KR19990053869A - Refining method of aluminum deoxidized steel - Google Patents

Abstract

The present invention relates to a ladle refining method of molten steel tapping with ladle; The purpose is to provide a method for refining aluminum deoxidized steel that can reduce the amount of nonmetallic inclusions in cast steel and prevent nozzle clogging.

In order to achieve the above object, the present invention, by tapping the molten steel into the ladle, and then injecting aluminum and subsidiary materials to determine the content of T.Fe + MnO and the ratio of CaO / SiO ₂ / Al ₂ O ₃ in the ladle slag In the method of refining aluminum deoxidized steel, including the step of tapping the refined molten steel with a tundish after refinement by strong bubbling while adjusting, the content of T.Fe + MnO in the ladle slag is 15% by weight or less, In addition, strongly bubbling at a ratio of CaO / SiO ₂ / Al ₂ O ₃ to 0.25-0.45; The present invention relates to a method for refining aluminum deoxidized steel, comprising: blowing 100-1000 g of Mg alone or a mixed Mg mixed powder at a rate of 10-100 g per minute into the strongly bubbled molten steel. .

Description

Refining method of aluminum deoxidized steel

The present invention relates to a ladle refining method of molten steel tapping with ladle, and more particularly to a method for refining aluminum deoxidized steel that can reduce the amount of non-metallic inclusions in the cast slab, and can prevent nozzle clogging. .

The production of steel materials under the modern mass production system is made by molten steel in a molten state, then cast into a certain shape to be solidified and then processed through rolling or the like. As a method of manufacturing molten steel in a molten state, it is roughly classified into a furnace- converter method and an electric furnace method.

The furnace-converting method is the most widely used today because it is relatively economical as a method of manufacturing pig iron by using iron ore as a raw material in a blast furnace and then secondarily treating it in a converter. On the other hand, in the electric furnace method, molten steel is manufactured to melt molten steel, so it is sensitive to market conditions such as the price of the product and the supply and demand of scrap raw materials, and high cost is limited in application. In recent years, however, mini-mill players have been developed in which the apparatus cost of subsequent casting processes is cheaper than before, and the steel production process by the electric furnace-mini mill continuous casting method is gradually expanding.

Molten steel produced in converters and electric furnaces is melted in molten steel storage containers called ladles and processed to suit subsequent casting processes. In the casting of molten steel, an ingot method and a continuous casting method are used. The ingot method has good quality characteristics of products, but it is limited to some special steels and high-grade steels because of the disadvantages of low productivity, low error rate, and low energy efficiency. On the other hand, the continuous casting method is widely used today because of the advantages of solving the disadvantages of the ingot method and controlling the quality of the product to be uniform.

Continuous casting molten steel has a carbon concentration of less than a certain level in order to prevent break-out, which is an accident that molten steel flows out of a mold during casting, and to prevent product defects such as pin-holes. Decarburization with oxygen, and the oxygen used during decarburization is deoxidized to aluminum in the ladle during the external refining process to produce aluminum de-oxidized steel (Al-Killed steel).

By the way, the cast steel obtained by continuously casting aluminum-kilted steel has a large amount of non-metallic inclusions in the surface layer portion, there is a problem that the error rate is reduced by removing the surface layer portion, these non-metallic inclusions are mainly composed of oxides, including alumina. In addition, the problem that casting is no longer difficult has been confirmed because non-metallic inclusions adhere to and grow on the inner wall of the refractory nozzles used in continuous casting operations. In particular, the mini-mill player has a very small thickness of the inner space of the nozzle compared to the conventional player, even if a little adhesion layer is formed, the continuous casting operation is stopped. Therefore, it is important to reduce the amount of nonmetallic inclusions in the steel to as low as possible in the pre-casting stage.

In general, the aluminum-kilted steel is added in an amount such that the aluminum concentration in the product may be 0.01 to 0.07% by weight for the purpose of improving the deoxidation properties of the molten steel and the mechanical properties of the steel. When aluminum is added to molten steel, an oxide is formed. The oxide is 90 ~ 95% separated and removed by the slag layer, but the oxide remaining in the steel becomes a non-metallic inclusion, which causes various defects in the subsequent process or use of the product, Since the mechanical properties are lowered, efforts are made to lower the amount of residual oxides as much as possible. Therefore, molten steel should be treated as slag with large absorption capacity of oxide-based nonmetallic inclusions.

Many reports and patents have been published for slag having high absorption capacity of oxide-based nonmetallic inclusions, and it is known that the degree of oxidation should be low so as to maintain a reducing atmosphere in any case. However, the degree of oxidation of the slag is increased as the amount of lower oxides such as iron oxide (FeO, Fe ₂ O _3, etc.), manganese oxide (MnO, etc.) increases, so it is included in the slag in contact with molten steel to reduce the oxide inclusions. The concentration of these lower oxides must be reduced.

The most widely used method for this purpose is a method of reducing the lower oxides and controlling the content of calcium oxide by adding a slag deoxidizer containing aluminum and the like. Currently, several patents have been applied or registered. 56122, 60757 and Korean Patent Application No. 92-10826.

According to the above patents, the concentration of T.Fe + MnO in the ladle slag is controlled to be 1.5 to 3% by weight or less for the purpose of reducing non-metallic inclusions in molten steel and improving cleanliness. In addition, the actual ladle slag has a relatively low content of FeO, MnO, rather, calcium oxide, silicon oxide, aluminum oxide, etc., the main component of the slag, the absorption capacity of the non-metallic inclusions of the slag depends on the composition ratio of these oxides, especially calcium oxide content It is known to vary. Considering the known techniques, the ratio of the weight percentage of calcium oxide / silicon oxide / aluminum oxide (hereinafter referred to as '(% CaO) / (% SiO ₂ ) / (% Al ₂ O ₃ )') is in the range of 0.25 to 0.45. .[0] It is known that the value is low, the amount of inclusions is small, the cleanliness of the steel is high, and the defect occurrence rate is low.

However, these known technologies suffer from the following problems in the actual process.

First, it is not easy to lower the T.Fe + MnO concentration to 1.5% level, as required by the known art using aluminum, and the ratio of aluminum injected to lower FeO and MnO in the slag is not used for its original purpose. not. This is because industrially used aluminum is bulky, heavier than slag, and easily penetrates into molten steel. Since aluminum in contact with molten steel dissolves into molten steel very quickly, the ratio of slag used to lower T.Fe + MnO decreases.

Second, when aluminum is added to molten steel, an oxide is formed. 90-95% of the produced oxide is separated and removed as a slag layer, but remains in the steel and becomes a non-metallic inclusion, which causes various defects in subsequent processes or products. Alumina, which degrades the mechanical properties of the steel, is still present in the steel.

Third, when the injected aluminum reduces FeO and MnO, alumina (Al ₂ O ₃ ) is produced. The requirement described above, that is, the ratio of (% CaO) / (% SiO ₂ ) / (% Al ₂ O ₃ ) is 0.25. To keep it in the range of 0.45, additional calcium oxide-containing materials such as quicklime should be added.

Finally, the refractory at the site of slag contact with slag in the ladle receiving slag and slag in the electric furnace is made of magnesia, and the slag composition with excellent inclusion absorption capacity (% CaO) / (% SiO ₂ ) / ( The slag fluidity is good in the ratio of% Al ₂ O ₃ ) in the range of 0.25 to 0.45, and the refractory erosion is severe because the contact with the magnesia refractory is easy.

Therefore, the present inventors have studied in various angles to improve the problem of aluminum added to reduce T.Fe + MnO as much as possible, and as a result, the T.Fe + MnO concentration is managed to a certain level or less, and the magnesium has excellent nonmetal removal ability. When the finishing process confirmed that the solution was possible, it came to propose this invention.

The present invention provides a method for refining aluminum deoxidized steel which can reduce the amount of nonmetallic inclusions in a cast steel and prevent nozzle clogging.

1 shows the effect of the sum of iron oxide and manganese oxide in slag on the oxygen concentration in steel;

It is a graph to represent;

2 shows the effect of T.Fe + MnO on aluminum or magnesium utilization.

It is a graph to represent;

3 is a product defective rate by inclusions according to the present invention and the conventional method

It is a graph.

In order to achieve the above object, the present invention, by tapping the molten steel into the ladle, and then injecting aluminum and subsidiary materials to determine the content of T.Fe + MnO and the ratio of CaO / SiO ₂ / Al ₂ O ₃ in the ladle slag In the method of refining aluminum deoxidized steel, including adjusting and refining by strongly bubbling and tapping the refined molten steel with a tundish, the content of T.Fe + MnO in the ladle slag is 15% by weight or less, In addition, strongly bubbling at a ratio of CaO / SiO ₂ / Al ₂ O ₃ to 0.25-0.45; Containing 100-1000g Mg alone or Mg mixed powder of molten steel in the steel bubbling molten steel at a rate of 10-100g per minute; is configured to include.

Hereinafter, the present invention will be described in detail.

In general, in an electric furnace or a converter, molten steel is first refined through a process such as decarburization, desulfurization, and delineation, followed by tapping into a ladle. Molten steel tapping with ladle is decarburized and deoxidized when decarburization is required to manage carbon concentration, or deoxidized immediately if decarburization is not needed.In this case, T.Fe + in the ladle slag is added by adding Al and secondary raw materials. The content of MnO and the ratio of CaO / SiO ₂ / Al ₂ O ₃ are controlled and strongly bubbled to remove nonmetallic inclusions of molten steel.

The present invention adjusts the ratio of CaO / SiO ₂ / Al ₂ O _{3 in} the ladle slag to 0.25-0.45 according to a conventional method, and reduces the content of Al added to lower the content of T.Fe + MnO. Instead of broadening the management range of Fe + MnO, Mg is introduced to effectively reduce nonmetallic inclusions by introducing Mg with a much higher vapor pressure than atmospheric pressure.

Therefore, according to the present invention, in order to prevent the ratio of (% CaO) / (% SiO ₂ ) / (% Al ₂ O ₃ ) from being reduced with a large amount of Al, there is no need to add calcium oxide such as quicklime. The production of non-metallic inclusions due to the addition of a large amount of aluminum is suppressed, and the magnesia component formed by the addition of Mg protects the magnesia refractory used in the ladle to reduce the erosion amount.

To this end, first, aluminum and subsidiary materials are added to the ladle slag so that the content of T.Fe + MnO in the ladle slag is 15% by weight or less, more preferably 3-15% by weight, and CaO / SiO ₂ / Strongly bubble the ratio of Al ₂ O ₃ to 0.25-0.45,

Here, limiting the ratio of CaO / SiO ₂ / Al ₂ O ₃ , which is a major component of ladle slag, to 0.25-0.45 is known to be effective in refining molten steel because of the low value of T. [O]. Because there is. In addition, the lower the content of T.Fe + MnO, the lower the T. [O] value in the steel, but about 15% is effective in reducing the heavy metal non-metallic inclusions, thereby broadening the management scope. And, 3% or more is to be 3% or more in the previous step because the utilization of Mg is high when 3% or less when Mg is added in the subsequent process as shown in FIG. Of course, managing the T.Fe + MnO content in this way is because it is possible to manage the T.Fe + MnO to an appropriate level by adding Mg in a subsequent process.

As described above, the ratio of T.Fe + MnO and CaO / SiO ₂ / Al ₂ O _{3 in} the ladle slag is strongly bubbled, and the ladle slag has a lower content of T.Fe + MnO as a lower oxide. High levels will contaminate molten steel. Therefore, it is necessary to further lower the T.Fe + MnO level in order to prevent contamination of molten steel by slag and to increase the inclusion absorption of slag.

The present invention blows 100-1000g Mg alone or Mg mixed powder per ton of molten steel at the rate of 10-100g per minute to the steel bubbling for this purpose.

Metal Mg has a very low solubility of about 0.05% by weight in molten steel at 1600 ° C, and vapor pressure is much higher than atmospheric pressure, so it does not cause any component change in molten steel even when blown into steel. When it reaches, it reacts easily with FeO, MnO, etc. in slag, and reduces the density | concentration.

This effect is apparent in the Mg input amount of more than 100g per ton of molten steel, and when the amount exceeds 1000g, the utilization efficiency is lowered. When Mg is injected into the molten steel, molten steel and slag are scattered by the vaporized Mg gas, but when the feed rate is 100 g or more per minute, the scattering is excessive and the workability deteriorates. Scattering of molten steel and slag decreases as the Mg feed rate decreases, but the time required for supplying a certain amount of Mg increases, which is inefficient, so the lower limit of the speed is limited to 10.

In addition, although Mg powder may be added independently, in order to prevent that the temperature of Mg powder increases excessively, it is good to add an oxide together, and these oxides are MgO and CaO. CaO shows an excellent metallurgical effect because it increases the concentration of calcium oxide in the slag.

At this time, the mixing ratio of the Mg-containing powder powder does not matter much, but Mg is more than 5% by weight and more preferably composed of one or two selected from the remaining MgO and CaO. This is because if the Mg is less than 5%, the feeding speed is increased and the powder may be scattered.

After refining as described above, tapping with a tundish, when using the molten steel for mini-mill continuous casting is required to clean molten steel to prevent nozzle clogging.

Therefore, Mg alone or Mg mixed powder is injected and Ca alone or Ca mixture is introduced into the steel which is strongly bubbling. The Ca reacts with Al ₂ O ₃ in the molten steel to form a CaO-Al ₂ O ₃ compound having a low melting point. Al ₂ O _{3 is} removed. The dose is preferably 0.2-1.0Kg per ton of molten steel, because the Ca alone or Ca mixture is less than 0.2kg, and the effect of clogging the nozzle is insufficient. This is because the error rate and utilization rate are lowered.

Hereinafter, the present invention will be described in detail through examples.

Example 1

In order to investigate the effect of T.Fe + MnO concentration in slag on the amount of nonmetallic inclusions in steel, experiments were conducted at 1600 ° C using a high-frequency air induction furnace. First, 30 kg of molten steel was dissolved, followed by deoxidation of molten steel with 0.15 kg of aluminum, and 2 kg of slag aid was added thereto. After slag was formed, a predetermined amount of aluminum chips were placed on top of the slag, and molten steel and slag specimens were collected and analyzed 12 minutes later (a conventional method).

The aluminum chip was added as described above, and then 30 g of magnesium powder was coated with iron shell and added to molten steel, and the experiment was conducted in the same manner (the present invention).

The results obtained in the experiment are shown in FIG. 1 as a relation between T.Fe + MnO concentration in slag and T. [0].

As shown in FIG. 1, as the T.Fe + MnO concentration decreases over the entire range of T.Fe + MnO concentration in slag, T. [0] may be decreased. In addition, when magnesium powder was used (invention), T. [0] was lower at the same T.Fe + MnO concentration than when aluminum was used alone (conventional method).

Oxygen present in steel is divided into dissolved oxygen dissolved in atomic state and oxygen in inclusion combined with oxide, depending on the type of existence. In aluminum deoxidized steel, dissolved oxygen is several ppm or less and does not change very much depending on the working conditions. Therefore, high oxygen concentration in steel means that the amount of oxygen in the oxide phase, or inclusions, is high, resulting in low cleanliness of steel and high defect occurrence rate. Therefore, it can be seen that the use of magnesium as in the present invention is more effective for removing non-metallic inclusions than when using aluminum alone.

Example 2

In order to quantitatively determine the ratio of aluminum and magnesium used to lower T.Fe + MnO in the slag, a high frequency induction melting furnace experiment was conducted at 1600 ° C.

The experimental method is similar to Example 1, and the results are shown in FIG. 2 as a relation between T.Fe + MnO concentration and utilization rate in slag after adding aluminum or magnesium. Here, the aluminum and magnesium utilization rate is calculated from the material resin using the amount of aluminum injected, molten steel, slag analysis values, and the like.

As the concentration of T.Fe + MnO in the slag decreases, the utilization rate of aluminum is higher than that of aluminum or magnesium used for slag deoxidation, but when it is 3.0 wt% or less, the aluminum utilization decreases rapidly. Therefore, slag deoxidation using magnesium as in the present invention can be said to be more effective at less than 3.0% by weight of the T.Fe + MnO concentration in the slag.

Example 3

Ladles containing 100-130 tons of molten steel and slabs of 1.5 to 3.0 tons at about 1600 ° C are continuously cast using molten steel refined as in the conventional method of Example 1, and the product defect rate by inclusions is investigated. The results are shown in FIG.

In addition, after the molten steel is deoxidized with aluminum into a ladle containing 120-150 tons of molten steel and slabs of 1.5 to 3.0 tons at about 1600 ° C, slag is added to the slag to form a predetermined amount of aluminum chips. ) Into the upper part of the slag, and then 100-1000 g of metal magnesium Mg powder per ton of molten steel alone or with CaO-containing material at a rate of 10 to 100 g per minute for the purpose of removing non-metallic inclusions and improving castability. Blown. Subsequently, thin slab was cast in a mini-mill continuous casting machine by refining molten steel by adding 0.2-1.0Kg of Ca alone or Ca mixture per ton of molten steel, and then examining the product defect rate by inclusions. The results are shown in FIG. 3.

As shown in FIG. 3, the defective rate of the product by inclusions was reduced by 50% or more.

As described above, according to the present invention, the amount of nonmetallic inclusions in the continuous cast can be reduced to obtain a clean steel, and the magnesia refractory material in which the magnesia component is used in the ladle can be protected. In particular, there is an effect that can prevent the clogging of the nozzle when playing the mini-mill.

Claims (4)

The molten steel is pulled out to the ladle, and then aluminum and auxiliary raw materials are added to the steel, and the steel is bubbling and refined while adjusting the content of T.Fe + MnO and the ratio of CaO / SiO ₂ / Al ₂ O ₃ in the ladle slag. In the method of refining aluminum deoxidized steel including the step of tapping the molten steel with a tundish, the content of T.Fe + MnO in the ladle slag is 15% by weight or less, and CaO / SiO ₂ / Al ₂ O ₃ Strongly bubbling the ratio of 0.25-0.45; And The method of refining aluminum deoxidized steel comprising the; step of blowing the Mg alone or Mg mixed powder of 100-1000g per ton of molten steel at a rate of 10-100g per minute to the strongly bubbled molten steel. The refining method according to claim 1, wherein the mixed Mg-containing powder is based on Mg, and one or two selected from MgO and CaO are mixed therein. The refining method according to claim 2, wherein the Mg-containing powder contains 5% by weight or more of Mg. The method according to any one of claims 1 to 3, wherein Mg alone or Mg mixed powder is blown and strongly bubbled, and then 0.2-1.0Kg of Ca alone or Ca mixture is added to molten steel, followed by tapping into tundish. Refining method characterized in that.