JP7067279B2 - Ladle refining method for molten steel - Google Patents

Description

The present invention relates to a method for refining a ladle of molten steel.

The molten steel primary smelted in a converter or electric furnace is stored in a ladle. Furthermore, as secondary refining, refining is performed on the molten steel in the ladle. The molten steel after secondary refining is mainly cast by continuous casting, and then hot-rolled to produce the desired product. Secondary refining is performed for the purpose of adjusting the composition of molten steel, floating separation of non-metal inclusions, uniform mixing of molten steel, heating and heating of molten steel, reduction of harmful impurities in molten steel, etc., according to the target quality of the product. In order to purify the steel highly, the molten steel is reduced in oxygen as a secondary refining.

As one of the secondary refining methods, a flux layer containing CaO is formed on the surface of the molten steel in the ladle, and the molten steel is agitated by blowing gas into the molten steel while energizing by immersing the current-carrying electrode in the flux layer. Secondary refining methods are known. The flux layer can be used as a refining agent to reduce oxygen (high cleaning) of the molten steel, and the temperature of the molten steel can be raised by energization heating. Hereinafter, this method is referred to as a ladle refining method for molten steel accompanied by energization heating.

Regarding the ladle refining of molten steel accompanied by energization heating, Patent Document 1 discloses an energization heating type ladle for refining. Three graphite electrodes are arranged near the center of the ladle, and a plurality of porous plugs are arranged at the bottom of the ladle for gas blowing. The lower tip of the electrode is immersed in a flux layer floating on the molten steel in the ladle and energized by a power feeding device to heat the flux and the molten steel. Inert gas is sent into the molten steel from the porous plug to stir the molten steel.

In ladle refining of molten steel accompanied by energization heating, a medium solvent containing CaO is added in order to form a flux layer. When CaO, a medium solvent, is melted in slag to reduce the concentration of Al ₂ O ₃ and SiO ₂ in the slag, deoxidation of Al and Si, which are deoxidizing elements having a strong affinity with O dissolved in molten steel. The reaction is activated. As a result, the deoxidation treatment of the molten steel proceeds. However, since CaO has a high melting point, it is necessary to melt it in the slag by reacting with the slag, and even if the stirring is simply strengthened, it takes a long time to melt and the deoxidation time becomes long. be.

US Pat. As a result, the slag-making agent is slagging extremely quickly, and the refining time can be shortened.
Patent Document 3 discloses a method of injecting a refining agent into a molten steel with an inert gas in an amount that does not disturb the generation of an arc through a dipping lance in a ladle refining method for molten steel accompanied by energization heating. ing. As a result, it is possible to reduce the sulfur content to the extremely low sulfur content more quickly and stably than before.

Japanese Unexamined Patent Publication No. 2001-040411 Japanese Unexamined Patent Publication No. 62-20216 Japanese Unexamined Patent Publication No. 2000-234119

When hypoxicizing molten steel by ladle refining of molten steel accompanied by energization heating to produce high-cleanliness steel, if the hypoxic treatment is performed using the method described in Patent Document 1, the target low oxygen of the molten steel is obtained. It took a long time to realize the hypoxia, and there was a problem that the target hypoxia could not be achieved within a predetermined time. In the methods described in Patent Documents 2 and 3, the medium solvent can be melted in a correspondingly short time, but it is necessary to further shorten the melting time. In addition, spraying and injection of the medium solvent using a lance is indispensable for promoting melting of the medium solvent and improving reactivity, and it is necessary to atomize the medium solvent by crushing it and repair the lance refractory, which increases the cost. There are also challenges.

INDUSTRIAL APPLICABILITY The present invention promotes the melting of the added flux and increases the hypoxicification reaction rate when the molten steel in the ladle is reduced in oxygen by refining the ladle of the molten steel accompanied by energization heating to produce high-cleanliness steel. It is an object of the present invention to provide a new and improved ladle refining method for molten steel.

That is, the gist of the present invention is as follows.
[1] A flux layer is formed on the surface of the molten steel in the ladle by adding a medium solvent containing CaO to the molten steel contained in the ladle from the auxiliary material inlet, and two or three in the center of the ladle. In the method of ladle refining of molten steel in which the electrode of a book is immersed in the flux layer and energized.
Gas blowing plugs are arranged at two locations on the bottom of the ladle to form a gas blowing _A plug and a gas blowing _B plug, respectively, and the flow rates of the gas blown from each of the gas blowing plugs are QA and QB, respectively. (In each case, the unit is NL / min / t).
In plan view, the circle circumscribed around the outer circumferences of all the two or three electrodes and having the minimum radius r is defined as the "circumscribed circle of the electrodes ", and the center position of the circumscribed circle of the electrodes is defined as C _E.
The radius of the bottom of the pan is _R , and the center position of the bottom of the pan is CO.
The center position of the gas blowing A plug is CA, the center position of the gas blowing _{B plug is C B ,} and the center position of the auxiliary material input port is C _G.
The distance between C _O and _CA is L _{O A} , the distance between _{C O} and C _B is L O B, the angle formed by CA- _CO -C _B is θ, and the angle formed by C _G -C _B - _CO is the angle formed by C G-C B-CO. Let φ, and let η be the angle ( _∠GOB ) formed by _{C G} -CO-CB.
Let H be the depth of the molten steel bath in the ladle, L _GE be the distance between C _G and C _E , and L be the distance between the line segment connecting C _O and C _B and C _G.
The gas blowing A plug, gas blowing B plug, and auxiliary raw material input port are arranged at positions that satisfy the following formulas (1) to (7).
A method for refining a ladle of molten steel, wherein Q _A and Q _B satisfy the following equations (8) to (10).
0.21H ≧ L (1)
45 ° ≧ φ (2)
L _GE > r (3)
90 ° ≧ η (4)
0.9R ≧ L _OB ＞ r (5)
180 ° ≧ θ ≧ 120 ° (6)
0.9R ≧ L _OA (except when L _OA is 0) (7)
0.3 ≤ Q _B ≤ 4.2 (8)
0.1 ≤ Q _A (9)
Q _B / Q _A ≧ 1.5 (10)
[2] The ladle refining method for molten steel according to [1], wherein the medium solvent is added from the auxiliary raw material input port before and during energization.
[3] The method for ladle refining of molten steel according to [1], wherein the medium solvent is added from the auxiliary raw material inlet before energization.

In the present invention, when flux is added to the molten steel in the ladle and oxygen is reduced by refining the ladle of the molten steel accompanied by energization heating to produce high-cleanliness steel, two gas blowing plugs are provided at the bottom of the ladle. By optimizing the position of the auxiliary raw material input port and the position of the gas blowing, it is possible to promote the stagnation (melting) of the flux during energization, and the deoxidation reaction rate for low oxygenation can be increased.

It is a top view which shows the preferable position of the electrode, the plug for gas blowing, and the auxiliary raw material input port in the ladle refining of this invention, (A) mainly shows the code of each part, (B) is mainly suitable arrangement of each part. The range is described. It is a figure explaining the ladle refining of this invention, (A) is the plan view which shows after the medium solvent injection and before the gas blowing start, (B) is the BB arrow cross-sectional view, (C) is the gas blowing start. A plan view showing the rear, (D) is a cross-sectional view taken along the line DD. It is a top view which shows the situation which the plug for gas blowing in the ladle refining, the position of the auxiliary raw material input port, and the gas blowing amount are out of the preferable range, (A) is the position of the auxiliary raw material input port, (B) (C) is the A plug. The arrangement positions (D) and (E) indicate situations in which the amount of gas blown out of the preferable range.

The present invention will be described with reference to FIGS. 1 to 3.
In the method for refining a ladle of molten steel of the present invention, a flux layer 6 containing CaO is formed on the surface of the ladle 5 in the ladle 1, and two or three electrodes 3 are immersed in the flux layer 6 in the center of the ladle. By energizing the molten steel, the molten steel is highly cleaned. For energization heating, a commonly used method can be used. That is, the flux and molten steel are heated by immersing the lower tip of the electrode 3 arranged in the upper part of the ladle in the flux layer 6 and energizing the electrode 3. As shown in FIG. 1, in the plan view of the pan 1, the circle circumscribed around the outer circumferences of all the two or three electrodes 3 is referred to as "the circumscribed circle 4 of the electrodes". When there are three electrodes 3 (see FIG. 1), the circumscribed circle 4 of the electrodes is fixed to one. When there are two electrodes 3, the one having the smallest radius is the circumscribed circle 4 of the electrodes. Therefore, the radius r of the circumscribed circle 4 of the electrode is expressed as "the minimum radius of the circumscribed circle of the electrode".

As described above, the electrode 3 is arranged in the center of the ladle because the molten steel in the ladle can be heated evenly. Here, the "center of the ladle" means that the center ( _CE ) of the circumscribed circles of the two or three electrodes is 0.1 × R or less from the center ( _CO ) of the bottom of the ladle (radius R). It means that it is in the range. Normally, the center position of the circumscribed circle of the electrode matches the center position of the bottom surface of the pan.
Further, in the present invention, as shown in FIGS. 1 and 2, gas blowing plugs 2 are arranged at two places on the bottom of the ladle 1, and the inert gas is blown into the molten steel from each of the gas blowing plugs 2. The molten steel in the ladle is agitated.

The molten steel that has been smelted by a primary smelting device such as a converter is put out in a ladle. At the time of steel removal, a part of the converter slag flows out to the ladle together with the molten steel, and a converter slag layer is formed on the surface of the molten steel in the ladle after the steel removal is completed. The converter slag is composed of CaO, SiO ₂ , Al ₂ O ₃ , FeO, MgO and the like. At the time of steel ejection, a deoxidizing agent such as Fe—Si or Al is added to deoxidize the free oxygen in the molten steel. SiO ₂ , Al ₂ O ₃ generated by deoxidation floats and is absorbed by the slag layer on the surface of the molten steel.

In ladle refining, a medium solvent containing CaO is added to the surface of the molten steel in the ladle, and the medium solvent is melted by energizing and heating with an electrode to form a flux layer, and gas is blown from the bottom of the ladle to form the molten steel. The flux layer is agitated. The highly oxidizing component such as FeO in the flux layer is reduced by the deoxidizing agent (Al or the like) contained in the molten steel. Further, by adding a medium solvent containing CaO as a main component, the content of SiO ₂ , Al ₂ O ₃ , FeO, MgO, etc. in the flux is reduced as compared with the concentration of the converter slag. As a result of the flux component becoming non-oxidizing and the concentration of Al ₂ O ₃ and SiO ₂ in the flux decreasing, O (free oxygen) remaining dissolved in the molten steel is combined with Al and Si. Then, the reaction that migrates as Al ₂ O ₃ or SiO ₂ into the flux is activated. As a result, the cleanliness of the molten steel progresses. This reaction proceeds at the interface between the molten flux layer and the molten steel. In order to promptly proceed with high cleaning of the entire molten steel in the ladle, it is important to quickly and sufficiently melt the medium solvent charged from the auxiliary material charging port.

The medium solvent charged from the auxiliary material charging port is mainly CaO, and since CaO has a high melting point, it melts only when the medium solvent is heated to a high temperature. In the present invention, in particular, the electrode is immersed in the flux layer to perform energization heating, and the medium solvent charged from the auxiliary material charging port reaches the high temperature region near the electrode and stays there, so that it is preferentially applied. It is heated to a high temperature and becomes a molten flux. The medium solvent that has not reached the vicinity of the electrode (for example, present in the vicinity of the inner wall of the ladle) remains at a relatively low temperature and cannot be melted quickly.

In the present invention, the gas blowing plugs are arranged at two places on the bottom of the ladle, and the gas blowing plugs are the gas blowing A plug and the gas blowing B plug, respectively. In the present invention, after the medium solvent made of CaO or the like is charged onto the surface of the molten steel from the auxiliary raw material input port, the medium solvent is conveyed to the vicinity of the electrode where the temperature becomes high and stays in the high temperature region to promote the melting of the medium solvent. Let me. It was decided to specify the conditions for the addition position of the medium solvent (the position of the auxiliary raw material input port) necessary for that purpose in relation to the arrangement positions of the two gas blowing plugs. Then, the placement position of the gas blowing A plug and the auxiliary raw material charging position is determined with reference to the placement position of the gas blowing B plug.

Here, as shown in FIG. 1, the center position of the bottom surface of the pan is _CO , the center position of the gas blowing A plug is CA, the center position of the gas blowing _{B plug is C B ,} and the center of the auxiliary raw material inlet. Let the position be C _G. Further, the minimum radius of the circumscribed circle of the electrode is r, the center position of the circumscribed circle of the electrode is _CE , the radius of the bottom surface of the ladle is R, and the depth of the molten steel bath in the ladle is H. The distance between CO and CA is L _OA , the distance between _CO and _{C B} is L _OB , and the distance between C _B and C _G is L _{B G.} Further, let L _GE be the distance between C _G and C _E , and L be the distance between the line segment connecting C _O and C _B and C _G. The angle formed by CA- _CO -CB (∠AOB) is θ, the angle formed by _{C G} - _{C B - CO} is φ, and the angle formed by _{C G} - _CO -CB is (∠GOB). ) Is η. Here, as the angle formed by CA _{- CO -} CB, a small angle of 180 ° or less and a large angle of 180 ° or more on the opposite side (the sum of both angles is 360 °) are recognized. Will be done. Here, θ, φ, and η all mean small angles of 180 ° or less.

<< Arrangement position of auxiliary raw material input port >>
When gas is blown from the gas blowing B plug 2B at the bottom of the ladle, the rising flow 8 of the molten steel is also formed by the rising of the blown bubbles 7 (see FIG. 2D). When the ascending flow 8 of the molten steel reaches the surface of the molten steel, a surface flow radiating from the surface of the molten steel is formed. In a plan view, a radial flow is formed centered on the center position C _B of the gas blowing B plug 2B. In the present invention, a medium solvent is charged from the auxiliary raw material charging port 10 toward the molten steel flow formed by gas blowing from the B plug 2B, and the charged medium solvent is moved on the molten steel flow to become a high temperature electrode. It is transported to the vicinity (region of the medium solvent 21 after spreading in FIG. 2C). For that purpose, it is preferable to arrange the center position C _G of the auxiliary raw material input port at the position between the center position C _B of the gas blowing B plug 2B and the circumscribed circle 4 of the electrode. Hereinafter, specific conditions for that purpose will be described.

When the molten steel bath depth of the ladle is H, it is known that the gas blown from the gas blowing plug 2 at the bottom of the ladle forms a swelling with a radius of 0.21 × H on the surface of the ladle. Hereinafter, this raised region is referred to as a bubble rising region 22, and is shown in FIGS. 1 to 3. Then, when the distance between the line segment connecting the center position C _O of the ladle and the center position C _B of the B plug and the center position C _G of the auxiliary raw material input port is L,
0.21H ≧ L (1)
If so, it was found that the molten steel flow from the B plug toward the circumscribed circle of the electrode is relatively strong in a plan view, and the charged auxiliary material moves to the high temperature electrode region. When the value of L is larger than 0.21H, the charged medium solvent does not direct toward the electrode, and the charged auxiliary raw material cannot be transported to the vicinity of the high temperature electrode (see FIG. 3A).

On the other hand, if the angle φ (∠GBO) formed by the center position C _G -B plug center position C _B -ladle center position C _O of the auxiliary material input port is too large, even if the above equation (1) is satisfied, Since the molten steel flow radiating from the center position of the B plug does not go toward the electrode, the charged auxiliary material cannot be transported to the vicinity of the high temperature electrode. put it here,
45 ° ≧ φ (2)
If so, the added auxiliary material moves to the high temperature electrode region in combination with the above equation (1).

In addition, when the auxiliary material input position is set near the electrode, the installation position of the energization equipment and the auxiliary material input equipment interferes, so the center position C _G of the auxiliary material input port is the electrode (2 or 3). It is preferable that the range is outside the circumscribed circle of the minimum radius r. That is,
L _GE > r (3)
Satisfy.
Since the center position C _G of the auxiliary raw material input port is arranged at the position between the center position C _B of the gas blowing B plug 2B and the circumscribed circle 4 of the electrode, the following equation (4) is established. ..
90 ° ≧ η (4)
The region satisfying the above equations (1), (2), (3), and (4) is shown as a suitable arrangement range 15 of the auxiliary raw material input port in FIG. 2 (B). The center position C _G of the auxiliary raw material input port may be within the suitable arrangement range 15 of the auxiliary raw material input port.

<< Arrangement position of B plug for gas blowing >>
In order to realize the above condition, it is necessary to arrange the gas blowing B plug at a position farther from the center position of the bottom surface of the pan than the radius r of the circumscribed circle of the electrode ( _LOB > r). Since the center position ( _CE ) of the circumscribed circle of the electrode is near or at the same position as the center position ( _CO ) of the bottom surface of the pan, the B plug for gas blowing is arranged outside the circumscribed circle of the electrode. .. When the B plug is placed inside the circumscribed circle of the electrode ( _LOB ≤ r), the auxiliary material input port is also placed inside the circumscribed circle of the electrode, and as described above, the energization equipment and the auxiliary material input equipment are installed. It will interfere. On the other hand, when the gas blown from the B plug becomes bubbles and rises in the molten steel, if it is too close to the ladle wall, it will come into contact with the ladle wall, the bubble rising position will become unstable, and the molten steel flow generated will be generated. It becomes unstable. Therefore, it is necessary to arrange the B plug at a position where 0.9R ≧ L _OB is also satisfied. That is, the following equation (5) 0.9R ≧ L _OB > r (5)
Satisfy.

<< Arrangement position of A plug for gas blowing >>
As described above, the medium solvent charged from the auxiliary raw material charging port is carried to the vicinity of the high temperature electrode by riding on the molten steel flow on the surface of the ladle formed by the gas blowing from the B plug. In order to melt the medium solvent quickly and efficiently, it is preferable to allow the medium solvent that has reached the vicinity of the electrode to stay in the vicinity of the electrode for as long as possible (see FIG. 2D). Therefore, in the present invention, the A plug is arranged at a position opposite to the B plug when viewed from the center of the bottom of the ladle. As a result, of the ladle surface molten steel flow formed by gas blowing from the A plug, the molten steel flow toward the electrode collides with the molten steel flow formed by the B plug, and the molten steel flow is retained near the electrode. Is possible. Specifically, by setting the angle θ (∠AOB) formed by the A plug center position CA _A -ladle center position C _O -B plug center position C _B to 120 ° or more, the molten steel flow is retained near the electrode. It becomes possible. When θ <120 °, the effect of damming the molten steel flow formed by the B plug in the vicinity of the electrode is small, and the effect of accumulating the medium solvent in the vicinity of the electrode is low (see FIG. 3B). On the other hand, according to the above definition of the angle θ, θ is 180 ° or less. That is,
180 ° ≧ θ ≧ 120 ° (6)
Satisfy.

Similar to the above B plug, for the gas blowing A plug, when the blown gas becomes bubbles and rises in the molten steel, if it is too close to the ladle wall surface, it will come into contact with the ladle wall and the bubble rising position will be. It becomes unstable and the molten steel flow generated becomes unstable (see FIG. 3C). Therefore, it is necessary to arrange the B plug at a position where 0.9R ≧ L _OA is also satisfied. That is,
0.9R ≧ L _OA (7)
Satisfy.
The region satisfying the above equations (5) and (6) is shown as a suitable arrangement range 16 of the gas blowing A plug in FIG. 2 (B). The center position CA of the A plug may be within the suitable arrangement range 16 of the _A plug for gas blowing.

<< Amount of gas blown from the gas blowing plug >>
FIGS. 2 (A) and 2 (B) show the positions of the medium solvent 20 immediately after the medium solvent is charged from the auxiliary raw material charging port 10. In the figure, neither electrode heating nor gas blowing has started yet. Then, as shown in FIGS. 2C and 2D, electrode heating and gas blowing are started, and the transfer of the medium solvent on the surface of the molten steel begins.
In the present invention, as described above, the medium solvent (medium solvent 20 immediately after charging) charged onto the surface of the molten steel from the auxiliary material charging port 10 is placed on the surface molten steel flow caused by the gas blowing from the gas blowing B plug 2B. Transport near high temperature electrodes. By setting the gas blowing amount QB from the _B plug 2B to 0.3 NL / min / t or more, the charged medium solvent can be effectively and quickly transported to the vicinity of the high temperature electrode. On the other hand, if the amount of gas blown from the _B plug QB is too large, the strength of bubble burst on the surface of the molten steel becomes strong, and the medium solvent may be scattered and lost. Therefore, the amount of gas blown from the _B plug QB is set to 4.2 NL / min / t or less. That is, the following equation (8) 0.3 ≤ Q _B ≤ 4.2 (8)
Satisfy.

The medium solvent conveyed by the molten steel flow by the gas blowing B plug needs to be dammed near the electrode by the molten steel flow on the molten steel surface caused by the gas blowing from the gas blowing A plug 2A. Therefore, the amount of gas blown from the A plug 2A Q _A should be 0.1 NL / min / t or more, and the following equation (9) 0.1 ≤ Q _A (9).
Satisfy. If Q _A is less than 0.1 NL / min / t, the medium solvent cannot be retained in the vicinity of the electrode (see FIG. 3 (D)).

On the other hand, if the gas blown amount QA from the _A plug is too large as compared with the gas blown amount QB from the _B plug, the medium solvent charged from the auxiliary raw material charging port 10 cannot reach the vicinity of the high temperature electrode. It stays near the B plug position (see FIG. 3 (E)). In the present invention, by setting the Q _B / Q _A ratio to 1.5 or more, the charged medium solvent can be conveyed in the vicinity of the high temperature electrode. That is,
Q _B / Q _A ≧ 1.5 (10)
Satisfy.

<< Suitable timing for adding medium solvent >>
In order to quickly melt the medium solvent charged in the ladle molten steel and promote the hypoxic reaction between the molten flux and the molten steel, it is preferable that the addition of the medium solvent is completed immediately after the start of energization as much as possible. This is because the energization time after the addition of the medium and solvent can be secured. Therefore, it is preferable to add CaO before and during energization.

In order to melt the medium solvent more quickly to form a molten flux, it is effective that the addition of the medium solvent is completed before energization.

The following shows the results of verifying the effectiveness of the method for deoxidizing molten steel by ladle refining of the present invention.
In a ladle accommodating 80 to 90 tons of molten steel, ladle refining of molten steel accompanied by energization heating was performed. A flux layer 6 containing CaO is formed on the surface of the molten steel 5 in the ladle 1, and three electrodes 3 are immersed in the flux layer 6 in the center of the ladle to energize the ladle to perform high cleaning treatment of the molten steel. .. The center position ( _CE ) of the circumscribed circle 4 of the electrode coincides with the center position ( _CO ) of the bottom surface of the pan. The molten steel bath depth H is about 2.3 m (0.21 H = 0.48 m), the circle-equivalent diameter D of the opening of the auxiliary material input port is 0.35 m, and the minimum radius r of the circumscribed circle 4 of the electrode is 0.6 m. be. As the gas blowing plug 2, the gas blowing A plug 2A and the gas blowing B plug 2B are arranged at the bottom of the ladle, and the gas blowing amount QA from the _A plug 2A and the gas blowing amount QB from the _B plug 2B. Blow in argon gas. Angle θ (∠AOB) formed by CA- _CO -CB, distance between _CO and C _B Ratio of L _OB to circumscribed circle radius r ( _{LO B} / r), distance between C _O and _{C B} Table 1 shows the distance L _OA (relationship with _R ) between L _OB , _CO and CA. Table 1 shows the gas injection amounts Q _A and Q _B. All η (∠GOB) are 90 ° or less.

The medium solvent is added from the auxiliary raw material input port 10 provided in the container lid 13. Angle φ (∠GBO) formed by C _G -C _B -C _O , distance between the line segment connecting the center position C _O of the pan and the center position C _B of the B plug and the center position C _G of the auxiliary material input port. Table 1 shows the relationship between L and 0.21H (= 0.48m) (0.21H-L), the distance between C _G and _CE , and the ratio of L _GE to the radius of the circumscribed circle of the electrode (L _GE / r). Shown in.

As shown in Table 1, _LGE / r = 1.1 to 1.8 and satisfy the equation (3), and the auxiliary raw material input port 10 is outside the electrode circumscribed circle 4. The angle φ (∠GBO) is any of 0 °, 45 °, and 60 °. Further, the distances L are 0 m, 0.48 m (= 0.21 × H), 0.74 m (= 0.32 × H), and 0.48 m and 0.48 m in the “0.21 H−L” display in Table 1, respectively. It is 0 m and -0.25 m. The conditions of φ = 45 ° and L = 0.21 × H (0 in the table) are conditions in which the charged solvent is most difficult to flow near the electrode within the scope of the present invention.

80 to 90 tons of molten steel that had been decarburized in a converter was taken out into the pan 1. At this time, about 1 ton of converter slag composed of CaO, SiO ₂ , Al ₂ O ₃ , FeO and the like flowed out. Al, which is a deoxidizing element, was added to the steel withdrawal. After that, the ladle 1 was transferred to a processing position where energization heat treatment was performed. The Al concentration in the molten steel at the start of energization heating is 0.01 to 0.10%, and the O concentration (total oxygen concentration) is 0.0045 to 0.0050%. After the transfer to the processing position, the container lid was attached, and the electrodes 3 (three) for energization were lowered into the converter outflow slag layer on the surface of the molten steel.

After that, gas was introduced from the two gas blowing plugs 2 at the bottom of the ladle, and the heat treatment by energization was started while stirring. The time for adding the medium solvent is after the ladle arrives at the processing position, before energization until the start of energization, and / or during energization. Energizing is the first half of the period from the start to the end of energization. Further, as a comparison, it was also performed when no energization was performed or when no medium solvent was added.

The amount of the medium solvent added is 5 kg (5 kg / t) per 1 ton of molten steel, the composition of the medium solvent is 90% pure CaO, and the rest are impurities that are inevitably mixed. The particle size of the medium solvent was such that the ratio of the particle size of 5 mm to 50 mm was 95% by mass or more. Further, under the condition that the addition was performed separately before energization and during energization, the addition amounts were 4 kg / t before energization and 1 kg / t during energization, respectively.

The energizing time and the gas stirring time through the gas blowing plug are 15 minutes.

Samples of molten steel were taken before and after energization, and the O concentration (total oxygen concentration) in the molten steel after energization stirring was evaluated. In this evaluation, the total oxygen concentration (mass%) of the molten steel after energization in Comparative Example 12 was set to 1.0, and other conditions were indexed. The index is
If it is 0.96 or more, ×,
If it is 0.84 or more and less than 0.96, △,
If it is 0.72 or more and less than 0.84, ○,
If it was less than 0.72, it was evaluated as ◎.

The results obtained are shown in Table 1. It should be noted that the underlined parameters in the table indicate that they are out of the scope of the present invention.

As shown in Table 1, if the test conditions were within the range of the present invention, the O concentration evaluations of Examples 1 to 9 were Δ, ◯, and ⊚. In particular, as specified in claim 2, the test No. 1 in which the medium solvent is added before and during energization. The O concentration evaluation of No. 8 was ◯, and the test No. 8 was evaluated. The O concentration was lower than 1-7. Further, as specified in claim 3, the test No. 1 in which the addition time of the medium solvent is before energization. The O concentration evaluation of 9 was ◎, and the test No. The O concentration was lower than 1-8.

On the other hand, the test No. 1 whose test conditions are outside the scope of the present invention. The O concentration evaluation of 10 to 19 was x. The following are the reasons why the O concentration was high under each condition.
Test No. In No. 10, the Al ₂ O ₃ concentration in the slag did not dilute because no medium solvent was added.

Test No. In No. 11, since θ (∠AOB) was 90 °, the flow of the medium solvent could not be stopped by the molten steel flow due to the blowing from the A plug, and the side beyond the vicinity of the electrode at the stage where the medium solvent did not melt. The spread medium solvent 21 was conveyed to the wall surface of the ladle (see FIG. 3B).
Test No. In No. 12, the distance between the B plug and the wall surface of the ladle was too close, so that the flow of air bubbles became unstable and the transport of the medium solvent was disturbed. Test No. In No. 13, the distance between the A plug and the wall surface of the ladle was too close, so that the flow of air bubbles became unstable, and the effect of blocking the flow from the B plug to the electrode was not stable (see FIG. 3 (C)).
Test No. In 14 and 15, the arrangement position of the auxiliary raw material input port was out of the appropriate range in relation to the B plug arrangement position, and the medium solvent could not be charged into the region where the molten steel flow toward the electrode was relatively strong (FIG. 3). (A)).

Test No. In No. 16, the flow rate from the B plug was too small, so that the molten steel flow for transporting the medium solvent could not be formed. Test No. In No. 17, since the flow rate from the A plug was too small, the medium solvent tended to pass in the vicinity of the electrode before the medium solvent conveyed by the flow from the B plug to the electrode proceeded to melt (FIG. 3 (FIG. 3). D) See).
Test No. In No. 18, Q _B / Q _A = 1, which was out of the appropriate range, the flow rates of the B plug and the A plug were in equilibrium, and the spread medium solvent 21 stayed in the vicinity of the B plug (FIG. 3). See (E)).

Test No. 19 is a case where the power is not supplied. It is highly probable that the molten steel flow was properly generated, but the solvent was not melted and the transport on the molten steel surface in the ladle was insufficient.

1 Ladle 2 Gas blowing plug (plug)
2A Gas blowing A plug 2B Gas blowing B plug 3 Electrode 4 Electrode circumscribed circle 5 Molten steel 6 Flux layer 7 Bubbles 8 Upflow 10 Auxiliary material input port 11 Molten steel surface 12 Intake pot wall surface 13 Container lid 15 Auxiliary material input port Suitable placement range 16 Suitable placement range of A plug for gas injection 20 Medium solvent immediately after injection 21 Medium solvent after spreading 22 Bubble rising region r Minimum radius of circumscribed circle of electrode R Radius of bottom surface of pan H Depth of molten steel in pan C _E Center position of the circumscribed circle of the electrode C _O Center position of the bottom of the pan C _A Center position of the A plug for gas blowing C _B Center position of the B plug for gas blowing _{C G} Center position of the auxiliary material input port θ CA- Angle formed by CO _- CB ( _∠AOB )
Angle formed by φ C _G -C _B -CO ( _∠GBO )
Distance between L _OA C _O and C _A Distance between L _O B C _O and C _B Distance between L B _G C _B and C _G Distance between L G _E C _G and C _E

Claims (3)

A flux layer is formed on the surface of the molten steel in the ladle by adding a medium solvent containing CaO to the molten steel contained in the ladle from the auxiliary raw material inlet, and two or three electrodes are provided in the center of the ladle. In the ladle refining method for molten steel, which is energized by immersing it in the flux layer.
Gas blowing plugs are arranged at two locations on the bottom of the ladle to form a gas blowing _A plug and a gas blowing _B plug, respectively, and the flow rates of the gas blown from each of the gas blowing plugs are QA and QB, respectively. (In each case, the unit is NL / min / t).
In plan view, the circle circumscribed around the outer circumferences of all the two or three electrodes and having the minimum radius r is defined as the "circumscribed circle of the electrodes ", and the center position of the circumscribed circle of the electrodes is defined as C _E.
The radius of the bottom of the pan is _R , and the center position of the bottom of the pan is CO.
The center position of the gas blowing A plug is CA, the center position of the gas blowing _{B plug is C B ,} and the center position of the auxiliary material input port is C _G.
The distance between C _O and _CA is L _{O A} , the distance between _{C O} and C _B is L O B, the angle formed by CA- _CO -C _B is θ, and the angle formed by C _G -C _B - _CO is the angle formed by C G-C B-CO. Let φ, and let η be the angle ( _∠GOB ) formed by _{C G} -CO-CB.
Let H be the depth of the molten steel bath in the ladle, L _GE be the distance between C _G and C _E , and L be the distance between the line segment connecting C _O and C _B and C _G.
The gas blowing A plug, gas blowing B plug, and auxiliary raw material input port are arranged at positions that satisfy the following formulas (1) to (7).
A method for refining a ladle of molten steel, wherein Q _A and Q _B satisfy the following equations (8) to (10).
0.21H ≧ L (1)
45 ° ≧ φ (2)
L _GE > r (3)
90 ° ≧ η (4)
0.9R ≧ L _OB ＞ r (5)
180 ° ≧ θ ≧ 120 ° (6)
0.9R ≧ L _OA (except when L _OA is 0) (7)
0.3 ≤ Q _B ≤ 4.2 (8)
0.1 ≤ Q _A (9)
Q _B / Q _A ≧ 1.5 (10) The ladle refining method for molten steel according to claim 1, wherein the medium solvent is added from the auxiliary raw material input port before and during energization. The ladle refining method for molten steel according to claim 1, wherein the medium solvent is added from the auxiliary raw material input port before energization.

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