JPH08290236A - Continuous cast slab manufacturing method - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a method for producing a multi-layer continuous cast slab, in which the alloy element concentration in the surface layer of the slab is higher than that in the inner layer in a continuous casting mold directly from molten steel. To do. [Structure] A continuous casting powder 10 containing an alloy element is used, and an electromagnetic stirrer 30 is installed in the upper part of the continuous casting mold 1 to melt and mix the alloy element in a horizontal cross section in the upper molten steel pool 3 in the mold. A stir flow 31 is formed, and a DC magnetic field 21 is applied in the width direction in the width direction below the stir flow 31 to form a braking region 5 in the molten steel pool. By supplying and casting, the multi-layered slab having a higher concentration of alloy elements in the surface layer of the slab than in the inner layer is produced. [Effect] By increasing the nickel concentration and the like in the surface layer portion, the surface defects due to copper peculiar to this steel type are suppressed, and the present invention is also applicable to casting of a slab having a small cross-sectional size.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a multi-layered continuous cast slab, in which the alloy element concentration in the surface layer of the slab is higher in the continuous casting mold directly from molten steel than in the slab.

[0002]

2. Description of the Related Art The present inventors have filed Japanese Patent Application No. 06-10436.
In No. 5, while using a continuous casting powder containing a predetermined alloy element, at a position a certain distance below the molten steel meniscus in the continuous casting mold, it is almost across the thickness of the slab across the width of the slab. Injection of molten steel with a constant composition while applying a DC magnetic field having a uniform magnetic flux density distribution,
Of the upper and lower molten steel pools divided by the DC magnetic field zone, continuous casting is performed in the upper molten steel pool through the continuous casting powder while mixing alloy elements, and the concentration of alloy elements in the surface layer is higher than that in the inner layer. A method for manufacturing the cast slab has been proposed.

[0003]

[Patent Document 1] Japanese Patent Application No. 06-10
According to the method proposed in No. 4365, it has become possible to manufacture a multi-layered slab having a higher concentration of alloying elements in the surface layer than in the inner layer. However, with this method, although the alloy elements added to the powder for continuous casting can be enriched in the surface layer of the slab, there is a problem in that the concentration distribution in the surface layer of the slab tends to become uneven.

The present invention solves the above-mentioned problems and provides a method for producing a continuous cast slab for achieving uniform alloy element concentration in the surface layer portion.

[0005]

The present invention uses a powder for continuous casting containing a predetermined alloy element,
With an electromagnetic stirrer installed in the upper part of the continuous casting mold,
A stir flow is formed in the horizontal cross section in the molten steel pool in the mold, and a DC magnetic field with a uniform magnetic flux density distribution in the width direction is applied below it to dampen the molten steel pool in the mold by applying it in the thickness direction of the slab. By forming a zone and casting molten steel under the direct current magnetic field while supplying molten steel with a dipping nozzle, it is possible to manufacture a multi-layered slab with a higher concentration of alloy element slab surface layer than in the inner layer. The method for producing a continuous cast slab is as follows.

In the method for producing a continuous cast slab described above, the amount of alloying element mixed into the inner layer portion of the slab is controlled by adjusting the positional relationship between the nozzle discharge hole position of the immersion nozzle and the DC magnetic field. It is characterized by.

Furthermore, in the above-mentioned continuous cast slab manufacturing method, the concentration distribution of alloy elements in the slab thickness direction in the slab surface layer portion is controlled by controlling the stirring strength of the electromagnetic stirrer. It is a thing.

[0008]

The operation of the present invention will be described below with reference to FIG.

First, a grain or powder of a predetermined alloy element 13
The powder 10 for continuous casting containing the above is continuously added to the entire molten metal surface from above the mold 1. Then, in the process in which the continuous casting powder 10 is melted on the meniscus in the mold 1, the particles or powder 13 of the predetermined element come into contact with the molten steel at the meniscus, and are mixed and diffused from the meniscus into the upper molten steel pool 3.

Next, it is necessary to make the concentration distribution of the upper molten steel pool 3 in the mold 1 uniform in the horizontal section.
In the molten steel pool having a rectangular cross section as shown in the inside, the flow pattern formed by the stirring flow 31 over a certain depth in the horizontal cross section has a more uniform concentration distribution than mixing by the vertical stirring flow. It is valid.

Therefore, an electromagnetic stirrer 30 is provided in the upper part of the continuous casting mold 1 and a stir flow 31 is formed in the molten steel pool in a horizontal cross section so that the concentration distribution of alloying elements in the stir zone 9 is made uniform. To do. In order to make the concentration distribution uniform, a stirring flow rate of at least 10 cm / sec or more is required.

Further, a DC magnetic field 21 having a uniform magnetic flux density distribution in the width direction is applied below the stirring area 9 in the thickness direction of the slab, so that the braking area 5 is formed in the molten steel pool.
To form. Further, in order to prevent the molten steel having a high alloy component concentration in the upper part of the mold from mixing into the lower molten steel pool 4 in the mold, the position of the discharge hole of the immersion nozzle 2 is important.

Basically, as the position of the discharge hole of the immersion nozzle 2 is lowered, the amount mixed into the lower molten steel pool 4 becomes smaller. Therefore, in order to minimize the mixing amount of the upper molten steel into the lower molten steel pool 4 in the mold, the nozzle discharge hole of the immersion nozzle 2 for supplying the molten steel into the mold 1 has a braking area formed by the DC magnetic field 21 thereof. It is provided below 5.

As a result, a molten steel pool having a higher concentration of alloying elements can be formed in the upper molten steel pool 3 below the braking area 5, and a lower molten steel pool 4 below the braking area 5 can be formed.
Then, it is possible to form the molten steel pool having the composition of the mother molten steel supplied into the mold 1 through the immersion nozzle 2.

In this way, the particles or powder 13 that has been sized according to the alloying element is contained in the continuous casting powder 10 in accordance with the concentration of the component to be added to the surface layer portion 6 of the cast slab. The molten steel component in the stirring area 9 can be adjusted to the concentration component.

In addition, a braking area 5 is formed in the molten steel pool below the stirring area 9 by the DC magnetic field 21, and the nozzle discharge holes of the immersion nozzle 2 are provided below the braking area 5 so that the braking area 5 is formed. The components of the alloy elements in the lower molten steel pool 4 below can be used as the components of the mother molten steel.

As a result, the added alloy elements are added to the stirring zone 9
It is possible to enclose the slab, and it is possible to cast a multi-layered slab in which the component concentration of the slab surface layer portion 6 is higher than that of the inner layer portion 7.

By controlling the strength of the stirring flow formed in the upper molten steel pool 3 in the mold by the electromagnetic stirring device 30, not only the strength of the stirring flow in the horizontal section of the pool but also above the braking area 5 is controlled. It is also possible to control the range of the stirring area 9 together. Therefore, it is also possible to control the component concentration distribution of the alloy element in the vertical direction in the upper molten steel pool 3 in the mold, that is, the component concentration distribution of the alloy element in the slab surface layer portion 6 in the thickness direction of the slab.

Further, in the present invention, the number of nozzle discharge holes of the immersion nozzle 2 may basically be one downward hole,
As shown in FIG. 2, by using the submerged nozzle 2 having two holes that are generally used and three discharge holes including one downward discharge hole, the lower part below the braking area 5 is used. The use of these dipping nozzles 2 is also effective in that these methods produce slabs of good internal quality, because the flow in the molten steel pool can be more stabilized.

[0020]

EXAMPLE A core center of an electromagnetic stirrer 30 is provided 150 mm below the level of the molten metal in the mold 1 so that a swirl flow can be formed in a horizontal cross section in the upper molten steel pool 3 in the mold 1, and In a continuous casting process (FIG. 1) provided with a DC magnetic field generator 20 capable of applying a DC magnetic field 21 having a uniform magnetic flux density distribution in the width direction 600 mm below the level in the thickness direction of the slab, Casting was performed under the conditions shown in Table 1. As for the nozzle discharge hole position of the immersion nozzle 2 for supplying molten steel into the mold 1, the distance from the molten metal level to the nozzle discharge hole was 700 mm.

[0021]

[Table 1]

Carbon powder was contained in the continuous casting powder 10 in an amount of 20% by weight and continuously supplied from above the mold 1 over the entire molten metal surface during casting. The result of investigating the carbon concentration distribution in the slab after casting is shown in FIG.

FIG. 3 (a) shows the carbon concentration distribution in the thickness direction of the slab at the center in the width direction, and (b) shows the carbon concentration distribution in the width direction of the slab at 10 mm from the surface. It can be seen that the carbon concentration is high, the carbon concentration inside the slab is the same as the carbon concentration inside the tundish, and that the carbon concentration is uniform in the width direction. Besides,
It is also clear that the carbon concentration distribution of the cast slab surface layer portion 6 changes depending on the stirring strength of the electromagnetic stirring device 30 (FIG. 4).

Further, the discharge hole position of the immersion nozzle and the amount of carbon mixed into the slab were investigated, and when the nozzle discharge hole was provided in the lower molten steel pool in the mold below the braking area, carbon It can be seen that the mixed amount is the smallest (Fig. 5).

Similar experiments were conducted for silicon, manganese, and nickel. As a result, a slab having a high alloy element concentration only in the slab surface layer portion 6 was produced regardless of the type of additive element. I was able to.

[0026]

EFFECTS OF THE INVENTION According to the method for producing a continuously cast slab according to the present invention, a continuous casting powder containing an alloy element is used to form a stirring flow in the upper molten steel pool in the mold, and below the stirring flow in the thickness direction of the slab. Applying a DC magnetic field to form a braking zone in the molten steel pool,
By supplying molten steel below this DC magnetic field with a dipping nozzle, the inside of the slab is the molten steel component in the tundish, the concentration of alloying elements is high only in the surface layer of the slab, and its concentration distribution is A slab that is uniform in the width direction can be manufactured.

Therefore, this method is applied to steel types in which surface defects are likely to occur depending on the carbon concentration of the steel, such as medium carbon steel and ultra low carbon steel (medium carbon steel; vertical cracks and ultra low carbon that occur on the surface of the slab. Steel: It is possible to easily avoid not only casting conditions, but also surface defects of products as well as casting slabs by applying it to inclusions, surface defects of bubble systems and scale defects due to poor peelability of oxide scale. You can

When casting steel containing copper,
By adding nickel only to the surface layer of the cast slab, it is possible to suppress the occurrence of cracking defects peculiar to steel containing copper. Further, how high the surface layer thickness of the alloy element is formed in order to suppress these defects depends on the type of each defect and in which process it occurs.

Therefore, it is extremely effective that the concentration distribution in the thickness direction of the slab in the surface layer of the slab can be controlled because a layer having a high alloying element with a required thickness can be reliably formed. Furthermore, the present invention, regardless of the slab size,
It can also be applied to castings with small cross-section sizes such as blooms and billets.

[Brief description of drawings]

FIG. 1 illustrates a situation during casting when a downward single hole type immersion nozzle for carrying out the method of the present invention is used, (a)
FIG. 3 is a drawing showing the casting condition as seen from above the mold, FIG. 7 (b) is the structure of the molten steel pool in the mold in the vertical section, and FIG.

FIG. 2 illustrates a situation during casting when a two-hole type immersion nozzle for carrying out the method of the present invention is used. (A) is a casting situation viewed from above the mold, (b) is a molten steel pool in the mold. The structure in the vertical cross section, (c) is a drawing showing the state of the concentration distribution of the alloying element in the horizontal cross section of the slab.

FIG. 3 shows the results of investigating the carbon concentration distribution in the cast slab, where (a) is the carbon concentration distribution in the slab thickness direction at the center of the slab width direction, and (b) is the slab width direction. 2 is a drawing showing the distribution of carbon concentration in FIG.

FIG. 4 shows the results of investigating the carbon concentration distribution in the surface layer portion of the cast slab when casting was performed by changing the stirring strength of the electromagnetic stirrer, and the carbon concentration in the slab thickness direction at the widthwise center of the slab was shown. It is the drawing which showed distribution.

FIG. 5 is a drawing showing the results of investigating the amount of carbon mixed into the cast slab when casting was performed by changing the nozzle discharge hole position of the immersion nozzle.

[Explanation of symbols]

 1 Mold 2 Immersion Nozzle 3 Upper Molten Steel Pool 4 Lower Molten Steel Pool 5 Braking Area (= Transition Area of Upper and Lower Molten Steel Pool) 6 Surface Layer of Cast Piece 7 Inner Layer of Cast Piece 8 Concentration Transition Area of Cast Piece 9 Stirring Area 10 Powder for continuous casting 13 Grain or powder of alloy element 20 DC magnetic field generator 21 DC magnetic field 30 Electromagnetic stirrer 31 Stirring flow

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location B22D 11/16 B22D 11/16 Z

Claims (3)

[Claims] 1. A continuous casting powder containing a predetermined alloy element is used, and a stirring flow is formed in a horizontal cross section in the molten steel pool in the mold by an electromagnetic stirring device installed in the upper part of the continuous casting mold. And, a braking area is formed in the molten steel pool in the mold by applying a DC magnetic field having a uniform magnetic flux density distribution in the width direction below the molten steel by a dipping nozzle below the DC magnetic field area. The method for producing a continuous cast slab, which comprises producing a multi-layer slab having a higher concentration of alloy elements in the surface layer of the slab than that of the inner layer by casting while supplying the alloy element. 2. The amount of alloying elements mixed into the inner layer portion of the cast slab is controlled by adjusting the positional relationship between the nozzle discharge hole position of the immersion nozzle and the DC magnetic field. Continuous casting slab manufacturing method. 3. The concentration distribution of alloy elements in the thickness direction of the cast piece in the surface layer of the cast piece is controlled by controlling the stirring strength of the electromagnetic stirrer.
A method for producing a continuous cast slab as described.