JPH0784604B2 - Method for producing iron powder for sinter shrinkage body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing fine iron powder, particularly iron powder suitable for use in sintering contraction bodies such as injection sintering molding.

[Conventional technology]

Conventionally, as a method for obtaining an iron-based sintered body by a powder metallurgy method, a press molding method for parts such as automobiles has been widely adopted. This is an ultra-low carbon iron powder with a particle size of about 100 μm that is pressed together with graphite powder by a strong press to obtain a compact with a density of 80 to 90% of the theoretical density of iron, and then sinter at high temperature to obtain the final product. Is a way to get. Hereinafter, the molded body obtained by this method is referred to as a press shrinkable body in the present specification.

In recent years, an injection molding method has appeared as one of the molding methods in powder metallurgy. This is because fine powder with a particle size of 30 μm or less is mixed with thermoplastics and molded with an injection molding machine to make a molded body with a density of about 50% of iron, and sinter it like the press molding method, At that time, the fine iron powder agglomerates and the compact is largely shrunk, resulting in a higher density than that obtained by press molding, and a product with a density of about 95% of the theoretical density is obtained. Hereinafter, the sintered body obtained by this method is referred to as a sinter contractor, and the iron powder suitable for the sinter is referred to as an iron powder for a sinter contractor.

In order to obtain a sintered shrinkable body for improving the density by shrinking at a high temperature, fine powder having a large surface energy is required, and it is said that the average particle diameter thereof is preferably 30 μm or less, particularly 10 μm or less.

As a method for producing such fine iron powder, an atomizing method such as a water atomizing method in which a small amount of molten iron is dropped into high-pressure water of 500 Kg / cm ² or more and a gas atomizing method using a gas as a refrigerant, and the iron powder is heated at high temperature and high pressure. There is known a carbonyl method or the like in which liquid Fe (CO) ₅ is made to react with CO gas to produce iron powder by evaporating it.

However, in these conventional methods for producing fine iron powder, the water atomization method has a low yield because of large variation in particle size distribution, and in the gas atomization method, since gas is used as a refrigerant, it becomes spherical, but the heat capacity of the gas is small. It has drawbacks such as extremely low productivity and extremely high cost in the carbonyl method.

[Problems to be Solved by the Invention]

As a method of obtaining iron powder, "Metals Handbook Ninth Edition Vo" published by American Society for Metals
lume 7 Powder Metallurgy),
There is an example in which iron is crushed to around 100 μm by a dry ball mill.

However, in the case of using a dry ball mill, the pulverization efficiency sharply decreases as the powder becomes finer. Also, in the method of placing the powder on the high-speed gas stream and colliding the powder or the powder with the collision plate,
It requires a large amount of gas, has low productivity per hour, and is not cost-effective for powder metallurgy based on mass production.

On the other hand, wet pulverization is effective for producing fine powder of 30 μm or less. However, when water is used as the solvent, oxidation during pulverization and drying becomes a problem. Of course,
The problem of oxidation is avoided if an organic solvent that does not dissolve oxygen in the medium is used. However, the organic solvent is disadvantageous in terms of cost and has many restrictions in terms of work such as explosion-proof measures.

The problem to be solved by the present invention is to effectively utilize the oxidation, which has been a conventional problem when crushing means using water as a medium is adopted in the production of iron powder for a sintered shrinkable body.

[Means for Solving the Problems]

The present invention is different from the iron powder for press shrink bodies, which is premised on low carbon and low oxygen, and the iron powder for sintering shrink bodies does not necessarily need to lower carbon and oxygen, and these elements remained in a considerable amount. It was completed based on the knowledge that it can be put to practical use even in the state.

That is, in the case of a press-shrinkable body, low-carbon steel is premised to facilitate plastic deformation in the press step, and since it has already shrunk in the firing step, oxygen is not reacted and removed as CO gas, so carbon is not formed before forming. And oxygen levels need to be lowered.

On the other hand, in the case of a sinter shrinkage body, carbon and oxygen escape as reaction gases during firing and then shrink, so there is no problem if these elements remain in balance. And completed.

The present invention is summarized as follows. By finely crushing high carbon content white pig iron in water as a medium, drying and optionally subjecting it to heat treatment to obtain an oxygen content commensurate with the carbon content in iron. , A method for producing fine iron powder for a sintered contraction body for producing a sintered body having a predetermined carbon content. Further, the dried iron powder is subjected to a heat treatment to form an aggregate of fine iron powder having a carbon content of 0.5% or less, and the aggregate is ground to make the powder shape spherical suitable for injection molding. That is.

As the high carbon content molten iron, pig iron containing about 4% by weight of carbon can be used, and blast furnace pig iron from which sulfur, silicon, phosphorus, etc. are removed by a pretreatment is particularly preferable.
When molten high-carbon iron is poured into the jet stream, the high-carbon iron turns into white pig iron and becomes extremely hard, brittle, and easily crushed into particles of 3 mm or less.

When crushing the granular iron, the first crushing, that is, the coarse crushing may be either wet or dry.

However, it is crushed by a vibrating ball mill, agitation ball mill, etc., using water as a medium from around 100 μm where the crushing efficiency is different. The crushed slurry is dehydrated by a centrifuge, and the dehydrated cake is dried in a constant temperature bath.

Oxidation of the fine iron powder occurs in the grinding and drying process. As means for preventing the oxidation, addition of a rust preventive agent to water as a grinding medium and reduction of oxygen partial pressure in a dry atmosphere can be mentioned. That is, the oxidation amount of the iron powder after drying can be controlled by the conditions of the aqueous medium and the conditions of the dry atmosphere.

The carbon contained in the iron powder is carbon that has not been reacted with oxygen due to the release of oxygen in the oxide produced in the pulverization and drying steps as carbon dioxide gas in the subsequent heat treatment or sintering after molding. Remains. That is, since the amount of carbon in the iron powder is constant, the amount of carbon in the sintered body can be controlled by adjusting the amount of oxidation in the grinding and drying steps.

However, as the crushing progresses and the particle size decreases, the total surface area of the iron powder increases, and the amount of oxidation increases. Especially when the thickness is 10 μm or less, the amount of oxygen exceeds the carbon equivalent in the iron powder even if the oxidation is suppressed by the above-mentioned means, and even if a sintered body is formed, no carbon remains and the product is inferior in amount of oxygen. Therefore, in such a case, excess oxygen is hydrogen-reduced before the decarburization reaction and before the contraction.

In this way, it becomes possible to control the carbon content of the sintered body by leaving the oxygen content in balance with the carbon content in the iron powder.

The iron powder containing carbon obtained by the present invention can be molded using water as a medium. In addition, the addition of a few% of an organic binder at the time of molding is remarkably effective in increasing the deformability at the time of molding and the strength of the green molded body, and makes handling such as correction before molding and sintering remarkably easy. When water is used as the binder, it is almost removed in the drying process and there is no problem. However, even when the organic binder is used, the amount used is extremely small, so long-time degreasing is not necessary.

The iron powder that has been crushed and heat-treated as described above is angular and lacks fluidity as a powder, so it cannot be said that it is necessarily optimal for a molding method that requires fluidity such as injection molding, and the amount of binder is increased. It is necessary to secure liquidity by such means. For this type of application, it is desirable for the powder to have a shape close to a sphere.

When the crushed iron powder is heated to 650-700 ℃, it releases CO gas and the decarburization reaction starts. As the temperature rises, the reaction proceeds and both carbon and oxygen drop, but at the same time, an agglomeration phenomenon occurs in which the particles adhere to each other.

Therefore, as described above, the decarburization treatment is usually performed in a temperature range that does not cause aggregation, and the removal of the remaining oxygen and carbon depends on the sintering process.

However, if the temperature is raised so that the agglomeration proceeds, the carbon in the powder will drop and become soft iron.

The iron powder thus obtained, which has a high degree of purity, partially adheres to the surface of the particles to form a sintered body, and therefore a crushing step of unifying the iron particles to the original particle size is required. Naturally, the energy required for crushing is related to the hardness of the sintered body, and the hardness depends not only on the heat treatment temperature but also on the grain size of the raw material. For example, iron powder of about 100 μm can be crushed without heat treatment even at 1000 ° C, but fine powder of about 20 μm requires large energy for crushing even if the heat treatment temperature is 900 ° C. It also undergoes considerable plastic deformation.

The principle of crushing is divided into impact crushing, which is crushed by impact force, and grinding, which is crushed by frictional force. As an example of impact crushing a low carbon sintered body, the one crushed by a ball mill has an angular shape. When the treatment is further continued, the particles are considerably deformed, and some of them have a foil shape and have poor fluidity, which is unsuitable as a raw material for injection molding which is poured together with a binder.

However, when the soft iron powder or the sintered body thereof is crushed, the particles of the powder tend to be spherical. Therefore, if the stirring time is extended, or if a stronger sintered body is made and crushed and crushed, spheroidization proceeds. This is probably because the way plastic deformation of iron powder changes depending on the type of energy applied during crushing. That is, when an impact force is applied, the particles are slapped, but when a frictional force is applied, the particles rub against each other and become spherical. As a grinding device for this purpose, for example, a grinding mill, a crushing machine, etc. can be mentioned, whereby spherical fine iron powder can be obtained. However, this spheroidizing phenomenon occurs with iron having a carbon content of 0.4% by weight or less, and iron having a carbon content exceeding 0.5% by weight is hard.

〔Example〕

Example 1 After the pig iron discharged from the blast furnace was placed in a torpedo car for dephosphorization treatment, it was dropped into a large amount of water to obtain granular iron. The components are shown in the table and the particle size distribution is shown in FIG. This granular iron was crushed to 44 μm or less by a vibrating ball mill. Thereafter, 1% of a rust preventive agent was added and mixed with 1 part of water in a ratio of 1 crushed powder and finely crushed to an average of 9.2 μm by a stirring ball crusher. The particle size distribution is shown in FIG.

After drying the above fine powder slurry in a thermostat at 80 ° C in the atmosphere, add 20% water and 0.5% methyl cellulose (both weight ratio to iron powder), put it in a pipe with an inner diameter of 12 mm, and solidify it into a columnar shape. Made pellets. Drying the pellets,
After dehydration, the product was heated at 1200 ° C. for 1 hour in an atmosphere isolated from the air, and as a result, a sintered body having a density of 7.1 g / cm ³ was obtained. The carbon content of the iron powder before forming was 3.0% and oxygen was 3.2%, but the content of the sintered body was 0.32% and 0.05%, respectively.

Example 2 The finely pulverized and dried iron powder obtained in Example 1 was heated in a muffle furnace at 800 ° C. for 1 hour in an atmosphere isolated from the atmosphere, and subsequently in a hydrogen atmosphere at 850 ° C. for 1 hour, An iron powder aggregate having a bulk density of 2.8 was obtained.

The compact was roughly crushed to 5 mm or less, and then crushed with a mortar-type crusher in which a pot was rotated around a fixed rod.

The components of the fine iron powder thus obtained are shown in the table, the particle size distribution is shown in FIG. 3, and the electron micrograph is shown in FIG.

In order to investigate the fluidity of the fine iron powder, it was measured using a Hall type flow meter. As a result, the crushed iron powder without heat treatment did not pass through the 5.0 mm diameter.
It passes through 5.0mm diameter in 7 seconds / 50g and 40 seconds / in case of 2.63mm diameter
A result of 50 g was obtained and it was found that the fluidity was improved.

〔The invention's effect〕

 The following effects can be achieved by the present invention.

I. Fine iron powder for firing shrinkage can be manufactured at low cost.

B. The carbon content of the fired body can be adjusted by a process other than the melting stage.

C. A molding method using water as a medium becomes possible, and the degreasing process is simplified as compared with the molding method using an organic medium.

D. It is possible to mass-produce iron powder for injection molding, which was conventionally possible only with a low productivity process.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a particle size distribution of raw material granular iron in Example 1 of the present invention, and FIG. 2 shows a particle size distribution of fine iron powder in Example 1 of the present invention. FIG. 3 shows the particle size distribution of the fine iron powder treated with the crushing machine after the heat treatment in Example 2. Further, FIG. 4 is an electron micrograph (× 30) showing the particle shape of the fine iron powder treated by the crusher after the heat treatment in Example 2.
00).

Claims (2)

[Claims] 1. A molten iron having a high carbon content of 2% by weight or more is discharged into water to make white iron pig iron, and the white iron pig iron is used as a medium in which water has an average particle diameter of 20 μm or less. A method for producing an iron powder for a sinter shrinkage body, which comprises pulverizing into fine powder and then drying. 2. A high carbon content molten iron having a carbon content of 2% by weight or more is discharged into water to produce white pig iron granules, and the white iron pig iron granules have an average particle diameter of 20 μm or less with water as a medium. It is characterized by finely pulverizing it into fine particles and then drying it to form an oxide film on the surface, heating the aggregate of the fine powder to decarburize it to a carbon content of 0.4% or less, and further grinding the decarburized aggregate. And a method for producing an iron powder for a sinter shrinkage body.