JPH084885B2 - In-mold electromagnetic stirring method for continuous casting of slabs - Google Patents

Description

The present invention relates to a method for electromagnetic stirring in a mold in continuous casting of a slab.

[Prior Art] Continuous casting of a slab is carried out by cooling molten steel injected from a tundish through a dipping nozzle into a mold from the periphery by a mold wall to withdraw while forming and developing a solidified shell. At this time, as shown in FIG. 8, the molten steel injected from the tundish (not shown) is immersed in the casting nozzle 15 in the mold.
The molten steel flow 17 flowing out from the discharge hole 16 of
Upon colliding with 18, a downward flow 19 is generated, and this downward flow 19 becomes the main flow of the molten steel flow and deeply penetrates into the slab S.

According to such a continuous casting method, inclusions and bubbles such as argon gas caught in the downward flow 19 are trapped at the interface of the solidified shell and exposed as defects during rolling to form inclusion accumulation zones, which causes a problem. It is known.

And in recent years, in order to improve such a problem, the continuous casting method which applied electromagnetic stirring is performed.

For example, Japanese Patent Laid-Open No. 60-37251 (Japanese Patent Application No. 58-145765)
In the publication, an electromagnetic stirrer is separately provided on the left and right within a wide surface of a mold, and the stirring thrust direction of the electromagnetic stirrer that is split left and right is used by switching according to the steel type and casting conditions.
A method for improving the quality of the slab is described. But,
In this in-mold electromagnetic stirring method, since the electromagnetic stirring device is provided separately in the wide surface of the mold, the stirring pattern of molten steel increases, and the advantage of being able to select the stirring flow according to the steel type and casting conditions is However, it is not clear what kind of slab the stirring flow is intended to improve the quality of the slab, and the downflow that deeply penetrates into the slab S described above.
Since it does not suppress 19, there is a problem that inclusions entrained in the descending flow 19 and bubbles such as argon gas are not reduced.

On the other hand, the present applicant has conducted intensive research to solve the above problems, and provided an electromagnetic stirring device for generating thrust in the drawing direction of the slab within the wide surface of the mold, and the electromagnetic stirring in the same direction as the drawing direction of the slab. It was found that the inclusion accumulation zone is improved by applying thrust, and the application (Japanese Patent Application No. 63-24
No. 3639). However, it was then found that the inclusion accumulation zone may not be improved even by the electromagnetic stirring method in the mold according to this prior application, and in particular, in the case of using a variable width mold, there was a case where it was not improved depending on the slab size.

Therefore, as a result of further intensive research by the applicant, even when an electromagnetic stirrer for generating thrust in the drawing direction is provided in the wide surface of the mold and continuous casting is performed, the distribution of the stirring thrust in the wide width direction of the slab by the electromagnetic stirrer Does not correspond to the slab width, it is not possible to suppress the downflow 19 deeply penetrating into the slab S described above, and it is found that inclusions entrained in the downflow 19 and bubbles such as argon gas are not reduced. By improving the electromagnetic stirrer that generates thrust in the drawing direction of the slab on at least the inner wide surface of the inner and outer wide surfaces of the slab mold, the thrust in the drawing direction by the electromagnetic stirrer is adjusted to the left and right of the wide surface of the cast slab. The inventors have invented a method of electromagnetic stirring in a mold in continuous casting of a slab which is operated within a range excluding a predetermined length from the side edge of the above, and filed as Japanese Patent Application No. 1-164875.

[Problems to be Solved by the Invention]

By the way, applying the electromagnetic stirring method in a mold in continuous casting of a slab shown in Japanese Patent Application No. 1-164875 to a variable width mold, in a range excluding a predetermined length from the left and right side ends of the wide surface of the slab to be cast. In order to apply a certain amount of pulling force in the drawing direction, for example, in a variable width mold in the range of 800 to 1630 mm, an electromagnetic stirring device is installed at symmetrical positions of the width center axes of the inner and outer wide surfaces of the variable width mold M. 4 units each, 8 units in total (refer to Fig. 1) are installed. All the coils a, a'-d, d'of the electromagnetic stirrer are used up to the slab width of 1630 ~ 1100 mm, and the slab width is 1100 ~. Up to 800 mm, it is necessary to use only coils b, b ', c, c'for casting. Furthermore, the projecting flow 17,1 in the mold M
7'is not always uniform on the left and right, and is often non-uniform, so coils a, a ', b, b'and coils c, c', d, d'use different thrusts in the pulling direction. However, in order to satisfy such a condition, it is necessary to separately provide a power source for supplying a current to the coils a ′ to d ′ and control each coil independently. Therefore, the applicant of the present invention, as shown in FIG.
An example was tried in which a power supply device in which the transformer 3 and the cycloconverter 4 were combined was wired for each of the coils a to d '. However, even when wiring is performed as shown in this figure, the phases of the currents are not uniform among the coils a to d'due to the nonuniformity of the characteristics of the power supply devices and the slight difference in the frequency control signals. As shown in the figure (b), the magnetic flux is concentrated on the mold wall surface, and the magnetic flux leaks to the adjacent coil, and the magnetic flux distribution becomes uneven, and the required magnetic flux density cannot be obtained at the center of the mold thickness. , The downward flow 19 that deeply penetrates into the slab S described above.
A desired stirring flow for suppressing (see FIG. 8) could not be obtained, and it often occurred that inclusions entrained in the descending flow 19 and bubbles such as argon gas were not sufficiently reduced. Moreover, many power supply devices are required, and the equipment is large, which is not economical.

The present invention has been made based on the above-mentioned circumstances, and an object thereof is to provide at least two pairs of electromagnetic stirring devices that generate thrust in the slab drawing direction at symmetrical positions of the width center axes of the inner and outer wide surfaces. Using the variable width mold for slabs, the thrust in the pulling direction of the electromagnetic stirrer installed on both the inner and outer wide surfaces was removed by a predetermined length from the left and right side edges (narrow surface) of the wide surface of the slab to be cast. When acting on the range, while aligning the phase of the current flowing through each coil of each electromagnetic stirring device,
Obtain the required magnetic flux density at the center of the mold thickness, suppress the penetration depth of the molten steel flow that penetrates into the slab along the narrow surface of the mold,
An object of the present invention is to provide a method for electromagnetic stirring in a mold in continuous casting of a slab, which prevents inclusions or bubbles such as argon gas entrained in the molten steel flow from deeply penetrating into the slab.

[Means for solving the problem]

In order to achieve the above object, the electromagnetic stirring method in the mold in the continuous casting of the slab according to the present invention, the symmetric position of the width center axis of the inner and outer wide surfaces of the variable width mold for the slab, thrust in the drawing direction of the slab. At least two pairs of electromagnetic stirrers to be generated are provided, and the polarities of the magnetic poles of the coils of the electromagnetic stirrer are set to the adjacent coils so that the magnetic poles of the coils facing each other with the mold sandwiched are different poles. On the other hand, electromagnetic stirring of molten steel is performed by aligning the phases of the magnetic poles so that the magnetic poles have the same polarity.

[Work]

In the configuration of the present invention, the polarities of the magnetic poles of the coils of the electromagnetic stirring device are set such that, for coils facing each other with the mold sandwiched, for adjacent coils of the same wide surface, each magnetic pole is different. Since the phases of the magnetic poles are aligned so that the magnetic poles are the same pole, there is no problem such as leakage of magnetic flux to the adjacent coil and the magnetic flux distribution becomes relatively uniform, so the magnetic flux penetrates the mold and the thickness of the mold ( Since it passes through to the center (between the inner and outer wide surfaces), a large magnetic force can be applied to the molten steel flow flowing through the thickness center of the mold, which allows the molten steel flow to enter the slab along the narrow surface of the mold. The depth of penetration can be suppressed, and it is possible to prevent bubbles such as inclusions and argon gas caught in the molten steel flow from entering deep inside the slab.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory view of an apparatus applied to an electromagnetic stirring method in a mold in continuous casting of a slab of the present invention, and FIG. 2 is
It is a wiring diagram of the electromagnetic stirring device shown in FIG.

In the figure, M is a variable width mold, and at the position symmetrical to the width center axis of the inner and outer wide surfaces 1, 1'of the variable width mold M, an electromagnetic stirring device for generating thrust in the drawing direction of the slab S is provided. The coils a, a'to d, d'are provided so as to act from the left and right side ends (narrow surfaces) 2 of the wide surface of the slab to be cast toward the center except a predetermined length L. The coils a, a'-d, d'of the electromagnetic stirrer include transformers 3a-3d for the coils a, a ', coils b, b', coils c, c ', and coils d, d'. And 2-phase (u-phase, v-phase) cycloconverter 4a ~
A two-phase power supply device composed of 4b is wired, while one frequency setting device 5 is wired to the cycloconverters 4a to 4b. Further, the winding structure for the coils a, a 'to d, d' is, for example, as shown in FIG. 3, taking the coils a, a ', b, b' as an example. Use the coil a,
For a ', b, b', the top pole and the third pole of each coil are opposite poles, and the top pole and the third pole of the inner coils a and b or the outer coils a'and b'are The u-phase winding is formed so as to have the same pole, and the uppermost and third poles of the inner coil a, b and the outer coil a ′, b ′ facing each other have opposite poles. Although not shown in the drawings for the second and bottom poles, the v-phase winding is performed in the same manner as above.

Since the electromagnetic stirrer in the mold has the above-mentioned configuration, the frequency set value set in the frequency setting device 5 is the same signal value at the same timing as each cycloconverter.
The signals can be transmitted to 4a to 4b, so that the currents flowing through the u-phase and v-phase of the respective cycloconverters 4a to 4b are in the same phase and can be synchronized. That is, as shown in FIG.
The coils facing each other across the mold have different magnetic poles, and adjacent coils on the same wide surface have the same magnetic poles, so magnetic flux penetrates the mold, and the magnetic flux has a 90 ° phase difference. Cycloconverter with u-phase and v-phase AC power supplies
Due to 4a to 4b, for example, the N pole (or S pole) at the uppermost stage shown in FIG. 3 is repeatedly moved to the second stage, the third stage, and the lowermost stage, which are the slab drawing directions. With the movement of the molten steel, the molten steel in the mold also receives the thrust, and is moved and stirred in the drawing direction. FIG. 4 (a) shows a magnetic field distribution taking the coils a and a ′ as an example when the synchronization is achieved in this way. For comparison, FIG. 4B shows the magnetic field distribution when the conventional synchronization is not achieved, and the magnetic flux densities of the two in the mold thickness direction are shown in comparison with FIG. As is clear from these figures, in the former, the magnetic flux is sufficiently passed to the center of the mold thickness as compared with the latter, and its magnetic flux density is sufficiently large at the center of the mold thickness, which results in the narrow surface of the mold. The penetration depth of the molten steel flow penetrating into the slab along can be suppressed, and it is possible to prevent bubbles such as inclusions and argon gas caught in the molten steel flow from deeply entering the slab.

FIG. 6 shows the molten steel flow in the mold discharged from the discharge hole 8 of the immersion nozzle 7 when the size of the variable width mold M is set to 230 mm × 800 mm based on the electromagnetic stirrer in the mold having the above structure. The flow of molten steel in the mold shown in FIG.
The molten steel flow in the mold shown in Fig. 6 (b) is a comparison when the cycloconverters 4b and 4c are used and the coils b and b'and the coils c and c'are energized at 400 amps and 5 Hz. This is the flow of molten steel when the magnetic stirring was not used. As is clear from the figure, in the former case, as compared with the latter case, the discharge flow velocity of the molten steel flow 9 discharged from the discharge hole 8 of the immersion nozzle 7 toward the narrow surface direction is sufficiently reduced, and the variable width mold M Molten steel flow penetrating into slab S along a narrow surface
The penetration depth of 10 is suppressed.

FIG. 7 shows the molten steel flow in the mold discharged from the discharge hole 8 of the dipping nozzle 7 when the mold size of the variable width mold M is set to 230 mm × 1640 mm based on the electromagnetic stirrer in the mold having the above configuration. The molten steel flow in the mold shown in Fig. 7 (a) uses the cycloconverters 4a to 4b and the coils a, a '
And the coils d and d ′ are 200 amps, 5 Hz, the coils b and b ′ are 400 amps to the coils c and c ′, and 5 Hz is the molten steel flow. The molten steel flow in the mold shown in) is performed for comparison.The former is the molten steel flow in the case where the energization conditions are the same as above and the current phase of each coil is not synchronized, and the latter is electromagnetic stirring. This is the flow of molten steel when not used. Also in FIG. 7,
As is clear from the figure, the molten steel flow shown in FIG. 7 (b) is more compared with the molten steel flow shown in FIG. 7 (b) than the molten steel flow shown in FIG. 7 (c). In the molten steel flow shown in FIG. 7 (a), the discharge flow velocity of the molten steel flow 9 discharged from the discharge holes 8 of the immersion nozzle 7 toward the sandwiching surface direction is sufficiently reduced, and the molten steel flow is narrowed on the narrow surface of the variable-width mold M. The penetration depth of the molten steel flow 10 that penetrates into the slab S is suppressed.

Since FIGS. 6 and 7 are bilaterally symmetric when viewed from a wide surface, only the left half is shown and described.

〔The invention's effect〕

As described above, according to the electromagnetic stirring method in the mold in the continuous casting of the slab according to the present invention, the thrust in the pulling direction of the electromagnetic stirring device provided on the inner and outer wide surfaces of the variable width mold for the slab is the wide surface of the slab to be cast. Since it can be operated reliably within a range excluding a predetermined length from the left and right side ends (narrow surface) of the mold, the penetration depth of the molten steel flow entering the slab along the narrow surface of the mold is suppressed, and Entrapped inclusions and bubbles such as argon gas are prevented from deeply penetrating into the slab, and a high quality slab is obtained.

[Brief description of drawings]

FIG. 1 is an explanatory view of an apparatus applied to an electromagnetic stirring method in a mold in continuous casting of a slab of the present invention, and FIG.
FIG. 3 is a wiring diagram of the electromagnetic stirrer shown in the figure, FIG. 3 is an explanatory diagram of a coil winding structure of the electromagnetic stirrer according to the present invention, FIG. 4 is an explanatory diagram of a magnetic field distribution state of the coil, and FIG. FIG. 6 is a diagram showing the magnitude of magnetic flux density in the thickness direction of the mold, FIGS. 6 to 7 are explanatory diagrams showing the flow of molten steel in the mold, FIG. 8 is an explanatory diagram of the prior art, and FIG. 9 is a comparative example. 2 is a wiring diagram of the electromagnetic stirring device of FIG. 1,1 '…… Wide surface of mold 2 …… Narrow surface of mold 3a ～ 3d …… Transformer 4a ～ 4b …… Cycloconverter 5 …… Frequency setting device 6 …… Iron core 7 …… Immersion nozzle 8 …… Discharge hole 9: Molten steel flow discharged from the immersion nozzle 10: Molten steel flow penetrating into the slab along a narrow surface a to d '... Electromagnetic stirrer coil M: Variable width mold S: Slab

Claims (1)

[Claims] 1. At least two pairs of electromagnetic stirrers for generating thrust in the pulling direction of the slab are provided at symmetrical positions of the width center axes of the inner and outer wide surfaces of the variable width mold for the slab in the bending type continuous casting machine, The polarities of the magnetic poles of the coils of the electromagnetic stirrer should be such that the magnetic poles of the coils facing each other with the mold sandwiched are different from each other, and the magnetic poles of the coils adjacent to each other are the same. An electromagnetic stirring method in a mold for continuous casting of a slab, characterized in that molten steel is electromagnetically stirred with the magnetic poles aligned in phase.