JP6772736B2 - Ni-based heat-resistant alloy - Google Patents

Description

The present invention relates to a Ni-based heat resistant alloy.

In recent years, energy conservation, effective use of resources, and reduction of CO ₂ gas emissions for environmental conservation have become one of the solutions to energy problems, and have become important industrial policies. In the case of a power generation boiler that burns fossil fuels, a reactor for the chemical industry, etc., a highly efficient ultra-supercritical pressure boiler or reactor is advantageous. Along with this, new ultra-supercritical boilers with increased steam temperature and pressure are being installed all over the world to improve efficiency. Specifically, it is planned to raise the steam temperature, which was around 600 ° C until now, to 650 ° C or higher, and further to 700 ° C or higher.

Due to the high temperature and high pressure of steam, the heater tube of the boiler, the reaction furnace tube for the chemical industry, and the thick plate and forged products as heat and pressure resistant members should be raised to a temperature of 700 ° C or higher during actual operation. It becomes. Therefore, a material that is used for a long time in such a harsh environment is required to have not only high temperature strength and high temperature corrosion resistance, but also long-term metal structure stability, and good creep rupture ductility and creep fatigue resistance. To.

Further, in maintenance such as repair after long-term use, it is necessary to perform operations such as cutting, processing, and welding on a material that has changed over a long period of time. Therefore, the heat-resistant material is strongly required to have not only the characteristics as a new material but also the soundness as an aged material.

Patent Documents 1 to 8 disclose Ni-based alloys used in the harsh high-temperature environment as described above. In these documents, Mo and / or W is contained to enhance solid solution, and Al and Ti are contained to form an intermetallic compound, the γ'phase, specifically, Ni ₃ (Al, Ti). Utilizes precipitation strengthening. Of these, since the Ni-based alloys disclosed in Patent Documents 4 to 6 contain 28% or more of Cr, a large amount of α-Cr phase having a bcc structure is also precipitated.

Patent Document 9 discloses that the creep strength is improved by adjusting the matching strain of the Ni-based single crystal superalloy.

Patent Document 10 discloses an austenitic steel in which high-temperature strength is increased by containing a large amount of additive elements such as Mn and Cr.

Patent Document 11 discloses a Ni-based alloy containing a predetermined amount of Mo and W and further containing a predetermined amount of Nd and B. The document further describes that limiting the total content of Sb, Zn, and As improves hot workability and creep rupture strength at high temperatures.

Patent Document 12 discloses a Ni-based heat-resistant alloy in which the grain boundaries are reinforced with carbides or borides while defining the balance of the contents of Al, Ti, and Nb constituting the γ'phase that reinforces the inside of the grains. ing.

Patent Document 13 discloses a Ni-based heat-resistant alloy that defines the volume fraction of the gamma prime (γ') phase of the parent phase and the balance of the contents of Nb and Mo.

Japanese Unexamined Patent Publication No. 51-84726 Japanese Unexamined Patent Publication No. 51-84727 Japanese Unexamined Patent Publication No. 7-150277 Japanese Unexamined Patent Publication No. 7-216511 Japanese Unexamined Patent Publication No. 8-127748 Japanese Unexamined Patent Publication No. 8-218140 Japanese Unexamined Patent Publication No. 9-157779 Special Table 2002-518599 Japanese Unexamined Patent Publication No. 2003-49231 Japanese Unexamined Patent Publication No. 61-179834 International Publication No. 2010/0388826 Japanese Unexamined Patent Publication No. 2013-216939 Japanese Unexamined Patent Publication No. 2016-56436

In the Ni-based alloys disclosed in Patent Documents 1 to 8, the γ'phase and the α-Cr phase are precipitated. Therefore, the creep rupture ductility at high temperatures is lower than that of conventional austenitic steels and the like, and in particular, when used for a long period of time, aging occurs and the ductility is significantly reduced as compared with new materials.

The Ni-based alloy disclosed in Patent Document 9 is a single crystal alloy and cannot be used in applications where ductility and workability are required in structures such as steel pipe materials. The austenitic steel disclosed in Patent Document 10 cannot obtain high breaking strength when the Ni content is 50% or more. The Ni-based alloy disclosed in Patent Document 11 may have low creep strength and creep rupture ductility after long-term use at high temperature.

Further, in the Ni-based heat-resistant alloy disclosed in Patent Document 12, since the grain boundaries are reinforced with carbides or borides, the strength may decrease in a high temperature environment of 800 ° C. or higher. Although the Ni-based heat-resistant alloy disclosed in Patent Document 13 has excellent creep strength and creep rupture ductility, there is still room for further improvement in terms of long-term stability of the metal structure.

An object of the present invention is to solve the above problems and to provide a Ni-based heat-resistant alloy having excellent creep strength and creep rupture ductility.

The present invention has been made to solve the above problems, and the following Ni-based heat-resistant alloys are the gist of the present invention.

(1) The chemical composition is mass%
C: less than 0.01%,
Si: 1.0% or less,
Mn: 1.0% or less,
Cr: 15.0% or more and less than 28.0%,
Al: More than 0.5% and less than 2.5%,
Ti: 0.1 to 2.0%,
Nb: 0.1 to 2.0%,
B: 0.0005-0.01%,
Mo: More than 0% and less than 7.0%,
W: More than 0% and less than 14.0%,
Fe: less than 0.1%,
Co: 0-25.0%,
Zr: 0-0.2%,
V: 0-1.5%,
Hf: 0-1.0%,
Mg: 0-0.05%,
Ca: 0-0.05%,
Nd: 0-0.5%,
Y: 0-0.5%,
La: 0-0.5%,
Ce: 0-0.5%,
Ta: 0-8.0%,
Re: 0-8.0%,
Remaining : Ni and impurities,
Satisfy the following equations (i) to (iv),
Ni-based heat-resistant alloy.
6.0 ≤ Mo + 0.5 W ≤ 7.0 ... (i)
0.3 ≤ Al / (Al + Ti + Nb) ≤ 0.6 ... (ii)
0.2 ≤ Ti / (Al + Ti + Nb) ≤ 0.5 ... (iii)
0 <Nb / (Al + Ti + Nb) ≤ 0.5 ... (iv)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the alloy, and if it is not contained, it is set to zero.

(2) The chemical composition is mass%.
Co: More than 5.0% and less than 25.0%,
Zr: 0.005-0.2%,
V: 0.02-1.5%,
Hf: 0.005-1.0%,
Mg: 0.0008 to 0.05%,
Ca: 0.0008-0.05%,
Nd: 0.001 to 0.5%,
Y: 0.001 to 0.5%,
La: 0.001-0.5%,
Ce: 0.001-0.5%,
Ta: 0.01-8.0%, and
Re: 0.01-8.0%,
Contains one or more selected from,
The Ni-based heat-resistant alloy according to (1) above.

(3) The volume fraction of the γ'phase of the parent phase is 25.0% or less.
Grain boundary coverage is 70.0% or more,
The Ni-based heat-resistant alloy according to (1) or (2) above.

(4) The volume fraction of the σ phase of the grain boundary is 10.0% or less.
The volume fraction of the Laves phase of the mother phase is 5.0% or less.
The Ni-based heat-resistant alloy according to any one of (1) to (3) above.

According to the present invention, a Ni-based heat-resistant alloy having excellent creep strength and creep rupture ductility can be obtained.

It is a figure for demonstrating the grain boundary coverage. It is a micrograph for comparing the initial structure and the fracture structure of the alloy of this invention and the conventional alloy.

Based on the Ni-based heat-resistant alloy disclosed in Patent Document 13, the present inventors have earnestly studied a method for further improving creep strength and creep rupture ductility, and obtained the following findings.

When a creep rupture test was carried out using a conventional alloy and the steel structure after rupture was observed, the Laves phase changed to the σ phase at the grain boundary, and a needle-shaped Laves phase was precipitated in the grain.

From this, the inventors have come up with the idea that it is necessary to improve the tissue stability in order to further improve the creep strength and the creep rupture ductility.

Therefore, a creep rupture test was conducted on alloys having various chemical compositions, and the ruptured structure was observed. As a result, the Fe content was reduced as much as possible and the Mo equivalent (= Mo + 0.5W) was optimized to achieve a semi-stable phase. It has been found that the Loves phase is stabilized and the formation of the σ phase is suppressed, and as a result, the creep strength and the creep rupture elongation can be significantly improved.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. 1. Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: Less than 0.01% Carbon (C) is considered to be an effective element for forming carbides and improving creep strength. However, in the present invention, high temperature creep strength is realized by an intermetallic compound which is more stable than carbides at high temperature for a long time. Therefore, when the C content is high, carbides are precipitated at the grain boundaries, so that the amount of intermetallic compounds precipitated at the grain boundaries is reduced and the grain boundary stability cannot be maintained at high temperatures for a long time. Further, when the C content is increased, carbides are excessively precipitated and mechanical properties such as toughness are deteriorated. Furthermore, the weldability is also reduced. Therefore, the C content is set to less than 0.01%.

Si: 1.0% or less Silicon (Si) deoxidizes the alloy. However, when the Si content becomes excessive, the weldability and hot workability deteriorate. Therefore, the Si content is 1.0% or less. The Si content is preferably less than 1.0%, more preferably 0.8% or less, and even more preferably 0.5% or less. When the deoxidizing action is sufficiently secured by other elements, the lower limit of the Si content may not be set. When it is desired to stably obtain effects such as deoxidizing action, oxidation resistance and water vapor oxidation resistance, the Si content is preferably 0.01% or more, more preferably 0.02% or more. It is preferable, and more preferably 0.04% or more.

Mn: 1.0% or less Manganese (Mn) deoxidizes the alloy. Mn further fixes the impurity S as a sulfide to improve the hot workability of the alloy. On the other hand, when the Mn content is excessive, the formation of a spinel-type oxide film is promoted, and the oxidation resistance at high temperatures is lowered. Therefore, the Mn content is 1.0% or less. The Mn content is preferably less than 1.0%, more preferably 0.8% or less, and even more preferably 0.5% or less. When it is desired to stably obtain the effect of improving hot workability, the Mn content is preferably 0.01% or more, more preferably 0.02% or more, and 0.04% or more. It is more preferable to have.

Cr: 15.0% or more and less than 28.0% Chromium (Cr) enhances corrosion resistance such as oxidation resistance, water vapor oxidation resistance, and high temperature corrosion resistance of the alloy. Cr further combines with Nb to form an intermetallic compound and precipitates at the grain boundaries, improving the creep strength of the alloy. On the other hand, when the Cr content becomes excessive, the α-Cr phase and the σ phase are excessively precipitated and coarsened, so that creep voids are likely to be formed at the precipitate interface during long-term use. This reduces creep strength and creep rupture ductility, as well as hot workability. Therefore, the Cr content is 15.0% or more and less than 28.0%. From the viewpoint of the lower limit, the Cr content is preferably higher than 15.0%, more preferably 16.0% or more, and further preferably 18.0% or more. From the viewpoint of the upper limit, the Cr content is preferably 27.0% or less, and more preferably 26.0% or less.

Al: More than 0.5% and 2.5% or less Aluminum (Al) forms a γ'phase (Ni ₃ Al) and enhances creep strength. On the other hand, when the Al content becomes excessive, the precipitation temperature of the γ'phase rises, the volume fraction of the γ'phase at a high temperature increases, and the hot workability decreases. Therefore, the Al content is higher than 0.5% and less than 2.5%. From the viewpoint of the upper limit, the Al content is preferably less than 2.5%, more preferably 2.3% or less, and further preferably 2.2% or less. From the viewpoint of the lower limit, the Al content is preferably 0.6% or more, and more preferably 0.7% or more. In the present specification, the Al content is referred to as sol. It means the content of Al (acid-soluble Al).

Ti: 0.1 to 2.0%
Titanium (Ti) is an element that forms a γ'phase together with Al to increase the creep strength of the alloy. On the other hand, if the Ti content is excessive, the hot workability is lowered. Therefore, the Ti content is set to 0.1 to 2.0%. The Ti content is preferably 0.5% or more, more preferably 1.0% or more. The Ti content is preferably less than 2.0%, more preferably 1.8% or less, and even more preferably 1.7% or less.

Nb: 0.1 to 2.0%
Niobium (Nb) dissolves in the γ'phase to reinforce the γ'phase. Nb further solidifies and strengthens the matrix. Therefore, Nb increases the high temperature strength of the alloy. However, if the Nb content is too high, the γ'phase increases and the solid solution amount of Nb in the matrix (intragrain) also increases. Therefore, the inside of the grain is extremely strengthened. In addition, coarse Nb carbides are formed, weakening the structure. Therefore, the creep strength and ductility are lowered, and the hot workability is also lowered. Therefore, the Nb content is set to 0.1 to 2.0%. The Nb content is preferably higher than 0.1%, more preferably 0.2% or more, and even more preferably 0.5% or more. The Nb content is preferably less than 2.0%, more preferably 1.8% or less, and even more preferably 1.7% or less.

B: 0.0005-0.01%
Boron (B) strengthens grain boundaries and enhances the creep strength and creep rupture ductility of Ni-based heat-resistant alloys. On the other hand, when the B content becomes excessive, the weldability is lowered, and the creep strength and the creep rupture ductility are also lowered. Therefore, the B content is 0.0005 to 0.01%. From the viewpoint of the upper limit, the B content is preferably less than 0.01%, more preferably 0.009% or less, and further preferably 0.008% or less. From the viewpoint of the lower limit, the B content is preferably higher than 0.0005%, more preferably 0.001% or more, and further preferably 0.002% or more.

Mo: More than 0% and less than 7.0% Molybdenum (Mo) dissolves in the matrix phase and strengthens the solid solution to improve the creep strength of the alloy. As will be described later, in the present invention, the Mo equivalent must be within a predetermined range, but when the Mo content is 0%, the processability is deteriorated and the ductility is further lowered. On the other hand, if the Mo content is excessive, the hot workability is lowered. In addition, carbides or nitrides are formed to reduce the strength for a long time at high temperature at 800 ° C. or higher. Therefore, the Mo content is set to more than 0% and less than 7.0%. The Mo content is preferably 0.2% or more, and more preferably 0.5% or more. The Mo content is preferably 6.5% or less, more preferably 6.0% or less, and even more preferably 5.5% or less.

W: More than 0% and less than 14.0% Tungsten (W) dissolves in the matrix phase to improve the creep strength of the alloy by solid solution strengthening. In addition, the Laves phase is precipitated at the grain boundaries to improve the grain boundary strength. As will be described later, in the present invention, the Mo equivalent must be within a predetermined range, but when the W content is 0%, the Laves phase becomes unstable in thermal equilibrium and the grain boundary precipitation amount decreases. On the other hand, if the W content is excessive, the hot workability is lowered. Therefore, the W content is set to more than 0% and less than 14.0%. The W content is preferably 1.0% or more, and more preferably 2.0% or more. The W content is preferably 12.0% or less, and more preferably 10.0% or less.

Fe: Less than 0.1% Iron (Fe) is originally an element that enhances the hot workability of Ni-based heat-resistant alloys. However, in the present invention, by reducing the Fe content in the alloy, the Laves phase, which is a metastable phase, is stabilized and the formation of the σ phase is suppressed. Therefore, the Fe content is set to less than 0.1%.

In the chemical composition of the Ni-based heat-resistant alloy of the present invention, the balance is Ni and impurities. Here, the "impurity" is a component mixed by various factors of raw materials such as ore and scrap, and various factors in the manufacturing process when an alloy is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

The Ni-based heat-resistant alloy of the present invention may further contain one or more elements selected from Co, Zr, V, Hf, Mg, Ca, Nd, Y, La, Ce, Ta and Re. ..

Co: 0-25.0%
Cobalt (Co) is distributed into the γ phase and the γ'phase, and mainly acts as a solid solution strengthening element. Co further reduces the lattice constant by dissolving in γ'and lowers the matching lattice strain. Therefore, Co improves creep strength and creep rupture ductility. The amount of increase in elongation at break corresponds to the amount of decrease in matching lattice strain due to the inclusion of Co. Co further lowers the precipitation temperature of the σ phase, which is the embrittlement phase, and expands the two-phase region of γ + γ ′, which is excellent in the strength and ductility balance in the grain. Therefore, Co may be contained if necessary.

However, when the Co content becomes excessive, the effect of improving ductility is saturated. Further, the excess solid solution of Co significantly strengthens the matrix phase and lowers the hot workability. Therefore, the Co content is 25.0% or less. The Co content is preferably less than 25.0%, more preferably 22.0% or less, and even more preferably 20.0% or less. In order to stably obtain the above-mentioned effects, the Co content is preferably more than 5.0%, more preferably 7.0% or more, and further preferably 10.0% or more.

Zr: 0-0.2%
Zirconium (Zr) is distributed between the intragranular γ'phase and the grain boundaries, and stabilizes the intragranular γ'phase in the grain as well as Ti. At the grain boundaries, it acts as a grain boundary solid solution element in the same manner as B, and enhances the creep strength and creep rupture ductility of the Ni-based heat-resistant alloy. Therefore, Zr may be contained if necessary. However, when the Zr content becomes excessive, the inside of the grain is excessively strengthened by the Zr distributed in the grain, and the hot workability is lowered. Further, a part of Zr forms coarse (Zr, Nb) carbides as inclusions, and the creep strength of the alloy is lowered. Therefore, the Zr content is 0.2% or less. The Zr content is preferably less than 0.2%, more preferably 0.1% or less, and even more preferably 0.05% or less. In order to stably obtain the above-mentioned effects, the Zr content is preferably 0.005% or more, more preferably 0.01% or more, and further preferably 0.02% or more. preferable.

V: 0-1.5%
Since vanadium (V) is an element that forms a carbonitride or an intermetallic compound to increase creep strength, it may be contained as necessary. However, if the V content is excessive, ductility and toughness will decrease due to the occurrence of high temperature corrosion and the precipitation of the embrittled phase. Therefore, the V content is set to 1.5% or less. The V content is preferably less than 1.5%, more preferably 1.2% or less, and even more preferably 1.0% or less. In order to stably obtain the above-mentioned effects, the V content is preferably 0.02% or more, more preferably 0.04% or more, and further preferably 0.06% or more. preferable.

Hf: 0 to 1.0%
Since hafnium (Hf) is an element that mainly contributes to strengthening grain boundaries and enhances creep strength, it may be contained as necessary. However, when the Hf content becomes excessive, the hot workability and weldability deteriorate. Therefore, the Hf content is set to 1.0% or less. The Hf content is preferably less than 1.0%, more preferably 0.8% or less, and even more preferably 0.5% or less. In order to stably obtain the above-mentioned effects, the Hf content is preferably 0.005% or more, more preferably 0.008% or more, and further preferably 0.01% or more. preferable.

The above V and Hf may be contained alone or in combination of two. When V and Hf are contained, the preferable total content of these elements is 2.8% or less.

Mg: 0-0.05%
Ca: 0-0.05%
Since both magnesium (Mg) and calcium (Ca) are elements in which the impurity S is fixed as a sulfide to enhance hot workability, they may be contained as necessary. However, if the content of these elements is excessive, the cleanliness is lowered, and the hot workability and ductility are lowered. Therefore, both the Mg content and the Ca content are set to 0.05% or less. The Mg content and Ca content are preferably less than 0.05%, more preferably 0.02% or less, and even more preferably 0.01% or less. In order to stably obtain the above effects, at least one of the contents of Mg and Ca is preferably 0.0005% or more, more preferably 0.0008% or more, and 0.001% or more. Is more preferable.

Nd: 0-0.5%
Y: 0-0.5%
La: 0-0.5%
Ce: 0-0.5%
Neodymium (Nd), yttrium (Y), lanthanum (La) and cerium (Ce) all fix S as a sulfide to improve hot workability. These elements further enhance the adhesion of the Cr ₂ O ₃ protective film on the alloy surface, and in particular, enhance the oxidation resistance during repeated oxidation. These elements further enhance grain boundaries to increase creep strength and rupture strain. Therefore, these elements may be contained if necessary. However, when the content of these elements becomes excessive, inclusions such as oxides increase and hot workability and weldability deteriorate. Therefore, the contents of Nd, Y, La and Ce are all set to 0.5% or less. The content of each of these elements is preferably less than 0.5%, more preferably 0.3% or less, and even more preferably 0.15% or less. In order to stably obtain the above-mentioned effects, the content of any of these elements is preferably 0.001% or more, more preferably 0.002% or more, and 0.003% or more. Is more preferable.

The above-mentioned Mg, Ca, Nd, Y, La and Ce can be contained in only one kind or in combination of two or more kinds. The preferred total content of these elements is 0.94% or less.

Ta: 0-8.0%
Re: 0-8.0%
Both tantalum (Ta) and rhenium (Re) form a carbonitride and dissolve in the matrix phase to increase creep strength. These elements are further dissolved in the γ'phase to increase the high temperature strength. Therefore, these elements may be contained if necessary. However, excessive content of these elements reduces processability and mechanical properties. Therefore, the Ta content and the Re content are 8.0% or less, respectively. The Ta content and the Re content are each preferably less than 8.0%, more preferably 7.0% or less, and even more preferably 6.0% or less. In order to stably obtain the above-mentioned effects, at least one of the contents of Ta and Re is preferably 0.01% or more, more preferably 0.1% or more, and 0.5% or more. Is more preferable.

The above Ta and Re can be contained alone or in combination of two. The preferred total content of these elements is 14.0% or less.

6.0 ≤ Mo + 0.5 W ≤ 7.0 ... (i)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the alloy, and if it is not contained, it is set to zero.
In the above-mentioned component system, excellent creep strength and creep rupture ductility can be obtained by setting the Mo equivalent within the above range. Specifically, 800 ° C. obtained using Larson-Miller parameter method, the creep rupture time at 100MPa becomes ¹⁰⁵ hours or more, and 800 ° C., creep rupture ductility at 100MPa is more than 20%.

0.3 ≤ Al / (Al + Ti + Nb) ≤ 0.6 ... (ii)
0.2 ≤ Ti / (Al + Ti + Nb) ≤ 0.5 ... (iii)
0 <Nb / (Al + Ti + Nb) ≤ 0.5 ... (iv)
However, each element symbol in the above formula represents the content (mass%) of each element contained in the alloy, and if it is not contained, it is set to zero.
The above formulas (ii) to (iv) represent the ratio of the Al content, the Ti content and the Nb content to the total amount of the Al, Ti and Nb contents, respectively.

The Ni-based heat-resistant alloy according to the present invention satisfies the above formulas (ii) to (iv) in addition to having a chemical composition within the above range. When the above formulas (ii) to (iv) are satisfied, the balance between the individual solid solution effects of Ti and Nb and the strengthening of precipitation of the γ'phase is good. That is, in this case, excellent creep strength and creep rupture ductility can be obtained by a significant increase in strength due to precipitation strengthening and an increase in ductility and strength due to solid solution strengthening.

2. Structure The metal structure of the Ni-based heat-resistant alloy according to the present invention is not particularly limited, but it is preferable to satisfy the following conditions in order to stabilize the structure and secure excellent creep strength and creep rupture ductility. ..

Volume fraction of the γ'phase of the matrix: 25.0% or less The volume fraction of the γ'phase of the matrix (vol.%) Is the volume fraction of the γ'phase in the Ni grains in the matrix (γ phase). It means the volume fraction. Depending on the process, the γ'phase may also be precipitated at the grain boundaries, but in the present invention, only the γ'phase precipitated in the grains is targeted.

If the volume fraction of the γ'phase is too high, the matching lattice strain may increase and the in-grain ductility may decrease. Further, since the precipitation temperature of the γ'phase rises, the hot workability may decrease. Therefore, the volume fraction of the γ'phase is preferably 25.0% or less. By setting the volume fraction of the γ'phase within the above range, a balance between strength and ductility is maintained, and excellent high-temperature strength (creep strength), ductility and hot workability (break elongation) can be obtained. The lower limit of the volume fraction of the γ'phase of the parent phase is not particularly set, but if the volume fraction of the γ'phase is too low, the required creep strength may not be obtained. Therefore, the volume fraction of the γ'phase is preferably 10.0% or more.

The volume fraction of the γ'phase of the parent phase is measured by the following method. X-ray diffraction measurement is performed on the Ni-based heat-resistant alloy, and the volume fraction is analyzed by structural optimization by the X-ray Rietveld method using a two-phase model of a matrix phase (γ phase) and a γ'phase. Here, the γ'phase precipitated at the grain boundaries is in the form of a thin film and is sparse, so that it is not detected by X-ray diffraction measurement. That is, the γ'phase detected in the X-ray diffraction measurement is the γ'phase precipitated in the γ grains. Therefore, the volume fraction of the γ'phase can be obtained by analyzing the volume fraction by the above-mentioned X-ray diffraction method.

The volume fraction of the γ'phase can also be measured by image analysis using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). In this case, the area ratio of the γ'phase precipitated in the γ grain is obtained by image analysis, and the obtained area ratio is defined as the volume fraction of the γ'phase.

Grain boundary coverage: 70.0% or more The grain boundary coverage is the ratio (%) of the length of the grain boundaries covered by the precipitates to the total length of the grain boundaries of the crystal grains (γ phase). In the present invention, the state before the Ni-based heat-resistant alloy is put into use, that is, the grain boundary coverage measured at the time of the new material is adopted.

When the alloy is deformed, stress is concentrated at the grain boundaries. If the grain boundaries are covered with precipitates, this stress can be dispersed. Therefore, the higher the grain boundary coverage, the higher the strength of the alloy. When the grain boundary coverage is 70.0% or more, excellent creep strength can be obtained. The grain boundary coverage is preferably 80.0% or more. On the other hand, if the grain boundary coverage is too high, there are no sites for deformation, and the ductility may decrease depending on the volume fraction of the intragranular γ'phase. Therefore, the grain boundary coverage is preferably 90.0% or less.

The grain boundary coverage is calculated by the following method. Take a sample from any location on the Ni-based heat-resistant alloy. From the collected sample, observe a region of about 60 × 50 μm ² in 5 fields. A scanning electron microscope (SEM) is used for observation. FIG. 1 is a schematic view of the observed region. The total length L of the crystal grain boundaries in the region is measured. Then, the lengths A1 to An of each grain boundary portion (n in total) covered with the precipitate are measured. Based on the obtained L and A1 to An, the grain boundary coverage (%) in each region (five in total) is determined based on the following formula (A). The average of the obtained five grain boundary coverage is defined as the grain boundary coverage (%).
Grain boundary coverage = (A1 + A2 + A3 + ... + An) / L ... (A)

The precipitates deposited at the grain boundaries in this embodiment are mainly intermetallic compounds. However, in calculating the grain boundary coverage, if it is covered with some precipitate such as carbide or boride in addition to the intermetallic compound, it is covered with the precipitate in the above formula (A). Calculated as the grain boundary part.

Grain boundary σ phase body integration rate: 10.0% or less When the grain boundary σ phase body integration rate exceeds 10.0%, the change from the Laves phase to the σ phase progresses rapidly under high temperature environment. , Most of the Laves phase existing at the grain boundary changes to the σ phase. As a result, the grain boundary σ phase is coarsened at an early stage, voids are generated around the σ phase, and a non-precipitation zone is formed near the grain boundary to weaken the structure, which may lead to early breakage. Therefore, the volume fraction of the σ phase of the grain boundary is preferably 10.0% or less.

The volume fraction of the σ phase of the grain boundary can be measured by image analysis using energy selective backscattering (EsB) attached to the SEM. In this case, the area ratio of the σ phase of the grain boundary is obtained by image analysis, and the obtained area ratio is defined as the volume fraction.

3. 3. Manufacturing Method The manufacturing conditions of the Ni-based heat-resistant alloy according to the present invention are not particularly limited, but can be manufactured by using the manufacturing method shown below. In the example shown below, a method for manufacturing a Ni-based heat-resistant alloy tube will be described.

First, a material having the above chemical composition is prepared. The material is hollow billet. Hollow billets are manufactured, for example, by machining or vertical drilling. Hot extrusion is performed on the hollow billet.

As an example of hot extrusion, hot extrusion by the Eugene-Sejurne method will be described. First, the hollow billet is heated. The heated hollow billet is housed in the container of the hot extruder. A mandrel is inserted into the heart hole of the hollow billet housed in the container, and the hollow billet is pushed forward by the stem. A die is placed in front of the container. The hollow billet extruded forward by the stem is extruded tubular from between the die and the mandrel. A Ni-based heat-resistant alloy tube is manufactured by the above hot extrusion process. Cold working such as cold rolling and / or cold drawing may be further performed on the Ni-based heat-resistant alloy pipe after hot extrusion.

The produced alloy tube is subjected to solution heat treatment. The solution heat treatment is specifically carried out by soaking the alloy tube to 1000 to 1200 ° C. The holding time is not particularly limited, but is, for example, 1 to 2 hours.

The first aging heat treatment is performed on the solution-treated alloy tube. Specifically, the first aging heat treatment is carried out by soaking the alloy tube at 750 to 950 ° C. The holding time depends on the chemical composition of the alloy tube and the soaking temperature, but is, for example, 2 to 400 hours. When the content of each element of the Ni-based alloy is within the above range, intermetallic compounds are mainly precipitated at the grain boundaries by the first aging heat treatment. At this time, the soaking temperature and the holding time are adjusted so that the grain boundary coverage is 70% or more.

The γ'phase is also formed at the same time by the first aging heat treatment. However, depending on the conditions of the first aging heat treatment and the chemical composition of the Ni-based alloy, the γ'phase may not be precipitated or the amount of precipitation may be insufficient. In that case, after performing the above-mentioned first aging heat treatment, a second aging heat treatment may be carried out in order to precipitate the γ'phase. The second aging heat treatment is an arbitrary step. That is, when the γ'phase is sufficiently precipitated in the grains by the first aging heat treatment, it is not necessary to carry out the second aging heat treatment.

Specifically, the second aging heat treatment is carried out by soaking the alloy tube that has been subjected to the first aging heat treatment at 650 to 850 ° C. In the second aging heat treatment, the soaking temperature and the holding time are adjusted so that the volume fraction of the γ'phase is 25.0% or less.

In the above, a method for manufacturing an alloy tube has been described as a Ni-based heat-resistant alloy. However, the Ni-based heat-resistant alloy may be manufactured in a shape other than the pipe, and for example, the Ni-based heat-resistant alloy may be a plate.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Alloys having the chemical compositions shown in Table 1 were produced. Each alloy having the above chemical composition was melted in a high-frequency vacuum melting furnace to produce a 25 kg ingot. Each ingot was heated to 1160 ° C. The heated ingot was hot forged to produce a plate having a thickness of 25 mm. After the hot forging was completed, the plate material was air-cooled. The air-cooled plate material was further hot forged to produce a plate material having a thickness of 15 mm. Then, solution treatment and aging heat treatment (first aging heat treatment or first and second aging heat treatment) were carried out on each plate material having a thickness of 15 mm under the conditions shown in Table 2.

Next, the steel structure of each alloy was measured by the following procedure.

<Measurement of volume fraction of γ'phase of mother phase>
X-ray diffraction was performed on each alloy. The X-ray output voltage and current were 40 kV and 40 mA, and the target was Cu. From the obtained X-ray diffraction pattern, the structure was optimized by the Rietveld method, and the volume fraction of the γ'phase was obtained.

<Measurement of grain boundary coverage>
For each alloy, a region of about 60 × 50 μm ² was observed in 5 fields by SEM. Then, as shown in FIG. 1, after measuring the total length L of the crystal grain boundaries in the region, the lengths A1 to An of each grain boundary portion (a total of n) covered with the precipitate were measured. Based on the obtained L and A1 to An, the grain boundary coverage (%) in each region (five in total) was determined based on the following formula (A). The average of the obtained five grain boundary coverages was taken as the grain boundary coverage (%).
Grain boundary coverage = (A1 + A2 + A3 + ... + An) / L ... (A)

<Measurement of volume fraction of σ phase of grain boundary and Laves phase of parent phase>
The volume fractions of the σ phase of the grain boundaries and the Laves phase of the base metal were measured by image analysis using EsB. Specifically, the area ratios of the σ phase of the grain boundary and the Laves phase of the base material were obtained by image analysis, and the obtained area ratio was used as the volume fraction of each phase.

Subsequently, a creep rupture test was conducted according to the following procedure. A round bar tension with a diameter of 6 mm and a gauge distance of 30 mm parallel to the longitudinal direction from the center in the thickness direction of each plate material subjected to aging heat treatment (first aging heat treatment or first and second aging heat treatment). The test piece was prepared by machining. A creep rupture test was carried out using the prepared round bar tensile test piece.

The creep strength and creep rupture ductility of each alloy were evaluated by the creep rupture test as follows. Specifically, a creep rupture test was carried out in the air at 750 ° C. or 850 ° C. with a load of 130 MPa to determine the rupture time and rupture elongation. Further, the obtained rupture time was regressed by the Larson-Miller parameter (LMP) method, and the obtained value was used as an index of creep strength. The rupture elongation was used as an index of creep rupture ductility.

The results are also shown in Table 2.

As can be seen from Table 2, Test Nos. Which satisfy all the regulations of the chemical composition of the present invention. In 1 to 18, the rupture elongation was 20% or more and the LMP value was 26000 or more, resulting in excellent creep strength and creep rupture ductility. On the other hand, Test No. which is a comparative example. In 19 to 34, at least one of creep strength and creep rupture ductility was inferior.

FIG. 2 shows a micrograph of the initial structure and the fracture structure of the alloy of the present invention and the conventional alloy. Test No. of Comparative Example using the alloy 19 containing Fe. In No. 19, it can be seen that a large amount of needle-shaped Laves phase is precipitated in the grains after the fracture test. On the other hand, Test No. 3 using the alloy 3 having the chemical composition specified in the present invention. In No. 3, it is considered that the structure change was small before and after the rupture test, and as a result, the creep strength and the creep rupture ductility were improved.

Claims (4)

The chemical composition is mass%,
C: less than 0.01%,
Si: 1.0% or less,
Mn: 1.0% or less,
Cr: 15.0% or more and less than 28.0%,
Al: More than 0.5% and less than 2.5%,
Ti: 0.1 to 2.0%,
Nb: 0.1 to 2.0%,
B: 0.0005-0.01%,
Mo: More than 0% and less than 7.0%,
W: More than 0% and less than 14.0%,
Fe: less than 0.1%,
Co: 0-25.0%,
Zr: 0-0.2%,
V: 0-1.5%,
Hf: 0-1.0%,
Mg: 0-0.05%,
Ca: 0-0.05%,
Nd: 0-0.5%,
Y: 0-0.5%,
La: 0-0.5%,
Ce: 0-0.5%,
Ta: 0-8.0%,
Re: 0-8.0%,
Remaining: 50.0% or more of Ni and impurities,
Satisfy the following equations (i) to (iv),
Ni-based heat-resistant alloy.
6.0 ≤ Mo + 0.5 W ≤ 7.0 ... (i)
0.3 ≤ Al / (Al + Ti + Nb) ≤ 0.6 ... (ii)
0.2 ≤ Ti / (Al + Ti + Nb) ≤ 0.5 ... (iii)
0 <Nb / (Al + Ti + Nb) ≤ 0.5 ... (iv)
However, each element symbol in the above formula, to display the content (mass%) of each element contained in the alloy. When the chemical composition is mass%,
Co: More than 5.0% and less than 25.0%,
Zr: 0.005-0.2%,
V: 0.02-1.5%,
Hf: 0.005-1.0%,
Mg: 0.0008 to 0.05%,
Ca: 0.0008-0.05%,
Nd: 0.001 to 0.5%,
Y: 0.001 to 0.5%,
La: 0.001-0.5%,
Ce: 0.001-0.5%,
Ta: 0.01-8.0%, and
Re: 0.01-8.0%,
Contains one or more selected from,
The Ni-based heat-resistant alloy according to claim 1. The volume fraction of the γ'phase of the parent phase is 25.0% or less,
Grain boundary coverage is 70.0% or more,
The Ni-based heat-resistant alloy according to claim 1 or 2. The volume fraction of the σ phase of the grain boundary is 10.0% or less ,
The Ni-based heat-resistant alloy according to any one of claims 1 to 3.