JPH09300052A - Method for cleaning molten steel in tundish - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for quickly and efficiently reducing oxygen concn. in a tundish in order to prevent the oxidation of molten steel in the tundish with air being the principal cause of reducing the cleanliness of the molten steel. SOLUTION: In this cleaning method, at the time of reducing the oxygen concn. by blowing inert gas in the tundish before pouring in continuous casting of steel, the inner diameter of a nozzle 8 for blowing the inert gas is defined as <=40mm, and the dipping depth of the nozzle into the tundish 1 is defined as >=1/4 of the tundish depth. Also, a metal to be gasified at the temp. of the tundish is added into the tundish.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying molten steel in a tundish by preventing rapid oxidation of the molten steel in the air when the molten steel is poured into the tundish from a ladle in continuous casting of steel. Things.

[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 terms of 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 pouring molten steel from the ladle into the tundish, there is a problem of molten steel contamination due to air oxidation, and at present, the effect of removing inclusions in the tundish is not sufficiently exhibited. For this reason, for the purpose of preventing molten steel contamination in the tundish, for example, as described in JP-A-61-17345, an inert gas is blown into the tundish covered with a heat insulating material board at the beginning of injection. Thus, air oxidation of the injected molten steel is prevented.

[0003]

However, in order to inject molten steel into the tundish, a space for inserting a molten steel injection nozzle into the tundish lid is required.
Furthermore, considering the thermal deformation of the tundish lid and the tundish body, it is difficult to completely seal the tundish. In particular, in the present day when the size of the tundish is increasing, the influence of thermal deformation is great, and it is more difficult to seal it. Furthermore, the inert gas is blown into the tundish from the space around the molten steel injection nozzle or from the gas injection nozzle provided in the tundish lid, but this method is rather the injection point or tundish. There is a problem that air is entrapped through the gap between the lids and the oxidation of the molten steel is increased. For this reason, at present, the oxygen concentration in the tundish cannot be reduced to the extent that air oxidation can be prevented by the conventional sealing method.

In view of these problems, in order to prevent air oxidation of molten steel in the tundish, which is the main cause of deteriorating the cleanliness of molten steel, the present invention rapidly and oxygen concentration in the tundish. It is an object of the present invention to provide a method that can be efficiently reduced.

[0005]

SUMMARY OF THE INVENTION The gist of the present invention is, in continuous casting of steel, when an inert gas is blown into a tundish before injection to reduce the oxygen concentration, the inner diameter of the inert gas blow nozzle is 40 mm. The depth of penetration of the nozzle into the tundish is 1 of the tundish depth.
/ 4 or more is secured, and a method of cleaning molten steel in a tundish is characterized by adding to the tundish a metal that has a high bondability with oxygen and is gasified at a tundish temperature.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Generally, when pouring molten steel from a ladle into a tundish, the stirring energy of the pouring flow is very large until the long nozzle of the ladle is immersed below the surface of the molten metal. The molten steel surface area increases, and a large number of inclusions are generated due to the air oxidation shown in the reaction of the following formula (1). 4 Al (in molten steel) + 3O ₂ (in air) = 2Al ₂ O ₃ (inclusions) (1) Thus, the air oxidation rate at the initial injection is much faster than that in the steady state, and the contamination of molten steel in the tundish is It is the biggest cause.

In order to prevent air oxidation at the initial stage of pouring, the present inventors have examined a method of capping the tundish, replacing the inside of the tundish with Ar gas, and then starting pouring molten steel. However, it was found that the conventional method of blowing Ar gas into the tundish cannot reduce the oxygen concentration to such an extent that the oxidation of molten steel does not pose a problem industrially (oxygen concentration of 1.0% or less).

FIG. 1 is a diagram for explaining a conventional tundish sealing method. In order to replace the air 2 in the tundish 1 with Ar gas 3 prior to the molten steel injection,
Gas injection nozzle 8 is installed in injection hole 7 for injecting molten steel in ladle 5 through molten steel injection nozzle 6 and tundish lid 4, and Ar gas 3 is injected into tundish 1 from here. There is. However, the molten steel injection nozzle 6
Since it is difficult to close the gap between the injection hole 7 and the injection hole 7, and the tundish lid 4 is deformed by thermal stress, the tundish 1 cannot be completely sealed. Therefore, when the Ar gas 3 is blown into the tundish 1 from the gas blowing nozzle 8 installed on the tundish lid 4, the jet of Ar gas 3 entrains the air 2 from the outside, so that the oxygen concentration in the tundish 1 is sufficiently increased. It cannot be reduced.

The inventors of the present invention focused on the inner diameter and the penetration depth (distance from the upper end of the tundish to the nozzle tip) of the gas blowing nozzle in order to develop an Ar gas blowing method in which it is difficult to entrain air in the tundish. The influence of these conditions on the oxygen concentration reached in the tundish was investigated. The result is shown in FIG. Generally, the gas blowing nozzle is attached to the tundish lid, the nozzle penetration depth is close to 0, and the nozzle inner diameter is about 20 to 30 mm. Therefore, the conventional Ar gas blowing method cannot sufficiently reduce the oxygen concentration in the tundish. On the other hand, the present inventors set the oxygen concentration in the tundish to 1.0 by setting the penetration depth of the gas blowing nozzle to 1/4 or more of the tundish depth and setting the nozzle inner diameter to 40 mm or more. We have found that it can be reduced to less than%.

However, in order to reduce the oxygen concentration in the tundish with only Ar gas as in the present method, it takes about 5 minutes even if a relatively large amount of Ar gas is blown in, so that the time between refining and casting is high. Due to the waiting time, the operation was not always satisfactory. Therefore, a small amount of Ar
As a result of further studies on a method of sufficiently reducing the oxygen concentration in the tundish even in the case of gas, it is effective to add metallic Mg into the tundish in combination with the Ar gas blowing method of the present invention. I found that.

The boiling point of metallic Mg is about 1100 ° C.,
When added to the tundish (1300 ° C. or higher) before pouring molten steel, it becomes Mg vapor and reacts with oxygen in the air according to the following equation (2), so that the oxygen concentration in the tundish can be reduced. 2Mg (gas) + O ₂ (gas) = 2MgO (solid) (2)

However, if only the addition of metallic Mg is used, (2)
Due to the reaction of the formula, volume contraction corresponding to the amount of oxygen in the tundish occurs, and the inside of the tundish becomes a negative pressure,
Air flows from the outside into the tundish. In order to prevent the inflow of air, it is necessary to make up the volume of oxygen reduced by the addition of metal Mg with Ar gas to make the inside of the tundish a positive pressure. However, when Ar gas is blown into the tundish by the conventional method, air entrapment occurs, so that the oxygen concentration lowered by the addition of the metal Mg increases again. Therefore, the metal M
The oxygen concentration in the tundish can be efficiently reduced in a state where the entrainment of air is prevented by using the addition of g together with the above-mentioned Ar gas blowing method in which it is difficult to entrain air.

The inner diameter of the nozzle for suppressing the entrainment of air in the present invention is defined by the outlet, so that the middle is made a conventional thin tube,
A sufficient effect can be obtained by enlarging the diameter near the nozzle outlet. Further, the gas species that can be used in the present invention is not limited to Ar gas, and similar effects can be obtained with other inert gases. Further, the metal added to the tundish is not limited to Mg, but may be any metal such as Ca that is easily gasified at the temperature in the tundish and easily bonded to oxygen.

As described above, according to the present invention, even when the airtightness of the tundish is low, the oxygen concentration in the tundish can be efficiently and quickly reduced, and the vigorous air oxidation at the initial stage of injection can be prevented. Since it can be prevented, the quality of the slab can be significantly improved.

[0015]

The present invention will be described below with reference to examples and comparative examples. Example 1 As shown in FIG. 3, a tundish 1 (depth 1.2 × width 1.5 × length 6.0 m) with a capacity of 50 t was placed on a lid 4.
Then, a gas blowing nozzle 8 having an inner diameter of 45 mm is installed at a position 310 mm from the upper end 9 of the tundish.
Blowing at a flow rate of 0 Nm ³ / h and 1.0 kg of metallic Mg
Was added. Immediately after the addition of the metallic Mg, the oxygen concentration in the tundish became stable at 1% or less, and after 20 seconds from the start of blowing, the component C: 50 ppm, Si: 0.01
5%, Mn: 0.25%, P: 0.02%, S: 0.0
250 t of molten steel with 1%, Al: 0.035%, and a temperature of 1550 ° C. (in a tundish) was poured from a ladle at 20 t / min. At this time, the total oxygen content in the molten steel on the tundish exit side showed a constant value from the initial injection, and the total oxygen content was stable at 15 ppm.
Was secured. As a result, molten steel contamination could be reliably prevented, and no surface defects occurred in the product after rolling.

Comparative Example 1: As shown in FIG. 1, a capacity of 50 t
Tundish (depth 1.2 x width 1.5 x length 6.0
m) is covered, and a gas injection nozzle with an inner diameter of 30 mm is attached at a position 0 mm from the upper end of the tundish, and A
Injecting r gas at a flow rate of 250 Nm ³ / h and at the same time metal M
1.0 kg was added. Immediately after the addition of metallic Mg, the oxygen concentration in the tundish reached 3%, but immediately thereafter, the oxygen concentration rose to 8.5%. The oxygen concentration in the tundish did not fall below 8.5% even 10 minutes after the addition of the metal Mg, so that the component C: 50 ppm, Si: 0.015%, Mn: 0.
25%, P: 0.02%, S: 0.01%, Al: 0.
250 t of molten steel having a temperature of 035% and a temperature of 1550 ° C. (in a tundish) was poured from a ladle at 20 t / min. At this time, the total amount of oxygen in the molten steel on the outlet side of the tundish was 90 pp at the beginning of injection.
It reached m and then gradually decreased, but the final value reached was 50.
It was ppm. For this reason, molten steel contamination at the initial stage of pouring could not be prevented, and surface defects occurred in the product after rolling.

Comparative Example 2: As shown in FIG. 1, the capacity is 50 t.
Tundish 1 (depth 1.2 x width 1.5 x length 6.
(0 m), a gas blowing nozzle having an inner diameter of 45 mm was installed at a position 310 mm from the upper end of the tundish, and Ar gas was blown from the gas blowing nozzle at a flow rate of 250 Nm ³ / h before the injection. 5 minutes after the start of Ar gas injection, the oxygen concentration in the tundish was 1%.
Since it became the following, component C: 50ppm, Si: 0.01
5%, Mn: 0.25%, P: 0.02%, S: 0.0
250 t of molten steel with 1%, Al: 0.035%, and a temperature of 1550 ° C. (in a tundish) was poured from a ladle at 20 t / min. At this time, the total amount of oxygen in the molten steel on the outlet side of the tundish showed a constant value from the beginning of injection, and a stable total amount of 15 ppm could be secured. As a result, the contamination of molten steel could be prevented, and no surface defects occurred in the product after rolling. However, because a waiting time of 5 minutes was required to reduce the oxygen concentration in the tundish, the refining time of the next pot was long and the processing cost was high.

[0018]

As described above, according to the method for cleaning molten steel in the tundish of the present invention, the oxygen concentration in the tundish can be efficiently reduced in a short time, so that the operation can be performed without disturbing the operation. The quality of the slab is greatly improved because it is possible to reliably prevent the molten steel contamination at the initial stage of intense injection.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a conventional tundish sealing method.

FIG. 2 is a diagram showing a relationship between a nozzle entry depth / a tundish depth and an oxygen concentration reaching a tundish.

FIG. 3 is a diagram for explaining an implementation status of the present invention.

[Explanation of symbols]

 1: Tundish 2: Air 3: Ar gas 4: Tundish lid 5: Ladle 6: Molten steel injection nozzle 7: Injection hole 8: Gas blowing nozzle 9: Tundish top

Claims (1)

[Claims] 1. In continuous casting of steel, when an inert gas is blown into a tundish before injection to reduce the oxygen concentration, the inner diameter of an inert gas blowing nozzle is set to 40 mm or more, and the tundish of the nozzle is filled. The inside of the tundish is characterized in that the penetration depth to the tundish is secured to be 1/4 or more of the tundish depth and a metal that has a high bondability with oxygen and that is gasified at the tundish temperature is added to the tundish. Method for cleaning molten steel.