阅读说明：本技术 钢的连续铸造方法 (Method for continuously casting steel ) 是由 松井章敏 近藤裕计 菊池直树 于 2017-04-25 设计创作，主要内容包括：在将交流磁场向铸模内钢水施加而使铸模内钢水产生回旋搅拌流的连续铸造方法中,施加与浸渍喷嘴的浸渍深度及从铸模内钢水熔液面至交流磁场的峰值位置的距离相应的适当的交流磁通密度,制造出高品质的铸片。本发明的钢的连续铸造方法是经由在一对铸模长边的背面相对设置的交流磁场产生装置向铸模内钢水施加交流磁场而使铸模内钢水产生水平方向的回旋搅拌流的钢的连续铸造方法,相对的所述铸模长边彼此的间隔设为200～300mm,具有两个喷出孔的浸渍喷嘴的所述喷出孔的喷出角度设为向下5°至向下50°的范围,所述交流磁场的频率设为0.5Hz以上且3.0Hz以下,并且,根据所述交流磁场的峰值位置,将所述浸渍喷嘴的浸渍深度及所述交流磁场产生装置产生的交流磁场的峰值位置的磁通密度控制成规定的范围。(in a continuous casting method in which an AC magnetic field is applied to molten steel in a mold to generate a swirling flow of molten steel in the mold, an AC magnetic flux density is applied to an appropriate depth of immersion of an immersion nozzle and a distance from a molten steel surface in the mold to a peak position of the AC magnetic field, thereby producing a high-quality cast piece. The continuous casting method of steel according to the present invention is a continuous casting method of steel in which an alternating magnetic field is applied to molten steel in a mold via an alternating magnetic field generating device provided opposite to a back surface of a pair of mold long sides to generate a swirling stirring flow in a horizontal direction to the molten steel in the mold, an interval between the opposing mold long sides is set to 200 to 300mm, a discharge angle of the discharge hole of a dipping nozzle having two discharge holes is set to a range of 5 ° downward to 50 ° downward, a frequency of the alternating magnetic field is set to 0.5Hz or more and 3.0Hz or less, and a dipping depth of the dipping nozzle and a magnetic flux density at a peak position of the alternating magnetic field generated by the alternating magnetic field generating device are controlled to a predetermined range according to a peak position of the alternating magnetic field.)

1. a method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

The distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

The frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

The distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is set to 200mm or more and less than 300mm,

The immersion depth of the immersion nozzle, i.e., the distance from the molten steel level in the mold to the upper end of the discharge hole of the immersion nozzle, is set to 100mm or more and less than 200mm,

The magnetic flux density at the peak position of the alternating magnetic field is set to 0.040T or more and less than 0.060T.

2. A method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

the distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

the frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

The distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is set to 300mm or more and less than 400mm,

the immersion depth of the immersion nozzle, i.e., the distance from the molten steel level in the mold to the upper end of the discharge hole of the immersion nozzle, is set to 100mm or more and less than 300mm,

The magnetic flux density at the peak position of the alternating magnetic field is set to 0.060T or more and less than 0.080T.

3. A method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

The distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

the frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

The distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is set to 400mm or more and less than 500mm,

The immersion depth of the immersion nozzle, i.e., the distance from the molten steel level in the mold to the upper end of the discharge hole of the immersion nozzle, is set to 100mm or more and less than 300mm,

The magnetic flux density at the peak position of the alternating magnetic field is set to 0.080T or more and less than 0.100T.

4. A method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

The distance between the opposing long sides of the mold is set to 200 to 300mm,

the discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

The frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

The immersion depth of the immersion nozzle, that is, the distance from the molten steel surface in the mold to the upper end of the discharge hole of the immersion nozzle and the magnetic flux density at the peak position of the alternating-current magnetic field generated by the alternating-current magnetic field generating device are made to satisfy any one of the following three conditions (A), (B) and (C) based on the peak position of the alternating-current magnetic field,

condition (a): when the distance from the molten steel surface in the mold to the peak position of the AC magnetic field is 200mm or more and less than 300mm, the immersion depth of the immersion nozzle is 100mm or more and less than 200mm, and the magnetic flux density at the peak position of the AC magnetic field is 0.040T or more and less than 0.060T,

Condition (B): when the distance from the molten steel surface in the mold to the peak position of the AC magnetic field is 300mm or more and less than 400mm, the immersion depth of the immersion nozzle is 100mm or more and less than 300mm, and the magnetic flux density at the peak position of the AC magnetic field is 0.060T or more and less than 0.080T,

condition (C): when the distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is 400mm or more and less than 500mm, the immersion depth of the immersion nozzle is 100mm or more and less than 300mm, and the magnetic flux density at the peak position of the alternating magnetic field is 0.080T or more and less than 0.100T.

Technical Field

The present invention relates to a continuous casting method for steel, in which an alternating-current magnetic field is applied to molten steel in a mold, and the molten steel is continuously cast while controlling the flow of molten steel in the mold by the alternating-current magnetic field.

Background

In recent years, high-grade steel sheet products such as steel sheets for automobiles, steel sheets for cans, and high-performance thick steel sheets have been required to have high quality, and high quality is desired at the stage of slab casting produced by continuous casting. One of the qualities required for a flat cast slab (hereinafter, also simply referred to as "slab") is that the surface layer and the internal oxide-based non-metallic inclusions (hereinafter, simply referred to as "inclusions") of the slab are small.

As the insert trapped on the surface layer and inside of the cast slab, there are (1) a deoxidation product generated in a deoxidation step of molten steel based on aluminum or the like and suspended in the molten steel, (2) bubbles of argon gas blown into the molten steel through a tundish or an immersion nozzle, and (3) a mold powder in which a mold powder scattered on a molten steel melt surface in a mold is entrained in the molten steel and suspended. Since both of them become surface defects and internal defects at the product stage, it is important to reduce the number of inclusions trapped in the surface layer and the interior of the cast piece.

Conventionally, in order to prevent deoxidation products, molding powder, and argon bubbles in molten steel from being captured by a solidified shell and to prevent product defects caused by inclusions, a magnetic field is applied to molten steel in a mold, and the flow of molten steel is controlled by electromagnetic force generated by the magnetic field. Many proposals have been made regarding this technique.

for example, patent document 1 discloses the following technique: an alternating-current magnetic field is applied to a discharge flow from an immersion nozzle immersed in molten steel in a mold, and a braking force or a horizontal rotational force is applied to the discharge flow so that the molten steel flow rate at the molten steel surface in the mold is in a range of an adhesion critical flow rate of an insert or more and a entrainment critical flow rate of mold powder or less.

Patent document 2 discloses the following method: the upper end of an alternating-current magnetic field generating device is positioned at a position 20-60 mm below the molten steel surface in the casting mold, and a dipping nozzle with a downward angle of 1-30 DEG is used, so that the spraying flow from the dipping nozzle collides with a solidified shell in a range from the center of the alternating-current magnetic field generating device to 450mm below, thereby continuously casting the molten steel.

Further, patent document 3 discloses a method of: when a swirling stirring flow in the width direction of the mold is imparted to molten steel in the mold by the AC magnetic field generator, the discharge port of the immersion nozzle is provided at a position where the magnetic flux density at the discharge port becomes 50% or less of the maximum magnetic flux density of the AC magnetic field generator, and the molten steel is continuously cast.

prior art documents

Patent document

patent document 1: japanese patent laid-open publication No. 2003-320440

Patent document 2: japanese patent laid-open No. 2000-202603

Patent document 3: japanese patent laid-open No. 2001-047201

Disclosure of Invention

Problems to be solved by the invention

however, the above-described conventional techniques have the following problems.

That is, patent document 1 is a method of performing flow control by applying a braking force or a stirring force in a horizontal direction to a discharge flow from an immersion nozzle based on a value of a molten steel flow velocity at a molten steel surface in a mold, and therefore, some equipment for measuring or monitoring the molten steel flow velocity at the molten steel surface in the mold is required. In addition, when the installation position of the ac magnetic field generating device installed on the back surface of the mold is changed, the accuracy of the critical flow rate prediction formula may be deteriorated, and it is difficult to say that the technique can be applied to the ac magnetic field generating device installed at any position on the back surface of the mold.

Patent document 2 focuses on the position where the discharge flow from the immersion nozzle collides, but is limited to a case where the ac magnetic field generating device is provided in the vicinity of the molten steel level in the mold, and cannot cope with a case where the ac magnetic field generating device is provided below the molten steel level in the mold.

Patent document 3 is also limited to a case where the ac magnetic field generating device is provided in the vicinity of the molten steel level in the mold, as in patent document 2. Further, although the discharge port of the immersion nozzle is provided at a position of 50% or less of the maximum magnetic flux density, in this case, the intervening material or the like may intrude below the ac magnetic field generating device in order to direct the discharge flow from the immersion nozzle to a position below the ac magnetic field generating device, which may cause internal defects of the cast slab.

the present invention has been made in view of the above circumstances, and an object thereof is to provide a method for continuously casting steel, in which an ac magnetic field is applied to molten steel in a mold from an ac magnetic field generating device provided across a long side of the mold, and a swirling stirring flow is generated in the molten steel in the mold, wherein a suitable ac magnetic flux density is given according to a distance from a melt surface of the molten steel in the mold to a peak position of the ac magnetic field and an immersion depth of an immersion nozzle, thereby making it possible to produce high-quality cast pieces.

Means for solving the problems

The gist of the present invention for solving the above problems is as follows.

[1] a method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

The distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

The frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

the distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is set to 200mm or more and less than 300mm,

The immersion depth of the immersion nozzle (the distance from the molten steel level in the mold to the upper end of the discharge hole of the immersion nozzle) is set to 100mm or more and less than 200mm, and,

the magnetic flux density at the peak position of the alternating magnetic field is set to 0.040T or more and less than 0.060T.

[2] A method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

the distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

the frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

the distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is set to 300mm or more and less than 400mm,

The immersion depth of the immersion nozzle (the distance from the molten steel level in the mold to the upper end of the discharge hole of the immersion nozzle) is set to 100mm or more and less than 300mm, and,

The magnetic flux density at the peak position of the alternating magnetic field is set to 0.060T or more and less than 0.080T.

[3] A method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

The distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

The frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

The distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is set to 400mm or more and less than 500mm,

the immersion depth of the immersion nozzle (the distance from the molten steel level in the mold to the upper end of the discharge hole of the immersion nozzle) is set to 100mm or more and less than 300mm, and,

The magnetic flux density at the peak position of the alternating magnetic field is set to 0.080T or more and less than 0.100T.

[4] a method of continuously casting steel, in which a molten steel is poured into a continuous casting mold having an inner space in which a pair of mold long sides and a pair of mold short sides form a rectangle, and a solidified shell produced by solidifying the molten steel is drawn from the mold to produce a cast piece,

Applying an alternating-current magnetic field to molten steel in the mold via an alternating-current magnetic field generating device provided on the back surfaces of the pair of mold long sides so as to face each other with the mold long sides interposed therebetween, and generating a swirling stirring flow in the horizontal direction in the molten steel in the mold by the alternating-current magnetic field,

the distance between the opposing long sides of the mold is set to 200 to 300mm,

The discharge angle of the dip nozzle having two discharge holes for injecting molten steel into the internal space is set to a range of 5 DEG downward to 50 DEG downward,

the frequency of the alternating current magnetic field is set to be more than 0.5Hz and less than 3.0Hz,

the immersion depth of the immersion nozzle (distance from the molten steel surface in the mold to the upper end of the discharge hole of the immersion nozzle) and the magnetic flux density at the peak position of the alternating-current magnetic field generated by the alternating-current magnetic field generating device are made to satisfy any one of the following three conditions (a), (B), and (C) according to the peak position of the alternating-current magnetic field.

Condition (a): when the distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is 200mm or more and less than 300mm, the immersion depth of the immersion nozzle is 100mm or more and less than 200mm, and the magnetic flux density at the peak position of the alternating magnetic field is 0.040T or more and less than 0.060T.

condition (B): when the distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is 300mm or more and less than 400mm, the immersion depth of the immersion nozzle is 100mm or more and less than 300mm, and the magnetic flux density at the peak position of the alternating magnetic field is 0.060T or more and less than 0.080T.

Condition (C): when the distance from the molten steel surface in the mold to the peak position of the alternating magnetic field is 400mm or more and less than 500mm, the immersion depth of the immersion nozzle is 100mm or more and less than 300mm, and the magnetic flux density at the peak position of the alternating magnetic field is 0.080T or more and less than 0.100T.

effects of the invention

According to the present invention, since the ac magnetic field having an appropriate magnetic flux density according to the distance from the molten steel surface in the mold to the peak position of the ac magnetic field and the immersion depth of the immersion nozzle is applied to impart the swirling stirring flow to the molten steel in the mold, the trapping of the deoxidation products, argon bubbles, and mold powder into the solidified shell can be suppressed, and the production of high-quality cast pieces can be easily achieved.

Drawings

Fig. 1 is a view showing an example of an embodiment of the present invention, and is a schematic view of a mold part of a slab continuous casting machine.

Fig. 2 is an enlarged view of the immersion nozzle shown in fig. 1.

Detailed Description

Hereinafter, embodiments of the present invention will be described.

The present inventors have conducted experiments and studies on the flow of molten steel in a mold in a continuous casting method for steel in which an alternating-current magnetic field is applied to molten steel in the mold and a swirling stirring flow in a horizontal direction is generated in the molten steel in the mold by the alternating-current magnetic field. In the test, a test was performed on a state in which a molten steel discharge flow is discharged from each discharge hole toward the short side of the mold, in particular, a molten steel flow state in the mold when the peak position of the ac magnetic field and the immersion depth of the immersion nozzle are changed, using a mold having a pair of long sides of the mold and a pair of short sides of the mold and forming a rectangular inner space, and an immersion nozzle (hereinafter, also referred to as a "two-hole immersion nozzle") having 2 discharge holes at the center of the inner space.

here, the peak position of the alternating-current magnetic field is a position at which the maximum root mean square value per time period of a component orthogonal to the inner wall surface in the magnetic flux density of the alternating-current magnetic field at the inner wall surface of the mold surrounding the internal space of the mold becomes maximum along the inner wall surface. The immersion depth of the immersion nozzle is defined by the distance from the molten steel level (also referred to as a "meniscus") in the mold to the upper end of the discharge hole of the immersion nozzle.

In the experiment, the flow state of the low melting point alloy in the mold, the flow velocity distribution in the mold, and the like were examined at this time by changing the installation position of the alternating-current magnetic field generating device provided to face the long side and the back side of the mold and the installation position of the dipping nozzle, that is, the dipping depth, and by using numerical calculation and a low melting point alloy device of a real 1/4 size. As the low melting point alloy, a Bi-Pb-Sn-Cd alloy (melting point; 70 ℃ C.) was used.

As a result of the examination, it was found that there was an appropriate range of application of the magnetic flux density of the alternating-current magnetic field depending on the peak position of the alternating-current magnetic field and the immersion depth of the immersion nozzle. That is, it is found that the application conditions of the alternating-current magnetic field can be roughly classified into 3 types of modes of the conditions (a) to (C) according to the peak position of the alternating-current magnetic field and the immersion depth of the immersion nozzle. The results of the study are shown in Table 1. The peak position of the ac magnetic field is defined by the distance from the molten steel surface in the mold to the peak position of the ac magnetic field.

[ Table 1]

1; condition (A)

When the peak position of the AC magnetic field is 200mm or more and less than 300mm from the molten steel surface in the mold, the immersion depth of the double-hole immersion nozzle is 100mm or more and less than 200mm, and the magnetic flux density at the peak position of the AC magnetic field is 0.040T or more and less than 0.060T.

The magnetic flux density is defined by a calculated average value of an effective value (Root Mean Square value) of the magnetic flux density at a peak position of the magnetic flux density in the cast piece drawing direction among the magnetic flux densities in the normal direction at a position separated by 15mm from a plane forming an internal space of a mold copper plate on which an alternating-current magnetic field generating device is arranged behind the mold copper plate in a direction toward the internal space along the normal direction of the plane, the effective value being measured at an arbitrary pitch in the mold width direction. The measurement pitch in the mold width direction may be considered to be a pitch of a degree that can sufficiently represent the spatial profile of the magnetic flux density.

When the magnetic flux density is less than 0.040T, the swirling stirring force is weak, and therefore the effect of cleaning the solidified shell by argon bubbles or deoxidation products is hardly exhibited. On the other hand, when the magnetic flux density is 0.060T or more, the swirling stirring force is too strong, and therefore, the entrainment of the powder by the long die is promoted.

When the immersion depth of the immersion nozzle is less than 100mm, the distance between the molten steel level and the discharge flow in the mold is too short, and thus fluctuation of the molten steel level is easily promoted in the mold. When the immersion depth is 200mm or more, the cost of the refractory may increase due to the longer immersion nozzle body, or the immersion nozzle may be easily damaged in view of heat resistance and load resistance, and the operation cost may increase instead.

2; condition (B)

When the peak position of the AC magnetic field is 300mm or more and less than 400mm from the molten steel surface in the mold, the immersion depth of the double-hole immersion nozzle is 100mm or more and less than 300mm, and the magnetic flux density at the peak position of the AC magnetic field is 0.060T or more and less than 0.080T.

The peak position of the ac magnetic field is deeper than the condition (a) from the molten steel surface in the mold, and therefore a magnetic flux density stronger than the condition (a) is required. That is, when the magnetic flux density is less than 0.060T, the swirling stirring force is weak, and therefore, it is difficult to exhibit the effect of cleaning the argon bubbles or the deoxidation products from the solidified shell. On the other hand, when the magnetic flux density is 0.080T or more, the swirling stirring force is too strong, and therefore, the entrainment of the long mold powder is promoted.

when the immersion depth of the immersion nozzle is less than 100mm, the distance between the molten steel level and the discharge flow in the mold is too short, and thus fluctuation of the molten steel level is easily promoted in the mold. When the immersion depth is 300mm or more, the cost of the refractory may increase due to the longer main body of the immersion nozzle, or the immersion nozzle may be easily damaged in view of heat resistance and load resistance, and the operation cost may increase.

3; condition (C)

when the peak position of the AC magnetic field is not less than 400mm and less than 500mm from the molten steel level in the mold, the immersion depth of the double-hole immersion nozzle is not less than 100mm and less than 300mm, and the magnetic flux density at the peak position of the AC magnetic field is not less than 0.080T and less than 0.100T.

The peak position of the ac magnetic field is deeper from the molten steel surface in the mold than in the conditions (a) and (B), and therefore a higher magnetic flux density is required. That is, when the magnetic flux density is less than 0.080T, the swirling stirring force is weak, and thus it is difficult to exert the effect of cleaning the argon bubbles or the deoxidation products in the solidified shell. On the other hand, when the magnetic flux density is 0.100T or more, the swirling stirring force is too strong, and therefore, the entrainment of the powder by the long die is promoted.

When the immersion depth of the immersion nozzle is less than 100mm, the distance between the molten steel level and the discharge flow in the mold is too short, and thus fluctuation of the molten steel level is easily promoted in the mold. When the immersion depth is 300mm or more, the cost of the refractory may increase due to the longer main body of the immersion nozzle, or the immersion nozzle may be easily damaged in view of heat resistance and load resistance, and the operation cost may increase.

In the conditions (a) to (C), the discharge angle of the immersion nozzle used is set to a range from 5 ° downward to 50 ° downward. When the discharge angle is less than 5 ° downward, the ac magnetic field cannot be sufficiently applied to the discharge flow. On the other hand, when the discharge angle is larger than 50 ° downward, the downward flow of the discharge flow becomes excessively strong, and therefore, the deoxidation products and gas bubbles may intrude into a deep position in the casting direction, which may become an internal defect and a starting point of fracture in the forming process of the steel sheet.

in the present invention, the peak position of the alternating magnetic field is set to be 200mm or more and less than 500mm from the molten steel level in the mold. When the peak position of the ac magnetic field is less than 200mm from the molten steel surface in the mold, the immersion depth of the immersion nozzle must be set to a position shallower than the peak position of the ac magnetic field in order to cause the ac magnetic field to act on the discharge flow from the immersion nozzle, which causes operational restrictions and makes it impossible to effectively apply the ac magnetic field. When the peak position of the alternating magnetic field is set to a position separated by 500mm or more from the molten steel surface in the mold, a swirling stirring flow is applied to a region where the solidified shell grows, and the effect of cleaning the deoxidized product or argon bubbles in the solidified shell is poor.

The frequency of the AC magnetic field is 0.5 to 3.0Hz, preferably 1.0 to 2.0 Hz. When the frequency is less than 0.5Hz, the electromagnetic force generated by the ac magnetic field is applied intermittently, and the effect of cleaning the deoxidation product or the argon bubbles in the solidified shell is unstable. On the other hand, when the frequency exceeds 3.0Hz, attenuation of magnetic flux density by the mold or the solidified shell becomes large, and an ac magnetic field cannot be effectively applied to molten steel in the mold.

Hereinafter, a specific embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a schematic view showing an example of an embodiment of the present invention, showing a mold part of a slab continuous casting machine, and fig. 2 is an enlarged view of a submerged nozzle shown in fig. 1.

In fig. 1 and 2, reference numeral 1 denotes molten steel, 2 denotes a solidified shell, 3 denotes a molten steel level in a mold, 4 denotes a discharge flow, 5 denotes a cast piece, 6 denotes a mold, 7 denotes a long side of the mold, 8 denotes a short side of the mold, 9 denotes a dipping nozzle, 10 denotes a discharge hole, 11 denotes an alternating-current magnetic field generating device, 12 denotes molding powder, and θ denotes a discharge angle of the dipping nozzle.

the mold 6 has a pair of opposing mold long sides 7 and a pair of opposing mold short sides 8 sandwiched between the mold long sides 7, and a rectangular inner space is formed by the pair of mold long sides 7 and the pair of mold short sides 8. A pair of ac magnetic field generators 11 disposed to face each other across the mold long side 7 is provided on the back surface of the mold long side 7. The interval between the opposing long sides of the mold is 200 to 300mm, the immersion nozzle 9 has 2 ejection holes 10, and the ejection angle (θ) of the ejection holes 10 ranges from 5 ° downward to 50 ° downward.

An immersion nozzle 9 is provided in the center of the rectangular internal space of the mold 6, and the molten steel 1 is poured into the internal space of the mold 6 by discharging the molten steel 1 from 2 discharge holes 10 toward the mold short sides 8 opposite to the respective discharge holes 10. The molten steel 1 poured into the internal space of the mold 6 is cooled by the mold long sides 7 and the mold short sides 8 to form a solidified shell 2. When a predetermined amount of molten steel 1 is poured into the internal space of the mold 6, the pinch rolls (not shown) are driven with the discharge holes 10 immersed in the molten steel 1 in the mold, and the drawing of the cast slab 5 having therein unsolidified molten steel 1 is started with the outer shell being the solidified shell 2. After the start of drawing, the molten steel level 3 in the mold is controlled to a substantially constant position, and the casting is conducted while the molten steel level is maintained at a substantially constant levelthe sheet drawing speed is increased to a predetermined cast sheet drawing speed. In FIG. 1, the dipping depth of the dipping nozzle 9 is represented by "L₁"means that the distance from the molten steel level 3 in the mold to the peak position of the AC magnetic field is represented by" L₂"means.

adding the mould powder 12 on the molten steel surface 3 in the casting mould. The mold powder 12 melts and can exert the effect of preventing oxidation of the molten steel 1 or flowing into between the solidified shell 2 and the mold 6 to serve as a lubricant. In order to prevent the adhesion of the deoxidation products suspended in the molten steel to the inner wall of the dipping nozzle, argon, nitrogen, or a mixed gas of argon and nitrogen is blown into the molten steel 1 flowing down through the dipping nozzle 9.

when the molten steel 1 is continuously cast in this manner, an alternating-current magnetic field is applied from the alternating-current magnetic field generator 11 to the molten steel 1 in the mold, and a swirling stirring flow in the horizontal direction is generated in the molten steel 1 in the mold. The frequency of the AC magnetic field is set to 0.5Hz to 3.0 Hz.

when an AC magnetic field is applied, the distance (L) from the molten steel surface (3) in the mold to the peak position of the AC magnetic field₂) When the depth is 200mm or more and less than 300mm (condition (A)), the depth (L) of immersion of the nozzle 9 is set to be₁) The magnetic flux density at the peak position of the AC magnetic field is set to be 0.040T or more and less than 0.060T, and is set to be 100mm or more and less than 200 mm.

further, the distance (L) from the molten steel level 3 in the mold to the peak position of the AC magnetic field₂) When the depth is 300mm or more and less than 400mm (condition (B)), the dipping depth (L) of the dipping nozzle 9 is set to₁) The magnetic flux density at the peak position of the AC magnetic field is set to be 0.060T or more and less than 0.080T.

Further, the distance (L) from the molten steel level 3 in the mold to the peak position of the AC magnetic field₂) When the depth is 400mm or more and less than 500mm (condition (C)), the dipping depth (L) of the dipping nozzle 9 is set to₁) The magnetic flux density at the peak position of the AC magnetic field is set to 0.080T or more and less than 0.100T.

The adjustment of the magnetic flux density at the peak position of the ac magnetic field is performed as follows. That is, the relationship between the electric power supplied to the ac magnetic field generator 11 and the magnetic flux density at the position 15mm away from the surface of the mold copper plate at the peak position of the ac magnetic field in the internal space of the mold 6 is measured in advance, and the electric power supplied to the ac magnetic field generator 11 is adjusted so that the magnetic flux density at the peak position of the ac magnetic field becomes a desired magnetic flux density.

as described above, according to the present invention, the distance (L) from the molten steel level 3 in the mold to the peak position of the AC magnetic field is applied₂) And the dipping depth (L) of the dipping nozzle₁) Since the ac magnetic field of appropriate magnetic flux density provides a swirling stirring flow to the molten steel in the mold, the trapping of the deoxidation products, argon bubbles, and the mold powder 12 into the solidification shell 2 can be suppressed, and the production of high-quality slab cast pieces can be easily achieved.