JPS6216266B2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Technical Field The present invention provides a high-performance, long-life cemented carbide for use in warm forging and hot forging tools. (b) Conventional technology and its problems Traditionally, die steel (SKD61) was mainly used for hot forging tools such as dies and punches. Die steel is prone to thermal cracking and deformation on its surface, so its lifespan is short and the dimensional accuracy of the product is poor.
Therefore, parts after hot forging require grinding. As a recent trend, development is progressing in the direction of precision forging using warm forging and hot forging and omitting post-processing. This requires high performance forging tools. The conventionally used hot die steel cannot achieve high accuracy because it suffers from severe surface roughness, deformation, and thermal cracking, and also because the steel has a large thermal expansion. Even SKH-51, which has high hardness at high temperatures, has problems with thermal expansion, thermal cracking, and cracks, making it difficult to expect a long life. Cemented carbide, whose thermal expansion at high temperatures is 1/2 that of steel, is desirable, but the cemented carbide currently used in the market has low thermal shock resistance and scorching properties, so it cannot be used under harsh conditions such as hot forging. It cannot withstand use. (C) Disclosure of the Invention The present invention is a tool for warm and hot forging that has improved heat cracking resistance and fracture resistance as a result of improving cemented carbide with high high temperature hardness and low coefficient of thermal expansion. This enabled us to develop a cemented carbide for hot plastic working. The gist of this application is that in a cemented carbide consisting of a hard layer and a binder phase, the hard phase is made of WC and the binder phase is
It consists of a ternary alloy of Co, Ni, and Cr, and the amount of the binder phase is
10 to 40% by weight, and the comparison between Co and Ni is 99≧
Co/Ni≧1, and the amount of Cr added is 0.1 to 3% by weight
It has been discovered that an alloy characterized by controlling the solidified grain size of the binder phase to 300μ or less by rapid cooling treatment is suitable for cemented carbide tools for hot forging. During warm and hot forging, the temperature of the workpiece is high, and the temperature of the mold surface rises rapidly due to heat generated by the deformation of the forging material. On the other hand, after the work is taken out, lubricant, cooling water, cooling oil, etc. are sprayed onto the mold surface, which causes damage to the tool surface due to rapid cooling and other thermal shocks. Note that warm here refers to a temperature of approximately 500 to 800°C, and term hot refers to a range of approximately 800 to 1100°C. The present invention can be used even under adverse conditions such as rapid thermal cycles, a decrease in high-temperature hardness due to an increase in tool surface temperature, high-temperature toughness required during forging, corrosion of the tool surface due to cooling water, and wear due to workpieces. This is the discovery of cemented carbide. For example, a rolling roll used for steel wire rolling is known as a cemented carbide used in hot rolling, but this roll is used under uniform load during rolling, and is used under a strong impact like the hot forging tool of the present application. From the fact that is not added, it can be inferred how severe the conditions of use as referred to in the present application are. The present inventors found that when WC was used as the hard phase and Co was used as the binder phase, the hardness decreased significantly at high temperatures as shown in Figure 1, and the high-temperature toughness value decreased as shown in Figure 2. This is based on new knowledge. In other words, Ni-
It is said that the addition of Cr reduces the eyelids, but
Under the condition of 99≧Co/Ni≧1, high strength can be maintained without reducing the toughness of the alloy.
Addition of Cr can reduce roughening of the cemented carbide tool surface caused by cooling water, but if it is less than 0.1% it is ineffective, and if it exceeds 3% by weight, the strength will decrease, so a range of 0.1 to 3% by weight is desirable. . The above compositional studies solved the high-temperature hardness, high-temperature toughness, and surface roughness necessary for hot forged tools, but the following measures were discovered as countermeasures against thermal cracking. In warm forging and hot forging, thermal cracks that form on the die surface grow further due to the impact during forging. Therefore, the occurrence of surface cracks must be prevented as much as possible.
In the present invention, as a result of intensive investigation of the crack generation mechanism during forging, it was found that the initial crack depth and crack density are influenced by the solidified particle size of the binder phase, the uniform dispersion of Co/Ni, and the concentration distribution of Cr. did. The present inventors found that WC-Co-Ni-Cr alloys are suitable for hot forging, but as a result of investigating various ways to improve their heat cracking resistance, they discovered that WC-Co-Ni-Cr alloys are suitable for hot forging. The solidification particle size can be controlled by cooling to a temperature of 0.3.
It has been discovered that thermal cracks that occur during forging can be prevented by controlling the thickness to less than mm. The solidified particle size is significantly affected by the cooling rate of the sintered body, and varies depending on, for example, the amount of gas inflow, the distance between the heating element and the object to be cooled, and the like. For example, by using He, Hz, etc., which have high thermal conductivity, the cooling rate can be greatly improved, and as a result, a cemented carbide having a solidified grain size of 0.1 mm or less can be obtained. The required solidified grain size varies depending on the application, but for example, as a tool for hot plastic working, a tool with a solidified grain size of 0.1 mm or less will have better properties. If the solidified particle size of the binder phase is increased to 0.3 mm or more, deep thermal cracks will become visible, and if it becomes even larger, there is a risk of major damage. The solidified particle size of cemented carbide can be easily detected by ion etching or chemical corrosion. Figure 3 shows the solidified structure of the cemented carbide under a microscope. The difference in shading is due to the crystal orientation. The material obtained by the present invention can be used for the die, punch, knockout, ejector pin, shear blade, roll, etc. of the plastic working tool for hot and warm forging thus obtained. (d) Example 1 WC, Co, Ni and Cr powders were weighed and WC-
A mixture of 12% by weight Co, 8% by weight Ni and 2% by weight Cr was mixed and pulverized. This is the outer diameter of 100
It was formed into a cylindrical shape with a diameter of 40 mm, an inner diameter of 40 mm, and a height of 80 mm, heated to 1400° C. in a vacuum furnace at 10 ^-2 Torr over 5 hours, and held at that temperature for an additional hour. After holding,
The frame charged in the furnace was moved from the heating section to the cooling section at once, _N2 gas was introduced into the furnace, and the alloy (A), which was made by rapidly solidifying cemented carbide, was held at 1400℃ for 1 hour. Alloy (B) was produced under the same conditions as above, after which the heating power source was cut off, the alloy was cooled to 1000° C. in a furnace, and then N ₂ gas was introduced. The alloy thus obtained was press-fitted into a steel shank and processed to produce a forging die for a bevel gear. The forging material was heated to 1100℃ and the bevel gear was warm-forged at a forging pressure of 200 tons. When die steel was used as a mold, the life span was 5,000 pieces, but the cemented carbide of the present invention (A) had a lifespan of 50,000 pieces, and (B) had a life of 10,000 pieces. Cemented carbide molds are more expensive than conventional die steel molds, but if you take that into consideration, it is not worth it in (B), but it is well worth using cemented carbide in (A). I found out. The solidified grain size of the cemented carbide obtained in this example was investigated by ion etching, and the average result (A) was 150.
It was μm. On the other hand, (B) was 300 μm. The die life was shortened due to thermal cracks occurring on the inner surface of the mold, and chipping occurred at the surface edges of the thermal cracks, resulting in poor product shape and surface scratches. It was found that the cemented carbide (A) of the present invention has a long life because the crack depth is shallow and the crack opening is small. However, in cemented carbide (B), the cracks are deep and the cracks open, eventually causing chipping. Also, in areas where the impact is large, the cracks extend and the cemented carbide breaks. In the case of die steel, the surface is severely roughened, thermal cracks and die deformation are observed, and the product shape and surface condition are deteriorated. Example 2 A cemented carbide was produced using the same method as in Example 1 with the formulations shown in Table 1. This is (A) in Example 1.
A forging die for a bevel gear was made by rapid cooling in the same manner as above, and warm forging was performed. 【table】

[Brief explanation of the drawing]

FIG. 1 shows the temperature characteristics of hardness of various cemented carbide alloys, and FIG. 2 shows the temperature characteristics of impact values of various cemented carbide alloys. Figure 3 shows a 30x micrograph. In the figure 1...WC-16Co, 2...WC-8Co-7Ni
-1Cr, 3...WC-25Co, 4...WC-15Co,
5...WC-12Co, 6...WC-14Co-1.5Ni-
0.5Cr, 7...WC−14Co−4Ni−2Cr.

Claims (1)

[Claims] 1. In a cemented carbide comprising a hard phase and a binder phase,
The hard phase consists of WC, the binder phase consists of Co, Ni, and Cr, and the range is 99≧Co/Ni≧1, and Cr
is 0.1 to 3% by weight, the amount of the binder phase is 10 to 40% by weight, and the solidified crystal grains are 0.3 mm or less. 2 In cemented carbide consisting of a hard phase and a binder phase,
The hard phase consists of WC, the binder phase consists of Co, Ni, and Cr, and the range is 99≧Co/Ni≧1, and Cr
is 0.1 to 3% by weight, and the amount of the binder phase is 10 to 40% by weight. Hot plastic processing characterized by rapid cooling by a gas body from the sintering temperature during the sintering process of cemented carbide in which the amount of the binder phase is 10 to 40% by weight. Manufacturing method of cemented carbide for use.