US3220891A

US3220891A - Annealing sheet metal coils and product

Info

Publication number: US3220891A
Application number: US164697A
Authority: US
Inventors: Reginald K Templeton; Earl E Stegen
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1962-01-08
Filing date: 1962-01-08
Publication date: 1965-11-30
Anticipated expiration: 1982-11-30

Description

United States Patent Otiice 3,220,891;- Patented Nov. 3f), 1965 3.220,391 ANNEAING SHEET METAL CGILS AND PRDUCT Reginald K. Templeton, Tenafiy, NJ., and Earl E. Stegen, Lakewood, Ghio, assignors to Aluminum Company of America. Pittsburgh, Pa., a corporation of Pennsylvania Filed Ilan. 8, 1962, Ser. No. 164,697 8 Claims. (Cl. 14S-11.5)

This invention is concerned with improvements in strip or sheet metal in coiled form, and particularly relates to aluminum sheet coils as a finished product having irnproved and value-enhancing physical and metallurgical characteristics, and to a method for producing the same.

The term aluminum as used herein is intended to indicate aluminum base alloys of the nonheat-treatable or non-precipitation hardening type wherein increased strength is imparted thereto by cold work, namely, such as those which contain small amounts of manganese and/or magnesium. In coiled sheet form, such alloys have an inherent tendency to develop coarse grains when the coils are conventionally batch furnace annealed to obtain the fully annealed or dead soft condition in the coiled sheet. This tendency is pronounced in the case of an alloy containing from 0.8 to 1.5% manganese, as impurities, a maximum of 0.6% silicon, 0.7% iron, 0.2% copper, 0.1% zinc, and the remainder essentially aluminum. The present invention has been found to be especially applicable to and has been extensively tested and is hereinafter described particularly in connection with this alloy in attaining surprisingly finer grain size in fully annealed coiled sheet thereof. However, it is to be understood that the invention is not limited to this alloy, but may be used to advantage in treating the class of alloys mentioned above and specifically including aluminum base alloys consisting essentially of aluminum, 0.25 to 1.5% magnesium and less than 0.15% silicon and 0.20% iron as impurities, with or without up to 0.5% manganese, which are especially adapted to the production of articles which receive a decorative finish.

The primary object of the invention is to produce aluminum sheet coils which are sized to customer or fabricator requirements in regards to inside and outside diameters and to sheet width and thickness or gage and which possess good quality sheet edges and surface finish substantially free from coiling scratches and, in particular, the coiled sheet is fully annealed and of substantially uniform fine grain structure throughout the extent thereof with a refined surface texture and increased resistance to orange peel effect so as materially to reduce the need for and costs of excessive butiing or other surface finishing operation on products or articles fabricated therefrom.

Another object of the invention is to provide an improved and practical mill practice for heating aluminum sheet metal coils sized to customer specifications throughout the entire mass thereof to a given annealing temperature at heating rates conducive .to producing a fine grain size in the coiled sheet.

Aluminum manufacturers can readily produce cold rolled aluminum strip or sheet metal of final thickness or gage by regular fabricating practices, the material because of its long length customarily being tightly coiled to facilitate its handling. It is known as coiled sheet. From this as-fabricated coiled sheet stock, the needs of customers for coiled sheet in the annealed temper and in narrower widths and coil sizes or diameters, as desired and ordered by fabricators for use in their fabricating operations, are supplied. This requires that annealing and shearing or slitting operations be effected on the sheet stock. Batch furnace annealing is presently available and customarily used for annealing coiled sheet. A

batch of sheet coils on a suitable carrier is charged into the furnace and heated to the desired annealing temperature, usually between 650 F.-900 F., and allowed to soak for an interval of several hours to assure that the entire mass of each coil reaches the annealing temperature, then the coils are allowed to cool to room temperature. Either before or after annealing, slitting of the sheet stock into the desired narrow widths and rewinding the same into the required coil sizes are accomplished by an unwind-slit-rewind operation which is a conventional and well-known practice. If it is carried out before annealing, sheet coils sized to customer requirements and with good quality sheared edges are readily produced, but the further requirement of fine grain and a dead soft condition will be lacking in the finished sheet coils. On the other hand, if the stock coils are first annealed and then slit and rewound, not only is ine grain lacking, but the sheet metal edges and surfaces are usually of undesired poor quality because of difficulties presented in avoiding formation of sharp burrs and excessive roughness when slitting the metal in the soft temper and in avoiding surface scratching during rewinding into a tight coil. Moreover, because of the varying degree of cold work put into the metal as a consequence of unwinding, slitting and recoiling after annealing it, the desired uniform full annealed temper may no longer be present.

In addition to a wide range in diameter or size, width and gage which is required for the finished sheet coils to meet customer requirements, it is now highly desired and required that the metal have a tine grain size so as principally to obviate or substantially reduce objectionable orange peel effect-which is a surface roughening encountered in forming produ-cts or articles, particularly by deep drawing and sharp bending, from metal stock that has a coarse grain size-and the necessity for excessive surface finishing or buiiing on the fabricated articles. Typically, fully annealed fine grained sheet, free from surface blemishes and excessively rough edges, is desired in widths from about .400 inch to 24 inches, in thickness from about .010 to .081 inch and in coils having a radial wall thickness of at least about 4 inches. Usually the coils are formed with an LD. (inside diameter) of from about 6 to 20 inches and an O D. (outside diameter) from about 14 to 36 inches, thus having an end area from about to 1000 square inches and Weighing up to about 2400 pounds, such material having had sufficient percentage reduction to finish thickness to promote development of a fine grained structure. However, the available batch furnace anneal practice, chiefly because of the low rate of heating and long time required to bring the entire mass of each sheet coil to the required minimum temperature does not produce or develop the fine grain required in the coiled metal; the grain size obtainable, as determined by the conventional ASTM comparative method of determination, being from about 35 to 125 grains per square millimeter. Hence, the finished sheet coils heretofore produced and sold as a mill product have been lacking in desirable characteristics for forming and other operations which are affected by grain size.

By the practice of the present invention, however, it has been found possible to produce aluminum sheet coils having all the characteristics set forth hereabove, 'especially a marked improvement in grain size which has been found to be at least about 300 grains per square millimeter, or better. Generally stated, the sheet coil production comprises uncoiling, slitting and rewinding finish gag'e cold rolled coiled sheet stock in its work-hardened temper, thereby providing the sheared sheet with relatively smooth edges substantially free from rough burrs and retaining the mill finish on the coiled sheet intact because its hard condition effectively resists surface scratching during re- 3 winding into a tight coil having a size and width conformed to customer requirements. After being thus produced, these sized sheet coils are subjected to fast heating through the temperature at which `recrystallization starts to a preselected annealing temperature with heating rates rapid enough to produce a fine grained structure in the annealed sheet. Preferably, the heating procedure comprises electrically inductively heating by transverse ilux either one or both of the edges of th'e sheet in a sheet coil to a sensible depth with respect to the sheet width and heating the remainder of the sheet width by thermal conduction from the hot edge or edges, so as to effect rapid through heating of the coiled metal by induction and conduction along each of the turns or convolutions thereof. Thereby, full use is made of the thermal Iconductivity property of the metal and the time for through heating of the coiled sheet to the selected minimum or annealing temperature is quite brief and is a matter of minutes, depending chieiiy on sheet width, for a given higher edge temperature, so that recrystallization and annealing is 'eifected without excessive grain growth.

A better understanding of the invention will be had from consideration of the following description thereof, taken in conjunction with the accompanying drawing which illustrates diagrammatically a form and arrangement of mechanism by which the process may be practiced, and wherein:

FIG. l represents a view in side elevation and partly in section of coiled sheet heating and handling apparatus and showing a sheet coil in heating position between opposed induction heating coils, the dotted lines indicating a handling position for the sheet coil;

FIG. 2 is a fragmentary top plan view taken on the line II-II of FIG. l;

FIG. 3 is a graphic illustration of asymmetrical forces between the metal coil and induction coils when their electrical centers are excessively offset and illustrating why substantial centering of the coils is desirable;

` FIG. 4 is a fragmentary enlarged cross-sectional view illustrating the relationship between the lower induction coil and the sheet coil during heating and depicting in a general way the flux eld pattern therebetween and the heat zone induced in the adjacent side face or sheet edge of the sheet coil, and

FIG. 5 is a schematic electrical diagram of a suitable control for controlling metal coil heating time.

The induction heating process requires the use of an induction coil through which alternating current is passed and relatively close inductive coupling between it and the work to be heated. For the present invention, as illustrated in FIGS. 1 and 2, two duplicate induction coils and 11 of liat or pancake form are provided, each suitably supported, as by being retained in a recessed face of an insulating block 12, suitably of an asbestos bercement composition, and covered by a thin sheet cove-r 13 of that insulating material. Each coil consists essentially of a tight, spirally wound copp'er bar conductor with a passageway through the length thereof for flow of cooling water and suitable insulation between the coil turns to prevent short-circuiting, all as is well known in the art.

Since the inside diameter and the outside diameter of the induction coils and of the sheet coils to be heated need to be related and the latter vary quite widely, the induction coils may be provided with power taps 14, as diagrammatically illustrated, at desired spaced intervals along a radius thereof. These taps extend from the back of each coil and outwardly from one side of its supporting block, so as to be adapted for the selective connection therewith of the ilexible cable terminals 15, which cables are connected to the circuit conductors 16 of a variable voltage alternating 'electric current power supply source, not shown. The induction coils are connected in series in order that the current iiow therein will be the same. Also, each coil may be provided with several inlet and outlet leads (not shown) for the cooling water at different points along its length, as desired. The power source may be of desired frequency and voltage, a commercial 60 cycle, A.C. supply source being suitable and desired because of its availability and for the greater depth of penetration of the induced heating currents it affords. Suitably, as by using a tapped transformer or a variable voltage alternator, the voltage supplied to the induction coils may be selected f-or varying the power input and heating time. Simply by connecting the power terminals 15 to selected taps 14 of the

coils

10 and 11, the electrically effective or active portion thereof in a radial direction can be adjusted to be slightly in excess of the radial wall thickness of a metal coil to be heated for any range in O.D. and I.D. of the metal coils which the induction coils are designed to accommodate. Exemplifying this, induction coils or inductors of about 4 inches I.D. and about 38 inches O.D. with power taps on approximately 2 inch increments in diameter have been found effective for processing sheet coil sizes from about 6 inches LD. to about 36 inches O.D. Larger or smaller ranges in sheet coil diameters may be accommodated by providing sets of inductors of appr0- priate dimensions. For inducing a more uniform temperature radially in a sheet coil, its O.D. should be slightly smaller and its LD. slightly larger than the O.D. and I.D., respectively, of the active portion of the induction coils. This relationship is readily obtained by the tap selection provided, an operator measuring the sheet coil and selecting the proper taps accordingly.

As shown in FIG. l, a sheet coil 17, suitably banded with an outside tie band 18 to prevent unwinding, as is customary with finished coils, is disposed for heat treatment in a sandwich position between the two

induction coils

10 and 11. Upper coil 11 is adapted for vertical movement, as by means of an over-head supported power cylinder 19. Since the metal coils, such as 17, are too heavy and hot, when heated, to be man-handled, the lower induction coil may suitably be mounted on carrier 20, adapted for translation along track 21 by power cylinder 22 between heating station A and handling station B. At the latter position, any suitably handling means, such as a jaw grab or a vacuum cup gripper 23 and hoist 24 therefor, suspended from a mono-rail carrier 25, may be provided. By these instrumentalities, each sheet coil, such as 17, may readily be brought sidewise (axis vertical) to and set down on end and substantially centered on induction coil 10, then moved by carrier 20 into position under the upper induction coil 11, an adjustable stop 26 engageable by the carrier assuring axial alignment between the lower and upper induction coils. The latter is then moved down against the top end or side edge of coil 17 so that the sheet coil is clamped in heating position and held against shifting under the action of the electromagnetic forces present during heating.

With the sheet coil smaller than the induction coils, but with their electrical centers excessively offset, as shown in FIG. 3, these forces have unbalanced components, as indicated by the direction of the inclined arrows, which could be capable of shifting the sheet coil laterally of the induction coils. To minimize this, centering of the sheet coil to Within 1A to 1/2 inch with respect to the induction coils should be effected. The cover of coil l() may be scored or provided with a series of markings, such as circles, to assist the operator in centering the sheet coil. Thereby, more uniform radial heating of the sheet coil is realized and reduced axial clamping pressure on the sheet coil is eective to prevent its movement with respect to the induction coils.

Referring to FIG. 4, which shows the heating operation to better advantage with respect to induction coil 10 and the cont-iguous side edge or end of sheet coil 17, it will be noted that the turns or convolutions of the sheet coil are perpendicular to the flat plane of the induction coil. Only so much of coil 10 is activated, by tap selection, as above described, as will cause the activated portion to slightly exceed the radial Wall thickness of coil 17 to the inside and outside, as shown. Alternating current and cooling water are passed continuously through the induction coil during a heating operation. This primary current sets up an alternating magnetic field about the induction coil. The ux lines of force of this field magnetically interlink with and cut generally transversely across the turns of the sheet coil and hence generally perpendicularly to their surfaces, as schematically indicated by the curved loops. This transverse flux pattern induces electrical potentials in each turn or convolution of the sheet coil 17 and these induced voltages, in turn, cause induced currents to flow and produce a heat pattern in the coiled sheet edges in accordance with current concentration or density and the resultant 12R losses. The induced or eddy currents which produce the heat in the sheet coil turns may flow within the cross-sectional area of each turn parallel to the sheet surfaces and some may also ow across the lapped ends of successive turns, but of an intensity insufficient to cause arcing between turns.

The induced currents effect quick edgewise heating of the coiled sheet to a depth inwardly from the edge that is dependent upon the frequency of the power supply. For the 60 cycle supply preferably used, the effective depth of this induced internal heat zone, as indicated by dotdash line 30, is approximately six-tenths of an inch for aluminum. The same action occurs at the opposite side edge or end of the sheet coil by the induction coil 11. From these two internal heat zones, the remainder of the sheet coil width to the center or mid-point thereof is heated by conduction heat flow through the sheet metal. Since the heat energy is transferred electromagnetically into the end areas of the sheet coil, the power concentration or amount of heat per square inch or other unit of coil end area can be made many times greater than can otherwise be obtained. A temperature gradient is thus established for conduction heat ow inwardly from both end areas to effect rapid heating of the remainder of the coiled sheet width. Poor heat conduction across a physical discontinuity or restricted heat transmission through a metal surface, as is present in other heating methods, is eliminated. In consequence, through heating of the sheet coil to the desired minimum heat treating or annealing temperature in a given time is readily accomplished simply by adjusting the power input to the nductors.

In the annealing of non-ferrous metals, the workhardened condition of cold rolled aluminum sheet in its finished gage being a prime example, it is known that the distorted, fragmented and broken original grains of the metal, under a definite temperature, are replaced by new strain-free grains by recrystallization. These are very smal lat iirst, but they grow by adsorbing neighboring grains. Slow heating rates and long heating or soaking periods tend to promote grain growth. The conventional furnace anneal practice cannot avoid slow heating rates and extended soaking periods, usually measured in hours, so that only the customary coarse grain size in the range of from about 35 to 125 grains per square millimeter can be produced in the coiled sheet metal, although it has had sufficient percentage reduction to finish gage to permit a liner grain structure. Principally, the grain size is dependent on heating rate and time at or above the temperature at which recrystallization begins. Endwise induction and conduction heating of the sheet coils, as above described, enables them to be thru-heated to the minimum temperature at faster rates and hence in shorter times, measured in minutes, than is possible with externally applied heat. Moreover, heating is terminated within a matter of minutes thereby minimizing soaking at the annealing temperature. In consequence, the important result is realized of achieving a substantially uniform fine grain size and the attendant advantages thereof, a grain size of a least about 300 grains per square millimeter (gr./ mm?) being attained, but mostly the superior grain size of from 450 to 600 gr./mm.2 or better is achieved, as op- 6 posed to the coarse grain size of from 35 to 125 gr./mm.2 obtained for identical coils subjected to conventional furnace anneal.

It has been found that a short heating time up to about 2O minutes maximum and depending on sheet coil width is required in order to produce the fine grained condition in the coiled sheet. Annealing temperature of about 650 F. at the center of the sheet coil width is desired to assure complete anneal, While keeping edge temperature Within a range of about 670 F. to l000 F., the upper limit preferably being about 900 F.; but it may go up to close to the temperature of incipient edge melting, if necessary. This edge temperature range permits considerable latitude in heating rates and times in processing coils of a given width. The heating rate is a function of power input to or power expenditure in the activated portion of the induction coils and the heating time or period is a function of sheet coil width. Hence, coils of a given width can be yheated at different inch per minute rates and the heating time made relatively long or short, by varying the power input to the induction coils, so long as the rate is such as to stay within the top time limit of about 20 minutes. This is indicated in the following table:

TYPICAL SHEET COILS [Dimensions in inches-time in minutes] Satisfactory Heating Avg. Heat- Width I.D. O.D. Tinle IKL/min. ing Time in Min/In. of Width l. 10 26 2-3. 5 5. 3 2. 5 2 6 24 4-8 5. 25 2. 5 4 9 18 4-20 1-. 2 2. 5 6 l2 33 6-20 1-. 3 2. 5 13 3l 7. 520 1. 33-. 5 2. 0 l2 35 10-20 1. 4-. 7 1. 4 12 33 12. 5-20 l. 411-. 9 1. 1 14 30 15. 5-20 l. 42-1. 1 0. 9 18 32 17. 5-20 1. 37-1. 2 0. 83

As the table Shows, the heating time for sheet coils up to 24 inches wide is quite short; namely, within a period of .about 2 minutes to 20 minutes depending upon sheet coil width, while the heating rate ranges from about 2 tenths to l5 tenths inch of width per minute depending upon heating time utilized for a given width sheet coil. In the heating time ranges indicated, all sheet coils reach a mid-width or center temperature of about 650 F. to 1000 F. at the time of power cut-off.

rThese are temperatures reached when the power is cut off, temperature recordings indicating that the respective edge and center temperatures rise progressively during the heating cycle. For several minutes thereafter, there is a leveling off of the differential temperature condition within the heated sheet coil, so that the center temperature rises somewhat above the preselected 650 mark.

It has been found that a higher edge temperature is reached when coils of a given width are heated at the higher rate than at the lower rate, due perhaps to the higher temperature differential established with the higher rate of heat input when the heating time is made as short as possible. In the ranges shown above, very advantageous results with respect to grain size are obtained, a grain size of at least about 300 gr./mm.2 being produced. Surprisingly, it has been found that, although the heating rate and period may be varied, as indicated, in processing coils of a given width, the resultant grain size produced remains substantially the same. This gives rise to a definite operating advantage in regards to the induction coils, as is hereafter pointed out.

It has been found from extensive test data that maintained power input to the series connected induction coils Within the range of between about 70 and 375 watts per square inch of sheet coil end area for a time within the range of between about .6 and 4.5 minutes per inch of sheet coil width, mutually related to stay within the time range for a given coil width, will result in fully annealed iine grained coiled sheet with substantially uniform grain size throughout the entire she/et coil. The grain size encompasses the above-stated range with the superior grain size in most of the processed coils.

In general, neglecting radiation losses, heating time is inversely proportional to power input land directly proportional to sheet coil width. Tap selection correlates the induction coils to sheet coil end area. Proper heating of any of the sheet coils, therefore, can be effected by inductively developing heat in both end areas of a sheet coil at the rate of about 190 to 390 watts per square inch of such end areas per minute per inch of coil width.

The sheet coils also can be properly heated on the basis of heating about 11.5 pounds per killowatt hour, provided the energy is expended at a time rate of about .75 to 3 minutes per inch of sheet coil width. Generally, it has been found practical to heat sheet coils up through 8 inches wideherein denoted narrow sheet coils-by using an average heating time of about 2.5 minutes per inch of width. For the wider coils to 24 inches-herein denoted wide sheet ycoils-With about 20 minutes maximum, this average heating time per inch of width decreases progressively accordingly to T/ W (T being time in minutes and W coil width in inches). This is shown in the right hand column on the above table. These relatively slow heating rates are advantageous from the standpoint of reducing the kilowatt loading and stresses on the induction coils, resulting in their prolonged service life. In all cases, the power input in kilowatts to the induction coils is selected, by voltage adjustment, to be suiiicient to satisfy the heat requirements of a given sheet coil and cifect fast heating thereof to the minimum or annealing temperature in a predetermined short time within the top time limit of about 20 minutes. Actual operating tests will best indicate the kilowatt input to be used. Typical electrical parameters with various sheet coil 12, heating rates and heating times can appropriately be as follows:

TABLE II Sheet Coils (Inches) Volts Amp KW. Kw./sq. Heating (Ini- (I ni- (Iniiu Imin. Time tial) tial) tial) in. (Min.) Width I.D. 0.1).

10 26 122 1, 800 108 232 1. 94 6 24% 123 l, 866 118 385 5. 9 9 18 57 1, 278 27. 1 321 18 6 36 296 1, 902 228 194 8. 4 6 36 262 1, 728 200 378 18. 7 6 24% 140 1, 782 100 303 16.3 12 33 208 1, 605 136 278 15 12 32 250 1, 845 184 239 18 13 31 374 3, 330 510 309 7. 5 30 165 1, 718 113 240 20 123s 34% e 467 3, 003 555 183 9. 75 12% 341%6 328 2, 170 288 223 18 12% 33 403 3, 030 504 236 12. 5 12 33 348 2, G70 372 254 18 The input power is applied continuously during the sheet coil heating period or cycle and is terminated suitably on a kilowatt hour or a time basis, as desired. FIG. 5 illustrates the latter in a conventional circuit. Timer T controls the length of a heating cycle and is set in accordance with the Value obtained by calculation or as a result of initial tests with the set of induction coils 1li-11, voltage settings and sheet coil sizes. A given time cycle and voltage setting, as set on voltage changer unit VC, transfers a given number of B.t.u.s into the coil being heated to bring it to desired temperature. At time-out, power flow to the induction coils is terminated. A push switch S enables the operator to start the heating cycle when ready. Other control circuits, apparent to those skilled in the art, may be used, as desired.

The comparative method of grain size determination, hereinabove mentioned, is simply a Visual test. Samples are taken at various intervals throughout a sheet coil, polished and etched, then, using a low power microscope, are compared with standard samples of known grain sizes. the grain sizes produced in the induction annealed sheet coils have been compared with those developed in furnace annealed duplicate sheet coils.

The following examples are illustrative of the invention and the results produced in regards to grain size.

Example l A group of 3003 aluminum alloy sheet coils of .025 gage sheet, 6 I.D. x 26" O.D., in widths from 1 through 6, were inductively heated endwise to annealing temperature of 650 F. -using a heating time of about 2.5 minutes per inch of coil width. One coil of each width was furnace annealed at 850 F. for 3 hours in an air atmosphere furnace. Grain size determination revealed that the samples taken from the induction annealed coils showed a substantially uniform grain size of at least 450 gr./mm.2 versus a grain count of gr./mm.2 for the furnace annealed control coils.

Example I1 In another test to evaluate difference in gage and heating times and yet have fine grain, five coils of 3003 aluminum alloy and of each gage of .011 inch, .025 inch, .O40 inch and .081 inch, 6 LD. x 26" O.D. X 4 wide, were inductively heated, one coil of each gage, using heating times of about 6.5 minutes, 8.5 minutes, 12 minutes, 15.5 minutes and 18 minutes, respectively, to a mid-width temperature of 650 F., the edge temperature ranging from about 670 F. to 800 F. Grain Counts made from samples from these coils showed a grain size of at least 450 gr./mm.2, except for .040 and .081 gage material heated in 12 and 18 minutes which showed a grain size of 300 gr./mm.2. However, Vthe duplicate control coils furnace annealed at 850 F. for three hours showed a grain size of 75 gr./mm.2 for the .011, .040 and .081 material and of 125 gr./mm.2 for the .025 material.

Example III In this test, .032 coiled 3003 aluminum alloy sheet, in coils approximately l2 LD. x 33 O.D. and respectively 6, 10, 14 and 18 wide, were used. Coils of each width were annealed in 18 minutes, in minimum time without melting the edges of the coiled sheet, and in an intermediate heating time, using the induction heating method. Control coils of each width were annealed by (1) 2 hour soak at 650 F. in controlled `atmosphere furnace maintained .at 6.75 F. and (2) 3 hour soak at 850 F. in air atmosphere open flame furnace controlled at 925 F. until metal reached 850 F. The grain size evaluation for these coils indicated the following results:

Heating Grain For Control Coils Coil Width Times Size,

(minutes) gin/mm.Z

Surprisingly, it has been found from the foregoing and many other tests that wide variation in heating time within the top limit of 20 minutes for coils of a given width does not appear to alter the grain size produced. It has been found and is to be expected, of course, that grain size may vary somewhat from lot to lot even employing the same heating rate due, in part, to tolerance variables in alloy composition, reductions, prior heating, etc., encountered under commercial production conditions. This is known to be true of the conventional furnace anneal practices. However, using fast heating for final annealing results in developing fine grain of at least about 300 gr./mm.2 in the coiled aluminum sheet In all of the test work on `the instant invention,

of inherently coarse grain alloys. Mechanical properties and earing characteiistics of the fast annealed material are found to be comparable to those of furnace annealed material. However, samples of both types of material drawn to small radius cups and also formed into finished articles showed that the fine grain material had much greater resistance to orange peel effect and had a rened surface texture an-d a higher sheen to the finish than is the case with furnace annealed material.

After cooling, the annealed coils need no further processing and can be taken directly to a shipping room for delivery to the customer.

In carrying out the induction heating, it may not be necessary to use two induction coils in opposed relationship for some of the sheet coil Widths. It is contemplated that a pressure member can be used in place of one of the induction coils to serve as a clamp to hold a sheet coil against the other induction coil, the latter putting the heat into the sheet coil only at one end and the remainder of the lsheet coil width being heated by conduction. In such a case, the maximum width of sheet coil to be heated will be restricted to about half the width that can be properly heated when using the two induction coils. It is further apparent that two duplicate sheet coils may be coaxially abutted or stacked between the opposed induction coils and be simultaneously heated from their outer ends, the combined width of the two coils not to exceed 24 inches. Abutting more than two sheet coils for a heating operation is not feasible to attain rapid heating and fine grain since the joint between abutting coil faces will interpose excessive resistance to the ow of heat from the outside coils to the intermediate coil. This diiiiculty is obviously not present when only two sheet coils are stacked.

What is claimed is:

l. In the production of coiled aluminum sheet metal as a fine grained annealed product, which material is an aluminum base alloy of the nonheat-treatable type and has the tendency to develop coarse grains when batch furnace annealed, the method which compirses:

forming said material into a tight sheet coil having a radial wall thickness lof at least about 4 inches and a width up to l2 inches; developing heat inductively in at least one end of said sheet coil throughout the entire end area thereof by subjecting said end to an annular radially elongated alternating magnetic field Whose flux cuts generally transversely across the sheet coil turns and induces eddy currents in each of them which heats the same throughout said end area; maintaining said induction heat development to furnish heat in said sheet coil end for conduction through the remainder of the sheet coil width and bring said sheet coil to annealing temperature of about 650 F.;

preselecting the rate vof said inductive heat development in accordance with said sheet coil width so as to reach said annealing temperature within a period of from about 2 to 20 minutes whereby a substantially uniform line grain is developed in the annealed material throughout the extent thereof of at least about 300 grains per square millimeter; and

terminating said heat development when said temperature is attained, lwhereupon the annealed sheet coil is removed from heating position and allowed to cool to room temperature.

2. In the production of coiled aluminum sheet metal as a fine grained annealed product, which material is an aluminum base alloy of the nonheat-treatable type and has the tendency to develop coarse grain-s when batch furnace annealed, the method which comprises:

providing tightly wound coils of said material having an end area of from about 125 to 1000 square inches and a width up to 24 inches;

developing heat inductively in the entire end area of each end of one of said sheet coils by subjecting said ends individually to annular radially elongated alternating magnetic fields of substantially the same intensity Whose ux cuts generally transversely across the sheet coil turns and induces in them eddy currents which cause substantially uniform heating of each end area of the sheet coil;

hold-ing said sheet coil against bodily movement in axial and radial directions during heating; maintaining said inductive heat development furnish heat in both ends of the sheet coil for conductive flow inwardly to the center of the width thereof and bring the same to annealing temperature of about 650 F.;

preselecting the rate of said inductive heat development in dependence upon the sheet coil width and end area so as to reach said temperature within a period of from about 2 to 2O minutes, whereby a substantially uniform fine grain is developed in the annealed material throughout the extent thereof of at least about 300 grains per square millimeter; and

terminating said heat development when said temperature is attained, whereupon the annealed sheet coil is removed from heating position and allowed to cool to room temperature.

3. In the production of coiled aluminum sheet as a fine grained fully annealed product from coiled sheet stock of an aluminum base alloy of the nonheat-treatable type and cold rolled to iinal gage, the method Iwhich comprises:

unwinding, slitting and rewinding said sheet stock in its work-hardened temper to form closely wound sheet coils having a radial wall thickness of at least about 4 inches and a width up to 24 inches;

disposing and holding a sheet coil in a heating position;mi

inductively developing alternating currents of ubstantially the same intensity in the entire endare/aS of each end of said sheet coil to heat the same to a depth of about six-tenths of an inch at a rate/of between about 190 to about 390 watts per square inch yof said end areas per minute per inch of sheet coil width, continuously maintained to bring the sheet coil to annealing temperature of about 650 F. at the center of its width with-in a period of from about 2 to 20 minutes depending upon the sheet coil width and develop a substantially uniform fine grain in said coiled sheet throughout the extent thereof of at least about 300 grains per square millimeter, and thereafter discontinuing coil heating, removing the annealed sheet coil from heating position and allowing it to cool to room temperature.

4. In the production of coiled aluminum sheet as a fine grained fully annealed product from coiled sheet stock of an aluminum base alloy of the nonheat-treatable type and cold rolled to vfinal gage, the method which comprises:

unwinding, slitting and rewinding said sheet stock in its work-hardened temper to form closely wound iinished sheet coils having an end area of from about to 1000 square inches;

coaxially abutting and holding a pair of duplicate sheet coils in a heating position;

inductively developing yalternating currents of substantially the same intensity in the entire outer end areas of said pair of sheet coils to heat said areas to a depth of about six-tenths of an inch at a rate of between about to about 390 watts per square inch of said end areas per minute per inch of the combined widths of said sheet coils, continuously maintained to bring both sheet coils to annealing temperature of about 650 F. throughout the entire mass thereof within a period of about 2 to 2O minutes depending unon their combined widths and develop a substantially uniform ne grain in both sheet coils throughout the extent thereof of at least about 300 grains per square millimeter,

the combined width of said sheet coils being heated being less than 24 inches, and thereafter discontinuing development of heat in said pair of sheet coils, re-

1 l moving them from heating position and allowing them to cool to room temperature. 5. In the production of coiled aluminum sheet as -a fine grained fully annealed product from coiled sheet stock cold rolled to final gage, the method which comprises:

providing said sheet stock as an alloy containing from 0.8 to 1.5% manganese, and as impurities a maximum of 0.6% silicon, 0.7% iron, 0.2% copper, 0.1% zinc, and the remainder essentially aluminum;

unwinding, slitting and rewinding said sheet stock in its work-hardened temper to form closely woundV finished sheet coils having an end area of from about 125 to 1000 square inches and a Width up to 24 inches;

disposing and holding a sheet coil in a heating position sandwiched substantially coaxially between two flat spiral induction coils; serially passing alternating current through areas of said induction coils opposite and slightly exceeding the sheet coil end areas thereby inducing eddy currents in and heating of the entire end area of each end of said sheet coil to a depth of about six-tenths of an inch at a heat input rate of from about 70 to 375 watts per square inch of said end areas;

maintaining said heat input for an average period of from about eight-tenths of a minute to about 2.5 minutes per inch of sheet coil width for heating the balance of the sheet coil width to the center thereof to annealing temperature of about 650 F. by conduction heat flow inwardly from the said end areas;

mutually relating said heat input rate and heating period so that the temperature rise in the sheet coil takes place at a rapid rate and said annealing temperature is attained within a time of from about 2 to minutes depending upon sheet coil width, whereby a substantially uniform fine grain is produced in said coiled sheet throughout the extent thereof from about 450 to 600 grains per square millimeter, and terminating said heat input when said temperature is reached, removing the annealed sheet coil from heating position and allowing it to cool to room temperature. 6. The method of producing aluminum sheet coils as a fine grained fully annealed product from coiled sheet stock of an aluminum base allow of the nonheat-treatable type and cold rolled to final gage, the steps comprising:

unwinding, slitting and rewinding said sheet stock in its Work-hardened temper to form closely wound sheet coils having an end area of from about 125 t-o 1000 square inches and a width up to 24 inches;

providing a pair of spirally wound, Hat induction coils having a face area larger than sheet coils with said 1000 square inch end area;

disposing and holding a sheet coil sandwiched substantially coaxially between said induction coils for induction heating;

selecting radial portions of said induction coils for energization in accordance with and slightly exceeding the radial wall thickness of said sheet coil to the inside and outside thereof;

serially passing alternating current power continuously through said selected portions of said induction coils at a Wattage of between about to 375 watts per square inch of said sheet coil end areas for a time of between about 0.7 to 4.5 minutes per inch of sheet coil width and thereby rapidly heating said sheet coil end areas by induction to a temperature of between about 670 F. to 1000 F. and the remainder of its width by heat conduction inwardly from said end areas to a mid-width annealing temperature of about 650 F.;

mutually relating said wattage and said time in dependence upon said sheet coil width to attain said temperatures within a period of between about 2 to 20 minutes and thereby producing a substantially uniform fine grain in the annealed sheet metal'of between about 450 to 600 grains per square millimeter throughout the entire extent thereof; and

terminating said power flow when said mid-width temperature is reached, whereupon the annealed sheet coil is removed from heating position and allowed to cool to room temperature.

7. A finished, coiled-sheet mill produce comprising a continuous length of aluminum sheet of an -aluminum base alloy of the nonheat-treatable type in tight coil form and annealed according to the process described in claim 6, said finished sheet coil having:

edges and surfaces substantially free from excessive burrs and coiling scratches, respectively;

a substantially uniform fine grain size of at least about 300 grains per square millimeter throughout the entire extent thereof; and

a high resistance to orange peel effect affording a refined surface texture and high sheen to articles produced from said sheet.

S. A finished, coiled-sheet mill product comprising a continuous length of aluminum sheet of an alloy containing from 0.8 to 1.5 manganese and as impurities a maximum of 0.6% silicon, 0.7% iron, 0.2% copper, 0.1% zinc, and the remainder essentially aluminum, and annealed according to the process described in claim S, said coiled sheet-mill product having:

a substantially uniform fine grain size of from about 450 to 600 grains per square millimeter throughout the entire extent thereof, and

a high resistance to orangey peel effect affording a refined surface texture and high sheen to articles produced from said sheet.

References Cited bythe Examiner UNITED STATES PATENTS 12/1939 LSchlup 148-154 4/1951 Stordy 266-5 UNITED STATES PATENT oEEICE CERTIFICATE 0F CORRECTION Patent No. 3,220,891 November 30, 1965 Reginald K. Templeton et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 47, after "650 F." insert and edge temperature ranges between about 670 F.

Signed and sealed this 20th day of September 1966.

(SEAL) Attest:

ERNEST W. SW'IDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents

Claims

1. IN THE PRODUCTION OF COILED ALUMINUM SHEET METAL AS A FINE GRAINED ANNEALED PRODUCT, WHICH MATERIAL IS AN ALUMINUM BASE ALLOY OF NONHEAT-TREATABLE TYPE AND HAS THE TENDENCY TO DEVELOP COARSE GRAINS WHEN BATCH FURNACE ANNEALED, THE METHOD WHICH COMPRISES: FORMING SAID MATERIAL INTO A TIGHT SHEET COIL HAVING A RADIAL WALL THICKNESS OF AT LEAST ABOUT 4 INCHES AND A WIDTH UP TO 12 INCHES; DEVELOPING HEAT INDUCTIVELY IN AT LEAST ONE END OF SAID SHEET COIL THROUGHOUT THE ENTIRE END AREA THEREOF BY SUBJECTING SAID END TO AN ANNULAR RADICALLY ELONGATED ALTERNATING MAGNETIC FIELD WHOSE FLUX CUTS GENERALLY TRANSVERSELY ACROSS THE SHEET COIL TURNS AND INDUCES EDDY CURRENTS IN EACH OF THEM WHICH HEATS THE SAME THROUGHOUT SAID END AREA; MAINTAINING SAID INDUCTION HEAT DEVELOPMENT TO FURNISH HEAT IN SAID SHEET COIL END FOR CONDUCTION THROUGH THE REMAINDER OF THE SHEET COIL WIDTH AND BRING SAID SHEET COIL TO ANNEALING TEMPERATURE OF ABOUT 650*F.; PRESELECTING THE RATE OF SAID INDUCTIVE HEAT DEVELOPMENT IN ACCORDANCE WITH SAID SHET COIL WIDTH SO AS TO REACH SAID ANNEALING TEMPERATURE WITHIN A PERIOD OF FROM ABOUT 2 TO 20 MINUTES WHEREBY A SUBSTANTIALLY UNIFORM FINE GRAIN IS DEVELOPED IN THE ANNEALED MATERIAL THROUGHOUT THE EXTENT THEREOF OF AT LEAST ABOUT 300 GRAINS PER SQUARE MILLIMETER; AND TERMINATING SAID HEAT DEVELOPING WHEN SAID TEMPERATURE IS ATTAINED, WHEREUPON THE ANNEALED SHEET COIL IS REMOVED FROM HEATING POSITION AND ALLOWED TO COOL TO ROOM TEMPERATURE.