JPH06182516A - Method for continuously casting thin cast slab - Google Patents

Abstract

PURPOSE:To prevent internal crack by executing the rolling reduction with rolls at the upper surface side of a cast slab arranged just below a mold and making the cooling rate on the upper surface of the cast slab more than the cooling rate on the lower surface of the cast slab at the rolling reduction position. CONSTITUTION:At first, the rolling reduction is executed to a cast slab 2 obtd. by pouring a molten metal into the mold 3 and passing it through this mold at the rolling reduction zone. Cylinders 9, 10 arranged in a static zone succeeding to a rolling reduction zone are used supplementally to perfectly execute the rolling reduction. A cooling section where cooling rate is different between the upper surface and the lower surface is arranged over both of the rolling reduction zone and the static zone. The suitable cooling rate ratio of the upper surface to the lower surface of the cast slab is set to be 3:2-2:1. In the case of being 3:2 or below this cooling rate ratio, the cooling rate of the upper surface of the cast slab 2 is so insufficient that the preventing effect for the internal crack is reduced. On the other hand, in the case of being 2:1 or higher, the temp. difference between the upper surface and the lower surface of the cast slab 2 is so excessive that the surface crack caused by trapezoid deformation is easily 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 continuously casting thin slabs of good quality, and more particularly to a method for continuously casting thin slabs by unsolidification reduction.

[0002]

2. Description of the Related Art In recent years, thin slabs are produced in a continuous casting process against the backdrop of facilitation of the production of slabs with good quality properties due to remarkable progress in refining technology and casting technology, and the increase in labor-saving and energy-saving ideas. By making the hot rolling process abbreviated by manufacturing, attempts to directly and continuously manufacture thin sheet materials from molten metal have been made not only for non-ferrous metals with relatively low melting points but also for ferrous metals. became.

The following methods have been proposed so far as means for continuously casting thin metal slabs. (A) A continuous casting method using a belt type wall surface moving mold (vertical or horizontal). (B) Continuous casting method using a modified cross-section mold called "SMS (Schlehmann-Jimmark) type"
60-158955, JP 62-220249, JP 62
-203651, JP-A-62-203652).

In this SMS type continuous casting method, the top of the mold spreads out at the center of its long side to form a pouring section, and the pouring section of the long side is squeezed as it goes down to the slab discharge side. The present invention is characterized in that continuous casting is performed by the upper and lower open molds, which are the inverted triangular or rectangular (not shown) transition surfaces.

However, the "method of using the belt type wall surface moving mold" among them has a problem that the maintenance cost and the running cost are high due to the difficulty of cooling the belt, and the "immersion nozzle" is used in this type of mold.
Since it was difficult to dispose of, it was impossible to perform degassing pouring, and it was difficult to maintain the surface quality.

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 to gradually reduce the cross-sectional area of the slab in the mold, but this frictional resistance causes It has been pointed out that the wear of the inner surface of the mold is severe and the life of the mold is shortened. Moreover, because of the mold structure in which the cross section is gradually reduced, the fluctuation of the molten metal level is amplified by the mold oscillation, which adversely affects the quality of the slab. There was also a problem.

As described above, the conventional thin cast continuous casting method is
There are many unsolved problems from the viewpoint of stable production of thin slabs of sufficiently satisfactory quality under good workability, and the result is hot work especially in the industrial production of iron-based metal sheet materials. The current situation is that the rolling process has not reached the point where it can be largely omitted.

[0008]

As another means of obtaining a thin slab, a method is conceivable in which the thickness of the slab is reduced by pressing it before the inside of the slab is completely solidified. This is a method of reducing the thickness of the slab by rolling the roll apron band in which there is an unsolidified portion inside the slab. However, when the roll is rolled down, a tensile force acts on the solidification interface in the rolling position in the longitudinal direction of the slab, the solidification interface cracks in the width direction, and the concentrated molten steel enters. Becomes

It is an object of the present invention to develop a continuous casting method for thin cast pieces, which is a method of producing a thin cast piece by rolling down a cast piece having an unsolidified layer with less internal cracking.

[0010]

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 a tensile force in the longitudinal direction of the slab deviates from the solid-liquid coexistence region where the solid fraction is 0.8 to 0.99, the rolling strains are not integrated and internal cracking occurs. It gets harder. Therefore, it was considered effective to increase the cooling rate in order to reduce the solid-liquid coexistence region in the rolling reduction range. That is, the temperature gradient in the slab is increased to reduce the solid-liquid coexistence region.

However, if the cooling rate of the cast slab is too fast, the deformation resistance of the cast slab becomes too high and the reduction becomes difficult, so that the original intended thin cast slab cannot be obtained.

On the other hand, the method of rolling down the slab with a roll is best by uniformly rolling down the slab from the upper and lower surfaces. However, since it is very difficult to maintain the pass line of the slab, the roll on the lower surface is usually used. Is generally arranged at a constant R, and rolling is generally performed only by the roll on the upper surface. In this case, most of the short side portion is solidified but not yet solidified, so that most of the short side portion is pressed toward the upper shell. Therefore, we have found that it is only necessary to suppress the occurrence of internal cracks in the solidified shell on the upper surface of the slab.

As a result of comprehensively examining the above matters, the tensile strain acting on the solidification interface is integrated by increasing the cooling of the upper surface of the slab higher than that of the lower surface of the slab and increasing the temperature gradient on the upper surface side in the slab. It was found that internal cracking can be prevented by reducing the amount.

The gist of the present invention lies in the following method. A continuous casting method of a thin slab to produce a thin slab by pressing down the slab with an unsolidified layer, and performing the reduction with the roll on the upper surface side of the slab arranged immediately below the mold, the reduction position The continuous casting method for thin slabs, wherein the cooling of the upper surface of the slab is stronger than the cooling of the lower surface of the slab.

[0015]

[Function] The preferable cooling strength ratio between the upper surface and the lower surface of the slab is 3: 2.
~ 2: 1. When this cooling strength ratio is 3: 2 or less, the upper surface of the cast slab is not sufficiently cooled, and the effect of preventing internal cracking intended by the present invention becomes small. On the other hand, when the cooling strength ratio between the upper surface and the lower surface of the slab is 2: 1 or more, the difference in the cooling degree between the upper surface and the lower surface of the slab is too large, and the trapezoidal deformation and the trapezoidal deformation due to the difference in the shrinkage amount of the slab are caused. Is likely to cause surface cracking, and insufficient cooling of the lower surface tends to cause internal cracking due to increase in bulging strain. Therefore, it is desirable that the cooling strength ratio between the upper surface and the lower surface of the slab is 2: 1 or more.

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 by FIG. 2 showing the relationship between the solidification shell thickness of the slab and the distance from the meniscus. FIG. 1 shows a conceptual diagram of a continuous casting apparatus used in the manufacturing process of a thin cast piece by the unsolidified reduction.
The rolling is performed by a roll on the upper surface side of the cast slab, and the rolling zone is provided immediately below the mold. When the cooling rate is increased, the temperature gradient in the solidified shell is increased as shown in FIG. The range in which the internal strain is integrated is, as described above, the range of the solid fraction 0.8 to 0.99, and if the reduction zone is constant, the larger the temperature gradient is, the smaller the integration range becomes. The amount of tensile strain will be small. Therefore, even if the amount of reduction is the same, internal cracks are less likely to occur.

[0017]

EXAMPLE A multi-point straightening type continuous casting apparatus 1 comprising a driving roll 4, a pressing roll 5 and a pinch roll 6 shown in FIG.
The cast piece 2 poured into the mold 3 and passed through the mold 3 is first pressed in the pressing zone. In a normal state, the slab 2 is rolled down only by the two sets of cylinders 7.8 arranged in the rolling zone, but when the deformation resistance of the material is large and the speed of the slab becomes slow, the full rolling is performed. I won't let you know
The pre-arranged cylinders 9 and 10 are used in combination in the settling zone following the reduction zone to complete the reduction. In the present invention, the vertical differential cooling section is set over both the reduction zone and the statically determined zone.

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

[0019]

[Table 1]

In Table 2, the No. 1 strand is cooled with the same amount of water on the upper and lower surfaces of the cast slab as in the conventional case, and for No. 2 strand, the amount of cooling water on the upper surface of the slab is twice that of the lower surface according to the method of the present invention. As a result, the state of occurrence of internal cracks was compared between both strands. Further, in Table 3, the No. 1 strand was cooled with the same amount of cooling water on the upper and lower surfaces of the cast piece as in the conventional case, and the No. 2 strand was cooled according to the method of the present invention with a ratio of the amount of cooling water on the upper surface of the cast piece to the lower surface of the cast piece of 3 : 2, the occurrence of internal cracks was compared between both strands. The total amount of cooling water for all strands was 4200 liters / min.

[0021]

[Table 2]

[0022]

[Table 3]

FIG. 3A shows that the No. 1 strand produced by the conventional method in which the ratio of the cooling water amount on the upper surface and the lower surface of the slab is 1: 1 and the ratio of the cooling water amount on the upper surface and the lower surface of the slab is 2: 1. The No. 2 strand produced by the method of the present invention was subjected to S-printing of the longitudinal section of the slab, and based on this result, the following formula 1
The internal crack code represented by is calculated, and the internal crack occurrence status is compared by the value.

FIG. 3B shows that the No. 1 strand produced by the conventional method in which the cooling water amount ratio of the upper surface and lower surface of the slab is 1: 1 and the cooling water amount ratio of the upper surface and lower surface of the slab is 3: 2. The No. 2 strand manufactured by the method of the present invention is shown in FIG.
S-printing of the vertical section of the slab was carried out in the same manner as in, and the internal crack code represented by the following formula 1 was obtained based on this result,
The state of occurrence of internal cracks is compared by that value.

[0025]

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

In each case, the No. 1 strand of the conventional method has a severe internal crack and the value of the internal crack code is high, but the No. 2 strand using the method of the present invention has an internal crack core. -Low value, almost no internal cracking, and a thin cast piece with a thickness of 60 mm could be manufactured. From the above test results, the effectiveness of the method of the present invention was confirmed, and it became possible to manufacture a thin cast piece by the non-solidification rolling method.

[0027]

According to the method of the present invention, a thin slab having an unsolidified portion is pressed to produce a thin slab, which makes it possible to produce a thin slab with good quality without internal cracking. Moreover, it has become possible to greatly omit the hot rolling process.

[Brief description of drawings]

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

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

FIG. 3 (A) A longitudinal section of a slab of No. 1 strand produced by the conventional method and No. 2 strand produced by the method of the present invention in which the ratio of the cooling water amount on the upper surface and the lower surface of the slab is 2: 1. FIG. 3 is a diagram in which the S-printing is performed, the internal crack code represented by the following Expression 1 is obtained based on the result, and the internal crack occurrence states are compared by the value. (B) N produced by the method of the present invention in which the ratio of the cooling water amount of the No. 1 strand produced 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, S-printing of the longitudinal section of the slab was carried out. Based on this result, the internal crack code expressed by the following equation 1 was determined, and the internal crack occurrence status was compared by that value. is there.

[Explanation of symbols]

 1 Continuous casting device 2 Cast slab 3 Mold 4 Drive roll 5 Reduction roll 6 Pinch roll 7 Cylinder-8 Cylinder-9 Cylinder-10 Cylinder-

Claims (1)

[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 rolling is performed by a roll on the upper surface of the slab arranged immediately below the mold. The continuous casting method for thin cast pieces is characterized in that the cooling of the upper surface of the cast piece at the rolling position is made stronger than the cooling of the lower surface of the cast piece.