JP2009249667A - Desulfurization refining method for molten - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a refining process for desulfurization treatment more easily, efficiently and stably without using an LF apparatus or a vacuum degassing apparatus having a high equipment cost and processing cost and without adversely affecting the environment. Provide a method.
When desulfurizing and refining molten iron, a desulfurizing agent is added as a first step to perform desulfurization, and a part or all of the desulfurized slag generated in the first step covering the surface of the molten iron is left as a second step, The upper surface of the slag is irradiated as a plasma gas with hydrogen gas or argon gas containing 1% by volume or more of hydrogen gas. Further, a flux substantially free of fluorine is used as a desulfurizing agent. Furthermore, the generated slag is used again as a desulfurizing agent.
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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, and part or all of the desulfurization slag produced in the first step covering the surface of the molten iron is left as the second step. A method for desulfurizing and refining molten iron, comprising irradiating an upper surface of the slag with argon gas containing 1% by volume or more of gas or hydrogen gas as plasma gas.
(2) The method for desulfurizing and refining molten iron according to (1) above, wherein a flux containing substantially no fluorine is used as a desulfurizing agent.
(3) The method for desulfurizing and refining molten iron according to (1) or (2), wherein the slag generated by the method for desulfurizing and refining molten iron according to (1) or (2) is used as the desulfurizing agent.

  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, when the present inventors conducted various desulfurization experiments and sprayed a gas containing hydrogen on the slag containing sulfur after the desulfurization treatment, the oxygen activity of the molten iron was reduced by reducing the oxygen partial pressure of the atmosphere. And the above formula (A) was promoted, and at the same time, it was found that the reaction represented by the following formula (B) has an extremely high vaporization desulfurization ability.
(S) + H ₂ → H ₂ S ↑ (B)

Furthermore, when hydrogen is used as an extremely high temperature plasma gas, which is said to be 10,000 ° C., hydrogen is converted into a single atom as shown in the following formula (C) and reacted with S in the slag, and even a low concentration hydrogen gas is vaporized and desulfurized. It has also been found that progresses significantly.
(S) + 2H → H ₂ S ↑ (C)
The present invention utilizes the vaporization desulfurization reaction from slag by hydrogen.

  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 it can be easily put into an argon gas atmosphere that is advantageous for desulfurization by covering with a dipping-type lid (immersion tube). Moreover, since a hydrogen-containing gas is sprayed by a plasma arc, it is desirable that it is a ladle in terms of equipment structure.

  After charging the 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 by the desulfurization treatment as described above.

  Next, as the second step, a part or all of the desulfurized slag generated in the first step is left, and the upper surface of the slag is irradiated with hydrogen gas or argon gas containing hydrogen gas as a plasma gas. Here, when only a part of the desulfurized slag is left, it is sufficient that the slag covers at least the entire surface of the molten iron.

  Since sulfur in molten iron has a strong affinity with iron, vapor desulfurization does not proceed even when hydrogen gas is blown, but sulfur in slag is rapidly vaporized by the reaction of the above formula (B). Further, when slag is irradiated with hydrogen gas or hydrogen-containing gas as plasma gas, the vaporization desulfurization reaction is further accelerated by the reaction of the above formula (C), and extremely low sulfur steel can be produced. In addition, due to 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.

  The present inventors have found from various plasma desulfurization experiments that the vaporization desulfurization rate is greatly improved at a hydrogen concentration of 1% by volume or more. From the viewpoint of the vaporization desulfurization rate, it is desirable that the hydrogen concentration is high, and the higher the hydrogen concentration is, the more the plasma output can be increased. It is an embodiment. On the other hand, from the viewpoint of gas cost and power cost, it is desirable that the hydrogen concentration is low, and the necessary plasma capacity can be reduced, so that the target sulfur concentration after processing, the required processing time, and the plasma power source capacity are 1 to 100 volumes. %, The hydrogen concentration can be selected as appropriate.

  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.

  As the plasma equipment, hydrogen can be dissociated into single atoms in an ultra-high temperature plasma stream, so the plasma system may be either transitional or non-transitional. In the transitional type, the torch side is either positive or negative But it ’s okay. In the case of the transfer type, an arc is formed between the plasma torch and the molten metal. Therefore, hydrogen gas or a hydrogen-containing gas is introduced into the plasma arc as a plasma gas and irradiated to the slag. In the case of the non-migration type, hydrogen gas or hydrogen-containing gas passes through a plasma arc formed in the plasma torch to become plasma gas, and this plasma gas is irradiated onto the slag. In addition, as the hydrogen-containing gas, it is preferable to use argon gas from the viewpoint of preventing nitrogen absorption and gas cost.

  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.

  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.

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 0.021 to 0.025 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 10 minutes after the upward addition of quicklime powder was performed. Thereafter, the impeller was pulled up, and as a second step, a plasma torch installed in the KR apparatus was inserted, and hydrogen and argon mixed gas having different hydrogen concentrations were sprayed on the slag upper surface at a supply rate of 15 Nm ³ / hour for 5 minutes. The plasma device was a direct current transfer type with a power capacity of 2 MW, and the torch side was a negative electrode. In the second step, 100 Nl of argon gas was stirred from a porous plug installed at the bottom of the hot metal pan.

  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 0.002% by 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. Nos. 7 and 8 indicate that the hydrogen concentration in the plasma gas is too low. No. 9 was not irradiated with plasma gas. Since 10 discharged all the slag, S was not sufficiently reduced after the treatment.

(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 0.010 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 hot metal. 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 10 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, a plasma torch installed in the CAS apparatus was inserted, and hydrogen and argon mixed gases having different hydrogen concentrations were sprayed on the upper surface of the slag inside the dip tube at a supply rate of 15 Nm ³ / hour for 5 minutes. The plasma device was a direct current transfer type with a power capacity of 2 MW, and the torch side was a negative electrode. Also in the second step, argon gas bottom blowing was continued.

  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 was stably less than 0.001% by mass, and it was confirmed that an extremely low sulfur steel could be produced.

  On the other hand, Comparative Example No. Nos. 7 and 8 indicate that the hydrogen concentration in the plasma gas is too low. No. 9 was not irradiated with plasma gas. Since 10 discharged all the slag, S was not sufficiently reduced after the treatment.

Claims (3)

  When desulfurizing and refining molten iron, the desulfurization agent is added as a first step to desulfurize, and the desulfurization slag produced in the first step covering the surface of the molten iron is left as the second step, leaving hydrogen gas or hydrogen. A method for desulfurizing and refining molten iron, comprising irradiating an upper surface of the slag with an argon gas containing 1% by volume or more of a gas as a plasma gas.   2. The method for desulfurizing and refining molten iron according to claim 1, wherein a flux containing substantially no fluorine is used as the desulfurizing agent.   3. The method for desulfurizing and purifying molten iron according to claim 1 or 2, wherein the slag generated by the method for desulfurizing and refining molten iron according to claim 1 or 2 is used as the desulfurizing agent.