JP2002239692A - Method for continuously casting small cross section aluminum-killed steel cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cast a small cross section aluminum-killed steel cast slab without deteriorating cleanliness of the cast slab while preventing clogging of a nozzle when the above cast slab to which no Ca adding treatment is performed is cast. SOLUTION: When the small cross section aluminum-killed steel cast slab 6 having <=0.1 m2 cross sectional area of the cast slab is continuously cast, the atmosphere in a tundish 3 is shut off from the outer air and the molten metal 1 being applied with Al-deoxidation and substantially containing no Ca is poured into the tundish. While dipping a tundish nozzle 7 where a cap nozzle 8 is disposed at the upper part, into the molten steel in a mold 4 covered with mold powder 5 and blowing inert gas in the range of 20-50 Nl/min per 1 m2 of the cross sectional area of the mold from the lower part of the cap nozzle, the molten steel in the tundish is cast into 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 small-section cast slab of aluminum-killed steel, and more particularly to a continuous casting method.
The present invention relates to a continuous casting method capable of casting aluminum-killed molten steel that has not been subjected to the addition treatment without causing nozzle clogging of a tundish nozzle and without impairing the cleanliness of the molten steel.

[0002]

2. Description of the Related Art In continuous casting of steel, a tundish nozzle is provided at the bottom of a tundish, and molten steel injected into a tundish from a ladle is injected into a mold through a tundish nozzle. I have. On the inner surface of this tundish nozzle, Al
High-melting-point deoxidation products such as Al ₂ O ₃ and TiO _{2 in} molten steel due to deoxidation or Ti deoxidation adhere and deposit, so-called “nozzle clogging” occurs. This nozzle clogging not only adversely affects the slab quality, but in severe cases, necessitates stopping the pouring operation itself.

[0003] Particularly, when casting aluminum killed steel in a continuous caster having a small cross section such as a billet continuous caster,
Since the inside diameter of the tundish nozzle is small, nozzle clogging is likely to occur, and preventing nozzle clogging with Al ₂ O ₃ is extremely important for performing a stable casting operation. Therefore, conventionally, when casting a small-section slab of aluminum-killed steel, Ca is added to molten steel, and the generated Al ₂ O ₃ is transformed into a CaO-Al ₂ O ₃ -based low melting point composite oxide, and a nozzle is formed. Methods have been implemented to prevent clogging. However, when Ca is added, Ca remains in the steel, and there is a problem that the impact characteristics of the Ca deteriorate the mechanical properties of the steel material.

Accordingly, the present inventors have studied a method for preventing nozzle clogging without adding Ca, and first described in
No. 253807 (hereinafter referred to as “prior art 1”) has been proposed. The casting method according to the prior art 1 is a method generally performed in casting of a large-section slab, that is, a method in which an inert gas is blown into the inner surface of a tundish nozzle to prevent adhesion and deposition of Al ₂ O _3. In particular, in the casting of small-section cast slabs, in order to prevent the molten steel level in the mold from fluctuating due to the blowing gas, the mold powder is caught and the cleanliness of the molten steel is prevented from deteriorating. with pass limit the molten steel flow below 0.55 t / min, the tundish blown inside the nozzle inert gas blowing mold cross-sectional area 1 m ² per flow 35 to 75
The range is limited to Nl / min, and the shape of the tundish nozzle is limited to an appropriate shape.

[0005]

However, it was found from the results of the subsequent casting that the casting method according to Prior Art 1 had the following problems. That is, when the blowing flow rate of the inert gas is near the upper limit of the limited range, especially when the cross section of the slab is 0.03 m ² or less, the molten steel level in the mold may fluctuate. If the gas blowing flow rate is near the lower limit of the limited range, nozzle clogging due to Al ₂ O ₃ may not be effectively prevented, and stable casting is not always possible.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a slab having a cross section area of 0.1 m.
When casting small section aluminum killed steel slabs of ² or less,
a Without adding treatment, even if an inert gas is blown into the tundish nozzle, it is possible to suppress the fluctuation of the molten metal level in the mold, without impairing the cleanliness of the slab, and without causing nozzle clogging. And to provide a continuous casting method capable of performing stable casting.

[0007]

Means for Solving the Problems The present inventors have intensively studied to solve the above-mentioned problems. As a result, in the tundish of the stopper type molten steel flow control system which is generally employed in continuous casting of a small section, it is assumed that Al ₂ O ₃ in the molten steel adheres and accumulates on the upper side of the cap nozzle. It was also found that no nozzle clogging occurred.
Since this part has a high molten steel flow rate, Al ₂ O ₃
Probably because the sediment did not grow. The cap nozzle is a refractory nozzle that is fitted and installed on the upper part of the tundish nozzle, and is a part that is joined to a stopper to control the flow rate of molten steel.

Therefore, using a continuous casting machine shown in an embodiment to be described later, the lower half of the cap nozzle is made of porous brick, while the upper half of the cap nozzle is made of a dense refractory material. Aluminum killed steel was cast at various flow rates, and the fluctuation of the molten metal level in the mold and the thickness of the Al ₂ O ₃ adhesion layer in the tundish nozzle were investigated. FIG. 1 shows the results of the investigation.

FIG. 1 shows that in casting aluminum killed steel,
Ar flow rate Q (N
l / min) and the ratio Q / S of the mold cross-sectional area S (m ² )
(Nl / min · m ² ) and the molten metal level variation Δ
FIG. 6 is a diagram showing a relationship between h (mm) and a thickness d (mm) of an Al ₂ O ₃ adhesion layer in a cap nozzle. As is clear from FIG. 1, as the value of the ratio Q / S increases, Al
_{Although the} thickness d of the ₂ O ₃ adhesion layer decreases, the variation Δh of the molten metal level in the molten steel in the mold is small and good when the ratio Q / S is small, but increases with an increase in the ratio Q / S. It turns out that it gets worse.

[0010] Then, d ≦ 7 m, which is a condition for limiting the thickness d of the Al ₂ O ₃ adhesion layer to prevent casting trouble due to nozzle clogging, obtained from a number of operation test results.
m, and Δh ≦ 10 which is a limiting condition of the level change Δh of the molten metal in order to prevent the inclusion defect of the product from being caused by the entrainment of the mold powder due to the level change of the molten steel in the mold.
mm, the ratio Q / S is set to 20 to 50N.
It was found that it was necessary to control within the range of 1 / min · m ² . In the above-mentioned prior art 1, since Ar was blown from the entire cap nozzle, the Ar blowing flow rate at the lower portion of the cap nozzle where nozzle clogging is likely to occur becomes relatively small, so that Ar blowing for preventing nozzle clogging is performed. It is considered that the lower limit of the flow rate was as high as 35 Nl / min per 1 m ^{2 of} the mold cross-sectional area.

The present invention has been made based on the above findings.
And the continuation of the small-section aluminum-killed steel slab according to the first invention
The casting method has a slab cross-sectional area of 0.1 m.^Two The following small section
In continuous casting of Lumikilled steel slab,
The atmosphere is shut off from the outside air, and Al
Injected molten steel that is acidified and contains substantially no Ca
Tundish with a cap nozzle at the top
Immerse the nozzle in molten steel in a mold covered with mold powder
While moving the mold sideways from below the cap nozzle.
Cross section 1m^Two Within the range of 20-50 Nl / min
While blowing inert gas, this tundish nose
The molten steel in the tundish into the mold
It is characterized by the following. In addition, the small device according to the second invention
The continuous casting method of a cross section aluminum-killed steel slab is the first invention
In, the slab cross-sectional area is 0.03m ^Two The following
It is characterized by the following.

According to the present invention, A
Since the inert gas such as r is blown, nozzle clogging of the tundish nozzle can be prevented with a small flow rate of the blown inert gas. Further, since the flow rate of the blown inert gas is small, the fluctuation of the molten steel level in the mold is suppressed, the entrainment of the mold powder is suppressed, and the cleanliness of the slab is not impaired.

Nozzle in continuous casting of aluminum killed steel
The blockage is 0.1m in cross section of slab ^Two In the following cases,
Easy to occur due to the small inside diameter of the dish nozzle
It becomes. Therefore, the cross-sectional area is 0.1m^Two The following aluminum keys
The effect of the present invention is exhibited when continuously casting cast steel slabs
You. In particular, the slab cross-sectional area is 0.03m^Two When the following
Vulnerable clogging easily and blowing inert gas
The variation in the molten metal level caused by the slab becomes large, so that the cross section area of the slab is 0.
03m^Two Originally developed during continuous casting of the following aluminum-killed steel slabs
Preferably, light is applied.

Further, in the present invention, aluminum-killed steel contains Ca
Since no treatment is performed, and therefore, the aluminum-killed steel slab does not substantially contain Ca, it is possible to solve the problem that the impact characteristics of the steel material caused by Ca are deteriorated.
Also, since Al deoxidation is performed before injecting molten steel into the tundish, the floating and separation of Al ₂ O ₃ , which is a deoxidation product, is actively performed in a secondary refining furnace such as an RH degassing facility. By accelerating, the cleanliness of the molten steel can be enhanced in advance.

Further, according to the present invention, since the atmosphere in the tundish is isolated from the outside air, air oxidation in the tundish is prevented, generation of new Al ₂ O _{3 in} the molten steel is prevented, and Clogging is less likely to occur. Further, since the molten steel in the mold is covered with mold powder to prevent oxidation of the molten steel by air, the cleanliness of the molten steel can be improved. Furthermore, since the tundish nozzle is immersed in the molten steel in the mold, while preventing air oxidation of the molten steel,
Fluctuation of the molten steel level in the mold can be suppressed.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a schematic explanatory view of a continuous casting machine showing one embodiment of the present invention.

As shown in FIG. 2, a tundish 3 composed of an iron shell 11 and a refractory 12 inside thereof is mounted on a tundish car (not shown) at a predetermined position above a mold 4. Are located. At the bottom of the tundish 3, a tundish nozzle 7 that fits with the refractory 12 is disposed, and the molten steel outflow hole 1 from the tundish 3 to the mold 4 is provided.
8 are formed. A stopper 9 is arranged above the tundish nozzle 7 to open and close the molten steel outflow hole 18. The position of the stopper 9 is controlled by the stopper opening control device 10, and the amount of molten steel flowing out to the mold 4 is appropriately controlled. Stopper opening control device 1
Numeral 0 receives the measurement signal of the eddy current level gauge 13 disposed immediately above the mold 4 and appropriately controls the amount of molten steel flowing out to the mold 4 according to the level of the molten steel 1 in the mold 4. .

The upper opening of the tundish 3 is covered with a tundish cover 15. An inert gas such as Ar is supplied into the tundish 3 from an inert gas supply pipe 16 penetrating the tundish cover 15. In addition, the inside of the tundish 3 can be shut off from the outside air to provide an inert gas atmosphere. Inside the tundish 3, a weir 1 for promoting floating and separation of Al ₂ O ₃ existing in the molten steel 1.
The tundish 3 is provided with a plasma heating torch 20 for heating the molten steel 1 penetrating the tundish cover 15. By operating the plasma heating torch 20 as needed,
The temperature of molten steel 1 in tundish 3 is controlled within a predetermined range.

FIG. 3 is a schematic vertical sectional view of a main part of the tundish nozzle 7. As shown in FIG. 3, a cap nozzle 8 (also referred to as an upper nozzle) is fitted into an upper portion of the tundish nozzle 7, and the cap nozzle 8 is connected to an inert gas introduction pipe 14. The upper portion 8a of the cap nozzle 8 is formed of a dense refractory material such as high alumina, and the lower portion 8b of the cap nozzle 8 is formed of a refractory porous brick. The inert gas is blown from the lower portion 8b of the cap nozzle 8 into the molten steel outflow hole 18. here,
The lower portion 8b of the cap nozzle 8 is defined to mean a lower side than a portion where the cap nozzle 8 and the stopper 9 are joined, and in the present invention, the lower portion 8b defined in this manner is used.
At least 50% or more of the inner peripheral surface is made of a refractory porous brick. It is not necessary to completely change the refractory material between the upper part 8a and the lower part 8b, and the two refractory materials may be mixed with each other.

Using the continuous casting machine configured as described above, an aluminum killed steel slab is manufactured as follows.
First, a molten steel 1 is taken from a steelmaking furnace (not shown) such as a converter, and a ladle 2 is provided.
After adding a predetermined alloying element, Al is added to perform Al deoxidation. Thereafter, the ladle 2 containing the molten steel 1 is placed at a predetermined position above the tundish 3, and the molten steel 1 in the ladle 2 is poured into the tundish 3. In addition, from the viewpoint of preventing nozzle clogging of the tundish nozzle 7, A
1 After deoxidation, for example, RH vacuum degassing equipment (not shown)
The molten steel 1 is subjected to a stirring treatment with a flux, a stirring treatment with a flux in a ladle refining facility (not shown), and the like, and the deoxidized product A
It is preferable that l ₂ O ₃ is separated and removed from the molten steel 1 and the molten steel 1 is cleaned.

When the molten steel 1 is injected into the tundish 3, the long nozzle 1 whose tip penetrates the tundish cover 15 in order to prevent the formation of Al ₂ O ₃ by air oxidation.
7 is provided at the bottom of the ladle 2, and the tip of the long nozzle 17 is immersed in the molten steel 1 in the tundish 3, and the air is shut off and injected. During the casting, the inert gas supply pipe 16 is used.
To supply an inert gas such as Ar from the above to make the inside of the tundish 3 an inert gas atmosphere. Before the injection into the tundish 3, an inert gas such as Ar gas may be supplied from the inert gas supply pipe 16 to make the inside of the tundish 3 an inert gas atmosphere.

When the molten steel 1 in the tundish 3 reaches a predetermined amount, the stopper 9 is raised by the stopper opening control device 10 and the molten steel 1 is introduced into the mold 4 through the molten steel outflow hole 18.
Inject. The molten steel 1 injected into the mold 4 is cooled and solidified to form a slab 6. The cross-sectional area of the slab 6 is 0.1 m ² or less, preferably 0.03 m ² or less. Then, the tip of the tundish nozzle 7 is immersed in the molten steel 1 in the mold 4, and when the level of the molten steel 1 in the mold 4 rises to a predetermined position, the mold powder 5 is added into the mold 4. At the same time, the drawing of the slab 6 is started.
Before and after the start of drawing of the slab 6, an inert gas such as Ar is supplied from the inert gas introducing pipe 14 and from the lower portion 8b of the cap nozzle 8 to ^{20 to} 50 Nl / min per 1 m ² of the cross-sectional area of the mold 4. The molten steel 1 in the tundish 3 is cast into the mold 4 while blowing an inert gas within the range of (1). Then, while maintaining this state, continuous casting of aluminum killed steel is continued.

At this time, the cross-sectional area of the slab 6 is 0.03 m ²
In the following cases, it is preferable to reduce the amount of inert gas blown from the viewpoint of minimizing fluctuations in the molten steel surface of the molten steel 1 in the mold 4, specifically, 20 to 35 Nl per 1 m ² of the cross-sectional area of the mold 4. / Min.

The nozzle clogging of the tundish nozzle 7 during the casting of aluminum killed steel occurs when Al ₂ O ₃ suspended in the molten steel 1 adheres and accumulates on the inner peripheral surface near the lower portion 8 b of the cap nozzle 8. In the present invention, since inert gas is blown into the molten steel 1 from the lower portion 8b of the cap nozzle 8, where Al ₂ O ₃ is likely to adhere and accumulate to cause nozzle clogging, stagnation of molten steel flow in this portion is eliminated. , A
it becomes possible to prevent adhesion and accumulation of l ₂ O _3.

By producing the aluminum-killed steel slab 6 having a slab cross-sectional area of 0.1 m ² or less as described above, the tundish nozzle 7 is excellent in cleanliness without clogging of the nozzle. The slab 6 can be manufactured stably.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation results of the present invention will be described below using a continuous casting machine shown in FIG. Table 1 shows the casting conditions of the present invention example, which is a continuous casting method within the scope of the present invention, and the comparative example, which is a continuous casting method outside the scope of the present invention. The continuous casting machine used in both the present invention example and the comparative example is a curved 6-strand billet continuous casting machine having a ladle capacity of 250 t of molten steel and a tundish capacity of 50 molten steel.
At time t, an aluminum-killed steel having a C content of 0.2 mass% and an Al content of 0.03 mass% was cast while Ar was blown into the tundish during casting to shut off the atmosphere in the tundish from outside air.

The material of the tundish nozzle used was Al ₂ O ₃ -graphite, and Ar was blown as inert gas from the porous brick portion of the cap nozzle. However, in Comparative Examples 1 to 3, the entire cap nozzle was formed of a porous brick. The difference between the casting conditions according to the present invention and the casting conditions of each comparative example is that in Comparative Example 1, Ar was blown from the entire cap nozzle, and in Comparative Example 2, Ar was blown from the entire cap nozzle, and that Ar was blown. Comparative Example 3 was that Ar was blown from the entire cap nozzle.
Is that the Ar blowing amount is larger than the range of the present invention,
Other casting conditions are within the scope of the present invention.

[0028]

[Table 1]

Table 1 shows the results of evaluation of nozzle clogging of the tundish nozzle and powder surface defects on the surface layer of the slab during continuous casting in the inventive examples and comparative examples. In the evaluation of nozzle clogging, the mark ○ indicates that no nozzle clogging has occurred, the mark X indicates that nozzle clogging has occurred, and in the evaluation of powdery surface defects on the surface layer of the slab, the mark ○ indicates powdery inclusions. Without slab care,
The crosses indicate that the cast slabs were cleaned with powdery inclusions.

As is apparent from Table 1, in the production of round billet slabs having a small cross-sectional size, if even one casting condition is out of the range of the present invention, the intended purpose cannot be achieved. It has been found that the intended purpose can be achieved when the casting conditions are within the scope of the present invention.

[0031]

As described above, according to the present invention, in continuous casting of small-section aluminum-killed steel slab having a slab cross-sectional area of 0.1 m ² or less, such as billet, Ca
Al-killed steel slab with small cross-section stably without adding treatment, preventing Al ₂ O ₃ from adhering to the inner surface of the tundish nozzle, and without generating powdery defects on the surface layer of the slab As a result, an aluminum-killed steel small-section slab having high cleanliness and excellent mechanical properties can be obtained by a continuous casting method, and an industrially useful effect is brought about.

[Brief description of the drawings]

FIG. 1 shows a ratio Q / S between an Ar flow rate Q blown from the lower half of a cap nozzle and a mold cross-sectional area S during casting of an aluminum killed steel, a molten metal level variation Δh of the molten steel in the mold, and it is a diagram showing the relationship between al ₂ O ₃ deposition layer thickness d.

FIG. 2 is a schematic explanatory view of a continuous casting machine showing one embodiment of the present invention.

FIG. 3 is a schematic vertical sectional view of a main part of a tundish nozzle used in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Molten steel 2 Ladle 3 Tundish 4 Mold 5 Mold powder 6 Cast piece 7 Tundish nozzle 8 Cap nozzle 9 Stopper 10 Stopper opening control device 13 Eddy flow level gauge 14 Inert gas introduction pipe

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Osako 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Kimio Inagaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor: Sho Ishizaka 1-2-1, Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 4E004 HA02 NB02 NC01

Claims (2)

[Claims] In the continuous casting of a small-section aluminum-killed steel slab having a slab cross-sectional area of 0.1 m ² or less, the atmosphere in the tundish is cut off from the outside air,
Deoxidation is performed and molten steel substantially free of Ca is injected, and a tundish nozzle having a cap nozzle provided thereon is immersed in molten steel in a mold covered with mold powder, and the cap nozzle is while blowing an inert gas within the mold cross-sectional area 1 m ² per 20~50Nl / min from the bottom, a small section aluminum killed to the molten steel in the tundish through the tundish nozzle, characterized in that the cast into the mold A continuous casting method for steel slabs. 2. The method for continuously casting a small-section aluminum-killed steel slab according to claim 1, wherein the cross-sectional area of the slab is 0.03.
continuous casting method of the small section aluminum killed steel slab, characterized in that the m ² or less.