JPH0689363B2 - High strength alloy steel powder for powder metallurgy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an alloy steel powder for powder metallurgy having high compressibility and high strength, which is used for manufacturing various sintered machine parts such as gears and bearing parts. Regarding The alloy steel powder of the present invention is compression-molded into a desired shape and then sintered (bonding of alloy powders),
Then, it is processed into various parts by HIP (hot isostatic pressing).

[Conventional technology]

In recent years, there has been an increasing demand for higher strength of sintered parts, and in response to this demand, various high-strength sintered materials have been developed by methods such as alloying and densification.

As one of the methods for obtaining this high-strength sintered material, pure iron powder is used as a main raw material, and fine powders of elemental elements for alloys such as Ni, Cu, Mo, etc. are mixed with this, and the alloy elements are solid-dissolved during sintering. There is a so-called premix method. However, in this premix method, the iron powder and the fine powder for alloy are separated and segregated due to the difference in specific gravity during press forming, or the fine powder for alloy is not sufficiently diffused during sintering, so that the structure is not good. There is a problem in that the strength is made uniform, and as a result, variations in strength and dimensions occur.

In addition, as another method, due to the development of the atomizing method,
An alloy steel powder in which alloy elements such as Ni, Cu, No, etc. are dissolved in Fe has been produced, and a so-called pre-alloy method using this alloy steel powder has been proposed. In the case of this alloy steel powder,
Although the problem of inhomogeneity of the structure can be solved, the hardness of the particles is increased, so that the compressibility is lowered, so that a high density sintered material cannot be obtained and sufficient strength cannot be obtained.

Therefore, as another method for solving the above problems and obtaining a high-density, high-strength sintered material, as described in, for example, Japanese Patent Publication No. 45-9649, Ni, Cu, No. There is so-called partially diffused alloy steel powder in which fine powders of elemental elements such as the above are partially diffused and adhered (see FIG. 2). According to the method described in this publication, the steel powder has a compressibility equivalent to that of pure iron powder and has high sintered body strength.

[Problems to be solved by the invention]

However, according to the method described in the above publication, for example, 6 to 10
When manufacturing a high Ni alloy steel powder containing wt% oNi, the melting point of this alloying element is high, and the diffusion rate into the iron powder is slow, so many undiffused alloying elements remain,
Sufficient strength cannot be obtained. Further, in order to sufficiently diffuse, it is necessary to sinter at a high temperature for a long time, which causes a problem of high cost.

Therefore, the inventors of the present invention, as a high-strength alloy steel powder for powder metallurgy capable of solving the above-mentioned problems, a fine alloy powder obtained by previously alloying high purity pure iron powder with two or more elements of Ni, Cu and No. We are developing a product that diffuses and adheres (see Japanese Patent Application No. 62-136934).

As shown in FIG. 1, the alloy steel powder according to the above development is pure iron powder to which any one of Ni-Cu powder, Ni-Mo powder, Ni-Cu-Mo powder, etc. is diffused and adhered. is there. In addition to the above-mentioned alloy fine powder, a simple powder of the above element may be dispersed and adhered to the alloy steel powder. Here, the diffusion adhesion (diffusion bonding) does not mean that the alloy fine powder is completely dissolved, but the Cu component of the alloy fine powder diffuses into the iron powder, and at the interface between the two, Alloyed
It means that they are attached in this state.

In the alloy steel powder according to the above development, since the alloy fine powder of Ni, Cu, Mo is made to diffuse and adhere, the melting point can be lowered as compared with the case of fine powder of these simple elements, and the high temperature long It is possible to sufficiently diffuse in a short period of time without requiring time sintering and improve the sintering strength.

However, according to subsequent experimental research, even if the above alloy fine powder is adopted, the above effect does not sufficiently appear depending on the ratio of the components to the mother powder, or conversely, the variation in dimensional change of the product becomes large. It turns out that there may be problems.

Therefore, the present invention has been made in view of the above situation, and realizes sure and sufficient diffusion by improving the sintered body strength by diffusing and adhering the alloy fine powder so as to be in the optimum component range of the additive element. It is also an object of the present invention to provide a high-strength alloy steel powder for powder metallurgy, which is capable of suppressing variations in dimensional change rate.

[Means for solving problems]

The present invention, C: 0.01 wt% or less, Si: 0.02 wt% or less, Mn: 0.10
wt% or less, P: 0.01 wt% or less, S: 0.010 wt% or less, O: 0.15w
High purity pure iron powder consisting of the balance iron and unavoidable impurities at t% or less, at least one pre-alloyed composite alloy fine powder of Ni-Cu, Ni-Mo, Ni-Cu-Mo, the above pure iron A high-strength alloy steel powder for a sintered alloy, characterized by being diffused and adhered to the powder so that Ni: 6 to 8 wt%, Cu: 0 to 2 wt% and Mo: 0.5 to 1.0 wt% Mo.

Here, the reasons for limiting each constituent element in the present invention will be described.

(1) The composition of pure iron powder, which is the mother powder, is C: 0.01 wt% or less, S
i: 0.02wt% or less, Mn: 0.10wt% or less, P: 0.01wt% or less,
The reason why the allowable limit is defined as S: 0.010 wt% or less and O: 0.15 wt% or less and the balance being iron and unavoidable impurities is to ensure excellent compressibility.

Si, Mn: A small amount of Si or Mn is added in order to deoxidize molten steel in the process of producing pure iron powder, but these elements having a strong affinity with O are oxidized during water atomization and oxidized inclusions And remains in the iron powder to impair the compressibility. Therefore, as the amount of Mn and Si added increases, the amount of oxidizing inclusions also increases, and even in the subsequent reduction process, these oxidizing inclusions remain without being reduced and reduce the compressibility, so Si is 0.
It is desirable to reduce the amount to 02 wt% or less and the Mn to 0.1 wt% or less as much as possible.

P, S: If P, S remain during refining of molten steel, the iron powder particles are hardened and the compressibility is reduced. When the amount of P and S is large, the particles do not become soft even after the reduction treatment. In order to prevent the iron powder particles from hardening, both P and S were set to 0.010 wt% or less.

C, O: This C, O can be reduced by decarburization and deoxidation in the reduction process of heating in a reducing atmosphere, but if a large amount of C remains in the iron powder after reduction, it will be compressed. Therefore, the content of C is set to 0.01 wt% or less. Also, if the amount of O is large, not only will the compressibility decrease, but
O is set to 0.15 wt% or less because it reduces the yield of graphite powder mixed and used in the usual powder metallurgy method and causes the variation of the structure.

(2) An alloy fine powder obtained by pre-alloying two or more kinds of elements among Ni, Cu and Mo is added to the above mother powder.
The reason for diffusing and adhering so as to be -8 wt%, Cu 0-2 wt% and Mo 0.5-1.0 wt% is as follows.

Ni has the effect of improving toughness and hardenability, and Mo enhances hardenability and prevents softening during quenching and tempering. Also Cu
Has the effect of improving strength.

In this case, if the Ni content is less than 6 wt%, the strength is insufficient, while if it exceeds 8 wt%, the strength deteriorates due to an increase in retained austenite. If Mo is less than 0.5 wt%, the effect of improving hardenability cannot be obtained, and if it exceeds 1.0 wt%, toughness (impact value) is obtained.
Is reduced. Further, when Cu exceeds 2%, the variation (standard deviation) of the dimensional change rate during sintering is significantly increased. Therefore N
i: 6 to 8 wt%, Cu: 0 to 2 wt%, Mo: 0.5 to 1.0 wt% are required.

As described above, when Ni and Mo are solid-dissolved in iron powder,
The strength of sintered or heat-treated materials can be significantly improved.
However, Ni and Mo have a high melting point and a slow diffusion rate into the iron powder. Therefore, a high temperature and long time sintering process is required to obtain sufficient diffusion. Also, when Ni, Cu, Mo are used as they are as elemental element powders, they occur with the probability that each elemental powder exists at a certain place, or only one certain elemental powder exists, and systematically occurs. It becomes uneven. Furthermore, if Cu is added as a simple substance powder, abnormal expansion due to Cu elution occurs, and the dimensional accuracy of the product deteriorates.

Therefore, in the present invention, alloy fine powder in which two or more kinds of elements among Ni, Cu and Mo are alloyed in advance, for example, Ni-Mo powder, Ni-Cu powder, or Ni-Cu-Mo powder is used. This alloyed fine powder has a lower melting point than the case of a single powder of Ni and Mo and can improve the diffusibility.

Further, by adding Cu as an alloy fine powder, there is no abnormal expansion due to elution of Cu which is observed when a simple substance is added, so that the dimensional accuracy is improved, the density after sintering is increased, and the strength is improved.

Here, the component ratio of the alloy fine powder itself used in the present invention is not particularly limited, but the following composition is preferable.

Ni-Mo system: The high melting point of Mo can be lowered by using Ni-Mo alloy powder, and the diffusibility into iron powder can be improved. However, when Mo is 50 wt% or more, this melting point lowering effect almost disappears, so Mo is preferably 50 wt% or less.

Ni-Cu system: In this system, all solid solution type alloy powder is obtained,
As the percentage increases, the melting point can be lowered and diffusion into the iron powder can be facilitated. However, if the Cu content is 40 wt% or more, the size after sintering expands and the effect is lost in terms of practicality. Therefore, Cu is preferably 40 wt% or less.

Ni-Cu-Mo system: In this system, Mo is
The composition is preferably 50 wt% or less and Cu is 40 wt% or less.

The alloy steel powder of the present invention can be manufactured, for example, by the following method. That is, the high-purity pure iron powder and the alloy fine powder are wet-mixed in an organic solvent, and then the mixed powder is reduced in a reducing atmosphere at a temperature higher than 75 ° C and lower than 1000 ° C. It is characterized in that the alloy fine powder and the elemental element fine powder are diffused and adhered to the pure iron powder by annealing.

Here, in the above manufacturing method, the pure iron powder and the alloy powder are wet-mixed in an organic solvent in order to uniformly and sufficiently adhere the alloy powder to the surface of the pure iron powder. To mix the metal powders, the metal powders are charged into a double-cone mixer in which the bottom surfaces of a pair of conical cones are connected together,
There is a dry mixing method in which the cone is rotated around a horizontal axis, but in this method, the metal powders are easily separated into layers due to the difference in specific gravity, and uniform mixing is difficult. On the other hand, in the above-mentioned manufacturing method, for example, the wet mixing method in which the alloy powder is dispersed in the organic solvent such as ethyl alcohol and the pure iron powder of the mother powder are mixed and stirred is used. The powder is uniformly dispersed and adheres in a uniformly dispersed state due to the wettability of the thin film of the solvent formed on the surface of each powder. Therefore, the powders do not separate due to the difference in specific gravity as in the dry method. As a result, the diffusion adhesion in the subsequent reduction step is performed uniformly and sufficiently, and the sintering strength is improved.

When the wet mixing method is industrialized, a certain amount of time may elapse between the mixing and the next reduction step, the solvent may evaporate, and the adhered alloy powder may be separated. Therefore, in order to maintain the above adhered state, it is desirable to add a binder such as a resin to the solvent.

The reason why the reduction annealing temperature is set to 750 to 1000 ° C. is as follows.

When the reduction annealing temperature is lower than 750 ° C., the reduction cake does not become hard and a high apparent density is obtained, and the compact density is excellent, but the sintering strength is slightly low. This is probably because the treatment temperature is low and the diffusion and adhesion (alloying) of the alloy powder is small. On the other hand, if the temperature is higher than 1000 ° C, the density of the compact will be low. It is considered that this is because the diffusion and solid solution of the alloying elements into the iron powder proceed, the iron powder particles become hard, and the compressibility decreases.

[Action]

According to the high-strength alloy steel powder for powder metallurgy according to the present invention, Ni-Cu, Ni as a strengthening powder to be diffused and adhered to high-purity pure iron powder
-Mo, Ni-Cu-Mo at least one pre-alloyed composite alloy fine powder was adopted, so the melting point of the alloy fine powder is lower than that of the fine powder of the alloying element alone, so diffusion to iron powder That is, alloying is performed easily and surely.

Further, when adding the alloying elements, the component ratio of the alloying elements to the pure iron powder was regulated within a predetermined range,
The effect of improving strength by Ni and Cu (see Example 1) and the effect of improving toughness (impact value) by Mo (see Example 2) are obtained, and abnormal expansion due to Cu elution observed when Cu alone is added can be suppressed. The effect (see Example 3) is obtained.

〔Example〕

 Examples of the present invention will be described below.

Example 1 In this example, the effect of improving the strength of the sintered body of the present invention is confirmed, in which fine powder obtained by previously alloying Ni with Cu is diffused and adhered to pure iron powder so that Ni falls within a predetermined component range. In order to do so, the following procedure was performed.

Ni-Cu alloy fine powder and Ni, Cu, Mo simple substance powder are wet-mixed with high-purity pure iron powder so as to have the compositions shown in Tables A to F, and then the mixed powder is mixed with AX gas (ammonia decomposition gas). −H ₂ : 7
5%, N ₂ : 25%) atmosphere, reduction annealing treatment was performed at 850 ° C. for 30 minutes, and then crushed. Further, for comparison, only Ni, Cu and Mo simple substance powders were used, wet-mixed so as to have compositions shown in G to I of Table 1, reduction annealing heat treatment, and granulation.

After adding 0.6 wt% of graphite powder and 0.75 wt% of zinc stearate powder as a lubricant to each of the disintegrated steel powders, V
The mixture was mixed for 30 minutes with a mold mixer, and this mixed powder was molded with a mold at a pressure of 6 TON / cm ² to obtain a molded product of 10 × 10 × 55 mm.

The molded body was sintered in an AX gas atmosphere at 1120 ° C for 30 minutes. A tensile test piece having a parallel portion of 6φ was formed from this sintered body, and the tensile strength of this was measured.

The data obtained from the above experiments are shown in Table 2.

As is clear from the table, the steel powders according to the present invention (A to F)
Column), the comparative example steel powders (columns G to I) have a slightly lower Ni content, and the method using the pre-alloyed Ni-Cu alloy fine powder of the present invention is a simple substance of Ni and Cu. It can be seen that the diffusion adhesion is superior to the case of using the powder.

For example, looking at 8Ni-1.5 Cu-1.0 Mo (columns B, E, H),
The strength of the sintered body is B>E> H, which shows that the strength is improved by increasing the use ratio of the alloy fine powder. Further, it can be understood that the strength becomes the highest when Ni is 8% and the strength is remarkably lowered when it is less than 6% and more than 10%.

Example 2 This example is for confirming the toughness improving effect by adding Mo within the range of the present invention. According to the procedure of Example 1 described above, a test with a Mo content of 0.5 to 2.0 wt% is performed. A piece was prepared, a Charpy impact test (without notch) was performed, and the impact value was measured. The sintered body density of this test piece is 7.20 g / cm.
^{Was 3} .

The measurement results are shown in FIG. As is clear from the figure,
It can be seen that when the Mo addition amount is 0.5 and 1.0 wt%, the impact value is at a high level, but when it exceeds 1.0 wt%, it suddenly decreases.

Example 3 This example is for confirming the effect of suppressing the variation in the dimensional change rate by diffusing and adhering fine powder alloyed with Cu to pure iron powder within the scope of the present invention. According to the procedure of 1), the content of Cu added in the form of Ni-Cu alloy is 0 to
A 2.5 wt% test piece (64Φ to 24Φ) was prepared and the standard deviation of the dimensional change rate during sintering was determined. For comparison, the same measurement was also performed on the test piece in which Cu was added alone so as to have the above content.

The measurement results are shown in FIG. As is clear from the figure,
When Cu is added in the form of Ni-Cu alloy, the standard deviation of the dimensional change rate is smaller at any content than when Cu is added alone. The addition amount is 2.0 wt%
Beyond, the standard deviation of the dimensional change rate rapidly increases not only when Cu is added alone but also when Ni-Cu alloy is added.

〔The invention's effect〕

As described above, according to the high-strength alloy steel powder for powder metallurgy according to the present invention, at least one pre-alloyed composite alloy fine powder among Ni-Cu, Ni-Mo, and Ni-Cu-Mo, respectively, 6 ~
Since it was diffused and adhered to high-purity pure iron powder so as to have a composition of 8 wt%, 0 to 2 wt%, 0.5 to 1.0 wt%, the melting point of the fine alloy powder was lower than that of the alloy element alone, so it was diffused into the iron powder. That is, alloying can be facilitated, the compressibility can be improved, and the strength of the sintered body can be greatly improved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a state in which the alloy powder is diffused and adhered to pure iron powder of the alloy steel powder according to the present invention, and FIG. 2 is a schematic diagram of a conventional alloy steel powder, FIGS. 3 and 4. Is a diagram for explaining the effect of the present invention, FIG. 3 is a Mo addition amount-impact value characteristic diagram,
FIG. 4 is a characteristic diagram of Cu addition amount-dimensional change rate standard deviation.

Claims (1)

[Claims] 1. C: 0.01 wt% or less, Si: 0.02 wt% or less, Mn: 0.
10wt% or less, P: 0.01wt% or less, S: 0.010wt% or less, O: 0.1
High-purity pure iron powder consisting of balance Fe and unavoidable impurities at 5 wt% or less, Ni-Cu, Ni-Mo, Ni-Cu-Mo at least one pre-alloyed composite alloy fine powder, Ni: 6 ~ 8wt
%, Cu: 0 to 2 wt%, Mo: 0.5 to 1.0 wt% by diffusion bonding (diffusion bonding), a high strength steel powder for powder metallurgy.