JP2018188685A - Ladle refining method of molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel and improved ladle refining method of a molten steel capable of efficiently proceeding a refining reaction between a flux and a molten steel, when a molten steel in the ladle is desulfurized by the ladle refining of the molten steel accompanying electric heating to produce a very low sulfur steel.SOLUTION: In a ladle refining method of a molten steel in which a flux layer 4 containing CaO is formed on a surface of a molten steel 3 in a ladle 1, and an electrode is dipped in a flux layer to energize, a gas blowing plug 2 is arranged at two positions on a bottom part of the ladle 1, and, regarding a flow rate of a gas blown from each of the gas blowing plugs, when a flow rate having larger gas flow rate is assigned to Qand the other gas flow rate is assigned to Q(unit for both is NL/min/t), Qand Qsatisfy the following (1) to (3) formulas.1.74≤Q≤4.50 (1), 0.1≤Q(2), and Q/Q≥3.0 (3).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 current-carrying 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.

  Regarding the ladle refining of molten steel accompanied by electric heating, Patent Literature 1 discloses an electric 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. 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. Has disclosed a method of blowing an inert gas with a difference in the gas flow rate. In the invention described in this document, the gas is blown so that the total flow rate of the gas blown from the two bottom blowing plugs is 0.8 to 1.8 NL / min / ton.

  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. In the invention described in this document, the total gas flow rate blown from the two bottom blowing plugs is 0.90 to 1.25 NL / min / ton in the first refining treatment and degassing treatment (first half), and the degassing treatment. In the second half of the refining process, gas is blown so as to be 0.45 to 0.70 NL / min / ton.

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 invention is capable of efficiently proceeding a refining reaction between flux and molten steel when producing ultra-low-sulfur steel by desulfurizing molten steel in a ladle by ladle refining of molten steel accompanied by electric heating. An object of the present invention is to provide a new and improved ladle refining method for molten steel.

That is, the gist of the present invention is as follows.
(1) 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 the electrode is immersed in the flux layer and energized.
Two gas injection plugs are arranged at the bottom of the ladle. Regarding the flow rate of gas injected from each of the gas injection plugs, the gas flow rate with the larger gas flow rate is Q _B , and the other gas flow rate is Q _A. (both units NL / min / t) and, Q _a, Q _B is the following (1) to (3) and satisfies the equation, ladle refining method of molten steel.
1.74 ≦ Q _B ≦ 4.50 (1)
0.1 ≦ Q _A (2)
Q _B / Q _A ≧ 3.0 (3)
(2) The ladle refining method for molten steel according to (1) above, wherein the thickness of the flux layer is 100 mm or more and 200 mm or less.
(3) The distance between the two gas blowing plug centers is 0.5R or more with the radius of the bottom of the ladle being R, and the distance from the ladle wall surface is 0.1R or more at any gas blowing plug center. The method for refining a ladle of molten steel as described in (1) or (2) above, wherein
(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.

  In the present invention, when producing ultra-low-sulfur steel by desulfurizing molten steel in a ladle by ladle refining of molten steel with energization heating, gas blowing plugs are arranged at two locations at the bottom of the ladle, By keeping the flow rate of the gas blown from each of the blow plugs within a predetermined range, stirring of the molten steel in the ladle can be promoted while promoting the entrainment of the flux layer, and refining between the flux and the molten steel. The reaction can proceed quickly.

It is sectional drawing which shows the condition which implements the ladle refining of this invention. It is sectional drawing which shows a condition when one gas blowing amount from the ladle bottom part is excessive. It is a perspective sectional view showing the situation when the ratio (Q _B / Q _A ) of the amount of gas blown from the ladle bottom two places is too small.

  As shown in FIG. 1, in the ladle refining method of the molten steel 3 of this invention, the flux layer 4 containing CaO is formed in the molten steel surface in the ladle 1, and an electrode (not shown) is immersed in the flux layer. Molten steel desulfurization is performed by energizing. 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 disposed in the upper portion of the ladle in the flux layer and energizing the electrode. Moreover, in this invention, the plug 2 for gas blowing is arrange | positioned at two places in the bottom part of the ladle 1, and the molten steel in a ladle is stirred by blowing inactive gas in each molten steel from the said gas blowing plug 2 each. .

  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 necessary to dissolve the flux layer sufficiently, and to quickly replace the molten steel that has undergone desulfurization with the molten steel that has not undergone desulfurization in the molten steel in the ladle. What is important is to increase the reaction interface area between the molten steel and the flux layer interface. In the present invention, in particular, attention has been paid to increasing the reaction interfacial area between the molten steel and the flux layer interface in order to proceed the desulfurization process to the very low sulfur by ladle refining of the molten steel. As shown in FIG. 1, the gas blown from the gas blowing plug 2 at the bottom of the ladle becomes a bubble 5, and the bubble 5 rises in the molten steel in the ladle and forms an upward flow 6 of the molten steel. When the molten steel ascending flow 6 with bubbles reaches the molten steel surface, it turns into a transverse flow 7 along the lower surface of the flux layer, and at that time, a part of the flux is suspended in the molten steel by a shearing force. As a result, the flux enters the molten steel as droplets 10, and the contact interface area between the molten steel and the molten flux increases. Thereby, the desulfurization reaction between molten steel and a flux is accelerated | stimulated.

In order to achieve the above object, in the present invention, the gas flow rate of the gas blown from each of the two gas blowing plugs arranged at the bottom of the ladle (the gas blowing B plug 2B) is larger. The flow rate is Q _B , and the gas flow rate of the other (gas plug A plug 2A) is Q _A (both units are NL / min / t), and Q _A and Q _B are the following formulas (1) to (3) It is characterized by satisfying.
1.74 ≦ Q _B ≦ 4.50 (1)
0.1 ≦ Q _A (2)
Q _B / Q _A ≧ 3.0 (3)

In the present invention, when the gas is blown from the two gas blowing plugs (2A, 2B), a difference is given to the gas flow rate. Then, by the gas blowing (gas flow rate = Q _B ) of the one with the larger gas flow rate (gas blowing B plug 2B), the flux droplet formation on the molten steel surface and the molten steel 3 of the flux droplet 10 are the objects of the present invention. To promote suspension. By setting the gas flow rate Q _B to 1.74 NL / min / t or more, a sufficient ascending flow 6 of the molten steel is formed, the flux layer is entrained on the surface of the molten steel, and the molten steel and the molten flux are mutually suspended and sufficient. Since an amount of flux droplets 10 is mixed in the molten steel, the contact interface area between the molten steel and the molten flux increases, and the desulfurization reaction between the molten steel and the flux is promoted (FIG. 1). When the gas flow rate Q _B is less than 1.74 NL / min / t, it is considered that the molten flux is in the form of droplets, but the amount is considered to be small, and the intended refining The end time of the reaction is delayed compared to the case of Q _B ≧ 1.74 NL / min / t. That is, by forming the left side inequality of the above formula (1), it is possible to sufficiently achieve the droplet formation of the flux necessary for the desulfurization reaction.

When the gas flow rate Q _B exceeds 4.50 NL / min / t, the flux entrainment frequency in the molten steel is saturated, while the flux on the molten steel surface is unevenly distributed on the ladle wall surface 12 as shown in FIG. This causes the unmelted flux 15 to be generated at the uneven distribution location. Also, when the upward flow containing bubbles reaches the surface of the molten steel, the area where the molten steel is exposed without being covered by the flux layer becomes excessive, so oxygen and nitrogen inevitably mixed in the atmospheric gas above the ladle. Adverse effects (condensation, nitrogen pickup, etc.) due to contact with molten steel occur. Therefore, in the present invention, the right side inequality of the above equation (1) is defined for the gas flow rate Q _B having the larger gas flow rate.

Next, the gas blowing (gas flow rate = Q _A ) of the gas blowing plug (gas blowing A plug 2A) having the smaller gas flow rate will be described.

  When the stirring gas according to the formula (1) is blown from only one plug (B plug 2B for blowing gas), the flux droplet 10 is generated by entrainment of the flux melted by energization heating, but the molten steel is formed at the bottom of the ladle. The stagnation part of a flow will arise and stirring on the molten steel side will become inadequate. For example, in the case of desulfurization refining, the sulfur concentration in the molten steel is rapidly reduced (mainly in the vicinity of the molten flux / molten steel interface) due to the reaction with the entrained molten flux above the plug according to the formula (1). On the other hand, molten steel having a relatively high sulfur concentration stays in the molten steel at the bottom of the ladle (except for the vicinity of the B plug 2B for gas blowing according to the formula (1)). As a result, the processing time required to reduce the impurity concentration at the bottom of the ladle to the target value or less is extended. Therefore, in addition to the plug according to the formula (1) (gas blowing B plug 2B), by providing a second plug (gas blowing A plug 2A), the stagnation part of the molten steel flow is eliminated, and the reaction progress degree is increased. Different molten steel can be replaced, and the reaction rate can be improved. At this time, if the flow rate of the gas blown from the gas plug A plug 2A is less than 0.10 (NL / min / t), the stirring effect of the molten steel becomes insufficient. Therefore, in the present invention, the flow rate of the stirring gas per plug is defined as 0.10 (NL / min / t) or more per molten steel weight. Note that the upper limit of the gas flow rate blown from the gas blowing A plug 2A is 1.50 (NL / min / t) when considered including the expression (3) described later.

The ratio (Q _B / Q _A ) between the gas flow rate Q _A blown from the gas blowing A plug 2A and the gas flow rate Q _B blown from the gas blowing B plug 2B will be described. The upward flow 6 generated as the gas is blown becomes a transverse flow that flows radially in the vicinity of the molten steel surface. In the case where gas is blown from two gas blowing plugs as in the present invention, the flow rate ratio (Q _B / Q _A ) of the stirring gas flow rate introduced from the two gas blowing plugs is less than 3.0. If it exists, it will become easy to generate | occur | produce the collision of the transverse flow 7 which arises by the stirring from both plugs. When the flow of molten steel in the opposite direction collides with the molten flux, the flow velocity of the transverse flow 7 cannot be increased, and the amount of flux suspension in the molten steel due to the shearing force of the transverse flow decreases (FIG. 3). Therefore, in the present invention, the above equation (3) is defined, and the flow rate ratio is set to 3.0 or more. The upper limit value of the flow rate ratio is not particularly required, but is 45.0 when the flow rate from the plug is set to the upper limit of the expression (1) and the lower limit of the expression (2), respectively.

When a third plug is disposed in addition to the plug of Q _B according to the equation (1) and Q _A according to the equation (2), the gas amount Q _C (NL) equal to or less than Q _A (NL / min / t). / Min / t) is disposed, and the expression (3) to be described later is
Q _B / (Q _A + Q _C ) ≧ 3.0
The present invention does not prevent the arrangement of three or more plugs.

The effect of improving the desulfurization refining efficiency obtained by setting the gas flow rate Q _B from the gas blowing B plug 2B to 1.74 NL / min / t or more becomes significant when the flux is in a molten state. This is because when the flux is not yet hatched, the movement of impurities into the solid flux is extremely slow and the reaction rate is low. Therefore, from the viewpoint of ensuring the melted state of the flux, the gas blowing of the present invention is preferably during energization or immediately after energization. “Immediately after energization” refers to 0 minutes to 20 minutes after energization. This is because the melting state of most or all of the flux layer can be secured usually within 20 minutes.

  In ladle refining of molten steel, refining not only for desulfurization refining but also for the purpose of reducing inclusions may be performed. When it is necessary to reduce inclusions, if the entrained flux reaches the specified level after the refining reaction, the flow of the stirring gas is reduced and the levitating process is performed. The use efficiency can be improved. In addition, when the flow rate of the stirring gas is set to 1.74 (NL / min / t) or more as described above, the entire refining process (the stirring process and the floating process of the flux of the present invention) is performed when the flux is floated. A reduction in time can be realized.

  For the gas blowing of the present invention, it is preferable to use a plug provided at the bottom of the ladle that enables stirring of the molten steel in contact with the bottom of the ladle. However, since the molten flux can be formed into droplets even with the immersion lance, the problem of the present invention can be solved to a certain level even with the immersion lance.

  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 this invention, it is preferable in the thickness of the flux layer formed in the molten steel surface in a ladle being 100 mm or more and 200 mm or less. When the thickness of the flux layer is less than 100 mm, the molten flux covering the molten steel surface may be interrupted by the molten metal surface undulation caused by gas blowing, leading to a limitation of the molten flux droplet formation region. For this reason, it is preferable that the thickness of a flux layer is 100 mm or more.
On the other hand, when the thickness of the flux layer exceeds 200 mm, a temperature difference tends to occur in the thickness direction of the flux layer. In some cases, the undegraded (unmelted) portion of the surface of the flux layer may be caused, and the effective use of the flux for refining is limited, and the droplet formation of the molten flux is also limited.
Therefore, in the present invention, the thickness of the flux layer is preferably 100 mm or more and 200 mm or less. In order to set the thickness of the flux layer to 100 mm or more and 200 mm or less, calculate the required flux input amount from the flux volume calculated from the dimensions of the ladle and the flux density at the assumed flux composition, or input The amount of flux input may be determined based on the correlation between the amount and the flux thickness (past results), or the amount of input 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 of the flux layer attached to the steel rod pulled out from the ladle. 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 distance between the two gas blowing plug centers is 0.5 R or more with the radius of the bottom of the ladle being R, and any of the gas blowing plug centers has a distance from the ladle wall surface. It is 0.1R or more.
In the present invention, in addition to gas blowing from the gas blowing B plug 2B, gas is blown also from the gas blowing A plug 2A, thereby eliminating the stagnation of the molten steel flow at the bottom of the ladle. When the radius of the ladle bottom is R, and the distance between the two plug centers is less than 0.5 R, the effect of eliminating the stagnation of the molten steel flow at the ladle bottom is limited, and the effect of the present invention is It cannot be obtained at maximum. On the other hand, when the distance between the ladle wall surface and the center of the plug is less than 0.1 R, the ladle wall surface and the plug are close to each other, and a part of the blown gas floats while contacting the wall surface ( Since the gas blows between the flux and the wall surface, the effect of the present invention cannot be obtained at the maximum. Therefore, in the present invention, the distance between the centers of the two plugs is preferably 0.5R or more, and the distance between the ladle wall surface and the plug center is preferably 0.1R 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. 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 the amount of molten steel in the ladle is 270 t or more.
In ladle refining treatment of molten steel, the refining time (for example, desulfurization treatment time) becomes longer as the amount of molten steel in the ladle increases. Compared to the conventional ladle refining method, by adopting the ladle refining method of the present invention, the refining time can be shortened by eliminating the stagnation part at the bottom of the ladle while causing the melting flux entrainment phenomenon. It becomes possible. Furthermore, according to the knowledge of the present inventors, the refining time can be further shortened by setting the amount of molten steel in the ladle to 130 t or more, and further to 270 t or more. In general, the larger the ladle molten steel, the smaller the static hot water surface area per ton of molten steel, and it is estimated that the effect of increasing the interfacial area due to the entrainment phenomenon tends to be effective.

  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 this example, an electrically heated type molten steel desulfurization treatment was performed. As a desulfurization treatment target, 110 to 120 t of coarse molten steel was placed in a ladle, and flux was added to the top of the coarse molten steel. The flux is mainly CaO and contains Al ₂ O ₃ , SiO _{2 and the} like. Thereafter, the electrode was immersed in the flux to start energization, and gas stirring was started. Ar gas was used as the stirring gas and was blown through a gas blowing plug provided at the bottom of the ladle.

  Two gas blowing plugs were installed at the bottom of the ladle as shown in FIG. The smaller value of the distance between the plug centers and the distance from the ladle wall surface to the plug center at each level was set to a numerical value shown in Table 1 with the ratio to the radius R of the ladle bottom.

  After visually confirming that most of the flux was melted by energization, a sample for molten steel component analysis at the start of the treatment was collected, and desulfurization treatment was started in which bottom blowing stirring was performed while energization was performed. Energization was continued during the desulfurization treatment. A sample (after treatment) for molten steel component analysis was taken again after 15 minutes from the start of treatment. The molten steel S content was analyzed at two times after the start of the treatment and after the treatment (except for Comparative Example 7), and the difference between the two was defined as the desulfurization amount (ppm). The amount of CaO added as flux (per ton of molten steel) is the CaO unit amount (kg / t), the desulfurization amount per CaO unit amount is the desulfurization rate (ppm / (CaO amount kg / t)), and based on the desulfurization rate It was evaluated whether or not the amount of CaO used was reduced.

  For the examples and comparative examples shown in Table 1 below, desulfurization of molten steel was performed under the above conditions, and the amount of CaO used based on the amount of desulfurization per unit amount of CaO (desulfurization rate (ppm / (CaO amount kg / t))). It was evaluated whether or not was reduced. Comparative Example 7 is an example in which no energization was performed, and the evaluation was performed with the amount of change from the time when the bottom blowing stirring was started to 15 minutes after the predetermined flux was charged. Such evaluation is based on Comparative Example 5 (100%), when the desulfurization rate is 15% or more higher than the standard, and when the increase in the desulfurization rate is 10% or more and less than 15% from the standard, ◎, desulfurization When the rate increase was 5% or more and less than 10% from the standard, it was evaluated as ◯, and when the desulfurization rate was equal to (less than ± 5%) or worsened as Comparative Example 2, it was evaluated as x.

  It should be noted that the same effect was obtained even when the start of treatment was set as the end of energization and the above-described gas injection was performed after the end of energization.

Comparative Example 6 is a condition where the evaluation value can be improved by 4% more stably than Comparative Example 5, but the evaluation effect is x because the effect of improving the desulfurization rate is less than 5%. This is thought to be due to the effect of forming droplets of the molten flux although Q _{B of} Comparative Example 6 is less than the lower limit of the present invention, but the floating treatment after treatment (floating of the flux suspended in the molten steel) Including the treatment) leads to an increase in the refining time as compared with Comparative Example 5.

[Example 2]
Using Example 8 described in Table 1 of Example 1 above and the gas blowing conditions and plug conditions of Comparative Example 2, respectively, “Example A” and “Comparative Example B” were used, and the amounts of molten steel were 60 to 80 t, 130 t, and 270 t. When the low-sulfur steel (S: 15 ppm or less) was melted under the three conditions, the time required for reducing the S concentration in the molten steel from 60 ppm to 15 ppm (desulfurization time) was compared. The average time in “Example A condition, molten steel amount 60 to 80 t” was 1.0 (reference), and other times were displayed as ratios relative to the reference. The results are shown in Table 2. The range and average value of the desulfurization time ratio are listed.

In the comparison of the average time of the desulfurization required time, it can be seen that the refining of Example A was completed in a shorter time than that of Comparative Example B under any molten steel amount condition, and the effect of the present invention was obtained. Furthermore, when the improvement effect of Example A with respect to the comparative example B is contrasted for every amount of molten steel in a ladle, it can read as follows.
(1) Molten steel amount: 60 to 80 t: Although the average time is improved, the effect may be unclear when compared with the time range.
(2) Molten steel amount 130t: Improvement in average time. Differences were observed even when compared over time.
(3) Molten steel amount 270 t: Improvement in average time. A clear improvement effect was also seen in the comparison of time ranges.
That is, it has been clarified that the effect of shortening the refining time when compared with the conventional method becomes more remarkable when the amount of molten steel in the ladle is 130 t or more, and further 270 t or more.

1 Ladle 2 Plug for gas injection (plug)
2A A plug for gas blowing 2B B plug for gas blowing 3 Molten steel 4 Flux layer 5 Bubbles 6 Upflow 7 Cross flow 10 Droplet 11 Molten steel surface 12 Ladle wall surface 15 Unmelted flux

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 the electrode is immersed in the flux layer and energized.
Two gas injection plugs are arranged at the bottom of the ladle. Regarding the flow rate of gas injected from each of the gas injection plugs, the gas flow rate with the larger gas flow rate is Q _B , and the other gas flow rate is Q _A. (both units NL / min / t) and, Q _a, Q _B is the following (1) to (3) and satisfies the equation, ladle refining method of molten steel.
1.74 ≦ Q _B ≦ 4.50 (1)
0.1 ≦ Q _A (2)
Q _B / Q _A ≧ 3.0 (3)   The ladle refining method for molten steel according to claim 1, wherein the thickness of the flux layer is 100 mm or more and 200 mm or less.   The distance between the two gas blowing plug centers is 0.5R or more with the radius of the bottom of the ladle being R, and the distance from the ladle wall surface is 0.1R or more at any gas blowing plug center. The ladle refining method of the molten steel according to claim 1 or 2, characterized by the above.   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.

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