JP5826138B2 - Tough cemented carbide and coated cemented carbide - Google Patents

Description

  The present invention relates to a tough cemented carbide and a coated cemented carbide.

  Conventionally, a cemented carbide in which tungsten carbide (WC) particles and cobalt (Co) as a binding metal are mixed and sintered at an appropriate ratio is known. Cemented Carbide made by mixing and sintering WC particles and Co is used as a material for manufacturing cemented carbide tools such as cutting tools and molds because of its high hardness and high strength. ing.

  In recent cold or hot forging processes, an increasingly heavy load and a long life are required for a cemented carbide mold. The same applies to cutting tools, and the development of coated cemented carbide (Coated Cemented Carbide) is in line with this requirement. As the cutting conditions are increased in speed and load, a coated cemented carbide base material is required to withstand hardness, plastic deformation resistance and high strength. These cemented carbides are widely used for cutting tools, dies, mining tools, wear-resistant tools and so on.

Patent Document 1 describes that the addition of chromium (Cr) or chromium carbide (usually Cr ₃ C ₂ ) to cemented carbide can be expected to suppress WC growth and improve corrosion resistance.

Patent Document 2 describes that Cr ₃ C ₂ solid-solution strengthens the binder phase and improves the wear resistance of the cemented carbide cutting blade.

In Patent Document 3, a cemented carbide in which a hard phase mainly composed of WC is dispersed in a binding phase of an iron group metal (Fe, Co, Ni) containing Co is heated to a heating temperature of 1200 to 1300 ° C. after immediately by quenching, the lattice constants of Co becomes higher 3.570A, but transverse rupture strength is not changed, there is a description that can be improved impact strength.

In Non-Patent Document 1, in the cemented carbide of WC—Cr3C2-15% Co, the addition of Cr3C2 up to 2% by mass does not change the transverse strength (Transverse-Rupture-Strength; TRS), but more than that. There is a description that the amount of addition decreases considerably. In a WC / Co alloy, a region where neither η phase (Co3W3C) nor free carbon is generated is generally called a healthy phase region. According to Non-Patent Document 1, the upper limit of the lattice constant of the corresponding Co phase (fcc) is about 3 .573 to 3.572 and the lower limit is about 3.552 to 3.553. Further, it is stated that when Cr is added, the upper limit value is slightly lowered and the lower limit value is increased. If the vicinity of the upper limit is exceeded, the risk of the occurrence of η phase increases. The generation of η phase must be avoided because it causes a fatal decrease in strength, and when approaching the lower limit value, free carbon is likely to be generated, and there is a risk of performance degradation. Accordingly, the region where neither the η phase nor free carbon is generated is generally known as a healthy phase region and is well known. However, quality control of cemented carbide is usually performed with strict adherence to this healthy phase region.

Japanese Examined Patent Publication No. 62-56224 Japanese Patent No. 3175077 Japanese Patent No. 4537501

"Structure and mechanical properties of WC-Cr3C2-15% Co cemented carbide", Hisashi Suzuki, Kei Tokumoto, "Powder and Powder Metallurgy", Vol. 31, No. 2, February 1984 "Fatigue of WC-12% Co alloy sintered by hot isostatic pressing" Yuji Fujiwara, Fumihiro Ueda, Hiroaki Masatomi, Hisashi Suzuki, "Powder and Powder Metallurgy", Vol. 27, No. 6, August 1980

In Patent Document 3, after heating the WC-Co alloy (not including Cr), by quenching immediately with decreasing the Co phase hexagonal close-packed structure (hcp), the lattice constants of Co 3. Although the bending strength is not changed, the impact strength can be improved.
However, it states that rapid cooling at a cooling rate of 1000 ° C./min or more is necessary to improve such performance. As is well known, the lattice constant of Co is 3.570 mm or more is an area where there is a risk of η phase generation. This document also states that rapid cooling of 1000 ° C./min or more is necessary for improving the performance in order to avoid the occurrence of this η phase. For the first time, the rapid quenching prevents the generation of η phase and provides a healthy structure with a high lattice constant. In general, in order to avoid the generation of η phase, it is considered that the lattice constant should be avoided very carefully in the vicinity of 3.570 mm, and the region exceeding 3.570 mm should be avoided. Such quenching can be applied to small products, specimens, or alloys with high Co content and low hardness, but cracks may occur when trying to apply to alloy products with high hardness that are generally used in practice. Even if the occurrence of this can be avoided, there is a risk that the reliability may be lowered due to residual internal stress or the like. Further, looking at the lattice constants of the examples, the average value is 3.573 mm, which is only in the vicinity of or in the vicinity of the danger region of the occurrence of η phase that deviates from the healthy phase. Although the η phase is avoided by ultra-rapid cooling, the cooling rate varies slightly depending on the location even if the cooling rate changes slightly, or even if it is a large product, the generation of the η phase is a concern. Moreover, the chromium (Cr) which is the object of the present application is not contained, and of course, there is no mention about the influence on the cemented carbide and the effect when Cr is added and Cr is contained. The present invention does not intend to realize a large lattice constant while preventing the generation of η phase by using such ultrafast cooling, and the lattice constant is generally less than or equal to around 3.570Å, as described above. This is a method that realizes high performance without risking the occurrence of η phase and the risk associated with ultra rapid cooling, which is made possible by the addition of Cr.
From the conventional technical point of view, the addition of chromium (Cr) or chromium carbide (usually Cr3C2) is expected to suppress WC grain growth, improve corrosion resistance, and improve wear resistance of special cutting blades. However, the bending strength (TRS) is hardly changed by adding a small amount, but it is said that the bending strength (TRS) is lowered when the addition amount is increased (Non-Patent Document 1). ).

  Cobalt (Co) has a transformation point near 420 ° C., fcc is stable above that temperature, and hcp is stable below that temperature. However, in the cemented carbide, since there are many carbide phases such as WC and few Co phases, the transformation is suppressed, and a lot of fcc remains even when the temperature is lowered to 420 ° C. or room temperature from the high sintering temperature, Usually, fcc and hcp are mixed. Since the fcc structure is more ductile than the hcp structure, it is considered that fcc is more suitable for the Co phase (bonding phase) that binds carbides such as WC that are hard but poor in ductility. Further, although the cemented carbide is required to have fatigue strength in many applications, according to Non-Patent Document 2, when fatigue progresses, fcc transforms into hcp, and the hcp / fcc ratio increases and does not eventually break. It is estimated that.

  In view of such circumstances, the object of the present invention relates to a cemented carbide containing tungsten carbide (WC), cobalt (Co) and chromium (Cr), while containing a small amount of chromium (Cr), compared with the conventional case. An object of the present invention is to provide a tough cemented carbide and a coated cemented carbide that have increased bending strength and compressive strength, as well as increased fatigue strength.

  The tough cemented carbide of the present invention contains tungsten carbide (WC), cobalt (Co) and chromium (Cr), WC has an average particle size of 1.5 to 20.0 μm, Vickers hardness of 650 to 1650 Hv, Co is contained in an amount of 4 to 30% by mass, Cr is contained in an amount of 2 to 18% by mass of the Co content, the balance is made of WC and inevitable impurities, and Cr is dissolved in Co in order to reduce Co in the hcp structure. The X-ray diffraction image (2θ = 44.3 degrees) of the fcc (111) plane of the Co phase using Cu—Kα rays and the X-ray diffraction image (2θ = 44.9) of the (002) plane of hcp. The half-value width of the X-ray diffraction images overlapping each other is 0.42 degrees or less. Here, the X-ray diffraction image is an X-ray diffraction image at a diffraction angle 2θ using Cu-Kα rays.

  In the present invention, Cr is solid-solved in Co to strengthen the Co phase (also referred to as γ phase) and to improve toughness, thereby improving the strength of the cemented carbide. In strengthening and reforming the Co phase, it is difficult to contribute to improving the strength of the cemented carbide unless the thickness of the Co layer between the WC particles is thick to some extent. Therefore, the effect of improving the strength is expected when the Co mass% is above a certain level, and the effect of improving the strength is expected when the WC particles are large when the Co mass% is small. The effect of the addition of a small amount of Cr in the cemented carbide was examined in detail. Then, it was discovered that the addition of Cr makes it easy to increase the fcc / hcp ratio of the Co phase, and an alloy that increases the fcc / hcp ratio by utilizing this Cr addition effect has its bending strength and compression. It has been found that strength such as strength is improved, and has found a significant improvement in performance in impact resistant tools, wear resistant tools and cutting tools using this alloy compared to conventional products. According to the experiment relating to the present invention, when the average particle size of WC is less than 1.5 μm, no improvement was observed due to the reduced thickness of the Co layer. Therefore, when the average particle size of WC was 1.5 μm or more, the bending strength and compressive strength were investigated by changing Co mass%. When the Co mass% was 4 mass% or less or the Vickers hardness exceeded 1650 Hv, the thickness of the Co layer was too small, so there was no improvement effect. Further, when the Vickers hardness is less than 650 Hv, the practicality of the cemented carbide is almost absent.

In this invention, the ratio with respect to Co content of Cr is set to 2-18 mass%. This is because the Cr effect does not appear when the content is less than 2% by mass, and the Cr3C2 crystal appears when the content is greater than 18% by mass, and the strength of the cemented carbide decreases. According to the experimental results to be described later, it has been found that the cemented carbide according to the present invention has higher compressive strength and greater fracture toughness than conventional products, and the cemented carbide according to the present invention is It has been found that the Vickers hardness is increased and the bending strength is increased as compared with the conventional product. The tough cemented carbide of the present invention is produced by a general powder metallurgy method as described in paragraph 0023. The present invention cannot achieve high performance by realizing a large Co lattice constant exceeding the sound region by utilizing the ultra rapid cooling as in Patent Document 3. Accordingly, the lattice constant of Co is preferably less than or equal to around 3.570 mm where there is little risk of η phase. On the other hand, in Table 1 of Example 1, the lattice constant of Sample A2 is 3.570 、, there is no η phase, high bending strength, and high performance is realized. Therefore, this patent added Cr, and Co phase (fcc) The lattice constant is 3.570 mm or less.

  In the present invention, a part of the WC includes carbides of transition metals such as TiC, TaC, NbC, HfC, ZrC, and VC, carbonitrides of these transition metals, or double carbides of these transition metals including W or W. It is characterized in that it is replaced with any one or more of these transition metal double carbonitrides.

  According to the experiment relating to the present invention described later, an improvement in the bending strength was recognized, and an improvement in wear resistance during cutting was observed.

  The present invention is characterized in that a de-β layer is formed on the surface of the alloy, and the thickness of the de-β layer is 1 to 30 µm.

The cemented carbide in which the β-free layer is formed is used as a base material for the coated cemented carbide. This base material improves the strength and toughness of the coated cemented carbide, and helps to extend the life and reliability of the cutting tool. However, as a drawback, cutting edge of the cutting edge (plastic deformation at high temperature) is likely to occur in cutting of high hardness steel and high speed cutting of general steel, so that the use in these cutting fields is limited.
It has been found that the cutting edge sagging can be improved by using a coated cemented carbide made of a cemented carbide with a de-β layer according to the present invention as a base material. Here, the de-β layer is a layer having no β phase, and is a layer having a high Co content and a slightly lower hardness but excellent strength and toughness. Since the coating material is ceramic and is hard and brittle, this de-beta layer compensates for the brittleness of the coating material and improves the toughness of the cutting tool. On the other hand, in the cutting of high hardness steel, the sag of the cutting edge is likely to occur because the de-β layer is soft. According to the present invention, Cr dissolves in Co in the de-β layer to improve the strength at high temperature, and the cutting edge sag is improved. In the present invention, the thickness of the de-β layer is set to 1 to 30 μm as an effective range of the de-β layer.

In the present invention, a method using an X-ray diffraction image is employed as means for quantitatively capturing the Co fcc / hcp ratio, and Cu is used as a target. The measurement surface of the alloy to be measured was polished to a mirror surface by polishing 50 μm after grinding. It is known that at a depth of 10-30 μm from the surface of the ground surface, the hcp / fcc ratio is increased by replacing part of the fcc of the Co phase with hcp due to residual stress and strain caused by normal grinding. In order to avoid the influence of this ground surface layer, in this patent, the half surface width and the lattice constant were measured by removing the ground surface by polishing 50 μm or more and measuring the mirror finished surface. The X-ray diffraction image of the (111) plane of the fcc of the Co phase has 2θ of 44.3 degrees, and the X-ray diffraction pattern of the (002) plane of hcp has 2θ of 44.9 degrees. Normally, the diffraction intensity of the X-ray diffraction image of the (111) plane of fcc is higher, but when the X-ray diffraction images of the (002) plane of hcp overlap, the half width of the overlapped X-ray diffraction image is The larger hcp is, the larger it is. In the present specification, the above-described overlapping X-ray diffraction images are described as fchp diffraction images for convenience. According to the experimental results, the strength of the alloy whose half width of the fchp diffraction image is 0.43 degrees or more is inferior to that of the alloy whose half width of the fchp diffraction image is 0.42 degrees or less.
In the X-ray diffraction image of the Co phase shown in FIG. 1, in the tough cemented carbide of the present invention, the half width of the fchp diffraction image is small as shown in FIGS. 1 (a) and 1 (c). It can be seen that the fcc / hcp ratio is increased. On the other hand, in the conventional cemented carbide, the half width of the fchp diffraction image is large and the fcc / hcp ratio is small as shown in FIGS. I understand. Here, in the present specification, the half-width of the fchp diffraction image obtained by the above measurement method is 0.42 degrees or less as the high-performance tough cemented carbide in the present invention. The alloys shown in FIGS. 1 (a) and 1 (b) contain 20% by mass of Co, and the alloys shown in FIGS. 1 (c) and 1 (d) contain 9% by mass of Co.

  According to the present invention, even when the fchp half-value width is 0.42 degrees or less, toughness and heat resistance strength are improved by adding Cr and increasing solid elements such as Cr (including W). The fcc lattice constant is measured as a method for measuring the amount of solid elements in the Co phase. As the fcc lattice constant of the Co phase is larger, the content of the solid element is increased and the strength and heat resistance are improved. The present invention is characterized in that the solid solution amount of Cr and W in Co is increased so that the fcc lattice constant of the Co phase is 3.560% or more. When the fcc lattice constant of the Co phase is 3.550 or less, the effect of improving strength and heat resistance is small.

  According to these inventions, according to the use of the cemented carbide, cutting tools and molds having good workability optimized for hardness, bending strength, compatibility with the work material, mining tools, It becomes a wear-resistant tool.

The powder metallurgy method is applied to the method for manufacturing the cemented carbide according to the present invention. For example, an appropriate amount of WC, Co, and Cr powders are mixed in advance, the mixed powder is press-molded, and then heated to an appropriate temperature in a vacuum to be sintered to obtain a sintered body. The sintered body may be subjected to hot isostatic pressing (HIP treatment). Alternatively, sintering and HIP treatment can be performed simultaneously in the same furnace using a SinterHIP furnace. Also, for quality control, the general concept of cemented carbide manufactured by powder metallurgy is applied, such as manufacturing without η phase and free carbon (so as to maintain a healthy phase range). .

  According to the present invention, the strength of the cemented carbide is improved by solid solution of Cr in Co to strengthen the Co phase and improve toughness. In the strengthening / modification of the Co phase, if the thickness of the Co layer between the WC particles is not thick to some extent, it is difficult to contribute to the improvement of the strength of the cemented carbide. It was set to 1.5 μm or more. However, cemented carbides with a proportion of Co exceeding 30% by mass and cemented carbides with an average WC particle size exceeding 20.0 μm are not practical in practice, so the upper limit of Co% by mass is 30% by mass. %, And the upper limit of the average particle size of WC is 20.0 μm.

According to the present invention, strength improvement can be expected while having the same hardness as a known alloy, so high performance can be expected in various application fields, for example, a cold alloy in which a cemented carbide with a relatively high Co content is used. Forging dies exhibit a life that is about twice as long as conventional products. Cold forging dies are often used in the automotive parts industry, the electronic parts industry, and the like, which will contribute to the cost reduction of these parts. According to the present invention, not only the strength at normal temperature but also the strength at high temperature can be expected, and the wear resistance of the cutting tool is improved.
In particular, a coated cemented carbide using a base material on which a β-free layer is formed contributes to prevention of cutting edge sag during cutting, which is a defect of the β-free layer. In addition, in cemented carbides such as mining tools that have large WC grains or a relatively large Co content, it is possible to expect reduction in fractures and improvement in wear resistance, thus realizing a long tool life.

It is an X-ray diffraction image of the Co phase in various cemented carbides, (a) and (c) are X-ray diffraction images of the Co phase of the tough cemented carbide of the embodiment to which the present invention is applied, (b) (D) is an X-ray diffraction image of the Co phase of the cemented carbide of the conventional example. It is the image which imaged the structure | tissue of the tough cemented carbide alloy of embodiment to which this invention was applied with the optical metal microscope. It is the image which imaged the structure | tissue of the conventional cemented carbide with the optical metal microscope. It is a top view which illustrates a cross joint. It is the image which observed the state after the lifetime test of the cross joint consisting of the tough cemented carbide of the said embodiment with the microscope. It is the image which observed the state after the life test of the cross joint which consists of the said conventional cemented carbide alloy with the microscope. It is sectional drawing which illustrates the cold forge metal mold | die for volt | bolts and nuts.

  Hereinafter, the best mode for carrying out the present invention will be described based on examples. It should be noted that the present invention is not limited to the following embodiments, and known modifications can be made within a range substantially the same as or equivalent to the present invention.

(Embodiment 1)
For wear resistant tools such as cold or hot forging dies and tools, or mining tools, WC powder having an average particle size of 1.5 to 20.0 μm is used, and Co powder is 7 to 25 mass. % Blended. Cr is blended by 2 to 18% by weight with respect to Co. For addition of Cr, Cr ₃ C ₂ powder is preferably used. Cr powder can also be used. However, it is necessary to pay attention to the fact that the surface of the Cr powder is oxidized and the carbon adjustment may be difficult in a cemented carbide with a small Co%. Although it is expensive, CrN can also be used.

These WC powder, Co powder, and Cr ₃ C ₂ powder are weighed and placed in a ball mill or attritor together with an organic solvent (alcohol, acetone, hexane, etc.) and wet mixed. Thereafter, the organic solvent is removed by evaporation, and the mixed powder is dried. Mass production can be improved by drying these powders with a spray dryer and granulating them at the same time.

A lubricant (paraffin, polyethylene glycol, camphor, etc.) for facilitating pressing is mixed with the powder mixture, and the powder mixture is put into a mold suitable for the shape of the product and pressed. Thereafter, sintering is performed in a vacuum at a temperature in the range of 1280 ° C to 1500 ° C. Sintering includes vacuum sintering, HIP treatment after vacuum sintering, and SinterHIP for performing sintering and HIP treatment in the same furnace, depending on the composition and use. The optimum sintering conditions are selected according to the composition, shape and application. And after sintering, electric discharge machining, grinding, and polishing are processed in this order to complete a tool such as a mold.
As one of the methods for controlling the fchp half-value width to 0.42 degrees or less, the cooling rate from the heating temperature heated to 1200 to 1500 ° C. to the cooling temperature of 50 to 200 ° C. is 10 ° C. / Min or more is desirable. Here, the heating temperature includes a temperature of 1280 ° C. or more and 1500 ° C. or less as a sintering temperature. Although there are other methods for controlling the fchp half-width, it has not been studied.

As a practical method of setting the fcc lattice constant of the Co phase to 3.560 mm or more and the half width of the fchp diffraction image to 0.42 degrees or less, the heating temperature or sintering temperature of the cemented carbide is 1200 ° C. or more and 1500 ° C. From the following temperatures to 800 ° C or lower and 500 ° C or higher, rapid cooling is performed as primary cooling, and the temperature of the product to be heat-treated is maintained for a certain period of time within the cooling temperature range of primary cooling so that the primary cooling is performed. The inventors have found a two-stage cooling method in which the temperature is rapidly cooled from the temperature to 200 ° C. or less as secondary cooling. Here, the cooling rate of the primary cooling is preferably 25 ° C./min or more, and the cooling rate of the secondary cooling is preferably 5 ° C./min or more. Furthermore, the cooling rate of primary cooling is preferably 30 ° C./min or more, and the cooling rate of secondary cooling is preferably 10 ° C./min or more.
On the other hand, even if the cemented carbide is continuously rapidly cooled from the range of 1200 ° C. to 1500 ° C. to the range of 50 ° C. to 200 ° C., the above two-stage cooling method and the corresponding half width A lattice constant is obtained. However, it can be said that the above two-stage cooling is more versatile considering that the residual stress on the product is reduced. Further, as a method for setting the lattice constant to 3.560 or more, there is a method of making the bonded carbon of carbides such as WC insufficient in addition to the above. However, with the method of lacking the bonded carbon of the carbide, it is not possible to stably realize the lattice constant of 3.560 or more, and it is considered difficult with the current technology. When measuring an X-ray diffraction image, it is necessary to remove the grinding-affected layer remaining at a depth of 10-30 μm from the grinding surface. For this reason, it is desirable to polish it to a mirror surface by polishing 50 μm or more in the depth direction from the surface of the ground surface in the cemented carbide.

(Embodiment 2)
WC powder having an average particle size of 1.5 to 7.0 μm is used, and 4 to 15 mass% of Co powder is blended. Cr is blended by 2 to 18% by weight with respect to Co. For addition of Cr, Cr ₃ C ₂ powder is preferably used. Depending on the application, titanium (Ti), tantalum (Ta), niobium (Nb), hafnium (Hf), zirconium (Zr), vanadium (V) carbide, carbonitride, or tungsten (W) is included. Any one or more of these transition metal double carbides or double carbonitrides are blended. The method for controlling weighing, wet mixing, pressing, sintering, and the half width of the fchp image to 0.42 degrees or less, and the method for controlling the lattice constant to 3.560 mm or more are the same as in the first embodiment. This composition is mainly used for cemented carbide for cutting tools. There are many things that can be used directly as a cutting tool after sintering, but they may be further ground into a high-precision tool.

(Embodiment 3)
When manufacturing the base material of the coated cemented carbide in which the de-β layer is formed, a small amount of nitride or carbonitride is added to the blended powder of Embodiment 2. The method of controlling the weighing, wet mixing, pressing, sintering, and the half width of the fchp image to 0.42 degrees or less, and the method of controlling the lattice constant to 3.560 mm or more are the same as in the first embodiment. . By the action of this added nitrogen, a deβ layer can be developed during vacuum sintering. A known CVD method is applied as a coating method for producing the coated cemented carbide according to the present invention, and the coating conditions are as usual.

  Since the implementation of the present invention can be carried out more stably when the cooling rate from a temperature of 1200 ° C. or higher is higher, in the examples shown below, the cooling rate is controlled within a predetermined speed range for experiments. However, it is considered that there is still a margin in the lower limit value of the cooling rate, and it is considered that the fcc / hcp ratio can be increased even at a cooling rate slower than the following examples, and the present invention can be implemented.

Example 1
Examples of cold forging dies are described below.

Table 1 shows the composition ratios of the inventive products A1 and A2, the comparative product A3, and the conventional product B in mass%. Here, WC having an average particle size of 3 μm was used. Attritor mixing was performed for 6 hours in alcohol at the above blending ratio. After the alcohol was evaporated and the powder was dried, pressing was performed at a pressing pressure of 1 ton / cm ² , and sintering was performed in a SinterHIP furnace under a sintering temperature of 1380 ° C. and a pressure of 1 MPa (10 bar). Invention product A1 and conventional product B were cooled at a cooling rate of 12 ° C./min from the sintering temperature to 200 ° C. Inventive product A2 is heated to 1250 ° C. after sintering, primarily cooled from 1250 ° C. to 650 ° C. at a cooling rate of 30 ° C./min, held at 650 ° C. for 10 minutes, and temperature 200 Secondary cooling to 10 ° C. was performed at a cooling rate of 10 ° C./min. Comparative product A3 did not adjust the cooling rate from the sintering temperature to 200 ° C., but the actual cooling rate was 4 ° C./min. For each sample, physical characteristics such as bending strength (TRS) were measured. The results are shown in Table 2.

As shown in Table 2, invention products A1 and A2 are superior in bending strength (TRS), Vickers hardness and compression strength to conventional product B not containing Cr. Inventive products A1 and A2 have better bending strength (TRS) and compressive strength than comparative product A3, which contains Cr and has a large half-value width of the fchp image. A1 and A2 also have Co lattice constants of 3.555 and 3.570, respectively, which are larger than A3 and B. A2 has a small half width and a lattice constant close to the upper limit allowed at 3.570 mm, but both the bending strength and hardness are larger than others.

  Next, a cold forging die was manufactured using the above-described invention products A1 and A2 and the conventional product B. Then, a cross joint was manufactured by closed forging and a life test was performed.

  FIG. 4 is a plan view showing the cross joint. A portion surrounded by a one-dot chain line in the figure is a portion where stress is easily concentrated due to its structure. FIG. 5 is an image obtained by observing a state of a cross joint made of a mold using the tough cemented carbide A1 of the above embodiment after a life test with a microscope, and there was no abnormality even after 300,000 shots. FIG. 6 is an image obtained by observing a state of a cross joint made of a mold using the conventional cemented carbide B after a life test with a microscope, and chipping occurred in 150,000 shots. Similarly, in the cross joint made of a mold using the tough cemented carbide A2 of the above embodiment, there was no abnormality even after 300,000 shots. Invention A2 was in a state where a longer life could be expected than Invention A1. From this, it can be said that the mold life made of the inventive products A1 and A2 is twice as long as the life of the mold made of the conventional product B. In the practical test, the reason why the performance is more than doubled is not only because the TRS or the compressive strength is a little high, but because the fatigue strength can be greatly improved by increasing the fcc / hcp ratio. .

(Example 2)
Examples of cutting tools are described below.

Table 3 shows the composition ratios of Invention C1, Comparative Product C2 and Conventional Product D in mass%. Here, WC having an average particle size of 2 μm was used. Attritor mixing was performed for 6 hours in alcohol at the above blending ratio. After the alcohol was evaporated and the powder was dried, pressing was performed at a pressing pressure of 1 ton / cm ² and vacuum sintering was performed at a temperature of 1400 ° C. Invention C1 was cooled from 1300 ° C. to 200 ° C. at a cooling rate of 12 ° C./min. The comparative product C2 and the conventional product D were cooled from 1300 ° C. to 200 ° C. at a cooling rate of 4 ° C./min. For each sample, physical characteristics such as bending strength (TRS) were measured. The results are shown in Table 4.

As shown in Table 4, the inventive product C1 has the same Vickers hardness as compared with the comparative product C2 and the conventional product D, but has a higher bending strength (TRS). SNMA432 chips were manufactured from the alloys of Invention C1, Comparative Product C2, and Conventional Product D, respectively, and a cutting test using a lathe was performed under the following conditions.
As the cutting conditions, the work material is SCM3, and the cutting speed v = 100.
mm / min, cutting depth d = 2 mm, feed rate f = 0.4 mm / rev, cutting time is 30 minutes.
As a result, the wear level of the chip made of the inventive product C1 is about 2/3 compared to the wear level of the chip made of the comparative product C2 and the wear level of the chip made of the conventional product D. It was found that the abrasion was excellent.

(Example 3)
An example of a coated cemented carbide cutting tool using a base material on which a de-β layer is formed will be described below.

Table 5 shows the composition ratios of Invention E and Conventional Product F in mass%. Here, WC having an average particle size of 4 μm was used. Attritor mixing was performed for 6 hours in alcohol at the above blending ratio. After the alcohol is evaporated and the powder is dried, pressing is performed at a pressing pressure of 1 ton / cm 2, vacuum sintering is performed at a sintering temperature of 1400 ° C., and a cemented carbide having a de-β layer with a thickness of 10 μm is prototyped did. Here, the cooling rate from the sintering temperature to 200 ° C. was 10 ° C./min for both the inventive product E and the conventional product F. Then, after measuring the hardness of each sample, TiCN was coated on the base material with a thickness of 7 μm, and the cutting edge sagging was compared by a cutting test.
As cutting conditions, the shape of the coated cemented carbide is SNM A 432, the workpiece is SNCM439, the cutting speed v = 140 mm / min, the cutting depth d = 2 mm, the feed speed f = 0.7 mm / rev, and the cutting time. 40 minutes. The results are shown in Table 6 below.

  As shown in Table 6, it can be seen that the inventive product E has the same Vickers hardness as the conventional product F, but the inventive product E has improved cutting edge sagging.

Example 4
The relationship between the Co content and the Vickers hardness, bending strength, compressive strength, fracture toughness, and half-value width of the fchp diffraction image was compared between the tough cemented carbide of the present invention and the conventional cemented carbide. . The results are shown in Table 7 below. Here, WC having an average particle size of 3 μm was used.

Table 7, for each of the inventions A (A11-A15) and conventional B (B10-B1 4), Vickers hardness, transverse rupture strength, compressive strength, a measure of the half value width of the fracture toughness and fchp diffraction pattern It is.

According to Table 7, the inventive product A (A11-A15) has a compressive strength of about 0.3 to 0.4 [GPa] greater than the conventional product B (B10-B1 4 ). . Inventive product A has a fracture toughness of about 2 to 6 [MN · m1 / 2] larger than that of conventional product B, and the difference in fracture toughness tends to increase as the Co content increases. Is seen.

According to Table 7, the inventive product A (A11-A15) has a Vickers hardness of 80-140 when the Co content is in the range of 9-25 wt%, compared to the conventional product B (B10-B1 4 ). [Hv] is a large value. In addition, the inventive product A has a greater bending strength of about 0.2 to 0.3 [GPa] when the Co content is in the range of 9 to 25 wt% compared to the conventional product B. .

  Among the cemented carbides shown in Table 7, the structure of the cemented carbide was imaged with an optical metal microscope for those having a Co content of 25 wt%. FIG. 2 is an image obtained by imaging the structure of the tough cemented carbide of the invention with an optical metal microscope. FIG. 3 is an image obtained by imaging the structure of a conventional cemented carbide with an optical metal microscope.

(Example 5)
Examples of cold forging dies for bolts and nuts will be described below.

Table 8 shows the composition ratios of invention products G and H in mass%. Here, WC having an average particle size of 3 μm was used. Sample H was mixed with attritor in alcohol at the above blending ratio for 6 hours. After the alcohol was evaporated and the powder was dried, pressing was performed at a pressing pressure of 1 ton / cm ² , and sintering was performed in a SinterHIP furnace under a sintering temperature of 1380 ° C. and a pressure of 1 MPa (10 bar). Sample G is the above-mentioned sample H sintered at 1380 ° C., reheated at 1280 ° C., primarily cooled from 1280 ° C. to 540 ° C. at a cooling rate of 35 ° C./min, and held at 540 ° C. for 30 minutes. Then, secondary cooling was performed from 500 ° C. to 200 ° C. at a cooling rate of 8 ° C./min. For each of the inventive products G and H, physical properties such as compressive strength and fracture toughness were measured. The results are shown in Table 9.

  As shown in Table 9, the sample G is superior in compressive strength and fracture toughness to the sample H which contains Cr and has a large half width of the fchp image.

  Next, a cold forging die was produced using each of the above-described invention products G and H. FIG. 7 is a view illustrating a cold forging die for bolts and nuts, FIG. 7 (a) is a sectional view seen from above, and FIG. 7 (b) is a sectional view seen from the side. is there. The workpiece is S25C. A reinforcing ring is attached to the outer periphery of the cemented carbide mold (FIG. 7). Bolts were manufactured by forward extrusion forging, and the number of bolts manufactured in each mold was compared. The point in time when the crack was generated from the hexagonal corner of the manufactured bolt was judged as the life of the mold, and the manufacturing was stopped. The results are shown in Table 10.

  As is clear from Table 10, the life of the mold using the tough cemented carbide sample G of the above embodiment is 1.5 times longer than that of the mold using the sample H. You can say that. The reason why 1.5 times or more high performance can be demonstrated in practical tests is not only excellent in compressive strength and fracture toughness, but also greatly improves fatigue strength by increasing the fcc / hcp ratio. It was because it was made. In addition, it is considered that the increase in the heat resistance strength by increasing the solid solution amount of Cr and W in Co contributes to a longer life.

Claims (5)

  Contains tungsten carbide (WC), cobalt (Co) and chromium (Cr), WC has an average particle size of 1.5 to 20.0 μm, Vickers hardness of 650 to 1650 Hv, and Co of 4 to 30% by mass. , Cr contains 2 to 18% by mass of Co content, the balance consists of WC and inevitable impurities, and in order to reduce Co in the hcp structure, Cr is dissolved in Co and Cu-Kα rays are used. X-ray diffraction in which the X-ray diffraction image (2θ = 44.3 degrees) of the fcc (111) plane of the Co phase overlapped with the X-ray diffraction image (2θ = 44.9 degrees) of the (002) plane of hcp A tough cemented carbide having a half width of an image of 0.42 degrees or less and a lattice constant of a Co phase (fcc) of 3.570 Å or less.   The tough cemented carbide according to claim 1, wherein the amount of solid solution of Cr or W in Co is increased so that the fcc lattice constant of the Co phase is 3.560 or more. In claim 1 or 2, a part of WC is TiC, TaC, NbC, HfC, ZrC, VC transition metal carbide or carbonitride of these transition metals, or double carbide of these transition metals including W or W. A tough cemented carbide comprising one or more of these transition metal double carbonitrides.   4. A tough cemented carbide according to claim 3, wherein a de-β layer is formed on the alloy surface, and the thickness of the de-β layer is 1 to 30 µm. A coated cemented carbide comprising the tough cemented carbide according to any one of claims 1 to 4 as a base material.