WC-based cemented carbide

Abstract

This WC-based cemented carbide, which is suitable for a substrate of a surface-coated cutting tool or the like, contains 6.0-10.0 mass% of Co, 0.08-0.90 mass% of Cr (where the ratio Cr content (mass%) / Co content (mass%) is 10% or lower), 0.0-3.8 mass% of M (where M is one or more of V, Ta, Nb, Ti, and Zr), and 4.5-7.5 mass% of C, the balance being W and unavoidable impurities. The WC-based cemented carbide has a binder phase, a hard phase, and a γ phase, the binder phase containing Co as the main component, the hard phase containing a carbide of W as the main component, and the γ phase containing a carbide of M as the main component. In crystal particles constituting the hard phase, the grain diameter C99 (μm) for 99% cumulative particle count is 3.30 or lower, and the ratio C99 / C50 (grain diameter C99 (μm) for 99% cumulative particle count to grain diameter C50 (μm) for 50% cumulative particle count) is 4.80-6.50. The proportion (L) of the phase interface length along which the hard phase and the binder phase are in contact with respect to the total phase interface length of the hard phase is 35% or greater.

Description

WC-based cemented carbide

[0001] The present invention relates to a WC-based cemented carbide. This application claims priority from Japanese Patent Application No. 2023-183208, filed on October 25, 2023. The entire contents of the Japanese patent application are incorporated herein by reference.

[0002] WC-based cemented carbide is used, for example, as the substrate of a cutting tool, or the substrate of a surface-coated cutting tool (hereinafter sometimes referred to as a coated tool). ２ O ３ It has a vapor-deposited layer of a TiCN layer or TiCN layer as a coating layer, and is used for cutting carbon steel, cast iron, alloy steel, etc.

[0003] In cutting processes where the cutting edge becomes hot, such as cutting stainless steel, the cutting edge of a coated tool may undergo plastic deformation, shortening the tool life. On the other hand, increasing the hardness of the substrate to suppress this plastic deformation results in a decrease in chipping resistance. Therefore, a substrate made of a WC-based cemented carbide that combines resistance to both plastic deformation and chipping has been sought, and the following proposals have been made, for example:

[0004] Patent Document 1 describes a WC-based cemented carbide alloy that includes a hard phase containing tungsten carbide particles and a binder phase whose main component is an iron-group element including cobalt, and that satisfies B / A≦0.05, where A is the number of the tungsten carbide particles and B is the number of tungsten carbide particles that have one or less contact points with other tungsten carbide particles, and the alloy is said to have excellent resistance to plastic deformation.

[0005] Patent Document 2 also describes a steel sheet containing 10 to 13 mass % of Co, 2 to 8% of Cr, 0.2 to 0.5 mass % of at least one of TaC and NbC, and the balance being WC, with a hardness of 88.6 to 89.5 HRA, and a WC cumulative grain size of 80% diameter D ８０ and cumulative particle size 20% diameter D ２０ Ratio D ８０ / D ２０ is 2.0≦D ８０ / D ２０ ≦4.0, and the above D８０ The publication describes a WC-based cemented carbide having a thickness of 4.0 to 4.86 μm and a WC adhesion degree c of 0.36≦c≦0.43, which is said to have excellent chipping resistance.

[0006] Patent No. 6256415 Patent No. 6774645

[0007] The present invention has been made in view of the above circumstances and proposals, and an object of the present invention is to provide a WC-based cemented carbide that provides excellent resistance to plastic deformation and chipping resistance, particularly when used as a substrate for cutting tools and coated tools.

[0008] A WC-based cemented carbide according to one embodiment of the present invention contains 6.0 to 10.0 mass% Co, 0.08 to 0.90 mass% Cr (with the proviso that the Cr content (mass%) / Co content (mass%) is 10% or less), 0.0 to 3.8 mass% M (M is one or more of V, Ta, Nb, Ti, and Zr), 4.5 to 7.5 mass% C, and the balance being W and inevitable impurities, and has a binder phase, a hard phase, and a γ phase, in which the binder phase is mainly composed of Co, the hard phase is mainly composed of W carbide, and the γ phase is mainly composed of M carbide, The crystal grains constituting the hard phase have a cumulative particle number 99% particle diameter C99 (μm) of 3.30 or less, a ratio of the cumulative particle number 99% particle diameter C99 (μm) to the cumulative particle number 50% particle diameter C50 (μm), C99 / C50, of 4.80 or more to 6.50 or less, and a ratio (L) of the interface length in contact with the crystal grains constituting the binder phase to the total interface length of the crystal grains constituting the hard phase is 35% or more.

[0009] The WC-based cemented carbide may satisfy the following condition (1).

[0010] (1) Rockwell hardness (HRA) is 88.8 or more and 90.6 or less.

[0011] The WC-based cemented carbide according to the embodiment provides excellent resistance to plastic deformation and chipping, particularly when used as a substrate for cutting tools and coated tools.

[0012] The present inventors have conducted extensive research into cemented carbide alloys that provide excellent resistance to plastic deformation and fracture when used as a substrate for a coated tool, and first recognized the following points (1) and (2) regarding the WC-based cemented carbide alloys described in Patent Documents 1 and 2.

[0013] (1) The WC-based cemented carbide described in Patent Document 1 reaches the end of its tool life early when subjected to interrupted cutting. (2) The WC-based cemented carbide described in Patent Document 2 lacks resistance to plastic deformation when subjected to high-speed, high-feed cutting.

[0014] The inventors of the present invention conducted further extensive research. As a result, they obtained the following findings and developed the present invention: (a) The plastic deformation resistance of a WC-based cemented carbide varies depending on the grain size of the crystal grains that make up the hard phase, and a smaller grain size contributes to improving the plastic deformation resistance of the WC-based cemented carbide. Therefore, by having a larger amount of crystal grains that make up the hard phase with a small grain size than conventionally, the plastic deformation resistance is improved. (b) By appropriately dispersing the crystal grains that make up this hard phase in the WC-based cemented carbide and suppressing contact between the crystal grains that make up the hard phase, stress concentration at the contact points is suppressed, and chipping resistance is improved.

[0015] The WC-based cemented carbide according to an embodiment of the present invention will be described in detail below to explain the present invention. In this specification and claims, when a numerical range is expressed as "A to B" (A and B are both numerical values), the range includes an upper limit (B) and a lower limit (A). When a unit is stated only for the upper limit (B), the upper limit (B) and the lower limit (A) have the same unit. Furthermore, the composition of a compound not expressed using a formula is not limited to a stoichiometric composition, but includes all conventionally known compositions with atomic ratios.

[0016] 1. Composition The composition of the WC-based cemented carbide is Co: 6.0 to 10.0 mass%, Cr: 0.08 to 0.90 mass% (however, the Cr content (mass%) / Co content (mass%) is 10% or less), M: 0.0 to 3.8 mass% (M is one or more of V, Ta, Nb, Ti, and Zr), C: 4.5 to 7.5 mass%, with the balance being W and unavoidable impurities. Each component will be explained below.

[0017] (1) When the Co content is 6.0 to 10.0 mass%, the WC-based cemented carbide has excellent plastic deformation resistance when used as a coated tool or a tool substrate for the coated tool. Co is mainly present in the binder phase and is the main component of the binder phase (accounting for 50 atomic % or more of all components contained in the binder phase). The binder phase is composed of crystal grains with an fcc structure and an hcp structure. The Co content is more preferably 7.0 to 9.6 mass %.

[0018] (2) Cr Cr dissolves in Co, the main component of the binder phase, and inhibits the growth of W carbides contained in the crystal grains that form the hard phase, refining the W carbides and reducing the size of these crystal grains. This function is insufficient when the Cr content is less than 0.08% by mass. On the other hand, when the Cr content exceeds 10% relative to the Co content, a composite carbide of Cr and W precipitates, reducing the toughness of the WC-based cemented carbide. Furthermore, since this composite carbide acts as a starting point for fracture generation, the Cr content (mass %) / Co content (mass %) ratio is set to 10% or less. Since the upper limit of the Co content is 10.0% by mass, the upper limit of the Cr content is set to 0.90% by mass to allow for some leeway.

[0019] (3) M (one or more selected from V, Ta, Nb, Ti, and Zr) M does not need to be contained (its content may be 0.0% by mass). However, if contained, it dissolves in Co, the main component of the binder phase, and functions to increase the hardness of the WC-based cemented carbide. Furthermore, M forms carbides (not limited to stoichiometric compositions) and becomes the main component of the γ phase (M accounts for 50 atomic % or more of all components contained in the γ phase). If its content exceeds 3.8% by mass, it reduces the toughness of the WC-based cemented carbide and becomes the starting point for chipping. Therefore, the upper limit of the M content is set to 3.8% by mass.

[0020] (4) C C is contained to form carbides, and is mainly contained in the hard phase and the γ phase. If the C content is 4.5 mass% or more and 7.5 mass% or less, carbides are formed in the hard phase and the γ phase in just the right amount.

[0021] (5) W W is the main component of the hard phase. That is, W accounts for 50 atomic % or more of all components contained in the hard phase. W exists mainly as carbides (mostly WC, but including W carbides not limited to the stoichiometric composition) in the hard phase.

[0022] (6) Inevitable Impurities Raw materials may contain unintended impurities, and there is also a possibility that unintended impurities may be mixed in during the manufacturing process. These impurities are called unavoidable impurities. The content of the unavoidable impurities is preferably 0.3% by mass or less when the entire WC-based cemented carbide is taken as 100% by mass.

[0023] 2. Structure WC-based cemented carbide has a binder phase, a hard phase, and a γ phase. That is, a free carbon phase, an η phase, etc. may be unintentionally generated. However, in the following explanation, it is assumed that only the binder phase, the hard phase, and the γ phase are present. In the claims and the specification, the constituents of each of the binder phase, the hard phase, and the γ phase are referred to as crystal grains, and their size is referred to as the crystal grain size. Here, the crystal grain size refers to the circle equivalent diameter.

[0024] (1) Binder phase The binder phase is mainly composed of Co, and is composed of crystal grains that may contain W and C, which are hard phase components, M, Cr, and inevitable impurities contained in γ phase.When these components other than Co exist in the crystal grains that constitute the binder phase, they are considered to be in a solid solution state.This crystal grain has an fcc structure and an hcp structure.The method for identifying the crystal grains that constitute the binder phase will be described later.

[0025] (2) Hard phase The hard phase is mainly composed of W carbide and is composed of crystal grains that may contain binder phase components, γ phase components, Cr and inevitable impurities. These crystal grains have a hcp structure. The method for identifying the crystal grains that make up the hard phase will be described later.

[0026] In the hard phase, the ratio of the cumulative particle number 50% particle diameter C50 (μm) of the crystal particles constituting the hard phase to the cumulative particle number 99% particle diameter C99 (μm), i.e., C99 / C50, is preferably 4.80 or more and 6.50 or less. The reason for this is that if C99 / C50 is less than 4.80, the fine crystal particles constituting the hard phase are insufficient, resulting in a decrease in plastic deformation resistance. On the other hand, if it exceeds 6.50, the grain size difference (particle size difference) between the coarse and fine crystal particles constituting the hard phase increases, causing the crystal particles constituting the hard phase to become too large, resulting in a decrease in the plastic deformation resistance of the WC-based cemented carbide. C99 / C50 is more preferably 5.30 or more and 6.30 or less. The C50 value is preferably 0.68 μm or less, more preferably 0.60 μm or less. Here, there is no restriction on the lower limit of the cumulative particle number 50% particle diameter C50 (μm) as long as C99 / C50 is 4.80 or more and 6.50 or less. However, when produced according to an example of the production method described below, the lower limit is about 0.38 μm. That is, the cumulative particle number 50% particle diameter C50 (μm) is preferably 0.38 to 0.68, and more preferably 0.38 to 0.60.

[0027] Furthermore, the cumulative particle number 99% particle diameter C99 (μm) is preferably 3.30 μm or less. If it exceeds 3.30 μm, the plastic deformation resistance of the WC-based cemented carbide decreases. The cumulative particle number 99% particle diameter C99 (μm) is more preferably 3.20 μm or less. Here, there is no restriction on the lower limit of the cumulative particle number 99% particle diameter C99 (μm) as long as C99 / C50 is 4.80 or more and 6.50 or less. However, when manufactured according to an example of the manufacturing method described below, the lower limit of the cumulative particle number 99% particle diameter C99 (μm) is approximately 2.50 μm. That is, the cumulative particle number 99% particle diameter C99 (μm) is preferably 2.50 to 3.30, more preferably 2.50 to 3.20.

[0028] The crystal grains constituting the hard phase are in contact with the crystal grains constituting the binder phase, the γ phase, and other hard phases. Here, the ratio (L) of the interface length in contact with the crystal grains constituting the binder phase to the total interface length of the crystal grains constituting the hard phase is preferably 35% or more. If this ratio is less than 35%, the crystal grains constituting the hard phase will have increased contact with each other, which will easily cause stress concentration and reduce the chipping resistance of the WC-based cemented carbide. L is more preferably 40% or more. There are no particular restrictions on the upper limit of L, but when manufactured according to an example of the manufacturing method described below, the upper limit is approximately 55%. That is, L is preferably 35 to 55%, and even more preferably 40 to 55%.

[0029] In addition, as long as the proportion of the interface length of the crystal grains constituting the hard phase that are in contact with the crystal grains constituting the binder phase in the phase interface length is 35% or more, there are no restrictions on the proportion of the crystal grains constituting the hard phase that are in contact with the crystal grains constituting the γ phase or the crystal grains constituting phases that are generated unintentionally during the manufacturing process (free carbon phase, η phase, etc.).

[0030] (3) γ Phase The γ phase is mainly composed of M carbides (not limited to the stoichiometric composition), and may contain binder phase components, hard phase components, Cr, and inevitable impurities. The γ phase is composed of crystal grains having an fcc structure. A method for identifying the crystal grains that make up the γ phase will be described later.

[0031] 3. Rockwell Hardness It is more preferable that the WC-based cemented carbide has a Rockwell hardness (HRA) of 88.8 or more and 90.4 or less. If the Rockwell hardness falls within this range, the plastic deformation resistance and chipping resistance are more reliably improved. The Rockwell hardness (HRA) is measured in accordance with ISO 3738-1:1982 and ISO 3738-2:1988.

[0032] 4. Method for distinguishing the binder phase, hard phase, and γ phase, as well as the crystal grains that compose them, and measurement of the particle size of the cumulative particle number The binder phase, hard phase, and γ phase, as well as the crystal grains that compose them, are distinguished as follows, and the particle size of the cumulative particle number is measured.

[0033] (1) Any surface or cross section of a cemented carbide is machined to a smooth surface by removing any minute irregularities that would interfere with EBSD measurement. To measure the grain size of the crystal grains constituting each phase, multiple fields of view (e.g., two fields of view), each measuring, for example, 90 μm (vertical) × 120 μm (horizontal), are set on the machined surface. Each of these multiple fields of view is observed using a field emission scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and an electron backscatter diffraction (EBSD) measurement device (e.g., an OIM Data Collection manufactured by AMETEK) at an acceleration voltage of 15 kV with a measurement point spacing of 0.1 μm, and the EBSD pattern and EDS data are simultaneously captured.

[0034] The surface or cross section is processed using, for example, a focused ion beam device (FIB device).

[0035] (2) Subsequently, the measurement data is read into software for analyzing the EBSD measurement results, for example, OIM Analysis ver. 7.3.1 manufactured by EDAX / TSL, and the EDS count values ​​obtained from each measurement point inside the crystal grain corresponding to each element for each crystal grain are averaged to obtain the EDS measurement value for each element of each crystal grain, and the composition of each crystal grain is derived from the obtained measurement values.

[0036] (3) According to the definition of crystal grains mentioned above, identify which phase each crystal grain is contained in. That is, the hard phase is the aggregate of crystal grains that are identified as having hcp structure from EBSD pattern and that W carbide accounts for 50 atomic % or more. The binder phase is identified as the aggregate of crystal grains with hcp crystal structure or fcc crystal structure, but γ phase is also identified as the aggregate of crystal grains with fcc crystal structure, so the EBSD measurement result alone cannot distinguish between the two. Therefore, the aggregate of crystal grains that have the EDS count value of M (V, Ti, Nb, Ta or Zr) obtained by EDS measurement is identified as γ phase, and the aggregate of crystal grains that are identified as the aggregate of crystal grains with the remaining fcc crystal structure is identified as binder phase.

[0037] (4) When adjacent measurement points are judged to be the same phase, if the difference in orientation obtained from each measurement point is 5 degrees or more, the boundary (midpoint) between those two measurement points is considered to be the interface of the crystal grain.

[0038] (5) EDS measurement is again performed on each of the crystal particles identified in (3) as crystal particles constituting the hard phase, γ phase, and binder phase, and it is confirmed that the crystal particles identified as crystal particles constituting the hard phase are made up of W carbide at 50 atomic % or more, the crystal particles identified as crystal particles constituting the γ phase are made up of M carbide at 50 atomic % or more, and the crystal particles identified as crystal particles constituting the binder phase are made up of Co at 50 atomic % or more.

[0039] (6) Based on the results of (5) above, a graph is created in which the horizontal axis represents the particle diameter of the crystal particles constituting the hard phase, and the vertical axis represents the cumulative number ratio of the crystal particles constituting the hard phase corresponding to each particle diameter on the horizontal axis ((cumulative number of crystal particles constituting the hard phase up to each particle diameter) / (total number of crystal particles constituting the hard phase) × 100).Then, the cumulative particle number 50% particle diameter C50 (μm) at which the cumulative number ratio reaches 50% and the cumulative particle number 99% particle diameter C99 (μm) at which the cumulative number ratio reaches 99% are determined for each observation field, and the results are arithmetically averaged to calculate C99 / C50.

[0040] 5. Method for distinguishing the binder phase, hard phase, and γ phase, as well as the crystal grains that compose them, and measurement of interface length and calculation of interface length ratio The binder phase, hard phase, and γ phase, as well as the crystal grains that compose them, are distinguished as follows, and the interface length of the crystal grains that compose each phase is measured.

[0041] (1) Any surface or cross section of a cemented carbide is machined to a smooth surface by removing any minute irregularities that would interfere with EBSD measurement. To measure the grain size of the crystal grains constituting each phase, multiple fields of view (e.g., four or five fields of view), each measuring, for example, 37 μm (vertical) × 47 μm (horizontal), are set on the machined surface. Each of these multiple fields of view is observed using a field emission scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and an electron backscatter diffraction (EBSD) measurement device (e.g., an OIM Data Collection manufactured by AMETEK) at an accelerating voltage of 15 kV with a measurement point spacing of 0.1 μm, and the EBSD pattern and EDS data are simultaneously captured.

[0042] (2) Subsequently, the measurement data is read into software for analyzing the EBSD measurement results, for example, OIM Analysis ver. 7.3.1 manufactured by EDAX / TSL, and the EDS count values ​​obtained from each measurement point inside the crystal grain corresponding to each element for each crystal grain are averaged to obtain the EDS measurement value for each element of each crystal grain, and the composition of each crystal grain is derived from the obtained measurement values.

[0043] (3) According to the definition of crystal grains mentioned above, identify which phase each crystal grain is contained in. That is, the hard phase is the aggregate of crystal grains that are identified as having hcp structure from EBSD pattern and that W carbide accounts for 50 atomic % or more. The binder phase is identified as the aggregate of crystal grains with hcp crystal structure or fcc crystal structure, but γ phase is also identified as the aggregate of crystal grains with fcc crystal structure, so the EBSD measurement result alone cannot distinguish between the two. Therefore, the aggregate of crystal grains that have the EDS count value of M (V, Ti, Nb, Ta or Zr) obtained by EDS measurement is identified as γ phase, and the aggregate of crystal grains that are identified as the aggregate of crystal grains with the remaining fcc crystal structure is identified as binder phase.

[0044] (4) When adjacent measurement points are judged to be the same phase, if the difference in orientation obtained from each measurement point is 5 degrees or more, the boundary (midpoint) between those two measurement points is considered to be the interface of the crystal grain.

[0045] (5) Based on the results of (3), the total interface length of the crystal grains constituting the hard phase and the interface length where the crystal grains constituting the hard phase and the crystal grains constituting the binder phase come into contact are calculated for each of the multiple fields of view. Software for analyzing the EBSD measurement results, such as OIM Analysis ver. 7.3.1 manufactured by EDAX / TSL, can be used to calculate the interface length.

[0046] Then, the measured values ​​obtained for each of the multiple measurement fields are arithmetically averaged to calculate the ratio of the interface length between the crystal grains constituting the hard phase and the crystal grains constituting the binder phase to the total interface length of the crystal grains constituting the hard phase.

[0047] 6. Composition Measurement Method The contents of W, Co, Cr, M (V, Ta, Nb, Ti, Zr), C, etc. are measured using an electron beam microanalyzer. A WC-based cemented carbide sample is prepared with a mirror-finished surface (a surface or cross section of the cemented carbide sample is machined to remove minute irregularities and achieve a smooth surface so as not to interfere with electron beam microanalyzer measurement). The sample is then inserted into the electron beam microanalyzer and irradiated with electron beams. For example, three observation fields measuring 100 μm (vertical) × 100 μm (horizontal) are set, and the contained components and their amounts are measured from the characteristic X-rays obtained from each observation field, and the arithmetic mean of the measurement results is calculated.

[0048] 7. Manufacturing Method An example of the manufacturing method will be described. In the manufacturing method, after preparing predetermined raw material powders, (1) a mixing step, (2) a molding step, (3) a sintering step, (4) a finishing step, and an optional (5) coating step are carried out in this order. Each step will be described below.

[0049] (1) Mixing Step In the WC-based cemented carbide according to this embodiment, it is preferable to add WC powder pulverized into fine particles, not to raise the sintering temperature in order to suppress the grain growth of the WC particles, and to disperse the WC particles well by allowing Co to penetrate. Therefore, it is recommended to perform the following two-stage mixing.

[0050] 1) First Mixing Step The first mixing step is a step for pre-pulverizing a portion of the WC powder to be used in order to obtain pulverized WC powder into fine particles. In the first mixing step, 20 to 50 mass% of the WC powder to be used is mixed using a known method such as an attritor or a ball mill. The mixing conditions using an attritor are a rotation speed of 40 to 90 min －１ The preferred conditions are a quantity of cemented carbide balls of 100 to 300 kg and a mixing time of 70 to 240 minutes. To obtain a WC-based cemented carbide alloy having a Rockwell hardness of 88.8 to 90.4, the mixing time is more preferably 120 to 240 minutes.

[0051] 2) Second Mixing Step In the second mixing step, the remaining WC powder, binder phase powder, and γ phase powder are added to the mixed powder obtained in the first mixing step so as to obtain the composition of the WC-based cemented carbide to be produced, and the mixture is mixed using a known method such as an attritor or a ball mill. The mixing conditions using an attritor are a rotation speed of 40 to 90 min －１ The preferred conditions are a quantity of cemented carbide balls of 200 to 500 kg and a mixing time of 70 to 420 minutes. To obtain a WC-based cemented carbide alloy having a Rockwell hardness of 88.8 to 90.4, the mixing time is more preferably 240 to 420 minutes.

[0052] The raw powder of the WC-based cemented carbide that has undergone these two mixing steps contains a large amount of fine WC powder that has been pulverized by mixing and contributes to improving plastic deformation resistance. As a result, the value of the cumulative 50% particle diameter C50 of the crystal particles that make up the hard phase becomes small, and the value of the cumulative 50% particle diameter C99 of the cumulative 99% particle diameter C99 to the cumulative 50% particle diameter C50, C99 / C50, can be set to 4.80 or more and 6.50 or less.

[0053] Furthermore, the two-stage mixing forms new surfaces on the surfaces of the WC particles, which makes it easier for Co to exist around them and improves sinterability. Therefore, even if the upper limit temperature of the sintering step described below is set to a relatively low temperature of 1,430°C, which can suppress the growth of the hard phase, it is possible to obtain a WC-based cemented carbide in which Co is well distributed around the hard phase and the crystal particles that make up the hard phase are dispersed, and it is possible to make the 99% cumulative particle size C99 of the crystal particles that make up the hard phase 3.30 μm or less and L 35% or more.

[0054] (2) Molding Step The raw material powder that has been mixed is molded into a substrate of a predetermined shape (for example, an insert shape of CNMG120408). Molding can be performed by press molding at a pressure of, for example, 100 MPa.

[0055] (3) Sintering process This is the process of sintering the molded raw material. In the sintering process, the maximum holding temperature is set to 1350 to 1430°C, and the sintering is performed for 10 minutes to prevent excessive grain growth of the crystal grains that make up the hard phase. －１ The material is held in a vacuum atmosphere of 100 Pa or less for 50 to 120 minutes. There are no restrictions on the cooling rate, but it is more preferable to cool from the maximum holding temperature to 1100 to 1200 °C at a cooling rate of 35 °C / min. If the cooling rate is slower than this rate, the grain growth of the crystal grains that make up the hard phase may be promoted even in the cooling process, causing coarsening. Therefore, the generation of coarse-grained WC due to abnormal grain growth of the crystal grains can be suppressed by cooling. The upper limit of the cooling rate was derived from the limitations of the equipment at the time of filing.

[0056] (4) Finishing process The molded body that has undergone the sintering process is ground to a predetermined shape (an insert shape of CNMG120408 defined in ISO standard 1832:2017 can be exemplified, but is not limited to this, and may be the shape of a drill, milling cutter, end mill, etc.).

[0057] (5) Coating step: A predetermined coating layer is formed by a known means. The WC-based cemented carbide according to this embodiment can be processed into a substrate of a predetermined shape and used as a cutting tool as is, but it may also be coated with a hard layer by a known method to form a surface-coated cutting tool.

[0058] The above description includes the following additional features: (Additional Note 1) Co: 6.0 to 10.0 mass%, Cr: 0.08 to 0.90 mass% (however, the Cr content (mass%) / Co content (mass%) is 10% or less), M: 0.0 to 3.8 mass% (M is one or more of V, Ta, Nb, Ti, and Zr), C: 4.5 to 7.5 mass%, and the balance is W and inevitable impurities, and has a binder phase, a hard phase, and a γ phase, the binder phase containing Co as a main component, the hard phase containing W carbide as a main component, and the γ phase containing M carbide as a main component, A WC-based cemented carbide characterized in that the crystal grains constituting the hard phase have a cumulative particle number 99% particle diameter C99 (μm) of 3.30 or less, the cumulative particle number 99% particle diameter C99 (μm) relative to the cumulative particle number 50% particle diameter C50 (μm), C99 / C50, of 4.80 to 6.50, and a ratio (L) of the phase interface length where the hard phase contacts the binder phase to the total phase interface length of the hard phase is 35% or more. (Appendix 2) The WC-based cemented carbide according to claim 1, characterized in that it has a Rockwell hardness (HRA) of 88.8 to 90.6. (Appendix 3) The WC-based cemented carbide according to Appendices 1 or 2, characterized in that the crystal grains constituting the hard phase have a cumulative particle number 50% particle diameter C50 (μm) of 0.68 or less. (Appendix 4) The WC-based cemented carbide according to any one of Appendices 1 to 3, characterized in that a ratio (L) of a phase interface length where the hard phase and the binder phase come into contact with each other to a total phase interface length of the hard phase is 40% or more.

[0059] The use of the cemented carbide for cutting tools of the present invention as a cutting tool substrate will be specifically described with reference to examples, but the present invention is not limited to these examples.

[0060] 1. Production of Examples (1) Raw Material Powders Raw material powders were: WC powder with an average particle size shown in Table 1; Co powder with an average particle size of 1.0 μm; Cr powder with an average particle size of 1.6 μm. ３ C ２Powder, TiC powder with an average particle size of 1.0 μm, TaC powder with an average particle size of 1.9 μm, NbC powder with an average particle size of 1.1 μm, ZrC powder with an average particle size of 1.2 μm, and VC powder with an average particle size of 1.7 μm were prepared and blended according to the blending compositions shown in A to G and A' to E' in Table 1. Here, the average particle size of each raw material powder is the Fischer diameter measured by the air permeability method using a Fischer subsieve sizer.

[0061] (2) Mixing, Molding, Sintering, and Finishing Steps As described above, the mixing, molding, sintering, and finishing steps were performed in this order. In the first mixing step, to obtain WC powder pulverized into fine particles, the WC powder was mixed and pulverized at the ratios (mass%) and for the mixing time shown in Tables 2 and 3, depending on the average particle size of the WC powder shown in Table 1. In the second mixing step, the WC powder that had undergone the first mixing step, the remaining WC powder that had not undergone the first mixing step (100 - the ratio (mass%) of the WC powder in the first mixing step), the hard phase raw material powder, and the binder phase raw material powder were mixed for the mixing time shown in Tables 2 and 3. The raw material powder after the second mixing step was press-molded into an insert shape of CNMG120408 at a pressure of 100 MPa. Thereafter, sintering was carried out at the holding temperatures shown in Tables 2 and 3 as the sintering process (some examples were cooled at the cooling rates shown in Table 3), followed by grinding as a finishing process to obtain the desired insert shape of CNMG120408. The results of analyzing this insert using the above-mentioned method are shown in Table 4.

[0062] 2. Manufacturing of Comparative Examples For comparison, raw material powders were prepared with the compositions shown in Table 1, A to E, G, and B'. The mixing, molding, sintering, and finishing processes were then performed in sequence. In the mixing process, WC powders were mixed in the proportions shown in Tables 2 and 3 in the first mixing step. (In Table 2, comparative examples in which the first mixing step is marked "-" did not include the first mixing step, but only the second mixing step.) After the mixing step, the raw material powder was press-molded into CNMG120408-shaped inserts at a pressure of 100 MPa. Sintering was then performed at the holding temperatures shown in Tables 2 and 3 (some comparative examples were cooled at the cooling rates shown in Table 3). Subsequently, grinding was performed as a finishing process to obtain the desired CNMG120408-shaped inserts. The results of analyzing these inserts using the aforementioned method are shown in Table 5.

[0063] 3. Formation of Coating Layers Then, by chemical vapor deposition, each of the examples and comparative examples was coated with a TiN layer having an average thickness of 0.2 μm, a TiCN layer having an average thickness of 3.5 μm, and an Al layer having an average thickness of 2.5 μm, in that order from the insert surface. ２ O ３ A coating layer of TiN having an average thickness of 0.2 μm was formed.

[0064]

[0065] In Table 1, "-" indicates that the component was not blended.

[0066]

[0067] In Table 2, "-" indicates that the first mixing step was not performed.

[0068]

[0069]

[0070]

[0071] In Tables 4 and 5, "-" indicates that the component in question was not contained. It was also confirmed that the content of unavoidable impurities in all Examples and Comparative Examples was 0.3% by mass or less when the entire WC-based cemented carbide was taken as 100% by mass.

[0072] Next, cutting tests were conducted for Examples 1 to 16 and Comparative Examples 1' to 9' under the following cutting conditions 1 to 4 for 15 minutes each. Every minute after the start of the cutting test, the presence or absence of chipping and peeling due to plastic deformation was visually observed, and the flank wear width was measured. When the flank wear width exceeded 0.2 mm, the flank wear width at the time elapsed from the start of the test (measurement time) that exceeded 0.2 mm was linearly approximated to the flank wear width at the measurement time immediately preceding that time to determine the elapsed time from the start of the test that gave a flank wear width of 0.2 mm (the time to the end of the service life). The results are shown in Tables 6 to 9, respectively.

[0073] Cutting condition 1 (high-speed, high-feed cutting) Workpiece: SUS304 round bar with an outer diameter of 100 mm Cutting speed: 150 m / s Depth of cut: 1.5 mm Feed per revolution: 0.3 mm Wet cutting

[0074] Cutting conditions 2 Workpiece: SUS316 round bar with outer diameter of 100 mm Cutting speed: 150 m / s Depth of cut: 2.0 mm Feed per revolution: 0.2 mm Wet cutting

[0075] Cutting conditions 3 Workpiece: SUS630 round bar with outer diameter of 100 mm Cutting speed: 120 m / s Depth of cut: 2.0 mm Feed per revolution: 0.15 mm Wet cutting

[0076] Cutting condition 4 (intermittent cutting) Workpiece: SUS304, regular hexagonal prism-shaped square bar with an outer diameter of 80 mm Cutting speed: 100 m / s Depth of cut: 2.0 mm Feed per revolution: 0.3 mm

[0077]

[0078]

[0079]

[0080]

[0081] As is clear from Tables 6 to 9, all of the Examples had little flank wear, no chipping or peeling due to plastic deformation, and demonstrated excellent cutting performance even in interrupted cutting and high-speed, high-feed cutting. In contrast, all of the Comparative Examples experienced at least one of chipping or peeling due to plastic deformation before the end of the cutting test time (15 minutes), and the flank wear width reached 0.2 mm, reaching the end of their service life.

[0082] The above-disclosed embodiments are merely illustrative in all respects and are not restrictive. The scope of the present invention is defined by the claims, not by the above-disclosed embodiments, and is intended to include any modifications equivalent to the claims and within the scope thereof.

Claims

1. Co: 6.0 to 10.0 mass%, Cr: 0.08 to 0.90 mass% (however, Cr content (mass%) / Co content (mass%) is 10% or less), M: 0.0 to 3.8 mass% (M is one or more of V, Ta, Nb, Ti, and Zr), C: 4.5 to 7.5 mass%, and the balance is W and inevitable impurities; it has a binder phase, a hard phase, and a γ phase, the binder phase is mainly composed of Co, the hard phase is mainly composed of carbides of W, and the γ phase is mainly composed of carbides of M, and the crystal grains constituting the hard phase have an accumulated particle number 99% grain size C99 (μm) of 3.30 or less, and the accumulated particle number 99% grain size C99 (μm) relative to the accumulated particle number 50% grain size C50 (μm), C99 / C50, is 4.80 or more and 6.50 or less, a ratio (L) of a phase interface length at which the hard phase and the binder phase come into contact with each other to a total phase interface length of the hard phase of 35% or more.

2. The WC-based cemented carbide according to claim 1, characterized in that its Rockwell hardness (HRA) is 88.8 or more and 90.6 or less.

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