JPH0740019A - Method for reducing center segregation in cast slab in continuous casting for steel - Google Patents

Abstract

PURPOSE:To reduce the center segregation, such as C, P, S, in a cast slab obtd. by continuous casting for steel. CONSTITUTION:Electromagnetic stirring devices 7a, 7b, 7c are arranged at the inner part of a mold 1, on the way of a strand and just before squeezing device 8, respectively. The electromagnetic stirring is executed to the unsolidified molten steel in the cast slab 2 in the prescribed magnetic flux density, and also the squeezing is continuously executed at the step before completion of the solidification becoming <=80mm unsolidified thickness in the cast slab 2 by using the reciprocative moving type squeezing device 8. As the squeezed part becomes equi-axed crystals, the formation of white band is restrained and also, as the squeezed solute elements are diffused into the whole unsolidified molten steel, the segregation can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing center segregation generated in a slab obtained by continuous casting of steel.

[0002]

2. Description of the Related Art The center segregation of continuous cast slabs of steel is a positive segregation due to the concentration of molten steel components such as C, Mn, P and S at the center of the thickness which is the final solidified part of the slab. It appears and is one of the quality defects that cannot be avoided by the conventional continuous casting method.
The center segregation generation mechanism is that the holes generated by solidification shrinkage of the solidification tip of the slab obtained by continuous casting and bulging of the solidification shell become the vacuum suction force, sucking the concentrated molten steel into the solidification tip and It is considered that the positive segregation remains in the center of the thickness. Such a conventional technique for reducing center segregation that appears in slabs such as blooms and slabs obtained by continuous casting is a low-temperature casting method in which the degree of superheat of molten steel is reduced and cast into a mold, molten steel or strands in the mold. A method of obtaining equiaxed crystals by electromagnetically stirring the molten steel inside has become widespread. All of these methods are intended to disperse segregation by increasing the proportion of equiaxed crystals in the solidified structure, and to suppress the central segregation from becoming a high proportion, but each has advantages and disadvantages, Even microsegregation could not be prevented and the effect of improving segregation was not always sufficient.

Japanese Unexamined Patent Publication No. 54-107831 discloses a method of reducing center segregation by a roll light rolling method. This method attempts to reduce the center segregation by compensating for the volumetric shrinkage of molten steel in the mold during the solidification process by compensating the molten steel in the vicinity of the solidification completion point of the slab by rolling reduction to suppress the concentrated molten steel occurring at the final stage of solidification. Is. This method consists in rolling down the slab with a roll, but with rolling down, the unsolidified portion is reduced by only about 10% of the amount of reduction applied to the surface of the slab, and The movement cannot be blocked and the center segregation of the cast slab is not sufficiently reduced.

That is, it is extremely difficult to adjust the amount of reduction to an appropriate amount in the region where solidification of molten steel in a slab in the final stage of solidification is unstable. If the amount of reduction by the roll is too large, it is inevitable that the solidification interface is cracked due to the stress distribution, which has a fatal drawback that the quality of the cast piece is adversely affected. Further, as a method for advantageously preventing the center segregation of the slab, Japanese Patent Laid-Open No. 63-183765 proposes to subject the slab obtained by continuous casting to forging at a stage before the solidification completion point. ing. In this proposal, at the stage before the solidification completion point of the slab obtained by continuous casting, the solidification interface and the solidification interface are pressure-bonded by forging using a forging device, and the solidification at the end of solidification is performed. By moving the molten steel upward, central segregation is reduced. By this means, it is possible to reduce internal cracking of the slab and positive segregation of the center, but since the solidified shell remains a columnar crystal or dendrite structure, molten steel flow at the solidification interface is likely to occur. For this reason, since all the unsolidified concentrated molten steel between the solidification completed parts is discharged, a negative segregation zone is formed in the center and a white band is formed, which impairs the advantage of preventing center segregation. .

Further, in JP-A-60-148651, an electromagnetic stirrer is provided in an incomplete solidification region on the upstream side, which is considerably distant from the solidification completion point of the slab in the continuous casting of steel, so that the slab is incomplete. A method of electromagnetically stirring the molten steel in the solidification region and using a reciprocating forging device installed near the solidification completion point of the slab to continuously forge the slab before the solidification is completed is proposed. Has been done.

In the prior art of Japanese Patent Laid-Open Publication No. 60-148651, electromagnetic stirring is performed on the upstream side far away from the position of the forging machine, and fine equiaxed crystals are obtained. It is suggested that unsolidified molten steel with a high solute concentration between trees is difficult to be squeezed out easily and that the center segregation is improved more effectively without forming a negative segregation zone, compared to the case where the columnar crystal is is doing.

[0007]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method by continuous forging pressure proposed in Japanese Patent No. 3765 is to perform forging pressure before the incompletely solidified region in which the unsolidified molten steel in the center of the slab remains remains completely solidified, and to make it into a completely solid phase or a state close to it. . Therefore, the unsolidified molten steel with incompletely solidified solute in the forging portion is forcibly squeezed out from the forging portion toward the incompletely solidified region and mixed with the molten steel in this portion.

At this time, the unsolidified molten steel having a high solute concentration squeezed out does not sufficiently mix with the unsolidified molten steel in the incompletely solidified region, and the incomplete solidification of the slab having a low solid phase ratio (solid phase ratio). An area of unsolidified molten steel having a high solute concentration is formed while re-melting the solid phase near the center of the area. In this unsolidified molten steel region, the solute concentration is further enriched by the unsolidified molten steel having a high solute concentration which is newly squeezed out as the forging pressure by casting progresses, and changes in the casting direction. As a result, the solute concentration at the center of the cast slab after the forging pressure changed highly with the progress of the forging pressure due to casting, and the desired center segregation reducing effect could not be obtained in some cases.

Further, there is a case where a fine equiaxed crystal cannot be obtained only by performing electromagnetic stirring by the electromagnetic stirring device provided on the upstream side, which is composed of the forging device proposed in the above-mentioned JP-A-60-148651. However, when casting for a long time, the solute concentration at the center of the slab after the forging changes with the progress of the forging pressure due to the casting, and the desired effect of reducing center segregation may not be obtained.

The present invention improves the method for reducing the center segregation of cast slabs by combining the above-mentioned conventional electromagnetic stirrer and forging device, and can reduce not only positive segregation but also white band due to negative segregation. It is an object of the present invention to provide a method for reducing center segregation of a slab in continuous casting of.

[0011]

The present invention for achieving the above object is to provide an electromagnetic stirrer upstream of a solidification completion point of a slab in continuous casting of steel to provide an incomplete solidification region of the slab. Electromagnetic stirring of the molten steel, and a reciprocating forging device provided in the vicinity of the solidification completion point of the slab are provided, and the slab is continuously forged in the stage before the solidification of the slab is completed. In the method of reducing the center segregation, the electromagnetic stirrer is placed in at least one place selected from the inside of the mold, the middle of the strand in which the slab is in the incompletely solidified region, and immediately before the forging device. The electromagnetic stirrer inside the mold is MEMS, the electromagnetic stirrer in the middle of the strand is SEMS, and the electromagnetic stirrer immediately before the forging device is F.
When using EMS, an electromagnetic force is applied under the conditions of the magnetic flux density B (Gauss) shown in Table 1 below, and electromagnetic stirring is performed.
The unsolidified thickness of the cast piece is 80 using the reciprocating forging device.
A method for reducing center segregation of a slab in continuous casting of steel, which is characterized by performing continuous forging at a stage before completion of solidification of less than mm.

[0012]

[Table 2]

[0013]

FIG. 1 shows a continuous casting machine equipped with an electromagnetic stirrer on the upstream side of a forging device suitable for applying the present invention, in which a slab 2 from a mold 1 is supported by a support roll 3 and a pinch roll. 4 is drawn out, and a pair of upper and lower molds 5 provided in the forging device 8 continuously perform forging by reciprocating motion of the unsolidified end portion (crater end) of the cast piece 2 and each hydraulic cylinder 6. In the state where such forging is performed, C, Mn, P, S that adversely affect center segregation
Molten steel in which solute elements such as is concentrated is pushed up to the upstream diffusion region by the large pressure by the die 5, while the solidified shell 2a of the slab 2 is continuously pressure-bonded, which results in casting with reduced center segregation. It becomes one piece.

However, such a slab 2 is formed by using the mold 5.
When the large-scale reduction of the unsolidified end portion of the steel is continuously performed, the solute element-enriched molten steel discharged without solidification is accumulated in the diffusion region defined upstream of the forging portion, and the continuous reduction length of the slab 2 is reduced. Since the concentration increases with the increase, the effect of reducing center segregation gradually decreases. Therefore, in the present invention, in order to prevent solute elements such as C, Mn, P, and S from diffusing and accumulating in the molten steel in the diffusion region, an electromagnetic stirrer 7 is provided upstream of the solidification completion point of the cast piece 2. The unsolidified molten steel in the provided slab 2 is electromagnetically stirred.

Specifically, an electromagnetic stirrer 7a is provided inside the mold 1, an electromagnetic stirrer 7b is provided in the middle of the strand of the moving path of the slab 2 in the incomplete solidification region of the slab 2, and a forging device is also provided. An electromagnetic stirrer 7c is provided immediately in front of 8. By stirring the molten steel in the diffusion region in which the solute elements are retained by these electromagnetic stirring devices 7a, 7b, 7c, the solute elements such as C, Mn, P, S in the molten steel can be applied to the entire unsolidified molten steel in the slab 2. It diffuses as uniformly as possible. Application of electromagnetic force in the mold 1 (MEM
S), application in the middle of the strand (SEMS), and application in front of the forging device 8 (FEMS) reduce central segregation in a stable state without forming a white band due to negative segregation on the slab 2. The application of these electromagnetic forces is effective alone, but the effect is further increased when used in an appropriate combination.

Electromagnetic stirring device (MEMS) 7 in the mold 1
a, electromagnetic stirrer (SEMS) 7 on the way of moving slab
b, the magnetic flux density B (Gauss), the unsolidified thickness (mm) of the portion to be forged by the forging device 8 and the forging pressure when the electromagnetic force is applied individually to the electromagnetic stirrer (FEMS) 7c before the forging device. The subsequent relationship with the slab center segregation ratio (C / Co) is shown in FIGS. However, Co indicates the solute concentration of the injected molten steel, and C indicates the concentration of the shaft core after forging.

FIG. 2 shows the application conditions of the magnetic flux density B of the (MEMS) 7a in the mold 1 and the unsolidified thickness (mm) of the forged part slab.
The relationship with the carbon segregation ratio (C / Co) of the slab center after forging is shown. From FIG. 2, increasing the magnetic flux density B of the (MEMS) 7a promotes the formation of the equiaxed crystal of the solidified shell 2a, so that the slab center segregation ratio (C / Co) is set within the allowable range 0.9 ≦ C / Co ≦ 1.1. It can be seen that the range of unsolidified thickness (mm) in the slab that can be obtained is widened and a more stable center segregation ratio (C / Co) can be achieved. In the case of MEMS, the magnetic flux density B is 500
Above Gauss, the effect is almost saturated, and the white band suppression effect is constant.

FIG. 3 shows a structure provided in the middle of the strand (SEM
S) The relationship between the application condition of the magnetic flux density B of 7b, the unsolidified thickness (mm) of the forging part cast, and the cast center segregation ratio (C / Co) after forging is shown. As shown in FIG. 3, by increasing the magnetic flux density B of (SEMS) 7b, the formation of equiaxed crystals is promoted and the slab center segregation ratio (C / Co) can be set within the allowable range 0.9 ≦ C / Co ≦ 1.1. The range of unsolidified thickness (mm) in the slab that can be expanded
It can be seen that a center segregation ratio (C / Co) close to 1.0 can be achieved. In the case of SEMS, when the magnetic flux density B is 1000 gauss or more, the effect is saturated and the white band suppressing effect becomes constant. It was over 500 gauss in MEMS, but S
EMS requires over 1000 gauss.

FIG. 4 shows the final (FE
The relationship between the application condition of the magnetic flux density B of MS) 7c, the unsolidified thickness (mm) of the slab at the forging part, and the slab center segregation ratio (C / Co) after the forging is shown. Similarly from FIG. 4, (FEMS) 7c
If the magnetic flux density B of is increased, the center segregation ratio (C / Co) of the slab can be set within the allowable range 0.9 ≦ C / Co ≦ 1.1. It can be seen that the center segregation ratio (C / Co) can be achieved. In the case of FEMS, when the magnetic flux density B is about 500 gauss, an effect almost equal to 1000 Gauss of SEMS is obtained, and by applying more than 500 gauss, the white band suppressing effect is saturated.

The (MEMS) 7a in the mold, the (SEMS) 7b in the middle of the strand, and the final (F
Table 2 shows the application conditions of the magnetic flux density B for electromagnetically stirring the unsolidified molten steel in the slab, either alone or in combination with EMS) 7b.
Shown in. For comparison, Table 2 also shows the case where each electromagnetic stirring device is not used. In this case, the unsolidified thickness of the forged part cast piece by the forging device is 80 mm in accordance with the upper limit of the limiting condition of 80 mm or less, and the upper and lower solidified interfaces are continuously forged by pressurizing and pressing the cast piece of the part. Was crimped. In Table 2, ◯ indicates the center segregation ratio (C / Co) of the slab.
Shows the case where it is within the allowable range of 0.9 ≦ (C / Co) ≦ 1.1,
The mark x indicates the case where a white band is formed outside the allowable range.

[0021]

[Table 3]

Based on the results in Table 2, the MEMS inside the mold
And SEMS in the middle of the strand and the final FEMS immediately before the forging machine are used alone or in combination, and the center segregation ratio (C / Co) of the slab is 0.9 ≦ (C / Co) ≤ 1.1, the conditions for applying the magnetic flux density B are summarized in Table 3.

[0023]

[Table 4]

When the strength, which is larger than the value of the magnetic flux density B shown in Table 3, is applied alone or in combination with MEMS, SEMS or FEMS, the center segregation ratio is allowed in a region where the unsolidified thickness is large as compared with the conventional case without EMS. Is a range
By setting 0.9 ≦ C / Co ≦ 1.1, it is possible to reduce the formation of white band.

[0025]

EXAMPLES C: 0.80 wt%, Si: 0.8 wt%, Mn: 0.5 wt
%, P: 0.005 wt%, S: 0.008 wt%, Al: 0.04 wt%,
Fig. 1 is a continuous drawing of molten steel at a casting speed of 0.75m / min in order to obtain a bloom with a slab size of 400 x 560mm, which is a material for hard steel wire rods. Using an electromagnetic stirrer (SEMS) 7b [arranged at a position of about 9 m upstream of the forging machine 10] in the middle of the strand and an electromagnetic stirrer (FEMS) 7c immediately before the forging machine,
(SEMS) 7b has a magnetic flux density of 1000 gauss and (F
Electromagnetic force is applied to the EMS) 7c with a magnetic flux density of 500 gauss to electromagnetically stir the unsolidified molten steel in the slab 2, and the upper and lower molds 5 provided in the forging device 8 with the unsolidified thickness of 80 mm are provided. A large reduction was performed with a thickness reduction of 110 mm.

FIG. 5 shows castings in which electromagnetic force is applied using the EMS according to the present invention under the above conditions and large reduction is performed using the forging device 8 and when only large reduction is performed by the forging device 8 without EMS. The C distribution in the thickness direction of the piece is shown.
As shown in FIG. 5, white bands due to negative segregation are generated in a slab only under unsolidified large pressure, whereas the present invention using an electromagnetic stirrer EMS shows a substantially uniform carbon distribution without white bands. It has been found.

[0027]

As described above, in the present invention, the electromagnetic stirrer is arranged in at least one position selected from the inside of the mold, the middle of the strand, and immediately before the forging device, and the Electromagnetic stirring of unsolidified molten steel and solidification thickness of 80 mm
In the stage before the completion of solidification, which will be described below, large reduction is performed by using a forging device, so that a slab with less positive segregation and negative segregation can be continuously cast, and a desired effect of improving center segregation is achieved.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a continuous casting machine suitable for applying electromagnetic stirring and forging processing according to the present invention.

FIG. 2 Magnetic flux density B of electromagnetic stirring device MEMS in the mold
(Gauss), the unsolidified thickness (mm) of the forging part cast piece, and a diagram showing the relationship between the center segregation ratio (C / Co).

FIG. 3 shows the magnetic flux density B (Gauss) of the electromagnetic stirrer SEMS in the middle of the strand, and the unsolidified thickness (mm) of the forged thick slab,
It is a diagram which shows the relationship with a center segregation ratio (C / Co).

FIG. 4 is a diagram showing the relationship between the magnetic flux density B (Gauss) of the electromagnetic stirrer FEMS immediately before the forging device, the unsolidified thickness (mm) of the forged product cast, and the center segregation ratio (C / Co). is there.

FIG. 5 is a diagram showing a comparison of center segregation ratio (C / Co) in the thickness direction of a slab between a forging method of the present invention using EMS and a conventional forging method not using EMS.

[Explanation of symbols]

 1 Mold 2 Cast Piece 3 Support Roll 4 Pinch Roll 5 Mold 6 Hydraulic Cylinder 7 Electromagnetic Stirrer 8 Forging Device

Claims (1)

[Claims] 1. An electromagnetic stirrer is provided upstream of a solidification completion point of a slab in continuous casting of steel to electromagnetically stir molten steel in an incompletely solidified region of the slab and near the solidification completion point of the slab. In the method of reducing the center segregation of the slab by continuously forging in the stage before the solidification of the slab, the electromagnetic stirrer is used as a mold. Inside, the slab in the middle of the strand in the incompletely solidified region, and arranged at least at one place selected from just before the forging device, the electromagnetic stirring device inside the mold as the MEMS, the middle of the strand When the electromagnetic stirrer is SEMS and the electromagnetic stirrer immediately before the forging device is FEMS, electromagnetic force is applied under the conditions of the magnetic flux density B (Gauss) shown in the following table to stir electromagnetically,
The unsolidified thickness of the cast piece is 80 using the reciprocating forging device.
A method for reducing center segregation of a slab in continuous casting of steel, which comprises continuously performing forging processing in a stage before completion of solidification of less than mm. [Table 1]