U.S. patent application number 15/516935 was filed with the patent office on 2017-10-19 for low iron loss grain oriented electrical steel sheet and method for manufacturing the same.
The applicant listed for this patent is JFE Steel Corporation. Invention is credited to Ryuichi Suehiro, Toshito Takamiya, Takashi Terashima, Masanori Uesaka, Makoto Watanabe.
Application Number | 20170298467 15/516935 |
Document ID | / |
Family ID | 55653112 |
Filed Date | 2017-10-19 |
United States Patent
Application |
20170298467 |
Kind Code |
A1 |
Suehiro; Ryuichi ; et
al. |
October 19, 2017 |
LOW IRON LOSS GRAIN ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR
MANUFACTURING THE SAME
Abstract
A grain oriented electrical steel sheet is subjected to a
temperature holding treatment at a temperature T of 250-600.degree.
C. for 1-10 seconds in the primary recrystallization annealing and
heated from temperature T to 700.degree. C. at not less than
80.degree. C./s and from 700.degree. C. to a soaking temperature at
not more than 15.degree. C./s, wherein an oxygen potential from
700.degree. C. to the soaking temperature is 0.2-0.4 and an oxygen
potential during the soaking is 0.3-0.5 and an area ratio of
secondary recrystallized grains is not less than 90% when an angle
.alpha. deviated from {110}<001> ideal orientation is less
than 6.5.degree. and an area ratio is not less than 75% when a
deviation angle is less than 2.5.degree. and an average length [L]
in the rolling direction is not more than 20 mm and an average
value [.beta.] of the angle .beta. is
15.63.times.[.beta.]+[L]<44.06.
Inventors: |
Suehiro; Ryuichi; (Tokyo,
JP) ; Terashima; Takashi; (Tokyo, JP) ;
Takamiya; Toshito; (Tokyo, JP) ; Watanabe;
Makoto; (Tokyo, JP) ; Uesaka; Masanori;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE Steel Corporation |
Chiyoda-ku |
|
JP |
|
|
Family ID: |
55653112 |
Appl. No.: |
15/516935 |
Filed: |
October 5, 2015 |
PCT Filed: |
October 5, 2015 |
PCT NO: |
PCT/JP2015/078173 |
371 Date: |
April 5, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/00 20130101;
C21D 8/1294 20130101; C21D 8/1272 20130101; C22C 38/54 20130101;
C22C 38/002 20130101; C21D 8/005 20130101; C21D 2201/05 20130101;
C21D 9/46 20130101; H01F 1/14775 20130101; C22C 38/02 20130101;
C22C 38/60 20130101; C21D 8/1288 20130101; C22C 38/16 20130101;
C22C 38/008 20130101; C21D 8/1261 20130101; C22C 38/001 20130101;
C22C 38/06 20130101; C22C 38/12 20130101; C22C 38/04 20130101 |
International
Class: |
C21D 9/46 20060101
C21D009/46; C22C 38/54 20060101 C22C038/54; C22C 38/16 20060101
C22C038/16; C22C 38/12 20060101 C22C038/12; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C21D 8/00 20060101
C21D008/00; C22C 38/00 20060101 C22C038/00; C22C 38/00 20060101
C22C038/00; C22C 38/00 20060101 C22C038/00; C21D 8/12 20060101
C21D008/12; C21D 8/12 20060101 C21D008/12; C21D 8/12 20060101
C21D008/12; C22C 38/02 20060101 C22C038/02; C22C 38/60 20060101
C22C038/60 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 6, 2014 |
JP |
2014-205426 |
Claims
1-5. (canceled)
6. A grain oriented electrical steel sheet having a chemical
composition comprising Si: 2.5-5.0 mass % and Mn: 0.01-0.8 mass %
and the remainder being Fe and inevitable impurities, wherein
continuous or discontinuous linear grooves or linear strain regions
are formed on one surface or both surfaces of the steel sheet in a
direction crossing a rolling direction at an interval d in the
rolling direction of 1-10 mm and a forsterite film and a tension
coating are formed on the both surfaces of the steel sheet, wherein
an area ratio S.sub..alpha.6.5 of secondary recrystallized grains
occupied in the surface of the steel sheet is not less than 90%
when an absolute value of an angle .alpha. deviated from {110
}<001> ideal orientation around a direction perpendicular to
a rolling face is less than 6.5.degree. and an area ratio
S.sub..beta.2.5 of secondary recrystallized grains occupied in the
surface of the steel sheet is not less than 75% when an absolute
value of an angle .beta. deviated from {110}<001> ideal
orientation around a widthwise direction is less than 2.5.degree.,
and an average length [L] (mm) of the secondary recrystallized
grains in the rolling direction and an average value [.beta.] of
the angle .beta. (.degree.) satisfy equations (1) and (2):
15.63.times.[.beta.]+[L]<44.06 (1) [L].ltoreq.20 (2).
7. The grain oriented electrical steel sheet according to claim 6,
containing one or more selected from Cr: 0.01-0.50 mass %, Cu:
0.01-0.50 mass %, P: 0.005-0.50 mass %, Ni: 0.010-1.50 mass %, Sb:
0.005-0.50 mass %, Sn: 0.005-0.50 mass %, Bi: 0.005-0.50 mass %,
Mo: 0.005-0.10 mass %, B: 0.0002-0.0025 mass %, Te: 0.0005-0.010
mass %, Nb: 0.0010-0.010 mass %, V: 0.001-0.010 mass % and Ta:
0.001-0.010 mass % in addition to the above chemical
composition.
8. A method of manufacturing a grain oriented electrical steel
sheet as claimed in claim 6, comprising: hot rolling a steel slab
having a chemical composition comprising C: 0.002-0.10 mass %, Si:
2.5-5.0 mass %, Mn: 0.01-0.8 mass %, Al: 0.010-0.050 mass % and N:
0.003-0.020 mass % and the remainder being Fe and inevitable
impurities to form a hot rolled sheet, subjecting the hot rolled
sheet to one cold rolling or two or more cold rollings interposing
an intermediate annealing therebetween after hot band annealing or
without hot band annealing to form a cold rolled sheet having a
final thickness, subjecting the cold rolled sheet to a primary
recrystallization annealing, applying an annealing separator to the
surface of the steel sheet, subjecting the sheet to a finish
annealing and forming a tension coating, wherein the sheet is
subjected to a temperature holding treatment at any temperature T
of 250-600.degree. C. for 1-10 seconds in a heating process of the
primary recrystallization annealing and then heated from the
temperature T to 700.degree. C. at a heating rate of not less than
80.degree. C./s, and a ratio (I.sub.max/I.sub.min) of a maximum
value I.sub.max in an emission intensity profile in a depth
direction of Si to a minimum value I.sub.min found in a position
deeper than the maximum value I.sub.max when the steel sheet
surface after the primary recrystallization annealing is observed
by a glow discharge optical emission spectrometry is not less than
1.5, and continuous or discontinuous linear grooves or linear
strain regions are formed on one surface or both surfaces of the
steel sheet in a direction crossing the rolling direction at an
interval d in the rolling direction of 1-10 mm in any process after
the cold rolling.
9. The method according to claim 8, wherein the steel slab contains
one or two selected from Se: 0.003-0.030 mass % and 5: 0.002-0.030
mass % in addition to the above chemical composition.
10. The method according to claim 8, wherein the steel slab
contains one or more selected from Cr: 0.01-0.50 mass %, Cu:
0.01-0.50 mass %, P: 0.005-0.50 mass %, Ni: 0.010-1.50 mass %, Sb:
0.005-0.50 mass %, Sn: 0.005-0.50 mass %, Bi: 0.005-0.50 mass %,
Mo: 0.005-0.10 mass %, B: 0.0002-0.0025 mass %, Te: 0.0005-0.010
mass %, Nb: 0.0010-0.010 mass %, V: 0.001-0.010 mass % and Ta:
0.001-0.010 mass % in addition to the above chemical
composition.
11. The method according to claim 9, wherein the steel slab
contains one or more selected from Cr: 0.01-0.50 mass %, Cu:
0.01-0.50 mass %, P: 0.005-0.50 mass %, Ni: 0.010-1.50 mass %, Sb:
0.005-0.50 mass %, Sn: 0.005-0.50 mass %, Bi: 0.005-0.50 mass %,
Mo: 0.005-0.10 mass %, B: 0.0002-0.0025 mass %, Te: 0.0005-0.010
mass %, Nb: 0.0010-0.010 mass %, V: 0.001-0.010 mass % and Ta:
0.001-0.010 mass % in addition to the above chemical composition.
Description
TECHNICAL FIELD
[0001] This disclosure relates to a grain oriented electrical steel
sheet suitable for use as an iron core material of transformers and
the like and having excellent magnetic properties, particularly an
iron loss property and a method for manufacturing the same.
RELATED
[0002] Grain oriented electrical steel sheets are magnetic
materials mainly used as iron core materials for transformers,
power generators, rotary machines and so on and are demanded to be
low in the energy loss (iron loss) generated in the inside of the
iron core by excitation.
[0003] As one method of decreasing the iron loss of the grain
oriented electrical steel sheet, there is a technique wherein Goss
orientation of crystal grains ({110}<001>) is highly aligned
in one direction toward the rolling direction of the steel sheet to
realize a high permeability. That technique utilizes a phenomenon
called as secondary recrystallization in which crystal grains of a
specified orientation, or Goss orientation are coarsely grown while
consuming crystal grains of the other orientations. By the
secondary recrystallization is directed <001> orientation as
an axis of easy magnetization of iron toward the rolling direction,
whereby permeability in the rolling direction is significantly
improved and hysteresis loss is reduced.
[0004] However, crystal grains having an orientation deviated from
the ideal Goss orientation are also generated by the secondary
recrystallization so that an industrially produced grain oriented
steel sheet becomes a polycrystalline body having some orientation
scatterings. To this end, proper control of the orientation
scatterings is an important development subject in the grain
oriented electrical steel sheet. For example, JP-A-2001-192785
discloses that excellent magnetic properties are obtained by
sharpening an angle .alpha. deviated from {110}<001> ideal
orientation around a direction perpendicular to the rolling face
(ND, thickness direction) in the whole of secondary recrystallized
grains to not more than an appropriate value and suppressing
variation of an angle .beta. deviated from {110}<001> around
a direction perpendicular to the rolling direction (TD, widthwise
direction). In that technique, however, the secondary
recrystallized grains becomes enormous so that eddy current loss is
not sufficiently reduced and the decrease of iron loss is critical
though hysteresis loss property is excellent.
[0005] Therefore, methods of decreasing the iron loss by
controlling a factor other than the orientation scatterings of the
secondary recrystallized grains are examined, one of which is a
technique wherein secondary recrystallized grain size is refined to
make a magnetic domain width small and decrease eddy current loss.
For example, Japanese Patent No. 2983128 proposes a technique
wherein a grain size after the secondary recrystallization is
refined by heating to a temperature of not lower than 700.degree.
C. at a heating rate of not less than 100.degree. C./s in the
heating process for decarburization annealing. Also, there is a
technique wherein magnetic domains are refined to decrease the eddy
current loss by intentionally forming strain regions in a direction
crossing to the rolling direction of the steel sheet surface or
periodically forming portions removed from the surface layer of the
steel sheet (grooves) in the rolling direction. For example,
Japanese Patent No. 4510757 proposes a technique of decreasing the
iron loss by irradiating a laser to the surface of the grain
oriented electrical steel sheet after finish annealing to refine
the magnetic domains, JP-B-S62-053579 proposes a technique of
decreasing the iron loss by applying a pressure to the grain
oriented electrical steel sheet after finish annealing to form
grooves in an iron matrix portion and refine magnetic domains and
then performing strain-relief annealing, and JP-A-2013-077380
proposes a technique wherein the iron loss property is improved by
subjecting to a magnetic domain refining treatment while making the
secondary recrystallized grain size to not less than 10 mm and
highly sharpening an average value of angle to not more than
2.degree..
[0006] As mentioned above, the iron loss property of the grain
oriented electrical steel sheet has been largely improved by
applying the technique of forming grooves or strain regions in the
surface of the steel sheet to attain magnetic domain refining.
However, the margin of improvement of the iron loss property by the
above techniques is not yet sufficient in view of the recent
demands for improvement of the iron loss property so that further
improvement is required.
[0007] It could therefore be helpful to provide a grain oriented
electrical steel sheet having a better iron loss property and
propose an advantageous manufacturing method thereof.
SUMMARY
[0008] The magnetic domain refining technique of forming the groove
or strain region in the surface of the steel sheet utilizes an idea
that the width of the main magnetic domain is decreased to mitigate
a high energy state generated in the locally introduced groove part
or strain region part to thereby decrease the eddy current loss.
That is, when the groove is introduced, a magnetic pole is
generated in the groove part, while when strain region is
introduced, a magnetic domain structure called as closure domain is
generated in the strain region part, whereby the high energy state
is caused so that the phenomenon of making the width of the main
magnetic domain narrow is utilized for mitigating the high energy
state. On the other hand, the technique of refining the secondary
recrystallized grains can be considered to be refining of domains
using grain boundaries as a generation site of the magnetic
pole.
[0009] To this end, the effect by the magnetic domain refining
treatment of forming the groove or strain region has been
considered to be the same as the effect by refining the secondary
recrystallized grains so that when the magnetic domain refining
treatment is performed to form the grooves or strain regions in the
steel sheet, the secondary recrystallized grains may be coarse and
hence the refining of the secondary recrystallized grains is not
performed.
[0010] We found that even when the magnetic domain refining
treatment of forming the grooves or strain regions in the steel
sheet surface is applied to further improve the magnetic properties
of the grain oriented electrical steel sheet, it is effective to
refine the secondary recrystallized grains, and particularly better
magnetic properties (iron loss property) are stably obtained by
controlling an average value [.beta.] of the angle .beta. deviated
from {110}<001> ideal orientation of the secondary
recrystallized grains around the widthwise direction to a proper
range depending on the secondary recrystallized grain size.
[0011] We thus provide a grain oriented electrical steel sheet
having a chemical composition comprising Si: 2.5-5.0 mass % and Mn:
0.01-0.8 mass % and the remainder being Fe and inevitable
impurities, wherein continuous or discontinuous linear grooves or
linear strain regions are formed on one surface or both surfaces of
the steel sheet in a direction crossing the rolling direction at an
interval d in the rolling direction of 1-10 mm and a forsterite
film and a tension coating are formed on the both surfaces of the
steel sheet, characterized in that an area ratio S.sub..alpha.6.5
of secondary recrystallized grains occupied in the surface of the
steel sheet is not less than 90% when an absolute value of an angle
.alpha. deviated from {110}<001> ideal orientation around a
direction perpendicular to a rolling face is less than 6.5.degree.
and an area ratio S.sub..beta.2.5 of secondary recrystallized
grains occupied in the surface of the steel sheet is not less than
75% when an absolute value of an angle .beta. deviated from
{110}<001> ideal orientation around a widthwise direction is
less than 2.5.degree., and an average length [L] (mm) of the
secondary recrystallized grains in the rolling direction and an
average value [.beta.] of the angle .beta. (.degree.) satisfy
equations (1) and (2):
15.63.times.[.beta.]+[L]<44.06 (1)
[L].ltoreq.20 (2).
[0012] The grain oriented electrical steel sheet is characterized
by containing one or more selected from Cr: 0.01-0.50 mass %, Cu:
0.01-0.50 mass %, P: 0.005-0.50 mass %, Ni: 0.010-1.50 mass %, Sb:
0.005-0.50 mass %, Sn: 0.005-0.50 mass %, Bi: 0.005-0.50 mass %,
Mo: 0.005-0.10 mass %, B: 0.0002-0.0025 mass %, Te: 0.0005-0.010
mass %, Nb: 0.0010-0.010 mass %, V: 0.001-0.010 mass % and Ta:
0.001-0.010 mass % in addition to the above chemical
composition.
[0013] Also, we provide a method of manufacturing the
aforementioned grain oriented electrical steel sheet comprising a
series of steps of hot rolling a steel slab having a chemical
composition comprising C: 0.002-0.10 mass %, Si: 2.5-5.0 mass %,
Mn: 0.01-0.8 mass %, Al: 0.010-0.050 mass % and N: 0.003-0.020 mass
% and the remainder being Fe and inevitable impurities to form a
hot rolled sheet, subjecting the hot rolled sheet to one cold
rolling or two or more cold rollings interposing an intermediate
annealing therebetween after hot band annealing or without hot band
annealing to form a cold rolled sheet having a final thickness,
subjecting the cold rolled sheet to a primary recrystallization
annealing, applying an annealing separator to the surface of the
steel sheet, subjecting the sheet to a finish annealing and forming
a tension coating, characterized in that the sheet is subjected to
a temperature holding treatment at any temperature T within a range
of 250-600.degree. C. for 1-10 seconds in a heating process of the
primary recrystallization annealing and then heated from the
temperature T to 700.degree. C. at a heating rate of not less than
80.degree. C./s, and a ratio (I.sub.max/I.sub.min) of a maximum
value I.sub.max in an emission intensity profile in a depth
direction of Si to a minimum value I.sub.min found in a position
deeper than the maximum value I.sub.max when the steel sheet
surface after the primary recrystallization annealing is observed
by a glow discharge optical emission spectrometry is not less than
1.5, and continuous or discontinuous linear grooves or linear
strain regions are formed on one surface or both surfaces of the
steel sheet in a direction crossing the rolling direction at an
interval d in the rolling direction of 1-10 mm in any process after
the cold rolling.
[0014] The steel slab used in the method is characterized by
containing one or two selected from Se: 0.003-0.030 mass % and S:
0.002-0.030 mass % in addition to the above chemical
composition.
[0015] The steel slab used in the method is characterized by
containing one or more selected from Cr: 0.01-0.50 mass %, Cu:
0.01-0.50 mass %, P: 0.005-0.50 mass %, Ni: 0.010-1.50 mass %, Sb:
0.005-0.50 mass %, Sn: 0.005-0.50 mass %, Bi: 0.005-0.50 mass %,
Mo: 0.005-0.10 mass %, B: 0.0002-0.0025 mass %, Te: 0.0005-0.010
mass %, Nb: 0.0010-0.010 mass %, V: 0.001-0.010 mass % and Ta:
0.001-0.010 mass % in addition to the above chemical
composition.
[0016] When the magnetic domain refining treatment is performed by
forming the linear grooves or strain regions on the surface of the
grain oriented electrical steel sheet, the effect of improving the
iron loss property by the magnetic domain refining can be developed
maximally by controlling the grain size and crystal orientation of
the secondary recrystallized grains to proper ranges so that it is
possible to provide grain oriented electrical steel sheets having
an iron loss lower than that of the conventional sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a graph showing an influence of an average value
[.beta.] of an angle .beta. deviated from {110}<001> ideal
orientation of secondary recrystallized grains around a widthwise
direction and an average length [L] of secondary recrystallized
grains in a rolling direction upon an iron loss W.sub.17/50.
[0018] FIG. 2 is a graph showing a relation between an area ratio
S.sub..alpha.6.5 of secondary recrystallized grains having a
deviation angle .alpha. of less than 6.5.degree. and an iron loss
W.sub.17/50.
[0019] FIG. 3 is a graph showing a relation between an area ratio
S.sub..beta.2.5 of secondary recrystallized grains having a
deviation angle .beta. of less than 2.5.degree. and an iron loss
W.sub.17/50.
[0020] FIG. 4 is a graph showing an influence of an area ratio
S.sub..alpha.6.5 of secondary recrystallized grains having a
deviation angle .alpha. of less than 6.5.degree. and an area ratio
S.sub..beta.2.5 of secondary recrystallized grains having a
deviation angle .beta. of less than 2.5.degree. upon an iron loss
W.sub.17/50.
[0021] FIG. 5 is an explanatory diagram of a method for determining
a ratio (I.sub.max/I.sub.min) of maximum value I.sub.max to minimum
value I.sub.min in an emission intensity profile in a depth
direction of Si.
DETAILED DESCRIPTION
[0022] It is first necessary that the magnetic domain refining
treatment is performed by forming linear grooves or linear strain
regions on one surface or both surfaces of the steel sheet to
decrease an iron loss. The linear grooves or strain regions formed
on the steel sheet surface for the magnetic domain refining are
introduced in a direction intersecting at an angle near to
90.degree. with respect to the rolling direction. As the
intersecting angle becomes smaller, the effect of improving the
iron loss property by the magnetic domain refining becomes smaller
so that it is desirable to be a range of 90-60.degree.. Moreover,
the grooves may be formed in a continuous linear form or may be
formed in a discontinuous linear form repeating a specified unit
such as dashed line or dot sequence.
[0023] The interval d of the linear grooves or linear strain
regions in the rolling direction of the steel sheet during the
magnetic domain refining treatment is necessary to be a range of
1-10 mm. When the interval exceeds 10 mm, the effect by the
magnetic domain refining is not obtained sufficiently, while when
it is less than 1 mm, a ratio of groove or strain region parts
occupied in the steel sheet becomes larger and hence an apparent
magnetic flux density lowers and hysteresis loss increases.
Preferably, it is 2-8 mm.
[0024] It is necessary that grain size and crystal orientation of
secondary recrystallized grains are controlled to proper ranges
described later to decrease the iron loss.
[0025] Various grain oriented electrical sheets are manufactured by
forming continuous linear grooves of 80 .mu.m width and 25 .mu.m
depth on one surface of a grain oriented electrical steel sheet
containing Si of 3.4 mass % at an intersecting angle of 70.degree.
with respect to the rolling direction and at an interval d in the
rolling direction of 3.5 mm and forming a forsterite film and a
phosphate-based glass tension coating on both surfaces of the steel
sheet, from which cut out test specimens of 100 mm width and 300 mm
length in the rolling direction as a lengthwise direction. With
respect to these test specimens are measured an angle .alpha.
deviated from {110}<001> ideal orientation of secondary
recrystallized grains around a direction perpendicular to a rolling
face, an angle .beta. deviated from {110}<001> ideal
orientation of secondary recrystallized grains around a widthwise
direction, an average length [L] of secondary recrystallization in
the rolling direction and an iron loss W.sub.17/50.
[0026] The iron loss W.sub.17/50 is an iron loss value of the each
test specimen measured by a method described in JIS C2556.
[0027] Each of the deviation angle .alpha. and deviation angle
.beta. is an average value of each of an angle .alpha. deviated
from {110}<001> ideal orientation of secondary recrystallized
grains around a direction perpendicular to a rolling face, an angle
.beta. deviated from {110}<001> ideal orientation of
secondary recrystallized grains around a widthwise direction
measured over the whole of the test specimen at a pitch of 2 mm in
widthwise direction and lengthwise direction with a general-purpose
X-ray diffraction apparatus.
[0028] The average length [L] of secondary recrystallized grains in
the rolling direction is an average grain size determined by
removing the films from the surface of the test specimen after the
measurement of the iron loss, drawing straight lines extending in
the rolling direction at a pitch of 5 mm in widthwise direction and
dividing a length of the straight line by the number of grain
boundaries crossing the straight line.
[0029] FIG. 1 shows an influence of an average value [.beta.] of
the deviation angle .beta. and an average length [L] in the rolling
direction of the secondary recrystallized grains upon an iron loss
W.sub.17/50. As seen from this figure, in the test specimen showing
such a good property that the iron loss W.sub.17/50 is less than
0.71 W/kg, the average length [L] (mm) of the secondary
recrystallized grains in the rolling direction and the average
value [.beta.] (.degree.) of the angle are in ranges satisfying
equations (1) and (2):
15.63.times.[.beta.]+[L]<44.06 (1)
[L].ltoreq.20 (2).
[0030] However, the test specimens having an iron loss W.sub.17/50
of not less than 0.71 W/kg also exist within the above ranges.
Therefore, a relation between an area ratio S.sub..alpha.6.5 of
crystal grains at a deviation angle .alpha. of not more than
6.5.degree. and an iron loss W.sub.17/50 and a relation between an
area ratio S.sub..beta.2.5 of crystal grains at a deviation angle
.beta. of not more than 2.5.degree. and an iron loss W.sub.17/50
are investigated, and the results thereof are shown in FIGS. 2 and
3.
[0031] Here, the area ratio S.sub..alpha.6.5 and the area ratio
S.sub..beta.2.5 are a ratio (%) of measuring points at a deviation
angle .alpha. of not more than 6.5.degree. and a ratio (%) of
measuring points at a deviation angle of not more than 2.5.degree.,
respectively, when each point measured at a pitch of 2 mm over the
full face of the test specimen is assumed as a measuring point of
one crystal grain.
[0032] As seen from these figures, the iron loss W.sub.17/50 is
correlated with the area ratio S.sub..alpha.6.5 and the area ratio
S.sub..beta.2.5, and as the area ratios become high, the iron loss
is decreased. Now, a relation among the iron loss W.sub.17/50, area
ratio S.sub..alpha.6.5 and area ratio S.sub..beta.2.5 in the test
specimens shown in FIG. 1 with an average length [L] of secondary
recrystallized grains in the rolling direction and an average value
[.beta.] of an angle .beta. satisfying the ranges of equations (1)
and (2) is shown in FIG. 4. As seen from this figure, the test
specimens showing such a good property that the iron loss
W.sub.17/50 is less than 0.71 W/kg have an area ratio
S.sub..alpha.6.5 of not less than 90% and an area ratio
S.sub..beta.2.5 of not less than 75%.
[0033] As seen from the above results, so that the grain oriented
electrical steel sheet has a good iron loss property, it is
necessary that the average length [L] of the secondary
recrystallized grains in the rolling direction and the average
value [.beta.] of the angle have ranges satisfying equations (1)
and (2) and further the area ratio S.sub..alpha.6.5 is not less
than 90% and the area ratio S.sub..beta.2.5 is not less than 75%.
Preferably, the value in right-hand side of equation (1) is not
more than 40, and the value in right-hand side of equation (2) is
not more than 18, and the area ratio S.sub..alpha.6.5 is not less
than 93%, and the area ratio S.sub..beta.2.5 is not less than
80%.
[0034] The reason why the good iron loss property is obtained by
controlling the grain size and crystal orientation of the secondary
recrystallization to the above ranges is not clear sufficiently,
but is considered as follows.
[0035] In the grain oriented electrical steel sheet subjected to
the magnetic domain refining treatment, when the secondary
recrystallization is sufficiently large as compared to the repeated
interval d of the formed linear grooves or strain regions in the
rolling direction, the magnetic domain refining effect by the grain
boundary hardly appears. However, as the size of the secondary
recrystallization comes close to the interval d to some extent, the
grain boundary intersecting with the rolling direction is started
to indicate an effect similar to the case of performing additional
magnetic domain refining treatment, whereby the eddy current loss
is further decreased to decrease the iron loss. We believe that the
above effect is developed in the magnetic domain refining treatment
rendering the interval d in the rolling direction into a range of
1-10 mm when the average length [L] of the secondary recrystallized
grains in the rolling direction is not more than 20 mm or satisfies
equation (2).
[0036] Moreover, the above effect is not obtained simply by
narrowing the interval d of the magnetic domain refining treatment
in the rolling direction. This is considered due to the fact that
regions subjected to the magnetic domain refining treatment
(groove, strain region) are large in the total volume as compared
to the grain boundaries and the iron matrix is not existent in the
case of the grooves and the magnetic permeability in the rolling
direction is decreased by strain in case of the strain region and
hence the apparent magnetic flux density lowers and the hysteresis
loss increases.
[0037] As the average length [L] of the secondary recrystallized
grains in the rolling direction becomes longer, the magnetic domain
refining effect obtained by grain boundaries crossing to the
rolling direction becomes weak so that it is necessary to
supplement a deteriorated amount of the iron loss associated
therewith by sharpening the crystal orientation. That is,
hysteresis loss is decreased by reducing the angle deviated from
{110}<001> ideal orientation of secondary recrystallized
grains around a widthwise direction, and further lancet domains
(region having a magnetic moment in the widthwise direction for
decreasing magnetostatic energy generated when the angle .beta. is
deviated at some degrees) are decreased to suppress the increase of
the magnetic domain width, whereby eddy current loss can be
decreased. Therefore, it is necessary to make the average value
[.beta.] of the deviation angle .beta. small according to equation
(1) as the average length [L] of the secondary recrystallized
grains in the rolling direction becomes longer.
[0038] The reason why there is a lower limit in each of the area
ratio S.sub..alpha.6.5 of secondary recrystallized grains at the
deviation angle .alpha. of not more than 6.5.degree. and the area
ratio S.sub..beta.2.5 of secondary recrystallized grains at the
deviation angle .beta. of not more than 2.5.degree. is considered
as follows.
[0039] Even if the average value [.alpha.] of the angle .alpha. and
the average value [.beta.] of the angle .beta. are small, when
crystal grains having an orientation largely deviated from Goss
orientation are contained in the secondary recrystallized grains at
an amount larger than a constant value, the magnetic properties are
deteriorated and the iron loss in the whole of the steel sheet is
increased. To this end, even when the average length in the rolling
direction [L] and average value [.beta.] of the deviation angle in
the secondary recrystallized grains satisfy equations (1) and (2),
if the area ratio S.sub..alpha.6.5 or S.sub..beta.2.5 is low, good
magnetic properties cannot be obtained as shown in FIGS. 2-4.
[0040] Therefore, the deviation angle .alpha. and deviation angle
.beta. of the secondary recrystallized grains are necessary to be
sharpened to a certain extent or more in the rolling direction, and
critical points thereof are to be 90% in S.sub..alpha.6.5 and 75%
in S.sub..beta.2.5.
[0041] In the actual manufacture of the grain-oriented electrical
steel sheets, it is effective to increase the heating rate of the
primary recrystallization annealing or the primary
recrystallization annealing combined with decarburization annealing
for reducing the average length [L] of the secondary recrystallized
grains in the rolling direction. When rapid heating is performed in
the heating process for the primary recrystallization annealing,
the number of primary recrystallized grains having Goss orientation
is increased in the structure of the steel sheet after the primary
recrystallization annealing and, hence, the grain size of the
secondary recrystallized grains after subsequent finish annealing
can be refined.
[0042] Concretely, the rapid heating treatment has an effect of
suppressing the development of <111>//ND orientation in the
recrystallization texture to promote generation of Goss oriented
grains ({110}<001>) as a nucleus for secondary
recrystallization. In general, <111>//ND orientation is at a
state that strain energy stored is high because much strain is
introduced in the cold rolling as compared to the other
orientations. To this end, recrystallization is preferentially
caused from the rolled texture of <111>//ND orientation
having a high stored strain energy in the primary recrystallization
annealing of heating at a usual heating rate (about 20.degree.
C./s). In this recrystallization, <111>//ND oriented grains
are usually generated from the rolled texture of <111>//ND
orientation so that main orientation of the texture after the
recrystallization is, <111>//ND orientation.
[0043] However, when the rapid heating is performed, the steel
sheet reaches to a higher temperature in a short time so that the
stored strain energy is relatively low, and recrystallization is
caused from Goss orientation having a high recrystallization
starting temperature as compared to <111>//ND orientation
grains so that <111>//ND orientation after the
recrystallization is relatively decreased and the number of Goss
oriented grains ({110}<001>) increases. As the number of Goss
oriented grains becomes high, many Goss oriented grains are
generated even in the secondary recrystallization and the secondary
recrystallized grains are refined to decrease the iron loss. It is
necessary to heat a zone of 500-700.degree. C. in the heating
process at a heating rate of not less than 80.degree. C./s to
obtain such an effect. Preferably, it is not less than 120.degree.
C./s.
[0044] Also, when warm rolling is performed as the cold rolling, it
is effective for refining the secondary recrystallized grains
because the introduction of deformation band (shear band) into the
crystal grains through the rolling is promoted and Goss orientation
angle surrounded by a region having a large strain is formed in the
deformation band.
[0045] Next, a technique of finely precipitating an inhibitor in
steel to control the secondary recrystallization is effective to
render the area ratio S.sub..alpha.6.5 into not less than 90% and
the area ratio S.sub..beta.2.5 into not less than 75% in addition
that the above [L] and the average value [.beta.] of the deviation
angle .beta. satisfy equations (1) and (2) through the sharpening
of the crystal orientation in the secondary recrystallized grains.
As the inhibitor may be used one or more selected from well-known
AlN, MnS, MnSe and so on, but is not limited thereto.
[0046] Also, it is effective to increase a rolling reduction of the
final cold rolling to sharpen the secondary recrystallization
orientation. As the rolling reduction of the final cold rolling is
increased, integration degrees of {111}<112> orientation as
one of <111>//ND orientation and {12 4 1}<148>
orientation are increased in the texture after the primary
recrystallization. Since a crystal grain boundary among crystal
grains having the two orientations and Goss oriented grains is
large in the mobility as compared to the other crystal grain
boundaries, preferential growth of Goss oriented grains is promoted
in the finish annealing. As a result, the sharpness of the
secondary recrystallization orientation into Goss orientation is
improved. However, when the rolling reduction is too increased, the
secondary recrystallization of Goss orientation becomes unstable.
Therefore, a rolling reduction in the final cold rolling is 85-94%.
Preferably, it is 87-92%.
[0047] As the rolling reduction of the final cold rolling is
increased, the integration degrees to {111}<112> orientation
and {12 4 1}<148> orientation are increased in the primary
recrystallization texture, while Goss orientation is decreased so
that the secondary recrystallized grains are coarsened. However, it
is necessary to hold the grain size and crystal orientation of the
secondary recrystallized grains at a proper balancing state so that
the coarsening is not favorable. To refine the secondary
recrystallized grains, the aforementioned rapid heating in the
primary recrystallization annealing is effective, but when the
rolling reduction in the final cold rolling exceeds 85%, it is
difficult to ensure sufficient number of Goss oriented grains only
by controlling the heating rate in the temperature zone of
500-700.degree. C.
[0048] In addition to the aforementioned rapid heating in the
heating process of the primary recrystallization annealing,
therefore, it is necessary that a temperature holding treatment is
performed at any temperature T of 250-600.degree. C. in the heating
process for 1-10 seconds, while a zone from the holding temperature
T to 700.degree. C. is heated at a heating rate of not less than
80.degree. C./s.
[0049] The reason is as follows.
[0050] When the temperature holding treatment is performed by
holding a temperature zone causing the recovery on the way of the
rapid heating (250-600.degree. C.) for a given time,
<111>//ND orientation having a high strain energy
preferentially causes the recovery. To this end, a driving force of
causing recrystallization by <111>//ND orientation produced
from the rolled texture of <111>//ND orientation is lowered
selectively, and hence recrystallization is caused by the other
orientations. As a result, the number of Goss oriented grains is
relatively increased after primary recrystallization. When the
holding temperature is lower than 250.degree. C. or the holding
time is less than 1 second, the recovery amount is lacking and the
above effect is not obtained. On the other hand, when the holding
temperature exceeds 600.degree. C. or the holding time exceeds 10
seconds, the recovery is caused in a wider range and the
recrystallization is not caused, and the recovered texture retains
as it is. As a result, a texture different from the above primary
recrystallization texture is formed, which badly affects the
secondary recrystallization and decreases the iron loss property.
Therefore, it is necessary to perform the temperature holding
treatment at any temperature of 250-600.degree. C. in the heating
process of the primary recrystallization annealing for a time of
1-10 seconds.
[0051] It is necessary to heat the zone of 500-700.degree. C. in
the heating process at a heating rate of not less than 80.degree.
C./s for increasing the number of Goss oriented grains as
previously mentioned. However, the holding temperature T (any
temperature of 250-600.degree. C.) is lower than 700.degree. C.
Therefore, the heating rate is necessary to be 80.degree. C./s even
in a zone from the holding temperature T to 700.degree. C.
Preferably, it is not less than 120.degree. C./s.
[0052] To obtain the grain-oriented electrical steel sheet
establishing the refining of the secondary recrystallized grains
and the adjustment of the deviation angles .alpha. and .beta., only
the aforementioned method is insufficient, and further it is
necessary to take means for increasing the integration degree of
secondary recrystallization orientation. Concretely, it is
necessary that an average heating rate from 700.degree. C. attained
in the heating process of the primary recrystallization annealing
to soaking is not more than 15.degree. C./s, and an oxygen
potential P.sub.H2O/P.sub.H2 of an atmosphere in a zone from
700.degree. C. to soaking is 0.2-0.4, and an oxygen potential
P.sub.H2O/P.sub.H2 in a soaking zone is 0.3-0.5.
[0053] The reason is as follows.
[0054] In a higher temperature zone of the primary
recrystallization annealing, particularly a temperature zone of not
lower than 700.degree. C., an internal oxide layer mainly composed
of SiO.sub.2 is usually formed on the surface layer of the steel
sheet by keeping the atmosphere at an oxidizing nature. The
internal oxide layer is a ground for reacting with an annealing
separator mainly composed of MgO in the subsequent finish annealing
to form a forsterite film, while it has an effect of preventing
such a nitriding that nitrogen in the atmosphere penetrates into
the steel sheet on the way of the finish annealing and suppresses
decomposition of AlN as an inhibitor. When the decomposition of AlN
is blocked by nitriding, the secondary recrystallization selecting
only Goss orientation is blocked and, hence, grains having an
orientation deviated from Goss orientation are subjected to
secondary recrystallization.
[0055] The effect of suppressing the nitriding is largely affected
by the structure of the internal oxide layer. That is, the
structure of the internal oxide layer effective to suppress
penetration of nitrogen is such a structure that SiO.sub.2 is
laminar or finely spherical and is concentrated in a position of a
specified depth of the internal oxide layer (Si enriched). When the
internal oxide layer has such a structure, it effectively blocks
the diffusion of nitrogen penetrated from the surface layer of the
steel sheet during the finish annealing into the inside of the
steel sheet and suppresses the nitriding.
[0056] The internal oxide layer having the above structure can be
judged from an enriching level of Si in the oxide layer.
Concretely, it is considered that the surface of the steel sheet
after the primary recrystallization annealing is analyzed by a
glow-discharge optical emission spectrometry device GDS to obtain a
concentration distribution of Si in the depth direction (emission
intensity profile), and as a value of an intensity ratio
(I.sub.max/I.sub.min) of a maximum emission intensity I.sub.max of
Si in the above emission intensity profile of Si to a minimum
emission intensity I.sub.min of Si presented in a position deeper
than the maximum intensity I.sub.max becomes larger, enrichment of
Si in the oxide layer is promoted to provide a structure suitable
for suppressing the penetration of nitrogen. The value
(I.sub.max/I.sub.min) of the internal oxide layer effective to
suppress nitriding is not less than 1.5. Moreover, the preferable
value (I.sub.max/I.sub.min) is not less than 1.55.
[0057] Here, the measure of I.sub.max/I.sub.min is described
below.
[0058] The surface of the steel sheet sample after primary
recrystallization annealing is analyzed with the high-frequency
glow-discharge optical emission spectrometry device to measure
emission intensities of Si from outermost surface at one-side of
the sample to a sufficiently deep region in a direction toward a
center of the sheet thickness, and the maximum emission intensity
I.sub.max of Si and minimum emission intensity I.sub.min of Si
presented in a position deeper than the maximum emission intensity
I.sub.max are determined from the thus obtained Si profile to
calculate I.sub.max/I.sub.min. The measurement up to the
sufficiently deeper position means that as shown in FIG. 5, when an
emission intensity distribution of Fe in a depth direction from the
surface of the steel sheet is measured together with Si and an
emission intensity of Fe at a measuring time t in a region deeper
than Fe absent layer existing in the surface layer portion in which
the emission intensity of Fe is increased and converged to a
certain value is I.sub.Fe (t) and a minimum time of an emission
intensity I.sub.Fe (2t) of Fe at a measuring time 2t within a range
of .+-.3% to the above emission intensity I.sub.Fe (t) is t.sub.0,
the measurement is continued at a time of 2 times or more of
t.sub.0.
[0059] To form the internal oxide layer having an enriched Si, an
atmosphere at a temperature zone of not lower than 700.degree. C.
starting formation of the internal oxide layer is made to a
relatively low oxidizing nature and slow heating is performed.
Concretely, it is desirable that an oxygen potential
P.sub.H2O/P.sub.H2 of the atmosphere from 700.degree. C. to the
soaking temperature is within a range of 0.2-0.4 and a heating rate
in the above temperature range is not more than 15.degree. C./s.
When the oxygen potential P.sub.H2O/P.sub.H2 of the atmosphere is
too high exceeding 0.4 or when the heating rate exceeds 15.degree.
C./s and the higher temperature is attained in a short time,
formation of the internal oxide layer is rapidly promoted and,
hence, the structure of SiO.sub.2 is changed from the laminar or
finely spherical form to a coarse spherical or dendrite form to
decrease the enrichment of Si. In contrast, when the oxygen
potential P.sub.H2O/P.sub.H2 of the atmosphere is less than 0.2,
the internal oxide layer is not formed sufficiently up to the
arrival in the soaking, and the formation of the internal oxide
layer is rapidly promoted during the soaking so that the structure
becomes still coarse spherical or dendrite. Preferably, the oxygen
potential P.sub.H2O/P.sub.H2 of the atmosphere in the above
temperature zone is a range of 0.25-0.35, and the heating rate of
the zone is not more than 10.degree. C./s.
[0060] Further, the oxidizing nature of the atmosphere during the
soaking is important and, hence, the oxygen potential
P.sub.H2O/P.sub.H2 of the atmosphere during the soaking is
necessary to be 0.3-0.5. When the oxygen potential
P.sub.H2O/P.sub.H2 is less than 0.3, the formation of the internal
oxide layer is not promoted and the enrichment of Si is not caused.
On the other hand, when it exceeds 0.5, the formation of the
internal oxide layer is rapidly promoted. In any case, formation of
the internal oxide layer associated with the proper enrichment of
Si cannot be performed. The preferable oxygen potential
P.sub.H2O/P.sub.H2 during the soaking is 0.35-0.45.
[0061] Next, the grain-oriented electrical steel sheet is necessary
to be provided on the surface of the steel sheet with a forsterite
film and a tension coating (insulation coating) for decreasing the
iron loss.
[0062] The forsterite film can be formed by applying an annealing
separator composed mainly of MgO to the surface of the steel sheet
after decarburization annealing and drying it and then subjecting
to a finish annealing. The forsterite film has an insulating
property and an action of applying tensile stress to the surface of
the steel sheet in the rolling direction to narrow the magnetic
domain width and decrease the eddy current loss.
[0063] Also, the tension coating (insulation coating) can be
obtained by applying a coating solution containing, for example,
phosphate-chromate-colloidal silica to the surface of the steel
sheet after the finish annealing and baking at a temperature of
about 800.degree. C., which has an action of increasing the
insulating property of the steel sheet surface and applying tensile
stress to the steel sheet surface in the rolling direction to
narrow the magnetic domain width and decrease the eddy current loss
like the forsterite film.
[0064] The tension applied to the steel sheet surface by these
coatings is preferable to be 4.8-36 MPa per one side surface of the
steel sheet from a viewpoint of effectively decreasing the eddy
current loss. The magnification of the tension applied can be
measured from a warping amount of the steel sheet when the coating
on the one side surface of the steel sheet is removed by pickling
or the like after the formation of the tension coating.
[0065] Moreover, the forsterite film is formed from a subscale
formed on the steel sheet surface during decarburization annealing
and composed mainly of silica as a raw material in the finish
annealing so that it is necessary to form a proper amount of the
subscale to ensure the insulating property and the adhesiveness of
the forsterite film to the steel sheet. When a coating weight
converted to oxygen is 0.30 g/m.sup.2, the subscale is too small
and the amount of the forsterite formed is insufficient and the
insulating property and adhesiveness of the coating are lowered. On
the other hand, when it exceeds 0.75 g/m.sup.2, the amount of
forsterite formed becomes too large to bring about the decrease of
a space factor in the lamination of steel sheets. Therefore, it is
preferable to restrict the coating weight converted to oxygen after
the decarburization annealing to a 0.30-0.75 g/m.sup.2. More
preferably, it is 0.40-0.60 g/m.sup.2.
[0066] There will be described the method of manufacturing the
grain-oriented electrical steel sheet below.
[0067] The grain-oriented electrical steel sheet is manufactured by
hot rolling a raw steel material (slab) adjusted to a predetermined
chemical composition described below to form a hot rolled sheet,
subjecting to one cold rolling or two or more cold rollings
interposing an intermediate annealing therebetween after a hot band
annealing or without hot band annealing to form a cold rolled sheet
with a final thickness, subjecting to a primary recrystallization
annealing or to a primary recrystallization annealing combined with
decarburization annealing, applying an annealing separator to the
steel sheet surface, subjecting to a finish annealing and forming
an insulation coating, while performing a magnetic domain refining
treatment at any step after the cold rolling.
[0068] The raw steel material (slab) used in the manufacture of the
grain-oriented electrical steel sheet is necessary to contain Si of
not less than 2.5 mass % for increasing a specific resistance of a
product sheet (steel sheet after the finish annealing) to decrease
the eddy current loss. When it is less than 2.5 mass %, the eddy
current loss cannot be decreased and good iron loss property is not
obtained. On the other hand, when it is contained exceeding 5 mass
%, it is difficult to perform cold rolling and a risk such as sheet
fracture or the like increases. Therefore, Si content is 2.5-5 mass
%. Preferably, it is 2.8-4.3 mass %.
[0069] Also, the slab is necessary to contain C and Mn within
ranges of C: 0.002-0.10 mass % and Mn: 0.01-0.8 mass %,
respectively, in addition to Si.
[0070] C has an effect of strengthening grain boundaries to
suppress slab breakage and is necessary to be contained in an
amount of not less than 0.002 mass %. On the other hand, C is
necessary to be decreased to not more than 0.0050 mass % at a stage
of a product sheet not to cause magnetic aging. If C content in the
raw steel material exceeds 0.1 mass %, there is a fear that the
material cannot be decarburized sufficiently even in the
decarburization annealing. Preferably, the C content of the raw
steel material is 0.01-0.09 mass %.
[0071] Also, Mn is necessary to be contained in an amount of not
less than 0.01 mass % to prevent hot embrittlement and ensure good
hot workability. However, when it exceeds 0.8 mass %, the above
effect is saturated and the magnetic flux density is decreased.
Preferably, Mn content is 0.02-0.5 mass %.
[0072] Further, the slab used as a raw material for the
grain-oriented electrical steel sheet is necessary to contain Al
and N as an ingredient forming an inhibitor of Al: 0.010-0.050 mass
% and N: 0.003-0.020 mass %, respectively, to cause secondary
recrystallization to increase integration degree into Goss
orientation. When Al is less than 0.050 mass % or when N is less
than 0.003 mass %, formation of AlN is insufficient and the
integration degree of Goss orientation is lowered. On the other
hand, when Al exceeds 0.050 mass % or when N exceeds 0.02 mass %,
the amount of AlN formed becomes excessive and the secondary
recrystallization of Goss orientation is blocked. Therefore, the Al
and N contents are necessary to be the above ranges. The ranges are
preferably Al: 0.015-0.035 mass % and N: 0.005-0.015 mass %.
Moreover, when AlN is used as an inhibitor, N may be contained in
an amount required for the secondary recrystallization in the
melting of steel, or may be contained in an amount required for the
secondary recrystallization by subjecting to nitriding at any step
from the cold rolling to the finish annealing for the secondary
recrystallization.
[0073] As an inhibitor other than AlN can be mentioned MnSe and
MnS. In the case of using such an inhibitor, S and Se are
preferable to be contained within ranges of Se: 0.003-0.030 mass %
and S: 0.002-0.03 mass %, respectively. More preferably, they are
within ranges of Se: 0.005-0.025 mass % and S: 0.002-0.01 mass %.
Moreover, the inhibitors of MnSe and MnS are preferable to be used
together with AlN. Also, MnSe and MnS may be used alone or may be
used together.
[0074] Moreover, the slab may contain one or more selected from Cr,
Cu and P within ranges of Cr: 0.01-0.50 mass %, Cu: 0.01-0.50 mass
% and P: 0.005-0.50 mass % for the purpose of further decreasing
the iron loss. Further, it may contain one or more selected from
Ni, Sb, Sn, Bi, Mo, B, Te, Nb, V and Ta within ranges of Ni:
0.010-1.50 mass %, Sb: 0.005-0.50 mass %, Sn: 0.005-0.50 mass %,
Bi: 0.005-0.50 mass %, Mo: 0.005-0.10 mass %, B: 0.0002-0.0025 mass
%, Te: 0.0005-0.010 mass %, Nb: 0.0010-0.010 mass %, V: 0.001-0.010
mass % and Ta: 0.001-0.010 mass % for the purpose of increasing the
magnetic flux density.
[0075] The slab is preferable to be produced by melting a steel
having the above chemical composition through a usual refining
process and further performing a usual ingot making-blooming method
or continuous casting method. Thereafter, the slab is hot rolled by
reheating to a temperature of about 1400.degree. C. according to
the usual manner. However, when AlN is used as an inhibitor and
nitriding is performed on the way of the production process, the
reheating temperature may be made lower than the above value.
[0076] Then, the hot rolled sheet obtained by hot rolling is
subjected to a hot band annealing, if necessary. The temperature of
this hot band annealing is preferable to be 800-1150.degree. C. for
providing good magnetic properties. When it is lower than
800.degree. C., a band structure formed by hot rolling is retained
and it becomes difficult to provide primary recrystallization
structure of neat grains and hence growth of secondary
recrystallized grains is blocked. On the other hand, when it
exceeds 1150.degree. C., the grain size after the hot band
annealing is too coarsened and it is difficult to provide primary
recrystallization structure of neat grains.
[0077] The steel sheet after the hot rolling or after the hot band
annealing followed to the hot rolling is subjected to one cold
rolling or two or more cold rollings interposing an intermediate
annealing therebetween to form a cold rolled sheet with a final
sheet thickness. An annealing temperature of the intermediate
annealing is preferably 900-1200.degree. C. When it is lower than
900.degree. C., the recrystallized grains after the intermediate
annealing become finer and further Goss nuclei in the primary
recrystallization structure are decreased to deteriorate the
magnetic properties of a product sheet. On the other hand, when it
exceeds 1200.degree. C., the crystal grains become too coarse and
it is difficult to provide primary recrystallization structure of
neat grains like the hot band annealing.
[0078] In the cold rolling to the final sheet thickness (final cold
rolling), a rolling reduction is necessary to be 85-94% for
controlling grain size and crystal orientation of secondary
recrystallized grains to proper ranges as previously mentioned.
Preferably, it is 87-92%.
[0079] The cold rolled sheet with the final sheet thickness is then
subjected to a primary recrystallization annealing combined with
decarburization annealing.
[0080] An annealing temperature of the primary recrystallization
annealing is preferably 800-900.degree. C. from a viewpoint of
rapidly promoting decarburization reaction in the case of combining
with decarburization annealing. Even in a case of C: not more than
0.005 mass % not requiring decarburization, therefore, it is
necessary to perform an annealing in the above atmosphere for
ensuring a subscale layer required for the formation of forsterite.
In this regard, C in the steel sheet after the decarburization
annealing is necessary to be not more than 0.0050 mass % from a
viewpoint of the prevention of magnetic aging. Preferably, it is
not more than 0.0030 mass %. Moreover, the primary
recrystallization annealing and the decarburization annealing may
be performed separately.
[0081] It is further important that a temperature holding treatment
of holding any temperature T of 250-600.degree. C. for 1-10 seconds
is performed in the heating process of the primary
recrystallization annealing and thereafter the heating is performed
at a heating rate of not less than 80.degree. C./s from the holding
temperature T to 700.degree. C. as previously mentioned. Moreover,
the holding temperature in the temperature holding treatment is not
indispensable to be constant, and a temperature change of not more
than .+-.10.degree. C./s may be supposed to be constant because the
effect similar to the temperature holding is obtained.
[0082] In the primary recrystallization annealing, it is further
necessary to form an internal oxide layer effective for the control
of nitriding during the finish annealing. Concretely, it is
necessary that a ratio (I.sub.max/I.sub.min) of a maximum value
I.sub.max to a minimum value I.sub.min presented in a position
deeper than the maximum value I.sub.max in an emission intensity
profile of Si in a depth direction when the steel sheet surface
after the primary recrystallization annealing is analyzed by a
glow-discharge optical emission spectrometry (GDS) is not less than
1.5 for formation of the internal oxide layer. To this end, it is
necessary that the heating is performed from 700.degree. C. to a
soaking temperature in an atmosphere having an oxygen potential
P.sub.H2O/P.sub.H2 of 0.2-0.4 at a heating rate of not more than
15.degree. C./s and further an oxygen potential P.sub.H2O/P.sub.H2
in the soaking is 0.3-0.5.
[0083] The steel sheet subjected to the primary recrystallization
annealing is subjected to a finish annealing after an annealing
separator composed mainly of MgO is applied and dried onto the
steel sheet surface to form a forsterite film on the steel sheet
surface. In the finish annealing, it is preferable that secondary
recrystallization is generated and completed by keeping at a
temperature of about 800-1050.degree. C. for not less than 20 hours
and then a temperature is raised to about 1200.degree. C. for
subjecting to a purification treatment. By performing the
purification treatment are decreased Al, N, S and Se as an
inhibitor forming ingredient added to the raw slab to an inevitable
impurity level in an iron matrix after the removal of coatings from
a surface of a product sheet, whereby the magnetic properties are
more improved.
[0084] Thereafter, the steel sheet after the finish annealing is
subjected to a shape correction by flattening annealing after the
unreacted annealing separator adhered to the steel sheet surface is
removed by water washing, brushing, pickling or the like, which is
effective for decreasing the iron loss. It is because the finish
annealing is usually performed in a coil form so that the
properties are deteriorated due to winding curl of the coil in the
measurement of the iron loss.
[0085] Furthermore, the steel sheet is necessary to form an
insulation coating on the steel sheet surface in the flattening
annealing or before or after thereof. The insulation coating is
necessary to be a tension coating applying tension to the steel
sheet for decreasing the iron loss. For example, it is preferable
to apply an insulation coating made of the aforementioned
phosphate-chromate-colloidal silica.
[0086] The steel sheet is necessary to be subjected to a magnetic
domain refining treatment for further decreasing the iron loss.
When grooves are formed on the steel sheet surface as a method of
the magnetic domain refining treatment, it is preferable that a
width of the groove is 20-250 .mu.m and a depth of the groove is
2-15% of the sheet thickness. When the width is too narrow or the
depth is too shallow, the magnetic domain refining effect cannot be
obtained sufficiently. Moreover, the method of forming the groove
is not particularly limited, and the formation may be performed,
for example, by etching on one side face or both faces of the steel
sheet, knurling with geared rolls, laser irradiation or the like at
any step after the final cold rolling to a final sheet
thickness.
[0087] When strain regions are introduced into the steel sheet
surface as a method of the magnetic domain refining treatment, the
introduction method of the strain region is not particularly
limited, and methods such as laser irradiation, electron beam
irradiation, plasma jet spraying, ion beam spraying and so on may
be used. The strain regions introduced by these methods are
preferable to be formed after the finish annealing because recovery
is caused by annealing at a higher temperature to lose the magnetic
domain refining effect.
[0088] Moreover, whether or not the magnetic domains are refined by
the formation of the grooves or the introduction of the strain
regions can be confirmed by the formation of closure domain
extending along a line direction in linear portion of the
strain-introduced steel sheet surface. The closure domain can be
easily observed without the removal of the coatings from the steel
sheet surface by a Bitter method wherein a magnetic colloid
solution is dropped onto the steel sheet surface or with a
commercially available magnet viewer utilizing the same. As a
matter of course, there can be used an observation method with a
Kerr-effect microscope using a magneto-optical effect, a
transmission electron microscope using electrons as a probe, a
spin-polarized scanning type electron microscope or the like. If
the closure domain is not formed, the magnetic domain refining
effect cannot be obtained and hence the sufficient effect of
decreasing the iron loss cannot be obtained.
EXAMPLE 1
[0089] A steel slab having a chemical composition comprising C:
0.070 mass %, Si: 3.50 mass %, Mn: 0.12 mass %, Al: 0.025 mass %,
N: 0.012 mass % and the remainder being Fe and inevitable
impurities is produced by a continuous casting method, reheated by
induction heating to a temperature of 1415.degree. C., and hot
rolled to form a hot rolled sheet of 2.5 mm in thickness. Then, the
hot rolled sheet is subjected to a hot band annealing at
1000.degree. C. for 50 seconds, cold rolled to an intermediate
thickness of 1.9 mm, subjected to an intermediate annealing at
1100.degree. C. for 25 seconds, and finally cold rolled to form a
cold rolled sheet having a sheet thickness of 0.23 mm (final cold
rolling reduction: 87.9%).
[0090] Next, continuous linear grooves having a width of 70 .mu.m
and a depth of 28 .mu.m are formed on one side of the cold rolled
sheet at an angle of 75.degree. crossing to the rolling direction
and an interval d in the rolling direction of 3 mm by electrolytic
etching.
[0091] Next, the cold rolled sheet is subjected to a primary
recrystallization annealing combined with decarburization annealing
by soaking at 850.degree. C. for 120 seconds. In this case,
conditions of a temperature holding treatment performed at a
temperature T in the heating process and a heating rate from the
holding temperature T to 700.degree. C. are variously changed as
shown in Table 1. Further, heating from 700.degree. C. to a soaking
temperature of 850.degree. C. is performed at a heating rate of
10.degree. C./s in an atmosphere having an oxygen potential
P.sub.H2O/P.sub.H2 of 0.30, and an oxygen potential
P.sub.H2O/P.sub.H2 of an atmosphere in the soaking process (in
decarburization annealing) is 0.39.
[0092] Then, a sample is cut out from a widthwise center portion of
the steel sheet after the primary recrystallization annealing and
an emission intensity of Si in a direction from a one-side
outermost surface of the sample toward a center of the sheet
thickness is measured with a high-frequency glow-discharge emission
spectrometry device GDS (System 3860 made by Rigaku Corporation).
From the thus obtained emission intensity profile of Si in the
thickness direction is determined I.sub.max/I.sub.min by the
aforementioned method. As a result, a value of I.sub.max/I.sub.min
is within a range of 1.6-1.7 in all of the steel sheets after the
primary recrystallization annealing. Moreover, the analysis with
GDS and measurement of I.sub.max/I.sub.min even in subsequent
examples are the same as mentioned above.
[0093] Then, the steel sheet after the primary recrystallization
annealing is subjected to a finish annealing by purification
treatment at 1200.degree. C. for 10 hours after the steel sheet
surface is coated with an annealing separator composed mainly of
MgO and dried and subjected to secondary recrystallization.
Moreover, an atmosphere in the finish annealing is H.sub.2 in the
keeping of 1200.degree. C. for the purification treatment and
N.sub.2 in the temperature rising and dropping.
[0094] Finally, a tension insulation coating composed mainly of
magnesium phosphate containing colloidal silica is applied onto
both surfaces of the steel sheet after the finish annealing at a
coating weight of 5 g/m.sup.2 per one surface and baked to obtain a
product coil.
[0095] From a longitudinal center portion of the thus obtained
product coil are cut out 10 test specimens of 100 mm
width.times.300 mm length in the rolling direction as a lengthwise
direction per widthwise direction to measure an iron loss
W.sub.17/50 by a method described in JIS C2556.
[0096] Also, crystal orientations of the secondary recrystallized
grains in the test specimens after the measurement of iron loss are
measured over a whole surface at a pitch of 2 mm in the widthwise
direction and the rolling direction by an X-ray diffraction device
to determine an average value [.beta.] of a deviation angle .beta.,
an area ratio S.sub..alpha.6.5 of crystal grains having a deviation
angle .alpha. of not more than 6.5.degree. and an area ratio
S.sub..beta.2.5 of crystal grains having a deviation angle of not
more than 2.5.degree..
[0097] Further, the insulation coating and forsterite film are
removed from the surface of the test specimen after the measurement
of iron loss to expose crystal grain boundaries and straight line
extending in the rolling direction is drawn at a pitch of 5 mm to
measure the number of grain boundaries crossing the straight line,
from which is determined an average length [L] of secondary
recrystallized grains in the rolling direction.
[0098] The measured results are also shown in Table 1. As seen from
this table, the iron loss property is excellent in all of the
grain-oriented electrical steel sheets controlled by properly
adjusting the conditions of the temperature holding treatment on
the way of the heating in the primary recrystallization annealing
(temperature T, time) and the heating rate from the holding
temperature T to 700.degree. C. and satisfying the average length
in rolling direction [L] and crystal orientation ([.beta.],
S.sub..alpha.6.5, S.sub..beta.2.5) of secondary recrystallized
grains.
TABLE-US-00001 TABLE 1 Production condition Properties of product
Holding Holding Heating rate from [L] [.beta.] Left side of
S.sub..alpha.6.5 S.sub..beta.2.5 Iron loss No temperature
T(.degree. C.) time (s) T to 700.degree. C. (.degree. C./s) (mm)
(.degree.) equation (1) (%) (%) W.sub.17/50 (W/kg) Remarks 1 -- 0
50 23 2.01 54.42 89.4 69.5 0.726 Comparative Example 2 500 2 50 21
1.94 51.32 90.6 72.3 0.717 Comparative Example 3 -- 0 100 15 1.88
44.38 88.4 76.5 0.718 Comparative Example 4 200 2 100 18 1.87 47.23
88.9 76.8 0.712 Comparative Example 5 300 2 100 10 1.86 39.07 90.7
77.1 0.691 Example 6 500 2 100 13 1.85 41.92 91.3 78.7 0.694
Example 7 650 2 100 18 1.86 47.07 94.1 76.9 0.743 Comparative
Example 8 300 7 100 12 1.88 41.38 91.2 76.2 0.694 Example 9 500 7
100 14 1.86 43.07 91.4 77.5 0.695 Example 10 300 12 100 18 1.91
47.85 90.5 73.4 0.724 Comparative Example 11 500 12 100 17 1.89
46.54 91.6 75.6 0.721 Comparative Example 12 -- 0 150 15 1.86 44.07
88.1 75.8 0.731 Comparative Example 13 300 2 150 9 1.83 37.60 91.2
76.1 0.682 Example 14 500 2 150 11 1.82 39.45 91.5 79.3 0.684
Example 15 300 7 150 10 1.85 38.92 91.8 77.6 0.686 Example 16 500 7
150 11 1.85 39.92 92.1 77.2 0.691 Example 17 300 12 150 15 1.86
44.07 92.5 76.9 0.722 Comparative Example 18 500 12 150 17 1.87
46.23 92.7 77.1 0.718 Comparative Example 19 -- 0 300 11 1.93 41.17
87.8 73.3 0.712 Comparative Example 20 300 2 300 7 1.87 36.23 90.2
76.2 0.677 Example 21 500 2 300 8 1.88 37.38 90.3 75.8 0.681
Example 22 300 7 300 8 1.88 37.38 90.6 75.7 0.683 Example 23 500 7
300 9 1.88 38.38 90.8 75.9 0.687 Example 24 300 12 300 13 1.89
42.54 91.2 74.6 0.717 Comparative Example 25 500 12 300 14 1.88
43.38 92.1 74.4 0.719 Comparative Example
EXAMPLE 2
[0099] A steel slab having a chemical composition comprising C:
0.080 mass %, Si: 3.3 mass %, Mn: 0.12 mass %, Al: 0.025 mass %, N:
0.012 mass % and the remainder being Fe and inevitable impurities
is produced by a continuous casting method, reheated by induction
heating to a temperature of 1400.degree. C., and hot rolled to form
a hot rolled sheet of 2.6 mm in thickness, which is subjected to a
hot band annealing at 1000.degree. C. for 50 seconds, cold rolled
to an intermediate thickness of 1.8 mm, subjected to an
intermediate annealing at 1100.degree. C. for 30 seconds, and
finally cold rolled at a rolling reduction of 89.4% to form a cold
rolled sheet having a sheet thickness of 0.23 mm.
[0100] Then, the cold rolled sheet is subjected to a primary
recrystallization annealing combined with decarburization annealing
at 840.degree. C. for 120 seconds. In this case, a temperature
holding treatment is performed at a temperature of 400.degree. C.
for 1.5 seconds on the way of the heating process, and thereafter
the heating is performed from 400.degree. C. to 700.degree. C. at a
heating rate of 150.degree. C./s and then a heating rate from
700.degree. C. to a soaking temperature of 840.degree. C., an
oxygen potential P.sub.H2O/P.sub.H2 of an atmosphere during this
zone and an oxygen potential P.sub.H2O/P.sub.H2 of an atmosphere in
the soaking process are changed into various conditions shown in
Table 2. Also, a sample is cut out from a widthwise center portion
of the steel sheet after the primary recrystallization annealing to
measure I.sub.max/I.sub.min in the same manner as in Example 1.
[0101] Next, the steel sheet after the primary recrystallization
annealing is coated on its surface with an annealing separator
composed mainly of MgO, dried, subjected to a secondary
recrystallization and further to a finish annealing by purification
treatment at 1200.degree. C. for 10 hours. Moreover, an atmosphere
in the finish annealing is H.sub.2 in the keeping of 1200.degree.
C. for the purification treatment and N.sub.2 in the temperature
rising and dropping.
[0102] Then, a tension insulation coating composed mainly of
magnesium phosphate containing colloidal silica is applied and
baked onto both surfaces of the steel sheet after the finish
annealing at a coating weight of 5 g/m.sup.2 per one side
surface.
[0103] Finally, a magnetic domain refining treatment is performed
by continuously irradiating CO.sub.2 laser onto the one side
surface of the steel sheet at an angle of 80.degree. crossing to
the rolling direction and an interval d in the rolling direction of
6 mm under conditions of an output of 100 W, a beam focusing
diameter of 210 .mu.m and a scanning rate of 10 m/s to form linear
strain regions, whereby a product coil is obtained. Moreover, a
magnetic domain structure of the steel sheet surface is observed
with a Bitter method after the magnetic domain refining treatment,
from which the formation of closure domains is confirmed in the
laser irradiated portion.
[0104] From a longitudinal center portion of the thus obtained
product coil are cut out 10 test specimens of 100 mm
width.times.300 mm length in the rolling direction as a lengthwise
direction per widthwise direction to measure an iron loss
W.sub.17/50 by a method described in JIS C2556.
[0105] The measured results are also shown in Table 2. As seen from
this table, the iron loss property is excellent in all of the
grain-oriented electrical steel sheets wherein I.sub.max/I.sub.min,
average length in rolling direction [L] and crystal orientation
([.beta.], S.sub..alpha.6.5, S.sub..beta.2.5) of secondary
recrystallized grains satisfy our conditions.
TABLE-US-00002 TABLE 2 Production conditions Properties of product
Heating rate P.sub.H2O/P.sub.H2 Iron loss from 700 to from
700.degree. C. to P.sub.H2O/P.sub.H2 of I.sub.max/ [L] [.beta.]
Left side of S.sub..alpha.6.5 S.sub..beta.2.5 W.sub.17/50 No
850.degree. C. (.degree. C./s) 850.degree. C. soaking zone
I.sub.min (mm) (.degree.) equation (1) (%) (%) (W/kg) Remarks 1 5
0.15 0.25 1.47 21 1.87 50.23 91.2 76.2 0.722 Comparative Example 2
5 0.15 0.45 1.34 20 1.95 50.48 88.8 73.5 0.718 Comparative Example
3 5 0.25 0.25 1.45 17 1.88 46.38 90.7 75.7 0.709 Comparative
Example 4 5 0.25 0.35 1.64 12 1.76 39.51 91.4 77.6 0.678 Example 5
5 0.25 0.45 1.57 14 1.78 41.82 91.0 77.3 0.681 Example 6 5 0.25
0.55 1.48 19 1.81 47.29 88.6 76.4 0.712 Comparative Example 7 5
0.35 0.25 1.44 18 1.86 47.07 89.5 75.8 0.708 Comparative Example 8
5 0.35 0.35 1.59 13 1.83 41.60 91.1 76.3 0.683 Example 9 5 0.35
0.45 1.56 14 1.85 42.92 90.2 76.1 0.686 Example 10 5 0.35 0.55 1.43
19 1.89 48.54 89.5 75.6 0.704 Comparative Example 11 5 0.45 0.25
1.47 23 1.87 52.23 87.2 76.1 0.723 Comparative Example 12 5 0.45
0.45 1.38 22 1.88 51.38 88.5 75.9 0.717 Comparative Example 13 10
0.15 0.25 1.46 18 1.86 47.07 90.1 76.3 0.711 Comparative Example 14
10 0.25 0.25 1.48 17 1.88 46.38 91.2 75.4 0.715 Comparative Example
15 10 0.25 0.35 1.56 13 1.83 41.60 90.8 76.3 0.695 Example 16 10
0.25 0.45 1.58 14 1.81 42.29 90.6 77.1 0.692 Example 17 10 0.45
0.45 1.56 16 1.92 46.01 90.2 73.5 0.703 Comparative Example 18 20
0.15 0.25 1.45 17 1.85 45.92 91.6 75.6 0.702 Comparative Example 19
20 0.25 0.35 1.43 19 1.83 47.60 90.8 75.8 0.706 Comparative Example
20 20 0.35 0.35 1.40 21 1.84 49.76 89.4 76.1 0.713 Comparative
Example 21 20 0.45 0.45 1.37 22 1.86 51.07 88.0 75.4 0.715
Comparative Example
EXAMPLE 3
[0106] A steel slab having a chemical composition comprising C:
0.080 mass %, Si: 3.40 mass %, Mn: 0.10 mass %, Al: 0.024 mass %,
N: 0.080 mass % and the remainder being Fe and inevitable
impurities is produced by a continuous casting method, reheated by
induction heating to a temperature of 1420.degree. C., and hot
rolled to form a hot rolled sheet of 2.4 mm in thickness, which is
subjected to a hot band annealing at 1100.degree. C. for 40
seconds, cold rolled to a thickness of 1.7 mm, subjected to an
intermediate annealing at 1100.degree. C. for 25 seconds, and
finally cold rolled at a rolling reduction of 86.4% to form a cold
rolled sheet having a sheet thickness of 0.23 mm.
[0107] Then, the cold rolled sheet is subjected to a primary
recrystallization annealing combined with decarburization annealing
at 845.degree. C. for 100 seconds. In this case, a temperature
holding treatment is performed at a temperature of 500.degree. C.
for 3 seconds on the way of the heating process, and thereafter the
heating is performed from 500.degree. C. to 700.degree. C. at a
heating rate of 200.degree. C./s and then a zone from 700.degree.
C. to a soaking temperature of 845.degree. C. is heated at a
heating rate of not more than 8.degree. C./s in an atmosphere
having an oxygen potential PH2O/PH2 of 0.24 and a soaking treatment
is performed in an atmosphere having an oxygen potential PH2O/PH2
of 0.33. A sample is cut out from a widthwise center portion of the
steel sheet after the primary recrystallization annealing to
measure Imax/Imin in the same manner as in Example 1, and as a
result, the measured value is 1.68.
[0108] Next, the steel sheet after the primary recrystallization
annealing is coated on its surface with an annealing separator
composed mainly of MgO, dried, subjected to a secondary
recrystallization and further to a finish annealing by purification
treatment at 1200.degree. C. for 10 hours. Moreover, an atmosphere
in the finish annealing is H2 in the keeping of 1200.degree. C. for
the purification treatment and N2 in the temperature rising and
dropping.
[0109] Finally, a tension insulation coating composed mainly of
magnesium phosphate containing colloidal silica is applied and
baked onto both surfaces of the steel sheet after the finish
annealing at a coating weight of 5 g/m2 per one side surface.
[0110] In the manufacture of the product coil, three magnetic
domain refining treatments of groove formation, laser irradiation
and electron beam irradiation shown in Table 3 are performed on the
way of the manufacturing process. Concretely, continuously linear
grooves having a width of 75 .mu.m and a depth of 25 .mu.m are
formed on the one side surface of the steel sheet after the final
cold rolling by electrolytic etching at an angle of 80.degree.
crossing to the rolling direction by changing an interval d in the
rolling direction as shown in Table 3. In the case of laser
irradiation, CO2 laser is continuously irradiated onto the one side
surface of the product coil at an angle of 80.degree. crossing to
the rolling direction under conditions of an output of 120 W, a
beam focusing diameter of 220 .mu.m and a scanning rate of 12 m/s
by changing an interval d in the rolling direction as shown in
Table 3, whereby linear strain is introduced into the steel sheet
surface. In the case of electron beam irradiation, electron beams
are irradiated linearly and continuously onto the one side surface
of the product coil with an electron beam acceleration device at an
acceleration voltage of 70 kV under a vacuum of 0.1 Pa, a beam
current of 15 mA and an angle of 80.degree. crossing to the rolling
direction by changing an interval d in the rolling direction as
shown in Table 3, whereby linear strain is introduced into the
steel sheet surface. In the case of the laser irradiation and
electron beam irradiation, we confirmed that closure domains are
formed in the laser irradiated portion when the magnetic domain
structure of the steel sheet surface is observed by a Bitter method
after the magnetic domain refining treatment.
[0111] From a longitudinal center portion of the thus obtained
product coil are cut out 10 test specimens of 100 mm
width.times.300 mm length in the rolling direction as a lengthwise
direction per widthwise direction to measure an iron loss W17/50 by
a method described in JIS C2556.
[0112] Also, crystal orientations of secondary recrystallized
grains in the test specimens after the measurement of iron loss are
measured over a whole surface at a pitch of 2 mm in the widthwise
direction and the rolling direction by an X-ray diffraction device
to determine an average value [.beta.] of a deviation angle .beta.,
an area ratio S.alpha.6.5 of crystal grains having a deviation
angle .alpha. of not more than 6.5.degree. and an area ratio
S.beta.2.5 of crystal grains having a deviation angle .beta. of not
more than 2.5.degree..
[0113] Further, the insulation coating and forsterite film are
removed from the surface of the test specimen after the measurement
of iron loss to expose crystal grain boundaries, and straight line
extending in the rolling direction is drawn at a pitch of 5 mm to
measure the number of grain boundaries crossing the straight line,
from which is determined an average length [L] of secondary
recrystallized grains in the rolling direction.
[0114] The measured results are also shown in Table 3. As seen from
this table, the iron loss property is excellent in all of the
grain-oriented electrical steel sheets wherein the interval d of
the magnetic domain refining treatment in the rolling direction
satisfies our condition.
TABLE-US-00003 TABLE 3 Production conditions Magnetic Interval d
domain of magnetic Iron loss refining domain W.sub.17/50 No method
refining(mm) (W/kg) Remarks 1 None -- 0.810 Comparative Example 2
Groove 0.5 0.706 Comparative Example 3 formation 3.0 0.677 Example
4 6.0 0.681 Example 5 9.0 0.693 Example 6 12.0 0.709 Comparative
Example 7 Laser 0.5 0.705 Comparative Example 8 irradiation 3.0
0.681 Example 9 6.0 0.676 Example 10 9.0 0.683 Example 11 12.0
0.705 Comparative Example 12 Electric beam 0.5 0.705 Comparative
Example 13 irradiation 3.0 0.681 Example 14 6.0 0.673 Example 15
9.0 0.682 Example 16 12.0 0.706 Comparative Example
EXAMPLE 4
[0115] An Si-containing steel slab having a chemical composition
shown in Table 4 is produced by a continuous casting method, heated
by an induction heating to a temperature of 1420.degree. C. and hot
rolled to form a hot rolled sheet of 2.4 mm in thickness, which is
subjected to a hot band annealing at 1100.degree. C. for 40
seconds, cold rolled to a thickness of 1.7 mm, subjected to an
intermediate annealing at 1100.degree. C. for 25 seconds, and
finally cold rolled at a rolling reduction of 86.4% to form a cold
rolled sheet having a sheet thickness of 0.23 mm.
[0116] After continuous grooves with a width of 75 .mu.m and a
depth of 25 .mu.m are formed on the one side surface of the cold
rolled sheet at an angle of 75.degree. from the rolling direction
and an interval d in the rolling direction of 3 mm by electrolytic
etching, the sheet is subjected to a primary recrystallization
annealing combined with decarburization annealing at 850.degree. C.
for 170 seconds. In this case, a temperature holding treatment is
performed at a temperature of 300.degree. C. for 2 seconds on the
way of the heating process, and thereafter the heating is performed
to 700.degree. C. at a heating rate of 100.degree. C./s and then a
zone from 700.degree. C. to a soaking temperature of 850.degree. C.
is heated at a heating rate of 5.degree. C./s in an atmosphere
having an oxygen potential P.sub.H2O/P.sub.H2 of 0.25 and a soaking
treatment is performed in an atmosphere having an oxygen potential
P.sub.H2O/P.sub.H2 of 0.35. Moreover, a sample is cut out from a
widthwise center portion of the steel sheet after the primary
recrystallization annealing to measure I.sub.max/I.sub.min in the
same manner as in Example 1, and as a result, the measured value is
1.65.
[0117] Next, the steel sheet is coated on its surface with an
annealing separator composed mainly of MgO, dried, subjected to a
secondary recrystallization and further to a finish annealing by
purification treatment at 1200.degree. C. for 10 hours. An
atmosphere in the finish annealing is H.sub.2 in the keeping of
1200.degree. C. for the purification treatment and N.sub.2 in the
temperature rising including secondary recrystallization and in the
temperature dropping. Then, an insulation tension coating composed
mainly of magnesium phosphate containing colloidal silica is
applied and baked onto both surfaces of the steel sheet after the
finish annealing at a coating weight of 5 g/m.sup.2 per one side
surface.
[0118] From a longitudinal center portion of the thus obtained
product coil are cut out 10 test specimens of 100 mm
width.times.300 mm length in the rolling direction as a lengthwise
direction per widthwise direction to measure an iron loss
W.sub.17/50 by a method described in JIS C2556.
[0119] Also, crystal orientations of secondary recrystallized
grains in the test specimens after the measurement of iron loss are
measured over a whole surface at a pitch of 2 mm in the widthwise
direction and the rolling direction by an X-ray diffraction device
to determine an average value [.beta.] of a deviation angle .beta.,
an area ratio S.sub..alpha.6.5 of crystal grains having a deviation
angle .alpha. of not more than 6.5.degree. and an area ratio
S.sub..beta.2.5 of crystal grains having a deviation angle .beta.
of not more than 2.5.degree..
[0120] Further, the insulation coating and forsterite film are
removed from the surface of the test specimen after the measurement
of iron loss to expose crystal grain boundaries, and straight line
extending in the rolling direction is drawn at a pitch of 5 mm to
measure the number of grain boundaries crossing the straight line,
from which is determined an average length [L] of secondary
recrystallized grains in the rolling direction.
[0121] The measured results are also shown in Table 4. As seen from
this table, the iron loss property is excellent in all of the
grain-oriented electrical steel sheets wherein the chemical
composition of the steel slab, I.sub.max/I.sub.min, average length
in rolling direction [L] and crystal orientation ([.beta.],
S.sub..alpha.6.5, S.sub..beta.2.5) of secondary recrystallized
grains satisfy our conditions.
TABLE-US-00004 TABLE 4 Properties of product Left side Iron loss
Production conditions I.sub.max/ [L] [.beta.] of equation
S.sub..alpha.6.5 S.sub..beta.2.5 W.sub.17/50 No C Si Mn Al N S Se
Others I.sub.min (mm) (.degree.) (1) (%) (%) (W/kg) Remarks 1 0.15
3.2 0.07 0.032 0.0021 -- -- -- 1.68 18 2.29 53.79 89.2 71.2 0.812
Comparative Example 2 0.07 3.3 1.20 0.014 0.006 -- -- -- 1.71 12
2.03 43.73 88.3 77.4 0.765 Comparative Example 3 0.05 3.3 0.21
0.063 0.011 -- -- -- 1.75 5 3.12 53.77 62.6 58.2 0.746 Comparative
Example 4 0.06 3.4 0.15 0.0072 0.017 -- -- -- 1.70 8 2.60 48.64
68.2 54.6 0.732 Comparative Example 5 0.08 3.5 0.22 0.031 0.032 --
-- -- 1.66 25 3.60 81.27 78.5 61.2 0.901 Comparative Example 6 0.06
3.3 0.08 0.022 0.0091 -- -- -- 1.65 9 2.01 40.42 91.7 77.6 0.691
Example 7 0.07 3.2 0.12 0.019 0.014 -- -- -- 1.73 12 1.86 41.07
93.5 76.5 0.684 Example 8 0.06 3.2 0.10 0.026 0.0085 0.005 -- --
1.72 13 1.82 41.45 92.4 76.2 0.692 Example 9 0.06 3.2 0.10 0.026
0.0085 -- 0.02 -- 1.68 15 1.83 43.60 92.8 75.8 0.688 Example 10
0.06 3.2 0.10 0.026 0.0085 0.005 0.01 -- 1.69 14 1.81 42.29 93.6
76.0 0.693 Example 11 0.06 3.2 0.08 0.026 0.0085 -- -- Cr: 0.02,
1.71 10 1.79 37.98 94.1 76.5 0.681 Example Ni: 0.02, Bi: 0.008, B:
0.001 12 0.06 3.2 0.08 0.026 0.0085 -- -- Cu: 0.05, 1.68 11 1.76
38.51 94.2 77.6 0.683 Example Sb: 0.03, Mo: 0.01, Te: 0.002 13 0.05
3.2 0.08 0.026 0.0085 -- -- P: 0.03, 1.65 9 1.77 36.67 93.9 77.1
0.679 Example Sn: 0.03, Nb: 0.003, V: 0.005, Ta: 0.003
* * * * *