Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-06-15
2001-07-10
Lin, Kuang Y. (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S428000
Reexamination Certificate
active
06257315
ABSTRACT:
TECHNICAL FIELD
This invention relates to the casting of steel strip.
It is known to cast metal strip by continuous casting in a twin roll caster. In this technique 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 or series of smaller vessels 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 closures, for example side plates or dams held in sliding engagement with 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.
Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals. One particular problem encountered in the casting of aluminium killed steel in a twin roll strip caster is the propensity for molten steel to produce solid inclusions, in particular inclusions which contain aluminates. Such inclusions can affect the surface quality of the strip as well as having the tendency to block any small casting passages in the metal delivery system. This has led to the use of manganese/silicon killed steels as an alternative, such as described in our New Zealand Patent Application 270147. However, such silicon/manganese killed steels have inherently a significantly higher oxygen content than aluminium killed steels and this, along with the ability of oxides present in the molten steel to be reduced, gives rise to problems in casters in which the casting pool is in contact with refractory materials which contain carbon. This may arise where the delivery nozzle formed of a carbon containing refractory material dips into the casting pool and where the pool confining side closures are formed wholly or in part of such a refractory material, for example alumina graphite. The exposure of the casting pool to carbon containing refractories causes the pool to be disturbed by carbon monoxide bubbles generated by reactions between carbon in the submerged delivery nozzle and oxygen containing compounds in the molten metal of the casting pool. More particularly, ferrous oxide or other oxides in the slag present in the casting pool react with carbon to be reduced to iron or other metals respectively. The pool disturbance caused by the carbon monoxide bubbles from such reduction leads to the formation of discrete waves in the casting pool which are reflected in the cast strip as depressions in the strip surface. These defects are commonly referred to as meniscus marks. Moreover, carbon leaching from the refractory material exposed to the casting pool is enhanced.
It should be noted that in casting aluminium killed steels the aluminates present in the molten metal are not readily reduced and in fact carbon cannot reduce same under such casting conditions.
Our International Patent Application PCT/AU96/00244 describes a proposal to address this problem by the controlled addition of sulphur to the silicon/manganese killed steel melt at least in the start-up phase of a casting operation. However, the controlled addition of sulphur to the steel adds complexity to the process and results in the production of steel with high sulphur content which may not generally be acceptable to all markets. By the present invention the problem is addressed by modifying the chemical composition of the refractory material exposed to the casting pool rather than that of the steel melt.
Our U.S. patent application Ser. No. 958908 describes how the refractory material of the metal delivery nozzle may be selected to minimise reaction with the oxygen containing compounds in the casting pool. The same refractory material may also be used in the construction of the pool confining side closures. For example, the side closures may be comprised of components made of alumina graphite in which the graphite has a purity of at least 96%. Simple alumina graphite side closures have suffered from poor abrasion resistance and excessive wear. This problem is avoided by the use of boron nitride closures which have superior abrasion resistance and which also do not react chemically with the oxygen containing compounds in the pool. However, baron nitride closures are very expensive to produce.
Recent developments in the design of the pool confining side closures have addressed the abrasion resistance and wear problems to permit the use of refractories such as alumina graphite. In particular, there have been proposals for multi-part or composite pool confining side closures with wear faces or pads applied to backing plates and with lubricant applicators to apply lubricant to the wear faces during operation of the caster. One such proposal is described in International Patent Publication WO 98/35775 of Nippon Steel Corporation. The present invention is particularly applicable to strip casters with composite side closures of this general kind although it could also be applied to side closures in the form of simple refractory plates.
DISCLOSURE OF THE INVENTION
According to the invention there is provided method of continuously casting steel strip of the kind in which molten steel is introduced into the nip between a pair of parallel casting rolls to create a casting pool of molten steel supported on casting surfaces of the rolls immediately above the nip and the casting rolls are rotated to deliver a solidified steel strip downwardly from the nip, wherein the ends of the pool are confined by side closures comprised of bodies of refractory material in contact with the molten steel of the pool, said refractory material comprising a major proportion of a refractory aggregate and a minor proportion of graphite in the range of 10 to 30% by weight and an anti-oxidant additive being aluminium or an alloy thereof, and wherein the graphite has a purity of at least 96%.
The invention also provides apparatus for casting steel strip, comprising a pair of parallel casting rolls forming a nip between them, an elongate delivery nozzle disposed above and extending along the nip between the casting rolls for delivery of molten steel into the nip to form a casting pool of molten steel supported on casting surfaces of the rolls above the nip, side closures to confine the two ends of the pool and means to rotate the rolls to produce a solidified steel strip passing downwardly from the nip, wherein the side closures are comprised of bodies of refractory material to contact the molten steel of the pool which refractory material comprises a major proportion of a refractory aggregate and a minor proportion of graphite in the range of 10 to 30% by weight and an anti-oxidant additive being aluminium or an alloy thereof, and wherein the graphite has a purity of at least 96%.
Preferably the purity of the graphite is of the order of 98% or higher.
Preferably the anti-oxidant additive contains the metal aluminium.
Preferably further the amount of the anti-oxidant additive in the refractory material is around 2% by weight.
It is preferred that the proportion of graphite be of the order of 20 to 24%.
The refractory aggregate may comprise any one or more of the compounds alumina, magnesia, zirconia and spinel. However, it is preferable that the aggregate be comprised mainly of alumina.
Preferably, any additives are such that the refractory material is essentially free of sodium.
The refractory aggregate will generally be selected on
Ishikawajima-Harima Heavy Industries Company Ltd.
Kerins John C.
Lin Kuang Y.
Miles & Stockbridge P.C.
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