CA1069669A - Method and apparatus for producing completely recrystallized metal sheet - Google Patents

Abstract

ABSTRACT OF THE INVENTION:
A method and apparatus for producing extremely fine-grained aluminum sheet in a casting machine having a pair of parallel casting rolls, a pouring tip on the entrance of the rolls, and means for driv-ing the rolls. Molten aluminum is poured through the tip into the space between the rolls, and the rolls are driven at a speed such that solidi-fication of the metal is completed at a point ahead of the centerline of the rolls, and the frozen metal is then hot-rolled down to the thickness of the roll spacing, During this hot-rolling, the metal is heavily stress-ed internally by being reduced at least 33% of the thickness at the point of solidification, and this destroys the "as cast" crystal structure and causes complete recrystallization to take place, As compared with con-ventional roll casters, the present caster has larger-diameter casting rolls, which are driven at a faster speed, and the tip is set back far-ther from the rolls centerline. The resultant cast product is vastly su-perior to anything produced by prior casters.

Description

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~he present invention pertains to apparatus for continuously cast-ing aluminum and its alloys, and the invention is more particularly con-verned with a new and improved form of roll caster and method of oper-ating the s ame .
~he roll casting machine is characterized by a pair of parallel casting rolls which are spaced apart slightly to receive molten metal be-tween them, a pouring tip fitted snugly into the converging space between said casting rolls on the entrance side thereof, and means for driving said rolls. ~he rolls are usually water-cooled to chill the molten metal and solidify the same. A good example of the prior roll caster described above is shown and described in U.S.Patent No.2,790,216, which issued Aprii 30, 1957, to J. L. Hunter.
The Hunte r continuous casting machine has 24-inch diamete r rolls and produces 6. 5 mm. thick strip of the softer aluminum alloys (e. g., alloy No. 1100, or example) at the rate of 100 to 115 cm per minute.
In the Hunter caster, complete solidification of the molten metal takes place ~lightly ahead of the centerline of the rolls, and this solidified metal is then reduced in thickness by some 15% to 20% as the metal ad-vances through the diminishing space between the rolls, until it passes through the roll cente rline, whe re the roll spacing is at the minimum. ~ -~hus, the Hunter caster provides simultaneous casting, soli~ification, and a slight amount of hot rolling, which produces a crystal grain struc-ture that is essentially "as cast" structure, except that the dendrites have been laid down somewhat, and are oriented at an acute angle to the sur-face, due to the rolling action.
~his typical orientation of the crystal structure gave the metal pro-duced by the Hunter caster certain advantages over that produced by other continuous strip casting machines, such as "band casters~', but the metal still suffered from many of the handicaps inherent in the "as cast" struc-ture, particularly where subsequent cold work was relatively slight. For ~ .

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1Q6966i9 example, deep drawing of heavy gauge metal frequently results in severe "earing" of the metal. However, for any application where cold work was sufficient, as in rolling foil, the traditional Hunter cast metal was of excellent quality, and its relatively large dendrite crystal structure was no handicap.
Before going on to the present invention, it might be well to digress for a moment to discuss what happens to any crystalline metal structure (particularly aluminum and its alloys~ during casting, hot working, cold working, and annealing. In conventional casting processes, molten metal 10 is usually poured into or through a mold. Cooling of the molten metal and subsequent solidification is obtained primarily through the mold walls and later by cooling the metal walls, as with water sprays or air blasts.
Ihe resulting "as cast" crystalline structure comprises a relatively thin skin of small-grain structure along the outer surface due t o the violent "chill" of the moldi the said skin surrounding the main bcd y of large, needle-shaped dendrite crystals forming the body of the casting; and j there being a cer~tral inner area where the dendrites, which grow perpen-dicular to the mold surface, meet. Ihis central area is usually an area of heavy segregation of impurities. ~he grain structure obtained on a ; 20 ~ "band caster" (e. g., the Hazelett caster) is very similar to the grain structure described above, since the heat tran~fer and metal solidifica-tion follow the same general pattern.
Ihe particular grain structure described above (usually referred to .. ~: .
as "as cast" structure), is not suitable for most applications, and to ob-tain a grain structure suitable for commercial application, the "as cast"
f~ structure mu~t be completely destroyed and regenerated through a cycle of ~deformation (hot or cold rolling) and heat treatment, which produces a phenomenon known as "recrystallization".
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i ~-" 1069669 When a crystalline metal structure is subjected to sufficient inter-nal stress, the original crystalline structure is fractured. If the material is heated (either instantaneously with the internal stressing or at a later time) to the recrystallization temperature (which, in the case of alumin-um alloys will usually be in the range of 345 to 400C), "center s of recrystallization" are formed along the fractured grain boundaries. Ihe higher the internal stresse s, the more centers of recrystallization are formed, and the finer the ultimate grain size. Ihe higher the tempera-ture to which the stressed metal is exposed, the quicker the recrystall-ization takes place. Ihere is also a relationship between stresses re-quired at different temperatures to trigger the recrystallization phenom-,, enon, as heat increases the molecular and crystalline mobility. Ihe fin-est grain size i9 achieved with heaviest internal stresses )to produce the largest number of center~ of recrystallization) and heating the metal to an elevated temperature just sufficient to give enough time for the newly formed grains to "take over" the full metal volume. If the metal iB ex-posed to the higH temperature beyond the optimum time interval, the re is a tendency of the larger grains to absorb the smaller grains, with Z the result that the grain structure becomes larger and coarser.
RecrystallizatiOn is customarily achieved by either of two processes (1) cold rolling, followed by heat treatment; or (2) hot rolling.
In the cold rolling process, hot rolled sheet, with its given grain !~ structure, is cold rolled at varying degrees, usually 35% to90% total reduction, depending on the metal alloy and the product. Ihe hot rolled grain structure is crushed, and heavy internal stresses are imparted to the metal, but no recrystallization takes place (under normal circumstances becauee the temperature during the cold rolling cycle is too low, and the 1"
metal is in a "frozen" state. Ihe metal is then heat-treated, or anneal-ed, by raising the temperature to a sufficiently high level to cause centers of recrystallization to form, New grains then start to grow around these 4_ : : - :,: , - . . ~ . :. . . . .
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` ~06~9 centers, and if the exposure to high temperature is sufficiently long, the new grain will completely replace the old grain, and the metal will be completely recry~tallized.
Hot rolling is usually done to transform cast metal ingots, or slabs, into a thinner sheet product, which may be the finished product, or it may be cold-rolled to finish gauge. The chief benefit of hot rolling is that there is a considerable economy due to energy savings and to reduction of equipment size. If hot rolling is performed at sufficiently high temperatures, and if the reduction of a particular rolling pass is sufficient to impart to the metal sufficient internal stresses, then a recrystallization cycle is triggered during and immediately after the rolling cycle.
The original grain structure has a great deal of in-fluence on the final structure, and to eliminate all of the adverse effects from the "as cast" structure ~low ductility, elongation, drawability, etc.), the metal must go through an extremely heavy cycle of hot and/or cold work, and repeated ; recrystallization cycles, until the metal has been completely recrystallized down to the finest possible grain size.
The conventional Hunter casting machine, and all other casting machines known to me at this time, produce what is basically an "as cast" structure, with all of the disadvant-ages and adverse physical characteristics of "as cast" metal.
Metal sheet or strip produced by these machines must be completely recrystallized by a combination of hot and/or cold rolling, together with heat treatment, all of which require expensive equipment, consumption of large amounts of energy, and high labor cost.
In one particular aspect the present invention provides a method of continuously producing metal to form a sheet or strip having an exceedingly fine grain structure which is ~ ' .

~.o69669 substantially equivalent to that of hot-rolled sheet or strip, said method comprising the following steps, namely, delivering molten metal by way of a pouring tip having an exit end i.nto a space which is bounded by the exit end of said pouring tip .-. : :
and by the curbed outer surfaces of two parallel cooled castin rolls the diameter of each of which is not less than 91.44 cm and which are spaced apart at their nip which is coincident with the plane which contains their respective axes of rotatio~
and synchronously driving said rolls in opposite directions of rotation and at a speed such that the metal is cooled and solidifies completely at a point which is located between the exit end of said pouring tip and said nip and which is so :~:
pos-itioned that the solidified metal is subjected to a hot-roll thickness reduction falling within the range extending from about 33% to about 57%.
In another particular aspect the present invention pro- . :
vides apparatus for continuously producing metal to form a sheet or strip having an exceedingly fine grain structure which is substantially equivalent to that of hot-rolled sheet or strip comprislng parallel casting rolls each of which has a diameter of not less than 91.44 cm and which are spaced apart to form a nip which is coincident with the plane con-taining their respective axes of rotation; means for cooling the rolls; means for synchronously driving the rolls in opposite directions of rotation; and a pouring tip having an exit end fitting snugly between the curved outer surfaces of said rolls at the entrance side of the nip and forming there-with the space into which the molten metal is to be introduced.
In a further particular aspect the present invention provides a method of continuously producing metal to form a sheet or strip having an exceedingly fine grain structure whicl .

is substantially equivalent to that of hot-rolled sheet or :
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iO69669 strip, said method comprising the following step~, namely, delivering molten metal by way of a pouring tip having an exit end into a space which is bounded by the exit end of said pouring tip and by the curved outer surfaces of two parallel cooled casting rolls the diameter of each of which is not less than 91.44 cm and which are spaced apart at their nip which is coincident with the plane which contains their res-pective axes of rotation; and synchronously driving said rolls in opposite directions of rotation and at a speed such that the metal is cooled and solidifies completely at a point which is located between the exit end of said pouring tip and said nip which is so positioned that the solidified metal is sub-~ècted to a hot-roll thickness reduction falling within the range extending from about 33% to about 57% while maintaining the temperature of the solidified metal between said point and said nip close to the melting point to maximize recrystall-ization.
So that the invention will be more clearly understood and further features thereof made apparent, a rolling mill embody-ing the invention and the method of the invention will now be described with reference to the accompanying drawing, which shows a fragmentary sectional view through the casting rolls at the point where the pouring tip projects into the space between the rolls.
The roll casting machine consists of two casting rolls 10 and 12, arranged one above the other, and parallel to one another, with a space between them at the roll centerline A--A.
The ends of the rolls are rotatably supported in bearing blocks which are mounted on a suitable frame. The rolls 10, 12 are water-cooled, and suitable means is provided for circulating liquid coolant through the ro]ls.

Fitting snugly into the converging space between the 5b-1~6~06~
casting rolls on the entrance side thereof, is a pouring tip 14 made of heat-resistant material having insulation propertie and also non-wettable by molten metal. The top and bottom surfaces of the tip 14 are formed with a cylindrical curvature at 16 and 18 to lie snugly against the outer surfaces of the respective rolls. An internal passageway 20 is provided in the pouring tip, and this passageway opens out at the tip end 22 into the space between the rolls.
The rolls 10, 12 are driven synchronously in the direction shown by the arrows, with the top roll 10 turning in the counterclockwise jl/~, -6-1~ .
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~0~9~69 direction, and the bottom roll 12 turning clockwise. With the rolls turn-ing as shown, molten metal from the pouring tip is carried through the ~pace between the rolls, being solidified and hot rolled in the process, and issuing from the machine on the exit side of the rolls as a solid sheet, or strip, 24.
Ihe radius of the rolls is designated R; the distance that the tip 22 is set back from the roll centerline A--A is Ll; and the roll spac-ing at the centerline A--A is Il. Preferably, the roll radius R is 46 cm;
the tip setback Ll has varied from about 60 to 76 mm; and the roll spac-10 ing Il is 6. 35 mm. Ihese dimensions can be increased or decreasedwithin certain limits, and will vary with different alloys of aluminum, or w ith different metals, such as zinc, for example, However, certain re-lation-hips must be maintained in order to practice the invention. I hese relationships will be given presently.
When the rolls 10, 12 are turning at the cptimum speed, molten metal freezes solidly across from one roll surface to the other at the vertical plane 26, shown at distance Lz back from the roll~enterline A--A. Distance L2 ha~ been determined by empirical means to be approx-imately, 6 of Ll for soft aluminum alloys, and therefore if Ll is 60 mm, A 20 L2 is ~ mm. Ihe thickness of the metal at the point 26 is designated12, and is typically about 9. 5 mm.
Another dimensional ratio that distinguishes the present invention from the prior rnachines is the ratio of the thickness of the finished strip (~1) to the thickness of the tip end 22 of the pouring spout 14. Ihe tip end 22 is approximately 14. 3 mm across (measured vertically in the drawing), and therefore the thickness Il of the finished strip is slightly less than half the thickness of the spout tip 22. Ihus, the reduction in ' thickness from the tip end 22 of the spout to the finished strip (Il) is - greater than 2, whereas in the Hunter caster and in other prior roll 30 casters, the ratio has been appreciably less than 2 --more on the order : ~ , , , -of 1. 5 or less. While this may appear to be a small difference, the resulting difference in the grain structure cf the strip produced by the two machines is surprisingly and unexpectedly large.
As the molten metal flows from the pouring tip 14, it fills the converging space between the casting rolls 10, 12 and starts immediately to freeze at the area of contact with the roll surfaces. Ihe thickness of the frozen metal on each roll surface increases as the rolls carry the metal toward the centerline A--A, and at point 26, the metal has soli-dified across the entire space between the ro11s. From point 26 to the roll centerline A--A, the frozen metal, which has already acquired the dendritic crystal structure of "as cast" metal, is reduced in thickness by hot rolling, ~he reduction in thickness i9 from 9. 5 to 6. 35 mm, which i~ approximately a 33% reduction. Ihis is substantially greater than the 15% to Z0% reduction of the Hunter caster, and exert~ extremely high inte mal stress on the hot metal, cau6ing the dend rites to fracture and creating a large multitude of recrystallization center~. ~he tempera-ture of the metal between points 26 and the centerline A--A is in the neighborhood of 510-538 C, and the roll force required to produce the internal stresse~ necessary to fracture the dendrite cry~tals and to 20 create the maximum number of recrystallization centers at thi~ temper-ature i8 only a fraction of the roll force that would be required at a lower temperature. At the same time, the speed o recrystallization is at it~ maximum, as the temperature of the metal is close to the melt-ing point.
hus~ the present invention realizes the perfect solution for con-tinuou91y casting strip of the highe9t quality, and that i9 to simultaneou9-~ .
ly cast, solidify, heavily hot roll, and recrystallize the metal. ~his isaccomplished by destroying the dendritic "as cast" crystal structure at the instant of its formation, and then replacing the "as cast~' structure , ' ' " 10696~9 with a completely recrystallized new grain structure. ~he finished strip 24 has the extremely fine-grained, fully recrystallized structure that is otherwise formed only in metal that has been heavily hot-rolled after cas ting .
In order for the apparatus to be effective, it is necessary that certain conditions exist. For soft aluminum alloys (e. g. ,1100) the thick-ness ~2 of metal at point 26 should be equal to or greater than 1. 5 times the dimension ~1. Excellent results have been obtained when casting 6. 35 mm thick strip of this alloy, using a ratio of L2 approximately 10 equal to or slightly less than 4. It is important that the pouring tip 22 be set back a substantial distance from the roll centerline A--A in order to allow the molten metal to freeze solidly across by the time it reaches point 26. Roll speed is important, as too slow roll speed will allow the metal to freeze solidly across, ahead of point 26, and this would greatly increase the roll-separating force, possibly leading to breakage of the rolls, ~he optimum roll speed with the dimensions shown is about . 6 rpm.
At this roll speed, and with the dimensions shown, the ratio of 1 is approximate ly equal to 10 .
One important feature of the invention is the use of large-diameter 20 rolls 10 and 12. Rolls 10 are preferably 91. 44 cm in diameter, whereas the Hunter caster has always been made with 60. 96 cm diameter rolls.
~he difference between 60. 96 cm diameter rolls and 91. 44 cm diameter rolls might seem to be almost without significanceJ yet the fact is that the larger diameter rolls of the present invention produce a dramatic and totally unexpected improvement in the grain structure of the finished product, in addition to providing a casting machine having the structural strength to stand up under the stresses that are produced. 1' Small diameter rolls require less force than larger diamete r rolls to achieve a given reduction. Small rolls lessen the separating force for _9 _ two reasons: (1) the area of contact is less, so that, with a given pressure, the total force required is less; and (2) the pressure builds up to a lower peak because of the shorter distance through which friction acts. The 91.44 cm diameter rolls of the invention exert a considerably greater pressure on the metal, and use more power for a given reduction, as compared with the 60.96 cm diameter rolls of the Hunter caster.
The additional power that goes into hot rolling is what causes the greatly increased internal stress within the metal that fractures and crushes the dendrite crystals and sets up the extremely large number of recrystallization centers. Thus, the large-diameter casting rolls constitutes the means by which a relatively large amount of power is expended in hot rolling the metal to achieve a reduction of the order of 37%
to 50%, so as to produce the high-level internal stresslng necessary for complete recrystallization of the metal. At the same time, the increased diameter of the rolls gives them greater strength and rigidity to resist bending under the increased roll-separating force.
As stated earlier, the tip set-back Ll on the machine shown and described herein is preferably about 6.35 cm. This distance has been experimentally increased to 7.62 cm or more, with the same 6.35 mm dimension for Tl, which increased the ratio Ll to 12. However, when Ll was increased to 7.62 cm it was deemed advisable to increase the rotational speed bf the rolls somewhat to avoid excessive roll-spreading force, due to the fact that the freezing point 26 might otherwise move back further from the roll centerline A--A causing T2 to increase to about 11.12 mm estimated distance. This would result in a hot-roll reduction tTl~ of 57%, which is a fairly (T2 ) o - heavy reduction, and about the maximum that can be done without going to an excessively heavy and expensive roll construction.

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By speeding up the rolls to approximately .8 rpm, the f~eezing point 26 was found to be approximately at the same distance from the roll centerline A--A as before (i.e., L2 = approx- `
imately 3.8 cm) and Tz = approximately 1.5 Tl'. -With all other parameters remaining constant, L2 is increased by slowing down the rotational speed of the rolls, and is decreased by speeding up the rolls. The higher the roll speed, the greater the output. However, roll speed shoul~
preferably not be increased beyond the point where the metal freezes solidly across from one roll to the other at a point 26 where T2 is appreciably less than 1.5 times T,.
The point 26 where the metal freezes solidly across will also be changed by increasing or decreasing the rate of heat transfer from the molten metal to the rolls, which is a function of the thermal conductivity of the metal forming the roll shell. Thus, rolls having a copper shell would produce extremely fast chilling action, and this would have to be compensated for by driving the rolls at a faster speed, or by reducing the tip set-back L, so that the tip end 22 is closer to the freezing point 26. In that case, L2 might have a considerably larger value than .6 Ll.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of continuously producing metal to form a sheet or strip having an exceedingly fine grain structure which is substantially equivalent to that of hot-rolled sheet or strip, said method comprising the following steps, namely, delivering molten metal by way of a pouring tip having an exit end into a space which is bounded by the exit end of said pouring tip and by the curved outer surfaces of two parallel cooled casting rolls the diameter of each of which is not less than 91.44 cm and which are spaced apart at their nip which is coincident with the plane which contains their respective axes of rotation; and synchronously driving said rolls in opposite directions of rotation and at a speed such that the metal is cooled and solidifies completely at a point which is located between the exit end of said pouring tip and said nip and which is so positioned that the solidified metal is subjected to a hot-roll thickness reduction falling within the range extending from about 33% to about 57%.

2. A method as claimed in Claim 1, wherein said point is at a distance L2 from the nip and wherein the surfaces of the rolls are spaced apart at said point by a distance T2, the ratio L2/T2 being substantially 4.

3. A method as claimed in Claim 1 or Claim 2, wherein said speed is at least about 0.6 r.p.m.

4. Apparatus for continuously producing metal to form a sheet or strip having an exceedingly fine grain structure which is substantially equivalent to that of hot-rolled sheet or strip comprising parallel casting rolls each of which has a diameter of not less than 91.44 cm and which are spaced apart to form a nip which is coincident with the plane containing their respective axes of rotation; means for cooling the rolls;

means for synchronously driving the rolls in opposite directions of rotation; and a pouring tip having an exit end fitting snugly between the curved outer surfaces of said rolls at the entrance side of the nip and forming therewith the space into which the molten metal is to be introduced.

5. An apparatus as claimed in Claim 4, wherein the nip is substantially 6.35 mm measured in said plane.

6. An apparatus as claimed in Claim 5, wherein said exit end of the pouring tip is positioned at a distance from said nip falling within the range from 60 mm to 76 mm.

7. An apparatus as claimed in any one of Claims 4 to 6, wherein the rolls are spaced apart at the nip by a distance T1 and wherein the exit end of the pouring tip is located at a distance L1 from said nip, the ratio L1/T1 being substantially 10.

8. An apparatus as claimed in Claim 4, wherein the distance which separates the rolls at said nip, said distance being measured in said plane, is not more than 55% of the distance between the surfaces of the rolls at the exit end of said tip, the second-mentioned distance being measured in a second plane which is parallel to the first-mentioned plane.

9. A method of continuously producing metal to form a sheet or strip having an exceedingly fine grain structure which is substantially equivalent to that of hot-rolled sheet or strip, said method comprising the following steps, namely, delivering molten metal by way of a pouring tip having an exit end into a space which is bounded by the exit end of said pouring tip and by the curved outer surfaces of two parallel cooled casting rolls the diameter of each of which is not less than 91.44 cm and which are spaced apart at their nip which is coincident with the plane which contains their respective axes of rotation; and synchronously driving said rolls in opposite directions of rotation and at a speed such that the metal is cooled and solidifies completely at a point which is located between the exit end of said pouring tip and said nip which is so positioned that the solidified metal is subjected to a hot-roll thickness reduction falling within the range extending from about 33% to about 57% while maintaining the temperature of the solidified metal between said point and said nip close to the melting point to maximize recrystallization.

10. A method as claimed in Claim 9, wherein said temperature is in the range of 510°-538°C.