JPH07278725A - Production of sintered steel having excellent machinability - Google Patents

Abstract

PURPOSE:To improve the machinability of a sintered steel obtd. by specifying the contents of Mn and S of raw material iron and steel powder and adding powders of the sulfides of specific metals and graphite to the raw material powder at the time of producing the sintered steel by a powder metallurgical process using pure iron powder or alloy steel power as the raw material. CONSTITUTION:The Mn content in the iron and steel powder of the raw material is specified to 0.1% and the S content to 0.08 to 0.30% at the time of producing the sintered steel member by the powder metallurgical process using the pure iron powder or alloy steel powder produced by a water atomization as the raw material. In addition, >=1 kinds of the powders of the metal sulfides, such as MOS2, WS2 and SnS, having an average grain size 3 to 5mum and the powder of the graphite are added and mixed at 0.4 to 5% and lead stearate as a lubricant is added and mixed at about 1% to and with the powder. Such powder mixture is compression molded to a desired shape and is then sintered in a nitrogen atmosphere contg. hydrogen. Even though the FeS in the raw material is made to remain without reducing by the presence of the metal sulfide of the MOS2 system in spite of sintering in hydrogen, the carburization by the graphite is prohibited and, therefore, the sintered steel which is not degraded in the machinability by the presence of the graphite is obtd.

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 usually manufactured by adding Cu powder, graphite powder, etc. to iron powder, compacting it in a mold and sintering it.
It is used for producing sintered machine parts and the like having a density of 5.0 to 7.2 g / cm ³ . 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. It is said that these S and MnS have the effect of facilitating the fracture of chips, or that they form a thin cutting edge of S and MnS on the tool and improve the machinability by the lubricating action on the rake face of the tool. But the normal Mn
In order to improve machinability by adding S, a large amount of Mn
In addition to the need to add S, there is a risk of adverse effects such as sooting during sintering.

For example, in Japanese Patent Publication No. 3-25481,
Powder metallurgy in which 0.1 to 0.5% by weight (hereinafter abbreviated as%) of pure iron powder containing 0.03 to 0.07% of S is added to pure iron powder containing Mn, Si, C, etc. Iron powder for use is disclosed. On the other hand, the present inventors, in Japanese Patent Application No. 5-337325, Japanese Patent Application No. 5-336076,
When the atomized iron powder for powder metallurgy containing S, Cr, and Mn is sintered, residual graphite is present in the pores of the sintered steel, and MnS within 5 μm exists in the sintered steel grains and grain boundaries. We have proposed a technology different from the conventional one in which a sintered steel that works effectively and has excellent machinability can be easily obtained.

[0005]

However, in the iron powder for powder metallurgy containing S including these, when sintered in an atmosphere containing hydrogen, S is reduced / removed and the machinability is remarkably deteriorated. There was a problem of doing. In view of such drawbacks of the prior art, the present invention aims to provide a method for producing a sintered steel having excellent machinability even when the atomized iron powder for powder metallurgy is sintered in an atmosphere containing hydrogen. And

[0006]

Means for Solving the Problems The present inventors have found that with respect to atomized iron powder for powder metallurgy containing Mn of less than 0.1% and containing S, reduction in machinability when sintered in an atmosphere containing hydrogen. The cause was examined thoroughly. S is 0.08 to 0.
When the atomized iron powder for powder metallurgy containing 3% was sintered in a nitrogen atmosphere containing hydrogen, the S content and the amount of residual graphite were reduced as compared with the sintered steel sintered in pure nitrogen. Also
Assuming that FeS was reduced with hydrogen, the reduction amount of S during sintering was calculated, and it was in good agreement with the analytical value. From these facts, it was found that FeS in atomized iron powder or on the surface is directly involved in the formation of residual graphite.

During sintering of atomized iron powder for powder metallurgy containing 0.08 to 0.3% of S in a nitrogen atmosphere containing hydrogen,
As a result of quenching and analysis of residual graphite, it was found that residual graphite was formed by preventing carburization during sintering. When sintered in a nitrogen atmosphere containing hydrogen in this way, FeS contained in the powder is reduced and removed by hydrogen, and as a result of losing the effect of blocking carburization during sintering, the amount of carburization of graphite increases and It was found that the amount of residual graphite in the steel is reduced and the machinability is reduced.

Further, when Mn of the molten steel is set to a usual level of 0.10 to 0.7%, no graphite remains even if S is added,
It was found that the effect of improving the machinability could not be obtained. It was also found that reducing Mn to less than 0.1% is a necessary condition for obtaining residual graphite that improves machinability. For the production of sintered steel with excellent machinability even when sintered in an atmosphere containing hydrogen using atomized iron powder for powder metallurgy containing Mn less than 0.1% and S containing 0.08 to 0.3%. For, it is necessary to prevent FeS contained in the powder from being reduced by hydrogen during sintering.

The reduction reaction of sulfide with hydrogen is 2MS + 2H ₂ = 2H ₂ S + 2M (1). By adding sulfides such as MoS ₂ , WS ₂ and SnS whose equilibrium partial pressure of H ₂ S is larger than that of FeS in Eq. (1), this sulfide is preferentially reduced over FeS during sintering. Therefore, FeS contained in the powder is less likely to be reduced. Fe
It was discovered that if S remains, carburization by graphite powder is prevented, residual graphite does not decrease, and machinability does not decrease.

The following techniques have been publicly announced / published as a method of mixing and adding MoS ₂ and WS ₂ to iron powder, molding and sintering the mixture to produce a sintered steel having excellent machinability. Japanese Patent Publication Sho 58-5
In Japanese Patent No. 7505, 1.5-5% C, 10-30% Cr, 15
-40% Mo, 10% or less Co, the balance being essentially iron alloy powder with Ni powder 0.5-5%, graphite powder 0.6-1.5%, Mo
0.5 to 2% of one or two of S ₂ powder and WS ₂ powder,
Ferromanganese powder or metallic manganese powder in Mn amount of 0.
A method for producing a material having excellent wear resistance and machinability by mixing 5 to 3% and molding and sintering is disclosed.
These MoS ₂ and WS ₂ are decomposed during sintering, and the decomposed S is Mn, Fe and Ni.
It is said that it reacts with to form sulfides and improves machinability.

Also in each of the above-mentioned techniques, the added Mo is added.
S₂, WS₂Itself or MoS₂, WS ₂Raw by decomposition of
The decomposed S formed into sulfides formed by reacting with Mn, Fe and Ni
We are aiming to improve cutting performance. On the other hand, the present invention
In order to improve the machinability of graphite remaining in the pores of sintered steel.
The present invention focuses on the MoS of the present invention.₂, WS₂The powder is
FeS necessary to prevent carburization and leave graphite
Atoma containing S to protect against hydrogen reduction during sintering
Each of the above-mentioned technologies is mixed and added to Izu iron powder.
Is a completely different technology.

JP-A-62-167864, JP-A-63-20431
In Japanese Patent Laid-Open Publication No. 2003-242242, an iron powder or an iron-based alloy powder is mixed with a sintering-accelerating powder, a graphite powder, and one or more kinds of powders selected from MnS, S, Se, and MoS ₂ and then molded and sintered. A free-cutting sintered steel and its manufacturing method are disclosed. Mo
S ₂ is added for the purpose of improving machinability. JP 48
No. -80409 discloses a free-cutting sintered steel to which WS ₂ powder is added as a mixed powder. The WS ₂ powder is said to act as a lubricant during cutting to improve machinability.

In Japanese Patent Laid-Open No. 64-79301, Fe-Mn is used.
A method of partially alloying MoS ₂ having a particle size of 30 μm or less in the alloy powder without completely forming a solid solution is disclosed. During sintering, S in MoS ₂ combines with Mn of Fe-Mn alloy powder to form MnS and improves machinability. In the present invention, as described above, iron powder containing a specific range of Mn and S and a specific sulfide form a residual graphite in the sintered steel even in sintering in an atmosphere containing hydrogen by a combination of mixing. This significantly improves the machinability and is different from the technique disclosed in Japanese Patent Laid-Open No. 64-79301.

That is, the present invention is a powder metallurgy method for obtaining a sintered steel by molding and sintering using pure iron powder or alloy steel powder, and powder metallurgy containing less than 0.1% Mn and 0.08 to 0.3% S. For iron powder for use, MoS _{2 with an} average particle size of 0.3 to 5 μm
0.05 to 0.5%, WS ₂ 0.05 to 0.5%, SnS 0.05 to 0.5%
One or more selected from the above and 0.4 to 5% of graphite powder are mixed, the mixed powder is compression-molded, and sintered in an atmosphere containing hydrogen. Is the way.

[0015]

[Operation] According to the present invention, the amount of Mn is limited to less than 0.1%,
Atomized iron powder for powder metallurgy containing 0.08 to 0.3% of S was added to sulfide powder with an equilibrium partial pressure larger than FeS in a hydrogen-containing atmosphere, graphite powder and, if necessary, Cu powder 0.5 to 4
%, Residual graphite is produced in the pores of the sintered steel, and sintered steel having excellent machinability can be manufactured.

Next, the reasons for defining the components of the present invention will be described. First, in order to exert the effect of the present invention, the composition of atomized iron powder for powder metallurgy, especially the Mn and S 2 contents, is very important and is defined as follows. The Mn of atomized iron powder for powder metallurgy is limited to less than 0.1%. Mn to 0.
It is difficult to reduce it to less than 1% with ordinary electric furnaces, and special refining methods are required. When Mn is 0.1% or more, Mn in the powder is S
FeS in iron powder is reduced, and therefore residual graphite in the sintered steel is reduced, since it combines with MnS to form MnS. Even if MnS, which is a free-cutting component, increases, it does not reach the effect of improving the machinability of residual graphite, so the machinability does not improve as expected.

S in atomized iron powder for powder metallurgy is the FeS source in the iron powder. This FeS suppresses carburization during sintering and allows graphite to remain in the sintered steel, improving the machinability of the sintered steel. The reason for defining the S content in the range of 0.08% by weight to 0.3% is as follows. If S is less than 0.08%, there is little residual graphite and the machinability is not improved. S is 0.3
If it exceeds%, soot is likely to be generated during the sintering, and there is a concern that the sintering furnace will be damaged. The preferable range is 0.1 to 0.2%.

The atomized iron powder for powder metallurgy may be so-called pure iron powder or alloy steel powder as long as it satisfies the above requirements. The powders to be added to and mixed with the atomized iron powder for powder metallurgy before forming / sintering must satisfy the following requirements in order to fulfill their respective roles. Mo
Powder S _2, WS _2, SnS, when sintered in an atmosphere containing hydrogen, is reduced preferentially by hydrogen than FeS equilibrium partial pressure is greater because the iron powder in the FeS contained in iron powder It is added to make it difficult to reduce. Addition of these powders does not reduce residual graphite and machinability. Mo
The reason for defining S _2, WS _2, SnS to 0.05% to 0.5% respectively is as follows. The lower limit was determined from the viewpoint of machinability, and the upper limit was determined from the viewpoint of cost.

That is, when the content is less than 0.05%,
FeS is reduced, and as a result, the residual graphite in the sintered steel is reduced and the machinability is reduced. If it exceeds 0.5%, the sintering furnace may be contaminated. The preferred range is 0.
1 to 0.3% The average particle size of the powder of MoS ₂ , WS ₂ and SnS is 0.3 to 5 μm. If it is less than 0.3 μm, the rate of reduction by hydrogen during sintering is too fast, so that it undergoes reductive decomposition before reaching the maximum temperature retention, and the effect of protecting FeS in the powder is lost. Therefore, the residual graphite of the sintered steel decreases and the machinability deteriorates. If it exceeds 5 μm, it is difficult to distribute it evenly on the surface of the iron powder, and during cutting, a portion with little residual graphite is formed and the machinability deteriorates. The preferred range is
0.5 to 3 μm.

The graphite powder is necessary as a residual graphite source in the sintered steel for improving the machinability, and is further dissolved in iron to enhance the strength. The reason for defining the graphite powder to 0.4 to 5% is as follows. This is because if it is less than 0.4%, the strength decreases, and if it exceeds 5%, pro-eutectoid cementite precipitates and the machinability deteriorates. The preferable range is 0.8 to 2%. From the viewpoint of machinability, 0.8% or more is preferable, and from the viewpoint of graphite powder cost, 2% or less is preferable.

In order to increase the strength of the sintered steel, it is desirable to form and sinter by using a mixed powder obtained by further mixing Cu powder in an amount of 0.5 to 4% as is usually done.

[0022]

【Example】

Example 1 Table 1 shows the chemical compositions of the six types of iron powder used. These iron powders are raw powders obtained by spraying molten steel with water 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.

[0023]

[Table 1]

Iron powders Nos. 1, 2 and 3 were provided with zinc stearate 1%, and at least one of MoS ₂ powder, WS ₂ powder and SnS powder, copper powder and graphite powder. After being mixed in the composition shown in Fig. 4, the mixture was molded to have a green compact density of 6.85 g / cm ³ and sintered in a nitrogen stream containing 10% of hydrogen at 1130 ° C for 20 minutes. The gas flow rate was 5 Nl / min per 1 kg of compact. The tensile strength and the Charpy impact value of the obtained sintered body were measured.

The machinability was evaluated by using a disc shape having an outer diameter of 60φ, a height of 10 mm, a green compact density of 6.85 g / cm ^3, and sintering under 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 could be machined under the m / rev condition (the average value of three drills). The residual graphite amount of the sintered body was quantified by infrared absorption method after filtering the nitric acid-dissolved residue with a glass filter.

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

[Table 4]

As shown in Examples 1 to 45 of the present invention in Tables 2, 3, and 4, iron powder Nos. 1, 2, and 3 contained MoS ₂ 0.05 to 0.5% and WS ₂
One or more selected from 0.05 to 0.5%, SnS 0.05 to 0.5%, Cu powder 0.5 to 4%, graphite powder 0.4 to 5%,
It can be seen that by molding and sintering the mixed powder with the balance being iron powder, excellent machinability can be obtained even if sintering is performed in an atmosphere containing hydrogen. Iron powder No. 1 with Fe-2.0% Cu-1.
When mixed and molded with 0% Gr-1% ZnSt and sintered at 1130 ° C for 20 minutes in a nitrogen stream containing 10% hydrogen, the number of drill holes is 15
The amount of residual graphite was 0.03%. Comparative Examples 1, 3,
5, 13, 15, 17, 25, 27, 29, MoS ₂ powder,
If the amount of WS ₂ powder or SnS powder added is less than 0.05%, the residual graphite is small and the machinability is poor. Comparative Examples 2, 4, 6, 14, 16, 1
As shown in 8, 26, 28 and 30, MoS ₂ powder, WS ₂ powder, SnS
When the amount of one or more of the powders added exceeded 0.5%, soot was generated in the sintered body, and there was a concern that the sintering furnace would be contaminated. Comparative example
As shown in 11, 23, and 35, when the particle diameter of sulfide is less than 0.3 μm, or when the particle diameter of sulfide is 5 as in Comparative Examples 12, 24, and 36.
If it exceeds μm, the machinability deteriorates. Comparative Examples 7 and 19,
As shown in 31, when the amount of added graphite is less than 0.4%, the strength is low, and there is little residual graphite, resulting in poor machinability. Comparative Examples 8 and 2
As shown in 0 and 32, if the amount of added graphite exceeds 5%, the machinability deteriorates due to proeutectoid cementite.

As shown in Table 5, as shown in Comparative Examples 37 and 38, Mn contained in the atomized iron powder for powder metallurgy is 0.1% or more,
Alternatively, when S is less than 0.03%, the residual graphite after sintering is small and the machinability is poor. Contained in iron powder as shown in Comparative Example 39
If the S content exceeds 0.3%, soot is generated in the sintered steel, and there was a concern that the sintering furnace would be contaminated. Furthermore, as shown in Comparative Examples 9, 21, and 33, when the amount of added Cu is less than 0.5%, the strength is low.
As shown in 2, 34, when the added Cu amount exceeds 4%, the impact value deteriorates.

[0031]

[Table 5]

Example 2 Table 6 shows the chemical composition of 1% Cr-0.3% Mo alloy steel powder used in the examples of the present invention and the comparative examples. These alloy steel powders were produced by drying raw powder obtained by spraying molten steel with water in a nitrogen atmosphere at 140 ° C for 60 minutes, reducing it in a pure hydrogen atmosphere at 930 ° C for 20 minutes, and then pulverizing and classifying. .

In the same manner as in Example 1, the composition shown in Table 7 was used.
Minute sintering. After sintering, tensile strength was measured in the same manner as in Example 1,
The Charpy impact value, the machinability, and the amount of residual graphite were measured.

[0034]

[Table 6]

[0035]

[Table 7]

As shown in Table 7 and Examples 46 to 58 of the present invention, iron
MoS to powder No. 7₂ 0.05 to 0.5%, WS ₂0.05 to 0.5%, Sn
S One or more selected from 0.05 to 0.5%, graphite powder 0.4 to
Molded and baked mixed powder containing 5% and the balance iron powder
By binding, sintering in an atmosphere containing hydrogen
Also, it can be seen that excellent machinability is obtained. Iron powder No. 7
Was mixed with Fe-1.0% Gr-1.0% Znst and mixed, and then hydrogen was added to 10%.
Drilled when sintered at 1150 ° C for 20 minutes in a nitrogen stream containing nitrogen
The number was 1, and the amount of residual graphite was 0.01%.

As shown in Comparative Examples 40, 42 and 44, MoS₂Powder,
WS₂Less than 0.05% of powder and SnS powder has less residual graphite
Machinability is poor. MoS as shown in Comparative Examples 41, 43, 45₂powder
End, WS ₂Powder, SnS powder 1 or more total exceeds 0.5%
Then, soot is generated in the sintered steel and there is a concern that the sintering furnace may be contaminated.
It was As shown in Comparative Examples 51 and 52, the particle size of sulfide is 0.3 μm.
If it is less than 5 μm or exceeds 5 μm, the machinability will deteriorate. Comparison
As shown in Example 46, when the amount of added graphite is less than 0.4%, the strength is low.
In addition, the residual graphite is small and the machinability is poor. Shown in Comparative Example 47
If the amount of added graphite exceeds 5%, the machinability deteriorates.
It

In powder as shown in Comparative Examples 48 and 49
When Mn is 0.1% or more or S is less than 0.3%, the residual graphite after sintering is small and the machinability is poor. As shown in Comparative Example 50, when the amount of S contained in the powder exceeded 0.3%, soot was generated in the sintered steel, and there was a concern that the sintering furnace would be contaminated.

[0039]

According to the method of the present invention, even when the atomized iron powder for powder metallurgy containing S is sintered in an atmosphere containing hydrogen, it is possible to easily produce a sintered steel having excellent machinability.

Claims (1)

[Claims] 1. Molding using pure iron powder or alloy steel powder
In the powder metallurgy method of obtaining a sintered steel by sintering, iron powder for powder metallurgy containing less than 0.1% Mn and 0.08 to 0.3% S in weight% is added to MoS _{2 having an} average particle size of 0.3 to 5 μm, respectively. 0.0
5 to 0.5%, WS ₂ 0.05 to 0.5%, one or more selected from SnS 0.05 to 0.5% and graphite powder 0.4 to 5% are mixed,
A method for producing a sintered steel having excellent machinability, which comprises compression-molding the mixed powder and sintering the mixture in an atmosphere containing hydrogen.