WC-based cemented carbide

Abstract

To provide a WC-based cemented carbide suitable for a cutting tool in which the wear of a flank is suppressed in a milling process of steel and the like.SOLUTION: A WC-based cemented carbide contains Co of 5 mass% or more and 12 mass% or less, at least one of Ta and Nb of 1 mass% or more and 15 mass% or less in total, and Ti of 1 mass% or more and 10 mass% or less, with the balance being WC and inevitable impurities. The WC particles have an average particle size of 0.7 μm or more and 2.5 μm or less, and the WC particles have an integrated circularity of 0.480 or more and 0.520 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to WC-based cemented carbides.

Background Art

[0002] WC-based cemented carbides such as WC-Ti-(Ta,Nb)-Co series are alloys excellent in heat resistance and weld resistance, and are thus used as tool materials for cutting workpieces such as steel. In order to improve the cutting performance of cutting tools using such WC-based cemented carbides, for example, proposals as described in Patent Document 1 have been made.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present invention has been made in view of the above circumstances, and an object thereof is to provide a WC-based cemented carbide suitable for a cutting tool in which flank wear is more suppressed in milling of workpieces such as steel.

Means for Solving the Problems

[0005] The WC-based cemented carbide according to an embodiment of the present invention contains Co in an amount of 5% by mass or more and 12% by mass or less, at least one of Ta and Nb in a total amount of 1% by mass or more and 15% by mass or less, and Ti in an amount of 1% by mass or more and 10% by mass or less, the balance being composed of WC and unavoidable impurities, the average particle size of the WC particles being 0.7 μm or more and 2.5 μm or less, and the integrated circularity of the WC particles being 0.480 or more and 0.520 or less.

[0006] Further, the WC-based cemented carbide according to an embodiment of the present invention The HRA may be 89.5 or more and 91.5 or less.

Advantages of the Invention

[0007] According to the above, it is possible to provide a WC-based cemented carbide suitable for a cutting tool in which flank wear is more suppressed in milling of steel or the like.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0009] The inventor has found that for a WC-Ti-(Ta,Nb)-Co-based WC cemented carbide, when the integrated circularity of WC is within a predetermined range, the variation in toughness within the cemented carbide structure is small, and flank wear is easily suppressed in milling of steel or the like.

[0010] Hereinafter, embodiments of the present invention will be described in detail. In this specification, with respect to WC constituting the WC-based cemented carbide, the notations "WC" or "WC particles" are used, but these notations are not intended to clearly distinguish between WC and WC particles.

[0011] 1. Composition The composition of the WC-based cemented carbide will be described.

[0012] (1) Co Co forms a binding phase that connects WC particles, which are hard phases, carbide phases, and composite carbide phases generated by the addition of Ta, Nb, and Ti, and is an element that imparts high toughness to WC-based cemented carbides. If the Co content becomes too low, the toughness and strength of the WC-based cemented carbide decrease, and the sinterability of the WC-based cemented carbide deteriorates. As a result, voids and Co pools are likely to occur, making it difficult to maintain toughness and strength. Furthermore, if the Co content decreases, the range of a sound structure in which decarburized phases and free carbon do not precipitate becomes narrow, making it difficult to control carbon in mass production. On the other hand, if the Co content becomes too high, the hardness decreases.

[0013] Based on the above, the Co content is preferably 5% by mass or more and 12% by mass or less, more preferably 8% by mass or more and 10% by mass or less.

[0014] (2) Ta and Nb It is preferable to contain either one or both of Ta and Nb. These elements are added as carbides to improve the heat resistance of WC-based cemented carbides. The total content of Ta and Nb (content as metal, not as carbide) is preferably 1% by mass or more and 15% by mass or less. The reason is that if it is less than 1% by mass, the improvement in heat resistance described above cannot be recognized. On the other hand, if it exceeds 15% by mass, the toughness and sinterability of the WC-based cemented carbide decrease. In addition, the total content of Ta and Nb is preferably 5% by mass or more and 10% by mass or less.

[0015] (3) Ti Ti is added as a carbide to enhance the heat resistance of WC-based cemented carbides. On the other hand, if the Ti content becomes too high, the toughness and sinterability decrease. Therefore, the Ti content (content as metal, not as carbide) is preferably 1% by mass or more and 10% by mass or less, more preferably 2% by mass or more and 8% by mass or less.

[0016] 2. Particle shape of WC The particle shape of WC will be described.

[0017] (1) Average particle size The average particle size of WC is preferably 0.7 μm or more and 2.5 μm or less. By having the WC average particle size within this range, a cutting tool excellent in the balance between wear resistance and toughness can be obtained in the machining of steel and the like. Here, the particle size is the diameter of a circle that gives an area equal to the particle area of WC for 2000 or more WC particles when observing the cross-section of the WC-based cemented carbide. In a graph where the particle size distribution is plotted on the horizontal axis and the diameter of this circle is plotted on the vertical axis, the diameter when the cumulative frequency of the particle size distribution reaches 50% is defined as the average particle size. This average particle size is more preferably 0.8 μm or more and 2.0 μm or less.

[0018] (2) Integrated circularity The circularity of WC particles is defined as 4πS / L, where S is the area and L is the perimeter length of each WC particle for 2000 or more WC particles. 2 The circularity of WC approaches 1 as its shape becomes closer to a perfect circle. The integrated circularity is calculated from the circularity and the particle size distribution. The particle size distribution is based on the number, and the WC particles with finer particle sizes are counted in order. The particle size at the occupancy rate N% with respect to the total quantity is defined as the cumulative frequency N%. Then, the average circularity of all particles up to the cumulative frequency N% is defined as the integrated circularity at N%. The integrated circularity in the range of 10% or more and 90% or less of the cumulative frequency of the WC particle size distribution is preferably 0.480 or more and 0.520 or less, and more preferably 0.490 or more and 0.510 or less. The reason is that when the integrated circularity is within this range, the WC with angular shapes such as triangles and quadrilaterals decreases. As a result, the uniformity of toughness improves inside the WC-based cemented carbide, and flank wear is easily suppressed in the milling of steel and the like.

[0019] For the measurement of circularity and particle size distribution, a backscattered electron diffraction detector (for example, OIM of TSL Solutions Co., Ltd., hereinafter referred to as EBSD) equipped with an electron probe microanalyzer (for example, JXA-8350F of JEOL Ltd., hereinafter referred to as EPMA) is used. An inverse pole figure is obtained by EBSD, and individual grain boundaries are determined by regarding regions where the misorientation value is greater than 5° as separate particles. The same process is performed for all WC particles within the measurement region to obtain the circularity and particle size distribution.

[0020] Coarse WC particles with grain growth during the sintering process become triangular or square, so they tend to have a small circularity. Considering this, the cumulative frequency and integrated circularity of the WC particle size distribution When the cumulative frequency is 20%, the integrated circularity is 0.500 or more and 0.510 or less, When the cumulative frequency is 50%, the integrated circularity is 0.495 or more and 0.505 or less, When the cumulative frequency is 80%, the integrated circularity is 0.485 or more and 0.495 or less It is more preferable to satisfy the above.

[0021] Also, as will be described in the manufacturing method below, in order to obtain the above integrated circularity, it is preferable to post-add coarse WC raw material powder during the mixing of the raw material powders.

[0022] 3. Others The WC-based cemented carbide preferably has an HRA of 89.5 or more and 91.5 or less. By setting the HRA within this range, it is possible to balance the hardness and toughness, which are in an antinomic relationship, in milling processes such as for steel.

[0023] 4. Manufacturing Method As an example of the manufacturing method, the following can be shown. That is, it is preferable to use WC raw material powders with an average particle size of 4.0 to 6.0 μm, particularly two types of WC raw material powders (fine grains and coarse grains) with a narrow particle size distribution width, progressing towards single crystallization, and different particle size distributions (average particle sizes). When mixing the raw material powders, it is preferable to mix the fine-grained WC raw material powder with other raw material powders for a predetermined time, and then add (post-add) the coarse-grained WC raw material powder and mix.

[0024] By post-adding the coarse-grained WC raw material powder, the shape of the WC raw material powder can be easily maintained even after mixing and sintering without over-grinding the coarse-grained WC raw material powder, and the integrated circularity of the cemented carbide can be increased. For mixing, an attritor, ball mill, etc. can be used. Sintering is preferably carried out by pressure sintering at 1400 to 1480 °C and holding for several hours.

Examples

[0025] As the WC raw material powders, two types were used: a powder with an average particle size of 4.5 μm (30 mass%) and a powder with an average particle size of 4.0 μm (70 mass%). As the Co raw material powder, a powder with an average particle size of 1.5 μm was used. As the TaC raw material powder and TiC raw material powder, powders with an average particle size of 1 μm were used respectively. These raw material powders were blended to have the compositions shown in Table 1. The total mass of the raw material powders was set to 200 kg, and wet mixing was carried out using a production-scale attritor. During mixing, a small amount of C powder to supplement the carbon consumed during the sintering process and a binder powder for molding were added.

[0026] In the example, first, powders other than the WC powder with an average particle size of 4.5 μm were mixed for 4.5 hours, and then the WC powder with an average particle size of 4.5 μm was post-added and mixed for an additional 2 hours. In the comparative example, the same raw material powders as in the example were prepared, and all the raw material powders were simultaneously charged into the attritor and mixed for 6 hours. After mixing, in both the example and the comparative example, drying was carried out using a spray dryer to produce granulated powder of each composition, and inserts were formed by press molding. Then, sintering was carried out at 1400 °C for 1 hour to produce a medium-carbon WC-based cemented carbide in which no decarburized phase and free carbon were precipitated. Table 1 shows the compositions, etc. of the WC-based cemented carbides according to the example and the comparative example produced.

[0027]

Table 1

[0028] As is clear from Table 1, it was confirmed that the examples and the comparative examples had the same hardness since the values of HRA were almost equal, and the holding force and saturation magnetization were also almost equal.

[0029] The produced WC-based cemented carbide was mirror-finished, and microstructure observation was carried out using EPMA (JXA-8530F manufactured by JEOL). Fig. 1 shows the microstructure observation photograph of this example (magnification: 3000 times, scale length: 10 μm), and Fig. 2 shows the microstructure observation photograph of the comparative example (magnification: 3000 times, scale length: 10 μm).

[0030] Then, using EBSD (Electron Back Scatter Diffraction) and image analysis software, 2000 WC particles were measured, and the particle size distribution equivalent to a circle and the integrated circularity of the WC particles were obtained. Fig. 3 shows the particle size distribution of the WC particles, and Fig. 4 shows the measurement results of the integrated circularity.

[0031] The average particle size obtained from Fig. 3 was almost the same for the examples and the comparative examples as shown in Table 1. On the other hand, regarding the integrated circularity, as shown in Fig. 4, the examples were in the range from 0.480 to 0.520, and the comparative examples were in the range from 0.465 to 0.490. It was confirmed that the integrated circularity of the examples was larger than that of the comparative examples.

[0032] Next, in order to evaluate the variation in toughness in the WC-based cemented carbide, the crack length was measured. That is, with respect to the mirror-polished surface of the WC-based cemented carbide, a Vickers indenter was linearly pressed into 10 places so that the center-to-center distance of each indentation was 3 to 4 mm under the following conditions, and the lengths (L1 to L4) of the cracks generated at the four corners of the indentations were measured. The measurement results of the crack length are shown in Table 2 for the examples and in Table 3 for the comparative examples, respectively.

[0033] (Condition for pressing the Vickers indenter) Testing machine: AVK type manufactured by Akashi Seisakusho Pressing force: 50 kgf (approx. 490 N) Pressing hold time: 15 seconds

[0034] The maximum value, minimum value, average value, standard deviation, for all of the lengths (L1 to L4) of the cracks that occurred at the four corners of each indentation The difference between the maximum value and the minimum value of the total value of the crack lengths (L1 to L4) for each indentation And The difference between the maximum value and the minimum value among the crack lengths (L1 to L4) for each indentation, the average value of this difference, and the difference between the maximum value and the minimum value of this difference Are shown in Table 4

[0035] [Table 2]

[0036] [Table 3]

[0037] [Table 4]

[0038] As is clear from Table 4, in any of the standard deviation, the difference between the maximum value and the minimum value of the total value of the crack lengths (L1 to L4), and the difference between the maximum value and the minimum value among the crack lengths (L1 to L4), the average value of this difference, and the difference between the maximum value and the minimum value of this difference, the values for the examples are smaller, indicating that the variation in crack length is small, that is, the variation in toughness is small

[0039] Subsequently, for the WC-based cemented carbides of the examples and comparative examples, coated inserts with an AlTiN coating having an average thickness of about 4 μm were produced by the PVD method (referred to as the example tool and the comparative example tool, respectively). Cutting evaluation was carried out under the following conditions. Three tests were conducted under the same conditions, and the maximum flank wear width after cutting and its variation were evaluated. The results of the cutting evaluation are shown in Table 5.

[0040] (Cutting evaluation conditions) · Cutter model number: ASRT5063R-4 · Insert model number: WDNW140520 · Number of cutting edges: 1 · Cutting method: Face milling · Workpiece material: S50C (220 HB) · Depth of cut: Axial direction, 1 mm, Radial direction, 42 mm · Cutting speed: 180 m / min · Feed per cutting edge: 1.5 mm / edge · Protrusion amount: 100 mm · Cutting length: 37 m

[0041]

Table 5

[0042] As is clear from Table 5, the tool of this example had a smaller maximum flank wear width and a smaller difference between the maximum and minimum values than the tool of the comparative example. Therefore, it can be said that the WC-based cemented carbide of the example had more stable toughness than the WC-based cemented carbide of the comparative example, and the maximum flank wear width and variation were improved.

Claims

1. It contains 5 mass% or more and 12 mass% or less of Co, a total of 1 mass% or more and 15 mass% or less of at least one of Ta and Nb, 1 mass% or more and 10 mass% or less of Ti, with the balance consisting of WC and inevitable impurities, the average particle size of the WC particles is 0.7 μm or more and 2.5 μm or less, and the integrated circularity of the WC particles is 0.480 or more and 0.520 or less. A WC-based cemented carbide characterized by this.

2. The WC-based cemented carbide according to claim 1, characterized in that HRA is 89.5 or more and 91.5 or less.

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