JPH08187563A - Continuous casting method applying electromagnetic force - Google Patents

Abstract

PURPOSE: To improve the lubricity between a mold and a cast slab and to cast a cast slab having little surface fault by impressing high frequency induction electromagnetic force to near a meniscus part of molten metal from the outer part of the mold and also, from the inner part of the mold. CONSTITUTION: When Lorentz's force acts in the direction of getting away from a coil by the high frequency induction magnetic field impressed from the outer part of the mold, at the time of negative strip, the molten steel 7 is allowed to overflow from the tip part of shell 3 to form newly the shell. At this time, since the shell is formed at the position distant from the mold 1 by the electromagnetic force directed inward, the interval for flowing of the powder 4 between the mold 1 and the solidified shell 3 is widened and tensile stress loaded to the shell by the movement of the mold 1 at the time of the negative strip is reduced. Further, Joule heat caused by induction current is given to the meniscus part with the high frequency induction magnetic force impressed from the inner part of the mold, and the meniscus part marked with oscillation mark is heated and the depth of the mark is made to be shallow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to continuous casting of molten metal such as steel, and more particularly to a continuous casting technique for improving the lubrication of a mold and a slab and producing a slab with few surface defects.

[0002]

2. Description of the Related Art In a continuous casting method for metal such as steel,
A major problem is to reduce the friction between the slab and the mold to prevent seizing of the slab or prevent breakout accidents. In order to reduce the friction between the slab and the mold, casting is usually performed while vibrating the mold vertically.

Usually, the waveform of the mold vibration is a sine waveform,
The oscillation condition including this waveform is appropriately changed according to the casting condition. As a special method, Japanese Patent Publication 4-79
By adopting the non-sinusoidal waveform disclosed in Japanese Patent Publication No. 744, the negative strip (hereinafter referred to as NS) period is secured at a certain level or more, while the mold rising speed is decreased during the positive strip (hereinafter referred to as PS) period. By that,
It improves the lubrication between the mold and the slab and reduces the friction between the mold and the slab.

[0004]

As a method of appropriately changing the above oscillation conditions according to casting conditions,
From the viewpoint of applying compressive force to the solidified shell produced in continuous casting, for example, as shown in Iron and Steel vol.60 (1974) No.7, P763, a negative strip time ratio (hereinafter referred to as NSR) of a certain value or more is set. It is necessary to secure it. From past experience, if the NSR was 30% or more, there were many operational problems because breakout occurred due to insufficient compressive force applied to the slab.

Furthermore, in order to secure NSR of 30% and cast speed of 3 m / min from the necessity of applying a constant compressive force to the shell, it is necessary to increase the frequency or amplitude with the increase of cast speed. was there. For example, (1) 2 at casting speed of 3 m / min and amplitude of ± 4 mm
A frequency of 00 cpm is required. (2) Casting speed 4 m / min, 2 when amplitude ± 4 mm
A frequency of 70 cpm is required. (3) Casting speed 5 m / min, 3 when amplitude ± 4 mm
A frequency of 38 cpm is required. (4) Casting speed 6 m / min, 4 when the amplitude is ± 4 mm
A frequency of 08 cpm is required.

However, the frequency cannot be increased so much in view of the mechanical rigidity of the continuous casting machine. Therefore, the frequency that satisfies the above cannot be ensured, and therefore it is difficult to secure stable operation at a casting speed of 3 m / min or more except for a small slab of 100 mm square or less. Further, as the speed increases, the lubrication between the mold and the slab becomes insufficient. If lubrication is insufficient, frictional force increases and breakout, which is a trouble in operation, occurs.

In the NS period and the PS period, as shown in FIG. 6 (a), the period when the mold descending speed is faster than the casting speed in one cycle of mold vibration is called the NS period, and the other periods are P period.
It is called the S period and the NS time ratio (hereinafter referred to as NSR)
Is expressed by the following formula.

When NSR = {t _N / (t _P + t _N )} × 100 and the mold vibration has a sinusoidal waveform, it can be expressed as follows. NSR = {1- [cos ^-1 (-Vc / 2πAf) / π]} × 100 where t _N ; time of negative strip in one cycle t _P ; time of positive strip in one cycle Vc; casting speed A; Amplitude of mold vibration f; Frequency of mold vibration

Further, in the continuous casting method, casting is performed by adding mold powder onto the molten metal. In addition to the lubrication of the mold and the slab, the powder has various functions such as a heat retaining function for the molten metal and capturing of inclusions in the molten metal after the floating. In order to improve the lubrication of the mold and the slab, the powder having a low viscosity was selected and the crystallization temperature was lowered.

However, there is a limit to the improvement of lubrication only by changing the physical properties of the powder and the oscillation conditions, and a casting method utilizing electromagnetic force as described below has been proposed. That is, as disclosed in JP-A-52-32824, a magnetic field is applied from the outside of the mold to push the tip of the shell, and an electromagnetic force is applied so that the slab separates from the mold, whereby the mold and the slab are separated. This is a method to widen the gap and make it easier for the powder to flow. However, with such continuous application of a magnetic field, sufficiently stable slab properties could not be obtained.

[0011]

In view of such circumstances, the present inventors have considered that in continuous casting using electromagnetic force, the coil is provided not only on the outside of the mold but also on the inside, that is, near the molten metal surface. A method of arranging coils and applying a high frequency magnetic field was found.

At the same time, in synchronization with or in relation to the vibration cycle of the mold, the electromagnetic force of each of the outer coil (hereinafter referred to as the outer coil) and the inner coil (hereinafter referred to as the inner coil) (hereinafter referred to as the outer electromagnetic force and the inner electromagnetic force) will be described. ) Is applied in the negative strip period so that the internal electromagnetic force> the external electromagnetic force, and in the positive strip period, the external magnetic force> the internal electromagnetic force so that the magnetic field is applied.

By such a method, high-frequency electromagnetic force can be concentrated on the surface of the molten metal, so that Lorentz force and Joule heat can be effectively generated at the oscillation mark or the tip of the nail, and the casting The surface quality of the piece can be improved. Further, the above means is particularly effective when the NSR is 20% or less.

[0014]

The electromagnetic coil of the present invention is arranged both outside and inside the mold, and a high frequency electromagnetic force is applied near the meniscus in the mold. As shown in FIG. 1, when a high-frequency magnetic field is applied around the molten metal, an induced current is generated in the molten metal, and the interaction between the induced current and the applied magnetic field causes a Lorentz force, that is, an electromagnetic force in a direction repulsing the coil. Occurs.

At the same time, Joule heat due to the induced current is also generated. The present invention utilizes this Lorentz force and Joule heat. High frequency magnetic field (1000 cycles /
This is because the stirring force is sufficiently smaller than that of a low frequency magnetic field (less than 1000 cycles / second), and only the effect of magnetic pressure can be expected.

The reason why the low frequency magnetic field is not used in the present invention is that in the case of the low frequency magnetic field, not only the pressure due to the electromagnetic force but also the instability of the molten metal surface for generating a large stirring force are promoted. On the other hand, in the case of a high frequency magnetic field, the stirring force is sufficiently small and the effect of pressure and induction Joule heat due to electromagnetic force can be expected.

FIG. 2 (a) is a vertical sectional view showing the arrangement of the mold and the electromagnetic coil, and FIG. 2 (b) is a plan view. The electromagnetic coil on the outside of the mold (referred to as the outer coil) mainly bends inside the solidification shell due to the Lorentz force, and the electromagnetic coil on the inside of the mold (referred to as the inner coil) prevents the Lorentz force from acting on the solidification shell. This is the position, and has the effect of mainly applying Joule heat due to the induced current.

That is, when the Lorentz force acts in the direction away from the coil by the high-frequency magnetic field applied from the outside of the mold, molten steel overflows from the tip of the shell to form a new shell during the negative strip. , At this time, due to the inward electromagnetic force, the shell is formed at a position away from the mold, so that the interval of powder inflow between the mold and the solidification shell is widened, and it is added to the shell by the movement of the mold during the negative strip. Tensile stress is reduced.

In addition, Joule heat due to an induced current is applied to the meniscus portion by a high frequency electromagnetic force applied from the inside of the mold, the meniscus portion where an oscillation mark is formed is heated, and a solidification delay occurs, causing oscillation. The so-called nail depth of the mark is made shallow. As described above, the external electromagnetic force and the internal electromagnetic force have different effects, and each may be applied alone, or the two can be used together to improve the quality of the cast slab.

Further, it is more desirable to control the application timing and intensity of the current to the outer coil and the inner coil in the following manner in synchronization with or in association with the vibration cycle of the mold.
During the negative strip period, control is performed so that (internal electromagnetic force)> (external electromagnetic force). During the positive strip period, control is performed so that (internal electromagnetic force) <(external electromagnetic force). By applying such a high frequency magnetic field, the electromagnetic force is applied more effectively, and the above-mentioned effect is further enhanced.

Conventionally, in order to prevent breakout, the negative strip ratio must be at least 20% or more, preferably 30% or more, but when the above means is adopted, the negative strip ratio is 20%.
It can be: That is, the casting speed of continuous casting can be increased more than ever before.

[0022]

EXAMPLE A longitudinal sectional view of an example of the present invention is shown in FIG.
A plan view is shown in FIG. Molten steel is poured into the mold 1 from the ladle via the tundish 8 and the immersion nozzle 2.
There are the following cases in the form of the coil and the mold for applying a magnetic field from the outside into the mold.

For example, when a coil is wound on the outside of the mold as shown in FIG. 3 and a slit perpendicular to the winding direction of the coil is cut in the middle of the mold, the coil is wound on the outside of the mold as shown in FIG. In some cases, a slit is cut in the mold in a direction perpendicular to the coil winding direction up to the upper part of the mold, and as shown in FIG. 5, the coil is incorporated inside the mold.

The slit portion shown in FIGS. 3 and 4 is a normal space and is extremely narrow so that molten steel does not enter the slit. However, for example, a refractory should be inserted into this slit portion. Is desirable. In this embodiment, FIG.
Using a mold (inside dimension: short side 180 mm long side 400 mm) with the slit shown in Fig. 1 cut to the top of the mold, the outer coil has 4 turns, the inner coil has 1 turn, and the outer coil and the inner coil each have different power sources. Are connected,
The application timing of each current can be changed according to the vibration timing of the mold.

The coil is preferably made of, for example, water-cooled copper or copper alloy. Theoretically, the larger the number of turns, the higher the magnetic flux density with the same coil current. However, the larger the number of turns, the higher the impedance of the coil. Therefore, the secondary voltage of the power supply (coil current)
There is a disadvantage that it is necessary to raise the value. Therefore, in the actual machine, the number of turns may be determined from a comprehensive viewpoint of 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, 3
The maximum coil current value for both the outer coil and the inner coil was 8000A. FIG. 6 shows the manner of applying the high frequency current. With this method, the timing of mold vibration and current application to the outer coil and the inner coil can be variously changed. In FIG. 6, (a) is a mold vibration wave type, and (b)-
(E) shows a mode of current application.

(B) is a case where a current is continuously applied to the outer coil regardless of the mold vibration, (c) is a case where only the PS period is applied only to the outer coil, and (d) is an NS only to the outer coil. Applied only during the first period, (e) applied current continuously to the inner coil regardless of mold vibration, (f)
Indicates that only the NS period was applied to only the inner coil, and (g) shows that only the PS period was applied to only the inner coil.

Table 1 shows examples of the various combinations carried out. In FIG. 6, it is possible to pass a very small amount of current when the power is cut off, and the same effect as when the power is not supplied is obtained.

The steel types cast in the examples have a carbon concentration of 0.1.
% Carbon steel, the molten steel overheating temperature in the tundish was adjusted to be 25 ° C. in each example. The powder used is a low-viscosity, low-melting point powder, which is advantageous for lubrication as shown in Table 2. At the end of casting, the powder is stopped inside the mold, and after cooling, it is taken out and put on the surface of the cast. It was calculated from the thickness of the adhered powder.

[0030]

[Table 1]

[0031]

[Table 2]

FIG. 7 shows the case in Table 1 and the application conditions shown in case, and the test results with the mold vibration shown in Table 3. Current is applied only to the outer coil, and the relationship between the coil current and the powder consumption is shown. It is shown. When a current was applied to the outer coil over the entire area of the mold vibration, it was confirmed that the powder consumption was increased by increasing the current.

[0033]

[Table 3]

On the other hand, even when a current was applied to the outer coil only in the PS period, almost the same effect was confirmed, and the result was not much different from that when continuously energized. This indicates that during the mold vibration, the powder preferentially flows between the mold and the solidified shell in the PS period. That is, it is considered that when the outer coil is energized, the Lorentz force acts on the solidified shell in the direction away from the coil, and the thickness of the inflowing powder increases.

PS to which a high-frequency magnetic field is continuously applied is PS.
The reason is not much different from that of the solid phase only, when the outer coil is energized in the NS phase, the solidification shell already has strength to some extent, and even when Lorentz force is applied to this, the solidification shell does not move and the powder It is considered that it contributes little to the inflow. Therefore, it is not necessary to energize the outer coil during the NS period.

FIG. 8 shows the case in Table 1 and the application conditions shown in case, and the test results under the mold vibration conditions shown in Table 3, showing that the current is applied only to the coil in the mold and the depth of the oscillation mark This shows the effect on the depth of the recess from the surface). When the coil current is increased, the depth of the oscillation mark decreases, but the current value exceeds 5000A in the case of applying only in the NS period, and the effect of reducing the oscillation mark becomes remarkable, but the value of continuous application gradually decreases. Just do it, the effect is small.

The reason for this is that since the electromagnetic force of the inner coil acts vertically downward on the solidified shell, the electromagnetic force applied to the inner coil over the entire area causes the electromagnetic force to act vertically downward on the solidified shell during the PS period. Since the meniscus remains bent, there is a Joule heat effect, but it is considered that the oscillation mark depth does not become too shallow.

Table 4 is an application pattern shown in case of Table 1, that is, the outer coil is applied in the PS period and the inner coil is N-shaped.
It is a method of applying in the S period, the coil current value is a summary of the results carried out under the condition of a constant value of 5000A for both the outer and inner coils, and Table 4 shows the casting conditions when actually casting, It shows the results of a survey of powder consumption and oscillation mark depth.

The mold used was the mold shown in FIG. 4, and the distortion rate of the non-sinusoidal waveform was all 40%.
Adopted more than 20% which is generally required. Here, the distortion rate of the non-sine waveform α = (t ₁ −t ₀ ) × 100 / t
It is ₀ . Here, t ₁ is the time until the displacement in the sinusoidal vibration changes from zero to the maximum value, t ₀ : the time until the displacement in the non-sinusoidal vibration changes from 0 to the maximum value, and t ₁ > t ₀
Is.

[0040]

[Table 4]

Table 4 shows, as a comparison, the powder consumption in the absence of the high-frequency magnetic field under the same casting conditions, and the examination result of the oscillation mark depth. By using electromagnetic force, powder consumption increased and oscillation mark depth decreased at various casting speeds, and stable casting and slabs with good surface quality were obtained.

Next, using only a sine wave, the NSR is 20%.
Examples of the present invention will be shown below. The powder used is the low-viscosity, low-melting-point powder shown in Table 2 which is advantageous for lubrication. As the pattern of applying the high frequency electromagnetic force, the pattern of FIG. 6C is applied to the outer coil, and the pattern of FIG. 6F is applied to the inner coil. That is, it is a pattern in which a current is applied to the outer coil in the PS period and a current is applied to the inner coil in the NS period.

Table 5 shows the results of casting conditions, powder consumption, and oscillation mark depth during actual casting. The mold used in this case is the one shown in FIG. 4 in which a slit is cut to the upper part of the mold.
As is clear from Table 5, in the casting method using the method of the present invention, stable casting can be achieved even at 5 m / min which cannot be achieved by conventional slab casting.

[0044]

[Table 5]

[0045]

As described above, in continuous casting using high frequency electromagnetic force, by placing an appropriate coil and applying an electromagnetic force by applying a magnetic field, stable powder lubrication can be secured even during high speed casting, It is possible to obtain a slab with extremely few surface defects without any operational trouble. As a result, a slab capable of maintenance-free rolling can be stably manufactured, and the effects such as an improvement in slab yield and a reduction in manufacturing cost are extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing Lorentz force applied to molten metal by a high frequency current.

FIG. 2 is a view showing a mold provided with a high frequency electromagnetic force coil according to the present invention.

FIG. 3 is a view showing an aspect of a mold according to the present invention.

FIG. 4 is a diagram showing an embodiment of a mold according to the present invention.

FIG. 5 is a view showing an aspect of a mold according to the present invention.

FIG. 6 is a diagram showing a mode of applying a current to an outer coil and an inner coil in the present invention.

FIG. 7 is a diagram showing a relationship between a current value of an outer coil and a powder consumption amount.

FIG. 8 is a diagram showing a relationship between a current value of an inner coil and an oscillation mark depth.

[Explanation of symbols]

 1 Mold 2 Immersion Nozzle 3 Solidifying Shell 4 Mold Powder 5 Outer Coil 6 Inner Coil 7 Molten Metal 8 Tundish 10 Molten Metal 11 Electromagnetic Coil 12 High Frequency Current 13 Induction Current 14 Lorentz Force

Claims (3)

[Claims] 1. A method of continuously casting a molten metal while applying a negative strip and a positive strip to a slab using a mold vibration, wherein a high frequency electromagnetic force (hereinafter A continuous casting method applying an electromagnetic force characterized by applying an external electromagnetic force) and / or a high frequency electromagnetic force (hereinafter referred to as an internal electromagnetic force) inside the mold. 2. The mold negative strip timing (hereinafter referred to as NS period) is applied with the internal electromagnetic force larger than the internal electromagnetic force, while the mold vibration positive strip period (hereinafter referred to as PS period). Said), the external electromagnetic force is made larger than the internal electromagnetic force. The continuous casting method to which the electromagnetic force is applied according to claim 1. 3. The continuous casting method applying electromagnetic force according to claim 1, wherein the time ratio of the negative strip of the mold is less than 20%.