JP2554105B2 - Method for preventing blockage of immersion nozzle in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention has hitherto attempted to prevent gas from being blown into an immersion nozzle of continuous casting to prevent clogging of the nozzle. This is done to prevent product quality defects.

<Prior Art> In continuous casting of molten steel, an immersion nozzle that forms a flow path of an injection flow between a tundish and a mold is used, and a method of blowing gas into the nozzle is used to prevent clogging of the nozzle. Although adopted, the gas at this time does not completely float above the bath surface in the mold and a part of it is trapped in the slab along with inclusions, resulting in product quality defects.
The apparatus shown in FIG. 1 is used for the nozzle, and as the blowing gas, argon gas whose melting in molten steel is almost negligible and nitrogen gas having a low melting rate are mainly used. This method uses argon gas (hereinafter abbreviated as Ar) flowing in the nozzle to clean the inner wall of the nozzle.
This is an extremely effective method for preventing the adhesion of alumina (hereinafter abbreviated as Al ₂ O ₃ ) and the solidification adhesion of molten steel to the inner wall of the nozzle, and preventing clogging of the nozzle during casting. This method is an excellent technique for stably and continuously performing continuous casting of steel deoxidized with aluminum (hereinafter abbreviated as Al), in other words, substantially containing Al, and is widely used industrially. Has been. In this method, most of Ar or N ₂ gas blown into the nozzle flows down together with the molten steel in the nozzle and flows into the mold as bubbles. These bubbles will float above the steel bath in the mold. However, only a small part of these bubbles loses the opportunity to float above the steel bath, penetrates deeply into the steel bath in the mold together with the injection flow, and casts during solidification in the mold or in the region thereafter. May be trapped in a piece of solidified shell. These trapped bubbles are found near the surface of the slab when the slab after casting is broken and investigated.
When observed in more detail, these bubbles are accompanied by inclusions such as Al ₂ O ₃ and, even if they become a product of a cold-rolled steel sheet through the subsequent rolling process, the traces remain and become a defect of the product.

The degree of trapping of bubbles in the slab as described above is greatly influenced by the inflow mode of the injection flow into the mold and the casting speed, and optimization is performed by changing the shape of the nozzle tip to improve the flow mode of the injection flow. It is being appreciated. However, when the casting speed is increased to improve the productivity, the amount of bubbles trapped in the slab increases, and the problem has not been completely solved.

<Problems to be Solved by the Invention> According to the conventional method, the present invention has a drawback that air bubbles accompanied by inclusions are trapped in the solidified shell of the slab as described above and a surface defect is likely to occur in the product. Therefore, it is an object of the present invention to overcome these drawbacks and to provide a method for preventing blockage of an immersion nozzle that can prevent surface defects of products.

<Means for Solving Problems> The present invention is based on an observation result of a bath surface when various gases are blown into molten steel from the bottom of a large ladle having a steel bath depth of 3 m or more. It was born. That is, when Ar gas or N ₂ gas that has been conventionally used is blown into the molten steel from the bottom, it floats in the bath, and it is observed that it reaches the bath surface and is diffused into the atmosphere, but H ₂ gas and CO gas Alternatively, the present invention has been made based on the phenomenon that even if a hydrocarbon gas such as C ₃ H ₈ is blown into molten steel, levitation to the bath surface is not observed and that it is absorbed in the bath during the levitation process in the bath. It is a thing.

The present invention blows a reducing and soluble gas into the immersion nozzle between the tundish and the mold of the high speed continuous casting of 1.6 m / min or more, and involves inclusions that prevent clogging of the nozzle and trap in the solidified shell. a method for reducing bubbles, H ₂ gas as the gas, the only soluble gases CO gas or hydrocarbon gas, alone or blockage preventing method of continuous casting immersion nozzle, wherein the blowing a mixed gas in, is there.

<Processes and actions leading to invention> H ₂ gas, CO gas or hydrocarbon gas such as C ₃ H ₈ is C,
Since it consists of O and H elements, it has a high solubility in molten steel and a high dissolution rate, and dissolves in molten steel in the following reaction in a relatively short time.

H ₂ → 2 H CO → C + O C ₃ H ₈ → 3 C + 8 H Al It is also rational thermodynamically that these reactions proceed in deoxidized molten steel and ordinary molten steel with a hydrogen concentration of 5 ppm or less. Target.

Based on the above results, an experiment was conducted in the apparatus shown in FIG. 1 using H ₂ gas or CO gas, which is a combination of C, O and H elements, as a gas for preventing nozzle clogging. When observing the bath surface in the casting mold during casting, the amount of gas bubbles floating on the bath surface is significantly reduced compared to the conventional Ar gas and N ₂ gas, and it is extremely Only a small amount floats, and most of the other gases are dissolved and absorbed in the molten steel while flowing down in the nozzle or flowing from the nozzle into the mold and floating in the bath. It was considered. The above results indicate that among the bubbles that have flowed into the bath in the mold, the opportunity to float above the bath is eliminated, and the bubbles that are deeply caught in the bath in the mold also dissolve in the molten steel as described above. It is considered that the air bubbles trapped in the slab are significantly reduced as they are absorbed, and the product defects due to the trapped air bubbles are significantly reduced as described later.

By the way, H ₂ gas consisting of a combination of C, O and H elements, C
If the dissolution of O gas or hydrocarbon gas is very fast and occurs instantaneously at the same time as the blowing, it is the same as not blowing the gas, and the effect of preventing nozzle clogging will not be exhibited, but the prevention of nozzle clogging will be prevented. Regarding the effect, when a gas flow rate slightly higher than Ar gas is used as compared with the conventional Ar gas method, H ₂ gas and C consisting of a combination of C, O, and H elements are used.
It was found that even O gas had a sufficient blocking prevention effect.
This is because the molten steel in the nozzle flows downward at a rate of 1 to 3 m / s, and the bubbles also flow downward along with this flow, so the stay time of the bubbles in the nozzle is as short as 0.5 seconds or less, which is large. It is considered that the bubbles in the part exert the effect of preventing nozzle clogging without melting, flow from the nozzle into the molten steel of the mold, and dissolve in the molten steel during the floating process in the bath.

<Examples> Examples will be described below.

In a two-strand slab continuous caster, the inner angle is 70 mm and it is made of alumina graphite.
Using a reverse Y-shaped immersion nozzle of 30 degrees, 260 mm x 130
Low carbon aluminum killed steel was injected into two molds having a cross section of 0 mm at a rate of 4 ton / min (1.69 m / min) per mold. Under this condition, one nozzle has a solubility in molten steel from the top.
24 ppm H ₂ gas is blown at a rate of 10 to 30 N / min, and the other mold nozzle is blown with Ar gas at a rate of 10 to 20 N / min, whose solubility in molten steel is not substantial by the conventional method. To prevent this. Under this condition, 5 pots of molten steel in a ladle holding 270 tons of molten steel were continuously poured. Both the strand using the H ₂ gas blowing nozzle and the strand using the Ar gas blowing nozzle, which consist of H element having a high solubility and a high melting rate in molten steel, do not cause nozzle clogging and can achieve stable casting with the casting speed maintained at a predetermined value. It was possible. Further, after casting, both nozzles were collected and the inner wall was observed, but the adhesion state of Al ₂ O ₃ was the same, and no difference between H ₂ gas and Ar gas was observed. Furthermore, these cast slabs were hot-rolled and cold-rolled to obtain steel plate products with a thickness of 0.7 mm.The occurrence rate of surface defects in the products due to trapped bubbles was determined by the solubility in molten steel. 0.24 in strand slab using H ₂ gas nozzle that consists of H element with high dissolution rate
%, 0.76 for strand slab with Ar gas nozzle
%, And it is clear that the use of H ₂ gas composed of H element having a high solubility in molten steel and a high dissolution rate according to the present invention reduces it to about 1/3.

Further, when hydrogen gas is blown into the nozzle, it is feared that the hydrogen concentration of the molten steel will increase.
It was clarified that the hydrogen concentration increased by 0.5 ppm or less compared to the case of using gas, which was not a level that affected the quality of cold-rolled steel sheet. This result almost coincides with the hydrogen concentration increase amount calculated as the total amount of the hydrogen gas used was absorbed in the molten steel.

Similarly, a comparative experiment was conducted between Ar gas and CO gas consisting of C and O elements, which has a very high solubility in deoxidized molten steel. In this case as well, there was no nozzle clogging in both the CO gas nozzle and the Ar gas nozzle, and it was clarified that CO gas has the same nozzle clogging prevention effect as Ar gas. Also,
Similarly, the quality of a 0.6 mm thick cold rolled steel sheet was investigated, and in the product of strand slab using a nozzle using CO gas consisting of C and O elements, surface defects were generated compared with those of Ar gas. The rate became about 1/3. In this case, CO gas is decomposed into carbon and oxygen and absorbed, and it is expected that the concentration of carbon and oxygen in the slab will increase when CO gas is used. Substantially no increase in carbon concentration was observed in the strips. Oxygen concentration in the slab
Although it was 17 to 26 ppm, it was an increase of 1 to 2 ppm as compared with the strand using the Ar nozzle, and it was not recognized as a substantial increase in concentration. The CO used for this value as well
It is estimated that the amount of carbon and oxygen concentration increase calculated as if the total amount of gas was absorbed is about 3 ppm, so considering that not all of the gas used dissolves, the actual concentration of carbon and oxygen It is clear that no increase occurs.

The above explanation is based on H, which has a high solubility in molten steel and a high dissolution rate.
C which has very high solubility in H ₂ gas consisting of elements and molten steel
Although the explanation has been made by taking the CO gas consisting of the elements O and O as an example, a hydrocarbon gas such as C ₃ H ₈ consisting of C and H elements, which has a high solubility in molten steel and a high dissolution rate, can also be used. It is also possible to use a mixture of these soluble gases, and the soluble gas used in the present invention is not limited to CO gas and H ₂ gas.

Further, regarding the blowing of the soluble gas, the method of blowing into the nozzle portion has been described, but the present invention is not limited to this, and a method of causing the soluble gas to flow into the nozzle portion from above the nozzle inlet together with the dissolved flow is also possible. Applicable.

<Effects of the Invention> According to the present invention, the prevention of immersion nozzle blockage can be made to the same extent as in the case of using Ar gas or N ₂ gas in the conventional method,
Furthermore, it was possible to prevent the surface defects of the product that were generated by gas bubbles trapped in the solidified shell and entering the slab, which could not be completely prevented by the conventional method.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an apparatus for explaining the present invention. 1 ... Tundish, 2 ... Molten steel, 3 ... Immersion nozzle, 4 ... Gas pipe, 5 ... Mold, 6 ... Solidified shell.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Seiji Watanabe 1 Kawasaki-cho, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Ei Kitaoka 1 Kawasaki-cho, Chiba City Chiba Prefecture Kawasaki Steel Co., Ltd. Shiki Company Technology Research Headquarters (72) Inventor Akihiko Namba 1 Kawasaki-cho, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd.Chemical Company Technology Research Headquarters (72) Inventor Toshikazu Sakuraya 1 Kawasaki-cho, Chiba City Kawasaki Steel Co., Ltd. Technical Research Division (56) References JP-A-62-38747 (JP, A)

Claims (1)

(57) [Claims] 1. A reducible and soluble gas is blown into an immersion nozzle between a tundish and a mold for high speed continuous casting of 1.6 m / min or more to prevent clogging of the nozzle and to prevent inclusions trapped in a solidified shell. A method for reducing entrained bubbles, which comprises blowing H ₂ gas, CO gas or hydrocarbon gas as the gas alone or as a mixed gas, to prevent clogging of an immersion nozzle in continuous casting.