JPH04325621A - Method for smelting extremely low carbon steel - Google Patents

Abstract

PURPOSE:To provide a method for quickly mass-smelting a steel having extremely low carbon and extremely low contents of gaseous impurities of nitrogen, hydrogen, etc., in a vacuum degassing treatment device of RH, DH, etc. CONSTITUTION:In the method for smelting the extremely low carbon steel by executing refining while sucking up the molten steel into a vacuum vessel, inert gas is injected like compression waves from one or two or more of immersion pipe tuyeres in the vacuum vessel. Further, injecting gas pressures in two or more of the immersion pipe tuyeres are made in two or more different steps at high and low, and by changing bubble arrival distance in each tuyere, fine bubbles can be formed and further, since the bubble arrival distance can be adjusted, the high speed treatment of the extremely low carbon steel can be attained with the increases of the circulating molten steel flow rate and 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. Molten steel 10 in molten steel ladle 11 is sucked into vacuum chamber 8 by inert gas from gas blowing nozzle 2 provided in 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. To solve these problems, as disclosed in Japanese Patent Application Laid-Open No. 2-213410, inert gas is blown into the lower side wall of the degassing tank through a plurality of small-diameter tuyeres to provide effective stirring to the molten steel and increase the reaction interface area. There is a way to blow in without clogging.

Furthermore, as disclosed in Japanese Patent Application Laid-Open No. 2-80507, a plurality of gas blowing tuyeres with different diameters are provided in the immersion pipe to control the distance that bubbles reach and improve the ability to circulate the gas, that is, the gas lift force. There is a method to increase the reflux amount. Furthermore, as disclosed in Japanese Unexamined Patent Publication No. 2-173205, an ultrasonic vibrator is provided around the immersion tube near the gas injection tuyere of the immersion tube to make the blown inert gas finer and improve the gas lift effect. There is also.

[0008]

[Problems to be Solved by the Invention] However, in the conventional method, when making the diameter of the blown gas bubbles finer, it is necessary to either reduce the diameter of the blowing tuyere or blow the blown gas under high pressure.
Either the injected gas can be atomized using ultrasonic waves, etc. If the diameter of the blowing tuyere is reduced to 3 mm or less, although it becomes finer, stable blowing over a long period of time is difficult due to problems such as nozzle clogging. In addition, the blowing pressure increases as the diameter becomes smaller, leading to an increase in equipment costs due to higher pressure in the piping. Furthermore, when blowing bubbles into fine particles using ultrasonic waves, there are problems to be solved, such as the heat resistance 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
A method for producing ultra-low carbon steel in which molten steel is sucked up into a vacuum tank such as H for scouring, and is characterized by blowing inert gas in a dense wave form from one or more immersion tube tuyeres of the vacuum tank. do. In addition, the blowing gas pressure of two or more immersion tube tuyeres can be adjusted to high or low.
It is characterized by having different stages or higher stages, and changing the bubble reach distance depending on the tuyere.

0010

[Function] Figure 3 shows the immersion pipe tuyere 1 installed on the side wall of the riser pipe.
4 is a cross-sectional view showing a refractory, and 5 is an iron skin. When gas is blown using the conventional method, as shown in FIG. 3, when the gas 15 leaves the immersion tube tuyere 1 into the molten metal 10 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 immersion tube 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, and FIG. 1 shows an immersion tube section to which one example of the devices is applied. As shown in FIG. 1, orthogonal circular pipe nozzles are used as inert gas injection nozzles for reflux, and compression waves are generated in the blown gas stream by blowing gas at several atmospheres or more. That is, the gas supply side piping 20 of the nozzle 2 and the piping 22 leading to the immersion tube tuyere 1 are crossed and connected, and the gas supply side piping 20 is connected to the crossed portion 23.
The resonance tube 21 is further extended and closed. 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. Furthermore, in the conventional method, as the flow rate from the tuyere of the immersion tube is increased, the effect of improving the decarburization reaction rate becomes almost nonexistent after a certain point. This is thought to be due to the fact that bubbles tend to collide and coalesce and become larger, and the so-called air curtain phenomenon occurs, causing the bubbles to blow upward from the tuyere without reaching the inner part of the tuyere. .

On the other hand, the above-mentioned problem can be solved by the method of the present invention in which the pressure of the blown gas at two or more tuyeres of the immersion tube is varied in two or more levels of high and low levels. In other words, FIG. 5 is a view of the riser pipe viewed from above, and FIG. 6 is a cross-sectional view taken along line A-A'. It is being This prevents air bubbles from concentrating in one place within the riser pipe. The reach distance L of the bubble is determined by the gas flow velocity being V, the tuyer diameter being D,
When the coefficient is k, the relationship is as shown in the following equation.

[Math 2]

Therefore, by providing the low-pressure side blowing pipe 26 and the high-pressure side blowing pipe 27 and changing the flow velocity V, the bubble reach distance can be changed. In the gas blowing nozzle of the present invention, changing the length l of the resonant tube changes the frequency of compression waves and changes the bubble size, so if l is adjustable, the bubble size and bubble reach distance are independent. can be changed to

By adjusting the air bubble reach distance of each of the plurality of nozzles in this way and suppressing the air curtain phenomenon, it is possible to increase the flow rate of circulating molten steel and increase the gas-liquid interface by making the bubbles finer, thereby producing ultra-low carbon steel of several ppm. This enables high-speed processing. This method can be applied to immersion tubes such as RH type vacuum degassing equipment and DH type vacuum degassing equipment, which are well known as vacuum degassing furnaces.

[0018]

【Example】

Example 1 Molten steel 25 with a carbon content of 400 ppm that has been blown in a converter
0t was treated with an RH degasser to which the method of the present invention was applied. The diameter of the dip tube is 600 mm, and the tuyere is 3.5
There are 8 pieces of millimeter diameter. The tuyere gas pressure was 7 atmospheres (gauge pressure), and argon gas was blown at 2500 l/min. The airflow Mach number is approximately 2, and the compression wave frequency is 20kHz.
It is. The results are shown in Table 1 as Example 1 along with a comparative example in which the gas flow was not made into a corrugated shape.

[Table 1]

Example 2 The experiment was carried out under the same conditions as in Example 1, except that the blown gas pressure was changed to two levels, high and low, using the tuyere. That is, the eight tuyeres were divided into four tuyeres each, and the pressure was set at 5 atm on the low pressure side and 9 atm on the high pressure side. The flow rate is Mach 1.7 on the low pressure side and Mach 2 on the high pressure side.
．． It is 3. The results are shown in Table 1 as Example 2. As shown in Table 1, when secondary smelting of molten steel is performed using this method, the recirculation flow rate of molten steel increases and the blown gas becomes finer, thereby promoting the decarburization reaction and reducing the amount of molten steel to 10 ppm in a shorter time compared to the conventional method. We were able to produce the following ultra-low carbon steel.

[0020]

[Effects of the Invention] As described above, according to the present invention, it is possible to uniformly blow fine air bubbles in the cross-sectional direction of the immersion tube than in the immersion tube of a vacuum degassing tank, thereby increasing the circulation amount of molten steel and inert gas. By increasing the gas-liquid interface area, it is possible to quickly reduce the carbon content of molten steel to extremely low carbon. Furthermore, by making the bubbles finer, it is possible to reduce the amount of adhesion to the walls of the vacuum chamber, making it possible to stably produce ultra-low carbon steel.

[Brief explanation of the drawing]

[Fig. 1] A cross-sectional view of a riser pipe equipped with a device for applying 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
] A top view of a riser pipe equipped with a device for applying the gas blowing method of the present invention

[Figure 6] A-A' sectional view in Figure 5

Claims (2)

[Claims] Claim 1: In a method for producing ultra-low carbon steel in which molten steel is sucked up into a vacuum tank and smelted, an inert gas is blown into the vacuum tank in a dense wave form from one or more immersion tube tuyeres. Characteristic ultra-low carbon steel melting method. 2. The ultra-low temperature control device according to claim 1, wherein the blowing gas pressure of two or more tuyeres of the immersion tube is varied in two or more levels of high and low levels, and the air bubble reach distance is varied depending on the tuyeres. Carbon steel melting method.