JPH07100595A - Method for continuously casting thin cast slab - Google Patents

Abstract

PURPOSE:To uniformize the solidified shell and to prevent the surface defect in a thin cast slab by dipping a turnover flow preventing plate into gap between a flat nozzle and a mold wall surface. CONSTITUTION:Molten steel 2 in a tundish 1 is controlled by sliding nozzle devices 3 and poured from nozzles 4 for supplying the molten steel. The turnover flow preventing plates 7 are dipped into the position near a meniscus 5 between respective short sides 8 and nozzles 4 for supplying the molten steel. This dipping depth is set so as to satisfy a prescribed condition by a positional adjusting device 6 of the turnover flow preventing plate 7. The turnover flow preventing plate 7 may be made of a material having refractoriness to molten steel, e.g. a refractory brick material or a ceramic is desirable. By this method, the turnover of a spouting flow in the mold is restrained and the shell at <=10mm the shell thickness can be uniformized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting thin slabs, and more particularly, to suppress the generation of a reverse flow of molten steel discharge flow generated in a mold to homogenize a solidified shell and to crack a slab surface. Regarding how to prevent.

[0002]

2. Description of the Related Art Conventionally, a continuous casting method for thin slabs has been attracting attention from the viewpoint of energy saving and cost saving, and the development thereof has been actively promoted and the practical application has been studied. This is because the thin slab enables the conventional hot rolling to be omitted, the continuous casting process is directly connected to the rolling process, and the most ideal thin steel plate manufacturing process ever achieved is realized. .

Usually, in this continuous casting method for thin cast pieces, molten steel is stored in a ladle from a steelmaking furnace, conveyed to a continuous casting machine, and temporarily stored in a tundish which is an intermediate container.
From here, it is poured into a continuous casting mold. An immersion type nozzle has been conventionally used as a pouring nozzle for this mold.
The appearance shape of this immersion type nozzle is cylindrical or rectangular, but in the case of a thin cast piece, a rectangular nozzle (flat nozzle) is generally used because it is a flat mold.

In this immersion type nozzle, it is important to supply a molten steel to the mold while maintaining a constant discharge flow rate during casting, and therefore the nozzle shape has been actively developed. However, along with the recent demand for higher casting speed, it is also necessary to develop technology that pays attention to shell attack by the discharge flow in order to stabilize the solidified shell. For example, as a publicly known technique in this field, Japanese Patent Laid-Open No. 1-293942
Japanese Patent Publication discloses a method of improving the molten steel supply in the center of the width by disposing a flat nozzle. In addition, JP-A-63
No. 177945 discloses a method of providing a shielding plate in the width direction of a thin cast piece. The former is to prevent the double surface of the slab, and is not to control the reversal flow due to the discharge flow to improve the vertical crack of the slab. In the latter case, such a shielding plate cannot be inserted when the thin cast piece is extremely thin.

Molten steel supplied from a normal immersion type nozzle becomes a discharge flow in the mold and forms a reverse flow in the space with the wall surface of the mold. This inversion flow bends the fast molten steel main stream to attack the solidified shell, which hinders the growth of the shell and promotes nonuniformity. Recently, it has been discovered that this causes vertical cracks in the cast slabs, and as this has been clarified, it has become clear that the control of this reversal flow greatly affects the improvement of the vertical cracks in the cast slabs. It was In addition to the demand for higher quality in products, there has been a demand for improvement of slab quality, and in particular, measures against slab vertical cracking.

[0006]

SUMMARY OF THE INVENTION The present inventors have clarified the mechanism of generation of reversal flow due to the discharge flow of a rectangular nozzle in a continuous casting method for thin cast pieces, for the purpose of solving the above conventional problems, A pouring method that maintains a uniform discharge flow without attacking the shell was investigated. That is,
In normal pouring, a swirling flow of molten steel is generated in the space between the mold wall surface and the nozzle, and this becomes a reverse flow having a vector opposite to that of the molten steel discharge flow. Once this occurs, the reversal flow itself and the downward discharge flow are bent by the moment of the reversal flow and collide with the shell of the mold wall, resulting in uneven growth of the shell, which causes longitudinal cracking of the slab. Cause.

According to the present invention, it is possible to prevent the generation of this reverse flow, to eliminate the influence of the downward main flow having a high velocity on the discharge flow, to make the shell uniform, and to prevent the vertical crack of the slab. A continuous casting method for thin cast pieces is provided.

[0008]

Means for Solving the Problems The present invention is to solve the above problems, and the gist thereof is (1)
In the continuous casting method of a thin cast piece in which molten steel is poured by a flat nozzle that forms a space between a mold wall surface and a flat nozzle, the flat nozzle is provided in a space formed between the flat nozzle near the meniscus and the mold wall surface. A thin cast piece characterized in that a pair of opposed reversal flow prevention plates are immersed close to both short sides of the shell to suppress molten steel flow and to make the shell uniform in a portion where the shell thickness is 10 mm or less. Is a continuous casting method of

(2) Immersion depth L of the reverse flow prevention plate
Is the distance between the nozzle and the short side A−50 ≦ immersion depth L ≦ (10
/ K) ^{1 / n} V (mm), where k is the solidification coefficient, V is the casting rate, and n is a constant, and the relationship of reverse flow prevention plate thickness ≤ mold thickness -20 (mm) is satisfied. (1) The method for continuously casting thin slabs, (3) The method for continuously casting thin slabs according to (1) or (2), wherein the reverse flow prevention plate has a heating device. ) The continuous casting method for a thin slab according to any one of (1) to (3), wherein the reverse flow prevention plate has heat retaining or heat insulating properties. .

[0010]

The operation of the present invention will be described below. The present invention pays attention to a discharge flow of molten steel supplied from an immersion type nozzle in a mold, finds that a reverse flow is formed in a space between a mold wall surface and a nozzle, and realizes a means for controlling this. Is. As the reversal flow is generated closer to the meniscus, the properties of the slab are greatly affected. Further, the tendency of the fast main stream of molten steel to be bent and attack the solidified shell also has a greater effect in the region where the shell is thinner. The inventors of the present invention can prevent vertical cracking of the slab caused by such a direct reversal flow by controlling the space size between the immersion depth of the nozzle and the mold wall surface. I found it. In other words, it has been found that it is the greatest requirement that the reverse flow enters this space.

That is, at a normal immersion depth, a swirl of molten steel is generated in the gap space between the mold wall surface and the nozzle,
This becomes a reverse flow having a vector opposite to that of the molten steel discharge flow, and the downward main discharge flow is bent by the moment of the reverse flow and collides with the shell on the mold wall surface. In order to prevent this, it is essential to immerse the reverse flow prevention plate in the space between the nozzle and the wall surface of the mold to straighten the main flow of the discharge flow. The present invention achieves that the reverse flow prevention plate is placed in the space where the reverse flow is generated to rectify the discharge flow, and the reverse flow is shielded to eliminate the influence on the downward discharge flow of the reverse flow.

FIG. 6 shows a conventional reverse flow situation. In this figure, the reverse flow 15 is generated and the main flow 16 of the discharge flow is generated.
And the shell attack is generated by the reverse flow and the bent main flow. On the other hand, in the present invention, the reverse flow prevention plate 7 is immersed in the position where the reverse flow is generated, the discharge flow is rectified in order to suppress the generation of the reverse flow, and the generated reverse flow is shielded. To prevent. The immersion depth of the reverse flow prevention plate 7 defines the minimum immersion depth from the meniscus so that the immersion depth L ≧ nozzle-short piece distance A-50 (mm) is satisfied. The reason for this limitation is that the immersion depth is the distance between the nozzle and the short piece A-50.
When it is less than (mm), a reverse flow is generated, and there is no effect of blocking the reverse flow, and the main flow of the discharge flow is narrowed by the reverse flow and a shell attack occurs.

Regarding the requirements of the present invention, the case where the number of nozzles is one will be described here, but the same effect can be obtained even when the number of nozzles is N, and the same effect can be obtained. The number of nozzles is not limited. FIG. 1 shows the situation of the discharge flow in the case of one flat nozzle (rectangular nozzle) of the present invention. That is, when the immersion depth L ≧ nozzle-short side distance A−50 (mm) is satisfied, no reversal flow is generated and therefore the main flow 13 is not affected. As a result, the main flow 13 of the discharge flow exhibits a wide flow velocity distribution, and the reversal flow is completely shielded by the reversal flow prevention plate 7 so that shell attack does not occur.

Further, in the present invention, the immersion depth L≤ (10 /
k) ^{1 / n} V (mm), where k is the solidification coefficient, V is the casting speed, and n is a constant, the following relationships are satisfied, but this does not affect the shell attack when the shell thickness is 10 mm or more. It is based on that. Hereinafter, the method of the present invention will be described in more detail with reference to the drawings of the embodiments of the present invention.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The schematic construction of the apparatus of the present invention will be described with reference to FIGS. 2 shows a rectangular nozzle 1
In the case of using a book, the molten steel 2 of the tundish 1 is controlled by the sliding nozzle device 3 and is poured from the molten steel supply nozzle 4. The reverse flow prevention plate 7 is immersed in the vicinity of the meniscus 5 between the short side 8 and the molten steel supply nozzle 4. The immersion depth is set by the position adjusting device 6 of the reverse flow prevention plate 7 so as to satisfy the immersion depth requirement of the present invention. It should be noted that the reverse flow prevention plate 7 of the present invention may be a substance having a refractory degree to molten steel, and for example, refractory brick or ceramics is preferable.

FIG. 3 shows a case of casting with two nozzles. FIG. 4 is a side view of the device of the present invention. The reverse flow prevention plate 7 is attached to the tip of the rod 16, and when the number of nozzles is two, the reverse flow prevention plate 7 is set to 1 between both nozzles.
A method of attaching to the rod 16 of the book in a portal shape with a space formed in the intermediate portion is advantageous. In this case, the gate-shaped double reversal flow prevention plate 7 prevents the reverse flow of both nozzles.
Further, the rod 16 is attached to the arm 15 and is rotatable up and down and left and right. In addition, it is preferable that the reverse flow prevention plate 7 can stand by in a state in which it is rotated around the continuous casting machine by the arm 15 and the cylinder 14 and escaped when it is not in operation.

In the present invention, the effects of one nozzle and two nozzles are almost the same, and it can be said that the effects are basically the same even if the number of nozzles is increased. Next, an example of the work procedure of the device of the present invention will be described. Although it is waiting at the start of the first continuous casting, it is heated to a predetermined temperature during this period as needed. With the start of casting, at the appropriate time, the cylinder is lowered, and a predetermined position is determined by a stopper device or the like to complete the setting of the reverse flow prevention plate in the mold. This stopper device may be installed at a plurality of positions on the arm and the cylinder, if necessary.

As another embodiment of the device of the present invention, the rod 16 to which the reverse flow prevention plate 7 is fixedly supported may be directly attached to and suspended from the tundish 1, and in this case, it is always It will move together with the tundish. Further, since the reverse flow prevention plate of the device of the present invention is immersed, there is a risk of the influence of the temperature drop of the molten steel in the vicinity of the meniscus, and if this temperature drop occurs, a phenomenon commonly referred to as Kawai tension occurs, which causes another phenomenon. The cause of defects may occur. In order to prevent this, in the present invention, a device having a heating and / or heat retaining / insulating function is provided inside or outside the reverse flow prevention plate, and the reverse flow prevention plate can be maintained at a predetermined temperature before insertion. It is necessary to prevent the temperature drop of molten steel due to the insertion of the reverse flow prevention plate as much as possible.

FIG. 5 shows the results of investigations on the occurrence of surface defects on thin cast pieces in the case of using one nozzle in the embodiment of the present invention. As casting conditions, steel type (medium carbon steel, low carbon steel), size (600 to 1800 mm W x 30 to 120 mm t), nozzle (300 to 800 mm W x 10 to 100 mm t), depth ( 50 to 300 mm), size (100 to 400 mm W x10
~ 100 mm t) was used. From this figure, it is shown that the embodiment of the present invention shows a remarkable effect and has a good surface defect level as compared with the surface defect index of the cast piece in which the reverse flow of the conventional method is generated. As described above, according to the present invention, the discharge flow is rectified, and the generated reversal flow is shielded. As a result, the surface defects of the thin slab are extremely reduced as compared with the conventional generation of the reversal flow. You can see that it will be improved.

[0020]

As described above, according to the present invention, it is possible to suppress the reverse flow due to the discharge flow in the mold or to eliminate the reverse flow, thereby improving the uniform growth of the solidified shell, (EN) It is possible to provide a continuous casting method that makes it possible to obtain a thin slab that prevents surface defects that have occurred in the thin slab.

[Brief description of drawings]

FIG. 1 is a schematic view showing a situation of a discharge flow and a reverse flow in a mold according to the present invention.

FIG. 2 is a schematic view showing the immersion state when using one rectangular nozzle according to the embodiment of the present invention.

FIG. 3 is a schematic view showing the immersion state when using two rectangular nozzles according to an embodiment of the present invention.

FIG. 4 is a schematic view showing a fixed state of a reverse flow prevention plate according to an embodiment of the present invention.

FIG. 5 is a graph showing measurement results of surface defects in thin cast pieces of an example of the present invention and a conventional method.

FIG. 6 is a schematic view showing a situation of a discharge flow and a reverse flow in a mold of a conventional method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Tundish 2 ... Molten steel 3 ... Sliding nozzle device (flow rate control device) 4 ... Molten steel supply nozzle 5 ... Meniscus 6 ... Position adjusting device 7 ... Reverse flow prevention plate 8 ... Short side 9 ... Thin cast piece 10 ... Cylinder 11 ... Arm 12 ... Rod 13 ... Main flow of discharge flow 14 ... Controlled reverse flow 15 ... Reverse flow 16 ... Main flow of discharge flow

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takemasa Ono 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd. Technical Development Division (72) Inventor Satoshi Horioka 1-chome Nishinosu, Oita-shi, Oita Made in Japan Oita Steel Works, Ltd.

Claims (4)

[Claims] 1. A method for continuously casting a thin slab, in which molten steel is poured by a flat nozzle which forms a space between a wall surface of a mold and a mold, and a space formed between the flat nozzle near the meniscus and the wall surface of the mold. In the above, a pair of opposed reverse flow prevention plates are soaked in proximity to both short sides of the flat nozzle to suppress molten steel flow and to make the shell uniform in a portion where the shell thickness is 10 mm or less. Continuous casting method for thin cast pieces. 2. The immersion depth L of the reverse flow prevention plate is such that the distance between the nozzle and the short side is A-50 ≦ immersion depth L ≦ (10 / k).
^{1 / n} V (mm), where k is the solidification coefficient, V is the casting speed,
The method for continuously casting thin cast pieces according to claim 1, wherein n is a constant, and the following relationship is satisfied, and the thickness of the reverse flow prevention plate ≦ the mold thickness−20 (mm). 3. The continuous casting method for thin cast pieces according to claim 1, wherein the reverse flow prevention plate comprises a heating device. 4. The method for continuously casting thin cast pieces according to claim 1, wherein the reverse flow prevention plate has heat retaining or heat insulating properties.