JP3504649B2 - Method and apparatus for electromagnetic stirring in a continuous casting mold - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is to reduce defects by applying electromagnetic force to molten steel in a mold in a continuous casting machine for producing a slab slab, which is a raw material for rolling a steel sheet, to control flow. The present invention relates to an electromagnetic stirring method capable of obtaining a large slab slab and an apparatus therefor.

[0002]

2. Description of the Related Art In continuous casting of steel slab, a moving magnetic field type electromagnetic stirring coil (hereinafter referred to as "linear motor") for generating electromagnetic stirring force along a mold having a rectangular cross section is installed and moved to molten steel. A magnetic field is applied to swirl stir the molten steel near the meniscus (electromagnetic stirring).
The main purposes of performing such electromagnetic stirring are as follows (a) and (b). (A) By flushing the solidification interface immediately below the molten metal surface where solidification starts, bubble defects and inclusion defects in the surface layer of the cast slab are reduced. (B) The occurrence of crack defects is prevented by making the molten steel temperature uniform and making the thickness of the solidified shell of the slab uniform just below the molten metal surface where solidification starts.

FIG. 1 is a schematic explanatory view showing a state of molten steel flow in a mold in conventional slab continuous casting, and FIG. 2 is a sectional view taken along the line AA of FIG. As shown in FIGS. 1 and 2, this continuous casting machine is provided with a pair of linear motors along the long side of the mold, and from the dipping nozzle installed in the center of the mold to both short sides. The molten steel flow discharged into the mold toward the mold is agitated while being accelerated by these linear motors. Further, in the configuration shown in FIGS. 1 and 2, the pair of linear motors accelerates the molten steel in the vicinity of the mold wall in the horizontal direction and in the opposite directions.

However, in a slab continuous casting machine for producing a material for steel plates, it is difficult to smoothly swirl and agitate the molten steel in the mold because the horizontal cross-sectional shape of the space in the mold is flat. Further, as a result of the stirring flow by the electromagnetic stirring device interfering with the discharge flow of the molten steel discharged from the immersion nozzle, a complicated flow is generated in the mold, the flow velocity distribution of the molten steel is not always uniform, and it is locally There are regions where the flow velocity becomes excessively high or low.

When an area where the molten steel flow velocity is excessively generated in the vicinity of the molten steel surface, the molten steel surface vibrates violently, and the molten steel scene is used for the purpose of heat retention, air oxidation prevention, and ensuring lubricity between the cast slab and the mold. The floating flux is entrained in the molten steel, which causes the problem of inclusion defects in the slab. On the other hand, when a region where the molten steel flow velocity is too low occurs, a sufficient solidified shell rinsing effect cannot be obtained in that region, resulting in frequent occurrence of bubble defects and inclusion defects.

Further, if the flow velocity distribution becomes non-uniform, the thickness of the solidified shell of the slab will also become non-uniform, and thermal strain will concentrate at the portion where the thickness of the solidified shell is thin, and cracks in the surface layer of the slab will be sufficiently reduced. Can not. Further, when the shape of the molten metal surface is wavy due to the flow velocity distribution, there is a problem that the flux thickness is likely to be insufficient at the raised portion, and the operation becomes unstable.

In the continuous casting of slabs, various techniques have been proposed so far from the viewpoint of achieving uniform stirring of molten steel. For example, in JP-A-7-9098,
A technique is disclosed in which the discharge hole of the immersion nozzle is made straight downward to reduce the interference between the discharge flow from the immersion nozzle and the stirring flow by the linear motor. However, in the conventional electromagnetic stirring technology for continuous slab casting, since the thrust (stirring force) acting on the molten steel by electromagnetic stirring is not originally uniformly exerted in the long side direction of the mold, even if such a technique is applied, The molten steel stirring flow velocity distribution cannot be made sufficiently uniform, which is not a fundamental solution.

On the other hand, in Japanese Patent Laid-Open No. 6-605, an attempt is made to realize smooth molten steel stirring by dividing the coil of the linear motor into a large number on each of the long sides of the mold so that the thrust can be controlled separately. Techniques for doing so are disclosed. However, in order to divide the coil of the linear motor into a large number in the long-side direction and control them individually, a large number of low-frequency power supplies are required, increasing the equipment cost and increasing the number of electrical connectors for the energizing cables. Therefore, the maintainability at the time of mold replacement is deteriorated. In addition, in order to properly control a large number of linear motors, it is necessary to accurately grasp the molten steel flow velocity in the mold, which is a control index, but it is difficult to directly measure the molten steel flow velocity, and it is an index necessary for proper control. Is hard to get.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of these circumstances, and its purpose is to appropriately control the flow of molten steel in a mold in continuous casting to minimize the occurrence of defects. It is an object of the present invention to provide an electromagnetic stirring method capable of producing a reduced slab slab and a useful apparatus for carrying out such a method.

[0010]

Means for Solving the Problems The electromagnetic stirring method of the present invention that has been able to solve the above-mentioned problems is to use a dipping nozzle installed in the center of a mold of a continuous casting machine for producing a slab slab having a rectangular cross section. In a method of electromagnetically stirring molten steel discharged into a mold with a linear motor, at least one molten steel near each long side of the mold is provided in a substantially horizontal direction by a linear motor provided along each of the long sides of the mold. Frequency f1 of the moving magnetic field generated by the linear motor installed in one long-side mold
(Hz) and the frequency f2 (Hz) of the moving magnetic field generated by the linear motor installed on the other long-side mold are different, and at least one of the following (I) and (II) is satisfied. While operating, it has a gist. (I) The frequencies f1 and f2 are set to satisfy the following expression (1). 0.1 / P ≦ | f1-f2 | <5.0 (1) However, P represents the pole pitch (m) of the linear motor.
(II) Difference in molten steel flow along each long side mold during casting,
Or, measure the level difference of molten steel surface near each short side wall caused by the difference of molten steel flow, and make sure that the current value flowing to the linear motor along the long side mold with the slower flow velocity is relatively increased. To do.

The pole pitch means the interval between adjacent N and S poles in a linear motor. For example,
In a linear motor in which electric coils for passing alternating currents having phases substantially different by 60 degrees are arranged in the acceleration direction, the pole pitch is equal to the interval of three electric coils. Further, in a linear motor in which electric coils for passing alternating currents whose phases are substantially different by 90 degrees are arranged in the acceleration direction, the pole pitch is equal to the interval of two electric coils.

In each of the above electromagnetic stirring methods, (a) when the frequency at which the thrust acting on the molten steel becomes maximum is fm, the frequency fm and the frequencies f1 and f2 satisfy the following equation (2). In this way, (b) the magnitude of the thrust F1 acting on the molten steel by the linear motor installed on one long-side mold and the thrust acting on the molten steel by the linear motor installed on the other long-side mold. It is preferable to operate so that the size F2 satisfies the following expression (3). f1 <fm <f2 (2) | F1-F2 | / (F1 + F2) ≦ 0.05 (3)

On the other hand, the electromagnetic stirrer of the present invention which has achieved the above-mentioned object means that the dipping nozzle installed at the center of the mold of a continuous casting machine for producing a slab slab with a rectangular cross section is used to obtain both short sides. In a device configured to electromagnetically stir a molten steel flow discharged into a mold toward a side while being accelerated by a linear motor, at least one linear motor is provided along each long side of the mold, The molten steel in the vicinity of the side mold is configured to be accelerated in a substantially horizontal direction and in mutually opposite directions, and the frequency f1 (Hz) of the moving magnetic field generated by the linear motor installed in one long side mold and the other Frequency f of moving magnetic field generated by linear motor installed in long side mold
The gist is that it is configured so that 2 (Hz) is different.

If a plurality of such electromagnetic stirring devices are installed,
Multiple slab slabs can be manufactured simultaneously. Also, as a specific configuration when adopting such a configuration,
As shown in FIG. 11 described later, a low-frequency power source that generates a current of frequency f1 supplies power to a linear motor installed in one long-side mold of a plurality of electromagnetic stirrers, and another that generates a current of frequency f2. A configuration in which a low-frequency power source supplies power to a linear motor installed in the other long-side mold of a plurality of electromagnetic stirrers can be mentioned.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The function and effect of the present invention will be described below along with the background of the completion of the present invention. First, the present inventors investigated the cause of non-uniformity of the flow velocity distribution in order to solve the problems in the conventional electromagnetic stirring technology.

In the electromagnetic stirring so far, it is general that the moving magnetic fields generated by the linear motors installed on the opposing long side molds are operated at the same frequency. For example, in the device configuration shown in FIG.
Frequency f1 of the moving magnetic field generated by the pair of linear motors
An example of the magnetic flux density distribution measurement results when f2 is the same and 3.0 Hz is shown in FIG. At this time, the length of the long side mold is 1800 mm. In addition, the magnetic flux density indicates the peak value of the component in the short side direction of the mold, and the measurement position is 15 mm from the mold wall on the long side (the lower long side mold wall in FIG. 1) that generates a moving magnetic field that moves to the right. And the center position of the height of the linear motor.

As is clear from this result, it is understood that the magnetic flux density greatly changes depending on the position in the long side direction of the mold. In addition, the position where the magnetic flux density is maximum corresponds to the position where the moving magnetic fields generated by the linear motors constantly strengthen each other.
The position where the magnetic flux density is minimum corresponds to the position where the moving magnetic fields generated by the linear motors always weaken, and the interval between the maximum positions and the minimum positions is considered to be almost equal to the pole pitch P of the linear motor. Was given.

In this way, there are places where the magnetic fields are always strengthened and the magnetic flux density becomes maximum, and where the magnetic fields are always weakened and the magnetic flux density becomes minimum, the frequencies f1 and f2 of the respective linear motors are the same. It was thought that it was due to that. That is, in the case of the frequency f1 = f2, one (first) long side is the N pole and the other (second) long side is the S pole at a certain moment, and the two sides face each other. After the half cycle (after 0.5 / f1 second), the first long side is S
The poles and the second long sides face each other as N poles and face each other, resulting in continuous strengthening of the magnetic field. Also, at a certain moment, the first long side,
At a position where both the second long sides face each other as N poles and weaken the magnetic field, after half a period, both the first and second long sides face each other as S poles and continue to weaken the magnetic field. Becomes Thus, in the conventional electromagnetic stirring method,
Since the moving magnetic field generated by the linear motor installed on each long side changes synchronously at the same frequency, the magnetic fields interfere with each other,
The magnetic flux density changes depending on the position in the long side direction of the mold.

FIG. 4 is a plan view showing an example of the calculation result of the thrust distribution acting on the molten steel when the conventional electromagnetic stirring method is carried out. The thrust distribution does not form one large vortex in the entire molten steel in the mold having a rectangular horizontal section, but has a winding distribution at the pole pitch. The reason why the thrust twists in this way is that the magnetic flux density is distributed as shown in FIG. 3, so that the thrust acts not only in the long side direction of the mold but also in the short side direction of the mold, and the direction of the thrust in the short side direction of the mold. It is thought that this is due to the fact that they are alternately reversed depending on the position in the long side direction of the mold.

FIG. 5 is a graph showing an example of the calculation result of the molten steel stirring flow velocity distribution in the mold by the thrust acting on the molten steel when the conventional electromagnetic stirring method is carried out. This result shows that the position of the short-side mold is set so that the long-side length W is 1350 mm, and the flow velocity is 50 mm directly below the molten steel surface and 10 mm from the wall of the long-side mold on the side that generates thrust to the right. Is a component in the long side direction of. This calculation is for the purpose of evaluating how smoothly the electromagnetic stirrer itself can stir the molten steel in a state where the stirring flow and the discharge flow do not interfere with each other, as in the case of using the immersion nozzle that discharges the molten steel downward. , No molten steel discharge flow from immersion nozzle (Vc = 0m /
min)). Even under such a calculation condition, the calculation result of the stirring flow velocity has a non-uniform distribution in the long side direction of the mold, and it can be seen that there is a region where the flow velocity is slow. Further, the region where the stirring flow velocity was slow almost corresponded to the region where the magnetic flux density was low.

From these examination results, the main reason why it is difficult to obtain a uniform flow velocity distribution by the conventional electromagnetic stirrer in the slab continuous casting is that the moving magnetic field generated by the linear motor installed in each long side mold is synchronized. It turned out to be because they interfere with each other. The intervals between the maximum magnetic flux density positions and the minimum magnetic flux density positions are substantially equal to the pole pitch P of the linear motor. Therefore, the number of poles is increased and the pole pitch P is reduced to make the thrust distribution uniform. Can also be considered.
However, when the pole pitch P is reduced, the thrust generated at the same magnetic flux density and the same frequency is proportionally reduced, so that there is a limit to the reduction of the pole pitch. From this point of view, in the continuous casting mold having the long side length of about 2000 mm, the pole pitch P is preferably about 500 mm and the number of poles is about 4.

Based on the above-mentioned examination results, the present inventors
If the frequencies f1 and f2 of the moving magnetic field generated by the linear motors installed in the long-side molds facing each other in the continuous slab casting are made different (f1 ≠ f2), the distribution of the magnetic flux density in the long-side direction can be made uniform and the molds can be made uniform. The present invention has been completed by finding that the thrust force distribution in the short side direction can be smoothed and the stirring flow velocity distribution by electromagnetic stirring can be made uniform.

A device for carrying out the present invention is basically similar to the device shown in FIG.
For example, by providing different low frequency power supplies (ie,
At least two), the frequency f1 (Hz) of the moving magnetic field generated by the linear motor installed in one long-side mold and the frequency f2 (Hz) of the moving magnetic field generated by the linear motor installed in the other long-side mold It is configured differently.
The electromagnetic stirrer shown in FIG. 1 has a configuration in which a pair of linear motors are provided along the long side of the mold, but in the apparatus for carrying out the method of the present invention,
Not limited to such a configuration, at least one linear motor is provided along each of the long sides of the mold, and the molten steel in the vicinity of the long side of the mold is configured to be accelerated substantially horizontally and in opposite directions at different frequencies. I'm good. However, if the number of linear motors provided along each of the long sides increases, a large number of low-frequency power sources are required as described in the above-mentioned prior art, which increases the facility cost and increases the electricity consumption of the power cable. Since the number of connectors increases and maintainability at the time of mold replacement is deteriorated, about 1 to 2 is suitable.

In carrying out the present invention, the form of the immersion nozzle in the continuous casting machine is not particularly limited, and as shown in FIGS. 1 and 2, the immersion nozzle installed in the center of the mold. From the structure that discharges the molten steel flow from both sides toward both short sides, the discharge hole of the immersion nozzle is straight downward, or the discharge hole position of the immersion nozzle is below the lower end of the linear motor iron core, The present invention can be applied to a structure in which the interference between the discharge flow from the immersion nozzle and the stirring flow by the linear motor is reduced. In short, if it is a form that accelerates the molten steel or molten steel flow discharged from the immersion nozzle into the mold,
The present invention is applicable.

The present inventors have found that the frequency f1 of the moving magnetic field (leftward acceleration in FIG. 1) of the linear motor provided on one long side.
Was set to 2.5 Hz and the frequency f2 of the moving magnetic field (rightward acceleration in FIG. 1) of the linear motor provided on the other side was set to 3.0 Hz, and the magnetic flux density distribution was measured when electromagnetic stirring was performed. An example of the result is shown in FIG. In this graph, FIG.
Similarly, the magnetic flux density shows the peak value of the component in the short side direction of the mold, the measurement position is 15 mm from the long side mold wall, and the center position of the height of the linear motor.

As is apparent from the results of FIG. 6, the change in the magnetic flux density depending on the position in the long side direction of the mold is reduced as compared with the case of the conventional electromagnetic stirring (FIG. 3), and the magnetic flux density distribution is more uniform. You can see that it has been made. The reason why these results were obtained could be considered as follows. That is, as a result of changing the frequencies f1 and f2 of the moving magnetic field generated by the linear motor installed in each long-side mold (f1 ≠ f
2) Compared with the undulation period (1 / | f1-f2 |) of the moving magnetic field, the position where the moving magnetic field strengthens (antinode) and the position where the moving magnetic field weakens change over time. The result is that the moving magnetic fields do not interfere with each other when time averaged over a sufficiently long time.

In other words, when the frequencies of the linear motors are different (f1 ≠ f2), the first long side becomes the N pole and the second long side becomes the S pole at a certain moment. Face to face,
At the position where the magnetic fields are strengthened, after the half cycle of the first moving magnetic field (1/2 · f1 second), the first long side changes to the S pole, but the half cycle of the second moving magnetic field (1 The second long side does not become a perfect N pole because a time different from (/ 2 · f2) has elapsed. As a result, the position where the magnetic fields are strengthened moves to a position different from the position where the magnetic fields are strengthened first, and the position where the magnetic fields are strengthened first is not the position where the magnetic fields are strengthened. When time passes, weakening and strengthening of the magnetic field are repeated at the same position. For this reason, when averaged over a long time, the magnetic fields do not strengthen and weaken each other, and in fact, there is no interference between the moving magnetic fields.

FIG. 7 schematically shows an example of a thrust distribution acting on molten steel when the electromagnetic stirring method of the present invention is carried out. As is clear from this result, as the magnetic flux density distribution is made uniform, the thrust in the short side direction of the mold hardly acts, and the thrust distribution shows one large vortex in the entire molten steel in the mold whose horizontal section is rectangular. Will be formed.

FIG. 8 shows an example of the calculation result of the molten steel stirring flow velocity distribution in the mold when the electromagnetic stirring method of the present invention is carried out. As in the case of FIG. 5, this result shows the flow velocity mold long side direction component at a position 10 mm from the long side mold wall just below the molten steel molten metal surface, and does not consider the molten steel discharge flow from the immersion nozzle. . In the conventional electromagnetic stirring method, the flow velocity is slowed almost corresponding to the region where the magnetic flux density is low (FIG. 5), whereas in the electromagnetic stirring method of the present invention, the flow velocity distribution is uniform. I understand.

The present invention operates while making the frequencies f1 and f2 different, but the object of the present invention cannot be achieved only by satisfying these conditions, and the difference between the frequencies f1 and f2 is not achieved. It is necessary to control the absolute value of | f1-f2 | If this value | f1-f2 | is too small, the magnitude of the moving speed at the position (node) that weakens the magnetic flux density becomes smaller than the magnitude of the stirring flow velocity of the molten steel,
The magnetic flux density distribution at each moment affects the stirring flow velocity. As a result, even if it can be judged that the magnetic flux density distribution is uniformized when averaged for a long time, a sufficient flow velocity uniformization effect is not exhibited in continuous casting in which solidification is constantly progressing.

When the above value | f1-f2 | is in the proper range, the position where the magnetic flux density becomes minimum moves faster than the molten steel flow velocity, so the stirring flow velocity has almost no influence on the magnetic flux density distribution at each moment. It will not be done. At this time, since the stirring flow velocity is governed by the magnetic flux density distribution averaged over a long period of time,
The stirring flow velocity distribution is made uniform.

The stirring flow velocity required to wash away air bubbles and inclusions having a diameter of 300 μm, which cause surface defects on the thin plate, by electromagnetic stirring is about 0.10 (m / s), so The size V needs to be V ≧ 0.10 (m / s). Further, the frequency f1 and the phase velocity V1 = 2.
A quasi-magnetic field composed of a moving magnetic field that moves in one direction at P · f1 (m / s) and a moving magnetic field that moves in the opposite direction at frequency f2 and phase velocity V2 = −2 · P · f2 (m / s). The magnitude V of the moving velocity of a node in a standing wave magnetic field is V = P · | f
It is expressed as 1-f2 | (m / s). From these conditions, |
It is necessary to satisfy f1-f2 | ≧ 0.10 / P (Hz). On the other hand, if the value | f1-f2 | is too large, the frequency f
Since the difference between 1, f2 and the frequency fm at which the thrust is maximized becomes large, the stirring efficiency will decrease, so
It is necessary to set f1-f2 | <5.0 (Hz).

In order to realize more uniform agitation, it is desirable to set the thrust by the linear motor to be substantially equal in one long side mold and the other long side mold. The thrust by each linear motor is F1 and F2 respectively.
Then, the thrusts F1 and F2 are the magnetic flux density measurement results B1 and B2 at the center in the long side direction of the mold, at the center of the height of the linear motor, and at the same distance from the surface of the long side mold.
(T), each frequency f1, f2 (Hz), pole pitch P
According to (m), it is possible to estimate as in the following equations (4) and (5) using a common proportional constant k. F1 = kP · f1 × B1 ² …… (4) F2 = kP · f2 × B2 ² …… (5)

As shown in the equations (4) and (5), the thrusts F1 and F2 are proportional to the pole pitch P, the frequencies f1 and f2, and the square of the magnetic flux density. Therefore, if the magnetic flux density is the same, a higher thrust can be obtained as the frequencies f1 and f2 are higher. However, as the frequencies f1 and f2 increase, the attenuation of the magnetic field by the mold copper plate increases, and the magnetic flux density obtained by passing the same current decreases. Due to these effects, generally, in electromagnetic stirring in a mold, as shown in FIG. 9, there is a frequency fm at which the thrust is maximized at a constant current. For this reason, the frequency f1 of the moving magnetic field of one long-side mold is made smaller than fm, and the frequency f2 of the moving magnetic field of the other long-side mold is made larger than fm [Equation (2)]. It is possible to set the two frequencies so that the thrusts F1 and F2 are equal when a current having the same current value is applied. Also,
It is also possible to separately set the current value of each electromagnetic stirrer so that the thrusts F1 and F2 are equal.

In FIG. 10, by changing F2 while keeping F1 constant, (F1-F2) / (F1 + F2)
It is a graph which shows the calculation result of the average stirring flow velocity in the vicinity of each long side mold when is changed. As is clear from this result, when the value of | F1-F2 | / (F1 + F2) is larger than 0.05, the difference in the average flow velocity increases rapidly, so that the frequency should be set so as to satisfy the following equation (3). It is desirable to set f1 and f2. | F1-F2 | / (F1 + F2) ≦ 0.05 (3) From the expressions (4) and (5), the value on the left side of the expression (3) is the value of the proportional constant k. Regardless of (6) below
It can be estimated from the magnetic flux density measurement result by the formula. | F1-F2 | / (F1 + F2) = | f1 × B1 ² −f2 × B2 ² | / (f1 × B1 ² + f2 × B2 ² ) (6)

According to the electromagnetic stirring method of the present invention, at least two low-frequency power sources need to be installed for one mold, and wiring is required between the linear motors installed on each long-side mold. There is no. Therefore, the number of low-frequency power sources required does not increase and the number of electrical connectors required is smaller than in the case where a plurality of divided linear motors are installed in one long-side mold. As a result, maintainability at the time of mold replacement is improved.

Further, since the magnetic flux density distribution itself is made more uniform than in the conventional electromagnetic stirring method, the molten steel flow velocity distribution in the mold cannot be accurately grasped because the currents of a plurality of linear motors are complicatedly controlled. Also, the molten steel flow velocity distribution in the mold can be made uniform. Therefore, it is possible to surely reduce the inclusion defect and the crack defect of the slab.

FIG. 11 is a schematic explanatory view showing one embodiment of the electromagnetic stirrer of the present invention. In this device,
A low-frequency power source that generates a current of frequency f1 supplies power to a linear motor installed in one long-side mold of a plurality of electromagnetic stirrers, and a plurality of electromagnetic waves is generated from another low-frequency power source that generates a current of frequency f2. It is configured to supply power to a linear motor installed in the other long-side mold of the stirrer. In this way, by installing a plurality of electromagnetic stirrers, a plurality of slab cast pieces can be manufactured at the same time.

In carrying out the present invention, the difference in molten steel flow along the respective long side molds in the mold, or the difference in molten steel level near each short side wall caused by the difference in molten steel flow, was measured. The object of the present invention can also be achieved by adopting a configuration in which the current value applied to the linear motor along the long-side mold having a low flow velocity is controlled to be relatively increased. That is, by relatively increasing the value of the current flowing through the linear motor along the long-side mold on the low flow velocity side, the flow of the molten steel flow accelerated by this linear motor becomes faster, and the molten steel in both linear motors becomes faster. The flow can be made uniform. As a result, thrust can be automatically controlled to be in an appropriate range, and stagnation occurs by preventing an extreme imbalance in the molten steel flow rate caused by the difference in the frequency of the linear motor along each slab mold. Will disappear.

In the above description, "relatively increasing the current value" means, of course, increasing the current value applied to the linear motor along the long-side mold having the lower flow velocity.
It is also intended to include reducing the value of the current flowing through the other linear motor.

The object of the present invention can be achieved by such a configuration alone, but even when such a configuration is adopted, the frequencies f1 and f2 of the moving magnetic fields generated by both linear motors are set to the above (1), (2). ) Is satisfied, and it is also useful that the thrusts F1 and F2 satisfy the formula (3), whereby the effect of the present invention is more remarkably exhibited. .

Hereinafter, the present invention will be described in more detail with reference to Examples. The following Examples are not intended to limit the scope of the present invention, and any modification of the design of the present invention can be made without departing from the gist of the preceding and the following. Are included in the technical scope of.

[0043]

EXAMPLE 1 In the apparatus configuration shown in FIG. 1, the frequency f1 of the moving magnetic field generated by the linear motor installed in one of the long-side molds.
At a frequency of 2.5 Hz and a moving magnetic field frequency f2 generated by a linear motor installed on the other long-side mold of 3.5 Hz to perform electromagnetic stirring to produce a slab slab for a zinc-plated thin steel sheet. The occurrence of defects was investigated. At this time, the frequency f when the thrust acting on the molten steel becomes maximum
m is 2.9 Hz, and the pole pitch P is 0.45 m. Further, the frequencies f1 and f2 are set to be the same (f1
= F2 = 3.0 Hz) The magnetic stirrer was also investigated and the magnetic stirrer was not carried out.

The results are shown in FIG. The occurrence frequency of inclusion defects was indicated by an index (inclusion defect index) when 1 was set without electromagnetic stirring. As is clear from this result, it is understood that the electromagnetic stirring method of the present invention is extremely effective in reducing inclusion defects.

Example 2 In the apparatus configuration shown in FIG. 11, a linear motor installed in one long side mold of a plurality of electromagnetic stirrers is supplied with a current of 2.6 Hz from one low frequency power source, and When a linear motor installed in the other long-side mold in a plurality of electromagnetic stirrers is supplied with a current of frequency 3.0 from another low-frequency power source to perform electromagnetic stirring, and a slab slab for thick plates is manufactured. The occurrence of slab cracking was investigated. At this time, the frequency fm when the thrust acting on the molten steel becomes maximum is 2.8 Hz, and the pole pitch P is 0.
It is 40 m. Further, similarly to the above-mentioned Example 1, investigations were also conducted when the frequencies f1 and f2 were the same (f1 = f2 = 3.0 Hz) and the magnetic stirring was not performed.

The results are shown in FIG. The frequency of occurrence of slab cracking was indicated by an index (cast slab cracking defect index) where 1 was set without electromagnetic stirring. As is clear from these results, it is understood that the electromagnetic stirring method of the present invention is extremely effective in reducing slab crack defects.

Embodiment 3 In the apparatus configuration shown in FIG. 14, the frequency of each linear motor provided along each long side of the mold is set to f1.
= 2.9 Hz and f2 = 4.0 Hz. The level of molten metal on both short sides is measured every second every second by a thermocouple group embedded in the vicinity of the molten metal of each short-side mold during casting, and the molten metal level on both short sides is measured every 1 m of slab withdrawal length. The average value of the differences was calculated. In FIG. 14, for convenience of explanation, the linear motors are shown as the east side linear motor and the west side linear motor, respectively, and both short sides are shown as the south side and the north side, respectively.

The relationship between the average value of the molten metal level difference on both short sides and the current increase / decrease value of each linear motor is set in advance as shown in FIG. 15, and the molten metal level is determined for each 1 m of the slab drawing length. The current value of each linear motor was increased or decreased according to the average value of the differences. At this time, the current set value was calculated by adding the current increase / decrease value based on the average value of the molten metal level difference from 1 m before the cast slab drawing length to the present to the past current set value. However, the current set value this time is the current upper limit value (650
When it exceeds A), 650 A is energized, and when it is smaller than the current lower limit value set to 600 A, the lower limit value of 600 A is energized.

FIG. 16 shows the transition of the current value of each linear motor from the start of the current control until the slab is pulled out by 20 m, and FIG. 17 shows the transition of the average value of the molten metal level. as a result,
At the time of starting the current control, a current of 650 A is applied to each linear motor, and the average level of the molten metal is the frequency f1 =
The south short side corresponding to the stirring downstream side of the 4.0 Hz western linear motor was 15 mm lower than the north short side. This is considered to correspond to the fact that the molten steel flow velocity along the east long side mold is slower than that on the west side.

However, after the current control is started, the current value of the east side linear motor is gradually decreased, and as a result, the current of the west side linear motor having a slow molten steel flow velocity is relatively increased, and the level of the molten metal on the short sides of the north and south sides is relatively increased. The difference was gradually disappearing. After the slab of about 15 m was pulled out from the start of the control, the current values of both the east and west linear motors became almost constant, and the difference between the molten metal surface levels on the north and south short sides could be stabilized to almost zero.

[0051]

EFFECTS OF THE INVENTION The present invention is configured as described above, and it is possible to reduce the nonuniformity of the stirring flow rate due to the nonuniformity of the magnetic flux density distribution in the electromagnetic stirring in the continuous slab casting. The effect of reducing defects and cracking defects is improved, and slab cast pieces with few defects can be stably manufactured. In addition, since the linear motor of one long-side mold need only be supplied with one system of current (when three-phase AC current is to be supplied, power is supplied with three electric cables), it is divided into a plurality of parts in the long-side direction. As compared with the case of using a linear motor, the maintainability at the time of mold replacement is also high.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing a state of a molten steel flow in a mold in conventional slab continuous casting.

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 is a graph showing an example of a magnetic flux density distribution measurement result in the vicinity of a long side mold wall in a conventional electromagnetic stirrer.

FIG. 4 is a plan view showing a thrust distribution in molten steel in a conventional electromagnetic stirrer.

FIG. 5 is a graph showing an example of calculation results of molten steel agitation flow velocity distribution in a mold by a thrust acting on a molten steel by a conventional electromagnetic stirring device.

FIG. 6 is a graph showing an example of measurement results of magnetic flux density distribution near the long side mold wall by electromagnetic stirring of the present invention.

FIG. 7 is an explanatory view schematically showing an example of a thrust distribution acting on molten steel when the electromagnetic stirring method of the present invention is carried out.

FIG. 8 is a graph showing an example of calculation results of molten steel stirring flow velocity distribution in a mold when the electromagnetic stirring method of the present invention is carried out.

FIG. 9 is a graph showing the relationship between the frequency f1 of each linear motor and the thrust force.

FIG. 10 is a graph showing a calculation result of an average stirring flow velocity in the vicinity of each long-side mold when (F1-F2) / (F1 + F2) is changed.

FIG. 11 is a schematic explanatory view showing one form of an electromagnetic stirring device of the present invention.

FIG. 12 is a graph showing the effect of various electromagnetic stirring methods on the occurrence of inclusion defects in a zinc plated thin steel sheet.

FIG. 13 is a graph showing the effect of various electromagnetic stirring methods on the occurrence of crack defects in thick plate cast pieces.

FIG. 14 is a schematic explanatory view showing another embodiment of the electromagnetic stirrer of the present invention.

FIG. 15 is a graph showing an example of setting the current increase / decrease value of each linear motor in the third embodiment.

FIG. 16 is a graph showing the transition of the current value from the start of current control until the slab is pulled out by 20 m in Example 3.

FIG. 17 is a graph showing a transition of a molten metal level average value from the start of current control to pulling out a slab 20 m in Example 3.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiyuki Hosokawa 1-5-5 Takatsukadai, Nishi-ku, Kobe City Kobe Steel Research Institute, Kobe Steel, Ltd. (56) Reference JP-A-58-90358 (JP, A) ) JP-A-7-256412 (JP, A) JP-A-9-131046 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/04 311 B22D 11/115

Claims (8)

(57) [Claims] 1. A method of electromagnetically stirring molten steel discharged into a mold from a dipping nozzle installed in the center of a mold of a continuous casting machine for producing a slab slab having a rectangular cross section while accelerating by a linear motor and electromagnetically stirring the molten steel. At least one linear motor is provided along each of the long sides of the mold to accelerate molten steel in the vicinity of the long side of the mold in a substantially horizontal direction and in opposite directions, and a linear motor installed in one of the long sides of the mold. The frequency f1 (Hz) of the moving magnetic field generated by is different from the frequency f2 (Hz) of the moving magnetic field generated by the linear motor installed on the other long-side mold, and these frequencies f1 and f2 are as follows ( An electromagnetic stirring method in a continuous casting mold, characterized in that the operation is performed so as to satisfy the formula (1). 0.1 / P ≦ | f1-f2 | <5.0 (1) However, P represents the pole pitch (m) of the linear motor. 2. A method of electromagnetically stirring molten steel discharged into a mold from a dipping nozzle installed in the mold center of a continuous casting machine for producing a slab slab having a rectangular cross section while accelerating by a linear motor and electromagnetically stirring the molten steel. At least one linear motor is provided along each of the long sides of the mold to accelerate molten steel in the vicinity of the long side of the mold in a substantially horizontal direction and in opposite directions, and a linear motor installed in one of the long sides of the mold. The moving magnetic field frequency f1 (Hz) generated by is different from the moving magnetic field frequency f2 (Hz) generated by the linear motor installed on the other long-side mold, and The difference in molten steel flow along the molten steel, or the difference in molten steel level near each short side wall caused by the difference in molten steel flow, is measured, and the current value to the linear motor along the long side mold on the slow flow side is measured. phase An electromagnetic stirring method in a continuous casting mold, characterized in that the operations are performed so as to increase relative to each other. 3. A method of electromagnetically stirring molten steel discharged into a mold from a dipping nozzle installed in the center of the mold of a continuous casting machine for producing a slab slab having a rectangular cross section while accelerating with a linear motor. At least one linear motor is provided along each of the long sides of the mold to accelerate molten steel in the vicinity of the long side of the mold in a substantially horizontal direction and in opposite directions, and a linear motor installed in one of the long sides of the mold. The frequency f1 (Hz) of the moving magnetic field generated by is different from the frequency f2 (Hz) of the moving magnetic field generated by the linear motor installed on the other long-side mold, and these frequencies f1 and f2 are as follows ( While satisfying the formula (1), the difference in molten steel flow along the respective long-side molds during casting, or the difference in molten steel level near each short-side wall caused by the difference in molten steel flow is measured, and the flow velocity is An electromagnetic stirring method in a continuous casting mold, characterized in that the linear motor along the slow-side long side mold is operated such that the value of the current flowing through the linear motor is relatively increased. 0.1 / P ≦ | f1-f2 | <5.0 (1) However, P represents the pole pitch (m) of the linear motor. 4. When the frequency when the thrust acting on the molten steel becomes maximum is fm, the frequency fm and the frequencies f1 and f2 are operated so as to satisfy the following equation (2). 3. The electromagnetic stirring method according to any one of 3 above. f1 <fm <f2 (2) 5. A thrust amount F1 applied to molten steel by a linear motor installed on one long side mold and a thrust amount F2 exerted on molten steel by a linear motor installed on the other long side mold are as follows: The electromagnetic stirring method according to any one of claims 1 to 4, which is operated so as to satisfy expression (3). | F1-F2 | / (F1 + F2) ≦ 0.05 (3) 6. A structure in which molten steel discharged into a mold from a dipping nozzle installed in the center of a mold of a continuous casting machine for manufacturing a slab slab having a rectangular cross section is electromagnetically stirred while being accelerated by a linear motor. In the apparatus described above, at least one linear motor is provided along each of the long sides of the mold to accelerate molten steel in the vicinity of the long side of the mold in a substantially horizontal direction and in opposite directions. F1 of the moving magnetic field generated by the linear motor installed in the long side mold
(Hz) and the frequency f2 (Hz) of the moving magnetic field generated by the linear motor installed on the other long-side mold are different from each other, the electromagnetic stirrer in the continuous casting mold. 7. An electromagnetic stirrer configured to install a plurality of electromagnetic stirrers according to claim 6 and simultaneously manufacture a plurality of slab cast pieces. 8. A low-frequency power source that generates a current of frequency f2 while supplying power to a linear motor installed in one long-side mold of a plurality of electromagnetic stirrers from a low-frequency power source that generates a current of frequency f1. 8. The electromagnetic stirrer according to claim 7, which is configured to supply power to a linear motor installed in the other long-side mold of the plurality of electromagnetic stirrers.