U.S. patent application number 10/509647 was filed with the patent office on 2006-03-30 for high tensile steel excellent in high temperature strength and method for production thereof.
Invention is credited to Tatsuya Kumagai, Yasushi Mizutani, Tadayoshi Okada, Yoshio Terada, Ryuji Uemori, Yoshiyuki Watanabe.
Application Number | 20060065335 10/509647 |
Document ID | / |
Family ID | 36097659 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060065335 |
Kind Code |
A1 |
Mizutani; Yasushi ; et
al. |
March 30, 2006 |
High tensile steel excellent in high temperature strength and
method for production thereof
Abstract
The present invention relates to a high-tensile-strength low
alloy carbon steel (in the form of a steel sheet, a steel pipe, a
section steel or a wire rod), for a building structure, said
high-tensile-strength steel being excellent in high temperature
strength during a relatively short span of about one hour in the
temperature from 600.degree. C. to 800.degree. C. and being used in
the field of building construction, civil engineering, an offshore
structure, shipbuilding, a reservoir tank or the like; and, more
specifically, is a high-tensile-strength steel excellent in high
temperature strength, characterized by containing, in mass, C at
not less than 0.005% to less than 0.08%, Si at not more than 0.5%,
Mn at 0.1 to 1.6%, P at not more than 0.02%, S at not more than
0.01%, Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than
0.025%, B at 0.0005 to 0.003%, Al at not more than 0.06%, and N at
not more than 0.006%, with the balance consisting of Fe and
unavoidable impurities and satisfying the expression
p.gtoreq.-0.0029.times.T+2.48 when the steel temperature T
(.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature.
Inventors: |
Mizutani; Yasushi; (Chiba,
JP) ; Uemori; Ryuji; (Chiba, JP) ; Kumagai;
Tatsuya; (Futtsu-shi, Chiba, JP) ; Okada;
Tadayoshi; (Chiba, JP) ; Watanabe; Yoshiyuki;
(Chiba, JP) ; Terada; Yoshio; (Chiba, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
36097659 |
Appl. No.: |
10/509647 |
Filed: |
March 28, 2003 |
PCT Filed: |
March 28, 2003 |
PCT NO: |
PCT/JP03/04040 |
371 Date: |
November 2, 2005 |
Current U.S.
Class: |
148/654 ;
148/328; 420/123 |
Current CPC
Class: |
C21D 2211/005 20130101;
C22C 38/02 20130101; C21D 8/0226 20130101; C22C 38/004 20130101;
C21D 8/0426 20130101; C22C 38/12 20130101; C22C 38/06 20130101;
C22C 38/14 20130101; C21D 8/0463 20130101; C21D 8/0263 20130101;
C21D 2211/002 20130101; C22C 38/04 20130101 |
Class at
Publication: |
148/654 ;
148/328; 420/123 |
International
Class: |
C22C 38/12 20060101
C22C038/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2002 |
JP |
200294834 |
Claims
1. A high-tensile-strength steel excellent in high temperature
strength, characterized by containing, in mass, C at not less than
0.005% to less than 0.08%, Si at not more than 0.5%, Mn at 0.1 to
1.6%, P at not more than 0.02%, S at not more than 0.01%, Mo at 0.1
to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than 0.025%, B at
0.0005 to 0.003%, Al at not more than 0.06%, and N at not more than
0.006%, with the balance consisting of Fe and unavoidable
impurities.
2. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, characterized by said steel
satisfying the expression p.gtoreq.-0.0029.times.T+2.48 when the
steel temperature T (.degree. C.) is within the range from
600.degree. C. to 800.degree. C., wherein p is a stress drop ratio
(a yield stress at a high temperature/a yield stress at room
temperature) that is obtained by a yield stress normalized by using
a yield stress at room temperature.
3. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, characterized in that: said steel
comprising a single structure composed of bainite or a composite
structure composed of ferrite and bainite at room temperature; the
temperature (Ac.sub.1) at which said structure reversely transforms
into austenite during high temperature heating corresponding to a
fire higher than 800.degree. C.; and said steel satisfies the
expression p.gtoreq.-0.0029.times.T+2.48 when the steel temperature
T (.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature.
4. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, characterized in that, in the high
temperature range from 600.degree. C. to 800.degree. C.: said steel
has such a strength as to satisfy the expression
p.gtoreq.-0.0029.times.T+2.48 when the steel temperature T
(.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature; said steel has a structure wherein the
temperature (Ac.sub.1) at which a single structure composed of
bainite or a composite structure composed of ferrite and bainite at
room temperature reversely transforms into austenite during high
temperature heating corresponding to a fire is higher than
800.degree. C.; one or more of carbonitrides precipitated phases
thermodynamically stable in said single structure composed of
bainite or in said composite structure composed of ferrite and
bainite is not less than 5.times.10.sup.-4 in terms of a molar
fraction; and the total amount of Mo, Nb and Ti that dissolve in
the ferrite structure is not less than 1.times.10.sup.-3 in terms
of a molar concentration.
5. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, characterized in that, in the high
temperature range from 600.degree. C. to 800.degree. C.: said steel
has such a strength as to satisfy the expression
p.gtoreq.-0.0029.times.T+2.48 when the steel temperature T
(.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature; said steel has a structure wherein the
temperature (Ac.sub.1) at which a single structure composed of
bainite or a composite structure composed of ferrite and bainite at
room temperature reversely transforms into austenite during high
temperature heating corresponding to a fire higher than 800.degree.
C.; the average circle equivalent diameter of prior austenite
grains in said steel is not more than 120 .mu.m; one or more of
carbonitrides precipitated phases thermodynamically stable in said
single structure composed of bainite or in said composite structure
composed of ferrite and bainite is not less than 5.times.10.sup.-4
in terms of a molar fraction; and the total amount of Mo, Nb and Ti
that dissolve in the ferrite structure is not less than
1.times.10.sup.-3 in terms of a molar concentration.
6. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, characterized in that the weld
cracking susceptibility index PCM of said steel defined by the
following expression is not more than 0.20%;
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+v/10+5.times.B.
7. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, wherein the steel further
containing, in mass, one or more of Ni at 0.05 to 1.0%, Cu at 0.05
to 1.0%, Cr at 0.05 to 1.0%, and V at 0.01 to 0.1%.
8. A high-tensile-strength steel excellent in high temperature
strength according to claim 1, wherein the steel further
containing, in mass: one or more of Ni at 0.05 to 1.0%, Cu at 0.05
to 1.0%, Cr at 0.05 to 1.0%, and v at 0.01 to 0.1%; and
additionally one or more of Ca at 0.0005 to 0.004%, REM at 0.0005
to 0.004%, and Mg at 0.0001 to 0.006%.
9. A high-tensile-strength steel excellent in high temperature
strength according to claim 7, characterized in that, in the high
temperature range from 600.degree. C. to 800.degree. C.: said steel
has such a strength as to satisfy the expression
p.gtoreq.-0.0029.times.T+2.48 when the steel temperature T
(.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature; said steel has a structure wherein the
temperature (Ac.sub.1) at which a single structure composed of
bainite or a composite structure composed of ferrite and bainite at
room temperature reversely transforms into austenite during high
temperature heating corresponding to a fire higher than 800.degree.
C.; the average circle equivalent diameter of prior austenite
grains in said steel is not more than 120 .mu.m; one or more of
carbonitrides precipitated phases thermodynamically stable in said
single structure composed of bainite or in said composite structure
composed of ferrite and bainite is not less than 5.times.10.sup.-4
in terms of a molar fraction; and the total amount of Mo, Nb and Ti
that dissolve in the ferrite structure is not less than
1.times.10.sup.-3 in terms of a molar concentration.
10. A method for producing a high-tensile-strength steel excellent
in high temperature strength, characterized by comprising the steps
of: reheating a casting or a slab having a steel composition
according to claim 1 in the temperature range from 1,100.degree. C.
to 1,250.degree. C.; hot rolling it in the temperature range of not
lower than 850.degree. C. while controlling the cumulative
reduction ratio in the temperature range of not higher than
1,100.degree. C. to not less than 30%; finishing the hot rolling,
cooling the hot-rolled steel sheet at a cooling rate of not lower
than 0.3 K/sec. from the temperature of not lower than 800.degree.
C. to the temperature of not higher than 650.degree. C.; and thus
making the microstructure of the steel comprising a single
structure composed of bainite or a composite structure composed of
ferrite and bainite.
11. A high-tensile-strength steel excellent in high temperature
strength, characterized by the steel comprising, in mass, C at not
less than 0.005% to less than 0.08%, Si at not more than 0.5%, Mn
at 0.1 to 1.6%, P at not more than 0.02%, S at not more than 0.01%,
Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than 0.025%,
B at 0.0005 to 0.003%, Al at not more than 0.06%, and N at not more
than 0.006%, with the balance consisting of Fe and unavoidable
impurities; having a structure wherein the temperature (Aci) at
which a composite structure composed of ferrite and bainite, the
composite structure having a bainite fraction being in the range
from 20 to 95% at room temperature, reversely transforms into
austenite during high temperature heating corresponding to a fire
is higher than 800.degree. C.; and having a low yield ratio.
12. A high-tensile-strength steel excellent in high temperature
strength according to claim 11, wherein the steel further
containing, in mass, one or more of Ni at 0.05 to 1.0%, Cu at 0.05
to 1.0%, Cr at 0.05 to 1.0%, and v at 0.01 to 0.1%.
13. A high-tensile-strength steel excellent in high temperature
strength according to claim 11, wherein the steel further
containing, in mass: one or more of Ni at 0.05 to 1.0%, Cu at 0.05
to 1.0%, Cr at 0.05 to 1.0%, and V at 0.01 to 0.1%; and
additionally one or more of Ca at 0.0005 to 0.004%, REM at 0.0005
to 0.004%, and Mg of 0.0001 to 0.006%.
14. A method for producing a high-tensile-strength steel excellent
in high temperature strength, characterized by comprising the steps
of: reheating a casting or a slab having a steel composition
according to claim 11 in the temperature range from 1,100.degree.
C. to 1,250.degree. C.; hot rolling it in the temperature of not
lower than 850.degree. C. while controlling the cumulative
reduction ratio in the temperature of not higher than 1,100.degree.
C. to not less than 30%; finishing the hot rolling, cooling the
resultant hot-rolled steel sheet at a cooling rate of not lower
than 0.3 K/sec. from the temperature of not lower than 800.degree.
C. to the temperature of not higher than 650.degree. C.; thus
making the microstructure of the steel comprising a single
structure composed of bainite or a composite structure composed of
ferrite and bainite; forming a microstructure wherein the
temperature (Ac.sub.1) at which a composite structure composed of
ferrite and bainite, the composite structure having a bainite
fraction being in the range from 20 to 95% at room temperature,
reversely transforms into austenite during high temperature heating
corresponding to a fire is higher than 800.degree. C.; and securing
a low yield ratio.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
high-tensile-strength low alloy carbon steel (in the form of a
steel sheet, a steel pipe, a section steel or a wire rod), for a
building structure, the high-tensile-strength steel showing an
excellent high temperature strength during a relatively short span
of about one hour in the temperature range from 600.degree. C. to
800.degree. C. and being used for a general structure in the field
of building construction, civil engineering, an offshore structure,
shipbuilding, a reservoir tank or the like.
BACKGROUND ART
[0002] In the field of building construction, civil engineering or
the like, for example, steel standardized by JIS, etc. are widely
used as steel for various structures. Here, the allowable
temperature of an ordinary steel for a building structure is
550.degree. C. because the strength thereof begins to lower at a
temperature of about 350.degree. C.
[0003] For that reason, in order to secure safety from a fire in
the case where an above-mentioned steel material is used in
architecture such as a building, an office, a dwelling, a
multi-level car parking tower or the like, the application of
sufficient fire-resistant coating to the steel is required and the
laws related to architectures stipulate that the temperature of the
steel material should not rise to 350.degree. C. or higher during a
fire.
[0004] The reason for the above regulation is that the proof stress
of the above-mentioned steel at about 350.degree. C. becomes about
two-thirds of that at room temperature and thus it falls short of
the required strength. When steel is used in a building structure,
a fire-resistant coating is applied thereto so that the temperature
of the steel may not reach 350.degree. C. during a fire. Therefore,
the cost for the fire-resistant coating goes up in comparison with
the cost of the steel and a large increase in the construction cost
is inevitable.
[0005] Solving the above problems, Japanese Unexamined Patent
Publication Nos. H2-77523 and H10-68044, for example, disclose that
a steels usable at a temperature of not lower than 600.degree. C.
is generally called "a fire-resistant steel." As an example of the
relevant invention, Japanese Unexamined Patent Publication No.
H2-77523 proposes a fire-resistant steel having such a high
temperature strength that the yield strength thereof at 600.degree.
C. is not less than two-thirds (about 70%) of that at room
temperature. Further, in other examples of the invention related to
a fire-resistant steel withstanding a temperature of 600.degree.
C., the generally adopted criterion is that a yield strength at
600.degree. C. is not less than two-thirds of that at room
temperature.
[0006] However, in case of a fire-resistant steel withstanding a
temperature of 700.degree. C. or 800.degree. C., no general rules
are specified at present with regard to the specification of a high
temperature strength (the ratio of a yield strength at a high
temperature to that at room temperature). For example, Japanese
Unexamined Patent Publication No. H2-77523 discloses a steel, to
which considerable amounts of Mo and Nb are added, that can secure
a proof stress at 600.degree. C. of not less than 70% of the proof
stress at room temperature, but it does not describe a proof stress
at 700.degree. C. or 800.degree. C.
[0007] Furthermore, where the proof stress of a steel at
600.degree. C. is only about 70% of that at room temperature,
considering a temperature rise during a fire, though it is possible
to reduce the amount of fire-resistant coating, a building
structure to which a steel not coated with fire-resistant coating
is applicable is limited to an architecture having an open space
such as a multi-level car parking tower or an atrium and therefore
the application of the steel not coated with a fire-resistant
coating is substantially limited.
[0008] Japanese unexamined Patent Publication No. H10-68044
discloses a technology that secures a proof stress at 700.degree.
C. being not less than 56% of the proof stress at room temperature
by making the microstructure of a steel, to which considerable
amounts of Mo and Nb are added, composed of a bainite structure,
but it does not describe a proof stress at 800.degree. C.
[0009] Consequently, though a steel securing a high temperature
strength at about 600.degree. C. has already been used in the
market and a steel securing a certain strength at 700.degree. C.
has been invented in the above examples, it has been difficult to
stably produce a practically usable steel that is capable of
securing a high temperature strength at a temperature of
700.degree. C. or 800.degree. C.
[0010] Meanwhile, the present inventors have recently disclosed a
fire-resistant steel withstanding 850.degree. C. in Japanese
Unexamined Patent Publication No. 2002-105585. The invented steel
secures effective precipitates even at a high temperature and
obtains fire resistance at 850.degree. C. by adding comparatively
large amounts of alloying elements such as Al, Ti, etc. However, it
is not suitable for the steel to be applied to a welded
structure.
[0011] As it has been explained above, when an ordinary steel is
applied to architecture, as the high temperature strength is low.
The ordinary steel has not been used without a fire-resistant
coating or with a thin fire-resistant coating, and therefore it has
had to be coated with an expensive fire-resistant coating.
[0012] In addition, even in a fire-resistant steel, the guaranteed
fire-resistant temperature has been 600.degree. C. to 700.degree.
C. at the highest and therefore the development of a steel that can
be used at a temperature of 700.degree. C. or 800.degree. C.
without the application of fire-resistant coating and thus allows a
fire-resistant coating process to be eliminated, has long been
sought for.
DISCLOSURE OF THE INVENTION
[0013] The object of the present invention is to provide: a
high-tensile-strength steel that is excellent in high temperature
strength in the temperature range from 600.degree. C. to
800.degree. C. and in weldability and is used in the field of
building construction, civil engineering or the like; and a
production method that makes it possible to stably supply the steel
in an industrial scale. The gist of the present invention is as
follows:
[0014] (1) A high-tensile-strength steel excellent in high
temperature strength, characterized by containing, in mass, C at
not less than 0.005% to less than 0.08%, Si at not more than 0.5%,
Mn at 0.1 to 1.6%, P at not more than 0.02%, S at not more than
0.01%, Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than
0.025%, B at 0.0005 to 0.003%, Al at not more than 0.0.6%, and N at
not more than 0.006%, with the balance consisting of Fe and
unavoidable impurities.
[0015] (2) A high-tensile-strength steel excellent in high
temperature strength according to the item (1), characterized-by
said steel satisfying the expression
p.gtoreq.-0.0029.times.T+2.48+when the steel temperature T
(.degree. C.) is within the range from 600.degree. C. to
800.degree. c., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature.
[0016] (3) A high-tensile-strength steel excellent in high
temperature strength according to the item (1), characterized in
that; said steel comprising a single structure composed of bainite
or a composite structure composed of ferrite and bainite at room
temperature; the temperature (Ac.sub.1) at which said structure
reversely transforms into austenite during high temperature heating
corresponding to a fire is higher than 800.degree. C.; and said
steel satisfies the expression p.gtoreq.-0.0029.times.T+2.48 when
the steel temperature T (.degree. C.) is within the range from
600.degree. C. to 800.degree. C., wherein p is a stress drop ratio
(a yield stress at a high temperature/a yield stress at room
temperature) that is obtained by converting a yield stress
normalized by using a yield stress at room temperature.
[0017] (4) A high-tensile-strength steel excellent in high
temperature strength according to the item (1), characterized in
that, in the high temperature range from 600.degree. C. to
800.degree. C.: said steel has such a strength as to satisfy the
expression p.gtoreq.-0.0029.times.T+2.48 when the steel temperature
T (.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature; said steel has a structure wherein the
temperature (Ac.sub.1) at which a single structure composed of
bainite or a composite structure composed of ferrite and bainite at
room temperature reversely transforms into austenite during high
temperature heating corresponding to a fire is higher than
800.degree. C.; one ore more of carbonitrides precipitated phases
thermodynamically stable in said single structure composed of
bainite or in said composite structure composed of ferrite and
bainite is not less than 5.times.10.sup.-4 in terms of a molar
fraction; and the total amount of Mo, Nb and Ti that dissolve in
the ferrite structure is not less than 1.times.10.sup.-3 in terms
of a molar concentration.
[0018] (5) A high-tensile-strength steel excellent in high
temperature strength according to the item (1), characterized in
that, in the high temperature range from 600.degree. C. to
800.degree. C.: said steel has such a strength as to satisfy the
expression p.gtoreq.-0.0029.times.T+2.48 when the steel temperature
T (.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature; said steel has a structure wherein the
temperature (Ac.sub.1) at which a single structure composed of
bainite or a composite structure composed of ferrite and bainite at
room temperature reversely transforms into austenite during high
temperature heating corresponding to a fire is higher than
800.degree. C.; the average circle equivalent diameter of prior
austenite grains in said steel is not more than 120 .mu.m; one or
more of carbonitrides precipitated phases thermodynamically stable
in said single structure composed of bainite or in said composite
structure composed of ferrite and bainite is not less than
5.times.10.sup.-4 in terms of a molar fraction; and the total
amount of Mo, Nb and Ti that dissolve in the ferrite structure is
not less than 1.times.10.sup.-3 in terms of a molar
concentration.
[0019] (6) A high-tensile-strength steel excellent in high
temperature strength according to any one of the items (1) to (5),
characterized in that the weld cracking susceptibility index PCM of
said steel defined by the following expression is not more than
0.20%;
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5.times.B.
[0020] (7) A high-tensile-strength steel excellent in high
temperature strength according to any one of the items (1) to (6),
wherein the steel further containing, in mass, one or more of Ni at
0.05 to 1.0%, Cu at 0.05 to 1.0%, Cr at 0.05 to 1.0%, and V at 0.01
to 0.1%.
[0021] (8) A high-tensile-strength steel excellent in high
temperature strength according to any one of the items (1) to (7),
wherein the steel further containing, in mass: one or more of Ni at
0.05 to 1.0%, Cu at 0.05 to 1.0%, Cr at 0.05 to 1.0%, and V at 0.01
to 0.1%; and additionally one or more of Ca at 0.0005 to 0.004%,
REM at 0.0005 to 0.004%, and Mg at 0.0001 to 0.006%.
[0022] (9) A high-tensile-strength steel excellent in high
temperature strength according to the item (7) or (8),
characterized in that, in the high temperature range from
600.degree. C. to 800.degree. C.: said steel has such a strength as
to satisfy the expression p.gtoreq.-0.0029.times.T+2.48 when the
steel temperature T (.degree. C.) is within the range from
600.degree. C. to 800.degree. C., wherein p is a stress drop ratio
(a yield stress at a high temperature/a yield stress at room
temperature) that is obtained by converting a yield stress
normalized by using a yield stress at room temperature; said steel
has a structure wherein the temperature (Ac.sub.1) at which a
single structure composed of bainite or a composite structure
composed of ferrite and bainite at room temperature reversely
transforms into austenite during high temperature heating
corresponding to a fire is higher than 800.degree. C.; the average
circle equivalent diameter of prior austenite grains in said steel
is not more than 120 .mu.m; one or more of carbonitrides
precipitated phases thermodynamically stable in said single
structure composed of bainite or in said composite structure
composed of ferrite and bainite is not less than 5.times.10.sup.-4
in terms of a molar fraction; and the total amount of Mo, Nb and Ti
that dissolve in the ferrite structure is not less than
1.times.10.sup.-3 in terms of a molar concentration.
[0023] (10) A method for producing a high-tensile-strength steel
excellent in high temperature strength, characterized by comprising
the steps of: reheating a casting or a slab having a steel
composition according to any one of the items (1) to (9) in the
temperature range from 1,100.degree. C. to 1,250.degree. C.; hot
rolling it in the temperature range of not lower than 850.degree.
C. while controlling the cumulative reduction ratio in the
temperature range of not higher than 1,100.degree. C. to not less
than 30%; finishing the hot rolling, cooling the hot-rolled steel
sheet at a cooling rate of not lower than 0.3 K/sec. from the
temperature range of not lower than 800.degree. C. to the
temperature range of not higher than 650.degree. C.; and thus
making the microstructure of the steel comprising a single
structure composed of bainite or a composite structure composed of
ferrite and bainite.
[0024] (11) A high-tensile-strength steel excellent in high
temperature strength, characterized by: comprising, in mass, C at
not less than 0.005% to less than 0.08%, Si at not more than 0.5%,
Mn at 0.1 to 1.6%, P at not more than 0.02%, S at not more than
0.01%, Mo at 0.1 to 1.5%, Nb at 0.03 to 0.3%, Ti at not more than
0.025%, B at 0.0005 to 0.003%, Al at not more than 0.06%, and N at
not more than 0.006%, with the balance consisting of Fe and
unavoidable impurities; having a structure wherein the temperature
(Ac.sub.1) at which a composite structure composed of ferrite and
bainite, the composite structure having a bainite fraction being in
the range from 20 to 95% at room temperature, reversely transforms
into austenite during high temperature heating corresponding to a
fire is higher than 800.degree. C.; and having a low yield
ratio.
[0025] (12) A high-tensile-strength steel excellent in high
temperature strength according to the item (11), wherein the steel
further containing, in mass, one or more of Ni at 0.05 to 1.0%, Cu
at 0.05 to 1.0%, Cr at 0-05 to 1.0%, and V at 0.01 to 0.1%.
[0026] (13) A high-tensile-strength steel excellent in high
temperature strength according to the item (11) or (12), wherein
the steel further containing, in mass: one or more of Ni at 0.05 to
1.0%, Cu at 0.05 to, 1.0%, Cr at 0.05 to 1.0%, and V at 0.01 to
0.1%; and additionally one or more of Ca at 0.0005 to 0.004%, REM
at 0.0005 to 0.004%, and Mg at 0.0001 to 0.006%.
[0027] (14) A method for producing a high-tensile-strength steel
excellent in high temperature strength, characterized by comprising
the steps of: reheating an ingot or a slab having a steel
composition according to any one of the items (11) to (13) in the
temperature range from 1,100.degree. C. to 1,250.degree. C.; hot
rolling it in the temperature of not lower than 850.degree. C.
while controlling the cumulative reduction ratio in the temperature
of not higher than 1,100.degree. C. to not less than 30%; finishing
the hot rolling, cooling the hot-rolled steel sheet at a cooling
rate of not lower than 0.3 K/sec. from the temperature of not lower
than 800.degree. C. to the temperature of not higher than
650.degree. C.; thus making the microstructure of the steel
comprising a single structure composed of bainite or a composite
structure composed of ferrite and bainite; forming a structure
wherein the temperature (Ac.sub.1) at which a microcomposite
structure composed of ferrite and bainite, the composite structure
having a bainite fraction being in the range from 20 to 95% at room
temperature, reversely transforms into austenite during high
temperature heating corresponding to a fire is higher than
800.degree. C.; and securing a low yield ratio.
BEST MODE FOR CARRYING OUT THE INVENTION
[0028] The present inventors have proposed steels excellent in high
temperature strength at 600.degree. C. and 700.degree. C. and the
steels excellent in high temperature strength at 600.degree. C.
have already been used in various fields including building
construction. However, there is a very strong demand in the market
for a steel withstanding a still higher temperature. At the same
time, there also is a strong demand for a steel excellent in high
temperature strength to have a still higher strength.
[0029] In a fire resistance design, a steel is well accepted as
long as the steel maintains high strength for the duration of a
fire. That is, it is not necessary to consider such long lasting
strength as required of a conventional heat-resistant steel and a
steel is well accepted as long as the yield strength of the steel
is maintained for a relatively short time at a high temperature.
For example, a steel can be sufficiently used as a fire-resistant
steel withstanding 800.degree. C. as long as the yield strength of
the steel is secured for a short retention time of about 30 minutes
at a high temperature of 800.degree. C.
[0030] The performance of a conventional fire-resistant steel has
been regulated so that a yield strength at a high temperature is
not less than two-thirds of that at room temperature. However,
considering the fact that the range of the strength of a steel in
the actual design of a steel construction is about 0.2 to 0.4 time
the lower limit of the yield strength at room temperature, it is
necessary for the steel to satisfy the expression
p.gtoreq.-0.0029.times.T+2.48 when the steel temperature T
(.degree. C.) is within the range from 600.degree. C. to
800.degree. C., wherein p is a stress drop ratio (a yield stress at
a high temperature/a yield stress at room temperature) that is
obtained by converting a yield stress normalized by using a yield
stress at room temperature.
[0031] In order to further enhance high temperature strength, it is
effective to promote the precipitation of carbonitrides that are
stable at a high temperature and make a microstructure consist of
bainite by the combined addition of Mo and Nb. In order to enhance
strength at room temperature and emphasize the properties as a
high-tensile-strength steel, a microstructure may be made composed
of a single structure of bainite.
[0032] However, since a strength at room temperature increases as
the fraction of hard bainite increases, when the upper limit of a
yield ratio (YR) is regulated, it is desirable to make the
microstructure of a steel comprising a single structure composed of
bainite or a composite structure composed of ferrite and bainite
that has an adequate bainite fraction, in accordance with the
required properties including a strength at room temperature.
[0033] In order to produce a proper microstructure and obtain a
strength in the prescribed range at room temperature, it is
effective to lower a C content. A low C content has the effects of
enhancing the thermodynamic stability of bainite or a composite
structure composed of ferrite and bainite at a high temperature and
also raising the temperature (Ac.sub.1) at which a structure
reversely transforms into austenite. However, in this case, it has
been clarified that the microstructure and the steel properties are
apt to be influenced by rolling conditions and subsequent cooling
conditions and a stable production is hardly obtained.
[0034] To address the problems, the present inventors investigated
the control of a microstructure and the enhancement of high
temperature strength, as a result, found that an appropriate the
amount B addition was effective for the stabilization of
production, and established the present invention.
[0035] A steel in this category is generally required to have such
weldability as required of a conventional steel for a welded
structure since the steel may be used for a welded structure, and
therefore it has been a very difficult challenge to achieve such a
steel excellent in strength at a high temperature of 700.degree. C.
to 800.degree. C.
[0036] The present inventors carried out intensive studies to solve
the problem, and found that, in order to obtain a high temperature
strength in the temperature range from 700.degree. C. to
800.degree. C., it was effective to enhance precipitation hardening
of a steel by the combined addition of alloying elements such as
Mo, Nb, V, Ti, etc., in order to increase dislocation density by
making a microstructure composed of bainite, and further to delay
the recovery of the dislocation by dissolved Mo, Nb and V and
somewhat by dissolved Ti.
[0037] The present inventors further found that, in order to
simultaneously secure all of a strength at 700.degree. C. to
800.degree. C., a strength at room temperature, and a desired a
stress drop ratio p from room temperature to a high temperature, it
was important to make a microstructure comprising a composite
structure composed of ferrite and bainite or a single structure
composed of bainite and, at the same time, to obtain the thermal
stability of the matrix structure at a high temperature and the
adequate effects of conformable precipitation hardening and
dislocation recovery delay by controlling the amounts of alloying
elements addition in appropriate amounts. Furthermore, in order to
secure a low yield ratio, it is necessary to make a microstructure
comprising an adequate composite structure composed of ferrite and
bainite.
[0038] In general, the yield strength of a steel begins to drop
sharply from a temperature close to 450.degree. C. This is because,
as a temperature rises, thermal activation energy drops and
resistance to dislocation slip movement, which has been effective
at a low temperature, becomes ineffective.
[0039] Generally speaking, Cr carbide, Mo carbide and the like,
which are utilized for strengthening a steel in a temperature of
around lower than 700.degree. C., function as effective resistance
to dislocation slip movement up to a high temperature of about
600.degree. C., but they dissolve again at a high temperature of
800.degree. C. or so and therefore can scarcely maintain the
strengthening effect.
[0040] The present inventors investigated single or composite
structures of various precipitates having higher stability at a
high temperature. As a result, it was found that precipitates
formed by combining Mo with Nb, Ti and V had high stability at a
high temperature and also a high strengthening effect at
700.degree. C. to 800.degree. C. That is, precipitates formed by
combining Mo with Nb, Ti and V precipitate finely during reheating,
for example during temperature rise at a fire, by: adding
appropriate amounts of Mo, Nb, Ti and V; keeping a heating
temperature high at hot rolling; thus making those elements
dissolve sufficiently; also introducing proper structure after hot
rolling having a high dislocation density; and, by so doing,
securing precipitation sites where precipitates can occur.
[0041] Even such composite precipitates grow and coarsen while a
steel is retained at 700.degree. C. to 800.degree. C. and the
strengthening effect decreases before long. However, when such
composite precipitates exist densely and finely dispersed manner, a
desired level of yield strength can sufficiently be obtained at
700.degree. C. to 800.degree. C. as long as the retention time is
about 30 minutes.
[0042] In addition, Mo, Nb, V and Ti dissolved in a BCC phase are
effective for the delay of dislocation recovery and have the effect
of raising the temperature at which a sharp drop of yield strength
commences. The present inventors obtained the following discovery
as a result of variously studying in detail the effect of those
high temperature strengthening factors on yield stress at
700.degree. C. to 800.degree. C. That is, in order that a steel
satisfies the expression p.gtoreq.-0.0029.times.T+2.48 when the
steel temperature T (.degree. C.) is within the range from
700.degree. C. to 800.degree. C., namely the stress drop ratio is
not less than 45% at 700.degree. C. and not less than 16% at
800.degree. C., wherein p is a yield stress drop ratio from room
temperature to a high temperature (a yield stress at a high
temperature/a yield stress at room temperature), it is necessary
that, in the temperature range, carbonitrides compositely
containing Mo, Nb, V and Ti are not less than 5.times.10.sup.-4 in
terms of a molar fraction and the total amount of Mo, Nb, V and Ti
that dissolve in a BCC phase is not less than 1.times.10.sup.-3 in
terms of a molar concentration.
[0043] The composition of a composite carbonitride precipitates
that are important for the securing a high temperature strength can
easily be identified by analysis with, for example, an electron
microscope or an EDX. The amount of a thermodynamically stable
precipitates that are formed equilibriously and the amounts of
alloying elements that dissolve in a BCC phase can easily be
calculated from the amounts of the alloying elements addition by
using a commercially available software for a thermodynamic
computation database or the like.
[0044] However, even when precipitates themselves are stable, if a
base steel transforms due to a temperature rise, the coherency
between the precipitates and the matrix is lost, they become
incoherent, and thus the strengthening function of the precipitates
deteriorates sharply. That is, in order to make use of the
strengthening effect of composite precipitates that are stable even
at a high temperature, it is essential for a steel that the base
steel structure of the matrix does not transform even at
800.degree. C. which is a designed temperature.
[0045] For that reason, concretely, it is necessary to control the
Ac.sub.1 transformation temperature of a steel to not lower than
800.degree. C. by adjusting alloying elements, for example by
lowering the amount of Mn addition that has a function of forming
austenite.
[0046] Further, the concept of the present invention is to enhance
strengthening at a high temperature by utilizing precipitates and
dissolved elements and, thus, the amounts of addition alloying
elements, such as Cr, Mn and Mo, that have so far been added
abundantly to a conventional steel for high temperature use can
rather be restrained at a low level. Therefore, it is possible to
design alloy addition so as not to deteriorate weldability.
[0047] Note that, as a steel comprising a single structure composed
of bainite has a high strength, the condition of a low yield ratio
that is required of a steel for building construction cannot
necessarily be satisfied. In order to cope with that, when a low
yield ratio is required of a steel according to the present
invention, a microstructure is made comprising composite structure
composed of ferrite and bainite and the bainite fraction is
controlled in the range from 20 to 95%. The reason is that an
excessive ferrite fraction in a microstructure makes it difficult
to secure strength both at room temperature and at a high
temperature by increasing the amounts of alloying elements
addition.
[0048] The reasons for regulating the components in the present
invention are explained hereunder. Here, % means a percent in terms
of mass.
[0049] C is an element that affects the properties of a steel most
conspicuously and is essential for the formation of composite
precipitates (carbides) with Mo, Nb, Ti and V. Therefore, a C
amount of at least 0.005% is necessary. If a C amount is less than
the amount, the is strength of a steel is insufficient. However,
when C is added in excess of 0.08%, the Ac.sub.1 transformation
temperature lowers, and therefore strength at 800.degree. C. is
hardly obtained and toughness also deteriorates. For those reasons,
a C amount is limited in the range from 0.005 to 0.08%. Further, it
is preferable to limit a C amount to less than 0.04% in order to
keep the matrix composed of ferrite and bainite thermodynamically
stable during high temperature heating corresponding to a fire, to
maintain the coherency of the matrix with carbonitride precipitates
compositely containing MO, Nb, V and Ti, and thus secure the
strengthening effect.
[0050] Si is an element contained in a steel as a deoxidizing agent
and is effective in enhancing the strength of a steel at room
temperature as it has a function of strengthening a steel by acting
as substitutional solution hardening. However, Si does not have the
effect of enhancing strength particularly at a high temperature
exceeding 600.degree. C. If Si is added abundantly, weldability and
HAZ toughness deteriorate, and therefore the upper limit of an Si
amount is limited to 0.5%. A steel can be deoxidized only by Ti
and/or Al, and therefore it is preferable that an Si amount is as
low as possible from the viewpoint of HAZ toughness and
hardenability. Therefore, Si may not necessarily be added.
[0051] Mn is an element indispensable for securing strength and
toughness. Mn is a substitutional solution hardening element and
therefore it is effective for the enhancement of strength at room
temperature. However, Mn does slight contribution to increase high
temperature strength exceeding 600.degree. C. For that reason, in a
steel containing a comparatively large amount of Mo, such as a
steel according to the present invention, an Mn amount is limited
to not more than 1.6% from the viewpoint of the improvement of
weldability, namely the lowering of a PCM value. To control the
upper limit of a Mn amount to a low level is advantageous also from
the viewpoint of the control of the segregation at the center of a
continuously cast slab. Further, in order to control the Ac.sub.1
transformation temperature to not lower than 800.degree. C., the
addition of Mn must be restrained and it is desirable to set the
upper limit at 0.9%. The lower limit of an Mn amount is not
particularly regulated, but it is desirable to add Mn by not less
than 0.1% from the viewpoint of the securing of the strength and
toughness of a steel.
[0052] In order to obtain an appropriate fraction of a bainite
structure, a cooling rate must be not lower than 0.3 K/sec. in the
temperature range from 800.degree. C. to 650.degree. C. after the
completion of hot rolling. That is, a comparatively thin steel
sheet less than about 25 mm in thickness must be produced through
an air cooling process or an accelerated cooling (water cooling)
process, and a comparatively thick steel sheet more than about 25
mm in thickness must be produced through an accelerated cooling
(water cooling) process.
[0053] P is an impurity in a steel according to the present
invention and the reduction of a P amount tends to reduce
intergranular fracture at a HAZ. Therefore, it is preferable that a
P amount is as small as possible. When a P amount is high, the low
temperature toughness of a steel and a weld deteriorate. For that
reason, the upper limit of a P amount is set at 0.02%.
[0054] S, like P, is an impurity in a steel according to the
present invention and therefore it is preferable that an S amount
is as small as possible from the viewpoint of the low temperature
toughness of a base steel. When an S amount is high, the low
temperature toughness of a base steel and a weld deteriorate. For
that reason, the upper limit of an S amount is set at 0.01%.
[0055] Mo is a basic element that constitutes composite
precipitates which enhance high temperature strength and thus is an
essential element in a steel according to the present invention. It
is necessary to add Mo by not less than 0.1% in order to obtain
composite precipitates formed by combining Mo with Nb and Ti or
composite precipitates formed by combining Mo with Nb, Ti and V at
a high density and thus enhance high temperature strength. On the
other hand, when Mo is added in excess of 1.5%, the uniformity of
the properties of a steel is hardly controlled, the toughness of a
weld heat-affected zone deteriorates, and also the economical
efficiency is lost. For those reasons, an addition amount of Mo is
limited in the range from 0.1 to 1.5%, preferably from 0.2 to
1.1%.
[0056] Nb is an element that contributes important roles in
securing strength at a high temperature of 700.degree. C. or
800.degree. C. in a steel according to the present invention to
which a comparatively large amount of Mo is added. Firstly, in
general, Nb is an element that is useful for raising the
recrystallization temperature of austenite and exhibiting the
effect of controlled rolling during hot rolling to the maximum.
Secondly, Nb contributes to the grain size refinement of austenite
in a heated steel at reheating prior to hot rolling, normalizing or
quenching.
[0057] In addition, Nb has the effect of enhancing strength by
precipitation hardening, and also contributes to the enhancement of
high temperature strength by the combined addition with Mo. When
the amount of Nb addition is less than 0.03%, the effect of
precipitation hardening is insufficient in the temperature range
from 700.degree. C. to 800.degree. C., and therefore it is
preferable to add Nb by not less than 0.1%. On the other hand, when
an Nb amount exceeds 0.2%, the toughness of a steel may
deteriorate, and therefore the upper limit of an Nb amount is set
at 0.3%. Consequently, a Nb amount is limited in the range from
0.03 to 0.3%.
[0058] Ti, like Nb, is also effective for the enhancement of high
temperature strength. When severe toughness is required of a steel
and a weld in particular, it is desirable to add Ti. The reason is
that, when an Al amount is small (not more than 0.003%, for
example), Ti forms precipitates mainly composed of Ti.sub.2O.sub.3
by combining with O, the precipitates act as nuclei for forming
intragranular transformed ferrite, and that improves toughness at a
weld. Another reason is that Ti forms TiN in the slab by combining
with N, restrains the coarsening of .gamma. grains during
reheating, and thus is effective for the microstructure refinement
after hot rolling, and further that fine TiN remaining in a steel
sheet refines microstructure of a heat-affected zone at welding. A
Ti amount of at least 0.005% is necessary in order to secure those
effects. However, when a Ti amount is excessive, Ti forms TiC and
then deteriorates low temperature toughness and weldability. For
that reason, a Ti amount is preferably not more than 0.02%; the
upper limit thereof is 0.025%.
[0059] B is very important in controlling strength through the
control of the fraction of bainite formed. That is, B is effective
in improving hardenability by segregating at the grain boundaries
of austenite and restraining ferrite formation, and forming bainite
stably even when a cooling rate is comparatively low as in air
cooling. A B amount of at least 0.0005% is necessary in order to
secure the above effects. However, if an addition amount of B is
excessive, not only the effect of improving hardenability is
saturated but also B precipitates that cause the embrittlement of
prior austenite grain boundaries and are detrimental to toughness
may form. Therefore, the upper limit of a B amount is set at
0.003%.
[0060] Al is an element generally contained in a steel as a de
oxidizing agent. However, only Si or Ti can play the role of
deoxidization sufficiently and thus the lower limit of an Al amount
is not specified in the present invention (including the case of an
Al amount is zero). On the other hand, if an Al amount is
excessive, not only the cleanliness of a steel deteriorates but
also the toughness of a weld metal deteriorates. Therefore, the
upper limit of an Al amount is set at 0.06%.
[0061] N is an element that is contained in a steel as an
unavoidable impurity and the lower limit of an N amount is not
particularly specified. However, the increase of an N amount is
extremely detrimental to toughness at a HAZ and weldability.
Therefore, the upper limit thereof is set at 0.006% in a steel
according to the present invention.
[0062] Next, the reasons for specifying the addition and the ranges
of the addition amounts of Ni, Cu, Cr, V, Ca, REM and Mg, which may
be contained in a steel as occasional demands, are explained
hereunder. The main object of further adding those elements in
addition to the basic components is to improve properties such as
strength, toughness and the like with the excellent features of a
steel according to the present invention not harmed. Accordingly,
the addition amounts thereof should be restricted
spontaneously.
[0063] Ni enhances the strength and toughness of a steel while
weldability and toughness at a HAZ are not badly affected. In order
to secure those effects, Ni must be added by at least not less than
0.05%. On the other hand, if Ni is added excessively, not only
economical efficiency is harmed but also weldability is adversely
affected, and therefore the upper limit of an Ni amount is set at
1.0%.
[0064] Cu exhibits almost the same effects and roles as Ni. An
excessive addition of Cu causes the deterioration of weldability
and the generation of Cu-induced cracks during hot rolling which
makes the production difficult, and therefore the upper limit of a
Cu amount is set at 1.0%. The lower limit of a Cu amount should be
the least amount in which a substantial effect is obtained and thus
is set at 0.05%.
[0065] Cr enhances both the strength and the toughness of a steel.
However, if an addition amount of Cr is excessive, the toughness
and weldability of both a base steel and a weld are deteriorated,
and therefore a Cr amount is limited in the range from 0.05 to
1.0%.
[0066] The aforementioned Cu, Ni and Cr are effective in not only
the strength and toughness of a steel but also the weather
resistance thereof. For those purposes, it is preferable to add the
elements within the range where weldability is not hindered.
[0067] V has almost the same function of composite precipitation as
Nb has, but the effect thereof is smaller than that of Nb. Further,
V influences hardenability and also contributes to the enhancement
of high temperature strength. The same effect as Nb is hardly
obtained with a V addition amount of less than 0.01%. On the other
hand, if the amount of V addition is excessive, the toughness of a
steel deteriorates sometimes. Therefore, the lower and upper limits
of a V amount in a steel according to the present invention are set
at 0.01% and 0.1%, respectively.
[0068] Ca and REM combine with S, which is an impurity, and have
the functions of enhancing toughness and restraining cracks induced
by dispersed hydrogen at a weld. However, if their amounts are
excessive, coarse inclusions are formed and they exert harmful
influence. Therefore, the adequate content of Ca or REM is 0.0005
to 0.004%.
[0069] Mg has the functions of, restraining the growth of austenite
grains and fractionizing them at a heat-affected zone, and enhances
toughness at a weld. In order to secure those effects, a Mg
addition of not less than 0.0001% is necessary. On the other hand,
if the Mg addition increases, the degree of the effects to the
increase of the addition amount decreases and economical efficiency
is harmed. Therefore, the upper limit of an Mg amount is set at
0.006%.
[0070] Now, it is also an effective means for improving the
properties of a steel according to the present invention to secure
high temperature strength by adding an appropriate amount of W in
the same effect as the addition of Mo, Nb and V. A W amount of at
least 0.01% is necessary in order to obtain the effect. However, if
a W amount exceeds 1%, the effect is saturated and, therefore, the
upper limit thereof is set at 1% from the economical efficiency
viewpoint.
[0071] Further, in order to secure a low cracking susceptibility at
room temperature and make welding without preheating viable, a PCM
value is limited to not more than 0.20%. PCM is an index that
represents weldability and, as a PCM value decreases, weldability
improves. In a steel according to the present invention, an
excellent weldability can be secured as long as a PCM value is not
more than 0.20%. Here, the weld cracking susceptibility index. PCM
is defined by the following expression;
PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5.times.B.
[0072] In addition, the diameter of prior austenite grains in a
finally transformed structure is limited to not larger than 150
.mu.m in terms of an average circle-equivalent diameter at the
position in the depth of one-fourth of the sheet thickness on a
cross section in the direction of the final hot rolling of a steel
sheet. The reason is that a prior austenite grain diameter
significantly influences toughness together with a microstructure
and it is very important and essential to control the prior
austenite grain diameter to as small as possible, particularly in
order to enhance the toughness of such a Mo-added steel according
to the present invention. The reason for limiting a prior austenite
grain diameter as stated above, which has been clarified on the
basis of the results of the experiments that have been carried out
by the present inventors with production conditions variously
changed, is that toughness comparable with that of a steel having a
lower Mo amount than a steel according to the present invention can
be secured as long as the prior austenite grain diameter is not
larger than 120 .mu.m in terms of an average circle-equivalent
diameter. Here, there are not a few cases where prior austenite
grains are hard to identify in those cases, an average
circle-equivalent diameter can be obtained by: using a notched
impact test piece with that is cut out from a position the center
of which is in the depth of one-fourth of the sheet thickness in a
direction perpendicular to the final hot rolling direction of a
steel sheet, for example a JIS 22202 No. 4 test piece (with 2 mm
V-notch); defining a unit of fractured faces caused by the brittle
fracture of the test piece at a sufficiently low temperature as an
effective grain diameter that can be regarded as a prior austenite
grain diameter; and measuring the average circle-equivalent
diameter of the units. In those cases too, the value must be not
larger than 150 .mu.m.
[0073] With regard to a method for producing a
high-tensile-strength steel excellent in high temperature strength
according to the present invention, it is preferable that a
reheating temperature is high when a slab or an ingot is rolled in
order to sufficiently dissolve Mo, Nb, Ti and V. However, the
reheating temperature is limited in the range from 1,100.degree. C.
to 1,250.degree. C. from the viewpoint of the securement of the
toughness of a steel.
[0074] Thereafter, the reheated slab or ingot is subjected to hot
rolling while an cumulative reduction ratio of not less than 30%
relative to the finish-rolled sheet thickness is secured in a
temperature range of not higher than 1,100.degree. C., and then the
hot rolling is completed at a temperature not lower than
850.degree. C. If reduction in a low temperature range is
excessive, ferrite transformation is accelerated, a ferrite
fraction becomes excessive, thus strength is hardly secured,
further Nb, Ti and v precipitate as carbides during the hot
rolling, and thus necessary amounts of dissolved Mo, Nb, Ti and v
are not obtained. For those reasons, the lower limit of a hot
rolling finishing temperature is 850.degree. c.. On the other hand,
if hot rolling is completed at a temperature exceeding
1,100.degree. C., toughness is insufficient, and therefore the
upper limit of a hot rolling finishing temperature is set at
1,100.degree. C.
[0075] After the completion of the hot rolling, the resultant steel
sheet is cooled at an average cooling rate of not less than 0.3
K/sec., which is measured on the surface of the steel sheet, in the
temperature range from not lower than 800.degree. C. to not higher
than 650.degree. C. in terms of the temperature of the steel sheet
surface. The object is to obtain a microstructure, after hot
rolling, that abundantly contains deformation bands and
dislocations acting as the sites of precipitation, and then, by
freezing those with water cooling, to obtain composite precipitates
at a high density, the composite precipitates being formed by
combining Mo with Nb, Ti and V and, during reheating, being kept
fine and coherent to the matrix.
[0076] Note that, even though a steel according to the present
invention is reheated after it is produced to a temperature not
higher than the Ac.sub.1 transformation temperature with the aim of
dehydrogenation or the like, the features of the steel according to
the present invention are not harmed at all.
[0077] A steel sheet may be subjected to tempering treatment in a
temperature of not higher than 500.degree. C. for not longer than
30 minutes after water cooling.
[0078] Further, a steel according to the present invention can
sufficiently enjoy the advantages even when it is used in the form
of such a steel as a heavy steel plate, a steel pipe, a steel
sheet, a section steel or the like.
EMBODIMENT
[0079] Steel sheets (15 to 50 mm in thickness) having various steel
components were produced through the processes of a converter,
continuous casting and plate rolling, and the strength, toughness,
yield strength at 700.degree. C. and 800.degree. C., occurrence of
root cracks during the y-crack test without preheating (at room
temperature) and the like of the resultant steel sheets were
investigated.
[0080] The steel components of the invention steels together with
the comparative steels are shown in Tables 1 and 2, the production
conditions and the microstructures of the steel sheets in Table 3,
and the results of investigating the various properties in Table
4.
[0081] In cases of the invention steels Nos. 1 to 9, all the
microstructures comprising the composite structures composed of
ferrite and bainite and the average circle-equivalent diameters of
prior austenite grains were not larger than 120 .mu.m. Thus
obtained yield strength ratios were excellent; 64% and 23% at
700.degree. C. and 800.degree. C., respectively.
[0082] In cases of the invention steels NOS. 10 to 18, each of the
microstructures comprising a single structure composed of bainite
or a composite structure composed of ferrite and bainite and the
average circle-equivalent diameters of prior austenite grains were
not larger than 120 .mu.m. Thus obtained yield strength ratios were
excellent; 61% and 25% at 700.degree. C. and 800.degree. C.,
respectively.
[0083] In case of the comparative steel No. 19, the C amount was
excessive and the temperature Ac.sub.1 at which the structure begun
to reversely transform into austenite was not higher than
800.degree. C. Therefore, though the high strength was obtained at
room temperature, the ratio (p) of the yield strength at the high
temperature to that at room temperature was less than the value
defined by the expression -0.0029.times.T+2.48.
[0084] In case of the comparative steel No. 20, the C amount was
insufficient, the yield strength was insufficient as a steel of 490
MPa class, the amount of the composite carbonitrides formed in the
high temperature of not lower than 600.degree. C. was less than
5.times.10.sup.-4, and also the ratio (p) of the yield strength at
the high temperature to that at room temperature was less than the
value defined by the expression -0.0029.times.T+2.48.
[0085] In case of the comparative steel No. 21, the Mn amount
exceeds 1.6%, therefore the Ac.sub.1 temperature was lower than
800.degree. C., and the ratio (p) of the yield strength at the high
temperature to that at room temperature is less than the value
defined by the expression -0.0029.times.T+2.48 in the temperature
range of not lower than 700.degree. C.
[0086] In case of the comparative steel No. 22, the Mn amount was
less than 0.1%, therefore the effect of solution hardening was
insufficient at room temperature, and thus the yield strength and
the tensile strength at room temperature were lower than the
relevant lower limits of the standard values of a 490 MPa class
steel.
[0087] In case of the comparative steel No. 23, the P amount
exceeds 0.02%, and therefore both the ductile-brittle transition
temperature of the base steel and the absorbed energy of the
reproduced HAZ at 0.degree. C. deteriorate.
[0088] In case of the comparative steel No. 24, the S amount
exceeds 0.01%, and therefore both the ductile-brittle transition
temperature of the base steel and the absorbed energy of the
reproduced HAZ at 0.degree. C. deteriorated, similarly to the
comparative steel No. 23.
[0089] In case of the comparative steel No. 25, the amount of Mo
dissolved in both the carbonitrides precipitated phases and the BCC
phase was insufficient due to the insufficient amount of Mo
addition, and therefore the resultant ratio of yield strength at a
high temperature of 800.degree. C. to that at room temperature was
as low as 15% though the strength at room temperature was good.
[0090] In case of the comparative steel No. 26, the Mo amount is
excessive and, therefore, the unevenness of the base steel
properties increases and the root cracks occurred in the y-crack
test without preheating in spite of the fact that the weld cracking
susceptibility index PCM was 0.18%. In addition, the absorbed
energy of the reproduced HAZ was low.
[0091] In case of the comparative steel No. 27, the Nb amount was
insufficient, the effect of precipitation hardening is not obtained
sufficiently at 700.degree. C. and 800.degree. C., and therefore
the ratio (p) of the yield strength at the high temperature to that
at room temperature was less than the value defined by the
expression -0.0029.times.T+2.48.
[0092] In case of the comparative steel No. 28, the Nb amount was
excessive, and therefore the absorbed energy of the reproduced HAZ
was low though the high temperature strength is enhanced.
[0093] In case of the comparative steel No. 29, the .gamma. grains
were coarse, and therefore the absorbed energy of the reproduced
HAZ was low.
[0094] In case of the comparative steel No. 30, the Ti amount was
excessive, and therefore both the ductileness-brittleness
transition temperature of the steel and the absorbed energy of the
reproduced HAZ deteriorated.
[0095] In case of the comparative steel No. 31, the addition amount
of B was insufficient, a sufficient hardenability cannot be
obtained, the bainite fraction of the microstructure was too small,
and therefore the yield strength at room temperature was lower than
the lower limit of the standard value of a 490 MPa class steel.
[0096] In case of the comparative steel No. 32, the addition amount
of B was excessive, and therefore the ductile-brittle transition
temperature of the base steel was close to 0.degree. C. and the
absorbed energy of the reproduced HAZ is low.
[0097] In case of the comparative steel No. 33, the Al amount
exceeded 0.06% and, therefore, the ductile-brittle transition
temperature of the base steel was close to 0.degree. C. and the
toughness of the reproduced HAZ was low.
[0098] In case of the comparative steel No. 34, the N amount
exceeded 0.006%, and therefore the toughness of the reproduced HAZ
was low.
[0099] In case of the comparative steel No. 35, the PCM value
exceeded 0.20% and the root cracks occurred in the y-crack test
without preheating. In addition, the absorbed energy of the
reproduced HAZ was low.
[0100] In case of the comparative steel No. 36, the reheating
temperature was lower than 1,100.degree. C., and therefore the
added alloying elements did not dissolve in austenite during the
reheating, a sufficient precipitation hardening effect was not
obtained, and the ratio (p) of the yield strength at the high
temperature to that at room temperature was less than the value
defined by the expression -0.0029.times.T+2.48, though both the
yield strength and the tensile strength at room temperature were
good.
[0101] In case of the comparative steel No. 37, the reheating
temperature exceeded 1,250.degree. C., and therefore austenite
grains became coarsen during the reheating and the absorbed energy
of the reproduced HAZ was low.
[0102] In case of the comparative steel No. 38, the cumulative
reduction ratio at not higher than 1,100.degree. C. was less than
30%, and therefore the prior austenite grains were coarse and the
toughness of the reproduced HAZ was low.
[0103] In case of the comparative steel No. 39, the hot rolling was
applied in a temperature of lower than 850.degree. C., and
therefore the precipitation of Nb, Ti and V was accelerated, a
sufficient precipitation hardening was not obtained, and the ratio
(p) of the yield strength at the high temperature to that at room
temperature was less than the value defined by the expression
-0.0029.times.T+2.48, though the strength at room temperature
fulfills the standard of a 490 MPa class steel.
[0104] In case of the comparative steel No. 40, the reheating
temperature was as high as 1,250.degree. C., and therefore the
austenite grains after the completion of the hot rolling were
coarse; larger than 120 .mu.m, and the toughness of the base steel
was low.
[0105] In case of the comparative steel No. 41, though it was
attempted to raise the strength at room temperature by applying the
water cooling after the hot rolling, the cooling rate in the
vicinity of the .gamma.-.alpha. transformation temperature was
insufficient at the portion in the depth of one-fourth of the sheet
thickness because of the large sheet thickness. Therefore, the
ferrite fraction was excessive (the ferrite fraction exceeding 80%
and the bainite fraction being less than 20%), the solid solution
strengthening effect at room temperature was insufficient, and thus
the tensile strength at room temperature was lower than the lower
limit of the standard value of a 490 MPa class steel for building
construction.
[0106] In case of the-comparative steel No. 42, the sheet thickness
was thicker than 25 mm, and therefore it was attempted to secure a
cooling rate of not less than 0.3 K/sec. by applying-accelerated
cooling. However, the temperature at the start of water cooling was
lower than 700.degree. C., the cooling rate during the time period
from the completion of the-hot rolling to the start of the cooling
(at 690.degree. C.) is less than 0.3 K/sec., and the transformation
of ferrite proceeds before the start of the water cooling. As a
result, the bainite fraction was less than 20% and the tensile
strength at room temperature was lower than 490 MPa. TABLE-US-00001
TABLE 1 Chemical components (mass %) Classification Steel No. C Si
Mn P S Mo Nb B Al N Invention steel 1 0.018 0.33 0.15 0.0061 0.0026
1.29 0.040 10 0.031 30 2 0.010 0.14 0.18 0.0042 0.0025 0.80 0.039
12 0.004 53 3 0.008 0.12 0.33 0.0075 0.0028 0.50 0.120 25 0.035 34
4 0.016 0.12 0.30 0.0034 0.0077 1.10 0.040 11 0.033 32 5 0.025 0.10
0.38 0.0041 0.0040 1.12 0.036 6 0.003 42 6 0.018 0.14 0.20 0.0083
0.0050 0.60 0.050 10 0.004 26 7 0.013 0.19 0.40 0.0015 0.0033 0.40
0.140 11 0.020 52 8 0.016 0.08 0.29 0.0039 0.0049 0.50 0.056 10
0.035 26 9 0.017 0.15 0.22 0.0062 0.0065 1.10 0.055 11 0.022 47 10
0.018 0.33 0.55 0.0061 0.0026 1.29 0.040 10 0.031 30 11 0.033 0.09
0.70 0.0075 0.0033 1.20 0.055 11 0.020 52 12 0.016 0.12 0.60 0.0034
0.0077 1.10 0.040 11 0.033 32 13 0.040 0.11 1.35 0.0042 0.0055 0.45
0.120 16 0.006 45 14 0.049 0.04 0.45 0.0041 0.0061 1.18 0.039 12
0.044 29 15 0.028 0.04 1.49 0.0070 0.0050 1.10 0.025 12 0.012 37 16
0.027 0.05 0.50 0.0059 0.0055 1.40 0.040 9 0.004 38 17 0.018 0.05
1.20 0.0084 0.0030 0.70 0.077 26 0.030 33 18 0.032 0.04 0.60 0.0052
0.0025 1.30 0.050 11 0.030 29 Chemical components (mass %) 1) 2)
Classification Steel No. Ni Cu Cr Ti V Ca REM Mg PCM Ceq Invention
steel 1 0.128 0.379 2 0.52 0.007 0.109 0.350 3 0.30 0.015 0.089
0.253 4 0.020 0.0015 0.114 0.346 5 0.009 0.033 0.128 0.375 6 0.058
0.097 0.261 7 0.61 0.012 0.082 0.203 8 0.021 0.045 0.0030 0.076
0.196 9 0.015 0.112 0.335 10 0.148 0.446 11 0.61 0.167 0.469 12
0.020 0.0015 0.129 0.396 13 0.015 0.045 0.0018 0.154 0.385 14
0.0011 0.158 0.421 15 0.012 0.060 0.189 0.557 16 0.30 0.167 0.462
17 0.011 0.0015 0.139 0.395 18 0.66 0.012 0.189 0.459 1) PCM = C +
Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5 .times. B
2) Ceq = C + Mn/6 + Si/24 + Ni/40 + Cr/5 + Mo/4 + V/14 *B and N are
expressed in terms of ppm.
[0107] TABLE-US-00002 TABLE 2 Chemical components (mass %)
Classification Steel No. C Si Mn P S Mo Nb B Al N Comparative Steel
19 0.082 0.10 0.38 0.0040 0.0032 0.80 0.048 10 0.003 42 20 0.004
0.15 0.28 0.0041 0.0025 0.60 0.045 12 0.004 53 21 0.015 0.05 1.93
0.0049 0.0040 1.12 0.038 6 0.003 42 22 0.010 0.12 0.90 0.0042
0.0028 0.80 0.039 10 0.004 53 23 0.019 0.14 0.21 0.0220 0.0050 1.10
0.052 10 0.004 26 24 0.014 0.20 0.50 0.0002 0.0120 1.30 0.077 18
0.030 33 25 0.016 0.12 0.30 0.0039 0.0077 0.25 0.040 11 0.033 32 26
0.014 0.20 0.80 0.0082 0.0030 1.60 0.076 15 0.030 33 27 0.018 0.18
0.60 0.0053 0.0026 1.26 0.024 0 0.000 44 28 0.022 0.14 0.78 0.0061
0.0049 1.06 0.160 8 0.004 24 29 0.018 0.18 0.72 0.0052 0.0025 1.24
0.033 8 0.008 44 30 0.016 0.08 0.40 0.0034 0.0047 1.01 0.056 10
0.035 26 31 0.025 0.10 0.51 0.0040 0.0041 1.12 0.038 3 0.003 42 32
0.012 0.12 0.33 0.0072 0.0027 0.60 0.090 34 0.035 34 33 0.016 0.08
3.29 0.0036 0.0049 1.01 0.056 10 0.065 26 34 0.011 0.14 0.22 0.0042
0.0020 1.10 0.039 12 0.004 53 35 0.020 0.28 0.64 0.0050 0.0025 1.21
0.050 18 0.030 29 36 0.016 0.14 0.62 0.0082 0.0051 1.20 0.055 15
0.007 26 37 0.014 0.16 1.20 0.0093 0.0000 1.18 0.040 9 0.006 36 38
0.014 0.20 1.20 0.0081 0.0080 1.18 0.040 9 0.006 36 39 0.014 0.20
1.20 0.0081 0.0080 1.18 0.048 9 0.006 36 40 0.008 0.12 0.33 0.0073
0.0042 0.40 0.080 20 0.035 20 41 0.018 0.15 0.55 0.0061 0.0038 1.32
0.055 15 0.004 40 42 0.016 0.08 0.48 0.0052 0.0025 0.90 0.050 11
0.030 29 Chemical components (mass %) 1) 2) Classification Steel
No. Ni Cu Cr Ti V Ca REM Mg PCM Ceq Comparative Steel 19 0.009
0.163 0.350 20 0.069 0.207 21 0.009 0.177 0.572 22 0.55 0.007 0.145
0.475 23 0.012 0.050 0.118 0.338 24 0.042 0.0016 0.146 0.434 25
0.020 0.057 0.134 26 0.011 0.044 0.0015 0.179 0.559 27 0.008 0.142
0.441 28 0.014 0.140 0.423 29 0.008 0.148 0.461 30 0.028 0.020
0.111 0.339 31 0.011 0.030 0.133 0.396 32 0.32 0.106 0.286 33 0.021
0.0016 0.156 0.487 34 0.49 0.007 0.131 0.427 35 0.40 0.35 0.50
0.012 0.040 0.204 0.554 36 0.015 0.059 0.145 0.429 37 0.010 0.163
0.516 38 0.33 0.169 0.526 39 0.33 0.012 0.169 0.526 40 0.010 0.065
0.168 41 0.012 0.146 0.446 42 0.66 0.141 0.324
[0108] TABLE-US-00003 TABLE 3 Cumulative Accelerated Accelerated
Finish hot reduction cooling cooling Reheating rolling ratio at
start stop Classi- Steel temperature temperature 1,000.degree. C.
or temperature temperature Sheet fication No. (.degree. C.)
(.degree. C.) lower (%) (.degree. C.) (.degree. C.) thickness (mm)
Invention 1 1,150 880 70 -- -- 25 steel 2 1,200 900 60 -- -- 15 3
1,100 880 50 850 450 40 4 1,150 910 70 -- -- 20 5 1,100 870 50 --
-- 25 6 1,100 900 40 680 495 50 7 1,100 970 30 820 500 30 8 1,100
950 50 820 500 32 9 1,150 880 60 -- -- 18 10 1,150 870 70 -- -- 25
11 1,100 1,000 30 -- -- 30 12 1,150 960 65 -- -- 20 13 1,100 920 50
850 580 50 14 1,100 900 50 850 480 40 15 1,100 880 60 820 650 65 16
1,100 900 60 860 600 32 17 1,150 860 60 810 590 28 18 1,150 960 50
900 620 45 2) Dissolved 4) Occurrence of Bainite 1) Composite
element 3) Prior root cracks during fraction in Ac.sub.1
carbonitride amount in austenite y-crack Classi- Steel micro-
temperature amount BCC phase grain test without fication No.
structure (%) (.degree. C.) (.times.10.sup.3) (.times.10.sup.3)
diameter (.mu.m) preheating Invention 1 45 891 1.35 7.06 55 No
crack steel 2 62 877 0.57 4.62 72 No crack 3 41 829 0.82 2.92 45 No
crack 4 40 833 1.03 6.24 56 No crack 5 59 815 0.62 2.00 88 No crack
6 46 863 1.00 4.47 43 No crack 7 63 803 1.40 2.33 51 No crack 8 44
839 1.06 2.84 66 No crack 9 50 854 1.12 6.20 55 No crack 10 85 815
1.33 7.08 55 No crack 11 73 805 2.73 5.90 51 No crack 12 55 821
1.03 6.24 56 No crack 13 85 805 1.84 1.92 82 No crack 14 75 812
4.08 4.93 59 No crack 15 100 632 0.73 4.85 76 No crack 16 81 828
2.27 7.22 78 No crack 17 88 808 1.20 3.96 73 No crack 18 89 817
2.46 6.65 62 No crack Cumulative Accelerated Accelerated Finish hot
reduction cooling cooling Reheating rolling ratio at start stop
Classi- Steel temperature temperature 1,000.degree. C. or
temperature temperature Sheet fication No. (.degree. C.) (.degree.
C.) lower (%) (.degree. C.) (.degree. C.) thickness (mm) Compara-
19 1,150 950 60 -- -- 18 tive 20 1,150 925 50 -- -- 15 steel 21
1,150 940 50 -- -- 20 22 1,150 900 35 -- -- 25 23 1,100 875 40 820
550 40 24 1,100 920 50 -- -- 27 25 1,050 915 50 -- -- 16 26 1,050
960 60 -- -- 15 27 1,100 950 50 -- -- 22 28 1,100 920 60 -- -- 25
29 1,150 930 60 880 550 25 30 1,150 925 60 880 580 45 31 1,100 940
60 -- -- 18 32 1,150 970 60 -- -- 16 33 1,100 890 60 -- -- 16 34
1,200 915 55 900 595 50 35 1,100 920 60 880 550 35 36 980 880 50
850 550 25 37 1,280 995 40 -- -- 25 38 1,200 980 25 -- -- 16 39
1,100 830 70 -- -- 16 40 1,250 960 50 -- -- 25 41 1,150 960 60 850
600 70 42 1,100 900 60 790 445 40 2) Dissolved 4) Occurrence of
Bainite 1) Composite element 3) Prior root cracks during fraction
in Ac.sub.1 carbonitride amount in austenite y-crack Classi- Steel
micro- temperature amount BCC phase grain test without fication No.
structure (%) (.degree. C.) (.times.10.sup.3) (.times.10.sup.3)
diameter (.mu.m) preheating Compara- 19 100 810 0.63 68 No crack
tive 20 25 834 0.45 3.49 52 No crack steel 21 100 774 0.78 6.39 87
No crack 22 45 805 0.56 4.62 48 No crack 23 52 842 1.11 2.29 55 No
crack 24 64 810 1.18 7.47 83 No crack 25 58 837 0.74 1.41 52 No
crack 26 100 812 1.14 9.26 62 Cracking 27 52 823 1.19 6.87 74 No
crack 28 66 812 2.50 6.21 84 No crack 29 70 809 1.19 6.94 135 No
crack 30 54 816 1.23 5.81 42 No crack 31 15 802 1.69 5.88 64 No
crack 32 69 828 1.04 3.46 58 No crack 33 55 908 1.15 5.79 72 No
crack 34 52 858 0.65 6.34 81 No crack 35 48 834 1.38 6.69 67
Cracking 36 58 807 1.13 6.84 58 No crack 37 70 812 0.88 6.74 124 No
crack 38 68 808 0.90 6.73 145 No crack 39 62 815 0.91 6.74 53 No
crack 40 100 824 0.79 2.33 162 No crack 41 15 807 1.24 7.42 86 No
crack 42 10 825 0.92 5.09 91 No crack 1) Thermodynamically
calculated value of molar fraction in phase at 700.degree. C. 2)
Thermodynamically calculated value of molar fraction at 700.degree.
C. 3) Average circle-equivalent diameter of prior austenite grains
at the position in the depth of one-fourth of sheet thickness on
cross section in the direction of the final rolling of steel sheet
4) JIS Z 3158: Oblique y-shaped weld cracking test
[0109] TABLE-US-00004 TABLE 4 Strength at room 800.degree. C.
temperature 700.degree. C. 4) Resultant 1) Yield 2) Tensile Yield
3) Yield Yield yield 5) Toughness of Classi- Steel strength
strength Yield vTrs strength strength strength strength reproduced
fication No. (MPa) (MPa) ratio (%) (.degree. C.) (MPa) Ratio (%)
(MPa) Ratio (%) HAZ .sub.vE.sub.o (J) Invention 1 366 499 73 -51
236 64 85 23 220 steel 2 409 530 77 -40 267 65 94 23 210 3 353 489
72 -32 232 66 86 24 199 4 348 486 72 -35 225 65 81 23 187 5 408 530
77 -37 263 64 93 23 225 6 362 496 73 -40 237 65 85 24 218 7 421 539
78 -35 274 65 97 23 155 8 357 492 73 -41 233 65 84 24 230 9 375 506
74 -33 246 65 88 24 224 10 516 699 74 -51 337 65 135 26 250 11 521
689 76 -45 374 72 137 26 205 12 468 686 68 -45 325 69 121 26 227 13
535 723 74 -42 327 61 121 23 238 14 483 729 66 -40 335 69 124 26
241 15 551 680 81 -42 377 68 136 25 254 16 492 703 70 -43 346 70
128 26 271 17 524 721 73 -46 386 74 143 27 242 18 506 699 72 -52
434 68 128 25 227 Strength at room temperature 700.degree. C.
800.degree. C. 1) Yield 2) Tensile Yield 3) Yield Yield 4) Yield 5)
Toughness of Classi- Steel strength strength Yield vTrs strength
strength strength strength reproduced fication No. (MPa) (MPa)
ratio (%) (.degree. C.) (MPa) Ratio (%) (MPa) Ratio (%) HAZ
.sub.vE.sub.o (J) Compara- 19 516 610 85 -30 180 35 65 13 198 tive
20 304 453 67 -41 128 42 44 14 210 steel 21 621 755 82 -35 272 44
89 14 188 22 320 465 69 -28 201 63 84 26 225 23 380 509 75 -1 249
66 89 23 18 24 420 539 78 -5 278 66 101 24 22 25 402 525 77 -34 181
45 62 15 165 26 554 670 83 -45 351 63 123 22 25 27 383 511 75 -30
168 44 52 14 217 28 424 542 78 -25 281 66 101 24 25 29 438 552 79
-40 245 56 82 19 22 30 388 515 75 -2 253 65 90 23 21 31 276 433 64
-21 175 63 66 24 188 32 437 551 79 -6 288 66 103 24 15 33 390 517
76 -1 255 65 91 23 38 34 379 508 75 -25 248 65 88 23 21 35 373 504
74 -28 243 65 88 24 42 36 398 523 76 -41 172 43 54 14 220 37 435
550 79 -40 285 66 100 23 24 38 431 547 79 -32 282 65 99 23 22 39
413 533 77 -39 178 43 58 14 215 40 530 620 85 -45 350 66 122 23 198
41 291 444 66 -38 193 66 73 25 206 42 291 444 66 -38 193 66 73 25
206 1) Yield strength at room temperature .gtoreq. 325 MPa 2)
Tensile strength at room temperature .gtoreq. 490 MPa 3) Y.S ratio
of yield strength at 700.degree. C. to that at room temperature (p)
.gtoreq. 45% 4) Y.S ratio of yield strength at 800.degree. C. to
that at room temperature (p) .gtoreq. 16% 5) PT: 1,400.degree. C.,
.DELTA.t8/5 = 99S, .sub.vE.sub.o .gtoreq. 27J
INDUSTRIAL APPLICABILITY
[0110] A steel that has a specific chemical components and is
produced by a method according to the present invention: has a
microstructure comprising a composite structure composed of ferrite
and bainite or a single structure composed of bainite; is a
high-tensile-strength steel having a strength of not lower than 490
MPa at room temperature; has the feature of satisfying the
expression p.gtoreq.-0.0029.times.T+2.48 when the steel material
temperature T (.degree. C.) is in the temperature range from
600.degree. C. to 800.degree. C., wherein p is the ratio of a
stress at a high temperature to that at room temperature (a yield
stress at a high temperature/a yield stress at room temperature);
thus has the properties required of a fire-resistant steel for
building construction; and is an entirely novel steel with
qualities beyond those of all previous steels.
* * * * *