JP2019163536A - Low thermal expansion alloy and method for producing the same - Google Patents

Abstract

To provide a low thermal expansion alloy having a low thermal expansion coefficient and high Young's modulus, and further having high strength and excellent ductility.SOLUTION: A low thermal expansion alloy has a chemical composition containing, in mass%, at least one selected from the group consisting of C: 0.20-2.00%, Mn: 0.05-2.00%, V: 0.80-10.00%, Ni: 30.00-40.00%, Si: more than 0 to 0.50% and Al: more than 0 to 0.100%, Co: 0-10.00%, and Cr: 0-3.00%, with the balance being Fe and impurities, and satisfies formula (1) and formula (2). In the low thermal expansion alloy, vanadium carbides constitute 2.5-12.5 vol.%; the vanadium carbides with an equivalent circle diameter of less than 200 nm is 50/μmor more; and coarse vanadium carbides with an equivalent circle diameter of 1.0 μm or more have an average equivalent circle diameter of less than 2.8 μm. 30.00≤Ni+Co≤40.00 (1), -0.50<V-50.94/12.01×C<2.00 (2).SELECTED DRAWING: Figure 1

Description

  The present invention relates to an alloy and a manufacturing method thereof, and more particularly to a low thermal expansion alloy and a manufacturing method thereof.

  Invar (trademark) alloy is known as a low thermal expansion alloy. Invar alloys have a low coefficient of thermal expansion in the range of room temperature to 300 ° C. due to spontaneous volume magnetostriction (Invar effect). Therefore, the dimensions are not easily changed even under the influence of heat. Invar alloys are used for members of devices that require high dimensional accuracy, such as machine tools and precision measuring instruments.

  However, the Invar alloy has a small coefficient of thermal expansion but a low Young's modulus and tensile strength. For example, Young's modulus is about 140 GPa, which is as low as about 2/3 of general steel. Therefore, it is difficult to use an Invar alloy for a member that requires rigidity and strength.

  Low thermal expansion alloys having high rigidity or high strength while maintaining a low coefficient of thermal expansion are proposed in Japanese Patent Application Laid-Open No. 2015-178672 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2017-172444 (Patent Document 2). Has been.

  The low thermal expansion alloy described in Patent Document 1 aims at a low thermal expansion coefficient and a high Young's modulus. The low thermal expansion alloy of this document is mass%, C: 0.2-2.0%, Si: 0.05-1.0%, Mn: 0.05-2.0%, Al: 0.01 -0.14%, V: 0.8-10.0%, Ni: 30.0-40.0%, Co: 0-10.0%, and at least one of Nb and Ti: 0-4 0.0%, with the balance being Fe and impurities and having a chemical composition satisfying 30.0 ≦ Ni + Co ≦ 40.0. The low thermal expansion alloy contains a specific carbide containing any one of V, Nb, and Ti in a volume ratio of 2.5 to 12.5%. Thereby, the alloy which has a low thermal expansion coefficient and a high Young's modulus is obtained, and the Young's modulus of 150-171 GPa is implement | achieved in the Example.

The low thermal expansion alloy described in Patent Document 2 aims at a low thermal expansion coefficient, a high Young's modulus, and a high tensile strength. The low thermal expansion alloy of this document is mass%, C: 0.2-2.0%, Mn: 0.05-2.0%, V: 0.8-10.0%, Ni: 30.0 ~ 10.0% selected from the group consisting of 40.0%, Si: 0.5% or less, and Al: 0.1% or less, Co: 0 to 10.0%, and Cr: 0 to 3.0 %, The balance is made of Fe and impurities, and has a chemical composition satisfying the formulas (1) and (2). Furthermore, it contains 2.5 to 12.5% by volume of vanadium carbide and contains 10 vanadium carbides having an equivalent circle diameter of less than 200 nm / μm ² or more. Here, Formula (1): 30.0 ≦ Ni + Co ≦ 40.0, Formula (2): −0.5 <V−50.94 / 12.01 × C <2.0. Thereby, a low low thermal expansion coefficient, a high Young's modulus, and a high tensile strength are obtained. In the examples, a Young's modulus of 150 to 168 GPa and a tensile strength of 800 to 910 MPa are realized.

Japanese Patent Laying-Open No. 2015-178672 JP 2017-172044 A

E. A. Owen, E .; L. Yates, and A.A. H. Sully: Proc. Phys. Soc. 49,323 (1937) Eremenko V.E. N. , Kharkova A .; M.M. , Velikanova T .; Y. : Isolational section of the vanadium-rhenium-carbon system at 1950 ° C. Dopovidi Akademii Nauk Ukrains'koi RSR, Seriya A: Fiziko-Matemachini ta Tekhini Nauki 5 (1984) 83-85 (in Ukrainian)

  The low thermal expansion alloys proposed in Patent Literature 1 and Patent Literature 2 have a low coefficient of thermal expansion and a high Young's modulus, and the low thermal expansion alloy proposed in Patent Literature 2 further has a high tensile strength. Recently, there has been a demand for a low thermal expansion alloy having higher strength while maintaining a low thermal expansion coefficient and a high Young's modulus. Recently, ductility is also required for low thermal expansion alloys.

  An object of the present disclosure is to provide a low thermal expansion alloy having a low coefficient of thermal expansion and a high Young's modulus, and further having high strength and excellent ductility.

The low thermal expansion alloy according to the present disclosure is:
Chemical composition is mass%,
C: 0.20 to 2.00%
Mn: 0.05-2.00%
V: 0.80 to 10.00%,
Ni: 30.00-40.00%,
One or more selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%;
Co: 0 to 10.00%,
Cr: 0 to 3.00%, and
Balance: Fe and impurities,
Satisfying the equations (1) and (2),
In the low thermal expansion alloy,
The volume fraction of vanadium carbide is 2.5 to 12.5%,
Fine vanadium carbide having an equivalent circle diameter of less than 200 nm is 50 pieces / μm ² or more,
The average equivalent circle diameter of coarse vanadium carbide having an equivalent circle diameter of 1.0 μm or more is less than 2.8 μm.
30.00 ≦ Ni + Co ≦ 40.00 (1)
−0.50 <V−50.94 / 12.01 × C <2.00 (2)
Here, the content (mass%) of the corresponding element is substituted into the element symbols of the formulas (1) and (2).

A method for producing a low thermal expansion alloy according to the present disclosure includes:
The chemical composition is
% By mass
C: 0.20 to 2.00%
Mn: 0.05-2.00%
V: 0.80 to 10.00%,
Ni: 30.00-40.00%,
One or more selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%;
Co: 0 to 10.00%,
Cr: 0 to 3.00%, and
Balance: Fe and impurities,
A hot forging process in which an alloy material is manufactured by hot forging after heating a material satisfying the formula (1) and the formula (2) to 1150 to 1300 ° C., in the hot forging process The cumulative reduction rate is 30.0% or more, the reduction rate per pass until the cumulative reduction rate becomes 30.0% is 5.0% or more, and the temperature of the material during hot forging is 900 ° C. A hot forging process,
A solution treatment step for carrying out a solution treatment for holding at 1000 to 1300 ° C. for 0.5 hours or more with respect to the alloy material after the hot forging step;
A cold working step of performing cold working at a cold working rate of 20.0% or more on the alloy material after the solution treatment step;
An aging heat treatment step of performing an aging heat treatment by holding the alloy material after the cold working step at 500 to 800 ° C. for 0.5 hour or more.

  The low thermal expansion alloy according to the present disclosure has a low coefficient of thermal expansion and a high Young's modulus, and further has high strength and excellent ductility. The low thermal expansion alloy manufacturing method according to the present disclosure can manufacture a low thermal expansion alloy having the above-described configuration.

FIG. 1 is a diagram showing a heat pattern in each step of the method for producing a low thermal expansion alloy of the present embodiment and a schematic diagram of a microstructure observation photograph after each step. FIG. 2 is a schematic diagram for explaining the rolling reduction rate and the cumulative rolling reduction rate in the hot forging process during the manufacturing process of the present embodiment.

  The inventor investigated and examined the thermal expansion coefficient, Young's modulus, tensile strength, and ductility of the low thermal expansion alloy. As a result, the present inventor obtained the following knowledge.

[About thermal expansion coefficient]
Ni increases the spontaneous volume magnetostriction of the alloy and, as a result, reduces the thermal expansion coefficient of the alloy. If Ni content is 30.00-40.00 mass%, the thermal expansion coefficient of an alloy will become low. Furthermore, Co can replace Ni. That is, Co also increases the spontaneous volume magnetostriction of the alloy and, as a result, reduces the thermal expansion coefficient of the alloy. In particular, the chemical composition is mass%, C: 0.20 to 2.00%, Mn: 0.05 to 2.00%, V: 0.80 to 10.00%, Ni: 30.00 to 40 One or more selected from the group consisting of 0.000%, Si: more than 0 to 0.50% and Al: more than 0 to 0.100%, Co: 0 to 10.00%, Cr: 0 to 3.00 %, And the balance: Fe and impurities, further satisfying the formula (1), the thermal expansion coefficient of the alloy becomes low.
30.00 ≦ Ni + Co ≦ 40.00 (1)
Here, the content (mass%) of the corresponding element is substituted into the element symbol of the formula (1).

[About Young's modulus]
In order to increase the Young's modulus of the alloy, it is effective to use an element of Group 4-6 in the periodic table (hereinafter referred to as a specific element). When the specific element exists in a solid solution state, the Young's modulus of the alloy becomes high. However, when the solid solution amount of the specific element exceeds a certain amount, the thermal expansion coefficient of the alloy increases rapidly. On the other hand, the Young's modulus of the alloy can also be increased by combining the specific element with the precipitate. However, in this case, the thermal expansion coefficient of the alloy increases due to thermal expansion of the precipitate.

  Therefore, the present inventor has studied to increase the Young's modulus of the alloy not by these methods but by a method of dispersing a compound having a low coefficient of thermal expansion and a high Young's modulus.

  Vanadium carbide (VC) has a low coefficient of thermal expansion and a high Young's modulus. Further, the vanadium carbide easily crystallizes or precipitates in the process where the molten alloy solidifies. Therefore, in this embodiment, vanadium carbide (VC) is used as a compound that increases the Young's modulus of the alloy having the above chemical composition. Specifically, the chemical composition is mass%, C: 0.20 to 2.00%, Mn: 0.05 to 2.00%, V: 0.80 to 10.00%, Ni: 30. One or more selected from the group consisting of 00 to 40.00%, Si: more than 0 to 0.50% and Al: more than 0 to 0.100%, Co: 0 to 10.00%, Cr: 0 to 0 In an alloy consisting of 3.00% and the balance: Fe and impurities and satisfying the formula (1), the vanadium carbide (VC) in the alloy is made 2.5 to 12.5% by volume. In this case, the Young's modulus increases in the alloy having the above chemical composition that satisfies the formula (1).

[Strength]
In the alloy having the chemical composition satisfying the formula (1), as described above, the Young's modulus has a correlation with the volume fraction of vanadium carbide. On the other hand, in the alloy having the chemical composition satisfying the formula (1), among the vanadium carbides in the steel, coarse vanadium carbides having an equivalent circle diameter of 1.0 μm or more are unlikely to contribute to the improvement of the strength of the alloy. On the other hand, fine vanadium carbide having an equivalent circle diameter of less than 200 nm contributes strongly to improving the strength of the alloy. Therefore, if the number density (pieces / μm ² ) of fine vanadium carbide having an equivalent circle diameter of less than 200 nm is increased, the strength is considered to increase dramatically. Hereinafter, vanadium carbide having an equivalent circle diameter of less than 200 nm is also referred to as “fine vanadium carbide”. A vanadium carbide having an equivalent circle diameter of 1.0 μm or more is referred to as “coarse vanadium carbide”. In addition, a circle equivalent diameter means the diameter when the area of vanadium carbide is converted into a circle as described later.

In order to increase the number density of fine vanadium carbide (pieces / μm ² ) in the alloy having the above chemical composition satisfying the formula (1), a solution treatment at a high temperature is performed on the alloy after hot working. It is conceivable that the vanadium carbide in the alloy is dissolved to some extent and then a large amount of fine vanadium carbide is precipitated by aging heat treatment.

  However, vanadium carbide is a compound that is stable even at high temperatures. And in the alloy of the above-mentioned chemical composition, when a liquid alloy solidifies in a casting process, it crystallizes or precipitates and coarse vanadium carbide is formed. Therefore, normally, even in a heat treatment at a high temperature such as a solution treatment, coarse vanadium carbide is hardly dissolved in the alloy. Therefore, even if an aging heat treatment is performed after the solution treatment, the fine vanadium carbide is not easily precipitated in the alloy.

Therefore, the present inventor has studied a method for dissolving the coarse vanadium carbide crystallized or precipitated in the alloy so that the fine vanadium carbide is sufficiently precipitated. As a result, the present inventor solidified a part of coarse vanadium carbide by solution treatment by appropriately adjusting the V content and the C content in the alloy having the above chemical composition satisfying the formula (1). It was found that it can be dissolved. Specifically, in the above chemical composition satisfying the formula (1), by further satisfying the formula (2), a part of coarse vanadium carbide crystallized or precipitated in the alloy is dissolved in the solution treatment. It becomes easy to do. As a result, in the subsequent aging heat treatment, fine vanadium carbide having an equivalent circle diameter of less than 200 nm can be precipitated to some extent.
−0.50 <V−50.94 / 12.01 × C <2.00 (2)
The element content (mass%) of the corresponding element is substituted into the element symbol of the formula (2).

[Examination of further increase in strength and ductility]
However, even in a low thermal expansion alloy having a chemical composition satisfying the formulas (1) and (2), as described in Patent Document 2, the number density of fine vanadium carbide having an equivalent circle diameter of less than 200 nm is 10 to 10. It is about 23 / μm ² . In this case, the tensile strength remains at about 910 MPa at the maximum.

Therefore, the present inventor has examined further improvement in strength. As described above, in the alloy having the above chemical composition satisfying the formulas (1) and (2), the total volume ratio of vanadium carbide strongly influences the Young's modulus. On the other hand, regarding the strength, coarse vanadium carbide hardly affects, and the number density of fine vanadium carbide having an equivalent circle diameter of less than 200 nm (pieces / μm ² ) strongly influences. Therefore, the present inventor considered that even if the total volume fraction of vanadium carbide that increases the Young's modulus is the same, the strength can be further increased if the number density of fine vanadium carbide can be further increased. .

  Therefore, the present inventor considered to further increase the number density of fine vanadium carbide by the following method. Cold working is performed after the solution treatment and before the aging heat treatment, and an appropriate strain (dislocation) is introduced into the alloy before the aging heat treatment. In this case, the introduced dislocations become vanadium carbide precipitation sites during the aging heat treatment. Therefore, the number density of fine vanadium carbide precipitated by aging heat treatment can be dramatically increased.

  Then, based on the said examination result, cold processing was implemented and the increase in the number density of fine vanadium carbide was tried. Specifically, after the solution treatment, cold working was performed, and after the cold working, an aging heat treatment was performed to produce a low thermal expansion alloy. However, in this case, it was found that although the number density of the fine vanadium carbide can be increased, the tensile strength of the low thermal expansion alloy is lowered and the ductility is sometimes lowered. Therefore, the present inventor further examined the cause of the decrease in tensile strength and ductility of the low thermal expansion alloy when cold working is performed.

  As a result of the examination, the cause of the decrease in tensile strength was considered as follows. When casting an alloy having the above chemical composition that satisfies the formulas (1) and (2), columnar or massive coarse vanadium carbides crystallize or precipitate in the solidification stage during casting. Once coarse vanadium carbide is produced by casting, a part of each coarse vanadium carbide is dissolved in the solution treatment, but the amount of the solid solution is limited. That is, once coarse vanadium carbide is produced by casting, the coarse vanadium carbide retains most of its size (although it will dissolve somewhat by satisfying equation (2)). If cold working is performed with the coarse vanadium carbide remaining in the alloy, it is considered that fine cracks are generated at the interface between the coarse vanadium carbide and the parent phase. Therefore, even if the aging heat treatment is performed after introducing strain by cold working and the number density of fine vanadium carbide is increased dramatically, if the coarse vanadium carbide remaining in the alloy is too large, The tensile strength and ductility of the expanded alloy are reduced. More specifically, when the elongation (breaking elongation) of the low thermal expansion alloy is used as an index of ductility, even if the number density of fine vanadium carbide is sufficient, if the average equivalent circle diameter of coarse vanadium carbide is too large, The product of the tensile strength and the elongation of the expanded alloy is reduced, and the tensile strength and ductility are reduced.

  Based on the above examination, in order to increase both the tensile strength and ductility of the low thermal expansion alloy, the present inventor crushed coarse vanadium carbide produced during solidification during hot forging, and obtained the size of coarse vanadium carbide. I thought that it was effective to make it smaller. If the size of the coarse vanadium carbide is reduced, the coarse vanadium can be prevented from becoming the starting point of cracking during processing. As a result, a decrease in tensile strength and ductility can be suppressed.

Therefore, as an example of the manufacturing method, if the rolling reduction rate per pass until the cumulative rolling reduction reaches 30.0% is set to 5.0% or more in the hot forging step, the coarse vanadium generated during solidification is sufficient. In a low thermal expansion alloy having a chemical composition satisfying formulas (1) and (2) and a total volume fraction of vanadium carbide of 2.5 to 12.5%, the equivalent circle diameter is 1.0 μm or more. The present inventors have found that the average equivalent circle diameter of the coarse vanadium carbide is less than 2.8 μm, and that the coarse vanadium can sufficiently suppress the starting point of cracks during processing. Furthermore, by setting the cold working rate during cold working to 20.0% or more, it has a chemical composition satisfying the formulas (1) and (2), and the total volume ratio of vanadium carbide is 2.5 to 2.5. In a 12.5% low thermal expansion alloy, the number density of vanadium carbide having an equivalent circle diameter of less than 200 nm can be 50 pieces / μm ² or more. As a result, the tensile strength can be increased and ductility can be increased. Also found that can be enhanced. In addition, the above-mentioned example of a manufacturing method is an example to the last. Therefore, the vanadium having the total volume fraction of vanadium carbide of 2.5 to 12.5% and the equivalent circle diameter of less than 200 nm in the alloy having the above chemical composition satisfying the formulas (1) and (2) by another method. The number density of carbides may be 50 / μm ² or more, and the average equivalent circle diameter of coarse vanadium carbide may be less than 2.8 μm.

  The gist of the low thermal expansion alloy according to the present embodiment completed based on the above technical idea is as follows.

The low thermal expansion alloy of [1]
Chemical composition is mass%,
C: 0.20 to 2.00%
Mn: 0.05-2.00%
V: 0.80 to 10.00%,
Ni: 30.00-40.00%,
One or more selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%;
Co: 0 to 10.00%,
Cr: 0 to 3.00%, and
Balance: Fe and impurities,
Satisfying the equations (1) and (2),
In the low thermal expansion alloy,
The volume fraction of vanadium carbide is 2.5 to 12.5%,
Fine vanadium carbide having an equivalent circle diameter of less than 200 nm is 50 pieces / μm ² or more,
The average equivalent circle diameter of coarse vanadium carbide having an equivalent circle diameter of 1.0 μm or more is less than 2.8 μm.
30.00 ≦ Ni + Co ≦ 40.00 (1)
−0.50 <V−50.94 / 12.01 × C <2.00 (2)
Here, the content (mass%) of the corresponding element is substituted into the element symbols of the formulas (1) and (2).

The low thermal expansion alloy of [2] is the low thermal expansion alloy of [1],
The chemical composition is
Co: 0.10 to 10.00% is contained.

The low thermal expansion alloy according to [3] is the low thermal expansion alloy according to [1] or [2],
The chemical composition is
Cr: 1.00 to 3.00% is contained.

The method for producing the low thermal expansion alloy of [4]
Chemical composition is mass%,
C: 0.20 to 2.00%
Mn: 0.05-2.00%
V: 0.80 to 10.00%,
Ni: 30.00-40.00%,
One or more selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%;
Co: 0 to 10.00%,
Cr: 0 to 3.00%, and
Balance: Fe and impurities,
A hot forging process in which an alloy material is manufactured by hot forging after heating a material satisfying the formula (1) and the formula (2) to 1150 to 1300 ° C., in the hot forging process The cumulative reduction rate is 30.0% or more, the reduction rate per pass until the cumulative reduction rate becomes 30.0% is 5.0% or more, and the temperature of the material during hot forging is 900 ° C. A hot forging process,
A solution treatment step for carrying out a solution treatment for holding at 1000 to 1300 ° C. for 0.5 hours or more with respect to the alloy material after the hot forging step;
A cold working step of performing cold working at a cold working rate of 20.0% or more on the alloy material after the solution treatment step;
An aging heat treatment step of performing an aging heat treatment by holding the alloy material after the cold working step at 500 to 800 ° C. for 0.5 hour or more.

The method for producing a low thermal expansion alloy according to [5] is the method for producing a low thermal expansion alloy according to [4],
The chemical composition is
Co: 0.10 to 10.00% is contained.

The method for producing a low thermal expansion alloy according to [6] is the method for producing a low thermal expansion alloy according to [4] or [5],
The chemical composition is
Cr: 1.00 to 3.00% is contained.

  Hereinafter, the low thermal expansion alloy of this embodiment and the manufacturing method of the low thermal expansion alloy will be described in detail. Hereinafter, “%” in the chemical composition means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the low thermal expansion alloy of this embodiment contains the following elements.

C: 0.20 to 2.00%
Carbon (C) combines with vanadium (V) to form vanadium carbide. The Young's modulus of vanadium carbide is high. Furthermore, the thermal expansion coefficient of vanadium carbide is low, about half that of austenite. Therefore, vanadium carbide can increase the Young's modulus while suppressing an increase in the thermal expansion coefficient of the alloy. If the C content is less than 0.20%, this effect cannot be obtained. On the other hand, if the C content exceeds 2.00%, C dissolves in the austenite which is the parent phase. If C dissolves in the matrix, the thermal expansion coefficient will increase. Therefore, the C content is 0.20 to 2.00%. The minimum with preferable C content is 0.30%, More preferably, it is 0.40%, More preferably, it is 0.80%. The upper limit with preferable C content is 1.80%, More preferably, it is 1.60%, More preferably, it is 1.20%.

Mn: 0.05 to 2.00%
Manganese (Mn) combines with the impurity sulfur (S) to improve the hot workability of the alloy. If the Mn content is less than 0.05%, this effect cannot be obtained. On the other hand, if the Mn content exceeds 2.00%, the spontaneous volume magnetostriction of the alloy decreases. As a result, the thermal expansion coefficient of the alloy is increased. Therefore, the Mn content is 0.05 to 2.00%. The minimum with preferable Mn content is 0.06%, More preferably, it is 0.08%, More preferably, it is 0.15%. The upper limit with preferable Mn content is 1.70%, More preferably, it is 1.50%, More preferably, it is 1.00%.

V: 0.80 to 10.00%
Vanadium (V) combines with carbon (C) and crystallizes or precipitates in the alloy as vanadium carbide. Thereby, the Young's modulus of the alloy can be increased while suppressing an increase in the thermal expansion coefficient of the alloy. Furthermore, by precipitating as fine vanadium carbide having an equivalent circle diameter of 200 nm, the tensile strength can be increased. If the V content is less than 0.80%, this effect cannot be obtained. On the other hand, if the V content exceeds 10.00%, V is excessively dissolved in the matrix phase, and as a result, the thermal expansion coefficient of the alloy increases. If the V content exceeds 10.00%, the vanadium carbide further coarsens, and as a result, the ductility of the alloy decreases. Therefore, the V content is 0.80 to 10.00%. The minimum with preferable V content is 1.00%, More preferably, it is 1.60%, More preferably, it is 3.20%. The upper limit with preferable V content is 9.00%, More preferably, it is 8.00%, More preferably, it is 6.00%.

Ni: 30.00-40.00%
Nickel (Ni) increases the spontaneous volume magnetostriction of the alloy and, as a result, reduces the thermal expansion coefficient of the alloy. If the Ni content is less than 30.00%, this effect cannot be obtained. On the other hand, if the Ni content exceeds 40.00%, the thermal expansion coefficient of the alloy increases on the contrary. Therefore, the Ni content is 30.00 to 40.00%. The minimum with preferable Ni content is 31.00%, More preferably, it is 32.00%, More preferably, it is 33.00%. The upper limit with preferable Ni content is 39.00%, More preferably, it is 38.00%, More preferably, it is 35.00%.

  The chemical composition of the low thermal expansion alloy further includes at least one selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%.

Si: more than 0 to 0.50%
Silicon (Si) deoxidizes the alloy. If Si is contained even a little, a deoxidizing effect can be obtained to some extent. However, if the Si content exceeds 0.50%, the spontaneous volume magnetostriction of the alloy decreases and the thermal expansion coefficient of the alloy increases. Therefore, the Si content is more than 0 to 0.50%. The minimum with preferable Si content is 0.01%, More preferably, it is 0.05%. The upper limit with preferable Si content is 0.30%, More preferably, it is 0.20%.

Al: more than 0 to 0.100%
Aluminum (Al) deoxidizes the alloy. If Al is contained even a little, a deoxidation effect can be obtained to some extent. However, if the Al content exceeds 0.100%, the spontaneous volume magnetostriction of the alloy decreases. As a result, the thermal expansion coefficient of the alloy is increased. Therefore, the Al content is more than 0 to 0.100%. The minimum with preferable Al content is 0.010%, More preferably, it is 0.020%, More preferably, it is 0.030%. The upper limit with preferable Al content is less than 0.090%, More preferably, it is 0.050%. In the present embodiment, the Al content is the total Al content.

  The balance of the chemical composition of the low thermal expansion alloy of this embodiment is Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or production environment as raw materials when the alloy is industrially produced, and do not adversely affect the low thermal expansion alloy of the present embodiment. Means what is allowed. Impurities are, for example, phosphorus (P), sulfur (S), nitrogen (N), oxygen (O) and the like. The contents of P, S, N and O are, for example, P: 0.02% or less, S: 0.005% or less, N: 0.02% or less, and O: 0.01% or less.

[Arbitrary elements]
The chemical composition of the low thermal expansion alloy of the present embodiment may further contain Co instead of a part of Fe.

Co: 0 to 10.00%
Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When contained, Co, like Ni, lowers the thermal expansion coefficient of the alloy. However, if the Co content exceeds 10.00%, the thermal expansion coefficient will increase. Therefore, the Co content is 0 to 10.00%. The minimum with preferable Co content is more than 0%, More preferably, it is 0.10%, More preferably, it is 2.00%, More preferably, it is 4.00%. The upper limit with preferable Co content is 9.00%, More preferably, it is 8.00%, More preferably, it is 6.00%.

  The chemical composition of the low thermal expansion alloy of this embodiment may further contain Cr instead of part of Fe.

Cr: 0 to 3.00%
Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When contained, Cr dissolves in the alloy and increases the Young's modulus of the alloy. However, if the Cr content exceeds 3.00%, the thermal expansion coefficient increases due to Cr excessively dissolved in the matrix. Therefore, the Cr content is 0 to 3.00%. The minimum with preferable Cr content is more than 0%, More preferably, it is 1.00%, More preferably, it is 1.50%. The upper limit with preferable Cr content is 2.50%, More preferably, it is 2.00%.

[Regarding Formula (1)]
The chemical composition of the low thermal expansion alloy of the present embodiment further satisfies the formula (1).
30.00 ≦ Ni + Co ≦ 40.00 (1)
The element content (mass%) of the corresponding element is substituted into the element symbol of the formula (1).

  Define F1 = Ni + Co. F1 is the total content of Ni and Co in the alloy. As described above, both Ni and Co lower the thermal expansion coefficient in the alloy having the above chemical composition. If F1 is less than 30.00, a sufficiently low thermal expansion coefficient cannot be obtained in an alloy having the above chemical composition. On the other hand, even if F1 exceeds 40.00, a sufficiently low thermal expansion coefficient cannot be obtained in the above chemical composition. In the said chemical composition, if F1 is 30.00-40.00, a sufficiently low thermal expansion coefficient will be obtained. The minimum with preferable F1 is 32.00, More preferably, it is 33.00. The preferable upper limit of F1 is 38.00.

[Regarding Formula (2)]
The chemical composition further satisfies formula (2).
−0.50 <V−50.94 / 12.01 × C <2.00 (2)
The element content (mass%) of the corresponding element is substituted into the element symbol of the formula (2).

It is defined as F2 = V−50.94 / 12.01 × C. 50.94 is the atomic weight of V and 12.01 is the atomic weight of C. F2 represents the relationship between the V content and the C content in the alloy. The vanadium carbide in the alloy having the chemical composition satisfying the formula (1) is almost VC, and other vanadium carbides (composite carbides and the like) can be ignored. As described above, vanadium carbide is a compound that is stable even at high temperatures. Therefore, it is difficult to solidify vanadium carbide in the alloy by a solution treatment described later. However, in the above chemical composition satisfying the formula (1), if F2 satisfies the formula (2), a part of the vanadium carbide in the alloy can be dissolved by the solution treatment. The solid solution V and C are precipitated in the alloy to some extent as fine vanadium carbide having an equivalent circle shape of less than 200 nm by cold working and aging heat treatment performed after the solution treatment. In the above chemical composition satisfying the formula (1), F2 satisfies the formula (2), which is an essential condition for setting the vanadium carbide having an equivalent circle diameter of less than 200 nm to 50 pieces / μm ² or more in the present embodiment. One.

  If F2 is 2.00 or more, the amount of V dissolved is too large. On the other hand, if F2 is −0.50 or less, the amount of C dissolved is too large. In any case, the vanadium carbide is stabilized, and therefore the vanadium carbide is hardly dissolved even when the solution treatment is performed. Therefore, even if an aging heat treatment is performed after the solution treatment, fine vanadium carbide cannot be sufficiently precipitated. In this case, an alloy having sufficient tensile strength cannot be obtained. Therefore, F2 is more than −0.50 and less than 2.00. The lower limit of F2 is preferably -0.10. The upper limit of F2 is preferably 1.00.

[Volume ratio of vanadium carbide]
In the low thermal expansion alloy of this embodiment, the volume fraction of vanadium carbide is 2.5 to 12.5%. If the alloy contains a sufficient volume fraction of vanadium carbide, the Young's modulus of the alloy is increased. Vanadium carbide crystallizes or precipitates in the alloy when the molten metal solidifies. Vanadium carbide is also an alloy having the above-described chemical composition satisfying the formulas (1) and (2), and a part of the alloy is solid-solubilized during the solution treatment, and the solution treatment and the aging heat treatment after the cold working are performed. Precipitates finely.

  The Young's modulus of vanadium carbide is very high. Therefore, the vanadium carbide can increase the Young's modulus of the low thermal expansion alloy having the above chemical composition that satisfies the formulas (1) and (2). In the low thermal expansion alloy having the above chemical composition satisfying the formulas (1) and (2), the Young's modulus depends not on the size (equivalent circle diameter) or number of vanadium carbides but on the volume ratio (%). Specifically, if the volume fraction of vanadium carbide in the alloy having the chemical composition satisfying the formulas (1) and (2) is less than 2.5%, the Young's modulus of the low thermal expansion alloy is low. In addition, in the case of the said chemical composition satisfy | filling Formula (1) and Formula (2), the upper limit of the volume fraction of vanadium carbide is 12.5% at most. Therefore, in this embodiment, the volume ratio of vanadium carbide is 2.5 to 12.5%.

  If the total volume fraction of vanadium carbide in the alloy is 2.5 to 12.5%, the Young's modulus of the alloy having the above chemical composition that satisfies the formulas (1) and (2) can be sufficiently increased.

  A preferable lower limit of the volume fraction of vanadium carbide in the alloy having the above chemical composition satisfying the formulas (1) and (2) is 5.9%. A preferable upper limit of the total volume ratio of vanadium carbide in the alloy having the above chemical composition satisfying the formulas (1) and (2) is 10.0%.

The total volume fraction of vanadium carbide is measured by the following method. Collect specimens from any location on the low thermal expansion alloy. For example, when the low thermal expansion alloy is a plate material, the sample material is collected from the center position of the plate width. When the low thermal expansion alloy is cylindrical (bar material), the specimen is collected from the R / 2 position (the center position of the radius R of the cross section) in the cross section perpendicular to the longitudinal direction of the low thermal expansion alloy. If the low thermal expansion alloy is a tube, collect the specimen from the center of the wall thickness. The size of the test material is, for example, 10 mm in length, 10 mm in width, and 10 mm in thickness. The collected sample is electrolyzed using a 10% AA electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte). The current during electrolysis is 20 mA / cm ² .

  The electrolytic solution obtained by the electrolysis is filtered through a filter having a pore size of 200 nm, and the mass of the residue is measured. In the alloy having the above chemical composition satisfying the formulas (1) and (2), there are negligibly few inclusions and precipitates other than the vanadium carbide. Therefore, it can be considered that the residue obtained by filtering with the filter is all vanadium carbide. That is, in this specification, the residue obtained by filtering with the filter is all vanadium carbide. The amount of electrolysis is determined from the total mass of the test material before electrolysis and the total mass of the test material after electrolysis. And the mass of the residue obtained by filtering with a filter is calculated | required. As described above, all the residues obtained are regarded as vanadium carbide. The molar fraction of vanadium carbide is calculated from the mass of electrolysis and the mass of the residue. Next, the volume fraction (volume%) of the vanadium carbide is calculated based on the lattice constant of the matrix (low thermal expansion alloy) and the lattice constant of the vanadium carbide, using the obtained mole fraction. In this specification, the lattice constant of the matrix (low thermal expansion alloy) uses 3.59% of Invar alloy of Non-Patent Document 1. The lattice constant of vanadium carbide is 4.17% of VC of Non-Patent Document 2.

  In the above method for measuring the total volume fraction of vanadium carbide, vanadium carbide having an equivalent circle diameter of less than 200 nm is not substantially contained in the residue (that is, the residue is vanadium carbide having an equivalent circle diameter of 200 nm or more). However, the volume ratio of vanadium carbide having an equivalent circle diameter of less than 200 nm is remarkably small, and can be ignored in the total volume ratio of vanadium carbide. Therefore, in this specification, the total volume ratio measured by the above method is defined as the total volume ratio (%) of vanadium carbide.

[Vanadium carbide in low thermal expansion alloys]
In the low thermal expansion alloy of the present embodiment, vanadium carbide having an equivalent circle diameter of less than 200 nm and coarse vanadium carbide having an equivalent circle diameter of 1.0 μm or more are mixed. That is, the vanadium carbide in the low thermal expansion alloy of the present embodiment includes vanadium carbide (fine vanadium carbide) having an equivalent circle diameter of less than 200 nm and vanadium carbide (coarse vanadium carbide) having an equivalent circle diameter of 1.0 μm or more. .

  Fine vanadium carbide increases the strength of the low thermal expansion alloy by precipitation strengthening. Specifically, if the number density of fine vanadium carbide is large, the tensile strength is increased. On the other hand, coarse vanadium carbide affects the tensile strength and ductility. Specifically, if the average equivalent-circle diameter of the coarse vanadium carbide is too large, the coarse vanadium tends to be the starting point of cracking, and the ductility (elongation) decreases. If the ductility (elongation) decreases, the tensile strength also decreases. As a result, tensile strength × elongation (MPa ·%), which is an index of balance between strength and ductility, decreases. Therefore, if the average equivalent circle diameter of coarse vanadium is small, both tensile strength and ductility, that is, tensile strength × elongation can be increased.

  Hereinafter, the number density of fine vanadium carbide and the average equivalent circle diameter of coarse vanadium carbide in the low thermal expansion alloy of the present embodiment will be described.

[Number density of vanadium carbide having an equivalent circle diameter of less than 200 nm]
In the low thermal expansion alloy of the present embodiment, the number density of vanadium carbide (fine vanadium carbide) having an equivalent circle diameter of less than 200 nm is 50 / μm ² or more. The lower limit of the equivalent circle diameter of the fine vanadium carbide is 1 nm. Here, the equivalent circle diameter means a diameter when the area of the specified vanadium carbide is converted into a circle in observation with a transmission electron microscope described later.

In a low thermal expansion alloy that satisfies the formulas (1) and (2), the total volume fraction of vanadium carbide is 2.5 to 12.5%, and the average equivalent circle diameter of coarse vanadium carbide is less than 2.8 μm, If the number density of the vanadium carbide is 50 / μm ² or more, the strength can be dramatically increased while maintaining a low thermal expansion coefficient and a high Young's modulus. In the low thermal expansion alloy of this embodiment, by setting the number density of fine vanadium carbide to 50 pieces / μm ² or more, which could not be realized in the past, the content of each element in the chemical composition is within the range of this embodiment. In the low thermal expansion alloy that satisfies the formulas (1) and (2), the total volume fraction of vanadium carbide is 2.5 to 12.5%, and the average equivalent circle diameter of the coarse vanadium carbide is less than 2.8 μm A tensile strength of 950 MPa or more can be realized.

If the fine vanadium carbide is less than 50 / μm ² , the content of each element in the chemical composition is within the range of the present embodiment, and satisfies the formulas (1) and (2), and the coarse vanadium carbide. Even if the average equivalent circle diameter is less than 2.8 μm, the tensile strength of the low thermal expansion alloy is less than 950 MPa. Therefore, the fine vanadium carbide in the low thermal expansion alloy is 50 pieces / μm ² or more. The fine vanadium carbide in the low thermal expansion alloy is preferably 60 pieces / μm ² or more. The upper limit of the number density of fine vanadium carbide in the low thermal expansion alloy is not particularly limited, but is, for example, 1500 / μm ² .

The number density of fine vanadium carbide is measured by the following method. Collect specimens from any location on the low thermal expansion alloy. For example, when the low thermal expansion alloy is a plate material, the sample material is collected from the center position of the plate width. When the low thermal expansion alloy is cylindrical (bar material), the specimen is collected from the R / 2 position (the center position of the radius R of the cross section) in the cross section perpendicular to the longitudinal direction of the low thermal expansion alloy. If the low thermal expansion alloy is a tube, collect the specimen from the center of the wall thickness. The specimen is mechanically polished to a thickness of 70 μm. Further, the surface (observation surface) of the test material is polished by a twin jet polishing method (electrolytic solution: methanol perchlorate (perchloric acid 10%, methanol 90%)). Microscopic observation is carried out on the observation surface of the polished specimen using a transmission electron microscope. The observation visual field is 300 nm × 500 nm. A dark field image formed using a diffraction spot of fine vanadium carbide is obtained. The number density of fine vanadium carbide having an equivalent circle diameter of 1 nm or more and less than 200 nm is measured by image software. The number of fine vanadium carbides is divided by the field area of the dark field image to obtain the number density (pieces / μm ² ) of the fine vanadium carbides of this embodiment. The equivalent circle diameter of fine vanadium carbide that can be specified by this method is 1 nm or more.

[Average equivalent circle diameter of vanadium carbide with equivalent circle diameter of 1.0 μm or more]
In the low thermal expansion alloy of this embodiment, the average particle diameter of vanadium carbide (coarse vanadium carbide) having an equivalent circle diameter of 1.0 μm or more is less than 2.8 μm. The content of each element in the chemical composition is within the range of the present embodiment, satisfies the formulas (1) and (2), the total volume ratio of vanadium carbide is 2.5 to 12.5%, and fine vanadium. Even if the number of carbides is 50 / μm ² or more, if the average particle size of the coarse vanadium carbide is 2.8 μm or more, the ductility is lowered even if the strength can be increased by the fine vanadium carbide. As a result, tensile strength × elongation (MPa ·%) decreases.

The content of each element in the chemical composition is within the range of the present embodiment, satisfies the formulas (1) and (2), the total volume ratio of vanadium carbide is 2.5 to 12.5%, and fine vanadium. When the number of carbides is 50 / μm ² or more and the average particle size of the coarse vanadium carbide is less than 2.8 μm, the coarse vanadium carbide is less likely to be a starting point of cracking, and ductility (elongation) is improved. As a result, not only the strength but also the ductility (elongation) is increased. Specifically, the tensile strength × elongation (MPa ·%) is 5000 MPa ·% or more.

  The average equivalent circle diameter of coarse vanadium carbide is measured by the following method. A test material having a surface parallel to the longitudinal direction of the low thermal expansion alloy is collected. The longitudinal direction corresponds to the forging direction, for example. For example, when the low thermal expansion alloy is a plate material, the sample material is collected from the center position of the plate width. When the low thermal expansion alloy is cylindrical (bar material), the specimen is collected from the R / 2 position (the center position of the radius R of the cross section) in the cross section perpendicular to the longitudinal direction of the low thermal expansion alloy. If the low thermal expansion alloy is a tube, collect the specimen from the center of the wall thickness. Of the surfaces of the test material, the surface parallel to the longitudinal direction is taken as the observation surface. The observation surface of the specimen is polished with emily paper, and then buffed with diamond. A reflected electron image is obtained using a scanning electron microscope with respect to arbitrary 28 visual fields in the observation surface of the specimen after buffing. Each field of view is 180 μm × 420 μm. A photographic image of each field of view is generated, and binarization processing is performed to distinguish the precipitate from the parent phase. Precipitates and matrix can be clearly distinguished by contrast. Furthermore, it can be considered that precipitates other than vanadium carbide are not present in the microstructure of the low thermal expansion alloy of the present embodiment. Therefore, vanadium carbide can be specified by the binarization process.

  The equivalent circle diameter of each specified vanadium carbide is determined. And the vanadium carbide | carbonized_material whose circular equivalent diameter is 1.0 micrometer or more is specified. The average equivalent circle diameter of the vanadium carbide having an equivalent circle diameter of 1.0 μm or more, which is specified in 28 visual fields, is obtained. The average equivalent circle diameter is a value obtained by rounding off the second decimal place (that is, the first decimal place value).

As described above, in the low thermal expansion alloy of the present embodiment, each element in the chemical composition is within the range of the present embodiment, satisfies the formulas (1) and (2), and the vanadium carbide in the alloy The volume ratio is 2.5 to 12.5%, the number density of fine vanadium carbide is 50 pieces / μm ² or more, and the average equivalent circle diameter of coarse vanadium carbide is less than 2.8 μm. Therefore, the low thermal expansion alloy of the present embodiment not only has a high Young's modulus while maintaining a low thermal expansion coefficient, but also has a high tensile strength and an excellent ductility. Specifically, in the low thermal expansion alloy of the present embodiment having the above-described configuration, the thermal expansion coefficient is 4.0 × 10 ⁻⁶ / ° C. or less, the Young's modulus is 150 GPa or more, and the tensile strength is 950 MPa or more. Yes, the product of tensile strength and elongation is 5000 MPa ·% or more.

[Measurement method of thermal expansion coefficient]
Here, the thermal expansion coefficient of the low thermal expansion alloy of the present embodiment can be obtained by the following method. Test specimens are taken from any location on the low thermal expansion alloy. For example, when the low thermal expansion alloy is a plate material, a test piece is collected from the center position of the plate width. When the low thermal expansion alloy is cylindrical (bar), a test piece is taken from the R / 2 position (the center position of the radius R of the cross section) in the cross section perpendicular to the longitudinal direction of the low thermal expansion alloy. If the low thermal expansion alloy is a tube, take a specimen from the center of the wall thickness. The test piece has a cylindrical shape with a diameter of 3 mm and a length of 15 mm. A thermal expansion coefficient is calculated | required based on JISZ2285 (2003) using a test piece. A horizontal differential expansion mechanical analyzer is used to measure the thermal expansion coefficient. Specifically, the test piece is heated at a rate of 5 ° C./min, and a thermal expansion coefficient of 30 to 100 ° C. is obtained at a 1 ° C. pitch. Let the average of the calculated | required thermal expansion coefficient be the thermal expansion coefficient ( ^x10 < ^-6 > / degreeC) of the low thermal expansion alloy of this embodiment.

[Measurement method of Young's modulus]
The Young's modulus of the low thermal expansion alloy of this embodiment can be obtained by the following method. Test specimens are taken from any location on the low thermal expansion alloy. For example, when the low thermal expansion alloy is a plate material, a test piece is collected from the center position of the plate width. When the low thermal expansion alloy is cylindrical (bar), a test piece is taken from the R / 2 position (the center position of the radius R of the cross section) in the cross section perpendicular to the longitudinal direction of the low thermal expansion alloy. If the low thermal expansion alloy is a tube, take a specimen from the center of the wall thickness. The test piece has a length of 60 mm, a width of 10 mm, and a thickness of 1.5 mm. Using the test material, Young's modulus at room temperature (20 ° C. ± 15 ° C.) is measured according to JIS Z 2280 (1993). In the measurement of Young's modulus, a measuring device using a transverse resonance method is used.

[Measurement method of tensile strength (TS) and elongation (EL)]
The tensile strength TS (MPa) and elongation EL (%) of the low thermal expansion alloy of this embodiment are determined by the following method. Test specimens are taken from any location on the low thermal expansion alloy. For example, when the low thermal expansion alloy is a plate material, a test piece is collected from the center position of the plate width. When the low thermal expansion alloy is cylindrical (bar), a test piece is taken from the R / 2 position (the center position of the radius R of the cross section) in the cross section perpendicular to the longitudinal direction of the low thermal expansion alloy. If the low thermal expansion alloy is a tube, take a specimen from the center of the wall thickness. The test piece is a tensile test piece having a parallel part length of 65 mm and a parallel part diameter of 6 mm. The parallel part length is parallel to the longitudinal direction (forging direction) of the low thermal expansion alloy. Using the collected tensile test piece, a tensile test is performed at room temperature (20 ° C. ± 15 ° C.) and in the atmosphere in accordance with JIS Z 2241 (2011) to obtain a stress-strain curve. Tensile strength TS (MPa) and elongation EL (%) are determined from the obtained stress-strain curve. In this specification, the elongation is the elongation at break. Based on the obtained tensile strength TS and elongation EL, a product (TS × EL) of the tensile strength TS and the elongation EL is obtained.

[Production method]
An example of the manufacturing method of the low thermal expansion alloy of this embodiment is demonstrated below. In addition, the low thermal expansion alloy of this embodiment is not limited to the following manufacturing methods. The above chemical composition satisfying formula (1) and formula (2), wherein the total volume fraction of vanadium carbide is 2.5 to 12.5%, and the number density of fine vanadium carbide having an equivalent circle diameter of less than 200 nm is The production method is not particularly limited as long as a low thermal expansion alloy having an average equivalent circle diameter of less than 2.8 μm of coarse vanadium carbide having a circle equivalent diameter of 1.0 μm or more and 50 pieces / μm ² or more can be produced.

  FIG. 1 is a diagram showing a heat pattern as an example of the manufacturing process of the low thermal expansion alloy of the present embodiment and a schematic diagram of the microstructure of the alloy after the completion of each process. With reference to FIG. 1, the manufacturing method of the low thermal expansion alloy of this embodiment has a casting process (S1), a hot forging process (S2), a solution treatment process (S3), and cold work as an example. A process (S4) and an aging heat treatment process (S5) are included. Hereinafter, each step will be described.

[Casting process (S1)]
In the casting step (S1), an alloy material (cast material) having a chemical composition satisfying the expressions (1) and (2) is manufactured. The casting method may be an ingot casting method or a continuous casting method. In the case of a chemical composition satisfying the formulas (1) and (2), coarse vanadium carbides crystallize or precipitate in the casting process. Therefore, as shown in the microstructure (A) in FIG. 1, the material includes a large number of columnar or massive coarse vanadium carbides 200 together with the matrix 100.

[Hot forging process (S2)]
In the hot forging step (S2), hot forging is performed on the material produced by the casting step to produce an alloy material. The hot forging step (S2) not only forms the shape of the low thermal expansion alloy, but also has a role of crushing and reducing the coarse vanadium carbide 200 in the material. Specifically, the material is heated to a heating temperature T2 = 1150 to 1300 ° C.

Hot forging is performed on the heated material. Hot forging may be forging or upsetting. Specifically, the material is reduced by a plurality of passes. Here, “pass” means that the material is squeezed once using a hot forging machine. The rolling reduction Pn (%) per pass is defined as follows. Here, n means the number of passes, and the reduction rate of the first pass is denoted as P1, and the reduction rate of the second pass following the first pass is denoted as P2.
Pn = {1− (thickness in the reduction direction of the material after the nth pass reduction / thickness in the reduction direction of the material before the nth pass reduction)} × 100
Here, the thickness in the rolling direction means the thickness Tn (mm) in the rolling direction in the cross section 10 including the rolling direction P in the material 1 shown in FIG.

Further, the cumulative reduction ratio Pt (%) in the hot forging process is defined as follows.
Cumulative rolling reduction Pt = (1−cross-sectional area in the section including the rolling direction of the alloy material after hot forging / cross-sectional area in the section including the rolling direction of the material before hot forging) × 100
Here, the cross section in the rolling direction means the cross section 10 shown in FIG. In addition, when a raw material is not a rectangular parallelepiped shape, let the cross-sectional area in the cross section containing the rolling direction of the raw material before hot forging be a cross-sectional area of the minimum cross section among the cross sections 10 in the raw material before hot forging.

  In this manufacturing method, the cumulative reduction ratio in the hot forging step is set to 30.0% or more. Furthermore, the rolling reduction rate in each pass in the hot forging step until the cumulative rolling reduction rate becomes 30.0% is set to 5.0% or more. Furthermore, the temperature of the cast material during hot forging is set to 900 ° C. or higher.

  In the casting step (S1), if a material having a chemical composition satisfying the formula (1) and the formula (2) is manufactured, the columnar or massive coarse vanadium carbide 200 is formed in the material in the casting step (S1) as described above. Crystallizes or precipitates. Therefore, it cannot be avoided that the material contains columnar or massive coarse vanadium carbide 200. Therefore, in the present manufacturing method, in the hot forging process, rolling is performed under the above conditions, and the coarse vanadium carbide 200 in the alloy material is crushed by rolling per pass. Thereby, in the alloy material after the hot working step (S2), as shown in the microstructure (B) of FIG. 1, coarse vanadium carbide 200 is crushed and coarse vanadium carbide 210 smaller than coarse vanadium carbide 200 is obtained. It becomes.

  In addition, the temperature of the raw material during hot forging (that is, from the start of hot forging to completion of hot forging) is maintained in a known temperature range, for example, 900 ° C. or more. Here, the temperature of the cast material during hot forging means the surface temperature of the material. The temperature of the raw material during hot forging can be measured by thermometers installed on the entry side and the exit side of the hot forging device. During hot forging, if the temperature of the material is likely to be below 900 ° C., the material is charged into a heating furnace or a soaking furnace and heated again before the temperature of the material is below 900 ° C. That is, the material may be heated during the pass during hot forging.

  Through the above steps, hot forging is performed on the material to produce an alloy material.

[Solution Treatment Step (S3)]
In the solution treatment step (S3), the solution treatment is performed on the alloy material after the hot forging step (S2). By performing the solution treatment, as shown in the microstructure (C) of FIG. 1, the coarse vanadium carbide in the alloy material is solidified from a part of the coarse vanadium carbide 210 immediately after the hot forging step (S2). To obtain coarse vanadium carbide 220. In the solution treatment, the alloy material is heated to a solution treatment temperature T3 of 1000 to 1300 ° C. and held at the solution treatment temperature T3. The holding time is, for example, 0.5 hours or longer. The upper limit of the holding time is not particularly limited, but is 100 hours, for example, considering the manufacturing cost.

  In the present specification, the solution treatment temperature T3 (° C.) means the furnace temperature (° C.) of the heat treatment furnace. The furnace temperature of the heat treatment furnace can be measured by a thermometer arranged inside the heat treatment furnace.

  The hot forging step (S2) is performed under the above-described conditions to produce an alloy material, and the solution treatment is performed on the alloy material under the above-described conditions. Thereby, a part of each coarse vanadium carbide 210 is solid-dissolved to become coarse vanadium carbide 220, and the solid-dissolved vanadium carbide is converted into fine vanadium carbide 250 (microstructure in FIG. 1) by subsequent cold working and aging heat treatment. Many precipitates as shown in FIG.

If the solution treatment temperature T3 is less than 1000 ° C., a part of the coarse vanadium carbide 210 is not sufficiently dissolved. In this case, even if a cold working process and an aging heat treatment process described later are performed, the fine vanadium carbide 250 having an equivalent circle diameter of less than 200 nm does not sufficiently precipitate, and the number density of the fine vanadium carbide is less than 50 / μm ^2. It becomes. On the other hand, if the solution treatment temperature T3 is higher than 1300 ° C, the alloy material is likely to partially melt. Therefore, the solution treatment temperature T3 is set to 1000 to 1300 ° C. In addition, after holding time passes at the said solution treatment temperature, an alloy material is rapidly cooled. The rapid cooling is, for example, water cooling.

[Cold working process (S4)]
In the cold working step (S4), cold working is performed on the alloy material after the solution treatment step (S3). As cold working, cold forging may be performed, cold drawing may be performed, or cold rolling may be performed. The cumulative cold working rate in cold working is 20.0% or more. Here, the cumulative cold work rate CW is defined by the following equation.
Cold work rate CW = (1−Cross sectional area of alloy material after cold working process / Cross sectional area of alloy material before cold working process) × 100
In the cold work rate CW, the cross-sectional area of the alloy material means an area of a cross section perpendicular to the longitudinal direction (axial direction) of the alloy material.

In this manufacturing method, the cold working step (S4) is performed on the alloy material after the solution treatment step (S3) to introduce strain into the alloy material. This strain (dislocation) functions as a precipitation site for the fine vanadium carbide 250 in the next aging heat treatment step (S5). Therefore, the number density (pieces / μm ² ) of the fine vanadium carbide 250 precipitated in the next aging heat treatment step (S5) is remarkably increased as compared with the case where the cold working step (S4) is not performed.

If the cold work rate is less than 20.0%, the introduction of dislocations as precipitation sites for the fine vanadium carbide 250 is insufficient. Therefore, even if the hot working step (S2) that satisfies the above conditions is performed, the fine vanadium carbide 250 is not sufficiently precipitated, and the number density of the fine vanadium carbide 250 is less than 50 / μm ² . If the cold working rate is 20.0% or more, the number density of fine vanadium carbide 250 in the alloy after the aging heat treatment step (S5) is 50 / μm on the premise that the conditions are satisfied in other manufacturing steps. ² or more. Furthermore, in the hot forging step, the rolling reduction in each pass in hot forging until the cumulative rolling reduction reaches 30.0% is set to 5% or more. Thereby, the coarse vanadium carbide 200 is crushed. Therefore, it is possible to suppress a reduction in ductility due to the remaining coarse vanadium carbide.

[Aging heat treatment step (S5)]
In the aging heat treatment step (S5), the aging heat treatment step (S5) is performed on the alloy material after the cold working step to precipitate fine vanadium carbide in the alloy material. In the aging heat treatment step (S5), the aging heat treatment temperature T5 is set to 500 to 800 ° C., and the holding time at the aging heat treatment temperature T5 is set to 0.5 hours or more. The upper limit of the holding time is not particularly limited, but is 500 hours, for example, considering the manufacturing cost.

  In this specification, the aging heat treatment temperature T5 (° C.) means the furnace temperature (° C.) of a heat treatment furnace for performing the aging heat treatment. The furnace temperature of the heat treatment furnace can be measured by a thermometer arranged inside the heat treatment furnace.

An aging heat treatment is performed on the alloy material after the cold working step (S4) under the above-described conditions. By satisfying the conditions in the hot working step (S2), the coarse vanadium carbide 200 in the cast material is crushed to produce a coarse vanadium carbide 210 smaller than the coarse vanadium carbide 200, and further a solution treatment step (S3). Thus, a part of coarse vanadium carbide 210 is dissolved to produce coarse vanadium carbide 220. Further, after introducing strain (dislocation) into the alloy material by the cold working step (S4), aging heat treatment under the above conditions is performed. In this case, the alloy material subjected to the aging heat treatment is a state in which the coarse vanadium carbide 200 is made sufficiently small by the hot working step (S2) and the solution treatment step (S3) and a part of the vanadium carbide 200 is dissolved. In addition, dislocations functioning as precipitation sites for the fine vanadium carbide 250 are sufficiently introduced. Therefore, as a result of the aging heat treatment under the above-described conditions, a large number of fine vanadium carbides 250 are generated together with the coarse vanadium carbides 220 in the alloy material as shown in the microstructure (D) in FIG. Specifically, in the alloy material after the aging heat treatment step (S5), the average equivalent circle diameter of coarse vanadium carbide having an equivalent circle diameter of 1.0 μm or more is less than 2.8 μm, and the equivalent circle diameter is less than 200 nm. The number density of the fine vanadium carbide becomes 50 pieces / μm ² or more.

If the aging heat treatment temperature T5 is less than 500 ° C or higher than 800 ° C, fine vanadium carbide is not sufficiently precipitated. In this case, the strength of the alloy is not sufficiently high, and the tensile strength is less than 950 MPa. If the aging heat treatment time is less than 0.5 hours, fine vanadium carbide is not sufficiently precipitated. In this case, the number density of fine vanadium carbide having an equivalent circle diameter of less than 200 nm is less than 50 / μm ² .

The low thermal expansion alloy of this embodiment can be manufactured by the above manufacturing process. As described above, the chemical composition satisfying the formula (1) and the formula (2), the total volume ratio of the vanadium carbide is 2.5 to 12.5%, and the equivalent circle diameter is less than 200 nm. If a low thermal expansion alloy in which the number density of vanadium carbide is 50 pieces / μm ² or more and the average equivalent circle diameter of coarse vanadium carbide having a circle equivalent diameter of 1.0 μm or more is less than 2.8 μm can be manufactured, The method for producing the low thermal expansion alloy is not particularly limited.

  Hereinafter, the effect of one aspect of the low thermal expansion alloy of the present embodiment will be described more specifically by way of examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the low thermal expansion alloy of the present embodiment. Therefore, the low thermal expansion alloy of this embodiment is not limited to this one condition example.

  Test materials having chemical compositions shown in Table 1 were prepared.

  “-” In Table 1 means that the corresponding element content was less than the detection limit. In any of the test numbers, the P content as an impurity is 0.02% or less, the S content is 0.005% or less, the N content is 0.02% or less, and O The content was 0.01% or less. Each test material was induction-melted in vacuum to produce a cylindrical ingot having a diameter of 30 kg and a diameter of 100 mm. After the manufactured ingot is heated to a heating temperature T2 = 1200 ° C., hot forging (forging) is carried out to produce a plate material with a thickness of 16 mm for test numbers other than test number 19, An alloy material having a thickness of 75 mm was manufactured. The ingot temperatures from the start of hot forging to the completion of hot forging for each test number were all 900 ° C. or higher. In each pass in the hot forging process, the reduction ratio Pn was obtained. Moreover, the cumulative reduction ratio Pt in the hot forging was obtained. Furthermore, the temperature of the ingot at the time of hot forging was measured with a thermometer arranged in the hot forging machine.

  In Table 2, the cumulative reduction rate “Pt” column shows the cumulative reduction rate Pt (%) for each test number. In Table 2, the reduction ratio “Pn” column, “◯” indicates that the reduction ratio Pn in each pass until the cumulative reduction ratio reached 30.0% was 5.0% or more. Indicates. “X” indicates that any of the rolling reduction ratios Pn in each pass until the cumulative rolling reduction ratio reaches 30.0% was less than 5.0%.

  Solution treatment was performed on the plate material after the hot forging step. Table 2 shows the solution treatment temperature (° C.) and the retention time (hr) at the solution treatment temperature T3 for each test number. Cold processing was implemented with respect to the board | plate material after a solution treatment process. Specifically, cold rolling was performed on the plate material after the solution treatment. The cold working rate CW in the cold working is shown in the “CW” column of Table 2. When the “CW” column is “0”, it indicates that cold working has not been performed. Aging heat treatment was performed on the plate material after the cold working process. Table 2 shows the aging heat treatment temperature T5 (° C.) in the aging heat treatment and the holding time (hr) at the aging heat treatment temperature. In Test No. 31, no aging heat treatment was performed (both described as “-” in the aging heat treatment temperature T5 column and the retention time column in Table 2).

  The low thermal expansion alloy of each test number was manufactured by the above manufacturing process.

[Evaluation test]
[Measurement test of total volume fraction of vanadium carbide]
A test material having a length of 10 mm, a width of 10 mm, and a thickness of 10 mm was produced from the center portion of the plate width of the manufactured low thermal expansion alloy (plate material) of each test number. The collected specimens were electrolyzed using a 10% AA electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte). The current during electrolysis was 20 mA / cm ² . The electrolytic solution obtained by electrolysis was filtered through a filter having a pore size of 200 nm, and the mass of the residue was measured. Here, all the residues obtained by filtering with the filter were considered to be vanadium carbide. From the total mass of the test material before electrolysis and the total mass of the test material after electrolysis, the mass of the electrolysis amount was determined. The molar fraction of vanadium carbide was calculated from the mass of electrolysis and the mass of the residue. Next, the total volume fraction (% by volume) of the vanadium carbide was calculated based on the lattice constant of the matrix (low thermal expansion alloy) and the lattice constant of the vanadium carbide using the determined mole fraction. Here, the lattice constant of the matrix (low thermal expansion alloy) was 3.59 and the lattice constant of vanadium carbide was 4.17. Table 3 shows the total volume ratio (% by volume) of the obtained vanadium carbide.

[Measurement test of number density of fine vanadium carbide]
A test material was collected from the center of the width of the manufactured low thermal expansion alloy (plate material) of each test number. The specimen was mechanically polished to a thickness of 70 μm. Furthermore, the surface (observation surface) of the test material was polished by a twin jet polishing method (electrolytic solution: methanol perchlorate (perchloric acid 10%, methanol 90%)). Microscopic observation was carried out on the observation surface of the polished specimen using a transmission electron microscope. The observation visual field was 300 nm × 500 nm. A dark field image formed using a diffraction spot of fine vanadium carbide was obtained. The number density of fine vanadium carbide having an equivalent circle diameter of 1 nm or more and less than 200 nm was measured by image software. The number of fine vanadium carbides was divided by the field area of the dark field image to obtain the number density (pieces / μm ² ) of the fine vanadium carbides of this embodiment. The number density of the obtained fine vanadium carbide is shown in Table 3.

[Average equivalent circular diameter of coarse vanadium carbide]
A test material was collected from the center portion of the plate width of the manufactured low thermal expansion alloy (plate material) of each test number so that the observation surface was parallel to the forging direction and the plate thickness direction. The observation surface of the test material was polished with Emily paper, and then buffed using diamond. Reflected electron images were obtained using a scanning electron microscope for any 28 fields in the observation surface of the polished specimen. Each field of view was 180 μm × 420 μm. The obtained backscattered electron image was binarized using image analysis software to distinguish the precipitate from the matrix. Further, in the microstructure of the low thermal expansion alloy having the chemical composition shown in Table 1, it could be considered that no precipitate other than vanadium carbide was present. Therefore, vanadium carbide could be specified by the binarization process. The equivalent circle diameter of each identified vanadium carbide was determined. And the vanadium carbide | carbonized_material whose circular equivalent diameter is 1.0 micrometer or more was specified. The average equivalent circle diameter of vanadium carbide having an equivalent circle diameter of 1.0 μm or more, which was specified in 28 visual fields, was determined. The average equivalent circle diameter was rounded off to the second decimal place. Table 3 shows the equivalent-circle diameter (μm) of the obtained coarse vanadium carbide.

[Thermal expansion coefficient measurement test]
A cylindrical test piece having a diameter of 3 mm and a length of 15 mm was produced from the center portion of the plate width of the manufactured low thermal expansion alloy (plate material) of each test number. The longitudinal direction of the test piece was parallel to the forging direction. The thermal expansion coefficient was calculated | required using the test piece. Specifically, a horizontal differential expansion mechanical analyzer was used for measuring the thermal expansion coefficient. The test piece was heated at a rate of 5 ° C./min to obtain an average coefficient of thermal expansion of 30 to 100 ° C. The results are shown in Table 3.

[Young's modulus measurement test]
A test piece having a length of 60 mm, a width of 10 mm, and a thickness of 1.5 mm was produced from the plate width central portion of the manufactured low thermal expansion alloy (plate material) of each test number. The Young's modulus was determined using the test piece. Specifically, the Young's modulus was measured using a lateral resonance measuring device. The Young's modulus was determined based on JIS Z 2280 (1993). Table 3 shows the measured Young's modulus.

[Tensile test]
A round bar tensile test piece having a parallel part diameter of 6 mm and a parallel part length of 65 mm was prepared from the center part of the plate width of the manufactured low thermal expansion alloy (plate material) of each test number. The parallel part was parallel to the hot forging direction. A strain gauge was affixed to the produced tensile test piece. Thereafter, a tensile test was performed at room temperature and in the air using a tensile test piece to obtain a stress-strain curve. Using the obtained stress-strain curve, the tensile strength TS (MPa) and the elongation (breaking elongation) EL (%) were determined. Furthermore, the product (TS × EL) of the obtained tensile strength TS and elongation EL was determined. Table 3 shows the obtained tensile strength TS (MPa) and the product TS × EL (MPa ·%) of tensile strength and elongation.

[Evaluation results]
Referring to Tables 1 to 3, the chemical compositions of the alloys of test numbers 1 to 11 were appropriate and satisfied the formulas (1) and (2). Further, the total volume ratio of vanadium carbide is 2.5 to 12.5%, the number density of fine vanadium carbide having an equivalent circle diameter of less than 200 nm is 50 pieces / μm ² or more, and the equivalent circle diameter is 1.0 μm. The average equivalent circular diameter of the coarse vanadium carbide was less than 2.8 μm. Therefore, the thermal expansion coefficients of these test numbers were as low as 4 × 10 ⁻⁶ / ° C. or less, and the Young's modulus was as high as 150 GPa or more. Furthermore, the tensile strength was 950 MPa or more, and the product of tensile strength and elongation (TS × EL) was 5000 or more.

  On the other hand, the C content and V content of the alloy of test number 12 were low. Therefore, the total volume fraction of vanadium carbide of the alloy of test number 12 was less than 2.5%. As a result, the alloy of test number 16 had a Young's modulus of less than 150 GPa.

The Ni content of the test number 13 alloy was high. Therefore, the thermal expansion coefficient of the alloy of test number 13 exceeded 4.00 × 10 ⁻⁶ / ° C.

The Ni content of the alloy of test number 14 was low. Therefore, the thermal expansion coefficient of the alloy of test number 14 exceeded 4.00 × 10 ⁻⁶ / ° C.

The Cr content of the test number 15 alloy was high. As a result, the thermal expansion coefficient of test number 15 exceeded 4.00 × 10 ⁻⁶ / ° C.

Although the chemical composition of test number 16 was appropriate, the F1 value was too high and did not satisfy the formula (1). As a result, the thermal expansion coefficient of test number 16 exceeded 4.00 × 10 ⁻⁶ / ° C.

Although the chemical composition of the test number 17 alloy was appropriate, F2 was less than the lower limit of the formula (2). Therefore, the number density of fine vanadium carbide in the alloy of test number 17 was less than 50 / μm ² . As a result, the tensile strength of the alloy of test number 17 was less than 950 MPa.

Although the chemical composition of the alloy of test number 18 was appropriate, F2 exceeded the upper limit of formula (2). Therefore, the number density of fine vanadium carbide of the alloy of test number 18 was less than 50 / μm ² . As a result, the tensile strength of the alloy of test number 18 was less than 950 MPa.

  Although the chemical composition of the test No. 19 alloy was appropriate, the cumulative rolling reduction Pt in the hot working process was less than 30.0%. Therefore, the average equivalent circle diameter of the coarse vanadium carbide was 2.8 μm or more. As a result, the tensile strength of the alloy of test number 19 was less than 950 MPa, and the product of tensile strength and elongation (TS × EL) was less than 5000.

  In test numbers 20 and 21, although the chemical composition of the alloy was appropriate, there was a pass that was less than 5% in the reduction rate in each pass until the cumulative reduction rate reached 30.0%. Therefore, the average equivalent circle diameter of the coarse vanadium carbide was 2.8 μm or more. As a result, the tensile strengths of the alloys of test numbers 20 and 21 were less than 950 MPa, and the product of tensile strength and elongation (TS × EL) was less than 5000.

Although the chemical composition of the alloy of the test number 22 was appropriate and satisfy | filled Formula (1) and Formula (2), solution treatment temperature was less than 1000 degreeC. Therefore, the number density of the fine vanadium carbide was less than 50 / μm ² . As a result, the tensile strength was less than 950 MPa.

In test numbers 23 to 25, although the chemical composition of the alloy was appropriate, the cold working process was not performed. Furthermore, in the test numbers 26 to 28, the cold working rate in the cold working process was less than 20.0%. Therefore, the number density of the fine vanadium carbide was less than 50 / μm ² . As a result, the tensile strength was less than 950 MPa.

In Test No. 29, the chemical composition of the alloy was appropriate, but the aging heat treatment temperature exceeded 800 ° C. Therefore, the number density of the fine vanadium carbide was less than 50 / μm ² . As a result, the tensile strength was less than 950 MPa.

In Test No. 30, the chemical composition of the alloy was appropriate, but the aging heat treatment temperature was less than 500 ° C. Therefore, the number density of the fine vanadium carbide was less than 50 / μm ² . As a result, the tensile strength was less than 950 MPa.

Although the chemical composition of the alloy of test number 31 was appropriate, no aging heat treatment was performed. Therefore, the number density of the fine vanadium carbide was less than 50 / μm ² . As a result, the tensile strength was less than 950 MPa.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (6)

A low thermal expansion alloy,
Chemical composition is mass%,
C: 0.20 to 2.00%
Mn: 0.05-2.00%
V: 0.80 to 10.00%,
Ni: 30.00-40.00%,
One or more selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%;
Co: 0 to 10.00%,
Cr: 0 to 3.00%, and
Balance: Fe and impurities,
Satisfying the equations (1) and (2),
In the low thermal expansion alloy,
The volume fraction of vanadium carbide is 2.5 to 12.5%,
Fine vanadium carbide having an equivalent circle diameter of less than 200 nm is 50 pieces / μm ² or more,
The average equivalent circle diameter of coarse vanadium carbide having an equivalent circle diameter of 1.0 μm or more is less than 2.8 μm.
Low thermal expansion alloy.
30.00 ≦ Ni + Co ≦ 40.00 (1)
−0.50 <V−50.94 / 12.01 × C <2.00 (2)
Here, the content (mass%) of the corresponding element is substituted into the element symbols of the formulas (1) and (2). The low thermal expansion alloy according to claim 1,
The chemical composition is
Co: 0.10 to 10.00% is contained,
Low thermal expansion alloy. The low thermal expansion alloy according to claim 1 or 2,
The chemical composition is
Containing Cr: 1.00 to 3.00%,
Low thermal expansion alloy. Chemical composition is mass%,
C: 0.20 to 2.00%
Mn: 0.05-2.00%
V: 0.80 to 10.00%,
Ni: 30.00-40.00%,
One or more selected from the group consisting of Si: more than 0 to 0.50% and Al: more than 0 to 0.100%;
Co: 0 to 10.00%,
Cr: 0 to 3.00%, and
Balance: Fe and impurities,
A hot forging process in which an alloy material is manufactured by hot forging after heating a material satisfying the formula (1) and the formula (2) to 1150 to 1300 ° C., in the hot forging process The cumulative reduction rate is 30.0% or more, the reduction rate per pass until the cumulative reduction rate becomes 30.0% is 5.0% or more, and the temperature of the material during hot forging is 900 ° C. A hot forging process,
A solution treatment step for carrying out a solution treatment for holding at 1000 to 1300 ° C. for 0.5 hours or more with respect to the alloy material after the hot forging step;
A cold working step of performing cold working at a cold working rate of 20.0% or more on the alloy material after the solution treatment step;
With respect to the alloy material after the cold working step, an aging heat treatment step of performing an aging heat treatment by holding at 500 to 800 ° C. for 0.5 hours or more,
A method for producing a low thermal expansion alloy. It is a manufacturing method of the low thermal expansion alloy of Claim 4, Comprising:
The chemical composition is
Co: 0.10 to 10.00% is contained,
A method for producing a low thermal expansion alloy. A method for producing a low thermal expansion alloy according to claim 4 or 5, wherein
The chemical composition is
Containing Cr: 1.00 to 3.00%,
A method for producing a low thermal expansion alloy.

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