JPS589803B2 - Vacuum evacuation method of shaft furnace for reduction annealing of steel powder and flange joint for connection between furnaces - Google Patents

Description

[Detailed Description of the Invention] This invention provides iron-based metal powder (pure iron powder,
Contains any alloy steel powder.

The present invention relates to an evacuation method and apparatus for reduction annealing of steel powder (hereinafter referred to as steel powder).

Conventionally, the production of steel powder has been carried out through the process of water atomization of molten steel and gas reduction (or coarse reduction and pulverization of iron ore, mill scale, etc. using coke and gas reduction), and the gas reduction method has been used as the final reduction process. .

However, in the case of this gas reduction method, it is difficult to deoxidize to a sufficiently low oxygen level with steel powder alloyed with highly oxidizing elements such as Mn and Cr.

In addition, in recent years, with the advent of sintering technology, there has been a demand for inexpensive steel powder for obtaining high-density materials with excellent hardenability and toughness.

In order to meet this demand, it has become necessary to produce steel powder that is alloyed mainly with Mn and Cr and has a sufficiently low oxygen content.

On the other hand, the present inventors previously proposed
He invented the method disclosed in No. 8.

Briefly, this invention consists of a shaft furnace that is maintained in a vacuum atmosphere and has a preheating furnace and an induction heating furnace that are vertically arranged in sequence. The raw steel powder is sequentially cut out, and the raw steel powder is first turned into a cylindrical sintered cake in the preheating furnace, and then the remaining part is subjected to deoxidation, decarburization, and sintering reactions in the induction heating furnace, thereby forming the steel powder. This is a method of continuous reduction annealing.

In short, the invention disclosed therein is: (1) Carbon is incorporated in the raw steel powder in advance (regardless of alloy carbon or mixed carbon), and the molar ratio of carbon and oxygen is adjusted appropriately according to the purpose. , using the interior carbon as a reducing agent.

(2) Performing reduction annealing of the raw material steel powder in a vacuum atmosphere.

(3) Applying induction heating to the reduction annealing of the raw material steel powder described above to directly generate heat in the raw material steel powder itself, which is the object to be reduced.

(4) For reduction annealing of the raw steel powder, a shaft furnace is used, which consists of a preheating furnace for indirectly heating the raw material steel powder and an induction heating furnace for directly heating the raw material steel powder. After forming a shell-shaped sintered cake, it is efficiently deoxidized, decarburized, and sintered in an induction heating furnace, and then subjected to continuous reduction annealing.

It consists of the above four items.

As described above, the invention of JP-A-52-110208 allows extremely effective deoxidation of steel powder alloyed with Mn, Cr, etc.

However, new problems were encountered in subsequent research.

That is, the method disclosed in JP-A-52-110208 uses a method in which the reduction product gas generated inside the shaft furnace is exhausted both from above the preheating zone and from below the induction heating zone.

However, in this method, the surface of the final deoxidized cake (cylindrical) after induction heating was reoxidized, and a reoxidized layer was formed in which the amount of oxygen gradually decreased toward the inside of the cake.

It was found that the formation of this reoxidation layer was mainly due to the reduction product gas generated in the shaft furnace.

There are two types of reduction product gas generated in this shaft furnace, one of which is the gas generated in the preheating zone, which is CO gas containing a small amount of CO2 (usually several percent CO2). Contains.

), the other is nearly 100 percent pure CO gas generated in the induction heating zone.

Of these, a particular problem is CO2 in the gas generated in the preheating zone, and the cake that has been deoxidized is easily reoxidized by this CO2 gas.

In addition, it is conceivable that the CO gas also undergoes a decomposition reaction on the surface of the cake during cooling, and the resulting CO2 gas reoxidizes the surface of the cake.

Therefore, in order to prevent re-oxidation of the cake surface, the first step is to evacuate the cake using a powerful pump, but it is clear that this method does not essentially solve the problem.

This is because as long as conventional vacuum evacuation is continued, the following problems will remain even if a powerful pump is used.

The conventional vacuum evacuation methods are: 1) A method in which the entire volume is evacuated from above the preheating zone.

2) A method in which the entire amount is exhausted from below the induction heating zone.

3) A method in which a portion is exhausted from above the preheating zone and the remaining portion from below the induction heating zone.

There were three ways.

However, in case 1), since a large amount of gas is sucked upward, the supplied powder is blown up at the upper part of the preheating tube, which not only causes problems in powder supply, but also causes sintering during the preheating process. This method has drawbacks such as being a cause of poor solidification and, in turn, being a contributing cause of troubles in the induction heating process.

In addition, when exhaust method 2) is used, blowing up of the powder above the preheating tube can be completely prevented, but since the gas containing CO2 generated in the preheating zone flows downward, the above 1) As in the case of , the surface of the deoxidized cake is re-oxidized, which is undesirable.

On the other hand, method 3) is quite effective, and since the gas generated in the upper half of the preheating zone is exhausted from the lower half of the preheating zone and the gas generated in the induction heating zone is exhausted from below the induction heating zone, the powder is removed from the upper part of the preheating tube. In addition to almost preventing the blow-up of
This method was far more effective than methods 1) and 2), as the reoxidation of the deoxidized cake surface during cooling could be considerably reduced.

However, since the gas generated in the lower half of the preheating zone is exhausted via the bottom of the device, some CO contained in this gas is
2, the surface of the sintered deoxidized cake is still reoxidized.

An object of the present invention is to overcome the above-mentioned drawbacks of the prior art associated with evacuation of a shaft furnace for reduction annealing of steel powder, and to provide a means for evacuation from an intermediate point between a preheating zone and an induction heating zone. This was solved by

The details of the method and apparatus of this invention will be explained below.

FIG. 1 is a diagram showing an example of a shaft furnace and a vacuum evacuation system used in carrying out the present invention.

In this figure, reference numeral 1 denotes a powder supply section, which includes a hopper 5, a feeder 6, a powder stock line detection device (feeler) 27 in a preheating tube, an upper tank 26, and the like.

2 is a preheating zone, which is composed of a furnace body 7, a preheating tube 8, and the like.

3 is an induction heating zone, which includes a primary coil 12 for excitation;
It is composed of a secondary coil 13 for heating, a fireproof heat insulating member 14, an upper flange joint 24 for fixing the coil, a lower flange joint 25 for fixing the coil, and the like.

Below the induction heating furnace, there is a cake processing section 4, including a pinch roll 17 for lowering the cake, a cake cutter 18, a shooter 19, a chute guide 20, a packet lifter 21 for transferring the cake, a lower tank 16, and a cake cooling chamber 23.
, and a flapper valve 22 are provided.

In addition, this shaft furnace has a flange joint 15 that connects the preheating zone 2 and induction heating zone 3, and a preheating tube 8 that connects the preheating zone 2 and the induction heating zone 3.
a support rod 9 that is raised inside and used to support the raw material powder and bake the seed cake, and a backing member 10 at its tip;
It is equipped with a laminated mat 11, etc.

Furthermore, in addition to this series of functional equipment, a vacuum pump is provided to exhaust the gas generated within the furnace.

This vacuum pump requires at least two series, but preferably three series as shown in FIG.

In addition, although MB in the figure means a mechanical booster pump and RP means an oil rotary pump, a steam ejector may be provided in place of the mechanical booster pump, or an oil diffusion pump may be provided in the vacuum pump system.

Also, in Figure 1, the vacuum pump system is MB1 + RP1, MB2.
+RP2 and RP3, but RP3 may be combined with MB3 if necessary.

Next, the specific operating method will be briefly explained.

The raw steel powder, which is successively cut out from the hopper 5 into the preheating tube 8 via the feeder 6, gradually becomes a cylindrical shell-shaped sintered cake while continuously descending inside the preheating tube, and reaches the induction heating zone 3.

The cake undergoes even stronger deoxidation and decarburization in this induction heating zone 3, and sintering progresses to the core, and then the pinch roll 1
7 and cut by a cutter 18, then transferred to a cake cooling chamber 23 and cooled.

The cooled cake is taken out of the device and pulverized by a pulverizer, and is turned into product powder through processes such as classification and magnetic separation.

In the shaft furnace operation as described above, the present invention provides a shaft for vacuum evacuation. In addition to maintaining a structure in which the reduction product gas generated in the furnace can be exhausted from the upper part of the preheating zone 2 and the lower side of the induction heating zone 3 as in the conventional case, there is also an intermediate connection between the two zones 2 and 3. It was also designed to exhaust air from the

In addition, the present invention allows evacuation to be performed only from such intermediate connection portions, which provides an effect superior to that of the prior art.

The reason for this is that the gas containing CO2 generated in the preheating zone is immediately exhausted from the flange joint 15 for connecting between furnaces directly below the preheating zone 2, so that gas is released from the bottom of the shaft furnace including the induction heating zone 3. It can prevent it from penetrating in half.

On the other hand, the pure CO gas generated in the induction heating zone 3 is also exhausted from the flange joint 15 for connecting the furnaces directly above the induction heating zone 3, thereby regenerating the deoxidized cake that has passed through the induction heating zone 3. This is because it can prevent oxidation.

Furthermore, in order to implement this invention method even more effectively, there are three main vacuum pump series, two of which are for constant exhaust,
It is desirable that one series be used only when necessary.

To explain this with reference to Fig. 1, MB2 and RP2 are used for exhaust from the upper part of the preheating zone 2, and MB1 and RP1 are used for exhaust from the flange joint between the furnaces and the lower part of the induction heating zone 3. By separating and exhausting the internally generated reduction product gas into two systems, it is possible to almost completely prevent reoxidation of the deoxidized cake.

In other words, the CO generated in the relatively low temperature area above preheating zone 2
If the gas with a high CO2 content is exhausted from the upper part of the preheating zone 2, and the gas with a relatively low CO2 content generated in the lower part of the preheating zone 2 is exhausted mainly from the flange joint 15 for connecting between furnaces, the deoxidized cake can be reoxidized. Prevention is entirely possible.

If such an exhaust method is adopted, the exhaust direction of the gas generated in the upper part of the preheating zone 2 and the induction heating zone 3 will be opposite to the direction in which the cake descends.

Furthermore, although the gas generated at the bottom of the preheating zone 2 is in the same direction as the cake descending, it is immediately exhausted from the connection between the furnaces, so the deoxidized cake that has passed through the induction heating zone 3 is not reoxidized. There is almost no risk of it happening.

In Fig. 1, the exhaust pump RP3 in the third series is not used to exhaust the reduction product gas generated in this shaft furnace, but to exhaust the air inside the hopper when raw material powder is additionally charged into the hopper 5 during operation. It is used for exhausting air from the cooling chamber, or for exhausting the air in the cooling chamber when the cake is taken out from the cake cooling chamber 23 (at this time, the flapper valve 22 is closed). It is maintained.

Next, the exhaust device used when carrying out the above-mentioned vacuum evacuation method, that is, the flange joint 15 for connecting between furnaces will be explained.

This flange joint 15 for connecting between furnaces is configured to perform the following functions.

1) The vacuum of the shaft furnace is maintained by connecting the outer circumferential surface 28 of the lower end of the preheating tube and the secondary heating coil 13 of the induction heating furnace via the upper flange joint 24 for fixing the coil.

2) Thermal expansion caused by external heating of the preheating tube 8 of the preheating zone 2 is absorbed by the flange joint 15 for connecting between furnaces, which is slidably engaged.

3) Provide an exhaust pipe 31 on the flange joint 15 for connecting between furnaces,
By connecting this to the vacuum piping, the in-furnace reduction generated gas is evacuated from the flange joint 15 for inter-furnace connection.

4) Preventing the lower end of the preheating tube from wobbling due to the suspension support of the upper part.

Next, to explain its structure, this flange joint 15
connects the preheating tube 8 and the heating secondary coil 13, and is installed on the upper flange joint 24 for fixing the coil and tightened with bolts, and is also attached to the furnace support plate 30 via bolts 32. Fix it.

Thus, the preheating tube 8 fitted into the flange joint 15 is slidably and airtightly fitted.

In the body of this joint 15, there are two concentric spaces: an annular exhaust passage 33 and an annular cooling water passage 3.
4 is provided.

Moreover, the sliding surface with the preheating tube 8 has three annular grooves 36, 3.
7 and 38 are missing, and the upper and lower 2 of the annular grooves are missing.
O-rings are fitted into the pieces 36 and 38 for vacuum sealing, and the remaining annular groove 37 in the middle is a grease groove for smooth sliding. High purity inert gas (N2, Ar, He)
A constant positive pressure (normal gauge pressure of 5 kg/cm2 or less) is sufficient between the two O-rings.

) to prevent vacuum breakdown due to poor sliding of the preheating tube 8, etc.

Note that the cooling water passages 34, 34' through which water is circulated through the cooling introduction pipes 40, 40' are provided to prevent the O-ring from burning out and to prevent the O-ring from seizing at the joint with the upper flange joint 24 for fixing the coil. .

Further, an exhaust pipe 31 is connected to the annular exhaust passage 33, and a plurality of notched passages 41 are provided on the inner wall facing the inside of the furnace, and the annular exhaust passage 31 is connected to the inside of the shaft furnace.
It is connected to 3.

Next, a method of evacuation using the above-described flange joint 15 for connecting between furnaces will be explained along with examples.

This example is the result of testing how much the oxygen content differs in the deoxidized cake and the product powder obtained by pulverizing it, depending on the vacuum evacuation method.

Note that Table 1 shows that the raw material powder has an oxygen content of 1.44% and a carbon content of 1.27%. S%Mn, -1%Cr-0.25
% Mo composition was subjected to reduction treatment using the apparatus shown in FIG. 1 at a preheating furnace temperature of 1100°C and an induction heating temperature of approximately 1350°C.

Symbols A, B, C, and D in Table 1 indicate the positions of the cross-section of the deoxidized cake. For a cake with a diameter of 170 mmφ, A is the outermost surface, B is the position 10 mm inside from the surface, and C is the position of the cake. A position 40 mm inside from the surface, D means the center of the cake.

The -60 mesh powder in the table is a powder obtained by pulverizing the entire cake with a diameter of 170 mmφ.

Each embodiment will be described below based on FIG.

Example 1 This example is based on the method of this invention, and includes valves K and L.
, D were opened, and the MB2 and RP2 pump groups were used to evacuate from above the preheating zone.At the same time, the valve F was opened and the MB1 and RP1 pumps were used to evacuate the furnace from the flange joint.

At that time, except for the flapper valve 22, all other valves were operated in a closed state.

As a result, the amount of oxygen on the outermost surface of the cake was 980 ppm, which is less than 1000 ppm, and the degree of reoxidation (A/D value in Table 1) on the cake surface was smaller than in Comparative Examples 3 to 5.

Furthermore, since positions C and D showed approximately the same amount of oxygen, it is considered that they were not affected by reoxidation.

Furthermore, the oxygen content of the -60 mesh powder was less than 500 ppm, demonstrating the remarkable effect of the method of the present invention.

Example 2 Similar to Example 1, this is an example belonging to the method of this invention, in which valves K, L, and D were opened and MB2, R was heated from above the preheating zone.
While evacuating with the P2 pump group, at the same time, valve F was fully opened and valve A was half open, and the MB, RP1 pump group was used to evacuate from the flange joint for connecting the furnaces and from below the induction heating zone.

Except for the flapper valve 22, the other valves were fully closed.

In this case, the amount of oxygen on the outermost surface of the cake was 590 ppm, which is close to 500 ppm, and the degree of reoxidation on the cake surface and -
The oxygen content of the 60 mesh powder showed the lowest value among Examples 1-5.

Embodiment 3 This embodiment is based on a conventional vacuum evacuation method, and is an example in which the entire amount of the reduction product gas generated in the shaft furnace is evacuated downward.

That is, valves K, L, E and valve A are opened, all other valves except flapper valve 22 are closed, and the induction heating zone is heated from below by using two vacuum pump groups MB2, RP2 and MB1, RP1 in combination. Exhausted.

As a result, the reoxidation of the deoxidized cake was the worst among the five examples, and even the D position in the center of the cake showed an oxygen amount of 3420 ppm.

Therefore, the oxygen content of -6.0 mesh powder is also 4620p
pm, and the average deoxidation rate was as low as about 70%.

Example 4 Similar to Example 3, this is a vacuum evacuation example based on the conventional example.
In this example, the entire amount of reduction product gas generated inside the shaft furnace was exhausted from above the preheating zone.

That is, valves K, L, D and valve B are opened, all other valves except flapper valve 22 are fully closed, and exhaust is exhausted from above the preheating zone using two vacuum pump groups MB2, RP2 and MB1, RP1 in combination. This is what was done.

As a result, the powder was blown up considerably and the upper tank 2
Steel powder was scattered in the interior of the 6, to the extent that it was partially deposited.

Although the degree of reoxidation of the cake was somewhat improved compared to Example 3, the results were largely the same.

Embodiment 5 This example is also an example using a vacuum evacuation method belonging to the conventional example, but in this example, two vacuum pump groups are used for evacuation from above the preheating zone and from below the induction heating zone. Used separately.

That is, with valves K, L, and D open, pump groups MB2 and R
While exhausting air from above the preheating zone at P2, on the other hand, valve A was opened to simultaneously exhaust air from below the induction heating zone using pump groups MB1 and RP1.

As a result, the reoxidation of the deoxidized cake was significantly improved, but
Nevertheless, the reoxidation has reached around position C, and the amount of oxygen on the outermost surface of the cake exceeds 1000 ppm, and the -60 mesh powder also has more than 500 ppm.
This was insufficient compared to Examples 1 and 2.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view showing an example of a shaft furnace for implementing the method of the present invention, FIG. 2 is a cutaway view of a flange for connecting between furnaces of the shaft furnace, and FIGS. 3 and 4 are They are a XX sectional view and an enlarged sectional view of a flange joint for connecting between furnaces. 1... Powder supply section, 2... Preheating zone,
3...Induction heating zone, 4...Cake processing section, 5...Hopper, 6...Feeder, 7...Preheating furnace main body , 8... Preheating tube, 9... Support rod, 10... Backing member, 11... Laminated mat, 12... Excitation 13... Secondary coil for heating, 14... Fireproof insulation member, 15... Flange joint for connection between furnaces, 16...・Lower tank,
17...Pinch roll, 18...Cake cutter, 19...Shooter, 20...
... Chute guide, 21 ... Packet lifter, 22 ... Flap valve, 23 ...
・Cake cooling chamber, 24... Upper flange joint for fixing the coil, 25... Lower flange joint for fixing the coil, 26... Upper tank, 27...
・Feeler, 30...Furnace support plate, 31...
...Exhaust pipe, 32...Bolt, 33...
... Annular exhaust passage, 34, 34'... Annular cooling water passage, 36, 37, 38... Annular groove,
39... Gas introduction pipe, 40, 40'...
・Cooling water introduction pipe, 41...Notch passage.

Claims (1)

[Scope of Claims] 1. Raw material steel powder to be loaded with carbon is sequentially cut into a shaft furnace maintained in a vacuum atmosphere in which a preheating zone and an induction heating zone are vertically arranged, and a cylinder is heated in the preheating furnace. After forming a shell-shaped sintered cake, the remaining part undergoes deoxidation, decarburization, and sintering reactions in the induction heating furnace.During continuous reduction annealing of the steel powder, reduction products generated inside the shaft furnace. A method for evacuation of a shaft furnace for reduction annealing of steel powder, characterized in that gas is exhausted from a connection point between a preheating zone and an induction heating zone. 2. In a shaft furnace consisting of a preheating furnace and an induction heating furnace arranged vertically in order, the furnaces are connected airtightly, and a concentric annular cooling water passage and an annular exhaust passage are provided in the body, and The annular exhaust passage includes an exhaust pipe connected to the exhaust device and a cutout passage for communicating with the inside of the furnace,
Furthermore, the inner circumferential surface that airtightly and slidably connects the preheating furnace is configured with two annular grooves for housing O-rings and an annular groove for storing grease for inert gas injection sandwiched between the annular grooves. A flange joint for connecting between furnaces.