CA1109772A - Process of producing an electrically insulative glass film on silicon steel - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process of providing an electrically insulative glass film on cold reduced silicon steel strip or sheet stock which will have a magnetic perme-ability greater than 1850 at 796 A/m after a final high temperature anneal which develops cube-on-edge orientation, which comprises applying an aqueous slurry containing magnesia, at least one boron compound, and up to 20% titanium oxide, based on the weight of magnesia, drying the coating, and subjecting the coated strip or sheet to said final anneal, the boron content being proportioned within the range of about 0.07% to about 0.30%, based on the weight of magnesia, in accordance with the particle size distribution and citric acid activity of the magnesia, and inversely to the bulk density of the dried coating. Improved core loss characteristics are obtained without sacrifice of perme-ability values.

Description

l~C~772 This invention relates to the production of cube-on-edge oriented silicon steel strip or sheet material having very high magnetic permeability (greater than 1850 at 796 A/m) and more particularly to a process of providing the strip or sheet material with a thin, continuous, electrically insulative glass film, The process of the invention involves proportioning the amount of boron in a magnesia annealing separator com-position, in relation to the particle size distribution, citric acid activity (as hereinafter defined), and surface area of the magnesia, whereby to obtain improved core loss charactPristics and improved glass film formation, while maintaining very high permea~ility, in the production of silicon steel strip and sheet stock having the cube-on-edge orientation.
The production of silicon steel strip or sheet material havin~ very hiqh permeability is disclosed in United States Patents 3,873,381 and 3,855,019 In No. 3,873,381 critical amounts of boron and nitrogen are added to the silicon steel melt~ along with the conventional additions of ~anganese and sulfur (or selenium). to obtain very high permeability. No. 3,855fO19 discloses a copper addition to a silicon steel melt, in which aluminum nitride is also present as a primary grain growth inhibitor, to obtain improved permeability vaiues.
- United States Patent No. 3,676,227 issued July 11, 'ii3~

1972, to F. Matsumoto et al, (assigned to Nippon Steel Corporation) aiscloses a process of producing cube-on-edge oriented silicon steel, containing less than 4~ silicon and 0.010~ to 0.065~ acid soluble aluminum, having very high permeability and "low iron loss" ~i.e~, low core 108~
which includes applying an a~nealing separator composition to the surfaces of cold reduced, decarburized silicon steel stock, drying the separator composition, and - subjecting the stock to a final anneal at a temperature 1~ above 1000C for more than 5 hours ln hydrogen or nitrogen~
The annealing separator may be magnesium oxide, calcium oxide, aluminum oxide, titanium dioxide, or mixtures thereof, and contains from 0.01% to 1.0% by weight boron or a boron compound, based on the weight of annealing - 15 separator.
United States Patent 3,700,506 discloses - the addition of a boron compound to a magnetic separatoE
composition, which also contains titanium, manganese - and sulfur, for use on a silicon steel containing aluminum nitride.
- ~ The addition of boron compound~ to magnesium oxide anneallng separator compositions is al50 disclosed ... .
in British Patent 1,398,504; in United State~ Patents `- 3,583,887, and 3,841,925 assigned to Morton Norwich , . .
Products, Inc.; ~n United States Patents 3,697,322, 3,735,879, 3,932,202, 3,941,621 and 3,945,862 assigned to Merck & Co., Inc.; and in United States Paten~ 4,010,050.
A number of the above patents are concerned with improving ~he electrically insulative glass film formation and Franklin resiativity, by means of boron , 11C~9772 compound additions, in silicon steel stocks which would have permeabilities less than 1850 at 796 A/m.
Such materials ordinarily do not contain significant amounts of acid soluble aluminum, and hence do not relate to the same technology required of very high permeability (i.e., greater than 1~50 at 796 A/m) material, for reasons explained hereinafter.
It has been found that a magnesia from a Japanese source, containing about 0 08~ boron based on the weight of magnesia, produced excellent glass film coated silicon steels both from the standpo nt of the physical properties of the qlass film and the magnetic properties of the final silicon steel stock, in the produc-tion of very high permeability, cube-oneedge oriented material.
However, it was found to be impossible to reproduce these results, and to obtain consistently high permeability, low core loss, and good glas~ film properties ;n magnesia from other sources with boron additions of the same magnitude.~ Investigations showed that variations in sodium, calcium and chloride contents of the magnesias had little effect. On the other hand, variations in citric acid activity and surface area were found to have a pronounced effect, Despite the fact that hydration rate and surface area were thus known to affect the performance of magnesia, it was found that specifying a particular citric acid activity and surface area range still did not achieve uniformly reproducible results particularly with respect to magnesias from different commercial $ources.
Even different batches of magnesia from the same commercial ~, ' ; .

11`~977Z
.

source were found to cause difficulties of one type or another despite citric acid activity and surface area values which, according to prior art teachings, should have been optimum.
In view of the above back~round it is evident that attempts to improve magnetic properties of very ~~~' high permeability silicon steel by addition of about 0.08% boron to a commercial source of mQgneSia were at best only occasionally successful, and performance was unpredictable.
It is a principal object of the present invention to provide a process which solves the above problems in the production of silicon steels ha~ing very high permeability in accordance with any of the - 15 above United States Patents.
-i Briefly, it is known that boron, added to a magnesia coatin~ is volatile at the final high annealing tem~erature (at or about 1200C), with part of the ~oron dif~using inwardly through the surfaces of the silicon steel stock~ and the remainder escaping in~
to the annealin~ atmosphere where it is ;neffective, It has now been found that the ~mount of boron which ; volatilizes into the annealing atmosphere is a direct function of the bulk density, or packing factor of the , 25 dried magnesia coating. (It will be understood that the coating is applied as an aqueous slurry by any conventional method such as dipping, spraying, metering rolls, or the like, and is then dried at relatively low heat.) The bulk density or packing factor is in turn llOg77Z

directly dependent on the particle size distribution and degree of hydration of the magnesia.
~ lthough the citric acid activity of magnesia can be controlled, at least within broad limits, when pro-duced commercially in large quantities the particle sizedistribution is dependent on the particular manner of pro-duction and cannot be easily varied by commercial producers.
In view of th~s, the no~el concept of the present invention is to compensate for different particle size distri~utions by proportioning the amount of boron addition in relation to par-ticle size distribution, surface area and citric acid activity.
~ In other words, the boron addition is made inversely proportional - to the bulk density or packing factor of the dried magnesia coating.
Because of the thinness of the dried coatinq and relative roughness of the surface of the silicon ; steel stock, it is not possible to determine the bulk density of the dried coating with currently available - equipment or techniques. Hence the boron addition is proportionea to the three parameters which most directly affect the bulk density, viz., particle size distribution, surface area and degree of hydration, as determined ~y citric acid activity.
It has been found that good adherence of a dried magnesia coating on silicon steel surfaces usually produces a coating with a high ~ulk density which will thus require a relatively low boron addition It has further been found that the tightness or tension in the winding of a coil during the final high temperature anneal can affect the amount of boron required.

, .

~1~977Z

Loose laps or convolutions allow more boron to escape into the annealing atmosphere.
In connection with hydration, it will be recognized that formation of magnesium hydroxide lowers the density and changes the morphology of the original magnesia particles. The water of hydration is not driven off by the relatively low heat used in the drying of the coating. However, this water is driven off by heating to a higher temperature, such as occurs in the high temperature final anneal, thus increasing the porosity of the magnesia coating. This is the reason for the direct effect of the degree of hydration on bulk density.
With respect to the effect of particle size distribution on packing or bulk density, reference may be made to Fig. 3.2 of "Introduction to Ceramics" by W. D. Kingery, J. Wiley & Sons, Inc. (1960).
` According to the invention there is provided a method of improving the core loss characteristics of 20 cube-on-edge oriented silicon steel strip and sheet stock which will have a magnetic permeability greater than 18~0 at 796 A/m after a final high temperature anneal in a reducing atmosphere, which comprises adding - a boron compound to an aqueous magnesia slurry, applying 25 said slurry to the surfaces of the stock and drying the - so applied coating prior to the final anneal, characterized by adding the boron compound to provide a total boron content within the range of 0.07% to 0.30%, based on the weight of the magnesia, in inverse 30 proportion to the bulk density of the dried coating, whereby to cause a uniform amount of boron to diffuse ,~ .

`` 11`t~9772 inwardly through the magnesia coating during said final anneal, irrespective of the amount of boron volatillzed into the annealing atmosphere from the coating.
:. Reference is made to the accompanying drawings ~ S wherein:
: FIG 1 is a graphic representation of particle size distribution of a prior art magnesia from a first source, FIG, 2 is a graphic representation of particle size distribution of a magnesia from a second sourcet and FIG. 3 is a graphic representation of particle size distribution of a magnesia from a third source.
A preferred process of the invention for the production of silicon steel strip and sheet stock which will have a magnetic permeability greater than 1850 at 796 ~/m after a final high temperature anneal, includes the steps of providing a cold reduced decarburized silicon steel strip and sheet stock containing from 2% to 4% silicon, 0.01% to 0.15~ manganese, 0.002% to 0.005% carbon, 0.01% to 0.03 20 sulfur, up to 0.010% boron, 0 005% to 0,010% nitrogen, 0 010% to 0.065% acid soluble aluminum~ and balance iron ;: plus incidental impurities, applying to the surfaces of the stock an aqueous slurry comprising magnesium oxide, at ` least one boron compound, and up to 20%- titanium dioxide based on the weight of magnesium oxide, drying the so applied slurry by heating to a temperature sufficient to evaporate . .

. ;

11`~977Z

the water and leave a dried'coating on the surfaces, and annealing the coated stock in a non-oxidizing atmosphere at a temperature of about 1095 to about 1260C, whereby to form an insulative film and to develop a cube on-edge orienta-tion by secondary recrystallization, characterized in that the amount of said boron compound in the slurry is proportioned to ~~~
provide a total boron content ranging from 0.07% to 0.30~ by ` weight, based on the weight of magnesium oxide, in accordance with the particle size distribution, surface area and citric acid activity of the magnesium oxide. The process of the invention will result in production of a thin, continuous glass film and will improve the core loss characteristics while retaining the very high permeability of the stock.
It should be understood that a thin continuous glass film is advantageous in promoti`ng improved magnetic quality, better space factor, bette~ magnetostriction, and better adherence. Additionally, where applying a ~- secGndary coating such as the type disclosed in United States Patent 3,8~0,37~ to James D. Evans, a glass film must be thin and continuous in order to obtain good adherence of the secondary coating and to permit the tension-im~arting characteristics to be realized.
The thickness of the dried magnesia coating cannot be accurately determined for the same reasons explained above with respect to determination of bulk aensity of the coating. Accordingly, the coating-weight of the dried coating is used for control purposes, and , ," .

" 113[)9772 a dried coating which will form a continuous thin glass film having the above described advantageous properties will be formed with a dried coating weight of 6.3 to 15.65 grams per square meter ~or a magnesia having a citric acid activity of greater than 50 seconds.
A cold .educed decarburized silicon steel strip and sheet stock may be prepared by a conventional process wherein a suitable melt is cast as ingots or continuously cast into slab form If cast as ingots the steel is bloomed and slabbed in conventional manner, and the slabs are hot rolled to intermediate thickness from a temperature of about 1260 to about l400C, with annealing after hot rolling. The hot mill scale is then removed, and the material is cold rolled to final gauge in one or more stages, followed by decarburization in a hvdrogen atmosphere If the steel is continuously cast into slab form having a columnar grain structure, the method disclosed in United States Patent No. 3~764,406, to M,F, Littmann, is preferably followed, In this process, a continuous~cast slab having a thickness of about 10 to about 30 centimeters is heated to a temperature between 750 and 1250C. and initially hot reduced with a reduction in thickness of 5% to 50~, prior to reheating the slab to a temperature between about 1260 and 1400C for conventional hot rolling, The hot rolling, annealing~
- cold reduction and decarburization then follow in the manner described above.
The cold reduced and decarburized material is then coated with an aqueous magnesia slurry by dipping, '` ;
: ' ' lt) `~`
spraying, or metering rolls and dried by heating to a temperature on the order of about 200 - 300C to obtain a dried coating weight of 6.3 to 15.6 grams per square meter. The coated strip or sheet material is then subjected to a final high temperat~re anneal, which may be a box ~` anneal or an open coil anneal.
. j A convenient aqueous slurry concentration, ` when using metering rolls, ranges between n . 096 and 0.192 gram of magnesia per milliliter of water up to 20~
titanium dioxide, and preferably about 5%, may be added to the slurry based on the weight of magnesia.
The final high temperature anneal during which --the cube-on-edge orientation is produced by secondary recrys-tallization in known manner is carried out at about 1095 to - 15 about 1260C in a reducing atmosphere. It will be understood ~- that the magnesia reacts with silicon in the steel to form a glass film in this anneal. The heat-up portion of the final anneal is preferably conducted in a nitrogen-hydrogen atmosphere in order to optimize formation of nitrides which act as grain growth inhibitors. The final portion of the anneal, which includes soaking at temperature and ; cool-down, is preferably conducted in hydro~en since this is,known for Purification of the steel to promote secondary recrystallization.-The type of boron compound and the point at ; which it is added has been found to be of no particular significance. Boric acid, calcium borate, or other commonly available boron compounds may thus be used. The compounds may be added to the magnesia before or during the processing thereof, or may be added a~ter an aqueous slurry has been formed. It is also pcsslble to apply a boron compound to ' 1 1 11`~7Z
t the strip surfaces before applying the magnesia slurry therèto. Accordingly, the term "adding a boron compound to an aqueous magnesia slurry", as used herein, is to be construed as brO~d enough to cover addition or application of the boron compound at any stage prior to application of the slurry o the silicon steel stock surfaces.
Referring to FIG. 1 the particle size distri-bution of a magnesia of the first source is illustrated.
It will be noted that two peaks or humps occur with about 10~ of the particIes between 5 and 10 microns and about 22% between 0.8 and 2 microns. Magnesia from this source typically exhibits particle size distribution a~ follows, inJweight percent' 8-10% between 5 and 10 microns 3G-40% between 5 and 2 microns 20-30% between 2 and 1 microns 18-40% less than 1 micron.
, A particle size distribution of the type illustrated in FIG. 1 approaches a two-component system as shown in FIG. 3.2 of the above mentioned "Introduction to Ceramics". Hence the magnesia of FIG. 1 forms a relatively dense dried coating. Magnesia can be obtained from this source having a citric acid activity of greater - than 50 seconds. A nominal 0.08% boron addition has been __ round to give excellent results.
FIG. 2 represents particle size distribution ^ in a magnesia from a second source in which there is , some spread of sizes among relatively large and relatively small particles, but with a great preponderance less than 1 micron. Typical particle size distributions from this source are as follows:

.

Z
.` , ,, ; ,.

0-5~ between 5 and 10 microns 5-10% between 5 and 2 microns 5-10% between 2 and 1 microns : 5 ~ 75-90% less than 1 micron, with 50-60~ between 0.3 and 0.5 micron.
A magnesia of the type shown in FIG. 2 forms a less dense dried coating than that of FIG. 1. Accordi~gly, -i it has been found that about 0010% to about O.lS~ boron, based on the weight of magnesia, i8 needed for magnesia of the type of FIG. 2, with a citric acid activity greater ~ - than 50 seconds, in order to compensate for the boron lost r, ~ into the annealing atmosphere during the final anneal, when : th,e particle size distribution is from 75~ to 90% less than 1 micron. Citric acid activity may range from about 55 to about 120 seconds for magnesia from this source.
- FIG. 3 represents the particle size distribution of a magnesia from a third source. It will be noted that there is very little "scatter" into relatively large and small particle sizes and that a great preponderance lies within the size range of 2 to 5 microns. Typical particle size distribution for magnesias from this source are as follows:
0-5% between 5 and 10 microns 80-90~ between 5 and 2 microns : 25 10-20% between 2 and 1 micrans 0% less than 1 micron.
It was found that the bulk density of dried coa~i~gs of the magnesia shown in FIG. 3 was relatively low and less ~han either of those of FIGS. 1 and 2.
Accordingly, it was found that a boron content of about 0.15% to 0.20~ was necess~ry~ together with a citric , ?

11~977~

acid activity greater than 50 seconds, in order to compensate for boxon lost into the annealing atmosphere, when the particle size distribution is from 80% to 90% between 2 and 5 microns. Citric acid activity may range from about 60 to about 200 seconds for magnesia fxom this source.
Although less critical than citric acid activity and particle size distribution, ~urface area of the m~gnesia is nevertheless of importance in controlling the activity or hydration rate of the magnesia. Very finely divided material ` 10 with consequent high surface area tends to hydrate rapidly with consequent undesirable effect on the bulk density of the dried coating, as explained above. Material having too coarse a ,particle size and a very low su.face area tends to settle out of the aqueous slurry and does not readily undergo reaction with silica, during the final high temperature anneal, to form a thin~ continuous glass film. It has been found that a surface area between about 10 and about 20 square meters per gram gives excellent results, in combination with the other parameters of the present process.
The composition of the silicon steel set forth above is generally conventional and has been found to be critical in order to o~tain optimum magnetic properties.
The presence of manganese sulfide and aluminum nitride - within the specified ranges are necessary for preferential grain growth during the final high temperature anneal, which may have a total duration of about 8 to about 30 hours. ~lthough not required, boron may be added to the silicon steel melt along with nitrogen in critical amounts, in accordailce with the teachings of United States Patent 3,873,381 is~ued to J. M. Jackson. These boron :

- 1'1 97~Z

and nitrogen additions to the steel melt are for the purpose of controlling grain growth during the primary grain growth stage of the final anneal.
On the other hand, United States Patent 3,700,506 discloses the addition o~ a boroncompound to a magnetic separator compo~ition, which also contains tit2nium, manganese, and sulfur or sëlenium, in order to control.
secondary grain growth dur~ng ~he final anneal, in a silicon steel containing aluminum nitride as a primary grain growth inhibitor.
The presence of aluminum in the silicon steel results in the formation of a ~mall amount of aluminum oxide on,the surfaces of the silicon steel, which makes formation of a thin, adhe~ent and continuous glass film more difficult.
However, the addition of titanium dioxide within the range of about 5% to 20~ minimi2es this difficulty.
A series of tests have been made, all of which were conducted with cold reduced, decarburized silicon steel strip stock having a composition within the ranges of about 2~ to ~0 about 4~ silicon, about 0.01~ to about 0.15~ manganese, about 0.002~ to about 0.005% carbon, about 0.01% to about 0.03%
sulfur, about 0.005~ to about 0.010% nitrogen, about 0.010~ to about 0.065% acid soluble aluminum, up ~o about 0.010~ boron ~ and balance iron plus incidental impurities.
The magnesia ~rom the first source (FIG. 1) was used throughout the tests as a ~tandard for comparison since it has been succes~fully used for several years at a nominal boron content of ~bout 0.08% (total).
Test data are set for~h in the Ta~les herein.
Table I conta~ the scurce des;~nationst citric acid activity, surface area, coating weight (dried coating) and boron content of the various samples, It will be understood that the source designations refer to the three sources which are plotted in FIGS, 1-3 with respect to particle size distribution.
Table II summarizes the magnetic properties of coated and annealed coils of Table I samples. All values reported in Table II represent averages of front and back specimens of coils corrected to a thickness of 11.6 mils, All the magnesia coating slurries contained 5% titanium dioxide, based on the weight of magnesia, and slurry concentration ranged from 0.085 to 0.121 gram magnesia per milliliter o~-water.
The citric acid activity is a measure of the hydra-tion rate of magnesium oxide and is determined by measuring the time required for a given weiqht of a magnesia . .
, to provide hydroxyl ions suff;`cient to neutralize a given weight of citric acid, The test is the same as that reported in United States Patent 3,841,925, viz,s 1, 100 ml of 0.400 normal aqueous citric acid ; 20 containing 2 ml o~ 1~ phenolphthalein indicator i5 brought to 30C in an 8 ounce wide mouth jar, The jar is ~itted with a - screw cap and a magnetic stirrer bar.
~ . 2. Magnesia weighing 2,00g is admitted to the jar, - and a stop watch is started at the same instant.
.
3. As soon as the magnesia sample is added the lid is screwed on the jar. At the 5 second point the jar and contents are vigorously shaken. Shaking is terminated at the 10 second point, 4. At the 10 second point the sample is placed on a magnetic stirrer assembly. Mechanical stirring should produce a vortex about 2cm deep at the center when the inside '' ;

11~)9772 diameter of the jar is 6cm.
5. The stop watch is stopped at the instant the suspension turns pink, and the time is noted. This time in seconds is the citric acid activity.
It is evident that a low value represents a relatively active magnesia, i.e., one whiçh hydrates rapidly. The rate of hydration is of greater significance than the eventual degree of hydration, although a high rate usually al~o indicates a high degree of hydration at equilibrium.
Sample B, from Source 2, presented no problems with respect to coating. The slurry wet the strip and produced a smooth even coating on both surfaces. Hydration of the magne,sla in the aqueous slurry did not occur readily, and the adherence of the dried coating was good. The magnetic ; 15 properties were comparable to those of its control Sample A, from Source 1, thus indicating that the boron content ofjO.12%

: :
for Sample B was close to the optimum.
Sample D, from Source 3, which also contalned 0.12%
boron, also developed no coating problems. The slurry wet both surfaces well and produced an excellent dried coating, although the adherence of the dried coating was fair, rather than good.
The glass film after the final anneal was smooth, continuous , and light gr~y in appearance. ~owever, the core loss of Sample D
did not duplicate that of its control Sample C, from Source 1, the difference of 0.047 watts/kg being considered significant.
The permeability was also somewhat lower than that of the control Sample C. It is therefore evident that the optimum boron content for Sample D would be greater than 0.12~ boron.
Samples F and G from Source 2 were the same 3Q magnesia containing 0.07~ boron appli d a~ ~wo different coating :1109772 weights of 7.6 and 20.8 g/m2, respectively. These indicate that coating weight is a variable which can effect final magnetic properties. The low coating weight of Sample F
produced a thin, discontinuous glass film containing only a few small sulfide particles. The thicker glas~ film of Sample G contained a large number of large sulfide particles, and the subsurface silica particles were large and relatively few in number. However, neither of Samples F and G duplicated its control Sample E (from Source 1) in core loss ~aluos or in permeability. This indicated that th~ boron level of 0.07%
was insufficient.

TABLE I
Total Surface ~ Boron C.A.A. a~ea Coating Weight (based ~n Sample Source (seconds) (m /g~ (g/m2)_ wt. MgO) A 1 67 13.5 11.64 0.08 B 2 65 24.0 17 0.12 C 1 67 13.5 11.64 0.0 D 3 57 10 13 0.12 E 1 67 13.5 11.64 0.08 F 2 36 30.0 7.6 0.07

2 36 30.0 20.8 0.07 TABLE II
.
Core Loss Sample Source ~wat;ts~kg) Permeability 1.7 Tesla at 796A/m A 1 1~388 1916 B 2 1.397 1918 C ~ 1.388 1916 ~ 3 1.435 1910 E 1 1.418 1922 F 2 1.438 1917 G ; ~ 1.485 1916 ill:)9~7Z

: `

Variation in magnetic properties as a function of boron content (at several citric acid activity values) was shown by laboratory evaluations of several magnesias from the second source, having the particle size distribution of FIG. 2. These results are summarized in Table III. A
magnesia from the first source (Sample H, ~ource 1) was used as a control for comparison with Samples J and K, while :~ another~magnesia batch from the first source (Sample L) was i used as a control for comparison with Samples M through R.
The data of Table III (averages corrected to 11.6 mils gauge) indicate that although the 0.08% boron content of Sample J (from the second source) duplicated the magnetic properties ; of the control Sample H, substantially better magnetic : properties were obtained at a boron level of 0.13% in Sample K.
Samples M and P indicate that a boron level of 0.077% was insuficient to duplicate the magnetic properties of the control (Sample L), and that a boron level of about 0.1%
to 0.12% is necessary. However, a compari~on of Samples N and ~` O (C.A.A. of 80 seconds3with Samples Q and ~ (C.A.A. of 36 seconds~ shows that at a higher citric acid activity less boron - is needed to duplicate the magnetic properties of the control.
- Moreov~r, in the case of Samples N and O, better core loss was obtained at the 0.1% boron level than at 0.12%. This shows that an optimum range exists for any given citric acid activity and particle size distribution, and that boron contents below or above the optimum affect magnetic properties adversely.

97 7~

TABLE III

Total C.A.A. ~watts/kg) Permeability Sample Source ~Boron (Seconds) 1..7 Tesla at 796 A/m H 1 0.077 59 1.485 1918 J- 2 0.08 62 1.479 1920 K 2 0.13 62 1.420 1930 L 1 0.08 62 1.535 1919 M 2 0.077 80 1.605 1912 N 2 ~.10 80 1.485 1932 O 2 0.12 80 1.545 1927 P 2 0.077 36 1.595 1915 Q 2 0.10 36 1.579 1919 R ! 2 0.12 36 1.511 1931 . .
.~ Accordingly, the boron range of 0.10% to 0.30% is -~ to be considered critical, and this is demonstrated by further laboratory evaluations performed on a magnesia from the second source, having a ci~ric acid activity of 72 seconds, to which . boron~was added in amounts of 0.03%, 0.08%, 0.15%, 0.20%, 0.25%, : and 0.30%. respectively, based on the weight of magnesia. The . magnetic properties of the specimens were as follows:
Core Loss Total (watts/kg Permeability % Boron 1.7 Tesla at 796 A/m , 0.03 1.488 1930 0.08 1.436 1936 0.~5 1.450 1927 - 0.20 1.450 1917 0.25 1.462 1913 - 0.30 1.608 1926 ` ` 11~977Z

It is evident that the optimum boron range for the above sample was from 0,0~ to 0,20%, In addition to magnetic properties, a number of other factors are of importance in the ~ormation of an electrically insulative glass film, Among these are the viscosity of the magnesia slurry, wettability of the stock surfaces by the aqueous slurry, adherence of the dried coating, and thickness, smoothness and physical appearance of the qlass film.
Viscosity ordinarily is not a problem for slurry concentrations ranging between 0,096 and 0.192 grams of magnesia per milliliter of water, unless the hydration ,. .
rate is high. Under these conditions~ the viscosity gradually increases during the course of a run as the magnesia graduaily hydrates to a greater extent. This results in excessive ignition losses of the dried coating and an undesirable thick glass film, This can be avoidied in the practice of the present invention by insuring a ~- citric acid activity of greater than 50 seconds, which will reduce the hydration rate sufficiently. When viscosity increases, it is more difficult to get a smooth even as dried magnesia coating, Streaking of the coat;`ng may res~lt with high vtscosity~
Adherence of the dried coating apparently is a function of porosity, which again is affected by particle size distribution and citric acid activity, When these parameters are controlled in accordance with the present invention, adherence Qf the dried coating has been found to be satisfactory in all instances.

Thickness of the glass film and the reasons for li~V977Z
.

control thereof have been discussed above. It should suffice to reiterate that control of particle size dis-tribution and citric acid activity in accordance with the invention results in the formation of a desirable thin, continuous glass film. Similarly, smoothness of the glas~-metal interface is attained either directly or indirectly by control of these parameters.
- With respect to physical appearance of the film, discoloration usually occurs as a result of iron oxide formation. Excessive water pre~ent in the coating during the final anneal will usually produce a porous glass film which will not protect the steel and will not prevent iron oxide formation. Again th~iR i5 controlled in the practice of the present invention by providing a citric acid activity of greater than 50 seconds, thereby minimizing hydration.
In summary, for a magnesia having a particle . :
size distribution typical of that illustrated in FIG. 2 and a citric acid activity of greater than 50 seconds to about 120 seconds, a boron addition of about 0.~0~ to about 0.15~, based on the weight of magnesia, gives excellent results. For a magnesia having a particle size distribution typical of that illustrated in FIG. 3 and a citric acid activity of greater than 50 to about 200 seconds, a boron addition of about 0.15% to about 0.20%, based on the weight of magnesium oxide, gives excellent results.
In its broad aspect the invention provides a process for the production of fiilicon steel strip and she~t stock having a magnetic permeability greatex than 1109~7Z

1850 at 796 A/m, including the steps of providing a cold`reduced, decarburized silicon steel strip and sheet stock containing ~rom about 2% to about 4% silicon .
and from about 0.01% to about 0,065~ acid Roluble aluminum, ~ applying to the surfaces of the ~tock an aqueous slurry comprising magnesium oxide, at least one boron compound, and up to 20% by weight titanium dioxide (based on the;
.. weight of magnesium oxide)~ drying the 80 applied slurry to form a dried coating, and subjecting the coated stock to a final high temperature anneal, whereby to form a glass f$1m and to develop in ths stock a cube-on-edge orientation by secondary recrystallization, and pro-po~tioning the total boron content within the range of about 0.07% to about 0.3~ by weight, based on the weight of magnesium oxide, in accordance with the particle size distribution and citric acid activity of the magnesium oxide, whereby to improve the core loss characteristics while obtaining very high magnetic permeability in the - stock.
Preferably the final high temperature anneal is conduc.ted in a reducing atmosphere at a temperature of about 1095 to about 1260C, for a period of time up . - to about 30 hours. The preferred silicon steel composition, - in cold reduced and decarburized condition, consists , essentially of~ in weight percent~ from abou~ 2~ to about 4% silicon, about Q.01% to about 0.15% manganese, about - 0.002% to about 0.005% carbon, about 0.01~ to about 0.03% sulfur, about 0.005% to about 0.010~ nitrogen, about 0.010% to about 0.065% acid soluble aluminum, and balance iron plus incidental impurities.

Claims (10)

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

1. A method of improving the core loss characteristics of cube-on-edge oriented silicon steel strip and sheet stock which will have a magnetic permeability greater than 1850 at 796 A/m after a final, high temperature anneal in a reducing atmosphere, which comprises adding a boron compound to an aqueous magnesia slurry, applying said slurry to the surfaces of said stock and drying the so applied coating prior to said final anneal, characterized by adding said boron compound to provide a total boron content within the range of 0.07% to 0.30%, based on the weight of magnesia ! in inverse proportion to the bulk density of the dried coating, whereby to cause a uniform amount of boron to diffuse inwardly through said magnesia coating during said final anneal, irrespective of the amount of boron volatilized into the annealing atmosphere from said coating.

2, A process for the production of silicon steel strip and sheet stock having a magnetic permeability greater than 1850 at 796 A/m, including the steps of providing a cold reduced, decarburized silicon steel strip and sheet stock containing from 2% to 4% silicon and from 0.01% to 0.065%
acid soluble aluminum, applying to the surfaces of said stock an aqueous slurry comprising magnesium oxide, at least one boron compound, and up to 20% by weight titanium dioxide based on the weight of magnesium oxide, drying the so applied slurry on said surfaces to form a dried coating, and subjecting the coated stock to a final high temperature anneal, whereby to form a glass film and to develop in said stock a cube-on-edge orientation by secondary recrystallization, characterized by proportioning the total boron content of said slurry within the range of 0.07% to 0.30% by weight, based on the weight of magnesium oxide, in accordance with the particle size distribution and citric acid activity of said magnesium oxide, whereby to improve the core loss characteristics while obtaining very high magnetic permeability in said stock.

3. The process claimed in claim 1 or 2, characterized in that said silicon steel strip stock consists essentially of from 2% to 4% silicon, 0.01% to 0.15% Manganese, 0.002% to 0.005% carbon, 0.01% to 0.03% sulfur, 0.005% to 0.010% nitrogen, 0.10% to 0.065% acid soluble aluminum, and balance iron plus incidental impurities.

4. The process claimed in claim 1 or 2, characterized in that said slurry is applied at a rate sufficient to produce a dried coating weight of 6.3 to 15.65 grams per square meter, and that the citric acid activity of said magnesium oxide is greater than 50 seconds.

5. The process claimed in claim 1 or 2, characterized in that boron is added within the range of 0.10%
to 0.15%, based on the weight of magnesium oxide, when the citric acid activity is from greater than 50 to about 120 seconds, and when the particle size distribution of the magnesium oxide is from 75% to 90% less than 1 micron.

6. The process claimed in claim 1 or 2, characterized in that boron is added within the range of 0.10% to 0.15%, based on the weight of magnesium oxide, when the citric acid activity of said magnesium oxide is from greater than 50 to about 120 seconds, and when the particle size distribution thereof is as follows:
0-5% between 5 and 10 microns 5-10% between 5 and 2 microns 5-10% between 2 and 1 microns 75-90% less than 1 micron.

7. The process claimed in claim 1 or 2, characterized in that boron is added within the range of 0.15% to 0.20%, based on the weight of magnesium oxide, when the citric acid activity is from greater than 50 to about 200 seconds, and when the particle size distribution of the magnesium oxide is from 80% to 90% between 2 and 5 microns.

8. The process claimed in claim 1 or 2, characterized in that boron is added within the range of 0.15% to 0.20%, based on the weight of magnesium oxide, when the citric acid activity of said magnesium oxide is from greater than 50 to about 200 seconds, and when the par-ticle size distribution thereof is as follows:
0-5% between 5 and 10 microns 80-90% between 5 and 2 microns 10-20% between 2 and 1 microns 0% less than 1 micron

9. The method claimed in claim 1 or 2, characterized in that slurry contains from 5% to 20%
titanium dioxide, based on the weight of magnesia.

10. The method claimed in claim 1 or 2, characterized in that the surface area of said magnesia is from 10 to 20 square meters per gram.