Strip casting employing non-contact heat absorbers

Details Strip casting employing non-contact heat absorbers Strip casting employing non-contact heat absorbers

1997-09-10

1999-10-05

Batten, Jr., J. Reed

Metal founding

Process

Shaping liquid metal against a forming surface

164417, 164428, 164477, B22D 1106, B22D 1122, B22D 11124

Patent

active

059608560

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to continuous casting of metal strip in a strip caster, particularly a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or damn held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
After leaving the caster the hot strip may be passed to a coiler on which it is wound into a coil. Before proceeding to the coiler it may be subjected to in-line treatment such as controlled temperature reduction, reduction rolling, full heat treatment or a combination of such treatment steps. The coiler and any in-line treatment apparatus generally applies substantial tension to the strip which must be resisted. Moreover, it is necessary to accommodate differences between the casting speed of the twin roll caster and the speed of subsequent in-line processing and coiling. Substantial differences in those speeds may develop particularly during initial start up and until steady state casting speed is achieved. In order to meet these requirements it has been proposed to allow the hot strip leaving the caster to hang unhindered in an unrestrained loop from which is passes through one or more sets of pinch rolls into a tensioned part of the line in which the strip may be subjected to further processing and/or coiling. The pinch rolls provide resistance to the tension generated by the down line equipment and are also intended to feed the strip into the down lying equipment.
Particularly in the casting of steel strip, it is common to enclose the strip leaving the strip caster within a sealed enclosure for scale control purposes. The strip may, for example, be passed through a sealed enclosure charged with an inert atmosphere to inhibit the build up of scale or it may be passed through a sealed enclosure from which oxygen is extracted by oxidation of the strip passing through it in the manner described in our Australian Patent Application No 42235/96.
One particular problem encountered in the direct casting of thin metal strip is that solidification of molten metal in the central part of the strip is generally not completed at the time that the strip leaves the caster. The strip leaving the caster has a central mushy zone which continues to solidify, so giving up heat of solidification which causes reheating of the solidified metal with consequent weakening and thinning of the solidified outer parts of the strip. This effect is particularly severe when casting steel strip in a twin roll caster as the strip leaves the nip at very high temperatures of the order of 1400.degree. C. and there is a substantial central mushy zone which does not solidify until some time after the strip has exited the nip and has lost contact with the chilled casting rolls.
In the case where the strip exiting the nip hangs in an unrestrained loop, the newly formed strip near the nip is required to support a substantial part of the weight of the loop and the weakening of the solidified outer parts of the strip due to reheating caused by continuing solidification of the central mushy zone can be quite sufficient to cause transverse cracking and even complete rupture of the strip in this re

REFERENCES:
patent: 5584337 (1996-12-01), Nakashima et al.
patent: 5720335 (1998-02-01), Osada et al.
patent: 5762126 (1998-06-01), Assefpour-Dezfully et al.
Patent Abstracts of Japan, M-777, p. 131, JP 63-207461 A (Nippon Steel Corp) Aug. 26, 1988.
ASM Handbook, vol. 15 "Casting", 1988, p. 312.

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