US3161225A

US3161225A - Method for obtaining flat and stress-free magnetic strip

Info

Publication number: US3161225A
Application number: US163283A
Authority: US
Inventors: Chester E Ward; Victor W Carpenter
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1961-12-29
Filing date: 1961-12-29
Publication date: 1964-12-15
Anticipated expiration: 1981-12-15
Also published as: DE1275561B; SE317694B; BE625879A; GB951749A

Description

Dec. 15, 1964 c. E. WARD ETAL 3,161,225

METHOD FOR OBTAINING FLAT AND STRESS-FREE MAGNETIC STRIP Filed Dec. 29, 1961 2 Sheets-Sheet 1 Fig. 1

4 I F1g.2 5

I0 4 5 CHESTER E. WA IZD EL Z O BY were/2 W CARPENTER,

Fig. 3 ATTORNEYS.

Dec. 15, 1964 c, E. WARD ETAL 3,161,225

METHOD FOR OBTAINING FLAT AND STRESS-FREE MAGNETIC STRIP Filed Dec. 29. 1961 2 Sheets-Sheet 2 15 75 /5q /4 760 l6 l6 Fig. 5

INVENTOR.

CHESTER E. WARD AND V/croe W CARPENTER,

ATTORNEYS,

United States Patent Ofifice 3,161,225 Patented Dec. 15, 1964 3,161,225 METHOD FOR GBTAKNENG FLAT AND STRESS-FREE MAGNETEQ STRIP (Ihester E. Ward and Victor W. Uarpenter, Middletown,

@hio, assimors to Arrnco Steel (Iorporation, ,Middletown, Qhio, a corporation of @hio Filed Dec. 2h, i961, Ser. No. 163,283 tjlairns. (Cl. 153-86) This invention has to do with the production of magnetic strip materials which require extreme flatness and freedom from stress, and relates more specifically to the removal of coil set and the minimization of stresses,

both mechanical and thermal, encountered in the production of magnetic strip materials.

In the thermal flattening of magnetic strip material, two distinct problems are encountered The first is strip shape. Invariably, the coils to be processed exhibit a lack of the flatness required in laminations for transformers and other applications of magnetic material. While various methods have hitherto been proposed to flatten the strip, such as hot roller leveling and the use of catenary units, these methods rely heavily upon tension and the resultant elongation to flatten the material and are designed primarily to remove such irregularities as longitudinal troughs, buckles, and the like commonly encountered in rolled and heat treated metallic strip. Such methods are effective in flattening strip but the operation often is so severe that all of the deleterious effects of mechanical stresses are not removed and the material is then unsatisfactory for magnetic applications.

The second important problem encountered in the production of magnetic strip material is stress. The stresses involved are mechanical stresses, such as those occasioned by the physical processing of the strip, and thermal stress resulting from fast and non-uniform cooling of the strip after the heating step in the processing. These two types of stresses can be complementary and under extreme circumstances result in permanent impairment of magnetic properties, i.e., original properties are not recoverable with heat treatment for stress-relief. Even if not per manently damaging, residual stresses from processing may be unduly difiicult to remove with stress-relief annealing and will, if not completely removed, cause decreased etticiency in the magnetic application. Consequently, it is highly desirable that prior processing stresses, inclusive of those introduced by flattening of the strip, be kept to an absolute minimum.

Accordingly, a principal object of the instant invention is the provision of procedures by means of which both coil set and the stresses introduced during processing of the magnetic strip material may be effectively eliminated, thereby providing strip having extreme flatness without any appreciable loss in magnetic properties.

A further object of the instant invention is the provision of a simple and inexpensive procedure by means of which coil set or longitudinal curvature can be effectively removed from magnetic strip material without inducing mechanical stresses which cannot be relieved in the same operation. I

Still a further object of the invention is the provision of a technique for obtaining extreme flatness in magnetic strip material while at the same time relieving both mechanical and thermal stresses introduced in the strip during prior processing or as an incident to the removal of coil set, such techniques embodying the passage of the strip material through a heat treating furnace wherein a controlled reverse curvature is introduced into the strip to offset coil set, such reverse curvature being introduced while the strip is being subjected to a stress relieving heat treatment and advanced through the furnace with a minimum of stress-producing tension and flexing of the strip.

The foregoing together with other objects of the instant invention which will appear hereinafter or which will be apparent to the skilled worker in the art upon reading this specification, are accomplished by those procedures and method steps which will be described more fully hereinafter.

Reference is now made to the accompanying drawings wherein:

FIGURE 1 is a schematic representation of one form of apparatus suitable for carrying out the procedures of the instant invention.

FIGURE 2 is a schematic diagram similar to FIGURE 1 illustrating an alternative form of apparatus.

FIGURE 3 is a schematic diagram of another form of apparatus.

FIGURE 4 is a schematic diagram of still another form of apparatus having a two part pass line.

FIGURE 5 is a schematic diagram of yet another form of apparatus.

The flattening procedures of the instant invention are based upon the theory that if strip containing coil set (longitudinal curvature) is flattened, transverse curvature develops approximately in the amount R =R /a, where R =transverse radius of curvature, R =longitudinal radius of curvature and U=POlSSOn S ratio. Poissons ratio is deimcd as the ratio of the transverse contraction of a strained test specimen to its longitudinal elongation. Since a metal strip does not remain fiat or planar in cross-section when thes trip is straightened, but rather forms a trough with the edges high, it has been found that a very slight reverse bend will restore the planar condition of the cross-section. Such reverse bend can be readily induced by causing the strip to pass over a roll positioned to deflect the strip from its path of travel in a single direction only in opposition to the coil set of the strip. To be effective, such reverse bending must be carried out at a temperature where the strip material deforms plastically with case. It has been found that where this is done an essentially flat surface will be developed without material damage to the magnetic properties of the strip. This statement should be qualified by pointing out that the plastic deformation which can be tolerated appears to apply only to deformation incurred by the single direction bending operation of the instant invention. That is, the stresses induced by the single reverse bend at the annealing temperature will be self-healing, Whereas it has been found that even very small amounts of plastic strain of the order of 0.05% elongation caused by multiple bending or by tension will result in irrecoverable damage to the magnetic properties of the strip.

Referring now to FIGURE 1 of the drawings, a thermal-flattening furnace is indicated at 1, such furnace being of conventional length which, while variable, may be on the order of feet. The leading end of the furnace, i.e., approximately the first 60 feet, is for heating and soaking; whereas the remainder of the furnace is for cooling. The strip 2 is conducted through the furnace on a series of spaced apart hearth rolls 3 which are preferably driven, the strip being fed into the furnace from a pay oil reel 4 and removed by means of rewind reel 5. It will be understood, of course, that the magnetic strip material will often be coated with a thin layer of a composition capable of reacting with the surface material of the strip while at an elevated temperature to produce an adherent solid insulating film of high resistivity on the surfaces of the strip material. For example, box annealed coils of silicon steel strip may be first uncoiled and passed through a scrubber and then into a coating tank where a coating composition, such as an aqueous phosphoric acid solution, is applied and.

inch, preferably approaching 100 lbs. per square inch. The reduction in tension is greatly facilitated by driving the hearth rolls.. Desirably, the tensile stress exerted on the strip should be insufiicient in combination with thermal stress to case creep or plastic elongation of the strip. Stresses of a magnitude to cause creep or plastic trated in FIGURE 1, the reverse bending of the strip can be induced -by elevating a single roll 6 above the pass line of the hearth rolls 3. Preferably, the roll 6-which can.

be referred to as a flattening rollalso will be driven and may comprise a 4 inch 'diarneter'cylindrical roll.

elongation have been found unnecessary to eflect flattening in accordance with the instant invention, and in ad-v dition they cause irreparable damage to the magnetic properties of the strip.

Turning now to FIGURE 2 of the drawings, there is illustrated a modification of the invention wherein the single reverse bending of the strip is induced by means of a series of adjacent flattening rolls 7, '8 and 9 displaced upwardly from the pass line of the strip, as: defined by the hearth rolls 3. As before, the purpose of this arrangement is to introduce a single bend in the strip in In an exemplary embodiment of the invention, the flattening roll'was positioned approximately feet from the exit end of the heating and soaking zone of the furnace. The reverse bending which takes place is unidirectional, that is, the strip is deflected from the pass line in a single direction only, and while the strip returns to the pass line 'upon passage over the flattening roll, it is not bent in more than one direction nor more than one time.

This is important since multiple bending of the strip, whether it be in the same direction or in opposite directions, as where the strip would be fed over and under adjacent rolls, acts to introduce undesirable mechanical stresses which can permanently impair the magnetic properties of, thestrip.

The amount of flattening rollelevation above the passline of the strip will vary depending upon the diameter of the flattening roll; however, where a 4 inch diameter contacts the strip and hence the greater the timethe transverse cross-section is planar. It. will also be evident that, fora given roll elevation, the smaller the roll. diameter the sharper the bend in the strip. Consequently, the amount of penetration will vary depending upon numerous factors; and the combinations of roll size and roll penetration that will produce. satisfactory-results are numerous. a r

It 'is also contemplated, that the: flattening roll will'be movably mounted so that it may be automatically. moved 7 relative to the pass line of the 'strip'in accordance with" the diameter; of the coil being unwound That is, as

1 to proportionally increase the degree of reverselbending i the strip, las defined by the sets of-

rolls

15 and 16, but the strip is unwound from the coil the diameter of the 3 coil will decrease with each succeeding convolution, and] the coil set and resultant transverse curvature of the strip 5 will increase accordingly Consequently, it is desirable opposition to its coil set. In effect, the cluster of flattening rolls 7, 8 and 9 define a single large diameter bending surface which enables the user to more closely calibrate the corrective deformation of the strip.

. It should be evident that, instead of elevating one or more of the'flattening rolls, the same effect may be brought about by depressing one or more rolls below the pass line of the strip, in which event the strip will be payed off and recoiled from the top of the coil instead of from the bottom of the coil.

It should be understood that the term singlereverse bend as used in the description and claims does not preclude some flexing of the strip in the direction of original 'stripcurvature or coil 'set as an incident of the bending operation. In the embodiments of ,the inventionillustrated in FIGURES 1 and2, some flexing ofthe strip in the direction of coil set takes place as the strip moves upwardly about the flattening roll and then returns to the hearth rolls 3. While such flexing of the strip has not been found to adversely affect the'eifectiveness of the flattening rolls in removing coil set, other furnace arrangements may be. employed in which flexing of the strip in the direction of coil set is eliminated entirely or materially reduced. a

Foriexample, as, seen in FIGURE 3, the

coils

4 and 5 may be placed beneath the furnace and the strip lead over a series" of hearth rolls 10 arranged to define an inverted U-shaped' path of travel. With this arrangement, there .is no flexing of the strip in the direction of coil set as itpasses through the furnace. Alternatively, the coils may be placed to the sides or above the furnace and the U-shaped rolls oriented accordingly.

In the embodiment of the invention illustrated in FIG- URE 4, the'str'ip is taken over a series of hearth rolls 11 .to an elevated flattening roll 12, but in this'instance the pass line of the strip beyond the elevated flattening roll is raised .to the level of the flattening roll 12, the strip 7 being conveyed from the furnace on raised hearth rolls 13.

A further form of the invention is illustrated in FIG URE 5 wherein the hearth roll 14 lies in the pass line of gle reverse bendingof the strip without elevating or dcto compensate for the progressivelyiincreasing coil set.

This may be readily accomplished by a screw jack mounting5of the. flatteningroll; with the screw driven in timed;

and calibrated relation to th pam t s 1 being unwound and its feeding "speed.-

a r Y strip to advance it through'ihe' furnace. For-"optimum results, the tensionlshouldbe, essentially norg r'eater thanv that required to advancethe strip. Here again, it'is impossible to state precise tensions, although ideally the pressingany of the rolls. The 'strip will pass over or under the isolated roll 14 depending upon whether the strip is payed off and reeled in from the bottom or top of the coil, respectively, It may be noted that while 'asinglefree 'catenary within the hot zone of :a furnace has been used for manytyears, it has not "been'pre'viously known to position a hearth roll in'such a mannerjthat one or two shorter catenaries-,may be formed and' con trolledto efiectfthe desired reverse bending of the-strip.

As should now be"apparent, the instant invention contemplates proceduresbyfmeans of which extreme flatness ,eanb'e produced in magnetic strip material, such asv grain tensile stress jshould be wellsbelow r1000, lbs;.fp'er square aori ented silicon steel, by bending the "strip in one direction only-in opposition to coil setwhile at stress relieving temperature and in the absence of excessive tensile stresses. The flatness characteristics of the strip are greatly enhanced while at the same time the type of magnetic damage which results from mechanically induced stresses is essentially eliminated. It will be understood that deformations of the strip, such as wavy edges, sag buckles, and the like, will be minimized by processing controls ahead of the thermal flattening treatment. Thermal stresses, such as those induced by fast and nonuniform cooling of the strip, will be controlled in the cooling section of the furnace. If desired, a cooling hood may be provided in order to provide more gradual cooling of the strip.

The instant procedures are particularly applicable to the treatment of coated strip since the strip is deformed only in one direction and is contacted only on one side. Consequently, the scratching, cracking, and flaking of the insulating'films during the processing and handling of the strip is minimized, thereby providing on the magnetic strip materials more uniform insulating films having high dielectric properties.

Having thus described the invention in certain exemplary embodiments and with the understanding that modification may be made therein without departing from its spirit and purpose, what it is desired to secure and protect by Letters Patent is:

1. A process for flattening magnetic metallic strip material wherein the strip, as an incident of its processing, has been coiled and box annealed, said process comprising unwinding .the coil strip and passing it through a heat treating furnace and subjecting it to a stress relieving heat treatment and, as said strip passes through said furnace, introducing a single reverse bend in said strip in opposition to the coil set thereof by contacting one side only of the strip and deflecting it in the direction opposite to the coil set, said strip being advanced through the furnace at a tension insufiicient to produce creep or plastic elongation of the said strip which would impair its magnetic properties.

2. The process claimed in claim 1 wherein the pass line of said strip is defined by a series of spaced apart hearth rolls transversely disposed with respect to the path of travel of the strip and upon which the strip is supported and advanced through the furnace, and wherein the single reverse bend in said strip is effected by means of at least one flattening roll positioned to penetrate the pass line of said strip.

3. The process claimed in claim 2 wherein the penetration of said flattening roll is progressively increased in proportion to the decrease in the diameter of the coil being treated as the strip material making up said coil is payed out.

4. The process claimed in claim 3 wherein said flattening roll has a diameter on the order of 4 inches and is mounted to selectively penetrate the pass line of said strip by a distance of from about 2 inches to 6 inches.

5. A process for the thermal flattening of grain oriented silicon steel to remove coil set and at the same time minimize mechanical stresses introduced therein by such processing, said process comprising the steps of unwinding the coil strip and passing it through a heat treating furnace and, while said strip is passing through said furnace, subjecting it to a single reverse bend in opposition to the coil set of said strip by contacting one side only of the strip and deflecting it in the direction opposite to the coil set, said strip being advanced through said furnace at a tension insufficient to produce creep or plastic elongation of the strip which would impair its magnetic properties.

6. The process claimed in claim 5 wherein said stress relieving heat treatment is at a temperature effective to provide plastic bending of the strip with case.

7. The process claimed in claim 6 wherein the strip enters the furnace along a first pass line, wherein the single reverse bend is introduced by means of at least one flattening roll displaced vertically from said first pass line, and wherein said strip, upon passage beyond said flattening roll, is conveyed along a second pass line in essentially horizontal alignment with said flattening roll.

8. The process claimed in claim 6 wherein said strip is uncoiled from beneath the furnace and is passed upwardly through the furnace in an inverted U-shaped path of travel.

9. The process claimed in claim 6 wherein said strip is passed through the furnace while supported on a series of spaced apart hearth rolls defining an essentially horizontally disposed pass line, wherein at least one of said hearth rolls is separated from the adjoining hearth rolls by a distance sutficient to define a catenary therebetween.

References Cited in the file of this patent UNITED STATES PATENTS 2,868,702 Brennan Jan. 13, 1959 2,922,460 Schwendenwein Jan. 26, 1960 2,980,561 Ford et al Apr. 18, 1961

Claims

1. A PROCESS FOR FLATTENING MAGNETIC METALLIC STRIP MATERIAL WHEREIN THE STRIP, AS AN INCIDENT OF ITS PROCESSING, HAS BEEN COILED AND BOX ANNEALED, SAID PROCESS COMPRISING UNWINDING THE COIL STRIP AND PASSING IT THROUGH A HEAT TREATING FURNACE AND SUBJECTING IT TO A STRESS RELIEVING HEAT TREATMENT AND, AS SAID STRIP PASSES THROUGH SAID FURNACE, INTRODUCING A SINGLE REVERSE BEND IN SAID STRIP IN OPPOSITION TO THE COIL SET THEREOF BY CONTACTING ONE SIDE ONLY OF THE STRIP AND DEFLECTING IT IN THE DIRECTION OPPOSITE TO THE COIL SET, SAID STRIP BEING ADVANCED THROUGH THE FURNACE AT A TENSION INSUFFICIENT TO PRODUCE CREEP OR PLASTIC ELONGATION OF THE SAID STRIP WHICH WOULD IMPAIR ITS MAGNETIC PROPERTIES.