JPH06599A - Method for continuously casting aluminum-killed steel for cold rolling - Google Patents

Abstract

PURPOSE:To enable continuous casting of an ultra low carbon aluminum-killed steel having excellent quality by adding a specific quantity of Ca into molten steel, using the inner wall of a tundish nozzle, immersion nozzle, etc., with the refractory having a specific component and dispensing with gas blowing into the nozzle part. CONSTITUTION:At the time of pouring the molten steel 2 into the tundish 6 from a ladle 4, Ca-Si alloy is added from an injection lance 8 while carrying with argon gas flow, Ca quantity is added in >=5ppm - <10ppm concn. The molten steel 2 overflows weirs 20, 22 through an opening part 18 at the lower end of a weir 16 being around a pouring nozzle 14 from the ladle 4 and reaches to the tundish nozzle 24 and is cast into a mold 30 through the immersion nozzle 28. The inner walls of the tundish nozzle 24 and the immersion nozzle 28 are made of refractory containing >=15% CaO. Further, the blowing off of the inert gas into these nozzles is not entirely executed. By this method, Al2O3 of inclusion in the molten steel is not deposited and stuck to the nozzle part and blow hole in the cast slab is not developed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting aluminum-killed steel for cold rolling, and in particular, the inner wall of the submerged nozzle can be closed without blowing an inert gas into the inner wall. The present invention relates to a continuous casting method for ultra low carbon aluminum killed steel for cold rolling.

[0002]

2. Description of the Related Art Recent advances in secondary refining techniques such as vacuum degassing techniques have enabled continuous casting of ultra-low carbon aluminum killed steel. Degassing technology made this possible, and as a result, it became possible to manufacture high-cleanliness steel with few non-metallic inclusions. However, when an extremely low carbon cold rolled steel sheet is annealed, the surface of the annealed steel sheet has a width of 1 to 4 mm and a width of 1 to 6
It was found that a cm raised portion was generated. This raised portion is a so-called "blister defect", which often occurs especially in the case of an ultra-low carbon steel having a carbon content of 0.015% by weight or less. It was a cause of a large drop in the stay. The cause of this blistering defect is
It has been found that the influence of non-metallic inclusions in steel, especially Al ₂ O ₃ , is great.

In the case of ultra-low carbon steel, since there is a marked difference in the deformation strength between the matrix of base metal and inclusions, when cold rolling is performed, the portion of the matrix that is soft with respect to hard Al ₂ O _{3 is used} . Has a larger elongation, and as a result, a void is formed at the boundary between the two. The void is proportional to the size of the Al ₂ O _3, the more the voids increases the larger the Al ₂ O ₃ inclusions. However when annealed cold-rolled steel sheet having such voids, the but of H ₂ atmosphere gas AX gas or the like into the void from entering, because the solubility of H ₂ in particular during quenching cooling is reduced, of H ₂ in the void The partial pressure rises and the surface of the steel sheet in the vicinity of Al ₂ O ₃ swells, resulting in what is called a “blister defect”. Such a "blister defect" does not occur when the void is small, that is, when the size of the Al ₂ O ₃ inclusion is small, because the increase in the internal pressure is small. Furthermore, according to the study of the trap to the Al ₂ O ₃ in the steel to produce this blister defects, the majority dipping a part of Al ₂ O ₃ aggregated adhered to the immersion nozzle are mainly composed of Al ₂ O ₃ It has been found to have been carried away from the nozzle surface. Therefore, in order to prevent this blistering defect, it is most important to prevent Al ₂ O ₃ from adhering to the immersion nozzle.

From this point of view, conventionally, an inert gas was blown into the immersion nozzle, and Ca or Ca-Si was added into the molten steel.
It was added Ca alloy etc., also suppress aggregation deposition amount of the Al ₂ O ₃ inclusions and a low melting point of CaO-Al ₂ O ₃ inclusions, Al ₂ O ₃ to the immersion nozzle by its melting point lowering amount Techniques for doing so are disclosed. For example, in Japanese Patent Laid-Open No. 61-276756, "2 to 40 wt% of steel is added by adding Ca or a Ca alloy to a molten aluminum killed steel containing C≤0.015 wt% at the melting stage or continuous casting. A method for preventing blistering defects in an ultra-low carbon cold-rolled steel sheet is characterized by treating ppm ultra-low carbon so as to form CaO-Al ₂ O ₃ -based inclusions. "

Further, in Japanese Patent Laid-Open No. 1-99761, "When continuously casting aluminum killed steel, a refractory cylinder whose lower end is immersed in molten steel of a tundish within a distance of 1 m from the mounting center position of a tundish nozzle is arranged. However, 5 to 20 ppm of Ca is added to the amount of molten steel passing through the tundish nozzle in the refractory cylinder, and a continuous casting method of aluminum killed steel is disclosed. However, in the method of blowing an inert gas such as argon into the immersion nozzle, the blown inert gas is trapped by the solidified shell in the mold, there is a problem that becomes the origin of blowholes or swelling defects of the slab, and Ca or Ca
The method of adding the O alloy has a problem that if the amount of Ca added to the steel exceeds 10 ppm in the steel material, rust tends to occur, and on the other hand, the immersion nozzle A
To prevent blockage by l ₂ O ₃ aggregated attachment may antinomic problem with many 20~50ppm as Ca concentration in the steel is required.

[0006]

The requirements for continuous casting of ultra-low carbon aluminum killed steel for cold rolling, which the present invention aims to achieve, are as follows. (A) The amount of Ca added to molten steel should be minimized, and should not exceed 10 ppm in order not to cause rusting of steel materials. (B) Moreover, the added Ca has a low melting point of CaO--Al ₂ O ₃
To prevent Al ₂ O _{3 from} adhering to the dipping nozzle by forming a system compound. (C) Inner surfaces of the upper nozzle, sliding nozzle and immersion nozzle of the tundish through which the Ca-containing molten steel passes,
To increase the content of CaO to facilitate the formation of a CaO-Al ₂ O ₃ -based low melting point compound. (D) Completely eliminate the blowing of inert gas into the nozzle. An object of the present invention is to provide an effective continuous casting method of aluminum killed steel for cold rolling which can satisfy the above requirements.

[0007]

The gist of the present invention is as follows. That is, (1) In the continuous casting method for ultra-low carbon aluminum killed steel for cold rolling, Ca is 5 ppm in the ultra-low carbon aluminum killed molten steel.
And a step of adding to a concentration of less than 10 ppm,
The upper wall of the tundish for continuous casting, the sliding nozzle, and the inner wall of the dipping nozzle had a CaO content of 15
% Refractory, and continuous casting of aluminum killed steel for cold rolling characterized in that no inert gas is blown into the upper nozzle, the sliding nozzle and the dipping nozzle. Method. (2) In the immersion nozzle during continuous casting of the ultra-low carbon aluminum killed molten steel, a low melting point compound such as 12CaO.7Al ₂ O ₃ is formed to prevent Al ₂ O ₃ from adhering to the inner wall of the immersion nozzle. The continuous casting method for cold rolled aluminum killed steel according to (1) above.

As described in the above-mentioned section of the prior art, when aluminum-killed steel is continuously cast, the Al ₂ O ₃ -based oxide, which is a deoxidation product, agglomerates and adheres to the inner wall of the dipping nozzle, and the casting is continued. As a result, it is well known that the following measures have been conventionally taken to prevent nozzle clogging because the immersion nozzle is clogged and it becomes impossible to supply molten steel from the tundish to the mold. (A) Inert gas is blown into the nozzle. (B) Add Ca into the ladle (Japanese Patent Laid-Open No. 58-1544).
47) (C) Add Ca grains or Ca powder from a ladle to the injection hole of the tundish. (D) Add Ca alloy from the ladle to the injection hole of the tundish with a wire. (Japanese Patent Application Laid-Open No. 61-1457) However, in the case of the conventional method, although any of the above-mentioned methods still has a problem as described above, it is unavoidable to use these methods together, and therefore the nozzle is used today. An epoch-making method in which no inert gas is blown into the inside has not yet been found. However, the essential constituents of the present invention exert a synergistic effect on each other and make this possible, and the reasons for limiting the constituents of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a case of two strands of continuous casting aluminum-killed steel for cold rolling according to the present invention, in which a Ca-Si alloy is added by Ar by air into molten steel when pouring into a tundish from a ladle. FIG. 2 is an enlarged cross-sectional view showing the situation during casting from the tundish nozzle through the immersion nozzle into the mold. First, an operating procedure for continuous casting of ultra low carbon aluminum killed steel according to the present invention will be described. The ultra-low carbon molten steel 2 that has been refined in a converter and, if necessary, decarburized by a vacuum degassing device such as RH, and the secondary refining of degassing has been completely deoxidized with aluminum and stored in a ladle 4. It is carried into the continuous casting machine at. C according to the invention
The addition of a or Ca alloy may be done by using a Ca alloy wire or the like in the ladle 4, but in FIG. 1, it is provided near the tundish 6 when the molten steel 2 is poured from the ladle 4 into the tundish 6. It shows the case where the Ca-Si alloy is added to the molten steel injection flow 2A by the injection lance 8 by pneumatic feeding of argon. The tundish 6 has a tundish cover 10 for cooling the molten steel 2 contained therein and for preventing oxidation, and further has an Ar filling hole 12 for filling the inert gas by the deaeration operation to strengthen the oxidation prevention. Further, a partition weir 16 is provided around the injection nozzle 14 from the ladle 4 to the tundish 6 to shut off the outside air, and the injected molten steel 2 avoids mixing of the slag 3 and an opening 18 at the lower end of the weir 16. Through weir 2
0,22 and reaches the tundish nozzle 24, which is usually called the upper nozzle, and the sliding nozzle 26
And is cast into the mold 30 via the immersion nozzle 28. The aluminum-killed steel slab 32 formed by the mold 30 is guided by the guide roll 38 and pulled downward.

FIG. 2 shows the upper nozzle 24 of the tundish 6.
FIG. 3 is an enlarged cross-sectional view showing a state where molten steel 2 is cast into a mold 30 through a sliding nozzle 26 and a dipping nozzle 28. Conventionally, an inert gas such as Ar is externally supplied from a tundish upper nozzle 24 by a lance 34. Although the blown Al ₂ O ₃ was prevented from aggregating and adhering into the injected molten steel 2, the lance 34 is not necessary because no inert gas is blown in the present invention. The molten steel 2 cast in the mold 30 is rapidly cooled by a water-cooled copper plate to form a solidified shell 36 from a portion in contact with the mold 30 and is withdrawn as a cast piece 32. According to the present invention, the inert gas is not blown into the nozzle portion. Since it does not, the surface of the mold 30 is quiet and the mold powder 3
It was possible to obtain a slab slab 32 of excellent quality with very few inclusions having no inclusions.

In the present invention, the concentration of Ca added to the molten steel 2 is limited to 5 ppm or more and less than 10 ppm for the following reason. That is, by adding Ca to the molten steel 2 and reacting partially oxidized CaO with Al ₂ O ₃ in the steel, 12 CaO.
To form a low melting point compound of 7Al ₂ O ₃ ,
Ca is insufficient CaO with too small in Ca5ppm, difficult to make a compound 12CaO · 7Al ₂ O ₃ of the low melting point, because easily migrate to CaO · 2Al ₂ O ₃ side of the high-melting compound. Also C
When the content of a exceeds 10 ppm, it promotes the generation of rust in the cold rolled steel sheet, which is not preferable. As a result, the added amount of Ca is 5ppm
It is limited to less than 10 ppm. Although any method may be used for adding Ca, it is natural that since Ca is an expensive element, the method with the highest addition yield should be selected.

[0012] Then, the inner wall of the nozzle 24, the sliding nozzle 26 and the immersion nozzle 28 on the tundish 6 to be used in the present invention, should be CaO content of 15% or more CaO-ZrO ₂ -C refractories However, it was found from the following experiments by the present inventors that this constituent factor has a very important relation to the effect of the present invention together with the constituent factor of Ca addition amount of 5 ppm or more and less than 10 ppm. That is, the inventors of the present invention added ultra low carbon aluminum killed steel with a Ca concentration of 8 p
When Ca was added so as to be pm and continuous casting was performed, Al ₂ O ₃ remarkably adhered to the nozzle portion and clogged. As a result of investigating the cause, it was found that a high melting point compound of CaO.2Al ₂ O ₃ was formed in the molten steel. At that time, the refractory materials on the inner wall of the upper nozzle 24, the sliding nozzle 26 and the immersion nozzle 28 were All used alumina graphite. Therefore, the present inventors have decided that this is CaO-Zr.
By changing to O ₂ -C refractory, the same Ca concentration of 8
When continuous casting was performed at ppm, it was possible to smoothly perform continuous casting without clogging of the nozzle portion and blowing of inert gas at all.

As a result of further research on the above experimental results,
In order to react the Al ₂ O ₃ -based compound adhering to the working surface of the nozzle refractory with CaO to generate a low-melting 12CaO · 7Al ₂ O ₃ compound, CaO in the nozzle refractory is used.
It was found that the content should be at least 15% or more. The yield of addition of Ca to molten steel is about 15%, but the yield increases if conditions such as complete degassing are good. Therefore, in the present invention, the minimum of 5 ppm or more and 10 ppm from the risk of rusting of steel materials.
In order to facilitate the formation of the low melting point compound 12CaO.7Al ₂ O ₃ by compensating for the insufficient Ca concentration, the content of CaO in the refractory on the inner wall of the nozzle is 1
It was found that if it is less than 5%, it is insufficient, and at least 15% or more is required. The upper limit of the CaO content in the refractory material on the inner wall of the nozzle is not particularly limited, but the melting loss rate becomes high as the CaO content increases, and therefore 40% or less is preferable.

[0014]

The reason for limiting the two important constituent elements of the present invention has been described, but these two constituent elements have a very close correlation with each other. That is, the amount of Ca added to the aluminum-killed molten steel is limited to 5 ppm or more and less than 10 ppm. The amount of Ca added is generally 20 to 30 ppm although it depends on the oxygen concentration and S concentration in the steel to prevent nozzle clogging in conventional continuous casting. From the point of view of the above, the Ca limited amount of the present invention is insufficient to prevent nozzle clogging by the Al ₂ O ₃ based oxide. In the present invention, the CaO content on the working surface of the refractory in the nozzle portion through which the molten steel containing Ca passes this Ca deficiency is set to 15% or more, so that the Al ₂ O ₃ -based oxide cohesively adhered to the inner wall of the refractory member in the nozzle portion The above reaction produces 12CaO.7Al ₂ O ₃ having a melting point of 1455 ° C. 12CaO + 7Al ₂ O ₃ → 12CaO.7Al ₂ O ₃ CaO alone has a melting point of 2600 ° C., and Al ₂ O _{3 has} a melting point of 202.
Considering that the temperature is 0 ° C., 12CaO · 7Al ₂ O ₃ has an extremely low melting point, which is lower than the temperature of the molten steel itself, and therefore naturally does not precipitate and adhere. In the present invention, the inner wall of the tundish upper nozzle 24, the sliding nozzle 26, and the dipping nozzle 28 in the nozzle portion is limited to be a refractory material having a CaO content of 15% or more. the CaO content of 15% or more CaO-ZrO ₂ -C refractories can be used. The use of this refractory material in the nozzle portion is also proposed in JP-A-4-28462, but the inventors of the present invention have found that the use of this refractory material alone has little effect of preventing Al ₂ O ₃ from adhering. It is clear from the experimental results of. Incidentally, JP-A-4-2846.
According to No. 2, according to the above-mentioned ZrO ₂ —CaO—C refractory, the immersion nozzle which makes the active SiO ₂ content 0.3 wt% or less is Al.
_It is said that it is effective in preventing ₂ O ₃ adhered and blocked. The present invention is
Ultra low carbon aluminum-killed molten steel in a Ca 5 ppm or more as nuclear, was added to a concentration of less than 10 ppm, and Tan CaO-ZrO ₂ -C refractories of the nozzle inner wall CaO content of 15% or more of the dishes and However, this is an epoch-making technology in which the two constituent elements exert a synergistic effect on each other to completely eliminate the blowing of the inert gas into the nozzle portion.

[Example] C: 0.0020%, Mn: 0.30%, Al: 0.030
% Ultra-low carbon aluminum killed steel 250t melted in a 250t converter with a cross-sectional dimension of 220mm x 1200
It was cast with a mm slab continuous casting machine. Example 1 In a 250-ton ladle of the above-mentioned ultra-low carbon aluminum killed molten steel, a wire of a Ca alloy containing 30% of Ca was fed into the molten steel, and the Ca wire 52 was adjusted so that the pure Ca content was 8 ppm.
Kg was added. Immediately after the addition of Ca, the tundish 6, the ladle 4, and the molten steel 2 were placed in the space formed by Ar as shown in FIG.
Gas was sealed and continuous casting was performed under so-called deaeration. During this casting, no inert gas was blown into the nozzle portion from the lance 34 in FIG. 2, but no Al ₂ O ₃ cohesive adhesion was observed and the 250 t slab casting could be completed smoothly.

Example 2 In the same method as shown in FIG. 1, Ca-Si grains were pneumatically added to a molten steel flow 2A from a ladle 4 to a tundish 6 under an inert atmosphere of Ar, and 250 t of molten steel 2 was added. Ca content of 8pp
The amount of Ca-Si alloy used, which was evenly charged so as to be m, was 370 kg with respect to 250 t of molten steel, and the Ca yield was 18%. In this case also, in FIG. 2, no inert gas was blown into the nozzle portion from the lance 34, but Al ₂ O ₃ aggregated in the nozzle portions of the upper nozzle 24, the sliding nozzle 26, and the dipping nozzle 28. No adhesion was observed at all, and 250 ton slab continuous casting could be completed. In each of Examples 1 and 2, CaO containing 15% or more of CaO in the inner wall of each of the upper nozzle 24, the sliding nozzle 26, and the immersion nozzle 28 shown in FIG.
It was -ZrO ₂ -C refractories. The measured analysis components were as follows. _{CaO: 21.3% ZrO 2: 49.6} % C: 26.8%

[0017]

INDUSTRIAL APPLICABILITY According to the present invention, in continuous casting of ultra-low carbon aluminum killed steel for cold rolling, (a) Ca is added to the molten steel so as to have a concentration of 5 ppm or more and less than 10 ppm. (B) a nozzle on the tundish, the sliding nozzle, and CaO-ZrO ₂ -C refractories inner wall a content more than 15% CaO of the immersion nozzle. (C) No inert gas is blown into the nozzle. The new continuous casting method consisting of the above-mentioned constituents (a), (b), and (c) was completed, and the following effects could be obtained. (A) Even if no inert gas is blown into the nozzle,
No clogging of the nozzle part due to cohesive adhesion of Al ₂ O ₃ based oxide was observed. (B) No rusting of steel was observed in the Ca-added steel. (C) As a result of no blowing of inert gas, no blowholes are generated in the slab, and the molten metal surface in the mold at the time of casting calms down, so there is no entrainment of mold powder or non-metallic inclusions. A small number of slab products were obtained. (D) Since the amount of Al ₂ O ₃ -based nonmetallic inclusions is extremely small, so-called blistering defects are extremely small. (E) Due to the above, the defect occurrence rate of the cold-rolled steel sheet became 0.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a continuous casting apparatus for cold rolling ultra-low carbon aluminum killed steel used in an example of the present invention.

FIG. 2 is an enlarged cross-sectional view showing nozzle portions of the upper nozzle, the sliding nozzle and the immersion nozzle of the tundish of FIG.

[Explanation of symbols]

 2 Molten Steel 3 Slag 4 Ladle 6 Tundish 8 Ca Alloy Injection 10 Tundish Cover 12 Ar Filling Hole 14 Ladle Injecting Nozzle 16 Partition Weir 18 Opening 20, 22 Weir 24 Tundish Nozzle (Upper Nozzle) 26 Sliding Nozzle 28 Dipping Nozzle 30 Mold 31 Mold powder 32 Cast slab 34 Ar blowing lance 36 Solidified shell

Claims (2)

[Claims] 1. A continuous casting method for ultra low carbon aluminum killed steel for cold rolling, wherein the ultra low carbon aluminum killed molten steel is Ca.
Is added so as to have a concentration of 5 ppm or more and less than 10 ppm, and the inner wall of the upper nozzle, the sliding nozzle and the dipping nozzle for the continuous casting tundish is made of a refractory material having a CaO content of 15% or more. A continuous casting method of cold-rolled aluminum-killed steel, characterized in that no inert gas is blown into the upper nozzle, the sliding nozzle and the dipping nozzle. 2. In the immersion nozzle during continuous casting of the ultra low carbon aluminum killed molten steel, 12CaO.7Al ₂ O ₃
A continuous casting method for cold-rolled aluminum-killed steel according to claim 1, wherein Al ₂ O ₃ is prevented from adhering to the inner wall of the immersion nozzle by forming a low melting point compound such as Al.