US3347718A - Method for improving the magnetic properties of ferrous sheets - Google Patents

Description

United States Patent Ofiice 3,347,718 Patented Oct. 17, 1967 These and other objects of the invention which will be set forth hereinafter, or will be apparent to one skilled in the art upon reading these specifications, are accomplished by that procedure and in those materials of which certain exemplary embodiments will now be described.

As will later be more fully outlined, the invention is applicable to a wide variety of ferro-magnetic materials.

While the permeabilities and other magnetic properties of ferrous magnetic materials will of course vary with composition and other gross characteristics of the material including grain size, the kind and degree of grain orientation (it any) and the like, the present invention is based upon the discovery of hitherto unknown effects of the surface condition of magnetic sheet stocks, which effects are essentially independent of the said gross characteristics,

in that they occur in useful measure despite changes in the gross characteristics. The benefits of this invention are attained in materials having thicknesses greater than about 10 mils.

Hitherto in the ferro-magnetic art improvements in permeability and other aspects of performance were sought in changes in the gross characteristics of the iron or alloy sheet stocks employed. The prior art is replete with patents having to do with composition and with processing factors such as the kind and character of hot rolling treatments, the extent and nature of cold rolling reductions, and the number, temperatures and atmospheres of heat treatments as determining grain size, and the kind and degree of grain orientation. Remarkable improvements in the permeabilities and core loss characteristics of ferrous magnetic sheets have been made. At the same time the factor of cost enters into the availability and use of ferro-magnetic stocks, so that a very large proportion of the sheet stocks used is made up of magnetic ingot irons, low alloy sheets and even materials classifiable as silicon-irons which can be produced cheaply and which are essentially devoid of preferential grain orientation. The present invention is applicable to such materials as well as to silicon-irons in the cube-on-edge orientation, nickel irons containing approximately 20% to 80% nickel, and others.

It has hitherto been understood that the matter of space factor in a core or magnetic structure made up of stacked laminae of magnetic sheet material has an effect upon the performance of the structure. While the use on the laminae of coatings of insulative substance to impart interlamination resistivity affects space factor, it has nevertheless been understood that, all other conditions being equal, a high degree of surface roughness on the sheet stocks may make for a poorer core loss factor. However, the ferro-magnetic sheet stocks to which this invention pertains have not hitherto had a surface smoothness better than 10 to 20 microinches as measured on the profilometer.

Expected improvements in space factor would not justify the cost of attaining a greater surface smoothness; and, as will later be pointed out, the magnetic improvements which result from the practice of this invention are'of a kind different from the benefits which could be premised upon an improvement in space factor.

Many of the magnetic sheet materials of the types to which this invention is addressed have hitherto regularly been made with surfaces rougher than the 10 to 20 microinches mentioned above, for various reasons:

(a) A stock which has been hot rolled to gauge will have a rough surface Within the meaning of this application whether or not the hot mill scale has been removed from the surfaces of the stock, as by pickling.

(b) Pickling can, in and of itself, leave the surface of the stock rough within the meaning of this application.

(c) Stocks which have been cold rolled have imparted to their surfaces the surface characteristics of the rolls of the cold mill or mills used to process them. For the materials to which this invention applies these rolls are ground to desired contours by procedures which leave their surfaces characterized by roughness within the meaning of this application. Moreover, as the rolls wear, their surfaces increase in roughness.

(d) It has hitherto been thought that a rough surface on ferrous magnetic sheet stocks of the types hereinabove set forth was desirable because under some circumstances it facilitates rolling, because it facilitates the feeding of small punched blanks in automatic presses, and because it minimizes sticking during the customers strain relieving annealing operation following punching. As a consequence artificially roughened rolls have been used in the cold mills, e. g. Pangborn or shot blasted rolls.

So far as is known, only two classes of ferromagnetic sheet stocks, of any kind, have been produced in the practical art with very smooth surfaces. The first of these classes is that of exceedingly thin materials which be cause of their thinness must be produced on precision mills having very small working rolls. The working rolls of such mills are generally made with highly polished surfaces. The second of these classes is that of siliconiron sheet stocks which are made to have a cubic texture, i.e. a [001] grain orientation by Millers Indices, by processes involving the production in the stock of a number of grains having the cubic texture, and then subjecting the stock to a secondary recrystallization which proceeds basically by the phenomenon of surface energy. The annealing atmosphere used for the secondary recrystallization contains a minute quantity of a polar compound which shifts the order of the energy levels of grains having different orientations; and the Kohler US. Patent No. 3,090,711 teaches that the cubic texture cannot be attained in high degree unless the sheet stock being treated has a smooth surface, which apparently affects the surface energy of the grains. But if any other or different magnetic effects are attained in products of the two classes just mentioned (as to which there is no evidence), such effects could not be observed or detected. Since the products can be made only in the Ways indicated, there is no basis of comparison.

Moreover, in the production of ferrous magnetic stocks having the cube-on-edge texture, i.e. [001] orientation by Millers Indices, the secondary recrystallization proceeds by grain boundary phenomena, and is not controlled by the obtaining of the desired orientation and the roughness or smoothness of the surfaces of the sheet stock.

It has now been discovered, however, in accordance with the present invention that certain new and unobvious 3 elfects may be obtained if the ferrous sheet stocks of the types set forth hereinafter have exceedingly smooth surfaces in their final forms as sold and used.

By a smooth surface on the magnetic sheet stock is meant a smoothness which will measure not more than about. 5 microinches on the profilometer. Preferably the sheet or strip stock of this invention will give a profilometer reading range from less than one to about 5 microinches.

The magnetic benefits which are derived from the practice of the invention are most apparent at the higher inductions, by which is meant conditions of operation of a core at inductions from about the knee of the magnetization curve up to saturation. For cube-on-edge stock and the variations thereof, high induction operation would be generally in the range of '14 to 19 kilogausses; while the knee of the curve for non-oriented silicon steel is at about 11 kilogausses. Magnetic ingot iron, low alloy steel, and non-oriented nickel-iron of the 48% nickel type have knees at about 6 kilogausses. Grain-oriented nickel-iron has a knee at about 14 kilogausses. These values cannot be expressed more definitely because they will vary with the degree of orientation in the material, the amount of residual stress and other known factors.

The improvement in magnetic qualities obtained through the practice of the invention include an improvement in permeability and a reduction in core loss at high inductions as above defined. These phenomena were hitherto unknown.

As has been stated, the prior art has gone to the pro-.

duction of rough surfaces (and in most cases deliberately roughened surfaces) on ferrous magnetic sheet stocks of the types to which this invention relates, The cost and other advantages during rolling, as well as the provision of laminations or layers which could not stick together during the customers anneal, far outweighed any advantage which could be predicted on an improved space factor, assuming that an improved space factor could be achieved.

Without wishing to be bound by theory, it is now believed that the remarkable improvements at high inductions noted with the materials of this invention are due principally to improved domain orientation and decreased excitation requirements for establishing fiux near the surface. Ifthe surface of the layers or laminations in a core are characterized by roughness, when the core is magnetized the flux lines near the surfaces of the laminations tend to follow the contour of the surfaces. This results in longer paths for flux to travel as well in crowding and non-uniform flux density along the metal axis. Thus, where the cross sectional area of a metal sheet is smallest, i.e. at a section joining the valleys on the surfaces, there will be a constriction which impedes the flux flow; and particularly at high inductions, the flux will attempt to bridge the air gap between high points on the rough surfaces, resulting in increased excitation requirements. Discontinuities in surface layers in rough surfaced stock establish unfavorable domain structures (through demagnetization effects) which increase hysteresis loss and result in increased core loss in addition to the increased losses imposed by constricting the flux.

While there is some advantage in having a smoother than ordinary surface in DC. applications, the principal.

advantages of the invention have been found in AC, applications. At a low frequency such as 60 cycles the effects on core loss are most pronounced at inductions above the knee of the magnetization curve. As frequency is increased the apparent inductions at which the effects are realized are lowered because of the skin effect phenomenon.

Briefly, in the practice of this invention ferrous magnetic materials of the types herein taught are provided withvvery smooth surfaces. This may be done at any stage in their manufacture providing treatments at a subsequent stage do not produce or recreate a rough condition, as later explained. For. reasons which will also be given later the formation of the smooth surfaces-upon the ferrous magnetic sheet stock is ordinarily accomplished prior to the treatment which develops the ultimate magnetic characteristics in the products.

The new and unobvious effects attained by the practice of the invention depend upon the presence of th smooth surface as defined on the magnetic material in its final condition. Insofar as is known, the smoothness characteristic of the surface does not control or vary the effect of any of the steps by which the product is reduced to gauge or heat treated for the development of what is ordinarily called its ultimate magnetic characteristics, or

the obtaining of any characteristics or perfection of orien tation in oriented stocks as. defined herein. In other words, the novel effects hereinafter taught could be obtained in theory by smoothing the surfaces of the stock after the stock is otherwise in finished condition. The principal objection to such a procedure would be the presence of stresses that permanently deteriorate the magnetic properties through undesired changes in crystallographic lattice or grain structures.

Sensitivity of the magnetic properties to stressproduced changes in lattice or grain structure dictates to a considerable extent the stage at which the smoothing operation may be applied. The necessary surface smoothness can be attained in the laboratory for the extremely sensitive cube-on-edge oriented material even after development of the grain orientation.

Various surface finishes for cold rolls are recognized in the industry. Rolls which are buffed as that term is understood in the trade, will have a surface smoothness reading from about 0.2 to 1.0 microinch on the profilometer. Grinding the rolls with a cork or shellacbonded abrasive wheel, as also known in the .art, -will impart a surface smoothness measuring from about one to about five microinches on the profilometer. Grinding the rolls with a fine grit grinding wheel may also be employed; but the grit should pass through a sieve (US. Standard Sieve Series) having an opening of 0.0035 in. or 0.088 mm. Higher grit numbers of 300 to 400 are generally used in making wheels which impart to the roll surface a smoothness of two microinches or less.

Where a ferrous magnetic sheet stock has been carried down substantially to final gauge by hot rolling and has been pickled for the removal of the hot mill scale, a light cold rolling reduction of less than 10% will ordinarily be found sufficient to impart to the stock the desired degree of smoothnessswhere the stock is carried down to gauge by cold rolling, very smooth cold mill rolls need to be used only for the last pass or passes. The amount of cold rolling reduction required to convert a rough surface to a smooth within the meaning of this application will of course depend to some extent on the degree of roughness of the original surface; but in any event it will be small. In general, cold rolling treatments with smooth rolls as herein defined will produce thedesired smoothness of the sheet stock surfaces at reductions of less than 5%. It may be noted that the amount of reduction required with the smooth surfaced rolls may be of the orderof the reductions taken in temper rolling or skin passing steps which form a part of some routings. Also it is generally somewhat less than the cold rolling made for critical straining procedures, sometimes employed for the purpose of growing large grains.

In the manufacture of non-oriented stockswhich are to be punched into laminations, it would be possible to practice a skin passing with smooth rolls after the development of final magnetic properties as hitherto understood in the art, since the customary anneal practiced by the purchaser of the stock after the stamping operation would be sufiicient to relieve the skin passing strains as well as the strains arising from the punching operation. However, even here, the employment of an extra processing step is undesirable cost-wise, and for this reason the treatment employed to provide the stock with smooth surfaces preferably precedes the final anneal and preferably involves the use of smooth rolls for some terminal part of a cold rolling step already comprised within the routing. In the case of stocks having a high degree of cube-on-edge orientation it is additionally advisable to perform the smoothing operation prior to the ultimate heat treatment which controls the perfection of the orientation, since the introduction of cold rolling strains into the material thereafter may adversely affect the perfection of the orientation.

The invention is applicable to and may be practiced upon, or in the course of the manufacture of, ferrous magnetic sheet stocks greater than about mils in thickness, and of the following classes or grades which are well recognized in the art:

(a) magnetic ingot iron,

(b) low alloy steel,

(c) non-oriented silicon-iron,

(d) silicon-iron having cube-on-edge grain orientation,

(e) nickel-iron magnetic alloys containing from to 80% nickel and especially alloys containing about equal parts of nickel and iron,

(f) aluminum-iron alloys of oriented grades containing up to about 20% of aluminum.

As previously stated, the composition of the ferrous metal or alloy does not constitute a limitation on the invention, inasmuch as the benfits derived from the practices herein taught do not depend upon any specific composition.

The invention thus is applicable to any of the grades of iron or iron alloys listed above as these are understood in the art. For the purpose of the claims which follows, there is now given a list of ingredient percentages by weight for these grades, it being understood that these are typical and indicative of the general understanding of the art, but are not limiting:

Oriented silicon-iron:

Si 24 C .015-.303" Mn .05-.15 S max .03 Al max .01

Magnetic properties meet AISI Standards for Grades M5 to M8.

Non-oriented silicon steel:

S1 0.5-5 C .03-.10 Mn max 0.3 S max .03 Al max 0.5

Magnetic properties meet AISI Standards for Grades M14 to M43.

Low alloy steel:

The above is a typical analysis. The total value of elements other than iron is less than 0.1%.

Nickel-irons containing both lesser and greater amounts of nickel and other alloying elements are also improved by the practice of this invention.

The term silicon-irons is generally applied to ferrous materials containing about /2% to about 5% silicon. The materials of the present invention may contain silicon up to the practical limit of workability, which for stocks hot rolled to substantially final gauge may be as high as 5%. Stocks intended for substantial reduction in gauge by cold rolling may contain silicon up to the practical limit of cold workability, which is currently in the neighborhood of 3.5%.

The ferrous magnetic stocks of this invention, as will already be apparent, may be unoriented (random), or the silicon-irons may have the cube-on-edge orientation or variations thereof in which the cube faces, while still lying at a significant angle to the plane of the surface of the sheet stock, depart more or less from a 45 angle to that surface, or cubic texture in the case of nickel-iron, which tends to assume that texture even Without secondary recrystallization. Orientations of the described char- 'acter for the designated materials, or the lack of any preferred orientation, do not affect the obtaining of the benefits of the invention as such.

The magnetic benefits are affected in degree only by the gross factors mentioned above and by others. By and large, greater magnetic benefits will be attained in the case of oriented stocks than in the cases of non-oriented stocks because, inter alia, oriented stocks have higher general permeabilities. The thickness or gauge of the ferrous magnetic stocks is another factor, since it will be obvious that for a given degree of roughness, the volume of material affected by the smoothing operation will be greater as the number of laminations present in a given core thickness is increased. The obtaining of the benefits of this invention is likewise not dependent upon the routing by which the ferrous magnetic sheet stock is made, excepting for the attainment of the described surface condition in the ultimate product. The sheet stock may be hot rolled to substantially final gauge, or it may be hot rolled to an intermediate gauge and then cold rolled to final gauge in one or more stages, with or without intermediate annealing. Normally a ferrous magnetic sheet stock will be given a final heat treatment.

In short, the present invention is applicable to all of the procedures current in the art for making ferrous magnetic sheet stocks of the classes indicated, and the practice of the teachings of this application does not preclude the use of any of those steps hitherto known for improving the characteristics of such magnetic sheet stocks, including but without limitation the nature and temperature of the stages of hot rolling and coiling, the number and extent of cold rolling treatments, and the nature, atmospheres and temperatures of all heat treatments, whether for softening, decarburization, primary or secondary recrystallization, or for any other purpose.

The nature and extent of the benefits derived from the practice of the invention can best be illustrated by certain examples. Examples I and II relates respectively to non-oriented iron stocks and to silicon-iron stocks having a high degree of cube-on-edge orientation.

7 8 Example I TABLE II.EFFEOT OF summon ROUGHNESS ON MAG- Samples of ferrous magnetic material were made in ff id ifigdggg g iggg SAME MATERI- accordance with the teachings of the copending application of Robert W. Easton et al., Ser. No. 229,203, filed Magnemmg Force, H Oct. 8, 1962, now Patent No. 3,180,767, by a procedure (oersteds) which involved hot rolling, pickling, and then cold rolling with a reduction of substantially 40% to. 80%. The fer- Sulfa Rollghnessi Difference in Micromches Magnetizmg Force rous magnetic material was substantially devoid of silicon Due to Surface (containing no more than about 0.1% thereof) and had 55 5 a carbon content not greater thanabout 0.005%. The samples were from the same heat and were treated idengggg g BT Space Factor, Percent tically including cold rolling to gauge with Pangborn rolls. A portion of the samples were then machined to 97A 994 produce a smooth surface. Profilometer measurements r showed roughness values of about 55 microinches for-the Type Surface Omteds Percent first mentioned group of samples, and values of about 5 microinches for the second group of samples. The samples Pangbom Ground were nominally mils in thickness.

The apparent induction to saturate the rough material 152 152 0 leveled off at about 21,250 gausses, whereas the induction 238 5% in the smooth surfaces samples reached 21,475 gausses 830 730 100 before leveling off. The following tables indicate the per- 'ggg g8 28 formance of the material:

TABLE I.EFFEOT OF SURFACE ROUGHNESS ON MAG- NETIC INDUCTION IN SILICON-FREE MAGNETIC MA- TERIAL AT GIVEN LEVELS OF MAGNETIZING FORCE Example II Intrinsic InductionBi Total Induction, BT

(Gausses (Gausses) Cube-on-edge oriented stock containing about 3% silicon was made by a current process involving hot rolling surface Roughnessmfimmches to an intermediate gauge cold rolling, annealing, again cold rolling, and developing, the ultimate magnetlc char- Magnetizing 5 55 5 Difference H acteristics of the product by a final high temperature Force,H Due to 30 anneal. The details of these steps and other steps in the (Oersmds): Space Factor Percent 2:35: 1 routing are not germane here for thereason that all samples came from the same heat and were treated in 4 99.4 914 4 exactly the same way, with these exceptions:

(a) Two sets of samples were produced. One set of Type surface 40 samples was cold rolled to .011 inch before the final anneal and the other set of samples was cold rolled to yangbom Ground pangbom Ground .009 inch, and therefore, had a lesser thickness. In each set of samples, some were rolled between Pangborn rolls 18,100 18,100 18,200 18,200 0 so as to give a rough surface, while others were rolled 35 5 45 in a mill having smooth rolls as defined above. Thus, 211110 211240 5 1 21:940 there were produced for comparative tests rough and 338 31.2 33 328 323% $8 smooth samples at the heavier thickness and rough and 211250. 21460 24, 250 241460 210 Smooth samplfis atlhe lighter g (b) The results obtained on composite 12-strip Ep- BT:B1+H. 5O Steins are as follows:

TABLE III Smooth Rolls Paugborn Rolls Nominal Thickness 1 Ga P10;60 P15;60 P17;60 Perm! Ga Il0;60 P15;60 P17;60 Perm. 11 10. 7 .214 .487 .695 1,822 10. 3 .216 .511 766 1, s03 9 9.1 .199 .461 .693 1, 798 8.5 .198 .481 .747 1, 730

1 Mierometer measurement. 2 By weight. 3 Permeability at H= 10 oersteds.

Core losses are measured at various power levels and frequencies; e.g. P10z60 signifies a test induction of 10 kilogausses at a test frequency of 60 cycles per second.

Using the final thickness by weight as means for estimating the eiTect of final thickness or permeability, the test data showed that those with the rougher surfaces had about 10 points lower permea'bilities at H=10 oersted. When tested at 10 kilogausses, no appreciable diiference was observed between the rough and smooth samples. However, when the several samples were tested at 17 kilogausses, after adjusting for thickness, the rough surfaced samples showed a core loss .065 watts per pound (9.4%) greater than the smooth surfaced samples. When tested at 15 kilogausses, the rough surfaced samples showed a core loss .026 watts per pound (5.3%) higher than the smooth surfaced samples.

The differences in the surface characteristics of the samples were measured by the profilometer. The rough samples showed a value of about 50 microinches while the smooth surfaced samples showed a value of about 5 microinches.

Modifications may be made in the invention without departing from the spirit of it. The invention having been 10 described in certain exemplary embodiments, what is claimed as new and desired to be secured by Letters Patent is:

A process of cold rolling ferrous magnetic sheet stock selected from the group consisting of magnetic ingot iron, low alloy steel, non-oriented silicon-iron, silicon-iron having a cube-on-edge type of orientation, nickel-iron magnetic alloys, and oriented aluminum-iron alloys containing up to about 20% aluminum, to a final thickness of about 10 mils or greater, and a smoothness of no more than about 5 microinches, and then heat treating the stock to produce therein an improved core loss and permeability at high inductions from about the knee of the magnetization curve up to saturation.

References Cited UNITED STATES PATENTS 3/1927 Brace 148-111 5/1963 Kohler 148111