JP4360239B2 - Method for desulfurization of molten steel in vacuum degassing equipment - Google Patents

Description

  The present invention relates to a method for desulfurizing molten steel in a vacuum degassing facility.

In recent years, there has been an increasing demand for the production of high-grade steels with less impurities than before, and in particular, low-sulfur steels with a low sulfur content are required. In the production of low-sulfur steel, desulfurization treatment is performed in the hot metal stage before the decarburization process in the converter, but in so-called very low-sulfur steel with a sulfur concentration of 0.001 mass% or less, Furthermore, desulfurization is also performed in the molten steel stage after the steel from the converter. As a desulfurizing agent for molten steel, generally, a desulfurizing agent using quick lime (CaO) as a main component and containing alumina (Al ₂ O ₃ ), fluorite (CaF ₂ ) or the like as a melting point depressant is used. ing.

  This desulfurization treatment of molten steel has heretofore been performed in a so-called ladle refining furnace that is equipped with a heating means, a stirring means, and an injection means and is performed at atmospheric pressure. By the way, in high-grade varieties such as ultra-low sulfur steel, degassing treatment is necessary for the purpose of dehydrogenation or cleaning of molten steel. It was desulfurized in a ladle refining furnace, and then dehydrogenated in a vacuum degassing facility, and manufactured in two ladle facilities in a ladle. However, in order to solve problems such as the complexity of transporting between two secondary refining facilities and the waste of double investment of facilities, desulfurization treatment is performed in a vacuum degassing facility that was mainly dehydrogenated. Many attempts have been made to simplify the manufacturing process.

For example, Patent Document 1 proposes a method of using a limestone-added limestone as a desulfurizing agent and blowing the desulfurizing agent over molten steel in a vacuum tank of an RH vacuum degassing apparatus. . Further, in Patent Document 2, the molten steel is vigorously stirred in the RH vacuum degassing apparatus, and the refractory of the vacuum degassing equipment becomes severely melted by adding a desulfurizing agent containing fluorite, thereby preventing this. Therefore, desulfurization using a desulfurization agent obtained by adding MgO to a CaO—CaF ₂ desulfurization agent has been proposed. However, desulfurization ability falls by adding MgO.

However, in any case, a desulfurization agent mainly composed of CaO needs to add alumina or fluorite to lower the melting point of CaO in order to promote the desulfurization reaction. An increase in the amount of slag due to the addition and a decrease in the molten steel temperature due to this, or a refractory melting due to the addition of fluorite is inevitable. Therefore, as a desulfurization method for preventing these problems, Patent Document 3 discloses that an agglomerated desulfurization agent containing Al and MgO is added and added onto molten steel in a vacuum tank of a vacuum degassing facility. A method of processing has been proposed. The desulfurization treatment method according to Patent Document 3 is a method in which MgO is reduced by Al, the generated Mg gas is reacted with sulfur in molten steel, and desulfurized.
JP-A-5-239534 Japanese Patent Publication No. 1-49772 JP-A-11-193416

  However, the method of Patent Document 3 also has some problems, and the main ones are as follows. That is, in Patent Document 3, an agglomerated desulfurization agent is placed on top so as not to be exhausted by the exhaust device of the vacuum degassing equipment, and the desulfurization reaction occurs mainly on the surface of the bulk desulfurization agent, and the reaction The interfacial area is small and the desulfurization rate is not always fast. In addition, the agglomeration increases the floating speed of the desulfurizing agent in the molten steel, contributing to the reduction of the reaction time. From these, desulfurization efficiency is not necessarily high, and there is room for further improvement.

  The present invention has been made in view of the above circumstances, and its object is to desulfurize molten steel using Mg gas generated from MgO reduced by Al during refining of vacuum degassing equipment. An object of the present invention is to provide a method for desulfurizing molten steel, which can greatly increase the reaction interfacial area and the reaction time compared to the conventional method and can efficiently desulfurize the molten steel.

The desulfurization treatment method of the molten steel in the vacuum degassing equipment according to the first invention for solving the above-mentioned problem, when performing the desulfurization treatment of the molten steel in the vacuum degassing equipment, the CaO concentration of the slag on the molten steel in the ladle, While adjusting the basicity (CaO / SiO ₂ ) of the slag to be 2.0 or more, the Al concentration of the molten steel refined in the vacuum degassing equipment is adjusted to 0.05 mass% or more, and then A powdery MgO source is sprayed onto the molten steel surface under reduced pressure using an upper blowing lance, and Al in the molten steel reacts with MgO in the MgO source to generate Mg gas, and this Mg gas desulfurizes the molten steel. It is characterized by doing.

In the desulfurization treatment method of the molten steel in the vacuum degassing facility according to the second invention, when performing the desulfurization treatment of the molten steel in the vacuum degassing facility, the CaO concentration of the slag on the molten steel in the ladle is determined by the basicity of the slag (CaO / SiO ₂ ) is adjusted to 2.0 or more, and while continuously adding an Al source to the molten steel refined in the vacuum degassing facility, the top of the molten steel under reduced pressure is used. A powdery MgO source is sprayed on the molten metal surface, Al dissolved in molten steel reacts with MgO in the MgO source to generate Mg gas, and the molten steel is desulfurized by this Mg gas. .

In the desulfurization treatment method for molten steel in the vacuum degassing facility according to the third invention, when performing desulfurization treatment of the molten steel in the vacuum degassing facility, the CaO concentration of the slag on the molten steel in the ladle is determined by the basicity of the slag (CaO / SiO ₂ ) is adjusted to 2.0 or more, and a powdery MgO source and powdery powder are used on the molten steel surface under reduced pressure refined in the vacuum degassing equipment using an upper blowing lance. The Al source is sprayed simultaneously, Al dissolved in the molten steel reacts with MgO in the MgO source to generate Mg gas, and the molten steel is desulfurized by this Mg gas.

  The method for desulfurizing molten steel in the vacuum degassing facility according to the fourth invention is characterized in that, in the first invention to the third invention, the size of the MgO source has an average particle size of 1 mm or less. It is.

  In the present invention, since the MgO source is sprayed and added to the molten steel surface under reduced pressure through the top blowing lance at a high speed, even if the MgO source is powder, it is exhausted by the exhaust device of the vacuum degassing equipment. It can be added to the molten steel. Since the MgO source is added as a powder having a large specific surface area, the reaction interface area of the MgO source is large. Therefore, Al added in advance in the molten steel or the Al source added simultaneously with the MgO source and MgO are as follows. According to the formula (1), Mg gas is generated by reacting rapidly, and the generated Mg gas reacts with sulfur in the molten steel according to the following formula (2) to form MgS. It is possible to perform desulfurization treatment well.

In addition, since the MgO source is a powder, the ascending speed in the molten steel is slower than in the case of a lump, and the MgO source stays in the molten steel for a long time, so the reaction time becomes longer and the desulfurization reaction becomes more efficient. To do. Furthermore, since a high melting point desulfurization agent mainly composed of MgO is used, the refractory melt resistance of the vacuum degassing equipment is greatly suppressed as compared with the case where a CaO—CaF ₂ -based desulfurization agent is used. The The average particle size of the MgO source is preferably 1 mm or less from the viewpoint of increasing the reaction interface area.

  According to the present invention, since the powdery MgO source is used as the desulfurization agent, the reaction interface area and the reaction time are greatly increased as compared with the case of using the bulk MgO source, and the amount of generated Mg gas is reduced. Since it increases, molten steel can be efficiently desulfurized. Further, unlike desulfurization using a CaO-based desulfurizing agent, a hatching accelerator such as alumina or fluorite is not required, and since inexpensive MgO or Al is used, the desulfurizing agent unit cost can be reduced. Further, unlike the desulfurization reaction using a CaO-based desulfurization agent, it is not necessary to hatch the desulfurization agent, and the reduction of MgO by Al is an exothermic reaction, so that the temperature drop of the molten steel due to the desulfurization treatment can be reduced.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic cross-sectional view of an RH vacuum degassing apparatus used when carrying out the desulfurization treatment according to the present invention.

  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.

  The molten steel 3 obtained by the converter decarburization refining of the hot metal that has been discharged from the blast furnace and desulfurized in the hot metal stage is discharged from the converter to the ladle 2, and the ladle 2 containing the molten steel 3 is subjected to RH vacuum. Transport to degassing device 1. In the RH vacuum degassing apparatus 1, 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 in the ladle 2. Then, Ar gas is blown into the rising side dip tube 8 from the reflux gas blowing tube 10 as a reflux gas, and the inside of the vacuum chamber 5 is evacuated by an exhaust device (not shown) connected to the duct 11 to be evacuated. The inside of the tank 5 is depressurized. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 in the ladle 2 ascends the rising side dip tube 8 together with Ar gas blown from the reflux gas blowing tube 10 and flows into the vacuum chamber 5, and then A flow returning to the ladle 2 via the descending side dip pipe 9, that is, a so-called recirculation flow is formed and RH vacuum degassing is performed.

  During this RH vacuum degassing, a powdered MgO source is sprayed from the top blowing lance 13 together with the carrier gas toward the molten steel 3 in the vacuum chamber 5 (also referred to as “projection”), and desulfurized to the molten steel 3. Apply processing. As the carrier gas, it is preferable to use a rare gas such as Ar gas so as not to interfere with the desulfurization reaction which is a reduction reaction. In this case, when it is necessary to subject the molten steel 3 to vacuum decarburization, it is preferable to perform vacuum decarburization before desulfurization. While the desulfurization treatment is a reduction reaction, the vacuum decarburization treatment is an oxidation reaction, and at the end of RH vacuum degassing refining, the molten steel 3 is generally in a deoxidized state. This is because the reductant used in the desulfurization process, that is, the deoxidizer, is oxidized in the vacuum decarburization process, and thus is wasted. However, this does not apply in the case where inconvenience occurs such as a problem in quality.

  In order to desulfurize the molten steel 3 with the powdery MgO source, Al is required as shown in the above-described formulas (1) and (2). Therefore, the Al source is charged into the molten steel 3. The following three methods can be used as a method for adding the Al source.

  In the first method, before projecting a powdery MgO source, an Al source such as a massive metal Al is introduced into the molten steel 3 in the vacuum chamber 5 from the raw material inlet 12 to obtain an amount necessary for the desulfurization reaction. In this method, Al is contained in the molten steel 3. The present inventors have confirmed that the amount of Al necessary for the desulfurization reaction can be secured if the Al concentration in the molten steel is 0.05 mass% or more. The second method is a method in which an Al source such as lump metal Al is introduced into the molten steel 3 in the vacuum chamber 5 from the raw material inlet 12 at the same time as the projection of the powdery MgO source. The third method is a method of projecting a powdered Al source from the top blowing lance 13 toward the molten steel 3 in the vacuum chamber 5 simultaneously with the powdered MgO source. Whatever method is used, the powdery MgO source projected at a high speed from the top blowing lance 13 is mixed with the molten steel 3 and flows into the ladle 2 together with the molten steel 3 circulated from the vacuum chamber 5 to the ladle 2. . In the meantime, it reacts with Al dissolved in the molten steel 3 to generate Mg gas, and this Mg gas reacts with sulfur in the molten steel 3 so that it can be efficiently desulfurized. Also in the second method and the third method, Al enough to deoxidize the molten steel 3 may be added in advance.

The MgO source is not particularly limited. For example, natural magnesia obtained by pyrolyzing brucite (Mg (OH) ₂ ) and magnesite (MgCO ₃ ), or seawater magnesia obtained from seawater. Can be suitably used. Various MgO precursors that are thermally decomposed up to 900 ° C. to become MgO may be used as the MgO source. For example, Mg (OH) ₂ or MgCO ₃ can be mentioned. Dolomite (MgCO ₃ · CaCO ₃ ) may be added. The MgO concentration of the MgO source powder is preferably 88 mass% or more. If it is less than 88 mass%, the component which does not contribute to generation of Mg gas becomes excessive, which is not preferable. The size of the MgO source is preferably an average particle size of 1 mm or less in order to increase the interfacial area and promote the desulfurization reaction.

  The powdery Al source projected from the top blowing lance 13 simultaneously with the MgO source powder is not particularly limited. As the powdered Al source, atomized powder obtained by atomizing aluminum melt with gas, powder generated when polishing and cutting aluminum alloy, aluminum dross powder generated when melting and regenerating aluminum scrap, etc. It can be used suitably. As for the size of the powdery Al source, it is preferable to set the average particle size to 1 mm or less as in the case of the MgO source in order to perform smooth projection.

  MgS, which is a reaction product of Mg gas and sulfur contained in the molten steel 3, finally floats up from the molten steel 3 and is absorbed by the slag 4. However, when the oxygen potential of the slag 4 is high, MgS is changed to MgO according to the following equation (3), so that a phenomenon that sulfur returns to the molten steel 3, so-called sulfurization occurs, and desulfurization efficiency decreases.

To prevent this resulfurization, the CaO concentration of the slag 4, as the basicity of the slag 4 (CaO / SiO ₂₎ of 2.0 or more, it adjusts. Increasing the basicity decreases the oxygen potential, and CaO and MgS react according to the following formula (4) to form stable CaS, thereby preventing sulfurization.

Further, in order to prevent sulfurization, CaO or CaCO ₃ as a main component may be added from the raw material inlet 12 after projecting MgO powder, or may be mixed with MgO powder and simultaneously projected. Good. When CaO or CaCO ₃ is mixed with the MgO powder, CaO or CaCO ₃ is preferably set to an amount of 5 mass% or more of the added amount of MgO. Furthermore, in order to reduce the oxygen potential of the slag 4, an Al source may be added to the slag 4, and the slag 4 may be reduced with Al.

  After projecting a predetermined amount of MgO powder, the molten steel 3 is circulated for several minutes to several tens of minutes for desulfurization, and if the sulfur concentration of the molten steel 3 falls to the target value, Al, C, Si, Mn, A component adjusting agent such as Nb or Ti is introduced into the molten steel 3 from the raw material inlet 12 in a state where the reflux is continued, and the components of the molten steel 3 are adjusted. Then, the inside of the vacuum chamber 5 is returned to the atmospheric pressure and RH vacuum desorption is performed. Finish gas refining. The dehydrogenation process, the denitrogenation process and the cleaning process are performed simultaneously with the desulfurization process by circulating the molten steel 3.

As described above, according to the present invention, the MgO source is sprayed and added to the molten steel surface under reduced pressure through the top blowing lance 13 at a high speed. Therefore, even if the MgO source is powdery, RH vacuum desorption is performed. It is not exhausted by the exhaust device of the gas device 1 but can be added into the molten steel 3. Since the MgO source is added as a powder having a large specific surface area, the reaction interface area of the MgO source is increased, and the molten steel 3 can be efficiently desulfurized. Further, since a high melting point desulfurization agent mainly composed of MgO is used, it is not necessary to hatch the desulfurization agent, and the refractory melt of the RH vacuum degassing apparatus 1 has a CaO—CaF ₂ -based desulfurization agent. It is greatly suppressed compared with the case of using.

  In the above description, the example in which the RH vacuum degassing apparatus 1 is used as the vacuum degassing equipment has been described. However, the present invention is not limited to the RH vacuum degassing apparatus 1. The present invention can also be applied to a degassing apparatus, a VOD facility, a VAD facility, and the like.

  Using the RH vacuum degassing apparatus shown in FIG. 1, a test for desulfurization by projecting MgO powder onto molten steel was performed. About 250 tons of molten steel discharged from the converter to the ladle was deoxidized with metal Al at the time of steel discharge, and this molten steel was conveyed to the RH vacuum degasser. While adjusting the sulfur concentration in the molten steel to about 35 ppm at the time of steel output, quick lime was introduced into the ladle at the time of steel output to adjust the basicity of the slag to 2.0 or more. In the RH vacuum degassing apparatus, desulfurization treatment was performed by three methods of levels 1 to 3.

  In Level 1, if a molten steel recirculation is formed in the RH vacuum degassing apparatus, after adding bulk metal Al from the raw material inlet and adjusting the Al concentration in the molten steel to 0.05 mass%, the average particle size becomes About 3 minutes from the start of RH vacuum degassing and refining a mixture of powdered MgO of 1.0 mm or less and powdered CaO having an average particle size of 1.0 mm or less (MgO mass: CaO mass = 9: 1) Nitrogen gas was sprayed onto the molten steel surface through the top blowing lance as the transfer gas at an addition rate of 70 kg / min.

  In Level 2, when a molten steel recirculation is formed in the RH vacuum degassing apparatus and approximately 3 minutes have elapsed from the start of the RH vacuum degassing refining, when bulk metal Al is added from the raw material inlet at an addition rate of 30 kg / min. At the same time, a mixture (MgO mass: CaO mass = 9: 1) of powdered MgO having an average particle size of 1.0 mm or less and powdered CaO having an average particle size of 1.0 mm or less is obtained from the top blowing lance. Ar gas was sprayed on the molten steel surface as a carrier gas at an addition rate of 70 kg / min.

  In level 3, a molten steel recirculation is formed in the RH vacuum degassing apparatus, and after about 3 minutes from the start of the RH vacuum degassing refining, powdered MgO having an average particle diameter of 1.0 mm or less and an average particle diameter When a mixture of powdered CaO with a particle size of 1.0 mm or less (MgO mass: CaO mass = 9: 1) is sprayed from the top blowing lance onto the molten steel surface at an addition rate of 70 kg / min, Atomized Al powder having an average particle size of 1.0 mm or less was sprayed onto the molten steel surface at an addition rate of 26 kg / min. That is, a mixture of MgO powder, CaO powder, and Al powder was sprayed from the top blowing lance onto the molten steel surface at an addition rate of 96 kg / min using Ar gas as a carrier gas.

  At each level, the projection was stopped when the projected amount of the mixture of MgO powder and CaO powder reached 4 kg (total amount: 1000 kg) per ton of molten steel. At level 2, the metal Al was also stopped when the projection from the top lance was stopped. The recirculation was continued even after the projection was stopped, and the vacuum degassing refining was completed when 30 minutes had elapsed from the start of the treatment in the RH vacuum degassing apparatus. During this time, analytical samples were taken from the molten steel about every 5 minutes to investigate the transition of sulfur concentration. Of the tests in which the basic unit of the desulfurizing agent is adjusted to 4 kg per ton of molten steel, the level 1 test is indicated as Invention Example 1, the level 2 test is indicated as Invention Example 2, and the level 3 test is indicated as Invention Example 3. . Under the above conditions, the amount of metal Al added in Invention Example 2 is about 430 kg, and the amount of atomized Al powder added in Invention Example 3 is 370 kg.

For comparison, a CaO—CaF ₂ -based desulfurization agent was blown from a top lance and nitrogen gas was projected as a carrier gas to desulfurize the test, and a mixture of MgO powder and Al powder was agglomerated. A level 5 test was conducted in which the desulfurization agent formed into briquettes was placed on the molten steel from the raw material inlet and desulfurized, and the transition of the sulfur concentration of the molten steel was also investigated at levels 4 and 5. The addition start timing of the desulfurizing agent was the same as the levels 1 to 3. At level 4, the basic unit of the desulfurizing agent was adjusted to levels 1 to 3, and at level 5, the basic unit converted to MgO in the desulfurizing agent was adjusted to levels 1 to 3. Among the tests in which the basic unit of the desulfurizing agent is adjusted to levels 1 to 3, the test of level 4 is indicated as Comparative Example 1, and the test of level 5 is indicated as Comparative Example 2.

  Table 1 shows the operating conditions and the desulfurization rate in Examples 1-3 of the present invention and Comparative Examples 1-2. The desulfurization rate is the difference between the sulfur concentration in the molten steel before the start of the desulfurization treatment and the sulfur concentration in the molten steel at the end of the desulfurization treatment, expressed as a percentage with respect to the sulfur concentration in the molten steel before the start of the desulfurization treatment. Moreover, in FIG. 2, transition of the sulfur concentration in the molten steel investigated in this invention examples 1-3 and comparative examples 1-2 is shown.

  As shown in Table 1 and FIG. 2, in Invention Examples 1 to 3, the sulfur concentration in the molten steel was stably 10 ppm or less, and a high desulfurization rate of 75% or more was obtained. On the other hand, in Comparative Example 1, the sulfur concentration of the molten steel remained at about 25 ppm, and the desulfurization rate was 24%. In Comparative Example 2, the desulfurization rate was significantly improved as compared with Comparative Example 1, but was lower than the desulfurization rates of Invention Examples 1 to 3.

  Moreover, in the level 1-5, the test which changed the usage-amount of the desulfurization agent per charge was implemented, respectively, and the damage rate of the lower tank refractory of the RH vacuum degassing apparatus was investigated. The results are shown in Table 2 and FIG.

As shown in Table 2 and FIG. 3, in Levels 1 to 3 and Level 5 using MgO powder as a desulfurizing agent, the lower tank of the RH vacuum degassing device is compared to Level 4 using a CaO—CaF ₂ -based desulfurizing agent. It was confirmed that the damage rate of refractories was greatly reduced.

It is a schematic sectional drawing of the RH vacuum degassing apparatus used when implementing this invention. It is a figure which shows transition of the sulfur concentration in the molten steel investigated in Example 1. FIG. It is a figure which shows the damage rate of the lower tank refractory of the RH vacuum degassing apparatus investigated in Example 1. FIG.

Explanation of symbols

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 (4)

When performing the desulfurization treatment of the molten steel in the vacuum degassing equipment, the CaO concentration of the slag on the molten steel in the ladle is adjusted so that the basicity (CaO / SiO ₂ ) of the slag becomes 2.0 or more, and The Al concentration of the molten steel refined in the vacuum degassing equipment is adjusted to 0.05 mass% or more, and then a powdered MgO source is sprayed on the molten steel surface under reduced pressure using an upper blowing lance. A method for desulfurizing molten steel in a vacuum degassing facility, comprising reacting Al with MgO in a MgO source to generate Mg gas, and desulfurizing the molten steel with the Mg gas. When performing the desulfurization treatment of the molten steel in the vacuum degassing equipment, the CaO concentration of the slag on the molten steel in the ladle is adjusted so that the basicity (CaO / SiO ₂ ) of the slag becomes 2.0 or more, and While continuously adding an Al source to molten steel refined in a vacuum degassing facility, a powdered MgO source was sprayed onto the molten steel surface under reduced pressure using an upper blowing lance, and the dissolved Al and A method for desulfurizing a molten steel in a vacuum degassing facility, characterized by reacting MgO in an MgO source to generate Mg gas and desulfurizing the molten steel with the Mg gas. When performing the desulfurization treatment of the molten steel in the vacuum degassing equipment, the CaO concentration of the slag on the molten steel in the ladle is adjusted so that the basicity (CaO / SiO ₂ ) of the slag becomes 2.0 or more, and A powdered MgO source and a powdered Al source are simultaneously sprayed onto the molten steel surface under reduced pressure, which is refined in a vacuum degassing facility, using an upper blowing lance, and the dissolved Al and MgO sources in the molten steel A method for desulfurizing a molten steel in a vacuum degassing facility, characterized in that Mg gas is produced by reacting with MgO and the molten steel is desulfurized with the Mg gas.   The method for desulfurizing molten steel in a vacuum degassing facility according to any one of claims 1 to 3, wherein the MgO source has an average particle size of 1 mm or less.

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