Method of producing silicon-iron sheet material with boron addition, and product

Addition of as little as six parts per million boron to the magnesia final anneal coating on silicon-iron strip containing only about 1.5 ppm insures secondary recrystallization while the addition of 70 ppm boron to the coating on a strip containing 10 to 15 ppm boron results in substantial reduction in losses without affecting permeability of the final product when the nitrogen content of the alloy is in the 80 to 90 ppm range.

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 silicon-iron sheet through the use in the 
electrically-insulating coating on a boron-containing silicon-iron 
magnetic sheet of small amounts of boron in critical proportion to the 
boron and nitrogen contents of the sheet. 
CROSS REFERENCE 
This invention is related to the invention disclosed and claimed in U.S. 
patent application Ser. No. 677,147 now abandoned, filed Apr. 15, 1976, in 
the name of Carl M. Maucione for "Method of Producing Silicon-Iron Sheet 
Material With Boron Addition, and Product" assigned to the assignee hereof 
and directed to the novel concept of incorporating in the final anneal 
coating on silicon-iron sheet material a relatively very small amount of 
boron to cause secondary recrystallization of the alloy during the final 
anneal or to produce a final product having substantially improved 
permeability. 
BACKGROUND OF THE INVENTION 
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 alloy with 
about 2.2 to 4.5 percent 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 percent 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 percent 
reduction in thickness, and given a final or texture-producing annealing 
treatment. 
As disclosed and claimed in U.S. Pat. No. 3,905,842, issued Sept. 16, 1975 
to Herbert E. Grenoble and assigned to the assignee hereof, the magnetic 
properties of such sheet materials can be very considerably improved by 
incorporating boron in the metal so that it is present there 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. Pat. No. 3,905,843, issued Sept. 16, 
1975 to Howard C. Fiedler and assigned to the assignee hereof, that such 
use of boron in the metal in proportion to nitrogen 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 is set forth in U.S. Pat. No. 3,957,546 
of Howard C. Fiedler assigned to the assignee hereof, which defines a 
process of direct cold rolling to the final gauge from the hot band stage 
with consistently good magnetic properties in the final product through 
maintenance of boron-nitrogen and manganese-sulfur ratios in the metal 
within certain critical ranges. 
SUMMARY OF THE INVENTION 
I have discovered that new and important results and advantages in addition 
to those set forth in referenced patent application Ser. No. 677,147 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 electrically-insulating 
coating on the sheet to another particular range. Further, I have found 
that it is essential to these new results and advantages that the total 
boron in the alloy and the coating thereon be limited to a certain 
maximum. Still further, I have 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, these new results can be 
obtained regularly and routinely. 
Specifically, I have found that 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 six 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. 
Still another discovery which I have made is that, 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. 
I have further found that such increases in coating boron content do not 
materially affect grain size of the final product sheet material. This is 
surprising in view of the disclosure by Matsumato et al in U.S. Pat. No. 
3,676,277 that such additions result in secondary grains somewhat smaller 
than normal and have no effect upon the permeability of the ultimate 
product. 
Still another finding that I have made is that the ratio of manganese to 
sulfur which is limited to 2.1 in the process disclosed and claimed in 
referenced U.S. Pat. No. 3,957,546 can run as high as 2.5 in accordance 
with the present invention with consistently good end-product magnetic 
properties. 
As set out in referenced patent application Ser. No. 677,147, the 
requirements of this invention in respect to the proportions of boron in 
the alloy and its coating, the nitrogen content of the alloy and the 
manganese and sulfur content of the alloy can all be met without 
difficulty. Again, as stated in the copending patent application, one has 
the choice of applying the boron with the magnesia slurry or other coating 
material in similar form, or the coating may be provided as disclosed in 
U.S. Pat. No. 3,054,732 (issued Sept. 18, 1962 to McQuade and assigned to 
the assignee hereof) and the coated sheet metal then contacted with an 
aqueous solution of a suitable boron compound. The latter procedure can 
take the form of a dipping operation or the aqueous solution may be 
brushed or sprayed on the coating, as desired. Suitable boron sources for 
this purpose include H.sub.3 BO.sub.3 and Na.sub.2 B.sub.4 O.sub.7, but it 
will be understood that other boron-containing compounds may be used 
individually or in a mixture and preferably in solution of water or other 
suitable vehicle to insure easy, uniform distribution over the coating 
surface. 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 product aspects. The product is a decarburized, coated, 
cold-rolled sheet or strip of final gauge thickness which contains boron 
that in criticl proportion to and in combination with the boron in the 
coating and the nitrogen in the metal will enable the development of the 
desired magnetic properties through secondary recrystallization during the 
final anneal. The process by which this coated sheet is produced is 
likewise novel as is the overall process of producing the final desired 
sheet material from a silicon-iron metal through a new combination of 
method steps including new critical boron and nitrogen proportioning 
steps. 
Briefly described, in its article aspect this invention takes 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 
thin, 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. 
Similarly described, the method of this invention 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.

DETAILED DESCRIPTION OF THE INVENTION 
In carrying out this invention, one may provide the intermediate sheet 
product described above by preparing a silicon-iron melt of the requiried 
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, 
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. 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. Pat. No. 3,054,732, referenced 
above, a uniform coating of Mg(OH).sub.2 about 0.5 mil thick 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 of the order of 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 to cause secondary grain growth which begins at 
about 950.degree. C. As the temperature is raised at about 50.degree. C. 
per hour to 1000.degree. C., the recrystallization process is completed 
and heating may be carried on to up to 1175.degree. C. if desired to 
insure complete removal of residual carbon, sulfur and nitrogen. 
The following illustrative, but not limiting, examples of my 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 
Twelve laboratory heats were melted in an air induction furnace under an 
argon cover using eletrolytic iron and 98 percent ferrosilicon, all 
containing 3.1 percent silicon, 0.1 percent copper and 0.03 percent 
chromium. The same amount of sulfur (0.024 percent) as iron sulfide was 
added to each heat, the sulfur analyses range 0.033 percent down to 0.019 
percent with an average of 0.026 percent. 
Slices 1.75 inch thick were cut from ingots cast from these melts and were 
hot rolled from 1200.degree. C. in six passes to a thickness of about 90 
mils. Following pickling, the hot band samples were heat treated at 
950.degree. C., the time between 930 and 950.degree. 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 I: 
TABLE I 
______________________________________ 
Composition as Determined on Cold-Rolled Strip 
Heat ppm B ppm N ppm O % Mn % S % C 
______________________________________ 
1 &lt; 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 percent by heating at 800.degree. C. in 70.degree. F. dew point 
hydrogen. The carburized 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 percent 
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 or 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 40.degree. C. per hour from 800.degree. C. to 1175.degree. C. 
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 II and in FIGS. 1 and 2: 
TABLE II 
______________________________________ 
Magnetic Properties Afer Final Anneal in Hydrogen 
MgO Only MgO+15ppm B MgO+30ppm B 
Heat ppm B 17kB .mu.10H 
17kB .mu.10H 
17kB .mu.10H 
______________________________________ 
1 &lt; 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 II and FIG. 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 FIG. 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 FIG. 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 behavior of both laboratory and mill heats. 
EXAMPLE II 
In another experiment designed to test the capabilities of this new 
process, a commerical melt was prepared using BOF silicon-iron as 
described in referenced U.S. Pat. No. 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 I 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 FIG. 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 800.degree. C. and 
heated at the rate of 40.degree. C. per hour to the maximum temperature of 
1175.degree. 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 FIG. 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 FIG. 3 that boron additions to the coating of 
boron-containing silicon-iron result in sufficient 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 III 
In another experiment like that described in Example I, 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 alone resulted in the 
highest heat nitrogen contents. The heats all contained 3.1 percent 
silicon, 0.1 percent copper and 0.03 percent 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 III: 
TABLE III 
______________________________________ 
Composition of Heats As Determined on Cold-Rolled Strip 
Heat % Mn % S % C 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 percent carbon at 800.degree. C. in hydrogen (dew point about 
70.degree. 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 percent. Analyses of the coatings for boron prior to the 
final anneal indicated that the concentrations increased linearly by 
approximately 12 ppm boron (on the basis of strip rather than coating 
weight) for each such 0.5 percent increment. The final anneal consisted of 
heating at 40.degree. C. per hour from 800.degree. to 1175.degree. 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 FIGS. 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 FIGS. 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 gain 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.