US2850999A - Method of making a coated embossed steel sheet - Google Patents

Description

Sept? 9, 1958 R. KAPLAN ET AL METHOD OF MAKING A COATED EMBOSSED STEEL SHEET Filed Aug. 19, 1955 RoaER-r KAPLAN EDWARD N. SIENKO United States Patent METHOD OF MAKING A COATED EMBOSSED STEEL SHEET Application August 19, 1955, Serial No. 529,537

1 Claim. (Cl. 113-120) This invention relates to the treatment of metal sheets and products obtained thereby, and more particularly,

to the treatment of coated metal sheets and products obtained thereby which may be used for decorative or structural purposes.

The instant invention is particularly concerned with the handling of sheets of the so-called rust resistant metals (i. e. copper, aluminum, stainless steel, etc.) and carbon steel sheets, although the instant invention may be used with any type of metal sheet, particularly any type of metal sheet that possesses cold formability at least as good as the carbon steel sheets here employed. The rust resistant metals have been used in sheet form for decorative purposes and/or structural purposes for a number of years. The rust resistant sheets such as aluminum and stainless steel sheets have also been embossed in small strips to provide ornamental sheets. sheets have been coated, after they are embossed, with various coating materials and, although the coating materials appear to be rather well anchored because of the irregular embossed surfaces of these metals problems are often presented in connection with completeness of the coating or continuity of the coating because of the many fine grooves or cuts in the embossed metal. Adherence of the coatings is sometimes impaired also because of bending of the embossed materials.

To the best of our knowledge others in the industry have not heretofore embossed carbon steel sheet; but the problems of applying a coating to embossed carbon steel sheet also include the problem of applying a complete or continuous coating to cover the steel surface at all points. This is particularly important with respect to carbon steel, because carbon steel is not a rust resistant metal and a coating applied thereto should provide protection against corrosion as well as the desired ornamental efi'ect.

As is well known, there are a number of other problems involved in the handling of sheet steel, usually in the form of hot rolled sheets of carbon steel, i. e., steel wherein the principal alloying element is carbon or steel of the SAE 1000 series. One of the properties of particular importance in SAE 1000 series steel is cold formability. Cold forming or cold working of steel is contrasted to hot working in the mechanical treatment of steel in that such working is carried out below the critical range. The cold working of steel involves a number of operations including cold forming, which may involve merely bending or stamping, or it may involve the relatively more difiicult operation of drawing.

In drawing a generally flat sheet of metal is subjected to bending combined with a shearing force, but the metal having good drawing quality yields to the shearing tions.

These force to the extent that it is deformed in drawing rather substantially less brittle. In general;drawtsgarivaivs the formation of a dish-shaped article from a generally flat metal sheet and this operation is contrasted to mere stamping or bending in that the metal having deep drawing quality is capable of being drawn perhaps four inches using a piece one square foot in area. Extra deep drawing may involve the formation of as much as an eight inch depression in a one square foot sheet of metal.

It is also well known that the concentration of forces is so great in the cold drawing operation that metallurgical changes are effected in certain steels, usually to the extent that'brittleness may be imparted to higher carbon steels. On the other hand, higher carbon steels such as SAE 1020 have increased rigidity or strength so as to resist cold drawing and make the operation much more difficult from the point of view of forces applied as well as wear and tear on the dies. In general, it has been the practice in industry for some time to use carbon steels in the series SAE 1006 to SAE 1015 (i. e., having about 0.05-0.15% C and about 0.300.60% Mn) for drawing automobile body and fender stock, lamps, oil pans, and a number of other deep drawing opera- Steel of this type, usually referred to as mild steel, possesses very good ductility or the ability to withstand cold deformation, but it possesses such ductility at a sacrifice in strength or rigidity.

Coatings applied to such carbon steels of the type which adhere rather well to the carbon steel tend to separate therefrom upon bending or subsequent treatment of the carbon steel, and particularly upon drawing or actual cold forming of the carbon steel. The same is true of coatings applied to the rust resistant metal sheets which are subsequently cold formed or drawn. There are a vast number of coatings which may be applied to such metal sheets, but in general the coatings with which we are here concerned are coatings which are formed of materials having a certain amount of deformability as contrasted to the very great rigidity of a porcelain enameling coating, for example. Such coatings are generally recognized in the art as flexible or deformable coatings, such as electroplated metal, resin coatings eitheras natural resins in paints or varnishes or as synthetic resins alone or in paints or varnishes, and the like. Such coatings are deformable at least to the extent that they are no more brittle than the carbon steel sheets here employed (or they are no more brittle in the form of thin coating films than the carbon .steel sheet here employed). In other words, theese materials are capable of deformation at least to the same extent (and usually much more easily) as the metal sheet itself. Notwithstanding this fact it appears that such coatings tend to separate from the metal backing when the same is bent or otherwise formed into a desired shape. I

The instant invention affords a solution to many problems confrontingthe workers in this art. A key to the instant invention resides in the concept of applying such deformable flexible coatings to the metal sheeting as an initial treatment, followed by embossing of the coated sheet, before drawing and/ or other secondary operations The embossing of the coated sheet apparently brings about a number of unique and important advantages,

some of the more striking including ease of drawing, and

substantially better adherence ofthe coating itself insubsequent operation such as drawing.

An important object of the instant invention is to provide an improved method of handling metal sheets, and particularly in the application of coatingsithereto, and further to provide improved coated metal sheets and articles formed ther'efrom. It is a further object oflthe instant invention to provide an improved process which comprises applying a thin coherent filmusf deformable coating" material to "a metal sheet, followed by embossing the coated sheet; and to provide an improved product resulting therefrom.

Other objects, features and advantages of the instant invention will become apparent to those skilled in the art from the following detailed disclosure thereof and the drawings attached hereto and made a part hereof.

On the drawings:

Figure 1 is a detail view in section of a metal sheet having a coating applied to one side thereof;

Figure 2 is a top plan view of one modification of an embossed coated metal sheet embodying the instant invention;

Figure 3 is a sectional elevational view of an embossed coated sheet of the instant invention (such as that shown in Figure 2) which has been drawn to form a generally dish-shaped panel, for example, for a refrigerator door or the like; and

Figure 4 is an enlarged sectional detail view taken substantially along the line IV-IV of Figure 2.

As shown onthe drawings:

In Figure l, the reference numeral indicates generally a metal sheet 10a having a coating 11 on one side thereof. The metal sheet 10a is a fiat sheet of substantially uniform thickness and the coating 11 is a flat coating of substantially uniform thickness. An initial step in the method of the instant invention involves applying the coating 11 to the fiat metal sheet 10a by any of a number of methods which will be described hereinafter. Next, the sheet 10 is embossed to provide a sheet 10' of Figures 2 and 4.

Referring to Figures 2 and 4, it will be noted that the sheet 10' has a multitude of alternating bosses or raised portions 11a, 11b, 116, etc. surrounded by valleys or depressions 12a, 12b, 12c, etc. on the coated face thereof. The coating 11' has conforming peaks 13a, 1311, etc. and valleys or depressions 14a, 14b, etc. Because of the greater ease of deformability of the coating 11, the thickness of the coating 11' may not necessarily remain uniform throughout, but the embossing process serves to effectively trap the coating 11 so that an appreciable thickness thereof remains over the entire surface. There are also depressions 15a, 15b, etc. and bosses 16a, 16b, on the rear or uncoated side of the metal sheet 10a corresponding to opposed bosses or depressions as the case may be. For example, the boss 11a on the coated side has a corresponding depression 15a on the uncoated side and the depression 12b on the coated side has a corresponding boss 16a on the uncoated side. This is the general outline of the cross-sectional shape of the embossed sheet 10', although the positioning of the bosses and recesses on the opposite faces thereof need not be positioned with such precise regularity or uniformity, so long as the recesses on one side are opposed generally to the bosses on the opposite side. In fact, it is generally preferable to present a surface in the form of the coating 11' as shown in Figure 2 wherein the positioning of the bosses appears to be a random positioning such as in simulated stucco, rather than a perfectly uniform positioning. Such an arrangement is generally more ornamental and attractive to the observer. Actual designs presenting greater uniformity or symmetry in the positioning of the bosses may be used, however, to substantially the same advantage in the practice of the instant invention.

In carrying out the coating step in the practice of the invention, it will be appreciated that any of a number of well known coating methods may be employed and any of a number of well known coating materials may be employed. The materials which may be employed in coatings include resinous or plastic materials such as natural resins in rosin-base or cellulosic derivative-base paints, varnishes or lacquers, with or without pigments, and synthetic resin-base paints, varnishes or lacquers, with or without pigmentation. Also, the coating material may be a metal applied by electro-plating, for ex ample. In general, the coatings may be applied in thicknesses ranging from a practical minimum of about 0.001 inch in order to cover the surface of the material with a coherent film to a practical maximum of about 0.010 inch, above which results in unnecessary loss of the coating material during the embossing step. The coating material is a deformable material having a degree of toughness suflicient to permit deformation of the extent here obtained. The extent of the deformation here obtained will be described in connection with the embcssing step proper; and the material employed in the instant coating composition should be capable of deformation to this extent without breaking (i. e., the material should not be so brittle that it breaks when undergoing such deformation). The material is thus not more brittle than the carbon steel sheet (which is probably the most brittle form of metal base material used in the practice of the invention).

In carrying out the embossing step, it has already been mentioned that carbon steel sheet may be used as the base material. Such sheet may have about 0.05- 0.30% C, and preferably has only about 0.30-0.90% Mn. This involves the steels within the range SAE 1006 to SAE 1030, except for SAE 1019, 1022, 1024, and 1027 which have higher Mn contents (of as much as 1.65% Mn). Preferably, the Mn content is 0.25- 0.60%, using C contents as high as 0.30%; and in many instances the greatest advantages of the instant invention are obtained using steels within the range SAE 1006 to SAE 1015 (ODS-0.15% C and 0.25-0.60% Mn).

The advantages obtained using carbon steel sheet are quite unique in that a protective coating for carbon steel sheet is required in order to prevent excessive corrosion, as by rusting. There are a number of structural advantages obtained from embossing a plain carbon steel sheet; but the embossed sheet must then be painted or coated in most instances to prevent corrosion thereof. Although coating films will in many instances adhere very well to an embossed carbon steel sheet, the application of such coatings is often complicated by the difficulties in applying coatings so as to completely cover the metal in the various crevices on the surface thereof without employing so much coating material that the openings or crevices in the surface are substantially filled by the coating material. This tends to alter the appearance and the con tour of the surface. Instead, in the practice of the instant invention the coating is applied in a uniform thin film before the embossing process and the coating remains on the surface after the embossing process in a correspondingly thin film which is forced into each of the various crevices or openings and which completely covers the metal therein. Also, substantially better adherence between the coating and the metal is obtained if the embossing step is carried out subsequently, in accordance with the teachings of the instant invention. This advantage is also obtained in the use of the so-called rust resistant metals; and, of course, protective coatings are often applied to the so-called rust resistant metals also because of peculiar corrosion problems which may arise.

The thickness of the sheets of metal backing material employed in the practice of the instant invention may range from a maximum of about 0.1 inch to a practical minimum of about 0.01 inch. Preferably 18 gauge (0.050 inch thickness) or less is used.

In general, the embossing operation is a cold working or cold forming operation. The coated sheet is passed between matched hard steel embossing rolls, at cold working temperatures preferably, and the embossing rolls are provided with a multitude of small bosses matingly aligned on the two rolls so as to avoid having the bosses on one roll directly opposite the bosses on the other roll at the embossing nip therebetween through which the sheet passes. The bosses, in order to obtain the full benefit of the instant invention, have an averageheight or extend an average distance from the'roll periphery of about 0.010 to about 0.014 inch for embossing stock of 0.035 inch thickness (or for that matter for embossing stock over the entire operativerange hereinbefor'e set forth, the size of the bosses being optionally altered to conform somewhatto variations in thickness of the sheet). Expressed in other terms, the bosses are approximately 4 to /2 of the sheet thickness in height and preferably about 20 to 45% of the sheet thickness in height. The distances a, d (Figure 4) betweenthe tops of the bosses and the depressions in the sheet are substantially the same as the proportions just given for the bosses on the embossing rolls, although the distances just mentioned on the sheet may be slightlyless if completely elfective'embossing is not accomplished because of spacing between the rolls or reduced pressure at the nip.

p The embossing operation (with the possible exception of'the overall pressures used) is substantially the same for each of the various coating materials and each of the various coated base sheets hereinbefore described. The embossingprocess may thus be carried out using carbon steel sheeting of carbon content'up to as high as perhaps SAE 1052 (about 52% carbon and as high as 1.55%

Mn), but there are practical limitations such as wear and tear on'the embossing rolls or dies which would subtract to an appreciable extent from the overall advantages of the instant invention if too hard a carbon steel were used. Also, the relatively poor cold formability of such high qualityif it can be drawn four inches per square foot. The drawing operation itself involves applying suitably formed male and female dies to the sheet material under pressure (at less than the critical temperature for cold drawing) in order to effect deformation of the sheet to form the dish-shaped article. As mentioned, drawing of the sheet to the ultimate shape need not be accomplished in a single drawing step but may be accomplished through series of successively deeper draws, with heat treatment in-between such steps (if a refractory coating such as an electro-plated metal has been applied).

Referring to Figure 3, it will be seen that an embossed sheet such as the sheet 10' (Figure 2) may be drawn to C and high Mn steels would necessitate greater embossing operating pressures and greater care in carrying out the operation, the operation in suchcases necessarily results in metallurgical changes in these high C-Mn steels which require subsequen t heat treatment. In any event, however, the embossed steelsh'eets of the invention afford unusualadvantages in cold forming. This is also the case in connection with embos sed rust resistant metal sheets. As an example, the embossingstep effects a definite increase in rigidity or strength of approximately 25 to over unembossed stock; but contrary to expectations this increase in rigidity greatly facilitates rather than making more difficult cold forming operations. In addition, the coatings further facilitate cold forming operations and permit cold forming operations without effectively losing the coating on the surface of the embossed material. Wear and tear on the die is reduced presumably because there is less metal-to-metal contact and a better chance for lubrication (rather than increased as might be expected because of the supposedly roughened metal surface of the embossed workpiece). This afi'ords an advantage even in the case of the higher carbon steels, if coated with a suitably refractory coating, which may have to be heat treated once or several times during any sort of cold forming operation.

The drawing operation which is employed to particular advantage in the instant invention is, of course, a standard drawing operation of the type well known to those skilled in the art. The differences here involved include greater ease of drawing, apparently better lubrication between the die and the workpiece, less wear and tear on the die, a retention of the embossed contour of the workpiece during the drawing or forming operation, and a retention of the embossed contour of the coating on the base metal during the drawing or forming operation. In other respects, the drawing operation is the same as an ordinary commercial operation. Drawing itself is a well known art and need not be described herein in detail. For the sake of distinguishing from ordinary bending, stamping or cutting operations drawing could probably best be defined as involving the application of forces to the workpiece that are comparable to forces at least suflicient to make a two-inch depression in a square foot of the workpiece sheet. Expressed in other terms, drawing involves the shaping of a sheet into a dish-shaped article or an article having a bowed contour; and the workers in the art generally consider a material has good deep drawing form a generally dish-shaped panel member 17 wherein the central portion 17a is recessed about four inches, as the dimension r indicates, and a flange-like portion 17b is retained around the periphery. The drawing operation is accomplished in an ordinary drawing press so as to provide a dish-shaped front panel member 17 for a door, such as an ice box door. In Figure 4, the contour of the coated embossed sheet 10 is shown.

For example, sheets of material are prepared for embossing, according to the instant invention, by carrying out the following procedures: I

(1) Using-20 gauge SAE 1010-steel sheeting as a cathode, plating is carried out using a chrome plating bath (having 280 g./l. CrO and 2.8 g./l. H SO content) maintained at 130 F.,withcathode current density; of 200 a. s. f. (amperes per square foot) and an anode current density of a. s. f., using ale'ad anode, and plating for 10 minutes.

(2) Using 20 gauge SAE 1015 steel sheeting as a cathode, plating is carried out in an electrolyte containing zinc (content: 4.5 oz./gal. of Zn, 12.5 oz./gal. of NaCN, 10.5 oz./ gal. of NaOH) maintained at 95 F. with a cathode current density of 20 a. s. f. and a (zinc) anode current density of 10 a. s..f., 'and plating for 10 minutes.

(3) Using about 0.050 in; 18-8 stainless steel sheeting as a cathode, plating is carried out in an electrolyte containing copper (content: 3 oz./ gal. of Cu, 4 oz./gal. of Cu(CN) 6 oz./gal. of NaCN) maintained at F. with a cathode current density of 40 a.'s. f. and a (copper) anode current density 'of20 a, s..f.; and plating for 10 minutes.

(4) Using 18 gauge SAE 1010 steel sheeting as a cathode, plating is carried out in an electrolyte containing cadmium (content: 3 oz./ gal. of Cd, 12 oz./ gal. of NaCN, 3 oz./ gal. of NaOH) maintained at 90 F. using a cathode current density of 20 a. s. f., and a (cadmium) anode current density of 10 a. s. f., and plating for 10 minutes.

The foregoing metal plated sheets are embossed in the manner hereinbefore described by passing the same through a nip defined by matched hard steel embossing rolls at cold working temperatures, the rolls having bosses thereon of an average height of 0.012 inch; and the resulting embossed sheets have superior cold formability and can be drawn to form panels such as the panel shown in Figure 3. In these embossed sheets, it is noted that the metal plating adheres well during the embossing operation and during subsequent cold forming operations such as the drawing operation.

Substantially the same results are obtained using sheets of other rust-proof metals such as aluminum and plating with the aforementioned plating baths or plating baths depositing other metal coatings. Also, silver coatings deposited chemically (as in the formation of mirrors) may be applied to the carbon steel sheets or the rust-proof metal sheets used herein and the resulting coated sheets may be embossed to obtain the advantages of the instant invention.

Among the non-metallic coatings which may be employed in the practice of the instant invention those of the greatest significance are the natural and/ or synthetic resin coatings. Among this group the so-called elastomers are preferred. The resin or plastic elastomers are well known materials to those skilled in the art possessing generally elastomeric properties comparable to that of natural rubber. Such elastomers include rubber, chlorinated rubber, rubber-synthetic resin admixtures, synthetic rubbers (i. e. butadiene-styrene, isoprene, chloroprene, butadiene-acrylonitrile copolymers), flexible (or unsaturated) polyester resins, vinyl chloride polymers, vinyl chloride-vinylidene chloride copolymers, vinyl chloride-vinyl acetate copolymers, and the like. These resin or elastomer coatings may be applied per so as by flame-spraying, which is particularly effective with polyethylene and polytetrafluoroethylene (which are well known elastomers), or the resins may be applied in solution in organic solvents with subsequent baking operations to remove the solvent, or the resins may be applied in emulsion form in aqueous media also with subsequent drying to remove the carrier. Solutions or emulsions of the resins may be applied by spraying, dipping, painting, or the like. Special treatments for the metal surface prior to the application of such resins may also be employed, such as the bonderizing, or Parkerizing processes. Pigments may be included or omitted as desired, but since decorative effect is an important feature in many uses of the invention, various colored pigments are usually included so that an elastomer base paint is preferred in many instances.

Specific examples of coatings include the following:

Vinyl chloride-vinylidene chloride commercial grade medium copolymer is applied in a film 0.05 in. thick on 18 gauge SAE 1010 sheeting from a mineral spirits solution sprayed on followed by baking at 350 F. to dry and cure the coating.

(6) Paint containing green pigments and vinyl chloride-vinylidene chloride commercial grade medium copolymer is applied in a film 0.05 in. thick on 20 gauge SAE 1010 sheeting, from a commercial aqueous emulsion followed by baking at 350 F. to dry and cure the coating.

(7) Polyethylene is flame-sprayed on 0.050 in. thick 18-8 stainless steel sheet to provide a continuous film of 0.03 in. average thickness.

(8) Polytetrafluoroethylene is flame-sprayed on 0.050 in. aluminum sheeting to provide a continuous film of 0.03 in. average thickness.

(9) Commercial flexible (unsaturated) polyester resin (i. e. ethylene glycol-propylene glycol-phthalate) in varnolene solution is painted onto 20 gauge SAE 1010 sheeting and cured at 200 F. to obtain a film of 0.05 in. average thickness.

Each of the foregoing coated sheets is embossed in the manner hereinbefore described (i. e., using embossing rolls having bosses thereon of an average height of 0.012 inch) and it is found that the embossed sheets may be bent back and forth without causing the resin coating thereon to crack or separate from the metal. Also, the embossed sheets are cold formed in panels such as the panel shown in Figure 3 by drawing and it is found that the drawing process is simplified even though the embossed panels have greater rigidity, and the coating is not harmfully affected by the drawing process.

It .will be understood that modifications and variations may be efiected without departing from the spirit and scope of the novel concepts of the present invention.

We claim as our invention:

A method which comprises applying a thin coherent 0.001 to 0.01 inch thick film of deformable thermoplastic synthetic elastomeric resinous coating material to 0.01 to 0.1 inch thick carbon steel sheet, pressure embossing the coated sheet by passing the coated sheet through a press nip defined by embossing rolls having bosses thereon having a height of 0.1 to 0.5 times the sheet thickness, and then cold drawing the embossed coated sheet without eradicating the embossing.

References Cited in the file of this patent UNITED STATES PATENTS 1,499,985 Kirsch July 1,1924 1,773,926 Michael Aug. 26, 1930 1,892,754 Tarbox Jan. 3, 1933 1,934,256 Bronson Nov. 7, 1933 2,114,150 Rodman Apr. 12, 1938 2,120,461 Copeman June 14, 1938 2,122,537 Pfetfer July 5, 1938 2,166,226 Vehko July 18, 1939 2,387,919 Lose Oct. 30, 1945 2,735,390 Engel Feb. 21, 1956 OTHER REFERENCES Pages 102, 103 of Product Engineering, March 1949.

Images