Desulfurization method for molten steel

Abstract

To provide a more reaction-efficient method for desulfurizing molten steel. [Solution] A method for desulfurizing molten steel in which a desulfurizing agent is blown from the top of an RH-type vacuum degassing apparatus, wherein the desulfurizing agent is a powder containing 50% by mass or more of CaCO3 and having an average particle size of 50 μm or more and 500 μm or less, and the powder supply rate of the desulfurizing agent is set to W PB (kg / min), when the molten steel flow rate is Q (tons / min), the relative W PB Desulfurization is performed so that / Q is 1.50 or less.

Description

[Technical Field]

[0001] This invention relates to a method for desulfurizing molten steel in secondary refining. [Background technology]

[0002] In recent years, the performance levels required for steel products have been increasing, and thorough removal of impurities such as S and C from molten steel is necessary. For example, regarding S, in order to produce ultra-low sulfur steel, a common method is to desulfurize the molten steel surface by spraying a desulfurizing agent powder mainly composed of CaO along with a carrier gas in a reflux-type vacuum degassing apparatus such as an RH.

[0003] On the other hand, when powder is blown upwards into the vacuum chamber of a vacuum degassing apparatus, much of the powder is scattered into the exhaust system, and even if the powder reaches the surface of the molten steel, it is difficult to break the surface tension of the molten steel and penetrate into the interior. Therefore, Patent Document 1 discloses a technique in which a desulfurizing agent is blown from an upward-blowing lance into a vacuum degassing apparatus, in which a desulfurizing agent is a uniform mixture of a certain proportion of a powdery desulfurizing agent mainly composed of CaO, a powdery gas generating substance made of metal carbon oxide, and a metal powder having deoxidizing power, and the desulfurizing agent is a briquette with a particle size of 1 mm or more. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-188728 [Non-patent literature]

[0005] [Non-Patent Document 1] T. Kuwabara, K. Umezawa, K. Mori and H. Watanabe: Trans. Iron Steel Inst. Jpn., 28(1988), 305 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the technology described in Patent Document 1 uses a desulfurizing agent with large particle size, resulting in a small reaction interface area and insufficient reaction efficiency in desulfurization.

[0007] In view of the aforementioned problems, the present invention aims to provide a method for desulfurizing molten steel with higher reaction efficiency. [Means for solving the problem]

[0008] The inventors of the present invention have diligently studied methods to allow the desulfurizing agent to penetrate deep into the molten steel and improve reaction efficiency when spraying the desulfurizing agent onto the molten steel from an upper blowing lance under reduced pressure in an RH-type vacuum degassing apparatus. They focused on CaCO3, which is less expensive than CaO, as a component of the desulfurizing agent. Since CaCO3 has a higher specific gravity than CaO, when used as the main component of the desulfurizing agent, it penetrates deeper into the molten steel compared to when CaO is the main component. Therefore, the particle size of the desulfurizing agent can be made smaller in order to increase the reaction interface area.

[0009] On the other hand, since the decomposition reaction of CaCO3 is an endothermic reaction, when CaCO3 powder is sprayed onto molten steel, the CaCO3 decomposes, resulting in heat loss. Furthermore, when the decomposition reaction of CaCO3 occurs, CO and O are produced, which can cause the desulfurization reaction in equation (3) below to proceed to the left, potentially inhibiting the desulfurization reaction. CaO + S = CaS + O (3)

[0010] Therefore, the inventors conducted desulfurization tests through experiments and found that the effect of heat loss was very small, and that this effect could be further reduced by including deoxidizing components such as metallic aluminum in the desulfurizing agent. Furthermore, it was found that the effect of improving reaction efficiency by using CaCO3 as the main component, and the effect of improving reaction efficiency by circulating and stirring the molten steel with reflux gas, were greater than the inhibition of the desulfurization reaction by the generation of CO and O due to the decomposition reaction of CaCO3.

[0011] The present invention is as follows. [1] A method for desulfurizing molten steel by blowing a desulfurizing agent from an upper lance to desulfurize the molten steel in a RH type vacuum degassing apparatus, using a powder containing 50% by mass or more of CaCO3 and having an average particle size of 50 μm or more and 500 μm or less as the desulfurizing agent, characterized in that the desulfurizing agent is sprayed onto the molten steel surface under conditions satisfying the following formulas (1) and (2). W PB / Q ≦ 1.50 ··· (1) Q = 11.4G 1 / 3 D LEG 4 / 3 {ln(P1 / P2)} 1 / 3 ··· (2) In the formulas, W PB represents the powder supply rate (kg / min) of the desulfurizing agent, Q represents the molten steel circulation rate (ton / min). G represents the flow rate of the circulating gas (Nl / min), and D LEG represents the inner diameter (m) of the immersion tube. Also, P1 represents the pressure (kPa) at the gas injection position of the circulating gas, and P2 represents the pressure (kPa) in the vacuum chamber.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a method for desulfurizing molten steel with higher reaction efficiency.

Brief Description of the Drawings

[0013] [Figure 1] It is a diagram for explaining the outline of a RH type vacuum degassing apparatus.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram for explaining a state in which a desulfurizing agent is sprayed from an upper lance in a RH type vacuum degassing apparatus. First, the immersion tube 5 of the RH-type vacuum degassing apparatus 1 is immersed in the ladle 4 containing the molten steel 6. Then, a vacuum is drawn out in the vacuum chamber 3 to create a reduced pressure inside the vacuum chamber 3. Tuyeres (not shown) are provided on the inner side of the immersion tube 5, and under reduced pressure conditions, a circulating gas 7 such as Ar gas is blown into the molten steel 6 from the tuyeres to circulate the molten steel 6.

[0015] Furthermore, an upward-blowing lance 2 is installed at the top of the RH-type vacuum degassing apparatus 1. While the molten steel 6 is circulating inside the immersion tube 5, the desulfurization treatment is performed by blowing the desulfurizing agent powder 8, along with the carrier gas, from the upward-blowing lance 2 toward the surface of the molten steel. The shape of the upward-blowing lance 2 is not particularly limited, but for example, an upward-blowing lance with a Laval nozzle can be used.

[0016] First, let's explain the desulfurizing agent sprayed during the desulfurization process. In this embodiment, a desulfurizing agent containing 50% by mass or more of CaCO3 as the main component is used, as CaCO3 is cheaper than CaO and has a higher specific gravity than CaO. Because CaCO3 has a higher specific gravity than CaO, when used as the main component of the desulfurizing agent, it penetrates more easily into the molten steel compared to when CaO is the main component. Furthermore, CaCO3 decomposes thermally in the molten steel to become fine CaO. Therefore, even if a larger particle size is sprayed on top to facilitate penetration, it can decompose in the molten steel, increasing the reaction interface area and promoting powder penetration and reaction efficiency. If the CaCO3 content is less than 50% by mass, powder penetration and reaction efficiency cannot be sufficiently promoted. There is no particular upper limit on the proportion of CaCO3, and a desulfurizing agent containing 100% by mass of CaCO3 may also be used.

[0017] Furthermore, it is preferable that the desulfurizing agent also contains other components that promote the desulfurization effect, such as components that have a deoxidizing effect. Specifically, it is preferable to include one or more selected from the group consisting of CaO, Al2O3, MgO, CaF2, Na2O, and metal powders (or alloys) that have a deoxidizing effect. Among these, it is more preferable to include CaO, which reacts directly with S in molten steel. The following describes each component.

[0018] As mentioned above, CaO is a component that causes the desulfurization reaction through the reaction of equation (3), and if CaO is included in the desulfurizing agent, the desulfurization reaction can be promoted without generating heat loss. Al2O3 is a component that improves the liquid phase ratio of the slag, and if Al2O3 is included in the desulfurizing agent, the reaction between slag and metal is promoted, and as a result the desulfurization reaction is promoted. MgO has the effect of protecting the refractory material in the vacuum chamber when molten steel is circulated.

[0019] CaF2 is a component that reduces the viscosity and liquidity of slag. When CaF2 is included in a desulfurizing agent, it promotes the slag-metal reaction, similar to Al2O3, and as a result, the desulfurization reaction is accelerated. Na2O is a component that enhances the desulfurization ability of slag. When Na2O is included in a desulfurizing agent, it can complement the desulfurization ability of fine CaO particles decomposed from CaCO3.

[0020] Furthermore, a metal powder with deoxidative properties is a metal powder that possesses deoxidative ability, such as Al. When metallic Al is included in the desulfurizing agent, the metallic Al reacts with oxygen in the molten steel, causing the reaction to proceed to the right in equation (3), thereby promoting the desulfurization reaction. In addition, the exothermic reaction during deoxidation compensates for heat loss, preventing a decrease in the molten steel temperature. Note that an alloy may be used instead of a single metal powder; for example, an Al alloy may be included in the desulfurizing agent.

[0021] Regarding the particle size of the desulfurizing agent, in order for the desulfurizing agent, which mainly consists of CaCO3, to penetrate into the molten steel and for the desulfurization reaction to proceed with high reaction efficiency, the average particle size should be between 50 μm and 500 μm. If the average particle size is less than 50 μm, the amount scattered into the exhaust system increases, and it becomes difficult for the powder to penetrate into the molten steel due to top blowing, resulting in a decrease in reaction efficiency. Also, if the average particle size exceeds 500 μm, the acceleration of the powder due to top blowing decreases, making it difficult for the powder to penetrate into the molten steel. Furthermore, since the reaction interface area of ​​the powder becomes smaller, even if CaCO3 decomposes in the molten steel, the reaction interface area of ​​the resulting CaO also becomes smaller, resulting in a decrease in reaction efficiency. Preferably, the average particle size is between 100 μm and 300 μm.

[0022] Next, the spraying conditions of the desulfurizer with the above-described components and particle sizes will be described. In the present embodiment, in the RH-type vacuum degassing apparatus, the circulating gas is sprayed from the immersion tube to circulate the molten steel, so that the desulfurizer does not deposit on the surface of the molten steel, and further, the thermal decomposition of the desulfurizer before it enters the molten steel is suppressed. Therefore, when the powder supply rate of the desulfurizer is W PB (kg / min) and the molten steel circulation rate is Q (ton / min), the desulfurization treatment is performed so as to satisfy the following formula (1). W PB / Q ≦ 1.50 ···(1)

[0023] Here, the molten steel circulation rate Q can be calculated by the following formula (2) described in Non-Patent Document 1. Q = 11.4G 1 / 3 D LEG 4 / 3 {ln(P1 / P2)} 1 / 3 ···(2)

[0024] (In formula (2), G represents the flow rate of the circulating gas (Nl / min), and D LEG represents the inner diameter of the immersion tube (m). Also, P1 represents the pressure (kPa) at the gas injection position of the circulating gas, and P2 represents the pressure (kPa) in the vacuum chamber.)

[0025] When the ratio W PB / Q exceeds 1.50, the circulation of the molten steel becomes insufficient with respect to the powder supply amount, and a part of the desulfurizer deposits on the surface of the molten steel, resulting in a decrease in the reaction efficiency. Note that the lower limit of the ratio W PB / Q is not particularly limited. However, since the wear of the refractory becomes large when the molten steel circulation rate Q is too large, the molten steel circulation rate Q is preferably 250 ton / min or less. Also, when the powder supply rate W PB is too small, the desulfurization treatment time becomes too long. Therefore, the powder supply rate W PB is preferably 50 kg / min or more.)

[0026] Furthermore, the pressure P2 inside the vacuum chamber during desulfurization is preferably reduced to 10 kPa or less to prevent poor acceleration when spraying the desulfurizing agent. The type of carrier gas used when spraying the desulfurizing agent from the top-blowing lance under reduced pressure is not particularly limited, but from the viewpoint of operational stability and processing costs, rare gas elements are preferred, and the use of Ar gas is more preferable. In this embodiment, the flow rate of the carrier gas is adjusted so that the powder supply rate satisfies equation (1). However, if the flow rate of the carrier gas is too low, the force of the carrier gas jet decreases, reducing the acceleration effect of the desulfurizing agent and making it difficult for the desulfurizing agent to penetrate into the molten steel. Also, if the flow rate of the carrier gas is too high, the molten steel will scatter inside the vacuum chamber due to the carrier gas jet, increasing the load on the equipment. From the above viewpoint, the flow rate of the carrier gas is 200 to 800 Nm 3 It is preferable to use / min. [Examples]

[0027] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0028] A ladle containing 300 tons of molten steel is placed below the RH-type vacuum degassing unit, and the inner diameter of the immersion tube D is set from above. LEG Two 0.8m immersion tubes were immersed in molten steel, and the pressure inside the vacuum chamber was reduced to the pressure P2 shown in Table 1. Then, Ar gas was blown into the molten steel as the recirculating gas, and the molten steel was circulated. The recirculating gas flow rate G at this time was 2000 Nl / min, and the pressure P1 at the gas injection point of the recirculating gas was 114.6 kPa. The composition of the molten steel before desulfurization was 0.03-0.06 mass% C, 0.02-0.20 mass% Si, 0.1-1.5 mass% Mn, 26-33 ppm S, and 0.01-0.20 mass% Al, and the molten steel temperature was 1590-1610°C.

[0029] Next, using Ar gas as the carrier gas, desulfurization was performed by blowing desulfurizing agent powder from an overhead lance toward the molten steel surface. The desulfurization time was set to 5-7 minutes. After the desulfurization treatment, the sulfur concentration in the molten steel was measured to confirm the desulfurization effect. To confirm the effect, the desulfurization k value, expressed by the following equation (4), was used as an indicator of the reaction efficiency of desulfurization. Desulfurization k value = ln([%S] 脱硫前 / [%S] 脱硫後 ) / Desulfurizing agent cost per unit ···(4)

[0030] (4) [%S] in equation 脱硫前 [%S] 脱硫後 The values ​​(ppm) represent the sulfur concentration in the molten steel before desulfurization, and the desulfurizing agent unit cost (kg / ton) represents the amount of powder supplied during desulfurization divided by the mass of molten steel. The desulfurization k value was evaluated as having achieved the effect of the invention when it was 0.170 (1 / (kg / ton)) or higher. The experimental results are shown in Table 1.

[0031] [Table 1]

[0032] Underlined text in the table indicates conditions outside the scope of the present invention. Experimental results showed that for all Ch.No.1 to 10, almost no heat loss due to CaCO3 was observed before and after desulfurization treatment. For Ch.No.1 to 5, the desulfurization treatment was performed under conditions where the proportion of CaCO3 in the desulfurizing agent was 50% by mass or more, the average particle size was in the range of 50 to 500 μm, and equation (1) was satisfied, resulting in a high desulfurization k value and high reaction efficiency in desulfurization.

[0033] On the other hand, in the case of Ch.No.6 and 7, the desulfurization treatment was carried out under conditions that did not satisfy equation (1), resulting in an excess supply of powder relative to the molten steel circulation flow rate. This caused some of the desulfurizing agent to accumulate on the surface of the molten steel, resulting in low reaction efficiency during desulfurization. In addition, in the case of Ch.No.8, the proportion of CaCO3 in the desulfurizing agent was too low, resulting in a low specific gravity of the entire desulfurizing agent. This prevented the desulfurizing agent powder from penetrating sufficiently into the molten steel, resulting in low reaction efficiency during desulfurization.

[0034] In Ch.No.9, the average particle size of the desulfurizing agent powder was too large, resulting in a small reaction interface area and low reaction efficiency during desulfurization. Similarly, in Ch.No.10, the average particle size of the desulfurizing agent powder was too small, preventing the powder from penetrating sufficiently into the molten steel, resulting in low reaction efficiency during desulfurization. [Explanation of symbols]

[0035] 1. RH-type vacuum degassing apparatus 2. Upward-blowing lance 3 Vacuum chamber 4 Ladle 5 dip tube 6 Molten steel 7. Circulating gas 8 Powder

Claims

[Claim 1] A method for desulfurizing molten steel in which a desulfurizing agent is blown upward from an upward blowing lance in an RH-type vacuum degassing apparatus, As the desulfurizing agent, CaCO 3 Using a powder containing 50% by mass or more of and having an average particle size of 50 μm or more and 500 μm or less, A method for desulfurizing molten steel, characterized by spraying the desulfurizing agent onto the molten steel surface under conditions that satisfy the following equations (1) and (2). W PB / Q≦1.50 ・・・(1) Q=11.4G 1 / 3 D LEG 4 / 3 {ln(P 1 / P 2 )} 1 / 3 ・・・(2) where W PB represents the powder supply rate (kg / min) of the desulfurizing agent, Q represents the molten steel circulation rate (ton / min). G represents the flow rate of the circulating gas (Nl / min), and D LEG represents the inner diameter (m) of the immersion tube. Also, P 1 represents the gas injection position pressure (kPa) of the circulating gas, and P 2 represents the pressure (kPa) in the vacuum chamber.

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