JPH0833963A - Method for continuously casting thin cast slab - Google Patents

Abstract

PURPOSE:To enable the continuous casting to be performed uniformly while solidifying a thin cast slab without causing a longitudinal crack in the thin cast slab by uniformizing the extents of size of the eddies of four turnover flows developed at the time of pouring molten steel and making the turnover flows a confluent flow at the center of a flat nozzle. CONSTITUTION:In the casting of the thin cast slab by a twin belt type continuous casting machine, the molten steel 7 is poured into a mold 1 through two flat nozzles 9a, 9b. The two flat nozzles 9a, 9b are arranged in parallel to the long side of the mold 1 toward the short side in series aligning the thickness center of the nozzles to the thickness center of the mold 1 and the width center position (x) of the respective flat nozzles 9a, 9b and the immersion depth (y) into the molten steel 7 are set within the specific range. The extents of size 2R of respective four turnover flows A, B, C, D developed at the time of pouring the molten steel 7 are made equal to each other to make these turnover flows confluent flow at the center of the respective flat nozzles 9a, 9b, and the thin cast slab without longitudinal crack can continuously be cast.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixed mold type continuous casting method used in the production of thin slab slabs in the field of steel.
The present invention relates to a twin belt type continuous casting method.

[0002]

2. Description of the Related Art For example, in the field of iron and steel, attempts have been made to put a continuous casting inline hot finish rolling process into practical use from the viewpoints of cutting down heat consumption, omitting steps, improving productivity, and so on. There is an increasing demand for continuously casting thin slab slabs of 30 to 100 mm level.
In recent years, in order to meet such demands, in addition to the known fixed mold type continuous casting method, for example, Japanese Patent Laid-Open No. 1-25
A twin belt type continuous casting method as disclosed in Japanese Patent No. 4354 is also proposed.

In general, when a thin slab slab is continuously cast by such a continuous casting method, the gap between the nozzle and the mold is narrowed, so that the flow of molten steel causes the solidified shell to attack, resulting in It is known that a significant delay in solidification occurs and vertical cracks are likely to occur in a thin slab slab obtained by casting. In continuous casting of such a thin slab slab, a flat nozzle is used as the injection nozzle from the viewpoint of uniformly injecting molten steel into the mold, and as the slit length increases, structural durability and dimensional accuracy are sufficiently ensured. Since it can not be done, width 100 ~ 800 mm
The two flat nozzles may be arranged in a row in the mold width direction.

However, heretofore, there has been found no concrete disclosure of the optimum arrangement of nozzles when two flat nozzles are used. Therefore, conventionally, there is no definitive method capable of preventing the occurrence of vertical cracks in thin slab slabs obtained by continuous casting, and the current situation is that deterioration of the quality of thin slab slabs due to vertical cracks cannot be avoided.

[0005]

SUMMARY OF THE INVENTION The present invention optimally arranges two flat nozzles having a width of 100 to 800 mm having a downward slit in a mold and injecting molten steel into the mold to cause vertical cracking. Provided is a continuous casting method of a thin slab slab capable of obtaining a thin slab slab with a thickness of 30 to 100 mm.

[0006]

The first invention of the present invention is as follows:
In a continuous casting method of a thin slab slab using two flattened nozzles having downward slits as a molten steel injection nozzle, the size of the four reversal flow vortices generated during molten steel injection is made uniform and the reversal flow is flattened. By confluence at the center of the nozzle, uniform solidification, is a continuous casting method of thin slab slab characterized by preventing vertical cracking of the thin slab slab, the second invention is the first In the continuous casting method of a thin slab cast piece using two flat nozzles having downward slits as molten steel injection nozzles into a mold, Place the flat nozzle 2 toward the mold short side so that the thickness center of the nozzle matches the mold thickness center.
These are arranged in tandem, and each flat nozzle is soaked at a width center position from the short side of the mold to a position of ¼ ± 50 mm of the mold width W and at a depth of 30 mm to × 2/3 mm or less of the flat nozzle width. Thereby, four reverse flows generated at the time of pouring molten steel are generated, the sizes of the vortices are made uniform, and the reverse flows are merged at the center of the flat nozzle, which is a continuous casting method for a thin slab cast piece.

[0007]

In the present invention, the flat nozzle having the downward slit is used as the nozzle for injecting the molten steel into the mold.
By using this and optimally arranging it, the vortices of the four reverse flows generated during molten steel injection are made uniform, and the reverse flows are merged at the center of the flat nozzle, so that solidification can be made uniform. A thin slab slab of good quality without vertical cracks can be stably continuously cast. INDUSTRIAL APPLICABILITY The present invention is particularly suitable for continuous casting of thin-walled slabs having a thickness of 30 to 100 mm and is more suitable.

The present invention will be described below. The inventors of the present invention, for example, in the twin-belt continuous casting method or the fixed-type continuous casting method, which is attracting attention as a high-speed continuous casting method for thin slab slabs, obtain a thin slab slab with a thickness of 75 mm or less in the vertical direction. Since the occurrence of cracks was observed, various experiments and analyzes were repeated for the case of using two flat nozzles in order to know the mechanism of the occurrence.

In this experiment, the vortex generation mode (vortex number, vortex diameter) of the molten steel reversal flow was determined from the inclination of dendrite in the solidification structure of the cast slab and the direction of molten steel flow,
It is obtained by estimating the flow velocity. As a result of this experiment and analysis, it was confirmed that the vertical cracks of the thin slab slab were generated in relation to the vortex shape of the molten steel reversal flow generated when the molten steel was injected into the mold and the immersion depth of the flat nozzle.

FIG. 1A shows the relationship between the width center position of the flat nozzle and the vertical crack occurrence rate of the thin slab cast. From this figure, (1) If the center of the flat nozzle width is close to the short side of the mold, as shown in (b), the diameters of the two inner vortices in the four vortices become large, and the thin slab on the edge side of the flat nozzle. Vertical cracking occurs in the slab. (2) When the flat nozzle width center is in the vicinity of 1/4 of the mold width, the diameters of the four vortices are the same, and vertical cracks do not occur in the thin slab cast as shown in FIG. (3) If the center of the flat nozzle width is close to the center of the mold width, the diameters of the two outer vortices in the four vortices become large and vertical cracks occur on the edge side of the flat nozzle, as shown in FIG. .

FIG. 2A shows the relationship between the immersion depth of the flat nozzle and the incidence rate of vertical cracks. From this figure, (4) The immersion depth of the flat nozzle is calculated as follows: flat nozzle width x 2/3
If the thickness is more than mm, as shown in Fig. (b), the diameter of the vortex is uniform, but only two vortices are generated, and vertical cracking occurs in the thin slab slab. (5) The immersion depth of the flat nozzle is the width of the flat nozzle × 2/3
If the thickness is not less than mm, vertical cracking does not occur in the thin slab cast as shown in FIG.

The present invention has been conceived and completed based on such knowledge, and is influenced by the molten steel reversal flow generated when the molten steel is poured into the mold, and is likely to cause non-uniform solidification. When continuously casting thin slab slabs,
When arranging two flat nozzles having downward slits in the mold, for example, as shown in FIG. 4, the distance x between the center position of the width of the two flat nozzles and the mold short side and the immersion depth y are set. By equalizing the sizes of the diameters 2R of the vortices A, B, C, D of the four reverse flows generated in the mold at the time of pouring molten steel, and by merging the reverse flows at the center of the flat nozzle, It is characterized by preventing vertical cracking of thin slab cast pieces. When continuously casting slab slabs having a thickness of 100 mm or more, the nozzle / mould interval is sufficiently wide, and uneven solidification and vertical cracking caused by molten steel flow do not pose a serious problem.

[0013]

EXAMPLE The present invention will be described below with reference to FIG. 1 together with an example of the structure of a mold. This embodiment is applied to a case where a thin slab cast piece is cast by a twin belt type continuous casting machine. In this twin-belt type continuous casting machine, as shown in FIG. 3, for example, the mold 1 is wound between two pulleys 2a and 2b and a tension adjusting pulley 2c whose position is controlled by a tension control device 3, and is endlessly rotated. It is formed by a pair of belts 4a and 4b having a cooling structure 4w for cooling and a pair of moving short side blocks 5a (5b) consisting of a large number of copper blocks that move endlessly between the both side ends of the pair of belts. , Operating the sliding nozzles 8a, 8b in which the molten steel 7 of the tundish 6 is installed in the tundish,
Molten steel is injected into the mold 1 through the two flat nozzles 9a and 9b.

The belts 4a and 4b and the movable short side blocks 5a and 5b forming the mold 1 are cooled, and the molten steel 7 is cooled and solidified here, and the supporting rolls disposed below the mold 1 are cooled. It is vertically-curved-horizontally supported by 10 and pulled out to be carried out as a thin slab cast piece 11 in a subsequent process. Two flat nozzles 9a, 9b
Have a downward slit and are arranged in a row in the width direction of the mold 1.

In the twin-belt type continuous casting machine thus constructed, the continuous casting operation according to the present invention and the continuous casting operation outside the scope of the present invention were carried out. Table 2 shows the occurrence of vertical cracks in the slab obtained by this continuous casting. Casting operating conditions Casting steel type: Low carbon steel {Component composition (wt%) is shown in Table 1} Casting speed: 5-10 m / min Mold size: Thickness 75 mm, width 1300 mm, vertical length 3000
mm Nozzle: 2 flat nozzles Size: Thickness 30 (slit thickness 10) mm Arrangement: Flat nozzle thickness center line coincides with mold thickness center line

[0016]

[Table 1]

[0017]

[Table 2]

In Table 2, (1) to (5) are results obtained by changing the center positions of the two flat nozzles. The distance x between the flat nozzle width center position and the mold short side is 1/4 ± of the mold width W.
In Examples (1) to (3) of the present invention in the range of 50 mm, no vertical cracking occurred.

On the other hand, the distance x is 1/4 of the mold width W.
In Comparative Examples (4) to (5) outside the range of ± 50 mm, vertical cracking occurred. (6) to (9) are 325 mm where the distance x between the two flat nozzle width centers and the mold short side is appropriate.
This is the result of changing the immersion depth y of the flat nozzle in the case of. Immersion depth y is 30 to 200 mm (flat nozzle width x
Example (6) of the present invention in the range of 2/3 or less)
In (7), no vertical cracking has occurred. On the other hand, in Comparative Example (8) in which the immersion depth y is 20 mm, the molten steel flow in the meniscus is unstable and the vertical cracks occur because the immersion depth is shallow. Further, in the comparative example in which the immersion depth is 300 mm (flat nozzle width × 2/3 or more), the vortex of the reverse flow of molten steel becomes two large vortices as shown in FIG. It has occurred.

(10) to (11) are the results of changing the immersion depth y using a flat nozzle having a width of 250 mm. In Example (10) of the present invention in which the immersion depth y was 150 mm (flat nozzle width × 2/3 or less), vertical cracking did not occur. On the other hand, in Comparative Example (11) in which the immersion depth y was 200 mm (flat nozzle width × 2/3 or more), vertical cracking occurred.

In this embodiment, the present invention is applied to the twin belt type continuous casting method, but the present invention is also applied to the continuous casting method using another mold in which the inner surface of the mold part is a vertical surface. When applied, the same effect can be obtained.

[0022]

According to the present invention, two flat nozzles having downward slits are used as the nozzles for injecting the molten steel into the mold, and these nozzles are optimally arranged to determine the size of the vortex of the reverse flow generated at the time of injecting the molten steel. By making the flow uniform and merging the reversal flows at the center of the flat nozzle, the solidification can be made uniform, and thus a thin slab cast piece with good quality without vertical cracks can be stably continuously cast. INDUSTRIAL APPLICABILITY The present invention is particularly suitable for continuous casting of thin cast slabs having a thickness of 30 to 100 mm and is more suitable.

[Brief description of drawings]

FIG. 1 (a) is an explanatory view showing a relationship between a center position of a flat nozzle width and a vertical crack occurrence rate when two flat nozzles are arranged in a continuous casting method of a thin slab cast piece. (B) ~
FIG. 3D is an explanatory view showing an example of the vortex morphology of the reverse flow of the molten steel injected into the mold.

FIG. 2 (a) is an explanatory diagram showing the relationship between the immersion depth of the flat nozzle and the vertical crack occurrence rate when two flat nozzles are arranged in the continuous casting method for a thin slab cast. (B),
FIG. 6C is an explanatory view showing a morphological example of a vortex of a reverse flow of molten steel injected into a mold.

FIG. 3A is a schematic side sectional view showing the structure of a twin-belt continuous casting machine according to an embodiment of the present invention. FIG. 6B is a schematic explanatory view of a cross section taken along the line Aa-Ab in FIG.

FIG. 4 is a schematic explanatory view of the arrangement of two flat nozzles in the present invention.

[Explanation of symbols]

 1 Mold 2a, 2b Pulley 3 Tension adjusting device 4a, 4b Belt 5a, 5b Moving short side block 6 Tundish 7 Molten steel 8a, 8b Sliding nozzle 9a, 9b Flat nozzle 10 Support roll 11 Thin slab cast piece

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Nakamura 1 Nishinosu, Oita City, Oita Prefecture Shin-Nippon Steel Co., Ltd. Oita Works

Claims (2)

[Claims] 1. A continuous casting method for a thin slab slab using two flat nozzles having downward slits as a nozzle for injecting molten steel into a mold, in which the sizes of four reversal flow vortices generated during molten steel injection are made uniform. A continuous casting method for thin slab slabs, characterized by uniformizing solidification and preventing vertical cracking of thin slab slabs by merging reverse flows at the center of a flat nozzle. 2. In a continuous casting method for thin slab slabs, which uses two flat nozzles having downward slits as molten steel injection nozzles, the flat nozzle is such that its thickness center coincides with the mold thickness center. And arrange two nozzles in parallel toward the mold short side, and set the width center position of each nozzle to the position of 1/4 ± 50 mm of the mold width W from the mold short side, and 30 mm to 2/3 mm of the flat nozzle width. A thin slab slab characterized by causing four reversal flows generated at the time of pouring molten steel to make the vortices uniform and merging the reversal flows at the center of the flat nozzle by immersing the reversal flows in the following depths. Continuous casting method.