JP2019210505A - Ladle-refining method for molten steel - Google Patents

Abstract

To provide a new improved ladle-refining method for a molten steel capable of promoting fusion of an added flux and making efficient use of the flux in the production of an extremely-low sulfur steel by ladle-refining and desulfurization of the molten steel in the ladle accompanying electric heating.SOLUTION: A ladle-refining method for a molten steel includes forming a flux layer 6 containing CaO on the surface of the molten steel 5 in a ladle 1 to turn on electricity by immersing two or three electrodes 3 in the flux layer 6. The plug 2 for gas blowing is arranged at two places of the bottom of the ladle 1, that is, arranged at one place (a center location C) in the inside of an electrode circumcircle 4 circumscribed on all outer periphery of two or three electrodes and at another one place (a center location C) in the outside. The angle θ formed by the center C-Cof C-circumcircle is made to be 90° to 180°. Further, the gas flow rates (Q, Q) from the plugs 2A, 2B for gas blowing are made to be in the suitable ranges of the following expressions: 2.33≤Q/Q... (4), Q≤4.50 ... (5) and 0.20≤Q... (6).SELECTED DRAWING: Figure 1

Description

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

  Molten steel primarily refined in a converter or electric furnace is stored in a ladle. Furthermore, as secondary refining, refining is performed on molten steel in the ladle. The molten steel after the secondary refining is cast mainly by continuous casting, and further, hot rolling or the like is performed to produce the target product. Secondary refining is performed for the purpose of adjusting the components of molten steel, flotation and separation of non-metallic inclusions, uniform mixing of molten steel, heating and heating of molten steel, and reduction of harmful impurities in molten steel according to the target quality of the product. In order to reduce the sulfur (S) concentration in the molten steel, molten steel desulfurization is performed as secondary refining.

  As one of the methods of secondary refining, 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 a gas into the molten steel while immersing the energizing electrode in the flux layer and energizing it. Secondary refining methods are known. The molten steel can be desulfurized using the flux layer as a desulfurizing agent, and the molten steel can be heated by electric heating. Hereinafter, this method is referred to as a ladle refining method for molten steel with energization heating.

With regard to ladle refining of molten steel with current heating, Patent Literature 1 discloses a current heating type refining ladle. Three electrodes made of graphite are arranged on the lid covering the top of the ladle, and a porous plug for gas blowing is arranged at the bottom of the ladle. The three electrodes are arranged 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 is energized by a power supply device to heat the flux and molten steel. In the case of using two or three electrodes, a circle circumscribing the outer periphery of all the two or three electrodes is referred to as an “electrode circumcircle”.
Further, an inert gas is fed into the molten steel from the porous plug, and the molten steel is agitated. Conventionally, there was only one porous plug arranged at the bottom of the ladle. However, 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 arranged at the bottom of the ladle. Are arranged unevenly on one side. An inert gas having the same flow rate is blown into the molten steel from the plurality of porous plugs.

  Patent Document 2 discloses a ladle refining method that can be applied to ladle refining of molten steel with energization heating, and two bottom blowing plugs are arranged at the bottom of the ladle. Is a method in which an inert gas is blown with a difference in gas flow rate from the gas, and a technique for 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. Moreover, about the plug, although the position of a pan center-plug is 680 mm and the position of 900 mm about a pot with a diameter of 2784-3188 mm (all are described in Table 1 of patent document 2), it uses several electrodes. When energization is performed and the center of the circumscribed circle of the plurality of electrodes coincides with the center of the pan bottom, the position of the plug is often outside the circumscribed circle of the electrode.

  Patent Document 3 discloses a method for producing a high cleanliness steel in which a first refining process, a degassing process, and a second refining process are performed in this order on the molten steel that has been produced. In the refining process and the degassing process, the inert gas is blown from the two bottom blowing plugs, and the inert gas is blown with a difference in the gas flow rate from the two bottom blowing plugs. As a refining process, a ladle refining method for molten steel with energization heating is used. Particularly in the first refining process, the main purpose is desulfurization. In addition, the position of the plug of patent document 3 describes the position where the position of the pan center to the plug is 680 mm and 900 mm (both are described in Table 1 of patent document 3) for the pot having a diameter of 2784 to 3188 mm as in the above-mentioned patent document 2. doing.

Japanese Patent Laid-Open No. 2001-040411 JP 2011-214083 A JP 2011-214084 A

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

  The present invention promotes the melting of the added flux and efficiently utilizes the flux when producing ultra-low sulfur steel by desulfurizing the molten steel in the ladle by ladle refining of the molten steel with current heating. It is an object of the present invention to provide a new and improved ladle refining method for molten steel that can be used.

That is, the gist of the present invention is as follows.
(1) In a ladle refining method for molten steel, 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 at the center of the ladle and energized.
Gas blowing plugs are arranged at two locations on the bottom of the ladle. Regarding the flow rate of gas blown from each of the gas blowing plugs, the gas flow rate of the gas blowing B plug with the larger gas flow rate is Q _B , The gas flow rate of the A plug for gas injection is Q _A (both units are NL / min / t),
In plan view, a circle circumscribing the outer periphery of all the two or three electrodes is referred to as an “electrode circumcircle”, the minimum radius of the circumcircle of the electrode is r, and the center position of the circumcircle of the electrode is C _O , The center position of the A plug for gas blowing is C _A , the center position of the B plug for gas blowing is C _B , the distance between C _O and C _A is L _OA , and the distance between C _O and C _B is L _OB , The angle formed by C _A -C _O -C _B is θ,
The gas blowing A plug and the gas blowing B plug are arranged at positions satisfying the following formulas (1) to (3),
_A ladle refining method for molten steel, wherein Q _A and Q _B satisfy the following expressions (4) to (6):
0 ≦ L _OA ≦ r (1)
L _OB > r (2)
90 ° ≦ θ ≦ 180 ° (3)
2.33 ≦ Q _B / Q _A (4)
Q _B ≦ 4.50 (5)
0.20 ≦ Q _A (6)
(2) The ladle refining method for molten steel according to (1) above, wherein a thickness of the flux layer converted into a molten state is set to 100 mm or more and 200 mm or less.
(3) In the above (1) or (2), the radius of the bottom of the ladle is R, and the center position of the B plug for gas blowing is 0.1R or more from the ladle wall surface. The ladle refining method of the molten steel as described.
(4) The molten steel ladle refining method according to any one of (1) to (3) above, wherein the amount of molten steel in the ladle is 130 t or more.
(5) The molten steel ladle refining method according to any one of (1) to (3) above, wherein the amount of molten steel in the ladle is 270 t or more.

  The present invention, when producing ultra-low-sulfur steel by desulfurizing the molten steel in the ladle by ladle refining with current heating, arranges gas blowing plugs at two locations on the bottom of the ladle, By blowing an appropriate amount of gas from the appropriate gas blowing position, hatching (melting) of the flux during energization can be promoted, and the utilization efficiency of the flux can be improved.

It is a figure shown about the ladle refining method of this invention, (A) is a top view, (B) is BB arrow sectional drawing. It is a diagram showing the flow of ladle surface when the gas flow rate Q _A is low. Gas flow rate Q _B is a diagram showing a ladle cross section when it is excessive. Is a diagram showing the flow of ladle surface when the gas flow rate Q _B / Q _A is too small. It is a figure which shows the flow of the ladle surface in the suitable gas flow rate of this invention.

As shown in FIG. 1, in the ladle refining method of the molten steel of the present invention, a flux layer 6 containing CaO is formed on the surface of the molten steel 5 in the ladle 1, and two or three electrodes are formed in the center of the ladle. 3 is immersed in the flux layer 6 and energized to perform molten steel desulfurization. As for the electric heating, a usual method can be used. That is, the flux and molten steel are heated by immersing the lower tip of the electrode 3 disposed in the upper portion of the ladle in the flux layer 6 and energizing the electrode 3. As shown in FIG. 1A, in the plan view of the ladle 1, a circle circumscribing the outer periphery of all of the two or three electrodes 3 is referred to as “electrode circumcircle 4” as described above. When there are three electrodes 3 (see FIG. 1A), the circumscribed circle 4 of the electrodes is determined as one. When there are two electrodes 3, the circumscribed circle 4 of the electrode has the smallest radius. 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”.
The reason why the electrode 3 is arranged at the center of the ladle as described above is that 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 within 0.1 × R from the center of the bottom of the ladle (radius R). Yes.
Moreover, in this invention, as shown in FIG.1 (B), the plug 2 for gas blowing is arrange | positioned in two places at the bottom part of the ladle 1, and inactive gas is blown in into molten steel from each of the said plug 2 for gas blowing. To stir the molten steel in the ladle.

In 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 contained S 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 quickly proceed with desulfurization of 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.
Especially in this invention, an electrode is immersed in a flux layer and it heats by electricity, The flux located in the vicinity of an electrode is heated preferentially to high temperature, and turns into a molten flux. The flux that exists at a location away from the electrode (for example, in the vicinity of the inner wall of the ladle) remains at a relatively low temperature, and a temperature difference of the flux occurs in the plane direction in the ladle. In order to achieve the refining purpose of the ladle refining, it is preferable that the flux is melted. Therefore, the flux on the surface of the molten steel is preferably melted in a wide range (including the entire region in the ladle plane direction). As long as there is little exchange of flux between the vicinity of the electrode and the position away from the electrode, a certain amount of time is required to ensure the melting of a wide range of flux, so it is necessary to extend the refining process time. The time to advance the refining reaction that is a part of is limited.

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

In order to achieve the effect of achieving faster hatching and refining reaction by spreading the flux heated by the electric current in the vicinity of the electrode over the entire surface of the molten steel, a gas blowing plug placed at the bottom of the ladle is used. Provided in at least two places. Then, in the arrangement position of the gas blowing plug in a plan view, one of the two locations is arranged in the vicinity immediately below the electrode, and the other is provided at the peripheral edge away from the electrode, and from each gas blowing plug It is necessary to control the gas flow rate. Specifically, in plan view, the gas blowing B plug 2B having the larger gas flow rate is disposed outside the circumscribed circle 4 of the electrode, and the gas blowing A plug 2A having the smaller gas flow rate is disposed on the circumscribed circle of the electrode. 4 is arranged inside. That is, as shown in FIG. 1A, the radius of the circumscribed circle 4 of the electrode is r, the center position of the circumscribed circle of the electrode is C _O , the center position of the gas blowing A plug is C _A , and the gas blowing B plug is The center position of C is C _B , the distance between C _O and C _A is L _OA , the distance between C _O and C _B is L _OB , and the gas injection A plug 2A and the gas injection B plug 2B are arranged as follows. The position satisfies the expressions (1) and (2).
0 ≦ L _OA ≦ r (1)
L _OB > r (2)
Further, the center position of the circumscribed circle 4 electrodes and C _O, the angle formed by the C _A -C _O -C _B θ, the position that satisfies the following formula (3). The angle formed by C _A -C _O -C _B includes an angle of 180 ° or less and an angle exceeding 180 ° on the opposite side. Of these, an angle of 180 ° or less is referred to as angle θ here. To do.
90 ° ≦ θ ≦ 180 ° (3)

Further, when the gas flow rate of the gas blowing B plug with the larger gas flow rate is Q _B , and the gas flow rate of the other gas blowing A plug is Q _A (both units are NL / min / t), Q _A , Q _B has a preferable range.

First, when the stirring gas flow rate Q _A introduced from the A plug for gas blowing located inside the circumscribed circle of the electrode is less than 0.20 (NL / min / t), as shown in FIG. 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 This is the region where the temperature is highest when heated at. Further, the bubble rising region 22 is illustrated. Specifically, the flow direction of the high-temperature flux (flux layer Y region 21Y) tends to be fixed in one direction by gas blowing in the gas blowing B plug 2B located outside the circumscribed circle 4 of the electrode. The high temperature flux (flux layer Y region 21Y) tends to be unevenly distributed locally on the inner wall of the pan. Therefore, the effect of dispersing the high-temperature flux due to the stirring gas in the gas blowing A plug 2 </ b> A cannot be obtained, and the unmelted flux 15 may adhere to the ladle wall surface 12. Therefore, the following equation (6) is defined.
0.20 ≦ Q _A (6)

Next, if the stirring gas flow rate Q _B introduced from the gas blowing B plug 2B exceeds 4.50 (NL / min / t), the high-temperature flux on the molten steel surface becomes unevenly distributed on the ladle inner wall. The ladle inner wall has a temperature lower than that of the electrode energization area, and as shown in FIG. 3, the flux is unmelted / undagulated, so that an unmelted flux 15 is generated on the ladle wall surface 12. Therefore, the following equation (5) is defined.
Q _B ≦ 4.50 (5)

Furthermore, when the ratio Q _B / Q _{A of} the stirring 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 are antagonized, and the high temperature flux (flux layer Y Substitution of the region 21Y) does not occur sufficiently (see FIG. 4). Therefore, the following equation (4) is defined.
2.33 ≦ Q _B / Q _A (4)

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

Note that the upper limit value of Q _B / Q _A , the upper limit value of Q _A , and the lower limit value of Q _B are automatically determined from the equations (4) to (6). That is, the upper limit value of Q _B / Q _A is 22.5 calculated from the lower limit value, the upper limit of the Q _B of Q _A. 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. Furthermore, since the lower limit value of Q _B is 2.33 × Q _A ≦ Q _B and 0.20 ≦ Q _{A according} to the equation (1), 0.466 ≦ Q _B is satisfied.

  As described above, in the present invention, gas blowing for stirring molten steel is performed by blowing from a 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 plugs 2 for blowing gas. As for gas blowing into molten steel, a method is also known in which a dipping lance is dipped from the surface of the molten steel and gas is blown from the tip of the dipping lance. However, when an immersion lance is used for gas blowing in the present invention, there will be a lance that prevents flux movement on the surface of the molten steel, and the effects of the present invention cannot be obtained. Therefore, as described above, the gas blowing is defined as using a gas blowing plug provided at the bottom of the ladle.

  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 a desulfurization ability. If the CaO content in the flux layer is 25% by mass or more, desulfurization can be suitably performed. More preferably, it is 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 to a molten state; hereinafter also referred to as converted thickness) is preferably 100 mm or more and 200 mm or less. When 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 due to convection promotion 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 convection, which causes the occurrence of an unflanged (unmelted) portion of the surface 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 set the converted thickness of the flux layer to 100 mm or more and 200 mm or less, the flux volume calculated from the dimensions of the ladle and the required flux input amount from the flux density at the assumed flux composition, The flux input amount may be determined based on the correlation (past performance) between the input amount and the flux thickness (all melting premise), or the input amount may be adjusted while measuring the flux thickness.

  Here, it is difficult to measure the thickness of the flux layer during the refining process. For this reason, the thickness of the flux layer defined here is the thickness of the flux layer after the refining process, in a state where the gas blowing into the molten steel is stopped and the molten steel is left still. This flux layer thickness can be obtained, for example, by inserting a steel rod into the ladle and then measuring the thickness (melted thickness) of the flux layer attached to the steel rod pulled out from the ladle. If the measurement is performed in the state where the flux is completely melted (the state in which there is no unhatched flux), a value corresponding to the converted thickness can be measured. Even if a refining process such as a desulfurization process is performed, the thickness of the flux layer after feeding the flux does not change substantially before and after the refining.

Preferably, in the present invention, the radius of the bottom of the ladle is R, and the center position C _B of the B plug for gas blowing is 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 close to each other, 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 the gas floats between the flux and the wall surface. Therefore, in the present invention, the distance between the ladle wall surface and the central position of the gas blowing B plug is preferably 0.1 R or more. In addition, what is necessary is just to employ | adopt the radius of the equivalent circle which has the same area as the bottom face of a ladle as R, when the bottom face of a ladle is not a perfect circle. Further, when the gas blowing portion of the plug is not at the center of the plug body, the center of the gas blowing portion may be regarded as the center of the plug. When the plug body or the gas blowing portion of the plug is not a perfect circle, the center of gravity of the horizontal section of the gas blowing 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 t or more. More preferably, it is set to 270 t or more. As the amount of molten steel in the ladle increases, the ladle opening area increases and the amount of flux used for scouring also increases, so 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 to be heated, The melting time of the flux can be shortened. According to the knowledge of the present inventors, the amount of molten steel in the ladle is preferably 130 t or more, and more preferably 270 t or more, since the effect of shortening the flux melting time when compared with the conventional method becomes more remarkable.

  Hereinafter, the result verified about the effectiveness of the desulfurization method of the molten steel by the ladle refining of this invention is shown.

[Example 1]
In the present Example 1, the electric heating type molten steel desulfurization process was performed (refer FIG. 1). The region in contact with the molten steel at the bottom of the ladle is a circle having a radius R of 1.4 m (approximately 1.5 m at the surface of the molten steel). Three electrodes 3 for electric heating were arranged. The three electrodes 3 are arranged at the vertex positions of the equilateral triangle. A circle circumscribing the outer periphery of all three electrodes 3 is a circumcircle 4 of the electrodes. The center position C _O of the circumscribed circle of the electrode is the same position as the center of the ladle bottom in plan view, and the radius r of the circumscribed circle 4 of the electrode is 0.6 m.

  The stirring gas was blown into the molten steel 5 in the ladle 1 via a gas blowing plug 2 provided at the bottom of the ladle or a blowing lance (not shown) inserted from above the ladle. When the gas blowing plug 2 was used, two places (a gas blowing A plug 2A and a gas blowing B plug 2B) were installed at the bottom of the ladle 1. When using a blow lance, two pieces 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 regarded as being equivalent to the center of the plug, and the lance position was determined. The distance from the steel bath surface to the tip of the lance (immersion depth) was set to 75% of the bath depth of the steel bath.

In a plan view, regarding the positional relationship between the center position C _{A of} the gas blowing _A plug and the center position C _B of the gas blowing B plug at each level, the distance between C _O and C _A is set to L _OA , C _O and C the distance between _B and L _OB, the angle formed by C _a -C _O -C _B and theta, and set to the respective numerical values shown in table 1. In the case of using a blowing lance, the expression is based on the plug for gas blowing.

As a desulfurization treatment target, 110 to 120 t of coarse molten steel was accommodated in the ladle 1, and flux was introduced into the upper part of the coarse molten steel to form a flux layer 6. The supplied flux is mainly CaO and contains Al ₂ O ₃ , SiO _{2 and the} like. Thereafter, 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 is described in the table in terms of molten flux.

  In order to confirm the melted state of the flux, after applying the flux for 10 minutes, the melt flux thickness was measured at three locations about 100 mm away from the inner wall of the pan. The melt flux thickness was determined by immersing an iron rod to the molten steel, lifting the rod after the molten steel immersion part of the rod was melted, and determining the melt flux thickness based on the length of the melt flux adhering to the rod. . The average value of the measured melt flux thicknesses at the three locations was calculated as “average melt flux thickness”. In Comparative Example 7, the thickness of the molten flux was measured after stirring for 10 minutes without conducting electricity.

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

When the above melt flux index was 95% or more, it was evaluated as ☆, when it was 90% or more, ◎, when it was 80% or more, ○, and when it was less than 80%, it was evaluated as ×. Of the comparative examples, the highest melt flux index was in Comparative Example 1, and the melt flux index was 80%. Therefore, the above threshold value (80%) was determined on the premise that the conditions under which melting could be promoted than Comparative Example 1 were acceptable. When the melt flux index is less than 80%, it has been confirmed that the efficiency in the subsequent desulfurization treatment is insufficient.
In addition, no significant deterioration in electrical conductivity was observed at a gas flow rate as small as that of the example of the present invention.

  In addition, even when the treatment start was set to the end of energization and the treatment was performed under the above-described gas blowing conditions after the end of energization, the same tendency was obtained in the improvement of the flux melting state.

[Example 2]
Using the gas injection conditions and plug conditions of Invention Example 7 and Comparative Example 1 listed in Table 1 of Example 1 above, “Invention Example A” and “Comparative Example B”, respectively, the amounts of molten steel are 60 to 80 t, 130 t. The electric heating type molten steel desulfurization treatment was performed under the conditions of three conditions of 270 t.
The regions in contact with the molten steel at the bottom of the ladle are each circular with a radius R of 1.2, 1.4, and 1.8 m (approximately 1.3, 1.6, and 2.0 m at the surface of the molten steel). . Three electrodes 3 for electric heating were arranged. The three electrodes 3 are arranged at the vertex positions of the equilateral triangle. A circle circumscribing the outer periphery of all three electrodes 3 is a circumcircle 4 of the electrodes. The center position C _O of the circumscribed circle of the electrode is the same position as the center of the ladle bottom in plan view, and the radii r of the circumscribed circle 4 of the electrode are 0.5, 0.6, and 0.8 m, respectively. is there.

  The same energization and gas blowing process as in Example 1 was performed, and after 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 to obtain a melt flux index. In each of the three types of molten steel, 3 to 6 treatments were performed, and the average value, minimum value, and maximum value of the melt flux index were obtained. The average value of the melt flux index of Comparative Example B is defined as a reference value 1.00, and the maximum value and minimum value of Comparative Example B, and the average value, maximum value, and minimum value of Invention Example A are expressed as a percentage of the reference value It was shown in 2.

In the comparison of the melt flux index, it can be seen that the invention example A is more melted than the comparative example B in any molten steel amount condition, and the effect of the present invention is found. Furthermore, when the improvement effect of the invention example A with respect to the comparative example B is contrasted for every molten steel amount in a ladle, it can be read as follows.
That is, first, when the molten steel amount is 60 to 80 t, although the average value of the melt flux index is improved, the effect may be unclear when compared with the variation range. Secondly, the molten steel amount 130t is improved in average value. Even when compared within the range of variation, differences are observed. Third, the average value of the molten steel amount 270 t is improved. A clear improvement effect is also seen in the comparison of variation ranges.
As described above, it has been clarified that when the amount of molten steel in the ladle is 130 t or more, and further 270 t or more, the effect of promoting flux melting when compared with the conventional method becomes more remarkable.

1 Ladle 2 Plug for gas injection (plug)
2A Gas plug A plug 2B Gas blow B plug 3 Electrode 4 Electrode circumscribed circle 5 Molten steel 6 Flux layer 7 Bubbles 8 Upflow 9 Cross flow 11 Molten steel surface 12 Ladle wall surface 15 Unmelted flux 21X Flux layer X region 21Y center position of the center position C _B gas blowing for B plug a plug blowing center position C _a gas of the circumscribed circle of radius C _O electrodes of the circumscribed circle of the flux layer Y region 21Z flux layer Z region 22 bubble rising region r electrodes θ Angle formed by C _A -C _O -C _B

Claims (5)

In the ladle refining method for molten steel, 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 at the center of the ladle and energized.
Gas blowing plugs are arranged at two locations on the bottom of the ladle. Regarding the flow rate of gas blown from each of the gas blowing plugs, the gas flow rate of the gas blowing B plug with the larger gas flow rate is Q _B , The gas flow rate of the A plug for gas injection is Q _A (both units are NL / min / t),
In plan view, a circle circumscribing the outer periphery of all the two or three electrodes is referred to as an “electrode circumcircle”, the minimum radius of the circumcircle of the electrode is r, and the center position of the circumcircle of the electrode is C _O , The center position of the A plug for gas blowing is C _A , the center position of the B plug for gas blowing is C _B , the distance between C _O and C _A is L _OA , and the distance between C _O and C _B is L _OB , The angle formed by C _A -C _O -C _B is θ,
The gas blowing A plug and the gas blowing B plug are arranged at positions satisfying the following formulas (1) to (3),
_A ladle refining method for molten steel, wherein Q _A and Q _B satisfy the following expressions (4) to (6):
0 ≦ L _OA ≦ r (1)
L _OB > r (2)
90 ° ≦ θ ≦ 180 ° (3)
2.33 ≦ Q _B / Q _A (4)
Q _B ≦ 4.50 (5)
0.20 ≦ Q _A (6)   The ladle refining method for molten steel according to claim 1, wherein a thickness of the flux layer converted into a molten state is set to 100 mm or more and 200 mm or less.   The molten steel according to claim 1 or 2, wherein a radius of the bottom of the ladle is R, and a center position of the B plug for gas blowing is 0.1R or more from the ladle wall surface. Ladle refining method.   The molten steel ladle refining method according to any one of claims 1 to 3, wherein the amount of molten steel in the ladle is 130 t or more.   The molten steel ladle refining method according to any one of claims 1 to 3, wherein the amount of molten steel in the ladle is 270 t or more.

Images