JP4848656B2 - Method and apparatus for continuous casting of steel - Google Patents

Description

  The present invention relates to a steel continuous casting method and apparatus, and more particularly to a steel continuous casting method and apparatus capable of reducing segregation at the center of a slab, so-called center segregation.

  In continuous casting, as shown in FIG. 1A, the molten steel 10 is poured into a water-cooled mold 18 from a ladle (not shown) through a tundish 14 and an immersion nozzle 16, and then a slab ( The slab is continuously produced while drawing out 20 in the casting direction and applying cooling. First, in the first mold part, the slab is removed from heat by coming into contact with the mold 18 to form an initial solidified shell. Thereafter, the slab 20 that has passed through the mold 18 is gradually pulled out in the casting direction by the pinch roll 22. At that time, the slab 20 is cooled to gradually thicken the solidified shell and complete solidification within the length of the continuous casting machine.

  Thereafter, by cutting the slab into a predetermined length by the torch cutter 24, the slab can be manufactured stably and in large quantities.

  When macro segregation occurs at the thickness center of the slab during the continuous casting, the homogeneity of the steel is impaired and the product becomes out of specification. Thus, reducing the center segregation of a slab is an extremely important problem in producing a hydrogen-resistant cracked steel such as a high-strength line pipe. For this reason, conventionally, many methods for preventing center segregation have been reported.

  Conventionally, in order to prevent center segregation in continuous casting operations, it is possible to lightly reduce the slab at the end of solidification (light reduction method) or increase the equiaxed crystal zone at the center of the slab. Has been adopted. The means for increasing the equiaxed crystal zone is further classified into a low temperature casting method and an electromagnetic stirring method (Non-patent Document 1).

  Among these methods, the light reduction method is a method of suppressing the flow of molten steel by reducing the volume shrinkage accompanying solidification at the end of solidification.

  The low temperature casting method and the electromagnetic stirring method are methods for dispersing segregation by increasing the equiaxed crystal zone at the center of the slab.

  In the low temperature casting method, casting is performed with the superheat degree of the molten steel (difference between the molten steel temperature and the liquidus temperature) as low as possible to produce equiaxed crystals at the initial stage of solidification.

  In the electromagnetic stirring method, the molten steel is flowed by electromagnetic stirring to shear the tip of the dendrite to generate solidified nuclei, thereby increasing the equiaxed crystals and stirring the equiaxed crystals to form crystals. Arrangement is made with good filling properties (arrangement that minimizes the gap between crystals) to solidify. For example, Patent Document 1 describes that an alternating magnetic field is applied, and Patent Document 2 describes that after applying a static magnetic field to an initial solidified slab drawn from a mold to form a solidified structure in a columnar crystal, It is described that light reduction is applied to the slab immediately before the final solidification stage while applying a static magnetic field, and Patent Document 3 applies a static magnetic field near the solidification interface within 5 minutes from the start of casting to promote dendrite growth. After that, the dendrite is broken by electromagnetic stirring, and equiaxed crystals are deposited on the axial center of the slab. Patent Document 4 describes that a low-frequency alternating static magnetic field is applied. Yes.

  In addition, the applicant has proposed in Patent Document 5 that a static magnetic field and an alternating magnetic field are superimposed and applied to a mold.

Uchibori; 126th, 127th, Nishi Memorial Technology Course "Present and Future of Highly Clean Steel", November, December 1988, p. 17-19, Japan Iron and Steel Association Japanese Patent Laid-Open No. 9-206897 JP-A-6-608 Japanese Patent Laid-Open No. 4-319053 JP-A-2-274350 JP 2003-103349 A

  However, each of the above conventional techniques has problems.

  That is, in the light reduction method, the crystal structure of the solidified structure at the final solidification position is such that the columnar crystal grows to the center position on the upper surface side of the slab, while the solidification that has occurred in the mold or before final solidification on the lower surface side. There are crystals in a form called branched columnar crystals formed by the nucleation of sediment and deposition. Thus, if the upper and lower crystal forms are different, the solidified shells collide with each other at the time of final solidification to form a single slab, so that the crystals are not arranged without gaps, so that the crystal filling property is poor. An unsolidified phase (concentrated molten steel) enters a part (a part having a gap), and island-like or granular segregation can be generated in this part (the degree of segregation increases as the area of the segregated grains increases).

  In addition, the low-temperature casting method has a problem in performing stable casting operation because casting is performed at a low superheat temperature of molten steel. That is, the clogging of the immersion nozzle occurs, the surface of the molten steel in the mold is covered, or inclusions are caught in the molten steel and the quality of the slab is remarkably deteriorated.

  On the other hand, in the electromagnetic stirring method, if the stirring flow rate is increased, equiaxed crystals increase, but in the region where the molten steel flows, the distribution of the solute at the solid-liquid interface changes, so if the stirring flow rate is increased too much, A negative segregation band (concentration range lower than the average concentration) called white band occurs. In this method, the segregated grains are reduced and dispersed by stirring, but the degree to which the segregated grains can be subdivided is not sufficient.

  Furthermore, a combination of a static magnetic field or a moving magnetic field as disclosed in Patent Documents 1 to 3 or a dendrite segmentation by a low-frequency AC magnetic field as disclosed in Patent Document 4 does not cause a temperature field. It has become uniform and is still insufficient for reducing the central segregation that satisfies the recent quality improvement needs.

  Further, Patent Document 5 aims to prevent surface defects and cannot reduce center segregation.

  An object of the present invention is to provide a steel continuous casting method and apparatus capable of producing a slab that does not cause the problems as in the prior art and has very little center segregation.

In order to achieve the above object, in the present invention, in order to suppress the flow of the internal molten steel while breaking the dendrite on the solidification front surface, a static magnetic field acting through the slab in the thickness direction from both sides of the slab Then, an alternating magnetic field (a moving magnetic field, an oscillating magnetic field, or a magnetic field in which the vibration peak position moves) is superimposed on the same position in the casting direction of the slab, and a magnetic flux is applied to the central part over the entire width of the slab .

  The appropriate solid phase ratio to be applied is effective in a wide range of 0.1 to 0.8 as the solid phase ratio at the center. At a low solid fraction such as 0.1, the effect of uniforming the temperature distribution by suppressing the flow of molten steel with a static magnetic field is large, whereas in the region with a high solid fraction, an alternating magnetic field or an oscillating static magnetic field The effect of increasing the equiaxed crystal ratio due to dendrite fracture is large. The optimum frequency changes depending on the difference in steel type and casting speed, but by applying the AC magnetic field and the DC magnetic field in combination at the same position, the above two synergistic effects result in a wide solid phase ratio range in the intermediate region. Thus, a greater segregation reduction effect can be obtained than when an AC magnetic field or a DC magnetic field is individually applied.

  When an alternating magnetic field is applied, the molten steel is agitated and the temperature field becomes non-uniform, but by combining a static magnetic field, the temperature field can be made uniform, so that both dendrite fracture and homogenization can be achieved.

  In the present invention, both the upper surface and the lower surface of the center part of the slab can be solidified with the same crystal form, and the crystal form of the center part of the slab can be converted into a columnar crystal. Therefore, the gaps between the crystals when the solidified shells (both columnar crystals) on both sides of the upper and lower surfaces collide with each other are smaller than when the equiaxed crystals are generated. For this reason, the effect of reducing the center segregation is greater than when the equiaxed crystal is generated.

  When solidified while applying a static magnetic field, columnar crystals tend to develop. The reason is that when a static magnetic field is applied to the slab drawn from the mold, the flow of the molten steel is suppressed, so that the heat transfer in the process of cooling the molten steel is changed from the heat transfer by convection to the heat transfer mainly by conduction. change. For this reason, in the vicinity of the solidification interface of the slab, a temperature gradient is formed in the thickness direction, and at the same time, thermal diffusion is reduced, so that the heating degree of the molten steel is kept long. As a result, even if the solidified nuclei generated in the mold have settled, as described above, if the heating degree of the molten steel is sufficiently secured, the solidified nuclei are easily dissolved during the sedimentation process, The growth proceeds from the solidified shell only to become columnar crystals.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the first embodiment of the present invention, as shown in FIG. 1 (A), a static magnetic field as shown in FIG. 1 (B) is applied at a position of an appropriate solid phase ratio during casting below the mold 18. For example, a static magnetic field generating device 30 made of a DC coil or a permanent magnet, an AC magnetic field, for example, a comb-like core 42, and a coil (not shown) wound around the core. The AC magnetic field generator 40 made up of a large number of electromagnets is provided in an overlapping manner.

  As schematically shown in FIG. 2, the static magnetic field generating device 30 suppresses the bulk flow rate and makes the temperature uniform, thereby growing dendrites.

  On the other hand, the AC magnetic field generator 40 is a moving magnetic field that moves over a wide range in the width direction of the slab 20, for example, as shown in FIG. 3 (A), or as shown in FIG. The dendrite is broken by applying an oscillating magnetic field that vibrates in the width direction in the field, or a magnetic field whose peak position of vibration moves as shown in FIG.

  In this way, by overlapping the static magnetic field and the alternating magnetic field at the same position of the slab and vibrating the solidification interface while suppressing the bulk flow rate, the dendrite can be fractured and equiaxed to reduce segregation. .

  In the first embodiment, as the AC magnetic field generator 40, a coil wound around the comb-shaped core 42 is used. However, the configuration of the AC magnetic field generator is not limited to this.

  Next, a second embodiment of the present invention in which the AC magnetic field generator is not required by scanning the static magnetic field generator 30 will be described.

  In the present embodiment, as shown in FIG. 4, a plurality of static magnetic field generators made of, for example, permanent magnets 32 are provided in the width direction of the slab 20 (four in the figure), and these are installed on the plate of the slab 20 on the spot. By oscillating in the direction perpendicular to the thickness direction (left and right width direction in the figure), the same effect as the oscillating magnetic field by the electromagnet in FIG. 3B is obtained.

  The configuration can be simplified by vibrating the base (not shown) to which the permanent magnet 32 is fixed without vibrating the permanent magnet 32 individually.

  Further, the scanning method of the permanent magnet 32 is not limited to that corresponding to FIG. 3B, and scanning may be performed as shown in FIG.

  In the present embodiment, an expensive alternating-current magnetic field generator that combines magnetic poles and coils having a complicated shape as in the first embodiment is unnecessary, and can be configured at low cost. In particular, when the static magnetic field generator is composed of permanent magnets, a power source and wiring for a DC coil are not necessary. A DC coil may be used in place of the permanent magnet 32.

  Further, the scanning direction of the permanent magnet 32 (static magnetic field) is not limited to the width direction of the cast slab 20 (left and right direction in FIG. 4), and as shown in FIG. The scanning may be performed in an arbitrary direction perpendicular to the plate thickness direction of the slab 20. Furthermore, the scanning direction of the static magnetic field is not limited to one direction. For example, the dendrites can be more reliably broken by combining scanning in the width direction and casting direction.

  Medium carbon steel was cast with a curved bloom continuous caster (mold size 300 × 400 mm), and the center segregation of the slab was evaluated. Segregation was evaluated by polishing and macro-etching the C cross-section of the slab, quantifying the area of the center segregation portion, standardizing as 0 where segregation was hardly observed, and 1 as having the strongest segregation. .

  In general, it is known that the center segregation deteriorates with an increase in casting speed, but it was confirmed that the degree of segregation was improved by the method of the present invention even when the casting speed was increased.

1A is a general configuration diagram of the first embodiment of the present invention, and FIG. 1B is a cross-sectional view showing the configuration of a magnetic field generator. Longitudinal section showing the effect of static magnetic field Cross-sectional view showing the effect of AC magnetic field The perspective view which shows the structure of the static magnetic field generator used in 2nd Embodiment of this invention. Longitudinal sectional view showing the operation of a modification of the static magnetic field generator Vertical sectional view showing the operation of another modification of the static magnetic field generator

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Molten steel 14 ... Tundish 16 ... Immersion nozzle 18 ... Mold 20 ... Slab 22 ... Pinch roll 24 ... Torch cutter 30 ... Static magnetic field generator 32 ... Permanent magnet 40 ... AC magnetic field generator 42 ... Core

Claims (5)

In below the mold bottom, during casting, a static magnetic field acting through from both sides of the slab the slab in the thickness direction, by superimposing the AC magnetic field in the casting direction the same position of the slab, over the entire width of the slab A continuous casting method of steel, wherein casting is performed while applying magnetic flux to the center .   The continuous casting method for steel according to claim 1, wherein the alternating magnetic field is a moving magnetic field.   2. The steel continuous casting method according to claim 1, wherein the alternating magnetic field is an oscillating magnetic field.   2. The steel continuous casting method according to claim 1, wherein the AC magnetic field is a magnetic field in which a vibration peak position moves. A static magnetic field generator and an alternating magnetic field generator for superimposing a static magnetic field acting through the slab from both sides of the slab in the thickness direction and an alternating magnetic field at the same position in the casting direction of the slab below the lower end of the mold Is a continuous casting apparatus for steel, characterized in that it is provided over the entire width of the slab .

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