JPH09150243A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the defect caused by inclusion in a cast slab by preventing descending flow concentrated along a short side frame of a mold without causing clogging state of molten steel flow. SOLUTION: In a continuous casting method, by which electromagnets having almost the same width of a long side frame to the long side frame 1 side at the mold in the rectangular cross sectional mold are arranged opposite to the magnetic poles 7 and electromagnetic force is acted to the molten steel flow spouted in the mold from an immersion nozzle 3 to control the molten steel fluid, the electromagnets to be arranged at the long side frame 1 side of the mold are arranged so that its magnetic pole is arranged with the height difference at the center part of the width direction of the long side frame. Further, this height difference is made to 30-50mm. Then, the frequency of current impressed to the electromagnet is set to >=1Hz and the magnetic field is made magnetic field static type AC magnetic field.

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 for steel in which molten steel flow discharged into a mold is controlled by electromagnetic force.

[0002]

2. Description of the Related Art In continuous slab casting for thin steel sheets, high-speed casting technology has been actively developed in order to improve production capacity. In high-speed casting, the molten steel injected from the dipping nozzle into the mold has a large discharge speed, and the molten steel discharge flow collides with the short side of the mold to generate a molten steel flow in two vertical directions along the short side of the mold. .

For this reason, the deoxidation product mainly composed of alumina contained in the molten steel penetrates deeply from the mold together with the downward molten steel flow, and the upward molten steel flow causes the molten metal surface in the mold to be violent. As a result, the mold powder is caught and trapped in the solidified shell, and these become defects in inclusion properties in the product.

[0004] The inclusion defect in the slab due to the molten steel flow becomes remarkable as the casting speed increases, and is serious in the hot direct rolling process, which requires high speed casting for the production of high temperature slab. It becomes a problem.

As a countermeasure against this, many attempts have been proposed to control molten steel flow (molten steel discharge flow) by utilizing electromagnetic induction force.

For example, in Japanese Patent Laid-Open No. 3-258442 (hereinafter, referred to as Prior Art 1), a DC electromagnet is arranged over the entire width in the width direction on the long side of the mold, with its magnetic poles facing each other.
A method is disclosed in which when the molten steel discharge flow from the immersion nozzle passes through a magnetic field, the discharge flow is braked to stop the momentum. As a result, the molten steel flow after passing through the magnetic field is uniformly damped over the entire width of the long side, so that the electromagnet is locally arranged in the width direction of the long side of the mold (for example, JP-A-2-75455). , It is possible to prevent unsteady turbulence of molten steel flow (a state in which molten steel flow is generated in various directions) near the discharge port immediately after discharge, which is observed when the electromagnetic force is too strong and the braking force is excessive. it can.

Further, JP-A-1-150450 (hereinafter, referred to as
According to the prior art 2, instead of the DC magnetic field that always generates a unidirectional static induction current as in the prior art 1,
A method is disclosed in which a magnetic field static AC magnetic field is provided at a position 1.5 to 4 meters below the immersion nozzle to apply a low frequency AC of less than 1 Hz. According to this, the direction of the magnetic field and the direction of the induced current alternately change by 180 degrees, but the direction of the magnetic braking force does not change, and the direction acts in the direction opposite to the molten steel discharge flow, so that the molten steel becomes similar to the DC magnetic field. You can brake the flow.

In addition to this, as a method for controlling molten steel flow in a mold using a low frequency alternating magnetic field, a technique of applying an alternating current in the range of 0.1 to 60 Hz to a moving magnetic field in which the magnetic field temporally moves in the width direction of the mold. Many (hereinafter referred to as Prior Art 3) have also been proposed (for example, Japanese Patent Laid-Open No. 63-212051) and put into practical use.

[0009]

However, in the method according to the prior art 1, since the molten steel discharge flow is subjected to the opposite braking force over the entire width, the molten steel flow from the immersion nozzle is not smoothly discharged in the casting direction. , A kind of blockage. In such a state and when the braking force is too strong, the molten steel flow may cause the molten metal surface in the mold to ramp up, and the mold powder may be caught.

Since the magnetic poles are the same on the long side of one of the molds and are arranged opposite to each other, the molten steel flow on the short side of the mold becomes a descending flow when it collides with the short side of the mold. Since the direction of the induced current and the direction of the molten steel flow are parallel to the downward direction, the magnetic braking force does not work. For this reason, the downflow is concentrated along the short side of the mold to form a rapid molten steel flow, which penetrates deeply and the inclusion physical property defect in the widthwise center of the slab is improved, but the inclusion properties near the short side are improved. There are many defects.

In the method of applying the AC magnetic field according to the prior art 2, since a static induction current is not generated, the downward molten steel flow along the short side of the mold is not reduced and the downward flow is not promoted. However, the absolute value of the magnetic braking force changes from a value proportional to the maximum value of the given magnetic flux density to zero. Therefore, in the case of a low-frequency AC magnetic field, the magnetic poles are switched while the inertial force applied to the braked molten steel flow is not lost, so that the molten steel flow velocity fluctuates at that frequency. As a result, in the high-speed casting in which the fluctuation of the molten steel flow rate reaches the molten metal surface in the mold, the entrainment of the mold powder occurs and the inclusion physical property defect is caused.

In prior art 3, the molten steel stirring technique is applied to the molten steel flow control in the mold, but the braking force acts only in the moving direction of the magnetic field, and the braking force acts on the molten steel flow to cause inclusion property defects. It is insufficient as an improved method for controlling molten steel flow. Furthermore, if the electromagnetic force is too strong, an unsteady flow occurs or a stir flow is generated by the moving magnetic field, so if the balance of the discharge flow velocity from the immersion nozzle and the magnetic field strength is disturbed, powder entrainment is promoted and intervention occurs. It causes physical property defects.

The present invention has been proposed to solve the above-mentioned problems in the prior art. In a continuous casting method in which electromagnetic force is applied to control the flow of molten steel in a mold, a closed state of molten steel flow is provided. Therefore, the present invention proposes a continuous casting method capable of preventing a downward flow concentrated along the shorter side of the mold and reducing inclusion physical defect in the slab. In addition, we propose a highly reliable continuous casting method that applies an AC magnetic field of an appropriate frequency, which does not affect the molten steel flow velocity due to frequency to the molten metal in the mold.

[0014]

The invention according to claim 1 is to dispose an electromagnet having a width of a long side on the long side of a rectangular cross-section mold, with its magnetic poles facing each other, and from the dipping nozzle to the inside of the mold. In a continuous casting method in which an electromagnetic force is applied to the molten steel flow discharged to control molten steel flow, an electromagnet arranged on the long side of the mold has a height difference in the casting direction at the center of the width direction of the long side. And a magnetic pole is arranged.

On the long side of the rectangular cross-section mold, electromagnets having a width substantially equal to the width of the long side are arranged with their magnetic poles facing each other, so that a magnetic field is formed in the thickness direction of the mold over the entire width of the long side. The electromagnet is arranged below the immersion nozzle in order to control the molten steel flow by applying an electromagnetic force to the molten steel flow discharged from the immersion nozzle into the mold.

As a result, when the molten steel flow passes through this magnetic field, a magnetic braking force acts in the direction opposite to the molten steel discharge flow over the entire long side width, and the molten steel discharge speed is reduced. And the inclusion property defects in the slab are reduced. Further, since the electromagnets are arranged over the entire width in the width direction of the long side, local disturbance of the molten steel discharge flow does not occur.

At this time, since the electromagnets arranged on the long side of the mold have magnetic poles arranged at a height difference in the casting direction at the center portion in the width direction of the long side, the electromagnets are arranged at the center portion in the casting direction. Depending on the size of the height difference, a region where no magnetic field is formed or a region where the magnetic flux density is low (this is called a valley of the magnetic field) is formed.

Since the molten steel flow passing through such an area is not subjected to braking or passes with a small braking force (is discharged in the casting direction), the conventional electromagnets are arranged over the entire width of the long side. The blockage state (stuck phenomenon of molten steel flow) that occurs in the technology is eliminated, and the fluctuation of the molten metal level in the mold due to this is prevented.

Further, since the resistance passing through the above-mentioned region is small and the resistance easily passes, even when a static magnetic field is applied or when the magnetic field is too strong, a sharp downward flow concentrated along the short side of the mold described above. Is less likely to occur, and this downward flow is not promoted.

Further, since the molten steel flow passing through this region is once braked and decelerated, and the discharge direction is substantially horizontal, the depth of penetration of the molten steel flow is greatly reduced.

The invention according to claim 2 is the continuous casting method, wherein the height difference of the magnetic poles is 50 mm or more and 300 mm or less.

The present inventors conducted a casting test by changing the height difference of the magnetic poles of the electromagnets provided in the casting direction, and investigated the relationship between the height difference and the number of intervening physical property defects in the slab. As a result,
It was confirmed that by providing the height difference of 50 mm or more and 300 mm or less, the inclusion physical defect can be reduced to 1/2 or less as compared with the conventional technique in which the height difference is zero and the height difference is zero. .

When the difference in height is less than 50 mm, the effect of reducing the defects of inclusions is recognized as compared with the prior art, although it is slight. However, as in the case where there is no difference in height, the above-mentioned closed state or a rapid downward flow occurs along the shorter side of the mold, and the expected reduction effect cannot be obtained. When the height difference exceeds 300 mm, most of the molten steel flow is not braked, or the braking force is small and the braking force does not work effectively, and the expected reduction effect cannot be obtained.

According to the third aspect of the invention, the magnetic poles of the electromagnet are arranged with a height difference, and in addition, the frequency of the current applied to the electromagnet is an alternating current of 1 Hz or more, and the magnetic field is a static magnetic field type alternating magnetic field. It is a continuous casting method characterized by the above.

In order to obtain a more reliable continuous casting method in addition to providing height differences in the magnetic poles of the electromagnets, the inventors have replaced the DC static magnetic field method with the magnetic field not moving in the long side direction of the mold. Uses a static AC magnetic field. However, in order to prevent the fluctuation of the molten metal level in the mold, which is seen in the low-frequency AC magnetic field system of less than 1 Hz in the prior art, the casting test was performed by changing the frequency applied to this AC magnetic field, and the molten metal level fluctuation in the mold was carried out. Was investigated. FIG. 1 shows the result.

FIG. 1 shows a cast thickness of 220 mm and a cast width of 1200.
With a slab of mm, a height difference of 150 mm is provided, and the upper end of the upper magnetic pole of the electromagnet is arranged so as to match the lower end of the immersion nozzle.
Under the casting conditions of a casting speed of 2.0 m / min and a magnetic flux density of 0.2 Tesla (hereinafter referred to as T), the frequency of the applied AC magnetic field is changed within a range of 0.5 to 5 Hz to change the molten metal level in the mold. The amount is measured with an eddy current range finder.

As is clear from the figure, when the frequency is less than 1 Hz, the lower the frequency, the more rapidly the level fluctuation amount increases, and a large level fluctuation amount of 2.5 mm or more was observed. With the above, the fluctuation of the molten metal level was drastically reduced. It was confirmed that the fluctuation of the molten metal surface in the vicinity of the short side of the mold promotes the inclusion of the mold powder, and that the inclusions decrease when the frequency is 1 Hz or higher.

From the above, it has been clarified that by applying a static magnetic field of a magnetic field static type having a frequency of 1 Hz or more, it is possible to prevent the fluctuation of the molten metal surface in the mold due to the fluctuation of the molten steel flow caused by the magnetic braking.

The magnetic field static AC magnetic field is not a moving magnetic field used for a linear motor or the like, and the magnetic poles change with time, but the magnetic field does not move in the long side or short side direction of the mold with time. The magnetic field refers to the height difference between the magnetic poles, which is the distance between the center positions of the two magnetic poles in the casting direction.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 schematically shows an embodiment of an electromagnet arranging method according to the present invention in which magnetic poles have height differences in the casting direction, and a control state of molten steel flow by the method.

FIG. 2A is a schematic plan view of the periphery of the mold, FIG.
(B) is a side perspective view around the mold, 1 is the long side of the mold,
2 is a short side of the mold, 3 is a dipping nozzle, 4 is a mold powder layer, 5 is a molten metal surface in the mold, 6 is a return yoke, 7 is a magnetic pole,
Reference numeral 8 indicates an exciting coil, and reference numerals N and S indicate magnetic poles.

As shown in FIG. 2 (A), the mold unit has a rectangular cross section, and is made of a copper plate whose inside is water-cooled. The mold long side 1 (called the mold width direction) and the mold short side 2 (mold thickness direction). That said) and.

The electromagnet has a pair of magnetic poles 7 on the outside of the long side 1 of the mold, and a pair of magnetic poles 7 divided to the left and right at the center in the width direction of the mold. A total of four magnetic poles 7 are provided.

The end of the magnetic pole 7 in the mold width direction is the long side 1 of the mold.
It is arranged to the outside of the end of. The magnetic poles 7 are arranged facing each other at the same height in the casting direction. The magnetic poles 7 divided into left and right on the long side of the mold are connected by a return yoke 6 which is horizontal and has a height difference in the casting direction to form one magnetic circuit.

In the exciting coil 8, a power source is wired (neither is shown) so that a direct current or an alternating current can flow. By passing a direct current or an alternating current,
As shown in FIG. 2 (A), different magnetic poles (N pole, S pole) are provided between the magnetic poles 7 in the mold width direction, and different magnetic poles (N pole, S pole) are also provided between the facing magnetic poles 7 in the mold thickness direction. As a result, an induced current is generated in the molten steel in the mold.

A static magnetic field whose magnetic poles do not change when a DC current is passed, and each magnetic pole changes 180 degrees at each frequency when an AC current is passed, but the magnetic field changes temporally in the long side direction of the mold. A stationary AC magnetic field is formed that does not move to the magnetic field. The dotted lines in FIG. 2B indicate magnetic fields 9a and 9b formed by passing a direct current or an alternating current.
Is shown.

An immersion nozzle 3 is arranged substantially at the center of the long side 1 of the mold. The magnetic poles 7 in the mold width direction are arranged with a predetermined gap in the mold width direction and a predetermined height difference (upper magnetic pole 7a, lower magnetic pole 7b) in the casting direction with the immersion nozzle 3 interposed therebetween.

In this embodiment, the height of the upper end of the upper magnetic pole 7a is arranged so as to coincide with the lower end of the dipping nozzle 3, and the height difference is designed to be variable in the casting direction in the range of 0 mm to a maximum of 350 mm. .

The submerged nozzle 3 has a shape having a basin at the bottom and has two discharge ports having downward angles. When molten steel is discharged from the discharge port in the direction of the arrow, the molten steel flow enters the magnetic fields 9a and 9b, but receives a magnetic braking force (Lorentz force) in the direction opposite to the molten steel flow.

FIG. 2 (B) schematically shows the path of the molten steel flow subjected to the magnetic braking force. The thick line shows the main flow with high flow rate, and the thin line shows the sub flow with low flow rate. On the left side of FIG. 2 (B), since the upper magnetic field 9a and the ejection port are close to each other,
A braking force acts on the molten steel flow immediately after discharge, but the energy of the discharged molten steel flow is large, and after entering the upper magnetic field 9a, it is braked. At this time, since the space between the upper magnetic field 9a and the lower magnetic field 9b is open, the main flow passes through this space and is discharged. On the other hand, the sub-flow rises after colliding with the short side, but likewise receives the braking force and is damped, so that the molten metal surface 5 in the mold on the left side does not run away.

On the right side of FIG. 2 (B), since the lower magnetic field 9b and the discharge port are separated from each other, the discharge flow is decelerated, so that the penetration depth of the molten steel flow into the lower magnetic field 9b is on the left side. It is less than that of, but the molten steel flow is sufficiently damped. In this case also, since the space between the upper magnetic field 9a and the lower magnetic field 9b is open, the main flow passes through this space and is discharged.

On either side, the main flow is the upper magnetic field 9
Since it passes through the valley of the magnetic field formed between a and the lower magnetic field 9b, the molten steel flow is not blocked,
Further, even when a DC magnetic field is formed, a downward flow along the short side is unlikely to occur. Further, the main flow is braked and decelerated, and since it is a molten steel flow in the lateral direction, the penetration depth in the casting direction becomes shallow, and the inclusion physical property defect is unlikely to occur near the short side. At the same time, since it passes through the valley of the magnetic field, the molten metal surface 5 in the mold is not violated, and the powdery inclusion defect is unlikely to occur.

The height difference between the magnetic poles of the electromagnet and the horizontal distance in the mold width direction may be selected from appropriate values with few inclusion defects due to conditions such as casting speed, magnetic field strength, and casting size. In this embodiment, the immersion nozzle is sandwiched between the magnetic poles 7 in the mold width direction (between the magnetic poles 7 which are divided into right and left), but a predetermined interval is provided, but when the casting speed is low or the height difference is large. In some cases, this interval may be zero. In this case, a valley of the magnetic field is formed in the horizontal direction between the upper magnetic field 9a and the lower magnetic field 9b, and the molten steel flow passes through this valley having a low resistance, and therefore the same effect can be obtained.

Further, in the case of high speed casting in which the casting speed exceeds 1.5 m / min, the molten metal surface 5 in the mold may be violent due to the rising sub-flow after colliding with the short side. In such a case, in addition to the above-mentioned height difference, it is desirable to form a magnetic field static AC magnetic field having a frequency of 1 Hz or higher.

[0045]

EXAMPLE In order to confirm the effect of the present invention, the vertical portion 2.5
Using a vertical bending type continuous casting machine having m, an electromagnet was arranged around the mold as shown in FIG. 2, and a test for controlling molten steel flow in the mold was conducted.

(Confirmation test 1: Investigation of influence of height difference of magnetic poles)
In this test, low carbon aluminum killed steel is used, the casting size is 1200 mm width, 220 mm thickness, and the casting speed is 1.0 m.
/ Min and 2.0m / min, and two levels of magnetic flux density of 0.1T and 0.2T, the upper end of the upper magnetic pole is aligned with the lower end of the immersion nozzle, and the height difference of the magnetic pole is 25m.
The frequency was constant at 0.5 Hz in the range of m to 350 mm.

In the conventional example for comparison, there is no difference in height of the magnetic poles, a DC magnetic field (frequency is 0 Hz) is set, the upper end of the magnetic pole is set 200 mm below the lower end of the immersion nozzle, and the magnetic flux density is constant at 0.2T. did.

The method for investigating inclusions in the test material was to count the number of inclusion defects per unit area on the surface of the cold rolled coil which is the final product. In the quality evaluation method, the number of inclusion defects in the conventional example was set to 1.0, and the number of inclusion defects in the example was indexed (this is called an inclusion defect index) for evaluation.

FIG. 3 shows the survey results measured in this way. As shown in FIG. 3, in the height difference range of 50 to 300 mm, a good result that the inclusion defect index was 0.5 or less was obtained. On the other hand, the height difference is 25 mm and 350 mm
Then, the result of the inclusion defect index exceeded 0.5,
The effect was recognized as compared with the conventional example. The effect of casting speed is
A lower inclusion index was obtained at a casting speed of 1.0 m / min than at a casting speed of 2.0 m / min in the range where the height of the magnetic pole was about 200 mm or less, but the height difference of the magnetic pole was about 250 mm. With the above, the casting speed is 2.0 m / m.
On the contrary, the better results were obtained. From this, it was found that increasing the height difference of the magnetic poles with increasing casting speed is more effective for quality, and that there is an appropriate height difference depending on the casting speed.

The influence of the magnetic flux density is 0.2 compared to 0.1T.
T has a height difference of 25 to 350 mm over the entire range,
A product having a lower inclusion defect index was obtained.

(Confirmation test 2: Investigation of influence of frequency) In this test, the height difference between the upper magnetic pole and the lower magnetic pole was set to 150 mm,
The frequency of the applied magnetic field is 0Hz (DC magnetic field), 0.5H
A total of 5 levels of z, 1.0 Hz, 2.0 Hz, and 5.0 Hz were set, and the other conditions were the same as those of the confirmation test 1, and the test was performed.

In the conventional example, there was no height difference between the magnetic poles, a direct current magnetic field (frequency was 0 Hz), the upper end of the magnetic pole was set 200 mm below the lower end of the immersion nozzle, and the magnetic flux density was constant at 0.2T.

The inclusion index was measured and investigated in the same manner as in the confirmation test 1. FIG. 4 shows the results. As shown in FIG.
In the range of 5.0 Hz or less, the inclusion defect index is 0.
A favorable result of 4 or less was obtained, which was significantly reduced as compared with the conventional example, and a favorable result was obtained. In addition, at a frequency of 1.0 Hz or higher, as compared with less than 1.0 Hz, the inclusion defect index is
Lower results were obtained.

[0054]

According to the present invention, the height difference of the magnetic poles of the electromagnet is provided to prevent the molten steel flow from being blocked and the downward flow concentrated along the shorter side of the mold, thereby reducing the inclusion property defects in the slab. it can. Also, the height difference is 50 mm or more, 300 mm
By restricting the following, inclusion property defects in the slab can be further reduced.

Further, in addition to the height difference, by using a static AC magnetic field of 1 Hz or more, the fluctuation level of the molten metal in the mold is reduced, and the inclusion defect in the cast piece can be further reduced.
A stable and highly reliable slab can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram in which a relationship between a frequency and an amount of fluctuation of a molten metal surface in the vicinity of a short side is investigated when a static AC magnetic field is adopted.

FIG. 2 is a diagram schematically showing an embodiment of an electromagnet arranging method according to the present invention and a molten steel flow control state.

FIG. 3 is a diagram investigating the relationship between the height difference of the magnetic poles of the electromagnet and the inclusion defect index.

FIG. 4 is a diagram investigating the relationship between the frequency of a magnetic field and the inclusion defect index.

[Explanation of symbols]

 1 Mold Long Side 2 Mold Short Side 3 Immersion Nozzle 4 Mold Powder Layer 5 Mold Surface 6 Return Yoke 7 Magnetic Pole 8 Excitation Coil

Claims (3)

[Claims] 1. A rectangular cross-section mold is provided with electromagnets on the long side of the mold, the width of which is approximately equal to the width of the long side, with their magnetic poles facing each other.
In a continuous casting method that controls the molten steel flow by applying an electromagnetic force to the molten steel flow discharged from the immersion nozzle into the mold, the electromagnet arranged on the long side of the mold is cast at the center in the width direction of the long side of the mold. A continuous casting method, wherein magnetic poles are arranged with a height difference in the direction. 2. The height difference is 50 mm or more and 300 mm.
The continuous casting method according to claim 1, wherein: 3. The continuous casting method according to claim 1, wherein the frequency of the voltage applied to the electromagnet is an alternating current of 1 Hz or higher, and the magnetic field is a static magnetic field of the magnetic field static type.