CA1117465A - Coated silicon-iron product and process therefor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A boron-containing electrical steel sheet is provided with an electrolytically-deposited boron-containing Mg (OH)2 primary coating about 0.05 mil thick and a secondary coating of electrolytically-deposited Mg(OH)2 about 0.15 mil thick.

Description

~74~ RD--8153 The present invention relates generally to the art of producing electrical steel and is more particularly con-cerned with a novel method of electrolytically depositing on a boron-containing a silicon-iron magnetic sheet a coating of boron-containing electrically-insulating material in which the boron is concentrated in close proximity to the metal surface. This invention is also particularly concerned with the new and useful product of that process.
This invention is related to the invention disclosed and claimed in Canadian Patent Application Serial No.
301,722, filed April 21, 1978 in the names of Ronald H. Arendt and Patrick F. Aubourg entitled, "Silicon-Iron Production and Composition and Process Therefor" and assigned to the assignee hereof and directed to the novel concept of electrolytically code-positing Mg(BO2)2 and Mg(OH)2 on boron-containing electrical steel strip or sheet material.
This invention is also related to the invention disclosed and claimed in Canadian Patent Application Serial No. 299,138 filed March 17, 1978 in the names of Ronald H. Arendt and Matthew J. Curran entitled, "Coated Silicon-Iron Product and Process Therefor" and assigned to the assignee hereof and directed to the novel concept of providing a calcium metaborate coating on silicon-iron sheet or strip material by an electrolytic deposition process.
Following the discovery by Grenoble (U.S. Patent No.
3,905,842 dated September 16, 1975, and assigned to the assignee hereof) that boron is effective in small but critical amounts and in critical proportion to nitrogen in silicon-iron to promote secondary recrystallization during the final texture-developing anneal, Maucione (Canadian Pat., ~74~i5 Application Serial No. 275,369 dated, April 1, 1977, and also assigned to the assignee hereof) found that the presence of a very small amount of boron in the coating on such a boron-containing steel further promotes secondary recrystal-lization and development of still better magnetic properties in the ultimate product. Macuione further found that the presence of boron in the coating can cause secondary re-crystallization to take place when it otherwise would not, and also discovered that the presence of boron in the in-10 sulating 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.
In the practice of Maucione's teachings, boron has been incorporated in the refractory oxide coating, usually magnesium hydroxide [Mg(OH)2], provided in accordance with the process disclosed and claimed by McQuade in U.S.
Patent No. 3,054,732 dated September 18, 1962 by a dipping operation or by brushing a solution of a suitable boron compound on the coating, or even spraying it on.
Then, through the discovery by Arendt and Aubourg that a boron-containing compound and Mg(OH)2 can be electro-lytically codeposited as described and claimed in their patent application referred to above, it ~ me possible to exercise better control over the amount of boron in-corporated in the insulating coating and also to distribute the boron more uniformly throughout the coating. As another important advantage of this codeposition method, the resulting product has a surface which is more amenable to fabrication operations customarily involved in the use of electrical steel.
In accordance with our discoveries to be described, it is possible to enhance substantially the beneficial e ~c~s ill ~46S RD-8153 on the ultimate sheet product of boron in the coating without losing the foregoing advantages of the Maucione and the Arendt and Aubourg inventions. In particular, we have found how to achieve at ~e the heretofore conflicting objectives of using coating boron 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 def~
lower level to avoid de~e~mcnLal effects of boron on the ultimate product. Specifically, we have 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 primary coating. Further, we have discovered that by providing a somewhat heavier or thicker 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 through these discoveries and the invention based thereon.
We have further found that while the overall thick-ness 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 thicnkess sufficient to provide the requisite amount of boron for the egress-blocking purpose. Con-sequently, 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.

-- ~1174~5 RD-8153 Additionally, we have 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 insted, as set forth in U.S. Patent No. 3,054,732 dated September 18, 1962 - McQuade.
Briefly described, then, the new method of this invention comprises the steps of providing a boron-containing electri-cal steel, electrolyzing a solid MgO-buffered aqueous solut-ion of magnesium acetate and magnesium metaborate of pH
8.0-9 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 65C and thereby covering the sheet with a boron-containing adherent electrically-insulating but relatively thin coating, and then electroly-zing 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~, and thereafter subjecting the resulting double-coated sheet to a final heat treatment to develop (llO) [001] secondary recrystal-lization texture in the silicon-iron sheet.
Similarly described, the article of this invention is the double-coated primary recrystallized product of this process having a primary boron-containing coating 0.02 to 0.07 mil thick and a secondary substantially boron-free coating about 0.10 to 0.18 mil thick.
As illustrated by the drawing accompanying and forming a part of this specification, this invention is carried out 111746~ RD-8153 using a boron-containing electrical steel sheet substrate and applying thereto a comparatively thin primary coating of suitable refractory material having electrically-insulating characteristics and containing magnesium meta-borate 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 andhot rolling to intermediate thicnkess. 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 incidential impurities including carbon, aluminum, copper and oxygen. Following anneal, the hot band is cold rolled with or without intermediate anneal to final gauge thicnkess and then decarburized.
The resulting fine-grained, primary recrystallized, silicon-iron sheet material in whatever manner produced is processed to provide the essential boron-containing primary coating and the secondary coating of this in-vention in preparation for the final texture-developing anneal. Processing at this point involves electrolyti-cally codepositing Mg(OH)2 and a boron comppound source as disclosed and claimed in referenced Canadian patent application Serial No. 30/, 7~ dated ~,/ ~J,J9 With the sheet material connected as a cathode and the circuit as described in referenced U.S. Patent No.3,054,732 dated September 18, 1962 and immersed in an electrolyte 111746~; RD-8153 as clescribed above, a uniform thickness coating (suitably about 0.02 to 0.07 and preferably about 0.05 mil) of Mg(O~2 XMg(BO2)2-YH2O, X = C l, Y = 0-15, is formed over that parl: of the sheet surface in contact with the electrolyte.
~he 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. At the outset of the electrolytic codeposition step, the electrolyte is at a temperature above about 65C, preferably about 90C to 95C, and throughout the period that codeposition is conducted the electrolyte is maintained at such elevated temperature.
As the next step of this invention method, the roEulting 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. $his step is suitably carried out as disclosed and claimed in referenced U.S. Patent No. 3,054,732, so that the thickness of the duplex coating is in the range stated above. Additionally, in the preferred practice of this invention ~secondary 111746~ RD-8153 coating will contain little or no boron, it being a purpose of this invention 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 relaiively 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 this invention, 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 950C. As the temperature is raised at about 50C per hour to 1000C, 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 our novel process 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 I
Eleven-mil strips of silicon-iron of the following composition were prepared as described in U.S. Patent No.
3,905,843 dated September 16, 1975, referred to above:
Carbon 0.030%
Manganese 0.035%
Sulfur 0.031%
Boron 0.0010%
Nitrogen 0.0050%
Copper 0.24%

Aluminum 0.005%
Iron Remainder lli ~4~5 RD-8153 From this melt composition, 10.2 mil sheets were produced in a ~eries of hot rolling passes followed by pickling and annealing of the intermediate thickness sheet material (about 100 mils) and cold rolling to 60 mils thick-ness, whereupon the material was reheated and cold rolled again to final thickness and the cold-worked sheet was given a decarburizing heat treatment at 800C for eigth minutes in hydrogen (room temperature dew point).
Epstein strips were cut from the strip to provide seven packs for utlimate 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 dated September 18, 1962, 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 1175C
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 1~1 74~ RD-8153 in distilled water. The amount of boric acid added to the slurry was 0.4317 mole per liter calculated as set forth in Canadian application Serial No. ~l~ 7~-~

dated ,~pr ~ / a/, / ~ 7~
As shown in Table I, the thickness of the primary andsecondary coatings of the strips of the six duplex coated 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)2coating, while the second designates the mass (i.e., relative thickness) of the secondary coating.
TABLE I

Coatings Loss, Watts Per Pound Masses Pack 15 kG 17 kG ~10 Oe Mg Per Strip 1 0.556 0.930 1755 0.00 83.00

2 0.498 ~.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 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 ~ 6 ~ RD-8153 The ratio of MgO to B2O3 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.
In another experimental test of this invention process 10.0 mil Epstein strips like those of Example I (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 II
-Loss, Watts/Pound B 3 g Mg(OH)2 Pack 15 kG 17 kG ~10 Oe Mg/Strip Mg/Strip 1 0.494 0.740 18523.70 87.70 2 0.476 0.677 18977.20 77.00 3 0.475 0.675 188813.90 69.10 4 0.483 0.674 189516.20 64.40 0.498 0.689 188820.57 74.59 6 0.468 0.656 189421.90 63.20 7 0.464 0.661 188723.70 63.70 8 0.621 1.000 172032.67 51.37 9 0.517 0.761 183752.15 36.47 1117~5 RD-8153 TABLE III

Loss, ~atts/Pound B2 3 Mg (OH)2 P _ 15 kG 17 kG ~10 Oe (Mg/Strip) Mq/Strip 1 0.465 0.675 18834.20 86.00 2 0.467 0.663 18977.10 76.30 3 0.472 0.665 190011.40 65.50 4 0.467 0.655 190412.70 62.30 0.467 0.664 189115.10 62.80 6 0.492 0.694 189316.03 70.20 7 0.462 0.649 190619.90 66.60 8 0.500 0.707 187237.04 49.15 9 0.518 0.727 185248.60 37.33 EXAMPLE III

The process of this invention was again tested in a manner similar to that described in Examples I and II
except that all thirty-one samples from different steel heat/coil combinations were processed in parallel by the present invention process (1.5 MgO.B2O3 primary coat) and by the pxocedure set forth in Example III of the Canadian patent application Serial No. 275,369 dated April/1/1977.
While the steel samples were of variable magnetic quality the average results were as follows:

Loss at 17 kG

Watts x 10 /Pound 10 Oe Maucione EX. III 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 111746~ RD-8153 in ounces per square foot of steel strip surface and that 0.0275 oz/ft = 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.

Claims (8)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method of producing grain-oriented silicon-iron sheet which comprises the steps of providing a finegrained primary-recrystallized silicon-iron sheet containing 2.2 to 4.5 percent silicon, between about three and 50 parts per million boron, and between about 15 and 95 parts per million nitrogen in the ratio to boron of one to 15 parts per part of boron, electrolyzing a solid MgO-containing 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 about 65°C and thereby covering the sheet with a boron-containing adherent electrically-insulating but relatively thin coating of Mg(OH)2, and then electrolyzing a solid MgO-containing aqueous solution consisting essentially of magnesium acetate with the resulting coated sheet arranged as the cathode in the said magnesium acetate solution and thereby covering the boron-containing Mg(OH)2 coating with a substantially thicker Mg(OH)2 coating, 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.

2. The method of claim 1, in which the electrolyzing is carried out with an electrolyte consisting of an aqueous magnesium acetate-borate solution containing solid magnesium hydroxide.

3. The method of claim 2, in which the electrolyte is a 0.2 molar solution of magnesium acetate and has a pH from 8.0 to 8.5.

4. The method of claim 1, in which the boron-containing initial or primary coating is about 0.02 to 0.07 mil thick and the Mg(OH)2 secondary layer or overcoat is about 0.10 to 0.18 mil thick.

5. The method of claim 4, in which the total thickness of the electrically-insulating coatings is between about 0.10 and 0.40 mil.

6. The method of claim 4, in which the total thickness of the two electrolytically-deposited coatings is about 0.20 mil.

7. The method of claim 3, in which the electrolyte is maintained at a temperature between about 90°C and 95°C through-out the period of electrolytic codeposition.

8. The double-coated primary-recrystallized silicon-iron sheet of the method of claim 1.