JP7590695B2 - cBN sintered body - Google Patents

Description

The present invention relates to cubic boron nitride sintered bodies (hereinafter sometimes referred to as cBN sintered bodies), which are hard composite materials.

Although cBN sintered compacts are less hard than diamond, they have the property of low reactivity with Fe-based and Ni-based materials, and are therefore also used as cutting tools. Proposals have been made to improve the performance of cBN sintered compacts.

For example, Patent Document 1 describes a cBN sintered body that contains 40-85% by volume of cBN, at least one metal selected from Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ni, and Al, a binder phase consisting of at least one carbide, nitride, carbonitride, boride, oxide, and mutual solid solutions of these metals, and the remainder is unavoidable impurities, with Mo content of 0.2-3.0% by mass, Ni content of 0.2-3.0% by mass, and Ta content of 0.5-3.0% by mass, and that is said to have excellent wear resistance and chipping resistance when used as a cutting tool.

For example, Patent Document 2 describes polycrystalline cBN in which protruding borides are formed on the surface of the cBN, with an average particle size of 4-8 μm and a volume ratio of 70-85%; the borides are formed within 100 nm of the surface of the cubic boron nitride particles and are 0.001-1 μm in size; this polycrystalline cBN is said to have excellent impact resistance and wear resistance.

Patent No. 5614460 Patent No. 6715349

The present invention has been made in consideration of the above circumstances and proposals, and aims to obtain a cBN sintered body with improved wear resistance and chipping resistance, for example, when used as a cutting tool for high-hardness steel.

The cubic boron nitride sintered body according to the embodiment of the present invention is
It has cubic boron nitride particles and a binder phase,
The content of the cubic boron nitride particles is 30 to 85 volume %,
The binder phase includes any one of the four combinations of transition metal compounds: VB and VC, TaB and TaC, NbB and NbC, and CrB and CrC;
The transition metal element constituting the transition metal compound group is contained in an amount of 0.1 to 0.8 atomic %;
When the 111 diffraction ray intensity of a metal boride in the transition metal compound group is I _MB and the 111 diffraction ray intensity of a metal carbide is I _MC , 1.0≦I _MB /I _MC ≦3.0.

The cubic boron nitride sintered body according to the above embodiment may satisfy one or more of the following items (1) and (2) :

(1) The content of cubic boron nitride particles is 40 to 80 volume percent.
(2) The average particle size of the cubic boron nitride particles is 2.0 to 8.0 μm.

When the cubic boron nitride sintered body is used as a cutting tool, it is possible to further improve the wear resistance and chipping resistance.

FIG. 2 is a schematic diagram showing the structure of a cBN sintered body according to an embodiment of the present invention.

As a result of examining the proposals described in the above-mentioned patent documents, the present inventors have
1) Carbon, an impurity (unavoidable impurity) that is unintentionally mixed in during the manufacturing process, adheres to the cBN particle interfaces, reducing strength.
2) The content ratio of compounds or mutual solid solutions in the binder phase is not specified, so depending on the compounds contained or the content ratio of the compounds, the cutting conditions under which high-hardness steel is subjected to high loads may not be able to be endured.
I recognized that.

The inventors therefore conducted extensive research based on the above findings to obtain a cBN sintered body with improved wear resistance and chipping resistance. As a result, they obtained the following findings 1) to 5).

1) When an appropriate amount of orthorhombic metal boride is produced by heat treatment from the hexagonal metal diboride, which is added as a raw material powder to form the binder phase of the cBN sintered body, the wear resistance and chipping resistance of the cBN sintered body are improved.

2) However, simply generating metal borides is not sufficient to improve the wear resistance and chipping resistance of cBN sintered bodies.

3) In order to solve the problem 2) above, it is preferable that the carbon inevitably contained in the orthorhombic metal boride during the formation of the orthorhombic metal boride does not exist as simple carbon but exists as a cubic metal carbide.
However, since the cubic metal carbide itself is an embrittlement material, if too much is produced, the wear resistance and fracture resistance will decrease, and therefore unless the amount produced is controlled, the wear resistance and fracture resistance will not improve.

4) To achieve this, the binder phase must contain one of the four combinations of components: VB and VC, TaB and TaC, NbB and NbC, and CrB and CrC.

5) As mentioned above, metal carbides are embrittles, and therefore if too much is produced, the wear resistance and chipping resistance will decrease. On the other hand, orthorhombic metal borides are less susceptible to wear when cutting high-hardness steel, but if too much is produced, chipping resistance will decrease. Therefore, it is necessary to control the amounts of metal borides and metal carbides produced to an appropriate ratio, and as an indicator of this, it is possible to quantitatively evaluate the amounts produced by comparing the intensity of the 111 diffraction line in the transition metal compound group.

Hereinafter, the cBN sintered body according to the embodiment of the present invention will be described in detail.
In the present specification and claims, when a numerical range is expressed as "A to B" (A and B are both numerical values), this is equivalent to "A or more and B or less," and the range includes an upper limit (B) and a lower limit (A). The units of the upper limit and the lower limit are the same.
Furthermore, the numerical values include tolerances.

In this embodiment, as shown in FIG. 1, there is provided cBN particles (1) and a binder phase (2), in which metal carbide particles (3) and metal boride particles (4) selected from any one of four combinations of VB and VC, TaB and TaC, NbB and NbC, and CrB and CrC are dispersed.
These will be explained below.

1. Cubic boron nitride (cBN) particles The average particle size of cBN particles and the content of cBN particles in a cBN sintered compact will be described.

(1) Average Particle Size The average particle size of the cBN particles used in this embodiment is not particularly limited, but is more preferably in the range of 2.0 to 8.0 μm.

The reason for this is that, in addition to the fact that the inclusion of cBN particles in the sintered body enhances chipping resistance, if the average particle size is 2.0 to 8.0 μm, when used as a cutting tool, not only is chipping and chipping caused by the uneven shape of the cutting edge caused by cBN particles falling off the tool surface more reliably suppressed, but also the propagation of cracks that develop from the interface between the cBN particles and the binder phase caused by stress applied to the cutting edge during use, or cracks that develop as cBN particles break, is more suppressed, resulting in better chipping resistance. An average particle size of 3.0 to 6.0 μm is even more preferable, and 3.5 to 5.0 μm is even more preferable.

Here, the average particle size of the cBN particles can be determined as follows.
The cross section of the cBN sintered body is mirror-finished, and the mirror-finished surface is observed with a scanning electron microscope (hereinafter referred to as SEM) to obtain a secondary electron image. Next, the cBN grains in the obtained image are extracted by image processing, and the average grain size, which will be described later, is calculated based on the maximum length of each grain determined by image analysis.

When extracting the cBN particle portion of the image through image processing, in order to clearly distinguish between the cBN particles and the bonding phase, the image is displayed in monochrome with 256 gradations, with 0 being black and 255 being white, and binarization is performed so that the cBN particles appear black using an image with pixel values where the ratio of the pixel values of the cBN particle portion to the pixel values of the bonding phase portion is 2 or more.

In addition, it is desirable to select an area of approximately 0.5 μm x 0.5 μm as the area for determining the pixel value of the cBN particle portion, and to use the average value determined from at least three different locations within the same image area as the aforementioned pixel value of cBN.

After the binarization process, the parts of the cBN particles that are thought to be in contact with each other are separated, for example, by using the watershed method to separate the cBN particles that are thought to be in contact with each other.

The parts of the image obtained after the binarization process described above that correspond to the cBN particles (black parts) are subjected to particle analysis, and the maximum length of each particle obtained is taken as the diameter of each particle. In particle analysis to determine the maximum length, the Feret's diameter for one cBN particle is calculated, and the larger of the two lengths obtained is taken as the maximum length, and this value is taken as the diameter of each particle.

Assuming that each particle is an ideal sphere with this diameter, the cumulative volume is calculated as the volume of each particle, and a graph is drawn based on this cumulative volume with the vertical axis representing volume percentage (%) and the horizontal axis representing diameter (μm), and the diameter when the volume percentage is 50% is taken as the average particle size of the cBN particles. This is done for the three observation areas, and the average value is taken as the average particle size (μm) of the cBN particles.

When performing this particle analysis, the length (μm) per pixel is set using the scale value known in advance from the SEM. For the observation area used in image processing, a viewing area of approximately 40 μm x 40 μm is desirable when the average particle size of the cBN particles is approximately 4.0 μm.

(2) Content Ratio The content ratio (volume %) of cBN particles in the cBN sintered compact is more preferably 40 to 80 volume %.

The reason for this is that if the content is less than 40% by volume, there is little hard material in the cBN sintered body, and when used as a cutting tool, the chipping resistance may decrease, while if it exceeds 80% by volume, voids that can become the starting points for cracks may be generated in the cBN sintered body, and the chipping resistance may decrease. A cBN particle content of 50 to 75% by volume is even more preferable.

The cBN particle content in a cBN sintered body can be determined as follows. That is, the cross-sectional structure of the cBN sintered body is observed by SEM, the cBN particle portion in the obtained secondary electron image is extracted by image processing, the area occupied by the cBN particles is calculated by image analysis, and the images of at least three observation regions are processed and the average value obtained is taken as the cBN particle content (volume %). When the average cBN particle content is about 50 volume %, a viewing area of about 40 μm x 40 μm is preferable as the observation region used for image processing.

2. Binder Phase The binder phase preferably contains any one of the four combinations of transition metal compounds: VB and VC, TaB and TaC, NbB and NbC, and CrB and CrC. There are no particular limitations on other components.

Other components include one or more of Al borides, nitrides, and oxides, and one or more of Ti borides, nitrides, carbides, and carbonitrides.

With regard to the borides and carbides of the transition metal compound group, when the 111 diffraction ray intensity of the metal boride is I _MB and the 111 diffraction ray intensity of the metal carbide is I _MC , the wear resistance and chipping resistance are improved when 1.0≦I _MB /I _MC ≦3.0.

Here, the I _MB and I _MC are measured using Cu-Kα rays as an X-ray source, and the wavelength of the X-rays is 1.54 Å.

The content of the transition metal element in the transition metal compound group is not particularly limited, but is preferably 0.1 to 0.8 atomic %. If the content is within this range, the above-mentioned object can be reliably achieved.
Here, the atomic % refers to the ratio of the transition metal atoms contained in the transition metal compound group to all atoms contained in the cBN sintered body.

The transition metal element content in the cBN sintered body is determined by analyzing the cBN sintered body using an electron probe micro analyzer (EPMA), and the detected elements are analyzed using the ZAF quantitative analysis method to obtain the transition metal element content (atomic %) in the cBN sintered body.

Next, examples will be described. However, the present invention is not limited to these examples.

This example was manufactured according to the following steps (1) to (4).

(1) To obtain the metal borides of the transition metal compound group, a metal diboride powder having an average particle size of 0.1 μm to 6.0 μm (one type selected from VB ₂ , NbB ₂ , TaB ₂ , and CrB ₂ shown in Table 1) was prepared as a raw material powder, which was then filled into a container lined with cemented carbide together with cemented carbide balls and acetone. After the container was closed with a lid, the powder was pulverized using a ball mill, and the mixed slurry was dried and classified using a centrifugal separator to obtain a metal diboride raw material powder having an average particle size of 20 to 300 nm.

(2) A Ti compound powder (at least one selected from the group consisting of TiN powder, TiC powder, TiCN powder, and _TiAl3 powder shown in Table 1), the metal diboride raw material powder (shown as _MB2 in Table 1) previously prepared in the above (1), and cBN powder as a hard phase raw material were blended, wet mixed, and dried to obtain a sintered raw material powder.

(3) The obtained sintered body raw material powder was press molded at a molding pressure of 1 MPa to dimensions of 50 mm diameter x 1.5 mm thickness, and then this molded body was pre-sintered by holding it at a predetermined temperature in the range of 900 to 1300°C in a vacuum atmosphere with a pressure of 1 Pa or less. The main purpose of the pre-sintering was to remove the solvent that was present during the wet mixing.

(4) The compact produced in (3) was placed in an ultra-high pressure sintering apparatus and subjected to ultra-high pressure sintering under conditions of a pressure of 5 GPa, a temperature of 1200 to 1400°C, and a holding time of 30 minutes, to obtain the sintered bodies of the present invention (Examples 1 to 12) shown in Table 2.

For comparison, comparative sintered bodies (Comparative Examples 1' to 8') were prepared by the same processes as (1) to (4), which were either ones that did not have any compound of the transition metal compound group in the binder phase, ones that had only metal borides or only metal carbides as the transition metal compounds, ones that did not have borides and carbides of the same transition metal, and ones that did not satisfy 1.0≦I _MB /I _MC ≦3.0.

Next, the above-prepared Examples 1 to 12 and Comparative Examples 1' to 8' were cut to the specified dimensions using a wire electric discharge machine. This was brazed to the brazing portion (corner portion) of a WC-based cemented carbide insert body having a composition of Co: 5 mass%, TaC: 5 mass%, WC: remainder, and an insert shape conforming to ISO standard CNGA120408, using a brazing filler material of Ag alloy having a composition of Cu: 26 mass%, Ti: 5 mass%, Ag: remainder, and the upper and lower surfaces and outer circumference were polished and honed to produce the cBN-based ultra-high pressure sintered compact cutting tools of the present invention (Examples 1 to 12) and the comparative cBN-based ultra-high pressure sintered compact cutting tools (Comparative Examples 1' to 8') shown in Table 2, which have an insert shape conforming to ISO standard CNGA120408.

Table 2 shows the I _MB /I _MC ratios, atomic % of transition metal elements, content of cBN particles, and average particle size measured by the above-mentioned method for Examples 1 to 12 and Comparative Examples 1' to 8'.

Next, cutting was carried out on Examples 1 to 12 and Comparative Examples 1' to 8' under the following cutting conditions, and the tool life (cutting time) until chipping occurred was measured.
<Cutting conditions>
Work material: Carburized steel (JIS SCR420, hardness: HRC58-62) Round bar Cutting speed: 250m/min
Cut: 0.2 mm
Feed: 0.1 mm/rev
Dry cutting tests of high hardness steel were conducted under the above conditions.

The time until the cutting edge of each tool was chipped was defined as the tool life, and the cutting edge was observed every 30 seconds of cutting time to check for the presence or absence of chipping on the cutting edge, and the time until the tool life was determined.
Table 3 shows the results of the cutting test.

The results shown in Table 3 show that in all of the Examples, tool life was extended without sudden chipping of the cutting edge compared to all of the Comparative Examples, and toughness was improved, demonstrating excellent chipping resistance even in continuous cutting of hardened steel.

When the cBN sintered body of the present invention is used as a cutting tool, it exhibits excellent chipping resistance over long periods of use without chipping, and the tool life is extended, so it can fully respond to the need to improve the performance of cutting equipment, as well as to reduce labor, energy, and costs in cutting.

1 cBN particles 2 Binding phase 3 Metal carbide particles 4 Metal boride particles

Claims (3)

A cubic boron nitride sintered body having cubic boron nitride particles and a binder phase,
The content of the cubic boron nitride particles is 30 to 85 volume %,
The binder phase includes any one of the four combinations of transition metal compounds: VB and VC, TaB and TaC, NbB and NbC, and CrB and CrC;
The transition metal element constituting the transition metal compound group is contained in an amount of 0.1 to 0.8 atomic %;
A cubic boron nitride sintered body, characterized in that, when the 111 diffraction ray intensity of the metal boride in the transition metal compound group is I _MB and the 111 diffraction ray intensity of the metal carbide is I _MC , 1.0≦I _MB /I _MC ≦3.0. 2. The cubic boron nitride sintered body according to claim 1 , wherein the content of said cubic boron nitride particles is 40 to 80 volume %. 3. The cubic boron nitride sintered body according to claim 1, wherein the cubic boron nitride particles have an average particle size of 2.0 to 8.0 μm.

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