JPH0310703B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for producing tool steel, particularly iron-based sintered tool steel which requires high mechanical properties. [Prior Art] In recent years, the production of high-speed tool steel (JIS SKH), cold work tool steel (JIS SKD), etc. using a sintering method has gradually become popular. These are made by molding alloy powder by water atomization using a press molding method or the like and then sintering it in a vacuum or in an inert gas to achieve true density.
In other words, starting material is powder produced by water atomization, which has a relatively high O ₂ content but has excellent moldability.
Appropriate graphite powder is added to this and the oxide is reduced by CO gas, and the eutectic carbide in the powder is melted by a eutectic reaction during the sintering process, and this partial liquid phase is utilized. This makes true densification possible. This method is attracting attention as a process that significantly reduces the number of steps because it uses alloy powder and can obtain a shape similar to the final product. [Problems to be solved by the invention] However, in these conventional manufacturing methods, since the sintering temperature coincides with the melting temperature of the eutectic carbide, densification and coarsening of the structure proceed simultaneously, which inevitably deteriorates the mechanical properties. is lower than that of conventionally cast materials, and furthermore, the enlargement of eutectic carbides causes the depletion of alloying elements in the matrix, which has the disadvantage of low hardness after quenching and tempering, making it difficult to use as wear-resistant parts. Although it had been used, it was considered unsuitable for cutting tools that required toughness and hardness. [Means for Solving the Problems] The present invention focuses on the above problems, and provides a compound that dissociates under the atmosphere and temperature conditions during sintering, and some of the constituent elements in the compound after dissociation are gaseous. The remaining part is removed from the sintered product by adding only 0.5 to 2.0% by weight (the same applies hereinafter) to the alloy powder of an elemental compound that does not deteriorate its properties even if it remains in the sintered product. The present invention aims to provide an iron-based sintered tool steel that can have a fine grain structure and has mechanical properties equivalent to or better than conventional cast materials. In other words, the present invention is a nitride of Si, Cr, Mo, and V, which are the constituent elements of JIS SKH and SKD.
Add 0.5 to 2.0% of one or more of Si ₃ N ₄ , VN, CrN, Mo ₂ N, a substance that dissociates in a vacuum atmosphere of 1000°C or higher and 10 ^-1 Torr or higher, to the alloy powder and mix. It is characterized by being sintered. Here, Si ₃ N ₄ , VN, CrN, and Mo ₂ N dissociate during sintering and are removed from the sintered product as gaseous substances, and some of them remain as solid solutions as metallic elements such as Si, V, Cr, and Mo. . [Examples] The present invention will be illustrated below using Examples, including comparisons with conventional sintered materials and ingot materials. (Example 1) C0.91%, Si0.23%, Mn0.2 equivalent to JIS SKH9
%, Cr4.10%, W5.95%, Mo5.02%, V1.98%,
A -100 mesh alloy powder consisting of the balance Fe and unavoidable impurities was produced by water atomization. The _O2 content in the dry state after atomization is
It was 3200ppm. Add this O ₂ content to Co
Graphite powder was added at 0.24% as the stoichiometric amount of carbon for reduction in gas form and heated at 1000℃ x 3Hr in vacuum.
A reduction process was carried out. Analysis of the O ₂ content after reduction revealed that it was 960 ppm, and the C content was 0.83%.
Stearic acid is then added to the reduced powder as a lubricant.
Si ₃ N ₄ , VN, CrN, with 0.7% Zn and purity of 99.0% or more
Add Mo ₂ N powder at the mixing ratio shown in Table 1, mix in a ball mill, and then press using a normal press.
Molding was carried out at a pressure of 6 tonf/cm ² . This molded body
1255℃ at a heating rate of 200℃/Hr in a vacuum of 10 ^-2 Torr
After that, the temperature was raised to 100 mL, and then maintained for 1 hour and cooled in the furnace. After holding these samples at 860°C for 3 hours, the cooling rate was 20
Fully annealed at ℃/Hr, then 1200
After cooling in a vacuum furnace at ℃ x 20 min, quenching was performed by injecting 3 bar N ₂ gas. The cooling rate at this time was a half-cooling time of 3 minutes. This material was tempered twice at 560℃ x 1 hour in the atmosphere, and 5.0w x 3.0t x 40 pit specimens were cut out.
The results of the bending test and hardness test were
Shown in the table. Table 1 shows the results of a bending test and a hardness test using test pieces cut from a molten material (JIS SKH9) 30φ steel bar and subjected to the same heat treatment as above for comparison. Ta. Also, Fig. 1 shows the conventional sintered material, and Fig. 2 shows the conventional sintered material.
Microstructure after sintering of Si ₃ N ₄ 1.2% additive material (×400)
are shown respectively.

【table】

[Table] Conventional sintered material (No.
17) bending strength and hardness are respectively 270Kg/mm ² and H _R
C63.1, bending strength and hardness of melted material (No. 18)
350Kg/mm ² , which is a lower value compared to H _R C66.1. The cause of this is presumed to be that the eutectic carbide becomes coarse and unstable in shape, and stress tends to concentrate as shown in FIG. 1, and that the austenite crystal grain size is coarse. In contrast, in the materials of the present invention containing Si ₃ N ₄ , VN, CrN, and Mo ₂ N, no effect was observed when 0.2% was added, but improvements in bending strength and hardness were observed when 0.7% was added. It is clear that it has a significant effect on the increase. Among them, the material with 1.2% Si ₃ N ₄ addition showed a relationship between bending strength and hardness that exceeded that of the ingot material. As is clear from FIG. 2, this is because the eutectic carbide has become extremely spheroidal and fine. The effect of addition decreases in the order of Si ₃₄ , VN, Mo ₂ N, and CrN. However, in both materials with an additive amount of 1.7%, the degree of improvement in bending strength and hardness is slower than in materials with an additive amount of 1.2%, and there may be an upper limit to the effect of the additive amount. It was inferred. Table 1 also shows changes in the composition of the material after sintering due to an increase in the amount of each substance added. Metal element and N contents increase with increasing addition amount. The increase in N was smaller than the stoichiometrically calculated value, and it is estimated that a considerable amount of N was discharged from the sintered body as a gaseous substance. In JIS SKH and SKD, N is mostly
Combines with carbide-forming elements such as Cr, V, Mo, W, etc.
It is known as an element that exists in steel in the form of carbonitride, and some of it is dissolved in the matrix, and is known as an element that is effective in increasing wear resistance and improving tempering hardness.
There is no problem in increasing the range of materials according to the present invention. Si,
Metal elements such as V, Cr, and Mo are added as alloying elements, so a slight increase does not pose a problem. In addition, the existence form of the added substances in the sintered body was investigated using X-ray diffraction method and X-ray microanalyzer (EPMA), but it was found that Si ₃ N ₄ , VN, CrN,
None of the MoN substances were detected, and it was determined that they were dissociated and diffused during sintering. Example 2 C1.53%, Si0.31%, Mn0.43 equivalent to JIS SKD11
%, Cr 12.28%, Mo 1.16%, V 0.33%, balance Fe and unavoidable impurities. -100 mesh alloy powder was produced by water atomization method. 0.2% graphite powder was added to this powder and after mixing, the same reduction treatment as in Example 1 was performed. Analysis of the O ₂ content after reduction revealed that it was 760 ppm and the C content was 1.55%.
As in Example 1, Si ₃ N ₄ ,
Powders of Cr ₂ N, VN, and Mo ₂ N were added and mixed.
In the case of SKD11, since the contents of Mo and V are trace amounts, in this example, the upper limit of the amount of VN and Mo ₂ N added was set at 1.2%, and the film was formed at a pressure of 6 tonf/cm ² . Sintering conditions are 1200℃ in a vacuum atmosphere of 10 ^-2 Torr.
x 1 hour, after sintering, held at 1000°C for 1 hour, air-cooled and quenched, tempered twice at 530°C x 1 hour, cut out the same bending test piece as in Example 1, and conducted a bending test. . The results are shown in Table 2. The comparative materials used were 30φ pieces cut from additive-free material (No. 33) and melted material (No. 34) and subjected to the same heat treatment.

[Table] In the case of this example (SKD11), Si ₃ N ₄ , VN, CrN,
Improvements in bending strength and hardness were observed for all Mo ₂ N additives of 0.7%, 1.2%, and 1.7%.
The effect of adding 1.2% Si ₃ N ₄ is remarkable. (Example 3) Example 1 for representative steels of JIS SKH and SKD,
Si ₃ N ₄ , which had a remarkable addition effect even with 2, was added to 1
% was added and the bending strength and hardness were compared with that of a material without additives. SKH uses Mo-based SKH58 (1.03C−0.2Si
−0.30Mn−4.2Cr−1.73W−8.56Mo−1.98V),
SKH57 (1.23C−0.31Si−0.38Mn−4.03Cr−
10.58W−3.35Mo−3.51V−9.75Co), and SKD1 (2.15C−0.38Si−0.53Mn−13.50Cr) as SKD.
-100 mesh alloy powder was produced by the water atomization method, and a true density material was produced by the same process as in Examples 1 and 2, and a comparative experiment was conducted. Table 3 shows the results.

〔Effect of the invention〕

As described above, the present invention can obtain a shape similar to the final product, and has excellent effects in terms of yield, finishing efficiency, reduction in the number of steps, etc., but compared to conventional ingot materials in terms of mechanical properties. For the inferior iron-based sintered material
By adding 0.5 to 2.0% of one or more of Si ₃ N ₄ , VN, CrN, and Mo ₂ N, it is possible to refine the structure and increase the toughness and hardness to a level comparable to that of ingot material. It has great effects as a tool steel.

[Brief explanation of the drawing]

Figure 1 is a micrograph showing the micrometallic structure after sintering of a conventional sintered material equivalent to JIS SKH9, and Figure 2 is a micrograph showing the micrometallic structure after sintering of conventional sintered material equivalent to JIS SKH9.
It is a micrograph showing the micrometallic structure after sintering of a Si ₃ N ₄ additive material to JIS SKH9.

Claims (1)

[Claims] 1 Weight% C0.6-2.5%, Cr2-15%, W20% or less, Mo10% or less, V6.0% or less, Co12% or less,
In the manufacturing method of iron-based tool steel containing 1.5% or less Si, 1.0% or less Mn, and the balance consisting of Fe and unavoidable impurities, alloy powder with a composition close to the final composition is produced.
An iron-based material characterized by adding 0.5 to 2.0% by weight of one or more of Si ₃ N ₄ , VN, Cr ₂ N, and Mo ₂ N and then sintering to refine the structure. Method of manufacturing sintered tool steel.