JP7047605B2 - 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. Desulfurization of molten steel is performed as secondary refining for the purpose of reducing the concentration of sulfur (S) in the molten steel.

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 desulfurization treatment of molten steel is possible using the flux layer as a desulfurizing agent, 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 on the lid covering the upper part of the ladle, and a porous plug for blowing gas is arranged on the bottom of the ladle. The three electrodes are located near the center of the ladle and concentrically with the outer circumference of the ladle. 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. When two or three electrodes are used, the circle circumscribed around the outer periphery of all the two or three electrodes is referred to here as the "circumscribed circle of the electrodes".
In addition, the inert gas is sent into the molten steel from the porous plug to stir the molten steel. Conventionally, only one porous plug is arranged at the bottom of the ladle, but in the invention described in Patent Document 1, a plurality of porous plugs are arranged at the bottom of the ladle, and the plurality of porous plugs are the bottom of the ladle. They are arranged unevenly on one side. The same flow rate of the inert gas is blown into the molten steel from the plurality of porous plugs.

Patent Document 2 is a ladle refining method that can be applied to ladle refining of molten steel accompanied by energization heating, in which two bottom blowing plugs are arranged at the bottom of the ladle and two bottom blowing plugs are provided. It is a method of injecting an inert gas with a difference in the gas flow rate from the above, and a technique of suppressing aggregation and coarsening of inclusions by controlling the flow rate ratio is disclosed. The invention described in Patent Document 2 does not require energization or immediately after energization. Regarding the plug, for a pot having a diameter of 2784 to 3188 mm, the positions from the center of the pot to the plug are described as 680 mm and 900 mm (both are described in Table 1 of Patent Document 2), but a plurality of electrodes are used. In the case of energization, when the center of the circumscribed circle of the plurality of electrodes coincides with the center of the bottom of the pot, the position of the plug is often outside the circumscribed circle of the electrode.

Patent Document 3 discloses a method for producing high-cleanliness steel, in which the first refining treatment, the degassing treatment, and the second refining treatment are performed in this order on the molten steel produced. In the refining treatment and the degassing treatment, the inert gas is blown from the two bottom-blowing plugs, and the inert gas is blown with a difference in the gas flow rates from the two bottom-blowing plugs. As a refining process, a ladle refining method for molten steel accompanied by energization heating is used. In particular, in the first refining treatment, the main purpose is desulfurization treatment. The position of the plug of Patent Document 3 is the same as that of Patent Document 2, and the positions of the pot center to the plug of the pot having a diameter of 2784 to 3188 mm are 680 mm and 900 mm (both are described in Table 1 of Patent Document 3). are doing.

Japanese Unexamined Patent Publication No. 2001-040411 Japanese Unexamined Patent Publication No. 2011-214083 Japanese Unexamined Patent Publication No. 2011-214084

When desulfurizing the molten steel in the ladle by ladle refining of the molten steel accompanied by energization heating to produce ultra-low sulfur steel, the target is to perform the desulfurization treatment using the methods described in Patent Documents 1 to 3. It takes a long time to realize the ultra-low desulfurization of the molten steel, and there is a problem that the target ultra-low sulfurization cannot be achieved within a predetermined time. The present inventors have found that the methods described in Patent Documents 1 to 3 may take time to melt the added flux.

The present invention is to promote the melting of the added flux and efficiently utilize the flux when desulfurizing the molten steel in the ladle by the ladle refining of the molten steel accompanied by energization heating to produce ultra-low sulfur steel. It is an object of the present invention to provide a new and improved method for ladle refining of molten steel.

That is, the gist of the present invention is as follows.
(1) In a method for refining a ladle of molten steel in which a flux layer containing CaO is formed on the surface of the molten steel in the ladle, and two or three electrodes are immersed in the flux layer in the center of the ladle to energize.
Gas blowing plugs are arranged at two places on the bottom of the pan, and the gas flow rate of the gas blown from each of the gas blowing plugs is Q _B , the gas flow rate of the gas blowing B plug having the larger gas flow rate, and the other. The gas flow rate of the _A plug for gas blowing is QA (both units are NL / min / t).
In a plan view, a circle circumscribed around the outer circumferences of all the two or three electrodes and having a minimum radius r is defined as the "circumscribed circle of the electrodes", and the center position of the circumscribed circle of the electrodes is _CO and gas. The center position of the blowing A plug is CA, the center position of the gas blowing B plug is C _B , the distance between _CO and _CA is L _OA , the distance between _CO and _{C B} is L _OB , and C. Let θ be the angle formed by _A -C _O -C _B.
The gas blowing A plug and the gas blowing B plug are arranged at positions that satisfy the following equations (1) to (3).
A method for refining a ladle of molten steel, wherein Q _A and Q _B satisfy the following equations (4) to (6).
0 ≤ L _OA ≤ r (except when L _OA is 0) (1)
L _OB > r (2)
90 ° ≤ θ ≤ 180 ° (3)
2.33 ≤ Q _B / Q _A (4)
QB ≤ _4.50 (5)
0.20 ≤ Q _A (6)
(2) The method for ladle refining of molten steel according to (1) above, wherein the thickness of the flux layer converted into a molten state is 100 mm or more and 200 mm or less.
(3) The radius of the bottom of the ladle is R, and the center position of the gas blowing B plug is 0.1R or more from the wall surface of the ladle. The described method for ladle refining of molten steel.
(4) The method for refining a ladle of molten steel according to any one of (1) to (3) above, wherein the amount of molten steel in the ladle is 130 tons or more.
(5) The method for refining a ladle of molten steel according to any one of (1) to (3) above, wherein the amount of molten steel in the ladle is 270 tons or more.

In the present invention, when the molten steel in the ladle is desulfurized by the ladle refining of the molten steel accompanied by energization heating to produce ultra-low sulfur steel, gas blowing plugs are arranged at two places on the bottom of the ladle, which is appropriate. By blowing an appropriate amount of gas from a suitable gas blowing position, it is possible to promote the slagging (melting) of the flux during energization, and the efficiency of flux utilization can be improved.

It is a figure which shows the ladle refining method of this invention, (A) is a plan view, (B) is a sectional view taken along the line BB. It is a figure which shows the flow of the surface of a ladle when the gas flow rate _QA is small. It is a figure which shows the cross section of a ladle when a gas flow rate Q _B is excessive. It is a figure which shows the flow of the surface of a ladle when the gas flow rate ratio Q _B / Q _A is too small. It is a figure which shows the flow of the surface of a ladle at a suitable gas flow rate of this invention.

As shown in FIG. 1, 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 are formed in the center of the ladle. Molten steel desulfurization is performed by immersing 3 in the flux layer 6 and energizing it. 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 (A), in the plan view of the ladle 1, the circle circumscribing the outer periphery of all of the two or three electrodes 3 is referred to as the “electrode circumscribed circle 4 of the electrodes” as described above. When there are three electrodes 3 (see FIG. 1A), 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 of the circumscribed circle of the two or three electrodes is in the range of 0.1 × R or less from the center of the bottom of the ladle (radius R). There is.
Further, in the present invention, as shown in FIG. 1 (B), the 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.

In the ladle refining, a flux layer containing CaO is formed on the surface of the molten steel in the ladle, the flux layer is melted, and the S contained in the molten steel reacts with the components in the flux and moves into the flux layer. , The desulfurization reaction proceeds. The desulfurization reaction proceeds at the interface between the molten steel and the flux layer. In order to rapidly desulfurize the entire molten steel in the ladle, it is important to sufficiently dissolve the flux layer in the entire area in the plane direction of the ladle.
In the present invention, in particular, the electrode is immersed in the flux layer to perform energization heating, and the flux located in the vicinity of the electrode is preferentially heated to a high temperature to become a molten flux. The flux existing at a place away from the electrode (for example, near the inner wall of the ladle) remains relatively low temperature, and a temperature difference of the flux occurs in the plane direction inside the ladle. In order to achieve the refining purpose of the ladle refining, it is preferable that the flux is melted. Therefore, it is preferable that the flux on the surface of the molten steel is melted in a wide range (including the entire area in the plane direction in the ladle). As long as the flux exchange between the vicinity of the electrode and the position away from the electrode is small, it takes a certain amount of time to secure the melting of the flux over a wide range, so it is necessary to extend the refining processing time, and the refining processing time. The time to proceed with the refining reaction that is part of is limited.

As a method of quickly melting the flux, the area of the flux layer 6 on the surface of the ladle that is energized and heated by controlling the introduction position and the flow rate of the stirring gas blown from the gas blowing plug 2 at the bottom of the ladle. Alternatively, in the area immediately after energization that has been energized and heated, the flux is higher than in other regions, but by promoting mixed replacement of the high-temperature flux and the low-temperature flux in other regions, the entire flux on the surface of the molten steel can be removed. The present inventors paid attention to melting.

In order to obtain the effect of realizing faster slagging and refining reaction by spreading the flux heated by energization near the electrode over the entire surface of the molten steel, the gas blowing plug placed at the bottom of the ladle is used. Provide at least two places. Then, in the arrangement position of the gas blowing plug in the plan view, one of the two places is arranged in the vicinity directly under the electrode, and the other is provided on the peripheral edge away from the electrode, and from each gas blowing plug. It is necessary to control the gas flow rate. Specifically, in a plan view, the gas blowing B plug 2B having the larger gas flow rate is arranged outside the circumscribed circle 4 of the electrode, and the gas blowing A plug 2A on the side having the smaller gas flow rate is placed in the circumscribed circle of the electrode. Place it inside 4. That is, as shown in FIG. 1 (A), the radius of the circumscribed circle 4 of the electrode is r, the center position of the circumscribed circle of the electrode is _CO , the center position of the gas blowing _A plug is CA, and the gas blowing B plug. The center position of is C _B , the distance between _{CO and CA is L OA, the distance between C O and C B is L OB ,} and the placement positions of A plug 2A for gas blowing and B plug 2B for gas blowing are as follows. A position that satisfies the equations (1) and (2).
0 ≤ L _OA ≤ r (1)
L _OB > r (2)
Further, the center position of the circumscribed circle 4 of the electrode is set to _CO , and the angle formed by CA _{- CO -} CB is set to θ, and the position satisfying the following equation (3). The angles formed by CA _{- CO -} CB include an angle of 180 ° or less and an angle of more than 180 ° on the opposite side, of which the angle of 180 ° or less is referred to as the angle θ here. do.
90 ° ≤ θ ≤ 180 ° (3)

Further, when the gas flow rate of the gas blowing _B plug having the larger gas flow rate is QB and the gas flow rate of the other gas blowing _A plug is QA (both units are NL / min / t), _QA . , _QB has a suitable range.

First, when the agitated gas flow rate QA introduced from the gas blowing _A plug located inside the circumscribed circle of the electrode is less than 0.20 (NL / min / t), as shown in FIG. 2, with respect to the flux. Therefore, sufficient stirring power cannot be given. In FIG. 2, the flux layer is color-coded into three regions. The white flux layer X region 21X is a region where the temperature is relatively low, the irregularly hatched flux layer Z region 21Z is a region where the temperature is higher than the flux layer X region, and the dot-hatched flux layer Y region 21Y is an electrode. It is the region with the highest temperature heated by. Further, the bubble rising region 22 is shown in the figure. Specifically, the flow direction of the high-temperature flux (flux layer Y region 21Y) tends to be fixed in one direction by gas blowing with the gas blowing B plug 2B located outside the circumscribed circle 4 of the electrode. , High temperature flux (flux layer Y region 21Y) tends to be unevenly distributed locally on the inner wall of the pot. Therefore, the effect of dispersing the high-temperature flux by the stirring gas in the gas blowing A plug 2A cannot be obtained, and the unmelted flux 15 may adhere to the ladle wall surface 12. Therefore, it was decided to specify the following equation (6).
0.20 ≤ Q _A (6)

Next, if the agitated gas flow rate QB introduced from the gas blowing _B plug 2B exceeds 4.50 (NL / min / t), the high-temperature flux on the molten steel surface causes uneven distribution on the inner wall of the ladle. The temperature of the inner wall of the ladle is lower than that of the electrode energized area, and as shown in FIG. 3, the flux is unmelted and unstained, so that the unmelted flux 15 is generated on the ladle wall surface 12. Therefore, it was decided to specify the following equation (5).
QB ≤ _4.50 (5)

Further, when the ratio Q _B / Q _A of the agitated gas flow rate is less than 2.33, the flow from the gas blowing A plug 2A and the flow from the gas blowing B plug 2B antagonize each other, and the high temperature flux (flux layer Y) The substitution of the region 21Y) does not occur sufficiently (see FIG. 4). Therefore, it was decided to specify the following equation (4).
2.33 ≤ Q _B / Q _A (4)

As described above, in the present invention, the flow rate of the agitated gas during energization is defined as the equations (4) to (6). As a result, as shown in FIG. 5, the flux sufficiently melted at a high temperature near the electrode spreads over the entire area in the plane direction in the ladle, and the flux layer 6 in the entire area is in a molten state, which promotes the ladle refining reaction. Can be realized.

From equations (4) to (6), the upper limit of Q _B / Q _A , the upper limit of Q _A , and the lower limit of Q _B are automatically determined. That is, the upper limit of Q _B / Q _A is 22.5 calculated from the lower limit of Q _A and the upper limit of Q _B. Further, since the upper limit value of Q _A is 2.33 × Q _A ≦ Q _B ≦ 4.50 from the equation (1), Q _A ≦ 1.93. Further, the lower limit of Q _B is 2.33 × Q _A ≦ Q _B from the equation (1), and 0.20 ≦ Q _A , so 0.466 ≦ Q _B.

As described above, in the present invention, the gas blowing for stirring the molten steel is performed by blowing from the gas blowing plug provided at the bottom of the ladle. The flux on the surface of the molten steel is moved over the entire surface of the molten steel by blowing gas from the two gas blowing plugs 2. As for the gas blowing into the molten steel, a method of immersing the immersion lance from the surface of the molten steel and blowing the gas from the tip of the immersion lance is also known. However, when the immersion lance is used for injecting gas in the present invention, the lance that hinders the movement of the flux exists on the surface of the molten steel, and the effect of the present invention cannot be obtained. Therefore, as described above, it is stipulated that the gas blowing plug used at the bottom of the ladle is used for gas blowing.

In the present invention, a flux layer containing CaO is formed on the surface of the molten steel in the ladle, and the molten steel is desulfurized by the flux layer. The CaO component in the flux has desulfurization ability. When the CaO content in the flux layer is 25% by mass or more, desulfurization can be preferably performed. It is more preferably 30% or more.

In the present invention, the thickness of the flux layer formed on the surface of the molten steel in the ladle (thickness converted into a molten state; hereinafter, also referred to as converted thickness) is preferably 100 mm or more and 200 mm or less. If the converted thickness of the flux layer is less than 100 mm, it is difficult to obtain positive convection in the flux layer, and it is difficult to obtain desulfurization promotion by promoting convection in the flux layer, which is a feature of the present invention.
On the other hand, when the converted thickness of the flux exceeds 200 mm, a temperature difference is likely to occur in the thickness direction of the flux layer even if the flux layer is convected, which causes the generation of an unstained (unmelted) portion of the surface layer of the flux layer. There is a possibility.
Therefore, in the present invention, it is preferable that the converted thickness of the flux layer is 100 mm or more and 200 mm or less. In order to make the equivalent thickness of the flux layer 100 mm or more and 200 mm or less, the required flux input amount can be calculated from the flux volume calculated from the dimensions of the pan and the flux density in the assumed flux composition. The flux input amount may be determined based on the correlation (past results) between the input amount and the flux thickness (premise of total melting), or the input amount may be adjusted while measuring the flux thickness.

Here, the thickness of the flux layer is difficult to measure during the refining process. Therefore, the thickness of the flux layer specified here is the thickness of the flux layer after the refining treatment and in a state where the gas blowing into the molten steel is stopped and the molten steel is allowed to stand still. This flux layer thickness can be obtained, for example, by inserting a steel rod into a ladle and then measuring the thickness (melt thickness) of the flux layer attached to the steel rod pulled out from the ladle. If the measurement is performed in a state where all the flux is melted (a state in which there is no unpolluted flux), a value equivalent to the converted thickness can be measured. Even if refining treatment such as desulfurization treatment is performed, the thickness of the flux layer after the flux is added does not substantially change before and after refining.

In the present invention, the radius of the bottom of the ladle is preferably R, and the center position CB of the gas blowing _B plug has a distance of 0.1 R or more from the ladle wall surface 12. When the distance between the ladle wall surface 12 and the center position C _B of the gas blowing B plug is less than 0.1R, the ladle wall surface and the plug are in close proximity, and a part of the blown gas comes into contact with the wall surface. However, the effect of the present invention cannot be obtained at the maximum because it floats (because the gas blows through between the flux and the wall surface). Therefore, in the present invention, the distance between the wall surface of the ladle and the center position of the gas blowing B plug is preferably 0.1 R or more. If the bottom surface of the ladle is not a perfect circle, the radius of a corresponding circle having the same area as the bottom surface of the ladle may be adopted as R. Further, when the gas blowing portion of the plug is not in the center of the plug body, the center of the gas blowing portion may be regarded as the center of the plug. If the plug body or the gas blowout portion of the plug is not a perfect circle, the center of gravity of the horizontal cross section of the gas blowout portion may be regarded as the center of the plug.

In the present invention, the amount of molten steel in the ladle is preferably 130 tons or more. More preferably, it is 270 tons or more. As the amount of molten steel in the ladle increases, the opening area of the ladle increases and the amount of flux used for refining also increases, so that the melting time of the flux becomes longer. According to the present invention, it is possible to appropriately control the circulation of the flux on the molten steel surface, circulate the high temperature molten flux in the vicinity of the electrode over the entire bath surface, and supply the low temperature flux in the vicinity of the electrode for heating. It is possible to shorten the melting time of the flux. According to the findings of the present inventors, it is preferable that the amount of molten steel in the ladle is 130 tons or more, further 270 tons or more, because the effect of shortening the flux melting time as compared with the conventional method becomes more remarkable.

The following shows the results of verifying the effectiveness of the desulfurization method for molten steel by ladle refining of the present invention.

[Example 1]
In Example 1, an energization heating type molten steel desulfurization treatment was performed (see FIG. 1). The region of the bottom of the ladle that comes into contact with the molten steel is a circle with a radius R of 1.4 m (the radius is approximately 1.5 m at the surface position of the molten steel). Three electrodes 3 for energization heating were arranged. The three electrodes 3 are arranged at the apex positions of an equilateral triangle. The circle circumscribed around the outer circumferences of all three electrodes 3 is the circumscribed circle 4 of the electrodes. The center position _CO of the circumscribed circle of the electrode is the same position as the center of the bottom of the pan in a plan view, and the radius r of the circumscribed circle 4 of the electrode is 0.6 m.

The agitated gas was blown into the molten steel 5 in the ladle 1 through the gas blowing plug 2 provided at the bottom of the ladle or the blowing lance (not shown) inserted from above the ladle. When using the gas blowing plug 2, two places (gas blowing A plug 2A and gas blowing B plug 2B) were installed at the bottom of the ladle 1. When using a blow-in lance, two lances were inserted into the molten steel from above the ladle. Each lance was provided with four gas introduction holes horizontally (perpendicular to the lance insertion direction). The center of the lance was considered to be equivalent to the center of the plug, and the position of the lance was determined. The distance (immersion depth) from the steel bath surface to the tip of the lance was set to 75% of the bath depth of the steel bath.

In the plan view, regarding the positional relationship between the center position CA of the gas blowing _A plug and the center position C _B of the gas blowing _B plug at each level, the distance between _CO and CA is L _OA , _CO and C. The distance between _B was L _OB , the angle formed by CA- _{CO - CB} was θ, and the values shown in Table 1 were set respectively. Even when a blow lance is used, the expression is similar to that of a gas blow plug.

110 to 120 tons of crude molten steel was housed in a pan 1 as a desulfurization target, and flux was poured into the upper portion of the crude molten steel to form a flux layer 6. The charged flux is mainly CaO and contains Al ₂ O ₃ , SiO ₂ and the like. Then, the electrode 3 was immersed in the flux layer 6 to start energization, and gas stirring was started. As for the amount of flux input, the thickness (converted thickness) of the flux layer 6 in terms of molten flux is shown in the table.

In order to confirm the molten state of the flux, the flux was applied and then energized for 10 minutes, and then the thickness of the molten flux was measured at three points about 100 mm away from the inner wall of the pot. The melt flux thickness was determined by immersing a thin iron rod in the molten steel, pulling up the thin rod after the molten steel immersion part of the thin rod had melted, and determining the thickness of the molten flux based on the length of the molten flux adhering to the thin rod. .. The average value of the measured melt flux thicknesses at the three locations was calculated and used as the "average melt flux thickness". In Comparative Example 7, the melt flux thickness was measured after stirring for 10 minutes without energization.

For each example, the molten flux thickness was calculated on the assumption that the total amount of the added flux was melted, and used as the "converted total flux thickness". The average molten flux thickness measured above was divided by the converted total flux thickness and displayed as%, and used as an index of the molten state (hereinafter referred to as a molten flux index (%)). The examples of the present invention and comparative examples shown in Table 1 below were evaluated using the above-mentioned melt flux index. In Table 1, numerical values and items outside the scope of the present invention are underlined.

When the melt flux index was 95% or more, it was evaluated as ☆, when it was 90% or more, it was evaluated as ⊚, when it was 80% or more, it was evaluated as ◯, and when it was less than 80%, it was evaluated as ×. Among the comparative examples, the one having the highest melt flux index was Comparative Example 1, in which the melt flux index was 80%. Therefore, the above threshold value (80%) was determined on the premise that the condition that the melting could be promoted was passed as compared with Comparative Example 1. When the melt flux index is less than 80%, it is confirmed that the efficiency of the subsequent desulfurization treatment is insufficient.
In addition, no significant deterioration in electrical conductivity was observed at a small gas flow rate as in the examples of the present invention.

Even if the treatment was performed under the above-mentioned gas blowing conditions after the energization was completed with the start of the treatment as the end of energization, the same tendency was obtained for the improvement of the melting state of the flux.

[Example 2]
Using the gas blowing conditions and plug conditions of Example 7 of the present invention and Comparative Example 1 shown in Table 1 of Example 1, “Example A” and “Comparative Example B” are used, respectively, and the amount of molten steel is 60 to 80 t and 130 t. Under the three conditions of 270t, the energization heating type molten steel desulfurization treatment was performed.
The regions of the bottom of the ladle that come into contact with the molten steel are circular with radii R of 1.2, 1.4, and 1.8 m (radius is approximately 1.3, 1.6, 2.0 m at the surface position of the molten steel). .. Three electrodes 3 for energization heating were arranged. The three electrodes 3 are arranged at the apex positions of an equilateral triangle. The circle circumscribed around the outer circumferences of all three electrodes 3 is the circumscribed circle 4 of the electrodes. The center position _CO of the circumscribed circle of the electrode is the same position as the center of the bottom of the pan in a plan view, and the radii r of the circumscribed circle 4 of the electrode are 0.5, 0.6, and 0.8 m, respectively. be.

The same energization and gas blowing treatment as in Example 1 was performed, and after performing the energization treatment for 10 minutes from the start of flux addition, the melt flux thickness was measured and the average melt flux thickness was calculated. The converted total flux thickness was calculated in the same manner as in Example 1, and the molten flux index was obtained. The treatment was performed 3 to 6 times for each of the three types of molten steel, and the average value, the minimum value, and the maximum value of the molten flux index were obtained. The average value of the molten flux index of Comparative Example B is set to the reference value 1.00, and the maximum and minimum values of Comparative Example B and the average value, maximum value, and minimum value of Example A of the present invention are shown as ratios to the reference value. Shown in 2.

In the comparison of the molten flux index, it is shown that the melting of Example A of the present invention is more advanced than that of Comparative Example B under any of the conditions of the amount of molten steel, and it can be seen that the effect of the present invention is obtained. Furthermore, comparing the improvement effect of Example A of the present invention with respect to Comparative Example B for each amount of molten steel in the ladle, it can be read as follows.
That is, first, when the amount of molten steel is 60 to 80 tons, the average value of the molten flux index is improved, but the effect may be unclear when compared within the variation range. Secondly, the amount of molten steel 130t is improved on average. Differences can be seen even when compared within the range of variation. Thirdly, at the molten steel amount of 270 tons, the average value is improved. The improvement effect can be clearly seen in the comparison of the variation range.
As described above, it has been clarified that when the amount of molten steel in the ladle is 130 tons or more and further 270 tons or more, the effect of promoting flux melting as compared with the conventional method becomes more remarkable.

1 Ladle 2 Gas blowing plug (plug)
2A A plug for gas blowing 2B B plug for gas blowing 3 Electrode 4 Electrode circumscribed circle 5 Molten steel 6 Flux layer 7 Bubbles 8 Upflow 9 Transverse flow 11 Molten steel surface 12 Take pot wall surface 15 Unmelted flux 21X Flux layer X region 21Y Flux layer Y region 21Z Flux layer Z region 22 Bubble rise region r Radius of the circumscribed circle of the electrode C _O Center position of the circumscribed circle of the electrode C _A Center position of the gas blowing A plug C _B Center position of the gas blowing B plug θ Angle formed by CA _{- CO -} CB

Claims (5)

In the ladle refining method of molten steel, in which a flux layer containing CaO is formed on the surface of the molten steel in the ladle and two or three electrodes are immersed in the flux layer in the center of the ladle to energize.
Gas blowing plugs are arranged at two places on the bottom of the pan, and the gas flow rate of the gas blown from each of the gas blowing plugs is Q _B , the gas flow rate of the gas blowing B plug having the larger gas flow rate, and the other. The gas flow rate of the _A plug for gas blowing is QA (both units are NL / min / t).
In a plan view, a circle circumscribed around the outer circumferences of all the two or three electrodes and having a minimum radius r is defined as the "circumscribed circle of the electrodes", and the center position of the circumscribed circle of the electrodes is _CO and gas. The center position of the blowing A plug is CA, the center position of the gas blowing B plug is C _B , the distance between _CO and _CA is L _OA , the distance between _CO and _{C B} is L _OB , and C. Let θ be the angle formed by _A -C _O -C _B.
The gas blowing A plug and the gas blowing B plug are arranged at positions that satisfy the following equations (1) to (3).
A method for refining a ladle of molten steel, wherein Q _A and Q _B satisfy the following equations (4) to (6).
0 ≤ L _OA ≤ r (except when L _OA is 0) (1)
L _OB > r (2)
90 ° ≤ θ ≤ 180 ° (3)
2.33 ≤ Q _B / Q _A (4)
QB ≤ _4.50 (5)
0.20 ≤ Q _A (6) The method for ladle refining of molten steel according to claim 1, wherein the thickness of the flux layer converted into a molten state is 100 mm or more and 200 mm or less. The molten steel according to claim 1 or 2, wherein the radius of the bottom of the ladle is R, and the center position of the gas blowing B plug is 0.1R or more from the wall surface of the ladle. Ladle refining method. The method for refining a ladle of molten steel according to any one of claims 1 to 3, wherein the amount of molten steel in the ladle is 130 tons or more. The method for refining a ladle of molten steel according to any one of claims 1 to 3, wherein the amount of molten steel in the ladle is 270 tons or more.

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