JPH09122833A - Method for concentrating solute on surface layer of cast slab in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control the quality on surface layer of a cast slab by concentrating a solute element added into molten steel in a mold onto the surface layer of the cast slab in continuous casting. SOLUTION: In producing the cast slab by pouring the molten steel 4 into the mold 1, a molten steel partition plate 3 is arranged at the upper part of discharging hole 12 of an immersion nozzle 2 in the mold, and DC magnetic field is impressed in the horizontal direction to the molten steel in the mold existing at the lower part from the molten steel partition plate by using magnets 6. The molten steel at the upper part from the molten steel partition plate is stirred by using an electromagnetic stirring device 5 arranged at the upper part of the mold and the allay element is added into the molten steel at the upper part from the molten steel partition plate to concentrate the adding element on the surface of the cast slab.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuous casting method for molten steel.

[0002]

2. Description of the Related Art Changing the solute concentration in the surface layer of a slab in continuous casting is extremely useful for controlling the material quality of the surface layer of a steel sheet. Conventionally, a method of adding an alloying element to molten steel in a mold of a continuous casting machine by adding a wire is well known, but in this method, the additional element is dispersed throughout the cast piece, and the alloying element is concentrated only in the surface layer of the cast piece. It cannot be turned into As a method of thickening the solute on the surface layer of the cast slab, JP-A-55-704 is known.
51 and JP-A-55-70450,
A method is disclosed in which an alloy is added to the molten steel between a partition wall and a partition wall placed parallel to the mold wall. However, even with this method, the alloy elements added to the mold are mixed by the discharge flow of the immersion nozzle and dispersed inside the cast piece, and it is difficult to concentrate the alloy element only on the surface layer of the cast piece.

[0003]

As described above, there is currently no method for concentrating elements only in the surface layer of a slab by adding elements to the molten steel in the mold, and the surface layer of the slab is stable and at low cost. There is a demand for a method of enriching a desired element.

An object of the present invention is to provide a method for concentrating the solute element added to the molten steel in the mold in the surface layer of the slab in continuous casting.

[0005]

As a result of various investigations to solve the above problems, the inventors of the present invention have found that the molten steel partition plate is provided by the action of the molten steel partition plate installed above the discharge hole of the immersion nozzle and the action of the DC magnetic field. By suppressing the mixing of molten steel in the upper and lower parts of the plate and stirring the molten steel in the upper part of the molten steel partition plate with an electromagnetic stirrer, the alloying elements added to the molten steel in the upper part are mixed and diffused in the lower molten steel. However, it was found that only the surface layer of the slab was thickened.

The gist of the present invention is, in continuous casting of steel,
When molten steel is poured into the mold to produce a slab, a molten steel partition plate is installed above the discharge hole of the immersion nozzle in the mold.
In addition, a direct current magnetic field is applied to the molten steel in the mold below the molten steel partition plate in the horizontal direction, and the molten steel above the molten steel partition plate is agitated using the electromagnetic stirrer installed on the upper part of the mold to melt the molten steel partition plate. The continuous casting method is characterized in that the additive element is concentrated in the surface layer of the slab by adding the alloying element to the molten steel above.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram of a molten steel partition plate 3 and an electromagnetic stirrer 5 at the upper part of a discharge hole of a dipping nozzle when pouring molten steel 4 into a mold 1 through a dipping nozzle 2 in continuous casting. The top view at the time of installing is shown.

FIG. 2 is a sectional view taken along the line A--A in FIG. 1. In continuous casting, the molten steel partition plate 3 is installed above the discharge hole 12 of the immersion nozzle 2, and the electromagnetic stirrer 5 and the magnet 6 are installed. The schematic diagram when the additive element is concentrated on the surface layer of the cast slab is shown.

During casting, the molten steel 4 flowing in the molten steel flow direction 7 as shown by the arrow and flowing into the mold 1 through the discharge holes 12 of the immersion nozzle 2 flows out to a region below the molten steel partition plate 3. The molten steel 4 in the mold is solidified by heat removal from the molten steel to the mold to form a solidified shell 8, which is drawn downward at a certain casting speed as indicated by the arrow moving direction 9 of the slab. Be done. The molten steel partition plate can suppress mixing of molten steel in the upper and lower portions of this plate to some extent, but when the casting speed is high and the nozzle flow velocity of molten steel is high, or the space between the molten steel partition plate 3 and the solidified shell 8 is large. in case of,
Molten steel flows vertically between the molten steel partition plate and the solidified shell,
It is not possible to completely suppress the mixing of the molten steel in the lower part and the molten steel in the upper part of the molten steel partition plate. At this time, when a direct current magnetic field is applied to the molten steel below the molten steel partition plate by the magnet 6, the molten steel receives an electromagnetic force in the direction opposite to the direction of motion and the flow velocity is reduced. Therefore, the molten steel flow velocity in the lower portion of the molten steel partition plate is reduced. In addition to reducing the flow rate of molten steel flowing from the lower part of the molten steel partition plate to the upper region, the mixing of molten steel in the upper and lower parts of the molten steel partition plate can be suppressed to a minimum, and it is added to the molten steel in the upper part. Mixing and diffusion of alloy elements into the lower molten steel can be suppressed, and the additive elements in the upper part of the mold can be efficiently concentrated in the surface layer of the slab.

When the iron wire 11 containing the element to be added is immersed in the molten steel above the molten steel partition plate 3, the iron wire is melted and the additive element is mixed with the molten steel in the upper region.
At this time, if the molten steel above the molten steel partition plate is stirred with an electromagnetic stirring device installed in the upper part of the mold, the melting of the iron wire is promoted, and insufficient melting of the iron wire can be prevented,
The added element is uniformly dissolved in the molten steel above. Since the molten steel containing the additional element is solidified in the mold portion above the molten steel partition plate, the solute element can be concentrated in the surface layer of the cast piece up to the thickness of the solidified shell at the position of the partition plate.
The slab surface layer thickness at which the additive elements can be concentrated becomes thicker as the casting speed is slower and as the distance from the molten steel surface in the mold to the molten steel partition plate is longer. By changing these values, the thickness of the solute-enriched layer on the surface of the slab can be variously changed. Unless the molten steel above the molten steel partition plate is electromagnetically stirred, the additive elements are not uniformly mixed in the molten steel, and as a result, the surface concentration of the slab becomes uneven at the circumferential position of the slab.

As a method of adding an element to the molten steel above the molten steel partition plate 3, in addition to the above-mentioned wire addition method, particles containing an element to be added are added to the mold flux 1
It is also possible to dissolve the additive element from the molten mold flux in contact with the molten steel surface into the molten steel by mixing it with 0 and pouring it into the molten steel surface.

When the molten steel partition plate 3 is not installed and no DC magnetic field is applied by the magnet 6, the solute elements added to the wire are mixed by the molten steel flow caused by the discharge flow of the immersion nozzle,
Since it is dispersed throughout the molten steel pool, the additive element concentration is dispersed throughout the slab. Therefore, it is important to partition the upper region to which the element is added and the lower region into which the discharge flow flows by the molten steel partition plate, and to suppress the mixing of the molten steel in the upper region and the lower region by the action of the DC magnetic field.

According to the present invention, due to the action of the molten steel partition plate and the action of the DC magnetic field, the mixing of the molten steel in the upper region added with the element and the molten steel in the lower region flowing from the dipping nozzle is extremely reduced, and further, the upper portion of the mold is It is possible to efficiently add the solute element to the surface layer of the slab by agitating the molten steel with the electromagnetic stirrer described above to uniformly dissolve the additive element.

[0014]

[Example] Bloom cast (thickness 300 mm, width 500 m
An experiment was conducted in which the alloy element was concentrated on the surface layer of m). In a continuous casting machine using a normal vibration copper mold, mold flux was used and molten steel of carbon steel containing 0.7 mass% of carbon as a main component was poured into the mold using an immersion nozzle. As the casting conditions, the casting speed was 0.7 m / min, the degree of superheat of molten steel in the tundish was in the range of about 10 to 30 ° C., and water spray was adopted for the secondary cooling of the cast piece after the mold part.

The molten steel partition plate has a length of 200 mm and a width of 400.
mm, 50 mm thick, made of alumina graphite.
As shown in FIG. 3, the dipping nozzle was fixed to the upper part of the discharge hole and was installed so as to be dipped at a position about 400 mm below the molten steel meniscus.

In order to stir the molten steel in the upper part of the mold, as shown in FIG. 2, a moving magnetic field type electromagnetic stirrer is installed in the upper part of the mold (center position of the electromagnet in the longitudinal direction of the slab: 1 from the molten steel level).
00 mm down, coil current: 200 amps) was used.

An electromagnet is used to apply a DC magnetic field, and as shown in FIG. 2, it is placed below the molten steel partition plate, and the magnets have dimensions such that the length in the width direction of the cast piece is 800 mm and the length in the length direction of the cast piece is long. Is 200 mm, and the center position of the magnetic pole is 5 from the molten steel surface.
It was installed so that the position was 00 mm. The experiment was conducted under the condition that the magnetic field strength at the central position between the magnetic poles of the electromagnets installed on both sides of the mold was about 0.1 tesla.

Regarding the method of adding the elements, a method of continuously adding an Mn-containing iron wire into the molten steel in the mold was tested. The Mn concentration in the molten steel flowing from the dipping nozzle to the lower region in the mold was about 0.5 mass%, and the Mn concentration in the cast slab surface layer was about 0.7 m due to the addition of wire.
The amount of Mn in the iron wire and the wire feed rate were adjusted so as to increase to ass%. Further, with respect to each of the molten steel partition plate, the electromagnetic stirrer, and the application of the DC magnetic field by the electromagnet, an experiment was performed with and without use, and the effects were compared.

In order to investigate the degree of concentration of the additive element in the surface layer of the cast piece, in the steady part of the cast piece obtained in the casting experiment,
The Mn concentration was analyzed at a position of 10 mm from the surface of the slab in the thickness direction of the slab (representative position of the surface layer of the slab where the additive element was concentrated) and at a position of 75 mm (representative position inside the slab).

As a result, (1) when there is no electromagnetic stirring,
When both the molten steel partition plate and the DC magnetic field are not used, the Mn concentration of the slab is about 0.53 without distinction between the slab surface layer and the inside.
It was 5 to 0.538 mass%, and it was found that the additive element could not be concentrated only in the surface layer of the cast piece, and the added element was dispersed in the entire cast piece. When the molten steel partition plate and the DC magnetic field were used independently, the Mn concentrations in the surface layer of the slab were 0.64 to 0.70 mass% and 0.65, respectively.
Is 0.70 mass%, and when both the molten steel partition plate and the direct current magnetic field are used at the same time, the Mn concentration of the slab surface layer is 0.67 to 0.70 mass%, and when the molten steel partition plate and the direct current magnetic field are used together, the molten steel It was found that the mixing of molten steel in the lower part and the upper part of the partition plate can be reduced, and although the concentration of the additive element in the surface layer of the cast slab slightly fluctuates, the concentration of the additive element in the surface layer of the slab becomes high.

Further, (2) under the use of electromagnetic stirring,
When both the molten steel partition plate and the DC magnetic field were used, the fluctuation of the Mn concentration in the surface layer of the slab was further reduced to about 0.69 to 0.7.
This is a fluctuation of 0 mass%, and the concentration of the additive element in the surface layer of the slab is changed because the iron wire added to the molten steel above the molten steel partition plate by the electromagnetic stirring is melted and the additive element is well mixed in the molten steel. It was found that the concentration of additive elements on the surface layer of the slab can be improved very well.

[0022]

EFFECTS OF THE INVENTION By carrying out the present invention, it is possible to suppress the elements added in the mold from being dispersed in the entire slab, and to stably and inexpensively concentrate the additional elements on the surface layer of the slab.

[Brief description of the drawings]

FIG. 1 is a plan view showing the relationship among an immersion nozzle, a mold, a molten steel partition plate, and an electromagnetic stirrer.

2 is a cross-sectional view taken along the line AA in FIG. 1, in which a molten steel partition plate is installed above the discharge hole of the immersion nozzle in continuous casting, and an additive element is added to the surface layer of the slab using an electromagnetic stirrer and a magnet. It is a schematic diagram at the time of making it concentrated.

[Explanation of symbols]

 1 Mold 2 Immersion Nozzle 3 Molten Steel Partition Plate 4 Molten Steel 5 Electromagnetic Stirrer 6 Magnet 7 Flow Direction of Molten Steel 8 Solidification Shell 9 Moving Direction of Cast Piece 10 Mold Flux 11 Wire 12 Nozzle Discharge Hole

Claims (1)

[Claims] 1. In continuous casting of steel, when molten steel is poured into a mold to produce a slab, the molten steel partition plate is installed above the discharge hole of the immersion nozzle in the mold, and the molten steel partition plate is also provided. A direct current magnetic field is applied horizontally to the molten steel in the mold below, and the electromagnetic stirrer installed at the top of the mold is used to stir the molten steel above the molten steel partition plate so that it is above the molten steel partition plate. A continuous casting method, which comprises adding an alloying element to molten steel to concentrate the additive element on a surface layer of a slab.