JP3191594B2 - Continuous casting method using electromagnetic force - Google Patents

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 of a metal such as steel,
A major problem is to reduce the friction between the slab and the mold to prevent seizure of the slab or prevent a breakout accident. In order to reduce the friction between the slab and the mold, casting is usually performed while the mold is vibrated up and down.

[0003] 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, 4-79
By adopting the non-sinusoidal waveform disclosed in Japanese Patent No. 744, the rising speed of the mold is reduced during the positive strip (hereinafter, referred to as PS) while the negative strip (hereinafter, referred to as NS) is maintained at a certain level or more. By that
It improves lubrication between the mold and the slab and reduces friction between the mold and the slab.

[0004]

As a method for appropriately changing the oscillation conditions according to the casting conditions, there are the following methods.
From the viewpoint of applying a compressive force to the solidified shell formed in continuous casting, for example, as shown in Iron and Steel vol.60 (1974) No. Need to secure. According to past experience, if the NSR is not more than 30%, breakout occurs due to insufficient compression force applied to the slab, and there are many operational problems.

[0005] Further, in order to secure the NSR of 30% and increase the casting speed to 3 m / min from the necessity of applying a constant compressive force to the shell, it is necessary to increase the frequency or amplitude as the casting speed increases. was there. For example, (1) When the casting speed is 3 m / min and the amplitude is ± 4 mm, 2
A frequency of 00 cpm is required. (2) Casting speed 4m / min, amplitude ± 4mm, 2
A frequency of 70 cpm is required. (3) Casting speed: 5 m / min, amplitude: ± 4 mm, 3
A frequency of 38 cpm is required. (4) For casting speed 6m / min, amplitude ± 4mm, 4
A frequency of 08 cpm is required.

However, the frequency cannot be increased so much in view of the mechanical rigidity of the continuous caster. Therefore, it was not possible to secure a frequency that satisfies the above, and it was difficult to secure a stable operation at a casting speed of 3 m / min or more, except for small cast pieces having a size of, for example, 100 mm square or less. In addition, lubrication between the mold and the slab becomes insufficient with the increase in speed. Insufficient lubrication increases the frictional force and causes breakouts, which are operational problems.

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

When NSR = {t _N / (t _P + t _N )} × 100 and the mold vibration has a sine 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

In the continuous casting method, casting is performed by adding a mold powder onto a molten metal. In addition to lubrication of the mold and the slab, the powder has various functions such as a function of keeping the molten metal warm and trapping of inclusions in the molten metal after floating. In order to improve the lubrication of the mold and the slab, powders having low viscosity were selected or the crystallization temperature was lowered.

However, there is a limit in improving lubrication only by changing the physical properties of the powder and the oscillation conditions, and the following casting method using an electromagnetic force has been proposed. That is, as shown in Japanese Patent Application Laid-Open No. 52-32824, a magnetic field is applied from the outside of the mold to press the tip of the shell, and an electromagnetic force is applied so that the slab separates from the mold. This is a method to widen the gap and make it easier for the powder to flow. However, continuous application of such a magnetic field did not provide sufficiently stable slab properties.

[0011]

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

Simultaneously or synchronously with or related to the vibration cycle of the mold, the respective electromagnetic forces (hereinafter referred to as an external electromagnetic force and an internal electromagnetic force) of an outer coil (hereinafter referred to as an outer coil) and an inner coil (hereinafter referred to as an inner coil) are respectively referred to. This is a method of applying a magnetic field so that the application timing of ()) is such that the internal electromagnetic force> the external electromagnetic force in the negative strip period and the external electromagnetic force> the internal electromagnetic force in the positive strip period.

According to this method, a high-frequency electromagnetic force can be applied to the surface of the molten metal in a concentrated manner, so that the Lorentz force and Joule heat can be effectively generated at the tip of the oscillation mark or the claw. 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 on both the outside and the inside of 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 the Lorentz force, that is, the electromagnetic force to repel the coil. Occurs.

At the same time, Joule heat is also generated by the induced current. The present invention utilizes this Lorentz force and Joule heat. High frequency magnetic field (1000 cycles /
(Seconds or more) is because the stirring force is sufficiently small compared to the low-frequency magnetic field (less than 1000 cycles / second), and only the effect of the 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 is promoted. On the other hand, in the case of a high-frequency magnetic field, the stirring force is sufficiently small, and the effects of pressure and induced Joule heat by the electromagnetic force can be expected.

FIG. 2A is a longitudinal sectional view showing the arrangement of the mold and the electromagnetic coil, and FIG. 2B is a plan view. The electromagnetic coil outside the mold (referred to as outer coil) has a bending action to the inside of the solidified shell mainly due to Lorentz force, and the electromagnetic coil inside the mold (referred to as inner coil) is hard to apply the Lorentz force to the solidified shell. Position, which mainly acts to impart Joule heat by induced current.

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

In addition, Joule heat is applied to the meniscus by an induced current due to the high-frequency electromagnetic force applied from the inside of the mold, and the meniscus where an oscillation mark is formed is heated. The so-called nail depth of the mark is reduced. As described above, the external electromagnetic force and the internal electromagnetic force have different effects, and each may be applied alone, or the quality of the slab can be improved by using both together.

Further, it is more desirable to control the timing and intensity of current application to the outer coil and the inner coil in the following manner in synchronization with or related to the vibration cycle of the mold.
In the negative strip period, control is performed so that (internal electromagnetic force)> (external electromagnetic force). In 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 more effectively applied, 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. However, when the above means is adopted, the negative strip ratio is reduced to 20%.
It can be: That is, the casting speed of the continuous casting can be increased as compared with the conventional case.

[0022]

FIG. 2A is a longitudinal sectional view of an embodiment of the present invention.
FIG. 2B shows a plan view. Molten steel is injected into the mold 1 from the ladle via the tundish 8 and the immersion nozzle 2.
Embodiments of the coil and the mold for applying a magnetic field from outside to the mold include the following cases.

For example, when a coil is wound around the outside of the mold as shown in FIG. 3 and a slit in the direction perpendicular to the winding direction of the coil is cut in the middle of the mold, the coil is wound around 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 winding direction of the coil up to the top of the mold, or the coil is incorporated inside the mold as shown in FIG.

The slits shown in FIGS. 3 and 4 are usually spaces and are extremely narrow so that molten steel does not enter into the slits. For example, a refractory must be inserted into the slits. Is desirable. In this embodiment, FIG.
Using a mold with a structure shown in (1), in which the slit is cut up to the top of the mold (inner dimension: short side 180 mm, long side 400 mm), the outer coil has 4 turns, the inner coil has 1 turn, and the outer coil and the inner coil have different power sources. 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 number of turns of the coil is such that the larger the number of turns, the higher the magnetic flux density at the same coil current, but the larger the number of turns, the greater the impedance of the coil, so the secondary voltage of the power supply (coil current)
However, there is a disadvantage that it is necessary to increase the height. Thus, in the actual machine, the number of turns may be determined from the 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
00KW, and the maximum coil current value of both the outer coil and the inner coil was 8000A. FIG. 6 shows the manner of applying the high-frequency current. In this method, it is possible to variously change the timing of the vibration of the mold and the application of the current to the outer coil and the inner coil. In FIG. 6, (a) shows a mold vibration wave type, and (b) to
(E) shows the mode of current application.

(B) is a case where a current is continuously applied only to the outer coil irrespective of the mold vibration, (c) is a case where only the PS period is applied to the outer coil only, and (d) is NS where only the outer coil is applied to the NS. (E) only the inner coil is continuously applied irrespective of mold vibration, (f)
Shows the case where only the NS phase was applied to only the inner coil, and (g) shows the case where only the PS phase was applied to only the inner coil.

Table 1 shows working examples of these various combinations. In FIG. 6, a very small amount of current can be supplied during the period when the power is turned off, and the same effect as in the case where no power is supplied can be obtained.

The steel type cast in the examples had a carbon concentration of 0.1.
%, The superheating temperature of the molten steel in the tundish was adjusted to 25 ° C. in each example. As shown in Table 2, the powder used is a low-viscosity, low-melting-point powder that is advantageous for lubrication. At the end of casting, the slab is suspended in the mold at the end of casting, taken out after cooling, and removed from the surface of the slab. It was obtained by calculation from the thickness of the attached powder.

[0030]

[Table 1]

[0031]

[Table 2]

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

[0033]

[Table 3]

On the other hand, 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 the current was continuously supplied. This indicates that the powder is preferentially flowing between the mold and the solidified shell during the PS phase during mold vibration. That is, it is considered that the energization of the outer coil causes a Lorentz force to act on the solidified shell in a direction away from the coil, thereby increasing the thickness of the flowing powder.

The one to which a high-frequency magnetic field is continuously applied is PS
The reason is not much different from that of the period only when the outer coil is energized during the NS period, the solidified shell already has some strength, and even if Lorentz force is applied to it, the solidified shell does not move and the powder does not move. This is probably because it hardly contributes to the inflow. Therefore, there is little need to energize the outer coil during the NS period.

FIG. 8 shows the test results under the application conditions shown in Table 1 and under the mold vibration conditions shown in Table 3. The current was applied only to the coil in the mold, and the depth of the oscillation mark (cast slab) was obtained. (Depth of the recess from the surface). When the coil current is increased, the depth of the oscillation mark decreases. However, when the voltage is applied only during the NS period, the current value exceeds 5000 A, and the effect of reducing the depth of the oscillation mark becomes remarkable. Only effect, little effect.

The reason is that the electromagnetic force of the inner coil acts vertically downward on the solidified shell, so that the electromagnetic force applied vertically to the inner coil acts on the solidified shell vertically downward also in the PS phase. Since the meniscus is still bent, it is considered that the oscillation mark depth does not become too shallow, though there is a Joule heat effect.

Table 4 shows the application patterns shown in the case of Table 1, that is, the outer coil is applied in the PS period, and the inner coil is applied in the N period.
In the method applied in the S phase, the coil current value is a summary of the results obtained under the condition of a constant value of 5000 A for both the outer and inner coils, and Table 4 shows the casting conditions at the time of actual casting. It shows the results of surveys on powder consumption and oscillation mark depth.

The mold used was the mold shown in FIG. 4 and the distortion rate of all non-sinusoidal waveforms was 40%.
Employed over 20%, which is generally required. Here, the distortion rate α of the non-sine waveform = (t ₁ −t ₀ ) × 100 / t
It is ₀ . Here, t ₁ is the time required for the displacement in the sine wave vibration to go from zero to the maximum value, t _{0 is} the time required for the displacement in the non-sine wave vibration to go from zero to the maximum value, and t ₁ > t ₀
It is.

[0040]

[Table 4]

Table 4 shows comparison results of powder consumption and oscillation mark depth without a high-frequency magnetic field under the same casting conditions. At various casting speeds due to the use of electromagnetic force, the powder consumption increased and the oscillation mark depth decreased, resulting in stable casting and slabs with good surface properties.

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

Table 5 shows the results of the casting conditions, powder consumption, and oscillation mark depth when casting was actually performed. The mold used in this case is a mold in which a slit is cut up to the top of the mold shown in FIG.
As is evident from Table 5, in the casting method using the method of the present invention, stable casting could 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 arranging appropriate coils and applying electromagnetic force by applying a magnetic field, stable powder lubrication can be ensured even during high-speed casting. A slab with extremely few surface defects can be obtained without any trouble in operation. As a result, slabs that can be maintained without rolling can be stably manufactured, and the effects thereof are extremely large, such as improvement in slab yield and reduction in manufacturing cost.

[Brief description of the drawings]

FIG. 1 is a diagram showing Lorentz force applied to a 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 one embodiment of a mold according to the present invention.

FIG. 4 is a view showing one embodiment of a mold according to the present invention.

FIG. 5 is a view showing one embodiment 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]

 REFERENCE SIGNS LIST 1 mold 2 immersion nozzle 3 solidified 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 induced current 14 Lorentz force

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-111552 (JP, A) JP-A-3-90255 (JP, A) JP-A-59-104257 (JP, A) JP-A 64-64 83348 (JP, A) JP-A-8-168851 (JP, A) JP-A-7-96360 (JP, A) JP-A-8-257693 (JP, A) JP-A-62-107848 (JP, A) JP-A-58-53354 (JP, A) WO 96/5926 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/07 B22D 11/04 311 B22D 11/11 B22D 11/115 B22D 11/16 105

Claims (4)

(57) [Claims] In a method of continuously casting a molten metal while giving a negative strip and a positive strip to a slab using a mold vibration, a high-frequency electromagnetic force (hereinafter, referred to as “hereafter”) is applied to the vicinity of a meniscus portion of the molten metal from outside the mold. applying an external called electromagnetic force), further, a continuous casting method which applies an electromagnetic force and applying high-frequency electromagnetic force (hereinafter referred to internal electromagnetic force) inside the mold. 2. At the time of the negative strip of the mold (hereinafter, referred to as NS period), the inner electromagnetic force is applied larger than the outer electromagnetic force, and at the time of the positive strip of the mold vibration (hereinafter, PS period). 2. The continuous casting method according to claim 1, wherein the external electromagnetic force is made larger than the internal electromagnetic force. 3. A negative strike on a slab using mold vibration.
Molten metal while giving lip and positive strip
The continuous casting of the molten metal in the meniscus portion of the molten metal
High frequency electromagnetic force (hereinafter referred to as external
Magnetic force) and high-frequency electromagnetic force (hereinafter
And lower inner electromagnetic force), and at the time of the negative strip of the mold (hereinafter referred to as NS period), only the inner electromagnetic force is applied without applying the outer electromagnetic force. A continuous casting method characterized by applying only the external electromagnetic force without applying the internal electromagnetic force during the period of the positive strip of vibration (hereinafter referred to as PS period). 4. The continuous casting method using an electromagnetic force according to claim 1, wherein a time ratio of the negative strip of the mold is less than 20%.