US2697869A - Process for making aluminum coated material - Google Patents

Description

Dec. 28, 1954 w gs o ETAL 2,697,869

PROCESS FOR MAKING ALUMINUM COATED MATERIAL Filed April 25, 1948 ITs. 2

, INVENTORS Walter E Kingston Igor M Zararine harlesflfiussert firm +6 M4 H1191 orney United States Patent '0 PROCESS FOR-MAKING ALUMIN UM: COATED i MATERIAL-f Walter E. Kingston, Port. Washingtonpand -lgor. N. Zavarine, Great Neck, N.'-Y., and Charles E.Bussert Middletown, Ohio; said 'Bussert-assignor to Armco Steel: Corpcratiom-a corporation ;of-l1io,- and-said Kingston and said :Zavarine assignors to Sylvania Elec-.

tric Products,ilnc., New York, N. Y.', a corporati0n-.-of -1 Massachusetts Application April 23, 1948',*Serial;No. 22,814

1 Claim; (Cl. 29--189) The present invention relates to .a method :of coat-' ing'metallic stock, and 'in'particular to the'production":

ofan'adherent iron-"aluminumalloy on a ferrous sheet.

Advantageously, relatively ductile sheet stock.may; be produced-by the present process; which maybe formed."

and drawn into components for radio tubesr'.

It is readily possible to coat ironor mildsteel sheet or strip' stockwith aluminum by" hot'dipping," as byfollowing the methods and- .using'the apparatus-of the" United States SendZirnir Patent '2,1 10,893.

Since. the processes of this'patentzinclude a heat treat ment which may be carried on as a softening anneal, the. hot coated products, as produced, may"bechar:' acterized .;by good 'formability and good 'idrawability;,, and the" adherence of'the aluminum *coatingis sufficient to permit these. operations.

However, for the attainment of certain thin gauges, and for the. attainment of certain other. gauges with the greatesrpossible 1 convenience and economy; it is at present found necessary to 'reducetthe thickness of the coatedproduct' aftenit has been.coated. Also, as will" hereinafter appear, certain :final. products require a thick-' ness'of coating"substantiallyfless than. thexminimum ping procedures. uct-willbe-subsequentlyx cold rolled, and: it has been 40 thickness which canat presentbe appliedlin hot dip- In .these instances the coated prod found'that the coating produced-as set forth above readily withstands such cold rolling'treatments'."

The result ofcold rolling will be; a workihardening of 'the 11'011 or steelrstock in proportion'tor the amount of cold work *performedon it, and 1his:inturn"will 5 Ord nary destroy the. forming and drawing characteristics of"thematerial; Since practically all productswhich would befabricated fromsuch stock would require either.

drawing'or forming; a heat treatment'in. the nature'of a softening ganneal is'indicated; But .a heat treatmentata temperature" high-enough; to produce-softening of the-iron or-steel within "a givenlength of 'time;:will within-the 'same length of itime cause'thealuminum,

coating to alloy'fully,with the-metalxof the'rsheet or strip: stock.

It has hitherto been suggested" thatthe inclusion'of" certainfelements such as silicon or. others in. the molten coatingmetal Wlll, to a degree at least, control :thefor mation=of 'iron-aluminum."alloysrl Such.:.elements, and

othersy-appear to have a measurable effect in controlling alloying during the. formation of a coating by hot'dip-' an rannealing treatment-Int 'any temperature for a sufa ficientolengthtof;1ime.;to ;produce..;softe'ning in' a commercial fashion without at the same time; alloying .the aluminum with lhQlrOnl' The additioniof' silicon, magnesium-,=. .or other: substances to the. aluminum bath 'used -I' 1n.tcoat1ng mayrbezdesirable for other reasons, andwhere herein'twe referzto aluminum. we desire to beunderstood asrrincludinginot .only. .the; substantially pure metalbut" also: aluminum "containing :minor amounts of other sub- 7 stances-including those mentionedabdvez So far as we knowpthesionly successfuliiinstances of 2,697,869 PatentedvDem-sZS, .1954

the annealing of iron' orzsteel sheets previously coated with aluminum, carried on in'such a way as not to produce alloyingyare to be found in the German-Feran process in which layers of'iron' and'aluminum are joined by rolling together to form a plate which is then reduced to'final thin gauge by such an exceedingly great amount of cold work; say-a cold reduction of the order of 98-per cent, that recrystallization willoccur 'at an abnormally low temperature. Under these circumstances it has been found possible to produce softening without :completely alloyingthealuminum with the iron. Suchexcessive reductions are not economical as they must be carried out on small mills and in small plates, nor do they form part of thefprocesses'to which this application relates. V

For some uses where a brightsurface ofaluminum is desired a complete alloying of the aluminum coatingwith the iron' of the base cannot 'be' tolerated. For other uses, thebarefact that an. iron or steel'prod net 'is coated with a coating .of aluminum-iron .alloy insteadof'aluminummight 'not necessarily be disadvantageous. It will be understood that. the aluminumironlalloysoproduced is grey or black. in color, and. for'some uses this is desirable. For example, cookie pans andother metal baking trays formed of iron or steel sheet metal having a bright aluminum coating are not satisfactory. By reason of the highcheatreflectivity of the .coating, insufiicient heat .is transferred through the pan to the-bottom surface of the dough or batter to produce proper ccooking on the bottom side. within the length of time in whichthe top'surface of the dough is browned by. direct radiation fromheatin'g elements and fromzrefiection from the pans "above. Again, in the formation: of anodes :for radio tubes a darktsurface having a'high.black body radiationcoeflicient is'require'd.

It has been generally recognized that the efficiency' of certain elements of an electron.discharge tube "are increased if the surfaces of these elements .are black.- ened so as toincrease their. heat radiation. generally known that such blackened materials will have a smallersecondary emission as compared to bright In the case of anodes. made from the productofthe German Feran process it has been: the practice. to

produce by heat treatment an alloying of the aluminum. withlthe iron .in the .finished, fabricated anodes.

anodes.- Ithas always. been found, however, that when a stockhot-dipped in aluminum has been subsequently-cold reduced to a lightergauge and heat treated sufficiently to soften the base :metal and transform the coatingcompletely toiron-aluminum alloy,- this alloy is brittle. andof poor adherence so that the stock could not be formed or drawn without destroying and flakingsawaywthe alloy coating, leaving .the base .metal un-, protected.

While we shall describe our invention :in. connection with a process involving substantial. amounts of cold rolling, it willrbe understood that in instances where thetrequired thicknesses of the base stock and the required thickness of coating can "otherwise be obtained, it mayzstillbe desirable; to provide a completely alloyed coating priorto forming or drawing, and the invention herein described is"not" limited to cold rollingsexcept asset forth hereinafter.

The principal objects of "our invention lie in the solution of problems briefly discussed above, and other objects, which will'be set forth-hereinafter or will be apparent to one skilledin the artupon reading the. dis closures which follow: Th'e'severalobjects of our in\ vention are 'attained'by that procedure'and in "that Hot dipped aluminum coatings alloyed with the base metal have similarly been found to have excellent black 'body characteristics which make-them suitable for radlo tube product of which we shall now describe certain exemplary embodiments.

In the drawings:

Fig. 1 is a photomicroscopic view of an aluminum coated steel strip showing fine surface cracks which do not penetrate to the base metal,

Fig. 2 is a photomicroscopic view of an aluminum coated steel strip showing large surface cracks penetrating to the base metal.

We have discovered that under certain circumstances, hereinafter explained at length, it is readily possible to coat an iron or steel stock with aluminum, then, as may be necessary to obtain the desired thinness of product and coating, cold reduce the coated product, and finally develop desired degrees of drawability or formability therein by a heat treatment which at the same time will produce an alloying of the aluminum with the iron of the base, and that it is also possible to produce thereby a product in which the aluminum alloy layer is so firmly bound to the base that the stock may be subjected to drastic forming operations or to repeated drastic drawing operations without loss or essential disruption of the coating.

The particular requirements which we have discovered must be met in the practice of our process to result in this novel product are:

1. In the final stage of the product, the aluminumiron alloy coating layer must be very thinless than certain maximum values.

2. The time and temperature of the heat treatment must be so limited and coordinated that the base metal will be softened and the coating completely alloyed with out the formation of non-adherent iron-aluminum alloys.

We shall now take up in detail the requirements for the production of sheet or strip metal of the kind just described.

The base metal sheet or strip stock may be of iron or mild steel, either rimming or killed, as these terms are understood in the art. Other ferrous base metals may be used, however, as this is not a limitation of the invention.

The base metal sheet or strip stock should be coated with a sound and tightly adherent coating of aluminum, which as set forth above may if desired contain minor amounts of other alloying ingredients. The coating is preferably done by the processes and with the use of apparatus set forth in the Sendzimir patents mentioned above, or may be accomplished otherwise.

The aluminum-coated iron or steel strip stock may then be cold rolled to final guage. In order to meet the maximum coating-thickness requirement, the aluminumcoated product when at final gauge and prior to alloying should bear on any coated face an aluminum coating not greater than substantially between .0003 inch and .0001 inch in thickness, average. The thickness of coating may be determined either by microscopic methods, or by chemical methods in which the average weight of coating per unit area of surface represents a certain thickness.

In the aluminum coated and cold rolled condition a weight of .50 ounce of coating per square foot of sheet is approximately equivalent to a .001" coating thickness on each side of the sheet. Similarly .050 ounce of coating represents .0001 inch thickness. These figures are approximately correct for either pure aluminum or aluminum containing 9 percent silicon. The chemical method of determining the coating thickness by stripping the aluminum coating from the sheet stock before subjecting it to heat treatment at the range at which alloying takes place is a preferred method for determining the thickness of coating.

While somewhat thicker coatings may be employed, the maximum range given is one within and below which a satisfactory adherent and formable or drawable alloy coating can dependably be produced in accordance with our teachings. This is irrespective of the specific final thickness of the base sheet or strip stock, although in certain circumstances the thickness of the coating within the range set forth may advantageously be varied with the thickness of the final gauge of the base stock. By way of example, the thickness of an iron or steel sheet or strip stock intended for use in the fabrication of anodes for radio tubes will be of the order of .005 inch. The thickness of an iron or steel sheet or strip stock intended for fabrication into cookie pans may be of the order of .05 inch. In the case of such thin materials as those used in radio tubes it has been found that the specific thickness of the ultimate iron-aluminum alloy layer has an appreciable effect on the stiffness of the annealed product. Hence in commercial operations our preference is to manufacture coated sheet or strip stock of the thinner type with aluminum coatings near the lower end of the coating thickness range, while heavier base stocks may be manufactured with aluminum coatings near the higher end of the coating thickness range.

The final annealing or heat treatment which forms the second requirement of our process is very important; but it will be understood by the skilled worker in the art that variations are contemplated depending upon the particular qualities desired in the final material. A cold reduced, coated product will not be very satisfactorily formable or drawable without a softening heat treatment; but formability usually requires a greater degree of softness and a larger grain size than those which produce maximum drawability. The final heat treatment may be in the nature of a box anneal or in the nature of an open or strand anneal, and it may be varied in known ways to obtain the desired characteristics in the base metal, within the limits hereinafter set forth. The softening produced is a function of time and temperature, higher temperatures requiring less time as is well understood. Moreover the rapidity of softening will generally depend upon the percentage of cold reduction practiced on the coated product.

Approximately the lowest feasible temperature for softening the base metal or alloying the coating will be around 1050 degrees F., at which temperature the time required to produce the necessary softness for a forming or drawing stock will be of the order of hours, indicating the practice of a box anneal. As the temperature rises, the time for softening decreases rapidly as does also the time required to cause the aluminum layer to alloy fully with iron from the base. Thus at 1550 degrees F. satisfactory softness and full alloying will occur in a matter of seconds or minutes depending on the thickness of the base metal. For example, we have found that for base metal of .005 inch thickness only 17 seconds are required while for .050 inch thickness approximately 2 minutes are needed. This indicates the feasibility of a strand or open anneal at this and other temperatures down to about 1200 degrees F. below which we have found the required time to be unduly long.

In general excellent results can be attained with the range of coated materials set forth herein by open annealing treatments between substantially 1450 and 1750 F. for times between 7 minutes and 17 seconds.

The qualities of the alloy layer may be impaired by overheating. Thus at temperatures higher than about 1800 degrees F. the alloy coating tends to take on a grayish appearance and to become markedly less adherent.

Microscopic examination leads us to believe that this phenomenon is due to the further diffusion of iron into the alloyed coating with a resultant increase in the thickness of the alloy layer accomplished by a decrease in its adherence. In general, we prefer to confine the annealing temperatures to those not higher than about 1750 degrees F. The qualities of the coating may also be impaired by holding the material for too great lengths of time at lower temperatures. Thus where full alloying occurs in 17 seconds at 1550 degrees F. a strand or open anneal is advisable since a box anneal will unduly prolong the time at which the material is held at high temperature. Where the time for softening to a desired degree is measured at a particular temperature in hours, we prefer a box anneal; but where a temperature is employed which will produce complete alloying in a time measured in minutes or seconds we prefer an open or strand anneal. Thechoice of temperatures and times may largely be determined by the desired qualities of the finished product, it being understood that prolonged holding at elevated temperatures promotes grain growth and formability. By formability as used herein we mean the ability of the sheet or strip stock to take a sharp bend without undue springback, and by drawability we mean the property of the material to become deformed by stretching in suitable drawing dies. While the greatest possible degree for formability, all other conditions being equal, can be attained by prolonged holding at an elevated temperature, satisfactory degrees of formability for most uses can be attained in open or continuous anneals at somewhat higher temperatures.

meanness.

I By (an open: or.econtinuoussanneala'is:rmeant: a zheatc treatment;in' whieh";thezmateri or.-.strip-. iformriis carried: through an:elongatedufurnacee r The atmosphere 'rduring the'i heatu treatment is 1110131:- Ofi? controlling rimpontancem oxidizing atmospheres :may be employed especially in continuousaor.:openranneals?but ournpreference-is rfO1.f3;ll atmosphereuof momoxidizing, inert gas,=such as -nitrogen,;or of any :commerciallya-avaih ablenreducinge atmosphere; including hydrogen; r: dissociatecl ammonia; the so calledd-x. gas and-tmany: others. It is not necessary in the practice of our process to employ wlhateis commonly; known as a bright; annealing atmosp eree:

Coldtrollingof the aluminumcoateddron or steel stock when -requiredvmay 'be. carried out- .with anyu-suitable. equipment..-;

In practice with; present hot dipped rcoatings cold .'reduction-isIequired-if the 'aluminum'coating': is to :bemade thin enough; and substantial 2cold-r0lling is further -of.-advantageqin introducing enough strain into the. base metal. to-.enable''-it to'tber-softeneduat .relatively. lower temperatures.

In .=t-he.-coating-of iron or steel stock .with aluminum by.

1 single+thicknessrsheetm ho'tsdipping; for 'our purpose'rit is highly desirable that thev coating bemcontrolled-:as touniformity. Moreover thetthieknesses of coatingsaapplied by hot dipping methodsare circumscribed and cannot be greatlywvariedriftuniw formity is to be achieved. For example, aluminum coatings of controlled uniformity can be applied roughly between .001 and .002 .inch in thickness,v whilewiped coatings, i. e., coatings treated by the-wipingmeans disclosed in U. S. Patent No. 2,398,034 to Oganowski issued April 9,'.-1946,-.'or byflother suitable wiping means, can nbezmade on ;the ordertof about .0005 inch in thickness. It'will-"be evident thatvsubstantial cold reduction--of such a coated pnoduct 'will-fbe' required to reduce thethickness of such coatings xto the rangeof substantially .0003 inch-10.0001 inch::0r less set forth above. There-isin generalia practical upper limitato the thickness of strip stock whichmay. beiapassed through coatingapparatusiofhthe kindsw re' ferred to above, which lies at around..125 to .150 inch: On the other hand considerations ofreconomy in .the use of apparatus and mechanical limitations .of the apparatus lie against the coating of exceedingly thin stocks; and in any event it will be found helpful to practice substantial colduwork onvthe coated StOCkS- itlvOIdfiIlitO redu'cetthe thinnest practical coatings to thethickness" rangegrequired for the production of adherent alloy layers.

It will now be evident that the operator should calculate back from the desired final thickness of base stock and final thickness of aluminum coating to determine a suitable initial thickness of coated stock, initial thickness of coating, and amount of cold reduction required to produce the finished product. However, it is readily possible for final products in which the thickness of the base iron or steel sheet or strip lies roughly between .08 and .005 inch with aluminum layers or coatings lying substantially between .0003 and .0001 inch in thickness to determine the thicknesses of starting sheet or strip suitable for aluminum coating, thicknesses of initial aluminum coatings and degrees of cold reduction to be performed on the coated product which will meet the requirements as set forth herein. By Way of example, we now give representative procedures which we may employ in producing satisfactorily softened strip with alloyed coating, where the strips are .005, .05 and .08 inch in thickness.

Example 1 Hot roll to .080 inch Cold reduce to .040 inch Aluminum coat .001 inch each surface Cold reduce to .005 inch Final aluminum thickness .00013 inch each surface Open anneal at 1550 degrees F. for 17 seconds in dissociated NH atmosphere.

Example 2 Hot roll to .180 inch Cold reduce to .100 inch Aluminum coat and wipe to .0005 inch each surface Cold reduce to .05 "inch Final aluminum thickness .00025 inch each surface Open anneal at 1475 degrees F. for 4 minutes in an atmosphere of dissociated NHs.

Cir

6 v Ex'ampie Ho'tf. roll i'to' r150. 'in'ch j: Aluminum 'coat' and. wipeqto .0005 inch 'eachsurface Cold reduce '"to .08 "inch Final aluminum thickness .00027lincheach surface. Open annealat 147 5 degrees" Fffor 7 minutes;

It will be-runderstood thanitheseexexemplaryrproceduresi'i are illustrative and". not:=limiting; rand'tthatr thei tconditionsv? set'forth he'reinmay.be:attainedrby substantial: departures therefrom: 1 The :finaliriheat-rztreatments win 2 the: examples: 1: above produce 1116a desired'zadherentrcoatingsi and a sufir-r'n 0161111: softness of r the materiale fonnforming and? drawing purposes as well as the :desired adherent blackened coats: mg. 1

With special reference. :to. :electrorr tube 'rstripigit may be said: that under; present usage; any": strip v:or; sheet: material"preparedizfor. use :aszrelectrodermaterial 2i .anza electronic tube must be soft enough so that it will rnotr: cause e cessive wearnon the: :shapingttools; butrofxfar greater -'1mportance;---it must -retain fits LICOIIlPlflXl shape (as :31 observedin multitudinous :numbers ofashapes 70f anodes-)1 after forming-within=aneextremely ClOSGFtO1613I1C6.

A physical ".testing zmachinenzto describe:whethernthezz material meets this specification is the bend tester desigw nated as ASTM-B+15541T. For. the: PHBPOSC.OfFUS1Hg ourz shapingrtoolsn -W6wl'1aV6. foundthat: anode materials? should have :a -deflection= anglerof: .35 degreesror'less, as i measured '.-by ;-th-is=. bend tester; and" "this :zanglesvalue; is i .1 specified for material -iforsthis' use; though sometoleramcerm in: this limitingefigure, rusually; up to "40 degrees:maximum'nri is allowed foncontinuous production :of imaterialn The: materialiJafterncold rolling: to :therrdesiredt-sizen' usually 1 about e .005" -inch..- showsw'a :bend: .testanglei .con-rwsiderablygreater thant-35 degreeswregardless ofthethick-ii ness, .percent-a'geI of aluminum: tor percentage of reduction: in thiekness,-:-anddobviously-@mustbe softened-:"prionrzto use; Further,- this softening: mustxbe=consistently dupliecablemfrom -a production standpoint to allow-continuous :fiow of the materialathtough the:anodetforming.machines?' and reliably; to. attain the: desiredvbend'ttestx:

Tmp;,.F.' 00028 100025" 100033 100036" .6005": O006 Degrees Degrees Degrees Degrees Degrees Degrees The values in inches are for the thickness of the aluminum on strip .005 inch in thickness.

The following table of bend test angles shows that as the aluminum alloy thickness increases, it becomes progressively more difficult to soften the material sufficiently even though the time or temperature be increased.

The lower practical thickness of aluminum is determined by the factors of increased gas evolution of the electrode in the electronic tube and by the commercial unfeasibility of cladding steel with very thin layers of aluminum by a hot dip method.

The upper temperature limit for treating the .005 inch strip for use as electrode material is dictated by two effects. The first of these is as follows: If two pieces of the strip from the same sheet, are simultaneously annealed and blackened, one at 1250 degrees and the other at 1650 degrees, a difference in behaviour with respect to forming properties is noted.

Material transformed (annealed and blackened) at the lower temperature when bent around a sharp radius will show closely spaced, very fine cracks on the surface, none of which penetrate to the base metal. Note Fig. 1 of the drawings which shows a photomicroscopic view of the above-described aluminum clad steel strip. However, the material transformed at 1650 degrees will show widely spaced, wide cracks which penetrate somewhat into the metal. Note Fig. 2 which shows a photomicroscopic view of the aluminum coated strip treated at 1650 degrees F. Material heated to temperatures much above 1650 degrees F. will crack through the strip if exposed to the critical stresses of an electrode forming operation.

The second effect relating to the upper temperature limit is the surface condition of the coating. Deviations from a black body condition cause a decided loss in the radiation characteristics of the electrode, which are of utmost importance to efiicient operation of the electronic valve.

We have found the black color of the surface is due to an iron aluminum compound formation coupled with a thin film of black iron oxide, though we do not wish to limit the scope of this invention to the exact nature of the blackened surface. We have found that the blackened surface can be brightened considerably to a light grey by heat treating in a dry hydrogen gas atmosphere (dew-pointminus 30 degrees C.), at 1650 degrees F. or in a less reducing atmosphere at higher temperatures, such as 1700 degrees to 2000 degrees F. Obviously, from an electrode standpoint, this brightening is undesirable. Inasmuch as the heat treating (simultaneous annealing and blackening) is preferably carried out in an easily controllable reducing type atmosphere (as is well known, strongly oxidizing atmospheres tend to form flaky, poorly adherent oxide scales on iron), it may be desirable to restrict the temperature to 1650 degrees F. or below. If the conditions as set forth above are met, a sheet or strip product is produced which is coated with a dark or black layer comprising or consisting principally of an iron-aluminum alloy. It is a discrete layer, very thin, and sharply demarked from the iron itself. This layer is tough and adherent so that the product may be formed or drawn repeatedly without flaking of the coating. The iron or steel sheet or strip base stock will have the desired qualities for forming or drawing, and these operations will not affect the integrity of the coating. The product will have the other desirable characteristics set forth hereinabove.

Modifications may be made in our invention without departing from the spirit of it. Having thus described our invention in certain illustrative embodiments, what we claim as new and desire to secure by Letters Patent 1s:

A process of producing a coated ferrous sheet gauge stock which comprises hot rolling ferrous metal to thicknesses substantially between .180 and .080 inch, cold rolling it to thicknesses substantially between .100 and .040- inch, coating the cold rolled stock with aluminum in thickness substantially between .002 and .0005 inch on any coated surface, cold rolling the stock to thicknesses between substantially .06 and .005 inch whereby to re duce the aluminum coating thereon to between substantially .0003 and .0001 inch on any coated surface and thereafter heat treating the coated stock wherein said heat treatment is an open anneal, the temperatures lying within the range substantially between 1450 and 1750 degrees F. for a time varying directly with thickness of base metal and inversely with temperature between substantially 7 minutes and 17 seconds.

References Cited in the file of this patent 30 UNITED STATES PATENTS Number Name Date 1,656,892 Shover Jan. 17, 1928 1,792,377 Jordan Feb. 10, 1931 35 1,877,569 Falkenthal Sept. 13, 1932 1,982,563 Wimmer Nov. 27, 1934 2,082,622 Fink June 1, 1937 2,110,893 Sendzimir Mar. 15, 1938 2,170,361 Whitfield Aug. 22, 1939 40 2,197,622 Sendzimir Apr. 16, 1940 2,406,245 Oganowski et al Aug. 20, 1946 2,444,422 Bradford July 6, 1948 FOREIGN PATENTS Number Country Date 210,377 Switzerland Sept. 16, 1940 526,525 Great Britain Sept. 19, 1940

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