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(11) | EP 1 018 382 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
| published in accordance with Art. 158(3) EPC |
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| (54) | METHOD AND DEVICE FOR CONTINUOUS CASTING |
| (57) A continuous casting device comprising (i) a non-rolling area 400 to 1,000 mm long
which is disposed immediately below a mold, the mold having an aperture which is 50
to 120 mm wide at its outlet, (ii) a group of rollers for rolling a cast strip in
an unsolidified state, the group of rollers being provided in the downstream of the
non-rolling area, and (iii) a group of rollers for rolling the cast strip further
in a semisolidified state that there is no flow of melted steel in its central portion
but it has not completely solidified yet. The cast strip, which is 50 to 120 mm in
thickness at the outlet of the mold, is rolled in the unsolidified state and further
rolled in the semisolidified state to be reduced to thickness of 40 mm or less. Cast
strips can be rolled thinner than those by the conventional unsolidified rolling method.
Besides, better quality of cast strips is attained. |
Technical Field
[0001] The present invention relates to a continuous casting method and a continuous casting device. A thin cast strip produced by a continuous casting device is rolled thinner into a steel sheet by rolling mills in the downstream. If a cast strip is made thin enough in the casting process, the downstream rolling process can be made short, which reduces the cost of the rolling equipment. The object of the present invention is to provide a continuous casting method and a continuous casting device for producing such thin cast strips.
Background Art
[0002] To produce thin cast strips, a narrow aperture is made in the mold. However, the molding aperture cannot be made as narrow as it is desired to be because of the thickness of the nozzle of melted steel. A molding aperture of even width through its length, or depth, can be made as narrow as 90 mm. If the aperture is made narrower than 90 mm, stable casting cannot be made. Accordingly, some molds have an aperture which is wide at its inlet and narrows, towards its outlet, to 50 to 120 mm.
[0003] To make a cast strip thinner than the outlet of the aperture of a mold, the unsolidified rolling method has been used, whereby a strip drawn out of the mold and having flowing melted steel inside is rolled (prior art I).
[0004] However, because the rolling of a cast strip by the unsolidified rolling method has to be completed before the two face (as distinguished from "side") portions of its solidified shell meet each other, the cast strip cannot be made thinner than the total thickness of the two face portions of its solidified shell in the rolling area. If the rolling of a cast strip is started immediately below the mold to complete the rolling before the shell grows thick and thereby attain a thin cast strip, rolling the cast strip immediately below the mold disturbs the contact between the cooling plates of the mold and the solidified shell, causing uneven solidification, which in turn causes a breakout or surface cracks. If the reduction per roller is made large to raise the rolling-completion point upstream, inner cracks occur. Accordingly, the reduction per roller has to be 10 mm or less, and the rolling-completion point has to be 1,900 mm or more below the surface of melted steel in the mold. Under such restrictions, it has, as a matter of fact, been impossible to complete the rolling of a cast strip before its solidified shell grows thick. Accordingly, a method of producing thin enough cast strips continuously has not been made available yet.
[0005] On the other hand, disclosed in the Japanese Unexamined Patent Application No. 164460/H8 (1996) is a method by which a cast strip is rolled with rollers of a large diameter only after the completion of its solidification. In its unsolidified area, the cast strip is not rolled, but just guided by guide rollers (prior art II).
[0006] Because even small reduction of a completely solidified cast strip requires a large pressure and rollers of a large diameter, the device materializing the above method becomes large and costly. Besides, cast strips by the method are liable to develop troubles, which reduces their quality. For example, cracks can occur in such a cast strip due to the decrease in temperature of its corners while it is being rolled.
[0007] In accordance with the above, the object of the present invention is to provide a continuous casting method and a continuous casting device for producing cast strips which are thinner, and whose quality is higher, than those produced by the conventional unsolidified rolling method.
Disclosure of Invention
[0008] In accordance with the first invention, there is provided a continuous casting method of producing cast strips of thickness of 40 mm or less in a continuous casting device. The method comprises (i) the step of rolling a cast strip, which is 50 to 120 mm in thickness at the outlet of a mold, in an unsolidified state that its solidified shell has grown to a certain thickness and melted steel is flowing in its central portion and (ii) the step of rolling the cast strip further in a semisolidified state that there is no flow of melted steel in its central portion but it has not completely solidified yet.
[0009] In accordance with the second invention, there is provided a continuous casting device, which comprises (i) a non-rolling area 400 to 1,000 mm long which is disposed immediately below a mold, the mold having an aperture which is 50 to 120 mm wide at its outlet, (ii) a group of rollers for rolling a cast strip in an unsolidified state, the group of rollers being provided in the downstream of the non-rolling area, and (iii) a group of rollers for rolling the cast strip further in a semisolidified state that there is no flow of melted steel in its central portion but it has not completely solidified yet.
Brief Description of Drawings
[0010]
Fig. 1 shows the general arrangement of the mold and rollers of an embodiment of continuous casting device of the present invention.
Fig. 2 is a graph showing the relation between the temperature of a cast strip and its deformation resistance.
Best Mode for carrying out the Invention
[0011] In Fig. 1, the numeral 1 indicates a mold; 2, support rollers in a non-rolling area 2A; 3, rolling rollers in an unsolidified rolling area 3A; and 4, rolling rollers in a semisolidified rolling area 4A.
[0012] The two faces of the aperture of the mold 1, corresponding to the two faces of a cast strip A, may be parallel with each other, or may be tilted to each other so as to make the outlet narrower than the inlet. In either case, the outlet of the aperture is 50 to 120 mm wide. In other words, the spacing between the two faces of the aperture is 50 to 120 mm at the outlet.
[0013] The non-rolling area 2A is directly under the mold 1 and have a length of 400 to 1,000 mm. The cast strip A is guided by the support rollers 2 but not rolled in the non-rolling area 2A.
[0014] The melted steel poured into the mold 1 is cooled in it, and a solidified shell S is formed around the melted steel, or cast strip A. Then, its solidification is promoted by water spray, etc. in the non-rolling area 2A.
[0015] In the unsolidified rolling area 3A below the non-rolling area 2A, the rollers 3 roll the cast strip A in an unsolidified state. In the unsolidified state, melted steel is flowing in the central portion of the cast strip A and, therefore, the cast strip A can be rolled by small pressure. Because the required rolling pressure is small, rollers 3 of a small diameter can be disposed with small spacing between them and, hence, the cast strip A is less liable to develop bulging due to the static pressure in the melted steel.
[0016] While the rollers 3 are rolling the cast strip A to reduce its thickness in the unsolidified rolling area 3A, the cast strip A cools naturally, thickening its shell S.
[0017] The two face (as distinguished from "side") portions of the shell S of the cast strip A meet each other at a point 6 on the bottom of the unsolidified rolling area 3A, where the cast strip A completes its solidification.
[0018] In the semisolidified rolling area 4A under the solidification-completing point 6, the cast strip A has no melted steel flowing in its central portion but has not completely solidified yet. In the semisolidified rolling area 4A, the mean temperature of a section (the ?????? section) of the cast strip A is over 1,200º C, or the temperature of the center of the cast strip A is over 1,350º-1,400º C, and the mean temperature of its solidified shell S is 1,250º to 1,300º C. It has been found by the inventor(s) that a cast strip A in such a semisolidified state can be rolled with a pressure far smaller than the reaction which occurs in the cast strip A if it is rolled in a solidified state.
[0019] As shown in Fig. 2, the mean temperature of the solidified shell of a cast strip in the solidified rolling area, where the cast strip is rolled in a solidified state, of the prior art II is about 1,100º C, and the deformation resistance is 4 kg/mm2. On the other hand, the deformation resistance in the semisolidified rolling area 4A in accordance with the present invention is 1 kg/mm2 or less because the mean temperature of the solidified shell in the area is 1,250º to 1,300º C. Thus, in accordance with the present invention, the deformation resistance is reduced to a quarter.
[0020] Accordingly, rollers 4 of a diameter as small as that of the rollers 3 in the unsolidified rolling area 3A can be used in the semisolidified area 4A and, hence, the rollers 4 can be disposed with small spacing between them. Therefore, the freshly formed solidified shell does not bulge. Besides, the cast strip A does not develop cracks in it because it is rolled in a high temperature area where a rolling temperature (1,100ºC or more) above the embrittlement temperature range can be secured.
[0021] Used in the unsolidified and semisolidified rolling areas 3A and 4A are rolling segments each comprising small-diameter rollers which are disposed with small spacing between them. The rolling segments are so configured that they can be raised and lowered, and tilted. The thickness of the cast strip A after the rolling in the unsolidified and semisolidified rolling areas 3A and 4A can freely be set by changing the magnitudes of up-and-down movement and tilting movement of the rolling segments.
[0022] On the other hand, due to fluctuations in the casting speed, the solidification-completing point 6 moves up and down, which causes the length of the unsolidified rolling area 3A and the semisolidified rolling area 4A to change. However, because the deformation resistance in the cast strip A in the semisolidified rolling area 4A does not rise suddenly, a prescribed thickness of the cast strip A can be secured notwithstanding such fluctuations.
[0023] Although the continuous casting device shown in Fig. 1 is a vertical type, it can also be a curved type with the same mechanism.
Industrial Applicability
1. A continuous casting method of producing cast strips of thickness of 40 mm or less
in a continuous casting device, comprising the steps of:
rolling a cast strip, which is 50 to 120 mm in thickness at the outlet of a mold, in an unsolidified state that its solidified shell has grown to a certain thickness and melted steel is flowing in its central portion; and
rolling the cast strip further in a semisolidified state that there is no flow of melted steel in its central portion but it has not completely solidified yet.
2. A continuous casting device comprising:
a non-rolling area 400 to 1,000 mm long which is disposed immediately below a mold, the mold having an aperture which is 50 to 120 mm wide at its outlet;
a group of rollers for rolling a cast strip in an unsolidified state, the group of rollers being provided in the downstream of the non-rolling area; and
a group of rollers for rolling the cast strip further in a semisolidified state that there is no flow of melted steel in its central portion but it has not completely solidified yet.