JP2001058247A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method, by which a thin cast slab excellent in the surface quality and the inner quality can be obtd. at a low producing cost. SOLUTION: This continuous casting method is executed by using a vertical- bending type continuous caster, of which the length of the vertical part 4 below the outlet of a mold is 1.0-2.5 m and of which the curving radius R at the bending part below the vertical part is 3-4 m and in the range of the vertical part below the outlet of the mold or the range of the bending part, starting the rolling reduction of the cast slab with plural rolling reduction roll pairs 5. The rolling reduction is applied to the cast slab having the rectangular sectional shape having the thickness at the outlet of the mold of 100-150 mm to obtain the thin cast slab having 30 mm to <100 mm thickness.

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 of steel capable of obtaining a thin slab of excellent surface quality and internal quality at a low production cost.

[0002]

2. Description of the Related Art In the production of a hot coil wound from a hot-rolled steel strip, a continuous casting method of a thin slab is combined with a rolling method using a simple hot rolling facility arranged on a casting line. Methods have been put to practical use. The rolling material of this simple hot rolling equipment has a thickness of 50 to 100.
mm thin slab is used.

As a method of obtaining a thin cast piece having a thickness of about 50 mm, the shape of the long side of the mold inlet at the position where the immersion nozzle is inserted is bulged outward, and the bulge is reduced along with the casting direction. The slab thickness is 50 at the mold exit
There is a method of casting a rectangular thin slab of about mm. However, in this method, since the surface layer portion of the slab is deformed in the mold, cracks are easily generated on the slab surface.

[0004] As a countermeasure against this, when using a mold having the same thickness of the slab at the inlet and the outlet as generally used, the thickness of the slab at the outlet of the mold is reduced to 50 mm.
It is difficult to continuously cast a thin slab of about mm. This is because long-time casting is difficult because the thickness of the immersion nozzle cannot be increased. Therefore, the lower limit of the thickness of the slab at the exit of the mold that can be practically cast is about 90 mm.
Therefore, a thin slab having a thickness of about 90 mm is cast at the exit of the mold, and while the unsolidified molten steel is present at the center of the thickness of the slab, the slab is reduced to a thickness of about 50 mm (not yet). Coagulation pressure) method.

In recent years, a hot coil manufactured by a combination of such a continuous casting method of thin cast slabs and a rolling method using a simple hot rolling equipment has been required to have excellent quality such as automotive parts. It is being applied to other products. Thin cast slabs, which are materials for such automobile parts, are required to be excellent in both surface quality and internal quality and to be inexpensive in manufacturing costs.

In order to obtain a thin slab of excellent surface quality,
It is required to use a mold in which the thickness of the slab at the inlet and the outlet is substantially the same. Since the surface layer of the slab is not deformed in the mold, a thin slab with excellent surface quality can be obtained. Further, in order to obtain a thin slab having excellent internal quality, it is required that the thickness of the slab in the mold at the time of casting be greater than about 90 mm. This is because oxides and the like in the molten steel in the mold float up and are easily adsorbed by the molten powder.

On the other hand, from the viewpoint of reducing the manufacturing cost, it is required to obtain a thin slab having a thickness of less than 50 mm by unsolidification reduction in order to make the hot rolling equipment simple equipment.

For example, when the thickness of the slab at the exit of the mold is 10
In order to cast a thin slab thicker than 0 mm and thinner than 50 mm by unsolidification reduction, it is necessary to reduce the slab below the mold outlet. This is because in a region further downstream than the outlet of the mold, solidification is progressing, and it is difficult to obtain a thin slab having a thickness smaller than 50 mm due to unsolidification pressure.

As a method of rolling down a slab at a vertical portion below a mold outlet, Japanese Patent Application Laid-Open No. 63-264250 discloses a method in which a slab having a thickness of 40 to 50 mm at a mold outlet is placed in a region below the mold outlet. A method has been proposed in which an extremely thin slab of 2 to 25 mm is cast down. With one reduction roll pair,
This is a method of rolling down a slab including an unsolidified portion.

However, according to this method, a large stress generated at the time of rolling extends to the solidified shell in the mold, and cracks are generated on the surface of the slab, so that a slab with good surface quality cannot be obtained. Further, the solidified shell breaks and breakout is likely to occur.
In addition, a cast piece having good internal quality cannot be obtained. In addition, 1
Since a large reduction is performed by the two reduction roll pairs, the volume change of the unsolidified portion inside the slab per unit time in the reduction region is large, so that the molten steel level in the mold tends to be unstable.

Thus, the thickness of the slab at the exit of the mold is 1
At present, there is no technique for casting a thin slab having a thickness of less than 50 mm, which is thicker than 00 mm and is less than 50 mm.

[0012]

SUMMARY OF THE INVENTION The present invention relates to a method for rolling down a slab including an unsolidified portion, and at a low production cost,
An object of the present invention is to provide a continuous casting method capable of obtaining a thin slab having excellent surface quality and internal quality.

[0013]

The gist of the present invention is that the length of the vertical portion below the mold outlet is 1.0 to 2.5 m, and the radius of curvature of the curved portion after the vertical portion is 3 to 4 m. Using a vertical bending type continuous casting machine, in the region of the vertical portion or the curved portion below the mold outlet, a continuous casting method of starting the reduction of the slab by a plurality of reduction roll pairs, the cross-sectional shape is rectangular A continuous casting method of steel in which a slab having a thickness of 100 to 150 mm at a mold outlet is reduced to obtain a thin slab having a thickness of 30 mm or more and less than 100 mm.

The present inventors have solved the above-mentioned problems of the present invention as follows. (A) In the method of the present invention, a slab having a rectangular horizontal cross section and a thickness at the exit of the mold of 100 to 150 mm is cast. At this time, it is desirable to use a mold in which the thickness and the shape of the slab at the inlet and the outlet are substantially the same.

When a mold having the same thickness of the slab at the entrance and the exit of the mold is used, the surface layer of the slab is not deformed in the mold. Therefore, cracks are less likely to occur on the slab surface,
A slab with better surface quality is obtained.

Also, since a thick slab having a rectangular cross section and a mold outlet thickness of 100 to 150 mm is cast, oxides and the like in the molten steel float in the molten steel in the mold and become molten powder. It is trapped and easily removed outside the molten steel system. Therefore, a slab of excellent internal quality is obtained.

(B) In the method of the present invention, the reduction of the slab is started in the region of the vertical portion or the region of the curved portion below the outlet of the mold using a vertical bending type continuous casting machine. Since the reduction of the slab is started from an arbitrary position immediately after the mold is removed, even if a thick slab such as 100 to 150 mm in thickness of the mold outlet is cast, a thin slab having a thickness of up to 30 mm is obtained. be able to. Since a thin slab having a thin thickness of up to 30 mm is used as a raw material, the number of stands of the hot rolling equipment can be reduced, and therefore, the manufacturing cost can be reduced.

(C) In the method of the present invention, since the slab is reduced by a plurality of reduction roll pairs, the thickness of the mold outlet is 100 m.
Even when a thin slab having a thickness of about 30 mm is obtained by rolling down a slab of about m, the rolling strain applied to the slab can be reduced by one pair of rolls. for that reason,
It is possible to prevent the occurrence of cracks on the slab surface during rolling down,
A slab of excellent surface quality is obtained.

Further, by reducing the slab by a plurality of reduction roll pairs, it is possible to reduce the volume change of the unsolidified portion inside the slab per unit time in the reduction region. Stabilizes the hot water level.

[0020]

FIG. 1 is a schematic diagram of an example of an apparatus for implementing the method of the present invention. In the method of the present invention, in the vertical bending type continuous casting machine, the reduction of the slab is started in the region of the vertical portion or the region of the curved portion below the outlet of the mold. FIG. 1 shows an example in which the reduction of the slab is started in the region of the vertical portion below the outlet of the mold.

The molten steel 3 is injected into the mold 1 through the immersion nozzle 2 and solidifies in contact with the inner surface of the mold to form a solidified shell 8. In the vertical portion 4 below the outlet of the mold, the solidified shell, that is, the slab 10 including the unsolidified portion 9 is supported and guided by the guide roll 7 and then reduced by the pair of reduction rolls 5. The pressed slab is pulled out by a plurality of pinch rolls 6 via a plurality of guide rolls 7. The symbol R shown in the drawing indicates that the radius of the curved portion after the vertical portion is R.

In the method of the present invention, the length of the vertical portion 4 is 1.
0 to 2.5 m, the bending radius of the bending portion after the vertical portion is 3 to 4
m vertical bending type continuous casting machine is used.

If the length of the vertical portion is less than 1.0 m, the time required for oxides and the like in the molten steel to float in the molten steel is insufficient.
It is difficult to remove oxides and the like in the molten steel out of the molten steel system. Therefore, it is difficult to obtain a slab of excellent internal quality. When the length of the vertical portion exceeds 2.5 m, the size of the continuous casting machine becomes large.

If the bending radius R of the bending portion after the vertical portion is less than 3 m, the bending strain when bending the slab becomes large,
Cracks easily occur on the slab surface. On the other hand, if it exceeds 4 m, the size of the continuous casting machine becomes large. Further, by setting the length of the vertical portion to 1.0 to 2.5 m and the bending radius of the bending portion to 3 to 4 m, the height of the continuous casting machine (the sum of the bending radius and the vertical length) becomes 4 to 6.5 m, so that a continuous casting machine with a low machine height can be obtained.

In the method of the present invention, the reduction of the slab is started by a plurality of reduction roll pairs in the region of the vertical portion or the region of the curved portion below the outlet of the mold.

The area where the slab starts rolling is in the area of the vertical portion or the curved portion below the mold outlet. When the casting speed is low, it is desirable to start the reduction in the region of the vertical portion, and when the casting speed is high, the reduction may be started in the curved portion.

It is desirable that the position where the reduction of the slab is completed is up to 3 m below the outlet of the mold. If the position where the reduction is completed exceeds 3 m below the mold outlet, it becomes difficult to obtain a thin slab having a thickness of 30 mm by the reduction.

When the slab is reduced by a plurality of pairs of reduction rolls, it is preferable to use two to five pairs of reduction rolls and to uniformly reduce the amount of reduction to perform reduction. By such reduction,
Since the rolling strain applied to the slab can be reduced, a slab with excellent surface quality can be obtained.

In the method of the present invention, a slab having a rectangular cross section and a thickness at the exit of the mold of 100 to 150 mm is cast.

When the thickness is less than 100 mm, oxides and the like in the molten steel are unlikely to float in the molten steel in the mold, so that slabs having poor internal quality tend to be formed. On the other hand, if it exceeds 150 mm, it is necessary to increase the number of reduction roll pairs used for reduction in order to reduce the processing strain applied to the slab during the reduction, and the size of the continuous casting machine increases. Therefore, the thickness of the slab at the exit of the mold is set to 100 to 150 mm. 110
~ 120 mm is more desirable.

In the method of the present invention, the thickness is 100 to 150.
mm, the thickness of the slab is reduced from 30 mm to 100 mm.
Cast a thin slab less than mm.

When a relatively thick slab having a thickness of less than 100 mm and around 100 mm after the reduction is obtained in performing the above-mentioned reduction, the thickness is 150 mm or less and 150 m or less.
m to reduce the relatively thick slab in the vicinity, and to obtain a relatively thin slab in the vicinity of 30 mm with a thickness after the reduction of 30 mm or more, a relatively thick slab of 100 mm or more in the vicinity of 100 mm Reduce the thin slab.

When rolling down, the amount of rolling down is desirably 70 mm or less. If the rolling reduction exceeds 70 mm, it is necessary to plastically deform the short sides at both ends in the width direction of the solidified slab, and the deformation amount is large, so that the slab surface is liable to crack.

By setting the thickness after the rolling to 30 mm, the number of stands of the hot rolling equipment at the time of rolling can be reduced, and therefore, the manufacturing cost can be reduced.
In a slab having a thickness of less than 30 mm after rolling, the temperature drop of the slab after casting is large, and the energy loss is large. On the other hand, if it is 100 mm or more, the hot rolling equipment on the same casting line becomes a large-sized equipment, and the manufacturing cost increases.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the apparatus configuration shown in FIG.
A casting test was performed using a vertical bending type continuous casting machine having a bending radius of 0 m and a bending radius of the bending portion of 3.5 m. The captain is 12m.
In each test, a mold was used in which the thickness of the slab at the inlet and the outlet was almost the same. In the test of the present invention example and the test of some comparative examples, in the region of the vertical portion below the mold outlet, a region in the range of 1.0 to 2.0 m from the mold outlet, that is,
The slab was reduced in the range of 1.0 m downstream from the middle of the vertical portion having a length of 2.0 m. At this time, the final roll pair is the final roll in the vertical portion. In addition, in the tests of some comparative examples, in the region of the curved portion, 2.
The slab was reduced in a range of 0 to 3.0 m, that is, in a range of 1.0 m of the curved portion immediately after the vertical portion. When the slab was reduced, reduction was performed using three or five reduction roll pairs. The amount of reduction of each of the three or five reduction roll pairs was equalized.

Using a low-carbon steel having a carbon content of 0.05% by weight, three tests were continuously performed in each test. The amount of slab per heat is about 150 t.

During the casting, the state of the work of rolling the slab was observed. The occurrence of cracks on the surface of the slab obtained from each heat was visually observed. Further, a slab sample having a length of 1 m in the casting direction was sampled from each heat, and the total oxygen content of the sample sampled from the 1/4 thickness position was analyzed at the center of the width of the cross section. The average value of the three heats was taken as the total oxygen content of the slab in the test. The cleanliness of the steel was evaluated from these total oxygen values. Table 1 shows test conditions and test results.

[0038]

[Table 1]

Test No. of the present invention example 1 and No. 1 In No. 2, the slab thickness at the exit of the mold was 120 mm, and the casting was performed at a speed of 3 or 8 m / min. Each was reduced by five pairs of reduction rolls. Test No. of casting speed 3m / min. In No. 1, the thickness equivalent of the slab of 40 mm was reduced, and the slab thickness after the reduction was set to 80 mm. Test N at a casting speed of 8 m / min
o. In No. 2, the thickness equivalent of the slab of 60 mm was reduced, and the slab thickness after the reduction was set to 60 mm. In each case, the reduction was started in the region of the vertical portion below the outlet of the mold, and the slab was reduced in the range of 1.0 to 2.0 m from the outlet of the mold.

Test No. 1 and No. 1 In No. 2, in all cases, the rolling operation could be performed smoothly, and the surface quality of the slab was good. The total oxygen content of the slab was 15 to 18 ppm, and the slab had good cleanliness.

Test No. of the present invention example In No. 3, the slab thickness at the exit of the mold was 100 mm, and the casting was performed at a speed of 10 m / min. Start the reduction in the vertical area below the mold outlet,
The slab was reduced in a range of 1.0 to 2.0 m from the mold outlet. It was reduced by five pairs of reduction rolls. The slab thickness of the slab after reduction is reduced by 30 m
m. The rolling operation could be performed smoothly, and the surface quality of the slab was good. The total oxygen content of the slab was 19 ppm, indicating that the slab had good cleanliness.

Test No. of Comparative Example In No. 4, the thickness of the slab at the outlet of the mold was set to 90 mm, excluding the condition of the thickness of the slab at the outlet of the mold specified by the method of the present invention, and casting was performed at a speed of 10 m / min. The reduction was started in the region of the vertical portion below the mold outlet, and the slab was reduced in the region within a range of 1.0 to 2.0 m from the mold outlet. It was reduced by three pairs of reduction rolls. 60m
m was reduced by an amount equivalent to the thickness of the slab, and the slab thickness after the reduction was reduced to 30 mm. Although a few cracks occurred on the slab surface, the slab was a crack that was not problematic for hot rolling using the slab as a raw material. However, the total oxygen content of the slab was 42 ppm, and the slab was extremely poor in cleanliness.

Test No. of Comparative Example 5 and test no. 6
Then, the test was performed at a slab thickness of 90 mm outside the lower limit of the range of the slab thickness at the mold exit defined by the method of the present invention. Casting was performed at a speed of 3 m / min or 10 m / min. Initiate the reduction in the area of the bend below the mold outlet, from 2.0 to 2.0 from the mold outlet.
The pressure was reduced in the area of 3.0 m.

Test No. 3 in which the casting speed was 3 m / min. In No. 5, since the casting speed was low and the reduction region was far away from the molten steel meniscus, only the 25 mm slab thickness could be reduced, and the slab thickness after reduction could not be reduced to less than 65 mm. . The rolling operation was smooth and the surface quality of the slab was good.
mm, the total oxygen content of the slab is 40 ppm,
The slab had poor cleanliness.

Test No. 1 having a casting speed of 10 m / min. 6
Although the casting speed is high, since the rolling region is far away from the molten steel meniscus, it is possible to reduce only the thickness of a 50 mm slab, and the slab thickness after the reduction is reduced by 40 mm.
I couldn't roll down more thinly. Also, the rolling work is going well,
Although the surface quality of the slab was good, the total oxygen content of the slab was 64 ppm because the thickness at the mold exit was as thin as 90 mm, and the slab was poor in cleanliness.

[0046]

According to the method of the present invention, when a thin slab having a thickness of 30 mm or more and less than 100 mm is cast by rolling down a slab including an unsolidified portion, excellent surface quality can be obtained at a low production cost. And a thin slab of internal quality can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an example of an apparatus when performing the method of the present invention.

[Explanation of symbols]

 1: Mold 2: Immersion nozzle 3: Molten steel 4: Vertical portion directly below the mold 5: Roller pair 6: Pinch roll 7: Guide roll 8: Solidified shell 9: Unsolidified portion 10: Cast piece R: Curved radius

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masafumi Hanao 4-5-233 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (72) Inventor Yuki Kikuchi 4 Kitahama, Chuo-ku, Osaka-shi, Osaka No.5-33, Sumitomo Metal Industries, Ltd. F-term (reference) 4E002 AA04 BD02 4E004 NA01

Claims (1)

[Claims] 1. The length of a vertical portion below a mold outlet is 1.0 m.
Not less than 2.5 m, using a vertical bending type continuous casting machine in which the bending radius of the bending portion after the vertical portion is 3 m or more and 4 m or less,
A continuous casting method in which the reduction of a slab is started by a plurality of reduction roll pairs in a region of a vertical portion or a curved portion below a mold outlet, wherein a cross-sectional shape is rectangular and a thickness at a mold outlet is 100 mm or more. A continuous casting method for steel, characterized in that a slab having a thickness of 150 mm or less is reduced to obtain a thin slab having a thickness of 30 mm or more and less than 100 mm.