JPS6229481B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high carbon alloy steel powder that has excellent sinterability, that is, a wide sintering temperature range, when used as a raw material powder for powder metallurgy. Typically, wear-resistant parts for automobiles and valve train parts, such as valve seats and rocker arm tips, cutting tools such as throw-away tips, hobs, drills, taps, and dies, wear-resistant tools such as rolling rolls and die rolls, and When manufacturing various molds by powder metallurgy, W, W,
High carbon alloy steel powders containing carbide-forming elements such as Mo, V, Nb, and Cr have been used. Generally, as the above-mentioned high carbon alloy steel powder, those formed by a water spray method are widely used from the viewpoint of manufacturing cost and mass productivity. On the other hand, high carbon alloy steel powder formed by the water spray method is not only hard because it is rapidly cooled from the molten metal, but also has an oxide layer formed on its surface.
0.6 to 1.2% by weight (all percentages below indicate weight%)
Because it contains a high content of oxygen, it is annealed and reduced in a vacuum, inert gas, or reducing gas atmosphere for the purpose of annealing and deoxidation.
In this case, the constituent components of the high carbon alloy steel powder are
Cr, V, Nb, etc. have a strong affinity with oxygen, and therefore, annealing reduction treatment is performed at a relatively high temperature within the temperature range of 900 to 1100° C. in order to reduce these elements. However, in high carbon alloy steel powder annealed and reduced under the above conditions, although the oxygen content is reduced to 0.05 to 0.1%, carbide precipitation and grain growth are significant, and the average grain size is usually 0.5 to 1.5 μm. It's summery. Therefore, when sintering is performed using this high carbon alloy steel powder as a raw material powder, the content of carbon and alloy components in the matrix decreases due to the formation of the coarse carbides, resulting in deterioration of sinterability. Therefore, high-temperature sintering is unavoidable in order to obtain a high-density sintered body, and in this case, the upper limit temperature for sintering is naturally limited, so the sintering temperature range is extremely narrow. As a result, not only the characteristics of the sintered body tend to vary, but also the growth of crystal grains is significant, leading to problems such as a decrease in the mechanical strength of the sintered body. It was something that happened. Therefore, from the above-mentioned viewpoint, the present inventors conducted research to obtain a high carbon alloy steel powder that exhibits excellent sinterability when used as a raw material powder for powder metallurgy. Content is 0.6
~1.2% by weight of high-carbon alloy steel powder formed by water spraying in an atmosphere of vacuum, inert gas, or reducing gas.
When annealing reduction treatment is performed under conditions of heating and holding at a predetermined temperature within the temperature range of 650 to 850°C for a predetermined period of time, the average grain size of the precipitated carbide becomes 0.3 μm or less, so the carbon and alloy components in the matrix are As a result, the supersaturated austenitic state generated during rapid solidification before annealing reduction is maintained at a high content, and as mentioned above, the reduction by the carbon contained in the alloy steel powder is maintained for low temperature annealing reduction. The reaction does not proceed completely, resulting in a high carbon alloy steel powder with 0.1 to 0.5% oxygen remaining, and when this high carbon alloy steel powder is used as a raw material powder for powder metallurgy, the high carbon content in the base It exhibits excellent sinterability due to the melting point lowering effect of the carbon content and alloy components, and the oxides remaining on the surface during sintering react with the carbon in the high carbon alloy steel powder and reducing atmospheric gas. As a result, the powder surface becomes activated due to its reduction, and as a result, it exhibits excellent sinterability, and as a result, it can be sintered at a relatively low temperature, that is, it can be sintered at a high density and fine structure over a wide sintering temperature range. They found that it was possible to obtain solid bodies. This invention was made based on the above knowledge, and the reason why the annealing reduction temperature is limited to 650 to 850°C is that at a temperature below 650°C, the oxygen content is 0.5
If the temperature exceeds 850℃, carbide precipitation and Grain growth rapidly accelerates and coarse carbides are formed, which not only reduces the content of carbon and alloy components in the matrix and deteriorates sinterability, but also causes This is based on the reason that the surface-activated sintering effect based on the reduction of the oxide also decreases. Next, the method of the present invention will be specifically explained using examples. Example After melting 10 kg of high carbon alloy steel having the compositions shown in Table 1 using the usual melting method, these molten metals were heated at a predetermined temperature in an inert gas atmosphere. , Spray water under the conditions of water pressure: 200Kg/cm ² and spray angle: 80 degrees to form a powder.
Next, methods 1 to 28 of the present invention and comparative methods 1 to 12 were carried out by subjecting these powders to annealing reduction treatment under the conditions shown in Table 1, respectively. Note that Comparative Methods 1 to 12 were carried out under conditions where the annealing reduction temperature was outside the range of the present invention. The carbon content, oxygen content, and average carbide particle size of the various high carbon alloy steel powders obtained as a result were measured, and the measurement results are shown in Table 1. Furthermore, in order to evaluate the sinterability of various high carbon alloy steel powders obtained from this process, after pulverization and mixing under normal conditions, a green compact with dimensions of 5 mm x 5 mm x 10 mm was prepared at a pressure of 5 ton/cm ² . molded into a shape, then in a vacuum,

【table】

[Table] Sintered at 1220°C, 1230°C, 1240°C, 1250°C, 1260°C, and 1270°C for 60 minutes.
The density of the sintered body after sintering was measured at each sintering temperature, and the density ratio was calculated. From the results shown in Table 1, methods 1 to 28 of the present invention
When using high carbon alloy steel powder produced by Comparative Methods 1, 3, 5, 7, and 7, a high-density sintered body can be obtained over a wide sintering temperature range. When using the high carbon alloy steel powders produced by 9 and 11, high-density sintered bodies could not be obtained at any sintering temperature due to the too high oxygen content of the powder. , and when high carbon alloy steel powders produced by Comparative Methods 2, 4, 6, 10, and 12 were used, the content of carbon and alloy components in the matrix was reduced due to the formation of coarse carbides. Not only does the sinterability deteriorate due to a decrease in the amount of powder, but the sinterability also deteriorates due to a decrease in the sintering activation effect due to a decrease in the oxygen content in the powder.
It is clear that dense sintered bodies can only be obtained in a very narrow sintering temperature range on the high side. As described above, according to the method of the present invention, it is possible to produce a high carbon alloy steel powder with excellent sinterability that can produce a high-density sintered body over a wide sintering temperature range, and However, by using this high carbon alloy steel powder, low-temperature sintering becomes possible, which not only makes it possible to produce a high-density sintered body with a uniform microstructure, but also makes it possible to produce a high-density sintered body with a wide sintering temperature range. This eliminates variations in the properties of the sintered body and brings about industrially useful effects such as improved yield.

Claims (1)

[Claims] 1 A high carbon alloy steel powder formed by a water spray method with an oxygen content of 0.6 to 1.2% by weight is used as a raw material, and it is heated at 650 to 850 °C in a vacuum, inert gas, or reducing gas atmosphere. Annealing and reduction treatment is performed under the conditions of heating and holding at a predetermined temperature within the temperature range, and the oxygen content is reduced to 0.1 to 0.5% by weight through a reduction reaction with the carbon contained in the alloy steel powder, and the average grain size of the carbide is reduced. 1. A method for producing high carbon alloy steel powder with excellent sinterability, characterized by producing high carbon alloy steel powder having a diameter of 0.3 μm or less.