JP3158936B2 - High frequency coil for continuous casting and continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting high-frequency coil for concentrating a magnetic field at the tip of a solidified shell and a continuous casting method using the continuous casting high-frequency coil in continuous casting of molten metal such as steel. is there.

[0002]

2. Description of the Related Art On a surface of a slab produced by a continuous casting method of molten steel, a surface defect called an oscillation mark is generated due to vibration of a mold. The oscillation mark may cause lateral cracks in the cast slab, or a segregation layer may be formed at a valley of the oscillation mark, and the segregation layer may remain on the surface of the steel sheet product and become a surface defect. For this reason, it is required that the depth of the oscillation mark be reduced as much as possible.

Conventionally, as a method of reducing the depth of the oscillation mark, there are methods such as increasing the frequency of the mold and changing the vibration waveform (using a non-sine waveform).

However, each of these methods is a method of shortening the time during which the downward speed of the mold vibration becomes higher than the moving speed of the slab (casting speed), that is, the time during the negative strip period. Also in this method, there is a limit in reducing the depth of the oscillation mark.

[0005] In continuous casting, mold powder is used to reduce the depth of the oscillation mark and to improve the lubricity of the mold and the slab. The corner of the mold short side and the mold long side is cooled by the mold in two directions, so that the thickness of the molten layer of the mold powder is reduced, and the molten metal discharge flow from the tundish immersion nozzle collides with the mold short side. As a result, an upward flow occurs, and the short side of the mold tends to have a bare metal surface, so that the thickness of the mold powder is reduced. For this reason, the surface properties of the slab are inferior at the short side of the slab, particularly at the corner, as compared with the long side of the slab.

As a method for solving these problems, Japanese Patent Laid-Open Publication No. HEI 5-115952 discloses a method as shown in FIG.
A method has been proposed in which a high-frequency coil is installed inside a mold to apply an electromagnetic force to molten metal. In FIG. 8, 1 is a mold, 2 is an immersion nozzle, 3 is a high-frequency coil, 4 is a high-frequency coil support jig, 5 is molten steel, 6 is mold powder, 7
Is a tundish, 8 is cooling water, 9 is a solidified shell, and 10 is a short side mold.

The molten steel is poured from a ladle into a tundish 7 and then into a mold 1 that has been cooled with water 8 through a dipping nozzle 2. Also, on the surface of molten steel 5 in mold 1,
The mold powder 6 is sprayed, and the high-frequency coil 3 is disposed above the mold powder 6. The high frequency coil 3 is a short side mold 10
And is supported by a high-frequency coil support jig 4 installed thereon. In this method, the slab width and the length of the high-frequency coil have substantially the same length, an electromagnetic force is applied to the entire slab width by the high-frequency coil 3, and the high-frequency coil is installed near the surface of the molten metal and By applying strong electromagnetic force, slabs with few surface defects are manufactured.

[0008]

However, when the technique disclosed in Japanese Patent Application Laid-Open No. H5-115952 is applied, the cross-sectional shape of the high-frequency coil 3 on the molten metal becomes as shown in FIG.
As shown in (c), since the molten metal surface in the mold fluctuates and the high-frequency coil 3 cannot approach the molten metal surface, the magnetic field applied to the solidified shell is weakened. was there.

[0009] The present invention has been made in view of such circumstances, and by concentrating the electromagnetic force more strongly on the solidified shell of the molten metal, the lubricity of the mold powder is improved, and the oscillation mark is formed. High circumference for continuous casting that can be made shallow and produce slabs with few surface defects
An object of the present invention is to provide a wave coil and a continuous casting method of molten metal using the coil .

[0010]

The high-frequency coil for continuous casting of the present invention is a high-frequency coil for continuous casting which applies a high-frequency magnetic field to the surface of a molten metal inside a mold, and has a cross-sectional shape of an upper portion. Write than the width of the side of the width of the lower side have a narrow cross-section, prior to
It is arranged vertically above the surface of the molten metal, and
The method is characterized in that a high-frequency magnetic field is applied to the surface of the molten metal .

Further, the continuous casting method of the present invention is a continuous casting method in which a high-frequency magnetic field is applied to the surface of a molten metal inside a mold. Using a coil, a high-frequency current is strongly applied during the negative strip period of the mold vibration and weakly applied during the positive strip period, or strongly applied only during the negative strip period.

[0012]

The present inventors have studied the shape of the high-frequency coil in order to concentrate the high-frequency magnetic field on the solidified shell of the molten metal.

As shown in FIG. 5, a mold 1 and a high-frequency coil 3
The probe 11 for measuring the magnetic field was arranged between the two. The tip of the magnetic field measurement probe 11 is separated from the mold 1 by 5 mm,
This is a position indicated by A in the figure at a position near both the molten metal surface 12 and the solidified shell 9. The location of the high-frequency coil 3 on the molten metal was such that the distance from the molten metal surface 12 to the lower end of the high-frequency coil 3 was 50 mm. This distance was determined from the thickness of the mold powder and the amplitude length of the mold. The high-frequency coil is generally rectangular, but the test was performed by changing the shape of the high-frequency coil in various ways. FIG. 6 shows the magnetic field strength at the location A and the cross-sectional shape index of the high-frequency coil (X / X) where X is the width of the upper end surface of the high-frequency coil and Y is the width of the lower end surface of the high-frequency coil.
It is a graph which shows the relationship with Y). As is clear from this figure, the magnetic field strength at the location A increases as the sectional shape index of the high-frequency coil is larger than 1, and the magnetic field strength decreases as the sectional shape index is smaller than 1.

FIGS. 7A and 7B show lines of magnetic force of the high-frequency coil. FIG. 7A shows the high-frequency coil of the present invention, and FIG. 7B shows the conventional high-frequency coil. The arrow in the figure is the line of magnetic force 13, and ● indicates the strongest point B of the magnetic field. As is clear from this figure, in the coil of the present invention, the strongest point B of the magnetic field is close to the solidified shell side.

That is, even if the coil is separated by a distance in consideration of the fluctuation of the molten metal surface, the coil of the present invention can concentrate the electromagnetic force on the solidified shell because the magnetic field is strong.

When a high-frequency magnetic field is applied to the molten metal, Joule heat is generated in the solidified shell by an induced current. The Joule heat heats the molten metal surface where the oscillation mark is formed, and this heating causes a solidification delay,
The depth of the oscillation mark is reduced, and the surface quality of the cast piece can be improved. In addition, the interaction between the magnetic force and the induced current causes the solidified shell tip to bend inward, increasing the space between the mold and the solidified shell where the mold powder flows, increasing the consumption of the mold powder, and increasing the lubrication of the mold powder. And the tensile stress on the solidified shell is reduced.

Since the oscillation mark is generated in the negative strip period, the application timing of the high-frequency coil is strong during the negative strip period and weak during the positive strip period, or is applied only during the negative strip period, in accordance with the oscillation period of the mold. By doing so, the effect can be further expected.

[0018]

An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view of a continuous casting mold used in an embodiment of the present invention, in which (a) is a plan view, (b) is a side view of XX of (a), and (c) is ( It is sectional drawing of YY of a). In the figure, the same parts as those in FIG.

The dimensions of the mold 1 are such that the short side width is 220 mm and the long side width is 950 mm. The cross-sectional shape of the high frequency coil of the present invention is 10 mm on the upper side, 5 mm on the lower side, and 10 mm in height.
In the conventional high-frequency coil, the cross-sectional shape is 10 mm on the upper side,
The lower side is 10 mm and the height is 10 mm. The distance between the high-frequency coil 3 and the mold 1 is 10 m in the present invention and the conventional example.
m.

In the present invention, the high-frequency coil 3 has one turn and is connected to a power supply (not shown), so that the application timing can be changed in accordance with the vibration cycle of the mold.

The number of turns of the high-frequency coil is theoretically higher when the number of turns is larger, and the magnetic flux density is higher at the same high-frequency coil current. However, as the number of turns increases, the impedance of the high-frequency coil increases. High frequency coil current)
However, there is a disadvantage that it is necessary to increase In an actual machine, the number of turns may be determined by comprehensively judging the effect obtained by increasing the number of turns and the disadvantage of increasing the voltage.

The high frequency oscillator of the power supply has a frequency of 10 KHz and an output of 300 KW, and the maximum high frequency coil current value is 8000
A.

The steel type used was low-carbon steel having a carbon concentration of 0.04%, and the degree of superheat of molten steel in the tundish was adjusted to 20 ° C. to 30 ° C. in each of the examples. As the used mold powder, a low-viscosity, low-melting-point mold powder advantageous for lubrication as shown in Table 1 was used.

The consumption of mold powder is obtained by dividing the consumption of mold powder used during casting by the surface area of the slab. For the depth of the oscillation mark, a cast slab was cut out, and the depth of the oscillation mark was measured from a cross section in the casting direction.

[0026]

[Table 1]

FIG. 2 is a graph showing a vibration velocity waveform of the mold used in the embodiment of the present invention and a pattern of application of a high-frequency magnetic field. FIG. 2A shows a mold vibration velocity of a sine waveform.
(B) is the mold vibration velocity of the non-sine waveform. A period in which the mold descending speed is higher than the casting speed in one cycle is referred to as a negative strip period (hereinafter, referred to as an NS period), and a period in which the mold descending speed is lower than the casting speed is referred to as a positive strip period (hereinafter, referred to as a PS period).

Incidentally, the current application pattern of the high-frequency coil in synchronism with the vibration of the mold is as follows:
The period was applied intensified.: The high-frequency coil current was not applied in the PS period but applied only in the NS period. In addition, the PS period was strongly reduced in the NS period, the PS period was applied only, or the entire period. Is applied continuously.

FIG. 2C shows the current application pattern described above. FIG. 3 shows a casting speed of 2.0 m.
/ Min, mold frequency 142 cpm, mold amplitude width ± 4 mm,
The mold vibration waveform is such that under a certain condition of a sine wave, the current application pattern of the high-frequency coil is applied only in the NS period shown in FIG.
And (c) shows the relationship between the current value of the high-frequency coil and the depth of the oscillation mark of the slab when only the PS phase is applied. Although the oscillation mark depth decreases when the current value of the high-frequency coil increases in each application condition, N
When only the S period was applied, the current value of the high-frequency coil was 500
After 0 A, the depth of the oscillation mark is significantly reduced, but in the case of continuous application and the case where only the PS period is applied, the reduction of the oscillation mark depth is small.

The reason for this is that the electromagnetic force of the high-frequency coil inside the mold acts vertically downward on the solidification shell, so that the electromagnetic force acts vertically downward on the solidification shell also in the PS phase during solidification. Because the shell remains bent, the oscillation mark depth does not become very shallow, although there is a Joule heat effect. Also, since the oscillation mark is generated in the NS phase, the oscillation mark generated only in the PS phase generates an oscillation mark. It is considered that the effect of the electromagnetic force at the time is small.

The same effect as that of the application pattern shown in FIG. 2 was obtained with the application pattern shown in FIG.

Table 2 summarizes the results of the application of the high-frequency coil current under the condition of a constant value of 6000 A in the application pattern shown in FIG. 2, and the distortion rate of all non-sine waveforms is 40%. It was adopted.

Table 2 shows that the conventional high-frequency coil having a rectangular cross-sectional shape under the same casting conditions, and the high-frequency coil of the embodiment in which the cross-sectional shape on the lower side is smaller than the width on the upper side (X / Y = 2) and mold powder consumption,
The relationship between the oscillation mark depths is shown in comparison. As is clear from this table, in the present invention, compared with the conventional example, the consumption of the mold powder is increased, the depth of the oscillation mark is reduced, and a cast piece having good surface properties can be obtained.

[0034]

[Table 2]

FIG. 4 is a schematic view of a continuous casting mold according to another embodiment of the present invention. 1 is characterized in that a notch 14 is provided in the upper part of the mold 1. In this example,
Since the high-frequency coil is closer to the mold, a stronger electromagnetic force can be concentrated on the tip of the solidified shell than in the above-described embodiment.

[0036]

According to the present invention, the high-frequency coil of the present invention has a cross-sectional shape that is narrower on the lower side than on the upper side, and by controlling the application pattern of the high-frequency coil, it is possible to obtain a mold powder that is stable even during high-speed casting. Lubrication can be ensured, and a slab with extremely few surface defects can be obtained without operational trouble.

That is, a slab which can be maintained without rolling can be manufactured stably, and the yield can be improved and the manufacturing cost can be reduced.
The effect is very large.

[Brief description of the drawings]

FIG. 1 is a schematic view of a continuous casting mold used in an embodiment of the present invention, (a) is a plan view, (b) is a side view of XX of (a), (c) is (a) It is sectional drawing of YY.

FIG. 2 is a graph showing a vibration velocity waveform and a pattern of application of a high-frequency magnetic field of a mold according to an embodiment of the present invention.

FIG. 3 is a graph showing a relationship between a current value of a high-frequency coil and a depth of an oscillation mark of a slab according to an embodiment of the present invention.

FIG. 4 is a schematic view of a continuous casting mold according to another embodiment of the present invention, in which (a) is a plan view, (b) is a side view of XX of (a), and (c) is a view of (a). It is sectional drawing of YY.

FIG. 5 is a schematic diagram showing a method of measuring a magnetic field of a high-frequency coil.

FIG. 6 is a graph showing a relationship between a cross-sectional shape index of a high-frequency coil and a magnetic field strength.

FIG. 7 is a schematic view showing the behavior of the high-frequency coil magnetic field lines;
(A) is a coil according to the present invention, and (b) is a conventional coil.

FIG. 8 is a schematic view of a continuous casting mold used in a conventional example.
(A) is a plan view, (b) is a side view of XX of (a),
(C) is a sectional view taken along line YY of (a).

[Explanation of symbols]

 Reference Signs List 1 mold 2 immersion nozzle 3 high-frequency coil 4 high-frequency coil support jig 5 molten steel 6 mold powder 7 tundish 8 cooling water 9 solidified shell 10 short-side mold 11 probe for magnetic field measurement 12 molten metal surface 13 lines of magnetic force 13

Continuation of the front page (56) References JP-A-3-90255 (JP, A) JP-A-5-115595 (JP, A) JP-A-7-290197 (JP, A) JP-A-5-146852 (JP) , A) Full-fledged 1986-67960 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/07 B22D 11/16 105 B22D 11/04 311

Claims (2)

(57) [Claims] 1. A high-frequency coil for continuous casting to apply a high-frequency magnetic field to the molten metal melt surface of the mold inner, have a narrow cross-sectional shape toward the lower side in the width than the width of the sectional shape <br/> upper side ,
It is arranged vertically above the surface of the molten metal, and above the surface of the molten metal.
A high-frequency coil for continuous casting characterized by applying a high-frequency magnetic field to the surface of molten metal . 2. A continuous casting method in which a high-frequency magnetic field is applied to a surface of a molten metal inside a mold, wherein a high-frequency coil having a cross section whose width on a lower side is smaller than that on an upper side is used.
A continuous casting method characterized in that a high-frequency current is strongly applied during the negative strip period of the mold vibration and weakly applied during the positive strip period, or strongly applied only during the negative strip period.