US3857434A

US3857434A - Roll-couple, continuous-strip casting

Info

Publication number: US3857434A
Application number: US00343493A
Authority: US
Inventors: E Wondris
Original assignee: National Steel Corp
Current assignee: National Steel Corp
Priority date: 1973-03-21
Filing date: 1973-03-21
Publication date: 1974-12-31
Anticipated expiration: 1991-12-31

Abstract

Roll-couple, continuous-strip caster in which molten casting metal is continuously supplied from a reservoir to a position beneath the nip of the casting rolls. The supplied molten metal flows in a path upwardly towards the roll nip, then past each of the casting rolls toward overflow weirs to clean the surface of the melt in front of each roll. The overflow metal is recirculated; it is collected, reheated under controlled conditions, filtered, and returned to the supply reservoir. The static head of metal in the supply reservoir controls the recirculation rate. The system provides for improved strip quality, facilitates maintenance of constant heat interchange conditions, and improves solidification characteristics and rate.

Description

States Patent [191 Wondris ROLL-COUPLE, CONTINUOUS-STRIP CASTING Primary ExaminerR. Spencer Annear Attorney, Agent, or FirmShanley, ONeil and Baker 5 7] ABSTRACT Roll-couple, continuous-strip caster in which molten casting metal is continuously supplied from a reservoir to a position beneath the nip of the casting rolls. The supplied molten metal flows in a path upwardly towards the roll nip, then past each of the casting rolls toward overflow weirs to clean the surface of the melt in front of each roll. The overflow metal is recirculated; it is collected, reheated under controlled conditions, filtered, and returned to the supply reservoir. The static head of metal in the supply reservoir controls the recirculation rate. The system provides for improved strip quality, facilitates maintenance of constant heat interchange conditions, and improves solidification characteristics and rate.

26 Claims, 4 Drawing Figures sum 1 0F 2 I I I I I I I 4 I I I I I I I I PATENIEUBEC3 1 I974

sum

2 or 2 DRIVE FIG. 2

ROLL-COUPLE, CONTINUOUS-STRIP CASTING BACKGROUND OF THE INVENTION In continuous-strip casters employing a roll couple (i.e., a pair of cooperating casting drums) there is a problem in that oxidation on the melt surface tends to produce a poor, jagged surface on the cast strip. This oxide layer forms notwithstanding such preventive techniques as the maintenance of an inert atmosphere over the melt. This is because it is not feasible to effect complete exclusion of oxygen, and such casting metals as aluminum have a very high affinity for whatever oxygen is in the vicinity.

There is another problem in the operation of such continuous-strip casters. This problem is the tendency of refractory and other nonmetallic inclusions to collect on the melt surface. The inclusions further contribute to poor casting quality, as a result of their becoming entrapped in solidifying metal and resultant incorporation in the cast strip.

A still further problem in prior art casters is the lack of maintenance of constant heat interchange conditions. Constant heat interchange conditions are needed for maintenance of a constant casting temperature, which is very important because casting temperature controls solidification rate and, therefore, controls casting speed.

Main objects of the invention are to overcome the foregoing problems.

Other advantages of the invention will appear from the following detailed description which, in connection with the accompanying schematic drawings, discloses a preferred embodiment ofthe invention for purposes of illustration only.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side cross-sectional view illustrating a continuous-strip casting system embodying principles of the invention.

FIG. 2 is aview taken on the section plane designated by line 2-2 in FIG. 1.

FIG. 3 is a cross-sectional view taken on the plane designated by line 33 in FIG. 2.

FIG. 4 is a cross-sectional view taken on the plane designated by line 4-4 in FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT In FIG. 1, continuous-strip caster comprises refractory walls defining a vessel 12 for molten casting metal. The metal can, for example, be aluminum. As used herein, the term aluminum embraces aluminum-based alloys as well as the pure metal. However, it will be appreciated that other metals can be used.

The walls of vessel 12 include back wall 14, front wall 15 (FIG. 2) and opposite side walls in the form of opposed weir members l6, l8. Weirs 16, 18 are provided for overflowing molten metal from vessel 12 into

catch basins

20, 22 respectively. Each weir extends from back wall 14 to front wall 15 of vessel 12.

A pair of roll members 24, 26 having parallel, horizontal axes of rotation are immersed in the molten metal over equal parts of their respective peripheries. The rolls sealably engage curved, refractory bearing members 28, 30 (FIG. 2) which respectively form portions of back and

front walls

14, 15. The portions of the lengths of the rolls which are carried by the bearings are of course not immersed in the molten metal. The bearing members can e.g., be made of the fibrous refractory sold under the trade name Marinite by the Johns-Manville Company, or of the ceramic sold under the trade name Fiberfrax by the Carborundum Company. Rolls 24, 26 are driven by motor 32 in counterrotating movement in the directions indicated by arrows 31 (FIG. 1). Coolant water is passed through passageways (e.g., 34) in each roll as shown by arrows as 35 (FIG. 2), to effect cooling of the roll circumference.

As a result of the cooling and rotative movements of the rolls, metal solidifies on the periphery of each roll. The metal solidifies in progressively increasing thicknesses from the respective roll dipping points 36, 38 (FIG. 1) to the nip of the rolls. The roll nip is generally indicated at 40, and is the region which is defined by the rolls at the location where each increment of roll surface comes into proximity with the other roll. In the roll nip, the solidified metal on each roll is merged with solidified metal on the other roll and formed into continuous strip 42 in the usual way. Typical strip thicknesses are of the order of 0.25 0.50 inch. Strip 42 is trained upwardly from rolls 24, 26 and around guide roller 44 to coiling equipment (not shown).

The roll couple formed by rolls 24, 26 is positioned centrally between opposed weirs 16, 18. Each weir is spaced across an unobstructed portion of the surface of the melt from the dipping point of the associated roll a distance W which is equal to the distance of the other weir from the dipping point of its associated roll.

Feed means which include refractory walls 46 defining a reservoir 48, and which include conduit 50, are provided for continuously supplying molten metal to vessel 12. Conduit 50, which communicates reservoir 48 and vessel 12, discharges molten metal into vessel 12 at a location which is beneath roll nip 40 and which is spaced vertically therefrom. Stated differently, roll nip 40 is in superposed, vertically spaced relation to exit opening 52 of conduit 50. As used herein, the term beneath means directly below.

In the embodiment illustrated, conduit 50 discharges molten metal into vessel 12 in a horizontal direction through a circular exit opening 52. Baffle 54 is positioned in vessel 12 in confronting relation to the conduit exit opening for deflecting discharged molten metal in a direction upwardly towards roll nip 40. Baffle 54 also promotes temperature uniformity in the melt by effecting a distribution of the incoming molten metal. Conduit exit opening 52 is located symmetrically with respect to the immersed length of rolls 24, 26 (FIG. 2). That is, the exit opening is positioned with its central point at a location equidistantly spaced from each end of the immersed length of the rolls. The overflow weirs are coextensive with the immersed length of the rolls.

In the structure described, there is a flow path established for the molten metal which flow path is pictorially depicted by the directional arrows, e.g., 56, in FIG. 1. The molten metal enters vessel 12 from conduit 50 and is deflected upwardly by baffle 54 toward roll nip 40. Because of flow resistance offered by the rolls, the flow splits into two branches. One branch passes upwardly beside roll 26 to weir l8, and the other branch passes upwardly beside roll 24 to weir 16. This flow pattern is highly advantageous because it provides for continuous cleaning of the melt at the dipping point of each drum. That is to say, the flowing molten metal continuously carries away any oxide layer which forms on the melt surface at the roll dipping point, and also carries away any nonmetallic inclusions. The oxides and inclusions are swept over the weirs.

Distance W of each weir from the dipping point of the associated roll is important. If the distance is too large, there will be no cleaning action at the clipping point of the roll because the flowing molten metal will bypass the roll to provide a zone of stagnant flow at the clipping point. If distance W is too small, the melt surface could freeze over because of the combined cooling effects of the roll, the weir, and the hood atmosphere. Accordingly, distance W is established at a magnitude which establishes a molten metal flow path which cleans the surface of the molten metal contiguous to the roll, but which maintains the metal surface at the weir in molten condition. For example, in a caster using rolls 8.25 inches in diameter, distance W can be about 5 inches.

The flow path of the molten metal in the vessel is also advantageous in improving solidification characteristics and rate. That is to say, the flow path assures that the coolest molten metal is at the surface of the melt, where each increment of roll first contacts the melt. This promotes quicker solidification for fine grain structure in the casting and for higher casting speeds. It also tends to enhance the grip of the embryo strips on the rolls, which grips are established by immersion of the rolls at a melt-contact angle which is in excess of 180 of the roll periphery.

The flow path of the molten metal also makes the temperature distribution in vessel 12 symmetrical over each roll, promoting soundness in the cast structure by preventing voids. The flow pattern also promotes maintenance ofa relatively uniform temperature throughout the melt, and this in combination with the maintenance of a constant melt level by the weirs promotes maintenance of the constant heat interchange conditions which are necessary for maintenance of a constant casting temperature. As indicated hereinabove, this is important because casting temperature controls solidification rate and therefore controls casting speed.

The molten metal overflowing weirs 16, 18 is recirculated through reservoir 48 by a recirculating system which includes

catch basins

20, 22 and which includes conduits 58, 60. These conduits respectively conduct molten metal from the catch basins by gravity flow to collector vessel 62. In collector vessel 62 the collected molten metal is thermally conditioned by heating with a heat exchange coil 64, or other heating system, to replace heat loss in transit around the molten metal flow circuit. It will be appreciated that in certain circumstances it can be desirable to thermally condition the metal in collector vessel 62 by cooling instead of heating, in order to attain desired temperature for recycling into the casting vessel 12. This can occur, for example, where heat is applied to the recirculating molten metal before it enters collector vessel 62.

The thermally conditioned molten metal is transferred from collector vessel 62 to reservoir 48, which in turn transfers it to casting vessel 12 through conduit 50. Transfer to reservoir 48 is effected by pump 66, which can be of any suitable type of conventional design. The flow rate accomplished by pump 66 is varied and controlled and this for example can be effected by varying the speed of a conventional variable-speed motor 68 which drives pump 66. Molten metal passes from pump 66 to reservoir 48 through conduit 70. Filter 72 in conduit can remove oxide particles and nonmetallic inclusions from the recirculating metal. Where the metal being cast is aluminum, the filter material can for example be alumina screening. Make-up casting metal can be supplied to collector vessel 62, or directly to reservoir 48.

Walls 46 of reservoir 48 are higher than weirs l6, l8, so that the rate of supply of molten metal from reservoir 48 to casting vessel 12 can be controlled by controlling the height of molten metal in reservoir 48. Stated otherwise, the rate of supply of molten metal, and thus the molten metal recirculation rate, can be controlled by the static head of molten metal in reservoir 48. The level of molten metal in reservoir 48 is controlled by controlling the rate of flow of metal to the reservoir by controlling the speed of operation of pump 66.

In order to keep oxidation of the molten metal at a minimum, casting vessel 12 and catch

basins

20, 22 are covered by protective-atmosphere hood 74. An inert gas (e.g., argon) is supplied to hood 74 through conduits 76. Collector vessel 62 and reservoir 48 are similarly provided with protective atmospheres under hoods 78, respectively. Inert gas is supplied to these hoods through

conduits

82, 84 respectively.

The recirculation system effects molten metal savings which make it possible to provide the above-described molten metal flow pattern in casting vessel 12, and its concomitant advantages. Moreover, recirculation makes it possible to exercise close temperature control, thereby facilitating maintenance of the constant heat interchange conditions needed for maintenance of the constant casting temperature which is important in controlling solidification rate and casting speed. Thus, the flow path in casting vessel 12 and the recirculation system coact to provide a clean, uniform-temperature melt for improved quality in the cast strip and for rapid operation of the strip caster.

A summary of operation of the system is as follows: A stream of molten metal is discharged into casting vessel 112 through conduit 50 and is deflected upwardly by baffle 54 toward roll nip 40. The upflowing molten metal stream divides into two branches, with each branch flowing toward a weir. Each branch stream sweeps oxides and nonmetallic inclusions away from the vicinity of the associated roll and over the respective weir. The overflow metal is collected, reheated under controlled conditions in collector vessel 62, cleaned by filter 72, and returned to feed reservoir 48. Pump 66 controls the static head of metal in reservoir 48, to thereby control the rate of recirculation of the molten metal.

Continuous-strip casting systems according to the invention are highly advantageous. Metal flow away from the rolls carries the oxide layer and nonmetallic inclusions away from each roll dipping point, which avoids jagged surfaces on the strip and avoids entrapment of inclusions in the strip. The flow, which is countercurrent with respect to the direction of rotation of each roll, improves solidification characteristics and rate by providing for fast solidification, which promotes fine grain structure and fast casting speeds Temperature is symmetrical over each roll, for symmetrical solidification on the rolls and resultant sound castings. Constant heat interchange conditions are maintained by the constant melt level established by the weirs, by maintenance of a relatively uniform temperature throughout the melt, and by continuous supply of molten metal at controlled temperature. Maintenance of constant heat interchange condition makes it possible to maintain constant casting temperatures and solidification rates, and, therefore, constant casting speeds, for sustained production of sound cast strips. And all the foregoing are achieved with maximum economy of casting metal.

Although the invention has been described in connection with a preferred embodiment, many modifications can be made as will be apparent to those skilled in the art. For example, the conduit exit opening could discharge molten metal into the casting vessel in a vertical direction instead of horizontal, so that no baffle such as 54 is necessary. Or, the exit opening could take the form of an elongated orifice or slot extending along the roll nip. Or, a plurality of exit openings could be used positioned symmetrically with respect to the center of the immersed length of the rolls. Or, the weirs could be made to be adjustable in height. Many other modifications could be made. Accordingly, reference will be made to the appended claims for definition of the scope of the invention.

1 claim:

1. Continous-strip casting apparatus, comprising means including walls defining a vessel for molten material having a surface,

strip-forming means including a pair of roll members having length,

means mounting the roll members to provide a roll nip therebetween and to position the roll members in the vessel with the peripheries of the roll members being partially immersed in the molten material along at least a portion of the length of the roll members,

the walls of the vessel comprising means including a pair of opposed weir members for overflowing molten material from the vessel,

the roll members being positioned between the weir members,

each weir member being spaced across an unob structed portion of the surface of the molten material from an adjacent roll member,

and conduit means for feeding molten material into the vessel below the surface of the molten material for flow upwardly toward the roll nip and toward the unobstructed portion of the surface between each weir and the adjacent roll member.

2. Apparatus as defined in claim 1, in which the conduit means discharges molten material into the vessel at a location spaced beneath the roll nip.

3. Apparatus as defined in claim 1,

the conduit means having an exit opening which is positioned symmetrically with respect to the immersed length of the roll members.

4. Apparatus as defined in claim 1,

including a reservoir for molten material communicating with a conduit means, and

means for controlling the height of molten material in the reservoir to control the rate of supply of molten material from the reservoir to the vessel.

5. Apparatus as defined in claim 1,

each weir member being spaced from the associated roll member a distance of a magnitude to establish a molten material flow path which cleans the surface of the molten material contiguous to the roll member and which maintains the surface of the molten material at the weir member in molten condition.

6. Apparatus as defined in claim 1,

the weir members being coextensive with the immersed length of the roll members.

7. Apparatus as defined in claim 1, including recirculating means for recirculating overflowed molten material to the vessel.

8. Apparatus as defined in claim 7,

the recirculating means including heat exchange means for thermally conditioning recirculating molten material.

9. Apparatus as defined in claim 8,

the recirculating means inclulding means including a second vessel for collecting overflowed molten material,

the heat exchange means including means for thermally conditioning molten material collected in the second vessel,

the recirculating means also including means for transferring thermally conditioned molten material to the first-named vessel.

10. Apparatus as defined in claim 8, including means for filtering recirculating molten material.

11. Continous-strip casting apparatus, comprising means including walls defining a vessel for molten metal having a surface,

strip-forming means including a pair of roll members having length,

the roll members having peripheries which are partially immersed in the molten metal along at least a portion of the length of the roll members,

the walls of the vessel comprising means including a pair of opposed weir members for overflowing molten metal from the vessel,

the roll members being positioned between the weir members,

each weir member being spaced across the surface of the molten metal from a roll member,

feed means including walls defining a reservoir for continuously supplying molten metal to the vessel,

the feed means including conduit means communicating the reservoir and the vessel,

the pair of roll members defining a roll nip,

the conduit means discharging molten metal into the vessel at a location spaced beneath the roll nip, and

means for directing discharged molten metal upwardly toward the roll nip.

12. Apparatus as defined in claim 11, including recirculating means for recirculating overflowed molten metal to the reservoir.

13. Apparatus as defined in claim 12,

each weir member being spaced from the associated roll member a distance of a magnitude to establish a molten metal flow path which cleans the surface of the molten metal contiguous to the roll member and which maintains the surface of the molten metal at the weir member in molten condition.

14. Apparatus as defined in claim 13,

the recirculating means including means including a second vessel for collecting overflowed molten metal,

heat exchange means for thermally conditioning molten metal collected in the second vessel, and

transfer means for transferring thermally conditioned molten metal to the reservoir.

l5. Apparatus as defined in claim 14,

16. Apparatus as defined in claim 15,

17. Apparatus as defined in claim 16,

the heat exchange means being heating means.

18. Apparatus as defined in claim 17, including means for filtering recirculating molten metal.

19. Apparatus as defined in claim 18,

the walls of the reservoir being higher than the weir members,

the transfer means including flow-control means for controlling the height of molten metal in the reservoir to control the rate of supply of molten metal from the reservoir to the first-named vessel.

20. Apparatus as defined in claim 19,

the flow-control means including a pump.

21. Apparatus as defined in claim 20, in which the discharged molten metal is directed upwardly toward the roll nip by baffle means located in the first-named vessel. 22. Continuous-strip casting process utilizing a casting apparatus of the type having a vessel adapted to contain molten metal to be cast and a pair of roll members providing a roll nip and having peripheries which are partially immersed in the molten metal contained in the vessel with each roll being spaced from an adjacent wall of the vessel, the process comprising the steps of:

continuously supplying molten metal to the vessel at the location in the vessel spaced beneath the surface of the molten metal contained within the vessel, passing the supplied molten metal in a direction upwardly toward the roll nip and in a direction toward the space between each roll and an adjacent wall of the vessel, and overflowing molten metal from the space between each roll and an adjacent wall of the vessel over the adjacent wall to without the vessel. 23. Process as defined in claim 22, including the step of recirculating overflowed molten metal to the vessel.

24. Process as defined in claim 22, the metal being aluminum.

25. Process defined in claim 22, collecting overflowed molten metal in a reservoir, thermally conditioning collected molten metal, and transferring thermally conditioned molten metal from the reservoir to the vessel. 26. Process as defined in claim 25, including the step of controlling the height of molten metal in the reservoir to control the rate of supply of molten metal from the reservoir to the vessel.

* l l =l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,857,434 Dated December 31, 1974 Inventor(s) Erich F. Wondris It is certified that error appears in the ,above -identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 6, "condition". should be --condi,tions.-'-.

Column 6, line 19, "inclulding" should be --including.

Column 8, line 7, r "the (first occurrence) should be -a--;

line 23, after "22," insert -including the steps Of 'I Signed and sealed this 4th day of March 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C.- MASON I Commissioner of Patents Attesting Officer v and Trademarks FORM PO-1050 (10-69) I I QSCOMNFDC 60376-[369 9 U.S. GOVERNMENT PRINTING OFFlcE: 199 OJ5-33"

Claims

1. Continous-strip casting apparatus, comprising means including walls defining a vessel for molten material having a surface, strip-forming means including a pair of roll members having length, means mounting the roll members to provide a roll nip therebetween and to position the roll members in the vessel with the peripheries of the roll members being partially immersed in the molten material along at least a portion of the length of the roll members, the walls of the vessel comprising means including a pair of opposed weir members for overflowing molten material from the vessel, the roll members being positioned between the weir members, each weir member being spaced across an unobstructed portion of the surface of the molten material from an adjacent roll member, and conduit means for feeding molten material into the vessel below the surface of the molten material for flow upwardly toward the roll nip and toward the unobstructed portion of the surface between each weir and the adjacent roll member.

3. Apparatus as defined in claim 1, the conduit means having an exit opening which is positioned symmetrically with respect to the immersed length of the roll members.

4. Apparatus as defined in claim 1, including a reservoir for molten material communicating with a conduit means, and means for controlling the height of molten material in the reservoir to control the rate of supply of molten material from the reservoir to the vessel.

5. Apparatus as defined in claim 1, each weir member being spaced from the associated roll member a distance of a magnitude to establish a molten material flow path which cleans the surface of the molten material contiguous to the roll member and which maintains the surface of the molten material at the weir member in molten condition.

6. Apparatus as defined in claim 1, the weir members being coextensive with the immersed length of the roll members.

8. Apparatus as defined in claim 7, the recirculating means including heat exchange means for thermally conditioning recirculating molten material.

9. Apparatus as defined in claim 8, the recirculating means inclulding means including a second vessel for collecting overflowed molten material, the heat exchange means including means for thermally conditioning molten material collected in the second vessel, the recirculating means also including means for transferring thermally conditioned molten material to the first-named vessel.

11. Continous-strip casting apparatus, comprising means including walls defining a vessel for molten metal having a surface, strip-forming means including a pair of roll members having length, the roll members having peripheries which are partially immersed in the molten metal along at least a portion of the length of the roll members, the walls of the vessel comprising means including a pair of opposed weir members for overflowing molten metal from the vessel, the roll members being positioned between the weir members, each weir member being spaced across the surface of the molten metal from a roll member, feed means including walls defining a reservoir for continuously supplying molten metal to the vessel, the feed means including cOnduit means communicating the reservoir and the vessel, the pair of roll members defining a roll nip, the conduit means discharging molten metal into the vessel at a location spaced beneath the roll nip, and means for directing discharged molten metal upwardly toward the roll nip.

13. Apparatus as defined in claim 12, each weir member being spaced from the associated roll member a distance of a magnitude to establish a molten metal flow path which cleans the surface of the molten metal contiguous to the roll member and which maintains the surface of the molten metal at the weir member in molten condition.

14. Apparatus as defined in claim 13, the recirculating means including means including a second vessel for collecting overflowed molten metal, heat exchange means for thermally conditioning molten metal collected in the second vessel, and transfer means for transferring thermally conditioned molten metal to the reservoir.

15. Apparatus as defined in claim 14, the weir members being coextensive with the immersed length of the roll members.

16. Apparatus as defined in claim 15, the conduit means having an exit opening which is positioned symmetrically with respect to the immersed length of the roll members.

17. Apparatus as defined in claim 16, the heat exchange means being heating means.

19. Apparatus as defined in claim 18, the walls of the reservoir being higher than the weir members, the transfer means including flow-control means for controlling the height of molten metal in the reservoir to control the rate of supply of molten metal from the reservoir to the first-named vessel.

20. Apparatus as defined in claim 19, the flow-control means including a pump.

21. Apparatus as defined in claim 20, in which the discharged molten metal is directed upwardly toward the roll nip by baffle means located in the first-named vessel.

22. Continuous-strip casting process utilizing a casting apparatus of the type having a vessel adapted to contain molten metal to be cast and a pair of roll members providing a roll nip and having peripheries which are partially immersed in the molten metal contained in the vessel with each roll being spaced from an adjacent wall of the vessel, the process comprising the steps of: continuously supplying molten metal to the vessel at the location in the vessel spaced beneath the surface of the molten metal contained within the vessel, passing the supplied molten metal in a direction upwardly toward the roll nip and in a direction toward the space between each roll and an adjacent wall of the vessel, and overflowing molten metal from the space between each roll and an adjacent wall of the vessel over the adjacent wall to without the vessel.

23. Process as defined in claim 22, including the step of recirculating overflowed molten metal to the vessel.

24. Process as defined in claim 22, the metal being aluminum.

25. Process defined in claim 22, collecting overflowed molten metal in a reservoir, thermally conditioning collected molten metal, and transferring thermally conditioned molten metal from the reservoir to the vessel.

26. Process as defined in claim 25, including the step of controlling the height of molten metal in the reservoir to control the rate of supply of molten metal from the reservoir to the vessel.