CA1117464A - Coated silicon-iron product and process therefor - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A relatively thin coat of calcium metaborate is electro-lytically-applied directly to the surface of a boron-containing electrical steel and a substantially thicker overcoat of Mg(OH)2 is electrolytically-deposited on the calcium meta-borate coating to provide a duplex coating about 0.2 mil thick.

Description

1~174~4 RD-865S

The present invention relates generally to the art of producing electrical steel and is more particularly concerned with a novel method of electrolytically depositing a boron-containing electrically-insulating coating on a boron-containing silicon-iron magnetic sheet, and with the unique coated silicon-iron product made by that method.
This invention is related to the invention disclosed and claimed in Canadian Patent Application Serial No.
299,139 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 on a boron-containing electrical steel a "duplex"
refractory electrically-insulating coating of a primary layer of an electrolytically-deposited layer of a boron-containing Mg(OH)2 and a somewhat thicker secondary layer or overcoat of electrolytically-deposited Mg(OH)2.
This invention is also related to the invention dis-closed and claimed in Canadian 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 codepositing Mg(BO2)2 and Mg(OH)2 on boron-containing electrical steel sheet or strip material.
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 amount and in critical proportion to nitrogen in silicon-iron to promote secondary recrystallization during the final texture-developing anneal, Maucione (Canadian 1~74~4 RD-8655 Application Serial No. 275,369 dated April 1, 1977 and 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 re-crystallization and development of still better magnetic properties in the ultimate product. Maucione further 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 in-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 became possible to exercise better control over the amount of boron in-corporated in the insulating coating and also to more uniformly distribute the boron through 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 effect 1~17~64 RD-8655 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 'nave found that a coating of calcium metaborate [Ca(BO2)2] can be applied electrolytically through the use of a homogeneous solution of calcium acetate and boric acid buffered with solid Ca(BO2)2 at a temperature of at least 65C and preferably about 95C.
Further, we have found that Ca(BO2)2 in coating form is effective to prevent egress of boron from the silicon-iron sheet during the final anneal and that by providing a coating of Mg(OH)2 over the Ca(BO2)2 coating it is possible to retain the boron in place in proximity to the sheet surface during the critical early stages of the final anneal even though the overcoating itself containslittle or no boron at all.
Thus, the boron essential to the secondary recrystallization of the silicon-iron during the texture-developing anneal is retained in the metal without having the offsetting disadvantages arise of either a coating which is not thick enough to provide the insulating property required or a coating which contains a total quantity of boron which is detrimental to the magnetic properties of the ultimate silicon-iron sheet product.
We have discovered in addition that the primary coating of this invention serves effectively to getter sulfur in the silicon-iron substrate to enhance the magnetic prop-erties of the ultimate sheet material.
Still further, we have found that satisfactory coatings and the new advantages of this invention can be consistently produced and obtained only if the temperatures of the electrolytes are maintained throughout the deposition operations at 65C or higher and preferably about 90-95C.

This invention has both method and article or product ..~., ..~.
f ~.

~ ~17 ~6 4 RD-8655 aspects, the method centering in the novel feature of sequen-tial Ca(BO2)2 and Mg(OH)2 electrolytic deposition. The product is the Mg(OH)2- and Ca(BO2)2 - coated boron-containing silicon-iron body produced by the sequential deposition process of this invention.
Briefly described, then, the new method of this invention comprises the steps of providing a boron-containing electri-cal steel, electrolyzing a Ca(BO2)2- buffered aqueous solution of calcium acetate and boric acid of pH less than 7.0 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 65C and thereby covering the sheet with an adherent, relatively thin, electrically-insulating coating of Ca(BO2)2, then with the resulting coated sheet arranged as the cathode in an aqueous solution consisting essentially of magnesium acetate buffered with solid MgO
electrolytically depositing a substantially thicker coating of Mg(OH)2 on the said coated sheet, and thereafter subject-ing the resulting double-coated sheet to a final heat treatment, subjecting the resulting coates sheet to a final heat treatment of develop (llO) [001] secondary re-crystallization texture in the silicon-iron sheet.
Similarly described, the article of this invention is the duplex-coated, primary recrystallized, boron-containing silicon-iron sheet product of the sequential deposition steps of the process of this invention.
As illustrated by the drawing accompanying and for-ming a part of this specification, this invention is carried out using a boron-containing electrical steel sheet sub-strate and applying thereto a substantially uniform, re-latively thin coating of Ca(BO2)2 and then applying a Mg(OH~2 coating of somewhat greater thickness to the result-i~7464 RD-8655 ing coated sheet material. 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 incidential im-purities 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 de-carburized.
The resulting fine-grained, primary-recrystallized, silicon-iron sheet material in whatever manner produced is processed to provide the essential boron-containing 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 including an inventive process of electrolytically depositing an in-itial coating of Ca(BO2)2 and then a secondary and some-what heavier or thicker coating of Mg(OH)2. With the sheet materia] connected as a cathode and the circuit as des-cribed in the above-referenced U.S. Patent No. 3,054,732 dated September 18, 1962, and immersed in an electrolyte of calcium acetate and boric acid buffered with solid Ca(BO2)2, a coating of Ca(BO2)2 of substantially uniform thickness is formed over the entire surface of the sheet in contact with the electrolyte. A coating of Mg(OH)2 is similarly electrolytically deposited over the Ca(BO2)2 coating to a total thickness between about 0.10 and 0.40 mil, ~ 6 4 RD-8655 the Ca(BO2)2 initial coat being the thinner of the two at between about 0.02 and 0.07 mil.
As the final step of the process of this invention, the double- or duplex-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 recrystal-lization process is completed and heating may be carried on to up to 1175C 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 in-vention:
EXAMPLE I
Eleven-mil strips of silicon-iron of the following composition were prepared as described in U.S. Patent 3,095,843 dated July 2, 1963 referred to above:
2Q Carbon 0.030%
Manganese 0.035%
Sulfur 0.031%
Boron 0.0010%
Nitrogen 0.0050%
Copper 0.24%
Aluminum 0.005~
Iron Remainder From this melt composition, 10.7 mil sheets were produced in a series of hot rolling passes followed by pickl-ing and annealing of the intermediate thickess sheet material (about 100 mils) and cold rolling to 60 mils thickness, where-upon the material was reheated and cold rolled again to final 1117'~4 RD-8655 thickness and the cold-worked sheet was given a decarburiz-ing heat treatment at 800C for eight minutes in hydrogen (room temperature dew point).
~ pstein strips cut from the sheet to provide nine Epstein packs were immersed in an electrolyte prepared by adding boric and Ca(BO2)2 to a 10% aqueous solution of calcium acetate in distilled water. Sufficient boric acid was added to the solution to raise the pH to about 7Ø

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 varying lengths of time to provide Ca(sO2)2 coatings of the thickness set forth in Table I. On removal of the strips from the primary coating electrolytes, they were placed in magnesium acetate electrolytes as described in U. S. Patent No. 3,045,732 dated September 18, 1962, the coated strips again being cathodes in electric circuits and eight volts again being applied across the terminals until the coating mass of Mg(OH~2 deposited on the strips was as also indicated in Table I.

After annealing in hydrogen for about eight hours at 1175C, the duplex-coated strips had the magnetic properties stated in Table I.

11~74~4 RD-8655 TAsLE I
Coating Densities (mg/strip) Losses, mwpp Pack ~10 Oe 15 kG 17 kGCa(BO ) Mg(OH)

2 2 2 1 1889 500 645 2.2 72.3 2 1891 503 651 4.9 68.2

3 1890 489 632 6.1 72.2

4 1887 498 651 8.8 92.7 1885 505 660 9.6 62.8 6 1886 501 649 9.8 62.0 7 1890 489 629 12.0 60.4 8 1889 503 657 15.4 47.0 9 1885 511 676 21.8 34.6 As is apparent from comparision of these data with those out in Tables I and II of Canadian patent application Serial No.~99,J39 dated ~rcl /7,/97~ the results obtained through the process of the present invention are generally the same as those obtained through the use of the Mg(OH)2 ~ Mg(BO2)2 duplex coating method of the in-vention of that companion case of ours.
The process of this invention was tested again in a similar manner except that a total of 25 randomly selected Epstein packs from different steel heats and coils were treated as described in Example I to provide the initial Ca(BO2)2 coating and the heavier Mg(OH)2 overcoat in the ranges of density (i.e., thickness) of mill practice involving the procedure of Example III of Maucione Canadian application Serial No. 275,369 dated April 1, 1977. Thus, each strip had a total coating density of 0.025 to 0.030 ounce per square foot (steel) of which the primary Ca(BO2)2 coat constituted about 15%. Also, the final anneal was that of usual practice involving heating in hydrogen to cause ~li74~4 RD-8655 secondary grain growth starting at about 950C, the tem-perature being raised about 50C per hour to 1000C when recrystallization is completed and going on up to 1175C
as stated in Example I. The magnetic properties of the resulting strips are compared in Table II with those of strip processed in accordance with the aforesaid Example III of the Maucione patent application.

TABLE II

Losses, mwpp ~10 Oe 17 kG

Maucione EX. III Process 1881 704 Present Invention Process 1891 704 Those skilled in the art will recognize that coating weight or thickness is commonly expressed in terms of den-sity in ounces per square foot of steel strip surface and that 0.0275 oz/ft = 77 milligrams per Epstein strip.
Further, it is generally understood that 77 mg/Epstein strip corresponds to uniform coating thickness of 0.05 mil.

Claims (9)

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 fine-grained, 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 an aqueous solution consisting essentially of calcium acetate and boric acid with solid Ca(BO2)2 with the sili-con-iron sheet being arranged as the cathode in said solution and said solution being at a temperature of at least about 65°C
and thereby covering the sheet with an adherent electrically-insulating coating of Ca(BO2)2, then electrolyzing an aqueous solu-tion consisting essentially of solid magnesia-buffered magnesium acetate with the resulting coated sheet arranged as the cathode in said magnesium acetate solution and said magnesium acetate solution being at a temperature of at least about 65°C and thereby covering the Ca(BO2)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 boron content of the silicon-iron sheet is between about 10 and 30 parts per million.

3. The method of claim 1, in which the boron content of the silicon-iron sheet is about 10 parts per million and the nitrogen content of said sheet is about 30 parts per million.

4. The method of claim 1, in which said solution of calcium acetate and boric acid and said magnesium acetate solution are maintained at a temperature between about 90°C
and 95°C during the period of electrolytic codeposition.

5. The method of claim 1, in which the Ca(BO2)2 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.

6. The method of claim 5, in which the total thickness of the two coatings is between about 0.10 and 0.40 mil.

7. The method of claim 5, in which the total thickness of the two coatings is about 0.20 mil.

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

9. The double-coated primary-recrystallized silicon-iron sheet product of the method of claim 5.