JP2003171741A - Iron based powder for warm compacting, and warm compacting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide iron based powder for powder metallurgy by which the defect of the conventional raw material powder for powder metallurgy that the increase of the density of a green compact in warm compacting is insufficient is improved, the strength of a final sintered compact obtained by sintering for the green compact is increased, and a product having more excellent mechanical strengths can be produced, and to provide a compacting method therefor. <P>SOLUTION: The iron based powder consists of iron powder, alloy steel powder or the like, and is used as the main raw material for powder metallurgy. The iron based powder contains the components of, by mass, ≤0.02% C, ≤0.10% Si, ≤0.30% Mn, ≤0.02% P, and ≤0.20% O, and the concentration of solid solution nitrogen is controlled in ≤10 ppm by mass. Further, the iron based powder is subjected to warm compacting at any temperature in the range of 80 to 250°C. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of so-called "warm molding", in which a raw material powder for powder metallurgy is compression-molded in a warm state to obtain a high-density green compact, and in particular,
The present invention relates to improving the properties of an iron-based powder, which is a main raw material for warm compaction, by which a green compact can be further densified to obtain a final compact having more excellent mechanical properties.

[0002]

2. Description of the Related Art In the following description, the indication simply by "%" means the mass ratio unless otherwise specified.

In the powder metallurgy method using an iron-based powder such as iron powder or alloy steel powder as a main raw material, the iron-based powder adjusted to a predetermined grain size is used as a raw material and compression-molded by a die to obtain a green compact. Thereafter, the green compact is further sintered in a heating furnace to obtain a product sintered body, which is widely applied as a manufacturing method excellent in productivity.

The product sintered body produced in this way is
When applied to parts where strength is particularly important, such as mechanical structural parts, it is effective to make the product density as high as possible. It is important to.

Therefore, as a method of increasing the density of the green compact, the yield stress of the iron-based powder is reduced by compression molding in an intermediate temperature region that exceeds room temperature and is not extremely higher than the hot temperature region. A method by so-called "warm molding" has been developed to improve the density of the green compact even at low molding pressure and to maintain the lubrication function of the lubricant to prevent mold wear. Has been proposed. For example, a proposal of a lubricant suitable for warm forming (for example, Japanese Patent Laid-Open No. 2-15002, JP-B-7)
-103404), and so-called die lubrication molding, vibration molding, and other proposals regarding process improvement (for example, Japanese Patent Laid-Open No. 9-2.
7901 and JP-A 2000-144208).

Although these improved techniques have made it possible to obtain a green compact having a considerably high density, it is necessary to improve the compressibility of the iron-based powder itself in order to achieve higher density. That is, even in the warm forming, the iron-based powder for cold forming is diverted and used as it is.
Therefore, when using a conventional iron-based powder premised on molding at room temperature, the problem remains that the compressibility of the iron-based powder itself is not sufficient in molding at temperatures above room temperature. The point has not been sufficiently examined, and no proposal has been made yet for an iron-based powder having properties suitable for warm compaction as the raw material powder itself.

[0007]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to improve the drawback of the conventional powder metallurgy raw material powder that the densification of the green compact in warm compacting is not sufficient, and to improve the high density compaction. Achieved high strength by sintering powder,
It is an object of the present invention to provide an iron-based powder for warm compaction and a warm compaction method for producing a powder compact having a higher density than ever before in order to produce a product having excellent mechanical strength.

[0008]

[Means for Solving the Problems] In order to further improve the density of the green compact in the warm compacting, the present inventors intend to improve the compressibility of the iron-based powder in the temperature range in which the warm compacting is performed. From the standpoint that this is necessary, we have eagerly pursued research and development.

That is, in the field of cold forging of steel materials, the inventors of the present invention have been subject to age hardening during deformation when the concentration of solute nitrogen in the steel material is high in the temperature range of 150 to 500 ° C. to cause deformation resistance. That becomes larger and less likely to be deformed (eg, Hyakusaki et al .: Materials and Processes, vo
l. 11 (1998), p. 1219, Kawakami et al .: R & D
Kobe Steel Technical Report, vol. 34 (1984), p. 73)
Focusing on, the effect of solid solution nitrogen concentration was investigated and examined in detail for the iron-based powder for powder metallurgy, and the present invention was completed based on the result. In cold forging of steel, etc., a prismatic billet such as a billet is formed by rolling it down with a roll or the like, whereas in the case of powder compacting in powder metallurgy, fine powdery metal particles are further lubricated. The agent is added and pressure molding is performed using a mold. In this way, since the molding methods of both are very different,
The iron-based powder of the present invention is not one that could be easily invented simply from the relationship between the solid solution nitrogen concentration in the steel material and the age hardening,
It was completed for the first time after adding detailed consideration / examination based on the experimental results and the like of the examples described later.

The gist of the present invention is as follows.

The invention of claim 1 is an iron-based powder used as a main raw material for warm compacting, wherein C: 0.
02% or less, Si: 0.10% or less, Mn: 0.30%
Hereinafter, P: 0.02% or less, O: 0.20% or less of the component is included, and the concentration of solute nitrogen is 10 ppm in mass ratio.
The iron-based powder for warm compacting is characterized by the following.

The invention of claim 2 is a warm compacting method, characterized in that the iron-base powder for warm compacting according to claim 1 is compacted at any temperature between 80 and 250 ° C. .

[Operation] Here, the “iron-based powder” means all particles mainly composed of iron, which are usually used in powder metallurgy, and for example, particles of substantially pure iron (high-purity iron powder). ), Iron particles (alloy steel powder) containing alloying elements such as transition metals and / or other strengthening metals, and the like, but are not particularly limited thereto.

The solid solution nitrogen concentration in the iron-based powder is 1 by mass ratio.
By setting the content to 0 ppm or less, preferably 7 ppm or less, dynamic strain aging of the iron-based powder is prevented in the temperature range where warm forming is performed, the deformation resistance of the iron-based powder itself is reduced, and deformation is facilitated. Thereby, the density of the green compact can be further increased.

In addition, the components of the iron-based powder are C: in mass ratio.
0.02% or less, Si: 0.10% or less, Mn: 0.3
By setting the content to 0% or less, P: 0.02% or less, and O: 0.20% or less, the deformation resistance of the iron-based powder at the time of warm forming is reduced to facilitate deformation, and the green compact is baked. The mechanical properties of the molded product after binding can be made excellent.

That is, if the C concentration in iron is too high, most of the pearlite is formed and the deformation resistance becomes excessive. Therefore, the C concentration is 0.02% or less.

Further, Si easily forms an oxide in the melting / atomizing stage, and if the content is large, Si causes nozzle clogging during atomization. Therefore, the Si concentration is 0.10%
Below.

Further, Mn, like Si, causes nozzle clogging due to oxide formation, and also forms a solid solution in iron powder to harden the matrix, resulting in poor compressibility. Therefore, M
The n concentration is 0.30% or less.

P also forms a solid solution in the iron powder and hardens the matrix, so that it reduces the compressibility. Therefore, P concentration is 0.02
% Or less.

Further, O inhibits the compressibility of the iron powder, and reacts with the added graphite during sintering to lower the C of the sintered body and lower the strength. Therefore, the O concentration is 0.20
% Or less.

If the temperature of the warm compaction is too low, the effect of reducing the compressibility of the iron-based powder is low, while if it is too high, the effect of improving the compressibility of the iron-based powder is saturated and the heating energy is wasted. The temperature is preferably between 80 and 250 ° C, more preferably between 100 and 200 ° C, and even more preferably between 150 and 200 ° C.

That is, as will be shown in Examples described later, the compressibility of the iron-based powder begins to rise sharply at about 80 ° C. or higher when the molding temperature is raised from room temperature, but rises at 150 ° C. or higher. Slows down and peaks at about 200 ° C,
Although it tends to gradually decrease at higher temperatures, it is 250
This is because the high compressibility is still maintained even at ° C. The reason why the compressibility of the iron-based powder changes depending on the molding temperature is due to the combined effect of the effect of softening the metal structure and the effect of oxidizing and hardening the metal as the temperature increases.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The iron-based powder of the present invention may be any of iron powder, steel powder, alloy steel powder, etc., as long as it is mainly composed of iron, C: 0.02% or less, Si: 0. 10% or less,
Mn: 0.30% or less, P: 0.02% or less, O: 0.
The content of the component is 20% or less, and the solid solution nitrogen concentration is 10 ppm or less, preferably 70 ppm or less in mass ratio.

Here, as an iron-based powder satisfying the above component range excluding the concentration of solute nitrogen, there is, for example, a conventional high-purity iron powder (for example, trade name: Atmel 300NH, manufactured by Kobe Steel). Of the components of this conventional high-purity iron powder, for example, the C concentration is very low at around 10 ppm and is within the above-mentioned component range of the present invention (0.02% or less), but the solute nitrogen concentration is about 15 ppm. It exceeds the range (10 ppm or less) of the concentration of solid solution nitrogen required for the iron-based powder of the invention. This conventional high-purity iron powder uses raw materials with reduced impurities,
After smelting and pulverizing by atomizing with water at a pressure of 8 to 11 MPa, the powder is put into a reducing furnace in a pure hydrogen atmosphere for about 800
By repeating the step of reducing under the condition of ° C x about 6 hours twice, deoxidation, decarburization, denitrification, etc. are carried out to obtain a high-purity product.

Therefore, the solid solution nitrogen concentration of the present invention is 10 pp.
The iron-based powder having a particle size of m or less is obtained, for example, in the same pulverization process as that of the conventional product, and then the powder is placed in a reducing furnace in the same pure hydrogen atmosphere as that of the conventional product at about 800 ° C. for about 6 h. By repeating the reducing step under the conditions 3 to 5 times, which is larger than that of the conventional product, denitrification can be further advanced to manufacture. Alternatively, instead of or in addition to increasing the number of times of repeating the reduction step as compared with the conventional product, the reduction time may be extended as compared with the conventional product.

The particle size distribution of the iron-based powder of the present invention is not particularly limited, but the proportion of particles having a size of 250 μm or less, which is generally used in powder metallurgy, is 99 by the sieving method.
It is desirable to set it to be at least%.

The iron-based powder of the present invention thus obtained is molded into a green compact by the same molding method as the conventional product. For example, if necessary, a lubricant is added to and mixed with the iron-based powder of the present invention to preheat it to a predetermined temperature, and then heating is performed by an arbitrary method such as heating with a heater, Joule heating by energization, high-frequency heating, or infrared heating. Fill in the mold,
By performing compression molding at a predetermined temperature, a green compact having a higher density than that of the conventional product can be obtained. Also, since the iron-based powder itself has excellent compressibility, regardless of the type of lubricant,
Further, it is effective regardless of the type of process such as die lubrication molding and vibration molding.

[0028]

EXAMPLES The following comparative experiments were conducted to confirm the effects of the present invention.

A commercially available high-purity iron powder A (product name: Atmel 300N) is used as a conventional iron-based powder for comparison tests.
H, Kobe Steel Co., Ltd. and alloy steel powder B (Product name: 46F)
4H, manufactured by Kobe Steel Ltd.) were selected.
These components are shown in Table 1 and the particle size distribution is shown in Table 2. In addition,
The dissolved nitrogen concentrations of the high-purity iron powder A and the alloy steel powder B were 15 ppm and 12 ppm, respectively.

Here, the solid solution nitrogen concentration was measured by the following method. First, a nitrogen concentration measuring device (product name: TC-1
36, manufactured by LECO), and the total nitrogen concentration W ₁ (%) in the iron-based powder is measured by the inert gas fusion thermal conductivity method. Next, according to JIS G1228 (iron and steel-nitrogen determination method, ammonia distillation separation indophenol blue absorptiometry), the nitrogen content W of the nitrogen compound present in the iron-based powder is determined.
₂ (%) is measured. Then, the required solid solution nitrogen concentration W
_N (%) is obtained by W _N = W ₁ −W ₂ .

Then, in order to obtain an iron-based powder having a solid solution nitrogen concentration of 10 ppm or less as the iron-based powder of the present invention, the above-mentioned conventional iron-based powders A and B were used as mother powders, respectively, and pure in a small heating furnace. It was produced by performing denitrification treatment at 800 ° C. × 6 h × 1-2 times in a hydrogen atmosphere. On the other hand, in order to obtain an iron-based powder in which the concentration of solid solution nitrogen was intentionally increased as the iron-based powder of the comparative example, hydrogen-nitrogen mixture was performed in a small heating furnace using the above-described conventional iron-based powders A and B as mother powders. Gas (H _{2 by} volume ratio:
N ₂ = 1: 4) or 940 ° C. under a pure nitrogen gas atmosphere
It was produced by performing a nitrification treatment once for 6 hours. Table 3 shows a list of denitrification or nitrification treatment conditions and the concentration of solute nitrogen after the treatment of the iron-based powder for comparative tests produced in this manner.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

Next, using each of the iron-based powders for comparison tests shown in Table 3 as a raw material powder, the temperature was sequentially changed between room temperature and 250 ° C., and the green compact was molded at each constant temperature. Detailed molding conditions are as follows. That is, when molding is performed at a temperature between room temperature and 50 ° C., 0.8 mass% of zinc stearate is added to the raw material powder and the mixture is mixed in a V-type mixer for 30 minutes.
After mixing, preheat to a prescribed temperature, fill this mixed powder in a molding die heated to a prescribed temperature, and mold surface pressure 7 t / cm ²
31.8 mm in length × 12.7 mm in width ×
A green compact having a thickness of about 5 mm and a mass of 18 g was molded. Also,
When molding is performed at a temperature higher than 50 ° C., zinc stearate is added to the raw material powder in an amount of 0.1% by mass and mixed in a V-type mixer for 30 minutes and then preheated to a predetermined temperature. Lithium oxide is dispersed in alcohol and applied to the inner wall of the mold, and then supplied through a mold lubricating mold heated to a predetermined temperature to a molding mold also heated to a predetermined temperature, and a molding surface pressure of 7 t / cm ² Pressurized with, length 31.8
A green compact having a mass of 18 g and a rectangular parallelepiped shape having a size of mm × width 12.7 mm × thickness 5 mm was formed. In addition, the preheating of the mixed powder is performed by forming a hole of φ20 mm × 50 mm in the aluminum block for 10 to 20 mm.
This was carried out by a method in which individual pieces were provided, the holes were filled with weighed iron powder, the outer circumference of the aluminum block was covered with a panel heater, and heating was performed. The heating of the molding die and the die lubrication die was carried out by winding a band heater around each die and controlling the die temperature measured by a thermocouple within a range of ± 2 ° C.

The density and bending strength of the green compact thus formed were measured. Specifically, the density of the green compact was obtained by measuring the mass and dimensions of the green compact, calculating the volume from the dimensions, and dividing the mass by the volume. The bending strength is ISO33.
The strength was measured with a strength tester (product name: AUTOGRAPH, manufactured by Shimadzu Corp.) at a fulcrum distance in the longitudinal direction of the powder compact of 25 mm according to 25 (sintered metal material bending strength). In addition, the arithmetic mean value of the measured values of three green compact samples was adopted for both the density and the bending strength.

The measurement results are shown in FIGS.

1 and 2 show high-purity iron powder (No. 1 to 5 in Table 3) and alloy steel powder (No. 6 to No. 6 in Table 3), respectively.
It is a graph figure which shows the relationship of a solid-solution nitrogen concentration, a green compact density, and bending strength when 10) is shape | molded at room temperature (cold forming). 3 and 4 show high-purity iron powder (Nos. 1 to 5 in Table 3) and alloy steel powder (N in Table 3).
o. 6 to 10) is a graph showing the relationship between the solid solution nitrogen concentration, the green compact density, and the bending strength in the case of molding (warm molding) at 150 ° C. Further, FIG. 5 shows high purity iron powder with low solid solution nitrogen concentration (10 ppm or less) according to the present invention (Table 3).
No. It is a figure which shows the relationship between shaping | molding temperature and green compact density of the green compact which uses 2) or alloy steel powder (No. 7 of Table 3) as a raw material powder.

As shown in FIG. 1, when cold-forming high-purity iron powder, the density of the green compact is high not only when the concentration of solute nitrogen is lower than that of the conventional product (15 ppm) but also when it is high. In some cases, even if the solid solution nitrogen concentration is about 5 times that of the conventional product, the green compact density equivalent to that of the conventional product is obtained, and there is no clear correlation between the solid solution nitrogen concentration and the green compact density. . In addition, since the bending strength drops sharply when the concentration of solute nitrogen decreases,
It cannot always be said that the concentration of solute nitrogen is low. Further, as shown in FIG. 2, even when cold forming the alloy steel powder, the same tendency as in FIG. 1 is exhibited, and therefore it is not always preferable that the concentration of solute nitrogen is low.

On the other hand, as shown in FIG.
In the case of warm forming at 50 ° C., when the concentration of solute nitrogen becomes lower than that of the conventional product (15 ppm), the green compact density increases.
When the concentration of solute nitrogen is lower than that of the conventional product, the bending strength of the green compact is slightly lower than that of the conventional product, but the degree of decrease is not as great as in cold forming, and the density and bending strength of the green compact are Considering the balance, it was confirmed that the solid solution nitrogen concentration lower than the conventional product was superior to the conventional product. Further, as shown in FIG. 4, even when the alloy steel powder is warm-formed at 150 ° C., the tendency similar to that of FIG. 3 is exhibited. It was confirmed to be excellent.

Further, as shown in FIG. 5, when a high-purity iron powder or alloy steel powder having a solid solution nitrogen concentration of 10 ppm or less is used as the raw material powder, the density of the green compact increases from the room temperature to the molding temperature. At 80 ° C or higher, the temperature rises sharply, but at 150 ° C or higher, the degree of increase slows down and shows a peak at about 200 ° C. At higher temperatures, it tends to gradually decrease, but at 250 ° C, the density is still high. You can see that you are maintaining.

[0042]

As described in detail above, according to the present invention, it is possible to provide an iron-based powder having a much higher compressibility than conventional products in warm compaction. Further, the density of the green compact can be further increased as compared with the conventional product by warm forming the iron-based powder as a raw material powder at a predetermined temperature. As a result, it is possible to further improve the mechanical strength of the final molded body obtained by sintering this green compact.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the concentration of solute nitrogen, the green compact density, and the bending strength when high-purity iron powder is cold-formed.

FIG. 2 is a graph showing the relationship between the concentration of solute nitrogen, the green compact density and the bending strength when alloy steel powder is cold-formed.

FIG. 3 is a graph showing the relationship between the concentration of solute nitrogen, the green compact density and the bending strength when high-purity iron powder is warm-formed at 150 ° C.

FIG. 4 is a graph showing the relationship between the concentration of solute nitrogen, the density of green compacts, and the bending strength when alloy steel powder was warm-formed at 150 ° C.

FIG. 5 is a diagram showing a relationship between a molding temperature and a green compact density of a green compact of a low solid solution nitrogen concentration high-purity iron powder or alloy steel powder as a raw material powder according to the present invention.

Continued front page    (72) Inventor Tetsuya Sawayama             5-9-12 Kitashinagawa, Shinagawa-ku, Tokyo Stocks             Kobe Steel, Ltd. Tokyo head office (72) Inventor Keibun Hojo             2-3-3 Niihama, Arai-cho, Takasago, Hyogo Prefecture             Takasago Works, Kobe Steel, Ltd. F-term (reference) 4K018 AA25 BA15 CA01 CA02

Claims (2)

[Claims] 1. An iron-based powder used as a main raw material for warm compacting, wherein C: 0.02% or less by mass and S.
i: 0.10% or less, Mn: 0.30% or less, P: 0.
02% or less, O: 0.20% or less of the components, and
An iron-based powder for warm forming, wherein the concentration of solute nitrogen is 10 ppm or less in mass ratio. 2. A warm compacting method, characterized in that the iron-base powder for warm compaction according to claim 1 is compacted at any temperature between 80 and 250 ° C.