JP6268963B2 - Method for refining molten steel - Google Patents

Description

  The present invention relates to a method for refining molten steel using a recirculation-type vacuum degassing process, which is a secondary refining process in a steelmaking process, and by increasing the ring flow rate of the molten steel, the uniform mixing time is shortened, and the refining characteristics of vacuum treatment are The purpose is to provide a technique for improving the processing time.

  With the recent upgrading of steel materials, there is a demand for improving the removal efficiency of impurities (O, S, N, etc.) in steel materials and controlling the concentration of molten steel components in a narrower range. Furthermore, the removal of inclusions is also required in order to meet the high cleanliness melting of steel.

  In response to these requirements, the secondary refining process in the steelmaking process plays an important role. In particular, in a process using a recirculation type vacuum degassing apparatus which is a representative apparatus for secondary refining process, it is necessary to extend the processing time in order to satisfy these requirements. However, the extension of the treatment time causes problems such as a decrease in molten steel temperature and promotion of refractory wear in addition to reducing productivity.

  In order to cope with the upgrading of steel materials without extending the processing time, it is necessary to improve the refining characteristics in the reflux type vacuum degassing process. It is known that it can be achieved by increasing the ring flow rate as a solution for improving the refining characteristics in the reflux type vacuum degassing process. It is also known that the ring flow rate is influenced by the dip tube diameter and the gas blowing position together with the degree of vacuum and the reflux gas flow rate. However, the apparatus configuration of the recirculation type vacuum degassing apparatus is closely related to the cost, and the degree of vacuum, the recirculation gas flow rate, the dip tube diameter, and the gas blowing position are naturally limited, so that a large investment cannot be made. Thus, technical development relating to a method of blowing reflux gas has been made so far.

  Japanese Patent Application Laid-Open No. 62-142715 proposes a method of using tuyere bricks in which the gas blowing holes are inclined upwardly into the dip tube. When this method is used, the velocity component in the vertical upward direction of the gas promotes the rise of the molten steel, so the reflux velocity is increased.

  In JP-A-4-235212, a function in which the number of argon gas blown tuyere in the circumferential direction arranged in the riser of the dip tube is a dip tube inner diameter, an argon gas flow rate, and an argon gas blown tuyere inner diameter are satisfied. A method of providing is presented. When this method is used, the distance by which the discharge gas moves the molten steel in the horizontal direction is spread over the entire area in the circumferential direction of the dip tube, and the circulation velocity can be increased more efficiently.

  Japanese Patent Application Laid-Open No. 3-36209 discloses a method of turning a molten steel ascending in an ascending pipe by applying a turning force. Furthermore, in order to give a swirl flow, a method is proposed in which a plurality of gas blowing tuyere provided on the outer periphery of the riser pipe are respectively inclined to the direction toward the center point of the riser horizontal cross section. . When this method is used, shearing force is applied to the bubbles rising in the riser pipe to prevent an increase in diameter due to the coalescence of the bubbles, and the gas lift effect due to the bubbles is maximized, and the ring flow rate of the molten steel can be increased. I can do it.

Japanese Patent Laid-Open No. 62-142715 JP-A-4-235212 JP-A-3-36209

  However, the technique described above merely discloses the gas blowing technique around the tuyere and is not related to the direct improvement of the refining characteristics. For example, when the bath depth is relatively shallow, the circulation of the circulating gas bubbles flowing into the vacuum chamber becomes insufficient, so the kinetic energy transfer efficiency of the circulating gas is low, and the circulating gas flow rate is increased. However, there was a problem that the ring flow rate was almost constant.

  The present invention solves the problem of improving the refining characteristics even when the recirculation gas flow rate is the same, and further improving the refining characteristics as well as increasing the recirculation flow rate when the recirculation gas flow rate is increased.

  Accordingly, the inventor has the condition that the recirculation gas bubbles in the vacuum chamber can be dispersed and the recirculation gas flow rate can be increased even when the bath depth is relatively shallow, where the refining characteristics are likely to deteriorate. Was examined.

  One is to control the reflux gas flow rate within an appropriate range. When the reflux gas flow rate is excessively small, the bubbles are transmitted along the wall surface of the riser tube, the kinetic energy transmission efficiency associated with the rise of the bubbles is reduced, and the circulation flow rate is also reduced. On the other hand, when the reflux gas flow rate is excessively large, the reflux gas bubbles ejected from the tuyere reach the center of the riser tube, and the bubbles cannot be dispersed.

  Another is to disperse the bubbles in the riser. As a method therefor, there is a method in which an angle in the circumferential direction on the horizontal plane, that is, a horizontal angle is given to the straight line connecting the tuyere and the riser center to the reflux gas blowing tuyere. The gas jet blown by applying this horizontal angle shifts from the center direction of the riser, gives kinetic energy that generates a swirling flow in the riser, and applies centrifugal force to the bubbles to move the bubbles to the center of the riser To go to. For this reason, the bubbles in the vicinity of the inner surface of the riser tube are directed toward the center, and as a result, the bubbles are dispersed in the riser tube. However, if the horizontal angle is too small, the swirl flow will not develop, and if it is too large, the gas bubbles ejected from the tuyere will collide with the wall in the riser, suppressing the bubble rise and reducing the ring flow rate. To do.

  Thus, it can be seen that there is an appropriate range for the conditions of the recirculation gas flow rate and the horizontal angle applied to the recirculation gas blowing tuyere. Furthermore, when the bath depth is too shallow, the circulation of the circulating gas bubbles becomes insufficient, and the influence of the friction at the bottom of the tank becomes dominant, and the circulation flow rate does not increase. The ring flow rate can be increased without using.

  In order to clarify the appropriate range of these conditions, an experiment using a water model was performed and the ring flow rate was measured.

  The experimental apparatus consisted of an acrylic ladle containing 155 liters of water, an acrylic vacuum tank with an inner diameter of 330 mm and a riser pipe with an inner diameter of 100 mm and a descender pipe attached to the lower part. This experiment was conducted with 150 liters of water in an acrylic ladle.

  The ascending pipe and the descending pipe were immersed in the ladle, and the pressure was reduced so that the bath depth in the vacuum chamber was a predetermined depth. After completion of the decompression, a reflux gas was introduced to start the reflux. After confirming the stability of the reflux, an aqueous KCl solution was added into the vacuum chamber, and the change in electrical conductivity over time was continuously measured with an electrical conductivity meter installed in the ladle. The time from the start of the addition of the KCl aqueous solution to the convergence of the electric conductivity to a constant value was defined as the uniform mixing time. Concerning the convergence of the electrical conductivity, the relative difference of the measured increase in electrical conductivity relative to the increase in electrical conductivity according to the amount of KCl added was within ± 5%. The ring flow rate is a value obtained by dividing the amount of water by the uniform mixing time. The shorter the uniform mixing time, the larger the ring flow rate.

An index (G / (n · A _n · d) obtained by dividing the uniform mixing time (τ) and the reflux gas flow rate (G) by the number of tuyere (n), the tuyere cross-sectional area (A _n ), and the immersion pipe diameter (d _u ). The relationship with _u )) is shown in FIG. In FIG. 1, the normalized bath depth (h / D) obtained by dividing the bath depth (h) in the vacuum chamber by the inner diameter (D) of the vacuum chamber is classified in the range of 0.020 to 0.35.

As shown in FIGS. 1A and 1F, when h / D <0.025 or h / D> 0.30, even when the horizontal angle is changed, the uniform mixing time (G / (n · A The relationship of _n · d _u )) did not change. This is because when h / D = 0.020, the bath depth in the vacuum chamber is too shallow, and the flow in the vacuum chamber is governed by the frictional force received from the bottom of the vacuum chamber. This is because when h / D = 0.35, the bath depth in the vacuum chamber is sufficiently deep so that the circulating gas bubbles are dispersed in the vacuum chamber regardless of the horizontal angle.

When h / D is 0.025 to 0.30 as shown in FIG. 1 (b) to FIG. 1 (e), if θ = 15 ° to 60 °, compared to θ <15 ° and θ> 60 °, When G / (n · A _n · d _u ) was 50 or more, the uniform mixing time was shortened. This is because the dispersibility of the bubbles in the vacuum chamber is improved when the horizontal angle is given within an appropriate range. However, even in the range of θ = 15 ° to 60 °, the uniform mixing time was prolonged when G / (n · A _n · d _u )> 570. This is because the bubbles are unevenly distributed because the penetration distance of the circulating gas bubbles ejected from the tuyere into the water is extended and the centrifugal force due to the swirling flow in the riser draws the bubbles to the center. is there. Accordingly, the present invention is not simply trivial than only the proper range of the horizontal angle of the tuyere, feathers _{talkative, G / (n · A n} · the percentage of recirculated gas flow rate to the inner diameter of the tuyere cross-sectional area and riser The invention is finally completed by defining the range of d _u ).

  The liquid density differs between the water model and molten steel treatment. However, if the corrected fluid numbers shown in equation (i) are the same, the bubble motions are mechanically similar. That is, the appropriate conditions obtained with the water model can be used in the molten steel treatment. In the water model experiment, the corrected fluid number under appropriate conditions is 0.02 to 2.4.

Fr ′ = ρ _g · V _g / {(ρ ₁ −ρ _g ) · (g · d ₀ )} (i)
ρ _g : gas density (kg / m ³ ), ρ _l : fluid density (kg / m ³ ), V _g : bubble ejection speed (m / s),
g: Gravitational acceleration (m / s ² ), d ₀ : tuyere diameter (m)
In view of the above results, the present invention that can increase the ring flow rate by increasing the reflux gas flow rate even when the bath depth in the vacuum chamber is relatively shallow is as follows.

The present invention is a recirculation type vacuum degassing apparatus, in which the bath depth (h) of the molten steel in the vacuum chamber satisfies the relationship of the formula (1) with respect to the inner diameter (D) of the vacuum chamber. Adjust the recirculation gas blowing tuyere provided in the riser attached to the tube so that it is at an angle of 15 ° to 60 ° horizontally with respect to the straight line connecting the tuyere and the center of the dip tube. (N) The index (G / (n · An · du)) divided by the tuyere cross-sectional area (An) and the rising pipe diameter (du) is adjusted so as to satisfy the expression (2). Method for refining molten steel.

0.025 ≦ h / D ≦ 0.30 (1)
h: Bath depth in the vacuum chamber (m), D: Vacuum chamber inner diameter (m)
50 ≦ G / (n · A _n · d _u ) ≦ 570 (2)
G: recirculation gas flow rate (Nm ³ / s), n: number of tuyere (−), A _n : cross-sectional area of recirculation gas blowing tuyere (m ² ), _du : rising pipe diameter (m)

  According to the present invention, even when the bath depth in the vacuum tank of the recirculation type vacuum degassing apparatus is relatively shallow, the recirculation gas flow rate can be increased to increase the recirculation flow rate, thereby improving the refining characteristics. be able to. Here, the refining characteristics include degassing performance and deoxidizing performance.

FIG. 1 is a diagram showing a relationship between a uniform mixing time of a water model and G / (n · A _n · d _u ) of the reflux type vacuum degassing apparatus used in the present invention. (F) is a figure when h / D is 0.020, 0.025, 0.050, 0.10, 0.30, and 0.35, respectively. FIG. 2 is a diagram showing a schematic configuration of a reflux-type vacuum degassing apparatus used in the present invention, FIG. 1 (a) is a longitudinal sectional view, and FIG. 2 (b) is an A diagram in FIG. 2 (a). FIG.

  The form at the time of using the recirculation type vacuum degassing process which is the refining method of the molten steel which concerns on this invention is demonstrated.

(1) Apparatus Configuration FIG. 2 is a diagram showing a schematic configuration of a reflux type vacuum degassing apparatus used in the present invention, FIG. 2 (a) is a longitudinal sectional view, and FIG. 2 (b) is a diagram of FIG. It is an AA arrow line view of (a). The recirculation-type vacuum degassing apparatus is composed of a vacuum chamber 1, and a rising pipe 2 and a downfalling pipe 3 attached to the bottom of the vacuum tank 1, and the rising pipe 2 is provided with a recirculation gas blowing tuyere 4. . The upper part of the vacuum chamber 1 is connected to a vacuum exhaust device 5. When vacuum processing of molten steel is performed, the ascending pipe 2 and the descending pipe 3 are immersed in the molten steel 7 in the pan 6. The surface of the molten steel 7 is covered with slag 8.

  The recirculation-type vacuum degassing apparatus used in the present invention may be the one used in a normal apparatus other than the recirculation gas blowing tuyere provided in the rising pipe. However, the inner diameter (D) of the vacuum chamber is preferably 0.9 m or more. If the inner diameter of the vacuum chamber is too small, even if the bubbles are sufficiently dispersed in the vacuum chamber, the bubbles collide with each other as they expand and blow-through occurs, resulting in insufficient transmission of kinetic energy. This is because there are cases where the

The inner diameter (d _u ) of the dip tube is preferably in the range of the formula (ii) according to the amount of molten steel to be processed. When the inner diameter of the dip tube is excessively small compared to the amount of molten steel, the flow rate is suppressed by the frictional force generated on the inner surface of the dip tube by the molten steel flowing in the dip tube, and the ring flow rate is reduced. This is because when the inner diameter of the dip tube is excessively large compared to the amount of molten steel, the molten steel temperature may be lowered by cooling the refractory in the dip tube.

0.0012 · W + 0.12 ≦ d _u ≦ 0.0022 · W + 0.27 (ii)
W: amount of molten steel (t)
The installation height of the reflux gas blowing tuyere is preferably in the range of 1/5 to 1/2 of the dip tube length from the lower end of the dip tube. This is because when the installation height of the tuyere is excessively low, the frequency of collision coalescence of the circulating gas bubbles tends to increase. Moreover, it is because the transmission efficiency of the kinetic energy of the rise of the circulating gas bubble with respect to molten steel will fall easily when installation height is too high.

  The number (n) of the reflux gas blowing tuyere is preferably 8 or more and 24 or less. When the number is less than 8, the flow rate of the reflux gas per hole becomes too large, and the bubble jet ejected from the reflux gas blowing tuyere becomes unstable, and the molten steel flow toward the refractory around the reflux gas blowing tuyere This is because bubbles may be generated and bubbles may rise in the vicinity of the refractory, resulting in insufficient bubble dispersion. When there are more than 24, the distance between adjacent tuyere becomes short, and coalescence of bubbles ejected from the adjacent tuyere tends to occur. That is, the bubbles may not be sufficiently dispersed.

  The reflux gas blowing tuyere is provided with a horizontal angle of 15 ° to 60 ° in the circumferential direction on the horizontal plane with respect to a straight line connecting the reflux gas blowing tuyere and the center of the rising pipe. However, the shape and material of the tuyere other than this need not be changed from the shape and material normally used.

  This horizontal angle may be obtained by aligning all the recirculating gas blowing tuyere at the same angle or by changing the angle individually. In this case, it is necessary to be within a range of 15 ° to 60 °. is there.

The cross-sectional area (A _n ) per hole of the recirculating gas blowing tuyere is preferably 1.7 × 10 ^{−6 to} 5.0 × 10 ⁻⁵ m ² . If it is smaller than 1.7 × 10 ⁻⁶ m ^{2, the} circulating gas bubbles may collect at the center of the dip tube and the bubbles may not be dispersed. If it is larger than 5.0 × 10 ⁻⁵ m ^{2, the} circulating gas is unevenly distributed on the wall surface of the dip tube, and the circulating gas bubbles become large, so that the kinetic energy transfer efficiency associated with the rising of the bubbles may be reduced.

In the present invention, the reflux gas flow rate (G) is increased by increasing the flow rate. The reflux gas flow rate depends on the number of tuyere (n), tuyere cross-sectional area (A _n ) and rising pipe diameter (d _u ) specific to the reflux type vacuum degassing apparatus, and depends on the reflux type vacuum degassing apparatus. Any flow rate that satisfies equation (2) is acceptable. For this reason, it is not necessary to define a particularly preferable range.

The present invention adjusts the circulating gas blowing tuyere to a predetermined angle under the condition that the bath depth (h) of the molten steel in the vacuum chamber satisfies the relationship of the formula (1), and sets the circulating gas flow rate to the number of tuyere. The index (G / (n · A _n · d _u )) divided by (n), the tuyere cross-sectional area (A _n ) and the rising pipe diameter (d _u ) needs to satisfy the formula (2). However, even if the recirculation gas flow rate is simply increased to increase the recirculation flow rate, the recirculation gas bubbles reach the center of the riser tube, and the recirculation gas flow cannot be increased without being able to disperse the bubbles. In order to increase the recirculation flow rate, the recirculation type vacuum degassing apparatus has its own number of tuyere (n), tuyere cross-sectional area (A _n ) and rising pipe diameter (d _u ). It is necessary to determine the reflux gas flow rate. As described above, these inherent values greatly contribute to the dispersibility of the circulating gas bubbles and the kinetic energy transfer efficiency associated with the rising of the circulating gas bubbles. Accordingly, the reflux gas flow rate (G) takes into consideration all of the tuyere number (n), tuyere cross-sectional area (A _n ) and rising pipe diameter (d _u ) inherent to the reflux-type vacuum degassing apparatus (n · A). It is necessary that the flow rate of the reflux gas flow rate (G) with respect to ( _n · d _u ) satisfies the equation (2).

(2) Processing method The molten steel accommodated in the ladle after the converter steel is transferred to the reflux type vacuum degassing apparatus, and the vacuum processing of the molten steel of the present invention is started. However, ladle refining for the purpose of slag reforming and desulfurization may be performed before the vacuum treatment that is the method for refining molten steel of the present invention.

In the vacuum treatment of the molten steel, the riser pipe and the downfall pipe are immersed in the molten steel in the ladle while introducing the reflux gas from the tuyere of the reflux gas. The reflux gas flow rate is adjusted so that the value obtained by dividing the reflux gas flow rate (G) by the number of tuyere (n), the tuyere cross-sectional area (A _n ), and the rising pipe diameter (d _u ) satisfies the scope of the present invention. . At this time, the modified fluid number is preferably 0.02 to 2.4. Thereafter, the inside of the vacuum chamber is depressurized by a vacuum exhaust device such as a steam ejector pump, and the molten steel in the ladle is sucked into the vacuum chamber. The atmospheric pressure in the vacuum chamber is preferably 67 to 4000 Pa. If it is less than 67 Pa, the splash which is the molten steel scattering generate | occur | produced from the molten steel surface becomes intense, and the loss of molten steel may become large. If it exceeds 4000 Pa, the expansion of the bubbles may be insufficient, and the transfer efficiency of the kinetic energy for rising the bubbles may be reduced.

  When the pressure in the vacuum chamber is reduced, the bath depth in the vacuum chamber, which is geometrically calculated from the apparatus shape and the pressure in the vacuum chamber, satisfies the ratio of the bath depth (h) and the vacuum chamber inner diameter (D) of the present invention. Adjust the immersion depth of the dip tube. In the present invention, the bath depth (h) is the depth from the upper end of the dip tube to the steel bath surface when left standing. About the range of bath depth, since it will be a comparatively shallow range if it is a range which satisfies Formula (1) in relation to the internal diameter of a vacuum chamber, it is not necessary to prescribe | regulate a especially preferable range.

  As the vacuum in the vacuum chamber progresses, the rising energy associated with the rising of the circulating gas bubbles blown from the circulating gas blowing tuyere is transmitted to the molten steel, and an upward flow develops in the molten steel in the rising pipe, and downwards in the downcomer pipe The flow develops. Thereby, the recirculation | flow which is a flow through which molten steel circulates in a vacuum chamber and a ladle arises. Processing such as deoxidation, degassing, component adjustment, and temperature adjustment is performed during the reflux.

  In performing the secondary refining treatment of the molten steel, while performing the treatment using the reflux type vacuum degassing treatment device having the above-mentioned device configuration, the metal Cu was added as a tracer, and then a sample was taken and Cu was collected. The uniform mixing time was measured from the increasing behavior of the concentration.

The vacuum processing of the molten steel 160 to 300 t was performed in the following processing order. Molten steel in the ladle with the recirculation gas introduced into the recirculation gas blowing tuyere (8 to 16 holes, cross-sectional area per hole is 3.14 × 10 ^{−6 to} 2.0 × 10 ⁻⁵ m ² ) The riser (inner diameter 500-700 mm) and the downcomer (inner diameter 500-700 mm) were immersed in After immersion, the inside of the vacuum chamber was depressurized to 67-1300 Pa using a steam ejector pump. Under the present circumstances, the immersion depth of the dip tube was adjusted so that the bath depth in a vacuum chamber might satisfy | fill Formula (1).

An alloy was added to adjust the molten steel components within the specified range after decompression. Simultaneously with the addition of this alloy, metallic Cu was added as a tracer. After addition of the alloy and the metal Cu, a sample was taken and subjected to chemical analysis to investigate changes in Cu concentration over time. The time when the increment of the Cu concentration calculated from the Cu concentration obtained by the analysis was within ± 5% with respect to the added Cu concentration was defined as the uniform mixing time. Table 1 shows the measurement results of the uniform mixing time. The test is a test No. 1 that satisfies the processing conditions specified by the present invention. 1 and test no. 2 and test no. Test No. 1 in which h / D of No. 1 was changed to 0.020. 3, test no. Test No. 1 in which θ of 1 was changed to 0 °. 4, test no. Test No. 1 in which θ of 1 was changed to 75 °. 5, Test No. No. 2 G / (n · A _n · d _u ) was changed to 590. 6. Test No. under conditions where the bath depth is sufficiently deep. 7 was carried out.

In order to prove the usefulness of G / (n · A _n · d _u ), which is an index of the present invention, in Table 1, “Tatsuro Kuwahara et al .: Iron and Steel, vol. 73 (1987), S176” The known ring flow rate estimation formula (iii), the uniform mixing time estimated using the relational formula (iv) between the uniform mixing time and the ring flow rate, and the corrected fluid number are also shown.

Q = 11.4 · G ^1/3 · d _u ^4/3 · {ln (P ₁ / P ₀ )} ^1/3 (iii)
τ = W / Q (iv)
Q: ring flow rate of molten steel (t / min), _du : inner diameter of dip tube (m), G: flow rate of reflux gas (Nl / min), P ₀ : atmospheric pressure in vacuum chamber (Pa), P ₁ : reflux gas Molten steel static pressure (Pa) at the blowing tuyere position, τ: Estimated uniform mixing time (s), W: Molten steel amount (t)

Test No. 1 and test no. Comparison of 3 shows that when h / D is below the range defined by the present invention, the uniform mixing time is prolonged. Test No. 1 and test no. 4 or test no. When 5 is compared, it can be seen that when θ is outside the range defined by the present invention, the uniform mixing time is prolonged. Test No. 2 and test no. When comparing 6 with G / (n · A _n · d _u ) exceeding the range defined by the present invention, it can be seen that the uniform mixing time is prolonged. Test No. In No. 7, since the bath depth is sufficiently deep, even when both θ and G / (n · A _n · d _u ) are outside the scope of the present invention, the uniform mixing time is short.

  Compared to the estimated uniform mixing time, test no. 1, no. In the inventive example of No. 2, this is shortened. 3-No. In 6, it became longer than the estimated uniform mixing time. Test No. with sufficiently deep bath 7 was almost equal to the estimated uniform mixing time.

Therefore, when the bath depth is relatively shallow, the uniform mixing time can be shortened by setting θ and G / (n · A _n · d _u ) within the range defined by the present invention. At this time, the ring flow rate also increases.

  The method for refining molten steel according to the present invention defines an appropriate range of bath depth, horizontal angle, and reflux gas flow rate in the vacuum chamber according to the shape of the reflux type vacuum degassing apparatus, and uniform mixing is performed when this range is satisfied. This is a method that makes it possible to shorten the time.

  If the uniform mixing time can be shortened, the concentration concentration generated in the reflux vacuum degassing apparatus is reduced with the progress of the refining reaction, and the refining reaction such as deoxidation and degassing is promoted. Furthermore, the time during which the alloy added for component adjustment is mixed and homogenized throughout the apparatus is shortened, and the processing time is shortened. As described above, since the processing time is shortened as a whole with the reduction of the uniform mixing time, the heat dissipation loss is reduced, the temperature drop is suppressed, and the cost required for the temperature rise can be reduced.

  Therefore, the present invention is an effective method in the refining treatment of molten steel.

Claims (1)

In the reflux type vacuum degassing apparatus, the bath depth (h) of the molten steel in the vacuum chamber was attached to the bottom of the vacuum chamber in a condition satisfying the relationship of the expression (1) with respect to the inner diameter (D) of the vacuum chamber. The recirculation gas blowing tuyere provided in the riser is adjusted so that it is at an angle of 15 ° to 60 ° horizontally with respect to the straight line connecting the tuyere and the center of the dip tube. The index (G / (n · A _n · d _u )) divided by the tuyere cross-sectional area (A _n ) and the rising pipe diameter (d _u ) is adjusted so as to satisfy the expression (2). Method for refining molten steel.
0.025 ≦ h / D ≦ 0.30 (1)
h: Bath depth in the vacuum chamber (m), D: Vacuum chamber inner diameter (m)
50 ≦ G / (n · An · du) ≦ 570 (2)
G: recirculation gas flow rate (Nm ³ / s), n: number of tuyere (−), An: cross-sectional area of recirculation gas blowing tuyere (m ² ), du: rising pipe diameter (m)

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