GB1584455A

GB1584455A - Method of producing silicon-iron sheet and a product thereof

Info

Publication number: GB1584455A
Application number: GB1568977A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1976-04-15
Filing date: 1977-04-15
Publication date: 1981-02-11
Also published as: BR7702460A; PL115481B1; YU98477A; SE7703456L; FR2348277B1; JPS52141415A; AU513065B2; FR2348277A1; AU2431777A; IT1125734B

Description

(54) METHOD OF PRODUCING SILICON-IRON SHEET AND A PRODUCT THEREOF (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 12305, 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 novel method of producing singly-oriented siliconiron sheet through the use of small amounts of boron in the electrically-insulating coating on a boron-containing silicon-iron magnetic sheet and through the use of small amounts of boron in the coating in critical proportion to both the boron and nitrogen contents of the sheet.

The sheet materials to which this invention is directed are usually referred to in the art as "electrical" silicon steels or, more properly, silicon-irons and are ordinarily composed principally of iron alloyed with about 2.2 to 4.5 per cent silicon and relatively minor amounts of various impurities and very small amounts of carbon. These products are of the "cube-on-edge" type, more than about 70 per cent of their crystal structure being oriented in the (110)[001] texture, as described in Miller Indices terms.

Such grain-oriented silicon-iron sheet products are currently made commercially by the sequence of hot rolling, heat treating, cold rolling, heat treating, again cold rolling and then final heat treating to decarburize, desulfurize and recrystallize. Ingots are conventionally hot-worked into a strip or sheet-like configuration less than 0.150 inch in thickness, referred to as "hot-rolled band." The hot-rolled band is then cold rolled with appropriate intermediate annealing treatment to the finished sheet or strip thickness usually involving at least a 50 per cent reduction in thickness, and given a final or texture-producing annealing treatment.

As disclosed and claimed in U.S. Patent No. 3,905,842 the magnetic properties of such sheet materials can be very considerably improved by incorporating boron in the metal so that it is present there in critical proportion to the nitrogen content of the metal at the time of the final or texture-developing anneal. As stated in that patent. the amount of boron required to produce that result is quite small but highly critical.

Similarly. it is disclosed in U.S. Patent No..3.905,843. that such use of boron in the metal in proportion to nitrogen while maintaining the ratio of manganese to sulfur at less than 2.1 will enable the corresponding substantial improvement in magnetic properties of a product made by the process including cold rolling in two stages, including an intermediate anneal.

Still another related disclosure concerning the use of small but critical amounts of boron in silicon-iron are shown in UK Patent Specification No. 1519919. This specification describes the concept of cold rolling hot-rolled silicon-iron sheet directly to final thickness without an intermediate heat treatment through the use of small but critical amounts of boron and by maintaining the ratio of manganese to sulfur in the metal at less than 1.8.

The present invention provides a method of producing grain-oriented silicon iron sheet which comprises the steps of: A. providing a fine-grained, primary-recrystallized silicon iron sheet containing from 2.2 to 4.5 per cent silicon between 1.5 to 50 parts per million boron and from 30 to 90 parts per million nitrogen in a ration to boron of from 1.0 to 15 parts per part of boron B. Covering the sheet with an adherent electrically-insulating coating containing boron, and C. subjecting the coated sheet to a final heat treatment to develop (110) [001] secon dary recrystallization texture in the silicon iron sheet.

The present invention also provides an electrically-insulated magnetic sheet material comprising a fine-grained primary recrystallized, magnetic, silicon iron sheet containing from 2.2 to 4.5 per cent silicon, from 1.5 to 50 parts per million boron and from 30 to 90 parts per million nitrogen and having thereon a boron-containing coating of a waterinsoluble hydroxide of calcium, magnesium manganese or aluminium.

Under certain conditions the presence of boron in the usual electrically-insulating coating on silicon iron sheet material can have a beneficial effect upon the secondary recrystallization of the metal to develop the (110) [001] texture and special magnetic propoerties associated with it. The presence of a very small amount of boron in the coating during the final anneal results in the development of substantially better magnetic properties than would otherwise be produced. It can, in fact, cause secondary recrystallization to take place when otherwise it would not. It has also been determined, however, that the present of boron in the insulating coating during the final anneal is not effective in this respect if there is substantially no boron present in the metal itself at the outset of the final anneal. It follows, however, that by virtue of this invention one can substantially reduce the amount of boron added to the ladle in accordance with the foregoing two patents -and- patent application without penalty to the desirable properties of the ultimate silicon iron sheet product attributable to the present of boron during the final anneal.

In addition, it has been found that advantageous results can be obtained consistently by limiting the amount of boron in a cold rolled and decarburized silicon iron sheet to one particular range and by limiting the amount of boron available in the electricallyinsulating coating on the sheet to another particular range. Further, it has been found that for such results the total boron in the alloy and the coating thereon be limited to a certain maximum. Still further, it has been found that by proportioning the alloy nitrogen content in a particular manner to the middle and upper ranges of total boron content of the alloy and its coating, such results can be obtained regularly and routinely.

Secondary recrystallization can consistently be obtained during the final anneal of silicon-iron containing as little as 1.5 ppm boron when there is available in the electrically-insulating coating thereon as little as 6.0 ppm boron. Additionally, much greater total amounts of boron in the alloy and its coating will likewise consistently result in products having superior magnetic properties, providing that the total boron does not exceed about 90 ppm and also providing that the alloy boron content does not exceed about 50 ppm as the final anneal is begun. Still further, when the boron content of the alloy plus that available in the coating exceeds about 40 ppm, the nitrogen content of the alloy should be greater than about 70 ppm and preferably in the range from 80 to 90 ppm for consistently good results in terms of the magnetic properties of the ultimate singly-oriented. silicon-iron, magnetic sheet product.

Except for the extremities of the above critical range of boron available in the coating, increases in the boron content of the coating result in significant decreases in losses in the finished sheet product and, to a lesser degree, result in improvement in the superior permeability of the product.

The ratio of manganese to sulfur which is limited to 2.1 in the process disclosed and claimed in U.K. Patent Specification No. 1519919 can run as high as 2.5 in accordance with the present invention with consistently good end-product magnetic properties.

These discoveries are surprising, especially in view of the fact that quite different results are obtained when boron is added to the final anneal coating on silicon-iron sheet containing no boron. Thus, according to U.S. Patent No. 3,676,227 such additions result in smaller secondary grains than the average but no improvement in permeability, whereas grain size is not diminished while permeability is substantially improved by the present invention process.

While the amount of boron in the coating necessary to produce the new results is both critical and quite small, it is not a difficult requirement to meet. In fact, one has the choice of applying the boron with the Mg(OH)2 or other similar electrically-insulating coating material in slurry form or, alternatively, forming the coating as disclosed in U.S.

Patent No. 3,054,732 and then contacting the coated sheet metal with an aqueous solution of a boron compound. The latter procedure may take the form of a dipping operation or the aqueous solution may be brushed on the coating or even sprayed on, if desired.

Additionally, H3BO3 and Nazi407 are desirable boron sources according to this invention, and their use for this purpose is contemplated either individually or in combination. Further, those skilled in the art will understand that other boron sources compatible with the final anneal environment for the purpose of this invention may also or alternatively be used in the coating. It will be understood that a basic requirement of the boron source in the coating is that it be decomposable under the conditions of the final anneal so that the boron can diffuse into the alloy surface to produce the new results and advantages set out above.

From the foregoing, it will be understood that this invention has both method and article or product aspects. The product can be a fine-grained, primary-recrystallized, magnetic, silicon iron sheet of final gauge thickness having a boron-containing coating of the reaction product of silicon and magnesium hydroxide or the like. By virtue of the content of boron, nitrogen, manganese and sulfur in the sheet and the boron in the coating, the silicon iron sheet can be converted to the singly oriented state in which it will have valuable magnetic properties but may not contain much, if any, of the boron which enabled the development of those properties during the final through secondary recrystallization.

Additionally, the product can be a decarburized coated, cold-rolled sheet or strip of final gauge thickness which contains boron that in critical proportion to and in combination with the boron in the coating and the nitrogen in the metal which will enable the development of the desired magnetic properties through secondary recrystallization during the final anneal.

The product preferably has a thin, tightly-adhering, boron-containing coating of the water-insoluble hydroxide of calcium, magnesium, manganese or aluminium. Preferably, the amount of boron in the coating should be between about 25 to 150 parts per million on the basis of the silicon iron sheet substrate, and for optimum results in terms of limiting core losses should be between 50 and 80 ppm on the same basis. Further. these ranges apply independently of the boron content of the silicon iron sheet substrate as long as the latter is within the three of 50 ppm range stated above.

Additionally, the article aspect may take the form of an electrically insulated magnetic sheet of fine grained primary-recrystallized, magnetic silicon iron which contains between about 1.5 to 50 ppm boron and between about 30 and 90 ppm nitrogen and has a tightly-adhering water insoluble metal hydroxide coating containing between about six and 90 ppm boron proportioned to the boron content of the alloy sheet so that the total amount is between about 7.5 and 90 ppm.

The method of this invention in a preferred embodiment comprises the steps of providing this intermediate sheet product and subjecting it to a final heat treatment to develop the cube-on-edge secondary recrystallization in it.

The present invention will be further described by way of example only with reference to data gathered during certain of the experiments described below which are graphically illustrated in the accompanying drawings, in which Figure 1 is a chart on which permeability is plotted against cold-rolled strip boron content, the three curves showing the effects of boron additions to the strip magnesia coating; Figure 2 is a chart on which improvement in losses is plotted against cold-rolled strip boron content. the curve showing the effect of brushing the magnesia coating with dilute boric acid solution prior to the final anneal; Figure 3 is a chart on which both permeability and losses are plotted against maximum boron content available to the strip from the coating, the three curves showing relatively small improvement in permeability with increases in coating boron and somewhat greater improvements in losses over the same boron range; Figure 4 is another chart on which permeability is plotted against maximum boron available to the strip from its coating, the four curves showing the effect of increasing metal nitrogen content from 42 to 84 ppm and one of them also showing the effect of increasing the metal boron content from about nine to 50 ppm; Figure 5 is a chart like that of Figure 4 for metal specimens of 40% greater sulfur content; and Figure 6 is a chart like those of Figures 4 and 5, the curves representing data collected in tests of metal processed in accordance with this invention having still greater sulfur content.

In carrying out this invention, one may provide the intermediate sheet products described above by preparing a silicon-iron melt of the required chemistry, and then casting and hot rolling to intermediate thickness. Thus, the melt on pouring can contain from 2.2 to 4.5 per cent silicon, manganese and sulfur in amounts in a ratio of manganese to sulfur less than 2.5, from three to 50 ppm boron and 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, aluminium, copper and oxygen.

Alternatively, the melt on pouring can contain from 2.2 to 4.5 percent silicon and from about 1.5 to 50 ppm boron and about 30 to 90 ppm nitrogen in the ratio range to boron of one to 15 parts to one, manganese up to about 0.10 per cent and sulfur up to a ratio of 2.5 parts of manganese per part of sulfur, the remainder being iron and small amounts of incidental impurities as stated above. In either case, 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 boron-containing coating of this invention in preparation for the final texture-developing anneal, Preferably, the coating step is accomplished electrolytically as described in U.S. Patent No. 3,054,732, referenced above, a coating of between about 0.2 to 0.5 mil thick of Mg(OH)2 thereby being applied to the sheet. The coated sheet is then dipped in aqueous solution of boric acid or sodium borate or other suitable boron compound solution which is preferably relatively dilute, containing five to 10 grams per liter of the boron compound.

As the final step of the process 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 1000"C, the recrystallization process is completed and heating may be carried on to up to 1175"C if desired to ensure complete removal of residual carbon, sulfur and nitrogen.

The following illustrative, but not limiting, examples of the 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 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 % Boron 0.0010% Nitrogen 0.0050% Copper 0.24 % Aluminum 0.005 % Iron Remainder From this melt composition, 10.8-mil and 13.6-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). Cold rolling was then carried on to 60 mils thickness, whereupon the material was heated and cold rolled again to final thickness and the coldworked sheet was given a decarburizing heat treatment at 800"C for eight minutes in hydrogen (room temperature dew point).

Epstein strips cut from the sheets were provided with a coating of Mg(OH)2 of 0.2-mil thickness as described in U.S. Patent No. 3,054,732 - McQuade, particularly Example II thereof.

Three of each of the 10.8-mil and 13.6-mil strips were selected for tests of this invention process, one of each group being a control sample and so not being provided with boron in the magnesia coating. Another of each group was dipped in a five-gram-perliter solution of sodium borate for 15 seconds, while the third was dipped in a tengram-per-liter solution of sodium borate for 15 seconds. The six strips were then annealed at 1 1600C in hydrogen for five hours. The magnetic properties of the resulting strip materials are set forth in Table I: TABLE I Na2B407 Dipping Solution MWPP (Coated) tL at IOH Sample (gm/l) 15 kG 17 kG (Coated) l1-lH0 0 598 898 1799 ll-1H5 5 687 972 1806 ll-1H 10 10 594 840 1881 14-1H 0 0 710 1050 1743 14-1H 5 5 864 1240 1707 14-1H 10 10 740 1040 1801 ll-lB 0 0 661 1000 1729 ll-1B 5 5 646 908 1834 ll-1B 10 10 663 992 1747 14-1B 0 0 665 998 1767 14-1B 5 5 725 1060 1797 14-1B 10 10 760 1084 1778 EXAMPLE II Two Epstein packs of additional strips of 10.71 mil and 10.77-mil sheet materials were prepared and electrolytically-coated as described in Example I and then immersed in a 7.5 gram-per-liter aqueous solution of Na2B4O, for 15 seconds. Epstein packs of the resulting strips were subjected to the final anneal of Example I with the results indicated in Table II: TABLE II MWPP Pack 15 16.3 17 CL at 10 H 1H 584 714 808 1842 Lab Anneal 1H 581 715 807 1834 Lab Anneal EXAMPLE III In another experiment involving the process of this invention, a commercial melt prepared through the use of BOF silicon-iron as described in above U.S. Patent No.

3.905.843 was used, its ladle composition being: Silicon 3.10 % Copper 0.26 % Manganese 0.032 % Sulfur 0.014 % Carbon 0.024 % Boron 0.0015% Nitrogen 0.0035% Hot rolling and direct cold rolling to final gauge thickness about 11 mils were carried out. Cold-rolled material was decarburized and provided with a magnesia coating in accordance with McQuade U.S. Patent Number 3.054,732 and then dipped in solution consisting of 142 gallons of 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 coated strips were then annealed at 2 1 500F in dry hydrogen for three hours.

The ultimate, finally-annealed specimens were found to have good magnetic properties. permeability being 1905 Gausses per oersted (in a l0-oersted field) with losses measuring 0.468 and 0.629 watts per pound at 15,000 and 17,000 gausses. respectively.

EXAMPLE IV In still another test of this invention a mill heat was prepared as above described of the following ladle composition: Silicon 3.15% Copper 0.26 Manganese 0.32 Sulfur 0.14 Carbon 0.26 Phosphorus 0.005 Chromium 0.06 Mn/S = 2.29 Nickel 0.091 Titanium 0.004 Tin 0.011 Boron 0.0011 Nitrogen 0.0035 Iron Balance Again, hot rolling and direct cold rolling to final gauge thickness (10.6 mils) were conducted.

This material was final normalized and electrolytically coated with 0.2 mill magnesia per the McQuade patent and "Mill dipped" in a 1% boric acid solution prepared as described in example III.

Epstein pack specimens from several coils were redipped in a laboratory one per cent boric acid solution. Two other specimens from each coil were redipped, respectively, in two per cent and three per cent boric acid solutions in the laboratory. On analysis, the boron contents of the coatings were found as set forth in Table III which also lists the magnetic properties measured in these strips following annealing as Epstein packs at 21500F in dry hydrogen for three hours.

TABLE III Coating 17 kG Loss Boron * Lot mwpp CL lOH mg/strip Final 656 1876 -0 Normalize Mill Dip 692 1872 0.68 1% 674 1909 1.24 2% 707 1885 1.72 3% 705 1887 2.20 2 Final 670 1886 -0 Normalize Mill Dip 640 1900 1.57 1%, 649 1912 2.06 2% 659 1921 2.86 3% 711 1906 2.88 3 Final 656 1870 -0 Normalize Mill Dip 643 1886 0.89 1% 653 1909 1.33 2% 658 1907 2.13 3% 688 1886 2.50 * One milligram per Epstein strip =- 50 parts per million silicon-iron equivalent.

EXAMPLE V Twelve laboratory heats were melted in an air induction furnace under an argon cover using electrolytic iron and 98 per cent ferrosilicon, all containing 3.1 per cent silicon, 0.1 per cent copper and 0.03 per cent chromium. The same amount of sulfur (0.024 per cent) as iron sulfide was added to each heat, the sulfur analyses range 0.033 per cent down to 0.019 per cent with an average of 0.026 per cent.

Slices 1.75 inch thick were cut from ingots cast from these melts and were hot rolled rom 1200"C in siz passes to a thickness of about 90 mils. Following pickling, the hot band samples were heat treated at 9500C, the time between 930 and 950"C being about three minutes. The hot bands were then cold rolled directly to 10.8 mils and analyzed with the results set forth in Table IV: TABLE IV Composition as Determined on Cold-Rolled Strip Heat ppm B ppmN ppmO %Mn %S 1 < 1 68 70 0.034 0.025 0.038 2 1.2 -- -- .036 .033 .037 3 1.6,1.4 -- -- .035 .025 .038 4 1.8 -- -- .034 .019 .040 5 2.4,3.1 49 76 .035 .029 .033 6 5.6 48 90 .035 .025 .044 7 6.9 46 88 .036 .030 .037 8 7.4 52 115 .036 .021 .039 9 14 50 98 .035 .031 .036 10 24 46 96 .036 .024 .038 11 26,25 62 70 .036 .022 .040 12 29 47 69 .035 .023 .038 Epstein-size strips of the cold-rolled material were decarburized to about 0.007 per cent by heating at 8000C in 70"F dew point hydrogen. The decarburized strips were brushed with milk of magnesia to a weight gain of about 40 milligrams per strip and boron additions were made to some of the magnesia coated strips using either a 0.5 or 1.0 per cent boric acid solution which deposited sufficient boron on the coating that if it were all taken up by the silicon-iron, the boron content of the metal would be increased by 15 to 30 ppm, respectively. The resulting coated strips including both those brushed with the boric acid solution and those not so treated, were subjected to a final anneal consisting of heating at 400C per hour from 800"C to 1 1750C in dry hydrogen and holding at the latter temperature for three hours.

Magnetic properties of the ultimate products of the foregoing process of this invention and those representing the control specimens are set forth in Table V and in Figures 1 and 2: TABLE V Magnetic Properties After Final Anneal in Hydrogen MgO Only MgO+15ppmB MgO +30ppm B Heat ppm B 17kB ,uwIOH 17kB ,uIOH 17kB ,uIOH 1 < 1 -- 1383 -- 1402 -- 1394 2 1.2 -- 1432 1322 1483 -- 1467 3 1.5 1136 1664 730 1873 1000 1678 4 1.8 929 1751 739 1876 1094 1655 5 2.7 771 1849 725 1881 940 1730 6 5.6 750 1887 -- -- 741 1856 7 6.9 696 1892 678 1908 -- - 8 7.4 749 1890 702 1898 755 1845 9 14 747 1891 701 1900 870 1768 10 24 813 1844 736 1869 1322 1536 11 26 754 1873 690 1900 803 1805 12 29 -- 1472 -- 1423 -- 1406 Table V and Figure 1 illustrate that the effect of boron additions to the coating on the permeability. Providing boron in the coating in amount representing a total theoretically available to the alloy of 15 ppm greatly enhances the magnetic properties. particularly those of the alloys initially containing only 1.5 or 1.8 ppm boron. Doubling the coating boron content was formed to consistently reduce the permeability of the ultimate strip product. Degradation of permeability of the high boron strip specimens might be rationalized in terms of an imbalance in the boron/nitrogen ratio, as evidenced by the very different results obtained in the high nitrogen heat (No. 11).

Curve A of Figure 1 represents those data obtained with specimens having magnesia coatings untreated with boron solution, while Curves B and C represent, respectively, data obtained with specimens bearing magnesia coatings treated. with boron-containing solution providing 15 ppm and 30 ppm total boron on the basis of the alloy in each instance. The improvement in loss resulting from boron coating additions is illustrated in Figure 2. The large improvement for the two lowest boron alloys is due mainly to the improved permeability while the approximately 50 mwpp improvement of the higher boron content alloys with little or no change in permeability is typical behaviour of both laboratory and mill heats.

EXAMPLE VI In another experiment designed to test the capabilities of this new process, a commercial melt was prepared using BOF silicon-iron as described in referenced U.S. Patent 3,905,843, the melt having the following ladle analyses: Silicon 3.10 % Copper 0.29 % Manganese 0.033 % Sulfur 0.019 % Carbon 0.024 % Boron 0.0015% Nitrogen 0.0058% Strips were cut from the cold rolled and decarburized sheet to provide Epstein packs, some of the strips being provided with magnesia coating as described in Example V and then being brushed with boric acid solution as therein described to provide varying amounts of boron from about 10 to about 90 ppm, as illustrated in Figure 3. Others of the strips were coated with magnesia in which boric acid was premixed to provide varying amounts of available boron ranging from 10 to 70 ppm as shown in the drawing.

Epstein Packs made of these prepared specimens and others coated but not borated were loaded into the retort for final anneal at 8000C and heated at the rate of 40"C per hour to the maximum temperature of 1175"C, which was held for four hours. The resulting annealed finished test specimens were subjected to tests of their magnetic properties with the results indicated in Figure 3. In addition. the grain size of a number of the specimens were measured. The grain size of the control sample in which there was no boron available to the silicon-iron coating was 9.8 mm. while that of the 15 ppm boron coating was 11.3 mm. that of the 30 ppm boron coating was 10.4 mm, and that of the 60 ppm boron coating was 11.5 mm. This latter data stands in contrast to that disclosed in the prior art to the effect that reduction in grain size from 12 mm to 4 mm results in losses reduction approximating 50 mwpp.

It is apparent from Figure 3 that boron additions to the coating of boron-containing silicon-iron result in significant reduction in losses and somewhat less change in permeability. i.e.. relatively slight increases in permeability. over the range virtually from 5 to 10 ppm to 90 ppm of boron available to the silicon-iron from the coating.

EXAMPLE VII In another experiment like that described in Example V. 11 laboratory heats were prepared in an air induction furnace either under an argon cover with argon bubbled through the melt prior to pouring. or with nitrogen used for the cover. the bubble. or both. The use of argon alone gave the lowest heat nitrogen contents and the use of nitrogen along resulted in the highest heat nitrogen contents. the heats all contained 3.1 per cent silicon. 0.1 per cent copper and 0.03 per cent chromium. Cold-rolled strip prepared as described in Example I from each of the heats were found on analysis to have the composition set forth in Table VI: TABLE VI Composition of Heats As Determined on Cold-Rolled Strip Heat %Mn %S O/oC ppm B ppm N 20 0.025 0.011 0.036 7.8 42 21 0.027 0.010 0.035 6.2 48 22 0.025 0.010 0.033 9.2 84 30 0.025 0.010 0.033 51.0 84 23 0.027 0.013 0.036 6.7 43 24 0.025 0.014 0.029 7.2 55 25 0.025 0.013 0.030 7.7 62 26 0.025 0.013 0.030 8.2 72 27 0.024 0.017 0.030 6.3 42 28 0.024 0.018 0.032 5.9 60 29 0.025 0.019 0.031 6.7 93 Epstein-size strips of the cold rolled materials were decarb

Claims

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

TABLE VI Composition of Heats As Determined on Cold-Rolled Strip Heat %Mn %S O/oC ppm B ppm N

20 0.025 0.011 0.036 7.8 42

21 0.027 0.010 0.035 6.2 48

22 0.025 0.010 0.033 9.2 84

30 0.025 0.010 0.033 51.0 84

23 0.027 0.013 0.036 6.7 43

24 0.025 0.014 0.029 7.2 55

25 0.025 0.013 0.030 7.7 62

26 0.025 0.013 0.030 8.2 72

27 0.024 0.017 0.030 6.3 42

28 0.024 0.018 0.032 5.9 60

29 0.025 0.019 0.031 6.7 93 Epstein-size strips of the cold rolled materials were decarburized to less than 0.01 per cent carbon at 800"C in hydrogen (dew point about 70"F). The strips were then brushed with milk of magnesia for a weight gain of about 40 mg per strip and boron additions were made to a number of the strip coatings by boric acid solutions of concentrations in multiples of 0.5 per cent. Analyses of the coatings for boron prior to the final anneal indicated that the concentration increased linearly by approximately 12 ppm boron (on the basis of strip rather than coating weight) for each such 0.5 per cent increment. The final anneal consisted of heating at 40"C per hour from 800" to 1175"C and holding for three hours.

The permeabilities of Epstein Packs annealed with and without the boric acid additions to the strip coatings are illustrated in Figures 4, 5 and 6 where the heats are grouped according to sulfur content, as shown. The nitrogen contents of the strips prior to final anneal are also indicated on the drawings. In addition, in Figures 4-6, measured losses are entered adjacent to a number of the appropriate data points. All the heats were found to have improved properties through boron additions to the coating, the most dramatic improvement occurring with the low sulfur, high nitrogen heat. Without an addition of boron to the coating. all four low sulfur heats primarily undergo normal grain growth but with boron added to the coating, the high nitrogen heat undergoes complete secondary recrystallization. The tendency for high nitrogen heats to develop high permeabilities is also evident with heats of intermediate sulfur content: the lowest nitrogen heat is poorer than any of the others in the group.

It was also observed that adverse effects such as blisters and slivers commonly associated with relatively high nitrogen contents in silicon-iron were not evident in any of the speciments of this experiment.

Throughout this specification and the appended claims, as well as in the drawings, the boron content of the coating is expressed in terms of the total amount theoretically available to the silicon-iron strip bearing the coatings. In actual practice a small fraction of such boron will diffuse into the strip during the final anneal prior to completion of secondary recrystallization. Depending upon the circumstances that fraction may be as high as approximately one-half of the content of low boron content coatings.

WHAT WE CLAIM IS: 1. A method of producing grain-oriented silicon-iron sheet which comprises the steps of: A. providing a fine-grained, primary-recrystallized, silicon-iron sheet containing: from 2.2 to 4.5 per cent silicon between 1.5 to 50 parts per million boron and from

30 to 90 parts per million nitrogen in a ratio to boron of from 1.0 to 15 parts per part of boron B. covering the sheet with an adherent electrically-insulating coating containing boron, and C. subjecting the coated sheet to a final heat treatment to develop (110) [001] secon dary recrystallization texture in the silicon-iron sheet.
2. A method as claimed in claim 1 in which the siliconitron sheet contains manganese and sulphur in the ratio of up to 2.5 parts of manganese per part of sulphur.
3. A method as claimed in claim I or claim 2 in which the boron in the coating is in the form of boric acid.
4. A method as claimed in claim I or claim 2 in which the boron in the coating is in the form of sodium borate.
5. A method as claimed in any one of the preceding claims including the steps of

electrolytically forming an electrically-insulating coating on the sheet and then contacting the coated sheet with an aqueous solution of a boron compound.
6. A method as claimed in claim 5 in which the aqueous solution contains about five grams-per-litre of Na2B407.
7. A method as claimed in claim 5 in which the aqueous solution contains about ten grams-per-litre of Na2B407.
8. A method as claimed in claim 5 in which the boron compound is boric acid and the solution contains from one to 15 grams-per-litre of said acid.
9. A method as claimed in any one of claims 1 to 4 in which the silicon-iron sheet is provided with the boron-containing coating by first brushing the sheet with milk of magnesia and brushing the resulting magnesia coating with an aqueous boric acid solution.
10. A method as claimed in claim 9 in which the aqueous solution contains about five grams-per-litre of Na2B407.
11. A method as claimed in claim 9 in which the aqueous solution contains about ten grams-per-litre of Na2B4O7.
12. A method as claimed in claim 9 in which the boron compound is boric acid and the solution contains from one to 15 grams-per-litre of said acid.
13. A method as claimed in any one of claims 1 to 8 wherein the silicon iron sheet contains 3 to 35 parts per million boron.
14. A method as claimed in any one of claims 1 to 8 wherein the silicon iron sheet contains 3 to 50 parts per million boron and the coating contains 25 to 150 parts per million boron.
15. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 wherein the silicon iron sheet contains 1.5 to 35 parts per million boron and the coating contains 7.5 to 90 parts per million boron.
16. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 wherein the silicon iron sheet contains 1.5 to 50 parts per million boron and the coating contains 7.5 to 90 parts per million boron.
17. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 wherein the total amount of boron in the sheet and boron in the coating available to the sheet is from 7.5 to 90 parts per million.
18. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 in which the boron content of the silicon-iron sheet is about 1.5 parts per million and the coating contains about 15 parts per million boron on the basis of the silicon iron sheet.
19. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 in which the boron content of the silicon iron sheet is about 6.9 parts per million and the coating contains about 15 parts per million boron on the basis of the silicon iron sheet.
20. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 in which the silicon iron sheet contains about 10 parts per million boron, and about 30 parts per million nitrogen, and the coating contains between 10 to 70 parts per million boron on the basis of the silicon iron sheet.
21. A method as claimed in any one of claims 1 to 4 and claims 9 to 12 in which the silicon iron sheet contains about 50 parts per million boron, and 80 to 90 parts per million nitrogen and the coating contains between 10 and 40 parts per million boron on the basis of the silicon iron sheet.
22. A method as claimed in any one of claims 1 to 8 in which the boron content of the coating is equivalent to 50 to 80 parts per million on the basis of the silicon iron sheet.
23. An electrically-insulated magnetic sheet material comprising a fine-grained primary recrystallized, magnetic, silicon iron sheet containing from 2.2 to 4.5 per cent silicon, from 1.5 to 50 parts per million boron and from 30 and 90 parts per million nitrogen and having thereon a boron-containing coating of a water-insoluble hydroxide of cal cium. magnesium manganese or aluminium.
24. A sheet material as claimed in claim 23 wherein the silicon iron sheet contains 3 to 35 parts per million boron.
25. A sheet material as claimed in claim 23 wherein the silicon iron sheet contains 3 to 50 parts per million boron.
26. A sheet material as claimed in claim 23 wherein the silicon iron sheet contains 1.5 to 35 parts per million boron. said coating containing 5 to 90 parts per million boron on the basis of the silicon iron sheet, the total amount of boron in the sheet and coating being between 7.5 and 90 parts per million.
27. A sheet material as claimed in claim 23 wherein the silicon iron sheet contains 1.5 to 50 parts per million boron, said coating containing 6 to 90 parts per million boron on the basis of the silicon iron sheet, the total amount of boron in the sheet and coating being between 7.5 and 90 parts per million.
28. A magnetic sheet as claimed in claim 23 wherein said coating contains from 6.0 to 90 parts per million boron on the basis of the silicon-iron sheet and so proportioned to the boron content of said sheet that the total amount of boron in the sheet and boron in the coating is from 7.5 to 90 parts per million.
29. A sheet material as claimed in claim 23, in which the coating is an electrolytic Mg(OH)2 coating, the silicon iron sheet contains about 10 parts per million boron and about 30 parts per million nitrogen, and the coating contains of from 50 to 80 parts per million boron on the basis of the silicon iron sheet.
30. A sheet material as claimed in claim 23 in which the boron content of the silicon sheet is about 1.5 parts per million, the nitrogen content of the said sheet is less than 55 parts per million nitrogen and the coating is electrolytically-deposited magnesium hydroxide containing about 15 parts per million boron on the basis of the silicon iron sheet.
31. A sheet material as claimed in claim 23 in which the total boron content of the silicon iron sheet and the boron available in the coating is between 6.5 and 90 parts per million and the nitrogen content of the metal is between about 80 and 90 parts per million.
32. A sheet material as claimed in claim 23 which contains about 50 parts per million boron and from 80 to 90 parts per million nitrogen, and the coating contains from 10 and about 40 parts per million boron on the basis of the silicon iron sheet.
33. A method of producing grain-oriented silicon iron sheet as claimed in claim 1 substantially as hereinbefore described in any one of Examples I to IV.
34. A method of producing grain-oriented silicon iron sheet as claimed in claim 1 substantially as hereinbefore described in any one of Examples V to VII.
35. An electrically-insulated magnetic sheet material as claimed in claim 23 substantially as hereinbefore described in any one of Examples I to IV.
36. An electrically-insulated magnetic sheet material as claimed in claim 23 substantially as hereinbefore described in any one of Examples V to VII.
37. A grain oriented silicon sheet when produced by a method as claimed in any one of claims 1 to 8, 14, 22 and 33.
38. A grain oriented silicon sheet when produced by a method as claimed in any one of claims 1 to 4, 9 to 12, 15, 17 to 20 and 34.
39. A grain oriented silicon sheet when produced by a method as claimed in claim 13.
40. A grain oriented silicon sheet when produced by a method as claimed in claim 16.
41. A grain oriented silicon sheet as claimed in claim 23 substantially as hereinbefore described.