US3761253A - Steel for electrical applications and novel article - Google Patents

Abstract

A DUCTILE NON-ORIENTED ELECTRICAL-SHEET STEEL CONTAINING UP TO 0.025% CARBON, 1.5 TO 2.7% SILICON 2.2 TO 5.2% ALUMINUM WITH THE BALANCE IRON AND NORMAL IMPURITIES, AND HAVING THE COMBINED SILICON AND ALUMINUM CONTENT SUFFICIENT TO SATISFY THE EQUATION:

(PERCENT SI)+0.37 (PERCENT AL)$3.7

AND AN ALUMINUM CONTENT EQUAL TO OR GREATER THAN THE SILICON CONTENT. UPON A CONTROLLED ATMOSPHERE ANNEAL, A COMPLEX OXIDE OF IRON, SILICON AND ALUMINUM IS FORED ON THE SHEET SURFACE WHICH PROVIDES A HIGH DEGREE OF ELECTRICAL RESISTANCE ACROSS THE SURFACE OF THE SHEET.

Description

L. J- REGITZ Sept. 25, 1973 STEEL FOR ELECTRICAL APPLICATIONS AND NOVEL ARTICLE Filed aanqzo, 1972 COLD ROLLAB/L/TY L/M/Z'y-COMMERC/AL EQUIPMENT 5 m R h/ M wm Mm E .1 V m m WW TE w Mm ME UMV MM P m ON 6 R A 0 8 .MA n m L United States Patent ()ffice 3,7fil,253 Patented Sept. 25, 1973 3,761,253 STEEL FOR ELECTRICAL APPLICATIONS AN NDVEL ARTICLE 1 Lester J. Regitz, Penn Township, Allegheny County, Pa., assignor to United States Steel Corporation Continuation-impart of application Ser. No. 882,729, Dec.

5, 1969, now Patent No. 3,657,024, which is a continuation-in-part of abandoned application Ser. No. 591,982, Nov. 4, 1966. This application Jan. 20, 1972, Ser. No. 219,377

Int. Cl. C22c 37/10, 39/02 US. Cl. 75--124 3 Claims ABSTRACT OF THE DISCLOSURE A ductile non-oriented electrical-sheet steel containing up to 0.025% carbon, 1.5 to 2.7% silicon, 2.2 to 5.2% aluminum with the balance iron and normal impurities, and having the combined silicon and aluminum content sufficient to satisfy the equation:

(Percent Si) +0.37(Percent A1)g3.7

and an aluminum content equal to or greater than the silicon content. Upon a controlled atmosphere anneal, a complex oxide of iron, silicon and aluminum is formed on the sheet surface which provides a high degree of electrical resistance across the surface of the sheet.

This application is a continuation-in-part of application Ser. No. 882,729, filed Dec. 5, 1969, Pat. No. 3,657,- 024, which was a continuation-in-part of application Ser. No. 591,982, filed Nov. 4, 1966, now abandoned.

Steel for electrical sheet applications is ferromagnetic. Ferromagnetic materials may be divided into soft and hard, or permanent magnet, ferromagnetic categories. The soft ferromagnetic compositions are widely used as core materials since they are capable of being magnetized to high magnetic field densities in electrical devices such as motors, generators, power transformers, saturable core reactors, relays and similar equipment, with low energy losses incurred upon magnetization. Generally, these core materials are employed in the form of punched or sheared polycrystalline sheet laminations which are stacked to form the desired geometric shape or volume of the core.

The cores may be formed from punched, sheared or slit strips of polycrystalline sheet-soft ferromagnetic material that is Wound into concentric layers or stacked and bent to assume the desired geometric shape.

At the present time, a widely used material for electrical applications is a steel containing up to about 5% silicon, the balance being substantially all iron plus the usual incidental steelmaking impurities. Alloys containing up to about 8% aluminum, balance iron, or up to 5% molybdenum, balance iron, have also been used for electrical-sheet steel applications. The alloying elements are thought to be beneficial because they increase the electrical resistivity of the material, thereby reducing eddy currents.

In electrical-sheet steel, the crystalline structure is body centered cubic at operating temperatures. Such materials may be easily magnetized in a crystallographic direction parallel to a cube edge or a unit cell of the body centered cubic lattice.

For some applications, it is desirable to employ nonoriented steels, i.e., steels in which the grains have no significant alignment or preferred orientation of their respective cube edges. Polycrystalline sheet steel satisfying this condition is characterized by having equal magnetic induction or flux density for a given magnetizing force in any direction in the polycrystalline sheet material, and therefore, an equal permeability in any direction. Unfortunately, these steels which contain high alloy content to insure low core losses are very difficult to fabricate into desirable polycrystalline sheet forms due to inability to cold roll these relatively brittle materials satisfactorily. As a result, steels containing about 3.5 to 5% silicon are generally manufactured into electrical-sheet material by a relatively expensive series of operations involving successive hot rolling and long box annealing operations. The relatively rough surface resulting from such hot rolling causes a decreased and inferior magnetic permeability. Moreover, many of these materials must be sheared since their excessive room temperature brittleness precludes economical punching for use in laminations. In addition, the brittleness of these materials limits their production to relatively short sheets that can be conveniently heated for successive hot rolling operations, or which can be manufactured into coils by joining the relatively short sheet sections end to end into coil form. The difiiculties due to brittleness can be overcome by reducing the alloy content. However, while this results in a material that can be cold reduced and punched, such steels have a lower quality (higher core losses) due to the increase in eddy current losses accompanying the decrease in alloy content.

Iron-aluminum alloys also exhibit brittleness which is believed to be due to the presence of excessive, even when low, amounts of carbon. It is very difficult to remove carbon in the solid state during processing of these alloys after solidification without incurring severe and detrimental oxidation of the alloy.

It is an object of the present invention to provide a high-quality, non-oriented, polycrystalline sheet steel of soft ferromagnetic material which is sufiiciently ductile to permit manufacture into coil form yet possesses the core loss and permeability properties typical of a brittle high alloy electrical-sheet steel. Such steel, in accordance with the invention, has a nonoriented body centered cubic polycrystalline structure, a preferment of orientation not greater than that available in non-oriented polycrystalline soft ferromagnetic materials and magnetic properties at least as good as such available steels, and has sufficient ductility to permit cold rolling and punching.

Another advantage of my steel composition is that the magnetic properties do not deteriorate with increased sheet thickness as much as they do with other electricalsheet steels. Thus, the core loss properties of this steel are substantially better than conventional electrical grade steels at greater thickness as well.

An additional benefit of the steel composition provided in accordance with the invention is that it lends itself to the formation on its surface of an electrically non-conducting coating when exposed to controlled atmospheres during annealing operations. The coating so produced has a superior resistance and is desirable to further reduce core loss due to eddy currents.

In accordance with the invention, there is provided a novel steel composition suitable for ductile, non-oriented electrical-sheet steel applications which consists essentially of up to about 0.025% carbon, 1.5 to 2.7% silicon, and 2.2 to 5.2% aluminum, the balance iron and normal steelmaking impurities, with the combined silicon and aluminum content satisfying the equation:

(Percent Si) +0.37(Percent Al)53.7

and the aluminum content being equal to or greater than the silicon content. Another embodiment of the invention comprises a sheet of the aforementioned steel composition containing a surface coating of a complex iron-aluminum-silicon oxide, preferably less than 1 mil thick. Uninterrupted coatings in accordance with the invention of less than 0.0005-inch improve the surface resistance substantially.

the total alloy content, silicon plus aluminum, does not exceed the relationship:

(Percent Si) +0.37(Percent Al) 3.7

In summary, therefore, most of the above composition limits are very critical if the sheet is to have sufficient ductiltiy to permit cold rollability on commercial equipment, and yet retain commercially acceptable magnetic properties. Specifically, in order to obtain the essential degree of ductility, it is critical that (1) the total alloy content, silicon and aluminum, must satisfy the above noted equation; (2) the silicon content must not exceed 2.7%; and (3) the aluminum content must not be less than the silicon content. Moreover, in order to obtain magnetic properties equal to or superior to prior art commercial steels, the silicon content must be at least 1.5%. Since the aluminum content cannot be less than the silicon content, it is therefore essential that the aluminum content must also be at least 1.5% for material to be cold rolled on laboratory mills, and at least 2.2% to yield sheet material with acceptable magnetic properties after cold rolling on production facilities. And, as the prior art has shown, the carbon content must be minimized in order to optimize magnetic properties. The attached figure is a Fe-Si-Al ternary phase diagram illustrating the above compoition limits as shown in the shaded area.

The resulting ductility effected by this invention is greater than the ductility of iron-silicon alloys of comparable alloy content. The increase in ductility is accomplished without impairing resistivity and saturation magnetization, and at the same time initial and maximum permeabilities increase. It is apparent that greater ductility is an important consideration in cold Working these steels and the working significantly affects the final magnetic properties, particularly the core loss and permeability at given induction. Moreover, cold working is more economical than hot working here. A preferred composition in accordance with the invention contains up to about 0.015% carbon, 2 to 2.7% silicon, 2.2 to 4% aluminum and a combined silicon and aluminum content of between 4.2 and 6.7%, the balance substantially iron and normal impurities.

In the preferred method of fabricating steel of the abovementioned compoistion into sheets and coils suitable for the manufacture of electrical equipment components, clean ingots of the composition are first brought to temperature of between about 2100 and 2500 F., more desirably between 2200 and 2350 F., by soaking and rolled to slabs, usually less than 8-inches, i.e. between 5- and 7-inches thick, at a finishing temperature between 1600 and 2000 F., preferably 1700 and 1800 F. The slabs may be slow cooled if desired or if necessary due to the slab thickness. The slabs are reheated to 2100 to 2500 F., preferably 2200 to 2350 F., and hot rolled to sheet having a thickness between 0.06 and 0.12 inch at a finishing temperature of 1600 to 1800 F. The hot-rolled sheet is water cooled by spraying to a temperature below about 1700 F., and may be cooled to room temperature.

Hot-rolled sheets so produced may then be pickled to remove scale and cold rolled to final thickness directly and without intermediate annealing, if desired. Conventional processing may also include an annealing after pickling, prior to cold reduction. The cold reduced sheet may then be box annealed or continuously annealed in appropriate furnaces at between 1400 and 2100 F., a preferred temperature between 1600 and 2000 F. is desirable, depending upon the mode of annnealing. The annealing time at temperature may vary although generally annnealing is performed for a time sufficient to promote recrystallization, some grain growth when desired and some purifiction.

An improvement in the aforementioned manufacturing process to produce a product in accordance with the invention involves performing the aforementioned final annealing in a hydrogen or nitrogen atmosphere, or mixtures thereof, or any inert gas atmosphere with a dew point between +30 and 26 F., and preferably less than 8 F. The dew point may be adjusted, however, to allow some purification, particularly of carbon, while not allowing appreciable oxidation of the alloy, particularly oxidation of the aluminum contained in the alloy. By using atmosphere containing a limited quantity of water vapor as measured by the dewpoint of the gas, a continuously coherent and adherent coating composed of alumina, silica and iron oxide is produced on the surface of the sheet which provides a superior level of resistance to the fiow of electrical current through the surface of the sheet.

The following examples will aid in understanding the invention:

Steel containing 1.5 to 5.2% aluminum, 1.5 to 3.5% silicon and the balance substantially iron and the normal steel-making impurities of the compositions shown in Table I were vacuum melted by processes which simulated melting in open hearth or basic oxygen steelmaking facilities followed by vacuum carbon deoxidizing and then cast into 3 x 8 x 14-inch ingots. After reheating to a temperature between 2200 and 2350 F., the ingots were hot rolled to l-inch slabs at a finishing temperature of between 1700 and 1750 F. and slowly cooled in insulating vermiculite. After conditioning and reheating to 2250 F., the slabs were hot rolled to a thickness of 0.080-inch at a finishing temperature between 1600 and 1700 F. Following hot rolling, the sheet was cooled either by quenching to room temperature in a water spray or by quenching to a temperature between 1250 and 1300 F. accomplished by holding in a furnace at 1200 F. for 1 hour. The latter process is preferred to simulate commercially employed coiling practice. The samples were then pickled, cold rolled to 0.014-inch, sheared and annealed at the various times and temperatures shown in Table II.

Sample numbers 1 through 12 were laboratory samples processed as described above. Samples 13 through 16 were of commercially produced AISI grades of electrical sheet steel, and are included for comparison. Samples 17 and 18 were samples taken from the product of a 30-ton heat made on commercial production facilities.

TABLE I Sample numbers r AISI M14 grade.

2 AISI M15 grade.

S Si Cu AISI M17 grade.

3 Maximum. AISI M19 grade.

TABLE II.SAMPLE DESIGNATION AND PROCESSING TREATMENTS EMPLOYED IN THEIR PRODUCTION Nominal composition, Annealing treatment percent Tempera- Al Si Treatment prior to final anneal Time ture, F. Atmosphere 1. 5 3. 25 Hot rolled, quenched, cold reduced 1 10 minutes 1,800 N -15% Hz 2 3 do do 1,800 N215% H2 3 do 1, 800 N215% Hz 4 2 .....d0. do 1,800 Nr15% Hg 3 2 Hot rolled, coiled, cold reduced 1 do 1,800 Ive-15% H2 4 2 Same (including footnotes) 1, 800 N215% H;

1.5 .3. 25 do 1,800 Hz 3 2 1,800 H2 1. 5 1,600 H215% Na 2 1,600 N215% Hz 3 2 1,600 N215% H2 4 2 ..do. --do 1,600 N-r15% H2 4. Hot rolled, commercial M14 non-oriented Hot rolled 0 4. 25 Hot rolled, commercial M15 non-or ented do 0 4 Hot rolled, commercial M17 non-oriented..-

2. 5 2. 5 Hot rolled, slow cooled, cold reduced 4 minutes 1, 830 N;

2.5 2.5 do. 3minutes 1,900 N;

1 Single cold reduction.

2 Double cold reduction with intermediate anneal. 3 Simulated by slow cooling from 1,200 F to room temperature.

Annealing was accomplished in either nitrogen, hydrogen or a mixed nitrogen atmosphere containing 3 to 15% hydrogen and having a dewpoint between about +6 and --26 F. as shown in Table III.

tion Test. The coating was obtained by annealing under the conditions shown in the table for each sample. However, the surface coating could be developed through anneals at lower temperatures and shorter times with appropriate increases in the dewpoint of the atmosphere.

TABLE IV MAGNETIC PROPERTIES OF SAMPLES Maximum 10 kg. kg. torque, Franklin thousands insulation Core loss, Core loss, of dyne- Ampere at w./lb./60 Perm. w./lb,/60 Penn c1n./cm. 250 psi 0.620 7, 750 1.39 1,200 16 e 0.05 0.645 7, 500 1.47 1, 145 20 0.016 0.580 8, 300 1.80 1, 220 13 l 0.09 0.640 6,000 1.49 450 22 0. 02 0. 640 7, 500 1. 44 918 ND N D 0. 647 2, 875 1.60 215 ND ND 0. 643 6,150 1. 46 892 ND ND 0.625 5,850 1.40 1,015 ND ND 0.51 8,100 1.10 1,064 25 ND 0. 57 7, 576 1. 26 938 23 ND 0. 59 5, 100 1. 32 857 19 ND 0. 62 5, 025 1. 31 714 20 ND 0. 532 3, 409 l. 22 378 37 b 0. 56 0.549 4, 379 1. 29 377 49 b 0. 5O 0. 602 4, 456 1. 34 412 57 b 0.47 0. 519 6, 897 1. 25 600 44 0. 56 0.470 7, 937 1. 1, 230 34 d 0.25

a After annealing at 2,000 F. in Nz-3% Eli-8 F. dew point, for 8 hours.

b Separate core plating and annealing operations.

v After annealing at 1,830 F. in nitrogen with a 26 F. dew point.

d After annealing at 1,900 F. in nitrogen with 2. +20 F. dew point.

No'rE.ND=Not determined.

TABLE IIL-DEWPOINT RANGES IN EXPERIMENTAL ANNEALS Dew point Temperature, F. Length of time range Magnetic properties are not always well-developed in anneals conducted at a dewpoint of +6 F., but the surface coating was satisfactory. Dewpoints of less than about -8 F. consistently produce superior coatings. The sheets thus produced were tested for core loss and permeability at 10 and 15 kg. and at 60 cycles and the resulting values corrected for the specific gravity of the samples being treated. The corrected data for successful combinations of composition and processing are shown in Table IV along with the results of typical torque magnetometer data and data reflecting the resistance of the surface coating as measured by the well-known Franklin Insula- Annealing can be accomplished at any temperature between 1400 and 2200 F. for a period of 3 minutes to 8 hours as necessary to develop suitable electrical properties. The preferred temperature is 1600 to 2000 F. with the optimum conditions being between 1800 and 1900 F. for 3 to 6 minutes for continuous anneal or for 8 hours for a box annealing. Alloys developed effective coatings at all annealing times and temperatures used within the dewpoint range of +3 to -26 F.

As mentioned above, steels in accordance with the invention exhibit substantial superior core loss properties in thicker sheet sizes than conventional electrical grade steels. The data in Table V, for example, shows how the core loss properties decrease (i.e. increase in value) with sheet thickness. Even at thicknesses of 18 and 25 mils the core loss of steel in accordane with the invention is almost 50% less than the other steels. Sample 2A corresponds in composition to sample 2 of Table I. However, it was treated by double cold reduction with intermediate and final anneals in an atmosphere of 15 H in N for 10 minutes at 1800 F.

TABLE v.-10 KG. CORE LOSS VARIATION WITH 1 Not available.

All samples containing from 1.5 to 3.5% silicon and 1.5 to 6% aluminum where the total content was from 3 to 7.5%, yielded the desired magnetic properties and ductility when processed with laboratory equipment where no end-tension is applied. However, when experimentation was advanced to commercial production equipment, more limiting critical composition limits were shown to be necessary. Specifically, some compositions within the above range cannot be cold rolled on commercial equipment without breaking, despite extreme precautions to prevent breakage. Evaluation of these experiences revealed that in order to insure sufficient ductility to be cold rollable on commercial production equipment without breakage, it is necessary to confine the silicon content to the range 1.5 to 2.7%, and to assure that the aluminum content is equal to or greater than the silicon content. Therefore, even though the samples within the first discussed specified ranges do provide superior magnetic properties and improved ductility, the ductility is still not sufficiently improved to permit commercial cold rolling unless the silicon is below 2.7%, i.e. Within the range 1.5 to 2.7% and the aluminum to silicon ratio is at least unity.

To understand the differences realized between laboratory cold rolling and commercial cold rolling, it must first be noted that additions of silicon and/ or aluminum to the steel causes a reduction of both ductility and malleability. Both are reduced sufficiently to limit the alloy content of cold-rollable iron-silicon-aluminum alloys. The effect of aluminum additions on such reduction of cold-rollability is not as pronounced as the effect of silicon additions, as is evidenced by lines on the attached figure, representing alloy limits of cold-rollability in the laboratory and on production facilities, which indicate higher total alloy contents, and hence superior magnetic properties for alloys in which aluminum content is high, relative to silicon content. With this in mind, it is noted that laboratory coldrolling is performed on small sample sheets without end tensioning, hence, malleability is the primary controlling factor. In commercial cold-rolling, however, the equipment includes end-tensioning means applied to the strip being rolled to facilitate easier cold-reduction and to provide precise, tight and neat coils of cold rolled steel. On commercial equipment, therefore, malleability and ductility are both factors. The effect of reduction of ductility is additive to the effect of reduction of malleability causing the total alloy content to be reduced for cold-rollability on production facilities, relative to such limits for laboratory facilities. Also appearing in the attached figure are minimum alloy contents permitting the superior magnetic properties displayed by alloys of this invention.

It is apparent from the above that various changes may be made without departing from the invention. Accordingly, the scope thereof should be limited only by the appended claims.

I claim:

1. A ductile electrical sheet steel having a non-oriented polycrystalline structure consisting essentially of up to about 0.025,% carbon, 1.5 to 2.7% silicon, 2.2 to 5.2% aluminum, the combined silicon and aluminum content satisfying the equation:

(Percent Si) +0.37(Percent A1) 53.7

with the aluminum content being at least equal to the silicon content, and the balance substantially iron and normal impurities.

2. A steel according to claim 1 in which the carbon content is below about 0.015%.

3. A ductile electrical sheet steel having a non-oriented polycrystalline structure consisting essentially of up to about 0.025% carbon and silicon and aluminum contents within the shaded area in the attached figure.

References Cited UNITED STATES PATENTS 842,043 1/ 1907 Hadfield -l24 2,193,768 3/1940 Masumoto 75--124 HYLAND BIZOT, Primary Examiner

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