JPH01268815A - Vacuum degassing treatment of molten steel - Google Patents

Abstract

PURPOSE:To increase the purity of steel and to reduce the amt. of a material having an undesirable compsn. by separately controlling the blowing pressures of systems for blowing reflux gas into upper and lower gas feeding nozzles fitted to a riser. CONSTITUTION:Plural gas feeding nozzles 8 are fitted to a riser 2a. The moment that vacuum degassing treatment is started the blowing pressures of systems 8a', 8b' for blowing reflux gas into the upper and lower gas feeding nozzles 8a, 8b are separately controlled. For example, the pressure of reflux gas blown into the upper nozzle 8a is made lower and that blown into the lower nozzle 8b is made higher. The purity of steel is increased and the amt. of a material having the undesirable compsn. can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for vacuum degassing treatment of molten steel, particularly to a method for vacuum degassing treatment of molten steel using an RH circulation type degassing device.

<Conventional technology> With the increasing demand for high-purity steel such as high-purity steel, the importance of so-called vacuum degassing equipment, which sucks up molten steel from an immersion tube into a vacuum degassing tank and degass it, is increasing. There is a strong desire to improve this ability.

Among these, RH circulation degassing equipment has two immersion pipes, that is, a rising pipe and a descending pipe, and circulates molten steel by blowing inert gas such as argon gas from the rising pipe.
Achieving this capacity improvement has become a major challenge.

As shown in FIG. 5, the conventional RH circulation type vacuum degassing apparatus depressurizes the inside of the vacuum degassing tank 1 by immersing the rising pipe 2a and the descending pipe 2b in the molten w43 in the ladle 4, thereby removing the molten steel 3. Inert gas is blown into the gas inlet 8 for suction and circulation, and the molten steel 3 is guided into the vacuum degassing tank 1 using the gas flotation blade, and is lowered through the downcomer pipe 2b to circulate and degas the molten steel. It is something to do. 5 is the iron skin of the vacuum degassing tank 1, 6
8 indicates the scattered molten steel, and 8' indicates the gas injection system.

Many proposals have been made to improve the performance of the RH reflux degasser as described above, but these proposals can be broadly classified into the following two types.

■Those that aim to improve degassing processing by increasing the temperature-controlled flow rate (hereinafter referred to as the recirculation flow rate) that passes through the vacuum chamber.

■Promotes the degassing reaction of molten steel in a vacuum chamber to improve degassing efficiency.

Conventional examples of (1) increasing the recirculation amount include means for enlarging the inner diameter of the immersion pipe and means for blowing recirculation gas through an inlet provided near the center of the horizontal section at the lower end of the riser pipe (Japanese Patent Laid-Open No. 51-6).
103) has been proposed.

Furthermore, as a conventional example of (2) promoting the molten steel degassing reaction, a method has been proposed in which inert gas is injected from a stirring gas blowing pipe provided on the wall of the vacuum degassing tank (see Japanese Patent Application Laid-Open No. 73716/1983). has been done.

<Problems to be Solved by the Invention> However, in the conventional examples as described above, it is difficult to sufficiently improve the degassing processing ability.

That is, among the conventional methods described above, the means for enlarging the inner diameter of the immersion tube is limited by the arrangement of other related equipment, and the promotion of the degassing reaction is insufficient even though it requires major remodeling of the equipment.

Furthermore, the method proposed in Japanese Patent Application Laid-Open No. 51-6103 has the problem that the blowing pipe is immersed in molten steel, so the service life is short, the blowing pipe must be replaced more frequently, and the operating rate is lowered.

Furthermore, the method disclosed in JP-A-58-73716 has a problem in that the gas inlet is provided on the wall of the vacuum degassing tank, and therefore the flow of molten steel in the degassing tank is not sufficiently agitated. Ta.

1. Changes in molten steel gas components during vacuum degassing treatment
As an example, FIG. 6 shows the change in carbon concentration in steel when 200 tons of undeoxidized steel is vacuum decarburized. As shown in Figure 6, the degassing reaction in the vacuum degassing tank is generally active from the start of the degassing process to the middle stage of the process, so the degassing rate is controlled by the recirculation flow rate. At the final stage, the degassing reaction in the vacuum degassing tank stagnates, so the degassing rate comes to be controlled by the magnitude of the degassing reaction rather than the recirculation amount.

In other words, among the conventional techniques mentioned above, (2) which increases the recirculation amount is an effective means from the start of degassing treatment to the middle stage of the process, and (2) which promotes the molten steel degassing reaction is an effective means from the middle stage to the end of degassing treatment. However, when applied individually, the effect is relatively small.

The present invention solves the above-mentioned conventional problems, and makes it possible to increase the recirculation flow rate and promote the molten steel degassing reaction over the entire treatment period from the start of vacuum degassing treatment to the end of the treatment by making occasional improvements to existing equipment. The object of the present invention is to provide a method for vacuum degassing of molten steel.

<Means for Solving the Problems> In order to achieve the above object, a method for blowing gas into an RH type degassing device, that is, a riser pipe, as disclosed in Japanese Patent Application No. 159699/1987, which the present applicant previously filed, as a means for increasing the recirculation amount. In a molten steel recirculation degassing device equipped with a downcomer pipe and a plurality of molten steel recirculation gas injection ports in the riser pipe, the riser pipe has at least two reflux gas injection systems, each of which has its own injection pressure. The present invention was completed as a result of repeated experiments on means for controlling the degassing reaction in the vacuum degassing tank and further promoting the degassing reaction in the vacuum degassing tank.

EMBODIMENT OF THE INVENTION Hereinafter, the present invention will be explained based on FIGS. 1 and 2, which correspond to embodiments.

In the one shown in Fig. 1, the riser pipe 2 of the vacuum degassing device
A plurality of gas blowing ports 8 for molten steel circulation are provided in a, and at least two gas blowing systems are provided to the gas blowing ports 8, and the blowing pressure of each of the gas blowing systems is adjusted at the start of the vacuum degassing process. The above gas supply port 8 is controlled individually.
Molten steel 3 in ladle 4 is removed by blowing inert gas from
is guided into the vacuum degassing tank 1 via the riser pipe 2a and the descender pipe 2
b to the ladle 4 for vacuum degassing treatment, and at the stage when the degassing reaction due to the vacuum degassing treatment has stagnated, in addition to blowing inert gas from the gas blowing port 8. Start blowing an inert gas into the molten steel 3 in the vacuum degassing tank 1 from a gas blowing immersion nozzle 10 provided near the center of the bottom of the vacuum degassing tank 1 to perform vacuum degassing treatment. It is characterized by:

In addition, in the case shown in FIG. 2, when the degassing reaction due to the vacuum degassing treatment has stagnated, the submerged nozzle 10 for gas injection provided near the center of the bottom of the vacuum degassing tank 1 shown in FIG. Instead, the vacuum degassing process is performed by starting blowing an inert gas onto the surface of the melt 113 in the vacuum degassing tank 1 from a gas blowing lance 11 installed in the vacuum degassing tank 1. There is.

<Function> Next, the function will be explained with reference to FIGS. 1 and 2. 1st
As shown in FIG. 2 and FIG. 2, at the same time as the start of the vacuum degassing process, the reflux gas blowing system 8a' to the upper gas supply port 8a of the plurality of gas supply ports 8 provided in the riser pipe 2a and the lower gas supply port 8a' The blowing pressure of the recirculation gas blowing system 8b' to the gas blowing port 8b is individually controlled.

For example, by lowering the pressure at the upper gas inlet 8a and increasing the pressure at the lower gas inlet 8b, the riser pipe 2
The reach distance of the inert gas blown into the molten steel in the horizontal cross section in a is adjusted respectively, and the bubbles are combined as a whole into the riser pipe 2a.
This increases the flow rate of the molten steel due to the rise of bubbles.

In the case shown in FIG. 1, when the degassing reaction due to the vacuum degassing treatment has stagnated, the inside of the vacuum degassing tank 1 is injected from the immersion nozzle 10 installed near the center of the bottom of the vacuum degassing tank 1. The purpose is to start blowing inert gas into the molten steel 3 to promote the degassing reaction of the molten steel 3 in the vacuum degassing tank 1. 7 shows a gas blowing system to the immersion nozzle 10.

In addition, in the case shown in FIG. 2, when the degassing reaction due to the vacuum degassing treatment has stagnated, the gas injection lance 11 installed in the vacuum degassing tank 1 is connected to the vacuum degassing tank 1.
The purpose is to start spraying an inert gas onto the molten steel 3 inside to promote the degassing reaction.

Reference numeral 9 indicates a gas supply system to the lance 11, and 12 indicates a sealing portion of the lance 11.

<Example> Below, a preferred example of the present invention will be described based on the drawings.

As shown in Figure 1, the riser pipe 2a is provided with two systems, an upper gas inlet 8a and a lower gas inlet 8b, each of which controls the pressure individually. Pressure from O to 9
．． A recirculation gas blowing system 8a' which can be controlled within a range of 9 kg/cd and a recirculation gas blowing system 8b whose source pressure can be controlled within a range of 30 to 50 kg/d are connected.

Further, two porous plugs 10 are installed near the center of the bottom of the vacuum degassing tank 1 as immersion nozzles.

An experiment was conducted using 200 tons of undeoxidized molten steel for vacuum degassing treatment. At the start of the vacuum degassing process, argon gas is supplied at 1500 Ml/m from the upper gas inlet 8a.

A vacuum degassing process was performed by blowing 100 ONZ/m from the lower gas blowing port 8b.

Then, after the middle stage of decarburization when the decarburization reaction due to the vacuum degassing process has stagnated, that is, in the final stage of the decarburization process after 7 minutes from the start of the vacuum degassing process, it is installed near the center of the bottom of the vacuum degassing tank 1. total amount of 10 from the two porous plugs 10
Vacuum degassing treatment was performed by blowing argon gas at a rate of 0ONZ/m.

The transition results of the carbon concentration in steel due to the vacuum degassing treatment of the above-mentioned method of the present invention are referred to as the method 1 of the present invention, and the conventional method shown in Fig. 5 and the gas injection system to the riser pipe are 2 systems and the pressure is controlled individually. Figure 3 also shows a comparative method that only requires As is clear from FIG. 3, according to the method of the present invention, the decarburization rate is significantly improved compared to the conventional method as well as the comparative method, and the decarburization rate can be improved throughout the entire period.

Note that the number of blowing systems for the recirculation gas is not limited to two, and three or more systems may be used to more precisely control the pressure.
The pressure control range does not need to be limited to the above range, and can be determined according to the internal diameter of the riser pipe of the vacuum degassing device.Also, the immersion nozzle installed in the vacuum degassing tank is not limited to porous plugs. (The type and number of thin tube collection nozzles can be selected as appropriate, even if it is just a pipe, as long as it can withstand use.)

11■1 As shown in Fig. 2, a lance 11 for spraying gas onto the molten steel bath surface of the vacuum degassing tank 1 is installed, and a gas blowing port 8a and a 8b is provided, and each has a blowing source pressure of θ to 9.9 kg/c.
A recirculation gas blowing system 8a' that can control the blowing source pressure within a range of 30 to 50 kg/cd is connected to a recirculating gas blowing system 8b' that can control the blowing source pressure within a range of 30 to 50 kg/cd.

An experiment was conducted using 200 tons of undeoxidized molten steel for vacuum degassing treatment. At the same time as the start of the vacuum degassing process, argon gas was blown at 1500 Ml/m from the gas inlet 8a and 100 ONZ/am from the gas inlet 8b to perform the vacuum degassing process.

Then, after 7 minutes from the start of processing, it melts inside the vacuum chamber 1! 143
Lance 1 installed at a distance of 1.5 m from the surface of
Vacuum degassing treatment was carried out by blowing argon gas at an initial velocity of 580 m/s at a rate of about 36 ONrrf/hr.

The results of the change in carbon concentration in steel due to the vacuum degassing treatment of the method of the present invention are summarized as Method 2 of the present invention, which combines the conventional method and a comparative method in which the gas injection system into the riser pipe is controlled individually as two systems. This is shown in Figure 4.

As is clear from FIG. 4, method 2 of the present invention also improves the degassing rate throughout the entire degassing treatment period. In Example 2, a Laval nozzle lance with a throat diameter of 10φ was used. In this way, the steel bath surface inside the vacuum chamber 1 is stirred and the melt is scattered! In order to increase A6, it is more effective to increase the flow velocity on the bath surface, and as a lance nozzle, it is desirable to use a Laval nozzle whose initial velocity can be higher than the speed of sound.

<Effects of the Invention> As explained above, according to the present invention, the degassing rate can be improved over the entire period of vacuum degassing treatment, the vacuum degassing treatment time can be shortened, and the gas components in a can be reduced. It is possible to reduce the

In addition, by shortening the vacuum degassing treatment time and lowering the steel tapping temperature in the converter, the life of the refractories in the converter and ladle can be extended, and productivity can be improved.Furthermore, by reducing the gas content in the steel in a short time, It is possible to improve the purity of steel and reduce the number of materials with defective components by reducing the variation in the components after treatment, so it has great industrial utility.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a vacuum degassing apparatus according to a first embodiment of the present invention, FIG. 2 is a sectional view showing a vacuum degassing apparatus according to a second embodiment of the present invention, and FIGS. The figure is a graph showing the change in carbon concentration (ppm) in steel during degassing treatment.
The figure is a sectional view showing a conventional vacuum degassing apparatus, and FIG. 6 is a graph showing the change in carbon concentration (ppm) in steel during degassing treatment by the conventional method. DESCRIPTION OF SYMBOLS 1... Vacuum degassing tank, 2a... Ascending pipe, 2b... Descending pipe, 3... Molten steel, 4... Ladle, 8a, 8b... Gas supply port, 8a', 8b'...Gas blowing system, 10...Immersed nozzle for gas blowing, 11...Lance for gas blowing. Patent applicant Kawasaki Steel Corporation Figure 1 Figure 2 Figure 3 Figure 4 Degassing treatment time (minutes) Figure 5 Figure 6 De-gassing treatment time (minutes)

Claims (1)

[Claims] 1. A plurality of gas injection ports for molten steel circulation are provided in the riser pipe of the vacuum degassing device, and at least two gas injection systems are provided to the gas injection ports, and as soon as the vacuum degassing process starts, The molten steel in the ladle is guided into the vacuum degassing tank via the riser pipe by blowing inert gas through the gas blowing port while individually controlling the blowing pressure of each of the gas blowing systems mentioned above. reflux into the ladle through a vacuum degassing treatment,
At the stage when the degassing reaction due to the vacuum degassing treatment has stagnated, in addition to the inert gas injection from the gas injection port, a submerged nozzle for gas injection installed near the center of the bottom of the vacuum degassing tank is used. A method for vacuum degassing treatment of molten steel, characterized in that the vacuum degassing treatment is performed by starting the injection of an inert gas into the molten steel in the vacuum degassing tank. 2. Inert gas is blown onto the molten steel surface in the vacuum degassing tank from a gas blowing lance installed inside the vacuum degassing tank instead of the immersed nozzle for gas blowing installed near the center of the bottom of the vacuum degassing tank. 2. The method for vacuum degassing treatment of molten steel according to claim 1, wherein the vacuum degassing treatment is carried out after starting the deposition.