JPH0696182B2 - Continuous casting method for thin slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention directly manufactures a thin slab from a molten metal supplied to a mold made of a circulating body, at least one side of which moves in synchronization with a drawing speed of the slab. In particular, the present invention seeks to avoid inflow of molten metal around the bath surface.

There are various types such as a belt type, a block type, a single roll type or a twin roll type as a continuous casting machine having a circulating body that moves in synchronization with the withdrawal speed of the slab, for example, a belt type continuous casting machine or a belt caster. Is a narrowing method, and while maintaining a gap for holding a casting material such as a molten metal or a solidified shell over a predetermined distance, a pair of opposed long side faces that rotate around each other through a plurality of guide rolls. The casting space is formed by confining the four sides by the metal belts supporting the metal belts and the side plates for the short side surfaces which are in close contact with each other between the both metal belts. Then, molten metal is injected into the casting space through an injection nozzle (immersion nozzle) to manufacture a thin cast piece.

In such a synchronous continuous casting method, the mold powder used in a general continuous casting method is rarely used. Because the mold is a synchronous type, a circulating agent (mold powder) between the mold and the solidification shell is unnecessary, and it is difficult to make the mold powder uniformly flow in the width direction of the mold, and if it flows unevenly. This is because the thickness of the slab cannot be made constant. However, the degassing casting which is generally performed with the mold powder is necessary also in the synchronous continuous casting method, and for this purpose, it is advantageous to fill the space above the bath surface of the molten metal with an inert gas.

(Prior Art) Japanese Patent Application Laid-Open No. 62-130749 describes that in the general continuous casting, the vicinity of a meniscus is hermetically sealed and an inert gas is blown inside. However, it is very difficult to seal the vicinity of the meniscus in synchronous continuous casting. For example, in a belt caster, a coating material is applied to the belt in order to uniformly cool the belt, reduce the heat load on the belt, and reduce the frictional force between the shell and the belt due to the solidification shrinkage of the slab, but a cover for sealing is used. If you stick it to the belt, the coating material will come off. Therefore, it was attempted to create a gap between the cover and the belt and to shield this gap with a fireproof cloth or caul wool suspended from the cover side, but it was impossible to completely seal it.

(Problems to be Solved by the Invention) An object of the present invention is to propose a method for always maintaining a space above a bath surface of a molten metal in an inert atmosphere, and thereby realizing open-gas casting.

(Means for Solving the Problems) The present invention is intended to continuously cast a slab by supplying molten metal to a mold made of a circulating body, at least one side of which moves in synchronism with the slab drawing speed. In this case, the molten metal bath surface including the main part of the mold is covered with a cover, an inert gas is blown into the cover, and the gas in the cover is sucked through the gap between the mold and the cover. is there.

Now, the present invention will be specifically described with reference to FIG.

The illustrated synchronous belt caster is a cast metal (a molten metal or a casting material such as its solidified shell) over a predetermined distance.
And a metal belt 2 for supporting a pair of long side surfaces arranged opposite to each other, which rotate around each of a plurality of guide rolls 1a, 1b, 1c, while maintaining a gap for holding the two metal belts. A mold is formed by confining the four sides with the upper and lower converging short side plates 3 for supporting a pair of short side faces that are in close contact with each other and close to each side edge.

When molten metal is supplied to the casting space from the immersion nozzle 4,
The molten metal contacted with the metal belt 2 cooled by the cooling pad 5 is drawn downward while forming a solidified shell to form a cast piece.

In addition, a cover 6 for covering the bath surface of the molten metal supplied to the mold is arranged, and an inert gas (for example, Ar gas, N ₂ gas, etc.) is blown from the nozzle 7 inserted in the cover 6 to create an atmosphere in the cover 6. Inactivate. Further, a sub-chamber 9 that communicates with the gap 8 between the cover 6 and the metal belt 2 is provided, and an exhaust fan or the like in the sub-chamber 9 is used to constantly suck the inert gas through the gap 8 and the inside of the cover 6 The atmosphere is prevented from entering the inside and the inert atmosphere in the cover 6 is maintained.

In addition to the belt type continuous casting machine of FIG. 1, the present invention is not limited to the Hazley continuous casting machine and the Als swiss type block continuous casting machine in which the short side is synchronized with the withdrawal of the slab in the same manner as the long side.
It is also advantageously suitable for twin-roll and single-roll continuous casters.

(Function) When the bath surface of the molten metal supplied to the mold is covered with the cover to be hermetically sealed, the problem is that the air entrained by the cover from the cover and the mold (for example, a metal belt) is trapped in the gap between the cover and the mold. It is to invade. For example, regarding the inclusion of outside air when there is a gap between the circulating belt and the cover,
It will be described below.

Considering the boundary layer formed on a flat plate when it moves in a stationary fluid, the boundary layer thickness δ
Can be expressed by the following equation (1) from the Reynolds number Re of the fluid and the length l of the flat plate.

In the belt caster shown in FIG. 2, i. Belt speed, u = 0.9 m / min, ii. Belt length, l = 0.3 m iii. Air (40 ° C.) viscosity, ν = 0.175 × At 10 ⁻⁴ m ² / s, the boundary layer thickness is δ≈7 mm, that is, when there is no cover on the belt, an air layer having a thickness of about 7 mm on the belt is wound into the mold by the movement of the belt.

If a thin slab with a width of 1 m is to be cast, the cover is arranged with an average gap d between the belt and an inert gas of 0.8 Nm ³ in the cover.
The increase in the atmosphere O ₂ concentration in the cover when blowing in at / min is as shown in FIG. The velocity u of the air flowing into the cover was obtained from the position shown in FIG. 4, assuming that the air entering through the gap d uniformly flows.

From the figure, it can be seen that the O ₂ concentration increases by 0.4% if the gap is about 1 mm. This increase in O ₂ gas in the atmosphere leads to an increase in non-metallic inclusions, for example, Al in molten steel is changed to Al ₂ O ₃ .

Even if the gap between the cover and the belt is small as described above, the outside air is trapped and has an adverse effect. Therefore, by sucking the gas inside the cover from outside, it is possible to form a flow that opposes the entrained flow of the outside air. , It is extremely effective in always inactivating the atmosphere in the cover.

(Example) Using a belt type continuous casting machine shown in FIG. 1 in which the molten steel bath surface was covered with a cover, a low carbon aluminum killed steel was cast at a speed of 9 m / m.
When casting a slab with a width of 1000 mm and a thickness of 30 mm at in,
Ar gas was blown into the cover according to the following conditions to degas the molten steel bath surface.

Note Internal volume of cover: 0.3m ³ Ar gas injection rate: 0.8Nm ³ / min Gap d: 1mm Ar gas suction rate: 0.3Nm ³ / min O ₂ concentration of the atmosphere in the cover during 20 heats of the above casting The measurement results are shown in FIG.

As a comparison, the same casting and the same measurement were performed even when Ar gas was not sucked from the outside of the cover. The results are also shown in the figure.

As is clear from the figure, the O ₂ concentration of the atmosphere in the cover could be suppressed to approximately 0.1% by the method according to the present invention, whereas it could not be suppressed to 0.5% or less in the comparative example.

(Effect of the Invention) According to the present invention, since the non-metallic inclusions can be reliably reduced by reliably oxidizing the molten metal on the bath surface, it is possible to provide a slab with good surface quality without surface cracks and high internal quality, Further, the life of the circulating body in the continuous casting machine can be extended.

[Brief description of drawings]

FIG. 1 is a schematic view showing a belt caster suitable for use in the present invention, FIG. 2 is an enlarged view of a belt and a cover, and FIG. 3 shows a relation between a gap between the belt and the cover and O ₂ concentration in the cover. A graph, FIG. 4 is an explanatory diagram showing how to obtain the gap d, and FIG. 5 is a graph showing the relationship with the O ₂ concentration in the cover in each operation example. 1a, 1b, 1c …… Guide roll 2 …… Metal belt 3 …… Short side plate 4 …… Immersion nozzle 5 …… Cooling pad 6 …… Cover, 7 …… Nozzle 8 …… Gap, 9 …… Sub Room

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoaki Kimura 3-1-1 Sachimachi, Hitachi City, Ibaraki Hitachi Ltd. Hitachi factory (56) References JP-A-63-177945 (JP, A) JP-A-2-224850 (JP, A)

Claims (1)

[Claims] 1. A molten metal containing a main part of a mold for continuously casting the molten metal by supplying the molten metal to a mold composed of a circulating body, at least one side of which moves in synchronism with the speed of drawing the molten slab. The method for continuous casting of thin cast pieces, characterized in that the bath surface is covered with a cover, an inert gas is blown into the cover, and the gas in the cover is sucked through a gap between the mold and the cover.