U.S. patent application number 13/988738 was filed with the patent office on 2013-11-14 for manufacture method of oriented silicon steel having good magnetic performance.
The applicant listed for this patent is Deyang Hu, Jie Huang, Weizhong Jin, Guobao Li, Kanyi Shen, Huande Sun, Meihong Wu, Guohua Yang. Invention is credited to Deyang Hu, Jie Huang, Weizhong Jin, Guobao Li, Kanyi Shen, Huande Sun, Meihong Wu, Guohua Yang.
Application Number | 20130299049 13/988738 |
Document ID | / |
Family ID | 49547706 |
Filed Date | 2013-11-14 |
United States Patent
Application |
20130299049 |
Kind Code |
A1 |
Wu; Meihong ; et
al. |
November 14, 2013 |
MANUFACTURE METHOD OF ORIENTED SILICON STEEL HAVING GOOD MAGNETIC
PERFORMANCE
Abstract
A method for manufacturing a grain-oriented silicon steel having
excellent magnetic performance, comprising steps as follows
1)conventionally melting and casting into a steel blank; 2) heating
the steel blank and hot rolling the same into a strip of steel;
3)normalizing process; carrying out the normalizing process having
two stages, wherein the strip is firstly heated to
1100.about.1200.degree. C., then cooled to 900.about.1000.degree.
C. within 50.about.200 s; and next, the strip is rapidly cooled in
water having a temperature of 10-100; in this period, a tension
force is applied to the strip of steel, the strip of steel in the
temperature range of 900 .degree. C..about.500.degree. C. has a
stress of 1.about.200N/mm.sup.2; 4)cold rolling, i.e. carrying out
a primary cold rolling, or a double cold rolling with intermediate
annealing; 5)carrying out primary recrystallizing annealing, then
coating an annealing separator, whose main composition is MgO, to
carry out final product annealing comprising secondary
recrystallizing annealing and purifying annealing. The invention
optimizes the content and distribution of martensite in the steel
plate after normalization by adjusting the tension force applied to
the steel plate while normalization transformation, so as to make
the content of martensite in the range ensuring a better magnetic
performance of the final product and to optimize the magnetic
performance of final products.
Inventors: |
Wu; Meihong; (Shanghai,
CN) ; Jin; Weizhong; (Shanghai, CN) ; Sun;
Huande; (Shanghai, CN) ; Yang; Guohua;
(Shanghai, CN) ; Shen; Kanyi; (Shanghai, CN)
; Huang; Jie; (Shanghai, CN) ; Hu; Deyang;
(Shanghai, CN) ; Li; Guobao; (Shanghai,
CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wu; Meihong
Jin; Weizhong
Sun; Huande
Yang; Guohua
Shen; Kanyi
Huang; Jie
Hu; Deyang
Li; Guobao |
Shanghai
Shanghai
Shanghai
Shanghai
Shanghai
Shanghai
Shanghai
Shanghai |
|
CN
CN
CN
CN
CN
CN
CN
CN |
|
|
Family ID: |
49547706 |
Appl. No.: |
13/988738 |
Filed: |
April 28, 2011 |
PCT Filed: |
April 28, 2011 |
PCT NO: |
PCT/CN2011/073419 |
371 Date: |
July 23, 2013 |
Current U.S.
Class: |
148/546 |
Current CPC
Class: |
C22C 38/02 20130101;
C21D 8/005 20130101 |
Class at
Publication: |
148/546 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 26, 2010 |
CN |
20100561051.3 |
Claims
1. A method for manufacturing a grain-oriented silicon steel having
magnetic performance, comprising steps as follows: 1) melting and
casting to form a steel blank; 2) heating the steel blank and hot
rolling the steel blank to a strip of steel; 3) normalizing the
strip by heating the strip to 1100.about.1200.degree. C., and then
cooling the strip to 900.about.1000.degree. C. in 50.about.200 s;
and next, the rapidly cooling the strip in water having a
temperature of 10.about.100.degree. C.; in this period, applying a
tension force to the strip of steel, the strip of steel in a
temperature range of 900.degree. C..about.500.degree. C. having a
stress of 1.about.200N/mm.sup.2; carrying out a primary cold
rolling, or a double cold rolling with intermediate annealing; 5)
carrying out a primary recrystallizing annealing, then coating an
annealing separator, whose main composition is MgO to carry out
annealing to a final product, which annealing comprises secondary
recrystallizing annealing and purifying annealing.
2. The method for manufacturing a grain-oriented silicon steel
having magnetic performance according to claim 1, characterized in
that the tension force is applied to the strip of steel by
disposing a tension roller in a normalizing furnace or varying
front and rear tension rollers.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for manufacturing
a grain-oriented silicon steel, especially to a method for
manufacturing a grain-oriented silicon steel with excellent
magnetic performance.
DESCRIPTION OF THE PRIOR ART
[0002] Oriented silicon steel is an indispensable and important
soft magnetic alloy in electric, electronic and military
industries, which is mainly utilized for the iron core for
transformer, as well as the electric generator and large electric
machine and like. It is desired that the grain-oriented silicon
steel has excellent magnetic performance, especially degradation of
iron loss.
[0003] Oriented silicon steel may have excellent magnetic
performance in a rolling direction by utilizing a secondary
re-crystallizing technology, making Goss texture (Goss texture:
{110} means that crystal face is parallel to rolling plane,
<001>means that crystal direction is parallel to rolling
direction) to undergo an abnormal grain growth so as to merge
grains in other orientations.
[0004] A traditional method for manufacturing a grain-oriented
silicon steel having high magnetic induction is as follows. A steel
blank is heated to a temperature of 1350.degree. C. to 1400.degree.
C. in a special high temperature heating furnace, then the
temperature is maintained for more than 1 h, so as to facilitate
the sufficient solid solution of impurities of AlN, MnS or MnSe,
and then the steel blank is rolled, the roll-finishing temperature
is over 950.degree. C., the hot-rolled steel strip is coiled after
being rapidly splashed and cooled with water. In the following
normalizing process, fine and diffusive second phase particles
(namely, a grain growth inhibitor) are separated out from the body
of the steel, pickling is carried out to the hot-rolled steel after
normalization to remove a ferric oxide skin from its surface. After
being further cold rolled to a thickness of a final product, the
steel sheet is subjected to decarburizing and annealing process to
reduce [C] content in steel sheet to the extent that will not
affect the magnetic property of the final product (.ltoreq.30ppm),
and then an annealing separator, whose main composition is MgO, is
coated on the steel sheet to carry out high temperature annealing,
and the steel sheet is subjected to a secondary recrystallization
to form an under coating of Mg.sub.2SiO.sub.4 as well as purify the
steel, and finally, the steel sheet is coated with an insulation
coating, stretched and annealed, and thus the product of the
grain-oriented silicon steel with high performance that has high
magnetic induction, low iron loss and good insulation is
obtained.
[0005] The following problems come with the above manufacturing
method. [0006] 1. heating temperature is high, and burning loss of
the steel blank is great; [0007] 2. the heating furnace shall be
repaired frequently, and the production efficiency would be low;
[0008] 3. hot rolling temperature is high, and flange creak of hot
rolling is large.
[0009] In order to solve these problems, some foreign companies
grope and develop some methods for manufacturing grain-oriented
silicon steels at a relatively low temperature for heating the
steel blanks, for example:
[0010] 1.a method for manufacturing grain-oriented silicon steel at
an intermediate temperature
[0011] Some steel mills, such as Russian Novolipetsk Iron &
Steel Corporation (NLMK), and VIZ etc., utilize an intermediate
temperature oriented silicon steel manufacturing technology, the
steel-blank-heating temperature is 1200-1300.degree. C., chemical
composition contains a relatively high content of Cu (0.4%-0.7%),
while MN and CuS are used as inhibitors. This method can avoid
several problems due to heat steel blank in high temperature, the
disadvantage is that only general oriented silicon steels can be
manufactured.
[0012] 2. A method of heating steel blank and nitriding at low
temperature
[0013] When cold rolled sheets pass a decarburizing and annealing
furnace, NH.sub.3 is induced to nitride the interior of the steel
sheets to form an acquired obtained type inhibitor. By utilizing
this method, the steel-blank-heating temperature can be reduced to
be lower than 1250.degree. C., and the method can be utilize to
produce not only general oriented silicon steel but also oriented
silicon steel with high magnetic.
[0014] 3. A method of manufacturing grain-oriented silicon steels
without inhibitors
[0015] When in smelting, materials are controlled to be highly
purified, the contents of Se, S, N, O are controlled to be lower
than 30 ppm to eliminate any influence due to segregation of Se, S,
N, O etc. Thus, the grain-oriented silicon steel can be
manufactured by utilizing difference between travel speeds of high
energy grain boundary and other grain boundaries.
[0016] M. Barisoni et al. propose that steel sheet is cooled to
800.about.850.degree. C. at a speed of 20.degree. C./s is after
being normalized, then the steel sheet is quenched at a cooling
speed of 100.degree.C./s, so as to form dispersed martensite phase
whose volumetric percentage is about 8%, and hardness H.gtoreq.600
(the hardness of steel plate matrix H.gtoreq.230), as well as to
segregate out a great amount of AlN of about 10 nm. Martensite is
formed to make stored energy increased, and accordingly the stored
energy after cold rolling is increased, while the stored energy
will make {110} grain to recrystallize and grow more easily in
decarburizing and annealing process, and {110} composition after
subjected to decarburizing and annealing is strengthened, and thus
the magnetic performance of the final product is improved.
[0017] Martensite phase transition can be induced by rapidly
cooling (quenching), which is named as thermally induced martensite
phase transition. Also, Martensite phase transition can be induced
due to stress or strain, which is named as stress or strain induced
martensite phase transition. In view of free energy of phase
transition, work by which stress induces the martensite phase
transition is identical to the free energy variation by which the
phase transition is driven. Therefore, the driving force of
martensite phase transition is composed of two parts, i.e., a
chemical driving force and a mechanical driving force.
[0018] In a status of stress, the temperature of martensite phase
transition decreases. When at or below Curie temperature
(770.degree. C.), the grain-oriented silicon steel presents
spontaneous ferromagnetic elongation, which can partly counteract
automatic contraction in volume when cooling, so as to increase the
decrease of the temperature of martensite phase transition.
[0019] Martensite phase transition goes through two phases of
nucleation and growth.
[0020] As can be seen in accordance with the solid state phase
transition theory, by importing deformation stored energy,
nucleation rate of martensite is greatly increased, whose extent
may reach tens of order of magnitude to hundreds of order of
magnitude stored energy does not greatly influence the growing
speed of crystal nucleus of martensite.
[0021] In U.S. Pat. No. 3,959,033, an amount of martensite is
controlled by controlling normalizing process after hot rolling,
especially by controlling the cooling speed from
700.about.900.degree. C. to the room temperature in the normalizing
process, and finally, the magnetic performance of the final product
is improved. The disadvantage of this patent is that it is
difficult to achieve consistency in cooling speed in a direction of
plate thickness, which results in inhomogeneous of distribution of
martensite in a direction of plate thickness; because there exists
this inhomogeneous, it is difficult to achieve an effective control
to the amount of martensite. Further, in this patent, water is
utilized to control a cooling speed from 700.about.900.degree. C.
to the room temperature, firstly the control is likely to be
limited by site conditions, for example air temperature, damage or
obstruction of nozzle , which may render cooling speed unstable;
and secondly, the temperature of steel sheets cannot be accurately
measured due to artificial factors, it is difficult to achieve an
accurate control, and accordingly it is difficult to achieve a fine
tuning of cooling speed.
SUMMARY OF THE INVENTION
[0022] The object of the present invention is to provide a method
for manufacturing a grain-oriented silicon steel with excellent
magnetic performance, in which the content of martensite in steel
plate and distribution thereof after normalizing can be optimized
by adjusting the stress in the steel sheet in normalizing phase
transition, so as to enable the content of martensite is in the
range that a better magnetic performance of the final product can
be obtained, and an optimization in the magnetic performance of the
final product is realized.
[0023] In order to obtain the above-described object, the technical
solution of the present invention is that
[0024] A method for manufacturing oriented silicon steels with good
magnetic performance, comprising steps as follows: [0025] 1)
conventionally melting, casting to form a steel blank; [0026] 2)
heating the steel blank being heated and hot rolling the steel
blank to a strip of steel; [0027] 3) normalizing process [0028]
carrying out the normalizing process having two stages, wherein the
strip is firstly heated to 1100.about.1200.degree. C., then is
cooled to 900.about.4000 .degree. C. in 50.about.200s, next, the
strip is rapidly cooled in water having a temperature of
10.about.100.degree. C. ,in this period, a tension force is applied
to the strip of the steel, the strip of steel in a temperature
range of 900.degree. C..about.500.degree. C. has a stress of
1.about.200N/mm.sup.2; [0029] 4) cold rolling; [0030] carrying out
a primary cold rolling, or a double cold rolling with an
intermediate annealing; [0031] 5) carrying out primary
recrystallizing annealing, then coating an annealing separator,
whose main composition is MgO to carry out annealing to a final
product which annealing comprises secondary recrystallizing
annealing and purifying annealing.
[0032] Further, the tension force can be applied to the strip of
steel by disposing a tension roller within a normalizing furnace or
varying front and rear tension rollers.
[0033] In accordance with the present invention, by adjusting the
stress in the steel sheet in the normalizing phase transition, the
stress or strain induces the martensite phase to be transited, so
as to achieve reasonable and effective control on the amount of the
martensite in the steel sheet after normalizing, and finally, the
magnetic performance of the final product is improved. In
accordance with the present invention, a relatively homogeneous
martensite structure can be derived in the direction of the
thickness of the steel sheet.
[0034] Due to utilize a tension control, limit due to the site
conditions is fewer, for a sample sheet with same thickness, the
desired amount of martensite can be obtained stably, while the
tension control is quantified with a little human factor, so that
it is more easy to control accurately, and fine tuning can be
achieved.
[0035] By controlling the stress in the hot rolled sheet in the
normalizing phase transition, the amount of martensite after
normalizing is optimized so as to make the content of the
martensite in normalized steel sheet in a range that a better
magnetic performance of the final product can be obtained, and
finally, a better magnetic performance of the final product is
obtained.
[0036] The reasons why an appropriate content of martensite will be
helpful to improve magnetic performance B.sub.8of the final product
are as follows.
[0037] (1)Because there exists the martensite, which makes the
stored energy improved, after cold rolling, the stored energy is
increased, which facilitates the recrystallization and growth of
the (110) grain in decarburizing and annealing process, the content
of (110) composition increases, the magnetic performance can be
improved.
[0038] (2)Because there exists the martensite, after cold rolling
as well as decarburizing and annealling, the amount of high angle
grain boundary increases, which assists the Goss texture to merge
grains in other orientations, which facilitates the secondary
recrystallization.
[0039] (3) After the martensite is cold rolled as well as
decarburized and annealed, y fiber texture is formed in the
material, which facilitates the process of the secondary
recrystallization. Because of the above-analyzed relative factors,
improvement in degree of grain orientation of the final product can
be achieved, the magnetic performance B.sub.8 of the final product
is improved.
[0040] If the composition of steel sheets is identical, conditions
of manufacturing processes are identical and methods for measuring
martensite amount are identical, the amounts of martensite in the
sheets are identical. So, the relation between the martensite
amount and the magnetic performance of the final product can be
calculated in advance in accordance with the amount of martensite
in the steel sheet after normalizing and before cold rolling
measured by the same measuring method in the sample sheet that is
produced in advance, a target range of the amount of martensite in
the steel sheet after normalizing and before cold rolling can be
calculated.
[0041] As the means for controlling the amount of martensite, there
are the following three ways.
[0042] (1) The content of martensite is varied by varying the
stress in the steel sheet in the phase transition so as to vary the
nucleation number of martensite in the phase transition.
[0043] (2) The content of martensite is varied by varying a highest
temperature of normalizing to vary the amount of austenite at the
highest temperature.
[0044] (3) The content of martensite is varied by varying a speed
of secondary cooling when normalizing. The measured value of the
amount of martensite in the steel sheet after normalizing is
compared with a target value, according to the difference
therebetween, the stress (1.about.200N/mm.sup.2) of the steel plate
in the normalizing phase transition (in a range of 900.degree. C.
to 500.degree. C.) is varied by at least one of adjusting the
tension roller disposed within the furnace or varying winding
tension, a purpose of optimization of the content and distribution
of martensite in the steel sheet after normalizing can be achieved,
the amount of martensite is in the range that a better magnetic
performance of the final product can be obtained.
[0045] The steps (1), (2), (3) and (4) in the method in accordance
with the present invention are all general technical means for
manufacturing the grain-oriented silicon steel, and the description
thereof will be omitted.
[0046] The advantages of the present invention are as follows: In
accordance with the present invention, a reasonable and effective
control on the amount of martensite in the steel sheet after
normalizing is realized, which finally improves the magnetic
performance of the final product, by adjusting the stress in the
steel sheet in normalizing phase transition so that the tension
force or strain induces the phase of martensite to transit.
[0047] The present invention can obtain relatively homogeneous
martensite texture in the direction of plate thickness, and can
perform the fine tuning with respect to the content of martensite
as desired.
[0048] The present invention utilizes the tension control with few
limits due to the site condition, and with respect to the sample
plates having the same thickness, the desired amount of martensite
can be obtained stably; the tension control is more quantified,
influence of artificial factors is few, it is easy to conduct an
accurate control, and the fine tuning can be realized.
BRIEF DESCRIPTION OF DRAWINGS
[0049] FIG. 1 is a view showing relation between the content of
martensite (vol %) with magnetic performance B.sub.8 of a final
product with respect to a grain-oriented silicon steel, which is
normalized, in accordance with the present invention.
[0050] FIG. 2 is a schematic view showing the distribution of
martensite vs. sheet thickness at a transverse section of oriented
silicon steel in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0051] Hereinafter, the present invention will be described in
connection with embodiments.
Embodiment 1
[0052] Steel sheets, which contain a variety of compositions, are
normalized.
[0053] The main compositions of steel sheet are as shown in Table
1.
TABLE-US-00001 TABLE 1 (% by weight) NO. Si C Als N Mn S 1 3.03
0.0456 0.0264 0.0078 0.12 <0.0060 2 3.22 0.0507 0.0261 0.0081
0.12 <0.0060 3 3.41 0.0542 0.0269 0.0083 0.12 <0.0060
[0054] The steel sheet, which comprises the above-described
compositions, is heated to 1200.degree. C., which temperature is
held preserved for 180 minutes. Then, the steel sheet is directly
rolled to 2.0 mm. Two-stage normalizing process is carried out to
the sheet which is hot rolled. Firstly, the steel sheet is heated
to 1200.degree. C. , then cooled to 900.degree. C. within 200 s,
and next, the steel sheet is rapidly cooled in water having the
temperature of 100.degree. C. . Stress (1.about.200N/mm.sup.2) in
the steel sheet at the normalizing phase transition (in a range of
900.degree. C. to 500.degree. C.) can be varied by at least one of
adjusting a tension roller disposed within the furnace or varying
front and rear tension rollers, so as to optimize of the content
and distribution of martensite in the normalized sheet within a
range that a better magnetic performance range can be achieved.
[0055] After being pickled, a single-stage cold rolling is carried
out to the steel sheet for 5 rolling passes, wherein the third and
fourth passes are at 220.degree. C., and the steel sheet is pressed
to have a thickness of 0.30 mm. Decarburization and nitride
annealing is carried out to the cold rolled sheet at 850.degree. C.
After nitriding, an annealing separator, whose main composition is
MgO, is coated on the surface of the sheet, being heated to
1220.degree. C. in an atmosphere of 25% N.sub.2 and 75% H.sub.2,
then the atmosphere is changed to pure H.sub.2, and the sheet is
preserved in this temperature for 30 hours.
[0056] The content of martensite after normalization, tension force
applied to the steel sheet in phase transition and magnetic
performance are shown in Table 2.
TABLE-US-00002 TABLE 2 Applied Tension Force and Magnetic
Performance of the final Product Content of Applied martensite
tension force composition (% by area) (N/mm.sup.2) B.sub.8 (T) NO.
1 Comparative 2.9 0 1.87 Example 1 Embodiment 1 8.8 30 1.93 NO. 2
Comparative 3.2 0 1.87 Example 2 Embodiment 2 10.7 40 1.92 NO. 3
Comparative 25 60 1.86 Example 3 Embodiment 3 9.2 20 1.92
Embodiment 2
[0057] The main chemical compositions of the steel sheet are Si
3.05% by weight, C 0.060% by weight, Als 0.0290% by weight, N
0.0077% by weight, Mn 0.13% by weight and S<0.006% by
weight.
[0058] The steel sheet, which contains the above-described
compositions, is heated to 1200.degree. C., which temperature is
held for 180 minutes. Then, the steel sheet is directly rolled to
2.0 mm. Two-stage normalizing process is carried out to the hot
rolled sheet, firstly, the steel sheet is heated to 1100.degree.
C., and then cooled to 1000.degree. C. in 50 s, and next, the steel
sheet is rapidly cooled in water having the temperature of
50.degree. C. Stress (1.about.200N/mm.sup.2) in steel sheet in the
normalizing phase transition (in 900.degree. C. to 500.degree. C.)
can be varied by at least one of adjusting a tension roller
disposed within furnace or varying a winding tension, so as to
optimize the content and distribution of martensite in the
normalized sheet within a range that a better magnetic performance
range can be achieved.
[0059] After being pickled, a single-stage cold rolling is carried
out to the steel sheet for 5 rolling passes, wherein the third and
fourth passes are at 220.degree. C., and the steel sheet is pressed
to have a thickness of 0.30 mm. Decarburization and nitride
annealing is carried out to the cold rolled strip at 850.degree. C.
.After nitriding, an annealing separator, whose main composition is
MgO, is coated on the surface of the sheet, being heated to
1220.degree. C. in an atmosphere of 25% N.sub.2 and 75% H.sub.2,
then the atmosphere is changed to pure H.sub.2, and the sheet is
preserved in this temperature for 30 hours.
[0060] The content of martensite after normalization, tension force
applied to the steel sheet in phase transition and magnetic
performance are shown in Table 3.
TABLE-US-00003 TABLE 3 Applied Tension Force and Magnetic
Performance of the final Product Applied tension Content of force
martensite (N/mm.sup.2) B.sub.8 (T) Comparative 20 50 1.86 Example
Embodiment 8 15 1.92
Embodiment 3
[0061] The main chemical compositions of the steel sheet are Si 2.9
wt %, C 0.048 wt %, Als 0.0255 wt %, N 0.0073 wt %, Mn 0.10 wt %
and S<0.006 wt %.
[0062] The steel sheet, which contains the above-described
compositions, is heated to 1200.degree. C., which temperature is
held for 180 minutes. Then, the steel sheet is directly rolled to
2.0 mm. Two-stage normalizing process is carried out to the hot
rolled sheet, firstly, the steel sheet is heated to 1100.degree.
C., and then cooled to 900.degree. C. in 100 s. Next, the steel
sheet is quick cooled in water having the temperature of 80.degree.
C. Stress (1.about.200N/mm.sup.2) in the steel sheet in the
normalizing phase transition (in the range of 900.degree. C. to
500.degree. C.) can be varied by at least one of adjusting a
tension roller disposed within furnace or varying a winding
tension, so as optimize the content and distribution of martensite
in the normalized sheet within a range that a better magnetic
performance can be achieved.
[0063] After the steel sheet is pickled, a single-stage cold
rolling is carried out to the sheet for 5 rolling passes, wherein
the third and fourth passes are at 220.degree. C., and the steel
sheet is pressed to have a thickness of 0.30 mm. Decarburization
and nitride annealing is carried out to the cold rolled sheet at
850.degree. C. .After nitriding, an annealing separator, whose main
composition is MgO, is coated on the surface of the sheet, being
heated to 1220.degree. C. in an atmosphere of 25% N.sub.2 and 75%
H.sub.2, then the atmosphere is changed to pure H.sub.2, and the
sheet is preserved in this temperature for 30 hours.
[0064] The content of martensite after normalization, the tension
force applied to the steel sheet in the phase transition and
magnetic performance are shown in Table 4.
TABLE-US-00004 TABLE 4 Applied Tension Force and Magnetic
Performance of the final Product Applied tension Content of force
martensite (N/mm.sup.2) B.sub.8 (T) Comparative 1.5 0 1.85 Example
Embodiment 9 18 1.93
Embodiment 4
[0065] The main chemical compositions of the steel sheet are Si
3.41% by weight, C 0.0542% by weight, Als 0.0269% by weight, N
0.0083% by weight, Mn 0.12% by weight and S<0.006% by
weight.
[0066] The steel sheet, which contains the above-described
compositions, is heated to 1200.degree. C., which temperature is
held for 180 minutes. Then, the steel sheet is directly rolled to
2.0 mm. Normalizing annealing is carried out by means of the method
described below, respectively.
[0067] Firstly, the steel sheet is heated to 1180.degree. C., and
then cooled to 920.degree. C. in 200 s, and next, the steel sheet
is rapidly cooled in water having a temperature of 100 .degree.
C.
[0068] (1) a tension force of 60N/mm.sup.2 is applied to the steel
sheet during the cooling period (Comparative Example);
[0069] (2)a tension force of 20N/mm.sup.2 is applied to the steel
sheet during a cooling period (900 .degree. C.-500 .degree. C.). ,
so as to keep the content of normalized martensite in a range that
the excellent magnetic performance of the final product can be
obtained (Embodiment).
[0070] After the steel sheet pickled, the single-stage cold rolling
is carried out to the sheet for 5 rolling passes, wherein the third
and fourth passes are at 220.degree. C., the steel sheet is pressed
to have a thickness of 0.30 mm. Decarburization and nitride
annealing are carried out to the cold rolled strip at 850.degree.
C. .After nitriding, an annealing separator, whose main composition
is MgO, is coated on the surface of the sheet, being heated to
1220.degree. C. in an atmosphere of 25% N.sub.2 and 75% H.sub.2,
then the atmosphere is changed into pure H.sub.2, and the sheet is
preserved in the temperature for 30 hours.
[0071] Results are shown in Table 5.
TABLE-US-00005 TABLE 5 Applied Tension Force and Magnetic
Performance of the final Product Applied tension Content of force
martensite (N/mm.sup.2) B.sub.8 (T) Comparative 25 60 1.86 Example
Embodiment 9.2 20 1.92
[0072] The distributions of martensite vs. the sheet thickness in
the transverse section of Comparative Example and Embodiment are
shown in FIG. 2.
[0073] As can be seen from the figure, a relatively homogeneous
martensite texture in the sheet-thickness direction can be obtained
by means of the tension control. For a sample plate with the same
thickness, the desired amount of martensite can be obtained stably;
a better magnetic performance of the final product can be
obtained.
* * * * *