U.S. patent application number 10/606081 was filed with the patent office on 2004-01-08 for structural fe-cr steel sheet, manufacturing method thereof, and structural shaped steel.
This patent application is currently assigned to JFE Steel Corporation, a corporation of Japan. Invention is credited to Furukimi, Osamu, Kasamo, Toshihiro, Matsuo, Noriyuki, Nakashima, Hiroyuki, Ota, Hiroki, Shigemi, Masato, Ujiro, Takumi.
Application Number | 20040003876 10/606081 |
Document ID | / |
Family ID | 29720288 |
Filed Date | 2004-01-08 |
United States Patent
Application |
20040003876 |
Kind Code |
A1 |
Ota, Hiroki ; et
al. |
January 8, 2004 |
Structural Fe-Cr steel sheet, manufacturing method thereof, and
structural shaped steel
Abstract
A structural Fe--Cr steel sheet and a manufacturing method
thereof are provided, the steel sheet having an as-hot rolled
tensile strength of about 400 to about 450 MPa and generating no
embrittlement in welded portions even when rapid heating and
cooling is performed by electric resistance welding. In addition,
structural shaped steel is also provided which is formed from the
steel sheet described above by using electric resistance
welding.
Inventors: |
Ota, Hiroki; (Chiba, JP)
; Ujiro, Takumi; (Chiba, JP) ; Furukimi,
Osamu; (Chiba, JP) ; Matsuo, Noriyuki; (Chiba,
JP) ; Nakashima, Hiroyuki; (Chiba, JP) ;
Shigemi, Masato; (Chiba, JP) ; Kasamo, Toshihiro;
(Chiba, JP) |
Correspondence
Address: |
IP DEPARTMENT OF PIPER RUDNICK LLP
3400 TWO LOGAN SQUARE
18TH AND ARCH STREETS
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE Steel Corporation, a
corporation of Japan
Tokyo
JP
|
Family ID: |
29720288 |
Appl. No.: |
10/606081 |
Filed: |
June 25, 2003 |
Current U.S.
Class: |
148/602 ;
148/609; 420/60 |
Current CPC
Class: |
C22C 38/04 20130101;
C22C 38/46 20130101; C22C 38/42 20130101; C22C 38/02 20130101; C21D
8/0263 20130101; C22C 38/004 20130101; C21D 8/0226 20130101 |
Class at
Publication: |
148/602 ;
148/609; 420/60 |
International
Class: |
C22C 038/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 4, 2002 |
JP |
2002-195479 |
Claims
What is claimed is:
1. A structural Fe--Cr steel sheet comprising: about 0.0025 to
about 0.010 mass % of C; about 0.0025 to about 0.010 mass % of N;
about 0.015 mass % or less of C+N; about 0.01 to about 1.0 mass %
of Si; about 0.01 to about 0.30 mass % of Mn; about 0.04 mass % or
less of P; about 0.03 mass % or less of S; about 8 mass % to less
than about 10 mass % of Cr; about 0.01 to about 1.0 mass % of Cu;
about 0.01 to about 1.0 mass % of Ni; about 0.01 to about 0.20 mass
% of V; about 0.05 mass % or less of Al; and the balance being Fe
and incidental impurities, wherein the tensile strength is about
400 to about 450 MPa.
2. The structural Fe--Cr steel sheet according to claim 1, further
comprising about 1.0 mass % or less of Mo.
3. A method for manufacturing a structural Fe--Cr steel sheet,
comprising: heating a steel raw material to a temperature of about
1,100 to about 1,280.degree. C., which comprises about 0.0025 to
about 0.010 mass % of C; about 0.0025 to about 0.010 mass % of N;
about 0.015 mass % or less of C+N; about 0.01 to about 1.0 mass %
of Si; about 0.01 to about 0.30 mass % of Mn; about 0.04 mass % or
less of P; about 0.03 mass % or less of S; about 8 mass % to less
than about 10 mass % of Cr; about 0.01 to about 1.0 mass % of Cu;
about 0.01 to about 1.0 mass % of Ni; about 0.01 to about 0.20 mass
% of V; about 0.05 mass % or less of Al; and the balance being Fe
and incidental impurities; hot rolling the steel raw material into
a steel sheet; finishing the hot rolling at a temperature of more
than about 930.degree. C.; coiling the steel sheet at a temperature
of more than about 810.degree. C. to form a coil; and cooling the
coil at an average cooling rate of about 2.degree. C./min or less
from about 800 to about 400.degree. C., which is an average cooling
rate of inside the coil, to obtain a tensile strength of about 400
to about 450 MPa.
4. The manufacturing method according to claim 3, wherein the hot
rolling comprises rough rolling with at least one pass with a
reduction in thickness of about 30% or more at a temperature of
more than about 1,000.degree. C.
5. The manufacturing method according to claim 3, wherein the steel
raw material further comprises about 1.0 mass % or less of Mo.
6. The manufacturing method according to claim 5, wherein the hot
rolling comprises rough rolling with at least one pass with a
reduction in thickness of about 30% or more at a temperature of
more than about 1,000.degree. C.
7. The manufacturing method according to claim 3, wherein, during
cooling, the average cooling rate of about 2.degree. C./min or less
from about 800 to about 400.degree. C. is a cooling rate of
substantially every point of the entire coil.
8. The manufacturing method according to claim 4, wherein, during
cooling, the average cooling rate of about 2.degree. C./min or less
from about 800 to about 400.degree. C. is a cooling rate of
substantially every point of the entire coil.
9. The manufacturing method according to claim 5, wherein, during
cooling, the average cooling rate of about 2.degree. C./min or less
from about 800 to about 400.degree. C. is a cooling rate of
substantially every point of the entire coil.
10. The manufacturing method according to claim 6, wherein, during
cooling, the average cooling rate of about 2.degree. C./min or less
from about 800 to about 400.degree. C. is a cooling rate of
substantially every point of the entire coil.
11. The manufacturing method according to claim 7, wherein the
cooling is performed with a heat insulating box, a heat insulating
cover, or a heat insulating furnace.
12. The manufacturing method according to claim 8, wherein the
cooling is performed with a heat insulating box, a heat insulating
cover, or a heat insulating furnace.
13. The manufacturing method according to claim 9, wherein the
cooling is performed with a heat insulating box, a heat insulating
cover, or a heat insulating furnace.
14. The manufacturing method according to claim 10, wherein the
cooling is performed with a heat insulating box, a heat insulating
cover, or a heat insulating furnace.
15. Structural shaped steel formed by electric resistance welding
using the steel sheet according claim 1.
16. Structural shaped steel formed by electric resistance welding
using the steel sheet according claim 2.
17. Structural shaped steel formed by electric resistance welding
using the steel sheet formed by the manufacturing method according
to claim 3.
18. Structural shaped steel formed by electric resistance welding
using the steel sheet formed by the manufacturing method according
to claim 4.
19. Structural shaped steel formed by electric resistance welding
using the steel sheet formed by the manufacturing method according
to claim 7.
20. Structural shaped steel formed by electric resistance welding
using the steel sheet formed by the manufacturing method according
to claim 11.
Description
BACKGROUND
[0001] 1. Field of the Invention
[0002] This invention relates to structural Fe--Cr steel sheets,
which have a strength equivalent to that of SS400 defined by
Japanese Industrial Standard (JIS) G 3101 (1995) and which are used
for civil engineering and architectural structures such as bridges
and housing structures, requiring superior corrosion resistance,
durability, weldability, and characteristics of welded portions.
More particularly, this invention relates to a structural Fe--Cr
steel sheet in which variations in strength in a coil thereof does
not substantially exist and in which deteriorates in the strength
of welded portions does not substantially occur even when
fabrication is performed by electric resistance welding which
causes extremely rapid heating and cooling; to a manufacturing
method of the structural Fe--Cr steel sheet described above; and to
a structural shaped steel manufactured therefrom.
[0003] 2. Description of the Related Art
[0004] In addition to strength, corrosion resistance and durability
have been required for civil engineering and architectural
structures. Accordingly, as materials for use in the application
described above, primarily used are ordinary steel such as SS400
defined by JIS G 3101 (1995) and SN400B defined by JIS G 3136
(1994); high tensile steel such as SM490 defined by JIS G 3106
(1999); and materials formed of the steel described above
processed, for example, by coating, plating, and cationic
electrodeposition (electrophoretic deposition) coating. In
addition, as materials used for general architectural structures,
various shaped steel, such as welded lightweight H-shaped steel
defined by JIS G 3353 (1990) including SWH400, have been used. In
addition, in recent years, concomitant with diversification of
design and increasing attention to environmental concern, studies
on the use of various types of materials have been carried out.
[0005] Among the various types of materials, Fe--Cr steel, which
has superior corrosion resistance and higher design performance,
has drawn intensive attention as a very attractive candidate in
view of life cycle cost (LCC). The reason for this is that the
maintenance cost of the steel described above for plating
treatment, anti-rusting coating, touch-up treatment after punching
or welding, and the like, which primarily relate to preventing rust
generation, is not substantially required.
[0006] Among various types of Fe--Cr steel, described above, as a
civil engineering and architectural structural material, an
austenitic stainless steel such as SUS304A defined by JIS G 4321
(2000) has been studied, which has been most widely used because of
its material strength, corrosion resistance, easy weldability,
toughness of welded portions, and easy availability. This
austenitic stainless steel has sufficient characteristics used as a
civil engineering and architectural material in view of the
strength, corrosion resistance, fire resistance, toughness of
welded portions and the like. However, since the austenitic
stainless steel contains a large amount of alloying elements such
as nickel (Ni) and chromium (Cr), the price is considerably high as
compared to that of ordinary steel. Hence, it has been difficult to
substitute the stainless steel described above for conventional
ordinary steel, high tensile steel, and materials formed of the
steel mentioned above processed by plating or coating, and as a
result, a problem has occurred in that the application of the
austenitic stainless steel has been extremely limited.
[0007] To solve the problem described above, improvement of
martensitic stainless steel, such as SUS410 or SUS410S, defined by
JIS G 4304 (1999), containing no expensive Ni and a relatively
small amount of Cr, has been performed to develop a material for
use in civil engineering and architectural application. The
martensitic stainless steel has advantages in that there is no
concern about a embrittlement, 475.degree. C. embrittlement, and
the like, which are problems for a high Cr alloy, and that stress
corrosion cracking, which is a problem for austenitic stainless
steel, does not substantially occur under the conditions in which a
chloride is present.
[0008] For example, martensitic stainless steel for welded
structures, having improved characteristics of welded portions, has
been disclosed in Japanese Examined Patent Publication No.
51-13463, in which the contents of Cr, Ni, silicon (Si), and
manganese (Mn) are 10 to 18 wt %, 0.1 to 3.4 wt %, 1.0 wt % or
less, and 4.0 wt % or less, respectively. Further, the contents of
carbon (C) and nitrogen (N) are decreased to 0.03 wt % or less and
0.02 wt % or less, respectively; and a massive martensitic
structure is generated in a welding heat-affected zone. In
addition, structural martensitic stainless steel, having high
toughness of a welded portion and superior workability, has been
disclosed in Japanese Examined Patent Publication No. 57-28738, in
which the contents of Cr, Si, and Mn are 10 to 13.5 wt %, 0.5 wt %
or less, and 1.0 to 3.5 wt %, respectively; the contents of C and N
are decreased to 0.02 wt % or less and 0.02 wt % or less,
respectively; and the content of Ni is limited to less than 0.1 wt
% so that preheating before and post heating after welding are not
required.
[0009] In Japanese Unexamined Patent Publication No. 2002-53938, a
technique has been disclosed for improving initial rust resistance,
workability, and weldability of an alloy, in which cobalt (Co),
vanadium (V), and tungsten (W) are particularly added in
combination to a Fe--Cr alloy containing Cr in the range of more
than 8 mass % to less than 15 mass %, and in addition, increase of
Ni, copper (Cu), Cr, molybdenum (Mo), and the like, addition of
titanium (Ti) and niobium (Nb), and excessive decrease of C and N
are not performed. However, the steel materials disclosed in
Japanese Examined Patent Publication Nos. 51-13463 and 57-28738,
have an excessively high as-hot rolled strength, annealing must be
performed after hot rolling, and hence there have been problems
with cost and smooth delivery. In addition, the technique disclosed
in Japanese Unexamined Patent Publication No. 2002-53938, the
addition of Co, V, and W in combination must be performed. Also,
annealing of the hot rolled sheet is recommended for softening.
[0010] Accordingly, development of cost-reduction techniques, for
example, for decreasing the amount of an alloy element or by
omitting an annealing step of a hot rolled sheet, has been
implemented. For example, in Japanese Unexamined Patent Publication
No. 11-302737, a technique for omitting annealing of a hot rolled
sheet has been disclosed, in which a steel raw material, containing
8 to 16 wt % of Cr, 0.05 to 1.5 wt % of Si, and 0.05 to 1.5 wt % of
Mn, and containing C, N, and C+N at decreased contents of 0.005 to
0.1 wt %, 0.05 wt % or less, and 0.1 wt % or less, respectively, is
heated to 1,100 to 1,250.degree. C., and is then hot rolled. After
hot rolling is finished at 800.degree. C. or more, coiling is
performed at 700.degree. C. or more, and cooling is then performed
at an average cooling rate of 5.degree. C./min or less. However,
the steel material disclosed in Japanese Unexamined Patent
Publication No. 11-302737 has a tensile strength of more than 450
MPa, and hence when the material described above is formed into
shaped steel or pipes or is processed by secondary elaboration,
drilling, and the like, it is difficult to used the production line
designed for SS400, which has been processed, without any
enhancement of the line.
[0011] In addition, although having superior arc weldability such
as MIG using a welding rod, the steel material formed by the
conventional technique described above does not have sufficient
measures against hardening and embrittlement problems of welded
portions which is rapidly heated and cooled, for example, by
electric resistance welding. For example, a technique for
manufacturing structural welded lightweight H-shaped steel has been
disclosed in Japanese Unexamined Patent Publication No. 2-305939,
in which a steel material, containing 3.5 to less than 10.5 wt % of
Cr, 0.01 to 1.0 wt % of Si, and 0.01 to 2.5 wt % of Mn, and
containing C and N at decreased contents of 0.001 to 0.1 wt % and
0.001 to 0.10 wt %, respectively, is welded by electric resistance
welding in a non-oxidizing atmosphere or in a reducing flame
shield. However, according to the technique described above, when
welding is performed in the air, a so-called "penetrator", an oxide
generated by heating during the welding, is not removed but that it
remains, and as a result, a problem may arise in that breakage
occurs at welded portions in a step of applying a tensile force.
Accordingly, another problem may arise in that facilities for
controlling the atmosphere become necessary.
[0012] As described above, since many hot rolled Fe--Cr steel
sheets formed by conventional techniques have an as-hot rolled
tensile strength of more than 450 MPa, a problem may arise when
production facilities, which have been used for manufacturing
shaped steel using SS400, are used without any enhancement. In
particular, the front and rear ends in the longitudinal direction
and edge portions in the width direction of a hot rolled coil must
be cut away since the strength thereof is largely increased, and as
a result, the production yield is unfortunately decreased. In
addition, since the steel sheets manufactured by conventional
techniques do not have sufficient measures against hardening and
embrittlement problems of welded portions which are rapidly heated
and cooled by electric resistance welding or the like, a problem
may arise when the steel sheets described above are used as a raw
material for forming welded lightweight H-shaped steel and electric
resistance welded (ERW) tubes by electric resistance welding.
[0013] In consideration of the problems described above, it would
be advantageous to provide a structural Fe--Cr steel sheet and an
inexpensive manufacturing method thereof, the Fe--Cr steel sheet
having an as-hot rolled tensile strength, that is, a tensile
strength of a hot rolled sheet obtained without annealing, of 400
to 450 MPa in the entire longitudinal and width directions of a
coil of the steel sheet, and generating no embrittlement in welded
portions even when rapid heating and cooling are performed by
electric resistance welding. In addition, it would be advantageous
to provide structural shaped steel which is formed by electric
resistance welding using the steel sheet described above.
SUMMARY OF THE INVENTION
[0014] Accordingly, a summary of the invention is as follows.
[0015] In accordance with one aspect of the invention, a structural
Fe--Cr steel sheet comprises: about 0.0025 to about 0.010 mass % of
C; about 0.0025 to about 0.010 mass % of N; about 0.015 mass % or
less of C+N; about 0.01 to about 1.0 mass % of Si; about 0.01 to
about 0.30 mass % of Mn; about 0.04 mass % or less of phosphorous
(P); about 0.03 mass % or less of sulfur (S); about 8 mass % to
less than about 10 mass % of Cr; about 0.01 to about 1.0 mass % of
Cu; about 0.01 to about 1.0 mass % of Ni; about 0.01 to about 0.20
mass % of V; about 0.05 mass % or less of aluminum (Al); and the
balance being iron (Fe) and incidental impurities, wherein the
tensile strength is about 400 to about 450 MPa.
[0016] The structural Fe--Cr steel sheet of the invention may
further comprise about 1.0 mass % or less of Mo.
[0017] According to another aspect of the invention, a method for
manufacturing a structural Fe--Cr steel sheet comprises: heating a
steel raw material to a temperature of about 1,100 to about
1,280.degree. C., which comprises about 0.0025 to about 0.010 mass
% of C; about 0.0025 to about 0.010 mass % of N; about 0.015 mass %
or less of C+N; about 0.01 to about 1.0 mass % of Si; about 0.01 to
about 0.30 mass % of Mn; about 0.04 mass % or less of P; about 0.03
mass % or less of S; about 8 mass % to less than about 10 mass % of
Cr; about 0.01 to about 1.0 mass % of Cu; about 0.01 to about 1.0
mass % of Ni; about 0.01 to about 0.20 mass % of V; about 0.05 mass
% or less of Al; and the balance being Fe and incidental
impurities: hot rolling the steel raw material into a steel sheet;
finishing the hot rolling at a temperature of more than about
930.degree. C.; coiling the steel sheet at a temperature of more
than about 810.degree. C. to form a coil; and cooling the coil at
an average cooling rate of about 2.degree. C./min or less from
about 800 to about 400.degree. C., which is an average cooling rate
of inside the coil, to obtain a tensile strength of about 400 to
about 450 MPa.
[0018] In the manufacturing method described above, when higher
corrosion resistance is required, the steel raw material may
further comprise about 1.0 mass % or less of Mo.
[0019] In the manufacturing method of the invention, described
above, hot rolling may comprise rough rolling at least one pass
with a reduction in thickness of about 30% or more at a temperature
of more than about 1,000.degree. C.
[0020] In the cooling the coil step of the manufacturing method of
the invention, the average cooling rate of about 2.degree. C./min
or less from about 800 to about 400.degree. C. is preferably a
cooling rate of every point of the entire coil, and in addition,
the cooling the coil step is preferably performed by using one of a
heat insulating box, a heat insulating cover, and a heat insulating
furnace.
[0021] According to another aspect of the invention, structural
shaped steel is formed by electric resistance welding using the
steel sheet described above or the steel sheet formed by the
manufacturing method described above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a graph showing an example of results obtained by
calculating the temperature history of a hot rolled coil after
coiling;
[0023] FIG. 2 is a graph showing an example of results obtained by
calculating the temperature history of a hot rolled coil which is
covered with a heat insulating cover after coiling;
[0024] FIG. 3 is an example of the heat insulating cover; and
[0025] FIG. 4 is a graph showing cooling curves shown in FIG. 2 and
a curve of a cooling rate of 2.degree. C./min together with CCT
diagrams.
DETAILED DESCRIPTION
[0026] We focused on a low alloy steel containing Cr from about 8
mass % to less than about 10 mass % and carried out an intensive
study of compositions of the steel, having an as-hot rolled
strength of abut 400 to about 450 MPa and superior electric
resistance weldability, and the manufacturing method thereof to
obtain a material having corrosion resistance necessary for civil
engineering and architectural structures and to realize cost
reduction. As a result, it was found that in a steel sheet
containing Cr from about 8 mass % to less than about 10 mass %,
since a fine martensitic structure is formed in welding
heat-affected zones, to prevent embrittlement of the welded
portions, it is important to prevent the martensitic structure in
the heat-affected zone from being excessively hardened.
[0027] The hardness of a martensite phase largely depends on the
content of C and N dissolved in a steel material. Accordingly, by a
conventional technique using a welding method such as arc welding
in which a cooling rate after welding is relatively slow, the
content of C and N in a steel sheet can be decreased, and in
addition, by annealing a hot rolled steel sheet, a steel can be
obtained having a strength of 400 to 450 MPa, superior workability
and weldability, and in addition, having excellent toughness of a
welded portion. However, when electric resistance welding, which
has been used for manufacturing welded lightweight H-shaped steel
or an electric resistance welded tube, is applied to a conventional
steel sheet, hardening in a heat-affected zone considerably occurs,
a problem may arise in that sufficient balance between strength and
toughness cannot be obtained. In particular, the embrittlement
described above occurs most considerably in a portion heated to 800
to 900.degree. C. in welding.
[0028] The reasons for this have been believed as described below.
A conventional structual steel in a temperature range of from 800
to 900.degree. C. has a two-phase structure of ferrite
phase(.alpha.)+austenite phase(.gamma.). However, the contents of C
and N dissolved in the a and y phases are very different from each
other. In portions made of the .gamma. phase, the contents of C and
N are increased as compared to those in a single .gamma. phase. The
.gamma. phase containing such concentrated C and N is transformed
to a hard martensite phase during cooling after welding, and as a
result, embrittlement occurs in welded portions. However, by
general arc welding, since the vicinity of the welded portion is
air-cooled (spontaneously cooled) after welding, hardening of the
martensite phase described above does not occur much. On the other
hand, by electric resistance welding, rapid heating and cooling
considerably occur concomitant with welding, and in addition, when
the vicinity of welding portions is water-cooled to prevent
overheating of peripheral devices such as a welding tip, since a
steel sheet in the vicinity of welding portions is very rapidly
cooled immediately after welding to form a hard martensite phase,
embrittlement considerably occurs. Accordingly, the control of
composition and microstructure of the steel material becomes
increasingly important for a steel material processed by electric
resistance welding.
[0029] To solve the problem of embrittlement of welded portions, we
first tried to decrease the contents of C and N. However, an
excessive decrease in C and N did not only cause a decrease in
martensite production ability of the welding heat-affected zone,
but also produced so-called "coarse" and "large" ferritic grains,
and as a result, the characteristics of the welded portion were
degraded. In addition, in the case in which a strong element, such
as Ti or Nb, forming a carbonitride was added, the contents of the
dissolved C and N were also excessively decreased, and as a result,
the same result was unfortunately obtained.
[0030] Accordingly, to improve the electric resistance welding
properties, we considered that it is necessary to improve the
microstructure of a ferrite phase+martensite phase, which is
generated by heating to the two-phase temperature region of
.alpha.+.gamma. in welding followed by cooling. Hence, we carried
out a further intensive study focusing on two aspects, that is,
improvement in toughness of the ferrite phase by forming finer
ferritic crystal grains of a parent material in addition to
prevention of hardening of the martensite phase. As a result, we
discovered that by adding an appropriate amount of V, in addition
to decreasing the contents of C and N, the increase in hardness of
the martensite phase generated at the two-phase temperature region
can be suppressed. In addition, by performing rough rolling at
least one pass with an reduction in thickness of about 30% or more
in hot rolling, we also discovered that a finer ferrite structure,
which is the parent material, can be obtained and that, as a
result, embrittlement which occurs at the two-phase temperature
region by electric resistance welding can be significantly
improved. In addition, among the components of the steel, in
particular, in addition to decreased Cr and Mn, when an appropriate
amount of Cu is added, we discovered that generation of a
penetrator remaining in the welded potion can be suppressed and
that even in air, electrical resistance welding can be
advantageously performed.
[0031] Further, we carried out an intensive study of a method for
obtaining a coil having an as-hot rolled strength of 400 to 450 MPa
in the entire longitudinal and width directions thereof. First, in
order to accurately grasp the cooling rate of the coil, a
thermocouple was actually fixed to a coil which was hot rolled, and
the temperature thereof was measured with time. Based on the
results, the cooling rate at each position of the coil was
calculated. The calculation was performed assuming that the coil
weight was 12,300 kg, the coil width was 1,450 mm, the inside
diameter was 760 mm, the coiling temperature was 850.degree. C.,
and the outdoor air temperature was 20.degree. C.
[0032] One example of the results is shown in FIG. 1. As can be
seen from FIG. 1, at a coldest point Tmin (an edge portion in the
width direction of the outermost portion of the coil, hereinafter
referred to as a "coil coldest point"), the temperature was
decreased to approximately 400.degree. C. for just approximately 30
minutes, and it was understood that cooling was performed at a high
speed of approximately 13.degree. C./min between 800 to 400.degree.
C. As a result, it is believed that at the front and rear ends in
the longitudinal direction (inner portion and outer portion) of the
coil and the edge portions in the width direction thereof, many
hard phases, such as a martensite phase and a bainite phase, are
formed and are further hardened.
[0033] Accordingly, we collected metallurgical data, such as
continuous cooling transformation diagrams (CCT diagram),
isothermal time-temperature-transformation diagrams (TIT diagram),
and the like, of an alloy containing Cr in the range of from 8 mass
% to less than 10 mass %, and the transformation behavior was
investigated when heat insulation was performed during the cooling.
To prevent the front and the rear end portions in the longitudinal
direction of the coil and the edge portions in the width direction
thereof from being hardened, heat insulation is started by using a
certain type of means after coiling is completed and before the
portions described above are cooled to less than about 400.degree.
C. so that the average cooling rate from about 800 to 400.degree.
C. is controlled to be 2.degree. C./min or less by a recuperation
effect of increasing temperature using the heat inside the coil and
an effect of slow cooling obtained by suppressing heat dissipation
using the heat insulation.
[0034] As a result, it was found that desired softening can be
achieved in the entire longitudinal and width directions of an
as-hot rolled coil. In the invention, the average cooling rate does
not mean that an average cooling rate of about 2.degree. C./min or
less must be controlled at every moment from about 800 to about
400.degree. C., but it means that the time for cooling from about
800 to about 400.degree. C. is controlled to be about 200 minutes
or more so that an average cooling rate of about 2.degree. C./min
or less is obtained.
[0035] FIG. 2 shows an example of results obtained by calculating
the coil temperature with time in the case in which, as shown in
FIG. 3, a coil was covered with a heat insulating cover after 30
minutes from the completion of the coiling, wherein the heat
insulating cover was lined with an insulating material 100 mm thick
and was used as one heat insulating means. From FIG. 2, it is
understood that by using the heat insulating cover, since even the
coldest point Tmin of the coil was cooled from about 800 to about
400.degree. C. for about 400 minutes or more, cooling can be
performed at an average cooling rate of about 1.degree. C./min or
less.
[0036] In addition, in FIG. 4, the cooling curves shown in FIG. 2,
a curve showing a cooling rate of 2.degree. C./min, and CCT
diagrams are shown all together. Marks F, B, and M in FIG. 4
indicate generation regions of a ferrite phase, a bainite phase,
and a martensite phase, respectively. In the case in which the
cooling is continuously performed at a constant cooling rate, when
the cooling rate from about 800 to about 400.degree. C. is set to
about 2.degree. C./min or less, that is, when the cooling from
about 800 to about 400.degree. C. is performed for about 12,000
seconds (about 200 minutes) or more, it is understood that a soft
single ferrite phase structure can be obtained with no generation
of a bainite phase. In addition, when the heat insulation is
performed, an optional position of the coil is processed by the
temperature history represented by the region surrounded by Tmax
and Tmin. However, when the heat insulation is performed before the
temperature, even at the coldest point shown by the line of Tmin,
is decreased to less than about 400.degree. C., it is understood
that the generation of a hard martensite phase can be substantially
suppressed. Furthermore, it is also understood that a bainite phase
generated in a part of the coil by cooling performed before the
heat insulation can be transformed to tempered bainite or a ferrite
phase by tempering because of the recuperation effect after the
heat insulation, and that as a result, softening can be achieved.
Accordingly, when the insulating cover is used, by the
manufacturing method of the invention, a Fe--Cr steel sheet for use
in architectural structure applications can be provided at a
reasonable price.
[0037] Hereinafter, selected embodiments of the invention will be
described in detail.
[0038] First, the reason the composition of the steel sheet of the
invention is limited will be described.
[0039] C: about 0.0025 to about 0.010 mass %, N: about 0.0025 to
about 0.010 mass %, and C+N: about 0.015 mass % or less
[0040] The welding heat-affected zone of the steel according to the
invention forms a fme martensitic structure. The contents of C and
N have a large influence on the hardness of the martensite phase
generated in the welding heat-affected zone. It is effective to
decrease the contents of C and N to improve the toughness and the
workability of the welding heat-affected zone so as to prevent weld
cracking, as has been known. However, in addition to the increase
in refining cost, excessive decrease in C and N decreases the
martensite production ability of the welding heat-affected zone,
facilitates the generation of coarse and large ferritic grains, and
as a result, considerably decreases the toughness of welded
portions. Hence, the contents of C and N are each set to about
0.0025 mass % or more. On the other hand, the upper limits of the
contents of C, N, and C+N are set to about 0.010 mass %, about
0.010 mass %, and about 0.015 mass %, respectively to prevent
extreme increase in hardness of the martensite phase generated in
the welding heat-affected zone and to prevent the embrittlement
thereof. Preferable ranges of the contents of C and N are about
0.003 to about 0.008 mass % and about 0.0030 to about 0.0060 mass
%, respectively. In particular, when electric resistance welding is
performed in air, the content of N is preferably set to about 0.006
mass % or less. In addition, the content of C is more preferably in
the range of from about 0.003 to about 0.005 mass %.
[0041] Si: about 0.01 to about 1.0 mass %
[0042] Si is an element added to be used as a deoxidizing agent and
also to increase strength. Sufficient deoxidizing effect cannot be
obtained when the content is less than about 0.01 mass %. On the
other hand, when the content is excessively high such as about 1.0
mass % or more, in addition to the decrease in toughness and
workability, the martensite production ability of the welding
heat-affected zone is decreased. Accordingly, the content of Si is
set to the range of from about 0.01 to about 1.0 mass %. The
content is particularly preferable in the range of from about 0.1
to about 0.5 mass %.
[0043] Mn: about 0.01 to about 0.30 mass %
[0044] Mn is an element stabilizing an austenite phase (.gamma.
phase) and allowing the welding heat-affected zone to have a
martensitic structure, thereby contributing to improvement in
toughness of the welded portion. However, when the content is
excessively high, the ratio of an as-hot rolled hard phase is
increased, and as a result, the targeted tensile strength (about
400 to about 450 MPa) cannot be obtained. In addition, the hardness
of the martensite generated at the two-phase temperature region by
electric resistance welding is increased to cause the embrittlement
thereof. Further, MnS is formed to decrease the corrosion
resistance. Accordingly, the upper limit of the Mn content is set
to about 0.30 mass %. On the other hand, the lower limit of the Mn
content is set to about 0.01 mass % since Mn is an effective
deoxidizing agent as is Si. A particularly preferable range is
about 0.10 to about 0.30 mass %.
[0045] P: about 0.04 mass % or less
[0046] P is a hazardous element which does not only decrease
hot-workability, formability, and toughness, but also degrades
corrosion resistance. In particular, when the content of P is more
than about 0.04 mass %, since the influence thereof becomes
significant, the content is limited to about 0.04 mass % or less. A
more preferable content is about 0.030 mass % or less.
[0047] S: about 0.03 mass % or less
[0048] 038 S reacts with Mn to form MnS, thereby causing a decrease
in corrosion resistance and durability. In addition, S is a
hazardous element which exists locally in crystal grain boundaries
to facilitate grain boundary embrittlement, and hence the content
of S is preferably decreased as much as possible. In particular,
when the content is more than 0.03 mass %, the adverse influence
becomes significant, and hence the content is limited to about 0.03
mass % or less. A more preferable content is about 0.008 mass % or
less.
[0049] Cr: about 8 mass % to less than about 10 mass %
[0050] Cr is an effective element for improving corrosion
resistance. However, when the content is less than about 8 mass %,
sufficient corrosion resistance cannot be reliably obtained. On the
other hand, when the content of Cr is increased to about 10 mass %
or more, the cost is inevitably increased, and in addition, it
becomes difficult to obtain the desired as-hot rolled strength.
Hence, the content is limited to the range of from about 8 mass %
to less than about 10 mass %.
[0051] Cu: about 0.01 to about 1.0 mass %
[0052] Cu is an effective element for improving corrosion
resistance and is added for improving the life of architectural
structures and the like. In addition, in particular, Cu is an
element which is positively used to perform electric resistance
welding in air. The reason the remaining penetrator, which is
generated in welding, can be reduced by the addition of Cu has not
been clearly understood. However, we believed that, in addition to
the decrease in amount of an element, such as Cr or Mn, which is
likely to form an oxide in the welded portion, when an appropriate
amount of Cu, which is a noble element (being unlikely to be
ionized as compared to iron, or having a higher standard electrode
potential than that of iron), is added, generation of an oxide in
the welded portion can be suppressed. However, when the addition is
less than about 0.01 mass %, the effect described above cannot be
satisfactorily obtained, and on the other hand, when the content of
Cu is excessively high such as more than about 1.0 mass %, in
addition to the increase in cost, hot-cracking sensitivity is
enhanced. Hence the embrittlement may occur during hot rolling in
some cases. Accordingly, the addition is limited to the range of
from about 0.01 to about 1.0 mass %. The lower limit of Cu is
preferably set to about 0.1 mass % at which an apparent effect of
improving the corrosion resistance can be obtained, and on the
other hand, the upper limit is preferably set to about 0.7 mass %
to prevent hot cracking and to obtain good workability.
[0053] Ni: about 0.01 to about 1.0 mass %
[0054] Ni is an element which improves ductility and toughness. Ni
is used to improve the toughness of the welding heat-affected zone
and, in addition, to improve anti-rusting properties. In addition,
hot cracking which occurs during hot rolling when Cu is added can
be effectively prevented by addition of Ni. However, when the
content of Ni is less than about 0.01 mass %, the effect described
above is not so significant, and on the other hand, when the
content of Ni is more than about 1.0 mass %, the effect described
above is saturated, and the material is hardened or the cost is
increased. Accordingly, the amount of Ni is limited to the range of
from about 0.01 to about 1.0 mass %.
[0055] V: about 0.01 to about 0.20 mass %
[0056] V is a very important element, and by addition of an
appropriate amount thereof, embrittlement of the welding
heat-affected zone, caused by electric resistance welding, can be
prevented. In addition, formation of coarse and large ferritic
crystal grains can also be prevented. However, the effect described
above is not so significant when the content is less than about
0.01 mass %. On the other hand, when the content is more than 0.20
mass %, the martensite production ability of the welding
heat-affected zone is considerably decreased, the toughness of the
welded portion is decreased, and in addition, it becomes difficult
to obtain a desired as-hot rolled tensile strength (about 400 to
about 450 MPa). Accordingly, the content of V is limited to the
range of from about 0.01 to about 0.20 mass %. A preferred amount
is about 0.03 to about 0.20 mass %.
[0057] Although the mechanism in which embrittlement of the welding
heat-affected zone is suppressed by the addition of V has not been
clearly understood, it has been considered as follows. When an
element such as Ti or Nb, having a strong affinity for C and N, is
added, since the carbonitride thereof is formed and precipitated,
the amounts of dissolved C and N are considerably decreased, and as
a result, the martensite production ability of the welding
heat-affected zone is significantly decreased. On the other hand,
when V is added, since the affinity thereof for C and N is not so
strong as compared to that of Ti or Nb, in a portion heated to a y
single-phase temperature region or more, a significant decrease in
the amount of dissolved C and N does not occur, and hence the
martensite production ability of this portion can be sufficiently
ensured. On the other hand, in a portion heated to the two-phase
temperature region, since the carbonitride of V in this temperature
region is stable, and the amounts of dissolved C and N are
decreased to a low level, hardening of the martensite phase caused
by increase in concentration of dissolved C and N in the .gamma.
phase is unlikely to occur. As a result, without decreasing the
martensite production ability of the portion heated to the .gamma.
single-phase temperature region or more, the hardness of the
martensite phase formed at the two-phase temperature region can be
decreased to a lower level, and as a result, over the entire region
of the welding heat-affected zone, a superior toughness can be
obtained.
[0058] Al: about 0.05 mass % or less
[0059] Al is not only effective as a deoxidizing agent but also can
contribute to improvement in bending workability of a steel sheet.
To obtain the effect described above, the amount of about 0.003
mass % or more must be added. However, when the amount is increased
to more than about 0.05 mass %, inclusion particles are increased,
and as a result, the mechanical characteristics are degraded
thereby. Accordingly, the amount of Al is limited to about 0.05
mass % or less. In addition, in particular, Al may not be
contained/included at all.
[0060] Mo: about 1.0 mass % or less
[0061] Mo is also an effective element which can improve corrosion
resistance. It may be added whenever desired. The amount of about
0.03 mass % or more is added to obtain the effect desribed above.
However, when the amount is increased to more than about 1.0 mass
%, workability is considerably degraded, and in addition, a desired
as-hot rolled tensile strength (about 400 to about 450 MPa) cannot
be obtained. Accordingly, the amount of Mo is limited to about 1.0
mass % or less. In addition, in view of the balance among corrosion
resistance, strength, and workability, the amount is preferably in
the range of from about 0.1 to about 0.5 mass %.
[0062] Next, the characteristics of the steel sheet of the
invention will be described.
[0063] 047 The steel sheet of the invention must have a tensile
strength in the range of from about 400 to about 450 MPa.
Heretofore, shaped steel used for civil engineering and
architectural structures has been manufactured by primarily
processing SS400 steel, and to utilize the same production line as
that for SS400, the steel must have strength and workability
equivalent to those of SS400. That is, when the tensile strength is
more than about 450 MPa, it is not preferable since work load
applied to the production line of the shaped steel is increased,
the facilities must be enhanced thereby, and in addition, the
workability is also degraded. On the other hand, when the strength
is less than about 400 MPa, an excessive deformation may occur when
the shaped sheet is fabricated, and in addition, the strength
necessary used as a finished product may not be obtained in some
cases.
[0064] Next, a method for manufacturing a Fe--Cr steel, according
to the invention will be described.
[0065] After molten steel having the composition according to the
invention is formed by a generally known melting furnace such as a
converter or an electric furnace, refining is performed by a known
refining method, such as a vacuum degassing (RH) method, a vacuum
oxygen decarburization (VOD) method, an argon oxygen
decarburization (AOD) method, or the like, and next, by a
continuous casting method or a ingot making method, a steel slab
(steel raw material) is formed. In this case, the thickness of the
slab is preferably about 100 mm or more to reliably ensure the
reduction in thickness in hot rough rolling described later.
[0066] Next, after the steel slab is heated to a temperature of
about 1,100 to about 1,280.degree. C., hot rolling is performed,
thereby forming a hot rolled steel sheet. The slab heating
temperature is preferably high from the viewpoint that softening is
facilitated by self-annealing after the completion of coiling.
However, when it is more than about 1,280.degree. C., slab sagging
considerably may occur in some cases, coarse and larger crystal
grains are formed, and as a result, the toughness of the hot rolled
sheet is decreased. On the other hand, when the heating temperature
is less than about 1,100.degree. C., it becomes difficult to
perform hot rolling at a finishing temperature of more than about
930.degree. C. Hence, the heating temperature is preferably in the
range of from about 1,100 to about 1,250.degree. C.
[0067] In a step of hot rough rolling of the invention, rolling
with a reduction in thickness of about 30% or more is preferably
performed at least one pass in a temperature range of more than
about 1,000.degree. C. The reason for this is that by this rolling
with a high reduction in thickness, the crystal structure of the
steel sheet becomes finer to suppress the decrease in toughness of
the parent material. The decrease in toughness of the parent
material, described above, is caused by formation of coarse and
large ferritic crystal grains, which primarily occurs in the
central portion in the longitudinal direction of the coil when the
cooling rate is decreased by heat insulation after the completion
of coiling. (Heat insulation will be described later.)
[0068] In addition, the hot rough rolling with a high reduction in
thickness also has an effect of improving the toughness of a
portion heated by electric resistance welding to the two-phase
temperature region of a ferrite phase (.alpha.) and an austenite
phase (.gamma.). The reason for this is that the martensite at the
two-phase temperature region is generated in ferritic crystal
boundaries of the steel sheet, and when this is excessively
hardened, sites generating cracks are formed, and embrittlement
occurs. Accordingly, when the ferritic structure as a matrix is
made to have a finer structure to improve the toughness thereof,
propagation of cracks can be prevented, and hence embrittlement can
be suppressed.
[0069] The reason for this is that although the steel sheet is an
austenite (.gamma.) single phase at a temperature of more than
1,000.degree. C., when the reduction in thickness per pass is set
to about 30% or more, the number of sites generating the ferrite
phase is increased, and hence the finer crystal grains can be
obtained. In addition, the reason the rolling temperature is set to
more than about 1,000.degree. C. in this case is that the finish
temperature of hot rolling is also set to more than about
930.degree. C.
[0070] A final temperature in finish rolling following the hot
rough rolling is set to more than about 930.degree. C., and the
coiling temperature after the rolling is set to more than about
810.degree. C. to facilitate the softening by a self-annealing
effect obtained after the completion of coiling. The reason the
final temperature of the finish rolling is set to more than
930.degree. C. is to prevent the formation of a deformed ferrite
phase by rolling in the two-phase temperature region of a ferrite
phase (.alpha.) and an austenite phase (.gamma.) and to ensure a
coiling temperature of more than about 810.degree. C. In addition,
the reason the coiling temperature is set to more than about
810.degree. C. is that by maintaining the high temperature inside
the coil, the recuperation effect can be easily obtained when heat
insulation is performed after completion of coiling. In addition,
the coiling temperature must be set to more than about 810.degree.
C. to obtain a temperature of 400.degree. C. or more at the edge
portions in the width direction of the coil when the heat
insulation is started.
[0071] In addition, to obtain the targeted strength, the cooling
time for the coil from about 800 to about 400.degree. C. must be
set to about 200 minutes or more after the completion of the
coiling so that the cooling is performed at an average cooling rate
of about 2.degree. C./min or less. By the average cooling rate
described above, the steel sheet structure can be formed of a
ferrite single phase (partly including a carbonitride), a tempered
bainite single phase, or a tempered bainitic+ferritic structure,
and as a result, the growth of a hard martensite phase can be
perfectly prevented.
[0072] In this embodiment, the average cooling rate inside the coil
is a cooling rate measured inside the coil, that is, measured at a
position in the vicinity of the center in the longitudinal
direction of the coil and at a distance of about 50 mm or more from
the edge in the sheet width direction. The measurement may be
performed by inserting a thermocouple into the coil. Alternatively,
the rate may be estimated by using an equation based on the surface
temperature of the coil.
[0073] In the case described above, inside the coil described
above, the steel sheet after the completion of coiling can be
relatively easily cooled at an average cooling rate of about
2.degree. C./min or less. However, at the front end portion (inner
portion) the rear end portion (outer portion) in the longitudinal
of the coil, and the end portions (edge portions) of the coil in
the width direction, the average cooling rate is likely to be more
than about 2.degree. C./min, and as a result, a bainite phase or a
martensite phase is easily generated to form a hard structure.
Accordingly, the portions described above must be cut away, thereby
causing the problem of a decrease in production yield.
[0074] To solve the problem described above, the invention provides
a method in which heat insulation is started before the temperature
of the coil after the completion of coiling is decreased to less
than about 400.degree. C., and in which by using the recuperation
effect obtained by this heat insulation, the cooling time from
about 800 to about 400.degree. C., which is a temperature
substantially at every position of the coil, is set to about 200
minutes or more so as to obtain an average cooling rate of about
2.degree. C./min or less. By the heat insulation described above,
the end portions in the longitudinal and width direction of the
coil can be sufficienty annealed, and as a result, the coil can
obtain the targeted strength in the entire width and longitudinal
directions thereof. The average cooling rate is more preferably set
to about 1.degree. C./min or less. In this case, since the coldest
point of the coil corresponds to each of the two end portions in
the width direction of the outermost portion of the coil, when a
thermocouple is welded to this position, the cooling rate can be
measured. In addition, temperature measurement may be performed by
a radiation thermometer.
[0075] As a heat insulating method, for example, there may be
mentioned a method in which the coil is covered with a heat
insulating cover made of iron and lined with a heat insulating
material inside thereof; a method in which the coil is placed in a
heat insulating box formed by digging a pit and adhering a heat
insulating material to the walls thereof and, if necessary, the
coil is covered with a heat insulating cover; or a method using a
device provided with a heating function, and in consideration of
individual production facilities, heat insulating devices suitable
thereto may be selectively used. In addition, in consideration of
cooling performed from the lower side of the coil, possible
measures, for example, in which the coil is placed on a heat
insulating material, must be taken whenever necessary. In addition,
in particular, to the front and the rear end portions in the
longitudinal direction of the coil and the two end portions in the
width direction thereof, which are very rapidly cooled after the
completion of coiling, induction heating or the like may be
additionally used.
[0076] By using the heat insulating method described above, without
performing annealing of a hot rolled steel sheet, a steel sheet can
be obtained having an as-hot rolled tensile strength of about 400
to about 450 MPa in the entire longitudinal and width directions of
the coil, and hence the problem of the conventional technique in
which the front and the rear ends in the longitudinal direction of
the coil must be cut away and/or the edge portions in the width
direction of thereof must be largely trimmed can be suppressed.
Hence the decrease in production yield can be suppressed.
Accordingly, significant cost reduction can be obtained. In
addition, since the tensile strength is made equivalent to that of
SS400 steel, machining such as bending and drilling can be
performed in the same production line as that used for SS400.
[0077] In addition to superior workability and toughness in an
as-hot rolled state, the hot rolled steel sheet of the invention
also has superior characteristics in which embrittlement of the
welding heat-affected zone does not occur even by using electric
resistance welding which causes rapid heating and cooling in
welding. The steel sheet of the invention, which is in an as-hot
rolled state, can be used, and in addition, it can also be used
after being processed by skinpass rolling for shape compensation,
whenever necessary; shotblasting, pickling, or the like for
removing scale; or polishing for obtaining a desired surface
condition. Furthermore, whenever necessary, the steel sheet can be
used after being processed by application of an anti-rusting agent
or the like. When pickling is performed, to improve the pickling
performance, annealing may be performed for the hot rolled steel
sheet.
[0078] The steel sheet of the invention can be applied to various
types of shaped steel, which are formed by bending machining, roll
forming, and the like, and are suitably used for shaped steel for
civil engineering and architectural structures, and in particular,
for housing structures. In addition, the steel sheet of the
invention can be used as a material for shaped steel formed by
various welding techniques such as arc welding, and in particular,
since embrittlement of the welded portion caused by rapid heating
and cooling does not occur at all, the invention is preferably
applied to manufacturing of welded lightweight H-shaped steel,
electric resistance welded (ERW) tubes, square pipes, and the like
formed by electric resistance welding using induction heating or
direct electric heating.
[0079] Furthermore, the steel sheet of the invention may also be
used as a material for various structures such as containers, coal
wagons, and bus frames by effectively using the characteristics
thereof. In addition, the steel sheet having the composition of the
invention may also be applied to various steel materials, such as
thick steel sheets formed by hot rolling, shaped steel, and steel
bars, for use in civil engineering and architectural fields.
EXAMPLES
Example 1
[0080] Steel having the composition shown in Table 1 was formed
into steel slabs 200 mm thick by melting through a
converter-secondary refining step followed by continuous casting.
After being reheated to 1,170.degree. C., these steel slabs were
processed by rough rolling with seven passes under the conditions
shown in Table 2 in which the reduction in thickness at the sixth
pass was set to 20 to 45% and those of the other passes were each
set to less than 30%, were then processed by finish rolling with
seven passes at a finish rolling temperature of 940 to
1,050.degree. C. to form hot rolled steel sheets 4.5 mm and 6.0 mm
thick, and were coiled at a temperature of 815 to 910.degree. C. to
form coils, followed by air cooling. In addition, by adjusting the
coil weight of some coils, the cooling rate was changed. For
example, by forming coils from a small lot so as to decrease the
weight of each coil, the cooling rate can be increased. The coils
formed by coiling were each provided with a thermocouple on the
side of the coil that is measured at a position in the vicinity of
the center in the longitudinal direction of the coil and at a
distance of 50 mm or more from the edge in the sheet width
direction to measure the cooling rate.
[0081] After the hot rolled steel sheet was cooled and then
processed by shotblasting and pickling for scale removal, tensile
test pieces (JIS NO. 5) were obtained therefrom along the rolling
direction and in the vicinity of the position at which the
temperature was measured, and the 0.2% proof stress, tensile
strength, yield ratio, and elongation were measured. After this
coil was slit, welded H-shaped steel was formed therefrom by
electric resistance welding, the H-shaped steel having a web height
of 300 mm, a flange width of 150 mm, a web thickness of 4.5 mm, and
a flange thickness of 6.0 mm. In manufacturing the H-shaped steel,
a web material was sequentially brought into contact with the
central portions in the width direction of two flange materials,
followed by electric resistance welding. Welding was performed
under the conditions in which the atmosphere was air or purged with
a nitrogen gas, the electrical power was 330 to 370 kW, and the
welding speed was 20 to 40 m/min. From this welded H-shaped steel,
H-shaped welded tensile test pieces, having a width of 35 mm along
the welding direction, in accordance with JIS G 3353 were obtained
by cutting, and each test piece was held at the two flange portions
and was then pulled, thereby measuring the tensile strength and the
breaking position. In this test, it is necessary that the H-shaped
steel be broken not at the welded portion, but at the web portion
and have a desired strength.
[0082] The results are shown in Table 2. According to the steel
sheet of the invention, the strength was equivalent to that of
SS400 or SN400B, the strength of the H-shaped steel formed of the
steel sheet described above was also equivalent to that of SWH400,
embrittlement of the welded portion caused by electric resistance
welding did not occur at all, and every breakage occurred at the
web portion. In addition, by welding performed even in air,
superior welding could be performed, and breakage of the welded
portion caused by a remaining penetrator did not occur at all. On
the other hand, according to comparative examples, which were out
of the scope of the invention, the targeted strength (400 to 450
MPa) could not be obtained, and in the tensile characteristic test,
breakage occurred at the welded portion, and in addition, a
sufficient strength could not be obtained.
[0083] In particular, sample No. 10 had a steel sheet strength
within the desired range. However, since rough rolling with a high
reduction in thickness was not performed, embrittlement of the
welded portion formed by electric resistance welding considerably
occurred, breakage occurred at the welded portion of the H-shaped
steel in the tensile characteristic test, and the desired strength
thereof could not be obtained. Sample No. 11 was cooled at a high
cooling rate after hot rolling, and as a result, the desired
strength could not be obtained. Since sample No. 14 and sample No.
15 had an excessive C content and an excessive C+N content,
respectively, embrittlement of the welded portion formed by
electric resistance welding at the two-phase temperature region
considerably occurred, and as a result, after formation of the
H-shaped steel, the desired strength thereof could not be obtained.
Sample No. 16 contained a small amount of Cu, and due to the
influence of a remaining penetrator, in the tensile characteristic
test of the H-shaped steel, breakage occurred at the welded
portion. Since sample No. 17 contained a small amount of V,
embrittlement of the welded portion formed by electric resistance
welding occurred concomitant with the formation of coarse and large
ferritic crystal grains, and breakage occurred at the welded
potion. Since sample No. 18 contained a large amount of Mn,
hardening occurred in the heat-affected zone by electric resistance
welding, and in the tensile characteristic test of the H-shaped
steel, breakage occurred at the welded portion.
Example 2
[0084] Steel having the composition shown in Table 3 was formed
into steel slabs 200 mm thick by melting through a
converter-secondary refining step followed by continuous casting.
After being reheated to 1,170 to 1,220.degree. C., these steel
slabs were processed by rough rolling with seven passes under the
conditions shown in Table 4 in which the reduction in thickness at
the sixth pass was set to 30 to 45% and those of the other passes
were each set to less than 30%, were then processed by finish
rolling with seven passes at a finish rolling temperature of 940 to
1,050.degree. C. to form hot rolled steel sheets 4.5 mm and 6.0 mm
thick, and were coiled at a temperature of 815 to 910.degree. C. to
form coils. The coils thus formed were conveyed to a heat
insulating yard, the inside of which was covered with a heat
insulating material, and were each covered with a heat insulating
cover, the inside of which was lined with a heat insulating
material 100 mm thick, whereby heat insulation was performed. The
measurement of the cooling rate of the coil was performed by a
thermocouple welded to the vicinity of the edge of the outermost
side of the coil. In addition, by adjusting the coil weight of some
coils or by changing the thickness of the insulating material, the
cooling rate was changed. From the edge portion in the width
direction of the outermost portion of the hot rolled coil and from
the 1/4 portion in the width direction thereof, test pieces in
accordance with JIS NO. 5 were obtained by cutting, and the tensile
characteristic test was performed. The tensile direction was in the
rolling direction.
[0085] The results are shown in Table 4. According to the steel
sheet of the invention processed by slow cooling using the heat
insulating cover, the strength was equivalent to that of SS400 or
SN400B, hardening in the vicinity of the edge portion of the
outermost portion of the coil, which was the coldest point, did not
substantially occur, and the targeted strength (400 to 450 MPa)
could be obtained. On the other hand, according to comparative
examples, which were out of the scope of the invention, the
strength in the vicinity of the edge portion was particularly
increased, and in comparative examples in which the composition was
out of the scope of the invention, the targeted strength could not
be obtained even at the 1/4 width portion inside from the edge of
the coil in the width direction. In particular, since sample No. 30
was cooled at a high cooling rate after the completion of coiling,
the desired strength could not be obtained at the edge portion. In
sample No. 31, the desired strength could not be obtained at both
the edge portion in the width direction and the 1/4 width portion
for the same reason as described above. In addition, since the
content of C of sample No. 34, the content of N of sample No. 35,
and the content of C+N of sample No. 36 were out of the range of
the invention, the desired strength could not be obtained.
Furthermore, since the content of Cu of sample No. 37, the content
of V of sample No. 38, and the content of Mn of sample No. 39 were
out of the range of the invention, the desired strength could not
be obtained.
[0086] As has thus been described, according to the invention, by
appropriately combining the composition of the steel sheet, the hot
rolling conditions, and the cooling conditions after hot rolling, a
structural Fe--Cr steel sheet can be obtained which has an as-hot
rolled strength equivalent to the strength of SS400 and which does
not cause hardening at the front and the rear end portions in the
longitudinal direction of the coil and at the edge portions in the
sheet width direction thereof, and as a result, in the conventional
production line, various shaped steel can be manufactured using the
steel sheet described above. In addition, since the steel sheet of
the invention can be fabricated by a welding method in which rapid
heating and cooling are performed, structural shaped steel can be
manufactured by electric resistance welding. Furthermore, the steel
sheet of the invention has sufficient corrosion resistance and
durability used for civil engineering and architectural structures,
the reduction in life cycle cost can be achieved, and hence the
industrial and commercial values are very significant.
1TABLE 1 Steel Components (mass %) mark C Si Mn P S Al Cr N Cu Ni V
Mo C + N Remarks A 0.0060 0.22 0.24 0.030 0.003 0.009 9.42 0.0046
0.51 0.20 0.08 -- 0.0106 Example B 0.0025 0.20 0.27 0.025 0.006
0.010 9.96 0.0060 0.40 0.30 0.06 -- 0.0085 Example C 0.0100 0.20
0.05 0.027 0.009 0.008 8.04 0.0026 0.46 0.18 0.03 -- 0.0126 Example
D 0.0054 0.21 0.30 0.026 0.004 0.020 9.44 0.0042 0.70 0.21 0.05 --
0.0096 Example E 0.0060 0.30 0.27 0.030 0.002 0.009 9.06 0.0044
0.30 0.13 0.20 -- 0.0104 Example F 0.0060 0.10 0.26 0.010 0.001
0.001 8.97 0.0039 0.45 0.20 0.09 -- 0.0099 Example G 0.0065 0.22
0.24 0.015 0.003 0.009 8.80 0.0035 0.41 0.21 0.08 -- 0.0100 Example
H 0.0094 0.46 0.15 0.030 0.003 0.010 9.42 0.0055 0.30 0.03 0.12 --
0.0149 Example I 0.0051 0.22 0.26 0.030 0.003 0.011 9.10 0.0044
0.05 0.20 0.04 0.33 0.0095 Example J 0.0055 0.20 0.25 0.029 0.005
0.014 9.23 0.0040 0.30 0.19 0.05 0.08 0.0095 Example K 0.0110 0.21
0.25 0.035 0.006 0.010 8.89 0.0039 0.34 0.22 0.04 -- 0.0149
Comparative example L 0.0095 0.30 0.15 0.031 0.007 0.011 9.33
0.0058 0.30 0.01 0.04 -- 0.0153 Comparative example M 0.0089 0.20
0.24 0.030 0.004 0.013 9.27 0.0044 <0.01 0.21 0.08 -- 0.0133
Comparative example N 0.0092 0.25 0.25 0.029 0.003 0.012 9.29
0.0055 0.40 0.30 <0.01 -- 0.0147 Comparative example O 0.0069
0.24 0.50 0.030 0.003 0.014 8.95 0.0060 0.31 0.20 0.08 -- 0.0129
Comparative example Note: Column with underline is out of the scope
of the present invention
[0087]
2 TABLE 2 Manufactoring conditions Tensile Hot-rolling conditions
Steel sheet characteristics for H-shaped steel characteristics
Roughing Average cooling (4.5 mm thick, L direction, JIS No.5
Welding of H-shaped steel reduction in Finish rolling Coiling rate
from Tensile electrical Welding Tensile Steel thickness at
temperature temperature 800 to 400.degree. C. 0.2% proof strength
Yield ratio power speed strength Breaking No. mark 6th pass (%)
(.degree. C.) (.degree..degree.C.) (.degree. C./min) stress (MPa)
(MPa) (%) Elongation (%) (kW) (m/min) Atmosphere (MPa) position
Remarks 1 A 35 980 830 0.6 252 410 61 40 340 30 Air 420 Web Example
2 A 35 1000 850 0.6 260 410 63 41 370 35 Purged 410 Web Example
with nitrogen 3 B 30 980 820 2.0 245 402 61 42 360 35 Air 404 Web
Example 4 C 35 1050 900 0.6 250 417 60 40 360 35 Purged 417 Web
Example with nitrogen 5 D 45 940 815 0.6 280 421 67 40 360 30 Air
420 Web Example 6 E 35 970 830 0.6 295 444 66 38 355 30 Air 440 Web
Example 7 F 40 980 850 0.6 275 420 65 40 350 35 Air 421 Web Example
8 G 35 1000 900 0.6 297 433 69 39 350 35 Air 440 Web Example 9 H 35
980 850 0.3 300 448 67 38 330 20 Purged 440 Web Example with
nitrogen 10 H 20 980 840 0.6 304 442 69 39 350 30 Purged 310 Welded
Comp. with portion Ex nitroen 11 H 35 980 840 2.5 351 495 71 32 350
30 Air 505 Web Camp. Ex 12 I 35 990 880 0.6 305 440 69 38 360 35
Purged 450 Web Example with nitrogen 13 J 35 970 850 0.6 280 432 65
38 370 35 Air 420 Web Example 14 K 35 1050 910 0.6 321 450 71 34
360 30 Air 325 Welded Comp. portion Ex 15 L 35 980 820 0.6 330 461
72 33 330 20 Purged 333 Welded Camp. with portion Ex nitrogen 16 M
35 970 820 0.6 280 443 63 36 355 30 Air 385 Welded Comp. portion Ex
17 N 35 980 830 0.6 299 430 70 36 350 25 Air 346 Welded Comp.
portion Ex 18 Q 35 980 830 0.6 297 418 71 39 350 30 Air 370 Welded
Comp. portion Ex Note: Column with underline is out of the scope of
the present invention
[0088]
3TABLE 3 Steel Components (mass %) mark C Si Mn P S Al Cr N Cu Ni V
Mo C + N Remarks AA 0.0046 0.20 0.23 0.029 0.004 0.010 9.44 0.0060
0.52 0.18 0.07 -- 0.0106 Example BB 0.0037 0.21 0.26 0.024 0.005
0.008 9.97 0.0068 0.41 0.30 0.05 -- 0.0105 Example CC 0.0104 0.19
0.06 0.025 0.008 0.008 8.03 0.0025 0.45 0.20 0.03 -- 0.0129 Example
DD 0.0044 0.21 0.30 0.028 0.006 0.021 9.40 0.0040 0.66 0.20 0.01 --
0.0084 Example EE 0.0048 0.30 0.26 0.032 0.006 0.010 9.14 0.0045
0.35 0.14 0.18 -- 0.0093 Example FF 0.0059 0.10 0.27 0.011 0.001
0.001 8.99 0.0040 0.45 0.20 0.09 -- 0.0099 Example GG 0.0066 0.22
0.22 0.014 0.003 0.008 8.80 0.0036 0.40 0.20 0.06 -- 0.0102 Example
HH 0.0094 0.45 0.14 0.029 0.005 0.010 9.40 0.0056 0.30 0.03 0.10 --
0.0150 Example II 0.0051 0.21 0.24 0.030 0.004 0.012 9.12 0.0047
0.05 0.19 0.04 0.31 0.0098 Example JJ 0.0054 0.20 0.26 0.028 0.005
0.013 9.22 0.0039 0.28 0.20 0.05 0.08 0.0093 Example KK 0.0110 0.22
0.25 0.035 0.007 0.009 8.89 0.0044 0.33 0.21 0.03 -- 0.0154
Comparative example LL 0.0058 0.22 0.26 0.035 0.005 0.009 9.10
0.0105 0.45 0.20 0.04 -- 0.0163 Comparative example MM 0.0094 0.30
0.14 0.030 0.006 0.012 9.30 0.0066 0.31 0.01 0.03 -- 0.0160
Comparative example NN 0.0090 0.20 0.23 0.029 0.005 0.012 9.25
0.0045 1.20 0.20 0.05 -- 0.0135 Comparative example OO 0.0088 0.24
0.22 0.028 0.006 0.011 9.33 0.0052 0.40 0.30 0.30 -- 0.0140
Comparative example PP 0.0065 0.23 0.51 0.033 0.005 0.013 8.95
0.0059 0.32 0.20 0.08 -- 0.0124 Comparative example Note: Column
with underline is out of the scope of the present invention
[0089]
4 TABLE 4 Hot-rolling conditions Edge Average portion cooling Steel
sheet Roughing tem- rate Steel sheet characteristics of edge
characteristics of 1/4 width reduction Finish perature from portion
(L direction, JIS No. 5) portion (L direction, JIS No. 5) in
rolling Coiling at start of 800 to 0.2% 0.2% thickness tem- tem-
heat 400.degree. C. proof Tensile Yield proof Tensile Yield Steel
at 6.sup.th perature perature insulation (.degree. C./ stress
strength ratio Elonga- stress strength ratio Elonga- No. mark pass
(%) (.degree. C.) (.degree. C.) (.degree. C.) min) (MPa) (MPa) (%)
tion (%) (MPa) (MPa) (%) tion (%) Remarks 21 AA 35 1000 850 500 0.3
256 414 62 42 245 404 61 44 Example 22 AA 35 980 830 410 0.4 298
444 67 42 255 405 63 45 Example 23 BB 30 980 820 450 1.0 255 410 62
45 236 400 59 48 Example 24 CC 35 1050 900 550 0.3 270 420 64 43
248 415 60 45 Example 25 DD 45 940 815 500 0.5 277 422 66 41 270
418 65 42 Example 26 EE 35 970 830 480 0.4 298 450 66 40 286 440 65
43 Example 27 FF 40 980 850 500 0.3 280 438 64 42 270 414 65 44
Example 28 GG 35 1000 880 530 0.4 288 434 66 40 280 430 65 42
Example 29 HH 35 980 840 560 0.5 298 446 67 40 280 443 63 43
Example 30 HH 35 980 840 510 2.5 350 490 71 31 303 445 68 34 Comp.
Ex 31 HH 35 980 840 350 13.0 450 602 75 25 352 504 70 34 Comp. Ex
32 II 35 1000 880 540 0.4 320 447 72 40 280 430 65 42 Example 33 JJ
35 980 850 500 0.3 285 435 66 40 265 422 63 42 Example 34 KK 35
1050 910 500 0.4 380 522 73 31 332 472 70 35 Comp. Ex 35 LL 35 970
840 510 0.3 368 510 72 33 303 480 63 37 Comp. Ex 36 MM 35 980 830
480 0.3 377 515 73 29 333 455 73 34 Comp. Ex 37 NN 35 980 820 450
0.4 390 536 73 31 350 480 73 35 Comp. Ex 38 OO 35 990 830 520 0.4
367 502 73 30 325 460 71 37 Comp. Ex 39 PP 35 980 830 490 0.5 359
505 71 31 330 460 72 38 Comp. Ex Note: Column with underline is out
of the scope of the present invention
* * * * *