JPH08332553A - Continuous casting method - Google Patents

Abstract

PURPOSE: To reduce a slab defect caused by inclusion, bubble, etc., in molten steel by arranging an electromagnetic stirring device on short side walls of a mold in continuous casting of a steel slab. CONSTITUTION: In the continuous casting of the steel slab, one pair of the electromagnetic stirring devices 5 are arranged to one pair of the short side walls 4 of the mold for slab continuous casting composed of one pair of the short side walls 4 and one pair of long side walls 3 or one pair of short side surface sides just below the mold. Flow pattern at a meniscus part is changed by forcedly giving the molten steel fluid in the same direction as the casting direction to the molten steel discharging flow from an immersion nozzle 2 and the defect caused by the entrapment of powder on the molten steel surface into the slab is prevented. In this result, since the divided ascending flow developed by the molten steel discharging flow from the immersion nozzle 2 which collides against solidified shell 7 on the short side surface, is not entirely developed or remarkably weakened, the circulating flow in a meniscus is not developed. Further, rising of the molten steel surface near the short side surface is out of the problem.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method capable of effectively reducing slab defects caused by inclusions and bubbles in continuous casting of steel slabs.

[0002]

Air bubbles trapped under the epidermis of a steel slab during continuous casting of a steel slab are mainly caused by an inert gas (Ar gas) blown into the molten steel to prevent the immersion nozzle from clogging. Material is generated by deoxidation of molten steel, and the main cause is alumina (Al ₂ O ₃ ) remaining in the steel, continuous casting powder on the molten metal surface in the mold, and slag from ladle and tundish.

When a slab containing such bubbles and inclusions is rolled into a coil in a subsequent process, surface defects such as beard and sliver, or serious product defects such as blister defects. In particular, as shown in FIG. 3, the powdery inclusion defect caused by the continuous casting powder being trapped on the front surface of the solidified shell is caused by the molten steel discharge flow (white arrow) from the immersion nozzle 2 as shown in FIG. This is caused by the scraping A of the continuous casting powder 8 due to the branch upward flow generated by the collision with the No. 7 and the entrainment B of the continuous casting powder 8 due to the generation of vortices. The solid arrow indicates the direction of the molten steel flow, and the dotted arrow indicates the direction of the weak molten steel flow.

When the branched upflow is strong and the swelling C on the surface of the molten metal is remarkable, the powder melting layer is locally depleted, which causes local insufficiency of the powder melting layer into the meniscus in the mold. This leads to insufficient internal lubrication. In addition, uneven heat removal in the meniscus portion and poor cooling of the corner portion occur, leading to slab surface cracks and corner cracks. In FIG. 3, 1 is molten steel, 4 is a mold short side wall, and 6 is a short side support roll.

As described above, the flow in the mold caused by the molten steel discharge flow from the immersion nozzle has a great influence on the generation of defects in the slab.

[0006]

[Patent Document 1] JP-A-59-101262, JP-A-59-101263, and JP-A-59-104258 disclose a method of casting molten steel in a mold in continuous casting of a steel slab. Vertical stirring devices that electromagnetically stir in the same or opposite directions are disposed on the pair of opposing long side walls. However, when the electromagnetic stirrer is arranged in this way, it is not possible to obtain a sufficient effect in preventing the rise of the molten metal surface near the short side surface of the mold.

Japanese Unexamined Patent Publication (Kokai) No. 4-322859 discloses a molten metal flow control method for continuous casting in order to accurately detect and suppress an abnormality (one-sided flow) of molten steel flow in the mold. An electromagnetic stirrer is installed on a pair of opposing long side walls,
This is a method of electromagnetic stirring according to the degree of one-way flow in the same or opposite direction to the discharge flow direction from the immersion nozzle. With this method, it is difficult to set the strength balance between the discharge flow and the electromagnetic stirring provided on the long side wall. That is, when the discharge flow is excessively suppressed, the temperature of the molten metal in the mold becomes low, and when it is remarkable, solidified steel (decker) is generated on the molten metal surface. Further, if the discharge flow is excessively promoted, the rise of the molten metal surface near the short side surface increases as described above.

Therefore, in order to suppress the one-way flow, it is necessary to finely adjust the strength of the electromagnetic stirrers arranged on both sides of the immersion nozzle. Japanese Unexamined Patent Publication No. 1-228645 discloses a method of preventing vertical cracks on the surface of a solidified shell by horizontally flowing molten steel along the inner peripheral surface of the solidified shell in an area of at least 200 mm from immediately below the meniscus in a mold. Has been done. However, this method is excellent in uniformizing the heat removal of the meniscus portion, but as shown in FIG. 4, in the vicinity of the corner where the horizontal molten steel flow parallel to the long side of the mold collides with the solidified shell of the short side. However, there is a drawback that an accumulation portion D of inclusions and bubbles is generated. In addition, in FIG. 4, (a) is a schematic plan view of the continuous casting apparatus, (b) is a schematic sectional view thereof, 3 is a long side wall of the mold,
5 is an electromagnetic stirrer.

An object of the present invention is to provide a continuous casting method capable of effectively reducing slab defects caused by inclusions and bubbles in the conventional continuous casting of steel slabs.

[0010]

DISCLOSURE OF THE INVENTION The present invention, in continuous casting of a steel slab, comprises a pair of short side walls of a continuous casting mold for a slab consisting of a pair of short side walls and a pair of long side walls, or a pair immediately below the mold. In the continuous casting method, a pair of electromagnetic stirrers are disposed on the short side surface side of the above, and the flow of molten steel discharged from the immersion nozzle is controlled.

[0011]

The present inventors conducted a full-scale water model experiment in order to solve the above-mentioned problems caused by the flow of molten steel discharged from the immersion nozzle and the resulting flow in the mold. The following findings were obtained from this experiment. That is, the phenomenon in which the flow in the mold is unbalanced on both the left and right sides of the immersion nozzle occurs even when there is no difference in the flow velocity or flow rate of the discharge flow of the immersion nozzle, and 1) as shown in FIGS. This occurs only when such a meniscus circulation flow E is formed. 2) It has a periodicity and the phases are opposite to each other. 3) The cycle is about 15 seconds to 1 minute, throughput {molten steel injection amount per unit time (t / min)}, mold width, immersion nozzle shape (discharge port diameter, discharge port angle), inertness blown into molten steel It depends on the presence or absence of gas. 4) When the throughput is high and the width is narrow, the meniscus circulation flow is easily formed, and the uneven flow in the mold is severe. Therefore, the maximum flow velocity on the surface of the meniscus and the flow velocity fluctuation amount increase, and powder entrainment is likely to occur.

That is, the cause of the uneven flow in the continuous mold is the periodical movement of the meniscus circulating flow (vortex), and the periodical movement of the circulating flow causes the gap between the immersion nozzle and the long side wall of the mold to pass through the long side wall. It was found to be amplified by the interaction between the side-by-side flow of the submerged nozzle flowing horizontally along the horizontal direction and the lateral flow formed after the lower part of the mold. The present invention is based on this finding.

According to the present invention, as shown in FIGS. 1 (a) and 1 (b), an electromagnetic stirrer 5 is arranged in a pair of mold short side walls 4, or as shown in FIG. A pair of electromagnetic stirrers 5 are arranged on the side of the short side to apply a flow to the molten steel in the mold or immediately below the mold in the same direction as the casting direction along the side of the short side. As a result, the rising flow of molten steel discharged from the immersion nozzle 2 collides with the solidified shell 7 on the short side surface and is not weakened or significantly weakened, so that the meniscus circulation flow E does not occur. Moreover, the rise of the molten metal near the short side surface does not become a problem.

Therefore, it is possible to prevent slab defects due to improper flow in the mold as shown in FIG.

[0015]

EXAMPLE A slab having a thickness of 260 mm and a width of 2400 mm was cast at 1.00 m / min using a slab molten steel continuous casting device equipped with an electromagnetic stirrer as shown in FIG. The molten steel component in this case is% by weight, C: 0.12%, Si: 0.30%, Mn: 1.45%, P:
It contained 0.008%, S: 0.010%, Al: 0.028%, and the casting amount was 140 t.

The same molten steel was cast under the same conditions with other strands in the absence of an electromagnetic stirrer, and the surface and internal properties of the obtained cast pieces were compared. The stirring strength of the electromagnetic stirrer is based on the data obtained in advance for the same steel type, and the current value is set to 0.4 m / s in front of the short-side solidified shell at the center of the casting direction of the machine. The frequency is set. Frequency of cracks at the corners of the slab surface and thickness 3 mm x 200 mm x 260
Occurrence of powdery defects by 10 x 10 mm slice samples up to 50 mm from the short side of the slab and examined by X-ray transmission method (10
The average value of the number of sheets was compared with that with or without applying electromagnetic stirring, and as shown in Table 1, a remarkable improvement effect was observed by the present invention.

[0017]

[Table 1]

The corner cracking index in Table 1 is a value (pieces / m) obtained by counting the number of corner cracking of the slab and converting it per unit length without electromagnetic stirring.
It was indexed as 1.0, and the powder inclusions index is all powder inclusions of 50μ or more found in 20 slice samples taken from near both short side surfaces of the slab in the middle part of the casting period. It is counted and indexed as 1.0 without electromagnetic stirring.

According to the present invention, an electromagnetic stirrer is provided on the short side wall of the mold or on the short side face immediately below the mold, and the forced molten steel flow is applied in the same direction as the casting direction to change the flow pattern of the meniscus portion. By doing so, the main purpose is to prevent defects due to trapping of the molten metal surface powder in the slab, but the method of the present invention can also be used in the following cases. 1) When alumina inclusions, etc. adhere to the outlet of the immersion nozzle and the flow rate changes at the outlet, causing a drift in the molten steel flow in the mold, the stirring force of the electromagnetic stirrer on both short side faces is changed. By changing them independently, it is possible to mitigate the drift so that there is practically no damage. The drift can be detected by using a thermoelectric meter or a heat flow meter embedded in the mold steel sheet. 2) When the temperature of the meniscus in the mold becomes low and deckle occurs on the molten metal surface, an appropriate molten steel flow is applied in the direction opposite to the casting direction in order to supply heat to the molten metal surface. 3) In order to soften the collision of the outlet flow from the dipping nozzle with the short-sided surface solidified shell, forced flow in the same direction as the casting direction and in the opposite direction is alternately applied at a cycle of several seconds to several tens of seconds.

[0020]

EFFECTS OF THE INVENTION According to the present invention, the flow of molten steel discharge flow in continuous casting of steel slabs is controlled, the downward flow of the mold is mainly used, and the slab defects caused by the inclusion of continuous casting powder and air bubbles are drastically reduced. be able to.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an implementation status of the present invention.

FIG. 2 is an explanatory diagram showing another embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a conventional molten steel flow state.

FIG. 4 is an explanatory diagram showing a conventional molten steel flow state.

FIG. 5 is an explanatory diagram showing a conventional molten steel flow state.

[Explanation of symbols]

 1 Molten Steel 2 Immersion Nozzle 3 Mold Long Side Wall 4 Mold Short Side Wall 5 Electromagnetic Stirrer 6 Short Side Support Roll 7 Short Side Solidified Shell 8 Continuous Cast Powder

Claims (1)

[Claims] 1. In continuous casting of a steel slab, a pair of short side walls of a continuous casting mold of a slab consisting of a pair of short side walls and a pair of long side walls or a pair of short side faces immediately below the mold. A continuous casting method characterized in that an electromagnetic stirrer is provided to control the flow of molten steel discharged from an immersion nozzle.