JPH0780615A - Production of continuously cast slab having excellent internal quality - Google Patents

Abstract

PURPOSE:To stick the central cavity by pressing and to obtain a continuously cast slab having excellent internal quality without internal crack by continuously casting a low carbon molten steel and executing rolling reduction in a specific condition of the rolling reduction during the unsolidified condition in the center part of the cast slab at the end period of the solidification. CONSTITUTION:Molten steel having $0.18C is continuously cast and one time of the rolling reduction is executed at 2-5% rolling reduction ratio to the part having 90-98% solid phase ratio fS in the center part of the cast slab at the end period of the solidification with rolling reduction rolls having the diameter of 2-5 times of the cast slab thickness. In the case of being <90% the solid phase ratio fS, the internal crack is developed because the solidified shell is weak, and in the case of being >98%, the center cavity is not stuck by pressing because the rolling reduction is not affected to the center. Further, in the case of being >5% the rolling reduction ratio, negative segregation is developed, and in the case of being <2%, the center cavity is not stuck by pressing. Further, in the case of being >=2 the ratio of the roll diameter and the cast slab thickness, the cross sectional area compressive rate and the deformed rate are made to be large and contribute to the sticking of the central cavity by pressing, but in the case of being >=5 this ratio, the bulging in the cast slab between the rolling reduction rolls is developed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuous cast slab having excellent internal quality, and more particularly to a method for producing a continuous cast slab in which a central cavity of low carbon killed steel is crimped. .

[0002]

2. Description of the Related Art The center cavity of a continuously cast slab is one in which solidification shrinkage holes remain. In particular, in low carbon killed steel, the growth of columnar crystals is remarkable due to the narrow solid-liquid coexistence region, and the formation of equiaxed crystals is small.Therefore, solidification shrinkage causes voids to form in the central part, which is the central part. Remain as pipe-shaped continuous holes. This hole is the slab of the subsequent
It remains even after the rolling process, resulting in ultrasonic flaws in the product.

As described above, since the central cavity causes internal defects in the product, various methods for eliminating it have been proposed. One of them is a method of preventing the formation of a central cavity.This method electromagnetically stirs the molten steel in the mold to generate a large amount of equiaxed crystals, and further electromagnetically stirs the residual molten steel at the end of solidification to obtain a temperature gradient of the residual molten steel. By reducing solidification to cause solidification to proceed almost simultaneously to confine and disperse pores between crystal grains, thereby suppressing the generation of continuous or intermittent pores.

Another method is a method of crimping the generated central cavity in the next step, which is a method of crimping the central cavity by using the central strong reduction rolling technique at the time of lumping or rolling. In addition, a combination of the above two methods is also performed.

[0005]

However, none of the methods can completely eliminate the harmful effects of the central cavity. In order to crimp the central cavity that has been generated once, a large amount of reduction is required to apply a force to the center of the slab. Therefore, even if the reduction is performed after completion of solidification or in the agglomeration or rolling process, the reduction effect on the central portion of the slab where the cavity is present is small, and a large amount of equipment is required to obtain a reduction amount to compensate for this.

On the other hand, in continuous casting, if the reduction is carried out by the end of the solidification, the reduction effect on the center of the slab becomes relatively good. However, when the solid fraction at the center of the slab (hereinafter referred to as fs) is small, if a large amount of reduction is performed, a large tensile strain (internal strain) is generated in the solidification interface and in the vicinity of the strengthless solidification shell, Internal cracks easily occur. Further, if the reduction amount is reduced, the central cavity is not crimped.

The present invention has been made in order to solve the above problems, and is a continuous cast slab excellent in internal quality in which the central cavity is pressure-bonded by performing reduction while the center of the slab is not solidified. It aims at providing the manufacturing method of.

[0008]

[Means for Solving the Problems] Molten steel with C ≦ 0.18% is continuously cast, and at the final stage of solidification of the cast slab, a portion having a solid fraction of 90 to 98% at the center of the cast slab is It is a method for producing a continuously cast slab having excellent internal quality in which a reduction roll having a diameter of 2 to 5 times the diameter is pressed once at a reduction rate of 2 to 5%.

[0009]

In carbon steel, there is a relationship as shown in FIG. 1 between the carbon concentration in steel and the equiaxed crystal ratio, and the equiaxed crystal ratio tends to increase as the carbon concentration increases. As described above, when the production of equiaxed crystals is reduced during the solidification process, the central cavity is easily generated. Therefore, in the present invention, the C content is set to 0.
Limited to 18% or less.

FIG. 2 shows the relationship between fs (solid phase ratio at the center of the slab) and the rolling reduction rate, which influence the internal properties of the slab when the final solidification stage of the slab is rolled once. As is clear from the figure, internal cracking occurs when the fs is less than 90% because the solidified shell is weak, and when fs exceeds 98%, the reduction does not work to the center and the central cavity is not crimped. Even if fs is 90 to 98%, if the reduction rate exceeds 5%, negative segregation occurs and the reduction rate is 2%.
If it is less than%, the central cavity is not crimped. Therefore,
In the present invention, fs is 90 to 98% and the rolling reduction rate is 2 to 5%.
%, And it was decided to roll down once.

Next, the reason why the diameter of the reduction roll is limited to 2 to 5 times the thickness of the slab in the present invention will be described. Figure 3 was obtained by numerical analysis by the finite element method, and the final solidification part of a 300 mm thick slab had the same reduction rate (about 2.7%) and the roll diameters were 400 mm, 600 mm, 10 mm.
It shows changes in the internal strain of the slab, the amount of compression of the cross-sectional area of the unsolidified portion, and the amount of deformation of the unsolidified portion when the pressure is changed to 00 mm and reduced.

As is apparent from FIG. 3, if the roll diameter is more than double the ratio of the thickness of the cast piece to the same rolling reduction ratio, the amount of compression of the cross-sectional area of the unsolidified portion and the amount of deformation of the unsolidified portion are increased. It greatly increases and greatly contributes to the pressure bonding of the central cavity, but if it is less than twice, the amount of compression of the cross-sectional area of the unsolidified portion and the amount of deformation of the unsolidified portion are small, and the pressure bonding effect of the central cavity cannot be expected. For this reason, the diameter of the reduction roll is set to 2 times the thickness of the slab.
Limited to more than double. On the other hand, the upper limit of the diameter of the reduction roll is limited to 5 times or less the thickness of the slab, because if the thickness is 5 times or more, the spacing between the reduction rolls becomes too large and bulging of the slab occurs between the reduction rolls. This is because the action of reducing the final stage of coagulation cannot be obtained sufficiently.

[0013]

EXAMPLES Examples of the present invention will be described below. Molten steel having the chemical composition shown in Table 1 was cast with a continuous casting machine to a slab thickness of 300 mm, and the final stage of solidification was reduced once under the conditions shown in Table 2. The pressure-bonded state of the center cavity of the cast piece thus obtained, the presence or absence of internal cracks, and the ultrasonic flaw-failure index of the product are also shown in Table 2.

[0014]

[Table 1]

[0015]

[Table 2]

As shown in Table 2, No. 1 of the method of the present invention,
In Nos. 2 and 3, the central cavity was crimped, there was no internal cracking, and the ultrasonic testing failure index of the product was 0. On the other hand, in Comparative Example No. 4, the solid fraction in the center of the slab is low, so internal cracking occurs, and the pressure bonding in the center cavity is not good
The ultrasonic testing failure index of the product was 20. No. 5 is the reduction work after the solidification of the center of the slab, and No. 6 has the reduction work rate of 0, so the center cavity is not pressure-bonded in all cases, and the ultrasonic flaw rejection index in the product Was 15.

[0017]

As is apparent from the above description,
According to the present invention, it is possible to obtain a continuously cast slab with excellent internal quality, in which the central cavity is pressure-bonded and has no internal cracking.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between carbon concentration in steel and equiaxed crystal ratio.

FIG. 2 is a diagram showing a relationship between fs that affects the internal properties of a cast piece and a rolling reduction rate.

FIG. 3 is a diagram showing a relationship between a roll diameter obtained by numerical analysis by a finite element method, an internal strain of a slab, a sectional area compression amount of a non-solidified portion, and a deformation amount of the non-solidified portion.

Claims (1)

[Claims] 1. A molten steel having C ≦ 0.18% is continuously cast, and at the final stage of solidification of the slab, a portion having a solid fraction of 90 to 98% in the central portion of the slab is 2 to the thickness of the slab. A method for producing a continuously cast slab having excellent internal quality, characterized in that a reduction roll having a diameter of 5 times is used to perform reduction once at a reduction rate of 2 to 5%.