JPH091292A - Continuous casting of thin slabs - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a continuous casting method for thin slabs using a modified cross-section mold and unsolidified reduction. [Structure] A thin slab of a certain size is cast using a mold having a flat octagonal horizontal cross section and a movable wall, and the thin slab is pressed in a non-solidified state by a guide roll immediately below the mold to form a thin cast. A method for continuously casting thin slabs in which the horizontal cross-sectional shape of the slab is shaped into a rectangle. In this method, the width and thickness can be changed during casting. [Effect] It is possible to stably manufacture a thin slab having a rectangular cross section without internal cracks.

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.

[0002]

2. Description of the Related Art In recent years, due to the fact that the production of slabs with good quality has been facilitated by the remarkable progress of refining technology and casting technology, and the idea of labor saving and energy saving has been increased, the hot rolling process has been greatly reduced. It seems that attempts to directly and continuously produce thin cast pieces (thin plate materials) from molten metal without omission or hot rolling will be applied to not only non-ferrous metals with relatively low melting points but also ferrous metals. Has become.

The following methods (1) to (3) have been proposed so far as means for continuously casting the thin cast pieces.

(1) Continuous casting method using a belt type wall surface moving mold (2) Continuous casting method using a modified cross-section mold (SMS method) (3) Twin roll type continuous casting method There are problems such as. In other words, in the belt type continuous casting method of (1) above, maintenance costs and running costs are high due to difficulty in cooling the belt, and degassing by the immersion nozzle, which tends to be difficult to dispose with this type of mold. Without casting it is very difficult to maintain surface quality. (2)
In the continuous casting method using the modified cross-section mold, in order to gradually reduce the cross-sectional area in the mold toward the casting direction, a large frictional force is generated between the mold inner surface and the slab surface,
This frictional resistance causes slab surface flaws and causes severe wear on the inner surface of the mold, resulting in shortened mold life. In the twin roll continuous casting method of (3), the molten metal flows so strongly that the inclusions are difficult to float and separate during roll pressure in the unsolidified portion, and segregation easily occurs.

As described above, the conventional continuous casting method of thin slabs has many unsolved problems from the viewpoint of stably producing thin slabs of sufficiently satisfactory quality with good workability. However, it has not reached the level of being able to substitute for the conventional method involving hot rolling particularly in the industrial production of iron-based metal sheet materials.

In particular, the hot water supply method for a thin mold is the biggest problem, and various proposals have been made for this.

The methods are classified into the following (1) and (2).

(1) Hot water supply using a modified cross-section mold (JP-A-60
-158955, JP-A-62-220249, JP-A-2-295659, and JP-A-2-500501) (2) Hot water supply by flattening the immersion nozzle (JP-A-58-74258)
, JP-A-60-12264, JP-A-60-130456, JP-A-62-1
97252, JP-A-62-292255, JP-A-63-53900) However, when the horizontal and cross-sectional shapes of the mold are modified, there is a problem of surface defects of the slab and the mold as described above. Also in the flat dipping nozzle hot water supply method, there are problems such as increase in discharge flow velocity and non-uniformity, and nozzle clogging.

[0009]

In the casting method in which the thickness of the mold is directly set as the target thickness of the cast piece as a means for producing wide and thin cast pieces, the shape of the dipping nozzle is set to the conventional thick-walled cylindrical shape. Since it is difficult to flatten the nozzle, the width of the nozzle discharge hole is narrowed to 20 to 30 mm, nozzle clogging and one-way flow easily occur, and stable hot water supply cannot be performed. Further, the molten steel discharge flow velocity from the nozzle is 10 to 20.
cm / sec becomes very large, and one-sided flow easily occurs.

That is, in order to flatten the shape of the immersion nozzle, it is necessary to make the wall thickness thinner and wider than the shape of a normal immersion nozzle, and in addition to a large heat radiation area, an area through which molten steel inevitably passes. Is also thinned, and nozzle clogging is likely to occur.

Further, since the bad influence of one-sided flow due to nozzle clogging is large, fluctuations in the molten metal surface and powder entrainment are likely to occur, resulting in deterioration of the quality of the slab.

[0012] Particularly, at the time of low flow rate hot water supply to wide and thin slabs, the flow of molten steel discharged from a normal dipping nozzle does not reach the vicinity of the short side wall of the mold, solidification of the molten metal surface occurs, and poor powder slag formation and nozzle Problems such as chipping are more likely to occur.

On the other hand, as a means for producing wide and thin slabs, in the method in which the mold cross section is deformed (reduced) toward the casting direction, and the immersion nozzle is of the conventional cylindrical shape, extra slag is applied to the surface of the slab. There is a concern that such frictional force may be applied to the surface of the mold, and at the same time, the mold itself may be loaded and a flaw may be generated on the inner surface of the mold to affect the life of the mold. In particular, it is said that it is very difficult to cast peritectic steel, which is highly susceptible to cracking, using this modified cross-section mold.

Further, in the above method, internal cracking easily occurs near the short side of the slab due to the unsolidified reduction. FIG. 6 is a cross-sectional view showing the shape of a slab before and after the reduction by a conventional rectangular horizontal section mold (parallel mold). As shown in the figure, since the short side of the slab is rolled, buckling causes the short side of the slab to deform in a convex shape. These defects lead to deterioration of internal quality and surface defects during rolling in the subsequent process.

The present invention has been made to solve the above problems, and an object of the present invention is to cast in an unsolidified state by casting using an octagonal horizontal cross-section mold whose width and thickness can be changed. It is an object of the present invention to provide a continuous casting method for producing a thin cast piece having a rectangular horizontal cross section by pressing the piece.

[0016]

The gist of the present invention is as follows.
It is in the continuous casting method of thin cast pieces of (1) and (2).

(1) A method for continuously casting thin slabs using a mold having a flat octagonal horizontal cross section, wherein a mold having the following conditions (1) to (4) is used to obtain a flat octagonal horizontal cross section: Continuous casting of thin slab characterized by shaping thin horizontal slab into rectangular shape by rolling thin slab with dimensions and pressing the thin slab with a guide roll just below the mold in unsolidified state. Casting method (hereinafter referred to as the first method of the present invention).

In the flat octagonal horizontal section, the thickness on the short side wall side of the mold is thin, the thickness on the long side wall side is thick, and the transitional portion from the thin portion to the thick portion changes linearly. .

The short side wall of the mold is a movable wall having a trapezoidal horizontal cross section that is movable in the width direction.

One of the long side walls of the mold is a movable wall that is movable in the thickness direction.

(2) At the start of casting, according to the first method of the present invention described in (1) above, a relatively wide and thick thin cast piece of a certain size is cast, and the thin cast piece in a non-solidified state is guided directly below the mold. While reducing the width and thickness during casting while shaping the horizontal section of the thin slab into a rectangular shape by pressing down.
And then cast a narrow thin slab with a rectangular horizontal cross section, and by subjecting it to reduction in the unsolidified state, the thin slab characterized by shaping the horizontal cross section of the thin slab into a rectangle. Continuous casting method (hereinafter, referred to as the second method of the present invention).

In the above item (1), the term "linear" means "a straight line or a substantially straight line that does not hinder the change of width or pulling out of a slab," and the above item (2) means "relatively". "Compare after width change" means respectively.

[0023]

The structure of the mold used in the method of the present invention will be described with reference to FIG. FIG. 1 is a horizontal sectional view of this mold.

The mold 3 used in the method of the present invention is composed of long side walls (copper plates) 1 and 1 and short side walls (copper plates) 2 and 2 of a water cooling structure, and each wall is independent. The horizontal cross-sectional shape of the mold 3 is a flat octagonal shape as shown in the figure, and the thickness T3 of the mold short side walls 2 and 2 is thin, and the thickness T1 of the central portion of the long side walls 1 and 1 is large. The transitional portion from the thin portion to the thick portion changes linearly.

The horizontal cross-sectional shape of the short side walls 2 and 2 is a trapezoidal shape as shown in the figure. The short side walls 2 and 2 are movable walls provided with normal width reduction mechanisms 4 and 4 that enable width reduction during casting. Therefore, one of the long side walls 1 and 1 is a movable wall provided with a similar normal thickness reducing mechanism (not shown), and at the time of width reduction, one of the long side walls 1 and 1 is simultaneously moved to reduce the thickness. It is necessary to change the width by changing the mold thickness at the same time. The reason why the horizontal cross-sectional shape of the short side walls 2 and 2 is limited to the trapezoidal shape is that the thickness decreases as the width is changed as described above. However, it is possible to change the width within this decrease and the further decreased thickness. This is because In FIG. 1, W1 is the mold width.

In the casting according to the first method of the present invention, the casting mold 3 shown in FIG. 1 is used to cast a thin slab of a certain size having a flat octagonal horizontal sectional shape. The immersion nozzle 5 is used to supply the molten metal to the mold 3. In FIG. 1, t1
Is the distance between the immersion nozzle 5 and the central portion of the long side walls 1 and 1 of the mold.

The preferable conditions of the mold, the dipping nozzle, and the casting speed are as follows.

Mold width (W1): 1000 to 1800 mm Thickness of mold central part (T1): 100 to 150 mm Length in the width direction of the transitional portion which changes linearly: 200 to
400 m (width of the thickness T1 portion = width of the central portion of the mold: 20)
0 to 1400 mm) Immersion nozzle: cylinder with outer diameter of 40 to 60 mm or regular square tube shape, or thickness of 40 to 60 mm, width of 80 to 150
mm flat tube shape 2 discharge holes (inner diameter 20 to 30 mm), downward 20 to 4
Distance between the 5 ° immersion nozzle and the center of the long side wall of the mold (t1
): 20-55 mm Casting speed: 3.0-7.0 m / min For example, target thickness 70 mm, width 1000-1800 mm
When casting thin steel slabs of No. 2 at a casting speed of 5.0 m / min, the mold shape is such that when the width of the mold central portion is 1000 mm, the thickness of the central portion of the mold (T1) is 100 mm, and both short sides The thickness (T3) is a flat octagon with a target slab thickness of 70 mm. The shape of the immersion nozzle has an outer diameter or thickness of 60 mm,
A two-hole nozzle having a discharge angle of 45 ° downward (inner diameter 30 mm) is used, and at this time, the separation distance t1 is 20 mm.

In the first method of the present invention, only the central portion on the long side of the thin cast piece having the flat octagonal horizontal cross-sectional shape obtained by the above casting, that is, only the thick T1 portion is directly under the mold in the unsolidified state. By rolling down with the guide rolls of 3 to 3 stages to 5 to 5 stages provided in, the horizontal cross-sectional shape is shaped into a rectangle while maintaining the short side thickness T3 of the thin cast piece. This example will be described with reference to FIGS.

FIG. 2 is a schematic vertical sectional view for explaining an example of the rolling-down method. As shown in the figure, the three-to-three-tiered guide rolls 6 provided immediately below the mold 3 are used to sequentially reduce the unsolidified pressure from both sides of the long side of the thin slab 7 so that the thickness of the central portion becomes from T1 to T3. Give. The thickness T3 is the thickness T3 on the side of the short side wall of the mold.

For example, casting with T1 set to 100 mm,
When the reduction casting is completed as a thin slab shaped into a rectangular shape with a thickness (T3) of 70 mm, the total reduction amount is 30
mm uncoagulated reduction, 30mm reduction pattern is 1
It is desirable to have a top strong reduction pattern of 5 mm → 10 mm → 5 mm. This is because, according to the strain integration model, strain is less accumulated in the top strong reduction, so that the occurrence of internal cracks in the thin cast piece can be suppressed.

FIG. 3 is a horizontal sectional view showing an example of changes in the shape of the slab before and after the reduction. According to the above-mentioned reduction method, since there is no rolling of the short side of the slab, the shape of the short side remains rectangular even after reduction as shown in Fig. 3, and surface cracking occurs during rolling in the post process. Therefore, a good coil can be manufactured.

The horizontal sectional shape of the mold is a flat octagon,
In the case of the first method of the present invention in which a slab having the same cross-sectional shape is manufactured and the unsolidified pressure is reduced as shown in FIG. 3, only the collapse of the solidified shell occurs during the unsolidified pressure, and the short side shape is Three
It remains rectangular after reduction as shown in. As a result,
As shown in FIG. 6, internal cracks that occur during convex deformation due to buckling during rolling of the short side during unsolidification reduction from a rectangular cross-section slab cast from a mold whose horizontal section is originally rectangular , Can be suppressed.

Further, in order to prevent the above-mentioned convex deformation under the unsolidified pressure from the rectangular slab as shown in FIG. 6,
Although an appropriate rolling device for the short side such as a hydraulic cylinder is also required, the rolling in the method of the present invention does not involve rolling of the short side, and no special rolling device is required. There is also an advantage that shaping under coagulation can be performed.

In the second method of the present invention, using the mold shown in FIG. 1, according to the first method of the present invention at the start of casting, a fixed dimension having a flat octagonal horizontal cross-sectional shape and after the subsequent width change. A thin slab that is wider and thicker than that is cast, and the slab is pressed by a guide roll immediately below the mold in the unsolidified state by the method shown in FIGS. 2 and 3. And change the width during casting,
Continuously cast a narrow thin slab with a rectangular horizontal cross section, and by similarly pressing this slab with a guide roll directly below the mold in the unsolidified state, the horizontal cross section of the slab is further shaped into a rectangle. A thin cast piece is obtained. In this case, it is the same as the casting and rolling method from the start of casting to the width change, and the first method of the present invention. After that, the width is changed during casting. The shape of the mold in this case will be described with reference to FIG.

FIG. 4 is a horizontal sectional view showing the mold after changing the width in the case of carrying out the second method of the present invention. In the width change, the short side walls 2 and 2 are moved until there is no transitional portion where the thickness of the mold 3 changes linearly or more, and the mold width W1 is reduced to W2, and the long side walls 1 and 1 are changed. Either one is moved so that the mold thickness is reduced from T1 to D2, and the horizontal cross-sectional shape of the mold 3 is made rectangular.

Since it is necessary to install the immersion nozzle 5 at the thickness center of the mold 3 at the same time as the thickness changes, the position of the immersion nozzle 5, that is, the position of the tundish (not shown) is changed at the same time as the width is changed. .

In order to reduce the width of the slab, it is common to change the width from a wide width to a narrow width because of the physical distribution in the rolling process. Second of the present invention
Also in the method, as described above, the thickness is changed from the thick one to the thin one.

The preferable conditions of the mold after the width change, the dipping nozzle and the casting speed are as follows.

Mold width (W2): 200 to 1400 mm Thickness of mold center (D2): 90 to 100 mm Distance between immersion nozzle and center of long side wall of mold (t2
): 15 to 25 mm Casting speed: 3.0 to 7.0 m / min Even in this case, the immersion nozzle 5 and the mold long side wall 1, 1
The distance t2 from the center of the mold can be at least 15 mm, and casting is possible without causing problems such as nozzle breakage due to solidification (skinning) of molten steel between the mold 3 and the immersion nozzle 5. .

One of the problems in casting a thin slab of steel having a thickness of about 90 to 120 mm is breakage of the immersion nozzle due to solidification of the molten metal surface at the start of casting. However, the second aspect of the present invention
By applying the method, it is possible to solve this problem. That is, the thickness of the slab at the start of casting is 10 at the center of the mold.
0 mm or a little more than this is desirable, and as the casting is continued by the second method of the present invention, the cast piece width becomes narrow and the cast piece thickness becomes thin. It is more relaxed than when using a 70 mm parallel (rectangular horizontal section) mold.

If a dipping nozzle having an outer diameter of 40 mm is used in the above method, a thin cast piece having a thickness of 70 mm can be manufactured.
If a conventional parallel mold is used, the distance (t2) between the immersion nozzle and the center of the long side wall of the mold is 5 mm or less, and skinning occurs and it is difficult to continue casting. Therefore, the outer diameter of the immersion nozzle must be 30 mm or less, and it is substantially difficult to supply hot water from the tundish.

After the width change described above, the obtained thin thin strip having a rectangular horizontal cross section was subjected to the same unsolidification reduction method as described above,
Further, it is a narrow thin slab shaped into a rectangular shape.

As the mold width decreases, the slab thickness also decreases, and the guide rolls directly below the mold are set so as to have a constant thickness interval, and no reduction device is provided.
Does not follow the decrease in the thickness of the slab. For this reason, the slab bulges due to lack of support, but the slab is squeezed between the guide rolls of 3 to 3 stages to 5 to 5 stages and unsolidified to the target thin slab thickness. It doesn't become a big problem.

Further, in this case, the horizontal cross section of the unsolidified slab after the width change is changed from a rectangular shape to a rectangular shape, but the original short piece thickness (D2) is smaller than that when the conventional parallel mold is used. Since it is small, it is possible to reduce the convex side as shown in FIG.

The method of the present invention can be applied to non-ferrous metals such as copper and aluminum having a relatively low melting point regardless of the type and steel type except for the horizontal continuous casting machine.

[0047]

【Example】

(Test 1) A low carbon aluminum killed steel having the composition shown in Table 1 was cast under the following conditions using a 2-strand curved slab continuous casting machine.

[0048]

[Table 1]

Inventive Example 1 The mold of FIG. 1 was used for one strand (W1 = 1800 m, T1 = 100 mm, T3 = 7).
0 mm, t1 = 25 mm) The outer diameter of the dipping nozzle is 40 mm. The unsolidified pressure at the center of the long side of the slab is as shown in FIG.
0mm, T3 = 70mm, rolling pattern is 15mm → 1
(Comparative example 1: using a conventional parallel mold for 2 strands (thickness 70 mm, width 1800 mm, t1 = 20 m)
m) Immersion nozzle outer diameter is 30 mm No unsolidified rolling at the center of the long side of the cast piece Casting speed: 5.0 m / min for both strands As a result, 100 ton ladle on the one strand side to which the first method of the present invention is applied It was possible to cast a maximum of 7 consecutively.
On the other hand, on the two-strand side of the conventional mold, the outer diameter of the dipping nozzle was only allowed up to 30 mm, and the nozzle was clogged as early as the completion of casting of 100 tons of molten steel, making it impossible to continue casting.

(Test 2) Using the continuous casting machine of Test 1,
A low carbon aluminum killed steel having the composition shown in Table 1 was cast under the following conditions.

Example 2 of the present invention: 1 strand with the same conditions as Example 1 of the present invention Comparative Example 2: 2 strands with the same conditions as the above-mentioned Example 2 of the present invention with respect to the mold and immersion nozzle. Method (T1 = 10
0 mm, T3 = 70 mm, rolling pattern is 5 mm → 10
mm → 15 mm bottom pressure reduction) Comparative Example 3: Using a conventional parallel mold for 2 strands (thickness 100 mm, width 1800 mm, t1 = 25 m)
m) The unsolidified reduction in the central part of the long side of the slab is the same top strong reduction as in the above-mentioned Inventive Example 2 Casting speed: 5.0 m / min in both cases, thin after the unsolidified reduction obtained in Inventive Example 2 and Comparative Example 3 When the cross-sectional shapes of the slabs are compared, in the present invention example 2, the shape is close to a rectangle as shown in FIG. 3, whereas in the comparative example 3, the short side is a convex shape as shown in FIG. Was presented. As a result of subjecting these slabs to rolling in a post-process for comparison, in the case of Comparative Example 3, surface defects frequently occurred in the vicinity of the short side of the rolled material, but in the case of the present invention 2, surface defects occurred. I confirmed that there is no.

When the bottom strong reduction of Comparative Example 2 was applied, internal cracking occurred not only in the corners of the short pieces but also in the center of the long pieces.

FIG. 5 is a diagram showing the difference in the internal crack occurrence state between the present invention example 2 and the comparative examples 2 and 3. As shown in the figure, in the present invention example 2, no internal cracks were generated, whereas in the comparative example 3, internal cracks were found near the corners. Comparative example 2 in which a strong bottom reduction was applied
Then, the tendency was remarkable.

(Test 3) Using the continuous casting machine of Test 1,
A low carbon aluminum killed steel having the composition shown in Table 1 was cast under the following conditions.

Inventive Example 3 A 1: 1 strand was cast under the same conditions as in Inventive Examples 1 and 2 in the initial stage of casting, the width was changed during casting as follows, and the rolling condition thereafter was 1
A thin cast piece having a minimum width of 1000 mm and a thickness of 70 mm was manufactured as a top strong reduction pattern of 5 mm → 10 mm → 5 mm.

W1 = 1800 mm → W2 = 1000 mm (5 steps for each 200 mm) T1 = 100 mm → D2 = 70 mm Comparative Example 4: A parallel mold capable of changing the width of two strands was used, and Comparative Example 3 was used at the initial casting stage. Casting was performed under the same conditions, the width was changed during casting as described below, and then, with the same top strong reduction pattern as in Inventive Example 3, the width was 1000 m.
m, a thin slab having a thickness of 40 mm was produced.

Width = 1800 mm → 1000 mm Thickness = 100 mm → 70 mm Comparative Example 5: A parallel mold whose width can be changed in two strands is used, and in the initial stage of casting, thickness = 70 mm, width = 1.
800 mm, t1 = 10 mm, other conditions were cast under the same conditions as in Comparative Example 3, only the width was changed during casting as described below, and thereafter the unsolidified rolling was not performed, and the width was 1000 mm,
A thin cast piece having a thickness of 70 mm was manufactured.

Width = 1800 mm → 1000 mm Casting speed: 5.0 m / min in all cases Before changing the width of Inventive Example 3, a thickness of 7 without internal cracking.
A thin slab having a width of 0 mm and a width of 1800 mm was obtained. After changing the width, the short piece became a convex shape, but the protruding amount was 5 m
As a result, a thin cast piece having a thickness of 70 mm and a width of 1000 mm, which was as small as m and had no internal cracks, was obtained.

Before the width change of Comparative Example 4, the thickness is 70 mm,
Although a thin cast piece having a width of 1800 mm was obtained, it became a convex shape with a large protruding amount of the short piece of 15 mm due to the unsolidified pressure.

Furthermore, internal cracking occurred near the corners of the cast slab. After changing the width, the thickness is 70m due to unsolidified reduction.
Although a thin slab with m and a width of 1000 mm was obtained, the short slab shape and internal cracking of the slab were not as good as those before the width change.

In Comparative Example 5, the superheated degree of molten steel (ΔT) is low immediately after the start of casting and the casting speed is slow, so that the flow of molten steel in the mold is small. Therefore, in this example, the separation distance (t1) between the immersion nozzle and the center of the long side wall of the mold is as small as 10 mm. In this example, skinning (solidification) occurs between the inner wall of the mold and the immersion nozzle immediately after the start of casting, It became difficult to continue.

[0062]

EFFECTS OF THE INVENTION According to the unsolidified continuous casting method using a flat octagonal irregular cross-section mold of the present invention, it is possible to stably produce a thin cast piece having a rectangular cross section without internal cracks.

[Brief description of the drawings]

FIG. 1 is a horizontal sectional view of a mold used in the method of the present invention.

FIG. 2 is a schematic vertical cross-sectional view for explaining an example of a method for rolling down unsolidified slab just below a mold used in the method of the present invention.

FIG. 3 is a cross-sectional view showing a shape of a slab before and after unsolidified reduction by the method of the present invention.

FIG. 4 is a horizontal cross-sectional view after changing the width of the mold used in the method of the present invention.

FIG. 5 is a diagram showing a difference in the state of occurrence of internal cracks in a thin cast piece.

FIG. 6 is a cross-sectional view showing a shape of a slab before and after unsolidified pressure in the case of a conventional rectangular horizontal cross-section mold.

[Explanation of symbols]

1: long side wall, 2: short side wall, 3: mold, 4:
Width reduction mechanism, 5: dipping nozzle, 6: guide roll, 7: thin slab

Claims (2)

[Claims] 1. A method for continuously casting thin slabs using a mold having a flat octagonal horizontal cross section, wherein the flat octagonal horizontal cross section has a small thickness on the short side wall side of the mold and a center on the long side wall side. The thickness of the part is thick, and the transitional part from the thin part to the thick part changes linearly, and the trapezoidal horizontal movable wall that is movable in the width direction as the mold short side wall is Using a mold with two movable walls, each of which is movable in the thickness direction,
Forming a thin slab of a certain size having a flat octagonal horizontal cross section, and shaping the thin slab into a rectangular horizontal cross section by pressing the thin slab with a guide roll directly below the mold in the unsolidified state. A method for continuously casting thin slabs. 2. A method for continuously casting thin slabs using a mold having a flat octagonal horizontal cross section, wherein the flat octagonal horizontal cross section is thin on the short side wall side of the mold and is centered on the long side wall side. The thickness of the part is thick, and the transitional part from the thin part to the thick part changes linearly, and the trapezoidal horizontal movable wall that is movable in the width direction as the mold short side wall is Using a mold with two movable walls, each of which is movable in the thickness direction,
At the start of casting, a comparatively wide and thick thin slab with a flat octagonal horizontal cross section is cast, and in a non-solidified state, the thin slab is pressed down by a guide roll just below the mold to obtain a horizontal cross section of the thin slab. While shaping into a rectangular shape, the width and the thickness are reduced during casting, and then a narrow thin slab with a rectangular horizontal cross section is cast, and further reduction is performed in the unsolidified state by further reducing the slab. The method for continuously casting thin slabs, which comprises shaping the horizontal cross-sectional shape of the above into a rectangle.