JP3257546B2 - Steel continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention suppresses white bands (negative segregation) in the surface layer of a slab, secures equiaxed zones in the slab, and stably produces a high-quality continuous cast slab. The present invention relates to a continuous casting method of steel that can be manufactured at low cost.

[0002]

2. Description of the Related Art Conventionally, for the purpose of improving the surface layer portion and internal quality of a slab, for example, the following continuous casting method using electromagnetic stirring has been proposed.

In Japanese Patent Application Laid-Open No. 6-606, the center of a coil for electromagnetic stirring is positioned near the meniscus so as to form a meniscus molten steel flow circulating in one direction in a mold.
Meniscus molten steel flow rate in the mold is 10-60 cm /
A method has been proposed in which a moving magnetic field is applied so that the surface property is improved so as to improve the surface properties.

In Japanese Patent Application Laid-Open No. 9-47853, the Lorentz force in an initial acceleration stage in which the molten steel flows inward from one short side to the long side so that the flow velocity of the molten steel is 20 to 60 cm / sec, and the other is from the inside to the other. In order to improve the quality, it defines the ratio to the Lorentz force in the later acceleration stage to move toward the short side, removes inclusions on the solidified shell, prevents powder entrainment of the molten metal surface, and improves quality. I have.

In Japanese Patent Application Laid-Open No. 8-187557, molten steel is supplied in a casting direction using a cylindrical single-hole type nozzle having no bottom, and the flow velocity of the molten steel surface at the meniscus is increased by 10 to 30.
By controlling by setting upper and lower limits of cm / sec, in addition to suppressing solidification (skinning) at the meniscus, reducing inclusions and foamy defects, powder entrapment defects caused by excessive meniscus flow of molten steel The one which suppresses is proposed.

[0006] However, if the stirring intensity of the electromagnetic stirring is too high, powder seizure defects increase, and negative segregation occurs at the solidification interface during stirring (hereinafter referred to as white band).
Problem.

[0007] The white band interferes with the hardenability of the steel material and the mechanical properties of the surface layer portion. Therefore, when a member having severe mechanical strength specifications of the slab surface layer portion is manufactured by a continuous casting machine, It is necessary to suppress generation of a white band in the surface layer of the slab as much as possible.

For the purpose of suppressing the generation of such white bands, for example, the following continuous casting method using electromagnetic stirring has been proposed.

In Japanese Patent Publication No. 63-48619, the position of the electromagnetic stirrer is specified, and the product of the magnetic flux density and the stirring time at the interface between the unsolidified portion and the solidified portion of the slab is regulated. Those that suppress generation have been proposed.

Japanese Patent Publication No. 62-47103 discloses that the maximum equiaxed zone is formed with a constant stirring force by arranging an electromagnetic stirring position within a range in which the cross-sectional area solidification rate of a bloom slab satisfies a specified value. A device that can generate the white band and suppresses the generation of the white band has been proposed.

Japanese Patent Application Laid-Open No. 53-45627 discloses a method in which the electromagnetic stirring position, magnetic flux density, and casting speed are adjusted to refine the solidification structure, improve the internal properties of steel, and suppress the generation of white bands. Has been proposed.

[0012]

As described above, the white band generated by excessively increasing the stirring force impairs the hardenability of the steel material and the mechanical properties of the surface layer. Therefore, in the continuous casting method of steel using electromagnetic stirring, in principle, it is desirable to impart electromagnetic stirring to unsolidified molten steel to promote equiaxed crystallization of the solidified structure, but not to generate a white band. .

However, in the conventional continuous casting method proposed above, the electromagnetic stirring conditions must be changed according to the type of steel to be cast. Therefore, when casting various types of steel with one continuous casting machine, a wide variety of methods are required. In order to produce high quality slabs of steel types, it is necessary to apply appropriate electromagnetic stirring conditions to each steel type. Therefore, when many steel types are cast under the same electromagnetic stirring conditions as in the related art, there is a problem that a harmful defect occurs in a solidified structure depending on the steel type.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. In a single continuous casting machine, when casting a plurality of steel types having different harmful effects of segregation, the casting speed is high even if the casting speed is high. Provided is a steel continuous casting method capable of suppressing generation of a white band in one surface layer portion, securing an equiaxed crystal zone in a central portion of a slab, and performing continuous casting stably and efficiently. The purpose is to:

[0015]

Means for Solving the Problems The present inventors have conducted various experiments to achieve the above object. As a result, even if the electromagnetic stirring strength is the same, the carbon concentration in the molten steel (hereinafter referred to as the molten steel It was found that the higher the carbon content of the steel, the higher the white band was formed under the surface of the slab. And
The relationship between the carbon concentration in the molten steel, the flow rate of the molten steel, and the formation ratio of the white band was clarified by experiments, and it was also found that the formation of the white band when the electromagnetic stirring was applied in the mold was dependent on the solidification speed of the solidified shell.

The method for continuous casting of steel according to the present invention is based on the above-mentioned findings. An electromagnet coil for applying a moving magnetic field to molten steel is installed in a mold for continuous casting, and a 20 mm-thick steel sheet is formed in the mold. In the area where the following solidified shells form,
The average molten steel flow rate at the solidified shell interface is defined as U (cm / sec).
c), the average solidification speed of the solidified shell is V (cm / sec)
When a, have row a magnetic stir to molten steel flow velocity satisfies the following formula perpendicular to the casting direction, Hua of the slab surface portion
And the equiaxed zone in the center of the slab is confirmed.
To keep . 5.0 ≦ U ≦ (19.2 × V ^0.195 ) / carbon concentration in molten steel However, carbon concentration (%) in molten steel ≦ 1.0 (0.1 to 0.
18% is excluded), and V = 0.001~0.50cm / sec.

By doing so, the white band at the surface layer of the slab can be suppressed, and the equiaxed crystal zone at the center of the slab can be secured. Thus, a plurality of steel types having different harmful effects of segregation can be cast stably and efficiently.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is implemented, for example, as follows. FIG. 1A shows continuous casting of a round billet, and FIG.
(B) shows the situation when performing slab continuous casting, respectively. An electromagnet coil 3 for applying a moving magnetic field to the molten steel 2 is installed in these molds. In addition, molten steel 2
Are respectively poured into the mold through the immersion nozzle 4.

The conditions for applying a magnetic field to the electromagnet coil 3 are as follows:
The frequency is 1 to 10 Hz, the current value is 100 to 800 A, the moving magnetic field is 0 to 1000 Tesla (1 G = 100 μT), and the moving magnetic field is applied to the molten steel 2 using the electromagnetic coil 3 installed in the mold. At this time, a flow of the molten steel 2 in the horizontal direction along the solidified shell, that is, a swirling flow in the circumferential direction in the continuous casting of the round billet, and a swirling flow from the short side to the long side in the continuous casting of the slab are induced.

When the flow of the molten steel 2 is induced, the vicinity of the interface of the unsolidified molten steel in the solidified shell (hereinafter, referred to as the “solidified front”) is subjected to a washing effect, and the liquid phase concentration on the solidified front is reduced. The solute concentration on the solid layer side of the solidification front decreases in accordance with the equilibrium distribution coefficient of the molten steel 2. Also, if the molten steel flow velocity near the solidified shell is excessively increased, that is, if the stirring intensity of the molten steel 2 by the electromagnetic stirring is large, a white band (negative segregation zone) is generated in the solidified shell affected by the stirring. become.

[0021] Since the white band interferes with the hardenability of the steel material and the mechanical properties of the surface layer, it is required to reduce the stirring strength of the molten steel by electromagnetic stirring in order to suppress this.

On the other hand, by performing electromagnetic stirring,
As described above, it is well known that there are effects of improving slab quality, such as decreasing porosity at the center of the slab, increasing the number of equiaxed crystal zones, and removing inclusions and bubbles from the surface layer of the slab. In the experiments of the present inventors, the equiaxed crystal ratio at the center of the slab was 8.0%.
It has been found that securing the above is effective in suppressing the positive segregation in the center and reducing the porosity.

Therefore, when using electromagnetic stirring, it is necessary to accurately know the limit value of the stirring intensity at which a white band is not generated, and it is necessary to perform the electromagnetic stirring so as to induce the flow of molten steel at the solidified shell interface as much as the limit value or less. Guarantees the quality of the slab.

The present inventors cast various steel types having a carbon concentration in molten steel by using a round billet continuous casting machine, and investigated the segregation degree C _i / C ₀ of the surface layer of the slab during electromagnetic stirring. did. Incidentally, C _i is the molten steel in the carbon concentration of the average within the slab surface portion 20 mm (%), C ₀ is the molten steel in the carbon concentration in the mother material (%) means respectively.

FIG. 2 shows the average molten steel flow velocity U (cm / sec) near the solidified shell generated when electromagnetic stirring is performed on molten steel having different carbon concentrations in molten steel in the mold.
The relationship with the carbon concentration in molten steel is shown. Here, the average flow velocity is the average flow velocity of the circular billet continuous casting shown in FIG. 1A, and the average flow velocity swirled in the circumferential direction. In the case of the continuous casting of the slab shown in FIG. , Mean mean flow velocity swirled along the short side-long side.

The flow rate of molten steel in the mold was measured by a Karman vortex flow meter. An outline of this measurement principle is shown in FIG.
Shown in In FIG. 6, when the molten steel flow passes near the sialon rod 5, the Karman vortex 2a surrounds the sialon rod 5.
Is generated, and the Karman vortex 2 a vibrates the diaphragm 6 via the sialon rod 5. After the frequency analysis of the left-right vibration of the vibration plate 6, the measurement is performed by converting the vibration into a flow velocity.

As a result of our experiments, the segregation degree C _i / C
₀ found that as the carbon concentration in the molten steel increased, it decreased linearly regardless of the average molten steel flow velocity near the solidified shell as shown in FIG. FIG. 3 shows the segregation degree C _i / C
The relationship between ₀ and the average solidification rate V (cm / sec) of molten steel is shown. The average solidification speed V was measured from the interval between the secondary dendrite arms of the slab. As shown in FIG. 3, experiments show that the segregation degree C _i / C _{0 at the} surface layer of the slab increases exponentially with respect to the average solidification velocity V regardless of the average molten steel flow velocity near the unsolidified shell. found.

As described above, from FIGS. 2 and 3, the segregation degree C _i / C
As a result of obtaining the relationship between ₀ and the carbon concentration in the molten steel of the steel type, the average molten steel flow velocity U and the average solidification velocity V of the molten steel by multiple regression,
It has been found that the relationship of the following equation (1) is established. C _i / C ₀ = 1− [1.04 × 10 ⁻³ × {(carbon concentration in molten steel × U) / V ^0.195 }] (1) However, in this equation (1), the carbon concentration in molten steel is 1.0. %
Hereinafter, the average solidification speed V is 0.001 to 0.50 cm /
It is satisfied in the range of sec.

Normally, there is no steel type in which the carbon concentration in molten steel exceeds 1.0%, and the average solidification speed V is usually 0.00%.
It is in the range of 1 to 0.5 cm / sec.

Further, the present inventors have found from the above experiments that in order to secure the mechanical strength of the surface layer of the slab, that is, to suppress the white band, the relationship between the carbon concentration in the molten steel and the flow rate U of the molten steel was determined. , It was found that the segregation degree C _i / C ₀ should be 0.98 or more.

In addition, the equiaxed crystal ratio at the center of the slab is 8.0%.
In order to ensure the above, that is, to suppress the central positive segregation and reduce the porosity, as shown in FIG. 4, the relationship between the average molten steel flow velocity U and the equiaxed crystallinity at the center of the slab, The average molten steel flow rate U at 5.0 cm / sec
c or more, desirably an average flow velocity U of 10.0 cm / sec.
It was found that the above should be done.

Therefore, in the continuous casting method of steel using electromagnetic stirring, in order to suppress the white band on the surface layer of the slab and to secure the equiaxed crystal at the center of the slab, the average molten steel flow rate in the mold U Can be defined in the range of the following equation by modifying the above equation (1). 5.0 ≦ U ≦ (19.2 × V ^0.195 ) / carbon concentration in molten steel However, carbon concentration (%) in molten steel ≦ 1.0 V = 0.001 to 0.50 cm / sec.

As described above, by setting the average molten steel flow velocity U within the specified range of the above equation based on the carbon concentration in the molten steel and the average solidification velocity V, as shown in FIG. It can be suppressed.

[0034]

EXAMPLES Examples performed to confirm the effects of the steel casting method of the present invention will be described below. Tables 1, 2
Is obtained by continuously casting round billet slabs using a continuous casting mold having a circular cross section with a diameter of 225 mm at a casting speed of 2.0 m / min and an oscillation stroke of 4.0 mm. These are the conditions and results.

Table 3 shows that a continuous casting mold having a rectangular cross section with a long side length of 1000 mm and a short side length of 300 mm was used, the casting speed was 1.4 m / min, and the oscillation stroke was 5. It is each condition and result when a slab slab is continuously cast under the casting condition of 0 mm.

Both the round billet slab and the slab slab are ordinary steel, and the main components in the molten steel are C: 0.01 to 1.0.
%, Si: 0.01 to 3.0%, Mn: 0.01 to 2.0
%, P: 0.001 to 0.5%, S: 0.0001 to
0.05%. The solidification rate of the solidified shell was controlled by changing the temperature of the cooling water in the mold and the flow rate of the cooling water.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

As shown in Tables 1 to 3, A, B, D, E, G, H, J, L, N,
As for Q, R, S, V, and W, the segregation degree C _i / C ₀ in the surface layer of the slab is 0.98 or more, and therefore, an effect of suppressing a white band is observed, and the equiaxed crystal ratio is also 8. 0%
As a result, the slab quality was improved.

On the other hand, C, which is out of the specified range of the present invention,
For F, I, K, M, O, P, T, U, and X, the equiaxed crystal ratio was 8.0% or more, but the segregation degree C _i / C ₀ in the slab surface layer was 0.1%. Since it is smaller than 98, a white band was generated, and the shortage of mechanical strength of the surface layer portion of the slab was reduced.

In addition to the results of Tables 1 to 3, in addition to the results of a plurality of experiments, as an example, the carbon concentration in the molten steel and the average molten steel flow rate when the average solidification velocity V is 0.02 cm / sec. FIG. 5 shows the relationship with U. As is clear from the figure, the formation of a white band was reliably suppressed by continuous casting characterized by performing electromagnetic stirring suitable for the method of the present invention.

[0043]

As described above, according to the continuous casting method of the present invention, the average flow velocity U of the molten steel by the electromagnetic stirring is always optimally defined based on the carbon concentration in the molten steel and the average solidification velocity V. Therefore, even in the case of casting a large number of steel types having different harmful effects of segregation in one continuous casting machine, it is possible to surely suppress the generation of white bands on the surface layer of the slab, and at the center of the slab. The continuous casting can be stably and efficiently performed by securing the equiaxed crystal zone.

[Brief description of the drawings]

FIG. 1 shows a continuous casting state of steel using electromagnetic stirring.
(A) is a figure which shows the continuous casting situation of a round billet, (b) is a figure which shows the slab continuous casting situation, respectively.

FIG. 2 is a diagram showing the relationship between the carbon concentration in molten steel and the degree of segregation.

FIG. 3 is a diagram showing the relationship between the average solidification rate and the degree of segregation.

FIG. 4 is a diagram showing the relationship between the average molten steel flow velocity and the equiaxed crystal ratio at the center of a slab.

FIG. 5 is a graph showing the relationship between the carbon concentration in molten steel and the average molten steel flow velocity.

FIG. 6 is a view for explaining the principle of measuring the flow rate of molten steel.

[Explanation of symbols]

 2 Molten steel 3 Electromagnetic coil 4 Immersion nozzle

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-1-228645 (JP, A) JP-A-8-187557 (JP, A) JP-A-53-45627 (JP, A) 606 (JP, A) JP-A-11-33687 (JP, A) JP-A-9-47853 (JP, A) JP-A 8-309495 (JP, A) JP-A 8-174164 (JP, A) JP-A-63-60056 (JP, A) JP-A-61-135465 (JP, A) JP-A-58-100955 (JP, A) JP-A-57-142751 (JP, A) JP-A-60-54251 (JP, A) JP-A-2001-25848 (JP, A) JP-A-11-188464 (JP, A) JP-A-7-112252 (JP, A) JP-B-63-48619 (JP, B2) JP-B 62-47103 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/115

Claims (1)

(57) [Claims] When a molten steel is poured into a casting mold for continuous casting through an immersion nozzle to produce a cast slab, a method of continuous casting steel in which electromagnetic stirring is performed on unsolidified molten steel is performed. In a region where a solidified shell having a thickness of 20 mm or less is formed, the average molten steel flow rate at the solidified shell interface is U (cm / sec),
When the average solidification rate of the solidified shell was set to V (cm / sec), the molten steel flow velocity perpendicular to the casting direction have rows magnetic stirring so as to satisfy the following expression, white bars of the slab surface portion
A continuous casting method for steel, characterized in that the metallization is suppressed and an equiaxed zone at the center of the slab is secured . 5.0 ≦ U ≦ (19.2 × V ^0.195 ) / carbon concentration in molten steel However, carbon concentration (%) in molten steel ≦ 1.0 (0.1 to 0.
18% is excluded), and V = 0.001~0.50cm / sec.