JPH0931443A - 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 ratio of constituting elements 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 having a molar ratio of boron to nitrogen (B/N molar ratio) of 0.95-1.00 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, whose B/N ratio is controlled, at high temperature under high pressure in a stable region of the cubic boron nitride.

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 240236, it is produced by crushing a cBN sintered body synthesized by a non-catalytic 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 severe operating conditions such as heavy grinding and high speed grinding, and especially to reduce initial grinding resistance and wear resistance. An object of the present invention is to provide an abrasive grain capable of producing a large grinding wheel.

The inventors of the present invention have conducted various studies to develop abrasive grains having stable and low grinding resistance and high wear resistance, and as a result, the molar ratio of boron to nitrogen (hereinafter referred to as B / N molar ratio) has been found. Controlled Polycrystalline cBN Abrasive Grains and Single Crystal c
The present invention has been completed by finding that a grindstone using abrasive grains containing BN abrasive grains has stable grinding resistance and is excellent in wear resistance.

That is, with a grindstone containing various polycrystalline type cBN abrasive grains having different B / N molar ratios and single crystal type cBN abrasive grains at various ratios, heavy grinding is actually performed, and the surface of the grindstone is ground before and after grinding. As a result of observing the state of each protruding abrasive grain, (1) the polycrystalline cBN abrasive grains having a B / N molar ratio of 0.95 or more and less than 1.00 were not worn or destroyed. , (2) B / N molar ratio is 0.95 or more and 1.00
Single-crystal cBN surrounded by polycrystalline abrasive grains that is less than
It was found that the abrasive grains did not suffer wear or major destruction compared with the case where it was used alone, and the sharp edge was maintained. Further, as described in detail in Examples, compared with a grindstone using a single-crystal polycrystalline cBN abrasive grain or a single-crystal cBN abrasive grain having a B / N molar ratio of 0.95 or more and less than 1.00 Has extremely small grinding resistance and is stable, and wear resistance is B /
Polycrystalline type c with N molar ratio of 0.95 or more and less than 1.00
This was found to be comparable to the case where BN abrasive grains were used alone.

[0010]

That is, the present invention is based on B
/ N molar ratio is 0.95 or more and less than 1.00, and it is an abrasive grain characterized by including a polycrystal type cBN abrasive grain and a single crystal type cBN abrasive grain.

[0011]

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

In the present invention, the nitrogen content of the polycrystalline cBN abrasive grains can be quantified by melting and decomposing it in an impulse furnace in an inert gas atmosphere, and analyzing the generated nitrogen gas by gas analysis. The boron content can be quantified by mixing polycrystalline cBN abrasive grains with an alkali, heating and melting it, dissolving it in hydrochloric acid, and subjecting a sample extracted by the methanol distillation method to alkali neutralization titration.

In the present invention, B of the polycrystalline cBN abrasive grain is used.
The reason why the / N molar ratio is limited to 0.95 or more and less than 1.00 is that the polycrystalline cBN abrasive grains having other B / N molar ratios have low toughness. This is because when used as a grindstone, the wear resistance is significantly reduced.

The B / N molar ratio used in the present invention is 0.95.
As a method of obtaining the polycrystalline cBN abrasive grains having a size of not less than 1.00, for example, there are the following methods. That is, as described above, the polycrystalline cBN abrasive grains are obtained by crushing a cBN sintered body synthesized by a non-catalyst direct conversion method without using a catalyst into a desired particle size.
As for the method of synthesizing the sintered body, as described in, for example, Japanese Patent Laid-Open No. 6-240236, pyrolytic boron nitride is added to cB.
That is, the treatment is performed under high temperature / high pressure, which is a stable region of N. In order to obtain polycrystalline cBN abrasive grains having a desired B / N molar ratio, the B / N molar ratio of the raw material may be controlled as described in JP-A-6-240236.

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 are prepared by mixing the above-mentioned polycrystalline cBN abrasive grains having a B / N molar ratio of 0.95 or more and less than 1.00 and single crystal cBN abrasive grains by a general method. It can be manufactured. 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 grinding, the protrusion heights of both are almost the same. During grinding, a shocking force acts on the tips of these abrasive grains due to collision with the work material. When grinding is started, the same impact force acts on both at the initial stage. However,
Among them, the single crystal type cBN abrasive grains are prone to cleavage fracture, so that the tip portion is destroyed and the protrusion height becomes low. On the other hand, since the polycrystalline cBN abrasive grains have high strength, the protrusion height is maintained to some extent. Since the polycrystalline cBN abrasive particles protruding from the single crystal type cBN abrasive particles are present around the single crystal type cBN abrasive particles, the impact force acting on the single crystal type cBN abrasive particles is reduced, and the tip portion of the single crystal type cBN abrasive particles is reduced. The destruction of will 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 4 Using the method described in JP-A-6-240236, polycrystalline cBN abrasive grains having various B / N molar ratios were produced as follows.

That is, various commercially available pyrolytic boron nitrides having a purity of 99.0% or more and a B / N molar ratio of 0.94 to 1.01 were left in a nitrogen atmosphere at a temperature of 200 ° C. for 60 minutes, and then the nitrogen atmosphere The lower part was sealed in a molybdenum foil bag and used as a raw material.

Using 99.9% or more of high-purity carbon as a heater for heating, a sleeve made of a molded body of high-purity NaCl powder was placed inside the heater to assemble a reaction cell. This reaction cell was loaded into a flat belt type ultrahigh pressure and high temperature generator, and the temperature was 2100 ° C. and the pressure was 7.3 GPa.
Various B / N ratios can be obtained by the direct conversion method of treating for 60 minutes.
A polycrystalline cBN sintered body having a molar ratio was synthesized. This is crushed with a roll crusher and then classified to 80/1
The 00 mesh abrasive grains were selected.

From this abrasive grain, 1.2 g was sampled by the method of JIS R6003, mixed with an alkali and heated and melted. After dissolving the melt solidified at room temperature with hydrochloric acid, a sample was extracted using a methanol distillation method, and the extract was alkali-titrated to perform a gravimetric analysis of boron in the abrasive grains. On the other hand, 1.2 g of separately collected abrasive grains was melted and decomposed in an impulse furnace in an inert gas atmosphere, and the generated nitrogen gas was analyzed using a gas chromatograph to analyze nitrogen in the abrasive grains. From the above results, the B / N molar ratio of the polycrystalline cBN abrasive grains was calculated. Table 1 shows the results.

Next, a commercially available product as a single crystal type cBN abrasive grain ["Borazon TYPE manufactured by General Electric Co., Ltd.
I "(particle size 80/100 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.).

150 carats were 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 150. 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 10 μ
m.

[0029]

[Table 1] (Note) Initial value of grinding resistance: 10 mm ³ / mm Value after grinding Medium value of grinding resistance: 9000 mm ³ / mm Value after grinding Late value of grinding resistance: 18,000 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 abrasion resistance.

Claims (1)

[Claims] 1. The molar ratio of boron to nitrogen is 0.95.
An abrasive grain comprising polycrystalline cubic boron nitride abrasive grains having a grain size of 1.00 or more and less than 1.00 and single crystal cubic boron nitride abrasive grains.