JPH09262651A - Method for reducing non-metallic inclusion in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the inclusion on the surface and in the inner part of a cast slab even in the case of being quick casting speed by casting plural kinds of steels having different harmfulness of the inclusion, in a continuous casting. SOLUTION: Electromagnets 7 for electromagnetic stirring are arranged on a mold part 1 at the upper part from the lower end part of an immersion nozzle 2, and electromagnetic devices 8 for impressing shifting magnetic field and static magnetic field to molten steel are arranged on the mold part at the lower part from the lower end of the immersion nozzle. The non-metallic inclusion in the cast slab is reduced by electromagnetic-stirring the molten steel at both of the upper part and the lower part in the mold or by electromagnetic-stirring to the upper part and impressing the static magnetic field to the lower part according to the kind of steel and the casting speed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuous casting method for molten steel.

[0002]

2. Description of the Related Art In continuous casting of steel, various steel types are cast by one continuous casting machine. Among the steel types for thin plates, there are steel types for which it is desired to reduce relatively large non-metallic inclusions and bubbles existing near the surface of the slab as much as possible in order to improve the appearance and quality of the thin plate products. There are steel types that want to reduce inclusions inside the slab as much as possible in order to improve the workability, and the present situation is that these steel types are cast by one and the same continuous casting machine.

[0003] Conventionally, as a method of reducing inclusions near the surface of a slab in the mold, molten steel flow is applied to the tip of the solidification shell in the mold, and the cleaning effect suppresses trapping of inclusions in the solidification shell. Therefore, a method is known in which a moving magnetic field is applied to molten steel by an electromagnet installed on the upper part of the casting mold to induce horizontal stirring and flow along the tip of the solidified shell. Wishhei 4-159
Japanese Patent No. 802 discloses a method of applying a moving magnetic field to molten steel in a mold and stirring the molten steel. In order to prevent surface flaws of thin plate products such as sliver flaws, it depends on the thickness of the slab and the thickness of the thin sheet product, but relatively large inclusions and bubbles in the thickness area up to about 25 mm from the surface of the slab should be used. Must be reduced. Therefore, when the casting speed is slow, it is possible to reduce the inclusions in the surface layer of the slab with the electromagnet installed on the upper part of the mold, but when the casting speed is high, the stirring effect by the electromagnet on the upper part of the mold can be exhibited. Since the surface layer becomes thin, it is not possible to completely prevent the flaws on the surface of the thin plate.

On the other hand, in order to reduce the non-metallic inclusions in the slab, a static magnetic field is applied to the molten steel in the mold to reduce the penetration depth of the downward flow that flows downward from the discharge hole of the immersion nozzle. A method of suppressing inclusions in molten steel from being trapped inside the slab and applying a static magnetic field to the molten steel to brake the molten steel flow is disclosed in Japanese Patent Application No. 62-241439 and Japanese Patent Application No. 4241439. No. 127,938.

However, with the same single continuous casting machine, inclusions and bubbles in the surface layer of the slab can be reduced to a level without problems, and inclusions inside the slab to a level without problems, depending on the steel type and casting speed. The reality is that there is no casting method that can be reduced.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention In a single continuous casting machine, a plurality of steel types having different harmfulness of inclusions are cast to reduce the amount of inclusions on the surface layer of the slab even if the casting speed is high. The problem is to stably perform efficient continuous casting that reduces the amount of inclusions inside the slab.

[0007]

As a result of various studies to solve the above-mentioned problems, the inventors of the present invention installed an electromagnet device for applying a moving magnetic field to molten steel on the upper part of the mold, while By installing an electromagnet device that can apply a moving magnetic field to stir molten steel by one electromagnet device or apply a static magnetic field to brake molten steel, inclusions on the surface layer of the slab can be reduced, and the slab can be reduced. It has been found that inclusions inside can also be reduced.

The gist of the present invention is, in continuous casting of steel,
When producing molten slab by pouring molten steel into the mold through the immersion nozzle, an electromagnetic stirring electromagnet is installed in the mold part above the lower end of the immersion nozzle, and a mold below the lower end of the immersion nozzle. An electromagnetic device for applying a moving magnetic field and a static magnetic field to the molten steel is installed in the part to electromagnetically stir both the upper and lower molten steel in the mold or electromagnetically stir the upper part according to the steel type and casting speed. The lower part is a continuous casting method characterized by applying a static magnetic field to reduce non-metallic inclusions in the slab.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is for applying a moving magnetic field to the molten steel at the upper part of the casting mold when pouring the molten steel 4 into the mold 1 through the discharge hole 3 of the immersion nozzle 2 in continuous casting. 2 is a vertical cross-sectional view when the electromagnet device 7 is installed, and the electromagnet device 8 for applying a moving magnetic field and a static magnetic field to molten steel is installed below the mold part. The discharge hole of the immersion nozzle is usually downward from the horizontal direction, and when the electromagnet device is not operated, the nozzle discharge flow of the molten steel poured into the mold is directed to the solidified shell 6 on the short piece side of the slab. Upon collision, the flow is divided into a downward flow that flows downward from the discharge hole of the nozzle and an upward flow that flows upward from the discharge hole. The molten steel is solidified by heat removal to the mold, and the solidified shell is continuously drawn downward. If the velocity of the downward flow is too high, the depth of penetration of the discharge flow becomes deep, which is disadvantageous for the floating removal of non-metallic inclusions, and many inclusions remain inside the slab, adversely affecting the quality of steel products. Exert.

In the case of ultra-low carbon steel having a carbon concentration of about 30-40 ppm or less, relatively large inclusions or bubbles in the surface layer of the slab may cause surface flaws in the thin plate product. It is necessary to reduce objects and bubbles as much as possible. Therefore, when a moving magnetic field is applied to the molten steel using the electromagnet 7 at the top of the mold shown in FIG. 1, horizontal flow of the molten steel along the solidification shell is induced, and inclusions and bubbles are washed away near the solidification shell and solidification occurs. The entrapment of relatively large inclusions and bubbles in the shell is reduced. If the electromagnetic stirring in the upper part of the mold is too strong, the meniscus-shaped casting flux of molten steel will be caught in the molten steel, causing non-metallic inclusions in the slab to increase. In this case, the molten steel flow velocity due to the electromagnetic stirring must be suppressed to a flow velocity of about 0.4-0.6 m / s or less, which does not cause the adverse effect of flux entrainment. As the casting speed increases, the thickness of the solidified shell formed in the casting mold becomes thinner, and the thickness of the surface layer of the cast slab that can enjoy the effect of reducing inclusions and bubbles by the electromagnet device 7 at the top of the casting mold becomes thinner. Therefore, for ultra-low carbon steel, in which inclusions and bubbles on the surface of the slab are important to remove, in the case of a high casting speed, in addition to the upper electromagnet 7, the lower electromagnet device 8 is also used to apply a moving magnetic field to the molten steel. When molten steel flow is applied along the solidified shell, inclusions and bubbles up to a thickness of about 30 mm on the surface of the slab can be reduced, and surface defects on the thin plate can be prevented. In addition,
Inclusions inside the slab are relatively unlikely to adversely affect sheet products.

On the other hand, such as low-carbon aluminum killed steel used in tin products, steel grades that must strictly reduce not only large inclusions on the surface of the slab but also non-metallic inclusions inside the slab are cast. In this case, when the inclusions and bubbles on the surface of the slab are reduced by using the electromagnet device above the mold, and at the same time, a static magnetic field is applied to the molten steel using the electromagnet device 8 below the mold,
Electromagnetic force acts on the molten steel flowing in the static magnetic field in the direction opposite to the flowing direction of the molten steel, and the flow velocity of the molten steel decreases. For this reason, the penetration depth of the discharge flow is significantly reduced, non-metallic inclusions do not penetrate deep into the molten steel pool, and floating removal to the meniscus is promoted.

FIG. 2 is a schematic diagram of a horizontal cross section taken along the line AA in FIG. 1, and a pair of electromagnets 8 and 8'arranged opposite to each other with a constant space sandwiching the mold 1 under the mold. Indicates. The configurations and functions of the electromagnets 8 and 8'are the same. The electromagnet 8 includes an iron core 9 and a magnetic pole 1 branched from the iron core.
1. A plurality of coils 10 wound around an iron core, and a coil 12 wound around an iron portion of a branched magnetic pole. As described below, a moving magnetic field or a static magnetic field can be applied to the molten steel in the mold by changing the current flowing through the coils 10 and 12.

Regarding the method of applying the moving magnetic field, in FIG. 2, three adjacent coils u, v, w of the coil 10 are used.
When a three-phase alternating current having a phase difference of 120 degrees is applied to the molten steel in the mold from the tips of the magnetic poles 11 according to the time-dependent change of the current flowing in the coils u, v, and w. The magnetic field that changes with time causes the moving magnetic field to act on the molten steel near the magnetic poles, and a flow 13 of the molten steel is generated by the action of this moving magnetic field. In the same manner, the molten steel flow 13 'can be generated on the opposite side of the mold, and the molten steel in the mold is agitated. Similarly, the electromagnet having the structure shown in FIG. 2 can be installed on the upper part of the mold, and a moving magnetic field can be applied to the molten steel on the upper part of the mold to stir the molten steel.

When applying a static magnetic field to the molten steel, in the case of the electromagnet device shown in FIG. 2, there is a method of supplying a direct current to the coil 12, and by changing the direction of the direct current to be supplied to the coil 12, As shown in FIG. 3, the polarity of the magnetic pole is N
Alternating arrangements of poles and south poles are possible. The magnetic field is N
Since the magnetic poles from the pole to the S pole are arranged so that the magnetic poles on the opposite side of the mold are alternately arranged with the N pole and the S pole, a static magnetic field from the N pole to the S pole can be applied to the molten steel in the mold. When the molten steel flows in, the electromagnetic force acts on the side opposite to the flowing direction, and the flow of the molten steel is suppressed.

[0015]

EXAMPLES In continuous casting of slabs, electromagnets were installed on the upper and lower parts of the mold as shown in FIG. 1, and the effect of applying a moving magnetic field on the upper and lower parts of the mold on nonmetallic inclusions in the slab, An experiment was conducted to examine the effect when a moving magnetic field was applied to the upper part of the mold and a static magnetic field was applied to the lower part. In a continuous casting machine using an ordinary copper mold, in a casting experiment using mold flux, the size of the slab slab was 170 mm in thickness and 800 mm in width, the length of the mold was 800 mm, and the position of the nozzle discharge hole was from the meniscus. It is 250 mm below. Regarding the electromagnet on the upper part of the mold, the center position of the electromagnet in the casting direction was 100 mm from the meniscus, and a three-phase alternating current of about 500 A was applied to the electromagnet to apply a moving magnetic field. On the other hand, the electromagnet at the bottom of the mold is installed so that the center position of the magnet is 400 mm below the meniscus. When a moving magnetic field is applied, a current of about 500 A is applied to the coil, and when a static magnetic field is applied. A magnetic field having a magnetic field strength of about 0.3 tesla was generated by applying a direct current to the coil 12 shown in FIG.

Casting using ultra-low carbon steel having a carbon concentration of about 30 ppm and investigating the effect of stirring molten steel by applying a moving magnetic field at the top and bottom of the mold on the amount of non-metallic inclusions on the surface of the slab at different casting speeds. An experiment was conducted. After the casting experiment, a plane parallel to the surface of the slab was observed with a microscope at intervals of 5 mm from the surface of the long side of the slab, and the number (inclusions / cm ² ) of inclusions of 10 μm or more in the surface layer of the slab was examined. as a result,
When the casting speed is 1 m / min, the thickness of the surface layer of the cast slab, which reduces the number of inclusions to about 1.0 pieces / cm ² or less,
When only the upper electromagnet is operated, it is about 25 mm thick, and when the upper and lower electromagnets are operated, it is about 30 mm thick. It is better to use both upper and lower electromagnets.
It was found that the surface area of the slab where the amount of inclusions must be reduced becomes thicker. It should be noted that the amount of inclusions inside the cast, which is not affected by the electromagnet, is about 1.5 to 2.0 pieces / c.
It was about m ² . Moreover, the casting speed is 1.6 m / min.
In the case of high speed, the number of inclusions is reduced to about 1.0 pieces / cm ² or less, and the thickness of the surface layer of the cast slab is about 15 mm when only the upper electromagnet is operated. When the electromagnet is operated, the thickness will be about 25 mm, and even if the casting speed increases, both the upper and lower electromagnets can be used to reduce the amount of inclusions and secure a thick slab surface area. I understood.

Next, using aluminum-killed steel with a carbon concentration of 0.01%, molten steel stirring was performed by applying a moving magnetic field at the upper part of the mold, while a static magnetic field was applied at the lower part of the mold to perform a casting experiment. The amount of inclusions in the slab was investigated. The casting speed was set to 1.6 m / min, and the difference in the amount of inclusions depending on whether or not the electromagnet was used was examined. As a result, 2 from the surface of the slab
The amount of inclusions on the surface of the cast slab up to 0 mm is large at about 1.5 pieces / cm ² when the electromagnets at the upper and lower parts are not operated, compared with 0.85 pieces / cm ² when the electromagnet is operated. It was confirmed that the amount of inclusions on the surface of the cast slab, which is considered to be due to the effect of electromagnetic stirring on the upper part, was reduced. Regarding the amount of inclusions inside the slab, the amount of inclusions at the position of ¼ thickness on the upper surface side of the slab is 2.1 pieces / cm ² when the upper and lower electromagnets do not operate. When the upper and lower electromagnets are activated, the number is reduced to 1.3 / cm ² , and the static magnetic field applied by the lower electromagnet prevents the molten steel discharge flow from penetrating deeply due to the static magnetic field applied. It was found that the inclusions were less trapped in the mold and the floating removal of the inclusions in the mold was promoted.

As described above, depending on the steel type and casting speed,
It is possible to reduce inclusions on the surface of the slab and inclusions inside the slab by stirring the molten steel with the electromagnet above the mold and applying a moving magnetic field or static magnetic field to the molten steel using the electromagnet below the mold. It turned out that

[0019]

According to the present invention, in one continuous casting machine, even if the casting speed is high, inclusions on the surface layer of the slab are reduced, and inclusions inside the slab are reduced, Efficient continuous casting can be stably performed.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing a relationship among an immersion nozzle, a mold, and an electromagnet device.

FIG. 2 is a plan view of a position AA in FIG. 1, and is a schematic view of an electromagnet device installed with a mold sandwiched therebetween.

FIG. 3 is a schematic diagram in which the polarities of the magnetic poles of the electromagnet are alternately symmetrical on both sides of the mold.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Immersion nozzle 3 Discharge hole 4 Molten steel 5 Direction of molten steel flow 6 Solidification shell 7 Electromagnet for electromagnetic stirring 8, 8'Combined electromagnet for applying moving magnetic field and static magnetic field 9 Iron core 10 Coil 11 Magnetic pole 12 Coil 13, 13 ' Molten steel stirring direction

Claims (1)

[Claims] 1. In continuous casting of steel, when molten steel is poured into a mold through an immersion nozzle to produce a slab, an electromagnet for electromagnetic stirring is installed in a mold part above a lower end of the immersion nozzle. In addition, an electromagnet device for applying a moving magnetic field and a static magnetic field to the molten steel is installed in the mold portion below the lower end of the immersion nozzle, and the molten steel at both the upper and lower portions in the mold is electromagnetically affected according to the steel type and casting speed. A continuous casting method characterized by stirring or by electromagnetically stirring the upper part and applying a static magnetic field to the lower part to reduce non-metallic inclusions in the cast slab.