JPH07214255A - Continuous casting operation method with lessened slag inclusion and tundish therefor - Google Patents

Abstract

PURPOSE:To prevent inclusion of slag into a tundish by specifying the supply rate of molten steel in a ladle for restoring the molten steel level in the tundish after ladle exchange of continuous casting within a range shown by specific equation. CONSTITUTION:The molten steel 6 is supplied via the ladle nozzle 1 into a tundish stirring chamber 5 formed by partitioning the inside of the tundish under casting by gates 3. This molten steel 6 is supplied via an injection nozzle 4 into a casting mold while its flow rate is adjusted by a stopper 2. At this time, the molten steel 6 is supplied at such a rate at which the supply rate of the molten steel 6 in the ladle for restoring the molten steel level in the tundish after the ladle exchange of continuous casting attains the rate within the range specified by the equation epsilon/H<220[epsilon:(0.5rhoQu<2>)/V, epsilon: stirring energy (w/m<3>), rho: molten steel density (kg/m<3>), Q: supply rate (m<3>/s), U: passing speed at a long nozzle (m/s),V: stirring space volume (m<3>), H: depth of stirring space bath (m)]. As a result, the inclusion of the slag floating on the molten steel surface into the molten steel 6 is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents the entrainment of slag in a tundish in a continuous steel casting process,
The present invention relates to technology for preventing product defects caused by slag and non-metallic inclusions.

[0002]

2. Description of the Related Art As is well known, in the continuous casting method, a tundish is installed directly above a mold, and a slab for supplying molten steel to the mold is manufactured.

Further, the residence time in the tundish is lengthened and the inclusions are floated and separated to clean the steel.

However, since the dropping flow from the pan is fast, the pouring flow cannot be calmed down only by the weir provided in the ordinary tundish, and the tundish slag is entrained in the vicinity of the long nozzle in the tundish, and the tundish is further absorbed. Local fast flow through the bottom to the mold (direct flow)
Exists.

As described above, since the tundish slag is entrained and the direct flow is present, and the inclusions thus entrained are very fine, a sufficient cleaning effect cannot be obtained. Is the actual situation.

Various proposals have been made to clean the steel by eliminating the entrainment of the tundish slag and the direct flow.

[0007] For example, as disclosed in Japanese Patent Laid-Open No. 63-126656, a weir having a large flow resistance is installed in the tundish to prevent the falling flow from the pot and to moderate the flow in the downstream region. A method of promoting floating, and as shown in Japanese Patent Laid-Open No. 63-1407745, the reverse flow generated after the falling flow from the pot collides with the bottom of the tundish is calmed by electromagnetic force to eliminate the naked hot water near the nozzle. There is an injection method that actively prevents secondary oxidation of molten steel.

There is also a method of using a weir and an electromagnetic force in combination, as disclosed in Japanese Patent Laid-Open No. 63-132752.

[0009]

However, since these methods focus on calming the flow, they lack operational stability. Further, these methods are more effective than prevention of inclusion of inclusions in molten steel. Has the problem of improving the floating separation characteristics after being caught up, or has problems such as the main stream being able to be calmed down but still having a fast flow around it, and a sufficient cleaning effect was not obtained.

That is, as described above, in the method in which the weir is provided in the tundish, in order to obtain a slow uniform flow, the hole diameter of the weir is made small to greatly increase the flow resistance, so that a flow causing local stagnation is generated. It has been set up.

However, as a result, insufficient preheating of the tundish weir directly leads to operational troubles, and clogging of pores and uneven flow are apt to occur, resulting in unstable operation.

Further, at the end of the operation, since the flow resistance at the weir is too large, the passing flow rate is drastically reduced as the molten steel head is lowered, and it is necessary to take measures such as extending the casting time or stopping the casting midway. However, a decrease in yield is inevitable.

In the latter method, an electromagnetic force is locally applied to the main flow of the falling flow to calm it down, but a flow (direct flow) still unevenly distributed in the depth and width directions in the tundish is generated. It exists and it is impossible to obtain sufficient levitation effect of inclusions.

Further, even when the weir and the electromagnetic force can be used under the best conditions, the conventional method has no effect on the inclusion of inclusions in the molten steel and the separation of the inclusions mixed in. However, since the inclusions were too small, the buoyancy was small and a sufficient cleaning effect could not be expected.

The present invention intends to radically solve the problems of the above-mentioned conventional method, and provides a steel casting operation method and a tundish thereof for producing a slab having excellent cleanliness.

[0016]

According to the present invention, a space for accommodating molten steel supplied from a ladle is installed at a position of a ladle nozzle discharge port of a pouring part in a tundish when continuously casting steel. , The volume and depth of the stirring region are set so that the ratio of the stirring energy generated by the molten steel supplied from the ladle nozzle into the space and the bath depth of the space is within the range of formula (1). thing.

Alternatively, in the continuous casting operation using a tundish in which a space for accommodating the molten steel supplied from the ladle is installed at the position of the ladle nozzle discharge port during continuous casting of the steel, after the continuous casting ladle is replaced. It is to supply the ladle molten steel for the recovery of the tundish molten metal surface level within the range of the formula (1).

[0018]

[Equation 3] ε / H ² <20 (1)

[0019]

(4) ε: (0.5ρQU ² ) / V

Ε: Stirring energy (w / m ³ ) ρ: Molten steel density (kg / m ³ ) Q: Supply amount (m ³ / s) U: Long nozzle passage speed (m / s) V: Stirring space volume ( m ³ ) H: bath depth of stirring space (m)

[0021]

The present invention will be described with the following actions. FIG. 1 is a view for explaining the situation inside the tundish when the volume and bath depth of the inflow chamber of the tundish are defined according to the present invention.

FIG. 2 is a diagram for explaining a state in which the slag floating on the molten metal surface is caught in the molten steel when the weirs in the central portion of the tundish are narrow and the stirring chamber has a small volume.

Molten steel (6) is supplied through a ladle nozzle (1) into a tundish stirring chamber (5) partitioned by a weir (3) in the tundish during casting.

Further, the molten steel is supplied into the mold through the injection nozzle (4) while adjusting the flow rate with the stopper (2).

In this case, when the volume of the stirring area in the tundish is small or the bath depth is shallow, the slag (7) floating on the molten steel surface of the stirring chamber in the molten metal drop is in the form of droplets in the molten steel. It becomes a slag (8) and is caught.

Alternatively, as shown in FIG. 3, when the ladle level of the tundish decreases when the ladle for continuous casting is changed, the molten steel is supplied from the ladle so as to raise the molten metal level of the tundish to the steady part level. When the amount is suddenly increased, the slag floating on the molten steel surface of the stirring chamber is also involved.

In order to prevent this, according to the present invention, the volume or bath depth of the stirring chamber of the tundish is deepened to attenuate the molten steel flow velocity supplied from the ladle nozzle, but as shown in FIG. 4, it floats on the molten steel surface. The amount of slag entrained in the molten steel depends on the entrainment parameter (ε / H ² ) calculated from the volume of the stirring chamber and the bath depth. Therefore, the volume of the stirring chamber and the bath depth need to be within the ranges of the above parameters.

Further, when the ladle of the next charge moves to the upper part of the tundish at the time of replacing the ladle of the continuous casting, the ladle nozzle is opened to increase the molten steel supply rate when the molten metal level of the tundish is increased to the steady part level. Is adjusted so that the molten steel in the tundish is supplied even during the exchange of the ladle in the unsteady part.

Therefore, even if the volume of the stirring area and the level of the molten metal are sufficient to suppress the entrainment of the steady portion, the entrainment occurs in the unsteady portion.

Further, as for the molten steel supply amount in the unsteady portion, the slag entrainment amount depends on the stirring energy in the stirring region and the bath depth as shown in FIG.

After the entrainment of the tundish slag can be suppressed by the above method, it is possible to improve such as preventing the air oxidation in the tundish by suspending the fusible slag above the tundish.

[0032]

Example 1 A slab size is 980 mm wide and 250 m thick
m, a low-carbon aluminum killed steel was continuously cast at a casting speed of 1.0 m / min. In this casting, the tundish width was 10 m, the width between the long sides of the tundish was 1 m, and the bath depth was 1 as shown in FIG.
Casting was performed using a tundish having a stirring chamber separated by a weir interval of 1 m in the center of the m tundish.

A ladle nozzle having a 100 mm inner diameter was used. However, a large number of defects were detected in the cast slab, and a tundish slag component was recognized by the inspection of the defective part.

Therefore, according to the present invention, casting was performed by expanding the weir spacing in the tundish to 1.6 m. as a result,
The occurrence of defects in the slag diameter could be prevented.

[0035]

Example 2 A slab size is 980 mm wide and 250 m thick
m, low-carbon aluminum killed steel was continuously cast at a casting speed of 1.0 m / min. In this casting, the width was 10 m and the distance between the long sides was 1 m.
The casting was performed using a tundish having a stirring chamber divided by a weir space of 3 m in the central part, and a tundish having a bath depth of 1.2 m in the stationary part.

A ladle nozzle having a 100 mm inner diameter was used. As a result, at a steady state of 1.7 t / min, a healthy slab without slag entrainment was obtained, but when the ladle in the non-steady section was replaced, the tundish bath depth dropped to about 0.9 m. Furthermore, in order to recover the surface of the tundish, it is 3t / mi, which is about twice as much as the stationary part.
Since the molten steel was supplied at a rate of n, a large amount of tundish slag was involved and many defects were generated.

Therefore, according to the present invention, the amount of molten metal poured during the recovery of the tundish surface is reduced to 2 t / min to reduce the recovery rate of the surface, and as a result, the slag entrainment can be suppressed.

[0038]

EFFECTS OF THE INVENTION By implementing the present invention, it becomes possible to stably produce high quality cast slabs by suppressing slag entrainment within the stirring amount region of the long nozzle immersion part in the tundish.

[Brief description of drawings]

FIG. 1 is a side view of a tundish in the case where casting is performed by setting a volume of a stirring region, a bath depth and a pouring amount in the tundish according to the present invention.

FIG. 2 is a diagram illustrating that the slag floating on the molten metal surface is caught in the stirring region when the weir space is narrowed.

FIG. 3 is a diagram illustrating that slag entrainment occurs when the molten metal level of the tundish decreases when the ladle for continuous casting is replaced.

FIG. 4 is a diagram illustrating that the amount of slag entrained in a tundish is represented by stirring energy and bath depth.

[Explanation of symbols]

 1 ladle nozzle 2 stopper 3 tundish weir 4 injection nozzle 5 tundish stirring chamber 6 ladle supply molten steel 7 floating slag 8 slag droplets

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Kimura 1-1, Toibata-cho, Tobata-ku, Kitakyushu City Nippon Steel Corporation Yawata Works Co., Ltd.

Claims (2)

[Claims] 1. A continuous casting operation using a tundish in which a space for accommodating molten steel supplied from a ladle is provided at a ladle nozzle discharge port when continuously casting steel. The continuous casting operation method is characterized by supplying the ladle molten steel for recovering the tundish molten metal surface level within the range of the formula (1). 2. When continuously casting steel, a space for accommodating molten steel supplied from the ladle is provided at the position of the ladle nozzle discharge port of the pouring part in the tundish, and the ladle nozzle is further provided in the space. For continuous casting, characterized in that the volume and depth of the stirring region are set so that the ratio of the stirring energy generated by the molten steel supplied from the tank to the bath depth of the space is within the range of formula (1). Tundish. ## EQU1 ## ε / H ² <20 Equation (1) ε = (0.5ρQU ² ) / V ε: Stirring energy (w / m ³ ) ρ: Molten steel density (kg / m ³ ) Q: Supply amount (m ³ / s) U: Long nozzle passage speed (m / s) V: Agitation space volume (m ³ ) H: Agitation space bath depth (m)