JP6547734B2 - Method of manufacturing low-sulfur steel - Google Patents

Description

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

  In recent years, there has been an increasing demand for high-purity steel melts containing few impurities in order to improve material properties accompanying high added value of steel and expanded usage of steel materials. In particular, there is a high demand for low-sulfur steel having a low content of sulfur (S), which is an element that impairs the toughness of steel materials. A steel having a low sulfur content is produced by desulfurization treatment in a hot metal stage with high desulfurization efficiency, and then decarburizing this 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 with stability alone. Therefore, in the production of low-sulfur steel such as high-grade electrical steel sheet and steel for line pipes, desulfurization treatment has been performed even at the molten steel stage using a ladle smelting furnace after refining in a converter.

  However, if the ladle refining furnace does not have a refining function under reduced pressure and degassing treatment such as dehydrogenation or denitrification is required, vacuum such as ladle refining furnace and RH vacuum degassing device It must be refined with both refining furnaces. As a result, not only operation problems such as a decrease in molten steel temperature, a decrease in working efficiency and an increase in lead time from tapping to casting but also an installation problem that two secondary smelting furnaces are required . Therefore, for the purpose of integration of the secondary refining furnace and simplification of the secondary refining process, a method of desulfurizing treatment in a vacuum refining furnace such as an RH vacuum degassing apparatus has come to be performed.

  The desulfurization treatment method with RH vacuum degassing apparatus is generally a method of introducing a desulfurization agent onto molten steel in a vacuum tank from a raw material inlet provided in the vacuum tank and stirring the molten steel and the desulfurization agent However, there is a disadvantage that the desulfurization agent yield is poor due to the influence of the desulfurization agent that has been input 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 interface area is reduced, which is disadvantageous in terms of reaction efficiency.

  Therefore, in the desulfurization treatment of molten steel in a vacuum smelting furnace, a number of proposals have been made for the purpose of enhancing the addition yield of the desulfurizing agent and efficiently desulfurizing the molten steel.

  Patent Document 1 proposes a method of blowing a powder desulfurization agent into molten steel together with a carrier gas through a desulfurization agent injection tuyere provided below the surface of the molten steel in the lower part of the vacuum tank of the RH vacuum degassing apparatus. . However, in the method proposed in Patent Document 1, the blow-in tuyere is a consumable item, and maintenance and inspection of the blow-in tuyere are necessary, which not only increases the manufacturing cost but also blows the carrier gas. The decrease in molten steel temperature due to In addition, during periods when the desulfurization agent is not blown in, it is necessary to flow a rare gas in order to prevent the molten steel from infiltrating and clogging the desulfurization agent blowing tuyere. And the degree of vacuum in the vacuum chamber is reduced.

Patent Document 2 discloses a method of desulfurizing molten steel by spraying a desulfurizing agent from an upper blowing lance installed in a vacuum tank of an RH vacuum degassing device with a carrier gas onto the molten steel in the vacuum tank (a solid substance is bathed with a carrier gas Spraying on a surface is also called "projection"). As in Patent Document 2, in the desulfurization treatment performed by spraying a desulfurizing agent from the upper-blown lance onto molten steel, a CaO-CaF ₂ -based desulfurizing agent is usually used in order to promote the degradation of the desulfurizing agent. However, when using a desulfurizing agent containing CaF ₂ , there is a problem that the processing cost of the generated desulfurizing slag is increased due to the environmental problem of elution of fluorine (F), and by CaF ₂ in the desulfurizing agent, There is a problem that significant melting damage occurs in the inner wall of the ladle and the vacuum tank, and the dip tube, and the cost of the refractory increases.

Patent Document 3 proposes a fluorine-free CaO-Al ₂ O ₃ -based desulfurizing agent having a CaO content of 60% by mass or more and an Al ₂ O ₃ content of 5 to 40%. By using the CaO-Al ₂ O ₃ desulfurization agent, remarkable erosion of the refractory is prevented, but in the range of 5 to 40% of the Al ₂ O ₃ content, the CaO-Al ₂ O ₃ desulfurization agent is used. The melting point is high, the rate of aging of the added CaO-Al ₂ O ₃ -based desulfurization agent is slow, and efficient desulfurization can not be performed. Although it is necessary to perform pre-melting treatment (pre-melt treatment) on the CaO-Al ₂ O ₃ -based desulfurization agent in order to rapidly cool at the molten steel temperature, Patent Document 3 describes the CaO-Al ₂ O ₃ -based desulfurization agent Has not been pre-melted.

  According to Patent Document 4 and Patent Document 5, the desulfurizing agent, the fuel gas and the oxygen gas are sprayed toward the molten steel bath surface in the vacuum chamber from the upper blowing lance installed in the vacuum chamber of the RH vacuum degassing device, A method has been proposed in which a desulfurizing agent is heated or melted by a burner flame formed below the tip of the top-blowing lance by combustion, and desulfurizing treatment of molten steel with the heated or melted desulfurizing agent. By heating or melting the desulfurizing agent with the burner flame, the desulfurization reaction is promoted and a decrease in the molten steel temperature is prevented.

  By the way, the desulfurization process of molten steel progresses by the reaction of following (5) Formula. This reaction is a substitution reaction of oxygen (O) and sulfur (S), and the desulfurization reaction shown in the equation (5) stagnates when the oxygen content in the molten steel increases.

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

In Patent Document 6, resulfurization after desulfurization treatment with slag in a ladle (resulfurization is a phenomenon in which CaS formed by the desulfurization reaction is oxidized to CaO and the sulfur concentration in molten steel increases) is disclosed. In order to prevent the desulfurization of the molten steel in the RH vacuum degassing apparatus, after adding the desulfurizing agent, MgO, Al ₂ O ₃ , CaO, or ZrO is added to the circulating molten steel or ladle or both. _A powdery flux consisting of one or more of ₂ and having a melting point equal to or higher than the desulfurization treatment temperature is added, the flux layer is formed under the slag layer in the ladle, and the molten steel after desulfurization treatment is Methods have been proposed to prevent contact with in-pan slag.

  Patent Document 6 discloses that if the sum of the FeO concentration and the MnO concentration in the slag exceeds 5% by mass, resulfurization becomes remarkable, but a burner flame is sprayed with a heated or melted desulfurizing agent to melt the molten steel. From the Patent Document 6, it can not be estimated from what amount of combustion oxygen gas is supplied when desulfurizing treatment that re-curing becomes remarkable.

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

As proposed in Patent Documents 4 and 5, a burner flame is formed below the tip of the upper blowing lance installed in the vacuum tank of a vacuum smelting furnace such as an RH vacuum degassing apparatus, and the burner flame heats the desulfurizing agent or By desulfurizing the molten steel by projecting the molten desulfurizing agent by melting, heating or melting, efficient desulfurization processing became possible at the same time as suppressing the lowering of the molten steel temperature, but to obtain a highly efficient desulfurization rate It has not been clarified yet about means for securing the cleanliness of the molten steel when a large amount of Al ₂ O ₃ is formed in the molten steel due to the optimum aluminum addition amount in the above and oxygen gas for combustion.

  The present invention has been made in view of the above circumstances, and the object of the present invention is to form a burner flame below the tip of the upper blowing lance installed in the vacuum tank of a vacuum smelting furnace, and heat the desulfurizing agent with this burner flame. Or when desulfurizing the molten steel by spraying molten, heated or molten desulfurizing agent on the bath surface of molten steel to produce low-sulfur steel, desulfurization can be performed at a high desulfurization rate, and oxide-based nonmetal intercalation It is an object of the present invention to provide a method for producing a low-sulfur steel which can produce a highly clean liquid steel having few substances (hereinafter, also simply referred to as "inclusions").

The gist of the present invention for solving the above-mentioned subject is as follows.
[1] When the molten steel in the converter produced by the decarburizing treatment is discharged from the converter to the ladle, aluminum is added to the ladle within the range satisfying the following equation (1) to make the molten steel Deacidified,
Thereafter, the ladle containing the molten steel is conveyed to a vacuum smelting furnace, and a burner flame is formed below the tip of the upper blowing lance installed in the vacuum tank of the vacuum smelting furnace, and the burner flame generates CaO-Al ₂ O While heating the _3- system pre-melt desulfurization agent, the CaO-Al ₂ O ₃ -based pre-melt desulfurization agent is sprayed onto the molten steel bath surface in the vacuum tank together with the transport gas from the upper blowing lance to desulfurize the molten steel,
After the processing of the molten steel in the vacuum smelting furnace is completed, a ladle containing the molten steel desulfurized by the period from the end of the processing in the vacuum smelting furnace to the start of continuous casting in the continuous casting facility of the next step Let stand for 10 minutes or more (stationary means making the ladle stand without forcibly stirring the molten steel contained in the ladle), and then perform continuous casting of 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 equation (1), a _of represents the oxygen content in molten steel (mass ppm) at the end point of converter decarburization treatment, and [% Al] _LL is a steel product manufactured from low-sulfur steel to be manufactured The lower limit value (mass%) of the aluminum content specification value, W _{LD -Al} is the amount of added aluminum (kg / t) to the molten steel at the time of tapping.
[2] After tapping the steel from the converter to the ladle, add aluminum for slag reduction within the range satisfying the following equation (2) onto the slag in the ladle containing the molten steel, and 3) The method for producing low-sulfur steel according to the above-mentioned [1], characterized in that quick lime for reforming the slag component is added in a range satisfying the formula.
0.12 × ln (a _of ) −0.5 ≦ _Wslag−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 equations (2) and (3), a _of is the oxygen content in molten steel (mass ppm) at the end point of converter decarburization treatment, W _slag-Al is the addition amount of aluminum for slag reduction ( kg / t), _Wslag-CaO is the addition amount (kg / t) of quick lime for slag component modification.
[3] The burner flame is a flame formed by burning a fuel gas with oxygen gas, and the desulfurization treatment is performed when the supply amount of the oxygen gas forming the burner flame is 0.5 Nm ³ / t or more. After completion, the lime-containing auxiliary material is added to the molten steel in the vacuum tank in the range satisfying the following equation (4), the low-sulfur steel according to the above [1] or [2] 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, the CaO-Al ₂ O ₃ -based pre-melt desulfurizing agent is sprayed onto the molten steel in the vacuum tank while heated by the burner flame formed below the tip of the 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 converter steel removal to keep the aluminum content in the molten steel high and to suppress the increase in oxygen activity in the molten steel, so the progress of the desulfurization reaction Is promoted to stably produce low-sulfur steel. In addition, after the desulfurization treatment, the ladle containing the desulfurized molten steel is allowed to stand for 10 minutes or more (stationization means that the ladle can be stopped without forcibly stirring the molten steel contained in the ladle. Then, since the molten steel in the ladle is continuously cast, Al ₂ O ₃ in the molten steel generated by the burner flame can be sufficiently floated in the slag, thereby making the oxide-based nonmetal It is possible to stably melt low-sulfur steel having a high degree of cleanliness with few inclusions.

It is a schematic longitudinal cross-sectional view of one example of 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 or a transfer container such as a hot metal pot or torpedo car, and the received hot metal is transferred to a steel making process for decarburizing treatment. During this transportation, the hot metal is usually subjected to a hot metal pretreatment such as desulfurization and dephosphorization, and the present invention has a specification value of the sulfur content of the steel product of 0.0030 mass% or less From the viewpoint of low sulfur steel, it is essential to carry out desulfurization treatment on hot metal. The molten metal is charged into a converter, and the charged molten metal is deoxidized by oxygen blowing. After the decarburization treatment, the molten steel in the converter is decarburized and the molten steel in the converter is discharged to a ladle.

At the time of tapping from the converter to the ladle, a manganese-based alloy iron or ferrosilicon alloy iron for adjusting the molten steel component and aluminum for deoxidizing the molten steel are added to the ladle. In order to reduce the amount of SiO ₂ and MnO formed in the 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 dissolve oxygen After reduction, it is preferable to add manganese-based alloy iron or ferrosilicon alloy iron. In addition, the amount of added aluminum removes oxygen in the molten steel, secures the lower limit value of the aluminum content standard value of the steel product, and performs the desulfurization treatment efficiently, the following equation (1) is It should 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 equation (1), a _of represents the oxygen content in molten steel (mass ppm) at the end point of converter decarburization treatment, and [% Al] _LL is a steel product manufactured from low-sulfur steel to be manufactured The lower limit value (mass%) of the aluminum content specification value, W _{LD -Al} is the amount of added aluminum (kg / t) to the molten steel at the time of tapping. When the aluminum to be added is metal aluminum, the addition amount of metal aluminum may be in the range of W _LD-Al , and when the aluminum to be added is an aluminum-containing alloy such as an Al-Fe alloy, the aluminum of the aluminum-containing alloy The addition amount of the pure portion may be in the range of W _LD -Al. Here, "kg / t" represents the added amount (kg) per ton of molten steel.

In addition, the slag in the converter (referred to as "converter slag") generated at the time of decarburization treatment at the converter is mixed with the molten steel at the time of tapping steel and discharged to the ladle, and it is contained in the ladle after tapping steel. 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 The converter slag is reduced by adding a deoxidizing agent such as aluminum or aluminum dross on the slag in the ladle to reduce the corrosion property, and on 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 to dilute FeO and MnO in the converter slag. This quicklime is referred to as quicklime for reforming the slag component.

  The addition amount of aluminum for slag reduction is preferably in the range satisfying the following equation (2), and the addition amount of quick lime for reforming the slag component satisfies the following equation (3) It is preferable to be in the range.

0.12 × ln (a _of ) −0.5 ≦ _Wslag−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 equations (2) and (3), a _of is the oxygen content in molten steel (mass ppm) at the end point of converter decarburization treatment, W _slag-Al is the addition amount of aluminum for slag reduction ( kg / t), _Wslag-CaO is the addition amount (kg / t) of quick lime for slag component modification. In addition, when using aluminum dross as an aluminum source for slag reduction, it is added according to the aluminum purity of aluminum dross so that the aluminum content in aluminum dross may satisfy the range of (2) type.

  After the addition of aluminum for slag reduction and quick lime for slag component modification, in order to fully react these added materials with the slag in the ladle, it is installed in the immersion lance or ladle bottom immersed in molten steel It is preferable to blow in a rare gas from the obtained porous plug and stir the molten steel, aluminum for reducing the slag and quick lime for reforming the slag component.

  Thereafter, the ladle containing the molten steel is transferred to a vacuum smelting furnace such as an RH vacuum degassing apparatus, a DH vacuum degassing apparatus, a VAD furnace, a VOD furnace, and the like. The molten steel used is not limited to the molten steel obtained by decarburizing the molten iron from the blast furnace in the converter, and may be a molten steel in which iron scraps and the like are melted and refined in the electric furnace.

  Among vacuum smelting furnaces that can be used in the present invention, there are RH vacuum degassing apparatus, DH vacuum degassing apparatus, VAD furnace, VOD furnace, etc. Among them, the most representative of them is RH vacuum degassing It is an apparatus. Therefore, the RH vacuum degassing apparatus shown in FIG. 1 will be described as an example.

  In FIG. 1, 1 is an RH vacuum degassing apparatus, 2 is a ladle, 3 is molten steel, 4 is slag, 5 is a vacuum tank, 6 is an upper tank, 7 is a lower tank, 8 is a rising side dip pipe, 9 is a descending A side immersion pipe 10 is a reflux gas blowing pipe 11 is a duct 12 is a raw material inlet, 13 is an upper blowing lance, and the vacuum tank 5 is composed of an upper tank 6 and a lower tank 7. Further, the upper blowing lance 13 can move up and down in the vacuum tank, and the fuel gas and the oxygen-containing gas for fuel gas combustion are injected from the upper blowing lance 13 to lower the tip of the upper blowing lance 13 downward. A burner flame is formed, and a powdery desulfurizing agent is sprayed to 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 the oxygen-containing gas as the carrier gas. As fuel gas, hydrocarbon gas such as methane gas, propane gas and LNG is used, and as oxygen-containing gas for fuel gas combustion, oxygen gas (pure oxygen), mixed gas of rare gas and oxygen gas, Use air, oxygen-enriched air, etc.

  In the RH vacuum degassing apparatus 1 of this configuration, the molten steel 3 is subjected to the desulfurization treatment as follows.

  The ladle 2 accommodating the molten steel 3 is conveyed immediately below the vacuum tank 5. In the inside of the ladle 2, a part of the slag 4 generated by the refining in the converter or the electric furnace is mixed, and the bath surface of the molten steel 3 is covered. The conveyed ladle 2 is raised by a lifting device (not shown), and the rising side immersion pipe 8 and the descending side immersion pipe 9 are immersed in the molten steel 3 accommodated in the ladle 2. Then, argon gas is blown as reflux gas into the inside of the rising side immersion pipe 8 from the reflux gas injection pipe 10, and the inside of the vacuum tank 5 is exhausted by an exhaust device (not shown) connected to the duct 11. The pressure in the vacuum chamber 5 is reduced. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 stored in the ladle 2 is lifted with the argon gas by the gas lift effect by the argon gas blown from the reflux gas blowing tube 10 to vacuum After flowing into the inside of the tank 5 and thereafter returning to the ladle 2 via the downside dip tube 9, a so-called reflux is formed to apply RH vacuum degassing to the molten steel 3.

  During this RH vacuum degassing, the aluminum content in the molten steel before desulfurization treatment is analyzed, and if the aluminum content falls below the lower limit of the aluminum content specification 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 in a range which 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, from the upper blow lance 13, a powdery lime-based desulfurization agent, a fuel gas, and an oxygen-containing gas for fuel gas combustion together with the transport gas (generally using a rare gas) are contained in the vacuum tank 5. The fuel gas is burned with an oxygen-containing gas to form a burner flame below the tip of the upper lance 13, and the burner flame heats the powdery lime-based desulfurizing agent with this burner flame. The molten steel 3 is sprayed with the molten steel 3 (referred to as “burner projection”), and the molten steel 3 is desulfurized. As a lime-based desulfurization agent, CaO-containing pre-melted (premelted) containing no CaF ₂ and having a CaO content of 55 to 75% by mass and an Al ₂ O ₃ content of 25 to 45% by mass An Al ₂ O ₃ -based desulfurizing agent (referred to as “pre-melt desulfurizing agent”) is used.

  If the degree of vacuum inside the vacuum tank 5 is increased during desulfurization processing, the attenuation of the jetted gas velocity from the upper blowing lance 13 is reduced, so the molten steel 3 of the jetted gas is obtained even when the flow rate of the transfer gas is constant. The dynamic pressure of the gas on the bath surface is high, which 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 desulfurizing agent attracted to the exhaust system will increase, so the degree of vacuum of the vacuum chamber 5 is preferably adjusted to 20 to 100 torr (2.7 to 13.3 kPa).

Although the CaO-Al ₂ O ₃ -based desulfurizing agent used does not contain CaF ₂ , this is necessary to prevent melting of the refractory and to reduce the price of the desulfurizing agent. . If the CaO content of the CaO-Al ₂ O ₃ -based desulfurization agent is less than 55% by mass, it is not preferable because the desulfurization ability is poor, and if the CaO content exceeds 75% by mass, the melting point of the desulfurization agent is high. It is not preferable because it does not melt in the burner flame, does not melt quickly even if it penetrates into the molten steel, and the desulfurization reaction is delayed.

From the viewpoint of reaction efficiency, the particle size of the CaO-Al ₂ O ₃ -based desulfurizing agent used is preferably less than 1 mm in particle diameter, and more preferably 80% or more by mass ratio. On the other hand, from the viewpoint of reducing the amount sucked into the exhaust system, it is desirable for the fine powder content to be small. Therefore, it is preferable that the particle size less than 10 μm is less than 10% by mass ratio, and the particle size less than 50 μm is 10% More preferably, Incidentally, SiO ₂ up to 5 wt% as an impurity in the _CaO-Al 2 _{O 3} based desulfurizing agent is acceptable. When the SiO ₂ content exceeds 5% by mass, the desulfurization ability is unfavorably reduced.

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

  The MgO source introduced into the inside of the vacuum tank 5 flows from the downside dip pipe 9 to the molten steel 3 accommodated in the ladle 2 along with the molten steel flow, and floats up in the molten steel in the ladle to be removed. A high melting point barrier layer 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, can maintain an atmosphere of low oxygen potential suitable for desulfurization, and also prevent resulfurization. it can.

  In addition to magnesia clinker, MgO-based refractory scraps can also be used as the MgO source. 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 introduced, the upper limit is preferably 5 kg / t, more preferably 3 kg / t.

The upper blowing lance 13 is provided at the axial center of the lance, and a passage through which the CaO-Al ₂ O ₃ -based desulfurizing agent sprayed to the molten steel 3 passes along with the carrier gas, and the CaO-Al ₂ O _{3 at} the tip of this passage. A flame can be formed by a central hole for injecting a desulfurization agent, a passage through which an oxygen-containing gas for fuel gas combustion passes, a passage through which a fuel gas passes, and an oxygen-containing gas for fuel gas combustion and a fuel gas An upper-blowing lance consisting of a peripheral hole, or a central hole that ejects a CaO-Al ₂ O ₃ -based desulfurizing agent using an oxygen-containing gas as a carrier gas, and an upper-blowing lance where the fuel gas merges with the tip of this central hole Any of the above may be used.

When the sulfur content of the molten steel 3 falls below the target value, the supply of the CaO-Al ₂ O ₃ -based desulfurizing agent, the fuel gas, and the oxygen-containing gas for fuel gas combustion is stopped to complete the desulfurization treatment. After completion of the desulfurization treatment, adjustment of alloy components is carried out while refluxing the molten steel 3, and then RH vacuum degassing refining is finished.

However, when the desulfurization treatment uses oxygen gas as the oxygen-containing gas for fuel gas combustion and the supply amount of oxygen gas for fuel gas combustion from the upper blowing lance 13 is 0.5 Nm ³ / t or more. In the molten steel, a large amount of Al ₂ O ₃ is generated in the molten steel, which may deteriorate the cleanliness of the molten steel 3. Al ₂ O ₃ inclusions in the steel cause defects such as surface cracks in the steel product during rolling in a post process. Therefore, when the supply amount of 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 completed, lime-containing secondary such as light-burning dolomite or quick lime It is preferable to add the raw material to the molten steel 3 in the vacuum tank through the raw material inlet 12 and allow the molten steel 3 to reflux for 5 minutes or more after the addition.

  It is preferable that the addition amount of lime-containing auxiliary materials such as light-burned dolomite and quicklime be in the range satisfying the following equation (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 it with the molten steel 3, CaO in the lime-containing auxiliary material and Al ₂ O ₃ in the molten steel are combined, and the CaO-Al ₂ O ₃ inclusions formed by this combination The melting point lowers and easily floats, and floats in the molten steel and is removed in the slag. The refluxing time of the molten steel 3 is preferably 5 minutes or more, more preferably 7 minutes or more to exert this effect. However, if the length is too long, the temperature of the molten steel 3 drops, so the upper limit is preferably 10 minutes.

  After processing of molten steel 3 in RH vacuum degassing apparatus 1 is completed, desulfurized molten steel 3 is stored from the end of processing in RH vacuum degassing apparatus 1 to the start of continuous casting in the continuous casting facility of the next process Let the ladle 2 stand for more than 10 minutes. "Standing" indicates that the ladle 2 stands still without forcibly stirring the molten steel 3 stored in the ladle.

  By leaving the ladle 2 containing the molten steel 3 after desulfurization treatment to stand for 10 minutes or more, inclusions suspended and present in the molten steel 3 float up in the molten steel and reach the slag 4 and absorbed into the slag 4 And the cleanliness of the molten steel 3 is improved. The standing place of the ladle 2 is a carriage provided in the RH vacuum degassing apparatus 1 or even around the RH vacuum degassing apparatus 1 or continuous casting of the ladle 2 containing the molten steel 3 in the next step It may be transported to a facility and may be around the continuous casting facility or in a continuous casting facility itself such as a swing tower.

  After the ladle 2 accommodating the molten steel 3 after the desulfurization treatment is allowed to stand for 10 minutes or more to float inclusions in the molten steel on the slag 4, continuous casting of the molten steel 3 is started in a continuous casting facility. The standing time for exerting the standing effect is required for 10 minutes or more, preferably 15 minutes or more. However, if the length is too long, the temperature of the molten steel 3 drops, so the upper limit is preferably 25 minutes. If the standing time is less than 10 minutes, the inclusions in the molten steel do not rise sufficiently and the quality deteriorates.

As described above, according to the present invention, the CaO-Al ₂ O ₃ -based pre-melt desulfurizing agent is heated while being heated by the burner flame formed below the tip of the upper blowing lance 13 provided at the top of the vacuum chamber 5. When desulfurizing the molten steel 3 by blowing on the molten steel 3 in the tank, an appropriate amount of aluminum is added after the converter is discharged to keep the aluminum content in the molten steel high and to keep the oxygen activity in the molten steel Since the increase is suppressed, the progress of the desulfurization reaction is promoted, and the low-sulfur steel can be stably manufactured. Also, after the desulfurization treatment, the ladle 2 accommodating 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, so Al ₂ in the molten steel formed by the burner flame O ₃ can be sufficiently floated in the slag, and this makes it possible to stably melt low-sulfur steel having a high degree of cleanliness with less oxide nonmetallic inclusions.

  Although the above description has been described using the RH vacuum degassing apparatus 1 as the vacuum smelting furnace, the present invention is not limited to the RH vacuum degassing apparatus 1 and it is possible to use DH if the upper blowing lance 13 is provided. A vacuum degassing apparatus, a VAD furnace, a VOD furnace, etc. can also be implemented along the above description.

  Change the amount of metal aluminum added to the ladle at the time of tapping steel from the converter to the ladle, and change the standing time after desulfurization treatment in the RH vacuum degassing device A test was conducted to investigate the influence of the addition amount on the desulfurization efficiency and the influence of the standing time after the desulfurization treatment on the cleanliness (Test No. 1 to 15).

  About 300 tons of molten steel extracted from the converter was transported 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 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 It was mass% and the molten steel temperature was 1600 to 1650 ° C. The lance height of the top blowing lance at the desulfurization treatment was 6 m.

After adding manganese-based alloy iron, ferrosilicon alloy iron, metallic aluminum, etc. to the molten steel delivered from the converter, this molten steel is transported to the RH vacuum degassing apparatus, and the molten steel temperature is measured as necessary. Before the addition of the desulfurizing agent, it was confirmed whether the required molten steel temperature could be secured. After that, when sufficient aluminum concentration in molten steel is not ensured only with the amount of aluminum added at the time of steel extraction, metallic aluminum is added to adjust the aluminum concentration in molten steel, and then inserted above the top of the vacuum tank The tip of the lance is fixed at a position 6 m from the molten steel bath surface, LNG is supplied as the fuel gas, and oxygen gas is supplied as the oxygen-containing gas for fuel gas combustion, and a burner flame is provided below the tip of the top blowing lance. 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. 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 argon gas flow rate for reflux is 3000 NL / Min. The addition amount of quick lime for slag component reforming at the time of tapping was 3 kg / t. The place where the ladle containing the molten steel after desulfurization treatment was placed was the swing tower of the continuous casting facility. That is, the ladle was allowed to stand for a predetermined time in a state of being put on the swing tower. Moreover, the lower limit of the aluminum content specification value of the steel type (iron and steel product) which tested was 0.015 mass%, and the upper limit was 0.035 mass%. Table 1 shows the operating conditions and results in each test.

  Table 1 shows the following. The desulfurization index in Table 1 is obtained by indexing the desulfurization rate (sulfur concentration reduction amount / initial sulfur concentration) per unit of desulfurization agent, and the larger the desulfurization index, the faster the desulfurization reaction. Indicates In addition, inclusions in molten steel immediately after the completion of RH vacuum degassing were evaluated and indicated as an inclusion index. The inclusion index indicates that the larger the number, the more inclusions.

  The test No. 1 which did not form a burner flame had a molten steel temperature drop before and after the desulfurization agent projection was 37 ° C., whereas the test Nos. 2 to 15 which formed a burner flame had a molten steel temperature drop of 25 to 5 The temperature was 27 ° C., and the molten steel temperature drop was suppressed more than in Test No. 1. This is what the heat of the burner flame heats up to the molten steel. In addition, test No. 1 had a desulfurization index of 0.4, whereas test Nos. 2 to 15 had a high desulfurization index of 0.7 to 0.8. It is considered that this is because the desulfurizing agent is sprayed onto the burner to promote the degreasing agent's aging and melting, thereby promoting the desulfurization reaction.

In addition, in the test numbers 4 to 9 and 13 to 15 in which the added amount of aluminum (W _LD-Al ) at the time of steel tapping from the converter satisfies the formula (1), the processing time of RH vacuum degassing refining is 29 to 31 minutes Met. This is due to the fact that the processing time is shorter compared to the other tests because it is not necessary to add metallic aluminum in the RH vacuum degassing apparatus. In Test Nos. 2 and 3 in which the aluminum addition amount (W _LD-Al ) at the time of tapping the converter is smaller than the range of Formula (1), the processing time of RH vacuum degassing refining is 35 to 36 minutes, and the test No. It became longer than 4-9 and 13-15. This is because in Test Nos. 2 and 3, metal aluminum was added in the RH vacuum degassing apparatus to adjust the aluminum concentration in the molten steel. In the test numbers 10, 11 and 12 where the aluminum addition amount (W _LD-Al ) at the time of tapping the converter is larger than the range of formula (1), the processing time of RH vacuum degassing refining is 33 to 38 minutes, It became longer than test numbers 4-9 and 13-15. This is because the concentration of aluminum added to the converter steel (W _{LD -Al} ) was too large, and the aluminum content in the molten steel was higher than the upper limit of the aluminum content specification value. This is because the molten steel in the vacuum chamber was fed with a gas device and the dealumination treatment was performed.

  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 is low compared to the test numbers 13 and 14 in which the standing time was 10 minutes or less. The It is considered that this is because the inclusions in the molten steel float on the slag and the cleanliness of the molten steel is increased by securing a sufficient standing time.

  A test is conducted to investigate the effect on desulfurization reaction by changing the addition amount of aluminum for slag reduction and the addition amount of quick lime for slag component modification added on the slag in the ladle after tapping steel from the converter It carried out (test number 21-29).

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

  The following can be seen from Table 2.

The addition amount of aluminum for slag reduction ( _Wslag-Al ) satisfies the equation (2), and the addition amount of quick lime for reforming the slag component ( _Wslag-CaO ) satisfies the equation (3), and In Test Nos. 22 and 23 where the molten steel was stirred after addition of aluminum for slag reduction and quick lime for slag component modification, there is no dissolution loss of the immersion pipe, and the desulfurization index is 1.0 to 1.1, and desulfurization The reaction was promoted.

On the other hand, the addition amount ( _Wslag-Al ) of aluminum for slag reduction is smaller than the formula (2) or the addition amount ( _Wslag-CaO ) of quicklime for reforming the slag component is (3) In the test numbers 21, 26 and 27 less than the formula, the desulfurization index was 0.7 to 0.8, and the desulfurization efficiency was reduced compared to the test numbers 22 and 23. This is considered to be due to the fact that in Test No. 21, the amount of addition of aluminum for slag reduction was small and the degree of oxidation of slag was high, and in Test Nos. 26 and 27, the amount of addition of quicklime for reforming the slag component was It was considered that because of low content, the basicity of the slag did not become sufficiently high, and there was a lot of resurgence from slag to molten steel.

  In the test numbers 24 and 25, although the desulfurization index was high, the treatment time was 34 to 35 minutes and was longer than the test numbers 22 and 23. This is because the addition amount of aluminum for slag reduction is too large, and a part of the aluminum for slag reduction contributes to the increase of the aluminum concentration in the molten steel, and the concentration exceeds the upper limit of the aluminum specification. The cause is that the molten steel in the tank is subjected to acidification to carry out dealumination treatment.

  In Test No. 28, although the desulfurization index was high, the melting loss of the immersion tube increased. This is considered to be due to the fact that the dissolution loss of the immersion pipe has increased because the addition amount of the quick lime for reforming the slag component is too large.

  In Test No. 29, after adding aluminum for slag reduction and quick lime for reforming slag component, the molten steel was not stirred, so the reaction between quick lime for slag reduction aluminum and slag component and slag was sufficient. It is considered that the desulfurization rate was low, without the effect of adding aluminum for slag reduction and quick lime for slag component reforming.

When desulfurizing treatment of about 300 tons of molten steel is performed by the RH vacuum degassing apparatus shown in FIG. 1 in the same manner as in Example 1, the oxygen gas for fuel gas combustion is obtained by the desulfurizing process by the RH vacuum degassing apparatus. When the supply amount was 0.5 Nm ³ / t, after the supply of oxygen gas was finished, a test was conducted to add light-baked dolomite or quick lime as a lime-containing auxiliary raw material (Test No. 31 to 41). The CaO content in the calcined dolomite is 60% by mass (CaO content ratio = 0.6).

In all tests, the aluminum addition amount (W _LD-Al ) for molten steel deoxidation at the time of tapping from the converter is 1.5 kg / t which satisfies the formula (1), and desulfurization with an RH vacuum degassing apparatus The treatment conditions are the same as Example 1, such as the desulfurizing agent addition rate, lance height, and vacuum degree, the addition amount of aluminum for slag reduction ( _Wslag-Al ) is 0.4 kg / t, and the slag component The amount of quick lime added for modification ( _{Wslag-CaO 2} ) was 3.5 kg / t.

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

  From Table 3, the following can be seen. The desulfurization index was 1.5 in all of the test numbers 31 to 41. The inclusion index is 0.1 in the test numbers 33 to 39 in which the added amount of light-burned dolomite or quicklime added after the oxygen gas supply to the molten steel in the RH vacuum degassing apparatus is completed satisfies the formula (4) 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 quick lime does not satisfy the formula (4), the inclusion index is 0.5 to 0.6, which is higher than the test numbers 33 to 39. It became a high value. This is because Test No. 31 and 32 do not sufficiently capture Al ₂ O ₃ in the steel due to the small addition amount of lightening dolomite and quick lime, and Test No. 40 and 41 show that lightening dolomite or quick lime It is considered that CaO-based inclusions remain in the steel because the amount added is too large.

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 descending side immersion pipe 10 reflux gas injection pipe 11 duct 12 raw material inlet 13 top blow lance

Claims (3)

When the molten steel in the converter produced by the decarburizing treatment is discharged from the converter to the ladle, aluminum is added to the ladle within the range satisfying the following equation (1) to deoxidize the molten steel. Process
Thereafter, the ladle containing the molten steel is conveyed to a vacuum smelting furnace, and a burner flame is formed below the tip of the upper blowing lance installed in the vacuum tank of the vacuum smelting furnace, and the burner flame generates CaO-Al ₂ O While heating the _3- system pre-melt desulfurization agent, the CaO-Al ₂ O ₃ -based pre-melt desulfurization agent is sprayed onto the molten steel bath surface in the vacuum tank together with the transport gas from the upper blowing lance to desulfurize the molten steel,
After the processing of the molten steel in the vacuum smelting furnace is completed, a ladle containing the molten steel desulfurized by the period from the end of the processing in the vacuum smelting furnace to the start of continuous casting in the continuous casting facility of the next step Let stand for 10 minutes or more (stationary means making the ladle stand without forcibly stirring the molten steel contained in the ladle), and then perform continuous casting of 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 equation (1), a _of represents the oxygen content in molten steel (mass ppm) at the end point of converter decarburization treatment, and [% Al] _LL is a steel product manufactured from low-sulfur steel to be manufactured The lower limit value (mass%) of the aluminum content specification value, W _{LD -Al} is the amount of added aluminum (kg / t) to the molten steel at the time of tapping. After tapping steel from the converter to the ladle, aluminum for slag reduction is added on the slag in the ladle containing the molten steel within the range satisfying the following equation (2), and the following equation (3) The method for producing low-sulfur steel according to claim 1, characterized in that quick lime for reforming the slag component is added in a range satisfying the following conditions.
0.12 × ln (a _of ) −0.5 ≦ _Wslag−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 equations (2) and (3), a _of is the oxygen content in molten steel (mass ppm) at the end point of converter decarburization treatment, W _slag-Al is the addition amount of aluminum for slag reduction ( kg / t), _Wslag-CaO is the addition amount (kg / t) of quick lime for slag component modification. The burner flame is a flame formed by burning a fuel gas with oxygen gas, and 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 completed. 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 equation (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.

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