US3052936A

US3052936A - Method of continuously casting metals

Info

Publication number: US3052936A
Application number: US613347A
Authority: US
Inventors: Hamilton Newell
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1956-10-01
Filing date: 1956-10-01
Publication date: 1962-09-11
Anticipated expiration: 1979-09-11
Also published as: GB835084A

Description

P 1962 N. HAMILTON 3,052,936

METHOD OF CONTINUOUSLY CASTING METALS Filed Oct. 1, 1956 ATTORNEY 32 INVENTOR. z' Newell Hamilton Unite States atent dice 3,952,935 Patented Sept. 11, 1962 3,052,936 METHGD (3F CON 1h JUOUSLY CASTING METALS Nowell Hamilton, Beaver Falls, Pa., assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Oct. 1, 18 56, Ser. No. 613,347 2 Claims. (Cl. 22-2091) This invention relates to the continuous casting of metals and alloys, and more particularly to a method of continuously casting ferrous metals wherein an additive is utilized to reduce the formation of metal oxides on the molten metal surface within the mold, to fluidize the slag on the surface of the metal in the mold, and to form a coating on the mold wall which is non-wetting with respect to the molten metal being cast.

In the continuous casting art many patents have suggested the use of additives such as lubricants, or have suggested methods for regulating the character of the mold atmosphere adjacent the upper surface of the metal being cast in the mold so as to avoid the formation of metal oxides. Many of the additives have apparently performed the dual purposes of combining with the oxygen in the gases above the molten metal surface of the mold, and forming a film between the mold wall and the metal being cast.

Many of the lubricants and/ or controlled atmosphere additions have included hydrocarbons which have been introduced in solid or liquid or gaseous forms. For example, methane has been introduced into the mold cavity adjacent the entering stream of molten metal where the temperatures in the vicinity of the molten metal have cracked the gases to release free hydrogen and carbon. At least some of the hydrogen combines with the oxygen present in the mold cavity to form water vapor, while the free carbon, at least in part, is deposited on the surface of the mold. Hydrocarbons have also been introduced into the mold cavity in the form of liquids, such as oil, and as a solid either in liquid or gaseous suspension, or in the form of a powder. Whenever hydrocarbons are used, the water vapor formed by the presence of moisture in the hydrocarbon or as a result of the dissociation of the hydrocarbon, has contributed to the formation of cavities in or adjacent the surface of the cast ingot. Apparently, the water vapor formed in the cavity condenses upon the relatively cool surface of the mold wall and when contacted with the hot metal is again dissociated with hydrogen and oxygen, or steam, or some combination of the three being driven into the surface of the casting so as to injure the high quality desired of the cast ingot.

Materials other than hydrocarbons have been proposed to control the atmosphere within the mold cavity and to avoid the presence of free oxygen in the immediate vicinity of the hot molten metal. Such materials are exemplified by gases such as argon which displaces oxygen due to the difierence in its specific gravity. However, most of the gases proposed in the art have been used in conjunction with some form of hydrocarbon, which has been introduced in a solid, liquid or gaseous state.

In accordance with the present invention, I provide a material for addition to the cavity of a continuous casting mold which not only avoids the formation of metal oxides on the molten metal surfaces of the metal being cast by combining with the oxygen to form a harmless compound, but also provides a suitable coating on the surface of the continuous casting mold which has a non-Wetting characteristic with respect to the metal being cast. In addition, the material is effective in fiuidizing slag which may be present on the surface of the molten metal within the mold, so that the slag will have a tendency to deposit as extremely thin films on the exterior surface of the cast metal and will not substantially interfere with the heat transfer effects of the casting mold, and adversely effect the surface quality of the cast product.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

The drawing is a schematic elevation, partly in section, of a continuous casting unit constructed and arranged according to the present invention.

In the continuous casting of metals, such as ferrous metals and alloys, molten metal is delivered to the open upper end of an upright casting mold wherein the metal is solidified. The solidified or partially solidified metal is withdrawn from the lower end of the casting mold by a withdrawal mechanism which not only is operative to withdraw the embryo casting but is also arranged for variable speed operation so as to control the rate of withdrawal of the casting from the casting mold. Ordinarily, the casting is cut to length at a position beneath the withdrawal mechanism so that the casting can be conveniently handled for subsequent disposal.

As shown in the drawing, a casting zone, which is rep resented as a Water-cooled substantially upright mold 10 is supplied with molten metal from an external source such as a furnace or the like (not shown). The molten metal in passing to the mold 10 is directed through a substantially closed flow path wherein floating slag on the molten metal stream is separated from the molten metal and the metal is thereafter discharged over a casting lip into the mold. As shown, the molten metal stream passing through a delivery channel 11 is discharged to a closed tundish 12. The tundish is arranged for positional adjustment and is provided with a depending bafile 13 intermediate its length. The bafile 13 extends downwardly from the tundish roof 14 to a spaced position above the bottom 15 of the tundish. With this construction, the molten metal must pass under the lower end 16 of the bafile 13 and any slag or other impurities are retained upstream of the bafile so that the slag may be periodically removed. The tundish illustrated is provided with a fixed axis 17 adjacent its discharge end 21 and is further provided with a vertical adjusting means adjacent its molten metal inlet end 21. In the embodiment shown, vertical adjustment of the inlet end 21 of the tundish is obtained by angular movement of an eccentric 22 about a shaft 23. Rotation of the eccentric raises and lowers the inlet end of the tundish where the movement may be accomplished by means of a motor attached to the shaft 23. he tundish is provided with a weir discharge lip 24 so that the molten metal flows over the weir and descends under the force of gravity through an opening 25 in the bottom of the tundish and into the open upper end of the mold 10. As shown, a flexible sealing means 26 is provided between the bottom of the tundish and the upper end of the mold so as to 3 exclude infiltration of air or other oxygen containing gases into the mold cavity.

The mold assembly may be of the type disclosed in US. Patent 2,590,311 wherein cooling water is introduced into the mold through a pipe 27 at super-atmospheric pressure, and is discharged through cooling fluid flow passageways on the exterior side of a mold liner. The molten metal entering the casting zone is cooled by heat exchange through the mold liner to the cooling water wherein the cooling effect of the casting zone and the rate of molten metal introduction is coordinated with the cooling water flow so that a self-sustaining shell is formed on the casting before the casting is withdrawn from the lower end of the mold. Thereafter, the embryo casting 30 is passed downwardly through a pair of pinch-rolls 31 which engage the surface of the casting so as to regulate the rate of casting withdrawal. In accordance with good practice, it is desirable to provide means (not shown) for supporting the embryo casting leaving the mold as to avoid swelling and skin rupture of the embryo casting.

The pinch-rolls are driven in the usual manner by a variable. speed motor connected with a set of gears (now shown) so that the speed of rotation of the rolls 31 may be regulated. immediately below the pinch-roll mechanism is positioned a cutting torch 32 of the oxy-acetylene type, which is arranged to sever the casting into predetermined lengths for subsequent handling.

While the flow channel of the molten metal entering the casting zone of the unit is covered to avoid infiltration of oxygen, nevertheless, some oxygen will be drawn into the mold cavity of the casting zone and encourage the formation of metal oxides on the surface of the molten metal. When casting ferrous alloys, the presence of metal oxides and slag on the molten metal pool maintained within the mold has a tendency to produce inclusions on the surface of the casting produced in the unit. Any metallic oxides or other impurities present in the mold cavity have a tendency to agglomerate and to be withdrawn from the surface of the molten metal pool in relatively large masses which form an insulating envelope or insert on part of the casting wall. Such inclusions are not only detrimental to the quality of the casting but also interfere with rapid cooling and solidification of the metal.

In accordance with the present invention, an additive material is introduced into the mold cavity so as to avoid the formation of metal oxides therein and to fluidize the oxides that may be present on the surface of the molten metal pool. The additive of the present invention also has the property of forming a parting agent between the inner surfacerof the mold and the exterior surface of the casting.

A very desirable, and practical, additive material consists of cryolite which may be added to the mold cavity in the form of a dried powder. As shown in the drawing, the additive may advantageously be introduced to the mold cavity by means of a screw feeder 34 which withdraws the powder from a storage hopper 35 for delivery to the mold at a desirable rate, as determined by the operation of a screw feeder motor 36. The cryolite powder discharged from the screw feeder falls by gravity through an upright connecting conduit 37 which is extended through the roof of the tundish and is positioned in substantially vertical alignment with the axis of the continuous casting mold 10.

The cryolite powder is preferably pre-dried to avoid the inclusion of mechanical moisture in the powder and is delivered to the mold in a pulverulent or powdered form having a fineness of, for example, 100 to 200 mesh, or finer. A particularly satisfactory fineness for the purpose indicated is a powder prepared to a fineness of minus 100 and plus 200 mesh, i.e. all of the powder will pass through a 100 mesh U.S.S. screen and be retained upon a 200 mesh U.S.S. screen. With the additive introduced as a' fine powder from above the stream of metal, the

powder is dispersed throughout the mold cavity and is maintained in a turbulent suspension due to the eddy currents created by the incoming molten metal stream. The cryolite is introduced into the mold cavity at a rate substantially equal to the ratio by weight of 1 part of cryolite to from 25,000 to 40,000 par-ts of molten metal. The amount of cryolite may vary to some extent depending upon the type of molten metal being cast. For example, slightly more cryolite may be used when casting stainless steels of, for example, an 18-8 composition, than when casting low carbon steel.

While the additive is shown as being introduced in powdered form from a mechanical feeder, it will be understood the powder may be introduced in a gaseous or liquid suspension, as for example, entrained in an inert gas stream.

Cryolite is a double fluoride of sodium and aluminum having the general formula 3NaF-AlF Binary compounds of sodium fluoride and aluminum fluoride having a different relationship than cryolite have a substantially equal effect as an additive in the casting of ferrous metal, and other binary metal and fluorine compounds may also be used as an additive. For example, magnesium and potassium can be combined with fluorine to accomplish the advantageous results described with a cryolite additive. Likewise, sodium and aluminum may be combined with other of the halogens to form binary compounds usuable as additive materials in a continuous casting mold.

When casting ferrous metals, the molten metal is delivered to the mold 10 in a substantially uniform stream, quantatively regulated from the source. With the delivery of molten metal to the mold the feeder is operated to deliver a substantially uniform stream of additive through the tube. The additive, being in a dry powder state floats in the atmosphere of the mold cavity and is agitated by the falling stream of metal so that the material is well dispersed between the upper level of the molten metal within the mold and the depending baffle with the greatest density of marterial occurring in the mold cavity.

When cryolite or another sodium aluminum fluoride is used as the additive, a small amount of the aluminum apparently dissociates from the compound to form alumina so that oxygen in the mold atmosphere does not appreciably combine with the molten iron to form iron oxides. Any iron oxides or other slag impurities present on the molten metal surface is fluidizedby the additive and is easily accommodated on the surface of the casting without serious damage to the quality of the casting. Some portion of the cryolite will deposit on the surface of the mold walls and will form a protective film between the mold surface and the molten metal of the casting. ctual operations have indicated that the cryolite film remains at least in part on the wall of the mold and a cryolite film has been found on the lower wall portion of the mold when any contact will be between the mold and the solidified shell of the casting.

While in accordance with the provisions of the statues I have illustrated and described herein the best form and mold of operation of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of continuously casting ferrous metal which comprises the steps of introducing molten metal into a casting zone to form a pool of molten metal in the upper portion of said casting zone, cooling said molten ferrous metal in said casting zone to form an embryo ingot, withdrawing the embryo ingot from said casting zone, and delivering an additive material to the casting zone to form a suspension of said additive in the gaseous atmosphere above said pool of molten metal therein, said additive material consisting of substantially dry cryolite having a size range between 100 and 200 U.S.S. mesh size and added to the casting zone in a Weight ratio of the order of 1 part of cryolite to between 25,000 and 40,000 parts of molten metal.

2. The method of continuously casting ferrous metal which comprises the steps of introducing molten metal into a casting zone to form a pool of molten metal in 10 the upper portion of said casting zone, cooling said molten ferrous metal in said casting zone to form an embryo ingot, Withdrawing the embryo ingot from said casting zone, and delivering an additive material to the casting zone to form a suspension of said additive in the gaseous atmosphere above said pool of molten metal therein, said additive material essentially including a substantially dry compound of sodium and aluminum floride having a size range between 100 and 200 U.S.S. mesh size and added to the casting zone in a weight ratio of the order of 1 part of said compound to between 25,000 and 40,000 parts of molten metal.

References Cited in the file of this patent UNITED STATES PATENTS 48,483 Everett June 27, 1865 230,369 Walker July 20, 1880 2,376,518 Spence May 22, 1945 2,510,155 Tanczyn June 6, 1950 2,590,311 Harter et a1 Mar. 25, 1952 FOREIGN PATENTS 513,473 Canada June 7, 1955 OTHER REFERENCES Metal Industry, July 25, 1947, pages 71 and 72. Foundry Trade Journal, Jan. 21, 1932, pages 59 and 60.

Claims

1. THE METHOD OF CONTINUOUSLY CASTING FERROUS METAL WHICH COMPRISES THE STEPS OF INTRODUCING MOLTEN METAL INTO A CASTING ZONE TO FORM A POOL OF MOLTEN METAL IN THE UPPER PORTION OF SAID CASTING ZONE, COOLING SAID MOLTEN FERROUS METAL IN SAID CASTING ZONE TO FORM AN EMBRYO INGOT, WITHDRAWING THE EMBRYO INGOT FROM SAID CASTING ZONE, AND DELIVERING AN ADDITIVE MATERIAL TO THE