JPH05212511A - Method and device for improving internal quality in continuously cast slab - Google Patents

Abstract

PURPOSE:To restrain the formation of equiaxed crystal, prevent the deterioration of quality caused by inclusions and obtain a cast slab having excellent internal quality by acting magnetic field to molten steel in a mold, stirring, acting static magnetic field to the molten steel at downstream side from the magnetic field acting position and braking the descending flow according to the stirring. CONSTITUTION:In addition to an electromagnetic stirring device 3 in a mold and an electromagnetic stirring device 6 for secondary cooling zone, an electromagnet 4 is newly arranged just below the mold and Tesla static magnetic field is added in the thickness direction of a cast slab and the braking force is given to the descending flow developed with the stirring in the mold. The stirring is given at 40-50cm/sec molten steel flow speed in the mold and at 20-50cm/sec of the electromagnetic stirring in the secondary cooling zone. Therefore, to the descending flow of the molten steel generated from action of a rotating or shifting magnetic field generating device 3, by giving the static magnetic field in the direction crossing at the right angle just below the descending flow with the static magnetic field generating device 4, the braking force can be acted to the descending molten steel flow.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for improving the internal quality of carbon steel slabs produced by continuous casting.

[0002]

2. Description of the Related Art Conventionally, as a method for accelerating the equiaxed crystallization of a solidification structure in a continuous casting method, there is electromagnetic stirring in a mold, a secondary cooling zone, and a molten steel solidification final stage (Japanese Patent Application No. 58-25).
1455).

The effect of electromagnetic stirring is described in Iron and Steel 1987 (1981) No. 8 P1287 and P1297.

In particular, it has been generally recognized that electromagnetic stirring in the mold has a remarkable effect on the equiaxed crystallization of the solidification structure and the prevention of deckle formation in the mold.

However, even when these electromagnetic stirrings are applied, the equiaxed crystal ratio of the slab greatly varies in steel types in which equiaxed crystal formation is difficult, and the target equiaxed crystal ratio is often not secured sufficiently. ..

In both the rotary type electromagnetic stirrer and the moving type electromagnetic stirrer, the molten steel flow horizontally driven by the electromagnetic force changes its direction at the inner wall of the mold, causing a flow upstream and downstream of the strand.

The swelling of the molten metal surface due to the flow toward the upstream is a phenomenon well known to operators, while the flow toward the downstream is caused by the columnar crystal structure observed in the longitudinal section of the slab of meniscus. It is an event that can be recognized from the fact that it grows to the side.

When the present inventors analyzed the relationship between equiaxed crystallization of solidification structure and quality of inclusions and operating conditions, the larger the inclination of the columnar crystal structure observed in the longitudinal section of the slab toward the upstream side, the greater the castability. It was confirmed that the equiaxed crystal ratio of the piece decreased and the number of inclusion defects detected in the center of the steel bar product by ultrasonic flaw detection tended to increase.

In particular, it is worth noting that if the columnar crystals have different inclinations in a slab cast under almost the same operating conditions, there will be a large difference in the equiaxed crystallization of the slab and the inclusion level at the center of the slab. ..

Such an increase in the inclination of the columnar crystal structure of the longitudinal section of the cast slab to the upstream side indicates that the flow velocity of the downflow existing at the tip of the columnar dendrite during solidification increased.

Therefore, from the above, the downward flow induced by electromagnetic stirring in the mold has a bad influence on the equiaxed crystallization of the solidification structure and the quality of inclusions in the center of the slab, and the downward flow is severe. Then, it is judged that the equiaxed crystallization is hindered and the inclusion level is lowered.

[0012]

DISCLOSURE OF THE INVENTION The present invention suppresses equiaxed crystallization of a solidification structure by a downward flow induced by electromagnetic stirring in a mold when applying electromagnetic stirring in a mold by a continuous casting method to produce a slab. It is an object of the present invention to provide a method for producing an excellent quality ingot slab that prevents deterioration of the quality of inclusions and inclusions, achieves sufficient equiaxed crystallization, and has less segregation and porosity.

[0013]

There are the following two methods and devices as means for solving the above problems.

First, when a slab is manufactured by the continuous casting method, an electromagnetic force acts on the molten steel in the mold to stir it, and an electromagnetic force is exerted downstream from the stirring position to dampen the molten steel flow. A method and apparatus for improving the internal quality of a continuous cast slab, which is characterized by the above.

The other is that when a cast slab is manufactured by the continuous casting method, the molten steel in the mold is stirred by applying an electromagnetic force, and the molten steel flow is damped by applying an electromagnetic force downstream from the stirring position. Further, the present invention provides a method and apparatus for improving the internal quality of a continuous cast slab, which comprises stirring the molten steel by applying an electromagnetic force to the molten steel at a position downstream thereof.

[0016]

The present inventors have found that the downward flow induced by electromagnetic stirring in the mold suppresses equiaxed crystallization of the solidification structure and deteriorates the quality of inclusions, and solves the problem caused by this downward flow. Various studies were made on the method.

The reason why the equiaxed crystallization of the solidification structure is inhibited by the downward flow generated by stirring by electromagnetic stirring in the mold having a remarkable effect of promoting equiaxed crystallization is that the molten steel flow at a relatively high temperature near the meniscus is stirred. Is believed to invade into the strands along the solidification shell, at which time the supercooling degree of the solidification front necessary for nucleation of equiaxed crystals disappears and the embrio that becomes the nucleus of equiaxed crystals is redissolved. ..

On the other hand, the level of inclusions at the center of the slab deteriorates due to the downward flow generated by electromagnetic stirring in the mold, because the inclusions that have entered through the nozzle discharge holes and the entrained powder are separated by levitation in the vicinity of the meniscus. It is presumed that it is not accompanied by the descending flow and penetrates deeply into the strand.

It is considered that the problems such as suppression of equiaxed crystallization and reduction of inclusion level due to this downflow can be solved by suppressing the downflow caused by electromagnetic stirring in the mold by some method. As one method, the method of applying an electromagnetic force so as to brake the downward flow at a position downstream of the stirring position in the mold was conceived.

Conventionally, it has been attempted to provide an electromagnet on a mold to brake a nozzle jet (Sugizawa et al .: Materials and Processes, CAMP-ISIJ Vol. 4 (1981)-
1281), the purpose is to prevent fluctuations in the molten metal surface of the meniscus and to prevent entrainment of non-metallic inclusions, and to prevent the entry of a high temperature downward flow due to electromagnetic stirring in the mold, and not to promote equiaxed crystallization of cast slabs.

Further, when an electromagnet for damping the downward flow of molten steel is installed in the mold, it is difficult to apply electromagnetic stirring in the mold, which has a remarkable effect on equiaxed crystallization of the slab. It is expected that equiaxed crystals required for quality improvement cannot be stably secured.

FIG. 1 shows an embodiment of the downflow suppressing method.

A static magnetic field is applied by the static magnetic field generator 4 to the downward flow of the molten steel generated by the action of the rotating or moving magnetic field generator 3 in a direction perpendicular to the downward flow of the molten steel, thereby exerting a braking force on the descending molten steel flow. Can be made

Regarding the direction in which the static magnetic field is applied, the direction in which the static magnetic field is applied in the direction parallel to the short side of the slab is considered to be the most effective because the magnetic field is less attenuated.

Further, if it is too close to the electromagnetic stirring position in the mold, the momentum of the downward flow is large, and it is necessary to considerably increase the magnetic field strength for braking it, and the position where the braking force is applied is in the mold. Even if it is too far from the stirring position, the solidified shell becomes thick and the damping in the shell becomes large, so that the magnetic field strength necessary for damping the molten steel flow is considerably increased.

Therefore, as for the position to apply the braking force, the most appropriate position should be selected in combination with both.

Further, by applying the braking force in several steps in the strand direction, more reliable suppression of the downward flow can be realized.

Further, even if a moving magnetic field is applied so as to drive molten steel in the direction opposite to the downward flow, that is, toward the upstream side and the downward flow is attenuated, the same effect as when the static magnetic field is applied is expected. it can.

Further, the method of using a moving magnetic field and the method of suppressing a downward flow by adding a static magnetic field may be combined.

When the above-mentioned static magnetic field or moving magnetic field is applied to suppress the downward flow induced by electromagnetic stirring in the mold, the supercooling degree of the molten steel in the strand is not lost by the high temperature flow, and is maintained. Since the embryo, which is the nucleus of the equiaxed crystal, is not redissolved, the equiaxed crystallization of the solidified structure further progresses, and the downward flow is suppressed, so that it is expected that the inclusions penetrating into the inside of the strand can be greatly reduced.

Further, when the molten steel is stirred around the casting shaft in the secondary cooling zone again under the condition that there is almost no downward flow from the meniscus and is not affected by the high temperature flow from the meniscus, the solidification front The solute-enriched layer, which had decreased the degree of supercooling of molten steel, was diluted by the mother molten steel, the degree of supercooling of molten steel on the solidification front increased, and the area of supercooling expanded, resulting in equiaxed crystallization of the solidification structure. Is further promoted.

[0032]

EXAMPLES The present invention will be further described based on examples.

In order to confirm the effect of the present invention, S48C which is a refractory equiaxed steel grade was cast using a curved continuous casting machine having a slab cross section size of 162 mm × 162 mm.

In the present embodiment, the tundish molten steel has a superheat of 25 to 45 ° C. and a casting speed of 2.0 to 2.5 m.
The range was / min.

FIG. 1 shows an embodiment of the present invention. In addition to the existing electromagnetic stirrer 3 in the mold and the electromagnetic stirrer 6 in the secondary cooling zone, an electromagnet 4 is newly provided immediately below the mold 1 in the thickness direction of the cast piece.
A static magnetic field of Tesla was applied to apply a braking force to the downward flow generated by stirring in the mold.

With stirring in the mold, the molten steel flow rate is 40 to 50 cm.
/ Sec, the electromagnetic stirring in the secondary cooling zone was applied at 20 to 30 cm / sec.

In order to understand the effect of improving the quality of the cast product by electromagnetically damping the downward flow induced by the electromagnetic stirring in the mold,
Braking force was applied with one strand of the continuous casting machine, and the other strand was cast without cutting the electromagnet to give the braking force.

Further, the level at which the electromagnetic stirring in the secondary cooling zone is applied when the electromagnetic stirring in the mold and the electromagnetic braking are applied to the strand of the present invention is also set.

The solidification structure was evaluated by etch printing in the longitudinal section of the slab, and the inclusion level in the center of the slab was evaluated by ultrasonic flaw detection in the center of the rolled steel product.

FIG. 2 shows the results of an investigation on the relationship between the molten steel heating degree in the tundish and the equiaxed crystal ratio on the upper surface side of the slab defined by the thickness ratio.

From this figure, it is seen that when electromagnetic braking is added at the lower end of the mold than when only electromagnetic stirring in the mold is added, electromagnetic stirring in the secondary cooling zone is further added The equiaxed crystal ratio is improved.

In the two methods of the present invention, a remarkable effect of promoting equiaxed crystallization is recognized especially on the high superheat side as compared with the case of only electromagnetic stirring in the mold.

When the two-stage electromagnetic stirring in the mold and the secondary cooling zone is applied, the equiaxed crystal ratio on the upper surface side is almost equal to or slightly improved from the level when only electromagnetic stirring in the mold is used. Therefore, it can be seen that the method of the present invention has a considerably excellent effect of promoting equiaxed crystallization even when compared with the two-stage electromagnetic stirring.

FIG. 3 shows the results of investigation by ultrasonic flaw detection on inclusions in the center of the product obtained by rolling a cast steel bar.

The product inclusion defect index on the vertical axis of this figure is an index corresponding to the ratio of products in which inclusions having a size exceeding the allowable level exist. The larger the index, the worse the inclusion level. Shows.

From the figure, the level of inclusions at the center of the product is improved compared to the case where the present invention is applied and the downward flow due to electromagnetic stirring in the mold is suppressed.

[0047]

As described in detail above, by applying the present invention, the equiaxed crystal ratio on the upper surface side of the carbon steel slab is greatly improved and the variation is reduced, resulting in reduction of center segregation and center porosity. However, as a result of the inclusion level at the center of the slab also increasing, the quality of the slab is greatly improved.

On the other hand, according to the present invention, it is not necessary to severely lower the superheat degree of TD molten steel in order to secure the target equiaxed crystal ratio, and nozzle clogging or inclusions generated in the low temperature casting which has been conventionally carried out to secure equiaxed crystal. Level deterioration is also avoided, and further stable operation and improved slab quality are achieved.

[Brief description of drawings]

FIG. 1 is an elevational view showing an embodiment of the present invention in an actual machine test.

FIG. 2 is a diagram showing the results of an examination of the relationship between the degree of heating of molten steel in a tundish and the equiaxed crystal ratio of the upper surface of the slab, showing the effect of promoting equiaxed crystallization of the present invention.

FIG. 3 is a result of investigating the inclusion level at the center of the product, showing the effect of improving the inclusion level at the center of the slab according to the present invention.

[Explanation of symbols]

 1 Tundish 2 Mold 3 Electromagnetic stirrer in mold 4 Electromagnet for static magnetic field addition 5 Secondary cooling zone 6 Electromagnetic stirrer for secondary cooling zone

Claims (4)

[Claims] 1. When manufacturing a slab by a continuous casting method, a magnetic field is applied to the molten steel in the mold to generate a stirring flow around the casting axis in the molten steel, and a magnetic field is applied to the molten steel at a position downstream from the magnetic field acting position. Is applied to stop the downward flow associated with the stirring at the position, thereby improving the internal quality of the continuous cast slab. 2. When producing a slab by a continuous casting method, a magnetic field is applied to the molten steel in the mold to generate a stirring flow around the casting axis in the molten steel, and a magnetic field is applied to the molten steel at a position downstream from the magnetic field acting position. Is applied to brake the downward flow associated with the stirring at the position, and a magnetic field is applied to the molten steel at a position downstream from the braking magnetic field application position to generate an agitated flow around the casting axis in the molten steel. Method for improving the internal quality of continuous cast slabs. 3. In a continuous casting machine, a magnetic field generator for generating a stirring flow around the casting axis in molten steel in a mold, and a downward flow of molten steel accompanying the stirring at a position downstream of the position of the stirring magnetic field generator. A molten steel flow control device for a continuous casting machine, comprising: 4. In a continuous casting machine, a magnetic field generator for generating a stirring flow around the casting axis in molten steel in a mold, and a downward flow of molten steel accompanying the stirring at a position downstream of the position of the stirring magnetic field generator. A magnetic field generator for braking the
A molten steel flow control device for a continuous casting machine, further comprising a magnetic field generation device for generating a stirring flow around the casting axis in the molten steel at a position downstream of the braking magnetic field generation device position.