Denitrification treatment method of molten steel

Abstract

To provide a denitrification treatment method of molten steel with which a denitrification treatment can efficiently be performed without excessively using Mg.SOLUTION: In a denitrification treatment method of molten steel in which a denitrification treatment is performed by adding a refining agent comprising Mg to molten steel under reduced pressure, the refining agent is added so as to satisfy 0.03>[Mg]≥-0.0006[Al]2+0.0011[Al]+0.0003 and 0.0020>[O]≥0.0002 immediately after the completion of the addition of the refining agent, where [Mg], [Al], and [O] are the concentrations of Mg, Al, and O (all in mass%) in the molten steel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for denitrifying molten steel to efficiently perform denitrification. [Background technology]

[0002] Because AlN precipitates reduce the toughness of steel, the nitrogen concentration in molten steel is reduced during secondary refining. Various measures have been taken to generate bubbles to improve the denitrification rate during secondary refining, but this poses the problem of atmospheric intrusion and nitrogen pickup under reduced pressure. Therefore, a method has been proposed in which Mg, a volatile element, is added to molten steel in a vacuum degassing process to generate bubbles and accelerate denitrification, thereby achieving a faster denitrification rate than nitrogen absorption by atmospheric intrusion.

[0003] Patent Document 1 discloses a method for desulfurization and denitrification by spraying powder containing a deoxidizing agent such as Mg onto molten steel, while Patent Document 2 discloses a method for denitrification by adding an element such as Mg that has a lower boiling point than molten steel to generate fine bubbles. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-322431 [Patent Document 2] Japanese Patent Application Publication No. 2023-127879 [Non-patent literature]

[0005] [Non-Patent Document 1] Todoroki et al.: ISIJ Int., 50(2010), 50. Summary of the Invention [Problem to be solved by the invention]

[0006] However, since Mg is a highly volatile metal and has a low solubility in molten steel, it is necessary to add a large amount of Mg to achieve sufficient denitrification treatment using the methods described in Patent Documents 1 and 2. However, adding an excessive amount of Mg increases costs, and furthermore, the excess Mg evaporates and remains in the refining vessel, resulting in a problem of increased maintenance burden.

[0007] In view of the above-mentioned problems, an object of the present invention is to provide a method for denitrifying molten steel that can efficiently perform denitrification without using excessive amounts of Mg. [Means for solving the problem]

[0008] The present inventors have conducted extensive research into methods for achieving efficient denitrification without adding excessive amounts of Mg. Focusing on the composition of molten steel, they investigated the Mg dissolution rate and denitrification rate by spraying Mg-containing powder onto the surface of molten steel under vacuum with a carrier gas using experimental and commercial equipment. In these experiments, the oxide species coexisting in the system were varied to change the equilibrium oxygen concentration and the molten steel composition. The results revealed that the Mg dissolution rate in molten steel varies significantly depending on the chemical potentials of Mg, Al, and O in the molten steel. From these experimental results, the present inventors discovered that by appropriately controlling these chemical potentials, the Mg dissolution rate can be increased, resulting in a high denitrification rate.

[0009] The present invention is as follows. [1] A method for denitrifying molten steel by adding a refining agent containing Mg to molten steel under reduced pressure, A method for denitrification of molten steel, characterized in that, when the Mg concentration in molten steel is [Mg], the Al concentration is [Al], and the O concentration is [O] (all in mass%), the refining agent is added immediately after completion of addition so as to satisfy the following formulas (1) and (2): 0.03>[Mg]≧-0.0006[Al] 2 +0.0011[Al]+0.0003 (1) 0.0020>[O]≧0.0002 (2) [Effects of the Invention]

[0010] According to the present invention, denitrification can be carried out efficiently without using an excessive amount of Mg. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a graph showing the relationship between the Al concentration and the Mg concentration in molten steel. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings, in which: First, the mechanism by which the dissolution rate of Mg increases will be described.

[0013] First, the mechanism that affects the dissolution rate of Mg is thought to involve the reactions of the following equations (3) and (4). Mg + O = MgO (3) MgO + Al = Mg + Al2O3 (4)

[0014] In the equilibrium condition of Equation (3), when the activity of MgO is high, the concentration product on the left side increases, improving the Mg dissolution rate. However, when the O concentration in the molten steel is high, the reaction proceeds to the right, decreasing the Mg dissolution rate. Furthermore, according to the reaction in Equation (4), Al in the molten steel reacts with MgO, providing a source of Mg. In other words, it can be seen that the Mg dissolution rate is correlated with the O and Al concentrations in the molten steel. Therefore, the inventors conducted experiments by varying the types of refractories and refining agents in the refining vessel, taking these equilibrium reactions and the activity of each component into consideration. They also investigated the appropriate potential in the molten steel using the Fe-Al-Mg-Ca-O phase stability diagram described in Non-Patent Document 1.

[0015] First, in the experiment, 15 kg of electrolytic iron was heated to 1600°C and melted using various refractory materials in a 15 kg vacuum melting furnace. After adjusting the composition to a high Al and high Si composition, the pressure was reduced to 0.4 kPa. Then, CaO-15% Mg powder, or powder containing 50% MgO added to CaO-15% Mg, was top-blown from the lance as a refining agent. The composition of the molten iron was then investigated by sampling.

[0016] Figure 1 is a diagram showing the relationship between the Al concentration and the Mg concentration in molten steel. According to the Fe-Al-Mg-Ca-O phase stability diagram described in Non-Patent Document 1, as the Mg concentration increases, the stable region is divided into the CaO-Al2O3 liquid phase region, the spinel (MgO Al2O3) stable region, and the MgO stable region, and curve 11 in Figure 1 roughly coincides with the boundary between the CaO-Al2O3 liquid phase region and the spinel stable region.

[0017] The experimental results showed that when an MgO refractory was used, the composition of the molten steel was in the spinel stability region above curve 11 in Figure 1, and in this case, the activity of MgO was kept constant at the equilibrium of equation (3), and the Mg dissolution rate was also stable. In addition, partial dissolution of the MgO from the refractory shifted the equilibrium of reaction (4) to the right, making it easier for Mg to be supplied. Furthermore, when CaO-Mg-MgO powder was used, the equilibrium of reaction (4) also shifted to the right, and the Mg dissolution rate was also high.

[0018] On the other hand, when a ZrO2 refractory was used, the composition of the molten steel was below curve 11 in Figure 1, in the CaO-Al2O3 liquid phase stable region. In this case, the activity of MgO was low, and the Mg oxidation reaction (3) was dominant, resulting in a low Mg dissolution rate. Furthermore, when this ZrO2 refractory was used, even if CaO-Mg-MgO powder was used, the reaction (4) proceeded to the left because the composition was in the liquid phase stable region, and it was presumed that the Mg dissolution rate did not increase.

[0019] From the above experimental results, it was found that, based on the reactions of equations (3) and (4), it is important to add refining agents so that the composition of molten steel is in the spinel stability region or MgO stability region in order to carry out efficient denitrification treatment. Furthermore, it was found that the O concentration in molten steel also affects the Mg dissolution rate according to the equilibrium condition of equation (3), so it is necessary to set an upper limit for the O concentration.

[0020] In the method for denitrification of molten steel according to this embodiment, when a refining agent containing Mg is added under reduced pressure in secondary refining, the Mg concentration in the molten steel is adjusted to [Mg], the Al concentration to [Al], and the O concentration to [O] (all in mass%) so as to satisfy the following equations (1) and (2) immediately after the addition of the refining agent is completed. 0.03>[Mg]≧-0.0006[Al] 2 +0.0011[Al]+0.0003 (1) 0.0020>[O]≧0.0002 (2)

[0021] The lower limit in equation (1) is a quadratic function of the Al concentration, represented by curve 11 in Figure 1. If the Mg concentration falls below this lower limit, the CaO-Al2O3 liquid phase becomes stable, and Mg is easily consumed in the oxidation reaction, reducing the Mg dissolution rate and the denitrification rate. On the other hand, line 12 in Figure 1 represents the upper limit of the Mg concentration. Even if the Mg concentration is 0.03 mass% or higher, the Mg dissolution rate and denitrification rate are high, but the amount of Mg volatilization increases, increasing the maintenance burden on the refining vessel. Therefore, the Mg concentration is set to less than 0.03 mass%. Preferably, the Mg concentration is set to 0.015 mass% or less, and more preferably, to 0.01 mass% or less.

[0022] Furthermore, as mentioned above, when the O concentration is 0.0020% by mass or higher, the equilibrium in equation (3) shifts to the right, and Mg is consumed in the deoxidation reaction, resulting in a decrease in the Mg dissolution rate. On the other hand, the lower limit represents the realistic O concentration limit achieved by the deoxidation treatment carried out before the addition of the refining agent for the denitrification treatment, and it would be very costly to reduce the O concentration to less than 0.0002% by mass.

[0023] Next, we will explain a specific method for controlling the conditions so that the above-mentioned equations (1) and (2) are satisfied. First, an immersion tube is immersed in molten steel in a vacuum degassing treatment device such as an RH device, and the vacuum chamber is evacuated to reduce the pressure. Then, under reduced pressure (for example, less than 10 kPa), a powder of a refining agent containing Mg is sprayed onto the surface of the molten steel together with a carrier gas such as Ar to perform denitrification treatment. Note that this method is not limited to RH-type vacuum degassing treatment devices, and can be applied to various commonly used vacuum degassing processes.

[0024] In addition, before spraying the refining agent, deoxidation is performed by adding metallic Al to the molten steel. The amount of Al added during deoxidation varies depending on the target steel type, but deoxidation must be performed to the extent that the O concentration in the molten steel falls within the range of equation (2) immediately after adding the refining agent. Then, a refining agent containing Mg is added depending on the Al concentration in the molten steel after deoxidation. At this time, the amount of Mg volatilization is predicted from past operational results, and the amount of refining agent added is determined so that it falls within the range of equation (1).

[0025] In order to generate fine bubbles and perform denitrification, the refining agent must contain Mg. However, Mg may be contained as metallic Mg or as an Mg alloy. When Mg alloys are contained in the refining agent, evaporation can be suppressed and yield can be improved. Examples of Mg alloys that can be used include Mg-Si alloys and Mg-Ni alloys. If the Mg concentration in molten steel is the same, metallic Mg and Mg alloys have the same effect of generating Mg bubbles. However, when using Mg alloys, it is preferable to use them depending on the type of steel to be finally produced. For example, when using an Mg-Si alloy as the Mg alloy, Si remains in the molten steel, so it is preferable to use it when producing a steel type with a high Si concentration.

[0026] Furthermore, the refining agent may contain a substance that has not only a denitrification effect but also a desulfurization effect. For example, CaO functions as a nucleation site for Mg bubbles and reduces the sulfur concentration in molten steel as a desulfurization agent. Furthermore, the presence of Mg, a deoxidizing element, near CaO also promotes desulfurization. When the refining agent contains CaO, it is preferable that the content of CaO in the refining agent be 50 to 95 mass%. Note that, in addition to CaO, calcium aluminate or a mixture of CaO and CaF2 can also be used. Furthermore, as mentioned above, MgO may be contained. By including MgO in the refining agent, the dissolution rate of Mg can be further increased. When the refining agent contains MgO, it is preferable that the content of MgO in the refining agent be 5 to 30 mass%.

[0027] When adding the refining agent as a powder, it is sprayed onto the molten steel together with the carrier gas from a top-blowing lance. However, if the particle size of the refining agent is too small, some of it may be discharged as dust, which may result in a decrease in yield. On the other hand, if the particle size is too large, the specific surface area of ​​the powder becomes small, which tends to slightly reduce the denitrification effect. Therefore, it is preferable that the particle size of the refining agent has a median diameter of 5 to 300 μm. In addition, it is preferable that the powder spray rate be 100 to 500 g / min / ton of molten steel.

[0028] The method of adding the refining agent is not limited to adding the powder by top blowing, but may also be powder injection, alloy wire addition, or adding lumps from a hopper. [Example]

[0029] Next, an example of the present invention will be described, but the conditions are merely examples 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 in various ways to achieve the object of the present invention without departing from the gist of the present invention.

[0030] A ladle containing 280 t of molten steel tapped from a converter was transferred to an RH vacuum degasser, where an immersion tube was immersed in the molten steel. The vacuum vessel was then evacuated to 0.7 kPa. The molten steel composition at the time of tapping from the converter was, by mass, 0.15% C, 0.30% Si, 0.40% Mn, 0.020% P, 0.010% S, 0.008% Al, and 0.008% N, and the molten steel temperature was 1600–1650°C. Ar gas was then flowed through the side of the immersion tube as a reflux gas to reflux the molten steel. Al was then added according to the molten steel temperature, and the temperature was raised by oxygen spraying, after which the molten steel was refluxed for at least 10 minutes. A refining agent powder was then sprayed onto the molten steel surface along with Ar gas as a carrier gas from a refractory lance installed above the molten steel in the vacuum vessel. The refining agent used was 85% CaO-15% metallic Mg by mass. Table 1 shows the amounts of metallic Al and metallic Mg added.

[0031] The amount of alloy to be added was determined using the following method. The Al and Mg yields and O concentrations in the molten steel were estimated from conventional operation under the same conditions, and the amounts of Al and Mg to be added were determined from these estimates. During the vacuum refining process, molten steel samples were collected as needed, and their components were quantified using rapid analysis. Table 1 shows the concentrations of Mg, Al, and O in the molten steel immediately after the refining agent spraying was completed. The post-treatment [N] in Table 1 represents the N concentration immediately after the refining agent spraying was completed, and the denitrification rate in Table 1 was calculated by dividing the difference between the N concentration immediately after the refining agent spraying and the N concentration before the treatment by the refining time. The pH of the collected dust water was also measured, and a pH of 12 or higher was considered to indicate that a large amount of Mg-containing dust remained in the vacuum vessel.

[0032] [Table 1]

[0033] In Comparative Example 1, the O concentration in the molten steel immediately after the addition of the refining agent was too high, causing the Mg in the refining agent to be lost as MgO through oxidation, resulting in insufficient bubble generation and a low denitrification rate. In Comparative Examples 2, 3, and 5, the Mg concentration in the molten steel immediately after the addition of the refining agent was lower than the lower limit of formula (1). The Mg dissolution rate was low, and much Mg was consumed as MgO, resulting in insufficient bubble generation and a low denitrification rate. In Comparative Example 4, the Mg concentration in the molten steel immediately after the addition of the refining agent was too high. This resulted in the generation of excess bubbles exceeding the amount required for denitrification, resulting in a large amount of Mg-containing dust remaining in the vacuum tank. As a result, the pH of the collected water rose, necessitating a water treatment process.

[0034] In contrast, in Examples 6 to 10, the proportion of Mg contributing to the denitrification reaction could be increased by appropriately controlling the concentrations of Al, Mg, and O in the molten steel, and a high denitrification rate could be obtained. Furthermore, because the amount of Mg added was appropriate, little Mg remained in the vacuum vessel in any case, and additional water treatment was not required. [Explanation of symbols]

[0035] 11 curve 12 straight line

Claims

[Claim 1] A method for denitrifying molten steel by adding a refining agent containing Mg to molten steel under reduced pressure, A method for denitrification of molten steel, characterized in that, when the Mg concentration in molten steel is [Mg], the Al concentration is [Al], and the O concentration is [O] (all in mass%), the refining agent is added immediately after completion of addition of the refining agent so as to satisfy the following formulas (1) and (2): 0.03>[Mg]≧-0.0006[Al] 2 +0.0011[Al]+0.0003 ・・・(1) 0.0020>[O]≧0.0002...(2)

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