JP2016108639A - Less resulfurization molten pig iron desulfurization method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a less resulfurization molten pig iron desulfurization method that can reduce the resulfurization formed during the desulfurization processing of the molten pig iron and caused by the fine desulfurized slag suspended in the molten pig iron.SOLUTION: The method includes using a desulfurizing agent having an average particle size of 50 μm or more and 90 μm or less as a desulfurizing agent when desulfurizing a molten pig iron by adding a desulfurizing agent from above to the bath surface of a molten pig iron 3 within a treatment vessel 2 agitated by an agitation blade 4 of a mechanical agitation type desulfurization equipment.SELECTED DRAWING: Figure 1

Description

  The present invention is produced during the desulfurization treatment of hot metal in a steelmaking process in which molten steel is made from molten iron by performing dephosphorization treatment or decarburization refining in the next step for hot metal subjected to hot metal pretreatment such as desulfurization treatment. In addition, the present invention relates to a hot metal desulfurization method with less resulfurization that can reduce the resulfurization caused by desulfurization slag remaining in the processing vessel by being suspended in the hot metal after the desulfurization treatment.

  In recent years, the ratio of steel grades of extremely low sulfur and / or extremely low phosphorus has been increased due to an increase in needs for high purity and high functionality of steel materials. Under such circumstances, in the steelmaking process for melting steel, it is necessary to develop a technology for melting extremely low sulfur and / or extremely low phosphorus steel without causing an increase in cost or an increase in the amount of slag generated. It has become.

  In general, low-sulfur steel and ultra-low-sulfur steel are subjected to desulfurization treatment at the hot metal stage, and desulfurization slag having a high sulfur content generated by this desulfurization treatment is discharged from the treatment vessel. Dephosphorization treatment and decarburization refining are performed in the steelmaking process. In this case, most of the desulfurized slag is discharged from the processing vessel, but the fine desulfurized slag suspended in the hot metal is carried over to the next process or attached to the side wall of the processing container and carried over to the next process. Or The desulfurization treatment is a reduction reaction, whereas the dephosphorization treatment and decarburization refining in the next step are oxidation reactions. Therefore, the sulfur contained in the desulfurization slag carried over to the next step is oxidized to the molten iron or Returning to the molten steel, so-called “resulfurization” occurs in which the sulfur concentration in the molten iron or in the molten steel is increased.

  If the sulfur concentration in hot metal or molten steel becomes high due to sulfidation, and the sulfur component standards cannot be satisfied, secondary definitive refining is performed after decarburization refining in the converter to remove sulfur in the molten steel. It will be necessary. The desulfurization of molten steel performed in secondary refining is not only more expensive than the desulfurization treatment of hot metal, but the productivity is reduced because it is necessary to extend the refining time in order to remove the increased amount of sulfur. .

  Conventionally, the hot metal desulfurization treatment includes a method of injecting a CaO-based desulfurizing agent into the hot metal, a method of stirring and mixing the CaO-based desulfurizing agent and the hot metal using a mechanical stirring desulfurization apparatus, or a metal Mg-based desulfurizing agent. In general, a method of injecting into a hot metal is used. There is also a method (projection addition) in which a CaO-based desulfurizing agent is added to the bath surface of hot metal being stirred by a stirring blade together with a carrier gas through an upper blowing lance.

  In these desulfurization treatments, the desulfurizing agent is dispersed in the hot metal by injection or mechanical stirring. In order to promote the desulfurization reaction and improve the desulfurization efficiency, it is effective to use a small-diameter desulfurization agent having a large surface area. Thus, for example, Patent Document 1 discloses a method using a desulfurization agent having a particle size of 30 μm or more and less than 60 μm and having a powder composition ratio of 50% or more as a desulfurization agent for injection. Patent Document 2 discloses a method of using CaO powder in which fine powder having a particle size of 150 μm or less is 90% by weight or more when performing projection addition, and Patent Document 3 discloses a particle size as a lime-based desulfurizing agent. A method using 30 to 400 μm is disclosed. These techniques determine the optimum particle size for improving the desulfurization efficiency in each desulfurization method.

Also disclosed is a method of using an aluminum-based flux as a medium solvent for improving the desulfurization efficiency or as a deoxidizer. For example, Patent Document 4 discloses a desulfurization agent in which the ratio of Al ₂ O ₃ is 33 to 37.7% by weight in the mixing ratio of CaO and Al ₂ O ₃ . Further, it is disclosed that a desulfurizing agent added with 5 to 20% by mass of aluminum dross containing 50% by weight or more of Al ₂ O ₃ is disclosed in the case of projecting addition. The content of these Al ₂ O ₃ defines the optimal amount of Al ₂ O ₃ in the desulfurizing agent for improving the desulfurization efficiency.

JP 2006-241502 A JP 2008-50659 A JP 2011-149087 A Japanese Patent Application Laid-Open No. 09-3515

  In the prior art described above, in order to improve the reaction efficiency of the desulfurizing agent, it is important to disperse the desulfurizing agent in the hot metal. When the dispersion state is good, the desulfurizing reaction is performed efficiently. However, when the dispersion state is good and the fine desulfurizing agent is suspended in the hot metal, there is a problem that slag having a small particle size is generated and it is difficult to float from the hot metal. With respect to this problem, the hot metal is allowed to stand for a long time after the desulfurization treatment, whereby the fine desulfurization agent suspended in the hot metal can be floated on the hot metal bath surface and removed from the processing vessel. However, standing for a long time leads to a decrease in productivity and a decrease in hot metal temperature, so such a treatment is not performed in a process.

  In addition, a technique of blowing a stirring gas into molten steel and promoting the floating of nonmetallic inclusions suspended in the molten steel is also widely performed. Therefore, after desulfurization treatment, gas injection is performed in the hot metal to promote floating separation of the fine desulfurization slag that is suspended, and condensing gas to the hot metal after desulfurization treatment can be reduced. When an injection lance is used to blow the hot metal, there arises a problem that the hot metal temperature decreases and the hot metal amount decreases due to the immersion of the injection lance. In addition, when the desulfurization treatment is performed regularly by injecting the desulfurizing agent into the hot metal, a new stirring gas injection lance is not required, but the desulfurization treatment is performed using a mechanical stirring desulfurization device. In such a case, there is a problem that a new injection lance for the stirring gas is required.

  Therefore, it is desired to develop a technique for promoting the floating separation of the fine desulfurization agent suspended in the hot metal after the desulfurization treatment and reducing the resulfurization of the hot metal after the desulfurization treatment without using gas injection.

  The present invention has been made in view of the above circumstances. The purpose of the present invention is to perform molten iron from molten iron by performing dephosphorization and decarburization refining in the next step on the molten iron subjected to desulfurization treatment. An object of the present invention is to propose a hot metal desulfurization method with less reflow that can reduce the resulfurization caused by the fine desulfurization slag that is generated during the desulfurization treatment of the hot metal in the steelmaking process to be produced.

  The inventors diligently studied to overcome the above-mentioned problems of the prior art and realize the object. As a result, the inventors have developed the present invention according to the following summary configuration. That is, the present invention adds an average particle as the desulfurizing agent when desulfurizing the hot metal by adding a desulfurizing agent from above onto the bath surface of the hot metal in the processing vessel stirred by the stirring blade of the mechanical stirring desulfurization apparatus. It is a hot metal desulfurization method with little desulfurization characterized by using a desulfurization agent having a diameter of 50 μm or more and 90 μm or less.

ここで、(Ａｌ_２Ｏ_３)は、脱硫スラグ中のアルミナ含有量（mass％）、（ＣａＯ）は、脱硫スラグ中の石灰（ＣａＯ）含有量（mass％）であること、
（２）１００μｍ以下が８０mass％以上、かつ、５０μｍ以下が５０mass％以上存在している脱硫剤を用いること、
を採用することにより好ましい解決手段を提供できるものと考えられる。 Further, the present invention according to the above configuration further includes
(1) Aluminum system added during desulfurization treatment so that the ratio of the lime (CaO) content and the alumina (Al ₂ O ₃ ) content in the slag after desulfurization treatment of hot metal satisfies the following formula (1) The method of desulfurizing hot metal desulfurization according to claim 1, wherein the amount of deoxidizer is determined:

Here, (Al ₂ O ₃ ) is the alumina content (mass%) in the desulfurized slag, (CaO) is the lime (CaO) content (mass%) in the desulfurized slag,
(2) Use a desulfurization agent in which 100 μm or less is present at 80 mass% or more and 50 μm or less is present at 50 mass% or more,
It is considered that a preferable solution can be provided by adopting.

  According to the present invention, in the steelmaking process in which molten steel is produced from molten iron by performing dephosphorization treatment or decarburization refining in the next step, the desulfurization treatment is performed on the molten iron subjected to desulfurization treatment. Since the particle size of the desulfurized slag remaining in the processing vessel is increased by suspending it in the hot metal later, and the slag is promoted to float, most of the desulfurized slag that causes resulfurization can be removed. Thus, it is possible to reliably prevent desulfurization in dephosphorization treatment and decarburization refining. This also enables ultra-low sulfur steel to be melted without desulfurization as secondary refining at the molten steel stage, greatly reducing production costs and improving productivity compared to conventional methods. Is possible.

It is the side schematic diagram explaining an example of the apparatus which enforces the hot metal desulfurization method of this invention.

Hereinafter, the present invention will be specifically described.
In hot metal desulfurization treatment using a CaO-based desulfurizing agent, it is important to stir the CaO-based desulfurizing agent and hot metal in a processing vessel and disperse the CaO-based desulfurizing agent in the hot metal in order to increase the reaction interface area. . At this time, the sulfur in the hot metal reacts with the CaO-based desulfurization agent dispersed in the hot metal in accordance with the reaction formula of “CaO + S → CaS + O” to produce desulfurized slag containing CaS and having a high sulfur concentration. The desulfurization slag floats on the hot metal bath surface at the end of the desulfurization process, and the hot metal bath surface is covered with the desulfurization slag. The desulfurized slag is discharged from the processing container by a slag scraper after the desulfurization process (this process is hereinafter referred to as “desulfurization slag discharge process”). Thereafter, the hot metal in the processing vessel is conveyed to the next dephosphorization process or decarburization refining process.

  Since the ascending speed of desulfurized slag in the hot metal is proportional to the particle size of the desulfurized slag in accordance with Stokes' law, the fine desulfurized slag suspended in the hot metal has a slow ascent rate and is suspended in the hot metal. The desulfurization process is finished as it is. Further, the desulfurization slag adhering to the inner wall of the processing vessel is also difficult to float and remains as it is at the end of the desulfurization process. Therefore, most of the fine desulfurization slag suspended in the hot metal and the desulfurization slag adhering to the inner wall of the processing container remain in the processing container without being discharged from the processing container in the desulfurization slag discharging step.

  Since the reaction in the dephosphorization treatment and decarburization refining in the next step is an oxidation reaction, the desulfurization slag remaining in the treatment vessel without being discharged from the treatment vessel in the desulfurization slag discharge step is dephosphorization treatment step or decarburization refining step When it is carried over, CaS in the desulfurized slag is oxidized to produce CaO, sulfur (S) dissociated from CaS is transferred to hot metal or molten steel, and sulfurization occurs in which the sulfur concentration in the molten iron or molten steel increases. .

  The present invention has been made to prevent this resulfurization, and defines processing conditions for optimizing the slag particle size so that the rising of the desulfurized slag generated after the desulfurization treatment is promoted. The particle size of the desulfurized slag produced by the process increases, and the desulfurized slag suspended in the hot metal can be floated on the hot metal bath surface by the slag discharging process, so that most of the desulfurized slag is discharged from the processing vessel. Can do. As a result, desulfurization slag carried over to the next dephosphorization process and decarburization refining process is reduced, and resulfurization is suppressed.

Hot metal desulfurization is performed by immersing the impeller in hot metal contained in a ladle-type treatment vessel such as a hot metal ladle or charging pan, and rotating the impeller to agitate the hot metal and the CaO-based desulfurizing agent. This is carried out using a desulfurization apparatus. Examples of the CaO-based desulfurization agent to be used include quick lime (CaO), limestone (CaCO ₃ ), slaked lime (Ca (OH) ₂ ), dolomite (CaO-MgO), fluorite (CaF ₂ ), and aluminum soot (Al + Al). ₂ O ₃ ) or the like mixed with about 5 to 30 mass% of CaO hatching accelerator can be used, and this is, for example, molten iron stirred by a stirring blade with a carrier gas via an upper blowing lance. Add to the bath surface.

  As a result of investigating the relationship between the particle size of the desulfurizing agent and the particle size of the generated desulfurized slag, the inventors found that the smaller the average particle size of the desulfurized agent, the larger the particle size of the generated desulfurized slag. . However, it was also confirmed that when the average particle size of the desulfurizing agent was decreased, the yield of the desulfurizing agent was lowered and the desulfurization rate was deteriorated.

Furthermore, as a result of investigating the relationship between the composition of the desulfurizing agent and the particle size of the generated desulfurizing slag, the ratio of alumina (Al ₂ O ₃ ) and lime (CaO) generated in the desulfurizing agent (hereinafter referred to as “alumina ratio”) It has been found that the particle size of the desulfurized slag to be generated increases as the number of However, it was also confirmed that when the alumina ratio becomes too large, the melt phase in the slag increases, the slag particle size becomes too large, the reaction interfacial area decreases, and the desulfurization rate decreases. On the other hand, it was found that when the alumina ratio was too low, the oxygen concentration in the hot metal increased due to insufficient deoxidation, and the desulfurization rate decreased.

  From these results, the desulfurization efficiency in the hot metal desulfurization method, in which the lime-based desulfurization agent is added to the hot metal bath surface that is being stirred by the mechanical stirring type desulfurization apparatus through the upper blowing lance together with the transport gas. In addition, the present inventors have found that there is an optimum particle size for a desulfurizing agent that can increase the particle size of slag after treatment to a particle size that can rise to the hot metal surface by the slag discharging process. That is, in the present invention, it is specified that a desulfurizing agent having an average particle size of 50 μm or more and 90 μm or less is used.

  The particle size of the desulfurizing agent and the particle size of the desulfurizing slag to be generated depend on the method of adding the desulfurizing agent. In the method of injecting the CaO-based desulfurizing agent into the hot metal, the desulfurizing agent can be used even when a small-diameter desulfurizing agent is used. Although the yield does not deteriorate, the slag particle size obtained does not increase even when the same small-diameter desulfurization agent is used because the added desulfurization agent does not agglomerate as compared with the mechanical stirring type hot metal desulfurization method.

  In addition, when a mechanical stirring type desulfurization apparatus is used, a method of adding a desulfurizing agent from the charging port onto the molten iron being stirred (upper addition) and a stirring blade with a carrier gas through an upper blowing lance In the method of adding to the hot metal bath surface (projection addition), even if a desulfurizing agent having the same particle size is used, the yield of the desulfurizing agent and the obtained slag particle size are different. Specifically, in the case of addition over the top, when a small-diameter desulfurizing agent is used, the addition yield of the desulfurizing agent is lowered, and desulfurization is adversely affected. On the other hand, in the case of projection addition, it is possible to add a small-diameter desulfurizing agent into the hot metal, and since a mechanically stirred desulfurization apparatus is used, the desulfurizing agent is agglomerated and the resulting desulfurized slag is Increase diameter. Therefore, in a preferred embodiment of the present invention, the desulfurization agent is projected and added using a mechanical stirring desulfurization apparatus.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic side view showing an embodiment of the desulfurization treatment of the present invention, and shows an example in which a ladle type hot metal ladle is used as a processing container for containing hot metal. As the processing vessel, since a desulfurization process is performed by a mechanical stirring type desulfurization apparatus, a ladle type processing vessel is optimal as shown in FIG. 1, but a torpedo car can also be used. Hereinafter, an example in which a hot metal ladle is used as the processing container will be described.

  In the example shown in FIG. 1, the hot metal is discharged from the blast furnace, the hot metal 3 is received by the hot metal ladle 2 mounted on the carriage 1, and the received hot metal 3 is conveyed to a mechanical stirring type desulfurization apparatus. In addition, when receiving with a torpedo car, it is desirable to transfer to a ladle type processing container prior to the desulfurization process. The hot metal 3 to be subjected to the desulfurization treatment of the present invention may be any component, and for example, desiliconization treatment or dephosphorization treatment may be performed in advance. The above desiliconization treatment is a treatment of mainly removing Si in the hot metal by adding an oxygen source such as oxygen gas or iron ore to the molten iron 3 prior to the dephosphorization treatment in order to efficiently perform the dephosphorization treatment. That means.

  The mechanical stirring type desulfurization apparatus is equipped with a stirring blade 4 made of refractory for immersing and burying in a hot metal 3 accommodated in a hot metal ladle 2 and rotating to stir the hot metal 3. The elevating device (not shown) moves up and down in a substantially vertical direction, and the rotating device (not shown) turns. Further, in the mechanical stirring type desulfurization apparatus, an upper blowing lance 5 for adding a desulfurizing agent and / or a deoxidizing agent by blowing up toward the hot metal 3 in the hot metal ladle 2, a desulfurizing agent and / or a deoxidizing agent. A charging port 6 is provided for adding the agent on the bath surface of the hot metal 3 in the hot metal pan 2. Further, an exhaust duct port (not shown) connected to a dust collector (not shown) is provided at an upper position of the hot metal ladle 2 so that gas and dust generated during the desulfurization process are discharged. .

  The top blowing lance 5 includes a hopper 7 containing a desulfurizing agent and a rotary feeder 11 for quantitatively cutting out from the hopper 7, a hopper 9 containing deoxidizer and a rotary feeder 12 for quantitatively cutting out from the hopper 9 The desulfurizing agent and / or the deoxidizing agent can be independently supplied at an arbitrary timing together with the carrier gas from the upper blowing lance 5. Of course, it has the structure which can supply simultaneously and can also blow up only the gas for conveyance.

  Similarly, the charging port 6 includes a hopper 9 that contains the desulfurizing agent and a rotary feeder 13 for quantitatively cutting out from the hopper 9, and a hopper 10 that contains the deoxidizer and the rotary feeder for quantitatively cutting out from the hopper 10. The desulfurizing agent and / or the deoxidizing agent can be supplied independently from each other at an arbitrary timing from the charging port 6.

As the desulfurizing agent, not only a CaO-based desulfurizing agent but also various desulfurizing agents such as a calcium carbide-based desulfurizing agent, a soda-based desulfurizing agent, and metallic Mg can be used. It is preferable to use a desulfurizing agent. In addition, it is preferable to use only a CaO-based desulfurization agent without using a fluorine source such as fluorite, because environmental measures and the reuse of generated slag are easy. As the CaO-based desulfurization agent, quick lime (CaO), dolomite (MgCO ₃ · CaCO ₃ ), slaked lime (Ca (OH) ₂ ), limestone (CaCO ₃ ) and the like can be used. In the present invention, when the desulfurizing agent is added by top blowing or when the desulfurizing agent is forcibly entrained in the hot metal 3 by the top blowing gas, sufficient desulfurization can be achieved without using a fluorine source. However, a desulfurization agent in which fluorine is inevitably mixed as an impurity component may be used.

  Here, the size of the CaO-based desulfurization agent to be blown up can be selected according to the size of the top blowing lance 5 to be used, the top blowing condition, etc., but in the range where the addition yield does not decrease, In order to increase the reaction interfacial area and increase the particle size of the desulfurized slag, it is preferable to use a lime-based desulfurizing agent in which 100 µm or less is 80 mass% or more and 50 µm or less is present in 50 mass% or more. .

As the deoxidizer, it is preferable to use an Al-based one, and for example, metal Al or aluminum dross powder available at low cost is desirable. Further, it may be atomized powder obtained by atomizing an aluminum melt with gas, or cutting powder generated when an aluminum alloy is polished and cut. These may be added by spraying on the surface of the hot metal 3 together with the carrier gas, or may be added from the inlet separately from the lime-based desulfurizing agent. The Al-based, Al ₂ O _3, or the deoxidation agent is generated by oxidation during desulfurization process, Al 2 _O 3 was contained in the deoxidizing agent prior to the _addition, occurs in the pretreatment step, the processing vessel The total of Al ₂ O _{3 in} the slag remaining in the slag becomes Al ₂ O ₃ in the desulfurized slag. Since the amount of Al ₂ O _{3 and the} amount of CaO in the desulfurized slag is an important factor for determining the particle size of the desulfurized slag to be generated, the lime (CaO) content and the alumina (Al It is preferable to determine the addition amount of the aluminum-based deoxidizer during the desulfurization treatment so that the ratio of the ₂ O ₃ ) content satisfies the following formula (1).

  In performing the desulfurization treatment using the mechanical stirring desulfurization apparatus, first, the position of the carriage 1 on which the hot metal ladle 2 is mounted is adjusted so that the position of the stirring blade 4 is approximately the center of the hot metal ladle 2, and then the stirring is performed. The blade 4 is lowered and immersed in the hot metal 3. If the stirring blade 4 is immersed in the molten iron 3, the stirring blade 4 starts to turn, and the speed is increased to a predetermined rotational speed. When the rotational speed of the stirring blade 4 reaches a predetermined rotational speed, the rotary feeder 11 is started, and the desulfurizing agent in the hopper 7 is directed from the upper blowing lance 5 to the bath surface of the hot metal 3 together with the conveying gas. Add by spraying. As the carrier gas, a reducing gas, an inert gas, or a non-oxidizing gas can be used. Since the desulfurization reaction is promoted as the speed of spraying on the surface of the hot metal 3 is increased, the flow rate of the transport gas is adjusted so that the flow velocity of the transport gas colliding with the bath surface of the hot metal 3 becomes 10 m / sec or more. It is preferable.

  In order to accelerate the desulfurization reaction, the deoxidizer in the hopper 8 is a rotary feeder in parallel with the above-described top blowing addition, before or after top blowing addition, or over the entire duration of the desulfurization treatment period. 12 is preferably supplied from the top blowing lance 5 into the hot metal ladle 2. In the above-described example, the desulfurizing agent and / or deoxidizing agent is supplied from the top blowing lance 5 to the hot metal 3, but the desulfurizing agent in the hopper 9 and the deoxidizing agent in the hopper 10 are used as the rotary feeders 13, 14. It can also be performed through the insertion port 6 by driving.

  After stirring for a predetermined time, the number of revolutions of the stirring blade 4 is decreased and stopped, and then the stirring blade 4 is raised and placed on the hot metal ladle 2 on standby. The generated slag floats to cover the hot metal surface, and the desulfurization process of the hot metal 3 is finished in a stationary state. After the desulfurization treatment, the generated slag is discharged from the hot metal ladle 2 and conveyed to the next refining process.

  By performing the desulfurization treatment on the hot metal 3 in this manner, even when a fine-grain desulfurization agent is used, scattering during addition is reduced and the addition yield of the desulfurization agent is improved. Further, since the fine-grain desulfurization agent has a large reaction interface area, the desulfurization reaction is promoted and the desulfurization rate is improved. Furthermore, since the particle size of the desulfurized slag to be generated is large and the slag floating after the desulfurization process is promoted, the slag dispersed in the hot metal floats on the hot metal surface before the desulfurized slag is discharged and removed. Is done. The desulfurization slag can be removed from the processing container by tilting the processing container to the extent that molten iron does not flow out and drawing it mechanically using a slag scraper or by suction / removal using a vacuum slag removing device. Or the like can be used. After the desulfurization slag is discharged, it is preferable to add a heat retaining agent in the processing container in order to prevent the hot metal temperature from decreasing.

  In addition, it is preferable that the hot metal to be used is a hot metal melted in a blast furnace or a shaft furnace, and desiliconization treatment or dephosphorization treatment may be performed before the desulfurization treatment. In the case of hot metal previously subjected to dephosphorization, the next process is a decarburization refining process in the converter, and therefore, the hot metal after the desulfurization slag removal process is transported to the converter that performs decarburization refining. In addition, when the hot metal dephosphorization treatment is performed as a preliminary treatment after the desulfurization treatment, since the next step is a dephosphorization treatment step, the hot metal after the desulfurization slag removal treatment is transported to a facility for performing the dephosphorization treatment. In addition, you may perform the removal process of desulfurization slag just before the process of the following process.

  As described above, according to the present invention, in the process of producing molten steel from hot metal by performing dephosphorization treatment and decarburization refining in the next step on the hot metal subjected to desulfurization treatment, it is generated at the time of desulfurization treatment of hot metal. The desulfurization slag remaining in the processing vessel is suspended by suspending in the hot metal after the desulfurization treatment, etc. until the slag removal processing is carried out, and the hot metal discharged from the processing vessel is dephosphorized in the next step. Since it is used for treatment and decarburization refining, most of the desulfurization slag that causes resulfurization is removed when dephosphorization treatment and decarburization refining, reducing resulfurization in dephosphorization treatment and decarburization refining. can do.

  After desulfurizing the hot metal and removing the generated desulfurized slag from the hot metal ladle, dephosphorization and decarburization refining are performed in the converter of the next process, and the product sulfur concentration standard is 0.0024 mass% or less. Sulfur steel was melted. Specifically, the hot metal contained in the hot metal ladle is desulfurized, and after the generated desulfurized slag is removed from the hot metal hot pot, the hot metal is charged from the hot metal hot pot into the charging pan, and further from the charging pan to the converter. After charging and dephosphorizing in the converter, the hot water was again discharged into the charging pot, the charging pot was recharged into the converter, and decarburization refining was performed in the converter.

  In the above desulfurization treatment, lime-based desulfurization agents having different average particle sizes (average particle size: Comparative Example 1 (40 μm), Invention Example 1 (50 μm), Invention Example 2 (60 μm), Invention Example 3 (90 μm), Comparison Using Example 2 (100 μm) and Comparative Example 3 (250 μm), the desulfurization treatment was performed for 150 charges each, and the slag after the dephosphorization treatment was collected to obtain the average particle size of the desulfurization slag, and the desulfurization rate, The amount of sulfur was investigated.

ここで、D_a：平均粒径（mm）、d_i : それぞれの粒径範囲における平均粒子径（篩目中央値）(mm)但し「45μｍ以下」の篩目中央値は22.5μｍとし、「1000μｍ以上」は（２）式計算に算入しない。また、w_i ：それぞれの篩上の脱硫剤重量(kg)である。 Here, the average particle size of the desulfurizing agent is 45 μm or less, 45 to 75 μm, 75 to 100 μm, 100 to 125 μm, 125 to 150 μm, 150 to 300 μm, 300 to 500 μm, 500 to 1000 μm, 1000 μm or more. The particle size distribution was measured. The average particle size was determined by the formula (2).

Here, D _a : average particle diameter (mm), d _i : average particle diameter in each particle size range (medium screen average value) (mm) However, the median screen size of “45 μm or less” is 22.5 μm, “1000 μm or more” is not included in the calculation of equation (2). W _i is the weight (kg) of the desulfurizing agent on each sieve.

A specific hot metal desulfurization treatment method was performed using a mechanical stirring desulfurization apparatus, and CaO—CaF ₂ was used as a CaO-based desulfurization agent to be added. All the impellers used have four blades with a rotating diameter of 1.4 m and a height of 0.8 m, and the blades have no inclination angle.

  The chemical components before desulfurization treatment of the hot metal used were C: 3.5 to 5.0 mass%, Si: 0.1 to 0.3 mass%, S: 0.025 to 0.035 mass%, P: 0.10. The hot metal temperature before desulfurization treatment was in the range of 1250 to 1350 ° C. at ˜0.15 mass%. In the desulfurization treatment, a hot metal ladle capable of storing 250 to 350 tons of hot metal as a processing vessel was used, and the amount of hot metal to be processed was about 300 tons. The amount of desulfurizing agent used was 5.0 to 7.5 kg / molten iron-ton. The stirring time was constant. As a deoxidizer, aluminum dross powder was added into the hot metal before the desulfurizer was added. At this time, the amount of aluminum dross powder added was determined so that the alumina ratio in the slag after treatment was in the range of 0.1 to 0.15. In addition, no desulfurizing agent was used, and during the dephosphorization process, no auxiliary material was added which would cause an increase in the S concentration in the hot metal. After the removal process of the desulfurization slag from the hot metal ladle and after the dephosphorization treatment, a hot metal sample was taken and the sulfur concentration in the hot metal was analyzed.

  Table 1 shows the average particle size of the desulfurizing agent used, the particle size index of the slag obtained after the treatment, the desulfurization rate, and the amount of resulfurization. Here, the above-mentioned amount of resulfurization is the difference between the sulfur concentration after the dephosphorization treatment and the sulfur concentration after removing the desulfurized slag from the hot metal ladle. The average slag particle size index is an index when the average slag particle size in the treatment using a desulfurizing agent having an average particle size of 250 μm is 1.0. The results are shown in Table 1 below.

  From the results in Table 1, the following can be understood. That is, the smaller the average particle size of the desulfurizing agent used, the larger the slag particle size after treatment. In Invention Examples 1 to 3, in which the average particle size of the desulfurizing agent is 90 μm or less, the desulfurization having an average particle size of 250 μm. When the agent was used (Comparative Example 4), the slag particle size was 2.9 times or more larger than that of the slag, and the amount of resulfurization was as low as 0.0008 mass% or less. On the other hand, in the case of using a desulfurizing agent having an average particle size of 100 μm or more (Comparative Examples 2 and 3), the amount of resulfurization is 0.0025 mass% and 0.0052 mass%, respectively, and the sulfur concentration at the time of converter steel is standard value. There was a need to carry out desulfurization refining on the molten steel in the ladle in the ladle refining equipment (LF equipment) after the steel was discharged from the converter. When a desulfurizing agent having an average particle size of 40 μm was used (Comparative Example 1), the slag particle size was large and the amount of resulfurization was as low as 0.0005 mass%, but the desulfurization rate was 77.1%. And low. This is because the particle size of the desulfurizing agent is too fine and the yield of addition is reduced.

  The average particle size is 60 μm and 90 μm, both using a lime-based desulfurization agent within the scope of the present invention, and changing the addition amount of aluminum dross powder as a deoxidizer, thereby changing the alumina ratio in the slag after treatment. The desulfurization treatment was carried out, followed by dephosphorization treatment in a converter, decarburization refining, and experiments for melting low-sulfur steel were carried out for 50 charges each. At that time, the slag after the desulfurization treatment was collected, the average particle diameter of the desulfurized slag was obtained, and the desulfurization rate and the amount of sulfurization were investigated. The order of the refining treatment and the desulfurization treatment method are the same as those in the first embodiment. Further, during the dephosphorization process, no auxiliary material that causes an increase in the S concentration in the hot metal was added. After removing the desulfurized slag from the hot metal ladle and after the dephosphorization treatment, a hot metal sample was taken and analyzed for the sulfur concentration in the molten steel. Table 2 below shows the alumina ratio of the slag obtained after the desulfurization treatment, the particle size index of the slag, the desulfurization rate, and the amount of sulfurization. In addition, the average particle diameter index of slag is an index when the slag particle diameter in the treatment using the desulfurizing agent having an average particle diameter of 250 μm is 1.0 as in the case of Example 1.

  The following can be seen from the results in Table 2. That is, in both cases where the average particle size of the desulfurizing agent is 60 μm and 90 μm, the slag particle size after the desulfurization treatment tends to be larger than when a desulfurizing agent having an average particle size of 250 μm is used. The particle size of slag is increased more than 5 times. Further, the amount of sulfurization was as low as 0.001 mass% or less. In particular, in Invention Examples 5 to 8 and 12 to 15 in which the alumina ratio in the slag after the desulfurization treatment is in the range of 0.05 to 0.2, [S] after the desulfurization treatment is 10 mass ppm (0.0001 mass%). The desulfurization rate was as high as 97.2% or more. However, in Invention Examples 4, 9 to 11, 16, and 17 in which the alumina ratio in the treated slag is less than 0.05 and 0.25 or more, [S] after the desulfurization treatment is 0.0013 to 0.0018 mass. % And the desulfurization rate was below 95%. The reason for this is that when the alumina ratio in the slag after the desulfurization treatment is less than 0.05, the oxygen concentration in the hot metal during the desulfurization treatment tends to be high, and the desulfurization rate is slightly deteriorated due to insufficient deoxidation. It is thought that. On the other hand, in the inventive examples 9, 10, 16, and 17 in which the alumina ratio is 0.25 or more, since the slag particle size index after treatment is 6 times or more and slag having a considerably large particle size is generated, the reaction interface area is It is considered that the desulfurization rate decreased due to the decrease.

  From this result, it has been found that resulfurization can be prevented while performing desulfurization treatment with a higher desulfurization rate when the alumina ratio in the slag after desulfurization treatment is in the range of 0.05 to 0.2.

  The average particle size is 55 to 70 μm, but after desulfurizing the hot metal using lime-based desulfurizing agents with different particle size distribution, dephosphorization and decarburization refining in the converter, the S concentration is 0.0024 mass% Experiments for melting the following low-sulfur steel were performed 50 charges each. At this time, the slag after the desulfurization treatment was collected, the average particle diameter of the desulfurization slag was obtained, and the desulfurization rate and the amount of sulfurization were investigated. The order of the refining treatment and the test method are the same as those in Example 1. Further, during the dephosphorization process, no auxiliary material that causes an increase in the S concentration in the hot metal was added. After the removal process of the desulfurization slag from the hot metal ladle and after the dephosphorization process, hot metal samples were collected and analyzed for the sulfur concentration in the hot metal (molten steel). The alumina ratio in the slag after the desulfurization treatment was in the range of 0.09 to 0.16. Table 3 shows the average slag particle size index, the average desulfurization rate, and the average amount of resulfurization obtained after the desulfurization treatment. In addition, the average particle diameter of slag was shown as an index when the particle diameter of slag in the treatment using a desulfurizing agent having an average particle diameter of 250 μm was set to 1.0, as in Example 1.

  The following can be understood from the results of Table 3. That is, in any case, the slag particle size is 2.5 times or more larger than the slag when the desulfurizing agent having an average particle size of 250 μm is used, and the amount of sulfurization is 0.0009 mass% or less. It was low. In particular, when using a desulfurization agent in which 100 μm or less is 80 mass% or more and 50 μm or less is present in 50 mass% or more (Invention Examples 18 and 19), the desulfurization rate is as high as 98.0% or more, And the amount of resulfurization was as low as 0.0005 mass%, which was the best result.

  As described above, in the hot metal desulfurization method of the present invention, it was confirmed that the sulfur concentration in the molten steel after decarburization refining can be controlled to 0.0024 mass% or less of the standard value of the low-sulfur steel type with full charge. Therefore, in the hot metal desulfurization method of the present invention, desulfurization refining in a ladle refining facility (LF facility) after steelmaking can be completely omitted.

  According to the hot metal desulfurization method of the present invention with less recirculation, desulfurization treatment in a converter and desulfurization in decarburization refining can be reduced, whereby desulfurization refining as secondary refining is performed in the molten steel stage. Without this, it is possible to melt ultra-low-sulfurized steel, and as a result, it is possible to significantly reduce production costs and improve productivity compared to conventional steelmaking. It can be suitably applied to refining.

1 Cart 2 Hot metal ladle 3 Hot metal 4 Stirrer blade 5 Top blowing lance 6 Input port 7, 8, 9, 10 Hopper 11, 12, 13, 14 Rotary feeder

Claims (3)

  When desulfurizing the hot metal by adding a desulfurizing agent from above onto the bath surface of the hot metal in the processing vessel agitated by the stirring blades of the mechanical stirring desulfurization apparatus, the average particle size of the desulfurizing agent is 50 μm or more and 90 μm or less. A hot metal desulfurization method with less resulfurization, characterized by using a desulfurization agent. Aluminum-based deoxidation added during desulfurization treatment so that the ratio of lime (CaO) content and alumina (Al ₂ O ₃ ) content in the slag after desulfurization treatment of hot metal satisfies the following formula (1) The hot metal desulfurization method with less resulfurization according to claim 1, wherein the additive amount of the agent is determined:

Here, (Al ₂ O ₃ ) is the alumina content (mass%) in the desulfurized slag, and (CaO) is the lime (CaO) content (mass%) in the desulfurized slag.   3. The hot metal desulfurization method with less resulfurization according to claim 1 or 2, wherein a desulfurizing agent having 100 μm or less of 80 mass% or more and 50 μm or less of 50 mass% or more is used.

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