JP3100497B2 - Continuous casting of thin slabs - 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 for thin slabs, and more particularly to a method for suppressing the occurrence of a reversal flow of a molten steel discharge flow generated in a mold so as to make a solidified shell uniform and to crack the slab surface. On how to prevent.

[0002]

2. Description of the Related Art Conventionally, a continuous casting method of thin cast slabs has attracted attention from the viewpoint of energy saving and cost saving, and has been actively developed and studied for practical use. This is because the thin cast slab enables the simple omission of the conventional hot rolling, and the continuous casting process is directly connected to the rolling process, so that the most ideal thin steel sheet manufacturing process that has never been realized is realized. .

[0003] Usually, in this continuous casting method of thin slabs, molten steel is stored in a ladle from a steelmaking furnace, transported to a continuous casting machine, and temporarily stored in a tundish as an intermediate container.
From here it is poured into a continuous casting mold. An immersion type nozzle has conventionally been used as a pouring nozzle for the mold.
The external shape of the 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 molten steel to a mold while maintaining a constant discharge flow rate during casting. For this reason, the shape of the nozzle has been actively developed. However, with the recent need for higher casting speeds, it is also necessary to develop a technology that focuses on the shell attack by the discharge flow in order to stabilize the solidified shell. For example, as a known technique in this field, Japanese Patent Application Laid-Open No. 1-293942.
There is disclosed in Japanese Patent Application Laid-Open Publication No. HEI 9-15064 a method of improving the supply of molten steel at the center in the width by disposing a flat nozzle. Also, JP-A-63
JP-177945 discloses a method of providing a shielding plate in the width direction of a thin slab. The former prevents double slabs of the slab, but does not control the reversal flow due to the discharge flow to improve the vertical cracks in the slab. In the latter case, when the thickness of the thin cast piece is extremely thin, such a shielding plate cannot be inserted.

[0005] 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 mold wall surface. Due to this reversal flow, the fast main flow of molten steel is bent and attacks the solidified shell, where the growth of the shell is inhibited and unevenness is promoted. Recently, it has been found that this causes vertical cracks in the slab, and as this clarification progresses, it has been found that the control of this reversal flow has a great effect on the improvement of the vertical cracks in the slab. Was. In addition to the demand for higher quality of products, improvement of cast slab quality, particularly measures against vertical slab cracks have been desired.

[0006]

SUMMARY OF THE INVENTION In order to solve the above-mentioned conventional problems, the present inventors have clarified the mechanism of generation of a reversal flow due to a discharge flow of a rectangular nozzle in a continuous casting method of a thin slab. A pouring method to maintain a uniform discharge flow without causing an attack on the shell was studied. That is,
In ordinary pouring, a vortex of molten steel is generated in a gap space between the mold wall surface and the nozzle, and this becomes a reverse flow having a vector in the opposite direction to the molten steel discharge flow. Once this occurs, the reversing flow itself and the downward discharge flow are bent by the moment of the reversing flow and collide with the shell on the mold wall, resulting in non-uniform shell growth, which leads to vertical cracks in the slab. Cause.

The present invention prevents the occurrence of this reverse flow, eliminates the influence of the main flow having a high downward speed on the discharge flow, achieves a uniform shell, and prevents vertical cracks in the slab. To provide a continuous casting method for thin cast slabs.

[0008]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and its gist is as follows:
In a continuous casting method of a thin slab in which molten steel is poured by a flat nozzle which forms and immerses a space between a mold wall and a flat nozzle, the flat nozzle is provided in a space formed between the flat nozzle near the meniscus and the mold wall. A thin cast slab characterized by immersing a pair of opposing reverse flow prevention plates in the vicinity of both short sides to suppress molten steel flow and to make the shell uniform at a portion having a shell thickness of 10 mm or less. Continuous casting method,

(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 speed, n is a constant, and the thickness of the reverse flow prevention plate ≦ the mold thickness−20 (mm). (3) The continuous casting method of thin slab according to (1) or (2), wherein (3) the reverse flow prevention plate comprises 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 retention or heat insulation properties. .

[0010]

The operation of the present invention will be described below. The present invention pays attention to the discharge flow of molten steel supplied from the immersion type nozzle in the mold, finds out that a reverse flow is formed in the space between the mold wall surface and the nozzle, and realizes a means for controlling this. It is. The more this reversal flow occurs near the meniscus, the more greatly it affects the slab properties. In addition, the tendency of the rapid flow of molten steel to bend and attack the solidified shell has a greater effect in a region where the shell is thinner. The present inventors have found that in order to prevent such vertical cracks in the slab caused by the direct reversal flow, it can be achieved by controlling the space depth between the nozzle immersion depth and the mold wall surface. I found it. That is, it has been found that the greatest requirement is that the inversion flow enters this space.

That is, at a normal immersion depth, a vortex of molten steel is generated in the gap between the mold wall surface and the nozzle.
This is a reverse flow having a vector opposite to 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 mold wall to rectify the main flow of the discharge flow. In the present invention, a reverse flow prevention plate is placed in the space where the reverse flow is generated, the discharge flow is rectified, and the reverse flow is shielded to eliminate the influence on the downward discharge flow of the reverse flow.

FIG. 6 shows a state of a conventional reverse flow. In this figure, a reverse flow 15 is generated and the main flow 16 of the discharge flow is generated.
And the shell attack occurs due to 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. It is 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 ≧ the distance between the nozzle and the short piece A-50 (mm). The reason for this limitation is that the immersion depth is a distance between the nozzle and the short piece A-50.
If 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.

[0013] The requirements of the present invention will be described here for the case of one nozzle, but the same effect can be obtained by making the same requirement for the case of N nozzles. It does not limit the number of nozzles. FIG. 1 shows the state of the discharge flow in the case of one flat nozzle (rectangular nozzle) of the present invention. That is, when the immersion depth L ≧ the distance between the nozzle and the short side A−50 (mm) is satisfied, no reverse flow occurs, and therefore, there is no influence on the main flow 13. As a result, the main flow 13 of the discharge flow exhibits a spread flow velocity distribution, and the reverse flow is completely shielded by the reverse flow prevention plate 7, so that no shell attack occurs.

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, which satisfies the relationship, but this is not affected by the shell attack when the shell thickness is 10 mm or more. It is based on 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 A schematic configuration of an apparatus according to the present invention will be described with reference to FIGS. FIG. 2 shows a rectangular nozzle 1
In the case where a book is used, the molten steel 2 of the tundish 1 is controlled by a sliding nozzle device 3 and poured from a molten steel supply nozzle 4. The reverse flow prevention plate 7 is immersed near 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. The reverse flow prevention plate 7 of the present invention may be a substance having a degree of fire resistance to molten steel, and is preferably made of, for example, refractory bricks or ceramics.

FIG. 3 shows a case where casting is performed using 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. When two nozzles are provided, the reverse flow prevention plate 7 is provided between the two nozzles.
It is advantageous to use a method in which a space is formed at the intermediate part in a portal shape and attached to the rod 16 of the book. In this case, the double reverse flow prevention plate 7 prevents the reverse flow of both nozzles.
Further, the rod 16 is engaged with the arm 15 and is rotatable vertically and horizontally. In addition, it is preferable that the reverse flow prevention plate 7 be able to stand by in a state in which it is rotated near the continuous casting machine by the arm 15 and the cylinder 14 and escapes when 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 further increased. Next, an example of a work procedure of the apparatus of the present invention will be described. Initially, it is on standby at the start of continuous casting, during which time it is heated as necessary to a predetermined temperature. At the appropriate time when the casting is started, the cylinder is lowered by the cylinder, a predetermined position is determined by a stopper device or the like, and the setting of the reverse flow prevention plate in the mold is completed. This stopper device may be installed at a plurality of positions on the arm and the cylinder as necessary.

As another embodiment of the device according to the present invention, the rod 16 on which the reverse flow prevention plate 7 is fixedly supported may be of the type in which the rod 16 is directly engaged with and suspended from the tundish 1. It will move together with the tundesh. In addition, since the reverse flow prevention plate of the apparatus of the present invention is immersed, there is a concern that the temperature of the molten steel decreases in the vicinity of the meniscus, and if this temperature drop occurs, a phenomenon generally referred to as “kawa tension” occurs. The cause of the defect may occur. In order to prevent this, in the present invention, a device having a heating and / or heat insulation / insulation function is provided on the inner or outer surface of the reverse flow prevention plate so that the reverse flow prevention plate can be maintained at a predetermined temperature before insertion. In addition, it is necessary to prevent the temperature of the molten steel from being lowered by inserting the reverse flow prevention plate as much as possible.

FIG. 5 shows the results of a survey on the occurrence of surface defects in thin slabs in the case of one nozzle in the embodiment of the present invention. As casting conditions, steel type (medium carbon steel, low carbon steel), size (600-1800mm W x 30-120mm t), nozzle (300-800mm W x 10-100mm t), depth prevention ( 50-300 mm), size (100-400 mm W x10
100100 mm t) was used. This figure shows that the embodiment of the present invention has a remarkable effect and a good surface defect level as compared with the surface defect index of the slab where the reversal flow has occurred in the conventional method. As described above, according to the present invention, the discharge flow is rectified, and the generated reverse flow is shielded, so that the surface defects of the thin slab are extremely reduced as compared with the conventional generation of the reverse flow. It can be seen that it is improved.

[0020]

As described above, according to the present invention, it is possible to suppress or eliminate the reversal flow due to the discharge flow in the mold and improve the uniform growth of the solidified shell. It has been made possible to provide a continuous casting method capable of obtaining a thin slab in which surface defects occurring in the thin slab have been prevented.

[Brief description of the drawings]

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

FIG. 2 is a schematic view showing a dipping situation when one rectangular nozzle according to an embodiment of the present invention is used.

FIG. 3 is a schematic diagram showing a immersion situation when two rectangular nozzles according to an embodiment of the present invention are used.

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

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

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

[Description of Signs] 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 slab DESCRIPTION OF SYMBOLS 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

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takemasa Ohno 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Satoshi Horioka No. 1 Nishinosu, Oaza-shi, Oita-shi, Oita New Japan (56) References JP-A-3-66450 (JP, A) JP-A-6-246402 (JP, A) JP-A-6-190511 (JP, A) JP-A-6 -142852 (JP, A) JP-A-3-207557 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/103 B22D 11/06 340 B22D 11/10 330

Claims (4)

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