CA1241178A - Method and apparatus for continuous casting of crystalline strip - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method is provided for directly casting molten metal from of the exit end of a casting vessel onto a moving casting surface to form a continuous strip of crystalline metal. The surface tension of the molten metal flowing from the exit forms the top, edge and bottom surfaces of the strip being cast to provide good strip surface quality, edges and structure. An apparatus is also provided including a casting vessel having a molten metal receiving end and an exit end from which a fully developed uniform flow of molten metal leaves through a U-shaped structure onto a moving casting surface. A direct cast strip product is also provided.

Description

Express Mail No. B6584176l P~ENT

J

MET~OD AND APPARATUS FOR
CONTINUOUS CASTING OF CRYSTALLINE STRIP

This invention relates to method and apparatus for direct casting of me~al alloys from ~olten metal to continuous strip. More particularly, it relates to feeding molten metal through an open casting vessel outlet to solidify continuous s strip of desired thickness on a moving casting surface.
In conventional production of metal strip, such methods may include the ~teps of casting the molten metal into an ingot or billet or slab form, then typically includes one or more stages of hot rolling and cold rolling, as well as pickling and annealing at any of various stages of the process in order to produce the desired strip thicknes~ and quality. The cost of producing continuous strip, particularly in as cast gauges ranging from 0.010 inch to 0.100 inch (0.0254 to 0.254 cm) could be reduced by eliminating some of the processing steps of conventional methods. The as-cast strip could be processed conventionally, by cold rolling, pickling and annealing to final gauges of 0.002-0.040 inch.
rhere are known a wide variety of methods and apparatus ~or the production of directly cast strip. Typical of such *~

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methods are those which include spraying molt n metal through a metering orifice across a gap to a rapidly moving quenching surface such as a wheel or continuous belt; methods which partially submerge a rotating quenching surface into a p~ol of molten m~tal; methods which U52 horizontal link belts as quenching ~ubctrates upon which molten metal flows for 501idification; and methods of casting with twin casting rolls having a p~ol of molten metal therebetween.
Direct casting of me~al.~ through an orifice has long been attempted for the commercial production of strip with good quality and structure. U.S. Patent 112,054 dated February 21, 1871 discloses a method of manufacturing flat solder wire from molten metal forced through an orifice and onto a ro;ating casting surfaca. Similarly, U.S. Patent 905,758, issued December 1, 190~, discloses a method of drawing molten metal out of an outlet at the lower end of a vessel and onto a casting surface.
British Patent 24,320, dated October 24, 1910, discloses a method of producing sheet or strip from molten metal flowing through a tube channel having at least one sid~ in contact with the moving casting surface. ~epresentative of a more recent system is U.S.
Patent 3,522,836 - King, issued August 4, 1970, which discloses a method of maintaining a convex meniscus projecting from a nozzle and moving a surface past the nozzle orifice outlet to continuously draw off material and solidify as a continuous product. The molten material is maintained in static equilibrium ` at thP outlet and gravitationally maintained in continuous contact with the moving surface. U.S. Patent 4,221,257 -Narasimhan, issued September 9, 1980, relates to a method of forci~g molten metal under pressure through a slotted nozzle onto the surface of a moving chill body.

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The orifice-type casting systems are generally restricted to light gauge materials as cast usually on the order of less than about 0.010 inch (0.0254 cm) i~ thickness. Such system appear to be gauge-limited for the moving quenching surface appears to be limited in the material which it can solidify and carry away as it is delivered from the noz~le orifice. Such systems behave as a molten metal pump and transfer excess molten metal from the orifice to the quenching surface in a molten state with more heat than can be extracted to provide a suitable strip. By reducing the delivery rate of the metal and/or by i~creasing the velocity of the quenching surface, such a conditiion can be overcome,-how2ver, a reduction in gauge will result .
When crystalline strip i5 attempted to be produced at the high cpeeds associated with the orifice type casting systems, poorer quality usually results. As molten metal is sprayed upon a high-speed quenching surface or is flowed out full width on a slower-moving horizontal belt, it rapidly moves away from the source of the supply in a still partially molten state. It is this condition that lead~ to the deterioration in quality, for as the strip rapidly solidifies from the quenching surface side of the strip, shrinkage occurs which can only be moderated by a resh supply of molten metal. Without such a fresh supply of molten metal, cracks quickly develop within the structure of the strip and greatly reduce its physical properties. ~ttempts have been made to improve the nozzle geometry to overcome the problems associated with orifice-type casting as shown in U.S. Patents 4,274,473, issued June 23, 1931 and 4,290,476, issued September 22, 1981. ~ disadvantage of the orifice-type casting is that the orifice meters out an amount of molten metal which, in effect, ~ 2 4~

determines the gauge of the strip. Furthermore, relativaly high pressure heads used in order to supply enough molten metal to the orifice and a relatively sn~ll standoff distance from the casting wheel for containment of the molten metal also limits the strip gauge.
Thicker strip can be produced on a single quenching surface such aq by dipping a slowly rotating quenching wheel into a static supply o molten metal to permit the solidification of a much thicker stripO Molten metal solidifies on the surface of this wheel and continues to thicken at a predictable rate until it immerges from this bath of molten metal or it separates from the surface. The fresh s~pply of molte~ metal avoids the cracking generally associated with solidification of a finite layer such as in orifice-type casting. Furthermore, an extremely steep thermal gradient between this molten pool and the solidification front also leads directly to a more uniform internal structure and superior upper surface quality. A
drawback from such a dip system come~ from the difficulty of keeping molten metal from solidifying upon the edges of the slightly submerged quenching wheel and having a tendency to cast a channel-like structure. Furthermore, there is the added difficulty of insuring uniform contact between the solidifying strip and the surface of the quenching wheel as it enters the molten pool, and results in poor surace quality on the cast side of the strip. Such di~ficulties lead to spot variations in strip gauge, wherein lighter gauge sections are produced where intimate contact is reduced or lost.
Other direct casting processes have been proposed, but have not developed into commercial processes. For example, pouxing of molten metal on the top of a moving casting wheel produces strip of nonuniform gauge, poor edges and unacceptable quality. U.S~ Patent 993,904, dated May 30, 1911, discloses an apparatus including a molten metal first vessel with a gravity discharge outlet opening into the lower part of a tray like S second ve~el below the level of molten metal therein. The molten metal passes out of the second vessel through an overflow to deliver molten metal to a casting wheel. U.S. Patent 3,381,739, is~ued May 7, 1968, discloses a method of forming sheet or strip material by flowing liquid about a surface which is wetted and bridging the distance to the mo~ing casting surface on which it solidifies.
Wha~ is needed is a ~ethod useful in commercial productio~ ~or direct casting strip having surface guality comparable to or better than conventionally-produced strip. The method and apparatus of direct casting hould produce strip which is superior to orifice-type casting, as well as other k~own direct casting processes including dip-cast systems, horizontal link belt quenching systems, and twin casting rolls. It is an objective that the method and apparatus overcome the di~advantages of known direct casting methods. Furthermore, what is needed is a method and apparatus to permit the direct casting of relatively thick strip on the order of greater than 0.010 inch (0.0254 cm) and up to about 0.100 inch (0.254 cm) or more. It is desirable that the factors contributing to shrinking and cracking of direct cast strip be minimized or eliminated in order to provide improved surface quality and structure of strip.
Furthermore, a method and apparatus is desirable which is suitable for commercial production of strip at reduced cost and to facilitate production of new alloys. The direct cast strip ~2~

should have good suxface quality, edges and structure and properties at least as good as conventio~ally cast strip.
SUM~ARY OF THE INVENTION
I~ accordance with the pre~ent invention, a method is provided for directly casting molten metal to continuous strip of crystalline material. The method include~ supplying molten metal to a casting vessel having a receiving end and exit end being adjacent a casting surface moving generally upwardly past the exit end. The m~lten metal is fed from the receiving end to the exit end to provide a pool of molten metal having a substantially uniform flow and free upper urface in the exit end. The molten metal flows from the exit end onto the casting surface such that across the width of the exit end of the casting ves~el a ~ubsta~tially uniform flow of molten metal is pre~ented to the ca~ting surface. The surface tension of the flowing metal formsall the surfaces of the strip being ca~t. The top surface tension of the free surface of molten metal pool forms the top of the cast strip, and the surface tension of the molten metal leaving the sides of the exit end forms the edges of the cast s~rip. The surface tension of the molten metal leaving the bottom of the exit end maintains a meniscus between an inside surface of the bottom of the exit end and the casting surface to form the bottom of the cast strip. The depth of the molten metal in the exit end and distance between the vessel and casting surface are con~rolled to maintain the surface tension. The as-cast strip is removed from the casting surface.
An apparatus is also provided for directly casting molten metal to continuous strip of crystalline material comprising a movable casting surface, a casting vessel and a means for supplying molten metal to the casting vessel. The casting vessel has a receiving end, an exit end having a generally U~shaped structure adjacent the casting surface and having edges thereof substantinally parallel thereto and an intermediate section to facilitate a substa~tially uniform flow of molten metal from the receiving end to the exit end. The U-shaped structure of the exit end has a bottom wall and diverging inside sidewall surfaces opening upwardly and having the width between the inside surfaces being about aq wide as the strip to be cast. The e~it end has a fixed width along the bottom wall between the inside surfaces and a uniform cro ~-sectional area over a lenqth sufficient to provide a substantially uniform flow of molten metal from the exit end~
The casting surf-ace is movable generally upwardly past the exit end of the casting ves~el at a distance of between 0.005 to a . 060 inch (0.013 to 0.152 cm) thererom at a speed of 20 to S~0 feet per minute.
~ continuous direct cast ~trip product made in accordance with the present invention is also provided.

Figure 1 is a schematic of a strip casting apparatus of the present invention.
Figure 2 i9 an elevated view in cross section of a casting vessel of the present invention.
Figure 2a i9 a detailed elevation view of Figure 2.

Figure 2b is another detailed view of Figure 2.
Figure 3 is a top view of a casting vessel of Figure 2.
Flgure 3a is an end view of the casting vessel of Figure 3.

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Figure 4 is a~ elevation view in cross section of a preferred embodiment of a casting vessel of the present invention.
Figure S is a top view of a preferred embodiment of the casting vessel of Figure 4.
Figure 6 is an enlarged elevated view of a preferred embodiment of the exit end of a casting vessel of the present invention.
Figure 7 is a photomicrograph of typical Type 304 alloy as-cast strip of the present i~vention.
Figure 8 is a photomicrogr ph of a typical Type 304 alloy csnventionally produced hot-roll band.
DETAI~E~ DESCRIPTION OF TEE PREFERRED EMBODIME~TS
Figure 1 generally illust rates casting apparatus 10 including transfer vessel 12 and feed tundish 14 for supplying molten m~tal to casting ve~sel 18 for directly casting molten metal on a casting surface 20 to produce continuous product in strip or 3heet form 15. Molten metal 19 is supplied from vessel 12 to tundish 14 to casting vessel 18 in a conventional manner.
Sto~per rod 16 or other quitable means may control the flow of molten metal to casting vessel 18 such as through spo~lt 17.
Casting vessel 18 is shown substantially horizontal having a receiving end and an exit end disposed adjacent to the casting surface 20.
The supply of molten metal 19 through the casting vessel 18 may be accomplished by any suitable conventional methods and apparatus o~ vessels, tundishes, or molten metal pumps, for example. Vessel 12 and feed tundish 14 may be of known design and should be suitable for supplying an adequate amount of molten metal to casting vessel 18 for strip generation at the quenching wheel.
Casting surface 20 may also be conventional and may take the form of a continuous belt, or a casting wheel.
Preferably a casting wheel is used. The composition of the ca~ting ~urface doe not appear to be critical to the present invention, although some ~urfaces may provide better results than others. The method and apparatus of the present invention have been u~ed with casting surfaces of copper, carbon steel and stainless steel. It is important that the casting surface be movable past the casting vessel at co~trolled speeds and be able to provide desired quenching rates to extract sufficient heat for solidifyin~ the molten metal into strip form. The casting surface 20 is mo~able past casting vessel 18 at speeds which may range from 20 to S00 feet per minute, preferably 50 to 300 feet per minute ~FPM), which is suitable for commercial production of crystalline material. The casting surface 20 should be sufficiently cool in order to provide a quenching of the molten metal to extract heat from the molten metal for solidification of strip of crystalline form. The quench rates provided by casting surface 20 of apparatu~ 10 are less than 10,000C per second and typically preferably less than 2000C per second.
Two important aspects of the casting surface are that it have a direction of movement generally upwardly past the exit end of vessel 18 and a free surface molten metal pool in exit end 26. The free surface o the molten metal pool in exit end 26 is essential to development of good top surface quality of the cast strip. By "freet', it is meant that the top surface is unconfined by structure, i.e., not in contact with vessel structure and free ~24~ 1~7B

to seek its own level between receiving section 22 and exit end 26. Generally, the path is oriented at an included angle ~ from about 0 to 135 from the horizontal and in the direction of metal flow as measured between the direction of metal flow at the free ~urface of molten metal in the exit end and the direction of movemen of the casting surface at the free surface in the exit end of casting vessel 18. For a casting wheel, the path of the casting surface is tangent to the free surface at ~he exit end of vessel 18. Preferably, the a~gle is between 0 and 45 from the horizontal. For a -asting wheel, preferably, the ve~sel is adjacent a position in an upper quadrant of the wheel when the free surface of molten metal is near the crown of the casting wheel, the angle is at about the 0 positio~.
Casting ve~sel 18 is essential to the method and apparatus 10 of the present invention and i5 better shown in Figure 2 which iq an elevation view of the vessel 18. Casting vessel 18 is disposed adjacent casting surface 20, preferably is substantially horizontal, and is composed of heat insulative and refractory material described below. This arrangement is necessary for providing the re~uired uniform and fully-developed flow of molten metal to the casting surface 20. Vessel 18 includes a receiving end 22 at a rearward section and an exit end 26. Preferably, receiving end 22 and exit end 26 have substantially the same cross-sectional area or exit end 26 has a greater cross-sectional area as measured perpendicular to the direction of metal flow from the receiving end 22 to exit end 26.
Receiving end 22 is shown deeper than exit end 26 which facilitates receiving molten metal 19 such as from supply spout 17 and for developing a flow of molten metal to exit end 26.

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Exit end 26 of vessel 18 has a generally U-shaped structure defined by a bottom wall portion 2~ and sidewalls 30, as is shown in Figure 3. Sidewalls 30 may have vertical inside wall inside surface~ 31, but preferably, the surfaceq 31 of sidewalls 30 of the U-~haped structure diverge to open upwardly to facilitate metal flow. The slight taper tends to improve metal flow from exit end 26, but too great a taper ~ay cause a lo~s of surface te~sion control and flooding of molten metal. A
taper of less than 10 psr side and preferably 1-5 is provided.

Exit e~d 26 includes bottom wall 28 which has a generally planar inside portion having a length sufficient to provide a substantially uniform flow of molten metal from the e~it. Preferably, the length of the pianar wall portion as measured in the direction of metal flow is at least equ~l to the depth of molten metal pool to be contained in e~it end 26. More preferably, the ratio of length to depth is at least l:l or greater. Exit end 26 preferably has fixed or uniform dimensioins o width and height throughout the length of the planar inside surface of bottom wall 28 to define a uniform cross-sectional area in exit end 26. The width of the exit end 26 as measured between the inside surfaces 31 of sidewalls 30 along the free surface of molten metal pool is about as wide as the strip to be cast. Preferably, exit end 26 is positioned adjacent casting surface 20 with the ends or edges of the sidewalls 30 and bottom wall 28 defining the U-shaped structure being substantially parallel to the casting surface.
To facilitate transition flow between receiving section 22 and exit end 26, an intermediate section 24 communicating between the re~eiving end 22 and the exit end 26 should be provided in order to have a substantially uniorm flow at exit end 26. Preferably, intermediate section 24 maintains substantially uniform cross-sectional area throughout its length for receiving section 22 to exit end 26. Intermediate section 24 shown in Figure 3 has a gradually increasing width from the receiving end 22 to exit end 26 and, as shown in Figure 2, a gradually decreasing depth ~o as to maintain a substantially uniform cross-sectional area throughout it length. Intermediate section 24 may be provided with a tapered bottom wall 32 which gradually decreases the depth of the vessel 18 from the xeceiving end 22 to the exit end 26. Similarly, intermediate section 24 may have at least one ~idewall 34 which fans outwardly in order to provide a gradually increasing width from the narrower receiving end 22 to the wider exit end 26. Figure 2 is a top view of casting vessel 18 illustrating the widening of sidewall 34 of intermediate section 24.
Figure 2 al o illustrates that weirs or weir plates 36 may be used in casting vessel 18 such as in an intermediate section 24 or near where ~ection 24 merges into exit end 26 in order to further acilitate development of uniform flow. Weir plates 36 should be made of a refractory or heat-resistant material which is also resistant to corrosion by molten metal.
Kaowool refractory board, treated with a diluted colloidal silica suspension has proven satisfactory. Weirs 36 may extend across the entire width or a portion of the width of casting vessel 18.
~s shown in Figure 2, preferably, the molten metal level in the receiving end 22 of casting vessel 18 is at about the same level as the molten metal in exit end 26. Weirs 36 are useful for baffling or dampening the flow in order to facilltate development of a uniform fully-developed flow and to restrain movement of surface oxides and slag.

Figures 2a and 2b illustrate ~he use of surface tension of the flowing molten metal to form the surfaces of the s~rip being cast. Figure 2a is a detailed elevation view in partial cross section of exit end 26 adjacent casting surface 20. Molten metal flowing from the exit end 26 forms and maintains a meniscus 35 be~ween the inside surface of bottom wall 28 of the U-shaped structure and the casting surface. The surface tension forming meniscus 35 form~ the bottom of the strip 15 being cast. The surface tension of the free surface of the molten metal pool in exit end 26 forms a curvilin2ar portion 39 on the top of the molten metal in the U ~haped structure as it forms the strip product.
Figure 2b illustrates exi~ end 26 adjacent casting surface 20 showing solidifying metal 19 therebetween in a view lS from under exit end 26. The surface tension of the molten metal 19 forms the convex surfaces or meniscus 37 between exit end 26 and casting surface 20 at the inside surface 31 of sidewalls 30 near bottom wall 28.
A preferred embodiment of casting vesse} 18 is shown in the elevation and top views of Figures 4 and 5, respectively.
Vessel 18 is shown having an outer metal support shell 38, a re~ractory insulation 40, and a liner 42 which defines the internal surface of the casting vessel 18 and which is in contact with molten metal during casting. The construction of vessel 18 should be made from refractory material which is heat insulative and resistant to molten metal corrosion. The casting vessel may be secured to some suitable table or means to orient and position the vessel at the desired casting position on the casting surface or wheel 20. The exit end 26 of casting vessel 18 should have the front face or edges 33 of sidewalls 30 and bottom wall 28 :~2~

which define and form the U-shaped structure contoured to the casting surface. This can be done simply by using 60 or 100-grit silicon carbide grinding papers held betwee~ the casting surface and the vessel assembly and rubbing tbe ~aper against the vessel 18 to make the edges parallel to the wheel~ The ront surface 33 of the casting vessel 18 may then be brush coated with zirconia cement and allowed to dry be~ore casting.
Figures 4 and 5 illustrate a preferred embodiment of the casting vessel 18 of the present invention which is useful for casting strips of 4 inches, and up to about 13 inches and may be useful up to 48 inches wide. The metal support shell 38 may be used depending upon the type of material used for the insulation layer 40. Insulation layer 40 may be a foamed-ceramic cement insulation which would need an external support such as a metal support shell 38. In the alternative, if a standard refractory bricX or block i9 used and cemented together into the desired shapes and then carved to achieve the desired inner and outer dimension~, then the outer shell 38 is not necessary.
The vessel 18 m2y also be a monolithic shape formed from castable ceramic material. Liner 42 on the internal surface of casting ve~sel 18 is also made of an insulating refractory molten metal resistant material. It has been found that an insulating blanket of a high alumina fiber-silicate composition is useful, such as Fiberfrax brand material, that has been saturated in a diluted colloidal silica suspension and contoured within the casting vessel 18 and then dried prior to actual use.

Figures 4 and 5 also show a rear overflow element 44 including a rearwardly-sloping surface 45 extending from the inner surface of casting vessel 18 to the outer walls of vessel 18. The heisht of the overflow element 44 determines the maximum 7~

depth of molten metal that may be contained in the receiving end 22 and, accordingly, the depth of the molten metal in the exit end 26 of casting ves el 18. Overflow element 44 facilitates control of the molten metal level in the casting vessei 18 which is essential to gauge and quality control of the ca t strip.
Also shown in Figure 4 iq a casting vessel 18 which may optionally include a cover a~sembly 46 in the vicinity of intermediate section 24 of casting vessel 18. Cover 46 includes downwardly extending walls 48 and 50 joined by a bottom surface 52. The downwardly-extending walls 48 and 50 are similar to the weir plates shown in Figure 2. Cover 46 is ge~erally composed of a refractory insulative material resistant to molten metal.
Co~er 46 may comprise a liner 42, a refractory insulation layer 40 and an outer metal shell 3~ having a similar manner of construction as is casting vessel 18. The cover 46 may extend across the entire width or part of the width of casting vessel 18 in the vicinity of intermediate section 24. It is important that the presence of a cover 46, which is useful for retaining the heat in the molten metal in the casting vessel 18, does not contact the molten metal in the receiving end 22 and exit end 26 in order to maintain the free ~urface in the pool in exit end 26.
The cover also can extend over portions or all of rear receiving section 22 to contain a protective atmosphere therein.
Figure 6 illustrate~ another embodiment wherein the exit end 26 of vessel 18 is provided with a means for providing a non-oxidizing atmosphere in a zone defined above the molten metal across the width of th~ U-shaped structure of exit end adjacent to casting surface 20 together with a means for radiantly cooling the molten metal in that zone. The two features may be present separately or in combination.

~2~ 8 Means for providing a non-oxidizing atmosphere provides a protective cover or blanket of inert or reducing gases in a zone about the molten metal in the U-shaped structure of exit end 26. The gases minimize or prevent the buildup or formation of slag and oxides on the top surface of the molten metal, which oxide could be ca~t into the cast strip. The non-oxidizing atmosphere may be sta~ic, or a recirculating atmosphere.
Preferably, a non-contacti~g cover over the zone above the molten metal pool at the exit end 26 of casting vessel 18 and at least ~ 10 one gas nozzle or a series of nozzles 56 provides a continuous ; flow of inert or reducing gas counter to the direction of the cast strip.~ Preferably the gas i~ introduced qo that it impinges in the zone on the top of the molten metal liguid pool where the strip is emerging. The embodiment m3y provide a protective cover for ~ealing the zone over the molten metal pool containing a blanket of inert or reducing gases directed into streams of gases to push any oxide away from the forming of the strip. The series of narrow gas nozzles 56 i~ positioned along the width of the casting strip so that streams or jets of gas impact the zone wherein the ~trip emerges from the liquid pool. Nozzles 56 are directed counter to the casting o the strip at an angle to the plane of the formed stxip, preferably about 20~-30. The gas blanket may be a gas selected from the group consi~ting of hydrogen, argon, helium, and nitrogen in order to minimize the oxides that may be formed duxing casting. The velocity of the gases from nozzle 56 should be quite low, for higher velocities may cause a disturbance in the upper surface of the molten metal pool and result in damage to the cast strip.
Means for radiantly cooling the molten metal in the 30 zone may include providing a coolant in the vicinity of the zone to facilitate extraction of heat from the top surface of the molten metal. The coolant may be provided by a panel of tubes or pipes 54 located above the molten liquid to remove radiated heat from the molten metal. Water or other fluid may be used as a coolant. Preferably, a cover is provided which includes a series of water~cooled tubes 54 ~ealed to the top of the casting vessel 18 with refractory m~terial ~nd cement. Radiant cooling of the top surface of molten metal as it flows from the U-shaped structure of exit end 26 on~o casting surface improves the heat extraction from the top surface of the solidifying moltesl metal to improve as-cast strip top surface quality and structur~ by controlling the growth o dendritic structure in the strip.
Preferably, means for providing a non-oxidi~ing atmosphere and the means for radiantly cooling are used in combination. ~ non-contacting cover for sealing th~ zone over the molten metal at the exit end 26 incllldes a cooling means to remove radiated heat from the molten metal and a non-oxidizing atmosphere means. Preferably, the cover includes a series of water-cooled tubes 54 and a series of gas nozzles 56. The inert gases in this embodiment are cooled by the tubes 54 which further facilitate removal of the radlant heat. The cover containing the cooling tubes 54 seals the zone to reduce oxide or slag formation which could be deposited on the strip product.
In the operation of the casting apparatus of the present invention, vessel 12, tundish 14 and casting vessel 18 are preheated to operating temperatures prior to introducing molten metal into the casting vessel 18 for the production of strip material. ~ny conventional heating means should be suitable and may be used.~ ~n air-acetylene or air-natural gas heating lance positioned in the receiving end 22, as well as ~2a¢~71~3 providing a preheat front cover for the front edges of the casting vessel U-shaped structure which will be placed adjacent the ~asting surfac~ 20. Normal preheating temperatures for c~asting molten stainless steel may be on the order of 1900-2000F. After the minimum preheat levels desired are reached, the heating lances are removed ancl the vessel 18 is positioned adjacent the casting surface at a preset standoff distance ~uch as between 5 and 20 mils.
In commencing the method of directly casting alloy from molten metal to continuous strip, molten metal 19 is supplied from a bulk transfer ladle or vessel 12 to a feed tundish 14 and thereafter to the casting vessel 18 which is oriented ~ubstantially horizonta~ly. The flow of molten metal from feed tundish 14 to casting vessel 18 may be controlled and regulated by valve mean~ such as stopper rod 16 and through spout 17 into the rear feed section or receiving end 22 of casting vessel 18.
~s vessel 18 begins to fill with molten metal, the molten metal begins to flow in a dire~tion toward the exit end of the vessel and flows through an intermediate section 24 and the exit end 26 as qhown in Figure 2. Casting vessel 18 permits the molten metal to flow 50 as to feed the molten metal to the exit end 26 of vessel 18. Casting ves~el 18 may include weirs 36 such as shown in Figure 2 to dampen and ba~fle the flow of molten metal 19 in order to facilitate a uniform fully-developed flow in exit end 26.
The molten metal preferably maintains a substantially uniform cross-sectional area of flow from the receiving end 22 through ~he exit end 26. Generally exit end 26 is wider than the receiving end 22 and the ~-shaped structure has a width which approximates the width of the strip to be cast. Casting vessel 18 has a casting volume having tapered and fanned intermediate 7~

section. Casting vessel 18 is designed to prevent cross flows of molten me~al within the vessel while developing a uniform turbulent flow from exi~ end 26 across the width of the U-shaped tructure il~ end 26 ~uch that the fully-developed flow has the bulk of the velocities in the direction of flow from the receiving end 22 to the exit end 26. The level of molten metal in exit end 26 is about the same as the level in receiving end 22, although the depth of the molten metal will be less in exit end 26. The molten metal continues to flow from the exit end 25 onto the moving casting surface 20 such that across the width of the U-~haped structure of exit end, a substantially uniform flow of molten metal is presented to the casting surface 20. The molten metal in exit end 26 h~s a top surface tension and the molten metal leaving the opening has edge surface tension which form, in part, the top and edges~ respectively, of the cast strip 15. The bottom surface is formed from surface tension in the form of a meniscus between the bottom inside surface of the U-~haped structure and the ca~ting surface.
Though there is no intent to be bound by theory, it appears that the solidification of the molten metal leaving the exit end of vessel 18 commences with the molten metal contacting the castiny surface as it leaves the bottom of the U-shaped opening of exit end 26 of vessel 18. The strip is solidified from the pool of molten metal available to the casting surface at the exit end of vessel 18 and forms a thickness wherein the solidifying strip is continually presented with an oversupply of molten metal until leaving the exit end 26 of vessel 18. Such a pool of molten metal is believed to form a substantial part of the strip thickness as it contacts the moving casting surface 20 with only a minor portion of the strip thickness resulting from ~2~ 7~

molten metal solidified as it was pulled out of the vessel 18 adjacent the top curvilinear surface tension portion 39. It i9 estimated that more than 70% and probably more than about 80% of the strip thickness results from the pool of molten metal provided adjacent the meniscus 35. The molten metal solidifies from the bottom of molten metal pool provided to the casting surface from the bottom of the U-shaped structure of exit end 26 of vessel 18.
Casting surface 20 moves past casting vessel 18 in a generally upward direction from the bottom of the U-shaped opening of of exit end 26 to the open top o~ the opening. The po ition of vessel 18 on the casting surface 20 and the speed of the casting surface are predetermined factors in order to achieve the quality and gauge of the cast strip. If the casting surface 20 is a casting wheel, then the vessel 18 is positianed, preferably t on an upper quadrant of the casting wheel.
By the method of the present invention, there is an important control of several factors which results in the abili~y to cast desired gauges of metal strip ranging from a . ol to 0.06 inch with good surface guality and, edges and structure. The control of molten metal flow onto the casting surface, the speed of the casting surface, the solidification from the bottom of the molten metal pool, and the controlled depth of molten metal in the pool and standoff distance from the casting surface to maintain the surface tension of the molten metal are important interrelating factors.
In order to better understand the present invention, the following examples are presented.
xample_I
A casting vessel having the structure generally as .7~31 show~ in the Figure 2 but having only one weir plate 36 near the f..'`~`. exit end 26 was constructed from hardened blocks of Kaowool~
refractory, which is an alumina-silica composition material. It was treated by soaking it with a colloidal silica suspension S dried overnight at 250F and then fired for 1 hour at 2000F in air. After the blocks were cut and shaped, they were coated with a thin layer of Xaowool cement. The vessel was shaped to the contour of the wheel and then the U-shaped structure ends were coated with a thin layer of a zirconia cement. A weir of similar composition was used. The casting vessel was then heated with alr-acetylene lances. The vessel 18 was about 8.75 inches long from the receiving end 22 to the exit end 26 and was about 6.5 inches wide at the receiving end 22 and about 4 inches wide at bottom wall 28 at exit end 26. Molten metal of Type 304 alloy was tapped at 1580C, supplied to the vessel 18 and maintained at a level of about 1.75 inches deep in the receiving end 22 and the molten metal was about 0.75 inch~s deep in the U-shaped structure in the exit end 26 of vessel 18. A casting surface was a copper casting wheel having a width of 7 inches and a diameter of about 36 inches which provided cooling on the order of less than 2000C/sec. The casting wheel was rotated at a speed of about 250 to 300 feet per minute past the exit end of vessel 18 and spaced about 40 mils therefrom at an angle ~ of about 40.
The U-shaped structure of the vessel had diverging or tapered inside surface 31 of sidewalls 30 of exit end 26 opening upwardly.
The taper was on the order of about 3 per inside surface. Run 25 of about 100 pounds was cast according to the present invention and resulted in successful production of strip having a width of about 4 inches and a uniform thickness of from 16 to 18 rf c~ ~ a r k .

~2~ 7~3 mils having smooth and uniform upper and lower surfaces as-cast and flat edges ~howing no signs of raygedness or curls.
~2~
~ casting vessel having a structure generally as shown in Figure 4 was constructed having a RoawooL refractory and alumina bubble refractory insulation 40 in a metal shell 38. The liner 42 was made of Fiber~rax material, 0.5 inch (1.27 cm) thick at eight pounds per cubic foot which was saturated with a diluted colloidal silica suspension and then dried prior to use. The ves el 18 outside dimPnsions wexe about 15 inches long and 18 inches wide at the exit end vessel 18 had a sligh~ increasing cro~-sectional area to exit end 26. Weir plate 36 was made and positioned similar to Example I and cemented between ; 15 sidewalls of ve3sel 18. The inside surfaces 31 of sid~walls 30 were also tapered or diverging on the order of about 3 per surface. The casting ve~sel was set at a standoff distance of about 35 mil~ at an angle of about 0 for the free surface of the molten metal wa~ near the crown of the casting wheel.
500-pound Run 84-97 of molten metal of Type 304 was cast according to the present inventiion on a casting surface of a low carbon steel seamless pipe having a 12.75-inch outside diameter, a 0.375-inch wall thicknes~, 48 inches wide and internally spray water cooled. The casting wheel was rotated at about 200 FPM at the start of the casting for 10-15 seconds to facilitate flushing of the initial metal flow and then slowed to 100 FPM for the duration of the Run. The molten metal maintained a depth of about 2 inches (5.08 cm) in the exit end 26 and 2.75 inches l6.98 cm) in the receiving end 22.
The vessel 18 also included a cover having a means for radiantly cooling and means for providing a helium atmosphere as shown in Figure 6. The cooling was effected by circulated water at about 3 gallons per minute through copper tubing having a 0.375-inch outside diameter.
The as-cast strip wa abou~ 13 inches wide~ and having 5' a uniform thickness of about 45 mils and having good upper ~urface quality which wa~ uniform, smooth and crack free. The as-cast strip was then conventionally processed by pickling in a nitric/hypofluxoic acid, cold rolling about 50% reduction, annealing at 1950 for 5 mi~utes, pickli~g again in a similar manner, and then cold rolling to 5 mils and annealed. The room temperature mechanical propertie~ of the annealed as-cast sa~ples are shown below in comparison to typical propertie~ of conventionally produced Type 304 annsaled hot-roll ban~.
Table 0.2~Elongation in Samples Ten~ile Strength Yield Strength 2 inch.
(KSI) (RSI) (~) _ _ ___ 1 104.6 44.6 52.0

2 100.~ 40.8 50.0

3 100.3 40.8 49.0

4 100.0 40.0 52.5 7 102.8 42.0 55.0 8 102.0 42.0 57.5 ~ 103.6 44.0 52.0 105.2 44.0 s4.s Type 304 alloy conventionally produced may have typical or average room temperature mechanical properties of annealed hot-roll band of 101.1 RSI tensile strength, 43.8 KSI yield strength and 57~ elongation in 2 inches.

Figure 7 is a photomicrograph of as-cast strip of the present invention showing the typical internal structure from Run 84-52. The Type 304 alloy, shown at lOOX ~agnification, illustrates the typical as-cast structure of small columnar cells oriented in the direction of strip thickness, i.e., top to bottom surfaces. This direction generally conformc; to the direction of heat extraction from the strip as it solidifie~ The method and apparatus of the present invention controls the growth of the dendritic structure in the strip to produce an as-cast strip which can be conventionally processed into finished strip having properties comparable to or better than conventionally pxoduced strip product.
Figure 8 illu~trates a typical structure of a lS conventionally produced hot-roll band of Type 304 alloy at lOOX
magnification.
lt is observed that the ~ethod and apparat~s of the present invention results in even better strip structure and quality as the gauge of the strip product increases and as the width of the strip increases. The tendency of edge curl in the ~trip product cast in 4 to 6-inch widths appears to no longer be present in the wider widths up to 13 inches. The method and apparatus of the present invention provides an uncomplicated and direct method for casting crystalline metal strip or sheet from molten metal to continuous strip. The shrinking and cracking problems of finite film solidification are eliminated and a relatively thick strip of quality comparable to or better than conventional production methods is provided.
The methods and apparatus appear useful for various metal- and alloys, including stainless steels and silicon steels.

2~
.

Claims (23)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Method of directly casting molten metal to continuous strip of crystalline metal, comprising:
supplying molten metal to a receiving end of a casting vessel having a receiving end and an exit end, the exit end being adjacent to a casting surface and having edges thereof parallel to the casting surface and having a substantially uniform cross-sectional area over a length;
moving the cooling casting surface generally upwardly past the exit end;
feeding molten metal with a gradually increasing width of flow while maintaining a substantially uniform cross-sectional area of a slightly increasing cross-sectional area of flow from the receiving end to the exit to provide a pool of molten metal having a substantially uniform flow and a free upper surface in the exit end of the casting vessel;
flowing molten metal from the exit end onto the moving casting surface with a substantially uniform flow across the width of the exit, the surface tension of the flowing metal forming all the surfaces of the strip to be sent, the surface tension of the free surface of the molten pool forming the top of the cast strip, the surface tension of the molten metal leaving the bottom of the exit end maintaining a meniscus between an inside surface of the
1. Claim 1 continued....
   
   bottom of the exit end and the casting surface to form the bottom of the cast strip; the surface tension of the molten metal leaving two sides of the exit end forming the edges of the cast strip and being about as wide as the strip being cast;
   controlling the depth of molten metal in the exit of the vessel and the distance between the exit end and the casting surface to maintain the surface tension of the molten metal on the top, bottom and sides; and removing cast strip from the moving casting surface.
2. 2. The method of claim 1 wherein said feeding of molten metal in the casting vessel includes providing molten metal in the receiving end at the same level as in the exit end.
3. 3. The method of claim 1 includes baffling the flow of molten metal in the casting vessel to facilitate development of uniform flow at the exit end.
4. 4. The method of claim 1 includes moving the casting surface past a generally U-shaped structure of the exit end of the vessel in a direction generally from the bottom of the U-shaped structure to the top of the exit end.
5. 5. The method of claim 1 wherein the direction of the upwardly moving casting surface at the exit end of the vessel is at an angle of from about 0' to 135' as measured from the horizontal in the direction of metal flow at the free surface.
6. 6. The method of claim 5 wherein the angle is from 0°
   to 45° from the horizontal.
7. 7. The method of claim 1 including controlling the thickness of the strip by the speed of the casting surface and the depth of molten metal in the exit end of the casting vessel.
8. 8. The method of claim 1 including cooling the molten metal after contact with the casting surface at a rate of less than 2000° per second to effect solidification of crystalline metal.
9. 9. The method of claim 8 wherein the cooling is at a rate of less than 1500°C per second.
10. 10. The method of claim 1 wherein moving the casting surface is at a speed of 20 to 500 feet per minute.
11. 11. The method of claim 1 wherein providing the casting surface parallel to edges of the exit end and at a distance of between 0.005 to 0.060 inches from the exit end.
12. 12. The method of claim 1 wherein removing strip from the casting surface having a uniform strip width, which ranges from 4 to 48 inches.
13. 13. The method of claim 1 wherein removing strip having a uniform thickness which ranges from 0.010 to 0.100 inch.
14. 14. Method of directly casting molten metal to continuous strip of crystalline metal, comprising:
    supplying molten metal to a receiving end of a substantially horizontal casting vessel having a receiving end and an exit end, the exit end being adjacent to a casting surface and having a generally U-shaped structure and edges thereof parallel to the surface and spaced therefrom at a distance of between 0.005 to 0.060 inch and having a substantially uniform cross-sectional area over a length;
    moving the casting surface generally upwardly past-the exit end at a speed of 20 to 500 feet per minute in a path at an angle of 0° to 135° as measured between the free surface in the direction of metal flow and the direction of the casting surface at the exit end of the casting vessel;
    feeding molten metal with a gradually increasing width of flow from the receiving end to the exit end while maintaining a substantially uniform cross-sectional area or a slightly increasing cross-sectional area of flow from the receiving end to the exit end to provide a pool of molten metal having a substantially uniform flow and free upper surface in the exit end;
    flowing molten metal from the U-shaped structure of the exit end onto the moving casting surface with a substantially uniform flow across the width of the exit, the Claim 13 continued....
    
    surface tension of the flowing metal forming the surfaces of the strip being cast, the surface tension of the free surface of the flowing molten metal forming the top of the cast strip, the surface tension of the molten metal leaving two sides of the exit end forming the edges of the cast strip and being about as wide as the strip being cast and the surface tension of the molten metal leaving the bottom of the U-shaped structure maintaining a meniscus between an inside surface of the bottom of the exit end and the casting surface to form the bottom of the cast strip;
    cooling the molten metal upon contact with the casting surface at a rate of less than 2000°C per second to effect solidification of crystalline metal;
    controlling the depth of molten metal in the exit end of the vessel and the distance between the exit end and the casting surface to maintain the surface tension of the molten metal on the top, bottom and sides; and removing cast strip from the moving casting surface.
    
    15. An apparatus for directly casting molten metal to continuous strip of crystalline material, comprising:
    movable casting surface casting vessel having a receiving end, an exit end, and an intermediate section therebetween, said vessel having
15. Claim 15 continued....
    
    a cross-sectional area from receiving end to exit end being substantially uniform or slightly greater at the exit end;
    means for supplying molten metal to the receiving end of the casting vessel;
    said casting vessel having an exit end having a generally U-shaped structure adjacent the casting surface and edges thereof substantially parallel thereto, and an intermediate section having a gradually increasing width to the exit end to facilitate a substantially uniform flow of molten metal from the receiving end to the exit end, the U-shaped structure of the exit end having a planar bottom wall and diverging inside sidewalls opening upwardly and having a width between the inside surfaces about as wide as the strip to be cast, the exit end having fixed width along the bottom wall between the inside surfaces of the sidewalls and a substantially uniform cross-sectional area over a length sufficient to provide a substantially uniform flow of molten metal from the exit end;
    said casting surface movable generally upwardly past the exit end of the casting vessel at a distance of between 0.005 to 0.060 inches therefrom and at a speed of 20 to 500 feet per minute.
16. 16. The apparatus of claim 15 wherein the casting vessel includes a means for maintaining the molten metal level in the receiving end and exit end at the same level.
17. 17. The apparatus of claim 15 wherein the intermediate section communicating between the receiving end and exit end has a gradually decreasing depth so as to maintain a substantially uniform cross-sectional area throughout the length of the casting vessel.
18. 18. The apparatus of claim 15 including baffles in the casting vessel to facilitate development of uniform flow.
19. 19. The apparatus of claim 15 wherein the casting surface has a direction of movement upwardly at an angle of from 0° to 135° from the horizontal in the direction of metal flow at the free surface.
20. 20. The apparatus of claim 19 wherein the exit end of the vessel is adjacent a position on the casting wheel forming an angle of 0° - 45°.
21. 21. The apparatus of claim 15 wherein the casting surface comprises the peripheral surface of a rotating casting wheel.
22. 22. The apparatus of claim 21 wherein the exit end of the casting vessel is adjacent a position in an upper quadrant of the casting wheel.
    
    23. An apparatus for directly casting molten metal to continuous strip of crystalline, comprising:
    movable casting surface:
    substantially horizontal casting vessel;
    
    Claim 23 continued....
    
    means for supplying molten metal to the casting vessel;
    said casting vessel having a receiving end, an exit end having a generally U-shaped structure adjacent the casting surface and substantially parallel thereto, and an intermediate section communicating between the receiving end and the exit end, the intermediate section having a gradually increasing width to the exit end and a gradually decreasing depth to the exit end so as to maintain a substantially uniform cross-sectional area or a slightly greater cross-sectional area from the receiving end to the exit end of the vessel;
    the exit end having a bottom wall and diverging sidewalls opening upwardly defining the U-shaped structure having a width between the inside surfaces of the sidewalls being about as wide as the strip to be cast, the width of the bottom wall between the inside surface of the sidewalls being fixed to provide a substantially uniform cross-sectional area over a length at least equal to the depth of the molten metal therein, the length being sufficient to provide a substantially uniform flow of molten metal from the exit, said casting surface movable generally upwardly past the exit end of the casting vessel at a distance of between 0.005 to 0.060 inches therefrom and said surface
23. Claim 23 continued....
    
    having an upward path forming an angle of 0 to 135° between the direction of metal flow at the free surface and the direction of casting surface movement at the free surface, the casting surface moving at a speed of 20 to 500 feet per minute.