JP7824323B2 - Cemented Carbide and Cutting Tools - Google Patents

Description

The disclosed embodiments relate to cemented carbide and cutting tools.

In recent years, there has been an increasing need for micromachining tools for electronic devices, such as extremely small diameter drills for drilling holes in printed circuit boards (hereinafter also referred to as PWB drills) (see, for example, Patent Document 1).

In addition, printed circuit boards, which are the components to be processed, have become lighter, thinner, shorter, and smaller in recent years, and as the demand for higher electrical reliability increases, there is a demand for higher heat resistance, heat dissipation, and rigidity. To meet these demands, efforts are being made to increase the inorganic filler content in the glass fiber reinforced resin that makes up printed circuit boards, and to increase the density of the glass fibers.

Japanese Patent Application Laid-Open No. 2021-122934

The cemented carbide disclosed herein has a hard phase containing W and C and a binder phase containing an iron-group metal. Furthermore, the fractal dimension of the dispersed state of the binder phase is 0.7 or greater.

FIG. 1 is a diagram schematically illustrating the configuration of a PWB drill according to an embodiment. FIG. 2 is a diagram showing a binarized image of an SEM photograph of a cemented carbide alloy that was fired at a temperature rise rate of 2° C./min. FIG. 3 is a diagram showing a binarized image of an SEM photograph of a cemented carbide alloy that was fired at a temperature rise rate of 6° C./min. FIG. 4 is a diagram showing the relationship between the rate of temperature rise during firing treatment, the fractal dimension of the dispersed state of the binder phase, and the particle size of the hard phase. FIG. 5 is a diagram showing the relationship between the fractal dimension of the dispersed state of the binder phase and the standard deviation of Vickers hardness at multiple points. FIG. 6 is a diagram showing the relationship between the fractal dimension of the dispersed state of the binder phase and the process capability index CPK of the degree of deviation of hole positions.

Embodiments of the cemented carbide and cutting tool disclosed herein will be described below with reference to the accompanying drawings. Note that the present invention is not limited to the embodiments described below.

In recent years, there has been an increasing need for micromachining tools for electronic devices, such as extremely small diameter drills for drilling holes in printed circuit boards (hereinafter also referred to as PWB drills).

In addition, printed circuit boards, which are the components to be processed, have become lighter, thinner, shorter, and smaller in recent years, and as the demand for higher electrical reliability increases, there is a demand for higher heat resistance, heat dissipation, and rigidity. To meet these demands, efforts are being made to increase the inorganic filler content in the glass fiber reinforced resin that makes up printed circuit boards, and to increase the density of the glass fibers.

However, with the above-mentioned conventional technology, as the amount of inorganic filler or glass fiber in a printed circuit board increases, the cutting edge of the PWB drill can become more prone to uneven wear when cutting the printed circuit board. This can lead to problems such as poor hole positioning accuracy, increased roughness of the inner wall surface, and breakage of the PWB drill.

Therefore, there is hope for the realization of technology that can overcome the above-mentioned problems and reduce uneven wear during cutting in PWB drills.

<PWB drill>
First, the configuration of a PWB drill 1 according to an embodiment will be described with reference to Fig. 1. Fig. 1 is a diagram schematically showing the configuration of a PWB drill 1 according to an embodiment.

As shown in Figure 1, the PWB drill 1 according to the embodiment has a cutting portion 2, a shank portion 3, and a connecting portion 4. The cutting portion 2 has a long cylindrical shape, and the tip of this cylindrical shape is sharpened. In addition, one or more spiral chip discharge grooves 2a are formed on the side of the cutting portion 2, extending from the tip end toward the base end.

The cutting portion 2 of the embodiment is composed of a cemented carbide alloy having a hard phase containing W (tungsten) and C (carbon) and a binder phase containing an iron group metal (at least one of Co (cobalt), Ni (nickel), and Fe (iron)).

The shank portion 3 has, for example, a cylindrical shape with an outer diameter larger than that of the cutting portion 2. The shank portion 3 is made of, for example, stainless steel. This allows for a reduction in the manufacturing cost of the PWB drill 1 compared to when the entire PWB drill 1 is made of cemented carbide.

The connecting portion 4 connects the cutting portion 2 and the shank portion 3, which have different outer diameters. The connecting portion 4 has a first tapered portion 4a, a second tapered portion 4b, and a cylindrical portion 4c.

The first tapered portion 4a is integrally formed with the shank portion 3, and its outer diameter gradually decreases toward the tip. In other words, the first tapered portion 4a is made of the same material (such as stainless steel) as the shank portion 3.

The second tapered portion 4b is integrally formed with the cutting portion 2, and its outer diameter gradually increases as it moves away from the tip. In other words, the second tapered portion 4b is made of the same material (carbide alloy) as the cutting portion 2.

A portion of the cylindrical portion 4c is integral with the first tapered portion 4a, and the remainder is integral with the second tapered portion 4b. The cemented carbide and stainless steel are joined at the joining surface 4d located on the cylindrical portion 4c, connecting the cutting portion 2 and the shank portion 3.

Note that the PWB drill 1 of the embodiment is not limited to the example shown in Figure 1, and may have any configuration as long as at least the cutting portion 2 is made of cemented carbide.

In this embodiment, the fractal dimension of the dispersion state of the binder phase in the hard phase in the cemented carbide constituting the cutting portion 2 is 0.7 or greater. The value of the fractal dimension of the dispersion state of the binder phase indicates the dispersion state of the binder phase in the sintered body, so the higher the value of the fractal dimension of the dispersion of the binder phase, the more uniform the dispersion of the binder phase.

The uniformity of binder phase dispersion correlates with the variation in the Vickers hardness of the sintered body. Specifically, the more uniform the binder phase dispersion, the smaller the variation in the Vickers hardness of the sintered body from a microscopic perspective, and the smaller the standard deviation. Therefore, having a high fractal dimension value makes the binder phase dispersion more uniform, reducing the formation of uneven areas in the hard phase in the cemented carbide.

Therefore, according to the embodiment, the variation in hardness within the cemented carbide can be reduced, thereby reducing uneven wear during cutting.

In addition, in an embodiment, the cemented carbide constituting the cutting portion 2 may have a fractal dimension of the dispersion state of the binder phase in the hard phase of 0.8 or more. This further reduces the formation of non-uniform portions in the hard phase in the cemented carbide.

Therefore, according to the embodiment, the variation in hardness within the cemented carbide can be further reduced, thereby further reducing uneven wear during cutting.

In an embodiment, the standard deviation of the Vickers hardness measured at multiple points in a 400 μm ² area of the cemented carbide constituting the cutting portion 2 may be 2.5 or less. By reducing the variation in hardness in a small area such as the cutting edge, uneven wear during cutting can be further reduced.

In addition, in an embodiment, the degree of deviation in hole position when drilling a hole in a component such as a printed circuit board using the tip of the cutting portion 2 may be 2.45 or more in terms of process capability index CPK. In this way, improved hole position accuracy can further reduce uneven wear during cutting, thereby achieving a longer life for the PWB drill 1.

The following provides a detailed description of examples of the present disclosure. Note that the examples described below focus on cemented carbide alloys that use cobalt as the binder phase, but the present disclosure is not limited to the following examples.

First, various powders were blended to a total weight of 100%: 75-98.9% tungsten carbide (WC) powder with an average particle size of 0.1-1 μm, 1-20% cobalt powder, 0.1-5% carbide powder of at least one element selected from Groups 4A, 5A, and 6A of the periodic table, and other unavoidable impurities. In this disclosure, metallic tungsten (W) powder or carbon black (C) may also be blended if desired.

Next, water or an organic solvent and, if desired, an organic binder are added to the prepared mixed powder and mixed, then pulverized and mixed using known methods such as a ball mill or a vibrating mill, and then dried in a spray dryer to form granules.

Next, this granular mixed powder was molded into the desired cutting tool shape as shown in Figure 1 using known molding methods such as press molding, casting, extrusion molding, and cold isostatic pressing.

Next, this compact was sintered in a sintering furnace in a vacuum or in a non-oxidizing atmosphere such as Ar or _N2 at a sintering temperature of 1350 to 1450°C for 0.5 to 5 hours, and then subjected to HIP sintering at a temperature 5 to 50°C lower than the sintering temperature, at a pressure of 5 to 20 MPa, and for 0.5 to 3 hours to produce cemented carbide alloys for cutting tools of the examples and comparative examples.

In this disclosure, the maximum firing temperature itself was all kept the same, but the heating rate just before the firing temperature reached the maximum temperature was varied to control the crystalline state within the cemented carbide. In the comparative example, the firing temperature was allowed to reach the maximum temperature without changing the heating rate.

Specifically, for the cemented carbide of the example, the temperature of the firing furnace was first increased at a heating rate of V1, and then the heating rate was changed to V2 just before the maximum temperature was reached. At this time, the heating rate V2 was set to be smaller than the heating rate V1. Furthermore, for the cemented carbide of the example, the firing temperature was allowed to reach the maximum temperature at a constant heating rate V0. More specifically, the heating rates V0 and V1 were set to 8 (°C/min). Furthermore, three cemented carbide alloys were prepared as examples, with heating rates V2 of 2 (°C/min), 4 (°C/min), and 6 (°C/min).

Here, changing the heating rate immediately before the firing temperature changes the heating temperature when it reaches approximately 80-99% of the maximum temperature. For example, if the maximum temperature is 1400°C and the heating rate is changed when the firing temperature reaches 1358°C, the heating temperature will change when it reaches 97% of the maximum temperature (= 1358°C/1400°C). In this case, it can be said that the heating rate immediately before the firing temperature changes.

In this disclosure, the crystalline state within the cemented carbide alloy is controlled by changing the rate of temperature increase immediately before the firing temperature reaches its maximum temperature, but control of the crystalline state is not limited to this method. If the fractal dimension of the dispersion state of the binder phase within the hard phase can be increased, the crystalline state within the cemented carbide alloy may be controlled, for example, by changing the particle size distribution of the mixed powder, or by adding an organic substance different from the organic substance contained in the solvent when adding water or an organic solvent to the mixed powder and mixing it.

In the present disclosure, if desired, a coating film may be formed on the surface of the prepared cemented carbide substrate, such as titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), titanium aluminum composite nitride (TiAlN), or aluminum oxide (Al ₂ O ₃ ).

Furthermore, well-known film-forming methods such as chemical vapor deposition (thermal CVD, plasma CVD, organic CVD, catalytic CVD, etc.) and physical vapor deposition (ion plating, sputtering, etc.) can be used to form such coating films.

Next, designated areas of the resulting cemented carbide for cutting tools were polished, and the polished surface was photographed using a field emission scanning electron microscope (FE-SEM) manufactured by JEOL under the following conditions: magnification: 2000x, acceleration voltage: 10 (kV), brightness: 581, contrast: 4630, with five fields of view per sample.

The photographs were then analyzed using the computer software "ImageJ" (developed by Wayne Rasband, National Institutes of Health). Specifically, the photographs were first imported into ImageJ, and the hard and bonded phases were binarized using "Trainable Weka Segmentation" in ImageJ.

For this binarization process, one photograph was selected, and at least four areas were extracted from the Co (black areas) and the hard phase (other areas), and the binarization conditions were determined. These conditions were then applied to the remaining four photographs, and binarization was performed on each of the five photographs.

Next, this binarized image was subjected to fractal analysis using the box counting method with "Fractal Box Count..." in ImageJ. The box sizes for division were 5, 6, 7, 8, 9, and 10. The fractal value was calculated for each of the five photographs, and the five obtained fractal values were averaged to determine the fractal dimension of the cemented carbide being evaluated.

In addition, using ImageJ, the average circle equivalent diameter (hereinafter also referred to as particle diameter) of the crystal grain size of the hard phase in the photograph was calculated.

The resulting cemented carbide for cutting tools was subjected to micro-area hardness measurement using a micro Vickers hardness tester. Specifically, the hardness was measured at 100 locations spaced approximately ² μm apart in a measurement area of 400 μm (e.g., 20 μm square) using a micro Vickers hardness tester. The same test was repeated for five fields of view per sample.

Then, the hardness distributions of the multiple Vickers hardness measurements obtained were plotted as a histogram and approximated with a normal distribution, and the standard deviation of the Vickers hardness was calculated.

The resulting cemented carbide for cutting tools was also evaluated for hole position deviation when drilling holes in a component using the process capability index (CPK). Specifically, six PWB drills 1 with a drill diameter of 150 μm (μmΦ) and a printed wiring board substrate were prepared. Next, the prepared PWB drills 1 were used to drill 10,000 holes per drill into the prepared printed wiring board substrate.

Next, the distance between the drilled hole and the target center position is measured and used as hole position accuracy data.Then, the average value and standard deviation of the measured hole position accuracy data are calculated.Finally, the process capability index CPK is calculated using the calculated average value and standard deviation values using the following formula (1).
CPK = (standard value - average value) / (3 x standard deviation) (1)

In this disclosure, the process capability index CPK is calculated using a standard value of 75 (μm). Furthermore, in this disclosure, the process capability index CPK for six drills was measured and then averaged to determine the overall process capability index CPK.

Figure 2 shows a binarized image of an SEM photograph of a cemented carbide alloy that was fired at a heating rate of 2 (°C/min), and Figure 3 shows a binarized image of an SEM photograph of a cemented carbide alloy that was fired at a heating rate of 6 (°C/min).

As shown in Figures 2 and 3, by reducing the heating rate until the maximum temperature is reached, the dispersion state of the binder phase (black part) is improved.

Figure 4 shows the relationship between the heating rate during firing treatment, the fractal dimension of the dispersed state of the binder phase, and the particle size of the hard phase.

As shown in Figure 4, by setting the heating rate V2 until the maximum temperature is reached lower than the initial heating rate V1, the fractal dimension, which indicates the dispersion state of the binder phase, is improved (i.e., the dispersion state of the binder phase is improved). In particular, it can be seen that the fractal dimension increases as the heating rate V2 decreases.

When the heating rate V2 is smaller than the heating rate V1, it is possible to ensure a longer firing time just before reaching the maximum temperature, which has a significant impact on the dispersion state of the binder phase, while avoiding an excessive increase in the time required for the entire firing process. This is thought to promote uniformity in the dispersion state of the binder phase and make it easier to efficiently increase the fractal dimension. In particular, the smaller the heating rate V2, the longer the firing time just before reaching the maximum temperature. This is thought to promote more uniformity in the dispersion state of the binder phase and make it easier to increase the fractal dimension.

The heating rate V2 may be constant or may be varied. For example, the heating rate V2 may be composed of a first stage indicated by a constant heating rate V21 and a second stage indicated by a constant heating rate V22 (but at a value different from V21). In this case, the heating rate V22 may be smaller than the heating rate V21. When the heating rate V2 gradually decreases in this way, a longer firing time can be ensured just before reaching the maximum temperature, which has a significant impact on the dispersion state of the binder phase. This is thought to further promote uniform dispersion of the binder phase and make it easier to increase the fractal dimension.

Furthermore, when the heating rate V2 is composed of multiple stages as described above, and the heating rate in each stage is constant, temperature control of the firing furnace is easier than when the heating rate V2 changes continuously. Therefore, for example, it is easier to avoid variations in fractal dimension due to the position of the cemented carbide in the firing furnace and variations in the firing process depending on the operator. In other words, the fractal dimension of the cemented carbide is stable, resulting in good reproducibility.

Furthermore, in this disclosure, since the maximum temperature itself is kept constant, it can be seen that even if the heating rate just before reaching the maximum temperature is changed, the particle size of the hard phase remains almost unchanged. This makes it possible to control the dispersion state of the binder phase without changing the hardness characteristics of the cemented carbide.

Figure 5 shows the relationship between the fractal dimension of the binder phase dispersion state and the standard deviation of Vickers hardness at multiple points. As shown in Figure 5, by setting the fractal dimension to 0.7 or more, the standard deviation of Vickers hardness can be reduced to a favorable value.

In other words, in an embodiment, by setting the fractal dimension to 0.7 or more, the hardness distribution in a microscopic area can be made uniform, thereby reducing uneven wear during cutting.

In addition, in an embodiment, by setting the fractal dimension to 0.8 or more, the standard deviation of Vickers hardness can be reduced to an even better value. This makes it possible to further uniformize the hardness distribution in a microscopic area, thereby further reducing uneven wear during cutting.

In this disclosure, the upper limit of the fractal dimension may be 2 or less, preferably 1 or less, from the standpoint of manufacturing efficiency, etc.

In addition, in an embodiment, by setting the standard deviation of the Vickers hardness at multiple points to 2.5 or less, the hardness distribution in a microscopic area can be made uniform, thereby reducing uneven wear during cutting.

Figure 6 shows the relationship between the fractal dimension of the dispersion state of the binder phase and the process capability index CPK of the degree of hole position deviation. As shown in Figure 6, by setting the fractal dimension to 0.7 or more, the process capability index CPK can be improved to a good value.

In other words, in the embodiment, the fractal dimension of the dispersed state of the binder phase is set to 0.7 or more, thereby improving the accuracy of the hole position. Therefore, according to the embodiment, uneven wear during cutting can be reduced, thereby achieving a longer life for the PWB drill 1.

In addition, in the embodiment, by setting the fractal dimension of the dispersed state of the binder phase to 0.8 or more, the process capability index CPK can be improved to an even better value. Therefore, according to the embodiment, the life of the PWB drill 1 can be further extended.

In addition, in an embodiment, by setting the process capability index CPK to 2.45 or more, uneven wear during cutting can be reduced, thereby achieving a longer life for the PWB drill 1.

The above describes an embodiment of the present invention, but the present invention is not limited to the above embodiment and various modifications are possible without departing from the spirit of the present invention. For example, while the above embodiment shows an example in which the above-mentioned cemented carbide is used in a PWB drill, the present disclosure is not limited to such an example and the above-mentioned cemented carbide may be used in various cutting tools such as chips.

Further advantages and other aspects may readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described above. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and equivalents thereof.

1 PWB drill 2 Cutting part 3 Shank part 4 Connecting part

Claims (5)

a hard phase containing W and C;
a binder phase containing an iron group metal;
and
The cemented carbide, wherein the binder phase has a fractal dimension in a dispersed state of 0.7 or more and 2 or less . The cemented carbide according to claim 1, wherein the binder phase has a fractal dimension of 0.8 or more in a dispersed state. The cemented carbide according to claim 1 or 2, wherein the standard deviation of Vickers hardness measured at multiple points in a 400 (μm ² ) area is 2.5 or less. A cutting tool comprising the cemented carbide according to claim 1 or 2 in a long cylindrical shape, one end of which is joined to a shank portion, and the other end of which is sharpened. The cutting tool according to claim 4 , wherein a degree of deviation of a hole position when a hole is drilled in a member by the other end portion is 2.45 or more in terms of a process capability index CPK.