JPH08257715A - Continuous casting method - Google Patents

Abstract

PURPOSE: To enable the restraint of porosity by dividing rolls in the width direction of a cast slab, forming the roll diameter in the width corresponding to a prescribed solid phase ratio, executing the rolling reduction having the ratio compensating the shrinkage of vol. or higher at the time of solidifying and continuing the rolling reduction from the prescribed value of the solid phase ratio to perfectly solidifying. CONSTITUTION: The solidifying shrinkage rate in unsolidified part at the center part of the cast slab 1 at the inlet of a rolling reduction part, is obtd. and the necessary rolling reduction rate is obtd. from the material balance in the cross section. The rolling reduction can be executed preferentially to the center part of the cast slab 1 by using the divided rolling reduction roll 6 fixed to a segment frame 4 with a bearing 5. Since the rolling reduction is not directly executed to the bearing 5 part but the displacement having the same degree as the rolling reduction with the rolls 6 is executed, the rolling reduction can be executed only in the unsolidified width at the center part of the cast slab 1. The roll diameter in the width corresponding to the part having <=0.7 solid phase ratio in the cast slab 1 is formed to large to compensate the shrinkage of vol. or more and the rolling reduction is continued from 0.3 solid phase ratio in the cast slab to the perfect solidification and then, the porosity can be restrained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for suppressing the generation of center porosity of a slab in a steel continuous casting process.

[0002]

2. Description of the Related Art Center porosity and center segregation of a slab in a continuous casting process are caused by shrinkage during solidification. As a technique for preventing center segregation of a slab in a continuous casting process, it is disclosed in Japanese Patent Publication No. 3-6855. There is a method by light reduction of the slab. At the end of solidification, since the fluidity of molten steel becomes small,
Since the molten steel corresponding to the volume reduction due to solidification shrinkage is not supplied from the upstream, a void is generated in the central portion of the slab. This void is called center porosity. As a measure for preventing center porosity, there is a light reduction method of preventing molten steel flow by reducing the volume of the slab corresponding to the solidification shrinkage at the final stage of solidification by applying a light reduction to the slab. As a measure for center porosity, in a continuous casting machine, a roll located at the final solidification stage from the central solid fraction of the slab to 0.3 to the fully solidified position is used as a slab reduction roll, and the slab is deformed by light reduction. A method was developed.

In slab continuous casting, the width of the slab may exceed 2 m, and the rolls are bent because the distance between the bearings becomes large in an integral roll. When the roll bends, a sufficient force is not applied to the width center portion of the slab, and a predetermined slab deformation cannot be obtained at the slab center portion, which originally requires reduction. As a countermeasure against this, improvement of the roll strength is effective, but there is a restriction on the roll interval from the viewpoint of preventing bulging between the rolls, and it has been impossible to increase the roll diameter in order to increase the strength. In order to solve this, a technique for dividing rolls into at least two pieces in the width direction and providing a bearing to prevent roll deflection is developed.

[0004]

With the center porosity control method based on the light reduction of the split rolls, it is possible to evenly reduce the wide surface of a slab such as a slab having a large flatness ratio, thereby improving the center porosity. It was However, when the thickness of the cast piece is increased to improve the productivity, the deformation resistance of the solidified portion on the short side is large, and a predetermined reduction amount cannot be obtained.

An object of the present invention is to solve the above problems and to provide a continuous casting method capable of effectively improving the center porosity of a slab in a steel continuous casting process.

[0006]

SUMMARY OF THE INVENTION The present invention is a continuous casting apparatus having a mold and one or a plurality of light reduction segments along a direction in which a cast piece is drawn from the mold, wherein at least the light reduction segment is provided. Some have rolls divided into at least two or more in the width direction of the cast slab, and the roll diameter is changed to other widths corresponding to the region where the solid fraction of the slab is 0.7 or less. The diameter of the slab is larger than that of the part, and the increase of the diameter corresponds to more than the amount of reduction capable of compensating for the volume contraction at the end of solidification. This is a continuous casting method characterized by rolling down.

[0007]

The function will be described below with reference to the drawings. In FIG. 1A,
FIG. 3 is a cross-sectional view of a cast slab at the inlet of the draft zone, which has a non-solidified portion 2 in the center of the cast slab 7. When the amount of solidification shrinkage that occurs when this portion solidifies is calculated and the material balance in the cross section when there is no molten steel flow is considered, the amount of solidification shrinkage corresponding to the amount of solidification shrinkage 3 required by the roll shown in FIG. Is required.

FIG. 2 shows a schematic view of a roll under light pressure according to the present invention. By using this reduction roll 6, it is possible to preferentially reduce the central portion of the cast slab 7. Although the bearing 5 portion provided on the split roll does not directly perform the rolling reduction, the bearing 5 is displaced by the strength of the solidified portion 1 on the long side, which is equivalent to the rolling reduction. That is, it is not necessary to deform the entire width of the cast slab 7, and only the portion having the width corresponding to the required area may be volume-compensated with the required reduction amount. For that purpose, it was found that the solidification profile of the slab was obtained in the casting direction and the unsolidified width of the central portion was reduced by a roll.

Further, it has been found that the boundary between the unsolidified portion and the solidified portion is the solid fraction at the central portion of the thickness of the slab, and the optimum solid fraction of the molten steel is 0.7. It was If the rolling width of the roll is made narrower than this, the rolling of the end portion becomes insufficient and the center porosity becomes worse. Further, when the rolling width of the roll is set to have a solid phase ratio of 0.7 or more, the rolling efficiency is reduced by the deformation resistance of the solid phase portion of the end portion,
The deterioration of the center porosity is not so remarkable as compared with the case where the reduction width is narrow.

[0010]

[Example] Regarding a slab having a width of 2200 mm and a thickness of 300 mm,
The width of the unsolidified region in the region where the solid fraction at the center of the thickness of the cast strip at the inlet of the rolling zone is 0.7 or less is 1800 mm. This width changes depending on the casting speed and the secondary cooling conditions,
By keeping the operating conditions constant, the required roll reduction width can be determined in terms of equipment. FIG. 3 shows the center porosity evaluation index in the case where the central solid fraction of 0.3 to the completely solidified position is reduced by a dividing roll having a reduction width of 1800 mm and when the conventional dividing roll is used in the same condition. Here, the center porosity evaluation index is an index of the size of the center porosity and the number multiplied by the number of each size, and the larger the value, the worse the center porosity. In the present invention, it can be seen that the center porosity can be suppressed to one fifth of the conventional method.

[0011]

As a result of the method of the present invention, the center porosity can be suppressed in each step as compared with the conventional method, and as a result, the defects generated in the central portion of the final product after rolling are reduced and the yield of the final product is improved.

[Brief description of drawings]

FIG. 1 is a schematic view of a cross section of a slab and deformation of the slab due to light reduction of the present invention.

FIG. 2 is a schematic view of a light reduction roll according to the present invention.

FIG. 3 is a diagram showing the effect of improving the center porosity score according to the present invention.

[Explanation of symbols]

 1 Cross-section of cast piece (solidified portion) 2 Cross-section of cast piece (non-solidified portion) 3 Area equivalent to solidification shrinkage 4 Segment frame 5 Bearing 6 Rolling roll 7 Cast piece

Claims (1)

[Claims] 1. A continuous casting apparatus having a mold and one or more light reduction segments along a direction of withdrawing a cast piece from the mold, wherein at least a part of the light reduction segment is cast. It has a roll divided into at least two or more in the width direction of the slab, and the solid fraction of the slab is 0.7.
For the width corresponding to the following region, the roll diameter is larger than that of the other parts, and the diameter has an increment corresponding to a reduction amount or more capable of compensating for the volume contraction in the final stage of solidification. A continuous casting method characterized by rolling down a phase ratio of 0.3 to a completely solidified position.