WO2015129382A1 - Continuous steel casting method - Google Patents

Abstract

The purpose of the present invention is to provide a continuous steel casting method that further curbs pinhole defects. The present invention is a continuous steel casting method in which, when the value resulting from averaging the Lorentz force density component in a direction that is parallel to a long side of a mold (11) in the range in which an iron core (13a) that is a constituent element of an electromagnetic stirring device (13) is present is denoted by Lx (N/m³) and the value resulting from averaging the Lorentz force density component in a direction that is parallel to a short side of the mold (11) in the range in which the iron core (13a) is present is denoted by Ly (N/m³), the relationship between an effective Lorentz force density (F) (N/m³) that is calculated using [F = Lx - α ∙ Ly] and the current frequency (Hz) of the electromagnetic stirring device (13) is determined, and continuous casting of steel is performed using an electromagnetic stirring current frequency that is in the range of 0.9Fmax from the maximum value (Fmax) of the actual Lorentz force density (F).

Description

Steel continuous casting method

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

As a main cause of deteriorating the quality of the slab surface layer produced by continuous casting, there is a pinhole defect. This pinhole defect is generated when Ar gas blown into the immersion nozzle enters the molten steel in the mold and is captured by the solidified shell in order to suppress the clogging of the immersion nozzle during continuous casting.

As a method for suppressing the pinhole defect, it is effective to install an electromagnetic stirring device in the mold. The operating factors of this electromagnetic stirring device include molten steel flow velocity, immersion nozzle, molten steel throughput, Lorentz force, and the like.

For example, the following technologies are disclosed as those that make these operating factors within an appropriate range.

For example, Patent Document 1 discloses a technique for setting the electromagnetic stirring flow rate at the meniscus position to 10 to 60 cm / s in order to reduce the surface defect occurrence rate of the obtained slab.

Further, in Patent Document 2, bubbles are formed in the solidified shell using parameters such as the distance between the immersion nozzle and the long side of the mold, the distance in the casting direction of the molten steel discharge hole of the immersion nozzle, the amount of molten steel throughput, and the magnetic flux density at the solidification interface. A technique is disclosed in which the surface defect of a slab resulting from the adhesion of slabs 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 electromagnetic force in the direction parallel to the mold long side is 3000 to 12000 N / in order to promote the floating of Ar gas bubbles and avoid the entrainment of mold powder in the molten steel. m ^3, a local value in a direction parallel to the mold short side -2000 ~ 2000N / ^{m 3,} the local value of the vertically downward direction exerts an electromagnetic force so that the -1000 ~ 1000N / ^{m 3} discloses a technique .

By applying the techniques disclosed in Patent Documents 1 to 3, pinhole defects are suppressed to some extent. However, there are no pinhole defects. As the surface quality of steel sheets required by users is becoming more and more strict, a technology for further suppressing pinhole defects is required.

In continuous casting of steel, the electromagnetic stirrer is the most effective device for suppressing pinhole defects. Even in the techniques disclosed in Patent Documents 1 to 3, the electromagnetic force generated by the electromagnetic stirring device and the appropriate range of the molten steel flow velocity generated by the electromagnetic force are studied in detail.

Here, the electromagnetic stirring device is a device that generates Lorentz force in the molten steel in the mold and causes the molten steel to flow. The Lorentz force is generated only in molten steel having electrical conductivity, and is not generated in an extremely low conductivity such as Ar gas bubbles (generally called an insulator).

Therefore, the Ar gas bubbles move in the opposite direction relative to the molten steel in the mold. In other words, as shown in FIG. 8, the electromagnetic force generated by the electromagnetic stirrer includes a negative component that causes Ar gas bubbles to gather on the surface of the slab and increases pinhole defects.

The component of the electromagnetic force that collects Ar gas bubbles contained in the molten metal on the surface of the slab is called “electromagnetic repulsive force” or “electromagnetic Archimedes force” and is described in detail in Non-Patent Document 1. In FIG. 8, 1 is a mold wall surface, 2 is a solidified shell, 3 is a solidified interface, 4 is a bubble of Ar gas, and an arrow heading from the lower side to the upper side of the drawing indicates the Lorentz force and the lower side from the upper side of the drawing. An arrow pointing to the side indicates electromagnetic repulsion. Non-Patent Document 2 discloses a thermal fluid simulation in consideration of Lorentz force density acting on molten steel in continuous casting.

JP-A-6-605 JP 2007-216288 A JP 2010-240687 A

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 electromagnetic stirring of molten steel in a mold during continuous casting of steel, in the case of the prior art, focusing on the electromagnetic repulsion generated by the electromagnetic stirring device, a suitable electromagnetic stirring condition is determined. There was no idea to do.

The present invention further suppresses the pinhole defect by determining the best current frequency of the electromagnetic stirring device so that the electromagnetic repulsive force generated when electromagnetically stirring the molten steel in the mold can be reduced as much as possible. It is aimed.

The present invention has been made on the basis of the results of the study of the inventor described below,
In continuous casting of steel using an electromagnetic stirrer installed in the mold,
Lx (N / m ³ ) is a value obtained by averaging the Lorentz force density component in the direction parallel to the mold long side in a range where the iron core that is a component of the electromagnetic stirrer exists.
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 where the iron 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 stirrer is obtained,
The most important feature is that the frequency of the electromagnetic stirring current in the range of the maximum value Fmax to 0.9Fmax of the effective Lorentz force density F is used.
F = Lx-α · Ly
In the above formula, α is a coefficient (= 3 to 7) indicating the adverse effect of electromagnetic repulsion.

In the present invention, since the optimum current frequency of the electromagnetic stirrer is determined so that the electromagnetic repulsive force generated when electromagnetically stirring the molten steel in the mold can be reduced as much as possible, Ar gas bubbles are brought close to the slab surface layer. Collecting can be suppressed as much as possible.

According to the present invention, since it is possible to suppress Ar gas bubbles from being gathered together on the surface of the slab as much as possible, pinhole defects can be further suppressed as compared to the continuous casting method of steel using the prior art. it can.

It is a figure explaining the casting_mold | template and electromagnetic stirring apparatus which are used for the continuous casting method of steel of this invention, and is the figure which looked at the casting_mold | template from the upper direction. It is the figure which showed distribution of the Lorentz force density in the cast piece drawing direction center position of the iron core core obtained by numerical analysis simulation. It is the figure which showed the relationship between the value Lx and the current frequency which averaged the Lorentz force density component in the direction parallel to the mold long side in the range where the iron core of the electromagnetic stirrer exists. It is the figure which showed the relationship between the value Ly and the current frequency which averaged the Lorentz force density component in the direction parallel to the mold short side in the range where the iron core of the electromagnetic stirrer exists. It is the figure which showed the relationship between Ly / Lx and a current frequency. It is the figure which showed the result of having investigated the change of the number of pinholes per unit area (pieces / m < ² >) in the solidification interface by a numerical frequency by numerical analysis. It is the figure which showed the frequency dependence of the effective Lorentz force density F when the coefficient (alpha) which shows the bad influence degree of an electromagnetic repulsion is set to 5. It is a figure explaining electromagnetic repulsion.

The object of the present invention is to further suppress pinhole defects by determining the best current frequency of the electromagnetic stirring device so that the electromagnetic repulsion generated when electromagnetically stirring the molten steel in the mold can be made as small as possible. Realized.

The inventors have studied in detail the electromagnetic repulsive force generated in the mold when operating a continuous casting machine with an electromagnetic stirrer installed in the mold. As a result, it is possible to reduce pinhole defects by suppressing the electromagnetic repulsive force. I found out.

As a result of further investigation by the inventors on the electromagnetic force application method that suppresses the electromagnetic repulsive force so that the Ar gas bubbles do not approach the vicinity of the solidification interface, there is an appropriate current frequency when applying the electromagnetic force. It became clear to do.

The mold and electromagnetic stirring device used in the above examination are the same as those described in Patent Document 3 with a general shape and polarity as shown in FIG. 1 when the mold is viewed from above. In FIG. 1, 11 is a copper mold (hereinafter also simply referred to as a mold), 12 is an immersion nozzle, 13 is an electromagnetic stirrer, 13a is an iron core core constituting the electromagnetic stirrer 13, and 13aa is formed on the iron core 13. The teeth portion 13b is a winding wound around the outer periphery of the iron core 13a.

FIG. 2 shows the Lorentz force density distribution at the center position in the slab drawing direction of the iron core obtained by numerical analysis simulation. Here, the Lorentz force density means an electromagnetic force (N / m ³ ) per unit molten steel volume.

The Lorentz force density distribution shown in FIG. 2 is a slab size of width 1200 mm × thickness 250 mm, the thickness of the copper plate forming the mold is 25 mm, and the conductivity of the mold is 1.9 × 10 ⁷ S / m. It is the result of having performed an analysis simulation.

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

As is clear from FIG. 2, the Lorentz force along the wall surface of the mold has many components facing the inside of the mold. Such a Lorentz force directed toward the inside of the mold acts as an electromagnetic repulsive force directed toward the wall surface of the mold against Ar gas bubbles. That is, Ar gas bubbles are transported to the vicinity of the solidified shell interface by electromagnetic repulsion, and pinhole defects increase.

The Lorentz force density distribution 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 electromagnetic stirrer, the effect of suppressing the pinhole defect is obtained by the cleaning effect of the pinhole trapped at the solidified shell interface, while the solidified shell is obtained by the electromagnetic repulsion. As the number of Ar gas bubbles toward the interface increases, pinhole defects increase.

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

FIG. 3 shows a value Lx (N / m ³ ) obtained by averaging the Lorentz force density component in the direction parallel to the long side of the mold in the range where the iron core of the electromagnetic stirrer exists, and the current frequency (Hz). FIG. The value Lx in the direction parallel to the mold long side was calculated with the Lorentz force in the same direction as the turning direction of the molten steel by electromagnetic stirring as positive and the Lorentz force in the opposite direction as negative.

Specifically, in FIG. 2, in the region above the center of the short side of the mold, the Lorentz force density in the left direction of the page is positive, the Lorentz force density in the right direction of the page is negative, and the center of the short side of the mold is In the area below the paper surface, the Lorentz force density in the right direction on the paper surface was calculated as positive and the Lorentz force density in the left direction on the paper surface was calculated as negative.

From FIG. 3, the maximum value of the value Lx in the direction parallel to the long side of the mold is in the range of the current frequency of 2.3 to 2.5 Hz. A current frequency of 3 to 2.5 Hz should be selected.

FIG. 4 shows the relationship between the value Ly (N / m ³ ) obtained by averaging the Lorentz force density component in the direction parallel to the short side of the mold in the range where the iron core exists and the current frequency (Hz). FIG. The value Ly in the direction parallel to the mold short side was calculated with the Lorentz force density facing the inside of the mold as positive and the Lorentz force density facing the outside of the mold as negative.

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

That is, the value Ly in the direction parallel to the mold short side is a component of the Lorentz force density in which the molten steel in the mold is directed from the wall surface on the long side of the mold toward the center of the short side, Represents the electromagnetic repulsion toward the wall. FIG. 4 reveals that the value Ly in the direction parallel to the short side of the mold increases as the current frequency of the electromagnetic stirrer increases.

FIG. 5 shows a ratio Ly / Lx of the value Ly in the direction parallel to the mold short side to the value Lx in the direction parallel to the mold long side. From FIG. 5, it can be seen that the smaller the value of Ly / Lx, the smaller the electromagnetic repulsion component of the Lorentz force density generated in the molten steel in the mold.

4 and 5 show that reducing the current frequency is effective for reducing the electromagnetic repulsion. Further, FIG. 3 shows that the value Lx in the direction parallel to the long side of the mold needs to be set to a certain level or more in order to secure the stirring flow rate by electromagnetic stirring. As a result of examining a fluid analysis simulation described later, it was confirmed that the Lorentz force was insufficient when the current frequency was 0.4 Hz or less.

From the above, there should be the most appropriate current frequency from the current frequency at which the value Lx in the direction parallel to the mold long side is maximum to the current frequency at which electromagnetic stirring is inappropriate. The current frequency was investigated from numerical simulation of electromagnetic field and fluid.

The electromagnetic field simulation was performed by calculating the Lorentz force density distribution generated in the molten steel by the electromagnetic stirrer by the method as described above. A fluid simulation was 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 calculation of molten steel flow, heat transfer, solidification, and Ar gas bubbles was performed.

Information such as the flow velocity, solidification rate, Ar gas bubble distribution, etc. in the molten steel of the continuous casting machine can be obtained by the thermal fluid simulation by the method described in Non-Patent Document 2. Therefore, how to evaluate the Ar gas bubbles trapped in the solidified shell becomes a problem.

As described in Patent Document 1, it is known that if there is a molten steel flow velocity of 10 to 60 cm / s at the solidification interface, Ar gas bubbles are not trapped by the solidification shell. That is, when the molten steel flow velocity at the solidification interface is equal to or lower than the flow velocity at which Ar gas bubbles are trapped (hereinafter referred to as trap flow velocity), calculation is performed that Ar gas bubbles present at the position are trapped. Good.

The above-mentioned capture flow rate threshold is generally said to be 20 cm / s, but the exact value is unknown. In addition, it is considered unnatural to calculate that the molten steel flow rate is not captured at 19.9 cm / s and Ar gas bubbles are captured by the solidified shell at 20.1 cm / s.

Therefore, the inventor has devised a method for evaluating the probability that Ar gas bubbles are trapped in the solidified shell as a continuous function as shown in the following formula (1). Here, P _g (−) is the probability that Ar gas bubbles are trapped in the solidified shell, C ₀ is a constant, and U (m / s) is the molten steel flow velocity at the solidification interface.

When the constant C ₀ in the following formula (1) is set to 100, the capture probability P _g when the molten steel flow rate is 20 cm / s is 10 ⁻⁸ or less. This is the probability that one of the 1 million Ar gas bubbles is trapped by the solidified shell, and is a value that is regarded as zero in the numerical analysis simulation. It should be noted that the value of C ₀ used in the numerical analysis simulation is suitably 10 to 1000.

The rate η _g (number / m ³ · s) at which Ar gas bubbles are trapped in the solidified shell is determined by the number density n _g (number / m ³ ) of Ar gas bubbles at the solidification interface and the solidification rate R _s (1 / s). And the capture probability P _g (−), it is expressed as the following formula (2).

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

The number density S _g (number / m ³ ) of Ar gas bubbles in the solidified shell obtained from the above formula (3) was averaged over time to evaluate the number of Ar gas bubbles. At that time, it is considered that the trapping flow rate naturally changes depending on the bubble diameter of Ar gas, but the relationship is unknown. Therefore, the diameter of the main Ar gas bubbles existing in the mold of the continuous casting machine was set to 1 mm for investigation. Further, the range of 2 mm from the slab surface layer was evaluated as a range in which Ar gas bubbles having a diameter of 1 mm affect the slab surface.

FIG. 6 shows the result of examining the relationship between the current frequency and the number of pinholes per unit area (pieces / m ² ) by numerical analysis.

From FIG. 6, the number of pinholes decreases when the current frequency is 1.2 Hz, and the pinhole when the current frequency is 0.8 Hz or less, compared to when the current frequency is 2.3 Hz, where the Lorentz force density is maximum. It became clear that the number increased greatly.

When the current frequency is 1.2 Hz, the number of pinholes per unit area at the solidification interface is 43 (pieces / m ² ), which is the minimum because the Lorentz force density for electromagnetic stirring decreases. This is because the effect of reducing the Ar gas bubbles near the mold wall surface by reducing the repulsive force is great. However, when the current frequency is lowered below 1.2 Hz, the Lorentz force density for stirring the molten steel in the mold becomes insufficient, and therefore pinholes increase.

In general, the current frequency at which the Lorentz force density is maximized is selected as the current frequency of the electromagnetic agitator. In the electromagnetic agitator shown in FIG. It is 2.3 Hz that can be read. As shown in FIG. 6, the number of pinholes in the case of a current frequency of 2.3 Hz selected by the prior art is 57 (pieces / m ² ). Therefore, it can be seen that the pinhole defect can be suppressed more than the conventional technique, as shown in FIG. 6, in the current frequency range of 0.9 Hz to 2.3 Hz.

Therefore, the inventor has determined the number of pinholes in the conventional case when the slab size is 1200 mm wide × 250 mm thick, the thickness of the copper mold is 25 mm, and the conductivity of the copper mold is 1.9 × 10 ⁷ S / m. As a result, it was found that an appropriate frequency range that can be further suppressed is 0.9 to 2.3 Hz.

The fluid analysis for evaluating such pinholes requires a relatively long time compared to the electromagnetic field analysis. Therefore, the inventor examined a method for selecting an optimum frequency from the result of electromagnetic field analysis.

For the number of pinholes, the Lorentz force Lx (N / m ³ ) required for electromagnetic stirring acts as a positive factor, and the electromagnetic repulsive force Ly (N / m ³ ) acts as a negative factor. Therefore, the effective Lorentz force density F (N / m ³ ) is defined as shown by the following formula (4). Here, α is a coefficient indicating the adverse effect of electromagnetic repulsion.

Since α is a coefficient indicating the degree of adverse effect in the direction parallel to the mold short side, the degree of influence varies depending on the length of the mold short side. As a general continuous casting machine, the inventor examined α in which the evaluation by the above formula (4) is equivalent to the evaluation shown in FIG. 6 with respect to the mold short side length of 200 mm to 300 mm. As a result, it was found that it is appropriate to set α in the range of 3-7. When α is less than 3, the Lorentz force parallel to the mold short side is underestimated. When α exceeds 7, the Lorentz force parallel to the mold short side is overestimated.

FIG. 7 is a graph showing the frequency dependence of the effective Lorentz force density F (N / m ³ ) when the coefficient α indicating the adverse effect of electromagnetic repulsion is 5. FIG. 7 shows that the effective Lorentz force density F (N / m ³ ) has the maximum value when the current frequency is 1.2 Hz.

From FIG. 3 and FIG. 6, it is possible to suppress the pinhole defect as compared with the prior art in the range of the current frequency from 0.9 Hz to 2.3 Hz. This range is the effective Lorentz force density F, which is the maximum value. This corresponds to a range from Fmax to 0.9Fmax (current frequency is 0.9 to 2.0 Hz). Thus, by using the above formula (4), the best frequency of the electromagnetic stirring device can be determined from the result of only the electromagnetic field analysis.

The present invention has been made based on the above examination results by the inventors,
In continuous casting of steel using an electromagnetic stirrer installed in the mold,
Lx (N / m ³ ) is a value obtained by averaging the Lorentz force density component in the direction parallel to the mold long side in the range where the iron core that is a component of the electromagnetic stirrer exists.
When the value obtained by averaging the Lorentz force density component in the direction parallel to the mold short side in the range where the iron core is present is Ly (N / m ³ ),
Obtain 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,
This is a continuous casting method of steel using a frequency of electromagnetic stirring current in a range of the maximum value Fmax to 0.9Fmax of the effective Lorentz force density F.

According to the present invention, the best current frequency of the electromagnetic stirrer that can reduce the electromagnetic repulsion generated when the molten steel in the mold is electromagnetically stirred as much as possible can be determined from the result of the electromagnetic field analysis alone. Therefore, Ar gas bubbles can be prevented from being collected on the surface of the slab as much as possible, and pinhole defects can be further suppressed.

Of course, the present invention is not limited to the above-described example, and it is needless to say that the embodiments may be appropriately changed within the scope of the technical idea described in the claims.

The inventor performed the fluid simulation by the method described in Non-Patent Document 2, but it goes without saying that the thermal fluid simulation is not limited to the method described in Non-Patent Document 2.

DESCRIPTION OF SYMBOLS 11 ... Mold 13 ... Electromagnetic stirring apparatus 13a ... Iron core

Claims (1)

In continuous casting of steel using an electromagnetic stirrer installed in the mold,
Lx (N / m ³ ) is a value obtained by averaging the Lorentz force density component in the direction parallel to the mold long side in the range where the iron core that is a component of the electromagnetic stirrer exists.
When the value obtained by averaging the Lorentz force density component in the direction parallel to the mold short side in the range where the iron 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 stirrer is obtained,
A continuous casting method of steel, wherein a frequency of an electromagnetic stirring current in a range of the maximum value Fmax to 0.9Fmax of the effective Lorentz force density F is used.
F = Lx-α · Ly
Where α is a coefficient (= 3 to 7) indicating the adverse effect of electromagnetic repulsion.

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