JPH08269613A - Low thermal expansion cast iron and manufacturing method thereof - Google Patents

Abstract

(57) [Abstract] [Purpose] Especially as-cast material (as cast) without necessarily undergoing heat treatment such as quenching from high temperature
The present invention provides a low thermal expansion cast iron in which Ni segregation is effectively reduced and the low thermal expansion property is further improved even in the state of material), and a method for producing the same. [Structure] A low-thermal-expansion cast iron with a high Ni content having a thermal expansion coefficient of 4 × 10 ^-6 / ° C or less in the temperature range from room temperature to 100 ° C., the as-cast material was cooled from room temperature to liquid nitrogen temperature. It is characterized in that the area ratio at which the austenite matrix structure is transformed into the martensite structure is 15% or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low thermal expansion cast iron and a method for manufacturing the same, and it is possible to effectively reduce Ni segregation as it is as cast without performing a complicated heat treatment and to improve the low thermal expansion property. Further, it relates to an improved low thermal expansion cast iron and a method for producing the same.

[0002]

2. Description of the Related Art As is well known, cast iron has excellent castability, can be formed into a wide variety of complicated shapes, is easy to cut, and is relatively inexpensive in terms of materials and melting. Since it has the advantage that it can be easily manufactured even in a small factory, it is widely used as a basic material in various industrial fields.

By the way, recently, along with the development of the electronics industry, the optical industry, etc., machine tools, measuring instruments, molding dies, and other manufacturing machines related thereto are
Higher dimensional accuracy is required,
The demand for materials that keep thermal expansion and thermal deformation of precision parts as low as possible is increasing rapidly.

Conventionally, as a metal material having a small coefficient of thermal expansion near room temperature, about 36% Ni as shown in Table 1 below is used.
-Fe invariant steel (Invar alloy) or about 30% Ni-5% C
O-Fe super invariant steel (Super Invar alloy) and the like are known.

[0005]

[Table 1]

However, the above-mentioned various types of invariant steels have the disadvantages of poor machinability, inability to efficiently manufacture products with high dimensional accuracy, and poor castability, which easily causes defects, and therefore, they are not suitable for various applications. The situation is not sufficient to meet the technical requirements.

In order to solve the above drawbacks, carbon and silicon are added to a binary alloy such as the above Invar alloy or a ternary alloy such as a Super Invar alloy to form cast iron, which has machinability and castability. Attention is focused on materials that have improved. Table 1 shows low thermal expansion cast iron known as Niresist D5 from a long time ago, nobinite cast iron as low thermal expansion cast iron developed in the last few years, and Japanese Patent Laid-Open No. 30728/1975.
An example of the composition of cast iron disclosed in the publication is also shown.

Among the various materials exemplified in Table 1 above, low thermal expansion cast iron containing elements necessary for cast iron formation such as carbon has a thermal expansion coefficient higher than that of Invar alloy or Super Invar alloy containing no such elements. The coefficient is large. The reason is that Fe-Ni (-Co) is added because elements such as C, Si and Mn necessary for forming cast iron are added.
It is thought that this is because the Invar composition is out of the range and the specific Invar effect is impaired. A major cause of further increase in the coefficient of thermal expansion is Ni at the microstructure level.
It is considered that this is because the concentration gradient (Ni segregation) occurs. The increase in the coefficient of thermal expansion due to the above-mentioned Ni segregation becomes particularly remarkable when the cooling rate is relatively slow during casting or heat treatment, and becomes a serious problem particularly in the case of a cast product having a large wall thickness. .

Conventionally, as a method for reducing the Ni segregation, there has been used a method for reducing the Ni segregation by rapidly cooling the cast iron material after the solution treatment in the temperature range of 750 to 950 ° C.

[0010]

However, the above method has a problem of causing deformation due to heat treatment.
That is, since the low thermal expansion cast iron alloy containing high Ni has a smaller thermal conductivity than general cast iron, when the cast iron material is rapidly cooled by quenching it in water or oil, the inside of the cast iron material is the surface layer part. It is difficult to secure a sufficient cooling rate as compared with. As a result, a difference in cooling rate between the surface layer portion and the inside of the cast iron material causes a time lag in the elastoplastic deformability, resulting in a large residual stress. In addition, this residual stress is released over machining and over time,
There is a problem that causes a dimensional change with time of a cast product used for a long period of time.

Particularly in the case of a cast product having a large variation in wall thickness, there is a problem that the residual strain distribution becomes non-uniform, the deformation mode becomes more complicated, and it becomes difficult to maintain the dimensional accuracy. Therefore, the above-described measures for reducing Ni segregation by the quenching heat treatment are limited to cast products having a small wall thickness and a simple shape, and have a problem that they cannot be applied to large products.

Originally, since the low thermal expansion cast iron material is mainly intended to improve the dimensional accuracy, the heat treatment often requires a step of gradually cooling the cast iron material in a heat treatment furnace to remove residual stress. . However, when the conventional low thermal expansion cast iron is gradually cooled by the heat treatment for removing the stress, Ni segregation reoccurs, and the thermal expansion coefficient increases anyway.

In recent years, as the size and precision of various equipment have rapidly increased, the conventional low thermal expansion cast iron as described above frequently cannot sufficiently cope with mechanical strength and hardness. ing. For example, the degree of integration of semiconductors has been remarkably increasing in recent years, and Si which constitutes a semiconductor element has been increased.
Higher accuracy is required for the flatness of the wafer. On the other hand, Si wafers are becoming larger year by year, and the era of 4 to 5 inch diameter to 8 inch wafers is being entered. Under these circumstances, a polishing surface plate made of low thermal expansion cast iron is being used for processing Si wafers, but it is necessary to increase the size of the polishing surface plate as the size of the Si wafer increases. In order to suppress the thermal deformation of the board as much as possible and realize highly accurate polishing, a polishing surface plate made of a material having a low thermal expansion property is required.

The present invention has been made in order to solve the problems of conventional casting materials in response to the above technical requirements, and in particular, a heat treatment such as quenching for rapidly cooling from a high temperature state. As-cast material (as c
It is an object of the present invention to provide a low thermal expansion cast iron in which Ni segregation is effectively reduced even in the state of (ast material) and the low thermal expansion property is further improved, and a manufacturing method thereof.

[0015]

In order to achieve the above object, the inventors of the present application have clarified the relationship between the cause of Ni segregation and other constituent elements, and have obtained the following findings.

First, the cause of Ni segregation will be described. Fe-with a Ni content of 25-40% by weight
When carbon is contained in the Ni alloy in an amount of about 0.8 to 0.9%, graphite is formed in the metal structure even in the state of as cast material. In this case, carbon atoms existing in the matrix in the vicinity of the graphite diffuse in the graphite direction as the solidification proceeds during casting. Therefore, as shown in FIG. 2, the concentration of carbon dissolved in the matrix is A concentration gradient is generated such that it is lower as it is closer to graphite and higher as it is farther from graphite. Here, since Ni and C in iron tend to mutually reduce the solubility, a concentration gradient opposite to that of Ni is generated corresponding to the concentration gradient of carbon, and a distribution of low Ni concentration in a region apart from graphite is obtained. Formed, and this concentration distribution is N
i will be segregated. As a result, even if the amount of Ni that exerts the Invar effect as the average concentration of the entire cast material, when observed microscopically, there are many portions that deviate from the so-called Invar composition, and low thermal expansion is considered to deteriorate. To be

As described above, since the cause of Ni segregation, which is a factor inhibiting low thermal expansion, is the C concentration gradient,
The inventors of the present application have found that the concentration gradient of C can be effectively reduced by suppressing the concentration of solid solution carbon itself remaining in the metal matrix to be low by the following method.

One is to further promote graphitization to reduce the concentration of solute carbon. As a concrete method,
A method of using an alloy containing a rare earth element as an inoculum is effective. Rare earth elements are highly oxidative and form graphite-forming nuclei in the early stages of solidification of cast iron to promote graphitization. However, it was also confirmed by experiments that if the amount of oxygen in the molten metal is too high, the oxide of the rare earth element will grow rapidly and coarsely in the molten metal and will not serve as an inoculum.

This is similar to ordinary cast iron in Si and M.
If the total content of n is 2.0% or more, the amount of oxygen is controlled to a low value of about 50 ppm or less even if dissolved in the air, but in the low thermal expansion cast iron of the present invention, the total amount of Si and Mn is It is 1.8% or less, and the amount of oxygen in the molten metal is about 100 to 1
Since it is as high as 50 ppm, deoxidation treatment is necessary to obtain a proper inoculation effect.

In the case of spheroidal graphite type low thermal expansion cast iron, graphite spheroidizing treatment is performed by adding Mg or Ca, so that the oxygen concentration is reduced to 30 to 60 ppm and a sufficient inoculating effect by the rare earth element is exhibited. Was confirmed.

On the other hand, in the case of flake graphite type cast iron which is not subjected to the spheroidizing treatment, Fe-Zr alloy, Fe-Ti alloy and other alloys are added to the rare earth element at the same time or in advance to add these elements. It is possible to obtain a deoxidizing effect due to, and it is possible to sufficiently enhance the inoculation action. As the deoxidizing element, an iron alloy containing elements such as Zr, Ti, Nb, Ta and Hf is effective.

Further, the deoxidizing element has not only a deoxidizing effect, but also an effect that the oxide itself becomes a graphite forming nucleus. Then, it was confirmed that the inoculation effect is exhibited under the condition that the amount of oxygen in the molten metal is about 30 to 60 ppm, similar to the inoculation with the rare earth element.

In order to effectively obtain the inoculation effect by adding a rare earth element or a carbide forming element described later to the molten metal, in addition to the condition that the amount of oxygen is appropriate as described above, the addition time is to solidify as much as possible. It is preferable to be just before. That is, in the casting step, addition in the ladle or in the mold is preferable.

Further, as another method for reducing the solute carbon in the matrix, it was found that forming carbide within the grain is effective.

In the metallographic structure in which the graphite is deposited, the region separated from the graphite is a gap between resinous crystals (dendritic phase) when expressed as a solidified structure. It was found that this region is a part that becomes the final solidification part in terms of microstructure next to the graphite itself and the graphite peripheral part, and is a part where certain solute elements are easily segregated. The inventor of the present application pays attention to this point, and by adding a specific amount of a specific carbide-forming element having a strong affinity for carbon to the material, the solid solution carbon present in the dendrite gap is consumed by the formation of carbide. Let
It was found that the carbon concentration gradient can be eliminated as a whole and at the same time the Ni concentration gradient can be eliminated to effectively prevent Ni segregation.

Further, in order to eliminate the Ni segregation, it is important to disperse the precipitated carbide in the crystal grains of the matrix, and to identify the element which easily forms the carbide which can take such a dispersed form. This is very important.

With respect to this point, elements such as Cr, V, Mo and W, which are conventionally known as carbide forming elements,
Since coarse carbides are easily formed not in the crystal grains but in the crystal grain boundaries, the effect of reducing Ni segregation is small, and ultimately improvement in low thermal expansion cannot be expected. As a specific example, in the low thermal expansion cast iron disclosed in Japanese Patent Laid-Open No. 50-30728 in Table 1 above, V is an essential additive element, and Cr or Mo is supplementarily added for the purpose of improving strength. It is cast iron with carbides formed on the grain boundaries. Therefore, the effect of suppressing Ni segregation is not sufficient, and the coefficient of thermal expansion is high.

The present inventors have added N by adding various elements that tend to disperse in the matrix to form carbides.
The effect of reducing i segregation was compared and examined. As a result, when at least one of Ti, Zr, Hf, Nb, and Ta is added as a carbide-forming element, a carbide dispersed especially in the crystal grains is formed, Ni segregation is eliminated, and the as-cast material (asca
It was found that a cast iron material with a low coefficient of thermal expansion can be obtained even with the (st material).

Further, by subjecting the as-cast material containing the above-mentioned carbide-forming element to solution heat treatment under appropriate conditions, precipitation of carbide and graphitization can be further promoted, and the amount of solute carbon in the matrix can be reduced. At the same time, it was also found that the concentration gradient is further relaxed, Ni segregation can be further reduced, and more excellent low thermal expansion characteristics can be obtained. The present invention has been completed based on the above findings.

That is, the low thermal expansion cast iron according to the present invention has an effect of promoting graphitization by rare earth elements and Ti, Zr, Nb, T.
The effect of depleting the solid solution carbon in the matrix by the deoxidizing action, graphitization promoting action and carbide forming action of at least one carbide forming element selected from a and Hf and reducing Ni segregation is obtained. A low-thermal-expansion cast iron with a high Ni content having a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in a temperature range from room temperature to 100 ° C., when the as-cast material is cooled from room temperature to liquid nitrogen temperature. Area ratio of transformation of austenite matrix structure to martensite structure is 1
It is characterized by being 5% or less.

The above-mentioned Ni segregation can be confirmed by surface analysis of the structural components using EPMA (X-ray microanalyzer) or the like. As a simpler method, the casting alloy sample is dipped in liquid nitrogen. It can be confirmed by observing the structure cooled from room temperature to extremely low temperature with an optical microscope. That is, in an Fe-Ni-based alloy (or Fe-Ni-Co alloy) such as low thermal expansion cast iron or Invar alloy having a total amount of Ni + Co of about 30% or less, an austenite structure is obtained when cooled to a temperature of about -196 ° C or less. Transforms to martensite structure. The martensite structure generated by this transformation is stable at room temperature and has a property of returning to an austenite structure when heated to about 400 ° C. By this method, in the composition range in which the entire alloy structure becomes a martensite structure, that is, in the composition range in which the total content of Ni + Co is 28 to 30% by weight, the average thermal expansion coefficient in the temperature range from room temperature to 100 ° C is about Since it rapidly increases to 8 to 16 × 10 ⁻⁶ / ° C., the area ratio is set to 15% or less in the present invention.

In the above casting alloy, C is 0.3% by weight.
2.5% by weight or less, Si by 0.8% by weight or less, Mn
1.0% by weight or less, Mg or Ca 0.1% by weight
Hereinafter, Ni is 25% by weight or more and 40% by weight or less, and Co is 9%.
% By weight, provided that the total content of Ni and Co is 33% by weight or more and 43% by weight or less, 0.2% or less of rare earth elements and Ti, Nb, T in cast iron consisting of the balance Fe and impurities.
The composition is such that at least one of a, Zr, and Hf carbide forming elements is added in an amount of 0.1% by weight or more and 2.0% by weight or less.

Further, it is preferable that the casting alloy contains 1.0 to 2.5% by weight of C and has a metallic structure in which spheroidal graphite and carbide are mixed. In the composition of the above casting alloy, C is 0.8% by weight or more and 1.5% by weight or less, S
When i is set to 0.3% by weight or less and Mn is set to 0.3% by weight or less, an alloy further excellent in low thermal expansion can be obtained.

The method for producing low thermal expansion cast iron according to the present invention is
In a method for producing a low thermal expansion cast iron having a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in a temperature range from room temperature to 100 ° C., C is 0.3% by weight or more and 2.5% by weight or less and Si is 0.7% by weight or less. Wt% or less, Mn is 1.0 wt% or less, Ni is 2
5 wt% or more and 40 wt% or less, Co is 9.0 wt% or less, but the total amount of Ni and Co is 33 wt% or more and 43 wt% or less, and the balance Fe and impurities are melted to prepare a molten metal. And Mg or C when tapping from the melting furnace
Fe alloy or Ni alloy containing a is added to spheroidize, and Ti, Zr, Nb, T is added immediately before pouring into the mold.
an alloy containing at least one carbide forming element selected from a and Hf as an inoculant and a rare earth element are added to precipitate graphite and carbide in the grains during solidification during casting, thereby reducing the concentration of solute carbon as much as possible. It is characterized by reducing Ni segregation. In this manufacturing method, in the case of flake graphite type low thermal expansion cast iron that does not require spheroidizing treatment, only the addition treatment of Mg or Ca is omitted.
The subsequent steps and the effects obtained are the same as in the case of the spheroidal graphite type low thermal expansion cast iron.

It is also preferable to subject the cast and solidified casting alloy to a solution heat treatment at a temperature of 700 to 900 ° C. to promote the precipitation of at least one of carbide and graphite.

In the cast alloy having the above composition, when the carbon content exceeds 0.8 to 0.9% by weight, a metallic structure in which graphite crystallizes is obtained, but the carbon content is 0.3. Graphite usually does not crystallize in as cast material in the range of up to 0.8%. However, when the rare earth element is added in the range of about 0.01 to 0.2%, if the carbon content is 0.6% or more, crystallization of graphite can be obtained. The carbon content is 0.5% or less, which is a structure in which graphite does not crystallize and is deposited only with carbide. Even in this case, in the solidification process, first, the austenite phase having a low carbon concentration becomes a solid phase, and further the carbon concentration of the austenite phase that grows as the solidification progresses increases. As a result, although not as remarkable as in the case of an alloy having a graphite structure, a concentration distribution is formed in which the carbon concentration in the dendrite gap is also high, but the Ni concentration is low, though not as remarkable.

Therefore, even when the carbon content is small and a graphite structure is not formed, by adding the above-mentioned carbide forming element, the carbide is formed in the region where the carbon concentration is high and the concentration gradient of the carbon concentration distribution is relaxed. , Ni segregation can be suppressed.

It has been confirmed that the effect of suppressing the Ni segregation by the addition of the carbide forming element as described above is considerably effective even in the as-cast material (ascast material). However, it is possible to further enhance the effect by subjecting the as-cast material to an appropriate solution heat treatment. That is, the low thermal expansion cast iron alloy of the present invention to which a carbide forming element is added is 700 to 900 ° C.
By heating to the temperature of 2, the precipitation of carbide and the graphitization can be promoted, the solid solution carbon in the matrix is reduced, and at the same time, the concentration gradient thereof is also reduced and Ni segregation is reduced.

A heating time of the solution heat treatment of 1 to 5 hours is appropriate. In the cooling operation after the heat treatment, Ni segregation is reduced as the cooling rate is increased as much as possible, but an appropriate cooling condition is selected in consideration of the occurrence of residual stress due to rapid cooling. As a cooling method,
Generally, fan air cooling is suitable.

Next, the reasons for limiting the composition of the low thermal expansion cast iron according to the present invention will be described in detail below. That is, in the casting alloy constituting the low thermal expansion cast iron of the present invention, C is 0.3% by weight.
Above 2.5 wt%, Si below 0.8 wt%, Mn
1.0 wt% or less, Ni 25 wt% or more 40 wt%
Hereinafter, Co is 9.0% by weight or less, but the total amount of Ni and Co is 33% by weight or more and 43% by weight or less, the rare earth element is 0.2% by weight or less, and Mg or Ca is 0.1% by weight or less, 0.1 wt% or more and 2.0 wt% or less of at least one kind of carbide forming element selected from Ti, Zr, Hf, Nb and Ta, S of 0.003 to 0.2 wt%, balance Fe and impurities Consists of.

Generally, Al and A are used as the above-mentioned impurity elements.
g, As, B, Bi, Cd, Ce, Cr, Cu, I,
K, Li, Mo, P, Pb, Pt, Se, Sn, V,
Elements such as W, Y and Zn may be contained in an amount of 0.03% by weight or less.

By adjusting the composition as described above, in the state of as-cast material (as cast material), from room temperature (RT) to 10
It is possible to obtain a low thermal expansion cast alloy having a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in the temperature range of 0 ° C.

In the above composition, under the condition that the secondary graphitization heat treatment is not necessary, the more preferable component range of C is 1.0% by weight or more and 2.5% by weight or less. In this component range, graphite is crystallized in the as cast state and the amount of solid solution carbon in the matrix is sufficiently low, so that Ni segregation due to carbide formation is reduced and a cast alloy with a low thermal expansion coefficient is obtained.

A more preferable composition range of C, Si, and Mn is C1.0% or more and 1.5% or less and Si0.3 by weight.
% And Mn 0.3% or less. According to this composition range, even in the as-cast material, according to the temperature range from room temperature to 100 ° C., in the as-cast material, the thermal expansion coefficient in the temperature range from room temperature to 100 ° C. is 2.5 × 10 ⁻⁶ / It is possible to obtain cast iron having a low thermal expansion of ℃ or less.

The reasons for limiting the composition range of each component will be described more specifically below.

In the low thermal expansion cast iron according to the present invention, nickel (Ni) is a main component element that exerts the "Invar effect" and contributes to the reduction of the thermal expansion coefficient near room temperature. The Invar effect is effectively obtained when the Ni content in iron is set in the range of 25 to 40% by weight. When the Ni content is out of the above range, the coefficient of thermal expansion increases in all cases. The more preferable range of the Ni content is 28 to 36% by weight.
Is.

Cobalt (Co) is an element that further reduces the thermal expansion coefficient by the synergistic effect with Ni when the total content of Ni and Co is 33 to 43% by weight.
However, if the Co content exceeds 9% by weight, the temperature range exhibiting low thermal expansion tends to expand to the high temperature side, but the thermal expansion coefficient near room temperature increases greatly. Therefore, the Co content is set to 9% by weight or less.

Carbon (C) is a component that crystallizes graphite in low expansion cast iron and imparts castability, cutting workability, vibration damping properties, and the like. The carbon that has not become graphite exists as carbide and solid solution carbon. The present invention is characterized in that Ni segregation is suppressed by precipitating carbides in the crystal grains of the matrix in the metal structure, and carbon is the most important constituent element. The rest becomes solute carbon, which causes an increase in the thermal expansion coefficient. Therefore, it is important to set the amount of solute carbon as low as possible.

In the present invention, the C content is 0.3% to 2.5% by weight. If the C content is less than 0.3% by weight, it is difficult to impart sufficient castability, machinability and vibration damping. Further, if the C content exceeds 2.5% by weight, the coefficient of thermal expansion increases. In addition, 0.3 to 1.0
In the range of wt%, graphite does not crystallize in the as-cast material without heat treatment and only carbides are formed.

In this case, the heat treatment for the purpose of secondary graphitization of the as-cast material improves the machinability and the low expansion property. Further, in the range of 1.0% to 2.5%, both graphite and carbide are formed as an as-cast material, and low thermal expansion casting excellent in castability, machinability, vibration damping and low expansion. An alloy is obtained. A more preferable C content range is 1.0 to 1.5% by weight, and by adding the carbide formation of the present invention to the conditions, it is possible to keep the solid solution carbon in the solidification phase low. Segregation of Ni can be suppressed to such an extent that it does not pose a problem. Carbon content in this range is as cas
The low-expansion property of the t material is close to that of the rapidly heat-treated material.

In the present invention, silicon (Si) plays a small role in graphitization such as graphite nucleation sites and carbon equivalent constituents as in general cast iron. In the low expansion cast iron of the present invention, Si is added for the purpose of obtaining an effect of suppressing oxidation during atmospheric melting. On the other hand, Si is an element that increases the coefficient of thermal expansion most in low-expansion cast iron, and it is desirable that the content be as low as possible, and the content is 0.8% by weight or less. It is preferably 0.3% or less.

Manganese (Mn) is a basic component of cast iron, and Mn functions as a deoxidizer and a component for improving strength and improving corrosion resistance. However, if the content is too large, the coefficient of thermal expansion increases as the solid solution content increases, so the content of Mn is 1.
It is 0% by weight or less. More preferably, it is 0.3% or less.

Magnesium (Mg) or calcium (Ca) is added as a spheroidizing component of graphite and a deoxidizer. Similar to Mn, in order to prevent an increase in the coefficient of thermal expansion, the upper limit of the content of Mg or Ca is 0.1% by weight.
Generally, Mg is mainly used, and after Ni-5% Mg alloy and Fe-5% Mg alloy are melted, they are added just before casting and reacted with the molten metal. The amount of Mg or Ca remaining in the cast iron after solidification is generally required to be 0.04 to 0.09% for spheroidizing of graphite, but if it is 0.01 to 0.03%, cohesive spheroidal graphite or CV cast iron graphite is required. The so-called spheroidized graphite is obtained. Further, when Mg and Ca have a residual amount of 0.01% or less only due to the deoxidizing effect, a flake graphite structure is obtained. The lower the spheroidization rate of graphite, the higher the proportion of carbon that becomes graphite in the total carbon content, and the lower the amount of solute carbon, the lower the expansion coefficient and the better the vibration damping characteristics. Strength tends to decrease.

The rare earth element forms reaction products with S and O in the molten metal, and these reaction products act as extremely effective graphite formation nuclei to promote graphitization. In general, rare earth elements are Ce, La, Nd,
It is added as an alloy composed of elements such as Pr. The graphitization promoting additive used in the present invention has the same effect whether it is added as a misch metal or an alloy containing a rare earth element. The amount of the rare earth element added needs to be appropriate depending on the amounts of S and O in the molten metal. Further, in order for the above reaction products (sulfides and oxides) to function as graphite formation nuclei, it is necessary to disperse them in the molten metal in a fine state, and when they become coarse, they only have a desulfurizing and deoxidizing effect. I will end up. That is, when the residual amount of the rare earth element is 0.2% or more, the graphite promoting action is lost.

Other impurities such as phosphorus (P) and sulfur (S)
Etc. are contained in practical cast iron, but P is not preferable for the purpose of the present invention, so it is controlled to 0.03% by weight or less. However, S reacts with the rare earth element and is necessary for promoting graphitization, and the content thereof is approximately in the range of 0.003 to 0.2%. It is preferably 0.04 to 0.1%.

Regarding the carbide forming element, titanium (T
At least one selected from i), zirconium (Zr), hafnium (Hf), niobium (Nb), and tantalum (Ta) is added in the range of 0.1 to 2.0% by weight. All of these elements are elements having a low free energy for carbide formation in iron alloys, and the carbides of these elements are more likely to nucleate than graphite and have good compatibility with the crystal lattice of the matrix and are precipitated in the grains. Even if the carbon content is 0.9% or less and graphite is not present, a structure in which only carbide is dispersed and precipitated in the crystal grains in the matrix can be obtained.

Here, in the conventional low thermal expansion cast iron containing no carbide forming element as described above, as shown in FIG. 2, the amount of solid solution carbon in the vicinity of graphite is the lowest, and it is in the middle of graphite and graphite (dendritic gap). ), The concentration gradient of solute carbon increases. Therefore, a concentration gradient is also formed in Ni that is repelled by the solute carbon, and a portion having a low Ni content occurs between graphite (dendritic gap) (reverse segregation).

However, according to the findings of the inventors of the present application, the above-mentioned carbide-forming elements are rather segregated between graphites to form carbides as shown in FIG. 1, thereby forming a solid solution carbon concentration gradient. It was found to have the effect of eliminating.
That is, by adding a specific carbide-forming element,
It has been found that improvement of low thermal expansion is realized by the effect of eliminating Ni segregation and the effect of reducing the amount of solute carbon.

Further, the low expansion cast iron is improved by graphite due to the austenite matrix peculiar to cast iron with high Ni content, and the defects with poor machinability are improved by graphite. It was also found that it also has the effect of reducing workability and improving workability.

The above-mentioned carbide-forming elements can be added alone or in combination, but the total amount is in the range of 0.1 to 2.0% by weight, depending on the amount of carbon. If the content of the carbide-forming element is less than 0.1% by weight, the carbide cannot be sufficiently precipitated, and the Ni segregation reducing effect and the solute carbon content reducing effect cannot be sufficiently obtained. Further, when the content of the carbide forming element exceeds 2.0%, the amount of the residue that does not contribute to the formation of the carbide increases and the coefficient of thermal expansion increases.

It is preferable that at least 75% or more (more preferably 80% or more) of the carbide forming element in the present invention is present as a precipitation phase, and most of the elements are consumed as a precipitation phase. This is because the solid solution content of the carbide forming element adversely affects the low thermal expansion property. As described above, in order to cause most of the carbide forming elements to exist as the precipitation phase and not to remain in the matrix, it is only necessary to obtain the limit amount from the composition of the carbides of each element and add the carbide forming element according to the amount.

[0062] For example, although in the case of Ti form a TiC, the density [rho _Ti of Ti with 4.54 g / cm ^3, since the density [rho _c of one of the C is 2.25 g / cm ^3, and Ti The density ratio of C is about 2.0 times. From this, it is desirable that the addition amount of Ti is about 2.0 times as large as about 1/3 of the residual carbon amount after graphitization.

Similarly, the density ratio of carbon content to other carbide forming elements is 3.8 for Nb / C, 7.4 for Ta / C, and Zr / C.
Is 2.9 and Hf / C is 5.9. Since both are MC type carbides (atomic weight ratio is 1: 1), the proper addition amount of the carbide forming element is, as shown in the following formula (4), 0.3x density ratio to the residual carbon amount. It is the multiplied amount. As for the residual carbon amount, when the total carbon amount is 0.8% by weight or less, as shown in the formula (2), almost all the carbon amount remains, and when it is 0.9% by weight or more, graphite is formed. The residual carbon amount can be obtained by an approximate calculation using the equation (3). These approximation formulas are calculation formulas found by the inventor of the present invention by a multiple regression analysis method for a large number of data.

[0064]

[Equation 1]

[Equation 2]

(Equation 3)

[Equation 4] In the carbon content range of the present invention, the maximum residual carbon content is about 0.8% by weight, so the maximum addition amount of the carbide forming element is 2.0% by weight of Ta. If more than this is added, the excess will form a solid solution in the matrix, increasing the coefficient of thermal expansion. Thus, by appropriately setting the addition amount of the carbide-forming element, the solid solution content of the carbide-forming element becomes extremely small, so that the low expansion property is not deteriorated.

Further, in the low thermal expansion cast iron according to the present invention, the area ratio of carbides precipitated in the metal structure in the grain structure is 0.3%.
It is preferably in the range of ˜30%. When the area ratio of precipitated carbide is less than 0.3%, strength, hardness, machinability,
The effect of improving the low expansion property is insufficient, and if it exceeds 30%, the effects of the thermal expansion coefficient and hardness of the carbide adversely affect and adversely affect the low expansion property and the machinability. More preferable area ratio of precipitated carbide is 0.5%
˜15%, more preferably 1.5% ˜
It is in the range of 5.0%.

The grain size of the carbide also affects the mechanical properties and machinability. The preferable range of the grain size of the carbide is 5 to 50 μm
And For that purpose, it can be controlled by the amount of carbon and the content of the carbide forming element. The above-mentioned composition range of the low expansion cast iron also takes this point into consideration.

The amount of spheroidal graphite precipitated in the low expansion cast iron in the present invention is preferably in the range of 0.5% to 15% in terms of area ratio in the metal structure. When the amount of precipitation is 15% or more, the strength of cast iron is adversely affected, and preferably 1
It is preferably 0% or less. Therefore, the upper limit of the amount of carbon is set to 2.5%.

The area ratio of the precipitated carbides is measured by the following method.

First, a micrograph of a cross section of polished low expansion cast iron is prepared. In order to clarify the precipitation state of carbide,
Etch with 10% aqua regia. The magnification of the micrograph is preferably about 200 times. The area ratio is defined by the following formula.

[0070]

(Equation 5) Recently, a method for measuring the total area of carbides and graphite has been performed using an image analysis device for micrographs.
It is also possible to calculate the area ratio by cutting out a photograph enlarged to a photograph of 00 mm × 200 mm or more separately from the carbide, the graphite, and the matrix structure and using the measured values of the respective weights.

Next, the heat treatment will be described.

The main purpose of the solution heat treatment carried out in the method of the present invention is to promote the formation of carbide and the growth of graphite. This is an important treatment operation especially when it is necessary to form secondary graphite when the amount of carbon is 08% by weight or less and graphite is not sufficiently formed with the as-cast material.

The cast iron of the component of the present invention has a carbon content of 0.3% to
With a composition of 1.0%, the cast structure has a structure in which carbides are dispersed and precipitated in the austenite matrix, or a slight amount of graphite is formed, resulting in poor machinability. Then, solution treatment is performed at a temperature of 700 to 900 ° C. to form secondary graphite. As the solution heat treatment time, it is necessary to determine an appropriate treatment time according to the thickness and shape of the cast product, but the time calculated by the following equation (5) is a tentative guide.

[0074]

(Equation 6) In the solution heat treatment, heating at 700 ° C. or higher is necessary to promote the above graphitization and carbide formation.
The reason why the upper limit is set to 900 ° C. is that heating at 900 ° C. or higher causes the carbide to decompose, so that the carbide forming element forms a solid solution and increases the thermal expansion coefficient.

[0075]

According to the low thermal expansion cast iron and the method for producing the same having the above structure, a rare earth element having an extremely excellent effect of promoting graphite formation is added as an inoculant, and a specific carbide forming element having a strong affinity with carbon is added. Since it is combined with the solid solution carbon present in the dendrite gaps and precipitated in the grains as carbides, the carbon concentration gradient can be eliminated and at the same time the Ni concentration gradient can be eliminated to effectively prevent Ni segregation. Therefore, an increase in the thermal expansion coefficient due to Ni segregation can be effectively prevented, and cast iron excellent in low thermal expansion characteristics can be provided without performing a quenching heat treatment that easily causes residual stress and strain.

Further, by subjecting the as-cast material to solution heat treatment, precipitation of carbide and graphitization can be further promoted,
At the same time that the amount of solute carbon in the matrix is further reduced, the concentration gradient is further alleviated, Ni segregation can be further reduced, and more excellent low thermal expansion characteristics can be obtained.

[0077]

EXAMPLES Examples of the low thermal expansion cast iron according to the present invention will be described below.

Examples 1 to 12 Each cast iron material having the composition shown in Table 2 below was melted using a high frequency electric furnace having a capacity of 100 kg, and
The casting alloys according to Examples 1 to 12 having a wall thickness of 25 mm, a height of 150 mm, a width of 200 mm and a weight of 6 kg were prepared by pouring in a sand mold.

[0079]

[Table 2]

The composition of each example is set by adding an appropriate amount of a carbide forming element selected from Ti, Zr, Hf, Nb and Ta to the basic composition of the cast iron material alone or in combination. . The amount of each carbide-forming element added is
The content was combined with 1/3 of the amount of residual carbon that was not graphitized during casting as a guide.

A test piece having a diameter of 5 mm and a length of 65 mm was prepared from the thus-prepared casting alloys of the respective examples, and the JI
A thermal expansion test was conducted in accordance with the thermal expansion test method specified in S G5511, "Iron-based low expansion cast iron product", and the as-cast material (as
The average thermal expansion coefficient of the cast alloy as the cast material) was measured in the temperature range from room temperature to 100 ° C. Further, the area ratio of carbides precipitated in the metal structure in the grains was measured using the image analysis device.

Further, the cast alloys of the respective examples were subjected to solution heat treatment at a temperature of 850 ° C. for 4 hours in an electric furnace and then air-cooled. The heat-treated materials thus obtained were averaged in the same manner as the as-cast material. The thermal expansion coefficient and the area ratio of the precipitated carbides were measured and the results shown in Table 3 below were obtained.

[0083]

[Table 3]

As is clear from the measurement results shown in Table 3,
In the low thermal expansion cast iron according to each example having a predetermined component range to which a carbide forming element is added, as cast
), The average coefficient of thermal expansion in the temperature range from room temperature to 100 ° C. is 4 × 10 ⁻⁶ / ° C. or less. Especially Example 1
It was found that in the compositions of to 4, the coefficient of thermal expansion near room temperature was 2 × 10 ⁻⁶ / ° C. or less even when as-cast, and excellent low thermal expansion characteristics were exhibited.

Further, it was found that the as-cast material of each example was subjected to solution heat treatment under the above conditions to further reduce the coefficient of thermal expansion. Since the above heat treatment is not a quenching treatment such as water quenching but a slow cooling treatment such that it is allowed to cool in the atmosphere, it has an advantage that excessive residual stress and strain do not occur in the cast product. Therefore, according to this example, it was proved that Ni segregation can be reduced and a cast iron excellent in low thermal expansion can be provided without performing quenching treatment.

Comparative Examples 1 to 12 On the other hand, each cast iron material having the composition shown in Table 4 below was
Each is melted using a high frequency electric furnace with a capacity of 100 kg and then poured into a sand mold to have a wall thickness of 25 mm and a height of 150 mm.
The cast alloys according to Comparative Examples 1 to 12 each having a width of 200 mm and a weight of 6 kg were prepared. The component composition of each comparative example is set to a condition deviating from the composition condition of the present invention.

[0087]

[Table 4]

Test pieces were prepared from the thus-prepared casting alloys of the comparative examples in the same manner as in the examples, and a thermal expansion test was carried out. The casting alloys as as cast materials were cooled from room temperature to 100 ° C. The average coefficient of thermal expansion in the temperature range up to was measured. Further, the area ratio of carbides precipitated in the metal structure in the grains was measured using the image analysis device.

Further, the cast alloys of the respective comparative examples were subjected to solution heat treatment in an electric furnace at a temperature of 850 ° C. for 4 hours in the same manner as in the example, and then air-cooled. Similarly, the average thermal expansion coefficient and the area ratio of the precipitated carbides were measured, and the results shown in Table 5 below were obtained.

[0090]

[Table 5]

As is clear from the measurement results shown in Table 5,
Those in which the carbide forming element specified in the present invention is not added,
It has been found that the cast iron material out of the composition range defined by the present invention has a thermal expansion coefficient of more than 4 × 10 ⁻⁶ / ° C. near room temperature in the as cast state.

Also, when the as-cast material of each of the above comparative examples is subjected to solution heat treatment under the same conditions as those of the examples, the coefficient of thermal expansion of the heat treated material of each of the comparative examples is the same as that of the heat treated material of the example. It was found to be a high value in comparison, and the low thermal expansion property was found to be insufficient.

Furthermore, the casting alloys according to Comparative Examples 9 to 12 were
It is an alloy to which Cr, V, Mo, W is added, not the one to which the carbide forming element specified in the present invention is added. However, since these elements form carbides at grain boundaries, it was confirmed that the effect of reducing Ni segregation in the dendrite gap is small and the effect of improving the thermal expansion coefficient is small.

[0094]

As described above, according to the low thermal expansion cast iron and the method for producing the same according to the present invention, it is possible to add a rare earth element having an extremely excellent effect of promoting graphite formation as an inoculant and to specify a strong affinity for carbon. Since the carbide forming element is added and is combined with the solid solution carbon present in the dendrite gaps to cause intragranular precipitation as carbide, the concentration gradient of carbon is eliminated and at the same time the concentration gradient of Ni is also eliminated.
i segregation can be effectively prevented. Therefore, an increase in the thermal expansion coefficient due to Ni segregation can be effectively prevented, and cast iron excellent in low thermal expansion characteristics can be provided without performing a quenching heat treatment that easily causes residual stress and strain.

Furthermore, by subjecting the as-cast material to solution heat treatment, precipitation of carbide and graphitization can be further promoted,
At the same time that the amount of solute carbon in the matrix is further reduced, the concentration gradient is further relaxed, Ni segregation can be further reduced, and more excellent low thermal expansion characteristics can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing Ni concentration and C concentration in a metal structure of low thermal expansion cast iron according to the present invention.

FIG. 2 is a schematic diagram showing Ni concentration and C concentration in the metallographic structure of conventional low thermal expansion cast iron.

Claims (6)

[Claims] 1. A low-thermal-expansion cast iron with a high Ni content having a coefficient of thermal expansion of 4 × 10 ⁻⁶ / ° C. or less in a temperature range from room temperature to 100 ° C., and the as-cast material is changed from room temperature to liquid nitrogen temperature. A low thermal expansion cast iron having an area ratio of 15% or less in which an austenite matrix structure is transformed into a martensite structure when cooled. 2. The low thermal expansion cast iron comprises C in an amount of 0.3 wt% to 2.5 wt%, Si in an amount of 0.8 wt% or less, Mn in an amount of 1.0 wt% or less, and Ni in an amount of 25 wt% to 40%. Wt% or less, Co 9.0 wt% or less, but the total amount of Ni and Co 33 wt% or more 43 wt% or less, Mg or Ca 0.1 wt% or less, rare earth elements 0.2 wt% Below, N
b, Ti, Zr, Ta and Hf, containing at least one carbide forming element in an amount of 2.0 wt% or less,
The low thermal expansion cast iron according to claim 1, wherein the balance is Fe and impurities. 3. The C content is 0.8% by weight or more and 1.5% by weight or less, the Si content is 0.3% by weight or less, and the Mn content is 0.3% by weight or less. The low thermal expansion cast iron according to claim 2. 4. A method for producing a low thermal expansion cast iron having a high Ni content and a thermal expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less in a temperature range from room temperature to 100 ° C., wherein C is 0.3% by weight or more and 2.5 or more. Wt% or less, Si 0.8 wt% or less, Mn 1.0 wt% or less, Ni 25 wt% or more 40 wt% or less, Co 9.0 wt% or less, but the total amount of Ni and Co Is 33% by weight or more and 43% by weight or less, a material containing the balance Fe and impurities is melted to prepare a molten metal, and Mg or Ca remaining after casting is 0.1% by weight or less (including O).
After adding an alloy of Mg or Ca to the molten metal under the condition that the following conditions are satisfied, the rare earth element and Nb, Ti, Ta, Zr,
A method for producing cast iron having a low thermal expansion, which comprises adding an inoculum containing at least one kind of a carbide-forming element such as Hf immediately before casting and then casting. 5. The method for producing low thermal expansion cast iron according to claim 4, wherein the inoculation in the mold is performed by adding the inoculant in the mold. 6. The method for producing low thermal expansion cast iron according to claim 4, wherein the casting is performed in a mold equipped with a chiller or in a mold or a graphite mold.