JP2000000648A - Method and apparatus for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus for continuously casting a steel, with which suitable fluid can be secured according to the place even in the case that the place is anywhere in a molten metal pool part and thus, entrapment of mold powder and sticking of inclusion and bubble onto a solidified shell can effectively be prevented. SOLUTION: An electromagnetic brake coil 1 supplied with DC and a low frequency electromagnetic stirring coil 2 supplied with AC, are independently provided. A magnetic field, that the intensity and/or the direction with time are changed and this average value with time is not zero by using a magnetic field generating device winding these coils on the same iron core 3, is impressed on the unsolidified molten metal in a strand of a continuous caster.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for continuously casting steel, and more particularly, to a method and an apparatus for continuously casting steel in which molten steel flow is controlled by a magnetic field.

[0002]

2. Description of the Related Art In continuous casting of steel, prevention of entrapment of mold powder in the molten metal surface, prevention of product defects (UT defects, blisters, blisters, slivers) due to inclusions and bubbles, and solidification of inclusions and bubbles. Prevention of adhesion to
In order to promote uniform growth of the solidified shell, prevent cracking due to non-uniform solidification, or improve the solidified structure, the flow of molten steel is controlled (braking or stirring) by a magnetic field.

[0003] As an electromagnetic braking technique for suppressing excessive molten steel flow, for example, Japanese Patent Application Laid-Open No. 57-17356 discloses a technique.
An electromagnet is installed in the mold of a continuous slab caster, and a magnetic field is applied in the direction perpendicular to the molten steel discharge jet from the immersion nozzle, and the Lorentz force generated by the interaction between the current and the magnetic field induced in the molten steel A method is proposed in which the flow of the molten steel is damped with the above, and the discharge jet is suppressed from penetrating deeply into the molten steel pool, thereby preventing entrainment of the mold powder and promoting the floating of inclusions brought into the molten steel. Have been.

Japanese Patent Laid-Open No. 2-284750 discloses an effective method for further developing the above method and changing the operating conditions such as casting speed, slab width, immersion nozzle shape, and meniscus position. A method has been proposed in which a static magnetic field over the entire width of a mold is applied to the upper and lower portions of a discharge port of an immersion nozzle. In addition, as an example of electromagnetic stirring to prevent stagnation of molten steel flow, low-frequency movement to the meniscus part was performed to eliminate insufficient heat supply to the powder and prevent inclusions from being trapped by the initially solidified shell. Japanese Patent Laid-Open Publication No. Hei 2-37946 proposes a method of applying a magnetic field to flow molten steel.

Also, in order to prevent longitudinal cracking of the surface of the solidified shell of medium carbon steel, a method of flowing molten steel at a speed of 40 to 120 cm / s along the inner peripheral surface of the solidified shell in a region near the meniscus by an electromagnetic stirrer. Has been proposed in JP-A-1-228645. Furthermore, as an example of combining the electromagnetic brake and electromagnetic stirring, a static magnetic field is applied to the discharge flow from the immersion nozzle to reduce the molten steel discharge flow rate and promote the floating of large inclusions,
Downstream, a method of stirring the molten steel in the horizontal direction by an electromagnetic stirrer to prevent small inclusions from being trapped in the solidified shell (Japanese Patent Application Laid-Open No. 193755/1986).
While performing electromagnetic stirring so that a molten steel flow velocity of 4 m / s is obtained, a uniform static magnetic field is applied in the width direction of the slab at a place between 1.5 m below the meniscus and the vertical part of the continuous casting machine, A method of producing a cast slab having excellent cleanliness by suppressing the accumulation of inclusions on the surface layer and the inner layer portion (Japanese Patent Laid-Open No. 23803/1993), and a method of applying a low-frequency moving magnetic field at a meniscus position to cause a meniscus flow rate A method of reducing surface defects and internal defects of cast slabs by applying a uniform static magnetic field in the width direction above and below the immersion nozzle discharge port to suppress the flow of the injected molten steel ( JP-A-5-154620 is known.

Further, a method of applying a moving magnetic field or a static magnetic field in a mold by winding a plurality of coils around the same iron core, and switching between direct current and three-phase alternating current to flow through the coils (Japanese Unexamined Patent Application Publication No. 262650, JP-A-9-262651) are known.

[0007]

A problem that remains in continuous casting is to further increase throughput and casting speed in order to improve productivity. However, when the throughput or the casting speed is increased, the discharge speed of the molten steel from the immersion nozzle is increased, the flow speed near the meniscus is increased, and powder is entrained, and inclusions and the like enter the deep portion of the continuous casting machine. Therefore, it is necessary to attenuate both the upward flow and the downward flow from the discharge flow, and to provide an appropriate flow on the front surface of the solidified shell such as the meniscus to prevent inclusions from being caught by the solidified shell. Also, depending on the type of continuous caster, it may be necessary to cast extremely wide slabs at a low speed.In this case, in order to prevent stagnation of flow, both the upper and lower sides of the mold are required. Both need to give a moderate flow.

However, in the method disclosed in Japanese Patent Application Laid-Open No. 2-37946, in which a low-frequency moving magnetic field is applied to the meniscus to apply a flow to the molten steel, the discharge descending flow cannot be attenuated, and is carried from the immersion nozzle. It is impossible to prevent the inclusions or powder once caught in the continuous caster from entering. In addition, when applied to low speed casting of an extremely wide cast slab, the flow of molten steel becomes too strong, resulting in powder being caught, and the once caught powder enters the continuous casting machine.

Further, Japanese Patent Application Laid-Open No. 2-284750 discloses
Even when applying a static magnetic field over the entire width of the mold to the upper and lower parts of the discharge port of the immersion nozzle, when casting at high speed, if the magnetic field is increased to prevent the inclusion of inclusions, the flow at the meniscus will be too weak. When stagnation occurs, inclusions are trapped in the solidified shell. JP-A-61-193755
In the method disclosed in Japanese Patent Application Publication No. JP-A-2003-115, the static magnetic field is applied to a position surrounding the discharge flow, and the moving magnetic field is applied downstream of the static magnetic field. In other words, it is impossible to prevent a phenomenon in which the flow velocity near the meniscus is increased and powder is entrained.

The method disclosed in Japanese Patent Application Laid-Open No. Hei 5-23803, in which a moving magnetic field is applied in a mold and a uniform static magnetic field is applied under the mold in the width direction, can prevent powder entrainment when casting at high speed. Instead, inclusions may be trapped in the solidified shell at the stagnation of the vertical flow. In the method disclosed in JP-A-5-154620, in which a moving magnetic field is applied to a meniscus portion and a uniform static magnetic field is applied in a width direction above and below a nozzle discharge port, a solidified shell of an inclusion passing through a lower static magnetic field is also used. Has no effect on adhesion to

JP-A-9-262650 and JP-A-9-262650
In the method disclosed in Japanese Patent No. 262651 and applying a moving magnetic field or a static magnetic field in a mold by a pair of electromagnetic forces by changing the type of current to DC or three-phase AC, braking or stirring is performed at a certain place. Only one can be implemented. The problem with these molten steel flow control methods is that when attempting to brake or stir the flow at a certain location, only one of them can be performed.

In the method disclosed in Japanese Patent Application Laid-Open No. 5-154620, it is possible to apply a moving magnetic field to the meniscus portion and a uniform static magnetic field in the width direction above and below the nozzle orifice. In both locations, both agitation and braking of the flow cannot be achieved. With this method, it is possible to keep the flow at the meniscus within an appropriate range to some extent, but originally there should be an appropriate flow range not only in the width direction and casting direction of the mold but also in the thickness direction. Therefore, for example, in the vicinity of the molten metal surface, an appropriate flow is given to the front surface of the initially solidified shell to prevent the adhesion of inclusions, and the flow is braked at the part in contact with the inner mold powder to prevent the entrainment of the powder. Flow control, such as doing, is ideal.

An object of the present invention is to realize a flow control of molten steel that is close to the ideal, and an object of the present invention is to ensure a proper flow according to the location at any location in the molten steel pool, and It is an object of the present invention to provide a method and an apparatus for continuously casting steel, which can effectively prevent powder entrainment and adhesion of inclusions and bubbles to a solidified shell.

[0014]

SUMMARY OF THE INVENTION The present invention relates to a method for continuously casting steel, which comprises applying a magnetic field whose strength and / or direction fluctuates with time and whose time average value is not zero in an unsolidified molten steel in a continuous casting machine strand. And a method for continuously casting steel. The frequency of the fluctuation is 0.5Hz
Preferably, it is 10 Hz.

It is preferable that the time average value of the magnetic field and the fluctuation width around the average value are independently changed according to casting conditions. It is preferable that the time average value of the magnetic field and the fluctuation width around the average value are independently changed according to the position in the width direction and / or the casting direction. The location where the magnetic field is applied is preferably mainly in the mold, or mainly in the lower part of the mold, or mainly in the vicinity of the molten metal surface and in the lower part of the mold, or a combination of two or more of these.

Preferably, the location where the magnetic field is applied extends over substantially the entire width of the slab. Further, the present invention provides a steel continuous casting apparatus, in which an electromagnetic brake coil for supplying a direct current and a low-frequency electromagnetic stirring coil for supplying an alternating current are separately provided, and a magnetic field generator is formed by winding these coils around the same iron core. A continuous casting apparatus for steel, characterized by having at least one of

It is preferable that the magnetic field generator is mounted in the mold and / or below the mold. In the present invention, “magnetic field” and “magnetic field” have the same meaning, and “in the mold” means a distance measured from the meniscus position along the casting direction from the meniscus position as a starting point (referred to as a casting distance). ) The range up to 1 m, "the lower part of the mold" is the range from the position of 1 m to 5 m in the casting distance, and "the vicinity of the molten metal surface" is 0.2 in the casting direction from the meniscus position.
m.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In general, an electromagnetic force "vector f (hereinafter simply referred to as f)" has a current density "vector J (hereinafter simply referred to as J)" and a magnetic flux density "vector B (hereinafter simply referred to as B). )), The following formula is expressed as f = J × B (1). Here, "x" is a cross product (the same applies hereinafter).
The current density J is calculated by the following equation based on the electric conductivity σ, the electric field strength “vector E (hereinafter simply referred to as E)”, and the molten steel flow velocity “vector v (hereinafter simply referred to as v)”. J = σ (E + v × B) (2) [Ohm's law] When there is a time change of the magnetic field, the electric field strength E
Is given by time t, electric potential φ, and vector potential “vector A (hereinafter simply referred to as A)”.
The following equation E = −∇φ−∂A / ∂t (3) [Faraday's law]. Here, “∇” is a nabula (the same applies hereinafter), and the vector potential A satisfies the following equation: B = ∇ × A (4)

By substituting the equations (2) and (3) into the equation (1), the electromagnetic force f can be expressed by the following equation: f = σ [−Δφ × B + v × B × B− (ΔA / Δt) × B] (5) Equation (5) The second term on the right side represents the braking force, and the third term represents the stirring force. Note that the first term represents the force due to the return current.

According to the present invention, a magnetic field whose strength (magnitude, strength) and / or direction (direction) changes with time and whose time average value is not zero is applied to the molten steel. The flow contains the DC component of the magnetic field (DC magnetic field; equation
(5) The second term on the right-hand side acts as a brake, and in the slow flow, the AC moving component of the magnetic field (AC moving magnetic field; Equation (5)
Term) acts as a stirrer.

The frequency of the AC moving magnetic field is 0.5 Hz to 10 Hz.
Since the frequency is set to Hz, it is possible to freely control the molten steel flow at a desired place by changing the penetration depth of the magnetic force due to the skin effect. In other words, the higher the frequency, the more the stirring force can be applied to the vicinity of the surface.However, if the frequency exceeds 10 Hz, the magnetic force extends only to the solidified shell and the mold copper plate and the molten steel cannot be sufficiently stirred, so the upper limit of the frequency is set. 10 Hz. On the other hand, the lower the frequency, the more the magnetic force can spread to the inside of the molten steel. However, if the frequency is lower than 0.5 Hz, an effective stirring force cannot be obtained, so the lower limit of the frequency is set to 0.5 Hz. This frequency range belongs to the so-called low frequency range. Note that a more preferable frequency range is 1 Hz to 5 Hz.

As described above, the action of the AC moving magnetic field is stronger near the surface of the molten steel (the front surface of the solidified shell) and weaker as it is farther (inside). On the other hand, since the DC magnetic field acts regardless of whether it is on the surface or inside, from the equation (5), the effect of agitation can be increased in front of the solidified shell and the effect of braking can be enhanced inside. Further, in the present invention, the time average value of the strength of the magnetic field (fluctuation magnetic field) and the fluctuation width around the average value are determined by the casting conditions (steel type, slab width, casting speed, etc.), the width direction position, and the pouring direction position. The braking force and the agitation force can be made to match the values considered to be optimal for each condition and each position, and the desired molten steel can be obtained for each condition and each position. Flow can be imparted.

Also, since the application place of the fluctuating magnetic field is mainly in the mold, mainly in the lower part of the mold, mainly in the vicinity of the molten metal surface and in the lower part of the mold, or in a combination of two or more of them, It is possible to prevent inclusions and bubbles from adhering to the surface of the slab and to prevent the slab surface from cracking. In addition to the inclusion of mold powder on the molten metal surface and the inclusion of inclusions and bubbles in the initially solidified shell, the inclusion of inclusions and bubbles into the slab and the attachment of inclusions and bubbles to the inner layer of the slab are The combination of the above (1) and (2) produces a combined effect of the individual effects.

Further, since the application location of the fluctuating magnetic field is substantially the entire width of the slab, the above-mentioned effect can be obtained at any position in the width direction. Of course, since the magnetic field intensity distribution can be provided in the slab width direction, a desired molten steel flow corresponding to the position can be obtained at each position in the width direction. In the magnetic field generator of the present invention, an electromagnetic brake coil for supplying a direct current and a low-frequency electromagnetic stirring coil for supplying an alternating current are separately provided, and these coils are wound around a common iron core. A magnetic field that fluctuates with time can be generated from the iron core of the electromagnetic brake, and the electromagnetic brake and the electromagnetic stirring can be freely operated at the same position.

FIG. 1 is a three-dimensional view showing an example of a magnetic field generator according to the present invention. FIG. 3 shows a portion corresponding to one long side of the mold. As shown, the electromagnetic brake coil 1
The low frequency electromagnetic stirring coil 2 is wound around a common iron core 3. The iron core 3 is provided with a through hole 4 through which a low-frequency electromagnetic stirring coil 2 can be provided at each position in the width direction. The suffix “ _A ” is added for individual distinction.

FIG. 2 is a side sectional view showing a state where the magnetic field generator of FIG. 1 is mounted on a mold. In the figure, 5 is the long side of the mold (copper plate on the long side of the mold), 6 is the immersion nozzle, 7 is the solidified shell (solidified shell of the slab), 8 is the molten steel, 9 is the discharge port (nozzle discharge port), and 10 is the short mold. The side (short-side copper plate on the mold) 11 is a meniscus (fluid surface), and the same or corresponding parts as those in FIG. As shown in the figure, this magnetic field generating apparatus is suitable for a case where a magnetic field is applied in two steps in the vicinity of the molten metal surface and the lower part of the mold (two-step magnetic field application). In this state, by individually changing the current values of the electromagnetic brake coils 1 and 1 _A , the time average value of the magnetic field strength can be changed independently in the upper stage and the lower stage. , 2 _A , the time variation width of the magnetic field strength can be independently changed between the upper stage and the lower stage.

FIGS. 3 and 4 are plan sectional views each showing an example of a magnetic field generating apparatus according to the present invention, which is suitable for a case where a magnetic field is applied to the molten steel in the strand one step in the casting direction. In these drawings, the same or corresponding parts as those in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. In any of the examples, an electromagnetic brake coil 1 exclusively for DC (subscripts “ _B ” to “ _E ” are attached for individual distinction) and a low-frequency electromagnetic stirring coil 2 exclusively for AC are wound around a common iron core 3. The width direction of this step,
The DC magnetic field and the AC moving magnetic field can be superimposed on the molten steel 8 at any position in the thickness direction, and can be applied with the strength adjusted according to the position.

FIG. 5 is a waveform diagram comparing the magnetic fields generated by the method (c) of the present invention and the conventional method (conventional electromagnetic braking method (a), conventional electromagnetic stirring method (b)). The vertical axis in the figure is the x direction component of the magnetic flux density (the component in the direction parallel to the short side of the mold)
x, the horizontal axis represents the position in the mold width direction (the position on the coordinate axis taken parallel to the long side of the mold). In the conventional electromagnetic braking method in which a uniform static magnetic field is applied in the width direction, as shown in (a), only a DC magnetic field having a characteristic in which the same time value of Bx is constant in the width direction and does not change with time is generated. In this case, only the braking force acts and no stirring force is generated. On the other hand, in the conventional electromagnetic stirring method, as shown in (b), the same time value of Bx draws a sinusoidal waveform in the width direction, and only an AC moving magnetic field having a characteristic that this waveform moves in the width direction with time is generated. In places, agitation is sufficient but braking is insufficient. In contrast to these conventional methods, in the method of the present invention, as shown in (c), a DC magnetic field and an AC moving magnetic field are generated by superposition, so that both braking and stirring are sufficient at the same application location according to the situation. The magnetic field can be adjusted freely.

[0029]

[Example] A vertical bending type continuous casting machine was used to discharge 70
1.8m / min casting speed using a 2mm reverse Y-type two-hole immersion nozzle
When casting a low carbon aluminum killed steel slab having a width of 1500 mm and a thickness of 220 mm at a speed of 2.5 m / min, the magnetic field generator of the present invention illustrated in FIG. 1 was installed in a mold in the arrangement of FIG. The magnetic field strength (static magnetic field force) by the electromagnetic brake coil 1 is changed in the range of 1000 to 5000 Gauss, and the flow rate of the molten steel (meniscus flow rate) in the vicinity of the molten metal surface is reduced to 10 to 80 cm / s by the low frequency electromagnetic stirring coil 2. Examples were changed within the range.

The slab after casting is hot-rolled, and then cold-rolled to obtain a 1.0 mm thick × 1500 mm wide cold-rolled coil. The incidence of internal defects and the incidence of surface defects are determined by magnetic particle inspection and visual inspection. investigated. As shown in FIG. 6, instead of the magnetic field generator of FIG. 2, a low-frequency electromagnetic stirring coil 2 is provided in the vicinity of the molten metal surface, and an electromagnetic brake coil 1 is provided around the iron core 3 in the lower part of the mold. We install container independently,
Comparative Example 1 in which the magnetic field strength (static magnetic field force) by the electromagnetic brake coil 1 was changed in the range of 1000 to 5000 Gauss during casting, and the meniscus flow rate was changed in the range of 10 to 80 cm / s by the low frequency electromagnetic stirring coil 2. 7, only the low-frequency electromagnetic stirring coil 2 is installed in the vicinity of the molten metal surface, and the low-frequency electromagnetic stirring coil 2 reduces the meniscus flow rate from 0 to 80 cm.
For Comparative Example 2 changed in the range of / s, the same investigation as in the example was conducted.

Table 1 shows the results of these investigations. The defect occurrence rates in Table 1 are shown as relative ratios to Comparative Example 2 No. 1.
From Table 1, the defect generation rate in both the inside and the surface of Comparative Example 1 in which the molten steel was electromagnetically braked at the lower part of the mold was higher than Comparative Example 2 in which the molten steel was only electromagnetically stirred in the vicinity of the molten metal surface. It was low. Compared to Comparative Example 1, in the embodiment in which the electromagnetic stirring and the electromagnetic braking are overlapped in the vicinity of the molten metal surface and at the respective locations below the mold, the molten metal surface (inside) even at the same meniscus flow velocity
The internal flow rate and surface defects were further reduced because the flow velocity was reduced and powder entrainment was reduced.

[0032]

[Table 1]

[0033]

As described above, according to the present invention, a magnetic field generator is provided which comprises an electromagnetic brake coil for supplying a direct current and a low-frequency electromagnetic stirring coil for supplying an alternating current, and these coils are wound around the same iron core. Is used to apply a magnetic field whose strength and / or direction changes with time and whose time average value is not zero to the unsolidified molten steel in the continuous caster strand, so that a desired molten steel flow can be obtained, It has the effect of effectively reducing both surface defects and internal defects.It also eliminates the need for separate devices for electromagnetic braking and electromagnetic stirring as in the past, simplifying equipment and reducing costs. is there.

Also, the applied magnetic field is freely adjusted for the longitudinal portion of the slab (unsteady portion where the quality tends to deteriorate) at the time of changing the ladle where the casting speed changes, and the flow state is the same as that of the steady portion. , It is possible to prevent the quality of the non-stationary part from deteriorating.

[Brief description of the drawings]

FIG. 1 is a three-dimensional view showing an example of a magnetic field generator according to the present invention.

FIG. 2 is a side sectional view showing a state where the magnetic field generator of FIG. 1 is mounted on a mold.

FIG. 3 is a cross-sectional plan view showing an example of the magnetic field generating apparatus of the present invention suitable for providing a magnetic field applied to molten steel in a strand in one step in a casting direction.

FIG. 4 is a cross-sectional plan view showing an example of the magnetic field generator of the present invention suitable for providing a magnetic field applied to molten steel in a strand at one stage in the casting direction.

FIG. 5 is a waveform chart showing a comparison of magnetic fields generated by the method (c) of the present invention and the conventional method (the conventional electromagnetic braking method (a) and the conventional electromagnetic stirring method (b)).

FIG. 6 is a side sectional view showing a state where the magnetic field generator of Comparative Example 1 is mounted on a mold.

FIG. 7 is a side sectional view showing a state where the magnetic field generator of Comparative Example 2 is mounted on a mold.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electromagnetic brake coil 2 Low frequency electromagnetic stirring coil 3 Iron core 4 Drilling hole 5 Mold long side (Mold long side copper plate) 6 Immersion nozzle 7 Solidified shell (Slab solidified shell) 8 Molten steel 9 Discharge port (Nozzle discharge port) 10 Mold short Edge (Mold short side copper plate) 11 Meniscus (Fluid surface)

Claims (10)

[Claims] In a continuous casting method for steel, wherein a magnetic field is applied to unsolidified molten steel in a continuous casting machine strand, a magnetic field whose strength and / or direction fluctuates with time and whose time average value is not 0 is determined. A continuous casting method of steel, characterized by applying a voltage. 2. The continuous casting method according to claim 1, wherein the frequency of the fluctuation is 0.5 Hz to 10 Hz. 3. The continuous casting method according to claim 1, wherein the time average value of the magnetic field and a fluctuation range around the average value are independently changed according to casting conditions. 4. The magnetic head according to claim 1, wherein the time average value of the magnetic field and a fluctuation range around the average value are independently changed according to the position in the width direction and / or the casting direction. 3. The continuous casting method according to item 1. 5. The continuous casting method according to claim 1, wherein the place to which the magnetic field is applied is mainly in a mold. 6. The continuous casting method according to claim 1, wherein the place where the magnetic field is applied is mainly at a lower part of the mold. 7. The continuous casting method according to claim 1, wherein the location to which the magnetic field is applied is mainly in the vicinity of the molten metal surface and in the lower part of the mold. 8. The continuous casting method according to claim 1, wherein the location to which the magnetic field is applied covers substantially the entire width of the slab. 9. A continuous casting machine for steel provided with a magnetic field generator for applying a magnetic field to unsolidified molten steel in a continuous casting machine strand, the magnetic field generator comprising: an electromagnetic brake coil for supplying a direct current; A continuous casting apparatus for steel, comprising: a low-frequency electromagnetic stirring coil for energizing the steel; and a core wound in common with the low-frequency electromagnetic stirring coil. 10. The continuous casting apparatus according to claim 9, wherein the magnetic field generator is mounted inside and / or below the mold.