JPH07100603A - Method for cleaning molten steel in tundish - Google Patents

Abstract

PURPOSE:To prevent the contamination of molten steel caused by the entrapment of the air at the initial stage of casting and effectively execute the float-up and the removal of inclusion and slag by supplying the molten steel in a ladle into a tundish by a bottom pouring method, at the time of executing continuous casting. CONSTITUTION:A sliding nozzle 5 at the bottom part of the ladle is connected with a nozzle 4 for bottom pouring nozzle arranged at the side part of the tundish 1, and the molten steel in the ladle 2 in bottom-poured from a, pouring hole 6 at the bottom part of the tundish 1. By this constitution, the oxidation contamination of the molten steel caused by the entrapment of the air into the molten steel 3 in the tundish 1 can perfectly be prevented. Further, as the upward stream 7 is generated in the molten steel 3 at the molten steel pouring point in the tundish 1 by pouring the molten steel through the bottom, the inclusion and the slag in the molten steel 3 can quickly be floated up and separated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention prevents the contamination of molten steel due to air entrainment at the beginning of casting when starting the injection of molten steel from the ladle into the tundish in continuous casting of steel, and cleans the molten steel in the tundish. It is about how to plan.

[0002]

2. Description of the Related Art In continuous casting of steel, the tundish is located between the ladle and the mold, and is one of the most important parts in operation and quality.

Its functions are to control the amount of molten steel supplied into the mold, store molten steel, and separate and remove inclusions.

In particular, the function of removing inclusions has become an extremely important function as the quality of steel materials has become stricter in recent years.

However, when pouring molten steel into the tundish from the ladle, the problem of contamination of the molten steel due to air oxidation arises, so that the present condition is that the effect of removing inclusions in the tundish is not sufficiently exerted.

Therefore, various methods have been studied and put into practical use for the purpose of preventing molten steel contamination in the tundish.

For example, as described in Japanese Patent Laid-Open No. 59-1055, by blowing an inert gas into a tundish covered with a refractory board lid, it is possible to prevent air oxidation of injected molten steel. There is.

Further, in Japanese Unexamined Patent Publication No. 61-38749, the inclusion of Ca-based alloy or Ca-based flux from a ladle long nozzle for transferring molten steel from the ladle to the tundish is added to efficiently remove inclusions in the molten steel. The method of removing it has been proposed, and it has shown some effect.

[0009]

However, as for the method of covering the tundish with the refractory board lid and sealing with the inert gas, the tundish or the refractory board itself is deformed at a high temperature, and thus the tundish is prevented to the extent that air oxidation can be prevented. It is not possible to reduce the oxygen partial pressure in the dish.

The method of adding the Ca-based alloy or Ca-based flux from the long ladle nozzle has the effect of increasing the removal rate of inclusions, but since the floating inclusions are re-engaged by the injection flow, it is sufficient. It has not been possible to secure the cleanliness of molten steel.

In view of these problems, the present invention presents a method for cleaning molten steel in a tundish which prevents contamination of molten steel due to air entrainment at the beginning of the most intense casting and also improves the effect of removing inclusions in the tundish. The purpose is to do.

[0012]

Means for Solving the Problems The present invention, in continuous casting of steel, connects the bottom of a ladle and the bottom of a tundish with a bottom pouring nozzle equipped with a sliding nozzle, and pours molten steel in the ladle into a tundish. The present invention relates to a method for cleaning molten steel in a tundish, characterized in that

[0013]

[Operation] Generally, when injecting molten steel from the ladle into the tundish, the pouring flow entrains a large amount of oxygen in the atmosphere until the ladle long nozzle is immersed below the molten metal surface.
A large number of inclusions due to air oxidation, which is represented by the reaction of the equation (1), are formed in the molten steel.

[0014]

## STR1 ## 4 Al (in molten steel) + 3O ₂ (in air) = 2Al ₂ O ₃ (inclusions) (1)

In FIG. 1, the height h from the bottom of the tundish to the bottom of the ladle is changed, and the oxidation rate of Al is d [Al%] / d.
It is the result of investigating t.

As can be seen from the figure, as the height of the molten steel dropped increases, the oxidation rate of Al rapidly increases.

This is because oxygen is trapped in the molten steel as very fine bubbles, which is extremely faster than the oxidation rate at the time of steady injection and the reaction rate with slag, and is the main cause of molten steel contamination in the tundish. is there.

Therefore, the present inventors consider that in order to prevent the entrainment of air at the initial stage of injection, it is necessary to reduce the falling distance of the molten steel to the utmost limit, and as a method therefor, a method for injecting molten steel into the tundish. It was devised to change the conventional top pouring to bottom pouring.

That is, as shown in FIG. 2, when pouring molten steel 3 into the tundish 1 from the ladle 2, the lower pouring nozzle 4 provided beside the tundish 1 is provided with a sliding nozzle 5 at the bottom of the ladle. It is a method of connecting and pouring the molten steel 3 of the ladle 2 downward from the injection hole 6 at the bottom of the tundish 1.

As a result, it is possible to completely prevent the contamination of molten steel due to the entrainment of air in the molten steel. Further, since the upflow 7 of the molten steel is generated at the injection point of the tundish by the downward pouring, the inclusions and slag in the ladle can be quickly floated and separated in the tundish.

Further, when the heat retaining agent 8 is used in the tundish, the heat retaining agent was entangled by the molten steel flow at the time of top pouring and pouring, but this problem can be solved by pouring under the pouring.

Therefore, the present invention not only prevents the contamination of molten steel due to the entrainment of air at the initial stage of injection, but also has the effect of enhancing the function of removing inclusions in the tundish.

[0023]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples.

[0024]

[Embodiment 1] Component C: 50 pp was used by using the equipment for supplying molten steel from the ladle 2 to the tundish 1 shown in FIG.
m, Si: 0.015%, Mn: 0.25%, P: 0.
02%, S: 0.01%, Al: 0.035%, temperature 1
250 t of molten steel at 550 ° C. (in the tundish) was injected from the ladle 2 into the tundish 1 having a capacity of 50 t at a rate of 8 t / min through the lower pouring injection hole 6 of 100 mmφ provided at the bottom of the tundish.

At this time, the Al concentration in the molten steel at the initial point of the tundish injection did not decrease at all, and the total oxygen content in the molten steel was always constant at 20 ppm from the beginning of the injection.

By this, the contamination of molten steel could be reliably prevented. Further, the total oxygen content in the molten steel in the ladle was 60 ppm, and the effect of reducing the total oxygen content in the molten steel by 40 ppm at the tundish injection point was obtained.

Here, the total amount of oxygen in the molten steel corresponds to the amount of inclusions. As a result, no surface defects occurred in the rolled product.

[0028]

Comparative Example 1 Component C: 50 ppm, Si: 0.015
%, Mn: 0.25%, P: 0.02%, S: 0.01
%, Al: 0.035%, 250 t molten steel at a temperature of 1550 ° C. (in the tundish), an inner diameter of 100 mmφ from the ladle
Was poured into the tundish with a capacity of 50 t through the long nozzle of 8 t / min at a rate of 8 t / min in the usual manner.

At this time, the initial Al in the molten steel at the injection point
The concentration decreased to 0.007% and the total oxygen content in the molten steel was 14
Rose to 5 ppm.

The total amount of oxygen in the molten steel in the ladle is 60 pp.
It was m. Therefore, the effect of preventing molten steel contamination and the effect of removing inclusions at the initial stage of injection were not observed, and surface defects occurred in the product after rolling.

[0031]

As described above, according to the method for cleaning molten steel in the tundish of the present invention, it is possible to reliably prevent the most severe contamination of molten steel in the early stage of casting, and also to remove inclusions in the tundish. As a result, the quality of the slab is significantly improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the relationship between molten steel drop height and Al oxidation rate.

FIG. 2 is a diagram for explaining a method for cleaning molten steel in TD according to the present invention.

[Explanation of symbols]

 1 Tundish 2 Ladle 3 Molten steel 4 Nozzle for bottom pouring 5 Sliding nozzle at bottom of ladle 6 Injection hole 7 Upflow 8 Heat insulation

Claims (1)

[Claims] 1. When continuously casting steel, the bottom part of the ladle and the bottom part of the tundish are connected by a downward pouring nozzle with a sliding nozzle, and the molten steel in the ladle is supplied into the tundish by downward pouring. Method for cleaning molten steel in tundish.