JPH0931445A - Grinding grain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a grinding grain composed of a grinding grain of boron nitride of multicrystal cubic having a specific oxygen content and a grinding grain of boron nitride of monocrystal cubic, having a stable and small grinding resistance, excellent in abrasion resistance and useful for a metal-bond grind stone, an electro plated grind stone, etc. SOLUTION: This grinding grain is composed of (A) a boron nitride grinding grain of multicrystal cubic whose oxygen content is <=0.10mol% and (B) a boron nitride grinding grain of monocrystal cubic. For instance, the component B is contained in an amount of 20-90wt.% based on the total of the grinding grains. The component A is obtained e.g. by treating a thermally decomposed boron nitride as a raw material at high temperature under high pressure in a stable region of the cubic: boron nitride under the control of oxygen content in the raw material and in a reaction section.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to abrasive grains for a grinding wheel. Examples of grinding wheels to which the abrasive grains of the present invention are applied include heavy grinding for iron-based metal processing, metal bond grinding wheels for high-speed grinding, electrodeposition grinding wheels, and vitrified grinding wheels.

[0002]

2. Description of the Prior Art Cubic boron nitride (cBN), which is a high-pressure phase of boron nitride, has hardness and thermal conductivity second only to diamond, and has characteristics that diamond does not have if it does not react with iron-based metals. Utilization of iron-based metals as abrasive grains for grinding is being promoted.

[0003] In recent years, grinding has been directed toward labor saving and unmanned operation. Specific examples thereof are heavy grinding and high speed grinding, but under such severe grinding conditions, if the sharpness of the grindstone is poor, the grinding resistance increases and a heavy load is applied to the machine tool.
For this reason, machine tools have been made to have higher rigidity and higher performance, but this is not preferable from the viewpoint of labor saving, and a grindstone with good sharpness, stable and small grinding resistance, and excellent wear resistance is desired. There is.

Generally, the grinding resistance of a cBN grindstone is "cBN
It is known that it is extremely high in the early stages of use of the grinding wheel, as also introduced in "Wheel grinding technology" (published by the Industrial Research Board in 1988).

The cBN abrasive grains used for the cBN grindstone are roughly classified into two types, a polycrystalline type and a single crystal type. Since the polycrystalline cBN abrasive grains have a polycrystalline structure in which fine cBN crystal grains are firmly bonded to each other, a large breakage such as cleavage occurs like single crystal type cBN abrasive grains in which one particle is composed of a single crystal. It shows high strength without causing for that reason,
Shows excellent wear resistance when used as a grindstone. Polycrystalline cBN abrasive grains are disclosed in JP-B-63-44417 and JP-A-6-
As described in 240235, it is produced by crushing a cBN sintered body synthesized by a catalyst-free direct conversion method without using a catalyst into a desired particle size. However, even if such a polycrystalline cBN abrasive grain is actually used as a grindstone under severe conditions such as heavy grinding and high speed grinding, the abrasion resistance is excellent, but the initial grinding resistance after dressing is remarkable. There was the problem of becoming expensive.

On the other hand, the single crystal type cBN abrasive grain is basically inferior in wear resistance because one particle is composed of a single crystal, but it causes a large breakage such as cleavage, so that a sharp cleavage surface has a cutting edge. It is characterized by being easy to dry and excellent in sharpness. The single crystal cBN abrasive grains are produced from a cBN sintered body synthesized by using a catalyst as described in Japanese Patent Publication No. 38-14. However, such single-crystal cBN
Even if the abrasive grains are actually used as a grindstone under severe conditions such as heavy grinding and high-speed grinding, the initial grinding resistance after dressing is considerably high, and the abrasive grain strength is small and spills may occur, which may cause grinding. Along with the progress, the grinding resistance, which had once decreased, rises again, and there has been a problem that frequent redressing has to be performed.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a stable and small grinding resistance under heavy use conditions such as heavy grinding and high speed grinding, and particularly to reduce initial grinding resistance and wear resistance. An object is to provide abrasive grains capable of producing a large grinding wheel.

As a result of various investigations by the present inventors to develop an abrasive grain having a stable grinding resistance and a small abrasion resistance, a polycrystalline cBN abrasive grain in which the oxygen content is controlled and a single crystal type are obtained. The present invention has been completed by finding that a grindstone using abrasive grains containing cBN abrasive grains has a stable grinding resistance and a small abrasion resistance and is excellent in wear resistance.

That is, with a grindstone containing various polycrystalline type cBN abrasive grains having different oxygen contents and single crystal type cBN abrasive grains in various ratios, heavy grinding was actually carried out, and protrusions were made on the surface of the grindstone before and after grinding. As a result of observing the state of each of the present abrasive grains, (1) that the polycrystalline cBN abrasive grains having an oxygen content of 0.10 mol% or less have not been worn or destroyed (2)
The single crystal cBN abrasive grains surrounded by the polycrystalline cBN abrasive grains having an oxygen content of 0.10 mol% or less do not cause abrasion or large destruction as compared with the case where they are used alone, and have a sharp cutting edge. Was found to be maintained. Further, as will be described in detail in Examples, the initial grinding resistance is significantly smaller than that of a grindstone in which a polycrystalline cBN abrasive grain having an oxygen content of 0.10 mol% or less or a single crystal type cBN abrasive grain is used alone. It has been found that it is stable and has wear resistance comparable to the case where a polycrystalline cBN abrasive grain having an oxygen content of 0.10 mol% or less is used alone.

[0010]

That is, the present invention is an abrasive grain characterized by containing polycrystalline cBN abrasive grains having an oxygen content of 0.10 mol% or less and single crystal type cBN abrasive grains. .

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

In the present invention, the oxygen content of the polycrystalline cBN abrasive grains is made into carbon dioxide by, for example, melting and decomposing the polycrystalline cBN abrasive grains in an inert gas and reacting the generated oxygen with carbon. , Which can be analyzed using an impulse melting heat conduction method in which it is analyzed and quantified by gas chromatography. The presence of oxygen in the polycrystalline cBN abrasive grains can be determined, for example, by using both component analysis and structural analysis using a transmission electron microscope equipped with a micro-part analysis device.

In the present invention, the oxygen content of the polycrystalline cBN abrasive grains is limited to 0.10 mol% or less.
This is because the polycrystalline cBN abrasive grains having an oxygen content of more than 0 mol% have a low toughness, and the abrasion resistance is significantly reduced when the abrasive grains are mixed with the single crystal cBN abrasive grains.

The polycrystalline cBN abrasive grains having an oxygen content of 0.10 mol% or less used in the present invention can be obtained, for example, as follows. That is, polycrystalline cB
As described above, the N sintered body can be produced by crushing the cBN sintered body synthesized by the catalyst-free direct conversion method without using a catalyst into a desired particle size. A method for synthesizing the direct conversion cBN sintered body is widely known. For example, as described in Japanese Patent Laid-Open No. 6-240235, pyrolytic boron nitride is used at a high temperature / high temperature which is a stable region of cBN.
It is to synthesize by treating under high pressure. To obtain a polycrystalline cBN abrasive grain having a desired oxygen content,
For example, as described in JP-A-6-240235, controlling the raw materials and reaction chamber.

The single crystal type cBN abrasive grain used in the present invention is basically composed of one single crystal, but it does not mean only a complete single crystal but a twin crystal. It also includes particles in which large primary crystal particles of several tens of μm unit are combined.

Since the single crystal type cBN abrasive grains used in the present invention are widely and generally commercially available, they can be obtained on the market or as described in Japanese Patent Publication No. 38-14. In addition, a catalyst was added to hexagonal boron nitride to produce cB.
It can also be obtained by treating under a high temperature / high pressure which is a stable region of N 2.

The abrasive grains of the present invention have an oxygen content of 0.
Polycrystalline cBN abrasive grains of 10 mol% or less and single crystalline cBN
It can be produced by mixing with abrasive grains in a conventional manner. Upon mixing, it is preferable to use a mixer or a homogenizer because a uniform mixture can be obtained. Also, a third component such as diamond, alumina, or silicon carbide can be added.

As a mixing ratio of the polycrystalline cBN abrasive grains and the single crystal type cBN abrasive grains, the ratio of the single crystal type cBN abrasive grains to the whole abrasive grains is preferably 20 to 90% by weight. If the ratio of the single crystal type cBN abrasive grains is less than 20% by weight, the effect of improving the grinding resistance becomes small, and if it exceeds 90% by weight, the effect of improving the wear resistance becomes small.

[0019]

The reason why the grindstone using the abrasive grains of the present invention has a stable and small grinding resistance and is excellent in wear resistance is considered as follows.

First, the reason why the grinding resistance is stable and small is considered as follows. It is known that the grinding resistance is greatly affected by the sharpness of the tip of the abrasive grain on the surface of the grindstone that acts on the grinding, as described in, for example, "Grinding and Abrasive Machining" (published by Kyoritsu Shuppan, 1984). There is. That is, the sharper the tips of the abrasive grains present on the surface of the grindstone, the smaller the grinding resistance tends to be. The single crystal cBN abrasive grains tend to be sharp at the tip of the abrasive grains due to cleavage breakage, but since the strength is poor, large breakage occurs during grinding and the number of abrasive grains acting on grinding decreases. On the other hand, since the polycrystalline cBN abrasive grains have a large strength, the decrease in the number of working abrasive grains is small, but a large breakage is less likely to occur, and the abrasive grains are abraded, so that the shape is not sharp. When both abrasive grains coexist on the surface of the grindstone, the polycrystalline cBN abrasive grains with high strength protect the single crystal type cBN abrasive grains. Therefore, the cutting of the single crystal type cBN abrasive grains having a sharp cutting edge is performed. The decrease in the number of blades becomes smaller. At the same time, a larger load is applied to the tips of the polycrystalline cBN abrasive grains than in the case where only the polycrystalline cBN abrasive grains are used, so that moderate fracture occurs, which is unlikely to occur with ordinary polycrystalline cBN abrasive grains. It seems that the formation of sharp cutting edges occurs. As described above, when both abrasive grains coexist on the surface of the grindstone, it is considered that due to the synergistic effect of both abrasive grains, a large number of sharp abrasive grain cutting edges having excellent sharpness are generated and the grinding resistance is stably reduced.

Next, the reason for showing excellent wear resistance comparable to the case where the polycrystalline cBN abrasive grains are used alone is considered as follows. On the surface of a grindstone using the abrasive grains of the present invention, polycrystalline cBN abrasive grains having high strength and high wear resistance and single crystal type cBN abrasive grains having low strength are mixed, and a large number of abrasive grains It exists with a part protruding. Before the grinding, the protrusion heights of the both are almost the same, but during grinding, a shocking force acts on the tips of these abrasive grains due to the collision with the work material. When grinding is started, the same impact force acts on both at the initial stage. However,
The single crystal type cBN abrasive grains are prone to cleavage fracture and the tip portion is broken to lower the protrusion height, whereas the polycrystalline cBN abrasive grains have high strength and therefore the protrusion height is maintained to some extent. There is. Since the polycrystalline cBN abrasive grains protruding further than the single crystal cBN abrasive grains are present, the impact force acting on the single crystal cBN abrasive grains becomes small, and the tip portion of the single crystal cBN abrasive grains becomes smaller. Destruction does not proceed any further. Further, for the polycrystalline cBN abrasive grains, the load is reduced because the single crystal type cBN abrasive grains with excellent sharpness are present around the periphery while maintaining a certain protrusion height, and the synergistic effect of both abrasive grains is achieved. It is believed that this improves the wear resistance of the grindstone.

[0022]

EXAMPLES Next, the present invention will be described more specifically by way of examples.

Examples 1 to 8 Comparative Examples 1 to 2 Using the method described in Japanese Patent Laid-Open No. 6-240235, polycrystalline cBN abrasive grains having various oxygen contents were manufactured as follows.

That is, in order to deoxidize the raw material, a pyrolytic boron nitride having a purity of 99.9% or more is used.
After encapsulating in 9.9% or more high-purity carbon, it was heat-treated in a nitrogen atmosphere at a temperature of 800 ° C. for various times of 30 to 90 minutes. The obtained pyrolytic boron nitride was enclosed in carbon, placed in a reaction cell having a structure using the above-mentioned high-purity carbon as a heater for heating, and loaded in a flat-belt type ultra-high pressure and high temperature generator, at a temperature of 2100 ° C. and pressure. Polycrystalline cBN sintered bodies with various oxygen contents were synthesized by a direct conversion method of treating at 7.3 GPa for 60 minutes. This was crushed by a roll crusher and then classified to select 100/120 mesh abrasive grains.

From this abrasive grain, 1.2 g was sampled by the method of JIS R6003, put in an impulse furnace and heated, and oxygen generated by melting and decomposition was converted to carbon dioxide by carbon oxidation, which was quantified by gas chromatography. By doing so, the mol% of oxygen in the abrasive grains was calculated. Table 1 shows the results.

Next, as a single-crystal type cBN abrasive grain, a commercially available product ["Borazon TYPE manufactured by General Electric Co., Ltd.
I ”(particle size 100/120 mesh)] was obtained and mixed with the above-mentioned polycrystalline cBN abrasive grains at a desired mixing ratio (weight ratio) using a rocking mixer (manufactured by Aichi Denki Shoji Co., Ltd.).

100 carats was extracted from the mixed abrasive grains to prepare a metal bond grindstone having a diameter of 200 mm, a thickness of 10 mm and a concentration of 100. A grinding test was performed using this grindstone by a plane plunge cut method, and a grindstone axial current value and a grindstone wear amount in each grinding volume were measured as grinding resistance.
Table 1 shows the results. The work material used in the test was high speed tool steel SKH-51 (Rockwell hardness: 65).
And the grinding condition is a grinding wheel peripheral speed of 1800 m / mi
n, work material feed rate 9 m / min, grindstone cutting amount 15 μ
m.

Comparative Example 3 Polycrystalline cBN abrasive grains were manufactured and evaluated in the same manner as in Example 1 except that a pyrolytic boron nitride raw material that was not deoxidized was used. Table 1 shows the results.

[0029]

[Table 1] (Note) Initial value of grinding resistance: 10 mm ³ / mm Value after grinding Medium value of grinding resistance: 7,000 mm ³ / mm Value after grinding Late value of grinding resistance: 14,000 mm ³ / mm Value after grinding

[0030]

According to the abrasive grains of the present invention, it is possible to manufacture a high-performance grinding wheel having a stable grinding resistance and a small wear resistance.

Claims (1)

[Claims] 1. An abrasive grain comprising a polycrystalline cubic boron nitride abrasive grain having an oxygen content of 0.10 mol% or less and a single crystal cubic boron nitride abrasive grain.