JPS5952684B2 - Secondary refining method of molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a secondary refining method for refining molten steel discharged from a converter or the like into a ladle or the like outside the furnace.

Conventionally, molten steel that has been primarily refined in a converter or electric furnace is tapped from the furnace into a ladle, where alloys and deoxidizing agents are added to the molten steel to remove impurities such as oxygen and to achieve the desired purpose. Secondary refining is performed outside the furnace to adjust the composition of steel.

Various methods have been proposed as secondary refining methods.

These methods include a vacuum degassing method that uses a vacuum degassing device to prevent the molten steel from being oxidized by the atmosphere during processing, and simultaneously actively removes gases that are impurities in the molten steel; There are two methods: a molten steel stirring method that simply uses gas without using any equipment, and a molten steel stirring method.

The vacuum degassing method is represented by the RH reflux degassing method or the DH suction degassing method, and is an excellent secondary refining method, but it requires the use of large-scale vacuum equipment. , the equipment becomes large-scale, and equipment costs and processing costs become expensive.

For this reason, vacuum degassing methods have been implemented especially for the purpose of producing high-grade steel materials.

On the other hand, the molten steel stirring method is used for lower-grade steel materials compared to the vacuum degassing method, and is a so-called inert gas stirring method in which an inert gas is blown into the molten steel and the molten steel is stirred using floating blades of gas bubbles. A representative example of this is the method, which has low processing costs.

This method of stirring molten steel using an inert gas does not require any special equipment, as it only uses a gas blowing nozzle or porous plug, and the processing method is simple. This method has many characteristics as a method.

However, this method has weak stirring power compared to the vacuum degassing method mentioned above, and due to the principle of stirring, the interface between the slag and molten steel is most strongly stirred. The oxidizing slag that flows into the ladle with the molten steel has a tendency to react with the molten steel, which is difficult to prevent, so that during processing, for example, easily oxidized alloying elements such as aluminum can react with the slag and cause The disadvantage is that the concentration changes drastically and it is difficult to adjust it to the desired concentration.

Another problem is that the oxygen concentration at the end of the treatment does not drop sufficiently due to oxygen flowing in from the slag.

Furthermore, when the blowing gas flow rate is increased to strengthen the stirring power, the slag layer on the surface of the molten steel bath at the gas bubble release position is removed to the surrounding area, and the molten steel comes into direct contact with the atmosphere, causing the molten steel to be heated by the oxygen in the atmosphere. It has the disadvantage of being oxidized and reducing refining efficiency.

As a method to improve the above-mentioned drawbacks of the method of stirring molten steel using inert gas, as shown in FIG. Then, as shown in 4, air bubbles are blown into the molten metal 5 in the ladle 1, and the slag layer 7 is penetrated from the top into the molten metal in the refractory cylinder 6 ladle, so that the lower end of the cylinder 6 becomes the molten steel 5. A method has been proposed in which the molten steel surface without a slag layer is exposed inside the cylinder, and the blown gas escapes from the exposed molten metal surface inside the cylinder 6 into the atmosphere. .

According to this method, mixing and agitation between the two phases that occur when the blown gas bubbles pass through the interface between the slag and the molten steel can be prevented, so that reactions between the two phases can be effectively prevented.

Further, since the gas phase in the cylinder in contact with the molten steel bath becomes a non-oxidizing atmosphere due to the dissipation of the blown gas from the molten steel, reactions between the atmosphere and the molten steel can be prevented.

As mentioned above, the method shown in Figure 1 is an excellent method for preventing reactions between molten steel and slag or molten steel and the atmosphere, but it has no effect on improving stirring power compared to the conventional method, and is rather The immersed refractory cylinder attenuates the kinetic energy of the upward flow of molten steel caused by gas bubbles, resulting in problems such as reduced stirring power, and also makes it difficult to add alloying materials during processing, making vacuum degassing difficult. There are some drawbacks to this method.

The present invention is applicable to molten steel in secondary refining, where the purpose is not to degas H, M, etc. in molten steel, but to promote deoxidation reaction and equalize alloy component concentration and temperature by the stirring effect of molten steel. To provide a secondary refining method that can prevent reactions between slag or molten steel and the atmosphere, achieves a sufficiently low oxygen concentration after treatment, is excellent in deoxidizing treatment, has a good yield of additive alloys, and is easy to adjust components. With the goal.

The present inventors tested and investigated various stirring methods for secondary refining of molten steel, and invented an excellent secondary refining method of molten steel by utilizing a gas-dynamic stirring method.

The invention will be explained below with reference to the drawings.

In Figure 2, a refractory cylinder 6 is immersed in molten steel 5 in a ladle 1, and the upper end of this cylinder 6 is connected to a pressure fluctuation generator (not shown) through a pipe 8. The pressure of the gas phase inside the cylinder is increased or decreased, and accordingly, molten steel flows from the ladle 1 into the cylinder 6 during the depressurization period, and flows out from the cylinder 6 to the ladle 1 during the pressurization period. This shows a device configured to repeat.

The kinetic energy of the molten steel flowing out of the cylinder 5 described above is used as stirring force for the molten steel in the ladle, and the desired stirring force can be obtained by controlling the period and amplitude of the vibration motion of the molten steel.

As shown in Figure 3, there are three types of stirring methods according to the average pressure of the pressure fluctuation: (a) reduced pressure side, (b) normal pressure side, and (C) pressurized side. It is possible to change the processing method.

For example, when pouring molten steel from a ladle into a mold, it is often a problem that the low temperature molten steel that has been cooled by the refractory wall at the bottom of the ladle flows out at the start of pouring, making it impossible to obtain the desired pouring temperature. The injection nozzle may become clogged with low-temperature molten steel or become difficult to open.

In order to prevent such problems, we have confirmed that it is effective to stir the low-temperature molten steel at the bottom of the ladle by performing a treatment immediately before pouring as shown in Figure 3a.

It was also confirmed that the operating method shown in FIG. 3a is convenient when adding an alloy to a cylindrical self-melting steel from the upper part of the cylinder.

The reason for this is that some of the added alloy flows into the ladle in an undissolved state, and in this case, it is necessary to prevent it from floating to the ladle bath surface undissolved and reacting with the slag or the atmosphere. This is because it is advantageous for the molten steel flow containing unmelted alloy from the cylinder to reach the bottom of the ladle as much as possible.

Furthermore, in the operating method shown in Figure 3a, since the average value of pressure fluctuations in the gas phase is 1 atm or more, it is not necessary to use a pressure reducing device to generate pressure fluctuations, resulting in improved energy efficiency. It was confirmed that the system has the characteristic of being able to operate using only high-pressure gas.

On the other hand, the method shown in FIG. Inconveniences arise, such as the need to use a pump.

However, compared to the method shown in Fig. 3a, the method shown in Fig. 3C has a smaller amount of refractory bricks that are in contact with molten steel on both sides, and the construction cost of the refractory bricks forming the cylinder is lower. It has the advantage of having a long service life and reducing the cost of refractory bricks.

After all, each of the processing methods shown in FIGS. 3a to 3C has its own characteristics, and therefore, it is necessary to implement them in consideration of their respective characteristics depending on the processing purpose.

As mentioned above, a cylinder is immersed in the molten steel tapped into the ladle, and the molten steel in the ladle is stirred by raising and lowering the molten steel into the cylinder by applying pressure fluctuations to the gas in the cylinder. This provides various advantages as described below.

(1) The interface between the molten steel and the slag layer covering the surface of the self-molten steel bath in the ladle is not stirred.

This prevents the molten steel from reacting with the highly oxidized slag that is unavoidably injected into the ladle at the same time as the molten steel from the converter, reducing oxidation loss of the alloy during processing and reducing the oxygen concentration after processing. descend.

(2) The molten steel comes into direct contact with the gas phase only at the bath surface inside the cylinder, and by creating an inert gas atmosphere inside the cylinder, oxidation of the molten steel by the atmosphere can be prevented.

(3) By changing the period and amplitude of the pressure fluctuation of the gas phase inside the cylinder, any stirring force can be obtained, and by changing the depth of immersion in the cylinder, the molten steel at the desired depth in the ladle can be particularly strongly stirred. Can be stirred.

(4) Since there are no mechanically moving parts and the device is simple, the device construction cost is low and the processing cost is also low.

(5) Since there are no mechanically moving parts, it is easy to control the cycle of inflow and outflow of molten steel, and high-speed operation is easily possible by reducing the cycle to several seconds or less to increase the stirring force.

(6) Since it does not require a high vacuum of several mmHg or less like a normal vacuum degassing device, there is no need to use large-scale equipment, and pressurization and depressurization operations can be easily performed without time delay with a simple device. This makes it possible to increase the flow rate of the molten steel flowing out from the cylinder, which serves as the stirring force for the self-melting steel in the ladle, by a pressurizing operation that is particularly industrially easy.

As mentioned above, in order to increase the stirring effect by using the kinetic energy of the molten steel flowing out of the cylinder by effectively performing secondary refining of the molten steel using the gas-dynamic stirring method, it is necessary to increase the stirring effect by using the kinetic energy of the molten steel flowing out of the cylinder. , it is essential to reduce the outflow period and increase the flow velocity of the molten steel flowing out from the cylinder.

However, according to the results of a model experiment using water and an actual machine experiment using molten steel, it has been found that unnecessarily reducing the period causes the following disadvantages.

In other words, if the ascending movement of the molten steel in the cylinder is made too violent with a small period, the splash of molten steel grains will scatter from the molten steel bath surface in the cylinder, and the refractory wall in the part of the upper part of the cylinder that is not washed by the molten steel flow will be splashed. This causes inconveniences such as solidification and adhesion, eventually closing off the gas flow path.

Furthermore, in order to reduce the period of the vibration motion of the molten steel, the capacity of the pressure fluctuation generator inevitably becomes large.

As for the pressurization source, compressed gas of about 10 atmospheres can be used industrially at low cost, and no special problem arises in increasing the size of the pressure fluctuation generator.

However, as for the pressure reduction source, compared to the cylinder internal pressure of 1 to 2 atmospheres, the maximum pressure of the pressure reduction source is 0 atmosphere, and the pressure difference of at most 1 to 2 atmospheres is the driving force to reduce the pressure inside the cylinder. However, in order to reduce the pressure inside the cylinder in a short time, a large pressure reducing device is required, and the gas piping from the pressure fluctuation generator to the cylinder increases rapidly as the cycle time becomes shorter. It will be scale.

From the above points, as a way to efficiently improve the refining effect of molten steel while avoiding the problem of splash adhesion and increasing the size of the equipment, it is recommended to shorten the outflow period rather than reducing the period in which molten steel flows into the cylinder. It has been found that it is advantageous to increase the pressure fluctuation during pressurization to increase the outflow velocity from the cylinder, which serves as the energy for stirring the self-molten steel in the ladle.

Furthermore, the period during which the molten steel movement stops at the upper and lower limit positions of the vibration motion, where the flow direction of the cylindrical self-molten steel is reversed, plays no role in stirring the self-molten steel in the ladle, and the vibration The disadvantage is that the period is large, but if the period is several seconds or less and the above-mentioned splash solidification and adhesion becomes a problem, it is necessary to reduce the stopping time at the upper limit position to an extremely short period of several tenths of a second. On the other hand, the period during which the molten steel movement stops at the lower limit position is completely meaningless for stirring the molten steel; rather, the period during which the molten steel movement stops at the lower limit position is kept as short as possible, and the molten steel is suddenly stopped. There is an advantage in reversing the flow direction of the flow, and as a result of such a rapid reversal of the flow direction, the gas phase and the molten steel phase form a mixed phase state at the interface of both phases, which is advantageous when adding alloys. Understood.

In other words, the alloy ingots added from the top of the cylinder float to the bath surface of the cylindrical self-melting steel, but these alloy ingots easily get caught up in the molten steel if the direction of the molten steel flow rapidly reverses at the lower limit position. This is due to water flowing out from the cylinder into the ladle.

For this purpose, the above-mentioned stopping time needs to be within 0.5 seconds, preferably within 0.2 seconds.

The immersion position of the cylinder is not limited to the center of the ladle;
Considering the depth and diameter of the molten steel bath in the ladle, it is desirable to immerse it in a position where the maximum stirring force can be obtained.

Furthermore, if the complexity of the apparatus is not a concern, the stirring effect can be further improved by rotating the cylinder within the ladle.

Regarding the immersion depth of the cylinder 6, there are various methods as shown in Fig. 3 a to C.
From the viewpoint of erosion of the refractory due to the above molten steel temperature, it is desirable that the immersion depth be as small as possible.

The present inventors conducted various experiments regarding the minimum immersion depth and found that the minimum immersion depth is limited from the viewpoint of the refining effect of molten steel and the operational aspects.

That is, if the depth of immersion of the cylinder into the molten steel is too shallow, the flow of molten steel flowing out from the cylinder will entrain the slag on the surface of the molten steel bath, and the slag will be suspended in the molten steel.

For this reason, oxygen from the slag with a high oxygen concentration, which normally flows into the ladle together with the molten steel from a converter, becomes actively transferred into the molten steel, and the deoxidation rate decreases.

Furthermore, the oxygen concentration reached at the end of the treatment increases, and the refining efficiency decreases.

Furthermore, if the depth of immersion of the cylinder into the molten steel is too shallow, there is a risk that slag or atmospheric air may be partially sucked into the cylinder during the suction period.

If slag is inhaled, the refining efficiency with respect to deoxidation will be reduced for the reasons mentioned above, and if atmospheric air is inhaled, it will be difficult to control the movement of molten steel within the cylinder.

As a result of conducting experiments at various immersion depths under the above conditions, the preferable immersion depth is 1 point from the interface between slag and molten steel.
It turned out to be 0.00 mm or more.

The present invention will be described in detail below based on examples.

The outline of the equipment used in the experiment is shown in Figure 4. In the experimental equipment shown in the figure, the refractory cylinder 6 has an inner diameter of 300 mm, a length of 3500 mm, the thickness of the refractory is 200mm, and the outer side does not come into contact with molten steel. is surrounded by a steel plate 10 with a thickness of 15 mm.

Two lines of piping 11 and 12 are connected to the upper end of the cylinder 6, the piping 11 is connected to a high pressure N2 gas piping with an original pressure of 10 kg/mIt as a high pressure source, and the piping 12 has a vacuum level of 1 QQ torr.
It was connected to a steam ejector having a capacity of 780 kg air/hr to serve as a depressurization source.

Gas flow rate control valves 14, 15 and solenoid valves 16, 17 are arranged in the middle of each pipe, and a controller 18 with a built-in timer is used to alternately open and close the solenoid valves at a fixed period to reduce pressure fluctuations inside the cylinder. The vibration frequency was controlled.

Further, the amplitude of the pressure fluctuation was detected using a pressure detector 19 installed at the top of the cylinder, and the amplitude of the pressure fluctuation was controlled using the controller 18 by adjusting the opening degree of the flow rate control valves 14 and 15.

Approximately 100 tons of molten steel produced in an oxygen top-blown converter is held in an ordinary ladle, and the aforementioned cylinder is
Immerse it to the 0mm position and use the controller 18 to
The pressure inside the cylinder is set to 0 with a period of 2.5 seconds and an average pressure of 1 atm.
．． It was vibrated in the range of 5 to 2.5 atmospheres and treated for about 20 minutes.

Approximately 2.1 seconds out of the 2.5 second pressure fluctuation period is a decompression period,
The pressurization period was 0.4 seconds.

For 5 minutes from the start of the process, the gate 21 of the closed hopper 20 placed at the top of the cylinder is opened and the deoxidizing 1
60-100 kg of AI metal ingots with a diameter of 0-20 mm were added.

The composition of the treated molten steel is shown in Table 1.

This treated molten steel is a low-carbon aluminum guild steel for cold-rolled steel sheets, which tends to have many surface defects due to oxide inclusions.

In order to express the above-mentioned treatment effect, the oxygen concentration at the end of the treatment is shown in Table 2.

In the comparative example in Table 2, 100 to 25,017 ml of argon gas was supplied from the bottom of the ladle through a refractory porous plug.
This is an inert gas stirring treatment in which the gas is blown at a rate of about 20 minutes.

As is clear from Table 2, the average value of the oxygen concentration at the end of the treatment was lower than that of the comparative example, and the variation was small, indicating that it was effective in improving the cleanliness of molten steel.

Further, the yield of added AI is shown in Table 3, and compared to the above-mentioned comparative example, the A1 yield was improved by 7%, and its standard deviation was also improved by 3%, and the variation was small.

Next, the effect of improving the quality of the slab obtained by continuous casting of the above-mentioned steel type will be described.

Conventionally, in the above-mentioned steel types, alumina clusters, which are caused by the accumulation of alumina-based inclusions on the slab surface layer, have been a problem as surface defects of the slab due to oxide inclusions.

Therefore, the effect of the treatment on the frequency of appearance of alumina clusters on the slab surface was investigated.

Figure 5 shows the results for the slab during normal casting, excluding the initial and final stages of pouring.

The cluster score is evaluated by visually observing the slab surface and considering the number and size of clusters. A score of 1 means no defects in terms of quality, and a score of 5 requires full cleaning with a hot scarf to remove defects. .

A rating of 2 to 4 means partial care depending on the value.

As is clear from FIG. 5, the slab quality improvement effect is excellent, and the overall and partial maintenance rates are significantly reduced compared to the comparative example.

FIG. 6 shows the change over time in the number of alumina-based inclusions during treatment as a percentage of the value at the start of treatment.

The comparative example shown by the dotted line in FIG. 6 was carried out using the method shown in FIG.

In other words, a refractory cylinder with an inner diameter of 800 mm is approximately 500 m long.
The sample was immersed in molten steel, and the Ar gas blown from the bottom of the ladle was allowed to escape into the atmosphere within this refractory cylinder, creating an Ar gas atmosphere inside the cylinder.

As is clear from FIG. 6, the rate of reduction of inclusions is rapid, and the number of inclusions at the end of treatment is small, and the rate of removal of inclusions from molten steel is excellent.

Figure 7 shows the results of examining the effect of immersion depth on the deoxidizing effect by using the apparatus shown in the example, using the same molten steel as in the example, and changing only the immersion depth of the cylinder into the molten steel. The relationship between the oxygen concentration and immersion depth is shown below.

After about 20 minutes of treatment, a sample was taken from the molten steel at a certain position about 30 cm below the surface of the bath in the ladle, and the oxygen concentration was analyzed.

[Brief explanation of the drawing]

Fig. 1 is a schematic sectional view of a ladle showing a conventional inert gas stirring method, Fig. 2 is a schematic sectional view of an apparatus for carrying out the present invention, and Fig. 3 is a schematic sectional view of a ladle showing a conventional inert gas stirring method.
The figure is a schematic cross-sectional view for explaining the method of the present invention, Figure 4 is a schematic cross-sectional view of the apparatus used in the example of the present invention, and Figure 5 is a comparison between the present invention and the conventional method regarding the frequency of appearance of alumina clusters on the slab surface. Comparison diagram: Figure 6 is a comparison diagram of the present invention and the conventional method regarding the change over time in the number of alumina inclusions during treatment, and Figure 7 is a diagram showing the relationship between the immersion depth of a refractory cylinder in molten steel and the oxygen concentration. FIG. 1... Ladle, 5... Molten steel, 6...
... Refractory cylinder, 7... Slag layer, 8...
...Pipe, 10...Outer steel plate, 11...
High pressure gas piping, 12...Low pressure gas piping, 14.
... High pressure gas flow control valve, 15 ... Low pressure gas flow control valve, 16 ... High pressure gas solenoid valve,
17... Solenoid valve for low pressure gas, 18...
Pressure control controller, 19...pressure detector,
20... Hopper for adding alloy material, 21...
··Gate.

Claims (1)

[Claims] 1. Immerse the cylinder in the molten steel in a container holding molten steel so that the lower end of the cylinder is located at least 100 mm below the surface of the molten steel in the container, vary the gas pressure inside the cylinder, and add to the molten steel inside the cylinder. A method for secondary refining of molten steel, characterized in that the molten steel in the container is stirred by adding a substance and causing the molten steel in the container to flow into and out of a cylinder by the fluctuation of the gas pressure.