EP4129540A1 - Cutting tool made of wc-based cemented carbide - Google Patents

Abstract

A cutting tool made of WC-based cemented carbide and having a cutting edge comprises 8.0 to 14.0 mass% Co; 0.1 to 1.4 mass% Cr₃C₂; and 0.6 to 4.0 mass% at least one selected from the group consisting of TaC, NbC, TiC, and ZrC; the balance being WC and incidental impurities, wherein the following expressions hold: R=L1/L1+L2;R≥0.76−0.059×D×10/V−Vγ×0.06; and 1.0≤D≤4.0,where L1 is a total interfacial length between WC grains,L2 is a total interfacial length between the WC grains and binder phases and between the WC grains and γ phases,V is an area rate (%) of the binder phases;D is a mean diameter (µm) of the WC grains;Vγ is a theoretical volume rate of the γ phases; andR is a WC-WC interfacial length ratio.

Description

[Technical Field]
• The present invention relates to a cutting tool made of WC-based cemented carbide (hereinafter, also referred to as WC-based cemented carbide tool). This application claims priority benefit of Japanese Patent Application No. 2020-56624 filed on March 26, 2020 . The entire contents of the Japanese application are hereby incorporated by reference herein.
[Background Art]
• WC-based cemented carbide has high hardness and toughness, and WC-based cemented carbide tools including tool substrates made of the cemented carbide exhibit high wear resistance. Cutting tools accordingly have long service lives over long periods of use.
• Nevertheless, in recent years, various proposals have been made to further improve the cutting performance and the lives of WC-based cemented carbide tools, depending on the type of work material and cutting conditions.
• For example, PTL 1 discloses a cutting tool made of WC-based cemented carbide that satisfies the relation B/A ≤ 0.05 where A is the number of all WC grains and B is the number of WC grains having the number 1 or less of contact points with other WC grains. This cutting tool has improved plastic deformation resistance and long life in continuous wet cutting of carbon steel and stainless steel.
• PTL 2 discloses a cutting tool made of WC-based cemented carbide comprising 10-13 mass% Co, Cr in a content corresponding to 2 to 8% of the Co content, 0.2-0.5 mass% TaC and/or NbC, and the balance being WC, wherein the cutting tool has a hardness of 88.6 to 89.5 HRA, the D80/D20 ratio ranges from 2.0 to 4.0 where D80 is a cumulative 80% WC grain size and D20 is a cumulative 20% WC grain size on an area ratio on a polished surface, D80 ranges from 4.0 to 7.0 µm, and a WC adhesive ranges from 0.36 to 0.43. This cutting tool can prevent adhesion of work material and exhibits improved chipping resistance in cutting of difficult-to-cut materials, such as stainless steel.
• PTL 3 discloses a cutting tool made of WC-based cemented carbide that satisfies the relations: $$\mathrm{R}>\left(0.82-0.086\times \mathrm{D}\right)\times \left(10/\mathrm{V}\right)$$

  

  $$\mathrm{R}=\left(\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}1\right)/\left(\left(\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}1\right)+\left(\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}2\right)\right)$$

  

  $$0.6\le \mathrm{D}\le 1.5$$

  

  $$9\le \mathrm{V}\le 14$$

  

  where L1 is the total length of the WC-WC adhesive interfaces, L2 is the total length of the WC-Co adhesive interfaces, D is a grain diameter (µm) of WC at an area rate of 50% of WC, and V is a volume % of the binder phases. This tool exhibits improved thermoplastic deformation resistance and toughness in the cutting of Ni-based heat-resistant alloys.
[Citation List] [Patent Literature]
• [PTL 1] Japanese Patent No. 6256415
• [PTL 2] Japanese Unexamined Patent Publication No. 2017-88999
• [PTL 3] Japanese Unexamined Patent Publication No. 2017-179433
[Summary of Invention] [Technical Problem]
• An object of the present invention, which has been accomplished in view of the circumstances and the aforementioned proposal, is to provide a cutting tool made of WC-based cemented carbide that exhibits excellent cutting performance over a long period of use, especially in the cutting of stainless steel.
[Solution to Problem]
• A cutting tool made of WC-based cemented carbide according to the present invention comprises:
1. (1) 8.0 to 14.0 mass% Co;
   • 0.1 to 1.4 mass% Cr₃C₂; and
   • 0.6 to 4.0 mass% at least one carbide selected from the group consisting of TaC, NbC, TiC, and ZrC;
   • the balance being WC and incidental impurities, wherein
2. (2) the following expressions hold: $$\mathrm{R}=\left(\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}1\right)/\left(\left(\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}1\right)+\left(\mathrm{<figure-callout\; id="2"\; label="L"\; filenames="imgf0001.png"\; state="\{\{state\}\}">L</figure-callout>}2\right)\right);$$
   
   $$\mathrm{R}\ge \left(0.76-0.059\times \mathrm{D}\right)\times \left(10/\mathrm{V}\right)-\mathrm{V}\mathrm{\gamma }\times 0.06;$$
   
   and $$1.0\le \mathrm{D}\le 4.0,$$
   
   • where L1 is a total interfacial length between WC grains,
   • L2 is a total interfacial length between the WC grains and binder phases and between the WC grains and γ phases,
   • V is an area rate (%) of the binder phases;
   • D is a mean diameter (µm) of the WC grains;
   • Vγ is a theoretical volume rate of the γ phases; and
   • R is a WC-WC interfacial length ratio.
• The cutting tool made of WC-based cemented carbide according to the present invention may satisfy either or both the following conditions (1) and (2):
1. (1) The γ phases have an average grain size in the range of 0.2 µm to 4.0 µm; and
2. (2) A cutting edge has a coating layer.
[Advantageous Effects of Invention]
• According to the above configuration, the cutting tool made of WC-based cemented carbide exhibits excellent high temperature hardness, plastic deformation resistance, and chipping resistance in machining of stainless steel and other materials.
[Brief Description of Drawings]
• Figure 1 is a schematic diagram showing an example structure of a cutting tool made of WC-based cemented carbide according to an embodiment.
• Figure 2 is a schematic diagram of the side of the flank showing an example measurement of the amount of plastic deformation of the flank of the cutting edge.
[Description of Embodiment]
• The present inventors have extensively investigated cutting tools made of WC-based cemented carbide described in PTLs 1 to 3 and have found the following facts:
1. (1) In the cutting tool made of WC-based cemented carbide proposed in PTL 1, the number of contact points between WC-WC grains is controlled to improve the plastic deformation resistance of the cutting tool made of WC-based cemented carbide. However, the plastic deformation resistance is not sufficient.
2. (2) In the cutting tool made of WC-based cemented carbide proposed in PTL 2, the distribution of WC grain size is controlled to improve the fracture resistance and the adhesive resistance of the cutting tool made of WC-based cemented carbide. Since TaC and NbC are contained only to extents that these carbides are solid-soluble in the binder phases, the WC-WC grains don't be adhered by the γ-phases comprising these carbides and WC-WC grain boundary sliding don't be suppressed, resulting in insufficient high-temperature hardness and a short tool life in turning operations of stainless steel, for example.
3. (3) In the cutting tool made of WC-based cemented carbide proposed in PTL 3, the adhesive interfacial ratio between WC-WC grains is controlled to improve the plastic deformation resistance. The γ-phase component is not contained, the high-temperature hardness is insufficient, and the focus is only on increasing the interfacial length ratio between WC-WC grains; hence, the γ-phases may be insufficiently effective in bonding between WC-WC grains and suppressing grain boundary sliding of WC-WC grains, and plastic deformation resistance may be insufficient in cutting operations of stainless steel, for example.
• The inventors have made a series of intensive investigations based on these facts, and have finally found the following findings (1) to (4) on a cutting tool made of WC-based cemented carbide:
1. (1) At an increased ratio of the contact length (hereinafter referred to as "WC-WC interfacial length") at the interface between WC grains with high Young's modulus (hereinafter referred to as "WC-WC interface") in the presence of a predetermined amount of binder phases without a decrease in Co content that forms the binder phases, the WC grains act as a skeleton that prevents plastic deformation, and an appropriate amount and size of γ-phases is included in a cutting tool made of WC-based cemented carbide to reduce grain boundary sliding at the WC-WC interfaces and to improve the plastic deformation resistance of a cutting tool made of WC-based cemented carbide;
2. (2) An average size of the WC grains greater than a predetermined value reduces the climbing motion of the edge dislocations in the binder phases, and Cr₃C₂ included in the binder phases leads to solid solution strengthening of the binder phases and thus a further improvement in the plastic deformation resistance of a cutting tool made of WC-based cemented carbide;
3. (3) One or more of the γ-phase forming elements Ta, Nb, Ti, and Zr included in the binder phases lead to excellent oxidation resistance and high temperature hardness; and
4. (4) In the case that a cutting tool made of WC-based cemented carbide satisfying findings (1) to (3) is used for cutting of difficult-to-cut materials such as stainless steel, the plastic deformation resistance and high-temperature hardness of the cutting tool has been improved, which prevents abnormal damage such as chipping caused by deformation of the cutting edge of the tool and plastic deformation of the cutting edge of the tool, and the presence of γ phases improves oxidation resistance and high-temperature hardness, which extends the service life of the cutting tool.
• The inventors have discovered based on these findings that the aforementioned problems can be solved in the case that the WC-WC interfacial length ratio R of the "WC-WC interfacial length" to the sum of the "WC-WC interfacial lengths" and "the interfacial length of the WC-(binder phases and γ phases) (hereafter, "binder phases + γ phases") in a cutting tool made of WC-based cemented carbide has a specific relation between the area rate of the binder phases, the average WC grain diameter, and the theoretical volume rate of the γ phases.
• A cutting tool made of WC-based cemented carbide according to embodiments of the present invention will now be described.
• Throughout the specification and the claims, when a numerical range is expressed as "L to M" (L and M are both numerical values), the range includes the upper limit (M) and the lower limit (L). The units for the upper (M) and lower (L) limits are the same. Each numerical value has an error of measurement.
1. Content of each component
• The content of each component will now be described.
(1) Co
• Co is a primary constituent of the binder phase. A Co content less than 8.0 mass% leads to insufficient toughness of a cutting tool made of WC-based cemented carbide. A Co content exceeding 14.0 mass% leads to softening of the cutting tool, resulting undesirable hardness for cutting tools and thus pronounced deformation and wear. In conclusion, the Co content in WC-based cemented carbide cutting tools should preferably be 8.0 to 14.0 mass%, more preferably be 8.5 to 12.0 mass%.
• The binder phase may contain W, C, and other incidental impurities. The binder phase may further contain Cr and at least one of the metal elements Ta, Nb, Ti, and Zr constituting the γ phases. These elements may be present in the form of solid solution in Co.
• Co has two crystal structures, fcc and hcp structures. The binder phase containing W, C, Cr, and at least one of the metal elements Ta, Nb, Ti, and Zr that constitute the γ-phase also has fcc and hcp structures, like the main binder phase forming component Co. Thus, the binder phase regions with these two crystalline structures in a cutting tool made of WC-based cemented carbide are called binder phases (fcc) and binder phases (hcp), respectively.
(2) Cr₃C₂
• The content of Cr₃C₂ is preferably in a range of 0.1 to 1.4 mass%. Cr₃C₂ is dissolved in the form of elemental Cr in Co that primarily forms binding phase to solid solution strengthening Co and thus to enhance the strength of a cutting tool made of WC-based cemented carbide. A Cr₃C₂ content less than 0.1 mass% leads to insufficient solid-solution strengthen, whereas a Cr₃C₂ content exceeding 10% relative to the Co content may cause Cr in Cr₃C₂ to deposit as a complex carbide with W, resulting in a decrease in toughness and generation of start points of defects. In view of the upper limit of the Co content 14.0 mass%, the upper limit of the Cr₃C₂ content is determined to be 1.4 mass%, which corresponds to 10% of the upper limit of the Co content.
(3) TaC, NbC, TiC, and ZrC
• At least one of the carbides TaC, NbC, TiC, and ZrC should preferably be contained in a total content of 0.6 to 4.0 mass%. These components also constitute the γ-phases.
• Parts of these metal elements are dissolved in Co, which forms primary binder phases to increase high-temperature hardness of the binder phases. The residuals, which are not dissolved in the binder phases, of these metal elements are also present in the form of γ phases being carbide phases (that may further contain W besides the metal elements) to enhance oxidation resistance and resistance to crater wear and to adhere on the WC-WC interfaces to reduce grain boundary sliding at the WC-WC interfaces.
• A total content less than 0.6 mass% of these metal elements on the basis of carbides (1:1 compounds of metal elements and carbon) leads to insufficient effects as described above. A total content exceeding 4.0 mass% leads to ready formation of agglomerates. Accordingly, the total content is preferably in a range of 0.6 to 4.0 mass%.
• The average grain size of the γ-phase should more preferably be in a range of 0.2 to 4.0 µm to maintain a proper frequency of contact with WC grains. The average grain size of the γ-phases is calculated by mirror-polishing any surface or cross-section of a cutting tool made of WC-based cemented carbide, observing the polished surface with a scanning electron microscope (SEM), determining the area of at least 300 γ-phases by image analysis, calculating the diameter of a circle equal to that area, and taking the average of the calculated diameter. The mirror polishing can be performed, for example, with a focused ion beam (FIB) system, or a cross-section polisher system (CP system).
• The Cr₃C₂, TaC, NbC, TiC, and ZrC contents described above are determined as follows: The amounts of Cr, Ta, Nb, Ti, and Zr of a cutting tool made of WC-based cemented carbide are determined with an electron beam microanalyzer (EPMA) and then are converted into the amounts of corresponding carbides.
(4) WC
• WC is the remaining component and may contain unavoidable impurities that are unavoidably mixed during a manufacturing process.
• The WC content is determined assuming that W and C are bonded at 1:1.
(5) Incidental impurities
• As mentioned above, the remaining component and the binder phase may contain impurities unavoidably mixed during a manufacturing process, the outside amount of which should preferably be 0.3mass% or less of the total 100 mass% of a cutting tool made of WC-based cemented carbide.
2. Structure
• The sintered texture of the cutting tool made of WC-based cemented carbide is defined by a WC-WC interfacial length ratio (R), an area average grain diameter (D) (µm) of WC, the area rate (V) of the binder phases, and a theoretical volume rate of the γ phases.
• These parameters will now be described in detail.
(1) WC-WC interfacial length ratio (R)
• A high WC-WC interfacial length ratio (R) allows WC grains having high Young's modules to function as a skeleton that inhibits plastic deformation, resulting in a cutting tool made of WC-based cemented carbide having high resistance to plastic deformation.
• The WC-WC interfacial length ratio (R) is determined by the relation: R = (L1)/((L1+L2) where L1 is a total WC-WC interfacial length and L2 is a total interfacial length of the WC-(binder phase + γ phase). The total WC-WC interfacial length indicates the total interfacial length between adjoining WC grains, and the total interfacial length of the WC-(binder phase + γ phase) indicates the total interfacial length between WC grains and the adjoining binder phases and between WC grains and the adjoining γ phases.
• As mentioned above, the binder phases are composed mainly of Co in which W, C, Cr, and metal elements (Ta, Nb, Ti, and Zr) in the γ-phases are dissolved in the form of solid solution, and the γ-phases are composed of carbides of one or more of the metal elements Ta, Nb, Ti, and Zr.
• A method of measuring the WC-WC interfacial length L1 and the interfacial length L2 of WC-(binder phase + γ phase) will now be described with reference to Fig. 1.
• Any surface or cross-section of the cutting tool made of WC-based cemented carbide is ion-milled, and the milled surface is observed with a scanning electron microscope (SEM) equipped with a backscattered electron diffraction device (EBSD). In an observation field of 24 µm by 72 µm, measuring points (actually regular hexagonal areas) at intervals of 0.1 µm are two dimensionally irradiated with an electron beam. The number of fields of view are determined such that the number of WC grains (1) is 4000 or more. The EBSD pattern obtained by electron beam irradiation is used for identification of the WC grains (1), the binder phases (hcp) (3), the binder phases (fcc) (4), and the γ phases (5).
• Note that the γ-phases (5) having an fcc structure cannot be separated from the binder phases (fcc) (4) and thus are actually identified as binder phases (fcc) (4) + γ-phases (5).
• As shown in Fig. 1, when adjacent WC particles (1) have a misorientation angle of 2° to 180° from each other, the length of the interface is defined as a WC-WC interface (2) length L1,
  the length of the interface where WC (1) grains and binder phase (hcp) (3) are adjacent is defined as a WC-binder phase (hcp) interface (6) length L2-1, and the length of the interface where WC (1) grains and (binder phases (fcc) (4) + γ phases (5)) are adjacent is defined as a WC-(binder phases (fcc) + γ phases) interfacial (7) length L2-2.
• The WC-(binder phases + γ phases) interfacial length L2 is the sum of L2-1 and L2-2.
(2) Mean diameter (D) of WC grains
• WC grains preferably have a mean diameter D (µm) in a range of 1.0 µm to 4.0 µm. WC grains having such a mean diameter D enable the structure of a WC-based cemented carbide sinter to barely form plastic deformation due to climbing motion of edge dislocation at high temperatures. The more preferred mean diameter D (µm) of WC grains ranges from 1.6 µm to 3.0 µm.
• The mean diameter D (µm) of WC grains is determined by ion-milling any surface or cross-section of a cutting tool made of WC-based cemented carbide and by observing the machined surface with a SEM equipped with EBSD, as in the determination of the mean size of the γ phases. In detail, the areas of at least 4,000 individual WC grains in an observation region are measured by image analysis at measuring points every interval of 0.1 µm in two dimensions in a field of view of 24 µm by 72 µm. Individual WC grains are approximated to circles with the same areas, and the diameters and the percentage of area occupied by WC grains with that diameter are calculated. The mean diameter is obtained as the sum of the diameters of individual WC grains multiplied by the percentage of area.
(3) Area rate (V) of binder phase
• The area rate of the binder phase is determined assuming that the area rate of the binder phase in a two-dimensional plane is the same rate in the three-dimensional solid. Multiple fields of view (e.g., three fields of view) are selected on any surface or cross-sectional mirror-polished surface of a cutting tool made of WC-based cemented carbide, and each field of view is observed at 2000 to 3000x magnification with an electrolytic emission scanning electron microscope (FE-SEM) to take a backscattered electron image. The image is binarized by image processing to separate WC particles, γ phases, and binder phases, and the area rate of the binder phases to the entire field of view is determined.
(4) Theoretical volume rate of γ phase (Vγ)
• The theoretical volume rate Vγ of the γ-phase is defined as the sum of quotients by dividing the TaC, NbC, TiC, and ZrC contents (mass%) measured with a EPMA by their densities 14.4, 7.82, 4.92 and 6.66 (Refer to G. V. Samsonov (Author), I. M. Vujnicki (Author), Translation by Japan-Soviet News Agency, "Databook of High Melting Point Compounds" published on December 1977), respectively.
(5) Relation between WC-WC interfacial length ratio (R), area rate of binder phases (V), WC area-averaged grain size (D), and theoretical volume rate of γ phases (Vγ)
• The WC-WC interfacial length ratio (R) and the area rate of the binder phases (V) (%), the average WC area grain size (D) (µm), and the theoretical volume rate of the γ-phases (Vγ) preferably should satisfy the following relation: $$\mathrm{R}\ge \left(0.76-0.059\times \mathrm{D}\right)\times \left(10/\mathrm{V}\right)-\mathrm{V}\mathrm{\gamma }\times 0.06.$$

  
• A tool satisfying this relationship exhibits excellent plastic deformation resistance and chipping resistance.
• R usually has no upper limit. In preferred embodiment, R has an upper limit of 0.70, more preferably 0.60. R below the upper limit enables the micro-chipping of the cutting edge to be more reliably reduced.
3. Production
• The WC-based cemented carbide cutting tool of the present embodiment can be fabricated, for example, by the following steps (1) to (3):
(1) Powdered materials and compounding
• Two WC powders with different particle size distributions (WC powder with a first particle size distribution mode r1 (µm) and WC powder with a second particle size distribution mode r2 (µm) where r1 >r2) are blended into a specified composition and a specified particle size ratio, and a raw powder consisting of Co powder, and a Cr₃C₂ powder and then an additional raw powder containing one or more of the TaC powder, NbC powder, TiC powder and ZrC powder are added.
• The mixture is then mixed under conditions that do not apply a large crushing force, for example, by mixing using an attritor with a reduced amount of media, preferably using media-free mixers such as an ultrasonic homogenizer or a cyclone mixer, to prepare a mixed powder.
• Regarding the blending of the two WC powders, the ratio of the modes of the particle size distribution, i.e., r2/r1, should preferably satisfy 0.15 to 0.60.
(2) Preparation of compact and sinter
• The mixed powder is pressed to form a compact, which is sintered under a vacuum atmosphere at a heating temperature of 1300°C to 1450°C, more preferably 1300°C to 1400°C, and a heating holding time of 15 to 90 minutes, more preferably 15 to 60 minutes. This operation can prevent changes in shape and size distribution of WC grains due to grain growth. The sintering process is followed by a solid phase sintering process for 5 to 100 hours at 1100°C to 1200°C to enhance the binding force between WC grains due to grain boundary diffusion. This solid-phase sintering process may be performed immediately after sintering or once cooling is complete. The atmosphere may be inert gas, reducing gas atmosphere. The operation is preferably performed under vacuum.
(3) Post treatment
• The sinter is machined and ground to produce a cutting tool made of WC-based cemented carbide of desired size and shape.
(4) Formation of coating layer
• A coating layer of carbide, nitride, or carbonitride of Ti-Al or Al-Cr or of Al₂O₃ may be formed on at least the cutting edge of the cutting tool made of WC-based cemented carbide by a deposition process, such as PVD or CVD to prepare a surface-coated cutting tool made of WC-based cemented carbide.
• In the production of the surface-coated cutting tool made of WC-based cemented carbide, the coating layer and the coating process may be these well known to skilled in the art.
[Examples]
• The present invention will now be described by way of Examples, which should not be construed to limit the present invention.
• Cutting tools were prepared by the following procedures:
(1) Raw material powder and compounding
• The following raw powders for sintering were prepared: two WC powders having different particle size distributions (coarse WC powder having a first particle size distribution mode r1 (µm) and fine WC powder having a second particle size distribution mode r2 (µm)), Co powder, Cr₃C₂ powder, TaC powder, NbC powder, TiC powder, and ZrC powder.
• Table 1 shows the compositions (mass%) of several powders, and the particle size distribution modes of the two WC powders and the ratio of the modes. The mean particle diameter (D50) of each of the Co, Cr₃C₂, TaC, NbC, TiC, and ZrC powders was in a range of 1.0 µm to 3.0 µm. The incidental impurities were contained in an outside amount of 0.3 mass% or less to the total mass, i.e., 100 mass% of the cutting tool made of WC-based cemented carbide.
(2) Preparation of compact
• Raw powders for sintering according to the composition shown in Table 1 were wet-mixed in a media-free attritor at 50 rpm for 8 hours, and the mixture was dried, and then was pressed at a pressure of 100 MPa to produce green compacts.
(3) Sintering
• Each green compact was sintered under a vacuum atmosphere at a heating temperature (1340 to 1440°C) for a holding time (0.5 to 1.5 hours) shown in Table 3. The sinter was then subjected to continuous solid-phase sintering at 1100 to 1200°C for 10 to 100 hours to enhance the binding force between WC grains by diffusion at the grain boundaries.
(4) Post treatment
• The sinters were machined and ground to produce cutting tools 1 to 10 shown in Table 5 made of WC-based cemented carbide and having a CNMG120408-GM insert shape (hereafter referred to as "Examples 1 to 10").
• For comparison, cutting tools 11 to 18 made of WC-based cemented carbide shown in Table 6 were also produced (hereinafter referred to as Comparative Example 11 to 18).
• In the manufacturing process, the composition (mass%), mixing conditions, or sintering conditions were modified as shown in Tables 2 and 4. Incidental impurities were contained in an outside amount of 0.3 mass% or less to the total mass, i.e., 100 mass% of the cutting tool made of WC-based cemented carbide.
• The contents of Co, Cr, Ta, Nb, Ti, and Zr were measured by EPMA at 10 points on a cross section of a cutting tool made of WC-based cemented carbide in each of Examples 1 to 10 and Comparative Examples 11 to 18. The average was used as a content of each component. The results are shown in Tables 5 and 6.
• The Cr, Ta, Nb, Ti, and Zr contents are carbide equivalents.
• The cross sections of the cutting tools made of WC-based cemented carbide in Examples 1 to 10 and Comparative Examples 11 to 18 were observed with a SEM with EBSD by the method described above. The WC-WC interfacial length L1 and the WC-(binder phases + γ phases) interfacial length L2 were measured, and the WC-WC interfacial length ratio (R value) was calculated. The WC area-averaged grain size D (µm) was determined. The results are shown in Tables 5 and 6.
• The area rate V (%) of the binder phases was determined as follows: Three fields of view were selected in the cross section of each cutting tool made of WC-based cemented carbide in Examples 1 to 10 and Comparative Examples 11 to 18, one field of view containing 300 or more WC grains per cross section. The observed images were captured with a field emission scanning electron microscope (FE-SEM), were binarized by image processing, and were separated into WC grains, γ phases, and binder phases to calculate the area rate of the binder phases. The results are shown in Tables 5 and 6.
• The grain size of the γ-phases was measured on the longitudinal cross section of each cutting tool made of WC-based cemented carbide in Examples 1 to 10 and Comparative Examples 11 to 18 by the method described above. The results are shown in Tables 5 and 6.
• The theoretical volume rate of the γ-phases (Vγ) was as described above, and the results are shown in Tables 5 and 6. [Table 1]

  Example	Raw material powder	Composition (% by mass) & characteristics
  		Co	Cr3C2	TaC	NbC	TiC	ZrC	WC	r1 (µm)	r2 (µm)	r2/r1
  1	A	14.0	1.4	-	-	0.9	1.5	Balance	3.4	2.0	0.59
  2	B	10.2	0.7	1.0	1.0	1.0	1.0	Balance	3.0	1.0	0.33
  3	C	12.1	1.1	-	0.4	-	0.2	Ba lance	3.5	1.7	0.49
  4	D	9.0	0.5	1.7	-	0.2	-	Balance	6.3	2.5	0.40
  5	E	8.3	0.9	0.2	0.5	0.4	1.2	Ba lance	5.0	1.6	0.32
  6	F	8.5	0.8	1.0	1.4	-	0.4	Balance	5.4	1.3	0.24
  7	G	8.7	0.6	1.2	0.3	0.2	1.6	Balance	3. 2	1.5	0.47
  8	H	9.8	0.4	0.5	-	1.1	-	Balance	4. 5	1. 6	0.36
  9	I	11.4	0.8	0.2	0.2	0.2	-	Ba lance	2.6	1.3	0.50
  10	J	8.0	0.1	-	0.5	-	0.4	Ba lance	6.5	2.2	0.34

  "-": Not compounded. "WC" contains incidental impurities.

  [Table 2]

  Comparative Example	Raw material powder	Composition (% by mass) & characteristics
  		Co	Cr3C2	TaC	NbC	TiC	ZrC	WC	r1 (µm)	r2 (µm)	r2/r1
  11	a	9.1	0.3	1.5	1.6	1.2	1.3	Balance	7.1	1.0	0.14
  12	b	12.0	-	-	2.5	1.4	-	Ba lance	2.5	1.9	0.76
  13	c	10.2	0.2	-	0.1	-	-	Balance	3.2	0.4	0.13
  14	d	9.8	0.4	0.2	0.2	-	-	Ba lance	1.1	0.1	0.09
  15	e	7.8	0.1	1.2	-	0.6	0.2	Ba lance	6.3	0.2	0.03
  16	f	14.3	0.6	1.1	0.5	0.8	-	Balance	1.9	1.5	0.79
  17	g	8.4	1.5	0.6	-	-	-	Ba lance	6.8	4.5	0.66
  18	h	9.0	0.5	1.7	-	0.2	-	Balance	1.5	1.0	0.67

  - : Not compounded. "WC" contains incidental impurities.

  [Table 3]

  Example	Raw material powder	Sintering condition	Vacuum sintering		Solid phase sintering
  			Temp. (°C)	Time (hr)	Temp. (°C)	Time (hr)
  1	A	A	1340	1.0	1200	100
  2	B	B	1340	1.0	1160	50
  3	C	C	1350	1.0	1100	40
  4	D	D	1360	0.9	1130	60
  5	E	E	1440	1.4	1140	70
  6	F	F	1380	0.8	1200	10
  7	G	G	1360	0.5	1150	60
  8	H	H	1400	1.5	1110	45
  9	I	I	1340	1.0	1180	80
  10	J	J	1400	1.0	1190	35

  [Table 4]

  Comparative Example	Raw material powder	Sintering condition	Vacuum sintering		Solid phase sintering
  			Temp. (°C)	Time (hr)	Temp. (°C)	Time (hr)
  11	a	a	1500	0.5	-	-
  12	b	b	1450	1.0	1050	20
  13	c	c	1420	0.7	1150	100
  14	d	d	1360	1.6	950	60
  15	e	e	1520	0.8	1200	1
  16	f	f	1380	1.0	1150	2
  17	9	9	1430	1.2	1180	10
  18	h	h	1460	1.1	1100	2

  "-" : Not performed

  [Table 5]

  Example	Raw material powder	Sintering condition	Composition of sinter alloy (% by mass)							WC-WC interfacial length ratio (R) L1/(L1+L2)	Grain diameter of r phase (µm)	Theoretical volume rate of γ phase	WC area average grain diameter (D) (µm)	Binder phase area rate (V) (%)	R≧ (0.76-0.059×D) × (10/V) -V γ × 0.06
  			Co	Cr3C2	TaC	NbC	TiC	ZrC	WC
  1	A	A	14.0	1.4	-	-	0.9	1.5	Balance	0.35	1.7	0.41	1.0	21.2	Satisfied
  2	B	B	10.2	0.7	1.0	10	1.0	1.0	Balance	0.50	2.3	0.55	1.3	15.7	Satisfied
  3	C	C	12.1	1.1	-	0.4	-	0.2	Balance	0.40	1.9	0.07	1.6	19.1	Satisfied
  4	D	D	9.0	0.5	1.7	-	0.2	-	Ba lance	0.54	2.4	0.16	2.2	14.5	Satisfied
  5	E	E	8.3	0.9	0.2	0.5	0.4	1.2	Ba lance	0.59	3.8	0.34	4.0	13.2	Satisfied
  6	F	F	8.5	0.8	1.0	1.4	-	0.4	Balance	0.57	2.4	0.31	2.1	13.6	Satisfied
  7	G	G	8.7	0.6	1.2	0.3	0.2	1.6	Ba lance	0.56	1.9	0.40	1.9	13.8	Satisfied
  8	H	H	9.8	0.4	0.5	-	1.1	-	Balance	0.51	4.1	0.26	2.3	15.5	Satisfied
  9	I	I	11.4	0.8	0.2	0.2	0.2	-	Ba lance	0.39	2.1	0.08	2.0	18.1	Satisfied
  10	J	J	8.0	0.1	-	0.5	-	0.4	Balance	0.59	3.1	0.12	3.1	13.0	Satisfied

  "WC" contains incidental impurities.

  [Table 6]

  Comparative Example	Raw material powder	Sintering condition	Composition of sinter alloy (% by mass)							WC-WC interfacial length ratio (R) L1/ (L1+L2)	Grain diameter of r phase (µm)	Theoretical volume rate of r phase	WC area average grain diameter (D) (µm)	I Binder phase area rate (V) (%)	I R≧ (0. 76-0. 059 × D) × (10/V) -V γ ×0.06
  			Co	Cr 3C2	TaC	NbC	TiC	Z r C	WC
  11	a	a	9.1	0.3	1.5	1.6	1.2	1.3	Ba lance	0.32	3.3	0.75	3.7	14.0	Not satisfied
  12	b	b	12.0	-	-	2.5	1.4	-	Balance	0. 27	3.4	0.60	2.5	18.1	Not satisfied
  13	c	c	10.2	0.2	-	0.1	-	-	Balance	0.40	2.7	0.01	2.0	16.4	Satisfied
  14	d	d	9.8	0.4	0.2	0.2	-	-	Balance	0.33	3.2	0.04	0.9	15.8	Not satisfied
  15	e	e	7.8	0.1	1.2	-	0.6	0.2	Balance	0.40	3.9	0.24	3.5	12.6	Not satisfied
  16	f	f	14.3	0.6	1.1	0.5	0.8	-	Balance	0.20	2.7	0.30	1.6	21.9	Not satisfied
  17	9	g	8.4	1.5	0.6	-		-	Ba lance	0.45	3.4	0.04	4.2	13.7	Satisfied
  18	h	h	9.0	0.5	1.7	-	0.2	-	Ba lance	0.42	3.6	0.16	2.1	14.5	Not satisfied

  "WC" contains incidental impurities.

• Each cutting tool of Examples 1 to 10 and Comparative Examples 11 to 18 was screwed onto the tip of a tool steel bit with a fixture, and was subjected to the following wet continuous cutting test.
• Workpiece: JIS SUS304 (HB170) stainless steel round bar
• Cutting speed: 100m/min
• Depth of cut: 2.0 mm
• Feed: 0.7mm/rev
• Cutting time: 5 minutes
• Wet, water-soluble cutting fluid used
• The amount of plastic deformation of the flank of the cutting edge after the wet continuous cutting test was measured, and the state of wear of the cutting edge was observed. The amount of plastic deformation of the flank on main cutting edge (10) was calculated by drawing a line segment (11) on a ridge where the flank on main cutting edge (8) and the rake face (12) intersect at a sufficient distance from the cutting edge (9), extending the line segment in the direction of the cutting edge, and measuring the maximum distance between the extended line segment and the cutting edge ridge (in the vertical direction of the extended line segment). The amount of plastic deformation of the flank (10) of 0.04 mm or more was defined as the worn state of edge deformation (see Figure 2).
• Table 7 shows the results of these measurements. [Table 7]

  Example	Amount of plastic deformation of flank (mm)	State of wear	Comparative Example	Amount of plastic deformation of flank (mm)	State of wear
  1	0.038	Normal wear	11	0.054	Edge deformation
  2	0.022	Normal wear	12	0.059	Edge deformation
  3	0.029	Normal wear	13	0.045	Edge deformation
  4	0.017	Normal wear	14	0.135	Edge deformation
  5	0.016	Normal wear	15	-	Not measured due to chipping of edge
  6	0.013	Normal wear	16	0.110	Edge deformation
  7	0.013	Normal wear	17	-	Not measured due to chipping of edge
  8	0.023	Slight chipping on edge	18	0.044	Edge deformation
  9	0.030	Normal wear
  10	0.017	Slight chipping on edge

• Coating layers with an average thicknesses shown in Table 8 were formed on cutting edge surfaces of Examples 1 to 4 and Comparative Examples 11 to 14 by a PVD or CVD process to produce cutting tools of Examples 21 to 24 and Comparative Examples 31 to 34.
• Cutting tools of Examples 21 to 24 and Comparative Examples 31 to 34 were subjected to wet continuous cutting tests shown below to measure the plastic deformation of the flank of the cutting edge and to observe the worn state of the cutting edge.
Cutting conditions
• Workpiece: JIS SUS304 (HB170) stainless steel round bar
• Cutting speed: 150 m/min
• Depth of cut: 2.0mm
• Feed: 0.7mm/rev
• Cutting time: 5 minutes
• Wet, water-soluble cutting fluid used
• Table 9 shows the results of the cutting test. [Table 8]

  Type of tool		Coating layer (component, composition)	Deposition method	Average thickness of coating layer (µm)
  Example and Comparative Example	1	(Ti0.4Al0.6)N	PVD	3.0
  	2	(Ti0.5Al0.5)N	PVD	1.5
  	3	TiN (1 (µm) /TiCN (5 (µm) /Al2O3 (2 (µm)	CVD	8.0
  	4	TiN (0.5 (µm) /TiCN (4 (µm) /Al2O3 (1 (µm)	CVD	5.5

  (Note) Composition is represented by atmic ratio.

  [Table 9]

  Type of tool		Amount of plastic deformation of flank (mm)	State of wear of edge	Type of tool		Amount of plastic deformation of flank (mm)	State of wear of edge
  Example	21	0.035	Normal wear	Comparative Example	31	0.050	Edge deformation
  	22	0.020	Normal wear		32	0.054	Edge deformation
  	23	0.026	Normal wear		33	0.042	Edge deformation
  	24	0.015	Normal wear		34	0.108	Edge deformation

• The results of tests shown in Tables 7 and 9 demonstrate that all the Example tools exhibit excellent chipping resistance and plastic deformation resistance without chipping. In contrast, all the Comparative Example tools exhibit poor chipping resistance and plastic deformation resistance leading to the end of their service life in a short time.
• The disclosed embodiments are in all respects illustrative only and are not restrictive; the scope of the invention is indicated by the claims, not by the embodiments, and is intended to include all modifications within the meaning, scope, and scope equivalent to the claims, and all modifications within the scope of the claims are intended to be included.
[Reference Signs List]
• 1 WC grain
• 2 WC-WC interface
• 3 binder phase (hcp)
• 4 binder phase (fcc)
• 5 γ phase
• 6 interface between WC and binder phase (hcp)
• 7 interface between WC and (binder phase (fcc) + γ-phase)
• 8 flank on main cutting edge
• 9 cutting edge
• 10 amount of plastic deformation of flank
• 11 line segment extending the ridge line where the flank and rake face intersect
• 12 rake face

Claims (3)

1. A cutting tool made of WC-based cemented carbide, the cutting tool comprising:

   8.0 to 14.0 mass% Co;

   0.1 to 1.4 mass% Cr₃C₂; and

   0.6 to 4.0 mass% at least one selected from the group consisting of TaC, NbC, TiC, and ZrC;

   the balance being WC and incidental impurities, wherein

   the following expressions hold: $$\mathrm{R}=\left(\mathrm{L}1\right)/\left(\left(\mathrm{L}1\right)+\left(\mathrm{L}2\right)\right);$$

   

   $$\mathrm{R}\ge \left(0.76-0.059\times \mathrm{D}\right)\times \left(10/\mathrm{V}\right)-\mathrm{V}\mathrm{\gamma }\times 0.06;$$

   

   and $$1.0\le \mathrm{D}\le 4.0,$$

   

   where L1 is a total interfacial length between WC grains,

   L2 is a total interfacial length between the WC grains and binder phases and between the WC grains and γ phases,

   V is an area rate (%) of the binder phases;

   D is a mean diameter (µm) of the WC grains;

   Vγ is a theoretical volume rate of the γ phases; and

   R is a WC-WC interfacial length ratio.
2. The cutting tool made of WC-based cemented carbide set forth in claim 1, wherein the γ phases have a mean grain size in the range of 0.2 to 4.0 µm.
3. The cutting tool made of WC-based cemented carbide set forth in claim 1 or 2, wherein a cutting edge is coated by a coating layer.

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