JPH0598340A - Method and apparatus for producing extremely low carbon steel - Google Patents

Abstract

PURPOSE:To provide a method and an apparatus, in which decarbonize-treating time can be shortened at the time of executing the decarbonizing treatment of molten steel by using a vacuum degassing apparatus. CONSTITUTION:The method for producing an extremely low carbon steel is characterized in that after the pressure in the vacuum vessel reaches to <=20Torr during decarbonizing treatment before deoxidizing treatment, the pressure therein is returned back to >= the starting pressure of return-back + 5Torr and <=200Torr and again reduced to <=20Torr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for producing ultra low carbon steel.

[0002]

2. Description of the Related Art A vacuum degassing apparatus mainly comprises a vacuum chamber and a dipping pipe connected to a lower portion of the vacuum chamber for dipping in molten steel to be treated. As a typical example of this apparatus, an RH type vacuum degassing apparatus (circulation type vacuum degassing apparatus) in which an immersion pipe is divided into an ascending pipe for rising molten steel and a descending pipe for descending is known. By vacuum degassing, secondary refining of molten steel such as decarburization, deoxidation, dehydrogenation or denitrification of molten steel is performed.

In recent years, in cold rolled materials and electromagnetic materials, mainly in order to improve productivity by greatly speeding up the refining process, or to meet the demand for materials having excellent workability,
The need for ultra low carbon steel production is increasing. Generally, when melting ultra-low carbon steel, first decarburize as much as possible in a temporary refining furnace (converter, open furnace, electric furnace, etc.), and then vacuum until the desired carbon concentration is achieved. Decarburization is performed by using the degasser.

The final carbon concentration obtained by this vacuum degassing apparatus depends on the feed rate of unreacted molten steel in the ladle into the vacuum tank (hereinafter referred to as molten steel reflux rate), the molten steel stirring status in the vacuum tank. Determined by pressure change and decarburization treatment time. For this reason, various proposals have been made regarding methods for accelerating the decarburization reaction. First, regarding molten steel recirculation velocity, Japanese Patent Application Laid-Open No. 1-1
As proposed in Japanese Patent No. 32715 and Japanese Patent Laid-Open No. 64-79317, it has been attempted to increase the molten steel recirculation velocity by improving the circulating gas injection method or enlarging the dip pipe cross-sectional area. It was

Next, in order to perform sufficient stirring in the vacuum chamber,
In addition to a device (Japanese Patent Laid-Open No. 57-110611) in which a gas blowing tuyere is provided at a position between the opening ends of the immersion tube inside the tank of the RH vacuum tank, an inert gas is blown into an unspecified portion of the vacuum tank. A method in which a tuyere is installed (Japanese Patent Laid-Open No. 58-73716) and a method in combination with a linear motor (Japanese Patent Laid-Open No. 2-1)
No. 41521) has been proposed.

Regarding the pressure change in the vacuum chamber,
Decompression rate is 760 to 1 Torr on average 120 Torr
/ Min or more (Japanese Patent Laid-Open No. 60-63311) has been proposed. Further, it is a known fact that the final carbon concentration can be lowered by extending the decarburization treatment time.

In addition to the method described above, a method of blowing CO ₂ gas into molten steel during decarburization treatment (Japanese Patent Laid-Open No. 56-44711) has been proposed as a method of chemically promoting the reaction.

[0008]

However, the conventional methods are not always sufficient in terms of cost and quality. First, regarding the technique for increasing the molten steel recirculation velocity, the above-mentioned method has almost reached the limit, and there is almost no room for improvement.

Next, in the method of increasing the molten steel stirring force by blowing gas into the vacuum chamber, the amount of splash generated by the gas blowing increases and the amount of metal that adheres to the inner wall of the vacuum chamber also increases. In addition to being an obstacle when the alloy for adjusting the composition is added, the degree of vacuum on the surface of the molten steel in the vacuum tank is deteriorated due to the development of the metal, and the decarburization rate is reduced, which is not preferable.

Furthermore, the method of combining linear motors is not preferable in terms of cost because the linear motor device itself is expensive. Then, the average depressurization rate is 120
Although the method of setting Torr / min or more is effective in promoting decarburization in the first half of the treatment, the problem that the decarburization rate sharply decreases during the so-called decarburization stagnation period in the latter half of the treatment has not been solved.

In addition, the extension of the treatment time causes an increase in the molten steel temperature drop during the secondary refining treatment, so that it is necessary to raise the blow-off temperature of the primary refining furnace, which results in the cost of refractory and alloys for component adjustment. It wasn't always an effective way to go up. Finally, the method of promoting decarburization by blowing CO ₂ is not preferable for the same reason as the extension of the treatment time because the temperature drop amount during the decarburization treatment increases because C + CO ₂ = 2CO 3 is an endothermic reaction. ..

[0012]

The present invention has been made in order to advantageously solve the above-mentioned problems of the prior art. (1) Product carbon concentration of 40 by vacuum degassing method.
In the decarburization treatment for melting steel of less than ppm, during the decarburization treatment before the deoxidation treatment, after the pressure in the vacuum tank reaches 20 Torr or less, at least once, the re-compression start pressure + 5T
Re-pressurize from orr to 200 Torr or less and re-apply 20T
Ultra-low carbon steel melting method characterized by decompressing to less than orr, (2) Automatically controlling the vacuum tank internal pressure based on the vacuum tank internal pressure information during decarburizing before deoxidation (1) The method for melting ultra-low carbon steel according to (1), (3) the vacuum tank and the immersion pipe installed at the lower end thereof are immersed in the molten steel in the ladle to reduce the pressure in the vacuum tank, and to produce a vacuum. In a vacuum degassing device for degassing molten steel by sucking the molten steel into the tank and exposing the molten steel to a decompressed atmosphere, a pressure control tank connected to the vacuum tank or the exhaust system pipe, and this pressure control tank The gist is an apparatus for melting ultra-low carbon steel, which is characterized in that a pressure control valve in a vacuum tank is provided in a pipe in the middle of connecting a tank to a vacuum tank or an exhaust system pipe.

[0013]

The present invention will be described below with reference to the drawings.
This will be described in more detail. FIG. 1 is a sectional view schematically showing an example of the present invention. Two dip pipes 2 are provided in the lower part of a vacuum tank 1 connected to a vacuum exhaust system such as a steam ejector or a vacuum pump via a vacuum exhaust port 10, and the dipping pipes 2 are provided in a molten steel 4 to be treated in a ladle 3. The dip tube is submerged. By evacuating the inside of the vacuum chamber, the molten steel 4 is sucked up into the vacuum chamber 1, and the circulating gas is blown from one of the dipping pipes 2 through the circulating gas pipe 7 and the dipping pipe tuyere 8. Vacuum degassing such as decarburization and dehydrogenation is performed by continuously circulating the gas, and the vacuum chamber 1
The composition of the molten steel is adjusted by introducing the alloy for adjusting the composition through the alloy injection port 9 provided on the side wall of the steel.

When decarburization treatment is carried out by using an RH type degasser, the [C] content is usually 10 to 15 minutes after the treatment is started:
At 20 to 50 ppm, the so-called decarburization stagnation occurs, in which the decarburization reaction rate rapidly decreases. As a result of investigating the cause of the decarburization stagnation, the present inventors have revealed that the cause of the decarburization stagnation is that the pressure change in the vacuum chamber is stagnant. With respect to the capacity of a normal vacuum exhaust system, as the degree of vacuum increases (the pressure decreases), the total exhaust amount decreases, so that it becomes difficult to secure a necessary decompression rate in a region of 20 Torr or less. Therefore, once the degree of vacuum is reduced to 20 Torr or less, the pressure is restored to the pressure at which the decompression starts +5 torr or more, and the pressure is reduced by exhausting it again to secure the pressure change rate. It was found that decarburization up to carbon concentration was possible.

The present invention is effective in promoting the decarburization rate during the decarburization stagnation period. Therefore, the steel types to be melted are limited to those having a product carbon concentration of 40 ppm or less. Here, in order to decarburize in the region where the carbon concentration in the molten steel is 50 ppm or less, it is necessary to set the pressure in the tank to 50 Torr or less, so it is desirable to restore the pressure in the vacuum tank within the range of 50 Torr or less. .. According to the investigation conducted by the present inventors, the decarburization rate is increased when the pressure inside the vacuum chamber at the start of the pressure recovery is adjusted to the range of 1 to 10 Torr and the pressure inside the vacuum chamber after the pressure recovery is adjusted to the range of 10 to 30 Torr. Was most effective in

Further, if the recompression level (pressure difference between the start of recompression and the pressure after recompression) is 5 Torr or more, the intended purpose can be achieved, but if the recompression level is small, the required pressure change speed is secured. Since the time that can be set is shortened, the recompression level is 10To.
It is desirable to be rr or more. However, since these values depend on the relationship between the exhaust gas capacity of the degassing device and the total amount of gas, it goes without saying that the optimum pressure for starting re-compression and the pressure after re-compression differ depending on the degassing device. .. Furthermore, even with the same device, the leak gas amount of the total gas amount changes depending on the condition of the device (flange tightening condition, number of times the tank is used), and the generated gas amount also changes depending on the carbon concentration and oxygen concentration in the molten steel. In addition, in the case of a degasser that employs a steam ejector type vacuum exhaust device, the exhaust capacity changes depending on the cooling water temperature of the condenser. Changes.

Further, when the pressure in the vacuum chamber exceeds 200 Torr, the recirculation is stopped in the RH type degassing apparatus, which makes it difficult to continue the treatment. Therefore, the pressure after the recompression is at most 200 Tor.
rr or less. In addition, as shown in FIG. 1, the means for restoring the pressure is such that the vacuum tank internal pressure control valve 11
Is provided and the exhaust is temporarily stopped, so that the pressure in the tank rises with the passage of time.

Further, as shown in FIG. 2, the pressure recovery tank 1
If the pressure control valve 11 in the vacuum chamber is provided in the pipe connecting from 2 to the vacuum chamber or the exhaust system pipe, and the pressure in the vacuum chamber is adjusted by the operation of the valve, the time required for recompression can be shortened. Because of this (the decarburization rate in the repressurization process is not very high), rapid decarburization is effective. Here, since nitrogen gas, which is advantageous in terms of cost, is normally used as the gas for recompression, a special tank for decompression is provided separately as shown in FIG. Active gas, etc.
It is preferable to restore the pressure with a gas that does not pose a quality problem.
In such a case, it is possible to cope with this by installing the pressure adjusting tank 14 in addition to the normal recompression tank. However, in the ordinary melting of ultra-low carbon steel, the oxygen concentration in the molten steel is maintained at 250 ppm or more during the decarburization treatment,
It is a known fact that the increase in nitrogen concentration in molten steel is generally slight even if the pressure is restored by using nitrogen gas, because oxygen prevents nitrogen from penetrating into the molten steel. Needless to say, it is advantageous to properly use the devices shown in FIGS. 2 and 3 according to the type of steel to be melted.

Further, as shown in FIG. 4, the operation of the pressure control valve 11 in the vacuum chamber is controlled by the sensor 1 for detecting the pressure in the vacuum chamber.
By automatically controlling with the information obtained from No. 5, it becomes possible to control the valve operation timing and the range of valve opening and recuperation pressure under the optimum conditions according to the exhaust system capacity, and accelerate the decarburization speed. It is advantageous in that the effect can be efficiently enjoyed. Here, it goes without saying that the optimum combination of the pressure in the vacuum chamber at the time of starting the pressure recovery and after the pressure recovery is changed as described above, so that it is more effective if it is controlled by combining artificial intelligence and the like. ..

Further, even after the pressure is restored once, if the pressure is reduced to a certain extent, the pressure change becomes stagnant again. Therefore, it is effective to repeat the pressure restoration again. Here, it goes without saying that it is effective to provide a plurality of pressure-recovery tanks when the gas supply system capacity of the pressure-recovery tank is insufficient.

[0021]

[Example] In the RH-type vacuum degassing apparatus of the apparatus shown in FIG. 2, when performing decarburization treatment of molten steel, the rate of change in the pressure in the vacuum tank was changed during the decarburization stagnant period to determine the final carbon concentration. Tested to change. The operating conditions (common to the conventional and the present invention) at this time are: -carbon concentration before treatment: 370 ppm-amount of molten steel: 310 Ton-immersion pipe inner diameter: 650 mm-circulating gas flow rate: 2000 N1 / min gas type: argon-decarburization treatment time : 20 min.

FIG. 5 shows the change with time in the vacuum chamber pressure at this time. In the method of the present invention, 7 minutes after starting the treatment, 12
Minute, 17 minutes 3 times, the pressure in the vacuum chamber is 1.5T each
When the pressure reached to orr, the pressure in the vacuum chamber was restored to 20 Torr. As shown in FIG. 6, by performing the decarburization treatment with the method and apparatus of the present invention, the decarburization rate is accelerated particularly in the latter half of the decarburization treatment as compared with the conventional method, and a steel having a lower carbon concentration can be easily produced. Could be melted.

[0023]

As described above, according to the present invention, the decarburization treatment is performed in a state where the pressure change rate in the vacuum chamber is secured, and as a result, the decarburization stagnation phenomenon which is usually recognized is not generated. , Can decarburize to a desired carbon concentration.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an example of an embodiment of the present invention.

FIG. 2 is a diagram showing an example of the arrangement of pressure control valves for controlling the pressure in the vacuum chamber.

FIG. 3 is a diagram showing an embodiment for adjusting the pressure in the vacuum chamber, in which a tank for exclusive use of pressure restoration is provided separately from a normal pressure restoration tank.

FIG. 4 is a diagram showing an embodiment for automatically controlling the operation of the pressure control valve in the vacuum chamber with information from a sensor that detects the pressure in the vacuum chamber.

FIG. 5 is a diagram showing the relationship between the pressure in the vacuum chamber and the elapsed time after the start of processing.

FIG. 6 is a diagram showing the relationship between the carbon concentration in molten steel and the elapsed time after the start of treatment.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Naoto Tsutsumi 5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Co., Ltd. Nagoya Steel Works

Claims (3)

[Claims] 1. Product carbon concentration: 4 by vacuum degassing method
In the decarburization treatment for smelting steel of 0 ppm or less, during the decarburization treatment before the deoxidation treatment, after the pressure in the vacuum tank reached 20 Torr or less, at least once, the re-compression start pressure +5
Restore the pressure from above Torr to below 200 Torr, and re-apply 20
A method for melting ultra-low carbon steel, characterized in that the pressure is reduced to not more than Torr. 2. The method for melting ultra-low carbon steel according to claim 1, wherein the internal pressure of the vacuum chamber is automatically controlled based on the internal pressure information of the vacuum chamber during the decarburizing process before the deoxidizing process. 3. A vacuum chamber and a dip tube installed at its lower end,
In a vacuum degassing device for degassing molten steel by depressurizing the vacuum tank while being immersed in molten steel in a ladle, sucking molten steel in the vacuum tank, and exposing the molten steel to a decompressed atmosphere,
A pressure control tank connected to a vacuum tank or an exhaust system pipe, and a pole in which a pressure control valve in the vacuum tank is provided in a pipe in the middle of connecting the pressure control tank to the vacuum chamber or the exhaust system pipe. Low carbon steel melting equipment.