Note: Descriptions are shown in the official language in which they were submitted.
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:
12
.
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,
977Z
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
'' ;
16
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.
19
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
21
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
22
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.
