JPH08229648A - Continuous casting method for steel - Google Patents

Abstract

PURPOSE: To prevent the entrapment of mold powder and inclusion in high speed casting by impressing static electric field having a specific value of magnetic flux density to a mold with superconducting coils and also, electric current to molten steel in the mold and controlling the molten steel spouted flow. CONSTITUTION: The molten steel is supplied into the mold 1a, 1b from an immersion nozzle 2. The superconducting coils 3 are arranged to the long side walls 1b and the static magnetic field having >=0.5T is impressed to the molten steel. Simultaneously, the electric current is conducted to the electric conducting parts of electrodes 4 arranged at the upper part and the lower part of spouting hole 2a of the immersion nozzle 2 from the upper part to the lower part. The ascending flow developed by impressing the static magnetic field and conducting the electricity releases the molten steel flow spouted from the immersion nozzle 2. The molten steel is stirred in the mold, and the shortage of supplied heat to a meniscus can be eliminated. By this method, the uniform descending flow is obtd.

Description

Detailed Description of the Invention

[0001]

[Industrial application] In continuous casting of steel in which molten steel is supplied to a mold through a dipping nozzle connected to the bottom of a tundish, inclusions and bubbles are deep inside the mold as the molten steel supply per unit time increases. There is a defect that causes an internal defect of the slab to penetrate into the slab. In addition, reversal flow on the short side of the mold, especially upward flow, creates a raised portion in the meniscus part, which promotes fluctuations in the molten metal level and also causes mold powder to be entrained. .

[0002] The present invention, especially when performing high-speed casting in which the supply amount of molten steel exceeds twice the conventional amount, fluctuations in the molten metal level and entrainment of powder in the continuous casting mold,
Intrusion of inclusions and bubbles is suppressed as much as possible to improve the internal quality of the slab, and the surface quality of the slab is also improved to obtain stable cast slabs with improved internal and external qualities. To do.

[0003]

2. Description of the Related Art In order to control the jet flow of molten steel from an immersion nozzle, conventionally, the shape of the nozzle outlet has been modified,
It was common to reduce the injection rate of molten steel.

In the meantime, the conventional measures as described above have not yet completely prevented quality defects caused by inclusions contained in molten steel.

As a prior document showing a method for improving these defects, for example, in Japanese Patent Laid-Open No. 57-17356, a static magnetic field generator is installed in a continuous casting mold, whereby molten steel from a dipping nozzle is installed. Japanese Patent Application Laid-Open No. 2-284750 discloses a method of applying a static magnetic field to the entire surface of a continuous casting mold to thereby apply a braking force to the molten steel jet. .

In the former technique, when the jet flow of molten steel is braked, it changes its direction as if it hits a wall, but the energy of the jet flow cannot be dispersed to form a uniform flow. Moreover, it was not possible to obtain a satisfactory result because the jet flow escaped in the direction in which the static magnetic field was not acting.

On the other hand, the latter technique can make the jet flow of molten steel from the immersion nozzle uniform, and can improve the surface and internal quality of the cast slab to some extent. However, particularly in the case of performing high-speed casting in which the throughput of molten steel exceeds twice that of the conventional one, there are problems as described below, and there is still room for improvement.

That is, 1) Occurrence of uneven flow in the mold due to the molten steel jet from the immersion nozzle is unavoidable. 2) When the nozzle clogging of the immersion nozzle occurs when the molten steel jet speed is increased, the drift in the mold becomes large and stable continuous casting cannot be performed. 3) As the molten steel jet speed increases, the reversal flow on the short side of the mold also becomes faster, so the fluctuations in the molten metal surface become noticeable, and powder entrainment cannot be avoided. 4) Surface flaws (claw scratches) caused by oscillation frequently occur.

As a solution to such a problem, Japanese Unexamined Patent Publication No. 6-126399 proposes a method of applying a static magnetic field in a continuous casting mold and applying a direct current at that time. The technology proposed here has a drawback that it is difficult to apply it in high-speed casting where the throughput of molten steel exceeds twice that of the conventional technology, because the magnetic flux density of the static magnetic field applied to the mold is at most about 0.15T. .

Regarding the above technique, it is conceivable to increase the capacity of the device for applying the static magnetic field or increase the value of the applied current. However, in order to increase the capacity of the device, the size of the device itself becomes large. It is impossible to install it in the equipment of No.1, on the other hand, when the value of the applied current is increased, the power source becomes large, and since the current of several thousand A flows, the cable up to the electrode also becomes large accordingly. Moreover, the cables and electrodes generate heat due to Joule heat and cannot be used.

[0011]

SUMMARY OF THE INVENTION The object of the present invention is to solve the above-mentioned conventional problems that occur when high-speed continuous casting is carried out, and to achieve the DHCR method (Direct Hot Charg
ed Rolling) or CC-DR (Continuous Castin)
We are proposing a new continuous casting method that can stably supply maintenance-free slabs suitable for g rolling).

[0012]

According to the present invention, in supplying molten steel into a continuous casting mold through a submerged nozzle disposed in the continuous casting mold, a superconducting coil is provided between the opposing side walls of the continuous casting mold to form 0.5. A continuous casting method for steel, characterized in that a static magnetic field having a magnetic flux density of T or more is applied and an electric current is applied in the mold to control a molten steel jet flowing out from a discharge port of an immersion nozzle. Is preferably applied in the vicinity thereof including the discharge port of the immersion nozzle, in the entire region in the width direction of the continuous casting mold, or in the region above and / or below the discharge port of the immersion nozzle. .

Further, when an electric current is applied, between the opposing walls of the solidified shell on the exit side of the continuous casting mold (in continuous casting using a mold having a combination of long side walls and short side walls, the long side walls are It is desirable to arrange an electrode in the direction (along the direction) or in the vicinity of the discharge port of the immersion nozzle and apply a current thereto.

The continuous casting method according to the present invention is particularly suitable for high-speed casting in which the molten steel throughput is 6 ton / min or more.

[0015]

The present invention will be described in detail below with reference to the drawings.

1a and 1b show the construction of a continuous casting facility (continuous casting mold) suitable for carrying out the present invention.

In the figure, numeral 1 is a continuous casting mold comprising a pair of short side walls 1a and a pair of long side walls 1b, 2 is a dipping nozzle for supplying molten steel to the continuous casting mold 1, and 3 is continuous. A superconducting coil for applying a static magnetic field between the long side walls 1b of the casting mold 1, the superconducting coil 3 being on the back surface of the long side wall 1b and in the vicinity thereof including the discharge port 2a of the immersion nozzle 2. However, the superconducting coil 3 may be arranged in the entire region in the width direction of the mold or in the upper and / or lower region of the discharge port 2a of the dipping nozzle 2 as required.

Numeral 4 is an electrode for applying a current in the continuous casting mold 1. The electrode 4 is composed of a conductive portion 4a and an insulating portion 4b as shown in FIG.
When a porous immersion nozzle is used, the conductive portion 4 of the electrode 4
a is arranged at the upper part and the lower part of the discharge port 2a.

Using the continuous casting mold having the above-mentioned structure, a current is applied while applying a static magnetic field having a magnetic flux density of 0.5 T or more (from the upper conductive portion 4a to the lower portion of the discharge port 2a). Even when high-speed casting is performed such that an electric current is passed toward the conductive portion 4a, that is, an electric current is passed in the direction along the drawing direction of the cast slab) and the throughput of molten steel exceeds 6 ton / min, The flow velocity of the molten steel jet flowing out from the outlet becomes extremely small, and inclusions contained in the molten steel do not enter the bottom of the mold.

FIG. 3 shows an example in which the single-hole type immersion nozzle 2 is applied. In continuous casting using a mold having such a configuration, a static magnetic field having a magnetic flux density of 0.5 T or more is applied. By flowing the electric current i in the direction orthogonal to the long side wall 1b of the mold 1, the flow velocity of the molten steel jet can be reduced as in the case shown in FIG.

FIG. 4 shows that, in continuous casting using the single-hole type immersion nozzle 2, a static magnetic field is applied over the entire width of the lower end of the mold 1, and the space between the opposing walls of the solidified shell S immediately below the exit side of the mold 1 is increased. 2 shows an example of a structure in which a current is applied by the electrode rolls 5a and 5b.

In continuous casting using a mold having such a structure, the molten steel jet flowing out from the dipping nozzle 2 is canceled by an ascending flow generated by applying a static magnetic field and energizing, and The advantages are that the molten steel can be expected to be stirred, the shortage of heat supply to the meniscus, which is a drawback of the single-hole type immersion nozzle, can be solved, and a uniform downward flow can be obtained by the static magnetic field applied across the entire width. There is.

FIG. 5 shows that in continuous casting using the single-hole type immersion nozzle 2, a static magnetic field is applied to the entire width of the upper part (meniscus portion) of the mold 1 and a region in the vicinity of the discharge port of the immersion nozzle 2 and the length of the mold 1 is increased. This is an example of the case where the current i is passed in the direction orthogonal to the side wall 1b. If such continuous casting is performed, not only the flow velocity of the molten steel jet flow can be reduced, but also fluctuations in the molten metal level in the mold can be suppressed and calmed down.

The area to which the static magnetic field is applied and the area to which the current is applied vary depending on the difference in the structure of the immersion nozzle and the casting conditions, and are not limited to FIGS. 1 to 5 above.

FIG. 6 shows a specific structure of the superconducting coil.

The superconducting coil 3 has a helium tank, a radiation heat insulation shield, and a vacuum container which surrounds them and prevents heat from entering due to convection. The helium tank is connected to a liquid helium container, and the radiation heat insulation shield is connected to a liquid nitrogen container. There is.

The superconducting coil 3 is always cooled by liquid helium and kept at 268.9 ° C. or lower. Liquid nitrogen is constantly supplied to the radiation insulation shield from the liquid nitrogen container so that external heat does not reach the helium tank directly. Although not shown, each container has a refrigerator and has a mechanism in which each gas that has become a gas is cooled and liquefied again and collected in each container.

Since a superconducting coil as shown in FIG. 6 is used as the static magnetic field applying coil, a particularly high magnetic flux density (0.
(5 T or more) and iron core is not required, so it is possible to achieve weight reduction and compactness compared to the conventional normal conducting type coil. Further, it is not necessary to energize at all times, which is extremely advantageous in achieving energy saving.

FIG. 7 shows continuous casting of a low melting point alloy having almost the same characteristics as molten steel by using a single-hole type immersion nozzle (fluid and heat transfer calculation based on data obtained from an actual machine in advance to determine the lower end of the mold). The flow rate at which casting is possible is determined in advance,
It is the graph which showed the magnetic flux density of the static magnetic field and the current value in the case of being able to cast when it is below that value (casting model experiment).

When an electric current is applied in the continuous casting mold,
The current value at which the electrodes and cables self-heat and cannot withstand the operation is considered to be about 2000 A even if the heat transfer from the molten steel is taken into consideration.
It is possible to control the molten steel jet flow by applying a static magnetic field with a magnetic flux density of 0.5 T or more even if it is kept within the limit value of A, so that the throughput of molten steel reaches 6 to 10 ton / min. Casting can be easily dealt with. In the present invention, the current applied to the mold is preferably about 400 A to 2000 A from the viewpoints of self-heating of the cables and electrodes described above and the efficiency of the upward flow generated by the static magnetic field and the current.

FIG. 8 shows a case where ultra-low carbon steel is used and continuous casting is performed by varying the magnetic flux density of the static magnetic field applied in a continuous casting mold, and the obtained cast slab is finished into a cold rolled coil to form a coil defect. It shows the results of a survey on the occurrence status of the rate.

The defect occurrence rate of the coil is extremely reduced when the magnetic flux density is in the vicinity of 0.5 T, and the drift of the molten steel is suppressed especially when a current is applied in the mold.
The defect rate of the coil tends to be further reduced.

[0033]

Example: C: 10 to 15 ppm, Si: 0.008 to 0.005 wt%,
Mn: 0.15-0.2 wt%, P: 0.02-0.025 wt%, S: 0.0
08 to 0.012 wt%, Al: 0.025 to 0.035 wt%, T: 25 to 31
For molten steel with a ppm composition, the long side wall spacing (thickness of cast slab) is 220 mm, and the short side wall spacing (width of cast slab) is 16 mm.
A continuous casting machine equipped with a mold having the structure shown in FIGS. 1 to 5 with a superconducting coil (Nb-Ti wire) of 00 mm in length, 200 mm in length and 2000 mm in width on the back side of the long side wall is arranged. By applying and casting 7200 charges (260 tons per charge) under the following conditions, nozzle clogging of the immersion nozzle during casting, occurrence of breakout, internal quality of slab, surface quality (coil defect rate) Was investigated. The results are shown in Table 1 together with the results of the comparative example in which continuous casting was performed under the same conditions except that the static magnetic field was not applied.

Magnetic flux density: 1.0T Throughput of molten steel: 8 ton / min Applied current value at the electrode: 800 A a. Two-hole type immersion nozzle Nozzle diameter: 80 mm Discharge port size of immersion nozzle: 80 mm × 80 mm □ Discharge angle of immersion nozzle : Downward 20 ° Discharge nozzle position: 200 mm from the meniscus to the top of the nozzle discharge port Meniscus position: ± 20 mm from the top of the static magnetic field application coil
Position b. Single-hole nozzle Nozzle diameter: Inner diameter 80 mm Discharge nozzle position: 200 mm from meniscus to nozzle tip Meniscus position: ± 20 mm from top of static magnetic field application coil
Position of

[0035]

[Table 1]

As is clear from Table 1, in the continuous casting according to the present invention, the molten steel throughput was 8 ton / min.
Even in the longest casting, it is possible to reduce the entrainment of mold powder and the fluctuation of the molten metal surface, so it is possible to secure good quality both inside and outside. It was confirmed that it was possible.

[0037]

As described above, according to the present invention,
Even when performing high-speed casting such that the throughput of molten steel exceeds 6 ton / min, there is no involvement of mold powder or deep inclusions, and defects due to oscillation are reduced. Furthermore, since it is possible to avoid remelting of the solidified shell, it is possible to stably manufacture a cast slab with good quality both inside and outside.

[Brief description of drawings]

1A and 1B are configuration explanatory views of a continuous casting mold suitable for carrying out the present invention.

FIG. 2 is a view showing a main part of the mold shown in FIG.

3A and 3B are views showing another example of a continuous casting mold suitable for use in the practice of the present invention.

4A and 4B are views showing another example of a continuous casting mold suitable for carrying out the present invention.

5A and 5B are views showing another example of a continuous casting mold suitable for carrying out the present invention.

FIG. 6 is a diagram showing a specific configuration of a superconducting coil.

FIG. 7 is a diagram showing the relationship between magnetic flux density and current value.

FIG. 8 is a diagram showing a relationship between a magnetic flux density and a cold rolling coil defect rate.

[Explanation of symbols]

 1 Continuous casting mold 2 Immersion nozzle 3 Superconducting coil 4 Electrode 5a Electrode roll 5b Electrode roll

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Satoshi Idokawa 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Inside the Technical Research Laboratory, Kawasaki Steel Co., Ltd. Steel Engineering Co., Ltd.

Claims (6)

[Claims] 1. When supplying molten steel into a continuous casting mold through a dipping nozzle arranged in the continuous casting mold, a magnetic flux density of 0.5 T or more is generated between superposed coils between opposing side walls of the continuous casting mold. A continuous casting method for steel, which comprises applying a magnetic field and applying an electric current in the mold to control a molten steel jet flowing out from a discharge port of an immersion nozzle. 2. The method according to claim 1, wherein the static magnetic field is applied to the vicinity of the discharge nozzle of the immersion nozzle. 3. The method according to claim 1, wherein a static magnetic field is applied across the width of the continuous casting mold. 4. A method according to claim 1 or 3, wherein the static magnetic field is applied in the region above and / or below the outlet of the immersion nozzle. 5. An electric current is applied between the opposing walls of the solidified shell immediately below the exit side of the continuous casting mold.
The method according to 3 or 4. 6. The method according to claim 1, 2, 3 or 4, wherein an electrode is arranged in the vicinity of the discharge port of the immersion nozzle to apply an electric current.