JP2003103348A - Continuous casting method and facility for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture high quality metallic product by casting an ingot little in acquired bubble, a non-metallic inclusion and segregation of the ingot surface, a surface defect resulting from mold flux, having little inner inclusion. SOLUTION: By disposing three or more electromagnets 28 on the long side direction of a mold 10, and making the magnetic field, which generates at mutually adjacent coils 24, substantially reverse, a vibrational electromagnetic field substantially reversing the phase is made to act on a molten metal to make local excitation without causing the break of a dendrite on the front face of a solidified core by the electromagnetic force.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method and equipment for steel, and more particularly to improvement of molten steel flow in a mold by applying a magnetic field.

[0002]

2. Description of the Related Art In recent years, demands for improving the quality of steel products have become strict, especially for steel plates for automobiles, and there is an increasing demand for high-quality slabs having excellent cleanliness from the slab stage. Slab defects include those caused by inclusions and bubbles, and those caused by segregation of components in molten steel, and the flow in the mold is closely related to these, so many studies and inventions have been made. Came. As one of them, a method of controlling flow in a mold using a magnetic field is considered.

For example, in (A) a case where a direct current magnetic field is superimposed on a moving magnetic field, Japanese Patent Laid-Open No. 10-305353 discloses that two upper and lower magnetic poles opposed to each other with the long side of the mold interposed therebetween are arranged on the back side of the long side of the mold. (1) A magnetic field in which a DC static magnetic field and an AC moving magnetic field are superposed on a magnetic pole arranged on the lower side, or
(2) A method of controlling molten steel flow in a mold is disclosed in which a magnetic field in which a DC static magnetic field and an AC moving magnetic field are superposed is applied to a magnetic pole arranged on the upper side, and a DC static magnetic field is applied to a magnetic pole arranged on the lower side.

Japanese Patent No. 3067916 discloses an apparatus for controlling molten steel flow in a mold by causing appropriate AC electric currents for linear driving and DC electric currents for braking to flow in a plurality of electric coils.

Japanese Unexamined Patent Publication (Kokai) No. 5-154623 discloses a flow control method in a mold in which an AC moving magnetic field whose phase is shifted by 120 degrees and a DC static magnetic field are superposed.

In Japanese Patent Laid-Open No. 6-190520, a magnet placed above the discharge hole of the immersion nozzle causes a static magnetic field and a high-frequency magnetic field to overlap each other across the entire width direction, and a magnet placed below the discharge hole. Discloses a steel casting method in which a static magnetic field is applied.

Further, (B) as a combination of an upper DC magnetic field and a lower moving magnetic field, in JP-A-61-193755, a static magnetic field is applied to a position surrounding a molten steel flow discharged from an immersion nozzle, An electromagnetic stirring method is disclosed in which the flow velocity is reduced and an electromagnetic stirring device is installed at a position downstream of the static magnetic field to horizontally stir.

Further, (C) A combination of an upper moving magnetic field and a lower DC magnetic field is disclosed in Japanese Patent Laid-Open No. 6-226409, in which the center of the pole center is located between the molten metal surface and the discharge hole (downward 50 degrees or more). A casting method is disclosed in which a moving magnetic field is made to act by a magnet installed and a static magnetic field is made to act by a magnet placed below the immersion nozzle at the center of the pole core.

In Japanese Patent Laid-Open No. 9-262651, an electromagnetic stirring magnet is installed above the lower end of the immersion nozzle, and a magnet capable of applying a moving magnetic field and a static magnetic field is installed below the lower end of the immersion nozzle. A casting method is disclosed in which a static magnetic field and a moving magnetic field are selectively used according to the casting speed.

Japanese Patent Laid-Open No. 2000-271710 discloses that
When casting steel while blowing Ar gas into the immersion nozzle, a static magnetic field with a magnetic flux density of 0.1 tesla or more is applied to the molten steel flow immediately after exiting from the immersion nozzle, and the magnetic stirrer continuously continuously operates above it. A method for stirring or periodically changing the stirring direction is disclosed.

In Japanese Patent Laid-Open No. 61-140355, a static magnetic field is arranged on the long side of the mold so as to control the molten steel current supplied into the mold, and a moving magnetic field generator is arranged above. Then, a mold and a structure above the mold are disclosed in which the molten steel upper surface is made to flow from the center of the horizontal section to the short side.

In Japanese Unexamined Patent Publication (Kokai) No. 63-119959, an electromagnetic stirrer for causing horizontal flow of molten steel is provided on the upper part of the mold, and an electromagnetic brake for slowing down the discharge flow from the immersion nozzle is installed on the lower part of the mold. , A technique for controlling the discharge flow from an immersion nozzle is disclosed.

In Japanese Patent No. 2856960, a static magnetic field is used for the molten steel surface in a continuous mold, a straight nozzle is used as a continuous casting nozzle, and a progressive magnetic field is used for a discharge port.
A molten steel flow control technique using a static magnetic field is disclosed in the lower part thereof.

Further, (D) As a means for independently applying a direct magnetic field, in Japanese Patent Laid-Open No. 3-258442, an electromagnet having a length substantially the same as the long side is installed so as to face the long side of the mold. An electromagnetic brake that applies a static magnetic field is disclosed.

In Japanese Patent Laid-Open No. 8-19841, a magnetic pole bent or inclined toward the upper side of the mold from a predetermined position inside the center of the mold width or the short side of the mold to the vicinity of both ends is discharged from the dipping nozzle at the center of the width. There is disclosed a method of controlling molten steel flow in a mold by providing a direct current magnetic field or a low-frequency alternating current magnetic field installed below the hole.

In WO95 / 26243, a meniscus is prepared by applying a DC magnetic field having a substantially uniform magnetic flux density distribution over the entire width of the mold in the thickness direction of the mold and controlling the discharge flow from the immersion nozzle. A technique for controlling the flow velocity at 0.20 to 0.40 m / s is disclosed.

According to Japanese Patent Laid-Open No. 2-284750, a uniform static magnetic field in the thickness direction of the mold is applied to the entire width of the slab to act on the upper and lower portions of the discharge hole of the immersion nozzle to provide an effective braking force to the molten steel discharge flow. Techniques for applying and homogenizing the flow are disclosed.

Further, (E) As a means for applying a DC magnetic field or a moving magnetic field, Japanese Patent Laid-Open No. 9-262650 discloses a static magnetic field by flowing a DC current through a plurality of coils provided under the discharge holes of the immersion nozzle. A casting method is disclosed in which molten steel flow is controlled by applying a magnetic field or applying a moving magnetic field by applying an alternating current.

In "Materials and Processes", vol. 3 (1990), page 256, the discharge molten steel flow is braked (EML) by applying an AC moving magnetic field to the discharge flow from an immersion nozzle.
S) and acceleration (EMLA) are disclosed.

Further, (F) As a means for applying only a moving magnetic field, JP-A-8-19840 discloses that frequencies of 1 to 15 are used when controlling molten steel flow in a mold by electromagnetic induction.
A technique of applying a static alternating magnetic field of Hz to molten steel is disclosed.

"Iron and Steel" 66 (1980) p. 797 discloses a technique (M-EMS) for obtaining a horizontal molten steel swirl flow along a mold wall by electromagnetic stirring in a slab continuous casting machine. There is.

However, the techniques described in the above publications cannot prevent the inclusion of mold powder and the trapping of inclusions and bubbles at the solidification interface, so that the surface quality of the slab cannot be sufficiently improved. was there.

Further, (G) As an application of only an oscillating magnetic field, Japanese Patent No. 2917223 discloses that a low frequency alternating static magnetic field that does not move in time is applied to excite low frequency electromagnetic vibration immediately before solidification. Discloses a method of breaking columnar dendrites on the front surface of solidification and suspending them in the molten metal to refine the solidification structure and reduce center segregation, but the effect of reducing surface defects of the slab is small. .

[0024]

Due to the increasing needs for surface quality and the demand for cost reduction in recent years, a technique for further improving the quality of the surface of a slab is desired, and more effective flow control in the mold is required. Has become.

The present invention has been made in order to solve the above-mentioned conventional problems. It suppresses the entrainment of mold flux, improves the internal quality of the slab, and traps inclusions and bubbles in the solidification nuclei. An object of the present invention is to provide a continuous casting method and equipment for steel capable of suppressing and improving the surface quality of a slab.

[0026]

According to the present invention, at the time of continuous casting of steel, three or more electromagnets are arranged in the long side direction of the continuous casting mold to substantially reverse the magnetic field generated by adjacent coils. Thus, by applying an oscillating electromagnetic field in which the phase is substantially inverted to the molten steel and causing a local flow without causing the dendrites on the front surface of the solidification nuclei to be broken by an electromagnetic force, the above problems have been solved. is there.

In addition, the magnetic fields generated by the coils adjacent to each other are applied to the coils adjacent to each other by alternating currents having substantially opposite phases, or alternatively, the winding directions of the coils are reversed to form alternating currents having the same phase. When a current is applied, it is substantially inverted.

The maximum magnetic flux density of the alternating magnetic field is 1000
The frequency is less than Gauss and / or the frequency of the oscillating magnetic field is 1 Hz to 8 Hz.

The present invention also has, in a steel continuous casting facility, a mold comprising opposed long sides and short sides for holding and solidifying molten steel, and three or more magnetic poles in the long side direction of the mold. , The adjacent magnetic poles have different polarities and the polarities are inverted at a predetermined cycle, and the oscillating magnetic field generator for generating an oscillating magnetic field without macroscopic molten steel flow in the molten steel in the mold, It is a solution to the problem.

Further, the oscillating magnetic field generator is an electromagnet comprising a comb-teeth-shaped iron core having three or more comb teeth along the long side direction of the mold, and a coil arranged on each of the comb teeth. The coil is composed of an AC power supply for supplying an AC current having a predetermined frequency and a predetermined phase.

In the present invention, the flow velocity distribution in the thickness direction of the mold is defined. That is, in the vicinity of the center of the thickness, the flow velocity is reduced to suppress the entrainment of mold flux, and local flow is applied to the solidification interface close to the mold wall surface to prevent air bubbles and inclusions from being trapped and to prevent surface defects of the slab. Reduce.

As a method for this, it is necessary to devise a method of applying an alternating magnetic field, and as a result of carrying out model experiments and simulation calculations, the following conclusions were reached.

1. In the magnetic field in the thickness direction as shown in JP-A-6-190520, the Lorentz force was concentrated on the solidification interface or the molten steel surface by utilizing the skin effect of the alternating current. In order to concentrate the Lorentz force only on the solidification interface, it is necessary to control the magnetic field distribution.

2. As a method for this purpose, it is effective to arrange electromagnets whose phases are alternately inverted in the width direction and to alternate them. When vibrating the magnetic field in the thickness direction,
Since the electromagnetic force cannot be concentrated on the mold wall surface, that is, the solidification interface, it is necessary to vibrate the magnetic field in the width direction. Here, the phases of the currents flowing through the alternating electromagnets need to be substantially inverted, and for that purpose, the phases must be different by 130 ° or more.

3. The coil structure for this is shown in FIG.
As illustrated in FIG. 3, by winding a coil around the comb-teeth-shaped iron core 22 having three or more magnetic poles in the width direction and substantially inverting the phase of the currents between adjacent coils, the magnetic field in the width direction can be oscillated. it can. In the figure, 10 is a mold, 12 is a dipping nozzle, and 14 is molten steel (the shaded area is the low speed area).

4. If the frequency of the alternating current at this time is too low, sufficient flow will not be excited, and if it is too high, the molten steel will not follow the electromagnetic field, so the range of 1 Hz to 8 Hz is appropriate.

5. By using such an electromagnet, it is possible to induce a flow in the direction in which the molten metal is separated from the solidification front surface, and since the excited flow velocity is low, it is possible to obtain a cleaning effect on the solidification interface without breaking the dendrites. It was 2 (front view), FIG. 3 (horizontal sectional view taken along line III-III in FIG. 2), FIG. 4 (vertical sectional view taken along line IV-IV in FIG. 2)
In addition, regarding the case where the number of the magnetic poles 28 is four, the molten metal flow induced by the oscillating magnetic field of the present invention is schematically shown based on an example calculated by electromagnetic field analysis and flow analysis.

In the present invention, as shown in FIG. 5, the direction of the flow generated according to the Lorentz force F given by the following equation is the same,
Only the flow velocity v fluctuates in a half cycle of the applied current I.

F∝J × B (1) Here, J is an induced current and B is a magnetic field.

6. By reversing the winding direction of the coil, the phase of the magnetic field can be reversed even if the phase of the current is the same.

7. In Japanese Patent No. 2917223, by applying a low-frequency AC static magnetic field that does not move with time to excite low-frequency electromagnetic vibrations on the solidification front, the columnar dendrites on the solidification front are broken and suspended in the molten metal. , A method aiming at miniaturization of solidification structure and reduction of center segregation is disclosed. However, when a large electromagnetic force that breaks the dendrite is applied, the mold flux on the upper surface of the molten metal is involved and the surface quality is deteriorated. . Therefore,
The magnetic flux density of the alternating oscillating magnetic field is preferably less than 1000 Gauss. Depending on the coil arrangement, it may be possible to prevent the dendrite from breaking even at 1000 Gauss or more.

8. Further, in the method of Japanese Patent No. 2,917,223, dendrite fracture occurs and the columnar crystal structure changes to an equiaxed crystal structure. In the case of ultra-low carbon steel and the like, the columnar structure alone makes it easier to control as a texture during rolling, and therefore there is a problem that it becomes difficult to align the crystal orientations by making equiaxed crystal structure. Therefore, due to electromagnetic force,
It is important that the dendrites on the solidification front do not break.

From the above knowledge, by vibrating the magnetic field in the long side direction of the mold, the thickness of the slab and the flow in the casting direction are induced to give a flow that separates bubbles and inclusions from the solidification interface. It was concluded that it is effective to prevent trapping of bubbles and inclusions.

According to the present invention, it is possible to efficiently vibrate only the solidification interface and suppress the trapping of air bubbles and inclusions, so that the surface quality of the slab can be greatly improved.

[0045]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

An example of a steel continuous casting facility suitable for carrying out the present invention is shown in FIG. 6 in a schematic view of a horizontal section. In the figure, 10 is a mold, 12 is a dipping nozzle, 20 is an oscillating magnetic field generator, 22 is a comb-shaped iron core, 24 is a coil, 26a, 2
6b is an AC power source and 28 is a magnetic pole.

In the present invention, continuous casting is carried out while applying a magnetic field to the molten steel in the casting mold 10 having opposite long sides and short sides. The applied magnetic field is a magnetic field vibrating in the long side direction of the mold (hereinafter, also referred to as an oscillating magnetic field). The applied oscillating magnetic field is an alternating magnetic field whose application direction is the long side direction of the mold, and is a magnetic field which does not induce macroscopic flow of molten steel by periodically reversing its direction.

The oscillating magnetic field can be generated using, for example, an oscillating magnetic field generator 20 as shown in FIG. In the oscillating magnetic field generator 20 shown in FIG. 6, a comb tooth-shaped iron core 22 having three or more (12 in the figure) comb teeth is used in the long side direction of the mold, and the coils 24 are arranged on these comb teeth. To form the magnetic pole 28. The magnetic pole 28 adjusts the winding method of the coil and the alternating current flowing through the coil so that the adjacent magnetic poles have different polarities (N and S poles). In order to make the adjacent magnetic poles have different polarities (N and S poles), the windings of the adjacent magnetic poles are set in opposite directions, and the currents flowing in the coils are AC currents having the same phase and a predetermined frequency. Alternatively, it is preferable that the coils of adjacent magnetic poles are wound in the same direction, and the currents flowing through the coils are alternating currents having a predetermined frequency, which are out of phase with each other. The phase shift of the alternating current flowing through the adjacent magnetic poles is preferably 130 ° or more and 230 ° or less at which the phase is substantially inverted.

As the predetermined frequency of the alternating current,
The frequency is preferably 1 to 8 Hz, more preferably 3 to 6 Hz.
Hz. The example shown in FIG. 6 is a case where adjacent magnetic poles have different phases (substantially inverted phases) of alternating currents flowing through the coils with the winding directions of the coils being in the same direction. It is not limited to.

In the present invention, since the adjacent magnetic poles have different polarities, the electromagnetic force acting on the molten steel between the adjacent magnetic poles and the electromagnetic force acting on the molten steel between the adjacent magnetic poles are almost in the same direction. On the contrary, the macroscopic flow of molten steel is not induced. Further, in the present invention, since the current flowing through the coil is an alternating current, the polarities of the magnetic poles are reversed at a predetermined cycle, and vibration can be induced in the molten steel near the solidification interface in the width direction of the long side of the mold. This can prevent inclusions and bubbles from being trapped at the solidification interface, and can significantly improve the surface quality of the slab.

If the frequency of the alternating current flowing through the coil is less than 1 Hz, it is too low to induce sufficient flow. On the other hand, 8
If the frequency exceeds Hz, the molten steel will not follow the oscillating magnetic field, and the effect of magnetic field application will be reduced. Therefore, it is preferable that the frequency of the alternating current flowing through the coil is 1 to 8 Hz and the oscillation period of the oscillating magnetic field is 1/8 to 1 s.

In the present invention, the magnetic flux density of the applied oscillating magnetic field is preferably less than 1000 Gauss. When the magnetic flux density is 1000 gauss or more, not only the dendrite is broken, but also the fluctuation of the molten metal surface becomes large, and there is a problem that the inclusion of mold flux is promoted.

[0053]

The present invention will be described in more detail based on the following examples.

Approximately 300 tons of molten steel is melted in a converter and RH
An ultra-low carbon steel, Al killed steel, was processed to cast a slab with a continuous casting machine. Table 1 shows typical molten steel components.

[0055]

[Table 1]

The width of the slab is 1500 to 1700 m.
m, thickness 220 mm, throughput of molten steel 4-5
The range was ton / min.

Further, as shown in FIG. 1, as the coil structure, a comb-teeth-shaped iron core divided into 12 equal parts in the width direction was used, and they were arranged so as to generate a magnetic field whose phase was alternately inverted in the width direction. The maximum magnetic flux of the alternating magnetic field was 2000 gauss.

Table 2 shows the experimental conditions and the experimental results.

[0059]

[Table 2]

The surface segregation of the slab was evaluated by etching after slab grinding.
1m by visual observation ²Per segregation number
investigated. Also, visually observe the surface defects of the coil after cold rolling.
After inspecting and collecting a defect sample, analyze the defect
To investigate the number of defects due to mold flux
It was The amount of inclusions is slime from the position of 1/4 thickness of the slab.
After extracting the inclusions by the extraction method, the weight was measured. surface
Index for segregation, mold flux defects and inclusions
Of all the conditions, the worst one
It was set to 10, and it displayed with the linear ratio with respect to it.

As can be seen from Table 2, surface segregation, defects due to mold flux, bubbles, and non-metallic inclusions can be reduced by applying an oscillating magnetic field.

Here, if the strength of the oscillating magnetic field is too strong, the flux entrainment on the surface of the molten metal becomes large, deteriorating the surface quality, and if the frequency is too high, the molten metal cannot follow the magnetic field and the solidification interface Cleaning effect decreases, bubbles,
It is estimated that the number of inclusion defects is increasing.

When the C-section of the slab was macro-etched under all conditions except Comparative Example 4, the equiaxed crystal ratio did not change and the dendrite was not broken as compared with the case where no magnetic field was applied. Was confirmed.

In the above description, the comb-teeth-shaped iron core having 12 poles is used, but the number of magnetic poles and the shape of the iron core are not limited to this, and for example, even if the iron core is divided. I don't care.

[0065]

EFFECTS OF THE INVENTION According to the present invention, it is possible to cast slabs having few air bubbles, non-metallic inclusions and slab surface segregation, surface defects due to mold flux and internal inclusions, which makes it possible to obtain high quality metal products. Manufacture becomes possible.

[Brief description of drawings]

FIG. 1 is a horizontal sectional view schematically showing an electromagnet and a mold used in the present invention.

FIG. 2 is a front view schematically showing a calculation result by an electromagnetic field analysis and a flow analysis of a velocity vector of a molten metal flow induced by a magnetic field for explaining the principle of the present invention.

FIG. 3 is a horizontal sectional view taken along the line III-III in FIG.

FIG. 4 is a vertical sectional view taken along line IV-IV in FIG.

FIG. 5 is a diagram showing an example of a temporal change state of an applied current and a molten steel flow velocity in the present invention.

FIG. 6 is a horizontal sectional view schematically showing an embodiment of continuous casting equipment.

[Explanation of symbols]

10 ... Mold 12 ... Immersion nozzle 20 ... Oscillating magnetic field generator 22 ... Comb-shaped iron core 24 ... Coil 26a, 26b ... AC power supply 28 ... Magnetic pole

Claims (6)

[Claims] 1. An oscillating electromagnetic field in which the phase is substantially inverted in molten steel by arranging three or more electromagnets in the long side direction of the continuous casting mold and substantially reversing the magnetic field generated between adjacent coils. A continuous casting method for steel, which is characterized in that a local flow is induced without causing the dendrites in front of the solidification nuclei to be broken by an electromagnetic force. 2. A magnetic field generated by the adjacent coils,
The alternating currents having substantially opposite phases are passed through the coils adjacent to each other, or the alternating currents having the same phases are passed by reversing the winding directions of the coils to thereby substantially reverse the coils. Item 2. A continuous casting method for steel according to Item 1. 3. The continuous steel casting method according to claim 1, wherein the maximum magnetic flux density of the alternating magnetic field is less than 1000 Gauss. 4. The continuous casting method for steel according to claim 1, wherein the oscillating magnetic field has a frequency of 1 Hz to 8 Hz. 5. A mold comprising opposite long sides and short sides for holding and solidifying molten steel, and three or more magnetic poles in the long side direction of the mold, wherein adjacent magnetic poles have different polarities. An oscillating magnetic field generator for generating an oscillating magnetic field in which the polarity is reversed at a predetermined cycle and the molten steel in the mold does not have a macroscopic molten steel flow, and a continuous casting facility for steel. 6. An electromagnet, wherein the oscillating magnetic field generator comprises a comb-teeth-shaped iron core having three or more comb teeth along the long side direction of the mold, and a coil arranged on each of the comb teeth. The continuous casting equipment for steel according to claim 5, comprising an AC power supply for supplying an alternating current of a predetermined frequency and a predetermined phase to the coil.