JP7521194B2 - Ni-based alloy and its manufacturing method - Google Patents

Description

The present invention relates to a Ni-based alloy and a method for producing the same, and more specifically to a Ni-based alloy that can suppress the formation of freckle segregation without reducing production efficiency, and a method for producing the same.

"Ni-based alloy (or Ni-based superalloy)" refers to an alloy whose main component is Ni and which is solid solution strengthened and/or precipitation strengthened by adding Al, Ti, W, Mo, Ta, Cr, etc. Ni-based alloys are excellent in high-temperature strength, corrosion resistance, oxidation resistance, etc., and are therefore highly valued for use in the moving blades and stationary blades of aircraft jet engines and power-generating gas turbines, turbocharger turbines, etc. For this reason, various proposals have been made regarding such Ni-based alloys.

For example, Patent Document 1 states:
(A) 0.001<C<0.100mass%, 11.0≦Cr<19.0mass%, 0.5≦Co<22.0mass%, 0.5≦Fe<10.0mass%, Si≦0.1mass%, 2.0<Mo<5.0mass%, 1.0<W<5.0mass%, 2.5≦Mo+1/2W<5.5mass%, S≦0.010mass%, 0.3≦Nb<2.0mass%, 3.0<Al<6.5mass%, and 0.2≦Ti<2.49mass%, with the balance being Ni and unavoidable impurities;
(B) Disclosed is a Ni-base superalloy for hot forging that satisfies 0.2≦[Ti]/[Al]×10<4.0 and 8.5≦[Al]+[Ti]+[Nb]<13.0, where [M] is the atomic percentage of element M.
This document describes that by decreasing the amount of Ti and increasing the amount of Al, it is possible to achieve both good hot forgeability and good high-temperature strength properties.

In addition, Patent Document 2 states:
(A) 0.001<C<0.100mass%, 11≦Cr<19mass%, 5<Co<25mass%, 0.1≦Fe<4.0mass%, 2.0<Mo<5.0mass%, 1.0<W<5.0mass%, 2.0≦Nb<4.0mass%, 3.0<Al<5.0mass%, and 1.0<Ti<3.0mass%, with the balance being Ni and unavoidable impurities;
(B) Disclosed is a Ni-base superalloy for hot forging that satisfies 3.5≦([Ti]+[Nb])/[Al]×10<6.5 and 9.5≦[Al]+[Ti]+[Nb]<13.0, where [M] is the atomic percentage of element M.
The document describes that optimizing the contents of Al, Ti, and Nb can lower the solid solution temperature of the γ' phase, thereby enabling hot forging at low temperatures.

In addition, Patent Document 3 states:
(A) 0.001<C<0.100mass%, 11≦Cr<19mass%, 5<Co<25mass%, 0.1≦Fe<4.0mass%, 2.0<Mo<5.0mass%, 1.0<W<5.0mass%, 0.3≦Nb<4.0mass%, 3.0<Al<5.0mass%, 1.0<Ti<3.4mass%, and 0.01≦Ta<2.0mass%, with the balance being Ni and unavoidable impurities;
(B) Disclosed is a Ni-base superalloy for hot forging that satisfies 3.5≦([Ti]+[Nb])/[Al]×10<6.5 and 9.5≦[Al]+[Ti]+[Nb]<13.0, where [M] is the atomic percentage of element M.
The document describes that optimizing the contents of Al, Ti, and Nb can lower the solid solution temperature of the γ' phase, thereby enabling hot forging at low temperatures.

Furthermore, Non-Patent Document 1 discloses the results of a reproduction test of streak-like segregation (freckle segregation) using a horizontal unidirectional solidification test furnace.
The same document states:
(a) Segregation streaks grow in the direction of the sum of the solidification direction vector and the movement direction vector of the concentrated liquid phase driven by the specific gravity difference with the mother liquid phase;
(b) Ni-base superalloys are classified into two types, depending on the added components, namely, floating type in which segregation streaks extend upward from the solidification front, and sinking type in which segregation streaks extend downward from the solidification front; and
(c) The streak segregation tendency of Ni-base superalloys can be summarized by the method of using the ε×R ^1.1 value (ε: cooling rate, R: solidification rate) as the critical value for segregation formation;
is stated.

Ni-based alloys contain a large amount of alloying elements, so when they are slowly cooled during solidification, macrosegregation called freckles is likely to occur. Freckle segregation causes a decrease in the mechanical properties (e.g., tensile strength) of the material. For this reason, manufacturing methods that are less likely to cause segregation, such as electroslag remelting (ESR) and vacuum arc remelting (VAR), are generally used to manufacture Ni-based alloys. However, even when the ESR and VAR methods are used to manufacture Ni-based alloys, there is a problem in that the larger the size of the ingot, the more likely freckle segregation is to occur during solidification.

On the other hand, when the ESR method or the VAR method is used, if the melting rate of the consumable electrode is slowed down, the cooling rate of the molten metal is increased, and as a result, freckle segregation can be suppressed. However, this method results in an extremely slow solidification rate, and high production efficiency cannot be obtained.
Furthermore, the critical value at which freckle segregation occurs varies depending on the alloy composition. Therefore, in order to select manufacturing conditions that do not cause freckle segregation and can achieve high production efficiency, it is necessary to perform a unidirectional solidification test for each alloy composition and determine the critical value through experiments. However, such a method is extremely complicated. Even if the critical value is identified, it is often difficult to adopt manufacturing conditions that exceed the critical value (manufacturing conditions that are less likely to cause freckle segregation) in industrial-level production facilities.

JP 2015-129341 A JP 2017-145479 A JP 2017-145478 A

Iron and Steel, Vol. 95 (2009), No. 8, P613

An object of the present invention is to provide a Ni-based alloy capable of suppressing the formation of freckle segregation without reducing production efficiency, and a method for producing the same.
Another object of the present invention is to provide a Ni-based alloy in which the decrease in tensile strength caused by freckle segregation is minimal, and a method for producing the same.

In order to solve the above problems, the Ni-based alloy according to the present invention comprises:
0.05≦C≦0.10 mass%,
11.0≦Cr≦19.0mass%,
15.0≦Co≦22.0mass%,
0.5≦Fe≦10.0mass%,
2.5≦Mo≦5.0mass%,
2.0≦W≦5.0mass%,
0.3≦Nb≦2.0mass%,
3.0≦Al≦4.0 mass%, and
0.2≦Ti≦2.49mass%
The balance is Ni and unavoidable impurities.
The liquid phase density difference Δρ (=ρ ₀ −ρ _0.35 ) is greater than −0.050 and less than 0.015.
however,
ρ ₀ is the density (g/cm ³ ) of the mother liquid phase (solid fraction: zero) of the Ni alloy,
ρ _0.35 is the density (g/cm ³ ) of the concentrated liquid phase (solid fraction: 0.35) of the Ni alloy.

The method for producing a Ni-based alloy according to the present invention has the following features.
(1) The method for producing the Ni-based alloy includes the steps of:
a prediction formula acquisition step of acquiring in advance a prediction formula expressing the relationship between the liquid phase density difference Δρ (=ρ ₀ -ρ _0.35 ) of the Ni-based alloy and the critical value α (=V×R ^1.1 ) for the formation of freckle segregation, for cases where Δρ≧0 (floating type) and cases where Δρ<0 (sinking type);
A Δρ calculation step of calculating a liquid phase density difference Δρ _X of the alloy (X) to be produced based on the composition of the alloy (X);
A critical value calculation step of substituting the calculated _ΔρX into the prediction formula to calculate a critical value _αX for the formation of freckle segregation in the alloy (X);
and an alloy production process for producing the alloy ( _X ) under conditions in which the product of the cooling rate V' when the Ni-based alloy actually solidifies and the solidification rate R' when the Ni-based alloy actually solidifies to the power of 1.1 (=V'×R' ^1.1 ) is equal to or greater than αX.
however,
ρ ₀ is the density (g/cm ³ ) of the mother liquid phase (solid fraction: zero) of the Ni alloy,
ρ _0.35 is the density (g/cm ³ ) of the concentrated liquid phase (solid fraction: 0.35) of the Ni alloy,
V is the cooling rate (°C/min) when the Ni-based alloy solidifies,
R is the solidification rate (° C./min) when the Ni-based alloy solidifies.
(2) The alloy (X) is
0.05≦C≦0.10 mass%,
11.0≦Cr≦19.0mass%,
1.0≦Co≦22.0mass%,
0.5≦Fe≦10.0mass%,
2.5≦Mo≦5.0mass%,
1.0≦W≦5.0mass%,
0.3≦Nb≦2.0mass%,
3.0≦Al≦4.0 mass%, and
0.2≦Ti≦2.49mass%
and the balance being Ni and unavoidable impurities.
(3) The prediction formula acquisition step includes:
As the prediction formula when Δρ≧0, the following formula (3) is obtained in advance:
As the prediction formula when Δρ<0, the following formula (4) is acquired in advance:
The process includes:
V×R ^1.1 = 353.42Δρ+4.11 (3)
V×R ^1.1 = 167.64Δρ-0.29 (4)

When a Ni-based alloy is slowly solidified, the solute is distributed in a predetermined ratio between the solid and liquid phases as solidification progresses. As a result, a concentrated liquid phase with a density different from that of the mother liquid phase is generated during solidification. Freckle segregation is thought to develop with the liquid phase density difference Δρ between the mother liquid phase and the concentrated liquid phase as the driving force. In general, the larger the absolute value of Δρ, the larger the freckle segregation.

On the other hand, the alloying elements are broadly classified into those that increase Δρ, those that decrease Δρ, and those that have almost no effect on Δρ. Therefore, by increasing or decreasing the content of the other elements while maintaining the content of the elements essential for obtaining the desired characteristics, Δρ can be kept within a specified range. As a result, the formation of freckle segregation can be suppressed without impairing production efficiency and required characteristics (e.g., high-temperature tensile strength).

FIG. 1 is a graph showing the relationship between the liquid phase density difference Δρ and VR ^1.1 . 2A to 2F are diagrams showing the influence of the difference Δx in the content of C, Cr, Co, Fe, Mo, or W on the liquid phase density difference Δρ, respectively. 3A to 3D are diagrams showing the effect of the difference Δx in the content of Nb, Ti, Zr, or Al on the liquid phase density difference Δρ, respectively.

An embodiment of the present invention will be described in detail below.
[1. Ni-based alloy]
1.1. Composition
[1.1.1. Main constituent elements]
The Ni-based alloy according to the present invention has a liquid phase density difference Δρ within a predetermined range. The liquid phase density difference Δρ will be described in detail later. The composition of the Ni-based alloy according to the present invention is not particularly limited as long as the liquid phase density difference Δρ satisfies the condition described later.
The Ni-based alloy according to the present invention preferably contains the following elements, with the balance being Ni and unavoidable impurities. The types of the added elements, their component ranges, and the reasons for their limitations are as follows:

(1) 0.05≦C≦0.10mass%:
C combines with Cr, Nb, Ti, W, Mo, etc. to produce various carbides. Carbides with high solid solution temperatures (Nb-based and Ti-based carbides) have a pinning effect that allows them to withstand high temperatures. It suppresses the coarsening of crystal grains and contributes to improving hot workability. In addition, Cr-, Mo-, and W-based carbides precipitate at grain boundaries to strengthen the grain boundaries, improving mechanical properties. In order to obtain such an effect, the C content is preferably 0.05 mass% or more.

On the other hand, if the C content is excessive, the amount of carbides will be excessive. As a result, the structure will become non-uniform due to carbide segregation, and the hot workability and mechanical properties will deteriorate due to excessive precipitation of grain boundary carbides. Therefore, the C content is preferably 0.1 mass% or less.

(2) 11.0≦Cr≦19.0mass%:
Cr is an essential element for forming a protective oxide film _{of Cr2O3} and improving corrosion resistance and oxidation resistance. In addition, Cr combines with C to form _{Cr23C6 carbide} , In order to obtain such an effect, the Cr content is preferably 11.0 mass% or more. The Cr content is preferably 12.0 mass% or more, and more preferably 12.0 mass% or more. . 3 mass% or more.

On the other hand, Cr is a ferrite stabilizing element. Therefore, if the Cr content is excessive, austenite becomes unstable and the formation of embrittled phases such as σ phase and Laves phase is promoted. As a result, hot workability and mechanical properties such as strength and impact value are deteriorated. Therefore, the Cr content is preferably 19.0 mass% or less. The Cr content is preferably 17.0 mass% or less, and more preferably 16.0 mass% or less.

(3) 1.0≦Co≦22.0mass%:
Co dissolves in the austenite phase, which is the parent phase of the Ni-based alloy, to improve the workability. Co also promotes the precipitation of the γ' phase, improving high-temperature strength such as tensile properties. In order to obtain such an effect, the Co content is preferably 1.0 mass% or more, more preferably 15.0 mass% or more, and further preferably 19.0 mass% or more.
On the other hand, since Co is expensive, excessive addition of Co leads to high costs, and therefore the Co content is preferably 22.0 mass% or less.

(4) 0.5≦Fe≦10.0mass%:
Fe dissolves in the austenite phase, which is the parent phase of Ni-based alloys. A small amount of Fe does not affect the strength characteristics and workability. In addition, Fe can be mixed into the raw materials during alloy production. Although the Fe content may be high depending on the selection of raw materials, it leads to a reduction in raw material costs. In order to obtain such an effect, the Fe content is preferably 0.5 mass% or more. The Fe content is preferably 0.95 mass% or more, and more preferably 1.0 mass% or more.
On the other hand, if the Fe content is excessive, the strength decreases. Therefore, the Fe content is preferably 10.0 mass% or less. The Fe content is preferably 7.0 mass% or less, and more preferably 5.0 mass% or less. It is 0 mass% or less.

(5) 2.5≦Mo≦5.0mass%:
Mo is a solid solution strengthening element that dissolves in the austenite phase, which is the parent phase of Ni-based alloys, to strengthen the alloy. Mo also combines with C to form carbides, strengthening the grain boundaries. In order to obtain such an effect, the Mo content is preferably 2.5 mass% or more, and more preferably 3.0 mass% or more.
On the other hand, if the Mo content is excessive, it promotes the formation of harmful phases such as σ phase and Laves phase, and deteriorates hot workability and mechanical properties. Therefore, the Mo content should be 5.0 mass% or less. preferable.

(6) 1.0≦W≦5.0mass%:
W is a solution strengthening element that dissolves in the austenite phase, which is the parent phase of Ni-based alloys, to strengthen the alloy. W also combines with C to form carbides, strengthening the grain boundaries. In order to obtain such an effect, the W content is preferably 1.0 mass% or more. The W content is preferably 2.0 mass% or more, and more preferably 3.0 mass% or more. It is 2.5 mass% or more.
On the other hand, if the W content is excessive, it promotes the formation of harmful phases such as σ phase and Laves phase, and deteriorates hot workability and mechanical properties. Therefore, the W content is preferably 5.0 mass% or less. The W content is preferably 4.5 mass% or less, and more preferably 4.0 mass% or less.

(7) 0.3≦Nb≦2.0mass%:
Nb combines with C to form MC type carbides with a relatively high solution temperature. Therefore, when Nb is added, the pinning effect suppresses the coarsening of crystal grains after solution heat treatment, improving high-temperature strength properties and In addition, Nb, together with Ti, substitutes for the Al sites of the γ' phase (Ni ₃ Al), which is a strengthening phase, to form Ni ₃ (Al, Ti, Nb), which strengthens the γ' phase. In order to obtain such an effect, the Nb content is preferably 0.3 mass% or more. The Nb content is preferably 0.7 mass% or more, and more preferably 1. It is 0 mass% or more.

On the other hand, if the Nb content is excessive, the solution temperature of the γ' phase rises and hot workability decreases. In addition, the Laves phase, which is an embrittlement phase, is generated, resulting in a decrease in high-temperature strength. Therefore, the Nb content is preferably 2.0 mass% or less. The Nb content is preferably 1.5 mass% or less.

(8) 3.0≦Al≦4.0mass%:
Al acts as a generator of the γ' phase (Ni ₃ Al), which is a strengthening phase, and is an especially important element for improving high-temperature strength properties. In addition, Al increases the solid solution temperature of the γ' phase. However, compared with Nb and Ti, Al has a smaller effect on the rise in the solid solution temperature. On the contrary, Al has the effect of suppressing the rise in the solid solution temperature of the γ' phase while increasing the amount of γ' phase precipitated in the aging temperature range. Furthermore, Al combines with O to form a protective oxide film _made of _Al2O3 , which is also effective in improving corrosion resistance and oxidation resistance. The amount of Al is preferably 3.0 mass% or more. The Al content is preferably 3.5 mass% or more.

On the other hand, if the Al content is excessive, the solution temperature of the γ' phase will rise. In addition, the amount of γ' phase precipitation will increase, and there is a risk of the hot workability decreasing. Therefore, the Al content is preferably 4.0 mass% or less.

(9) 0.2≦Ti≦2.49mass%:
Ti combines with C to form MC type carbides, which have a relatively high solution temperature. Therefore, when Ti is added, the pinning effect suppresses the coarsening of crystal grains after solution heat treatment, improving high-temperature strength properties and In addition, Ti, together with Nb, substitutes for the Al sites of the γ' phase (Ni ₃ Al), which is a strengthening phase, to form Ni ₃ (Al, Ti, Nb), which strengthens the γ' phase. In order to obtain such an effect, the Ti content is preferably 0.2 mass% or more. The Ti content is preferably 0.23 mass% or more, and more preferably 0. It is 5 mass% or more.

On the other hand, if the Ti content is excessive, the solution temperature of the γ' phase rises and hot workability decreases. In addition, the Laves phase, which is an embrittlement phase, is generated, resulting in a decrease in high-temperature strength. Therefore, the Ti content is preferably 2.49 mass% or less. The Ti content is preferably 2.0 mass% or less.

[1.1.2. Sub-constituent elements]
In addition to the above-mentioned main constituent elements, the Ni-based alloy may further contain one or more of the following elements. The types of the added elements, their component ranges, and the reasons for their limitations are as follows: That is correct.

(10) 0.09≦Si≦0.15mass%:
When Si is added, a scale layer of Si oxide is formed, improving the oxidation resistance. In order to obtain such an effect, the Si content is preferably 0.09 mass% or more.
However, if the Si content is excessive, Si segregates, forming localized low melting points and deteriorating hot workability. Therefore, the Si content is preferably 0.15 mass% or less. The content is preferably 0.10 mass% or less.

(11) 1.0≦Ta≦2.0mass%:
Ta combines with C to form MC type carbides with a relatively high solution temperature. Therefore, when Ta is added, the pinning effect suppresses the coarsening of crystal grains after solution heat treatment, improving high-temperature strength properties and Ta, together with Nb and Ti, substitutes for the Al site of the γ' phase (Ni ₃ Al), which is a strengthening phase, forming Ni ₃ (Al, Ti, Nb, Ta). In order to obtain such an effect, the Ta content is preferably 1.0 mass% or more.

On the other hand, if the Ta content is excessive, the solution temperature of the γ' phase rises and hot workability decreases. In addition, the Laves phase, which is an embrittlement phase, is generated, resulting in a decrease in high-temperature strength. Therefore, the Ta content is preferably 2.0 mass% or less. The Ta content is preferably 1.5 mass% or less.

(12) 0.001≦B≦0.03mass%:
B segregates at the grain boundaries to strengthen the grain boundaries and improve the workability and mechanical properties. To obtain such effects, the B content is preferably 0.001 mass% or more. The amount is preferably 0.01 mass % or more.
On the other hand, if the B content is excessive, ductility is lost due to excessive segregation to grain boundaries, and hot workability is deteriorated. Therefore, the B content is preferably 0.03 mass% or less. The content is preferably 0.025 mass% or less, and more preferably 0.02 mass% or less.

(13) 0.04≦Zr≦0.1mass%:
Zr segregates at the grain boundaries to strengthen the grain boundaries and improve the workability and mechanical properties. To obtain such effects, the Zr content is preferably 0.04 mass% or more. The amount is preferably 0.045 mass% or more.
On the other hand, if the Zr content is excessive, ductility is lost due to excessive segregation to grain boundaries, and hot workability is deteriorated. Therefore, the Zr content is preferably 0.1 mass% or less.

[1.2. Liquid phase density difference]
[1.2.1. Preferred range of Δρ]
The Ni-based alloy according to the present invention has a liquid phase density difference Δρ of more than −0.050 and less than 0.015. Here, the “liquid phase density difference Δρ” is expressed by the following formula (1): The value represented is:
Δρ=ρ ₀ −ρ _0.35 …(1)
however,
ρ ₀ is the density (g/cm ³ ) of the mother liquid phase (solid fraction: zero) of the Ni alloy,
ρ _0.35 is the density (g/cm ³ ) of the concentrated liquid phase (solid fraction: 0.35) of the Ni alloy.

When a Ni-based alloy is slowly solidified, dendrite nuclei are formed on the casting wall, and the dendrites grow toward the inside of the mold. At this time, the solute is distributed to the solid phase and the liquid phase at a predetermined ratio. Usually, as solidification progresses, the solute is swept out of the solid phase, and the concentration of the solute in the liquid phase increases. As a result, a concentrated liquid phase having a density different from that of the mother liquid phase (solid fraction: zero) may be generated during solidification. The difference between the density of the mother liquid phase and the density of the concentrated liquid phase (liquid phase density difference) Δρ varies depending on the solid fraction. In the present invention, the density ρ _0.35 of the concentrated liquid phase when the solid fraction is 0.35 is used to calculate the liquid phase density difference Δρ. This is because Δρ is often maximum when the solid fraction is 0.35.

Freckle segregation is believed to develop with the liquid phase density difference Δρ between the mother liquid phase and the concentrated liquid phase as a driving force. Therefore, in general, the smaller the absolute value of Δρ, the more difficult it is for Freckle segregation to occur.
In order to produce an ingot having an industrial size (for example, a diameter of about 150 mm to 1000 mm) and free from freckle segregation using general-purpose production equipment, Δρ needs to be greater than −0.050 and less than 0.015.

[1.2.2. Relationship between Δρ and the critical value α of Freckle segregation]
The "critical value α for the formation of freckle segregation" refers to a value expressed by the following formula (2).
α=V×R ^1.1 …(2)
however,
V is the cooling rate (° C./min) when the Ni-based alloy solidifies,
R is the solidification speed (mm/min) when the Ni-based alloy solidifies.

It is known that there is a correlation between the critical value α (=V×R ^1.1 ) for the formation of freckle segregation and Δρ, and that when the cooling conditions during solidification are below the critical value α, freckle segregation is likely to form.
Therefore, even if the critical value α under actual manufacturing conditions is unknown, if Δρ can be known, it is possible to predict with some degree of accuracy whether or not freckle segregation will occur.
Furthermore, when Δρ is within the above-mentioned range, freckle segregation is unlikely to occur, but if freckle segregation still occurs, this indicates that the cooling conditions are below the critical value α. In such a case, the contents of the alloying elements should be changed so that the absolute value of Δρ becomes even smaller, or the manufacturing method and/or manufacturing conditions should be changed so that the cooling conditions exceed the critical value α (i.e., the cooling conditions become more rapid).

Various empirical formulas have been proposed to express the relationship between Δρ and α. However, in the past, it was common to discuss the relationship between Δρ and α without distinguishing whether the alloy was of the floating type (Δρ≧0) or the sinking type (Δρ<0).

In contrast, the correlation between Δρ and α differs significantly depending on whether the alloy is of the floating type (Δρ≧0) or the sinking type (Δρ<0). This was a finding first discovered by the present inventors. Therefore, if a prediction formula expressing the relationship between Δρ and α is obtained in advance for each of the floating type (Δρ≧0) and sinking type (Δρ<0), it is possible to predict the presence or absence of freckle segregation with a certain degree of accuracy.

The relationship (prediction equation) between Δρ and α generally varies with the type of alloy, but for a group of alloys that can be considered to have similar compositions and properties, a set of predictive equations can be used to estimate α.
Here, "a group of alloys that can be considered to have similar compositions and properties" means
(a) contain the same main constituent elements;
(b) have the same crystal structure;
(c) Alloys with similar physical and chemical properties.

For example, when the alloy is a Ni-based alloy containing the above-mentioned main constituent elements, if the Ni-based alloy is of the floating type (Δρ≧0), it is preferable to use the following formula (3) as the prediction formula. On the other hand, if the Ni-based alloy is of the sinking type (Δρ<0), it is preferable to use the following formula (4) as the prediction formula.
V×R ^1.1 = 353.42Δρ+4.11 (3)
V×R ^1.1 = 167.64Δρ-0.29 (4)
In reality, "V×R ^1.1 " does not take a negative value, but when Δρ takes a negative value, V×R ^1.1 is expressed as a negative value for the sake of convenience.

Since the equilibrium distribution coefficient of the solute is known, once the alloy composition is determined, Δρ can be calculated by theoretical calculation. On the other hand, the critical value α of an alloy having a certain composition can be obtained by a unidirectional solidification test (specifically, a vertical unidirectional solidification test is preferable, but not limited to this). Therefore,
(a) selecting a plurality of alloy compositions (preferably two or more) from a group of alloys that have significantly different Δρ, with Δρ≧0 and Δρ<0;
(b) performing a solidification test for each of the selected alloy compositions;
(c) A prediction formula can be obtained by performing linear regression on Δρ obtained from theoretical calculations and the critical value α (=V×R ^1.1 ) obtained from a coagulation test, separately for the cases where Δρ≧0 and Δρ<0.

[1.2.3. Method for adjusting Δρ]
In the Ni-based alloy, the alloying elements are:
(a) An element whose Δρ increases with an increase in its content (hereinafter also referred to as a “positive element”),
(b) elements whose Δρ decreases with increasing content (hereinafter also referred to as “negative elements”), and
(c) Elements whose content does not significantly affect Δρ even if it is increased or decreased (hereinafter, also referred to as “neutral elements”)
They can be broadly divided into:
Therefore, by increasing or decreasing the content of each alloying element depending on the purpose, Δρ can be maintained within a predetermined range, and by optimizing the selection of the alloying elements to be increased or decreased in content, the formation of freckle segregation can be suppressed without compromising production efficiency and required properties (e.g., high temperature tensile strength).

An example of a positive element is Al.
Examples of negative elements include Mo, Nb, Ti, Zr, C, and W (when the content is 2.5 mass% or more).
The neutral elements include, for example, Cr, Co, Fe, and W (when the content is 2.5 mass % or less).

[1.3. High temperature tensile strength]
By optimizing the contents of alloying elements, it is possible to obtain a Ni-based alloy that is substantially free of freckle segregation, and by subjecting such a Ni-based alloy to an appropriate heat treatment, it is possible to obtain a Ni-based alloy having a structure in which the γ' phase is precipitated in the matrix and having high high-temperature tensile strength.
Specifically, by optimizing the contents of each alloy element and the heat treatment conditions, the tensile strength at 650° C. becomes 1200 MPa or more.
By further optimizing the contents of the alloying elements, the tensile strength at 650° C. becomes 1300 MPa or more.

[2. Manufacturing method of Ni-based alloy]
The method for producing a Ni-based alloy according to the present invention comprises the steps of:
a prediction formula acquisition step of acquiring in advance a prediction formula expressing the relationship between the liquid phase density difference Δρ (=ρ ₀ -ρ _0.35 ) of the Ni-based alloy and the critical value α (=V×R ^1.1 ) for the formation of freckle segregation, for cases where Δρ≧0 (floating type) and cases where Δρ<0 (sinking type);
A Δρ calculation step of calculating a liquid phase density difference Δρ _X of the alloy (X) to be produced based on the composition of the alloy (X);
A critical value calculation step of substituting the calculated _ΔρX into the prediction formula to calculate a critical value _αX for the formation of freckle segregation in the alloy (X);
and an alloy production process for producing the alloy (X) under conditions where α is equal to or greater than the alloy ( _X ).
however,
ρ ₀ is the density (g/cm ³ ) of the mother liquid phase (solid fraction: zero) of the Ni alloy,
ρ _0.35 is the density (g/cm ³ ) of the concentrated liquid phase (solid fraction: 0.35) of the Ni alloy,
V is the cooling rate (°C/min) when the Ni-based alloy solidifies,
R is the solidification rate (° C./min) when the Ni-based alloy solidifies.

The alloy manufacturing process is specifically as follows:
(a) a melting and casting process in which raw materials mixed to obtain a predetermined composition are melted and cast under conditions in which the temperature is _αX or higher;
(b) a soaking treatment step in which the obtained ingot is subjected to a homogenization heat treatment;
(c) a hot working step of hot working the material after the homogenization heat treatment;
(d) a solution heat treatment step in which the material after hot working is subjected to solution heat treatment;
(e) an aging treatment step for performing aging treatment on the material after the solution heat treatment.

[2.1. Prediction formula acquisition process]
First, a prediction formula expressing the relationship between the liquid phase density difference Δρ (=ρ ₀ -ρ _0.35 ) of the Ni-based alloy and the critical value α (=V×R ^1.1 ) for the formation of freckle segregation is obtained in advance for each of the cases Δρ≧0 (floating type) and Δρ<0 (sinking type) (prediction formula obtaining step). Details of the prediction formula obtaining step are as described above, and therefore will not be described here.

[2.2. Δρ calculation process]
Next, the liquid phase density difference _ΔρX of the alloy (X) is calculated based on the composition of the alloy (X) to be produced (Δρ calculation step). The details of the Δρ calculation step are as described above. Therefore, the description will be omitted.

[2.3. Critical value calculation process]
Next, the calculated _ΔρX is substituted into the prediction formula to calculate the critical value _αX for the formation of freckle segregation in the alloy (X) (critical value calculation step). Details of the critical value calculation step are as described above, and therefore will not be described here.

[2.4. Alloy manufacturing process]
Next, the alloy (X) is produced under conditions where α is equal to or greater than the alloy ( _X ) (alloy production step).
[2.4.1. Melting and casting process]
First, the raw materials mixed to obtain a predetermined composition are melted and cast under conditions that give a temperature _{of αX} or higher (melting and casting process). The method and conditions for melting and casting are not particularly limited. The most suitable method and conditions can be selected depending on the purpose. Examples of the melting and casting method include electroslag remelting (ESR) method and vacuum arc remelting (VAR) method.

[2.4.2. Homogenization heat treatment process]
Next, the obtained ingot is subjected to a homogenization heat treatment. The homogenization heat treatment is performed to remove segregation that occurs during casting. The conditions of the homogenization heat treatment are not particularly limited as long as such an effect is achieved. Usually, the homogenization heat treatment is performed by heating and holding the ingot under the conditions of a temperature of 1100°C to 1220°C and a time of 10 hours or more.

[2.4.3. Hot forging process]
Next, the material after the homogenization heat treatment is hot forged. Hot forging is performed to destroy the coarse cast structure and refine the structure. The conditions of hot forging are not particularly limited as long as such effects are achieved. Usually, hot forging is performed by heating the material under conditions of 1100°C to 1160°C, forging under conditions of a forging end temperature of 1100°C, and then air cooling. Note that hot forging may be performed continuously after the homogenization heat treatment without cooling the material to room temperature.

[2.4.4. Solution heat treatment process]
Next, the material after hot forging is subjected to solution heat treatment. The solution heat treatment is performed to make the material into an austenite single phase. The conditions of the solution heat treatment are not particularly limited as long as such an effect is achieved. Usually, the solution heat treatment is performed by heating the material under the conditions of temperature: 1000°C to 1160°C x heating time: 3 hours to 5 hours, and then cooling it. Examples of the cooling method include air cooling, air blast cooling, oil cooling, and water cooling.

[2.4.5. Aging treatment process]
Next, the material after the solution heat treatment is subjected to aging treatment. The aging treatment is performed in order to precipitate the γ' phase in the matrix. The conditions of the aging treatment are not particularly limited as long as such an effect is achieved. Generally, the aging treatment is performed by heating the material at 700°C to 900°C for 10 hours to 24 hours. After the heat treatment, the material is cooled by air cooling.

[3. Action]
When a Ni-based alloy is slowly solidified, the solute is distributed between the solid and liquid phases in a predetermined ratio as the solidification proceeds. As a result, a concentrated liquid phase with a density different from that of the mother liquid phase is generated during solidification. Freckle segregation is thought to grow with the liquid phase density difference Δρ between the mother liquid phase and the concentrated liquid phase as the driving force. In general, the larger Δρ, the larger the Freckle segregation.

On the other hand, the alloying elements are broadly classified into those that increase Δρ, those that decrease Δρ, and those that have almost no effect on Δρ. Therefore, by increasing or decreasing the content of the other elements while maintaining the content of the elements essential for obtaining the desired characteristics, Δρ can be kept within a specified range. As a result, the formation of freckle segregation can be suppressed without impairing production efficiency and required characteristics (e.g., high-temperature tensile strength).

(Examples 1 to 24, Comparative Examples 1 to 7)
1. Preparation of Samples
50 kg of Ni-based alloy having the chemical composition shown in Table 1 was melted in a high-frequency induction furnace. The melted ingot was subjected to homogenization heat treatment at 1100 to 1220° C. for 16 hours. After that, it was hot forged into a bar having a diameter of 30 mm.
Next, the hot forged material was subjected to a solution heat treatment at 1000 to 1160° C. Furthermore, the material after the solution heat treatment was subjected to one or more stages of aging treatment at 700 to 900° C.

2. Test Method
[2.1. Calculation of liquid phase density difference Δρ]
Using thermal property calculation software: JMatPro (registered trademark), the density ρ ₀ of the mother liquid phase and the density ρ _0.35 of the concentrated liquid phase when the solid fraction is 0.35 were calculated. Furthermore, the liquid phase density difference Δρ was calculated from the density of the mother liquid phase and the density of the concentrated liquid phase.

2.2. Prediction of VR ^1.1
When the calculated liquid phase density difference Δρ was of the floating type (Δρ≧0), it was substituted into the above-mentioned formula (3) to obtain VR ^1.1 .
When the calculated liquid phase density difference Δρ was of the settling type (Δρ<0), it was substituted into the above-mentioned formula (4) to obtain VR ^1.1 .

[2.3. Presence or absence of freckle defects]
The prepared samples were cut, and the cut surfaces were subjected to macroscopic structural observation to check for the occurrence of freckle defects.
[2.4. High temperature tensile test]
The forged material was subjected to solution heat treatment and then one or more stages of aging treatment. Then, a sample with a parallel part diameter of 8 mm and a gauge length of 40 mm was prepared and subjected to a high-temperature tensile test in accordance with JIS G 0567. The test temperature was 650°C.

3. Results
The results are shown in Table 1. The relationship between the liquid phase density difference Δρ and VR ^1.1 is shown in Figure 1. The following can be seen from Table 1 and Figure 1.

(1) When the liquid phase density difference Δρ was greater than −0.050 and less than 0.015, an ingot free from freckle defects was obtained.
(2) When freckle defects were generated during casting, the high-temperature tensile strength was low even after aging treatment. On the other hand, when freckle defects were not generated during casting, the high-temperature tensile strength became 1200 MPa or more after aging treatment.

(Example 25)
1. Test Method
The composition of Example 1 was used as the base composition. For samples in which the contents of some elements contained in the base composition were increased or decreased, the liquid phase density difference Δρ was calculated by theoretical calculation. Furthermore, the critical value α (predicted value) of Freckle segregation was calculated using the above-mentioned formulas (3) and (4).

2. Results
It was estimated how Δρ would change when the content of element (M) deviated from the base composition by Δx (mass%). Figures 2(A) to 2(F) respectively show the effect of the difference Δx in the content of C, Cr, Co, Fe, Mo, or W on the liquid phase density difference Δρ. Figures 3(A) to 3(D) respectively show the effect of the difference Δx in the content of Nb, Ti, Zr, or Al on the liquid phase density difference Δρ. The following can be seen from Figures 2 and 3.

(1) The effect of Δx on Δρ varied greatly depending on the type of element (M). Elements (M) can be broadly classified into those in which Δρ increases with an increase in Δx, those in which Δρ decreases, and those in which Δρ does not increase or decrease.
(2) It was found that, among the elements (M), Al and Mo cause a large change in Δρ with a slight change in Δx.
(3) It was found that, among the elements (M), Mo, Nb, and Ti decrease Δρ with an increase in Δx.
(4) Among the elements (M), it was found that Cr, Co, Fe, and W show two different trends of increase and decrease in the amount of change in Δρ associated with a change in Δx from a negative value to a zero value, and in the amount of change in Δρ associated with a change in Δx from a zero value to a positive value.
(5) Among the elements (M), it was found that Al increases Δρ with an increase in Δx.

Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention.

The Ni-based alloy of the present invention can be used for turbine blades, stator blades, turbochargers, etc.

Claims (11)

0.05≦C≦0.10 mass%,
11.0≦Cr≦19.0mass%,
15.0≦Co≦22.0mass%,
0.5≦Fe≦10.0mass%,
2.5≦Mo≦5.0mass%,
2.0≦W≦5.0mass%,
0.3≦Nb≦2.0mass%,
3.0≦Al≦4.0 mass%, and
0.2≦Ti≦2.49mass%
The balance is Ni and unavoidable impurities.
A Ni-based alloy having a liquid phase density difference Δρ (=ρ ₀ −ρ _0.35 ) greater than −0.050 and less than 0.015.
however,
ρ ₀ is the density (g/cm ³ ) of the mother liquid phase (solid fraction: zero) of the Ni alloy,
ρ _0.35 is the density (g/cm ³ ) of the concentrated liquid phase (solid fraction: 0.35) of the Ni alloy. 0.09≦Si≦0.15 mass%,
0.001≦B≦0.03 mass%, and
0.04≦Zr≦0.1mass%
The Ni-based alloy according to claim 1, further comprising one or more elements selected from the group consisting of: The Ni-based alloy according to claim 1 or 2, further comprising 1.0≦Ta≦2.0 mass%. The Ni-based alloy according to any one of claims 1 to 3, having a tensile strength of 1200 MPa or more at 650°C. The Ni-based alloy according to any one of claims 1 to 4, having a tensile strength of 1300 MPa or more at 650°C. A method for producing a Ni-based alloy having the following configuration.
(1) The method for producing the Ni-based alloy includes the steps of:
a prediction formula acquisition step of acquiring in advance a prediction formula expressing the relationship between the liquid phase density difference Δρ (=ρ ₀ -ρ _0.35 ) of the Ni-based alloy and the critical value α (=V×R ^1.1 ) for the formation of freckle segregation, for cases where Δρ≧0 (floating type) and cases where Δρ<0 (sinking type);
A Δρ calculation step of calculating a liquid phase density difference Δρ _X of the alloy (X) to be produced based on the composition of the alloy (X);
A critical value calculation step of substituting the calculated _ΔρX into the prediction formula to calculate a critical value _αX for the formation of freckle segregation in the alloy (X);
and an alloy production process for producing the alloy ( _X ) under conditions in which the product of the cooling rate V' when the Ni-based alloy actually solidifies and the solidification rate R' when the Ni-based alloy actually solidifies to the power of 1.1 (=V'×R' ^1.1 ) is equal to or greater than αX.
however,
ρ ₀ is the density (g/cm ³ ) of the mother liquid phase (solid fraction: zero) of the Ni alloy,
ρ _0.35 is the density (g/cm ³ ) of the concentrated liquid phase (solid fraction: 0.35) of the Ni alloy,
V is the cooling rate (°C/min) when the Ni-based alloy solidifies,
R is the solidification rate (° C./min) when the Ni-based alloy solidifies.
(2) The alloy (X) is
0.05≦C≦0.10 mass%,
11.0≦Cr≦19.0mass%,
1.0≦Co≦22.0mass%,
0.5≦Fe≦10.0mass%,
2.5≦Mo≦5.0mass%,
1.0≦W≦5.0mass%,
0.3≦Nb≦2.0mass%,
3.0≦Al≦4.0 mass%, and
0.2≦Ti≦2.49mass%
and the balance being Ni and unavoidable impurities.
(3) The prediction formula acquisition step includes:
As the prediction formula when Δρ≧0, the following formula (3) is obtained in advance:
As the prediction formula when Δρ<0, the following formula (4) is acquired in advance:
The process includes:
V×R ^1.1 = 353.42Δρ+4.11 (3)
V×R ^1.1 = 167.64Δρ-0.29 (4) 7. The method for producing a Ni-based alloy according to claim 6 , wherein the alloy (X) has a liquid phase density difference Δρ (=ρ ₀ −ρ _0.35 ) that is greater than −0.050 and less than 0.015. The alloy (X) is
0.09≦Si≦0.15 mass%,
0.001≦B≦0.03 mass%, and
0.04≦Zr≦0.1mass%
The method for producing a Ni-based alloy according to claim 6 or 7, further comprising at least one element selected from the group consisting of: The method for producing a Ni-based alloy according to any one of claims 6 to 8, wherein the alloy (X) further contains 1.0 ≦ Ta ≦ 2.0 mass%. The method for producing a Ni-based alloy according to any one of claims 6 to 9, wherein the alloy (X) has a tensile strength of 1200 MPa or more at 650°C. The method for producing a Ni-based alloy according to any one of claims 6 to 10, wherein the alloy (X) has a tensile strength of 1300 MPa or more at 650°C.

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