Denitrification method for molten steel

Abstract

To provide a method for denitrifying molten steel that is inexpensive and highly efficient. [Solution] A method for denitrifying molten steel using a vacuum degassing apparatus under reduced pressure, wherein a mixed powder containing hydroxide of an alkali metal or alkaline earth metal and metallic aluminum is blown from an upper blowing lance toward the molten steel surface together with a carrier gas, or is injected into the molten steel together with a carrier gas.

Description

Technical Field

[0001] The present invention relates to a method for denitrifying molten steel using a vacuum degassing device.

Background Art

[0002] Conventionally, in order to remove nitrogen, which is an impurity gas component in molten steel, vacuum degassing treatment is performed using devices such as RH and REDA. Generally, in order to promote the degassing reaction, it is effective to increase the reaction interface area or promote mass transfer. By blowing gas into the molten steel to generate bubbles, the reaction interface area is increased or stirring is strengthened.

[0003] Therefore, a method is known in which a highly volatile metal element such as an alkali metal or an alkaline earth metal is sprayed onto molten steel as a powder to generate bubbles from the inside of the molten steel. In this case, since bubbles are generated by the gasification reaction, the reaction interface area increases and the denitrification reaction is promoted. On the other hand, this method has a problem that it is necessary to produce expensive metal powder, resulting in an increase in cost. Therefore, Patent Document 1 discloses a technique in which a powder of a hydroxide, which is cheaper than a single metal, is sprayed onto molten steel, and hydrogen bubbles are generated by the decomposition of the hydroxide to increase the reaction interface area.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As mentioned above, highly volatile metal powders are expensive, and furthermore, molten steel adsorbs nitrogen from the atmosphere when exposed to it, so there is a need for an inexpensive and more efficient denitrification method. In the case of the method described in Patent Document 1, although the reaction interface area is increased by the decomposition of hydroxide, the denitrification effect is insufficient, and furthermore, since the decomposition reaction of hydroxide is an endothermic reaction, there is a problem that energy loss is large.

[0006] In view of the aforementioned problems, the present invention aims to provide a method for denitrifying molten steel that is inexpensive and highly efficient. [Means for solving the problem]

[0007] The inventors of this invention conducted extensive research to increase the reaction interface area using an inexpensive method. As a result, they discovered that by using a mixed powder of alkali metal or alkaline earth metal hydroxides, which are cheaper than elemental metals, and metallic aluminum, the reaction interface area can be further increased not only by water vapor but also by the evaporation of highly volatile alkali metal or alkaline earth metals.

[0008] The present invention is as follows: [1] A method for denitrifying molten steel, comprising performing denitrification of molten steel under reduced pressure using a vacuum degassing apparatus, characterized in that a mixed powder containing an alkali metal or alkaline earth metal hydroxide and metallic aluminum is supplied to the molten steel. [2] The method for denitrifying molten steel according to [1] above, characterized in that the hydroxide is magnesium oxide. [3] The method for denitrifying molten steel according to [1] or [2] above, characterized in that the mixed powder is injected into the molten steel using a carrier gas. [Effects of the Invention]

[0009] According to the present invention, denitrification can be performed inexpensively and with high efficiency. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described in detail below. First, the immersion tube of the RH-type vacuum degassing apparatus is immersed in the molten steel in the ladle, and the vacuum chamber is evacuated to reduce pressure. Then, a circulating gas (an inert gas such as Ar) is passed through to circulate the molten steel in the vacuum chamber, and bubbles are generated by supplying the mixed powder described below together with a carrier gas (an inert gas such as Ar) under reduced pressure conditions (e.g., less than 10 kPa), thereby performing the denitrification treatment. When supplying the mixed powder to the molten steel, the mixed powder may be sprayed onto the molten steel surface from an overhead lance, or it may be injected into the molten steel, but from the viewpoint of yield and reaction efficiency, it is preferable to supply the mixed powder by injection.

[0011] In this embodiment, a mixed powder containing an inexpensive alkali metal or alkaline earth metal hydroxide and metallic aluminum is sprayed onto the molten steel. When the mixed powder is added to the molten steel, the mixture heats up, causing the following reactions to occur: in the case of alkali metal hydroxides, the reactions described in equations (1) and (2) below; and in the case of alkaline earth metal hydroxides, the reactions described in equations (3) and (4) below. 2MOH → M2O + H2O ... (1) 2MOH+2Al+2H2O→M2O+Al2O3+3H2...(2) M(OH)2 → MO + H2O ... (3) M(OH)2+2Al+2H2O→MO+Al2O3+3H2...(4) Here, M represents an alkali metal or alkaline earth metal.

[0012] Reactions (1) or (3) cause hydroxide to decompose in molten steel, generating water vapor (H2O) bubbles, which can increase the reaction interface area. Additionally, reactions (2) or (4) generate hydrogen, and these hydrogen bubbles can also increase the reaction interface area. On the other hand, reactions (2) or (4) produce oxides of alkali metals or alkaline earth metals, but in the vacuum chamber of a vacuum degassing apparatus, these oxides undergo the following reactions (5) or (6). 3M2O + 2Al → 6M + Al2O3 ... (5) 3MO + 2Al → 3M + Al2O3 ... (6)

[0013] Reactions (5) or (6) cause these oxides to decompose in molten steel, producing alkali metals or alkaline earth metals. Since these metals all have high vapor pressures, they contribute to increasing the reaction interface area as bubbles. Furthermore, while reactions (1) and (3) are endothermic, reactions (2), (4), (5), and (6) are exothermic. Therefore, the energy loss caused by the hydroxide reaction can be compensated for, and the effort and cost of additional processing to maintain the molten steel temperature can be reduced. In addition, alumina is produced by the reaction of hydroxides and oxides, but the amount of alumina recovered is less compared to when metallic aluminum is added separately and oxygen gas is blown in to raise the molten steel temperature in order to compensate for energy loss.

[0014] In this embodiment, the type of alkali metal or alkaline earth metal is not particularly limited as long as the mixed powder contains an inexpensive alkali metal or alkaline earth metal hydroxide and metallic aluminum. However, magnesium hydroxide is preferred as the hydroxide for the following reasons: Magnesium hydroxide is inexpensive and therefore readily available in large quantities. Furthermore, magnesium has higher elemental stability than sodium, potassium, and calcium, making it easier to handle the reaction product when using a mixed powder containing magnesium hydroxide. In addition, magnesium has a smaller atomic weight than calcium, another alkaline earth metal, resulting in a larger amount of gas being generated per unit weight, thus increasing the refining efficiency.

[0015] Furthermore, the closer the mixing ratio of alkali metal or alkaline earth metal hydroxide to metallic aluminum is to the molar ratio at which the reaction of equation (5) or (6) occurs, the greater the denitrification effect due to metal bubbles and the suppression of energy loss. In other words, in the case of a mixed powder containing alkali metal hydroxide or alkaline earth metal hydroxide and metallic aluminum, a molar ratio close to 3:2 is more effective. For these reasons, the mixing ratio of alkali metal or alkaline earth metal hydroxide to metallic aluminum is preferably 1.00 to 1.70 in molar ratio calculated as (OH) / Al.

[0016] In addition to the alkali metal or alkaline earth metal hydroxides and metallic aluminum mentioned above, other components may be included, provided they do not react with metallic aluminum. For example, CaO may be included in the mixed powder. The inclusion of CaO allows for the formation of fine bubbles by providing nucleation sites for alkali metal or alkaline earth metal bubbles on the surface of the CaO particles, thereby increasing the reaction interface area. Furthermore, since CaO also promotes desulfurization, a desulfurization effect can be obtained simultaneously.

[0017] The preferred input amount of the mixed powder varies depending on the (OH) / Al molar ratio described above, and it is preferable that the effective powder unit consumption (kg / t-steel) calculated by the following formula (7) or (8) is 1.2 kg / t-steel or more. When CaO or the like is included in addition to the hydroxides of alkali metals or alkaline earth metals and metallic aluminum, it is calculated as the effective powder unit consumption excluding the content of CaO or the like. Effective powder unit consumption (kg / t-steel) = Mixed powder unit consumption (kg / t-steel) / {(OH) / Al molar ratio} (when (OH) / Al molar ratio ≥ 1.00) ·· (7) Effective powder unit consumption (kg / t-steel) = Mixed powder unit consumption (kg / t-steel) × {(OH) / Al molar ratio} (when (OH) / Al molar ratio < 1.00) ·· (8)

[0018] Regarding the particle size of the mixed powder, when spraying the mixed powder together with the carrier gas (Ar gas) from the top-blown lance towards the molten steel under reduced pressure, if the particle size of the mixed powder is too small, a part will be discharged as dust, and the yield may decrease. On the other hand, if the particle size is too large, the specific surface area of the mixed powder will be small, and the denitrification effect tends to decrease slightly. Therefore, the particle size of the mixed powder is preferably 10 - 200 μm as the median diameter. On the other hand, when injecting the mixed powder into the molten steel together with the carrier gas (Ar gas) by injection, it is not easily discharged as dust, so the particle size of the mixed powder is preferably 5 - 200 μm as the median diameter.

[0019] If the flow rate of the carrier gas is too small, the powder will not reach the molten steel and the loss to the exhaust system will increase. On the other hand, if it is too large, the splashed molten steel will adhere to the furnace wall, leading to a decrease in yield and deterioration of cleanliness. Therefore, the flow rate of the carrier gas is preferably 5 - 20 Nm

[0018] / min. Regarding the powder supply rate of the mixed powder, if it is too small, the refining effect cannot be exerted. On the other hand, if it is too large, the refining agent will accumulate on the surface of the molten steel and the refining effect will decrease. Therefore, the powder supply rate is preferably 0.3 - 0.8 kg / (min·ton of molten steel).

[0020] In addition, since the mixed powder supplied to the molten steel contains metallic aluminum, if the oxygen concentration in the molten steel is too high, part of the metallic aluminum in the mixed powder will be consumed for deoxidation. Although it is possible to cope by increasing the ratio of metallic aluminum in the mixed powder according to the oxygen concentration in the molten steel, the ratio of its divalent oxide becomes small and the denitrification effect decreases. Therefore, before supplying the mixed powder, it is preferable that the total oxygen concentration [T-O] in the molten steel is 20 ppm or less, and more preferably the total oxygen concentration [T-O] is 13 ppm or less.

Example

[0021] Next, examples of the present invention will be described. However, these conditions are one example of conditions for confirming the feasibility and effects of the present invention, and the present invention is not limited to the description of this example. The present invention can be implemented by various means without departing from the gist of the present invention and achieving the object of the present invention.

[0022] A ladle containing 280 t of molten steel discharged from a converter was moved to an RH-type vacuum degassing device, the immersion tube was immersed in the molten steel, and the inside of the vacuum chamber was evacuated to a reduced pressure (10 kPa). Then, a mixed powder having the components shown in Table 1 was supplied to the molten steel together with a carrier gas by top blowing or injection to perform a denitrification treatment. The components of the molten steel before the denitrification treatment were, in mass%, C: 0.15%, Si: 0.30%, Mn: 0.40%, P: 0.020%, S: 0.010%, Al: 0.008%, N: 0.008%, T-O: 0.0018%, and the balance was Fe and impurities. Also, the molten steel temperature was always 1600 to 1650 °C. Table 1 shows the powder unit and effective powder unit of the mixed powder when the mixed powder was supplied to the molten steel. In the denitrification treatment, Ar gas was used as the carrier gas, and its flow rate was 350 Nm 3 / h, and the powder supply rate of the mixed powder was 180 kg / min. Further, the particle size of the mixed powder was all 50 μm in median diameter.

[0023] Furthermore, as an evaluation method, the nitrogen concentration in the molten steel was measured after the denitrification treatment, and the denitrification rate (ppm / min) was calculated by dividing the difference in nitrogen concentration before and after the denitrification treatment by the denitrification treatment time. The invention was evaluated as having achieved its effect if the denitrification rate was 2.0 ppm / min or higher. The experimental results are shown in Table 1.

[0024] [Table 1]

[0025] In the example tests No. 7 to No. 18, the denitrification rate was sufficiently high, and denitrification could be performed with high efficiency.

[0026] On the other hand, in comparative examples No. 1 and No. 2, only alkaline earth metal hydroxide powder was supplied to the molten steel, resulting in the generation of only hydrogen bubbles. Consequently, the amount of gas generated was less than in the examples, and the denitrification rate was lower. Furthermore, in comparative examples No. 3 and No. 4, since a mixed powder that did not contain alkali metal or alkaline earth metal hydroxides was supplied to the molten steel, almost no bubbles were generated, and the denitrification rate was close to zero. Furthermore, in comparative examples No. 5 and No. 6, a mixed powder containing oxides such as CaO or MgO but without metallic aluminum was supplied to the molten steel, resulting in the generation of only hydrogen bubbles. Consequently, the amount of gas generated was less than in the examples, and the denitrification rate was lower.

Claims

1. A method for denitrifying molten steel, comprising performing denitrification of molten steel under reduced pressure using a vacuum degassing apparatus, characterized in that a mixed powder containing an alkali metal or alkaline earth metal hydroxide and metallic aluminum is supplied to the molten steel.

2. The method for denitrifying molten steel according to claim 1, characterized in that the hydroxide is magnesium oxide.

3. The method for denitrifying molten steel according to claim 1 or 2, characterized in that the mixed powder is injected into the molten steel using a carrier gas.

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