JP5338124B2 - Method for desulfurizing and refining molten iron - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a refining method for highly efficiently and stably performing desulfurization treatment in a simpler manner without using an LF apparatus and a vacuum-degassing apparatus having high equipment cost and high treatment cost and without adversely affecting environment. <P>SOLUTION: In desulfurization refining of molten iron, hydrogen gas or inert gas containing &ge;30 vol.% hydrogen gas is blown from above desulfurization slag covering the molten iron surface, while performing desulfurization by adding an desulfurizing agent to the molten iron. In another embodiment, in the first step, the desulfurization is performed by adding the desulfurizing agent and in the second step, a part or the whole of the desulfurization slag covering the molten iron surface in the first step is left, and the hydrogen-containing gas is blown from above the left slag. Blowing the hydrogen-containing gas advances vaporization desulfurization from the desulfurization slag, so that the desulfurization from the molten iron into the slag continues, and excellent desulfurization ability is displayed. Further, as the desulfurizing agent, flux substantially containing no fluorine is used and furthermore, the generated slag is used again as the desulfurizing agent. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

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

Generally, in order to produce extremely low-sulfur steel (Sulfur concentration of molten steel [S] ≦ 14ppm) with carbon steel, first use soda ash, metallic Mg-based or lime-based desulfurizing agent at the hot metal stage to Pre-desulfurization is performed to reduce the sulfur concentration of the hot metal to about 20 to 50 ppm. Then, after decarburizing and refining the hot metal 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 Method of desulfurization by blowing desulfurizing agent in the atmosphere through a nozzle (3) Method of desulfurizing by circulating 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 is advanced 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.

  Further, the method using the VOD vacuum degassing tank has a problem that the desulfurization reaction efficiency is large because of the large stirring force, 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, from the start of desulfurization refining by adding a desulfurizing agent, desulfurization slag covering the surface of the molten iron contains 30% by volume or more of hydrogen gas or hydrogen gas from the top of the desulfurization slag A method for desulfurizing and refining molten iron, characterized by blowing gas.
(2) When desulfurizing and refining molten iron, desulfurization is performed by adding a desulfurizing agent as a first step, and part or all of the desulfurization slag of the first step covering the surface of the molten iron is left as a second step. A method for desulfurizing and refining molten iron, characterized by spraying hydrogen gas or an inert gas containing 30% by volume or more of hydrogen gas.
(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 desulfurization scouring method for molten iron according to any one of the above (1) to (3) is used as the desulfurization agent, according to any one of the above (1) to (3) Desulfurization smelting method for molten iron.

  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 vapor desulfurization ability.
(S) + H ₂ → H ₂ S ↑ (B)
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. Further, the ladle has an advantage that a dipping type cylindrical lid (immersion tube) is immersed in the molten metal surface, and the molten metal surface inside the immersion tube is easily made into an argon gas atmosphere advantageous for desulfurization. It is also possible to cover the entire upper surface of the ladle with an inert gas atmosphere over the entire molten metal surface in the ladle.

  After charging the above hot metal or molten steel into a 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 a second step, a part or all of the desulfurization slag generated in the first step is left, and an inert gas containing hydrogen is blown from a lance installed on the refining vessel. 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). 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.

  The present inventor has found from various desulfurization experiments that the vaporization desulfurization rate is greatly improved by using a gas having a hydrogen concentration of more than 30% by volume. From the viewpoint of vaporization desulfurization rate, a higher hydrogen concentration is desirable, and pure hydrogen gas is the best embodiment, but from the viewpoint of gas cost, a lower hydrogen concentration is desirable, and the target sulfur concentration after treatment The hydrogen concentration can be appropriately selected between 30 and 100% by volume depending on the required treatment time.

  In this case, the inert gas for diluting the hydrogen gas is preferably an inexpensive nitrogen gas because the denitrification proceeds in the subsequent decarburization and refining process when processing the hot metal, but when processing the molten steel, Argon gas is desirable to avoid nitrogen absorption.

  In the present invention, hydrogen gas or an inert gas containing hydrogen gas is blown from the top of the desulfurized slag covering the surface of the molten iron. That is, the sprayed gas is sprayed so as not to penetrate the desulfurization slag and reach the molten iron surface. Therefore, even when the molten iron is molten steel, an increase in the hydrogen concentration in the molten steel due to the sprayed hydrogen-containing gas does not occur.

  The desulfurization step using vaporization desulfurization in the second step may be performed simultaneously with the desulfurization step using the desulfurization agent that is the first step. That is, when the molten iron is desulfurized and refined without being divided into the first step and the second step, hydrogen gas or hydrogen gas is added from the upper part of the desulfurized slag covering the molten iron surface while adding a desulfurizing agent and performing desulfurization. It is good also as spraying the inert gas containing volume% or more.

  More preferably, after dividing into the first step and the second step, the S amount (mass%) in the molten iron in the first step is reduced to 20 to 70% of the pre-treatment S amount (mass%), It is more desirable to leave part or all of the desulfurization slag in the first step and perform vaporization desulfurization as the second step. 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 addition, when the inert gas containing hydrogen gas or hydrogen gas is blown from the upper part of the desulfurization slag that covers the surface of the molten iron according to claims 1 and 2, it is possible to stir the molten iron. Stirring is preferably performed by blowing argon gas from a porous plug at the bottom of the molten iron. In order to maintain the state where the molten iron front is substantially covered with the desulfurized slag, 0.1 (Nl / min) to 0. A blown gas flow rate of 5 (Nl / min) is preferred.

  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. This is because the S concentration in the desulfurized slag is lowered by the progress of vaporization desulfurization, so that sufficient desulphurization capability is provided without promoting the hatching by containing fluorine in the slag. 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 desulfurization and refining treatment was performed using a KR apparatus as mechanical stirring as the first step. The S concentration in the hot metal before desulfurization refining was 0.021 to 0.025 mass%. As the desulfurization refining agent, 5 kg of quick lime powder having a particle size of 1 mm or less was used per ton of hot metal. Mechanical stirring with an impeller for 10 minutes after the upward addition of quicklime powder was performed. Thereafter, the impeller is pulled up, and as the second step, an upper blowing lance installed in the KR apparatus is inserted, and hydrogen and nitrogen mixed gases having different hydrogen concentrations are sprayed on the slag upper surface at a supply rate of 30 Nm ³ / hour for 5 minutes. Was supplied with argon gas of 100 Nl / min (about 0.29 Nl / min / ton of molten iron) from a porous plug.

  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. No. Examples 1 to 6 are examples of the present invention. In each example, the S concentration after the treatment was stably less than 0.003 mass%, and it was confirmed that the S concentration in the slag after the treatment was also a low concentration that could be reused.

  No. 7 to 10 are comparative examples. No. Nos. 7 and 8 indicate that the hydrogen concentration in the blowing gas is less than the lower limit of the present invention. No. 9 does not spray a hydrogen-containing gas. No. 10 discharged the entire amount of desulfurized slag before gas blowing. Therefore, in all cases, S was high after treatment, and desulfurization could not proceed sufficiently.

(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%. As the desulfurization refining agent, a mixture of quick lime powder having a particle diameter 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 replacing the atmosphere in the dip tube with argon gas, as the first step, a refining agent is added from above and argon of 100 Nl / min (about 0.29 Nl / min / molten steel ton) from a porous plug installed at the bottom of the molten steel pan Stir with gas for 10 minutes. Thereafter, as a second step, an upper blowing lance installed in the CAS device was inserted, and hydrogen and argon mixed gas having different hydrogen concentrations were sprayed on the upper surface of the slag for 5 minutes at a supply rate of 30 Nm ³ / hour. In this case as well, stirring is performed with argon gas of 100 Nl / min (about 0.29 Nl / min / molten steel ton) from a porous plug installed at the bottom of the molten steel pan.

Example No. Only the level 1 is not divided into the first step and the second step, and an upper blowing lance is inserted from the start of desulfurization refining, and a mixed gas of hydrogen and argon is blown from the top at a supply rate of 15 Nm ³ / hour. Under the same conditions, desulfurization refining was performed while stirring the molten steel with argon gas from a porous plug.

  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. No. Examples 1 to 7 are examples of the present invention. In any of the examples, the S concentration after the treatment was stably less than 0.001% by mass, and it was confirmed that extremely low sulfur steel could be produced.

  No. 8 to 11 are comparative examples. No. Nos. 8 and 9 indicate that the hydrogen concentration in the blowing gas is less than the lower limit of the present invention. No. 10 does not spray a hydrogen-containing gas. Since No. 11 discharged | emitted all the desulfurization slag before gas blowing, all were high S after a process and desulfurization was not able to fully advance.

Claims (4)

When desulfurizing and refining molten iron, a desulfurizing agent is added and desulfurizing and refining is started , and then desulfurized slag covering the surface of the molten iron is sprayed with hydrogen gas or an inert gas containing 30% by volume or more of hydrogen gas from above the desulfurized slag. A method for desulfurizing and refining molten iron.   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, a part or all of the desulfurized slag in the first step covering the surface of the molten iron is left, and hydrogen gas is supplied from the top of the slag. Or the desulfurization refining method of the molten iron characterized by spraying the inert gas which contains hydrogen gas 30 volume% or more.   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.