JPH0772281B2 - Method for reducing atomized alloy steel powder through primary reduction and secondary finish reduction - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for reducing atomized alloy steel powder through primary reduction as well as secondary finishing reduction, and more specifically,
When reducing atomized alloy steel powder through primary reduction and finish reduction (secondary reduction) using atomized steel powder that contains carbon, which is difficult to reduce, such as Cr and Mn, in advance, this primary reduction The present invention relates to a method which does not cause the bumping phenomenon of the raw material powder even when the temperature is rapidly raised to the finishing reduction temperature range and can shorten the finishing reduction time.

In the following, the non-oxidizing atmosphere means a non-oxidizing gas atmosphere having a dew point of 10 ° C. or lower, and is an atmosphere composed of nitrogen (N) gas, argon (Ar) gas, etc. alone or a mixed gas thereof.

2. Description of the Related Art Demand for the sintered portion of iron powder has been increasing year by year, and in particular, there is a tendency for the demand for alloy steel powder for parts that require high strength and high toughness to increase.

As a strengthening alloy element, for example, water atomized steel powder obtained by pre-alloying inexpensive Cr and Mn has been developed as a raw material powder.
However, when this steel powder is gas-reduced, Cr and Mn have a strong affinity with oxygen, so that the amount of O contained in the steel powder cannot be reduced.

In recent years, a method for deoxidizing steel powder containing Cr and Mn has been developed, and as one of them, for example, a vacuum reduction method described in JP-B-58-10962 has been proposed. According to this method, low oxygen steel powder can be manufactured.

Further, JP-A-61-190004 proposes a reduction annealing furnace including a preheating chamber, a reduction annealing chamber and a cooling chamber. That is, in the preheating chamber, the raw material powder containing the non-reducing element and carbon is preheated and dried, and in the reduction annealing chamber, the preheated raw material powder is deoxidized by utilizing the carbon contained therein and then annealed. Then, in the cooling chamber, the steel powder after finish reduction annealing is cooled. Movable doors are provided at the boundaries of these chambers, each chamber is independent, and each chamber is provided with a decompression exhaust device, and preheating, finish reduction, and cooling are performed under reduced pressure. .

The following method has been proposed as one of the methods using the reduction annealing furnace having this structure. According to this method, the temperature is raised to 650 to 800 ° C. in a reduced pressure atmosphere of 20 Torr or less in a preheating chamber before the temperature is raised to the finishing reduction temperature of 800 to 1300 ° C.

Rapidly raising the temperature in this preheating chamber to improve productivity,
During this period, there is also a reduced pressure atmosphere of 20 Torr or less, so that gas is suddenly generated from the steel powder, and the raw material steel powder, especially the surface layer part of the partially sintered steel powder, rises, The phenomenon of bumping of steel powder occurs. When the bumping phenomenon of the steel powder occurs in the preheating chamber, the steel powder scatters in the chamber and damages the heater, which is not preferable.

In order to prevent this bumping phenomenon, the temperature must be slowly raised, for example, at a temperature of about 200 ° C./hour so that gas is not rapidly generated in the preheating chamber.

If the temperature is raised in this way, it takes a long time for preheating, and the preheating step in the preheating-finishing reduction annealing-cooling step is rate-determined, resulting in a decrease in productivity. In this respect, although it is possible to increase the size of the furnace itself in order to improve the productivity, it is necessary to evacuate or reduce the pressure of each chamber of the furnace.

Further, Japanese Patent Publication No. S58-10962 proposes a method of performing vacuum reduction in a reduction annealing chamber. However, C in steel powder,
When the amount of O is large, the amount of gas generated from steel powder is large,
In order to produce a low C and O steel powder, it is necessary to wait until the CO partial pressure in the reduction annealing chamber becomes low. Therefore, it takes a long time to produce a low C and O steel powder, and the productivity is reduced.

Further, Japanese Patent Laid-Open No. 64-25901 describes a method in which an easily reducible oxide in a raw material powder is primarily reduced and then the hardly reducible oxide is secondarily reduced. .

That is, by heating within a temperature range of 400 to 600 ° C. in a reducing atmosphere containing H ₂ (particularly, 50 at least up to 600 ° C.).
The temperature is raised at a heating rate of not more than ° C / min. ), The easily reducing oxide in the raw material powder is primarily gas-reduced. Next, in a vacuum atmosphere, an inert atmosphere or a reducing atmosphere, 1100-1300 ° C
Of the C and O contained in the raw material powder by heating within the temperature range of
By the reaction, decarburization, deoxidation and reduction of the non-reducible oxide are carried out to produce a low C, O low alloy steel powder for powder metallurgy.

However, the easily reducible oxide that forms the variation in the amount of O in the steel powder at the raw stage cannot reduce the variation in the amount of O in the steel powder even if it is reduced by the H ₂ reducing gas.

In addition, even with steel powder reduced with H ₂ etc. in this way,
The balance of C and O in steel powder is lost, and C and O in raw material powder
The decarburization and deoxidation reaction due to is limited and the reduction time cannot be shortened.

DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION The present invention aims to solve the above problems, and specifically, manufacture of steel powder for powder metallurgy comprising steps of preheating (primary reduction) -reduction annealing (secondary reduction) -cooling. In the method, the bumping phenomenon, which is the rate-determining factor in the preheating step and causes a decrease in productivity, is prevented, and at the same time, the amount of gas that is a rate-limiting factor in the reduction annealing (secondary reduction) step and causes a decrease in productivity is reduced. We propose possible reduction methods.

Means for Solving the Problem and Its Action That is, the method of the present invention, the atomized alloy steel powder raw material powder containing a non-reducing element and carbon is 20 Torr or less at a temperature of 800 ~ 1300 ℃ in a reduced pressure atmosphere Prior to the subsequent final reduction annealing, when heating the raw material powder to primarily reduce the easily reducible oxide in the raw material powder, the atmosphere at this time was set to a dew point of 10 ° C. or less, nitrogen or Keeping in a non-oxidizing atmosphere consisting of argon or nitrogen and argon,
It is characterized in that the bumping phenomenon of the surface layer portion of the raw material powder is prevented and the easily reducible oxide is primarily reduced by heating within a temperature range of 650 to 1200 ° C.

Therefore, the configuration and operation of these means will be described in detail as follows.

First, the present inventors have used atomized alloy steel powder containing carbon in addition to hard-to-reduce elements such as Cr and Mn as a raw material powder, and have earnestly studied deoxidation and decarburization steps for this raw material powder. , Especially in a reduced pressure atmosphere of 20 Torr or less
The bumping phenomenon which occurred in the preheating (primary reduction) process performed before the finish reduction annealing at the temperature of 0 to 1300 ℃ was studied.

As a result, when the raw material powder is heated to a temperature range of 650 to 1200 ° C. to temporarily reduce the easily reducible oxide contained in the raw material powder, the atmosphere is changed to a non-oxidizing atmosphere of nitrogen or argon. If held, it is possible to suppress the bumping phenomenon of the steel powder because it is in the state of normal pressure without depressurizing or vacuuming, and it is possible to reduce the amount of gas generated from the steel powder even during the finish reduction annealing process. Further, it was found that when the non-oxidizing atmosphere was a non-oxidizing atmosphere having a dew point of 10 ° C. or lower, the balance of C and O in the steel powder during the finish reduction annealing process could be properly maintained.

That is, the present inventors cause a bumping phenomenon of steel powder when the temperature is raised to 650 to 800 ° C. in a reduced pressure atmosphere in a preheating chamber for steel powder. To prevent this, the heating rate is set to about 200 ° C.
A method of slowly heating and raising the temperature at a time of / hour was adopted.

By this method, the bumping phenomenon of steel powder can be prevented to some extent. However, this heating and heating method requires a long time for reduction.

From this, the present inventors have studied the cause of the bumping phenomenon of the steel powder, the bumping phenomenon occurs when the temperature rises from room temperature to 650 ~ 800 ° C., steam gas generated from the steel powder And it was found to be due to CO gas. Especially, due to the temperature difference in the steel powder layer, steam gas and CO
There is a difference in the gas generation time, and the gas generated from the inside with this time difference causes the slightly sintered steel powder on the surface of the steel powder layer to be blown up and discharged outside the steel powder layer.

From this point, in the preheating process, if the water content and the C and O contents are adjusted and lowered in advance before the preheating and the primary reduction, it is not always necessary to reduce the temperature rising rate. Furthermore, if the amount of water, C, O, etc. is reduced before the finish reduction process, even if the temperature is rapidly raised to 800 ° C or higher,
The bumping phenomenon of steel powder during this temperature rising process is avoided.

Therefore, in the method of the present invention, the raw material powder is kept in the temperature range of 650 ° C. to 1200 ° C. while being kept in the temperature range of 800 to 1300 ° C. and prior to finish reduction annealing performed in a reduced pressure atmosphere of 20 Torr or less. When reducing precipitously and heating up to the final reduction temperature range and preheating, this atmosphere is adjusted to a non-oxidizing atmosphere consisting of nitrogen or argon or both without reducing the pressure, and this dew point is set to 10 ° C. adjust. That is, when heated in the preheating chamber to a range of 650 ° C to 1200 ° C and temporarily reduced, CO vapor generated is
Since the atmosphere is inert and there is no need to reduce the pressure, the rate of expansion is extremely small even when discharging to the surface layer, and CO
The steel powder will not be blown up by such as. Contrary to this,
In a depressurized or vacuum atmosphere, the discharge gas such as CO expands rapidly when discharged from the surface layer, the expansion rate is large, and the steel powder is pushed up by the discharge gas, and the bumping phenomenon occurs.

The non-oxidizing atmosphere is made of nitrogen and / or argon and has a dew point of 10 ° C or lower. For this reason, when the easily reducible oxide is temporarily reduced, for example, gas reduction such as H ₂ is not performed at all, and the reduction proceeds by decarburization and deoxidation reaction of C and O in the steel powder. However, the amount of C and the amount of O are reduced, and the amounts are made uniform.

Furthermore, since the dew point is 10 ° C. or lower, the balance of C and O in the steel after the primary reduction is maintained as shown in Example 2 and FIG. 1, and decarburization and deoxidation are performed in the secondary reduction finish reduction. The reaction proceeds smoothly, and low-C, low-O steel powder is obtained.

Further, in a non-oxidizing atmosphere of nitrogen, argon, etc., for example, a belt type annealing furnace of a type that allows raw material powder to pass through can be used, and a large amount can be processed.

The temperature range for the deoxidation and decarburization reaction of the primary reduction during this preheating was set to a temperature range of 650 to 1200 ° C. The reason for reducing the easily reducible oxide by C in steel below 650 ° C. Because there is no. This is because if the temperature exceeds 1200 ° C, the sintering of the packed bed of steel powder proceeds excessively, and a great amount of labor and a long time are required for the crushing performed in the subsequent finishing reduction process (secondary reduction).

The finish reduction annealing is performed in a reduced pressure atmosphere of 20 Torr or less, and the secondary reduction is performed while maintaining the temperature at 800 to 1300 ° C. That is, if the temperature is less than 800 ° C, the reduction of steel powder is incomplete,
If the temperature exceeds 0 ° C, the sintering will proceed too much and it will take a long time for subsequent crushing.

If finish reduction annealing is performed in an atmosphere exceeding 20 Torr, it is not possible to obtain low C and low O steel powder.

Examples Below, examples will be described.

Example 1. A water atomized steel powder containing the hardly-reducing elements Mn and Cr having the composition shown in Table 1 was heated at 650 ° C. for 60 minutes in an Ar atmosphere and subjected to primary reduction together with preheating. At this time, the bumping phenomenon of the steel powder did not occur at all.

Table 2 shows the amount of adsorbed water before and after the primary reduction and the amount of combined water.

As shown in Table 2, by performing the primary reduction,
The combined water and adsorbed water in the steel powder could be controlled to 0.01% or less.

After crushing the steel powder subjected to the primary reduction treatment with a hammer mill in this way, the temperature rising rate up to 800 ° C in a vacuum atmosphere of 20 Torr 400
Even when the temperature was raised at ° C / hour, the bumping phenomenon of the steel powder was not confirmed.

Example 2. Water atomized steel powder containing the hardly reducing elements Mn and Cr shown in Table 1 was preheated at 1100 ° C for 120 minutes by changing the dew point from -40 ° C to + 40 ° C in an N ₂ atmosphere. The primary reduction was also performed.

The amounts of C and O after the primary reduction were as shown in FIG. 1 and Table 3.

From Fig. 1 and Table 3, in the atmosphere when the primary reduction is performed in addition to the preheating, the dew point is 10 ° C or higher, especially at 40 ° C, C 0.01% and O 0.54%, C,
The balance of the amount of O is lost, and even from this point of view, if the atmosphere at the time of preheating (primary reduction) is a non-oxidizing atmosphere and the dew point is not higher than 10 ° C, deoxidation in finish reduction (secondary reduction),
It was found that the decarburization reaction did not proceed.

Example 3. Water atomized steel powder containing the hard-to-reduce elements Mn and Cr shown in Table 1 was heated in an Ar atmosphere at a temperature range of 500 to 1300 ° C. for 60 minutes to be preheated for primary reduction.

The amounts of C and O after the primary reduction are shown in FIG. 2 and Table 4. Oxygen is reduced by setting the primary reduction temperature to 650 ° C or higher.

At the high temperature of 1250 ° C. or higher in the primary reduction, the sintering proceeded too much, and it took a long time for the subsequent crushing.

After the steel powder thus primary reduced was crushed with a hammer mill, final reduction (secondary reduction) was performed at 1100 ° C. for 60 minutes in a vacuum atmosphere of 10 Torr. The amounts of C and O at that time are shown in Table 4. As shown in Table 4, within the scope of the present invention, the raw material powder was subjected to primary reduction to obtain C and O in a conventional vacuum atmosphere of 10 Torr.
It took 90 minutes or more to reduce C, but it was found that the amount of C and O could be reduced in a short time of 60 minutes.

Example 4. The steel powders in Table 1 were heated at 900 ° C. for 60 minutes in a N ₂ atmosphere to undergo a primary reduction treatment. Continue to 650-130 in a vacuum of 10 Torr
Final reduction (secondary reduction) was performed by heating at 0 ° C for 60 minutes. Table 5 shows the amounts of C and O after the primary and secondary reductions. As shown in Table 5, it was found that low oxygen, low carbon steel powder was obtained.

Example 5 In the same manner as in Example 3, the degree of vacuum of the steel powder subjected to the primary reduction treatment was set to 5
The final reduction (secondary reduction) was carried out at 1100 ° C. for 60 minutes while changing to ˜40 Torr. Table 6 shows the amounts of C and O at that time. As shown in Table 6, it was found that low oxygen, low carbon steel powder was obtained.

<Effects of the Invention> As described in detail above, according to the present invention, prior to finishing reduction, the raw material powder is 650 to 1 in a non-oxidizing atmosphere having a dew point of 10 ° C or less.
Since the easily reducible oxide is primarily reduced by heating in the temperature range of 200 ° C., water, C, and O can be sufficiently reduced during the primary reduction, and the bumping phenomenon does not occur at this time.

Further, the bumping phenomenon does not occur even when the temperature is rapidly raised to the temperature of the secondary reduction of the finish reduction.

C and O in the steel powder subjected to the primary reduction are balanced, and the primary reduction powder is 800 to 1200 ° C in a reduced pressure atmosphere of 20 Torr or less.
The finishing (secondary) reduction of (1) can proceed favorably and the productivity can be remarkably improved.

[Brief description of drawings]

FIG. 1 and FIG. 2 each show one embodiment according to the method of the present invention. FIG. 1 is a graph showing the relationship between the dew point of the atmosphere and the amounts of C and O during the primary reduction, and FIG. It is a graph which shows the relationship between temperature and the amount of C and O.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 64-25901 (JP, A) JP 4-9402 (JP, A)

Claims (1)

[Claims] 1. A raw material powder for atomized alloy steel powder containing a non-reducing element and carbon in a reduced pressure atmosphere of 20 Torr or less.
Prior to the secondary reduction annealing at a temperature of 0 to 1300 ° C., when the raw material powder is heated to primarily reduce the easily reducible oxide in the raw material powder, the atmosphere at this time is changed. , With a dew point of 10 ° C. or less, keeping in a non-oxidizing atmosphere composed of nitrogen or argon or nitrogen and argon to prevent the bumping phenomenon of the surface layer portion of the raw material powder.
~ A method of reducing atomized alloy steel powder through primary reduction and secondary finishing reduction, which comprises heating the temperature within a temperature range of 1200 ° C to primarily reduce the easily reducible oxide .