KR20160117586A - Continuous steel casting method - Google Patents

Abstract

SUMMARY OF THE INVENTION It is an object of the present invention to provide a continuous casting method of a steel further suppressing pinhole defects. (N / m < 3 >) obtained by averaging the Lorentz force density component in the direction parallel to the long side of the mold 11 within the range in which the iron core 13a, which is a component of the electromagnetic stirrer 13, (N / m < 3 >) obtained by averaging the Lorentz force density component in the direction parallel to the short side of the mold 11 in the range in which the core cores 13a are present, (Hz) of the electromagnetic stirrer 13 and the effective Lorentz force density F (N / m 3) calculated by the maximum Lorentz power density F The continuous casting of the steel is carried out using the frequency of the continuous casting of the steel.

Description

{Continuous casting method}

The present invention relates to a method for continuously casting a steel by optimally operating an electromagnetic stirring device provided in a mold.

As a main cause of deteriorating the quality of the surface of the cast slab produced by continuous casting, pin hole defects can be mentioned. This pinhole-type defect is generated when Ar gas blown into the immersion nozzle enters the molten steel in the mold and is trapped in the solidified shell in order to suppress the occlusion of the immersion nozzle during continuous casting.

As a method of suppressing the pinhole-type defect, it is effective to provide the mold with the electromagnetic stirring device. Examples of operating factors of the electromagnetic stirring apparatus include a molten steel flow rate, an immersion nozzle, a molten steel throughput, a Lorentz force, and the like.

For example, the following techniques are disclosed in order to set these operating factors in an appropriate range.

For example, Patent Document 1 discloses a technique in which the flow rate of the electron stirring at the meniscus position is set to 10 to 60 cm / s in order to reduce the surface defect occurrence rate of the obtained cast steel.

In Patent Document 2, bubbles are formed in the solidifying shell by using parameters such as the distance between the immersion nozzle and the mold longitudinal side, the distance in the casting direction of the molten steel discharging hole of the immersion nozzle, the amount of molten steel throughput and the magnetic flux density at the solidifying interface A surface defect of the cast steel caused by adhesion is set to a predetermined value or less. Patent Document 2 describes that the distance between the immersion nozzle and the mold long side is controlled by changing the shape of the immersion nozzle or the shape of the mold.

Patent Document 3 discloses that the average value of the electromagnetic force in the direction parallel to the mold long side is 3,000 to 12,000 N / m < 3 > to avoid the curling of the mold powder into the molten steel by promoting the rising of the bubbles of Ar gas, Direction is -2,000 to 2,000 N / m 3, and the vertical value in the vertical downward direction is -1000 to 1000 N / m 3.

By applying the techniques disclosed in the above Patent Documents 1 to 3, pinhole-type defects are suppressed to some extent. However, a pinhole defect is not a problem. Since the surface quality of a steel sheet required by a user is becoming increasingly strict, a technique for suppressing pinhole-forming defects is required.

In the continuous casting of steel, the electromagnetic stirrer is the most effective device for suppressing pinhole defects. In the technologies disclosed in the above Patent Documents 1 to 3, an appropriate range of the molten steel flow rate generated by the electromagnetic force generated by the electromagnetic stirring device or the electromagnetic force is studied in detail.

Here, the electromagnetic stirring apparatus is a device for generating Lorentz force in molten steel in a mold to flow molten steel. This Lorentz force is generated only in molten steel having conductivity, and does not occur in a material having a very low conductivity (generally called an insulator) such as bubbles of Ar gas.

Therefore, the bubbles of the Ar gas move in the opposite direction to the molten steel in the mold. That is, as shown in Fig. 8, the electromagnetic force generated by the electromagnetic stirring apparatus also includes a negative component which collects bubbles of Ar gas on the surface of the cast slab to increase the pinhole-type defects.

The component of the electromagnetic force collecting the bubbles of Ar gas contained in the molten metal on the surface of the cast steel is called "electron repulsion" or "electron Archimedean force" and is described in detail in Non-Patent Document 1. 8 indicates bubbles of the mold wall surface, 2 is the solidifying shell, 3 is the solidifying interface, and 4 is the bubbles of Ar gas. Arrows directed from the lower side to the upper side of the sheet indicate Lorentz force, Represents the electron repulsion force. Non-patent document 2 discloses a heat fluid simulation in which Lorentz force density acting on molten steel in continuous casting is taken into consideration.

Japanese Unexamined Patent Publication No. 6-605 Japanese Patent Application Laid-Open No. 2007-216288 Japanese Patent Application Laid-Open No. 2010-240687

Iron and Steel, Vol.83 (1997), No.1, p.30 ~ 35 K. Takatani: ISIJ International, Vol.43, 2003, No. 6, p.915-922

The problem to be solved by the present invention is that, in the case of the prior art, in the electromagnetic stirring of the molten steel in the mold when the steel is continuously cast, attention is paid to the electromagnetic repulsive force generated by the electromagnetic stirring apparatus, There was no idea.

An object of the present invention is to further suppress the pinhole-type defects by determining the current frequency which is the best of the electromagnetic stirring apparatus so that the repulsive force generated when the molten steel in the mold is electronically stirred can be made as small as possible.

The present invention has been made based on the examination results of the inventors described later. In continuous casting of steel using an electromagnetic stirring device provided in a mold,

The value obtained by averaging the Lorentz force density component in the direction parallel to the mold long side in the range in which the iron core serving as a component of the electromagnetic stirring apparatus exists is defined as Lx (N / m < 3 &

When the value obtained by averaging the Lorentz force density component in the direction parallel to the short side of the mold in the range in which the core core exists is Ly (N / m 3)

The relationship between the effective Lorentz force density F (N / m 3) calculated by the following formula and the current frequency (Hz) of the electromagnetic stirring apparatus is obtained,

And the frequency of the electromagnetic stirring current in the range of the maximum value Fmax to 0.9 Fmax of the effective Lorentz force density F is used.

F = Lx -? Ly

In the above equation,? Is a coefficient (= 3 to 7) indicating the degree of bad influence of the electron repulsion force.

In the present invention, since the optimum current frequency of the electromagnetic stirring apparatus is determined so that the repulsive force generated when the in-mold molten steel is electronically stirred can be minimized as much as possible, the accumulation of Ar gas bubbles in the surface layer can do.

According to the present invention, it is possible to suppress the gathering of Ar gas bubbles in the surface layer as much as possible, so that the pinhole defects can be further suppressed as compared with the continuous casting method of steel using the conventional technique.

Fig. 1 is a view for explaining a casting mold and an electromagnetic stirring device used in the continuous casting method of the present invention, and is a view of the casting mold viewed from above. Fig.
2 is a view showing the distribution of the Lorentz force density at the central position in the drawing direction of the core of the core obtained by the numerical simulation.
3 is a graph showing the relationship between the value Lx obtained by averaging the Lorentz force density component in the direction parallel to the mold longitudinal side in the range in which the core core of the electromagnetic stirrer exists and the current frequency.
4 is a graph showing the relationship between the value Ly obtained by averaging the Lorentz force density component in the direction parallel to the short side of the mold in the range in which the iron core of the electromagnetic stirring apparatus exists and the current frequency.
Fig. 5 is a diagram showing the relationship between Ly / Lx and the current frequency. Fig.
Fig. 6 is a diagram showing the result of examining a change in the number of pinholes (unit / m 2) per unit area at the solidification interface due to the current frequency by numerical analysis.
7 is a graph showing the frequency dependency of the effective Lorentz force density F when the coefficient alpha representing the degree of bad influence of the electromagnetic repulsive force is 5.
8 is a view for explaining the electronic repulsive force.

The present invention realizes the object of further suppressing pinhole-type defects by determining the current frequency which is the best of the electromagnetic stirring apparatus so that the repulsive force generated when the molten steel in the mold is electronically stirred can be made as small as possible.

The inventors of the present invention have studied in detail the electron repulsive force generated in the mold when operating the continuous casting machine provided with the electromagnetic stirrer in the mold. As a result, it has been found that pinhole defects can be reduced by suppressing the electron repulsion.

Further, as a result of further examination by the inventors of the electromagnetic force applying method for preventing the electron repulsion force from approaching the Ar gas bubble in the vicinity of the solidification interface, it has been found that there is an appropriate current frequency when applying the electromagnetic force.

The mold and the electromagnetic stirring apparatus used in the above-described examination are the same as those described in Patent Document 3 of the general shape and polarity shown in Fig. 1 when the mold is viewed from above. 1, reference numeral 11 denotes a copper mold (hereinafter simply referred to as a mold), 12 denotes an immersion nozzle, 13 denotes an electromagnetic stirring device, 13a denotes an iron core constituting the electromagnetic stirring device 13, 13aa denotes an iron core The formed tooth portion 13b is wound around the outer periphery of the core 13a.

Fig. 2 shows the distribution of the Lorentz force density at the central position in the pulling direction of the core of the core obtained by the numerical analysis simulation. Here, the Lorentz force density means the electromagnetic force per unit steel volume (N / m 3).

The distribution of the Lorentz force density shown in Fig. 2 is a size of a cast steel having a width of 1200 mm and a thickness of 250 mm. The thickness of the copper plate forming the mold is 25 mm and the conductivity of the mold is 1.9 x 10 ⁷ S / .

The Lorentz force density distribution shown in Fig. 2 has a distribution in which the molten steel in the mold is agitated counterclockwise, and a large Lorentz force is generated in the vicinity of the wall surface of the mold 11 along the long side direction of the mold 11 .

As apparent from Fig. 2, the Lorentz force along the wall surface of the mold has many components directed toward the inside of the mold. The Lorentz force toward the inside of the mold serves as an electron repulsive force toward the wall surface of the mold with respect to the bubbles of Ar gas. That is, the bubbles of the Ar gas are transported to the vicinity of the interface of the solidified shell by the electromagnetic repulsive force, and the pinhole defects are increased.

The distribution of the Lorentz force density does not change even when the EMS (Electro-Magnetic Stirrer) current value is increased. That is, when the flow rate is increased by increasing the current value of the electron stirring apparatus, the effect of suppressing the pinhole defects can be obtained by the cleaning effect of the pinholes trapped at the interface of the solidifying shell. On the other hand, As the bubble increases, the pinhole defect increases.

As a result of the inventors' review, it was very effective to change the current frequency of the electromagnetic stirring device in order to reduce the component toward the inside of the mold of the Lorentz force as described below.

3 is a graph showing the relationship between a value Lx (N / m 3) obtained by averaging the components of the Lorentz force density in the direction parallel to the mold longitudinal side in the range in which the iron core of the electromagnetic stirring apparatus exists and the current frequency (Hz) to be. The above-mentioned value Lx in the direction parallel to the mold long side was calculated by using the Lorentz force in the same direction as the turning direction of the molten steel by electron stirring and the Lorentz force in the opposite direction as a part.

Specifically, in FIG. 2, in the region above the short side center of the mold, the Lorentz force density in the leftward direction of the sheet is positive and the Lorentz force density in the right direction in the sheet is negative. In the region below the short side of the mold, , The Lorentz force density in the right-hand direction on the paper surface was calculated using the Lorentz force density on the left and right sides of the paper.

3, the maximum value of the value Lx in the direction parallel to the long side of the mold is in the range of 2.3 to 2.5 Hz, and in order to maximize the stirring flow rate, the current frequency of 2.3 to 2.5 Hz It should be selected.

4 is a graph showing the relationship between the value Ly (N / m 3) obtained by averaging the components of the Lorentz force density in the direction parallel to the short side of the mold in the range in which the core is present and the current frequency (Hz). The value Ly in a direction parallel to the short side of the mold was calculated by setting the Lorentz force density toward the inside of the mold to be positive and calculating the Lorentz force density toward the outside of the mold.

Specifically, in FIG. 2, in the region above the short side center of the mold, the downward Lorentz force density away from the wall surface on the long side of the mold is defined as positive, and in the region below the short side of the mold, And the upward Lorentz force density away from the wall surface of the long side of the first wall surface.

That is, the value Ly in the direction parallel to the short side of the mold is a component of the Lorentz force density in which the molten steel in the mold faces the center of the short side from the wall surface on the long side of the mold and the electron repulsion force of the bubbles of Ar gas toward the wall surface of the mold . It is clear from Fig. 4 that the value Ly in the direction parallel to the short side of the mold increases as the current frequency of the electromagnetic stirring apparatus becomes higher.

Fig. 5 shows the ratio Ly / Lx of the value Ly in the direction parallel to the short side of the mold to the value Lx in the direction parallel to the long side of the mold. It can be seen from Fig. 5 that the smaller the value of Ly / Lx, the smaller the electron repulsive force component of the Lorentz force density generated in molten steel in the mold.

It can be seen from Figs. 4 and 5 that it is effective to lower the current frequency in order to reduce the electron repulsive force. It is also understood from Fig. 3 that the above value Lx in the direction parallel to the mold longitudinal side needs to be set to some extent or more in order to ensure the stirring flow rate by the electromagnetic stirring. As a result of examining the fluid analysis simulation described below, it was confirmed that the Lorentz force was insufficient when the current frequency was 0.4 Hz or less.

From the above, there will be the most suitable current frequency from the current frequency at which the value Lx in the direction parallel to the mold long side becomes the maximum to the current frequency at which the electromagnetic stirring becomes inadequate, And a simulation was carried out.

The electromagnetic field simulation was performed by calculating the distribution of the Lorentz force density generated in the molten steel by the electromagnetic stirring apparatus in the manner as described above. Fluid simulations were performed using the obtained Lorentz force density, and the number of Ar gas bubbles trapped in the solidified shell was evaluated. The thermal fluid simulation was performed by the method described in Non-Patent Document 2, and the calculation of the molten steel flow, the heat transfer, the solidification, and the Ar gas bubbles were carried out.

Information of the flow velocity, solidification rate, distribution of Ar gas bubbles, etc. in the molten steel of the continuous casting machine can be obtained by the simulation of the fluid fluid according to the method described in the non-patent document 2. [ Therefore, how to evaluate the Ar gas bubbles trapped in the solidification shell becomes a problem.

As described in Patent Document 1, it is known that when there is a molten steel flow rate of 10 to 60 cm / s in the solidification interface, the Ar gas bubbles are not trapped in the solidification shell. That is, when the molten steel flow rate at the solidifying interface is equal to or lower than the flow velocity at which the Ar gas bubbles are captured (hereinafter, referred to as the trapped flow velocity), calculation may be performed such that the Ar gas bubbles existing at the above positions are captured.

Though the threshold value of the trapped flow rate is generally 20 cm / s, the exact value is unclear. In addition, it is considered unnatural to calculate that Ar gas bubbles are trapped in the solidified shell at 20.1 cm / s without the molten steel flow rate being 19.9 cm / s.

Thus, the inventor devised a method of evaluating the probability that the Ar gas bubble is trapped in the solidification shell as a continuous function expressed by the following formula (1). Where P _g (-) is the probability of the Ar gas bubble being trapped in the solidification shell, C ₀ is a constant, and U (m / s) is the melt velocity at the solidification interface.

When the constant C ₀ in the following formula (1) is 100, the trap probability P _g for a molten steel flow rate of 20 cm / s is 10 ^-8 or less. This is the probability that one out of a million Ar gas bubbles will be trapped in a solidified shell, and is a value considered zero in the numerical simulation. The value of C ₀ used in the numerical simulation is preferably 10 to 1000.

[Number 1]

The rate η _g ( _g / m 3 · s) at which the Ar gas bubble is trapped in the solidification shell depends on the number density n _g (number / m 3) of the Ar gas bubbles at the solidification interface, the solidification speed R _s (2) using the trap probability P _g (-).

[Number 2]

The number density S _g (number / m 3) of Ar gas bubbles in the solidification shell is calculated from the following equation (3). Here, U _s is the moving speed (m / s) of the solidifying shell in the slab withdrawing direction.

[Number 3]

The number density S _g (number / m 3) of Ar gas bubbles in the solidification shell obtained from the above formula (3) was time-averaged to evaluate the number of Ar gas bubbles. At this time, it is considered that the trapped flow rate changes naturally due to the bubble diameter of the Ar gas, but the relationship is unclear. Thus, the diameter of the main Ar gas bubbles existing in the mold of the continuous casting machine was set at 1 mm. A range of 2 mm from the surface of the cast steel was evaluated as a range in which the Ar gas bubble having a diameter of 1 mm affected the surface of the cast steel.

Fig. 6 shows the results of examining the relationship between the current frequency and the number of pinholes per unit area (unit / m 2) at the solidification interface by numerical analysis.

6, the number of pinholes is reduced when the current frequency is 1.2 Hz and the number of pinholes is significantly increased when the current frequency is 0.8 Hz or less, when the current frequency is 2.3 Hz, which is the maximum Lorentz force density, It became clear.

The reason why the number of pinholes per unit area at the solidification interface is 43 (number / m 2) when the current frequency is 1.2 Hz is that the Lorentz force density for electron stirring is lowered but the electron repulsion force is lowered, The effect of decreasing the Ar gas bubble is large. However, when the current frequency is lower than 1.2 Hz, pinholes increase because the Lorentz force density for stirring the molten steel in the mold is insufficient.

In general, the current frequency at which the Lorentz force density becomes maximum is selected as the current frequency of the electromagnetic stirring apparatus. In the electromagnetic stirring apparatus shown in Fig. 1, the current frequency at which the Lorentz force density becomes maximum is read Lt; / RTI > The number of pinholes in the case of the current frequency of 2.3 Hz selected by the conventional technique is 57 (number / m 2) as shown in FIG. Therefore, it is understood that the current frequency is in the range of 0.9 Hz to 2.3 Hz, as shown in Fig. 6, in which pinhole defects can be suppressed as compared with the prior art.

Therefore, the inventors have found that when the casting size is 1200 mm wide × 250 mm thick and the thickness of the copper mold is 25 mm and the conductivity of the copper mold is 1.9 × 10 ⁷ S / m, The appropriate frequency range that can be obtained is from 0.9 to 2.3 Hz.

The fluid analysis for evaluating such a pinhole requires a comparatively long time as compared with the electromagnetic field analysis. Thus, the inventors have studied a method of selecting an optimum frequency from the results of electromagnetic field analysis.

With regard to the number of pinholes, Lorentz force Lx (N / m 3) required for electron stirring acts as a positive factor and electron repulsion Ly (N / m 3) acts as a negative factor. Therefore, the effective Lorentz force density F (N / m 3) is defined as shown in the following equation (4). Here,? Is a coefficient indicating the degree of bad influence of the electron repulsion force.

[Number 4]

Since? Is a coefficient indicating the degree of bad influence in the direction parallel to the short side of the mold, the influence thereof varies depending on the length of the short side of the mold. As a general continuous casting machine, the present inventors have studied about an expression that the evaluation according to the above formula (4) is equivalent to the evaluation shown in Fig. 6 with respect to the short side length of the mold of 200 mm to 300 mm. As a result, it has been found that it is appropriate to set α to the range of 3 to 7. When? Is less than 3, the Lorentz force parallel to the short side of the mold is underestimated. When? Exceeds 7, the Lorentz force parallel to the short side of the mold is overestimated.

7 is a graph showing the frequency dependency of the effective Lorentz force density F (N / m 3) when the coefficient a indicating the degree of bad influence of the electromagnetic repulsive force is 5. From FIG. 7, it can be seen that the effective Lorentz force density F (N / m 3) becomes the maximum value when the current frequency is 1.2 Hz.

3 and 6 show that the current frequency is in the range of 0.9 Hz to 2.3 Hz and the effective Lorentz force density F is in the range of the maximum value Fmax to 0.9 Fmax (The current frequency is 0.9 to 2.0 Hz). As described above, by using the above equation (4), it is possible to determine the frequency of the electromagnetic stirring apparatus which is the best from the results of electromagnetic field analysis only.

The present invention is based on the above-described examination result by the inventor,

In the continuous casting of steel using the electromagnetic stirrer provided in the mold,

(N / m < 3 >) obtained by averaging Lorentz force density components in a direction parallel to the mold long side in a range in which an iron core serving as a component of the electromagnetic stirrer is present,

When the value obtained by averaging the Lorentz force density component in the direction parallel to the short side of the mold in the range in which the core core is present is Ly (N / m 3)

The relationship between the effective Lorentz force density F (N / m 3) calculated by the above formula (4) and the current frequency (Hz) of the electromagnetic stirring device is obtained,

And the frequency of the electromagnetic stirring current in the range of the maximum value Fmax to 0.9 Fmax of the effective Lorentz force density F is used.

According to the present invention, it is possible to determine the best current frequency of the electromagnetic stirring apparatus capable of minimizing the electromagnetic repulsive force generated when electromagnetic in-mold molten steel is stirred, from the result of electromagnetic field analysis only. Therefore, it is possible to suppress the gathering of bubbles of Ar gas in the surface layer of the cast as much as possible, and the pinhole defects can be further suppressed.

Needless to say, the present invention is not limited to the above-described example, and it goes without saying that the embodiments may be appropriately changed as long as they are within the scope of the technical idea described in the claims.

The inventor has performed the fluid simulation by the method described in the non-patent document 2, but it goes without saying that the method of performing the heat fluid simulation is not limited to the method described in the non-patent document 2. [

11: mold 13: electromagnetic stirrer
13a: core core

Claims (1)

In the continuous casting of steel using the electromagnetic stirrer provided in the mold,
(N / m < 3 >) obtained by averaging Lorentz force density components in a direction parallel to the mold long side in a range in which an iron core serving as a component of the electromagnetic stirrer is present,
When the value obtained by averaging the Lorentz force density component in the direction parallel to the short side of the mold in the range in which the core core is present is Ly (N / m 3)
The relationship between the effective Lorentz force density F (N / m 3) calculated by the following formula and the current frequency (Hz) of the electromagnetic stirring apparatus is obtained,
Wherein the frequency of the electromagnetic stirring current in the range of the maximum value Fmax to 0.9 Fmax of the effective Lorentz force density F is used.
F = Lx -? Ly
Where a: coefficient indicating the degree of bad influence of the electron repulsion (= 3 to 7).

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