JPH07223056A - Method for cleaning molten steel in tundish - Google Patents

Abstract

PURPOSE:To improve the quality of a cast slab by blowing inert gas from the bottom part of a tundish and also, coating the molten steel surface with a fusible insulating material, at the time of pouring the molten steel into a tundish from a ladle. CONSTITUTION:At the time of starting pouring of the molten steel 4 from the ladle 3 by beforehand putting the fusible insulating material 2 into the tundish 1, Ar bubble 9 from the bottom part 6 in the tundish 1 is blown into the molten steel 4. As a floculated aggregation of inclusion is promoted by stirring force of the Ar gas 9 blowing and also, the inclusion is stuck to the surrounding of the bubble itself, the Ar gas blowing has the effect quickening the float-up and separating speed of the inclusion. Further, the Ar gas blowing has the effect quickening the reducing speed of the total oxygen quantity (inclusion concn.) and the use of the fusible insulating material has the effect lowering the reaching value of the total oxygen quantity. By this method, the most vigorous contamination of the molten steel at the initial stage of the casting is quickly reduced and further, the removal effect of the inclusion in the tundish 1 can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the start of pouring molten steel into a tundish from a ladle in continuous casting of steel.
The present invention relates to a method for cleaning the molten steel in a tundish by promptly reducing the influence of molten steel contamination in the early stage of casting by increasing the inclusion removal rate.

[0002]

2. Description of the Related Art In continuous casting of steel, the tundish is located between the ladle and the mold, and is one of the most important parts in operation and quality.

Its functions are to control the amount of molten steel supplied into the mold, store molten steel, and separate and remove inclusions. In particular, the function of removing inclusions has become an extremely important function as the quality of steel materials has become stricter in recent years.

However, when the molten steel is poured into the tundish from the ladle, the problem of contamination of the molten steel due to air oxidation arises, and therefore, the effect of removing inclusions in the tundish is not sufficiently exerted at present.

Therefore, various methods have been studied and put to practical use for the purpose of preventing molten steel contamination in the tundish.

For example, as described in Japanese Patent Laid-Open No. 59-1055, by blowing an inert gas into a tundish covered with a refractory board lid, air oxidation of injected molten steel is prevented. There is.

Further, Japanese Patent Laid-Open No. 60-261651 discloses a method of reducing the amount of entrained air and suppressing air oxidation by limiting the rising speed of the molten metal in the tundish when pouring molten steel from a ladle, Japanese Patent Laid-Open No. 61-38749
Japanese Patent Publication proposes a method of efficiently removing inclusions in molten steel by adding a Ca-based alloy or Ca-based flux from a ladle long nozzle that transfers molten steel from a ladle to a tundish, or to some extent. Is effective.

[0008]

However, as for the method of covering the tundish with the refractory board lid and sealing with the inert gas, the tundish or the refractory board itself is deformed at a high temperature, and thus the tundish is prevented to the extent that air oxidation can be prevented. It is not possible to reduce the oxygen partial pressure in the dish.

Further, since the air bubbles entrained by the injection flow in the initial stage of casting are extremely fine, the reaction interfacial area becomes large, and the air oxidation rate is much faster than the injection flow in the steady state.

For this reason, the method of limiting the rate of rise of the molten metal in the tundish at the initial stage of casting has an effect of suppressing air oxidation to some extent, but is not a sufficient measure for preventing pollution.

Further, the method of adding the Ca-based alloy or the Ca-based flux from the long ladle nozzle has the effect of increasing the removal rate of inclusions, but since the floating inclusions are re-engaged by the injection flow, it is sufficient. It has not been possible to secure the cleanliness of molten steel.

In view of these problems, the present invention does not require large-scale equipment such as a refractory board lid, promptly reduces the most severe molten steel contamination at the initial stage of casting by a simple method, and further, it prevents inclusions in the tundish. The purpose of the present invention is to provide a method for cleaning molten steel in a tundish that also improves the removal effect.

[0013]

[Means for Solving the Problems]

(1) When injecting molten steel from a ladle into a tundish, an inert gas is blown from the bottom of the tundish, and the surface of the molten metal is coated with a meltable heat retaining agent.

(2) The method for cleaning molten steel in a tundish according to claim 1, wherein the blowing of the inert gas is performed from the bottom of the tundish just below the injection point.

[0015]

In general, when pouring molten steel from the ladle into the tundish, the pouring flow entrains a large amount of oxygen in the atmosphere until the long nozzle of the ladle is immersed below the surface of the molten metal. A large number of inclusions are generated due to the air oxidation shown by the reaction of the equation (1).

[0016]

## STR1 ## 4 Al (in molten steel) + 3O ₂ (in air) = 2Al ₂ O ₃ (inclusions) (1)

Further, since oxygen is entrained in molten steel as very fine bubbles, its oxidation rate is extremely higher than the oxidation rate at the time of steady injection and the reaction rate with slag, and the maximum contamination of molten steel in the tundish is caused. It is the cause.

Therefore, the inventors of the present invention conducted various studies on a method of promoting the floating separation of the inclusions generated by the air oxidation at the initial stage of casting and the inclusions brought in from the ladle. It was found that the problem can be solved by blowing an inert gas from the bottom of the dish and coating the molten metal surface with a meltable heat insulating agent.

FIG. 1 is a diagram for explaining the present invention in more detail. The meltable heat retaining agent 2 is placed in the tundish 1 in advance, and when the injection of the molten steel 4 from the ladle 3 is started, Ar bubbles 9 are blown into the molten steel 4 from the bottom 6 of the tundish.

The stirring force of the blowing of the Ar bubbles 9 promotes the agglomeration and coalescence of the inclusions, and the bubbles themselves also attach the inclusions to the surroundings. Therefore, the blowing of the Ar gas has the effect of increasing the floating separation speed of the inclusions. .

However, if the surface of the molten metal is not coated or if a solid heat retaining agent that cannot absorb the inclusions is used, the inclusions are re-engaged due to the rupture of the Ar bubbles 9 near the surface of the molten metal. Therefore, the ultimate value of the concentration of inclusions is not improved.

On the other hand, if the fusible heat retaining agent 2 which produces a liquid phase at a temperature of molten steel or lower is used, the absorbing ability of inclusions becomes high and the floating inclusions can be surely absorbed in the heat retaining agent 2, so that It is possible to improve the cleanliness by lowering the reached value of the substance concentration as compared with the conventional one.

FIG. 3 shows the results of the effect of Ar gas blowing and the effect of the heat retaining agent obtained by a crucible experiment.

As can be seen from the figure, the blowing of Ar gas has the effect of accelerating the rate of decrease of the total oxygen amount (inclusion concentration), and the use of the meltable heat insulating agent has the effect of decreasing the reached value of the total oxygen amount (inclusion concentration). have.

Further, since the meltable heat retaining agent uniformly coats the molten steel, it has an effect of suppressing pollution due to air oxidation.

Therefore, by using them together in the tundish, the effect of air oxidation at the initial stage of casting can be reduced and the effect of removing inclusions in the tundish can be improved.

As the meltable heat-retaining agent, one having a low viscosity and an excellent ability to absorb inclusions is effective. However, the meltable heat-retaining agent having a low viscosity violently dissolves the tundish refractory, so that the refractory meltdown It is necessary to reduce the viscosity within the range where does not matter.

Further, when the heat retaining agent contains a large amount of lower oxides such as SiO _2, it reacts with Al in the molten steel to form inclusions, which causes contamination of the molten steel from the heat retaining agent.

Therefore, it is effective to reduce the content of the lower oxide in the heat retaining agent as much as possible.

Here, as the meltable heat retaining agent, CaO + A
Those such as 1 ₂ O ₃ , CaO + Al ₂ O ₃ + MgO are convenient.

Regarding the method of adding the heat retaining agent, the same effect can be obtained by the method of adding the heat insulating agent on the molten metal surface at the start of the molten steel injection, in addition to the method of putting it in before the molten steel injection is started as described above.

On the other hand, the position of the inert gas blown at the bottom of the tundish is preferably between the molten steel injection point from the ladle to the tundish and the position of the immersion nozzle, and preferably the porous plug extends over the entire width of the tundish. 8 is provided and an inert gas such as Ar is blown therein.

The finer this inert gas is, the more effective it is for floating inclusions, and it is preferable to blow it as bubbles of 10 mmφ or less.

The flow rate of the inert gas blown is 5 to 1
It is preferably about 00 Nl / min.

That is, if it is less than 5 Nl / min, the effect of floating inclusions is small, and if it exceeds 100 Nl / min, the boiling of molten steel becomes too vigorous, which is not preferable.

In particular, if the gas is blown from the bottom of the tundish just below the injection point as shown in FIG. 2, the stirring force of the injection flow promotes the miniaturization of bubbles and improves the removal efficiency of inclusions.

Therefore, if there is no facility restriction,
The most efficient gas blowing method is to place the gas blowing position immediately below the injection point for cleaning molten steel.

Furthermore, it is possible to apply the present invention not only to the case where the molten steel is most heavily contaminated at the beginning of casting but also to the entire casting area, and it is an effective method for improving the quality of the entire cast piece. is there.

[0039]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples.

[0040]

Example 1 _{CaO: 50%, Al 2 O} 3: 50%, the tundish 1 (depth 1.0 × width 2.0 × length of capacity 50t shown in FIG. 1 the melting point 1400 ° C. meltable thermal insulation agent 8.0
m) was placed in advance. The tundish 1 is provided with a gas injection porous plug 8 connected to an inert gas supply pipe 7 at a bottom portion 6 thereof in the middle of a molten steel flow path leading to the immersion nozzle 10.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, temperature 1550 ° C. (in tundish) 250 t molten steel, ladle 3 to tundish 1
Injection was started at 25 t / min, and Ar gas with an initial bubble diameter of 10 mmφ was blown at 50 Nl / min from the porous plug (made of alumina) 8 of the tundish bottom 6.

After the molten steel 4 was filled in the tundish 1, the molten steel injection amount was reduced to 8 t / min and casting was started. At this time, the total oxygen content in the molten steel on the outlet side of the tundish drastically decreased from the initial stage of injection, and the total oxygen content of 20 ppm could be stably secured 2 minutes after the start of casting.

This ensures that molten steel contamination can be prevented,
No surface defects occurred in the rolled product.

[0044]

[Embodiment 2] This is an example of operation using the tundish 1 shown in FIG. 2. The tundish 1 has the same size as that of the first embodiment, and the inert gas blowing position is a long ladle provided on the ladle 3. It is located directly below the molten steel injection point from the nozzle 5.

CaO: 47.5%, Al ₂ O ₃ : 47.5
%, MgO: 5.0% Melt heat retention agent with melting point 1450 ° C. 2
Component C: 50 ppm, Si: 0.015%, Mn:
0.25%, P: 0.02%, S: 0.01%, Al:
250t of molten steel with 0.035% and temperature of 1550 ° C (in tundish) is 25t in ladle 3 and in tundish 1.
The addition was started on the molten metal surface at the same time when the injection was started at / min.

At the same time, Ar gas having an initial bubble diameter of 10 mmφ was blown at a rate of 50 Nl / min from the porous plug (made of alumina) 8 at the bottom 6 of the tundish 1.

After the tundish 1 is filled with molten steel,
The molten steel injection amount was reduced to 8 t / min, and injection was started from the immersion nozzle 10 into a mold (not shown) to start casting.

At this time, the total oxygen content in the molten steel on the tundish outlet side decreased sharply from the initial stage of injection, and a total oxygen content of 15 ppm could be stably secured 2 minutes after the start of casting.

With this, the contamination of molten steel can be surely prevented,
No surface defects occurred in the rolled product.

[0050]

Comparative Example 1 MgO: 100%, a solid heat insulator having a melting point of 2800 ° C. is shown in FIG.
It was placed in Tundish 1 in advance.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, temperature 1550 ° C. (in tundish) 250 t molten steel, ladle 3 to tundish 1
Injection was started at 25 t / min, and Ar gas blowing was not performed.

After the molten steel 4 was filled in the tundish 1, the molten steel injection amount was reduced to 8 t / min and casting was started.

The total oxygen content in the molten steel gradually decreased from 140 ppm to 60 ppm from the start of the injection to 18 minutes, but thereafter the decrease of the total oxygen content stopped and the final reached value was 60 p.
It was pm.

Therefore, the function of preventing molten steel contamination at the initial stage of injection and the effect of removing inclusions thereafter were not improved, and surface defects occurred in the product after rolling.

[0055]

COMPARATIVE EXAMPLE 2 MgO: 100%, a solid heat insulator having a melting point of 2800 ° C. is shown in FIG.
It was put in the tundish 1 of 1.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, temperature 1550 ° C. (in tundish) 250 t molten steel, ladle 3 to tundish 1
Injection was started at 25 t / min, and Ar gas 9 was blown into the tundish bottom 6 at a rate of 50 Nl / min with an initial bubble diameter of 10 mmφ from an alumina porous plug 8 having a diameter of 25 cm from just below the injection point.

After the molten steel 4 was filled in the tundish 1, the molten steel injection amount was reduced to 8 t / min and casting was started.

At this time, the total oxygen content in the molten steel on the outlet side of the tundish drastically decreased from the initial stage of injection and reached 60 ppm two minutes after the start of casting, but the total oxygen content did not decrease thereafter.

As a result, although the molten steel contamination preventing function at the initial stage of injection was obtained, the reached value of the total oxygen amount was high and the effect of removing inclusions was not improved. Therefore, surface defects occurred in the rolled product.

[0060]

COMPARATIVE EXAMPLE 3 CaO: 50%, Al ₂ O ₃ : 50%, and a meltable heat retaining agent 2 having a melting point of 1400 ° C. was placed in the tundish 1 of FIG. 1 in advance.

Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, temperature 1550 ° C. (in tundish) 250 t molten steel, ladle 3 to tundish 1
Injection was started at 25 t / min, and Ar gas blowing was not performed.

After the molten steel 4 was filled in the tundish 1, the molten steel injection amount was reduced to 8 t / min and casting was started.

At this time, the total amount of oxygen in the molten steel on the outlet side of the tundish gradually decreased and reached 20 ppm in about 22 minutes. Therefore, although the reached value of the total oxygen content was low and the effect of removing inclusions was improved, the rate of decrease of the total oxygen content was slow, and surface defects were generated after rolling in the slab in the early stage of casting.

[0064]

As described above, according to the method for cleaning molten steel in a tundish of the present invention, no large-scale equipment is required, the molten steel contamination at the earliest stage of casting is promptly reduced, and further in the tundish. Since the effect of removing inclusions can also be improved, the quality of the cast piece is significantly improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an example of a tundish used in Examples of the present invention.

FIG. 2 is an explanatory diagram showing an example of a tundish used in the examples of the present invention.

FIG. 3 is an explanatory view showing the effects of Ar gas blowing and use of a meltable heat retaining agent evaluated by a crucible test.

[Explanation of symbols]

 1 Tundish 2 Melt insulation agent 3 Ladle 4 Molten steel 5 Long nozzle 6 Tundish bottom 7 Inert gas supply pipe 8 Porous plug 9 Ar bubbles 10 Immersion nozzle

Claims (2)

[Claims] 1. Cleaning of molten steel in a tundish, characterized in that when pouring molten steel into a tundish from a ladle, an inert gas is blown from the bottom of the tundish and the molten metal surface is coated with a meltable heat retaining agent. Method. 2. The method for cleaning molten steel in a tundish according to claim 1, wherein the inert gas is blown from the bottom of the tundish just below the injection point.