US3441079A

US3441079A - Casting of aluminum ingots

Info

Publication number: US3441079A
Application number: US588940A
Authority: US
Inventors: Neil Burton Bryson
Original assignee: Aluminium Laboratories Ltd
Current assignee: Alcan Research and Development Ltd
Priority date: 1966-10-24
Filing date: 1966-10-24
Publication date: 1969-04-29
Anticipated expiration: 1986-04-29

Description

APYi2 1969 N. B BRYSON 3,441,079

CASTING OF ALUMINUM INGOTS Filed Oct. 24, 1965 Sheet of 3/ 0 WATER a x W Q INVENTOR.

April 69 N. B BRYSON 3,441,079

- C A S T I N G O F A L U M I N U M I N G O TS Filed on. 24. 1966 Sheet 4? of 2 INVENTOR. L 50/ ro/v BRKSO/V ATTORNEV United States Patent US. Cl. 16489 6 Claims ABSTRACT OF THE DISCLOSURE A continuous casting process for aluminum ingots wherein the emergent ingot is subjected to controlled cyclic cooling, for example, 3-12 inches initial length, to decrease the extent of bottom-bow and notch formation.

This invention relates to the continuous casting of aluminum ingots.

This is a continuation-in-part of my copending application Ser. No. 345,670 filed Feb. 18, 1964, and now abandoned.

Aluminum ingots are continuously cast by introducing molten aluminum into a shallow mold provided with a downwardly movable bottom. The mold is cooled by water and the aluminum in contact with the sides of the mold is solidified. A mass of aluminum is moved downwardly beneath the mold, supported by the movable bottom. The outer portions of the emerging mass are further cooled by streams of liquid water directed thereon. The casting is continued until a mass of the desired length (ingot) has been produced. This process is described in some detail in US. Patent No. 2,757,425.

The bottom end (butt) of ingots produced by the present process are bowed. This bottom-bow has roughly the shape of a segment of the surface of a sphere. More important the bow causes the bottom end of the ingot to notch, most prominent at the narrower sides of the ingot. The bow may produce such a poor shape and surface to the ingot that the butt must be cut off before the ingot can be rolled.

In many instances, the warping permits streams of molten metal to escape from the mold; the cooling water freezes these streams into icicles and end cold shuts.

These defects are reflected in the rolled slab; bottom bow as a long, narrow tail on the end of the slab; notching as a constriction in the width of the slab. In 9 in. thick ingots, these defects usually require the cropping of up to 'four feet from the end of the slab at /2 in. gauge. This loss is correspondingly greater on thicker ingots. An additional result of warping and notching on slabs rolled in reversing mills is the damage caused to the roll coating by the localized contact of the bowed butt face during the breakdown passes. End cold shuts and icicles are responsible for lost time required to condition them off the butt. In addition, severe cases cause ditficnlties with the removal of the ingots from the stool, and give rise to casting pit cleaning problems.

In continuous horizontal casting process, the same problems of bowing and other infirmities of the butt are present. i

3,441,079 Patented Apr. 29, 1969 ice FIGURE I shows an overall view of equipment suitable for continuous casting.

FIGURE II shows a schematic view of an embodiment of the casting equipment for carrying out the improved process.

FIGURE III shows a perspective view of the butt of an ingot as typically produced by the present commercial casting procedure.

FIGURE 1V is a section of FIG. III at plane IVIV. IIIFIGURE V is a partial section at plane VV of FIG.

Butt 50 in FIGURE III is the bottom end (the initially emergent portion) of the ingot 51. The bottom-bow is shown by distance 52. The bow 52 extends around the butt periphery of the ingot, as can be seen at 54 in FIG. IV. In most instances the lip 56 of the bow extends beyond the

vertical lines

58 and 60 etc. of the ingot as shown at 58' and 60' in FIG. IV. This is the objectionable thickening of the butt end of the ingot.

Cold shuts 62 and 64 are shown in FIGS. III and V. Icicles 66 etc. are shown at one side 68 of the ingot of FIG. III. These may also form on the longer side 70 of the ingot 51.

An object of the invention is a continuous casting process for aluminum ingots where the bottom-bow is decreased to an unobjectionable level. (A small bottom bow aids in the rolls getting a bite on the ingot.)

Other objects of the invention will be apparent from the detailed description of the invention.

Previously in the process of continuous casting of aluminum ingots a stream of molten aluminum is introduced into an open-end mold, which mold is provided with cooling means so as to solidify the molten aluminum at the sides of the mold; an ingot is moved outwardly by way of the open-end of said mold, and the emergent portion of said ingot is further cooled by applying a cooling means at a substantially constant rate thereto; this process is characterized by the formtaion of a considerable bottom bow on and notching of the emergent end of said ingot when said ingot has cooled to ambient temperature. The previous process is changed to the instant process by the improvement which consists essentially of: controlling the cooling rate of at least the initially emergent portion of the ingot, so as to decrease the extent of the bottombow and notching formation, by applying cooling water to said emergent portion in a controlled cyclical flow manner.

The invention is described in connection with the figures which form a part of this specification. There is shown in FIG. I a trough 3 by which molten aluminum may flow into a distributor basin 4. Supported in the distributor basin 4 adjacent and under the end of trough 3 is a filter generally indicated at 5.

Molten aluminum flows from the distributor basin 4 through valves 8 of conventional form, shown somewhat diagrammatically, into dip tubes 9 which convey the aluminum into continuous casting mold 10 through distributor 10a.

In the vertical continuous casting. process, the mold 10 is provided with a vertically movable bottom 11, shown simply as a plate supported by cables 11a which pass over pulleys 11b. When the casting of an ingot is started, the bottom 11 is raised to close the lower open end of the mold 10. Molten aluminum is poured into the mold through the dip tube 9 until the level rises enough to cover the end of the dip tube. As soon as the level of metal inside the mold has risen above the end of the dip tube, the bottom ill is lowered by a suitable motor (not shown) at a rate of speed appropriate for the particular alloy and ingot size being cast and the rate of flow of molten metal through the dip tube is adjusted so as to maintain the level of metal inside the mold 10. The outer portions of the ingot next to the sides of the mold are solidified by the time they pass the lower edge of the mold 10, and are effective to retain the molten inner parts of the ingot in place if the drop rate is properly controlled. The mold 10 and the ingot below the mold are cooled by water sprays 12. The rate of downward travel of the ingot is very slow, e.g., it is common to use rates in the neighborhood of 1 to 6 inches per minute. The process continues until an ingot of desired length is produced, whereupon the flow of molten aluminum is stopped.

It has been discovered that bottom bowing is caused by differential strains which are thermally induced in the mass during the ordinary rapid cooling of the butt as it emerges from the mold. In the process of the invention these thermal strains are effectively decreased by controlled cooling of the emerging mass in a particular manner.

In the particular embodiment of FIG. II cooling water from source is passed from line 16 through control valve 17 and line 18 into the cooling chamber 19 surrounding mold 20. Liquid water is practical, convenient, inexpensive and is therefore used as cooling medium. Openings 22 direct the water against the sides of (outer portions) mass 24 as it emerges from mold mass 24 is supported by stool cap 26 resting on plate 27. As influenced by cross-sectional area and width vs. thickness T relation thereof, the downward movement of cap 26 is maintained at a fixed rate after the mass has emerged a sufiicient distance.

The initial dropping speed must be slow to avoid icicles and is gradually increased to the desired rate which is dependent on the crosssectional shape of the ingot and to some extent the composition of the alloy being cast. The process of the invention allows a faster start-up while producing less bottom-bow 28 than in the conventional processes.

Programmed controller 3 1 includes means to open and close valve 17. The opening and closing may be virtually instantaneous to give a sharp on or off to the water flow. Or the valve may be opened or closed at pre-set rate to increase gradually the flow to the preset full-on amount and to decrease gradually the flow to the fulloff position. The term full-off includes a situation in which the water stream is directed away from the mass, i.e., does not strike the surface, as well as where the water is actually shut off by a valve. In some operations, it may be desirable to use a combination of these procedures.

The controlled cooling rate may be begun as mass 24, literally the top 26a of cap 26, passes below edge 33 of mold 20 or very shortly thereafter, such as one inch emergence. This is spoken of herein as initial emergence and the initially emergent portion of the mass. Benefits on the bottom bow and the start-up rate are obtained with even very short operation of the controlled cyclical cooling of the invention. On the other hand the controlled cyclical cooling may be continued until the entire length of ingot has emerged from mold 20. Also, the controlled flow can be begun as the molten metal enters the mold at the start of the casting operation.

The cyclic flow of water operation is also spoken of as pulsing or pulsating operation.

It has been observed with commercial size ingots that the extent of bottom bow decrease is dependent on the cross-sectional width W versus thickness T relation of the ingot (see FIG. III). The relatively square ingots can be 4 cast with a substantially flat bottom while the ingots approaching a relatively thin rectangular cross-section will always have some bow. However all cross-sections of the instant process will produce an ingot having less bowing than the conventional processes.

Commercial and pilot plant ingots generally are 4 inches to 18 inches thick and 18 inches to inches in width. The ingot length of the vertical cast ingots may be 6l8 feet. Hereinafter reference to commercial size ingots is intended to be understood in connection to the above dimensions, and especially the width and thickness dimensions, referred to hereafter as W and T respectively.

In most situations, the controlled cyclical flow cooling is sufficiently effective when carried out in the sharp on and off introduction of cooling water at the preselected valve opening, hereinafter referred to as water full-0n, which is nat necessarily the same as having the valve at full open position. In other words, the usual controlled cyclical fiow cooling manner consists of periods of full application of Water followed by periods of complete stoppage of water; each sequence of on and off being a cycle. These periods of time are necessarily timed in seconds or fraction thereof; usually each period falls in the range of about 0.1 to about 5 seconds.

With water at full-on or full-off, the periods form regular cycles of time of duration relation over the range of about 5/1 to about 1/5, where the numerator is the period of full application of water and the denominator is the period of complete stoppage of water. More commonly, these regular cycles extend over the range of about l/l to about 1/3 using an each period duration from about 0.5 to about 3 seconds, i.e., a total cycle time of about 1 to about 6 seconds.

With commercial size ingots of the common alloys,-

it has been observed that satisfactory bottom-bow decreases are obtained when the controlled cyclical flow is maintained for an emergence length (E in FIG. II) of ingot of about 3-12 inches or more commonly 4-l0 inches.

Generally after the controlled cyclical cooling has been applied to the optimum, or roughly so, emergence length, the controller stops the cycling and applies water at the maximum rate possible for the particular installation during the remainder of the casting of the ingot. Usually the maximum water rate is greater than the pro-selected water full-on rate used during the controlled cyclical cooling; but this is not neceessarily so. In one installation the water flow after cycling has been stopped ranges from 2 to 3 times that of the full-on rate.

While the invention has been described in connection with the embodiemnt of FIG. II, it is to be understood the scope is not limited thereto. The mold cooling and the programmed sub-mold cooling may be carried out using separate cooling streams. The programmed cooling may be continued over the entire length of the ingot. It may be desirable to employ one program for the first emergent length of the mass and one or more other programs for the remainder of the ingot length. Or a program may impose a gradual increase in a more or less continuous fashion in the amount of cooling as the emergent length increases. A combination of programmed cyclic cooling and programmed continuous cooling may be used in some situations. The programmed cooling may be continued after the ingot has emerged completely from the mold until the ingot is completely solid or even further cooled.

The term aluminum is used herein as a generic term and includes alloys in which aluminum is the principal component.

Illustration 1 Warping was studied on a commercial continuous casting apparatus using a 3 inch deep mold as illustrated in FIG. II.

A. In one series of tests ingots 9 inches by 32 inches cross-section were produced. The amount of warping was measured as the perpendicular distance from the lowermost surface at the 9' inch side of the ingot to the very lowermost bottom of the ingot. The warped surface was of more or less uniform curvature.

(1) In the standard commercial procedure, sub-mold water was directed at the emerging mass immediately that it appeared below the molda one-half inch distance r0ughly-at the standard rate of 108 gallons (US) per minute. Water temperature was about 30 C. The cooling water was directed continuously at this rate for the full length of the ingot product. The downward movement was at 3 inches per minute. The warpage was 1.75 inches.

(2) In this test, the water rate was decreased to 24 gallons (U.S.)/minute and this rate maintained for inches of the descending mass; then the rate was increased to 108 gallons/minute. The warpage was 0.9 inch.

(3) In this test, the water was cycled (pulsed) on a 3 second cycle with a quick acting valve giving full-on full-off operation with a 2 second full-on period and 1 second full-off period. The water rate was 108 gallons/ minute. The cycle cooling was made continuous after 6 inches of mass had emerged. The warpage was inch.

In Test 3, there was no notching of the butts and no icicles or end cold-shuts occurred. Ultra-sonic inspection showed the ingots to be sound. There were no significant macrostructural differences in the ingots of the three tests.

B. Another series of tests was carried out producing 7.5 inches by 51 inches ingots.

(4) Using the technique of Test 1 above, a warpage of 2.5 inches occurred.

(5) Using a water full on rate of 65 gallons/minute and 30 cycles/minute with the water on/water off ratio of l/2, the warpage was decreased to inchthe cycle cooling was switched to continuous at 7 inches of emergence and the remainder of the ingot cooled at a water rate of 108 gallons/ minute. In Test 5, there was no notching of the ingot or icicle formation.

C. Another series of 7.5 x 51 inch ingots was produced,

using another aluminum alloy. Under the operating conditions of Tests 4-5 the warpage was reduced from 3 inches at standard practice to /1. inch by the procedure of Test 5.

D. Another series of tests was carried out on 8 x 28 inch alloy ingots.

(6) In a standard operation, 72 gallons/minute of water was applied to the ingot continuously from emergence beneath the mold to its full length. The warpage was 1+ inches.

7. Cyclic water cooling was applied to the first 7 inches emerging and then continuously cooling for the remainder of the length of the ingot. A manual operation of the valve resulted in a three part cycle: water rate was increased from zero to 72 gallons/minute in /3 of the cycle; the water rate was decreased to zero again in /3 of the cycle; the water was full ofl? for one-third of the cyclea total cycle time of 2 seconds. The 7 inch emergence took 2 minutes. This operation resulted in a warpage of less than A inch.

In all of these tests, another advantage of programmed cooling was observed. In standard continuous direct chilling operation, the warping of the butt end caused a rise in the height of liquid metal in the mold and the float therein virtually shut-off the supply of molten metal into the mold. It was necessary to paddle molten metal out to the mold ends in order to maintain ingot production. During the cyclic cooling tests, this difiiculty was completely absent.

Illustration 2 In trials of the cyclic or pulsed water technique a three part pulsation cycle was used, the complete cycle occupying approximately 2 seconds; the water was turned completely off for one third of the cycle, raised to a maximum over the next third, and turned off over the last. The water flow was controlled b means of a diaphragm valve, operating similarly to a domestic tap; with the valve fully open the flow-rate was about 75 gal/min.

This cycle was used throughout the casting of a 7 /2" x 33" alloy ingot about 2 ft. long. Casting was started with a water flow-rate of 75 gal/min, at a ramspeed of 2 in./min.; as soon as the ingot reached the submold water, as indicated by the hiss as the water boiled, the pulsing cycle was commenced by turning off the water, and the ram-speed was increased over a period of about half a minute to 4 in./min. At the end of casting the pulsation was continued until the ingot was completely solid.

Samples for metallographic examination were taken from points through the thickness of the ingot near its center. This ingot had a fairly uniform structure throughout; the dendritic cell size was similar to that of conventional direct chill material, but there were far more intermetallic constitutents at the cell boundaries, the proportion being similar, so far as could be judged in view of the far smaller cell size, to that in permanent mould ingot. This association of a fine dendritic structure with nearequilibrium constitution is, so far as is known, unique in an as-cast ingot; with conventional continuous cooling a slow enough rate to produce near-equilibrium constitution results in a coarse structure, and a fast enough rate to give a fine structure results in supersaturation.

Small ingots were cut from the center and edge (includes as-cast surf-ace) of the as-cast pulse cooled ingot. These rolling ingots were preheated for 16 hours at 550 C., hot rolled to 0.189", cold rolled tothicknesses between 0.090" and 0.020" and finally annealed 1 hour at 400 C. Cups were pressed from the annealed sheet to assess the earing characteristics.

The sheet produced from ingot using the pulsed water technique not only had a low range of earing but the general level of earing (using a noninteranneal procedure) was lower than sheet from conventional continuously cooled ingot.

Illustration 3 (1) The following show commercial practice on 4 different aluminum metals.

Al metal: Cycle Pure metal 1 sec. on, /2 sec. off. 38 1 sec. on, 1 sec. off. 578 1 sec. on, 1 sec. off. D54S 1 sec. on, /2 see. off.

Slow starting speeds and the above cycles continue for the first 6" for pure metal and 3S and 4" for 57S type alloys. After that, the off cycle is reduced simultaneously with an increase in speed in 4 steps after each inch of casting. Thus, for pure metal and 38 type alloys the cycling is stopped and the speed returns to normal at 10" and for the 578 type alloys at 8".

During initial cycling the water volume is reduced about 20%. At this installation it was found that about 700 gallons of water per minute is the maximum than can be used during cycling for a multi-ingot casting operation.

(2) Cycling allows an easier, faster start (rise to full speed of the casting of the ingot), reduces bowing of the butt, and decreases the number of cold shuts.

(3) When casting D54S conventionally the ingot is repeatedly lifted a small distance off the stool caps by a build-up in steam pressure under the butt. This phenomenon occurs in the early stages of the drop. It has been found that cycling at the start noticeably reduces this bumping.

Illustration 4 Some large scale experimental castings are reported below.

Alloy D28 D28 D3S D2S Iugot cross-section, inches 18 x 54 18 x 76 18 x 72 7% x 44% Butt war-page: 1

Conventional starts (inches) 1%2 4 3-3% 2%3 Cycled starts 1% 1%2 %1 Starting procedure:

(a) Conventional Speed,

drop inches/min. 2% 1% 1% 3-4 Mold water gals.

min 155 240 190 150 (b) Cycled speed, drop inches/min 2 1% 1%1% 1%2% Mold water gals. (U.S.)/

min 155 250 190 150 y e Cycling started 2 (inches) Cycling period 2 (inches) 10 10 10 10 Casting practice:

Mold height, inches 3 3 3 3 Casting speed, inches] min 3% 3% 2% 5 Mold water gals (U.S.)/

min 255 340 340 180 l Warpage is defined as the distance between the lower corner of the narrow ingot face and the extreme bottom end. The figures shown represent typical values which were obtained with the practices listed above.

2 Expressed as inches of ingot cast below the mold, that is, when of ingot was cast cycling was started. Then when a total of of ingot had been cast below the mold cycling was stopped.

3 1 sec. on, 1 see. off.

4 1% sec. on, 1% sec. 01f.

Thus having described the invention, What is claimed is:

1. In the continuous casting of aluminum ingots Wherein a stream of molten aluminum is introduced continuously into an open-end mold provided with cooling means to solidify the molten aluminum at the sides of the mold to form a portion of an ingot; the ingot is moved continuously outwardly by Way of the open end of the mold; and the emergent portion of the ingot is cooled by applying a cooling medium at a substantially constant rate to the outer surfaces of the ingot outside the mold; which process is characterized by the formation of a considerabl bottom-row on the emergent end of the ingot and also considerable notching of the emergent end of the ingot, the improvement which consists of: applying cooling water to the emergent portion of the ingot in a controlled cyclical flow manner to control the cooling rate of at least that portion of the ingot initially emergent from the open end of the mold in such a manner that the thermal strains are effectively decreased in the emerging ingOt whereby the extent of the bottom-bow formation and notching formation in decreased.

2. The improved process of claim l-wherein said controlled cyclical flow cooling is maintained for an initial emergent length of commercial size ingot of about 3-12 inches.

3. The improved process of claim 1 wherein said controlled cyclical flow consists of periods of full application of water followed by periods of complete stoppage of water application, said periods being timed in seconds, and said periods being regular cycles of time of duration over the range of about 5/1 to about 1/5, where the numerator is the period of full application of water and the denominator is the period of complete stoppage of water.

4. The improved process of claim 3 wherein said period is from about 0.1 to about 5 seconds.

5. The improved process of claim 3 wherein said cycle relation is about 1/1 to about 1/ 3 and the period of time is from about 0.5 to about 3 seconds.

6. The process of continuous casting of commercial size aluminum ingots to produce ingots having a bottom row of substantially less curvature than the bottom row formed by previous casting procedures which process comprises:

introducing a stream of molten aluminum into an openend mold, said mold having a downwardly movable bottom and being provided with cooling means to solidify the aluminum at the sides of said mold to form a portion of an ingot having about the same cross-sectional area as said mold, said ingot being supported by said bottom;

cyclically directing liquid water against said ingot as it emerges from said mold, each cycle being water full-on to water full-oif in the ratio of from about 5/1 to about 1/5 and each period of time within said cycle being from about 0.5 to about 3 seconds; continuing said cycling of water until said ingot has emerged an initial distance about 3-12 inches; and thereafter continuously applying liquid water to the emergent ingot until an ingot of the desired length has been cast.

References Cited UNITED STATES PATENTS 2,726,430 12/ 1955 Rossi et al. 164-4 3,176,355 4/1965 Ljungstromer 164283 FOREIGN PATENTS 510,649 3/1955 Canada. 901,091 10/ 1944 France.

I. SPENCER OVERHOLSER, Primary Examiner.

R. S. ANNEAR, Assistant Examiner.

US. Cl. X.R. 164-15 4