%

Nickel (Ni) increases the nitridation resistance of the alloy material in a high temperature ammonia environment. If the content of Ni is less than 40.00%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of Ni is more than 80.00%, even if the contents of other elements are within the range of the present embodiment, intergranular cracking may occur in a high temperature ammonia environment.

Therefore, the content of Ni is 40.00 to 80.00%.

A preferable lower limit of the content of Ni is 43.00%, more preferably is 45.00%, further preferably is 50.00%, and further preferably is 55.00%.

A preferable upper limit of the content of Ni is 75.00%, more preferably is 70.00%, further preferably is 65.00%, and further preferably is 60.00%.

A preferable range of the content of Ni is, for example, 43.00 to 75.00%, more preferably is 45.00 to 70.00%, further preferably is 50.00 to 65.00%, and further preferably is 55.00 to 60.00%.

• • Cr: 10.00 to 35.00%

Chromium (Cr) increases the nitridation resistance of the alloy material in a high temperature ammonia environment. If the content of Cr is less than 10.00%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

On the other hand, if the content of Cr is more than 35.00%, the creep strength of the alloy material in a high temperature ammonia environment will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of Cr is 10.00 to 35.00%.

A preferable lower limit of the content of Cr is 12.00%, more preferably is 15.00%, and further preferably is 18.00%.

A preferable upper limit of the content of Cr is 33.00%, more preferably is 32.00%, further preferably is 31.00%, and further preferably is 30.00%.

A preferable range of the content of Cr is, for example, 12.00 to 33.00%, more preferably is 15.00 to 32.00%, further preferably is 18.00 to 31.00%, and further preferably is 18.00 to 30.00%.

The austenitic alloy material of the present embodiment also contains a first group.

[First Group]

One or more kinds of element selected from a group consisting of:

• • Sn: more than 0 to 0.1000%,
  • Zn: more than 0 to 0.0100%,
  • Pb: more than 0 to 0.0100%,
  • Sb: more than 0 to 0.0100%,
  • As: more than 0 to 0.0010%, and
  • Bi: more than 0 to 0.0010%.

Each of the elements Sn, Zn, Pb, Sb, As, and Bi suppress intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment. Hereunder, each element in the first group is described.

• • Sn: more than 0 to 0.1000%

Tin (Sn) does not have to be contained. That is, the content of Sn may be 0%.

When contained, that is, when the content of Sn is more than 0%, Sn segregates to grain boundaries during use of the alloy material in a high temperature ammonia environment. The segregated Sn suppresses the formation of precipitates at the grain boundaries and the segregation of P and S at the grain boundaries. By this means, grain boundaries are strengthened and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Sn is more than 0.1000%, Sn will excessively segregate to grain boundaries during use of the alloy material in a high temperature ammonia environment. In such case, the strength of the grain boundaries will, on the contrary, decrease. Consequently, even if the contents of other elements are within the range of the present embodiment, intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment will be promoted.

Therefore, the content of Sn is more than 0 to 0.1000%.

A preferable lower limit of the content of Sn is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

A preferable upper limit of the content of Sn is 0.0700%, more preferably is 0.0500%, and further preferably is 0.0450%.

A preferable range of the content of Sn is, for example, 0.0001 to 0.0700%, more preferably is 0.0002 to 0.0500%, and further preferably is 0.0003 to 0.0450%.

• • Zn: more than 0 to 0.0100%

Zinc (Zn) does not have to be contained. That is, the content of Zn may be 0%.

When contained, that is, when the content of Zn is more than 0%, Zn segregates to grain boundaries during use of the alloy material in a high temperature ammonia environment. The segregated Zn suppresses the formation of precipitates at the grain boundaries and the segregation of P and S at the grain boundaries. By this means, the grain boundaries are strengthened and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If even a small amount of Zn is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Zn is more than 0.0100%, Zn will excessively segregate to grain boundaries during use of the alloy material in a high temperature ammonia environment. In such case, the strength of the grain boundaries will, on the contrary, decrease. Consequently, even if the contents of other elements are within the range of the present embodiment, intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment will be promoted.

Therefore, the content of Zn is more than 0 to 0.0100%.

A preferable lower limit of the content of Zn is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0020%.

A preferable upper limit of the content of Zn is 0.0095%, more preferably is 0.0090%, and further preferably is 0.0080%.

A preferable range of the content of Zn is, for example, 0.0001 to 0.0095%, more preferably is 0.0010 to 0.0090%, and further preferably is 0.0020 to 0.0080%.

• • Pb: more than 0 to 0.0100%

Lead (Pb) does not have to be contained. That is, the content of Pb may be 0%.

When contained, that is, when the content of Pb is more than 0%, Pb segregates to grain boundaries during use of the alloy material in a high temperature ammonia environment. The segregated Pb suppresses the formation of precipitates at the grain boundaries and the segregation of P and S at the grain boundaries. By this means, the grain boundaries are strengthened and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If even a small amount of Pb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Pb is more than 0.0100%, Pb will excessively segregate to grain boundaries during use of the alloy material in a high temperature ammonia environment. In such case, the strength of the grain boundaries will, on the contrary, decrease. Consequently, even if the contents of other elements are within the range of the present embodiment, intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment will be promoted.

Therefore, the content of Pb is more than 0 to 0.0100%.

A preferable lower limit of the content of Pb is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0020%.

A preferable upper limit of the content of Pb is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.

A preferable range of the content of Pb is, for example, 0.0001 to 0.0090%, more preferably is 0.0010 to 0.0080%, and further preferably is 0.0020 to 0.0070%.

• • Sb: more than 0 to 0.0100%

Antimony (Sb) does not have to be contained. That is, the content of Sb may be 0%.

When contained, that is, when the content of Sb is more than 0%, Sb segregates to grain boundaries during use of the alloy material in a high temperature ammonia environment. The segregated Sb suppresses the formation of precipitates at the grain boundaries and the segregation of P and S at the grain boundaries. By this means, the grain boundaries are strengthened and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Sb is more than 0.0100%, Sb will excessively segregate to grain boundaries during use of the alloy material in a high temperature ammonia environment. In such case, the strength of the grain boundaries will, on the contrary, decrease. Consequently, even if the contents of other elements are within the range of the present embodiment, intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment will be promoted.

Therefore, the content of Sb is more than 0 to 0.0100%.

A preferable lower limit of the content of Sb is 0.0001%, more preferably is 0.0010%, and further preferably is 0.0015%.

A preferable upper limit of the content of Sb is 0.0090%, more preferably is 0.0080%, and further preferably is 0.0070%.

A preferable range of the content of Sb is, for example, 0.0001 to 0.0090%, more preferably is 0.0010 to 0.0080%, and further preferably is 0.0015 to 0.0070%.

• • As: more than 0 to 0.0010%

Arsenic (As) does not have to be contained. That is, the content of As may be 0%.

When contained, that is, when the content of As is more than 0%, As segregates to grain boundaries during use of the alloy material in a high temperature ammonia environment. The segregated As suppresses the formation of precipitates at the grain boundaries and the segregation of P and S at the grain boundaries. By this means, the grain boundaries are strengthened and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If even a small amount of As is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of As is more than 0.0010%, As will excessively segregate to grain boundaries during use of the alloy material in a high temperature ammonia environment. In such case, the strength of the grain boundaries will, on the contrary, decrease. Consequently, even if the contents of other elements are within the range of the present embodiment, intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment will be promoted.

Therefore, the content of As is more than 0 to 0.0010%.

A preferable lower limit of the content of As is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

A preferable upper limit of the content of As is 0.0009%, more preferably is 0.0008%, and further preferably is 0.0007%.

A preferable range of the content of As is, for example, 0.0001 to 0.0009%, more preferably is 0.0002 to 0.0008%, and further preferably is 0.0003 to 0.0007%.

• • Bi: more than 0 to 0.0010%

Bismuth (Bi) does not have to be contained. That is, the content of Bi may be 0%.

When contained, that is, when the content of Bi is more than 0%, Bi segregates to grain boundaries during use of the alloy material in a high temperature ammonia environment. The segregated Bi suppresses the formation of precipitates at the grain boundaries and the segregation of P and S at the grain boundaries. By this means, the grain boundaries are strengthened and intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment is suppressed. If even a small amount of Bi is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Bi is more than 0.0010%, Bi will excessively segregate to grain boundaries during use of the alloy material in a high temperature ammonia environment. In such case, the strength of the grain boundaries will, on the contrary, decrease. Consequently, even if the contents of other elements are within the range of the present embodiment, intergranular cracking in the vicinity of the outer layer of the alloy material in a high temperature ammonia environment will be promoted.

Therefore, the content of Bi is more than 0 to 0.0010%.

A preferable lower limit of the content of Bi is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

A preferable upper limit of the content of Bi is 0.0009%, more preferably is 0.0008%, and further preferably is 0.0007%.

A preferable range of the content of Bi is, for example, 0.0001 to 0.0009%, more preferably is 0.0002 to 0.0008%, and further preferably is 0.0003 to 0.0007%.

The austenitic alloy material of the present embodiment also contains one or more kinds of element selected from a group consisting of a second group to a fourth group.

[Second Group]

One or more kinds of element selected from a group consisting of:

• • Cu: more than 0 to 5.00%, and
  • Mo: more than 0 to 20.00%.

[Third Group]

One or more kinds of element selected from a group consisting of:

• • Co: more than 0 to 3.00%,
  • W: more than 0 to 7.00%,
  • Ti: more than 0 to 1.00%,
  • Nb: more than 0 to 0.10%,
  • V: more than 0 to 0.50%,
  • B: more than 0 to 0.0050%,
  • N: more than 0 to 0.200%, and
  • rare earth metal: more than 0 to 0.100%.

[Fourth Group]

One or more kinds of element selected from a group consisting of:

• • Al: more than 0 to 0.500%,
  • Ca: more than 0 to 0.0100%, and
  • Mg: more than 0 to 0.0150%.

Hereunder, each element of the second group to fourth group is described.

[(Second Group) Cu and Mo]

Cu and Mo each increase the nitridation resistance of the alloy material in a high temperature ammonia environment. Hereunder, each of these elements is described.

• • Cu: more than 0 to 5.00%

Copper (Cu) does not have to be contained. That is, the content of Cu may be 0%.

When contained, that is, when the content of Cu is more than 0%, Cu increases the nitridation resistance of the alloy material in a high temperature ammonia environment. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Cu is more than 5.00%, the creep ductility of the alloy material in a high temperature ammonia environment will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of Cu is more than 0 to 5.00%.

A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.50%, further preferably is 1.00%, and further preferably is 1.50%.

A preferable upper limit of the content of Cu is 4.50%, more preferably is 4.00%, further preferably is 3.50%, further preferably is 3.00%, and further preferably is 2.50%.

A preferable range of the content of Cu is, for example, 0.01 to 4.50%, more preferably is 0.05 to 4.00%, further preferably is 0.10 to 3.50%, further preferably is 0.50 to 3.00%, further preferably is 1.00 to 2.50%, and further preferably is 1.50 to 2.50%.

• • Mo: more than 0 to 20.00%

Molybdenum (Mo) does not have to be contained. That is, the content of Mo may be 0%.

When contained, that is, when the content of Mo is more than 0%, Mo increases the nitridation resistance of the alloy material in a high temperature ammonia environment. If even a small amount of Mo is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Mo is more than 20.00%, the hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of Mo is more than 0 to 20.00%.

A preferable lower limit of the content of Mo is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.50%, further preferably is 1.00%, further preferably is 1.50%, further preferably is 3.00%, further preferably is 5.00%, and further preferably is 8.00%.

A preferable upper limit of the content of Mo is 18.00%, more preferably is 16.00%, and further preferably is 14.00%.

A preferable range of the content of Mo is, for example, 0.01 to 18.00%, more preferably is 0.05 to 16.00%, further preferably is 0.10 to 14.00%, further preferably is 0.50 to 14.00%, further preferably is 1.00 to 14.00%, further preferably is 1.50 to 14.00%, further preferably is 3.00 to 14.00%, further preferably is 5.00 to 14.00%, and further preferably is 8.00 to 14.00%.

[(Third Group) Co, W, Ti, Nb, V, B, N, and Rare Earth Metal]

Each of the elements Co, W, Ti, Nb, V, B, N, and rare earth metal (REM) increases the creep strength of the alloy material in a high temperature ammonia environment. Hereunder, each of these elements is described.

• • Co: more than 0 to 3.00%

Cobalt (Co) does not have to be contained. That is, the content of Co may be 0%.

When contained, that is, when the content of Co is more than 0%, Co dissolves in the alloy material and increases the creep strength of the alloy material in a high temperature ammonia environment. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Co is more than 3.00%, the aforementioned advantageous effect will be saturated and the production cost will increase.

Therefore, the content of Co is more than 0 to 3.00%.

A preferable lower limit of the content of Co is 0.01%, more preferably is 0.05%, further preferably is 0.10%, and further preferably is 0.15%.

A preferable upper limit of the content of Co is 2.80%, more preferably is 2.50%, and further preferably is 2.00%.

A preferable range of the content of Co is, for example, 0.01 to 2.80%, more preferably is 0.05 to 2.50%, further preferably is 0.10 to 2.00%, and further preferably is 0.15 to 2.00%.

• • W: more than 0 to 7.00%

Tungsten (W) does not have to be contained. That is, the content of W may be 0%.

When contained, that is, when the content of W is more than 0%, W dissolves in the alloy material and increases the creep strength of the alloy material in a high temperature ammonia environment. W also forms precipitates during use of the alloy material in a high temperature ammonia environment and thereby increases the creep strength of the alloy material in a high temperature ammonia environment. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of W is more than 7.00%, the hot workability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of W is more than 0 to 7.00%.

A preferable lower limit of the content of W is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further preferably is 0.50%, further preferably is 1.00%, and further preferably is 2.00%.

A preferable upper limit of the content of W is 6.50%, more preferably is 6.00%, further preferably is 5.50%, further preferably is 5.00%, further preferably is 4.50%, further preferably is 4.00%, and further preferably is 3.50%.

A preferable range of the content of W is for example, 0.01 to 6.50%, more preferably is 0.05 to 6.00%, further preferably is 0.10 to 5.50%, further preferably is 0.50 to 5.00%, further preferably is 1.00 to 4.50%, further preferably is 2.00 to 4.00%, and further preferably is 2.00 to 3.50%.

• • Ti: more than 0 to 1.00%

Titanium (Ti) does not have to be contained. That is, the content of Ti may be 0%.

When contained, that is, when the content of Ti is more than 0%, Ti forms precipitates during use of the alloy material in a high temperature ammonia environment and thereby increases the creep strength of the alloy material in a high temperature ammonia environment. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Ti is more than 1.00%, the Ti precipitates will become coarse even if the contents of other elements are within the range of the present embodiment. In such case, the creep strength and toughness of the alloy material will decrease.

Therefore, the content of Ti is more than 0 to 1.00%.

A preferable lower limit of the content of Ti is 0.01%, more preferably is 0.02%, further preferably is 0.05%, and further preferably is 0.10%.

A preferable upper limit of the content of Ti is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, further preferably is 0.50%, further preferably is 0.45%, further preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.30%.

A preferable range of the content of Ti is, for example, 0.01 to 0.90%, more preferably is 0.02 to 0.80%, further preferably is 0.05 to 0.70%, further preferably is 0.10 to 0.60%, further preferably is 0.10 to 0.50%, further preferably is 0.10 to 0.45%, further preferably is 0.10 to 0.40%, further preferably is 0.10 to 0.35%, and further preferably is 0.10 to 0.30%.

• • Nb: more than 0 to 0.10%

Niobium (Nb) does not have to be contained. That is, the content of Nb may be 0%.

When contained, that is, when the content of Nb is more than 0%, Nb forms precipitates during use of the alloy material in a high temperature ammonia environment and thereby increases the creep strength of the alloy material in a high temperature ammonia environment. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Nb is more than 0.10%, the Nb precipitates will become coarse. In such case, the creep strength and toughness of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of Nb is more than 0 to 0.10%.

A preferable lower limit of the content of Nb is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.

A preferable upper limit of the content of Nb is 0.09%, more preferably is 0.08%, and further preferably is 0.07%.

A preferable range of the content of Nb is, for example, 0.01 to 0.09%, more preferably is 0.02 to 0.08%, and further preferably is 0.03 to 0.07%.

• • V: more than 0 to 0.50%

Vanadium (V) does not have to be contained. That is, the content of V may be 0%.

When contained, that is, when the content of V is more than 0%, V forms precipitates during use of the alloy material in a high temperature ammonia environment, and thereby increases the creep strength of the alloy material in a high temperature ammonia environment. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of V is more than 0.50%, V precipitates will become coarse. In such case, the creep strength and toughness of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of V is more than 0 to 0.50.

A preferable lower limit of the content of V is 0.01%, more preferably is 0.05%, and further preferably is 0.10%.

A preferable upper limit of the content of V is 0.45%, more preferably is 0.40%, and further preferably is 0.35%.

A preferable range of the content of V is, for example, 0.01 to 0.45%, more preferably is 0.05 to 0.40%, and further preferably is 0.10 to 0.35%.

• • B: more than 0 to 0.0050%

Boron (B) does not have to be contained. That is, the content of B may be 0%.

When contained, that is, when the content of B is more than 0%, B segregates to grain boundaries in a high temperature ammonia environment and strengthens the grain boundaries. Therefore, the creep strength of the alloy material in a high temperature ammonia environment increases. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of B is more than 0.0050%, the hot workability and weldability of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of B is more than 0 to 0.0050%.

A preferable lower limit of the content of B is 0.0001%, more preferably is 0.0002%, and further preferably is 0.0005%.

A preferable upper limit of the content of B is 0.0045%, more preferably is 0.0040%, and further preferably is 0.0035%.

A preferable range of the content of B is, for example, 0.0001 to 0.0045%, more preferably is 0.0002 to 0.0040%, and further preferably is 0.0005 to 0.0035%.

• • N: more than 0 to 0.200%

Nitrogen (N) does not have to be contained. That is, the content of N may be 0%.

When contained, that is, when the content of N is more than 0%, N dissolves in the alloy material and increases the creep strength of the alloy material in a high temperature ammonia environment. N also forms precipitates during use of the alloy material in a high temperature ammonia environment and thereby increases the creep strength of the alloy material in a high temperature ammonia environment. If even a small amount of N is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of N is more than 0.200%, even if the contents of other elements are within the range of the present embodiment, nitrides will excessively form during use of the alloy material in a high temperature ammonia environment. In such case, the creep ductility or toughness of the alloy material will decrease.

Therefore, the content of N is more than 0 to 0.200%.

A preferable lower limit of the content of N is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

A preferable upper limit of the content of N is 0.190%, more preferably is 0.160%, further preferably is 0.140%, and further preferably is 0.120%.

A preferable range of the content of N is, for example, 0.001 to 0.190%, more preferably is 0.005 to 0.160%, further preferably is 0.010 to 0.140%, and further preferably is 0.010 to 0.120%.

• • Rare earth metal: more than 0 to 0.100%

Rare earth metal (REM) does not have to be contained. That is, the content of REM may be 0%.

When contained, that is, when the content of REM is more than 0%, REM segregates to grain boundaries in a high temperature ammonia environment and strengthens the grain boundaries. Therefore, the creep strength of the alloy material in a high temperature ammonia environment increases. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of REM is more than 0.100%, inclusions such as oxides will be formed in the alloy material. Consequently, the creep strength of the alloy material will decrease even if the contents of other elements are within the range of the present embodiment.

Therefore, the content of REM is more than 0 to 0.100%.

A preferable lower limit of the content of REM is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

A preferable upper limit of the content of REM is 0.090%, more preferably is 0.070%, and further preferably is 0.055%.

A preferable range of the content of REM is, for example, 0.001 to 0.090%, more preferably is 0.005 to 0.070%, and further preferably is 0.010 to 0.055%.

In the present description, the term “REM” includes one or more kinds of element selected from a group consisting of Sc, Y, and lanthanoids (elements from La with atomic number 57 through Lu with atomic number 71), and the term “content of REM” means the total content (mass %) of these elements.

[(Fourth Group) Al, Ca and Mg]

Each of Al, Ca, and Mg deoxidizes the alloy in the process for producing the alloy material. Hereunder, each of these elements is described.

• • Al: more than 0 to 0.500%

Aluminum (Al) does not have to be contained. That is, the content of Al may be 0%.

When contained, that is, when the content of Al is more than 0%, Al deoxidizes the alloy in the process for producing the alloy material. If even a small amount of Al is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Al is more than 0.500%, even if the contents of other elements are within the range of the present embodiment, inclusions will excessively form and the creep strength and toughness of the alloy material will decrease.

Therefore, the content of Al is more than 0 to 0.500%.

A preferable lower limit of the content of Al is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

A preferable upper limit of the content of Al is 0.450%, more preferably is 0.400%, further preferably is 0.350%, and further preferably is 0.300%.

A preferable range of the content of Al is, for example, 0.001 to 0.450%, more preferably is 0.005 to 0.400%, further preferably is 0.010 to 0.350%, and further preferably is 0.010 to 0.300%.

• • Ca: more than 0 to 0.0100%

Calcium (Ca) does not have to be contained. That is, the content of Ca may be 0%.

When contained, that is, when the content of Ca is more than 0%, Ca deoxidizes the alloy in the process for producing the alloy material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Ca is more than 0.0100%, even if the contents of other elements are within the range of the present embodiment, inclusions will excessively form and the creep strength and toughness of the alloy material will decrease.

Therefore, the content of Ca is more than 0 to 0.0100%.

A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.

A preferable upper limit of the content of Ca is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, and further preferably is 0.0060%.

A preferable range of the content of Ca is, for example, 0.0001 to 0.0090%, more preferably is 0.0005 to 0.0080%, further preferably is 0.0010 to 0.0070%, and further preferably is 0.0010 to 0.0060%.

• • Mg: more than 0 to 0.0150%

Magnesium (Mg) does not have to be contained. That is, the content of Mg may be 0%.

When contained, that is, when the content of Mg is more than 0%, Mg deoxidizes the alloy in the process for producing the alloy material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent.

However, if the content of Mg is more than 0.0150%, even if the contents of other elements are within the range of the present embodiment, inclusions will excessively form and the creep strength and toughness of the alloy material will decrease.

Therefore, the content of Mg is more than 0 to 0.0150%.

A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0010%.

A preferable upper limit of the content of Mg is 0.0140%, more preferably is 0.0120%, further preferably is 0.0100%, and further preferably is 0.0080%.

A preferable range of the content of Mg is, for example, 0.0001 to 0.0140%, more preferably is 0.0005 to 0.0120%, further preferably is 0.0010 to 0.0100%, and further preferably is 0.0010 to 0.0080%.

The balance of the chemical composition of the austenitic alloy material according to the present embodiment is Fe and impurities. Here, the term “impurities” in the chemical composition means substances which are mixed in from ore and scrap used as the raw material or from the production environment or the like when industrially producing the austenitic alloy material, and which are substances that are not intentionally contained but are permitted within a range that does not adversely affect the austenitic alloy material according to the present embodiment.

Preferably, the content of Fe is more than 0 to 30.00%. Specifically, Fe increases the hot workability of a Ni-based alloy. If the content of Fe is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the content of Fe is too high, the corrosion resistance of the Ni-based alloy will decrease. Therefore, preferably the content of Fe is more than 0 to 30.00%.

A preferable lower limit of the content of Fe is 0.01%, more preferably is 0.20%, further preferably is 0.40%, and further preferably is 0.50%.

A preferable upper limit of the content of Fe is 29.00%, more preferably is 27.00%, further preferably is 25.00%, and further preferably is 23.00%.

A preferable range of the content of Fe is, for example, 0.01 to 29.00%, more preferably is 0.20 to 27.00%, further preferably is 0.40 to 25.00%, and further preferably is 0.50 to 23.00%.

[Regarding Preferable Chemical Composition of Austenitic Alloy Material]

Preferably, the chemical composition of the austenitic alloy material of the present embodiment is any one of the following first chemical composition to third chemical composition.

(First Chemical Composition)

A chemical composition that satisfies Feature 1, and contains C: 0.050% or less, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P: 0.025% or less, S: 0.010% or less, Cu: 2.00 to 4.00%, Ni: 44.00 to 50.00%, Cr: 20.00 to 25.00%, Mo: 5.00 to 7.00%, W: 2.00 to 5.00%, and Fe: 12.00 to 20.00%.

(Second Chemical Composition)

A chemical composition that satisfies Feature 1, and contains C: 0.150% or less, Si: 1.00 to 2.50%, Mn: 0.01 to 1.00%, P: 0.010% or less, S: 0.010% or less, Cu: 1.50 to 3.00%, Cr: 28.00 to 32.00%, Mo: 1.00 to 3.00%, Ti: 0.01 to 1.00%, and Fe: 2.00 to 6.00%.

(Third Chemical Composition)

A chemical composition that satisfies Feature 1, and contains C: 0.050% or less, Si: 0.01 to 0.50%, Mn: 0.01 to 0.50%, P: 0.030% or less, S: 0.015% or less, Cu: 0.01 to 0.50%, Cr: 27.00 to 31.00%, Fe: 7.00 to 15.00%, and Ni: 58.00 to 80.00%.

[(Feature 2) Regarding Fn1]

In the austenitic alloy material of the present embodiment, in addition, Fn1 defined by Formula (1) is less than 20:

Fn ⁢ 1 = 177.84 + 11.12 Si - 24.36 Mn - 8.11 Cu - 1.61 Cr - 1.78 Ni - 
 2.68 Mo ( 1 )

• • where, the content in percent by mass of the corresponding element is substituted for each symbol of an element in Formula (1). If an element is not contained, “0” is substituted the corresponding symbol of an element.

Fn1 is an index relating to the nitridation resistance of the alloy material in a high temperature ammonia environment. Among the elements in the chemical composition described above, Mn, Cu, Cr, Ni, and Mo increase the nitridation resistance of the austenitic alloy material in a high temperature ammonia environment. On the other hand, Si decreases the nitridation resistance of the austenitic alloy material in a high temperature ammonia environment. As shown in FIG. 1, if Fn1 is less than 20, the nitrided layer depth in a case where the alloy material is held at 600° C. for 25 hours in a 100% ammonia atmosphere will be 15.0 μm or less. Therefore, if Fn1 is less than 20, excellent nitridation resistance will be obtained in the austenitic alloy material when used in a high temperature ammonia environment.

A preferable upper limit of Fn1 is 19, more preferably is 18, further preferably is 17, further preferably is 16, further preferably is 13, and further preferably is 10. If Fn1 is 10 or less, the nitrided layer depth can be markedly reduced, and further excellent nitridation resistance will be obtained.

The lower limit of Fn1 is not particularly limited. The lower limit of Fn1 is, for example, 1, or for example is 2.

A preferable range of Fn1 is, for example, 1 to 19, more preferably is 2 to 18, further preferably is 2 to 17, further preferably is 2 to 16, further preferably is 2 to 13, and further preferably is 2 to 10.

Note that, the Fn1 value is to be an integer. That is, the Fn1 value is to be an integer obtained by rounding off the decimals of the determined value.

[(Feature 3) Regarding Fn2]

In the austenitic alloy material of the present embodiment, furthermore, Fn2 defined by Formula (2) is more than 21 and less than 50:

Fn ⁢ 2 = ( Sn + Zn + Pb + Sb + As + Bi ) × 10 3 ( 2 )

• • where, the content in percent by mass of the corresponding element is substituted for each symbol of an element in the formula. If an element is not contained, “0” is substituted for the corresponding symbol of an element.

Fn2 is an index relating to intergranular cracking in an outer layer of the alloy material in a high temperature ammonia environment. If Fn2 is 21 or less, even if the alloy material satisfies Feature 1 and Feature 2, intergranular cracking will easily occur in the outer layer of the alloy material in a high temperature ammonia environment. On the other hand, if Fn2 is 50 or more, Sn, Zn, Pb, Sb, As, and Bi will excessively segregate to grain boundaries, and the grain boundary strength will, on the contrary, decrease. Therefore, even if the alloy material satisfies Feature 1 and Feature 2, intergranular cracking will easily occur in the outer layer of the alloy material in a high temperature ammonia environment.

If Fn2 is more than 21 and less than 50, on the precondition that the alloy material satisfies Feature 1 and Feature 2, the occurrence of intergranular cracking in the outer layer of the alloy material in a high temperature ammonia environment can be sufficiently suppressed.

A preferable lower limit of Fn2 is 22, more preferably is 24, and further preferably is 26.

A preferable upper limit of Fn2 is 48, more preferably is 46, and further preferably is 44.

A preferable range of Fn2 is, for example, 22 to 48, more preferably is 24 to 46, and further preferably is 26 to 44.

Note that, the Fn2 value is to be an integer. That is, the Fn2 value is to be an integer obtained by rounding off the decimals of the determined value.

[Advantageous Effects of Austenitic Alloy Material of Present Embodiment]

As described above, the austenitic alloy material of the present embodiment satisfies Feature 1 to Feature 3. Therefore, in the austenitic alloy material of the present embodiment, during use in a high temperature ammonia environment, excellent nitridation resistance is obtained, and furthermore, the occurrence of intergranular cracking in the outer layer is sufficiently suppressed.

[Shape and Uses]

The shape of the austenitic alloy material of the present embodiment is not particularly limited. The austenitic alloy material of the present embodiment may be an alloy tube, may be a rod-like solid material, or may be an alloy plate. Further, the alloy tube may be a seamless tube or may be a welded tube.

The austenitic alloy material of the present embodiment can be widely applied to uses for which nitridation resistance is required. In particular, the austenitic alloy material of the present embodiment is suitable for use in high temperature ammonia environments. However, the austenitic alloy material of the present embodiment can also be applied to other uses besides use in a high temperature ammonia environment.

[Preferable Form of Austenitic Alloy Material of the Present Embodiment]

Preferably, the austenitic alloy material of the present embodiment satisfies Feature 1 to Feature 3, and also satisfies the following Feature 4.

(Feature 4)

When the average grain diameter in units of μm in the outer layer of the austenitic alloy material is defined as D_ave,

• • Fn3 defined by Formula (3) is more than 0.20, and
  • Fn4 defined by Formula (4) is 1000 to 5000.

Fn ⁢ 3 = Fn ⁢ 2 / D a ⁢ v ⁢ e ( 3 ) Fn ⁢ 4 = Fn ⁢ 2 × D a ⁢ v ⁢ e ( 4 )

Hereunder, Feature 4 is described.

Fn3 and Fn4 are indexes relating to intergranular cracking in the outer layer of the alloy material in a high temperature ammonia environment.

In a case where the total content of Sn, Zn, Pb, Sb, As, and Bi (in other words, Fn2) and the average grain diameter D_ave (μm) in the alloy material satisfy an appropriate relation, intergranular cracking in a high temperature ammonia environment is further suppressed. Hereunder, Fn3 and Fn4 are described.

[Regarding Fn3]

In a case where Fn2 satisfies Formula (2), if, in addition, Fn3 is more than 0.20, the total content of Sn, Zn, Pb, Sb, As, and Bi contained per unit grain boundary area will be even more appropriate. Consequently, intergranular cracking in a high temperature ammonia environment will be further suppressed.

Therefore, preferably Fn3 is more than 0.20.

A preferable lower limit of Fn3 is 0.21, more preferably is 0.22, and further preferably is 0.23.

The upper limit of Fn3 is not particularly limited. However, in a case where the austenitic alloy material satisfies Feature 1 to Feature 3, the upper limit of Fn3 is, for example, preferably 0.90, more preferably is 0.85, and further preferably is 0.82.

A preferable range of Fn3 is, for example, 0.21 to 0.90, more preferably is 0.22 to 0.85, and further preferably is 0.23 to 0.82.

Note that, the Fn3 value is to be a value to the second decimal place obtained by rounding off the third decimal place of the determined value.

[Regarding Fn4]

In addition, in a case where Fn2 satisfies Formula (2), when Fn4 is 1000 to 5000, the grain boundary area is an appropriate size and the total content of Sn, Zn, Pb, Sb, As, and Bi is an appropriate amount. In this case, not only the occurrence of cracks, but also the propagation of cracks can be more effectively suppressed. As a result, intergranular cracking in a high temperature ammonia environment is further suppressed.

Therefore, preferably, Fn4 is 1000 to 5000.

A preferable lower limit of Fn4 is 1100, and more preferably is 1200.

A preferable upper limit of Fn4 is 4900, more preferably is 4600, and further preferably is 4200.

A preferable range of Fn4 is, for example, 1100 to 4900, more preferably is 1200 to 4600, and further preferably is 1200 to 4200.

Note that, the Fn4 value is an integer.

[Method for Measuring Average Grain Diameter in Outer Layer of Austenitic Alloy Material]

The average grain diameter D_ave in the outer layer of the austenitic alloy material is measured by the following method.

A test specimen is taken from a cross section (L cross section) parallel to the rolling direction of the austenitic alloy material. The test specimen has a rectangular observation surface composed of a side with a length of 5 mm that corresponds to the surface of the alloy material, and a side of 5 mm in the depth direction from the surface in question. In other words, the observation surface is a rectangle with dimensions of 5 mm×5 mm. The size of the test specimen other than the observation surface is not particularly limited.

The test specimen is embedded in resin for microstructural observation. The observation surface of the resin-embedded test specimen is mirror polished. The observation surface after mirror polishing is subjected to etching using a mixed acid of hydrochloric acid and nitric acid to reveal the microstructure. An arbitrary 10 visual fields on the observation surface after etching are observed with an optical microscope at a magnification of 300×. Each visual field is set to a size of 1000 μm×1000 μm. In each visual field, grain diameters (μm) are determined in accordance with the intercept method described in JIS G 0551 (2020). The arithmetic average value of the grain diameters obtained in the 10 visual fields is defined as the average grain diameter D_ave (μm) in the outer layer of the austenitic alloy material.

[Method for Producing Austenitic Alloy Material of Present Embodiment]

One example of a method for producing the austenitic alloy material of the present embodiment will now be described. One example of a method for producing the austenitic alloy material of the present embodiment includes a blank preparation process, a hot working process, and a solution treatment process. Each process is described in detail hereunder.

[Blank Preparation Process]

In the blank preparation process, an alloy that satisfies Feature 1 to Feature 3 or satisfies Feature 1 to Feature 4 is melted. The alloy may be melted using an electric furnace, may be melted using an Ar—O₂ mixed gas bottom-blowing decarburization furnace (AOD furnace), or may be melted using a vacuum decarburization furnace (VOD furnace). The melted alloy may be made into an ingot by an ingot-making process, or may be made into a slab, a bloom, or a billet by a continuous casting process. As necessary, the slab, bloom, or ingot may be subjected to blooming to produce a billet. A blank (a slab, a bloom, or a billet) is produced by the above process.

[Hot Working Process]

In the hot working process, the produced blank is subjected to well-known hot working to produce an intermediate alloy material. The hot working may be hot forging, may be hot extrusion, or may be hot rolling. The method of hot working is not particularly limited, and it suffices to use a well-known method.

In a case where the end product is an alloy tube, for example, the Mannesmann process may be performed as hot working to produce a hollow shell as the intermediate alloy material. Further, the Ugine-Sejournet process or the Ehrhardt push bench process (that is, hot extrusion) may be performed as hot working to produce a hollow shell. In addition, the produced hollow shell may be subjected to hot rolling using a mandrel mill, a stretch reducing mill, a sizing mill or the like.

[Solution Treatment Process]

In the solution treatment process, the intermediate alloy material produced in the hot working process is subjected to a well-known solution treatment. For example, the intermediate alloy material is charged into a heat treatment furnace and held at a desired temperature, and thereafter is rapidly cooled. The solution treatment temperature is, for example, 1000 to 1300° C.

Note that, the grain diameter can be adjusted by adjusting the rolling reduction in the hot working process and the solution treatment temperature in the solution treatment process. The austenitic alloy material of the present embodiment is produced by the above production method.

[Regarding Other Processes]

The method for producing the austenitic alloy material of the present embodiment may include other processes in addition to the processes described above. For example, a cold working process may be performed on the intermediate alloy material at a timing that is after the hot working process and is prior to the solution treatment process. In the cold working process, the intermediate alloy material is subjected to the cold working. The cold working may be cold rolling or may be cold drawing. In this case, the intermediate alloy material can be processed into desired dimensions. For example, the intermediate alloy material after the solution treatment process may be subjected to cold working. In this case, the strength of the austenitic alloy material will increase.

Note that, in the foregoing description of the production method, a method for producing an alloy tube as one example of the austenitic alloy material has been described. However, the austenitic alloy material of the present embodiment may be another shape, such as a rod shape or a plate shape. A method for producing the austenitic alloy material that is a rod shape or a plate shape also includes, similarly to the production method described above, for example, a blank preparation process, a hot working process, and a solution treatment process, and in addition, a cold working process may be performed. Further, the production method described above is an example, and the austenitic alloy material of the present embodiment that satisfies Feature 1 to Feature 3 or satisfies Feature 1 to Feature 4 may also be produced by a different production method.

EXAMPLES

The advantageous effects of the austenitic alloy material of the present embodiment will now be described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effects of the austenitic alloy material of the present embodiment. Accordingly, the austenitic alloy material of the present embodiment is not limited to this one example of conditions.

Austenitic alloy materials having the chemical compositions shown in Table 1 (Table 1-1 and Table 1-2) were produced. Note that, the symbol in Table 1 indicates that the content of the corresponding element was at an impurity level or less.

TABLE 1-1
Test	Chemical Composition (unit is mass %; balance is impurities)

Number

C

Si

Mn

P

S

Ni

Cr

Fe

Sn

Zn

Pb

Sb

As

Bi

1	0.010	0.19	0.21	0.013	0.008	47.08	23.31	15.59	0.0200	—	0.0020	—	0.0007	—
2	0.009	0.21	0.19	0.003	0.001	45.66	22.74	18.13	—	0.0060	0.0090	0.0070	—	—
3	0.066	1.41	0.20	0.007	0.001	60.02	29.55	3.50	0.0400	—	—	—	—	—
4	0.059	1.63	0.40	0.002	0.002	60.13	29.80	3.91	0.0200	0.0089	—	—	—	—
5	0.019	0.35	0.50	0.008	0.001	58.61	29.23	10.85	0.0100	—	0.0089	0.0080	—	—
6	0.031	0.30	0.30	0.010	0.001	59.95	30.02	8.49	0.0300	—	—	—	—	—
7	0.010	0.05	0.10	0.007	0.001	57.03	15.47	6.48	—	0.0042	0.0089	0.0080	0.0007	0.0009
8	0.030	0.05	0.11	0.008	0.001	56.14	15.53	6.47	0.0100	0.0092	—	0.0075	—	—
9	0.055	1.82	2.47	0.041	0.017	49.85	22.19	22.16	0.0400	—	—	—	—	—
10	0.187	0.26	0.34	0.015	0.027	47.15	28.62	18.54	—	0.0077	0.0053	0.0086	—	—
11	0.096	0.63	0.15	0.026	0.001	66.49	22.58	1.03	0.0400	—	—	—	—	—
12	0.167	1.40	0.71	0.029	0.023	69.75	24.16	0.58	0.0200	0.0028	—	—	—	0.0008
13	0.068	0.62	1.45	0.033	0.023	67.22	16.53	13.99	0.0300	—	—	—	—	—
14	0.102	2.34	1.94	0.006	0.002	72.06	16.82	6.21	—	0.0088	0.0097	0.0050	0.0009	—
15	0.031	1.15	1.52	0.039	0.047	67.91	11.98	16.03	0.0400	—	—	—	—	—
16	0.088	1.05	1.97	0.023	0.026	46.09	25.82	20.50	0.0300	—	—	—	—	—
17	0.010	1.20	1.59	0.010	0.022	64.67	20.82	3.31	0.0400	—	—	—	—	—
18	0.010	2.42	1.23	0.020	0.030	53.98	22.69	3.23	0.0400	—	—	—	—	—
19	0.012	0.20	0.22	0.015	0.004	47.28	24.35	14.89	0.0052	0.0044	0.0019	0.0093	0.0004	0.0009
20	0.082	1.08	0.15	0.009	0.002	60.78	30.33	2.77	0.0043	0.0033	0.0083	0.0080	0.0001	0.0005
21	0.025	0.25	0.03	0.007	0.009	48.03	23.58	13.42	0.0195	0.0085	0.0059	0.0099	0.0005	0.0006
22	0.099	1.51	0.16	0.003	0.008	59.40	28.02	5.18	0.0201	0.0074	0.0067	0.0049	0.0008	0.0001
23	0.013	0.17	0.13	0.004	0.006	47.40	24.08	14.64	0.0078	0.0059	0.0026	0.0051	0.0002	0.0006
24	0.070	1.54	0.26	0.005	0.004	61.37	28.42	3.15	0.0069	0.0079	0.0050	0.0018	0.0005	0.0002
25	0.160	1.16	1.50	0.045	0.035	45.25	20.94	29.92	0.0003	0.0090	0.0086	0.0094	0.0009	0.0003
26	0.012	0.47	0.48	0.004	0.001	49.57	22.74	12.50	0.0151	0.0030	0.0053	0.0019	0.0003	0.0007
27	0.009	0.21	0.25	0.008	0.001	46.11	22.82	17.99	0.0139	0.0052	0.0012	0.0008	0.0008	0.0001
28	0.060	2.46	0.97	0.005	0.002	55.65	31.91	3.49	0.0068	0.0087	0.0059	0.0010	0.0001	0.0005
29	0.056	1.54	0.22	0.002	0.001	60.99	30.06	2.82	0.0117	0.0042	0.0066	0.0001	0.0008	0.0001
30	0.020	0.49	0.37	0.009	0.002	60.08	29.69	8.39	0.0208	0.0001	0.0018	0.0037	0.0001	0.0002
31	0.018	0.30	0.33	0.008	0.001	58.91	29.66	10.50	0.0069	0.0065	0.0043	0.0055	0.0004	0.0005
32	0.141	0.99	1.15	0.041	0.049	61.01	19.60	16.73	0.0168	0.0054	0.0063	0.0008	0.0002	0.0003
33	0.028	0.68	1.18	0.021	0.005	54.24	31.12	25.42	0.0271	0.0054	0.0078	0.0032	0.0007	0.0003
34	0.031	1.32	1.77	0.031	0.040	60.13	23.83	21.58	0.0055	0.0038	0.0050	0.0087	0.0001	0.0008
35	0.045	2.12	1.79	0.043	0.041	58.68	26.17	13.16	0.0153	0.0008	0.0034	0.0054	0.0002	0.0002
36	0.081	1.05	1.36	0.024	0.034	61.75	27.97	28.30	0.0170	0.0029	0.0070	0.0073	0.0009	0.0004
37	0.022	0.74	1.49	0.017	0.003	54.98	21.13	14.96	0.0138	0.0082	0.0011	0.0070	0.0004	0.0003
38	0.172	0.58	2.77	0.002	0.025	50.87	15.56	3.15	0.0157	0.0040	0.0084	0.0026	0.0007	0.0008
39	0.185	2.17	1.50	0.025	0.026	69.42	24.54	8.79	0.0158	0.0030	0.0088	0.0013	0.0001	0.0004
40	0.062	1.71	2.58	0.003	0.029	40.62	34.79	15.96	0.0176	0.0051	0.0024	0.0032	0.0008	0.0003
41	0.162	0.79	0.69	0.037	0.038	65.72	29.50	26.53	0.0041	0.0084	0.0045	0.0062	0.0006	0.0006
42	0.165	1.44	1.59	0.046	0.023	62.23	25.57	22.68	0.0025	0.0088	0.0087	0.0018	0.0007	0.0009
43	0.179	0.83	0.80	0.008	0.024	66.78	22.91	28.33	0.0126	0.0058	0.0048	0.0005	0.0001	0.0007
44	0.107	3.55	1.45	0.015	0.028	48.18	19.33	9.49	0.0100	0.0086	0.0065	0.0018	—	—
45	0.152	1.11	1.15	0.037	0.038	32.30	31.06	24.79	0.0300	0.0095	—	0.0033	0.0010	0.0005
46	0.091	0.13	0.04	0.011	0.007	82.88	13.82	1.82	0.0100	0.0095	0.0036	0.0091	0.0005	0.0009
47	0.141	2.84	0.45	0.039	0.043	63.51	8.92	5.89	0.0300	—	0.0042	0.0030	0.0003	—
48	0.168	1.62	1.52	0.030	0.033	41.17	26.35	17.22	—	—	—	0.0437	—	—
49	0.027	1.20	0.66	0.003	0.023	69.49	20.63	4.18	—	0.0020	0.0405	0.0007	—	—
50	0.109	0.68	0.99	0.004	0.024	53.07	14.66	17.15	—	0.0033	0.0062	0.0359	—	—
51	0.072	2.57	0.91	0.028	0.035	51.37	16.71	22.34	0.0300	—	—	—	—	0.0002
52	0.064	2.98	2.55	0.034	0.046	48.06	14.48	23.18	0.0200	0.0044	0.0078	—	0.0005	0.0007
53	0.125	1.42	0.19	0.048	0.026	60.81	33.03	0.90	0.0400	—	—	—	—	—
54	0.179	0.13	0.08	0.022	0.020	66.70	20.25	8.16	0.0800	0.0058	—	0.0080	0.0006	0.0009
55	0.174	1.86	0.51	0.029	0.027	65.66	18.19	5.93	0.0600	—	—	—	—	0.0007
56	0.171	1.31	1.34	0.007	0.012	48.23	25.44	19.22	0.0100	—	—	—	—	—
57	0.097	1.66	0.04	0.032	0.015	60.04	28.52	1.34	—	0.0045	0.0003	0.0031	—	0.0005
58	0.114	1.04	0.89	0.009	0.026	58.86	24.33	7.27	0.0100	—	—	—	0.0002	0.0004

TABLE 1-2
Test	Chemical Composition (unit is mass %; balance is impurities)

Number

Cu

Mo

Co

W

Ti

Nb

V

B

N

REM

Al

Ca

Mg

1	2.96	6.04	0.30	3.99	—	—	0.04	—	0.105	0.050	0.072	0.0052	—
2	3.06	5.90	0.03	3.85	0.04	0.01	0.01	—	0.079	—	0.060	0.0009	—
3	1.97	2.02	0.17	0.02	0.97	0.02	0.02	0.0012	0.011	—	0.008	—	—
4	1.95	1.92	0.02	0.01	0.43	0.01	0.04	0.0010	0.012	0.005	0.025	0.0005	—
5	0.02	0.01	—	—	0.24	—	—	—	0.005	—	0.140	—	0.0070
6	0.20	0.11	0.02	—	0.20	0.01	—	0.0010	0.033	—	0.130	—	0.0050
7	0.10	16.91	—	3.56	—	—	0.05	—	—	—	0.210	—	—
8	0.10	16.87	0.83	3.57	—	—	0.06	—	—	—	0.210	—	—
9	1.36	—	—	—	—	—	—	—	—	—	—	—	—
10	4.34	0.50	—	—	—	—	—	—	—	—	—	—	—
11	0.06	2.57	—	6.33	—	—	—	—	—	—	—	—	—
12	—	—	2.93	—	0.09	—	—	0.0019	0.135	—	—	—	—
13	—	—	—	—	—	—	—	—	—	—	0.039	—	—
14	—	—	—	—	—	—	—	—	—	—	0.488	0.0050	0.0068
15	1.03	—	—	—	—	0.03	—	—	0.189	—	—	0.0012	—
16	—	3.93	—	—	—	—	0.45	0.0025	—	0.011	—	0.0089	0.0047
17	—	—	1.58	5.97	0.44	—	—	—	—	—	0.340	—	—
18	1.36	8.74	—	6.15	—	0.05	—	—	0.004	0.035	—	0.0096	0.0005
19	3.61	5.93	—	2.63	0.10	—	0.34	—	0.081	—	0.319	—	—
20	2.24	1.35	—	—	0.51	—	0.35	—	0.152	—	0.170	—	—
21	3.45	5.79	—	4.26	0.51	—	0.38	—	0.034	—	0.178	—	—
22	2.58	1.55	—	—	0.81	—	0.21	—	0.086	—	0.345	—	—
23	3.63	5.64	—	3.90	0.17	—	0.06	—	0.094	—	0.037	—	—
24	2.26	1.75	—	—	0.58	—	0.07	—	0.113	—	0.388	—	—
25	2.74	3.48	1.40	5.62	0.53	0.06	0.05	0.0044	0.166	0.071	0.033	0.0001	0.0067
26	2.88	6.76	0.18	4.11	0.01	0.01	0.06	0.0005	0.103	0.004	0.078	0.0013	0.0015
27	2.92	5.88	—	3.78	—	—	—	—	—	—	—	—	—
28	2.03	2.97	0.02	0.04	0.24	0.03	0.06	0.0015	0.002	0.030	0.008	0.0004	0.0008
29	2.09	1.99	—	—	0.21	—	—	—	—	—	—	—	—
30	0.47	0.04	0.02	0.01	0.20	0.03	0.01	0.0003	0.030	0.002	0.102	0.0003	0.0070
31	0.25	—	—	—	—	—	—	—	—	—	—	—	—
32	1.66	—	—	—	—	—	—	—	—	—	—	—	—
33	—	2.08	—	—	—	—	—	—	—	—	—	—	—
34	—	—	1.06	—	—	—	—	—	—	—	—	—	—
35	—	—	—	4.54	—	—	—	—	—	—	—	—	—
36	—	—	—	—	0.62	—	—	—	—	—	—	—	—
37	—	—	—	—	—	0.06	—	—	—	—	—	—	—
38	—	—	—	—	—	—	0.48	—	—	—	—	—	—
39	—	—	—	—	—	—	—	0.0020	—	—	—	—	—
40	—	—	—	—	—	—	—	—	0.123	—	—	—	—
41	—	—	—	—	—	—	—	—	—	0.066	—	—	—
42	—	—	—	—	—	—	—	—	—	—	—	0.0056	—
43	—	—	—	—	—	—	—	—	—	—	—	—	0.0117
44	4.30	4.74	1.53	6.81	0.03	0.03	0.03	—	—	—	0.358	—	—
45	3.12	5.02	0.32	—	0.28	—	0.35	0.0030	0.082	0.038	0.104	0.0059	—
46	0.26	—	0.47	0.06	0.10	0.05	0.17	—	—	0.048	—	0.0054	—
47	3.53	11.72	1.00	1.70	0.08	—	0.09	0.0024	—	—	—	0.0094	—
48	2.76	6.75	1.43	0.42	0.02	—	0.17	0.0010	0.052	0.083	0.157	0.0061	—
49	0.12	0.08	2.48	—	0.37	0.01	0.24	—	0.159	0.030	0.257	—	—
50	—	11.74	0.91	—	0.35	0.01	—	0.0007	—	0.028	0.229	0.0039	—
51	2.52	—	2.37	—	0.24	0.08	0.18	0.0022	0.088	0.037	0.420	—	0.0004
52	0.72	—	1.72	5.21	0.22	0.06	0.27	0.0024	—	—	0.358	0.0086	—
53	—	0.33	0.44	2.01	0.49	—	0.06	0.0018	0.071	—	—	0.0032	—
54	1.67	0.44	1.27	—	0.45	0.02	—	—	0.081	—	0.434	—	0.0008
55	4.11	—	0.09	2.85	0.02	0.03	—	0.0014	0.038	0.056	0.357	0.0025	—
56	1.95	0.53	1.52	—	0.20	0.01	—	0.0023	—	0.040	—	0.0049	0.0061
57	3.93	—	1.33	2.27	0.34	—	—	0.0014	0.096	0.052	0.221	0.0040	0.0008
58	0.05	3.50	2.56	0.23	0.49	0.01	0.34	0.0050	0.123	0.074	0.054	0.0026	0.0009

The alloy of each test number was melted by a high-frequency induction vacuum melting method. Each melted alloy was used to produce a 30 kg ingot by an ingot-making process. The ingot of each test number was heated for two hours at 1200° C. After being heated, each ingot was subjected to hot forging to produce a rectangular bar having a cross section of 50 mm×55 mm. Each obtained rectangular bar was heated for 30 minutes at 1200° C., and thereafter was subjected to hot rolling to produce a hot-rolled material having a thickness of 15 mm. The obtained hot-rolled material was subjected to cold rolling to produce an intermediate alloy material (alloy plate) having a thickness of 10.5 mm. The intermediate alloy material was subjected to a solution treatment in which the intermediate alloy material was held for 10 minutes at a solution treatment temperature of 1150° C. Note that, water cooling was used as the cooling method after the intermediate alloy material was held at the solution treatment temperature. An austenitic alloy material (alloy plate) of each test number was produced by the production process described above.

[Evaluation Tests]

The produced austenitic alloy material of each test number was subjected to the following evaluation tests.

[Nitriding Resistance and Outer Layer Intergranular Cracking Evaluation Test]

The nitridation resistance and the outer layer intergranular cracking of the austenitic alloy material of each test number were evaluated by the following method.

A test specimen having a thickness of 3 mm, a width of 15 mm, and a length of 20 mm was cut out from the austenitic alloy material (alloy plate) of each test number. The surface of the test specimen was subjected to buff polishing, and thereafter the surface of the test specimen was degreased and finished. After being degreased and finished, the test specimen was suspended on a quartz jig using stainless steel wire and loaded into a box-shaped furnace. After being loaded into the furnace, the test specimen was held therein for 25 hours at 600° C. while passing atmospheric gas through the furnace. The atmospheric gas was 100% ammonia. The flow rate of the atmospheric gas was set to 500 mL/min.

After 25 hours had elapsed, the test specimen was used to prepare a structural observation test specimen including a cross section perpendicular to the longitudinal direction of the test specimen and the surface of the test specimen. In the structural observation test specimen, the aforementioned cross section was taken as the observation surface. On the observation surface, a rectangular visual field including the surface of the test specimen and which had a depth of 600 μm from the surface and a dimension of 800 μm in the width direction was observed at a magnitude of 300× using a scanning electron microscope (SEM), and a photographic image (backscattered electron image) was obtained. In addition, EDS analysis was performed in the rectangular visual field using an EDS apparatus attached to an SEM apparatus. Specifically, EDS analysis was performed in the depth direction from arbitrary points corresponding to the test specimen surface in the rectangular visual field. In the analysis results, a region where the content of N (mass %) was two times or more the content of N (mass %) of the base metal was identified as a nitrided layer. The depth of the region identified as the nitrided layer from the test specimen surface was defined as the nitrided layer depth (μm). The determined nitrided layer depth is shown in the column “Nitrided Layer Depth (μm)” in Table 2.

In addition, the presence or absence of intergranular cracking in an outer layer portion of the test specimen in the photographic image of the rectangular visual field of each test number was confirmed. If a region in which gaps with a length of 0.5 μm or more were confirmed was not present on the grain boundaries, it was determined that intergranular cracking was sufficiently suppressed (indicated by “E” (Excellent) in the column “Intergranular Cracking” in Table 2). If a region in which gaps with a length of 0.5 μm or more were confirmed was present on the grain boundaries, but a region in which gaps with a length of 1.0 μm or more were confirmed was not present, it was determined that intergranular cracking was suppressed (indicated by “G” (Good) in the column “Intergranular Cracking” in Table 2). If a region in which gaps with a length of 1 μm or more were confirmed was present on the grain boundaries, it was determined that intergranular cracking was not sufficiently suppressed (indicated by “B” (Bad) in the column “Intergranular Cracking” in Table 2). Note that, the length of the gaps refers to the total length of the gaps along the grain boundaries.

TABLE 2
						Nitrided
			Grain			Layer
Test			Diameter			Depth	Intergranular
Number	Fn1	Fn2	(μm)	Fn3	Fn4	(μm	Cracking	Remarks

1	13	23	82	0.28	1886	10.8	E	Inventive Example of Present Invention
2	17	22	61	0.36	1342	10.0	E	Inventive Example of Present Invention
3	13	40	73	0.55	2920	3.0	E	Inventive Example of Present Invention
4	10	29	66	0.44	1914	2.3	E	Inventive Example of Present Invention
5	18	27	120	0.23	3240	14.2	E	Inventive Example of Present Invention
6	17	30	131	0.23	3930	10.8	E	Inventive Example of Present Invention
7	3	23	83	0.28	1909	3.2	E	Inventive Example of Present Invention
8	5	27	70	0.39	1890	2.7	E	Inventive Example of Present Invention
9	2	40	101	0.40	4040	3.1	E	Inventive Example of Present Invention
10	6	22	97	0.23	2134	6.9	E	Inventive Example of Present Invention
11	19	40	100	0.40	4000	14.2	E	Inventive Example of Present Invention
12	13	24	47	0.51	1128	11.8	E	Inventive Example of Present Invention
13	3	30	82	0.37	2460	2.3	E	Inventive Example of Present Invention
14	1	24	115	0.21	2760	1.4	E	Inventive Example of Present Invention
15	5	40	49	0.82	1960	3.9	E	Inventive Example of Present Invention
16	7	30	132	0.23	3960	5.2	E	Inventive Example of Present Invention
17	4	40	76	0.53	3040	1.1	E	Inventive Example of Present Invention
18	8	40	104	0.38	4160	2.9	E	Inventive Example of Present Invention
19	6	22	130	0.17	2860	9.5	G	Inventive Example of Present Invention
20	7	25	127	0.20	3175	3.5	G	Inventive Example of Present Invention
21	13	45	120	0.38	5400	11.2	G	Inventive Example of Present Invention
22	15	40	128	0.31	5120	5.0	G	Inventive Example of Present Invention
23	9	22	41	0.54	902	10.7	G	Inventive Example of Present Invention
24	11	22	36	0.61	792	4.3	G	Inventive Example of Present Invention
25	8	28	106	0.26	2968	3.8	E	Inventive Example of Present Invention
26	5	26	104	0.25	2704	12.5	E	Inventive Example of Present Invention
27	16	22	105	0.21	2310	11.4	E	Inventive Example of Present Invention
28	7	23	112	0.21	2576	3.9	E	Inventive Example of Present Invention
29	10	24	92	0.26	2208	2.8	E	Inventive Example of Present Invention
30	16	27	79	0.34	2133	3.7	E	Inventive Example of Present Invention
31	18	24	58	0.41	1392	5.0	E	Inventive Example of Present Invention
32	7	30	135	0.22	4050	8.5	E	Inventive Example of Present Invention
33	4	44	103	0.43	4532	2.4	E	Inventive Example of Present Invention
34	4	24	102	0.24	2448	11.3	E	Inventive Example of Present Invention
35	11	25	49	0.51	1225	10.7	E	Inventive Example of Present Invention
36	1	35	79	0.44	2765	5.8	E	Inventive Example of Present Invention
37	18	31	75	0.41	2325	5.9	E	Inventive Example of Present Invention
38	1	32	46	0.70	1472	11.1	E	Inventive Example of Present Invention
39	2	29	85	0.34	2465	7.1	E	Inventive Example of Present Invention
40	6	29	140	0.21	4060	8.0	E	Inventive Example of Present Invention
41	5	24	61	0.39	1464	9.1	E	Inventive Example of Present Invention
42	3	23	50	0.46	1150	6.2	E	Inventive Example of Present Invention
43	12	24	87	0.28	2088	7.5	E	Inventive Example of Present Invention
44	18	27	126	0.21	3402	14.1	B	Comparative Example
45	16	44	68	0.65	2992	24.3	E	Comparative Example
46	6	34	98	0.35	3332	8.1	B	Comparative Example
47	11	38	62	0.61	2356	33.1	E	Comparative Example
48	3	44	105	0.42	4620	1.9	B	Comparative Example
49	17	43	72	0.60	3096	10.1	B	Comparative Example
50	12	45	110	0.41	4950	11.4	B	Comparative Example
51	45	30	95	0.32	2850	69.7	E	Comparative Example
52	34	33	111	0.30	3663	34.2	E	Comparative Example
53	27	40	67	0.60	2680	33.1	E	Comparative Example
54	11	95	76	1.25	7220	2.8	B	Comparative Example
55	7	61	93	0.66	5673	4.2	B	Comparative Example
56	16	10	42	0.24	420	11.0	B	Comparative Example
57	11	8	43	0.19	344	3.8	B	Comparative Example
58	14	11	134	0.08	1474	10.4	B	Comparative Example

[Test Results]

The test results are shown in Table 1 (Table 1-1 and Table 1-2) and Table 2. Referring to the respective tables, the austenitic alloy materials of Test Nos. 1 to 43 satisfied Feature 1 to Feature 3. Therefore, the nitrided layer depth was 15.0 μm or less and sufficient nitridation resistance was obtained in a high temperature ammonia environment. In addition, intergranular cracking was suppressed in a high temperature ammonia environment.

In particular, the austenitic alloy materials of Test Nos. 1 to 18 and Test Nos. 25 to 43 satisfied not only Feature 1 to Feature 3, but also satisfied Feature 4. Therefore, intergranular cracking was sufficiently suppressed in comparison to Test Nos. 19 to 24 which did not satisfy Feature 4.

On the other hand, in Test No. 44, the content of Si was too high. Therefore, intergranular cracking was not sufficiently suppressed in a high temperature ammonia environment.

In Test No. 45, the content of Ni was too low. Therefore, in a high temperature ammonia environment, the nitrided layer depth was more than 15.0 μm and sufficient nitridation resistance was not obtained.

In Test No. 46, the content of Ni was too high. Therefore, intergranular cracking was not sufficiently suppressed in a high temperature ammonia environment.

In Test No. 47, the content of Cr was too low. Therefore, in a high temperature ammonia environment, the nitrided layer depth was more than 15.0 μm and sufficient nitridation resistance was not obtained.

In Test Nos. 48 and 50, the content of Sb was too high. Therefore, intergranular cracking was not sufficiently suppressed in a high temperature ammonia environment.

In Test No. 49, the content of Pb was too high. Therefore, intergranular cracking was not sufficiently suppressed in a high temperature ammonia environment.

In Test Nos. 51 to 53, Fn1 was too high. Therefore, in a high temperature ammonia environment, the nitrided layer depth was more than 15.0 μm and sufficient nitridation resistance was not obtained.

In Test Nos. 54 and 55, Fn2 was too high. Therefore, intergranular cracking was not sufficiently suppressed in a high temperature ammonia environment.

In Test Nos. 56 to 58, Fn2 was too low. Therefore, intergranular cracking was not sufficiently suppressed in a high temperature ammonia environment.

An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range that does not depart from the gist of the present disclosure.