US3269007A

US3269007A - Method of restoring ductility to heavily cold worked sheet metal

Info

Publication number: US3269007A
Application number: US70642A
Authority: US
Inventors: John E O'donnell
Original assignee: Continental Can Co Inc
Current assignee: Continental Can Co Inc
Priority date: 1960-11-21
Filing date: 1960-11-21
Publication date: 1966-08-30
Anticipated expiration: 1983-08-30

Description

J. E. ODONNELL 3,269,007 METHOD OF RESTORINGDUG'IILITY TO HEAVILY COLD WORKED SHEET METAL Filed Nov. 21, 1960 Aug. 30, 1966 INVENTOR. JOHN E O Douuau.

BY Qw m AT T2 2 HEYS United States Patent 3,269,4l07 METHUD 0F RESTURHNG DUCTHHTY T0 HEAVTLY (IQLD WGRKED SHEET METAL John E. ODonnell, Highland, llntlL, assignor to Continental Can @ompany, line, New York, N.Y., a corporation of New York Filed Nov. 21, 196%, fier. No. 70,642 13 Claims. (Cl. 29-527) This invention relates in general to new and useful improvements in method of manufacturing sheet metal, and more particularly relates to a method of restoring ductility to heavily cold worked metal.

In the manufacture of certain types of sheet metal, particularly thin steel strip and sheet, the necessary reduction in thickness is obtained :by heavily cold rolling the starting metal stock. The resultant strip has been severely cold worked, frequently by more than 80% reduction, and although very hard and strong, it does not have adequate ductility for many uses. For example, if the ultimate product is to be tin plated steel for the making of cans, the hard rolled steel in conventional practise must be annealed and then temper rolled to a reduction of about only one percent. This very slight reduction imparts flatness and also masks the yield point elongation of such steel which otherwise would be detrimental to canmaking or other fabrication processes which might be employed and yet this minor reduction does not noticeably impair the ductility resulting from the prior anneal.

When such conventional tin plate is desired in thinner gauges, i.e. less than .008" thick, a variety of difficulties are encountered. For example, when thin rolled strips are pulled through the electrolytic tinning line, the applied tensile force used to pull the strip sometimes results in expensive fracture of the strip within the electroplating line. In order to avoid this difficulty, the steel industry has introduced a new type of tin plate to provide the desired thinner gauges of metal, which new type of plate is produced in such a manner that it is less costly, per unit area, to produce than heretofore.

The new type of tin plate is produced by additional substantial cold rolling reductions after the metal strip stock has been annealed and plated with tin. The initial cold rolling is to a somewhat thicker gauge than normal; followed by annealing, tinplating and a subsequent cold reduction of the order of 30 to 60 percent, resulting in a finished gauge of as low as 0.0044 inch, and even lower, if desired. Unfortunately, while the finished product has a desirable thinness and also desirable high strength and hardness, these qualities have been attained by an almost complete sacrifice of ductility and by a marked directionality of properties. Tensile testing of such material indicates a transverse elongation of the order of one percent or less in a two inch gauge length and a longitudinal elongation of from less than one to three percent.

When cans which have been made from this new type of tin plate have the rolling direction parallel to the can axis, frequent fracturing of the plate is encountered during manufacture. Fracture occurs primarily during forming of the side seam hook and during hanging of the can body or in the subsequent operation of double seaming a closure end on the flanged can body where fracture of the flange of the can body or of the double seamed portion of the can end may occur, thus causing can leakage. These can making operations are ones in which large calized plastic tensile strains may be introduced into the metal in a transverse direction. When these problems are avoided by having the can bodies made with the rolling direction of the tin plate in a circumferential direction, fractures of the cans have been encountered when, during rough handling, dents were produced in the body wall below the end seam seal. Therefore, in order to exploit fully this newly introduced type of tin plate for can making, some of the ductility of the metal must be restored. Thermal treatment, such as annealing after the second cold reduction is not practical since the tin coating which is already on the steel will alloy with the steel at the required steel annealing temperatures and there upon forming an excessive amount of undesirable hard and brittle tin-iron alloy.

The present invention is directed to a solution to the need for increasing the ductility of this new type of tin plate which has been given a substantial cold reduction after tinplating and one object of this invention to provide a novel method of mechanically treating such hard and brittle tin plate by employing an extremely simple operation which will increase the ductility of the hard and brittle plate, both longitudinally and transversely, with only minor reductions in the strength of the plate.

In the past, metal sheets to be formed into can bodies have been subjected to a grain breaking process wherein immediately prior to the shaping of the sheets in a can making process, the sheets are passed through a grain breaker to assure the formation of a perfectly round cylinder by preventing the formation of flutes when the can body cylinder is fabricated at high speeds. This grain breaker consists of several rolls disposed in vertical relation. However, while these rolls have produced the desired grain breaking and curvature of the indivdual can body blanks, the flexing of the sheets has been relatively light, and there has been no accompanying increase in ductility of the sheets so mechanically treated.

In the steel industry, in order to remove: ripples from steel sheets, as well as other steel shapes, it is common practise to pass the steel through leveling rollers. As the steel passes through the leveling rollers, it is alternatingly flexed with a resultant flattening of the metal. Such roller levelers used today have only 5 to 8 pairs of alternately spaced rollers of approximately 1 /2 to 3 inches in diameter. However, the steel industry does not flex the steel enough to substantially exceed the yield strength except at initially non-flat portions since the objective is simply to flatten or straighten and not to change any of the physical or mechanical properties of the material.

The present invention proposes broadly to use the principle of roller leveling, but to flex to a much greater degree by the use of a machine of much different design wherein, if rollers are used as is possible in one form of machine, there are many times the number of alternately spaced rollers and the rollers are of very much smaller diameter than in the conventional commercial roller levelers today available. Thus the operation is not roller leveling but rather multiple reversed flexing. The present invention is therefore intended to provide for a large number of flexures of the cold rolled metal, as compared to the customary small number of flexures in roller leveling and, in each of the many successive alternate flexings of the metal, at least one surface layer thereof will be successively stretched and compressed substantially beyond the yield strength of the metal. By so doing, it has been found that not only is the metal somewhat strain softened which is normally not at all desired in roller leveling, but also, there is a completely unexpected but marked percentage increase in ductility, which latter effect is highly desirable in the can making industry and other industries wherein there is a sharp bending of the metal. The resultant combination of very high strength with increased ductility is far better than that which is otherwise obtainable, ie by a lesser degree of original cold rolling or by the same cold Working and partial annealing.

In accordance with the foregoing, it is another object of the invention to provide a novel method of mechanically working previously heavily cold rolled metal to increase the suppleness thereof by both softening the metal and by increasing the ductility thereof with an attainment of a better combination of strength and ductility than is otherwise obtainable, the method consisting of the flexing of the thin strip or sheet metal in a longitudinal direction and out of the usual plane of the metal a large number of times, and during each flexing of the sheet, stressing the outer layers of the sheet substantially beyond the initial yield strength of the metal of the sheet.

Another object of the present invention is to provide a novel and simple method of increasing the ductility of severely cold rolled steel which has not been given any process anneal, the method consist-ing of flexing hard and brittle cold rolled steel for the required number of cycles about radii so chosen that the surfaces of the steel are alternately stretched and compressed substantially beyond the initial yield strength of the steel but below the stress at which damage from fatigue would initiate micro fractures in the steel.

Still another object of the invention is to provide a novel method of increasing the transverse ductility of a cold rolled metal, as well as the longitudinal ductility thereof, by alternatingly longitudinally tensioning and compressing the metal a large number of cycles and during each tensioning and compressing of the metal, stressing at least the outer layers of the metal substantially beyond the yield strength of the metal.

A further object of the invention is to provide a novel method of obtaining the necessary suppleness of tin plate which has been severely cold rolled after electroplating with tin and which will permit the use thereof by the can making industry, the method being an extremely simple and economically feasible one, and consisting solely of passing the hard and brittle tin plate through a series of rollers wherein as the tin plate passes around the individual rollers, it is flexed and portions thereof are alternatingly placed in compression and in tension, the degree of bending of the plate being suflicient to stress at least the outer layers of the plate substantially beyond the yield strength of the metal during each flexure of the plate, the multiple flexing of the plate providing for strain softening of the metal with an accompanying and unexpected increase in ductility in the metal sheet, not only longitudinally of the sheet but also transversely of the sheet, which increased ductility is desirable for the making of cans and other fabricated articles.

Another object of the invention is to provide a novel method of restoring the ductility of heavily cold worked metal strip and sheets, which method constitutes the alternate flexing of the metal first to place portions of a metal in compression and then in tension, and with each flexure of the metal, at least the outer layers thereof being stressed substantially beyond the yield strength of the metal, with the results of this flexing of the metal resulting in an unexpected increase in the longitudinal ductility of the metal with the increase in ductility being to some degree inversely proportional to the original ductility of the metal wherein when the metal is relatively ductile prior to treatment, there may be little or no increase in ductility, and the increase in longitudinal ductility being accompanied by an also unexpected, but highly desirable, increase in transverse ductility of the metal, the increase in transverse ductility of the metal being entirely independent of the longitudinal ductility of the metal with a metal :having an initial high longitudinal ductility and a low transverse ductility having none or a relatively low percentage increase in longitudinal ductility but a very high percentage increase in transverse ductility, whereas metal having low ductility both longitudinally and transversely have high percentage increases in ductility both longitudinally and transversely.

With the above, and other objects in view that will hereinafter appear, the nature of the invention will be more clearly understood by reference to the following detailed description, the appended claims and the several views illustrated in the accompanying drawing.

In the drawing:

FIGURE 1 is a schematic view showing the general steps in producing the new type of hard, cold rolled sheet metal but particularly tin plate suitable for use by the can making industry in accordance with this invention.

FIGURE 2 is a fragmentary elevational view of an edge of the relatively thick metal strip stock prepared for the production of this new type of hard, cold rolled sheet metal, particularly tin plate.

FIGURE 3 is an enlarged fragmentary elevational view similar to FIGURE 2, and shows the strip stock of FIG- URE 2 with metal coatings on opposite surfaces thereof.

FIGURE 4 is an enlarged fragmentary elevational View similar to FIGURES 2 and 3, and shows the hard, cold rolled tin plate in accordance with the invention.

FIGURE 5 is an enlarged fragmentary schematic view showing the manner in which two strips of metal stock may be simultaneously worked upon in accordance with this invention.

In FIGURE 1 of the drawings, there is schematically illustrated a process for producing hard, cold rolled tin plate which is suitable for use by the can making industry, the plate being produced in accordance with the invention. The desired metal stock is provided in strip form, with the strip being referred to by the letter S in FIGURE 1. The strip S is relatively thick as compared to the final product, as is best illustrated in FIGURE 1. The annealed strip S, of thickness approximately twice that desired in the final product, is passed through an electroplating bath 10 after it has been properly cleaned in a customary manner, not shown, with the result that coatings C are applied to opposite faces of the strip S. The coatings C are also relatively thick as compared to those desired in the final product. The coated strip is best illustrated in FIGURE 3. The coated strip S is then passed through a series of cold reduction rollers 11, 12, 13 wherein the thickness of the strip S is reduced by 30 to 60 percent. It is to be understood that the cold reduction rollers will vary in number in accordance with the amount of reduction desired and the amount of pressure exerted during the rolling operation. At the end of the rolling process, the strip is in the form of the new type of hard, cold rolled tin plate P which is now commercially available from a number of steel mills, e.g.-from the US. Steel Corp. under the designation Ferrolitef The plate P is formed of a metal strip S which has been reduced to the desired thickness and with one or two metal coatings, as desired, also of the desired thickness, the relatively thick coatings C illustrated in FIGURE 3 having been proportionately reduced during the cold rolling of the coated strip S.

The 30% to 60% cold rolled plate, as it comes from the cold reduction rollers, is relatively hard and brittle and therefore is only suitable for limited field of usage by the can industry. However, it has very great economic advantages, were the brittleness relieved by some operation. For every base box, i.e. 31,360 square inches, which is coated with tin at a gauge for example of .012", one obtains after cold rolling 50% a total of two base boxes of tin coated steel .006 thick. The low cost of rolling is such that doubling the area of product is accomplished at a much lower cost than would otherwise be possible. The steel industry has recognized the lower cost per unit area of this hard rolled tin plate,'as compared to conventional tin plate, by pricing it at 55 per base box below that of conventional plate of the same gauge. Since the hard rolled plate is much stronger than conventional plate, one can use a thinner gauge of the hard rolled plate with a further increment of savings of materials costs amounting to 15 per base box for every reduction in gauge of 0.00055 inch. The potential savings to the can industry attainable by the use of hard rolled plate add up to many millions of dollars. However, the full utilization of such plate, and attainment of related savings in metal costs, depend on the improvement in ductility of such hard and brittle plate which may be obtained by the practice of this invention.

There is another equally or perhaps more important field of application of this invention. The hard rolled tin plate referred to thus far has started as hot rolled steel strip which is cold rolled by 80 to 90% to perhaps double the desired final gauge, then annealed, coated with tin and again cold rolled to final gauge. It is possible instead to cold roll hot rolled strip directly to final gauge in a 5 or 6 stand tandem rolling mill. The cold reduction of 80 to 95% results in extremely hard and strong steel strip which ordinarily is far too brittle to be used in the unannealed state. However, by partially restoring the ductility of such material with the practise of this invention, it could be employed for many purposes in the hard, unannealed state, either as is or with a subsequently applied tin coating or thin coating of other metal or material. The high strength of such extremely hard rolled steel makes possible further reductions in gauge of metal for many fields of utilization, e.g. closures for can ends. Not only are there substantial savings in amount of material but the elimination of annealing and of conventional temper rolling result in savings of process costs in the production of this hard rolled metal strip.

In accordance with this invention, either of these two types of hard rolled plate P or any other type of hard, brittle plate may further be worked upon as part of the continuous forming thereof in the steel mill, or the plate P may be coiled subsequent to the cold reduction operation thereof and later worked upon in accordance with the invention either at the steel mill or in the can makers plant. For purposes of convenience, the working of the hard and strong but brittle cold rolled plate P in accordance with the invention has been illustrated as being part of a continuous operation.

In FIGURE 1, the hard, cold rolled plate P is illustrated as being passed through a special assembly of rollers, generally referred to by the numeral 14. This roller assembly 14 may be formed of a plurality of rollers 15 which extend transversely of the direction of movement of the plate P and which are arranged in two sets, the axes of one set of doubly backed up rollers, 15a, 15c, 15e, 15g, etc., lying in a flat plane while the axes of the second set of backed-up rollers, 15b, 15d, 15 15h etc., lie in a plane which is shown to be curved convexly towards the other plane.

When the alternating working rollers in each set, 15a, 15b, 15c, 15d, etc., are of very small diameter, they may be backed up by somewhat larger rollers in order to prevent deflections of these small working rollers when practising the multiple reversed flexing of very wide strip, e.g. up to 38 inches in width. Dual back-up rollers may be required to prevent deflection of small rollers not only from the plane of the strip but forward in the direction of strip travel. Suitable means (not shown) will be provided for adjusting the lower set of rollers, 1512, 15d, 15 15h, etc. to move this set of intermeshed rollers towards or away from the first set of rollers or to tilt the lower set of rollers relative to the upper set of rollers in accordance with the gauge of metal being processed. Thus, in passing through the roller assembly 14-, the hard, cold rolled plate P first passes under and around one roller 15a, and then over and around the next adjacent roller 15b. The hard, cold rolled plate P thus passes along a sinusoidal path and is first flexed in one direction and then in the other with the degree of flexing first increasing to a maximum and then decreasing towards the exit. As a result, during the passage of the hard, cold rolled plate P through the roller assembly 14, a given surface element of the plate P is repeatedly stressed first in compression and then in tension as the plate P is flexed.

When a single thickness of hard cold rolled strip is passed through the roller assembly 14, the convex portion of the plate P at each of the rollers 1'5 is in tension and the concave portion is in compression, with the neutral axis of the plate P being unaffected. When the plate passes from the one roller to the next roller, the tension and the compression therein are relieved, and are then reversed as it passes around the next roller 15', with the result that that portion of the plate P which was previously in tension now is in compression and that portion which was previously in compression is now in tension. Once again, there is no stressing along the neutral axis.

The relationship of the adjacent rollers 15 and the diameter thereof is such as to produce more than a mere roller leveling of the plate P. As the hard, cold rolled plate P passes around each of the rollers 15, it must be stressed to the extent that at least that surface layer of the plate P in tension remote from the roller is stressed well beyond the yield strength of the metal of the hard rolled plate P. Further, in the working of hard, cold rolled tin plate having severely cold worked steel strip as the base thereof, the surface layer of the steel strip under compression is also stressed well beyond the yield strength of the metal of the plate P. Tests indicate that the metal should be stressed beyond its yield strength to a depth of from 10 percent to 40 percent of the thickness of the plate P inwardly from each stressed facing layer thereof. Of course, the deeper the penetration of working within the range specified, the fewer are the number of cycles of reversed stress required to obtain the desired increase in ductility, but flexing obviously must be stopped prior to the initiation of any fatigue damage.

The depth of working beyond the yield strength upon each cycle of flexing about a specific radius may be computed from the formula below:

c=distance from neutral axis (center of thickness) to point where metal is at yield stress; i.e. section stressed only elastically.

t=half-thickness of metal strip ay=yield strength of metal E=modulus of elasticity of metal r=radius of bending or flexure Thus taking as an example, a hard rolled mild steel .006" thick and having a yield strength of 100,000 p.s.i., an elastic modulus of 30,000,000 p.s.i. and using a radius of bending or fiexure of .40 inch,

Therefore, for the stipulated conditions, the depth of working or the depth to which the metal is stressed beyond its yield strength is 56% of the half thickness or 28% of the total thickness of strip from each surface. Since each reversed flexure of an element will reduce the yield strength thereof, it is not determinable what percentage of the thickness would be stressed on each successive reversed flexure. However from the equation, it is evident that when ay is decreased, the depth of working will increase. It may be partly for this reason that many cycles of flexure are effective and desirable, assuming that the number of cycles of flexure are so limited that no fatigue damage such as cracking, is encountered.

In this connection, it should be noted that excessive repeated reversed flexures at a stress below the ultimate tensile strength of a metal may cause it to break. This phenomenon is known as fatigue and in the case of steels, the maximum repeated stress it will withstand indefinitely, i.e. to more than 10,000,000 cycles of flexure where adjusted to obtain the desired effect, data have been pri marily obtained based on ten passes through the roller assembly wherein there are 85 complete cycles of flexure; Typical results obtained are found in the following table.

TYPICAL PROPERTY CHANGES RESULTING FROM MULTIPLE REVERSED FLEXING [Testing directions] Longitudinal Transverse Item M lb. tin plate Ultimate Yield Elong, Ultimate Yield Elong, Tensile Strength, Percent Tensile Strength, Percent Strength, 1,000 p.s.i. in 2 inches Strength, 1,000 psi. in 2 inches 1,000 p.s.i. 1,000 p.s.i.

A. Annealed, then cold rolled 50% to .0066 gauge; as received 80. 4 85. 2 1. 2 98. 6 93. 1 1 13. Same as A., reverse flexed 85 cycles about 7/16 radius in two thicknesses of strip 78. 5 70. 4 5. 5 92. 86.0 1. 2 C. Annealed and then cold rolled 30% to .0066 gauge; as received. 67. 3 G5. 4 3.0 80. 2 76. 4 1 D. Same as 0., reverse flexed 85 cycles about 716 radius in two thicknesses of strip 60. 3 52. 0 8. 0 70. 3 65. 2 2. 3 E. .0044 MRTU conventional tin plate; rolled to gauge, continuous annealed, temper rolled by 1% 61- 3 60. 9 13. 3 69. 69. 5 0.5 F. Same as 15., reverse flexed 85 cycles about 716 radius, single thickness stock 61. 8 58. 4 13. 5 67. 7 67. 7 1. 7

For any given number of cycles, e.g. 10 or 100 or 1000, there will be a stress below the ultimate tensile strength which will be sufficient to cause fracture after the specified number of cycles of stressing. For the same number of cycles of stress, there will be a lower stress at which no fracture occurs but at which micro cracks are formed which would ultimately cause fracture, The line of stress to cause micro cracks vs. the number of cycles to form these at each stress is called a damage curve.

It is requisite, in the practise of this invention, that the number of cycles of stressing the surface layers beyond their yield strength by flexure should be less than the number of cycles to cause damage at the stress caused by that flexure. Ductility is increased by this invention when the combination of number of cycles of flexure and maximum flexural stress are so related that no fatigue damage occurs.

Tests have been conducted utilizing a roller assembly 14 wherein there are nine rollers in an upper assembly and eight rollers in a lower assembly. Each of these rollers 15a, 15b, etc. is 78 inch in diameter and adjacent rollers in each assembly are spaced one inch apart on centers with the spacing between the planes of the two sets of rollers being adjustable. However, the short distance between rollers in a given plane assembly and the relatively large roll diameter limits the degree of intermeshing of the two sets and therefore the degree of flexing attainable in very thin gauges of metal. This is a limitation imposed by the equipment available since one would prefer equipment which permitted the intermeshing of rollers as idealized in FIGURE 1. Apparatus designed to practice this invention commercially would be more like that shown schematically in FIGURE 1 and would have enough spacing between rollers in each assembly to permit intermeshing and thereby permit a greater degree of flexing.

In the test apparatus, hard, cold rolled tin plate has been passed through the multiple roller assembly a number of passes with the strip receiving 8% cycles of flexure for each pass. It was found that the increase in ductility of the strip of hard, cold rolled tin plate in going from one to ten passes was approximately equal to the increase on going from ten to 100 passes. Thus, while in general there was a continuing benefit, the benefit corresponded to the logarithmic increase in number of passes. There is obviously a commercial end point to the number of cycles of reversed fiexure to be employed by this new processing method. While this will probably be less than 85 complete cycles of fiexure with the flexure radius From the above table, it will be seen that the metal sheets, after being subjected to multiple reversed flexures in accordance with the invention, showed a completely unexpected and relatively large percentage increase in elongation, both longitudinally and transversely, with only a relatively small decrease in strength, the decrease in strength values which are already very high being insufficient to be a consideration in the manufacture of cans. It is also to be noted from items E and F in the foregoing table, that when the metal is relatively ductile, as in the longitudinal direction, there is very little percentage increase in the ductility thereof. On the other hand, even though the ductility of the sheet in the longitudinal direction is relatively great, when it is relatively low in a transverse direction and the sheet is worked upon in accordance with the invention, the longitudinal ductility remains substantially unaffected, but a marked percentage improvement is obtained in the transverse ductility.

At this time it is pointed out that the 50% cold reduced and multiple reverse flexed sheet of item B of the above table is both stronger and more ductile than the sheet of item C of the table which is of the same thick ness as the sheet of item B but has been cold reduced only 30%. This can be directly attributed to the fact that the sheet of item B has been subjected to multiple reversed flexures whereas the sheet of item C has not. Thus the more severe cold rolling followed by flexing has resulted in a novel and superior combination of strength and ductility, unobtainable prior to this invention.

From a practical standpoint, the direction of multiple reversed flexing will always be in the same direction as the rolling direction of the cold rolled plate, since the process would be performed either as a continuation of the plate production process or on coils of the plate. However, tests have been run on short strips of 50% cold rolled plate in a transverse direction, with the result that there was a big increase in transverse ductility and only a smaller increase in longitudinal ductility, contrasted to a larger percentage increase in the longitudinal direction and a smaller percentage increase in the transverse direction as shown by comparing items B and D of the table with their respective unflexed strips items A and C.

Particular reference has been made hereinabove to hard cold rolled tin plate which is now being manufactured by several steel mills. Also, as stated above, the hard cold rolled plate generally may be a tin coated metal strip. However, the invention is not so limited, in that it may be used equally as well in conjunction with either severely worked or somewhat brittle plain steel strip,

severely worked or somewhat brittle steel strips coated with coating metals and materials other than tin, and with other severely worked or somewhat brittle metal strips having properties similar to the new type of hard, cold rolled tin plate; i.e. insufficient ductility for general fabrication procedures such as the production of cans.

For example, whereas the data given previously were for tin plate which had been cold rolled, annealed, electrolytically tin coated and then cold rolled 50% (items A and B) or 30% (items C and D), it has been found that this invention is equally applicable to uncoated steel, i.e., black plate, which has been cold rolled to a reduction of 80% or more and not annealed. It has been found that if such conventional black plate, in lieu of being annealed, is given multiple reversed flexures in accordance with the present invention, the much higher tensile strength and yield strength of this very severely cold rolled steel may be largely retained together with the ductility thereof being sufficiently restored so that metal may be bent without cracking and is suitable for use, for example, for making can "bodies and can ends which can be joined to one another by double seaming as is done in the can making industry. Typical examples of results obtained through the multiple reversed flexing of unannealed steel plate may be found in the following table:

TYPICAL PROPERTY CHANGES RESULTING FROM MUL- hEVERSED FLEXING OF UNANNEALED METAL Ultimate Yield Elongation, Item Tensile Strength, Percent in Strength, 1,000 p.s.i. 2 inches 1,000 p.s.i.

A. Black plate of conventional analysis, cold rolled 85% to .0095 and not annealed 133 118 1. 7 B. Same as A., reverse flexed 170 cycles about Me radiusrolls. 112 100 3. C. Black plate of conventional analysis, 50% cold reduced to .0006", not annealed 86 85 1. 0 D. Same as 0., reverse flexed 170 cycles about liflradiusrolls. 78 70 5.0 E. .012" MC-TG Standard rephosphorized strong steel for fiat top beer can ends (annealed, 1% cold rolled) 73 66 14. 0

1 All tests made of samples with specimen axis in the rolling direction.

From the foregoing table, it will be apparent that the diminution in tensile and yield strength occurring upon multiple reversed flexures of the hard rolled steel plate is very much less than that resulting from the usual annealing followed by only temper rolling 1% thereof. The reversed flexing of the steel plate is a very simple operation as compared to annealing, with a resultant saving in cost of producing the metal stock and at the same time maintaining very much higher strengths. These higher strengths, obtained with sufficiently improved ductility to permit fabrication, permit the use of thinner metal with a further apprecable reduction in cost of manufacture of cans and other fabricated articles.

The improvement in ductility obtained by following the practise of this invention has been shown most dramatically not by tensile property measurements but rather by actual fabrication of metal so processed. For example, metal can end covers have been stamped out of the .0095 thick hard rolled black plate steel whose properties are given in the foregoing table. When the material Was in the as-rolled state of item A, each end cover as stamped out of the sheet, prior to any further fabrication, showed complete fracture of the metal where it was bent and stretched in a transverse direction, i.e. perpendicular to the rolling direction. However, the same material after flexing, item B, was fabricated into satisfactory can end covers and Was free of such fractures even at positions of tensile strain in the transverse direction. Not only could end cover units be stamped out of such multiple flexed sheet but it was possible to give such covers the more severe fabrication of double seaming to a flanged can body and obtain a particularly strong tightly seamed can end.

In the production of :tin or black plate, following the present invention, the hard rolled steel plate would be first multiple reverse flexed to increase the ductility thereof and then, if desired, it could be tin-plated by the usual electroplating or hot tinning methods. The use of such ultra strong but fabricable steel, as compared to that conventionally used by the can industry, will permit use of thinner steel, whether tinned or un-tinned. For example a 10 or 15% reduction in thickness of steel for can ends by use of multiple reverse flexed, hard rolled steel, permits tremendous saving in the quantity of steel used by the can industry. Not only is there this saving but the instant invention saves the time and process costs of annealing, which costs far-exceed those incurred by the multiple reversed flexure process here employed to make such hard 'rolled steel capable of being fabricated into containers and other articles.

Reference is now made to FIGURE 5, wherein it is shown that two strips or sheets of hard, cold rolled plates P may be Worked upon simultaneously in accordance with the invention. The plates P are illustrated as being separated by a spring steel sheet or strip 16, which spring steel strip 16 has the surfaces thereof roughened, preferably by sand blasting, to minimize slippage. As the two hard rolled plates P are passed through the roller assembly 14 with the spring steel strip 16, the strip of spring steel simulating the center section of a single strip being reverse flexed, the neutral axis of the assembly remains in the spring steel strip 16, with the result that as the assembly passes around one of the rollers 15, one of the hard rolled plates P Will be completely in tension throughout its thickness and the other will be completely in compression. However, in this case, fewer passes through the multiple reversed flexer are necessary. The tension and compression strains are graduated outwardly from the spring steel strip 16, and the outer surface layer of each of the hard rolled plates P is again stressed beyond the yield strength of the metal of the hard rolled plate P but to a much greater depth than when a single thickness of strip is processed.

It is to be noted that as the assembly of FIGURE 5 passes from one roller, e.g. 152, towards the next adjacent roller 15 of the roller assembly 14, the strain in the two hard rolled plates P will diminish until the strain reaches zero, after which time it will be reversed, and as the assembly passes around the next adjacent roller 15 the previously tensioned hard rolled plate P will be placed in compression and the previously compressed hard rolled plate P will be tensioned. Thus, when the two plates P are passed through the roller assembly 14 as an assembly with the spring steel strip 16, each of the hard rolled plates P will be alternately stretched and compressed as the hard rolled plates P are flexed about the rollers 15.. The magnitude of decreases in strength and improvements in'ductility obtained with the arrangement of FIGURE 5 were similar to those wherein a single hard rolled plate P is passed through the roller assembly 14. However, in this case fewer passes through the device, i.e. fewer total cycles of reversed flexure, are necessary because the alternate tensioning and compressing are effective to a greater depth, for the same radius of flexure. This is evident from the equation given previously, i.e.,

A l Depth of Working- 1 E't Since doubling the apparent thickness of strip increases the numerical value of t in this equation, the depth of Working is relatedly also increased. To take maximum advantage of this principle, slippage between the two outer strips being reverse flexed and the inner strip which contains the neutral axis of flexure, should be eliminated for example by sand blast roughening of the inner strip or by tack welding of the edges or other equivalent effective means.

At this time, it should be pointed out that although in FIGURE 5, a spring steel strip 16 has been provided to assure that the neutral axis will exist outside of the two strips P being flexed, this strip 16 can be omitted. Again, it would be desirable to prevent slippage at the two abutting faces of the two strips by use of tack welding or equivalent effective means.

In this case, all of the plastic working of the two strips will occur inward from their outer surfaces since the inner abutting surfaces would constitute the neutral axis of flexure. However this has by experiment been found to result in the desired improvement in ductility and in elimination of fractures during fabrication of such multiple flexed, hard rolled steel.

Not only is this process applicable to hard rolled steel, with or without tin coatings, other thin metallic coatings or with organic coatings, but it is also fully applicable to non-ferrous metals which have been so severely cold rolled that their ductility and associated fabrication characteristics are seriously impaired. Thus, the multiple reversed flexing process may be applied to hard rolled aluminum and alloys thereof, to hard rolled copper and alloys thereof, to hard rolled magnesium and alloys thereof and in general, to metals suffering from impaired ductility.

The advantageous results obtained with the reversed stress in the metal either longitudinally of the direction of rolling or transversely of the direction of reduction rolling have been specifically set forth hereinabove. However, the invention is not to be considered as limited to the reversed stressing of the metal in a single direction, either longitudinally or transversely of the direction of reduction rolling, in that highly desirable results are obtainable when the reversed stressing is alternatively applied to the metal in the direction of rolling and transverse thereto.

From the foregoing, it will be seen that novel and ad vantageous provision has been made for carrying out the desired end. However, attention is again directed to the fact that variations may be made in the example method and apparatus disclosed herein without departing from the spirit and scope of the invention, as defined in the appended claims.

I claim:

1. A method of increasing the ability of work hardened rolled metal strip and sheets cut therefrom to withstand fabrication with a reduction in brittleness of the metal and an increase in the ductility thereof, the method comprising the steps of first stressing the metal in a manner wherein the metal along the surface layer of the first side thereof is in tension and simultaneously the metal along the surface layer of the second side thereof is in compression with the metal along at least one surface layer thereof being stressed beyond the yield strength of the metal, then stressing the metal in a reverse manner wherein the metal along said first surface layer is in compression and the metal along the said second surface layer is in tension with the metal along at least one surface layer thereof being stressed beyond the yield strength of the metal and repeating the reversed stressing steps at least 25 times until the desired fabrication properties are obtained, the Work hardened rolled metal reversibly stressed beyond its yield strength extending inwardly to a depth ranging from 10% to 40% of the thickness of the rolled metal inwardly from each surface layer.

2. A method of increasing the ability of a metal work hardened through the cold rolling reduction thereof to withstand fabrication wherein the increase in fabrication properties is obtained with a better than normal combination of strength and ductility, the method comprising the steps of moving the metal in the direction of rolling,

and passing the hard rolled metal between a series of intermeshed rollers in two rows and in linear alternating relationship in the direction of sheet movement with the sheet first passing over a roller in one of said rows and then under a roller in the other of said rows to alternatingly and reversibly stress said metal, the rollers having such a diameter that said metal has at least one surface layer thereof stressed substantially beyond the yield strength of the metal as the sheet is alternatingly passed over and under the rollers, the metal reversely stressed beyond its yield point extending inwardly of the sheet to a depth ranging from 10% to 40% of the thickness of the sheet inwardly from each surface layer.

3. A method of increasing the ability of a metal work hardened through the cold rolling reduction thereof to withstand fabrication wherein the increase in fabrication properties is obtained with a better than normal combination of strength and ductility, the method comprising the steps of moving the metal in the direction of rolling and passing the hard rolled metal between a series of intermeshed rollers in two rows and in linear alternating relationship in the direction of sheet movement with the sheet first passing over a roller in one of said rows and then under a roller in the other of said rows to alternatingly and reversibly stress said metal, the rollers having such a diameter that said metal has at least one surface layer thereof stress beyond the yield strength of the metal as the sheet is alternatingly passed over and under the rollers, the hard rolled metal being passed between the series of intermeshed rollers together with another hard rolled metal in face-to-face relation thereto and preventing relative movement between the metals at least during the passage thereof through the rollers, each metal being thus first placed in tension and the second of the hard rolled metals in compression, then alternately plac ing the first of the hard rolled metals in compression and the second of the hard rolled metals in tension so that each hard rolled metal is stressed substantially beyond the yield strength of the metal.

4. A method of simultaneously increasing the ability of two hard rolled metals previously work hardened through the cold rolling reduction thereof to withstand fabrication wherein the increase in fabrication properties is obtained with a better than normal combination of strength and ductility, the method comprising the steps of providing a separator metal, assembling the two hard rolled metals on opposite sides of the separator metal in face-to-face engagement therewith, passing the assembled metals along a sinusoidal path, and preventing slippage between the hard rolled metals and the separator metal at least during the passage thereof along the sinusoidal path to first place the first of the hard rolled metals in tension and the second of the hard rolled metals in compression, then alternately placing the first of the hard rolled metals in compression and the second of the hard rolled metals in tension so that each hand rolled metal is stressed substantially beyond the yield strength, and repeating the steps.

5. The method of claim 4 wherein said separator metal first has the faces thereof roughened to form an interlock with the hard rolled metals.

6. The method of claim 4 wherein during the placing of the hard rolled metals in tension and compression, the neutral axis of the assembled metals remains in the separator metal so that when the hard rolled metals are alternatingly placed in tension and compression, said hard rolled metals will be completely in tension and compression, respectively.

7. The method of claim 4 wherein the separator metal is thinner than the hard rolled metals and during the placing of the hard rolled metals in tension and compression, the neutral axis of the assembled metals remains in the separator metal so that when the hard rolled metals are alternatingly placed in tension and compression, said hard rolled metals will be completely in tension and compression, respectively.

8. A method of increasing the ability of a metal sheet hardened through the cold rolling reduction thereof to withstand fabrication wherein the increase in fabrication properties is obtained with a better than normal combination of strength and ductility, the method comprising the steps of first progressively longitudinally stressing a full depth transverse area of the sheet to be substantially all in tension, then immediately thereafter longitudinally stressing the same transverse area to be substantially all in compression, and repeating the stressing steps.

9. The method of claim 8 wherein during each reversed stressing of the sheet one surface layer thereof is stressed substantially beyond the yield strength of the metal.

10. A method of forming a supple light gauge metal coated metal sheet having the ability to withstand fabrication, the method comprising the steps of providing a heavier gauge metal sheet, applying a relatively heavy metal coating on the metal sheet, rolling the heavier gauge coated metal sheet to reduce simultaneously the thickness of the heavier gauge metal sheet to the desired light gauge and the thickness of the metal coating during which rolling the strength of the metal of the metal sheet is increased and the metal undesirably work hardened, and then repeatedly alternatingly stressing the light gauge coated metal sheet placing the metal surface of the first side of the metal sheet in tension and simultaneously the metal surface of the second side of the metal sheet in compression with the metal along at least one surface thereof being stressed substantially beyond the yield strength of the metal, repeating the steps in order to make the light gauge, excessively work hardened, metal-coated metal sheet more ductile.

11. A method of forming a supple light gauge metal sheet having the ability to withstand fabrication, the method comprising the steps of severely cold rolling in a heavier gauge metal to the desired light gauge during which cold rolling to a reduction in excess of 80%, the strength of the met-a1 is increased and the metal is undesirably work hardened, and then repeatedly alternatingly stressing the light gauge metal sheet placing the metal surface of the first side of the metal sheet in tension and simultaneously the metal surface of the second side of the sheet in compression with the metal along at least one surface thereof being stressed substantially beyond the yield strength of the metal, repeating the steps in order to make the light gauge excessively work hardened sheet more ductile and then applying a thin metal coating to at least one surface of the light gauge metal.

12. A method of simultaneously increasing the ability of two hard rolled metal sheets previously work hardened through the cold rolling reduction thereof to withstand fabrication wherein the increase in fabrication properties is obtained with a better than normal combination of strength and ductility, the method comprising the steps of temporarily securing together the metal sheets in overlying relation, passing the assembled metal sheets between a series of intermeshed rollers arranged in two rows and in linear alternating relationship in the direction of composite sheet movement with the secured together sheets first passing over a roller in one of said rows and then under a roller in the other of said rows to first place a first one of the hard rolled metal sheets in tension and a second of the hard rolled metal sheets in compression, and then under a P011631 in the other of said rows of rollers to place the first of the hard rolled metal sheets in compression and the second of the hard rolled meta] sheets in tension so that each hard rolled metal sheet is stressed beyond the yield strength thereof, and then continuing to pass the secured sheets over and under the rollers of the two sets of rollers to thereby repeat the alternating tensioning and compressing thereof.

13. A method of increasing the ability of work hardened rolled metal strips and sheets cut therefrom to withstand fabrication with a reduction in the brittleness of the metal and an increase in the ductility thereof, the method comprising the steps of first stressing hard, brittle, cold worked metal in a manner wherein the metal along the surface layer of the first side thereof is in tension and simultaneously the metal along the surface layer of the second side thereof is in compression with the metal along at least one surface layer thereof being stressed beyond the yield strength of the metal, then stressing the metal in a reverse manner wherein the metal along said first surface layer is in compression and the metal along the said second surface layer is in tension with the metal along at least one surface layer thereof being stressed beyond the yield strength of the metal and repeating the reversed stressing steps at least twenty-five times until the hard brittle cold worked metal becomes ductile, the metal re versely stressed beyond this yield strength extending inwardly to a depth ranging from 10% to 40% of the thickness of the metal inwardly from each surface layer.

References Cited by the Examiner UNITED STATES PATENTS 234,193 11/1880 Mather 29--547 r 1,126,482 1/1915 Kinnear 2919 1,271,703 7/1918 Guibert 29546 1,584,499 5/1926 Zachhuber 153-86 1,649,705 11/1927 Kelley 15386 X 2,040,442 5/ 1936 Nieman.

2,060,400 11/1936 Nieman 153-86 X 2,347,904 5/1944 Greulich 29547 X 2,588,439 3/1952 Ward 153-86 X 2,593,460 4/1952 Johnson -60.7 3,031,750 5/1962 Von Planck 29-547 3,117,036 1/1964 Cleland et al 29547 X OTHER REFERENCES Metals Handbook, 1948 ed., A.S.M., p. 241.

The Iron Age, Metals Can Be Softened by Cold Work, Polakowski, Aug. 28, 1952, pp. 106-111.

JOHN F. CAMPBELL, Primary Examiner.

WHITMORE A. WILTZ, Examiner.

E. H. MARTIN, J. C. HOLMAN, Assistant Examiners.

Claims

1. A METHOD OF INCREASING THE ABILITY OF WORK HARDENED ROLLED METAL STRIP AND SHEETS CUT THEREFROM TO WITHSTAND FABRICATION WITH A REDUCTION IN BRITTLENESS OF THE METAL AND AN INCREASE IN THE DUCTILITY THEREOF, THE METHOD COMPRISING THE STEPS OF FIRST STRESSING THE METAL IN A MANNER WHEREIN THE METAL ALONG THE SURFACE LAYER OF THE FIRST SIDE THEREOF IS IN TENSION AND SIMULTANEOUSLY THE METAL ALONG THE SURFACE LAYER OF THE SECOND SIDE THEREOF IS IN COMPRESSION WITH THE METAL ALONG AT LEAST ONE SURFACE LAYER THEREOF BEING STRESSED BEYOND THE YIELD STRENGTH OF THE METAL, THEN STRESSING THE METAL IN A REVERSE MANNER WHEREIN THE METAL ALONG THE SAID SECOND SURFACE LAYER IS IN AND THE METAL ALONG THE SAID SECOND SURFACE LAYER IS IN TENSION WITH THE METAL ALONG AT LEAST ONE SURFACE LAYER THEREOF BEING STRESSED BEYOND THE YIELD STRENGTH OF THE METAL AND REPEATING THE REVERSED STRESSING STEPS AT LEAST 25 TIMES UNTIL THE DESIRED FABRICATION PROPERTIES ARE OBTAINED, THE WORK HARDENED ROLLED METAL REVERSIBLY STRESSED BEYOND ITS YIELD STRENGTH EXTENDING INWARDLY TO A DEPTH RANGING FROM 10% TO 40% OF THE THICKNESS OF THE ROLLED METAL INWARDLY FROM EACH SURFACE LAYER.