EP4692396A1 - Alloy material - Google Patents

Abstract

There is provided an alloy in which high creep strength, and excellent weld hot cracking resistance and hot workability are obtained. An alloy according to the present disclosure consists of, in mass%, C: 0.050 to 0.100%, Si: 1.00% or less, Mn: 1.50% or less, Cr: 19.00 to 23.00%, Ni: 30.00 to 35.00%, N: 0.010% or less, Al: 0.15 to 0.70%, Ti: 0.15 to 0.70%, B: 0.0001 to 0.0030%, Nb: 0.0010 to 0.5000%, Mo: 0.01 to 1.00%, and Ca: 0.0001 to 0.0200%, with the balance being Fe and impurities, and satisfies Formula (1) and Formula (2), and in a case where B ≤ 0.0010%, the alloy also satisfies Formula (3). $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$

Description

TECHNICAL FIELD
• The present disclosure relates to an alloy, and more particularly relates to an alloy which can be utilized in a high temperature environment.
BACKGROUND ART
• Alloys that are used in steam methane reformers, ethylene cracking furnaces, heating furnace pipes for petroleum refining and petrochemical plants, and polycrystalline silicon manufacturing equipment and the like are used in high temperature environments of 500 to 1000°C. Therefore, alloys that are used in such high temperature environments are required to have excellent corrosion resistance in a high temperature environment and high creep strength. Alloy 800, Alloy 800H, and Alloy 800HT are known as alloys for use in such high temperature environments.
• Alloy 800, Alloy 800H, and Alloy 800HT each contain large amounts of Cr and Ni. Therefore, these alloys are excellent in corrosion resistance at high temperature. These alloys also contain Al and Ti. Therefore, in these alloys, a gamma-prime (y') phase (Ni₃ (Al, Ti)) is formed during use in a high temperature environment. Because these alloys are precipitation-strengthened by formation of the y' phase, high creep strength is obtained in these alloys.
• In this connection, in the case of Alloy 800, Alloy 800H, and Alloy 800HT which contain Al and Ti, weld hot cracking is likely to occur in a heat affected zone (HAZ) during welding. Further, although these alloys are produced by being subjected to hot working in a temperature region of around 900°C, cracking is likely to occur due to embrittlement during hot working. Therefore, these alloys are required to have excellent weld hot cracking resistance and excellent hot workability.
• Techniques for increasing the weld hot cracking resistance of alloys containing Al and Ti are disclosed in Japanese Patent Application Publication No. 2022-163425 (Patent Literature 1), Japanese Patent Application Publication No. 2022-163585 (Patent Literature 2), and Japanese Patent Application Publication No. 2022-163586 (Patent Literature 3).
• Patent Literature 1 discloses an alloy having a chemical composition containing, in mass%, C: 0.15% or less, Si: 0.05 to 2.0%, Mn: 0.05 to 2.0%, P: 0.035% or less, S: 0.0015% or less, Cr: 16 to 30%, Ni: 18 to 50%, Al: 0.01 to 1.0%, Ti: 0.01 to 1.5%, N: 0.35% or less, O: 0.003% or less, Mo: 8% or less, Cu: 4% or less, Co: 3% or less, Ca: 0.0003 to 0.0050%, and Mg: 0.0045% or less, with the balance being Fe and impurities. In addition, a mass ratio of CaO, MgO, and Al₂O₃ in inclusions that is calculated based on an average Ca concentration, an average Mg concentration, and an average Al concentration of inclusions in which O or S is detected satisfies [CaO-0.6×MgO]/[CaO+MgO+Al₂O₃] ≥ 0.20. In Patent Literature 1, the mass ratio of oxide-based inclusions (CaO, MgO, and Al₂O₃) in the alloy is appropriately controlled. By this means, formation of coarse TiC is suppressed, and the weld hot cracking resistance of the alloy increases.
• Patent Literature 2 discloses an alloy having a chemical composition containing, in mass%, C: 0.15% or less, Si: 0.05 to 2.0%, Mn: 0.05 to 2.0%, P: 0.035% or less, S: 0.0015% or less, Cr: 16 to 30%, Ni: 18 to 50%, Al: 0.01 to 1.0%, Ti: 0.01 to 1.5%, N: 0.35% or less, O: 0.003% or less, Mo: 8% or less, Cu: 4% or less, Co: 3% or less, Ca: 0.0003 to 0.0050%, and Mg: 0.0060% or less, with the balance being Fe and impurities. In addition, the number density of TiC precipitates having an equivalent circular diameter of 1.0 µm or more and the content of Mg in the steel satisfy the following relation: number density (pieces/mm²) of TiC ≤ 463-9.5 × Mg concentration in the steel (ppm by mass). In Patent Literature 2, the amount of coarse TiC precipitates is appropriately controlled according to the Mg concentration in the alloy. By this means, the weld hot cracking resistance of the alloy increases.
• Patent Literature 3 discloses an alloy having a chemical composition containing, in mass%, C: 0.15% or less, Si: 0.05 to 2.0%, Mn: 0.05 to 2.0%, P: 0.035% or less, S: 0.0015% or less, O: 0.0020% or less, with the sum of O + S being 0.0020% or less, Cr: 16 to 30%, Ni: 18 to 50%, Al: 0.01 to 1.0%, Ti: 0.01 to 1.5%, N: 0.02% or less, Mo: 8% or less, Cu: 4% or less, Co: 3% or less, Ca: 0.0010 to 0.0050%, and Mg: 0.0010 to 0.0050%, with the balance being Fe and impurities. In addition, the average concentration of S in oxide-based inclusions and sulfide-based inclusions is 0.70% by mass or more. In Patent Literature 3, S that reduces grain boundary strength and the melting temperature of grain boundaries is immobilized in inclusions. By this means, the weld hot cracking resistance of the alloy increases.
• In addition, Japanese Patent Application Publication No. 2021-070838 (Patent Literature 4) discloses a technique for enhancing hot workability of an alloy containing Al and Ti.
• Patent Literature 4 discloses an alloy having a chemical composition containing, in mass%, C: 0.10% or less, Si: 0.05 to 1.0%, Mn: 0.05 to 2.0%, P: 0.035% or less, S: 0.0015% or less, Cr: 18 to 25%, Ni: 18 to 50%, Al: 0.05 to 1.0%, Ti: 0.15 to 1.5%, N: 0.02% or less, O: 0.003% or less, Mo: 5% or less, W: 2% or less, Cu: 3% or less, Co: 2.0% or less, Ca: 0.0003 to 0.005%, and Mg: 0.006% or less, with the balance being Fe and unavoidable impurities. Furthermore, the Ca/Al mass ratio in oxide-based inclusions is 1.0 to 15. In addition, excess Ca (ΔCa) that is calculated using the equation ΔCa = Ca-1.25×S-K(Ca/O)×O is 0.0003 to 0.0030%. In Patent Literature 4, ΔCa in the alloy is appropriately controlled. By this means, hot workability of the alloy increases.
CITATION LIST PATENT LITERATURE
• Patent Literature 1: Japanese Patent Application Publication No. 2022-163425
• Patent Literature 2: Japanese Patent Application Publication No. 2022-163585
• Patent Literature 3: Japanese Patent Application Publication No. 2022-163586
• Patent Literature 4: Japanese Patent Application Publication No. 2021-070838
SUMMARY OF INVENTION TECHNICAL PROBLEM
• In the alloys disclosed in Patent Literatures 1 to 3, although excellent weld hot cracking resistance can be obtained, there is no discussion regarding the hot workability of the alloys in Patent Literatures 1 to 3. In the alloy disclosed in Patent Literature 4, although excellent hot workability can be obtained, there is no discussion regarding weld hot cracking resistance.
• An objective of the present disclosure is to provide an alloy with which high creep strength is obtained, and in addition, excellent weld hot cracking resistance and excellent hot workability are obtained.
SOLUTION TO PROBLEM
• An alloy according to the present disclosure has a chemical composition consisting of, in mass%,
• C: 0.050 to 0.100%,
• Si: 1.00% or less,
• Mn: 1.50% or less,
• P: 0.035% or less,
• S: 0.0015% or less,
• Cr: 19.00 to 23.00%,
• Ni: 30.00 to 35.00%,
• N: 0.010% or less,
• Al: 0.15 to 0.70%,
• Ti: 0.15 to 0.70%,
• B: 0.0001 to 0.0030%,
• Nb: 0.0010 to 0.5000%,
• Mo: 0.01 to 1.00%,
• Ca: 0.0001 to 0.0200%,
• Ta: 0 to 0.50%,
• V: 0 to 1.00%,
• Zr: 0 to 0.100%,
• Hf: 0 to 0.10%,
• Cu: 0 to 1.00%,
• W: 0 to 1.00%,
• Co: 0 to 1.00%,
• rare earth metal: 0 to 0.1000%, and
• Mg: 0 to 0.0200%,
• with the balance being Fe and impurities, and
• satisfies Formula (1) and Formula (2),
• wherein:
  • when a content of B in the chemical composition is 0.0010% or less, the chemical composition further satisfies Formula (3): $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$
  • where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1) to Formula (3).
ADVANTAGEOUS EFFECTS OF INVENTION
• In the alloy according to the present disclosure, high creep strength is obtained, and furthermore, excellent weld hot cracking resistance and excellent hot workability are obtained.
DESCRIPTION OF EMBODIMENTS
• The present inventors have initially conducted studies from the viewpoint of the chemical composition with regard to an alloy in which high creep strength is obtained and, in addition, excellent weld hot cracking resistance and excellent hot workability are obtained. As a result, the present inventors have considered that if an alloy has a chemical composition consisting of, in mass%, C: 0.050 to 0.100%, Si: 1.00% or less, Mn: 1.50% or less, P: 0.035% or less, S: 0.0015% or less, Cr: 19.00 to 23.00%, Ni: 30.00 to 35.00%, N: 0.010% or less, Al: 0.15 to 0.70%, Ti: 0.15 to 0.70%, B: 0.0001 to 0.0030%, Nb: 0.0010 to 0.5000%, Mo: 0.01 to 1.00%, Ca: 0.0001 to 0.0200%, Ta: 0 to 0.50%, V: 0 to 1.00%, Zr: 0 to 0.100%, Hf: 0 to 0.10%, Cu: 0 to 1.00%, W: 0 to 1.00%, Co: 0 to 1.00%, rare earth metal: 0 to 0.1000%, and Mg: 0 to 0.0200%, with the balance being Fe and impurities, there is a possibility that the alloy can be used in a high temperature environment and that excellent weld hot cracking resistance and excellent hot workability can be obtained.
• Therefore, the present inventors have conducted further studies regarding the creep strength, weld hot cracking resistance, and hot workability of an alloy that satisfies the aforementioned chemical composition. As a result, the present inventors obtained the following findings.
[Regarding creep strength]
• The present inventors have investigated means for increasing creep strength in a high temperature environment in an alloy that satisfies the aforementioned chemical composition. As mentioned above, Al and Ti form a gamma-prime (y') phase (Ni₃ (Al, Ti)) in an alloy during use in a high temperature environment. The y' phase increases creep strength. Therefore, the total content of Al and Ti influences the creep strength. Specifically, in an alloy having the aforementioned chemical composition, if the following Formula (1) is satisfied, sufficient creep strength will be obtained. $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ where, a content of a corresponding element in the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (1).
[Regarding weld hot cracking resistance and hot workability]
• Among the elements in the aforementioned chemical composition, elements such as P, S, and Mg are likely to segregate to grain boundaries. If these elements segregate, the grain boundaries will become embrittled and hot workability at a temperature of around 900°C will decrease. B suppresses a decrease in hot workability caused by segregation of these elements to grain boundaries. Specifically, B segregates to grain boundaries and thereby suppresses segregation of elements other than B to the grain boundaries. Thus, the grain boundaries are strengthened by segregation of B. As a result, hot workability of the alloy increases.
• However, if B is contained, the solidification temperature of grain boundaries of a HAZ of the alloy that melted during the heating process during welding will decrease. Therefore, although B increases hot workability, B decreases the weld hot cracking resistance.
• Therefore, the present inventors have conducted studies regarding means that, while increasing hot workability by containing B, can also increase weld hot cracking resistance. As a result, the present inventors have discovered that in order to increase weld hot cracking resistance while also containing B, it is effective to (A) raise the eutectic melting temperature and solidification temperature of Ti-based precipitates that precipitate at grain boundaries, and (B) raise the solidification temperature of grain boundaries that melted due to a liquation by constitutional supercooling in the heating process during welding.
[Regarding (A): Raising the eutectic melting temperature and solidification temperature of Ti-based precipitates that precipitate at grain boundaries]
• In the present description, the term "Ti-based precipitates" means precipitates that contain Ti. Ti-based precipitates are mainly carbides that contain Ti. Ti-based precipitates may also contain elements other than Ti (for example, Si, Nb, and the like).
• C, Si, and Nb raise the eutectic melting temperature and solidification temperature of Ti-based precipitates. Specifically, C and Si increase the stability at high temperatures of Ti-based precipitates. Therefore, the eutectic melting temperature and solidification temperature of the Ti-based precipitates increase. Further, Nb is contained in Ti-based precipitates and raises the eutectic melting temperature and solidification temperature of the Ti-based precipitates. Therefore, C, Si, and Nb are elements that raise the eutectic melting temperature and solidification temperature of Ti-based precipitates that precipitate at grain boundaries.
[Regarding (B): Raising the solidification temperature of grain boundaries that melted due to a liquation by constitutional supercooling in the heating process during welding]
• Even in a case where the eutectic melting temperature and solidification temperature of Ti-based precipitates is high, if a liquation by constitutional supercooling occurs, weld hot cracking resistance will decrease. Specifically, during heating during welding, the alloy is subjected to rapid local heating. In this case, in the heated portion (HAZ) of the alloy, a eutectic melting reaction occurs between the Ti precipitates at the grain boundaries and the grain boundary region, and the Ti-based precipitates and grain boundaries liquefy at a temperature that is lower than their melting temperatures. Such a phenomenon is referred to as a "liquation by constitutional supercooling".
• Grain boundaries where a liquation by constitutional supercooling occurred act as a starting point for cracking. Therefore, to increase weld hot cracking resistance, it is effective to not only raise the eutectic melting temperature and solidification temperature of Ti-based precipitates, but also to raise the solidification temperature of grain boundaries where a liquation by constitutional supercooling occurred. Therefore, the present inventors have investigated means for increasing the solidification temperature of grain boundaries where a liquation by constitutional supercooling occurred. As a result, the present inventors have discovered that Mo, Ti, and B promote the occurrence of a liquation by constitutional supercooling at grain boundaries. When grain boundaries where a liquation by constitutional supercooling occurred contain Mo, Ti, or B, the solidification temperature of the grain boundaries where the liquation by constitutional supercooling occurred decreases. If the solidification temperature of grain boundaries where a liquation by constitutional supercooling occurred decreases, it is more likely for weld hot cracking to occur.
• Based on the findings described above, the present inventors have considered that weld hot cracking resistance will be increased by increasing the content of C, content of Si, and content of Nb to raise the eutectic melting temperature and solidification temperature of Ti-based precipitates in the alloy, and also reducing the content of Mo, content of Ti, and content of B to increase the solidification temperature of grain boundaries where a liquation by constitutional supercooling has occurred. Therefore, the present inventors have carried out further studies regarding the relation between these elements and weld hot cracking resistance. As a result, it has been revealed that if an alloy having the chemical composition described above also satisfies the following Formula (2), excellent weld hot cracking resistance and excellent hot workability will be obtained. $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ where, a content of a corresponding element in the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (2).
[Case where content of B is 0.0010% or less]
• However, even when alloys satisfied the aforementioned chemical composition and satisfied Formula (1) and Formula (2), if the content of B was 0.0010% or less, in some cases sufficient hot workability could not be obtained even if excellent weld hot cracking resistance was obtained. Therefore, the present inventors have conducted further studies regarding means that can achieve both excellent hot workability and excellent weld hot cracking resistance in a case where the content of B is low. As a result, the present inventors obtained the following finding.
• In an alloy having the aforementioned chemical composition, during hot working at approximately 900°C, in some cases Ti-based precipitates are present in the grains. The alloy is hardened by the Ti-based precipitates. As a result, hot workability of the alloy decreases. Therefore, in order to increase hot workability at approximately 900°C, it is effective to suppress hardening of the alloy caused by Ti-based precipitates.
• Thus, the present inventors have conducted studies regarding means for sufficiently suppressing hardening of an alloy by Ti-based precipitates during hot working in a case where the content of B is 0.0010% or less. As a result, the present inventors obtained the following finding.
• In the aforementioned hot working temperature range, C, Ti, Si, and Nb promote formation of Ti-based precipitates. C and Ti directly contribute to formation of Ti-based precipitates. Si increases the diffusion velocity of C and Ti, thereby contributing to formation of the Ti-based precipitates. Nb substitutes for Ti sites of Ti-based precipitates (i.e., lattice points occupied by Ti atoms in the Ti-based precipitates), and thereby promotes the formation of Ti-based precipitates containing Nb (composite precipitates). In addition, in the chemical composition described above, Mo strengthens the interior of the grains by solid-solution strengthening and thereby hardens the alloy even in the temperature range for hot working.
• On the basis of the above findings, the present inventors have considered that in a case where the content of B is 0.0010% or less, the hot workability at approximately 900°C can be increased by appropriately controlling the content of C, content of Ti, content of Si, content of Nb, and content of Mo.
• Therefore, based on findings described above, the present inventors have conducted further studies regarding the relation between the contents of the aforementioned elements (C, Si, Mo, Ti, and Nb) and hot workability in a case where the content of B is 0.0010% or less. As a result, it has been revealed that in an alloy that satisfies the aforementioned chemical composition and also satisfies Formula (1) and Formula (2), when the content of B is 0.0010% or less, excellent weld hot cracking resistance and excellent hot workability can both be achieved by also satisfying Formula (3): $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$ where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (3).
• The alloy of the present embodiment, which has been completed based on the above findings, is as follows.
• An alloy according to a first aspect has a chemical composition consisting of, in mass%,
• C: 0.050 to 0.100%,
• Si: 1.00% or less,
• Mn: 1.50% or less,
• P: 0.035% or less,
• S: 0.0015% or less,
• Cr: 19.00 to 23.00%,
• Ni: 30.00 to 35.00%,
• N: 0.010% or less,
• Al: 0.15 to 0.70%,
• Ti: 0.15 to 0.70%,
• B: 0.0001 to 0.0030%,
• Nb: 0.0010 to 0.5000%,
• Mo: 0.01 to 1.00%,
• Ca: 0.0001 to 0.0200%,
• Ta: 0 to 0.50%,
• V: 0 to 1.00%,
• Zr: 0 to 0.100%,
• Hf: 0 to 0.10%,
• Cu: 0 to 1.00%,
• W: 0 to 1.00%,
• Co: 0 to 1.00%,
• rare earth metal: 0 to 0.1000%, and
• Mg: 0 to 0.0200%,
• with the balance being Fe and impurities, and
• satisfying Formula (1) and Formula (2),
• wherein:
  • when a content of B in the chemical composition is 0.0010% or less, the chemical composition further satisfies Formula (3): $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$
  • where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1) to Formula (3).
• An alloy according to a second aspect is in accordance with the alloy of the first aspect, wherein the chemical composition contains one or more types of element selected from a group consisting of, in mass%:
• Ta: 0.01 to 0.50%,
• V: 0.01 to 1.00%,
• Zr: 0.001 to 0.100%,
• Hf: 0.01 to 0.10%,
• Cu: 0.01 to 1.00%,
• W: 0.01 to 1.00%,
• Co: 0.01 to 1.00%,
• rare earth metal: 0.0001 to 0.1000%, and
• Mg: 0.0001 to 0.0200%.
• An alloy according to a third aspect is in accordance with the alloy of the first or second aspect, wherein:
  the chemical composition contains, in mass%:
• B: more than 0.0010 to 0.0030%; and
• a content of Ti [Ti] in percent by mass in a residue obtained by an extraction residue method is less than 0.020%, or
• the [Ti] is 0.020% or more and a content of Nb [Nb] in percent by mass in the residue is 0.015% or more, and the [Ti] and the [Nb] satisfy Formula (4). $$\left[\mathrm{Ti}\right]+\left[\mathrm{Nb}\right]\ge 0.050$$
• An alloy according to a fourth aspect is in accordance with the alloy of the first or second aspect, wherein:
  the chemical composition contains, in mass%:
• B: 0.0001 to 0.0010%; and
• a content of Ti [Ti] in percent by mass in a residue obtained by an extraction residue method is 0.031% or less, or
• the [Ti] is more than 0.031%, and a content of Nb [Nb] in percent by mass in the residue is 0.3 × [Ti]% or more.
• Hereunder, the alloy of the present embodiment is described in detail. Note that, the symbol "%" in relation to elements means mass percent unless otherwise stated.
[Features of alloy of present embodiment]
• The alloy of the present embodiment satisfies the following Feature 1 to Feature 4.
(Feature 1)
• The chemical composition consists of, in mass%, C: 0.050 to 0.100%, Si: 1.00% or less, Mn: 1.50% or less, P: 0.035% or less, S: 0.0015% or less, Cr: 19.00 to 23.00%, Ni: 30.00 to 35.00%, N: 0.010% or less, Al: 0.15 to 0.70%, Ti: 0.15 to 0.70%, B: 0.0001 to 0.0030%, Nb: 0.0010 to 0.5000%, Mo: 0.01 to 1.00%, Ca: 0.0001 to 0.0200%, Ta: 0 to 0.50%, V: 0 to 1.00%, Zr: 0 to 0.100%, Hf: 0 to 0.10%, Cu: 0 to 1.00%, W: 0 to 1.00%, Co: 0 to 1.00%, rare earth metal: 0 to 0.1000%, and Mg: 0 to 0.0200%, with the balance being Fe and impurities.
(Feature 2)
• The chemical composition of Feature 1 also satisfies Formula (1): $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ where, the content of the corresponding element in the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (1).
(Feature 3)
• The chemical composition of Feature 1 also satisfies Formula (2): $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ where, the content of the corresponding element in the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (2).
(Feature 4)
• In a case where the content of B in the chemical composition of Feature 1 is 0.0010% or less, the chemical composition further satisfies Formula (3). $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$ where, the content of the corresponding element in the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (3).
• The alloy of the present embodiment satisfies the aforementioned Feature 1 to Feature 4. Therefore, in the alloy of the present embodiment, high creep strength is obtained, and furthermore, excellent weld hot cracking resistance and excellent hot workability are obtained. Hereunder, Feature 1 to Feature 4 are described.
[(Feature 1) Regarding chemical composition]
• The chemical composition of the alloy of the present embodiment contains the following elements.
C: 0.050 to 0.100%
• Carbon (C) increases the creep strength of the alloy in a high temperature environment. If the content of C is less than 0.050%, 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 C is more than 0.100%, C will form M₂₃C₆-type Cr carbides at grain boundaries. In such case, Cr-depleted zones will form at the grain boundaries. Therefore, even if the contents of other elements are within the range of the present embodiment, the stress relaxation cracking resistance of the alloy will decrease.
• Therefore, the content of C is 0.050 to 0.100%.
• A preferable lower limit of the content of C is 0.055%, more preferably is 0.060%, further preferably is 0.065%, and further preferably is 0.070%.
• A preferable upper limit of the content of C is 0.097%, more preferably is 0.095%, further preferably is 0.093%, further preferably is 0.090%, further preferably is 0.085%, and further preferably is 0.080%.
Si: 1.00% or less
• Silicon (Si) deoxidizes the alloy in the steelmaking process. Si also increases the oxidation resistance of the alloy in a high temperature environment. If even a small amount of Si is contained, the aforementioned advantageous effects will be obtained to a certain extent even when the contents of other elements are within the range of the present embodiment.
• However, if the content of Si is more than 1.00%, the weld hot cracking resistance and hot workability will decrease even if the contents of other elements are within the range of the present embodiment.
• Therefore, the content of Si is 1.00% or less.
• A preferable lower limit of the content of Si is more than 0%, more preferably is 0.01%, further preferably is 0.05%, further preferably is 0.10%, further preferably is 0.15%, and further preferably is 0.20%.
• A preferable upper limit of the content of Si is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.65%, further preferably is 0.60%, further preferably is 0.55%, and further preferably is 0.50%.
Mn: 1.50% or less
• Manganese (Mn) deoxidizes a weld zone of the alloy during welding. Mn also stabilizes austenite. If even a small amount of Mn is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Mn is more than 1.50%, sigma phase (σ phase) will easily form during use in a high temperature environment. The σ phase will reduce the toughness and creep ductility of the alloy in a high temperature environment. Therefore, the content of Mn is 1.50% or less.
• A preferable lower limit of the content of Mn is more than 0%, more preferably is 0.01%, further preferably is 0.05%, further preferably is 0.10%, further preferably is 0.40%, further preferably is 0.50%, and further preferably is 0.60%.
• A preferable upper limit of the content of Mn is 1.45%, more preferably is 1.40%, further preferably is 1.35%, further preferably is 1.30%, further preferably is 1.25%, and further preferably is 1.20%.
P: 0.035% or less
• Phosphorus (P) is an impurity that is unavoidably contained. That is, the content of P is more than 0%. P segregates to grain boundaries of the alloy during welding and reduces the stress relaxation cracking resistance. P also segregates to grain boundaries and decreases hot workability.
• Therefore, the content of P is 0.035% or less.
• The content of P is preferably as low as possible. However, excessively reducing the content of P will substantially increase the production cost. Therefore, when normal industrial manufacturing is taken 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 0.030%, more preferably is 0.025%, further preferably is 0.020%, and further preferably is 0.015%.
S: 0.0015% or less
• Sulfur (S) is an impurity that is unavoidably contained. That is, the content of S is more than 0%. S segregates to grain boundaries of the alloy during welding and during hot working, thereby reducing the weld hot cracking resistance and hot workability.
• Therefore, the content of S is 0.0015% or less.
• The content of S is preferably as low as possible. However, excessively reducing the content of S will substantially increase the production cost. Therefore, when normal industrial manufacturing is taken into consideration, a preferable lower limit of the content of S is more than 0%, more preferably is 0.0001%, and further preferably is 0.0002%.
• A preferable upper limit of the content of S is 0.0012%, more preferably is 0.0010%, further preferably is 0.0008%, and further preferably is 0.0006%.
Cr: 19.00 to 23.00%
• Chromium (Cr) increases the corrosion resistance of the alloy in a high temperature environment. If the content of Cr is less than 19.00%, 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 23.00%, the stability of austenite in a high temperature environment will decrease. In such case, the creep strength of the alloy will decrease even if the contents of other elements are within the range of the present embodiment.
• Therefore, the content of Cr is 19.00 to 23.00%.
• A preferable lower limit of the content of Cr is 19.20%, more preferably is 19.40%, and further preferably is 19.60%.
• A preferable upper limit of the content of Cr is 22.50%, more preferably is 22.00%, further preferably is 21.50%, further preferably is 21.00%, further preferably is 20.50%, and further preferably is 20.00%.
Ni: 30.00 to 35.00%
• Nickel (Ni) stabilizes austenite and increases the creep strength of the alloy in a high temperature environment. If the content of Ni is less than 30.00%, 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 35.00%, the aforementioned advantageous effect will be saturated. In addition, the raw material cost will increase.
• Therefore, the content of Ni is 30.00 to 35.00%.
• A preferable lower limit of the content of Ni is 30.20%, more preferably is 30.40%, further preferably is 30.60%, and further preferably is 30.80%.
• A preferable upper limit of the content of Ni is 34.70%, more preferably is 34.50%, further preferably is 34.00%, further preferably is 33.50%, further preferably is 33.00%, further preferably is 32.50%, further preferably is 32.00%, further preferably is 31.50%, and further preferably is 31.00%.
N: 0.010% or less
• Nitrogen (N) is an impurity. N dissolves in the matrix (parent phase) and stabilizes austenite. The dissolved N also forms fine nitrides in the alloy during use in a high temperature environment. The fine nitrides strengthen Cr-depleted zones, and therefore increase the stress relaxation cracking resistance of the alloy. The fine nitrides that are formed during use in a high temperature environment also increase the creep strength by precipitation strengthening. If even a small amount of N is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of N is more than 0.010%, Ti nitrides will excessively form and will coarsen. In such case, the weld hot cracking resistance of the alloy will decrease, and in addition, the toughness and hot workability will decrease. Therefore, the content of N is 0.010% or less.
• A preferable lower limit of the content of N is 0.001%.
• A preferable upper limit of the content of N is 0.007%, more preferably is 0.006%, and further preferably is 0.005%.
Al: 0.15 to 0.70%
• Aluminum (Al) deoxidizes the alloy in the steelmaking process. Al also increases the oxidation resistance of the alloy in a high temperature environment. In addition, Al forms a y' phase in high temperature environments and thereby increases the creep strength of the alloy in high temperature environments. If the content of Al is less than 0.15%, 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 Al is more than 0.70%, a large amount of y' phase will be formed during the process for producing the alloy. In this case, the hot workability during the process for producing the alloy will decrease even if the contents of other elements are within the range of the present embodiment.
• Therefore, the content of Al is 0.15 to 0.70%.
• A preferable lower limit of the content of Al is 0.17%, more preferably is 0.19%, further preferably is 0.21%, further preferably is 0.23%, and further preferably is 0.30%.
• A preferable upper limit of the content of Al is 0.65%, more preferably is 0.60%, further preferably is 0.57%, further preferably is 0.55%, further preferably is 0.53%, further preferably is 0.51%, further preferably is 0.45%, and further preferably is 0.40%.
• Note that, the term "content of Al" means the content (mass%) of so-called "total Al".
Ti: 0.15 to 0.70%
• Titanium (Ti) combines with Ni and Al in a high temperature environment to form a y' phase, and thereby increases the creep strength of the alloy in a high temperature environment. If the content of Ti is less than 0.15%, 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 Ti is more than 0.70%, Ti-based precipitates will coarsen, and a large amount of y' phase will be formed during the process for producing the alloy. In this case, even if the contents of other elements are within the range of the present embodiment, the weld hot cracking resistance will decrease and the hot workability will decrease.
Therefore, the content of Ti is 0.15 to 0.70%.
• A preferable lower limit of the content of Ti is 0.16%, more preferably is 0.17%, further preferably is 0.18%, and further preferably is 0.20%.
• A preferable upper limit of the content of Ti is 0.65%, more preferably is 0.60%, further preferably is 0.57%, further preferably is 0.55%, further preferably is 0.50%, and further preferably is 0.45%.
B: 0.0001 to 0.0030%
• Boron (B) segregates to grain boundaries in a high temperature environment of approximately 900°C, and increases the grain boundary strength. Therefore, the hot workability of the alloy increases. If the content of B is less than 0.0001%, 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 B is more than 0.0030%, B will lower the solidification temperature of grain boundaries that melted during heating during welding. In this case, the weld hot cracking resistance will decrease even if the contents of other elements are within the range of the present embodiment.
Therefore, the content of B is 0.0001 to 0.0030%.
• A preferable lower limit of the content of B is 0.0003%, more preferably is 0.0005%, and further preferably is 0.0010%.
• A preferable upper limit of the content of B is 0.0028%, more preferably is 0.0025%, further preferably is 0.0023%, and further preferably is 0.0020%.
Nb: 0.0010 to 0.5000%
• Niobium (Nb) is an element that, when Ti-based precipitates are contained, raises the eutectic melting temperature and solidification temperature of the Ti-based precipitates. As a result, the weld hot cracking resistance of the alloy increases. Nb also forms fine precipitates in the alloy in a high temperature environment, thereby increasing the creep strength of the alloy. If the content of Nb is less than 0.0010%, the aforementioned advantageous effects 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 Nb is more than 0.5000%, the hot workability of the alloy will decrease even if the contents of other elements are within the range of the present embodiment.
• Therefore, the content of Nb is 0.0010 to 0.5000%.
• A preferable lower limit of the content of Nb is 0.0020%, more preferably is 0.0050%, further preferably is 0.0080%, and further preferably is 0.0100%.
• A preferable upper limit of the content of Nb is 0.4500%, more preferably is 0.4000%, further preferably is 0.3000%, further preferably is 0.2500%, and further preferably is 0.2400%.
Mo: 0.01 to 1.00%
• Molybdenum (Mo) is an element which, by being contained together with B, strengthens grain boundaries by co-segregation and increases hot workability of the alloy. Even if the content of B is more than 0.0010%, if the content of Mo is less than 0.01%, the aforementioned advantageous effect will not be sufficiently obtained.
• On the other hand, if the content of Mo is more than 1.00%, intermetallic compounds such as Laves phases will form within the grains. In such case, the strength difference between the inside of the grains and the grain boundaries will be large. Consequently, the hot workability will decrease. Further, in a case where the content of B is 0.0010% or less, not only will the aforementioned advantageous effect decrease, the hardness within the grains will increase due to the solid-solution strengthening of Mo. Therefore, the hot workability at temperatures around 900°C will decrease.
• Therefore, the content of Mo is 0.01 to 1.00%.
• A preferable lower limit of the content of Mo is 0.02%, more preferably is 0.03%, further preferably is 0.04%, and further preferably is 0.05%.
• A preferable upper limit of the content of Mo is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.
Ca: 0.0001 to 0.0200%
• Calcium (Ca) immobilizes S (sulfur) as inclusions, and thereby increases the hot workability of the alloy. In addition, Ca suppresses grain-boundary segregation of S by immobilizing S. In this case, the weld hot cracking resistance increases. If the content of Ca is less than 0.0001%, the aforementioned advantageous effects 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 Ca is more than 0.0200%, the cleanliness of the alloy will decrease. In this case, even if the contents of other elements are within the range of the present embodiment, the hot workability of the alloy will, on the contrary, decrease.
• Therefore, the content of Ca is 0.0001 to 0.0200%.
• A preferable lower limit of the content of Ca is 0.0002%, more preferably is 0.0005%, and further preferably is 0.0010%.
• A preferable upper limit of the content of Ca is 0.0150%, more preferably is 0.0100%, further preferably is 0.0080%, further preferably is 0.0050%, further preferably is 0.0040%, and further preferably is 0.0030%.
• The balance of the chemical composition of the alloy according to the present embodiment is Fe and impurities. Here, the term "impurities" with respect to the chemical composition refers to substances which are mixed in from ore or scrap used as the raw material or from the production environment or the like when industrially producing the alloy, and which are permitted within a range that does not adversely affect the alloy of the present embodiment. Representative examples of impurities include Sn, As, Zn, Pb, Sb, Bi, and O (oxygen). The total content of these impurities is 0.10% or less.
[Optional Elements]
• The chemical composition of the alloy of the present embodiment may further contain, in lieu of a part of Fe, one or more types of element selected from the group consisting of:
• Ta: 0 to 0.50%,
• V: 0 to 1.00%,
• Zr: 0 to 0.100%,
• Hf: 0 to 0.10%,
• Cu: 0 to 1.00%,
• W: 0 to 1.00%,
• Co: 0 to 1.00%,
• rare earth metal: 0 to 0.1000%, and
• Mg: 0 to 0.0200%.
• Hereunder, these optional elements are described.
[First group: Regarding Ta, V, Zr, and Hf]
• The chemical composition of the alloy according to the present embodiment may further contain one or more types of element selected from a group consisting of the aforementioned Ta, V, Zr, and Hf in lieu of a part of Fe. These elements are optional elements. Each of these elements raises the eutectic melting temperature of Ti carbides and thereby stabilizes the Ti carbides and increases the weld hot cracking resistance.
Ta: 0 to 0.50%
• Tantalum (Ta) is an optional element, and does not have to be contained. In other words, the content of Ta may be 0%.
• When Ta is contained, that is, when the content of Ta is more than 0%, Ta combines with Ti and C and is contained in Ti-based precipitates. The eutectic melting temperature and solidification temperature of the Ti-based precipitates in which Ta is contained rise. Therefore, the solidification temperature of grain boundaries that melted during welding increases. As a result, the weld hot cracking resistance increases. If even a small amount of Ta is contained, the aforementioned advantageous effect will be obtained to a certain extent.
• However, if the content of Ta is more than 0.50%, during welding of the alloy, the weld hot cracking resistance in a heat affected zone of the alloy will decrease. If the content of Ta is more than 0.50%, furthermore, the hot workability will decrease.
• Therefore, the content of Ta is 0 to 0.50%.
• A preferable lower limit of the content of Ta is 0.01%, more preferably is 0.02%, further preferably is 0.05%, and further preferably is 0.08%.
• A preferable upper limit of the content of Ta is 0.45%, more preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.30%.
V: 0 to 1.00%
• Vanadium (V) is an optional element, and does not have to be contained. In other words, the content of V may be 0%.
• When V is contained, that is, when the content of V is more than 0%, V combines with Ti and C and is contained in Ti carbides. The eutectic melting temperature and solidification temperature of the Ti-based precipitates in which V is contained increase. Therefore, the solidification temperature of grain boundaries that melted during welding increases. As a result, the weld hot cracking resistance increases. 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 1.00%, during welding of the alloy, the weld hot cracking resistance in a heat affected zone of the alloy will decrease.
• Therefore, the content of V is 0 to 1.00%.
• A preferable lower limit of the content of V is 0.01%, more preferably is 0.02%, further preferably is 0.04%, and further preferably is 0.06%.
• A preferable upper limit of the content of V is 0.80%, more preferably is 0.50%, further preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.30%.
Zr: 0 to 0.100%
• Zirconium (Zr) is an optional element, and does not have to be contained. In other words, the content of Zr may be 0%.
• When Zr is contained, that is, when the content of Zr is more than 0%, Zr combines with Ti and C and is contained in Ti carbides. The eutectic melting temperature and solidification temperature of the Ti-based precipitates in which Zr is contained increase. Therefore, the solidification temperature of grain boundaries that melted during welding increases. As a result, the weld hot cracking resistance increases. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent.
• However, if the content of Zr is more than 0.100%, during welding of the alloy, the weld hot cracking resistance in a heat affected zone of the alloy will, on the contrary, decrease.
• Therefore, the content of Zr is 0 to 0.100%.
• A preferable lower limit of the content of Zr is 0.001%, and more preferably is 0.002%.
• A preferable upper limit of the content of Zr is 0.090%, more preferably is 0.080%, further preferably is 0.050%, further preferably is 0.030%, and further preferably is 0.010% or less.
Hf: 0 to 0.10%
• Hafnium (Hf) is an optional element, and does not have to be contained. In other words, the content of Hf may be 0%. When Hf is contained, that is, when the content of Hf is more than 0%, Hf combines with Ti and C and is contained in Ti-based precipitates. The eutectic melting temperature and solidification temperature of the Ti-based precipitates in which Hf is contained increase. Therefore, the solidification temperature of grain boundaries that melted during welding increases. As a result, the weld hot cracking resistance increases. If even a small amount of Hf is contained, the aforementioned advantageous effect will be obtained to a certain extent.
• However, if the content of Hf is more than 0.10%, during welding of the alloy, the weld hot cracking resistance in a heat affected zone of the alloy will, on the contrary, decrease.
• Therefore, the content of Hf is 0 to 0.10%.
• A preferable lower limit of the content of Hf is 0.01%, and more preferably is 0.02%.
• A preferable upper limit of the content of Hf is 0.09%, more preferably is 0.08%, further preferably is 0.07%, and further preferably is 0.06%.
[Second group: Regarding Cu, W, and Co]
• The chemical composition of the alloy according to the present embodiment may further contain one or more types of element selected from a group consisting of Cu, W, and Co in lieu of a part of Fe. These elements are optional elements, and each of these elements increases the creep strength of the alloy.
Cu: 0 to 1.00%
• Copper (Cu) is an optional element, and does not have to be contained. In other words, the content of Cu may be 0%.
• When Cu is contained, that is, when the content of Cu is more than 0%, during use of the alloy in a high temperature environment, Cu precipitates as a Cu phase in the grains. The creep strength of the alloy is increased by this precipitation strengthening. 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 1.00%, a Cu phase will excessively precipitate in the grains. In such case, the strength difference between the inside of the grains and the grain boundaries will be large. Consequently, the stress relaxation cracking resistance will decrease.
Therefore, the content of Cu is 0 to 1.00%.
• A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.02%, further preferably is 0.05%, further preferably is 0.10%, further preferably is 0.15%, and further preferably is 0.20%.
• A preferable upper limit of the content of Cu is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, further preferably is 0.55%, and further preferably is 0.50%.
W: 0 to 1.00%
• Tungsten (W) is an optional element, and does not have to be contained. In other words, the content of W may be 0%.
• When W is contained, that is, when the content of W is more than 0%, during use of the alloy in a high temperature environment, W increases the creep strength of the alloy by solid-solution strengthening. 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 1.00%, intermetallic compounds such as Laves phases will form within the grains. In such case, secondary induced precipitation hardening will increase and the strength difference between the inside of the grains and the grain boundaries will be large. Consequently, the stress relaxation cracking resistance will decrease.
• Therefore, the content of W is 0 to 1.00%.
• A preferable lower limit of the content of W is 0.01%, more preferably is 0.02%, further preferably is 0.03%, further preferably is 0.04%, further preferably is 0.05%, and further preferably is 0.10%.
• A preferable upper limit of the content of W is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.65%, further preferably is 0.60%, and further preferably is 0.50%.
Co: 0 to 1.00%
• Cobalt (Co) is an optional element, and does not have to be contained. In other words, the content of Co may be 0%.
• When Co is contained, that is, when the content of Co is more than 0%, Co stabilizes austenite and increases the creep strength of the alloy in a high temperature 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 1.00%, the raw material cost will increase.
• Therefore, the content of Co is 0 to 1.00%.
• A preferable lower limit of the content of Co is 0.01%, more preferably is 0.02%, further preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.10%.
• A preferable upper limit of the content of Co is 0.90%, more preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%, and further preferably is 0.50%.
[Third group: Regarding rare earth metal (REM)]
• The chemical composition of the alloy according to the present embodiment may further contain rare earth metal (REM) in lieu of a part of Fe.
Rare earth metal: 0 to 0.1000%
• Rare earth metal (REM) is an optional element, and does not have to be contained. In other words, the content of REM may be 0%.
• When REM is contained, that is, when the content of REM is more than 0%, REM immobilizes S (sulfur) as an inclusion, and thereby increases the hot workability of the alloy. REM also immobilizes S and thereby suppresses grain-boundary segregation of S. In such case, the weld hot cracking resistance increases. If even a small amount of REM is contained, the aforementioned advantageous effects will be obtained to a certain extent.
• However, if the content of REM is more than 0.1000%, the cleanliness of the alloy will decrease. In such case, the hot workability of the alloy will, on the contrary, decrease.
• Therefore, the content of REM is 0 to 0.1000%.
• A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0020%.
• A preferable upper limit of the content of REM is 0.0800%, more preferably is 0.0600%, and further preferably is 0.0400%.
• In the present description, the term "REM" includes at least one element or more among 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 of these elements.
[Fourth group: Regarding Mg]
• The chemical composition of the alloy according to the present embodiment may further contain Mg in lieu of a part of Fe.
Mg: 0 to 0.0200% or less
• Magnesium (Mg) is an impurity, and does not have to be contained. That is, the content of Mg may be 0%.
• If the content of Mg is more than 0.0200%, Mg will segregate to grain boundaries in a high temperature environment of approximately 900°C and will embrittle the grain boundaries. In such case, the hot workability of the alloy will decrease.
• Therefore, the content of Mg is 0 to 0.0200%.
• The content of Mg is preferably as low as possible. However, excessively reducing the content of Mg will substantially increase the production cost. Therefore, when normal industrial manufacturing is taken into consideration, a preferable lower limit of the content of Mg is more than 0%, more preferably is 0.0001%, and further preferably is 0.0002%.
• A preferable upper limit of the content of Mg is 0.0150%, more preferably is 0.0100%, further preferably is 0.0080%, further preferably is 0.0050%, and further preferably is 0.0040%.
[(Feature 2) Regarding Formula (1)]
• The chemical composition of the alloy of the present embodiment also satisfies Formula (1): $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ where, the content of the corresponding element of the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (1).
• Let F1 be defined as F1 = Al+Ti. F1 is an index of the amount of a y' phase formed during use of the alloy of the present embodiment in a high temperature environment. In the alloy of the present embodiment, a y' phase is formed during use in a high temperature environment. The creep strength of the alloy in a high temperature environment is increased by the y' phase.
• Even when the alloy satisfies Feature 1, Feature 3, and Feature 4, if F1 is 0.60 or less, a sufficient amount of a y' phase will not be formed in the alloy in a high temperature environment. In such case, the creep strength of the alloy in a high temperature environment will decrease.
• On the other hand, even when the alloy satisfies Feature 1, Feature 3, and Feature 4, if F1 is 1.20 or more, an excessively large amount of the y' phase will form in the alloy. In such case, the weld hot cracking resistance will decrease.
• Therefore, F1 is to be more than 0.60 to less than 1.20.
• A preferable lower limit of F1 is 0.61, more preferably is 0.62, further preferably is 0.64, further preferably is 0.66, further preferably is 0.68, and further preferably is 0.70.
• A preferable upper limit of F1 is 1.15, more preferably is 1.10, further preferably is 1.05, further preferably is 1.00, and further preferably is 0.95.
• The numerical value of F1 is to be a value to the second decimal place obtained by rounding off the third decimal place of the determined value.
[(Feature 3) Regarding Formula (2)]
• The chemical composition of the alloy of the present embodiment also satisfies Formula (2): $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ where, the content of the corresponding element of the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (2).
• Let F2 be defined as F2 = 3.3-41C-Si+2Mo+3Ti+245B-12Nb. F2 is an index of weld hot cracking resistance. In order to increase the hot workability in a case where hot working is performed at a temperature of approximately 900°C, the alloy of the present embodiment contains B. However, in an alloy having a chemical composition that satisfies Feature 1, the solidification temperature of grain boundaries of the alloy will decrease due to segregation of B to the grain boundaries. Consequently, the weld hot cracking resistance is likely to decrease. Therefore, in the alloy of the present embodiment, as mentioned above, (A) the eutectic melting temperature and solidification temperature of Ti-based precipitates that precipitate at grain boundaries are raised, and (B) the solidification temperature of grain boundaries that melted due to a liquation by constitutional supercooling in the heating process during welding is raised. As a result, the weld hot cracking resistance increases.
• Specifically, in a chemical composition that satisfies Feature 1, C, Si, and Nb raise the solidification temperature of Ti-based precipitates after melting. On the other hand, Mo, Ti, and B promote the occurrence of a liquation by constitutional supercooling and thereby lower the solidification temperature after melting of the grain boundaries. Therefore, by appropriately controlling the content of C, content of Si, content of Nb, content of Mo, content of Ti, and content of B, the aforementioned (A) and (B) are simultaneously achieved and the solidification temperature of the grain boundaries after melting during welding is raised, and as a result, the weld hot cracking resistance is increased. Even in a case where an alloy satisfies Feature 1, Feature 2, and Feature 4, the aforementioned advantageous effect will not be sufficiently obtained if F2 is more than 2.00. Therefore, F2 is 2.00 or less.
• A preferable upper limit of F2 is 1.97, more preferably is 1.95, further preferably is 1.93, and further preferably is 1.90.
• Although the lower limit of F2 is not particularly limited, a preferable lower limit is -7.00, more preferably is -6.00, further preferably is -5.00, and further preferably is -4.00.
• The numerical value of F2 is to be a value to the second decimal place obtained by rounding off the third decimal place of the determined value.
[(Feature 4) Regarding Formula (3)]
• When the content of B in the chemical composition that satisfies Feature 1 is 0.0010% or less, in addition, the chemical composition of the alloy of the present embodiment satisfies Formula (3): $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$ where, a content of a corresponding element in the chemical composition of the alloy is substituted in percent by mass for each symbol of an element in Formula (3).
• Let F3 be defined as F3 = 0.4+67C+1.3Si+5.5Mo+5.2Ti+13.4Nb. F3 is an index of hot workability in a case where hot working is performed at approximately 900°C.
• As mentioned above, B segregates to grain boundaries of the alloy and lowers the solidification temperature of the grain boundaries. Therefore, in an alloy that satisfies Feature 1 and contains B, the weld hot cracking resistance is likely to decrease. Therefore, in the present embodiment, by making the chemical composition of the alloy satisfy Formula (2), the solidification temperature of the grain boundaries after melting during welding is raised, thereby increasing the weld hot cracking resistance.
• In this connection, when performing welding on extremely thick-walled materials or when performing welding with large heat input, an alloy that has high weld hot cracking resistance is used. In such a case, the content of B in the alloy may be adjusted to 0.0010% or less.
• In a case where the content of B is set to 0.0010% or less, the weld hot cracking resistance will increase as a result of the reduction in the content of B. However, because the content of B is reduced, the hot workability will be likely to decrease. Therefore, in a case where the content of B in the chemical composition is set to 0.0010% or less, in the alloy of the present embodiment, the hot workability is increased by suppressing hardening of the alloy due to Ti-based precipitates.
• C, Si, Mo, Ti, and Nb promote formation of Ti-based precipitates. Therefore, in a case where the content of B in the chemical composition is set to 0.0010% or less, the value of F3 which consists of the content of C, content of Si, content of Mo, content of Ti, and content of Nb is appropriately adjusted to suppress the formation of Ti-based precipitates. By this means, the hot workability at around 900°C increases.
• Even in a case where an alloy satisfies Feature 1 to Feature 3, if F3 is more than 8.25, the aforementioned advantageous effect will not be sufficiently obtained. Therefore, F3 is 8.25 or less.
• A preferable upper limit of F3 is 8.20, more preferably is 8.15, further preferably is 8.13, and further preferably is 8.10.
• Although the lower limit of F3 is not particularly limited, a preferable lower limit is 4.60, more preferably is 4.80, further preferably is 5.00, and further preferably is 5.50.
• The numerical value of F3 is to be a value to the second decimal place obtained by rounding off the third decimal place of the determined value.
[Advantageous effects of alloy]
• The alloy of the present embodiment satisfies the aforementioned Feature 1 to Feature 4. As a result, the alloy of the present embodiment has sufficient creep strength in a high temperature environment, and can achieve both excellent weld hot cracking resistance and excellent hot workability.
[Microstructure and shape of alloy]
• The microstructure of the alloy of the present embodiment consists of austenite. Further, the shape of the alloy of the present embodiment is not particularly limited. The alloy may be an alloy pipe, or may be an alloy plate. The alloy may also be bar-shaped. Preferably the alloy of the present embodiment is an alloy pipe or an alloy plate.
[Preferred form of alloy of present embodiment when content of B is more than 0.0010%]
• In a chemical composition that satisfies Feature 1 to Feature 3, when the content of B is more than 0.0010% to 0.0030%, preferably the alloy of the present embodiment also satisfies the following Feature 5.
(Feature 5)
• A content of Ti [Ti] in percent by mass in a residue obtained by an extraction residue method is less than 0.020%, or
  the [Ti] is 0.020% or more and a content of Nb [Nb] in percent by mass in the residue is 0.015% or more, and the [Ti] and the [Nb] satisfy Formula (4). $$\left[\mathrm{Ti}\right]+\left[\mathrm{Nb}\right]\ge 0.050$$
• Feature 5 is described hereunder.
[(Feature 5) Relation between content of Ti [Ti] and content of Nb [Nb] in residue (Part 1)]
• In the alloy of the present embodiment, in a case where the content of B in the chemical composition is more than 0.0010 to 0.0030%, by satisfying Feature 1 to Feature 3, the alloy has sufficient creep strength in a high temperature environment, and can achieve both excellent weld hot cracking resistance and excellent hot workability.
• In this connection, in the alloy of the present embodiment, a certain amount of Ti-based precipitates are contained in the alloy before welding. Therefore, in the heating process when welding the alloy, Ti-based precipitates trapped at grain boundaries melt by eutectic reaction. Because the alloy of the present embodiment contains B, B segregates to the grain boundaries. Therefore, the solidification temperature of the grain boundaries after melting is lowered by B. As a result, the weld hot cracking resistance decreases.
• Therefore, in the alloy of the present embodiment, when the content of B is more than 0.0010 to 0.0030%, preferably, either one of the following two countermeasures, i.e., countermeasure I and countermeasure II, is adopted.
(Countermeasure I)
• The amount of Ti-based precipitates that are already present in the alloy is reduced as much as possible. By this means, eutectic melting at a low temperature of grain boundaries is suppressed, and the solidification temperature of the grain boundaries increases.
(Countermeasure II)
• Ti-based precipitates are caused to contain Nb to thereby stabilize the Ti-based precipitates up to a high temperature. In this case, the eutectic melting temperature of the grain boundaries containing Ti-based precipitates increases, and the solidification temperature of the grain boundaries also increases.
• Therefore, in a case where the content of B is more than 0.0010 to 0.0030%, in the alloy of the present embodiment, preferably either one of the following requirement I and requirement II is satisfied.
(Requirement I)
• The content of Ti [Ti] in the residue is less than 0.020%.
(Requirement II)
• [Ti] in a residue obtained by an extraction residue method is 0.020% or more, a content of Nb [Nb] in the residue is 0.015% or more, and [Ti] and [Nb] satisfy Formula (4). $$\left[\mathrm{Ti}\right]+\left[\mathrm{Nb}\right]\ge 0.050$$
[Regarding requirement I]
• In a case where the content of B is more than 0.0010 to 0.0030%, if the amount of Ti-based precipitates formed is small, weld hot cracking attributable to eutectic melting of Ti-based precipitates and grain boundaries will also be less likely to occur.
• According to requirement I, the content of Ti [Ti] in the residue is less than 0.020%. In this case, the amount of Ti-based precipitates formed in the alloy is sufficiently suppressed. Therefore, even more excellent weld hot cracking resistance is obtained.
• In the case of requirement I, a preferable upper limit of the content of Ti [Ti] in the residue is 0.019%, and more preferably is 0.017%.
[Regarding requirement II]
• In a case where the content of B is more than 0.0010 to 0.0030%, if a certain amount of Ti-based precipitates is formed, it is preferable to raise the solidification temperature after eutectic melting of Ti-based precipitates and grain boundaries. If the Ti-based precipitates in the alloy contain Nb, the Ti-based precipitates will be stabilized at high temperatures. Therefore, the solidification temperature of the grain boundaries after eutectic melting of Ti-based precipitates and grain boundaries can be increased.
• According to requirement II, [Ti] in the residue is 0.020% or more, the content of Nb [Nb] in the residue is 0.015% or more, and [Ti] and [Nb] satisfy Formula (4). $$\left[\mathrm{Ti}\right]+\left[\mathrm{Nb}\right]\ge 0.050$$
• In this case, among the Ti-based precipitates present in the alloy, the proportion of Ti-based precipitates that contain Nb is high. Therefore, the solidification temperature of the grain boundaries after eutectic melting of Ti-based precipitates and grain boundaries is sufficiently high. As a result, even more excellent weld hot cracking resistance is obtained.
• In the case of requirement II, a preferable lower limit of [Ti]+[Nb] in the residue is 0.052%, more preferably is 0.060%, further preferably is 0.070%, further preferably is 0.080%, and further preferably is 0.090%.
[Preferred form of alloy of present embodiment when content of B is 0.0010% or less]
• In a case where the content of B in the chemical composition that satisfies Feature 1 to Feature 4 is 0.0001 to 0.0010%, preferably the alloy of the present embodiment also satisfies the following Feature 6.
(Feature 6)
• A content of Ti [Ti] in percent by mass in a residue obtained by an extraction residue method is 0.031% or less, or
  [Ti] is more than 0.031%, and a content of Nb [Nb] in percent by mass in the residue is 0.3 × [Ti]% or more.
• Feature 6 is described hereunder.
[(Feature 6) Relation between [Ti] and [Nb] in residue (Part 2)]
• In a case where the content of B is 0.0001 to 0.0010%, preferably either one of the following requirement III and requirement IV is satisfied.
(Requirement III)
• A content of Ti [Ti] in the residue is 0.031% or less.
(Requirement IV)
• [Ti] in the residue is more than 0.031%, and a content of Nb [Nb] in the residue is 0.3 × [Ti]% or more.
[Regarding requirement III]
• In a case where the content of B is 0.0010% or less, in comparison to requirement (I), a decrease in the weld hot cracking resistance caused by the content of B is suppressed. Therefore, if [Ti] in the residue is 0.031% or less, formation of Ti precipitates in the alloy is sufficiently suppressed and even more excellent weld hot cracking resistance is obtained.
[Regarding requirement (IV)]
• Even in a case where the content of B is 0.0010% or less, if a certain amount of Ti-based precipitates is formed, the high temperature stability of the Ti-based precipitates is increased to raise the solidification temperature of the grain boundaries after eutectic melting of the Ti-based precipitates and the grain boundaries. Specifically, the proportion of Ti-based precipitates containing Nb in the alloy is increased.
• If [Ti] in the residue is more than 0.031% and the content of Nb [Nb] in the residue is 0.3 × [Ti]%, the proportion of Ti-based precipitates in which Nb is contained is sufficiently high. As a result, even more excellent weld hot cracking resistance is obtained.
[Method for measuring content of Ti [Ti] and content of Nb [Nb] in residue]
• [Ti] and [Nb] in a residue of the alloy is determined by the following extraction residue method.
• A test specimen is taken from the alloy. A cross section perpendicular to the longitudinal direction of the test specimen may be circular or may be rectangular.
• If the alloy is an alloy pipe, the test specimen is taken in a manner so that the center of a cross section perpendicular to the longitudinal direction of the test specimen is the center position of the wall thickness of the alloy pipe, and the longitudinal direction of the test specimen coincides with the pipe axis direction of the alloy pipe.
• If the alloy is an alloy plate, the test specimen is taken in a manner so that the center of a cross section perpendicular to the longitudinal direction of the test specimen is the center position of the plate width and the center position of the plate thickness of the alloy plate, and the longitudinal direction of the test specimen coincides with the longitudinal direction of the alloy plate.
• If the alloy is a round bar, the test specimen is taken in a manner so that the center of a cross section perpendicular to the longitudinal direction of the test specimen is an R/2 position (center position of a radius in a cross section perpendicular to the longitudinal direction of the round bar) of the round bar, and the longitudinal direction of the test specimen coincides with the longitudinal direction of the round bar.
• The surface of the taken test specimen is polished to remove approximately 50 µm of the surface by preliminary electropolishing to obtain a newly formed surface. The electropolished test specimen is then subjected to electrolyzation (main electrolyzation) using an electrolyte solution (10% acetylacetone + 1% tetraammonium + methanol). The electrolyte solution after the main electrolyzation is passed through a 0.2 µm filter to capture residue. The obtained residue is subjected to acid decomposition, and the Ti mass in the residue and the Nb mass in the residue are determined by ICP (inductively coupled plasma) emission spectrometry.
• In addition, the mass of the alloy electrolyzed by the main electrolyzation is determined. Specifically, the mass of the test specimen before the main electrolyzation and the mass of the test specimen after the main electrolyzation are measured. Then, a value obtained by subtracting the mass of the test specimen after the main electrolyzation from the mass of the test specimen before the main electrolyzation is defined as the alloy mass electrolyzed by the main electrolyzation.
• The Ti mass in the residue is divided by the alloy mass electrolyzed by the main electrolyzation to thereby determine the content of Ti [Ti] (mass%) in the residue. In addition, the Nb mass in the residue is divided by the alloy mass electrolyzed by the main electrolyzation to thereby determine the content of Nb [Nb] (mass%) in the residue.
[Method for producing alloy]
• A method for producing the alloy of the present embodiment will now be described. The production method described hereunder is one example of a method for producing the alloy of the present embodiment. Therefore, the alloy of the present embodiment may also be produced by a production method other than the production method described hereunder. However, the production method described hereunder is a preferable example of a method for producing the alloy of the present embodiment.
• A method for producing the alloy of the present embodiment includes the following processes.
• (Process 1) Preparation process
• (Process 2) Hot working process
• (Process 3) Cold working process
• (Process 4) Heat treatment process
• The above process 3 (cold working process) is an optional process, and does not have to be performed. Hereunder, each process is described.
[(Process 1) Preparation process]
• In the preparation process, a starting material having a chemical composition that satisfies the above Feature 1 to Feature 4 is prepared. The starting material may be supplied by a third party or may be produced. The starting material may be an ingot, or may be a slab, a bloom, or a billet.
• In the case of producing the starting material, the starting material is produced by the following method. A molten alloy that has the chemical composition described above is produced. The produced molten alloy is used to produce an ingot by an ingot-making process. The produced molten alloy may also be used to produce a slab, a bloom, or a billet by a continuous casting process. Hot working may be performed on the produced ingot, slab, or bloom to produce a billet. For example, hot forging may be performed on the ingot to produce a cylindrical billet, and the billet may be used as the starting material. In such case, although not particularly limited, the temperature of the starting material immediately before the start of the hot forging is, for example, 1100 to 1300°C. The method for cooling the starting material after hot forging is not particularly limited.
[(Process 2) Hot working process]
• In the hot working process, hot working is performed on the starting material prepared in the preparation process, to thereby produce an intermediate alloy. The intermediate alloy, for example, may be an alloy pipe, may be an alloy plate, or may be an alloy round bar.
• If the intermediate alloy is an alloy pipe, the following working is performed in the hot working process. First, a cylindrical starting material is prepared. A through-hole is formed along the central axis in the cylindrical starting material by machining. The cylindrical starting material in which the through-hole has been formed is heated. The heated cylindrical starting material is then subjected to a hot-extrusion process, which is typified by the Ugine-Sejournet process, to produce an intermediate alloy (alloy pipe).
• Further, instead of hot extrusion, an alloy pipe may be produced by performing piercing-rolling according to the Mannesmann process. In such case, the cylindrical starting material is heated. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. The heated cylindrical starting material is then subjected to piercing-rolling using a piercing machine. In the case of performing piercing-rolling, although not particularly limited, the piercing ratio is, for example, 1.0 to 4.0. The cylindrical starting material subjected to piercing-rolling is further subjected to hot rolling with a mandrel mill, a stretch reducing mill, a sizing mill or the like to produce a hollow blank (alloy pipe). Although not particularly limited, the cumulative reduction of area in the hot working process is, for example, 20 to 80%. In the case of producing an alloy pipe by hot working, preferably the temperature (finishing temperature) of the hollow blank immediately after completing the hot working is 800°C or more. Note that, the hot working process includes hot working at around 900°C.
• If the intermediate alloy is an alloy plate, one or a plurality of rolling mills equipped with a pair of work rolls is used in the hot working process. The starting material such as a slab is heated. The heated starting material is subjected to hot working using the rolling mill to produce an alloy plate. Although not particularly limited, the heating temperature of the starting material before hot rolling is, for example, 1100 to 1300°C. Note that, the hot working process includes hot working at around 900°C.
• If the intermediate alloy is a round bar, for example, one or a plurality of rolling mills equipped with a pair of work rolls is used in the hot working process. Grooves are formed in the pair of work rolls. The starting material such as a bloom is heated. The heated starting material is subjected to hot rolling using the rolling mill to produce a round bar. Although not particularly limited, the heating temperature of the starting material before hot rolling is, for example, 1100 to 1300°C. Note that, the hot working process includes hot working at around 900°C.
[(Process 3) Cold working process]
• A cold working process is performed as necessary. In other words, a cold working process does not have to be performed. In the case of performing a cold working process, cold working is performed on the intermediate alloy after the intermediate alloy has been subjected to a pickling treatment. If the intermediate alloy is an alloy pipe or an alloy bar, the cold working is, for example, cold drawing. If the intermediate alloy is an alloy plate, the cold working is, for example, cold rolling. Performing the cold working process allows the development of recrystallization and the uniformed grain size to occur. Although not particularly limited, the reduction of area in the cold working process is, for example, 10 to 90%.
[(Process 4) Heat treatment process]
• In the heat treatment process, the intermediate alloy after the hot working process or after the cold working process is subjected to a heat treatment to adjust the amount of dissolved Ti and the size of the grains in the alloy. A heat treatment temperature T1 is 1170 to 1300°C. Although not particularly limited, the holding time at the heat treatment temperature T1 is, for example, 5 to 30 minutes. After the holding time elapses, the intermediate alloy is rapidly cooled.
• The alloy of the present embodiment can be produced by the processes described above. The production method described above is one example of a method for producing the alloy of the present embodiment. Therefore, a method for producing the alloy of the present embodiment is not limited to the above production method. As long as Feature 1 to Feature 4 are satisfied, a method for producing the alloy is not limited to the production method described above.
[Preferable conditions in heat treatment process]
• Preferably, the heat treatment temperature T1 in the heat treatment process satisfies the following Formula (X): $$\mathrm{T}1\le 1600+33011\times {\left(\mathrm{Nb}\right)}^2-6995\times \left(\mathrm{Nb}\right)$$ where, the content of Nb in the chemical composition of the alloy is substituted in percent by mass for (Nb) in Formula (X).
• FA is defined as follows. $$\mathrm{FA}=1600+33011\times {\left(\mathrm{Nb}\right)}^2-6995\times \left(\mathrm{Nb}\right)$$
• FA is an index that affects the composition of precipitates in the alloy, and more specifically, is an index that affects the content of Ti [Ti] and content of Nb [Nb] in the residue.
• If the heat treatment temperature T1 is equal to or lower than FA, in a case where the content of B in the alloy is more than 0.0010 to 0.0030%, it is easier to satisfy Feature 5 (requirement (I) or requirement (II)). Further, in a case where the content of B in the alloy is 0.0001 to 0.0010%, it is easier to satisfy Feature 6 (requirement (III) or requirement (IV)).
[Method for producing welded joint of alloy]
• A welded joint of the alloy of the present embodiment can be produced by the following method.
• The alloy of the present embodiment is prepared as a base metal. A bevel is then formed in the prepared base metal. Specifically, a bevel is formed in an end of the base metal by a well-known processing method. The bevel shape may be a V shape, may be a U shape, may be an X shape, or may be a shape other than a V shape, a U shape or an X shape.
• Welding is performed on the prepared base metal to produce a welded joint. Specifically, two base metals in which a bevel has been formed are prepared. The bevels of the prepared base metals are butted together. The portion where the pair of bevels are butted together is then subjected to welding using a well-known welding consumable to thereby form a weld metal having the aforementioned chemical composition. The welding consumable is, for example, a welding consumable with the AWS classification: ER NiCr-3. However, the welding consumable is not limited to these examples.
• The welding method may be one in which the weld metal is produced by single-pass welding or by multi-pass welding. The welding methods include, for example, gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW), flux-cored arc welding (FCAW), gas metal arc welding (GMAW), and submerged arc welding (SAW). A welded joint of the alloy of the present embodiment can be produced by the above production process.
EXAMPLES
• The advantageous effects of the alloy 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 alloy of the present embodiment. Accordingly, the alloy of the present embodiment is not limited to this one example of conditions.
[Production of alloys]
• Ingots having the chemical compositions shown in Table 1A and Table 1B were produced. Each ingot was formed in a cylindrical shape with an outer diameter of 120 mm, and the mass of each ingot was 30 kg.
[Table 1A]
• TABLE 1A

  Test Number	Chemical Composition (unit is mass%; balance is Fe and impurities)
  	C	Si	Mn	P	S	Cr	Ni	N	Al	Ti	B
  1	0.068	0.22	1.22	0.013	0.0004	19.22	30.47	0.002	0.55	0.28	0.0016
  2	0.071	0.25	0.88	0.013	0.0003	19.73	30.36	0.002	0.51	0.16	0.0016
  3	0.069	0.23	0.99	0.012	0.0004	19.59	30.51	0.002	0.59	0.16	0.0017
  4	0.061	0.40	0.62	0.010	0.0005	19.05	30.50	0.002	0.39	0.56	0.0020
  5	0.072	0.15	0.71	0.012	0.0005	19.51	30.49	0.002	0.50	0.35	0.0014
  6	0.061	0.17	0.60	0.011	0.0004	19.39	30.44	0.002	0.57	0.54	0.0015
  7	0.068	0.18	0.60	0.012	0.0006	20.52	30.51	0.002	0.48	0.46	0.0015
  8	0.069	0.20	1.40	0.012	0.0004	19.22	30.34	0.002	0.58	0.18	0.0013
  9	0.071	0.24	0.68	0.011	0.0005	19.78	30.33	0.001	0.39	0.52	0.0020
  10	0.077	0.41	1.12	0.012	0.0005	19.25	30.48	0.002	0.38	0.53	0.0017
  11	0.073	0.29	0.59	0.012	0.0004	19.81	30.54	0.002	0.37	0.55	0.0018
  12	0.070	0.18	0.60	0.012	0.0005	19.37	30.35	0.002	0.43	0.57	0.0019
  13	0.072	0.62	1.22	0.011	0.0003	20.58	30.69	0.002	0.49	0.45	0.0015
  14	0.052	0.19	1.12	0.013	0.0004	19.91	30.45	0.002	0.42	0.34	0.0003
  15	0.086	0.13	1.21	0.010	0.0003	21.91	34.65	0.001	0.46	0.16	0.0001
  16	0.067	0.29	1.20	0.011	0.0005	19.70	30.28	0.003	0.49	0.45	0.0004
  17	0.058	0.30	1.19	0.011	0.0006	19.26	30.28	0.002	0.53	0.37	0.0006
  18	0.072	0.18	1.01	0.012	0.0004	19.78	30.34	0.001	0.37	0.39	0.0005
  19	0.074	0.13	1.22	0.011	0.0004	19.77	30.27	0.002	0.44	0.33	0.0004
  20	0.069	0.25	0.78	0.010	0.0006	19.18	30.30	0.002	0.40	0.37	0.0001
  21	0.071	0.19	0.65	0.012	0.0005	19.66	30.38	0.001	0.41	0.41	0.0006
  22	0.071	0.28	1.20	0.011	0.0005	19.80	30.46	0.002	0.59	0.18	0.0003
  23	0.077	0.29	1.13	0.011	0.0005	19.82	30.45	0.002	0.46	0.30	0.0004
  24	0.073	0.23	0.67	0.010	0.0003	19.46	30.29	0.002	0.53	0.36	0.0004
  25	0.069	0.25	1.04	0.012	0.0004	19.18	30.44	0.002	0.42	0.34	0.0002
  26	0.058	0.22	1.18	0.011	0.0005	19.16	30.81	0.001	0.32	0.30	0.0006
  27	0.069	0.10	1.22	0.010	0.0003	20.54	30.54	0.002	0.49	0.47	0.0001
  28	0.070	0.27	0.58	0.011	0.0006	19.91	30.41	0.002	0.39	0.43	0.0002
  29	0.074	0.19	0.66	0.010	0.0004	19.89	30.35	0.001	0.40	0.38	0.0003
  30	0.053	0.15	1.12	0.013	0.0004	19.81	30.38	0.002	0.43	0.31	0.0002
  31	0.059	0.11	1.22	0.012	0.0004	19.47	30.66	0.002	0.33	0.16	0.0001
  32	0.061	0.17	1.09	0.011	0.0005	19.39	30.43	0.001	0.31	0.16	0.0003
  33	0.056	0.10	1.30	0.013	0.0005	19.06	30.27	0.002	0.30	0.17	0.0014
  34	0.051	0.16	0.77	0.012	0.0004	19.64	30.41	0.002	0.38	0.15	0.0017
  35	0.052	0.46	1.20	0.012	0.0005	19.79	30.46	0.002	0.69	0.53	0.0001
  36	0.051	0.44	1.18	0.011	0.0004	19.58	30.38	0.001	0.68	0.54	0.0001
  37	0.065	0.53	1.40	0.011	0.0006	19.22	30.34	0.002	0.69	0.53	0.0013
  38	0.067	0.51	1.19	0.011	0.0003	19.19	30.43	0.002	0.68	0.55	0.0014
  39	0.055	0.46	1.36	0.013	0.0006	19.07	30.51	0.002	0.47	0.47	0.0003
  40	0.053	0.49	1.24	0.012	0.0005	19.11	30.38	0.004	0.46	0.48	0.0002
  41	0.061	0.16	0.61	0.013	0.0006	19.38	30.42	0.002	0.55	0.47	0.0019
  42	0.061	0.40	0.60	0.010	0.0004	19.52	30.45	0.001	0.39	0.56	0.0021
  43	0.057	0.09	1.19	0.011	0.0005	20.45	30.47	0.001	0.45	0.43	0.0015
  44	0.060	0.17	0.60	0.010	0.0005	19.25	30.23	0.001	0.53	0.46	0.0005
  45	0.068	0.40	1.20	0.013	0.0005	20.47	30.42	0.003	0.48	0.46	0.0001

[Table 1B]
• TABLE 1B

  Test Number	Chemical Composition (unit is mass%; balance is Fe and impurities)
  	Nb	Mo	Ca	Ta	v	Zr	Hf	Cu	w	Co	REM	Mg
  1	0.0700	0.14	0.0022	-	-	-	-	-	-	-	-	-
  2	0.0700	0.07	0.0019	-	-	-	-	-	-	-	-	-
  3	0.0060	0.11	0.0024	-	-	-	-	-	-	-	-	-
  4	0.1800	0.40	0.0021	-	-	-	-	-	-	-	-	-
  5	0.1700	0.28	0.0017	-	-	-	-	-	-	-	-	-
  6	0.2200	0.39	0.0019	0.01	-	-	-	-	-	-	-	-
  7	0.1300	0.09	0.0021	-	0.08	-	-	-	0.01	-	-	0.0018
  8	0.1400	0.08	0.0021	-	0.12	-	-	-	-	-	-	-
  9	0.1600	0.11	0.0019	-	-	0.002	-	-	-	-	-	-
  10	0.1900	0.08	0.0022	-	-	-	0.01	-	-	-	-	-
  11	0.1700	0.13	0.0020	-	-	-	-	0.16	-	-	-	-
  12	0.2000	0.02	0.0020	-	-	-	-	-	-	-	0.0150	0.0017
  13	0.1600	0.08	0.0024	-	-	-	-	-	-	-	-	-
  14	0.1100	0.04	0.0022	-	-	-	-	-	-	-	-	-
  15	0.0050	0.08	0.0022	-	-	-	-	0.12	-	-	-	-
  16	0.0030	0.08	0.0017	-	-	-	-	-	-	-	-	-
  17	0.0040	0.02	0.0025	-	-	-	-	-	-	-	-	-
  18	0.0020	0.03	0.0015	0.04	-	-	-	-	-	-	-	-
  19	0.0040	0.07	0.0019	-	0.18	-	-	-	-	-	-	-
  20	0.0050	0.08	0.0018	-	-	0.001	-	-	-	-	-	-
  21	0.0050	0.06	0.0023	-	-	-	0.01	-	-	-	-	-
  22	0.0030	0.08	0.0022	-	-	-	-	0.26	-	-	-	-
  23	0.0020	0.04	0.0018	-	-	-	-	-	0.22	-	-	-
  24	0.0040	0.05	0.0025	-	-	-	-	-	-	0.34	-	-
  25	0.0050	0.05	0.0016	-	-	-	-	-	-	-	0.0017	-
  26	0.0100	0.02	0.0019	-	-	-	-	-	-	-	-	0.0018
  27	0.0030	0.08	0.0018	-	-	-	-	-	-	-	-	0.0013
  28	0.0030	0.04	0.0020	-	0.08	-	-	0.29	-	-	-	-
  29	0.0060	0.08	0.0024	-	0.13	-	-	-	-	0.28	-	0.0005
  30	0.1400	0.03	0.0019	-	-	-	-	-	-	-	-	-
  31	0.0040	0.12	0.0027	-	-	-	-	-	-	-	-	-
  32	0.0020	0.14	0.0021	-	-	-	-	0.26	-	0.23	-	-
  33	0.1500	0.10	0.0017	-	-	-	-	-	-	-	-	-
  34	0.1200	0.08	0.0020	-	0.17	-	-	-	0.09	-	-	-
  35	0.0600	0.01	0.0016	-	-	-	-	-	-	-	-	-
  36	0.0500	0.01	0.0019	-	0.09	-	-	0.08	-	0.17	-	-
  37	0.0700	0.36	0.0015	-	-	-	-	-	-	-	-	-
  38	0.0700	0.37	0.0018	-	-	-	-	0.08	-	0.26	-	-
  39	0.0030	0.18	0.0023	-	-	-	-	-	-	-	-	-
  40	0.0020	0.14	0.0019	-	-	-	-	0.22	-	0.35	-	0.0003
  41	0.1000	0.39	0.0025	-	-	-	-	-	-	-	-	-
  42	0.1000	0.40	0.0018	-	-	-	-	-	-	-	-	-
  43	0.0050	0.08	0.0018	-	-	-	-	-	-	-	-	0.0023
  44	0.0990	0.02	0.0028	-	-	-	-	-	-	-	-	-
  45	0.0040	0.22	0.0019	-	-	-	-	-	-	-	-	0.0022

• The symbol "-" in Table 1B means that the content of the corresponding element was at the level of an impurity or less.
• A Gleeble test specimen that is described later was taken from each of the produced ingots. After the Gleeble test specimen was taken from the ingot, the ingot was subjected to hot forging to produce a starting material (alloy plate) having a thickness of 30 mm. The heating temperature of the ingot in the hot forging was 1100 to 1300°C. The produced starting material was subjected to a hot working process. Specifically, the starting material was heated in a heating furnace. The heating temperature in the hot working process was 1200°C. After being heated, the starting material was subjected to hot rolling to produce an intermediate alloy (alloy plate) having a thickness of 15 mm.
• The intermediate alloy was subjected to a heat treatment process. The heat treatment temperature T1 (°C) in the heat treatment process was as shown in the column "T1 (°C)" in Table 2. The holding time at the heat treatment temperature was 30 minutes. After the holding time elapsed, the intermediate alloy was water-cooled to normal temperature. An alloy (alloy plate) of each test number was produced by the above process.
[Table 2]
• TABLE 2

  Test Number	Type of B content	F1	F2	F3	T1 (°C)	FA	T1≤FA
  1	H	0.83	0.96	-	1300	1272	F
  2	H	0.67	0.31	-	1200	1272	T
  3	H	0.75	1.29	-	1250	1559	T
  4	H	0.95	1.21	-	1200	1410	T
  5	H	0.85	0.11	-	1200	1365	T
  6	H	1.11	0.76	-	1200	1659	T
  7	H	0.94	0.70	-	1200	1249	T
  8	H	0.76	-0.39	-	1200	1268	T
  9	H	0.91	0.50	-	1200	1326	T
  10	H	0.91	-0.38	-	1200	1463	T
  11	H	0.92	0.33	-	1200	1365	T
  12	H	1.00	0.07	-	1200	1521	T
  13	H	0.94	-0.31	-	1200	1326	T
  14	L	0.76	0.83	7.59	1250	1230	F
  15	L	0.62	0.25	7.67	1250	1566	T
  16	L	0.94	1.84	8.09	1200	1579	T
  17	L	0.90	1.87	6.76	1200	1573	T
  18	L	0.76	1.50	7.68	1200	1586	T
  19	L	0.77	1.32	7.68	1200	1573	T
  20	L	0.77	1.46	7.78	1200	1566	T
  21	L	0.82	1.64	7.93	1200	1566	T
  22	L	0.77	0.85	6.94	1200	1579	T
  23	L	0.76	0.91	7.74	1200	1586	T
  24	L	0.89	1.31	7.79	1200	1573	T
  25	L	0.76	1.33	7.46	1200	1566	T
  26	L	0.62	1.67	6.38	1200	1533	T
  27	L	0.96	1.93	8.08	1200	1579	T
  28	L	0.82	1.54	7.94	1200	1579	T
  29	L	0.78	1.38	8.10	1200	1559	T
  30	L	0.74	0.34	7.80	1200	1268	T
  31	L	0.49	1.47	6.04	1200	-	-
  32	L	0.47	1.44	6.34	1200	-	-
  33	H	0.47	0.16	-	1200	-	-
  34	H	0.53	0.64	-	1200	-	-
  35	L	1.22	1.62	8.10	1200	-	-
  36	L	1.22	1.83	7.92	1200	-	-
  37	H	1.22	1.89	-	1200	-	-
  38	H	1.23	1.94	-	1200	-	-
  39	L	0.94	2.39	8.16	1200	-	-
  40	L	0.94	2.38	7.88	1200	-	-
  41	H	1.02	2.09	-	1200	-	-
  42	H	0.95	2.19	-	1200	-	-
  43	H	0.88	2.63	-	1200	-	-
  44	L	0.99	1.02	8.47	1200	-	-
  45	L	0.94	1.91	9.13	1200	-	-

• Note that, "H" in the column "Type of B content" in Table 2 indicates that the content of B in the alloy was more than 0.0010 to 0.0030%. "L" indicates that the content of B in the alloy was 0.0001 to 0.0010%. The F1 value, F2 value, and F3 value of each test number are shown in the column "F1", column "F2", and column "F3" respectively. The FA value of each test number is shown in the column "FA". In the column "T1 ≤ FA", "T (True)" is shown in a case where T1 satisfies Formula (X), and "F (False)" is shown in a case where T1 does not satisfy Formula (X).
[Evaluation tests]
• The following evaluation tests were performed using the produced alloys.
• (Test 1) Test to measure content of Ti [Ti] and content of Nb [Nb] in residue of alloy
• (Test 2) Gleeble test (hot workability evaluation)
• (Test 3) Weld hot cracking resistance evaluation test
• (Test 4) Creep strength evaluation test
• Hereunder, each test is described.
[(Test 1) Test to measure content of Ti [Ti] and content of Nb [Nb] in residue of alloy]
• The content of Ti [Ti] (mass%) and the content of Nb [Nb] (mass%) in a residue were determined based on the method described above in the section [Method for measuring content of Ti [Ti] and content of Nb [Nb] in residue]. Note that, the test specimen was taken in a manner so that the center of a cross section perpendicular to the longitudinal direction of the test specimen was the center position of the plate width and the center position of the plate thickness of the alloy (alloy plate), and the longitudinal direction of the test specimen coincided with the longitudinal direction of the alloy (alloy plate). The size of the test specimen was 10 mm × 30 mm × the plate thickness (15 mm). The determined content of Ti [Ti] (mass%) and content of Nb [Nb] (mass%) are shown in the column "[Ti] (mass%)" and the column "[Nb] (mass%)" in Table 3. Further, the 0.3 × [Ti] value and the [Ti]+[Nb] value are shown in the column "0.3 × [Ti]" and the column "[Ti]+[Nb]" in Table 3.
[Table 3]
• TABLE 3

  Test Number	Type of B content	[Ti] (mass%)	[Nb] (mass%)	0.3×[Ti]	[Ti]+[Nb]	Requirement I	Requirement II	Requirement III	Requirement IV	Gleeble test	Weld Hot Cracking Resistance	Creep Strength	Remarks
  1	H	0.029	0.001	0.009	0.030			-	-	E	G	E	Inventive Example
  2	H	0.019	0.000	0.006	0.019	T		-	-	E	E	E	Inventive Example
  3	H	0.015	0.000	0.005	0.015	T		-	-	E	E	E	Inventive Example
  4	H	0.073	0.033	0.022	0.106		T	-	-	E	E	E	Inventive Example
  5	H	0.052	0.033	0.016	0.085		T	-	-	E	E	E	Inventive Example
  6	H	0.061	0.033	0.018	0.094		T	-	-	E	E	E	Inventive Example
  7	H	0.036	0.025	0.011	0.061		T	-	-	E	E	E	Inventive Example
  8	H	0.028	0.024	0.008	0.052		T	-	-	E	E	E	Inventive Example
  9	H	0.074	0.033	0.022	0.107		T	-	-	E	E	E	Inventive Example
  10	H	0.106	0.051	0.032	0.157		T	-	-	E	E	E	Inventive Example
  11	H	0.076	0.034	0.023	0.110		T	-	-	E	E	E	Inventive Example
  12	H	0.083	0.023	0.025	0.106		T	-	-	E	E	E	Inventive Example
  13	H	0.066	0.016	0.020	0.082		T	-	-	E	E	E	Inventive Example
  14	L	0.043	0.004	0.013	0.047	-	-			E	G	E	Inventive Example
  15	L	0.000	0.000	0.000	0.000	-	-	T		E	E	E	Inventive Example
  16	L	0.026	0.001	0.008	0.027	-	-	T		E	E	E	Inventive Example
  17	L	0.030	0.001	0.009	0.031	-	-	T		E	E	E	Inventive Example
  18	L	0.028	0.000	0.008	0.028	-	-	T		E	E	E	Inventive Example
  19	L	0.014	0.000	0.004	0.014	-	-	T		E	E	E	Inventive Example
  20	L	0.023	0.001	0.007	0.024	-	-	T		E	E	E	Inventive Example
  21	L	0.028	0.001	0.008	0.029	-	-	T		E	E	E	Inventive Example
  22	L	0.000	0.000	0.000	0.000	-	-	T		E	E	E	Inventive Example
  23	L	0.003	0.002	0.001	0.005	-	-	T		E	E	E	Inventive Example
  24	L	0.021	0.000	0.006	0.021	-	-	T		E	E	E	Inventive Example
  25	L	0.026	0.001	0.008	0.027	-	-	T		E	E	E	Inventive Example
  26	L	0.025	0.002	0.008	0.027	-	-	T		E	E	E	Inventive Example
  27	L	0.008	0.000	0.002	0.008	-	-	T		E	E	E	Inventive Example
  28	L	0.020	0.000	0.006	0.020	-	-	T		E	E	E	Inventive Example
  29	L	0.014	0.000	0.004	0.014	-	-	T		E	E	E	Inventive Example
  30	L	0.045	0.029	0.014	0.074	-	-		T	E	E	E	Inventive Example
  31	L	0.000	0.000	0.000	0.000	-	-	-	-	E	E	B	Comparative Example
  32	L	0.000	0.000	0.000	0.000	-	-	-	-	E	E	B	Comparative Example
  33	H	0.029	0.028	0.009	0.057	-	-	-	-	E	E	B	Comparative Example
  34	H	0.005	0.004	0.002	0.009	-	-	-	-	E	E	B	Comparative Example
  35	L	0.033	0.007	0.010	0.040	-	-	-	-	E	B	E	Comparative Example
  36	L	0.037	0.006	0.011	0.043	-	-	-	-	E	B	E	Comparative Example
  37	H	0.061	0.013	0.018	0.074	-	-	-	-	E	B	E	Comparative Example
  38	H	0.063	0.013	0.019	0.076	-	-	-	-	E	B	E	Comparative Example
  39	L	0.043	0.001	0.013	0.044	-	-	-	-	E	B	E	Comparative Example
  40	L	0.040	0.000	0.012	0.040	-	-	-	-	E	B	E	Comparative Example
  41	H	0.045	0.014	0.014	0.059	-	-	-	-	E	B	E	Comparative Example
  42	H	0.052	0.014	0.016	0.066	-	-	-	-	E	B	E	Comparative Example
  43	H	0.022	0.000	0.007	0.022	-	-	-	-	E	B	E	Comparative Example
  44	L	0.057	0.019	0.017	0.076	-	-	-	-	B	E	E	Comparative Example
  45	L	0.002	0.000	0.001	0.002	-	-	-	-	B	E	E	Comparative Example

• Note that, "H" in the column "Type of B content" in Table 3 indicates that the content of B in the alloy was more than 0.0010 to 0.0030%. "L" indicates that the content of B in the alloy was 0.0001 to 0.0010%. In the column "Requirement I", "T (True)" is shown in a case where the content of B in the alloy was more than 0.0010 to 0.0030% and requirement I was satisfied. In the column "Requirement II", "T (True)" is shown in a case where the content of B in the alloy was more than 0.0010 to 0.0030% and requirement II was satisfied. In the column "Requirement III", "T (True)" is shown in a case where the content of B in the alloy was 0.0001 to 0.0010% and requirement III was satisfied. In the column "Requirement IV", "T (True)" is shown in a case where the content of B in the alloy was 0.0001 to 0.0010% and requirement IV was satisfied.
[(Test 2) Gleeble test (hot workability evaluation)]
• In order to evaluate the hot workability of the alloy of each test number, the ruptured reduction of area at 900°C was measured by a Gleeble test machine.
• A Gleeble test specimen was taken from an R/2 position of an ingot (circular cylinder with an outer diameter of 120 mm) of each test number. Here, the term "R/2 position" means the center position of the radius in a cross section perpendicular to the axial direction of the ingot. The Gleeble test specimen was a cylindrical test specimen with a radius of 10 mm and a length of 120 mm. The central axis of the Gleeble test specimen was parallel with the axial direction of the ingot. Using a Gleeble test machine, the Gleeble test specimen was heated from room temperature to 1200°C in 60 seconds, and then held at 1200°C for 300 seconds. Thereafter, the Gleeble test specimen was cooled to 900°C at a cooling rate of 100°C/min using He gas, and held at 900°C for 10 seconds. After the holding time elapsed, a tensile test was carried out on the Gleeble test specimen at a displacement rate of 10 mm/sec to cause the Gleeble test specimen to rupture. The dimensions of the cross section of the Gleeble test specimen after rupture were measured, and the reduction of area (%) was calculated.
• If the reduction of area was 60% or more, the relevant test number was evaluated as being excellent in hot workability (indicated by "E (Excellent)" in the column "Gleeble test" in Table 3). On the other hand, if the reduction of area was less than 60%, it was evaluated that the hot workability of the relevant test number was low (indicated by "B (Bad)" in the column "Gleeble test" in Table 3).
[(Test 3) Weld hot cracking resistance evaluation test]
• From the alloy (alloy plate) of each test number, two test specimens that each had a plate thickness of 12 mm, a plate width of 40 mm, and a length of 300 mm were taken, which were each centered on the central position of the plate width and the central position of the plate thickness of the alloy (alloy plate). The two test specimens that were taken were subjected to a longitudinal Varestraint test described below.
• Specifically, melt run TIG welding (bead-on welding) was performed in the longitudinal direction at the central position of the plate width of each test specimen under welding conditions of a welding current of 200 A, a voltage of 12 V, and a speed of 15 cm/min. In the course of performing the TIG welding, bending stress was momentarily applied in parallel to the welding direction so that strain of 2% was applied to the outer layer.
• A portion including a place where a weld crack occurred due to application of the bending stress was cut out in a size that could be observed with an optical microscope. Regarding the size of the cut sample, the sample had a plate thickness of 12 mm, a plate width of 30 mm, and a length of 30 mm.
• Scale on the surface of the weld zone of the cut sample was removed by buffing. Thereafter, using an optical microscope at a magnification of ×100, whether or not a crack was present in the HAZ was determined, and in a case where a crack had occurred, the length of the crack was measured. Specifically, taking the boundary between the weld metal and the HAZ as the starting point, the length of a crack that propagated in a direction perpendicular to the welding direction (the length in the direction perpendicular to the welding direction) was measured. The lengths in the direction perpendicular to the welding direction of all cracks that had occurred in the test specimen were determined. The total of the lengths of those cracks was defined as the total crack length (mm). The total crack length was determined for two test specimens, respectively. The arithmetic average value of the determined total crack lengths was defined as the averaged total crack length.
• The averaged total crack length was evaluated as follows.
• Evaluation E: Averaged total crack length is 1.5 mm or less.
• Evaluation G (Good): Averaged total crack length is more than 1.5 mm to 2.0 mm or less.
• Evaluation B: Averaged total crack length is more than 2.0 mm.
• In the case of evaluation G or evaluation E, it was determined that the relevant test number was excellent in weld hot cracking resistance (indicated by "G" or "E" in the column "Weld Hot Cracking Resistance" in Table 3). On the other hand, in the case of evaluation B, it was determined that sufficient weld hot cracking resistance was not obtained (indicated by "B" in the column "Weld Hot Cracking Resistance" in Table 3).
[(Test 4) Creep strength evaluation test]
• The following creep strength evaluation test was performed on the alloy (alloy plate) of each test number.
• A creep rupture test specimen in accordance with JIS Z2271: 2019 was taken from a position located at the center position of the plate width and the center position of the plate thickness of the alloy (alloy plate) of each test number. The cross section perpendicular to the axial direction of the parallel portion of the creep rupture test specimen was circular. The parallel portion had an outer diameter of 6 mm and a length of 30 mm. The longitudinal direction of the creep rupture test specimen was parallel to the rolling direction of the alloy plate.
• A creep rupture test conforming to JIS Z2271: 2019 was carried out using the prepared creep rupture test specimen. Specifically, the creep rupture test specimen was heated to 700°C. Thereafter, the creep rupture test was carried out. The test stress was set to 80 MPa. In the test, the creep rupture time (hours) was determined.
• If the creep rupture time was 2000 hours or more, the relevant test number was evaluated as being excellent in creep strength (indicated by "E" in the column "Creep Strength" in Table 3). On the other hand, if the creep rupture time was less than 2000 hours, it was evaluated that the creep strength of the relevant test number was low (indicated by "B" in the column "Creep Strength" in Table 3).
[Test results]
• Referring to Table 1A, Table 1B, Table 2, and Table 3, in Test Nos. 1 to 30, in cases where the content of B was more than 0.0010 to 0.0030%, the alloys satisfied Feature 1 to Feature 3, and in cases where the content of B was 0.0001 to 0.0010%, the alloys satisfied Feature 1 to Feature 4. Therefore, in a high temperature environment, sufficient creep strength was obtained. In addition, excellent weld hot cracking resistance and excellent hot workability were obtained.
• Among Test Nos. 1 to 13 in which the content of B was more than 0.0010 to 0.0030%, Test Nos. 2 and 3 also satisfied requirement I of Feature 5, and Test Nos. 4 to 13 also satisfied requirement II of Feature 5. Therefore, the weld hot cracking resistance was even more excellent than in Test No. 1 which did not satisfy Feature 5.
• Among Test Nos. 14 to 30 in which the content of B was 0.0001 to 0.0010%, Test Nos. 15 to 29 satisfied requirement III of Feature 6, and Test No. 30 satisfied requirement IV of Feature 6. Therefore, the weld hot cracking resistance was even more excellent than in Test No. 14 which did not satisfy Feature 6.
• On the other hand, in Test Nos. 31 to 34, although the content of each element in the chemical composition was appropriate, F1 was too low. Therefore, sufficient creep strength was not obtained.
• In Test Nos. 35 to 38, although the content of each element in the chemical composition was appropriate, F1 was too high. Therefore, sufficient weld hot cracking resistance was not obtained.
• In Test Nos. 39 to 43, although the content of each element in the chemical composition was appropriate, F2 was too high. Therefore, sufficient weld hot cracking resistance was not obtained.
• In Test Nos. 44 and 45, although the content of each element in the chemical composition was appropriate, the content of B was 0.0010% or less, and F3 was too high. Therefore, sufficient hot workability was not obtained.
• An embodiment of the present invention has been described above. However, the embodiment described above is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above embodiment, and can be implemented by appropriately modifying the above embodiment within a range that does not depart from the gist of the present invention.

Claims (4)

1. An alloy having a chemical composition consisting of, in mass%,

   C: 0.050 to 0.100%,

   Si: 1.00% or less,

   Mn: 1.50% or less,

   P: 0.035% or less,

   S: 0.0015% or less,

   Cr: 19.00 to 23.00%,

   Ni: 30.00 to 35.00%,

   N: 0.010% or less,

   Al: 0.15 to 0.70%,

   Ti: 0.15 to 0.70%,

   B: 0.0001 to 0.0030%,

   Nb: 0.0010 to 0.5000%,

   Mo: 0.01 to 1.00%,

   Ca: 0.0001 to 0.0200%,

   Ta: 0 to 0.50%,

   V: 0 to 1.00%,

   Zr: 0 to 0.100%,

   Hf: 0 to 0.10%,

   Cu: 0 to 1.00%,

   W: 0 to 1.00%,

   Co: 0 to 1.00%,

   rare earth metal: 0 to 0.1000%, and

   Mg: 0 to 0.0200%,

   with the balance being Fe and impurities, and

   satisfying Formula (1) and Formula (2),

   wherein:

   when a content of B in the chemical composition is 0.0010% or less, the chemical composition further satisfies Formula (3): $$0.60<\mathrm{Al}+\mathrm{Ti}<1.20$$ $$3.3-41\mathrm{C}-\mathrm{Si}+2\mathrm{Mo}+3\mathrm{Ti}+245\mathrm{B}-12\mathrm{Nb}\le 2.00$$ $$0.4+67\mathrm{C}+1.3\mathrm{Si}+5.5\mathrm{Mo}+5.2\mathrm{Ti}+13.4\mathrm{Nb}\le 8.25$$

   where, a content of a corresponding element in the chemical composition is substituted in percent by mass for each symbol of an element in Formula (1) to Formula (3).
2. The alloy according to claim 1, wherein the chemical composition comprises one or more types of element selected from a group consisting of, in mass%:

   Ta: 0.01 to 0.50%,

   V: 0.01 to 1.00%,

   Zr: 0.001 to 0.100%,

   Hf: 0.01 to 0.10%,

   Cu: 0.01 to 1.00%,

   W: 0.01 to 1.00%,

   Co: 0.01 to 1.00%,

   rare earth metal: 0.0001 to 0.1000%, and

   Mg: 0.0001 to 0.0200%.
3. The alloy according to claim 1 or claim 2, wherein:

   the chemical composition comprises, in mass%,

   B: more than 0.0010 to 0.0030%; and

   a content of Ti [Ti] in percent by mass in a residue obtained by an extraction residue method is less than 0.020%, or

   the [Ti] is 0.020% or more and a content of Nb [Nb] in percent by mass in the residue is 0.015% or more, and the [Ti] and the [Nb] satisfy Formula (4). $$\left[\mathrm{Ti}\right]+\left[\mathrm{Nb}\right]\ge 0.050$$
4. The alloy according to claim 1 or claim 2, wherein:
   the chemical composition comprises, in mass%:

   B: 0.0001 to 0.0010%; and

   a content of Ti [Ti] in percent by mass in a residue obtained by an extraction residue method is 0.031% or less, or

   the [Ti] is more than 0.031%, and a content of Nb [Nb] in percent by mass in the residue is 0.3 × [Ti]% or more.