US2672429A - Electrical steel - Google Patents

Description

Patented Mar. 16, 1954 ELECTRICAL STEEL James E. Malloy, Bridgeport, Conn., assignor to "(The Stanley Works, New Britain, Conn, a corpora'tion'of Connecticut No Drawing. Application June 3, 1952, Serial No. 291,569

. l The present invention relates to electrical steels and more particularly to a method of mak in; electrical steels suitable for use in electronetic devices of various types such as motors, transformers and ballasts, for example, and which electrical steels are formed from ordinary low carbon rimming steels instead of silicon steels. 4

It is conventional to use electrical steels in electromagnetic devices as a core or the like for carrying the magnetic field. -To form a core it is customary to roll the steel into thin strips which are then die cut into desired configurations and then stacked and secured together such as by welding or riveting. It is essential that the metal from which the strips are formed possess electrical characteristics such that'it will carry the magnetic flux with a minimum of reluctance. If the reluctance of the steel used in the core is large,fwhich-is characteristic of most steels, there results a relatively-great power loss in the core which not only has-an adverse effect on the efficiency of the apparatus but which, being. dissipated in the form of heat, causes undesirable, if not dangerous,--temperature-rises in the apparatus; It'also is essentialthat the material forming the laminae have the-property of good blankability, which may be defined broadly as the ability or property of the metal strip to be cut by dies into the desired configuration without imperfections. ln'most designs of cores, the laminae have a very intricate configuration in order to meet desired specifications, and it is Well known that a'satisfactory core can be obtained only when the laminations are in closely stacked relationship. If the material from which the laminae are made does not have good blankability, then the edges will not be-free of burrs, tears, notches, etc. which'prevent close stacking of the laminations. 'The poorer the blankability of the material, the more diffic'ult becomes the stamping operation, with theresulting'need for frequent'interruption of the process in order to sharpen the dies. This down-time during which the dies are idle is an important cost factor.

In accordance with'the prior art, it has been believed that suitable electrical steels possessing the electrical specifications set up by theAmerican Society for Testing Materials or by manufacturers of electrical apparatus and also having suitable blankability properties coul-dbe at-- tained only by the production of strips from steels having 'a silicon content of'atleast .2%. For example, so-called field grade elec'trical steels, which are suitable for use in many 'inex pensive transformers, motors, :balltst's, etc., is a steel whichfwhen subjectedto the so-called Epstein Test at 60 'cycles and with thethicknessof the steel atizoztiincn; exhibits a. maximum watt loss of 2177 Watts perineum at kilomeans; (01. na -12 gausses and 6.41 watts at 15 kilogausses. The Epstein test is defined in A. S. T. M. specification A-34. In general, it has been necessary'to employ an electrical steel having a silicon content of the order or" .2 to .5% in order to meet these specifications.

In order to possess good blankability, a steel must have a relatively high Rockwell test rating of the order of at least 30 on the B scale and a relatively high yield point compared with its ultimate strength. Otherwise the steel laminae will have a tendency to burr, tear or notch, etc., particularly when out at high speeds on automatic machinery employing cutting dies.

In accordance with the present invention, I have found that by suitable treatment of ordinary low carbon rimming steels which have only incidental amounts of silicon of the order of .02% or less, it is possible to produce electrical steels meeting field grade specifications with respect to its electrical properties and also having satisfactory blankability properties whereby it may be utilized in place of steels having a relatively high silicon content. The importance of the invention from the standpoint of cost and I availability of the materials is, of course, apparent.

The phrase used herein to mean'ordinary or common steels like SAE No. 1008, for example, and similar steels which, when formed into billets or ingots, have not been killed by the addition of aluminum or silicon and which have a carbon content not exceeding .10%. -In general, it is preferred to utilize low carbon rimming steels having a carbon content of the orderof .06% or less since, while a lower carbon content may have some adverse effect on blankability, it has a very marked advantageous eifect upon the electrical properties.

In carrying out the method of the present invention, it is customary to begin with billets or slabs of ordinary low carbon rimming steel, as previously defined. In order to form the slabs or billets into strips, they. are first rolled byhot rolling procedure as commonly practiced in the art. The manner in Whichthehot rolling is carried out is not critical and conventional practice may be followed. The reduction in thickness accomplished by hot rolling may be any desired amount and, for convenience, I havefound it desirable to use a hot-rolled size having a thickness within the range of .060 to .125 inch. The manufacturers of electrical apparatus usually; specify electrical steel strips having a gauge within the range of about 22 to 30 (.031 to .014 inch in thickness). The Width of the strip of course may be anything desired within the capabilities of the mill. After hotrolling the strip is then pickled in low carbon rimming steels is the usual manner to remove undesirable scale produced -.during the hot rolling process. Such a step is .conventional and usually entails passing the strip through an acid bath, following which the strip is removed and rolled into coils for convenience in handling.

In accordance with the invention, following the hot rolling and pickling operation, the strip is then rolled in accordance with standard cold rolling mill procedure to within a critical size range slightly greater than the final thickness of gauge desired. I have found that this critical range is such that the strip is to thicker than the final desired gauge or thickness.

Following the cold rolling operation to within the critical degree specified, the steel strip is thenheat treated to remove the effects of the previous cold working. This may be accomplished either in a conventional normalizing step or by subjecting the strip to a process annealing, either continuous or pot annealing. For reasons which are not completely understood, I have found that electrical steel made in accordance with my invention and which has been heat treated at this stage by an annealing process, will frequently have superior electrical properties but this difference is not so material that the normalizing step cannot be used where more convenient. In the annealing operation the steel strip, preferably coiled for ease and convenience of .handling, is treated in a furnace which is maintained at a temperature below but close to the upper limit of the critical range so that the steel acquires a temperature within the range of approximately 1300 to 1625 F. In a conventional annealing process, the strip is permitted to remain in the furnace until the heat is uniform throughout, which depends of course on the temperature and the weight and form of the charge. The annealing operation is then followed by cooling as desired. When a conventional normalizing step is employed, the steel strip is continuously passed through a heating chamber maintained at a temperature above the critical range of the steel in question. Since the upper limit of the critical range for a rimming steelis approximately l625 R, a furnace temperature of the order of 1700 to 1900 F. is preferred. The strip is passed through the heating chamber at such rate that all parts of the strip will attain a temperature above the critical range, which rate will vary, of course, depending upon the temperature of the chamber and the size of the strip. After conventional air quenching, the strip is then coiled up for further processing.

By'reason of the above described normalizing or annealing heat treatment operation, whichever is selected, the cold rolled strip is relieved of all the effects of the previous cold working. Atthis stage the steel strip does not have the characteristics of an electrical steel and, being substantially free of any silicon content, would not'be expected to have such properties.

v.In accordance with my invention, the heat treated cold rolled strip is then again cold rolled in accordance with standard cold rolling mill procedure to further reduce the thickness of the strip in the critical amount of 10 to 25%, and preferably 15 to 18% of final thickness. As pointed out above, by controlling the extent of the first cold rolling step, it can be assured that thefinal critical reduction by cold rolling will result in the desired finished gauge or thickness. Following the final cold rolling step as just described, the strip, preferably in coil form, is then annealed by placing the same in a furnace heated to a temperature below but close to the upper limit of the critical range. The charge is re tained in the furnace until all parts of the charge reach a temperature within the range of approximately 1300 to 1625" F. The amount of time required of course will vary with the temperature and design of the furnace and the quantity and disposition of the load.

Following the final anneal, the strip may be cooled as desired and is then ready for stamping and use as a core material in electrical apparatus. The strip has now acquired electrical properties rendering the same suitable for use as an electrical steel in the class or grade known as field grade. The steel strip also possesses good blankability, as evidenced by a high Rockwell hardness rating and relatively high yield point compared to its ultimate strength. As a result, the strip may be conveniently cut into core laminae having desired configurations by the use of automatic die stamping apparatus with satisfactory freedom from burrs, tears, notches and the like. Since the dimensions of the strip are not changed in any way during the final anneal after the step of final cold rolling in the critical amount referred to, it will be apparent that, if desired, the cutting of the strip into laminae of desired configuration may be carried out prior to the final anneal and after cold rolling has been completed. Although it is contemplated that in most cases the user will prefer, as a matter of convenience, to stamp the strip into laminae after a final anneal by the supplier of the strip, nonetheless, there would be some advantage in following the alternate procedure referred to since the stamping or cutting of the strip inherently involves some cold working of the steel, particularly at the edge portions of the laminae, and even this slight amount of cold working will have a slight adverse effect on the electrical properties of the steel which can be alleviated by carrying out the final annealing step after stamping or cutting rather than prior thereto.

As stated above, I have found that the critical reduction in the final cold rolling step is within the range of 10 to 25 of the final gauge or thickness. In general, by carrying out the amount of cold rolling in the lower portion of the range, for example, in the range of 10 to 15%, somewhat better electrical properties are obtained but this is accomplished at the sacrifice of good blankability. Conversely, if the final cold rolling step is carried out within the upper portion of the range, say, between 18 and 25% of final thickness, there is obtained better blankability characteristics but at the sacrifice of electrical properties. Therefore, in general, it is preferred to operate within the more narrow intermediate range of 15 to 18% since this produces the best compromise between optimum electrical properties and blankability characteristics. In addition, taking into account that in commercial production there will be some non-uniformity throughout the strip and exact control is not always feasible, and in view of permissible variations in some of the factors such as the carbon content of the steel, the final gauge of the strip, etc.,it is preferred to specify that the final cold rolling step shall be carried out within the preferred range of 15 to 18%, thus keeping well within the 10 to 25% limitation and insuring more consistent results. As previously mentioned hereinbefore, the desired final gauge for electrical steel strips specified by manufacturers of electrical apparatus usually falls within the range of 201410.031" inch thickness. In accordance with "the accepted Standards of the industry as promulgated by the American Iron and Steel Institute "and published in its Steel Products Manual," section:13,the accepted standard I tolerancesfor thickness and crown of cold rolled stripin"the'gaugeherein referred to is plusor minus; approximately 6%. Therefore, in order to particularly point out-and distinctly claim the invention, thereduction in thickness. during the final cold rolling step will be' referred to in the claims as,approximately 15 to 18i per centjiby. which term is meanta reductionof 15" to 18%. taking into account standard tolerances orallowances ion deviations in thickness and crown from such exact percentage reduction. ..Whi1 itis ot de i ftql mit. hQinYs t to any particular theory of operation, nonetheless it is el v het e were descr b d. r t c amount of cold rolling following the first anneal or normalizing, and followed by thelast anneal as described, creates a critical structural change in the s teel which results in electrical steel properties with good blankability in the absence of characterizing amounts of silicon. I

. Byway of completeness, and'to assist in the understanding of the inven tion, the following specific examines are included to show various actual operations of the method of the present invention and the results obtained. These examples are given as illustrative only and are not to be construed: in alimiting sense since similarly satisfactory results can and have been obtained by carrying out the method of iny invention under other specific conditions of operation within the ranges set 'forth previously.

' Example 1 A low carbon rimming steelhaving an analysis of .06 carbon, .25 manganese, .009 phosphorus, .028 sulfur, less than .02 silicon andthe' remainder iron and residual amounts or traces of other elements" was formed into billets and then hot rolled to form a 'c-oilof strip material having a width of 53% inches and a thickness of .061 inch. Following hQl': Qro1ling.jthe strip was pickled and then cold rolled to a thickness of .022'i'nch. The "cold rolled strip was then normalized by running it through a furnace maintained at 1800 F. at the rate of 13 feet perininute with a slightly joxidizing atmosphere. After the normalizing step, the strip was thencold rolled to .022 inch (a redu ctionof %ib'a'sed on final thickness following which it annealed in a furnace at 1300 F. with a slightly "reducing atmosphere. Samples of the strip exhibited the following properties Core loss at 10 kilogau'sses 2.29watts/lb;

Core'loss at kilogausses 4.?9'waLtts/lb. Rockwell 3" scale 2 '1 42 Elongation in 2 inches 2 16% I Yield point 2 31,600 lbsj/sq'. in. Ultimate strength I c3200 lbsl/sql in. Grain size 3 3-4 {In all examples; core loss is determined by Epstein Test, ASTM Standard Method A-34 at 60 cycles. In all examples, physical properties are determined by ASTM Standard Methods. I

3 In all examples, the grain size is the number of grains per square inch viewed at 100 diameter magnification.

Example 2 In this example, the low carbon rimming steel h d a analy s; of... 96 carb n, 2. man a ese, .009 phosphorus, .03 sulfur, less than.02-si1icon and the remainder iron and residual' amountsior traces of other elements. The steel was first? hot rolled to-form a coil having a width of 4 inches and a thickness of .061 inch. Following hot rolling, the strip was pickled an'd then cold rolled .to a thickness of .028 inch. "The coil of cold rolled steel was then'anneald' in'a furnace heated. to a temperature of I475? F. with afslightly reducing atmosphere. Following the annealingstep; the strip was then cold rolled to a thickness of .025 inch (a reduction of 12%"ha'sed on final thickness) after which it was annealed at'l400' F. Samples of the strip exhibited the-following properties:

Core loss at 10 kilogausses i 211 watts/ lb. Core loss at 15 kilog-ausses 4.97 watts/lb. Rockwell B scale '34 Elongation in 2 inches 25% I lield point 24,000 lbs/sqiin. Ultimate strength 40,800 lbs/sqp'in. Gr'ain size; 3'4

Ere-ample 3 The steel treated in this example was "taken from the same heat as in the previous Example 2 and was-hot rolled and pickledin the: same manner. Following hot-rollingandpickling, the steel strip was then cold rolledtoa thickness of .029 inch and then normalizedfby' running ;it through a furnace maintained at 1800 Frat 'the rateof 13-feet per minute with a slightly oxidizing atmosphere. Following normalizingthesteel was cold rolled to a thickness.of .025 inch (a reduction of"16% basedpn final thickness) "and trien'amieaied at 1400 F. Thefollowihg'physical antlelec'trical properties werefo'u'nd:

Gore loss fill-l0 kilcgaussesnua 2.30 wattsylb. Core-loss at 15 kilogausses 5:32 watts/lb. Rock-Well B scale i 30 Elongation in 2 inches 28% Yield point. 23,-1'00 lbszfsqarin. Ul'timatestrength $39,200 1b's ./sq."in.

SiZe;. l 4

Example 4 In thisexample, the same'steelas in Example 3 was treated and exactly the same-procedure was followed except that,insteadof the normalizing'step, the strip was annealed at 1475 F. with a--sli=ghtly-reducing atmosphere. The resulting properties'were as follows:

Coreloss at 1 0 kilogau'sses i193 watts/lb. Coreloss at 15 kilogausses 4.86watt's/lb. Rockwell B scale i 29 Elongation in 2 inches -Q 30% Yield point 25",l00lbs:/sq. in. Ultimat strength "37,400 lbsL/sqiin. Grain size -l 5 Example 5 mosphere. The following were the properties of thestrip: .v Core loss at 10 kilogausse s 2.7i watts/lb.

The same steel was treated in this exampl as in the previous Example and it was hot rolled and pickled in exactly the same manner. Following hot rolling and pickling, the strip was cold rolled to .031 inch thickness and then normalized by running it through a furnace maintained at 1800"" F. with a slightly reducing atmosphere at the rate of 13 feet per minute. After normalizing, the strip was cold rolled to a thickness of .025 inch (a reduction of 24% based on final thickness) and was then annealed in a furnace at 1250 F. The resulting physical and electrical properties of the finished strip were as follows:

Core loss at kilogausses 2.72 watts/lb.

Core loss at kilogausses 5.85 watts/lb. Rockwell B scale 39 Elongation in 2 inches Yield point 28,400 lbs/sq. in. Ultimat strength 45,200 lbs/sq. in. Grain size 5-6 In the foregoing examples reference is made in some cases to the use of a slightly oxidizing or slightly reducing atmosphere during the normalizing and/or annealing steps. Where the atmosphere is not referred to, a substantially neutral atmosphere was used. It is conventional in the manufacture of silicon electrical steels to form the same with an oxide coating produced by an oxidizing atmosphere and most users require such a finish when employing silicon steels. In accordance with the present invention a slightly oxidizing atmosphere may be employed, if desired, to produce an electrical steel having such an oxidized finish, but it is an advantage of the invention that a non-oxidizing atmosphere may be employed resulting in a bright finish electrical steel without any apparent loss in electrical properties or suitability for use in core laminations and the like. As a matter of fact, a bright finish electrical steel prepared in accordance with the invention will not only be equal in electrical properties to that formed with an oxidized finish but, generally, will be even more satisfactory from a blankability standpoint.

It thus will be seen that electrical steel of field grade can be made from ordinary low carbon rimming steel in accordance with the present invention and that this steel, as evidenced by its Rockwell hardness and yield point strength, has good blankability characteristics rendering the same suitable for use commercially in the manuiacture of cores and the like. In fact, the good blankability properties of the steel are such that a die life of five to ten times that normally expected with electrical steels is not unusual. 'In addition, th electrical steel of the present invention does not undergo any age-hardening effects diminishing the electrical properties of the steel, thus assuring that electricalproducts made therefrom will perform efiiciently over long periods of time.

Otheradvantages' ofTthe invention willbe readily apparent to one'skilledin the art- Variations in the practice of the invention apparent to one skilled in the art are intended to be included within the scope of the invention.

- I claim as my invention:

1. A method of forming electrical steels in th form of thin strips having good blankability characteristics and electrical properties comparable to those of conventional field grade steels including a'watt loss equivalent to not more than about 2.77 watts per pound at 10 kilogausses and 6.14 watts per'pound. at 15 kilogauss'es in 24-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.10 percent and a maximum silicon content of about 0.02 percent, which comprises cold rolling a strip 2f said metal to within approximately 15 to 18 percent'of the desired final thickness, heat treating the strip to remove the effects of cold rolling, thereafter cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the strip at a temperature adjacent but below the upper limit of the critical range of the steel.

2. A method of forming electrical steels in the form of thin strips having a final maximum thickness of the order of about .01 to .03 inch, and having good blankability characteristics and electrical properties comparable to those of conventional field grade steels, including a watt loss equivalent to not more than about 2.77 watts per pound at 10 kilogausses and 6.14 watts per pound at 15 kilogausses in 24-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.06 percent and a maximum silicon content of approximately 0.02 percent, which comprises cold rolling a strip of said metal to within about 15 to 18 percent of the desired final thickness, heat treating the strip to remove the effects of cold rolling, thereafter cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the strip at a temperature adjacent but below the upper limit of the critical range of the steel.

.3. A method of forming electrical steels in the form of thin strips having good blankability characteristics and electrical properties comparable to those of conventional field grade steels including a watt loss equivalent to not more than about 2.77 watts per pound at 10 kilogausses and 6.14 watts per pound at 15 kilogausses in 24-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.10 percent and a maximum silicon content of about 0.02 percent, which comprises cold rolling a strip of said metal to within approximately 15 to 18 percent of the desired final thickness, subjecting said strip to a normalizing treatment at a temperature adjacent but abov the upper limit of the critical range of the steel, thereafter cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the strip at a temperature adjacent but below the upper limit of the critical range of the steel.

4. A method of forming electrical steels in the form of thin strips having good blankability characteristics and electrical properties comparable to those of conventional field grade steels including a watt loss equivalent to not more than about 2.77 watts per pound at 10 kilogausses and 6.14 watts per pound at 15-ki10gausses in 24-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.10 percent and a maximum silicon content of about 0.02 percent, which comprises coldrolling a strip of said metal to within approximately 15 to 18 percent of the desired final thickness, annealing said strip at a temperature adjacent but below the upper limit of the critical range of the steel, thereafter cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the strip at a temperature adjacent but below the upper limit of the critical range of the steel.

5. A method of forming electrical steels in the form of thin strips having good blankability characteristics and electrical properties comparable to those of conventional field grade steels includin a watt loss equivalent to not more than about 2.77 Watts per pound at 10 kilogausses and 6.14 watts per pound at kilogausses in z l-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.10 percent and a maximum silicon content of about 0.02 percent, which comprises cold rolling a strip of said metal to within approximately 1.5 to 18 percent of the desired final thickness, subjecting said strip to a normalizing treatment at a temperature exceeding 1625 degrees F., thereafter cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the strip at a temperature within the range of 1300 to 1825 degrees F.

6. A method of forming electrical steels in the form of thin strips having good blankability characteristics and electrical properties comparable to those of conventional field grade steels including a watt loss equivalent to not more than about 2.77 watts per pound at 10 kilogausses and 6.14 watts per pound at 15 kilogausses in 24-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.10 percent and a maximum silicon content of about 0.02 percent, which comprises cold rolling a strip of said metal to within approximately 15 to 18 percent of the desired final thickness, annealing said strip at a temperature within the range of 1300 to 1625 degrees F., thereafter cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the strip at a temperature within the range of 1300 to 1625 degrees F.

7. A method of forming electrical steels in the form of thin strips having a final maximum thickness of the order of about .01 to .03 inch, and having good blankability characteristics and electrical properties comparable to those of conventional field grade steels, including a watt loss equivalent to not more than about 2.77 watts per pound at 10 kilogausses and 6.14 watts per pound at 15 kilogausses in fa l-gauge thickness, from low carbon rimming steels having a maximum carbon content of about 0.10 percent and a maximum silicon content of about 0.02 percent, which comprises hot rolling a strip of said metal 'to a thickness greater than that of the desired final thickness, thereafter cold rolling said strip to reduce its thickness to approximately 15 to 18 percent of the desired final thickness, heating said strip to a temperature sufficient to remove the working stresses, cold rolling the said strip to reduce its thickness to the final desired thickness and thereafter annealing the resultant strip at a perature adjacent but below the upper limit of the critical range of the steel.

8. A method of forming steel laminae suitable for use in cores and the like and having electrical properties comparable to that of laminae formed of conventional field grade silicon from rimming steels having a maximum carbon content of 0.10 percent and a maximum silicon content of 0.02 percent comprising cold rolling a strip of said metal to within approximately 15 to 18 percent of the desired final thickness of the laminae, heat treating the strip to remove the effects of cold rolling, cold rolling the strip to reduce its thickness to the final desired thickness, thereafter annealing the resulting strip at a temperature adjacent but below the upper limit of the critical range of the steel, and then stamping the strip into laminae of desired configuration.

9. A method of forming steel laminae suitable for use in cores and the like and having electrical properties comparable to that of laminae formed of. conventional field grade silicon steels from rimming steels having a maximum carbon content of 0.10 percent and a maximum silicon content of 0.02 percent comprising cold rolling a strip of said metal to within approximately 15 to 18 per cent of a desired final thickness in the range of .01 to .03 inch, heat treating the strip toremove the effects of cold rolling, thereafter cold rolling the strip to reduce its thickness to the final desired thickness, stamping the strip into laminae of desired configuration, and then annealing the laminae at a temperature adjacent but below the upper limit of the critical range of the steel.

10. A low carbon rimming steel in strip form having good blankability characteristics and having electrical properties comparable to those of conventional field grade electrical steels, in cluding a watt loss equivalent to not more than 2.77 watts per pound at 10 kilogausses and 6.14 watts per pound at 15 kilogausses in 24-gauge thickness, said steel being obtained by cold rolling a strip of low carbon rimming steel to within References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,855,739 Duftschmid Apr. 26, 1932 2,377,922 Campbell et a1 June 12, 1945

Claims (1)

1. A METHOD OF FORMING ELECTRICAL STEELS IN THE FORM OF THIN STRIPS HAVING GOOD BLANKABILITY CHARACTERISTIC AND ELECTRICAL PROPERTIES COMPARABLE TO THOSE OF CONVENTIONAL FIELD GRADE STEELS INCLUDING A WATT LOSS EQUIVALENT TO NOT MORE THAN ABOUT 2.77 WATTS PER POUND AT 10 KILOGAUSSES AND 6.14 WATTS PER POUND AT 15 KILOGAUSSES IN 24-GAUGE THICKNESS, FROM LOW CARBON RIMMING STEELS HAVING A MAXIMUM CARBON CONTENT OF ABOUT 0.10 PERCENT AND A MAXIMUM SILICON CONTENT OF ABOUT 0.02 PERCENT, WHICH COMPRISES COLD ROLLING A STRIP OF SAID METAL TO WITHIN APPROXIMATELY 15 TO 18 PERCENT OF SAID DESIRED FINAL THICKNESS, HEAT TREATING THE STRIP TO REMOVE THE EFFECTS OF COLD ROLLING, THEREAFTER COLD ROLLING THE SAID STRIP TO REDUCE ITS THICKNESS TO THE FINAL DESIRED THICKNESS AND THEREAFTER ANNEALING THE STRIP AT A TEMPERATURE ADJACENT BUT BELOW THE UPPER LIMIT OF THE CRITICAL RANGE OF THE STEEL.