GB1578911A - Silicon-iron sheet production involving electrocoating - Google Patents

Description

(54) SILICON-IRON SHEET PRODUCTION INVOLVING ELECTROCOATING (71) We, GENERAL ELECTRIC COMPANY, a corporation organized and existing under the laws of the State of New York, United States of America, of 1 River Road, Schenectady 12345, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement::- The present invention relates generally to the art of producing electrical steel and is more particularly concerned with a method of electrolytically depositing a boron-containing electrically-insulating coating on a boron-containing silicon-iron magnetic sheet, and with an electrolyte composition for use in that method, as well as the uniquely coated silicon-iron product made by that method.

Following the discovery as described in U.S. Patent No. 3,905,842) that boron is effective in small but critical amount and in critical proportion to nitrogen in silicon-iron to promote secondary recrystallization during the final texturedeveloping anneal, U.S. Patent Application Serial No. 677,147, found that the presence of a very small amount of boron in the coating on such a boron-containing steel further promotes secondary recrystallization and development of still better magnetic properties in the ultimate product. It has been found that the presence of boron in the coating can cause secondary recrystallization to take place when it otherwise would not, and also discovered that the presence of boron in the insulating coating was not effective in causing or promoting secondary recrystallization in the absence of boron in the metal itself at the outset of the final anneal.

Heretofore, as described U.S. Serial No. 677,147, boron has been incorporated in the refractory oxide coating, usually magnesium hydroxide [Mg(OH)2], provided in accordance with the process disclosed and claimed in U.S. Patent No. 3,054,732 by a dipping operation or by brushing a solution of a suitable boron compound on the coating, or even spraying it on.

Accordingly, the present invention provides a method of producing grainoriented silicon-iron sheet which comprises the steps of providing a fine-grained, primary-recrystallized silicon-iron sheet containing 2.2 to 4.5 per cent by weight silicon, between three and 50 parts per million by weight boron, and between 15 and 95 parts per million by weight nitrogen in the ratio to boron of one to 15 parts per part by weight of boron, electrolyzing an aqueous solution consisting essentially of magnesium acetate and magnesium metaborate and containing magnesia with the silicon-iron sheet being arranged as the cathode in said solution and the said solution being at a temperature of at least 65"C and thereby covering the sheet with a boron-containing adherent electrically-insulating coating of Mg(OH)2 and subjecting the coated sheet to a final heat treatment to develop (110) 10011 secondary recrystallization texture in the silicon-iron sheet.

Thus it has been found possible to deposit a boron containing electrically insulating coating on a boron containing electrical steel. Such deposition according to the teaching of this invention may well lead to a number of important advantages. One advantage of this invention can be that the resulting product has a surface of substantially the same character as that of the coating produced by the process described in U.S. Patent No. 3,054,732 and quite different from those resulting from dipping, spraying or brushing operation. In particular, the coated article or product of this invention is more amenable to fabrication operations involved in customary uses of this electrical steel material in that its coating does not tend to flake or spall as the material in sheet or strip form is laminated or coiled and successive layers or courses are moved in sliding c6ntact with each other.

Additionally, a further advantage of this invention may be that it becomes possible to exercise better control over the amount of boron incorporated in the insulating coating and also to distribute the boron more uniformly throughout the coating. A still further advantage may be the ability to prevent premature egress of boron from the silicon iron sheet during the final anneal while at the same time limiting the boron content of the coating to a somewhat lower level to thereby avoid the detrimental effects of excess boron on the ultimate product. Yet a further advantage of this invention is the ability to further enhance the magnetic properties of the ultimate sheet material by providing a getter for sheet sulfur in the insulating coating itself.

In a first preferred embodiment of the present invention, it is possible to codeposit boron with magnesium hydroxide electrolytically, under certain special circumstances. Such simultaneous deposition of Mg(OH)2 and boron containing compound in intimate admixture, or more likely as a solid solution results in substantially more uniform distribution of boron throughout the coating than the prior dipping, brushing, and spraying methods. Additionally, it is possible to exercise better control over the amount of boron incorporated in the Mg(OH)2 coating, the proportions being established by the boron content of the electrolyte employed in the process, which is readily regulated.

This first preferred embodiment of the present invention is predicated upon the discovery that a solution of magnesium acetate and magnesium metaborate containing magnesia as a dispersed solid second phase can be electrolyzed to provide on a silicon-iron cathode a coating in which boron is both uniformly distributed and present in the desired amount. This codeposition process is also based on the discovery that satisfactory coatings can be produced consistently by this process only if the electrolyte temperature is above 65"C throughout the codeposition period. Additionally, it has been found that this new electrolyte can be prepared by adding the requisite amount of boron in the form of boric acid to magnesium acetate solution containing magnesia as a solid second phase.Still further, it has been found that continuous operations can be carried out by adding boric acid or other suitable source of boron (such as magnesium metaborate or magnesia containing boron in solid solution) to the electrolyte either continuously or intermittently to maintain the electrolyte magnesium metaborate content necessary to the deposition of a coating of the desired boron content.

Further, a second preferred embodiment which is a modification of the first embodiment of this invention attempts to realize the advantages of the first embodiment and to further achieve the heretofore conflicting objectives of using a boron coating to prevent premature egress of boron from the silicon-iron sheet during the final anneal while limiting the boron content of the coating to a somewhat lower level to avoid detrimental effects of excess boron on the ultimate product. Specifically, in this regard it has been discovered that the boron necessary to block loss of boron from the metal substrate too early in the final anneal can be provided in the form of a relatively thin first layer or primary coating.Yet further, it has been discovered that by providing a somewhat heavier or thicker second layer or overcoat containing little or no boron, the boron of the primary coating will be retained in place in proximity to the metal surface long enough to insure development of the desired secondary recrystallization texture in the silicon-iron sheet. Consequently, both the boron concentration requirement and the total coating boron content requirement are met.

It has been found that while the overall thickness of the duplex or combination coating of this invention is not sharply critical to the attainment of our new results, it is important that the boron-containing primary coating be of thickness sufficient to provide the requisite amount of boron for the egress-blocking purpose.

Consequently, the thinner the primary coating, the better in general and, in any event, it should not exceed about 0.07 mil in thickness because of the very limited mobility of boron at the relatively low temperatures prevailing during the critical early stage of the final anneal.

Additionally, it has been found that while it is possible to provide the secondary coating other than by the electrolytic method, it is not feasible in production operations as a practical matter for the reasons indicated above concerning subsequent fabrication operations. Still further, we have found that while Mg(OH)2 is preferred for this purpose, other refractory oxides may be used instead, as set forth in U.S. Patent No. 3,054,732.

Briefly described, with regard to the first preferred embodiment, the method of this invention comprises the steps of providing a boron-containing electrical steel, electrolyzing an aqueous solution of magnesium acetate and magnesium metaborate of pH 8.09.0 with the silicon-iron sheet material being arranged as the cathode in the solution and with the solution being at a temperature of at least about 65"C and thereby covering the sheet with a boron containing adherent electrically-insulating coating, and subjecting the resulting coated sheet to a final heat treatment to develop (110)[001] secondary recrystallization texture in the silicon-iron sheet.

Similarly described, the article aspect of the first preferred embodiment of this invention is the magnesia-coated, primary-recrystallized product of this process.

Finally, with regard to first preferred embodiment the composition of matter aspect of this invention is the new electrolyte used in this process which consists essentially of an aqueous solution of magnesium acetate and magnesium metaborate having a pH between 8.0 and 9.0 and containing magnesia as a solid second phase.

With regard to the second preferred embodiment of this invention, the method comprises the steps of providing a boron-containing electrical steel, electrolyzing a solid MgO-buffered aqueous solution of magnesium acetate and magnesium metaborate of pH 8.09.0 with the silicon-iron sheet material being arranged as the cathode in the solution and with the solution being at a temperature of at least about 65"C and thereby covering the sheet with a boron-containing adherent electrically-insulating but relatively thin coating, and then electrolyzing a solid MgO-buffered aqueous solution consisting essentially of magnesium acetate with the resulting coated sheet as the cathode in the magnesium acetate solution and thereby covering the boron-containing Mg(OH)2 coating with a substantially thicker coating of Mg(OH)2, and thereafter subjecting the resulting double-coated sheet to a final heat treatment to develop (110) [001] secondary recrystallization texture in the silicon-iron sheet.

Similarly described, the article aspect of the second preferred embodiment of this invention is a double-coated primary recrystallized product having a primary boron-containing coating 0.02 to 0.07 mils thick and a secondary substantially boron-free coating about 0.10 to 0.18 mil thick.

The present invention will be further described by way of example only with reference to the accompanying drawings, wherein: Figure 1 is an illustration of the ultimate silicon iron sheet in accordance with the first preferred embodiment of this invention.

Figure 2 is an illustration of the ultimate silicon iron sheet in accordance with the second preferred embodiment of this invention.

As illustrated in Figure 1, the first preferred embodiment of this invention is carried out using a boron-containing electrical steel sheet substrate and applying thereto a coating of substantially uniform thickness of suitable refractory material having electrically-insulating characteristics and containing magnesium metaborate substantially uniformly distributed throughout the coating. As the initial step in the process, the substrate metal sheet is provided by preparing a silicon-iron melt of the required chemistry and then casting and hot rolling to intermediate thickness.Thus, the melt on pouring will contain from 2.2 to 4.5 per cent silicon, manganese and sulfur in amounts in a ratio of manganese to sulfur less than 2.3, from about three to 50 parts per million boron and about 15 to 95 ppm nitrogen in the ratio range to boron of one and 15 parts to one, the remainder being iron and small amounts of incidental impurities including carbon, aluminum, copper and oxygen. Following anneal, the hot band is cold rolled with or without intermediate anneal to final gauge thickness and then decarburized.

The resulting fine-grained, primary recrystallized. silicon-iron sheet material in whatever manner produced is processed to provide the essential boroncontaining coating of this invention in preparation for the final texture-developing anneal. Processing at this point involves the critical use of the applicants' present discoveries and this invention process of electrolytically codepositing Mg(OH)2 and a boron compound source. With the sheet material connected as a cathode and the circuit as described in the above-referenced U.S. Patent No. 3,054,732 and immersed in an electrolyte of this invention as described above, a uniform thickness coating (suitably about 0.05 to 0.4 mil and preferably about 0.2 mil) of Mg(OH)2.XMg(BO2)2.YH2O, X=sl, XMg(BO2)2. YH2O, X=il,Y=015, is formed over that part of the sheet surface in contact with the electrolyte.

The electrolyte employed in this process is preferably prepared by adding boric acid to an aqueous magnesium acetate solution containing magnesia as a dispersed solid second phase. This magnesium acetate solution is suitably of 0.05 to 1.0 molar concentration and preferably about 0.2 molar strength. The pH of the electrolyte so produced will be between 8.0 and 9.0, reflecting the presence of excess magnesia. The amount of boric acid added is that which will provide the requisite boron content of the ultimate coating, which is preferably between 10 and 70 parts per million on the basis of the silicon-iron substrate, as disclosed and claimed in copending U.S. Patent Application Serial No. 677,146, filed April 15, 1976 and assigned to the assignee hereof.At the outset of the electrolytic codeposition step, the electrolyte is at a temperature above about 65"C, preferably about 9095 C, and throughout the period that codeposition is conducted the electrolyte is maintained at such elevated temperature.

As the final step of the process of the first preferred embodiment of this invention, the thus-coated sheet is heated in hydrogen or a mixture of nitrogen and hydrogen to cause secondary grain growth which begins at about 950"C. As the temperature is raised at about 50"C per hour to 10000C, the recrystallization process is completed and heating may be carried on to up to 11750C if desired to insure complete removal of residual carbon, sulfur and nitrogen.

The following illustrative, but not limiting, examples of the process with regard to the first preferred embodiment as actually carried out with the new results indicated above will further inform those skilled in the art of the nature and special utility of this invention: EXAMPLE 1 Eleven-mil strips of silicon-iron of the following composition were prepared as described in U.S.Patent No. 3,905,843, referred to above: Carbon 0.030 /O Manganese 0.035 /n Sulfur 0.031 /" Boron 0.0010 /" Nitrogen 0.0050 /n Copper 0.24 /" Aluminum 0.005 /" Iron Remainder From this melt composition, 10.8 mil sheets were produced in a series of hot rolling passes followed by pickling and annealing of the intermediate thickness sheet material (about 100 mils) and cold rolling to 60 mils thickness, whereupon the material was reheated and cold rolled again to final thickness and the cold-worked sheet was given a decarburizing heat treatment at 8000C for eight minutes in hydrogen (room temperature dew point).

Epstein strips cut from the sheet were immersed in an electrolyte prepared by adding boric acid to a slurry of magnesium acetate and magnesia in distilled water.

The amount of boric acid added to the slurry was that which would provide 50 parts per million on the basis of the silicon steel of each strip in an electrolytically applied coating of mass density about 0.0275 ounce per square foot of steel surface.

That amount in terms of concentration (moles per liter-') in the electrolyte was 0.0070 which represents 0.4317 liter-1 H3BO3 calculated as follows: M=desired mass density of electrolytically applied coating, oz.ft-2 (steel) b=boron content of coating on basis of steel G=thickness of strip (mils) then: [B]=bx 10-6xGx2.54x 10-3x7.65 =1.9431x10-8xbxG g cm-2 (steel) 28.35 M'=Mx =3.0516x10-2xM g cm2 (steel) (12x2.54)2 then: 2x 58.33 Q= 10.811 Assuming the coating composition to be MgoB203+ CB+3=boric acid concentration in the electrolyte, moles/liter-' C,+2=concentration of magnesium acetate per magnesium in electrolyte, moles/liter-' then:: 2CM g+2 Cub+3= Q-l Q=29.65 Cos,+2=0.2 moles liter-' 0.2 Cb+3= -=0.0070 moles liter-' 29.65-1.0 =0.4317 liter' H3BO3 The strips were made cathodes in electric circuits, eight volts being applied across the terminals at a current density of 90 amperes per square foot for the 40second duration of the electrolyzing period as 0.2 mil coatings of boron-containing Mg(OH)2 were formed over the entire surfaces of the strips. On removal of the strips from the electrolytes, their coatings were observed to be uniform in thickness (about 0.2 mil) and smooth and hard as is typical of those Mg(OH)2 coatings produced in accordance with the process of the referenced patent to McQuade (3,054,732).It was found on test that these coatings contained boron in the form of Mg(BO2)2. 12 H2O distributed substantially uniformly throughout and in amount closely approximating 50 parts per million on the basis of the silicon-iron substrates. Franklin insulation values for these samples were uniformly about 0.2 ampere following annealing in hydrogen at about 1175"C for eight hours.

EXAMPLE II In another similar test of the first preferred embodiment of this invention eleven-mil Epstein strips like those of Example I were coated by the procedure described just above except that the boric acid concentration of the coating solution or slurry was somewhat greater, the mass density of the electrolytically applied coating desired being only 0.004 oz.ft2 (steel). Thus, as calculated above, 0.0604 moles liter-' or 3.7337 g. liter-' H3BO3 was added to the slurry. The resulting coated strips had the characteristics described in Example I above except that the Franklin insulation values were not measured and the coatings were only about 0.03 mil in thickness, the deposition period being appropriately restricted to obtain that desired result as the other conditions stated above were approximately the same.

As illustrated in Figure 2, the second preferred embodiment of this invention is carried out using a boron-containing electrical steel substrate and applying thereto a comparatively thin primary coating of suitable refractory material having electrically-insulating characteristics and containing magnesium metaborate substantially uniformly distributed throughout the coating. Then a substantially thicker coating of suitable refractory material is applied to the coated electrical steel sheet. As the initial step in the process, the substrate metal sheet is provided by preparing a silicon-iron melt of the required chemistry and then casting and hot rolling to intermediate thickness.Thus, the melt on pouring will contain from 2.2 to 4.5 per cent silicon, manganese and sulfur in amounts in a ratio of manganese to sulfur less than 2.3, from about three to 50 parts per million boron and about 15 to 95 ppm nitrogen in the ratio range to boron of one and 15 parts to one, the remainder being iron and small amounts of incidental impurities including carbon, aluminum, copper and oxygen. Following anneal, the hot band is cold rolled with or without intermediate anneal to final gauge thickness and then decarburized.

The resulting fine-grained, primary recrystallized, silicon-iron sheet material in whatever manner produced is processed to provide the essential boroncontaining primary coating and the secondary coating of this invention in preparation for the final texture-developing anneal. Processing at this point involves electrolytically codepositing Mg(OH)2 and a boron compound source as disclosed with regard to the first preferred embodiment of this invention.

With the sheet material connected as a cathode and the circuit as described in referenced U.S. Patent No. 3,054,732 and immersed in an electrolyte as described above, a uniform thickness coating (suitably about 0.02 to 0.07 and preferably about 0.05 mil) of Mg(OH)2 XMg(BO2)2 YH2O, X=sl, Y--l5, is formed over that part of the sheet surface in contact with the electrolyte.

The electrolyte employed in this process is preferably prepared by adding boric acid to an aqueous magnesium acetate solution containing magnesia as a dispersed solid second phase. This magnesium acetate solution is suitably of 0.05 to 1.0 molar concentration and preferably about 0.2 molar strength. The pH of the electrolyte so produced will be between 8.0 and 9.0, reflecting the presence of excess magnesia. The amount of boric acid added is that which will provide the requisite boron content of the ultimate coating, which is preferably between 10 and 70 parts per million on the basis of the silicon-iron substrate, as disclosed and claimed in copending U.S. Patent Application Serial No. 677,146, filed April 15, 1976, and assigned to the assignee hereof.At the outset of the electrolytic codeposition step, the electrolyte is at a temperature above about 65"C, preferably about 9--95"C, and throughout the period that codeposition is conducted the electrolyte is maintained at such elevated temperature.

As the next step of the second preferred embodiment of this invention, the resulting coated sheet material is immersed in a solid MgO-buffered aqueous magnesium acetate solution as a cathode and an electric current is applied across the terminals to provide an electrolytic deposit of Mg(OH)2 over boron-containing primary coating. This step is suitably carried out as disclosed and claimed in U.S.

Patent No. 3,054,732 so that the thickness of the duplex coating is in the range stated above. Additionally, in the practice of the second preferred embodiment of this invention the secondary coating will contain little or no boron, it being a purpose of this embodiment to confine the boron to the primary coating so that it is present for its essential boron egress-blocking function and is not present in excess of that amount particularly in the secondary coating. Such excess boron at locations relatively removed from the sheet material surface will become comparatively mobile at the temperatures prevailing in the latter stages of the usual final or texture developing anneal.

As the final step of the process of the second preferred embodiment, the resulting duplex or combination coated sheet is heated in hydrogen or a mixture of nitrogen and hydrogen to cause secondary grain growth which begins at about 950"C. As the temperature is raised at about 50"C per hour to 10000C, the recrystallization process is completed and heating may be carried on to up to 1175 C if desired to insure complete removal of residual carbon, sulfur and nitrogen.

The following illustrative, but not limiting, examples of the process with regard to the second preferred embodiment as actually carried out with the new results indicated above will further inform those skilled in the art of the nature and special utility of this invention: EXAMPLE III Eleven-mil strips of silicon-iron of the following composition were prepared as described in U.S.Patent No. 3,905,843, referred to above: Carbon 0.030% Manganese 0.035% Sulfur 0.031 /O Boron 0.0010 / Nitrogen 0.0050 /O Copper 0.24% Aluminum 0.005% Iron Remainder From this melt composition, 10.2 mil sheets were produced in a series of hot rolling passes followed by pickling and annealing of the intermediate thickness sheet material (about 100 mils) and cold rolling to 60 mils thickness, whereupon the material was reheated and cold rolled again to final thickness and the cold-worked sheet was given a decarburizing heat treatment at 8000C for eight minutes in hydrogen (room temperature dew point).

Epstein strips were cut from the strip to provide seven packs for ultimate magnetic properties test in the usual way. The strips comprising one such pack were electrolytically coated with Mg(OH)2 through the use of a magnesium acetate electrolyte as described in U.S. Patent No. 3,054,732, the strips being cathodes in electric circuits and eight volts being applied across the terminals at current density of 90 amperes per square foot until the coating mass on each strip was about 83.00 milligrams, i.e., about 0.0285 ounce per square foot which is equivalent to about 0.22 mil average thickness over the entire surface area of the strip.Franklin insulation values for these strips as well as those of the other six packs of this experiment prepared as described below (allowance being made for minor thickness variations) were uniformly about 0.2 ampere following annealing in hydrogen at about 1175"C for eight hours.

The strips of the six other packs were likewise electrolytically coated except that in each instance two separate coatings were provided, the first being a Mg(BO2)2 coating and the second being a Mg(OH)2 coating like that of the first pack described just above. The electrolyte used for deposition of the primary coat was prepared by adding boric acid to a slurry of magnesium acetate and magnesia in distilled water. The amount of boric acid added to the slurry was 0.4317 mole per liter calculated as set forth with regard to the first preferred embodiment.

As shown in Table I, the thickness of the primary and secondary coatings of the strips of the six duplex-coated Epstein packs were varied in this experiment to determine the effects of relative thickness upon magnetic properties of the finished electrical steel product, total coating thickness being maintained reasonably constant through the individual strips of all seven packs. Also, as indicated in Table I, the first coating mass value refers in each instance to the primary Mg(BO2)2 coating, while the second designates the mass (i.e., relative thickness) of the secondary coating.

TABLE I Loss, Watts Coating Per Pound Masses Pack 15 kG 17 kG ,ul0 Oe Mg Per Strip 1 0.556 0.930 1755 0.00 83.00 2 0.498 0.689 1888 20.57 74.59 3 0.621 0.1000 1720 32.67 51.37 4 0.517 0.761 1837 52.15 36.47 5 0.492 0.694 1893 16.03 70.20 6 0.500 0.707 1872 37.04 49.15 7 0.518 0.727 1852 48.60 37.33 The ratio of MgO to B203 was from 2.5 to 3.0 in the primary coatings on the strips of Packs 2, 3 and 4, but only 1.5 to 2.0 in the corresponding borate coatings of Packs 5, 6 and 7.

EXAMPLE IV In another experimental test of the second preferred embodiment of this invention process 10.0 mil Epstein strips like those of Example III (i.e., of the same composition and processing history) were coated by the procedures described just above with the results set out in Tables II and III:: TABLE 11 2.5 MgO Loss, Watts/Pound B203 Mg(OH)2 Pack 15kG 17kG ,ulOOe Mg/Strip Mg/Strip 0.494 0.740 1852 3.70 87.70 2 0.476 0.677 1897 7.20 77.00 3 0.475 0.675 1888 13.90 69.10 4 0.483 0.674 1895 16.20 64.40 5 0.498 0.689 1888 20.57 74.59 6 0.468 0.656 1894 21.90 63.20 7 0.464 0.661 1887 23.70 63.70 8 0.621 1.000 1720 32.67 51.37 9 0.517 0.761 1837 52.15 36.47 TABLE III 1.5 MgO Loss, Watts/Pound B203 Mg(OH)2 Pack 15 kG 17 kG ,u10 Oe (Mg/Strip) Mg/Strip 1 0.465 0.675 1883 4.20 86.00 2 0.467 0.663 1897 7.10 76.30 3 0.472 0.665 1900 11.40 65.50 4 0.467 0.655 1904 12.70 62.30 5 0.467 0.664 1891 15.10 62.80 6 0.492 0.694 1893 16.03 70.20 7 0.462 0.649 1906 19.90 66.60 8 0.500 0.707 1872 37.04 49.15 9 0.518 0.727 1852 48.60 37.33 EXAMPLE V The process of the second preferred embodiment of this invention was again tested in a manner similar to that described in Examples III and IV except that all thirty-one samples from different steel heat/coil combinations were processed in parallel by the present invention process (1.5 MgO . B2Oa primary coat) and by the following procedure.Epstein strips of the cold-rolled material were decarburized by heating at 800"C in 50"C dew point hydrogen. The decarburized strips were provided with magnesia coatings about 0.2 mil thick by the electrocoating method of US Patent No. 3054732 and then dipped in a solution consisting of 142 gallons of "raw" water, 15 pounds of boric acid and four pints of ammonia. About 50 parts per million boron (steel equivalent) were thereby incorporated in the magnesia coating. The resulting strips were annealed at 21500F in dry hydrogen for three hours.

Loss at l7kG Wattsx 10Pound y10 Oe Prior Process 704 1870 Present Invention Process 697 1881 Those skilled in the art will recognize that coating weight or thickness is commonly expressed in terms of density in ounces per square foot of steel strip surface and that 0.0275 oz/ft2=-77 milligrams per Epstein strip. Further, it is understood generally that 77 mg/Epstein strip corresponds to a uniform coating thickness of 0.05 mil.

WHAT WE CLAIM IS: 1. A method of producing grain-oriented silicon-iron sheet which comprises the steps of providing a fine-grained, primary-recrystallized silicon-iron sheet containing 2.2 to 4.5 per cent by weight silicon, between three and 50 parts per million by weight boron, and between 15 and 95 parts per million by weight nitrogen in the ratio to boron of one to 15 parts per part by weight of boron, electrolyzing an aqueous solution consisting essentially of magnesium acetate and magnesium metaborate and containing magnesia with the silicon-iron sheet being

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (20)

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE IV In another experimental test of the second preferred embodiment of this invention process 10.0 mil Epstein strips like those of Example III (i.e., of the same composition and processing history) were coated by the procedures described just above with the results set out in Tables II and III:: TABLE 11 2.5 MgO Loss, Watts/Pound B203 Mg(OH)2 Pack 15kG 17kG ,ulOOe Mg/Strip Mg/Strip 0.494 0.740 1852 3.70 87.70 2 0.476 0.677 1897 7.20 77.00 3 0.475 0.675 1888 13.90 69.10 4 0.483 0.674 1895 16.20 64.40 5 0.498 0.689 1888 20.57 74.59 6 0.468 0.656 1894 21.90 63.20 7 0.464 0.661 1887 23.70 63.70 8 0.621 1.000 1720 32.67 51.37 9 0.517 0.761 1837 52.15 36.47 TABLE III 1.5 MgO Loss, Watts/Pound B203 Mg(OH)2 Pack 15 kG 17 kG ,u10 Oe (Mg/Strip) Mg/Strip 1 0.465 0.675 1883 4.20 86.00 2 0.467 0.663 1897 7.10 76.30 3 0.472 0.665 1900 11.40 65.50 4 0.467 0.655 1904 12.70 62.30 5 0.467 0.664 1891 15.10 62.80 6 0.492 0.694 1893 16.03 70.20 7 0.462 0.649 1906 19.90 66.60 8 0.500 0.707 1872 37.04 49.15 9 0.518 0.727 1852 48.60 37.33 EXAMPLE V The process of the second preferred embodiment of this invention was again tested in a manner similar to that described in Examples III and IV except that all thirty-one samples from different steel heat/coil combinations were processed in parallel by the present invention process (1.5 MgO . B2Oa primary coat) and by the following procedure.Epstein strips of the cold-rolled material were decarburized by heating at 800"C in 50"C dew point hydrogen. The decarburized strips were provided with magnesia coatings about 0.2 mil thick by the electrocoating method of US Patent No. 3054732 and then dipped in a solution consisting of 142 gallons of "raw" water, 15 pounds of boric acid and four pints of ammonia. About 50 parts per million boron (steel equivalent) were thereby incorporated in the magnesia coating. The resulting strips were annealed at 21500F in dry hydrogen for three hours. Loss at l7kG Wattsx 10Pound y10 Oe Prior Process 704 1870 Present Invention Process 697 1881 Those skilled in the art will recognize that coating weight or thickness is commonly expressed in terms of density in ounces per square foot of steel strip surface and that 0.0275 oz/ft2=-77 milligrams per Epstein strip. Further, it is understood generally that 77 mg/Epstein strip corresponds to a uniform coating thickness of 0.05 mil. WHAT WE CLAIM IS:

1. A method of producing grain-oriented silicon-iron sheet which comprises the steps of providing a fine-grained, primary-recrystallized silicon-iron sheet containing 2.2 to 4.5 per cent by weight silicon, between three and 50 parts per million by weight boron, and between 15 and 95 parts per million by weight nitrogen in the ratio to boron of one to 15 parts per part by weight of boron, electrolyzing an aqueous solution consisting essentially of magnesium acetate and magnesium metaborate and containing magnesia with the silicon-iron sheet being

arranged as the cathode in said solution and the said solution being at a temperature of at least 65"C and thereby covering the sheet with a boroncontaining adherent electrically-insulating coating of Mg(OH)2, and subjecting the coated sheet to a final heat treatment to develop (110) [001] secondary recrystallization texture in the silicon-iron sheet.

2. A method as claimed in claim I in which the boron content of the siliconiron sheet is between 10 and 30 parts per million.

3. A method as claimed in claim 1 in which the boron content of the siliconiron sheet is 10 parts per million and the nitrogen content of the said sheet is 30 parts per million.

4. A method of claim 1 in which the electrolyte is maintained at a temperature between 90 and 95"C throughout the period of electrolytic codeposition.

5. A method as claimed in any one of the preceding claims wherein the electrolyte has a pH of 8.0 to 9.0 and consists essentially of an aqueous solution of magnesium acetate and magnesium metaborate and containing magnesia as a solid second phase.

6. A method as claimed in claim 5 wherein the electrolyte is a 0.05 to 1.0 molar solution of magnesium acetate.

7. A method as claimed in claim 5 wherein the electrolyte is a 0.2 molar solution of magnesium acetate.

8. A magnesia-coated, primary-recrystallized silicon-iron sheet product of a method as claimed in any one of the preceding claims.

9. A modification of the method as claimed in claim I in which said aqueous solution is solid MgO buffered and wherein said sheet is first covered with a relatively thin boron-containing adherent electrically insulating coating of boroncontaining Mg(OH)2 and thereafter electrolyzing a solid MgO-buffered aqueous solution consisting essentially of magnesium acetate with the resulting coated sheet arranged as the cathode in the said magnesium acetate solution and thereby convering the boron-containing Mg(OH)2 coating with a substantially thicker Mg(OH)2 coating, said resulting double-coated sheet then being subject to a final heat treatment to develop (110) [001] secondary recrystallization texture in said silicon-iron sheet.

10. A method as claimed in claim 9 in which the electrolytic codeposition step is carried out with an electrolyte consisting of an aqueous magnesium acetateborate solution containing solid magnesium hydroxide.

Il. A method as claimed in claim 10 in which the electrolyte is a 0.2 molar solution of magnesium acetate of pH from 8.0 to 8.5.

12. A method as claimed in claim 9 in which the electrolyte is maintained at a temperature between 90 and 95"C throughout the period of electrolytic codeposition.

13. A method as claimed in claim 9 in which the boron containing initial or primary coating is 0.02 to 0.07 mil thick and the Mg(OH)2 secondary layer or overcoat is 0.10 to 0.18 mil thick.

14. A method as claimed in claim 13 in which the total thickness of the electrically-insulating coatings is between 0.10 and 0.40 mil thick.

15. A method as claimed in claim 13 in which the total thickness of the two electrolytically-deposited coatings is 0.20 mil.

16. A double-coated, primary-recrystallized silicon-iron sheet when produced by a method as claimed in any one of claims 9 to 15.

17. A method of producing a grain-oriented silicon-iron sheet as claimed in claim 1 substantially as hereinbefore described in Example I or Example 2.

18. A silicon-iron sheet produced by a method as claimed in claim 17.

19. A method of producing a grain-oriented silicon-iron sheet as claimed in claim 9 substantially as hereinbefore described in any one of Examples 3 to 5.

20. A silicon-iron sheet when produced by a method as claimed in claim 19.