JPS59211501A - Manufacture of steel powder - Google Patents

Abstract

PURPOSE:To obtain easily low carbon and low oxygen alloy steel powder by decarburizing steel powder contg. an easily oxidizable element obtd. by an oil atomizing method in an atmosphere contg. H2 and H2O, and reducing the powder in a nonoxidizing atmosphere contg. H2. CONSTITUTION:Molten steel contg. one or more kinds of easily oxidizable elements selected among Cr, Mn, V, Nb and Si is atomized with a nonoxidizing liq. as an atomizing medium. The resulting steel powder having <=0.2wt% O2 content and >=0.1wt% C content is decarburized in an atmosphere contg. H2O and H2, and it is reduced at 850-1,200 deg.C in a nonoxidizing atmosphere having <=-20 deg.C dew point.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention describes a method for producing alloy steel powder, more specifically, a method for producing alloy steel powder, in which easily oxidizable elements such as chromium and manganese obtained by an oil atomization method are
The present invention relates to a method for producing alloy steel powder containing more than

Steel powder, which is a raw material for powder metallurgy, is required to have a low oxygen content and, except for very special uses, a low carbon content. This is because oxygen and carbon in copper powder adversely affect the compressibility, formability, sinterability, and other properties of steel powder.

Conventionally, water atomization, gas atomization, and oil atomization are known as methods for producing steel powder. However, when attempting to produce alloy steel powder containing easily oxidizable elements as processed by the present invention, these conventional methods have the following problems.

British atomization method: Since water is used as the atomization medium, this method inevitably oxidizes the resulting powder during water atomization.

Moreover, the amount of oxidation is generally very large, and the addition of easily oxidizable elements is restricted in alloy component design.

In particular, chromium (Cr), manganese (Mn), vanadium (■), niobium (Nb), boron (B), silicon (S
When molten steel containing easily oxidizable elements such as i) is atomized with water, these elements are easily oxidized by water, which is a spraying medium, and therefore, it is difficult to use as a treatment to lower the oxygen content of steel powder. It is then necessary to subject the resulting steel powder to a reduction treatment. This reduction requires treatment at a high temperature and for a long time, for example, at a temperature of 1000° C. or higher for 1 to 2 hours, but even with such a reduction treatment, the carbon content remains at 0.
Only copper powder with an oxygen content of about 1% and 0.3% can be obtained.

When the carbon and oxygen contents are at this level, if you run,
The compressed density at a compacting pressure of 5 T/cmlJ is 6.7 g/c/4, and the properties as a steel powder for powder metallurgy remain insufficient.

Gas atomization method: This method uses an inert gas such as nitrogen or argon as an atomizing medium, and since oxidation is suppressed, the carbon and oxygen contents of the steel powder can be reduced to a desired level. On the other hand, the inert gas required in large quantities as a spray medium is expensive, so the cost is 1
It becomes 0 times or more. In addition, the cooling rate of gas cooling is slow, and the shape of the obtained copper powder tends to be spherical, resulting in poor formability and sinterability. Such copper powders are difficult to cold form and require expensive special forming methods.

For the above reasons, although the gas atomization method has been put into practical use for special purposes, it has hardly been adopted for producing steel powder for sintering or sinter-forging, which is the most common method in powder metallurgy.

Oil atomization method: The oil atomization method uses oil such as mineral oil or animal or vegetable oil as the atomizing medium, and compared to the water atomization method described above, oxidation does not occur during steel production. Even in the presence of easily oxidizable alloying elements, it is outstanding in that low-oxygen steel powder with an oxygen content of 0.2% or less can be obtained. However, since carburization from the spray medium to the steel powder occurs during atomization, decarburization must be performed in the next step.

When producing steel powder containing easily oxidizable elements, it is assumed that it is more advantageous to decarburize carburized material than to reduce oxidized material, but it is difficult to efficiently and easily prepare such copper powder. A decarburization treatment method had not been previously discovered.

The present inventors previously researched the decarburization conditions for steel powder containing easily oxidizable elements obtained by the oil atomization method, and found that the oil atomization method reduced oxygen to 0.2% or less and carbon to 0.1% or more. After decarburizing this under specific conditions in an H20H2-containing atmosphere and cooling it in a reducing atmosphere, the oxygen content is 0.2% or less and the carbon content is 0.1%. We have discovered that it is possible to efficiently and continuously produce the following low-carbon, low-oxygen alloy steel powder (Japanese Patent Application No. 57-189238
issue). In addition, in this specification, % is weight % unless otherwise specified.

A method consisting of a combination of oil atomization and subsequent decarburization is a very promising method in that it has better production efficiency than the water atomization method and can achieve both low oxygen and low carbon. There are the following difficulties in operating the plant. In other words, since decarburization using only hydrogen is slow and impractical, decarburization treatment is performed in an atmosphere containing hydrogen and steam, but since steam has oxidizing properties, it oxidizes more than Fe as decarburization progresses. Easy Cr, Mn, V, Nb
Since oxidation of easily oxidizable elements such as , B, and Si also progresses,
In order to achieve the target low carbon and low oxygen steel powder, there are restrictions on the decarburization treatment conditions. In other words, the range of decarburization treatment conditions is narrow, making operation control difficult.

Subsequent research by the present inventors revealed that the oil atomization method (H2 and H
Low carbon and low oxygen alloy steel powder can be obtained relatively easily by a two-stage treatment method in which decarburization treatment is performed in an atmosphere containing H20, followed by reduction treatment in a non-oxidizing atmosphere containing H2. The present invention was achieved based on the discovery that

According to the method of the present invention, copper powder with a carbon content of 0.03% or less and an oxygen content of 0.15% or less, which is significantly lower than copper powder obtained by water atomization, can be obtained. , when the carbon and oxygen contents are at this level, the compressed density of the copper powder is 6.9 g when the molding pressure is 5 T/od
/ca or more is good.

Here, the present invention provides Cr, Mn, ■, Nb, B and Si.
A molten steel containing one or more easily oxidizable elements selected from the following is pulverized using an atomizer using a non-oxidizing liquid as an atomizing medium, with an oxygen content of 0.2% by weight or less and a carbon content of 0.2% by weight. 1% by weight or more of copper powder is decarburized in an atmosphere containing at least H2O and H2, and then in a non-oxidizing atmosphere mainly composed of H2, 850°C≦t≦1200°C, Dp≦−20°C (However, t: atmospheric temperature, Dp: atmospheric dew point) This is a method for producing steel powder characterized in that the reduction treatment is carried out under the following conditions.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

The raw material steel powder used in the production method of the present invention is Cr
Molten steel containing at least one easily oxidizable element selected from the group consisting of XMn, V, Nb, B, and Si is pulverized by atomization using a non-oxidizing liquid such as mineral oil or animal or vegetable oil as a spray medium. It can be obtained by

#1filW of molten steel can be carried out by conventional methods,
Since molten steel contains the above-mentioned easily oxidizable elements, oxidation of such elements should be prevented as much as possible even during the melting stage.

Atomization is carried out using non-oxidizing liquids such as mineral oil, animal oil, vegetable oil, etc., as in conventional oil atomization methods, but the spray medium need only be essentially non-oxidizing as a whole;
It may also contain a small amount of oxidizing component such as water.

The oxygen content of copper powder obtained by oil atomization is 0.2
% or less, but this is because if the oxygen content is higher than this, some oxidation is unavoidable during the next decarburization process, so the oxygen content of the steel powder after decarburization is 0.3% or less. %
If this happens, as already mentioned with respect to the water atomization method, the subsequent reduction treatment will be troublesome, and furthermore, it will not be possible to achieve low oxygen levels even through reduction. Preferably, the oxygen content is as low as possible. By paying attention to the prevention of oxidation of the molten steel after deoxidizing treatment, especially during atomization, the oxygen content of the steel powder obtained by oil atomization can be kept to a fairly low level, 0.2% or less, An oxygen content of preferably 0.1% or less, particularly preferably 0.05% or less can be sufficiently obtained.

The carbon content of steel powder obtained by oil atomization is 0.1% or more, but such a large amount of carbon inevitably enters the steel powder due to carburization from the spray medium.

In the production method according to the present invention, steel powder obtained by oil atomization is first decarburized in an atmosphere containing H2 and H2O. This decarburization reduces the carbon content of steel powder to 0.0
It is preferable to lower it to 5% or less. H in the decarburizing atmosphere
Due to the presence of 2O, a slight increase in oxygen content is unavoidable, but the increase in oxygen content is also kept as low as possible. Oxygen contents of up to about 0.3%, preferably up to about 0.25%, are acceptable in this post-decarburization stage.

The decarburization treatment conditions are not particularly limited as long as the above carbon and oxygen contents can be achieved, but typical treatment conditions include a treatment temperature of 550°C or higher and 1200°C or lower, and an atmospheric dew point (Dp) of the working atmosphere. 15≦l) p≦35.

As described above, one of the characteristics of the method of the present invention is that, compared to the decarburization treatment conditions in the method of Japanese Patent Application No. 57-189238, the restrictions on the decarburization treatment conditions are looser and the treatment can be carried out relatively easily. .

In the present invention, the decarburized copper powder is then subjected to a reduction treatment in a non-oxidizing atmosphere mainly composed of H2 to reduce the oxygen content of the steel powder oxidized by the decarburization treatment to a desired level. let A trace amount of H2O is usually mixed in this non-oxidizing atmosphere, and an inert gas such as N2 or Ar may also be present as a diluent gas.

In this reduction treatment, the present inventors have found through experiments that the treatment temperature and the dew point (Dp) of the atmosphere have important effects in achieving the desired reduction.

Alloy steel powder obtained by the oil atomization method whose chemical composition and particle size are shown in Table 1 was decarburized at 930°C for 20 minutes in an H2O-H2' atmosphere with a dew point of 30°C. The chemical composition and particle size distribution of the steel powder after decarburization are also shown in Table 1. The resulting decarburized test steel powder was reduced in an H2 atmosphere containing a small amount of water vapor by varying the temperature and dew point of the atmosphere, and the effects of temperature and dew point on oxygen and carbon content were investigated. did. The experimental results are shown in Figure 1. In Figure 1, raw powder oxygen and raw powder carbon refer to after decarburization (
It represents the oxygen and carbon content of the sample steel powder (before reduction treatment).

The contents of an*p and S in Table 1 are each 0.025% or less.

As shown in Figure 1, the conditions under which reduction processing is possible (
In other words, the conditions under which it is possible to oxidize the oxygen content of the raw powder and lower it below Heril's level are determined by the processing temperature and the atmospheric dew point (D
It is determined from the relationship between both p). If the processing temperature is lower than 850°C, e.g. 830'C, a l) p of -40°C or less is required to achieve reduction, and at a processing temperature of 800°C, a p of -55°C is required. ) p is required.

However, in a non-oxidizing atmosphere containing H2 = 40
Obtaining a Dp below °C requires injecting a large amount of H2 gas into the furnace, and extremely low dew points (e.g.
This method involves various practical difficulties, such as the need to use H2 gas with a temperature of Dp -60°C or less, and is also relatively expensive, so it cannot be used in actual operations. Therefore, in the method of the present invention, the reduction treatment temperature is set at 850°C.
Limited to temperatures above ℃.

Regarding the dew point, from the data in Figure 1, the processing temperature is 85
It can be seen that when the temperature is 0°C or higher, reduction is possible if Dp is -20°C or lower. Therefore, in the present invention, the dew point of the reduction treatment atmosphere is limited to -20°C or less.

From FIG. 1, it can be seen that the higher the treatment temperature and the lower Dp, the greater the reduction M (amount of decrease in oxygen content).

On the other hand, if the steel powder is treated at an excessively high temperature, the adhesion of the steel powder particles to each other increases, making grinding difficult. Therefore, the influence of the reduction treatment temperature on the adhesion of the particles was investigated. The results are shown graphically in FIG. From this, it has been found that if the treatment temperature exceeds 1200°C, the mutual adhesion of the particles will rapidly become stronger, making it difficult to crush the particles after treatment, which is not a good idea. Therefore, the upper limit of the reduction treatment temperature was set to 1200°C. Incidentally, for the same reason, the decarburization treatment is also preferably carried out at a treatment temperature of 1200° C. or lower.

Regarding the carbon content, as shown in the graph of FIG. 1, the carbon content after the reduction treatment is lower than the carbon content of the raw powder. This decrease in carbon content is due to the decarburization reaction (C+820-GO+H2) caused by a trace amount of H20 in the atmosphere.
Alternatively, it is considered to be due to a decarburization reaction (C+2H2-CH4) caused by H2.

As described above, according to the two-stage treatment method of the present invention, when copper powder obtained by oil atomization is decarburized and then subjected to reduction treatment as the second stage treatment, the oxygen content is significantly reduced to the desired level. Moreover, since a substantial reduction in carbon content also occurs at the same time, it is no longer necessary to control the first stage decarburization process so tightly. In other words, a patent application filed in 1983 attempts to obtain the desired oxygen content and carbon content at the same time through decarburization treatment alone.
Compared to the method of No. 9238, the restrictions on the decarburization treatment conditions are significantly relaxed, the treatment can be carried out easily, and the subsequent reduction treatment is not subject to strict restrictions on the treatment conditions.

The invention will now be illustrated by examples.

Molten Mn-Cr low alloy steel on a canopy plate is pulverized by oil atomization using mineral oil as an atomizing medium, and the obtained steel powder is heated at 950°C in an atmosphere containing H2 and H20 with a dew point of 30°C.
Decarburization treatment was performed for 30 minutes. Next, the steel powder after the decarburization treatment was subjected to a reduction treatment in an H2 atmosphere containing a trace amount of 820 under the conditions shown in Table 2. Table 3 shows the chemical composition and particle size distribution of the steel powder as atomized (after atomization), after decarburization treatment, and after reduction treatment. As is clear from the results in Table 3, the carbon and oxygen contents after the reduction treatment were lower than after the decarburization treatment for all steel types A-C.
The target contents of carbon: 0.03% or less and oxygen: O, IS% or less were all sufficiently secured.

This example is an example of a steel powder containing one of 1, Nb, and B in addition to Mn and Cr. Steel powder was obtained by the oil atomization method using mineral oil as the spray medium in the same manner as in Example 1, and after being decarburized in the same manner, a trace amount of H was added under the reducing conditions shown in Table 4 below.
Reduction treatment was performed in an H2 atmosphere containing 2C. Table 5 shows the chemical composition and particle size distribution of each copper powder after atomization, decarburization treatment, and reduction treatment.

As can be seen from Table 5, the oxygen content of the steel powder after the reduction treatment was reduced by 0.02 to 0.05% compared to after the decarburization treatment, and the carbon content for all steel types: 0. 03%
Below, the desired decarburization and reduction with an oxygen content of 0.15% or less was achieved.

Table 4 Reduction processing conditions

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the reduction treatment temperature and atmospheric dew point (Dp) in the reduction treatment of decarburized copper powder and the carbon and oxygen content of the copper powder after reduction; It is a graph showing the relationship between treatment temperature and adhesion force of steel powder particles. Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Akira Hirose −

Claims (1)

[Claims] Molten steel containing one or more easily oxidizable elements selected from Cr, Mn-, V, Nb, B, and St is atomized using a non-oxidizing liquid as a spray medium. A steel powder obtained by pulverization with an oxygen content of 0.2% by weight or less and a carbon content of 1% by weight or more is treated with at least H2O.
Decarburization treatment is carried out in an atmosphere containing H2 and H2, and then H2
In a non-oxidizing atmosphere mainly composed of
A method for producing steel powder, characterized in that reduction treatment is carried out under the conditions of ≦1200°C, Dp ≦ -20°C (where t: ambient temperature, Dp: atmospheric dew point).