JPH0771625B2 - Finishing reduction method of alloy steel powder - Google Patents

Description

TECHNICAL FIELD The present invention relates to a finish reduction method for alloy steel powder, and more specifically, C
When pre-alloyed atomized steel powder containing carbon, which is a non-reducing element such as r, Mn, etc., is used as the raw material, and this is subjected to finish reduction annealing to produce alloy steel powder for sintering materials, prior to finish reduction. Alloy steel powder that can accelerate the temperature rising rate without causing bumping phenomenon of steel powder in the temperature rising process of raising the temperature to the finishing reduction temperature and can shorten the temperature rising time Related to the final reduction method.

2. Description of the Related Art The demand for sintered iron powder parts is increasing year by year, and in particular, the demand for alloy steel powder for parts that require high strength and high toughness tends to increase. As a strengthening alloy element,
For example, inexpensive non-reducing elements such as Cr and Mn are suitable,
When the steel powder pre-alloyed with Cr and Mn is produced by the water atomizing-gas reduction method, it is difficult to reduce the amount of O contained in the steel powder because these elements have a strong affinity with oxygen. It was As such a method for deoxidizing alloy steel powder containing Cr, Mn, etc., for example, alloy steel powder by vacuum reduction is proposed as described in JP-B-58-10962.
At present, it is possible to manufacture low oxygen steel powder.

Further, in the reduction annealing furnace used in producing the alloy steel powder, for example, as described in JP-A-61-190004, to preheat and dry the raw material powder containing the non-reducing element and carbon. Preheating chamber, a reduction annealing chamber for deoxidizing and decarburizing by using carbon contained in the alloy steel powder after the preheating, and a cooling chamber for cooling the reduction annealed steel powder Are partitioned and each of these chambers is arranged side by side and continuously, movable doors are provided at the boundaries of the respective chambers, and each chamber is configured as an independent space. Exhaust system is provided.

When reducing atomized steel powder using this reduction annealing furnace, before finishing heating to the reduction annealing temperature (800 to 1300 ° C) in the reduced annealing chamber in a reduced pressure atmosphere of 20 Torr or less, 650 to 800 in the preheating chamber. Raise the temperature to ℃.

However, when the temperature rise in the preheating chamber is rapidly heated to improve productivity, a gas is suddenly generated from the steel powder and the steel powder rises up (hereinafter referred to as bumping phenomenon). When this phenomenon occurs in the furnace, steel powder scatters in the furnace and damages the heater.

In order to prevent this bumping phenomenon, the temperature must be raised at a heating rate of, for example, about 200 ° C./hr so that gas is not rapidly generated in the preheating chamber. There is a problem that the preheating step in each of the steps of reduction annealing and cooling becomes rate-determining, which causes a decrease in productivity.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The present invention aims to solve the above problems, specifically, when raising the temperature to a reduction annealing temperature range prior to finish reduction annealing, a bumping phenomenon that causes a decrease in productivity. We propose a method that can be suppressed without generating and can shorten the heating time.

Means for Solving the Problem and Its Action That is, the method of the present invention, prior to this finish reduction, when producing a steel powder by finish reducing the raw material powder of atomized alloy steel powder containing a non-reducible element and carbon. Before heating the raw material powder to 650 to 800 ° C, raise the raw material powder to 500 to 760To
It is heated to a temperature range of 150 to 220 ° C in a reduced pressure atmosphere of less than rr and kept for 1 hour or more to remove adsorbed water and compound water in the raw material powder, and to 650 to 800 ° C prior to finish reduction. The temperature rise should exceed 200 ° C / hr in a reduced pressure atmosphere of 20 Torr or less and 30
It is characterized in that the temperature is raised at a heating rate of 0 ° C./hr or less.

Therefore, the configuration and action of these means will be described in detail.

The present inventors have conducted extensive studies on a method of suppressing the bumping phenomenon when performing deoxidation of atomized alloy steel powder raw material powders containing carbon, a non-reducing element such as Cr, Mn, etc. 150 ° C to 2 in a reduced pressure atmosphere of 500 to less than 760 Torr
It was found that, when heated to a temperature range of 20 ° C. and kept for 1 hour or more, the bumping phenomenon of steel powder can be suppressed as shown in Example 1 described later.

The present invention was established based on the results of this research by conducting further research.

According to the research conducted by the present inventors, conventionally, when the steel powder to be finish-reduced is heated to 650 to 800 ° C. in a depressurized atmosphere in the preheating chamber, the temperature rising rate is set to prevent the bumping phenomenon of the steel powder. The method of slowly heating at about 200 ° C / hr was adopted. However, although this method does not cause the bumping phenomenon of the steel powder, it takes time. Therefore, the present inventors further researched the cause of the bumping phenomenon of the steel powder. As a result, the bumping phenomenon that occurs when the temperature rises from room temperature to 650 to 800 ° C is caused by steam and CO gas, which are gases generated from steel powder, and due to the temperature difference in the steel powder packed bed, this steam and CO gas Therefore, it was found that the gas generated from inside causes a slight amount of gas generated from the inside to blow up the slightly-sintered steel powder on the surface and to go out of the steel powder packed layer.

Therefore, it has been found that if the water content and CO content of the steel powder are reduced before finish reduction, it is not always necessary to reduce the heating rate to prevent bumping as described above.

In the finish reduction annealing, the reduction of steel powder is incomplete at 800 ° C or lower, and the sintering proceeds at 1300 ° C or higher, so the finish reduction annealing is performed between 800 and 1300 ° C, but in an atmosphere exceeding 20 Torr at this time. When the final reduction annealing is performed in step 1, the progress of the reduction is slow, so the reduction is performed in a reduced pressure atmosphere of 20 Torr or less.

In the temperature increase prior to this finish reduction annealing, if the temperature increase rate is 200 ° C / hr or less, the productivity will decrease, and if it exceeds 300 ° C / hr, the bumping phenomenon will occur. 300 ° C /
Perform during less than hr.

In order to improve the efficiency of reduction, the temperature is raised to 650-800 ℃ in the preheating chamber.

In order to prevent the bumping phenomenon even if the temperature rising rate is further increased from the standpoint of increasing the productivity, the adsorbed water, iron hydroxide and compound water contained in the steel powder are reduced. However, when the raw material powder is dried by steam heating in a reduced pressure atmosphere as in the conventional example, adsorbed water and the like cannot be removed so as to prevent the bumping phenomenon.

On the other hand, in the method of the present invention, 150 to 200 ° C. in a reduced pressure atmosphere.
Is heated in this temperature range and kept in this heating temperature range for 1 hour or more. By such treatment, the water content such as adsorbed water in the steel powder is reduced to an extremely small level as in the embodiment described later, and the bumping phenomenon at the time of temperature rise can be prevented.

The reason why the temperature range is set to 150 ° C or higher is that adsorbed water, iron hydroxide and compound water cannot be sufficiently removed at 150 ° C or lower.

The upper limit is set to 220 ° C., because if it exceeds 220 ° C., the size of the device becomes large and the oxidation easily proceeds.

The drying time is set to 1 hour or more because if it is less than 1 hour, the adsorbed water, iron hydroxide and compound water cannot be sufficiently obtained.

The reason for limiting the dry atmosphere to less than 500 to less than 760 Torr is to avoid oxidation of the steel powder.

Examples Example 1. A pre-alloyed water atomized alloying steel powder containing Cr and Mn, which are hard to reduce, and carbon shown in Table 1 was heated in a vacuum dryer at a reduced pressure of 500 Torr. Then, at this time, the heating temperature and its holding time were changed as shown in Table 2 for the treatment.

When the adsorbed water, the combined water and the content in this case were measured,
It was as shown in Table 2.

The method for quantifying the adsorbed water and compound water in steel powder is JIS M 8
The method was carried out according to the method for determining combined water content in iron ore specified in 211.

As is clear from Table 2, 1 in a reduced pressure atmosphere of 500 Torr
By heating in the temperature range of 50 ° C. or higher for 1 hour or longer, the contents of adsorbed water and compound water in the water atomized alloy steel powder could be reduced.

Next, the steel powder from which such adsorbed water was removed was heated to 800 ° C. in a reduced pressure atmosphere of 20 Torr. The temperature raising rates at this time were changed to 200 ° C./hr, 300 ° C./hr, and 400 ° C./hr, respectively, and the conditions of occurrence of bumping at each temperature raising rate are shown in Table 3.

From Table 3, it was confirmed that the steel powder preheated in the present invention can suppress the bumping phenomenon even if the temperature rising rate is increased when the temperature is raised to the finishing reduction temperature range.

Example 2. Heating was performed for 1 hour at a heating temperature of 180 ° C. in the case where the atmosphere was a reduced pressure atmosphere of 500 Torr in a vacuum dryer. The other conditions are the same as those in the first embodiment.

After drying, the contents of adsorbed water and compound water are shown in Table 4 as Test No. 9 (reduced pressure atmosphere) and No. 10 (in air).

As is apparent from this case, the removal of the combined water was insufficient in the atmosphere (760 Torr).

<Effects of the Invention> As described above, in the method of the present invention, the raw material powder of the atomized alloy steel powder containing the non-reducing element and carbon is first added to the temperature range of 150 to 220 ° C. in the reduced pressure atmosphere of less than 760 Torr. It is heated to 1 hour and held for at least 1 hour to remove most of the adsorbed water and compound water, and then heated to the finishing reduction temperature range.
The heating rate is over 300 ℃ / hr and over ℃ / hr.

According to the present invention, prior to raising the temperature to the reduction temperature range,
Since the adsorbed water and the compounded water of the raw material powder are completely removed, the temperature rising speed can be increased to raise the temperature, and the temperature rising time can be shortened.

Claims (1)

[Claims] 1. When producing a steel powder by finish-reducing a raw material powder of atomized alloy steel powder containing a non-reducing element and carbon, the raw material powder is heated prior to the finish reduction to 650-8.
Before raising the temperature to 00 ° C, the raw material powder is heated to a temperature range of 150 to 220 ° C in a reduced pressure atmosphere of 500 to less than 760 Torr and kept for 1 hour or more to remove adsorbed water and compound water in the raw material powder. The temperature rise from 650 to 800 ℃ prior to finish reduction is 20
A finishing reduction method for alloy steel powder, which comprises heating at a heating rate of more than 200 ° C / hr and 300 ° C / hr or less in a reduced pressure atmosphere of Torr or less.