JPH08155617A - Method for continuously casting ti-containing extra-low carbon steel - Google Patents

Abstract

PURPOSE: To prevent the development of bulging defect and Al2 O3 -base deffect and to improve the productivity, in the continuous casting of Ti-containing extra-low carbon steel with a bending type slab continuous caster. CONSTITUTION: In a method for continuously casting the extra-low carbon steel containing Ti with the bending type slab continuous caster, without blowing argon gas from the upper nozzle 2 in a tundish 1, the fixed plate 3 of a sliding gate and an immersion nozzle 4, casting speed is made to be >=1.2m/min.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention does not blow argon gas from an upper nozzle of a tundish, a fixed plate of a sliding gate, and an immersion nozzle of an ultra low carbon steel containing Ti by a curved slab continuous casting machine. The present invention relates to a continuous casting method for Ti-containing ultra low carbon steel for continuous casting.

[0002]

2. Description of the Related Art In continuous casting, a dipping nozzle is usually used to feed molten steel from a tundish into a mold without oxidizing it. In this immersion nozzle, Al ₂ O _3, which is a precipitate in steel, adheres to the inner wall of the nozzle as the casting time elapses, and in severe cases, the nozzle may be clogged and the casting may have to be stopped. As means for eliminating the blockage of the immersion nozzle in this continuous casting, a method of removing these non-metallic inclusions by providing a weir in the tundish, disposing a gas sleeve or a porous nozzle or the like on the upper part of the immersion nozzle, A method to prevent non-metallic inclusions from precipitating in the immersion nozzle by blowing an inert gas such as argon gas from the inside, or to form concentric circles with the immersion nozzle inner hole A method is known in which a porous tubular refractory material is inserted into a main body of a dipping nozzle and an inert gas such as argon gas is blown from the inner wall of the porous refractory material (Japanese Patent Publication No. 58-3467).

However, the method of providing a weir in the tundish to remove the non-metallic inclusions is insufficient in terms of preventing the immersion nozzle from being blocked. In addition, an inert gas such as argon gas blown from the gas sleeve or porous nozzle provided on the upper part of the immersion nozzle or the porous refractory inner wall inserted in the immersion nozzle is the vertical type shown in FIG. In the continuous casting machine, the gas bubbles 42 mixed in the molten steel 41 float up to the surface in the mold 43 until solidified, but in the curved slab continuous casting machine shown in FIG. The bubbles 42 of the mixed gas are trapped at the interface of the solidified shell 44 during floating, and remain as they are in the slab as bubbles. The larger the air bubbles remaining in the slab, the more the air bubbles are not pressure-bonded after hot rolling or cold rolling and appear as swelling defects on the surface of the steel sheet.

The blistering defect is a defect which appears on the surface of the steel sheet after hot rolling and cold rolling, and has a width of 1 to 4 mm and a length of several meters.
It refers to a ridge that is raised to m, or these ridges of several mm are continuous in a dot shape and extend for 300 mm. This blistering defect reduces the carbon concentration in the steel sheet, for example,
In an ultra-low carbon steel having a carbon concentration of 50 ppm or less, it occurs particularly frequently in a steel type to which Ti is added to fix solid solution carbon in a product as a precipitate, resulting in a significant decrease in product yield. Therefore, in a curved slab continuous casting machine in which blistering defects are more likely to occur than in a vertical continuous casting machine, the casting speed is reduced to reduce the amount of argon in order to reduce the blistering defects of the ultra-low carbon steel containing Ti. Although we are trying to promote the floating of gas, we have not been able to significantly reduce blistering defects. In particular, when the slab width exceeds 1600 mm, the occurrence of swelling defects becomes remarkable.

The inert gas such as argon gas is blown to a greater or lesser extent for the purpose of preventing the immersion nozzle from being blocked and reducing Al ₂ O ₃ defects and powder defects. For example, in JP-A-2-37947 and JP-A-2-37948, in order to reduce Al ₂ O ₃ property defects and powder property defects, an appropriate region for the amount of argon gas blown in is determined according to the casting speed of molten steel. It has been shown that As described above, in the ultra-low carbon steel containing Ti, no example has been proposed in which no argon gas is blown.

Further, in JP-A-5-154629, from the viewpoint of preventing blistering defects, the amount of argon gas blown is greatly reduced, or nozzle clogging does not occur even when the amount of argon gas blown is not performed at all. As an immersion nozzle, the inner wall of the nozzle that comes into contact with molten steel has a silica content of 5%
The following discloses that the immersion nozzle body is made of a material having an aluminum titanate content of 10 to 90% and does not contain aluminum titanate.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The immersion nozzle disclosed in Japanese Patent No. 629 has the advantage that the nozzle is not clogged even if the amount of argon gas blown in is greatly reduced or no argon gas is blown in at all from the viewpoint of preventing blistering defects. In addition, by not blowing argon gas at all, it is possible to prevent the occurrence of blistering defects, but by stopping the blowing of argon gas, the floating of non-metallic inclusions is suppressed in the mold, and the non-metallic inclusions solidify at the interface. However, there is a drawback that the number of Al ₂ O ₃ defects is increased. In particular, in a curved slab continuous casting machine, non-metallic inclusions are easily trapped at the solidification interface during floating in the mold, the occurrence of Al ₂ O ₃ defects becomes remarkable, and the rate of product downgrade increases. It has drawbacks.

The object of the present invention is to solve the above-mentioned drawbacks of the prior art, and in a curved slab continuous casting machine, regardless of the size of the slab width, the occurrence of blistering defects can be significantly suppressed, and the Al ₂ O ₃ property can be improved. An object of the present invention is to provide a continuous casting method for Ti-containing ultra-low carbon steel capable of suppressing the occurrence of defects and improving productivity.

[0009]

[Means for Solving the Problems] The inventors of the present invention have conducted intensive studies on the mechanism of generation of blistering defects in order to achieve the above object. As a result, the blistering defect is caused by the fact that the argon gas blown into the molten steel is partly captured by the solidification interface during floating in the mold and remains in the slab as air bubbles, so continuous argonless casting Has been found to be effective. However, in argonless continuous casting,
Al ₂ O ₃ defects are increased without the effect of floating non-metallic inclusions by argon gas, but the casting speed is 1.2 m / m
It has been clarified that the occurrence of blistering defects and the occurrence of Al ₂ O ₃ -type defects can be suppressed by controlling the content to be not less than in, and the present invention has been reached.

That is, the present invention relates to a method of continuously casting an extremely low carbon steel containing Ti by a curved slab continuous casting machine, in which an argon gas is blown from an upper nozzle of a tundish, a fixed plate of a sliding gate and an immersion nozzle. This is a continuous casting method for Ti-containing ultra-low carbon steel, characterized in that the amount is eliminated and the casting speed is 1.2 m / min or more.

[0011]

According to the present invention, when the ultra low carbon steel containing Ti is continuously cast by the curved slab continuous casting machine, argon gas is blown from the upper nozzle of the tundish, the fixed plate of the sliding gate and the immersion nozzle. By eliminating the above and setting the casting speed to 1.2 m / min or more, the generation of blistering defects can be prevented by stopping the blowing of the argon gas, and as shown in FIG.
By increasing the casting speed, the amount of molten steel blown out from the immersion nozzle is increased, the molten steel 52 at high temperature is blown up to the upper surface of the molten steel in the mold 51, the formation of the solidified shell 53 on the upper surface is reduced, and Al ₂ O ₃ 54 is generated. The solidified shell 53 floats without being trapped on the inner surface thereof, and the generation of Al ₂ O ₃ -type defects can be prevented without blowing argon gas at all.

In the present invention, the casting speed is 1.2 m / m
In or more, when the casting speed is less than 1.2 m / min and no argon gas is blown,
As shown in (b), since the molten steel is supplied less due to the slower casting speed, the solidified shell 53 is generated up to the upper surface of the molten steel in the mold 51, and the floating Al ₂ O ₃ 54 and the like are captured on the lower surface of the solidified shell 53, and Al ₂ This is because the occurrence of O ₃ -type defects cannot be completely prevented. The upper limit of the casting speed is the upper limit speed at which breakout does not occur, which is determined by the curved continuous casting machine used. The ultra low carbon steel containing Ti in the present invention refers to an ultra low carbon steel having a Ti content of 200 to 300 ppm and a C content of 0.005% or less.

[0013]

EXAMPLE A Ti-containing ultra-low carbon steel having a representative chemical composition shown in Table 1 was melted and a curved slab continuous casting machine having a bending radius of 15000 mm was used.
The slab of 00mm is 0.7m / min, 1.0m / m
In, at a casting speed of 1.2 m / min, and with the amount of argon gas blown changed to 0 to 7 l / min, as shown in FIG. 3, the upper nozzle 2, the upper fixing plate 3 and the immersion of the tundish 1 were immersed. Continuously casting each while blowing from the nozzle 4, hot rolling each obtained slab, then cold rolling into a thin steel sheet, and the product rating index due to swelling defects and the product rating due to Al ₂ O ₃ property defects I calculated the index. The results are shown in FIGS. 1 and 2.

[0014]

[Table 1]

As shown in FIG. 1, the blistering defect is reduced by reducing the amount of argon gas blown,
In the case where the blowing of the argon gas was not carried out at all, even when the casting speed (Vc) was 1.2 m / min, there was no occurrence. Further, as shown in FIG. 2, the Al ₂ O ₃ property defects are increased by reducing the amount of argon gas blown, and are remarkably increased at the casting speed of 0.7 m / min, but the casting speed (Vc ) Is 1.2 m / min, even if no argon gas is blown,
Al ₂ O ₃ property defects can be kept low.

[0016]

As described above, according to the method of the present invention, when the Ti-containing ultra-low carbon steel is continuously cast by the curved slab continuous caster, the casting speed is 1. By setting the rate to 2 m / min or more, the occurrence of swelling defects can be significantly reduced, and Al ₂ O ₃ -type defects can be suppressed to none or low, and the product yield is significantly improved.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of argon gas blown for each casting speed and the product downgrade index due to blistering defects in the examples.

FIG. 2 is a graph showing the relationship between the amount of argon gas blown for each casting speed and the product downgrade index due to Al ₂ O ₃ property defects in Examples.

FIG. 3 is a schematic cross-sectional view for explaining an immersion nozzle including an upper nozzle of a tundish when carrying out the method of the present invention.

FIG. 4 is a schematic diagram for explaining generation of blistering defects in a mold, and FIG. 4 (a) is a schematic view of a curved slab continuous casting machine,
(B) The figure shows the case of a vertical continuous casting machine.

5A and 5B are schematic diagrams for explaining the casting speed and the occurrence of Al ₂ O ₃ property defects. FIG. 5A shows the case where the casting speed is high, and FIG. 5B shows the case where the casting speed is slow.

[Explanation of Codes] 1 Tundish 2 Upper nozzle 3 Upper fixing plate 4 Immersion nozzle 41, 52 Molten steel 42 Bubbles 43, 51 Mold 44, 53 Solidification shell 54 Al ₂ O ₃

Claims (1)

[Claims] 1. A method of continuously casting an ultra low carbon steel containing Ti by a curved slab continuous casting machine, eliminating argon gas blowing from an upper nozzle of a tundish, a fixed plate of a sliding gate, and an immersion nozzle, A continuous casting method for Ti-containing ultra-low carbon steel, characterized in that the casting speed is 1.2 m / min or more.