JPH05123836A - Method for improving cleanliness of cast slab in top and end parts in continuous casting strand - Google Patents

Abstract

PURPOSE:To prevent the entraping of flux and slag into molten steel part in a mold by dipping a cooling metal having through-holes in the vertical direction just below the molten steel surface in the mold at the time of completing continuous casting operation. CONSTITUTION:At the time of completing the pouring, the molten steel in a heat A forms the solidified phase 4. A tundish is taken off and the cooling metal 3 is dipped just below the molten metal surface L. As the cooling metal 3 has the through-holes to the vertical direction, this cooling metal can be clipped without pushing down the mold flux and the slag on the molten steel A surface into the molten steel. At this time, a solidified phase 5 is formed. New tundish is arranged and the molten steel in a heat B is poured. At this time, a solidified phase 6 is formed and at the initial stage of pouring the molten steel in the heat B, the molten steel entraping the mold flux and the slag does not invaded the molten steel in the heat A at the lower part of the cooling metal 3 and the cleanliness at the top part of the continuous cast strand is not deteriorated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to continuous casting operation.
At the end of the process, or when performing continuous casting of the same steel type after replacing the tundish, when the molten steel is poured into the mold from the tundish, the cleanliness of the molten steel part existing in the mold deteriorates. This method proposes a method of preventing, that is, preventing the entrainment of mold flux in the continuous cast strand top end.

Up to now, for example, in a continuous cast slab for cold-rolled steel sheets, the continuous cast strand top end slab is caused by inclusions at the stage of hot rolling / cold rolling to produce a coil or sheet. The rate of disqualification by the ultrasonic flaw detection test is high, and this improvement has been desired.

[0003]

2. Description of the Related Art Conventionally, in a continuous casting operation, a molten steel in a mold after the molten steel has been poured from a tundish into a mold, that is, a continuous cast strand top end is treated as a molten steel bar. Stir and cool. The molten steel is stirred with an Al rod, deoxidized and cooled. The slag on the molten steel is removed, and water is added to cool it. For example, as disclosed in JP-A-58-218344, a top treatment method in continuous casting, an iron plate is inserted below the surface of the molten steel and cooled. Are known. However, these are mainly intended to prevent bleeding that occurs when the continuous cast steel slab is drawn out, and they are not effective in preventing deterioration of the cleanliness of the continuous cast strand top slab. That is, in the above methods and, the mold flux, slag entrainment and suction by stirring and solidification shrinkage, and the method of not only there is a risk of water explosion, it is not possible to completely remove the slag and the like. From the above, suction of mold flux and slag by solidification shrinkage, and in the method of, when inserting the iron plate below the molten steel surface, push the mold flux and slag on the molten steel surface into the molten steel. There is a problem that the slag is caught in the molten steel and the cleanliness is deteriorated.

Further, when replacing the tundish in multiple casting of the same steel type, for example, "Handbook of Steel, 2, 2.
Maruzen Co., Ltd., P644 (Sho 54-10-15) "discloses a method of casting by adding the subsequent charge after the tundish exchange to the mold as it is, but at the initial stage of injection of the subsequent charge, Mold flux and slag together with the molten steel of the subsequent charge are caught in the molten steel of the preceding charge in the mold,
There is a problem that the cleanliness of this part is deteriorated.

[0005]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems in an advantageous manner. When the molten steel is poured from the tundish into the mold, the molten steel at the top end of the continuously cast strand in the mold is melted. With respect to the above, it is an object of the present invention to propose a method for preventing the entrainment of mold flux or slag into molten steel by devising the cooling treatment.

[0006]

The gist of the present invention is as follows. At the end of the continuous casting operation, after the molten steel was poured into the mold from the tundish, the tundish was removed, and then a cooling metal article having vertical through holes was immersed directly below the molten steel surface in the mold. It is a method for improving the cleanliness of the slab in the continuous casting strand top end characterized by

2. During continuous casting operation, in continuous casting of the same steel type that continuously performs continuous casting after tundish replacement, after pouring molten steel into the mold from the tundish, remove the tundish, then, A method for improving cleanliness of a slab in a continuous cast strand top end, characterized in that a cooling metal article having a through hole in a vertical direction is immersed immediately below the surface of the molten steel in the mold.

Here, the term "continuously cast strand top end" includes the strand top end of the preceding charge when the tundish is replaced in multiple castings of the same steel type, and the entrainment of mold flux means that the mold flux or slag is contained in molten steel. The phenomenon in which is mixed is a general term.

As the cooling metal fitting, it is sufficient that it has through-holes in the vertical direction, such as a steel plate having through-holes in the plate thickness direction, a combination of steel bars in a grid pattern, welded gold steel,
Any of a plurality of wire laths, metal laths, etc., which are superposed on one another, may be used.

[0010]

The greatest feature of the present invention resides in that, in the continuous cast strand top end, a cooling metal article having vertical through holes immediately below the surface of the molten steel in the mold is immersed. The effect of using the cooling metal article having the vertical through holes will be described below.

A. When the cooling metal article is dipped right below the molten steel surface, the mold flux or slag on the molten steel surface passes through the through holes in the vertical direction of the cooling metal material and floats on the molten metal surface.
Therefore, the mold flux and the slag are not pushed below the surface of the molten steel as in the case of immersing the plate-shaped cooling material.

B. Since the molten steel is not stirred and cooled, the mold flux and slag on the molten steel surface due to stirring are not involved.

C. By the immersion of the cooling metal object, the molten steel around this is removed from the heat and solidifies early, so that the mold flux and slag above the metal object due to subsequent solidification shrinkage are not sucked.

D. Immersing the cooling metal, when the molten steel after tundish exchange is injected when the periphery is in a solidified state, the molten steel in the state where the mold flux and slag are involved is blocked at the immersion part of the cooling metal, It does not affect molten steel below the cooling metal immersion part. Therefore, when molten steel is poured into the mold from the tundish, deterioration of cleanliness of the continuously cast strand top portion in the mold is prevented.

Next, an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a vertical cross-sectional view showing a solidified state of a mold and molten steel in the mold when a subsequent charge is injected after the tundish is exchanged when the cooling metal article according to the present invention is used.

From the tundish before replacement, charge A molten steel is poured into the mold 2, and at the end of this injection,
The molten steel of Charge A is in a state where the solidified phase 4 is formed. Then, after removing the tundish, the cooling metal object 3 is immersed directly below the molten steel surface L of the charge A. When this immersion is performed, the cooling metal object has a through hole in the vertical direction. Can be immersed without pressing mold flux or slag into molten steel. A solidified phase 5 is formed at the time when the cooling metal article is dipped.

The immersion position of the cooling metal fitting 3 may be located below the slag layer on the molten metal surface L.

As described above, by using the cooling metal article 3 having the through holes in the vertical direction without pushing the mold flux or slag into the molten steel, and by dipping the cooling metal article 3, the molten steel in this portion is removed. Since the solidification phase 5 is formed early by heat, the metal 3 caused by the subsequent solidification contraction
It is possible to prevent suction of mold flux and slag at the upper part.

Next, after immersing the cooling metal 3 directly below the molten steel surface L of the charge A, a new tundish to be replaced is installed at a regular position, and the charge B following the mold 2 from this tundish via the nozzle 1 is installed. Inject molten steel. At this point, the solidification of the molten steel further progresses and a solidification phase 6 is formed.

Therefore, at the initial stage of injecting the charge B molten steel from the tundish after the exchange, the molten steel enclosing the mold flux and the slag does not penetrate into the charge A molten steel below the cooling metal fitting 3, and this portion,
That is, the cleanliness of the continuous cast strand top portion is not deteriorated.

The size of the cooling metal article is preferably smaller than the internal size of the mold within the range of 30 to 50 mm.

[0022]

[Example] Using a continuous casting machine having two strands, mold dimensions: 1020 mm (width) x 260 mm (thickness), C: 0.05 wt
%, Si: 0.01 wt%, Mn: 0.20 wt%, Al: 0.030 wt
A continuous cast slab having a component composition of 10% was cast.

In casting the continuous cast slab, after the molten steel is poured into the mold from the tundish, the tundish is moved according to the method of the present invention, and then the cooling metal is placed at a position 100 mm directly below the molten steel surface in the mold. When immersed 1 minute after the completion of the molten steel injection and when no cooling metal was used as a comparative example,

In continuous casting of the same steel type, in which continuous casting is continuously performed after tundish exchange, in accordance with the method of the present invention, after completion of pouring molten steel into the mold from the tundish before exchange, the cooling metal article is the same as above. Was immersed just below the surface of the molten steel in the mold, and after 4 minutes, the molten steel was injected from the tundish after replacement, and as a comparative example, no cooling metal was used.

The cooling metal used here is the metal lath 6
The sheets are stacked and integrated into a single unit, and the dimensions are 920 x 200 mm, as shown in Fig. 2.

Each continuous cast strand top slab cast under the above conditions is finished into a cold rolled sheet coil with a sheet thickness of 1.5 mm by an ordinary hot rolling / cold rolling step, and subjected to an ultrasonic flaw detection test to obtain a rating rate index. Compared. These results are summarized in FIG.

Here, the rating rate indicates the rate at which the deep-drawing steel sheet was changed to a flat-use steel sheet because inclusions were detected by the ultrasonic flaw detection test.

As is apparent from FIG. 3, in the case where the molten steel is completely poured from the tundish into the mold and when the tundish is replaced, the conforming example of the method of the present invention has a significantly lower rating rate index than the comparative example. You can see that

[0029]

Industrial Applicability According to the present invention, molten steel is poured into a mold from a tundish or a tundish before replacement, and then a cooling metal article having through-holes in the vertical direction is immersed immediately below the surface of the molten steel in the mold. , To prevent entrainment of mold flux and slag into the molten steel portion in the mold and improve cleanliness, and the molten steel portion in the mold according to the present invention, that is, deterioration of the quality of the product produced from the continuous cast strand top portion Not, the stakes are greatly improved.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view showing a solidified state of a mold and molten steel in a mold when a subsequent charge is injected into the mold after a tundish is exchanged in a state where a cooling metal article according to the present invention is immersed in the molten steel.

FIG. 2 is a perspective view of a cooling metal piece used in an example.

FIG. 3 is a graph showing a disqualification rate index of a conforming example of the present invention and a comparative example.

[Explanation of symbols]

 1 Immersion nozzle 2 Mold 3 Cooling metal 4 Solidification phase 5 Solidification phase 6 Solidification phase A Molten steel of charge before tundish exchange B Molten steel of charge after tundish exchange L Melt steel surface of charge before tundish exchange

Claims (2)

[Claims] 1. At the end of the continuous casting operation, after the molten steel is poured into the mold from the tundish, the tundish is removed, and then a vertical through hole is formed directly below the molten steel surface in the mold. A method for improving cleanliness of a cast slab in a continuous cast strand top end, which comprises immersing a cooling metal having the same. 2. In continuous casting of the same steel type, which continuously performs continuous casting after tundish exchange during continuous casting operation,
After the molten steel is poured into the mold from the tundish, the tundish is removed, and then a cooling metal article having a through hole in the up-down direction is immersed immediately below the molten steel surface in the mold. A method for improving the cleanliness of cast slabs at the top end of cast strands.