JP2004202584A - Continuous casting method for high-quality cast piece - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for a high-quality cast piece which prevents defects and inner cracks brought about by inclusions in the surface layer and internal part of a cast piece and, at the same time, avoids the significant alteration of a continuous casting equipment and can easily be realized at low cost. <P>SOLUTION: In the continuous casting method for the high-quality cast piece which liquid steel 11 is introduced into a mold 13 through a dipping nozzle 15 and continuously withdrawn downward while it is solidified with the mold 13, a vertical part 19 is provided below a meniscus 20 of the liquid steel 11, and also a stream 22 circling along a peripheral wall near the meniscus 20 of the liquid steel 11 is provided through an electromagnetic stirring device 21 installed at a long side of the mold 13. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

The present invention relates to a continuous casting method for producing high-quality slabs such as slabs and blooms by casting molten steel in a mold.

In the conventional continuous casting method of molten steel, molten steel adjusted to a predetermined component is poured from a tundish into a mold having a water-cooled structure through a dipping nozzle, and the molten steel is gradually cooled from the surroundings by the mold and solidified. Form a shell. After the solidified shell is formed, cooling water is sprayed by a plurality of support rolls arranged below the mold and a secondary cooling water nozzle attached to the slab support roll group to promote solidification and solidify the cast metal. The strip is pulled out.
The secondary cooled slab is straightened at the end of the group of curved slab support rolls, cut into a predetermined size, and supplied to a subsequent step.
In this continuous casting method, it is important to cast a slab of excellent quality without defects, and it is necessary to improve the cleanliness by preventing the inclusion of inclusions in the molten steel to be cast and promoting the floating removal of the inclusions. In addition, it has been practiced to prevent internal cracks and the like of a slab during a casting process.
As for the inclusions that degrade the quality of the slab, as shown in Patent Document 1, a magnetic stirrer is installed outside the long mold piece, and the flow swirling inside the mold along the inner peripheral wall of the mold is described. It has been practiced to cast slabs having good surface cleanliness by forming inclusions to reduce inclusions trapped in the solidified shell.
In addition, for the floating removal of inclusions and internal cracks in cast slabs, as in Patent Document 2, a vertical portion is formed directly below the mold including the mold, and the inclusions in the molten steel are floated by the vertical portion. At the same time, the connection from the vertical portion to the bent portion is gradually and gradually bent by correcting the curvature at multiple points to relieve the strain on the slab and prevent the occurrence of cracks and the like inside the slab. .

JP-A-58-100955 JP-A-6-134558

However, in the method of casting a slab by forming a swirling flow inside the mold by installing an electromagnetic stirrer on the long mold piece described above, the inclusion in the solidified shell that solidifies at the beginning is reduced, and the surface layer is cleaned. Although a high degree of slab can be cast, there is a concern that conversely, inclusions may be concentrated inside the molten steel and cause internal defects of the slab. Inclusions concentrated inside the molten steel tend to be trapped at the solidification interface of the solidified shell inside and float on the upper inner surface of the bent portion of the curved slab support roll group when floating from below. The cast slab in which the inclusions are accumulated has a problem that it is turned into waste as a quality rejected material, thereby lowering the yield.
In addition, the inclusions in the molten steel are lifted and removed by the vertical part, and the connection from the vertical part to the bending part is relaxed by multi-point bending correction to reduce the straightening stress inside the slab and simultaneously prevent the slab internal cracking etc. In this method, the curvature of the bent portion of the group of slab support rolls is increased in order to correct the slab at multiple points. As a result, when applying the vertical part and multipoint straightening to the existing curved continuous casting equipment, it is necessary to update the mold and the mold support device, the slab support roll group, the building, etc. Increase.
Further, the vertical portion for floating and removing the inclusions in the molten steel needs to be 2.5 m or more, and the above problem becomes more remarkable as the vertical portion becomes larger. In addition, even in the case of multi-point curvature correction, since the correction is performed at a position where the critical strain of the slab is reduced due to the growth of the solidified shell, an internal crack is generated in a fragile portion inside. This has the same problem in the case of a general vertical bending type continuous casting apparatus that corrects a curve from a vertical portion of 2.5 m to a predetermined curvature.
In particular, in the case of curved continuous casting equipment, large-scale modification of the equipment is avoided, cost is maintained, defects caused by the surface layer and internal inclusions of the slab are eliminated, and at the same time internal cracks are generated. It is difficult to prevent and cast.

The present invention has been made in view of such circumstances, and prevents defects and internal cracks caused by the surface layer and internal inclusions of a slab, and at the same time, avoids large-scale remodeling of a continuous casting apparatus, and at a low cost. An object of the present invention is to provide a continuous casting method of high quality cast slab which can be easily realized.

The continuous casting method for high quality cast slabs according to claim 1, which meets the above-mentioned object, comprises pouring molten steel into a mold through a dip nozzle having discharge ports on both sides of a bottom, and continuously solidifying the molten steel by the mold. In the continuous casting method of high quality cast slabs to pull down
A vertical portion is provided below the mold so as to have a vertical distance of 0.7 m or more and less than 1.5 m below the meniscus of the molten steel, and the molten steel is passed through an electromagnetic stirrer provided on one long side of the mold. A flow swirling along the peripheral wall of the mold is provided near the meniscus.
Inclusions mixed in the molten steel are lifted and removed by a vertical portion provided below a meniscus (a molten metal surface of the molten steel) of the molten steel formed in the mold. On the other hand, by forming a swirling flow along the peripheral wall of the mold near the meniscus of the molten steel in the mold, while improving the cleanliness of the solidified shell of the surface layer, the inclusions mixed in the molten steel are aggregated and coalesced, and By floating removal at the vertical portion, the cleanliness of both the surface layer and the inner layer of the slab is improved.
Here, the vicinity of the meniscus of the molten steel is within a range of 500 mm below the mold from the meniscus (the molten metal surface of the molten steel) formed by the molten steel in the mold, and is preferably within a range of 400 mm below the mold.

And since the vertical part is less than 1.5 m from the meniscus of the molten steel, the vertical part removes inclusions mixed in the molten steel by floating, and corrects the bending from the vertical part to the bent part by limiting distortion. Since it can be performed in a large area, internal cracks generated in the slab can be suppressed.
If the vertical portion below the meniscus of the molten steel is 1.5 m or more, the straightening of the curve from the vertical portion to the bent portion becomes a region with a small critical strain value, causing internal cracks, and causing the mold and the mold supporting device. In addition, the cost of remodeling the slab support roll group and the like also increases. On the other hand, if the vertical portion from the meniscus of the molten steel is smaller than 0.7 m, there is a limit to the floating removal of inclusions mixed in the molten steel. For these reasons, the vertical portion below the meniscus of the molten steel is preferably 0.7 m to 1.4 m.

The continuous casting method of high quality cast slab according to claim 2 is the continuous casting method of high quality cast slab according to claim 1 , wherein the flow swirling along the peripheral wall of the mold is 0.2 to 0.6 m / sec. Because of the swirling flow of molten steel in the mold, the inclusions in the surface layer of the slab are cleaned and removed by the swirling flow, increasing the cleanliness, and increasing the buoyancy of the inclusions by aggregating the inclusions mixed in the molten steel. To remove the slab and improve the cleanliness of both the surface layer and the inner layer of the slab at the same time.
If the flow swirling along the peripheral wall of the mold is less than 0.2 m / sec, the cleanliness of the solidified shell forming the surface layer is reduced, and the function of agglomerating and integrating inclusions mixed in the molten steel is reduced. And the quality of the inner layer is reduced. If the flow swirling along the peripheral wall of the mold is greater than 0.6 m / sec, powder added to the upper part of the meniscus of molten steel or stagnation occurs in the swirling flow, and defects caused by inclusions and bubbles may occur. It is easy to occur. For this reason, the flow swirling along the mold peripheral wall is more preferably 0.3 to 0.5 m / sec.

In the method for continuously casting high quality cast slabs according to claims 1 and 2 , a vertical portion is provided below the meniscus of the molten steel, and the flow swirling along the peripheral wall of the mold near the meniscus of the molten steel via an electromagnetic stirring device. Since it is applied and cast, the inclusions mixed in the molten steel are lifted and removed, and the swirling flow along the mold peripheral wall improves the cleanliness of the solidified shell and agglomerates the inclusions mixed in the molten steel. By combining and floating by the vertical portion, the cleanliness of both the surface layer and the inner layer of the slab is improved.

And, since as less than 1.5m said vertical portion of the meniscus of the molten steel, thereby removing emerged inclusions mixed in molten steel by the vertical section, it the size of the critical strain curvature correction from the vertical portion to the bent portion This can be performed in the region, and internal cracks generated in the slab can be suppressed.

In the continuous casting method for high quality cast slab according to claim 2, since the flow swirling along the peripheral wall of the mold is set to 0.2 to 0.6 m / sec, the molten steel in the mold is swirled by the solidified shell by the swirling flow. In addition to increasing the cleanliness by removing the objects, the inclusions mixed in the molten steel by swirling flow are aggregated and coalesced and removed by floating, thereby improving the cleanliness of both the surface layer and the inner layer of the slab.

Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention.
FIG. 1 is a conceptual view of a continuous casting apparatus applied to a continuous casting method for high quality cast slabs according to an embodiment of the present invention. FIG. 2 is a conceptual view of the periphery of a mold as viewed from a meniscus surface in FIG. FIG. 4 is a diagram showing the relationship between the distance from the meniscus and the diameter of inclusions, and FIG. 4 is a diagram showing the relationship between the critical strain of the slab and the actual strain when straightening from the vertical portion to the bent portion is performed. 5 shows a case where a vertical distance of less than 1.5 m is provided to perform electromagnetic stirring in a mold (the present invention), a case where a vertical distance of less than 1.5 m is provided (comparative example), and a commonly used curved continuous casting. FIG. 11 is a diagram showing a comparison of internal defect indices in the case (conventional example).

As shown in FIG. 1, the continuous casting device 10 includes a tundish 12 for receiving and holding molten steel 11 from a ladle (not shown), and the molten steel 11 is provided at the bottom of the tundish 12. An immersion nozzle 15 having a discharge port 14 for pouring the mold 13 is provided. Further, the slab 18 is cooled by the support roll 16 and the curved support roll group 17 installed immediately below the mold 13, the slab 18 is pulled out in the direction of the arrow in the drawing, and the cast material that has passed through the curved support roll group 17 is cast. The piece 18 is straightened by a straightening device (not shown) without bending.
A vertical portion 19 is formed by the mold 13 and a support roll 16 provided immediately below the mold 13. The effective vertical distance during the casting of the vertical portion 19 is determined by a distance between the meniscus 20 of the molten steel 11 and the lower end of the support roll 16 (in the figure). (Indicated by a dotted line), and is 0.7 m or more and less than 1.5 m.

Further, in the vertical portion 19, the molten steel 11 is cooled by the mold 13 to form a solidified shell 11a, the solidified shell 11a is supported by the plurality of support rolls 16, and an attached cooling water spray nozzle (shown in FIG. By injecting the cooling water from the solidified shell 11) and cooling, the rigidity of the solidified shell 11a is ensured and the solidification is promoted. The curved support roll group 17 connected to the lower end of the support roll 16 is curved with a radius of curvature of 8 to 15 m, supports the cast slab 18 by a plurality of curved support rolls 17a, and has an attached cooling. Cooling water is injected from a water injection nozzle (not shown) to thicken the solidified shell 11a of the slab 18.
Further, an electromagnetic stirrer 21 for applying a magnetic field to the vicinity of the meniscus 20 of the molten steel 11 is provided outside the mold 13.
2, a plurality of electromagnetic stirrers 21a, 21b, 21c, and 21d are provided outside the mold long piece 13a of the mold 13, and the controller 23 controls the molten steel 11 as shown in FIG. To a predetermined moving magnetic field. The application of the moving magnetic field is performed by controlling the value of the current flowing through each of the electromagnetic stirring devices 21a to 21d and the phase moving speed to predetermined values by the control device 23. That is, a flow 22 that applies a moving magnetic field to a region 400 mm below the mold including the meniscus 20 (see FIG. 1) of the molten steel 11 poured from the immersion nozzle 15 having the discharge port 14 and turns along the mold peripheral wall. (Indicated by an arrow in FIG. 2). On the upper surface of the meniscus 20 of the molten steel 11, a powder layer (not shown) composed of a molten layer and an unmelted layer is formed for lubrication of the mold 13 and the solidified shell 11a, trapping of inclusions, prevention of oxidation, and the like. I have.

Next, a method for continuously casting high quality cast slabs according to an embodiment of the present invention will be described.
The molten steel 11 is received in a tundish 12 having a capacity of 40 tons, and 1.8 tons / min of molten steel 11 is poured into the mold 13 from the discharge port 14 of the immersion nozzle 15 so that the vertical portion 19 becomes 1.4 m. The solidified shell 11a was formed by cooling with the support roll 13 and the support roll 16 therebelow. At the same time, the current value of the electromagnetic stirrers 21a and 21c is 675 amperes, and the current of 202 to 675 amperes is applied to the electromagnetic stirrers 21b and 21d to apply a moving magnetic field from the meniscus 20 of the molten steel 11 to a range of 400 mm below the mold. A flow 22 swirling at 0.2 to 0.6 m / sec along the peripheral wall was provided.
FIG. 3 shows the present invention in which the effective vertical distance (hereinafter referred to as “vertical distance”) of the above-described vertical portion 19 is 1.4 m, and the swirling flow 22 is 0.2 to 0.6 m / sec. This is a result of examining the aggregation and the floating effect of inclusions with the mark (●) in the conventional case provided with the symbol, based on the size of the inclusions in the molten steel 11. Obviously, in the mark (●) where only a vertical portion is provided, inclusions mixed in the molten steel 11 float on the vertical portion with almost the initial grain size, and therefore the removal efficiency is poor and the vertical distance is small. Even at 2.5 m, the number of inclusions remains in the molten steel 11 at least twice the number of the present invention (（). On the other hand, in the present invention (○) where the vertical distance of the vertical portion 19 is set to 1.4 m and the swirling flow 22 is given, the inclusions are inert gas such as argon or other inert gas which is entrained in the molten steel 11. Since it is brought into contact with inclusions to promote agglomeration and trapping of air bubbles, the buoyancy is increased and the particles are positively floated and removed at the vertical portion 19, so that inclusions can be significantly reduced. Furthermore, the penetration depth of the inclusions is also reduced due to the decrease in the penetration depth of the downward flow of the molten steel 11, and the quality improvement can be achieved by reducing the inclusions in the surface layer and the inner layer of the slab 18.
In this way, by providing the swirling flow 22, inclusions having small to medium diameters and small buoyancy are positively agglomerated and trapped by the inert gas bubbles in the molten steel 11 and easily float. The cleanliness of the front and inner layers of the molten steel 11 can be improved, and the removal rate of inclusions having a diameter of 150 μm or more in the vertical portion 19 having a vertical distance of less than 1.5 m is about 65% or more.

The cast slab 18 pulled out of the vertical portion 19 in a vertical state is corrected to have a radius of 10 m by the plurality of curved support rolls 17 a of the curved support roll group 17, and descends in the curved support roll group 17.
FIG. 4 shows the relationship between the critical strain and the actual strain of the slab 18 when the curvature is corrected from the vertical portion 19 having a vertical distance of 1.4 m to a curvature having a radius of 10 m. In addition, the peak value of the actual strain is about 0.67, and the curvature is corrected to a radius of 10 m within the range of the critical strain, so that the internal crack of the slab 18 can be prevented. However, when the vertical distance of the vertical portion 19 exceeds 1.5 m, the critical distortion becomes 0.53 or less, and the actual case where the straightening is not performed from the vertical portion only when the slab 18 is supported by the bending support roll group 17. The difference from the distortion (indicated by the dotted line in FIG. 4) becomes smaller. The actual distortion due to the correction in this region (for example, the distance from the meniscus is 2.5 m) becomes large as shown by the mark (●) in the figure, and exceeds the limit distortion.
Further, the critical strain may be a commonly used one. For example, the critical strain at the position of each curved support roll 17a depends on the type of steel, slab temperature, casting speed, solidification thickness, amount of cooling water supplied to each roll position, and the like. The distortion may be estimated using a value obtained by an experiment, or may be predicted by calculation from a value obtained in advance by an experiment. The actual distortion caused by the straightening can be determined in the same manner based on the type of steel, the slab temperature, the casting speed, the solidification thickness, the amount of cooling water supplied to each roll position, and the like. The condition of the solidified shell 11a used for obtaining the critical strain and the actual strain is a solidification relational expression S = K × between the thickness S (mm) of the solidified shell 11a and the arrival time T (minute) of the molten steel 11 from the meniscus 20. There is a relationship of T ^1/2 , and a solidification coefficient K of 25 to 30 mm / M ^1/2 is used as a general value. Note that M represents minutes in time units.

From the relationship between the limit distortion and the actual distortion generated by the curvature correction, if the curvature is corrected at a position less than 1.5 m from the meniscus 20, it is possible to correct the curvature within the area within the limit distortion, and to 1.5 m or more. Then, since the strain approaches or exceeds the critical strain, it is understood that the brittle solidified shell 11a of the slab 18 is cracked and becomes an internal crack.
The actual strain indicated by a dotted line in FIG. 4 is obtained by using a curved continuous casting apparatus (not shown) under the same conditions, such as steel type, slab temperature, casting speed, solidification thickness, and the like, when the radius of curvature is 10 m. This is a distortion value due to slab bulging or the like when the piece 18 is curved and supported by the curved support roll group 17. Therefore, the actual distortion after performing the curvature correction is the same as the actual distortion, and therefore approximates the actual distortion indicated by the dotted line.

Next, an example of a continuous casting method for high quality cast slabs according to an embodiment of the present invention will be described.
The molten steel 11 is poured into a mold 13 having a length of 1200 mm and a thickness of 250 mm at a supply speed of 1.8 tons / minute, and a meniscus 20 is formed at a position 100 mm downward from the upper end of the mold 13 and a support roll is formed from the meniscus 20. The vertical portion 19 was 1.4 m up to the lower end of 16.
Further, electromagnetic stirring devices 21a, 21b, 21c, and 21d are installed outside the mold long piece 13a, and a current of 675 amps is passed through the electromagnetic stirring devices 21a and 21c to perform strong stirring, and electromagnetic stirring devices 21b and 21d are provided. Casting was performed at a speed of 1.2 m / min with the flow 22 swirling by passing a current of 202 to 675 amps at 0.3 m / sec. In addition, the slab 18 coming out of the vertical portion 19 was sequentially curved and corrected by the curved support roll group 17 having a curvature radius of 10 m in accordance with the curvature. Further, as a conventional example, the surface quality and the surface quality in the case where the curvature is sequentially corrected in accordance with the curvature by the curved support roll group 17 having the vertical distance of 2.5 m and the radius of curvature of 10 m at the same mold 13 and casting speed at the same casting speed, Table 1 compares the evaluations of the internal quality and the overall quality.
In the conventional example, since the vertical distance is large, an internal crack is generated when the curvature is corrected according to the curvature, and the internal quality is deteriorated (△), whereas in the present embodiment (1), there is no internal crack. Good results (（) were obtained. Furthermore, in the case of Example (1), the surface quality was good (O) because inclusions on the surface layer were removed by performing electromagnetic stirring, whereas in the conventional example, the inclusions remained on the surface layer and the surface quality was good. Defects occur (△) and the surface quality is considerably reduced. This example (1) is also excellent in overall quality (に つ い て).

FIG. 5 shows a case where the vertical distance of less than 1.5 m and the electromagnetic stirring in the mold are performed (the present invention), a case where a vertical distance of less than 1.5 m is provided (comparative example), and a commonly used curved continuous type. In each of the casting cases (conventional examples), a case is shown in which the internal defect in the case of the commonly used curved continuous casting is compared with an index of 100, where a vertical distance of less than 1.5 m is provided, and It can be seen that by performing the electromagnetic stirring, the inclusion of small inclusions (increase in size) or the promotion of inclusion trapping by inert gas bubbles and the floating effect by the vertical distance are remarkable.
The embodiments of the present invention have been described above. However, the present invention is not limited to these embodiments, and includes a range that does not depart from the gist of the present invention.
For example, in the embodiment, the support roll 16 is provided directly below the mold 13, but a cooling grit, a divided support roll, or the like can be used in addition to the above. Alternatively, a swirling flow may be formed by simple stirring using a plurality of electromagnetic stirring devices. The position of the meniscus of the molten steel 11 in the mold is 100 mm downward from the upper end, but may be set as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS It is a conceptual diagram of the continuous casting apparatus applied to the continuous casting method of the high quality cast piece which concerns on one Embodiment of this invention. FIG. 2 is a conceptual diagram of a periphery of a mold as viewed from a meniscus surface in FIG. 1. FIG. 4 is a diagram illustrating a relationship between a distance from a meniscus and a diameter of an inclusion. It is a figure which shows the relationship between the critical strain of a cast slab, and the actual strain at the time of performing correction from a perpendicular part to a bending part. The case where the vertical distance of less than 1.5 m and the electromagnetic stirring in the mold are performed (the present invention), the case where the vertical distance of less than 1.5 m is provided (Comparative Example), and the case where the curved continuous casting is generally used (conventional) It is a figure which shows the comparison of the internal defect index of Example).

Explanation of reference numerals

10: continuous casting apparatus, 11: molten steel, 11a: solidified shell, 12: tundish, 13: mold, 13a: mold long piece, 14: discharge port, 15: immersion nozzle, 16: support roll, 17: curved support roll Group, 17a: curved support roll, 18: cast slab, 19: vertical part, 20: meniscus, 21: electromagnetic stirrer, 21a to d: electromagnetic stirrer, 22: swirling flow, 23: controller

Claims (3)

In a continuous casting method of a high-quality cast slab in which molten steel is poured into a mold through an immersion nozzle and continuously drawn down while solidifying the molten steel by the mold, a vertical portion is provided downward from the meniscus of the molten steel. A method for continuously casting high quality cast slabs, wherein a flow of swirling along the peripheral wall of the mold is provided in the vicinity of the meniscus of the molten steel via an electromagnetic stirrer provided on one long side of the mold. 2. The method according to claim 1, wherein the vertical portion is less than 1.5 m from the meniscus of the molten steel. 3. The continuous casting method for high quality cast slabs according to claim 1, wherein a flow swirling along the peripheral wall of the mold is set to 0.2 to 0.6 m / sec.

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