JPH0729186B2 - Corner defect prevention method of thin slab slab in belt caster - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for preventing a corner defect of a thin slab cast piece in a belt caster, and particularly to a cast piece corner portion caused by narrowing of a solidified shell on the short side. The present invention aims to effectively prevent surface defects in the above.

(Prior Art) As a so-called belt caster for producing a thin slab cast piece having a thickness of about 30 mm directly from a molten metal, there is an apparatus as shown in FIG. In the figure, the numbers 1 and 1'are arranged opposite to each other, and a pair of movable belts that rotate around in synchronism with the withdrawal of the slab, 2 and 2'at both side edges of these belts 1 and 1 '. A pair of fixed short sides that are in close contact with the belt, and these form a casting space. In addition, 3, 3'is a movable belt
A cooling pad arranged on the back surface of 1,1 ', and a mechanism for cooling the movable belt 1,1' and then the molten steel 5 with cooling water 4 from the cooling pad 3,3 'to promote its solidification. It has become. In addition, 6 is a pouring nozzle, 7 is a thin slab cast piece, and 8 is a roll.

Here, the thickness of the thin slab in the casting space gradually decreases in the process of casting from the meniscus portion as shown in FIGS. 7 (a), (b), (C) and (d), and finally, The continuous casting method as described above is called a narrowing method because the thickness can be reduced to a thickness determined by the belt interval. In the figure, the number 9 is the narrowing-down area.

By the way, in the above-described narrowing-type continuous casting method, in the short side portion, the shell that started solidification immediately below the meniscus is gradually narrowed in its growth process, so that the shell deforms inward, and the molten steel enters the space after this deformation. It is known that corner surface defects occur in thin slab slabs as a result of repeated intrusion phenomena.

As a solution to the above-mentioned problem, for example, as shown in FIG. 8, a method of preventing the shell from growing and thus preventing solidification by covering the portion of the inner surface of the short side which comes into contact with the molten steel with a refractory material 10 (JP-A-58-218349) or a method of preventing solidification by resistance heating the inner surface of the short side with a nichrome wire or the like (JP-A-58-218353 and JP-A-58-21).
No. 8356) is proposed.

(Problems to be Solved by the Invention) However, since the short-side substrate is made of copper, and the inside thereof is water-cooled, even if the above-mentioned measures are taken, the solidification in the narrowed portion is still complete. It was impossible to prevent.

The present invention advantageously solves the above problems, and effectively prevents solidification in the narrowed portion of the inner surface of the short side, and can completely avoid the occurrence of surface defects in the corners of the cast slab. The purpose of the present invention is to propose a method for preventing corner defects of thin slab slabs.

(Means for Solving the Problem) That is, the present invention is directed to a pair of movable belts that are arranged opposite to each other and that rotate around in synchronism with the withdrawal of a cast piece, and the belts on both side edge portions of the belt. When casting a thin slab slab using the upper and lower narrowed belt casters that form a casting space with a pair of fixed short sides that are closely arranged, an induction coil placed on the back side of the short side of the narrowed pouring part. By interlinking the AC magnetic flux in the molten steel near the wall surface of the short side, an induced current is generated in the molten steel in the direction opposite to the direction of the current flowing in the induction coil. The electromagnetic force is generated in the molten steel by the action, and the electromagnetic force causes the molten steel near the short side to be cast while holding it without contacting the wall surface of the short side. It is a defect prevention method.

The present invention will be specifically described below.

FIG. 1 shows a perspective view of a short side having an induction coil attached to the back surface, and FIG. 2 shows a view taken along the line DD of FIG. 1, in which reference numeral 11 is an iron core and 12 is a coil. The induction coil 12 is set on the surface of the short side 2 opposite to the surface in contact with the molten steel from the upper end of the short side to the position where the molten steel is gradually narrowed to a desired thickness.

(Operation) When an alternating current is applied to the coil 12 from the power source 13, the magnetic flux passes through the short side 2 to the molten steel 5 as shown in FIG.
14 interlink. Now, assuming that the direction of the current flowing through the coil at a certain moment is as shown by the numbers 15 and 16 in FIG. 2, the direction of the current flowing through the coil from the law of electromagnetic induction due to the magnetic field interlinking with the molten steel 5. An electric current in the direction 17,18 opposite to that of 15,16 is generated in the molten steel.

In addition, the magnetic flux 14 reaching the molten steel surface 20 contacting the short side wall surface 19
Is attenuated according to an attenuation curve (see FIG. 3) determined by the electric conductivity σ of the molten steel, the magnetic permeability μ, and the frequency f of the current flowing through the coil. Therefore, the magnitude of the current density due to the induced current in the molten steel is As indicated by, in the figure, it becomes larger as it approaches the short side wall surface, and becomes smaller as it goes inward (indicated by the size of in FIG. 4).

Here, the electromagnetic force generated by the dielectric current in molten steel is
As shown in FIGS. 5 (a) and 5 (b), since the direction of the current is the same in the molten steel, a pinch force (contraction force) 21 is generated, while the magnetic flux (flux density) interlinking with the coil current and the molten steel A repulsive force 22 is generated between the short side and the molten metal due to the current flowing through. The magnitudes of the pinch force 21 and the repulsive force 22 are the largest on the molten steel surface in contact with the short side wall surface, and gradually decrease toward the inside of the molten steel.

Due to the action of the electromagnetic force as described above, the molten steel 5 has a short side wall surface.
It will be pushed inward from 19 and will not contact the wall surface, and the formation and growth of the shell will start immediately below the coil where electromagnetic force does not act. Therefore, if the coil 12 is installed in a range covering the entire narrowed portion 9 on the short side, solidification of molten steel in the narrowed portion 9 can be advantageously avoided.

As described above, by applying an alternating current to the coil installed on the back surface of the short side and generating an electromagnetic force on the molten steel, contact between the short side and the molten steel in the narrowed portion is advantageously avoided, so It is possible to completely prevent the surface defect in the corner portion of the thin slab slab, which may be caused by the shell grown in the narrowed portion.

(Example) Belt caster shown in Fig. 6 above-Roll diameter: 300 mm-Roll body length: 1.5 m-Belt thickness: 2 mm, casting speed: 10 mpm, thickness: 50 mm, width: 1.0 m thin slab A slab was produced.

At this time, an iron core and an induction coil as shown in FIG. 1 were installed on the back surface of the short side of the belt caster, and an alternating current of about 50 Hz was applied during the casting period.

No surface defects at the corners were observed in the thin slab cast thus obtained.

(Effects of the Invention) Thus, according to the present invention, conventionally, in the production of a thin slab cast using a belt caster, in the corner portion of the thin slab cast that was feared to occur due to the shell grown in the narrowed portion. Surface defects can be completely prevented.

Therefore, it is possible to reduce the strip edge cutting process in the slit line, which is conventionally required to remove the surface defect of the corner portion, and it is possible to reduce the manufacturing unit consumption and improve the product yield.

In addition, it is possible to completely prevent the breakout due to the boiling water on the short side.

[Brief description of drawings]

FIG. 1 is a perspective view of a short side with an induction coil attached to the back surface, FIG. 2 is a view taken in the direction of arrows D-D in FIG. 1, FIG. 3 is an attenuation curve diagram of magnetic flux density in molten steel, FIG. 4 is an explanatory diagram of an induced current generated in molten steel, FIGS. 5 (a) and 5 (b) are explanatory diagrams of an electromagnetic force generated in molten steel, and FIG. 6 is a perspective view of a typical belt caster. Figures 7, (a), (b), (c) and (d) show changes in the thickness of the short side in the narrowed portion, and Figure 8 shows the molten steel contact portion on the inner surface of the short side being refractory. It is a perspective view of the fixed short side covered with the thing. 1,1 '... Movable belt, 2,2' ... Fixed short side 3,3 '... Cooling pad, 4 ... Cooling water 5 ... Molten steel, 6 ... Pouring nozzle 7 ... Thin slab slab, 8 ... Roll 9 … Narrowed area, 10… Refractory 11… Iron core, 12… Induction coil 13… Power supply, 14… Magnetic flux 15, 16… Direction of current in coil 17, 18… Direction and distribution intensity of induced current in molten steel 19… Short side Wall surface, 20… Molten steel surface 21… Pinch force, 22… Repulsion force

Claims (1)

[Claims] 1. A pair of movable belts which are arranged opposite to each other and which rotate in synchronism with the withdrawal of a cast slab, and a pair of fixed short belts which are closely arranged with respect to the belt at both side edge portions of the belt. When casting a thin slab slab using the upper and lower narrowed belt casters that form the casting space with the sides,
By inducing an alternating current flux in the molten steel near the wall surface of the short side by an induction coil arranged on the back of the short side of the narrowed pouring part, an induced current in the direction opposite to the direction of the current flowing through the induction coil in the molten steel is generated. Then, an electromagnetic force is generated in the molten steel due to the interaction between this induced current and the alternating magnetic flux, and this electromagnetic force causes the molten steel near the short side to be cast while being held without contacting the short side wall surface. A method for preventing a corner defect of a thin slab slab in a belt caster.