Cemented carbide and cutting tool using the same

Abstract

To provide a cemented carbide and a cutting tool using the same, the cemented carbide being suitable for a cutting tool having superior durability in machining of high-hardness materials.SOLUTION: A cemented carbide of the present invention is a cemented carbide comprising a hard phase containing WC and a binder phase containing Co in an amount of 7 mass% or more and 13 mass% or less, wherein an average particle size of WC is in a range of 0.37 μm or more and 0.48 μm or less, and a coefficient of variation of WC particle size is in a range of 0.39 or more and 0.45 or less. A hardness of the cemented carbide is in a range of 92.5 HRA or more and 95 HRA or less on the Rockwell A scale. Furthermore, a value obtained by dividing the hardness of the cemented carbide by a transverse rupture strength of the cemented carbide is in a range of 19 or more and 29 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cemented carbide used as a material for cutting tools and the like, and to cutting tools. [Background technology]

[0002] In recent years, there has been an increasing demand for cutting tools suitable for cutting difficult-to-cut materials. There are various types of difficult-to-cut materials, and when cutting high-hardness steels such as hardened steel and die steel, the cutting tool itself must be highly wear-resistant (hard). Furthermore, for small-diameter cutting tools, breakage resistance (strength (transverse rupture strength)) is also required.

[0003] For example, Patent Document 1 discloses a cutting tool using a WC-based cemented carbide alloy that has high strength and small variation in transverse rupture strength, characterized in that the alloy contains an average WC grain size of 1.2 μm or less, 5 to 15% by weight of Co, and 0.4 to 1.5% by mass of one or more of Cr, V, Ta, and Nb, with the Co being uniformly dispersed throughout the alloy.

[0004] Furthermore, Patent Document 1 cites agglomerates of WC particles as the origin of fracture during measurement of transverse rupture strength (transverse rupture test), and describes that the agglomeration of WC is presumed to be caused by non-uniform Co concentration in the cemented carbide, and that by reducing the variation in Co concentration, it is possible to prevent the agglomeration of WC and reduce the variation in transverse rupture strength. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-120903 Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 1 does not make any specific (quantitative) reference to the wear resistance (hardness) of cemented carbide, and the inventors felt that it was necessary to determine the desirable ranges of wear resistance (hardness) and breakage resistance (transverse rupture strength) for cemented carbide suitable for cutting high-hardness materials.

[0007] Furthermore, the present invention focuses on the average grain size and grain size variation of WC, which forms the hard phase, as an element necessary for a cemented carbide to combine wear resistance (hardness) and breakage resistance (transverse rupture strength), and aims to provide an average WC grain size that achieves wear resistance (hardness) that can withstand cutting of high-hardness materials, while reducing the WC grain size variation to achieve breakage resistance (transverse rupture strength). Therefore, an object of the present invention is to provide a cemented carbide suitable for cutting tools with excellent durability when cutting high-hardness steel, and a cutting tool using the same. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, the present invention provides a cemented carbide having a hard phase containing WC with an average grain size in the range of 0.37 μm to 0.48 μm and a binder phase containing 7 to 13% Co by mass, and having a coefficient of variation of the WC grain size in the range of 0.39 to 0.45.

[0009] The hardness is set to a range of 92.5 HRA or more and 95 HRA or less on the Rockwell A scale. In this case, as an index representing the balance between wear resistance (hardness) and breakage resistance (transverse rupture strength), "Z" is defined as the value obtained by dividing the hardness value by the transverse rupture strength value of the cemented carbide, as shown in the following formula 1, and this "Z" value is set to a range of 19 or more and 29 or less. It is preferable that the Co contained in the binder phase is set to a range of 7 to 9% by mass. The invention of a cutting tool relates to a cutting tool using the above-mentioned cemented carbide as a base material. Z = (Hardness (HRA)) / (Transverse rupture strength (GPa)) (Equation 1) [Effects of the Invention]

[0010] The cemented carbide of the present invention has a hard phase containing WC and a binder phase containing Co in the range of 7% to 13% by mass, and by setting the average grain size of the WC in the range of 0.37 μm to 0.48 μm, cutting tools made from the cemented carbide of the present invention have achieved wear resistance (hardness) sufficient to withstand cutting of high-hardness materials.

[0011] In addition, by setting the coefficient of variation of the WC grain size in the range of 0.39 to 0.45, the breakage resistance (transverse rupture strength) required for cutting tools is improved. In other words, cutting tools made from the cemented carbide of the present invention have the effect of combining wear resistance (hardness) and breakage resistance (transverse rupture strength). [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a table showing the blending ratios of raw material powders of test pieces in Examples. [Figure 2] 1 is a SEM photograph of invention material 1 of an example. [Figure 3] 1 is an SEM photograph of Comparative Material 1 of the Example. [Figure 4] 1 is an SEM photograph of comparative material 2 of the example. [Figure 5] 1 is a table showing WC grain sizes and characteristic values ​​of test pieces of examples. [Figure 6] 1 is a table showing the average WC grain size of test pieces of examples. [Figure 7] 1 is a table showing the coefficient of variation of the WC grain size of test pieces of the examples. [Figure 8] 1 is a table showing the hardness of test pieces in examples. [Figure 9] 1 is a table showing the transverse rupture strength of test pieces of examples. [Figure 10] 1 is a table showing Z values ​​(hardness / transverse rupture strength) of test pieces in examples. [Figure 11] 1 is a graph showing the analysis results of WC particle diameters in an example. [Figure 12] 1 is a graph showing the measurement results of hardness and transverse rupture strength in Examples. DETAILED DESCRIPTION OF THE INVENTION

[0013] A cemented carbide according to one embodiment of the present invention will be described. The cemented carbide of the present invention is mainly composed of a hard phase containing WC and a binder phase containing Co. The hard phase and binder phase will be described in detail below.

[0014] (Hard phase) The hard phase in the cemented carbide of the present invention is mainly composed of WC (tungsten carbide), and if the average grain size is less than 0.37 μm, the risk of chipping of the cutting edge during cutting increases, while if it exceeds 0.48 μm, the hardness decreases, resulting in a decrease in wear resistance against the workpiece.

[0015] Therefore, the average particle size of WC was set to a range of 0.37 μm to 0.48 μm. The raw WC powder can be selected and used so that its size satisfies the WC average particle size range after sintering. At the same time, the coefficient of variation of the WC particle size was set to a range of 0.39 to 0.45.

[0016] The average grain size of the WC grains mentioned above was calculated using the following method. The polished cross-sectional structure of the bulk cemented carbide tool metal was observed using an SEM, and the resulting photograph was then binarized using the software "ImageJ." The WC grains were separated one by one and their diameters were measured to calculate the average grain size. The coefficient of variation of the WC grain size was calculated using the following equation 2. Coefficient of variation = (WC particle size variation (standard deviation)) / (WC average particle size) (Equation 2)

[0017] (bonded phase) The binder phase in the cemented carbide of the present invention is mainly composed of Co, and is contained in a range of 7% to 13% by mass. The binder phase content is set to 7 to 13% by mass because if the binder phase ratio is less than 7%, the risk of chipping of the cutting edge during cutting increases, and if the binder phase ratio exceeds 13%, the hardness decreases, and the wear resistance against the work material decreases.

[0018] Next, we will explain the method for manufacturing the cemented carbide of the present invention. First, raw material powders consisting of WC powder, Co powder, and a grain growth inhibitor are blended in a predetermined ratio. These raw material powders (mixed powder) are mixed and pulverized using equipment such as an attritor or ball mill, and then a molding aid such as wax is added to produce the mixed powder.

[0019] Next, these mixed powders are molded to produce a green compact, which is then sintered to obtain a sintered body. The sintering process involves a sintering temperature of 1340-1500°C, a sintering time of 30-180 minutes, and then a HIP treatment under optimal conditions of a holding temperature of 1340-1500°C, a holding pressure of 3-50 MPa, and a holding time of 30-150 minutes.

[0020] Prior to this step, the temperature may be gradually increased to around 700 to 1000°C and then maintained at that temperature for 30 to 180 minutes. Various grain growth inhibitors, such as VC and Cr3C2, can be used. VC is preferably used in the form of powder with a particle size in the range of 0.6 to 0.7 μm to more uniformly inhibit the grain growth of WC within the alloy. [Example]

[0021] Test pieces of the cemented carbide according to the present invention (hereinafter referred to as the inventive material) and the cemented carbide outside the present invention (hereinafter referred to as the comparative material) were prepared and subjected to material property evaluation tests, and the test results are described below. The material property evaluations performed in this test were focused on the following three material properties: ·WC particle size measurement Hardness measurement Transverse strength measurement

[0022] For the cemented carbide test pieces used in this test, raw material powders consisting of WC powder, Co powder, and VC and Cr3C2 as grain growth inhibitors were blended to prepare mixed powders according to the manufacturing method described above. Regarding the blending of raw material powders, four levels of test pieces were prepared according to the particle size and Co content of the WC powder, as shown in Figure 1: Inventive Materials 1 and 2, and Comparative Materials 1 and 2. The WC powder used for Inventive Materials 1, 2, and Comparative Material 1 had an average particle size of 0.5 μm, while the WC powder used for Comparative Material 2 had an average particle size of 0.8 μm.

[0023] Next, these mixed powders were molded to produce green compacts, which were then subjected to a sintering process to obtain sintered bodies. The sintering process involved holding the compacts at a predetermined temperature in the range of 1340-1430°C for 60 minutes in a vacuum atmosphere, followed by HIP treatment with the holding pressure adjusted to approximately 4-10 MPa, followed by cooling. Three lots of each of the cemented carbide alloys used in this test (invention materials 1 and 2 and comparison materials 1 and 2) were produced.

[0024] The WC particle size (mean value and coefficient of variation) was measured and calculated using the method described above (analysis using ImageJ and calculation using Equation 2). Hardness was measured using a Rockwell hardness tester, and the average of the values ​​measured at three points on each test piece was used. Transverse strength was measured using a three-point bending test with a jig with a span of 20 mm.

[0025] SEM photographs of the invention material 1 used in this example (material property evaluation test) are shown in Figure 2, SEM photographs of the comparative material 1 in Figure 3, and SEM photographs of the comparative material 2 in Figure 4. As shown in the SEM photographs in Figures 2 to 4, WC grains (white areas) and Co (black areas) can be seen in both the invention material and the comparative material. It can also be seen that the WC grain size in the invention material 1 shown in Figure 2 is smaller than the WC grain size in the comparative materials 1 and 2 shown in Figures 3 and 4.

[0026] The analysis results of WC grain size (calculated mean value and coefficient of variation) and the measurement results of hardness and transverse rupture strength are shown in Figure 5, the statistical results of each value are shown in Figures 6 to 10, and the resulting graphs of WC grain size (mean value, coefficient of variation), hardness, and Z are shown in Figures 11 and 12. Note that when calculating the range of each statistical value, a normal distribution was assumed, and the formula "3 sigma (3σ)", that is, "(mean) ± ((standard deviation) × 3)", was used to cover approximately 99.7% of the total distribution of the data.

[0027] (WC average particle size) The average grain size of WC in the cemented carbide produced in this example was in the range of 0.407 to 0.445 μm for Inventive Material 1 (n=3) and in the range of 0.388 to 0.417 μm for Inventive Material 2 (n=3), as shown in Figure 5. Similarly, it was in the range of 0.386 to 0.509 μm for Comparative Material 1 (n=3), and in the range of 0.578 to 0.682 μm for Comparative Material 2 (n=3).

[0028] Furthermore, taking into account the distribution (variation) of the measurement data, the range of the average WC particle size was 0.380 to 0.475 μm (mean: 0.427, standard deviation: 0.016) for Inventive Material 1 (n = 3), and 0.374 to 0.430 μm (mean: 0.402, standard deviation: 0.009) for Inventive Material 2 (n = 3), as shown in Figure 6. Similarly, the range was 0.282 to 0.593 μm (mean: 0.438, standard deviation: 0.052) for Comparative Material 1 (n = 3), and 0.604 to 0.630 μm (mean: 0.617, standard deviation: 0.004) for Comparative Material 2 (n = 3).

[0029] (Coefficient of variation of WC grain size) The coefficient of variation of the WC grain size for the aforementioned Inventive Materials 1 and 2 and Comparative Materials 1 and 2 was calculated using the aforementioned formula (2). As a result, for Inventive Material 1 (n=3), the coefficient was in the range of 0.425 to 0.430, and for Inventive Material 2 (n=3), the coefficient was in the range of 0.412 to 0.433, as shown in Figure 5. Similarly, for Comparative Material 1 (n=3), the coefficient was in the range of 0.442 to 0.449, and for Comparative Material 2 (n=3), the coefficient was in the range of 0.458 to 0.469.

[0030] Furthermore, the coefficient of variation range of the WC grain size, taking into account the distribution (variation) of the measurement data, was 0.421 to 0.434 (mean: 0.427, standard deviation: 0.002) for Inventive Material 1 (n = 3), and 0.391 to 0.448 (mean: 0.420, standard deviation: 0.009) for Inventive Material 2 (n = 3), as shown in Figure 7. Similarly, the coefficient of variation range was 0.436 to 0.456 (mean: 0.446, standard deviation: 0.003) for Comparative Material 1 (n = 3), and 0.451 to 0.478 (mean: 0.465, standard deviation: 0.004) for Comparative Material 2 (n = 3).

[0031] Therefore, it was found that for invention materials 1 and 2, the ranges of the WC average particle size taking into account the distribution (variation) of the measurement data shown in Figure 6 and the range of the coefficient of variation of the WC particle size taking into account the distribution (variation) of the measurement data shown in Figure 7 are, as shown in the shaded area in Figure 11, 0.370 μm or more and 0.480 μm or less for the WC average particle size, and 0.390 or more and 0.450 or less for the WC particle size coefficient of variation.

[0032] (Hardness) The hardness of the cemented carbide produced in this example was in the range of 93.4 to 93.7 HRA for Inventive Material 1 (n=3) and 92.7 to 92.9 HRA for Inventive Material 2 (n=3), as shown in Figure 5. Similarly, the hardness of Comparative Material 1 (n=3) was in the range of 92.8 to 93.0 HRA, and the hardness of Comparative Material 2 (n=3) was in the range of 91.5 to 91.8 HRA.

[0033] Furthermore, taking into account the distribution (variation) of the measurement data, the hardness range of the cemented carbide was 93.2 to 94.0 HRA for Inventive Material 1 (n = 3) (mean: 93.6, standard deviation: 0.1), and 92.6 to 93.0 HRA for Inventive Material 2 (n = 3) (mean: 92.8, standard deviation: 0.1), as shown in Figure 8. Similarly, Comparative Material 1 (n = 3) (mean: 92.7 to 93.2 HRA) (mean: 92.9, standard deviation: 0.1), and Comparative Material 2 (n = 3) (mean: 91.6, standard deviation: 0.1) (91.3 to 92.0 HRA).

[0034] (transverse rupture strength) The transverse rupture strength of the cemented carbide alloys produced in this example was in the range of 3.8 to 4.5 GPa for Inventive Material 1 (n = 3) and 4.5 GPa for Inventive Material 2 (n = 3), as shown in Figure 5. Similarly, the transverse rupture strength of Comparative Material 1 (n = 3) was in the range of 3.8 to 3.9 GPa, and the transverse rupture strength of Comparative Material 2 (n = 3) was in the range of 3.7 to 4.5 GPa.

[0035] (variation in transverse rupture strength) Furthermore, taking into account the distribution (variation) of the measurement data, the range of the transverse rupture strength of the cemented carbide alloys was 3.3 to 4.9 (average value: 4.1, standard deviation: 0.3) for Inventive Material 1 (n = 3), as shown in Figure 9, and 4.4 to 4.6 (average value: 4.5, standard deviation: 0) for Inventive Material 2 (n = 3). Similarly, Comparative Material 1 (n = 3) was in the range of 3.7 to 4.0 (average value: 3.9, standard deviation: 0), and Comparative Material 2 (n = 3) was in the range of 3.2 to 5.2 (average value: 4.2, standard deviation: 0.3).

[0036] (Z = hardness / transverse rupture strength) The hardness value (unit: HRA) of the above-mentioned cemented carbide was divided by the transverse rupture strength value (unit: GPa) to calculate "Z (unit: HRA / GPa)". As a result, the value for inventive material 1 (n=3) was in the range of 21.0 to 24.7, and the value for inventive material 2 (n=3) was in the range of 20.4 to 20.7, as shown in Figure 10. Similarly, the value for comparative material 1 (n=3) was in the range of 23.6 to 24.3, and the value for comparative material 2 (n=3) was in the range of 20.4 to 24.8.

[0037] Regarding the range of "Z," which takes into account the distribution (variation) of the measurement data, inventive material 1 (n=3) of this example was in the range of 19 to 28 (average value: 23, standard deviation: 1.5), and inventive material 2 (n=3) was in the range of 23 to 25 (average value: 24, standard deviation: 0.3), as shown in Figure 10. Similarly, comparative material 1 (n=3) was 21 (average value: 21, standard deviation: 0.1), and comparative material 2 (n=3) was in the range of 16 to 28 (average value: 22, standard deviation: 1.9).

[0038] Therefore, it was found that the ranges of hardness taking into account the distribution (variation) of the measurement data shown in Figure 8 and the range of transverse rupture strength taking into account the distribution (variation) of the measurement data shown in Figure 9 were in the range of 92.5 HRA or more and 95 HRA or less on the Rockwell A scale, as shown in the shaded area in Figure 12, and the value (Z) obtained by dividing the hardness value by the transverse rupture strength value was in the range of 19 or more and 29 or less.

[0039] From the above, as shown in Figures 6 and 7, Inventive Materials 1 and 2 have smaller average WC grain sizes and smaller coefficients of variation of WC grain size than Comparative Materials 1 and 2, indicating that fine WC grains are uniformly distributed. Furthermore, as shown in Figures 5, 8, 9, and 10, Inventive Materials 1 and 2 are found to have both high hardness and high transverse rupture strength.

[0040] Therefore, the results showed that inventive material 1 and inventive material 2 have a finer average WC particle size to achieve higher wear resistance (hardness) than comparative material 1 and comparative material 2, and a uniform WC particle size to achieve high breakage resistance (transverse rupture strength).

Claims

1. A cemented carbide having a hard phase containing WC and a binder phase containing Co in a range of 7% to 13% by mass, wherein the average grain size of the WC is in a range of 0.37 μm to 0.48 μm, and the coefficient of variation of the WC grain size is in a range of 0.39 to 0.

45.

2. 2. The cemented carbide according to claim 1, wherein the cemented carbide has a hardness in the range of 92.5 HRA to 95 HRA on the Rockwell A scale.

3. 3. The cemented carbide according to claim 2, wherein a value obtained by dividing the hardness value by the transverse rupture strength of the cemented carbide is in the range of 19 or more and 29 or less.

4. 4. The cemented carbide according to claim 3, wherein the Co content in the binder phase is in the range of 7% to 9% by mass.

5. A cutting tool made of the cemented carbide according to any one of claims 1 to 4.

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