JP5257793B2 - Drill and its manufacturing method - Google Patents

Description

  The present invention relates to a small-diameter drill made of a WC—Co-base cemented carbide and a method for producing the same. In particular, the present invention relates to a drill having excellent wear resistance.

  Conventionally, drilling made of a WC-Co based cemented carbide in which WC (tungsten carbide) is the main hard phase and this hard phase is bonded with a bonding phase such as Co (cobalt) is used for drilling (for example, Patent Document 1). In particular, for drilling a printed circuit board or the like, a drill having a small diameter (hereinafter referred to as a micro drill) having a drill diameter of 0.5 mm (500 μm) or less is used.

JP 2006-167856 A

  In recent years, the cutting environment has become increasingly severe. In the micro drill used for drilling the printed circuit board, the number of processes to be processed at a time by laminating a plurality of substrates is increased, or a material that is difficult to cut (hard-to-cut material) is used as a constituent material of the printed circuit board. , Increase the number of holes drilled in one micro drill, and so on. However, conventional micro drills cannot sufficiently meet the above requirements. One reason for this is that conventional micro drills have poor wear resistance.

  For example, it is conceivable to improve the wear resistance by making a cemented carbide added with an element having an effect of suppressing grain growth of WC and making the average grain size of WC very small such as less than 0.5 μm. However, since the particle size of WC in the cemented carbide is too fine, when Co, which is inferior in wear resistance at the time of cutting, falls off due to wear, the WC particles also easily fall off, and the effect of improving wear resistance is sufficient. Difficult to get to. In particular, when the workpiece (work material) is made of a difficult-to-cut material, the wear of Co having a low hardness is significant.

  Moreover, when the element which has the said grain growth inhibitory effect is only increased, the hardness of cemented carbide itself will become high and it will become easy to produce chipping.

  On the other hand, Patent Document 1 proposes reducing cutting resistance by removing Co on the drill surface. However, as a result of investigation by the present inventors, in the case of a micro drill, if Co is excessively dropped as described in Patent Document 1, it is easy to break and it is difficult to improve wear resistance. Obtained knowledge.

  Accordingly, one of the objects of the present invention is to provide a small-diameter drill excellent in wear resistance and a method for manufacturing the same.

  The present inventors have found that, in a small-diameter drill made of a cemented carbide, by specifying the extent and area where Co is reduced on the drill surface, it is particularly effective in improving wear resistance. As a result, the present invention has been completed.

  The drill of the present invention relates to a drill made of a cemented carbide mainly composed of WC and Co. This drill has a blade diameter of 0.5 mm or less, and is provided with a Co reducing layer at least on the surface side of the blade groove portion and an inner peripheral portion on the inside thereof. When the Co content in the inner peripheral portion is Co (i) (mass%) and the Co content in the Co reduced layer is Co (o) (mass%), the ratio of both Co (o) / Co ( i) is 50% or more and less than 80%, and the thickness of the Co reducing layer is less than 10 μm.

  According to this structure, it can be set as the drill which has high abrasion resistance by forming the Co reduction layer in which Co content was reduced moderately in the very thin area | region of the surface part of a drill.

  As one form of the drill of this invention, it is mentioned that the composition distribution of the said inner peripheral part of the drill radial direction is made substantially uniform.

  According to this configuration, since the composition distribution in the drill radial direction of the inner peripheral part, particularly the distribution of Co content, is substantially uniform, the composition greatly changes in the radial direction of the inner peripheral part that occupies most of the drill. There is no place, and the strength of the entire drill can be stabilized.

  As one form of the drill of the present invention, the cemented carbide contains Cr, and the ratio of Cr content (mass%) to Co content (mass%) in the cemented carbide is 4% or more. It may be less than 7%.

  With this configuration, the corrosion resistance of the cemented carbide can be appropriately adjusted, and the amount of Co removed can be easily adjusted when removing Co from the surface portion of the drill with an acid.

  As an embodiment of the drill of the present invention, the average particle diameter of WC in the cemented carbide may be 0.5 μm or more and 1.5 μm or less.

  According to this structure, it can be set as a drill with high abrasion resistance by comprising a drill with the cemented carbide alloy with a fine hard phase.

On the other hand, the manufacturing method of the drill of this invention is characterized by including the following processes.
A process of preparing a drill material made of a cemented carbide containing WC and Co as the main component and having a blade diameter of 0.5 mm or less.
A step of immersing at least a blade groove portion of this material in an acid to form a Co reducing layer on the surface side of the blade groove portion.
In this Co-reduced layer, the Co content in the Co-reduced layer is Co (o) (mass%), and the Co content in the inner periphery inside the Co-reduced layer is Co (i) (mass%). When the ratio Co (o) / Co (i) is 50% or more and less than 80%, the thickness is less than 10 μm.

  According to this structure, the drill of this invention can be manufactured by the simple method of immersing the raw material of a drill in an acid.

  As one form of the manufacturing method of the drill of this invention, it is mentioned that the said acid is a weak acid of pH 4-6.

  If a weak acid having a pH of 4 to 6 is used, excessive removal of Co from the surface portion of the drill can be suppressed, and the amount of Co removal in the Co reduction layer can be easily adjusted. Specific examples of weak acids include tartaric acid, citric acid, glutaric acid and the like.

  The drill of the present invention can improve wear resistance by providing an extremely thin Co reducing layer on the surface portion.

  Moreover, the manufacturing method of the drill of this invention can manufacture the drill of this invention easily.

It is a side view showing a drill of the present invention concerning an embodiment. 2 is a graph showing a composition distribution obtained by performing a line analysis on the cross section of Example A with EDX.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  The drill of the present invention is a drill with a predetermined small diameter composed of a WC-Co based cemented carbide. For example, as shown in FIG. 1, the drill 1 includes a blade groove portion 10 on the distal end side of the shank portion 12. The blade groove portion 10 is a portion having a spiral flute. A Co reducing layer is formed on the surface side of the blade groove portion 10, and an inner peripheral portion is provided on the inner side. Then, it is a feature of the drill of the present invention that the ratio of the Co content in the Co reducing layer and the inner peripheral portion and the thickness of the Co reducing layer are specified. Hereinafter, the constituent requirements of the drill of the present invention will be described in more detail, and the manufacturing method of the drill will also be described.

[drill]
<Type of drill>
<Blade diameter>
The blade diameter D (see FIG. 1) of the drill of the present invention is 0.5 mm or less. That is, a drill generally called a micro drill is an object of the present invention. In such a small-diameter drill, even if the Co-reducing layer described later is extremely thin, the presence of the Co-reducing layer greatly affects the cutting characteristics of the drill. For this reason, in such a small-diameter drill, by providing a predetermined Co-reducing layer, the wear resistance can be improved, and further, it is effective for suppressing chipping. The diameter of the drill may be, for example, 300 μm or less. In particular, it is considered that the effect of improving the wear resistance is high in a drill having a blade diameter of 100 μm or less in which the presence of the Co-reducing layer greatly affects the cutting characteristics.

《Blade shape》
The cutting edge shape of the drill is not particularly limited, and may be either a straight type or an undercut type. FIG. 1 shows an undercut type drill.

<Co reduction layer and inner circumference>
《Co content ratio》
When the Co content in the Co reduced layer is Co (o) (mass%) and the Co content in the inner periphery is Co (i) (mass%), the ratio Co (o) / Co (i) is 50% or more and less than 80%. By setting this ratio Co (o) / Co (i) to 50% or more, it is possible to avoid excessive Co detachment in the Co-reduced layer, and to appropriately retain WC in the binder phase containing Co, thereby improving resistance. An effect of improving wearability can be obtained. Further, by making this ratio Co (o) / Co (i) less than 80%, it is possible to expose a part of the WC particles of the hard phase from the binder phase mainly containing Co, and to improve the machinability. it can. In addition, since a relatively small amount of Co is present on the outermost surface of the drill, wear resistance can be improved and uneven wear of the drill can be suppressed. A more preferable lower limit of Co (o) / Co (i) is 60% or more.

  The measurement of the Co content in the Co reduced layer and the inner periphery can be performed by EDX surface analysis. More specifically, the Co content of the Co reduced layer may be determined by performing EDX analysis by irradiating an electron beam from the outer peripheral surface of the drill, and obtaining the Co content of the Co reduced layer from the result. In addition, the Co content of the inner peripheral portion may be obtained by forming a cross section of the drill, irradiating the central portion of the drill in this cross section with an electron beam, and determining the Co content of the inner peripheral portion from the analysis result. . The Co content in the inner peripheral portion may be obtained from any position as long as it is a place other than the Co reducing layer.

《Co reduction layer thickness》
The thickness of the Co reducing layer is less than 10 μm. By using such a thin Co-reducing layer, it is possible to suppress the falling of the hard phase particles accompanying the wear of Co during cutting, to improve the wear resistance of the drill and to ensure the toughness of the drill. A more preferable thickness of the Co reducing layer is 5 μm or less, and a more preferable thickness of the Co reducing layer is 3 μm or less.

The thickness of the Co reduced layer is estimated using the analysis result by EDX. In general, in the analysis by EDX, the penetration depth of the electron beam into the sample is about 0.5 to 3.0 μm although it depends on the acceleration voltage. Moreover, as shown in the Example mentioned later, when the cross-section of the drill was subjected to line analysis in the radial direction by EDX, no significant difference in Co content was observed between the surface side and the inner peripheral side of the drill. On the other hand, there was clearly a difference in Co content between the surface analysis in which the electron beam was irradiated from the outer peripheral surface of the drill and the surface analysis in which the electron beam was irradiated to the center of the drill cross section. Presuming from these results, the thickness of the Co-reducing layer is at most 10 μm, probably 5 μm or less, and further 3 μm or less.

  The thickness of the Co reducing layer may be less than 10 μm regardless of the blade diameter as long as it is a drill having the above-described blade diameter. However, if the blade diameter is reduced, it is considered that the effect of the Co reducing layer on the cutting characteristics of the drill is increased. Therefore, when the blade diameter is small, the thickness of the Co reducing layer may be reduced.

《Co reduction layer formation region》
The region where the Co reduction layer is formed is at least the entire blade groove. That is, the Co reducing layer is formed on the entire circumference from the root side (shank side) of the blade groove to the chisel edge at the tip. This blade groove part is a part directly involved in cutting, and the wear resistance of the drill can be improved by providing a Co reducing layer in the blade groove part. Note that the formation of the Co reducing layer in the shank portion does not directly affect the cutting characteristics, but the Co reducing layer may be formed on the surface side of the shank portion. As will be described later, considering the industrial productivity of the drill, the formation of the Co reduced layer is performed by immersing the entire drill material including not only the blade groove part but also the shank part in acid. In particular, a Co reducing layer is also formed in the shank portion.

《Composition distribution of inner circumference》
The composition distribution of the inner peripheral part is usually substantially uniform over the radial direction of the drill. That is, when surface analysis is performed by EDX on a plurality of locations arranged in the radial direction of the cross section of the drill, no significant difference is found in the composition at each analysis location. In other words, only the Co reduced layer has a relatively low Co content, and the portions other than the Co reduced layer have a substantially uniform composition. By providing such an inner peripheral portion, it is considered that the strength of the entire drill is stabilized.

<Composition of drill material>
The constituent material of this drill is a WC-Co based cemented carbide. For example, a drill is made of a cemented carbide made of a predetermined amount of Co and the balance of WC and inevitable impurities. Further, it may contain other additive elements such as Cr and V.

《WC》
WC occupies the constituent materials in the cemented carbide other than Co described below (the sum of Co and additive elements when there is an additive element). The average particle diameter of the WC is preferably 0.5 μm or more and 1.5 μm or less. If the average particle size is too small, less than 0.5 μm, the holding power of WC against Co is small, and the effect of improving wear resistance is small due to WC falling off during cutting. On the other hand, if the average particle size exceeds 1.5 μm, the effect of improving the wear resistance by using the fine hard phase is still small, and coarse WC may be the starting point of breakage. A more preferable average particle diameter of WC is 0.7 μm or more and 1.2 μm or less.

《Co》
The Co content is preferably 5 to 12% by mass. Containing 5% by mass or more of Co is effective in increasing drill toughness and suppressing chipping and breakage. Further, by containing Co of 12% by mass or less, a WC content can be relatively increased, and a drill having excellent wear resistance can be obtained. A more preferable Co content is 6.0% by mass or more and 8.0% by mass or less.

《Cr》
The cemented carbide constituting the drill preferably contains Cr. By containing a predetermined amount of Cr in the cemented carbide, it is possible to impart moderate corrosion resistance to the cemented carbide and facilitate the formation of the Co reduced layer. As will be described later, the Co reduced layer is formed by immersing a cemented carbide drill material in an acid. At this time, by adding appropriate corrosion resistance to the cemented carbide by adding Cr, excessive corrosion of the binder phase by the acid can be suppressed, and the Co-reducing layer defined by the present invention can be easily formed. Cr also effectively suppresses the growth of WC grains, reduces the generation of coarse WC, and stabilizes the manufacture of cemented carbides in which fine and uniform WC is uniformly dispersed. Contribute.

The Cr content is preferably such that the ratio Cr / Co of Cr content (mass%) to Co content (mass%) in the cemented carbide is 4% or more and less than 7%. If this ratio is less than 4%, the effect of improving the corrosion resistance of the cemented carbide is small. Conversely, if it exceeds 7%, the corrosion resistance is high, so the ratio Co (o) / Co (i) is within the appropriate range of Co. It is difficult to form a reduction layer. Further, if the ratio Cr / Co is 4% or more and less than 7%, the effect of suppressing grain growth of WC can be obtained. Note that Cr can be contained in the cemented carbide by using, for example, Cr ₃ C ₂ powder as a raw material powder. And as content with respect to the cemented carbide of Cr, about 0.3-0.8 mass% is suitable.

《V》
The cemented carbide constituting the drill of the present invention may further contain V in addition to Cr. V, like Cr, is highly effective in suppressing grain growth of WC, and by containing both Cr and V, grain growth of WC can be more effectively suppressed. The V content is preferably such that the total content of Cr and V in the cemented carbide is about 0.3 to 0.8 mass%. Below the lower limit, it is difficult to impart adequate corrosion resistance and grain growth suppressing effect to the cemented carbide, and it is difficult to form a Co-reducing layer with a ratio Co (o) / Co (i) in an appropriate range. On the other hand, if the upper limit is exceeded, V tends to precipitate as carbide, the hardness of the cemented carbide increases, and chipping tends to occur. V can be contained in the cemented carbide by using, for example, VC powder as a raw material powder. The content of the VC powder in the raw material powder or the content of V in the cemented carbide is preferably about 0 (including 0) to 0.3% by mass.

[Drill manufacturing method]
The manufacturing method of this drill has a preparation process of the drill raw material which has a blade groove part, and an immersion process which immerses this raw material in an acid and forms a Co reduction layer.

<Preparation of material>
Preparation of the material should just prepare the material of the drill which has a blade groove part. When producing this material, it is usually manufactured by the steps of raw material preparation → mixing and crushing of raw materials → drying → molding → sintering → processing.

<Preparation of raw material powder>
As the raw material powder, it is preferable to use a fine powder so that the particle diameter after sintering becomes a predetermined fine particle. WC is preferably WC powder having an average particle size smaller than the average particle size of WC in the sintered body. For example, WC powder having an average particle size of about 0.2 μm to 1.0 μm can be suitably used. Co is preferably used as fine as WC powder so that Co can be easily mixed with WC powder. Specifically, Co powder having an average particle size of 0.2 μm or more and 0.6 μm or less is preferable. If it is less than 0.2 μm, Co is easy to re-aggregate because Co is too small, and Co is not uniformly dispersed. As a result, sinterability decreases and sintering temperature increases due to decrease in sinterability. It is difficult to obtain a uniform particle size distribution by promoting grain growth. If it exceeds 0.6 μm, it is difficult to uniformly mix with the fine WC powder, and this causes a non-uniform presence of Co. In addition, Cr or V may be Cr ₃ C ₂ or VC powder as raw material powder.

<Mixing and grinding of raw material powder>
For mixing and crushing the raw material powder, mixing and crushing are performed by a crushing and dispersing machine having rotating blades such as an attritor, a ball mill, and a bead mill. In general, when the entire mixing and pulverization process is performed at high speed, Co agglomerates, the dispersion state becomes worse, and WC tends to grow, which may lead to uneven structure. On the other hand, if the mixing and pulverization process is performed at a low speed, the pulverization and mixing may be insufficient and the structure may become non-uniform.

<< Drying / Molding / Sintering / Processing >>
Drying, molding, sintering / processing, etc. can use conditions comparable to general conditions. For example, the molding is performed by pressurizing the dried raw material powder to form a round bar having a predetermined outer diameter. Examples of the sintering conditions include sintering temperature: 1360 to 1450 ° C., sintering time: 30 minutes or more and 120 minutes or less, vacuum sintering, Ar atmosphere sintering, or CO atmosphere sintering. It is preferable to perform HIP after this sintering. By performing HIP, the fine nests (pores) remaining in the sintered cemented carbide can be eliminated, and a dense cemented carbide can be obtained. Further, the obtained sintered material is processed into a drill shape by processing.

<Immersion of material in acid>
A predetermined Co reduction layer is formed by immersing the drill material in the acid. Adjustment of the Co content in the Co-reducing layer can be realized mainly by adjusting “acid type”, “acid temperature”, and “immersion time”. In addition, as described above, the adjustment of the Co content may include changing the Cr content.

<Types of acids>
As the acid for forming the Co reducing layer, weak acids such as tartaric acid, citric acid and glutaric acid are suitable. These are weak acids and can suppress excessive removal of Co from the cemented carbide. The index of acid strength is pH, but preferred pH of the acid used in the production method of the present invention is about 4-6. If the pH is lower than this lower limit, Co tends to be excessively removed. If the pH is higher than the upper limit, a Co reduced layer with an appropriate Co content cannot be formed or a Co reduced layer is formed. It is easy to cause troubles such as taking time.

《Acid temperature》
The acid temperature generally tends to promote the reduction of Co from the cemented carbide as the temperature increases. For simplicity, it is possible to immerse the material in an acid at room temperature, but considering the productivity, it is preferable to heat the acid to some extent. For example, use of an acid heated to about 40 to 45 ° C can be mentioned.

《Immersion time》
In general, the time for immersing the drill material in the acid tends to promote the reduction of Co from the cemented carbide as the length increases. Considering the productivity of the drill, the immersion time is preferably about 5 minutes or less, and more preferably 3 minutes or less.

《Immersion range》
The immersion range of the drill material is at least the region of the blade groove. By immersing the blade groove in an acid, a Co reduced layer can be formed over the entire surface. Of course, the entire drill material may be immersed in an acid, and the Co reducing layer may be formed on the entire surface of the drill including the shank portion. In consideration of the productivity of the drill, it is easier to immerse the whole in the acid than to immerse only a specific portion of the material in the acid.

<Others>
In addition, washing with water can be mentioned as a subsequent process of the dipping process. By washing with water, the acid attached to the material is removed. Thereby, further removal of Co is suppressed.

[Test Example 1]
A predetermined raw material powder was prepared to produce a drill material made of cemented carbide, and the obtained material was immersed in an acid under different conditions to produce a plurality of types of micro drills. And while analyzing the composition in the surface and inside of this drill, the drilling test was done with this drill and cutting evaluation was performed.

<Production conditions for drill material>
Co powder: 6% by mass, and the balance is substantially made of WC powder. If necessary, raw powder containing Cr ₃ C ₂ powder: 0.3-0.7% by mass, VC powder: 0.15% by mass is obtained with an attritor After mixing for 10 to 20 hours, the obtained mixed powder was dried and granulated with a spray dryer to prepare a granulated powder. When Cr ₃ C ₂ powder and VC powder were used, the content of each powder was such that the total content of Cr and V in the cemented carbide after sintering was 0.3 to 0.8 mass%. The obtained granulated powder was put into a mold and pressed to form a round bar-shaped molded body. The obtained molded body is placed in a sintering furnace and sintered in a vacuum at 1360 to 1450 ° C. for 2 hours. Further, the obtained sintered body was subjected to HIP in an Ar atmosphere at 1400 ° C. and 10 atm (about 10 MPa) to obtain a cemented carbide round bar. Then, a predetermined flute processing was performed on this round bar, and a drill material having a blade diameter of 0.2 mm and a shank diameter of 3 mm was manufactured.

<Pickling conditions>
Subsequently, the entire material is immersed in an acid to form a Co reduced layer in which Co is reduced on the surface side of the material. Specific pickling conditions are as shown in Table 1. The acid temperature was 45 ° C. for all.

<Composition analysis conditions>
The composition analysis of the obtained drill is a surface analysis by EDX. Specifically, the outer peripheral surface of the drill is irradiated with an electron beam, the composition of the irradiated region is analyzed, and further, the central portion (inner peripheral portion) of the cross section of the drill is irradiated with the electron beam. Analyze the composition. From the former, the Co content (Co (o)) of the Co-reducing layer can be detected, and from the latter, the Co content (Co (i)) in the inner periphery can be detected. Then, the ratio Co (o) / Co (i) between them was obtained by calculation. In addition, the content of Co in the entire drill (content of Co powder as raw material powder) and the content of Cr in the entire drill (content of Cr obtained from the content of Cr ₃ C ₂ powder as raw material powder) From this, the ratio of both contents (Cr / Co) was also determined. In addition, for reference, composition analysis was performed by linear analysis in the radial direction on the cross section of the drill.

<Drilling test conditions>
Work material: CCL (Copper clad laminate) 0.4mm thickness x 5 Rotation speed: 150000r.pm
Feed rate f: 5μm / rev.
Cutting oil: Not used (dry type)
Number of drilled holes: 3000 Evaluation criteria Wear area ratio: An end face image was acquired from the tip side of the drill. From this image, the area disappeared from the contour of the drill before the drilling test started, and the surface that remained after the drilling test The total of the area where wear is observed in the part is defined as the wear area, and the ratio of the wear area to the entire area of the end face is obtained.
Chipping: Check for chipping at the blade groove.

  Table 1 shows the drill conditions and Table 2 shows the drilling test results.

  As shown in Table 1, Examples A to C are pickling with tartaric acid, and the ratio Co (o) / Co (i) in the Co reduced layer is 50% or more and less than 80% regardless of the immersion time. Is in the range. In addition, the penetration depth of the electron beam into the sample in EDX is about 3 μm, and as shown in FIG. 2, in the line analysis for Example A, the Co content of the drill is close to the inner periphery and the outer periphery of the drill. Considering that no significant difference was observed, and that Examples B and C and Comparative Example 3 also had similar tendency graphs, the thickness of the Co-reduced layer was estimated to be less than 5 μm. Is done. In FIG. 2, the horizontal axis indicates the radial distance from the center of the drill, the vertical axis indicates the intensity of the detected characteristic X-ray, and the location where the tungsten waveform is greatly reduced near the right end of the graph is the material. The surface. From this graph, it is difficult to say that the Co content is significantly reduced in the region of 5 μm inside from the surface of the material compared to other regions.

  On the other hand, in Comparative Example 1 in which pickling with nitric acid was performed, the ratio Co (o) / Co (i) in the Co-reducing layer was small although the immersion time was shorter than in each Example. This is presumably because Co on the surface of the material is excessively removed by the strong acid. In Comparative Example 1, the Co reducing layer is also thick. The thickness of the Co-reducing layer is determined from the fact that significant displacement points were found in the Co content in the vicinity of the inner and outer peripheral surfaces of the drill as a result of line analysis. Furthermore, since the thickness of the Co reduction layer varied greatly, it can be seen that it is difficult to obtain a Co reduction layer with a stable Co reduction amount by pickling with nitric acid.

  Further, when Examples A to C and Comparative Example 3 are compared, the ratio Co (o) / Co (i) is made 50% or more and less than 80% when the ratio Cr / Co is about 4.0 to less than 7.0%. I understand that. This is presumably because the addition of Cr improved the corrosion resistance of the cemented carbide and reduced the amount of Co reduction when immersed in acid. For example, in Comparative Example 2, there is no difference in the Co content by linear analysis between the inner periphery and the outer periphery of the drill, and the Co reduction layer is thin, but the ratio Cr / Co is as low as 2.5%. The ratio Co (o) / Co (i) is also less than 50%.

  From the drilling test results of these samples, it can be seen that each example can be drilled without breakage, the wear area ratio is low, and no chipping occurs. On the other hand, Comparative Examples 1 and 2 having a small ratio Co (o) / Co (i) were broken during the test, and Comparative Example 3 having a large ratio Co (o) / Co (i) was found to have chipping.

  In addition, when the average particle diameter of WC was determined from the cross section of the drill of each sample, Examples A to C were all 0.5 μm or more and 1.5 μm or less. The average particle diameter was obtained by observing the polished cross section at a magnification of about 5000 times using an EBSD (Electron Back-Scatter Diffraction) method by FESEM (Field Emission Scanning Electron Microscope). More specifically, a plurality of arbitrary visual fields (for example, three visual fields) are selected with respect to the polished surface, and all the WC crystal grains existing in each visual field are classified by color (mapping) for each crystal orientation. To make it possible to visually grasp the crystal grain size. Image analysis is performed on the obtained mapping image, and the equivalent circle diameter of each particle area is obtained for all WCs present in a plurality of visual fields, and the equivalent circle diameter is defined as the WC particle diameter. Average the WC grain size to the average grain size of the cemented carbide.

[Test Example 2]
Next, a drill similar to that of Test Example 1 except for the composition of the cemented carbide was prepared to determine the ratio (Cr / Co) and ratio Co (o) / Co (i), and a similar cutting test was performed. The wear area ratio was determined. The method for obtaining each ratio is the same as in Test Example 1. In all samples, the average particle diameter of WC was 0.5 μm or more and 1.5 μm or less, and the pickling condition was pH 5 and immersed in tartaric acid at 45 ° C. for 1 minute. The thickness of the Co reducing layer of each sample is considered to be less than 5 μm. Table 3 shows the composition and test results of each drill.

  As is apparent from Table 3, when the ratio Co (o) / Co (i) is 50% or more and less than 80%, no breakage occurs and the wear area ratio is small. Further, the ratio Cr / Co is preferably 4% or more and less than 7%, particularly preferably 4% or more and less than 6%. When this ratio is less than 4%, wear on the drill surface is large and breakage tends to occur. On the contrary, when this ratio is 7% or more, the wear area ratio of the cutting edge of the drill tends to increase. Furthermore, it can be said that the total content of Cr and V in the cemented carbide after sintering is preferably 0.3 to 0.8% by mass, particularly preferably 0.3% by mass or more and less than 0.6% by mass. In addition, similar results can be expected for weak acids such as citric acid and glutaric acid.

  The above-described embodiment can be appropriately changed without departing from the gist of the present invention, and is not limited to the above-described configuration. For example, the composition of the cemented carbide, the average particle diameter of the raw material powder, and the like can be changed as appropriate, and a coating layer such as various ceramics may be formed on the surface of the drill.

  The drill of the present invention can be suitably used for drilling a printed circuit board.

1 Drill
10 Blade groove
12 Shank

Claims (6)

A drill made of a cemented carbide mainly composed of WC and Co,
Have a blade diameter of 0.5 mm or less,
A Co reducing layer is provided on the surface side of at least the entire blade groove portion of the drill, and an inner peripheral portion is provided on the inside thereof.
When the Co content in this Co-reduced layer is Co (o) (mass%) and the Co content in the inner periphery is Co (i) (mass%), the ratio of both Co (o) / Co (i) Is 64% or more and less than 80%,
A drill characterized in that the Co reducing layer has a thickness of 3 μm or less (excluding 0 μm) .   2. The drill according to claim 1, wherein a composition distribution in a drill radial direction of the inner peripheral portion is substantially uniform. The cemented carbide contains Cr,
3. The drill according to claim 1, wherein a ratio Cr / Co of Cr content (mass%) to Co content (mass%) in the cemented carbide is 4% or more and less than 7%.   The drill according to any one of claims 1 to 3, wherein an average particle diameter of WC in the cemented carbide is 0.5 µm or more and 1.5 µm or less. A process of preparing a drill material made of a cemented carbide mainly composed of WC and Co and having a blade diameter of 0.5 mm or less;
Immersing at least the entire blade groove portion of this material in an acid, and forming a Co reducing layer on the surface side of the blade groove portion,
The Co reducing layer is
When the Co content in the Co reduced layer is Co (o) (mass%) and the Co content in the inner peripheral part inside the Co reduced layer is Co (i) (mass%), the ratio of both Co (o ) / Co (i) is 64% or more and less than 80%,
A method for manufacturing a drill, characterized in that the thickness is 3 μm or less (excluding 0 μm) .   6. The drill manufacturing method according to claim 5, wherein the acid is a weak acid having a pH of 4 to 6.

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