U.S. patent application number 09/920749 was filed with the patent office on 2002-06-06 for non-oriented electromagnetic steel sheet having excellent magnetic properties after stress relief annealing and method of manufacturing the same.
This patent application is currently assigned to Kawasaki Steel Corporation. Invention is credited to Honda, Atsuhito, Kawano, Masaki, Ozaki, Yoshihiro.
Application Number | 20020066500 09/920749 |
Document ID | / |
Family ID | 32094348 |
Filed Date | 2002-06-06 |
United States Patent
Application |
20020066500 |
Kind Code |
A1 |
Kawano, Masaki ; et
al. |
June 6, 2002 |
Non-oriented electromagnetic steel sheet having excellent magnetic
properties after stress relief annealing and method of
manufacturing the same
Abstract
A finish annealed non-oriented electromagnetic steel sheet
includes about 0.01 wt % or less of C, greater than about 1.0 wt %
and at most about 3.5 wt % of Si, at least about 0.6 wt % and at
most about 3.0 wt % of Al, at least about 0.1 wt % and at most
about 2.0 wt % of Mn, at least about 2 ppm and at most about 80 ppm
of one or more rare earth metals (REM), a maximum content of Ti and
Zr being about 15 ppm and 80 ppm, respectively, wherein oxygen on
the surface layer of the steel sheet is 1.0 g/m.sup.2 or less.
Since the non-oriented electromagnetic steel sheet has desirable
mechanical properties resulting from the increased amounts of Si
and Al, a high magnetic flux density can be maintained without
sacrificing a punching property as well as very low iron loss can
be obtained even after stress relief annealing.
Inventors: |
Kawano, Masaki; (Okayama,
JP) ; Ozaki, Yoshihiro; (Chiba, JP) ; Honda,
Atsuhito; (Okayama, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
745 SOUTH 23RD STREET 2ND FLOOR
ARLINGTON
VA
22202
|
Assignee: |
Kawasaki Steel Corporation
|
Family ID: |
32094348 |
Appl. No.: |
09/920749 |
Filed: |
August 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09920749 |
Aug 3, 2001 |
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09240630 |
Feb 1, 1999 |
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6290783 |
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Current U.S.
Class: |
148/111 ;
148/309 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/001 20130101; C22C 38/008 20130101; H01F 1/16 20130101;
C22C 38/005 20130101; B82Y 25/00 20130101; C22C 38/04 20130101;
C21D 8/1261 20130101; C22C 38/004 20130101; C22C 38/002 20130101;
C22C 38/02 20130101; C21D 8/1272 20130101; C22C 38/60 20130101;
C22C 38/14 20130101 |
Class at
Publication: |
148/111 ;
148/309 |
International
Class: |
H01F 001/147 |
Claims
What is claimed is:
1. A non-oriented electromagnetic steel sheet, comprising about
0.01 wt % or less of C, greater than about 1.0 wt % and at most
about 3.5 wt % of Si, at least about 0.6 wt %. and at most about
3.0 wt % of Al, at least about 0.1 wt % and at most about 2.0 wt %
of Mn, at least about 2 ppm and at most about 80 ppm of one or more
rare earth metals (REM), a maximum content of Ti and Zr being about
15 ppm and 80 ppm, respectively, wherein oxygen on a metal surface
layer of the steel sheet is 1.0 g/m.sup.2 or less after finish
annealing.
2. The non-oriented electromagnetic steel sheet according to claim
1, further comprising at least about 0.002 wt % and at most about
0.1 wt % of at least one of Sb and Sn.
3. The non-oriented electromagnetic steel sheet according to claim
1, further comprising S, O and N in amounts of 20 ppm or less, 15
ppm or less and 300 ppm or less, respectively, wherein a ratio of
REM-containing inclusions coupled with nitride to REM-containing
inclusions having a diameter of at least about 1 .mu.m in the steel
sheet is 40% or more.
4. The non-oriented electromagnetic steel sheet according to claim
2, further comprising S, O and N in amounts of 20 ppm or less, 15
ppm or less and 300 ppm or less, respectively, wherein a ratio of
REM-containing inclusions coupled with nitride to REM-containing
inclusions having a diameter of at least about 1 .mu.m in the steel
sheet is 40% or more.
5. A method of manufacturing a non-oriented electromagnetic steel
sheet, comprising the steps of: hot rolling and cold rolling a
steel slab comprising about 0.01 wt % or less of C, greater than
about 1.0 wt % and at most about 3.5 wt % of Si, at least about 0.6
wt % and at most about 3.0 wt % of Al, at least about 0.1 wt % and
at most about 2.0 wt % of Mn, at least about 2 ppm and at most
about 80 ppm of one or more rare earth metals (REM), a maximum
content of Ti and Zr being about 15 ppm and 80 ppm, respectively;
and subjecting the thus rolled steel sheet to finish annealing by
adjusting at least one of a dew point and a gas atmosphere to
thereby control the amount of oxygen on the metal surface layer of
the steel sheet to 1.0 g/m.sup.2 or less.
6. The method of manufacturing a non-oriented electromagnetic steel
sheet according to claim 5, wherein the steel slab further
comprises at least about 0.002 wt % and at most about 0.1 wt % of
at least one of Sb and Sn.
7. The method of manufacturing a non-oriented electromagnetic steel
sheet according to claim 5, further comprising adding at least one
rare earth metal (REM) to a molten steel after S and O in the
molten steel are adjusted to 40 ppm or less and 25 ppm or less,
respectively, thereby to reduce S and O to 20 ppm or less and 15
ppm or less, respectively, and adjusting N to 30 ppm or less so
that a ratio of REM-containing inclusions coupled with nitride to
REM-containing inclusions having a diameter of at least about 1
.mu.m in the steel sheet is 40% or more after finish annealing.
8. The method of manufacturing a non-oriented electromagnetic steel
sheet according to claim 5, wherein the steel slab further
comprises at least about 0.002 wt % and at most about 0.1 wt % of
at least one of Sb and Sn, and further comprising adding REM to a
molten steel after S and O in the molten steel are adjusted to 40
ppm or less and 25 ppm or less, respectively thereby to reduce S
and O to 20 ppm or less and 15 ppm or less, respectively, and
adjusting N to 30 ppm or less so that a ratio of REM-containing
inclusions coupled with nitride to REM-containing inclusions having
a diameter of at least about 1 .mu.m in the steel sheet is 40% or
more after finish annealing.
9. The method of manufacturing a non-oriented electromagnetic steel
sheet according to claim 5, wherein the hot-rolled sheet is
annealed for at most about 40 seconds at a temperature of at least
about 700.degree. C. and at most about 1150.degree. C. after
hot-rolling.
10. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claim 6, wherein the hot-rolled sheet is
annealed for at most about 40 seconds at a temperature of at least
about 700.degree. C. and at most about 1150.degree. C. after
hot-rolling.
11. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claim 7, wherein the hot-rolled sheet is
annealed for at most about 40 seconds at a temperature of at least
about 700.degree. C. and at most about 1150.degree. C. after
hot-rolling.
12. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claim 8, wherein the hot-rolled sheet is
annealed for at most about 40 seconds at a temperature of at least
about 700.degree. C. and at most about 1150.degree. C. after
hot-rolling.
13. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claim 5, wherein the finish annealing is
performed for a soaking time of at most about 15 seconds at a
temperature of at least about 750.degree. C. and at most about
900.degree. C.
14. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claim 6, wherein the finish annealing is
performed for a soaking time of at most about 15 seconds at a
temperature of at least about 750.degree. C. and at most about
900.degree. C.
15. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claim 7, wherein the finish annealing is
performed for a soaking time of at most about 15 seconds at a
temperature of at least about 750.degree. C. and at most about
900.degree. C.
16. The method of manufacturing a non-oriented electromagnetic
steel sheet according to claims, wherein the finish annealing is
performed for a soaking time of at most about 15 seconds at a
temperature of at least about 750.degree. C. and at most about
900.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a non-oriented
electromagnetic steel sheet having excellent magnetic properties
after stress relief annealing and a method of manufacturing the
same.
[0003] 2. Description of the Related Art
[0004] Non-oriented electromagnetic steel sheets have been used as
the iron core materials of motors, transformers, and the like. It
is desirable to lower the iron loss of the non-oriented
electromagnetic steel sheet in order to increase the energy
efficiency of these devices.
[0005] Recently, it has become especially important to make the
motors more efficient. Accordingly, it is desired to improve the
magnetic properties of the non-oriented electromagnetic steel
sheet, in particular, to increase its magnetic flux density and
lower its iron loss. Also, the rotor unit thickness of a DC
brushless motor, for example, is reduced up to about 5 mm by
embedding a permanent magnet into a rotor. Accordingly, adequate
mechanical strength, which has not been important in conventional
small motors, is also required for the non-oriented electromagnetic
steel sheet, in addition to the magnetic properties. That is, there
is required an electromagnetic steel having excellent magnetic
properties and adequate mechanical strength as a material for small
motors of high efficiency.
[0006] As a means for reducing the iron loss of the non-oriented
electromagnetic steel sheet, there is available a method of
optimizing a grain size and a method of improving the specific
resistance of the steel sheet. That is, it is well known that the
iron loss is minimized by the grain size of about 150-200 .mu.m,
that the addition of Si or Al is effective to improve a specific
resistance, and that mechanical properties depend on Si and Al in
steel.
[0007] On the other hand, it is also well known that a problem
arises in that a saturation magnetic flux density is reduced and
the punching property of steel sheet is deteriorated when the
content of Si or Al is increased. In particular, the punching
property is a very important property for the non-oriented
electromagnetic steel sheet. Non-oriented electromagnetic steel
sheet is often used by users after it is punched to a prescribed
shape and then subjected to stress relief annealing. Since the
punched shape is complicated and requires accuracy, a precise
punching property is required for the non-oriented electromagnetic
steel sheet. The punching property is deteriorated by the increase
of the hardness and grain size of the electromagnetic steel sheet.
The increase of the hardness and grain size results from an
increase in the alloying components of the steel sheet or scales
formed on the surface of the steel sheet. For example, when Si
exceeds 1.0 wt % or when the grain size of a finished steel sheet
exceeds 40 .mu.m, a problem arises in that the punching property is
deteriorated.
[0008] Accordingly, the recent demand for higher motor efficiency
and adequate mechanical strength requires a material having an
excellent grain-growing property after stress relief annealing,
which thereby has a high magnetic flux density and very low final
iron loss without sacrificing its punching property.
[0009] This need can be met by sufficiently increasing the Si and
Al content thereby to coarsen the crystal grains. In particular, it
is preferred to increase the content of Al because it has less
effect on increased hardness. Also, it is preferred to coarsen
crystal grains because it reduces the iron loss after stress relief
annealing. More specifically, although Si and Al have the same
degree of specific resistance increasing effect, the Al content is
increased because the effect of Al per unit weight on the increase
of hardness is about one half that of Si. On the other hand,
although increasing the stress relief annealing temperature is
effective to coarsen crystal grains, the grain growing property
must be improved in a relatively low stress relief temperature
region of about 750.degree. C. at the highest, which is employed in
practice due to cost considerations.
[0010] Japanese Unexamined Patent Publication No. 8-3699 discloses
a low Si non-oriented electromagnetic steel sheet in which a Si
component is lowered to 1.0 wt % or less to obtain an excellent
growing property after stress relief annealing and a low final iron
loss. The grain growing property of the non-oriented
electromagnetic steel sheet is greatly improved by adding REM (rare
earth metals) to the steel and highly purifying the steel during
the steel-making process. High purity is accomplished by
suppressing the contents of Ti and Zr, which are elements contained
in a trace amount. Precipitates which deteriorate grain growth are
controlled by REM-addition or purification. According to the
publication, this works remarkably well; however, due to the low Si
content, the problem arises that the mechanical strength is
insufficient for some locations where the steel sheet is used and
iron loss of the sheet is insufficient to meet the need for a
greater reduction in iron loss of the cores.
[0011] Japanese Examined Patent Publication No. 61-4892 seeks to
improve magnetic properties by increasing the Al content. However,
although mechanical properties were improved by the increase of
only the Al content, the magnetic properties were greatly altered.
In particular, a low loss product could not be obtained stably
after stress relief annealing as described later. It has been found
by the inventors that the above problem is caused by nitriding
during the stress relief annealing.
[0012] In Japanese Unexamined Patent Publication No. 8-296007, it
is disclosed that the deterioration of the magnetic properties of a
steel sheet containing a large content of Al is suppressed by
controlling C contained in an insulating film, because the
deterioration is caused by nitriding in stress relief annealing.
According to the publication, however, although the change in
magnetic properties is reduced, the degree of reduction remains
insufficient and it is necessary to suppress the change
altogether.
SUMMARY OF THE INVENTION
[0013] Accordingly, it is an object of the present invention to
provide a non-oriented electromagnetic steel sheet having not only
excellent magnetic properties after stress relief annealing but
also excellent mechanical properties, and to propose an
advantageous manufacturing method for the non-oriented
electromagnetic steel sheet.
[0014] The inventors have investigated the levels to which Al and
Si should be set, on the premise that REM is added and a steel
sheet is highly purified in order to more greatly reduce iron loss
after stress relief annealing and to improve mechanical properties.
As a result, the inventors have confirmed that an increase in Al
content reduces iron loss without significantly deteriorating a
punching property, and is accordingly suitable for the improvement
of magnetic properties. However, a serious problem had still arisen
in that the magnetic properties were still altered after stress
relief annealing due to the increase in an Al content. As a result
of a diligent study to solve the above problem, the inventors have
newly found that it is very important, in a non-oriented
electromagnetic steel sheet whose iron loss is intended to be
reduced after stress relief annealing, to control surface scales
produced during finish annealing in addition to making the
components and precipitates in a steel adequate, in order to
simultaneously achieve good mechanical properties and the stable
improvement of iron loss after stress relief annealing by an
increase in Si and Al contents.
[0015] The present invention results from the above discovery.
[0016] According to the present invention, a non-oriented
electromagnetic steel sheet comprises at most about 0.01 wt % of C,
greater than 1.0 wt % and at most about 3.5 wt % of Si, at least
about 0.6 wt and at most about 3.0 wt % of Al, at least about 0.1
wt % and at most about 2.0 wt % of Mn, at least about 2 ppm and at
most about 80 ppm of REM, with Ti and Zr being suppressed to at
most about 15 ppm and 80 ppm, respectively, wherein the amount of
oxygen on the metal surface layer of the steel sheet is 1.0
g/m.sup.2 or less after finish annealing.
[0017] It is preferable that the non-oriented electromagnetic steel
sheet further comprises at least about 0.002 wt % and at most about
0.1 wt % of at least one of Sb and Sn.
[0018] It is advantageous for the stable improvement of magnetic,
properties that the non-oriented electromagnetic steel sheet
further comprises S, O and N suppressed to 20 ppm or less, 15 ppm
or less and 30 ppm or less, respectively, and that the ratio of the
number of REM-containing inclusions coupled with nitride to the
number of REM-containing inclusions having a diameter of at least
about 1 .mu.m in the steel sheet is 40% or more.
[0019] The non-oriented electromagnetic steel sheet is manufactured
by the steps of hot rolling and cold rolling a steel slab
comprising at most about 0.01 wt % of C, greater than 1.0 wt % and
at most about 3.5 wt % of Si, at least about 0.6 wt % and at most
about 3.0 wt % of Al, at least about 0.1 wt % and at most about 2.0
wt % of Mn, at least about 2 ppm and at most about 80 ppm of REM,
with Ti and Zr being suppressed to at most about 15 ppm and 80 ppm,
respectively, and subjecting the thus rolled steel sheet to finish
annealing by adjusting at least one of a dew point and a gas
atmosphere to thereby control the amount of oxygen on the metal
surface layer of the steel sheet to 1.0 g/m.sup.2 or less.
[0020] It is preferable that the steel slab used in this method
further comprises at least about 0.002 and at most about 0.1 wt %
of at least one of Sb and Sn.
[0021] In these manufacturing methods, it is advantageous to the
stable improvement of magnetic properties that when a molten steel
is made, REM is added after S and O in the molten steel is adjusted
to 40 ppm or less and 25 ppm or less, respectively, to thereby
suppress S and O to 20 ppm or less and 15 ppm or less,
respectively, as well as N is adjusted to 30 ppm or less so that
the ratio of the number of REM-containing inclusions coupled with
nitride to the number of REM-containing inclusions having a
diameter of at least about 1 .mu.m in the steel sheet is 40% or
more after finish annealing.
[0022] In the manufacturing methods of the present invention, it is
preferable that the hot-rolled sheet is annealed for 40 seconds or
less at 700.degree. C. or more to 1150.degree. C. or less after the
hot-rolling and that the finish annealing is performed in a soaking
time of 15 seconds or less at 750.degree. C. or more to 900.degree.
C. or less.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a conceptual view showing REM-containing
inclusions coupled with nitride.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] Embodiments of the present invention will be specifically
described below.
[0025] At first, the reasons for the disclosed contents of the
respective components will be described.
[0026] C: 0.01 wt % or less
[0027] Since C deteriorates magnetic properties by the
precipitation of carbide, it should be limited to 0.01 wt % or
less.
[0028] Si: greater than about 1.0 wt % and at most about 3.5 wt
%
[0029] Si is a component useful to reduce iron loss by increasing a
specific resistance. When the content of Si is 1.0 wt % or less,
reduction of the iron loss is insufficient and mechanical
properties do not improve. So the content of Si should be greater
than 1.0 wt %. By the increase of the Si content, the iron loss can
be reduced by increasing the specific resistance and mechanical
strength, for example, tensile strength and yield stress can be
increased. However, when the Si content is excessively increased,
hardness is excessively increased. Consequently, a punching
property is deteriorated. Furthermore a cold rolling becomes
difficult in manufacture. Therefore, it is necessary to set the Si
content to less than or equal to 3.5 wt %. In particular, it is
more preferable to set the SE content to more at least about 1.0 wt
% and at most about 2.5 wt %.
[0030] Mn: 0.1-2.0 wt %
[0031] Since Mn acts to fix S as coarse MnS, Mn should be contained
in an amount of 0.1 wt % or more and preferably 0.5 wt % or more.
On the other hand, since an excessive increase in the additive
amount of Mn deteriorates a magnetic flux density, Mn should be
contained in an amount of 2.0 wt % or less and preferably in an
amount of 1.5 wt % or less.
[0032] Al: at least about 0.6 wt % and at most about 3.0 wt %
[0033] Al is an important component in the present invention. Al is
an effective element to reduce iron loss in the same degree as Si
by increasing the specific resistance of a product. Moreover, Al
has a small hardening capability (a hardness increasing amount per
unit weight), about one half that of Si. Consequently, Al is an
effective element to suppress the hardening of a product and can
improve magnetic properties without sacrificing mechanical
properties. Furthermore, since Al is an element which forms
precipitates by nitriding, an increase in the additive amount of Al
can suppress the dispersion of fine AlN in a manufacturing process
and improve a grain growing property in subsequent
recrystallization annealing and stress relief annealing. Iron loss
is reduced by grain growth. The simultaneous addition of Al and REM
can improve the grain growing property greatly as described below.
Furthermore, Al has an effect for increasing crystal grains which
have [100] orientation. The [100] orientation is preferred in terms
of magnetic properties.
[0034] When Al is contained in an amount less than 0.6 wt %,
sufficient mechanical properties cannot be obtained. When Al is
contained in an amount greater than 3.5 wt %, a problem arises in
the punching property, the cold rolling ability in manufacture, and
the excessive deterioration of the magnetic flux density.
Therefore, the content of Al is set to at least about 0.6 wt %, at
most about 3.5 wt %, and preferably to 0.6 to 1.5 wt %.
[0035] REM: 2-80 ppm
[0036] The addition of one or more kinds of rare earth elements
(REM) in a total amount of 2-80 ppm can avoid the adverse affect of
Zr on the growth of grains in stress relief annealing. Zr is
inevitably contained in an amount of 5-80 ppm in a steel making
process executed on an industrial scale. Furthermore, it has been
confirmed that when Al is added in a large amount, the grain
growing property can be greatly improved by the further addition of
REM. It is predicted that this because the addition of REM changes
the precipitated state of other precipitates. Although the reason
is not apparent, it is believed that REM oxide and REM sulfide act
as a nucleating site when fine precipitates such as Zr nitride or
Al nitride precipitate. These effects are insufficient when REM is
contained in an amount less than 2 ppm, whereas the excessive
addition of REM increases the inclusions formed by REM and a
problem arises in that the grain growth is obstructed by REM
inclusions themselves. Accordingly, REM is contained in an amount
of 80 ppm or less and preferably in an amount less than 50 ppm.
[0037] Ti: 15 ppm or less
[0038] Ti is set to 15 ppm or less because it greatly deteriorates
iron loss by lowering the grain growing property in stress relief
annealing executed at a low temperature even if it is contained in
a very small amount. When Ti is set to 10 ppm or less, even better
reduction of iron loss can be obtained. It should be noted that the
addition of Ti alone is not so effective even if its content is set
to 15 ppm or less. On the other hand, the simultaneous addition of
Ti and REM can improve the grain growing property during
low-temperature stress relief annealing. Although the reason is not
apparent, it is believed that REM oxide and REM sulfide act as a
nucleus creating site when fine precipitates of such as Ti and the
like precipitate.
[0039] Zr: 80 ppm or less
[0040] It is preferable to reduce the content of Zr as much as
possible because Zr deteriorates the grain growing property in the
low temperature stress relief annealing even if it is contained in
a very small amount. However, it requires a remarkable increase of
production cost to stably set Zr to 5 ppm or less on an industrial
scale. Thus, in the present invention Zr is preferably set to 5-80
ppm. Zr becomes harmless in this range which can be stably achieved
industrially by the addition of REM. More specifically, a
remarkable effect for reducing iron loss can be obtained by setting
Zr content to 80 ppm or less in combination with the addition of
REM. Although the reason is not apparent, it is believed that REM
oxide and REM sulfide act as a nucleating site when fine
precipitates such as Zr nitride and Al nitride are formed.
[0041] Furthermore, the grain growing property can be improved by
controlling the form of the REM inclusions in a steel containing Al
in an amount of 0.6 wt % or more as described below. More
specifically, the grain growing property can be further improved by
keeping the ratio of the number of REM-containing inclusions
coupled with nitride to the number of REM-containing inclusions
having a diameter of at least about 1 .mu.m at 40% or more,
together with an 0 content of 15 ppm or less, a S content of 20 ppm
or less, and a N content of 30 ppm or less. The REM-containing
inclusions coupled with nitride are, for example, made by
REM-containing oxide and REM-containing sulfide coupled with
nitride such as AlN and the like. This example is shown in FIG. 1.
And the ratio of the number of this kind of the inclusions to the
number of the entire REM-containing inclusions is regulated as to
the REM-containing inclusions having the diameter of at least about
1 .mu.m.
[0042] The reason is not apparent why the grain growing property is
improved by the control of the form of the REM-containing
inclusions and the limitation of the O content, the S content and
the N content in the steel. However, it is believed that the grain
growing property is improved by the reduction of the oxide,
nitride, sulfide and the composite of them which form the
precipitates in the steel to their possible limits and growing the
nitride precipitate as large REM inclusions which do not affect the
grain growing property in the stress relief annealing.
[0043] Furthermore, the amount of oxide on the surface of a high Al
steel sheet can be reduced by including one or both of Sb and Sn in
a total amount at least about 0.002 wt % and at most about 0.1 wt
%. Sb and Sn are elements for suppressing surface oxidation. In
particular, when at least one of Sb and Sn is added to the steel
which contains basic components of the present invention, the
surface oxidation can be more effectively suppressed so that a
material having excellent properties can be stably obtained. When
Sn and Sb are contained in an amount less than 0.002 wt %, they are
not effective to suppress the surface oxidation. On the other hand,
when the Sn and Sb content exceeds 0.1 wt %, significant amounts of
Sn and Sb are segregated to the grain boundary, which obstructs
their movement. Consequently, the grain growing property in the
stress relief annealing is deteriorated. Accordingly, the Sb and Sn
content is set to 0.1 wt % or less.
[0044] Although the components other than the above components are
not particularly limited in the present invention, it is preferable
to limit the contents of the following components.
[0045] P: 0.2 wt % or less
[0046] P can be added to improve the punching property.
[0047] When, however, P content exceeds 0.2 wt %, a cold rolling
property is deteriorated. Therefore, it is preferable to add P in
an amount of 0.2 wt % or less.
[0048] S: 0.01 wt % or less
[0049] S forms MnS precipitate together with Mn. MnS obstructs the
movement of a magnetic domain wall and the growth of grains, so the
magnetic properties deteriorate. It is preferable to set the S
content to 0.01 wt % or less, the less the better.
[0050] N: 0.01 wt % or less
[0051] N generates nitrides which obstruct the movement of the
magnetic domain wall and the growth of the grains, so the magnetic
properties deteriorate. It is preferable also to set the N content
to 0.01 wt % or less, the less the better.
[0052] O: 50 ppm or less
[0053] When O is contained in an amount of more than 50 ppm, O
generates oxides which obstruct the movement of the magnetic domain
wall and the growth of the grains, so the magnetic properties
deteriorate. It is preferable to set an C content to 50 ppm or
less, the less the better.
[0054] Cu: 0.05 wt % or less
[0055] When Cu is contained in an amount exceeding 0.05 wt %, Cu
generates Cu sulfide which obstructs the movement of the magnetic
domain wall and the growth of the grains, so the magnetic
properties deteriorate. It is preferable to set a Cu content to
0.05 wt % or less, the less the better.
[0056] Nb: 0.005 wt % or less
[0057] Nb generates Nb carbide or Nb nitride which obstructs the
movement of the magnetic domain wall and the growth of the grains,
so the magnetic properties deteriorate. It is preferable to set a
Nb content to 0.005 wt % or less, the less the better.
[0058] B: 0.0005 wt % or less
[0059] B forms BN which obstructs the movement of the magnetic
domain wall and the growth of the grains, so the magnetic
properties deteriorate. It is preferable to set a B content to
0.0005. wt %or less, the less the better.
[0060] It is preferable to reduce elements such as V, Mo, Cr and
the like which are concerned in the formation of the precipitates
in steel such as oxide, nitride, sulfide and the like as much as
possible among the inevitable impurities in combination with the
reduction of the amounts of O, N and C.
[0061] In the present invention,,the amount of oxygen on the metal
surface layer of a steel sheet should be set to 1.0 g/m.sup.2 or
less on the completion of finish annealing after the
above-mentioned components are adjusted as described above.
[0062] The inventors carefully considered the effect of Al as
described above. When the inventors reduced iron loss by increasing
the amount of Al, the inventors confirmed that prescribed iron loss
could be obtained according to conventional knowledge. However, it
was difficult to obtain stable magnetic properties because the
magnetic properties were deviated after the stress relief
annealing. At first, the inventors supposed that the deviation of
magnetic properties was caused by the effect of impure elements and
studied the effect of them. But studying the effect of impurity
elements did not account for the deviation. After various
examinations, the inventors made the new discovery that a nitriding
phenomenon occurred during the stress relief annealing and was a
main factor of the deviation. This nitriding phenomenon depends
upon surface scales which were produced when the sheet was
subjected to the finish annealing in a steel containing Al in a
large amount. More specifically, it was discovered that when the
amount of oxygen on the metal surface layer exceeded 1.0 g/m.sup.2,
nitriding was remarkably accelerated during the stress relief
annealing which was executed after the finish annealing. As a
result, the scales caused a magnetism deteriorating phenomenon and
they were liable to be made by the increased amount of Al in the
finish annealing. Although the reason why the magnetism
deteriorating phenomenon was caused was not apparent, it was
believed that the form of surface scales affected the nitriding
phenomenon as the amount of oxygen increased.
[0063] The conditions for manufacturing a non-oriented
electromagnetic steel sheet according to the present invention and
also the reasons for those conditions will now be described.
[0064] At first, a molten steel is manufactured according to a
conventional steel-making process such as a converter degassing
process or the like. The molten steel is made into a slab by a
continuous casting process or a casting ingot-making process. In
order to set the ratio of the number of REM-containing inclusions
coupled with nitride to the number of REM-containing inclusions
having a diameter of at least about 1 .mu.m to 40% or more, O
content in the steel to 15 ppm or less, a S content to 20 ppm or
less and a N content to 30 ppm or less, which are the conditions to
further improve a grain growing property in stress relief
annealing, the following treatment is preferred:
[0065] (1) the O content in the molten steel is reduced to 25 ppm
or less by sufficiently performing degassing-Al deoxidizing;
[0066] (2) the S content is adjusted to 40 ppm or less by adding a
desulfurizing agent; and
[0067] (3) thereafter the S content is suppressed to 20 ppm or
less, the O content is suppressed to 15 ppm or less and the N
content is adjusted to 10 ppm or less by adding REM.
[0068] Although REMs are elements that form oxides and sulfides, it
is especially likely to couple with the oxygen in a steel. Thus,
the O content remaining in the steel must be sufficiently reduced
that enough desulfurizing can occur by formation of REM-sulfide.
More specifically, the coupling of REM with O can be reduced
sufficiently if the O content is set to 25 ppm or less in the steel
before the REM is added. Consequently, the sulfide can be
effectively created by REM. Since the thus created REM sulfide and
oxide partly float at the time when REM is added, the O content in
the steel is finally reduced to 15 ppm or less. Note, since the S
content should be finally reduced to 20 ppm or less through the.
desulfurization performed by the sulfide created by REM, the S
content should be reduced to about 40 ppm before the addition of
the REM. A desulfurizing agent such as an ordinary flux or the like
can be used. On the other hand, although it is preferable to set
the N content to about 40 ppm or less before the addition of REM,
it is sufficient that it is finally adjusted to 30 ppm or less.
Note, when the Al content in the steel is increased according to
the present invention, there occurs a deoxidizing effect by Al,
whereby the amount of oxygen in the steel is reduced before the
addition of REM.
[0069] Another object of setting the O and S contents to 25 ppm or
less and 40 ppm or less, respectively, before the addition of REM
is to set the ratio of the number of the REM-containing inclusions
coupled with nitride to the number of REM-containing inclusions
having a diameter of at least about 1 .mu.m in the steel to 40% or
more. It is not apparent why the ratio of the number of the
REM-containing inclusions coupled with nitride to the number of
REM-containing inclusions having a diameter of at least about 1
.mu.m can be set to 40% or more by the adjustment of the S and O
contents before the addition of REM; however, it is believed that
the N content is relatively increased by the reduction of the total
amounts of O and S in the steel which are coupled with REM before
the addition of REM so that the ratio of inclusions which are
combined with Ti nitride and Zr nitride is increased. The Ti
nitride and the Zr nitride are created when they are coupled with
nitride during the steel being solidified and cooled. The reason
why the grain growing property is improved by setting the ratio of
the number of the REM-containing inclusions coupled with nitrogen
to the number of REM-containing inclusions having a diameter of at
least about 1 .mu.m is as described above.
[0070] Subsequently the slab is hot rolled. The slab can be either
reheated and then hot rolled or directly hot rolled without
reheating. When a particularly high magnetic flux density is
needed, the aggregate structure can be improved by coarsening
crystal grains of a hot-rolled sheet by self-annealing performed
during hot-rolled sheet coiling after hot rolling. Either box
annealing (for example, 850.degree. C..times.1 hour) or continuous
annealing (for example, 950.degree. C..times.2 minutes) is suitable
as the hot-rolled sheet annealing.
[0071] The hot-rolled sheet is annealed continuously in a short
time from the view of cost reduction and productivity improvement.
But a high magnetic flux density cannot be obtained by conventional
techniques when a hot-rolled sheet is annealed for a short time
such as a soaking time of 30 seconds, because the crystal grains of
the hot-rolled sheet are not sufficiently coarsened. However, since
the grain growing property is improved by the present invention, a
high magnetic flux density can be obtained even if the hot-rolled
sheet is annealed for a relatively short time. The hot-rolled steel
sheet must conventionally be annealed for at least about 5 minutes
to obtain an excellent magnetic flux density. On the other hand,
since the steel composition of the present invention is greatly
improved in the grain growing property, the hot-rolled sheet can be
annealed in a time shortened to 40 seconds or less. At that time,
when the annealing temperature is less than about 750.degree. C.,
the effect of coarsening crystal grains of the hot-rolled sheet by
the annealing is small, whereas when the annealing temperature
exceeds about 1150.degree. C., the annealing becomes uneconomical.
Thus, it is preferable to set the annealing temperature from
750.degree. C. or more to 1150.degree. C. or less.
[0072] Next, the hot-rolled sheet is made as a product either by
cold rolling the hot-rolled sheet once so that it has the thickness
of the product and then finish annealing the cold-rolled sheet, or
by cold rolling the hot-rolled sheet twice with intermediate
annealing performed therebetween and then finish annealing the
cold-rolled sheet.
[0073] The finish annealing should be performed so that the amount
of oxygen on the metal surface layer of the steel sheet is
controlled to 1.0 g/m.sup.2 or less after the completion of the
finish annealing as described above. The amount of the oxygen on
the metal surface layer of the steel sheet is controlled by
adjusting at least one of the dew point and the gas atmosphere
during the finish annealing. Incidentally, it is advantageous to
set an oxygen potential represented by P(H.sub.2O)/P(H.sub.2) to
0.7 or less by adjusting, for example, either or both of the dew
point and the gas atmosphere so that the finish annealing can be
performed at 600.degree. C. or more and 1100.degree. C. or less.
P(H.sub.2O) represents a partial pressure of H.sub.2O gas and
P(H.sub.2) represents a partial pressure of H.sub.2 gas. The amount
of oxygen on the metal surface layer of the steel sheet is mainly
controlled by the atmosphere such as the dew point, a gas
composition and the like, although it also depends on an annealing
time, and it is sufficient to regulate the annealing time from the
viewpoint of productivity. Since the steel composition according to
the present invention is excellent in the grain growing property,
it is possible to perform the finish annealing in a short time.
That is, the finish annealing can be performed in 15 seconds or
less in the temperature range from 750.degree. C. or more and
900.degree. C. or less. Needless to say, the amount of oxygen on
the metal surface layer should be adjusted to 1.0 g/m.sup.2 also at
the time. As to the other finish annealing conditions, any of the
conditions for manufacturing a non-oriented electromagnetic steel
sheet excellent in iron loss after stress relief annealing is
applicable.
[0074] It is possible to form an insulating film on the surface of
the steel sheet and perform skin pass rolling of 2-10% after the
finish annealing by known methods. The same effect can be obtained
even if these processes are added.
EXAMPLES
Example 1
[0075] A slab containing the components shown in Table 1 was made
by continuous casting after it was subjected to a
converter-degassing process. The resulting slab was reheated and
then hot rolled to form a hot-rolled steel sheet. Thereafter, the
hot-rolled steel sheet was annealed for 25 seconds at 950.degree.
C. and rolled to form a cold-rolled steel sheet having a thickness
of 0.5 mm by cold rolling including pickling. Subsequently, the
cold-rolled steel sheet was finish annealed for 14 seconds at
800.degree. C. in various annealing atmospheres and an insulating
film was formed. In the finish annealing, a dew point was adjusted
in a mixed gas atmosphere containing 35% H.sub.2 and 65% N.sub.2 so
that the amount of oxygen on the surface layer of the steel sheet
was controlled. Thereafter, stress relief annealing was performed
for 2 hours at 750.degree. C. in a dry nitrogen atmosphere, and
magnetic properties were measured by a 25 cm Epstein method. Table
1 shows the result of the analysis of components, magnetic
properties measurement and punching property evaluation,
respectively. The punching property is evaluated by the burrs of a
punched piece edge after punching to a shape having a 30 mm
diameter with an SKD metal mold is performed 200,000 times. When a
punched product had a burr exceeding 20 .mu.m, the product was
evaluated as an insufficiently punched product and provided with a
mark x.
[0076] Mechanical strength was evaluated by the yield strength
y.sub.p of a product sheet. A product having yield strength y.sub.p
exceeding 300 N/mm.sup.2 was marked with o which meant acceptable,
whereas a product having yield strength less than 300 N/mm.sup.2
was marked with x which meant unacceptable.
[0077] This is also applicable to the respective examples which
will be described later.
1 TABLE 1 Evaluation Evaluation Component (after finish Amount
W.sub.13/30 Punchi Mechanical annealing) of (after ng property C
REM Ti Zr oxygen stress proper Yp No. (ppm) Si Al Mn P (ppm) (ppm)
(ppm) Sb Sn (g/m.sup.2) relief) ty* (N/mm.sup.2)** Reference 1 28
0.6 0.69 0.3 0.03 5 10 20 tr tr 0.1 2.9 .smallcircle. x Comparative
2 25 0.8 0.79 0.31 0.02 11 10 21 tr tr 0.1 2.85 .smallcircle. x
Comparative 3 23 1.21 0.25 0.28 0.03 5 11 32 tr tr 0.1 2.88
.smallcircle. x Comparative 4 22 1.22 0.75 0.29 0.02 11 10 21 tr tr
0.1 2.59 .smallcircle. .smallcircle. Inventive 5 24 1.21 2.28 0.32
0.01 4 12 19 tr tr 0.09 2.43 .smallcircle. .smallcircle. Inventive
6 20 1.85 2.21 0.33 0.03 8 9 19 tr tr 0.15 2.28 .smallcircle.
.smallcircle. Inventive 7 29 2.2 1.59 0.31 0.04 10 9 26 tr tr 0.25
2.28 .smallcircle. .smallcircle. Inventive 8 31 2.2 3.2 0.32 0.02 8
9 22 tr tr 8.19 0.29 x .smallcircle. Comparative 9 33 3.8 1.8 0.36
0.04 12 9 21 tr tr -- Cold -- -- Comparative rolling Example
impossibl e 10 32 1.23 0.7 0.81 0.05 15 9 23 tr tr 0.17 2.49
.smallcircle. .smallcircle. Inventive 11 32 1.26 1.6 2.3 0.03 11 9
21 tr tr 0.16 2.26 x .smallcircle. Comparative 12 122 1.2 1.6 0.32
0.04 8 9 22 tr tr 0.15 3.13 .smallcircle. .smallcircle. Comparative
13 22 1.2 1.6 0.31 0.03 7 6 21 tr tr 0.1 2.30 .smallcircle.
.smallcircle. Inventive 14 24 1.21 1.58 0.31 0.03 6 7 21 tr tr 0.13
2.39 .smallcircle. .smallcircle. Inventive 15 20 1.19 1.57 0.20
0.04 8 8 60 tr tr 0.11 2.42 .smallcircle. .smallcircle. Inventive
16 26 1.2 1.59 0.23 0.02 8 7 90 tr tr 0.13 2.99 .smallcircle.
.smallcircle. Comparative 17 36 1.22 1.52 0.33 0.02 1 or 6 30 tr tr
0.16 2.35 .smallcircle. .smallcircle. Inventive 18 36 1.22 1.52
0.33 0.03 22 6 30 tr tr 0.16 2.39 .smallcircle. .smallcircle.
Inventive 19 25 1.21 1.61 0.31 0.05 6 30 tr tr 0.19 2.41
.smallcircle. .smallcircle. Inventive 20 26 1.19 1.63 0.32 0.01 100
6 30 tr tr 0.2 2.91 .smallcircle. .smallcircle. Comparative 21 20
1.2 1.59 0.33 0.03 7 13 23 tr tr 0.3 2.4 .smallcircle.
.smallcircle. Inventive 22 25 1.21 1.50 0.35 0.04 6 20 21 tr tr
0.25 2.59 .smallcircle. .smallcircle. Comparative 23 29 1.22 1.04
0.34 0.01 1 or 10 24 tr tr 0.24 2.79 .smallcircle. .smallcircle.
Comparative 24 23 1.22 1.50 0.32 0.03 6 10 25 tr tr 0.8 2.39
.smallcircle. .smallcircle. Inventive 25 22 1.21 1.09 0.33 0.04 11
10 26 tr tr 1.3 2.79 .smallcircle. .smallcircle. Comparative 26 21
1.21 1.56 0.34 0.02 11 10 21 0.05 tr 0.08 2.33 .smallcircle.
.smallcircle. Inventive 27 22 1.15 1.58 0.32 0.03 12 9 22 tr 0.05
0.08 2.34 .smallcircle. .smallcircle. Inventive 28 24 1.16 1.59 0.3
0.02 13 8 26 0.05 0.05 0.06 2.31 .smallcircle. .smallcircle.
Inventive *.smallcircle.: good x: poor **O: Y.sub.p .gtoreq. 300
N/mm.sup.2 x: Y.sub.p <300 N/mm.sup.2
[0078] It is found from Table 1 that the comparative examples Nos.
1 and 2 which contain Si in a small amount and the comparative
example No. 3 which contained Al in a small amount could not
provide low iron loss, the comparative example No. 9 which
contained Si in an amount exceeding 3.5 wt % was broken during cold
rolling, the comparative example No. 8 which contained Al in an
amount exceeding 3.1 wt % was insufficiently punched, the
comparative example No. 11 which contained Mn in an amount
exceeding 2.0 wt % was also insufficiently punched, and the
comparative example No. 12 which contained C in an amount exceeding
0.01 wt % had deteriorated magnetic properties.
[0079] The comparative examples Nos. 17 and 23 which contained no
REM had magnetic properties of a low level even if they contained
Ti and Zr in amounts of 15 ppm or less and 80 ppm or less,
respectively, whereas when they contained REM and the Ti and Zr
were contained therein in amounts of 15 ppm or less and 80 ppm or
less, respectively, a remarkable iron loss reducing effect could be
obtained. Note, the comparative examples Nos. 22 and 16 contained
Ti and Zr in amounts exceeding 15 ppm and 80 ppm, respectively, and
had deteriorated magnetic properties.
[0080] Next, the inventive examples Nos. 13, 24 and the comparative
example No. 25 whose amounts of oxygen on the metal surface layer
of the steel sheet exceeded 1.0 g/m.sup.2 had deteriorated magnetic
properties after the stress relief annealing. Furthermore, the
inventive examples Nos. 26-28 had improved mechanical properties
because an amount of scales was reduced by the addition of Sb
and/or Sn.
Example 2
[0081] A slab containing the components shown in Table 2 was made
by continuous casting after it was subjected to a
converter-degassing process. CaO was added after the addition of
Al. In the above processes, CaO was added into molten steel after
Al was added, then REM was added, and the molten steel was stirred.
Subsequently, the resulting slab was reheated and then hot rolled
to form a hot-rolled steel sheet. Thereafter, the hot-rolled steel
sheet was annealed in 20 seconds at 950.degree. C. and rolled to
form a cold-rolled steel sheet having a thickness of 0.5 mm by cold
rolling including pickling. Subsequently, the cold-rolled steel
sheet was finish annealed for 9 seconds at 800.degree. C. and made
into products. In the finish annealing, a dew point was adjusted in
a mixed gas atmosphere containing 35% H.sub.2 and 65% N.sub.2 so
that the amount of oxygen on the surface layer of the steel sheet
was controlled. After the components of the resulting products were
analyzed and the inclusions of the products were examined, test
pieces were sampled and subjected to stress relief annealing for 2
hours at 750.degree. C. in a dry nitrogen atmosphere, and magnetic
properties were measured. The result of the investigation is
summarized in Table 2.
2TABLE 2 Composition (after finish annealing inclu- REM of (after
Ref- C (**) REM Ti Zr S O N sions S O oxygen stress er- No. ppm Si
Al Mn P ppm ppm ppm (ppm) (ppm) (ppm) (%) (ppm) (ppm) (g/m.sup.2)
relief) ence 1 23 1.23 1.61 0.31 0.03 9 6 10 25 18 33 26 50 18 0.1
2.38 In- ven- tive product 2 21 1.22 1.59 0.29 0.03 10 6 10 15 8 21
46 36 16 0.1 2.25 In- ven- tive product 3 22 1.21 1.6 0.3 0.03 11 6
11 16 8 23 30 56 18 0.1 2.35 In- ven- tive product 4 23 1.22 1.61
0.31 0.03 10 6 12 17 9 25 33 38 21 0.1 2.34 Inv- ven- tive product
5 22 1.21 1.58 0.3 0.03 11 5 15 45 17 22 23 62 26 0.1 2.36 In- ven-
tive product 6 23 1.22 1.6 0.29 0.02 9 6 16 19 9 33 45 31 19 0.1
2.35 In- ven- tive product 7 23 1.21 1.6 0.29 0.02 9 5 16 19 19 35
45 36 18 0.1 2.36 In- ven- tive product (*) Composition ratio of
REM inclusions: the ratio of the number of REM-containing
inclusions coupled with nitride which are occupied in the
REM-containing inclusions having a diameter of at least about 1
.mu.m (**) Inevitably mixed component
[0082] It is found from Table 2 that an inventive product No. 2
which was more excellent in iron loss after stress relief annealing
could be obtained by setting the amounts of S, O and N to 20, 15
and 30 ppm, respectively and setting the ratio of the number of
nitride REM inclusions contained in REM inclusions to 40% or more.
It can be found that the ratio of the number of the nitride REM
inclusions was set to 40% or more by setting the amount of S to 40
ppm or less and the amount of O to 25 ppm or less before the
addition of the REM.
Example 3
[0083] A slab containing the components shown in Table 3 was made
by continuous casting after it was subjected to a
converter-degassing process., The resulting slab was reheated and
then hot rolled to form a hot-rolled steel sheet. Thereafter, the
hot-rolled steel sheet was annealed in 25 seconds at 950.degree. C.
and rolled to form a cold-rolled steel sheet having a thickness of
0.5 mm by cold rolling including pickling. Subsequently, the
cold-rolled steel sheet was finish annealed for 20 seconds at
810.degree. C. in various annealing atmospheres and an insulating
film was formed. In the finish annealing, a dew point was adjusted
in a mixed gas atmosphere containing 35% H.sub.2 and 65% N.sub.2 to
prepare various oxygen potentials P(H.sub.2O)/P(H.sub.2) so that
the amount of oxygen on the surface layer of the steel sheet was
controlled. Thereafter, stress relief annealing was performed for 2
hours at 750.degree. C. in a dry nitrogen atmosphere, and magnetic
properties were measured by the 25 cm Epstein method. Table 3 shows
the results of the analysis of components, magnetism measurement
and evaluated punching property, respectively.
3 TABLE 3 Composition Annealing (*) (**) tempera- Amount of (After
Punching REM Ti Zr N P(H.sub.2O) ture oxygen stress property No. Si
Al Mn (ppm) (ppm) (ppm) (ppm) /P(H.sub.2) (.degree. C.) (g/m.sup.2)
relief) *** Reference 1 1.19 1.61 0.31 8 10 25 30 0.002 810 0.02
2.37 .smallcircle. Inventive Example 2 1.19 1.61 0.31 8 10 25 31
0.05 810 0.09 2.37 .smallcircle. Inventive Example 3 1.19 1.61 0.31
8 10 25 30 0.2 810 0.12 2.38 .smallcircle. Inventive Example 4 1.19
1.61 0.31 8 10 25 60 0.25 810 0.21 2.42 .smallcircle. Inventive
Example 5 1.19 1.61 0.31 8 10 25 70 0.5 810 0.41 2.44 .smallcircle.
Inventive Example 6 1.19 1.61 0.31 8 10 25 33 0.2 810 0.2 2.34
.smallcircle. Inventive Example 7 1.21 2.1 0.32 9 12 25 91 0.8 810
1.1 2.88 .smallcircle. Comparative Example 8 1.19 1.61 0.31 8 10 25
150 0.9 810 1.3 2.91 .smallcircle. Comparative Example 9 1.19 1.61
0.31 8 10 25 80 0.4 950 0.6 2.43 .smallcircle. Inventive Example 10
1.19 1.61 0.31 8 10 25 30 0.4 1150 1.3 2.71 .smallcircle.
Comparative Example 11 1.19 1.61 0.31 8 10 25 62 0.4 680 0.2 2.47
.smallcircle. Inventive Example 12 1.19 1.61 0.31 8 10 25 30 0.4
550 0.2 2.91 x Comparative Example (*) N: Amount of nitriding after
stress relief annealing (**) Amount of oxygen on metal surface
layer is changed .smallcircle.: good x: poor
[0084] It can be found from Table 3 that when the amount of oxygen
on the metal surface layer of the steel sheet having been subjected
to the finish annealing exceeded 1.0 g/m.sup.2, iron loss was
deteriorated. It can be assumed that the deterioration was caused
by nitriding in the stress relief annealing because the amount of N
abruptly increased at the time. The amount of oxygen on the metal
surface layer could be controlled to a preferable range by
regulating the oxygen potentials P(H.sub.2O)/P(H.sub.2) of the
atmosphere to 0.7 or less.
Example 4
[0085] A slab containing the components shown in Table 4 was made
by continuous casting after it was subjected to a
converter-degassing process. The resulting slab was reheated and
then hot rolled to form a hot-rolled steel sheet. Thereafter, the
hot-rolled steel sheet was annealed for 25 seconds-5 hours at
950.degree. C. and rolled to form a cold-rolled steel sheet having
a thickness of 0.5 mm by pickling/cold rolling. Subsequently, the
cold-rolled steel sheet was finish annealed for 9 seconds or 30
seconds at 800.degree. C. and made into products. In the finish
annealing, a dew point was adjusted in a mixed gas atmosphere
containing 35% H.sub.2 and 65% N.sub.2 to prepare various oxygen
potentials P(H.sub.2O)/P(H.sub.2)=0.002 so that the amount of
oxygen on the surface layer of the steel sheet was controlled to
0.02 gm.sup.2. After the components of the resulting products were
analyzed and the inclusions of the products were examined, test
pieces were sampled and subjected to stress relief annealing for 2
hours at 750.degree. C. in a dry nitrogen atmosphere, and magnetic
properties were measured. The result of the investigation is
summarized in Table 4.
4 TABLE 4 Manufacturing conditions Finish Anneal- annealing
Properties Composition ing Tem- Tem- Amount W.sub.15/90 (*) for hot
per- per- of (after REM Ti Zr N rolled a- a- oxygen B.sub.50 stress
No. Si Al Mn (ppm) (ppm) (ppm) (ppm) sheet ture Time ture Time
(g/m.sup.2) (T) relief) Reference 1 1.19 1.61 0.31 8 10 25 31 Done
950.degree. C. 25 sec 800.degree. C. 30 0.02 1.74 2.38 Inventive
sec Example 2 1.19 1.61 0.31 8 10 25 29 Done 950 CC 1.5 800.degree.
C. 30 0.02 1.75 2.39 Inventive min sec Example 3 1.19 1.61 0.31 8
10 25 30 Done 950.degree. C. 6.5 800.degree. C. 30 0.02 1.75 2.42
Inventive min sec Example 4 1.19 1.61 0.31 8 10 25 28 Done
83.degree. C. 5 hr 800.degree. C. 30 0.02 1.76 2.3 Inventive sec
Example 5 1.19 1.61 0.31 8 10 25 32 Not -- -- 800 CC 30 0.02 1.72
2.45 Inventive done sec Example 6 119 1.61 0.31 8 10 25 30 Done
50.degree. C. 25 sec 800.degree. C. 9 sec 0.02 1.74 2.39 Inventive
Example 7 1.2 1.59 0.32 0 17 20 30 Done 950.degree. C. 25 sec
800.degree. C. 30 0.02 1.72 2.91 Example sec Example 8 1.2 1.59
0.32 0 17 20 31 Done 950.degree. C. 1.5 800.degree. C. 30 0.02 1.71
2.9 Com- min sec parative Example 9 1.2 1.59 0.32 0 17 20 32 Done
95.degree. C. 6.5 800.degree. C. 30 0.02 1.71 2.89 Com- min sec
parative Example 10 1.2 1.59 0.32 0 17 20 29 Done 830.degree. C. 5
hr 800.degree. C. 30 0.02 1.73 2.78 Com- sec parative Example 11
1.2 1.59 0.32 0 17 20 28 Not -- -- 800.degree. C. 30 0.02 1.7 2.99
Com- done sec parative Example 12 1.2 1.59 0.32 0 17 20 29 Done
950.degree. C. 25 sec 800 C 9 sec 0.02 1.74 2.92 Com- parative
Example (*) N: Amount of nitriding after stress relief
[0086] It is found from Table 4 that since the inventive examples
to which REM was added and in which the amounts of Ti and Zr were
reduced were excellent in stress relief annealing, the properties
thereof were not deteriorated even if the hot-rolled steel sheet
was annealed for a relatively short time of 40 seconds or less at
950.degree. C., and the products, which were excellent particularly
in magnetic flux density as compared with the products to which no
REM was added, could be obtained. Furthermore, even if the
annealing time was set to 30 seconds and 9 seconds, there was no
difference between the grain growing properties. Therefore, it is
possible to perform the finish annealing for a short time of 15
seconds or less which is not conventionally employed.
[0087] It was discovered that the hot-rolled steel sheet annealing
time and the finish annealing time could be reduced by the
improvement of the grain growing property which was achieved by
controlling the amounts of REM, Ti, Zr and the like. As a result,
there is a large possibility that productivity can be greatly
improved by the present invention.
[0088] Since the non-oriented electromagnetic steel sheet provided
by the present invention includes preferable mechanical properties
realized by the increase of Si and Al, the high magnetic flux
density can be maintained without sacrificing the punching property
as well as the very low iron loss can be obtained even after stress
relief annealing. Accordingly, the non-oriented electromagnetic
steel sheet of the present invention is ideal as a material for
small high-efficiency motors suitable for use in household
electrical appliances and the like.
* * * * *