KR19990025232A - Refining method to improve the cleanliness of molten steel for ultra low carbon steel - Google Patents

Abstract

The present invention relates to a method for refining molten steel for producing ultra low carbon steel; The purpose is to provide a refining method that can improve the cleanliness of the steel by reducing the oxygen potential of the ladle slag during the refining operation in the RH vacuum degassing apparatus and at the same time improve the trapping ability of the steel inclusions.

In order to achieve the above object, the present invention, by depositing the RH vacuum degassing apparatus in the ladle in which molten steel and ladle slag is charged, the molten steel of the ladle is sucked into the RH vacuum degassing apparatus and decarburized, and then subsidiary material is added In the refining method of molten steel for ultra low carbon steel, including deoxidation by

The present invention relates to a refining method for improving the cleanliness of molten steel for ultra low carbon steel, which includes sucking less than 50% by weight of ladle slag together with molten steel and then deoxidizing by adding aluminum.

Description

Refining method to improve the cleanliness of molten steel for ultra low carbon steel

The present invention relates to a method for refining molten steel for producing ultra low carbon steel, and more particularly, to reduce the oxygen potential of the ladle slag during the refining operation in the RH vacuum degassing apparatus and at the same time improve the trapping ability of steel inclusions. The present invention relates to an RH refining method that can improve the cleanliness of steel.

In general, a method of refining molten steel using an RH vacuum degassing apparatus (hereinafter, simply referred to as 'RH') as shown in FIG. 1 is performed in an outside furnace refining process of a steelmaking process for producing ultra low carbon steel.

In this method, when the molten steel tapping out of the converter (not shown) reaches the ladle 1, the deposition pipe 3 of the RH 2 is first deposited on the molten steel 4 received on the ladle. At the same time, when the argon gas (Ar gas) is blown from the reflux gas supply device (5), the vacuum pump (5) is operated to depressurize the inside of the RH (2) to several tons (torr), and then the ladle (1). The molten steel 4 is sucked up inside the RH 2 by the pressure difference between the atmosphere inside and the RH 2, and the decarburization reaction of the following equation (1) proceeds to the molten steel 8 heated surface.

[C] + [0] = CO (g)

When the concentration of carbon in the molten steel is decarburized in an appropriate range by the reaction as in Equation 1 above, the secondary material is introduced into the RH 2 to proceed in the order of deoxidation of the remaining oxygen.

At this time, the ladle slag 9 hardly flows into the RH, and since the stirring between the molten steel 4 and the ladle slag 9 is insignificant even in the ladle 1, the ladle slag 9 and the molten steel 4. The reaction of the liver does not occur sufficiently. Therefore, when deoxidizing molten steel with aluminum, the reduction of iron and manganese oxide in slag does not occur sufficiently, and operation | movement is complete | finished with the oxygen potential of slag higher than molten steel. As a result, iron and manganese oxides in the ladle slag 9 continuously reoxidize molten steel deoxidized in a subsequent process after RH refining, ie, react with aluminum in the molten steel to produce alumina, which aggravates the cleanliness of the steel. It becomes the main cause to let. In order to suppress the generation of alumina by reoxidation of molten steel and improve cleanliness, the oxygen potential of slag, that is, the content of iron and manganese oxide, should be kept low.

Conventionally, as a method of lowering the oxygen potential of slag, there is a method of adding a deoxidizer such as aluminum to the slag as a secondary material, but in the RH refining operation, the ladle slag has almost no flow, so the deoxidizer and slag are not mixed smoothly. Does not react with slag, but rather oxidizes with oxygen in the air, making it difficult to expect the effect of adding an expensive deoxidizer.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and provides a refining method that can improve the cleanliness of molten steel by lowering the oxygen potential of slag during ultra low carbon steel production.

1 is a schematic view showing the operating situation in the conventional RH vacuum degassing apparatus

Figure 2 is a schematic diagram showing the refining operation in the RH vacuum degassing apparatus according to the present invention

3 shows the decarburization time and iron and manganese oxide according to the rate of absorption of ladle slag

It is a graph showing the reduction rate.

Explanation of symbols on the main parts of the drawings

1: Ladle 2: RH vacuum degassing device

3: immersion pipe 4: molten steel

5: reflux gas supply device 6: argon gas

7: vacuum pump 8: quenched molten steel

9: ladle slag 10: sucked ladle slag

In order to achieve the above object, the present invention, by depositing the RH vacuum degassing apparatus in the ladle in which the molten steel and ladle slag is charged, the molten steel of the ladle is sucked into the RH vacuum degassing apparatus and decarburized, and then the raw materials are added. In the refining method of molten steel for ultra low carbon steel, including deoxidation by

It is composed of sucking up less than 50% by weight of the ladle slag together with the molten steel, followed by adding and deoxidizing aluminum.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention is characterized by suppressing the reoxidation of molten steel by lowering the oxygen potential of the ladle slag by activating agitation of the molten steel and the ladle slag by drawing up an appropriate amount of the ladle slag into the RH.

In general, when the molten steel in the ladle sucks up to RH, when argon gas is blown from the rising pipe 3a of the deposition pipe 3 of RH shown in FIG. 1, the sucked molten steel 8 toward the down pipe 3b Since the apparent density of the lowered molten steel (8) is to repeat the circular motion to descend to the down pipe (3b) side.

At this time, the blown argon gas rises together with the molten steel which is sucked up, making the inside of the RH 2 very vigorous stirring. The present invention is to make the molten steel and the slag react by using the stirring characteristics in the RH.

That is, unlike decarburization and deoxidation refining by sucking only the molten steel in the ladle, as shown in FIG. 2, the slag 10 and the molten steel that are sucked up by sucking the ladle slag 4 together with the molten steel as shown in FIG. By reacting at RH, the oxygen potential of the sucked slag 10 is almost similar to that of molten steel. At this time, the most important thing is to make the amount of ladle slag sucked up in the ladle slag less than 50% by weight.

The reason is that if the slag of 50% by weight or more of the ladle slag is sucked into the RH, the reaction surface area between the molten steel and the reduced pressure atmosphere in the RH decreases, and the decarburizing ability deteriorates.

After sucking and decarburizing as described above, alumina is added to the deoxidizer inlet 11 in the usual manner to deoxidize. At this time, when alumina is added to RH immediately after alumina is added, cleanliness of molten steel is improved. Magnesium (MgO) and quicklime (CaO) are contained in light dolomite, and magnesia reduces the activity coefficients of iron and manganese oxides in slag after mixing wicking slag and residual slag, that is, reducing iron and manganese oxides. It is effective to prevent the reoxidation of molten steel by fixing so as not to. In addition, the quicklime increases the CaO content in the wicking slag, so that the slag can collect the alumina inclusions generated in the molten steel during aluminum deoxidation by 2CaO + 7Al ₂ O ₃ = 12CaO 7Al ₂ O ₃ reaction. There is an effect of improving the cleanliness of the molten steel by improving the.

In this case, it is preferable to add light dolomite at 250kg or less per ton of slag sucked into the RH refining furnace. If it exceeds 250kg, the concentration of MgO and CaO in the slag becomes too high and the viscosity of the slag rises accordingly. This is because the fluidity of the alumina decreases the absorption ability of the alumina, so that the cleanliness is worsened, and the slag adheres to the refining furnace wall, thereby deteriorating the workability.

After deoxidation as described above, the scouring furnace repressurizes the wicking slag 10 to mix with the slag of high oxygen potential remaining in the ladle, thereby lowering the oxygen potential of the entire slag on average. In addition, when the wicking slag is mixed into the ladle, it is introduced under the ladle residual slag and is positioned between the molten steel and the residual slag to prevent contact between the residual slag and the molten steel having a high oxygen potential to prevent reoxidation of the molten steel.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

Composition: 270 tons of molten steel with carbon: 0.03-0.04% or less, manganese: 0.05-0.15%, phosphorus: 0.08-0.012%, sulfur: 0.003-0.005%, CaO: 45-55%, Al ₂ A ladle with 2.5-3.5 tons of ladle slag with O ₃ : 30-40%, the sum of iron and manganese oxide: 5-15%, and SiO ₂ : 5% or less in a 250-ton RH refinery, The ladle slag was sucked up to the amount shown in Table 1 under reduced pressure at 0.1-1 torr, and molten steel was collected for component analysis. After deoxidation of the molten steel with aluminum, molten steel was collected while refluxing again, and after the completion of the RH refining operation, the ladle slag was collected and analyzed. At this time, the reflux gas flow rate was 200 Nm3 per hour. The reduction rate and decarburization time of iron and manganese oxide in the slag according to the slag absorption rate obtained by the above method are shown in Table 1 and FIG. 3.

Here, the slag absorption rate represents the ratio of the amount of delaminated slag into the RH refinery. In addition, the reduction rate of iron and manganese oxide is calculated as the following formula, the larger the absolute value means that the ability to lower the oxygen potential of the slag, that is, the cleanliness of the molten steel is improved.

Decarburization time means the time required to decarburize until the carbon concentration in molten steel reaches 30 ppm, and the larger the absolute value, the greater the decarburization capacity.

Slag absorption rate Reduction Rate of Iron and Manganese Oxides Decarburization time Conventional Example 1 0% 0% 19 minutes Inventive Example 1 10% 5% 19 minutes Inventive Example 2 20% 11% 18 minutes Inventive Example 3 30% 15% 20 minutes Inventive Example 4 40% 19% 20 minutes Comparative Example 1 50% 22% 24 minutes Comparative Example 2 60% 24% 29 minutes

As shown in Table 1 and Figure 3, the case of Inventive Example (1-4) and Comparative Example (1) in which the slag was sucked up than the conventional example that does not suck up the slag at all shows a high reduction rate of iron and manganese oxide, As the slag absorption rate was higher, the reduction rate was increased. The decarburization time is almost similar to that of Conventional Example (1-4) in which the slag absorption rate is less than 50%, but in Comparative Example (1-2) where the slag absorption rate is 50% or more, As a result, decarburization time increased drastically.

Example 2

In order to obtain the effect of improving the cleanliness of molten steel in the case of adding light dolomite into the RH refining furnace when the ladle slag is sucked into the RH refining furnace, the same method as in Example 1 was used. On the basis of the rate, light dolomite was added in the amounts shown in Table 2 immediately after aluminum deoxidation, and the total oxygen concentration in the steel, which is an indicator of cleanliness of the steel, was analyzed by taking specimens from molten steel continuously cast. The results obtained by the above method are shown in Table 2 together.

Addition of light dolomite (kg per ton slag) Total oxygen in steel (ppm) Conventional Example 2 ^* 12 Inventive Example 5 0 10 Inventive Example 6 50 10 Inventive Example 7 100 9 Inventive Example 8 150 8 Inventive Example 9 200 10 Comparative Example 3 250 13 Comparative Example 4 300 13 * Conventional Example 2 does not suck slag during RH refining

As shown in Table 2 above, it was found that the total oxygen in the steel decreases, that is, the cleanliness of the steel is improved by adding light dolomite (Invention Example (5-9)). However, it can be seen that the addition of more than 250kg of light bovine dolomite per ton of wicking slag is worse than the conventional method in the case of Comparative Example (3-4). This is because, as the amount of light dolomite used increases, the viscosity of the slag rises, which slows down the rate of trapping alumina.

As described above, the present invention is not only able to stably manufacture ultra low carbon steel by lowering the oxygen potential of slag and improving the trapping ability of alumina inclusions during the production of ultra carbon steel, but also has an excellent effect of improving the cleanliness of steel. .

Claims (2)

In the method of refining ultra-low carbon steel molten steel, which involves depositing a RH refining furnace on a ladle loaded with molten steel and ladle slag, drawing up the molten steel into the RH refining furnace, decarburizing it, and then deoxidizing by adding a subsidiary material. In Refining less than 50% by weight of the ladle slag together with the molten steel, and refining method for improving the cleanliness of the molten steel for ultra-low carbon steel, characterized in that it comprises a deoxidation by adding aluminum. The method for refining the cleanliness of molten steel for ultra low carbon steels according to claim 1, wherein light dolomite of 250 kg or less is added per ton of ladle slag sucked immediately after the aluminum injection.