JP2009249678A - Desulfurization refining method for molten - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a refining method for highly efficiently and stably performing desulfurization in a simpler and easier manner, without using an LF apparatus or vacuum degassing apparatus having high equipment cost and high treating cost and without adversely affecting environment. <P>SOLUTION: When molten iron is desulfurization-refined, in the first process, the molten iron is desulfurized by adding a desulfurizing agent thereto and in the second process a part or the whole of desulfurization slag generated at the first process and covering the molten iron surface is left, and oxygen gas or inert gas containing ≥81 vol% the oxygen gas is blown from above the left desulfurizationis slag, and S concentration in the molten iron after the second process is set to be lower than the S concentration after the first process, and further in the third process succeeding to the second process, the molten iron and the slag are deoxidized with a deoxidizing agent. Furthermore, as the desulfurizing agent, flux substantially containing no fluorine is used. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for desulfurizing and refining molten iron for melting ultra-low sulfur steel.

In general, in order to produce extremely low sulfur steel ([S] ≦ 14ppm in the molten steel sulfur concentration) with carbon steel, first, at the hot metal stage, soda ash, metallic Mg-based or lime-based desulfurizing agent is used to Pre-desulfurization is performed to reduce the sulfur concentration of the hot metal to about 20 to 50 ppm. Then, after the hot metal is decarburized and refined in a converter or the like, the obtained molten steel is further subjected to secondary refining to desulfurize to a final target sulfur concentration. The following methods are used for desulfurization performed in the secondary refining.
(1) A method of desulfurization using a so-called “LF device” consisting of a ladle, a lid, and a heating electrode, and performing desulfurization by heating with electric energy and slag-metal refining (2) Gas blowing into the molten steel held in the ladle A method of desulfurization by blowing a desulfurizing agent in the atmosphere through a nozzle (3) A method of desulfurizing the molten steel held in the ladle in a vacuum degassing tank such as RH and spraying the desulfurizing agent from above (4) A method in which molten steel held in a pan is set in a VOD vacuum degassing tank and desulfurized by vigorous stirring.

  As a method that does not use an LF device or a VOD vacuum degassing tank, the sulfur concentration is reduced to 10 to 35 ppm in advance in the hot metal pretreatment stage, and then 2 ppm or less by using premelt flux and controlling the Al concentration in the molten steel. There has also been proposed a method for producing a stable sulfur concentration (Patent Document 1).

  In addition, as a method for easily producing ultra-low sulfur steel, a method of adding CaO-based flux and Al while controlling the oxygen concentration in the free board low has been proposed (Patent Document 2).

  Patent Document 3 discloses a method in which vaporized desulfurization from slag proceeds in conjunction with desulfurization from molten iron to slag by blowing a gas whose equilibrium oxygen partial pressure is controlled to 0.2 to 0.8 atm to slag. Has been proposed.

  When producing ultra-low sulfur steel, it is common to use a desulfurization agent containing fluorine such as fluorite in order to enhance the desulfurization ability of slag.

JP-A-9-217110 JP 2004-107716 A JP-A-1-165709

  The method using the LF apparatus is a technique effective for heat insulation by melting the refining flux with electric power energy and covering the molten steel bath surface. In addition, it can be used even in refining fluxes that are difficult to melt, and the sulfur retention capacity (sulfide capacity) of slag can be increased, which has the advantage of high desulfurization reaction efficiency. However, when the LF apparatus is used, there is a problem in that not only the manufacturing cost increases because of the use of a large amount of electric energy, but also the melting time is long and the productivity is low.

  In addition, the method using a VOD vacuum degassing tank has a problem that the desulfurization reaction efficiency is large because the stirring force is large, but the melting time is long and the processing cost is high. Moreover, the problem that the melting loss of the refractory lining the ladle becomes remarkably large due to the strong stirring of the molten steel has occurred.

  In the method described in Patent Document 1, since two-stage refining in hot metal pretreatment and secondary refining is essential, the time and cost required for hot metal pretreatment are enormous. Further, when the achievement of the target is insufficient, there is a problem that further desulfurization treatment using the RH vacuum degassing tank, that is, two-stage desulfurization treatment is required only by secondary refining.

  In addition, in the method described in Patent Document 2, it is impossible to melt extremely low-sulfur steel with a sulfur concentration of 5 ppm or less, and since Al is used, the steel type and alumina inclusions that have Al concentration restrictions on the material are used. There was also a problem that it could not be applied to steel types that were not allowed to exist.

  In the method described in Patent Document 3, since desiliconization and dephosphorization are performed simultaneously with desulfurization, there is a limit in desulfurization capability, and there is a problem that it cannot be applied to low-sulfur steel with a sulfur concentration of less than 30 ppm.

  Furthermore, when the desulfurization process is performed with a flux containing fluorine that is generally used when manufacturing ultra-low sulfur steel, fluorine remains in the slag after the process. Therefore, in consideration of the influence of fluorine in the slag on the environment, it is required to suppress the use of a fluorine source in steel refining.

  The present invention provides a refining method for performing a desulfurization process more simply, efficiently and stably without using an LF apparatus or a vacuum degassing apparatus with high equipment costs and processing costs and without adversely affecting the environment. The task is to do.

In order to solve this problem, the gist of the present invention is as follows.
(1) When desulfurizing and refining molten iron, desulfurization is performed by adding a desulfurizing agent as the first step, leaving a part or all of the desulfurization slag of the first step covering the surface of the molten iron as the second step, A method for desulfurizing and refining molten iron, characterized in that oxygen gas or a gas containing 81% by volume or more of oxygen gas is sprayed to make the S concentration in molten iron after the second step lower than the S concentration after the first step.
(2) The desulfurization refining is performed on the molten steel after decarburization refining, and after performing the first step and the second step, the molten iron and slag are deoxidized by a deoxidizer as a third step. The method for desulfurizing and refining molten iron according to (1) above.
(3) The method for desulfurizing and refining molten iron according to (1) or (2) above, wherein a flux containing substantially no fluorine is used as a desulfurizing agent.
(4) The slag generated by the method for desulfurizing and smelting molten iron according to any one of (1) to (3) is used as the desulfurizing agent. The method for desulfurizing and smelting molten iron as described.

  According to the present invention, a stable ultra-low-sulfur steel is manufactured more easily, efficiently, and stably without using an LF apparatus or a vacuum degassing apparatus with high equipment costs and processing costs, and without adversely affecting the environment. It became possible.

In a normal desulfurization treatment, a CaO source is added, and the desulfurization reaction proceeds according to the following formula (A).
CaO + S → CaS + O (A)
In order to improve the reactivity with the flux and to improve the desulfurization ability of the slag, an alumina source or a fluorine source is mixed with CaO, or metal Al or the like is added to lower the oxygen activity in iron. Ingenuity has been made such as deoxidation, decompression of the atmosphere, and inert gas. In the hot metal stage, Mg may be desulfurized in the form of MgS while adding metal Mg, but MgS is unstable and is finally fixed in the slag in the form of CaS.

  In any case, the equilibrium sulfur concentration depends on the sulfur retention capacity (sulfide capacity) of slag and the oxygen activity in iron, and the desulfurization capacity with flux (slag) alone is limited.

On the other hand, the present inventors conducted various desulfurization experiments, and when high oxygen concentration gas was sprayed on slag containing sulfur after desulfurization treatment, extremely high vaporization desulfurization was achieved by the reaction represented by the following formula (B). It was found to have the ability.
CaS + O ₂ → CaO + SO ₂ ↑ (B)
The present invention makes the most efficient use of the vaporization desulfurization reaction from slag by oxygen.

  Hereinafter, details and preferred embodiments of the present invention will be described.

  In the present invention, hot metal or molten steel that has not undergone desulfurization treatment or has undergone some degree of preliminary desulfurization treatment is charged into a refining vessel. The smelting vessel may be a torpedo car, a converter, or a ladle. However, the present invention does not require a large free board as in the converter, and is characterized in that it can be implemented with a torpedo car or a ladle. Furthermore, the ladle has an advantage that a cylindrical lid (immersion pipe) of an immersion method is immersed in the molten metal surface, and the molten metal surface inside the immersion pipe is easily made into an inert gas atmosphere such as argon or nitrogen that is advantageous for desulfurization. It is also possible to make the entire molten metal surface in the ladle an inert gas atmosphere.

  After charging the above hot metal or molten steel into the refining vessel, quick lime, a mixture of quick lime and alumina source, a mixture of quick lime and metal Mg, a mixture of quick lime and metal Mg, a mixture of quick lime and fluorite A desulfurization treatment as the first step is performed while mixing the desulfurization agent and the molten iron by bottom blowing gas or mechanical stirring. The desulfurization in the first step is not particularly limited as long as S is contained in the slag on the molten iron as CaS by the desulfurization treatment as described above.

  Next, as a second step, a gas having a higher oxygen concentration is blown onto the slag than the lance installed on the refining vessel. Since sulfur in molten iron has a strong affinity with iron, vapor desulfurization does not proceed even when oxygen gas is blown, but sulfur in slag is rapidly vaporized by the reaction of the above formula (B). From the experiments by the present inventors, it has been found that the vaporization desulfurization rate is significantly improved by using a gas having an oxygen concentration of 81% by volume or more. By this vaporization desulfurization reaction, the S concentration in the molten iron is further reduced after the second step than after the first step. Setting the S concentration in the molten iron after the second step to 50% or less of the S concentration after the first step is a suitable condition for greatly exerting the effect of the present invention.

  In the second step, it is important to blow a gas having a high oxygen concentration on the slag. When the surface of the molten iron that is not covered with slag is exposed, when a gas with a high oxygen concentration is blown onto the exposed molten iron, the molten iron is oxidized, and the generated iron oxide moves into the slag and slag / metal. This increases the oxygen potential at the interface and prevents the progress of reductive desulfurization from metal to slag. The same applies when the gas breaks through the slag and directly contacts the molten iron under the slag. When argon gas blowing is used for stirring molten iron, when the blown argon gas rises, a molten iron exposed surface not covered with slag may be formed on the surface of the molten metal. When spraying a gas with a high oxygen concentration, it is important not to spray the gas onto the exposed surface of the molten iron. Further, if the injection pressure of the high oxygen concentration gas is too high, the gas will penetrate through the slag, so it is necessary to consider that the gas injection pressure is not too high. By adjusting the gas pressure while confirming the condition of the molten metal surface at the time of gas spraying, it is possible to spray gas on the slag.

  Moreover, it is desirable to implement a 2nd process process, after the S amount (mass%) in molten iron reduces to 20 to 70% of S amount (mass%) before a process. There are two reasons for this. The first point is that when the amount of S in molten iron exceeds 70% of the amount of S before treatment, that is, when the desulfurization rate is less than 30%, the vaporization desulfurization rate is low because the S concentration in the slag is still low, and sufficient vaporization is achieved. This means that the desulfurization effect cannot be obtained. The second point is that when the first step is continued until the S amount in the molten iron is less than 20% of the pre-treatment S amount, that is, the desulfurization rate exceeds 80%, the progress of desulfurization is slow or stagnant. It takes too much processing time.

  In the second step, when the molten iron to be treated is high-carbon molten iron containing molten iron (the C concentration of molten iron indicates 2.0 mass% or more and a saturated concentration or less), even if a gas having a high oxygen concentration is blown onto the slag, the carbon concentration Therefore, the oxygen activity at the interface between the molten iron and the slag is kept low, and the desulfurization from the molten iron to the slag further proceeds by the amount of sulfur released by vaporization from the slag, and the sulfur concentration is lowered to the extremely low sulfur region. The gas blown onto the slag is also the best embodiment when pure iron gas is desulfurized.

  On the other hand, when the molten iron to be processed is low-carbon molten iron containing molten steel (the C concentration of molten iron indicates less than 2.0% by mass), oxygen is permeated through the slag when a gas having a high oxygen concentration is blown, and the molten iron-slag interface Increased oxygen activity. In this case, vapor desulfurization from slag proceeds in the second step, but further desulfurization from molten iron to slag stagnate, and in some cases, part of sulfurization from slag to molten iron also proceeds. In this case, as a third step, it is desirable to stop blowing high oxygen concentration gas to the slag and add a deoxidizer to lower the oxygen activity of the molten iron and slag. Due to the decrease in the oxygen activity, the desulfurization ability of the slag is restored, and the total sulfur amount in the system decreases due to the vaporization desulfurization from the slag in the second step. The sulfur concentration decreases. Here, when S (mass%) in molten iron produces 0.0030 mass% (30 ppm) or less, since the resulfurization was recognized, when S produces 30 ppm or less, the third step may be performed. Is preferred.

  Here, it is desirable to use Al, which is a strong deoxidizing element, as a deoxidizing agent in terms of reducing the sulfur concentration, but there are restrictions on the presence of steel types and alumina inclusions that have Al concentration restrictions on the material. As for the steel type, Si, Mn, Ti, Zr, Ca and the like can be appropriately selected. In this case, the lower the oxygen concentration sprayed in the second step, the vaporization desulfurization rate is slightly reduced, but the vaporization desulfurization rate is significantly reduced from the viewpoint of reducing the deoxidizer basic unit in the third step. It is a preferred embodiment to dilute the blowing gas with an inert gas to an oxygen concentration of 81% by volume or higher.

  Note that the gas for diluting the oxygen gas in the second step is an inert gas. The inert gas is preferably a cheap nitrogen gas because denitrification proceeds in the subsequent decarburization and refining process when treating the hot metal, but when treating the molten steel, the argon gas is used to avoid nitrogen absorption. desirable.

  The present invention is also characterized in that a sufficiently high desulfurization ability can be obtained without substantially adding fluorine. The fact that it is not substantially added means that the elution of fluorine (F) is not recognized remarkably from the slag after desulfurization refining. According to the knowledge of the present inventors, F is 1 in the slag composition after refining. The case where it becomes below mass%. More preferably, F is 0.5% by mass or less.

  Furthermore, in the present invention, most of the sulfur is vaporized and escapes from the slag, so that the slag after desulfurization refining can be reused as a desulfurization agent for the next desulfurization refining treatment. By reusing it, the amount of new desulfurizing agent used and the amount of desulfurized slag slag discharged can be greatly reduced, and it has a significant effect on reducing the desulfurization treatment cost and desulfurization slag treatment cost. it can.

Example 1
The hot metal discharged from the blast furnace was charged into a hot metal ladle (350 tons) and subjected to desulfurization and refining treatment using a KR apparatus that was mechanical stirring. The S concentration in the hot metal before desulfurization refining was 0.020 to 0.024 mass%. In the first step, as the desulfurization refining agent, 5 kg of quick lime powder having a particle size of 1 mm or less was used per 1 ton of hot metal. Mechanical stirring with an impeller for 7 to 15 minutes was performed after the addition of quicklime powder. Thereafter, the impeller was pulled up, and as a second step, a lance for blowing oxygen gas was inserted, and oxygen and nitrogen mixed gases having different oxygen concentrations were blown onto the upper surface of the slag for 5 minutes at a supply rate of 30 Nm ³ / hour. In the second step, 100 Nl of argon gas was stirred from a porous plug installed at the bottom of the hot metal pan. Although the molten metal exposed surface that is not covered with slag is formed at the portion where the argon gas floats on the molten metal surface, the blowing direction was considered so that the blowing gas from the gas spray lance does not contact the molten metal exposed surface.

  The results of each example are shown in Table 1 together with the desulfurization treatment conditions. In addition, each average value shown in Table 1 averages the value in 10-20ch desulfurization process in each condition. In each of the examples of the present invention, the S concentration after the treatment is stably less than 0.003 mass%, and the S concentration in the slag after the treatment is also a low concentration that can be reused. confirmed.

  On the other hand, Comparative Example No. In Nos. 7 and 8, the oxygen concentration in the blowing gas was too low. No. 9 did not perform gas spraying in the second step, so the S concentration after treatment was not sufficiently reduced.

(Example 2)
The molten steel discharged from the converter was charged into a molten steel pan (350 tons), and desulfurization refining treatment was performed using a CAS apparatus having a dip tube. The CAS device is a cylindrical dip tube inserted from the top of the molten steel in the molten steel pan, the dip tube is immersed in the molten steel surface, argon gas is blown from the bottom of the ladle, and the argon gas floats inside the cylindrical dip tube. This is an apparatus for performing secondary refining such as addition of an alloy to the molten metal inside the dip tube after setting the inside of the dip tube to an argon gas atmosphere. The S concentration in the molten steel before desulfurization refining was 0.005 to 0.020 mass%. In the first step, as the desulfurization refining agent, a mixture of quick lime powder having a particle size of 1 mm or less and alumina powder in a mass ratio of 6: 4 was used at 5 kg per ton of molten steel . After substituting the atmosphere in the dip tube with argon gas, a refining agent was added onto the molten metal inside the dip tube from above, and stirred for 7 to 15 minutes with argon gas at 100 Nl / min from a porous plug installed at the bottom of the molten steel pan. Thereafter, as a second step, an oxygen gas spraying lance was inserted, and oxygen and argon mixed gases having different oxygen concentrations were sprayed onto the upper surface of the slag inside the dip tube at a supply rate of 15 Nm ³ / hour for 5 minutes. Also in the second step, argon gas bottom blowing was continued. Since the molten steel was exposed at the floating part of the argon gas inside the dip tube, the gas was sprayed in consideration of the spray gas not contacting the exposed molten steel. As a result, the sprayed gas contacted only the slag surface. At some levels, after the gas was blown, metal Al was added for deoxidation, and stirring was performed with 100 Nl / min bottom blown argon gas for 3 minutes.

  The results of each example are shown in Table 2 together with the desulfurization treatment conditions. In addition, each average value shown in Table 2 averages the value in the desulfurization process of 10-20ch in each condition. In all of the examples which are examples of the present invention, the S concentration after the treatment is stably less than 0.003% by mass, and at a level where deoxidation is performed after the high oxygen concentration gas is sprayed, an extremely low sulfur steel of 5 ppm or less It was confirmed that can be manufactured.

  On the other hand, Comparative Example No. In Nos. 11 and 12, the oxygen concentration in the blowing gas was too low. Since 13 and 14 did not perform gas spraying of the second step, the S concentration after the treatment was not sufficiently reduced.

Claims (4)

  When the molten iron is desulfurized and refined, a desulfurizing agent is added as a first step to perform desulfurization, and as a second step, part or all of the desulfurized slag of the first step covering the surface of the molten iron is left, and oxygen gas is supplied from the upper part of the slag Or the gas containing 81 volume% or more of oxygen gas is sprayed, S concentration in the molten iron after a 2nd process is made lower than the S concentration after a 1st process, The desulfurization refining method of the molten iron characterized by the above-mentioned.   The desulfurization refining is performed on the molten steel after decarburization refining, and after performing the first step and the second step, the molten iron and slag are deoxidized by a deoxidizing agent as a third step. The method for desulfurizing and refining molten iron according to 1.   The method for desulfurizing and refining molten iron according to claim 1 or 2, wherein a flux containing substantially no fluorine is used as the desulfurizing agent.   The method for desulfurizing and purifying molten iron according to any one of claims 1 to 3, wherein slag generated by the method for desulfurizing and refining molten iron according to any one of claims 1 to 3 is used as the desulfurizing agent.