EP1046442A1

EP1046442A1 - Method and machine for continuous casting of thin slabs

Info

Publication number: EP1046442A1
Application number: EP20000107872
Authority: EP
Inventors: Masahiro In Niihama Works Sumitomo Heavy Ikeda; Hirohisa Sumitomo Metal Industries Ltd. Kikuchi
Original assignee: Sumitomo Heavy Industries Ltd
Current assignee: Sumitomo Heavy Industries Ltd
Priority date: 1999-04-21
Filing date: 2000-04-12
Publication date: 2000-10-25
Also published as: JP2000301303A; JP3355311B2

Abstract

The object of the invention is to provide a continuous casting method and a continuous casting machine for producing thin-slab cast strips stably.

A continuous casting machine of a vertical bending type for casting thin-slab cast strips, wherein a liquid core reduction-roll segment 4 is provided so as to constitute rolls in the bending zone B below the mold 1. The liquid core reduction-roll segment 4 comprises a group of fixed rolls 8 disposed outside the guide roll radius being bent of a cast strip and a group of movable rolls 3 disposed inside the guide roll radius. The group of movable rolls 3 is supported by a tilting frame guide roll radius 7 with an oil-hydraulic cylinder 10 so that it is moved toward the group of fixed rolls 8 to roll the cast strip.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a continuous casting method and a continuous casting machine of thin-slab cast strips.
The thinner a thin-slab cast strip is cast, the simpler the latter liquid core reduction process becomes and the less the liquid core reduction equipment costs. Making the exit of the molding aperture of a mold narrow brings about direct effect of reducing the thickness of cast strips. However, as the molding aperture itself has to be wide enough for a nozzle to be inserted in it when melted steel is poured from the tundish into the mold through the nozzle, there is a limit to reducing the width of the molding aperture. Under the circumstances, molds have so far been given a molding aperture of which the exit is 90 mm wide or wider, and unsolidified slabs from them have been rolled to be reduced in their thickness.
Fig. 3 shows the basic configuration of a continuous casting machine to roll unsolidified slabs as described above.
The numeral 1 is a mold. A vertical zone A is provided immediately below the mold 1, and a bending zone B and a liquid core reduction zone C follow the vertical zone A. The bottom of the liquid core reduction zone C is the liquid core reduction completion point P, which is followed by guiding zones D and E.
A cast strip is cooled in the vertical zone A so as to develop its solidified shell to a certain extent. In the bending zone B where the cast strip is bent gradually, the guiding-zone radii of curvature r1 to r5 at the points of the rolls constituting the bending zone B are gradually reduced in the order of r1 to r5 so as to reduce the strain in the cast strip. Therefore, the bending zone B may be called "multipoint-bending zone."
The cast strip passed through the bending zone B and entered into the liquid core reduction zone C develops its solidified shell gradually, its center portion remaining unsolidified, and is rolled by liquid core reduction rolls to be reduced in its thickness.
The guide-zone radii of curvature at the points of the rolls in the liquid core reduction zone C are identical, and hence the zone C may be called "constant circular-arc zone." Liquid core reduction a cast strip while it has still an unsolidified portion in it reduces the liquid core reduction pressure required for the liquid core reduction and the radius required of the liquid core reduction rolls, allowing the simplification of construction around the liquid core reduction rolls.
The reason why the above continuous casting machine has a bending zone B and a liquid core reduction zone C separately and do the bending and the liquid core reduction of a cast strip separately is to prevent longitudinal cracks in and breakouts on the surfaces of the cast strip.
Namely, because either of the bending and the liquid core reduction of a cast strip causes stress in it, doing both at the same time causes more cracks in it. Therefore, it has been put under taboo to do both simultaneously. It has been technical common sense to do the bending and the liquid core reduction separately.
The vertical-and-bending-type continuous casting machine described above has a relatively long distance between the surface (meniscus) of melted steel in the mold and the constant circular-arc zone (liquid core reduction zone C). The longer the distance becomes, the thicker the shell of a cast strip grows, reducing the unsolidified portion in the cast strip and making the liquid core reduction of the cast strip difficult. To cope with this problem, the casting speed has to be raised.
The thickness of the solidified shell of a cast strip is generally given by the equation below: D = K(L/Vc )1/2 where

D =: thickness of solidified shell (mm)
V_c =: casting speed (m/min.)
L =: distance from meniscus (m)
K =: coefficient of solidification

K

For example, if the liquid core reduction completion point P is approximately four meters below the meniscus and the target thickness of a cast strip is 50 mm, the constant casting speed has to be about 5 m/min. or higher. If all kinds of steel require such a casting speed, the operational flexibility of the continuous casting machine is reduced and its operation becomes difficult.
In accordance with the above, the object of the present invention is to provide a continuous casting method and a continuous casting machine for producing thin-slab cast strips stably.

SUMMARY OF THE INVENTION

According to the first aspect of the present invention, there is provided a method of casting a thin-slab cast strip with a continuous casting machine of a vertical bending type. The method is characterized by bending a cast strip drawn out of the mold in the bending zone and liquid core reduction it simultaneously in an unsolidified state.
According to the second aspect of the present invention, there is provided the method of the first aspect, wherein said pre-solidification liquid core reduction of the cast strip is made by applying pressure to the cast strip from the inside of the guide roll radius being bent of the cast strip toward the outside of the guide roll radius.
According to the third aspect of the present invention, there is provided a continuous casting machine of a vertical bending type for casting thin-slab cast strips, wherein a liquid core reduction roll segment is provided so as to constitute rolls in the bending zone below the mold.
According to the fourth aspect of the present invention, there is provided the continuous casting machine of the third aspect with the following additional features. Its liquid core reduction roll segment comprises a group of fixed rolls disposed outside the guide roll radius being bent of a cast strip and a group of movable rolls disposed inside the guide roll radius. The group of movable rolls is supported by a tilting frame guide roll radius with an oil-hydraulic cylinder so as to be moved toward and away from the group of fixed rolls.
The advantages offered by the first aspect of the invention are as follows. By beginning simultaneously to bend and roll a cast strip without waiting for the completion of its bending and thereby finishing its presolidification liquid core reduction early, the distance between the meniscus and the liquid core reduction completion point can be shortened. It means that the cast strip is rolled while it has a thin shell and a large unsolidified pottion in it. Therefore, a prescribed reduction can be achieved at a relatively low casting speed. Accordingly, the minimum casting speed required is reduced, which increases the operational flexibility of the casting machine. Thus, it can be operated stably. Not-withstanding that a cast strip is rolled while it is being bent as described above, its pre-solidification liquid core reduction can be made without causing cracks in it, contrary to the common sense in the art. Though its reason has not been ascertained yet, it can be presumed that such behavior as makes strain due to bending and strain due to liquid core reduction overlap or intensify each other in a cast strip hardly occurs even when such strains occur simultaneously in the cast strip while its shell is still thin and it still have a large unsolidified portion in it.
The advantage offered by the second aspect of the invention is as follows. By making the presolidification liquid core reduction of a cast strip inside the guide roll radius being bent of the cast strip, inner cracks can be prevented well. Though its reason has not yet been ascertained well, it can be presumed that the bending and the liquid core reduction of a cast strip act so as to crush cracks in the fragile part, facing the unsolidified portion, of the shell inside the guide roll radius being bent.
The advantage offered by the third aspect of the invention is as follows. Because a liquid core reduction-roll segment is disposed in the bending zone, a cast strip can be rolled while it is being bent. In this way, the pre-solidification liquid core reduction of a cast strip can be made near the mold. Accordingly, this invention is suitable for effecting the invention of the first aspect.
The advantages offered by the fourth aspect of the invention are as follows. Because the reduction can be adjusted by changing the distance between the group of fixed rolls and the group of movable rolls, thin-slab cast strips of any thickness can be produced. In addition, because a cast strip is rolled by pressing the group of movable rolls inside the guide roll radius being bent of the cast strip toward the group of fixed rolls, bending strain in the cast strip can be reduced. Accordingly, this invention is suitable for effecting the invention of the second aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become more clearly appreciated from the following description in conjunction with the accompanying drawings, in which:
Fig. 1 is a schematic illustration of an embodiment of continuous casting machine of the present invention;
Fig. 2 is a side view of the liquid core reduction-roll segment of Fig. 1; and
Fig. 3 is a schematic illustration of a vertical-and -bending-type continuous casting machine of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, a preferred embodiment of the present invention will be described.
In Fig. 1, the numeral 1 is a mold. A vertical zone A, a bending zone B, a liquid core reduction zone C, and guiding zones D, E, and F are provided immediately below the mold 1 downward in the order of description. The drawing-out path of cast strips is bent, or curved, as shown in Fig. 1. The "inside of the bent guide roll radius" means hereinafter the side on which the center of curvature of the bent guide roll radius sits, and the "outside of the bent guide roll radius" means the opposite side.
The present invention is characterized by the bending zone B and the liquid core reduction zone C overlapping each other. The bending zone B comprises about six to ten pairs of rolls arranged along the drawing-out path ("a pair of rolls" means a set of a roll inside the bent guide roll radius and a roll outside the bent guide roll radius). The vertical zone A provided on the bending zone B has five pairs or so of rolls 2. Disposed below the vertical zone A is a liquid core reduction-roll segment 4 which comprises six pairs or so of liquid core reduction rolls 3 built in frame guide roll radiuss. The top five pairs or so of liquid core reduction rolls 3 of the liquid core reduction-roll segment 4 have two functions of bending and liquid core reduction a cast strip. Accordingly, the upper five pairs or so of rolls 3 are arranged so as to reduce the guiding-zone radius of curvature gradually, and they, at their fixed positions in the liquid core reduction-roll segment 4, bend a cast strip gradually. Namely, the upper five or so pairs of rolls 3 constitute a multipoint-bending zone.
The bottom two pairs or so of rolls 3 of the liquid core reduction-roll segment 4 have only the function of liquid core reduction a cast strip, and the position of the bottom pair of rolls 3 is the liquid core reduction-completion point P.
The liquid core reduction-roll segment 4 will now be described with reference to Fig. 2.
In Fig. 2, the numeral 5 is a fixed frame guide roll radius outside the bent guide roll radius of the drawing-out path; 6, an upper fixed frame guide roll radius inside the bent guide roll radius; 7, a lower tilting frame guide roll radius inside the bent guide roll radius.
The fixed frame guide roll radius 5 is fixed to an appropriate frame member of the continuous casting machine and a plurality of rolls 8 are arranged from the top to the bottom in and supported by the fixed frame guide roll radius 5.
The fixed frame guide roll radius 6 is fixed to a frame member inside the bent guide roll radius of the drawing-out path, and the rolls 2 described above are supported by the fixed frame guide roll radius 6 so as to be freely rotatable.
The tilting frame guide roll radius 7 is supported by a pivot pin 9 on the bottom of the fixed frame guide roll radius 6 so that it can tilt freely. A tilting oil-hydraulic cylinder 10 is fixed to the longitudinally middle portion of the tilting frame guide roll radius 7, and its piston rod 11 is connected, with a pin, to a bracket 12 which is fixed to the fixed frame guide roll radius 5. The liquid core reduction rolls 3 described above are supported by the tilting frame guide roll radius 7 so as to be rotatable freely.
Accordingly, by extending and contracting the tilting oil-hydraulic cylinder 10, the lower tilting frame guide roll radius 7 can be moved away from and toward the fixed frame guide roll radius 5.
In Fig. 2, a solid line shows the liquid core reduction position, and a dash-double dot line shows the nonliquid core reduction position, the tilting frame guide roll radius 7 being retreated from the liquid core reduction position by a distance "d".
While the tilting frame guide roll radius 7 is at the nonliquid core reduction position, the pairs of rolls 8 and 3 constitute a multipoint-bending zone to bend a cast strip gradually. When the tilting frame guide roll radius 7 is moved toward the fixed frame guide roll radius 5, the space between the rolls 8 and the rolls 3 is narrowed and, thereby, a cast strip is rolled. The re-duction can be continuously changed by changing the tilting distance "d".
In accordance with the present embodiment, because the pre-solidification liquid core reduction of a thin-slab cast strip can be made in an upper zone of which most part overlaps the bending zone, the minimum casting speed required for casting is reduced, which enables stable casting operation.
If the liquid core reduction-completion point P is about three meters below the meniscus and the target thickness of a cast strip is 50 mm, the minimum casting speed required is approximately 3.8 m/min. in accordance with the present embodiment, whereas it is about 5 m/min. in accordance with the prior art. Thus, the present embodiment brings about a large flexibility of casting speed and enables stable casting operation.
In accordance with the present embodiment, cast strips below 150 mm in thickness could be rolled in unsolidified states to thickness of 35-70 mm. In these cases, the liquid core reduction-completion point P was within four meters of the meniscus, and the continuous casting machine could be operated stably at casting speed of about 3 m/min. with reduction of 15 mm or less per roll.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The above embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

A method of casting a thin-slab cast strip with a continuous casting machine of a vertical bending type, the method characterized by bending the cast strip drawn out of a mold in a bending zone and liquid core reduction it simultaneously in an unsolidified state.
A method as claimed in claim 1, wherein said pre-solidification liquid core reduction of the cast strip is made by applying pressure to the cast strip from the inside of the guide roll radius being bent of the cast strip toward the outside of the guide roll radius.
A continuous casting machine of a vertical bending type for casting a thin-slab cast strip, wherein a liquid core reduction-roll segment is provided so as to constitute rolls in a bending zone below a mold.
A continuous casting machine as claimed in claim 3, wherein:

the liquid core reduction-roll segment comprises a group of fixed rolls disposed outside the guide roll radius being bent of a cast strip and a group of movable rolls disposed inside the guide roll radius; and

the group of movable rolls is supported by a tilting frame guide roll radius with an oil-hydraulic cylinder so as to be moved toward and away from the group of fixed rolls.