JPH07171663A - Continuous casting method for thin slabs - Google Patents

Abstract

(57) [Abstract] [Purpose] In the continuous casting method for thin slabs, pouring to prevent the reverse flow in the mold due to the molten steel discharge flow to prevent the main flow of the discharge flow from attacking the solidification shell. Provide a way. According to the present invention, the immersion depth of a flat nozzle is determined by geometric conditions such as a mold width, a nozzle width, and a distance between a nozzle and a mold short side, and a reverse flow of a reverse flow is suppressed in order to suppress a shell attack due to a discharge flow. The moment is minimized, and the nozzle immersion depth L ≧ [(nozzle-mold short piece distance A) +110] × 0.45 (mm) is satisfied, and the shell thickness is 10 mm. By so doing, the conventional phenomenon in which the molten steel main flow is narrowed by the reversing flow eliminates the influence of the reversing flow, has a tendency that the molten steel main flow spreads, and does not generate shell attack.

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 slabs, molten steel is stored in a ladle from a steelmaking furnace and conveyed to a continuous casting machine where it is temporarily stored in a tundish, which is an intermediate container, and then poured into a continuous casting mold. It will be hot water. 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, since it is a flat mold, a rectangular nozzle (flat nozzle) is generally used.

In this immersion type nozzle, it is important to supply 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 has become necessary to develop a technology that focuses on shell attack by the discharge flow in order to stabilize the solidified shell. For example, as a known technique in this field, Japanese Patent Laid-Open No. 1-29393
Japanese Unexamined Patent Publication No. 42-42 discloses a method of improving the molten steel supply at the center of the width by disposing a flat nozzle. This is to prevent the double surface of the slab and not to control the reversal flow due to the discharge flow to improve the vertical crack of the slab.

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, especially for vertical cracking.

[0005]

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 happens, the downward discharge flow is due to the moment of the reverse flow,
It is constantly bent and collides with the shell on the wall of the mold, resulting in uneven growth of the shell, which causes longitudinal cracking of the slab.

According to the present invention, by controlling the position of the reversal flow, further reducing the position of the reversal flow, and minimizing the moment, the influence of the downward downward flow of the main flow on the discharge flow is eliminated. The present invention provides a continuous casting method for thin slabs, which achieves the above-mentioned object, uniformizes the shell, and prevents vertical cracking of the slabs.

[0007]

Means for Solving the Problems The present invention is intended to solve the above-mentioned problems, and the gist thereof is (1) a molten steel is formed by a flat nozzle which forms a space between a wall surface of a mold and the surface of the mold. In the continuous casting method for pouring thin slabs, the immersion depth from the meniscus of the flat nozzle is determined based on the relationship between the mold width, the nozzle width and the geometrical conditions of the nozzle position, etc. Flow is a continuous casting method for thin slabs, characterized by minimizing the moment of reversal flow generated in the space between the flat nozzle and the mold,

(2) The relationship with the geometrical conditions is that the immersion depth L ≧ [(nozzle-mold short piece distance A) +110] ×
(1) The continuous casting method for a thin slab according to (1), which satisfies 0.45 (mm), and (3) the immersion depth is 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 is the continuous casting method of the thin slab described in (1), and (4) the rectangular nozzle is a straight type or a downward discharge nozzle with a corner having a curvature ( It is a continuous casting method of the thin cast piece described in 1).

[0009]

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. It was found that the size and center position of the vortex, which is the reversal flow, are closely related to the distance between the nozzle and the short side of the mold, and the larger this distance, the larger the vortex diameter and the deeper the generation position.
The present inventors have found that in order to prevent such vertical cracking of the slab that is directly caused by the reverse flow, it is possible to achieve it 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.

This is because at a normal immersion depth, a swirling flow of molten steel is generated in the space between the mold wall surface and the nozzle, and this becomes a reversing flow having a vector opposite to that of the molten steel discharge flow, which is the downward main flow. The flow is bent by the moment of the reversal flow and collides with the shell on the wall of the mold. To prevent this, it is essential to limit the size of the space between the nozzle and the wall surface of the mold. The present invention achieves elimination of the influence on the downward discharge flow by controlling the generation position of this reversal flow, making it as small as possible, and minimizing its moment.

FIG. 3 is a diagram showing a conventional reverse flow situation. In this figure, the reverse flow 4 is generated to narrow the main flow 5 of the discharge flow, and the main flow narrowed by the reverse flow attacks the shell. On the other hand, in the present invention, the minimum immersion depth from the meniscus is such that the relationship with the geometric conditions satisfies the immersion depth L ≧ [(nozzle-mold short piece distance A) +110] × 0.45 (mm). To limit the size. The reason for this limitation is that when the immersion depth is less than [(nozzle-mold short piece distance A) +110] × 0.45 (mm), the main flow of the discharge flow is narrowed by the reversal flow and shell attack occurs. Here, the constant 110 means that the immersion depth of the nozzle is always required to be 50 mm or more in order to prevent the generation of wrinkles on the surface of the slab due to the disorder of the molten metal surface. Further, the requirement of the present invention exerts its effect in the case of one injection nozzle. FIG. 1 shows the situation of the discharge flow in the case of one rectangular nozzle of the present invention. That is, the immersion depth L ≧
[(Nozzle-mold short piece distance A) + 110] x 0.45
When (mm) is satisfied, the flow has moved to a position where the main flow 7 of the discharge flow is not affected even if the reverse flow 6 is generated. Therefore, the main flow 7 of the discharge flow does not collide with the vortex and the vortex is used in reverse. It exhibits a divergent flow velocity distribution, and as a result, 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. The effects will be described in more detail below with reference to the drawings of the embodiments of the present invention.

[0013]

FIG. 2 shows the result of applying the present invention to an actual machine. FIG. 2 shows the case where one rectangular nozzle is used, and the casting conditions are as follows. (A) Steel type: Medium carbon steel Nozzle width: 600 mm Cast piece width: 1300 mm Casting speed: 4 to 10 m / min (b) Steel type: Medium carbon steel Nozzle width: 300 mm Cast piece width: 600 mm Casting speed: 4 to 10 m / min

Under the above conditions, the immersion depth L and the distance A between the nozzle and the mold wall surface were set as follows as requirements of the present invention, and the effects were confirmed. (A) Immersion depth L: 50-300 mm Nozzle-short side mold distance A: 330 mm (b) Immersion depth L: 50-200 mm Nozzle-short side mold distance A: 180 mm

After continuously casting thin slabs set as described above, the obtained slab (a) width 1300 mm x thickness 75 m
m, (b) Width 600 mm × thickness 75 mm was visually inspected for vertical surface cracks, and the results are shown in FIGS. The vertical crack occurrence rate on the vertical axis of FIG. 4 is a slab width of 1300 m.
m is the total length of the vertical cracks generated per 1 m of the thin cast piece under the conditions of the injection nozzle width of 600 mm. The horizontal axis of FIG. 4A shows the immersion depth L of the rectangular nozzle.

In the case of single injection with a nozzle width of 600 mm, the surface vertical crack occurrence rate is 0.0 m / m at a dipping depth of 200 mm, but the surface vertical crack occurrence rate is 0.1 at a dipping depth of 150 mm.
It becomes 005 to 0.015 m / m, and further the immersion depth becomes shallow, and the surface vertical crack occurrence rate shows a tendency to increase. Immersion depth L ≧ [(nozzle to
From the mold short piece distance A) +110] × 0.45 (mm), in the case of this example, the immersion depth L = 200 mm, and it can be seen that in this example, good results are shown within the range of the present invention.

Further, in FIG. 4B, the horizontal axis of FIG. 4A is a value obtained by dividing the immersion depth L by [(nozzle-mold short piece distance A) +110] according to the requirements of the present invention. It is displayed. From this figure [(Nozzle-mold short piece distance A) +
110] is 0.45 or more, the surface vertical crack occurrence rate of the thin cast piece is 0.0 m / m. The vertical crack occurrence rate in FIG. 5 shows the results under the conditions of a nozzle width of 300 mm and a slab width of 600 mm. In FIG. 5 (a), as in the case of (a) above, the deeper the immersion depth is, the lower the vertical crack occurrence rate is. Also, at the immersion depth of 100 mm, the vertical crack occurrence rate is 0.0 m / m. Indicates that Further, FIG. 5B shows the result of rearranging the vertical crack occurrence rate by a value obtained by dividing the immersion depth L by [(nozzle-mold short piece distance A) +110] as in the case of the above (a). Yes, as in the case of (a), when [(nozzle-mold short piece distance A) +110] is 0.45 or more, the vertical crack occurrence rate is 0.0 m / m. That is, it can be seen from the results of Examples (a) and (b) that a good cast piece can be obtained in the condition range of the present invention.

[0018]

As described above, according to the present invention, the reverse flow due to the discharge flow in the mold can be suppressed, or the position where the reverse flow is generated can be suppressed, and the uniform growth of the solidified shell is improved. (EN) It is possible to realize a continuous casting method for thin slabs, which prevents surface vertical cracking that has occurred in thin slabs.

[Brief description of drawings]

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

FIG. 2 is the immersion depth L of a rectangular nozzle according to an embodiment of the present invention.
It is a figure which shows the distance A between a nozzle and a short side casting mold wall.

FIG. 3 is a diagram showing a situation of a discharge flow and a reverse flow in a conventional mold.

FIG. 4 is a diagram showing a relationship between an immersion depth L and a vertical crack occurrence rate according to an embodiment of the present invention, and (b) an immersion depth L /
It is a figure which shows the relationship between [(nozzle-mold short piece distance A) +110] and a vertical crack occurrence rate.

FIG. 5 is a diagram showing a relationship between an immersion depth L and a vertical crack occurrence rate according to an embodiment of the present invention, and (b) an immersion depth L /
It is a figure which shows the relationship between [(nozzle-mold short piece distance A) +110] and a vertical crack occurrence rate.

[Explanation of symbols]

 1 ... Mold short side 2 ... Meniscus 3 ... Rectangular nozzle 4 ... Reverse flow 5 ... Main flow of discharge flow 6 ... Controlled reverse flow 7 ... Main flow of controlled discharge flow

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Noriyuki Kanai No. 1 Nishinosu, Oita City, Oita Prefecture, Nippon Steel Co., Ltd. Inside the Oita Steel Works, Nippon Steel Co., Ltd. (72) Atsushi Ishikawa, No. 1 Nishinozu, Oita City, Oita Prefecture New Japan Oita Steel Works, Ltd. (72) Toshiyuki Kajitani, No. 1 Nishinosu, Oita City, Oita Prefecture Shin-Nippon Steel Co., Ltd. Oita Works

Claims (4)

[Claims] 1. A method of continuously casting a thin slab in which molten steel is poured by a flat nozzle that forms a space between a wall surface of a mold and a mold, and the immersion depth from the meniscus of the flat nozzle is defined as the mold width, It is characterized in that the molten steel discharge flow minimizes the moment of the reversal flow generated in the space between the flat nozzle and the mold by determining it based on the relationship between the nozzle width and the geometrical conditions of the nozzle position. Continuous casting method of slab. 2. The relationship with the geometrical conditions is that the immersion depth L ≧ [(nozzle-mold short piece distance A) +110] × 0.
The method for continuously casting thin cast pieces according to claim 1, which satisfies 45 (mm). 3. The immersion depth is such that the immersion depth L ≦ (10 /
k) ^{1 / n} V (mm), where k is a solidification coefficient, V is a casting speed, and n is a constant. 4. The continuous casting method for thin cast pieces according to claim 1, wherein the rectangular nozzle is a straight type or a downward discharge nozzle with a corner having a curvature.