JPH07286230A - Production of sintered steel excellent in machinability - Google Patents

Abstract

PURPOSE:To produce a sintered steel having superior machinability even if prepared by sintering an iron powder for powder metallurgy in a hydrogen- containing atmosphere. CONSTITUTION:An iron powder for powder metallurgy, containing <0.1% Mn and 0.08-0.3% S, is mixed with 0.1-1.0% graphite powder of 25-120mum average grain size and 0.5-2% graphite powder of <=15mum average grain size. The resulting powder mixture is compacted and sintered.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sintered steel using water atomized steel powder for powder metallurgy, and more particularly to a method for producing a sintered steel having excellent machinability after sintering.

[0002]

2. Description of the Related Art Iron powder for powder metallurgy is manufactured by adding and mixing Cu powder, graphite powder, etc. to iron powder, compacting and sintering in a mold, and usually obtaining a density of 5.0 to 7.2 g / cm ³ . It is used to manufacture sintered machine parts and other products. The powder metallurgy method can manufacture a sintered body having a complicated shape with high dimensional accuracy, but when manufacturing a component with strict dimensional accuracy, cutting or drilling after sintering may be necessary.

[0003] Powder metallurgy products generally have inferior machinability and have a problem that tool life is shorter than ingot products, so that the cost during machining is high.
It is said that the deterioration of the machinability of the powder metallurgy product is caused by the intermittent cutting due to the pores contained in the powder metallurgy product or the increase of the cutting temperature due to the decrease of the thermal conductivity. In order to improve the machinability, free-cutting components such as S and MnS are often mixed with iron powder. These S and MnS are said to bring about the effect of facilitating the fracture of chips, or to improve the cutting property by forming a thin S, MnS cutting edge on the tool and lubricating the rake face of the tool. In Japanese Examined Patent Publication No. 3-25481, pure iron powder containing a small amount of Mn (0.1 to 0.5% by weight (hereinafter abbreviated as%)) and Si, C, etc. is further added with 0.03 to S.
A powder for powder metallurgy has been proposed in which a molten metal added with 0.07% is sprayed with water or gas.

In Japanese Patent Publication No. 61-40027,
As a method of precipitating free graphite in sintered steel, carbon powder having an average particle size of 20 to 350 μm as a raw material powder has an average particle size of 1 to 15 μm and suppresses carbon diffusion during sintering. One or two or more selected from the group consisting of Si, Sn and P plain powders having an action, and alloy powders containing 15% or more of these components, and an average particle size: 40 to 120 μm
Prepare one or both of Fe-based alloy powders having m, and use these raw material powders as carbon powder: 1 to 4%, and a powder having a carbon diffusion suppressing effect in a carbon diffusion suppressing component amount of 0.2.
~ 4%, one of Fe powder and Fe-based alloy powder, or a mixture composition (both by weight% or more) consisting of both and the rest, mixed, and mixed with either Fe powder or Fe-based alloy powder, or Both of them and the surface of the carbon powder are uniformly sprinkled with a powder having a carbon diffusion suppressing effect, and graphite is dispersed in the Fe substrate, and thereafter, the powder is press-molded and sintered according to a usual powder metallurgy method. A method for producing a free graphite-dispersed iron-based sintered sliding material is disclosed. There is a problem that the cost is high because plain or alloy powder thereof such as Si, Sn, and P is used for precipitating free graphite. Also, depending on Si, Sn, and P, the average particle size is 20-350.
In order to suppress the carburization of μm graphite powder, the sintered steel matrix has a relatively low carbon content, which causes a problem of low strength.

Japanese Patent Publication No. 5-63545 also discloses, as an iron-based sintered member having self-lubricating property, a sintered steel in which iron oxide and free graphite are dispersed in a matrix structure composed of ferrite and pearlite. ing. The amount of free graphite is preferably 0.5 to 3%, and it is said that free graphite is generated by adding graphite having a particle size larger than that of the solid solution graphite in the matrix.

[0006] Japanese Patent Publication No. 61-40027 and Japanese Patent Publication No. 5-63
All of the techniques disclosed in Japanese Patent No. 545 are techniques for dispersing graphite in an iron base, but the purpose is to apply the conventional self-lubricating action of graphite to sliding parts. It is not a technique relating to improvement of machinability which is an object of the present invention.

[0007]

However, when the S-containing iron powder for powder metallurgy produced by the atomization method using water including them is sintered in an atmosphere containing hydrogen, S is removed. As a result, there is a problem that the machinability is significantly reduced. In view of such drawbacks of the prior art, the present invention provides an inexpensive manufacturing method of sintered steel excellent in machinability even when the iron powder for powder metallurgy is sintered in an atmosphere containing hydrogen. To aim.

[0008]

Means for Solving the Problems The present inventors have found that when the iron powder for powder metallurgy produced by a spraying method using water containing S is sintered in an atmosphere containing hydrogen, the machinability is reduced. The cause was examined thoroughly. When atomized iron powder for powder metallurgy containing 0.03 to 0.3% S is sintered in a nitrogen atmosphere containing hydrogen, the amount of S and the amount of residual graphite are reduced as compared with the sintered steel sintered in pure nitrogen. Was there. Assuming that FeS is reduced by hydrogen, the reduction amount of S in the sintered steel was calculated, and it was in good agreement with the analytical value. From these facts, it was found that FeS existing in atomized iron powder or on the surface of iron powder is directly involved in the formation of residual graphite. Further, the atomized iron powder for powder metallurgy containing 0.03 to 0.3% of S produced by a spraying method using water was rapidly cooled during the sintering in a nitrogen atmosphere containing hydrogen, and the residual graphite was analyzed. It has been found that residual graphite forms as a result of the inhibition of carburization during sintering.

Therefore, when sintering is performed in a nitrogen atmosphere containing hydrogen as described above, FeS contained in the powder is reduced by hydrogen and the effect of inhibiting carburization during sintering disappears, and as a result, carburization proceeds and the surface of the sintered steel surface is reduced. It was found that the residual graphite of No. 3 decreased and the machinability deteriorated. In a factory where parts are manufactured by powder metallurgy, a nitrogen atmosphere containing hydrogen is often used for stabilizing the characteristics of parts or when using steel powder containing an easily oxidizable alloying element.

In order to produce a sintered steel having excellent machinability even when sintered in an atmosphere containing hydrogen, it is necessary to prevent FeS contained in the powder from being reduced by hydrogen during sintering, or
Alternatively, graphite having a slow carburizing rate during sintering may be added. Therefore, as a result of investigating various graphites from the latter point of view, graphite powder with an average particle size of 25 to 120 μm has a slow carburizing rate during sintering, and remains in the sintered steel in a sintering atmosphere containing hydrogen. It was discovered that it remains as graphite to improve the machinability and that the carburizing rate of graphite powder with an average particle size of 15 μm or less is fast. Therefore, the average particle size is 25 ~
It was found that the machinability can be improved without reducing the strength of the sintered body by using the graphite powder of 120 μm and the graphite powder having an average particle size of 15 μm or less. It was also found that the coexistence of S in iron powder enhances the carburizing inhibition effect.

The present invention is based on the above findings. That is, according to the present invention, in claim 1, in the powder metallurgy method of obtaining sintered steel by molding and sintering using pure iron powder or alloy steel powder, graphite powder having an average particle size of 25 to 120 μm is added to iron powder for powder metallurgy. Of 0.1 to 1.0% and 0.5 to 2% of graphite powder having an average particle size of 15 μm or less is molded and sintered, which is a method for producing a sintered steel having excellent machinability.
Atomized iron powder for powder metallurgy according to claim 2, which is used in a powder metallurgy method for obtaining a sintered steel by molding and sintering pure iron powder or alloy steel powder, containing less than 0.1% Mn and 0.08 to 0.3% S. , Graphite powder with an average particle size of 25 to 120 μm in an amount of 0.1 to 1.0
%, And 0.5 to 2% of graphite powder having an average particle size of 15 μm or less is mixed and molded and sintered to produce a sintered steel having excellent machinability.

[0012]

[Operation] According to the present invention, a graphite powder having an average particle diameter of 25 to 120 μm and a graphite powder having an average particle diameter of 15 μm or less are used in combination, so that the strength of the sintered body is not lowered and The machinability can be improved. Further, the coexistence of S 2 in the iron powder enhances the carburizing inhibition effect, so that the required amount of residual graphite can be secured in the surface layer portion of the sintered steel, and the machinability of the sintered steel can be further improved.

First, the limiting conditions for two types of large and small graphite powders will be described. All graphite to be added has an average particle size of 25 to 12
When graphite powder of 0 μm is used, carburization is slow, the amount of C contained in the sintered steel matrix is small, and the strength is low. However, the average particle size is 15μ
Since the graphite powder of m or less has a high carburizing rate during sintering, it can be additionally mixed to secure the C content to be contained in the sintered steel matrix and prevent the strength from decreasing. As described above, graphite powder having a fast carburization rate of 15 μm or less of average particle size secures the C content to be contained in the sintered steel matrix and prevents the strength from decreasing. By adding the graphite powder of No. 1 above, residual graphite is generated and the machinability can be improved. This is because only a part of the graphite powder of Fe-C system or Fe-Cu-C system, which is a normal sintered material, is made into large particle size graphite, and the price of large particle size graphite is small. Since it is almost the same as the particle size graphite, there is almost no increase in cost.

Therefore, according to the present invention, a mixed powder containing 0.1 to 1.0% of graphite powder having an average particle size of 25 to 120 μm and 0.5 to 2% of graphite powder having an average particle size of 15 μm or less is molded into a raw material powder metallurgy iron powder. Characterized by sintering. Average particle size 25-12
Graphite powder of 0 μm is added as a residual graphite source in sintered steel to improve machinability. If the average particle size is less than 25 μm, the carburizing speed is too fast, and the residual graphite is small, resulting in poor machinability. The reason for limiting the average particle size to 25 μm or more is that the larger the particle size is, the slower the carburizing rate is, and the larger the particle size of the residual graphite for improving the machinability is, the more the machinability is improved. If the average particle size exceeds 120 μm, uniform mixing is difficult to proceed, the distribution of residual graphite in the sintered steel becomes uneven, and the machinability partially deteriorates, resulting in a decrease in machinability. The preferred range is 50-100 μm.

Add the amount of graphite powder with an average particle size of 25 to 120 μm
The reason why the content is 0.1% or more is that if the content is less than 0.1%, there is little residual graphite and the machinability is not improved. The upper limit of the amount of graphite powder added was set to 1.0% because roughening of the sintered body occurs. The preferable addition range is 0.2 to 0.6%. Average particle size 1
Graphite powder of 5 μm or less is carburized during sintering and added to obtain desired strength. Graphite powder for carburizing must have an average particle size of 15 μm or less. If it exceeds 15 μm, carburization becomes slow and desired strength cannot be obtained. Average particle size 15μ
The reason why the addition amount of graphite powder of m or less is limited to 0.5 to 2% is that if less than 0.5%, the C content in the steel is small and the strength is poor, and if it exceeds 2%, proeutectoid cementite precipitates and the machinability deteriorates. Because it does. The preferred addition range is 0.8-2.0%
Is.

The amount of residual graphite remaining in the sintered steel is 0.1 to 0.3.
% Is preferred. If it is less than 0.1%, the machinability is poor, and if it exceeds 0.3%, the sintered steel tends to be rough. In order to exert the effect of residual graphite as described above, it is preferable that the iron powder for powder metallurgy has Mn of less than 0.1% and S of 0.08 to 0.3%. The preferred range of S content is 0.08%
The reason for limiting to 0.3% is as follows. 0.08 for S
S of iron powder containing ~ 0.3% is contained as a FeS source in the powder, and further suppresses carburization by graphite powder with an average particle size of 25 μm or more during sintering to further increase the residual graphite in the sintered steel. By doing so, the machinability is further improved. S 0.08
If it is less than%, there is no carburizing prevention effect based on FeS, and S
Compared with iron powder that does not contain iron, the increase in residual graphite is small and no further improvement in machinability is observed. If S is contained in an amount of more than 0.3%, soot is likely to be generated during sintering and the sintering furnace may be damaged. The preferred range of S is 0.1 to 0.2%.

The Mn of the iron powder for powder metallurgy produced by the spraying method using water is preferably less than 0.1%. When Mn is 0.1% or more, Mn in the powder easily bonds with S.
However, the effect of improving the machinability based on FeS described above is not exhibited, and the residual graphite is less increased than the iron powder containing no S, and no further improvement in machinability is observed. The iron powder for powder metallurgy to which the graphite of the present invention is mixed is not particularly limited, as long as it is an iron powder used in powder metallurgy, pure iron powder or alloy steel powder may be used.

[0018]

【Example】

(Examples and Comparative Examples of Claim 1) -1 Table 1 shows the chemical compositions of the four iron powders used. These iron powders are produced by water-spraying molten steel and the resulting raw powders in a nitrogen atmosphere.
It was dried at 40 ° C for 60 minutes, reduced in a pure hydrogen atmosphere at 930 ° C for 20 minutes, and then pulverized and classified to produce.

[0019]

[Table 1]

Iron powders Nos. 1 to 4 each contain 1% by weight of zinc stearate, copper powder, and two types of graphite powders having different average particle sizes.
After mixing with the composition shown in 5, mold to a green compact density of 6.85 g / cm ³ in a nitrogen stream containing 10% by volume of hydrogen at 1130 ° C 20
Minute sintering. In Tables 2 to 5, among the two types of graphite powder added, the addition amount and average particle size of the graphite powder having the larger particle size are shown as the added graphite amount 1 and the added graphite average particle size 1, respectively. The addition amount and average particle size of the graphite powder having the smaller particle size are shown as the addition graphite amount 2 and the addition graphite average particle size 2, respectively. Gas flow rate during sintering is 5 Nl / min per 1 kg of compact
Met. The tensile strength and the Charpy impact value of the obtained sintered steel were measured.

[0021]

[Table 2]

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

[Table 5]

The machinability was evaluated by using a disk shape having an outer diameter of 60φ and a height of 10 mm, a green compact density of 6.85 g / cm ^3, and sintering after the above conditions.
Using a HSS drill with a diameter of 1 mmφ, 10,000 rpm, 0.012 m
The tool life was evaluated as the average number of holes that were machined until the machining became impossible under the condition of m / rev (the average value of three drills). The residual graphite content of the sintered steel was quantified by infrared absorption method after filtering the nitric acid dissolution residue with a glass filter.

As shown in Examples 1 to 24 of the present invention in Tables 2 to 5,
Iron powder No. 1-4, Cu powder 0.5-4%, average particle size 25-120
By forming and sintering a mixed powder containing 0.1 to 1% of graphite powder of μm and 0.5 to 2% of graphite powder with an average particle size of 15 μm or less, excellent cutting can be achieved even when sintered in an atmosphere containing hydrogen. It can be seen that the sex is obtained. Iron powder No. 4 with Fe-2% Cu-
After mixing and molding with a mixture of 1.0% Gr-1.0% Znst (average particle size of graphite powder is 10 μm), it was mixed in a nitrogen stream containing 10% by volume of hydrogen.
When sintered at 30 ° C for 20 minutes, the number of drilled holes was 15 and the amount of residual graphite was 0.03%. Comparative Examples 1, 2, 8, 9, 15,
As shown in 16, 22, and 23, when the addition amount of the graphite powder with the larger particle size is less than 0.1% or the average particle size is less than 25 μm, the residual graphite is small and the machinability is poor. Comparative Examples 3, 10, 1
As shown in 7 and 24, the particle size of graphite with the larger particle size is 120
If it exceeds μm, the machinability deteriorates. Comparative Examples 4, 11, 18,
As shown in 25, the addition amount of graphite powder with a larger particle size
If it exceeds 1%, the sintered steel becomes rough. Comparative Examples 5 and 1
As shown in Nos. 2, 19, and 26, if the addition amount of the graphite powder with the smaller particle size is less than 0.5%, the tensile strength of the sintered steel decreases. As shown in Comparative Examples 6, 13, 20, and 27, if the amount of graphite powder added with a smaller particle size exceeds 2%, the machinability deteriorates. As shown in Comparative Examples 7, 14, 21, and 28, when the particle size of the smaller graphite powder exceeds 15 μm, the tensile strength is reduced. In addition,
In the present invention examples 5, 11, 17, 23 in which the added Cu amount is less than 0.5%, the tensile strength is low, and the present invention examples 6, 1 in which the added Cu amount exceeds 4%
Since the impact value deteriorates at 2, 18, and 24, the addition amount of Cu is 0.5
The range of up to 4% is desirable.

(Example of claim 1 and comparative example) -2 Table 6 shows the chemical composition of the iron powder used. These iron powders
Raw powder obtained by spraying molten steel with water at 60 ° C in a nitrogen atmosphere at 60 ° C
After drying for 20 minutes, reducing it in a pure hydrogen atmosphere at 930 ℃ for 20 minutes,
It was manufactured by crushing and classifying.

[0028]

[Table 6]

Iron powder Nos. 5 to 8 and 1 weight of zinc stearate
% And two types of graphite powders having different average particle diameters are mixed according to the formulations shown in Tables 7 to 10 and molded into a green compact density of 6.85 g / cm ³ in a nitrogen stream containing 10% by volume of hydrogen. Sintered at 1150 ° C for 30 minutes.

[0030]

[Table 7]

[0031]

[Table 8]

[0032]

[Table 9]

[0033]

[Table 10]

After sintering, the tensile strength, the Charpy impact value, and the like in the same manner as in (Example 1 of Claim 1 and Comparative Example) -1.
The machinability and the amount of residual graphite were measured. As shown in Tables 7 to 10 of the present invention, iron powders 5 to 8 have an average particle size of 25 to 12
By forming and sintering a mixed powder containing 0.1 to 1% of graphite powder of 0 μm and 0.5 to 2% of graphite powder having an average particle size of 15 μm or less, excellent machinability is obtained even when sintered in an atmosphere containing hydrogen. It can be seen that Iron powder No. 8 to Fe-2% Cu-1.
After mixing and molding with a mixture of 0% Gr-1.0% Znst (graphite powder has an average particle size of 10 μm) and sintering in a nitrogen atmosphere flow containing 10% by volume of hydrogen at 1130 ° C for 20 minutes, the number of drill holes is 2 And the amount of residual graphite was 0.03%. Comparative Examples 29, 30, 36,
As shown in 37, 43, 44, 50, 51, the addition amount of graphite powder with the larger particle size is less than 0.1%, or the average particle size is 25 μm.
If it is less than 1, the residual graphite is small and the machinability is poor. Comparative Example 31,
As shown in 38, 45 and 52, if the particle size of the graphite powder with the larger particle size exceeds 120 μm, the machinability deteriorates. Comparative Example 32,
As shown in 39, 46 and 53, the amount of graphite powder with the larger particle size
If it exceeds 1%, the surface of the sintered steel becomes rough. Comparative Example 33,
As shown in 40, 47, and 54, when the addition amount of the graphite powder with the smaller particle size is less than 0.5%, the tensile strength of the sintered steel decreases.
As shown in Comparative Examples 34, 41, 48 and 55, if the addition amount of the graphite powder with the smaller particle size exceeds 2%, the machinability deteriorates. As shown in Comparative Examples 35, 42, 49 and 56, when the particle size of the graphite powder having the smaller particle size exceeds 15 μm, the tensile strength decreases.

(Examples and Comparative Examples of Claim 2) -1 (Examples and Comparative Examples of Claim 1) -1 In order to show the influence of the composition of the iron powder, the results of the same compounding conditions are shown. Table 11
Are summarized in. That is, iron powder No. 1 to 4, copper powder 2% by weight, zinc stearate 1% by weight, average particle diameter 60 μ
0.4% m graphite powder and 0.8% graphite powder with an average particle size of 10 μm
% Of the mixed powder was molded and sintered under the same conditions as used in (Example 1 and Comparative Example 1), and the tensile strength, Charpy impact value, machinability, and residual graphite content were measured. .

[0036]

[Table 11]

Inventive Example 41: Mn less than 0.1%, S content 0.08
Sintered steel with ~ 3% iron powder has an S content of less than 0.08%, or
An increase in the amount of residual graphite and a further improvement in machinability were observed in comparison with the sintered steel of iron powder of Mn0.1% or more, and Mn of less than 0.1% Sn produced by a spray method using water was 0.08- It can be seen that the use of iron powder for powder metallurgy containing 0.3% is preferred. From Comparative Examples 57 and 58, in the case of a sintered steel of iron powder having an S content of less than 0.08% or Mn of 0.1% or more, the machinability is poor. As shown in Comparative Example 59, when iron powder No. 3 with an S content exceeding 0.3% was sintered, soot was generated in the sintered steel, and there was a concern that the sintering furnace would be contaminated.

(Example and Comparative Example of Claim 2) -2 (Example and Comparative Example of Claim 1) -2 In order to show the influence of the composition of the iron powder, the results of the same compounding conditions are shown. The results are summarized in Table 12. That is, 1% by weight of zinc stearate and 0.3% of graphite powder having an average particle size of 50 μm was added to iron powder Nos. 5 to 8.
%, A mixed powder containing 0.9% of graphite powder having an average particle size of 10 μm was molded and sintered under the same conditions as those used in (Example 1 and Comparative Example of claim 1) -2, tensile strength, Charpy impact value, cutting The properties and the amount of residual graphite were measured.

[0039]

[Table 12]

Inventive Example 42-Mn less than 0.1%, S 0.03-0.
Sintered steel with 3% iron powder has an S content of less than 0.08% or Mn
An increase in the amount of residual graphite compared to sintered steel with powder of 0.1% or more,
Further improvement in machinability is recognized, and it is understood that the use of iron powder for powder metallurgy containing Mn less than 0.1% and S 8 0.08 to 0.3% produced by a spraying method using water is preferable. In Comparative Examples 60 and 61, the machinability is poor in the case of the sintered steel of the powder in which the S content is less than 0.08% or Mn 0.1% or more. As shown in Comparative Example 62, when iron powder No. 7 with an S content exceeding 0.3% was sintered, soot was generated in the sintered steel, and there was a concern that the sintering furnace would be contaminated.

Typical examples are shown in Tables 11 and 12, and the results of Tables 2 to 5 and 1% Cr-for so-called pure iron powder are shown.
Comparing the results of Table 7 to Table 10 for 0.3% Mo alloy steel powder, the Mn content of the iron powder is less than 0.1% and the S content is 0.08.
It can be seen that limiting the content to ~ 0.3% is effective in further improving the machinability.

[0042]

EFFECTS OF THE INVENTION According to the present invention, it is possible to easily obtain a sintered steel having excellent machinability as compared with the conventional sintered steel whose machinability has been improved.

Claims (2)

[Claims] 1. Molding using pure iron powder or alloy steel powder
In the powder metallurgy method of obtaining sintered steel by sintering, 0.1 to 1.0 of graphite powder with an average particle size of 25 to 120 μm is added to iron powder for powder metallurgy.
% By weight, graphite powder with an average particle size of 15 μm or less 0.5-2% by weight
A method for producing a sintered steel having excellent machinability, which comprises molding and sintering a mixed powder containing a mixture of: 2. Molding using pure iron powder or alloy steel powder
In powder metallurgy to obtain sintered steel by sintering, Mn is 0.1
Atomized iron powder for powder metallurgy containing less than 10 wt% and 0.08 to 0.3 wt% S with graphite powder with an average particle size of 25 to 120 μm.
Graphite powder with a particle size of 0.1-1.0% and an average particle size of 15μm or less 0.5-2
A method for producing a sintered steel having excellent machinability, which comprises molding and sintering a mixed powder in which the weight% is mixed.