Patent Application
20050280192
This invention relates to making of steel, and more particularly to making of steel by continuous casting. The invention has application in making steel by casting steel strip.
In continuous casting of thin strip steel, molten steel is delivered to a caster from a ladle through a tundish and melt delivery system including a shroud, a delivery nozzle and sometimes a transition piece. These metal delivery components deliver molten steel at 1500 to 1600° C., or more, to the caster through refractories which are capable of withstanding the high temperatures in the process of casting molten steels. These refractories may also be preheated to the delivery temperature to avoid thermal shock when the molten steel is introduced through the delivery system. These metal delivery components are used in continuous casting by, among others, thick slab casters, thin slab casters and thin strip casters. Illustrative of the refractory materials for such metal delivery components particularly useful in thin strip casting are those described in U.S. Pat. Nos. 5,924,476 and 6,257,315.
In thin strip casting, molten metal is typically introduced between a pair of counter rotated horizontally positioned casting rolls to form a nip between them. The casting rolls are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip to produce a cast strip delivered downwardly from the nip. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which the metal flows through a delivery nozzle located above the nip to form a casting pool supported on the casting surfaces. The casting pool is supported on the casting rolls adjacent the nip and extending along the length of the nip. The casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls so as to dam the two ends of the casting pool against outflow.
A problem in such metal delivery systems is that the molten slag tends to stick to the refractories of the delivery system. This has been known to occur even when the refractories are pre-heated to the temperature of the molten steel. The buildup of slag that occurs on the refractories from the molten steel has a tendency to break away, and cause defects to form in the cast steel and on the surface of the cast steel. This is particularly true of the build up that occurs at the meniscus of the casting pool in thin strip casters.
We have found a refractory of particular composition that inhibits the buildup of the molten steel on refractories of a metal delivery system in continuous casting of steel. The refractory material for such thin strip steelmaking is comprised of 50 to 85% zirconia (ZrO2), 0 to 35% of silica (SiO2), 5 to 35% carbon (C), less than 5% alumina (Al2O3) and an anti-oxidant.1 The purity of the carbon used may be greater than 99.5%. The refractory material may be stabilized with lime or magnesium oxide, and typically in an amount less than about 28%. The refractory material has application in making delivery nozzles and transition piece nozzle blocks for use in making steel by continuous casting of steel strip. The refractory material may by used, for example, in making a delivery nozzle for use in making steel by continuous casting of steel strip.
1 All percentages are stated percent by weight.
Also disclosed is a method of continuous casting of steel strip comprising the steps of:
The refractory material may have a zirconia (ZrO2) content between 60 and 85%, and more specifically between 70 and 80% by weight. The refractory material may have a carbon content between 8 and 30%, and more specifically between 10 and 20% by weight. The zirconia may be stabilized or unstabilized, but again the refractory material may be stabilized with lime or magnesium oxide or a combination thereof to reduce wear on the refractory material during use.
The anti-oxidant inhibits oxidation of the other components of the refractory material, and may be any one or a combination of such materials that inhibits oxidation in the refractory material system. The anti-oxidant may be present in an amount up to about 10% by weight. Examples of antioxidants are, without limitation, Si metal, Al metal, silicon aluminum alloy and carbides such as boron carbide and silicon carbide.
In order that the invention may be more fully explained one particular embodiment is described with respect to continuously casting of steel strip and with reference to the accompanying drawing in which:
A twin-roll caster 11 comprises a main machine frame 21 which supports a pair of internally cooled casting rolls 22 having casting surfaces 22A. The casting rolls 22 are positioned laterally adjacent each other to form a nip 27 between them and through which cast steel strip may be formed. The twin-roll caster may be as illustrated in U.S. Pat. Nos. 5,184,668, 5,277,243 and 5,488,988, to which reference can be made for more detail.
Molten metal is supplied for the continuous casting of strip from the ladle (not shown) to a tundish 23. From the tundish 23 the molten metal is delivered through the metal delivery system by refractory lined shroud 24 to a transition piece nozzle block 25, which is also refractory lined. Stopper 18 is seated into refractory inlet 21 and attached to shroud 24 at connection 17 to regulate the flow of molten metal from the tundish 23 into the shroud 24. Stopper 18 is moveable to regulate the flow of molten metal from the tundish 23 into shroud 24.
Transition piece nozzle block 25 is configured to generally enclose the molten metal from exposure to the outside atmosphere, with an overflow 19 through which molten metal can flow should the metal in the transition piece reach a point of overflowing. The molten metal is delivered from the shroud 24 into transition piece 25 usually below the fill-line 16 of the molten metal in the transition piece to minimize exposure of the molten metal to air.
From the transition piece nozzle block 25, the molten metal is delivered to the casting pool 30 through a delivery nozzle 26 made of refractory material. The upper surface 31 of the casting pool 30 (generally referred to as the meniscus level) may rise above the lower end of the delivery nozzle 26 so that the lower end of the delivery nozzle is immersed within the casting pool 30, which is confined at the ends of the rolls by a pair of side closure dams or plates 28.
Casting pool 30 is positioned above the nip 27 between the casting rolls 22 supported on the casting roll surfaces 22A. The casting rolls 22 are driven to counter-rotate, and the casting rolls 22 are cooled internally, usually with circulation of water. As the casting rolls rotate, shells of metal solidify from the casting pool 30 on the moving casting roll surfaces 22A. The shells are in turn brought together at the nip 27 between casting rolls 22 to produce solidified strip 12 delivered downwardly from the nip 27.
The transition piece nozzle block 25 and the delivery nozzle 26 may be made of the refractory material of the present invention. The refractory material is comprised of 50 to 85% by weight zirconia, 0 to 35% by weight of silica, less than 5% by weight alumina, 5 to 35% by weight carbon and an anti-oxidant. The refractory material may have a zirconia (ZrO2) content between 60 and 85%, and more specifically between 70 and 80% by weight. The refractory material may have a carbon content between 8 and 30%, and more specifically between 10 and 20% by weight. The anti-oxidant may be up to about 10% by weight, and may be, for example, Si metal, Al metal, silicon aluminum alloy or a carbide such as boron carbide or silicon carbide. The refractory may be stabilized or unstabilized, but it may be stabilized with lime (CaO)Or magnesium oxide (MgO), or a combination thereof, to reduce wear on the refractory material during use in contact with molten steel. The amount of lime or magnesium oxide may be in an amount less than about 28% by weight.
An example of such refractory material may have the following composition:
The remainder of the chemical composition of the refractory may be other materials such as lime (e.g. 3%) which are purposefully added for stabilization, and impurities. The refractory in any case is a carbon bonded silica graphite.
Typical physical properties of the refractory of the specific composition are as follows:
The advantage of the refractory as described is that molten metal does not stick to the refractory and form slag as the molten metal flows through in contact with the refractory. Such slag usually collects at the meniscus of the casting pool 30, from where the slag breaks off and goes into the shells formed during solidification and into the strip 12 to produce defects in the strip and surface defects in the strip. With the zirconia carbon refractories of the present invention, such slag formation is inhibited and strip quality is improved in continuously making strip by the twin-roll caster.
While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Modifications may be made without departing from the spirit and scope of the invention.
