CA1155776A - Drawn and ironed cans with thin nickel coating and method of forming the cans - Google Patents

Abstract

DRAWN AND IRONED CANS WITH THIN NICKEL COATING
AND METHOD OF FORMING THE CANS
ABSTRACT OF THE DISCLOSURE
Seamless cans are drawn and ironed from steel sheet having a very thin coating of nickel on both sides. The steel may be flat rolled blackplate and the nickel may be plated onto the steel by drawing a continuous strip or sheet of the steel through a conventional electroplating bath to deposit the very thin nickel coating. The coated steel is cut into blanks which are drawn into cups, and the side wall of the cups are then ironed in the conventional manner to produce the desired thickness and height for the finished cans. The nickel coated steel may be chemically treated to increase storage life of the material before drawing and ironing. The drawn and ironed cans may also be chemically treated to enhance the adhesion of lacquer applied to the cans.

Description

1 1557~6 This application is a division of Canadian Application 339,723 filed November 13, 1979 and issued as Canadian Patent No. / /3~, c~ 7~- on ~ c,~3, ~8~.
Field of the Invention This invention relates to the forming of seamless coated metal cans by the draw and iron process, and more particularly to the forming of such cans from steelplate having a very thin coating of nickel plated on both sides.
Description of the Prior Art It is well-known to form seamless cans, and particularly beverage cans, by the draw and iron process.
This procedure involves drawing an initially flat metal blank into a cup having side and bottom wall thicknesses which are substantially equal, with the height of the cup being sub-stantially less than that of the final can and the diameter being substantially equal to that of the final can. The drawn cup is then supported on a punch and forced through one or more ironing dies or rings whose inside diameters are each smaller than the outside diameter of the cup passing there-through so that the ironing dies progressively reduce the thickness of the side wall of the drawn cup and force the metal along the punch to thereby increase the height of the can.
In the past, difficulty has been encountered in forming drawn and ironed can bodies from flat rolled steel.
The substantial forces required frequently resulted in tearing of the thinned side wall, or pushing the punch through the can bottom, in the absence of an adequate lubricating coating.
Accordingly, it has generally been found necessary to provide a lubricating coating of a softer metal, usual~y tin, on both sides of the steel base metal in order to successfully iron the side wall to the desired thickness. The tin coating ,.~

1~557.6 greatly increases the cost of the complete cans due both to the cost of the coating metal and the expense of applying it to the base steel.
While a lubricating coating of soft metal substantially increases the cost of a drawn and ironed can, there are certain advantages, in addition to facilitating drawing and ironing, to be obtained from such coatings. For example, cans drawn and ironed from tinplate, i.e., steel having a coating of tin on both sides, have a substantially increased corrosion resistance and the finished can has a brighter, more pleasing surface than cans formed from uncoated steel.
It has generally been found necessary to provide a coating of at least 0.25 lbs. of tin per base box of steel (quarter pound plate) in order to assure reliable solderability of the components in three-piece tinplate cans. Coating weights at least as great as for three-piece containers have generally been found necessary for drawn and ironed containers, and it is conventional to use half pound plate (0.5 lbs. tin per base box of steel) for producing commercial drawn and ironed tinplate cans. Half pound tinplate has a coating thickness of approximately 30 microinches on each side.
The heavy tin coating on tinplate used for drawing and ironing cans has been required both due to the substantial expansion of the surface of the steel forming the can side wall (with the consequent proportional reduction in coating metal thickness) and to the necessity of assuring a lubricating coating throughout the drawing and ironing process. There is a tendency for the tin to flow or be drawn from plateaus and be deposited in valleys of the base steel surface during the 1 ~ 557~6 drawing and ironing steps, resulting in the tin coating being extremely thin in some areas while in other areas it may actually be thicker than was originally plated onto the base metal. Insofar as formed can performance is concerned, substantial quantities of expensive tin are wasted because the coating on the finished can is no better than the thinnest area.
During ironing of tinplate, the tin may be so completely stripped from small areas of the base steel that direct steel-to-steel contact between the steel base of the tinplate and the ironing die may result. This tends to increase ironing loads and greatly shortens the useful life of the ironing dies. In an article entitled, "Surface Properties of Drawn-and-Ironed Tinplate Container", by P. R. Carter, et al., of United States Steel Corp., published at the International Tinplate Conference in Oct., 1976, it is stated that the tendency of tin to be stripped from the base steel during ironing of tinplate is reduced with increased tin coating weight. According to the authors, beverage containers drawn and ironed from quarter pound tinplate had only 80% of the side wall covered with tin at 1 inch from the bottom of the can, while only 30% was covered at a position 4.5 inches above the bottom. This coverage was stated to be increased to 90% and 70%, respectively, at the corresponding locations for cans drawn and ironed from half pound tinplate. While subsequent independent tests, employing more sophisticated measurement techniques, have indicated that the stated percentages of area covered were probably low, this U. S. Steel article does confirm the Xnown tendency of tin to segregate, or to be wiped from plateaus and be deposited in valleys of the base metal.

1 :~557 . 6 It has been proposed in U. S. Patent No. 3,655,349 to form a drawn and ironed steel can from a blank having a differential coating of lubricating metal on the opposed sides of the base metal. This patent states that the external surface of the side wall is subjected to an operation more closely approaching a drawing or wiping action as it passes through the ironing die whereas the inner surface is caused to flow along the outer surface of the punch without the wiping action. Accordingly, a thin coating of the lubricating metal is applied to the side of the base metal which becomes the inner surface of the finished can while a substantially thicker coating is required on the other side. In one experiment tabulated in the patent, beverage cans are stated to have been drawn and ironed from nickel plated steel blanks having a coating thickness of approximately 1 microinch on one surface and approximately 12.5 microinches on the other surface. A
footnote to the tabulated experimental data states that the nickel-plated cans were scored during ironing, a condition which is not acceptable in commercial cans.
U. S. Patent No. 3,295,936 discloses a conventional three-piece can formed from a steel plate having a thin coating of nickel plated thereon, with the nickel-plated surfaces being electrochemically treated in a solution of dichromate or chromic acid to deposit a film on the nickel to improve corrosion resistance and enhance lacquerability. The nickel coating is stated to be about 0.02 to 0.3 microns (about 0.8 to 11.8 microinches) in thickness.
Summary of the Invention In accordance with the present invention, a drawn and ironed can body is produced from a steel base metal having 1~557. 6 an extremely thin, substantially uniform coating of nickel plated on both sides of the base metal. By employing nickel as the coating metal rather than the more widely used tin, it has been found that, contrary to the teachings of the prior art, adequate lubricity and corrosion resistance are provided with coating thicknesses within the range of about 1/2 to 5 microinches before ironing, and that after the ironing operation the reduced coating thickness is more uniform than for a tin coating. This results in a substantial savings in coating metal and enables the use of the less expensive and more abundant nickel to replace the tin coating conventionally used on such drawn and ironed containers. The nickel coated steel may be chemically treated to increase storage life of the material, and the cans drawn and ironed from the material may also be chemically treated to enhance adhesion of the lacquer coating conventionally applied to such cans after forming.
BRIEF DESCRIPTION OF THE DRAWINGS

. _ The invention will be described hereinbelow with reference to the drawings, in which:
FIG. 1 is a fragmentary, enlarged sectional view of a coated steel blank embodying the present invention;
FIG. 2 is a sectional view of a drawing and ironing die gang and punch illustrated in the process of drawing and ironing a can from a blank of the type illustrated in FIG. l;
FIG. 3 is a bar graph illustrating the redistribution of coating metal of tinplate and nickelplate resulting from drawing and ironing; and 1 ~5577~

FIG. 4 is a graphic illustration of comparative punch loads required to draw cans from tinplate and nickel-plate.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In its broader aspects, the present invention involves plating onto both surfaces of a steel sheet 10 a thin coating 12 of nickel, cutting the coated steel sheet into blanks 14 (FIG. 2), forming the cut blanks into shallow cups by a drawing or drawing and redrawing operation, and subsequently ironing the side walls of the cups to reduce the thickness and increase the height of the side walls to form cans 16 having a height substantially greater than their diameter. The nickel-plated steel can be chemically treated in a dichromate or chromic acid solution, or other suitable chemical solution, to apply a protective coating 18 to the nickel to enhance storage life of the plated steel. The nickel coating on the steel substrate is very thin and may be in the range of about 0.5 to about 5.0 microinches, but preferably is within the range of about 1 to about 3 microinches in thickness.
The extremely thin coating enables the nickel to be applied at a high rate using conventional electroplating equipment and techniques. Surprisingly, it has been determined that such extremely thin nickel coating provides the necessary lubricity to enable uniform ironing of the side walls of the can without substantial rupturing or stripping of the thin coating. Further, the thin nickel coating enables ironing without excessive redistribution of the coating metal over the surface of the steel as has been found to occur during ironing of conventional tinplate. It has also been determined that the wall ironing ioad for nickel coated steel is slightly less 1~55'7.6 than for tin coated steel of comparable thickness. While coating thicknesses of up to 5 microinches reduce slightly the ironing loads and also slightly increase the corrosion resistance over coatings of about 0.5 to 1 microinch, such heavier coating thicknesses increase the cost of the finished can due both to the cost of the additional coating metal and the reduced line speed during the plating process. As shown below, tests have confirmed that an extremely thin nickel coating, when chemically treated to assure lacquer adhesion, provides the necessary corrosion resistance for commercial use in packaging of foods, beverages and the like. Nickel coating thicknesses greater than about 5 microinches and up to the thicknesses conventionally used for drawn and ironed tinplate do not produce results which are sufficiently improved over the results obtained from the thinner coatings to justify their use or which are required for drawn and ironed cans. While coating thicknesses as low as 0.5 microinch have been successfully employed, coating thicknesses of at least about 1 microinch are preferred in order to provide a margin for error and thereby reduce the criticality of control in the plating process.
The base steel sheet employed in accordance with the present invention can be flat rolled blackplate which, as is known, is the most economical of the available sheet metal container stocks. The chemical analysis and mechanical properties of a typical base steel successfully used to produce drawn and ironed cans in accordance with the present invention are as follows:

1~55776 CHEMICAL ANALYSIS MECHANICAL PROPERTIES
Carbon 0.061~ Yield strength39 ksi Manganese 0.31 % Ultimate strength 47.7ksi Sulfur 0.018~ Temper 51 R-30T
Phosphorous 0.004% Total elongation 26.8%
Silicon 0.013% Grain Size (ASTM) 8 Aluminum 0.061% Surface Roughness 66-86 (CLA) Iron & impurities balance Blackplate having the above properties has sufficient strength and dimension stability to provide a commercially acceptable can even when the side walls are reduced to a thickness substantially below that acceptable in cans produced from a softer, more expensive metal such as aluminum. For example, a side wall thickness, after ironing, of about 0.0025 to 0.0045 inches (about 0.065 to 0.115 mm) is adequate for a carbonated beverage container whereas the side walls of three-piece beverage containers normally range between about 0.006 to 0.008 inches (0.152 to 0.204 mm). Drawn and ironed aluminum cans may have a finished side wall thickness of from about 0.0055 to about 0.008 inches.
The base steel can be coated with nick~l employing conventional electroplating apparatus and techniques. Either a soluble or insoluble anode system can be used. A typical insoluble anode system can employ a plating solution containing 200g/1 of nickel sulfate and 40g/1 of boric acid and have a pH
within the range of about 3.5 to 4Ø Plating may take place at a current density of about 14.0 A/dm2, with the plating bath temperature being maintained at about 57C. A typical soluble anode system can employ a plating solution containing 75g/1 of nickel sulfate,75g/1 of nickel chloride, and 40g/1 of boric acid 1 ~S57 . 6 and have a p~ of about 2.5 to 3.5. The bath temperature and current density can be the same as in the insoluble anode system.
Referring to FIG. 2, the drawn and ironed cans 16 are formed by clamping the blanks 14 adjacent an opening in a drawing die 20 and forcing the blanks through the die by a punch or mandrel 22 to initially draw the blanks into cups.
The drawn cups can then be removed and ironed in a separate apparatus, but preferably are ironed in a continued downward movement of the punch 22 to force the cups through a succession of ironing dies illustrated at 24, 26, with the successive dies engaged having successively smaller openings to progressively reduce the thickness of the side wall 28 and increase the height of the cups to that desired for the finished cans. The bottom wall 30 of the cans remain substantially the same thickness as the original blank 14.
After a can passes through the lowermost ironing die, the punch 22 can continue its travel until the top edge of the can passes a stripper 32. The stripper engages and strips the can from the punch upon its return stroke. The apparatus employed in the draw ng and ironing operations can be conventional and accordingly is illustrated only schematically in FIG. 2.
In evaluating the feasibility of forming drawn and ironed cans from nickel-plated steel instead of the more commonly used tinplate, tests were conducted to compare the redistribution of the coating metal of flat rolled steel sheet having coatings of comparable thicknesses of tin and of nickel.
The coated blanks were drawn and redrawn to initially form a cup. Some redistribution of the coating metal was evident for 1~557, ~

both the nickel-coated and the tin-coated samples after the draw-redraw operation. The drawn-redrawn cups were then subjected to three ironing steps during which the redistribution of the coating metal was continued, i.e., the coating metal was to some extent wiped off the plateaus and deposited in the valleys of the base steel metal; however, examination of the finished cans thus formed with a scanning electron microscope revealed that there was more nickel remaining on the areas of low nickel content than there was tin in the region of low tin content. Initial testing was conducted with samples having coating thicknesses of approximately 3.5 microinches, with the results of the redistribution being illustrated in FIG. 3.
Thus, referring to FIG. 3, it is seen that there is less segregation of nickel to high and low concentrations, illustrating the lack of mobility of nickel as compared to tin, during the forming of tinplate and nickelplate into drawn and ironed containers. Subsequent testing has revealed that the resistance to segregation is equally effective for nickel coatings having a thickness within the range of 0.5 to 5 microinches.
Tests have also revealed that, for comparable coating thicknesses, smaller ironing loads are required to iron nickel-plate than tinplate. FIG. 4 shows the load on the punch during the second and third ironing steps for a plurality of coating thicknesses of tin and nickel. The reason for the lower ironing loads for nickelplate is believed to be that nickel does not segregate as much and therefore more coating metal is present on the thinly covered areas to act as a lubricant than is present on comparable tinplate material. The lower ironing loads reduces the tendency for the punch to tear the side walls of the 1 ~55776 can, or to punch through the bottom of the can during ironing.
Cans have been successfully drawn from nickel-plated blackplate having a nickel thickness ranging from 1/2 to 5 microinches. Testing shows that drawn and ironed cans made from 0.5 to 1 microinch nickel-plated steel possess satisfactory corrosion resistance and lacquer adhesion for use as a commercial carbonated beverage can or for other uses where the conventional tin-plated can is now employed when the nickel-plated steel cans are subjected to a chemical treatment, such as a conventional chromate treatment, after drawing and ironing.
The tested cans were lacquer coated using commercially available coating material such as a vinyl or epoxy coating conventionally employed with beverage cans.
Nickel-plated steel cans produced by a draw and redraw operation were subjected to food pack tests to determine lacquer adhesion and shelf life. Salt and citric acid packs and pork packs were employed to simulate conditions which might be encountered in commercial use for packaging foods and beverages.
Results of the tests confirmed that such nickel-plated steel cans having a chromic acid or dichromate coating on the nickel surface and coated wi~h a modified epoxy lacquer possessed excellent lacquer adhesion qualities, as indicated by the lack of underfilm corrosion in these test media. Similar cans coated with the same lacquer but without the chemical coating frequently showed signs of underfilm, indicating the need for a different, more adhering lacquer or the chemical treatment.
Flat panel tape pull tests and wedge bend tests conducted on salt and citric acid exposed panels were also conducted to confirm lacquer adhesion on the nickel-plated blackplate steel. These tests were conducted on panels coated - 1] -1 ~55776 with lacquer and cured at various temperatures ranging from 350F. to 410F. Various commercially available lacquer coating materials were used. These tests showed that the coatings could bè cured at temperatures of approximately 30F.
less than similar coatings on Weirchrome, a commercially available chrome-plated steel manufactured by Weirton Steel Division of National Steel Corporation and employed in the manufacture of containers. These tests also confirmed that lacquer adhesion was slightly better on those panels which were subjected to a chromate or chromic acid chemical treatment prior to the application of the lacquer.
Size 303 X 406 cans drawn and ironed from nickel-plated blackplate having a coating thickness, before drawing and ironing, of 1 microinch were tested to determine the affect of the chromate chemical treatment on vacuum loss. The fabricated cans were coated with an epoxy type spray coating and the packed cans were stored at 100F. The results of these tests were as follows:
VACUUM, in inches Hg.
PRODUCT ~ CHERRIES l CORN ¦T~MATO JUICE GREEN BEANS
_ Storage5 14 5 14 5 14 5 14 rimeweeks weeks weeks weeks weeks weeks weeks weeks Chemically Treated 12.7 9.2 15.2 13.219.2 18.215.7 8.2 Not _ ~ _ _ Chemically 8 4.7 6.7 9.2 18.7 17.2 9.7 10.5 Treated i l l l Tests were also conducted on cans of the size type just described to determine the effect of the pressure or absence of a chemical coating after ironing and before coating 1 ~557 . 6 with the spray type epoxy lacquer on the iron pickup of products stored at 100F. The results of these tests were as follows:

IRON PICKUP (PPM) PRODUCT CHERRIES CORN TOMATO JUICE GREEN BEANS
Storage5 14 5 14 5 14 5 14 ~imeweeks weeks weeks weeks weeks weeks weeks weeks Chemically rlc~t.~9.9 15.7 3.5 3.5 9.2 19.8 5.5 18.9 Chemically 19 0 24 4 8.1 14.1 11.7 30.0 7.0 26 7 Both of the above tests conducted on cans drawn and ironed from 1 microinch nickelplate indicates that better storage life is obtained by the chemical treatment when the spray type epoxy lacquer coating is employed.
While I have disclosed and described a preferred embodiment of my invention, I wish it understood that I do not intend to be restricted solely thereto, but rather that I do intend to include all embodiments thereof which would be apparent to one skilled in the art and which come within the spirit and scope of my invention.
I CLAIM:

Claims (6)

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

1. A nickel-plated steel can comprising, a bottom wall and a seamless side wall, said bottom and side walls being integrally formed by a drawing and ironing process whereby the side wall is ironed to a thickness substantially less than that of the bottom wall, said can being drawn and ironed from a sheet of flat rolled steel sheet having a coating of nickel plated on both sides thereof, the nickel coating having a thickness within the range of about 1/2 to about 5 microinches before being drawn and ironed.

2. The can according to claim 1 wherein the nickel coating has a thickness within the range of about 1 to about 3 microinches before being drawn and ironed.

3. The can according to claim 2 further comprising a thin dichromate or chromic acid coating on the surface of the nickel coating, said dichromate or chromic acid coat-ing being applied by emersing the nickel-plated steel in a solution of the coating material after it has been drawn and ironed.

4. The can according to claim 3 wherein said flat rolled steel sheet is blackplate.

5. The can according to claim 4 further comprising an organic lacquer coating applied to and cured on at least the inner surface of the can over the dichromate or chromic acid coating.

6. The can according to claim 3 wherein said can is drawn and ironed from flat rolled blackplate having said nickel plated thereon and having a thin dichromate or chromic acid coating applied to the nickel coating surface prior to being drawn and ironed.