JP2001009559A - Method and device for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attenuate upward/downward flow from a discharge opening, activate flow of molten steel at a solidification shell front face, and prevent occurrence of vortex or retention due to interference between an electromagnetic agitation swing flow and a discharge reverse rotation floating flow at a meniscus part by moving an alternating current magnetic field to be applied to upper/lower parts of an immersion nozzle discharge opening while it is overlaid with a direct current magnetic field from both ends of casting width towards a center symmetrically to right and left. SOLUTION: AC/DC electromagnets 7 are arranged at upper and lower portions of a discharge opening 14 of an immersion nozzle 2 in a mold 3, and the electromagnets 7 are constituted in a manner that an alternating magnetic filed moves towards an arrow 10, that is, it moves from both ends to the center of casting width symmetrically to right and left. Therefore, molten steel flow on the solidification shell front face can be significantly activated without distorting a molten steel flow pattern 9 due to the discharge flow. Thus, a wash-away effect of inclusions with the flow of molten steel is fully obtained, and defects are not caused by capturing non-metallic inclusions to a cast piece. Furthermore, it is preferable that frequency of the alternating field is 0.1-10 Hz.

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 apparatus for controlling molten steel flow by a magnetic field in continuous casting of steel.

[0002]

2. Description of the Related Art In continuous casting of steel, flow control of molten steel by a magnetic field is performed for the purpose of preventing entrapment of mold powder on a molten metal surface, preventing product defects due to inclusions and bubbles, and preventing uneven solidification. I have. JP-A-57-17356 discloses an electromagnetic brake technique for braking excessive molten steel flow.
Japanese Patent Laid-Open Publication No. H11-15064 proposes a method in which an electromagnet is installed in a mold of a continuous slab caster, and a magnetic field perpendicular to a discharge flow from an immersion nozzle is applied to brake the discharge flow.

As a further development of the above method, Japanese Patent Application Laid-Open No. 2-284750 proposes a method in which a static magnetic field over the entire width of a mold is applied to the upper and lower discharge ports of an immersion nozzle. As electromagnetic stirring for preventing stagnation of molten steel, Japanese Patent Application Laid-Open No. 2-37946 discloses a method in which a low-frequency moving magnetic field is applied to a meniscus to give flow to molten steel, thereby ensuring heat supply to powder and preventing inclusions from solidifying shell. It has been proposed to do so.

Japanese Patent Application Laid-Open No. 1-228645 proposes a method in which a medium carbon steel is caused to flow near the meniscus at a flow rate of molten steel of 40 to 120 cm / s by electromagnetic stirring in order to prevent longitudinal cracks. A method combining electromagnetic braking and electromagnetic stirring has also been proposed. In JP-A-61-193755, a static magnetic field is applied to the discharge flow of an immersion nozzle to promote the floating of large inclusions, and under that, small inclusions are trapped in the solidified shell by horizontal flow by electromagnetic stirring. A method to prevent this is shown. Japanese Patent Laid-Open No. 5-
In JP 23803, electromagnetic stirring is performed so that a molten steel flow of 0.1 to 0.4 m / s is obtained in the mold, and a uniform static magnetic field in the width direction is applied from 1.5 m below the meniscus to the vertical part of the continuous casting machine to remove the inclusions. It has been proposed to prevent intrusion. JP-A-5-15
Japanese Patent No. 4620 proposes a method of electromagnetically stirring a meniscus and applying a uniform static magnetic field in the width direction above and below a discharge port of an immersion nozzle.

Further, Japanese Patent Application Laid-Open No. 9-262650,
-262651 proposes a method of applying a moving AC magnetic field or a static magnetic field in a mold by switching between DC and three-phase AC to flow through a plurality of coils wound around the same iron core.
Japanese Patent Application Laid-Open No. Hei 10-305353 proposes a method in which a static magnetic field and a moving magnetic field are superimposed and applied to the upper and lower portions of an immersion nozzle discharge port.

[0006]

When the discharge speed of molten steel from the immersion nozzle is suddenly or steadily increased, the flow of molten steel in the vicinity of the molten metal surface increases, causing powder entrainment, and the short side wall of the mold. The descending flow in the vicinity also becomes large, and inclusions penetrate into the unsolidified molten steel bath deep inside the slab. Also, if the molten steel flow in front of the solidified shell is slow, the washing out of inclusions (hereinafter referred to as the Washing effect) due to the flow of molten steel is weakened, making it easier for the inclusions to be trapped in the solidified shell, and the initial solidification part In such a case, the claw-like solidified structure grows greatly due to a decrease in heat supply, causing the capture of powder and the capture of air bubbles and inclusions.

Therefore, it is important that the upward flow and the downward flow from the discharge flow attenuate them, and at the same time, the molten steel flow in front of the strong shell activates them. JP-A-2-37946, JP-A-1-228645 and JP-A-5-23803 each disclose the application of flow to the meniscus portion by electromagnetic stirring because there is no function to attenuate the upward flow and the downward flow from the discharge flow. It is difficult to prevent powder entrainment and inclusions from penetrating deep into the unsolidified molten steel bath in the slab.

Further, Japanese Patent Application Laid-Open Nos. Hei 2-37946 and 1-2286
45, JP-A-5-23803, JP-A-5-154620, JP-A-10-
In the means disclosed in Japanese Patent Publication No. 305353, in which the molten steel is swirled in the circumferential direction of the mold at the meniscus portion by electromagnetic stirring, the molten steel flow coming out of the discharge port of the immersion nozzle 2 is partially reversed as shown in FIG. The flow rising to the meniscus (discharge reversal floating upstream 5) collides with the swirling flow 4 due to electromagnetic stirring, which promotes the vortex 6A causing powder entrainment in the meniscus portion and the inclusion of inclusions in the solidified shell. Stagnation 6B is formed. Even if a static magnetic field is superimposed on the moving magnetic field as in the technique disclosed in Japanese Patent Application Laid-Open No. H10-305353, it is difficult to prevent the collision between the discharge reversal float 5 and the swirl flow 4.

The method of applying a static magnetic field over the entire width of the mold to the upper and lower portions of the discharge port of the immersion nozzle disclosed in Japanese Patent Application Laid-Open No. 2-284750 has no function of activating the flow of molten steel on the front surface of the solidified shell, resulting in poor Washing effect. The object of the present invention is
In order to solve the problems of the prior art, the upward flow and the downward flow from the discharge flow are attenuated, and at the same time, the molten steel flow on the front of the solidified shell is activated, and the electromagnetic stirring swirl flow at the meniscus portion and the discharge reversal floating upstream. An object of the present invention is to provide a method and an apparatus for continuously casting steel capable of preventing formation of vortices and stagnation due to interference.

[0010]

Means for Solving the Problems In order to achieve the above object, the present inventors first set a mold 3 as shown in FIG.
A number of experiments were conducted in which a DC electromagnet 8 was disposed at a position above and below the discharge port 14 of the immersion nozzle 2 and a DC magnetic field alone was applied in two stages in the casting thickness direction (perpendicular to the paper surface).
From the numerical calculation and the examination of the solidification structure of the slab, it was found that the molten steel flow pattern 9 caused by the discharge flow as shown in the figure was generated. According to this method, it has been found that the disturbance of the flow rate of the molten metal surface is reduced, so that the powder entrainment is significantly reduced, and the penetration of large inclusions into a deep position in the mold is also significantly reduced. However, due to laminar flow of the DC magnetic field,
It was confirmed that the molten steel flow on the front surface of the solidified shell became slow, the washing effect on the front surface of the solidified shell was not sufficiently obtained, and nonmetallic inclusions were trapped in the slab to cause defects.

Then, next, a means for activating the molten steel flow in front of the solidified shell without greatly distorting the discharge flow-induced molten steel flow pattern 9 was intensively investigated. As a result, as shown in FIG. Instead of the DC electromagnet 8 of FIG.
By using the AC / DC dual-purpose electromagnet 7 and configuring the AC / DC dual-purpose electromagnet 7 so that the AC magnetic field moves in the direction of arrow 10, that is, moves symmetrically from both ends of the casting width toward the center. It was found that the flow of molten steel in front of the solidified shell was significantly activated.

The present invention has been made based on this finding, and its gist lies in the following continuous casting method and apparatus for steel. (1) In a method for continuous casting while superimposing an AC magnetic field and a DC magnetic field on portions above and below an immersion nozzle discharge port of molten steel in a mold and applying the AC magnetic field and the DC magnetic field in a casting thickness direction, the AC magnetic field is applied to both ends of a casting width. A continuous casting method of steel, characterized in that the steel is moved symmetrically from the center toward the center.

(2) The frequency of the alternating magnetic field is 0.1 to 10 Hz.
(1) The continuous casting method for steel according to (1). (3) A coil that generates an alternating magnetic field that moves symmetrically from both ends of the casting width toward the center while applying a magnetic field to portions above and below the discharge port of the immersion nozzle in the molten steel in the mold. And a coil for generating a DC magnetic field are wound around a common iron core, and are disposed on both sides in the casting thickness direction of the mold so that the direction of the magnetic field matches the casting thickness direction.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, an AC / DC superimposed magnetic field is applied at two positions (two stages) in the casting direction (mold height direction), and the application direction is taken in the casting thickness direction (mold short side direction).
The point of application is the same as the conventional one, but in the present invention, the moving direction of the AC magnetic field is different from the conventional one. That is, in the related art, the AC magnetic field moves from one end of the casting width (the width of the long side wall of the mold) to the other end, whereas in the present invention, the AC magnetic field is symmetrical from both ends of the casting width to the center. Moving. In the conventional method of moving an AC magnetic field, even when a DC magnetic field (synonymous with a static magnetic field) is superimposed on the AC magnetic field, a horizontal swirling flow is generated along the circumferential direction of the mold as shown in FIG. However, it is difficult to prevent vortices and stagnation due to collision between the swirling flow and the upstream of the discharge reversal, and it is difficult to avoid entrapment of powder on the molten metal surface and trapping of bubbles and inclusions on the front surface of the solidified shell. .

In the present invention, the alternating magnetic field is moved symmetrically in the width direction with respect to the center of the casting width, so that the above-mentioned swirling flow does not occur. No loss, no eddies, no stagnation. The flows from the left and right energized by the alternating magnetic field (symmetrical moving alternating magnetic field) merge at the center of the casting width, while maintaining the undisturbed laminar flow state while maintaining the laminar flow state near the molten metal surface (meniscus). Experiments and calculations have confirmed that the flow decreases and the flow below the discharge port increases.

The AC magnetic field exerts a stirring force exceeding the braking force by the DC magnetic field on the casting thickness surface side (near the front surface of the solidified shell) due to the skin effect, and activates the flow in this portion, thereby causing bubbles and inclusions. Prevents capture in slabs. On the other hand, on the casting thickness center side, the stirring force due to the AC magnetic field is attenuated, and the braking force of the DC magnetic field mainly acts, so that the flow (upward and downward flows from the discharge flow) at this portion is attenuated,
Disturbance in the flow rate of the molten metal is suppressed, so that powder entrainment is prevented, and at the same time, the descending flow rate is reduced, so that deep penetration of large inclusions is also prevented.

In the present invention, the frequency of the AC magnetic field is set to 0.1 to 10
Hz is preferred. If this frequency is less than 0.1 Hz, it is difficult to provide a molten steel flow sufficient for the Washing effect to be exerted on the front surface of the solidified shell. Wa
This is because it is difficult to provide a flow of molten steel sufficient to exert the shing effect.

FIGS. 3A and 3B are schematic cross-sectional views showing an example of an apparatus suitable for carrying out the above method according to the present invention.
FIG. 3 is a schematic side sectional view. A pair of alternating current and direct current electromagnets 7
The mold 3 in which the immersion nozzle 2 is immersed is disposed so as to face each other on both sides in the casting thickness direction. The iron core (yoke) 12 of the AC / DC dual-purpose electromagnet 7 has upper and lower magnetic poles, and the upper and lower magnetic poles (upper and lower poles) are above and below the discharge port 14 of the immersion nozzle 2, respectively. Matched in the width direction. Note that the direction of winding of the DC coil 8A is such that the polarities of the magnetic poles facing each other on both sides of the mold 3 are complementary (one is N and the other is S).
Winding method.

The tip of each magnetic pole has a plurality of pairs (3 in this example).
The branch is divided into pairs, and an AC coil 11 is wound around each branch, and a DC coil 8A is wound around a common root of each branch. In this example, a three-phase alternating current flows through the AC coil 11. If the different phases of the three-phase alternating current are U-phase, V-phase, and W-phase, respectively, W phase, 2
The V-phase flows third, and the U-phase flows third. Thus, by arranging the different phases of the polyphase AC current symmetrically in the width direction with respect to the center of the casting width, the AC magnetic field generated by the polyphase AC current is directed in the direction indicated by arrow 10, namely, the casting width. Can be moved symmetrically in the direction toward the center from both ends.

Further, by winding the AC coil and the DC coil around the branch portion and the root portion of the same magnetic pole, it is possible to accurately set the application location of the AC / DC superimposed magnetic field,
It is also easy to adjust the strength and frequency of the DC magnetic field independently. In addition, from the viewpoint of making the molten steel flow on the front surface of the solidified shell 13 uniform in the casting width direction, it is preferable to set the number of branches at the tip of the magnetic pole in accordance with the casting width.

Further, from the viewpoint of uniformly activating the flow of molten steel in front of the solidified shell over the entire casting width, it is preferable that the AC / DC dual-purpose electromagnet be installed so as to cover the entire casting width as in this example.

[0022]

[Example] Low-carbon aluminum killed steel with a width of 1500 mm and a thickness of 220 mm was immersed in a vertical bending type continuous casting machine at a discharge angle of 15 ° downward from horizontal, at a casting speed of 1.2 m / min.
At the time of casting at 2.5 m / min, using the same apparatus as that shown in FIG. 3, casting was performed while applying a magnetic field to the mold part of the strand under various magnetic field application conditions shown in Table 1, and the casting was performed. For the cast slab, the surface defect index by the steel plate surface defect inspection after rolling and the work crack index by the inclusion-induced work crack inspection during the steel plate press working were investigated. The surface defect index and the processing crack index are based on the case where electromagnetic flow control is not performed.
The index is 1.0.

In Table 1, in the magnetic pole having the moving type of the A type, the width direction phase arrangement of the three-phase alternating current in FIG. 3 is shown in FIG. 3 in order to impart a horizontal swirling flow to the molten steel as in the prior art. Instead of the arrangement, U-phase, V-phase, W-phase, U-phase, V-phase,
The phase was W. The resulting AC magnetic field (referred to as an A-type AC magnetic field; corresponding to a conventional moving magnetic field) moves from one end to the other end of the casting width. On the other hand, in the magnetic pole having the movable type of B type, the width direction phase arrangement of the three-phase alternating current is changed to the left and right as shown in FIG. Symmetric. The generated AC magnetic field (referred to as a B-type AC magnetic field) moves symmetrically from both ends of the casting width toward the center.

In Table 1, the strength of the AC magnetic field is the effective value of the magnetic flux density at the position inside the mold copper plate when applied alone, and the strength of the DC magnetic field is the magnetic flux density value at the center position of the casting thickness when applied alone. Each is shown. Both AC and DC magnetic fields have zero strength
A pole other than T is a pole to which an AC / DC superimposed magnetic field is applied.
Conditions 1 to 5 in Table 1 are comparative examples outside the scope of the present invention, and condition 6 is an example within the scope of the present invention.

Table 1 shows the inspection results of the surface defect index and the work crack index. In addition, this investigation result is an average value of two investigation values according to casting speed conditions.

[0026]

[Table 1]

In the comparative example, the conditions are such that the A-type AC magnetic field and the DC magnetic field are applied singly or in an overlapping manner. In the case of only a DC magnetic field, heat supply of molten steel is insufficient, and a nail-like structure grows in the initially solidified portion. The nail-like structure bites the powder and increases the surface defect index. In the case of the A-type AC magnetic field alone, claw-like structure growth can be suppressed, but the electromagnetic braking force is poor, so that the inclusions penetrate deep into the unsolidified molten steel bath in the slab, and the swirl flow and discharge in the circumferential direction of the mold at the meniscus portion A vortex or stagnation is formed due to collision with the inverted floating upstream. The penetration of inclusions deep into the unsolidified molten steel bath in the slab increases the work crack index. The vortex causes powder entrainment, and the stagnation helps trap inclusions in the solidified shell, all of which increase the surface defect index. When a DC magnetic field is superimposed on an A-type AC magnetic field, deep penetration of inclusions can be suppressed, but eddies and stagnation cannot be eliminated. Therefore, in the comparative example, even under the best condition 5 in which an A-type AC magnetic field and a DC magnetic field are applied to both the upper and lower poles, the work crack index is reduced to 0.1 but the surface defect index is still high at 0.2.

On the other hand, in the embodiment, the condition 6 (the frequency is optimized from 2 Hz to 5 Hz) in which the B-type AC magnetic field is used instead of the A-type AC magnetic field in the condition 5 is adopted. Strengthening the washing effect and applying electromagnetic braking force to the center of the casting thickness to reduce the flow velocity of the molten steel flow (upward flow, downward flow from the discharge flow) and promote laminar flow,
Furthermore, since the generation of swirling flow in the meniscus portion was suppressed and the formation of vortices and stagnation there was eliminated, it was possible to reach a surface defect index and a work crack index of 0.05, which could not be reached in the comparative example.

[0029]

As described above, according to the present invention, in continuous casting of steel, the upward flow and the downward flow from the discharge flow are attenuated, and at the same time, the molten steel flow on the front surface of the solidified shell is activated. Since the formation of vortices and stagnation due to the interference between the flow and the discharge inversion floating upstream can be prevented, there is an excellent effect that a higher quality cast piece can be manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic side view showing a flow pattern of molten steel caused by a discharge flow by two-stage superposition of a symmetric moving AC magnetic field and a DC magnetic field.

FIG. 2 is a schematic side view showing a flow pattern of molten steel caused by a discharge flow by applying a DC magnetic field alone in two steps.

FIGS. 3A and 3B are schematic cross-sectional views showing an example of an apparatus according to the present invention.

FIG. 4 is a schematic plan cross-sectional view showing interference between a swirling flow due to electromagnetic stirring in a meniscus portion and a discharge reversal floating upstream.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 AC electromagnet 2 Immersion nozzle 3 Mold 4 Swirling flow 5 Discharge reversal floating upstream 6A Vortex 6B Stagnation 7 AC / DC dual-purpose electromagnet 8 DC electromagnet 8A DC coil 9 Discharge flow caused molten steel flow pattern 10 Shows the moving direction of AC magnetic field according to the present invention Arrow 11 AC coil 12 Iron core (yoke) 13 Solidified shell

Claims (3)

[Claims] 1. A method for continuous casting while superimposing an AC magnetic field and a DC magnetic field on portions above and below a discharge port of an immersion nozzle of a molten steel in a mold and applying the AC magnetic field and a DC magnetic field in a casting thickness direction. A continuous casting method of steel, wherein the steel is moved symmetrically from both ends toward the center. 2. The continuous casting method for steel according to claim 1, wherein the frequency of the alternating magnetic field is 0.1 to 10 Hz. 3. An apparatus for continuously casting a molten steel in a mold while applying a magnetic field to a portion above and below a discharge port of an immersion nozzle, generating an alternating magnetic field moving symmetrically from both ends of the casting width toward the center. Continuous casting of steel characterized in that a coil to be generated and a coil for generating a DC magnetic field are wound around a common iron core, and are disposed on both sides in the casting thickness direction of the mold so that the direction of the magnetic field and the casting thickness direction match. apparatus.