JP2008246534A - Steel continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an slab of good quality by efficiently applying an electromagnetic field to a solidified shell in the vicinity of a meniscus of molten steel in a mold to reduce an oscillation mark and promote floating of bubbles and inclusions.
SOLUTION: Continuous casting of molten steel 5 using a continuous casting mold 3 having a water cooling mechanism and not having a slit therethrough while applying a high frequency electromagnetic field from an electromagnetic coil 4 installed above a molten steel surface 9 in the mold. In doing so, casting is performed while controlling the molten metal surface position of the molten steel in the mold so that the distance between the lower end of the electromagnetic coil and the molten steel surface in the mold is 100 mm or less. In this case, the frequency of the electromagnetic field is preferably 2 to 35 kHz.
[Selection] Figure 1

Description

  The present invention relates to a continuous casting method of steel using electromagnetic force that enables high-frequency electromagnetic force to act on the molten steel near the molten steel surface in the mold from above the molten metal surface and improve the quality of the slab. is there.

  When molten steel is poured into a water-cooled mold and the solidified slab is continuously drawn out of the mold and continuous casting is performed, the slab solidification starts from the mold wall surface to form a thin solidified shell. And proceed while increasing the thickness of the solidified shell. Also, in order to promote lubrication between the mold and the solidified shell due to continuous drawing, the mold is vibrated and cast while forcing a flux (mold powder) between the mold and the solidified shell. . At this time, a large force is applied to the solidified shell, and various problems such as defects on the surface of the slab such as an oscillation mark due to deformation of the solidified shell and breakage (breakout) of the solidified shell occur. Such a problem becomes more remarkable as the casting speed increases.

  In order to solve these problems, it is necessary to reduce the frictional force between the mold and the solidified shell while maintaining the strength of the solidified shell, thereby increasing the soundness of the solidified shell. A continuous casting method using a control function of the inner hot metal surface shape has been studied.

  A method for applying a high-frequency electromagnetic field to a molten steel surface in a continuous casting mold (hereinafter also referred to as “meniscus”) by a single electromagnetic coil has been studied conventionally. For example, as shown in Patent Document 1, a continuous casting mold is used. In general, an electromagnetic coil is disposed on the outer periphery of the steel plate and a high-frequency electromagnetic field is applied to the meniscus portion of the molten steel. Patent Document 1 also shows an example in which a shielding screen is used as means for controlling the intensity of an electromagnetic field. In order to use various electromagnetic field effects, a plurality of electromagnetic coils may be arranged. For example, in Patent Document 2, the upper and lower two-stage electromagnetic coils are arranged on the outer periphery of the mold and placed on the back surface of a mold provided with a slit penetrating the upper part of the mold (also referred to as “comb mold”) to continuously cast molten steel. A method has been proposed. This technique uses two electromagnetic coils to simultaneously obtain different improvement effects, and the roles of each electromagnetic coil are different. Similarly, as shown in Patent Document 3, a method has been proposed in which an upper electromagnetic coil is disposed on the inner surface side of a mold above a meniscus.

  In these methods, an electromagnetic coil that generates a high-frequency electromagnetic field is installed on the back of the mold, so a slit that penetrates the mold or penetration of the electromagnetic field is possible to prevent attenuation of the high-frequency electromagnetic field in the mold. It is assumed that a special electromagnetic mold is used. However, when such a mold is used for casting a slab, the strength of the mold is remarkably reduced, and long-term continuous casting operation is difficult.

On the other hand, an AC electromagnetic field application method shown in Patent Document 4 has been proposed as a method of installing an electromagnetic coil on the inner surface side of the mold above the meniscus using a normal mold and applying an electromagnetic field from this electromagnetic coil. The applied electromagnetic field is only an alternating electromagnetic field, and it is not clear what frequency it is. The purpose of applying the electromagnetic field is mainly for the purpose of heating the molten steel and controlling the flow of the molten steel, not for the shape control function of the electromagnetic field.
JP-A-8-33959 JP-A-8-141710 JP-A-8-187563 Japanese Patent Laid-Open No. 7-256413

  The present invention has been made in view of the above circumstances. The object of the present invention is to prevent high-frequency electromagnetic field from being applied to a meniscus portion in a mold and continuously cast molten steel, thereby preventing deformation of the mold for a long period of time. Casting operation is possible, and a high-frequency electromagnetic field is efficiently applied to the solidified shell near the meniscus to reduce the oscillation mark and promote the rising of bubbles and inclusions. It is to provide a continuous casting method of steel that can be produced.

  The continuous casting method for steel according to the first aspect of the present invention for solving the above-described problems uses a continuous casting mold that has a water cooling mechanism and does not have a penetrating slit, and is an electromagnetic wave installed above the molten steel surface in the mold. When continuously casting molten steel while applying a high-frequency electromagnetic field from the coil, casting is performed while controlling the molten metal surface position of the molten steel in the mold so that the distance between the lower end of the electromagnetic coil and the molten steel surface in the mold is 100 mm or less. It is characterized by this.

  The continuous casting method for steel according to the second invention is characterized in that, in the first invention, the frequency of the electromagnetic field is 2 to 35 kHz.

  According to the present invention, in continuous casting using a normal mold, an electromagnetic field can be efficiently applied to the solidified shell near the meniscus in the mold, and the oscillation mark is reduced by the electromagnetic force induced from the electromagnetic field, Further, by appropriately controlling the frequency of the applied electromagnetic field, the floating separation of inclusions and bubbles is promoted, and the quality of the slab can be improved.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 1 and 2 are schematic views of a continuous casting machine mold part showing an example in which the present invention is applied to a steel slab continuous casting machine, and FIG. 1 shows an example in which an electromagnetic coil is installed above a mold, FIG. 2 shows an example in which the electromagnetic coil is installed on the inner surface side of the mold. 1 and 2 are views as seen from the thickness direction of the slab slab.

  1 and 2, molten steel 5 is injected from the tundish 1 into the mold 3 through the discharge holes 8 of the immersion nozzle 2 arranged at the bottom of the tundish 1. The mold 3 is cooled by cooling water, and the molten steel 5 injected into the mold 3 is cooled by the mold 3 to form a solidified shell 6 on the contact surface with the mold 3. And the slab which makes the solidified shell 6 the outer shell and the inside is the unsolidified molten steel 5 is continuously pulled out downward, and the position of the meniscus 9 of the molten steel in the mold is held at a substantially constant position. As the solidified shell 6 is drawn downward, the solidified shell thickness is increased by cooling with the mold 3. On the meniscus 9, a mold powder 7 having functions such as a lubricant for the solidified shell 6 and the mold 3, an antioxidant for insulating the meniscus 9 from the air, and a heat retaining agent is added. The mold 3 is a normal mold without a penetrating slit for interrupting the induced current due to electromagnetic force. Of course, the slit which is the flow path of the cooling water for cooling the mold is installed on the back side of the mold 3. Further, for the purpose of preventing the occurrence of defects due to solidification shrinkage and deformation of the solidified shell 6, a more stable casting can be performed by giving a taper to the mold 3.

  An electromagnetic coil 4 for applying a high-frequency electromagnetic field to the molten steel 5 in the mold is installed above the meniscus 9. In FIG. 1, the electromagnetic coil 4 is installed over the entire circumference of the mold 3 above the mold 3. In this case, from the viewpoint of bringing the electromagnetic coil 4 close to the meniscus 9, it is desirable to make the mold 3 and the electromagnetic coil 4 as close as possible, and they may be integrated with an insulating band or the like. Further, since the electromagnetic coil current flows inside the electromagnetic coil 4, it is desirable that the inner side of the electromagnetic coil 4 protrudes to the inner surface side of the mold 3 as shown in FIG. 1.

  If the distance between the electromagnetic coil 4 and the meniscus 9 is too large, the electromagnetic field will not effectively act on the molten steel bath. Therefore, the distance between the lower end of the electromagnetic coil 4 and the meniscus 9 must be 100 mm or less, preferably 50 mm or less. Thus, the position of the meniscus 9 is controlled. On the other hand, if the distance between the lower end of the electromagnetic coil 4 and the meniscus 9 is too small, such as 30 mm or less, the distance between the meniscus 9 and the upper end of the mold is also shortened, making it difficult to control the position of the meniscus 9. Since the mold powder 7 and the like supplied to 9 may overflow from the mold 3, it is preferable to secure a distance that does not cause a problem in operation.

  On the other hand, in FIG. 2, the electromagnetic coil 4 is installed above the meniscus 9 and at a position along the inner periphery of the mold 3 on the inner surface side. However, when the electromagnetic coil 4 is arranged on the inner surface side of the mold 3 as shown in FIG. 2, the coil current flows inside the electromagnetic coil 4 as described above, and therefore the effect of applying an electromagnetic field to the central portion of the meniscus 9 is increased. The electromagnetic field cannot be effectively applied to the initial solidified portion, that is, the solidified shell 6. Therefore, as shown in FIG. 1, it is preferable to install it above the mold 3. Also when the electromagnetic coil 4 is installed at the position shown in FIG. 2, the distance between the lower end of the electromagnetic coil 4 and the meniscus 9 needs to be 100 mm or less, and preferably the position of the meniscus 9 is controlled to be 50 mm or less. . Similarly to the above, the distance between the lower end of the electromagnetic coil 4 and the meniscus 9 is not too small, such as 30 mm or less.

  When the electromagnetic field is applied to the molten steel 5 in the mold in this way, the shape of the meniscus 9 is stabilized by the electromagnetic force, so that the molten metal surface disturbance in the initial solidified portion due to the mold vibration is suppressed, and is represented by an oscillation mark. Surface defects due to molten metal surface fluctuations are significantly reduced. At this time, the frequency of the electromagnetic field is preferably about 2 kHz or more. If it is less than 2 kHz, the penetration depth of the electromagnetic field into the molten steel will increase, and the molten metal surface disturbance will increase due to the increase in molten steel flow, so it will be difficult to obtain the effect of improving the surface properties.

  By applying an electromagnetic field (electromagnetic force), the surface layer of the molten steel 5 receives a surface pressure. Although this pressure works as a shape control action of the meniscus 9, in addition, an effect of promoting the rise of bubbles and inclusions by electromagnetic repulsion can be obtained. Here, these effects depend on the frequency of the electromagnetic field. That is, when the frequency is low, the penetration depth of the electromagnetic field increases and the range affected by the electromagnetic field increases, but the electromagnetic force itself decreases, and no force that can cause bubbles and inclusions to float is generated. The electromagnetic force works to induce molten steel flow, and the shape control effect is reduced accordingly. On the other hand, when the frequency is high, the electromagnetic force increases and the electromagnetic field concentrates on the surface layer of the molten steel 5 to increase the shape control effect, but the range affected by the electromagnetic field is narrowed.

  Here, as a result of examining the frequency of the electromagnetic field that works effectively on the surface layer of 2 to 3 mm, the present inventors found that there is a possibility that the effect of promoting the rise of bubbles and inclusions may be obtained in the case of 2 to 35 kHz. It was. If bubbles and inclusions can be accumulated on the surface of the molten steel 5 in this way, a high-quality slab can be obtained by removal with the molten mold powder 7 or surface treatment of the slab.

  In addition, this invention is not limited to the range of said description, A various change is possible. Although the above explanation is an example applied to a slab continuous casting machine, even if it is a bloom continuous casting machine or a billet continuous casting machine, and the cross section is other than a rectangle, the present invention is applied along the above. Can do. In general, the central axis of the electromagnetic coil 4 is generally parallel or coincident with the central axis of the mold 3, but there is no problem even if it is decentered. A coil may be arranged.

  A steel casting experiment was conducted using a continuous casting tester capable of casting a slab having a thickness of 100 mm and a width of 100 mm. The cast steel type was carbon steel having a carbon concentration of 0.05% by mass, and the degree of superheat of the molten steel in the tundish was 30 ° C. 100 kg of steel was melted in a high-frequency induction heating furnace, and after adjusting the components and temperature, the molten steel was injected into the water-cooled mold from the immersion nozzle through a tundish. The mold vibration conditions were a vibration frequency of 60 cpm and an amplitude of ± 4.5 mm, and casting was performed at a drawing speed of 0.8 m / min while pouring mold powder on the upper surface of the molten steel in the mold. The supply of molten steel from the tundish was controlled so that the meniscus position in the mold was 50 mm from the lower end of the electromagnetic coil.

  As shown in FIG. 1 described above, the electromagnetic coil was installed above the mold. The size of the electromagnetic coil is an inner diameter of 80 mm square, an outer diameter of 150 mm square, a height of 20 mm, and the number of turns is one turn.

  After casting was stabilized, an electromagnetic field was applied by an electromagnetic coil. The applied electromagnetic coil current was 9000 A, and the electromagnetic field frequency was changed to 5 levels in the range of 1 to 40 kHz (Tests 2 to 6). For comparison, a test without an electromagnetic field (Test 1) was also performed.

  The cast slab was visually observed to check for the presence of an oscillation mark. Further, stepping was performed every 1 mm from the surface of the slab, and the presence frequency of inclusions / bubbles up to a surface layer of 5 mm was measured and compared. Table 1 shows the frequency of each test and the survey results of slabs. The presence frequency of inclusions / bubbles in the slab was shown as an index with the frequency when there was no electromagnetic field (test 1) being 100.

  As shown in Table 1, in the slab without an electromagnetic field, there was a clear horizontal stripe-shaped depression, that is, an oscillation mark, but it was confirmed that the oscillation mark was reduced by application of the electromagnetic field. Further, in Tests 3 to 6 in which the frequency was 2 kHz or more, it was confirmed that inclusions and bubbles in the slab were less frequently, and that these were reduced by applying an electromagnetic field.

  A steel casting experiment was conducted using a continuous casting tester capable of casting a slab having a thickness of 100 mm and a width of 100 mm. The cast steel type was carbon steel having a carbon concentration of 0.05% by mass, and the degree of superheat of the molten steel in the tundish was 30 ° C. 100 kg of steel was melted in a high-frequency induction heating furnace, and after adjusting the components and temperature, the molten steel was injected into the water-cooled mold from the immersion nozzle through a tundish. The mold vibration conditions were a vibration frequency of 60 cpm and an amplitude of ± 4.5 mm, and casting was performed at a drawing speed of 0.8 m / min while pouring mold powder on the upper surface of the molten steel in the mold. The molten metal supply from the tundish was controlled so that the distance between the meniscus position in the mold and the lower end of the electromagnetic coil was a predetermined value in the range of 20 to 200 mm.

  The electromagnetic coil was installed at two levels: when installed above the mold as shown in FIG. 1 and when installed on the inner surface side of the mold as shown in FIG. As for the size of the electromagnetic coil, those installed above the mold have an inner diameter of 80 mm square, an outer diameter of 150 mm square, and a height of 20 mm, and those installed on the inner surface side of the mold have an inner diameter of 70 mm square, an outer diameter of 90 mm square, and a high The length is 40 mm and the number of windings is one turn for both.

  After casting was stabilized, an electromagnetic field was applied by an electromagnetic coil. The applied electromagnetic coil current was 9000 A, the electromagnetic field frequency was 10 kHz, and the distance between the meniscus position in the mold and the lower end of the electromagnetic coil was changed to 5 levels within a range of 20 to 200 mm (Tests 7 to 12). When the distance between the meniscus position in the mold and the lower end of the electromagnetic coil is 50 mm, comparison was made between the case where the electromagnetic coil was installed above the mold (Test 8) and the case where it was installed on the inner surface side of the mold (Test 12). .

  The cast slab was visually observed to check for the presence of an oscillation mark. Further, stepping was performed every 1 mm from the surface of the slab, and the presence frequency of inclusions / bubbles up to a surface layer of 5 mm was measured and compared. Table 2 shows the distance between the meniscus position in the mold and the lower end of the electromagnetic coil in each test, and the investigation results of the slab. The frequency of presence of inclusions / bubbles in the slab was shown as an index with the frequency of the distance between the lower end of the coil and the meniscus being 200 mm (test 11) being 100.

  As shown in Table 2, in Tests 7 to 9 and Test 12 in which the distance between the meniscus position in the mold and the lower end of the electromagnetic coil was 100 mm or less, it was confirmed that the oscillation mark was reduced by application of the electromagnetic field. However, in Test 7 in which the distance between the meniscus position in the mold and the lower end of the electromagnetic coil was 20 mm, it was difficult to control the level of the meniscus in the mold, and an overflow of mold powder occurred. On the other hand, in tests 10 to 11 in which the distance between the meniscus position in the mold and the lower end of the electromagnetic coil exceeded 100 mm, a clear oscillation mark was present. Further, in Tests 7 to 9 and Test 12 in which the distance between the meniscus position in the mold and the lower end of the electromagnetic coil is 100 mm or less, the existence frequency of inclusions and bubbles in the slab is low, and these are reduced by application of the electromagnetic field. Was confirmed.

  In Test 12 in which the electromagnetic coil was installed on the inner surface side of the mold, it was confirmed that the effect of improving the surface properties and inclusions / bubbles was smaller than in Test 8.

It is the schematic of the continuous casting machine mold part which shows the example which applied this invention to the steel slab continuous casting machine, and is a figure which shows the example which installed the electromagnetic coil above the casting_mold | template. It is the schematic of the continuous casting machine mold part which shows the example which applied this invention to the steel slab continuous casting machine, and is a figure which shows the example which installed the electromagnetic coil in the inner surface side of the casting_mold | template.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Tundish 2 Immersion nozzle 3 Mold 4 Electromagnetic coil 5 Molten steel 6 Solidified shell 7 Mold powder 8 Discharge hole 9 Meniscus

Claims (2)

  When continuously casting molten steel while applying a high-frequency electromagnetic field from an electromagnetic coil installed above the molten steel surface in the mold using a continuous casting mold having a water cooling mechanism and no slits penetrating the lower end of the electromagnetic coil A continuous casting method for steel, characterized in that casting is performed while controlling the surface position of the molten steel in the mold so that the distance between the molten steel surface and the molten steel surface in the mold is 100 mm or less.   The method of continuous casting of steel according to claim 1, wherein the frequency of the electromagnetic field is 2 to 35 kHz.

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