JPS589807B2 - Sintering reduction method of steel powder using shaft furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for sintering and reducing steel powder using a shaft furnace.

Steel powder, especially steel powder alloyed mainly with strongly oxidizing elements such as Mn and Cr, is often prepared by the water-atomization method as a conventional method, but it cannot be commercialized as it is by the powder metallurgy method. However, it cannot be used as a powder metallurgical material until it undergoes sintering reduction, which corresponds to the final reduction step conventionally performed after rough reduction such as mill scale.

However, such atomized steel powder cannot be made into sufficiently low-acid steel powder by the above-mentioned conventional finishing reduction process, so the demand for making it possible to perform sintering reduction commensurate with the atomized steel powder is as follows. It is painful now.

In this regard, the applicant previously proposed in Japanese Patent Application No. 51-26708 (see Japanese Unexamined Patent Publication No. 52-110208) that in order to overcome the above problem, the applicant incorporated carbon into the above-mentioned steel powder, and then The basic development result is that sintering self-reduction by high-frequency induction heating in a reduced-pressure atmosphere after preheating sintering of steel powder is advantageously applicable, and the shape furnace, or shaft furnace, is particularly suitable for its implementation. This has already been disclosed.

This invention was developed through practical operational experiments regarding sinter reduction of steel powder in shaft furnaces, and as a result of subsequent development research, newly discovered knowledge particularly unique to sinter reduction in shaft furnaces was developed. It is based on

This invention consists of a sequential vertical arrangement of a preheating furnace and a high frequency furnace,
When raw steel powder to be loaded with carbon is sequentially cut and charged into a shaft furnace maintained under reduced pressure and the steel powder is continuously sintered and reduced, an inert gas is used as the raw material steel powder. A method for sintering and reducing steel powder using a shaft furnace, characterized by using steel powder prepared by a water atomization method in an atmosphere, dehydrated and concentrated, and then subjected to a drying process in an inert gas. It is.

Here, the powder raw material to be subjected to sinter reduction under reduced pressure in a shaft furnace is the atomized steel powder mentioned above, and the carbon contained as a component is used as a reducing agent as it is, and if the carbon content is insufficient, Carbonaceous powder is mixed to contribute to the reduction reaction, but when the atomized steel powder that forms the main part of the powder raw material is powdered in an atmospheric environment according to the conventional water-atomization method, It has been found that the above sintering reduction process often involves the following problems.

That is, this powdering process begins with the preparation of molten steel adjusted to a predetermined alloy composition, and atomization is performed by the injection collision of a jet of water against the free-falling flow of the molten steel through a nozzle in a water tank. It is then sent to a concentration tank using a slurry pump, where it is dehydrated from about 30% moisture to about 8-10% moisture using a vibration dehydrator, and then dried using a means such as a steam dryer.
This process ends with the disintegration of powder agglomerates and classification and grading, but the oxygen content of the atomized steel powder obtained is approximately 1.5%, sometimes as high as 2 to 5%, and FeO+H20→α-FeO(
Because OH) (goethite) is generated, heating in the shaft furnace creates an oxidizing atmosphere inside the furnace and prevents the reduction reaction from occurring, and the steam pressure generated at this time increases the amount of material charged into the preheating furnace. There is a risk that the steel powder will blow up backwards, causing trouble in powder charging, and that the sintering process will be overflowed.

On the other hand, the steel material obtained by carrying out the atomization and drying steps as described above according to the present invention in an inert gas atmosphere, for example, nitrogen, does not cause agglomeration of particle agglomerates after first drying. Not only does it eliminate the need for a crushing step prior to classification, but the oxygen content of alloyed steel powder particles is generally less than 1%, often 0. 8% or less, which is particularly advantageous for the sintering reduction of the present invention, and furthermore, the formation of the above-mentioned goethite on the particle surface is minimal, which prevents the sintering reduction atmosphere from seeping out and the back blowing of the charging material in the preheating furnace. It has been confirmed that the occurrence of water vapor, which could be a cause, is extremely small and is extremely useful for the smooth progress of the sintering reaction of the present invention.

An example of a shaft furnace used for carrying out the present invention is shown in FIG.

In the figure, 1 indicates a preheating furnace, and 2 indicates a high frequency furnace, and these are arranged vertically in that order as shown in the figure.

The preheating furnace 1 is configured to burn C gas and other fuels between a core tube 3 made of a heat-resistant steel pipe that is slightly tapered downward and a refractory wall 4 that surrounds the core tube 3. Alternatively, it has a combustion chamber or a heating chamber 5 that performs external heating by resistance heating or the like, and 6 is an iron skin.

Although this example uses a C gas furnace, the burner and exhaust gas flue are not shown.

The high frequency furnace 2 has an induction heating coil 7 arranged around a one-turn secondary coil 9 connected to the furnace core tube 3 of the preheating furnace 1 via a sliding flange joint 8. An insulating inner cylinder 10 made of porcelain is housed inside.

A two-stage raw material hopper 12 is placed at the top of the preheating furnace 1 via a cutting device 11 .

In the raw material hopper 12, atomized steel powder prepared and dried in an inert atmosphere according to the present invention described above is stored as a raw material steel powder, and in the illustrated example, the atomized steel powder is first introduced into a fixed volume of a slide feeder 13, and then fed into a furnace core tube. An elevating feeler (feeler) that timely detects the charging level of raw steel powder in the
r) By operating the slide feeder 13 according to the stroke of 14, raw steel powder is intermittently fed into the furnace core tube
Cut it out and insert it inside.

The first charging of this raw steel powder is carried out in the preheating furnace 1 into the furnace core tube 3.
A rising type leading dummy 15 is provided to allow the deposition inside.

The leading dummy 15 is made up of a dummy head 16 that fits relatively loosely near the lower end of the furnace core tube 3, and a steel wool packing 17 superimposed on and fixed to the dummy head 16, which is held on a long elevating stem 18.

Although the elevating and lowering stem 18 is not shown, it is possible to control the lowering at a slow speed of several mm/min, for example, by means of a rope device, for example.

In the figure, reference numeral 19 denotes a pinch roll 20 that supports the weight of the sintered reduction cake obtained according to the present invention (hereinafter referred to as 1 cake) from the leading dummy 15 depending on its production.
21 is a discharge chute, 22 is a packet, 23 is an outlet door, and 24 is a cooling chamber for the I cake.

In the operation of the shaft furnace according to the present invention, the slide feeder 13 is first operated in the illustrated raised position of the leading dummy 15, and the raw steel powder in the raw material hopper 12 is transferred to the preheating furnace.
The first step is to cut out the material into the furnace core tube 3 and charge it intermittently.
Starting from the start, during the restoring process of the slide feeder 13, the filler 14 performs an empty stroke to repeat the cutting and charging operation of the slide feeder 13, and as a result, the raw steel powder is transferred to the leading dummy 17 in the core tube 3. When the columnar deposits are formed, the filler 14 detects this during the stroke, and as a result, the slide feeder 13 temporarily stops, while gas ignition occurs in the combustion chamber in the preheating furnace 1. is carried out, and by its external heating, the leading dummy 1
The columnar deposit held on the columnar deposit undergoes a sintering reaction radially inward from its outer periphery, and as a result, the columnar deposit grows so that a shell-shaped sintered region becomes thick.

This preheating humidity is generally in the range of 780 to 1200°C,
Appropriate selection is made depending on the composition of the raw steel powder, the scale of the equipment, and other conditions, and the above-mentioned shell-shaped sintered region is self-retaining.
Sintering is performed for a period of time necessary to support the weight of the raw material steel powder further deposited inside and above the steel powder, and to eliminate the fear of the raw powder flowing down due to crumbling.

At this time, according to a test in which the outer diameter of the columnar deposit was set to 160 mm, the wall thickness of the shell-shaped sintered region was approximately 30 mm, and therefore, the outer diameter of the inner unsintered part of about 100 mm was sufficient.

The columnar deposit containing this sintered region will be particularly referred to as a P cake.

Waiting for the above-mentioned P cake to be baked, the furnace humidity of the preheating furnace 1 is lowered once.

Note that this temperature drop amount is approximately 600°C when the preheating temperature is set to 1100°C.

This temperature drop is caused by the thermal contraction that occurs in the heat-resistant steel core tube 3, which causes radial tightening and compaction of the P cake, and when the preheating furnace is reheated, only the core tube 3 expands in advance, so that the P cake and This is necessary to create a separation between the inner wall of the furnace core tube 3 and to enable smooth descent of the P cake.

High-frequency induction heating is then applied to the P-cake that has started to descend in this way, but as described below, in the induction heating process, it is also necessary to heat the P-cake while gradually increasing the high-frequency input. be.

That is, after the above temperature drop, the temperature is 11, which is the predetermined preheating temperature.
The preheating furnace temperature is raised again to 00°C, and then the dummy head 16 of the leading dummy 15 lowers the P cake one after another to just below the heating zone of the high frequency furnace 2, during which time it is heated to the raw material steel powder charged subsequently. Preheating is performed, and cylindrical sintered regions are sequentially grown in the vertical direction.

Thus, P sequentially extends downward inside the shaft furnace.
Inside the cake, there is a primary coil 7 in the high frequency furnace 2.
A downwardly convex parabolic sintered-unsintered interface, ie, a sintering front, is formed with the upper end of the sintered surface as the bottom point.

After this sintering front reaches just above the heating zone of the high-frequency furnace 2, the high-frequency input is gradually increased to transform the P cake into the aforementioned cake.

This gradual increase in high frequency input can be achieved using the above example, i.e.
Good results were obtained by using a step method in which the I-cake of 0 mm was reached in 10-minute increments of about 2 KW, taking about an hour or more to reach a high number of KW.

Thereafter, the P cake produced in the preheating furnace 1 is lowered one after another to be continuously transformed into an I cake, and when the lower end reaches the level of the pinch roll 19, it is determined by the stem 18 of the leading dummy 15 that, for example, a detection contact is detected. This allows the load on the leading sill 15 to be supported by the pinch rolls 19, and then the leading sill 15 is rapidly lowered to the evacuation position. By activating one detection contact, the chute 21 is advanced from the previous evacuation position to a position directly below the pinch roll 19 and the cutter 20, and at the same time, the repetitive drive is started at an appropriate time. Roughly crush the cake using a cutter 20, then use a packet 22
can be guided within.

Although I did not mention it to avoid the complexity of the explanation, the reaction of 2MO+C → 2M+CO2 occurs mainly in the relatively low temperature region of the preheating furnace 1, and the reaction of MO+C occurs from the high temperature region of the preheating furnace 1 to the heating zone of the high frequency furnace 2. →The M+CO 2 reaction proceeds using the carbon contained in the raw material steel powder or the carbonaceous material added thereto as a reducing agent.

In order to eliminate such generated gas and promote the above-mentioned reduction reaction, it is necessary to provide vacuum system piping to each part of the shaft furnace to constantly exhaust the inside of the furnace.

That is, as shown in the illustrated example, for example, a mechanical booster MB1 and a rotary vacuum pump RP1 are arranged at the bottom b of the shaft furnace via a valve A, and this valve A and a mechanical booster M are connected to each other.
A separate valve B is provided between the furnace top t and the furnace bottom b, and a mechanical booster, for example, is connected to the furnace top via a valve D. Piping is provided between MB2 and the rotary vacuum pump RP2, and piping is also provided between the mechanical booster MB2 and valve D via valve E to the furnace bottom b.

This vacuum system performs the following valve operations at the time of starting and steady operation of the above-mentioned shaft furnace.

By opening the valve C halfway during the start operation, the degree of depressurization near the lower end of the core tube 3 of the preheating furnace 1 can be controlled from the hollow part M of the furnace bottom b or the sliding flange joint 8.
By making the pressure slightly weaker than that at the top of the furnace t, the difference in pressure between the upper and lower sides of the furnace core tube 3 can sometimes cause the columnar pile of raw steel powder to be blown away before the P cake is formed, causing the raw powder to flow down. This is to complete the situation.

In steady operation 1, the vacuum systems for furnace bottom b and furnace top t are made independent, and in steady operation 2, it is furnace bottom b that further strengthens the degree of depressurization at furnace bottom b. This is to avoid the risk of re-oxidation of the cake surface layer due to CO2 during the reaction.

In this way, it is possible to advantageously lead to the reliable production of P and ■ cake, but in the preheating furnace 1, the preliminary reduction of FeO is mainly carried out, and the
In the ~1400°C range, the subsequent reductions of MnO, Cr2O3, etc. proceed favorably, and in order to continue such reduction reactions, it is necessary to It is chosen so that it has already disappeared just before the heating zone.

This is because when the sintering front enters the high-frequency heating zone, even if the raw steel powder is in accordance with the present invention, the power of the high-frequency coil is lost due to the sintering reaction to the unsintered area, and the reduction reaction occurs. This is because it will be incomplete.

As described above, according to the present invention, troubles caused by the surface texture of atomized steel powder can be solved, which is a problem inherent to shaft furnaces, which is particularly problematic in the operation of shaft furnaces used for efficient sintering and reduction of steel powder. This can be solved advantageously, simply and reliably, without affecting the efficiency of the furnace operation in any way.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a shaft furnace showing an embodiment of the present invention. 1... Preheating furnace, 2... High frequency furnace, 3... Furnace core tube,
15... Leading dummy.

Claims (1)

[Claims] 1 A preheating furnace and a high-frequency furnace are sequentially arranged vertically, and raw steel powder to be loaded with carbon is sequentially cut and charged into a shaft furnace maintained under reduced pressure, and the steel powder is continuously sintered and reduced. In carrying out this process, the raw steel powder used is steel powder prepared by a water atomization method in an inert gas atmosphere, dehydrated and concentrated, and then subjected to a drying process in an inert gas atmosphere. A method for sintering and reducing steel powder using a shaft furnace, characterized by: