JP3236422B2 - Continuous casting method of steel using magnetic field - Google Patents

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 using a magnetic field, and in particular, in continuous casting of low C-Al killed steel and the like, UT defects, blisters, blisters, An object of the present invention is to reduce product defects such as sliver flaws and produce slabs of excellent quality.

[0002]

2. Description of the Related Art In general, in order to prevent the above-mentioned product defects, it is necessary to enhance the cleaning of molten steel, to improve the shape of the immersion nozzle to prevent inclusions and powder from being caught, In a casting machine, there is a method of promoting the floating of inclusions and bubbles in a mold by adopting a vertical portion.

[0003] Also, Iron and Steel Eng. May (1984)
In pages 41 to 47 and JP-A-57-17356, an electromagnet is arranged in a mold of a continuous slab casting machine, and a static magnetic field in a direction perpendicular to the molten steel discharge flow from the immersion nozzle into the mold is provided. When applied, the molten steel discharge flow is braked by Lorentz force generated by the interaction between the current induced in the molten steel and the static magnetic field, thereby preventing entrapment of mold powder and air bubbles, and causing entrainment in the molten steel. It has been proposed to promote the floating of inclusions, and as a method of further developing this technology, a method disclosed in Japanese Patent Application Laid-Open No. 2-284750 is known. These methods using a static magnetic field are effective in improving the quality of cast slabs by high-speed casting, because the Lorentz force increases as the casting speed increases, and are excellent continuous casting methods.

On the other hand, it is known that the surface properties of cast slabs are strongly affected by the initial solidification shell in the mold, and as a countermeasure, for example, in Japanese Patent Application Laid-Open No. 56-68565, A method of inductively heating the surface of molten steel by placing a flat coil directly above the molten metal surface and passing an alternating current has been proposed. According to this method, the heat input to the meniscus portion can be controlled independently of the control of the casting conditions, which is an advantageous method in that the molten steel surface can be uniformly heated.

[0005]

The above-mentioned method using a static magnetic field has a remarkable effect particularly in high-speed casting. However, when the casting speed must be temporarily reduced due to operational restrictions, the method using molten steel is difficult. If the degree of superheat (the difference between the temperature of the molten steel injected into the mold and the solidification temperature of the steel) is not sufficient, the heat supply to the meniscus decreases due to the low speed, and if the temperature of the meniscus decreases, the surface quality deteriorates There was a problem of inviting. That is, in the conventional method of applying a static magnetic field, when the casting speed is reduced, the control effect of the molten steel discharge flow by the Lorentz force becomes relatively small, and the effect of increasing the temperature of the molten steel caused by the application of the magnetic field also becomes small. No significant improvement in surface quality could be expected.

[0006] Of course, such a problem can be solved by controlling the temperature of the meniscus portion with an induction heating coil. However, in this method, an electromagnetic force acts on the surface of the molten steel due to the high-frequency magnetic field generated from the coil, so that a flow occurs on the surface of the molten metal. Moreover,
This phenomenon becomes more conspicuous when the electric power is increased in order to increase the heat supply, and causes a further disturbance of the molten metal surface, which leads to a problem of inducing powder entrainment.

Accordingly, an object of the present invention is to propose a continuous casting method that can advantageously solve the problem of molten metal surface disturbance caused when induction heating is performed on the surface of molten steel.

[0008]

According to the present invention, a magnetic field is generated by a magnetic pole disposed on a back surface of an opposite side wall of a continuous casting mold.
In controlling the flow of molten steel supplied from the immersion nozzle into the mold by the magnetic field to perform continuous casting of steel, the magnetic field is applied to the entire width direction of the continuous casting mold in the meniscus portion above the discharge port of the immersion nozzle. An upper magnetic field generated at
A lower magnetic field generated in the entire width direction of the continuous casting mold below the discharge port of the immersion nozzle is composed of two steps. Is a method of continuously casting steel using a magnetic field, wherein a static magnetic field is used, and the directions of an upper static magnetic field and a lower static magnetic field are opposite to each other. In addition, Ni-Cr-Fe alloy or Ni-Cr
-Co-based alloys are advantageously suitable.

FIG. 1 shows an example of equipment used in the method of the present invention. In FIG. 1, reference numeral 1 denotes a continuous casting mold including a pair of short side walls 1 a and long side walls 1 b, 2 denotes an immersion nozzle for supplying molten steel into the mold 1, 3 denotes a discharge port of the immersion nozzle 2,
Numeral 4 denotes an upper magnetic pole composed of a coil C and an iron core F for generating a magnetic field, and similarly, 5 denotes a lower magnetic pole. The upper magnetic pole 4 and the lower magnetic pole 5 have a length that covers the entire area in the width direction of the continuous casting mold 1. The upper magnetic pole 4 is located in the surface layer area of the molten steel above the discharge port 3 of the immersion nozzle 2 and in the lower area. The magnetic pole 5 generates a static magnetic field over the entire width of the mold in a region below the discharge port 3. Further, a high frequency current is supplied to the coil C of the upper magnetic pole 4 so as to be superimposed on the DC current in order to perform induction heating on the surface layer area of the molten steel, particularly on the meniscus portion. An example of yet another embodiment is shown in FIG. In FIG. 2, the upper magnetic pole 4 and the lower magnetic pole 5 are connected by a U-shaped iron core, there is no open magnetic pole, and the solenoid coils are independently disposed around the upper magnetic pole and the lower magnetic pole, respectively. DC current can be independently passed through the solenoid coil. In addition to the static magnetic field forming means, a gutter-like groove having a concave cross-sectional shape is provided at the pole tip of the upper magnetic pole 4, and a covered conductor for high-frequency current is provided along the groove. Thus, a high-frequency magnetic field coil C 'is formed.

The coil C in FIGS. 1 and 2 is arranged on the back of the mold 1. In a copper mold used for a normal continuous casting mold, the high frequency component of the upper magnetic pole is absorbed.
To prevent the induction heating efficiency of molten steel near the meniscus from decreasing and the molten steel surface not being heated, the mold is made of a material having low electric conductivity and high hot strength, for example, Ni-Cr-.
Inconel 718 which is Fe alloy and RENE which is Ni-Cr-Co alloy
41, UDIMET700, Waspaloy and the like are preferably used. It is because of such a mold configuration,
This is because the absorption of the high frequency component of the upper magnetic field can be suppressed, and the molten steel near the meniscus can be more effectively induction-heated.

As described above, according to the method of the present invention, the surface area of molten steel, particularly the meniscus portion, can be induction-heated without causing a large fluctuation in the molten metal level as in the prior art. Since the molten steel flow on the downstream side can also be suppressed, it is possible to reduce defects such as UT defects, blisters, blisters, and sliver flaws due to entrapment of mold powder and bubbles.

[0012]

According to the present invention, first, the molten steel is induction-heated from the back of the mold by the upper magnetic field generated by the upper magnetic pole, and the disturbance of the molten metal surface caused by the induction magnetic field is simultaneously suppressed by the DC magnetic field component.

[0013] Further, the static magnetic field generated by the lower magnetic pole promotes the floating separation of bubbles and inclusions entering the mold from the discharge port of the immersion nozzle.

Further, the directions of the static magnetic field components of the upper magnetic field and the lower magnetic field generated by each magnetic pole are made opposite to each other, so that the static magnetic field component becomes zero at the boundary between the two magnetic fields. By arranging the discharge port of the immersion nozzle near this boundary, as shown in FIG.
Is gradually decelerated and dispersed while flowing around a region where the static magnetic field component is zero and extends in the horizontal direction (the width direction of the mold). Here, the discharge jet 6 collides with the short side wall 1a, and becomes a branch flow 7 in both directions of the meniscus portion side and the mold deep portion side. These branch flows 7 enter the application region of the static magnetic field component over the entire width of the mold, where they are braked, so that the flow velocity of the molten steel at the meniscus portion and the downward flow speed to the deep portion of the mold are both reduced. As a result, mold powder is involved in the upper part of the mold, while intrusion of inclusions and air bubbles in the lower part of the mold,
Each is avoided and the surface quality as well as the internal quality of the slab is improved.

Here, generally, a magnetic flux φ (Wb) generated between magnetic poles of a static magnetic field generated by a solenoid has a magnetomotive force of F (Wb).
_m (AT), the magnetic resistance of the magnetic core is R _mc (AT / Wb), and the magnetic resistance in air is R _ma (AT / Wb), and φ = F _m / (R _mc + R _ma ) R _mc = 1 / μS F _m = NI where l: length of magnetic core (m), S: cross-sectional area of magnetic core (m
² ), μ: magnetic permeability of the magnetic core (H / m), N: number of coil turns (−), and I: coil current (A). Therefore, using a U-shaped magnetic core that does not have magnetic poles open to the air at both ends as the static magnetic field generator as the magnetic cores of the upper magnetic field and the lower magnetic field in common is a viewpoint of improving energy efficiency. From the viewpoint of not disturbing the lines of magnetic force applied perpendicular to the mold.

The reason why the directions of the upper static magnetic field and the lower static magnetic field are opposite to each other is that a magnetic flux component connected from the upper part to the lower part in the vicinity of the long side wall of the mold causes a vertical collision with the long side wall. This has the effect of damping the molten steel flow in the direction in which it flows, and reducing the incorporation of bubbles and inclusions into the initially solidified shell.

[0017]

[Example] Ultra-low carbon Al killed steel blown in a converter (C:
0.002 wt%) from a tundish (molten steel temperature: 1565 ° C) to various Ni-Cr-Fe alloys as shown in Table 1.
Feeding to a mold made of Ni-Cr-Co alloy, casting speed:
When continuously casting a slab having a width of 1200 mm and a thickness of 230 mm at 1.0 m / min, upper and lower magnetic fields are applied in the mold according to the conditions shown in FIG. 3 using the equipment shown in FIG. did. As a comparison, when a magnetic field was not applied using a Ni-Cr-Fe alloy mold (Comparative Example 1),
The same continuous casting was performed in the case where a static magnetic field was applied to the upper and lower portions in the mold (Comparative Example 2) and the case where induction heating was performed in the upper portion of the mold (Comparative Example 3). The respective execution conditions are as shown in Table 2.

[0018]

[Table 1]

[0019]

[Table 2] As shown in Table 1, Ni-Cr-Fe alloys or Ni-Cr alloys, which are particularly suitable as a casting material for continuous casting in the method of the present invention.
Cr-Co alloy has an electric conductivity of 8 to 9 × 10 ⁵ Ω ⁻¹ m ^−1.
It can be seen that this is nearly two orders of magnitude lower than that of the conventional mold material.

FIG. 4 shows the result of measurement of the temperature rise of the molten steel in the meniscus portion in each continuous casting as an index when the temperature rise in the example of the present invention is set to 1. From the figure, it is understood that the example of the present invention is not much different from Comparative Example 3 in which induction heating is performed.

Next, assuming that the fluctuation of the molten metal level in this case is 1 as the measurement result of Comparative Example 1, as shown in FIG. 5, the fluctuation of the molten metal level in Comparative Example 3 is increased. In the example, the level change of the molten metal level is extremely low.

The slab 10 obtained by each continuous casting is used.
Each one, hot rolling, then subjected to cold rolling, the results of measuring the number of streaky flaws, so-called sliver flaws, appeared on the steel sheet surface after rolling, the measurement results of Comparative Example 1 as 1, As shown in FIG. From the figure, it can be seen that although Comparative Example 3 is smaller than Comparative Example 1, the reduction as compared with the present invention is not achieved.

Further, the results of the investigation on the number of inclusions inside the slab were taken assuming that the measurement result of Comparative Example 1 was 1.
As shown in FIG. From the figure, it can be seen that the number of inclusions is significantly reduced in the example of the present invention.

In addition, as shown in the above figures, Inconel 718 (Ni-Cr) having a material with low electric conductivity and high hot strength
-Fe alloy) and RENE41, UDIMET700, Waspaloy (Ni-
Cr-Co alloys) were shown to be equally excellent mold materials.

[0025]

As described above, according to the present invention, the molten steel surface is induction-heated by superimposing a static magnetic field and a high-frequency magnetic field in the upper magnetic field over the entire width of the mold, and the molten steel surface is thereby disturbed. The surface quality of the slab can be improved, and the static magnetic field component of the lower magnetic field dampens and disperses the molten steel flow to promote the floating separation of inclusions and air bubbles. can do.

[Brief description of the drawings]

FIG. 1 is a schematic view of equipment used in the method of the present invention.

FIG. 2 is a schematic view showing another embodiment of the equipment used in the method of the present invention.

FIG. 3 is a graph showing a time change of a magnetic field intensity.

FIG. 4 is a graph showing a measurement result of a temperature rise on a molten steel surface.

FIG. 5 is a graph showing a measurement result of a molten metal level fluctuation amount.

FIG. 6 is a graph showing the measurement results of the number of occurrences of sliver flaws.

FIG. 7 is a graph showing a measurement result of the number of inclusions in a slab.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Continuous casting mold 4 Upper magnetic pole 1a Short side wall 5 Lower magnetic pole 1b Long side wall 6 Discharge jet 2 Immersion nozzle 7 Branch flow 3 Discharge port

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-293620 (JP, A) JP-A-5-154621 (JP, A) JP-A-5-154620 (JP, A) JP-A-5-154620 96351 (JP, A) JP-A-5-77007 (JP, A) JP-A-3-142049 (JP, A) JP-A-2-284750 (JP, A) JP-A-2-117756 (JP, A) JP-A-57-17356 (JP, A) JP-A-56-68565 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115 B22D 11/04 311 B22D 11 / 11

Claims (2)

(57) [Claims] 1. A magnetic field is generated by magnetic poles disposed on the back side of an opposite side wall of a continuous casting mold, and the magnetic field controls the flow of molten steel supplied from an immersion nozzle into the mold to perform continuous casting of steel. An upper magnetic field that generates the magnetic field in the entire width direction of the continuous casting mold above the discharge port of the immersion nozzle and a lower magnetic field that generates the magnetic field in the entire width direction of the continuous casting mold below the discharge port of the immersion nozzle And the upper magnetic field is superimposed with a static magnetic field and a high-frequency magnetic field.
A continuous casting method for steel using a magnetic field, characterized in that a static magnetic field is used as the lower magnetic field, and the directions of the upper static magnetic field and the lower static magnetic field are opposite to each other. 2. The method according to claim 1, wherein the continuous casting mold is made of a Ni—Cr—Fe alloy or a Ni—Cr—Co alloy.