JPH06172919A - Machine tool, precision measuring instrument and molding die using low thermal expansion cast iron - Google Patents

Abstract

PURPOSE:To obtain a machine tool or the like having high accuracy and excellent in function by constituting it of cast iron having a graphite structure, having a specified compsn. constituted of carbon of solid solution, Si, Ni, Co and Fe and having a low thermal expansion coefficient. CONSTITUTION:The compsn. of cast iron having a graphite structure in an austenitic base matrix is constituted of a one contg., by weight, 0.09 to 0.43% carbon of solid solution, <1.0% Si, 29 to 34% Ni and 4 to 8% Co and furthermore contg., at need, >=1.0% Mn and <=0.1% Mg, and the balance Fe. In this compsn., the cast iron whose thermal expansion coefficient in the temp. range of 0 to 200 deg.C is regulated to 4X10<-6>/ deg.C or below and simultaneously satisfying the characteristics such as castability, machinability and vibration absorption without deteriorating the mechanical strength can be obtd. By using this low thermal expansion cast iron, the machine tool, precision measuring instrument, molding die or the like having high accuracy and high performance can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic low thermal expansion cast iron, and particularly to a low thermal expansion cast iron which has a low coefficient of thermal expansion and which simultaneously satisfies characteristics such as castability, machinability, and vibration absorbing ability. The present invention relates to a used machine tool, a precision measuring instrument, and a molding die.

[0002]

As is well known, cast iron is widely used as a basic material for industry. The reason for this is that this material has good castability, can be formed into a wide variety of complicated shapes, is easy to cut, and the cost required for processing and melting the material is relatively low, and can be easily manufactured even in a small factory. This is because it has advantages such as

By the way, recently, starting with new materials,
Various organic and inorganic materials other than metals have been developed, and functional materials utilizing their respective characteristics are rapidly becoming popular.
In particular, with the development of the electronics industry, higher precision and superior function materials are required for machine tools, precision measuring instruments, molding dies, scientific instruments, and other manufacturing machinery related to them. It was

Cast iron has also been developed in which, in addition to the conventional materials and properties to meet the above requirements, a thermal expansion coefficient is reduced, vibration absorption capacity is increased, and heat resistance and corrosion resistance are added. ing. The typical one is Inver
Cast iron (36.5% Ni-Fe alloy), or Niresist D5 (ASTM A439 type D-5) which is an improved material thereof.
It is cast iron. The chemical composition of representative examples of these cast irons is shown in Table 1 below.

[0005]

[Table 1] The invar is one in which 34 to 37% of nickel is contained in iron (hereinafter, all component composition ratios are represented by weight%).
Thermal expansion coefficient is 1.5 near room temperature (0-200 ℃)
It has a low value of about × 10 ^-6 / ° C. The mechanism of low expansion of the invar alloy is based on a self-generated volume magnetostriction action generally called "inver effect".

The super invar is prepared by alloying 4 to 6% of cobalt in an iron-nickel substrate and has a coefficient of thermal expansion of 0.5 × 10 ⁻⁶ / ° C. near room temperature.
And has excellent characteristics lower than Invar.

However, the above-mentioned Inver and Super
Since the pine bar is low in castability, machinability, and vibration absorption ability, it is only put to practical use in a very narrow field.

Further, cast iron low expansion materials as shown in columns 3, 4 and 5 of Table 1 have been developed and put into practical use. For example, Niresist D5 is formed by alloying 34 to 36% nickel in iron containing carbon, silicon, and manganese, which are almost equivalent to those of general-purpose ductile cast iron, and the same amount of nickel as in the invar is added to cast iron having a graphite structure. By alloying, while maintaining the castability, machinability, and vibration isolation which are the advantages of cast iron, it also has heat and corrosion resistance and low expansion due to the "inver effect".

As a similar material, novinite cast iron is disclosed in Japanese Patent Publication No. 60-51547. This alloy cast iron is formed by alloying general-purpose ductile cast iron with nickel and cobalt in the same amount as that of the super-invar, so as to have both castability, machinability and low expansion. is there.

However, since the Niresist D5 and the nobinite cast iron contain carbon, silicon and manganese to the same extent as general-purpose ductile cast iron, the low expansion properties of the invar and superinver are impaired. ing.
That is, according to the actual measurement by the inventors of the present application, the respective thermal expansion coefficients are large values of 5 × 10 ⁻⁶ / ° C. and 4 × 10 ⁻⁶ / ° C.

However, the above cast iron alloy cannot sufficiently meet the recent demand for further reduction of the coefficient of thermal expansion, and for the recent precision equipment, high precision, mold materials for FRP, etc.,
Materials with even lower coefficients of thermal expansion are needed.

In order to meet the above-mentioned demand, the inventors of the present application have
In order to provide a material having a thermal expansion coefficient lower than the conventional 4 × 10 ⁻⁶ / ° C. and having castability, machinability, and vibration absorption ability, the content of each alloying element and the thermal expansion coefficient, mechanical The relationship with the properties was clarified by many experiments and statistical analysis methods, and a novel low thermal expansion cast iron was discovered.
Filed as No. 68249.

The low thermal expansion cast iron has the composition shown in the bottom column of Table 1. That is, in cast iron having austenitic base iron, the composition of the carbon is 1.0% or more and 3.5% or more.
Below, silicon 1.5% or less, nickel 32% or more 39.
By using cast iron containing 5% or less and 1.0% or more and less than 4% of cobalt and the total content of nickel and cobalt is 41% or less, (1) the coefficient of thermal expansion is 2 × 10 ^−6.
It was found for the first time that (2) a low thermal expansion material having low castability, machinability, vibration absorption ability and mechanical strength can be provided.

That is, the present inventors repeated various experiments and as a result, 1 to 3.5% of carbon, 32 to 39.
When 1 to 4% of cobalt is added to cast iron containing 5% and the amount of silicon added is set to 1.5% or less, preferably 1% or less, the coefficient of thermal expansion is very small, and the castability is It was discovered that cast iron with good workability can be obtained.

With the development of this low expansion cast iron, it has become possible to provide machined parts with higher precision, precision measuring instruments, and processed products such as molding dies.

[0016]

However, as the size and precision of the equipment have been further advanced, there is a situation in which conventional low thermal expansion cast iron cannot sufficiently cope with the situation. For example, with the development of communication technology such as satellite broadcasting in recent years, parabolic antennas used for transmitting and receiving equipment have become extremely large and have low thermal expansion, and also their processing accuracy, that is, castability, machinability, vibration absorption. It is required to have extremely high performance and mechanical strength. For example, as an antenna reflector,
Carbon fiber reinforced plastic (CFRP) having high rigidity and corrosion resistance is generally adopted. However,
The coefficient of thermal expansion of CFRP is extremely small, about 1.5 × 10 ^-6 / ° C. Therefore, in order to ensure high dimensional accuracy of the product even after molding, the molding die should have a similar coefficient of thermal expansion. It must be composed of the material that it has. Therefore, the coefficient of thermal expansion is smaller than that of the conventional one and is at least 1.
It is essential that the molding die be made of a material having a temperature of 5 × 10 ⁻⁶ / ° C. or less and excellent mechanical properties.

The present invention has been made to solve the above problems, and possesses further excellent castability, machinability and vibration absorbing ability as a machine tool, a precision measuring instrument and a molding die material. And the coefficient of thermal expansion is 0 to 20
4 × 10 ⁻⁶ / ° C. or less, preferably 3 at 0 ° C.
A machine tool, a precision measuring instrument, and a molding metal using low thermal expansion cast iron that simultaneously satisfy the characteristics of × 10 ^-6 / ° C or less, and more preferably a thermal expansion coefficient of 1.5 × 10 ^-6 / ° C or less. The purpose is to provide the mold.

[0018]

In order to improve the castability and machinability from the above viewpoints, the present invention has conducted a number of experiments on the minimum compositional conditions under which the graphite can be crystallized in the alloy structure under the assumption of casting. The above object is achieved by finding out through analysis and at the same time discovering optimum component conditions for obtaining low thermal expansion.

That is, the machine tool, the precision measuring instrument and the molding die according to the present invention are cast iron having a graphite structure in austenite base iron and solid solution carbon as a component composition expressed in% by weight. 09% or more and 0.43% or less, silicon less than 1.0%, nickel 29% or more and 34% or less, cobalt 4% or more and 8% or less, and the balance iron,
The coefficient of thermal expansion in the temperature range of 0 to 200 ° C. is 4 × 10
It is characterized by using cast iron having a low thermal expansion of ^-6 / ° C or less.

It is preferable to use cast iron containing 1.0% or less of manganese, preferably 0.5% or less, and 0.1% or less of magnesium in addition to the above component composition. The above-mentioned component composition range is set based on the following results obtained for the first time by various experiments and analyzes by the inventors.

First, as a first result, the relationship between the coefficient of thermal expansion and the content of each element was obtained, and the relationships of the following equations (1) and (2) were obtained.

Thermal expansion coefficient (× 10 ⁻⁶ / ° C.) = 14.905 + 0.1 [solid solution C amount] (%) + 1.49 × [Si amount] (%) −0.32 × [Ni amount] ( %) −0.70 × [Co amount] (%) + 1.35 × [Mn amount] (%) (1) Thermal expansion coefficient (× 10 ⁻⁶ / ° C.) = −2.14 + 1. 75 [Solid C content] (%) +2.11 x [Si content] (%) +0.14 x [Ni content] (%) +0.28 x [Co content] (%) +0.25 x [Mn content] ] (%) (2) By the way, as shown in FIG. 1, the relationship between the thermal expansion coefficient and the Ni content of the Fe-Ni alloy is such that the thermal expansion coefficient is about 36% when the Ni content is around 36%. It becomes the minimum. Therefore, the equation (1) is a relational expression obtained as a result of the analysis on the thermal expansion coefficient of each alloy element in the region where the Ni content is lower than the minimum point of the thermal expansion coefficient.

On the other hand, the expression (2) is a relational expression obtained by analysis of the thermal expansion coefficient of each alloying element in the region where the Ni content is higher than the minimum point.

Comparing the coefficients in the above equations (1) and (2), the coefficient of Si content (%) is the largest. That is, it can be seen that the silicon content has a positive correlation and has the greatest effect on the thermal expansion characteristics.

Therefore, it can be understood that a lower coefficient of thermal expansion can be obtained by reducing the amount of silicon as much as possible.

Regarding the effect of the carbon content in the Fe-Ni alloy on the coefficient of thermal expansion, it was conventionally thought that the total content of carbon contained had a great effect. However,
From the experiments conducted by the present inventors, it was discovered that it is not the entire carbon content that is affected, but only the amount of carbon that is in solid solution.

For the first time, it has been found that the thermal expansion characteristics can be further improved by reducing the silicon content and the solid solution carbon content within a predetermined range.

Next, as a second result, the relationship between the temperature and the coefficient of thermal expansion when the total content of Ni and Co is changed is as shown in FIG. 2, depending on the ratio of each Ni + Co content. The fact is that a bending point B at which the temperature dependence of the thermal expansion coefficient suddenly rises appears, and the temperature corresponding to the bending point B (hereinafter referred to as the bending point temperature) changes to the high temperature side.

That is, as is clear from FIG.
When the + Co amount increases, the bending point temperature shifts to the high temperature side, and as a result, the coefficient of thermal expansion increases in the practical temperature range from room temperature to 200 ° C. On the contrary, when the component composition is set such that the bending point temperature is 325 ° C. or lower, preferably 200 to 250 ° C., a low thermal expansion coefficient can be obtained in the practical temperature range (0 to 200 ° C.).

The inventors of the present invention experimentally determined the relationship between the inflection point temperature and the amount of each element and obtained the following equation (3).

Bending point temperature (° C.) = 22.5 × [Ni (%) + Co (%)] − 22 × Mn (%) − 600.3 (3) Mn is calculated from the equation (3). It was found that the addition makes it possible to shift the inflection point temperature to a lower temperature region.

Next, as a third result, it was found that by reducing the amount of solute carbon and the amount of carbide, the castability and the machinability were improved, and the vibration absorbing ability could be increased.

That is, carbon other than solid solution carbon exists as graphite or carbide. Among them, the larger the amount of crystallized graphite, the less shrinkage cavities during casting, the better the machinability, that is, the machinability, and the greater the vibration absorbing ability. On the other hand, when the carbide is deposited, it causes the generation of micro-cavities and the machinability is deteriorated. Therefore, it is important to reduce the amount of solute C and the amount of precipitated carbide as much as possible and increase the amount of crystallized graphite.

As a fourth result, a relational expression between the amount of solute carbon and mechanical strength was obtained as shown in the following equations (4) to (7).

Tensile strength (kgf / mm ² ) = 19.6 + 93 [amount of solid solution C] (%) (4) Proof strength (kgf / mm ² ) = 4.8 + 135.5 [solid solution C Amount] (%) (5) Young's modulus (kgf / mm ² ) = 6982.5 + 19750 [Solute C amount] (%) (6) Hardness (HB) = 128. 6 + 133 [solid solution C content] (%) (7) It is desirable to reduce the solid solution C content in order to lower the coefficient of thermal expansion from the equations (1) and (2). (4)
As is clear from the formula (7), it is necessary to increase the amount of solid solution C to some extent in order to improve the mechanical strength. Therefore, in order to satisfy both the low thermal expansion property and the good mechanical property at the same time. The optimal composition range of is determined.

Finally, as a fifth result, the relationship between the amount of solid-solved carbon and the total amount of contained carbon was conventionally considered to increase or decrease with a positive correlation, but according to the results of experiments by the present inventors, As shown in FIG. 3, it was confirmed for the first time that the amount of solute carbon decreases as the total amount of carbon increases.

This is because if the total amount of C is high, the amount of graphite crystallized in the initial stage of solidification increases, and the solid solution C in the vicinity thereof serves to provide sites for stable graphite. It is considered that the amount of C is reduced, and at the same time, the amount of C that becomes carbide is reduced. The relational expression between the amount of dissolved C and the total amount of C in FIG. 3 is shown in equation (8).

[Solution C content] (%) = 0.65-0.20 [total C content] (%) By substituting the relationship of the equation (8) into the equations (1) to (7), A relational expression between the carbon amount (total C amount) and each characteristic value is derived.

The composition of the low thermal expansion cast iron according to the present invention was determined based on the findings obtained from the above experimental results.

Next, the range of the content of each element and the reason for limiting the content will be described in more detail.

First, the carbon content is 1 to 3.5% by weight, preferably 1.2 to 3% by weight, and more preferably 2.2.
It is set to 2.3% by weight. Carbon in cast iron is divided into carbon crystallized as graphite and carbon solid-dissolved in iron. In order to improve the castability, machinability, and low thermal expansion which are the objects of the present invention, it is essential to increase the amount of crystallized graphite and decrease the amount of solute carbon as much as possible.

The relationship between the total carbon content in cast iron and the solid solution carbon content is clear from FIG. 3 and equation (8), and increasing the total carbon content is in line with the object of the present invention.

However, the amount of solute carbon and the amount of crystallized graphite have a great influence on the mechanical properties of the cast iron material. That is, the relationship between the Young's modulus and the total carbon content is obtained as the following equation (9) by substituting the equation (8) into the equation (6).

Young's modulus (kgf / mm ² ) = 19820-3950 [total carbon amount] (%) (9) That is, it is understood that the Young's modulus decreases as the total carbon amount increases.

By the way, the products to which the material of the present invention is applied include machine tools, precision measuring instruments and molding dies. When used as such a structural material, the Young's modulus is at least 9000 kgf / mm ^2. The value of the foundation is required.

Therefore, the total amount of carbon required from the equation (9) is 2.8% or less. Considering application to a structural member that can be used even with a Young's modulus as high as that of an aluminum alloy, the total carbon content can be increased to 3.5% as an upper limit value.

Further, as is clear from the results of the respective examples described later, when the total carbon amount is set in the range of 1.2 to 2.8%, the mechanical properties such as tensile strength are not particularly impaired, It is possible to obtain low thermal expansion cast iron that simultaneously satisfies a low thermal expansion coefficient and excellent castability, machinability, and vibration absorption ability. The solid solution carbon amount corresponding to the range of the total carbon amount at this time is calculated from the graph of the relational expression shown in FIG.
0.43% of the range, the range of solid solution carbon amount 0.09
It is extremely important to set the content in the range of 0.43% to 0.43% in order to simultaneously satisfy the required properties such as low thermal expansion, castability, machinability, and vibration absorbing ability of the cast iron according to the present invention.

The relationship between the coefficient of thermal expansion and each alloying element can be derived from the equations (1) and (8) as the following equation (10).

Thermal expansion coefficient (× 10 ⁻⁶ / ° C.) = 14.97−0.02 × [total C amount] (%) + 1.49 × [Si amount] (%) −0.32 × [Ni amount ] (%) −0.70 × [Co amount] (%) + 1.35 × [Mn amount] (%) (10) As is clear from the equation (10), the larger the total carbon amount is, Since a material having a low coefficient of thermal expansion can be obtained, it is desirable to set the total carbon content as high as possible. However, as is clear from the results shown in FIG. 3, when the total carbon content exceeds 3.5%, the solid solution carbon decreases, the mechanical strength decreases, and the castability decreases.

On the other hand, the lower limit of the total carbon content is determined from the relationship with the crystallinity of graphite and the coefficient of thermal expansion. That is, the lower limit of the total amount of carbon that provides a sound graphite composition is about 1%. If it is less than 1%, the formation of graphite nuclei at the time of solidification becomes insufficient, carbide is formed, and the machinability is greatly impaired.

Therefore, the total carbon content is set to 1% or more and 3.5% or less, preferably 2.0% or more and 3.0% or less. The amount of solute carbon is set to 0.09% or more and 0.43% or less.

Next, the silicon content is set to less than 1.0%. In the relational expression shown in the equation (10), the coefficient of the amount of silicon is the largest, and the amount of silicon has a great influence on the coefficient of thermal expansion. Therefore, the lower the amount of silicon, the lower the coefficient of thermal expansion.

Silicon is an element necessary for promoting graphite crystallization, but unlike general cast iron, the low thermal expansion cast iron according to the present invention contains about 30% of nickel, which is a graphitization promoting element. It was found that the minimum amount, for example 0.3% or more, which exhibits the inoculation effect, should be added. It was also confirmed that when graphite particles are used as an inoculant, a sufficient graphite composition can be obtained even if the amount of silicon is extremely small. However, iron-silicon alloys are used as inoculants in ordinary casting sites, and the addition amount in this case is 0.5% at the maximum.

Next, the manganese content is set to 1.0% or less. By adding manganese, the inflection point B shown in FIG. 2 shifts to the low temperature side, which has the effect of lowering the coefficient of thermal expansion in the practical temperature range from room temperature to 200 ° C.
However, like silicon, when the content exceeds 1%, the coefficient of thermal expansion increases conversely.

Therefore, the addition amount is set to 1.0% or less, preferably 0.5% or less.

Next, the Ni content is set to 29 to 34%. If the Ni content is less than 29% or more than 34%, the coefficient of thermal expansion will increase, so the Ni content is set to the above range.

The Co content is set in the range of 4-8%. When the Co content is less than 4%, the coefficient of thermal expansion increases, while when it exceeds 8%, the bending point shown in FIG. 2 shifts to the high temperature side, and the heat in the practical temperature range from room temperature to 200 ° C. It will increase the expansion coefficient.

Here, the proper range of Ni content and Co content is influenced by the contents of carbon, silicon and manganese. The Ni content that minimizes the coefficient of thermal expansion is
The result of the experiment is given by the following equation (11).

Ni content (%) at the minimum point = 35−0.29 × [Co content] (%) −6.0 [0.65-0.2 total C content] (%) + 0.57 [Mn Amount] (%) +0.45 [Si amount] (%) (11) Here, for the above reason, the total carbon amount is 1.5%, the silicon amount is 0%, and the manganese amount is 0%. Then, the Ni content (%) at the minimum point is given by the following equation (12).

Ni content (%) at the minimum point = 33−0.29 × [Co content] (%) (12) On the other hand, the total content of Ni and Co is shown in FIG. The temperature (bending point temperature θ) corresponding to the bending point B in the thermal expansion coefficient curve and the thermal expansion coefficient value are affected. In the range of the bending point temperature θ or less, the temperature change of the thermal expansion coefficient is small, while in the range of exceeding the bending point temperature θ, it greatly increases.

As a result of clarifying the relationship between the bending point temperature θ and the total content of Ni and Co by experiments, the following equation (13) was obtained.

Bending point temperature θ (° C.) = 22.5 × [Ni amount (%) + Co amount (%)] −600.7 (13) Here, for example, from room temperature to about 200 ° C. Assuming that the CFRP mold used in the temperature range is applied, and the bending point temperature θ is set to 200 to 250 ° C., N
The appropriate range of the total content of i and Co is given by the following equation (14).

Ni amount (%) + Co amount (%) = 36 to 38 (%) (14) From the relation with the above equations (14) and (12),
The optimum Ni amount is calculated to be 29 to 33% and the optimum Co amount is calculated to be 4 to 7%, and the component composition is set in this range.

Magnesium is an element necessary for spheroidizing and crystallizing graphite, and its content is 0.1.
It is set to be less than or equal to weight%. If the content exceeds 0.1%, carbides are formed, which is not preferable. Therefore, the magnesium content is preferably in the range of 0.04 to 0.1%.

[0065]

Embodiments of the present invention will be described below with reference to the drawings.

Example 1 A CFRP molding die as shown in FIGS. 4 and 5 was cast.

This mold has a length of 70 cm, a width of 65 cm, a thickness of 6 cm, and a weight of 130 kg. For melting, a high-frequency electric furnace having a capacity of 300 kg was used to melt the materials shown in Table 2 below.

[0068]

[Table 2] As shown in Table 3 below, the composition of the components is as follows: carbon 2.0%, silicon 0.15%, manganese 0.03%, nickel 30
%, Cobalt 6%, magnesium 0.05%, and the balance is an austenitic cast iron containing impurities.

Table 4 shows the results of measuring the respective characteristic values by collecting test pieces with a 1-inch sand mold for a key block. In Table 4, the coefficient of thermal expansion is 1.5 × 10 ⁻⁶ / ° C.,
Tensile strength 40kgf / mm ² , elongation 22%, Young's modulus 1
2000 kgf / mm ² was obtained.

The obtained mold is used in a step of press-molding a CFRP preform while heating it at 200 ° C. The thermal expansion coefficient of CFRP is 1.0 to 1.5 × 10 ^-6
Since it is / ° C, the dimensional accuracy of the CFRP product could be significantly improved by using the mold of this example having a value close to this coefficient value.

As described above, according to the cast iron having the component composition of this embodiment, the castability, machinability and mechanical properties which are almost the same as those of general cast iron are satisfied at the same time, and the low expansion close to that of the invar alloy is achieved. The coefficient can be obtained.

(Example 2) As shown in Table 3, the total C content was 2.8% and the Si content was 0.4%. The cast iron of this composition is one in the case of pursuing vibration absorbing ability. Ie all C
By increasing the amount to 2.8%, the damping capacity (Specific Dam
Ping Capacity) is 17%, which is 4 ~ 5 of general cast iron.
The vibration absorption capacity is doubled. Also, the hardness is HB125-13
It is about 5, which is as soft as an aluminum alloy. This is useful as a jig member for joining and capturing the mating material without damaging the mating material, in addition to the lubricating effect of graphite, and can be used for semiconductor and electronic manufacturing equipment that requires ultra-high accuracy.

As described above, it is possible to obtain a vibration absorbing ability 4 to 5 times that of general cast iron (FC30 material) and obtain the same softness as an aluminum alloy.

Example 3 As shown in Table 3, the carbon content was set as low as 1.20%. The other components were similar to those in the above example.

In this case, crystallization of graphite was observed although it was small, and as shown in Table 4, the workability was in an allowable range.

Example 4 As shown in Table 3, the silicon content was set as high as 0.9%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion was slightly high, but within the allowable range.

(Example 5) As shown in Table 3, the manganese content was set to a high 0.9%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion was slightly high, but within the allowable range.

(Example 6) As shown in Table 3, the manganese content was set to 0.7%. The other components were similar to those in the above example.

In this case as well, the coefficient of thermal expansion was within the allowable range.

In addition to the above-mentioned examples, various experiments were carried out within the scope of the present invention, and the same good characteristics as above were observed.

Comparative Example 1 As shown in Table 3, the carbon content was set to 0.71%, which was extremely low. The other components were similar to those in the above example.

In this case, as shown in Table 4, the workability, castability and vibration absorbing ability are poor.

Comparative Example 2 As shown in Table 3, the carbon content was set as high as 3.6%. The other components were similar to those in the above example.

In this case, as shown in Table 4, elongation and strength are lowered, and casting defects are large.

Comparative Example 3 As shown in Table 3, the silicon content was set as high as 1.2%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion is too high.

(Comparative Example 4) As shown in Table 3, the nickel content was set as low as 28.0%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion becomes high.

(Comparative Example 5) As shown in Table 3, the content of nickel was increased to 37.0%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion becomes high.

(Comparative Example 6) As shown in Table 3, the content of cobalt was lowered to 3.5%. The other contents were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion becomes high.

Comparative Example 7 As shown in Table 3, the cobalt content was increased to 8.2%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion becomes high.

Comparative Example 8 As shown in Table 3, the total content of nickel and cobalt was increased to 42.5%. The other components were similar to those in the above example.

In this case, as shown in Table 4, the coefficient of thermal expansion becomes high.

[0099]

[Table 3]

[0100]

[Table 4]

[0101]

As described above, the machine tool, the precision measuring instrument and the molding die of the present invention have a coefficient of thermal expansion of 1.5 to 3.0 × 10 ^-6 / ° C without impairing the mechanical strength. Has low thermal expansion characteristics, as well as castability and machinability comparable to general cast iron, and can absorb vibrations up to 4 to 5 times that of general cast iron, if necessary, softening like aluminum alloy. Since it is composed of cast iron that also has bulk, it is extremely useful because it can sufficiently cope with higher precision and higher functionality of equipment.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Ni content and a coefficient of thermal expansion.

FIG. 2 is a diagram showing the relationship between temperature and coefficient of thermal expansion, with the total amount of Ni and Co in Ni cast iron as a parameter.

FIG. 3 is a diagram showing the relationship between the total amount of carbon and the amount of solute carbon.

4 is a diagram showing the shape of a CFRP molding die cast in Example 1. FIG.

5 is a sectional view taken along the line IVb-IVb in FIG.

Claims (6)

[Claims] 1. In cast iron having a graphite structure in austenitic base iron, solid solution carbon is 0.09% or more and 0.43% or less, and silicon 1.0
%, Nickel 29% to 34%, cobalt 4%
A machine tool characterized by using a low thermal expansion cast iron having a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in the temperature range of 0 to 200 ° C., which is composed of the balance iron including 8% or less. 2. The composition of manganese is 1.0% or less,
The machine tool according to claim 1, wherein low thermal expansion cast iron containing 0.1% or less of magnesium is used. 3. In cast iron having a graphite structure in austenite base iron, solid solution carbon is 0.09% or more and 0.43% or less and silicon 1.0
%, Nickel 29% to 34%, cobalt 4%
A precision measuring instrument characterized by using a low thermal expansion cast iron having a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in the temperature range of 0 to 200 ° C., which is composed of the balance iron including 8% or less. 4. The composition of manganese is 1.0% or less,
The precision measuring device according to claim 3, wherein low thermal expansion cast iron containing 0.1% or less of magnesium is used. 5. In cast iron having graphite structure in austenitic base iron, solid solution carbon is 0.09% or more and 0.43% or less, and silicon is 1.0 as a component composition expressed in% by weight.
%, Nickel 29% to 34%, cobalt 4%
A molding die characterized by using a low thermal expansion cast iron having a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in a temperature range of 0 to 200 ° C., which is composed of the balance iron including 8% or less. 6. Manganese 1.0% or less as a component composition,
The molding die according to claim 5, wherein low thermal expansion cast iron containing 0.1% or less of magnesium is used.