JPS58721B2 - Method for preventing oxidation of water spray wet steel powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing oxidation of water-sprayed wet steel powder.

Water-sprayed steel powder is usually produced through the following steps.

First, steel materials such as scrap are melted in an electric furnace, their composition is adjusted, deoxidized, and the steel is tapped into an intermediate ladle at a temperature of about 1650°C, and then it falls naturally through a nozzle installed at the bottom of the intermediate ladle. When water pressurized by a high-pressure pump is injected at a predetermined angle into the flowing molten steel, the molten steel is pulverized and falls into a water tank located below the nozzle, where it is immediately cooled.

The amount of oxygen in the finely powdered copper powder varies slightly depending on the steel type, but it is usually 0.3 to 1.5 wt% (hereinafter referred to as wt%).
is simply written as %).

Subsequently, the steel powder is dehydrated and recovered as wet copper powder containing 5-12% moisture.

The wet copper powder is then dried to a moisture content of approximately 1% or less and stored for the next finishing reduction step.

By the way, the above-mentioned spraying step may be performed in a non-oxidizing atmosphere such as N2 or Ar gas, and in this case, steel powder with a relatively low oxygen content can be obtained.

However, since subsequent dehydration, transportation, storage, etc. after dehydration are performed in air, a considerable amount of air is involved.

Wet copper powder is naturally easy to oxidize, so it has a strong tendency to be easily oxidized by this entrained air, which not only increases the amount of oxygen in the steel powder but also causes large variations in the oxygen content in the copper powder. Become.

Therefore, in the subsequent final reduction step, it is necessary to measure the oxygen content in the copper powder in advance and change the reduction conditions according to the content.

Such changes in reduction conditions become extremely complicated to manage in continuous operation, and the powder metallurgical properties of copper powder once oxidized vary depending on the degree of oxidation. It is very disadvantageous that the properties of the products produced by this process vary due to variations in the properties of the steel powder.

Therefore, it is necessary to minimize the oxidation of the wet copper powder and to reduce the variation in oxidation, so the handling of the steel powder after dehydration must be strictly controlled.

However, even with such adequate management, an increase in the amount of oxygen is unavoidable, and especially in the event of a power outage or accident, wet copper powder cannot be dried immediately after dehydration, resulting in an increase in the amount of oxygen and an increase in the amount of oxygen. It is inevitable that the dispersion will increase.

Conventionally, as a measure to prevent oxidation in such cases, a small amount of an inhibitor was added to the spray water in advance, and after the copper powder was sprayed with Toya to prevent oxidation, the wet copper powder was further treated with non-oxidizing gas such as N2 or Ar gas. Methods such as storing it in a toxic gas are being considered.

However, since the addition of an inhibitor results in the addition of impurities to the copper powder, there is a possibility that the powder metallurgical properties of the copper powder will be adversely affected.

Furthermore, it is necessary to completely replace the air in the storage tank and in the wet steel powder packed bed with N2 or Ar gas, and such a replacement procedure is difficult.

Further, although this method naturally increases the cost for preventing oxidation, the effect is not sufficient.

The purpose of the present invention is to provide a method that eliminates and improves the drawbacks of the conventional methods, and involves immersing wet copper powder having an oxide film produced by a water spray method in water and storing it under oxidation suppression. In particular, the present invention provides a method for storing copper powder by immersing it in water, which is of great industrial significance and has a weight increase of 0.05% or less due to oxidation of copper powder.

Next, the present invention will be explained based on experimental data conducted by the inventors.

Within one hour after spraying, various wet copper powders produced by the water spray method were subjected to a dehydration method of applying mechanical vibrations of 2 to 500 Hz as proposed by the present inventors in JP-A-49-70860.

The moisture content of the wet copper powder after dehydration was about 11%.

Each of these dehydrated copper powders was washed twice with ethyl alcohol, water and alcohol were removed as much as possible, and the copper powder particle size after vacuum drying at 100°C for 4 hours was 160#, and chemical analysis revealed that the copper powder particle size was 160 #. The values are A1 to A6 of the above clothing respectively.
It was as follows.

Next, the top water after spraying was added to these wet copper powders, and 8
The sample was left to stand for a period of up to 1 day, and then dried using the method described above and analyzed for oxygen.

In addition, for comparison, we compared cases in which wet steel powder was left without any water added, and copper powder and mill scale reduced iron powder, which had been pickled with hydrochloric acid to remove the oxide film, were left to stand with water added to them. , we investigated changes in oxygen content.

FIG. 1 shows the relationship between the oxygen content and the standing period of wet copper powder 1 and 3.5 which were left to stand after being sprayed with -L water.

The amount of oxygen in each steel powder does not increase even after being left for as long as 8 days, and the amount of increase is less than 0.105%, making this an extremely excellent method for preventing oxidation of wet steel powder. One thing became clear.

When the water depth was 1 mm, the amount of oxygen increased only slightly compared to when the water depth was 60 mm.

These trends are hardly influenced by the composition of the copper powder.

Furthermore, within the scope of this experiment, the amount of oxygen in the wet copper powder before immersion had almost no effect on the increase in the amount of oxygen in the copper powder after water was added.

However, when the amount of oxygen is extremely small, the thickness of the oxide film becomes thinner, so it is thought that the weight increase due to oxidation becomes large.

If the amount of oxygen in the wet copper powder is large, there is no disadvantage in preventing oxidation, but it is not preferable because it increases the burden of finishing reduction in the next step.

Figure 2 shows for comparison the relationship between the amount of oxygen in wet copper powder without the addition of water and the storage period.

The amount of oxygen in each wet steel powder increased rapidly as the standing period increased, indicating that wet copper powder is extremely susceptible to oxidation.

This tendency was almost independent of the composition of the wet copper powder and the amount of oxygen in the wet copper powder.

In FIG. 2, the dispersion of the measured values is large.

From a comparison of FIG. 1 and FIG. 2, it became clear that immersion of wet steel powder in water as shown in FIG. 1 is an excellent method for preventing oxidation, and this led to the idea of the present invention.

Figures 3 and 4 show the results of wet steel powders 1 and 4 with no water added and with water added to a water depth of 60 mm, left for 4 days, classified with a sieve, and measured the amount of oxygen for each particle size. Each is shown below.

In wet steel powder 1 left without water addition, the amount of oxygen is extremely large for each particle size, especially +100# (+149μ) and -325
#(-44μ) copper powder has a large amount of oxygen.

Among these, it is thought that -325# powder has a large specific surface area, so oxidation is promoted.

In addition, the wet copper powder was oxidized with slight heat generation, so it aggregated into particles with a large diameter, and it is thought that these particles had a larger amount of oxygen than those that did not aggregate.

On the other hand, the amount of oxygen in the wet copper powder left after adding water was small regardless of the particle size, and the difference in the amount of oxygen between particle sizes was extremely small.

In this way, if left without water addition, not only will the average amount of oxygen increase, but there will also be a large difference in the amount of oxygen between particle sizes, and a large difference will appear in the powder metallurgical properties. It can be completely removed.

Almost similar results were obtained for steel immersion powder 4.

FIG. 5 shows the relationship between the amount of oxygen in the copper powder and the amount of dissolved oxygen in the added water.

Here, 300 g of wet copper powder was collected in a beaker with a diameter of 75 m, and the water that rose up after spraying with different amounts of dissolved oxygen was collected at a depth of 60 m.
The amount of oxygen after adding up to m and leaving it for 4 days is shown.

As the amount of dissolved oxygen increased, the amount of oxygen in the wet copper powder after standing increased, and the smaller the amount of dissolved oxygen, the better the results were obtained.

In the present invention, the increase in oxygen amount due to storage is reduced by 0.05%.
It was found that the amount of dissolved oxygen should be 8 ppm or less in order to achieve the following.

Figure 6 shows the influence of the temperature of the added water on the oxygen content of the wet copper powder.

Collect 30 M' of wet copper powder in a beaker with a diameter of 75 mm, add the top water after spraying to a depth of 60 mm, and heat at a room temperature of 35°C.
After being left at 55° C. and 75° C. for 4 days, it was dried by the method described above and subjected to oxygen analysis.

There was a tendency for the amount of oxygen in the copper powder to increase as the water temperature rose.

Therefore, the increase in the amount of oxygen in the copper powder due to the rise in water temperature is 0.0
It was found that in order to keep it below 5%, the water temperature should be kept below 55°C.

FIG. 7 shows the results of adding water to commercially available reduced iron powder and wet steel powder 1 and 6 which were pickled to reduce the amount of oxygen and left at room temperature for 4 days.

The amount of oxygen in each of these steel powders was significantly increased compared to copper powder with an oxide film.

Therefore, the effect of the oxide film on oxidation in water is large, and effective oxidation prevention by immersion in water is limited to wet copper powder with an oxide film such as Fed and Fe3O4 produced by water spraying. I understand.

FIG. 8 shows the results of X-ray diffraction of wet steel powder 6 after drying and sieving.

It was found that Fe3O4 was fixed in addition to α-Fe and γ-Fe, and the oxide film of the wet steel powder 6 was composed of an oxide mainly composed of Fe3O4.

In addition, iron oxides such as Fed, Fe2O3, and Fe0OH, oxides such as Mn, Al, and Si as impurities,
Mn, Ni, Si, AI, Cr, M added as alloy
The presence of oxides such as O, Co, or composite oxides of the above elements and iron is considered.

According to the experiment conducted by the present inventors, wet copper powder produced by the water spray method was heated to a temperature of 5 ppm or less with a dissolved oxygen content of 8 ppm or less.
We have newly discovered the fact that by immersing copper powder in water at 5°C or lower and storing it while suppressing oxidation, it is possible to prevent an increase in the amount of oxidation of copper powder to within an industrially valuable range of 0.05% or less. It was.

According to the present invention, wet copper powder with a water content of 5 to 12% is used after dehydrating slurry copper powder immediately after producing molten steel having a composition of carbon steel or alloy steel by a water spray method. be able to.

Next, in the present invention, it has been found that the amount of oxygen in the copper powder immersed in water as described above, that is, the presence of an oxide film on the surface of the wet copper powder, plays a major role in preventing oxidation of the copper powder after immersion in water.

It is generally known that steel oxidizes much less when immersed in water than when left in the presence of air and water vapor or air plus water.

However, when a copper powder product is simply immersed in water, oxidation is accelerated because the specific surface area of copper powder is much larger than that of steel, and it is not uncommon for the oxidation weight gain to reach 0.5% or more.

The presence of an oxide film, as in the method of the present invention, prevents oxidation that accompanies immersion and is therefore an effective method for preventing oxidation of wet steel powder.

As a result of various studies conducted by the present inventors regarding the range of the oxide film of wet copper powder, that is, the amount of oxygen, the inventors found that it is preferable to set the lower limit to 0.3% and the upper limit to 1.5%.

The reason why the lower limit of the oxygen content was set to 0.3% is as follows.

Wet copper powder has an oxide film with a substantially uniform thickness on its surface, and the thickness of this oxide film is almost determined by the amount of oxygen in the wet copper powder.

As mentioned above, the oxide film is effective in preventing oxidation of wet copper powder in water, but if the thickness is thin, it is not effective in preventing oxidation.

The reason for this is that defects such as cracks are likely to occur in the oxide film, and according to experimental results, the amount of oxygen in wet copper powder is 0.
．． If it is less than 3%, a satisfactory antioxidant effect cannot be obtained.

On the other hand, the reason why the upper limit of the oxygen amount is set to 1.5% is as follows.

If the amount of oxygen is large, the oxide film will become thicker, so there is no problem in preventing oxidation, but if the amount of oxygen exceeds 1.5%, the heating conditions for the subsequent final reduction process will be shifted to higher temperatures and longer times.
Furthermore, it is necessary to increase the amount of reducing gas used, which is not preferable.

Therefore, in order to effectively carry out the method of the present invention, good results can be obtained by regulating the oxygen content of the wet copper powder within the range of 0.3 to 1.5%.

Next, the particle size of wet copper powder suitable for the method of the present invention will be described.

As the particle size of wet copper powder becomes coarser, the specific surface area of wet copper powder decreases.
As can be easily estimated from the results of immersing steel materials in water,
Even when there is almost no oxide film on the surface or even when there is, the amount of oxidation per unit weight decreases due to the immersion of wet copper powder, resulting in the same tendency as in the case of steel materials.

On the other hand, when the particle size of wet copper powder becomes extremely fine, that is, −
When the amount of 325# (-44μ) powder exceeds a certain level, the specific surface area of the wet copper powder increases and becomes extremely susceptible to oxidation, causing problems.

From the above point of view, the present inventors investigated using wet copper powder having various particle size distributions, and found that the upper limit of the particle size was 42# (35#).
It was confirmed that the method of the present invention is effective for wet steel powder where the lower limit is about 55% or less of 325# (44μ).

When wet copper powder is immersed, it is sufficient that the wet copper powder is completely immersed, and the filling height of the copper powder and the distance from the surface of the copper powder filled layer to the water surface, that is, the water depth, may be any value.

The reason for this is that the air around the copper powder can be removed by immersing the wet copper powder in water, and the packing density of the copper powder can be extremely high.

It is preferable to increase the height of the copper powder filling and deepen the water depth.

This is because, when looking at the relationship between the steel powder filling height and the oxygen content of the copper powder, it is recognized that the oxygen content is somewhat higher in the surface layer of the packed bed.

Also, the shallower the water, the more they tend to be present.

Next, among the properties of the water to be added, the ones that matter are the amount of dissolved oxygen and the moisture content of the water.

First, we will discuss the reason why the former is set to 8 ppm or less.

Experimental results show that the lower the amount of dissolved oxygen, the better the results.
As the amount of oxygen increases, the amount of oxygen in the wet copper powder increases.

The limit for the amount of dissolved oxygen is determined as follows.

The amount of oxygen in the wet copper powder varies slightly depending on the type of steel and spraying conditions, but remains approximately constant.

However, even if these conditions are kept constant for spraying, these fluctuations cannot be avoided, and as a result, fluctuations in the amount of oxygen in the wet copper powder are also unavoidable.

By well controlling these conditions, it is possible to reduce the amount of oxygen variation in the wet copper powder to 0.05% or less.

Therefore, the oxidation weight increase due to standing in water is also 0.05.
It is advantageous to keep it below %.

According to the results of the first experiment, the lower the amount of dissolved oxygen, the better the results, and as the amount of dissolved oxygen increases, the amount of oxygen in the wet steel powder also increases.
When wet copper powder is immersed in water with a dissolved oxygen content exceeding 8 ppm, the amount of oxygen increase in the wet copper powder exceeds 0.05%.

On the other hand, when immersed in water exceeding 8 ppm, the increase in oxygen in the copper powder before final reduction exceeds 0.05%, so the increase in oxygen in the steel powder obtained by final reduction under certain conditions also increases. This results in a decrease in various properties, such as apparent density and compressibility, and abnormal dimensional changes during sintering.

That is, oxidation causes changes in the properties of copper powder products and increases the specific surface area, which is extremely disadvantageous in producing sintered products.

Therefore, controlling the amount of oxidation of wet copper powder is one of the most important things.

This is because the various properties of copper powder products vary significantly from the average value due to the oxidation of wet copper powder.

For these reasons, the amount of dissolved oxygen in water that should be used is 8pp.
It is advantageous to set it to m.

Next, the reason why the temperature of the added water was set to 55° C. or lower will be described.

Generally, the solubility of oxygen in air in water decreases as water temperature increases, so it is preferable to maintain a high water temperature in order to reduce the amount of dissolved oxygen.

On the other hand, the reaction rate between oxygen and iron increases as the water temperature increases.

Therefore, the degree of oxidation is determined by the balance between these two factors, and as mentioned above, in the wet copper powder used in the method of the present invention, the amount of oxygen increased by 0.05% due to storage in water.
In order to keep the temperature below, it is advantageous to keep the water temperature below 55°C.

Furthermore, other advantages have been found when implementing the method of the invention.

When wet copper powder is left to stand under hydrothermal addition, the powder particles tend to aggregate with each other as the wet copper powder oxidizes, forming large, hard lumps.

This tendency is almost always observed regardless of the length of the exposure period.

Once the wet copper powder has become agglomerated in this way, it must be thoroughly loosened and returned to its original fine powder form before being subjected to final reduction.

This is an additional step compared to the normal process and increases manufacturing costs, which is undesirable.

On the other hand, according to the method of the present invention, oxidation due to standing can be prevented and agglomeration of wet copper powder can be extremely reduced, so pulverization after drying is unnecessary.

As described above, according to the present invention, water-sprayed wet steel powder can be stored in an extremely efficient manner while being prevented from oxidizing.

[Brief explanation of drawings]

Figure 1 shows the relationship between the amount of oxygen in wet copper powder left after adding water and the length of time it was left. Figure 2 shows the relationship between the amount of oxygen and the length of time left in wet copper powder left without adding water. Figures 3 and 4 are diagrams showing the relationship between the amount of oxygen and particle size of wet copper powder, respectively.
Figure 6 shows the relationship between the amount of oxygen in wet steel powder and the amount of oxygen dissolved in added water, Figure 6 shows the relationship between the amount of oxygen in wet copper powder left at various water temperatures and the length of time it was left, and Figure 7 8 is a diagram showing the amount of oxygen before and after leaving reduced iron powder and pickled wet copper powder, and FIG. 8 is a diagram showing the results of X-ray diffraction of wet copper powder.

Claims (1)

[Claims] 1 Wet copper powder with an oxygen content of 0.3 to 1.5% by weight and a particle size of 44 to 350 μ obtained by the water spray method is placed in water with a dissolved oxygen content of 8 ppm or less and a temperature of 55°C or less. A method for preventing oxidation of water-sprayed wet steel powder, characterized by immersing it and storing it under oxidation-inhibited conditions.