JPH1099948A - Method for continuously casting steetl - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically reduce the defect on the surface layer part of a slab caused by oscillation mark by simultaneously impressing static magnetic field with superconducting coils in the case of continuously casting by supplying molten steel into a mold impressing an alternating magnetic field. SOLUTION: At the time of executing the continuous casting by supplying the molten steel 3 into the mold 1 through a nozzle 2 while impressing the alternating magnetic field by using the alternating magnetic field generating coils 5' to the mold 1 for continuous casting, the static magnetic field is impressed by using the supperconducting coils 6 at the same time of using the alternating magnetic field. At the time of impressing the static magnetic field so as to overlap with the alternating magnetic field, the oscillation mark is reduced in proportion to the intensities of the static magnetic field and the alternating magnetic field. Then, the depth of the oscillation mark can be reduced to almost zero and the reduction of the cost accompanied with the cleaning of the slab and the high productivity can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of steel, and more particularly to a continuous casting method of steel capable of maintaining high quality of a slab under a high speed casting operation.

[0002]

2. Description of the Related Art Conventionally, in continuous casting, a meniscus (free surface of molten steel in a mold) is prevented from being oxidized by introducing a molten flux onto molten steel in a mold, and the meniscus shape is maintained. In addition, when cooling is large in the meniscus portion, an initially solidified shell develops, which is one of the causes of surface defects. As countermeasures, the use of mild cooling powder has ensured the temperature at the meniscus portion and the flow of molten steel from the immersion nozzle.

However, in recent years, in order to respond to the demand for even higher quality defect-free slabs, the flow of molten steel is controlled by applying an electromagnetic field to the mold during continuous casting, thereby ensuring the meniscus temperature and the flow of the meniscus portion. Have been proposed and some have been put to practical use. For example, a method in which the flow rate of molten steel in a mold is controlled by applying a static magnetic field to the mold, and at the same time, the meniscus temperature (the surface temperature) is raised (see, for example, Ishii et al .: CAMP-ISIJ Vol.9 (1996) -206) ) Has been put to practical use. Also, a method in which a moving magnetic field is applied to the inside of the mold to give a flow to the meniscus to make the temperature uniform and to make the initial solidification uniform (for example, Fujisaki et al .: CAPM-ISIJ Vol.8 (1995)
-217, Okazawa et al .: See CAPM-ISIJ Vol.8 (1995) -218)). For this reason, in recent years, high productivity and high quality have been compatible.

However, there are still some surface layer defects (particularly defects caused by oscillation marks), and the flow of molten steel in the mold cannot be completely controlled. Therefore, a method that can better control the meniscus temperature and the flow of the meniscus portion at the same time is required,
In response to this, a method of applying an alternating magnetic field to the meniscus, applying a stir force to molten steel in the meniscus portion and generating Joule heat by a current and a magnetic field flowing only to the meniscus portion by a skin effect (for example, Fuji et al .: CAPM- ISIJ Vol.8 (1995) -215 etc.) is being developed. However, this method also has a problem that when a constant alternating magnetic field is continuously applied, the molten steel fluctuates in the meniscus portion, the meniscus portion becomes unstable, and the oscillation mark suddenly becomes deeper. Had to be eliminated.

In order to eliminate this instability, for example, Japanese Patent Application Publication No. WO9605926 discloses that when an alternating magnetic field is applied to a meniscus, the amplitude or frequency is changed at a constant period, and the alternating magnetic field is further changed to a square wave (rectangular wave). ) Or by applying a regular waveform such as a sine wave, the oscillation mark will be broken to some extent before the oscillation mark suddenly becomes unstable, and the oscillation mark that is lighter than the oscillation mark of the conventional continuous cast slab will be spaced at regular intervals. It has been proposed that the method be continued to be given. However, although it is lighter than the conventional continuous casting slab, it is still undeniable that the oscillation mark remains, and a further new method has been required.

In response to this, for example, Japanese Patent Application Laid-Open No. 8-155608 proposes a method of applying a double magnetic field to the meniscus. In this method, both the alternating magnetic field and the perpendicular magnetic field can be applied, and further, the direct current can be applied in a pulsed manner, so that the oscillation mark depth is reduced to a considerable extent. However, this is still not complete because oscillation marks of considerable depth still remain.

[0007] A method combining meniscus control and mold internal flow control has also been studied.
Japanese Patent Application Laid-Open No. 7-148555 discloses a method in which a static magnetic field is applied to the lower end of an immersion nozzle discharge port, and an alternating magnetic field is applied to a meniscus. Thereby, both sides of the flow inside the mold and the meniscus temperature / flow can be controlled. However, in this method, the reduction of the oscillation mark described above is not complete.

[0008] As described above, in the conventional casting method, a defect accompanying an oscillation mark is included in the surface layer portion of the slab, and thus the surface layer defect caused by steelmaking greatly affects the quality of the hot-rolled product. There are no other measures than eliminating a few millimeters of the surface of the slab by hot scarfing or the like in the slab stage at the slab stage, which has been a very serious problem in terms of product yield and maintenance costs. Although various measures have been proposed to solve this problem as described above, they have not been able to achieve a drastic solution. Therefore, the goal of high-speed casting while maintaining the surface layer of the slab at a high quality has not been reached.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art by taking a new measure capable of reducing the maximum depth of the oscillation mark to almost zero. It is an object of the present invention to provide a continuous casting method of steel in which surface defects caused by oscillation marks which have been generated are greatly reduced, and cost reduction and high productivity achieved by slab care and the like are achieved.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a continuous casting mold for supplying molten steel through a nozzle while applying an alternating magnetic field to the casting mold by using an alternating magnetic field generating coil. A continuous casting method for steel, wherein a static magnetic field is applied using a superconducting coil simultaneously with the magnetic field.

It is preferable that the static magnetic field is applied from outside the alternating magnetic field generating coil, and it is more preferable that the static magnetic field is applied including a meniscus. Further, in the present invention, it is preferable to use a shield that prevents the alternating magnetic field from entering the superconducting coil. The shield is preferably made of a material having a high conductivity when the frequency of the alternating magnetic field is high, and a material having a high magnetic permeability when the frequency of the alternating magnetic field is low.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have conducted intensive calculations and experiments on the state of molten steel temperature and flow at the meniscus, and as a result, the temperature and the flow are intricately entangled in the meniscus. The basic knowledge that the state is further complicated when is applied.

In particular, the application of an alternating magnetic field is a preferable direction in the sense that the Joule heat due to the induced current shortens the length of the initial solidified shell toward the inside of the mold. Inward, the meniscus is narrowed down, and at the same time, the molten steel in the meniscus also flows.If these parts work in an intricate manner and an unstable state occurs at a certain point, this unstable state occurs due to the relationship between the magnetic field and current. The situation is further amplified, creating the disadvantage of creating larger unstable parts.

This unstable amplification phenomenon cannot be sufficiently suppressed by a method of intermittent application of an alternating magnetic field (so-called pulse application) which has been conventionally experimentally attempted. As a result of examining the unstable amplification mechanism in more detail, it was found that there was a problem in the current flowing through the surface layer of the meniscus and the flow of molten steel due to the stirring of the meniscus. However, if all these are eliminated, the state of the casting will return to the conventional state, and as a result of further study to eliminate only the unstable amplification mechanism while leaving only good points, it is possible to significantly reduce the meniscus flow more than before. It has been found that it is effective, and for that purpose, it is not enough to apply the conventional alternating magnetic field alone, and it is extremely effective to superimpose and apply a static magnetic field thereon.

[0015] To give a specific example, the size in the mold is 0.4m.
(W) x 0.11m (Thickness) Test continuous casting machine Molten steel is supplied through a two-hole nozzle to the mold and cast continuously at a casting speed of 2.5m / min. T
(T: Tesla) alternating magnetic field (in this case, high frequency magnetic field) and 0.8T
When the static magnetic field is applied simultaneously, the maximum value of the oscillation mark depth is larger than when no magnetic field is applied.
From 800μm to 200μm, and maximum nail depth (depth from the slab surface to the tip of the initial solidified shell) is 2600μ
m to 400 μm.

However, when a static magnetic field of a normal level (about 0.3 T) is applied, the meniscus flow cannot be eliminated, and it is necessary to apply a strong magnetic field exceeding the normal level from the outside. Superconducting magnetic field generators capable of generating a strong magnetic field and sufficiently withstanding industrial use are being commercialized. The present invention has been completed by organically combining the above-mentioned new knowledge with the current superconducting technology.

FIG. 1 is a partial cross-sectional view of a main part of a continuous casting facility suitable for carrying out the present invention. In FIG. 1, 1 is a mold (mold), 2 is a nozzle (immersion nozzle), 3 is molten steel, and 3a Is a solidified shell, 4 is a mold powder, 5 is an alternating magnetic field generating coil, and 6 is a static magnetic field generator (superconducting coil). As shown in the drawing, the continuous casting method for steel according to the present invention supplies molten steel 3 into the casting mold 1 for continuous casting through the nozzle 2 while applying an alternating magnetic field to the casting mold 1 using the alternating magnetic field generating coil 5. In performing the continuous casting, the gist is that a static magnetic field is applied using the superconducting coil 6 simultaneously with the alternating magnetic field.

When the static magnetic field is superimposed on the alternating magnetic field and applied as described above, the oscillation mark is reduced in proportion to the strength of the static magnetic field and the alternating magnetic field, and the oscillation mark depth reaches the ultimate level which could not be achieved conventionally. Can be reduced to In the present invention, in order to prevent the alternating magnetic field from penetrating into the static magnetic field generator and to efficiently use the space around the mold, a static magnetic field is applied from outside the alternating magnetic field generating coil 5 as shown in FIG. It is preferable to apply.

Further, as shown in FIG. 2, it is more effective to apply the static magnetic field including the meniscus. Further, as shown in the figure, in the present invention, it is preferable to use a shield 7 for preventing an alternating magnetic field from entering the superconducting coil 6. The shield 7 is made of a material having a high electrical conductivity (electrical conductivity) when the alternating magnetic field is at a high frequency, and a material having a high magnetic permeability when the alternating magnetic field is at a low frequency, from the viewpoint of the magnetic penetration depth. It is more preferable to configure. Specifically, in the case of a high frequency, for example, Cu, Al, C or the like or a composite thereof is suitable. In the case of a low frequency, ferrite, silicon steel, pure iron, etc. are preferable because the penetration depth is large. At the same time, it is preferable that the shield 7 also serves as a heat shield from the mold 1. By using the shield 7, a high-quality slab can be cast more stably at a high speed, and it is economically advantageous. is there.

[0020]

【Example】

<Case A> C: 10 to 16 wtppm, Mn: 0.15 to 0.20 wt%,
P: 0.025 wt% or less, S: 0.015 wt% or less, Al: 0.025 to 0.
038wt%, Total O: 25-35wtppm, Tt (Tundish molten steel temperature) 1556-1565 ° C, using a vertical bending continuous caster with a vertical section of 3m, internal dimensions 260mmt x 1600mm
Nozzle diameter 70mmφ, discharge hole diameter 70mm × 80mm □, W mold
When casting at a rate of 260 tons / ch per charge through a two-hole immersion nozzle with a discharge hole downward angle of 20 °, no magnetic field is applied from outside to the mold (conventional example A1).
An alternating magnetic field of 5 to 2.0 kHz (800 A), 0.1 T (high-frequency magnetic field in this embodiment; the same applies hereinafter) is applied (conventional example A2), and a high-frequency magnetic field of 1.5 to 2.0 kHz (800 A), 0.1 T is applied near the meniscus. A magnetic field was applied under the three conditions of applying a static magnetic field of 1.0 T from the outside (Example A), and the rate of occurrence of surface defects caused by steelmaking after hot rolling of the obtained slab was determined. It is shown in Table 1.

As a superconducting coil for applying a static magnetic field, a superconducting coil using Nb ₃ Sn wire was used (the same applies hereinafter).

[0022]

[Table 1]

From Table 1, it is clear that Example A has a lower surface defect occurrence rate than Conventional Examples A1 and A2, and it can be seen that according to the present invention, the cost can be reduced while improving the quality. <Case B> C: 20 to 25 wtppm, Mn: 0.15 to 0.20 wt%,
P: 0.025 wt% or less, S: 0.015 wt% or less, Al: 0.025 to 0.
038 wt%, total O: 35 wtppm or less, having a composition of Tt of 15
Using a vertical bending continuous caster with a vertical section of 3m, a molten steel of 56 to 1565 ° C is cast into a mold of 260mmt x 1600mmW and the nozzle diameter is 70mm.
φ, discharge hole diameter 70mm × 80mm □, discharge hole downward angle 20 ° 2
When casting at 260 tons / ch per charge through a hole immersion nozzle, 2.0 mm is injected from the outside to the mold near the meniscus.
Apply a high-frequency magnetic field of ~ 5.0kHz (1200A), 0.2T (conventional example B), apply a high-frequency magnetic field of 2.0 ~ 5.0kHz (1200A), 0.2T near the meniscus and apply a static magnetic field of 1.0T from outside. (Example B1), 2.0 to 5.0 kHz (0.
A high-frequency magnetic field of 2T) was applied, and a static magnetic field of 1.0T including a meniscus was applied from the outside (Example B2). Table 2 shows the results of investigation of the incidence of surface defects.

[0024]

[Table 2]

From Table 2, it is clear that Example B1 has a lower incidence of surface defects than Conventional Example B, and that Example B2 has a lower incidence of surface defects than Example B1. It can be seen that, according to the application of the static magnetic field including the meniscus, which is a more preferable embodiment of the present invention, the cost can be reduced while further improving the quality. <Case C> A molten steel having a composition equivalent to low-carbon steel and ultra-low-carbon steel and having a Tt of 1553 to 1565 ° C., and a vertical bending continuous caster with a vertical portion of 3 m, and a nozzle having an inner dimension of 260 mmt × 1600 mmW. 260t per charge through a 2-hole immersion nozzle with a diameter of 70mmφ, discharge hole diameter 70mm × 80mm □, discharge hole downward angle of 20 °
When casting at on / ch, a high frequency magnetic field of 1.5 to 5.0 kHz (1200 A) and 0.2 T is applied to the mold from the outside near the meniscus, and a static magnetic field of 0.8 to 1.2 T including the meniscus is applied from outside. , Shield 7 shown in FIG. 2 (made of Cu (internal water cooling))
Table 3 shows the results obtained by applying the test under two conditions of “without installation” (Example C1) and “with installation” (Example C2). The quench rate is the number of quench cycles per cast charge (quench is
This is a phenomenon in which the superconducting state is broken for some reason in the superconducting coil), and here, it is used as an index of the operational stability of continuous casting.

[0026]

[Table 3]

From Table 3, it can be seen that the rate of occurrence of quench was determined in Example C1.
However, in Example C2, no quenching occurred, and the shield installation, which is a further preferred embodiment of the present invention, can more stably realize high quality and low cost in continuous casting. Is evident. In addition,
In the present embodiment, a specific example in the case where the alternating magnetic field has a high frequency is disclosed. However, it has been confirmed that the same effect as described above can be obtained by using a material having a high magnetic permeability for the shield even when the alternating magnetic field is at a low frequency.

[0028]

According to the present invention, surface layer defects caused by oscillation marks, which have conventionally occurred in continuous cast slabs, are greatly reduced, and cost reduction and high productivity due to slab maintenance and the like are achieved. It has a remarkable effect.

[Brief description of the drawings]

FIG. 1 is a half sectional view of a main part of a continuous casting facility suitable for carrying out the present invention.

FIG. 2 is a half cross-sectional view of a main part of a continuous casting facility more suitable for carrying out the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold (mold) 2 Nozzle (immersion nozzle) 3 Molten steel 3a Solidified shell 4 Mold powder 5 Alternating magnetic field generating coil 6 Static magnetic field generator (superconducting coil) 7 Shield

Claims (6)

[Claims] 1. A method for continuous casting by supplying molten steel through a nozzle while applying an alternating magnetic field to a mold for continuous casting while using an alternating magnetic field generating coil to perform continuous casting, using a superconducting coil simultaneously with the alternating magnetic field. A continuous casting method for steel, comprising applying a static magnetic field. 2. The continuous casting method according to claim 1, wherein the static magnetic field is applied from outside the alternating magnetic field generating coil. 3. The continuous casting method according to claim 2, wherein a static magnetic field is applied including the meniscus. 4. The continuous casting method according to claim 1, wherein a shield is used to prevent an alternating magnetic field from entering the superconducting coil. 5. The continuous casting method according to claim 4, wherein the shield is made of a material having high conductivity. 6. The continuous casting method according to claim 4, wherein the shield is made of a material having a high magnetic permeability.