JP2018100427A - Method for producing low sulfur steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a low sulfur steel reduced in inclusions at a high desulfurization rate.SOLUTION: Provided is a method where, when a molten steel in a converter is tapped into a ladle, aluminum is added in a range satisfying formula (1), the molten steel is subjected to deacidification treatment, thereafter, a burner flame is formed at the tip lower part of a top-blown lance 13 installed in a vacuum refining furnace, while heating a CaO-AlObased premelt desulfurizing agent with the burner flame, it is sprayed toward the molten steel surface to subject the molten steel 3 to desulfurizing treatment, the ladle 2 is allowed to stand for 10 min or higher in a period from the completion of the treatment in the vacuum treatment furnace to the start of continuous casting in continuous casting equipment, and continuous casting for the molten steel in the ladle is started. 0.0011×a+10×[%Al]+0.05≤W≤0.0011×a+10×[%Al]+1.30 (1)( adenotes the content of oxygen in the molten steel at the end point of decarburization treatment; [%Al]denotes the lower limit value of the standard value of the Al content in a steel product; and Wdenotes the content of Al added to the molten steel upon the tapping.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing low-sulfur steel by subjecting molten steel to desulfurization in a vacuum refining furnace such as an RH vacuum degassing apparatus.

  In recent years, there has been an increasing demand for high-purity steel smelting with less impurities in order to improve material properties associated with increased added value of steel and expanded use of steel materials. In particular, there is a high demand for low-sulfur steel with a low content of sulfur (S), an element that impairs the toughness of steel materials. Steel with a low sulfur content is manufactured by performing a desulfurization process in a hot metal stage with high desulfurization efficiency, and then decarburizing the hot metal in a converter. However, even if sulfur is reduced in the hot metal stage, sulfur is picked up during refining in the converter. For example, low sulfur steel with a sulfur content standard value of 0.0030% by mass or less is desulfurized in the hot metal stage. It is difficult to produce stably only by itself. Therefore, in the production of low-sulfur steel such as high-grade electrical steel sheets and steel for line pipes, desulfurization treatment has been performed at the molten steel stage using a ladle refining furnace after refining in a converter.

  However, the ladle smelting furnace does not have a refining function under reduced pressure, and when degassing such as dehydrogenation or denitrogenation is required, the ladle smelting furnace and the RH vacuum degassing equipment can It must be refined in both the smelting furnace. As a result, not only operational problems such as a decrease in molten steel temperature, a reduction in work efficiency, and an extension of the lead time from steelmaking to casting, but also a facility problem that two secondary refining furnaces are necessary are generated. . Thus, for the purpose of integrating the secondary smelting furnace and simplifying the secondary smelting process, a method of desulfurization treatment in a vacuum smelting furnace such as an RH vacuum degassing apparatus has been performed.

  A desulfurization treatment method using an RH vacuum degassing apparatus is generally a method in which a desulfurizing agent is introduced onto molten steel in a vacuum chamber from a raw material charging port provided in the vacuum chamber, and the molten steel and desulfurizing agent are agitated. However, there is a drawback that the yield of the desulfurizing agent is poor due to the influence of the desulfurizing agent being sucked into the exhaust system. If the particle size of the desulfurizing agent is increased, suction of the desulfurizing agent into the exhaust system can be prevented, but the desulfurization reaction interfacial area is reduced, which is disadvantageous in terms of reaction efficiency.

  Therefore, in the desulfurization treatment of molten steel in a vacuum refining furnace, many proposals have been made for the purpose of increasing the yield of adding a desulfurizing agent and efficiently desulfurizing the molten steel.

  Patent Document 1 proposes a method in which a powdered desulfurizing agent is blown into molten steel together with a carrier gas through a desulfurizing agent blowing tuyere provided below the surface of the molten steel bath in the lower part of the vacuum tank of the RH vacuum degassing apparatus. . However, in the method proposed in Patent Document 1, the blowing tuyere is a consumable item, and maintenance and inspection of the blowing tuyere is necessary, which not only increases the manufacturing cost but also blows the conveying gas. The drop of the molten steel temperature due to is a problem. In addition, during the period when the desulfurizing agent is not blown, it is necessary to flow a rare gas in order to prevent the molten steel from entering and closing the desulfurizing agent blowing tuyere. There is a problem that the degree of vacuum in the vacuum chamber is lowered.

Patent Document 2 discloses a method for desulfurizing molten steel by blowing a desulfurizing agent onto a molten steel in a vacuum tank together with a conveying gas from an upper blowing lance installed in a vacuum tank of an RH vacuum degassing apparatus (a solid substance is bathed together with a conveying gas). Spraying the surface is also called “projection”). In a desulfurization process performed by spraying a desulfurizing agent onto molten steel from an upper blowing lance as in Patent Document 2, a CaO—CaF ₂ -based desulfurizing agent is usually used to promote hatching of the desulfurizing agent. However, when a desulfurizing agent containing CaF ₂ is used, there is a problem that the processing cost of the generated desulfurized slag increases from the environmental problem of elution of fluorine (F), and due to the CaF ₂ in the desulfurizing agent, There is a problem that the inner wall of the ladle, the vacuum chamber, and the dip tube are significantly melted and the refractory cost is increased.

Patent Document 3 proposes a CaO—Al ₂ O ₃ desulfurization agent having a CaO content of 60% by mass or more, an Al ₂ O ₃ content of 5 to 40% and containing no fluorine. The use of the CaO—Al ₂ O ₃ desulfurizing agent prevents significant refractory melting, but when the Al ₂ O ₃ content is in the range of 5 to 40%, the CaO—Al ₂ O ₃ desulfurizing agent is used. The melting point is high, the hatching rate of the added CaO—Al ₂ O ₃ desulfurizing agent is slow, and efficient desulfurization cannot be performed. To rapidly slag formation is in the molten steel temperature, it is necessary to apply a pre-melt processed (pre-melt process) in _CaO-Al 2 _{O 3} based desulfurizing agent, Patent Document 3, _CaO-Al 2 _{O 3} based desulfurizing agent Is not pre-melted.

  In Patent Document 4 and Patent Document 5, a desulfurizing agent, a fuel gas, and an oxygen gas are sprayed from a top blowing lance installed in a vacuum tank of an RH vacuum degassing apparatus toward a molten steel bath surface in the vacuum tank. There has been proposed a method in which a desulfurizing agent is heated or melted with a burner flame formed below the tip of an upper blowing lance by combustion, and the molten steel is desulfurized with the heated or melted desulfurizing agent. By heating or melting the desulfurizing agent with a burner flame, the desulfurization reaction is promoted and the temperature of the molten steel is prevented from lowering.

  By the way, the desulfurization treatment of molten steel proceeds by the reaction of the following formula (5). This reaction is a substitution reaction between oxygen (O) and sulfur (S). When the oxygen content in the molten steel increases, the desulfurization reaction shown in the formula (5) stagnates.

CaO + S → CaS + O (5)
When a burner flame is formed in the vacuum chamber, a part of the oxygen gas supplied for combustion of the fuel gas or an oxidizing gas such as CO ₂ or H ₂ O generated by the combustion of the fuel gas Reaches the molten steel bath surface and reacts with aluminum (Al) in the molten steel added to reduce the oxygen content in the molten steel to produce aluminum oxide (Al ₂ O ₃ ). That is, when a burner flame is formed in the vacuum chamber, an increase in the aluminum content in the molten steel corresponding to the amount of aluminum added cannot be achieved. Therefore, the effect of reducing the oxygen content in the molten steel due to the addition of aluminum is reduced, the oxygen content in the molten steel is increased, that is, the activity of oxygen in the molten steel is increased, as shown in the equation (5) Desulfurization reaction stagnates. Further, there is a problem that the cleanliness of the molten steel is lowered by Al ₂ O ₃ generated by the reaction with the oxidizing gas.

Patent Document 6 discloses a resulfurization after a desulfurization treatment with slag in a ladle (resulfurization is a phenomenon in which CaS formed by a desulfurization reaction is oxidized to CaO, and the sulfur concentration in molten steel increases). In order to prevent this, in the desulfurization treatment of the molten steel in the RH vacuum degassing apparatus, MgO, Al ₂ O ₃ , CaO, ZrO is added to the molten steel in the circulating ladle or the molten steel in the ladle after adding the desulfurizing agent. ₂ is added, and a powdery flux having a melting point equal to or higher than the desulfurization treatment temperature is added to form the flux layer below the slag layer in the ladle, and the molten steel after desulfurization treatment and removal A method for preventing contact with the slag in the pan has been proposed.

  Patent Document 6 discloses that when the sum of the FeO concentration and the MnO concentration in the slag exceeds 5% by mass, the sulfurization becomes remarkable. However, the molten steel is sprayed with a desulfurizing agent heated or melted with a burner flame. It cannot be estimated from Patent Document 6 how much oxygen gas for combustion is supplied when desulfurization is performed, and resulfurization becomes significant.

Japanese Patent Laid-Open No. 61-130413 JP-A-5-311231 JP 2003-129122 A JP 7-41826 A JP 2012-172213 A JP-A-7-224318

As proposed in Patent Documents 4 and 5, a burner flame is formed below the tip of an upper blowing lance installed in a vacuum tank of a vacuum smelting furnace such as an RH vacuum degassing apparatus, and the desulfurizing agent is heated with this burner flame or By melting, heating or melting the desulfurizing agent to project the molten steel, the desulfurization treatment of the molten steel is suppressed, and at the same time an efficient desulfurization treatment is possible, but in order to obtain a highly efficient desulfurization rate The means for ensuring the cleanliness of the molten steel when a large amount of Al ₂ O ₃ is produced in the molten steel due to the optimal amount of aluminum added or the oxygen gas for combustion has not been clarified yet.

  The present invention has been made in view of the above circumstances, and its object is to form a burner flame under the tip of the upper blowing lance installed in the vacuum tank of the vacuum refining furnace, and heat the desulfurizing agent with this burner flame. Alternatively, when producing a low-sulfur steel by spraying molten, heated or melted desulfurization agent onto the surface of the molten steel to produce low-sulfur steel, it can be desulfurized at a high desulfurization rate, and oxide-based non-metallic intervening An object of the present invention is to provide a method for producing low-sulfur steel, which is capable of producing a molten steel having a small amount of material (hereinafter also simply referred to as “inclusions”) and having high cleanliness.

The gist of the present invention for solving the above problems is as follows.
[1] When the molten steel in the converter produced by the decarburization process is discharged from the converter to the ladle, aluminum is added to the ladle within the range that satisfies the following formula (1). Deoxidation treatment,
Thereafter, the ladle containing the molten steel is transported to a vacuum smelting furnace, a burner flame is formed below the tip of the upper blowing lance installed in the vacuum tank of the vacuum smelting furnace, and CaO-Al ₂ O is formed by the burner flame. while heating the _3-based pre-melt the desulfurizing agent, blowing toward the molten steel bath surface within the vacuum vessel was desulfurized molten steel together with carrier gas of CaO-Al ₂ O ₃ based pre-melt the desulfurizing agent from the upper lance,
A ladle containing the desulfurized molten steel in a period from the end of the processing in the vacuum refining furnace to the start of continuous casting in the continuous casting equipment in the next step after the processing of the molten steel in the vacuum refining furnace is completed. Let stand for 10 minutes or more (Standing is to stop the ladle without forcibly stirring the molten steel contained in the ladle), and then continuously casting the molten steel in the ladle. A method for producing low-sulfur steel, characterized by starting.
0.0011 × a _of + 10 × [% Al] _LL + 0.05 ≦ W _LD-Al ≦ 0.0011 × a _of + 10 × [% Al] _LL +1.30… (1)
However, in the formula (1), a _of is the oxygen content (mass ppm) in the molten steel at the end of converter decarburization treatment, and [% Al] _LL is the steel product produced from the low-sulfur steel to be produced. The lower limit (mass%) of the aluminum content standard value, W _LD-Al is the aluminum addition amount (kg / t) to the molten steel at the time of steel production.
[2] After the steel from the converter to the ladle, aluminum for slag reduction is added to the slag in the ladle containing the molten steel within the range satisfying the following formula (2). 3) The method for producing low-sulfur steel according to [1] above, wherein quick lime for slag component modification is added within a range satisfying the formula.
0.12 × ln (a _of ) −0.5 ≦ W _slag-Al ≦ 0.12 × ln (a _of ) −0.2 (2)
0.0031 × a _of + 1.5 ≦ W _slag-CaO ≦ 0.0031 × a _of +2.2… (3)
However, in the formulas (2) and (3), a _of is the oxygen content (mass ppm) in the molten steel at the end of the converter decarburization treatment, and W _slag-Al is the amount of aluminum added for slag reduction ( kg / t) and W _slag-CaO are the addition amount (kg / t) of quicklime for slag component modification.
[3] The burner flame is a flame formed by burning fuel gas with oxygen gas. When the supply amount of the oxygen gas forming the burner flame is 0.5 Nm ³ / t or more, desulfurization treatment is performed. After completion, the low-sulfur steel according to [1] or [2] above, wherein the lime-containing auxiliary material is added to the molten steel in the vacuum tank within a range satisfying the following expression (4): Production method.
0.3 ≦ W _RH-CaO × X _CaO ≦ 1.5 (4)
However, in the formula (4), W _RH-CaO is the addition amount (kg / t) of the lime-containing auxiliary material, and X _CaO is the CaO content ratio (−) of the lime-containing auxiliary material.

According to the present invention, a CaO—Al ₂ O ₃ -based premelt desulfurizing agent is sprayed onto molten steel in a vacuum tank while being heated by a burner flame formed below the tip of an upper blowing lance provided at the top of the vacuum tank. In the desulfurization treatment of steel, an appropriate amount of aluminum is added after the steel from the converter to keep the aluminum content in the molten steel high and suppress the increase in the oxygen activity in the molten steel. Is promoted, and low-sulfur steel can be stably produced. In addition, after the desulfurization treatment, the ladle containing the desulfurized molten steel is allowed to stand for 10 minutes or longer. (Standing means that the ladle held in the ladle is not forcibly agitated and left still. Then, since the molten steel in the ladle is continuously cast, the Al ₂ O ₃ in the molten steel generated by the burner flame can be sufficiently floated in the slag, and thereby the oxide-based nonmetal A low-sulfur steel with few inclusions and high cleanliness can be stably melted.

It is a schematic longitudinal cross-sectional view of one example of the RH vacuum degassing apparatus used when implementing this invention.

  Hereinafter, the present invention will be specifically described.

  The hot metal discharged from the blast furnace is received by a holding container such as a hot metal ladle or a torpedo car or a transport container, and the received hot metal is transported to a steel making process for decarburization. In the middle of this conveyance, the hot metal is usually subjected to hot metal pretreatment such as desulfurization treatment or dephosphorization treatment, and the present invention has a standard value of sulfur content of steel products of 0.0030% by mass or less. Since it is intended for low-sulfur steel, it is essential to carry out desulfurization treatment on hot metal. This hot metal is charged into a converter, and the molten iron is decarburized by oxygen blowing. After the decarburization treatment, the molten steel in the converter formed by decarburizing the hot metal is put into a ladle.

At the time of steel removal from the converter to the ladle, manganese-based alloy iron and ferrosilicon alloy iron for adjusting the molten steel components and aluminum for deoxidizing the molten steel are added into the ladle. In order to reduce the amount of SiO ₂ and MnO formed in molten steel by the addition of manganese-based alloy iron and ferrosilicon alloy iron, after the addition of aluminum, that is, the molten steel is deoxidized with aluminum to reduce dissolved oxygen. After the reduction, it is preferable to add manganese-based alloy iron or ferrosilicon alloy iron. In addition, the amount of aluminum added is the following formula (1) in order to remove oxygen in molten steel, to secure the lower limit value of the aluminum content standard value of steel products, and to perform efficient desulfurization treatment. It is necessary to be within the range of satisfaction.

0.0011 × a _of + 10 × [% Al] _LL + 0.05 ≦ W _LD-Al ≦ 0.0011 × a _of + 10 × [% Al] _LL +1.30… (1)
However, in the formula (1), a _of is the oxygen content (mass ppm) in the molten steel at the end of converter decarburization treatment, and [% Al] _LL is the steel product produced from the low-sulfur steel to be produced. The lower limit (mass%) of the aluminum content standard value, W _LD-Al is the aluminum addition amount (kg / t) to the molten steel at the time of steel production. When the aluminum to be added is metallic aluminum, the amount of metallic aluminum added may be in the range of W _{LD-Al. When} the aluminum to be added is an aluminum-containing alloy such as an Al-Fe alloy, the aluminum of the aluminum-containing alloy What is necessary is just to make it the addition amount of a pure part become the range of WLD _-Al . Here, “kg / t” represents the addition amount (kg) per ton of molten steel.

In addition, the slag in the converter (referred to as “converter slag”) generated during the decarburization process in the converter is mixed into the molten steel and discharged into the ladle at the time of steel output. Inevitably, converter slag is present on the molten steel. Converter slag is contained a large amount of FeO and MnO, the FeO and MnO reacts with aluminum in the molten steel, clean _Al 2 _{O 3} is formed in the molten steel, is generated _Al 2 _{O 3} of the molten steel In order to prevent this, the deoxidizer such as aluminum or aluminum dross is added to the slag in the ladle to reduce the converter slag, and the slag in the ladle. It is preferable to add quick lime (CaO) to increase the basicity (mass% CaO / mass% SiO ₂ ) of the converter slag and dilute FeO and MnO in the converter slag. This quicklime is called quicklime for slag component modification.

  The amount of aluminum added for slag reduction is preferably within the range satisfying the following equation (2), and the amount of quicklime added for slag component modification satisfies the following equation (3). It is preferable to be within the range.

0.12 × ln (a _of ) −0.5 ≦ W _slag-Al ≦ 0.12 × ln (a _of ) −0.2 (2)
0.0031 × a _of + 1.5 ≦ W _slag-CaO ≦ 0.0031 × a _of +2.2… (3)
However, in the formulas (2) and (3), a _of is the oxygen content (mass ppm) in the molten steel at the end of the converter decarburization treatment, and W _slag-Al is the amount of aluminum added for slag reduction ( kg / t) and W _slag-CaO are the addition amount (kg / t) of quicklime for slag component modification. In addition, when using aluminum dross as an aluminum source for slag reduction, it adds according to the aluminum pure content of aluminum dross so that the aluminum pure content in aluminum dross satisfies the range of Formula (2).

  After the addition of aluminum for slag reduction and quick lime for slag component modification, in order to sufficiently react these added substances with the slag in the ladle, it is installed at the bottom of the dipping lance or ladle immersed in molten steel. It is preferable to blow rare gas from the porous plug and stir the molten steel, aluminum for slag reduction, and quick lime for slag component modification.

  Thereafter, the ladle containing the molten steel is conveyed to a vacuum refining furnace such as an RH vacuum degassing apparatus, a DH vacuum degassing apparatus, a VAD furnace, or a VOD furnace. The molten steel to be used is not limited to the molten steel obtained by decarburizing the molten iron discharged from the blast furnace, but may be molten steel obtained by melting iron scrap or the like in an electric furnace.

  Examples of the vacuum smelting furnace that can be used in the present invention include an RH vacuum degassing apparatus, a DH vacuum degassing apparatus, a VAD furnace, and a VOD furnace. Device. Therefore, the RH vacuum degassing apparatus shown in FIG. 1 will be described as an example.

  In FIG. 1, 1 is a RH vacuum degassing device, 2 is a ladle, 3 is molten steel, 4 is a slag, 5 is a vacuum tank, 6 is an upper tank, 7 is a lower tank, 8 is a rising side dip tube, and 9 is a lowering A side dip tube, 10 is a circulating gas blowing tube, 11 is a duct, 12 is a raw material inlet, 13 is an upper blowing lance, and the vacuum chamber 5 is composed of an upper tank 6 and a lower tank 7. Further, the upper blowing lance 13 can be moved up and down in the vacuum chamber, and fuel gas and oxygen-containing gas for fuel gas combustion are injected from the upper blowing lance 13, and below the tip of the upper blowing lance 13. In addition, a burner flame is formed, and a powdery desulfurizing agent is sprayed onto the molten steel bath surface while being heated by the burner flame using a rare gas or an oxygen-containing gas as a carrier gas. The oxygen-containing gas for fuel gas combustion can also serve as an oxygen-containing gas as a carrier gas. As the fuel gas, a hydrocarbon gas such as methane gas, propane gas, or LNG is used. As the oxygen-containing gas for fuel gas combustion, oxygen gas (pure oxygen), a mixed gas of rare gas and oxygen gas, Use air, oxygen-enriched air, etc.

  In the RH vacuum degassing apparatus 1 having this configuration, the molten steel 3 is desulfurized as follows.

  The ladle 2 in which the molten steel 3 is stored is conveyed directly under the vacuum chamber 5. Inside the ladle 2 is partially mixed with slag 4 generated by refining in a converter or an electric furnace, and covers the bath surface of the molten steel 3. The conveyed ladle 2 is raised by an elevating device (not shown), and the ascending side dip tube 8 and the descending side dip tube 9 are immersed in the molten steel 3 accommodated in the ladle 2. Then, argon gas is blown into the rising side dip tube 8 from the reflux gas blowing tube 10 as the reflux gas, and the inside of the vacuum chamber 5 is evacuated by an exhaust device (not shown) connected to the duct 11. The pressure inside the vacuum chamber 5 is reduced. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 accommodated in the ladle 2 ascends the ascending side dip tube 8 together with the argon gas due to the gas lift effect by the argon gas blown from the reflux gas blow tube 10 and is vacuumed. After flowing into the tank 5 and returning to the ladle 2 via the descending side dip tube 9, a so-called recirculation is formed, and RH vacuum degassing is applied to the molten steel 3.

  During this RH vacuum degassing refining, the aluminum content in the molten steel before desulfurization is analyzed, and if the aluminum content is below the lower limit of the aluminum content standard value of steel products, the aluminum content in the molten steel However, aluminum is added to the molten steel 3 through the raw material inlet 12 within a range that is 0.005 mass% higher than the lower limit value of the aluminum content specification value and does not exceed the upper limit value of the aluminum content specification value.

Thereafter, a powdery lime-based desulfurization agent, a fuel gas, and an oxygen-containing gas for fuel gas combustion are supplied from the top blowing lance 13 together with a carrier gas (generally using a rare gas) to the inside of the vacuum chamber 5. This is burned toward the molten steel 3 and burned with an oxygen-containing gas to form a burner flame below the tip of the upper blowing lance 13 while heating the powdered lime-based desulfurization agent with this burner flame. The molten steel 3 is sprayed (referred to as “burner projection”), and the molten steel 3 is desulfurized. The lime-based desulfurizing agent, not containing CaF _2, CaO content of 55 to 75 wt%, _Al 2 _{O 3} content of 25 to 45 wt%, was subjected to pre-melt processed (pre-melt processed) CaO- An Al ₂ O ₃ desulfurizing agent (referred to as “pre-melt desulfurizing agent”) is used.

  When the degree of vacuum inside the vacuum chamber 5 is increased during the desulfurization process, the attenuation of the jet gas velocity from the top blowing lance 13 decreases, so even if the transport gas flow rate is constant, the molten steel 3 of the jet gas. This is advantageous from the viewpoint of improving the yield of the desulfurizing agent and promoting the desulfurization reaction. However, if the degree of vacuum is too high, the amount of desulfurization agent sucked into the exhaust system increases, so the degree of vacuum of the vacuum chamber 5 is preferably adjusted to 20 to 100 torr (2.7 to 13.3 kPa).

The CaO—Al ₂ O ₃ desulfurization agent used does not contain CaF ₂ , but this is necessary to prevent the refractory from being melted and to reduce the desulfurization agent price. . When the CaO content of the CaO—Al ₂ O ₃ desulfurization agent is less than 55% by mass, it is not preferable because the desulfurization ability is poor. On the other hand, when the CaO content exceeds 75% by mass, the melting point of the desulfurization agent is high. Therefore, it does not melt in the burner flame, and even if it enters into the molten steel, it does not melt quickly and the desulfurization reaction is delayed.

From the viewpoint of reaction efficiency, the CaO—Al ₂ O ₃ desulfurizing agent to be used preferably has a particle size of less than 1 mm, desirably less than 150 μm in weight ratio of 80% or more. On the other hand, from the viewpoint of reducing the amount sucked into the exhaust system, it is desirable that the amount of fine powder is small. Accordingly, the particle size of less than 10 μm is preferably less than 10% by mass, and the particle size of less than 50 μm is 10%. More preferably, it is less than. In addition, up to 5% by mass of SiO ₂ is acceptable as an impurity in the CaO—Al ₂ O ₃ desulfurization agent. When the SiO ₂ content exceeds 5% by mass, the desulfurization ability is lowered, which is not preferable.

Furthermore, in order to maintain the oxygen potential of the molten steel 3 during the desulfurization treatment at a low level and to effectively prevent the desulfurization after the desulfurization, a CaO—Al ₂ O ₃ desulfurization agent is used in the RH vacuum degassing apparatus 1. It is preferable to put an MgO source into the molten steel 3 inside the vacuum chamber 5 before performing the burner projection. As a method for charging the MgO source, either a method of charging from the raw material charging port 12 or a method of projecting from the upper blowing lance 13 that projects the CaO—Al ₂ O ₃ -based desulfurizing agent may be used. From the viewpoint of reducing the amount of suction to the system and using a massive magnesia clinker or the like, it is preferable to adopt a method of charging from the raw material charging port 12.

  The MgO source introduced into the vacuum chamber 5 flows from the descending side dip tube 9 into the molten steel 3 accommodated in the ladle 2 along with the molten steel flow, and floats in the molten steel in the ladle. A barrier layer having a high melting point is formed between the slag 4 present on the bath surface of the molten steel 3 in the pan and the molten steel 3. This barrier layer can prevent reoxidation of the molten steel 3 due to oxidizing components such as FeO and MnO contained in the slag 4, maintain a low oxygen potential atmosphere suitable for desulfurization, and prevent resulfurization. it can.

  As the MgO source, in addition to magnesia clinker, MgO-based refractory waste can be used. A preferable input amount of the MgO source for exhibiting the above effect is 1 kg / t or more, more preferably 1.5 kg / t or more. However, since a temperature drop of the molten steel 3 is caused when a large amount is added, the upper limit is preferably 5 kg / t, more preferably 3 kg / t.

The top blowing lance 13 is provided at the center of the lance with a passage through which CaO—Al ₂ O ₃ -based desulfurization agent sprayed onto the molten steel 3 together with the conveying gas, and at the tip of the passage, the CaO—Al ₂ O ₃ A flame can be formed by the center hole for ejecting the system desulfurization agent, the passage through which the oxygen-containing gas for fuel gas combustion, the passage through which the fuel gas passes, and the oxygen-containing gas and fuel gas for fuel gas combustion. An upper blowing lance comprising a peripheral hole, or a central hole for ejecting a CaO—Al ₂ O ₃ -based desulfurization agent using an oxygen-containing gas as a carrier gas, and an upper blowing lance in which fuel gas is merged at the tip of the central hole Any of these may be used.

When the sulfur content of the molten steel 3 is equal to or less than the target value, the supply of the CaO—Al ₂ O ₃ desulfurizing agent, the fuel gas, and the oxygen-containing gas for fuel gas combustion is stopped, and the desulfurization process is terminated. After completion of the desulfurization treatment, the alloy components are adjusted while circulating the molten steel 3, and then the RH vacuum degassing refining is completed.

However, when oxygen gas is used as the oxygen-containing gas for fuel gas combustion during the desulfurization process, and the supply amount of oxygen gas for fuel gas combustion from the top blowing lance 13 becomes 0.5 Nm ³ / t or more. May cause a large amount of Al ₂ O ₃ to be produced in the molten steel, which may deteriorate the cleanliness of the molten steel 3. Al ₂ O ₃ inclusions in steel cause defects such as surface cracks in steel products during subsequent rolling. Therefore, when the supply amount of the oxygen gas for fuel gas combustion is 0.5 Nm ³ / t or more, after the supply of the oxygen gas for fuel gas combustion is finished, the lime-containing auxiliary lime such as light burned dolomite and quick lime It is preferable to add the raw material to the molten steel 3 in the vacuum chamber through the raw material charging port 12 and to reflux the molten steel 3 for 5 minutes or more after the addition.

  The addition amount of the lime-containing auxiliary raw materials such as light-burned dolomite and quicklime is preferably within a range satisfying the following expression (4).

0.3 ≦ W _RH-CaO × X _CaO ≦ 1.5 (4)
However, in the formula (4), W _RH-CaO is the addition amount (kg / t) of the lime-containing auxiliary material, and X _CaO is the CaO content ratio (−) of the lime-containing auxiliary material.

By adding the lime-containing auxiliary material and stirring with the molten steel 3, CaO in the lime-containing auxiliary material and Al ₂ O ₃ in the molten steel are combined, and CaO—Al ₂ O ₃ -based inclusions generated by this combination Is easy to float due to a decrease in melting point, and floats in the molten steel and is removed into the slag. The preferable reflux time of the molten steel 3 for exhibiting this effect is 5 minutes or more, more preferably 7 minutes or more. However, since the temperature drop of the molten steel 3 will be caused if it is too long, the upper limit is preferably set to 10 minutes.

  After the processing of the molten steel 3 in the RH vacuum degassing apparatus 1, the desulfurized molten steel 3 is accommodated in the period from the end of the processing in the RH vacuum degassing apparatus 1 to the start of continuous casting in the continuous casting equipment of the next process. Leave the ladle 2 to stand for 10 minutes or more. “Standing” means that the ladle 2 is stopped without forcibly stirring the molten steel 3 accommodated in the ladle.

  By leaving the ladle 2 containing the molten steel 3 after the desulfurization treatment for 10 minutes or more, inclusions suspended in the molten steel 3 float up in the molten steel, reach the slag 4 and are absorbed by the slag 4. Thus, the cleanliness of the molten steel 3 is improved. The ladle 2 can be placed at a place where the RH vacuum degassing apparatus 1 is equipped with a cart or around the RH vacuum degassing apparatus 1 or the ladle 2 containing the molten steel 3 is continuously cast in the next process. It may be transported to the facility and may be the periphery of the continuous casting facility or the continuous casting facility itself such as a swing tower.

  The ladle 2 containing the desulfurized molten steel 3 is allowed to stand for 10 minutes or more to allow the inclusions in the molten steel to float on the slag 4, and then the continuous casting of the molten steel 3 is started in a continuous casting facility. The standing time for exhibiting the above-mentioned standing effect is 10 minutes or more, preferably 15 minutes or more. However, if the length is too long, the temperature of the molten steel 3 is lowered, so the upper limit is preferably set to 25 minutes. If the standing time is less than 10 minutes, the inclusions in the molten steel do not sufficiently float and the quality deteriorates.

As described above, according to the present invention, the CaO—Al ₂ O ₃ based premelt desulfurization agent is vacuum heated while being heated by the burner flame formed below the top end of the top blowing lance 13 provided at the top of the vacuum chamber 5. In performing desulfurization treatment of the molten steel 3 by spraying on the molten steel 3 in the tank, an appropriate amount of aluminum is added after the converter steel to keep the aluminum content in the molten steel high and the oxygen activity in the molten steel Since the rise is suppressed, the progress of the desulfurization reaction is promoted, and the low-sulfur steel can be stably produced. In addition, after the desulfurization treatment, the ladle 2 containing the desulfurized molten steel 3 is allowed to stand for 10 minutes or more, and then the molten steel 3 in the ladle is continuously cast. Therefore, Al ₂ in the molten steel generated by the burner flame is used. O ₃ can be sufficiently floated in the slag, and thereby, low-sulfur steel having a high degree of cleanliness and a low-sulfur steel with less oxide non-metallic inclusions can be stably melted.

  In addition, although the said description demonstrated by the example which used RH vacuum degassing apparatus 1 as a vacuum refining furnace, this invention is not restricted to RH vacuum degassing apparatus 1, If it has the top blowing lance 13, DH A vacuum degassing apparatus, a VAD furnace, a VOD furnace, or the like can also be implemented in accordance with the above description.

  Change the amount of metallic aluminum added to the ladle at the time of steel removal from the converter to the ladle, and change the standing time after desulfurization treatment in the RH vacuum degassing device, Tests were conducted to investigate the effect of the addition amount on the desulfurization efficiency and the effect of the standing time after the desulfurization treatment on the cleanliness (test numbers 1 to 15).

  About 300 tons of molten steel produced from the converter was conveyed to the RH vacuum degassing apparatus shown in FIG. 1, and the molten steel was desulfurized using the RH vacuum degassing apparatus. The molten steel composition before the RH vacuum degassing treatment has a carbon content of 0.08 to 0.10 mass%, a silicon content of 0.1 to 0.2 mass%, and a sulfur content of 0.0030 to 0.0032. The molten steel temperature was 1600 to 1650 ° C. The lance height of the top blowing lance during the desulfurization treatment was 6 m.

After adding manganese-based alloy iron, ferrosilicon alloy iron, metallic aluminum, etc. to the molten steel produced from the converter, this molten steel is transported to the RH vacuum degasser, and the molten steel temperature is measured as necessary. It was confirmed whether the necessary molten steel temperature could be secured before the addition of the desulfurizing agent. After that, if sufficient aluminum concentration in the molten steel is not ensured by the amount of aluminum added at the time of steel production, the aluminum concentration in the molten steel is adjusted by adding metallic aluminum, and then the top blow inserted from the top of the vacuum chamber The tip of the lance is fixed at a position 6 m from the surface of the molten steel bath, LNG is supplied as a fuel gas, and oxygen gas is supplied as an oxygen-containing gas for fuel gas combustion. CaO—Al ₂ O ₃ -based pre-melt desulfurization agent (CaO content; 70 mass%, Al ₂ O ₃ content; 30 mass%) at an addition rate of 175 kg / min using argon gas as the carrier gas The desulfurization treatment was performed by projecting 5 kg / t, and the desulfurization efficiency was evaluated in each test.

In all tests, the supply flow rate of LNG during desulfurization treatment is 400 Nm ³ / h, the supply flow rate of oxygen gas is 920 Nm ³ / h, the argon gas flow rate of the carrier gas is 400 Nm ³ / h, and the reflux argon gas flow rate is 3000 NL. / Min. The addition amount of quicklime for slag component modification at the time of steel output was 3 kg / t. The place where the ladle containing the molten steel after the desulfurization treatment was placed was a swing tower of continuous casting equipment. That is, the ladle was left standing for a predetermined time with the ladle placed on the swing tower. Moreover, the lower limit of the aluminum content specification value of the steel type (steel product) tested was 0.015 mass%, and the upper limit was 0.035 mass%. Table 1 shows the operation conditions and operation results in each test.

  Table 1 shows the following. The desulfurization index in Table 1 is an index of the desulfurization rate (sulfur concentration decrease / initial sulfur concentration) per unit of desulfurizing agent. The larger the desulfurization index, the more the desulfurization reaction is promoted. Indicates. Moreover, the inclusion in molten steel immediately after completion | finish of RH vacuum degassing refining was evaluated, and it showed as an inclusion index. This inclusion index indicates that the greater the value, the more inclusions.

  The test number 1 that did not form the burner flame had a molten steel temperature drop of 37 ° C. before and after the desulfurization agent projection, whereas the test numbers 2 to 15 that formed the burner flame had a molten steel temperature drop of 25 to 25 ° C. It was 27 degreeC and the molten steel temperature fall was suppressed rather than the test number 1. This is because the heat of the burner flame is applied to the molten steel. Test number 1 had a desulfurization index of 0.4, whereas test numbers 2 to 15 had a high desulfurization index of 0.7 to 0.8. This is thought to be because the hatching and melting of the desulfurizing agent was promoted and the desulfurization reaction was accelerated by projecting the desulfurizing agent with a burner.

In addition, in test numbers 4 to 9 and 13 to 15 in which the aluminum addition amount (W _LD-Al ) at the time of converter steelmaking satisfies the formula (1), the treatment time of RH vacuum degassing refining is 29 to 31 minutes. Met. This is because it is not necessary to add metallic aluminum in the RH vacuum degassing apparatus, and the processing time is shorter than in other tests. In test numbers 2 and 3 in which the amount of aluminum added (W _LD-Al ) at the time of steel conversion from the converter is less than the range of the formula (1), the treatment time for RH vacuum degassing is 35 to 36 minutes. It became long compared with 4-9 and 13-15. This is because, in Test Nos. 2 and 3, metallic aluminum was added with an RH vacuum degassing device in order to adjust the aluminum concentration in the molten steel. In test numbers 10, 11, and 12 in which the aluminum addition amount (W _LD-Al ) at the time of converter steelmaking is larger than the range of the formula (1), the treatment time of RH vacuum degassing refining is 33 to 38 minutes, It became long compared with test numbers 4-9 and 13-15. This is because the amount of aluminum added (W _LD-Al ) at the time of steel leaving the converter was too high, and the aluminum content in the molten steel was higher than the upper limit of the aluminum content standard value. This is due to the fact that the molten steel in the vacuum chamber was fed with a gas device and dealuminated.

  In addition, in the test numbers 2 to 12 and 15 in which the ladle containing the molten steel was allowed to stand for 10 minutes or more, the inclusion index was lower than the test numbers 13 and 14 in which the standing time was 10 minutes or less. It was. This is considered to be because the inclusion in the molten steel floated on the slag and the cleanliness of the molten steel was increased by ensuring a sufficient standing time.

  A test to investigate the effect on desulfurization reaction by changing the amount of aluminum for slag reduction added on the slag in the ladle and the amount of quick lime for reforming slag components after steel from the converter. (Test numbers 21 to 29).

About 300 tons of molten steel produced from the converter was transported to the RH vacuum degassing apparatus shown in FIG. 1, and the molten steel was desulfurized with the RH vacuum degassing apparatus. In all tests, the amount of aluminum added for deoxidation of molten steel (W _LD-Al ) at the time of converter steelmaking is within the range that satisfies the formula (1), and the desulfurization treatment in the RH vacuum degassing apparatus is desulfurization. The processing conditions such as the agent addition speed, the lance height, and the degree of vacuum were the same as those in Example 1. The lower limit of the aluminum content standard value of the tested steel type (steel product) is 0.015% by mass, the upper limit is 0.035% by mass, and the place where the ladle containing the molten steel after desulfurization treatment is placed Was a swing tower of continuous casting equipment, and the standing time was 12 minutes. Table 2 shows the operation conditions and operation results in each test.

  Table 2 shows the following.

The addition amount of aluminum for reducing _slag (W _slag-Al ) satisfies the formula (2), the addition amount of quick lime for modifying slag components (W _slag-CaO ) satisfies the formula (3), and In test numbers 22 and 23 in which the molten steel was stirred after the addition of aluminum for slag reduction and quick lime for slag component modification, there was no erosion loss of the dip tube, and the desulfurization index was 1.0 to 1.1. The reaction was promoted.

On the other hand, the added amount of aluminum for reducing _slag (W _slag-Al ) is less than the formula (2), or the added amount of quick lime for modifying slag components (W _slag-CaO ) is (3). In test numbers 21, 26, and 27 smaller than the equation, the desulfurization index was 0.7 to 0.8, and the desulfurization efficiency was lower than that in test numbers 22 and 23. This is considered to be due to the fact that the amount of aluminum added for slag reduction was small in Test No. 21 and the degree of oxidation of slag was high. In Test Nos. 26 and 27, the amount of quick lime for slag component modification was high. It was thought that the basicity of the slag was not sufficiently high because there were few, and there was much sulfurization from slag to molten steel.

  In test numbers 24 and 25, although the desulfurization index was high, the treatment time was 34 to 35 minutes, which was longer than that of test numbers 22 and 23. This is because the amount of aluminum added for slag reduction is too much, and a part of the aluminum for slag reduction contributed to the increase in the aluminum concentration in the molten steel, and the concentration exceeded the upper limit of the aluminum standard. This is because the dealumination was carried out by feeding the molten steel in the tank with acid.

  In Test No. 28, although the desulfurization index was high, the dip tube was damaged greatly. This is thought to be due to the fact that the amount of quick lime for modifying the slag component was too large, so that the dip tube melted out.

  Test No. 29 was not enough to stir the molten steel after the addition of aluminum for slag reduction and quick lime for slag component modification, so that the slag was sufficiently reacted with aluminum for slag reduction and quick lime for slag component modification. The effect of adding aluminum for reducing slag and quick lime for modifying slag components was not exhibited, and it is considered that the desulfurization rate was low.

As in Example 1, when about 300 tonnes of molten steel was desulfurized with the RH vacuum degassing apparatus shown in FIG. 1, the desulfurization with the RH vacuum degassing apparatus resulted in oxygen gas for fuel gas combustion. When the supply amount was 0.5 Nm ³ / t, after the supply of oxygen gas was completed, a test was conducted in which light-burned dolomite or quicklime was added as a lime-containing auxiliary material (test numbers 31 to 41). The CaO content in the light-burned dolomite is 60% by mass (CaO content ratio = 0.6).

In all the tests, the amount of aluminum added for deoxidation of molten steel (W _LD-Al ) at the time of converter steelmaking was 1.5 kg / t satisfying the formula (1), and desulfurization with an RH vacuum degasser. The treatment was performed under the same treatment conditions as in Example 1 such as the desulfurization agent addition speed, lance height, and vacuum degree, the amount of aluminum added for slag reduction (W _slag-Al ) was 0.4 kg / t, and the slag component The amount of quicklime for reforming (W _slag-CaO ) was 3.5 kg / t.

  The lower limit of the aluminum content standard value of the tested steel type (steel product) is 0.015% by mass, the upper limit is 0.035% by mass, and the place where the ladle containing the molten steel after desulfurization treatment is placed Was a swing tower of continuous casting equipment, and the standing time was 12 minutes. Table 3 shows the operation conditions and operation results in each test.

  Table 3 shows the following. In any of the test numbers 31 to 41, the desulfurization index was 1.5. In test numbers 33 to 39 in which the addition amount of light calcined dolomite or quick lime added after the supply of oxygen gas to the molten steel in the RH vacuum degassing apparatus satisfies the formula (4), the inclusion index is 0.1 to It was 0.3.

On the other hand, in the test numbers 31, 32, 40, and 41 in which the addition amount of light-burned dolomite or quicklime does not satisfy the formula (4), the inclusion index is 0.5 to 0.6, compared with the test numbers 33 to 39. It became a high value. In Test Nos. 31 and 32, the amount of light burned dolomite and quick lime was small, so that Al ₂ O ₃ in the steel could not be captured sufficiently. In Test Nos. 40 and 41, light burned dolomite or quick lime It is considered that CaO-based inclusions remained in the steel because the amount added was too large.

DESCRIPTION OF SYMBOLS 1 RH vacuum degassing apparatus 2 Ladle 3 Molten steel 4 Slag 5 Vacuum tank 6 Upper tank 7 Lower tank 8 Rising side immersion pipe 9 Lowering side immersion pipe 10 Recirculation gas blowing pipe 11 Duct 12 Raw material inlet 13 Upper blowing lance

Claims (3)

When the molten steel in the converter generated by the decarburization process is discharged from the converter to the ladle, aluminum is added to the ladle within the range satisfying the following formula (1) to deoxidize the molten steel. Process,
Thereafter, the ladle containing the molten steel is transported to a vacuum smelting furnace, a burner flame is formed below the tip of the upper blowing lance installed in the vacuum tank of the vacuum smelting furnace, and CaO-Al ₂ O is formed by the burner flame. while heating the _3-based pre-melt the desulfurizing agent, blowing toward the molten steel bath surface within the vacuum vessel was desulfurized molten steel together with carrier gas of CaO-Al ₂ O ₃ based pre-melt the desulfurizing agent from the upper lance,
A ladle containing the desulfurized molten steel in a period from the end of the processing in the vacuum refining furnace to the start of continuous casting in the continuous casting equipment in the next step after the processing of the molten steel in the vacuum refining furnace is completed. Let stand for 10 minutes or more (Standing is to stop the ladle without forcibly stirring the molten steel contained in the ladle), and then continuously casting the molten steel in the ladle. A method for producing low-sulfur steel, characterized by starting.
0.0011 × a _of + 10 × [% Al] _LL + 0.05 ≦ W _LD-Al ≦ 0.0011 × a _of + 10 × [% Al] _LL +1.30… (1)
However, in the formula (1), a _of is the oxygen content (mass ppm) in the molten steel at the end of converter decarburization treatment, and [% Al] _LL is the steel product produced from the low-sulfur steel to be produced. The lower limit (mass%) of the aluminum content standard value, W _LD-Al is the aluminum addition amount (kg / t) to the molten steel at the time of steel production. After the steel from the converter to the ladle, aluminum for slag reduction is added to the slag in the ladle containing the molten steel within the range that satisfies the following formula (2), and the following formula (3) The method for producing low-sulfur steel according to claim 1, wherein quick lime for slag component modification is added within a range that satisfies the above.
0.12 × ln (a _of ) −0.5 ≦ W _slag-Al ≦ 0.12 × ln (a _of ) −0.2 (2)
0.0031 × a _of + 1.5 ≦ W _slag-CaO ≦ 0.0031 × a _of +2.2… (3)
However, in the formulas (2) and (3), a _of is the oxygen content (mass ppm) in the molten steel at the end of the converter decarburization treatment, and W _slag-Al is the amount of aluminum added for slag reduction ( kg / t) and W _slag-CaO are the addition amount (kg / t) of quicklime for slag component modification. The burner flame is a flame formed by burning fuel gas with oxygen gas. When the supply amount of the oxygen gas forming the burner flame is 0.5 Nm ³ / t or more, after the desulfurization treatment is finished The method for producing low-sulfur steel according to claim 1 or 2, wherein the lime-containing auxiliary material is added to the molten steel in the vacuum tank within a range satisfying the following expression (4).
0.3 ≦ W _RH-CaO × X _CaO ≦ 1.5 (4)
However, in the formula (4), W _RH-CaO is the addition amount (kg / t) of the lime-containing auxiliary material, and X _CaO is the CaO content ratio (−) of the lime-containing auxiliary material.

Images