JPH081295A - Method for pouring molten steel into tundish for continuous casting - Google Patents

Abstract

PURPOSE:To surely prevent the entrapment of slag at the time of pouring molten steel into a tundish in continuous casting. CONSTITUTION:At the time of executing the continuous casting by pouring molten steel into the tundish from a ladle, the molten steel pouring rate is decreased to such an extent that the development of eddy current of the molten steel is prevented at the end stage of the molten steel pouring, and even such the entrapment of a small quantity of the slag into the molten metal that can't be detected is prevented from occurring. Successively, the slag is detected at the position of the molten steel pouring nozzle in the ladle with the descent of the slag layer in the ladle and the molten metal pouring nozzle is closed based on this detected result.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for pouring molten steel into a tundish for continuous casting.

[0002]

2. Description of the Related Art In continuous casting of steel, as is well known, molten steel is alternately poured from a plurality of ladles into a container having a buffer function, that is, a tundish, and the molten steel is poured from this tundish into a continuous casting mold to continuously cast the steel. To do. At the end of molten steel injection from the ladle into the tundish as described above, a vortex flow is generated in the molten steel layer above the molten steel injection nozzle at the bottom of the ladle, and part of the slag layer floating on the molten steel surface becomes vortex flow. It is caught and injected into the tundish together with molten steel, which causes quality defects of steel products. When the molten steel mixed with the slag is located at the molten steel injection nozzle, the electromagnetic slag outflow detector detects the slag and the nozzle is closed based on the detected slag.
No. 56 and JP-A-5-277686.
To detect such slag, generally, two coils are arranged on the outer circumference of the upper nozzle, an alternating current is applied to one coil to generate a magnetic field, and when the slag is mixed with the molten steel flow in the upper nozzle, the inside of the coil is detected. Induced electromotive force is generated in the other coil due to the change in the magnetic permeability of. The slag is detected by detecting this electromotive force.

[0003]

As described above, the slag outflow from the ladle causes a vortex flow in the molten steel layer above the molten steel injection nozzle at the end of molten steel injection, and the slag floating on the molten steel surface is drawn into the vortex flow. Since the molten steel swirl with a small diameter and a sharp shape is generated for a long time from the time when the amount of residual molten steel in the ladle injected into the tundish together with the molten steel decreases to a small amount, a small amount of slag continues to flow out accordingly. During this period, the ratio of the slag outflow rate to the molten steel outflow rate is almost constant. Next, when the molten steel residual amount becomes very small, the diameter of the vortex flow rapidly increases, and the ratio of the slag outflow amount to the molten steel outflow amount rapidly increases.

However, if the ratio of the outflow amount is almost constant from the time when the residual molten steel amount in the ladle is large to the time when it becomes small as described above, there is no abrupt change in the magnetic permeability in the slag detector coil, and the slag outflow occurs. On the other hand, when a large amount of slag is caught, a sudden change in magnetic permeability occurs and the slag is detected.Therefore, a large amount of slag flows out (injects) of molten steel containing a small amount of slag into the tundish. It will continue for a long time until it is caught and detected, and there is a problem that the quality of steel products is significantly reduced due to the inclusion of slag. The present invention has been made in order to advantageously solve such a problem, surely prevents a small amount of slag outflow for a long time, when the slag reaches the molten steel injection nozzle position almost completely the injection of molten steel. It is provided to prevent the slag from flowing out to the tundish by detecting and closing the molten steel injection nozzle based on the detection.

[0005]

SUMMARY OF THE INVENTION The present invention, which has solved the above problems, prevents molten steel eddy currents from being generated at the end of molten steel injection from a ladle when pouring molten steel from a ladle into a tundish for continuous casting. The molten steel injection amount is reduced as much as possible to prevent undetectable small amount of slag from mixing into the molten steel, and then the slag is detected at the molten steel injection nozzle position of the ladle as the slag layer in the ladle descends. The present invention relates to a method for injecting molten steel into a tundish for continuous casting, which is characterized in that the molten steel injecting nozzle is closed based on this result.

[0006]

The present invention quantitatively investigated the influence of the residual molten steel amount in the ladle and the molten steel injection amount on the generation of the molten steel swirl flow in the final stage of the ladle injection using a water model. As a result, as shown in FIG. 3, it was found that there was a region where molten steel vortex flow was not generated until the end of the injection. Then, by restricting the injection amount in the lower left area of the eddy current generation limit line in FIG. 3, the slag outflow due to the molten steel vortex flow is prevented until the end of the injection, and the detection coil of the electromagnetic slag outflow detector is attached at the end of the injection. It is possible to change the fluid flowing in the injection nozzle at the bottom of the ladle from molten steel to slag all at once.At this time, the magnetic permeability in the nozzle changes abruptly, making it possible to detect the outflow of slag. Close the nozzle to. This can reliably prevent the slag from flowing out.

By the combination of the throttle injection and the electromagnetic slag outflow detector, the amount of slag mixed in is greatly reduced and the early detection of slag is greatly improved. Even if the injection amount falls into the upper right region of the vortex flow generation limit line in Fig. 3 for some reason, if the injection amount is reduced to 3t / min or less at the end of injection (residual molten steel amount of 3t or less), an effect similar to that described above can be obtained. To be Since the eddy current generation limit line changes slightly depending on the shape of the ladle bottom and the position of the sliding nozzle plate, it is preferable to understand these relationships for each equipment specification.

[0008]

EXAMPLES Next, examples of the present invention will be given. In FIG. 1, steel is tapped from a converter (not shown) in a ladle 1, filled with molten steel 3 having a layer of floating slag 2 on its surface, and placed in a tundish 4 for continuous casting. Next, the nozzle opening / closing device including the upper plate 6 and the lower plate 7 of the molten steel injection nozzle 5 of the ladle 1 is opened, the molten steel 3 is injected into the tundish 4 through the long nozzle 8, and the tundish 4 is continuously provided at the lower part. When the molten steel 3 is injected into the continuous casting mold (not shown) for continuous casting, a vortex flow is generated in the molten steel 3 above the molten steel injection nozzle 5 at the end of the molten steel injection from the ladle 1, and electromagnetic slag outflow detection is performed. A controller for controlling the layer thickness of molten steel 3 (amount of molten steel) that entrains a small amount of slag 2 that cannot be detected by a vessel, and the amount of molten steel injected (amount of molten steel injection reduced) that can suppress the generation of eddies associated with a sequential decrease in the amount of molten steel. 9 is input, the amount of molten steel 3 in the ladle 1 is constantly measured by the load cell 10, and the result is led to the controller 9 to reach the end of molten steel injection and when the amount of molten steel in which vortex flow is generated, control is performed. Suppress eddy current generation from the vessel 9 The normal molten steel injection amount can be reduced to instruct the hydraulic unit 11 to move (slide) the lower plate 7 to reduce the molten steel injection amount, reduce the molten steel injection amount, and ensure the vortex flow. The casting is performed while suppressing the injection (discharging) to the tundish 4 due to the inclusion of the slag 2 in the above manner. Then, when the injection of the molten steel 3 in the ladle 1 is completed and the slag 2 descends to the nozzle 5 position, the detector 12 detects the slag 2 and guides this result to the controller 9 to the hydraulic unit 11. Instructing the nozzle 5 to be closed, the lower plate 7 is moved to close the nozzle 5. In this way, when the injection of the molten steel 3 is completed while suppressing the discharge of the slag 2 to the tundish 4, the ladle 1 is moved and a ladle filled with new molten steel is positioned to inject the molten steel as described above. Give.

Next, an operation example of the method of the present invention will be given together with a comparative example. Example of operation In a commonly used ladle with a capacity of 250t, C: 0.04%, M
n: 0.25%, P: 0.01%, S: 0.01%, SolAl: 0.04
%, Residual Fe and other impurities, molten steel at 1560 ° C was injected into the tundish at 12 t / min and cast (width 1000 mm, thickness 245 mm).
Continuous casting, the residual amount of molten steel in the ladle at the end of molten steel injection was 13t
When the layer thickness reaches 180 mm, the molten steel injection rate is reduced to 10 t / min, casting is performed while suppressing the molten steel vortex flow at the upper part of the molten steel injection nozzle, and then the injection rate is gradually reduced until the molten steel in the ladle is exhausted. Small injection amount decreased to 3t / min and residual amount of molten steel was 0
When it became, the slag was detected and the nozzle was closed.

When continuous casting (continuous casting by ladle exchange) is performed in this manner, as shown in FIG. 2, the slag layer thickness floating on the molten steel surface in the tundish is small, and continuous hot rolling is performed in the normal process after casting. When a tin plate with a thickness of 0.25 mm was manufactured by rolling-cold rolling-continuous annealing-tempered rolling, the yield of molten steel per ladle was 99.9%, and the product degradation rate due to quality defects such as surface defects due to slag inclusion was It brought an excellent effect of 0.11%.

Comparative Example Under the same conditions as in the above-mentioned example, the molten steel was injected at a rate of 12 t / min for continuous casting. At the molten steel injection nozzle position, the swirl depression of the molten steel became large and the entrained slag was detected and the nozzle was closed. As shown in FIG. 2, the thickness of the slag layer floating on the molten steel surface in the tundish was determined by the method of the present invention (operation example).
It is extremely larger than that of molten steel yield per ladle 99.5%,
The downgrade rate for products processed into tin plate was 0.68%.

[0012]

According to the method of the present invention, the slag outflow can be surely prevented and the molten steel can be poured completely, so that the quality of the slab can be greatly improved and the yield of the molten steel can be greatly improved. Further, since the method of the present invention does not require a large load in operation and does not require a new auxiliary device,
It is possible to obtain excellent effects such as being able to easily and surely on an industrial scale.

[Brief description of drawings]

FIG. 1 is a side view of a series of devices for explaining an example of the method of the present invention.

FIG. 2 is a chart showing the number of continuous castings and the slag layer thickness on the surface of molten steel in a tundish according to an operation example and a comparative example of the method of the present invention.

FIG. 3 is a table showing eddy current generation limit lines according to the method of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaru Ojima 5-3 Tokai-cho, Tokai City, Aichi Prefecture Nippon Steel Co., Ltd. Nagoya Steel Works

Claims (1)

[Claims] 1. When a molten steel is poured from a ladle into a tundish and continuously cast, the molten steel injection amount is reduced until the occurrence of molten steel vortex flow at the end of the molten steel injection from the ladle, and an undetectable small amount. Continuation characterized by preventing slag from mixing into molten steel, then detecting the slag at the molten steel injection nozzle position of the ladle as the slag layer in the ladle descends, and closing the molten steel injection nozzle based on this result Method of pouring molten steel into tundish for casting.