LOW THERMAL EXPANSION ALLOY

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2024-135356 filed on Aug. 14, 2024, the contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a low thermal expansion alloy, and more particularly to a low thermal expansion alloy that does not substantially contain Co and has a small thermal expansion coefficient in a wide temperature range.

BACKGROUND ART

The semiconductor market and the semiconductor-manufacturing apparatus market continue to grow with the progress of IT technology. In order to implement various requirements for final products, various sensor components for controlling semiconductor-manufacturing apparatuses are also required to have a high accuracy. For example, a pressure-sensitive body of a pressure sensor includes a diaphragm and a semiconductor formed on the diaphragm. When a pressure acts on the diaphragm, strain occurs in the semiconductor, and the pressure can be measured based on a magnitude of the strain. In order to measure the pressure with a high accuracy by using such a pressure sensor, it is necessary to reduce the strain that is caused by a difference in thermal expansion coefficient between the diaphragm and the semiconductor during changes in temperature.

Kovar (a Fe-29Ni-17Co alloy) has a low thermal expansion coefficient close to that of a semiconductor, and thus has been commonly used as a material for a diaphragm for a pressure sensor. In addition, Kovar has a high magnetic transformation point, and therefore, Kovar can maintain a thermal expansion coefficient equal to that of a semiconductor in a wide temperature range.

However, Kovar has a high content of expensive Co, and also has a low strength, which hinders the miniaturization of products. Therefore, resource risk and material cost of Kovar are considered to be problematic.

In order to solve this problem, various proposals have been made from the past.

For example, Patent Literature 1 discloses a low thermal expansion alloy containing Ni in an amount of 36 mass % to 40 mass %, Co in an amount of 1 mass % to 5 mass %, Ni+Co in an amount of 39 mass % to 42 mass %, with the balance being Fe and unavoidable impurities.

Patent Literature 1 discloses that an alloy having such a composition has a thermal expansion coefficient close to a thermal expansion coefficient of Si and has a magnetic transformation point of 300° C. or higher.

Patent Literature 2 discloses a high-strength, high-hardness, and low-thermal-expansion alloy containing C in an amount of 1.00 mass %, Si in an amount of 1.20 mass %, Ni in an amount of 28.97 mass %, Co in an amount of 5.29 mass %, Mn in an amount of 0.21 mass %, and Mg in an amount of 0.02 mass %, with the balance being Fe and unavoidable impurities, in which a part of austenite is transformed into martensite.

Patent Literature 2 discloses that when a part of austenite is transformed into martensite, the thermal expansion coefficient is slightly increased, but the strength, hardness, and thermal conductivity are improved.

Patent Literature 1 discloses a low thermal expansion alloy having a reduced Co content while maintaining a high magnetic transformation point and low thermal expansion properties. However, Patent Literature 1 does not mention the strength. In addition, about 5 mass % of Co is contained, and thus there is a problem of resource risk.

Patent Literature 2 discloses a low thermal expansion alloy having a high strength and a high hardness. However, the alloy described in Patent Literature 2 also contains about 5 mass % of Co, and thus the resource risk is high. Furthermore, martensite transformation may occur in a low-temperature environment, and low thermal expansion properties may be impaired. In addition, Patent Literature 2 does not mention a magnetic transformation point, and there is a concern about instability of thermal expansion properties in a high-temperature environment.

Furthermore, an example in which an alloy having a high tensile strength and stably exhibiting low thermal expansion properties in a wide temperature range has not been proposed in the related art.

CITATION LIST

Patent Literature

• Patent Literature 1: JPH09-324243A
• Patent Literature 2: JPH04-354848A

SUMMARY OF INVENTION

An object of the present invention is to provide a low thermal expansion alloy that stably exhibits low thermal expansion properties in a wide temperature range.

Another object of the present invention is to provide a low thermal expansion alloy that exhibits a high tensile strength in addition to the low thermal expansion properties.

In order to solve the above-described problem, a low thermal expansion alloy of the present invention consists of:

• • 0.10 mass %≤C≤0.40 mass %;
  • Si≤1.00 mass %;
  • 0.10 mass %≤Mn≤2.00 mass %;
  • P≤0.050 mass %;
  • S≤0.015 mass %;
  • 0.10 mass %≤Cu≤4.00 mass %;
  • 35.0 mass %≤Ni≤45.0 mass %;
  • 0.10 mass %≤V≤1.00 mass %;
  • 0 mass %≤Cr≤0.50 mass %;
  • 0 mass %≤Mo≤4.00 mass %;
  • 0 mass %≤Co≤0.50 mass %;
  • 0 mass %≤Al≤0.50 mass %;
  • 0 mass %≤Ti≤0.50 mass %;
  • 0 mass %≤Nb≤0.50 mass %;
  • 0 mass %≤W≤0.50 mass %;
  • 0 mass %≤Zr≤0.50 mass %;
  • 0 mass %≤Hf≤0.50 mass %;
  • 0 mass %≤Ta≤0.50 mass %;
  • 0 mass %≤B≤0.050 mass %;
  • 0 mass %≤Mg≤0.050 mass %;
  • 0 mass %≤Ca≤0.050 mass %; and
  • 0 mass %≤REM≤0.050 mass %,
  • with the balance being Fe and unavoidable impurities.

The low thermal expansion alloy satisfies the following formulas (1) and (2).

A ≥ 38. ( 1 ) 40.6 < B ≤ 4 ⁢ 4 . 5 ( 2 )
Here,

A = [ Ni ] + [ Co ] + 0.7 * [ Cu ] - [ Si ] - [ Mn ] - [ Cr ] - 0.5 * [ Mo ] - 0.5 * [ V ] , and B = [ Ni ] + 0.8 * [ Co ] + [ Cu ] + 6 * [ C ] + 1.1 * [ Si ] + [ Mn ] + 1.2 * [ Cr ] + 0.2 * [ Mo ] - 0.5 * [ V ] .

When C and V are added to the alloy in appropriate amounts and the component balance between V and C is adjusted, fine carbides are precipitated in the matrix. Therefore, the high strength can be achieved while preventing deterioration in the thermal expansion coefficient due to solid solution of the elements (particularly, C).

In addition, when the value A is optimized (particularly, components of Ni and Cu are optimized) and the manufacturing conditions are optimized, a magnetic transformation point of 280° C. or higher can be achieved. Therefore, the low thermal expansion property can be maintained in a high-temperature environment.

Furthermore, when the value B is optimized (in particular, components of Ni and Cu are optimized) and the manufacturing conditions are optimized, the thermal expansion coefficient can be maintained in a range of 3.5×10⁻⁶/° C. to 6.0×10⁻⁶/° C. in a wide temperature range.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described in detail.

[1. Low Thermal Expansion Alloy]

[1.1. Main Constituent Elements]

A low thermal expansion alloy according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows.

(1) 0.10 Mass %≤C≤0.40 Mass %:

C has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. In order to achieve such an effect, the content of C needs to be 0.10 mass % or more. The content of C is preferably 0.20 mass % or more.

On the other hand, in the case where the content of C is excessive, the toughness and ductility and the hot workability may decrease, or the thermal expansion coefficient may increase. Therefore, the content of C needs to be 0.40 mass % or less. The content of C is preferably 0.36 mass % or less.

(2) Si≤1.00 Mass %:

In the case where the content of Si is excessive, the thermal expansion coefficient may increase, and the magnetic transformation point may decrease. Therefore, the content of Si needs to be 1.00 mass % or less. The content of Si is preferably 0.70 mass % or less, more preferably 0.40 mass % or less, and further preferably 0.30 mass % or less.

The smaller the content of Si, the better. However, extreme reduction in the content of Si may cause an increase in manufacturing cost. Si also has an effect of improving the tensile strength by solid solution strengthening. Therefore, the content of Si needs to be 0.01 mass % or more.

(3) 0.10 Mass %≤Mn≤2.00 Mass %:

Mn has an effect of forming inclusions such as MnS and improving manufacturability (machinability) of the alloy. In order to achieve such effects, the content of Mn needs to be 0.10 mass % or more.

On the other hand, in the case where the content of Mn is excessive, the thermal expansion coefficient may increase, and the magnetic transformation point may decrease. Therefore, the content of Mn needs to be 2.00 mass % or less. The content of Mn is preferably 0.50 mass % or less.

(4) P≤0.050 Mass %:

In the case where the content of P is excessive, hot workability may decrease. Therefore, the content of P needs to be 0.050 mass % or less.

The smaller the content of P, the better. However, extreme reduction in the content of P may cause an increase in manufacturing cost. Therefore, the content of P may be 0.001 mass % or more.

(5) S≤0.015 Mass %:

In the case where the content of S is excessive, hot workability may decrease. Therefore, the content of S needs to be 0.015 mass % or less.

The smaller the content of S, the better. However, extreme reduction in the content of S may cause an increase in manufacturing cost. Therefore, the content of S may be 0.001 mass % or more.

(6) 0.10 Mass %≤Cu≤4.00 Mass %:

In a Fe—Ni-based alloy (particularly, a Fe—Ni-based alloy containing 30 mass % or more of Ni), Cu has an effect of reducing a thermal expansion coefficient and an effect of increasing a magnetic transformation point, similarly to Ni. In order to achieve such an effect, the content of Cu needs to be 0.10 mass % or more. The content of Cu is preferably 1.60 mass % or more, and more preferably 1.80 mass % or more.

In contrast, in the case where the content of Cu is excessive, the thermal expansion coefficient may increase. In addition, in the case where the content of Cu is excessive, component segregation may be promoted, and hot workability may decrease. Therefore, the content of Cu needs to be 4.00 mass % or less.

(7) 35.0 Mass %≤Ni≤45.0 Mass %:

In the Fe—Ni-based alloy, the thermal expansion coefficient is the smallest when the content of Ni is around 36 mass %. In the case where the content of Ni is more than 36 mass %, the thermal expansion coefficient increases.

On the other hand, the magnetic transformation point, which is the transition temperature of a ferromagnetic material and a paramagnetic material, tends to increase as the content of Ni increases. At a temperature that is higher than the magnetic transformation point, the thermal expansion coefficient significantly increases. In order to maintain a low thermal expansion coefficient even in a high-temperature range of 280° C. or higher, the content of Ni needs to be 35.0 mass % or more.

Ni is an expensive element. Therefore, in the case where the content of Ni is excessive, the raw material cost increases.

In addition, in the case where the content of Ni is excessive, the thermal expansion coefficient at a temperature equal to or lower than the magnetic transformation point may increase. Therefore, the content of Ni needs to be 45.0 mass % or less. The content of Ni is more preferably 40.0 mass % or less.

(8) 0.10 Mass %≤V≤1.00 Mass %:

V has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. In order to achieve such an effect, the content of V needs to be 0.10 mass % or more.

On the other hand, in the case where V is excessively added more than the amount necessary for precipitating carbides, the thermal expansion coefficient may increase, and the magnetic transformation point may decrease. Therefore, the content of V needs to be 1.00 mass % or less. The content of V is preferably 0.50 mass % or less.

(9) Unavoidable Impurities:

The low thermal expansion alloy according to the present invention may contain unavoidable impurities. Here, the “unavoidable impurities” are components mixed due to various factors such as a raw material and a production process when the low thermal expansion alloy is industrially produced, and the content thereof is within a range that does not adversely affect the properties of the low thermal expansion alloy according to the present invention.

Examples of the unavoidable impurities include the following elements in addition to Si, P, and S described above. In the case where the content of each of the following elements is equal to or less than the following upper limit, such elements are treated as “unavoidable impurities” in the low thermal expansion alloy according to the present invention.

The low thermal expansion alloy according to the present invention may contain any one of the following unavoidable impurities, or may contain two or more thereof. However, in order to maintain high properties of the low thermal expansion alloy, a total content of unavoidable impurities is preferably 1.0 mass % or less, and more preferably 0.5 mass % or less.

(9.1) Sn≤0.010 Mass %:

In the case where the content of Sn is excessive, hot workability may decrease. Therefore, the content of Sn is preferably 0.010 mass % or less.

The smaller the amount of Sn, the better. However, extreme reduction in the amount of Sn causes an increase in manufacturing cost. Therefore, the content of Sn may be more than 0 mass %.

(9.2) Zn≤0.010 Mass %:

In the case where the content of Zn is excessive, hot workability may decrease. Therefore, the content of Zn is preferably 0.010 mass % or less.

The smaller the content of Zn, the better. However, extreme reduction in the amount of Zn causes an increase in manufacturing cost. Therefore, the content of Zn may be more than 0 mass %.

(9.3) As≤0.010 Mass %:

In the case where the content of As is excessive, hot workability may decrease. Therefore, the content of As is preferably 0.010 mass % or less.

The smaller the content of As, the better. However, in the case where As and Cu coexist, the corrosion resistance of the alloy may be improved. Therefore, the content of As may be more than 0 mass %.

(9.4) Se≤0.010 Mass %:

In the case where the content of Se is excessive, hot workability may decrease. Therefore, the content of Se is preferably 0.010 mass % or less.

The smaller the content of Se, the better. However, an appropriate amount of Se has an effect of improving the machinability of the alloy. Therefore, the content of Se may be more than 0 mass %.

(9.5) Sb≤0.010 Mass %:

In the case where the content of Sb is excessive, the thermal expansion coefficient may increase. Therefore, the content of Sb is preferably 0.010 mass % or less.

The smaller the content of Sb, the better. However, in the case where Sb and Cu coexist, the corrosion resistance of the alloy may be improved. Therefore, the content of Sb may be more than 0 mass %.

(9.6) Ag≤0.010 Mass %:

In the case where the content of Ag is excessive, hot workability may be reduced. Therefore, the content of Ag is preferably 0.010 mass % or less.

The smaller the content of Ag, the better. However, extreme reduction in the content of Ag causes an increase in manufacturing cost. Therefore, the content of Ag may be more than 0 mass %.

(9.7) Bi≤0.0010 Mass %:

In the case where the content of Bi is excessive, hot workability may be reduced. Therefore, the content of Bi is preferably 0.0010 mass % or less.

The smaller the content of Bi, the better. However, extreme reduction in the content of Bi causes an increase in manufacturing cost. Therefore, the content of Bi may be more than 0 mass %.

(9.8) O≤0.050 Mass %:

In the case where the content of O is excessive, the thermal expansion coefficient of the steel may increase. Therefore, the content of O is preferably 0.050 mass % or less.

The smaller the content of O, the better. However, extreme reduction in the content of O causes an increase in manufacturing cost. Therefore, the content of O may be more than 0 mass %.

(9.9) N≤0.050 Mass %:

In the case where the content of N is excessive, the thermal expansion coefficient of the steel may increase. Therefore, the content of N is preferably 0.050 mass % or less.

The smaller the content of N, the better. However, extreme reduction in the content of N causes an increase in manufacturing cost. Therefore, the content of N may be more than 0 mass %.

[1.2. Sub Constituent Element]

The low thermal expansion alloy according to the present invention may further include one or two or more elements described below in addition to the main constituent elements and the unavoidable impurities described above. Types of added elements, component ranges thereof, and reasons for limitation thereof are as follows.

(1) 0 Mass %≤Cr≤0.50 Mass %:

The content of Cr may be 0 mass %. However, an extreme reduction in the content of Cr may cause an increase in manufacturing cost. In addition, Cr has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Cr is preferably 0.01 mass % or more.

On the other hand, in the case where the content of Cr is excessive, the thermal expansion coefficient may increase, and the magnetic transformation point may decrease. Therefore, the content of Cr is preferably 0.50 mass % or less.

(2) 0 Mass %≤Mo≤4.00 Mass %:

The content of Mo may be 0 mass %. However, an extreme reduction in the content of Mo may cause an increase in manufacturing cost. In addition, Mo has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Mo is preferably 0.01 mass % or more.

On the other hand, in the case where the content of Mo is excessive, the thermal expansion coefficient may increase, and the magnetic transformation point may decrease. Therefore, the content of Mo is preferably 4.00 mass % or less.

(3) 0≤Co≤0.50 Mass %:

The content of Co may be 0 mass %. However, an extreme reduction in the content of Co may cause an increase in manufacturing cost. Therefore, the content of Co is preferably 0.01 mass % or more.

On the other hand, Co is an expensive element, and therefore, an excessive content of Co may cause an increase in manufacturing cost. Therefore, the content of Co is preferably 0.50 mass % or less.

(4) 0 Mass %≤Al≤0.50 Mass %:

The content of Al may be 0 mass %. In addition to being a deoxidizing element, Al also has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Al is preferably more than 0 mass %.

On the other hand, in the case where the content of Al is excessive, the ductility may decrease and the thermal expansion coefficient may increase. Therefore, the content of Al is preferably 0.50 mass % or less.

(5) 0 Mass %≤Ti≤0.50 Mass %:

The content of Ti may be 0 mass %. Ti has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Ti is preferably more than 0 mass %.

On the other hand, in the case where the content of Ti is excessive, the ductility may decrease, and the thermal expansion coefficient may increase. Therefore, the content of Ti is preferably 0.50 mass % or less.

(6) 0 Mass %≤Nb≤0.50 Mass %:

The content of Nb may be 0 mass %. Nb has an effect of refining crystal grains and improving a tensile strength. Therefore, the content of Nb is preferably more than 0 mass %.

On the other hand, in the case where the content of Nb is excessive, the hot workability may decrease. Therefore, the content of Nb is preferably 0.50 mass % or less.

(7) 0 Mass %≤W≤0.50 Mass %:

The content of W may be 0 mass %. W has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of W is preferably more than 0 mass %.

On the other hand, in the case where the content of W is excessive, the ductility may decrease and the thermal expansion coefficient may increase. Therefore, the content of W is preferably 0.50 mass % or less.

(8) 0 Mass %≤Zr≤0.50 Mass %:

The content of Zr may be 0 mass %. In addition to being a deoxidizing element, Zr also has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Zr is preferably more than 0 mass %.

On the other hand, in the case where the content of Zr is excessive, the ductility may decrease and the thermal expansion coefficient may increase. Therefore, the content of Zr is preferably 0.50 mass % or less.

(9) 0≤Hf≤0.50 Mass %:

The content of Hf may be 0 mass %. Hf has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Hf is preferably more than 0 mass %.

On the other hand, in the case where the content of Hf is excessive, the ductility may decrease and the thermal expansion coefficient may increase. Therefore, the content of Hf is preferably 0.50 mass % or less.

(10) 0 Mass %≤Ta≤0.50 Mass %:

The content of Ta may be 0 mass %. Ta has an effect of improving a tensile strength by solid solution strengthening and precipitation strengthening by precipitation of carbides. Therefore, the content of Ta is preferably more than 0 mass %.

On the other hand, in the case where the content of Ta is excessive, the ductility may decrease and the thermal expansion coefficient may increase. Therefore, the content of Ta is preferably 0.50 mass % or less.

(11) 0 Mass %≤B≤0.050 Mass %:

The content of B may be 0 mass %. B has an effect of improving hot workability by grain boundary strengthening and an effect of improving grain boundary oxidation resistance. Therefore, the content of B is preferably more than 0 mass %.

On the other hand, in the case where the content of B is excessive, hot workability may be rather decreased. Therefore, the content of B is preferably 0.050 mass % or less.

(12) 0 Mass %≤Mg≤0.050 Mass %:

The content of Mg may be 0 mass %. Mg has an effect of fixing S to improve hot workability and an effect of reducing a thermal expansion coefficient. Therefore, the content of Mg is preferably more than 0 mass %.

On the other hand, in the case where the content of Mg is excessive, the hot workability may decrease. Therefore, the content of Mg is preferably 0.050 mass % or less.

(13) 0≤Ca≤0.050 Mass %:

The content of Ca may be 0 mass %. Ca has an effect of fixing S to improve hot workability and an effect of reducing a thermal expansion coefficient. Therefore, the content of Ca is preferably more than 0 mass %.

On the other hand, in the case where the content of Ca is excessive, the hot workability may be rather decreased. Therefore, the content of Ca is preferably 0.050 mass % or less.

(14) 0 Mass %≤REM≤0.050 Mass %:

The content of REM may be 0 mass %. REM has an effect of improving oxidation resistance in a high-temperature environment and expanding a usable temperature range. Therefore, the content of REM is preferably more than 0 mass %. Examples of REM include Nd, Pr, La, Ce, and Y.

On the other hand, an excessive content of REM may cause a significant increase in manufacturing cost. Therefore, the content of REM is preferably 0.050 mass % or less.

[1.3. Component Balance]

The low thermal expansion alloy according to the present invention needs to satisfy the following formulas (1) and (2).

A ≥ 38. ( 1 ) 40.6 < B ≤ 4 ⁢ 4 . 5 ( 2 )
Here,

A = [ Ni ] + [ Co ] + 0.7 * [ Cu ] - [ Si ] - [ Mn ] - [ Cr ] - 0.5 * [ Mo ] - 0.5 * [ V ] , B = [ Ni ] + 0.8 * [ Co ] + [ Cu ] + 6 * [ C ] + 1.1 * [ Si ] + [ Mn ] + 1.2 * [ Cr ] + 0.2 * [ Mo ] - 0.5 * [ V ] .

[1.3.1. Formula (1)]

The value Ain the formula (1) is an index correlated with the magnetic transformation point of the low thermal expansion alloy. When the components of the alloy are optimized so as to satisfy the formula (1) and the manufacturing conditions are optimized, the magnetic transformation point can be increased. In general, as the value A increases, the magnetic transformation point increases.

Specifically, when the components of the alloy are optimized so that the value A is 38.0 or more and the manufacturing conditions are optimized, the magnetic transformation point can be 280° C. or higher.

Similarly, when the components of the alloy are optimized so that the value Ais 38.5 or more and the manufacturing conditions are optimized, the magnetic transformation point can be 290° C. or higher.

Furthermore, when the components of the alloy are optimized so that the value A is 39.0 or more and the manufacturing conditions are optimized, the magnetic transformation point can be 300° C. or higher.

[1.3.2. Formula (2)]

The value B in the formula (2) represents an index correlated with the thermal expansion coefficient of the low thermal expansion alloy. When the components of the alloy are optimized so as to satisfy the formula (2) and the manufacturing conditions are optimized, the thermal expansion coefficient can be maintained at a value close to that of the semiconductor. In general, as the value B increases, the thermal expansion coefficient increases.

Specifically, when the components of the alloy are optimized so that the value B is more than 40.6 and 44.5 or less and the manufacturing conditions are optimized, the thermal expansion coefficient can be 3.5×10⁻⁶/° C. to 6.0×10⁻⁶/° C.

[1.4. Properties]

[1.4.1. Tensile Strength]

The “tensile strength” refers to a value obtained by performing a tensile test at room temperature (25° C.) in the atmosphere using a round bar-shaped No. 3 test piece having a parallel portion with a diameter of 6 mm in accordance with ASTM A370-17.

When C and V are added to the alloy in appropriate amounts and fine carbides are precipitated in the matrix, a high tensile strength is obtained. When the manufacturing conditions are optimized, the tensile strength of the alloy can be 500 MPa or more. When the manufacturing conditions are further optimized, the tensile strength can be 550 MPa or more, or 600 MPa or more.

[1.4.2. Thermal Expansion Coefficient]

In the present invention, the “thermal expansion coefficient” refers to an average linear thermal expansion coefficient from 30° C. to 100° C. measured by using a measurement device (TMA8310, manufactured by Rigaku), in accordance with ASTM E228-17.

When the content of Co is reduced and the components in the alloy are optimized so as to satisfy the above formula (2), the thermal expansion coefficient of the alloy can be maintained at a value close to that of a semiconductor in a wide temperature range.

When the manufacturing conditions are optimized, the thermal expansion coefficient of the alloy can be 3.5×10⁻⁶/° C. to 6.0×10⁻⁶/° C.

[1.4.3. Magnetic Transformation Point]

When the low thermal expansion alloy is heated to a temperature that is higher than the magnetic transformation point, the thermal expansion coefficient increases. In order to achieve low thermal expansion properties in a high-temperature environment, the higher the magnetic transformation point of the alloy, the better. When the components of the alloy are optimized so as to satisfy the above formula (1), the magnetic transformation point can be increased. When the manufacturing conditions are optimized, the magnetic transformation point of the alloy can be 280° C. or higher. When the manufacturing conditions are further optimized, the magnetic transformation point can be 290° C. or higher or 300° C. or higher.

[1.5. Structure]

As described below, the low thermal expansion alloy according to the present invention can be processed into various shapes through melting and casting, primary hot working, secondary hot working, annealing, and post-processing.

The low thermal expansion alloy according to the present invention may be in any state including an as-hot worked state, an as-annealed state, and a state in which necessary post-processing is performed after annealing. In order to dissolve coarse carbonitrides and reduce the manufacturing cost, the low thermal expansion alloy preferably includes a portion in an as-annealed state.

Here, “including a portion in an as-annealed state” means that (a) the entire low thermal expansion alloy is in an as-annealed state, or (b) a portion of the low thermal expansion alloy is subjected to necessary processing (for example, cutting) but the other portions are in an as-annealed state.

[1.6. Shape]

A shape of the low thermal expansion alloy in the present invention is not particularly limited, and an optimum shape can be selected according to a purpose thereof. Examples of the shape of the low thermal expansion alloy include a pipe, a bar, a wire rod, and a plate.

[2. Method for Manufacturing Low Thermal Expansion Alloy]

A method for manufacturing a low thermal expansion alloy according to the present invention includes: a melting and casting process of melting and casting raw materials blended so as to have a predetermined composition to obtain an ingot; a primary hot working process of performing primary hot working on the obtained ingot; a secondary hot working process of performing secondary hot working on the material subjected to the primary hot working; as necessary, an annealing process of performing annealing on the material subjected to the secondary hot working; and as necessary, a post-processing process of performing post-processing on the material subjected to the secondary hot working or the annealed material.

[2.1. Melting and Casting Process]

First, the raw materials blended so as to have a predetermined composition are melted and cast to obtain an ingot. Methods and conditions for melting and casting the raw material are not particularly limited, and optimum methods and conditions can be selected according to the purpose thereof. For manufacturing a molten metal, for example, an electric furnace, an argon oxygen decarburization (AOD) furnace, a vacuum oxygen decarburization (VOD) furnace, or the like can be used.

It should be noted that, if necessary, the obtained ingot may be subjected to a homogenization heat treatment for removing segregation.

[2.2. Primary Hot Working Process]

Next, a primary hot working is performed on the obtained ingot. The primary hot working is performed to destroy a coarse cast structure and refine the structure, and at the same time, convert the ingot into materials such as slabs, blooms, and billets. A primary hot working method is not particularly limited, and an optimal method can be selected according to the purpose. Examples of the primary hot working method include a hot forging and a hot rolling. A material such as a slab, a bloom, or a billet may be directly manufactured by continuous casting from the molten metal manufactured in the melting and casting process. In this case, the primary hot working process can be omitted.

[2.3. Secondary Hot Working Process]

Next, a secondary hot working is performed on the material subjected to the primary hot working. The secondary hot working is performed to finish the material subjected to the primary hot working into a final product shape (for example, a plate, a bar, a wire rod, or a pipe) or a shape close thereto. A secondary hot working method is not particularly limited, and an optimal method can be selected according to the purpose. Examples of the secondary hot working method include a hot rolling, a hot extrusion, and a hot piercing rolling.

Conditions of the secondary hot working are not particularly limited, and optimal conditions can be selected according to the purpose. In addition, the secondary hot working may be performed a plurality of times according to the purpose. The temperature at heating of the material performed before the secondary hot working is preferably 900° C. or higher and 1,300° C. or lower.

In addition, in the case where the secondary hot working is performed a plurality of times, the temperature of the material at the end of the finally performed secondary hot working is preferably 800° C. or higher and 1,200° C. or lower. This is to optimize crystal grains.

[2.4. Annealing Process]

Next, as necessary, annealing is performed on the material subjected to the secondary hot working. The annealing may be performed only once, or may be performed a plurality of times.

The annealing temperature may affect properties of the material. In the case where annealing is not performed or in the case where the annealing temperature is too low, a large amount of coarse carbonitrides may precipitate. As a result, the mechanical properties may deteriorate or the thermal expansion coefficient may increase. In addition, the crystal grain may be excessively refined, and the cuttability may decrease. Therefore, the annealing temperature is preferably 800° C. or higher. The annealing temperature is more preferably 900° C. or higher.

On the other hand, in the case where the annealing temperature is too high, the material may locally melt. In addition, since fine carbonitrides such as VC are completely dissolved, abnormal grain growth may occur, and mechanical properties may deteriorate. Therefore, the annealing temperature is preferably 1,300° C. or lower.

An optimal holding time at the annealing temperature can be selected according to the purpose. In general, as the annealing holding time increases, the number of fine crystal grains decreases. On the other hand, in the case where the holding time is lengthened more than necessary, the crystal grains may be excessively coarsened. The optimum holding time varies depending on the annealing temperature, but is usually 1 minute to 3 hours. After the holding time is ended, the material is cooled by water cooling, oil cooling, or air cooling, or at an equivalent cooling rate thereto.

The size of the crystal grains does not greatly affect the thermal expansion coefficient, but in the case where the crystal grains are too large, the mechanical properties may be adversely affected. The optimum grain size varies depending on the application, but is preferably #0 to #10. The grain size is more preferably #3 to #7.

[2.5. Post-Processing Process]

Next, as necessary, the post-processing is performed on the material subjected to the secondary hot working or the annealed material. Examples of the post-processing include cutting processing, welding, and cold working. A low thermal expansion alloy thus obtained is used for various applications.

[3. Effects]

When C and V are added to the alloy in appropriate amounts and the component balance between V and C is adjusted, fine carbides are precipitated in the matrix. Therefore, the high strength can be achieved while preventing deterioration in the thermal expansion coefficient due to solid solution of the elements (particularly, C).

In addition, when the value A is optimized (particularly, components of Ni and Cu are optimized) and the manufacturing conditions are optimized, a magnetic transformation point of 280° C. or higher can be achieved. Therefore, the low thermal expansion property can be maintained in a high-temperature environment.

Furthermore, when the value B is optimized (in particular, components of Ni and Cu are optimized) and the manufacturing conditions are optimized, the thermal expansion coefficient can be maintained in a range of 3.5×10⁻⁶/° C. to 6.0×10⁻⁶/° C. in a wide temperature range.

EXAMPLES

Examples 1 to 16 and Comparative Examples 1 to 20

[1. Preparation of Samples]

In a vacuum induction furnace, 5 kg of alloy having a composition shown in Tables 1 and 2 was melted and cast into an ingot. Thereafter, the hot forging and annealing are performed on the ingot to manufacture a bar material having a diameter of 15 mm.

TABLE 1
	Composition (mass %)
	C	Si	Mn	P	S	Cu	Ni	Cr	Mo
Example 1	0.23	0.10	0.20	0.002	0.001	2.01	39.1	0.4	2.15
Example 2	0.24	0.10	0.20	0.002	0.001	0.20	40.1	0.4	2.11
Example 3	0.23	0.10	0.20	0.003	0.000	1.99	39.1	0.4	0.01
Example 4	0.23	0.11	0.19	0.002	0.001	2.01	39.1	0.1	2.03
Example 5	0.23	0.11	0.20	0.003	0.001	2.00	39.1	0.1	0.01
Example 6	0.30	0.11	0.20	0.003	0.001	1.99	39.1	0.1	0.01
Example 7	0.23	0.10	0.20	0.003	0.001	3.90	36.0	0.1	0.01
Example 8	0.23	0.10	0.20	0.003	0.001	3.90	36.6	0.1	0.01
Example 9	0.23	0.70	0.19	0.002	0.001	2.01	39.1	0.1	2.03
Example 10	0.23	0.11	1.00	0.002	0.001	2.01	39.1	0.1	2.03
Example 11	0.23	0.11	0.19	0.002	0.001	1.70	39.1	0.1	2.03
Example 12	0.23	0.11	0.19	0.002	0.001	1.00	39.1	0.1	2.03
Example 13	0.24	0.09	0.20	0.001	0.001	2.20	38.8	0.3	2.14
Example 14	0.23	0.08	0.21	0.001	0.001	2.20	38.8	0.3	2.15
Example 15	0.25	0.09	0.20	0.001	0.001	2.20	38.8	0.3	2.14
Example 16	0.25	0.09	0.20	0.001	0.001	2.20	38.8	0.3	2.14

Composition (mass %)

		V	Co	O	N	Others	Value A	Value B
	Example 1	0.42	0.01	0.001	0.001		38.6	43.5
	Example 2	0.40	0.01	0.001	0.001		38.4	42.7
	Example 3	0.40	0.03	0.001	0.001		39.6	43.0
	Example 4	0.41	0.05	0.001	0.001		39.0	43.1
	Example 5	0.40	0.03	0.001	0.001		40.0	42.7
	Example 6	0.40	0.04	0.001	0.001		39.9	43.0
	Example 7	0.40	0.10	0.001	0.001		38.2	41.6
	Example 8	0.40	0.10	0.001	0.001		38.8	42.2
	Example 9	0.41	0.05	0.001	0.001		38.4	43.7
	Example 10	0.41	0.05	0.001	0.001		38.2	43.9
	Example 11	0.41	0.05	0.001	0.001		38.8	42.8
	Example 12	0.41	0.05	0.001	0.001		38.3	42.1
	Example 13	0.41	0.01	0.001	0.001	Al: 0.008	38.5	43.3
						Ti: 0.01
						W: 0.01
	Example 14	0.41	0.01	0.001	0.001	Mg: 0.008	38.5	43.3
						Ca: 0.002
						Zr: 0.003
						La: 0.002
	Example 15	0.41	0.01	0.001	0.001	B: 0.008	38.5	43.4
						Nb: 0.004
	Example 16	0.41	0.31	0.001	0.001	Ta: 0.03	38.8	43.6
						Hf: 0.05

	TABLE 2
	Composition (mass %)

	C	Si	Mn	P	S	Cu	Ni	Cr	Mo
Comp. Ex. 1	0.01	0.09	0.29	0.002	0.001	0.00	29.9	0.0	0.03
Comp. Ex. 2	0.00	0.12	0.09	0.002	0.001	0.01	41.2	0.0	0.02
Comp. Ex. 3	0.23	0.11	0.21	0.003	0.001	0.20	39.1	0.4	2.15
Comp. Ex. 4	0.22	0.14	0.25	0.003	0.001	0.00	37.7	0.4	2.06
Comp. Ex. 5	0.23	0.10	0.19	0.003	0.001	4.10	39.0	0.1	0.01
Comp. Ex. 6	0.30	0.10	0.20	0.003	0.000	4.10	39.1	0.1	0.01
Comp. Ex. 7	0.22	0.12	0.25	0.003	0.001	0.09	37.1	0.9	0.10
Comp. Ex. 8	0.20	1.62	0.61	0.001	0.002	2.50	38.6	0.4	0.03
Comp. Ex. 9	0.24	0.46	0.32	0.001	0.001	1.20	38.2	0.1	0.02
Comp. Ex. 10	0.27	0.10	0.24	0.006	0.002	0.18	37.9	0.2	2.03
Comp. Ex. 11	0.25	0.07	0.20	0.009	0.002	0.32	37.8	0.3	3.55
Comp. Ex. 12	0.33	0.29	0.22	0.013	0.012	0.38	38.0	0.2	2.52
Comp. Ex. 13	0.26	0.08	0.33	0.012	0.001	0.14	38.1	0.1	3.44
Comp. Ex. 14	0.28	0.05	0.28	0.010	0.003	0.28	37.5	0.2	2.28
Comp. Ex. 15	0.30	0.12	0.52	0.009	0.002	0.20	37.9	0.1	3.21
Comp. Ex. 16	0.28	0.09	0.20	0.001	0.009	0.02	37.9	0.2	0.22
Comp. Ex. 17	0.40	0.10	0.20	0.001	0.002	0.02	50.0	0.1	2.00
Comp. Ex. 18	0.40	0.10	0.50	0.001	0.002	0.05	38.5	0.5	1.50
Comp. Ex. 19	0.21	0.60	0.50	0.001	0.002	0.02	40	0.8	0.60
Comp. Ex. 20	0.22	0.10	0.20	0.001	0.002	0.02	38	0.8	1.40

Composition (mass %)

	V	Co	O	N	Others	Value A	Value B
Comp. Ex. 1	0.01	16.40	0.001	0.001		45.9	43.5
Comp. Ex. 2	0.01	0.10	0.001	0.001		41.1	41.6
Comp. Ex. 3	0.40	0.01	0.001	0.001		37.3	41.7
Comp. Ex. 4	0.36	0.07	0.001	0.001	Mg: 0.0049	35.8	40.1
Comp. Ex. 5	0.40	0.03	0.001	0.001		41.3	44.6
Comp. Ex. 6	0.40	0.04	0.001	0.001		41.4	45.2
Comp. Ex. 7	0.80	0.50	0.001	0.001	Al: 0.010	35.9	40.0
Comp. Ex. 8	0.60	0.02	0.001	0.001		37.4	44.9
Comp. Ex. 9	0.81	0.01	0.001	0.001		37.8	41.3
Comp. Ex. 10	0.82	0.02	0.001	0.004		36.1	40.3
Comp. Ex. 11	0.27	0.02	0.001	0.004		35.5	40.9
Comp. Ex. 12	0.95	0.02	0.001	0.003		35.8	41.2
Comp. Ex. 13	0.55	0.02	0.001	0.006		35.7	40.8
Comp. Ex. 14	0.93	0.02	0.001	0.006		35.5	40.1
Comp. Ex. 15	0.43	0.02	0.001	0.006		35.6	41.1
Comp. Ex. 16	0.66	0.02	0.001	0.001		37.0	39.9
Comp. Ex. 17	0.15	0.02	0.001	0.001		48.6	53.1
Comp. Ex. 18	0.15	0.02	0.001	0.001		36.6	42.4
Comp. Ex. 19	0.70	0.10	0.002	0.020		37.6	43.3
Comp. Ex. 20	1.00	0.20	0.001	0.001		35.9	40.6

[2. Test Method]

[2.1 Evaluation of Tensile Strength]

A round bar tensile test piece was taken from a central portion of each bar material. The parallel portion of the round bar tensile test piece was parallel to a longitudinal direction of the bar material. The diameter of the parallel portion was 6 mm. The tensile strength TS (MPa) was obtained by performing a tensile test on the round bar tensile test piece at normal temperature (25° C.) in the atmosphere in accordance with ASTM A370-17.

A case where the tensile strength TS (MPa) was 500 MPa or more was evaluated as “A (high strength)”, and a case where the tensile strength TS (MPa) was less than 500 MPa was evaluated as “B (not high strength)”.

[2.2 Evaluation of Average Linear Thermal Expansion Coefficient]

A round bar test piece was taken from a central portion of each bar material. The longitudinal direction of the round bar test piece was parallel to the longitudinal direction of the bar material. The obtained round bar test piece was used to measure the thermal expansion coefficient. The thermal expansion coefficient was measured in a temperature range of 20° C. to 800° C. by using a measurement device (TMA8310, manufactured by Rigaku), in accordance with ASTM E228-17.

A case where the average linear thermal expansion coefficient from 30° C. to 100° C. was 3.5×10⁻⁶/° C. or more and 6.0×10⁻⁶/° C. or less was evaluated as “A (good)”, and a case where the average linear thermal expansion coefficient was less than 3.5×10⁻⁶/° C. or more than 6.0×10⁻⁶/° C. was evaluated as “B (poor)”.

[2.3. Evaluation of Magnetic Transformation Point]

The magnetic transformation point was determined from a chart of displacement with respect to a test temperature, which was obtained during measurement of the thermal expansion coefficient. A bending point appearing in the chart was defined as the magnetic transformation point.

A case where the magnetic transformation point was 280° C. or higher and lower than 290° C. was evaluated as “C (good)”, a case where the magnetic transformation point was 290° C. or higher and lower than 300° C. was evaluated as “B (better)”, a case where the magnetic transformation point was 300° C. or higher was evaluated as “A (best)”, and a case where the magnetic transformation point was lower than 280° C. was evaluated as “D (poor)”.

[3. Results]

The results are shown in Table 3. The followings can be understood from Tables 1 to 3.

• • (1) In Comparative Examples 1 and 2, the tensile strength decreased. The reason is considered to be that the content of C and/or the content of V were/was small.
  • (2) In Comparative Example 3, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38.
  • (3) In Comparative Example 4, the magnetic transformation point was lower than 280° C. The reason is considered to be that the content of Cu was small and the value A was less than 38. In Comparative Example 4, the thermal expansion coefficient was less than 3.5×10⁻⁶/° C. The reason is considered to be that the value B was 40.6 or less.
  • (4) In Comparative Examples 5 and 6, the thermal expansion coefficient was more than 6.0×10⁻⁶/° C. The reason is considered to be that the content of Cu was excessive and the value B was more than 44.5.
  • (5) In Comparative Example 7, the magnetic transformation point was lower than 280° C. The reason is considered to be that the content of Cr was excessive and the value A was less than 38. In Comparative Example 7, the thermal expansion coefficient was less than 3.5×10⁻⁶/° C. The reason is considered to be that the content of Cr is excessive and the value B is 40.6 or less.
  • (6) In Comparative Example 8, the magnetic transformation point was lower than 280° C., and the reason is considered to be that the content of Si was excessive and the value A was less than 38. In Comparative Example 8, the thermal expansion coefficient was more than 6.0×10⁻⁶/° C. The reason is considered to be that the content of Si was excessive and the value B was more than 44.5.
  • (7) In Comparative Example 9, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38.
  • (8) In Comparative Example 10, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38. In Comparative Example 10, the thermal expansion coefficient was less than 3.5×10⁻⁶/° C. The reason is considered to be that the value B was 40.6 or less.
  • (9) In Comparative Examples 11 to 13, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38.
  • (10) In Comparative Example 14, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38. In Comparative Example 14, the thermal expansion coefficient was less than 3.5×10⁻⁶/° C. The reason is considered to be that the value B was 40.6 or less.
  • (11) In Comparative Example 15, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38.
  • (12) In Comparative Example 16, the magnetic transformation point was lower than 280° C. The reason is considered to be that the content of Cu was small and the value A was less than 38. In Comparative Example 17, the thermal expansion coefficient was less than 3.5×10⁻⁶/° C. The reason is considered to be that the value B was 40.6 or less.
  • (13) In Comparative Example 17, the thermal expansion coefficient was more than 6.0×10⁻⁶/° C. The reason is considered to be that the content of Ni was excessive and the value B was more than 44.6.
  • (14) In Comparative Example 18, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38.
  • (15) In Comparative Example 19, the magnetic transformation point was lower than 280° C. The reason is considered to be that the content of Cr was excessive and the value A was less than 38.
  • (16) In Comparative Example 20, the magnetic transformation point was lower than 280° C. The reason is considered to be that the value A was less than 38. In Comparative Example 20, the thermal expansion coefficient was less than 3.5×10⁻⁶/° C. The reason is considered to be that the value B was 40.6 or less.
  • (17) In Examples 1 to 16, the tensile strength was more than 600 MPa. In addition, in Examples 1 to 16, the magnetic transformation point was 280° C. or higher. Further, in Examples 1 to 16, the thermal expansion coefficient was 3.5×10⁻⁶/° C. to 6.0×10⁻⁶/° C.
  • (18) In Examples 3 to 6, the magnetic transformation point was 300° C. or higher. The reason is considered to be that the value A was 39.0 or more.

	TABLE 3
	Properties

		Magnetic	Average linear
	Tensile	transformation	thermal expansion
	strength	point	coefficient (×10−6/° C.)

Example 1	A	B	A
Example 2	A	B	A
Example 3	A	A	A
Example 4	A	A	A
Example 5	A	A	A
Example 6	A	A	A
Example 7	A	C	A
Example 8	A	B	A
Example 9	A	C	A
Example 10	A	C	A
Example 11	A	B	A
Example 12	A	C	A
Example 13	A	B	A
Example 14	A	B	A
Example 15	A	B	A
Example 16	A	B	A
Comp. Ex. 1	B	A	A
Comp. Ex. 2	B	A	A
Comp. Ex. 3	A	D	A
Comp. Ex. 4	A	D	B
Comp. Ex. 5	A	A	B
Comp. Ex. 6	A	A	B
Comp. Ex. 7	A	D	B
Comp. Ex. 8	A	D	B
Comp. Ex. 9	A	D	A
Comp. Ex. 10	A	D	B
Comp. Ex. 11	A	D	A
Comp. Ex. 12	A	D	A
Comp. Ex. 13	A	D	A
Comp. Ex. 14	A	D	B
Comp. Ex. 15	A	D	A
Comp. Ex. 16	A	D	B
Comp. Ex. 17	A	A	B
Comp. Ex. 18	A	D	A
Comp. Ex. 19	A	D	A
Comp. Ex. 20	A	D	B

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

INDUSTRIAL APPLICABILITY

The low thermal expansion alloy according to the present invention can be used for a diaphragm of a pressure sensor, a lead of a hermetic seal, and the like.