US2060400A - Method of and apparatus for treating sheet metal - Google Patents

Description

Nov. 10, 1936. H. w. NIEMAN 0 METHOD OF AND APPARATUS FOR TREATING SHEET METAL Filed Nov. 30, 1932 2 Sheets-Sheet l /2 O I j I l5 l5 /7 Z METHOD OF AND APPARATUS FOR TREATING SHEET METAL 2 Sheets-Shet 2 Nov. 10, 1936. H. w. NIEMAN Filed Nov. 30, 1932 gwuemtoz Patented Nov. 10, 1936 PATENT OFFICE METHOD OF AND APPARATUS FOR TREATING SHEET METAL Henry W. Nieman, Bethlehem,Pa., assignor to Bethlehem Steel Company, a corporation of Pennsylvania Application November 30, 1932, Serial No. 644,971

13 Claims.

The present invention relates to methods of and apparatus for conditioning mild steel sheets, strips, and analogous thin fiat articles for subsequent fabrication into articles of manufacture.

A large quantity of steel sheets or strips having a carbon composition in the neighborhood of {6% are rolled in the manufacture of tin plate, automobile sheets and the like, the reduction to final gauge being made either by hot or cold rolling. In either case the metal as it leaves the rolls is in a hard and unworkable condition and usually must be given an anneal. This renders the metal soft and ductile but at the same time introduces an undesirable quality, a tendency to flute or form stretcher strains upon subsequent fabrication.

Fluting and stretcher straining of annealed low-carbon sheet material may be designated by its peculiar and very undesirable property of yielding locally instead of over a broad area when deformed beyond its yield point. A mild steel sheet which bends sharply at one point instead of curving uniformly over a broader area when subjected to flexing forces is said to flute. A mild steel sheet, the surface of which apparently cracks when the sheet is subjected to a tension beyond its elastic limit, is said to stretcher strain."

When a sheet of such material is elongated in a testing machine, no permanent set takes place until the very definite yield point is reached.

Then a series of apparent cracks develops as the elongation continues, and these cracks, which are known as .stretcher strains, broaden out ,until they finally meet and cover the entire area, which then has a matte appearance quite different from the originally shiny appearance of the sheet. As the elongation is continued from this.

point on, the tension increases gradually, until the specimen necks out, and breakage finally results. If the test is halted when the stretcher strains have covered only a portion of the surface, it is found that the material, as measured in a strained area, is thinner than the original sheet. The phenomena of stretcher straining is particularly observed in the fabrication of automobile bodies and similar objects where the stretching of certain parts of the sheet by the dies is sufiicient to cause stretcher strains but not sufficient to cause these to coalesce into a relatively smooth surface. The resulting pattern is, of course, highly undesirable.

Various methods have heretofore been employed for the removal of the tendency to strain and flute from a mild steel annealed sheet. Thus,

it has been common to give the sheet, after annealing, a light cold rolling or planishing. While this method of eliminating the tendency of the sheet to flute upon fabrication is, in most instances when utilized, capable of producing the desired results, it is not always a practical or desirable method in that the cold rolling operation must be carried out very carefully to avoid warping the sheet or hardening it unduly and also by reason of the fact that a new surface is imparted to the sheet by. the rolls, which is often undesirable, for instance where the sheet has been previously tin-coated.

Again, it is known that mild steel annealed sheets may have their qualities improved, insofar as fiuting and stretcher straining are concerned, by passing them through roller levelers. It has been found, however, that the use of a roller leveler is not possible in connection with certain strips or sheets because of the very pronounced tendency of the sheets to flute or wrinkle in the leveler, and, moreover, with other steels which do not actually wrinkle there frequently are formed thereon multitudes of fine parallel'lines. These may be too fine for measurement but still deface the surface.

In accordance with tlfe present invention, I provide an improved method of and apparatus for treating annealed sheets, strips, or other fiat shapes of mild steel in such manner that the 30 tendency of such sheets to flute or form stretcher strains is entirely eliminated. I have discovered that a roller leveling process, which at the ordinary operating speeds of such pieces of apparatus would cause the sheets to flute, may be carried out at higher speeds without this occurring and that the final sheets are not only fiat and smooth but practically free fromall tendency to flute in subsequent fabrication. This process may be carried out in an ordinary roller leveler run at higher speed or in a special machine designed for this high speed undulation, the invention not being limited in this respect.

In carrying out the invention by undulating a sheet at high speed, I may cause an annealed sheet to be drawn through a roller leveler of ordinary and well-known type at a minimum speed in the neighborhood of one thousand feet per minute. At lower speeds of travel through the roller leveler, the same sheet will either flute 50 or have its surface impaired by the formation of multitudes of fine lines or striations. At the speed specified or at higher speeds, however,

I have discovered that all tendency of the sheet to flute or form stretcher strains in subsequent rolling operations is wholly eliminated and without defacing the sheet in the eliminating operation.

In the accompanying drawings, two forms of apparatus which I may employ are illustrated diagrammatically, the specific details of this apparatus being a matter of relatively minor importance.

In the drawings:

Figure 1 is a diagrammatic illustration of a roller leveler of the ordinary type and suitable for the practice of the process involving the undulation of the sheet at high speed;

Figure 2 is a diagrammatic illustration of a roller leveler of special type for the practice of the process involving the movement of the sheet through the leveler at high speeds;

Figure 3 is a diagrammatic illustration of a drum or reel from which the sheet wound thereon may be unwound at high speed; and

Figures 4 and 5 are diagrammatic illustrations of processes of producing tin plate using the principles of this invention.

The roller leveler machine shown in Figure 1 is of ordinary type, well-known in the steel industry, and it probably need only be said that the machine comprises a plurality of rollers 25, 25 disposed in sets in two parallel planes, the rollers of the two sets alternating with each other and the axis of the rollers being so disposed and their diameters so determined that any sheet 26 which is drawn through the roller leveler is given an undulatory movement, first in one direction and then in the opposite direc tion. While the use of machines of this type has been heretofore resorted to for the purpose of eliminating the tendency to' flute or form stretcher strains in fabrication which is possessed by mild annealed steel sheets, the satisfactory application of the process has been confined to materials which have this tendency to only a mild degree for if it is attempted with materials in which the tendency is very marked the process will develop flutes and thus give the surface a wrinkled appearance. According to invention, however, if the process is carried out at sufficiently high speed no such injurious effects take place.

The speeds necessary for this purpose depend on the quality and thickness of the sheets being treated. Thick hot roll sheets of large grain are the easiest to treat, in fact may even be treated in the usual way in the ordinary roller leveler, while thin fine-grained sheets produced by the cold rolling process are more diflicult, especially those having a high phosphorus content. In the case of material of tin plate gauge produced by cold rolling and then annealed, a speed of one thousand feet per minute has been found very satisfactory, whereas at lower speed, say of several hundred feet a minute, a fluting or wrinkling of the surface takes place. It may be found desirable, in order to still further increase the pliability of the sheet, to pass it through such an apparatus once and then repeat with the grain disposed at right angles to the original direction. The usual tendency in treating sheet material is to carry the process out thoroughly enough to render the material entirely smooth. but this may not always be necessary. Certain uses may not require such thorough treatment. A slight treatment may also be sufficient as one step in the process of manufacturing sheets. i

In the form of the apparatus shown in Figure 2, a pair of endless belts l0 and II are disposed minute. lustrated in Figure 3 of the drawings, where 27 over a series of rollers of varying diameter, some of which are backed up by backing rollers to prevent deflection. As a sheet of material I2 is introduced between the belts at the left-hand end of the machine, it is carried between the belts first over the rollers l3, which may be three inches in diameter, then over rollers l4, l5, l6, and I! of decreasing diameter, and finally over rollers l8, I9, 20, 2|, 22, 22, and 24 of increasing diameter, to the point of final discharge. In this machine the first five rollers serve to give the sheet the cold undulation for the removal of the tendency to flute or form stretcher strains, while the last six serve to straighten it out by the ordinary roller leveler action. The function of the belts employed is to' enable short sheets to be treated up to their extreme ends and also, in the case of tin or highly finished sheets,'to prevent damage to the surface. The belts may be omitted when continuous strips are 'to be treated.

In considering the problem of adapting the present invention to sheets of various kinds, the various elements which have bearing should be carefully considered, such for instance as the temperature at which the operation is carried out, the radius of curvature of the rollers of the machine and the number of such rollers, the speed of the operation, the composition of the metal, and the thickness of the metal.

Thus I have found that in the case of the treatment, carried out at room temperature, of an annealed sheet having a carbon content of .1%, a manganese content of 45%, a phosphorus content of 15%, and a thickness of .011 inch, the fiuting tendency was completely removed by passing the sheet through a roller leveler at a speed of approximately one thousand feet per minute. In this instance the machine had only five rollers 1 which gradually decreased in diameter, the first roller having a diameter of three inches and the last roller of one and one-quarter inches. In each individual case a certain amount of trial and experiment is necessary to achieve the best results.

I have also found that certain sheets may be conditioned, at least improvedin condition, by simply unwinding them from reels at high speed. Thus a mild steel annealed sheet which cannot be unwound from its reel at low speed'without objectionable fluting can be unwound without fiuting when removed at high speed, say in the neighborhood of one thousand linear feet per This operation is diagrammatically ilrepresents a reel from which strip material 29 is being unwound onto reel 28.

While the method which comprises the present invention is of general utility and may be made use of wherever it is desired to condition mild steel annealed sheets for subsequent fabrication, it is of special utility when employed in connection with processes of sheet manufacture involving tinning or galvanizing of the surfaces of the sheet. I have found that sheets which have been cold rolled to finished gauge, annealed, pickled, and tinned may be satisfactorily treated by one or the other of the before described cold undulation processes without injury to the sheet or defacing its surface, with the result that the tendency'to flute is removed without interfering with its excellent deep drawing properties.

Heretofore it has been thought possible to use the roller leveler to improve the physical properties of tinned sheets only where the sheets have been reduced to finished gauge by hot rolling processes. By my'improved method, however, I am enabled to materially improve the physical properties of drastically cold rolled and tinned sheets, so far as the tendency to flute or form stretcher strains is concerned, without impairment of the surfaces thereof. I have also found that, in many cases where tinned sheets are to be conditioned, a relatively mild undulatory treatment is suflicient to give the sheets the desired physical properties, as for instance by passing the sheet over two rollers only, at the stated speed, the sheet being thereby rapidly undulated first in one direction and then in the other.

As before pointed out, no satisfactory explanation has up until this time been advanced, making clear the reasons why mild steel sheets which have been annealed, and particularly those which have been produced by cold rolling, tend to flute or form stretcher strains in subsequent fabrication into manufactured articles. Neither is it possible at this time to advance a complete and satisfactory explanation of the theory underlying the processes of cold undulation above described and whereby such tendencies of the sheets are removed. It is clear, however, that treatment of a mild steel annealed sheet in accordance with the processes above outlined has the effect upon the material of lowering the yield point very materially and rendering it possible to flex or stretch the sheet after such treatment without causing fluting or stretcher strains. The invention is, therefore, of great importance to those engaged in the manufacture of sheets, strips, and the like. It can be demonstrated, that a piece of sheet metal which will flute badly if curved around a roller slowly may be bent around the same roller entirely without fluting if the bending operation is carried out with great speed. It can also be demonstrated that a sheet which will flute badly when drawn through a roller leveler at low speed can be drawn through the same leveler at very high speed without fluting. Hence it may be concluded that, in the treatment of sheet metal, the time element has an effect which may be taken advantage of in the conditioning treatment, and is taken advantage of when the sheets are drawn through a roller leveler, or are otherwise worked, at a relatively high speed.

The method of producing tinplate utilizing the above described principles may consist of the following steps: (1) Reduction to gauge by drastic cold rolling, (2) annealing, (3) tinning and (4) treating the tinned sheets to render the metal smooth working, that is free from the tendency to flute and stretcher strain. By this process a sheet is produced with qualities superior to that produced by any previous means and moreover the fewness of the steps involved makes the process exceptionally low in cost. By drastic cold rolling is meant a cold rolling process in which a substantial portion of the reduction is performed by cold rolls, as distinguished from processes where only a light cold rolling is employed to flatten the sheets or give them a smooth surface. In the latter case the actual reduction of gauge is only a few percent, whereas in the former the reduction is generally 30% or more. Such drastic cold rolling causes a thorough breaking up of the original grains into fibers, with the result that annealing can be carried out at a low temperature if desired, the grains finally resulting are small and equiaxed, and the product has a high ductility and freedom from directional properties. Furthermore, the sheet as it leaves the annealing furnace is as a rule very flat and has the highly polished surface due to the cold rolling. After the next step, which is tinning, the sheet is in appearance and ductility all that could be desired, but it has so strong a tendency to flute and stretcher strain that it is in' general unmarketable, until after it has received the treatment of the last step, which renders it smooth working. It is well recognized that the reduction of metal by drastic cold rolling produces 'a final product which is much superior to the ordinary hot rolled product in that it is fine grained and inherently ductile, but its strong tendency to fiuting and stretcher strains has in general made it necessary to meet the difficulty, to introduce a step of light cold rolling between the annealing and tinning. This,

series of operations, however, cannot produce the best results, since if the light cold rolling is of a very mild character, the sheet, although smooth working after this cold rolling, will set or age during the few seconds it is at the temperature of the tin pot, and thereafter show a tendency to fiuting and stretcher strains practically as marked as before the light cold rolling. If, on the other hand, the light cold rolling is sufficiently severe so that the metal will not revert during its passage through the tin pot, the ductility will be substantially reduced and a strong tendency will be introduced to fail by breaking along the direction of rolling. With my process, however, since the treatment which removes the tendency to flute and stretcher strain follows the tin pot, and the material is not subsequently subjected to heat, the good effects of the treatment will persist permanently, except as it will age very -'slowly, during the course of months or years even at room temperature.

Figs. 4 and 5 show in diagrammatic form processes of producing tin plate, following the principles of this invention. In Fig. 4 the hot rolled blanks are reduced in either one or many .and give the finished plate as shown at 36. The

steps of slitting, shearing, sorting, etc. are not represented in the diagram, but it will be understood that they may form steps of the process. In Fig. 5 the process is identical with that of Fig. 4, the steps being similarly numbered, with the additional step shown at 31 which is a-special treatment to remove wholly or in part the tendency to flute before the material enters the tin pot. This step may be desirable where the material has such a strong tendency to flute that it may form wrinkles in the tin pot. Such treatment may consist of the ordinary pinch pass through cold rolls, undulation as above described,

or any other treatment which will accomplish the The term normal surface" as smooth working means freedom from the claims means a speed which is substantially greater than speeds employed prior to my present invention to straighten cold or hot rolled steel sheets and/or in the treatment of 'hot rolled steel sheets to eliminate their tendency to flute or stretcher strain.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

-l. The method of conditioning a sheet reduced to gauge by drastic cold rolling which flutes readily, consisting of passing the same through a roller leveler at a speed of around one thousand feet per minute.

2. The method of conditioning a sheet reduced to gauge by drastic cold rolling which flutes readily, consisting of rapidly flexing the same about unidirectional axes and thereafter flexing the same rapidly about unidirectional axes disposed at an angle to the first mentioned axes.

3. The method of conditioning a sheet reduced to gauge by drastic cold rolling, which flutes readily, consisting of passing the same through a series of bending operations at high speed to produce a smooth sheet of substantially non-flut ing character.

4. The method of conditioning a mild steel annealed sheet for subsequent fabrication which comprises rapidly subjecting each portion thereof to repeated flexures, the radius of curvature of successive flexures gradually decreasing, and subsequently subjecting each portion of said sheet to repeated flexures, the radius of curvature of successive flexures gradually increasing.

5. A machine for conditioning a mild steel sheet for subsequent fabrication, comprising a series of parallel rollers adapted to act successively on opposite sides of said sheet, the successive rollers of said series decreasing in diameter from the entering and of said machine to an intermediate point and thereafter increasin in diameter toward the discharge end of said machine.

6. A method of producing tin plate comprising the following steps: reducing a blank by drastic cold working, annealing, tinning and subsequently roller leveling at high speed to produce a non-fluting product, of normal surface.

'7. A method of producing tin plate comprising the following steps: reducing a blank by drastic cold working, annealing, tinning and subjecting to a succession of bends at high speed in opposite directions of increasing severity.

8. A method of producing tin plate comprising the following steps: reducing a blank by drastic cold working, annealing, tinning and subjecting to a succession of bends in opposite directions of increasing severity followed by a succession of bends in opposite'directions of decreasing severity." l

9. A method of producing tin plate comprising the following steps: reducing a blank by drastic cold working, annealing, tinning and then subjecting to a succession of bends in opposite directions of increasing severity such bending being carried out at high speed to avoid fluting.

10. A method of producing tin plate comprising the following steps: reducing a blank by drastic cold working, annealing, tinning and subjecting to a succession of bends in opposite directions of increasing severity such bending being carried out at a speed sufliciently high to avoid fluting, followed by a succession of bends in opposite directions of decreasing severity.

11. The method of bending a sheet made by drastic cold working which flutes readily at ordinary bending speeds which consists in bending the same at high speed to avoid fluting.

, 12. The method of unwinding coiled material made by drastic cold working which flutes readily when unwound at ordinary speeds which consists in unwinding the same at high speed to avoid fluting.

13. The method of conditioning a tin-covered mild steel sheet reduced to gauge by drastic cold rolling which flutes readily comprising reversely flexing each portion thereof at high speed.

HENRY W. NIEMAN.

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