U.S. patent application number 12/300982 was filed with the patent office on 2009-07-23 for steel plate and steel plate coil.
This patent application is currently assigned to National Institute for Materials Science. Invention is credited to Tadanobu Inoue, Yuuji Kimura, Kotobu Nagai.
Application Number | 20090185943 12/300982 |
Document ID | / |
Family ID | 38693708 |
Filed Date | 2009-07-23 |
United States Patent
Application |
20090185943 |
Kind Code |
A1 |
Inoue; Tadanobu ; et
al. |
July 23, 2009 |
STEEL PLATE AND STEEL PLATE COIL
Abstract
In a steel plate having tensile strength of 1 GPa or more at the
normal temperature, the steel plate is made, stress decrement (SD)
after uniform elongation in the stress-strain diagram obtained by
using a tabular specimen of which is not less than
1.8.times.10.sup.2 MPa, yield ratio (YR) of which is preferably not
lower than 0.7, further preferably not lower than 0.8, and matrix
structure of which is composed of martensite or bainite, or mixture
of martensite and bainite substantially without proeutectoid
ferrite, whereby the tough-ductility is improved on a practically
utilizable level, and the steel plate becomes possible to be
supplied in large quantities by winding up into a coil.
Inventors: |
Inoue; Tadanobu; (Ibaraki,
JP) ; Kimura; Yuuji; (Ibaraki, JP) ; Nagai;
Kotobu; (Ibaraki, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
National Institute for Materials
Science
Tsukuba-shi, Ibaraki
JP
|
Family ID: |
38693708 |
Appl. No.: |
12/300982 |
Filed: |
April 12, 2007 |
PCT Filed: |
April 12, 2007 |
PCT NO: |
PCT/JP2007/058046 |
371 Date: |
February 18, 2009 |
Current U.S.
Class: |
420/8 |
Current CPC
Class: |
B21B 3/00 20130101; C21D
9/46 20130101; C22C 38/44 20130101 |
Class at
Publication: |
420/8 |
International
Class: |
C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2006 |
JP |
2006-137235 |
Claims
1. A steel plate characterized in that tensile strength is not
lower than 1 GPa at the normal temperature, and stress decrement
(SD) after uniform elongation in a stress-strain diagram obtained
by a tensile test using a tabular specimen is not lower than
1.8.times.10.sup.2 MPa.
2. The steel plate as set forth in claim 1, yield ratio (YR) of
which is not lower than 0.7.
3. The steel plate as set forth in claim 1, yield ratio (YR) of
which is not lower than 0.8.
4. The steel plate as set forth in claim 1, matrix structure of
which is composed of at least one of martensite and bainite, and
does not substantially contain proeutectoid ferrite.
5. A steel plate coil made by winding up the steel plate as set
forth in claim 1.
6.-11. (canceled)
12. The steel plate as set forth in claim 2, the matrix structure
of which is composed of at least one of martensite and bainite, and
does not substantially contain proeutectoid ferrite.
13. The steel plate as set forth in claim 3, the matrix structure
of which is composed of at least one of martensite and bainite, and
does not substantially contain proeutectoid ferrite.
14. A steel plate coil made by winding up the steel plate as set
forth in claim 2.
15. A steel plate coil made by winding up the steel plate as set
forth in claim 3.
16. A steel plate coil made by winding up the steel plate as set
forth in claim 4.
17. A steel plate coil made by winding up the steel plate as set
forth in claim 12.
18. A steel plate coil made by winding up the steel plate as set
forth in claim 13.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a high-utilizable
high-strength steel plate having tensile strength of 1 GPa or
above, a steel plate coil obtained by winding up the steel plate of
this kind and a method for producing such the steel plate and the
steel plate coil.
DESCRIPTION OF THE RELATED ART
[0002] A high-strength steel plate with tensile strength of several
hundreds of MPa is suggested (for example, refer to patent
documents 1 and 2).
[0003] Until now, workability of the steel plate has been
considered to be excellent in a steel well-balanced between
strength and elongation, and research & development have been
continued with a target for increasing total elongation in a case
of steel excellent in the tensile strength.
Patent document 1: JP, 06-35619,B Patent document 2: JP,
2005-97725,A
[0004] However, it has been impossible to obtain the steel plate
with practical tough-ductility (property combined with ductility
and toughness) and tensile strength of 1 GPa or more at the
ordinary temperature up to the present.
[0005] In general, the steel plate is supplied to the succeeding
process as a hot-rolled coil, and put to practical application
after cold working, cold rolling, and further heat treatment such
as annealing or so.
[0006] However, in the high-strength steel plate of this kind, a
considerable amount of cost and time are expended in process until
the rolling, and there is scatter in the strength and yield rate is
also inferior. Accordingly, it is the present situation that the
high-strength steel plate is not in a stage of mass application
even now.
SUMMARY OF THE INVENTION
[0007] The present invention is made in view of the aforementioned
situation of the conventional high-strength steel plate, and it is
an object of this invention to provide a steel plate having
tough-ductility for practical use and possible to be supplied in
large quantities as a steel plate coil and a method for producing
the same.
[0008] The present inventors, as a result of repeating assiduous
studies for achieving the above object, took note of stress
decrement (SD) after the uniform elongation in a stress-strain
diagram obtained by the tensile test using tabular specimens cut
out from steel plates, and found the object to be accomplished by
controlling the SD so as not to be less than predetermined value or
more.
[0009] The present invention is based on the above-mentioned
findings, the steel plate according to this invention is a steel
plate having the tensile strength not lower than 1 GPa at the
normal temperature, and characterized in that the stress decrement
(SD) after uniform elongation in a stress-strain diagram obtained
by a tensile test using a tabular specimen is not lower than
1.8.times.10.sup.2 MPa. Preferably, the steel plate is
characterized by having a yield ratio (YR) of 0.7 or above, more
preferably 0.8 or above, and further characterized by having matrix
structure composed of martensite or bainite, or mixture of
martensite and bainite, and not substantially containing
proeutectoid ferrite.
[0010] In the method for producing the steel plate according to
this invention, the steel plate is rolled with minimum strain rate
of not lower than 1.times.10/s at the same time of maintaining the
steel plate in an austenitic phase, and quenched to a temperature
lower than Ar1 point and not lower than Ms point, and preferably
quenched to a temperature lower than Ar1 point and not lower than
Ms point at average cooling rate of 1.times.10.degree. C./s or
more. Furthermore, the method is characterized in that obtained
steel plate is subjected to tempering or annealing at a temperature
of Ac1 point or below.
[0011] The steel plate coil according to this invention is made by
winding up the aforementioned steel plate. In the method for
producing the steel plate coil according to this invention, the
steel plate is rolled with minimum strain rate of not lower than
1.times.10/s at the same time of maintaining the steel plate in an
austenitic phase, and quenched to a temperature lower than Ar1
point and not lower than Ms point just before winding, and
preferably quenched to a temperature lower than Ar1 point and not
lower than Ms point at average cooling rate of 1.times.10.degree.
C./s or more from the exit of rolling. Furthermore, the method is
characterized in that obtained steel plate coil is subjected to
tempering or annealing at a temperature of Ac1 point or below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic diagram showing a stress-strain
diagram obtained by using tabular specimen.
[0013] FIG. 2 is a perspective schematic view illustrating rolling
process of the steel plate coil in examples.
[0014] FIG. 3 is a view illustrating cutting points of tensile
specimens for examining tensile strength variation in the cross
direction of the steel plate coil.
[0015] FIG. 4 (a).about.(c) are magnified photographs showing
fractured portions of tensile specimens according to examples 1, 6
and 16, respectively.
[0016] FIGS. 5 (d) and (e) are magnified photographs showing
fractured portions of tensile specimens according to comparative
examples 1 and 6 respectively, and FIG. 5 (f) is an illustration
showing camera angle of these photographs.
DETAILED DESCRIPTION OF THE INVENTION
[0017] Hereinafter, the steel plate and steel plate coil according
to this invention will be explained in detail together with a
production method thereof.
[0018] FIG. 1 is a schematic diagram showing a stress-strain
diagram, which is obtained by using tabular specimen (for example,
No. 5 specimen and No. 13 specimen specified in JIS Z 2201) cut out
from the steel plate according to this invention. In this diagram,
stress decrement (SD) is defined as the difference between tensile
strength (TS) and rapture stress.
[0019] In the steel plate according to this invention, the stress
decrement (SD) until the specimen is fractured after indicating
uniform elongation in the above-mentioned stress-strain diagram
takes value of 1.8.times.10.sup.2 MPa or more.
[0020] The steel plate according to this invention is excellent in
the tough-ductility (to be excellent in ductility and toughness at
the same time) as compared with high-strength TRIP (Transformation
Induced Plasticity) steel, as the SD value of the steel plate is
not lower than 1.8.times.10.sup.2 MPa.
[0021] Further, it becomes possible to wind up the steel plate into
a coil in the final process of the steel making, and becomes
possible to recoil (to roll again steel plate coiled once,
concretely skin pass, cold rolling and the like, for example)
afterward, therefore providability of the steel plate is remarkably
improved, and it becomes possible to supply the steel plates in
large quantities.
[0022] Although it is not always clear as to a reason for obtaining
such the effects in this invention, the SD value indicating stress
decrement after the uniform elongation of the steel plate according
to this invention is much better than that of conventional steels,
so that it seems that the aforementioned SD value has a
considerable influence on the tough-ductility in the steel plate
having tensile strength of 1 GPa or above.
[0023] In the conventional high-strength steel plates, much
importance has been attached to formability of plate materials
having TS (tensile strength) of 1 GPa or below, and special
attention has been paid to improve the uniform elongation by
strenuously reducing yield ratio YR=YS (yield strength: 0.2% yield
stress or lower yield point)/TS to a value lower than 0.7. However,
it is possible to increase the yield ratio YR to 0.7 or more in the
steel plate having TS of 1 GPa or above in the steel plate
according to this invention, whereby it becomes possible to improve
proof stress, impact absorbency and so on of the steel plate.
[0024] As a method for obtaining the steel plate of this invention
which is excellent in the tough-ductility by increasing SD value
and YR according to grain refining and fine dispersion of the
second phase particles such as carbides, a method is exemplified
for rolling the steel plate with predetermined strain rate of not
lower than 1.times.10/s, for example, at the same time of
maintaining the steel plate at a temperature within an austenite
range, and quenching the steel plate to a temperature lower than
Ar1 point and not lower than Ms point at average cooling rate of
1.times.10.degree. C./s or more desirably, thereby making matrix
structure of the steel plate so as to be composed of martensite
and/or bainite substantially without containing proeutectoid
ferrite.
[0025] The steel plate of this kind is desirable to be wound into
the form of steel plate coil, whereby it becomes possible to supply
the high strength steel plates in large quantities, and enable the
application in large quantities for various purposes.
[0026] Furthermore, such the steel plate coil is preferable to be
manufactured by winding up the steel plate immediately after
rolling, and possible to be manufactured by rolling with strain
rate of not lower than 1.times.10/s at the same time of maintaining
the steel plate in the austenite range, and quenching the steel
plate to a temperature lower than Ar1 point and not lower than Ms
point just before the winding.
[0027] In the case of quenching the steel plate to the temperature
lower than Ar1 point and not lower than Ms point just before the
winding, it is desirable to carry out the quenching at an average
cooling rate of 1.times.10.degree. C./s or above. In such the
manner, it is possible to obtain the high strength coiled steel
plate without dispersion of the strength in the cross direction,
and possible to further facilitate the application of the steel
plate.
[0028] It is preferable to further subject the steel plate and the
steel plate coil obtained by the aforementioned method to tempering
or annealing at a temperature of Ac1 point or below, thereby
improving the aforementioned properties and increase stability of
these properties.
[0029] It is necessary to pay attention not only to the
manufacturing condition but also to chemical compositions of the
steel in order to make the proeutectoid ferrite disappear and
obtain the matrix structure merely composed of martensite and/or
bainite with more certainty.
[0030] In this invention, it is certified that the directing steel
is easy to be obtained by rolling the steel containing not lower
than 0.1% of C, not lower than 0.15% of Mo, not lower than 0.5% of
Cr, not lower than 0.3% of Ni under the aforementioned conditions,
as a result of observing the difference of effects produced by the
rolling conditions concerning many steels having various chemical
compositions.
EXAMPLES
[0031] Hereinafter, this invention is further explained in detail
on basis of examples, but this invention is not limited to these
examples.
[0032] In the inventive examples and comparative examples described
below, it is necessary to allow for errors of at least 10.degree.
C. above and below in the measurement of temperatures of every kind
at the time of rolling, therefore it is proper to indicate the
respective temperature as a temperature zone of 25.degree. C. in
view of actual condition in the measurement of temperatures. Also
concerning the measured value of stress in the tensile test, it is
suitable to consider the measured value of not higher than 10 MPa
to be in the error range according to accuracy of specimen and
other measuring conditions.
[0033] In the finish rolling process as shown in FIG. 2, rolled
coils and steel plates were manufactured by rolling 12 kinds of
steel ingots having chemical compositions (mass %) shown in Table 1
under rolling conditions shown in Table 2.
[0034] At the time of manufacturing the steel plates, the rolling
was carried out by a rolling simulator of 1 stand general type. In
the Table 1, the respective steels contain Si, Mn, P, S and Cu
within ordinal ranges in addition to compositions described in the
table.
TABLE-US-00001 TABLE 1 Chemical Compositions (%) Steel C Ni Cr Mo
Fe A 0.38 1.02 1.94 0.65 bal. B 0.19 1.00 1.98 0.62 bal. C 0.40
2.01 2.00 1.03 bal. D 0.40 2.01 2.00 1.01 bal. E 0.21 2.02 2.01
1.01 bal. F 0.22 2.00 2.00 1.00 bal. G 0.34 0.09 1.03 0.17 bal. H
0.29 1.00 1.98 0.61 bal. I 0.22 -- 2.02 1.02 bal. J 0.39 1.00 2.03
1.03 bal. K 0.39 0.99 2.02 0.51 bal. L 0.40 1.00 2.00 0.70 bal.
[0035] The combination of respective steels and rolling conditions
is shown in Table 2 together with measured values of minimum strain
rates at the time of finish rolling, respective temperatures at the
inlet (start) and the outlet (end) of rolling, temperatures at the
position just before the winding and average cooling rates.
TABLE-US-00002 TABLE 2 Minimum Temp. just Starting Test strain
Temp. at Temp. at Cooling rate (.times.10.degree. C./s) before
Temp. of piece rate inlet outlet from outlet from outlet winding
cooling Tempering No. Steel Form (/s) (.times.25.degree. C.)
(.times.25.degree. C.) to winding down to 500.degree. C.
(.times.25.degree. C.) (.times.25.degree. C.) condition Remarks 1 A
coil 10 42 32 4 -- 21 -- -- Comp. ex. 1 2 A coil 6 42 33 5 -- 21 --
-- Comp. ex. 2 3 A coil 6 42 33 5 -- 20 -- -- Comp. ex. 3 4 A coil
6 42 36 5 -- 22 -- -- Comp. ex. 4 5 C plate 9 36 26 -- 0.4 -- 20 --
Comp. ex. 5 6 D plate 9 36 27 -- 0.4 -- 20 -- Comp. ex. 6 7 E plate
9 36 27 -- 0.4 -- 20 -- Comp. ex. 7 8 F plate 9 36 27 -- 0.4 -- 20
-- Comp. ex. 8 9 A coil 20 42 37 9 -- 14 -- Example 1 10 A coil 20
42 37 10 -- 13 -- Example 2 11 A coil 20 42 37 10 -- 13 500.degree.
C. .times. 1 h Example 3 12 A coil 20 42 37 10 -- 9 500.degree. C.
.times. 1 h Example 4 13 A coil 20 42 37 10 -- 13 600.degree. C.
.times. 1 h Example 5 14 A coil 20 42 37 10 -- 9 600.degree. C.
.times. 1 h Example 6 15 B coil 20 42 37 9 -- 14 -- Example 7 16 B
coil 20 42 37 9 -- 10 -- Example 8 17 B coil 20 42 37 9 -- 14
500.degree. C. .times. 1 h Example 9 18 B coil 20 42 37 9 -- 14
600.degree. C. .times. 1 h Example 10 19 G coil 20 42 32 6 -- 16 --
Example 11 20 C plate 10 44 30 -- 1 -- 19 -- Example 12 21 D plate
10 44 33 -- 1 -- 19 -- Example 13 22 E plate 10 44 32 -- 1 -- 19 --
Example 14 23 F plate 10 44 33 -- 1 -- 19 -- Example 15 24 H plate
20 36 28 -- 1 -- 14 -- Example 16 25 I plate 10 32 25 -- 1 -- 19 --
Example 17 26 I plate 10 36 27 -- 1 -- 19 -- Example 18 27 J plate
30 35 34 -- 1 -- 16 -- Example 19 28 K plate 30 35 33 -- 1 -- 16 --
Example 20 29 L plate 30 35 33 -- 1 -- 16 -- Example 21 30 B coil
20 43 38 6 -- 22 -- -- Example 22 31 B coil 20 43 39 6 -- 23 -- --
Example 23 32 B coil 20 43 38 6 -- 22 -- 550.degree. C. .times. 1 h
Example 24 33 B coil 20 43 39 6 -- 23 -- 550.degree. C. .times. 1 h
Example 25
[0036] In the examples of this invention, the rolled coils were
manufactured by cooling at average cooling rate not lower than
1.times.10.degree. C./s down to temperature just before the winding
from outlet temperature so as to wind up the rolled steel plates at
the temperature lower than Ar1 point and not lower than Ms
point.
[0037] Further, in examples (inventive examples) 3 to 6, 9, 10, 24
and 25, obtained rolled coils were subjected to tempering at
500.degree. C..about.600.degree. C. (not higher than Ac1 point) for
1 hour.
[0038] On the other side, in examples (inventive examples) 12 to
18, and comparative examples 5 to 8, steel materials with initial
thickness of 17 mm were rolled down to 3 mm through 5 passes in the
austenite range. In the examples 12 to 18, the steel plates were
manufactured by cooling them slowly down to the room temperature
after quenching them dawn to the temperature lower than Ar1 point
and not lower than Ms point at cooling rate of not lower than
1.times.10.degree. C./s. In examples (inventive examples) 19 to 21,
steel materials with initial thickness of 3.5 mm were rolled down
to 1.9 mm through 1 pass in the austenite range, and the steel
plates were manufactured by cooling them slowly down to the room
temperature after quenching them dawn to the temperature lower than
Ar1 point and not lower than Ms point at cooling rate of not lower
than 1.times.10.degree. C./s.
[0039] The rolled coils and the steel plates manufactured under the
condition shown in Table 2 did not substantially contain
proeutectoid ferrite, and matrix structures of them were composed
of martensite or bainite, or mixture of martensite and bainite.
[0040] No. 5 specimens (25 mm width) specified in JIS Z 2201 were
cut out from respective steel plate coils manufactured by the
aforementioned procedure, and No. 13B specimens (12.5 mm width)
specified similarly in JIS Z 2201 were cut out from respective
steel plate, from the center of width along the rolling direction
of respective plates. By using these specimens, tensile tests were
carried out in conformity to JIS Z 2241, thereby obtaining
stress-strain diagrams as shown in FIG. 1. Furthermore, yield
strength (YS: 0.2% proof stress or lower yield point), tensile
strength (TS) and stress decrement (SD) after uniform elongation
were obtained from the stress-strain diagram, respectively, and
yield ratio (YR) was calculated.
[0041] In order to further ascertain the level of dispersion of the
tensile strength, tensile specimens were cut out from the
respective positions of 1/2 W, 1/4 W and 1/8 W (W: width of rolled
coil) along the rolling direction as shown in FIG. 3, and tensile
tests were carried out by using these specimens. In this time,
dispersion of the tensile strength is given as a difference between
maximum and minimum values of obtained tensile strength. These
characteristic values are arranged respectively as shown in Table
3.
TABLE-US-00003 TABLE 3 Test Results of tensile test piece Thickness
Specimen Tempering TS SD Tough- Dispersion of No. Steel Form (mm)
JIS condition (GPa) YR (.times.10.sup.2 MPa) ductility TS (MPa)
Remarks 1 A coil 1.9 No. 5 -- 1.7 0.6 0.3 No good 87 Comp. ex. 1 2
A coil 1.9 No. 5 -- 1.7 0.6 0.6 No good 210 Comp. ex. 2 3 A coil
1.9 No. 5 -- 1.6 0.6 0.3 No good 64 Comp. ex. 3 4 A coil 1.9 No. 5
-- 1.5 0.6 0.7 No good 126 Comp. ex. 4 5 C plate 2.0 No. 13B -- 1.5
0.7 1.7 No good -- Comp. ex. 5 6 D plate 2.0 No. 13B -- 1.6 0.7 1.2
No good -- Comp. ex. 6 7 E plate 2.0 No. 13B -- 1.3 0.6 1.7 No good
-- Comp. ex. 7 8 F plate 2.0 No. 13B -- 1.4 0.6 1.7 No good --
Comp. ex. 8 9 A coil 2.1 No. 5 -- 1.6 0.9 3.8 Good 9 Example 1 10 A
coil 2.0 No. 5 -- 1.4 0.8 2.2 Good 8 Example 2 11 A coil 2.0 No. 5
500.degree. C. .times. 1 h 1.3 0.8 2.9 Good 7 Example 3 12 A coil
1.9 No. 5 500.degree. C. .times. 1 h 1.5 1.0 3.4 Good 6 Example 4
13 A coil 2.0 No. 5 600.degree. C. .times. 1 h 1.2 0.8 3.0 Good 2
Example 5 14 A coil 1.9 No. 5 600.degree. C. .times. 1 h 1.3 0.9
3.7 Good 2 Example 6 15 B coil 1.9 No. 5 -- 1.4 0.9 3.9 Good 5
Example 7 16 B coil 1.9 No. 5 -- 1.6 0.9 3.5 Good 9 Example 8 17 B
coil 1.9 No. 5 500.degree. C. .times. 1 h 1.3 0.9 4.4 Good 3
Example 9 18 B coil 1.9 No. 5 600.degree. C. .times. 1 h 1.2 1.0
4.6 Good 1 Example 10 19 G coil 1.9 No. 5 -- 1.0 0.9 2.8 Good 20
Example 11 20 C plate 2.0 No. 13B -- 1.4 0.8 2.6 Good -- Example 12
21 D plate 2.0 No. 13B -- 1.5 0.8 3.0 Good -- Example 13 22 E plate
2.0 No. 13B -- 1.2 0.7 2.2 Good -- Example 14 23 F plate 2.0 No.
13B -- 1.3 0.7 2.1 Good -- Example 15 24 H plate 1.9 No. 13B -- 1.3
0.8 2.3 Good -- Example 16 25 I plate 2.0 No. 13B -- 1.0 1.0 2.4
Good -- Example 17 26 I plate 2.0 No. 13B -- 1.1 0.8 3.3 Good --
Example 18 27 J plate 2.4 No. 13B -- 1.3 0.8 3.2 Good -- Example 19
28 K plate 2.5 No. 13B -- 1.2 0.8 2.8 Good -- Example 20 29 L plate
2.5 No. 13B -- 1.2 0.8 3.0 Good -- Example 21 30 B coil 1.9 No. 5
-- 1.2 0.8 2.0 Good 8 Example 22 31 B coil 1.9 No. 5 -- 1.1 0.8 2.1
Good 7 Example 23 32 B coil 1.9 No. 5 550.degree. C. .times. 1 h
1.2 0.8 2.5 Good 3 Example 24 33 B coil 1.9 No. 5 550.degree. C.
.times. 1 h 1.1 0.8 2.5 Good 2 Example 25
[0042] FIGS. 4 (a), (b) and (c) are magnified photographs showing
respective shape of fractured portions of the tensile specimens of
examples 1, 6 and 16 as representative examples of all. The
fractured portions of steel plates according to these examples are
characterized by large local construction (high reduction type of
area) as compared with the fractured portions in the specimens of
comparative examples 1 and 6 shown in FIGS. 5 (d) and (e),
therefore it is seen that the steel plates of the inventive
examples are excellent in the tough-ductility. These photographs
were taken from the side of respective specimens as shown in FIG. 5
(f).
[0043] As shown in Table 3, the steel plates and the coils
according to this invention which has SD values of
1.8.times.10.sup.2 MPa or more, have high strength of 1 GPa or
above in all cases, and they are high strength steel plates
excellent in the tough-ductility. Additionally, the steel plates of
examples 1 to 13, 16 to 25 have yield ratios of 0.8 or more, and
they are high strength steel plates excellent in proof stress and
also in tough-ductility. Furthermore, in the steel plate coils
according to this invention, dispersion of the tensile strength is
slight in the cross direction of the coil, the dispersion of the
tensile strength becomes further low by subjecting the coil to
tempering, and it is confirmed that stability of the properties is
improved by the tempering.
INDUSTRIAL APPLICABILITY
[0044] The steel plate according to the present invention is
possible to be widely applied as a steel plate for parts of
transportation equipments such as motor cars, air crafts, shipping
or so, and construction materials.
* * * * *