U.S. patent application number 10/481919 was filed with the patent office on 2004-08-05 for nonoriented electromagnetic steel sheet.
Invention is credited to Fujita, Akio, Honda, Atsuhito, Kawano, Masaki, Kohno, Masaaki.
Application Number | 20040149355 10/481919 |
Document ID | / |
Family ID | 19033747 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040149355 |
Kind Code |
A1 |
Kohno, Masaaki ; et
al. |
August 5, 2004 |
Nonoriented electromagnetic steel sheet
Abstract
The present invention provides a non-oriented electrical steel
sheet containing: 0-0.010% of C; at least one of Si and Al in a
total amount of 0.03% to 0.5%, or more than 0.5% to 2.5%; 0.5% or
less of Mn; 0.10% or more to 0.26% or less of P; 0.015% or less of
S; and 0.010% or less of N, on a mass percentage basis, wherein the
non-oriented electrical steel sheet has excellent dimensional
accuracy during a punching step. When the Si content is low, the
non-oriented electrical steel sheet has the excellent balance
between high magnetic flux density and low core loss. When the Si
content is medium or high, the non-oriented electrical steel sheet
has the excellent balance between high magnetic flux density and
high strength.
Inventors: |
Kohno, Masaaki; (Okayama,
JP) ; Kawano, Masaki; (Okayama, JP) ; Honda,
Atsuhito; (Okayama, JP) ; Fujita, Akio;
(Okayama, JP) |
Correspondence
Address: |
IP DEPARTMENT OF PIPER RUDNICK LLP
ONE LIBERTY PLACE, SUITE 4900
1650 MARKET ST
PHILADELPHIA
PA
19103
US
|
Family ID: |
19033747 |
Appl. No.: |
10/481919 |
Filed: |
December 24, 2003 |
PCT Filed: |
June 27, 2002 |
PCT NO: |
PCT/JP02/06458 |
Current U.S.
Class: |
148/111 ;
148/307; 148/651 |
Current CPC
Class: |
C22C 38/04 20130101;
H01F 1/14716 20130101; C21D 9/46 20130101; C22C 38/60 20130101;
H01F 1/16 20130101; C22C 38/008 20130101; C22C 38/002 20130101;
C22C 38/004 20130101; H01F 1/14775 20130101; C22C 38/02 20130101;
C21D 8/12 20130101; C22C 38/06 20130101; H01F 1/147 20130101 |
Class at
Publication: |
148/111 ;
148/651; 148/307 |
International
Class: |
C21D 008/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 28, 2001 |
JP |
2001-195832 |
Claims
1. A non-oriented electrical steel sheet containing: 0-0.010% of C;
at least one of Si and Al in a total amount of 0.03% to 0.5%; 0.5%
or less of Mn; 0.10% or more to 0.26% or less of P; 0.015% or less
of S; and 0.010% or less of N, on a mass percentage basis, the
remainder being fe and unavoidable impurities, and having: an
average grain size of 30 .mu.m or more to 80 .mu.m or less.
2. The non-oriented electrical steel sheet according to claim 1
further containing: at least one of Sb and Sn in a total amount of
0.40% or less on a mass percentage basis.
3. The non-oriented electrical steel sheet according to claim 1 or
2 further containing: 2.3% or less of Ni on a mass percentage
basis.
4. The non-oriented electrical steel sheet according to any one of
claims 1 to 3 further containing at least any one. of: 0.01% or
less of Ca; 0.005% or less of B; 0.1% or less of Cr; 0.1% or less
of Cu; and 0.1% or less of Mo, on a mass percentage basis.
5. The non-oriented electrical steel sheet according to any one of
claims 1 to 4 further having a thickness of 0.35 mm or less.
6. A non-oriented electrical steel sheet containing: 0-0.010% of C;
at least one of Si and Al in a total amount of more than 0.5% to
2.5%; 0.5% or less of Mn; 0.10% or more to 0.26% or less of P;
0.015% or less of S; 0.010% or less of N; and 2.3% or less of Ni
according to needs, on a mass percentage basis, the remainder being
Fe and unavoidable impurities, and satisfying: at least one of the
formula P.ltoreq.P.sub.A and the formula P.sub.F.ltoreq.0.26,
wherein P.sub.A=-0.2Si+0.12Mn-0.32Al+0.05Ni.sup.2+0.- 10Ni+0.36
(1); and P.sub.F=-0.34Si+0.20Mn-0.54Al+0.24Ni.sup.2+0.28Ni+0.7- 6
(2), where the unit of each element content is mass %.
7. The non-oriented electrical steel sheet according to claim 6
further containing: at least one of Sb and Sn in a total amount of
0.40% or less on a mass percentage basis.
8. The non-oriented electrical steel sheet according to claim 6 or
7 further containing at least any one of: 0.01% or less of Ca;
0.005% or less of B; 0.1% or less of Cr; 0.1% or less of Cu; and
0.1% or less of Mo, on a mass percentage basis.
9. A method for manufacturing a non-oriented electrical steel sheet
comprising the steps of: hot-rolling a steel slab having the
composition according to any one of claims 1 to 4 under the
conditions of a heating temperature in the single-phase austenite
region and a coiling temperature of 650.degree. C. or less; and
then descaling the hot-rolled sheet to cold-roll the descaled sheet
once or twice or more with an intermediate annealing sub-step
therebetween, and then finish-annealing the cold-rolled sheet at a
temperature of 700.degree. C. or more in the single-phase ferrite
region.
10. A method for manufacturing a non-oriented electrical steel
sheet comprising the steps of: hot-rolling a steel slab having
composition according to any one of claims 1 to 4 under the
conditions of a heating temperature in the single-phase austenite
region and a coiling temperature of 650.degree. C. or less;
annealing the hot-rolled sheet at a temperature of 900.degree. C.
or more in the single-phase ferrite region or at a temperature of
more than the Ac3 transformation point in the single-phase
austenite region if Ni is not contained or the Ni content is 1.0%
by mass or less; annealing the hot-rolled sheet at a temperature of
more than the Ac3 transformation point in the single-phase
austenite region if the Ni content is more than 1.0% to 2.3% by
mass or less; descaling the annealed sheet to cold-roll the
descaled sheet once or twice or more with an intermediate annealing
sub-step therebetween; and then finish-annealing the cold-rolled
sheet at a temperature of 700.degree. C. or more in the
single-phase ferrite region.
11. A method for manufacturing a non-oriented electrical steel
sheet comprising the steps of: hot-rolling a steel slab having
composition according to any one of claims 6 to 8 under the
conditions of a heating temperature of 1000-1200.degree. C. and a
coiling temperature of 650.degree. C. or less; annealing the
hot-rolled sheet according to needs; descaling the hot-rolled or
annealed sheet to cold-roll the descaled sheet once or twice or
more with an intermediate annealing sub-step therebetween; and then
finish-annealing the cold-rolled sheet.
Description
TECHNICAL FIELD
[0001] The present invention relates to non-oriented electrical
steel sheets used for iron core materials for electric apparatus.
In particular, the present invention relates to a non-oriented
electrical steel sheet suitable for an iron core material for
reluctance motors, IPM-type DC brushless motors, and the like, and
relates to a method for manufacturing the non-oriented electrical
steel sheet, wherein the reluctance and DC brushless motors need to
have high dimensional accuracy in punching together with high
magnetic flux density, and the DC brushless motors further need to
have high strength.
BACKGROUND ART
[0002] The non-oriented electrical steel sheet is of a soft
magnetic material mainly used for iron cores of electric apparatus
such as motors and transformers. In order to improve the efficiency
and in order to miniaturize the electric apparatus, the
non-oriented electrical steel sheet needs to have a small core loss
and a high magnetic flux density. In the field of electric motors,
magnetic characteristics of the non-oriented electrical steel sheet
for iron cores are being improved, that is, the core loss is being
lowered and the magnetic flux density is being increased.
Conventional AC induction motors, which are of an asynchronous
type, are being replaced with synchronous motors having high
efficiency, and high-performance motors are being increased,
rapidly.
[0003] Generally, a synchronous motor is classified into a DC
brushless motor type and a reluctance motor type, wherein the DC
brushless motor includes a surface permanent magnet (SPM) type and
an interior permanent magnet (IPM) type, and the reluctance motor
uses reluctance torque generated by the magnetic saliency of the
rotor and the stator. Particularly, in the reluctance motor, the
magnitude of the torque depends on the shapes of the rotor and the
stator, the gap between the rotor and the stator, and the magnetic
flux density of the materials. Accordingly, it is important for the
iron core material for the reluctance motor to have high magnetic
flux density and high dimensional accuracy in punching than other
motors.
[0004] As motors having an inverter have been increasing, the
rotational speed and the number of poles have been increasing in
order to improve the motor efficiency and the torque. Since these
factors raise the operating frequency, the non-oriented electrical
steel sheet, which is a motor material, needs to be improved in
magnetic characteristic not only at a conventional commercial
frequency (50-60 Hz) but also at a high frequency of 400 Hz or
higher.
[0005] In regard to the improvement of the magnetic flux density
and the core loss, various efforts have been made.
[0006] In order to reduce the core loss of the non-oriented
electrical steel sheet, the Si content is generally increased. For
example, top-grade non-oriented electrical steel sheets have an Si
content of about 3.5% by mass in some cases. However, when the Si
content is increased, the core loss is lowered but the magnetic
flux density is caused to decrease simultaneously.
[0007] On the other hand, since low-grade non-oriented electrical
steel sheets have a small Si content, a relatively high magnetic
flux density can be obtained. However, there is a problem in that
the core loss is large.
[0008] In order to improve the core loss of such low-Si steel, the
following technique is disclosed in Japanese Unexamined Patent
Application Publication No. 62-267421: In a non-oriented electrical
steel sheet having an Si content of 0.6% by mass or less and an Al
content of 0.15-0.60% by mass, the content of impurities such as C,
S, N, and O is lowered to reduce the quantity and the inhibition
factors of inclusions inhibiting the crystal grain growth to
promote the grain growth, thereby obtaining a small core loss.
However, in the grain growth of such low-Si steel, since a decrease
in strength arises, shear drop part and burr height of a punched
sheet become large at a punching step. Therefore, there is a
problem in that the punching properties such as the dimensional
accuracy are significantly lowered.
[0009] In order to adjust the hardness of the low-Si steel to
improve the punching properties, the following technique is
proposed: the P content is controlled in a range of about 0.08-0.1%
by mass. For example, the following technique is disclosed in
Japanese Unexamined Patent Application Publication No. 56-130425:
the punching properties are improved by adding P in an amount of
less than 0.2% by mass. Among techniques in which P is aggressively
added to low-Si steel, the following technique is disclosed in
Japanese Unexamined Patent Application Publication No. 2-66138: P
is added to Al-containing steel to improve the magnetic
characteristics with the combined effects of Al and P, wherein the
Si content in the steel is limited to 0.1% by mass or less and the
Al content is in the range of 0.1-1.0% by mass.
[0010] However, among the above techniques, the technique for
improving the punching properties by adding P is focused on
reducing the burrs by adjusting the hardness but does not make any
consideration for the dimensional accuracy in punching.
[0011] On the other hand, the interior permanent magnet-type DC
brushless motor needs to have high punching accuracy and magnetic
flux density in order to increase the torque and in order to
downsize the motor constitution. Furthermore, the electrical steel
sheet needs to have high strength in order to become possible to
rotate in higher speed of the rotor and in order to prevent the
interior permanent magnet from being detached. As described above,
high Si steel is advantageous from the point of view of the
strength but low Si content steel is preferred from the point of
view of the magnetic flux density. Thus, it is conventionally
difficult to obtain high strength together with high magnetic flux
density.
DISCLOSURE OF INVENTION
[0012] [Problems to be Solved by the Invention]
[0013] As described above, high magnetic flux density and low core
loss are characteristics that are commonly preferred for all the
applications of non-oriented electrical steel sheets, such as
various motors and transformers. Among the characteristics, for a
non-oriented electrical steel sheet used for reluctance motors, the
high magnetic flux density and high dimensional accuracy are
particularly important from the point of view of the operating
principle.
[0014] However, a non-oriented electrical steel sheet having the
following characteristics has not been found: excellent magnetic
characteristics such as high magnetic flux density and low core
loss, and superior punching properties that is, particularly high
dimensional accuracy. In addition to these characteristics, another
non-oriented electrical steel sheet further having the following
characteristic has also not been found: high strength required for
the interior permanent magnet-type DC brushless motor and the
like.
[0015] In addition to these magnetic characteristics and punching
properties, another non-oriented electrical steel sheet further
having the following characteristics has also not been found:
high-frequency characteristics adapted to the high-speed rotation
of recent motors and to multipolar motors.
[0016] The present invention has been developed in view of the
above situation, and it is an object of the present invention to
provide non-oriented electrical steel sheets suitable for iron
cores of motors and transformers and the like, and particularly to
provide the following non-oriented electrical steel sheets:
[0017] a non-oriented electrical steel sheet having superior
magnetic characteristics, that is, high magnetic flux density
together with low core loss more than ever, which are preferred for
iron core materials, used for reluctance motors and the like, that
need to have particularly high magnetic flux density and
dimensional accuracy and further having high dimensional accuracy
punching; and
[0018] another non-oriented electrical steel sheet having high
magnetic flux density and strength that is important to obtain
high-speed rotation and to prevent interior permanent magnets from
being detached and further having high dimensional accuracy in
punching.
[0019] It is another object of the present invention to provide a
method for manufacturing such non-oriented electrical steel
sheets.
[0020] Hereinafter, steel having a total Si and Al content of 0.03
to 0.5% by mass is referred to as low-Si steel, and steel having a
total Si and Al content of more than 0.5% by mass is referred to as
medium-to-high Si steel for convenience.
[0021] [Means for Solving the Problems]
[0022] As a result of the intensive research conducted in order to
obtain the above objects, the inventors have found that not only
excellent magnetic characteristics such as high magnetic flux
density and low core loss can be obtained but also dimensional
accuracy in punching is significantly improved when steel having a
small Si and Al content the same as that of low-Si steel and thus
essentially having high magnetic flux density is manufactured to
adjust the average crystal grain diameter within a predetermined
range and to add P to the resulting steel in an appropriate amount.
The inventors have also found that the addition of P in an
appropriate amount in addition to the adjustment of the total Si
and Al content within a range of more than 0.05% by. mass to about
2.5% by mass provides a greatly increased strength without reducing
the magnetic flux density, that is, unprecedented well-balanced
magnetic and strength characteristics can be obtained, in addition
to high dimensional accuracy in punching.
[0023] The present invention is based on the above findings.
[0024] The outline of the present invention is as follows.
[0025] 1. A non-oriented electrical steel sheet having excellent
magnetic properties and dimensional accuracy in punching
containing:
[0026] 0-0.010% of C;
[0027] at least one of Si and Al in a total amount of 0.03% to
0.5%;
[0028] 0.5% or less of Mn;
[0029] 0.10% or more to 0.26% or less of P;
[0030] 0.015% or less of S; and
[0031] 0.010% or less of N, on a mass percentage basis, the
remainder being Fe and unavoidable impurities, and having:
[0032] an average crystal grain diameter of 30 .mu.m or more to 80
.mu.m or less.
[0033] 2. In the above item 1, the non-oriented electrical steel
sheet having excellent magnetic properties and dimensional accuracy
in punching further containing:
[0034] at least one of Sb and Sn in a total amount of 0.40% or less
on a mass percentage basis.
[0035] 3. In the above item 1 or 2, the non-oriented electrical
steel sheet having excellent magnetic properties and dimensional
accuracy in punching further containing:
[0036] 2.3% or less of Ni on a mass percentage basis.
[0037] 4. In the above item 1, 2, or 3, the non-oriented electrical
steel sheet having excellent magnetic properties and dimensional
accuracy in punching further having a thickness of 0.35 mm or
less.
[0038] 5. A non-oriented electrical steel sheet having excellent
magnetic properties and dimensional accuracy in punching
containing:
[0039] 0-0.010% of C;
[0040] at least one of Si and Al in a total amount of more than
0.5% to 2.5%;
[0041] 0.5% or less of Mn;
[0042] 0.10% or more to 0.26% or less of P;
[0043] 0.015% or less of S;
[0044] 0.010% or less of N; and
[0045] 2.3% or less of Ni according to needs,
[0046] on a mass percentage basis, the remainder being Fe and
unavoidable impurities, and satisfying:
[0047] at least one of the formula P.ltoreq.P.sub.A and the formula
P.sub.F.ltoreq.0.26,
[0048] wherein
[0049] P.sub.A=-0.2Si+0.12Mn-0.32Al+0.05Ni.sup.2+0.10Ni+0.36 (1);
and
P.sub.F=-0.34Si+0.20Mn-0.54Al+0.24Ni.sup.2+0.28Ni+0.76 (2),
[0050] where the unit of each element content is mass %.
[0051] 6. In the above item 5, the non-oriented electrical steel
sheet having excellent magnetic properties and dimensional accuracy
in punching further containing:
[0052] at least one of Sb and/or Sn in a total amount of 0.40% or
less on a mass percentage basis.
[0053] The above steels may further contain at least one selected
from -the group consisting of 0.01% or less of Ca, 0.005% or less
of B, 0.1% or less of Cr, 0.1% or less of Cu, and 0.1% or less of
Mo as an auxiliary component.
[0054] 7. A method for manufacturing a non-oriented electrical
steel sheet having excellent magnetic properties and dimensional
accuracy in punching including the steps of:
[0055] hot-rolling a steel slab having the composition described in
any one of the above items 1-3 under the conditions of a heating
temperature in the single-phase austenite region and a coiling
temperature of 650.degree. C. or less; and then
[0056] descaling the hot-rolled sheet to cold-roll the descaled
sheet once-or twice or more with an intermediate annealing
therebetween to finish-anneal the cold-rolled sheet at a
temperature of 700.degree. C. or more in the single-phase ferrite
region.
[0057] 8. A method for manufacturing a non-oriented electrical
steel sheet having excellent magnetic properties and dimensional
accuracy in punching including the steps of:
[0058] hot-rolling a steel slab having composition described in any
one of the above items 1-3 under the conditions of a heating-
temperature in the single-phase austenite region and a coiling
temperature of 650.degree. C. or less;
[0059] annealing the hot-rolled sheet at a temperature of
900.degree. C. or more in the single-phase ferrite region or at a
temperature of more than the Ac3 transformation point in the
single-phase austenite region if Ni is not contained (0%) or the Ni
content is 1.0% by mass or less;
[0060] annealing the hot-rolled sheet at a temperature of more than
the Ac3 transformation temperature in the single-phase austenite
region if the Ni content is more than 1.0% to 2.3% by mass or
less;
[0061] descaling the annealed sheet to cold-roll the descaled sheet
once or twice or more with an intermediate annealing therebetween;
and then
[0062] finish-annealing the cold-rolled sheet at a temperature of
700.degree. C. or more in the single-phase ferrite region.
[0063] 9. A method for manufacturing a non-oriented electrical
steel sheet having excellent magnetic properties and dimensional
accuracy in punching including the steps of:
[0064] hot-rolling a steel slab having composition described in the
above item 5 or 6 under the conditions of a heating temperature of
1000 to 1200.degree. C. and a coiling temperature of 650.degree. C.
or less;
[0065] descaling the hot-rolled sheet to cold-roll the descaled
sheet once or twice or more with an intermediate annealing
therebetween; and then
[0066] finish-annealing the cold-rolled sheet.
[0067] In the method for manufacturing the electrical steel sheet
recited in the above item 9, hot rolled steel sheets may be
annealed after hot rolling.
[0068] In a method for manufacturing the electrical steel sheet
recited in any one of the above items 7,8 and 9, treatment for
providing an insulating coating may be performed after
finish-annealing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0069] FIG. 1 is a graph showing the effects of the Si content and
the P content on the relationship between the yield strength and
the punched hole diameter.
[0070] FIG. 2 is a graph showing the effects of the Si content and
the P content on the relationship between the yield strength and
the punching anisotropy.
[0071] FIG. 3 is a graph showing the effects of the Si content and
the P content on the relationship between the average grain size
and the punched hole diameter.
[0072] FIG. 4 is a graph showing the effects of the Si content and
the P content on the relationship between the average grain size
and the punching anisotropy.
[0073] FIG. 5 is a graph showing the effects of the Si content and
the P content on the relationship between the average grain size
and the core loss.
[0074] FIG. 6 is a graph showing the effects of the Si content and
the P content on the relationship between the average grain size
and the magnetic flux density.
[0075] FIG. 7 is a graph showing the effects of the Si content and
the P content on the relationship between the core loss and the
magnetic flux density.
[0076] FIG. 8 is a graph showing the effects of the Si content and
the P content on the occurrence of a delamination crack.
[0077] FIG. 9 is a graph showing the effects of the Si content and
the Ni content on the occurrence of the delamination crack.
[0078] FIG. 10 is a graph showing the effects of the Si content and
the Ni content on the relationship between the P content and the
punched hole diameter.
[0079] FIG. 11 is a graph showing the effects of the Si content and
the Ni content on the relationship between the P content and the
punching anisotropy.
[0080] FIG. 12 is a graph showing the effect of the P content on
the relationship between the tensile strength and the magnetic flux
density.
[0081] FIG. 13 is a graph showing the relationship between the
sheet thickness and the high-frequency core loss.
[0082] FIG. 14 is a graph showing the relationship between the
sheet thickness and the magnetic flux density.
BEST MODE FOR CARRYING OUT THE INVENTION
[0083] The result of the experiments conducted in order to make the
present invention will now be described. The unit % of the content
described below means the unit "mass %".
[0084] [Experiment 1]
[0085] In order to clarify the relationship between the composition
and the dimensional accuracy, during a punching process, of
non-oriented electrical steel sheets, the following steel ingots
were experimentally prepared: steel ingots having basic composition
including a C content of 0.0016-0.0028%, an Mn content of
0.20-0.22%, an Al content of 0.0007-0.0014%, an N content of
0.0012-0.0022%, and an Sb content of 0.03%, which are approximately
constant, and containing 0.02% of P and 0.03-1.49% of Si, wherein
the P content is constant and the Si content is varied; and steel
products having the above basic composition and containing
0.10-0.11% of Si and 0.02-0.29% of P, wherein the Si content is
approximately constant and.the P content is varied. These steel
products were heated to 1100.degree. C. for 60 minutes and then
hot-rolled so as to have a thickness of 2 mm. The hot-rolled sheets
were treated in a soaking process under the conditions of a
temperature of 600.degree. C. and a time of two hours, which
correspond to the coiling conditions, and then air cooled. The
resulting hot-rolled sheets were annealed at 900.degree. C. for 60
seconds, pickled, and cold-rolled so as to have a thickness of 0.5
mm. The cold-rolled sheets were finish-annealed at different
temperatures of 700-900.degree. C. to form recrystallized crystal
grains having different diameters. A semi-organic insulating
coating having an average thickness of 0.6 .mu.m was provided onto
each finish-annealed sheet and then baked, and the baked sheets
were used as samples for a punching test.
[0086] In each resulting sheet, a cross section in the thickness
direction and parallel to the rolling direction was observed to
obtain the average grain size corresponding to the diameter of a
circle, determined by the Jeffries method.
[0087] In the punching test, a circular die having a diameter of 21
mm was used and the clearance was set to 8% of the thickness. The
diameter (inner diameter) of a punched hole was measured in four
directions that make angles of 0.degree., 45.degree., 90.degree.,
and 135.degree. with respect to the rolling direction to determine
the average of the four obtained diameters. The difference between
the maximum and the minimum of the four diameters was determined to
be used as the index of punching anisotropy.
[0088] FIGS. 1 and 2 show the relationship between the result of
the above test and the yield strength (YP) obtained using tensile
specimens (JIS No. 5) obtained by cutting the above steel sheets in
the rolling direction.
[0089] As shown in FIGS. 1 and 2, as a whole, the soft samples
having low YP each have a large difference between the die diameter
and the punched hole diameter, and as YP increases, the punched
hole diameter becomes closer to the die diameter, that is, the
dimensional accuracy tends to improved. This tendency is considered
to be due to the effect that shear drop deformation arising during
the punching process is suppressed when the strength is large, as
conventionally known.
[0090] However, it is worthy to note that the samples of which the
strength is adjusted by changing the P content have superior
dimensional accuracy as compared with the conventional electrical
steel sheets of which the strength is adjusted by changing the Si
content, when both have substantially the same strength.
Furthermore, the above samples have a small difference between the
die diameter and the punched hole diameter, even in the relatively
low YP range (FIG. 1).
[0091] As shown in FIG. 2, in the steel sheets of which the
strength is adjusted by changing the Si content, as the strength
increases, the punched hole diameter becomes close to the die
diameter, however the anisotropy that is a difference between the
maximum diameter and the minimum diameter remains large. In
contrast, in the steel sheets of which the strength is adjusted by
changing the P content, the anisotropy of the shape of punched
holes is remedied.
[0092] FIG. 3 shows the relationship between the punched hole
diameter and the average crystal grain diameter of the
finish-annealed steel sheets, and FIG. 4 shows the relationship
between the anisotropy and the average crystal grain diameter
(average grain size).
[0093] As shown in FIGS. 3 and 4, in the steel sheets of which the
Si content is varied, the dimensional accuracy in punching and the
punching anisotropy are deteriorated when the grain diameter is
large. In contrast, in the steel sheet containing 0.13% or more of
P, the dimensional accuracy in punching and the punching anisotropy
are superior when the crystal grain diameter is large.
[0094] The phenomenon that the dimensional accuracy in punching and
the punching anisotropy are remedied when the P content is a
certain value or more is considered to be caused by the following
effects functioning in cooperation, even though the detail is not
clear:
[0095] (1) the effect that the strength is increased by adding P to
suppress the shear drop deformation arising during the punching
process;
[0096] (2) the effect that the punching fracture limit is lowered
by adding P, known to function as an element that makes steel
brittle, in a proper amount; and
[0097] (3) the effect that the grains having the {100} <uvw>
orientation in the texture of the finish-annealed sheets tend to
increase by adding P to remedy the anisotropy.
[0098] The result of studying the magnetic characteristics will now
be described.
[0099] The inventors have studied the relationship between the
manufacturing conditions and the magnetic characteristics using the
steel sheets essentially having high magnetic flux density by
minimizing the content of Si and Al, as much as possible, improving
the core loss but lowering the saturation magnetic flux
density.
[0100] FIG. 5 shows the relationship between the crystal grain
diameter and the core loss (W.sub.15/50: a value at a frequency of
50 Hz and a maximum magnetic flux density of 1.5 T) of the
finish-annealed sheets for the samples having a thickness of 0.5
mm, wherein the core loss is measured at a commercial frequency
band.
[0101] As shown in the figure, when the Si content is small, the
core loss is large because the electrical resistance is decreased.
However, since the core loss greatly changes depending on the
crystal grain diameter, the core loss is stable and small when the
crystal grain diameter is about 30 .mu.m or more. It is also
confirmed that the core loss is small when the grain diameter is
about 30 .mu.m or more in the case that the electrical resistance
is lowered by reducing the Al content.
[0102] However, in the case of a non-oriented electrical steel
sheet of the present invention, which is of a low grade in which
the Si and Al content is small, it has been customary that the
average crystal grain diameter of a finish-annealed sheet is
limited to 15-25 .mu.m. The reason is that the punching properties
are deteriorated due to the low strength when grain grows, as
indicated by the steel sheet sample (the symbol .circle-solid. in
the figure) having an Si content of 0.11% and a P content of 0.07%
shown in FIGS. 3 and 4.
[0103] In contrast, the steel sheets having a high P content have
superior dimensional accuracy in punching even when the grain size
is about 30 .mu.m or more.
[0104] FIG. 6 shows the relationship between the average crystal
grain diameter and the magnetic flux density of the steel sheets,
and FIG. 7 shows the relationship between the core loss and the
magnetic flux density. Herein, B.sub.50 represents the magnetic
flux density at a magnetizing force of 5000 A/m.
[0105] In the Si-containing samples, the core loss is improved but
the magnetic flux density is significantly decreased. In contrast,
in the P-containing samples, the magnetic flux density remains
large when the core loss is improved due to the growth of the
crystal grains.
[0106] Since P is an embrittling element, defects such as edge
cracks and delamination cracks mainly arise during a cold-rolling
process in some cases when the P content is large in the same
manner as for the present invention. The inventors have intensively
studied this phenomenon to obtain the following findings: when a
slab is heated to a temperature in the ferrite-austenite coexisting
region during a hot-rolling process, redistribution of P between
austenite grains and ferrite grains to cause the significant
segregation of P in the ferrite grains, thereby promoting the
embrittlement of the slab. In order to prevent such embrittlement,
the heating temperature of slabs during a hot-rolling step must be
in the single-phase austenite region (or the single-phase ferrite
region, if possible).
[0107] Since P is a ferrite-forming element, P has a function of
reducing the single-phase austenite area at around the heating
temperature of slabs. However, single-phase austenite can be
obtained when the Si content is small and the heating temperature
of slabs is 1000-1200.degree. C.
[0108] As described above, it is clear that the addition of P to
the low-Si steel sheets in an amount of about 0.1% or more is
extremely effective. Therefore, the positive addition of P to steel
sheets containing 0.5% or more of Si was investigated.
[0109] [Experiment 2]
[0110] Various types of steel products having the following
composition were prepared: a C content of 0.0013-0.0026, an Mn
content of 0.18-0.23%, an Al content of 0.0001-0.0011%, and an N
content of 0.0020-0.0029%, which are approximately constant, and an
Si content of 0.60-2.42 and a P content of 0.04-0.29%, wherein the
P and Si content are varied. These steel products were heated to
1100.degree. C. for 60 minutes and then hot-rolled so as to have a
thickness of 2 mm. The hot-rolled sheets were descaled, and then
cold-rolled so as to have a thickness of 0.50 mm. The following
defect arose depending on the steel composition: delamination
cracks which are in the hot-rolled steel sheets and are parallel to
the surfaces thereof. The result is shown in FIG. 8.
[0111] The mapping analysis of delamination crack portions was
performed using EPMA. As a result, it was observed that P is
segregated or concentrated at the delamination crack portions. The
segregation conditions of P were investigated in detail, and it
then became clear that a steel piece (slab) is reheated at a
temperature in the ferrite-austenite double phase region under a
soaking condition during a hot-rolling step, thereby causing P to
be distributed to the ferrite phase to be concentrated.
[0112] That is, it became clear that the single austenite phase
area is reduced since the content of Si and Al, which are
ferrite-forming elements, is large in a medium-to-high Si range and
therefore the ferrite-austenite double phase are readily formed at
a heating temperature conventionally applied.
[0113] The steel sheets containing more than 0.26% of P had
delamination cracks even if the steel sheets had any
composition.
[0114] Steel products having a various Si, Mn, Al, P content were
prepared in a laboratory to investigate the conditions of
suppressing-the segregation of P at a temperature range of about
1000-1200.degree. C. to the extent not to cause the rolling defect.
The above slab-reheating temperature is preferable in view of the
stable precipitation of carbides, nitrides, and sulfides contained
in the steel products.
[0115] Since the segregation due to phase distribution does not
arise under the condition that the slab-reheating temperature is in
the single-phase austenite region or in the single-phase ferrite
region, it is assumed that the formation of the delamination cracks
can be avoided if the P content is smaller than a certain value.
According to the above experiment, the P content needs to be 0.26%
or less.
[0116] The conditions for obtaining single-phase austenite in the
medium-to-high Si steel products were investigated.
[0117] As a result, in the steel products having an Si and Al total
content of more than 0.5%, it became clear that single-phase
austenite can be obtained when the P content satisfies the
following formula:
P.ltoreq.P.sub.A'
wherein
P.sub.A'=-0.2Si+0.12Mn-0.32Al+0.36 (1)'
[0118] (the unit of the content of Si, Mn, Al, and P is mass %).
Accordingly, when the above condition is satisfied and the
condition P.ltoreq.about 0.26% is also satisfied, the embrittlement
due to P can be suppressed.
[0119] The conditions for obtaining single-phase ferrite in the
medium-to-high Si steel products were investigated in the same
manner as the above it then became clear that single-phase ferrite
can be obtained when the P content satisfies the following
formula:
P.gtoreq.P.sub.F'
wherein
P.sub.F'=-0.34Si +0.20Mn-0.54Al+0.76 (2)'
[0120] (the unit of the content of Si, Mn, Al, and P is mass %).
Accordingly, when this condition is satisfied and the condition
P.ltoreq.about 0.26% is also satisfied, the embrittlement due to P
can be suppressed, too.
[0121] The conditions for achieving the following purpose were
investigated: to suppress the segregation of P when it is difficult
to heat a slab at a temperature in the single-phase austenite
region or in the single-phase ferrite region. When the
redistribution of P between the ferrite and austenite phases
arises, the P content in the ferrite phase corresponds to the above
P.sub.F'. As a result of the above investigation, it became clear
that the embrittlement due to P can be avoided when the above
P.sub.F' is about 0.26 or less.
[0122] The conditions for avoiding the embrittlement in the double
phase coexistent region and the conditions for avoiding the
embrittlement in the single-phase ferrite region can be combined
into the condition P.ltoreq.about 0.26% and the condition
P.sub.F'<about 0.26.
[0123] In summary, the conditions for avoiding the embrittlement
due to P are as follows: P.ltoreq.about 0.26%, and
P.ltoreq.P.sub.A' or P.sub.F'.ltoreq.about 0.26.
[0124] According to the above results, the following findings can
be obtained: steel sheets can be manufactured without causing
defects such as delamination cracks after a cold rolling step when
the steel sheets are heated at a temperature in the single-phase
austenite region or in the single-phase ferrite region during a hot
rolling step, and steel sheets can be manufactured when the Si
content and the Al content are relatively large, that is, the
quantity of P distributed to the ferrite phase is small, even if
the steel sheets are heated at a temperature in the
ferrite-austenite double phase region.
[0125] Furthermore, the inventors investigated such steel
composition that the single-phase austenite or single-phase ferrite
structure is formed in a slab-reheating temperature range (about
1000-1200.degree. C.) during a hot rolling step even if the P
content is about 0.1% or more.
[0126] As a result, it became clear that the addition of Ni is
effective in enhancing the austenite area at the hot-rolling
temperature of P-containing steel, wherein Ni is an element
suitable for improving the magnetic characteristics and suitable
for maintaining the strength.
[0127] [Experiment 3]
[0128] The following steel samples were prepared: steel products
having the basic composition including a C content of
0.0013-0.0026%, an Mn content of 0.18-0.23%, an Al content of
0.0007-0.0013%, an N content of 0.0014-0.0025%, and an P content of
0.16-0.18%, which are approximately constant, and containing
0.95-2.44% of Si and 0-2.20% of Ni, wherein the Si content and the
Ni content are varied. The steel samples were rolled so as to have
a thickness of 0.50 mm in the similar prosess as EXPERIMENT 2, and
the delamination cracks in the obtained cold-rolled sheets were
investigated. The results are shown in FIG. 9.
[0129] The steel sheets containing 1.1-1.5% of Si and no Ni have
cracks, but the steel sheets containing 1.1-1.5% of Si and Ni have
no cracks and thus the hot rolling is possible. On the other hand,
the steel sheets containing 1.95% of Si and no Ni and the steel
sheets containing 2.4% of Si and no Ni can be hot-rolled without
cracks. However, when such steel sheets contains Ni in a large
amount, the steel sheets have cracks in some cases. Thus, it is
clear that there is an appropriate range of the Ni content in order
to obtain the effect of Ni.
[0130] According to the above formulae, in consideration of the
effect of Ni, it became clear that the embrittlement due to P can
be avoided for the steel sheets having an Si and Al total content
of more than 0.5% in the following cases: the P content is about
0.26% or less and P.ltoreq.P.sub.A, wherein
P.sub.A=-0.2Si+0.12Mn-0.32Al+0.05Ni.sup.2+0.10Ni+0.36 (1); and
[0131] the P content is about 0.26% or less and
P.sub.F.ltoreq.about 0.26, wherein
P.sub.F=-0.34Si+0.20Mn-0.54Al+0.24Ni.sup.2+0.28Ni+0.76.ltoreq.P
(2).
[0132] In the former case, the slab-reheating temperature, which is
1000-1200.degree. C., is in the single-phase austenite region, and
in the latter case, the degree of the concentration of P is small
when the slab-reheating temperature is in the double phase region
or in the single-phase ferrite region.
[0133] In the above formulae, the unit of the content of Si, Mn,
Al, P, and Ni is mass %. The technical meanings of P.sub.F and
P.sub.A are the same as those of P.sub.F' and P.sub.A' described
above.
[0134] [Experiment 4]
[0135] The cold-rolled steel sheets, having a thickness of 0.50 mm,
prepared in Experiments 2 and 3 were finish-annealed, and a
semi-organic insulating coating having an average thickness of 0.6
.mu.m was provided onto each resulting steel sheet and then baked.
These samples were provided to the punching test according to the
procedure described in Experiment 1 to investigate the punched hole
diameters and the anisotropy thereof. The results are shown in
FIGS. 10 and 11. As shown in the figures, among the steel sheets
having an Si and Al total content of more than 0.5%, the steel
sheets satisfying the condition P.gtoreq.0.10% have superior
dimensional accuracy in punching. In the Ni-containing steel
sheets, the Ni content varies for 0.38% to 2.20%.
[0136] FIG. 12 shows the relationship between the magnetic flux
density B.sub.50 and the tensile strength TS of these samples. The
tensile strength was obtained from the same tensile test as in the
Experiment 1, and the magnetic flux density was also obtained
according to the procedure in the Experiment 1.
[0137] The steel sheets containing about 0.1% or more of P have
good balance between the magnetic flux density B.sub.50 and the
tensile strength TS, as compared with conventional electrical steel
sheets having a medium-to-high Si content (that is, Si+Al>0.5%).
Particularly, as the P content is increased, the tensile strength
is increased and the magnetic flux density is not decreased but
tended to increase. This is characteristic of the steel sheets as
compared with conventional electrical steel sheets to which alloy
elements such as Si and Al, which are not of a ferromagnetic
material, are added to increase the strength thereof, wherein such
a method causes a decrease in magnetic flux density.
[0138] These characteristics are suitable for rotor materials for
various rotary machines (motors and generators) such as DC
brushless motors and reluctance motors that need to have higher
motor torque, smaller size, and higher-speed rotation.
[0139] According to the above findings, in order to obtain superior
magnetic flux density together with dimensional accuracy in
punching, the Si, Al, P, and Ni content of steel are limited to the
following range. In addition, in case of low-Si steel, the average
grain size of finish-annealed sheets are limited to the following
range.
[0140] Total Content of One or Two of Si and Al in Low-Si Steel:
about 0.03-0.5%
[0141] Since Si and Al in steel have a deoxidizing function, Si and
Al are used as deoxidizing agents alone or in combination. In order
to exert the function, the alone Si or Al content or the Si and Al
total content must be about 0.03% or more. Si and Al have a
function of increasing the resistivity and a function of improving
the core loss. However, Si and Al decrease the saturation magnetic
flux density. Thus, the upper limit of the content thereof is
limited to 0.5%.
[0142] Total Content of One or Two of Si and Al in Medium-to-High
Si Steel: More Than 0.5% to about 2.5%
[0143] When steel needs to have high mechanical strength and low
core loss together with superior dimensional accuracy, the Si and
Al total content may exceed 0.5%. As described above, the
medium-to-high Si steel having a large P content has high
dimensional accuracy in punching and good balance between the
strength and the magnetic flux density, as compared with
conventional medium-to-high Si steel having a small P content.
However, when the Si and Al total content exceeds 2.5%, it is
difficult to cold-roll such steel by a method of the present
invention. Thus, the content is limited to the range from more than
0.5% to about 2.5%.
[0144] P Content: about 0.10% to about 0.26%
[0145] P is an especially important element in the present
invention. P has high ability to promote the formation of a solid
solution and therefore has a function of adjusting the steel
strength, as previously known. In low-Si and low-Al steel sheets,
which are relatively soft originally, since the average crystal
grain diameter must be about 30 .mu.m or more in order to obtain
low core loss in the present invention, there is a problem in that
the hardness is further decreased. P is essential to improve the
punching accuracy, that is, to suppress the increase of shear drops
and burrs caused by the insufficient strength of the steel sheets.
In addition to such an ability to increase the steel strength, P
has ability to decrease the rupture limit during a punching process
to reduce the total quantity of the punching deformation and
ability to increase the {100} <uvw> orientation in the
texture of finish-annealed sheets. Therefore, P can improve the
dimensional accuracy in punching with these effects.
[0146] Furthermore, P has the property of being able to increase
the strength of steel sheets and not to decrease the magnetic flux
density. In the medium-to-high Si steel, such effects can also be
obtained.
[0147] In order to obtain such effects, the P content must be about
0.10% or more. In contrast, P is originally an element that makes
steel brittle. Therefore, when the P content is excessively high,
edge cracks and delamination cracks are readily formed, thereby
lowering the productivity. In the present invention, high-P steel
can be manufactured by improving a manufacturing method thereof and
by adding Ni, wherein the production of such steel is
conventionally thought to be difficult. However, when the P content
exceeds 0.26%, the production of the high-P steel is difficult even
if a manufacturing method according to the present invention is
used. Thus, the P content is limited to the range from about 0.10%
to about 0.26%.
[0148] Ni Content: about 2.3% or Less (Ni can be Optionally
Contained)
[0149] Ni has not only a function of improving the texture of steel
to increase the magnetic flux density but also functions such as a
function of increasing the electrical resistance to decrease the
core loss and a function of increasing the strength of steel by
solid solution strengthening to reduce shear drops during a
punching process or so. Therefore, Ni can be positively added to
steel.
[0150] Since Ni is an element that contributes to form an austenite
phase, Ni has a function of extending the austenite region (the
.gamma.-loop in the phase diagram) at about 1000-1200.degree. C.,
wherein such a temperature range is suitable for heating a slab.
Particularly, for steel having an Si and Al total content of more
than 0.5%, Ni is effective in increasing the manufactural
stability. When this effect is used, low manufactural stability
during a hot-rolling step can be greatly improved, wherein such low
manufactural stability arises when the P content is high. That is,
in order to improve the manufactural stability of a high-P
steel-sheet, the excessive segregation of P must be suppressed
during a hot-rolling step, and therefore the slab-reheating
temperature must avoid the ferrite-austenite double phase region,
which is a key point. When the Si and Al total content exceeds
0.5%, the ferrite-austenite double phase is readily formed at the
slab-reheating temperature. However, because Ni has an effect of
extending the .gamma. region, the single austenite phase can be
obtained during a slab-reheating step even if the Si and Al total
content is in the above range.
[0151] However, when the Ni content exceeds about 2.3%, there is a
risk that the magnetic flux density is lowered because the
temperature at which transformation from the ferrite (.alpha.)
phase to the austenite (.gamma.) phase starts is decreased to cause
the austenite transformation to arise. Furthermore, in case of
low-Si steel sheet, it is difficult to obtain an average crystal
grain diameter of about 30 .mu.m or more at a finish-annealing
temperature lower than the transformation temperature, and thus,
the core loss is decreased. Thus, the Ni content should be about
2.3% or less. When Ni is added to steel, the Ni content is
preferably about 0.50% or more.
[0152] Average Crystal Grain Diameter of Finish-Annealed Sheet Made
of Low-Si Steel: from about 30 .mu.m to about 80 .mu.m
[0153] In a low-Si and low-Al non-oriented electrical steel sheet,
in order to obtain superior core loss property, a finish-annealed
sheet needs to have an average crystal grain diameter of 30 .mu.m
or more, as shown in FIG. 5. However, when the crystal grain
diameter exceeds about 80 .mu.m, further improved core loss cannot
be obtained. furthermore, steel products according to the present
invention are of transformable steel, and the single-phase ferrite
region suitable for recrystallization-annealing is in a range of
700-900.degree. C. Such a temperature range is relatively low as
compared with that of ferrite steel having a high Si content, and
therefore the excessive growth of crystal grains is disadvantageous
to the productivity of a continuous short-time annealing facility.
Thus, the upper limit of the crystal grain diameter is limited to
about 80 .mu.m.
[0154] In the medium-to-high Si steel, since the electrical
resistance is improved due to alloy, relatively low core loss can
be readily obtained. Thus, the crystal grain diameter is not
particularly limited and may be in an ordinary range. Generally,
the crystal grain diameter is about 20-200 .mu.m.
[0155] The inventors have studied a method for improving the
magnetic characteristics at a high frequency. Such characteristics
have recently become important because the high-speed rotation and
the increase of poles of motors have been advancing. As a result,
it became clear that reducing the thickness is effective and the
effect is particularly significant for low-Si steel. The experiment
that provides the above result will now be described.
[0156] [Experiment 5]
[0157] FIG. 13 shows the dependency of the coreless with the sheet
thickness, at 400 Hz, of the steel sheet containing 0.11% of Si and
0.18% of P, the steel sheet containing 0.95% of Si and 0.02% of P
and the steel sheet containing 2.0% of Si and 0.5% of Al.
[0158] As shown in the figure, in all the samples, it is clear that
the core loss at high frequency is improved because the
eddy-current loss is decreased when the thickness is reduced, and
that the effect of improving the core loss at high frequency by
reducing the thickness is more significant than that in the low-Si
steel.
[0159] However, conventionally, major non-oriented electrical steel
sheets have a thickness of 0.50 mm. Only some of the high-grade
non-oriented electrical steel sheets having a large content of Si
and Al, which are elements increasing the resistivity, have a
thickness smaller than 0.50 mm. There are no examples of such a
thinner non-oriented electrical steel sheets having a small content
of Si and Al.
[0160] FIG. 14 shows the dependency of the magnetic flux density
with the thickness of these steel sheets.
[0161] As shown in the figure, when the thickness is reduced, the
magnetic flux density is slightly decreased, wherein the degree of
the decrease is very small. The steel sheet having a smaller Si
content has a significantly larger magnetic flux density than that
of the other steel sheets all over the thickness range. For
applications such as driving motors for electric vehicles (EV) and
hybrid electric vehicles (HEV) in particular, a high-speed
rotation-type reluctance motor is being studied. In such an
application, high magnetic flux density and low core loss at high
frequency are important. Such characteristics can be obtained by
reducing the thickness of a low-Si and low-Al steel sheet,
essentially having high magnetic flux density, according to the
present invention.
[0162] As shown in FIG. 13, when the thickness is about 0.35 mm or
less, the effect of a reduction in thickness is significant. When
the thickness is about 0.30 mm or less, the effect is further
significant. Since the smaller thickness is more advantageous in
order to reduce the eddy-current loss, the lower limit of the
thickness is not particularly limited. However, when the thickness
is excessively lowered, the number of man-hours needed to stack the
cores is increased to raise the manufacturing cost and there is a
problem in that it is difficult to interlock the stacked cores.
Thus, the lower limit of the thickness is preferably about 0.10 mm
for general applications.
[0163] The reason for limiting the upper and lower limits of the
content of components other than Si, Al, and P in a steel sheet of
the present invention will now be described. C content: 0 to about
0.010%
[0164] Element C having an age-hardening function deteriorates the
magnetic characteristic (core loss) with the passage of time after
the production of the steel sheet. Since the degree of the
deterioration becomes significant when the C content exceeds about
0.010%, the C content is limited to 0.010% or less. In
consideration of the deterioration due to the age-hardening
function, smaller C content is more preferable. Thus, in the
present invention, the C content may include substantially zero
(below the lower limit of analysis).
[0165] Mn Content: about 0.5% or Less
[0166] Mn has a function of fixing S by reacting with S to form
MnS, thereby preventing the embrittlement caused by FeS during a
hot-rolling step. As the Mn content is increased, the resistivity
is increased to improve the core loss. In contrast, the increase of
the Mn content causes the decrease of the magnetic flux density.
Thus, the upper limit of the Mn content is limited to about
0.5%.
[0167] S Content: about 0.015% or Less
[0168] S is an unavoidable impurity. When FeS is precipitated, S
causes the embrittlement during a hot-rolling step, as described
above and fine particles made of precipitated FeS prevent grain
growth. In order to reduce the core loss, it is advantageous to
minimize the S content as much as possible. Since the deterioration
of the core loss becomes significant when the S content exceeds
0.015%, the upper limit thereof is limited to 0.015%. On the other
hand, S has a function of improving the shape of a sheared surface
during a punching step. Therefore, the extent of redusing S is
determined depending on the applications.
[0169] N Content: about 0.010% or Less
[0170] N is also an unavoidable impurity. Fine particles made of
precipitated AlN prevent crystal grain growth to increase the core
loss. Thus, the N content is limited to 0.010% or less.
[0171] In the above description, the essential components and the
components to be reduced are illustrated. In the present invention,
the following elements may be further contained in an appropriate
amount in order to improve the magnetic characteristics.
[0172] Sb and/or Sn Content: in a Total Amount of about 0.40% or
Less
[0173] Sb and Sn are located at the grain boundaries and have a
function of improving magnetic flux density and core loss by
preventing {111}-oriented recrystallized nucleus from being formed
at the grain boundaries during the recrystallization of steel. In
order to obtain this effect, the total content is preferably 0.01%
or more when these elements are contained alone or in combination.
However, if the content is excessively increased, the effect is not
greatly increased. When the content exceeds 0.40%, the
embrittlement arises to cause cracks during a cold-rolling step.
The total content is preferably 0.40% or less when the elements are
contained alone or in combination.
[0174] Other auxiliary components will now be described.
[0175] In the present invention, Ca can be contained in an amount
of about 0.01% or less, wherein Ca functions as a deoxidizing agent
and effectively captures S, which is an impurity, together with Mn.
Furthermore, B can be contained in an amount of about 0.005% or
less and Cr can be contained in an amount of about 0.1% or less in
order to suppress the oxidation and nitridation during stress
relief annealing.
[0176] Known elements such as Cu and Mo that do not deteriorate the
magnetic characteristics may be further contained, other than the
above-mentioned elements. In such a case, the effects of the
present invention are not deteriorated. In consideration of the
manufacturing cost, the content of each element is preferably about
0.1% or less.
[0177] Other elements such as Ti, Nb, and V that form carbonitrides
may be contained in a small amount and the content is preferably as
small as possible in order to assure low core loss.
[0178] As described above, in the medium-to-high content range of
Si, the excessive local segregation of P is suppressed to produce
steel having high P content in a reproducible manner by performing
the design for obtaining the following composition: the composition
in which either one of the single ferrite or austenite phase can be
obtained at a slab-reheating temperature; or the composition in
which the concentration of P distributed into the ferrite phase, in
which P is more readily concentrated, is suppressed when there is
the austenite-ferrite duplex phase.
[0179] Specifically, in order to suppress the excessive local
segregation of P at a slab-reheating temperature (about
1000-1200.degree. C.) which is suitable for allowing carbides,
nitrides, and sulfides, which are contained in the steel, to stably
precipitate, the following condition is satisfied:
[0180] the index P.sub.A expressed by the following formula:
P.sub.A=-0.2Si+0.12Mn-0.32Al+0.05Ni.sup.2+0.10Ni+0.36 (1) and
[0181] the P content satisfy the following condition:
[0182] P.ltoreq.P.sub.A; or
[0183] the index P.sub.F expressed by the following formula:
P.sub.F=-0.34Si+0.20Mn-0.54Al+0.24Ni.sup.2+0.28Ni+0.76 (2)
[0184] satisfies the following condition:
P.sub.F.ltoreq.about 0.26
[0185] where the unit of Si, Mn, Al, Ni, and P is mass %. In the
above formulae, the index P.sub.A corresponds to the upper limit of
the P content experimentally determined using various steel
products having a different content of Si, Mn, Al, and Ni so as to
obtain the single austenite phase at a temperature of about
1000-1200.degree. C. The index P.sub.F corresponds to the lower
limit of the P content-experimentally determined so as to obtain
the single ferrite phase.
[0186] Next, production conditions of the present invention will
now be described.
[0187] Molten steel having the above preferable composition is
prepared by a converter-refining method, an electric
furnace-melting method or the like to manufacture a slab by a
continuous casting method or an ingot-blooming method.
[0188] The slab is then heated and then hot-rolled. In order to
allow carbides, nitrides, and sulfides in the slab to stably
precipitate, the preferable temperature is about 1000-1200.degree.
C. As described above, the state of the phase is important in order
to suppress the excessive local segregation of P.
[0189] Since P is an element that contributes to form the ferrite
phase, P has a function of reducing the single-phase austenite
region close to the slab-reheating temperature. In the case of
low-Si steel, however, when the composition is within the scope of
the present invention, the single austenite phase can be obtained
at a slab-reheating temperature of about 1000-1200.degree. C.
Furthermore, in the case of medium-to-high Si steel, when the
composition satisfies the condition P.ltoreq.P.sub.A, the single
austenite phase can be obtained at a slab-reheating temperature of
about 1000-1200.degree. C. Furthermore, in the case of
medium-to-high Si steel, when the composition satisfies the
condition P.sub.F.ltoreq. about 0.26, the degree of the segregation
of P in the ferrite phase is in such a range that the embrittlement
can be avoided, even if the ferrite-austenite coexisting phase is
formed. When the slab is heated at a temperature in the
single-phase ferrite region, a steel sheet can be made without
forming delamination cracks if the P content is about 0.26% or
less.
[0190] In the present invention, the coiling temperature is also an
important factor in order to manufacture a high-P steel sheet. That
is, when the coiling temperature is high, iron phosphide
(Fe.sub.3P) is formed to deteriorate the bending properties and the
rolling properties of the hot-rolled sheet. Therefore, the coiling
temperature is about 650.degree. C. or less, preferably about
600.degree. C. or less, and more preferably about 550.degree. C. or
less. That is, the winding is preferably performed at a temperature
as low as possible. It is effective that the coil is acceleratingly
cooled by such a method that the coil is soaked in a water bath or
water is sprayed on the coil.
[0191] The hot-rolled coil is descaled by a pickling method or the
like and then subjected to a cold-rolling step. In order to further
increase the magnetic characteristics, the resulting hot-rolled
coil may be annealed.
[0192] In low-Si steel having an Si and Al total content of 0.5% or
less, the hot-rolled sheet is preferably annealed at a temperature
outside the ferrite-austenite coexisting region (two-phase
coexisting region). The reason is that the magnetic characteristics
such as the magnetic flux density are not improved because the
crystal grains cannot sufficiently grow at an annealing temperature
in the two-phase coexisting region. The suitable annealing
temperature of the hot-rolled sheets made of the low-Si steel will
now be described on an Ni content basis.
[0193] A steel sheet containing no Ni or a steel sheet having a
relatively small Ni content of 1.0% or less can be annealed at such
a temperature that is about 900.degree. C. and is within the
single-phase ferrite region in the same manner as that a hot-rolled
non-oriented electrical steel sheet is usually annealed. The
annealing temperature can be increased to such a temperature that
is higher than the Ac3 transformation point and in the single-phase
austenite region (preferably about 1050-1100.degree. C.). It is
important to avoid an annealing temperature (particularly about
950.degree. C.) in the duplex region, which is the intermediate
region between the two regions.
[0194] On the other hand, when the Ni content is more than 1.0% to
2.3%, which is relatively high, an annealing temperature of. about
900.degree. C. corresponds to the duplex region since the
austenite-forming temperature is lowered, thereby decreasing the
magnetic flux density. However, since the crystal grains cannot
sufficiently grow at an annealing temperature is 900.degree. C. or
less, single-phase ferrite region, high magnetic flux density
cannot be achieved. Thus, the annealing temperature of the
hot-rolled sheet having this content is limited to such a
temperature that is in the single-phase austenite region
(preferably about 1050-1100.degree. C.) which is the Ac3
transformation point or more.
[0195] In the medium-to-high Si steel sheet, the grain growth
during an annealing step is not an important factor as compared
with the low-Si steel sheet because low core loss can be achieved
if the grain diameter is small. Thus, the annealing temperature of
the hot-rolled sheet is not particularly limited and is preferably
700-1100.degree. C. usually.
[0196] Subsequently, the obtained coil is descaled and then cold or
warm-rolled once, or cold-rolled (or warm-rolled) twice or more
with an intermediate annealing step therebetween so as to have a
predetermined thickness.
[0197] The finish-annealing is then performed. In the case of the
low-Si steel sheet, the finish-annealing is preferably performed at
such a temperature that is 700.degree. C. or more and is in the
single-phase ferrite region. The reason is as follows: it is
difficult to make the crystal grains to uniformly grow so as to
have an average diameter of about 30 .mu.m or more when the
annealing temperature is less than 700.degree. C., and the texture
is deteriorated to decrease the magnetic flux density and to
increase the core loss when the annealing temperature exceeds the
single-phase ferrite region to form austenite grains.
[0198] In the medium-to-high Si steel sheet, the grain growth
during an annealing step is not an important factor as compared
with the low-Si steel sheet, as described above. Thus, the
finish-annealing temperature is not particularly limited and is
preferably 700-1100.degree. C. usually.
[0199] In the hot-rolled sheets and the cold-rolled sheets, the
temperature region in which single-phase ferrite or single-phase
austenite is formed can be obtained by observing samples with an
optical microscope, wherein the samples are prepared by heating and
then water-cooling the pieces of each steel sheet having certain
composition. Alternatively, the temperature region can be estimated
using a computational phase diagram obtained with a softwear for
thermodynamic calculation, for example, Thermo-Calc.TM..
[0200] After the finish-annealing, an insulating coating may be
provided onto the steel sheet in the same manner as for ordinary
non-oriented electrical steel sheets. The providing method is not
particularly limited. The following procedure is preferable: the
application of a coating solution and the baking treatment are
performed in that order.
[0201] The obtained coil is slit into strips having a desired width
and length. The strips are punched into pieces having shapes of
motor stators and rotors, and the resulting pieces are then stacked
to form products, by users. In some cases, stress relief annealing
will be carried out to these stacked cores (usually at 750.degree.
C. for 1-2 hours), and then used for manufacturing products.
EXAMPLES
Example 1
[0202] Each molten steels having the composition shown in Table 1
were experimentally casted. The obtained ingots were hot-rolled
into a sheet bar having a thickness of 30 mm. The sheet bar was
heated at 1100.degree. C. for 60 minutes and then hot-rolled so as
to have a thickness of 2 mm. The hot-rolled sheet was maintained at
600.degree. C. for two hours in a soaking step and was then air
cooled, wherein such conditions correspond to coiling conditions.
The hot-rolled sheet was annealed at 950.degree. C. for 60 second,
pickled, and then cold-rolled (once) so as to have a thickness of
0.50 mm. The cold-rolled sheet was finish-annealed at various
temperatures of 700-900.degree. C. to obtain different
recrystallized grain diameters. During the cold-rolling step, since
many delamination-cracks parallel to a sheet surface were formed in
the sample steel J in which P content exceeded invention range,
subsequent treatment and the evaluation were not performed.
[0203] The samples No. 56-59 were each prepared by the following
procedure: a sheet bar was hot-rolled and then cold-rolled twice
with an intermediate annealing step therebetween at 800.degree. C.,
without annealing the hot-rolled sheet.
[0204] A semi-organic insulating coating was provided onto the
finish-annealed sheet so as to have an average thickness of 0.6
.mu.m to form samples, which were used in various tests.
[0205] In a punching test, a circular die having a diameter of 21
mm was used and the clearance was set to 8% of the thickness. The
diameter (inner diameter) of a punched hole was measured in four
directions that make angles of 0.degree., 45.degree., 90.degree.,
and 135.degree. with respect to the rolling direction to determine
the average of the four obtained diameters. The difference between
the maximum and the minimum of the four diameters was measured to
use as the index of punching anisotropy.
[0206] The magnetic properties were measured by the Epstein method,
using rectangular specimens having a length of 180 mm and a width
of 30 mm, wherein the rolling direction makes an angle of 0.degree.
with respect to the longitudinal direction of one of the specimens
and makes an angle of 90.degree. with respect to the longitudinal
direction of another.
[0207] The yield point (YP) was measured by a tensile test method
at a crosshead speed of 10 mm/min. using a JIS No. 5 specimen of
which the longitudinal direction is parallel to the rolling
direction, and the upper yield stress was employed.
[0208] The obtained result is shown in Tables 2 and 3.
1 TABLE 1 Composition (mass %) Steel ID C Si Al Mn S P N Sb Sn A
0.0027 0.03 0.0008 0.21 0.0040 0.02 0.0015 0.030 <0.001 B 0.0026
0.10 0.0008 0.22 0.0035 0.02 0.0020 0.032 <0.001 C 0.0019 0.53
0.0012 0.22 0.0023 0.02 0.0018 0.030 <0.001 D 0.0019 0.95 0.0007
0.20 0.0033 0.02 0.0012 0.030 <0.001 E 0.0022 1.48 0.0014 0.21
0.0041 0.02 0.0022 0.033 <0.001 F 0.0016 0.11 0.0015 0.20 0.0074
0.07 0.0019 0.030 <0.001 G 0.0017 0.11 0.0008 0.21 0.0036 0.13
0.0022 0.031 <0.001 H 0.0023 0.11 0.0011 0.22 0.0022 0.18 0.0014
0.030 <0.001 I 0.0028 0.11 0.0006 0.22 0.0075 0.25 0.0018 0.031
<0.001 J 0.0016 0.11 0.0014 0.21 0.0060 0.29 0.0016 0.032
<0.001
[0209]
2TABLE 2 Punched Punched hole Grain Hole diameter Steel Diameter
B.sub.50 W.sub.15/50 YP Diameter Max - min No. ID (.mu.m) (T)
(W/kg) (MPa) (mm) (.mu.m) Remarks 1 A 11.3 1.818 9.79 311 20.979 17
CE*.sup.1 2 A 20.5 1.811 6.85 243 20.963 21 CE*.sup.1 3 A 28.2
1.807 5.90 214 20.959 28 CE*.sup.1 4 A 31.9 1.804 5.45 204 20.957
29 CE*.sup.1 5 A 42.8 1.797 5.09 182 20.952 25 CE*.sup.1 6 A 61.3
1.785 4.62 160 20.950 34 CE*.sup.1 7 B 10.8 1.808 10.23 322 20.981
16 CE*.sup.1 8 B 20.3 1.806 6.85 249 20.968 14 CE*.sup.1 9 B 26.8
1.801 5.99 223 20.961 18 CE*.sup.1 10 B 31.5 1.796 5.52 210 20.959
19 CE*.sup.1 11 B 46.2 1.786 4.94 183 20.954 23 CE*.sup.1 12 B 78.2
1.775 4.50 152 20.945 26 CE*.sup.1 13 C 9.3 1.786 10.95 375 20.985
9 CE*.sup.1 14 C 16.0 1.782 7.57 305 20.980 14 CE*.sup.1 15 C 33.6
1.771 5.22 236 20.970 13 CE*.sup.1 16 C 59.4 1.764 4.43 198 20.964
17 CE*.sup.1 17 C 78.9 1.757 4.25 183 20.957 31 CE*.sup.1 18 D 12.2
1.772 8.57 368 20.990 15 CE*.sup.1 19 D 23.5 1.767 5.88 297 20.977
12 CE*.sup.1 20 D 27.2 1.764 5.54 284 20.976 14 CE*.sup.1 21 D 42.8
1.758 4.56 249 20.968 12 CE*.sup.1 22 D 55.5 1.754 4.25 233 20.964
15 CE*.sup.1 23 D 64.9 1.746 4.20 224 20.962 16 CE*.sup.1 24 E 18.2
1.755 6.48 361 20.990 16 CE*.sup.1 25 E 26.8 1.752 5.20 324 20.986
15 CE*.sup.1 26 E 31.7 1.749 4.68 310 20.983 13 CE*.sup.1 27 E 45.6
1.741 4.18 284 20.980 14 CE*.sup.1 28 E 66.8 1.726 3.90 261 20.976
17 CE*.sup.1 *.sup.1CE represents the term "Comparative
Example".
[0210]
3TABLE 3 Punched Punched hole Grain Hole diameter Steel Diameter
B.sub.50 W.sub.15/50 YP Diameter Max - min No. ID (.mu.m) (T)
(W/kg) (Mpa) (mm) (.mu.m) Remarks 29 F 8.6 1.813 11.80 377 20.989 7
CE*.sup.1 30 F 26.5 1.811 5.99 246 20.975 11 CE*.sup.1 31 F 33.4
1.809 5.45 227 20.973 10 CE*.sup.1 32 F 52.0 1.802 4.77 196 20.966
14 CE*.sup.1 33 F 59.7 1.797 4.75 188 20.960 16 CE*.sup.1 34 G 11.3
1.817 9.69 363 20.995 6 CE*.sup.1 35 G 16.2 1.815 7.70 320 20.993 5
CE*.sup.1 36 G 26.5 1.814 5.95 271 20.989 6 CE*.sup.1 37 G 33.6
1.812 5.40 252 20.988 4 IE*.sup.2 38 G 43.8 1.808 4.95 233 20.986 5
IE*.sup.2 39 G 75.2 1.804 4.45 201 20.984 7 IE*.sup.2 40 H 11.3
1.822 9.66 384 20.995 3 CE*.sup.1 41 H 14.8 1.819 8.10 351 20.996 4
CE*.sup.1 42 H 23.0 1.819 6.34 305 20.995 5 CE*.sup.1 43 H 25.6
1.819 6.02 295 20.994 6 CE*.sup.1 44 H 35.6 1.816 5.26 269 20.993 4
IE*.sup.2 45 H 40.2 1.814 5.05 260 20.933 4 IE*.sup.2 46 H 56.8
1.813 4.62 237 20.992 3 IE*.sup.2 47 H 77.6 1.811 4.41 220 20.991 6
IE*.sup.2 48 I 10.8 1.826 9.93 420 20.994 3 CE*.sup.1 49 I 13.5
1.824 8.55 391 20.995 4 CE*.sup.1 50 I 26.8 1.821 5.77 321 20.996 5
CE*.sup.1 51 I 32.5 1.820 5.15 305 20.994 4 IE*.sup.2 52 I 40.8
1.818 4.94 288 20.993 5 IE*.sup.2 53 I 56.4 1.817 4.59 267 20.994 4
IE*.sup.2 54 I 60.5 1.816 4.53 263 20.992 4 IE*.sup.2 55 J Not
evaluated due to cracks CE*.sup.1 caused during a cold-rolled step
56 B 19.8 1.784 7.98 260 20.965 16 CE*.sup.1 57 B 39.4 1.761 5.22
199 20.953 21 CE*.sup.1 58 H 18.2 1.795 7.81 335 20.993 6 CE*.sup.1
59 H 35.6 1.816 5.26 269 20.993 4 1E*.sup.2 *.sup.1CE represents
the term "Comparative Example". *.sup.2IE represents the term
"Inventive Example".
[0211] In the steel products A-F (Sample No. 1-33, 56, and 57), the
P content does not satisfies the condition of the present
invention, and the strength varies depending on the Si content and
the crystal grain diameter. In such steel products, as the yield
stress YP increases, the punched hole diameter tends to become
close to the die diameter. However, the punching anisotropy remains
relatively large, that is, the anisotropy is about 10-20 .mu.m,
wherein the anisotropy corresponds to the difference between the
maximum and the minimum of the punched hole diameter. Furthermore,
there is a problem in that the magnetic flux density decreases as
the Si content increases.
[0212] In contrast, the steel products G-H according to the present
invention have low Si and Al content and contain a 0.10% of P or
more. Such steel products have a good punched hole shape and a
small punching anisotropy even if yield point YP is 350 MPa or
less, that is, the yield point YP is relatively small. Among them,
the steel products having an average crystal grain diameter of 30
.mu.m or more (Samples No. 37, 38, 39, 44, 45, 46, 47, 51, 52, 53,
54, and 59) are excellent in magnetic characteristic, that is, such
steel products stably have low core loss and high magnetic flux
density.
Example 2
[0213] Each steel having the composition shown in Table 4, was
experimentally casted. Obtained ingot was hot-rolled so as to have
a thickness of 2 mm in the same manner as that of Example 1. The
hot-rolled sheet was annealed at 1100.degree. C. for 30 seconds,
pickled, and then cold-rolled so as to have a thickness of 0.5 mm.
The cold-rolled sheet was finish-annealed at various temperatures
to obtain different recrystallized grain diameters, wherein the
various temperatures are 700.degree. C. or more and are in the
single-phase ferrite region.
[0214] Then, samples having a semi-organic insulating coating were
prepared in the same manner as that of Example 1. The samples were
used in various tests.
[0215] The obtained result is shown in Table 5.
[0216] The steel IDs K-M are such samples that the deoxidization
was performed by the Al content and decreasing the Si content. The
pair of the steel IDs N and O and the pair of the steel IDs Q and R
are samples prepared in order to evaluate the effect of Ni.
4 TABLE 4 Composition (mass %) Steel ID C Si Al Mn S Ni P N Sb Sn K
0.0011 0.01 0.32 0.25 0.0032 -- 0.05 0.0020 <0.001 0.044 L
0.0009 0.01 0.33 0.24 0.0039 -- 0.16 0.0021 <0.001 0.046 M
0.0019 0.02 0.31 0.22 0.0018 -- 0.24 0.0024 <0.001 <0.001 N
0.0033 0.21 0.23 0.15 0.0028 -- 0.16 0.0012 0.060 <0.001 O
0.0024 0.21 0.24 0.18 0.0016 1.23 0.16 0.0018 0.055 <0.001 P
0.0088 0.35 0.0011 0.35 0.0046 -- 0.05 0.0031 <0.001 <0.001 Q
0.0082 0.34 0.0007 0.33 0.0040 -- 0.19 0.0019 <0.001 <0.001 R
0.0080 0.35 0.0011 0.33 0.0051 0.95 0.19 0.0018 <0.001
<0.001
[0217]
5TABLE 5 Punched Punched hole Grain Hole diameter Steel Diameter
B.sub.50 W.sub.15/50 YP Diameter Max - min No. ID (.mu.m) (T)
(W/kg) (MPa) (mm) (.mu.m) Remarks 1 K 36.1 1.777 4.95 211 20.959 18
CE*.sup.1 2 K 61.3 1.769 4.27 177 20.950 26 CE*.sup.1 3 L 26.5
1.789 5.57 283 20.982 8 CE*.sup.1 4 L 34.2 1.785 4.98 262 20.985 7
IE*.sup.2 5 L 47.0 1.785 4.47 240 20.982 9 IE*.sup.2 6 M 12.5 1.777
8.63 396 20.995 6 CE*.sup.1 7 M 35.2 1.774 4.88 294 20.991 8
IE*.sup.2 8 M 70.2 1.768 4.06 250 20.992 9 IE*.sup.2 9 N 28.7 1.786
5.38 289 20.990 8 CE*.sup.1 10 N 36.2 1.785 4.89 270 20.989 9
IE*.sup.2 11 N 58.1 1.779 4.26 239 20.989 9 IE*.sup.2 12 O 6.8
1.807 12.96 484 20.995 2 CE*.sup.1 13 O 22.7 1.803 5.28 329 20.992
2 CE*.sup.1 14 O 32.1 1.803 4.37 299 20.992 3 IE*.sup.2 15 O 48.2
1.797 3.70 270 20.995 7 IE*.sup.2 16 P 43.0 1.768 4.88 218 20.966
16 CE*.sup.1 17 P 60.4 1.766 4.50 197 20.959 18 CE*.sup.1 18 P 72.0
1.769 4.38 187 20.961 21 CE*.sup.1 19 Q 18.4 1.766 6.99 348 20.987
7 CE*.sup.1 20 Q 44.8 1.766 4.76 273 20.977 8 IE*.sup.2 21 Q 75.1
1.775 4.29 243 20.984 8 IE*.sup.2 22 R 22.6 1.778 5.07 359 20.990 4
CE*.sup.1 23 R 39.7 1.784 3.75 313 20.992 5 IE*.sup.2 24 R 56.8
1.781 3.30 290 20.991 5 IE*.sup.2 *.sup.1CE represents the term
"Comparative Example". *.sup.2IE represents the term "Inventive
Example".
[0218] Such samples that have composition within the scope of the
present invention and have an average crystal grain diameter of 30
.mu.m or more, which is a proper value, have superior dimensional
accuracy in punching, low punching anisotropy, and further
excellent magnetic characteristics. From the comparison of the
steel products N and O and the comparison of the steel products Q
and R respectively, it is clear that the steel products O and R
containing Ni have a greatly increased magnetic flux density.
Example 3
[0219] The steel ID F having the composition shown in Table 1 and
the steel ID N and O having the composition shown in Table 4 were
experimentally hot-rolled to have a thickness of 2 mm in the same
manner as that of Example 1. Each obtained hot-rolled sheet was
annealed at 1100.degree. C. for 30 seconds, pickled, and then
cold-rolled so as to have a thickness of 0.50-0.2 mm. The obtained
cold-rolled sheet was finish-annealed at various temperatures that
is 700.degree. C. or more and is in the single-phase ferrite region
to control the recrystallized grain diameter in a range of 35-45
.mu.m.
[0220] Samples having a semi-organic insulating coating were
prepared in the same manner as that of Example 1. The samples were
used in various tests. For these samples, the core loss at high
frequency, that is, at 400 Hz, was measured.
[0221] The obtained result is shown-in Table 6.
6TABLE 6 Punched Punched hole Grain Hole diameter Steel Thickness
Diameter B.sub.50 W.sub.15/50 W.sub.15/400 YP Diameter Max - min
No. ID (mm) (.mu.m) (T) (W/kg) (W/kg) (MPa) (mm) (.mu.m) Remarks 1
F 0.50 38.2 1.807 4.84 171 217 20.968 14 Comparative Example 2 F
0.35 37.1 1.804 3.78 98 219 20.983 12 Comparative Example 3 F 0.20
42.8 1.803 3.41 58 209 20.989 11 Comparative Example 4 N 0.50 42.0
1.785 4.48 141 259 20.988 8 Inventive Example 5 N 0.35 37.9 1.783
3.39 71 267 20.993 7 Inventive Example 6 N 0.20 41.3 1.783 3.08 42
261 20.995 5 Inventive Example 7 O 0.50 37.4 1.802 3.86 89 288
20.992 4 Inventive Example 8 O 0.35 43.5 1.797 3.05 48 277 20.995 3
Inventive Example 9 O 0.20 35.2 1.798 2.78 29 292 20.995 3
Inventive Example
[0222] It is clear that the core loss tends to decrease
particularly at high frequency as the thickness decreases.
Furthermore, the dimensional accuracy in punching,tends to improve
when the thickness is reduced. The steel products N and O, of which
the composition is within the scope of the present invention, are
superior to the steel product F, which is a comparative example, in
such a tendency. Furthermore, the examples of the present invention
are superior in punching anisotropy at any thickness.
Example 4
[0223] Each steel having the composition shown in Table 7 was
experimentally casted to form an ingot. The obtained ingots were
treated at 1150.degree. C. for one hour in a soaking step and then
hot-rolled to form a sheet bar having a thickness of 30 mm.
[0224] The obtained sheet bar was heated at a temperature (SRT)
shown in Table 8 and held for one hour and then hot-rolled so as to
have a thickness of 2.0 mm. The hot-rolled sheet was treated under
the condition of a temperature of 580.degree. C. and a time of one
hour, which corresponds to the coiling condition, and then air
cooled. With some exception, the resulting hot-rolled sheets were
annealed under the conditions shown in Table 8. The annealed sheets
were pickled and then cold-rolled so as to have a thickness of 0.50
mm.
[0225] The machinability during a cold-rolling step was evaluated
by observing the state of each sheet that is being cold-rolled and
the texture in the cross section of the cold-rolled sheet during
the cold-rolling step. Many delamination cracks parallel to a sheet
surface were observed in the following steel products and samples:
the steel products (W, Z, a, c, d, k, and l) that have a high P
content (0.10% or more) and composition which is outside the scope
of the present invention, and the samples (No. 25 and 26) having
composition within the scope of the invention but having
slab-reheating temperature (SRT) or hot rolled sheet-coiling
temperature (CT) that are outside the scope of the present
invention. In some samples (No. 5, 19, and 25), separation by
delamination arose during the cold-rolling step, so that the cold
rolling could not proceed. The steel products and samples having
the above evaluations, are difficult to manufacture stably.
Therefore, for such steel products and samples, the subsequent
treatment and the evaluation were not conducted.
[0226] The cold-rolled sheets were finish-rolled at various
temperatures which are 700.degree. C. or more. A semi-organic
insulating coating was provided onto each cold-rolled sheet, and
the resulting cold-rolled sheets were provided to various tests.
JIS No. 5 specimens prepared by slitting the cold-rolled sheets in
parallel to the rolling direction were used for measuring the
strength. The specimens were tested at a crosshead speed of 10 mm/s
to obtain tensile strength (TS), which was evaluated. The result is
shown also in Table 8.
7TABLE 7 Steel Con- Com- Con- dition Steel po- dition P.sub.F
.ltoreq. ID sition C % Si % Al % Mn % S % Ni % P % N % Sb % Sn %
P.sub.A P.sub.A .gtoreq. P P.sub.F 0.26 Result*.sup.5 S CS*.sup.1
0.0018 0.60 0.0010 0.18 0.0041 0.00 0.05 0.0022 <0.001 <0.001
0.261 S*.sup.3 0.591 NS*.sup.4 Good T IS*.sup.2 0.0011 0.60 0.0011
0.19 0.0033 0.00 0.13 0.0032 <0.001 <0.001 0.262 S*.sup.3
0.593 NS*.sup.4 Good U IS*.sup.2 0.0014 0.60 0.0006 0.22 0.0028
0.00 0.19 0.0015 <0.001 <0.001 0.266 S*.sup.3 0.600 NS*.sup.4
Good V IS*.sup.2 0.0032 0.60 0.0005 0.18 0.0032 0.00 0.26 0.0018
<0.001 <0.001 0.261 S*.sup.3 0.592 NS*.sup.4 Good W CS*.sup.1
0.0031 0.63 0.0006 0.19 0.0041 0.00 0.29 0.0021 <0.001 <0.001
0.257 NS*.sup.4 0.585 NS*.sup.4 No good X CS*.sup.1 0.0011 1.02
0.0010 0.19 0.0032 0.00 0.04 0.0018 <0.001 0.023 0.178 S*.sup.3
0.451 NS*.sup.4 Good Y IS*.sup.2 0.0011 1.00 0.0011 0.21 0.0032
0.00 0.15 0.0020 <0.001 0.036 0.185 S*.sup.3 0.461 NS*.sup.4
Good Z CS*.sup.1 0.0011 0.98 0.0004 0.19 0.0032 0.00 0.21 0.0022
<0.001 0.025 0.187 NS*.sup.4 0.465 NS*.sup.4 No good a CS*.sup.1
0.0011 1.01 0.0006 0.18 0.0032 0.00 0.25 0.0025 <0.001 0.032
0.179 NS*.sup.4 0.452 NS*.sup.4 No good b CS*.sup.1 0.0019 1.52
0.0009 0.20 0.0050 0.00 0.04 0.0019 0.018 0.002 0.080 S*.sup.3
0.283 NS*.sup.4 Good c CS*.sup.1 0.0025 1.54 0.0011 0.19 0.0041
0.00 0.12 0.0012 0.022 <0.001 0.074 NS*.sup.4 0.274 NS*.sup.4 No
good d CS*.sup.1 0.0016 1.48 0.0008 0.22 0.0028 0.00 0.17 0.0031
0.023 <0.001 0.090 NS*.sup.4 0.300 NS*.sup.4 No good e IS*.sup.2
0.0018 1.63 0.0007 0.18 0.0019 0.00 0.19 0.0026 0.019 <0.001
0.056 NS*.sup.4 0.242 S*.sup.3 Good f IS*.sup.2 0.0024 1.60 0.0006
0.18 0.0032 0.00 0.25 0.0014 0.022 <0.001 0.061 NS*.sup.4 0.252
S*.sup.3 Good g CS*.sup.1 0.0008 2.18 0.25 0.20 0.0008 0.00 0.03
0.0018 <0.001 0.035 -0.132 NS*.sup.4 -0.076 S*.sup.3 Good h
IS*.sup.2 0.0011 2.20 0.26 0.18 0.0004 0.00 0.13 0.0022 0.002 0.036
-0.142 NS*.sup.4 -0.092 S*.sup.3 Good i IS*.sup.2 0.0016 2.11 0.25
0.18 0.0013 0.00 0.19 0.0021 <0.001 0.034 -0.120 NS*.sup.4
-0.056 S*.sup.3 Good j IS*.sup.2 0.0017 2.08 0.27 0.19 0.0017 0.00
0.24 0.0035 <0.001 0.032 -0.120 NS*.sup.4 -0.055 S*.sup.3 Good k
CS*.sup.1 0.0024 2.11 0.27 0.22 0.0023 0.00 0.29 0.0028 <0.001
0.035 -0.122 NS*.sup.4 -0.059 S*.sup.3 Good l CS*.sup.1 0.0026 1.50
0.0010 0.20 0.0032 0.50 0.18 0.0026 <0.001 0.022 0.146 NS*.sup.4
0.489 NS*.sup.4 No good m IS*.sup.2 0.0033 1.45 0.0005 0.19 0.0015
1.09 0.16 0.0021 0.002 0.021 0.261 S*.sup.3 0.894 NS*.sup.4 Good n
IS*.sup.2 0.0036 1.56 0.0010 0.19 0.0032 1.57 0.17 0.0019 <0.001
0.021 0.351 S*.sup.3 1.296 NS*.sup.4 Good o IS*.sup.2 0.0038 1.50
0.0006 0.21 0.0022 2.13 0.19 0.0025 <0.001 0.026 0.525 S*.sup.3
1.978 NS*.sup.4 Good *.sup.1The symbol CS (Comparative steel) means
that the composition is outside the scope of the present invention.
*.sup.2The symbol IS (Inventive steel) means that the composition
is within the scope of the present invention. *.sup.3The symbol S
means that the condition is satisfied. *.sup.4The symbol NS means
that the condition is not satisfied. *.sup.5Result of the
estimation by the formulae.
[0227]
8TABLE 8 Steel PHD*.sup.8 Con- Con- Com- max - dition dition Steel
po- Re- SRT CT HSAT*.sup.7 B50 YP TS PHD*.sup.8 min Produc- P.sub.A
.gtoreq. P.sub.F .ltoreq. No. ID sition marks (.degree. C.)
(.degree. C.) (.degree. C.) (T) (MPa) (MPa) (mm) (.mu.m) ibility
P.sub.A P P.sub.F 0.26 1 S CS*.sup.1 CE*.sup.5 1150 520 900 1.775
227 348 20.969 16 Possible 0.261 S*.sup.3 0.591 NS*.sup.4 2 T
IS*.sup.2 IE*.sup.6 1150 520 900 1.774 260 382 20.988 5 Possible
0.262 S*.sup.3 0.593 NS*.sup.4 3 U IS*.sup.2 IE*.sup.6 1150 520 900
1.776 286 410 20.991 6 Possible 0.266 S*.sup.3 0.600 NS*.sup.4 4 V
IS*.sup.2 IE*.sup.6 1150 520 900 1.777 315 439 20.993 5 Possible
0.261 S*.sup.3 0.592 NS*.sup.4 5 W CS*.sup.1 CE*.sup.5 1150 520 900
-- -- -- -- -- NP*.sup.9 0.257 NS*.sup.4 0.585 NS*.sup.4 6 X
CS*.sup.1 CE*.sup.5 1150 550 900 1.762 255 370 20.968 12 Possible
0.178 S*.sup.3 0.451 NS*.sup.4 7 Y IS*.sup.2 IE*.sup.6 1150 550 900
1.764 297 415 20.992 5 Possible 0.185 S*.sup.3 0.461 NS*.sup.4 8 Z
CS*.sup.1 CE*.sup.5 1150 550 900 -- -- -- -- -- NP*.sup.9 0.187
NS*.sup.4 0.465 NS*.sup.4 9 a CS*.sup.1 CE*.sup.5 1150 550 900 --
-- -- -- -- NP*.sup.9 0.179 NS*.sup.4 0.452 NS*.sup.4 10 b
CS*.sup.1 CE*.sup.5 1150 550 1100 1.733 291 400 20.974 15 Possible
0.080 S*.sup.3 0.283 NS*.sup.4 11 C CS*.sup.1 CE*.sup.5 1150 550
1100 -- -- -- -- -- NP*.sup.9 0.074 NS*.sup.4 0.274 NS*.sup.4 12 d
CS*.sup.1 CE*.sup.5 1150 550 1100 -- -- -- -- -- NP*.sup.9 0.090
NS*.sup.4 0.300 NS*.sup.4 13 e IS*.sup.2 IE*.sup.6 1150 550 1100
1.735 360 470 20.993 4 Possible 0.056 NS*.sup.4 0.242 S*.sup.3 14 f
IS*.sup.2 IE*.sup.6 1150 550 1100 1.733 383 494 20.992 3 Possible
0.061 NS*.sup.4 0.252 S*.sup.3 15 g CS*.sup.1 CE*.sup.5 1150 580
1000 1.703 338 444 20.990 15 Possible -0.132 NS*.sup.4 -0.076
S*.sup.3 16 h IS*.sup.2 IE*.sup.6 1150 580 1000 1.708 382 489
20.995 5 Possible -0.142 NS*.sup.4 -0.092 S*.sup.3 17 i IS*.sup.2
IE*.sup.6 1150 580 1000 1.709 400 509 20.996 3 Possible -0.120
NS*.sup.4 -0.056 S*.sup.3 18 J IS*.sup.2 IE*.sup.6 1150 580 1000
1.704 419 530 20.994 3 Possible -0.120 NS*.sup.4 -0.055 S*.sup.3 19
k CS*.sup.1 CE*.sup.5 1150 580 1000 -- -- -- -- -- NP*.sup.9 -0.122
NS*.sup.4 -0.059 S*.sup.3 20 l CS*.sup.1 CE*.sup.5 1150 550 1100 --
-- -- -- -- NP*.sup.9 0.146 NS*.sup.4 0.489 NS*.sup.4 21 m
IS*.sup.2 IE*.sup.6 1150 550 1100 1.742 353 465 20.996 3 Possible
0.261 S*.sup.3 0.894 NS*.sup.4 22 n IS*.sup.2 IE*.sup.6 1150 550
1100 1.748 372 483 20.995 4 Possible 0.351 S*.sup.3 1.296 NS*.sup.4
23 O IS*.sup.2 IE*.sup.6 1150 550 1100 1.751 383 495 20.996 4
Possible 0.525 S*.sup.3 1.978 NS*.sup.4 24 Y IS*.sup.2 IE*.sup.6
1100 550 -- 1.754 312 416 20.990 6 Possible 0.185 S*.sup.3 0.461
NS*.sup.4 25 Y IS*.sup.2 CE*.sup.5 1250 550 550 -- -- -- -- --
NP*.sup.9 0.185 S*.sup.3 0.461 NS*.sup.4 26 Y IS*.sup.2 CE*.sup.5
1150 700 700 -- -- -- -- -- NP*.sup.9 0.185 S*.sup.3 0.461
NS*.sup.4 *.sup.1The symbol CS (Comparative Steel) means that the
composition is outside the scope of the present invention.
*.sup.2The symbol IS (Inventive Steel) means that the composition
is within the scope of the present invention. *.sup.3The symbol S
means that the condition is satisfied. *.sup.4The symbol NS means
that the condition is not satisfied. *.sup.5The symbol CE
represents the term "Comparative Example". *.sup.6The symbol IE
represents the term "Inventive Example". *.sup.7The symbol HSAT
represents the hot-rolled sheet annealing temperature. *.sup.8The
symbol PHD represents the punched hole diameter. *.sup.9The symbol
NP means that the production is not possible due to delamination
cracks.
[0228] The samples (No. 2-4, 7, 13, 14, 16-18, and 21-24) having
composition that is within the scope of the present invention and
containing 0.1% or more of P have excellent dimensional accuracy in
punching in particular. That is, in the samples (No. 1, 6, 10, and
15) containing less than 0.1% of P, as the Si and Al total content
increases, the dimensional accuracy in punching tends to increase
but the punching anisotropy remains high. In contrast, it is clear
that the examples of the present invention are excellent in both
dimensional accuracy and punching anisotropy. Furthermore, the
examples of the present invention have magnetic flux density that
is the same as that or more than that of the comparative examples
having a P content of less than 0.1%. Even more, the examples have
high strength. That is, the examples have the excellent balance of
the strength and the magnetic flux density.
Example 5
[0229] The steel IDs M, N, and 0 having the -composition shown in
Table 4 were experimentally prepared. After the casting, sheet bars
having a thickness of 30 mm were obtained by hot rolling. The sheet
bars were heated at each temperature (SRT) shown in Table 9 for 60
minutes and hot-rolled so as to have a thickness of 2 mm. The
hot-rolled sheets were treated in a soaking process under the
conditions of each temperature (CT) and a time of one hour, which
correspond to the coiling conditions, and then air cooled. With
some exception, the hot-rolled sheets were then annealed at each
temperature shown in Table 9 for 60 seconds.
[0230] For the obtained hot-rolled sheets, the bending test was
conducted at room temperature (23.degree. C.). A specimen prepared
from hot-rolled sheet having a length of 100 mm and a width of 30
mm was used in the bending test, wherein the longitudinal direction
of the specimen is parallel to the rolling direction. The
repetitive bending test was conducted according to the method
defined in JIS-C 2550. In the bending test, the bending radius was
15 mm. Table 9 shows the number of times each specimen was bent
until cracks are formed on a surface.
[0231] Microstructures (phase) of each slab during the heating step
and each hot-rolled sheet during the annealing step were
investigated by the following procedure: each sheet bar and each
hot-rolled sheet are separately maintained at a predetermined
temperature (shown in Table 9) for a predetermined time (for one
hour when the slab is heated or for 60 seconds when the hot-rolled
sheet is annealed) and then quenched with water to fix the
microstructure during the heating step. The obtained microstructure
was observed with an optical microscope to determine the phase. The
result is also shown in Table 9.
[0232] The above hot-rolled sheets were pickled and then
cold-rolled (once) so as to have a thickness of 0.50 mm. The
cold-rolled sheets were checked if there are defects (delamination
cracks) due to the embrittlement arising during the cold-rolling
step. The cold-rolled sheets having no delamination cracks were
finish-annealed at various temperatures shown in Table 9. A
semi-organic insulating coating was then provided onto each
finish-annealed sheet in the same manner as that of Example 1 to
form samples, which were used in various tests. The obtained result
is shown in Table 9.
9TABLE 9 Grain Steel Steel Re- SRT Ms- CT HSAT*.sup.5 Ms- N-
FAT*.sup.10 size B50 W15/50 YP No. ID Composition marks (.degree.
C.) slab*.sup.4 (.degree. C.) (.degree. C.) hrs*.sup.6 bend*.sup.7
Producibility (.degree. C.) (.mu.m) (T) (W/kg) (MPa) 1 M IS*.sup.1
CE*.sup.2 1250 .alpha. and .gamma. 520 -- -- 4 NP-c*.sup.8 -- -- --
-- -- 2 M IS*.sup.1 IE*.sup.3 1150 single .gamma. 520 -- -- 30
Possible 800 46.2 1.765 4.47 275 3 M IS*.sup.1 IE*.sup.3 1050
single .gamma. 520 -- -- 28 Possible 800 38.2 1.762 4.75 288 4 M
IS*.sup.1 CE*.sup.2 950 .alpha. and .gamma. 520 -- -- 5 NP-c*.sup.8
-- -- -- -- -- 5 M IS*.sup.1 CE*.sup.2 1150 single .gamma. 720 900
single .alpha. 3 NP-b*.sup.9 850 45.1 1.762 6.83 288 6 N IS*.sup.1
IE*.sup.3 1150 single .gamma. 620 900 single .alpha. 17 Possible
850 55.2 1.764 4.31 242 7 N IS*.sup.1 CE*.sup.2 1150 single .gamma.
550 960 .alpha. and .gamma. 20 Possible 850 53.8 1.736 4.33 244 8 N
IS*.sup.1 IE*.sup.3 1150 single .gamma. 550 1100 single .gamma. 22
Possible 850 52.1 1.768 4.36 246 9 N IS*.sup.1 CE*.sup.2 1150
single .gamma. 500 900 single .alpha. 27 Possible 670 16.0 1.766
7.38 345 10 O IS*.sup.1 IE*.sup.3 1100 single .gamma. 600 1100
single .gamma. 26 Possible 800 36.5 1.777 4.12 290 11 O IS*.sup.1
IE*.sup.3 1100 single .gamma. 600 1000 single .gamma. 33 Possible
800 38.5 1.780 4.02 286 12 O IS*.sup.1 CE*.sup.2 1100 single
.gamma. 550 900 .alpha. and .gamma. 28 Possible 800 32.6 1.733 4.34
298 13 O IS*.sup.1 CE*.sup.2 1100 single .gamma. 550 800 single
.alpha. 24 Possible 800 36.8 1.733 4.10 289 *.sup.1The symbol IS
(Inventive Steel) means that the composition is within the scope of
the present invention. *.sup.2The symbol CE represents the term
"Comparative Example". *.sup.3The symbol IE represents the term
"Inventive Example". *.sup.4The symbol Ms-slab represents the
microstructure of a slab during a heating step. *.sup.5The symbol
HSAT represents the hot-rolled sheet annealing temperature.
*.sup.6The symbol Ms-hrs represents the microstructure of a
hot-rolled sheet during an annealing step. *.sup.7The symbol N-bend
represents the number of times a hot-rolled sheet is bent until
cracks are formed on a surface. *.sup.8The symbol NP-c means that
the production is not possible due to delamination cracks.
*.sup.9The symbol NP-b means that the production is not possible
due to the deterioration of the bending properties. *.sup.10The
symbol FAT represents the finish-annealing temperature.
[0233] In the samples (No. 2, 3, 6, 8, 10, and 1-1) having
composition. (low-Si steel) within the scope of the present
invention and having manufacturing conditions within the scope of
the present invention, steel sheets can be manufactured without
causing any problems and the characteristics are superior, even if
the P content is high.
[0234] In contrast, in the samples (No. 1 and 4) in which the
slab-reheating temperature is within duplex region, it is clear
that it is difficult to obtain the-products because defects due to
the embrittlement during the cold-rolled sheet step are readily
caused. In the sample (No. 5) in which the coiling temperature is
higher than 650.degree. C., the hot-rolled sheet has inferior
cold-rolling workability and the electrical steel sheet has also
inferior core loss. Furthermore, in the samples (No. 7 and 12) in
which the hot-rolled sheet-annealing temperature is in the
two-phase coexisting region and the sample (No. 13) in which the
hot-rolled sheet containing more than 1.0% by mass of Ni annealed
at a temperature in the single-phase alpha region, the obtained
electrical steel sheet have low magnetic flux density. Furthermore,
in the sample (No. 9) in which the finish-annealing temperature is
outside the scope of the present invention and is not sufficient to
form recrystallized crystal grains having a diameter of 30 .mu.m or
more, the magnetic characteristics are inferior.
[0235] Industrial Applicability
[0236] The present invention provides a non-oriented electrical
steel sheet having excellent magnetic characteristics such as high
magnetic flux density and low core loss and further having high
dimensional accuracy during a punching step and further provides
a-non-oriented electrical steel sheet having high strength, with
manufactural stablity.
[0237] A non-oriented electrical steel sheet of the present
invention is suitable for an iron core material for reluctance
motors and DC brushless motors that are of an interior permanent
magnet type, among iron core materials for various motors, wherein
the reluctance motors need to have high dimensional accuracy and
high magnetic flux density in combination, and the DC brushless
motors need to have high strength.
* * * * *