JPH04325622A - Production of ultralow carbon steel - Google Patents

Abstract

PURPOSE:To rapidly refine an ultralow carbon steel minimal in gaseous impurities, such as nitrogen and hydrogen, in large quantities in vacuum degassing treatment equipment by means of an RH process, a DH process, etc. CONSTITUTION:In an ultralow carbon steel refining method where a molten steel is sucked into a vacuum tank to undergo refining, fine bubbles can be formed by providing one or >=2 lower tank tuyeres on the side wall in the lower part of the vacuum tank and blowing inert gas into a state of compression waves, by which the high speed treatment for the ultralow carbon steel is made possible owing to the increase in the gas-liquid interface.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for quickly and large quantities of steel having extremely low carbon and containing very few gaseous impurities such as nitrogen and hydrogen in vacuum degassing equipment such as RH and DH. be.

0002

[Prior Art] From the viewpoint of increasing demands for high workability for cold-rolled steel sheets through continuous annealing processes and ensuring good plating properties for plated steel sheets, ultra-low carbon steel with a carbon content of 10 ppm to several ppm is suitable as a material. is now required. It is also known that in ferritic stainless steel, corrosion resistance can be dramatically improved by using extremely low carbon (30 ppm or less) and nitrogen (50 ppm or less).

Conventionally, ultra-low carbon steels have been produced using vacuum degassing equipment such as RH and DH. Figure 4 shows the conventional R
It is an explanatory view showing an H degassing device. The molten steel 10 in the molten steel ladle 11 is sucked into the vacuum chamber 8 by inert gas from the gas blowing nozzle 2 provided in the riser pipe 3. The molten steel in the vacuum tank returns to the molten steel ladle through the downcomer pipe 12, during which decarburization and degassing reactions take place.

[0004] These devices usually perform processing at a vacuum level of 1 Torr or less. The mechanism of decarbonization in vacuum degassing equipment is that because the partial pressure of CO in the vacuum chamber is extremely low (approximately 0.002 Torr or less), oxygen and carbon in the molten steel combine to form CO gas, which is discharged as exhaust gas. Carbon concentration decreases. Furthermore, by blowing an inert gas into the molten steel, the following reaction proceeds even at the inert gas bubble interface where the CO partial pressure is zero, so that the decarbonization reaction is promoted. Further, nitrogen and hydrogen are simultaneously removed from the steel as the partial pressure of these gases in the atmosphere decreases.

[Chemical formula 1]

[0005] The decarburization process in the current vacuum degassing equipment is based on the degree of vacuum in the tank and the use of argon gas for circulation of molten steel, and 400 ppm of carbon in molten steel is reduced to about 30 ppm in about 15 minutes.
It has the ability to reduce to pm. However, in order to meet the demand for mass production of ultra-low carbon steel, further decarburization promotion technology is needed, especially at 30p, which reduces the decarburization reaction rate.
It is essential in decarburization treatment below pm.

[0006] In order to further increase the decarburization reaction rate, it is important to increase the reaction interfacial area or to increase the amount of molten steel circulating in the vacuum degasser. However, if a large amount of argon gas is blown in to increase the amount of molten steel circulating, as shown in FIG. 4, the splash 9 in the tank becomes intense and the base metal adhesion 13 makes operation difficult. Furthermore, no matter how large the amount of inert gas blown from the immersion tube is, bubbles in the vacuum chamber concentrate near the riser tube, so it can be said that this is insufficient in terms of effectively utilizing the entire internal volume of the vacuum chamber.

[0007] Therefore, a method of increasing the reaction interfacial area by taking measures in a vacuum chamber to promote degassing has been considered. One method for this is to provide a tuyere on the side wall of the vacuum chamber and blow inert gas into it. In particular, in this case, it is preferable to set the tuyere diameter to 3 mm or less and set the blowing pressure to a high pressure of 20 kgf/cm2 or more. 2-217
It is disclosed in Japanese Patent No. 412. In addition, Japanese Patent Application Laid-Open No. 2-173204 discloses a technique in which an ultrasonic vibrator is installed at the bottom of the vacuum chamber in contact with the molten steel to crush and refine the bubbles already present in the vacuum chamber and generate new bubbles through the cavitation phenomenon. has been disclosed.

[0008]

[Problems to be Solved by the Invention] However, the method described above for blowing inert gas by providing tuyeres on the bottom side wall of the vacuum chamber requires blowing fine air bubbles throughout the vacuum chamber; Therefore, it is recommended to inject high pressure gas through a small diameter tuyere. However, on the other hand, small-diameter tuyeres are prone to clogging, and high-pressure gas equipment has problems in that equipment maintenance costs increase. Furthermore, the method of injecting ultrasonic waves into molten steel has problems that need to be resolved, such as the heat resistance and durability of the vibrator.

[0009]

[Means for Solving the Problems] The present invention is intended to advantageously solve the problems of the prior art, and provides RH, D
In a method for producing ultra-low carbon steel in which molten steel is sucked up into a vacuum tank such as H and smelted, one or more lower tank tuyeres are provided on the side wall of the lower part of the vacuum tank, and inert gas is blown in a dense wave pattern. It is characterized by being immersed.

0010

[Operation] First, one of the features of the present invention, in which the inert gas is blown into the tuyere in the form of dense waves, will be explained. Figure 3 shows the lower tank tuyere 1 installed in the lower part of the vacuum tank, the so-called lower tank.
5 is a cross-sectional view showing an iron skin, and 4 is a refractory. When gas is injected using the conventional method, as shown in FIG. 3, when the gas 15 leaves the molten metal 10 from the lower tank tuyere 1 as bubbles 16, they expand and the surface tension balance is no longer maintained. It breaks at certain places. However, according to the method of the present invention, as shown in FIG. 2, the gas 15 exits from the lower tank tuyere 1 in the form of a compression wave W, so that it becomes fine bubbles and separates into the molten metal. In other words, the pressure is high in dense areas and low in rough areas, so if the pressure amplitude of the compression wave is large enough, the flow is cut off at the tuyeres in rough areas due to a so-called wedge effect, forming fine bubbles. .

In order to carry out the method of the present invention, means for generating compression waves in the gas flow is required. Compression waves are nothing but sound waves, so simply put, a ``whistle'' can be placed in the middle of the pipe to the tuyere. However, in order to reliably make the bubbles smaller at the tuyere, the pressure difference between the density and density needs to be sufficiently large. Therefore, a mechanism is required to efficiently convert the gas flow into compression waves.

The present inventors have developed several devices for generating compressional waves, one example of which is shown in FIG. Gas supply side piping 20 of nozzle 2 and piping 2 leading to lower tank tuyere 1
2 are crossed and combined, and the gas supply side pipe 20 is extended from the crossed portion 23 and closed to form a resonance tube 21. On the other hand, the piping 22 leading to the immersion tube tuyere 1 is extended from the intersecting portion 23 and closed to form a space 24.

In this gas blowing nozzle, the inert gas 15 supplied from the gas supply pipe 20 at supersonic speed is released into a space expanded by a space 24 at the intersection 23 . This causes pressure instability and generates compression waves. At this time, since the resonance tube 21 is provided, a resonance phenomenon determined by the dimensions of the resonance tube 21 occurs, and a strong compressional wave is generated. The resonance frequency f is given by the following equation, where the speed of sound is c [m/s], and the length and diameter of the resonance tube are l and d [m], respectively.

[Math 1]

The size of the bubble becomes smaller as the resonance frequency of the gas increases, but the resonance frequency is usually 10 to 100k.
A frequency of around Hz is desirable. Also, the flow rate of the blown gas is 2 to 3
Mach is desirable.

FIG. 5 shows an RH for carrying out the method of the present invention.
FIG. 3 is a cross-sectional view taken along the riser pipe of the degasser. A lower tank tuyere 1 is provided in the lower part of the vacuum tank, the so-called lower tank 8A, and inert gas is blown into it in the form of compression waves. In this example of the device, the blowing nozzle connects the length of the resonance tube 21 to the piston mechanism 26.
and can be freely changed using an air cylinder 27. Note that the RH degassing device blows lift gas into the riser pipe 3, and the nozzles of this device also blow the lift gas in a coarse wave shape in this device example.

FIG. 6 is a diagram illustrating the implementation of the present invention as viewed from above inside the vacuum chamber. Eight lower chamber tuyeres are provided, and air bubbles are distributed throughout the vacuum chamber. The distance L that the bubbles reach has the following relationship, where V is the gas flow velocity, D is the tuyer diameter, and k is the coefficient.

[Math 2]

Therefore, by changing the blowing flow rate V, the bubble reach distance can be changed. In the gas injection nozzle of the present invention, changing the length l of the resonant tube changes the frequency of compression waves and changes the size of the bubbles, so as mentioned earlier, if l is adjustable, the size of the bubbles and The bubble reach distance varies independently.

[0018]

[Example] 200 ppm of carbon after blowing in a converter
250 tons of molten steel was treated with an RH degasser to which the method of the present invention was applied. There are 8 tuyeres in the lower tank, and the tuyere diameter is 3.5.
The tuyere gas pressure in millimeters is 9 atmospheres (gauge pressure). The argon gas flow rate was 3000 l/min, the air flow Mach number was 2.3, and the compression wave frequency was 20 kHz. In addition, lift gas is injected from the riser tuyeres, but this is a conventional method with an argon gas flow rate of 1500 l/min. The results are shown in Table 1 in comparison with the conventional technique in which argon gas is not blown from the lower tank tuyere. As can be seen, according to the method of the present invention, the decarburization reaction was promoted, and an extremely low carbon steel of 10 ppm or less could be produced in a shorter time than the conventional method.

[Table 1]

[0019]

[Effects of the Invention] According to the present invention, since the inert gas is blown into the lower part of the vacuum chamber in a wave pattern, fine air bubbles can be distributed throughout the vacuum chamber, and molten steel can be rapidly reduced to an extremely low carbon value. It is possible. Furthermore, by making the bubbles finer, splash can be reduced, and the amount of metal deposited in the lower tank can also be reduced.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a gas blowing nozzle for carrying out the gas blowing method of the present invention.

[Fig. 2] A diagram showing the state of bubble generation by the gas blowing method of the present invention.

[Fig. 3] Diagram showing the state of bubble generation by the conventional gas blowing method

[Figure 4] Cross-sectional view of a conventional RH vacuum degassing device

[Figure 5
] Cross-sectional view of a vacuum chamber for carrying out the gas blowing method of the present invention

[Figure 6] A top view of the inside of the vacuum chamber when implementing the present invention

Claims (1)

[Claims] Claim 1: In a method for producing ultra-low carbon steel in which molten steel is sucked up into a vacuum chamber and smelted, a
Alternatively, a method for producing ultra-low carbon steel, which is characterized by providing two or more lower tank tuyeres and blowing inert gas in a dense wave pattern.