JPH0672768A - Cubic boron nitride sintered compact for cutting tool - Google Patents

Abstract

PURPOSE:To ensure a long service life, high strength, high toughness and wear resistance by sintering high purity BN by a direct conversion method so that primary grains forming the resulting sintered compact are regulated to a prescribed size. CONSTITUTION:Low pressure phase BN having >=99wt.% purity such as pyrolytic BN is wrapped with Ta foil and put in a reaction chamber made of a high purity material not reacting with cubic BN. The BN in the chamber is then heated and held at 1,900-2,100 deg.C under desired pressure for <=120min to produce the objective cubic boron nitride sintered compact for a cutting tool having <=3.0mum average diameter of the primary grains.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cubic boron nitride sintered body used as a tool material for cutting such as drilling, especially as a tool material for cutting cast iron, cemented carbide, hardened steel and the like. Is.

[0002]

BACKGROUND OF THE INVENTION Cubic boron nitride (cBN), which is a high-pressure phase of boron nitride, has hardness and thermal conductivity second only to diamond, and has the characteristic that diamond does not react with iron-based metals. , Cemented carbides containing a large amount of iron-based metals and iron-based metals such as cobalt are being used as cutting tool materials.

In recent years, cutting work has tended to be highly efficient and unmanned. As a concrete method of high efficiency, heavy cutting,
Although it is high-speed cutting, under such severe cutting conditions, a large load is applied to the tool, especially the cutting edge of the tool, so a tool material with high strength is required. On the other hand, for unmanned use, a long-life tool material that does not wear even after long-term use and has excellent wear resistance that does not require frequent tool replacement is required.

However, a cBN sintered body tool which is capable of simultaneously satisfying the two functions of high strength and excellent wear resistance, particularly brittle but hard to have high hardness, such as cast iron, cemented carbide and hardened steel. Little has been developed that is suitable for cutting work materials. This is for the following reasons. (1) A conventional sintered body composed only of cBN can be obtained by sintering fine particles of cBN. Therefore, the hardness of the sintered body itself is very large, but the cBN particles are difficult to sinter, so that a high-strength material that can be used as a cutting tool material cannot be obtained. (2) In the conventional cBN sintered body for tools, in order to obtain high strength, a metal such as Al or Co, TiN or Ti is added to the cBN particles.
A composite obtained by adding a binder such as a carbide such as C or Al ₂ O ₃ , a nitride or an oxide and sintering the mixture has been used.
However, since such a composite contains a binder having a hardness smaller than that of cBN in the sintered body, the hardness of the sintered body itself becomes small, and the high hardness characteristic of cBN cannot be fully utilized.

[0005]

SUMMARY OF THE INVENTION In view of the above situation, the inventors of the present invention have made it possible to obtain a high degree of hardness even for processing of brittle but high hardness difficult-to-cut materials such as cast iron, cemented carbide, and hardened steel. As a result of various studies aimed at developing a cutting tool material that simultaneously satisfies the properties of strength, high toughness, and excellent wear resistance, the results show that the method of synthesizing the cBN sintered body, the purity and the microstructure are c
The present invention has been completed by discovering that the characteristics of the BN sintered body are deeply related to each other as shown below.

That is, when a direct conversion method is used instead of a sintering method in which a binder is generally used, a polycrystalline sintered body having a high hardness and composed of only cBN can be obtained. Among them, a cBN sintered body having a purity of 99% by weight or more has a particularly high strength, and if proper synthesis conditions are selected, the cBN primary crystal grains constituting the sintered body have a uniform size and a uniform fine structure. Obtained, cBN
The size of the primary crystal particles is closely related to the wear characteristics of the sintered body, and when the average size is 3.0 μm or less,
In particular, it has been found that when a difficult-to-cut material having brittleness but high hardness such as cast iron and cemented carbide is cut, it has excellent wear resistance.

[0007]

That is, the present invention comprises a cubic boron nitride sintered body having a purity of 99% by weight or more obtained by a direct conversion method, and the size of primary crystal grains constituting the sintered body. It is a cubic boron nitride sintered body for a cutting tool, which has an average thickness of 3.0 μm or less.

The present invention will be described in more detail below.

The direct conversion method in the present invention is a method of obtaining a cBN sintered body by direct phase transition between solids without using a catalyst or a binder. An example of this is Japanese Examined Japanese Patent Publication No. 63-394.
It can be obtained by treating pyrolytic boron nitride, which is one of low-pressure phase boron nitrides, at a high temperature and high pressure, which is a stable region of cBN. The low-pressure phase boron nitride referred to here has a structure in which hexagonal mesh planes formed by alternately bonding boron and nitrogen atoms are stacked, and specifically, a hexagonal nitride system. Boron (hBN), turbostratic boron nitride (tBN), and rhombohedral boron nitride (rBN) are used alone or as a mixture.

In the present invention, in order to obtain the above-mentioned cBN sintered body in which the high purity and the size of the primary crystal grains are controlled, the raw material, the reaction chamber for generating high temperature and high pressure, and the holding temperature and time are strictly controlled. This will be described later.

In the present invention, the reason for using the direct conversion method is (1) in the synthetic method using a catalyst or a binder, c
A high-hardness sintered body cannot be obtained because they remain as impurities in the grains of the primary crystal grains of the BN sintered body or in the grain boundaries. (2) In the synthesis method using a catalyst or a binder While the size of the primary crystal particles of the obtained cBN sintered body is only several μm or more, the size of the primary crystal particles can be variously changed by changing the synthesis conditions in the direct conversion method. It depends on what you can do.

In the present invention, c having a purity of 99% by weight or more is used.
The reason for using BN sintered body is that if the purity is lower than that, cB
This is because the strength and hardness of the N sintered body decrease. Therefore, even if the direct conversion method is used, components other than cBN, for example, a part of the low-pressure phase boron nitride as a raw material remains unconverted in the sintered body, or impurities from the reaction chamber are mixed. cB
Care must be taken not to produce a sintered body having N purity of less than 99% by weight.

The remaining amount of low-pressure phase boron nitride, which is one of the impurities, is determined by a conventional powder X-ray quantification method.
Specific diffraction lines of low-pressure phase boron nitride, such as d ₀₀₂ (hB
N and tBN) or d ₀₀₃ (for rBN) and cBN specific diffraction line intensities, eg (111)
It can be measured by measuring the ratio with the diffraction line intensity and comparing it with a calibration curve prepared in advance. For the calibration curve, a sample in which low-pressure phase boron nitride and cBN were mixed at a known weight% was used, and _d002 or d of low-pressure phase boron nitride was used.
It is prepared by measuring the ratio between the ₀₀₃ diffraction line intensity and the (111) diffraction line intensity of cBN.

Further, the amount of impurities such as carbon and metals mixed in from the reaction chamber and the like can be measured by a usual chemical analysis method or fluorescent X-ray.

In the present invention, the size of the primary crystal grains forming the cBN sintered body means the size of the cBN crystal grains forming the sintered body which is a polycrystalline body. The size may be called the primary particle size of the sintered body.

The average size of the primary crystal grains constituting the cBN sintered body can be measured, for example, as follows. However, in any of the measuring methods, a minute portion is analyzed, so it is desirable to measure at a plurality of locations so that the structure of the entire sintered body can be uniformly examined. (1) Using a transmission electron microscope with the sintered body as a thin piece,
A dark field image with a contrast corresponding to the size of the primary crystal particles is obtained by imaging using only specific diffraction lines, and a photograph of the dark field image (including many primary crystal particle images) is image analyzed. To measure. (2) The sintered body is broken, and the structure of the broken portion at the grain boundary is directly observed using a scanning atomic repulsive force microscope, and the obtained photograph is analyzed by image analysis. (3) Measured by etching the surface of the sintered body with hot-melt sodium carbonate, selectively etching grain boundary portions to make the surface uneven, and measuring the size of the unevenness using a surface roughness meter. To do.

In the present invention, the average size of the primary crystal grains constituting the cBN sintered body is defined to be 3.0 μm or less, because the average size of the primary particles exceeds 3.0 μm. This is because the toughness of the steel is reduced, and the wear resistance is significantly reduced when used as a cutting tool. The lower limit of the average primary crystal grain size is not particularly limited, but may be 0.1 μm, for example.

The cBN sintered body used in the present invention can be manufactured, for example, as follows. That is,
Basically, for example, as described in JP-B-63-394, it is obtained by treating pyrolytic boron nitride at a high temperature and high pressure, which is a stable region of cBN, but in the present invention, The raw materials, the reaction chamber that generates high temperature and high pressure, and the holding temperature and time are strictly controlled as follows.

First, it is necessary to use high-purity low-pressure phase boron nitride such as pyrolytic boron nitride as the raw material, and its purity is preferably 99.9% by weight or more. Also,
As the material of the reaction chamber, a high-purity material that does not react with cBN is used so that contamination does not occur in the high temperature / high pressure treatment process. Specifically, 99.9% by weight or more of semiconductor grade high-purity carbon is used as a heater for heating, a sleeve made of a molded body of high-purity NaCl powder is arranged inside the heater, and tantalum (Ta) foil is used. A reaction chamber structure is preferred in which the encapsulated low pressure phase boron nitride raw material is placed. With such a structure, Ta serves as a getter that absorbs impurities, so that diffusion of impurities from the carbon heater and the outside can be stopped by the Ta foil. Further, since NaCl has a low electric conductivity, by arranging it as a sleeve between Ta which is a good electric conductor and carbon, stable heating can be performed without bringing Ta and carbon into contact with each other.

The temperature, pressure and time of holding at high temperature and high pressure have a great influence on the purity of the obtained sintered body and the size of primary crystal grains. In order to obtain the primary crystal particles having an average size of 3.0 μm or less, the temperature is set to 1900 to 2100 at a pressure at which cBN is thermodynamically stable.
Must be ℃. If the temperature is less than 1900 ° C, the low-pressure phase boron nitride raw material is not completely converted to cBN, so that a sintered body having a purity of 99% by weight or more cannot be obtained. The average crystal grain size exceeds 3.0 μm.

The holding time under the above conditions is preferably 120 minutes or less. If it exceeds 120 minutes, the sintering of the once formed primary crystal particles of 3.0 μm or less proceeds, and the grain growth occurs to increase the size of the primary crystal particles.

[0022]

The reason why the cBN sintered body according to the present invention is suitable for cutting cast iron, cemented carbide, hardened steel, etc. by using it as a cutting tool material is considered as follows. .

When machining a metal having a not so high hardness, since the hardness difference between the tool and the work material is large, the cutting edge of the tool easily cuts into the work material, and even in intermittent cutting, the tool itself is not so much. It doesn't take much force. On the other hand, cast iron, cemented carbide, hardened steel, etc. are brittle but have high hardness, so that a large shearing force is shocked to the tip of the tool during processing. However, as in the present invention, the polycrystalline cBN sintered body having a purity of 99% by weight or more obtained by the direct conversion method has a large strength because the cBN particles are firmly bound to each other and no impurities are present at the grain boundaries. Moreover, since the average primary crystal grain size is 3.0 μm or less and the grain size is small and it has a uniform fine structure, crack propagation hardly occurs and the toughness is high. For these reasons, even long-hardness work materials can be machined with a long life.

Further, when the cBN sintered body of the present invention is used as a cutting tool, it is the reason that the wear rate of the cutting edge is small and the life is long, but when processing cast iron, cemented carbide, hardened steel, etc. It is considered that the abrasion of the tip of the cutting edge that occurs occurs due to the lack of primary crystal grains due to the impactive shearing force applied to the tip. However, since the cBN sintered body of the present invention has a high purity and almost no impurities are present in the grain boundaries, the bonding strength between primary crystal grains is large and the lack of grains is less likely to occur.
Even if chipping occurs, the average size of the primary crystal grains forming the sintered body is as small as 3.0 μm or less, so the amount of chipping at one time is small, the wear rate is small, and the life is long.

[0025]

EXAMPLES Next, the present invention will be described more specifically by way of Examples and Comparative Examples. Examples 1 to 4 and Comparative Examples 1 to 6 Commercially available pyrolytic boron nitride was used as a raw material, carbon having a purity of 99.9% by weight was used as a heater for heating, and the purity was 99.9% by weight.
The sleeve made of the above-mentioned molded body of NaCl powder is placed inside the heater, and the raw material is further wrapped with tantalum foil, and the flat belt type ultrahigh pressure and high temperature generator is filled with 1870.
The cBN sintered body was synthesized by a direct conversion method by treating at various temperatures of ˜2200 ° C. and a pressure of 7.7 GPa for 100 minutes.

The obtained sintered body was subjected to a step scanning speed of Cu-Kα2θ of 0.01 degree / minute using a powder X-ray diffractometer (manufactured by Rigaku Denki Co., Ltd.), and the low-pressure phase existing in the sintered body. The weight percentages of boron nitride and cBN were measured.
The measurement was performed by comparing the ratio of the intensity of the (002) diffraction line of low-pressure phase boron nitride and the intensity of the (111) diffraction line of cBN with a calibration curve prepared in advance. Further, a part of the sintered body was melted with sodium carbonate and chemically analyzed to measure the amount of metal impurities contained in the sintered body. The cBN purity of the sintered body was determined by combining these analytical values. The results are shown in Table 1.

On the other hand, a part of the sintered body is used as a thin piece and an image is formed using only a part of the (111) diffraction line by using a transmission electron microscope, and a contrast corresponding to the size of the primary crystal grain is obtained. A dark field image was obtained. The size of the primary crystal grains constituting the sintered body was obtained by analyzing the obtained photograph of the dark field image (including many primary crystal grain images) with an image analyzer (trade name "LA555" manufactured by Pierce). Was measured. In addition, the measurement was performed by arbitrarily selecting 10 fields of view so that the entire sintered body could be captured evenly. The results are shown in Table 1. In addition, cB obtained in Example 1
An electron micrograph of a dark field image (magnification: 8000 times) by a transmission electron microscope showing the particle structure of the primary crystal particles of the N sintered body is shown in FIG.

Next, from the cBN sintered body obtained above,
A chip blank for a tool was cut out by a grinding process using a diamond grindstone. This chip blank is mechanically clamped on the base of the cutting tool to make it a tool for cutting test,
Under the following conditions, a cutting test was performed using cast iron and cemented carbide as work materials. After the cutting test, the chipped state of the tool tip and the flank wear width were measured. The results are shown in Table 1.

In the case of cast iron Work material: FC25 Chip shape: TNGN332 Cutting oil: Yushiroken HDE30 Machine used: Hitachi NK25S-1100 Cutting speed: V = 700 m / min Feed: f = 0.1 mm / rev Depth of cut: d = 0. 1mm cutting time: 60min

In the case of cemented carbide Work material: WC-16% Co alloy (HRA84.5) Tip shape: TNGN332 Cutting oil: Yushiroken HDE80 (30 times dilution) Machine used: Hitachi NK25S-1100 Cutting speed: V = 20m / Min Feed: f = 0.2mm / rev Depth of cut: d = 0.5mm Cutting time: 20min

Comparative Example 7 A cutting test was conducted under the same conditions as in Example 1 using a commercially available cBN polycrystal sintered body containing a ceramics binder, and the chipped state of the tool tip and flank wear width after the test were carried out. Was measured. The results are shown in Table 1.

Comparative Example 8 Using a commercially available cBN polycrystalline sintered body containing a metal binder,
A cutting test was performed under the same conditions as in Example 3, and the chipped state of the tool tip and the flank wear width after the test were measured. The results are shown in Table 1.

[0033]

[Table 1]

[0034]

EFFECTS OF THE INVENTION The cBN sintered body of the present invention has sufficient strength to be suitable for cutting a work material having high hardness such as cast iron, cemented carbide, and hardened steel, and has a conventional strength. Compared with the cutting tool material, it can realize cutting processing with a much longer life.

[Brief description of drawings]

FIG. 1 is an electron micrograph of a dark field image (magnification: 8000 times) obtained by a transmission electron microscope showing the particle structure of primary crystal grains of the cBN sintered body obtained in Example 1.

Claims (1)

[Claims] 1. A cubic boron nitride sintered body having a purity of 99% by weight or more obtained by a direct conversion method, and an average size of primary crystal grains constituting the sintered body is 3.0 μm.
A cubic boron nitride sintered body for a cutting tool, characterized in that