JP2008303108A - Graphite material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a graphite material having high strength and high density, and further having excellent workability, and to provide a method of producing the graphite material. <P>SOLUTION: Disclosed is a graphite material having a fine structure composed of many graphite grains and pores. When the cross-section is observed with a scanning electron microscope, the number of the pores appearing in the cross-section is ≥250 pieces /per 6,000 μm<SP>2</SP>, the average area of the pores appearing in the cross-section is ≤5 μm<SP>2</SP>, and the average flattening ratio of the pores appearing in the cross-section is ≤0.55. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a graphite material and a method for producing the same, and more particularly to a graphite material suitable for a precisely machined member such as an electrode for electric discharge machining and a jig for electronic parts.

  Graphite material has excellent chemical stability, heat resistance, workability, etc., so it is used in many fields such as electrodes for electric discharge machining, jigs for glass sealing of electronic parts and brazing. . In recent years, along with miniaturization of home appliances and automobile parts, precision processing such as thin ribs, grooves, thin pins, and fine holes has been performed on dies used for die casting and plastic molding. In order to produce such a precise mold, an electric discharge machining electrode made of a graphite material capable of precise machining has been required.

  In order to use a graphite material as an electrode for electric discharge machining and to perform electric discharge machining into a precise shape such as a thin rib without damaging the electrode, the graphite material needs to have a certain level of strength. Further, in order to increase the dimensional accuracy of the mold to be processed, it is important that the graphite material is not deformed by heat and external force due to electric discharge machining.

As a high-strength and high-density graphite material suitable for such applications, it is known to use mesocarbon microbeads as a raw material (see, for example, Patent Document 1). In addition, as another method for producing a high-density and high-strength graphite material, using mesocarbon microbeads having a specific β-resin component amount, ash content, moisture content, volatile content, fixed carbon, and average particle diameter as raw materials It is disclosed that it is molded by a cold press and fired and graphitized at a predetermined temperature (for example, see Patent Document 2). Since the graphite material obtained by the manufacturing methods described in Patent Documents 1 and 2 has high strength and high density, there is an advantage that even if processed into a shape such as a thin rib, it is difficult to break.
JP-A-1-97523 JP-A-4-240022

  However, since the conventional graphite materials such as Patent Documents 1 and 2 have high strength and high density, there is a problem that the cutting resistance of the blade during processing is large and chipping is likely to occur. Further, since the cutting resistance of the blade is high, when processing thin ribs and thin pins, the graphite material is warped by the reaction force, and the accuracy of thickness and thickness is lowered. Furthermore, when machining the inner and bottom surfaces of a frame with a small corner R, a narrow groove, a deep narrow hole, etc. using an end mill or drill, the end mill or drill is warped, and high precision machining is not possible. It is a cause of breakage.

  In order to prevent these problems, it is possible in principle to reduce the amount of cutting once by the blade, but for this purpose, the blade feed speed is reduced or the blade rotation speed is increased. It is necessary to take measures such as In such a method, it is necessary to use a high-performance processing machine or blade having high rigidity that does not cause core blur even when rotating at high speed, and processing takes time.

  Furthermore, when conventional graphite materials are used for electrodes for electrical discharge machining for finishing, graphite materials generally have a relationship in which electrode consumption decreases as the Shore hardness increases, so graphite with a low graphitization temperature and high Shore hardness Materials were preferred, but graphite materials with high Shore hardness had problems such as high cutting resistance and fast blade consumption.

  The present invention has been made in view of the above problems, and provides a graphite material having high strength and high elastic modulus and excellent workability, and a method for producing such a graphite material. With the goal.

  As a result of intensive studies in view of the above problems, the present inventor has found that a graphite material having a specific structure is capable of breaking a cutter in precise processing such as thin ribs, thin pins, narrow grooves, and narrow holes, and the like. We found that it can be processed with high accuracy. That is, the means for solving the above problems are as follows.

(1) A graphite material having a fine structure composed of a large number of graphite particles and pores, and when the cross section is observed with a scanning electron microscope, 250 or more pores appearing in the cross section per 6000 μm ^2. The graphite material is characterized in that the average area is 5 μm ² or less, and the average flatness of pores appearing in the cross section is 0.55 or less.
(2) The graphite material as described in (1) above, having a bulk density of 1.78 to 1.86 g / cm ³ .
(3) The graphite material according to (1) or (2), which is for electric discharge machining.

Graphite material according to the present invention, pores appearing in cross section 6000 .mu.m ² per 250 or more, the average area of appearing pore 5 [mu] m ² or less in cross section, and the average aspect ratio of the pores appearing in section 0.55 below, fine Since it has a fine structure in which graphite particles and pores are uniformly distributed, it has high strength and high elastic modulus and excellent workability. Therefore, even when the graphite material according to the present invention is used for precision machining of a thin rib or the like using the electrode for electric discharge machining, the graphite material or the blade can be machined with high accuracy without being damaged. In addition, the graphite material according to the present invention can be finely processed and has low consumption during electric discharge machining, so that a mold having a fine pattern can be easily created, and for electric discharge machining in finishing. It is also advantageous as an electrode.

Hereinafter, embodiments of the graphite material according to the present invention will be described in detail.
The graphite material according to the present invention has a fine structure composed of a large number of graphite particles and pores. When the cross section of the graphite material was observed with a scanning microscope, pores appearing in cross-section, 6000 .mu.m ² per 250 or more, the average area of pores appearing on the cross-section is 5 [mu] m ² or less. In other words, the pores distributed in the graphite material are sufficiently small, and the number of pores present per unit volume of the graphite material is sufficiently large. For this reason, a smooth machined surface is obtained without dropping off in units of large particles during cutting. In addition, since the pores are very small compared to the normal processing shape applied to graphite material, the occurrence of breakage in fine pin processing due to dropout of particles, cracking and perforation in thin rib cutting processing is reduced. can do.

  Moreover, when the cross section of the graphite material according to the present invention is observed with a scanning microscope, the average flatness of pores appearing in the cross section is 0.55 or less. By setting the average flatness of the pores appearing in the cross section to 0.55 or less, the elastic modulus of the graphite material increases with respect to the compressive strength by the blade during processing, and the cutting waste generated during processing can be reduced. That is, the cutting resistance of the cutter is small and easy to process.

The relationship between the pore shape of the graphite material as described above and its workability is presumed to be due to the following mechanism.
When the graphite material is cut, a compressive force acts in the traveling direction of the blade. At this time, cutting is performed when the strain energy stored as the blade advances exceeds the energy required for destruction. In order to obtain a smooth machined surface, it is necessary to work while producing fine cutting powder, and it is important that destruction occurs before storing large strain energy.
In order not to store large strain energy, it is important that the compressive strength is small and the elastic modulus is large, that is, the diameter of the particles to be cut has a positive correlation with the compressive strength / elastic modulus. From the above, it can be seen that a graphite material having a higher elastic modulus is advantageous under a predetermined compressive strength in order to obtain a machined surface with a small diameter of the particles to be cut (finely detailed).

Next, the relationship between the elastic modulus of the graphite material and the shape of the pores will be explained. Generally, the elastic modulus of the graphite material is expressed by the following Knudsen's empirical formula.
E (P) = E (0) exp (-bP)
(E (P): elastic modulus, P: porosity, b: empirical constant)
The empirical constant b strongly depends on the shape of the pores, and when the shape of the pores is spherical, the value is small, and the value rapidly increases as the flat spheroid changes to a cracked pore shape. Is known (“Introduction to New Carbon Materials”, edited by the Carbon Materials Society of Japan). Therefore, it can be seen that a graphite material having a round shape (small flatness) is advantageous for increasing the elastic modulus.
From the above, it is considered that the relationship between the pore shape of the graphite material and its workability is derived. That is, since the pore shape is round (that is, the average flatness of pores appearing in the observation cross section is 0.55 or less), the elastic modulus of the graphite material can be increased, so that a finely machined surface can be obtained. And a graphite material excellent in workability can be obtained.

Next, regarding the compressive strength, even if the pores are flat spheroids or cracked pores, the pores are crushed by applying a compressive load, so the shape of the pores does not have a great influence. It can be seen that the effect of porosity is greater on the compressive strength.
When the porosity is small, the compressive strength becomes too high to be cut, and the unevenness of the processed surface becomes large. When the porosity is high, the compressive strength can be reduced, but the graphite material becomes soft, and therefore, even if it is subjected to fine processing, it tends to be broken or broken. Moreover, it becomes easy to wear even in electric discharge machining.

There is a high correlation between the porosity and the bulk density of the graphite material. When the same raw material is used and the same graphitization treatment is performed, the same porosity density is obtained with the same porosity.
In the present invention, the starting material and the graphitization temperature are performed within a limited range, although there are components that mainly undergo pitch coke, and components that are directly carbonized and graphitized, starting from pitch. Therefore, the bulk density of the graphite material according to the present invention has a preferable range, and its value is 1.78 to 1.86 g / cm ³ . In addition, the bulk density in this invention is obtained by measuring a volume and mass.

In the present invention, the number of pores appearing in the cross section, the average area, and the average flatness can be calculated by observing the graphite material with an electron microscope or the like. Specifically, first, the cut surface of the graphite material is processed by a CP (cross section polisher) method. It is obtained by observing with a FE-SEM after performing a flat milling process (45 °, 3 minutes) on the prepared cross section.
The analysis of the photographed image is binarized using image analysis software (Image J 1.37), and then the area of each void (pore appearing in the cross section) is calculated. Ellipse fitting is performed for each gap, and the flatness is calculated from the values of the major axis and minor axis.
The flatness is an ellipse (major axis-minor axis) / major axis fitted to a void (a pore appearing in a cross section).

  In addition, it is desirable to use SEM as described above for the measurement of the number of pores appearing in the cross section, the average area, and the average flatness. Sufficient resolution is obtained to determine the shape of micron-order pores, and the particle part is gray at a single concentration, the pore part is black for deep pores depending on the depth of the pores, and for shallow pores Is displayed in white so that pores and particles can be clearly distinguished.

  For the measurement of the number of pores appearing in the cross section, the average area, and the average flatness, it is preferable to use a graphite material not filled with resin. This is because if the graphite material is filled with resin, the resin is sealed in the open pores present in the graphite material, and the correct number and shape of the pores cannot be determined.

The maximum pore diameter (major axis) is preferably 20 μm or less. When the maximum pore diameter is 20 μm or less, cracks develop along the pores at the time of cutting. Therefore, the fine pins break, and thin ribs cause cracks and holes.
The maximum pore diameter can also be measured from the cross section observed with the SEM as described above. The diameter of the pores obtained from the cross-sectional observation of the SEM is different from the diameters of the pores and graphite particles obtained with a mercury intrusion porosimeter or the like. The former measures the actual size, while the latter measures the diameter of the inlet portion of the continuous pores.

The Shore hardness of the graphite material according to the present invention is preferably in the range of 55-80. When the Shore hardness is less than 55, particle dropout during electric discharge machining increases and electrode consumption increases, which is not suitable for an electric discharge machining electrode. If the Shore hardness exceeds 80, the cutting resistance of the blade increases during the cutting of the electrode, so that the blade is consumed quickly and the material is easily broken or applied during the processing.
Shore hardness can be measured according to JIS Z2246.

The specific resistance value of the graphite material according to the present invention is preferably 1000 to 2300 μΩcm. The specific resistance correlates with the Shore hardness of the graphite material. When the specific resistance decreases, the graphite material becomes soft. When it is 1000 μΩcm or less, the Shore hardness is less than 55, and the electrode wear becomes significant. In this case, even if a fine pattern is processed and used as an electrode, the electrode is consumed rapidly, and the processing accuracy cannot be transferred to the mold. When the specific resistance is 2300 μΩcm or more, when used as an electrode for electric discharge machining, abnormal discharge or the like may occur, and unevenness may easily occur on the processed surface of the workpiece.
The specific resistance value can be measured by the JIS R7222 voltage drop method.

  The graphite material of the present invention can be suitably used particularly for an electric discharge machining electrode for finishing. Rough machining is a rough machining of a mold, and no particularly fine machining is performed. The graphite material of the present invention can be subjected to fine pattern processing with high precision required in final finishing.

  Next, a method for producing a graphite material according to the present invention will be described. The graphite material according to the present invention is obtained by adding a carbonaceous fine powder to pitch and kneading, and then adjusting the volatile content while performing heat treatment at 400 to 500 ° C. to obtain a secondary raw material. Next, the obtained secondary raw material is pulverized while adjusting the particle size so as not to be excessively pulverized by a pulverizer having a function of removing fine powder having a small particle size, thereby obtaining secondary raw material powder. Next, it is molded into a rectangular parallelepiped by cold isostatic pressing (CIP molding), fired at about 1000 ° C. in a sintering furnace, and further graphitized at about 2500 ° C. in a graphitization furnace. A graphite material is obtained.

  The pitch used in the present invention is a coal-based or petroleum-based pitch, and may be a mixture thereof. Of these raw materials, it is desirable to use a coal-based pitch. In the case of a coal-based pitch, optical anisotropy hardly develops (a crystal does not easily develop in a needle shape), and a high-strength and highly elastic graphite material can be obtained.

The softening point of the pitch used in the present invention is desirably 50 ° C. or lower. When the temperature is 50 ° C. or higher, the viscosity at the time of kneading increases and the production becomes very difficult.
The carbonaceous fine powder used in the present invention is a core when the mesophase develops, and it is possible to use a carbonaceous powder such as carbon black, graphite fine powder, raw pitch coke fine powder, calcined pitch coke fine powder or the like. it can. The size of the fine powder is preferably 5 μm or less. This is because if a fine powder of 5 μm or more is used, it is difficult to control the particle size distribution when the secondary material obtained by kneading is pulverized, and the coarser side of the particle size distribution increases. The addition amount to the pitch is desirably 3 to 10% by weight. If the amount exceeds 10% by weight, the viscosity of the pitch will increase, making the production very difficult. When the amount is 3% by weight or less, the mosaic structure of coke cannot be sufficiently developed.

  In the heat treatment of the raw material, the temperature and time are adjusted so that the volatile content measured by JIS8812 is 6 to 12%, and a secondary raw material is obtained. When the volatile content is less than 6%, sufficient adhesion between the particles cannot be obtained, so that only a low-density graphite material can be obtained. In the case of 12% or more, the amount of hydrocarbon gas generated from the inside during firing is large, and it is easy to break, and the accumulated gas forms large pores.

The secondary raw material obtained by heat-treating the raw material is pulverized while controlling the particle size, and fine powder is removed from the obtained secondary raw material powder. The method of pulverization includes a method using a pulverizer equipped with an internal partial classifier, a method using a pulverizer plant equipped with a pulverizer and a precision airflow classifier, and a raw material crushed by the pulverizer separately with a precision airflow classifier. There is a method of adjusting the particle size.
The graphite material using the secondary raw material powder containing the fine powder is less likely to release the gas generated during firing and easily breaks. Furthermore, gas accumulates in the material and forms large pores.

  The secondary raw material powder preferably has a median diameter (DP-50: 50% integrated diameter) of 5 to 10 μm as measured with a laser diffraction particle size measuring instrument. The pores that normally exist between particles are often pores with sharp edges and large flatness, and when the size of the particles is large, the size and shape of the pores have a synergistic effect and the elastic modulus decreases. growing. When the median diameter is 10 μm or more, the elastic modulus decreases and the graphite material of the present invention cannot be obtained. Further, when the median diameter is 5 μm or less, the volatile matter generated from the compact of the secondary raw material powder during firing cannot be quickly discharged out of the raw material, and is easily broken. Furthermore, gas accumulates in the material and forms large pores.

Moreover, it is preferable that the secondary raw material powder has a particle size distribution range of 1 μm to 80 μm as measured with a laser diffraction particle size analyzer. If a raw material of 1 μm or less is included, the volatile matter generated from the molded body of the secondary raw material powder at the time of firing cannot be quickly discharged out of the raw material and is easily broken. Furthermore, gas accumulates in the material and forms large pores. When particles of 80 μm or more are contained, flat pores are easily formed near the outer periphery of the large particles and the interface between the large particles, the number of pores is reduced, and the average cross-sectional area is also reduced.
For example, LA-750 manufactured by HORIBA, Ltd. can be used as the laser diffraction particle size measuring instrument.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated more concretely, this invention is not limited to the following Example.

1. Production of graphite material (Examples 1-2)
After adding 5 parts by weight of calcined coke pulverized to an average of 2 μm to 95 parts by weight of a coal-based pitch with a softening point of 40 ° C., kneading, adjusting the volatile content while applying heat treatment at 415 ° C. The next raw material was obtained. Next, the mixture was pulverized by a pulverizer equipped with an inner classifier so as not to be excessively pulverized to obtain a secondary raw material powder. Next, after pressurizing at a pressure of 100 MPa with an isotropic isostatic press, firing was performed at a rate of temperature increase of about 5 ° C./hour up to 1000 ° C., and graphitization was performed at 2500 ° C.
Note that the secondary raw material powder obtained during the production did not contain powders of 1 μm or less and 80 μm or more in the particle size distribution measured with a laser diffraction particle size distribution meter.
Table 1 shows the characteristic values of the raw materials used, and Tables 2 and 3 show the characteristic values of the obtained graphite materials.

(Comparative Example 1)
A graphite material was produced in the same manner as in Examples 1 and 2 except that it was pulverized by a pulverizer having no inner partial classifier. In addition, the secondary raw material powder obtained during the production was not subjected to operations such as precision airflow classification, and the particle size distribution measured by a laser diffraction particle size distribution meter did not include powder of 80 μm or more. However, it contained 9.3% of powder of 1 μm or less.
Table 1 shows the characteristic values of the raw materials used, and Tables 2 and 3 show the characteristic values of the obtained graphite materials.

(Comparative Example 2)
After adding 65 parts by weight of calcined coke pulverized to an average of 14 μm to 35 parts by weight of a coal-based pitch having a softening point of 80 ° C., kneading, adjusting the volatile content while performing heat treatment at 250 ° C. The next raw material was obtained. Next, it was pulverized in a pulverization plant equipped with a pulverizer and a precision airflow classifier so as not to be excessively pulverized to obtain a secondary raw material powder. Next, after pressurizing at a pressure of 100 MPa with an isotropic isostatic press, firing was performed at a rate of temperature increase of about 5 ° C./hour up to 1000 ° C., and graphitization was performed at 2500 ° C.
In addition, the secondary raw material powder obtained in the middle of the production did not contain a powder of 1 μm or less in the particle size distribution measured with a laser diffraction particle size distribution meter, but contained about 3% of a powder of 80 μm or more. It was.
Table 1 shows the characteristic values of the raw materials used, and Tables 2 and 3 show the characteristic values of the obtained graphite materials.

2. Characteristic evaluation The following items were measured, and the characteristics of the graphite material obtained above were evaluated.
(Bulk density, shore hardness, specific resistance)
A φ8 × 60 mm test piece was extracted from the graphite material produced above, and the bulk density, Shore hardness, and specific resistance were measured and calculated by the above methods.

(Number of pores appearing in the cross section, average area, average flatness)
The number of pores appearing in the cross section, the average area, and the average flatness were calculated according to the following procedure.
(A) Sample rough polishing The test piece prepared above was cut into a cylindrical shape having a thickness of about 5 mm, and both surfaces of the sample were subjected to leveling treatment on both sides of the sample using a GATAN jig MODEL 623 and SiC water resistant abrasive paper # 2400. did. Next, it was fixed to a brass sample stage.
(B) CP processing
CP machining was performed at an acceleration voltage of 6 kV using SM09010 manufactured by JEOL.
(C) Milling Ar milling treatment was performed using a flat milling device E-3200 manufactured by Hitachi High-Tech Co., Ltd. at an acceleration voltage of 5 kV, 0.5 mA, a sample inclination angle of 45 °, and a milling time of 3 minutes.
(D) FE-SEM Observation The sample prepared as described above was observed at an acceleration voltage of 2 kV using an ultra-high resolution field emission scanning electron microscope S-4800 manufactured by Hitachi High-Tech.
(E) Image Analysis Using the analysis software ImageJ1.37 manufactured by the National Institutes of Health, the SEM image obtained above was analyzed. The observation magnification at this time was 2000 times, and after performing the noise reduction processing, the binarization processing of the plane portion / void (pore) portion was performed. It should be noted that the analysis target of voids (pores) exceeds 0.2 μm, from which it can be determined whether or not the voids (pores).
Area measurement and optimal ellipse fitting are performed on the void (pore) part obtained by binarization with image analysis software (ImageJ) and the number is counted, and the cross section is obtained from the value obtained by the above processing. The number of appearing pores, average area, and average flatness were calculated.

(Compressive strength)
Measurements were performed according to JIS R7222.

(Elastic modulus)
Measurements were performed according to JIS R7222.

3. Performance evaluation test The graphite material obtained by the Example and the comparative example was processed into a round bar of about φ70 × 100 mm. Processing was performed on a lathe with a depth of cut of 1 mm and a feed rate of 1 mm / rotation. The number of rotations of the lathe was 120 rpm. As the blade, TNGG160408R-A3 manufactured by Kyocera was used.
The cutting scraps thus obtained were collected, passed through a multistage vibrating sieve, and the median diameter (DP-50: 50% integrated diameter) was measured. In the case of a multistage vibrating sieve, the number of sieves that can be used for measurement is finite, so it is difficult to obtain an accurate median diameter value. A median diameter value was obtained by interpolating from the passing amount and the passing amount with respect to the opening of the uppermost sieve that could not pass 50% by weight. The workability of the graphite material was evaluated from the obtained DP-50 value. The smaller the value is, the better the workability is, and it is judged that there is little chipping and chipping. Table 2 shows the processability evaluation results of the examples and comparative examples.

As shown in Table 2, the graphite materials of Examples 1 and 2 included in the scope of the present invention are smaller in cutting waste than Comparative Examples 1 and 2, and therefore can be processed more precisely, and workability is improved. It turns out that it is excellent in.
Moreover, it can be seen from the cross-sectional photographs shown in FIGS. 5a to 5c and FIGS. 6a to 6c that the graphite material according to the present invention has a large number of uniformly distributed round pores having a relatively small size. On the other hand, it can be seen that the graphite materials of the comparative examples shown in FIGS. 7a to 7c and FIGS. 8a to 8c have few round pores and many relatively large ones.

  The present invention is less likely to cause chipping or chipping even when subjected to fine processing, and can be used for electric discharge machining electrodes having fine patterns, fine holes, pins, ribs, etc., jigs for electronic parts, etc. it can.

2 is a graph of particle size distribution of secondary raw material powder used in Example 1. FIG. It is a numerical value of the particle size distribution of the secondary raw material powder used in Example 1. 3 is a graph of particle size distribution of secondary raw material powder used in Example 2. FIG. 3 is a numerical value of particle size distribution of secondary raw material powder used in Example 2. FIG. 4 is a graph of particle size distribution of secondary raw material powder used in Comparative Example 1. It is a numerical value of the particle size distribution of the secondary raw material powder used in Comparative Example 1. 4 is a graph of particle size distribution of secondary raw material powder used in Comparative Example 2. It is a numerical value of the particle size distribution of the secondary raw material powder used in Comparative Example 2. 2 is a cross-sectional SEM photograph of the graphite material produced in Example 1. 2 is a binarized image obtained by subjecting a cross-sectional SEM photograph of the graphite material produced in Example 1 to image processing. 3 is an elliptical fit diagram of a binarized image obtained by image processing of a cross-sectional SEM photograph of the graphite material produced in Example 1. FIG. 3 is a cross-sectional SEM photograph of the graphite material produced in Example 2. 4 is a binarized image obtained by image processing of a cross-sectional SEM photograph of the graphite material produced in Example 2. FIG. 4 is an elliptical fit diagram of a binarized image obtained by image processing of a cross-sectional SEM photograph of the graphite material produced in Example 2. FIG. 3 is a cross-sectional SEM photograph of the graphite material produced in Comparative Example 1. 2 is a binarized image obtained by subjecting a cross-sectional SEM photograph of the graphite material produced in Comparative Example 1 to image processing. 4 is an elliptical fit diagram of a binarized image obtained by image processing of a cross-sectional SEM photograph of the graphite material produced in Comparative Example 1. FIG. 3 is a cross-sectional SEM photograph of the graphite material produced in Comparative Example 2. 4 is a binarized image obtained by subjecting a cross-sectional SEM photograph of the graphite material produced in Comparative Example 2 to image processing. 6 is an elliptical fit diagram of a binarized image obtained by image processing of a cross-sectional SEM photograph of the graphite material produced in Comparative Example 2. FIG.

Claims (3)

A graphite material having a fine structure composed of a large number of graphite particles and pores. When the cross section is observed with a scanning electron microscope, 250 or more pores appearing in the cross section per 6000 μm ² , and the average area of the pores appearing in the cross section graphite material but to 5 [mu] m ² or less, an average aspect ratio of the pores appearing in cross section, characterized in that 0.55 or less. The graphite material according to claim 1, wherein the bulk density is 1.78 to 1.86 g / cm ³ .   The graphite material according to claim 1 or 2, which is used for electric discharge machining.