JPS6310119B2 - - Google Patents

Description

[Detailed description of the invention] Cubic boron nitride (CBN) is second in hardness to diamond, has high thermal conductivity, and has excellent chemical stability at high temperatures, so it is attracting attention as a wear-resistant material for metal processing. It is used as grinding abrasive grains and cutting tools.

The inventors also paid attention to this CBN, and as a result of prototyping and researching various CBN sintered bodies, they invented a sintered body for tools that can maximize the excellent characteristics of CBN, and filed a patent application. (Japanese Patent Application Laid-Open No. 53-77811) This invention combines CBN crystals by forming a continuous phase of carbides, nitrides, borides, silicides, or mutual solid solution compounds of transition metals of groups 4a, 5a, and 6a of the periodic table. It has excellent heat resistance and abrasion resistance, and maintains high thermal conductivity even at high temperatures.
The present invention provides a sintered body for use in high-hardness tools that has particularly excellent thermal shock properties.

The inventors further conducted various cutting tests using these sintered bodies, and found that CBN is extremely effective even in low-speed cutting, where high-speed steel is used, due to its excellent welding resistance. discovered that it is a substance that

Therefore, CBN sintering using titanium nitride, a type of the invented sintered body, as a binder is used for so-called automatic lathe work of small precision parts that use high-speed steel or ultrafine-grained cemented carbide made of fine tungsten carbide. When we tested the structure, we found that it had excellent welding resistance, resulting in a beautiful glossy finished surface, but on the other hand, the dimensional accuracy was not very good, and there were some areas where the R shape should not be formed at the joints between the surfaces. Because it was said that the tool had an R shape, it did not show particularly superior performance compared to normally used tools.

As a result of conducting various investigations to clarify the cause of this, it was discovered that there was a problem with sharpness (obtaining a sharp cutting edge), which is a particularly necessary characteristic for tools used in such applications. Ta. That is, Figure 1,
Figure 2 shows ultrafine cemented carbide and the above test.
The results of an enlarged observation of the ground cutting edge of a CBN sintered body are shown, and it is clear that the test sintered body has a lot of edge loss and is different from ultrafine-grained cemented carbide. Therefore, it was thought that the problems such as the above-mentioned dimensional accuracy were caused by this edge sharpness.

Therefore, we thought that these problems could be overcome by making the CBN particles in the sintered body extremely fine, so we tried making a prototype of such a sintered body. Now, as a method for manufacturing CBN sintered bodies, for example, there is a method as shown in Japanese Patent Publication No. 52-43846.
CBN powder and Al, Co, Ni, which become catalysts for CBN synthesis,
There is a method of mixing Fe etc. in powder form, or placing these metal plates in contact with CBN powder and hot pressing under high temperature and high pressure conditions where CBN is stable.

However, in this method, sintering is performed in the presence of a liquid phase of these metals, so if the raw CBN powder is less than 1 μm in size, the surface energy is large, and grain growth of CBN particles occurs during sintering. The desired fine-grained sintered body cannot be obtained, and these metal phases in the sintered body have a high affinity with the workpiece, which inhibits the excellent adhesion resistance of CBN. So I don't like it.

As another method for producing a CBN sintered body, as mentioned above, the CBN sintered body is prepared by mixing a powder made of a ceramic material and a CBN powder, which was applied by the inventors.
There is a method of hot pressing at high temperature and high pressure, which is stable.

According to this method, grain growth of CBN particles in the sintered body does not occur, and a uniform and fine sintered body can be obtained, and the ceramic used as a binder generally has a good affinity with the work material. Poor, excellent welding resistance
It is thought that this will be a sintered body that takes advantage of the characteristics of CBN. Therefore, CBN with a particle size of 1μ or less is actually used as a raw material.
When particles and various ceramic powders were mixed and sintered at high temperature and pressure, most of the particles were smaller than 1μ.
We were able to obtain a dense sintered body with a fine microstructure in which CBN particles were uniformly dispersed. Using the various ceramic-bonded sintered bodies obtained, we again conducted tests using the automatic lathe process described above, and found that carbide containing WC as a main component or cemented carbide containing trace amounts of iron group metals. It was discovered that it showed the best performance when used as a binder. as a binding material
The reason why the one using WC showed the best performance is thought to be as follows.

In general, in automatic lathe work for small parts, the workpiece is a few mm to 30mm in diameter, and there are mechanical restrictions on the rotation speed of the workpiece, so the cutting speed cannot be freely selected, and many In some cases, it is necessary to cut at a low speed of less than 10 m/min. For this reason, the temperature of the cutting edge of the tool during cutting is low, and therefore tool wear is mainly due to mechanical scraping action, with very little wear due to chemical reactions. Furthermore, due to mechanical constraints, the amount of tool feed per revolution of the workpiece is also small, typically 0.1 to 0.5 when cutting with a carbide tool.
While the feed rate is generally mm/rev,
For automatic lathes, 0.01mm/rev or less is normal.
It is well known that even in the case of such minute feed, mechanical scraping wear is the main cause. For these reasons, it can be said that materials with high hardness, excellent strength, and toughness, rather than chemical stability, are suitable for tool materials used in applications such as automatic lathe work. WC has the highest hardness among various ceramic materials, and is well known as a material with excellent strength and toughness, and this is probably why products using WC as a binder showed excellent performance. .

In addition, the thermal expansion coefficient of WC, which is an important factor in making a composite sintered body with CBN, is almost close to that of CBN, and it does not leave any undesirable internal residual stress in the sintered body. However, it is suitable as a binder material.

In addition to WC, there is C (MoW), which has the same crystal structure as WC and is obtained by replacing part or most of W with Mo. One of the inventors, along with another researcher, conducted detailed research on the properties of cemented carbide using this compound, and found that, for example, the hardness of carbides expressed by (Mo ₇ W ₃ )C and (Mo ₅ W ₅ )C It was confirmed that properties such as rigidity, abrasion resistance, thermal conductivity, and thermal expansion coefficient are almost similar to WC. In the following explanation, only WC will be described, but in the present invention, this (MoW)C carbide can be used in the same way as WC.

The sintered body for tools of the present invention contains CBN particles of 1μ or less.
It is a sintered body with a uniform structure bonded by carbides mainly composed of WC of 1μ or less. The hard component, CBN particles, are extremely fine and uniformly dispersed, so when ground into a cutting tool, the cutting edge is extremely sharp and has no irregularities. In this way, in order to obtain good edge sharpness by grinding, the WC in the binder must also be 1μ or less. Furthermore, it is most preferable that the particle size of CBN and WC in the binder is 0.5μ or less. The fine structure not only improves the cutting edge but also has the advantage of improving toughness and strength, which are important for tools.

In addition, at cutting speeds of several tens of meters/min, which are common in automatic lathe work, a built-up cutting edge may develop on the cutting edge during cutting, the workpiece surface may be peeled off due to the accumulation of deposits, and the dimensional accuracy of the workpiece may deteriorate. may become worse,
CBN has a low affinity with the workpiece material and has excellent welding resistance, so it can achieve good dimensional accuracy and a beautiful finished surface. Furthermore, since WC, which has high strength and toughness, is used as the bonding material, it is resistant to mechanical scraping and wear, and is a strong sintered body with little breakage even when the cutting edge angle is sharp.

The CBN content in the sintered body of the present invention is 80~80 in terms of capacity.
It is 20% and can be changed depending on the application. In particular, for tools for interrupted cutting that require toughness and may sacrifice wear resistance to some extent, a tool with a large amount of bonding material is selected. When the CBN content is less than 20%, there seems to be little advantage in terms of cost and life of tools manufactured using ultra-high pressure equipment, such as the sintered body of the present invention.

Now, why does the sintered body having the composition of the present invention have a uniform structure consisting of ultrafine CBN particles of 1μ or less?
Furthermore, whether a sintered body with excellent tool performance could be obtained is estimated as follows.

CBN particles have extremely high hardness and are difficult to deform. Therefore, even if CBN particles are compressed under ultra-high pressure, voids remain between the particles. The finer the CBN particles, the higher the porosity, but in the sintered body of the present invention, as mentioned above, the CBN volume content is 80 to 20%.
There are always fine grains of WC between CBN particles, and by sintering these under ultra-high pressure, the WC deforms and a completely dense and uniform sintered body can be obtained. Furthermore, under ultra-high pressure, the diffusion rate of substances is lower than that under normal pressure, and in the sintered body of the present invention, CBN-CBN or WC-WC
It is thought that because there is little contact between particles, the grain growth of CBN and WC is suppressed, making it possible to obtain a fine-grained sintered body of 1 μm or less.

Furthermore, in the sintered body, CBN−WC−Co
A reaction occurs at the interface of CBN-WC and W boride is formed.
CBN has good adhesion at the interface, and CBN firmly adheres to WC, which is the binder phase, reducing the chance of CBN particles falling off during cutting. Therefore, the tool performance is very excellent.

However, if the W boride phase becomes too thick, these borides are brittle and cracks will occur in the boride, which will break at these points and CBN particles will easily fall off, resulting in poor tool performance. In this respect, if ultra-high pressure sintering technology is used, the diffusion rate will decrease as mentioned above, and the formation of W boride will be slowed down.
A sintered body with an optimum boride thickness can be easily obtained.

In carrying out the present invention, when an alloy is composed of such sub-micron CBN crystals and WC crystals, they are mixed in a wet ball mill using a cemented carbide ball and a pot lined with cemented carbide. is convenient. Alternatively, an attritor or vibrating mill may be used, which has almost the same function as a wet ball mill.

Since CBN is hard, a considerable amount of abrasion powder gets mixed in from the balls and lining. It is convenient to use this as it is as a binder component. In particular, it is more convenient if the ball and the inner lining have the same composition as the cemented carbide used to form the bonding material.

Although WC does not have to be the main component, it is most preferable to use WC from the standpoint of utilizing its excellent properties such as toughness and high thermal conductivity.

TiC as other carbide to replace part of WC,
ZrC, HfC, TaC, NbC, etc. can be used.

As the raw material CBN powder for the sintered body of the present invention, the particle size is
A material of 1 μm or more may be used by crushing it using a cemented carbide ball or pot as described above.

Fine CBN powder of 1μ or less and WC as in the present invention
If it is necessary to mix the ingredients uniformly, the ball mill method is most suitable as mentioned above, but
In this case, even if a ball or pot made of cemented carbide is used, the bonding metal contained in the cemented carbide will be mixed in. In this case, there is no problem if the amount of the bonding metal mixed in is small, but if it is too much, the bonding metal and CBN
reacts at low temperature, and if the bonding metal is M, then M-W
-During cutting to generate a large amount of B-based boride.
CBN particles fall off and tool performance deteriorates. If the amount of this mixture is large, the metal components can be dissolved and removed in a hydrochloric acid solution after pulverizing and mixing CBN and WC.

Since the raw material powder used in the present invention is extremely fine, it has a large amount of adsorbed gas. Therefore, it is usually necessary to heat, degas, and sinter in a vacuum at a temperature of 300° C. or higher.
Degassing at temperatures below 300°C requires a long time and is therefore impractical.

Examples will be described below.

Example 1 Using ultrafine CBN powder with a particle size of 1μ or less,
Grinding was carried out using acetone as a solvent using a WC-7%Co cemented carbide ball and a pot lined with a cemented carbide having the same composition. The amount of CBN added was 5 g, but after pulverizing for 100 hours, the weight increased to 9.3 g. This increased amount is the fine cemented carbide powder mixed in from the pots and balls. From this, the composition of this powder is estimated to contain 70% CBN by volume. When this powder was observed using a scanning electron microscope, it was found that all of the powder consisted of extremely fine powder of 1 μm or less in size. This powder was stamped and molded into a disk with a thickness of 1.5 mm and an outer diameter of 10 mm.

This was heated to 1000°C in a vacuum furnace to degas it. After degassing, use ultra-high pressure equipment to 55kb, 1370℃
It was held for 10 minutes and sintered.

When the obtained sintered body was polished with diamond paste and its structure was examined, it was found that
It was a sintered body of extremely fine particles consisting of CBN particles and WC of 0.5μ or less. In addition, when the composition of this sintered body was examined using X-rays, it was found that CBN-W.Co boride-
It was WC-Co. This was cut, one piece was brazed to a steel shank, and the cutting edge was ground with a diamond grindstone. Figure 3 shows an enlarged micrograph of the cutting edge. For comparison, a cutting tool was also created using a WC-based ultrafine-grained cemented carbide in the same shape.

These tools cut lead free-cutting steel at a speed of 30
A cutting test was conducted using water-insoluble cutting oil at m/min, depth of cut of 0.3 mm, and feed rate of 0.02 mm/rev. As a result, with ultra-fine grained cemented carbide, after 30 minutes of cutting time, the roundness of the right-angled corner of the cutting edge when viewed from the rake face direction was 20 μm.
In contrast, the sintered body of the present invention could be cut for 300 minutes to reach the same state.

Example 2 Using the same CBN raw powder as in Example 1 and cemented carbide balls and pots, the powder was pulverized for 150 hours. 5g
The amount of CBN powder introduced increased by 14.2g, and the total amount increased by 14.2g.
It was 19.2g. Estimating the composition of the powder from this, the volume is 60% CBN and the balance is WC-7%.
It is made of Co. The metal components were removed by pickling using a dilute hydrochloric acid solution.

After molding this powder, it was heated and degassed in the same manner as in Example 1. Separately, WC-10%Co thickness 3mm, diameter
A 10 mm disc and a Mo disc with a thickness of 0.05 mm and a diameter of 10 mm were prepared. A Mo disk was placed in contact with the stamped body containing degassed CBN, a cemented carbide disk was placed below it, and the whole was placed in an ultra-high pressure device and sintered under the same conditions as Example 1. . When the sintered body was cut and the cross section was observed, it was found that the 1 mm thick sintered body containing ultrafine CBN particles was firmly bonded to the cemented carbide disk through a 50 μ thick intermediate layer made of Mo carbide. was.

When the structure of this sintered body was examined using X-rays, it was found that
It was CBN-W bolide-WC. A cutting tool was prepared from this sintered body in the same manner as in Example 1, and an S45C round bar was cut. In addition, cutting speed 10~40m/
Cutting was performed for 120 minutes at min, depth of cut of 1 mm, and feed rate of 0.05 mm/rev. For comparison, cutting tests were also conducted on JIS classification P40 cemented carbide and ultra-fine grained cemented carbide. The results are shown in FIG. 4, and the sintered body of the present invention is the best at all speeds.

Example 3 CBN used in Example 1 using a ball and pot made of _{( Mo7W3} )C-10%Co-5%Ni alloy
4 g of powder and 1 g of TaC powder with a particle size of 3 μm were added and pulverized for 120 hours. The weight after crushing was 15g.

The mixed metal components were removed by pickling in the same manner as in Example 2. This powder has 65% CBN by volume.
(Mo ₇ W ₃ ) Contains 32% C and 3% TaC.
This powder was bonded to a cemented carbide disk via an intermediate layer of Mo carbide to create a sintered body in the same manner as in Example 2. The sintering conditions were 55 kb and held at 1450°C for 10 minutes.

The structure of the obtained sintered body is still fine grains of 1μ or less.
It was an ultrafine-grained alloy with a uniform structure consisting of CBN, (Mo ₇ W ₃ )C of less than 1 μm, and a small amount of TaC.

[Brief explanation of the drawing]

Figure 1 is for explaining the effects of the present invention.
This is an enlarged micrograph of the cutting edge of a cutting tool made from commercially available ultra-fine grained WC-based cemented carbide. Second
The figure is an enlarged micrograph of the cutting edge of a cutting tool made from a sintered body of CBN with a particle size of 3 to 6 μm and TiN as a binder. FIG. 3 is an enlarged micrograph of the cutting edge of a cutting tool made of the sintered body of the present invention.
FIG. 4 is a graph showing the results of a cutting test of the sintered body of the present invention and cemented carbide.

Claims (1)

[Claims] 1. Cubic boron nitride with a size of 1 μ or less, 80 to 20 by volume.
% and the remainder is 1 μ or less of WC, and is characterized by substantially all cubic boron nitride crystals not being adjacent to each other. . 2 Cubic boron nitride with a size of 1 μ or less is 80 to 20 by volume.
% and the remainder is 1μ or less, consisting of a (MoW) C carbide bonding phase having the same crystal structure as WC, and substantially all of the cubic boron nitride crystals are not adjacent to each other. Sintered body for small precision parts and micro-machining tools. 3 Cubic boron nitride with a size of 1 μ or less, 80 to 20 by volume
% and the remainder is less than 1μ of WC or (MoW) having the same crystal structure as the cemented carbide binder phase mainly composed of C carbide, and substantially all of the cubic boron nitride crystals are adjacent to each other. A sintered body for a precision small part micromachining tool according to claim 1. 4 Using a cemented carbide ball whose main component is WC and a pot lined with this cemented carbide, add WC carbide powder if necessary, crush the cubic boron nitride powder, and simultaneously remove the ball and lining material. After adding fine cemented carbide powder mixed in by abrasion and pulverizing these powders to less than 1 μm, if necessary, the bonding metal of the cemented carbide is removed by acid treatment, and this is made into powder. Alternatively, cubic boron nitride with a size of 1μ or less is formed by stamp molding, heated and degassed in a vacuum to a temperature of 300°C or higher, and then hot pressed at a high temperature and high pressure where the cubic boron nitride is stable. by volume
Manufacture of sintered bodies for precision small parts and micro-machining tools, which are composed of a carbide binder phase containing 80 to 20% WC with the remainder being 1μ or less, and in which virtually no cubic boron nitride crystals are adjacent to each other. Method. 5 Cemented carbide balls whose main component is (MoW),
Using the same pot lined with this cemented carbide, add (MoW) C carbide powder if necessary.
Cubic boron nitride powder is pulverized, and at the same time, fine cemented carbide powder mixed in from the balls and lining material due to wear is added. After pulverizing these powders to 1μ or less, acid treatment is performed if necessary. After removing the bonding metal of the cemented carbide, molding it into powder or stamping, heating it to a temperature of 300℃ or higher in a vacuum, and degassing it,
5. The method for producing a sintered body for small precision parts and micromachining tools according to claim 4, wherein cubic boron nitride is hot-pressed at a stable temperature and under high pressure. 6 Using a cemented carbide ball whose main component is WC or (MoW) C, which has the same crystal structure as this, and a pot lined with this cemented carbide,
If necessary, add WC or (MoW)C carbide powder, crush cubic boron nitride powder, and at the same time add fine cemented carbide powder mixed in by abrasion from the ball and lining material. After pulverizing the powder to less than 1 μm, it is molded into a powder form or molded by molding, heated in a vacuum to a temperature of 300℃ or higher, degassed, and then processed at high temperatures and high pressures where cubic boron nitride is stable. A method for producing a sintered body for a micromachining tool for precision small parts according to claim 4, which involves hot pressing.