U.S. patent application number 10/523880 was filed with the patent office on 2005-11-03 for cr steel for structural use and method for producing the same.
Invention is credited to Furukimi, Osamu, Ota, Hiroki, Shiokawa, Takashi, Ujiro, Takumi.
Application Number | 20050241737 10/523880 |
Document ID | / |
Family ID | 31972983 |
Filed Date | 2005-11-03 |
United States Patent
Application |
20050241737 |
Kind Code |
A1 |
Ujiro, Takumi ; et
al. |
November 3, 2005 |
Cr steel for structural use and method for producing the same
Abstract
To provide structural Cr-containing steel with excellent
low-temperature toughness and impact toughness, with low costs as
compared with stainless steel, and with sufficient corrosion
resistance. Specifically, this is structural Cr-containing steel
and a manufacturing method thereof, wherein the Cr-containing steel
contains C of 0.002 to 0.02%; N of 0.002 to 0.02%; Si of 0.05 to
1.0%; Mn of 0.05 to 1.0%; P of 0.04% or less; S of 0.02% or less;
Al of 0.001 to 0.1%; and Cr of 6.0 to 10.0%, further may contain Cu
of 0.1 to 1.0%, further may contain at least one of: Ni of 0.1 to
1.0%; and Mo of 0.1 to 1.0%, and further may contain at least one
of: Nb of 0.005 to 0.10%; and V of 0.005 to 0.20%, the balance are
formed of Fe and unavoidable impurities, and the Cr-concentration
in the surface layer of the steel is equal to or more than the
value wherein 1% is subtracted from the Cr-concentration within the
steel.
Inventors: |
Ujiro, Takumi; (Tokyo,
JP) ; Ota, Hiroki; (Tokyo, JP) ; Furukimi,
Osamu; (Tokyo, JP) ; Shiokawa, Takashi;
(Tokyo, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
220 5TH AVE FL 16
NEW YORK
NY
10001-7708
US
|
Family ID: |
31972983 |
Appl. No.: |
10/523880 |
Filed: |
March 1, 2005 |
PCT Filed: |
August 28, 2003 |
PCT NO: |
PCT/JP03/10908 |
Current U.S.
Class: |
148/650 ;
148/333 |
Current CPC
Class: |
C22C 38/18 20130101;
C22C 38/04 20130101; C21D 2261/00 20130101; C22C 38/004 20130101;
C21D 6/002 20130101; C22C 38/02 20130101; C21D 8/0278 20130101 |
Class at
Publication: |
148/650 ;
148/333 |
International
Class: |
C22C 038/18 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2002 |
JP |
2002-257229 |
Claims
1. Structural Cr-containing steel comprising: 0.002 to 0.02% by
mass of C; 0.002 to 0.02% by mass of N; 0.05 to 1.0% by mass of Si;
0.05 to 1.0% by mass of Mn; 0.04% by mass or less of P; 0.02% by
mass or less of S; 0.001 to 0.1% by mass; of Al 6.0 to 10.0% by
mass of Cr, and the balance being Fe and unavoidable impurities,
wherein the Cr-concentration in the surface layer of the steel is
equal to or more than the value wherein 1% by mass is subtracted
from the Cr-concentration within the steel.
2. Structural Cr-containing steel according to claim 1, further
comprising 0.1 to 1.0% by mass of Cu.
3. Structural Cr-containing steel according to claim 1, further
comprising at least one of: 0.1 to 1.0% by mass of Ni; and 0.1 to
1.0% by mass of Mo.
4. Structural Cr-containing steel according to claim 1, further
comprising at least one of: 0.005 to 0.10% by mass of Nb; and 0.005
to 0.20% by mass of V.
5. A manufacturing method for structural Cr-containing hot-rolled
steel comprising: a step wherein a steel material comprising: 0.002
to 0.02% by mass of C; 0.002 to 0.02% by mass of N; 0.05 to 1.0% by
mass of Si; 0.05 to 1.0% by mass of Mn; 0.04% by mass or less of P;
0.02% by mass or less of S; 0.001 to 0.1% by mass of Al; 6.0 to
10.0% by mass of Cr; and the balance being Fe and unavoidable
impurities, is formed into a steel strip by hot rolling after
reheating; wherein the steel surface is removed by a removal depth
of 10 to 200 .mu.m by descaling.
6. A manufacturing method for structural Cr-containing cold-rolled
steel, wherein following said descaling processing according to
claim 5, cold rolling, annealing cold-rolled steel, and pickling
are performed.
7. A manufacturing method for structural Cr-containing steel
according to claim 5, wherein said steel material further
comprising Cu of 0.1 to 1.0% by mass.
8. A manufacturing method for structural Cr-containing steel
according to claim 5, wherein said steel material further
comprising at least one of: 0.1 to 1.0% by mass of Ni; and 0.1 to
1.0% by mass of Mo.
9. A manufacturing method for structural Cr-containing steel
according to claim 5, wherein said steel material further
comprising at least one of: 0.005 to 0.10% by mass of Nb; and 0.005
to 0.20% by mass of V.
10. Structural Cr-containing steel according to claim 1, wherein
said steel is employed for freezing containers.
11. A manufacturing method for structural Cr-containing hot-rolled
steel according to claim 5, wherein said structural Cr-containing
steel is employed for frame material of freezing containers.
12. A manufacturing method for structural Cr-containing cold-rolled
steel according to claim 6, wherein said structural Cr-containing
steel is employed for external-wall material of freezing
containers.
13. A freezing container formed of said Cr-containing steel
according to claim 10, wherein formation of said freezing container
is made by forming and welding, and wherein the steel surface is
coated with dry-paint film thickness of 10 .mu.m or more.
14. A freezing container formed of said Cr-containing steel
manufactured with said manufacturing method according to claim 11,
wherein formation of said freezing container is made by forming and
welding, and wherein the steel surface is coated with dry-paint
film thickness of 10 .mu.m or more.
15. Structural Cr-containing steel according to claim 1, wherein
said steel is used for civil engineering and construction.
16. A manufacturing method for structural Cr-containing hot-rolled
steel according to claim 5, wherein said structural Cr-containing
hot-rolled steel is used for civil engineering and
construction.
17. A manufacturing method for structural Cr-containing cold-rolled
steel according to claim 6, wherein said structural Cr-containing
steel is used for civil engineering and construction.
18. Structural Cr-containing steel according to claim 2, further
comprising at least one of: 0.1 to 1.0% by mass of Ni; and 0.1 to
1.0% by mass of Mo.
19. Structural Cr-containing steel according to claim 2, further
comprising at least one of: 0.005 to 0.10% by mass of Nb; and 0.005
to 0.20% by mass of V.
20. Structural Cr-containing steel according to claim 3, further
comprising at least one of: 0.005 to 0.10% by mass of Nb; and 0.005
to 0.20% by mass of V.
21. Structural Cr-containing steel according to claim 18, further
comprising at least one of: 0.005 to 0.10% by mass of Nb; and 0.005
to 0.20% by mass of V.
Description
TECHNICAL FIELD
[0001] The present invention relates to structural Cr-containing
steel, particularly to Cr-containing steel employed for freezing
containers with excellent low-temperature toughness and impact
toughness, with low costs as compared with austenitic stainless
steel, and with sufficient corrosion resistance.
BACKGROUND ART
[0002] In recent years, demand for freezing containers is rapidly
increasing along with improvement in the human diet. The
performance required for the steel employed as a structural
material for freezing containers, which are mainly used for long
distance transport of provisions, includes high corrosion
resistance, high low-temperature toughness, and that in the event
that the structural material suffers impact the structural material
is not readily punctured, thereby preventing deterioration in
thermal insulation. The material employed for freezing containers
are roughly classified into frame material, external-wall material,
and internal-wall material. In most cases, the internal-wall
material is formed of cold-rolled annealed steel without painting,
and is preferably formed with low-temperature toughness, and
accordingly, in many cases, SUS304, which is austenitic stainless
steel, stipulated by JIS (Japanese Industrial Standard, which will
be referred to as "JIS" hereafter) G 4305 as a steel member is
employed. The aforementioned SUS304 exhibits excellent
low-temperature toughness, high elongation performance, small yield
ratio (yield stress/tensile strength), and a high work hardening
coefficient, thereby serving as stainless steel with excellent
impact toughness which has the advantage that the member formed
thereof is not readily punctured in the event that the member
suffers impact. However, SUS304 has the serious disadvantage of
high costs. On the other hand, the steel used for the frame
material and the external-wall material are assumed to be painted.
The cold-rolled annealed steel is employed as the external-wall
member, wherein while the SUS304, which is austenitic stainless
steel, is employed for high-grade freezing containers, ferritic or
martensitic stainless steel containing Cr of around 11% such as
SUS410L or SUS410S stipulated by JIS G 4305 is also employed due to
high costs of the aforementioned SUS304. The frame material is
formed of hot rolled annealed sheets, and particularly, in many
cases, are formed of 11%-Cr martensitic stainless steel with
reduced C and N.
[0003] As the 11%-Cr-containing steel employed for container
material, for example, martensitic stainless steel for welded
structural materials is known as disclosed in Japanese Examined
Patent Publication No. 51-13463, containing Cr of 10 to 18% by
weight, Ni of 0.1 to 3.4%, Si of 1.0% by weight or less, and Mn of
4.0% by weight or less, and furthermore, reducing C to 0.03% by
weight or less, and N to 0.02% by weight or less, so that massive
martensitic structure is formed in a welded heat-affected zone,
thereby improving ductility and toughness in the welded
heat-affected zone. Furthermore, structural martensitic stainless
steel is known as disclosed in Japanese Examined Patent Publication
No. 57-28738, containing Cr of 10 to 13.5% by weight, Si of 0.5% by
weight or less, and Mn of 1.0 to 3.5% by weight, reducing the
concentration of C to 0.02% by weight or less, and the
concentration of N to 0.02% by weight or less, and furthermore,
suppressing Ni to less than 0.1% by weight, thereby exhibiting
toughness and workability in a welded heat-affected zone without
pre-heating and post-heating before and after welding. The
aforementioned steel is employed for various structural materials
such as frame members of offshore containers and the like, as
disclosed in JOURNAL OF THE JAPAN WELDING SOCIETY, Vol. 57 (1988),
No. 6, p. 432. Such 11%-Cr stainless steel is employed in frame
members for container, or steel members for external-wall members,
due to relatively low costs thereof. Accordingly, there is great
demand for development of a technique for solving the problem of
poor low-temperature and poor impact toughness as compared with the
SUS304 which is austenitic stainless steel, and for enabling
further reduction of costs thereof by reducing Cr-concentration,
omission of annealing after hot rolling, and the like.
[0004] In order to solve the above-described problems,
building-structural ferritic stainless steel is known as disclosed
in Japanese Unexamined Patent Application Publication No.
11-302795, containing Cr of 8 to 16% by weight, Si of 0.05 to 1.5%
by weight, and Mn of 0.05 to 1.5% by weight, and furthermore,
suppressing C to 0.005 to 0.1%, N to 0.05% by weight or less, and
(C+N) to 0.1% by weight or less, wherein the components thereof are
adjusted such that martensitic structure is formed with a volume
ratio of 50% or more in a welded heat-affected zone. However, the
aforementioned steel disclosed in Japanese Unexamined Patent
Application Publication No. 11-302795 does not exhibit sufficient
low-temperature toughness required for freezing containers, and is
to be used without any processing after hot rolling, or is to be
used with heat treatment or pickling after hot rolling, and no
consideration has been given to the poor corrosion resistance after
painting.
[0005] Furthermore, a technique for forming hot-rolled steel
without annealing is disclosed in Japanese Unexamined Patent
Application Publication No. 11-302737, wherein steel containing Cr
of 8 to 16% by weight, Si of 0.05 to 1.5%, Mn of 0.05 to 1.5%, and
Ni of 0.05 to 1%, and furthermore, suppressing C to 0.005 to 0.1%,
N to 0.05% or less, and (C+N) to 0.1% or less, is heated at 1100 to
1250.degree. C., hot-rolling is completed under a temperature of
800.degree. C. or more, coiling is made under a temperature of
700.degree. C. or less, and cooling is performed with a cooling
rate of 5.degree. C. per minute or less, but with this technique,
the steel is to be used without any processing after hot rolling,
or is to be used with heat treatment or pickling after hot rolling,
and no consideration has been given to the poor corrosion
resistance after painting.
[0006] Furthermore, a technique for forming structural hot-rolled
steel without annealing is disclosed in Japanese Patent Application
No. 2003-141462 (corresponding to European Patent No. 03015110.4,
Filing Date: Jul. 3, 2003), which has been proposed by the present
inventors, wherein steel containing Cr of 8 to 10% by mass, Si of
0.01 to 1.0% by mass, Mn of 0.01 to 0.30% by mass, Cu of 0.01 to
1.0% by mass, Ni of 0.01 to 1.0% by mass, and V of 0.01 to 0.20% by
mass, is heated at 1100 to 1280.degree. C., hot-rolling is
completed under a temperature exceeding 930.degree. C., coiling is
made under a temperature exceeding 810.degree. C., and cooling is
performed with a cooling rate of 2.degree. C. per minute or less in
the temperature range between 800 to 400.degree. C. However, the
aforementioned steel member disclosed in Japanese Patent
Application No. 2003-141462 is to be used after pickling after hot
rolling without painting, and furthermore, no method has been known
for improving the surface properties by controlling pickling so as
to improve corrosion resistance after painting, which is an
essential component of the present invention.
[0007] In order to solve the above-described problems of the
conventional techniques, it is an object of the present invention
to provide structural Cr-containing steel, particularly to
Cr-containing steel employed for freezing containers, with
excellent low-temperature toughness and impact toughness, with low
costs as compared with the austenitic stainless steel, and with
sufficient corrosion resistance.
[0008] In most cases, various painting is made on the surface of
the steel members used for freezing containers from the perspective
of improvement of corrosion resistance, and in particular, from the
perspective of appearance. Accordingly, the corrosion resistance
after painting is an important performance, and it has been
revealed from the research by the present inventors that it is
necessary for the aforementioned steel material to exhibit
corrosion resistance wherein a steel sample subjected to formation
of cross-cuts after painting does not exhibit marked outflow rust
after 1000-hour salt spray testing.
DISCLOSURE OF INVENTION
[0009] In order to solve the above-described problems, the present
inventors intensely researched the influence of additive elements
upon the above-described properties of the Cr-containing steel, and
as a result, it has been revealed that the Cr-containing steel with
Cr-concentration of 6.0 to 10.0%, and with C-concentration of 0.02%
or less and N-concentration of 0.02% or less, exhibits both
sufficient corrosion resistance, toughness, and impact toughness,
required for structural Cr-containing steel, particularly required
for steel used for freezing containers, manufacturing can be made
with low costs as compared with austenitic stainless steel, and
furthermore, annealing for hot-rolled steel can be omitted, thereby
enabling manufacturing with lower costs. Furthermore, it has been
revealed that the method serves as important means for improving
corrosion resistance after painting, wherein the removal amount of
the steel surface layer in descaling of hot-rolled steel is
controlled so as to exhibit both the sufficient steel-surface
properties and the sufficient corrosion resistance after descaling.
The present invention has been made as follows based upon the
above-described information.
[0010] That is to say, structural Cr-containing steel according to
the present invention comprises: 0.002 to 0.02% by mass of C; 0.002
to 0.02% by mass of N; 0.05 to 1.0% by mass of Si; 0.05 to 1.0% by
mass of Mn; 0.04% by mass or less of P; 0.02% by mass or less of S;
0.001 to 0.1% by mass of Al; 6.0 to 10.0% by mass of Cr, and the
balance being Fe and unavoidable impurities, and the
Cr-concentration in the surface layer of the steel is equal to or
more than the value wherein 1% by mass is subtracted from the
Cr-concentration within the steel.
[0011] Also, the present invention is structural Cr-containing
steel which, in the above steel according to the invention, further
comprises Cu of 0.1 to 1.0% by mass.
[0012] Also, the present invention is structural Cr-containing
steel which, in the above steel according to the invention, further
comprises at least one of: 0.1 to 1.0% by mass of Ni; and 0.1 to
1.0% by mass of Mo.
[0013] Also, the present invention is structural Cr-containing
steel which, in the above steel according to the invention,
further-comprises at least one of: 0.005 to 0.10% by mass of Nb;
and 0.005 to 0.20% by mass of V.
[0014] Also, the present invention is a manufacturing method for
structural Cr-containing hot-rolled steel comprising: a step
wherein a steel material comprising: 0.002 to 0.02% by mass of C;
0.002 to 0.02% by mass of N; 0.05 to 1.0% by mass of Si; 0.05 to
1.0% by mass of Mn; 0.04% by mass or less of P; 0.02% by mass or
less of S; 0.001 to 0.1% by mass of Al; 6.0 to 10.0% by mass of Cr;
and the balance being Fe and unavoidable impurities, is formed into
a steel strip by hot rolling after reheating followed by a
descaling process;
[0015] wherein the steel surface is removed by a removal depth of
10 to 200 .mu.m by descaling.
[0016] Also, the present invention is a manufacturing method for
structural Cr-containing cold-rolled steel, wherein following the
descaling processing in the manufacturing method of the above steel
according to the invention, cold rolling, annealing cold-rolled
steel, and pickling are performed.
[0017] Also, the present invention is a manufacturing method for
structural Cr-containing steel which, in the manufacturing method
for the above steel according to the invention, further comprises
Cu of 0.1 to 1.0% by mass.
[0018] Also, the present invention is a manufacturing method for
structural Cr-containing steel which, in the manufacturing method
for the above steel according to the invention, further comprises
at least one of: 0.1 to 1.0% by mass of Ni; and 0.1 to 1.0% by mass
of Mo.
[0019] Also, the present invention is a manufacturing method for
structural Cr-containing steel which, in the manufacturing method
for the above steel according to the invention, further comprises
at least one of: Nb of 0.005 to 0.10% by mass; and V of 0.005 to
0.20% by mass.
[0020] Also, the present invention is structural Cr-containing
steel wherein the above steel according to the invention is be
employed for freezing containers.
[0021] Also, the present invention is a manufacturing method for
structural Cr-containing hot-rolled steel wherein, in the
manufacturing method for the above steel according to the
invention, the structural Cr-containing steel is employed for frame
members of freezing containers.
[0022] Also, the present invention is a manufacturing method for
structural Cr-containing cold-rolled steel wherein, in the
manufacturing method for the above steel according to the
invention, the structural Cr-containing steel is employed for
external-wall members of freezing containers.
[0023] Also, the present invention is a freezing container which is
formed of the above-described Cr-containing steel, wherein
formation of the freezing container is made by forming and welding,
and the steel surface is coated with dry-paint film thickness of 10
.mu.m or more.
[0024] Also, the present invention is Cr-containing steel wherein
the above-described structural steel according to the invention is
used for civil engineering and construction.
[0025] Also, the present invention is a manufacturing method for
structural Cr-containing hot-rolled steel, wherein the
above-described steel according to the invention is used for civil
engineering and construction.
[0026] Also, the present invention is a manufacturing method for
structural Cr-containing cold-rolled steel, wherein the
above-described steel according to the invention is used for civil
engineering and construction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a diagram which shows the relation between the
removal depth of the steel surface layer and rust area ratio after
SST (Salt Spray Testing).
[0028] FIG. 2A is a scanning electron micrograph which shows the
surface of the steel subjected to surface removal with the removal
depth of 8 .mu.m.
[0029] FIG. 2B is a scanning electron micrograph which shows the
surface of the steel subjected to surface removal with the removal
depth of 40 .mu.m.
[0030] FIG. 3A is a chart which shows concentration profiles of Fe
and Cr for the steel subjected to surface removal with the removal
depth of 8 .mu.m, in the thickness direction from the surface of
the steel by glow discharge optical emission spectroscopy.
[0031] FIG. 3B is a chart which shows concentration profiles of Fe
and Cr for the steel subjected to surface removal with the removal
amount of 40 .mu.m, in the thickness direction from the surface of
the steel by glow discharge optical emission spectroscopy.
[0032] FIG. 4 is a schematic diagram which shows a scale/steel
interface.
BEST MODE FOR CARRYING OUT THE INVENTION
[0033] Description will be made regarding arrangements according to
the present invention.
[0034] First, description will be made regarding the reasons that
the components of the alloy according to the present invention are
restricted to the above-described range. Note that "% by mass" will
be used as the unit of the concentration of the components, which
will be abbreviated to "%" hereafter.
[0035] (1) C: 0.002 to 0.02%
[0036] The lower the concentration of C is, the corrosion
resistance after painting is more preferably improved. The reason
is that generation of a Cr depleted layer due to precipitation of
carbonitride is suppressed. However, while an arrangement with the
concentration of C less than 0.002% exhibits shortage of strength,
an arrangement with the concentration of C exceeding 0.02% exhibits
shortage of toughness and ductility, leading to deterioration in
impact toughness. With the steel according to the present
invention, it is important that the steel is formed with reduced
concentration of C of 0.02% or less. Furthermore, with the present
embodiment, the steel is formed with the concentration of C of
0.02% or less, thereby enabling omission of annealing for
hot-rolled steel. Accordingly, with the present embodiment, the
steel is formed with the concentration of C in a range of 0.002 to
0.02%. The steel is preferably formed with the concentration of C
in a range of 0.003 to 0.013%, is more preferably formed with the
concentration of C in a range of 0.003 to 0.008%, and further
preferably formed with the concentration of C in a range of 0.003
to 0.005%, from the perspective of improvement of corrosion
resistance after painting.
[0037] (2) N: 0.002 to 0.02%
[0038] The lower the concentration of N is, the corrosion
resistance after painting is more preferably improved, in the same
way as with C. However, while an arrangement with the concentration
of N less than 0.002% exhibits shortage of strength, an arrangement
with the concentration of N exceeding 0.02% exhibits shortage of
toughness and ductility, leading to deterioration in impact
toughness. With the steel according to the present invention, it is
important that the steel is formed with reduced concentration of N
of 0.02% or less. Furthermore, with the present embodiment, the
steel is formed with the concentration of N of 0.02% or less,
thereby enabling omission of annealing for hot-rolled steel.
Accordingly, with the present embodiment, the steel is formed with
the concentration of N in a range of 0.002 to 0.02%. The steel is
preferably formed with the concentration of N in a range of 0.0030
to 0.0060% from the perspective of improvement of corrosion
resistance after painting.
[0039] (3) Si: 0.05 to 1.0%
[0040] While Si is an element which is effectively used as a
deoxidizing agent, the steel formed with the concentration of Si
less than 0.05% does not exhibit sufficient deoxidation, and
accordingly, the steel needs to be formed with the concentration of
Si of 0.05% or more. However, the steel with the concentration of
Si exceeding 1.0% exhibits shortage of toughness and ductility,
leading to deterioration in impact toughness. Accordingly, with the
present embodiment, the steel is formed with the concentration of
Si in a range of 0.05 to 1.0%. The steel is preferably formed with
the concentration of Si in a range of 0.1 to 0.5% from the
perspective of improvement of low-temperature toughness.
[0041] (4) Mn: 0.05 to 1.0%
[0042] While Mn is an element which is effectively used as a
deoxidizing agent in the same way as with Si, the steel formed with
the concentration of Mn less than 0.05% does not exhibit sufficient
deoxidation, and accordingly, the steel needs to be formed with the
concentration of Mn of 0.05% or more. However, the steel with the
concentration of Mn exceeding 1.0% exhibits deterioration in
corrosion resistance due to increased MnS-inclusion. Accordingly,
with the present embodiment, the steel is formed with the
concentration of Mn in a range of 0.05 to 1.0%. The steel is
preferably formed with the concentration of Mn in a range of 0.10
to 0.30% from the perspective of improvement of corrosion
resistance after painting.
[0043] (5) P: 0.04% or Less
[0044] P is an element which causes adverse effects upon corrosion
resistance, as well as upon mechanical properties such as
toughness, ductility, and the like, and particularly, the steel
with concentration of P exceeding 0.04% exhibits marked adverse
effects thereupon, and accordingly, the steel according to the
present embodiment is formed with restricted concentration of P of
0.04% or less. In particular, the steel required to exhibit high
corrosion resistance after painting is preferably formed with the
concentration of P of 0.02% or less.
[0045] (6) S: 0.02% or Less
[0046] S is combined with Mn to form MnS, leading to initial
rust-formation portions. Furthermore, S is an adverse-effect
element which causes intergranular segregation thereof, leading to
brittleness, and accordingly, the steel is preferably formed with
the concentration of S as low as possible. In particular, the steel
with the concentration of S exceeding 0.02% exhibits marked adverse
effects, and accordingly, the steel is formed with restricted
concentration of S of 0.02% or less. In particular, the steel
required to exhibit high corrosion resistance after painting is
preferably formed with the concentration of S of 0.006% or
less.
[0047] (7) Al: 0.001 to 0.1%
[0048] While Al is an element which is effectively used as a
deoxidizing agent, and furthermore has the advantage of improving
ductility by spheroidizing oxide, the steel formed with the
concentration of Al less than 0.001% does not exhibit the
aforementioned sufficient advantage, and accordingly, the steel
needs to be formed with the concentration of Al of 0.001% or more.
However, the steel with the concentration of Al exceeding 0.1%
exhibits deterioration in corrosion resistance due to increased
inclusions. Accordingly, the steel is formed with the concentration
of Al in a range of 0.001 to 0.1%. Note that the steel containing
excessive Al may cause formation of inclusions, leading to
deterioration in mechanical properties, and accordingly, the steel
is preferably formed with-the maximal concentration of 0.05% from
the perspective of workability of hot-rolled steel.
[0049] (8) Cr: 6.0 to 10.0%
[0050] Cr is a necessary element for maintaining corrosion
resistance required for materials for freezing containers, which is
the object of the present invention. The external-wall members for
the freezing containers are used with painting, and accordingly, is
not required to exhibit high corrosion resistance as compared with
SUS304, but the steel with the concentration of Cr less than 6.0%
does not exhibit sufficient corrosion resistance. However, the
steel with the concentration of Cr exceeding 10.0% exhibits
shortage of toughness and ductility, leading to deterioration in
impact toughness. As an important fact according to the present
invention, it has been revealed that steel with the concentration
of Cr in a range of 6.0% to 10.0% exhibits sufficient corrosion
resistance, as well as sufficient toughness and impact toughness,
required for materials for freezing containers. Furthermore, with
the present embodiment, the steel is formed with the concentration
of Cr of 10.0% or less, thereby enabling omission of annealing for
hot-rolled steel. Note that in order to form hot-rolled steel
without annealing with sufficient low-temperature toughness, the
steel is preferably formed with a concentration of Cr in a range of
6.0 to 9.5%. Furthermore, the steel is more preferably formed with
a concentration of Cr in a range of 6.0 to 9.0%.
[0051] While description has been made regarding basic components,
an arrangement may be made wherein the steel is formed with other
components for further improving corrosion resistance as
follows.
[0052] (9) Cu: 0.1 to 1.0%
[0053] Cu is an element which is effectively used for reducing
corrosion rate, thereby improving corrosion resistance, and
furthermore, has the advantage of suppressing crevice corrosion.
With the corrosion resistance after painting which is the object of
the present invention, corrosion in the crevice structure partially
subjected to exfoliation of painting is a serious problem, and
accordingly, the steel needing to exhibit high corrosion resistance
after painting is preferably formed with addition of Cu. However,
the steel with the concentration of Cu less than 0.1% does not
exhibit the aforementioned sufficient advantages, and on the other
hand, the steel with the concentration of Cu exceeding 1.0% readily
causes deterioration in ductility and impact toughness, and
furthermore, readily causes hot cracking during hot rolling.
Accordingly, the steel is preferably formed with the concentration
of Cu in a range of 0.1 to 1.0%. Note that the steel is preferably
formed with the maximal concentration of Cu of 0.7% from the
perspective of prevention of hot cracking, and workability.
[0054] (10) Ni: 0.1 to 1.0%
[0055] Ni is an element which is effectively used for reducing
corrosion rate, thereby improving corrosion resistance, as well.
Furthermore, Ni is a component which is effectively used for
improving toughness. However, the steel with the concentration of
Ni less than 0.1% does not exhibit the aforementioned sufficient
advantages, and on the other hand, Ni is an extremely high-cost
material, and accordingly, the steel formed with the concentration
of Ni exceeding 1.0% leads to high costs, and accordingly, the
steel is preferably formed with the concentration of Ni in a range
of 0.1% to 1.0%. Note that the steel is preferably formed with the
maximal concentration of Ni of 0.5% from the perspective of
prevention of excessive hardness and high costs.
[0056] (11) Mo: 1.0% or Less
[0057] Mo is an element which is effectively used for reducing
corrosion rate, thereby improving corrosion resistance. However,
the steel with the concentration of Mo less than 0.1% does not
exhibit the aforementioned sufficient advantages, and on the other
hand, Mo is an extremely high-cost material as with Ni, and
accordingly, the steel formed with the concentration of Mo
exceeding 1.0% leads to high costs, and furthermore exhibits
deterioration in ductility, and accordingly, the steel is
preferably formed with the concentration of Mo in a range of 0.1%
to 1.0%. Note that the steel is preferably formed with the
concentration of Mo in a range of 0.1% to 0.5% from the perspective
of balance between the corrosion resistance, and the strength and
workability.
[0058] (12) Nb: 0.005 to 0.10%
[0059] Addition of Nb leads to precipitation of Nb-carbonitride
during hot rolling, thereby suppressing growth of grains, and
thereby drastically reducing the size of the grains in the steel
after hot rolling. In particular, the steel required to exhibit
high low-temperature toughness is preferably formed with addition
of Nb. However, the steel with the concentration of Nb less than
0.005% does not exhibit the aforementioned sufficient advantage,
and on the other hand, the steel formed with the concentration of
Nb exceeding 0.10% exhibits deterioration in toughness at a welded
portion, and accordingly, addition of Nb is determined to be 0.005
to 0.10%. The steel is preferably formed with the maximal
concentration of Nb of 0.06% from the perspective of toughness at a
welded portion.
[0060] (13) V: 0.005 to 0.20%
[0061] Addition of V leads to precipitation of V-carbonitride or
V.sub.4C.sub.3 during hot rolling, and has the advantage of
reducing the size of the grains in the steel after hot rolling,
thereby exhibiting the advantage of improving low-temperature
toughness of steel as with Nb, but the steel with the concentration
of V less than 0.005% does not exhibit the aforementioned
sufficient advantages, and on the other hand, the steel with the
concentration of V exceeding 0.20% leads to the adverse effects of
deterioration in toughness of a welded portion and base
material.
[0062] Accordingly, the steel is formed with the concentration of V
in a range of 0.005% to 0.20%. Note that the steel is formed with
the maximal concentration of V of 0.15% from the perspective of
improvement of toughness of the base material.
[0063] The components other than the aforementioned ones include Fe
and unavoidable impurities.
[0064] (14) Microstructures in Steel
[0065] Next, description will be made regarding microstructures in
the steel. The steel manufactured with the technique according to
the present invention is substantially formed with a ferritic
single phase microstructure. While the steel subjected to cooling
after hot rolling may partially contain bainite, the steel
subjected to cold rolling and annealing substantially exhibits a
ferritic single phase microstructure. With the steel according to
the present invention, the components thereof are designed such
that the steel prior to processing, such as the steel after hot
rolling, the steel after cold rolling and annealing, or the like,
does not exhibit formation of a hard martensitic microstructure. On
the other hand, the components thereof are designed such that the
welded portions thereof are formed with a low-carbon, low-nitrogen
containing martensitic microstructure, thereby exhibiting an
excellent property of sufficient low-temperature toughness even
after assembly with welding.
[0066] (15) Manufacturing Method of the Steel
[0067] Next, description will be made regarding a manufacturing
method for the steel according to the present invention. First,
smelting processing is performed with a smelting furnace such as a
converter, an electric heating furnace, or the like, following
which the molten steel is subjected to refining so as to adjust the
components thereof to those according to the present invention with
the VOD method, the AOD method, the RH method, or the like, and
subsequently, the molten steel is formed into slabs with the
continuous casting method, or the casting slabbing method.
Subsequently, the slab is heated, and subjected to hot-rolling
processing so as to form a hot-rolled steel. Furthermore, an
arrangement may be made wherein the slab after casting is inserted
into a heating furnace prior to cooling to the room temperature, or
an arrangement may be made wherein the slab after casting is
directly subjected to hot rolling. While the slab-reheating
temperature for hot rolling at the time of reheating the slab is
not restricted to the particular one, the coiling temperature needs
to be set to a high temperature in order to omit the annealing
processing for the hot-rolled steel, and accordingly, the
slab-reheating temperature is preferably set to 1050.degree. C. or
more. On the other hand, processing performed under the reheating
temperature exceeding 1250.degree. C. leads to a problem of sagging
of the slab, as well as a problem of loss due to oxidization of the
surface of the slab. Furthermore, in some cases, this leads to a
problem of deterioration in workability in hot rolling due to
partial formation of 8-ferritic phase microstructure, depending
upon the components of the steel. While the reduction and
temperature conditions during roughing hot rolling are not
restricted to particular ones, at least one or more passes of
rolling with rolling reduction of 30% or more is preferably
performed. This high-reduction rolling leads to reduction of the
size of grains in the steel, thereby improving the low-temperature
toughness of the base material. The finishing temperature in hot
rolling is set to 900.degree. C. or more, and is preferably set to
a temperature exceeding 930.degree. C., from the perspective of
enhancement of softening after coil-winding. With the present
embodiment, the finishing temperature in hot rolling set to
900.degree. C. or more prevents formation of deformed ferrite
grains due to rolling in the .alpha.+.gamma. two phase region, and
furthermore, maintains the high coiling temperature, thereby
preventing formation of a hard martensitic phase microstructure
during cooling after coiling. The coiling temperature in hot
rolling is set to 800.degree. C. or more, and is preferably set to
810.degree. C. or more, from the perspective of softening after
coiling. Furthermore, an arrangement may be made wherein annealing
is performed for hot-rolled steel as necessary in the event that
the steel needs to be subjected to adjustment of strength after hot
rolling, or the like. Batch annealing or continuous annealing may
be performed under temperature of 600.degree. C. or more for the
aforementioned annealing of the hot-rolled steel. With the
aforementioned batch annealing, the annealing is preferably for one
hour or more. Subsequently, a scale layer and the surface of the
steel is removed by shot blasting, pickling, or the like. An
arrangement may be made wherein the steel after hot rolling, after
hot rolling and hot annealing, or after descaling, is subjected to
rolling process by skin pass rolling from the perspective of
adjustment of the shape.
[0068] (16) Removal Amount of the Steel Surface in Descaling
Processing
[0069] The removal amount of descaling processing is an important
factor having the great influence upon the corrosion resistance
after painting which is the object of the present invention. Here,
as shown in a schematic diagram in FIG. 4, "removal depth of the
steel surface 1" denotes the thickness in the depth direction from
a so-called scale/steel interface 2 including an internal oxide
layer 3 and a Cr depleted layer 4. A scale layer 5 is generally
formed of iron oxide and chromium oxide on the surface of the steel
after hot rolling, or after hot rolling and annealing, wherein a
spinel structure phase generally formed of Fe serves as the outer
layer thereof, and a spinel structure phase generally formed of Fe
and Cr serves as the inner layer thereof. It is known that in the
event that the steel is exposed to high temperature for a long time
after hot rolling and coiling or the like, Cr is dominantly
oxidized due to growth of the scale layer 5 near the steel, leading
to shortage of supply due to diffusion of Cr from the inside of the
steel, and leading to formation of Cr depleted layer 4 immediately
underneath the scale layer.
[0070] The remaining Cr depleted layer 4 on the surface of the
steel after descaling causes marked deterioration in corrosion
resistance, and accordingly, it is important for the descaling
processing to completely remove the Cr depleted layer 4 on the
surface of the steel. With the steel containing Cr of 11% or more,
i.e., the stainless steel, a fine-structured layer generally formed
of Cr.sub.2O.sub.3 serves as a continuous further-inner layer as to
the aforementioned spinel layer, leading to suppression of
diffusion of oxygen from the outside toward the inside of the
steel. Accordingly, the Cr depleted layer 4 is formed with the
thickness from the scale/steel interface being less than 10 .mu.m
at most. However, with the steel with the low Cr concentration of
10% or less as with the present invention, the Cr.sub.2O.sub.3
layer is not continuously formed, leading to marked diffusion of
oxygen from the outside, and leading to formation of the so-called
internal oxidized layer 3 as shown in FIG. 4. The internal oxide
layer 3 is formed due to elements with high oxygen affinity such as
Cr, Si, or the like, being dominantly oxidized, which can be
confirmed by observing dominant oxidation 6 in the grain-boundary
of the steel or oxide formation 7 inside the grains on the
cross-sectional sample of the steel. According to the present
invention, the processing is an important factor, wherein both the
internal oxide layer 3 formed on the inner side of the scale/steel
interface 2, and the Cr depleted layer 4 formed due to formation of
the aforementioned internal oxide layer 3, are removed by descaling
processing, thereby drastically improving corrosion resistance
after painting.
[0071] Furthermore, the present inventors have performed intense
study from the perspective of painting adhesion. As a result, it
has been revealed that the steel subjected to descaling by pickling
after hot rolling may suffer marked grain boundary erosion, leading
to deterioration in painting adhesion to the steel since sufficient
paint cannot flow into the eroded grain boundary portion due to
poor wettability of the high-viscosity paint. In particular, the
amount of Cr in the grain boundary is readily reduced in the
portion containing the Cr depleted layer as described above,
readily leading to grain boundary erosion in this portion.
Reduction of painting adhesion readily causes formation of the
crevice structure between the paint film and the steel, leading to
deterioration in corrosion resistance after painting.
[0072] The conditions for obtaining both the sufficient painting
adhesion and the corrosion resistance after painting have been
studied based upon the above-described information. FIG. 1 and
Table 1 show an-example of evaluated results of the corrosion
resistance of the steel and corrosion resistance after painting
over the removal amount of the steel surface, wherein
9%-Cr-containing hot-rolled steel manufactured in a plant is
employed as a sample, and is subjected to descaling by shot
blasting and pickling with sulfuric acid and hydrofluoric
acid/nitric acid in a laboratory. As can be understood from these
results, with the steel containing Cr of 6.0 to 10.0%, removal of
the steel surface of 10 .mu.m or more in the descaling step
improves painting adhesion and corrosion resistance after painting,
as well as improving corrosion resistance of the steel.
Furthermore, making the difference in Cr concentration, i.e.,
(Cr-concentration within the steel)-(Cr-concentration in the
surface layer of the steel), to be within 1%, improves corrosion
resistance after painting by reducing the roughness due to the
grain boundary erosion descried below, as well as improving
corrosion resistance on the surface of the steel. That is to say,
the steel subjected to removal processing with the removal depth of
10 .mu.m or more exhibits the difference in Cr concentration, i.e.,
(Cr-concentration within the steel)-(Cr-concentratio- n in the
surface layer of the steel), of 1% or less, and in this case,
exhibits excellent corrosion resistance after painting.
[0073] Here, the Cr-concentration within the steel means the
Cr-concentration near the middle portion in the thickness direction
of the steel, which is not affected by the Cr depleted layer, or
the Cr-concentration of the inside portion at the depth exceeding
200 .mu.m from the surface thereof in a case of hot-rolled steel or
hot-rolled annealed steel. In a case of cold-rolled annealed steel,
the Cr-concentration within the steel means the Cr-concentration at
the portion at the depth of t/4 or more, wherein t denotes the
thickness of the steel. The Cr-concentration can be measured with a
method such as EPMA, EDX, the analysis method using fluorescent
X-ray or the like, the solid emission spectroscopic analysis
method, the method wherein chemical solution of the steel is
subjected to quantitative analysis with the Inductively Coupled
Plasma-Atomic Emission (which will be referred to as "ICP method"
hereafter) or the titrimetric analysis, or the like. In a case of
employing EPMA analysis or the like, which obtains measurement
results of Cr-concentration at a particular portion, there is the
need to select the portion which is to be measured so as not to be
affected by segregation formed at middle portion in the thickness
direction of the steel.
[0074] FIG. 2 shows results obtained by observing the surface of
the steel subjected to the surface removal with the removal depth
of the steel surface of 8 .mu.m and 40 .mu.m using a scanning
electron microscope. With the example subjected to surface removal
with the removal amount of the steel surface of 8 .mu.m, the grain
boundary where dominant and marked erosion has occurred was
observed. On the other hand, with the example subjected to surface
removal with the removal amount of the steel surface of 40 .mu.m,
the grain boundary where marked erosion has occurred was not
observed.
[0075] FIG. 3 shows measurement results of the concentration
profiles of Fe and Cr in the thickness direction from the surface
of the steel by glow discharge optical emission spectroscopy (GDS).
While with the steel subjected to surface removal with the removal
depth of the steel surface of 8 .mu.m, the remaining Cr depleted
layer was observed near the surface of the steel, with the steel
subjected to surface removal with the removal depth of the steel
surface of 40 .mu.m, the remaining Cr depleted layer was not
observed. These steel samples were subjected to measurement of
Cr-concentration using the Electron Probe Micro Analyzer (EPMA),
wherein the measurement results were obtained that while
8-.mu.m-layer-removed steel exhibits reduction of Cr-concentration
by 2.5% by mass as compared with the Cr-concentration within the
steel (the concentration evaluated with the ICP method for the
steel sample subjected to surface removal with the removal amount
of the steel surface of 5900 .mu.m: 9% by mass),
40-.mu.m-layer-removed steel exhibits the Cr-concentration
generally equal to the Cr-concentration within the steel.
Furthermore, as the result of measurement of the whiteness of the
surface of the steel stipulated by JIS Z 8715, while
8-.mu.m-layer-removed steel exhibits the whiteness index of
approximately 62, 40-.mu.m-layer-removed steel exhibits the
whiteness index of 68. As a result of research of the whiteness of
various kinds of hot-rolled steel, in general, the hot-rolled steel
with the whiteness index of 65 or more does not contain marked
erosion of the grain boundary, thereby improving the corrosion
resistance of the steel after painting. Note that the whiteness was
measured using a spectrophotometer, wherein the CM-1000
manufactured by Minolta Corporation was employed.
[0076] As described above, it is assumed that in the event that the
removal depth of the steel surface does not reach 10 .mu.m, the
internal oxide layer 3 and the Cr depleted layer 4 are not
completely removed, leading to deterioration in painting adhesion
due to erosion of the grain boundary, and leading to deterioration
in corrosion resistance after painting, as well as leading to
insufficient corrosion resistance of the steel. Note that surface
removal with the removal depth of the steel surface exceeding 200
.mu.m may lead to problems of deterioration in corrosion resistance
and deterioration in the appearance of the steel, due to formation
of so-called smut adhered to the surface of the steel during
pickling, as well as leading to increased costs due to excessive
loss by descaling. The removal depth of the steel surface is more
preferably determined to 15 .mu.m or more, and is further
preferably determined to 20 .mu.m or more.
[0077] Control of the surface properties by removing the steel
surface of Cr-containing steel, and improvement of corrosion
resistance after painting by removing the internal oxide layer 3
and the Cr depleted layer 4, based upon the detailed information
obtained through intense study are the important factors of the
present invention, and as a result of the information through
intense study, it has been revealed that the steel is preferably
subjected to surface removal with the removal depth of 10 .mu.m or
more, is more preferably subjected to surface removal with the
removal depth of 15 .mu.m or more, and is further preferably
subjected to surface removal with the removal depth of 20 .mu.m or
more, while maintaining the Cr-concentration in the surface layer
of the steel of the value of (Cr-concentration within the steel-1%
by mass) or more, thereby markedly improving corrosion resistance
after painting.
[0078] Note that the corrosion resistance of the steel shown in
Table 1 was evaluated with the rust area ratio after 4-hours salt
spray test stipulated by JIS Z 2371, wherein a sample exhibiting a
rust area ratio of 20% or less is determined as a sample exhibiting
excellent corrosion resistance. On the other hand, the corrosion
resistance after painting was evaluated with the method wherein a
steel sample is coated with acrylic resin paint with an intended
dry film thickness of 50 .mu.m, is subjected to formation of
cross-cuts on the surface thereof, following which the steel sample
is subjected to 1000-hour salt spray testing stipulated by JIS Z
2371, and the steel sample wherein marked outflow rust such as
formation of a rust pool at the lower portion of the sample, is not
observed, is determined as a steel sample with excellent corrosion
resistance after painting. On the other hand, the specific
measurement method for obtaining the removal depth of the steel
surface is that the weight and size of the sample following
mechanical removal of the scale by shot blasting, and the weight
thereof after pickling, are measured, the difference in weight
therebetween is divided by the surface area of the sample so as to
calculate the removal amount of the steel (g/cm.sup.2), following
which the removal thickness (depth) (.mu.m) of the steel is
calculated using the density of the steel (7.8g/cm.sup.3).
[0079] Note that the descaling method for hot-rolled steel employed
in the present invention is not restricted to a particular one.
Various known methods which may be employed in the present
invention include mechanical removal methods by shot blasting,
brushing, or using a small-diameter roller, chemical removal
methods using hydrochloric acid, sulfuric acid, nitric acid,
hydrofluoric acid, hydrofluoric/nitric acid, ferric chloride
solution, and the like.
[0080] (17) Processing After Descaling
[0081] The steel after descaling described above may be employed as
the steel according to the present invention. Furthermore, an
arrangement may be employed as the steel according to the present
invention, wherein the steel after the aforementioned descaling is
subjected to cold rolling so as to be formed with a predetermined
thickness, following which the cold-rolled steel is subjected to
annealing and pickling. The surface of the cold-rolled annealed
steel is sufficiently smooth, and accordingly, it is assumed that
deterioration in corrosion resistance does not occur due to poor
painting adhesion described above, but in the event that
insufficient descaling has been performed after hot rolling, the
steel does not exhibit sufficient corrosion resistance after cold
rolling, as well. With the steel according to the present
invention, the internal oxide layer 3 and the Cr depleted layer 4
immediately underneath the scale are completely removed by
descaling for hot-rolled steel, thereby exhibiting sufficient
corrosion resistance even after cold rolling and annealing. Cold
rolling is preferably performed with rolling reduction of 30% or
more. The steel after cold rolling is preferably subjected to
annealing in order to soften the steel under annealing temperature
of 600.degree. C. or more. The steel after cold rolling and
annealing is subjected to pickling or similar processing, following
which various finishing processing may be performed stipulated by
JIS Z 4305. Giving consideration to the corrosion resistance after
painting, No. 2B finishing is preferably employed.
[0082] (18) Painting Method
[0083] Painting is made by spray painting, brush painting, or the
like, using various kinds of paints stipulated by JIS K 5500, such
as an acrylic resin paint, phthalic resin paint, epoxy resin paint,
polyurethane resin paint, or the like.
[0084] Furthermore, the steel may be coated with various kinds of
primers prior to painting for preventing initial rusting.
[0085] Furthermore, the steel is subjected to under-coating or
middle coating using various kinds of rust resisting paints or
resin paints, as necessary. Note that the steel according to the
present invention exhibits excellent adhesion between the surface
thereof and a top coating paint, as well as exhibiting the high
corrosion resistance of the steel itself as compared with common
steel, thereby enabling omission of primer coating, under-coating,
and middle coating, and thereby enabling directly coating of the
steel with high-viscosity top-coating paint. Giving consideration
to use for freezing containers, there is the need to coat the steel
with the paint-film thickness of 10 .mu.m or more for obtaining
sufficient corrosion resistance. Note that with the steel according
to the present invention, painting can be omitted, depending upon
the use thereof, e.g., use for residential structural materials,
and particularly for members which are not required to exhibit high
corrosion resistance.
[0086] (19) Target for the Mechanical Properties of the Steel
According to the Present Invention
[0087] Giving consideration to use for structural steel, the steel
is required to exhibit a Charpy impact value of 50 J/cm2 or more at
-25.degree. C. serving as an index value of the toughness. In
particular, giving consideration to use for materials for freezing
containers, or use for housing materials in the cold region, the
steel is preferably formed with a Charpy impact value of 80
J/cm.sup.2 or more at -25.degree. C.
[0088] While the steel is preferably formed so as to exhibit as
great an elongation value as possible in the tensile test, the
steel is required to exhibit the aforementioned elongation value of
30% or more to be formed in various kinds of shapes. Furthermore,
while the steel is preferably formed so as to exhibit yield ratio
as low as possible, wherein the yield ratio serves as an index
value representing the degree of difficulty in workability and
earthquake performance in a case of being employed as a housing
material, the steel is required to exhibit the aforementioned yield
ratio of 80% or less, and is preferably formed with the yield ratio
of 75% or less, in a case of being employed as a structural
material.
[0089] (20) Target for the Corrosion Resistance After Painting of
the Steel According to the Present Invention
[0090] In most cases, the steel for freezing containers is
subjected to various kinds of painting of the surface thereof from
the perspective of improvement of corrosion resistance, and
particularly from the perspective of appearance. Accordingly, the
corrosion resistance after painting is an important factor.
[0091] The present inventors performed a detailed comparative study
with regard to results of the corrosion resistance after painting
of the steel which has been used in actuality, and the results
obtained from the accelerated test by salt spraying, and as a
result, it has been revealed that in the event that the sample of
the steel subjected to painting, formation of cross-cuts, and
1000-hour salt spray testing, in that order, does not exhibit
marked outflow rust, the steel exhibits sufficient corrosion
resistance in practical use thereof. Accordingly, corrosion
resistance after painting was evaluated based upon the results
obtained from the 1000-hour salt spray testing.
EXAMPLES
[0092] The steel materials were subjected to casting with the
chemical composition shown in Table 2 so as to form a steel ingot
of 50 kg by vacuum melting, heated up to 1200.degree. C., and held
under this temperature for one hour, following which the steel was
subjected to hot rolling into hot-rolled steel with the thickness
of 4 mm. The half of the samples of the hot-rolled steel were
subjected to homogenizing annealing (hot-rolling annealing) under
650.degree. C. for ten hours. The hot-rolled steel without the
subsequent processing and the hot-rolled annealed steel were
subjected to descaling by shot blasting and subsequent pickling
using mixed acid such as mixture of hydrofluoric acid and nitric
acid so as to remove the steel surface by approximately 15 .mu.m.
Note that the measurement method for obtaining the removal amount
of the steel surface is that the weight and size of the sample
following mechanical removal of the scale by shot blasting, and the
weight thereof after pickling, are measured, the difference in
weight therebetween is divided by the surface area of the sample so
as to calculate the removal amount of the steel (g/cm.sup.2),
following which the removal thickness (depth) (.mu.m) of the steel
is calculated using the density of the steel (7.8 g/cm.sup.3). Note
that the steel was subjected to pickling using mixed acid formed of
hydrofluoric acid solution of 1 to 2% by mass and nitric acid
solution of 13 to 15% by mass under temperature of 40 to 60.degree.
C., and was repeatedly picked up for measuring the weight thereof
following pickling every 30 seconds, thereby obtaining desired
removal depth of the steel surface.
[0093] With the steel denoted by No. 1, the samples with various
kinds of the removal depth of the steel surface were prepared as
comparative examples. The Cr-concentration in the surface layer of
the steel was obtained for these samples by making EPMA analysis.
The aforementioned EPMA was performed with an accelerating voltage
of 15 kV. It is assumed that the information obtained under the
aforementioned conditions is generally reflected in the
concentration in the depth range between the surface thereof and a
depth of 0.5 .mu.m. With the steel denoted by No. 1, the steel was
subjected to surface removal with the removal depth of the steel
surface of 500 .mu.m, following which the steel sample was
subjected to quantitative analysis using the ICP method, whereby
the concentration was determined to be 9.1% by mass. While the
steel samples subjected to surface removal with the removal depth
of the steel surface of 5 .mu.m and 8 .mu.m exhibited
Cr-concentrations of 5.1% by mass and 6.6% by mass, respectively,
the steel sample subjected to surface removal with the removal
depth of the steel surface of 15 .mu.m exhibited 8.3% by mass,
which was equal to or more than the (Cr-concentration within the
steel-1)%.
[0094] Table 3 shows the results of Cr-concentration measurement
for other steel samples under the same conditions. In any case, the
steel sample subjected to surface removal with the removal depth of
the steel surface of 15 .mu.m exhibited Cr-concentration equal to
or more than the (Cr-concentration within the steel-1)%.
[0095] Plate samples of the size of (the
thickness.times.50.times.100 (mm)) were cut off from these
hot-rolled steel products, were coated with acrylic-silicone resin
paint (top coat with SILICOTECT AC manufactured by Kansai Paint
Co., Ltd.) with intended dry paint thickness of 50 .mu.m on the
surface by spray painting, X-shaped cross-cuts were formed on the
top layer including the aforementioned paint film, the prepared
samples were subjected to the 1000-hour salt spray testing
stipulated by JIS Z 2371 (5%-NaCl solution, 35.degree. C., and pH
6.5 to 7.2), and the time was measured wherein marked outflow rust
was generated, e.g., a rust pool was generated at the lower portion
of the sample. Note that the dry film thickness of the paint film
was measured by using an electromagnetic film thickness monitor and
by observing the cross-section with microscope. The measured dry
film thickness was approximately 50 .mu.m. Furthermore, both
surfaces of the steel sheet was ground by 0.75 mm so as to obtain
the thickness of 2.5 mm, then Charpy-impact test samples of a
sub-size wherein a V-shaped notch was formed with a depth of 2 mm
perpendicular to the rolling direction, were obtained stipulated by
JIS Z 2202, and the Charpy-impact value (J/cm.sup.2) was measured
under -25.degree. C. stipulated by JIS Z 2242. The results are
shown in Table 3. While the Charpy-impact value cannot be measured
for a cold-rolled steel sheet with a thickness of 2 mm or less
using a normal method, in general, the smaller the thickness of the
steel is, the greater the toughness thereof is (for example, see
JOURNAL OF THE JAPAN WELDING SOCIETY, Vol. 61, No. 8 (1992), p.
636), and since the cold-rolled steel has the advantage for
Charpy-impact value as compared with the hot-rolled steel from the
point of the microstructure, the cold-rolled steel exhibits
Charpy-impact value under -25.degree. C. equal to or greater than
that of the hot-rolled steel. Accordingly, in the event that the
hot-rolled steel with a great thickness exhibits sufficient
Charpy-impact value, the cold-rolled steal with small thickness
formed of the same material as with the aforementioned hot-rolled
steel exhibits a sufficient Charpy-impact value, as well. A
cold-rolled annealed sample with a thickness of 0.7 mm was actually
formed using the steel denoted by No. 2 without being subjected to
annealing after hot-rolling, wherein a V-shaped notch was formed
with a depth of 2 mm while maintaining the thickness of 0.7 mm, and
the Charpy absorbed energy was measured for the prepared sample
under -25.degree. C., using a small-sized Charpy tester (100
kgf=98N), and as a result, the excellent result of 150 J/cm.sup.2
was obtained.
[0096] Furthermore, the aforementioned hot-rolled steel subjected
to pickling was subjected to cold rolling so as to be formed with
thickness of 0.7 mm, following which the steel was subjected to
annealing under 750.degree. C. for one minute, and subsequently,
the steel was subjected to descaling by electro-pickling in a
neutral salt solution and nitric acid, whereby cold-rolled steel
products were obtained. The aforementioned electro-pickling in a
neutral salt solution was performed in a 20%-Na.sub.2SO.sub.4
solution under temperature of 70 to 80.degree. C. with the charge
amount of 100 to 200 C/dm.sup.2. On the other hand, the
aforementioned electro-pickling in a nitric acid was performed in a
10%-HNO.sub.3 solution under temperature of 50 to 60.degree. C.
with the charge amount of 20 to 40 C/dm.sup.2.
[0097] Tensile test samples were cut off from these cold-rolled
steel products in the rolling direction stipulated by JIS13B, and
tensile test was performed for these samples stipulated by JIS Z
2241, whereby the elongation and the yield ratio thereof were
measured. Furthermore, plate samples of the size of (the
thickness.times.50.times.100 (mm)) were cut off from these
cold-rolled steel products, were coated with acrylic-silicone resin
paint (top coat with SILICOTECT AC manufactured by Kansai Paint
Co., Ltd.) with intended dry paint thickness of 50 .mu.m by surface
spray painting, X-shaped cross-cuts were formed on the top layer
including the aforementioned paint film, the prepared samples were
subjected to the 1000-hour salt spray testing stipulated by JIS Z
2371 (5%-NaCl solution, 35.degree. C., and pH 6.5 to 7.2), and the
time was measured wherein marked outflow rust is generated, e.g., a
rust pool is generated at the lower portion of the sample. Note
that the dry film thickness of the paint film was measured by using
an electromagnetic film thickness monitor and by observing the
cross-section with microscope. The measured dry film thickness was
approximately 50 .mu.m. Table 3 shows these measurement
results.
[0098] As can be understood from Table 3, the steel samples 1
through 10, and the steel samples 18 and 19, according to the
present invention, exhibit toughness (Charpy impact value) of 50
J/cm.sup.2 or more, elongation of 33% or more, and the yield ratio
of 75% or less, regardless of whether or not annealing has been
performed for the hot-rolled steel. Furthermore, these samples
exhibit excellent corrosion resistance wherein outflow rust is not
generated after the 1000-hour salt spray testing. On the other
hand, with the steel samples 11 through 17 serving as comparative
examples having the steel composition out of the scope of the
present invention, at least one of the toughness, elongation, yield
ratio, and corrosion resistance, does not reach the sufficient
level, regardless of whether or not annealing has been performed
for the hot-rolled steel.
[0099] Note that the steel was manufactured with various kinds of
compositions according to the present invention using actual mass
production equipment, and it has been confirmed that the steel thus
manufactured has the advantages according to the present invention,
as well.
1 TABLE 1 Cr concentration in Removal the inside layer of depth of
steel sheet - Cr steel plate Corrosion Corrosion concentration in
the surface resistance resistance surface layer layer of steel
after thereof (.mu.m) sheet painting (% by mass) 5 NG NG 4.0 8 NG
NG 2.5 10 Good Good 1.0 14 Good Good 0.8 20 Good Good 0.4 40 Good
Good 0.0 80 Good Good 0.0 110 Good Good 0.0
[0100]
2TABLE 2 Steel No. C N Si Mn P S Al Cr Cu Ni Mo Nb V Notes 1 0.005
0.014 0.21 0.33 0.02 0.004 0.010 9.1 -- -- -- -- -- Present
invention 2 0.007 0.005 0.22 0.25 0.03 0.006 0.020 9.5 -- -- -- --
-- Present invention 3 0.011 0.008 0.30 0.10 0.02 0.003 0.010 8.8
-- -- -- -- -- Present invention 4 0.013 0.005 0.35 0.31 0.03 0.005
0.005 8.7 -- -- -- -- -- Present invention 5 0.005 0.006 0.26 0.27
0.03 0.002 0.020 9.2 -- -- -- -- -- Present invention 6 0.005 0.007
0.25 0.35 0.02 0.004 0.010 9.1 -- -- -- -- -- Present invention 7
0.006 0.005 0.23 0.32 0.02 0.004 0.010 9.3 0.55 -- -- -- -- Present
invention 8 0.008 0.003 0.22 0.26 0.02 0.005 0.020 8.5 -- 0.48 --
-- -- Present invention 9 0.003 0.012 0.40 0.23 0.03 0.008 0.020
6.5 -- -- 0.51 -- -- Present invention 10 0.005 0.015 0.10 0.40
0.03 0.006 0.010 8.1 0.25 0.18 0.22 -- -- Present invention 11
0.013 0.008 0.28 0.36 0.03 0.006 0.020 0.1 -- -- -- -- --
Comparative example 12 0.009 0.005 0.21 0.23 0.02 0.003 0.030 4.2
-- -- -- -- -- Comparative example 13 0.007 0.007 0.32 0.36 0.03
0.005 0.010 11.4 -- -- -- -- -- Comparative example 14 0.012 0.009
0.38 0.35 0.02 0.009 0.010 13.5 -- -- -- -- -- Comparative example
15 0.052 0.007 0.24 0.21 0.03 0.004 0.020 9.2 -- -- -- -- --
Comparative example 16 0.006 0.061 0.25 0.28 0.03 0.003 0.020 8.9
-- -- -- -- -- Comparative example 17 0.033 0.031 0.22 0.31 0.02
0.005 0.010 9.1 -- -- -- -- -- Comparative example 18 0.005 0.006
0.28 0.22 0.03 0.005 0.012 9.4 -- -- -- 0.043 -- Present invention
19 0.007 0.006 0.32 0.18 0.03 0.004 0.020 9.3 0.15 0.18 -- 0.060
0.05 Present invention
[0101]
3TABLE 3 Removal Properties of hot-rolled steel Properties of
cold-rolled depth (Hot-rolled annealed steel) annealed steel of Cr
Cr Time Time steel concentration concentration for for Annealing
sheet of of outflow Charpy outflow for hot- surface surfacelayer
inside layer rust to impact Yield rust to Steel rolled layer of
steel sheet of steel sheet occur value Elongation ratio occur No.
steel (.mu.m) (% by mass) (% by mass) (hour) (J/cm.sup.2) (%) (%)
(hour) Notes 1 Yes 15 8.2 9.1 >1000 90 35 65 >1000 Present
invention 2 Yes 15 8.7 9.5 >1000 120 36 63 >1000 Present
invention 3 Yes 15 8.0 8.8 >1000 100 35 68 >1000 Present
invention 4 Yes 15 7.8 8.7 >1000 100 35 70 >1000 Present
invention 5 Yes 15 8.3 9.2 >1000 110 37 65 >1000 Present
invention 6 Yes 15 8.3 9.1 >1000 120 37 66 >1000 Present
invention 7 Yes 15 8.5 9.3 >1000 100 38 62 >1000 Present
invention 8 Yes 15 7.7 8.5 >1000 130 36 65 >1000 Present
invention 9 Yes 15 6.0 6.5 >1000 90 35 70 >1000 Present
invention 10 Yes 15 7.3 8.1 >1000 80 35 68 >1000 Present
invention 11 Yes 15 <0.1 0.1 80 130 40 62 100 Comparative
example 12 Yes 15 3.8 4.2 350 130 39 63 400 Comparative example 13
Yes 15 11.0 11.4 >1000 30 30 77 >1000 Comparative example 14
Yes 15 13.3 13.5 >1000 20 29 80 >1000 Comparative example 15
Yes 15 8.4 9.2 >1000 20 29 78 >1000 Comparative example 16
Yes 15 8.0 8.9 >1000 20 30 76 >1000 Comparative example 17
Yes 15 8.2 9.1 >1000 30 30 77 >1000 Comparative example 1 No
15 8.3 9.1 >1000 90 34 66 >1000 Present invention 1 No 5 5.1
9.1 300 90 34 66 400 Comparative example 1 No 8 6.6 9.1 500 90 34
66 600 Comparative example 2 No 15 9.1 9.5 >1000 110 36 63
>1000 Present invention 3 No 15 8.1 8.8 >1000 90 35 70
>1000 Present invention 4 No 15 8.0 8.7 >1000 100 34 71
>1000 Present invention 5 No 15 8.4 9.2 >1000 100 35 67
>1000 Present invention 6 No 15 8.5 9.1 >1000 100 37 68
>1000 Present invention 7 No 15 8.8 9.3 >1000 100 38 64
>1000 Present invention 8 No 15 7.9 8.5 >1000 90 35 70
>1000 Present invention 9 No 15 6.1 6.5 >1000 80 34 70
>1000 Present invention 10 No 15 7.5 8.1 >1000 80 34 69
>1000 Present invention 11 No 15 <0.1 0.1 40 130 39 65 50
Comparative example 12 No 15 4.0 4.2 300 120 38 65 300 Comparative
example 13 No 15 11.3 11.4 >1000 20 27 82 >1000 Comparative
example 14 No 15 13.5 13.5 >1000 10 26 85 >1000 Comparative
example 15 No 15 8.5 9.2 >1000 10 28 82 >1000 Comparative
example 16 No 15 8.2 8.9 >1000 10 28 81 >1000 Comparative
example 17 No 15 8.5 9.1 >1000 20 27 81 >1000 Comparative
example 18 Yes 15 8.7 9.4 >1000 210 40 62 >1000 Present
invention 19 Yes 15 8.6 9.3 >1000 210 38 63 >1000 Present
invention 18 No 15 9.0 9.4 >1000 200 38 63 >1000 Present
invention 19 No 15 9.0 9.3 >1000 200 38 64 >1000 Present
invention
INDUSTRIAL APPLICABILITY
[0102] The steel according to the present invention also has the
great advantage of relatively low costs. The steel according to the
present invention is formed with low concentration of Cr, C, and N,
as compared with SUS304 or 11%-Cr-containing stainless steel, which
enables omission of annealing after hot rolling, thereby enabling
further cost reduction. On the other hand, the steel according to
the present invention may be applied to various kinds of structural
materials such as building materials due to excellent mechanical
properties and low costs thereof, and particularly, the steel
according to the present invention has the advantage of use in the
cold region.
[0103] The present invention provides cr-containing steel for
freezing containers, having the excellent advantages of sufficient
low-temperature toughness, impact toughness, and corrosion
resistance, and furthermore, has the advantage of low costs as
compared with stainless steel.
* * * * *