JP6164040B2 - Steel continuous casting method - Google Patents

Description

  The present invention relates to a method of controlling flow in a mold of continuous casting with electromagnetic force in order to produce a high-quality slab having few bubbles and inclusion system defects.

  The flow of molten steel in the mold in the continuous casting process greatly affects the slab quality. Therefore, how to control the flow of molten steel in the mold is extremely important. Since electromagnetic force can control the flow of molten steel in the mold in a non-contact manner, various methods have been studied and used in the past. A method of forming a swirling flow in a horizontal cross section near the molten metal surface in the mold by installing an electromagnetic stirring device on the back side of the long side of the mold and applying a thrust in the opposite direction on the opposite long sides is widely used. At that time, the positional relationship between the electromagnetic stirring device and the molten metal surface, the positional relationship between the electromagnetic stirring device and the immersion nozzle discharge hole for supplying molten steel into the mold from the tundish, the relationship between the flow rate of the molten steel discharged from the nozzle and the stirring flow rate In the past, various techniques have been disclosed. For example, Patent Document 1 discloses a method in which the discharge hole of the immersion nozzle is installed at a position where the magnetic flux density in the immersion nozzle discharge hole is 50% or less of the maximum magnetic flux density of the electromagnetic stirring device. Patent Document 2 discloses a method in which the discharge angle of the immersion nozzle is 35 ° or more and 75 ° or less, and the discharge port is positioned lower than the lower surface of the core (iron core) of the electromagnetic stirrer. .

The electromagnetic force acting on the molten steel is proportional to B ² f when expressed using the magnetic flux density B and the frequency f. Therefore, the magnetic flux density B in the mold is measured under the condition of each frequency f, B ² f is obtained, and the frequency that maximizes the value is generally selected. For example, FIG. 1 shows the result of investigating the relationship between the frequency applied in a small-scale experimental apparatus having an electromagnetic stirrer on the back of the mold and B ² f shown above. Incidentally, the vertical axis plots the value normalized by B ² f obtained from the measured values of the condition of 12 Hz. Under this condition, 12 Hz at which B ² f shows the maximum value is selected. Here, the frequency at which B ² f takes the maximum value exists because the induced current increases with the increase in frequency, so the electromagnetic force increases, but when the frequency becomes excessive, the induction loss due to the copper plate cannot be ignored. .

  In some cases, a brass plate is installed at a position of 15 mm from the inner wall surface of the mold, and the force acting on the brass plate is measured by using a strain gauge or the like by driving the electromagnetic stirrer, and casting is performed under the condition that this value is maximized. .

  The method described here is for obtaining electromagnetic force acting on molten steel based on magnetic flux density measurement results and thrust measurement results in the absence of molten steel. A sufficient study has not been made on how to select the frequency under the condition that the electromagnetic force distribution varies depending on the frequency and there is a solidified shell growing in the mold.

Japanese Patent Laid-Open No. 2001-47201 JP 2004-42062 A

Edited by Japan Iron and Steel Institute, “3rd Edition Steel Handbook II Steelmaking and Steelmaking”, page 619

  As described above, in the conventional technology, the interference between the stirring flow by electromagnetic stirring and the discharge flow from the submerged nozzle and the discharge reverse flow is geometrically arranged, and the magnetic flux density measurement results and thrust measurement results are measured offline. Originally, it investigated the positional relationship with the core height of the electromagnetic stirrer and the submerged nozzle discharge hole, considering the distribution of electromagnetic force in the actual molten steel in the mold and the presence of the solidified shell. It was not a thing. Therefore, the present invention takes into account the distribution of electromagnetic force in the actual molten steel in the mold and the presence of solidified shells, makes the most of the functions of electromagnetic stirring, creates the most preferable flow, and produces high-quality casting. The object is to provide a method of manufacturing a piece.

The configuration of the present invention is as follows.
(1) An electromagnetic stirrer that forms a swirling flow in a horizontal section is installed near the molten metal surface in the mold to control the flow in the mold during continuous casting. The mold copper plate thickness D _Cu , casting speed V _c , nozzle immersion depth The continuous casting method of steel, wherein the length L, the applied frequency f of the electromagnetic stirrer and the electrical conductivity σ of the copper plate are adjusted so as to satisfy the following relational expression.
D _Cu < √ (1 / (2σωμ)) <D _Cu + k√ (L / V _c ) (i)
Here, ω = 2πf is an angular frequency (rad / s), μ is a magnetic permeability (N / A ² ) in vacuum, and k is a solidified shell growth rate coefficient.
(2) In (1), a DC magnetic field generator that applies a DC magnetic field having a uniform magnetic flux density distribution in the width direction below the electromagnetic stirring device that forms a swirling flow in a horizontal section in the mold thickness direction A continuous casting method for steel characterized by being used in combination.

  By using the continuous casting method of the present invention, even if the electromagnetic stirrer is the same, by changing the frequency of energizing the coil, the stirring region can be operated only in the vicinity of the meniscus or the submerged nozzle discharge hole region can be operated. This makes it possible to make maximum use of the stirring area. Furthermore, in the method of casting by combining this method with a device that generates a DC magnetic field under the electromagnetic stirring device, a part of the stirring flow is particularly braked by the DC magnetic field by making maximum use of the stirring region. This is preferable because the stirring flow rate in the region is accelerated.

It is the result of investigating the relationship between frequency and B ² f with a simulator. The electromagnetic stirrer, the moving magnetic field, and the electromagnetic force formation state in the molten steel in the mold are schematically shown. FIG. 2 schematically shows a mechanism in which a difference occurs in the distribution in the depth direction of electromagnetic force in molten steel in a mold by changing the induction loss in the copper plate depending on the frequency. As a result of electromagnetic field analysis, it is shown how the electromagnetic force attenuates from the surface of the mold copper plate with respect to the electromagnetic force formed in the molten steel in the mold. The region where the electromagnetic force substantially acts in the molten steel by the attenuation of the electromagnetic force by the frequency and the solidified shell is schematically shown. The positional relationship of the electromagnetic stirring apparatus and immersion nozzle in a casting_mold | template is typically shown, (a) is a top view, (b) is front sectional drawing, (c) is side sectional drawing. The relationship between the frequency of the present invention and the skin depth is shown. The positional relationship in the method of controlling the flow in a casting_mold | template using an electromagnetic stirring apparatus and a DC magnetic field generator is shown typically. In the method of controlling the flow in the mold using an electromagnetic stirrer and a DC magnetic field generator, the distribution of the stirring flow velocity in the depth direction from the molten metal surface is schematically shown.

  As schematically shown in FIG. 2, by installing an electromagnetic stirrer on the back side of the long side of the mold, the mold thickness direction component By of the magnetic flux density, the vertical component Jz of the induced current, and the outer product of both of them in the molten steel in the mold By forming the width direction component Fx of the electromagnetic force obtained as a result, a propulsive force in the same direction as the moving magnetic field is applied to the molten steel. Furthermore, a method of forming a swirling flow in a horizontal section in the vicinity of the molten metal surface in the mold by applying a reverse propulsive force on the opposite long sides is widely used. Regarding the depth direction distribution of electromagnetic force in the stirring region formed in the molten steel, it has been considered that the iron core center is the strongest and has a vertically symmetric distribution. In addition, the thickness direction distribution from the mold surface has been considered to follow the skin depth determined by the electrical conductivity and frequency of the steel. However, detailed investigations by the inventors have found that the distribution of electromagnetic force in the molten steel depends on the frequency and the electrical conductivity of the copper plate.

  Hereinafter, the obtained knowledge will be described. The distribution of electromagnetic force in the molten steel is considered by dividing it into two parts: (1) depth direction distribution in the molten steel (z direction distribution in the figure) and (2) thickness direction distribution (y direction distribution). The copper plate between the electromagnetic stirring coil and the molten steel has a great influence on both.

  First, the depth direction distribution (z direction distribution in FIG. 2) of the electromagnetic force (1) will be described. As described above, it seems that the distribution of electromagnetic force in the depth direction is symmetric with respect to the iron core center. However, as schematically shown in FIG. It was found that the loss due to induction was different between the top and bottom, and the top loss was small. For this reason, the distribution of electromagnetic force is shifted to the molten metal surface side, that is, the peak of electromagnetic force is shifted upward from the coil center. The reason for this is explained as follows using FIG. In the case of an electromagnetic stirrer, the coil is wound vertically around the iron core, so that the current is applied in the vertical direction. Therefore, the current induced in the copper plate is also in the vertical direction of the copper plate. In the method of forming a swirling flow near the mold surface in the mold as in the present invention, an electromagnetic stirring device is installed near the mold surface. In addition, the height of the hot water surface in the mold is positioned about 50 mm to 200 mm from the upper end of the mold. Therefore, considering the distance between the coil center and the upper end and lower end of the copper plate, the distance between the lower end of the mold and the coil center is long, so that the induction loss becomes larger in the lower part where the induced current easily flows. Therefore, since the induction loss is large in the lower part at the upper part and the lower part, the peak position of the electromagnetic force shifts upward. This phenomenon becomes more prominent as the frequency is higher.

  On the other hand, when the frequency is relatively low, the induction loss is reduced as a whole as shown in the lower part of FIG. The effect of reducing the induction loss is great at the lower part of the coil where the induction is large. As a result, although both the upper and lower losses are smaller, the reduction is larger in the lower part, so that the peak of the electromagnetic force shifts downward. That is, the distribution of electromagnetic force in the depth direction varies depending on the frequency.

Next, the thickness direction distribution (y direction distribution in FIG. 2) of the electromagnetic force formed in molten steel with an electromagnetic stirring apparatus was investigated using electromagnetic field analysis software. In the evaluation, the electrical conductivity in the molten steel is the electrical conductivity of the molten steel (7.22 × 10 ⁵ S / m), the permeability is 4π × 10 ⁻⁷ N / A ² , and the applied frequency is 4.5 Hz. The y-direction distribution of the electromagnetic force (N / m ³ ) at the center of the coil in the depth direction (z direction in FIG. 2) was calculated. The results are shown in the plot (■) in FIG. The horizontal axis in FIG. 4 is the distance in the y direction from the surface of the solidified shell. Further, as a result of approximating the plot (■) as an exponential function, the solid line in FIG. 4 was obtained. The solid line is
F = 5944.2 × exp (−24.82 × y) (ii)
The correlation coefficient was R ² = 0.9898.

Conventionally, the distribution in the thickness y direction of the magnetic force B and current I in the molten steel in the mold excited by the electromagnetic stirrer is generally the electrical conductivity σ, permeability μ, applied magnetic field angular velocity of the conductor (molten steel in this case). The skin depth δ ₁ determined by ω was considered to be determined as follows. The skin depth δ means a distance at which the value attenuates to 1 / e.
B = B ₀ × exp (−y / δ ₁ ) (iii)
I = I ₀ × exp (−y / δ ₁ ) (iv)
δ ₁ = √ (2 / σωμ) (v)
Here, B ₀ and I ₀ are the magnetic force and current at y = 0. The vertical axis in FIG. 4 represents electromagnetic force, and electromagnetic force = magnetic field × current. Therefore, the electromagnetic force skin depth δ ₂ is ½ of the current and magnetic field skin depth δ ₁ .
δ ₂ = √ (1 / 2σωμ) (vi)

Therefore, for electromagnetic force F,
F = F ₀ × exp (−y / δ ₂ ) (vii)
Was plotted as a dashed line in the same FIG. Then, the ■ plot, which is the evaluation result of the electromagnetic field analysis software, proved that the electromagnetic force was attenuated at a much shorter distance than the broken line. FIG. 4 shows the result of calculating equation (vii) using the electric conductivity of a copper plate instead of molten steel as the electric conductivity used when calculating electromagnetic force skin depth δ ₂ of equation (vi). Is shown. From the analysis results, we found that the distribution in the thickness direction was determined by the skin depth determined by the electrical conductivity of the mold copper plate, not the molten steel. That is, the presence of the copper plate means that the electromagnetic force is concentrated near the copper plate. Since steel and copper have different electrical conductivities by about 20 times, the penetration depth of the electromagnetic field is the same as when a magnetic field having a frequency of about 20 times is applied in the vicinity of the mold copper plate. That is, the electromagnetic force formed in the molten steel through the copper plate by the electromagnetic stirrer is concentrated near the mold surface.

In addition, the solidified shell thickness increases as the depth direction distance z from the molten metal surface increases. As schematically shown in FIG. 5, depending on the distribution of electromagnetic force, an electromagnetic force may act in the solidified shell, but an electromagnetic force may not act in the molten steel inside the solidified shell. FIG. 5 shows the electromagnetic force skin depth δ ₂ ^H at a high frequency and the electromagnetic force skin depth δ ₂ ^L at a low frequency. According to FIG. 5, in the case of a low frequency, the electromagnetic force skin depth δ ₂ ^L is thicker than the thickness of the solidified shell throughout the depth direction of the mold (z direction in FIG. 2). In this case, in the lower half of the mold, the electromagnetic force skin depth δ ₂ ^H is thinner than the thickness of the solidified shell, and it is assumed that electromagnetic force does not reach the molten steel inside the solidified shell in this part. . For this reason, it is necessary to determine the application frequency and casting conditions of the electromagnetic stirring device in consideration of the skin depth of the electromagnetic force and the thickness of the solidified shell.

Based on the knowledge described above, the effect of electromagnetic stirring can be improved by casting after the stirring region formed in the molten steel in the mold becomes an appropriate region according to the casting conditions by the electromagnetic stirring device. Casting enjoyed to the fullest is possible. Specifically, it may be adjusted so as to satisfy the relational expression determined from the relationship among the mold copper plate electrical conductivity σ, the mold copper plate thickness D _Cu , the casting speed V _c , and the nozzle immersion depth L. In particular,
D _Cu <√ (1 / (2σωμ)) <D _Cu + k√ (L / V _c ) (i)
Here, ω = 2πf is an angular frequency (rad / s), f is an applied frequency (Hz) of an electromagnetic stirrer, μ is a vacuum permeability (N / A ² ), and k is a solidified shell growth rate coefficient.

Note that √ (1 / (2σωμ)) is a half value of the skin depth δ ₁ of the current or magnetic field, and means the skin depth δ ₂ of the electromagnetic force. The skin thickness is determined not from the slab surface but from the electromagnetic coil side surface of the mold copper plate as the origin. As described above, the mold copper plate electrical conductivity σ is used as the electrical conductivity for calculating the skin depth.

As described in Non-Patent Document 1, the solidification shell growth rate coefficient k varies depending on the thickness of the mold copper plate, the amount of cooling water, the secondary cooling conditions, the continuous casting powder used, and the like. The range is 2 cm / min ^1/2 . Specifically, it can be determined by adding sulfur to the molten steel in the mold during casting and collecting a sulfur print of the slab.

First, it is necessary to make the skin depth of the electromagnetic force thicker than the copper plate thickness D _Cu . This is an indispensable condition for the electromagnetic force to penetrate into the molten steel, and the maximum value of the frequency is defined by this condition. The above condition can be achieved by satisfying the inequality of the left side and the middle side of the formula (i).

Next, it is necessary to avoid as much as possible interference between the stirring flow of the molten steel by the electromagnetic stirring device and the nozzle discharge flow discharged from the immersion nozzle. This is because when a swirl flow is formed across the nozzle discharge flow, the flow is in the opposite direction to one of the swirl flows, causing interference. For this reason, it is preferable that the nozzle discharge flow be installed below the region where the swirl flow is formed. FIG. 6 schematically shows the positional relationship between the electromagnetic stirring device and the immersion nozzle in the mold. (A) is a plan view, (b) is a front sectional view, and (c) is a side sectional view. is there. As shown in FIG. 6C, the immersion depth from the upper end position of the immersion nozzle discharge hole to the molten metal surface is L. The solidified shell thickness D _SH at the immersion depth L is
D _SH = k√ (L / Vc) (viii)
Can be expressed as Here, V _c is a casting speed, and k is a solidified shell growth rate coefficient. If the solidified shell thickness D _SH at the immersion depth L is adjusted so that the electromagnetic force by the electromagnetic stirring device does not reach the molten steel inside the solidified shell, the molten steel stirring flow by the electromagnetic stirring device is discharged from the immersion nozzle. Interference with the nozzle discharge flow can be avoided. As described above, according to the analysis results using the electromagnetic field analysis software, it was found that the attenuation of electromagnetic force in the molten steel in the mold was close to that when using the electrical conductivity of the mold copper plate as the electrical conductivity. . Therefore, if the skin depth when the electrical conductivity of the copper plate is used for σ of δ ₂ = √ (1 / (2σωμ)) is less than D _Cu + D _SH , electromagnetic stirring is performed at the immersion depth L. Will not be effective. That is, it is only necessary to satisfy the inequality of the middle side and the right side of the formula (i). The frequency that satisfies this condition is the lower limit of the frequency.

An example of the result is shown in FIG. The horizontal axis in FIG. 7 is the applied frequency of the electromagnetic stirrer, and the vertical axis is the distance in the thickness direction from the electromagnetic coil side surface of the casting copper plate (indicated as “skin depth” in the figure). The thick solid line means the skin depth when the electrical conductivity of the copper plate is used for σ of δ ₂ = √ (1 / (2σωμ)). That is, it means that the electromagnetic force applied by the electromagnetic stirring device under the condition of thick line> thin line acts on the molten steel. Here, the electrical conductivity of the copper plate varies depending on the copper plate material (copper plate alloy type). As an example, when performing casting at a casting speed of 1 m / min and an immersion depth of 350 mm using a copper plate of ES40A (copper plate thickness is 25 mm), the middle side and the right side of the formula (i) are approximately at a frequency of 2.5 Hz. Since the left side and the middle side of the formula (i) are equal at a frequency of 5.5 Hz, the frequency greater than the frequency 2.5 Hz and less than 5.5 Hz satisfies the above-described relational expression. In the figure, lines with different distances from the surface of the hot water are added, but if you want to secure the stirring area up to the nozzle immersion depth, the frequency is 2.5 Hz, and the stirring is performed up to the bottom of the iron core of the electromagnetic stirrer. This indicates that it is necessary to select a frequency of 3 Hz when the region is desired. That is, by adjusting the frequency, it is possible to set a condition as to how far the molten steel pool in the mold is set as the stirring region. Conditions can be set by adjusting the electrical conductivity of the mold copper plate. That is, by using the present invention, the agitation region in the molten steel pool in the mold can be adjusted with a high degree of freedom in consideration of the presence of the solidified shell in the mold, resulting in the required slab quality. Accordingly, the flow in the mold can be created in the most preferable form.

  In the present invention, an electromagnetic stirring device that forms a swirling flow in a horizontal cross section is installed in the vicinity of the molten metal surface in the mold to control the flow in the continuous casting mold. Here, installing the electromagnetic stirrer in the vicinity of the hot water surface in the mold can be realized by providing the upper end of the iron core of the electromagnetic stirrer within a range of 50 mm above and below the hot water surface in the mold. Desirably, it is within half of the core thickness.

  Further, the present invention is combined with a device that generates a DC magnetic field below the electromagnetic stirring device as schematically shown in FIG. The present invention can also be applied to a casting method in which a swirl flow is applied within a horizontal section in the vicinity. Under the conditions in which this DC magnetic field generator is used in combination, as shown schematically in FIG. 9, a part of the stirring flow is braked by the DC magnetic field, so that the stirring range is limited. By lowering the frequency of the stirrer or lowering the electrical conductivity of the copper plate, the stirring flow can be applied over a wide range as much as possible.

  The ultra-low carbon steel was melted by refining in the converter, processing in the reflux type vacuum degassing equipment and alloy addition. This molten steel was cast into a slab having a thickness of 280 mm and a width of 1800 mm. Molds with different copper plate thicknesses and materials were prepared, and casting was performed under the conditions of different electromagnetic stirrer frequency, casting speed, and nozzle immersion depth. The electromagnetic stirrer iron core had a thickness in the casting direction (core thickness) of 0.2 m, and was installed so that the upper end of the iron core was at the position of the hot water surface in the mold. The electromagnetic stirring thrust was 80 mmFe / m. Further, casting was performed using a DC magnetic field generator that applies a DC magnetic field having a substantially uniform magnetic flux density distribution across the entire width of the mold in the mold thickness direction below the electromagnetic stirrer.

Conditions and results are shown in Table 1. In both cases, the casting speed was 1,1.5 and 2 m / min, and Ar gas was allowed to flow through the nozzle at 10 Nl / min. In estimating the solidified shell thickness, k: solidified shell growth rate coefficient (cm / min ^1/2 ) = 2.0 cm / min ^1/2 .

  Under the conditions indicated as “0.5 T” in the “DC magnetic field generator” column of Table 1, the DC magnetic field generator was also used, and the magnetic flux density of the applied DC magnetic field was 0.5 Tesla. The "-" in the column does not use a DC magnetic field generator.

  Regarding the number of bubbles / inclusions in the slab surface layer part, a sample having a total width of 200 mm in length in the casting direction was cut out from the upper and lower surfaces of the slab, and the bubbles / inclusions in the surface of the total width × 200 mm in length were 1 mm from the surface. Every 10 mm was ground, the number of bubbles / inclusions of 100 μm or more was investigated, and the sum of the number was indexed as the defect index. The condition in which the electromagnetic force is not applied is set to 10 and the ratio is displayed. The defect index of 4 or less is good and the defect index is good. In Table 1, the electrical conductivity of the copper plate is required when obtaining the electromagnetic force index. The values of electrical conductivity at room temperature shown in the lower column of Table 1 were used.

ＥＳ４０Ａ、ＥＳ５０Ａ、ＥＳ６０Ａは銅板材質を示し、数値が高くなるほど電気伝導度が高い。

ES40A, ES50A, and ES60A indicate copper plate materials, and the higher the numerical value, the higher the electrical conductivity.

In Table 1, Comparative Examples 1 to 4 were cast using the mold copper plate material ES40A, and the casting speed was 1 m / min, 1.5 m / min, and 2 m / min. At this time, in Comparative Examples 1 and 3, the frequency was set to 2 Hz. As a result, the defect index was large and many powder entrainments were observed. It was estimated as a result of entrainment due to interference between the nozzle discharge flow and the stirring flow. The electromagnetic force index was also larger than the “sum of the mold copper plate thickness and the solidified shell thickness at the nozzle immersion depth position”, and it was estimated that the stirring penetrated into the interior. On the other hand, in Comparative Examples 2 and 4, casting was performed under the condition of a frequency of 8 Hz, but the defect index was large. In this case, the electromagnetic force skin depth was less than the mold copper plate thickness, and the stirring was sufficient. It was estimated that the result was not working. In Comparative Examples 5 to 8, casting was performed using ES50A and ES60A, which have higher electrical conductivity of the copper plate. Moreover, the frequency of electromagnetic stirring was set to 8 Hz in Comparative Examples 5 and 6, and 6 Hz in Comparative Examples 7 and 8. In Comparative Examples 6 and 8, casting was performed using a DC magnetic field generator capable of applying a uniform magnetic field in the width direction in the mold thickness direction. In any of Comparative Examples 5 to 8, the skin depth of the electromagnetic force was equal to or less than the thickness of the copper plate, and the defect index increased due to insufficient stirring flow rate. The tendency was conspicuous under the conditions where the DC magnetic field generators of Comparative Examples 6 and 8 were applied. On the other hand, in Comparative Example 9, casting was performed under the conditions of Comparative Example 1 with a frequency of 3 Hz and an immersion depth of 200 mm. Under this condition, the skin depth δ ₂ of the electromagnetic force is not less than “the sum of the mold copper plate thickness D _Cu and the solidified shell thickness D _SH at the nozzle immersion depth position”. The force penetrated to the inside, causing interference with the nozzle discharge flow and increasing the defect index. On the other hand, in Invention Examples 1 to 8, casting was performed by changing the mold copper plate material, the casting speed, and the presence or absence of the DC magnetic field generator. At that time, the electromagnetic force penetration depth δ ₂ is larger than the copper plate thickness and less than “the sum of the mold copper plate thickness D _Cu and the solidified shell thickness D _SH at the nozzle immersion depth position”. It was a good defect index even under the conditions.

1 Mold 2 Immersion Nozzle 3 Electromagnetic Stirrer 4 Discharge Port 5 Iron Core
6 Coil 8 DC Magnetic Field Generator 10 Molten Steel 11 Discharge Flow 12 Swirl Flow 13 Moving Magnetic Field 14 Electromagnetic Force (Lorentz Force)

Claims (2)

In a continuous casting method of steel in which an electromagnetic stirrer that forms a swirling flow in a horizontal section is installed near the molten metal surface in the mold and the flow in the continuous casting mold is controlled, the mold copper plate thickness D _Cu , casting speed V _c , nozzle immersion depth The continuous casting method of steel, wherein the length L, the applied frequency f of the electromagnetic stirrer and the electrical conductivity σ of the copper plate are adjusted so as to satisfy the following relational expression.
D _Cu <√ (1 / (2σωμ)) <D _Cu + k√ (L / V _c ) (i)
Here, ω = 2πf is an angular frequency (rad / s), μ is a magnetic permeability (N / A ² ) in vacuum, and k is a solidified shell growth rate coefficient.   3. Use according to claim 1, in combination with a DC magnetic field generator for applying a DC magnetic field having a uniform magnetic flux density distribution in the width direction below the electromagnetic stirrer forming a swirl flow in a horizontal section in the mold thickness direction. A method for continuously casting steel, characterized by:

Images