JP4535493B2 - Cemented carbide for discharge electrodes - Google Patents

Description

  The present invention relates to a cemented carbide, and in particular, has excellent discharge characteristics and rigidity. When drilling holes with a diameter of 0.2 mm or less by electrical discharge machining on structural steel plates, high accuracy and a short time are required. The present invention relates to a cemented carbide for a discharge electrode suitable as a workable discharge electrode material.

In conventional electrical discharge machining, galphite, Cu, etc. have been used as the discharge electrode material used for engraving, and wire materials such as W and Cu-W alloys and high-pressure materials have been used as the discharge electrode material used for drilling. It was. However, as shown in Fig. 2 (b), when drilling holes with a minimum diameter of 0.2mm or less, the discharge electrode itself becomes an extremely fine wire, so that the difference in workpiece diameter due to the discharge stake during machining varies. However, there is a problem that the required hole accuracy cannot be obtained due to an increase in taper and an influence on processing accuracy. For example, if a copper-tungsten electrode is used on a structural steel plate with a thickness of 1.5 mm and a 0.2 mm hole is drilled, the taper will be 0.08 mm due to the difference in diameter and the accuracy within 0.01 mm due to the difference in diameter will be increased. It could not be applied to the required use. In order to solve such a problem, for example, as shown in Patent Document 1, in recent years, high-precision fine drilling or fine groove machining uses a lot of WC having high rigidity as a discharge electrode. Cemented carbide wire is applied. By using a cemented carbide wire as the discharge electrode, the required hole accuracy can be secured.
Japanese Patent Laid-Open No. 11-207526

However, in high-precision fine drilling by electric discharge machining, although WC has high rigidity, its electrical resistivity is extremely high compared to W, Cu and the like. For this reason, in the thing of patent document 1, the discharge characteristics made from a cemented carbide alloy had the fault that discharge characteristics were bad, the drilling speed became slow, and the drilling time became long. Table 1 shows the rigidity (yank ratio) and electrical resistivity of each of WC, Co, W, and Cu (Maruzen Co., Ltd., published on March 25, 1993, “Revised 2nd Edition Metal Data Handbook” P13) , P31, P275). In order to improve productivity, it is required to shorten the machining time, and as a future technical trend, further downsizing of holes and higher accuracy are required.

  The problem of the present invention is to solve the above-mentioned problems of the prior art, in order to meet the demand to use a cemented carbide for the electrode in high-precision fine drilling by electric discharge machining, without reducing the WC content, While maintaining rigidity, that is, while maintaining hole drilling accuracy, we improve the discharge characteristics by lowering the electrical resistivity, providing a cemented carbide that has better discharge characteristics than the current cemented carbide and can reduce the machining time There is to do.

  For this reason, the present invention has a composition in which the alloy component is wt%, Co: 3 to 7%, the total amount of one or two of TaC and NbC is 0.1 to 1%, and the balance is WC and inevitable impurities. The alloy carbon content ranges from the lower limit of the two-phase region of WC and Co to the upper limit of 0.05%, and the alloy structure of the two-phase region of WC and Co is the alloy structure, particle size or maximum particle length. It has an alloy structure containing free carbon of 5 μm or less, that is, the amount of alloy carbon in Co weight 3% decreases from 5.87% to 6.02%, and the amount of alloy carbon decreases by 0.04875% and 0.055% for each Co weight 1%, Cemented carbide for discharge electrodes, characterized in that it has an alloy structure including an alloy carbon amount range of 5.605% to 5.80% at a Co weight of 7% and an average particle diameter of WC is 1 μm or more. The above-described problems of the present invention have been solved.

  In the present invention, in high-precision fine drilling by electric discharge machining, while maintaining the high rigidity characteristic of the cemented carbide, that is, while ensuring the hole machining accuracy, the electric discharge characteristics are reduced by reducing the electrical resistivity. An improved cemented carbide that has better discharge characteristics than the current cemented carbide and that can shorten the processing time is provided. FIG. 2 (a) schematically shows an electrode wear pattern during electric discharge machining using a discharge electrode round bar made of a cemented carbide for discharge electrodes according to the present invention.

  Co: 3 to 7% by weight, Co: 3 to 7%, 1 to 2 of TaC and NbC, the total amount is 0.1 to 1%, the balance is composed of WC and inevitable impurities, and the amount of alloy carbon is WC The upper limit of the two-phase region of WC and Co is 0.05% above the upper limit of the two-phase region of WC and Co, and the alloy structure includes an alloy structure containing free carbon whose particle size or maximum length is 5 μm or less. In other words, the amount of alloy carbon at a Co weight of 3% decreases from 6.02% to 6.07% by 0.055% for each Co weight of 1%, and the amount of alloy carbon at a Co weight of 7% decreases from 5.80% to 5.85. It is more desirable to use a cemented carbide for a discharge electrode having an alloy structure including an alloy carbon amount range of up to 1% and having an average particle diameter of WC of 1 μm or more.

  Preferably, the electrical resistivity of the cemented carbide for discharge electrode is 20 μΩ · cm or less.

  In the high-precision fine drilling by electric discharge machining, the present inventors have reduced the electrical resistivity while maintaining the high rigidity characteristic of the cemented carbide, that is, while ensuring the hole machining accuracy. In order to achieve the task of improving the characteristics, we researched a cemented carbide that has better discharge characteristics than the current cemented carbide and that can reduce the processing time, Co weight%, alloy carbon content, alloy structure, WC The research focused on the relationship between average particle size, electrical resistivity, discharge characteristics, and drilling accuracy. Among these researches, the first is Co weight% and drilling accuracy, the second is the amount of alloy carbon, the third is the average grain size and electrical resistivity of WC, and the fourth is the morphology and electrical resistivity of the alloy structure. As a result, the present inventors have found a study result that there is a close relationship between the discharge characteristics and the discharge characteristics.

  The present invention has been made on the basis of such research results. The best embodiment of the cemented carbide for a discharge electrode for carrying out the present invention is such that the alloy component is Co: 3 to 7% by weight, TaC, The total amount of one or two of NbC is 0.1 to 1%, the balance is WC and inevitable impurities, and the amount of alloy carbon is 0.05% from the lower limit of the two-phase region of WC and Co. And the alloy structure has an alloy structure in a two-phase region of WC and Co or an alloy structure containing free carbon having a particle diameter or a maximum particle length of 5 μm or less, that is, symbol ac in FIG. -In the range surrounded by db, the amount of alloy carbon at Co weight of 3% decreases from 5.87% to 6.02%, and the amount of alloy carbon decreases by 0.04875% and 0.055% for each Co weight of 1%. It has an alloy structure including an alloy carbon content range in which the alloy carbon content at 7% ranges from 5.605% to 5.80%, and the average grain size of WC The child diameter is 1 μm or more. FIG. 1 was prepared based on the description of Maruzen Co., Ltd., issued February 20, 1986, Suzuki, “Cemented carbide and sintered hard material” P101, lines 18-30, FIG. 1.118.

  In the cemented carbide for discharge electrodes according to the present invention, the reason why Co is 3% or more by weight is that if Co is less than 3%, the rigidity can be further increased, but sintering becomes difficult, This is because nests are easily generated, the bending strength is reduced, and breakage is easily caused. The reason why Co is set to 7% or less by weight is that when Co exceeds 7%, the processing accuracy of the hole deteriorates due to the decrease in rigidity, and cannot be applied to the intended use. Also, the carbon content of the alloy ranges from the lower limit of the two-phase region of WC and Co to the upper limit of 0.05%, and the alloy structure is the two-phase region of WC and Co, or the particle diameter or maximum length is 5 μm or less. The reason for using free carbon is that it has been found that when free carbon crystallizes in the alloy structure, the electrical resistance is lowered, the discharge characteristics are excellent, and the processing time per hole is shortened. However, the reason for limiting to free carbon having a particle diameter or maximum length of 5 μm or less is that when the particle diameter or maximum length of free carbon exceeds 5 μm, the bending strength is remarkably lowered, and the grinding yield is also remarkably increased. This is because it was found that it deteriorated and there was a problem in practical use. The average particle size of WC is set to 1 μm or more. When the average particle size is less than 1 μm, the electrical resistivity increases, the discharge characteristics deteriorate, and the processing time per hole increases. Because.

  By the way, considering the discharge characteristics, the alloy carbon content is in the range from exceeding the upper limit of the two-phase region of WC and Co to exceeding the upper limit of 0.05%, and the alloy structure has a particle diameter or maximum length of 5 μm or less. An alloy structure including a free carbon, that is, an area surrounded by symbols aeffb in FIG. 1 is desirable. That is, the alloy composition is composed of 3 to 7% by weight of Co, 0.1 to 1% of the total amount of one or two of TaC and NbC, the balance of WC and inevitable impurities, and the amount of alloy carbon. Is a range from the upper limit of the two-phase region of WC and Co to 0.05% exceeding the upper limit of the two-phase region of WC and Co, and the alloy structure contains free carbon whose particle diameter or maximum length is 5 μm or less It has a structure, that is, the amount of alloy carbon in Co weight 3% decreases from 6.02% to 6.07% by 0.055% for each Co weight 1%, and the amount of alloy carbon in Co weight 7% is 5.80%. A cemented carbide for a discharge electrode having an alloy structure including an alloy carbon content range of ˜5.85% and having an average particle diameter of WC of 1 μm or more is desirable.

  Preferably, the electrical resistivity is 20 μΩ · cm or less. The reason why the electrical resistivity is set to 20 μΩ · cm or less is that when the electrical resistivity exceeds 20 μΩ · cm, the electrical resistivity is increased, the discharge characteristics are deteriorated, and the processing time per hole is increased.

Cemented carbides produce free carbon when the carbon content exceeds a preset value, and if the carbon content is insufficient, the alloy structure produces a η phase of WC + Co + Co ₃ W ₃ C, which in both cases has mechanical properties. Deteriorates (see Maruzen Co., Ltd., issued February 20, 1986, Suzuki, “Cemented carbides and sintered hard materials” P54, P101). Therefore, the most important thing in sintering cemented carbide is not to cause such a harmful phase. However, when the application is a discharge electrode, priority is given to low electrical resistivity and good discharge characteristics over mechanical properties.

The electrical resistivity varies depending on Co%, WC particle size, alloy carbon amount, and the like. The higher the Co%, the coarser the WC particle size, and the higher the amount of alloy carbon, the lower the electrical resistivity (March 1989, Sumitomo Electric No. 134, Hiroyuki Kakuya et al. Electrical and magnetic properties ”P210 to P215; Maruzen Co., Ltd., issued February 20, 1986, see Suzuki,“ Cemented carbides and sintered hard materials ”P63). The inventors paid attention to these points and studied a cemented carbide for a discharge electrode having a low electrical resistivity and a short drilling time. The inventors adjusted the alloy carbon content of the cemented carbide composed of Co: 5% by weight, TaC: 0.6%, the balance WC and inevitable impurities to adjust the two-phase region, free carbon, WC + Co + Co ₃ W ₃ C. A cemented carbide with a η-phase structure was fabricated and ground into a round bar with the required shape as an electrical discharge machining electrode, and then mounted on a drilling electrical discharge machine to measure the time required for machining per hole. As a result, it was found that when drilling was performed with a cemented carbide containing free carbon in the alloy structure, the processing time per hole was shortened. However, as described above, when free carbon is generated, the mechanical properties deteriorate, and there is a concern that the yield may be reduced due to bending during grinding. However, when the particle size or maximum length of free carbon is 5 μm or less, it can be seen that the grinding yield is almost the same as that of a two-phase alloy, and there are few practical difficulties. It was.

  At the same time, the electrical resistivity and the bending strength were also evaluated. It was found that the electrical resistivity when free carbon is included is lower than that in the case of the two-phase region. It was also found that the bending strength was small when the particle size or maximum length of free carbon was 5 μm or less, even when compared with an alloy in a two-phase region. It was also found that when the particle size or maximum length of free carbon exceeds 5 μm, the bending strength is significantly reduced as compared with the alloy in the two-phase region. Furthermore, it was found that the grinding yield was significantly deteriorated as compared with the two-phase alloy. Therefore, when the particle size or the maximum length of free carbon exceeds 5 μm, although the discharge characteristics are excellent, it is considered that there is a problem in practical use. Also, it was considered that there was a close relationship between bending strength and grinding yield.

  Next, the inventors adjusted the alloy carbon amount of the cemented carbide having the same composition, changed the electrical resistivity, and also conducted experiments on the time required for processing per hole. As a result, even in the case of a two-phase region that does not contain free carbon, if the electrical resistivity is 20 μΩ · cm or less, the processing time per hole is longer than that when free carbon is included. The difference was found to be slight.

The inventors manufactured alloys whose Co weight percentages were adjusted to 4, 5, 7, and 8%, respectively, were ground into a round bar of the required shape as an electrical discharge machining electrode, and then attached to a drilling electrical discharge machine Then, it was determined whether or not the hole machining accuracy satisfies the taper within 0.01 mm in diameter. As a result, it was found that the Co weight% was within 0.01 mm when 4, 5 and 7%, but exceeded 0.01 mm when 8%. The decrease in rigidity due to the decrease in WC content was considered as a factor.
In addition, the inventors set the average particle diameter of WC of the cemented carbide composed of Co: 7%, TaC: 0.6%, balance WC and inevitable impurities in weight% to 0.6 μm and 1.9 μm samples, respectively. After producing an alloy and grinding it to a discharge electrode round bar of the required shape as an electric discharge machining electrode, attach it to a drilling electric discharge machine, measure the time required for machining per hole, and measure the electrical resistivity at the same time. Went. As a result, it was found that the sample with an average particle diameter of 0.6 μm had an extremely long processing time and a high electric resistivity as compared with the sample with 1.9 μm.

As shown in the left half of Table 2, cemented carbide samples A to I having the composition, the amount of alloy carbon, and the WC average particle diameter were prepared by the following method. First, WC powder, TaC powder, Co powder, C powder, and W powder were prepared as raw material powders. Three types of powders of 0.7 μm, 1.1 μm, and 2.0 μm were prepared according to the target particle size after sintering. These powders were blended at a predetermined ratio, put into a ball mill, added with acetone, and uniformly mixed and ground by operation for 72 hours. After drying the homogenized mixed powder, the binder necessary for extrusion molding is added, and after the powder and binder are uniformly mixed by a kneader, it is formed into a discharge electrode round bar having an outer diameter of 0.6 mm by a flanger type extruder. After being molded and cut to a predetermined length, it was sintered into a cemented carbide by heating in a vacuum at 1380 to 1450 ° C. for 1 hour in a vacuum sintering furnace. The sintered cemented carbide is cooled to room temperature and then taken out from a vacuum sintering furnace. After mirror processing, it is observed with a metal microscope while corroding or not corroding with Murakami reagent (alkaline red blood salt solution). It was done.

The composition of cemented carbide showed the result of analyzing the sample by ICP method. The amount of alloy carbon shows the result of analyzing the sample with a carbon analyzer. The average particle diameter of WC was the average value measured from the secondary electron image photograph of the sample observed with an electron microscope. Determine the alloy structure by mirroring the sample and observing it with a metal microscope while observing with a Murakami reagent (alkaline red blood salt solution) or without corroding at a magnification of 200x (20x objective, 10x eyepiece). did. 3 to 6 show micrographs at a magnification of 200 times that photographed the surface alloy structure of each material in Table 2 of Example 2 after mirror finishing. Fig. 3 is a micrograph of a magnification of 200 times of the microstructure of alloy containing free carbon and WC + Co + C exceeding the particle size of material E. Fig. 4 is a particle size of materials A, C, D, H and I of 1 to 3 µm. Magnification 200x micrograph of alloy structure containing free carbon and WC + Co + C, Fig. 5 is a two-phase region of materials B and F, 200x micrograph of alloy structure containing WC + Co, Fig. 6 is material G FIG. 2 shows a 200 × magnification micrograph of an alloy structure containing η phase region and WC + Co + Co ₃ W ₃ C.

  Each material in Table 2 of Example 2 was mirror-finished, cut to a predetermined length, and then ground to a discharge electrode round bar having an outer diameter of 0.09 mm required as an electric discharge machining electrode by centerless grinding. A discharge electrode round bar with an outer diameter of 0.2 mm was ground to measure the bending strength and electrical resistivity. A discharge electrode round bar ground to 0.09mm and attached to a drilling electric discharge machine, 0.13mm drilling was performed. SCr430 plate with a thickness of 1.8mm was used as the workpiece. The right half of Table 2 shows the measured electrical resistivity, bending strength, time taken for processing per hole, hole processing accuracy, and grinding yield. The hole machining accuracy was judged by whether or not the accuracy within a taper of 0.01 mm was satisfied by the difference in diameter.

  As can be seen from Table 3, Sample A of the alloy of the present invention contains free carbon having a particle size of 1 to 3 μm. Compared with the sample F of the comparative example which is an alloy of the same composition and WC particle size and is also a two-phase region, the electrical resistivity is lower and the drilling time is shorter. There is no significant difference between the bending strength and the grinding yield. From this, the discharge characteristics are improved without deteriorating the grinding yield by including free carbon with a particle size of 1 to 3 μm. I understand. The sample E of the comparative example has the same composition and WC particle size as the sample A of the alloy of the present invention, but contains free carbon exceeding 5 μm. Compared with the sample A, the electrical resistivity and the drilling time are slightly better. Poor bending strength and grinding yield. From this, it can be seen that when free carbon exceeding 5 μm is included, the discharge characteristics are excellent, but there is a problem in practical use.

Samples F to I belonging to the range of the comparative example belong to the range of the alloy of the present invention, while the machining time per hole is the shortest at 39 seconds while maintaining the hole machining accuracy. In A to E, the machining time per hole is the longest at 36 seconds or less while maintaining the hole machining accuracy, and it can be seen that the machining efficiency is improved. Sample I of the comparative example was obtained by increasing the Co weight% of the sample D of the alloy of the present invention from 7% to 8%, which is required although the electrical resistivity is low and the drilling time is shortened. Drilling accuracy was not obtained. This is thought to be due to the lack of rigidity due to the decrease in WC content.
The sample H of the comparative example is obtained by refining the WC particle diameter of the sample D of the alloy of the present invention from 1.9 μm to 0.6 μm, and as a result, the electrical resistivity is high and the drilling time is long. It can be seen that as the WC particle size decreases, the electrical resistivity increases and the discharge characteristics deteriorate.
Sample F of the comparative example has the same composition, alloy structure (two-phase region), and WC particle diameter as the sample B of the alloy of the present invention, but has a high electrical resistivity. Compared with sample B, the processing time per hole is high. It is getting longer. This shows that it is important to lower the electrical resistivity in order to improve the discharge efficiency. As described above, according to the present invention, it is possible to provide a cemented carbide for a discharge electrode in which a drilling time is shortened and a highly accurate small-diameter hole can be drilled with high efficiency.

The graph which shows the relationship with the amount% of alloy carbon of a vertical axis | shaft and alloy structure with respect to the bonding layer Co amount% of a horizontal axis in a cemented carbide, respectively. (A) is explanatory drawing which shows the electrode consumption form of the Example of this invention goods, (b) shows the explanatory drawing which shows the electrode consumption form of the comparative example which is a conventional product, respectively. Magnification 200 times micrograph of an alloy structure containing free carbon and WC + Co + C having a particle diameter of more than 5 μm after mirror finishing of material E in Table 2 of Example 2. The magnification 200 times micrograph of the alloy structure containing free carbon and WC + Co + C having a particle diameter of 1 to 3 μm after mirror finishing of materials A, C, D, H and I in Table 2 of Example 2. Magnification 200 times micrograph of alloy structure containing two-phase region and WC + Co after mirror finishing of materials B and F in Table 2 of Example 2. Magnification 200 times micrograph of alloy structure containing η phase region and WC + Co + Co ₃ W ₃ C after mirror finishing of material G in Table 2 of Example 2.

Claims (4)

  Co: 3 to 7% by weight, Co: 3 to 7%, 1 to 2 of TaC and NbC, the total amount is 0.1 to 1%, the balance is composed of WC and inevitable impurities, and the amount of alloy carbon is WC In the range from the lower limit of the two-phase region of Co and Co to the upper limit of 0.05%, the alloy structure of the two-phase region of WC and Co or free carbon with a particle diameter or particle maximum length of 5 μm or less The alloy carbon content of Co is 3%. The alloy carbon content is reduced by 0.04875% and 0.055% for each Co weight of 1% from 5.87% to 6.02%. A cemented carbide for a discharge electrode having an alloy structure including an alloy carbon amount range of 5.605% to 5.80% and having an average particle diameter of WC of 1 μm or more.   Co: 3 to 7% by weight, Co: 3 to 7%, 1 to 2 of TaC and NbC, the total amount is 0.1 to 1%, the balance is composed of WC and inevitable impurities, and the amount of alloy carbon is WC The upper limit of the two-phase region of WC and Co is 0.05% above the upper limit of the two-phase region of WC and Co, and the alloy structure includes an alloy structure containing free carbon whose particle size or maximum length is 5 μm or less. In other words, the amount of alloy carbon at a Co weight of 3% decreases from 6.02% to 6.07% by 0.055% for each Co weight of 1%, and the amount of alloy carbon at a Co weight of 7% decreases from 5.80% to 5.85. A cemented carbide for a discharge electrode having an alloy structure including an alloy carbon amount range of up to 1% and an average particle diameter of WC of 1 μm or more.   The cemented carbide for discharge electrodes according to claim 1 or 1, wherein the cemented carbide for discharge electrodes has an electrical resistivity of 20 µΩ · cm or less.    The discharge electrode round bar which consists of the cemented carbide for discharge electrodes of Claim 1, Claim 2 or Claim 3 which has a diameter of 0.2 mm or less.