JPH08117939A - Method for blowing air bubbles into molten steel - Google Patents

Abstract

PURPOSE: To provide a method for blowing air bubbles capable of incorporating the fine air bubbles into molten steel in continuous casting of steel. CONSTITUTION: A tundish 1 for continuous casting of the steel is provided with passages having ruggednesses 16, 17 in the direction of flow between immersion nozzles 3 from a ladle long nozzle 2 in a tundish 1 in the continuous casting of steel to allow passage of the molten steel therethrough. Gas is blown into the tundish from the projecting parts 15, by which the fine bubbles are incorporated into the molten steel. As a result, the blowing of the fine bubbles into the molten metal is made possible. Gaseous components, such as hydrogen and nitrogen, and impurities, such as carbon and nonmetallic inclusions, in the molten metal are effectively removed by the fine bubbles.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for blowing bubbles which enables fine bubbles to be mixed into molten steel in continuous casting of steel.

[0002]

2. Description of the Related Art Gas bubbles are blown into a molten metal as a means for removing gas components such as hydrogen and nitrogen, carbon and non-metallic inclusions in the molten metal. It is effective to blow fine bubbles in order to actively perform the removal effect by blowing bubbles. As a means for producing fine bubbles in the tundish, a partition weir provided in the molten steel flow passage in the tundish as shown in JP-A-2-11256 makes the molten steel flow velocity fast and the vicinity of the partition weir. There has been proposed a method of obtaining fine bubbles by blowing a gas from.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the method disclosed in Japanese Patent Publication, it is necessary to reduce the flow passage area in order to secure the flow velocity, so that the pressure loss becomes large and it becomes difficult for the molten steel to pass through, and the amount of molten steel cannot be secured when casting at high speed. .

Further, if the flow passage area is small, the flow passage is easily clogged, and casting cannot be performed in a short time. Furthermore, since the operation for reducing the casting speed is usually performed at the beginning of casting or when the ladle is replaced, the molten steel flow velocity becomes slow and the bubbles become large. Therefore, it is necessary to generate fine bubbles at a slower flow rate.

[0005]

In order to solve the above-mentioned problems, the present invention provides molten steel by providing a passage having irregularities in the flow direction between a long nozzle of a ladle and a dipping nozzle in a tundish for continuous casting of steel. The present invention proposes a method of blowing bubbles characterized in that fine bubbles are uniformly mixed into the molten steel by letting gas pass through and passing gas through the convex portion.

[0006]

The method of the present invention will be described in detail with reference to FIG. A partition weir 4 is installed between the long nozzle 2 and the immersion nozzle 3 of the tundish 1 in continuous casting, and a hole 9 for flowing the molten steel 5 having irregularities in the flow direction is opened in the weir, and the inner wall of the hole is convex. When the porous refractory 10 into which the Ar gas is blown is installed in the refractory of the portion 16 and the Ar gas is blown into the refractory, bubbles 8 are generated in the molten steel. The flow 18 of molten steel flowing in the direction of the immersion nozzle is fast in this hole portion, and due to this flow, a shearing force is exerted to promote the separation of the bubbles and the bubbles become smaller.

As a result of various experiments, the inventors have found that the diameter of bubbles generated corresponds to the thickness of the boundary layer of the flow, which is determined by the average velocity of the cross-section of the holes. That is,
The bubbles blown from the porous refractory grow on the wall surface, but when the bubbles are smaller than the boundary layer where the flow is weak, the bubbles are difficult to separate. However, when the bubbles become larger than the boundary layer, the influence of the flow becomes strong and the bubbles on the wall surface are separated and separated into the molten steel. Therefore, the bubble diameter is almost the same as the boundary layer thickness. As a result, even if the average cross-sectional flow velocity is the same, it is possible to reduce the diameter of bubbles generated by reducing the boundary layer thickness.

On the other hand, as shown in FIG. 2, it is necessary to blow air bubbles having a diameter of 2 mm or less in order to float inclusions having a size of 50 μm or more, which are defective in the product, in the tundish. The boundary layer may have a thickness of 2 mm or less.

As shown in FIG. 3, the boundary layer becomes thicker as the distance from the tip of the flow channel becomes longer. Therefore, if a bubble blowing portion is provided in front of the flow channel, fine bubbles can be blown even at a low flow velocity. However, since this limits the blowing area,
When the amount of inclusions is large as in the initial stage of casting or when the ladle is replaced, the amount of bubbles necessary for removing the inclusions may not be blown. Therefore, when unevenness is provided in the flow path, the developed boundary layer peels off at the concave portion, and the boundary layer begins to develop again from the tip of the convex portion, so a thin area of the boundary layer can be secured over a large area, and fine air bubbles cannot be blown in. It will be possible.

The length of the convex portion must be smaller than the distance such that the boundary layer thickness is 2 mm or less. The height of the unevenness is preferably as large as possible, but it is necessary to make it at least as high as that of the boundary layer. Considering the reduction of unevenness due to adhesion of molten steel, slag, inclusions, melting loss, etc., it is desirable to secure at least 5 mm. It is desirable that the length of the recess be approximately the same as the height of the unevenness in order to sufficiently remove the boundary layer.

The weir may have a plurality of holes instead of one. In this case, it is necessary that each hole has a boundary layer thickness of 2 mm or less. The shape of the holes may be circular or rectangular.

The refractory material into which gas is blown may be embedded in a hole formed in the weir, or a cylinder having an inner hole body such as an immersion nozzle may be fitted as a hole in the weir. The means for blowing gas may be a single-tube or multi-tube nozzle or a porous refractory material.

[0013]

EXAMPLE As a first example, a molten steel 5 of low carbon aluminum killed steel injected at 10 ton / min from a ladle through a long nozzle 2 in a tundish 1 having a width of 40 cm and a depth of 80 cm in continuous casting of steel is immersed. It was poured into the mold 6 from the nozzle 3 to cast a slab 7 having a thickness of 245 mm and a width of 1200 mm.

[0014]

[Table 1]

As shown in FIG. 1, a refractory partition weir 4 is installed between the long nozzle and the dipping nozzle, and Table 1
A rectangular hole 9 through which the molten steel having a cross-sectional area capable of obtaining the cross-sectional average flow velocity shown in FIG. The bottom of the hole is made uneven, and the alumina-based porous refractory material 10 is embedded in the projection 16 to form 10N.
A 1 / min Ar gas was blown in. The size of bubbles rising from the dipping nozzle side of the weir was observed, and a molten steel sample was collected and the amount of inclusions was measured by the slime extraction method.

As shown in the results in Table 1, it was observed that bubbles having a diameter of 2 mm or less were generated, and it was confirmed that the bubbles became smaller as the cross-sectional area of the holes became smaller. In addition, the amount of inclusions is significantly reduced as compared with the case where no blowing is performed.

As a second embodiment, as shown in FIG. 4, it is divided into two parts, a first tank 11 of the long nozzle 2 part and a second tank 12 of the immersion nozzle 3 part in the continuous casting of steel, and the central part thereof. In the tundish 1 which is H-shaped when viewed from above, where the two tanks are connected at the connecting part 13 of the molten steel 5 of low carbon aluminum killed steel injected at 10 ton / min from the ladle through the long nozzle from the dipping nozzle. Mold 6
And cast into a slab 7 having a thickness of 245 mm and a width of 1200 mm.

As shown in FIG. 4, the connecting portion 13 of the two tanks is connected.
The weir was installed by embedding a cylindrical refractory material 14 having a cross-sectional area capable of obtaining the cross-sectional average flow velocity shown in Table 1. A ring-shaped porous refractory material 15 was embedded inside the cylindrical refractory material to form a convex portion 16, and Ar gas was blown from the porous refractory material. The size of bubbles rising from the dipping nozzle side of the weir was observed, and a molten steel sample was collected and the amount of inclusions was measured by the slime extraction method.

As shown in the results in Table 1, it was observed that bubbles having a diameter of 2 mm or less were generated. In addition, the amount of inclusions is significantly reduced compared to the case without blowing.

As a comparative example, as shown in FIG. 5, in a tundish 1 having a width of 40 cm and a depth of 80 cm in continuous casting of steel, 10 ton was passed from a ladle through a long nozzle 2.
The molten steel 5 of low carbon aluminum killed steel, which is injected at a flow rate of 1 / min, is injected from the dipping nozzle 3 into the mold 6, and the thickness is 245 mm and the width is 1
A 200 mm slab 7 was cast.

A refractory partition weir 4 is installed between the long nozzle and the dipping nozzle, and a rectangular hole 9 through which molten steel shown in Table 1 passes is formed, and an alumina porous refractory 10 is provided at the bottom of the hole. Buried and Ar gas was blown in. The size of bubbles rising from the dipping nozzle side of the weir was observed. Compared with the embodiment, only large bubbles can be obtained even with the same cross-sectional area, and the effect of reducing inclusions when not blown is not great. Moreover, when the cross-sectional area was reduced to obtain bubbles of 2 mm or less as in Comparative Example 2, casting could not be performed at a predetermined casting speed.

[0022]

EFFECTS OF THE INVENTION According to the present invention, the amount of molten steel cannot be secured in the case of high speed casting, and casting cannot be performed in a short time. The gas bubbles such as hydrogen and nitrogen in the molten metal and impurities such as carbon and non-metal inclusions can be efficiently removed by the fine bubbles produced by the present invention.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the effect of removing inclusions by the bubble diameter.

FIG. 3 is an explanatory diagram showing a relationship between a distance and a thickness of a boundary layer.

FIG. 4 is an explanatory diagram showing a second embodiment of the present invention.

FIG. 5 is an explanatory diagram showing a comparative example.

[Explanation of symbols]

1: Tundish, 2: Long nozzle, 3: Immersion nozzle, 4: Weir, 5: Molten steel, 6: Mold, 7: Cast slab, 8: Bubble, 9: Molten steel passage hole, 10: Porous refractory material, 11 : 1st tank, 12: 2nd tank, 13: connection part, 14: refractory cylinder, 15: ring-shaped porous refractory material, 16: convex part, 17: concave part, 18: molten steel flow.

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Claims (1)

[Claims] 1. In a tundish for continuous casting of steel, a passage having irregularities in the flow direction is provided between a long ladle nozzle and a dipping nozzle to pass molten steel and blow gas from the convex portion to form fine bubbles. A method for injecting bubbles into molten steel, characterized in that is mixed with molten steel.