JP4757661B2 - Vertical continuous casting method for large section slabs for thick steel plates - Google Patents

Description

  The present invention relates to a vertical continuous casting method for a large-section slab for a thick steel plate having a slab thickness of 380 mm or more.

  In vertical continuous casting of large-section slabs for thick steel plates, a number of rolls are vertically arranged at intervals corresponding to the slab thickness below the mold, and cooled while moving the slab downward at a low casting speed. Yes. When the slab thickness is 380 mm or more, the casting speed is a low speed of 0.2 m / min or less. As shown in Patent Document 1, the roll group on one side is a reference plane roll constituting a reference plane, and the roll group on the opposite side moves to reduce the roll interval with the reference plane in accordance with shrinkage in the thickness direction of the slab. It is a surface roll. Molten steel was poured into the mold from the tundish through the immersion nozzle, and the immersion nozzle was arranged at the center of the mold thickness, and the molten steel was evenly supplied to both sides of the mold.

However, large cross-section slabs with a slab thickness of 380 mm or more have slab defects called center segregation or zaku at the center, leading to a reduction in toughness. Thus, as shown in Patent Document 2, attempts have been made to prevent the center segregation by applying a rotating magnetic field to the discharge flow from the submerged nozzle. However, sufficient results have not yet been obtained.
Japanese Patent Laid-Open No. 2002-18561 JP-A-11-285788

  The present invention provides a vertical continuous casting method for a large-section slab that solves the above-mentioned conventional problems, refines the grain size of equiaxed crystals generated in the center of the large-section slab, and improves toughness. It is intended to provide.

  As a result of repeated studies to solve the above-mentioned problems, the present inventor is that the equiaxed crystal generated in the central portion of the large-section slab is coarse because the crystal nucleus is small and the casting speed is low. The reason for this is that equiaxed crystals grow while being cooled, and the fact that there are few crystal nuclei in the vertical continuous casting of large-section slabs is due to the pouring flow from the immersion nozzle located at the center of the mold thickness. It has been clarified that there is a cause in collision with only the solidified shell on the moving surface side.

  That is, as shown in FIG. 1, the reference surface roll 3 is disposed vertically below the mold, whereas the moving surface roll 4 is disposed slightly inclined by the shrinkage of the slab accompanying cooling. Has been. For this reason, the pouring flow from the immersion nozzle 2 arranged at the center of the thickness of the mold 1 tends to collide only with the solidified shell on the moving surface side, and it is considered that sufficient crystal nuclei are not formed.

The present invention has been completed on the basis of the above-described knowledge, and a large-section slab for a thick steel plate having a slab thickness of 380 mm or more, and a reference surface roll arranged vertically below the mold and going downward A continuous casting method in which casting is performed at a casting speed of 0.2 m / min or less by a vertical continuous casting machine provided with a moving surface roll disposed so as to be inclined so as to approach the reference surface roll. It is characterized by being cast by being installed perpendicularly to the meniscus at the center with respect to the actual slab thickness determined by the distance between the lowermost reference surface roll and the moving surface roll.

  Further, it is preferable to cast the molten steel in the tundish with respect to the liquidus temperature of 10 to 50 ° C., and it is preferable to cast the molten steel in the mold while stirring it using electromagnetic stirring in the mold.

According to the present invention, the position of the immersion nozzle is offset from the center of the thickness of the conventional mold to the reference surface side, and installed perpendicularly to the center portion with respect to the actual slab thickness determined by the distance between the lowermost reference surface roll and the moving surface roll. To do. As a result, the pouring flow from the immersion nozzle uniformly collides with the inner surfaces of the solidified shells on the reference surface side and the moving surface side, and a large number of equiaxed nuclei are generated in the molten metal. As a result, the grain size of the equiaxed crystal generated in the central portion of the large-section cast slab can be reduced, and the occurrence of zaku can be suppressed to improve toughness.

  Moreover, the equiaxed crystal ratio of a slab center part can be made into 20% or more by casting as the superheat degree with respect to the liquidus temperature of a tundish inner molten steel like Claim 2 as 10-50 degreeC. Further, by casting the molten steel in the mold using the electromagnetic stirring in the mold as in claim 3, the effect of refining the grain size of the equiaxed crystal can be enhanced.

Embodiments of the present invention will be described below.
FIG. 2 is a cross-sectional view of a vertical continuous casting machine illustrating an embodiment of the present invention. 1 is a mold, 2 is an immersion nozzle for pouring molten steel from a tundish into the mold 1, and 3 is below the mold 1 The arranged reference surface rolls 4 are opposed to the reference surface roll 3 and are moving surface rolls that are moved in accordance with the contraction amount of the slab. As shown in FIG. 2, the immersion nozzle 3 is installed perpendicular to the meniscus. As described above, the reference surface roll 3 is arranged vertically below the mold 1. However, the moving surface roll 4 is expected to contract the slab with cooling, and approaches the reference surface roll 3 as it goes downward. Are slightly inclined.

  For this reason, the center of the substantial slab thickness D at the lower part of the roll is shifted from the center of the thickness of the mold 1 to the reference surface roll 3 side. Therefore, in the present invention, the position of the immersion nozzle 2 for pouring the molten metal from the tundish (not shown) into the mold 1 is set so as to be the central portion with respect to the substantial slab thickness D shown in FIG. For this reason, the position of the immersion nozzle 2 is offset to the reference surface roll 3 side than before.

  As a result, in the vertical continuous casting method for a large-section slab according to the present invention, the pouring flow from the lower part of the immersion nozzle 2 enters the center of the solidified shell and collides almost uniformly with the reference surface side and the moving surface side. . Along with this, a larger number of equiaxed crystal nuclei are generated than before, so that the grain size of the equiaxed crystal generated in the central portion of the large-section slab during the cooling process can be made finer to improve toughness. Specifically, the grain size of the conventional equiaxed crystal was around 4 mm, but the present invention made it possible to reduce the size to about 1.5 mm.

  In addition, as shown in FIG. 3, the degree of superheat with respect to the liquidus temperature of the molten steel in the tundish and the equiaxed crystal ratio (the ratio of the equiaxed crystal zone thickness in the center of the slab thickness direction to the slab thickness) There is a correlation between them, and as shown in FIG. 4, when the equiaxed crystal ratio becomes 20% or less, the zaku volume ratio increases and the toughness decreases. For this reason, in order to make the zaku volume fraction less than 0.1%, which does not affect toughness reduction, the equiaxed crystal ratio needs to be 20% or more. For this purpose, the superheat degree of the tundish molten steel is 50 ° C or less. It is preferable that In addition, since there exists a possibility that continuous casting cannot be performed smoothly when this superheat degree becomes less than 10 degreeC, a preferable range is 10-50 degreeC like Claim 2.

  In addition, the nucleation of equiaxed crystals can be further promoted by casting the molten steel in the mold while stirring using the conventionally known electromagnetic stirring in the mold.

150 tons of carbon steel with a carbon concentration of 0.10 to 0.6% by mass, using a vertical continuous casting machine with a machine length of 10 m, a slab thickness of 380 mm while stirring the molten steel in the cross-section direction of the slab C with an electromagnetic stirring device in the mold A slab of ˜700 mm and a width of 1800 to 2400 mm was produced. The casting speed was 0.06 to 0.18 m / min, and the degree of superheat with respect to the liquidus temperature of the molten steel in the tundish was 10 to 70 ° C. There were two types of immersion nozzle positions: 1) the central part in the mold thickness direction and 2) the central part in the slab thickness direction. The results are summarized in Table 1.

The meanings of 1) to 5) in Table 1 are as follows.
1) The central part ± 2 mm in the mold thickness direction and the central part ± 2 mm in the slab thickness direction.
2) The degree of superheat with respect to the liquidus temperature of tundish molten steel.
3) The ratio of the equiaxed crystal zone thickness at the center of the slab thickness direction to the slab thickness.
4) A steel piece of width 200mm x length 150mm x thickness 20mm was cut out from the center of the slab width direction and the thickness center of the 1/4 width part, and a zaku was revealed by the X-ray transmission method.
5) The overall evaluation was “◯” when the equiaxed crystal ratio was 20% or more and the zaku area ratio was 0.1% or less.

  As is clear from the data of the above examples, according to the vertical continuous casting method for large-section slabs of the present invention, the slab thickness is generated at the center of a large-section slab for thick steel sheets having a thickness of 380 mm or more. It is possible to improve the toughness by reducing the grain size of the axial crystals.

It is sectional drawing which shows the conventional vertical type | mold continuous casting method. It is sectional drawing which shows the vertical type | mold continuous casting method of this invention. It is a graph which shows the relationship between molten steel superheat degree and an equiaxed crystal ratio. It is a graph which shows the relationship between equiaxed crystal ratio and zaku volume ratio.

Explanation of symbols

1 Mold 2 Immersion nozzle 3 Reference surface roll 4 Moving surface roll

Claims (3)

A reference surface roll vertically arranged below the mold, and a moving surface that is inclined so as to approach the reference surface roll as it goes downwards A continuous casting method in which a vertical continuous casting machine equipped with a roll is cast at a casting speed of 0.2 m / min or less, and the submerged nozzle is determined by the distance between the lowermost reference surface roll and the moving surface roll. A vertical continuous casting method for large-section slabs, wherein the casting is carried out by being installed perpendicularly to the meniscus at the center with respect to the thickness of the slab.   2. The vertical continuous casting method for large-section slabs for thick steel plates according to claim 1, wherein the superheating degree of the molten steel in the tundish is set to 10 to 50 [deg.] C. with respect to the liquidus temperature.   The vertical continuous casting method for a large-section slab for a thick steel plate according to claim 1 or 2, wherein the molten steel in the mold is cast while being stirred using electromagnetic stirring in the mold.

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