JP4288020B2 - Molten metal flow controller - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a molten metal flow control device, and more particularly to a molten metal flow control device capable of suppressing flow instability generated in a meniscus portion of a molten metal in a mold.
[0002]
[Prior art]
In continuous casting in which slabs or billets are produced from molten steel melted in a blast furnace, molten steel in a ladle is injected into a mold from a dipping nozzle through a tundish. Then, the molten steel is drawn from the bottom of the mold and formed into a slab or billet.
[0003]
During molding, if the molten steel in the mold is not uniform in the horizontal direction, surface cracks and shell fractures are likely to occur in the product. Therefore, a linear motor coil is installed at the upper end of the mold side wall to give circulation power to the molten steel. It has already been proposed.
In order to further improve the quality of the product, alternating current of high voltage and low voltage (including zero voltage) is repeated every predetermined period in order to promote the inclusion of powder added on the meniscus at the boundary between the molten steel and the mold. So-called pulse electromagnetic stirring, in which excitation by electric power is repeated, has also been proposed.
[0004]
FIG. 1 is a top view (A) and an XX sectional view (B) of a conventional mold for manufacturing a square billet having a side length of 1600 mm when pulsed electromagnetic stirring is applied. An electromagnetic stirring coil 11 is wound around the periphery of the mold 10 and an immersion nozzle 12 is installed at the center of the mold 10.
When pulse electromagnetic stirring is applied, the meniscus rises from both side walls of the mold 10 toward the immersion nozzle 12, so that a flow from the immersion nozzle 12 toward the mold peripheral wall is generated in the molten steel near the meniscus.
[0005]
And since the electromagnetic stirring coil 11 is excited by the alternating current power supply 13 which outputs alternating current power which repeats a high voltage and a low voltage alternately for every predetermined period, the speed of the flow of the molten steel which goes to the mold peripheral wall from the immersion nozzle 12 is temporal. As a result, irregularities occur in the meniscus, and surface cracks and shell breakage tend to occur in the billet.
Therefore, in order to suppress fluctuations in the flow at the meniscus, measures are taken such as making the immersion nozzle 12 thick, for example, 900 mm in diameter and reducing the meniscus surface area.
[0006]
FIG. 2 is a graph showing fluctuations from the reference level of the meniscus along the wall of a square mold having a dip nozzle of 900 mm in diameter and a side of 1600 mm, and the horizontal axis represents the inner periphery of the mold from A → B → C → D. → The distance that goes around in the order of A, and the vertical axis represents the fluctuation of the meniscus from the reference level.
That is, in this case, the maximum variation is +5 mm, the minimum variation is −5 mm, and the amplitude is about 10 mm.
[0007]
[Problems to be solved by the invention]
However, when the mold side length is increased to 2200 mm in order to increase the productivity of the billet, the maximum fluctuation is +15 mm, the minimum fluctuation is -20 mm, and the amplitude is 35 mm, which sufficiently suppresses fluctuations in the speed of the molten steel. It is difficult to avoid the deterioration of billet quality.
[0008]
This invention is made | formed in view of the said subject, Comprising: It aims at providing the flow control apparatus of the molten metal which can suppress the fluctuation | variation of the speed of the molten steel in the meniscus vicinity.
[0009]
[Means for Solving the Problems]
A molten metal flow control device according to the present invention includes a shared coil that generates AC magnetic flux and DC magnetic flux wound around the outer periphery of a mold, an AC power source that AC-excites the shared coil, and a DC power source that DC-excites the shared coil. If, comprising a, the DC power source, while the AC power is output AC power stops DC power state, and are not to supply the DC power while the AC power is stopped, and the DC power and the AC power is supplied alternately, the DC power is Ru der those supplied by direct current flowing in opposite directions alternately.
[0010]
The DC power supply can alternately supply DC power of reverse current while the AC power supply is stopped.
[0011]
It is also possible to provide a rest period in which the outputs of both the AC power source and the DC power source are suspended.
[0012]
The mold is a rectangle having a side length of LA [mm] and a thickness of LB [mm],
The molten metal flow control device according to any one of claims 1 to 3, wherein a diameter D [mm] of an immersion nozzle for injecting molten metal satisfies the following equation.
(L-2 × a) × 0.7 ≦ D ≦ (L-2 × a) × 1.3
However, L = (LA × LB) ^1/2 [mm]
a is the raised height of the molten metal when excited by AC power [mm]
[0016]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 3 is a configuration diagram of the molten metal flow control device according to the first embodiment of the present invention, in which (A) is a top view and (B) is a YY sectional view.
That is, the molten metal flow control device according to the first embodiment includes an electromagnetic stirring coil 31 in addition to the electromagnetic stirring coil 31 and the immersion nozzle 32 wound around a square mold 30 having a side length L of 2200 mm. Similar to the electromagnetic stirring coil 31, a DC coil 33 wound around a mold 30 is provided on the outside of the coil 31.
[0017]
The electromagnetic stirring coil 31 is intermittently excited by AC power supplied from the AC power supply 34, and the DC coil 33 is excited by DC power supplied from the DC power supply 35.
Since the molten metal rises in a convex shape around the immersion nozzle 32 by the alternating magnetic flux generated by the electromagnetic stirring coil 31 excited by the alternating current power, the molten metal flows from the immersion nozzle 32 toward the mold peripheral wall.
[0018]
However, since the alternating magnetic flux fluctuates in a pulse shape, the flow rate of the molten metal fluctuates with time. Therefore, in order to suppress this fluctuation, the DC coil 33 is excited with DC power.
FIG. 4 is an operation explanatory diagram of the molten metal flow control device according to the first embodiment of the present invention.
That is, the DC magnetic flux B _z generated by the DC coil 33 at the meniscus acts vertically upward on the molten metal. Further, the molten metal flows from the center of the mold 30 toward the peripheral wall at a velocity V _y . Therefore, a current I _x flowing parallel to the wall surface is induced in the molten metal in accordance with Fleming's right-hand rule.
[0019]
Then, in accordance with Fleming's left-hand rule due to the interaction between the DC magnetic flux B _z and the current I _x , a Lorentz force F _y is generated in the direction opposite to the velocity V _y , that is, from the peripheral wall of the mold 30 toward the center. Acts in the direction of deceleration.
That is, if the speed of the molten metal is large, the Lorentz force F _y is also large, and if the speed of the molten metal is small, the Lorentz force F _y is also small, and fluctuations in the speed of the molten metal are suppressed.
[0020]
FIG. 5 is an explanatory diagram of the DC coil installation effect, in which the horizontal axis represents time and the vertical axis represents the flow rate of molten metal. A straight line indicates a case where the DC coil is excited, and a broken line indicates a case where the DC coil is not excited.
That is, when the DC coil is excited, the flow fluctuation of the molten metal is suppressed. This is because when the velocity V _{y of the} molten metal is large, the current I _x becomes large, and as a result, the Lorentz force F _y becomes large. This is because when the velocity V _{y of the} molten metal is small, the current I _x becomes small, and as a result, the Lorentz force F _y becomes small.
[0021]
As is apparent from the above, the magnetic flux density generated by the DC coil may be about 0.05 to 0.6 Tesla as long as the fluctuation of the molten metal speed can be suppressed without disturbing the stirring of the molten metal by the AC coil.
It is also possible to reduce the power capacity of the AC power supply 34 by inserting a resonance capacitor 35 in parallel with the AC coil 31.
[0022]
In the first embodiment, the AC coil and the DC coil are separate coils. However, since the coils are both wound around the mold, the coil is made one by exciting with the power superposed of the AC power and the DC power. Is possible.
FIG. 6 is a configuration diagram of a molten metal flow control device according to a second embodiment of the present invention, in which (c) is a top view and (d) is a YY sectional view.
[0023]
In other words, the molten metal flow control device according to the second embodiment includes the common coil 61 and the immersion nozzle 32 wound around a square mold 30 having a side length L of 2200 mm.
The shared coil 61 is intermittently excited by AC power supplied from the AC power supply 34 and also excited by DC power supplied from the DC power supply 35.
[0024]
FIGS. 7 and 8 are explanatory views (No. 1) and (No. 2) of the excitation method of the shared coil, and (E) shows a case where the AC power source 34 and the DC power source 35 are simply connected in series. That is, the DC power source 35 constantly supplies DC power having a voltage V _DC , and the AC power source 34 intermittently supplies AC power having a _PP voltage of V _AC every 50 milliseconds.
[0025]
In this case, since the rated current of the shared coil 61 is determined from the sum of the alternating current and the direct current, the winding of the shared coil 61 must be thickened.
(F) is a case where alternating current power and direct current power are applied alternately, and the rated current of the common coil 61 is defined by the larger of the alternating current and direct current (usually alternating current). It is not necessary to thicken the windings more than necessary.
[0026]
However, in this case, since direct current always flows in one direction, among the plurality of switching elements included in the power supply, the element that becomes conductive and the element that becomes blocked are fixed, and heat generation of the element that becomes conductive is generated. I can't avoid becoming big.
(G) is for solving the above-described problem, and shows a case where alternating current power and direct current power are alternately applied and direct current is alternately supplied in the opposite direction.
[0027]
In this case,
V _DC → V _AC → −V _DC → V _AC → V _DC
By supplying power in this order, the heat generation of the switching elements can be made uniform.
(H) is a case where a rest period in which electric power is not supplied to the common coil is provided, and for example, when the hot water level is measured by an eddy current type hot water meter, the influence of excitation by AC power and DC power is eliminated. Is possible.
[0028]
It is obvious that the resonant capacitor 36 can also be applied in the second embodiment.
In order to further suppress fluctuations in the speed of the molten metal, it is effective to reduce the surface area of the meniscus by increasing the diameter of the immersion nozzle 32 to be equal to or greater than the rate of increase in the side length of the mold.
[0029]
That is, when the side length is increased from 1600 mm to 2000 mm, the injection time of the molten metal can be kept constant by increasing the diameter of the immersion nozzle from 900 mm to 1240 mm. By setting as described above, it is possible to further suppress fluctuations in the speed of the molten metal.
The molten steel excited by the AC coil rises from the inner wall of the mold 30 toward the immersion nozzle 32. Therefore, if the diameter D of the immersion nozzle 32 is determined by the following equation, fluctuations in the speed of the molten metal are effectively suppressed.
[0030]
(L-2 × a) × 0.7 ≦ D ≦ (L-2 × a) × 1.3
However, L = (LA × LB) ^1/2
LA is the mold side length LB is the mold wall thickness a is the raised height of the molten metal when alternating current is excited When the mold side length is 2200 mm and the thickness is 250 mm, the raised height a of the molten steel by alternating current excitation is Since it is about 250 mm, the diameter D of the immersion nozzle 32 is
D = 742−2 × 250 = 242
169 ≦ D ≦ 315
It becomes.
[0031]
FIG. 9 is an explanatory diagram of the effect of the present invention, in which the horizontal axis indicates the distance of the inner circumference of a square mold having a side length of 2200 mm in the order of A → B → C → D → A, and the vertical axis indicates the meniscus. It represents the change from the reference level. A broken line indicates a case where the diameter of the immersion nozzle 32 is 140 mm and the DC coil is not excited, and a solid line indicates a case where the diameter of the immersion nozzle 32 is 200 mm and the DC coil is excited.
[0032]
That is, the maximum amplitude of the meniscus when the diameter of the immersion nozzle 32 is 140 mm and the DC coil is not excited reaches 35 mm, but the maximum amplitude is suppressed to 15 mm when the diameter of the immersion nozzle 32 is 200 mm and the DC coil is excited. Is possible.
When the diameter of the immersion nozzle 32 is determined by the above equation, fluctuations in the flow rate of the molten metal are suppressed to some extent without exciting the DC coil.
[0033]
【The invention's effect】
According to the molten metal flow control device of the first aspect of the invention, a perpendicular direct current magnetic flux acts on the meniscus of the molten metal, and fluctuations in the speed of the molten metal from the mold center toward the peripheral wall can be suppressed.
In addition , since the AC magnetic flux and the DC magnetic flux are generated by one common coil, the configuration can be simplified.
[0034]
Furthermore, since the rated current of the shared coil has only to be determined by one of the AC power current and the DC power current, which is a large current, an increase in size of the shared coil can be prevented.
Furthermore , since the DC power is alternately supplied in the opposite direction, the heat generation of the switching elements in the power supply can be made uniform.
[0035]
According to the molten metal flow control device of the second invention, there is a period during which no electric power is supplied to the coil, so that the molten metal surface can be accurately measured.
According to the molten metal flow control device according to the third aspect of the invention, the diameter of the immersion nozzle is adjusted so that the minimum gap between the mold peripheral wall and the immersion nozzle is equal to the rising height of the molten metal even when the mold is enlarged. By determining, it is possible to further suppress fluctuations in the speed of the molten metal.
[Brief description of the drawings]
1A and 1B are a top view and a cross-sectional view taken along line XX of a conventional billet mold.
FIG. 2 is a graph showing fluctuations of a meniscus from a reference level.
FIG. 3 is a configuration diagram of a molten metal flow control device according to the first embodiment.
FIG. 4 is an operation explanatory diagram of the molten metal flow control device according to the first embodiment.
FIG. 5 is an explanatory diagram of a DC coil installation effect.
FIG. 6 is a configuration diagram of a molten metal flow control device according to a second embodiment.
FIG. 7 is an explanatory diagram (No. 1) of a common coil excitation method;
FIG. 8 is an explanatory diagram (No. 2) of a method for exciting a shared coil.
FIG. 9 is an explanatory diagram of the effect of the present invention.
[Explanation of symbols]
30 ... Mold 31 ... Coil for generating AC magnetic flux 32 ... Immersion nozzle 33 ... Coil for generating DC magnetic flux

Claims (3)

A shared coil that generates AC magnetic flux and DC magnetic flux wound around the outer periphery of the mold;
An AC power source for AC exciting the common coil;
A DC power source for DC excitation of the shared coil,
The DC power source, while the AC power is output AC power stops DC power state, and are not to supply the DC power while the AC power is stopped, and the AC power and The molten metal flow control device , wherein the DC power is alternately supplied and the DC power is supplied by a DC current that flows alternately in the opposite direction . The molten metal flow control device according to claim 1 , further comprising a rest period in which outputs of both the AC power source and the DC power source are suspended. The mold is a rectangle having a side length of LA [mm] and a thickness of LB [mm],
The molten metal flow control device according to claim 1 or 2 , wherein a diameter D [mm] of an immersion nozzle for injecting molten metal satisfies the following formula.
(L-2 × a) × 0.7 ≦ D ≦ (L-2 × a) × 1.3
However, L = (LA × LB) ^1/2 [mm]
a is the raised height of the molten metal when excited by AC power [mm]

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