JP3055413B2 - Method and apparatus for continuous casting of molten metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for greatly improving the casting speed in continuous casting of metal, particularly in continuous casting of steel, and consequently to an invention which significantly improves the performance of a continuous casting machine. .

[0002]

2. Description of the Related Art Various techniques have been developed for improving the casting speed in continuous casting of steel. However, when the casting speed is increased, the frequency of breakouts increases. Therefore, the development of technology for suppressing the frequency of breakouts has been a central issue. The technical content is a technique for smoothly performing the initial solidification in the mold, and has roughly the following two purposes.

[0003] The central task is to increase the mold powder inflow and reduce the friction between the solidified shell (hereinafter simply referred to as a shell) and the mold wall. .

[0004] Reducing the friction is to improve the lubrication between the shell and the mold wall and reduce the tensile stress applied to the shell during the vibration of the mold. For that purpose, it is important to increase the consumption of the mold powder. According to a conventional study (for example, Kitagawa et al., Japan Steel Pipe Technical Information Vol. 93 (1982), p. 18), the mold powder consumption Q (kg / m ² ) per unit casting surface area is obtained by the following equation (1). Can be

[0005] _{Q = k · t p · f} / Vc ---- (1) ^{_{where, t p = cos -1 (-Vc}} / 2πAf) / πf --- (2) k: constant t _p: positive Strip time f: Frequency (mold) A: Amplitude of mold vibration Vc: Casting speed

[0006] From equation (2), may be as large as possible a positive strip (hereinafter PS) Time t _p, To do this to reduce the mold amplitude A, it is necessary to reduce the frequency f.

On the other hand, in order to strengthen the shell and promote the formation of a sound shell, the time ratio of the negative strip (hereinafter referred to as NS) in the mold vibration (referred to as NSR) in the mold vibration is shown in FIG. Has turned out to be important. That is, if the shell tip is hot, the portion is fragile, so it is necessary to apply a compressive force to the shell during the NS period.

Here, NSR can be expressed by the following equation. NSR = 1- {1 / π · cos -1 (-Vc / 2 πAf)} = t N / (t p + t N) where, t _p: Time Positive strips during one cycle t _N: 1 cycle Negative strip time

Generally, a negative strip time ratio of 30% or more is required. If the time ratio is less than 30%, the fragile portion at the tip of the shell is broken in the middle of drawing, and constraining breakout tends to occur. However, as the casting speed increases as can be determined from FIG. 7, it becomes difficult to secure NSR.

That is, by increasing the amplitude A and the frequency f contrary to the PS time tp, the NSR can be prevented from being reduced even when the casting speed Vc is increased. Values have upper limits. Therefore, there is a problem that it is impossible to sufficiently secure the NSR.

On the other hand, the physical properties of the mold powder also affect the magnitude of Q. Generally, Q can be increased by decreasing the viscosity coefficient (η) of the powder, but the increase in Q is small and the effect of the casting speed Vc is much greater.

In general, when the value of Q is less than 0.3 kg / m ² , the frequency of breakout increases, so that it is necessary to improve Vc while securing a value higher than this.
However, because of the large influence of Vc, NS on an actual operation basis
It was difficult to increase Vc to 3.0 m / min or more while ensuring R of 30% or more and Q of 0.3 kg / m ² .

[0013]

As described in the section of the prior art, at the time of high speed casting, it is important to reduce the friction of the shell in the mold and to strengthen the compression of the shell tip. However, in this regard, as described above, there is a limit to mold vibration, and thus a limit to the direction of conventional technology development. Accordingly, an object of the present invention is to provide a new technique for controlling an initial solidified shell that does not rely only on mold vibration.

[0014]

Means for Solving the Problems The present inventors applied high-frequency electromagnetic force to bend the initial solidification shell generated near the meniscus in the mold inward from the mold wall, to increase the inflow of powder, and With the idea that casting at a higher speed can be performed at a higher speed than in the past by strengthening the compression at the tip, the following invention has been made.

(1) The first aspect of the present invention is a method for continuously casting molten metal, comprising the following steps. (A) Prepare a water-cooled continuous casting mold in which the upper part of the mold is comb-shaped and the lower part following this is integrally formed,
(B) Pouring molten metal into the mold while performing mold vibration, and continuously drawing out the formed slabs,
(C) A high-frequency electromagnetic force is applied to the molten metal in the vicinity of the meniscus level in the upper and lower stages from the outside of the mold to continuously cast the solidified shell formed in the vicinity of the meniscus while curving inward.

(2) In the invention of claim 2, the lower electromagnetic force of the two-stage high-frequency electromagnetic force is applied at the time of the positive strip of the mold vibration, and the upper electromagnetic force is applied at the time of the negative lip. A method for continuous casting of molten metal according to claim 1, which is operated.

(3) The invention according to claim 3 is the method for continuously casting molten metal according to claim 1 or 2, wherein a negative lip time ratio of mold vibration of the mold is 25% or less.

(4) The invention according to claim 4 is the method for continuously casting molten metal according to claim 1 or 2, wherein the molten metal is an alloy containing Fe.

(5) The invention according to claim 5 is a continuous casting mold provided with the following members. (A) a continuous casting mold made of copper or a copper alloy in which the upper part of the mold has a comb shape, the lower part is integrally formed, and the inside of each part can be water-cooled; (b) melting of the mold At the position corresponding to the level of the metal meniscus,
At least two high-frequency coils having the central axis of the mold as a central axis are provided in the vertical direction on the outer periphery of the mold.

[0020]

The present invention can be applied to continuous casting of all molten metals, but is particularly effective when applied to an alloy containing Fe.
This is because this alloy has a high melting point and usually cannot increase the casting speed. In order to solve the above-mentioned problem, as described in the claims, in the present invention, an electromagnetic force, particularly a high-frequency magnetic field, is applied to a vicinity of a meniscus (fluid surface) in a mold. As shown in FIG. 1, high-frequency magnetic field (above 1000 Hz / sec)
Is applied, an induced current is generated in the molten metal, and the interaction between the induced current and the applied magnetic field is used to generate Lorentz force, that is, an effect of generating a magnetic pressure in a direction to repel the coil.

The high-frequency magnetic field has a sufficiently small stirring force as compared with the low-frequency magnetic field, and only the effect of the magnetic pressure is expected. However, it is impossible to solidify while holding the molten steel only by the high-frequency magnetic field from the viewpoint of heat removal. Therefore, the initial solidification is controlled in combination with a conventional vibration mold.

That is, the above-mentioned increase in the amount of powder flowing in and the compression of the initial shell are reinforced by the application of two high-frequency magnetic fields. The reason why the low-frequency magnetic field is not used in the present invention is that the low-frequency magnetic field (1000 Hz /
In the case of (sec or less), not only the magnetic pressure but also a large stirring force is generated, and the fluctuation of the molten metal surface is promoted.

It is generally difficult to apply a high-frequency magnetic force from the outside of the mold to the inside of the mold because of the skin effect.
However, it becomes possible by using a cold crucible type template structure. That is, if a comb-shaped mold having a slit in the vertical direction is provided on the upper part of the mold, high-frequency magnetic force can be efficiently applied to the molten metal (FIG. 2B).

In this case, when a magnetic pressure acts in a direction away from the coil by a high-frequency magnetic field applied from the outside, the molten steel overflows from the tip of the shell in the case of a negative strip, and a new shell is formed. When the shell is formed at a position away from the mold due to the electromagnetic force acting inward, the gap between the powder and the solidified shell is widened, reducing the tensile stress applied to the shell due to mold movement during the negative strip period Is done.

Further, in the present invention, a high-frequency magnetic force by a high-frequency magnetic field is applied in accordance with the vibration behavior of the mold to further enhance the function of the negative strip and the function of the positive strip.

As shown in FIG. 2 (b), the apparatus of the present invention has a water-cooling coil installed outside a cold-crucible type, that is, a comb-shaped copper or copper alloy mold. Provided in two parts of the coil. Then, the meniscus is positioned near the upper end of the lower coil. A high frequency current is applied to the lower coil corresponding to the PS phase of the mold or continuously. By doing so, the amount of powder flowing into the PS phase can be increased.

It is more desirable to apply a high-frequency magnetic field only in the PS period, since oscillation of the oscillation mark will not be generated. Further, it is desirable to apply a high-frequency current to the upper coil at a time corresponding to the NS period. By doing so, the inward compression of the shell tip in the NS phase can be further reinforced.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. FIG. 2 shows a casting apparatus according to the present invention. FIG. 2A is an overall view, and FIG. 2B shows the structure of the mold 18 in detail.
100mm inner diameter, 160mm outer diameter, 500mmL length
Is a water-cooled copper or copper alloy mold, the upper part of which is comb-shaped, in that respect a cold crucible mold.

When a coil is provided in a direction perpendicular to the mold having such a shape and a high-frequency magnetic field is applied from outside, a high-frequency magnetic force can be efficiently introduced into the molten metal inside the mold.
The mold does not constitute the entire mold itself, but rather the comb mold is housed in a frame containing, for example, a steel water box. Further, although the embodiment is an example in which a cylindrical cast piece is cast, a rectangular mold may be used when casting a rectangular strip.

That is, the water-cooled coil is wound in the horizontal direction, and the slit is vertically about 250 m from the upper end of the mold.
m. A slit is not provided in a portion of the lower part of the mold sufficiently distant from the coil (a part below 250 mm from the upper end of the mold), and conduction may be provided in the circumferential direction of the mold. The molten metal does not leak to the outside from the slit. This is because a high high-frequency magnetic field exists in the comb-shaped slit portion. However, it is desirable to insert, for example, a refractory between the slits.

The reason for not providing a slit at the bottom of the mold is as follows.
This is for electrically connecting the water-cooled copper mold portion inside the comb mold near the coil. In addition, this portion is sufficiently away from the meniscus portion to which the magnetic field is applied, so that it is not necessary to apply the magnetic field. On the other hand, a water-cooled copper coil for generating a magnetic field is divided into an upper coil 44 and a lower coil 42. As an example, the upper coil had two turns, and the lower coil also had two turns.

The upper coil and the lower coil are connected to different power sources, respectively, and it is desirable to change the application timing of each current in accordance with the vibration timing of the mold. The number of turns of the coil is theoretically higher when the number of turns is larger, the magnetic flux density becomes higher at the same coil current. There is a point. Therefore,
In an actual machine, the number of turns is determined from the comprehensive viewpoint of the effect obtained by increasing the number of turns and the disadvantage of increasing the voltage.

FIG. 3 shows a mode of applying a high-frequency magnetic field. In this apparatus, it is possible to variously change the timing of mold vibration and current application to the upper coil and the lower coil. In FIG. 3, (a) is a mold vibration wave type, and (b) to
(E) shows the mode of current application. (B) is a case where current is continuously applied to only the lower coil irrespective of mold vibration, (c) is a case where current is applied only to the PS period of the lower coil,
(D) is a case where a current is continuously applied to only the upper coil irrespective of mold vibration, and (e) is a case where a current is applied to the upper coil only during the NS period. Table 1 summarizes these various combinations.

[0034]

[Table 1]

The applied high-frequency current has a frequency of 10 kHz,
The test was performed at a power of 300 kW and a coil current value of 8000 A for both the upper and lower coils. Other test conditions are shown in Table 2, but the main points will be described.

[0036]

[Table 2]

The mold speed was high speed casting of 4 m / min, the mold vibration was a sine wave, a stroke ± 4 mm, and a vibration frequency of 18
0Cpm, negative strip time ratio was set to _{(NSR) = (t N /} t p + t N) × 100 = 15%.

Here, the reason why the NSR was significantly reduced to less than 30% necessary for avoiding breakout in normal operation is to reduce the NSR and reduce the compressive force on the initial shell, which is disadvantageous. This is for making the effect of applying a high-frequency magnetic field at that time more remarkable. The cast steel type is an aluminum killed steel of C = 0.10%. Note that the method of the present invention exhibits its effect when the NSR is 25% or less. That is, the effect is exhibited when casting at a higher speed than before.

As shown in FIG. 2A, molten steel is injected from a ladle through a tundish (TD) 12 and a dipping nozzle 14 into a mold 18. The flow rate from the immersion nozzle 14 was controlled, and the level of the molten metal in the mold was set to the upper end of the lower coil 42, and the level of the molten metal was controlled with an accuracy of ± 2 to 3 mm. Also,
The mold powder 16 shown in Table 3 is used.
At the end of casting, the powder inflow Q was determined by calculating the thickness of the slag attached to the surface of the slab by taking the slab out of the slab after cooling it.

[0040]

[Table 3]

FIG. 4 shows the test results under the conditions shown in case 1 and in Table 1 and shows the relationship between the coil current I and the powder consumption Q by applying a current only to the lower coil. When a current was applied to the entire lower part of the mold during the vibration of the mold, it was confirmed that increasing the current I increased the consumption Q.

On the other hand, even when a current was applied to the lower coil only during 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 during the vibration of the mold, the powder preferentially flows during the PS phase during solidification with the mold. At this time, it is considered that high-frequency magnetic force acts on the inner periphery (in the direction away from the coil) by energizing the lower coil, and the thickness of the powder flowing in increases by pushing the molten metal back inward.

Also, the reason why the continuous application of the high-frequency magnetic force is not so different from that in the PS phase alone, is that the shell already has some strength even when the lower coil is energized in the NS phase, It is considered that the shell did not move even when the high-frequency magnetic force was applied, and it hardly contributed to powder inflow.

FIG. 5 also shows the results under the conditions shown in case 1 and 2 in Table 1. In the case where the coil was continuously applied, the disturbance of the oscillation mark (OSM) on the slab surface was caused by the disturbance of the coil current. Although it increased with the increase, in the case of only the PS phase, there was almost no disturbance of the oscillation mark.

In general, since the magnetic flux density is high in the slit portion and the slit portion corresponds to the position where the OSM is formed, the OSM is formed in the NS period. It is considered that the solidification delay was caused by Joule heat. On the other hand, in the case where only the PS period was applied, the coil was not energized at the time of forming the OSM (NS period), so that the shell was formed uniformly in the NS period, so that the oscillation was less likely to be disturbed. Can be

FIG. 6 shows the effect of the upper coil. Since the upper coil is located above the meniscus as described above, the meniscus receives a high-frequency magnetic force obliquely downward (toward the mold). As a result, the fragile portion at the tip of the initially solidified shell is pushed and bent in an oblique direction, and a compressive force acts inside the shell to supplement the compression by NS.

As described above, NSR at a casting speed of 4 m / min.
However, under the condition of 15%, the compression at the shell tip is insufficient, so that minute internal cracks frequently occur immediately below the OSM. This can evolve into the constraint of the shell during casting, and conventionally N
This was prevented by increasing the SR. FIG. 6 shows that this problem can be prevented by increasing the current in the upper coil, and shows that the frequency of internal cracking under OSM can be significantly reduced by energizing the upper coil during the NS period.

On the other hand, when the upper coil was continuously energized, the effect was hardly observed even when the coil current was increased. This is because high-frequency magnetic force works obliquely downward from the upper coil in the PS phase by continuously energizing, and the meniscus shape solidifies while being oblique,
This is probably because no compression force acts on the shell tip.

As a result, by supplying current of 5000 A or more to the upper and lower coils (the upper coil in the NS phase and the lower coil in the PS phase), N
Even under the unstable casting condition of SR = 15%, the frequency of breakout occurrence (frequency of occurrence indexed as BO) can be reduced to 1/10 of the conventional frequency.

The same effect was obtained by conducting a test in which the conditions of the upper and lower coils were changed under different conditions. However, when the upper coil was set as close to the meniscus as possible, the internal cracks immediately below the OSM were better. It has a great effect on preventing occurrence.

[0051]

According to the method and apparatus of the present invention, almost no breakout occurs even when the casting speed is increased more than before, so that the efficiency of casting can be remarkably increased and the slab having good surface properties can be obtained. Can be manufactured.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electromagnetic force generated between a molten metal and an applied high-frequency current.

FIGS. 2A and 2B are a schematic view of a casting method using an electromagnetic force according to the present invention, FIG. 2A and a view (b) showing a positional relationship between a mold and a coil.

FIG. 3 is a diagram showing a mold vibration and a timing of applying a current to an upper coil and a lower coil in the present invention.

FIG. 4 is a diagram showing a relationship between a current applied to a lower coil and a powder consumption according to the present invention.

FIG. 5 shows the current applied to the lower coil and the OS in the present invention.
It is a figure showing relation with disorder of M.

FIG. 6 is a diagram showing the relationship between the current applied to an upper coil and the frequency of occurrence of internal cracks immediately below an oscillation mark in the present invention.

FIG. 7 is a diagram showing the relationship between mold vibration and casting speed in conventional continuous casting.

[Explanation of symbols]

 2 molten metal 4 electromagnetic coil 42 lower electromagnetic coil 44 upper electromagnetic coil 6 high frequency current 8 induction current 10 Lorentz force 12 tundish 14 immersion nozzle 16 powder 18 mold

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI B22D 27/02 B22D 27/02 V (72) Inventor Shinichi Nishioka 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56 References JP-A-5-111552 (JP, A) JP-A-64-83348 (JP, A) JP-A-4-138843 (JP, A) JP-A-4-178247 (JP, A) 7-96360 (JP, A) JP-A-4-13443 (JP, A) JP-A-5-212512 (JP, A) JP-A-3-90255 (JP, A) JP-A-8-90183 (JP, A) A) JP-A-4-13442 (JP, A) JP-A-2-147150 (JP, A) JP-A-8-132189 (JP, A) JP-A-7-266004 (JP, A) JP-A-6 -246405 (JP, A) JP-A-7-223060 (JP, A) JP-A-6-320238 (JP, A) WO 96/5926 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/04 311 B22D 11/16 105 B22D 27/02 B22D 11/07

Claims (5)

(57) [Claims] 1. A method for continuously casting molten metal, comprising the following steps. (A) Prepare a water-cooled continuous casting mold in which the upper part of the mold is comb-shaped and the lower part following this is integrally formed,
(B) Pouring molten metal into the mold while performing mold vibration, and continuously drawing out the formed slabs,
(C) A high-frequency electromagnetic force is applied to the molten metal in the vicinity of the meniscus level in the upper and lower stages from the outside of the mold to continuously cast the solidified shell formed in the vicinity of the meniscus while curving inward. 2. The melting device according to claim 1, wherein the lower electromagnetic force of the two-stage high-frequency electromagnetic force is applied at the time of the positive strip of the mold vibration, and the upper electromagnetic force is applied at the time of the negative lip. A continuous casting method for metal. 3. The continuous casting method for molten metal according to claim 1, wherein a negative lip time ratio of mold vibration of the mold is 25% or less. 4. The continuous casting method for a molten metal according to claim 1, wherein the molten metal is an alloy containing Fe. 5. A continuous casting mold comprising the following members. (A) a continuous casting mold made of copper or a copper alloy in which the upper part of the mold has a comb shape, the lower part is integrally formed, and the inside of each part can be water-cooled; (b) melting of the mold At a position corresponding to the vicinity of the metal meniscus level, at least two high-frequency coils having the central axis of the mold as a central axis are provided in the vertical direction on the outer periphery of the mold.