JP2004042057A - Method and apparatus for continuously casting steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize solidification at the initial stage by casting while giving the oscillation to a mold with electromagnetic force and to give the lubricating action and the surface characteristic improving action between the mold and molten metal in a method for continuously casting the molten metal. <P>SOLUTION: In the continuous casting method, with which the molten metal is solidified while impressing AC current by disposing a coil on the outer periphery of the mold and giving pinch force to a meniscus part, conditions of the mold oscillation are adjusted so as to satisfy the following relationships (1), (2) and (3). t<SB>P</SB>≥ 0.2 × exp(-αP)...(1), t<SB>N</SB>≥ 0.1 × exp(-βP)...(2), S ≥ 4 × exp(-γP)...(3), wherein, t<SB>P</SB>: positive stripping time, t<SB>N</SB>: negative stripping time, S: oscillation stroke, P: raise-up height and α,β,γ: factors. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for continuously casting molten metal, and in particular, stabilizes initial solidification by performing casting while applying mold vibration by electromagnetic force to a mold, and has a lubricating action and a surface property improving action between the mold and the molten metal. It relates to a continuous casting method while applying.
[0002]
[Prior art]
Generally, in continuous casting of steel, powder is added to the upper surface of a molten metal pool in a mold, and the powder melted by heat from the molten metal is moved by a relative motion between a vertically vibrating mold and a solidified shell drawn at a constant speed. Flows into these gaps. The meniscus and the tip of the solidified shell are deformed by the dynamic pressure generated when the molten powder flows. Since this deformation is repeated at the cycle of the mold oscillation, periodic wrinkles called oscillation marks are formed on the slab surface. Although the formation of this regular minor oscillation mark does not significantly affect the casting operation or the stabilization of the slab surface quality, the formation of a deep and deep oscillation mark can This leads to the problem of slab surface defects such as increased air bubbles and inclusions.
[0003]
As a method for solving such a problem, for example, Japanese Unexamined Patent Application Publication No. 58-32824 discloses a method of injecting a molten metal 2 together with a lubricant 4 into a water-cooled mold 1 which vibrates at a constant cycle, as shown in FIG. In a continuous casting method in which the molten metal 2 is continuously drawn downward, an alternating current is continuously applied to the electromagnetic coil 5 provided around the mold 1 and the molten metal 2 is raised in a convex shape using an electromagnetic force generated by the alternating magnetic field. Discloses a method for improving the surface properties of a slab. JP-A-64-83348 discloses that an electromagnetic force is applied to a molten metal in a mold by an electromagnetic coil, and the electromagnetic force is applied intermittently by applying an alternating current generated in an induction coil provided around the mold. Then, in powder casting in which the meniscus portion is convexly curved to promote powder feeding along the inner surface of the mold, intermittently applying an AC magnetic field in a pulsed manner to intermittently apply an electromagnetic force between the mold and the slab. A continuous casting method that improves lubrication is disclosed. Further, as disclosed in WO96 / 05926, as shown in FIG. 5, an alternating current is applied to a solenoid-shaped electromagnetic coil 5 installed so as to surround a mold wall 1, and the molten metal solidified in the mold is solidified. 2, the molten metal 2 is raised in a convex shape while applying an electromagnetic force 22 acting in a direction separating from the mold wall 1 toward the molten metal side, which is determined by the directions of the induced current 20 and the induced magnetic field 21. A method is disclosed in which the amplitude or waveform of the alternating current is periodically changed by the waveform generator 24 to stabilize the initial solidification and improve the surface properties of the slab.
[0004]
However, in this method, the permissible condition of the mold oscillation is relaxed as compared with the conventional casting condition, but another problem of breakout occurs, and the effect of improving the surface properties is not so large. There is a problem that it cannot be obtained.
[0005]
[Problems to be solved by the invention]
The present invention provides a method for continuously casting molten metal, in which a mold is subjected to casting while applying mold vibration by electromagnetic force, thereby stabilizing initial solidification, and continuously casting steel to improve lubrication and surface properties between the mold and the molten metal. Provide a way to do it.
[0006]
[Means for Solving the Problems]
The present invention has been made to solve the above problems, and the gist thereof is as follows.
[0007]
(1) In a continuous casting method in which a coil is arranged on the outer periphery of a mold and solidified while applying a pinch force to a meniscus portion of a molten metal while applying an alternating current, the condition of the mold oscillation is satisfied so that the following relational expression is satisfied. A continuous casting method for steel characterized by being adjusted t _P ≧ 0.2 · exp (−αP) −−−−− (1)
t _N ≧ 0.1 · exp (−βP) −−−−− (2)
S ≧ 4 · exp (−γP) −−−−−−− (3)
Here, t _P : positive strip time, t _N : negative strip time, S: oscillation stroke, P: height of rise, α, β, γ: coefficient.
[0008]
(2) A continuous casting method for steel according to the above (1), wherein an alternating current to be applied to the coil disposed on the outer periphery of the mold is intermittently applied.
[0009]
(3) A continuous casting method for steel according to (1) or (2), wherein a DC magnetic field is applied near the meniscus while applying an AC current to the coil disposed on the outer periphery of the mold.
[0010]
(4) In a continuous casting method in which a coil is arranged on the outer periphery of a mold and solidification is performed while applying a pinch force to a meniscus portion of a molten metal while applying an alternating current, conditions for mold oscillation can be adjusted according to the applied electromagnetic force. A continuous casting apparatus for steel, characterized in that:
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings.
[0012]
FIG. 1 is a schematic cross section of a continuous casting apparatus for performing a method according to the present invention. An energizing coil 5 for passing an alternating current is provided around the water-cooled mold 1 so as to be buried in the water-cooled mold 1 so that an upper end thereof is located at the meniscus 3. During casting, molten metal is supplied into the water-cooled mold 1 from a tundish (not shown) above the water-cooled mold 1 via the immersion nozzle 6.
[0013]
The discharge direction of the molten metal from the immersion nozzle varies depending on the casting conditions, such as discharge toward the short side, discharge toward the long side and short side, discharge below the pool, and superposition of these. Although there is a direction, in the following description, an example in which the discharge direction of the immersion nozzle is the short side direction in slab casting will be described.
[0014]
In the following description, it is assumed that the horizontal cross-sectional shape of the water-cooled mold is a rectangular mold. Powder 4 is supplied as a lubricant to the surface of the molten metal 2 in the water-cooled mold 1, and the molten metal 2 is cooled while being oscillated by the vertical movement of the water-cooled mold 1, solidified and drawn downward as a slab. It is. At this time, when an AC current of several tens to several hundreds of Hz is applied to the energizing coil 5, an induction magnetic field B is generated in the mold, and an induction current J is induced. The electromagnetic force F acts on the molten metal 2 in the direction toward the center of the water-cooled mold 1 by the action of the induced current J and the induced magnetic field B, and the molten metal is raised in a convex shape in the vicinity of the meniscus 3 to lubricate the water-cooled mold 1. Solidification proceeds while maintaining smoothness.
[0015]
As a result, it is possible to reduce the oscillation mark and the like of the slab and improve the surface properties, but the effect is considered to be different depending on the oscillation condition, so the oscillation conditions were examined. FIG. 2 shows the relationship between mold oscillation conditions and casting speed. The positive strip time indicates a time zone in which the mold speed is lower than the slab pull-out speed, and the negative strip time indicates a time in which the mold speed is faster than the slab pull-out speed. Generally, since a correlation between the positive strip time and the powder consumption is recognized, it is said that it is necessary to secure a certain time or more in order to secure the powder consumption. On the other hand, as for the negative strip time, since the level change during the operation cannot be 0, the solidified shell once restrained by the mold is securely pushed into the solidified shell that has been drawn out and joined at the time of the level change, It is necessary to secure a certain time. In general, in order to reduce the depth of the oscillation mark, the stroke of the oscillation is shortened and the frequency is increased.However, if the frequency is too large, the deformation of the meniscus does not follow, and In some cases, the oscillation mark depth may be deeper, and it is known that there is an appropriate oscillation condition.
[0016]
Therefore, the casting was performed by changing the electromagnetic force applied to the vicinity of the meniscus portion and the mold oscillation conditions in various ways, and the relationship between occurrence of breakout, powder consumption, and oscillation mark depth was investigated. As a result, it was found that the higher the height of the electromagnetic force to be applied, that is, the higher the swelling height of the molten metal, the lower the minimum value for both the positive strip time and the negative strip time, and the stable casting range can be expressed by the following equation. did.
[0017]
The height of the swell is defined as shown in FIG. The difference between the level (FIG. 4B) near the center of the thickness when the electromagnetic force is applied and the level (FIG. 4A) when the electromagnetic force is not applied is defined as the swelling height. . For the measurement method, for example, it is possible to measure the shape of the molten metal level by immersing several copper-coated iron wires in the mold in the thickness direction and pulling them up. Height can be defined.
[0018]
t _P ≧ 0.2 · exp (−αP) −−−−− (1)
t _N ≧ 0.1 · exp (−βP) −−−−− (2)
The reason that the minimum value of the positive strip time and the negative strip time is reduced by the application of the electromagnetic force is that by applying the electromagnetic force, the gap between the meniscus portion of the molten metal and the mold is enlarged, so that the powder inflow amount is inevitable. Will increase. This is related to the applied electromagnetic force, that is, the height of the raised portion of the molten metal, and the larger the height of the raised portion of the molten metal, the larger the amount of expansion of the gap. Therefore, the positive strip time provided to secure the powder inflow amount can be shortened. Further, regarding the negative strip time, the possibility that the solidified shell is restrained by the mold is reduced, and as a result, the negative strip time provided to secure the pushing force to the solidified shell and avoid the restraint breakout is reduced. The minimum value can be shortened. Since the level change during casting cannot be reduced to zero during operation, some time is required for both the positive strip time and the negative strip time even under the condition where the electromagnetic force is applied. From the relationship.
[0019]
With respect to the relationship with the oscillation mark depth, it has been found that the relationship between the stroke at which the oscillation mark depth is locally increased and the electromagnetic force is expressed by the following equation. This is because when the electromagnetic force was not applied, the stroke was shortened and the deformation of the meniscus could not follow, and the oscillation mark was disturbed.However, the distance between the meniscus and the mold was expanded by applying the electromagnetic force. This makes it difficult for the influence of the deformation of the meniscus due to the oscillation to propagate, so that the allowable oscillation stroke can be made shorter by the electromagnetic force. In addition, within the same stroke, the depth of the oscillation mark could be reduced as the negative strip time was shorter under the condition represented by the expression (2).
[0020]
S ≧ 4 · exp (γP) −−−−− (3)
In addition, the same investigation was performed for the case where a DC magnetic field was also superimposed in the vicinity of the meniscus in the sense of intermittently applying the electromagnetic force and in the sense of preventing microscopic fluctuations in the molten metal surface in addition to the pinch force. By satisfying almost the same relational expression, stable casting was possible and the depth of the oscillation mark was reduced. In addition, the waveform of the oscillation is not limited to a commonly used sine wave because the relative speed between the mold and the slab is a problem. I do not care.
[0021]
【Example】
A solenoid coil having a height of 1500 mm is placed on the meniscus portion of a mold having a width of 1500 mm, a height of 880 mm and a thickness of 250 mm so that the upper end thereof is at the meniscus position, and has a depth of 300 mm from the meniscus position. An immersion nozzle having an outer diameter of 150 mm and an inner diameter of 90 mm was used. A sine wave alternating current with a frequency of 150 Hz is applied to the electromagnetic coil so that the height of the molten metal becomes 20 mm, and a casting speed: 1.5 m / min and a low carbon aluminum killed steel slab having a width of 1500 mm and a thickness of 250 mm are cast. did. As the mold oscillation conditions, casting was performed with a stroke of 8 mm and various vibration frequencies. The results are shown in FIGS. 3 (a), 3 (b) and 3 (c).
[0022]
As can be seen from FIGS. 3 (a), 3 (b) and 3 (c), a breakout occurs when the negative strip time is less than 0.1 second under the condition where no electromagnetic force is applied and when the positive strip time is less than 0.2 second. However, when an electromagnetic force was applied and the oscillation conditions were set according to the electromagnetic force, no breakout was observed. When casting was performed under the condition that the oscillation stroke was 3.5 mm without applying an electromagnetic force, disturbance of the oscillation mark was confirmed. However, the stroke was shortened according to the applied electromagnetic force and the negative strip time was reduced. Under the reduced conditions, no disturbance of the oscillation mark was observed and the depth of the oscillation mark was also reduced, so that a slab with extremely good surface properties could be obtained.
[0023]
In general, as the casting speed increases, the frequency is set to a high cycle, or if the frequency is still insufficient, the oscillation stroke is generally lengthened. Greatly expands. In addition, since the depth of the oscillation mark can be reduced, the casting speed can be increased while improving the quality under the surface of the slab. Furthermore, by applying the present invention to a method of casting a thin slab having a slab thickness of less than 100 mm at a high speed, the load of mold vibration can be greatly reduced, and high-speed casting can be performed stably.
[0024]
【The invention's effect】
As described above, the present invention applies a pinch force to the meniscus portion of the molten metal by applying an alternating current to the coil installed on the outer periphery of the mold, and the raised height of the molten metal is raised in a convex shape. By specifying the oscillation conditions and the oscillation conditions, it is possible to simultaneously suppress deformation due to oscillation of the meniscus portion and ensure lubrication, and it is possible to stably obtain a cast piece having good surface properties while preventing breakout.
[Brief description of the drawings]
FIG. 1 is a schematic view showing an embodiment of continuous casting according to the present invention.
FIG. 2 is a diagram showing a relationship between a negative strip time and a positive strip time of electromagnetic force in continuous casting according to the present invention.
3A is a diagram showing the relationship between the molten steel swell height and the negative strip time, FIG. 3B is a diagram showing the relationship between the molten steel swell height and the positive strip time, and FIG. It is a figure showing the relation with a stroke.
4A and 4B are diagrams showing the definition of the molten steel swelling height, wherein FIG. 4A shows the molten steel swelling height when no electromagnetic force is applied, and FIG. 4B shows the molten steel swelling height when an electromagnetic force is applied. FIG.

Claims (4)

In a continuous casting method in which a coil is arranged around the mold and solidification is performed while applying a pinch force to the meniscus portion of the molten metal while applying an alternating current, the conditions of the mold oscillation are adjusted so that the following relational expression is satisfied. Steel continuous casting method characterized by the following formula: t _P ≧ 0.2 · exp (−αP) --- (1)
t _N ≧ 0.1 · exp (−βP) −−−−− (2)
S ≧ 4 · exp (−γP) −−−−−−− (3)
Here, t _P : positive strip time, t _N : negative strip time, S: oscillation stroke, P: height of rise, α, β, γ: coefficient. 2. The continuous casting method for steel according to claim 1, wherein the alternating current applied to the coil disposed on the outer periphery of the mold is intermittently applied. 3. A continuous casting method for steel according to claim 1, wherein a direct current magnetic field is applied near the meniscus while applying an alternating current to the coil disposed on the outer periphery of the mold. In a continuous casting method in which a coil is arranged on the outer periphery of a mold and solidification is performed while applying a pinch force to a meniscus portion of a molten metal while applying an alternating current, conditions for mold oscillation can be adjusted according to an applied electromagnetic force. A continuous casting apparatus for steel.

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