Oriented magnetic steel sheets and manufacturing process therefor

An oriented magnetic steel sheet with a very low core loss and a process for manufacturing it at a lower cost are disclosed. The steel sheet consists essentially of Si: greater than 3.0% and at most 6.0%, Mn: greater than 2.0% and at moat 8.0%, sol. Al: 0.003-0.015%, with Si (%)-0.5.times.Mn (%).ltoreq.2.0 and the balance being Fe and incidental impurities, The amounts of C, N, and S as impurities are respectively at most 0.005%, at most 0.006%, and at most 0.01%. This steel sheet can be produced from a slab containing up to 0.01% C., up to 0.01% S and 0.001-0.010% N by (i) hot rolling the slab to obtain a hot-rolled steel sheet, (ii) cold rolling the hot-rolled steel sheet, as hot-rolled or after being subsequently annealed, one or more times with an intermediate annealing performed between successive stages of cold rolling to prepare a cold-rolled sheet, (iii) causing primary recrystallization by continuous annealing of the cold-rolled sheet, and (iv) finish annealing the continuously annealed steel sheet. The cold rolling may be carried out at a sheet temperature of 70.degree.-300.degree. C. The finish annealing preferably comprises causing secondary recrystallization by holding the annealed sheet in a temperature range of 825.degree.-925.degree. C. for 7-100 hours in a nitrogen-containing atmosphere, and holding the secondary-recrystallized sheet in a temperature range greater than 925.degree. C. and up to 1050.degree. C. for 4-100 hours in a hydrogen atmosphere to carry out purification annealing.

BACKGROUND OF THE INVENTION 
This invention relates to grain-oriented magnetic steel sheets or strips 
which are extensively used to make magnetic shields and cores in 
transformers, generators, and motors. The present invention also relates 
to a process for manufacturing such oriented steel sheets. 
Oriented steel sheets are soft magnetic materials that have a 
crystallographic orientation in which the {110}&lt;001&gt; orientation, 
generally referred to as the Goss orientation, is dominant. They have 
excellent excitation and core loss characteristics in the rolling 
direction. 
A typical process for producing oriented steel sheets comprises the steps 
of hot-rolling a slab of steel containing about 3.0% Si to obtain a 
hot-rolled sheet and then cold-rolling the hot-rolled sheet one or more 
times to attain a final sheet thickness, either immediately after hot 
rolling or after annealing the hot-rolled sheet. Intermediate annealing is 
conducted between successive stages of cold rolling. The sheet is then 
subjected to a continuous decarburization annealing to cause primary 
recrystallization, followed by application of a parting agent for 
preventing fusion or seizure, winding the sheet in a coil, and further 
performing finish annealing at a very high temperature of 
1100.degree.-1200.degree. C. The purpose of the finish annealing is 
two-fold; it is conducted to cause secondary recrystallization, thereby 
forming a textured structure in which integration in the Goss orientation 
is dominant, and it is also conducted to remove the precipitate, called an 
"inhibitor", which has been used to cause secondary recrystallization. The 
step of removing the precipitate is also known as "purification annealing" 
and may be regarded as an essential step for obtaining satisfactory 
magnetic characteristics. 
One major disadvantage of oriented magnetic steel sheets produced by the 
method described above is their extremely high cost since the production 
process involves special steps such as continuous decarburization 
annealing and finish annealing at extra-high temperatures of at least 
1100.degree. C. 
Various R&D efforts have been made with a view of solving this cost 
problem. For example, the present inventors developed an oriented magnetic 
steel sheet chiefly characterized by comprising 0.5-2.5% Si, 1.0-2.0% Mn, 
0.003-0.015% sol. Al, up to 0.01% C and 0.001-0.010% N, as well as a 
process for its production that did not need decarburization annealing but 
which was capable of low-temperature annealing (Japanese Published 
Unexamined Patent Application No. 1-119644/1989). That process is 
anticipated to make a great contribution to reducing the cost of oriented 
magnetic steel sheets by omitting the step of continuous decarburization 
annealing while lowering the temperature for finish annealing. 
Due to an ever-growing societal demand for energy conservation, there is a 
strong impetus to reduce the core loss of oriented magnetic steel sheets. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
oriented steel sheet and a process for its manufacture, the sheet having 
properties superior to those described in Japanese Published Unexamined 
Patent Application No. 1-119644/1989, described above. 
Another object of the present invention is to provide an oriented magnetic 
steel sheet with a very low core loss, as well as a process for its 
manufacture. 
Core losses can be roughly divided into hysteresis losses and eddy current 
losses. Hysteresis losses can be decreased by raising the degree of 
integration of the Goss orientation or by decreasing the level of 
impurities, while eddy current losses can be decreased by increasing the 
resistivity of the steel sheet and decreasing the sheet thickness. 
However, efforts to increase the integration of the Goss orientation or to 
decrease the levels of impurities have virtually reached practical limits. 
Although it is still possible to decrease eddy losses by decreasing the 
thickness of steel sheets, decreasing the sheet thickness inevitably 
results in increased manufacturing costs. 
The resistivity of a steel sheet can be increased by raising the Si 
content, but increases in the Si content results in a degradation in the 
workability of the steel sheet and cold rolling becomes difficult. 
Therefore, in actual practice, it is impractical to raise the Si content 
above 3.3%. For this reason, in Japanese Published Unexamined Patent 
Application No. 1-119644/1989, the Si content of a magnetic steel sheet is 
restricted to at most 2.5% by weight. Accordingly, attempts to decrease 
core losses by increasing the Si content to raise the resistivity have 
reached practical limits. 
As a result of investigations aimed to finding a method of increasing the 
resistivity of steel sheets without degrading workability, the present 
inventors made the following discoveries. 
(1) Even if the Si content of a steel sheet exceeds 3%, if the Mn content 
satisfies the formula 
EQU Si(%)-0.5.times.Mn(%).ltoreq.2.0 
decreases in workability can be restrained, and the occurrence of 
secondary recrystallization at the time of finish annealing can be 
stabilized. 
(2) The workability at the time of cold rolling of a steel containing Si 
and Mn in amounts satisfying the above formula can be enormously increased 
if the cold rolling is performed when the steel sheet is in a temperature 
range of 70.degree.-300.degree. C. 
(3) Like Si, Mn has the effect of increasing the resistivity of steel 
sheet, and it is extremely effective at decreasing core losses. 
(4) In a steel with a high content of Si and Mn, in order to initiate 
secondary recrystallization, it is effective in the first half of finish 
annealing to maintain the steel in an environment including N.sub.2 at a 
temperature of 825.degree.-925.degree. C., and in order to remove nitrides 
which function as inhibitors, it is effective in the last half of finish 
annealing to perform purification annealing in an H.sub.2 atmosphere at a 
temperature greater than 925.degree. C. and at most 1050.degree. C. 
Accordingly, an oriented magnetic steel sheet according to the present 
invention consists essentially of, on a weight basis, of 
Si: greater than 3.0% and at most 6.0%, 
Mn: greater than 2.0% and at most 8.0% 
sol. Ai: 0,003-0.015% 
with Si (%)-0.5.times.Mn (%).ltoreq.2.0 and a balance of Fe and incidental 
impurities, wherein the amounts of C, N, and S as impurities are 
C: at most 0.005%, 
N: at most 0.006%, and 
S: at most 0.01%. 
A manufacturing process for an oriented magnetic steel sheet according to 
the present invention comprises subjecting a steel slab having a 
composition consisting essentially of 
C: at most 0.01%, 
Si: greater than 3.0% and at most 6.0% and more preferably at most 4.0%, 
Mn: greater than 2.0% and at most 8.0% and more preferably at most 4.0% 
S: at most 0.01%, 
sol. Al: 0.003-0.015%1 
N: 0.001-0.010%, 
with Si (%)-0.5.times.Mn (%).ltoreq.2.0 and a balance of Fe and incidental 
impurities 
to the following steps: 
(i) hot rolling the slab to obtain a hot-rolled steel sheet; 
(ii) cold rolling the hot-rolled steel sheet, either as hot-rolled or after 
being subsequently annealed, one or more times with an intermediate 
annealing performed between successive stages of cold rolling to prepare a 
cold-rolled sheet; 
(iii) causing primary recrystallization by continuous annealing of the 
cold-rolled sheet; and 
(iv) performing finish annealing. 
The cold rolling step is preferably carried out such that the temperature 
of the sheet being cold rolled is 70.degree.-300.degree. C. 
The finish annealing is preferably carried out by causing secondary 
recrystallization by holding the annealed sheet in a temperature range of 
825.degree.-925.degree. C. for 7-100 hours in a nitrogen-containing 
atmosphere, and holding the secondary-recrystallized sheet in a 
temperature range above 925.degree. C. and up to 1050.degree. C. for 4-100 
hours in a hydrogen atmosphere to carry out purification annealing. 
A parting agent may be applied to the steel sheet after the continuous 
annealing and before the finish annealing. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A magnetic steel sheet according to the present invention is manufactured 
from a steel slab having a prescribed composition. The limits on the 
content of each component of this composition will be described below. 
(a) C and N 
The presence of carbon (C) and nitrogen (N) has an adverse effect on the 
properties of magnetic steel sheet. Therefore, in the finished steel 
sheet, it is necessary to limit the C content to at most 0.005% and the N 
content to at most 0.006%. Preferably, the C content is at most 0.003% and 
the N content likewise at most 0.003%. The reason for these limits is that 
C and N remaining in the final product form carbides and nitrides which 
obstruct domain-wall mobility, resulting in an increase in core loss. 
However, if the C content of the steel slab which serves as a raw material 
is made at most 0.01%, even if the annealing after the final stage of cold 
rolling is not decarburization annealing, secondary recrystallization 
during finish annealing is not impeded. It is also possible to decrease 
the C content to a desired level during purification annealing in the last 
half of finish annealing. Therefore, the C content in the steel slab is 
restricted to at most 0.01%. 
Nitrogen is necessary for forming inhibitor nitrides and should be present 
until after secondary recrystallization is completed. If the N content is 
less than 0.001% in the starting steel slab, the precipitation of nitrides 
is too small to provide the desired inhibitor effect. On the other hand, 
the effectiveness of N saturates when present in an amount exceeding 
0.010%. Hence, the range of 0.001-0.010% is preferable for the N content. 
The N content can also be reduced to a level of at most 0.006% during the 
purification annealing. 
(b) Si 
Silicon (Si) causes substantial effects on magnetic characteristics. The 
higher its content, the higher the electric resistance of the steel sheet 
and the lower the eddy-current loss, leading to a smaller core loss. 
However, if the Si content exceeds 6.0%, the workability decreases to make 
subsequent cold rolling difficult to achieve. Therefore, the upper limit 
on the Si content is preferably 6.0% and more preferably 4.0%. On the 
other hand, if the Si content is 3.0% or below, the electric resistance of 
the steel sheet is too low to reduce the core loss. Therefore, the Si 
content is preferably greater than 3.0% and preferably at most 6.0% and 
more preferably at most 4.0%. 
(c) Mn 
Manganese (Mn) is effective at causing .alpha.-.gamma. transformation in 
the slabs of high Si and extra-low carbon steels such as the steel of the 
present invention. That transformation promotes the refining and 
homogenization of the structure of the sheet being hot rolled. As a 
result, secondary recrystallization characterized by a higher degree of 
integration in the Goss orientation will occur in a stable way in the 
finish annealing, and the workability of a high-Si steel is improved. 
The development of .alpha.-.gamma. transformation is determined by the 
balance between the content of Si, which is a ferrite-forming element, and 
Mn, which is an austenite-forming element. Hence, a suitable content of 
each of Si and Mn is determined by the content of the other. In the 
present invention, Mn is contained in such an amount as to satisfy the 
condition Si (%)-0.5.times.Mn (%).ltoreq.2.0. This is necessary for 
causing the appropriate transformation in the hot-rolled sheet. In the 
case where Si is contained in an amount of greater than 3.0%, at least 
2.0% of Mn is necessary in order to satisfy the condition set forth above. 
Furthermore, when the Si content is 6.0%, the Mn content must be at least 
8.0%. However, an Mn content of greater than 8.0% results in a degradation 
in cold workability. Therefore, the upper limit on the Mn content is 
preferably 8.0% and more preferably 4.0%. 
Like Si, Mn is effective for raising the electrical resistance of a steel 
sheet. From the standpoint of decreasing core loss, the Mn content should 
be greater than 2.0%. Accordingly, the Mn content is preferably greater 
than 2.0% and preferably at most 8.0% and more preferably at most 4.0% and 
satisfies the formula Si(%)-0.5.times.Mn(%).ltoreq.2.0. 
(d) S 
Sulfur (S) combines with Mn to form MnS. In the present invention, AlN, 
(Al,Si)N, and Mn-containing nitrides are used as principal inhibitors. In 
other words, MnS which is used in ordinary oriented magnetic steel sheets 
is not used as a principal inhibitor in the present invention. Hence, 
there is no need to add S in large amounts. If large amounts of MnS grains 
remain in the product steel, its core loss characteristics will 
deteriorate. Further, the temperature for finish annealing is at most 
1050.degree. C. in the present invention, so one cannot expect a 
desulfurizing effect to occur in the step of purification annealing. 
Therefore, the S content is controlled to be no more than 0.010% whether 
in the final product or the starting steel slab. For reducing the core 
loss, the S content is preferably at most 0.005%. 
(e) Sol. Al 
Aluminum (Al) is an important element that forms nitrides such as Al N and 
(Al,Si)N, which are principal inhibitors playing an important role in the 
development of secondary recrystallization. If the Al content is less than 
0.003% in terms of sol. Al, the inhibitor effect will be inadequate. 
However, if the amount of sol. Al exceeds 0.015%, not only does the 
inhibitor level become excessive but it is also dispersed inappropriately, 
making it impossible to cause secondary recrystallization in a stable way. 
Next, the steps in manufacturing a steel sheet according to the present 
invention will be described. 
(f) First Step (hot rolling) 
The starting steel slab has the composition specified in the preceding 
paragraphs. It may be a slab produced by continuous casting of a molten 
steel that is prepared in a converter, an electric furnace, etc. and that 
is optionally subjected to any necessary treatment such as vacuum 
degassing, or it may be produced by blooming an ingot of that molten 
steel. The conditions for hot rolling are not limited in any particular 
way but preferably the heating temperature is 1150.degree.-1270.degree. C. 
and the finishing temperature is 700.degree.-900.degree. C. 
(g) Second Step (cold rolling) 
The hot-rolled steel sheet is cold-rolled either once or a plurality of 
times to achieve a predetermined thickness of the product sheet. In this 
case, annealing (generally referred to as "hot-rolled sheet annealing") 
may be performed prior to the start of cold rolling. Hot-rolled sheet 
annealing promotes the optimization of the state of dispersion of 
precipitates and the homogenization of the microstructure of the 
hot-rolled sheet due to recrystallization. Hence, it is effective at 
stabilizing the development of secondary recrystallization during finish 
annealing. 
If hot-rolled sheet annealing is to be accomplished by continuous 
annealing, soaking is preferably conducted at 700.degree.-1100.degree. C. 
and more preferably at 750.degree.-1100.degree. C. If annealing is to be 
performed by box annealing, soaking is preferably conducted at 
650.degree.-950.degree. C. 
If cold rolling is to be performed a plurality of times, an intermediate 
annealing step is performed between successive stages of cold rolling. 
This intermediate annealing is preferably conducted at a temperature of 
700.degree.-1000.degree. C. In order to attain a satisfactory structure of 
primary recrystallization by continuous annealing, the reduction in 
thickness to be achieved upon completion of the cold rolling is preferably 
40-90%, with even better results being effectively attained by a reduction 
of 60-90%. 
If the temperature of the steel sheet during cold rolling is at least 
70.degree. C., the workability of the steel sheet is improved, and the 
incidence of breakage during rolling is greatly decreased. The higher the 
temperature of the sheet during cold rolling the higher the greater the 
improvement in the cold rolling characteristics of the sheet. However, if 
the temperature of the steel sheet during cold rolling exceeds 300.degree. 
C., the surface of the sheet oxidizes, which is undesirable. Therefore, 
the temperature of the steel sheet during cold rolling is preferably 
70.degree.-300.degree. C. 
When a plurality of stages of cold rolling are performed, it is desirable 
that the steel sheet be within the above-described temperature range for 
each pass. It is necessary for the sheet temperature during cold rolling 
at least when the sheet has a thickness of 1.0 mm or above. 
(h) Third step (continuous annealing before finish annealing--primary 
recrystallization annealing) 
In order to insure that stable secondary recrystallization will occur in 
the finish annealing to be described below, primary recrystallization by 
rapid heating is necessary. For this purpose, continuous annealing is 
effective. The annealing temperature is preferably 
700.degree.-1000.degree. C. 
(i) Fourth step (finish annealing) 
Finish annealing is performed in order to produce secondary 
recrystallization and produce an integrated structure in which the Goss 
orientation is integrated. In the present invention, the finish annealing 
preferably consists of annealing (first annealing) in the first half of 
annealing in order to develop secondary recrystallization and subsequent 
annealing (second annealing) which is intended to remove precipitates 
(purification). 
To develop secondary recrystallization, annealing in a nitrogen-containing 
atmosphere is necessary. This is for preventing the occurrence of unstable 
secondary recrystallization due to the decrease in inhibitor nitrides upon 
denitration. One reason for this practice is in order to increase the 
precipitation of inhibitor nitrides by nitrogen absorption from the 
annealing atmosphere so as to induce the occurrence of secondary 
recrystallization that is characterized by a higher degree of integration 
in the Goss orientation. To meet this need, the content of N.sub.2 in the 
annealing atmosphere is preferably at least 10 vol % (it may be composed 
of 100 vol % N.sub.2). The non-N.sub.2 gaseous component of the annealing 
atmosphere may be H.sub.2 or Ar, with the former being more common. 
The effective temperature range for causing secondary recrystallization is 
825.degree.-925.degree. C. Below 825.degree. C., the inhibitors which are 
used have such a strong power of inhibiting grain growth that secondary 
recrystallization will not occur. On the other hand, the inhibitor effect 
is so weak in the temperature range exceeding 925.degree. C. that either 
secondary recrystallization characterized by a low degree of integration 
in the Goss orientation will occur, or alternatively the normal grains 
will grow to coarsen the grains of primary recrystallization. The 
temperature in the range of 825.degree.-925.degree. C. is held for at 
least 7 hours, but there is no advantage to holding it for more than 100 
hours, and doing so is economically disadvantageous. For these reasons, 
the first half of the finish annealing process (first annealing) is 
accomplished by holding the steel sheet at 825.degree.-925.degree. C. for 
7-100 hours in a nitrogen-containing atmosphere in order to cause 
secondary recrystallization. 
Once secondary recrystallization has occurred, the inhibitor nitrides are 
deleterious to magnetic characteristics and must be removed. This removal 
is performed by the second annealing comprising purification annealing. It 
can be effectively accomplished by annealing in an H.sub.2 atmosphere. An 
adequate effect can not be obtained at a temperature of 925.degree. C. and 
below, and more preferably the purification annealing is carried out a 
temperature of at least 950.degree. C. However, there is no advantage to 
employing a temperature exceeding 1050.degree. C. since the effect of 
annealing to remove nitrides saturates. The temperature for purification 
annealing must be held for at least 4 hours but holding for more than 100 
hours is unnecessary. Therefore, the second half of the finish annealing 
process (second annealing) is to be accomplished by performing 
purification annealing in a temperature range exceeding 925.degree. C. but 
not exceeding 1050.degree. C. for 4-100 hours in an H.sub.2 atmosphere. 
As in the process for producing conventional oriented magnetic steel 
sheets, a parting agent may be applied before finish annealing so as to 
prevent seizure that may occur during annealing. Steps to be adopted after 
finish annealing are also the same as in the case of conventional oriented 
magnetic steel sheets; after removing the parting agent, an insulating 
coat may be applied or flattening annealing may be carried out as required 
.

The present invention will be further described in conjunction with the 
following working examples which are presented merely for illustrative 
purposes. 
(Example 1) 
Steel slabs having the compositions given in Table 1 were prepared by a 
process consisting of melting in a converter, compositional adjustment by 
treatment under vacuum, and continuous casting. The slabs were hot rolled 
at an elevated temperature of 1250.degree. C. and finished to a thickness 
of 2.0 mm at 830.degree. C. The test steels had a much higher resistivity 
than conventional oriented magnetic steel sheets (with a resistivity of 
approximately 50 .mu..OMEGA.cm). The balance of Si and Mn was varied so as 
to maintain the resistivity substantially constant. Subsequently, the 
hot-rolled sheets were annealed by soaking at 880.degree. C. for 1 minute, 
descaled by pickling, and cold rolled to a thickness of 0.30 mm by one 
stage of rolling. 
Steels No. 1-3 which had compositions outside the range of the present 
invention developed cracks in the edge portions of the steel sheets during 
cold rolling and ended up breaking, so cold rolling to a desired thickness 
could not be carried out. In contrast, hot rolled Steels No. 4 and 5 
according to the present invention suffered no breakage and could be cold 
rolled to form steel sheets of a desired thickness. 
TABLE 1 
__________________________________________________________________________ 
Composition of stee slab (wt %) 
Run Fe + Si (%) - 
Resistivity 
No. 
C Si Mn S sol. Al 
N Impurities 
0.5 .times. Mn (%) 
(.mu..OMEGA. .multidot. cm) 
Remarks 
__________________________________________________________________________ 
1 0.0030 
4.56 
0.08 
0.003 
0.007 
0.0042 
Bal. 4.52 65 X 
2 0.0029 
4.02 
1.20 
0.003 
0.008 
0.0040 
" 3.42 66 X 
3 0.0030 
3.51 
2.25 
0.003 
0.007 
0.0038 
" 2.39 66 X 
4 0.0035 
3.22 
2.82 
0.003 
0.006 
0.0037 
" 1.81 66 .largecircle. 
5 0.0030 
3.10 
3.00 
0.003 
0.008 
0.0041 
" 1.60 65 .largecircle. 
__________________________________________________________________________ 
NOTE 
.largecircle.: Present Invention 
X: Comparative 
(Example 2) 
A cold rolled sheet (0.30 mm thick) obtained in Example 1 of Steel No. 5 
was subjected to continuous annealing by soaking at 880.degree. C. for 30 
seconds in a 75 vol % N.sub.2 +25 vol % H.sub.2 non-decarburizing 
atmosphere having a dew point of -20.degree. C. so as to cause primary 
recrystallization, followed by application of a parting agent and a finish 
annealing. The finish annealing process consisted of a first annealing 
performed by soaking in a 75 vol % N.sub.2 +25 vol % H.sub.2 atmosphere at 
885.degree. C. for 24 hours, changing the atmosphere to an H.sub.2 
atmosphere, and then second annealing consisting of purification annealing 
performed by soaking for 24 hours at the various temperatures shown in 
Table 2. The C and N contents of the resulting steel sheets and the 
magnetic characteristics in the rolling direction are also shown in Table 
2. 
As is clear form Table 2, Steels Nos. 2-7 according to the present 
invention had low core losses, and the core losses decreased as the C and 
N contents decreased. Furthermore, as can be seen from the test results 
for Steels Nos. 4-7, when the purification annealing in the last half of 
finish annealing is performed in the temperature range specified by the 
present invention, the C and N contents greatly decrease, and steel sheet 
having even lower core losses is obtained. 
TABLE 2 
______________________________________ 
2nd C and N levels, core loss 
Annealing and flux density of product 
Run Temperature 
C N W.sub.17/50 
B.sub.8 
No. (.degree.C.) 
(%) (%) (W/kg) (T) Remarks 
______________________________________ 
1 880 0.0025 0.0065 
1.30 1.78 X 
2 900 0.0020 0.0045 
1.22 1.79 .largecircle. 
3 920 0.0014 0.0030 
1.17 1.79 .largecircle. 
4 940 0.0009 0.0018 
1.05 1.81 .largecircle. 
5 960 0.0008 0.0015 
1.03 1.81 .largecircle. 
6 980 0.0007 0.0010 
1.02 1.82 .largecircle. 
7 1000 0.0007 0.0010 
1.00 1.82 .largecircle. 
______________________________________ 
NOTE 
.largecircle.: Present Invention 
X: Comparative 
[Example 3] 
Three steel types having substantially the same composition within the 
ranges specified by the present invention but differening with respect to 
the content of sol. Al (see Table 3) were hot-rolled under the same 
conditions as in Example 1 and each finished to a thickness of 2.3 mm. The 
hot-rolled sheets were descaled by pickling and subjected to box annealing 
by soaking at 800.degree. C. for 2 hours. Subsequently, each of the 
annealed sheets was cold-rolled to a thickness of 0.35 mm by one stage of 
rolling. 
Each of the cold-rolled sheets was subjected to continuous annealing by 
soaking at 875.degree. C. for 30 sec in a 80 vol % N.sub.2 +20 vol % 
H.sub.2 non-decarburizing atmosphere having a dew point of -25.degree. C. 
or below to cause primary recrystallization, followed by application of a 
parting agent and finish annealing. The finish annealing process consisted 
of soaking in a 75 vol % N.sub.2 +25 vol % H.sub.2 atmosphere at 
875.degree. C. for 24 hours, shifting to an H.sub.2 atmosphere, and 
purification annealing by soaking t 950.degree. C. for 24 hours. The C and 
N levels of the resulting steel sheets and their magnetic characteristics 
in the rolling direction are shown in Table 4. 
Steel No. 1 had a smaller amount of sol. Al than specified by the present 
invention. Even though the C and N contents were within the ranges of the 
present invention, on account of the weak inhibitor effect, secondary 
recrystallization characterized by integration in the Goss orientation 
could not be obtained and it had poor magnetic characteristics. Steel No. 
3 had a greater amount of sol. Al and a higher N content than specified by 
the present invention also had a high N content. No secondary 
recrystallization was found to have occurred, so Steel No. 3 was very poor 
with respect to both core loss and magnetic flux density. In contrast, 
Steel No. 2 corresponding to an example of the electrical steel sheet of 
the present invention exhibited excellent magnetic characteristics. 
TABLE 3 
______________________________________ 
Composition of steel slab (wt %) 
Run Fe + 
No. C Si Mn S Sol. Al 
N Impurities 
______________________________________ 
1 0.0025 3.21 3.22 0.005 
0.002 0.0037 
Bal. 
2 0.0027 3.20 3.20 0.005 
0.006 0.0035 
Bal. 
3 0.0029 3.20 3.21 0.005 
0.021 0.0033 
Bal. 
______________________________________ 
TABLE 4 
______________________________________ 
C and N levels, core loss 
and flux density of product 
Run C N W.sub.17/50 
B.sub.8 
No. (%) (%) (W/kg) (T) Remarks 
______________________________________ 
1 0.0008 0.0013 2.36 1.60 X 
2 0.0009 0.0015 1.12 1.80 .largecircle. 
3 0.0009 0.0062 4.10 1.52 X 
______________________________________ 
NOTE 
.largecircle.: Present Invention 
X: Comparative 
(Example 4) 
Steel slabs each consisting of 0.0050% C, 3.31% Si, 3.45% Mn, 0.0006% S, 
0.007% sol. Al, 0.0035% N, and a balance of Fe and incidental impurities 
were prepared by the same method as in Example 1. The slabs were hot 
rolled under the same conditions as in Example 1 and finished to a 
thickness of 2.3 mm. The hot rolled sheets were descaled by pickling, cold 
rolled to a thickness of 1.4 mm, subjected to intermediate annealing by 
soaking at 850.degree. C. for 1 min, and cold rolled to a thickness of 
0.27 mm. 
Subsequently, the cold rolled sheets were subjected to continuous annealing 
by soaking at 875.degree. C. for 30 sec in a 70 vol % N.sub.2 +30 vol % 
H.sub.2 non-decarburizing atmosphere having a dew point of -15.degree. C. 
or below to cause primary recrystallization. Thereafter, a parting agent 
was applied and finish annealing was conducted. 
The finish annealing was conducted under the three different conditions set 
forth in Table 5. The finish annealing process consisted of first 
annealing comprising soaking in a 50 vol % N.sub.2 +50 vol % H.sub.2 
atmosphere for the purpose of achieving secondary recrystallization and 
second annealing in an H.sub.2 atmosphere for the purpose of purification 
annealing. The C and N levels of the resulting steel sheets and their 
magnetic characteristics in the rolling direction are shown in Table 6. 
Steel No. 1 was subjected to first annealing using a soaking temperature 
higher than the range specified by the present invention. The inhibitor 
effect was weak, normal grain growth progressed, and secondary 
recrystallization did not take place, so even though the C and N contents 
were within the ranges specified by the present invention, Steel No. 1 had 
poor magnetic characteristics. Steel No. 3, which was subjected to the 
second annealing at a lower soaking temperature than specified by the 
present invention, experienced secondary recrystallization, but since the 
C and N contents were outside the ranges specified by the present 
invention, the magnetic characteristics were not satisfactory. In 
contrast, Steel No. 2 corresponding to an example of the present invention 
had excellent magnetic characteristics. 
TABLE 5 
______________________________________ 
Run Soaking condition 
Soaking condition 
No. for 1st annealing 
for 2nd annealing 
______________________________________ 
1 960.degree. C. .times. 24 h 
960.degree. C. .times. 24 h 
2 890.degree. C. .times. 24 h 
960.degree. C. .times. 24 h 
3 890.degree. C. .times. 24 h 
890.degree. C. .times. 24 h 
______________________________________ 
TABLE 6 
______________________________________ 
C and N levels, core loss 
and flux density of product 
Run C N W.sub.17/50 
B.sub.8 
No. (%) (%) (W/kg) (T) Remarks 
______________________________________ 
1 0.0008 0.0016 2.15 1.51 X 
2 0.0008 0.0017 0.92 1.80 .largecircle. 
3 0.0020 0.0062 1.27 1.78 X 
______________________________________ 
NOTE 
.largecircle.: Present Invention 
X: Comparative 
[Example 5] 
Steel slabs having the compositions shown in Table 7 were hot rolled to a 
thickness of 2.0 mm. In order to decrease core loss, these test steels had 
a much high resistivity that conventional oriented magnetic steel sheets, 
which generally have a resistivity of approximately 50 .mu..OMEGA.cm. The 
balance of Si and Mn in these steels was varied in a manner which 
maintained the resistivity substantially constant. 
Next, the steel sheets were subjected to continuous annealing by soaking at 
880.degree. C. for 1 minute followed by descaling by pickling. Then, the 
sheets were cold rolled to a thickness of 0.30 mm. The temperature of the 
steel sheets during cold rolling was adjusted by placing the steel sheets 
in coiled form prior to cold rolling into a box annealing furnace and 
heating the coiled sheets so that the temperature of the steel sheets at 
the time of cold rolling was 120.degree.-150.degree. C. 
Steels No. 1-3 which had compositions outside the range of the present 
invention developed cracks in the edge portions of the steel sheets during 
cold rolling and ended up breaking, so cold rolling to a desired thickness 
could not be carried out. In contrast, hot rolled Steels No. 4 and 5 
according to the present invention suffered no breakage and could be cold 
rolled to form steel sheets of a desired thickness. 
TABLE 7 
__________________________________________________________________________ 
Composition of stee slab (wt %) 
Run Fe + Si (%) - 
Resistivity 
No. 
C Si Mn S sol. Al 
N Impurities 
0.5 .times. Mn (%) 
(.mu..OMEGA. .multidot. cm) 
Remarks 
__________________________________________________________________________ 
1 0.0030 
6.46* 
0.08 
0.002 
0.008 
0.0042 
Bal. 6.42* 82 X 
2 0.0029 
5.45 
2.13 
0.002 
0.009 
0.0040 
" 4.39* 82 X 
3 0.0030 
4.51 
4.05 
0.002 
0.008 
0.0038 
" 2.49* 82 X 
4 0.0035 
4.02 
5.00 
0.002 
0.007 
0.0037 
" 1.52 82 .largecircle. 
5 0.0030 
3.85 
5.20 
0.002 
0.009 
0.0041 
" 1.25 81 .largecircle. 
__________________________________________________________________________ 
NOTE 
*: Outside the range of the present invention 
.largecircle.: Present Invention 
X: Comparative 
[Example 6] 
A cold rolled sheet (0.30 mm thick) obtained by the method of Example 5 and 
having the composition of Steel No. 4 in Table 7 was subjected to 
continuous annealing by soaking at 880.degree. C. for 30 seconds in an 75 
vol % N.sub.2 +25 vol % H.sub.2 non-decarburzing atmosphere having a dew 
point of -20.degree. C. so as to cause primary recrystallization, followed 
by application of a parting agent and a finish annealing. The finish 
annealing process consisted of a first annealing performed by soaking in a 
50 vol % N.sub.2 +50 vol % H.sub.2 atmosphere at 885.degree. C. for 24 
hours, changing the atmosphere to a 100% H.sub.2 atmosphere, and then 
second annealing consisting of purification annealing performed by soaking 
for 24 hours at the various temperatures shown in Table 8. The magnetic 
characteristics in the rolling direction of the resulting steel sheets are 
shown in Table 8. 
As is clear form Table 8, all the steel sheets had good magnetic 
characteristics, but when the purification annealing temperature in the 
last half of finish annealing was in the range defined by the present 
invention (Steels Nos. 4-7), the steel sheets had even lower core losses. 
TABLE 8 
______________________________________ 
Purification 
Annealing Core Loss Flux Density 
Run Temperature W.sub.15/50 
B.sub.8 
No. (.degree.C.) (W/kg) (T) 
______________________________________ 
1 880 0.97 1.60 
2 900 0.91 1.60 
3 920 0.80 1.61 
4 940 0.70 1.61 
5 960 0.69 1.62 
6 980 0.69 1.62 
7 1000 0.68 1.62 
______________________________________ 
[Example 7] 
Slabs of three steel types having substantially the same composition within 
the ranges specified by the present invention but differing with respect 
to the content of sol. Al (see Table 9) were hot-rolled and finished to a 
thickness of 2.3 mm. The hot-rolled sheets were descaled by pickling and 
subjected to box annealing by soaking at 800.degree. C. for 2 hours. The 
sheets were then heated to 130.degree. C. by induction heating and cold 
rolled to a thickness of 0.35 mm. 
Each of the cold-rolled sheets was subjected to continuous annealing by 
soaking at 875.degree. C. for 30 sec in a 80 vol % N.sub.2 +20 vol % 
H.sub.2 non-decarburizing atmosphere having a dew point of -25.degree. C. 
or below to cause primary recrystallization, followed by application of a 
parting agent and finish annealing. The finish annealing process consisted 
of soaking in a 75 vol % N.sub.2 25 vol % H.sub.2 atmosphere at 
875.degree. C. for 24 hours, shifting to an H.sub.2 atmosphere, and 
purification annealing by soaking at 950.degree. C. for 24 hours. The 
magnetic characteristics in the rolling direction of the resulting steel 
sheets are shown in Table 10. 
Steel No. 1 had a smaller amount of sol. Al than specified by the present 
invention. On account of the weak inhibitor effect, secondary 
recrystallization characterized by integration in the Goss orientation 
could not be obtained and it had poor magnetic characteristics. Steel No. 
3 had a greater amount of sol. Al than specified by the present invention. 
No secondary recrystallization was found to have occurred, so Steel No. 3 
had very poor magnetic characteristics. In contrast, Steel No. 2 
corresponding to an example of the electrical steel sheet of the present 
invention exhibited excellent magnetic characteristics. 
TABLE 9 
__________________________________________________________________________ 
Composition of steel slab (wt %) Si (%) 
No. 
C Si Mn S sol. Al 
N Fe + Impurities 
0.5 .times. Mn (%) 
__________________________________________________________________________ 
1 0.0025 
3.51 
4.22 
0.003 
0.002* 
0.0037 
Bal. 1.4 
2 0.0027 
3.50 
4.20 
0.003 
0.007 
0.0035 
Bal. 1.4 
3 0.0029 
3.50 
4.21 
0.003 
0.021* 
0.0033 
Bal. 1.4 
__________________________________________________________________________ 
NOTE: 
*: Outside the range of the present invention 
TABLE 10 
______________________________________ 
Core Loss Flux Density 
Run W.sub.15/50 B.sub.8 
No. (W/kg) (T) Remarks 
______________________________________ 
1 2.35 1.50 X 
2 0.82 1.70 .largecircle. 
3 3.95 1.42 X 
______________________________________ 
NOTE 
.largecircle.: Present Invention 
X: Comparative 
[Example 8] 
Steel slabs each consisting of 0.0050% C, 3.51% Si, 4.25% Mn, 0.0006% S, 
0.006% sol. Al, 0.0035% N, and a balance of Fe and incidental impurities 
were prepared by the same method as in Example 5 and hot rolled to a 
thickness of 2.3 mm. The hot rolled sheets were descaled by pickling, cold 
rolled to a thickness of 1.4 mm, subjected to intermediate annealing by 
soaking at 850.degree. C. for 1 min, and cold rolled to a thickness of 
0.27 mm. 
Subsequently, the cold rolled sheets were subjected to continuous annealing 
by soaking at 875.degree. C. for 30 sec in a 70 vol % N.sub.2 +30 vol % 
H.sub.2 non-decarburizing atmosphere having a dew point of -15.degree. C. 
or below to cause primary recrystallization. Thereafter, a parting agent 
was applied and finish annealing was conducted. 
The finish annealing was conducted under the three different conditions set 
forth in Table 11. The finish annealing process consisted of first 
annealing comprising soaking in a 50 vol % N.sub.2 +50 vol % H.sub.2 
atmosphere for the purpose of achieving secondary recrystallization and 
second annealing in an H.sub.2 atmosphere for the purpose of purification 
annealing. The magnetic characteristics in the rolling direction of the 
resulting steel sheets are shown in Table 12. 
Steel No. 1 was subjected to first annealing using a soaking temperature 
higher than the range specified by the present invention. The inhibitor 
effect was weak, normal grain growth progressed, and secondary 
recrystallization did not take place, so Steel No. 1 had poor magnetic 
characteristics. Steel No. 3, which was subjected to the second annealing 
at a lower soaking temperature than specified by the present invention, 
experienced secondary recrystallization, but adequate annealing did not 
take place, so the magnetic characteristics were not satisfactory. In 
contrast, Steel No. 2 corresponding to an example of the present invention 
had excellent magnetic characteristics. 
TABLE 11 
______________________________________ 
Run Soaking condition 
Soaking condition 
No. for 1st annealing 
for 2nd annealing 
______________________________________ 
1 960.degree. C. .times. 24 h 
960.degree. C. .times. 24 h 
2 890.degree. C. .times. 24 h 
960.degree. C. .times. 24 h 
3 890.degree. C. .times. 24 h 
890.degree. C. .times. 24 h 
______________________________________ 
TABLE 12 
______________________________________ 
Core Loss Flux Density 
Run W.sub.15/50 B.sub.8 
No. (W/kg) (T) Remarks 
______________________________________ 
1 2.15 1.41 X 
2 0.63 1.70 .largecircle. 
3 0.75 1.68 .largecircle. 
______________________________________ 
NOTE 
.largecircle.: Present Invention 
X: Comparative 
[Example 9] 
Hot rolled steel sheets having a thickness of 2 mm and the same composition 
as in Example 8 were subjected to annealing by soaking at 880.degree. C. 
for 1 minute, descaled by pickling, heated in an annealing furnace to the 
various temperatures shown in Table 13, and cold rolled to a thickness of 
0.30 mm. The percent of sheets in which breakage occurred during cold 
rolling is indicated in Table 13. 
As can be seen from Table 13, the incidence of breakage was extremely high 
for Steels Nos. 1 and 2 for which the temperature of the steel sheets 
during cold rolling was less than 70.degree. C. In contrast, when cold 
rolling was carried out in the temperature range specified by the present 
invention (Steels Nos. 4-5), there was virtually no breakage. 
TABLE 13 
______________________________________ 
Coil Temperature 
Run During Cold Rolling 
Breakage Ratio 
No. (.degree.C.) During Cold Rolling 
Remarks 
______________________________________ 
1 20* 15/30 X 
2 50* 9/30 X 
3 80 3/30 .largecircle. 
4 120 0/30 .largecircle. 
5 180 0/30 .largecircle. 
______________________________________ 
NOTE: 
*: Outside the range of the present invention 
Breakage Ratio = Number of Broken Sheets/Total Number of Cold Rolled 
Sheets 
.largecircle.: Present Invention 
X: Comparative 
As demonstrated by the above examples, the oriented magnetic steel sheet of 
the present invention has a very small core loss and can advantageously be 
used to make cores in transformers, generators and motors, and magnetic 
shields. 
Furthermore, such a steel sheet can be easily produced by the process of 
the present invention. Since this process includes neither a 
decarburization annealing step which takes a prolonged time nor a finish 
annealing step which is conducted at an extra-high temperature of 
1150.degree.-1200.degree. C., it is also advantageous from the viewpoint 
of lower manufacturing costs.