JP2013036062A - Method of desulfurizing molten steel and molten iron alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a desulfurizing method for improving a desulfurization rate in a desulfurizing process of reducing a sulfur concentration in molten metal including molten iron alloy such as molten steel and stainless steel by a slag-metal reaction using a CaO-SiOslag concerning a desulfurizing technology of a refractory sulfur steel.SOLUTION: A slag containing CaO, SiO, and AlOby 80% or more in total and one or more of MgO, CrO, MnO, and iron oxide by 20% or less as another component is formed on a surface of the molten metal, and the molten steel and the slag are agitated for a desulfurization treatment. At this time, after concentrations of CaO, SiO, and AlOin the slag are adjusted to simultaneously satisfy Formula (1): (%CaO)/(%AlO)≥2.3 and Formula (2): 0.4≤(%CaO)/(%SiO)≤3.5, and also the molten steel is adjusted to Si concentration [Si]≥0.1 mass% or Al concentration [sol. Al]≥0.005 mass%, BaO is added to the slag to satisfy BaO concentration in the slag as indicated by Formula (3): 4≤(%BaO)≤20.

Description

The present invention relates to a desulfurization method for molten steel and molten iron alloy, and specifically, relates to a desulfurization treatment for reducing the sulfur concentration in a molten metal in a refining treatment of a molten iron alloy such as molten steel and stainless steel such as steel refining, more particularly, to processing method to improve the desulfurization efficiency without using CaF ₂ and a large amount of Al.

  For carbon steel used in shipbuilding, construction or steel pipe, stainless steel used in boilers and gas tanks, or high alloy steel containing a large amount of Ni or Cr, it is required to further reduce the S concentration in the steel. S in steel forms non-metallic inclusions in steel such as MnS and causes various defects. Therefore, in order to demonstrate high characteristics and reliability even in harsh usage environments, the S concentration in steel must be reduced. Is required.

  Reduction of S concentration in steel is performed by performing desulfurization treatment in a molten state such as molten steel or molten iron alloy. For this reason, many desulfurization techniques for various molten iron alloys such as molten steel, molten stainless steel, high Cr molten steel, and high Mn molten steel have been developed.

Methods widely used as a desulfurization treatment (hereinafter simply referred to as desulfurization) of molten steel or molten iron alloy are roughly classified into the following two. Here, molten steel will be described as an example.
The first is a method of controlling the slag composition existing on the surface of the molten steel in the ladle and desulfurizing by a slag-metal reaction by stirring the molten steel with an inert gas or electromagnetic induction. This method is adopted in general inert gas blowing ladle refining and VOD.

  The second is a method in which desulfurization is performed by adding, spraying, or blowing a flux having excellent desulfurization capability to the molten steel and causing the molten steel and the flux to react directly. This method is adopted in flux blowing refining in the above-described inert gas ladle refining, addition in the tank to the molten steel in the RH vacuum tank, top blowing, or the like.

  In the first method, the desulfurization rate, which is an index of the desulfurization treatment capacity, can be increased by optimizing mainly the slag composition and the second method in the flux composition and addition method. Many developments have also been made.

In particular, it is well known that it is effective to add an appropriate amount of metals such as Ca and Mg and fluorides such as oxides, chlorides, CaF ₂ or Na compounds in order to enhance the desulfurization ability of slag and flux. ing. In addition, it is well known that Ba oxide BaO, which is an element similar to Ca and Mg, is also effective for desulfurization, and has been utilized particularly for dephosphorization technology in high alloy molten steel.

  Patent Document 1 describes the dephosphorization and desulfurization technology of Cr-containing steel, in which a flux containing barium chloride, barium oxide and chromium oxide is added to Cr-containing steel, and Patent Document 2 discloses a barium oxide to a molten high Mn iron alloy. In addition, Patent Document 3 discloses a denitrification slag in which alumina and BaO are controlled to an appropriate concentration ratio, respectively.

In these techniques, although the usage purpose and method of use of BaO are different, the ability of BaO is higher than the desulfurization ability and dephosphorization ability of CaO.
Furthermore, in recent years, in addition to improving desulfurization capacity, the use of CaF ₂ and Na ₂ CO ₃ is being restricted from the viewpoint of the environment. Patent Documents 4 and 5 disclose calcium aluminate-based desulfurization agents having a low melting point composition using BaO in order to compensate for a decrease in desulfurization capacity due to this restriction.

  Thus, conventionally, desulfurization technology capable of desulfurization utilizing the high reactivity of BaO and environmentally-friendly desulfurization agents have been developed. Moreover, in order to utilize the high reactivity of BaO, the 2nd method of adding the flux containing BaO directly to molten steel is widely used for the prior art.

  As described above, BaO-based fluxes have been developed, and in recent years, BaO-containing lime aluminate-based fluxes that are environmentally friendly have been developed.

JP 58-31011 A JP-A-61-272312 JP-A-64-42519 JP 2002-60832 A JP 2003-328022 A

Recently, in addition to the environmental considerations described above, the demand for high desulfurization rates is increasing for products that had previously been considered difficult to achieve high desulfurization rates due to the increase in required product performance. . For example, in the prior art, strong deoxidation using Al or Ca of molten steel or molten iron alloy or the use of CaO-Al ₂ O ₃ slag was premised. In order to meet this requirement, Al cannot be used, and in addition, CaO—SiO ₂ system is inferior in desulfurization power to CaO—Al ₂ O ₃ system in order to suppress Al contamination from slag. I have to use slag.

As described above, there is an increasing need to obtain a high desulfurization rate even with weak deoxidation such as Si deoxidation and CaO-SiO ₂ slag having a low Al ₂ O ₃ concentration instead of Al deoxidation.
However, as described above, the purpose of the BaO-containing flux that has been mainly developed so far was to avoid the use of CaF ₂ from the viewpoint of high Cr steel, high Mn dephosphorization desulfurization, or environment that is difficult to dephosphorize. It has not been possible to sufficiently cope with improving the desulfurization rate of molten steel, molten iron alloy, or CaO—SiO ₂ slag having a low Al ₂ O ₃ concentration.

The present invention has been made in view of the above-mentioned problems, and relates to a desulfurization technique for hardly desulfurized steel. Molten iron alloy such as molten steel and stainless steel is melted by a slag-metal reaction using CaO-SiO ₂ slag. It aims at providing the desulfurization method which improves a desulfurization rate in the desulfurization process which reduces the sulfur concentration in the inside.

Desulfurization of molten steel using oxide has been systematized academically using thermodynamics such as sulfide capacity and optical basicity, and as already explained, slag containing BaO and Na ₂ O ₃ It is well known that flux has a high desulfurization rate.

According to these past research findings, it is predicted that the desulfurization rate is improved by adding BaO in large amounts regardless of the basic component system such as CaO—Al ₂ O ₃ system and CaO—SiO ₂ system.
Therefore, we desulfurization experiments Al deoxidation S-containing molten steel with alumina saturated _CaO-Al 2 O 3 -BaO slag. The amount of molten steel is 10 kg, and the amount of slag is 200 g. In the experiment, CaO and Al ₂ O ₃ were added, and then BaO was added. The BaO addition time was set to 0 min. The change with time of the desulfurization rate was measured with the BaO concentration in the slag as two levels. The Al concentration in the molten steel before addition of BaO is 0.007 to 0.01% (unless otherwise specified in this specification, “%” in terms of concentration or chemical composition means “mass%”), and the Si concentration is 0. Adjusted to 1%. The desulfurization rate is defined by equation (5).
Desulfurization rate = (S concentration in the initial molten steel-S concentration in the molten steel at a certain time) / (S concentration in the initial molten steel) x 100 (5)

  The experimental results are shown graphically in FIG. As shown in the graph of FIG. 1, the final desulfurization rate is higher when the BaO concentration in the slag is increased, and this agrees well with the existing research findings that the desulfurization power improves as the BaO concentration increases.

  On the other hand, paying attention to the experimental results from 0 minutes to 40 minutes, the experiment with a lower BaO concentration has a higher desulfurization rate. From this, it can be seen that the higher the BaO concentration, the higher the desulfurization rate is obtained in the state of equilibrium or near equilibrium, but the rate is not necessarily higher as the BaO concentration is higher. The cause of this is thought to be due to kinetic factors such as slag viscosity and wettability or inclusion absorption in molten steel, but it is not clear.

  However, when considering production on an industrial scale, desulfurization rate is an important requirement in addition to equilibrium or near equilibrium. Therefore, the present inventors paid attention to the relationship between the desulfurization rate and the slag composition in the process of reaching the equilibrium, and intensively studied the optimum conditions for improving the desulfurization rate in a short time, and completed the present invention.

The present invention is as follows.
(1) On the surface of the molten metal, CaO, SiO ₂ , Al ₂ O ₃ is contained in total of 80% or more, and MgO, Cr ₂ O ₃ , MnO, one or two or more kinds of iron oxides are contained as other components. In a refining process in which a slag containing 20% or less in total is formed and the molten steel and slag are stirred and desulfurized,
Each concentration of CaO, SiO ₂ and Al ₂ O _{3 in} the slag is adjusted so as to satisfy the expressions (1) and (2) at the same time, and the Si concentration in the molten steel [Si] ≧ 0.1%, or Al concentration [sol. A method for desulfurizing molten steel and molten iron alloy, characterized by adding BaO to the slag so that the BaO concentration in the slag satisfies the formula (3) after adjusting to Al] ≧ 0.005%.

(% CaO) / (% Al ₂ O ₃ ) ≧ 2.3 (1)
0.4 ≦ (% CaO) / (% SiO ₂ ) ≦ 3.5 (2)
4 ≦ (% BaO) ≦ 20 (3)
In the formulas (1) to (3), (% MO) means the mass concentration of M oxide in the slag.
(2) The molten steel and molten iron alloy desulfurization method according to (1), wherein the BaO concentration in the slag satisfies the formula (4) indicating the relationship with the SiO ₂ concentration in the slag.
11.2 × ln (% SiO ₂ ) −30 ≦ (% BaO) ≦ 9.4 × ln (% SiO ₂ ) -19 (4)
(3) Desulfurization of molten steel and molten iron alloy according to (1) or (2), characterized in that BaO is added to slag after 60% and 85% of the total desulfurization treatment time. Method.

According to the present invention, the desulfurization rate can be significantly improved in the desulfurization treatment using the slag-metal reaction using CaO—SiO ₂ slag.

FIG. 1 is a graph showing the influence of the BaO concentration in slag on the temporal change of the desulfurization rate. FIG. 2 is a graph showing the relationship between the BaO concentration in the slag and the desulfurization rate. FIG. 3 is a graph showing the relationship between the SiO ₂ concentration and BaO concentration in the slag and the desulfurization rate. FIG. 4 is a graph showing the relationship between the BaO addition time index and the normalized desulfurization rate (C / S = 1, (% BaO) = 7-8%).

First, CaO, SiO ₂ , Al ₂ O ₃ is contained in a total of 80% by mass or more, and MgO, Cr ₂ O ₃ , MnO, or one or more of iron oxides in total as 20% by mass as other components. The reason for limiting to slag containing no more than% will be described.

In the desulfurization of molten steel or molten iron alloy, components such as CaO, SiO ₂ and Al ₂ O ₃ are generally used as the main components. Besides this, MgO, MnO, FeO, Cr ₂ O ₃ or REM oxides are used. Etc. If the components other than CaO, SiO ₂ and Al ₂ O ₃ become higher than 20%, the above-mentioned kinetic factors are considered to change greatly. Therefore, CaO, SiO ₂ and Al ₂ O ₃ are combined in total. It contains 80 mass% or more, and is limited to slag containing 20 mass% or less in total of MgO, Cr ₂ O ₃ , MnO, or one or more of iron oxides as other components.

Next, the reason why the slag composition is limited to the range defined by the above formulas (1) and (2) will be described.
As described above, the present invention targets the CaO—SiO ₂ system, which is difficult to desulfurize by the prior art, and aims to improve the desulfurization rate by appropriately adding BaO. Therefore, as a result of measuring the BaO addition effect using the CaO-SiO ₂ slag having a relatively low Al ₂ O ₃ concentration by the same method as described above, in the region represented by the formulas (1) and (2), the addition of BaO Although the effect was recognized, the effect was small in other areas. Then, the slag composition range which this invention makes object is limited to the range shown by (1) Formula and (2) Formula.

In the present invention, since a desulfurization process that utilizes combined slag Metal intermolecular reactions due to the slag of a small amount of BaO based on CaO-SiO ₂ slag, method of adding BaO after forming a CaO-SiO ₂ slag Is the most stable and repeatable. For example, when CaO, SiO ₂ and CaO are mixed and added, the amount of these oxides added to the molten steel in the ladle becomes very large. As a result, the influence of the dissolution rate of slag and the mixing rate of each oxide increases. End up.

Next, the reason for limiting equation (3) will be described.
As described with reference to the graph of FIG. 1, since the BaO concentration range is considered to have an appropriate range defined by kinetic factors, the CaO − defined by the equations (1) and (2) is used. The BaO concentration in the SiO ₂ —Al ₂ O ₃ slag was changed, and the change over time in the desulfurization rate was measured.

The measurement was performed by the following method. 10 kg of molten iron whose oxygen concentration and sulfur concentration were adjusted in advance was adjusted to 1873 K, and the S concentration in molten steel obtained by taking a sample from steel was taken as the S concentration in the initial molten steel. Next, CaO reagent, SiO ₂ reagent and Al ₂ O ₃ reagent were added on the molten steel surface while Ar gas was blown into the molten steel and stirred to form slag. The reagent addition amount was determined so that each oxide concentration in the slag satisfied the expressions (1) and (2). Next, a predetermined amount of barium carbonate reagent was added to adjust the BaO concentration in the slag. The slag weight after addition of BaO was adjusted to 200 g. The concentrations of Al and Si in the molten steel were the same as those in FIG. After adding the CaO, SiO ₂ , and Al ₂ O ₃ reagents, the mixture was held for 30 minutes. The holding time was determined based on the graph of FIG. After holding, samples were taken from the molten steel and slag, and the S concentration and slag composition in the molten steel were quantified.

The results are shown graphically in FIG. As shown in the graph of FIG. 2, regardless of the CaO / SiO ₂ ratio in the slag (hereinafter referred to as C / S), when the BaO concentration in the slag is less than 4%, the effect of the addition of BaO is not recognized, and abruptly occurs at 4% or more. The desulfurization rate is improved. Furthermore, when the BaO concentration exceeds 20%, the desulfurization rate slightly decreases. From this result, it is understood that the BaO concentration in the slag is appropriate to be 4% or more and 20% or less as shown in the equation (3). It can also be seen that the desulfurization rate is high and stable when the BaO concentration in the slag is in the range of 8% to 15%.

  Next, claim 2 will be described. As understood from the graph of FIG. 2, the desulfurization rate is improved by adding a constant BaO regardless of C / S, but at the same time, slag having a smaller C / S is desulfurized at the same BaO concentration. The rate is low. Therefore, the conditions for improving this were examined.

When paying attention to the graph of FIG. 2, in the experiment of C / S = 0.7, the difference from the experimental result of C / S = 2.5 is larger as the experimental result of the lower BaO concentration. In order to improve this, it is considered that the BaO concentration should be controlled according to the SiO ₂ concentration in the slag. The results of investigating this relationship are shown graphically in FIG.

  From the graph of FIG. 3, when the BaO concentration in the slag was less than 4%, the desulfurization rate R could not be made 70% or more. In the figure, black circles indicate results with a desulfurization rate of 80% or more and less than 93%, and white circles indicate results with a desulfurization rate of 93% or more.

It can be seen that the BaO concentration needs to be increased in accordance with the SiO ₂ concentration in the slag, and in order to obtain a high desulfurization rate, it is necessary to exceed the curve B in the graph of FIG. However, when the BaO concentration increases beyond the curve A, the desulfurization rate again becomes less than 93%. When the curves A and B are regressed, the following equations (5) and (6) are obtained. Therefore, it is necessary to satisfy the above expression (4).

In other words, pre-calculate the SiO ₂ concentration in the slag of SiO ₂ or the like the amount of generating SiO ₂ concentration in the slag in the additive amount of SiO ₂ and deoxidation, such as quartz sand, is BaO concentration in accordance with this calculation concentration (4) What is necessary is just to control BaO addition amount so that it may be satisfied.
11.2 × ln (% SiO ₂ ) −30 ≦ (% BaO) (5)
(% BaO) ≦ 9.4 × ln (% SiO ₂ ) -19 (6)
Finally, claim 3 will be described.

Since the present invention uses speed rather than equilibrium, it is considered that there is an optimum time for adding BaO to CaO—SiO ₂ slag. Then, it examined by the following method. The time for adding CaO—SiO ₂ slag not containing BaO was set to 0, and the mixture was held for 20 to 40 minutes while blowing an inert gas. Let this retention time be Te. Next, while holding, BaO is added to the slag using barium carbonate, and this addition time is Tb. A BaO addition time index A is defined by equation (7) as an index indicating the BaO addition time.

BaO addition time index A = Tb / Te × 100 (7)
Then, as a desulfurization capacity index, the desulfurization rate at A = 3 is defined as R1, and the desulfurization rate R at each A is defined as a standardized desulfurization rate by equation (8).

Standardized desulfurization rate = R / R1 (8)
FIG. 4 is a graph showing the relationship between A and the normalized desulfurization rate. It can be seen that when A <50%, almost the same desulfurization rate can be obtained regardless of the addition time, but when A is 60% or more and 85% or less, a higher desulfurization rate than when initially added is obtained. However, when A exceeds 85%, the desulfurization rate decreases. The reason why the desulfurization rate decreases when A becomes excessively large is that the reaction time for effective action of BaO is shortened, but the reason why the desulfurization rate increases when A is 60% or more and 85% or less is considered to be as follows. It is done.

  Considering thermodynamic equilibrium, the desulfurization rate is the same if the slag composition is the same. Therefore, in order to obtain the equilibrium desulfurization rate, BaO is also different from other components such as CaO as disclosed in Patent Document 4. It is most appropriate to add them simultaneously.

On the other hand, when the non-equilibrium state is utilized as in the present invention, it is presumed that the following mechanism is expressed. When CaO—SiO ₂ -based slag is added in advance, desulfurization proceeds with this slag, but when BaO is added in this state, BaO is present as BaO (s) until dissolved in the slag. At this time, the desulfurization rate temporarily increases due to a direct reaction between BaO (s) and the molten steel. However, when BaO (s) completely dissolves in the slag after a certain period of time, it becomes CaO—SiO ₂ —BaO slag, so the desulfurization rate decreases toward the equilibrium desulfurization rate. In the graph of FIG. 4, when A is 60% or more and 85% or less, it is estimated that a higher desulfurization rate than that predicted from thermodynamics is obtained due to a transient phenomenon from addition of BaO to dissolution.

From the above, in the slag-metal reaction desulfurization in which BaO is added to CaO—SiO ₂ slag that has been controlled to a predetermined composition in advance, the addition time of BaO is from the time when 60% of the total desulfurization treatment time has elapsed until the time when 85% has elapsed By doing so, the desulfurization rate can be further increased.

Note that the desulfurization total processing time refers to the CaO-SiO ₂ slag composition under molten steel agitation from the time of adjusting the concentration of claim 1, wherein to the molten steel completion of stirring. During the molten steel stirring, a refining operation that does not inhibit desulfurization, for example, addition of an alloy excluding sulfide or degassing may be performed.

  Moreover, although the above description was made | formed about molten steel, this invention is not limited to molten steel, It can process similarly with molten stainless steel and molten high alloy steel. Specifically, a molten metal having a Cr concentration of 40% or less, a Ni concentration of 45% or less, a total concentration of Cr and Ni of 60% or less, and a total of Cr, Ni and Fe of 95% or more can be processed.

Next, the present invention will be described more specifically by taking as an example an embodiment in which the present invention is implemented using a converter and an inert gas blowing refining apparatus under atmospheric pressure.

After decarburization refining in the converter, the molten steel is taken into the ladle. Si at the time of tapping, Mn, alloy and CaO, such as Al, may be added, such as SiO ₂ in the molten steel. At this time, the load of the next process can be reduced by adding the addition amounts of CaO and SiO ₂ so as to be within the specified range of the present invention.

  After adding a predetermined alloy and oxide, the ladle is transferred to a refining apparatus with an inert gas blown at atmospheric pressure. In addition, after carrying out vacuum degassing processing by RH etc. after converter steelmaking, you may perform refining by blowing in inert gas under atmospheric pressure.

In an inert gas blowing refining apparatus, the following treatment is performed while blowing an inert gas into molten steel.
First, an alloy necessary for adjusting the molten steel components is added. Furthermore, when performing the molten steel temperature rise process by methods, such as adding Al or Si to molten steel, and spraying oxidizing gas on molten steel, it implements before performing the desulfurization process by this invention. This is because if an oxidizing gas is used after desulfurization, a resulfurization phenomenon in which S moves from slag to molten steel occurs.

In addition, when the molten steel temperature rise treatment is performed using Al or Si, Al ₂ O ₃ or SiO ₂ is generated. Therefore, when the formulas (1) and (2) defined in the present invention cannot be satisfied after the temperature rise treatment, Needs to add CaO again to satisfy the formulas (1) and (2).

As described above, in the present invention, the slag composition is defined as the start of the desulfurization treatment when the slag composition satisfies the expressions (1) and (2) while the molten steel is being stirred. For example, when all of the CaO and SiO ₂ required for steel leaving the converter are added and the gas blowing and stirring is started immediately after the ladle is transferred, the treatment start with the inert gas blowing refining becomes the desulfurization treatment start and the temperature rises When the treatment is performed, the desulfurization treatment starts when the necessary CaO or the like is added after the temperature increase treatment.

  When it is necessary to add the alloy again for readjustment or fine adjustment of the component adjustment, it is desirable to add it in the first half of the desulfurization treatment, and it is more desirable to add it immediately after the start of the treatment. This is because desulfurization becomes unstable if the oxygen activity in the molten steel changes during the desulfurization treatment or if a slight amount of contamination from the additive occurs.

  After desulfurization is started, BaO is added to the slag. BaO may be added as a compound such as barium carbonate in addition to BaO. The addition time is set to the time from the time when 60% of the total desulfurization treatment time has elapsed until the time when 85% has passed, so that the highest desulfurization rate can be obtained. Therefore, it is desirable to add less than 90%.

Further, it may be added subsequently to CaO and SiO ₂ added, but it is necessary not to perform BaO added prior to the addition simultaneously with or addition of these CaO and SiO _2. The present invention, in order to kinetically improved by a small amount of BaO added desulfurization by CaO-SiO ₂ slag, the presence of CaO-SiO ₂ slag is assumed. Therefore, if BaO is performed simultaneously or in advance, the effect intended by the present invention may be reduced.

  BaO added as a BaO source in slag or Ba compounds such as barium carbonate may be in any shape such as powder of 100 mesh under, lump of several mm to several tens mm, etc., but addition device using free fall such as hopper In the case of using, it is desirable that it is a lump for suppressing scattering. When using a powdery Ba compound, it is desirable to add an inert gas or the like to the slag surface as a carrier gas by a method of spraying the slag surface via an upper blowing lance.

  The amount of slag used is preferably 20 kg / ton or more and 80 kg / ton or less. At 20 kg / ton or less, desulfurization is difficult due to the material balance. Further, if the amount of slag exceeds 80 kg / ton, the amount of each oxide added to satisfy the present invention increases, and it takes time to make the slag composition uniform, and the processing time becomes longer or desulfurized. The rate may become unstable.

  As a molten steel component at the time of adding BaO, Si concentration [Si] ≧ 0.1% or Al concentration [Al] ≧ 0.005% is required. If [Si] <0.1% or [Al] <0.005%, deoxidation may be insufficient and desulfurization may become unstable. [Al] <0.015% is desirable. When [Al] is excessively increased, BaO may be contained in some of the inclusions in the molten steel due to the reaction BaO → Ba + O.

The inert gas for stirring the molten steel and slag may be a gas blowing lance immersed in the molten steel, or may be blown into the molten steel from a porous brick or tuyere installed at the bottom of the ladle.
The inert gas flow rate during the desulfurization treatment is desirably 8 Nl / (min · molten ton) or more and 20 Nl / (min · molten ton) or less. If it is less than 8 Nl / (min · molten steel ton), uniform mixing of the added BaO into the slag may be delayed. On the other hand, if it exceeds 20 Nl / (min · molten steel ton), the slag is caught in the molten steel, and unintended BaO-containing inclusions may be generated in the steel.

  The desulfurization treatment time is preferably from 10 minutes to 30 minutes, and more preferably from 15 minutes to less than 20 minutes. If the treatment time is short, desulfurization may not proceed sufficiently. In addition, if the treatment time is long, the desulfurization rate may approach thermodynamic equilibrium and the desulfurization rate may decrease. Therefore, it is desirable to perform the processing time within an appropriate range as described above.

In the present invention, as described above, CaO, SiO ₂ , Al ₂ O ₃ is contained in a total of 80% by mass or more, and MgO, Cr ₂ O ₃ , MnO, one of iron oxides as the other components or It limited to the slag which contains 2 or more types in total 20 mass% or less.

Further, the MgO concentration is preferably 15% or less, the Cr ₂ O ₃ concentration is 1% or less, and the total of the iron oxide and MnO concentrations is 2% or less. If the concentration of MgO or Cr ₂ O ₃ is higher than these values, the slag fluidity decreases, so that the desulfurization rate can be further stabilized by reducing these concentrations. MnO and iron oxide are called lower oxides, and it is well known to inhibit desulfurization by increasing the oxygen activity at the slag metal interface. Therefore, these concentrations are desirably lower, and more desirably 2% or less in total. In the present invention, since sufficient desulfurization power can be obtained by using BaO, it is desirable that the slag does not contain Na ₂ CO ₃ or CaF ₂ from the viewpoint of environmental conservation.

  In the above description, the case where a converter and an inert gas blowing refining apparatus under atmospheric pressure are used is taken as an example. However, the present invention is not limited to this case, and VOD is used when an electric furnace and VOD are used. The present invention can be implemented. VOD implementation requirements are the same as in the case of the above-described inert gas blown refining apparatus under atmospheric pressure.

250 ton of molten steel decarburized in the converter was taken out into a ladle. A deoxidizing material such as Al and Si, CaO (quick lime), and SiO ₂ (silica sand) were added to the molten steel in the ladle at the time of steel removal to adjust the molten steel components and the slag composition.

The ladle was transferred to an Ar gas blowing refining apparatus under atmospheric pressure, and Ar gas was blown from the blowing lance immersed in the molten steel at 14 Nl / (min · molted steel ton) to stir the molten steel and slag. First, a temperature rise treatment of molten steel in which Al was added to the molten steel and oxygen gas was blown up was performed for 5 to 8 minutes. Thereafter, a molten steel composition was adjusted by adding a metal or alloy such as Mn, Ni, or Cr. Further, the amount of Al ₂ O _{3 and the} amount of SiO ₂ generated from the amount of Al and Si added and the Al concentration or Si concentration in the molten steel are calculated, and CaO is reconstituted to satisfy the equations (1) and (2). Added. The amount of slag was 18-22 kg / molten steel ton.

  The CaO re-addition time was set as the desulfurization treatment start time, and the desulfurization treatment was performed for 20 minutes from the start time. A predetermined amount of barium carbonate was added using the BaO addition time index A described above. For comparison, a treatment without adding barium carbonate was also performed. The desulfurization treatment was performed for 20 minutes, the gas blowing was finished, and the Ar gas blowing refining under atmospheric pressure was finished. The S concentration in the molten steel before the desulfurization treatment and the S concentration in the molten steel after the desulfurization treatment were analyzed, and the desulfurization rate was calculated by the method described above.

  The results are summarized in Table 1. The Al concentration and Si concentration shown in Table 1 are the concentrations before adding BaO.

  Test Nos. 1 to 4 are conditions satisfying claim 1, and a high desulfurization rate is obtained as compared with Test Nos. 12 to 16, which are comparative tests in which BaO was not added. In particular, in Example Test No. 1, which was the most difficult C / S test for desulfurization, the desulfurization rate was 77.8%, whereas in Comparative Example Test No., which was the highest C / S test superior to desulfurization. No. 15 has a desulfurization rate of 75.1%, and it can be seen that an example test in which desulfurization is most disadvantageous can obtain an equivalent or slightly higher desulfurization rate than a comparative test in which desulfurization is most advantageous. This indicates that desulfurization under conditions that have been difficult in the past has become possible as described above.

  Test numbers 5 to 7 are the results of claims 1 and 2, test numbers 8 and 9 are the results of satisfying claims 1 and 3, and test numbers 10 and 11 are the results of satisfying all claims. Further, a higher desulfurization rate than Test Nos. 1 to 4 was obtained.

Claims (3)

On the surface of the molten metal, CaO, SiO ₂ , Al ₂ O ₃ is contained in a total of 80% by mass or more, and MgO, Cr ₂ O ₃ , MnO, one or two or more of iron oxides are added in total as other components. In a refining process in which a slag containing 20% by mass or less is formed and the molten steel and slag are stirred and desulfurized,
Each concentration of CaO, SiO ₂ and Al ₂ O _{3 in} the slag is adjusted so as to satisfy the expressions (1) and (2) at the same time, and the Si concentration in the molten steel [Si] ≧ 0.1% by mass, Or Al concentration [sol. A method for desulfurizing molten steel and molten iron alloy, characterized by adding BaO to the slag so that the BaO concentration in the slag satisfies the formula (3) after adjusting to Al] ≧ 0.005 mass%.
(% CaO) / (% Al ₂ O ₃ ) ≧ 2.3 (1)
0.4 ≦ (% CaO) / (% SiO ₂ ) ≦ 3.5 (2)
4 ≦ (% BaO) ≦ 20 (3)
In the formulas (1) to (3), (% MO) means the mass concentration of M oxide in the slag. The molten steel and molten iron alloy desulfurization method according to claim 1, wherein the BaO concentration in the slag satisfies the formula (4) indicating a relationship with the SiO ₂ concentration in the slag.
11.2 × ln (% SiO ₂ ) −30 ≦ (% BaO) ≦ 9.4 × ln (% SiO ₂ ) -19 (4)   The method for desulfurizing molten steel and molten iron alloy according to claim 1 or 2, wherein BaO is added to the slag after 60% and 85% of the total desulfurization treatment time.

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