JP2015085370A - Continuous casting method of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of steel which provides a high quality slab hardly causing defect due to entrainment of minute air bubble or mold flux when casting a slab having a wide casting width with high through-put by the method of continuously casting steel while controlling molten steel flow in a casing mold by using electromagnetic force.SOLUTION: When continuously casting molten steel in such high efficiency conditions that a molten steel through-put Q is 6.0 to 7.5 ton/min, and a casting width is 1850 mm or more, a molten steel ejection angle of an immersion nozzle and an immersion depth and, further, the strength of vertical DC magnetic field and the strength of upper AC moving magnetic field are caused to fall into an optimal range and, moreover, the ratio R/Q of inactive gas blowing amount R from the immersion nozzle to molten steel through-put Q is satisfied by the relations:(15-Q)/3.75≤R/Q≤(16.5-Q)/3.75.

Description

  The present invention relates to a continuous casting method of a slab having a wide casting width, and relates to a method of continuously casting steel while controlling a flow of molten steel in a mold by electromagnetic force.

In recent years, quality requirements for high-grade steel products such as automobile steel plates and steel plates for cans have become stricter, and there has been a demand for higher quality from the slab manufacturing stage (continuous casting stage). One of the qualities required for slabs is a small amount of inclusions on the slab surface layer. The inclusions captured by the slab surface include the following (1) to (3). Since all of these become surface defects at the product stage, it is important to reduce the amount captured by the slab surface layer.
(1) Deoxidation products produced in the deoxidation process of molten steel with aluminum and suspended in the molten steel (2) Ar gas bubbles blown into the molten steel with a tundish or immersion nozzle (3) Molten steel in the mold Mold powder sprayed on the surface of the molten steel and suspended in molten steel

Conventionally, in order to prevent non-metallic inclusions, mold flux, and bubbles in molten steel from being trapped in the solidified shell and resulting in product defects, a magnetic field is applied to the molten steel flow in the mold and electromagnetic force generated by the magnetic field is used. The flow of molten steel has been controlled, and many proposals have been made regarding this technology.
For example, Patent Document 1 discloses a technique for braking a molten steel flow velocity by a static magnetic field. In this method, the molten steel flow discharged from the discharge hole of the immersion nozzle is braked by a DC magnetic field, so that non-metallic inclusions and mold flux accompanying the molten steel flow are not captured by the solidified shell.

Patent Document 2 discloses that a molten steel flow is braked by a DC magnetic field applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles that are opposed to each other with a long side of the mold interposed therebetween, and AC is applied to the upper magnetic pole or the lower magnetic pole. A method of applying a superimposed magnetic field is disclosed. In this method, the molten steel flow is braked by a DC magnetic field, and the cleaning effect of nonmetallic inclusions and the like at the solidified shell interface is obtained by stirring the molten steel by an AC magnetic field.
Patent Document 3 discloses a method in which a DC magnetic field is applied to each of a pair of upper magnetic poles and a pair of lower magnetic poles that are opposed to each other with a long side of the mold interposed therebetween, and an AC magnetic field is applied to the upper magnetic poles in a superimposed manner. A method is disclosed in which the strength of the magnetic field and the strength of the alternating magnetic field are set to a specific numerical range depending on the casting width and casting speed.

  On the other hand, in continuous casting of steel, it is necessary not only to ensure the quality of slabs but also to improve productivity. In order to improve productivity, it is necessary to increase the amount of molten steel per unit time injected into the mold, and specific measures for this include improving the casting speed and increasing the casting width. However, as a problem at that time, the discharge flow rate from the immersion nozzle increases, and the inclusions in the molten steel are carried to the deep part of the molten steel in the mold, which causes a product defect. In addition, when the casting width is wide, the molten steel flow velocity cannot be sufficiently obtained at both ends in the width direction, and a cleaning effect such as non-metallic inclusions cannot be sufficiently obtained. There is a problem that it increases.

Japanese Patent No. 2726096 JP-A-10-305353 Japanese Patent No. 4567715

With the stricter quality of steel plates for automobile outer plates in recent years, defects caused by entrapment of minute bubbles and mold flux that have not been a problem until now are becoming a problem. However, the continuous casting method as shown in FIG. In particular, alloyed hot-dip galvanized steel sheets are heated after hot-dip plating to diffuse the iron component of the base steel sheet into the galvanized layer, and the surface layer properties of the base steel sheet contribute to the quality of the alloyed hot-dip galvanized layer. A big influence. In other words, if there are bubbles or flux defects on the surface layer of the base steel plate, even if it is a small defect, unevenness in the thickness of the plating layer occurs, which appears as a streak defect on the surface, such as an automobile outer plate It cannot be used for such a demanding quality requirement.
Further, in Patent Document 3, although an appropriate magnetic field condition is disclosed according to the casting condition, the casting condition is limited to a casting width of less than 1850 mm, and a wide casting of 1850 mm or more is required to improve casting efficiency. If you do, you can't.

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and by using a method of continuously casting steel while controlling the flow of molten steel in a mold using electromagnetic force, a slab having a high throughput and a wide casting width is obtained. It is an object of the present invention to provide a continuous casting method capable of obtaining a high-quality slab with few defects due to inclusion of non-metallic inclusions, minute bubbles, and mold flux.

  In order to solve the above-mentioned problems, the present invention provides a molten steel discharge angle of a submerged nozzle when continuously casting molten steel under a high efficiency condition in which a molten steel throughput Q is 6.0 to 7.5 ton / min and a casting width is 1850 mm or more. The ratio R / Q of the inert gas blowing amount R from the immersion nozzle to the molten steel throughput Q is optimized after setting the immersion depth, the upper and lower DC magnetic field strength, and the upper AC moving magnetic field strength to the optimum ranges. The summary is as follows.

[1] A pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold are provided outside the mold, and the DC magnetic field peak position of the upper magnetic pole and the DC magnetic field peak position of the lower magnetic pole In between, using a continuous casting machine in which a molten steel discharge hole of a submerged nozzle for pouring molten steel is located, a DC magnetic field is applied to the pair of lower magnetic poles to brake the molten steel flow, and the pair of pairs A method of continuously casting steel while applying a DC magnetic field and an AC moving magnetic field to the upper magnetic pole while superimposing the molten steel flow and stirring the molten steel,
When casting a slab having a molten steel throughput Q of 6.0 to 7.5 ton / min and a casting width of 1850 mm or more, the molten steel discharge angle from the horizontal direction of the molten steel discharge hole is 15 to 35 °, and the immersion depth (however, , The distance from the meniscus to the molten steel discharge hole upper end) is 180 mm or more and less than 300 mm, the intensity of the AC moving magnetic field applied to the upper magnetic pole is 0.07 to 0.09 T, and the intensity of the DC magnetic field applied to the upper magnetic pole Is 0.25 to 0.32 T, the intensity of the DC magnetic field applied to the lower magnetic pole is 0.35 to 0.40 T, the amount R (NL / min) of the inert gas blown from the inner wall surface of the immersion nozzle, and the molten steel throughput. A steel continuous casting method, wherein a ratio R / Q with Q (ton / min) satisfies the following formula (1).
(15-Q) /3.75≦R/Q≦ (16.5-Q) /3.75 (1)

[2] In the continuous casting method of [1] above, the immersion nozzle has a nozzle inner diameter (however, a nozzle inner diameter at the position of the molten steel discharge hole) of 70 to 90 mm, and an opening area of each molten steel discharge hole is 3600 to 8100 mm ² . A method for continuous casting of steel, characterized in that there is.

  According to the present invention, in continuous casting of molten steel under a high efficiency condition in which the molten steel throughput Q is 6.0 to 7.5 ton / min and the casting width is 1850 mm or more, bubble defects due to fine bubbles, non-metallic inclusion physical properties High-quality slabs with very few defects and flux defects can be produced with high efficiency.

The longitudinal cross-sectional view which shows one Embodiment of the casting_mold | template and immersion nozzle of a continuous casting machine with which implementation of this invention is carried out Sectional view along the line II-II in FIG. The top view which shows typically one Embodiment of the upper magnetic pole provided with the magnetic pole for DC magnetic fields and AC magnetic pole which were mutually independent in the continuous casting machine with which implementation of this invention is carried out

In the present invention, a pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold are provided outside the mold, and the DC magnetic field peak position of the upper magnetic pole and the DC magnetic field peak of the lower magnetic pole are provided. Using a continuous casting machine in which a molten steel discharge hole of a submerged nozzle for pouring molten steel is positioned between the positions, a DC magnetic field is applied to the pair of lower magnetic poles to brake the molten steel flow, and the pair A method of continuously casting the steel while braking the molten steel flow and stirring the molten steel is used by applying a DC magnetic field and an AC moving magnetic field superimposed on the upper magnetic pole of the steel.
According to the present invention, in such a continuous casting method, in order to increase the productivity of the slab, the molten steel is cast under a high efficiency condition in which the molten steel throughput Q is 6.0 to 7.5 ton / min and the casting width is 1850 mm or more. . Here, molten steel throughput is the amount of molten steel cast per unit time.

In the continuous casting method in which flow control of molten steel is performed using the electromagnetic force as described above, the present inventors have achieved high efficiency with a molten steel throughput Q of 6.0 to 7.5 ton / min and a casting width of 1850 mm or more. When the molten steel was continuously cast under the conditions, the investigation on the molten steel flow in the mold was conducted. In this investigation, the flow velocity distribution in the mold was repeatedly obtained by numerical calculation and measurement of the flow velocity with a low-melting-point alloy (Bi, Pb, Sn, Cd alloy: melting point 70 ° C.) of an actual machine 1/4 size. As a result, a molten steel discharge angle from the horizontal direction of the molten steel discharge hole is 15 to 35 °, and an immersion depth (however, a distance from the meniscus to the upper end of the molten steel discharge hole) is 180 mm or more and less than 300 mm. The strength of the AC moving magnetic field applied to the upper magnetic pole is 0.07 to 0.09 T, the strength of the DC magnetic field applied to the upper magnetic pole is 0.25 to 0.32 T, and the strength of the DC magnetic field applied to the lower magnetic pole is 0.35 to 0 The ratio R / Q between the amount R (NL / min) of the inert gas blown from the inner wall surface of the immersion nozzle and the molten steel throughput Q (ton / min) satisfies the following formula (1). It has been found that high quality slabs with few defects can be obtained even under high efficiency conditions where the molten steel throughput Q is 6.0 to 7.5 ton / min and the casting width is 1850 mm or more by performing casting in this manner. .
(15-Q) /3.75≦R/Q≦ (16.5-Q) /3.75 (1)

  Here, when R / Q is smaller than the numerical value represented by the left side of the above equation (1), the flow velocity at both ends in the width direction of the wide material decreases, and the cleaning effect of nonmetallic inclusions and bubbles is poor. Thus, it was found that these were trapped in both end regions and became defects. When the defect distribution of the product cast under such conditions was investigated, it was found that the defect occurrence area was concentrated at the end in the width direction, which coincided with the above-described investigation result. On the other hand, it was found that when R / Q is larger than the value represented by the right side of the above formula (1), the surface flow velocity is increased, so that the mold powder is likely to be caught, resulting in a product defect. .

  The molten steel flow velocity control in the mold is generally performed by an electromagnetic force, but the electromagnetic force is a constant control in the mold width direction. In a wide and high throughput condition such as a cast width of 1850 mm or more and a molten steel throughput of 6.0 to 7.5 ton / min, the flow of inert gas blown from the immersion nozzle has a large effect on the flow in the mold, and the mold width direction It becomes a situation where the magnitude of the molten steel flow velocity is likely to occur. It is difficult to control such fluctuations in the molten steel flow rate only by electromagnetic force control that is constant in the width direction, and the amount of inert gas blown is managed in a fine range, and the molten steel flow rate in the mold is controlled more strictly. Thus, it has been clarified by the present inventors that high-quality slabs can be manufactured even under wide-width and high-throughput conditions.

Based on the above knowledge, in the present invention, when continuously casting a slab having a molten steel throughput Q of 6.0 to 7.5 ton / min and a casting width of 1850 mm or more, molten steel is discharged downward from the horizontal direction of the molten steel discharge hole. Using an immersion nozzle with an angle of 15 to 35 ° and an immersion depth (distance from the meniscus to the upper end of the molten steel discharge hole) of 180 mm or more and less than 300 mm, the strength of the AC moving magnetic field applied to the upper magnetic pole is 0.07 to 0 0.09T, the intensity of the DC magnetic field applied to the upper magnetic pole is 0.25 to 0.32 T, the intensity of the DC magnetic field applied to the lower magnetic pole is 0.35 to 0.40 T, and the inert gas from the inner wall surface of the immersion nozzle The steel is continuously cast so that the ratio R / Q of the blow rate R (NL / min) and the molten steel throughput Q (ton / min) satisfies the following formula (1).
(15-Q) /3.75≦R/Q≦ (16.5-Q) /3.75 (1)
In the present invention, the upper limit of the molten steel throughput Q is set to 7.5 ton / min. This is a high throughput exceeding 7.5 ton / min, even if the casting conditions of the present invention are used. This is because a quality slab cannot be obtained. Further, although there is no particular upper limit to the casting width of the slab, in general, about 2700 mm is a practical upper limit due to required product size and equipment restrictions.

Next, the reason for limiting the molten steel discharge angle of the immersion nozzle (the molten steel discharge angle downward from the horizontal direction of the molten steel discharge hole; hereinafter referred to as the molten steel discharge angle α), the immersion depth, and the magnetic field strength of the upper magnetic pole and the lower magnetic pole will be described. .
The flow state of the molten steel in the mold varies greatly depending on the immersion depth of the immersion nozzle and the molten steel discharge angle α. That is, the smaller the nozzle immersion depth, the more easily the influence of the flow state of the molten steel discharged from the immersion nozzle is transmitted to the molten steel surface (meniscus), while the lower the nozzle immersion depth, the greater the downward flow rate. Further, when the molten steel discharge angle α is increased, the molten steel downward flow is increased as compared with the molten steel upward flow, and when the molten steel discharge angle α is decreased, the reverse is achieved.

  Even if the nozzle immersion depth is too large or too small, the flow state of the molten steel in the mold changes greatly when the flow rate or flow velocity of the molten steel discharged from the immersion nozzle changes. Proper control becomes difficult. If the nozzle immersion depth is less than 180 mm, the molten steel surface (meniscus) will fluctuate directly when the flow rate or flow velocity of the molten steel discharged from the immersion nozzle changes, and the surface disturbance will increase and the mold flux will be entrained. On the other hand, at 300 mm or more, when the flow rate of the molten steel fluctuates, the flow rate downward tends to increase, and the non-metallic inclusions and bubbles tend to increase.

  When the molten steel discharge angle α exceeds 35 °, even if the molten steel descending flow is braked by the DC magnetic field of the lower magnetic pole, nonmetallic inclusions and bubbles are easily carried down by the molten steel descending mold and are easily captured by the solidified shell. . On the other hand, when the molten steel discharge angle α is less than 15 °, even if the molten steel upward flow is braked by a DC magnetic field, the turbulence of the molten steel surface cannot be appropriately controlled, and the mold flux is likely to be involved.

Further, if the strength of the DC magnetic field applied to the upper magnetic pole is less than 0.25 T, the braking effect of the molten steel upward flow by the DC magnetic field is insufficient, the fluctuation of the molten metal surface is large, and the mold flux is likely to be involved. On the other hand, when the strength of the DC magnetic field exceeds 0.32 T, the cleaning effect of non-metallic inclusions and the like due to the flow in the mold is reduced, so that they are easily captured by the solidified shell, leading to product defects.
In addition, when the strength of the DC magnetic field applied to the lower magnetic pole is less than 0.35 T, the braking effect of the molten steel descending flow by the DC magnetic field is insufficient, so that non-metallic inclusions and bubbles accompanying the molten steel descending flow downward. It becomes easy to get into and get caught by the solidified shell. On the other hand, when the strength of the DC magnetic field exceeds 0.40 T, the cleaning effect due to the downflow of the molten steel is reduced, so that nonmetallic inclusions and bubbles are easily captured by the solidified shell.

  The AC magnetic field applied to the upper magnetic pole so as to overlap the DC magnetic field is an AC moving magnetic field as will be described later. If the strength of this AC moving magnetic field is less than 0.07 T, the swirling flow caused by the AC magnetic field is likely to be interfered by the molten steel ascending flow, the solidification interface flow rate cannot be stably increased, and bubble defects are likely to occur. On the other hand, when the intensity of the AC moving magnetic field exceeds 0.09 T, the swirl stirring force becomes too strong, and the mold flux is likely to be caught.

  The nozzle inner diameter of the immersion nozzle (however, the nozzle inner diameter at the position of the molten steel discharge hole) is preferably 70 to 90 mm. When non-metallic inclusions such as alumina suspended in the molten steel partially adhere to the inside of the immersion nozzle, drift occurs in the molten steel discharged from the immersion nozzle (the flow velocity symmetry in the width direction becomes worse). In some cases, if the nozzle inner diameter is less than 70 mm, the drift may become extremely large in such a case. When such an extreme drift occurs, it becomes difficult to control the molten steel flow in the mold. On the other hand, adjustment of the amount of molten steel flowing in the immersion nozzle is performed by adjusting the opening of the sliding nozzle above the immersion nozzle. However, if the nozzle inner diameter exceeds 90 mm, there may be a portion where the molten steel is not filled inside the nozzle. In this case, as described above, extreme drift may occur, and it may be difficult to control the molten steel flow in the mold.

The opening area of each molten steel discharge hole of the immersion nozzle is preferable to be 3600~8100mm ^2. The reason why the opening area of the molten steel discharge hole affects the effect of the present invention is that if the opening area of the molten steel discharge hole is too small, the flow speed of the molten steel discharged from the molten steel discharge hole becomes too large, and conversely if the opening area is too large. This is because the molten steel flow velocity is too small, and in any case, it is difficult to optimize the flow velocity of the molten steel flow in the mold.

1 and 2 show an embodiment of a mold and an immersion nozzle of a continuous casting machine used for carrying out the present invention. FIG. 1 is a longitudinal sectional view of the mold and the immersion nozzle, and FIG. It is a figure (sectional drawing which follows the II-II line of FIG. 1).
In the figure, reference numeral 1 denotes a mold, and the mold 1 is constituted by a mold long side portion 10 (mold side wall) and a mold short side portion 11 (mold side wall) in a rectangular shape in a horizontal section.
Reference numeral 2 denotes an immersion nozzle, and molten steel in a tundish (not shown) installed above the mold 1 is injected into the mold 1 through the immersion nozzle 2. The immersion nozzle 2 has a bottom portion 21 at the lower end of a cylindrical nozzle body, and a pair of molten steel discharge holes 20 penetrates the side wall portion directly above the bottom portion 21 so as to face both mold short side portions 11. It is installed.

As described above, the immersion nozzle 2 has a molten steel discharge angle α of 15 to 35 ° downward from the horizontal direction of the molten steel discharge hole 20 and an immersion depth (however, the distance from the meniscus 6 to the upper end of the molten steel discharge hole 20). ) Is 180 mm or more and less than 300 mm, the nozzle inner diameter (however, the nozzle inner diameter at the position of the molten steel discharge hole 20) is 70 to 90 mm, and the opening area of each molten steel discharge hole 20 is preferably 3600 to 8100 mm ^2. .

In order to prevent non-metallic inclusions such as alumina in the molten steel from adhering to and accumulating on the inner wall surface of the immersion nozzle 2 and blocking the nozzle, Ar gas is provided in the gas flow path provided inside the nozzle body of the immersion nozzle 2. An inert gas such as is introduced, and this inert gas is blown into the nozzle from the inner wall surface of the nozzle.
Molten steel flowing into the immersion nozzle 2 from the tundish is discharged into the mold 1 from a pair of molten steel discharge holes 20 of the immersion nozzle 2. The discharged molten steel is cooled in the mold 1 to form a solidified shell 5 and is continuously drawn below the mold 1 to form a slab. A mold flux is added to the meniscus 6 in the mold 1 as a heat insulating agent for molten steel and a lubricant for the solidified shell 5 and the mold 1.

Further, the inert gas bubbles blown from the inner wall surface of the immersion nozzle 2 are discharged into the mold 1 from the molten steel discharge hole 20 together with the molten steel.
A pair of upper magnetic poles 3a and 3b and a pair of lower magnetic poles 4a and 4b that are opposed to each other with the long side of the mold interposed therebetween are provided on the outer side of the mold 1 (the back side of the mold side wall). The lower magnetic poles 4a and 4b are arranged along the entire width in the width direction of the mold long side portion 10, respectively.

  The upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b have a DC magnetic field peak position of the upper magnetic poles 3a and 3b in the vertical direction of the mold 1 (vertical vertical position: usually the vertical center of the upper magnetic poles 3a and 3b. Position) and the peak position of the DC magnetic field of the lower magnetic poles 4a and 4b (the peak position in the vertical direction: usually the center position in the vertical direction of the lower magnetic poles 4a and 4b). The In addition, the pair of upper magnetic poles 3 a and 3 b are usually arranged at positions that cover the meniscus 6.

  Since a DC magnetic field is applied to the upper magnetic poles 3a and 3b and the lower magnetic poles 4a and 4b, respectively, and an AC magnetic field (AC moving magnetic field) is applied to the upper magnetic poles 3a and 3b in a superimposed manner. Each of 3a and 3b includes a magnetic pole for DC magnetic field and a magnetic pole for AC magnetic field (both magnetic poles are each composed of an iron core portion and a coil). Thereby, each intensity | strength of the direct current magnetic field and alternating current magnetic field which are superimposed and applied can be selected arbitrarily. FIG. 3 is a plan view schematically showing an embodiment of such upper magnetic poles 3a and 3b, and a pair of alternating magnetic field magnetic poles 30a and 30b (= AC) on the outside of both mold long sides of the mold 1. (Magnetic field generator) is disposed, and a pair of DC magnetic poles 31a and 31b (= DC magnetic field generator) are disposed outside the magnetic field generator.

Further, the upper magnetic poles 3a and 3b may be provided with a DC magnetic field coil and an AC magnetic field coil with respect to a common iron core part. By providing the magnetic field coil, it is possible to arbitrarily select the strengths of the DC magnetic field and the AC magnetic field that are superimposed and applied. On the other hand, the lower magnetic poles 4a and 4b include an iron core portion and a DC magnetic field coil.
Moreover, the alternating magnetic field that is superimposed on the direct current magnetic field is an alternating current magnetic field. An AC moving magnetic field is a magnetic field obtained by energizing an arbitrary adjacent N coils with an AC current whose phase is shifted by 360 ° / N. Generally, N = 3 (position) because of high efficiency. A phase difference of 120 °) is used. Here, the reason why the AC magnetic field superimposed on the DC magnetic field is an AC moving magnetic field is to create a flow of the molten steel by the moving magnetic field in order to stir the molten steel in the mold. An AC magnetic field that is not an AC moving magnetic field, that is, a magnetic field in which the current phase of each coil is the same and the position of the magnetic flux does not move with time does not contribute to stirring of molten steel.

The molten steel discharged from the molten steel discharge hole 20 of the immersion nozzle 2 in the direction of the mold short side part collides with the solidified shell 5 generated on the front surface of the mold short side part 11 and is divided into a downward flow and an upward flow. A DC magnetic field is applied to each of the pair of upper magnetic poles 3a and 3b and the pair of lower magnetic poles 4a and 4b. The basic action of these magnetic poles is electromagnetic that acts on molten steel moving in the DC magnetic field. Using the force, the molten steel upward flow is braked (decelerated) by the DC magnetic field applied to the upper magnetic poles 3a, 3b, and the molten steel downward flow is braked (decelerated) by the DC magnetic field applied to the lower magnetic poles 4a, 4b. To do.
Further, in the pair of upper magnetic poles 3a and 3b, the AC moving magnetic field applied to be superimposed on the DC magnetic field swirls and stirs the meniscus molten steel in the horizontal direction, and the resulting molten steel flow causes non-solidification of the solidified shell interface. An effect of cleaning metal inclusions and bubbles can be obtained.

In the present invention, there are no particular restrictions on the thickness of the cast slab, the amount of inert gas blown from the inner wall surface of the immersion nozzle 2, the frequency of the alternating magnetic field applied to the upper magnetic pole, etc. From the viewpoint of controlling the above, the following is preferable.
The molten steel discharged from the molten steel discharge hole 20 of the immersion nozzle 2 is accompanied by bubbles, and if the slab thickness is too small, the molten steel flow discharged from the molten steel discharge hole 20 is directed to the solidified shell 5 on the long side of the mold. Approaching, the solidification interface bubble concentration becomes high, and the bubbles are easily trapped at the solidification shell interface. In particular, when the slab thickness is less than 220 mm, even if the electromagnetic flow control of the molten steel flow as in the present invention is performed, it is difficult to control the bubble distribution for the reasons described above. On the other hand, when the slab thickness exceeds 300 mm, the productivity of the hot rolling process is lowered. For this reason, it is preferable that the slab thickness cast is 220-300 mm.

  When the amount of inert gas blown from the inner wall surface of the immersion nozzle 2 increases, the concentration of bubbles in the solidified interface increases, and bubbles are easily trapped at the solidified shell interface. In particular, when the inert gas blowing rate exceeds 20 NL / min, even if the electromagnetic flow control of the molten steel flow as in the present invention is performed, it is difficult to control the bubble distribution for the reasons described above. On the other hand, if the amount of inert gas blown is too small, nozzle clogging is likely to occur, and on the other hand, drift is increased and control of the flow rate tends to be difficult. For this reason, it is preferable that the amount of inert gas blown from the inner wall surface of the immersion nozzle 2 is 3 to 25 NL / min.

  In addition, there is no particular restriction on the frequency of the AC magnetic field applied to the upper magnetic pole, but if the frequency is too low, the temporal change in the molten steel flow induced by the application of the AC magnetic field becomes large and the flow on the molten steel surface is disturbed. This is likely to cause the mold powder to be undissolved and to change the molten metal surface. On the other hand, if the frequency is excessive, the magnetic field strength used for stirring the molten steel is attenuated, and the stirring property tends to be lowered. From the above viewpoint, the frequency of the AC magnetic field applied to the upper magnetic pole is most preferably in the range of 1.5 to 5.0 Hz.

[Example 1]
1 to 3, a continuous casting machine, that is, a pair of upper magnetic poles and a pair of lower magnetic poles that are opposed to each other with the long side of the mold interposed therebetween, and a DC magnetic field of the upper magnetic poles. Applying a DC magnetic field to the pair of lower magnetic poles using a continuous casting machine in which a molten steel discharge hole of an immersion nozzle for injecting molten steel is positioned between the peak position and the DC magnetic field peak position of the lower magnetic pole About 300 tons of aluminum killed molten steel is cast by a continuous casting method that brakes the molten steel flow and applies a DC magnetic field and an AC moving magnetic field to the pair of upper magnetic poles to superimpose the molten steel flow and stir the molten steel. did.

The casting thickness (slab thickness) was 220 to 300 mm, the casting width (slab width) was 1850 to 2100 mm, and the molten steel throughput was 6.0 to 7.5 ton / min. The molten steel discharge angle α of the immersion nozzle was 15 to 35 °, and the immersion depth (however, the distance from the meniscus to the upper end of the molten steel discharge hole) was 180 mm or more and less than 300 mm. Each molten steel discharge hole of the immersion nozzle has a square shape with a side length of 80 mm (open area of each molten steel discharge hole 6400 mm ² ), and the nozzle inner diameter of the immersion nozzle (however, the nozzle inner diameter at the position of the molten steel discharge hole) is 80 mm. Further, the intensity of the AC moving magnetic field applied to the upper magnetic pole is 0.07 to 0.09 T, the intensity of the DC magnetic field applied to the upper magnetic pole is 0.25 to 0.32 T, and the intensity of the DC magnetic field applied to the lower magnetic pole is 0. The frequency of the alternating magnetic field applied to the upper magnetic pole was 3.3 Hz. Ar gas was used as the inert gas blown from the immersion nozzle, and casting was performed by changing the blow amount of Ar gas.

  The cast slab was hot-rolled and cold-rolled into a steel plate, and this steel plate was subjected to alloying hot dip galvanizing treatment. The surface defects of this alloyed hot-dip galvanized steel sheet are continuously measured with an on-line surface defect meter, from which the defect appearance, SEM analysis, ICP analysis, etc. are used to identify steelmaking defects (flux defects and bubble defects). The number of defects per coil length of 100 m was evaluated.

  The results are shown in Tables 1 to 3 together with the casting conditions. In Invention Examples 1 to 30, the number of product defects is as very small as 0.10 to 0.20 / 100 m, and good quality is obtained. On the other hand, in Comparative Examples 1 to 15, the ratio R / Q between the Ar gas blowing amount R (NL / min) and the molten steel throughput Q (ton / min) does not satisfy the condition of the formula (1). The number of product defects is as large as 0.52 to 0.65 / 100 m, and the quality is poor. Further, Comparative Examples 16 to 37 are the molten steel throughput, the molten steel discharge angle of the immersion nozzle, the immersion depth of the immersion nozzle, the strength of the AC moving magnetic field applied to the upper magnetic pole, the strength of the DC magnetic field applied to the upper magnetic pole, and the lower magnetic pole. Since any one of the strengths of the applied DC magnetic field does not satisfy the conditions of the present invention, the number of product defects is as large as 0.51 to 0.61 / 100 m, and the quality is poor.

[Example 2]
About 300 tons of aluminum killed molten steel was cast by the same continuous casting method as in [Example 1] above. The casting thickness (slab thickness) was 220 to 300 mm, the casting width (slab width) was 1850 to 2100 mm, and the molten steel throughput was 6.0 to 7.5 ton / min. The molten steel discharge angle α of the immersion nozzle was 15 to 35 °, and the immersion depth (however, the distance from the meniscus to the upper end of the molten steel discharge hole) was 180 mm or more and less than 300 mm. The intensity of the AC moving magnetic field applied to the upper magnetic pole is 0.07 to 0.09 T, the intensity of the DC magnetic field applied to the upper magnetic pole is 0.25 to 0.32 T, and the intensity of the DC magnetic field applied to the lower magnetic pole is 0.35. The frequency of the AC magnetic field applied to the upper magnetic pole was 3.3 Hz under the condition of .about.0.40 T. Ar gas was used as the inert gas blown from the immersion nozzle, and the amount of Ar gas blown was such that the formula (1) was satisfied according to the molten steel throughput. Each molten steel discharge hole of the immersion nozzle has a square shape with a side length of 50 to 100 mm, and the opening area of each molten steel discharge hole is changed in the range of 2500 to 10000 mm ² , and the nozzle inner diameter of the immersion nozzle (however, molten steel) Casting was performed while changing the nozzle inner diameter at the position of the discharge hole in the range of 60 to 100 mm.

  The cast slab was hot-rolled and cold-rolled into a steel plate, and this steel plate was subjected to alloying hot dip galvanizing treatment. The surface defects of this alloyed hot-dip galvanized steel sheet are continuously measured with an on-line surface defect meter, from which the defect appearance, SEM analysis, ICP analysis, etc. are used to identify steelmaking defects (flux defects and bubble defects). The number of defects per coil length of 100 m was evaluated.

The results are shown in Tables 4 and 5 together with the casting conditions. Among Invention Examples 31 to 48 implemented by changing the opening area of the molten steel discharge hole, those having an opening area of the molten steel discharge hole of 3600 to 8100 mm ² have a product defect number of 0.10 to 0.20 pieces / 100 m. Very little and good quality is obtained. On the other hand, when the opening area of the molten steel discharge hole is 2500 mm ² and 10000 mm ² , the number of defects of the product is slightly increased to 0.22 to 0.29 / 100 m, but these are sufficiently acceptable levels as a product.
Further, among the inventive examples 49 to 63 in which the nozzle inner diameter of the immersion nozzle is changed, those having a nozzle inner diameter of 70 to 90 mm have a very small number of product defects of 0.10 to 0.19 / 100 m, which is good. Quality is obtained. On the other hand, when the nozzle inner diameter is 60 mm and 100 mm, the number of product defects is slightly increased to 0.23 to 0.29 / 100 m, but the product is sufficiently acceptable.

DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3a, 3b Upper magnetic pole 4a, 4b Lower magnetic pole 5 Solidified shell 6 Meniscus 10 Mold long side 11 Mold short side 21 Bottom 20 Molten steel discharge hole 30a, 30b AC magnetic pole 31a, 31b DC magnetic pole

Claims (2)

A pair of upper magnetic poles and a pair of lower magnetic poles facing each other across the long side of the mold are provided outside the mold, and between the DC magnetic field peak position of the upper magnetic pole and the DC magnetic field peak position of the lower magnetic pole. The molten steel flow is braked by applying a DC magnetic field to the pair of lower magnetic poles using a continuous casting machine in which a molten steel discharge hole of a submerged nozzle for injecting molten steel is located, and to the pair of upper magnetic poles By applying a DC magnetic field and an AC moving magnetic field in a superimposed manner, a method of continuously casting steel while braking the molten steel flow and stirring the molten steel,
When casting a slab having a molten steel throughput Q of 6.0 to 7.5 ton / min and a casting width of 1850 mm or more, the molten steel discharge angle from the horizontal direction of the molten steel discharge hole is 15 to 35 °, and the immersion depth (however, , The distance from the meniscus to the molten steel discharge hole upper end) is 180 mm or more and less than 300 mm, the intensity of the AC moving magnetic field applied to the upper magnetic pole is 0.07 to 0.09 T, and the intensity of the DC magnetic field applied to the upper magnetic pole Is 0.25 to 0.32 T, the intensity of the DC magnetic field applied to the lower magnetic pole is 0.35 to 0.40 T, the amount R (NL / min) of the inert gas blown from the inner wall surface of the immersion nozzle, and the molten steel throughput. A steel continuous casting method, wherein a ratio R / Q with Q (ton / min) satisfies the following formula (1).
(15-Q) /3.75≦R/Q≦ (16.5-Q) /3.75 (1) ^2. The steel according to claim 1, wherein the immersion nozzle has a nozzle inner diameter (however, a nozzle inner diameter at a position of the molten steel discharge hole) of 70 to 90 mm, and an opening area of each molten steel discharge hole is 3600 to 8100 mm ^2. Continuous casting method.

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