JP2613525B2 - Continuous casting method of aluminum killed steel for cold rolling - Google Patents

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 an immersion nozzle can be closed without blowing an inert gas into the inner wall as in the prior art. The present invention relates to a continuous casting method of 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 made it possible to continuously cast ultra-low carbon aluminum killed steel. Degassing technology has made this possible, and as a result, it has become possible to produce high-purity steel with few nonmetallic 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 mm.
It was found that a raised portion of cm occurred. This raised portion is a so-called “bulging defect”, which often occurs particularly in the case of ultra-low carbon steel having a carbon content of 0.015% by weight or less. This caused a large drop in the number of The cause of this blister defect is
It has been found that the influence of nonmetallic inclusions in steel, particularly Al ₂ O ₃ , is significant.

[0003] In the case of ultra low carbon steel, due to the significant difference in the deformation strength of the inclusions base steel matrix, soft, part of the matrix with respect to the applying cold rolling, hard Al ₂ O ₃ 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, causing so-called “bulging defects”. Such a “bulge 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 rise in the internal pressure is small. Furthermore, according to a study on trapping in Al ₂ O ₃ steel which causes this blistering defect, most of Al ₂ O ₃ cohesively adhered to an immersion nozzle mainly composed of Al ₂ O ₃ was immersed. It has been found that they 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.

[0004] From this point of view, conventionally, an inert gas is blown into an immersion nozzle, and Ca or Ca-Si is introduced into molten steel.
And the above-mentioned Al ₂ O ₃ inclusions are made into low melting point CaO-Al ₂ O ₃ -based inclusions, and the amount of aggregation and attachment of Al ₂ O ₃ to the immersion nozzle is suppressed by the lowering of the melting point. A technique for performing this is disclosed. For example, Japanese Patent Application Laid-Open No. 61-276756 discloses that “adding Ca or a Ca alloy to an aluminum-killed molten steel containing C ≦ 0.015% by weight at the smelting stage or during continuous casting makes the steel 2-40% by weight. A method for preventing blistering defects in an ultra-low carbon cold rolled steel sheet, which is characterized in that a treatment is performed such that CaO-Al ₂ O ₃ -based inclusions are produced by leaving metal Ca of ppm.

[0005] Japanese Patent Application Laid-Open No. 1-99761 discloses that "in continuous casting of aluminum killed steel, a refractory cylinder having a lower end immersed in molten steel of a tundish at a distance within 1 m from a mounting center position of a tundish nozzle. And adding 5 to 20 ppm of Ca to the amount of molten steel passing through the tundish nozzle into the refractory cylinder. " However, the method of blowing an inert gas such as argon into the immersion nozzle has a problem that the blown inert gas is trapped in the solidified shell in the mold and becomes a starting point of blowholes or blistering defects in the slab, and Ca or Ca
In the method of adding an O alloy, if the amount of Ca added to the steel exceeds 10 ppm in the steel material, there is a problem that rust is easily generated.
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 the ultra-low carbon aluminum killed steel for cold rolling that the present invention aims to achieve are as follows. (B) The amount of Ca added to the molten steel should be minimized and should not exceed 10 ppm in order not to cause rusting of the steel material. (B) Moreover Addition Ca is a low-melting CaO-Al ₂ O ₃
Forming a system compound to prevent Al ₂ O _{3 from} adhering to the immersion nozzle. (C) The inner surface of the upper nozzle, sliding nozzle and immersion nozzle of the tundish through which the Ca-containing molten steel passes,
To increase the CaO content to facilitate the formation of a CaO-Al ₂ O ₃ -based low melting point compound. (D) Eliminate the blowing of inert gas into the nozzle. An object of the present invention is to provide an effective method for continuously casting aluminum killed steel for cold rolling that can satisfy the above requirements.

[0007]

The gist of the present invention is as follows. (1) In the continuous casting method of an ultra-low carbon aluminum killed steel for cooling, 5 pp.
m and less than 10 ppm, and the inner wall of the upper nozzle, sliding nozzle, and immersion nozzle of the continuous casting tundish has a CaO content of
A refractory of 15% or more and 40% or less , wherein no blowing of inert gas is performed at all into the upper nozzle, the sliding nozzle and the immersion nozzle. Casting method of aluminum killed steel for aluminum. (2) In the immersion nozzle during continuous casting of the extremely 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 of aluminum-killed steel for cold rolling according to the above (1).

As described in the section of the prior art, when aluminum-killed steel is continuously cast, Al ₂ O ₃ -based oxide as a deoxidized product coheres and adheres to the inner wall of the immersion nozzle, causing clogging and continuing the casting as it is. As a result, it is well known that the following countermeasures have been taken in the past to prevent nozzle blockage, because the immersion nozzle is blocked and molten steel cannot be supplied from the tundish to the mold. (A) Inert gas is blown into the nozzle. (B) Add Ca to the ladle (Japanese Patent Laid-Open No. 58-1544)
47) (C) Add Ca grains or Ca powder from the ladle to the injection hole of the tundish. (D) Add the Ca alloy from the ladle to the injection hole of the tundish with a wire. However, in the case of the conventional method, problems still remain with any of the above-described methods, as described above. An epoch-making method in which no inert gas is blown into the inside has not yet been found. However, the essential components of the present invention exert a synergistic effect mutually, and this is made possible. The reasons for limiting the respective components of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows the case of continuous casting of aluminum-killed steel for cold rolling according to the present invention in the case of two strands, in which a Ca-Si alloy is pneumatically added by Ar into molten steel when poured from a ladle into a tundish. FIG. 2 is an enlarged cross-sectional view showing a situation during casting from a tundish nozzle into a mold via an immersion nozzle. First, the operation procedure of continuous casting of ultra-low carbon aluminum killed steel according to the present invention will be described. The ultra-low carbon molten steel 2 which has been refined in a converter and, if necessary, decarburized and degassed by a vacuum degassing device such as RH, is completely deoxidized with aluminum and stored in a ladle 4. Is carried into the continuous casting apparatus. C according to the invention
Although the addition of a or Ca alloy may be added to the ladle 4 with a Ca alloy wire or the like, FIG. 1 shows that the molten steel 2 is provided near the tundish 6 when the molten steel 2 is poured into the tundish 6 from the ladle 4. The case where Ca-Si alloy is added to molten steel injection flow 2A by injection lance 8 by argon pneumatic feeding is shown. The tundish 6 has a tundish cover 10 for cooling the contained molten steel 2 and preventing oxidation, and further has an Ar filling hole 12 for filling an inert gas by a degassing operation to enhance 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 is opened at the lower end 18 of the weir 16 to avoid mixing of the slag 3. Through the weir 2
0, 22 to reach the tundish nozzle 24, commonly referred to as the upper nozzle, and the sliding nozzle 26
And it 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 out downward.

FIG. 2 shows the upper nozzle 24 of the tundish 6,
FIG. 3 is an enlarged cross-sectional view showing a situation in which molten steel 2 is cast into a mold 30 via a sliding nozzle 26 and a dipping nozzle 28, and conventionally, an inert gas such as Ar is externally supplied by a lance 34 from the upper nozzle 24 of the tundish. Although the blown-in Al ₂ O ₃ was prevented from agglomerating and adhering into the molten steel 2 to be injected, 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 quenched by a water-cooled copper plate and forms a solidified shell 36 from a portion in contact with the mold 30 and is drawn out as a slab 32. According to the present invention, the inert gas is blown into the nozzle portion. The mold surface is quiet and the mold powder 3
It was possible to obtain a slab cast piece 32 of excellent quality with a very small number of inclusions without any entanglement.

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 reasons. That is, Ca is added to the molten steel 2, and CaO partially oxidized and reacted with Al ₂ O ₃ in the steel to form 12 CaO ·
7Al ₂ O ₃ to form a low melting point compound,
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
If the content of a exceeds 10 ppm, it is not preferable because rust is promoted in the cold-rolled steel sheet. As a result, the added amount of Ca was 5 ppm
It was limited to less than 10 ppm. Although any method of adding Ca may be used, since Ca is an expensive element, it is natural that a method with the best 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 by the following experiments by the present inventors that this constituent element, together with the constituent element with a Ca content of 5 ppm or more and less than 10 ppm, has a very important relationship with the effect of the present invention. In other words, the present inventors added a Ca concentration of 8p to ultra-low carbon aluminum killed steel.
When Ca was added so as to reach pm and continuous casting was performed, Al ₂ O ₃ was remarkably adhered to the nozzle portion and closed. Investigation of the cause revealed that a high melting point compound of CaO.2Al ₂ O ₃ was formed in the molten steel. At this time, the inner wall refractories of the upper nozzle 24, the sliding nozzle 26 and the immersion nozzle 28 In each case, alumina graphite was used. Then, the present inventors set this to CaO-Zr.
By changing to an O ₂ -C refractory, the same Ca concentration of 8
When continuous casting was performed at ppm, the continuous casting was smoothly performed without clogging of the nozzle portion and without blowing any inert gas.

As a result of further studying the above experimental results, Al ₂ O ₃ -based compounds adhering to the working surface of the refractory in the nozzle were reacted with CaO to form a low melting point 12CaO · 7Al _2.
It has been found that the CaO content in the refractory at the nozzle should be at least 15% or more in order to generate the O ₃ compound. The addition yield of Ca into molten steel is about 15%, but if conditions such as complete deaeration are good, the yield will increase.
In the present invention, since the risk of rusting steel is limited to less than the minimum 5ppm or 10 ppm, to compensate for the lack of Ca concentration in order to facilitate the formation of 12CaO · 7Al ₂ O ₃ of low melting compounds , C in the refractory on the inner wall of the nozzle
If the aO content is less than 15%, it is insufficient.
It turned out to require more than%. The upper limit of the CaO content in the refractory on the inner wall of the nozzle portion is as follows.
Increases the erosion rate, so it is limited to 40% or less
I did .

[0014]

The reasons for limiting the two important components of the present invention have been described. These two components have a very close correlation with each other. That is, the addition of Ca to the aluminum-killed molten steel is limited to 5 ppm or more and less than 10 ppm. However, the amount of Ca varies depending on the oxygen concentration and S concentration in steel for preventing nozzle clogging in conventional continuous casting, but generally 20 to 30 ppm is required. In view of the above, the limited amount of Ca of the present invention is insufficient for preventing nozzle blockage due to Al ₂ O ₃ -based oxide. According to the present invention, the Al ₂ O ₃ -based oxide which coagulates and adheres to the inner wall of the refractory of the nozzle is reduced by increasing the CaO content of the working surface of the refractory at the nozzle where the Ca-containing molten steel passes through the Ca deficiency to 15% or more. To produce 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 and is lower than the temperature of the molten steel itself, so that it does not naturally precipitate and adhere. In the present invention, the inner walls of the upper tundish nozzle 24, the sliding nozzle 26, and the immersion nozzle 28 in the nozzle portion are limited to refractories 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 for the nozzle portion has also been proposed in Japanese Patent Application Laid-Open No. Hei 4-28462. However, the inventors of the present invention have found that the use of only this refractory alone has little effect of preventing Al ₂ O ₃ from adhering. This is clear from the experimental results. In addition, Japanese Patent Application Laid-Open No. 4-2846
According to No. ₂ , the immersion nozzle for reducing the content of active SiO ₂ to 0.3% by weight or less in the above ZrO ₂ —CaO—C refractory is Al.
_It is said that it is effective for preventing blockage of adhesion of ₂ O ₃ . The present invention
As described above, Ca is added to the extremely low carbon aluminum killed molten steel so as to have a concentration of 5 ppm or more and less than 10 ppm, and the inner wall of the nozzle portion of the tundish is made of a CaO-ZrO ₂ -C refractory having a CaO content of 15% or more. These two components have a synergistic effect, and complete the epoch-making technology of completely eliminating the blowing of the inert gas into the nozzle portion.

[Example] C: 0.0020%, Mn: 0.30%, Al: 0.030
% Of ultra low carbon aluminum killed steel melted in a 250 t converter having a component composition of
It was cast with a continuous slab caster of mm. Example 1 A 250 ton ladle of the above ultra-low carbon aluminum killed molten steel was used to feed a Ca alloy wire containing 30% of Ca into the molten steel, and a Ca wire 52 was added so that the Ca content was 8 ppm.
Kg was added. Immediately after the addition of Ca, the space formed by the tundish 6, the ladle 4, and the molten steel 2, as shown in FIG.
The 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 cohesion and adhesion of Al ₂ O ₃ was observed at all, and the slab casting of 250 t was completed smoothly.

Example 2 Ca-Si grains were pneumatically added to a molten steel stream 2A from a ladle 4 to a tundish 6 in an inert atmosphere of Ar by the same method as shown in FIG. Ca content of 8pp
The amount of the Ca—Si alloy charged uniformly so as to reach m was 370 kg for 250 tons of molten steel, and the Ca yield was 18%. In this case as well, no inert gas was blown into the nozzle portion from the lance 34 in FIG. 2, but Al ₂ O ₃ aggregated in the nozzle portions of the upper nozzle 24, the sliding nozzle 26, and the immersion nozzle 28. No adhesion was observed at all, and continuous slab casting of 250 t was completed. In each of the first and second embodiments, the inner wall 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 components were as follows. _{CaO: 21.3% ZrO 2: 49.6} % C: 26.8%

[0017]

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) The inner walls of the upper nozzle, the sliding nozzle, and the immersion nozzle of the tundish are made of a refractory having a CaO content of 15% or more and 40% or less. (C) No inert gas is blown into the nozzle. A new continuous casting method comprising the above components (a), (b) and (c) was completed, and the following effects were obtained. (A) Even if no inert gas is blown into the nozzle part,
No nozzle blockage due to cohesion and adhesion of the Al ₂ O ₃ -based oxide was observed. (B) No rusting of the steel material observed in the Ca-added steel was observed. (C) As a result of not blowing the inert gas at all, there is no blow hole in the slab, and the surface of the molten metal in the mold at the time of casting calms down. Less slab products were obtained. (D) Since there are very few Al ₂ O ₃ -based eaves metal inclusions, so-called blister defects are extremely small. (E) From the above, the defective rate of the cold-rolled steel material became zero.

[Brief description of the drawings]

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

FIG. 2 is an enlarged sectional view showing a nozzle portion of an upper nozzle, a sliding nozzle, and a dipping 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 injection 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 Slab 34 Ar blowing lance 36 Solidified shell

Claims (2)

(57) [Claims] 1. A method for continuously casting ultra-low carbon aluminum killed steel for cold rolling, comprising:
At a concentration of 5 ppm or more and less than 10 ppm, and the inner wall of the upper nozzle, sliding nozzle and immersion nozzle of the continuous casting tundish is C
a refractory having an aO content of 15% or more and 40% or less , wherein no inert gas is blown into the upper nozzle, the sliding nozzle and the immersion nozzle at all. Casting method of aluminum-killed steel for cold rolling. 2. In the immersion nozzle during continuous casting of the ultra-low carbon aluminum killed molten steel, 12CaO · 7Al ₂
Al ₂ O ₃ of the immersion continuous casting method of a cold-rolled for aluminum-killed steel according to the first term of the claims attached is prevented to the nozzle inner wall by forming a low-melting compound of O ₃ and the like.