JP5200380B2 - Desulfurization method for molten steel - Google Patents

Description

  The present invention relates to a method for desulfurizing molten steel, which is performed by adding a desulfurizing agent under reduced pressure to molten steel in a ladle that has been discharged from a decarburization refining furnace such as a converter.

  In recent years, the demand for high-purity steel is increasing compared to the prior art in order to improve the material properties accompanying the increase in added value of steel and the expansion of the use of steel materials. In order to meet this demand, extremely low sulfidation of molten steel is one important condition.

  In the melting of so-called “low-sulfur steel” with a sulfur concentration of about 0.003 to 0.010 mass%, it can be dealt with by performing a desulfurization process in the hot metal stage before the decarburization refining process in the converter. However, the so-called “ultra-low-sulfur steel” with a sulfur concentration of 0.0010% by mass or less, such as high-grade electrical steel sheets and high-grade line pipe steel sheets, cannot be dealt with only by the desulfurization process in the hot metal stage, but is desulfurized in the hot metal stage. After application, desulfurization treatment is also performed in the molten steel stage after the steel is removed from the converter. Conventionally, this desulfurization treatment of molten steel has been carried out in a so-called “ladder refining furnace” which is performed at atmospheric pressure and equipped with heating means, stirring means, flux injection means, and the like.

  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. Therefore, ultra-low sulfur steel is decarburized and refined in a converter or the like. After that, it is first desulfurized in a ladle refining furnace, and then dehydrogenated in a vacuum degassing facility such as an RH vacuum degassing device, and is manufactured through two secondary refining facilities in the ladle. It was. However, in order to solve problems such as the complexity of transporting between the two secondary refining facilities and the double investment of the facilities, desulfurization treatment should be performed in the vacuum degassing facilities that were mainly dehydrogenated. Many attempts have been made to simplify the manufacturing process.

For example, as a desulfurization method for an RH vacuum degassing apparatus that is most widely used as a vacuum degassing facility, Patent Document 1 discloses that the total concentration of iron oxide and manganese oxide in slag in a ladle is 5 mass%. It was proposed that the desulfurization treatment be performed by spraying a desulfurizing agent on the surface of the molten steel bath under reduced pressure using an upper blowing lance during refining in the RH vacuum degassing apparatus. After adjusting the total Fe concentration of the slag in the pan to 10 mass% or less, CaF ₂ or Al ₂ O containing CaO as the main component perpendicular to the surface of the molten steel circulating in the vacuum tank of the RH vacuum degasser It is proposed that a desulfurizing agent containing ₃ be sprayed through an upper blowing lance for desulfurization treatment. Patent Document 3 discloses a desulfurizing agent containing a metal having a strong affinity for oxygen, and a lance height of 1 to 3 m. The top blowing lance It is proposed that the desulfurization treatment be performed by spraying the molten steel bath surface under reduced pressure.

  In Patent Document 4, after adjusting the total concentration of iron oxide and manganese oxide in the slag in the ladle to 5% by mass or less, oxygen and aluminum are added to the molten steel in the vacuum chamber through an upper blowing lance, and the molten steel temperature is increased. It is proposed that the desulfurization treatment is performed by spraying a desulfurizing agent through the lance, and the slag composition in the ladle, the degree of vacuum in the vacuum chamber, and the lance height of the upper lance are disclosed in Patent Document 5. It has been proposed that the composition of the desulfurizing agent and the blowing speed of the desulfurizing agent are defined, and the desulfurizing treatment is performed by spraying the desulfurizing agent from the top blowing lance toward the molten steel under reduced pressure.

In Patent Document 6, the aluminum concentration in the molten steel is set to 0.100% by mass or more and desulfurization treatment is performed by spraying a desulfurizing agent from the top blowing lance onto the molten steel surface under reduced pressure. After desulfurization treatment, the aluminum in the molten steel is burned. It has been proposed to remove.
JP-A-5-214424 JP-A-5-171253 JP-A-6-73429 JP-A-5-287359 JP-A-9-170012 JP-A-6-299229

  In the RH vacuum degassing apparatus, it is difficult to stir the slag in the ladle. When desulfurization is performed in the RH vacuum degassing apparatus, as described in Patent Documents 1, 2, 4, and 5, Before reaching the RH vacuum degassing apparatus, it is necessary to reduce the oxygen potential of the slag in the ladle and adjust it to a slag suitable for the desulfurization process. As for the composition of the molten steel, since the desulfurization efficiency depends on the oxygen potential of the molten steel, Patent Document 6 proposes that the aluminum concentration in the molten steel be a high concentration of about 0.100% by mass. 1 suggests that the aluminum concentration in the molten steel be 0.025 to 0.053 mass%.

  As a result of various studies, the present inventors have found that components other than aluminum in molten steel also contribute to desulfurization efficiency when the molten steel is desulfurized with an RH vacuum degassing apparatus. However, conventionally, in order to improve the desulfurization efficiency as in Patent Documents 1, 2, 4, 5, and 6, it has been proposed to adjust the slag composition in the ladle and the aluminum concentration in the molten steel before the desulfurization treatment. However, no mention is made of other ingredients.

  In addition, Patent Documents 1 to 5 propose various conditions for the desulfurization method in the RH vacuum degassing apparatus. However, as in Patent Document 5, when the desulfurization agent blowing conditions and the slag composition are defined in detail. However, the following problems remain.

  That is, since the RH vacuum degassing apparatus does not have an electric heating apparatus for heating molten steel like a ladle refining furnace, it is easily affected by a temperature drop due to the addition of a desulfurizing agent. For this reason, it may be necessary to burn aluminum in the molten steel to raise the temperature of the molten steel. In addition, in the RH vacuum degassing apparatus, it may be necessary to perform not only a desulfurization process but also a vacuum decarburization process in addition to a gas component removal process such as a dehydrogenation process or a denitrogenation process. Among these processes, the vacuum decarburization process and the heat treatment by burning aluminum are oxidation reactions, and the oxygen potential of the molten steel is increased by supplying oxygen gas or the like. On the other hand, since the desulfurization treatment is a reduction reaction, the lower the oxygen potential of molten steel, the better.

Therefore, when the desulfurization treatment is followed by the vacuum decarburization treatment and the heat treatment by combustion of aluminum, the sulfur once transferred from the molten steel to the slag by the desulfurization treatment returns to the molten steel as the oxygen potential in the molten steel rises, so-called `` Because the “sulfurization reaction” occurs, the sulfur concentration of the molten steel cannot be lowered stably. On the other hand, when the desulfurization treatment is performed after the vacuum decarburization treatment and the heat treatment by combustion of aluminum, the oxygen potential of the slag is high, and the desulfurization agent added for the desulfurization treatment is oxidized and the desulfurization reaction does not proceed. In addition, there arises a problem that Al ₂ O ₃ generated when the molten steel is deoxidized for the desulfurization treatment reacts with the added desulfurization agent to reduce the desulfurization ability of the desulfurization agent. In particular, in the heat treatment by combustion of aluminum, a large amount of Al ₂ O ₃ is produced, which reduces the desulfurization ability of the desulfurizing agent.

  From these, when desulfurization treatment is carried out in a vacuum degassing facility, it is necessary to establish a desulfurization method that does not lower the desulfurization capacity in accordance with refining other than desulfurization treatment. There is no mention of any points.

  The present invention has been made in view of the above circumstances. The purpose of the present invention is to desulfurize molten steel in a vacuum degassing facility such as an RH vacuum degassing apparatus. To provide a method for desulfurizing molten steel that optimizes the order of refining in a degassing facility and can perform desulfurization processing much more efficiently than conventional methods.

  In order to achieve the above-mentioned problems, the present inventors, when desulfurizing the molten steel discharged from the converter with an RH vacuum degassing apparatus, the molten steel components at the time of steel extraction, the slag in the ladle after the steel output A test operation was carried out by changing the reforming conditions, the order of refining in the RH vacuum degassing apparatus, and the time between refining, and the effects of these conditions on the desulfurization treatment were investigated and examined.

As a result, in order to ensure a high desulfurization rate of 80% or more, the total slag in the slag before desulfurization treatment. It was found that the total concentration of Fe and manganese oxide needs to be adjusted to 5% by mass or less. In addition, the silicon concentration in the molten steel is also important. It was found that the total concentration of Fe and manganese oxide was adjusted to 5% by mass or less, and at the same time, the silicon concentration in the molten steel was required to be 0.10% by mass or more. In addition, the total in the slag. Fe is the total iron content of all iron oxides (FeO, Fe ₂ O ₃ etc.) in the slag.

Further, in the case where the molten steel temperature is insufficient and the RH vacuum degassing apparatus performs the heat treatment by the combustion of aluminum, after the heat treatment is completed, the vacuum degassing refining is continued for 3 minutes or more and the Al ₂ in the molten steel. It was found that desulfurization can be efficiently performed by adding a desulfurizing agent after sufficiently floating and separating O ₃ .

The present invention has been made based on the above examination results, and the desulfurization method for molten steel according to the present invention includes silicon in steel discharged from a decarburization refining furnace that performs decarburization refining under atmospheric pressure to a ladle. Alloy iron is added to adjust the silicon concentration of the molten steel in the ladle to 0.10% by mass or more, and an aluminum-containing slag modifier is added to the slag in the ladle after steelmaking. The total of the slag. The total concentration of Fe and manganese oxide is adjusted to 5% by mass or less, and then the ladle is transported to a vacuum degassing facility, where aluminum is added to the molten steel, and then the molten steel under reduced pressure. Oxygen gas is supplied to the surface through an upper blowing lance to burn aluminum in the molten steel to raise the temperature of the molten steel. Thereafter, the aluminum concentration dissolved in the molten steel after the molten steel is heated to 0.010% by mass or more while ensuring, allowed to proceed floating and separation of the molten steel than 3 minutes after completion of the supply of the oxygen gas from the molten steel Al ₂ O ₃ which was refluxing under reduced pressure was generated when the molten steel temperature heat for the molten steel temperature heat Thereafter, a desulfurizing agent is sprayed and added to the surface of the molten steel under reduced pressure of 50 torr or less together with a carrier gas through an upper blowing lance to desulfurize the molten steel.

According to the present invention, the total slag in the slag before the desulfurization treatment. Since the total concentration of Fe and manganese oxide is 5% by mass or less and the silicon concentration of the molten steel is 0.10% by mass or more, the oxygen potential of the molten steel and slag is lowered, and the molten steel can be efficiently desulfurized. it can. In addition, when the temperature of the molten steel is insufficient and the heat treatment by combustion of aluminum is performed in the vacuum degassing facility, the desulfurization treatment is started after refining the molten steel for 3 minutes or more under reduced pressure after the completion of the heat treatment. The floating and separation of Al ₂ O ₃ produced by the heat treatment proceeds, and the desulfurization agent can be efficiently desulfurized without being affected by the Al ₂ O ₃ and without degrading the desulfurization ability.

  Hereinafter, the present invention will be specifically described.

  The hot metal discharged from the blast furnace is received in a hot metal holding / conveying vessel such as a hot metal ladle or torpedo car, and transferred to a converter that performs decarburization and refining under atmospheric pressure in the next process. A desulfurization process is performed on the hot metal during the conveyance. This is because the type of steel that undergoes desulfurization treatment in the molten steel stage after converter refining is generally ultra-low-sulfurized steel, so desulfurization treatment is performed in advance in the hot metal stage to reduce the load of desulfurization treatment in the molten steel stage. Because. This desulfurization process in the hot metal stage includes a mechanical stirring type desulfurization method in which a CaO-based desulfurizing agent is added to the mechanically stirred hot metal, and an injection method (blowing) in which a CaO-based desulfurizing agent is blown into the hot metal together with a carrier gas. A conventional desulfurization method such as (Method) may be used. After the desulfurization treatment, the generated desulfurization slag is discharged from the hot metal holding / conveying vessel, and the hot metal is conveyed to the converter.

  This hot metal is charged into a converter and decarburized and refined with top blown oxygen, bottom blown oxygen, and the like. After completion of decarburization refining, the molten steel obtained by decarburization refining is discharged from the converter to the ladle. At the time of this steel removal, silicon-containing alloy iron such as Fe-Si alloy or Si-Mn alloy is added to the ladle, and the silicon concentration dissolved in the molten steel in the ladle becomes 0.10% by mass or more. Adjust as follows. Moreover, you may deoxidize molten steel by adding metallic aluminum in a ladle at the time of this steel extraction. In order to stabilize the oxygen potential of molten steel at a low level, the amount of metal aluminum added is preferably adjusted so that the amount of aluminum dissolved in the molten steel is 0.01% by mass or more. Furthermore, in order to dilute total Fe (hereinafter referred to as “T.Fe”) and manganese oxide in the slag, quick lime or a flux containing quick lime may be added at the time of steel production.

And after steeling out, the slag modifier which contains aluminum is added to the slag which exists on the molten steel in a ladle, T. in slag is added. The total concentration of Fe and manganese oxide is adjusted to 5% by mass or less. As the slag modifier containing aluminum, metallic aluminum alone, a mixture of metallic aluminum and quicklime, Al ash (also referred to as “aluminum dross”), and the like can be used. In particular, Al ash is suitable because it is inexpensive. Al ash is a mixture of metallic aluminum containing 30 to 50% by mass of metallic aluminum and Al ₂ O _3, and also contains other components.

  Next, the molten steel is transported to a vacuum degassing facility such as an RH vacuum degassing device, a DH vacuum degassing device, or a VOD furnace, and predetermined vacuum refining and desulfurization treatment are performed in the transported vacuum degassing facility.

  The molten steel subjected to the desulfurization treatment in the vacuum degassing equipment is not limited to the molten steel obtained by decarburizing and refining the molten iron discharged from the blast furnace. It may be molten steel decarburized and refined by an electric furnace. In that case, what is necessary is just to replace a converter with an electric furnace in the process after the steelmaking of the said description. In the case of molten steel melted in an electric furnace, the desulfurization process is performed only once.

  A typical equipment of the vacuum degassing equipment is an RH vacuum degassing apparatus. Hereinafter, an example of refining using an RH vacuum degassing equipment as the vacuum degassing equipment will be described. FIG. 1 shows an example of an RH vacuum degassing apparatus used in carrying out the present invention.

  As shown in FIG. 1, the RH vacuum degassing apparatus 1 includes a vacuum tank 5 including an upper tank 6 and a lower tank 7, an ascending-side dip pipe 8 and a descending-side dip pipe 9 provided below the lower tank 7. The upper tank 6 is provided with a duct 11 connected to an exhaust device (not shown), a raw material inlet 12, and an upper blowing lance 13 that is movable in the vertical direction inside the vacuum tank 5, The ascending-side dip tube 8 is provided with a circulating gas blowing tube 10. From the reflux gas blowing tube 10, Ar gas is blown into the rising side immersion tube 8 as the reflux gas.

  The top blowing lance 13 blows oxygen gas toward the molten steel 3 inside the vacuum chamber 5, or toward the molten steel 3 inside the vacuum chamber 5 using a desulfurizing agent as a non-oxidizing gas or a rare gas as a carrier gas. It is configured so that it can be sprayed. Naturally, only a non-oxidizing gas or a rare gas can be blown, or a mixed gas of a non-oxidizing gas, a rare gas, and oxygen gas can be blown.

  In the RH vacuum degassing apparatus 1 having such a configuration, the desulfurization treatment of the molten steel 3 is performed as follows.

  First, the ladle 2 in which the molten steel 3 is stored is conveyed directly under the vacuum chamber 5. The inside of the ladle 2 is generated by decarburization refining in a converter, electric furnace, etc. The slag 4 modified so that the total concentration of Fe and manganese oxide is 5 mass% or less covers the molten metal surface of the molten steel 3. Next, the 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, 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 connected to the duct 11. Depressurize the inside. When the inside of the vacuum chamber 5 is depressurized, the molten steel 3 accommodated 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. Then, a flow returning to the ladle 2 through the descending side dip tube 9, that is, a so-called “circular flow” is formed, and RH vacuum degassing is performed.

  During this RH vacuum degassing, a powdered desulfurization agent is sprayed from the top blowing lance 13 together with the carrier gas toward the molten steel 3 inside the vacuum chamber 5 (also referred to as “projection”), and the molten steel 3 The desulfurization treatment is performed. The desulfurizing agent to be used is not particularly required, but a CaO-based desulfurizing agent mainly composed of CaO is suitable because it is inexpensive. Specifically, quicklime simple substance or a mixture of quicklime and fluorite can be used.

  The projected desulfurizing agent is caught in the molten steel 3, reaches the ladle 2 through the descending side dip tube 9, floats in the ladle, and enters the slag 4. In the process until the desulfurization agent is mixed into the slag 4, sulfur in the molten steel reacts with the desulfurization agent, and the desulfurization reaction product is mixed into the slag 4 together with the desulfurization agent. The slag 4 is T.D. The total concentration of Fe and manganese oxide is 5% by mass or less, the oxygen potential is low, and no sulfurization occurs. Moreover, since the silicon concentration of the molten steel 3 is 0.10% by mass or more and the oxygen potential is low, the desulfurization reaction is promoted.

  When the desulfurization agent is projected onto the molten steel 3 inside the vacuum chamber 5 and subjected to desulfurization treatment, if the degree of vacuum inside the vacuum chamber 5 is increased (the pressure is decreased), the velocity of the gas blown out from the top blowing lance 13 is attenuated. Therefore, even when the transfer gas flow rate is constant, the gas dynamic pressure on the bath surface of the molten steel 3 of the jet gas is increased, the yield of the refining agent is improved, and at the same time, the desulfurization reaction at the projection position is promoted. Is advantageous. Therefore, the pressure inside the vacuum chamber 5 is preferably 50 torr (66.7 hPa) or less, and preferably 10 torr (13.3 hPa) or less if evacuation to a high vacuum is possible.

  When refining the molten steel 3 with the RH vacuum degassing apparatus 1, in addition to the original purpose, gas component removal treatment such as dehydrogenation treatment and denitrogenation treatment, not only desulfurization treatment, but also vacuum decarburization treatment, aluminum There is a case where it is necessary to perform a heat treatment by combustion (hereinafter, also simply referred to as “heat treatment”) and a component adjustment of the molten steel 3. Among these processes, the vacuum decarburization process and the heat treatment are oxidation reactions. In particular, in the case of the vacuum decarburization process, it is necessary to increase the oxygen potential of the molten steel 3 by applying oxygen gas or the like. On the other hand, since the desulfurization treatment is a reduction reaction, the lower the oxygen potential of the molten steel 3, the better.

  Therefore, if the vacuum decarburization treatment and the heat treatment by combustion of aluminum are carried out after the desulfurization treatment, the reducing agent used in the desulfurization treatment, that is, the deoxidizing agent, is oxidized and is wasted. Since the sulfur transferred from the slag 4 returns to the molten steel 3 as the oxygen potential increases during vacuum decarburization treatment and heat treatment, a so-called sulfite reaction occurs, so the sulfur concentration of the molten steel 3 is stably reduced. I can't.

Therefore, in the present invention, the desulfurization process is performed after the vacuum decarburization process, which is an oxidation reaction, and the heat treatment by the combustion of aluminum, and the component adjustment is performed after the desulfurization process. When both the vacuum decarburization treatment and the heat treatment are to be performed, the molten steel 3 is also heated in the vacuum decarburization treatment. Therefore, first, the vacuum decarburization treatment is performed, and then the aluminum is burned. A heat treatment is performed. When desulfurization treatment is performed after the heat treatment by combustion of aluminum, in order to reduce the influence of the Al ₂ O ₃ produced by the heat treatment on the desulfurization agent, specifically, after the heat treatment, the heat treatment is specifically performed. After stopping the supply of oxygen gas from the top blow lance 13 for feeding (acid delivery), the molten steel 3 is circulated for 3 minutes or more, and the desulfurization is performed after the floatation and separation of the Al ₂ O ₃ produced by the sublimation heat treatment. Start processing. Specifically, RH degassing refining is performed on the molten steel 3 as follows.

  (1) When accompanied by vacuum decarburization treatment: When the reflux of the molten steel 3 is started, first, vacuum decarburization treatment is performed by supplying oxygen gas from the top blowing lance 13 or the like. After the vacuum decarburization treatment, the molten steel 3 is deoxidized by adding metal aluminum from the raw material inlet 12 so that the aluminum dissolved in the molten steel 3 becomes 0.010 mass% or more, and then the top blow lance 13 The desulfurization treatment is carried out by spraying the desulfurization agent from After the desulfurization treatment, the alloy iron and the coolant are added from the raw material inlet 12 to adjust the components and temperature of the molten steel 3, and then the RH vacuum degassing refining is finished. The reason why the metal aluminum is added after the vacuum decarburization treatment is that aluminum in the molten steel is oxidized to become substantially zero by the vacuum decarburization treatment and compensates for this. The coolant is obtained by processing small thin steel plate scraps such as chopper scraps into a round shape.

  (2) When both the vacuum decarburization process and the heat treatment are accompanied: When the molten steel 3 starts to reflux, first, the vacuum decarburization process is performed by supplying oxygen gas from the top blowing lance 13. Then, after vacuum decarburization treatment, metal aluminum is introduced from the raw material inlet 12 to sufficiently increase the aluminum concentration in the molten steel, and in this state, oxygen gas is blown from the top blowing lance 13 toward the surface of the molten steel. Aluminum is oxidized and the molten steel 3 is heated using the heat generated by this oxidation reaction. After the end of this heat treatment, the molten steel 3 is deoxidized by adding metallic aluminum from the raw material inlet 12 so that the aluminum dissolved in the molten steel 3 becomes 0.010 mass% or more. After the heat treatment is completed, the molten steel 3 is circulated for 3 minutes or more, and then a desulfurizing agent is sprayed from the top blowing lance 13 to perform the desulfurization treatment. After the desulfurization treatment, the alloy iron and the coolant are added from the raw material inlet 12 to adjust the components and temperature of the molten steel 3, and then the RH vacuum degassing refining is finished. In addition, if the aluminum melt | dissolved in the molten steel 3 is ensured 0.010 mass% or more at the time of completion | finish of a heat treatment, the addition of the metal aluminum after completion | finish of a heat treatment is unnecessary.

  (3) When heat treatment is accompanied: When reflux of molten steel 3 is started, first, metal aluminum is introduced from raw material inlet 12 to sufficiently increase the aluminum concentration in the molten steel. Oxygen gas is blown toward the surface of the molten steel to oxidize aluminum in the molten steel, and the molten steel 3 is heated using heat generated by this oxidation reaction. After the end of this heat treatment, the molten steel 3 is deoxidized by adding metallic aluminum from the raw material inlet 12 so that the aluminum dissolved in the molten steel 3 becomes 0.010 mass% or more. After the heat treatment is completed, the molten steel 3 is circulated for 3 minutes or more, and then a desulfurizing agent is sprayed from the top blowing lance 13 to perform the desulfurization treatment. After the desulfurization treatment, the alloy iron and the coolant are added from the raw material inlet 12 to adjust the components and temperature of the molten steel 3, and then the RH vacuum degassing refining is finished. In addition, if the aluminum melt | dissolved in the molten steel 3 is ensured 0.010 mass% or more at the time of completion | finish of a heat treatment, the addition of the metal aluminum after completion | finish of a heat treatment is unnecessary.

  When neither vacuum decarburization processing nor heat-up heat treatment is performed, desulfurization processing is performed immediately after the reflux of the molten steel 3 is started, and then the components and temperature of the molten steel 3 are adjusted.

By performing the desulfurization treatment of the molten steel 3 in this manner, the oxygen potential of the molten steel 3 and the slag 4 is lowered, and the molten steel 3 can be efficiently desulfurized. In addition, when the temperature of the molten steel is insufficient and the heat treatment is performed by burning aluminum, the desulfurization process is started after the molten steel 3 is circulated for 3 minutes or more under reduced pressure after the heat treatment is completed. As a result, floating and separation of Al ₂ O ₃ proceeded, and the desulfurizing agent can be efficiently desulfurized without being affected by the Al ₂ O ₃ and without reducing the desulfurization ability.

  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 degassing apparatus, VOD facility, VAD facility, etc. can be implemented in accordance with the above description.

  The example which implemented the desulfurization method of this invention using the RH vacuum degassing apparatus shown in FIG. 1 is demonstrated.

  About 350 tons of molten steel decarburized and refined in a converter was put into a ladle. At the time of steeling, an Fe—Si alloy was added to the ladle so that the silicon concentration in the molten steel was 0.10% by mass or more. In addition, for comparison, the amount of Fe-Si alloy input was changed, and an operation in which the silicon concentration in the molten steel was less than 0.10% by mass was also performed. Further, at the time of steel output, metal aluminum was added to deoxidize the molten aluminum in the molten steel to 0.010% by mass or more, and 1000 kg of quick lime was added toward the steel output flow.

  After steeling, metal aluminum was added to the slag in the ladle as a slag modifier to improve the slag. For comparison, an operation in which metallic aluminum as a slag modifier was not added was also carried out, and the influence of the slag composition on the desulfurization rate was investigated. The desulfurization rate is the difference between the sulfur concentration in the molten steel before and after the desulfurization treatment, expressed as a percentage with respect to the sulfur concentration in the molten steel before the desulfurization treatment.

  Then, the ladle which accommodated the molten steel was conveyed to RH vacuum degassing apparatus. The molten steel before treatment by the RH vacuum degassing apparatus has a carbon concentration of 0.02 to 0.1 mass%, a silicon concentration of 0.01 to 0.43 mass%, and a sulfur concentration of 0.0025 to 0.0040 mass%. And the molten steel temperature was 1600-1650 degreeC.

  After starting the treatment in the RH vacuum degassing device, supply oxygen gas from the top blowing lance as needed, perform vacuum decarburization treatment, measure the molten steel temperature, and secure the necessary temperature before starting the desulfurization treatment Confirmed that it has been. The necessary temperature is a temperature determined for each processing condition in consideration of a temperature decrease with the progress of the desulfurization process and a temperature decrease due to the addition of the desulfurizing agent. If the temperature is insufficient, metal aluminum is added from the raw material inlet, oxygen gas is supplied from the top blowing lance to oxidize and burn aluminum in the molten steel, and the heat of combustion raises the temperature of the molten steel to a predetermined temperature. I let you.

  After the supply of oxygen gas was completed, the molten steel was circulated for 3 minutes or more, and then the desulfurization treatment was started. In addition, for comparison, an operation of starting the desulfurization treatment within 3 minutes from the end of the supply of oxygen gas was also performed, and the influence of the elapsed time after the end of the heat treatment on the desulfurization rate was investigated. In the desulfurization treatment, the tip position of the top blowing lance is set within a range of 1.5 to 2.5 m from the surface of the molten steel, the degree of vacuum in the vacuum chamber is adjusted to 50 torr (66.7 hPa) or less, and then Ar gas is discharged from the top blowing lance. Was carried out by projecting a CaO-based desulfurizing agent toward the molten steel surface in the vacuum chamber.

  Table 1 shows the T.V. in the slag upon arrival of the RH vacuum degasser. The total concentration of Fe and manganese oxide (MnO), the silicon concentration and sulfur concentration of the molten steel upon arrival of the RH vacuum degassing apparatus, the sulfur concentration in the molten steel at the end of RH degassing refining, and the desulfurization rate are shown. In addition, all the data shown in Table 1 are the data of the operation which ensured the molten steel recirculation time for 5 minutes after completion | finish of a heat-up heat treatment, when a heat-up heat treatment was implemented.

  FIG. 2 is a graph showing the relationship between the silicon concentration in the molten steel and the desulfurization rate when the RH vacuum degassing apparatus arrives, obtained in Invention Examples 1-10 and Comparative Examples 1-4. All of these data indicate the T.V. in the slag upon arrival at the RH vacuum degasser. The total concentration of Fe and MnO is data of 5 mass or less.

  As shown in FIG. 2, when the silicon concentration in the molten steel was less than 0.10% by mass (Comparative Examples 1 to 4), the desulfurization rate was less than 30%, but the silicon concentration in the molten steel was 0.00. In the case of 10% by mass or more, a high desulfurization rate of 80% or more could be obtained. If the silicon concentration in the molten steel is 0.10% by mass or more, an increase in the desulfurization rate with the increase in the silicon concentration is not observed. Therefore, the silicon concentration in the molten steel may be 0.10% by mass or more. There is no need to make it higher than necessary. What is necessary is just to determine from the specification of the steel product calculated | required in the range of 0.10 mass% or more.

  FIG. 3 is a graph showing T.V. in the slag upon arrival at the RH vacuum degassing apparatus obtained in Invention Examples 11-17 and Comparative Examples 5-12. It is a figure which shows the relationship between the sum total density | concentration of Fe and MnO, and a desulfurization rate. All of these data are data in which the silicon concentration in the molten steel upon arrival of the RH vacuum degassing apparatus is 0.10 mass or more.

  As shown in FIG. 3, TD in the slag upon arrival of the RH vacuum degasser. When the total concentration of Fe and MnO was 5% by mass or less (Examples 1 to 17 of the present invention), the desulfurization rate was 80% or more. When the total concentration of Fe and MnO is more than 5% by mass and 10% by mass or less (Comparative Examples 5 to 9), the desulfurization rate is slightly deteriorated to 60 to 70%, and further, the slag upon arrival at the RH vacuum degassing device Medium T. When the total concentration of Fe and MnO exceeded 12% by mass (Comparative Examples 10 to 12), the desulfurization rate was less than 40%.

  Table 2 shows the timing of the heat treatment by burning aluminum in the RH degassing refining, the reflux time from the end of the heat treatment to the start of the desulfurization treatment, the sulfur concentration in the molten steel, and the desulfurization rate in each process. The data shown in Table 2 all indicate that the silicon concentration in the molten steel at the arrival of the RH vacuum degassing apparatus is 0.10 mass or more, and the T.V. in the slag at the arrival of the RH vacuum degassing apparatus. It is data of an operation in which the total concentration of Fe and MnO is 5 mass or less.

  As shown in Table 2, in Comparative Examples 15 to 19 in which the heat treatment was performed after the desulfurization treatment, resulfurization occurred, and a high desulfurization rate of 80% or more could not be obtained.

  FIG. 4 shows the results from the completion of the acid feed for the heat treatment to the start of the desulfurization treatment obtained in the case where the heat treatment was performed before the desulfurization treatment (Examples 18 to 24 and Comparative Examples 13 to 14 of the present invention). It is a figure which shows the relationship between the reflux time after completion | finish of an acid (= recirculation | circulation time from the time of completion | finish of a heat-up heat processing to the start of a desulfurization process), and a desulfurization rate.

  As shown in FIG. 4, in the case where the reflux time after the end of acid feeding is less than 3 minutes (Comparative Examples 13 to 14), the desulfurization rate is as low as 60 to 70%. When desulfurization treatment was performed after securing, a high desulfurization rate of 80% or more was obtained. However, if a reflux time exceeding 10 minutes is ensured after completion of the heat treatment, the treatment time is increased. Therefore, it is preferable to ensure a reflux time of 3 to 10 minutes after the completion of the heat treatment.

  From the above results, the silicon concentration in the molten steel at the arrival of the RH vacuum degassing apparatus was set to 0.10 mass or more, and the T.O. When the total concentration of Fe and MnO is set to 5 mass or less, and when the heat treatment is performed by combustion of aluminum with the RH vacuum degassing apparatus, the desulfurization treatment is performed after the molten steel is circulated for 3 minutes or more after the heat treatment. Thus, it was found that a high desulfurization rate of 80% or more can be obtained.

It is a figure which shows the example of the RH vacuum degassing apparatus used when implementing this invention. It is a figure which shows the relationship between the silicon concentration in molten steel at the time of arrival of RH vacuum degassing apparatus, and a desulfurization rate. Slag in slag upon arrival of RH vacuum degasser It is a figure which shows the relationship between the sum total density | concentration of Fe and MnO, and a desulfurization rate. It is a figure which shows the relationship between the reflux time after completion | finish of acid feeding, and a desulfurization rate.

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 (1)

By introducing silicon-containing alloy iron into the ladle from the decarburizing and refining furnace that performs decarburization and refining at atmospheric pressure, the silicon concentration of the molten steel in the ladle is adjusted to 0.10 mass% or more. A slag modifier containing aluminum is added to the slag in the ladle after steel, and the total amount of the slag. The total concentration of Fe and manganese oxide is adjusted to 5% by mass or less, and then the ladle is transported to a vacuum degassing facility.
In the vacuum degassing equipment, aluminum is added to the molten steel, and then oxygen gas is supplied to the molten steel surface under reduced pressure through an upper blowing lance to burn the aluminum in the molten steel to raise the temperature of the molten steel. , while ensuring the concentration of aluminum to be dissolved in the molten steel after the molten steel temperature heat than 0.010 wt%, the molten steel or 3 minutes after completion of the supply of oxygen gas for the molten steel temperature heat to reflux under reduced pressure The floatation and separation of Al ₂ O ₃ generated during molten steel heating from the molten steel proceeds, and then a desulfurizing agent is sprayed together with the carrier gas through the top blowing lance toward the surface of the molten steel under reduced pressure of 50 torr or less. And desulfurizing the molten steel. A method for desulfurizing molten steel.

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