JP7057156B2 - Iron powder for powder metallurgy - Google Patents

Description

The present invention relates to iron powder for powder metallurgy.

Powder metallurgy is widely used to form metal parts by compacting metal powder and then sintering it. In powder metallurgy, in general, the mechanical strength of the obtained metal sintered body can be increased by increasing the density of the green compact, that is, reducing the porosity of the green compact. Further, in powder metallurgy, by increasing the strength of the green compact, the dimensional accuracy of the obtained metal sintered body can be improved and the yield can be increased.

According to JP-A-4-173901, the apparent density of iron powder for powder metallurgy, that is, the density of the green compact can be increased by relatively increasing the bulk specific gravity of the powder in a stationary state, but it is seen above a certain level. It is described that the strength of the green compact becomes insufficient when the hanging density is increased. However, as a result of verification by the present inventors, in the limit region where the decrease in the strength of the green compact begins to become a problem, the large-small relationship between the apparent density of the iron powder for powder metallurgy having the same composition and the density of the green compact It turned out that it is not uncommon for and to be reversed.

By increasing the molding pressure during compaction forming, the density of the green compact can be increased and the strength of the green compact can be increased, but the increase in the forming pressure is gold. Since the life of the mold is shortened and other inconveniences occur, the production efficiency of metal parts is lowered.

Japanese Unexamined Patent Publication No. 4-173901

In view of the above inconvenience, it is an object of the present invention to provide iron powder for powder metallurgy, which can obtain a high-strength sintered body.

The iron powder for powder metallurgy according to one aspect of the present invention made to solve the above problems has C of 0.005% by mass or less, Si of 0.030% by mass or less, and P of 0.020% by mass or less. S has 0.020% by mass or less, O is 0.15% by mass or less, the total of Mn, Ni, Mo and Cr is 3.0% by mass or less, and the balance is Fe and unavoidable impurities. The density is 3.90 g / cm ³ or more and 4.20 g / cm ³ or less.

By setting the tap density within the above range, the iron powder for powder metallurgy can easily rearrange the iron powder particles so as to be in the most densely packed state, and thus has excellent compressibility during compaction molding. , The strength of the finally obtained sintered body is high.

Here, the "tap density" is a value measured in accordance with JIS-Z2512 (2012).

As described above, the iron powder for powder metallurgy according to one aspect of the present invention has a high density of green compact and a high-strength sintered body can be obtained.

It is a graph which shows the relationship between the tap density of the iron powder for powder metallurgy, and the density of a green compact. It is a graph which shows the relationship between the tap density of iron powder for powder metallurgy, and the rattler value of a green compact.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

[Iron powder for powder metallurgy]
The iron powder for powder metallurgy according to the embodiment of the present invention contains C of 0.005% by mass or less, Si of 0.030% by mass or less, P of 0.020% by mass or less, Mn, Ni, Mo and Cr. The total is 3.0% by mass or less, the balance is Fe and unavoidable impurities, and the tap density is 3.90 g / cm ³ or more and 4.20 g / cm ³ or less.

<C (carbon)>
C is an element that dissolves and hardens the particles of the iron powder for powder metallurgy. C can also cure iron powder particles by combining with other impurities to form fine carbides. When the iron powder particles become hard, they are less likely to be deformed during the compaction molding, so that the moldability is lowered and the density of the compaction is lowered. Therefore, the upper limit of the C content in the iron powder for powder metallurgy is 0.005% by mass, preferably 0.003% by mass, and more preferably 0.002% by mass.

<Si (silicon)>
Si is an element that easily binds to oxygen and forms an oxide film on the particle surface of the iron powder for powder metallurgy. Since the oxide film made of Si is not easily reduced, the strength of the obtained sintered body is lowered. Further, since Si has an action of hardening iron powder particles, it lowers the compressibility (density and strength of green compact) of the iron powder for powder metallurgy. Therefore, the upper limit of the Si content is 0.030% by mass, preferably 0.020% by mass, and more preferably 0.015% by mass.

<P (phosphorus)>
P is an element that hardens iron powder particles and lowers compressibility. Therefore, the upper limit of the P content is 0.020% by mass, preferably 0.017% by mass, and even more preferably 0.015% by mass.

<S (sulfur)>
S is an element that hardens the iron powder particles and lowers the compressibility. Therefore, the upper limit of the S content is 0.020% by mass, preferably 0.015% by mass, and even more preferably 0.010% by mass.

<O (oxygen)>
O is an element that hardens iron powder particles and lowers compressibility. Therefore, the upper limit of the O content is 0.15% by mass, preferably 0.12% by mass, and even more preferably 0.10% by mass.

<Mn, Ni, Mo, Cr>
Mn, Ni, Mo and Cr are elements added to improve the strength of the sintered body obtained by solid-melting and compacting and sintering the iron powder for powder metallurgy. However, if the content of these elements becomes too large, the iron powder particles may become too hard and the compressibility may be insufficient. Therefore, the upper limit of the total content of Mn, Ni, Mo and Cr is 3.0% by mass, preferably 2.5% by mass, and even more preferably 2.0% by mass.

<Tap density>
The tap density is an index showing the ease of rearrangement of iron powder particles, and if the true specific gravity is constant, the larger the value, the denser the iron powder particles and the smaller the porosity, the more densely the iron powder particles are rearranged into a packed state. Cheap. Therefore, the larger the tap density, the higher the compressibility, the easier the powder compaction is, and the higher the density of the green compact can be obtained at a relatively low pressure. On the other hand, if the tap density becomes excessively large, the adhesiveness between the iron powder particles may be insufficient and the strength of the green compact obtained may be insufficient. Therefore, the lower limit of the tap density of the powder metallurgy iron powder is 3.90 g / cm ³ , preferably 3.95 g / cm ³ , and more preferably 3.97 g / cm ³ . On the other hand, the upper limit of the tap density of the powder metallurgy iron powder is 4.20 g / cm ³ , preferably 4.15 g / cm ³ , and more preferably 4.10 g / cm ³ .

<Particle size distribution>
The lower limit of the content of particles passing through the plain woven wire net having an average opening of 45 μm in the iron powder for powder metallurgy is preferably 10% by mass, more preferably 12% by mass. On the other hand, the upper limit of the content of particles passing through the plain woven wire net having an average opening of 45 μm in the iron powder for powder metallurgy is preferably 20% by mass, more preferably 18% by mass. If the content of particles passing through a plain woven wire net having an average opening of 45 μm in the iron powder for powder metallurgy is less than the above lower limit, the strength of the sintered body of the iron powder for powder metallurgy may be insufficient. On the contrary, when the content of the particles passing through the plain woven wire mesh having an average opening of 45 μm in the iron powder for powder metallurgy exceeds the above upper limit, the strength of the finally obtained green compact may be insufficient.

<Density of green compact>
The lower limit of the density of the green compact obtained when 0.75% by mass of zinc stearate is added to the powder metallurgy iron powder and the powder is formed at a molding pressure of 7 tf / cm ² is 7.20 g / cm ³ . Preferably, 7.22 g / cm ³ is more preferable. If the density of the green compact is less than the lower limit, the strength of the finally obtained sintered body may be insufficient.

<Strength of green compact>
The upper limit of the rattler value, which is an index of the strength of the green compact obtained when 0.75% by mass of zinc stearate is added to the iron powder for powder metallurgy and the molding pressure is 7 tf / cm ² , is 0. It is preferably .75%, more preferably 0.70%. If the rattler value of the green compact exceeds the upper limit, the strength of the green compact may be insufficient and the dimensional accuracy and yield of the sintered body may be insufficient. The "latler value" is a value measured in accordance with the JSPM standard 4-69.

<Manufacturing method>
The iron powder for powder metallurgy has a water atomizing step of injecting water into the molten iron prepared to the above composition to powder it, and a reducing step of heating the powder obtained in this water atomizing step in a reducing gas atmosphere. , It can be produced by a method including a crushing step of crushing the iron powder solidified in the reduction step.

(Water atomizing process)
In the water atomizing step, fine iron powder to be injected is obtained by injecting water onto the molten iron flowing down from the furnace. In this water atomizing step, the tap density of the obtained iron powder for powder metallurgy is adjusted within the above range by adjusting the water pressure of the injected water. Specifically, the higher the water pressure, the smaller the tap density of the iron powder for powder metallurgy.

(Reduction process)
In the reduction step, the iron powder oxidized in the water atomizing step is reduced by heating in a reducing gas environment.

As the reducing gas, for example, hydrogen gas, ammonia gas, butane gas can be used.

(Crushing process)
In the crushing step, the iron powder solidified into a cake by the reduction treatment is crushed by a mill. By sufficiently crushing the iron powder, the particle size distribution of the iron powder for powder metallurgy obtained is similar to the particle size distribution of the iron powder obtained in the water atomizing step, and a desired tap density is ensured.

As the mill used in this crushing step, for example, a hammer mill, a feather mill, or the like can be used.

Further, in this crushing step, it is preferable to classify the crushed iron powder with a wire mesh and re-inject large particles into the mill.

<Advantage>
By setting the tap density within the above range, the iron powder for powder metallurgy can easily rearrange the iron powder particles so that the apparent density becomes large, so that the iron powder is excellent in compressibility during powder forming and is excellent in compressibility. A green compact having sufficient strength can be obtained. Therefore, the iron powder for powder metallurgy can efficiently produce a sintered body having high strength.

[Other embodiments]
The above embodiment does not limit the configuration of the present invention. Therefore, the above-described embodiment can be omitted, replaced or added with components of each part of the above-described embodiment based on the description of the present specification and the common general technical knowledge, and all of them are construed to belong to the scope of the present invention. Should be.

Hereinafter, the present invention will be described in detail based on Examples, but the present invention is not limitedly interpreted based on the description of this Example.

Molten iron was prepared using an electric furnace and pulverized by a water atomization method in which water was sprayed onto the molten iron flowing down from the electric furnace. At this time, the pressure of the water to be sprayed was selected from three types: a low pressure of 30 to 60 kgf / cm ² , a medium pressure of 60 to 90 kgf / cm ² , and a high pressure of 90 to 120 kgf / cm ² . Next, the obtained iron powder was dehydrated and dried to remove coarse powder at 425 μm, and then subjected to a reduction treatment in a temperature range of 880 to 980 ° C. for 30 to 60 minutes in a decomposed ammonia gas atmosphere. Then, the iron powder that had been reduced and solidified into a cake shape was crushed by a hammer mill and a feather mill, and sifted with a wire net having an opening of 425 μm, 250 μm, or 180 μm. Obtained 1-9.

The prototype No. of the iron powder for powder metallurgy thus obtained. The compositions of 1-9 were analyzed. The contents of C and S were measured using a carbon / sulfur analyzer "CS-244" manufactured by LECO. The O content was measured using an oxygen / nitrogen analyzer "TC-400" manufactured by LECO. The content of elements other than C, S and O was measured using an ICP-issued analyzer "ICPV-5500" manufactured by Shimadzu Corporation. Prototype No. The analysis results of the compositions of 1 to 9 are shown in Table 1 below.

Furthermore, the prototype No. of iron powder for powder metallurgy. The particle size distribution of 1 to 9 and the tap density were measured. The particle size distribution was measured by a sieving test based on JIS-Z8815 (1994). The tap density was measured according to JIS-Z2512 (2012).

Prototype No. of iron powder for powder metallurgy. A powder obtained by adding 0.75% of zinc stearate as a lubricant to 1 to 9 and mixing them is powder-molded at a molding pressure of 7 tf / cm ² to prepare a columnar green compact having a diameter of 11.28 mm and a height of 10 mm. bottom. The density and rattler value of the obtained green compact were measured. The density of the green compact was measured according to JIS-Z2501 (2000). The rattler value of the green compact was measured according to the JSPM standard 4-69.

Prototype No. of iron powder for powder metallurgy. Particle size distribution of 1 to 9, tap density, and prototype No. of iron powder for powder metallurgy. The densities and rattler values of the green compacts 1 to 9 are summarized in Table 2 below.

Further, in FIG. 1, the prototype No. of iron powder for powder metallurgy is shown. The relationship between the tap density of 1 to 9 and the density of the green compact is shown, and FIG. 2 shows the prototype No. of iron powder for powder metallurgy. The relationship between the tap density of 1 to 9 and the rattler value of the green compact is shown.

As shown in the figure, it was confirmed that the tap density, the powder density and the rattler value are in a substantially proportional relationship. More specifically, the density of the green compact is 7.20 g / cm ³ or more, which provides sufficient strength after sintering, and the rattler value of the green compact is 0.75% or less, which is within the allowable range for cracking and chipping. It was confirmed that the tap density of the iron powder for powder metallurgy should be 3.90 g / cm ³ or more and 4.20 g / cm ³ or less.

The iron powder for powder metallurgy according to the present invention can be suitably used for manufacturing mechanical parts such as gears.

Claims (1)

C is 0.005% by mass or less,
Si is 0.030% by mass or less,
P is 0.020% by mass or less,
S is 0.020% by mass or less,
O is 0.15% by mass or less,
It has a composition in which the total of Mn, Ni, Mo and Cr is 3.0% by mass or less, and the balance is Fe and unavoidable impurities.
The tap density is 3.90 g / cm ³ or more and 4.20 g / cm ³ or less.
The content of particles passing through a plain weave wire mesh having an average opening of 45 μm is 10% by mass or more and 20% by mass or less .
An iron powder for powder metallurgy having a rattler value of 0.75% or less when zinc stearate is added in an amount of 0.75% by mass and molded at a molding pressure of 7 tf / cm ² .

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