JP2015145533A - Cemented carbide and working tool - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cemented carbide that has smaller bias of particle size distribution of hard phase particles as compared with that of the conventional ones and is excellent in abrasion resistance and defect resistance.SOLUTION: The cemented carbide is formed by binding a hard phase mainly composed of WC particles with a binder phase mainly composed of Co. An average particle size of WC particles constituting the hard phase of the cemented carbide is less than 0.4 μm and a degree of dispersion as an index showing uniformity of the WC particle size is 0.55 or less, preferably, 0.50 or less. [the degree of dispersion is a value obtained by dividing the half value width (circle equivalent diameter when the cumulative frequency is 75%-circle equivalent diameter when the cumulative frequency is 25%) by the average particle size (50% particle size) of the WC particles]. An average circularity of the WC particles is 0.68 or more, preferably, 0.75 or more. The cemented carbide has more excellent abrasion resistance and defect resistance than those of the conventional ones and consequently, and thereby can be suitably used for various working tools.

Description

  The present invention relates to a cemented carbide suitable for a material for a processing tool. In particular, the present invention relates to a cemented carbide used as a material for a processing tool used for micromachining such as a micro drill.

  Conventionally, a WC-based cemented carbide in which WC (tungsten carbide) particles are bonded with Co (cobalt) has been used as a material for processing tools. In particular, a cemented carbide having a WC particle having an average particle size of 1 μm or less as a hard phase, a so-called fine cemented carbide, is known to be excellent in strength and wear resistance.

  In order to make the WC particles in the cemented carbide fine, it is common to make the WC particles prepared when producing the cemented carbide fine. For example, in the example of Patent Document 1, a WC raw material powder having an average particle diameter of 0.5 μm, a Co raw material powder having an average particle diameter of 1 μm, and other additive elements are blended, and these raw material powders are pulverized with a ball mill for 48 hours. Mixed.

Japanese Patent No. 3762777

  However, the cemented carbide obtained by the production method described in Patent Document 1 may not exhibit sufficient wear resistance and fracture resistance depending on the application.

  For example, when a cemented carbide is used for a tool used for micromachining such as a micro drill, it is said that the smaller the WC particles contained in the cemented carbide, the better the cemented carbide will have better wear resistance and fracture resistance. It has been broken. However, as a result of the study by the present inventors, in the cemented carbide obtained by the conventional manufacturing method, the particle size distribution of the WC particles contained in the cemented carbide is biased. In some cases, it was deficient.

  The present invention has been made in view of the above circumstances, and one of its purposes is to provide a cemented carbide having a smaller deviation in the particle size distribution of the hard phase particles than the conventional one and having excellent wear resistance and fracture resistance. There is.

<Cemented carbide of the present invention>
The cemented carbide of the present invention is a cemented carbide in which the hard phase is bonded by a binder phase mainly composed of Co, and the average particle size of WC particles mainly constituting the hard phase is less than 0.4 μm, and The degree of dispersion, which is an index representing the uniformity of the WC particle diameter, is 0.55 or less.

  A cemented carbide that satisfies the above-mentioned definition has a smaller deviation in the particle size distribution of the hard phase particles than before, and therefore has excellent wear resistance and fracture resistance.

  Hereinafter, each structure of the cemented carbide of the present invention including the definition of the degree of dispersion will be described in detail.

≪Hard phase≫
The cemented carbide of the present invention is a WC-based cemented carbide containing the most WC particles as a hard phase. The hard phase substantially constitutes the remainder excluding the binder phase and inevitable impurities described later.

  As a compound constituting the hard phase other than WC, a compound of at least one metal selected from Group 4a, 5a and 6a elements of the periodic table and at least one element of carbon and nitrogen (however, WC is excluded) ), That is, one or two or more compounds selected from carbides of the above metals (excluding WC), nitrides, and carbonitrides. For example, TaC, (Ta, Nb) C, NbC, TiCN, etc. are representative. In the present invention, hard phase particles other than WC are collectively referred to as the second hard phase.

  The ratio of WC in the hard phase (other than WC / WC + WC) is preferably 85 to 97% by mass. In addition, when a hard phase is substantially comprised only from WC particle | grains, it will become a cemented carbide alloy excellent in heat cracking resistance, toughness, and intensity | strength.

  The average particle diameter of the WC particles in the cemented carbide is less than 0.4 μm. In this case, a cemented carbide having high toughness and strength can be obtained, and as a result, a cemented carbide excellent in wear resistance and fracture resistance can be obtained. If the hard phase particles are too small, the toughness tends to decrease, so the lower limit is preferably 0.05 μm. Here, the average particle diameter of the particles in the present specification including the average particle diameter of the WC particles is determined as follows. First, 500 or more sample particles are extracted, the area of each particle is obtained from the image of these sample particles, and the equivalent circle diameter corresponding to the area is calculated. Then, the equivalent circle diameter when the cumulative frequency of the sample particles is 50%, that is, the 50% particle size is defined as the average particle size of the particles.

[Dispersion frequency]
The degree of dispersion, which is an index representing the uniformity of the WC particle size, is a value obtained by dividing the half-value width of the particle size distribution of the WC particles by the average particle size of the WC particles. In order to explain the definition of the degree of dispersion, sample No. 2 and No. Reference is made to the particle size distribution graph 101 (FIG. 1). The horizontal axis of the graph is the equivalent circle diameter (nm) of WC particles, and the vertical axis is the cumulative frequency (%) of WC particles. The half-value width of the particle size distribution of the WC particles is a value (nm) obtained by subtracting the equivalent circle diameter when the cumulative frequency is 25% from the equivalent circle diameter when the cumulative frequency is 75%. For example, if the number of extracted WC particles is 1000, the equivalent circle diameter of the WC particles having the 750th size from the smaller one is subtracted from the circle equivalent diameter of the WC particles having the 250th size. The half-value width of the particle size distribution.

  It can be said that the smaller the value of the degree of dispersion, the smaller the deviation in the particle size distribution of the WC particles. In the cemented carbide of the present invention, the degree of dispersion is 0.55 or less. Furthermore, when the degree of dispersion of the WC particles is 0.50 or less, the wear resistance and fracture resistance of the cemented carbide are dramatically improved.

[Average circularity]
The WC particles preferably have a shape with rounded corners (that is, a shape with high sphericity) instead of an angular shape. As a result of the study by the present inventors, it has been found that the thermal diffusivity of the cemented carbide tends to improve as the shape of the hard phase particles in the cemented carbide becomes a nearly spherical shape. The thermal diffusivity is improved because the WC particles having a rounded shape tend to have a larger contact area between the WC particles having a higher thermal diffusivity than the WC particles having an angular shape. It is done. In addition, since the WC particles have a rounded shape, the surface area per unit volume is likely to be small, and the interface of WC-Co, in which heat is difficult to diffuse, becomes small as a whole cemented carbide. This is thought to be a factor that increases the thermal diffusivity. Furthermore, by using WC particles with a high degree of circularity, stress concentration when an external force is applied can be suppressed, and the strength and fracture toughness of the cemented carbide can be improved. The fact that the WC particles in the cemented carbide are nearly spherical is that, as shown in the embodiments described later, the cross section of the cemented carbide is observed, and the circularity of the WC particles (the WC particle equivalent circumference is measured WC). It can be evaluated by obtaining a numerical value divided by the circumference of the particle.

  The evaluation of the circularity of the entire WC particles in the cemented carbide may be performed, for example, by calculating the circularity of 500 or more WC particles and using the average circularity obtained by averaging them. In that case, the average circularity is preferably 0.68 or more, and more preferably 0.75 or more. As described above, the higher the average circularity, the higher the thermal diffusivity of the cemented carbide, and it is difficult for thermal cracking to occur in the cemented carbide. As a result, the strength and fracture toughness of the cemented carbide can be improved.

≪Binder phase≫
The binder phase contains the most Co. In addition to Co, other iron group elements such as Ni and Fe may be contained, but it is preferable that only Co or substantially Co and Ni are included. Note that “substantially composed” means a case where it is composed of Co, except for inevitable impurities, or a case where it is composed of Co and Ni, as well as a compound used as a raw material (WC, Cr ₃ C described later). ₂ ) is allowed to be dissolved.

  When Ni is contained in the binder phase, the thermal diffusivity of the cemented carbide tends to decrease. Therefore, considering only toughness and thermal diffusivity, only Co is preferable. Therefore, when Ni is contained in the binder phase, Ni / Co + Ni is preferably 25% by mass or less, and more preferably 10% by mass or less.

  The ratio of the binder phase in the cemented carbide (binder phase / hard phase + bond phase) is preferably 4.5 to 15% by mass or less. By setting this ratio to 4.5% by mass or more, the cemented carbide can have sufficient toughness, and the occurrence of thermal cracks in the cemented carbide can be suppressed. Moreover, by setting the ratio to 15% by mass or less, the cemented carbide can have sufficient hardness, and a cemented carbide that hardly wears can be obtained. In particular, when the total content of the binder phase is 6% by mass or more and 13% by mass or less, it is easy to become a dense cemented carbide with high sinterability, and can be provided with a high balance between high hardness and high toughness, This cemented carbide has a high thermal diffusivity and is excellent in wear resistance and toughness.

≪Other contained elements≫
The cemented carbide of the present invention can be composed of WC and Co (or Co + Ni or Fe) and unavoidable impurities, as well as WC and Co (or Co + Ni or Fe), the following additive elements, and unavoidable impurities. . Examples of the additive element include one or more elements selected from Cr, Ta, Nb, Zr, and Ti. These elements are allowed to be contained in a total amount of 0.05 to 5.0% by mass in the cemented carbide. These elements are elements added in the form of compound particles in the crushing step or the mixing step in order to suppress the grain growth of particles (mainly WC particles) constituting the hard phase. Even when the compound particles are sintered, the size and shape when used as a raw material are easily maintained. In particular, inclusion of an element having a grain growth inhibiting effect such as Cr within the above range is expected to reduce a decrease in strength due to coarsening of WC particles during sintering. Therefore, a processing tool including a base material made of a cemented carbide containing an element such as Cr described above has a processing performance (abrasion resistance, heat crack resistance) due to a synergistic effect of grain growth suppression effect and thermal diffusivity improvement. Expected to be high.

Of the additive elements listed above, Cr is particularly preferable. Moreover, it is more preferable to contain Cr and one or more elements of Ta and Nb. In the case of containing Ta and Nb, compared with the case of containing Ti and Zr, there is little decrease in thermal diffusivity and toughness, and the effect of suppressing grain growth is enhanced by a synergistic effect with Cr. When only one of the above additive elements is contained, it is preferable to contain only Cr, since the synergistic effect of the grain growth suppressing effect and the improvement of the thermal diffusivity is most likely to be expressed. 0.05-3 mass% is preferable. The said effect can be acquired, without reducing a thermal diffusivity by making content of the said addition element 0.05-5 mass% or less. In particular, when it contains only Cr, its content is more preferably 0.3 to 3% by mass, and when it contains Cr and an element such as Ta, the content of Cr is 0.05 to 3% by mass. % Is preferable, and the total content is more preferably 0.3 to 5% by mass. In order to allow the additive element to be present in the cemented carbide, it is possible to use a simple element as a raw material or a compound such as a carbide containing the metal element (for example, Cr ₃ C ₂ ). The compound used as the raw material exists as a compound in the cemented carbide as it is, forms a new composite compound, dissolves in the binder phase, or exists as a single element.

<Machining tools>
The cemented carbide of the present invention can be used as a material for various processing tools. If it is this invention processing tool using this invention cemented carbide, it is excellent in abrasion resistance and a fracture resistance.

<Manufacturing method of cemented carbide>
The cemented carbide of the present invention described above can be produced, for example, by the following production method. First, a conventional method for producing a cemented carbide will be described, and its problems will be pointed out. Next, a method for producing a cemented carbide invented by the present inventors will be described.

  Conventional cemented carbides are generally manufactured in the following steps: preparation of mixed raw material of hard phase raw material and binder phase raw material → crushing and mixing of mixed raw material → drying → molding → sintering (→ heat treatment as appropriate). The crushing / mixing is conventionally performed for a relatively long time (several hours to several tens of hours) using a ball mill or an attritor. This is because fine WC particles are very easily aggregated, and unless the aggregation is sufficiently crushed, coarse WC particles are easily formed in the cemented carbide.

  However, as a result of investigation by the present inventors, when pulverization / mixing for a long time was performed, some of the WC particles prepared as the hard phase raw material were pulverized to a size smaller than the prepared particle size. It was found that a large amount of very fine WC particles were generated. It is considered that the minute WC particles generated in a large amount cause a deviation in the particle size distribution of the hard phase particles in the obtained cemented carbide and reduce mechanical properties such as fracture toughness value of the cemented carbide. In addition, very fine WC particles generated by pulverization tend to be a growth source for Ostwald growth (where WC diffused during solution phase sintering reprecipitates as coarse WC particles). The generation of coarse WC particles is also considered to be one factor that deteriorates the mechanical properties of the cemented carbide. In addition, the finely pulverized WC particles are likely to have a shape having sharp corners, and there is a problem that such irregularly shaped (low circularity) WC particles are likely to be a growth source for Ostwald growth.

Based on the above findings, the present inventors propose a method for manufacturing a cemented carbide comprising the following steps.
A raw material preparation step of preparing a hard phase raw material powder made of WC powder having an average particle size of 0.4 μm or less and a binder phase raw material powder mainly composed of Co powder.
A slurry preparation step of preparing a slurry containing at least a hard phase raw material powder among the two types of raw material powders prepared in the raw material preparation step.
A crushing step of stirring the slurry to crush the aggregation of the powder contained in the slurry.
The raw material addition process which adds binder phase raw material powder to the slurry which passed through the crushing process, when the binder phase raw material powder is not included in the slurry in the slurry preparation step.
A mixing step of mixing the hard phase raw material powder and the binder phase raw material powder by stirring the slurry that has undergone the crushing step.
A drying step of drying the slurry that has undergone the mixing step.
A forming step of forming a mixed powder of the hard phase raw material powder and the binder phase raw material powder obtained in the drying step.
A sintering process for sintering the compact obtained in the molding process.
* However, at least one of the crushing process and the mixing process does not use an attritor or a collision type jet mill.

  According to the method for producing a cemented carbide, the cemented carbide can be produced so that the particle size of the WC powder prepared as the material of the cemented carbide is substantially maintained even after sintering. This is because only the crushing treatment of the hard phase raw material powder containing WC particles that easily aggregate is performed first, and then the binder phase raw material powder is added to and mixed with the hard phase raw material powder. At that time, at least one of the crushing step and the mixing step does not use an attritor or a collision type jet mill, that is, does not employ a strong stirring method with a large total energy amount used to stir the raw material. It is possible to suppress the WC particles from being finely pulverized before consolidation. As a result, the particle size distribution of the WC particles at the time of sintering hardly occurs, and the cemented carbide obtained by sintering can also suppress the particle size distribution of the hard phase particles. If a tool such as a microdrill is made of such a cemented carbide, it can be a tool having a longer life than the conventional ones, which is superior in toughness, fatigue limit and wear resistance.

  The significance of not using an attritor or a collision type jet mill in at least one of the crushing step and the mixing step is that the total energy input to the raw material for stirring the raw material in both steps is less than a predetermined value. This is to reduce the possibility of WC particles being crushed. However, since it is difficult to control the total energy input to the raw material through both steps, in the present invention, the combination of [cracking step / mixing step] is [weak stirring / strong stirring], [strong stirring / weak stirring]. The total energy does not exceed a predetermined value by setting “stirring” or “weak stirring / weak stirring”.

  The criterion for determining “strong stirring” is that when 1 kg of powder composed of WC particles with a Fisher diameter of 0.4 μm is stirred until it becomes powder composed of WC particles with a Fisher diameter of 0.2 μm, It may be determined whether or not the integrated power input is 2.0 kWh / kg or more. Attritors and collision-type jet mills have a very large amount of energy per unit time, and consume power that easily exceeds the above standards in a short time. On the other hand, medialess stirring methods such as single-hole jet mills, ultrasonic homogenizers, and cyclone mills can be classified as weak stirring methods. Even a stirring method using a medium can be classified as a weak stirring method because a ball mill or a bead mill has a smaller energy amount per unit time than an attritor or the like. However, even if a ball mill or the like is performed over a long period of time, there is a possibility that it can be classified as a strong stirring method, but in the method of manufacturing the cemented carbide of the present invention, the crushing step and the mixing step are separated, The processing time of each process is much shorter than before. Therefore, ball mills and the like are classified as weak stirring methods in the present invention.

  Hereinafter, each process of the manufacturing method of a cemented carbide will be described in more detail.

≪Raw material preparation process≫
In the preparation step, a hard phase raw material powder made of WC powder having an average particle size of 0.4 μm or less and a binder phase raw material powder mainly composed of Co powder are prepared.

  The average particle size of the WC particles prepared in the raw material preparation step is defined by the 50% particle size as described above. The WC particles preferably have a shape close to a smooth sphere with few sharp points. This is because deformed WC particles are likely to be a growth source for Ostwald growth.

  The binder phase raw material powder contains the largest amount of Co powder. In addition to Co powder, iron group elements such as Ni powder and Fe powder may be contained. The content of the binder phase raw material powder at the time of mixing (binder phase raw material powder / total raw materials) is preferably 4.5 to 15% by mass. In that case, a cemented carbide excellent in the balance of hardness, toughness, and fatigue limit can be produced. The content of Co in the binder phase raw material powder (Co powder / Co powder + powder other than Co) is preferably 75% by mass or more, and more preferably 90% by mass or more.

≪Slurry preparation process≫
Among the hard phase raw material powder and the binder phase raw material powder prepared in the preparation step, a slurry is prepared in which at least the hard phase raw material powder is dispersed in an alcohol solvent such as ethanol. The concentration of particles in the slurry is preferably 40 to 70% by mass. If it is the density | concentration of this range, when stirring a slurry by a post process, a slurry can be stirred uniformly.

≪Crushing process≫
In the crushing step, the slurry prepared in the slurry preparation step is stirred to crush the particles aggregated in the slurry. Here, since the WC particles contained in the hard phase raw material powder are more likely to aggregate due to neck gloss as they become finer, fine WC particles of 0.4 μm or less as prepared in the raw material preparation step are very likely to aggregate. Therefore, in this manufacturing method, a process of loosening the aggregated WC particles into individual WC particles, that is, crushing is performed. Here, conventionally, this crushing step and a mixing step, which will be described later, are generally performed simultaneously using an attritor or a ball mill, but in this manufacturing method, both steps are performed separately. Thereby, the crushing step of stirring the slurry can be completed in a short time, and the agglomerated WC particles can be reliably crushed while suppressing the WC particles from being crushed.

  This crushing step is preferably performed by a medialess stirring method excluding a collision type jet mill. Examples of the medialess stirring method include a single-hole jet mill, an ultrasonic homogenizer, and a cyclone mixer. These medialess stirring methods can suppress WC particles from being pulverized into fine particles. An attritor or a collision type jet mill may be used in the crushing step, but in that case, the attritor and the collision type jet mill must not be used in the subsequent mixing step.

≪Raw material addition process≫
The raw material adding step is a step performed when the binder phase raw material powder is not included in the slurry in the slurry preparing step. That is, this step is not necessary when both the hard phase raw material powder and the binder phase raw material powder are included in the slurry in the slurry preparation step. Note that since the binder phase raw material powder may also aggregate, it is preferably pulverized before being added to the slurry.

≪Mixing process≫
In the mixing step, the hard phase raw material powder and the binder phase raw material powder contained in the slurry are uniformly mixed by stirring the slurry that has undergone the crushing step. Unlike the conventional method, this mixing step is also performed in a step separate from the pulverization of the WC particles. Therefore, the mixing step can be completed in a short time, and the two raw material powders are homogeneous while suppressing the WC particles from being pulverized. Can be mixed.

  This mixing step is preferably performed by a medialess stirring method excluding a collision type jet mill, as in the crushing step. If it is a medialess stirring method, since it can suppress that WC particle | grains are overmilled, it can suppress that the particle size distribution of WC particle | grains after a mixing process becomes broad. In the mixing step, an attritor or a collision type jet mill may be used, but in that case, it is assumed that the attritor and the collision type jet mill are not used in the previous crushing step.

≪Drying process, molding process, sintering process≫
The said drying, shaping | molding, and sintering process can utilize the thing of general conditions. For example, a drying method using a spray dryer or the like can be used for the drying process. When the slurry contains a binder such as paraffin wax, it can be granulated by a drying process. Moreover, the press using a metal mold | die, the extrusion using a die | dye, etc. can be utilized for a formation process. Furthermore, sintering can be performed in a vacuum atmosphere under conditions of 1320 to 1500 ° C. × 1 to 2 hours. You may perform HIP (hot isostatic pressing) sintering for 1350-1400 degreeC x 0.5 to 4 hours after sintering.

≪Others≫
Between the raw material preparation step and the mixing step, the following second hard phase raw material powder and grain growth inhibitor may be added to the slurry. These second hard phase raw material powder or grain growth inhibitor may be added to the slurry in the slurry preparation step, for example, just before the crushing step, or during the crushing step, just before the mixing step, Alternatively, it may be added to the slurry during the mixing step.

[Second hard phase raw material powder]
The second hard phase raw material powder is a compound that becomes a part of the hard phase by sintering when contained in the slurry. As the second hard phase raw material powder, for example, a compound of at least one metal selected from Periodic Table 4a, 5a, and 6a group elements and at least one element of carbon and nitrogen (however, excluding WC) That is, the compound particle | grains which consist of 1 type, or 2 or more types of compounds selected from the carbide | carbonized_material of the said metal (however, except WC), nitride, and carbonitride can be mentioned. By adding these second hard phase raw material powders in the crushing step and the mixing step, the wear resistance of the finished cemented carbide can be improved. Specific examples of the compound include TaC, (Ta, Nb) C, NbC, and TiCN. In particular, the compound is desirably at least one of TaC and NbC.

  The total content of the second hard phase raw material powder in the total hard phase raw material powder (second hard phase raw material powder / hard phase raw material powder consisting of WC particles + second hard phase raw material powder) is the wear resistance of the cemented carbide. From the viewpoint of improving the thermal crack resistance, the content is preferably 0.1 to 2.0% by mass.

[Grain growth inhibitor]
The grain growth inhibitor is an element or a compound of the element that suppresses grain growth of WC particles. Examples of the grain growth inhibitor include V carbide (for example, VC) and Cr carbide (for example, Cr ₃ C ₂ ). Here, VC and _Cr 3 _{C 2} is also by the second hard phase material powders. Therefore, if VC or Cr ₃ C ₂ is selected as the second hard phase raw material powder, the same effect as that obtained by adding the grain growth inhibitor can be obtained. The total content of the grain growth inhibitor (grain growth inhibitor / total raw materials) is preferably 0.1 to 2.0% by mass.

  According to the cemented carbide of the present invention, it exhibits excellent wear resistance and fracture resistance as a material for processing tools.

It is a graph of the particle size distribution of WC particles for explaining the definition of the dispersity frequency, where the horizontal axis is the equivalent circle diameter of the WC particles in the cemented carbide and the vertical axis is the cumulative frequency of the WC particles.

  A plurality of WC-based cemented carbide samples were produced by different production methods, and various characteristics of the obtained cemented carbide were measured to evaluate the influence of each production method on the obtained cemented carbide.

<Sample manufacturing method>
The manufacturing method of each sample is basically performed as follows.
Preparatory process for preparing slurry of WC powder that will be hard phase of cemented carbide → Crushing process of WC powder in slurry → Mixing process of WC powder and powder that becomes binder phase after crushing → Slurry after mixing process Process of drying / molding / sintering Each process will be described in detail below.

≪Preparation of WC powder slurry≫
In preparing the cemented carbide, a WC powder having an average particle size (50% particle size) shown in “WC particle size” in the “Slurry” column of Table 2 below was prepared as a hard phase raw material powder. These WC powders were mixed with an ethanol solvent to prepare a slurry. The WC concentration in each slurry is shown in “WC concentration” in Table 2.

≪Crushing process of WC powder≫
Next, among the processes listed below, one of a ball mill (BM), an attritor (ATR), a single-hole jet mill (JM-1), a collision jet mill (JM-2), and an ultrasonic homogenizer (SSH) Was used to break up agglomeration of WC particles in each slurry. The pulverization method and the average particle diameter (50% particle diameter) of the WC particles after pulverization are shown in “Crushing method” and “WC particle diameter after pulverization” in Table 2 below.

[Ball mill: BM]
A cemented carbide medium having a diameter of 5 mm was used. The total weight of the media was 8 kg (number of media = about 1000) with respect to 2 L of slurry. The mixing time was 24 hours. The total energy amount for performing BM under this condition is 0.8 kWh / kg.

[Attritor: ATR]
A cemented carbide medium having a diameter of 5 mm was used. The total weight of the media was 10 kg (number of media = about 1250) per 2 L of slurry. The mixing time was 15 hours. The total energy amount for performing ATR under this condition is 3.0 kWh / kg.

[Bead mill: BSM]
A cemented carbide medium having a diameter of 1.3 mm was used. The total weight of the media was 5 kg (number of media = about 36000) with respect to 2 L of slurry. The mixing time was 5 hours. The total energy amount for performing BSM under this condition is 1.2 kWh / kg.

[Single-hole jet mill: JM-1]
The slurry is ejected 10 times at a pressure of 240 MPa from a diamond nozzle having a nozzle inner diameter of 0.1 mm. The total energy amount for performing JM-1 under this condition is 1.4 kWh / kg.

[Collision type jet mill: JM-2]
A pair of diamond nozzles having a nozzle inner diameter of 0.1 mm are arranged to face each other, and slurry injected from each diamond nozzle is collided 10 times at a pressure of 240 MPa. The total energy amount for performing JM-2 under this condition is 3.1 kWh / kg.

[Ultrasonic homogenizer: SSH]
A horn with a 20 mm diameter tip is inserted into a tank that stores 2 L of slurry kept at room temperature, and sonication is performed for 3 hours at an output of 1 kW. The total energy amount for performing SSH under this condition is 1.4 kWh / kg.

[Cyclone mixer: CM]
The process is performed for 4 hours by setting the gap between the stationary blades and the rotor blades facing each other to 3 mm and the peripheral speed of the rotor blades to 180 m / min. The total energy amount for performing CM under this condition is 0.5 kWh / kg.

≪Mixing process≫
With respect to the slurry that has undergone the above crushing step, the binder phase raw material powder listed below, the second hard phase raw material powder, and at least one kind of grain growth inhibitor, and the paraffin wax that becomes the granulating material, In addition, mixing was performed using any of the BM, ATR, JM-1, JM-2, SSH, BSM, and CM described above. Table 1 shows the mixing ratio between the WC contained in the slurry and the added binder phase raw material powder. The amount of paraffin wax added to the slurry is 1.5% by mass. The average particle diameter (50% particle diameter) of the WC particles after the mixing method and mixing is shown in “Mixing method” and “WC particle diameter after mixing” in Table 2 below.

[Binder phase raw material powder]
Co powder (average particle size: 0.3 μm)
Ni powder (average particle size: 0.5 μm)
[Second hard phase raw material powder]
TaC powder (average particle size: 0.3 μm)
NbC powder (average particle size: 0.33 μm)
[Grain growth inhibitor]
VC powder (average particle size: 0.4 μm)
Cr ₃ C ₂ powder (average particle size: 0.6 μm)

  In the mixing step, the binder phase raw material powder, the second hard phase raw material powder, and the grain growth inhibitor may be preliminarily pulverized dry or wet.

≪Drying, forming, sintering process≫
After the mixing, each slurry was dried and granulated with a spray dryer, and the obtained granulated powder was extruded into a round bar shape of φ2.6 mm. The pressed body was sintered in a vacuum atmosphere at 1360 ° C. for 1 hour, and then the sintered body was sintered at 1350 ° C. for 1 hour by HIP (hot isostatic pressing).

<Evaluation of each sample>
About each obtained cemented carbide, the content of W, Co, Ni, Cr, Ta, Nb, V, Zr and Ti, the average particle diameter of WC particles, and the average circularity were measured. Moreover, in order to grasp the variation in the particle size distribution of the WC particles in the cemented carbide, the degree of dispersion (described in detail later) was obtained. Furthermore, the hardness, toughness, fatigue limit of each cemented carbide, and the fracture life of micro drills made from these cemented carbides were measured.

≪Content of each element≫
When the contents (mass%) of W, Co, Ni, Cr, Ta, Nb, V, Zr and Ti were measured by EDX (Energy Dispersive X-ray Spectroscopy) analysis, they were almost the same as the amounts used for the raw materials. Met. Further, when the composition of the hard phase in the cemented carbide was examined by X-ray diffraction, the hard phase of any sample was substantially composed of WC particles. The composition analysis can also be measured by XPS (X-ray Photoelectron Spectroscopy) or SIMS (Secondary Ion Mass Spectrometry).

≪Average particle size and average circularity≫
The average particle size (μm) of the WC particles in the cemented carbide was calculated by image analysis of a photomicrograph taken with a field emission scanning electron microscope (FESEM). First, an arbitrary cross section is taken in the cemented carbide, and the cross section is mirror-wrapped. Then, 500 or more WC particles are identified (mapped) by image analysis software (Scion Image; Scion Co.) for an arbitrary plurality of fields of view (magnification of 50,000 times) in the cross section. The equivalent circle diameter of each WC particle was determined from the area of each mapped WC particle. Of the obtained equivalent circle diameter, the equivalent circle diameter when the cumulative frequency was 50% was defined as the average particle diameter of the WC particles. Further, the circularity of each WC particle was obtained by dividing the equivalent circle circumference of each mapped WC particle by the measured circumference of the WC particle, and the average circularity was obtained by averaging these circularities. . The average particle diameter and average circularity of these WC particles are shown in “WC particle diameter” and “WC circularity” in “after sintering” in Table 2.

≪Dispersion frequency≫
Next, as an index for grasping the uniformity of the WC particle size, a value (dispersion frequency) obtained by dividing the half-value width of the particle size distribution by the average particle size was obtained. In order to explain the definition of the degree of dispersion, FIG. 2 and No. 101 shows a particle size distribution graph. The horizontal axis of the graph is the equivalent circle diameter (nm) of WC particles, and the vertical axis is the cumulative frequency (%) of WC particles. The half-value width of the particle size distribution is a value (nm) obtained by subtracting the equivalent circle diameter when the cumulative frequency is 25% from the equivalent circle diameter when the cumulative frequency is 75%. For example, if the number of mapped WC particles is 1000, the circle equivalent diameter of the WC particles having the 250th size is subtracted from the circle equivalent diameter of the WC particles having the 750th size from the smaller one. The half-value width of the particle size distribution referred to here. The degree of dispersion is obtained by dividing the half width by the average particle size (= 50% particle size) of the WC particles. The results are shown in Table 3.

≪Vickers hardness and fracture toughness value≫
The obtained cemented carbide was mirror-wrapped and then measured for Vickers hardness according to JIS R 1607. Further, the fracture toughness value Kc was determined by an indentation-fracture method using a Vickers hardness meter. The results are shown in Table 3.

≪Fatigue limit≫
The obtained cemented carbide was polished with a # 800 diamond grindstone, and then the fatigue limit (the maximum test load capable of 107 rotations: MPa) was determined using a hunter type rotating bending fatigue tester (span: 280 mm). The results are shown in Table 3.

≪Breakage test≫
The obtained round bar-shaped sintered body was formed into a drill shape. Specifically, the outer circumference of the round bar was roughly polished to obtain a round bar with a diameter of φ2.0 mm, and finally a micro drill having a drill diameter of 0.1 mm and a blade length of 4.0 mm was produced with diamond abrasive grains. . Microdrills made of cemented carbide for each manufacturing method by subjecting the finished microdrill to dry drilling under the following conditions and measuring the number of times the microdrill is chipped and wears beyond tolerance The fracture life of was evaluated. The results are shown in Table 3.

[conditions]
Work material: A printed circuit board in which glass layers and epoxy resin layers are alternately laminated. The total number of layers is 20 and the total thickness is 2.4 mm.
Drill rotation speed: 250 krpm
Drill feed speed: 1.5m / min

≪Summary≫
From the above Tables 1 to 3, the following became clear.
1. When a weak stirring method (stirring method other than Attritor (ATR) and collision type jet mill (JM-2)) is used in the crushing step, the reduction width of the average particle diameter of the WC powder is small before and after crushing (Table 2). See).
2. When a weak stirring method is used in the mixing step, the reduction width of the average particle diameter of the WC powder is small before and after mixing (see Table 2).
3. When the reduction width of the average particle diameter of the WC powder is small for each step, the degree of dispersion is small (see Tables 2 and 3).
4). When the degree of dispersion is 0.55 or less (sample Nos. 1 to 23), the toughness, fatigue limit, and fracture life tend to be excellent (see Table 3).
5. In particular, when the degree of dispersion is 0.5 or less, the breakage life is dramatically extended.
6). When the circularity is 0.68 or more, the breakage life is extended. In particular, the circularity is preferably 0.75 or more, and the breakage life tends to be extended as the circularity increases.

  As described above, conventionally, the crushing / mixing step in one step is made into an independent crushing step and a mixing step, and at least one of the two steps is performed using a weak stirring method. It was revealed that Nos. 1 to 23 exhibited excellent toughness, fatigue limit and fracture life.

  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.

  The cemented carbide of the present invention can be suitably used as a material for a tool used for precision machining such as a micro drill, an end mill, and a ball end mill, and further for fine machining such as a mold.

For example, when a cemented carbide is used for a tool used for micromachining such as a micro drill, it is said that the smaller the WC particles contained in the cemented carbide, the better the cemented carbide will have better wear resistance and fracture resistance. It has been broken. However, as a result of the study by the present inventors, in the cemented carbide obtained by the conventional manufacturing method, the particle size distribution of the WC particles contained in the cemented carbide is broad (particle size that is larger or smaller than the particle size peak). since the WC particles is in the high state), there is a case that defects early utilizing such a micro drill.

The present invention has been made in view of the above circumstances, and one of its purposes is to provide a cemented carbide that is more excellent in wear resistance and fracture resistance than conventional ones.

A cemented carbide that satisfies the above requirements is superior in wear resistance and fracture resistance .

It can be said that the smaller the degree of dispersion, the sharper the particle size distribution of the WC particles (there are fewer WC particles having a particle size that is larger or smaller than the particle size peak) . In the cemented carbide according to one aspect of the present invention, the degree of dispersion is 0.55 or less. Furthermore, when the degree of dispersion of the WC particles is 0.50 or less, the wear resistance and fracture resistance of the cemented carbide are dramatically improved.

However, as a result of investigation by the present inventors, when pulverization / mixing for a long time was performed, some of the WC particles prepared as the hard phase raw material were pulverized to a size smaller than the prepared particle size. It was found that a large amount of very fine WC particles were generated. It is considered that the minute WC particles generated in a large amount make the particle size distribution of the hard phase particles in the obtained cemented carbide broad and reduce mechanical properties such as fracture toughness value of the cemented carbide. In addition, very fine WC particles generated by pulverization tend to be a growth source for Ostwald growth (where WC diffused during solution phase sintering reprecipitates as coarse WC particles). The generation of coarse WC particles is also considered to be one factor that deteriorates the mechanical properties of the cemented carbide. In addition, the finely pulverized WC particles are likely to have a shape having sharp corners, and there is a problem that such irregularly shaped (low circularity) WC particles are likely to be a growth source for Ostwald growth.

According to the method for producing a cemented carbide, the cemented carbide can be produced so that the particle size of the WC powder prepared as the material of the cemented carbide is substantially maintained even after sintering. This is because only the crushing treatment of the hard phase raw material powder containing WC particles that easily aggregate is performed first, and then the binder phase raw material powder is added to and mixed with the hard phase raw material powder. At that time, at least one of the crushing step and the mixing step does not use an attritor or a collision type jet mill, that is, does not employ a strong stirring method with a large total energy amount used to stir the raw material. It is possible to suppress the WC particles from being finely pulverized before consolidation. As a result, hardly becomes a particle size distribution of the WC particles is broad at the time of sintering, the particle size distribution of the hard phase particles in a cemented carbide obtained by sintering can be suppressed to become broad. If a tool such as a microdrill is made of such a cemented carbide, it can be a tool having a longer life than the conventional ones, which is superior in toughness, fatigue limit and wear resistance.

Claims (6)

A cemented carbide in which a hard phase mainly composed of WC particles is bonded by a binder phase mainly composed of Co,
The average particle size of the WC particles constituting the hard phase is less than 0.4 μm, and
A cemented carbide having a dispersity frequency of 0.55 or less, which is an index representing uniformity of WC particle diameter.
Here, the degree of dispersion is a value obtained by dividing the half-value width of the particle size distribution of the WC particles by the average particle size of the WC particles.   The cemented carbide according to claim 1, wherein the degree of dispersion is 0.50 or less.   The cemented carbide according to claim 1 or 2, wherein an average circularity of the WC particles is 0.68 or more.   The cemented carbide according to claim 3, wherein the average circularity is 0.75 or more.   The cemented carbide according to any one of claims 1 to 4, wherein the grain growth inhibitor includes at least one of V carbide and Cr carbide.   A machining tool manufactured using the cemented carbide according to any one of claims 1 to 5.

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