JPH09253807A - Method for continuously casting aluminum killed steel cast billet having small cross section - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the clogging of a nozzle and continuously cast a cast billet having high quality while restraining the variation of molten steel surface in a mold even in the case of being the mold having small cross section by blowing suitable inert gas from the nozzle without controlling inclusion form of Al2 O3 inclusion with Ca. SOLUTION: The flowing rate of the molten steel 15 passing through the nozzle 6c for casting the molten steel into the mold, is limited to <=0.55t/min, and the nozzle 6c uses the immersion type nozzle into the molten steel in the mold. Further, a contracting ratio to the nozzle diameter at a cap 11 part in the upper part of the nozzle 6c is made to >=0.25 and the molten steel is cast into the mold while blowing the inert gas, e.g. Ar gas 12 from the inner peripheral surface 11a of the cap 11 in the range of 35-75Nl/mm/m<2> of crossing sectional area of the mold.

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 producing a steel slab having a small cross section, and more particularly to C
The present invention relates to a method for continuously casting a sound steel slab containing an Al ₂ O ₃ inclusion that has not been subjected to a addition treatment, without clogging of a tundish nozzle.

[0002]

2. Description of the Related Art In continuous casting of steel, it is located between a ladle that holds molten steel to be cast while keeping it at a predetermined temperature and a mold of a continuous casting machine, and stably supplies molten steel to the mold. The tundish for this purpose is provided with a tundish nozzle for supplying molten steel to the mold at the bottom. This tundish nozzle (hereinafter referred to as "nozzle") allows molten steel to pass through the inside and causes Al ₂ O ₃ inclusions and TiO ₂ -based inclusions in the molten steel to agglomerate and accumulate on the inner surface, and in extreme cases. In some cases, the nozzle may be clogged and molten steel cannot be cast. The occurrence of such so-called nozzle clogging not only adversely affects the quality of the slab but also forces the casting operation itself to be stopped.

When a molten steel containing Al ₂ O ₃ inclusions generated by Al deoxidation is cast in a continuous casting machine such as a steel billet continuous casting machine for producing a slab having a small cross section size, Preventing the occurrence of nozzle clogging due to ₂ O ₃ inclusions is particularly important for stable casting work. Therefore, in the case of casting molten steel containing Al ₂ O ₃ inclusions, particularly when casting in a small cross-section mold,
A method of preventing nozzle clogging by changing the Al ₂ O ₃ inclusions to CaO-Al ₂ O ₃ inclusions which are low melting point compounds by the Ca addition treatment has been adopted.

On the other hand, when a molten steel containing Al ₂ O ₃ inclusions is cast in a continuous casting machine such as a slab continuous casting machine in which a molten steel passing flow rate is large in order to produce a slab having a large cross section size, Conventionally, an inert gas is blown into the casting molten steel flow from the inner surface of the upper nozzle of the tundish or the immersion nozzle of the tundish, and due to the cleaning effect of the inner surface by the inert gas bubbles, Al ₂ O ₃ inclusions are aggregated and deposited on the inner surface. Is being prevented. For example, Japanese Laid-Open Patent Publication No. 2-37948 discloses a casting speed of 1.0 to 10 in continuous casting of a slab.
In the case of within the range of 3.5 t / min, Ar gas was supplied from the tundish upper nozzle to 3.0 to 5.0 Nl / molten steel to
A method of blowing within the range of n (hereinafter referred to as "prior art 1") is disclosed.

[0005]

Conventionally, Al has been deoxidized.
When casting molten steel into a mold with a small cross section such as a billet
In order to solve the nozzle clogging problem described above,
As described above, the treatment for adding Ca to the molten steel has been performed. I
However, due to the addition of Ca, the internal impact of the mechanical properties of steel materials
There is a problem that the strike characteristics deteriorate. Therefore, Ca addition
Injecting inert gas into the nozzle without adopting the treatment method
The inner surface of the nozzle by _TwoO_ThreeInclusion
Attempts have been made to prevent the agglomeration / accumulation of water.
However, when casting billets, etc., the mold cross-sectional area
Is smaller than the prior art 1 and has a small cross section,
Inert gas blown through the sledge reaches the mold
Then, the molten metal level in the mold is greatly changed. This hot water
Movement may cause the powder in the mold to get caught in the slab.
It causes powdery defects in wood products.

Therefore, an object of the present invention is to suppress the fluctuation of the molten metal level in the mold as much as possible even if the inert gas is blown into the nozzle without the Ca addition treatment of the molten steel and the casting is performed by the small cross-section mold. Moreover, a continuous casting method for a small-section cast piece for producing a good-quality cast piece that does not cause nozzle clogging by performing proper blowing that does not cause Al ₂ O ₃ inclusions to aggregate and accumulate on the inner surface of the nozzle, is provided. To provide.

[0007]

From the above-mentioned viewpoints, the inventors of the present invention carry out a stable casting operation that does not cause nozzle clogging in producing a continuously cast slab having a small cross-section size of Al killed steel. We have conducted intensive research to develop a technology for producing sound slabs. As a result, as a nozzle of the tundish stopper-type molten steel outflow control device, a refractory brick nozzle that can blow gas into the molten steel flow from the cap at the top of the immersion nozzle is used, and the reduction ratio in the nozzle throttle shape of the cap is used. Is an appropriate value,
It was found that the intended purpose can be achieved by setting the blowing gas flow rate to an appropriate value according to the mold cross-sectional area and limiting the amount of molten steel passing through the nozzle per unit time.

The continuous casting method of the present invention is a nozzle for casting molten steel into a mold (hereinafter referred to as a "tundish nozzle") in the continuous casting method of a small-section aluminum-killed steel slab having a cross-sectional area of 0.1 m ² or less. The flow rate of molten steel passing through the tank is limited to 0.55 t / min or less, the tundish nozzle is an immersion type nozzle in the molten steel in the mold, and the nozzle diameter reduction ratio of the cap on the tundish nozzle is 0.
25 or more, and the inert gas is blown out from the inner peripheral surface of the cap. The flow rate of the inert gas is characterized by being limited to the range of 35 to 75 Nl / min per 1 m ^{2 of the} cross-sectional area of the mold. However, the "cap nozzle diameter reduction ratio" means the cap nozzle minimum diameter D _min.
And the maximum diameter D _max , D _min / D _max .

[0009]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a schematic explanatory view showing one embodiment of the present invention. As shown in FIG. 1, molten steel is poured into a tundish 3 through a ladle nozzle 4 from a ladle 2 containing a predetermined Al-killed molten steel 1 that has been tapped from a steelmaking furnace (not shown). After a predetermined amount of 5 is accumulated, the operation panel 6a of the molten steel outflow control device 6 is operated to raise the tundish stopper 6b by a predetermined amount, the molten steel 5 is cast into the mold 7 through the tundish nozzle 6c, and the slab 8 is continuously cast. To cast. The tundish nozzle 6c is an immersion type tundish in which the lower end is immersed in the molten steel 9 in the mold 7, and the upper surface of the molten steel 9 is covered with the mold powder 10.

FIG. 2 is a schematic vertical sectional view of an essential part of the immersion type tundish nozzle 6c. As shown in FIG.
A cap (also referred to as an upper nozzle) 11 is fitted on the upper part of the tundish nozzle 6c, and a thin metal tube 13 for supplying the Ar gas 12 is connected to the cap 11. The cap 11 is made of a refractory porous brick, and the Ar gas 14 is blown into the molten steel casting flow 15 from the inner peripheral surface 11 a of the cap 11. Here, the cap 11 is manufactured to have an appropriate ratio (D _min / D _max ≧ 0.25) of the nozzle minimum diameter D _min and the maximum diameter D _max .

In the present invention, the cross-sectional area of the mold is set to 0.1 m ² or less, the flow rate of the molten steel passing through the tundish nozzle is limited to 0.55 t / min or less, and the mold 7 of the molten steel 5 in the tundish 3 is set. Ar gas 14 is blown into the tundish nozzle 6c through the cap 11 from before the start of casting into the predetermined time after the completion of casting. The blowing of Ar gas was performed by a gas flow rate control device (not shown) at 35 to ⁷ per 1 m ² of the cross-sectional area of the mold.
The flow rate is controlled within the range of 5 Nl / min.

Next, the reason why the continuous casting method of the present invention is limited as described above will be explained. Nozzle clogging in continuous casting of aluminum killed steel has a mold cross-sectional area of 0.1.
This is especially likely to occur when casting is less than m ² . Therefore, it is effective when continuously casting a slab having a cross-sectional area of 0.1 m ² or less. Then, when the molten steel is cast into a small-section mold having a cross-sectional area of 0.1 m ² or less, if the casting speed of the molten steel exceeds 0.55 t / min, an inert gas is injected into the molten steel flow from the inner surface of the tundish nozzle. By blowing, it is difficult to prevent the aggregation and deposition of Al ₂ O ₃ inclusions on the inner surface of the tundish nozzle, and to suppress the fluctuation of the molten metal surface in the mold to be small. Therefore, when casting molten steel into a mold having a mold cross-sectional area of 0.1 m ² or less, the casting rate should be limited to 0.55 t / min or less.

Further, by making the tundish nozzle an immersion type nozzle in the molten steel, it is possible to suppress the fluctuation of the molten metal level in the mold smaller than that of the open type nozzle which is not immersed in the molten steel, and to oxidize the casting flow by air. It is advantageous for prevention and prevention of entrainment of powder in molten steel.

The clogging of the tundish nozzle at the time of casting the aluminum killed molten steel is caused by the fact that Al ₂ O ₃ inclusions suspended in the molten steel agglomerate and accumulate on the inner peripheral surface of the cap above the tundish nozzle to prevent the inflow of the molten steel to be cast. It is caused by Therefore, by blowing the inert gas into the molten steel flow from the inner surface of the cap, the stagnation of the molten steel flow in this portion is eliminated, and the Al ₂ O ₃ inclusions are aggregated and deposited (see FIG. 2).
It is advantageous to prevent the nozzle clogging with a gas injection amount as small as possible. The shape and dimensions of the cap are Al ₂
It is important to make O ₃ inclusions hard to aggregate and accumulate.

FIG. 3 shows the nozzle diameter reduction ratio of the cap above the tundish nozzle and the thickness of the Al ₂ O ₃ inclusions adhering layer thickness (m) in the casting of Al killed molten steel.
7 is a graph showing the relationship with m). The figure shows the flow rate Q (Nl / min · t) of Ar gas blown from the cap above the tundish nozzle and the cross-sectional area S of the mold.
The ratio Q / S to (m ² ) is 50 (Nl / min · t ·
m ² ). According to a large number of operation test results, in order to prevent casting trouble due to clogging of the tundish nozzle, the thickness of the Al ₂ O ₃ inclusion adhering layer was set to 7
It is necessary to set it to mm or less. Therefore, from the figure, the nozzle diameter reduction ratio of the tundish nozzle cap is
It is necessary to set it to 0.25 or more.

FIG. 4 shows the ratio Q of the flow rate Q (Nl / min) of Ar gas blown out from the inner peripheral surface of the cap at the upper part of the tundish nozzle and the mold cross-sectional area S (m ² ) in the casting of Al killed molten steel. / S (Nl / min
And m ² ) is a graph showing the relationship between the level variation Δh (mm) in the mold and the thickness d (mm) of the Al ₂ O ₃ inclusions adhering layer to the cap. The figure shows the case where the nozzle diameter reduction ratio of the cap is 0.33. From the figure,
As the value of the ratio Q / S of the flow rate of Ar gas to the cross-sectional area of the mold increases, the thickness d of the Al ₂ O ₃ inclusion adhering layer decreases, while the variation Δh of the molten metal level in the mold is It can be seen that when it is small, it is small and good, but it increases and deteriorates as the Q / S increases. And, the limiting condition of the Al ₂ O ₃ inclusion adhesion layer thickness d: d ≦ 7 mm and the fluctuation of the molten metal surface in the mold, which are obtained from many operation test results, in order to prevent casting trouble due to clogging of the tundish nozzle Restriction condition of molten metal level variation amount Δh to prevent generation of inclusion physical defect of the product due to entrainment of mold powder: In order to satisfy both of Δh ≦ 20 mm, the ratio Q / S of Ar gas flow rate and mold cross-sectional area
However, it is necessary to limit it to the range of 35 to 75 Nl / min · t).

[0017]

Next, the present invention will be described in more detail with reference to Examples. Table 1 shows continuous casting conditions in Examples which are continuous casting methods within the scope of the present invention and in Comparative Examples which are continuous casting methods outside the scope of the present invention. In each of the examples and comparative examples, the continuous casting machine is a curved type 6-strand billet continuous casting machine, the ladle capacity is 250t molten steel, the tundish capacity is 50t molten steel, and the casting steel type is C: 0.
9 wt.% Al killed steel.

[0018]

[Table 1]

Table 2 shows the evaluation results of the tundish nozzle clogging during continuous casting and the powder inclusion defect in the surface layer of the slab in the above Examples and Comparative Examples. In the evaluation of clogging of the tundish nozzle, o indicates that the tundish nozzle is not clogged, and x indicates that the tundish nozzle is clogged. Further, in the evaluation of the powdery inclusion defect in the surface layer of the slab, the mark ◯ indicates that the slab was not maintained due to the powdery inclusion, and the mark x was that the slab was maintained due to the powdery inclusion.

[0020]

[Table 2]

The following matters are clear from Tables 1 and 2. In the production of a round billet slab with a small cross-sectional size, even if one continuous casting condition is outside the scope of the present invention, the intended purpose cannot be achieved, but the continuous casting condition is within the scope of the present invention. In the case of, it turns out that the intended purpose can be achieved.

[0022]

As described above, according to the present invention, continuous casting of a small-section aluminum-killed steel slab such as a billet can be performed on the inner surface of the tundish nozzle without the addition of Ca to molten steel. It is possible to provide a continuous casting method for a small-section aluminum-killed steel slab that can prevent adhesion of Al ₂ O ₃ inclusions and can be performed without generating powdery inclusion defects in the surface layer of the slab. It has a useful effect.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing one embodiment of the present invention.

FIG. 2 is a schematic vertical sectional view of an essential part of a tundish nozzle used in the present invention.

FIG. 3 is a drawing of the cap nozzle diameter at the upper part of the tundish nozzle and the Al ₂
O ₃ is a graph showing the relationship between the inclusions adhering layer thickness.

FIG. 4 is a ratio Q / S of the flow rate Q of Ar gas blown from the inner peripheral surface of the cap on the upper part of the tundish nozzle and the cross-sectional area S of the mold in the casting of Al killed molten steel.
And the amount of fluctuation Δh in the mold surface and Al to the cap
It is a graph showing the relationship between the ₂ O ₃ inclusions adhesion layer thickness d.

[Explanation of symbols]

1 Molten Steel 2 Ladle 3 Tundish 4 Ladle Nozzle 5 Molten Steel 6 Molten Steel Outflow Control Device 6a Operation Panel 6b Tundish Stopper 6c Tundish Nozzle 7 Mold 8 Cast Piece 9 Molten Steel 10 Mold Powder 11 Cap (Upper Nozzle) 11a Inner Circumference Surface 12 Ar gas 13 Metal thin tube 14 Ar gas 15 Molten steel casting flow 16 Al ₂ O ₃ Aggregation / accumulation of inclusions 17 Tundish bottom 18 Ar gas 19 Plasma heating torch

Claims (1)

[Claims] 1. A method of continuously casting a small-section aluminum-killed steel slab having a cross-sectional area of 0.1 m ² or less, wherein a molten steel flow rate passing through a molten steel casting nozzle into a mold is 0.55 t.
/ Min is limited to below the nozzle and immersion nozzle into the mold molten steel, the nozzle diameter throttle ratio of the nozzle top of the cap is 0.25 or more, the molded cross-sectional area 1 m ² per 35～75Nl / A method for continuously casting a small-section aluminum-killed steel slab, characterized in that the molten steel is cast into the mold while blowing out an inert gas within a range of min from the inner peripheral surface of the cap.