JP2888071B2 - Thin slab continuous casting method - Google Patents

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 thin cast slabs of good quality, and more particularly to a continuous casting process for thin cast slabs under unsolidified pressure.

[0002]

2. Description of the Related Art In recent years, thin cast slabs have been produced in a continuous casting process due to the remarkable progress in refining and casting techniques, which has facilitated the production of cast slabs having good quality properties, and have increased the concept of energy saving and energy saving. By manufacturing, the hot-rolling process is largely omitted, and the attempt to directly and continuously produce a thin sheet material from a molten metal is performed so that not only nonferrous metals having a relatively low melting point but also ferrous metals. became.

The following methods have been proposed as means for continuously casting thin metal slabs. (A) Continuous casting method using a belt type wall moving mold (vertical or horizontal). (B) A continuous casting method using a modified cross-section mold called “SMS (Schlehmann-Dimark) type”
No. 60-158955, JP-A-62-220249, JP-A-62
-203651, JP-A-62-203652).

In this SMS type continuous casting method, the top of the mold is expanded at the center of its long side to form a pouring section, and the long side pouring section is drawn down as it goes down to the slab exit side. It is characterized in that it is continuously cast by an upper and lower open mold which is a transition surface of an inverted triangular or rectangular shape (not shown).

[0005] However, the "method using a belt-type wall-moving mold" among them has a problem that maintenance cost and running cost are high due to difficulty in cooling the belt, and a "immersion nozzle" is used in this type of mold.
However, there is a problem that it is difficult to maintain the surface quality due to the difficulty in dispensing the molten metal because of the difficulty in disposing.

In the "SMS type continuous casting method", a large frictional force is generated between the inner surface of the mold and the surface of the slab because the cross-sectional area of the slab is reduced gradually but gradually. It is pointed out that the wear of the mold inner surface is severe and the life of the mold is shortened.In addition, due to the mold structure whose cross section is gradually reduced, the fluctuation of the molten metal surface is amplified by mold oscillation, which adversely affects the quality of the slab. There were also problems.

[0007] As described above, the conventional thin slab continuous casting method is as follows.
There are many unsolved problems from the standpoint of stable production of thin slabs of satisfactory quality under good workability, and the results have been particularly high in the industrial production of iron-based sheet metal. At present, it has not reached a region where the rolling step can be largely omitted.

[0008]

As another means for obtaining a thin slab, there is a method of reducing the thickness of the slab by reducing the pressure before the inside of the slab is completely solidified. This is a method in which the thickness of the slab is reduced by rolling down the roller with a roller apron band having an unsolidified portion inside the slab. However, when the roll is rolled down, a tensile force acts in the longitudinal direction of the slab at the solidified interface at the rolled position, so that the solidified interface cracks in the width direction and the concentrated molten steel penetrates. Becomes

An object of the present invention is to develop a continuous casting method of a thin slab which produces a thin slab by rolling down a slab having an unsolidified layer with less occurrence of internal cracks.

[0010]

The internal cracks occur when the integrated value of the tensile strain acting on the solidification interface having a solid fraction of 0.8 to 0.99 exceeds a limit value. Therefore, if the rolling position for generating the tensile force in the longitudinal direction of the slab deviates from the solid-liquid coexistence region having a solid phase ratio of 0.8 to 0.99, each rolling strain is not integrated and internal cracks are generated. It becomes difficult. Therefore, it was considered effective to increase the cooling rate in order to reduce the solid-liquid coexistence area in the range where the reduction was performed. That is, the temperature gradient in the slab is increased to reduce the solid-liquid coexistence region.

However, if the cooling speed of the slab is too high, the deformation resistance of the slab becomes too high to reduce the slab, making it difficult to obtain a thin slab which is the original purpose.

On the other hand, the method of rolling the slab by a roll is best by uniformly reducing the slab from the upper and lower surfaces of the slab. However, it is very difficult to maintain the pass line of the slab. Is generally arranged at a constant R, and the pressure is reduced only by the roll on the upper surface. In this case, the short side portion is solidified but most of it is not solidified, so that most of the pressure is reduced to the upper shell side. Therefore, it was found that it was only necessary to suppress the occurrence of internal cracks in the slab upper surface solidification shell.

As a result of a comprehensive examination of the above matters, the cooling of the upper surface of the slab is made larger than that of the lower surface of the slab, and the temperature gradient on the upper surface side in the slab is increased, so that the integrated tensile strain acting on the solidification interface can be calculated. It has been found that internal cracks can be prevented with a reduced amount.

The gist of the present invention lies in the following method. A continuous casting method of a thin slab for producing a thin slab by rolling down a slab having an unsolidified layer, wherein the slab is rolled down by an upper surface side roll of a slab disposed immediately below a mold, and the rolling position is reduced. Ratio of the cooling strength of the slab upper surface to the lower surface of the slab is 3: 2 to 2: 1
A method for continuously casting thin slabs, wherein cooling of the upper surface of the slab is made stronger than cooling of the lower surface of the slab.

The ratio of the cooling strength between the upper and lower surfaces of the slab is 3: 2 to 2.
Must be 2: 1. When the cooling strength ratio is 3: 2 or less, the cooling of the upper surface of the slab is not sufficient, and the effect of preventing internal cracking intended by the present invention becomes small. On the other hand, if the cooling strength ratio between the upper and lower surfaces of the slab is 2: 1 or more, the difference in cooling degree between the upper and lower surfaces of the slab is too large, and trapezoidal deformation and trapezoidal deformation due to the difference in shrinkage of the slab. Surface cracks are likely to occur due to the above, and internal cracks due to an increase in bulging distortion are likely to occur due to insufficient cooling of the lower surface. Therefore, the cooling strength ratio between the upper surface and the lower surface of the slab is set to 2: 1 or less.

The reason why the integrated value of the tensile strain at the solidification interface decreases due to the increase in the cooling strength of the slab is explained with reference to FIG. 2 which shows the relationship between the thickness of the solidified shell of the slab and the distance from the meniscus. FIG. 1 is a conceptual diagram of a continuous casting apparatus used in a thin slab production process by unsolidification reduction.
The rolling is performed by a roll on the upper side of the slab, and the rolling zone is provided immediately below the mold. Increasing the cooling rate increases the temperature gradient in the solidified shell, as shown in FIG. The range in which the internal strain is integrated is, as described above, a range of the solid fraction of 0.8 to 0.99. If the reduction zone is constant, the larger the temperature gradient, the smaller the integration range and the total amount. The amount of tensile strain will be small. Therefore, even if the amount of reduction is the same, internal cracks hardly occur.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a multi-point straightening continuous casting apparatus 1 comprising a driving roll 4, a pressing roll 5, and a pinch roll 6 shown in FIG.
The cast slab 2 that has been poured into the mold 3 and passed through it is first reduced in the reduction zone. In a normal state, the slab 2 is reduced only by the two sets of cylinders 7.8 arranged in the reduction zone. However, when the deformation resistance of the material is large and the speed of the slab is reduced, complete reduction is performed. Because it is not
The cylinders 9 and 10 which are preliminarily arranged in the settling zone following the reduction zone are used in combination to complete the reduction. In the present invention, the vertical difference cooling section is set over both the reduction zone and the static setting zone.

In FIG. 1, the size is 100 mm thick × 1500 mm using a multipoint straightening continuous casting machine having a radius of curvature of 5.0 m.
A slab of width was cast at a casting speed of 5.0 m / min. The rolling zone was a roller apron zone immediately below the mold, and the rolling was performed from 100 mm thickness to 60 mm thickness. As the steel type, a medium carbon steel having a high crack susceptibility and having the components shown in Table 1 was used.

[0019]

[Table 1]

In Table 2, in the No. 1 strand, the same amount of water was cooled on the upper and lower surfaces of the slab as in the conventional case, and in the No. 2 strand, the cooling water amount on the slab upper surface was twice that of the lower surface in accordance with the method of the present invention. Then, the occurrence of internal cracks was compared between the two strands. In Table 3, the No. 1 strand was cooled with the same amount of cooling water on the upper and lower surfaces of the slab as before, and the No. 2 strand was cooled with the ratio of the cooling water amount between the upper surface and the lower surface of the slab according to the present invention. : 2, the occurrence of internal cracks was compared between both strands. In addition, all the strands were cooled at a total cooling water amount of 4200 liter / min.

[0021]

[Table 2]

[0022]

[Table 3]

FIG. 3 (A) shows that the ratio of the cooling water amount between the No. 1 strand manufactured by the conventional method and the cooling water amount between the upper surface and the lower surface of the slab is 2: 1. For each of the No. 2 strands manufactured by the method of the present invention, S-printing of the slab longitudinal section was performed.
Is obtained by calculating the internal cracking code represented by the following formula, and comparing the occurrence of internal cracking based on the value.

FIG. 3B shows that the ratio of the cooling water amount between the No. 1 strand manufactured by the conventional method and the cooling water amount between the upper surface and the lower surface of the slab is 3: 2, where the ratio of the cooling water amount between the upper surface and the lower surface of the slab is 1: 1. Each of the No. 2 strands manufactured by the method of the present invention is shown in FIG.
The S-print of the vertical section of the slab was performed in the same manner as described above, and based on the result, an internal crack code represented by the following equation 1 was obtained.
The values are used to compare the occurrence of internal cracks.

[0025]

[Equation 1] Internal crack code = (Total internal crack length (m)) /
(Total slab length (m))

In each case, although the internal crack code was high in the No. 1 strand of the conventional method and the internal crack code was high, the internal crack code was high in the No. 2 strand using the method of the present invention. The value of-was low, there was almost no occurrence of internal cracks, and a thin slab 60 mm thick could be produced. From the above test results, the effectiveness of the method of the present invention was confirmed, and it became possible to produce thin slabs by the unsolidified rolling method.

[0027]

According to the method of the present invention, a thin slab is produced by rolling down a slab having an unsolidified portion to produce a thin slab of good quality without internal cracks. In addition, it is possible to largely omit the hot rolling step.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a continuous casting apparatus used in the thin cast continuous casting method of the present invention.

FIG. 2 is a diagram showing a relationship between a solidified shell thickness of a slab and a distance from a meniscus.

(A) No. 1 strand manufactured by the conventional method and No. 2 strand manufactured by the method of the present invention in which the ratio of the cooling water amount of the upper surface to the lower surface of the slab is 2: 1. 5 is a diagram in which S-printing is performed, an internal cracking code represented by the following equation 1 is obtained based on the result, and the internal cracking occurrence state is compared based on the value. (B) N manufactured by the method of the present invention in which the ratio of the cooling water amount of the No. 1 strand manufactured by the conventional method to the upper surface and the lower surface of the slab is 3: 2.
o. For each of the 2 strands, an S-print of the vertical section of the slab was performed for each strand, and based on the results, an internal cracking code represented by the following equation 1 was obtained, and the internal cracking occurrence status was compared based on the values. is there.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Continuous casting apparatus 2 Cast piece 3 Mold 4 Driving roll 5 Roll-down roll 6 Pinch roll 7 Cylinder 8 Cylinder 9 Cylinder 10 Cylinder

Claims (1)

(57) [Claims] 1. A continuous casting method of a thin slab for producing a thin slab by rolling down a slab having an unsolidified layer, wherein the slab is rolled down by an upper surface side roll of a slab disposed immediately below a mold. The ratio of the cooling strength between the upper surface and the lower surface of the slab at the rolling position is 3: 2 to 2
A continuous casting method for thin slabs, wherein cooling of the upper surface of the slab is made stronger than that of the lower surface of the slab as 2: 1 .