JP2003320444A - Casting method for aluminum killed steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the quality of a slab by reducing the amount of large alumina inclusions in an aluminum killed steel slab. <P>SOLUTION: A refractory containing CaO content of 20 mass % or more is applied for 50% or more of the inner hole total area of the divided nozzles comprising an upper nozzle, a sliding nozzle, a lower nozzle, and an immersion nozzle or the like, which allows the alumina in molten steel to be absorbed. Suppressing the alumina from sticking on the inner hole prevents coalescence of alumina to suppress the large alumina inclusions from intruding in the slab. <P>COPYRIGHT: (C)2004,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for casting aluminum-killed steel, and more particularly to a nozzle used for casting aluminum-killed steel. 2. Description of the Related Art In the casting of aluminum-killed steel, alumina adhered to the inner surface of a casting nozzle (hereinafter referred to as "nozzle") is united into large inclusions, which are formed in a slab together with the molten steel flow. It is taken in and becomes a defect of the cast slab, deteriorating quality. Therefore, in recent years, aluminum-killed steel, which is cast as a high-grade steel such as a thin plate, has recently been used for pouring molten steel from a tundish (hereinafter referred to as TD) into a mold in continuous casting in accordance with stricter steel quality. Many efforts have been made to prevent alumina deposition on the inner bore of the nozzle. As an example of the countermeasure, there is a method of blowing argon gas into molten steel from the inner surface of a nozzle to physically prevent the adhesion of alumina. However, in this method, if the amount of argon gas blown is too large, bubbles are taken into the slab and become pinholes, resulting in defects. Therefore, the blowing of the argon gas cannot always be a sufficient countermeasure because the blowing amount of the gas is restricted. There is also a method in which the refractory material constituting the nozzle itself has a function of preventing adhesion of alumina. This is a method in which CaO is contained in a brick to react with alumina adhered thereto to generate a low melt. For example, Japanese Patent Publication No. 61-4 / 1986
No. 4836 discloses a casting nozzle using a refractory mainly composed of a raw material obtained by combining graphite and sintered calcia, electrofused calcia, or another ceramic raw material containing a CaO component. [0006] Usually, when casting steel, as a nozzle for injecting molten steel from a TD into a mold, a split type nozzle in which a plurality of split nozzles shown in FIG. 1 are combined is used. And a single type consisting of a single nozzle shown in FIG. The split type nozzle is a combination of an upper nozzle 2, a sliding nozzle 3, a lower nozzle 4, and an immersion nozzle 5 immersed in a mold 6, which are attached to the bottom opening of the tundish 1. The flow rate into the mold 6 is controlled by adjusting the degree of opening of the opening of the sliding nozzle 3. The excellent flow rate control stabilizes the molten metal surface, thus enabling stable casting and safety. It is also excellent in terms of point and is widely used. [0008] The single-type nozzle has a flow path from the bottom opening of the tundish 1 into the mold 6 formed by a single long immersion nozzle 8.
The degree of opening of the bottom opening of the tundish 1 is adjusted by the long stopper 7 disposed in the inside, and the flow rate into the mold 6 is controlled. When the material containing CaO is applied to the inner hole of a single type nozzle as shown in FIG. 2, the adhesion of alumina to the inner hole surface is certainly reduced. However, when applied to the split-type nozzle shown in FIG. 1, large alumina-based inclusions in the slab increase compared to when applied to the single-type nozzle, and the quality of the slab is improved. Did not contribute much. [0010] The present invention is to cast aluminum-killed steel which greatly reduces the content of large alumina-based inclusions in the slab even if the material containing CaO is applied not only to a single type but also to a split type nozzle. Provide a way to The method for casting aluminum-killed steel according to the present invention is characterized in that at least 50% of the total area of the inner holes of all nozzles used for injecting molten steel from a tundish into a mold is improved. And a nozzle formed of a refractory containing 20% by mass or more of CaO. In a casting method using a single type nozzle, molten steel passing through the nozzle and air hardly come into contact with each other.
In the split type, air flows in from the joints of the nozzles, and in particular, the SN needs to be slid during use, so it is difficult to seal between the surfaces, and air enters from between the surfaces. However, even if CaO-containing refractories are applied, the effect cannot be obtained. According to the present invention, a CaO-containing refractory is applied to each inner hole of a nozzle for injecting molten steel from a TD into a mold, and the refractories are used in combination to reduce the content of large alumina-based inclusions in a slab. As a result of the investigation, the content of large alumina-based inclusions in the slab was based on the finding that there was a very strong correlation between the total area of the inner hole of the nozzle and the amount of CaO in the refractory applied. . [0014] Aluminum-killed steel has aluminum dissolved in molten steel and is oxidized when contacted with air to produce alumina. The alumina thus produced is taken into the cast slab to become alumina-based inclusions. In the case of a split type nozzle divided by a plurality of nozzles, even if a CaO-containing refractory is applied to a specific nozzle, alumina adheres to a nozzle to which the CaO-containing refractory is not applied, and coalesces there. The enlarged alumina is taken into the slab. The refractory containing CaO has a function of lowering the melting point of the attached alumina and a function of adsorbing the alumina. For this reason, the refractories containing CaO are TD
By applying the method to a nozzle for injecting molten steel from a mold into the mold, the amount of alumina flowing into the mold can be reduced. When a refractory containing 20% by mass or more of CaO is applied to 50% or more of the inner holes of the nozzle,
The function of adsorbing the alumina of the refractory containing CaO and the low melt generated by the reaction of CaO and alumina exhibit a liquid phase, so that the inner wall surface becomes smooth, and the adhesion of alumina is prevented. Due to the synergistic effect with the action of preventing coalescence of alumina, the content of large alumina-based inclusions in the slab is dramatically reduced. In order to exert the synergistic effect, it is necessary to apply the refractory containing CaO to the inner hole of 50% or more of the total inner hole area of the nozzle. If it is less than 50%, the effect of reducing the amount of alumina flowing into the mold is small, and the improvement of the content of large alumina-based inclusions in the slab is small. It is preferably at least 60%, and most preferably, the entire inner surface of all nozzles is made of a CaO-containing refractory. However, if the use of a refractory containing CaO causes erosion, abrasion, etc. and causes a problem in use, it is important to select that part according to the use conditions, such as using a conventional refractory. It is. According to the present invention, the entire nozzle is composed of an upper nozzle and an immersion nozzle, or an SN and an immersion nozzle, or an upper nozzle and an SN and an immersion nozzle, and further, as shown in FIG. In the case of the so-called split type in which the CaO-containing refractory occupies 50% or more of the total inner hole area of the entire nozzle, even in the case of the so-called split type in which the In any case, it is applicable. Further, even if the refractory containing CaO is applied to a part of the inner hole of the nozzle, if it is applied so as to be 50% or more of the total area of the inner hole of the entire nozzle, a great improvement effect of the slab quality can be obtained. Can be The present invention can be similarly applied to a case including a nozzle in which the upper nozzle and the SN or the SN and the lower nozzle are integrated. When the CaO content of the refractory applied to the inner hole of the nozzle is less than 20% by mass, the alumina adsorption ability and the alumina adhesion preventing ability are small and the degree of improvement in the content of large alumina-based inclusions in the slab is reduced. Is small, so it is necessary to be 20% by mass or more. In terms of the effect of reducing the presence of large alumina inclusions in the cast piece, there is no upper limit for the amount of CaO contained in the refractory. However, when CaO increases, erosion increases or digestion becomes easier. Therefore, it is important to appropriately adjust according to use conditions. Under general casting conditions, a CaO content of about 60% by mass is sufficient. Examples of preferred refractories include MgO-C
aO-based refractory, MgO-CaO-C-based refractory, ZrO2
-CaO-based refractories, ZrO2-CaO-C-based refractories, and the like. In particular, MgO-CaO-based refractories, MgO-
CaO-C refractories are more preferable because of their excellent ability to adsorb alumina. It is important that the CaO-containing refractory is applied to at least the inner surface of each nozzle in contact with molten steel, and the same material may be used for portions other than the inner surface.
A commonly used refractory may be applied as it is. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to an embodiment in which the present invention is applied to a divided nozzle shown in FIG. [Table 1] Table 1 shows the composition of the material applied to each of the divided nozzles shown in FIG. The materials A and B shown in the table are CaO-containing materials according to the present invention, and the materials C and D are conventional general materials not containing CaO used for comparison. A refractory molded body was formed by molding, sintering, and processing the materials A to D into a sleeve having a thickness of 10 mm, and was inserted into the nozzle and bonded with mortar to obtain a test nozzle. Materials A and C were applied to the immersion nozzle, and B and D were applied to the upper nozzle, the sliding nozzle (SN), and the lower nozzle. Table 2 shows the surface area of the inner hole of each nozzle to which the CaO-containing refractory was applied. [Table 2] Combining these test nozzles with the upper nozzle, SN, lower nozzle, and immersion nozzle to form a series of nozzle structures for continuous casting, investigating the effect of the nozzle material in these combinations on the quality of slabs, The effect of applying CaO-containing refractories was investigated. Investigation was conducted by changing the combination of nozzles, casting aluminum killed steel under the casting conditions of 250 ton pot capacity, 45 ton TD capacity, and 1.0 to 1.3 m / min casting speed, and included in the obtained castings. The effect was examined by the number of large alumina-based inclusions having a size of 50 μm or more per area. [Table 3] Table 3 shows the results of the survey. In the same table, the number of each example was indicated by an index, with the number of large alumina-based inclusions in the slab obtained by the combination nozzle of Comparative Example 1 being 100. Therefore, the smaller the index, the smaller the size of large alumina-based inclusions and the better the quality of the slab. FIG. 3 is a table summarizing the results of Table 3, and shows the relationship between the area ratio of the CaO-containing refractory in the entire inner hole of the nozzle and the number of large alumina-based inclusions.
From the figure, it can be seen that when the area ratio of the CaO-containing refractory to the entire inner hole of the nozzle is 50% or more, the number of inclusions is greatly reduced, and the quality of the slab is improved. And, the quality of the slab becomes better as the area ratio of the CaO-containing refractory further increases, and it is understood that it is most preferable to apply the CaO-containing refractory to all the nozzle bores. Next, in the nozzle configuration shown in FIG.
The influence of the CaO content in the CaO-containing refractory on the quality of the slab was investigated. [Table 4] Table 4 shows the compositions of the CaO-containing refractories E to L in addition to A and B shown in Table 1. From these CaO-containing refractories, as in the case of the materials shown in Table 1 above, a thickness of 10 mm
A refractory molded body was formed, fired, and processed into a sleeve shape, and inserted into a nozzle and bonded with mortar to form a test nozzle. A, E, F, G, H to the immersion nozzle, B,
I, J, K, and L were applied to the upper nozzle, SN, and lower nozzle. The surface area of the inner hole of each nozzle is the same as in Table 2. Table 5 shows the results of investigating the quality of the slab by performing casting by combining these nozzles. The quality evaluation method is the same as in Table 3. [Table 5] The results in Table 5 are summarized in FIG. 4, and the relationship between the average CaO content of the refractory in the bore of the nozzle and the number of large alumina-based inclusions is shown. As shown in this figure, it is clear that when the average content of CaO in the bored refractory is 20% by mass or more, the quality of the slab is greatly improved. According to the present invention, during the casting of aluminum-killed steel, the alumina in the molten steel is adsorbed on the wall surface of the nozzle inner hole, and the low melt is formed by the reaction between alumina and CaO in the refractory. By suppressing the coalescence of alumina, the content of large alumina in the slab can be greatly reduced, and the production cost can be reduced by reducing the quality defect rate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an example of the structure of a split-type nozzle composed of a plurality of nozzles provided with SNs. FIG. 2 shows a structural example of a single type nozzle. FIG. 3 shows the relationship between the area ratio of the CaO-containing refractory in the entire inner hole of the nozzle and the number of large alumina-based inclusions. FIG. 4 shows the relationship between the average CaO content of the inner hole refractory of the nozzle and the number of large alumina-based inclusions. [Description of Signs] 1 Tundish 2 Upper nozzle 3 Sliding nozzle 4 Lower nozzle 5 Immersion nozzle 6 Mold 7 Long stopper 8 Immersion nozzle

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[Procedure amendment] [Date of submission] May 22, 2002 (2002.5.2)
2) [Procedure amendment 1] [Document name to be amended] Description [Item name to be amended] 0009 [Correction method] Change [Contents of amendment] The material containing CaO as shown in FIG. When applied to the bore of a single type nozzle, alumina deposition on the bore is certainly reduced. However, when applied to the nozzle of a part of the split type shown in FIG. 1, compared with the case of applying to the single type nozzle,
It was found that large-sized alumina inclusions in the slab increased and did not contribute much to the improvement of the quality of the slab.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Keisuke Asano             1-1 Higashihama-cho, Yawatanishi-ku, Kitakyushu-shi, Fukuoka             Kurosaki Harima Co., Ltd. (72) Inventor Toshiyuki Hokkii             1-1 Higashihama-cho, Yawatanishi-ku, Kitakyushu-shi, Fukuoka             Kurosaki Harima Co., Ltd. (72) Inventor Takeki Yoshitomi             1-1 Higashihama-cho, Yawatanishi-ku, Kitakyushu-shi, Fukuoka             Kurosaki Harima Co., Ltd. F-term (reference) 4E014 DA03

Claims (1)

Claims 1. A casting nozzle used for injecting molten steel from a tundish into a mold contains at least 50% of the total area of the inner hole of the casting nozzle and at least 20% by mass of CaO. A method of casting aluminum killed steel using a nozzle formed of refractory.