U.S. patent application number 14/780818 was filed with the patent office on 2016-03-03 for steel structure for hydrogen gas, mehtod for producing hydrogen storage tank, and method for producing hydrogen line pipe (as amended).
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Akihide Nagao, Shusaku Takagi.
Application Number | 20160060738 14/780818 |
Document ID | / |
Family ID | 51623199 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160060738 |
Kind Code |
A1 |
Nagao; Akihide ; et
al. |
March 3, 2016 |
STEEL STRUCTURE FOR HYDROGEN GAS, MEHTOD FOR PRODUCING HYDROGEN
STORAGE TANK, AND METHOD FOR PRODUCING HYDROGEN LINE PIPE (AS
AMENDED)
Abstract
Provided is a steel structure for hydrogen gas such as a
hydrogen storage tank or a hydrogen line pipe which achieves a
lower fatigue crack propagation rate in a high-pressure hydrogen
atmosphere than steels used in the related art and has high
hydrogen embrittlement resistance. The steel structure for hydrogen
gas, which has high hydrogen embrittlement resistance in
high-pressure hydrogen gas, has a steel microstructure including
any one of 10% to 95% of bainite on an area-ratio basis, 10% to 95%
of martensite on an area-ratio basis, and 10% to 95% of pearlite on
an area-ratio basis, with the balance being substantially
ferrite.
Inventors: |
Nagao; Akihide; (Tokyo,
JP) ; Takagi; Shusaku; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Tokyo
JP
|
Family ID: |
51623199 |
Appl. No.: |
14/780818 |
Filed: |
March 28, 2014 |
PCT Filed: |
March 28, 2014 |
PCT NO: |
PCT/JP2014/001833 |
371 Date: |
September 28, 2015 |
Current U.S.
Class: |
148/593 ;
148/663; 420/104; 420/105; 420/110; 420/120; 420/128; 420/83;
420/91 |
Current CPC
Class: |
C21D 6/002 20130101;
C22C 38/46 20130101; C21D 9/46 20130101; C22C 38/42 20130101; C22C
38/005 20130101; C22C 38/12 20130101; C21D 6/004 20130101; C22C
38/28 20130101; C21D 2211/009 20130101; C21D 6/001 20130101; C22C
38/24 20130101; C22C 38/44 20130101; C21D 9/14 20130101; C22C 38/50
20130101; C22C 38/54 20130101; C21D 9/085 20130101; C21D 2211/002
20130101; C22C 38/20 20130101; C21D 8/105 20130101; C21D 9/0068
20130101; C22C 38/22 20130101; C22C 38/40 20130101; C22C 38/04
20130101; C22C 38/14 20130101; C22C 38/001 20130101; C22C 38/38
20130101; C21D 2211/008 20130101; C22C 38/02 20130101; C21D 6/008
20130101; C22C 38/00 20130101; C22C 38/002 20130101; C22C 38/06
20130101; C21D 6/005 20130101; C22C 38/26 20130101; C22C 38/32
20130101; C22C 38/48 20130101 |
International
Class: |
C22C 38/54 20060101
C22C038/54; C21D 9/08 20060101 C21D009/08; C21D 9/00 20060101
C21D009/00; C21D 6/00 20060101 C21D006/00; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/38 20060101 C22C038/38; C22C 38/32 20060101
C22C038/32; C22C 38/28 20060101 C22C038/28; C22C 38/26 20060101
C22C038/26; C22C 38/24 20060101 C22C038/24; C22C 38/22 20060101
C22C038/22; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 8/10 20060101 C21D008/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2013 |
JP |
2013-075008 |
Mar 29, 2013 |
JP |
2013-075009 |
Mar 29, 2013 |
JP |
2013-075010 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
11. (canceled)
12. (canceled)
13. (canceled)
14. A steel structure for hydrogen gas, the steel structure
comprising a steel microstructure including any one of 10% to 95%
of bainite on an area-ratio basis, 10% to 95% of martensite on an
area-ratio basis, and 10% to 95% of pearlite on an area-ratio
basis, with the balance being substantially ferrite.
15. The steel structure for hydrogen gas according to claim 14, the
steel structure having a steel composition containing, by mass, C:
0.05% to 0.20%, Si: 0.05% to 0.50%, Mn: 0.5% to 2.0%, Al: 0.01% to
0.10%, N: 0.0005% to 0.008%, P: 0.05% or less, S: 0.01% or less,
and O: 0.01% or less, with the balance being Fe and inevitable
impurities, wherein the steel microstructure includes 10% to 95% of
bainite on an area-ratio basis with the balance being substantially
ferrite.
16. The steel structure for hydrogen gas according to claim 14, the
steel structure having a steel composition containing, by mass, C:
0.05% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.5% to 2.0%, Al: 0.01% to
0.10%, N: 0.0005% to 0.008%, P: 0.05% or less, S: 0.01% or less,
and O: 0.01% or less, with the balance being Fe and inevitable
impurities, wherein the steel microstructure includes 10% to 95% of
martensite on an area-ratio basis with the balance being
substantially ferrite.
17. The steel structure for hydrogen gas according to claim 14, the
steel structure having a steel composition containing, by mass, C:
0.05% to 0.10%, Si: 0.05% to 0.50%, Mn: 0.5% to 2.0%, Al: 0.01% to
0.10%, N: 0.0005% to 0.008%, P: 0.05% or less, S: 0.01% or less,
and O: 0.01% or less, with the balance being Fe and inevitable
impurities, wherein the steel microstructure includes 10% to 95% of
pearlite on an area-ratio basis with the balance being
substantially ferrite.
18. The steel structure for hydrogen gas according to any one of
claims 15 to 17, wherein the steel composition further contains at
least one group selected from the groups A and B consisting of:
Group A: one or more elements selected from Cu: 0.05% to 1.0%, Ni:
0.05% to 2.0%, Cr: 0.1% to 2.5%, Mo: 0.05% to 2.0%, Nb: 0.005% to
0.1%, V: 0.005% to 0.2%, Ti: 0.005% to 0.1%, W: 0.05% to 2.0%, and
B: 0.0005% to 0.005% by mass Group B: one or more elements selected
from Nd: 0.005% to 1.0%, Ca: 0.0005% to 0.005%, Mg: 0.0005% to
0.005%, and REM: 0.0005% to 0.005% by mass.
19. A method for producing the hydrogen line pipe, the method
comprising heating a steel material having the steel composition
according to any one of claims 15 to 17 to an Ac.sub.3
transformation temperature or more, followed by hot rolling; and
subsequently performing cooling from an Ar.sub.3 transformation
temperature or more to 600.degree. C. or less at a cooling rate of
1.degree. C./sec. to 200.degree. C./sec.
20. A method for producing the hydrogen line pipe, the method
comprising heating a steel material having the steel composition
according to any one of claims 15 to 17 to an Ac.sub.3
transformation temperature or more, followed by hot rolling;
performing quenching from an Ar.sub.3 transformation temperature or
more to 250.degree. C. or less at a cooling rate of 1.degree.
C./sec. to 200.degree. C./sec; and subsequently performing
tempering at an Ac.sub.1 transformation temperature or less.
21. A method for producing the hydrogen storage tank, the method
comprising forming a steel material having the steel composition
according to any one of claims 15 to 17 into a predetermined shape,
followed by heating to an Ac.sub.3 transformation temperature or
more; performing quenching from an Ar.sub.3 transformation
temperature or more to 250.degree. C. or less at a cooling rate of
0.5.degree. C./sec to 100.degree. C./sec; and subsequently
performing tempering at an Ac.sub.1 transformation temperature or
less.
22. A method for producing the hydrogen line pipe, the method
comprising heating a steel material having the steel composition
according to claim 18 to an Ac.sub.3 transformation temperature or
more, followed by hot rolling; and subsequently performing cooling
from an Ar.sub.3 transformation temperature or more to 600.degree.
C. or less at a cooling rate of 1.degree. C./sec. to 200.degree.
C./sec.
23. A method for producing the hydrogen line pipe, the method
comprising heating a steel material having the steel composition
according to claim 18 to an Ac.sub.3 transformation temperature or
more, followed by hot rolling; performing quenching from an
Ar.sub.3 transformation temperature or more to 250.degree. C. or
less at a cooling rate of 1.degree. C./sec. to 200.degree. C./sec;
and subsequently performing tempering at an Ac.sub.1 transformation
temperature or less.
24. A method for producing the hydrogen storage tank, the method
comprising forming a steel material having the steel composition
according to claim 18 into a predetermined shape, followed by
heating to an Ac.sub.3 transformation temperature or more;
performing quenching from an Ar.sub.3 transformation temperature or
more to 250.degree. C. or less at a cooling rate of 0.5.degree.
C./sec to 100.degree. C./sec; and subsequently performing tempering
at an Ac.sub.1 transformation temperature or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to a steel structure for
hydrogen gas, such as a hydrogen storage tank or a hydrogen line
pipe, which has high hydrogen embrittlement resistance in a
high-pressure hydrogen atmosphere, a method for producing such a
hydrogen storage tank, and a method for producing such a hydrogen
line pipe.
BACKGROUND ART
[0002] In recent years, worldwide attention has been focused on
hydrogen as a renewable energy source and as an energy source that
enables energy diversification to be achieved. In particular,
development of fuel-cell vehicles that use high-pressure hydrogen
as a fuel source has been strongly anticipated, and studies on the
development of fuel-cell vehicles have been carried out all over
the world. In some of the studies, a test for practical use has
already been conducted.
[0003] Fuel-cell vehicles run on hydrogen contained in a tank
mounted thereon instead of on gasoline. Thus, in order to spread
the use of fuel-cell vehicles, hydrogen stations, at which
refueling is performed instead of gas stations, are required. At a
hydrogen station, a hydrogen fuel tank mounted on a vehicle is
filled with hydrogen supplied from a hydrogen storage tank, which
is a vessel for hydrogen in which hydrogen is stored at a high
pressure. While the maximum filling pressure of a vehicle-mounted
hydrogen tank is currently 35 MPa, it is desired to increase the
maximum filling pressure to 70 MPa in order to increase the driving
ranges of fuel-cell vehicles to a level comparable to the driving
ranges of gasoline vehicles. Thus, it is required to store and
supply hydrogen with safety in such a high-pressure hydrogen
atmosphere. Accordingly, the pressure in a hydrogen storage tank
used in a hydrogen station is currently required to be 40 MPa. If
the maximum filling pressure is increased to 70 MPa, the pressure
in the hydrogen storage tank used in a hydrogen station would be
required to be 80 MPa. In other words, in such a case, the hydrogen
storage tank used in a hydrogen station would be subjected to an
80-MPa atmosphere.
[0004] However, it is known that entry of hydrogen into a low-alloy
steel causes embrittlement (i.e., hydrogen embrittlement) to occur.
In the case where the hydrogen pressure is about 15 MPa or less,
low-alloy steel plates having a sufficiently large thickness can be
used. However, a hydrogen pressure exceeding about 15 MPa increases
the risk of hydrogen embrittlement fracture that may occur during
service. Therefore, low-alloy steel materials are not used and, for
example, austenitic stainless steels such as SUS316L steel, which
are less likely to cause hydrogen embrittlement to occur than
low-alloy steels, are used instead.
[0005] Since steel materials such as SUS316L steel are expensive
and have low strengths, a hydrogen storage tank that is designed so
as to withstand a hydrogen pressure of 80 MPa needs to have a
considerably large thickness, which greatly increases the price of
such a hydrogen storage tank. Thus, development of a hydrogen
storage tank for hydrogen stations which is capable of withstanding
a pressure of 80 MPa at a lower cost has been anticipated.
[0006] In order to address the above-described issues, various
techniques for using low-alloy steels for producing a high-pressure
hydrogen storage tank have been studied. Patent Literature 1
proposes a steel for a high-pressure hydrogen atmosphere in which
non-diffusible hydrogen is produced by using a MnS-based or
Ca-based inclusion or VC as a hydrogen-trapping site in the steel
in order to reduce the risk of embrittlement that may be caused by
diffusible hydrogen. Patent Literature 2 and Patent Literature 3
propose a low-alloy high-strength steel having high resistance to
high-pressure hydrogen atmosphere embrittlement. The tensile
strength of the low-alloy high-strength steel is controlled within
a considerably narrow range of 900 to 950 MPa by performing a
tempering treatment at a relatively high temperature during
quenching and tempering of a Cr--Mo steel. Patent Literature 4
proposes a low-alloy steel for a high-pressure gaseous hydrogen
atmosphere in which a V--Mo-based carbide is used for increasing
tempering temperature in order to enhance resistance to hydrogen
atmosphere embrittlement. Patent Literature 5 proposes a steel for
hydrogen storage tank (or high-pressure hydrogen storage vessel)
which has high resistance to hydrogen. Large amounts of Mo and V
are added to the steel and, during production of steel plates,
stress-relief annealing is performed for long hours after a
normalizing treatment to cause a large amount of (Mo,V)C to
precipitate. Patent Literature 6 proposes a technique in which the
amount of hydrogen entry is reduced by reducing the sizes of
cementite particles and thereby the toughness of the base metal is
increased in order to reduce the risk of hydrogen embrittlement.
Patent Literature 7 proposes a technique in which formation of
coarse cementite particles and island-like martensite (i.e.,
martensite-austenite constituent (MA)) is suppressed and thereby
occurrences of hydrogen entry and ductility deterioration are
limited in order to reduce the risk of hydrogen embrittlement. The
fatigue crack propagation characteristics of ordinary low-alloy
steel materials are described in, for example, Non Patent
Literature 1 and Non Patent Literature 2.
CITATION LIST
Patent Literature
[0007] [PTL 1] Japanese Unexamined Patent Application Publication
No. 2005-2386 [0008] [PTL 2] Japanese Unexamined Patent Application
Publication No. 2009-46737 [0009] [PTL 3] Japanese Unexamined
Patent Application Publication No. 2009-275249 [0010] [PTL 4]
Japanese Unexamined Patent Application Publication No. 2009-74122
[0011] [PTL 5] Japanese Unexamined Patent Application Publication
No. 2010-37655 [0012] [PTL 6] Japanese Unexamined Patent
Application Publication No. 2012-107332 [0013] [PTL 7] Japanese
Unexamined Patent Application Publication No. 2012-107333
Non Patent Literature
[0013] [0014] [NPL 1] Yoru WADA: "Journal of the Hydrogen Energy
Systems Society of Japan", Vol. 35, No. 4 (2010), pp. 38-44 [0015]
[NPL 2] Taisuke MIYAMOTO et al.: "Transactions of The Japan Society
of Mechanical Engineers (Series A)", Vol. 78, No. 788 (2012), pp.
531-546
SUMMARY OF INVENTION
Technical Problem
[0016] A hydrogen storage tank, which is used in a particularly
high-pressure hydrogen atmosphere, is subjected to a cyclic stress
since the storage tank is repeatedly filled with hydrogen, which
makes it difficult to achieve a long service life. In order to
increase the service life, it is important to reduce fatigue crack
propagation rate. However, it has been impossible to reduce fatigue
crack propagation rate to a sufficient degree in the
above-described techniques of the related art.
[0017] Moreover, it is also desirable that steel structures for
hydrogen gas such as a hydrogen line pipe for hydrogen pipelines,
which have not always been used in a high-pressure hydrogen
atmosphere comparable to that in which a hydrogen storage tank is
used, have the same degree of safety as the hydrogen storage
tank.
[0018] The present invention has been developed in light of the
above-described fact. An object of the present invention is to
provide a steel structure for hydrogen gas such as a hydrogen
storage tank or a hydrogen line pipe which achieves a lower fatigue
crack propagation rate in a high-pressure hydrogen atmosphere than
steels used in the related art and has high hydrogen embrittlement
resistance.
Solution to Problem
[0019] From the above-described viewpoint, the inventors of the
present invention have conducted extensive studies of the hydrogen
embrittlement resistances of steel structures for hydrogen gas
having various microstructures in a high-pressure hydrogen gas. As
a result, the inventors have found that a steel structure for
hydrogen gas which has a steel microstructure including any one of
bainite, martensite, and pearlite with the balance being
substantially ferrite may have higher hydrogen embrittlement
resistance in high-pressure hydrogen gas than the materials used in
the related art which have a single-phase microstructure and
thereby a steel structure for hydrogen gas such as a hydrogen
storage tank or a hydrogen line pipe which has high hydrogen
embrittlement resistance may be produced.
[0020] Specifically, the inventors of the present invention have
found that a steel structure for hydrogen gas which has a steel
microstructure including a predetermined amount of bainite with the
balance being substantially ferrite may achieve higher hydrogen
embrittlement resistance in high-pressure hydrogen gas than the
materials used in the related art which have a single-phase
microstructure and have high hydrogen embrittlement resistance.
Note that the expression "steel microstructure including a
predetermined amount of bainite with the balance being
substantially ferrite" means that the steel microstructure is a
dual-phase microstructure substantially composed of ferrite and
bainite.
[0021] The inventors of the present invention have also found that
a steel structure for hydrogen gas which has a steel microstructure
including a predetermined amount of martensite with the balance
being substantially ferrite may achieve higher hydrogen
embrittlement resistance in high-pressure hydrogen gas than the
materials used in the related art which have a single-phase
microstructure and have high hydrogen embrittlement resistance.
Note that the expression "steel microstructure including a
predetermined amount of martensite with the balance being
substantially ferrite" means that the steel microstructure is a
dual-phase microstructure substantially composed of ferrite and
martensite.
[0022] The inventors of the present invention have further found
that a steel structure for hydrogen gas which has a steel
microstructure including a predetermined amount of pearlite with
the balance being substantially ferrite may achieve higher hydrogen
embrittlement resistance in high-pressure hydrogen gas than the
materials used in the related art which have a single-phase
microstructure and have high hydrogen embrittlement resistance.
Note that the expression "steel microstructure including a
predetermined amount of pearlite with the balance being
substantially ferrite" means that the steel microstructure is a
dual-phase microstructure substantially composed of ferrite and
pearlite.
[0023] The inventors of the present invention have conducted
further studies on the basis of the above-described findings. Thus,
the present invention was made. The summary of the present
invention is described below.
[0024] [1] A steel structure for hydrogen gas which has high
hydrogen embrittlement resistance in high-pressure hydrogen gas,
the steel structure having a steel microstructure including any one
of 10% to 95% of bainite on an area-ratio basis, 10% to 95% of
martensite on an area-ratio basis, and 10% to 95% of on an
area-ratio basis, with the balance being substantially ferrite.
[0025] [2] The steel structure for hydrogen gas described in [1]
which has high hydrogen embrittlement resistance in high-pressure
hydrogen gas, in which the steel microstructure includes 10% to 95%
of bainite on an area-ratio basis with the balance being
substantially ferrite.
[0026] [3] The steel structure for hydrogen gas described in [1]
which has high hydrogen embrittlement resistance in high-pressure
hydrogen gas, in which the steel microstructure includes 10% to 95%
of martensite on an area-ratio basis with the balance being
substantially ferrite.
[0027] [4] The steel structure for hydrogen gas described in [1]
which has high hydrogen embrittlement resistance in high-pressure
hydrogen gas, in which the steel microstructure includes 10% to 95%
of pearlite on an area-ratio basis with the balance being
substantially ferrite.
[0028] [5] The steel structure for hydrogen gas described in [2],
the steel structure having a steel composition containing, by mass,
C: 0.05% to 0.20%, Si: 0.05% to 0.50%, Mn: 0.5% to 2.0%, Al: 0.01%
to 0.10%, N: 0.0005% to 0.008%, P: 0.05% or less, S: 0.01% or less,
and O: 0.01% or less, with the balance being Fe and inevitable
impurities.
[0029] [6] The steel structure for hydrogen gas described in [3],
the steel structure having a steel composition containing, by mass,
C: 0.05% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.5% to 2.0%, Al: 0.01%
to 0.10%, N: 0.0005% to 0.008%, P: 0.05% or less, S: 0.01% or less,
and O: 0.01% or less, with the balance being Fe and inevitable
impurities.
[0030] [7] The steel structure for hydrogen gas described in [4],
the steel structure having a steel composition containing, by mass,
C: 0.05% to 0.10%, Si: 0.05% to 0.50%, Mn: 0.5% to 2.0%, Al: 0.01%
to 0.10%, N: 0.0005% to 0.008%, P: 0.05% or less, S: 0.01% or less,
and O: 0.01% or less, with the balance being Fe and inevitable
impurities.
[0031] [8] The steel structure for hydrogen gas described in any
one of [5] to [7], in which the steel composition further contains,
by mass, one or more elements selected from Cu: 0.05% to 1.0%, Ni:
0.05% to 2.0%, Cr: 0.1% to 2.5%, Mo: 0.05% to 2.0%, Nb: 0.005% to
0.1%, V: 0.005% to 0.2%, Ti: 0.005% to 0.1%, W: 0.05% to 2.0%, and
B: 0.0005% to 0.005%.
[0032] [9] The steel structure for hydrogen gas described in any
one of [5] to [8], in which the steel composition further contains,
by mass, one or more elements selected from Nd: 0.005% to 1.0%, Ca:
0.0005% to 0.005%, Mg: 0.0005% to 0.005%, and REM: 0.0005% to
0.005%.
[0033] [10] The steel structure for hydrogen gas described in any
one of [1] to [9], the steel structure being a hydrogen storage
tank or a hydrogen line pipe.
[0034] [11] A method for producing the hydrogen line pipe described
in [10] which has high hydrogen embrittlement resistance in
high-pressure hydrogen gas, the method including heating a steel
having the steel composition described in any one of [5] to [9] to
an Ac.sub.3 transformation temperature or more, followed by hot
rolling; and subsequently performing cooling from an Ar.sub.3
transformation temperature or more to 600.degree. C. or less at a
cooling rate of 1.degree. C./sec. to 200.degree. C./sec.
[0035] [12] A method for producing the hydrogen line pipe described
in [10] which has high hydrogen embrittlement resistance in
high-pressure hydrogen gas, the method including heating a steel
having the steel composition described in any one of [5] to [9] to
an Ac.sub.3 transformation temperature or more, followed by hot
rolling; performing quenching from an Ar.sub.3 transformation
temperature or more to 250.degree. C. or less at a cooling rate of
1.degree. C./sec. to 200.degree. C./sec; and subsequently
performing tempering at an Ac.sub.1 transformation temperature or
less.
[0036] [13] A method for producing the hydrogen storage tank
described in [10] which has high hydrogen embrittlement resistance
in high-pressure hydrogen gas, the method including forming a steel
material having the steel composition described in any one of [5]
to [9] into a predetermined shape, followed by heating to an
Ac.sub.3 transformation temperature or more; performing quenching
from an Ar.sub.3 transformation temperature or more to 250.degree.
C. or less at a cooling rate of 0.5.degree. C./sec. to 100.degree.
C./sec; and subsequently performing tempering at an Ac.sub.1
transformation temperature or less.
Advantageous Effects of Invention
[0037] According to the present invention, a steel structure for
hydrogen gas such as a hydrogen storage tank or a hydrogen line
pipe which has markedly higher hydrogen embrittlement resistance in
high-pressure hydrogen gas than those of the related art may be
produced, which is highly advantageous from an industrial
viewpoint.
DESCRIPTION OF EMBODIMENTS
[0038] The present invention is described specifically below.
[0039] The steel structure for hydrogen gas according to the
present invention has a steel microstructure including any one of
10% to 95% of bainite on an area-ratio basis, 10% to 95% of
martensite on an area-ratio basis, and 10% to 95% of pearlite on an
area-ratio basis, with the balance being substantially ferrite. In
other words, the steel structure for hydrogen gas according to the
present invention has any one of the following: a steel
microstructure including 10% to 95% of bainite on an area-ratio
basis with the balance being substantially ferrite; a steel
microstructure including 10% to 95% of martensite on an area-ratio
basis with the balance being substantially ferrite; and a steel
microstructure including 10% to 95% of pearlite on an area-ratio
basis with the balance being substantially ferrite.
[0040] The steel microstructure of the steel structure for hydrogen
gas according to the present invention is a dual-phase
microstructure substantially composed of soft ferrite and any one
of bainite, martensite, and pearlite, which are hard phases. The
soft ferrite and the hard phase are dispersed in the steel
microstructure of the steel structure for hydrogen gas according to
the present invention. Fatigue cracks stagnate, divert, and/or
split at the interface therebetween, which reduces fatigue crack
propagation rate and enables high hydrogen embrittlement resistance
to be achieved.
[0041] In the present invention, microstructure fraction may be
determined, for example, in the following manner. Nital etching is
performed in order to cause a microstructure to appear. An image of
the microstructure is captured using an optical microscope or an
SEM (scanning electron microscope). Each microstructure is
identified using the image, and the area ratio of the
microstructure is calculated.
[0042] The term "steel structure for hydrogen gas having high
hydrogen embrittlement resistance in high-pressure hydrogen gas"
used herein refers to a steel structure for hydrogen gas which has
a fatigue crack propagation rate of 1.0.times.10.sup.-6 (m/cycle)
or less at a stress intensity factor range .DELTA.K of 25
(MPam.sup.1/2) as described below. Examples of the steel structure
for hydrogen gas include a hydrogen storage tank and a hydrogen
line pipe.
[0043] The hydrogen storage tank, which is the steel structure for
hydrogen gas according to the present invention, is a storage tank
used in, for example, a hydrogen station as described above.
Examples of such a storage tank include storage tanks composed of
only the Type-1 steel material and storage tanks composed of the
Type-2 or Type-3 steel material wrapped with carbon fiber
reinforced plastic (CFRP). The terms "Type-1", "Type-2", and
"Type-3" used herein refer to the classification of the structures
of vessels described in standards pertaining to compressed natural
gas vehicle fuel containers, ISO11439, ANSI (American National
Standards Institute)/NGV (Natural Gas Vehicle), Container Safety
Rules-Exemplified Standard-Appendix-9 of High Pressure Gas Safety
Act, and the like. The pressure of hydrogen stored in the vessel is
about 35 MPa or about 70 MPa. Examples of the hydrogen line pipe,
which is the steel structure for hydrogen gas according to the
present invention, include a seamless steel line pipe and an UOE
steel line pipe. The hydrogen pressure is 5 MPa or more.
[0044] The present invention is described specifically below for
each of steel microstructures of the structure for hydrogen gas,
that is, 1) bainite and ferrite (Invention 1), 2) martensite and
ferrite (Invention 2), and 3) pearlite and ferrite (Invention
3).
1) Steel Microstructure Including Bainite with Balance being
Substantially Ferrite (Invention 1)
[0045] One of steel microstructures of the steel structure for
hydrogen gas according to the present invention is a steel
microstructure including 10% to 95% of bainite on an area-ratio
basis with the balance being substantially ferrite. This steel
microstructure of the steel structure for hydrogen gas according to
the present invention is a steel microstructure in which soft
ferrite and hard bainite are dispersed. In the steel structure for
hydrogen gas according to the present invention, fatigue cracks
stagnate, divert, and/or split in the vicinity of the interface
between the dispersed soft ferrite and hard bainite, which reduces
fatigue crack propagation rate and enables high hydrogen
embrittlement resistance to be achieved. The term "soft ferrite"
used herein refers to polygonal ferrite having a microstructure
having a hardness value of about 70 to 150 HV10, and the term "hard
bainite" used herein refers to either upper bainite (Type BI, BII,
or BIII) or lower bainite having a microstructure having a hardness
value of about 150 to 300 HV10. The term "HV10" refers to a Vickers
hardness measured at a testing force of 98 N in accordance with JIS
Z2244:2009 "Vickers hardness test-Test method".
[0046] The above-described effect becomes apparent when the area
ratio of a bainite microstructure to the entire microstructure is
10% to 95% and the balance is basically composed of ferrite, that
is, when the steel microstructure is a dual-phase microstructure
primarily composed of ferrite and bainite. Thus, in the present
invention, the steel structure for hydrogen gas has a steel
microstructure including 10% to 95% of a bainite microstructure on
an area-ratio basis with the balance being substantially a ferrite
microstructure. The area ratio of bainite is preferably 20% to 95%
and is more preferably 25% to 95%. The area ratio of bainite is
further preferably 30% to 70%. Further preferably, the area ratio
of bainite is 40% to 60%. The fatigue crack propagation rate
becomes the lowest when the area ratios of the ferrite
microstructure and the bainite microstructure are substantially
equal to each other. Specifically, the fatigue crack propagation
rate becomes the lowest when the proportion of the area ratio of
the bainite microstructure in the total area ratio of the ferrite
microstructure and the bainite microstructure, that is, namely, a
bainite-area-ratio proportion [Bainite-Area-Ratio Proportion: (Area
Ratio of Bainite Microstructure)/((Area Ratio of Ferrite
Microstructure)+(Area Ratio of Bainite Microstructure))], is 0.3 to
0.7. Thus, the bainite-area-ratio proportion is preferably 0.3 to
0.7 and is more preferably 0.4 to 0.6. Although the balance other
than the bainite microstructure is substantially ferrite,
microstructures other than bainite or ferrite (e.g., pearlite and
martensite) may be included in such a manner that the total area
ratio of the other microstructures is 2% or less. This is because
the advantageous effects of the present invention are not
substantially impaired when the total area ratio of the other
microstructures is 2% or less. In other words, the other
microstructures may be included in such a manner that the total
area ratio of bainite and ferrite is 98% or more.
[0047] A preferable steel composition of the steel structure for
hydrogen gas according to the present invention (Invention 1),
which has the above-described steel microstructure including 10% to
95% of bainite on an area-ratio basis with the balance being
substantially ferrite, is described below. Hereinafter, the
notation of "%" regarding compositions represents "% by mass"
unless otherwise specified.
C: 0.05% to 0.20%
[0048] Carbon (C) is added to a steel in order to ensure adequate
hardenability. However, this effect may become insufficient if the
C content is less than 0.05%. Accordingly, the C content is set to
0.05% or more and is preferably set to 0.08% or more. In
particular, in order to facilitate achieving the above-described
area ratio of bainite, the C content is preferably set to 0.10% or
more. However, if the C content exceeds 0.20%, the toughness of a
base metal and the toughness of a weld heat-affected zone may
become reduced and weldability may be significantly degraded.
Accordingly, the C content is set to 0.20% or less and is
preferably set to 0.17% or less. In particular, in order to
facilitate achieving the above-described area ratio of bainite, the
C content is preferably set to 0.15% or less. Thus, the C content
is limited to 0.05% to 0.20%.
Si: 0.05% to 0.50%
[0049] Silicon (Si) is added to a steel as an element that serves
as a deoxidizer in a steelmaking process and that ensures certain
hardenability. However, the effect may become insufficient if the
Si content is less than 0.05%. Accordingly, the Si content is set
to 0.05% or more and is preferably set to 0.08% or more. In
particular, in order to facilitate achieving the above-described
area ratio of bainite, the Si content is preferably set to 0.10% or
more. However, if the Si content exceeds 0.50%, embrittlement of
grain boundaries may occur, which leads to a reduction in
low-temperature toughness. Accordingly, the Si content is set to
0.50% or less and is preferably set to 0.45% or less. In
particular, in order to facilitate achieving the above-described
area ratio of bainite, the Si content is preferably set to 0.40% or
less. Thus, the Si content is limited to 0.05% to 0.50%.
Mn: 0.5% to 2.0%
[0050] Manganese (Mn) is added to a steel as an element that
ensures certain hardenability. However, this effect may become
insufficient if the Mn content is less than 0.5%. Accordingly, the
Mn content is set to 0.5% or more and is preferably set to 0.8% or
more. In particular, in order to facilitate achieving the
above-described area ratio of bainite, the Mn content is preferably
set to 1.0% or more. However, a Mn content exceeding 2.0% may
reduce grain boundary strength, which leads to a reduction in
low-temperature toughness. Accordingly, the Mn content is set to
2.0% or less and is preferably set to 1.8% or less. In particular,
in order to facilitate achieving the above-described area ratio of
bainite, the Mn content is preferably set to 1.5% or less. Thus,
the Mn content is limited to 0.5% to 2.0%.
Al: 0.01% to 0.10%
[0051] Aluminium (Al) is added to a steel as a deoxidizer. Al also
forms a fine precipitate of an Al-based nitride, which causes
pinning of austenite grains to occur during heating and thereby
limits coarsening of the grains. However, these effects may become
insufficient if the Al content is less than 0.01%. Accordingly, the
Al content is set to 0.01% or more and is preferably set to 0.02%
or more. However, an Al content exceeding 0.10% may increase the
risk of formation of surface flaws in a steel plate. Accordingly,
the Al content is set to 0.10% or less and is preferably set to
0.08% or less. Thus, the Al content is limited to 0.01% to
0.10%.
N: 0.0005% to 0.008%
[0052] Nitrogen (N) is added to a steel because it reacts with Nb,
Ti, Al, or the like to form a nitride and then forms a fine
precipitate, which causes pinning of austenite grains to occur
during heating and thereby limits coarsening of the grains. This
leads to an increase in low-temperature toughness. However, if the
N content is less than 0.0005%, the sizes of microstructures may
fail to be reduced to a sufficient degree. Accordingly, the N
content is set to 0.0005% or more and is preferably set to 0.002%
or more. However, a N content exceeding 0.008% may increase the
amount of dissolved N, which reduces the toughness of a base metal
and the toughness of a weld heat-affected zone. Accordingly, the N
content is set to 0.008% or less and is preferably set to 0.006% or
less. Thus, the N content is limited to 0.0005% to 0.008%.
P: 0.05% or Less
[0053] Phosphorus (P), which is an impurity element, is likely to
segregate at grain boundaries. A P content exceeding 0.05% may
reduce the grain boundary strength of adjacent grains, which leads
to a reduction in low-temperature toughness. Thus, the P content is
limited to 0.05% or less and is preferably set to 0.03% or
less.
S: 0.01% or Less
[0054] Sulfur (S), which is an impurity element, is likely to
segregate at grain boundaries and is likely to form MnS, which is a
nonmetallic inclusion. A S content exceeding 0.01% may reduce the
grain boundary strength of adjacent grains and increase the amount
of inclusion, which leads to a reduction in low-temperature
toughness. Thus, the S content is limited to 0.01% or less and is
preferably set to 0.005% or less.
O: 0.01% or Less
[0055] Oxygen (O) reacts with Al or the like to form an oxide and
thereby affects ease of shaping materials. An O content exceeding
0.01% may increase the amount of inclusion and reduce the ease of
shaping. Thus, the O content is limited to 0.01% or less and is
preferably set to 0.006% or less.
2) Steel Microstructure Including Martensite with Balance being
Substantially Ferrite (Invention 2)
[0056] One of steel microstructures of the steel structure for
hydrogen gas according to the present invention is a steel
microstructure including 10% to 95% of martensite on an area-ratio
basis with the balance being substantially ferrite. This steel
microstructure of the steel structure for hydrogen gas according to
the present invention is a steel microstructure in which soft
ferrite and hard martensite are dispersed. In the steel structure
for hydrogen gas according to the present invention, fatigue cracks
stagnate, divert, and/or split in the vicinity of the interface
between the dispersed soft ferrite and hard martensite, which
reduces fatigue crack propagation rate and enables high hydrogen
embrittlement resistance to be achieved. The term "soft ferrite"
used herein refers to polygonal ferrite having a microstructure
having a hardness value of about 70 to 150 HV10, and the term "hard
martensite" used herein refers to a microstructure having a
hardness value of about 200 to 600 HV10 which may, but does not
necessarily, include cementite.
[0057] The above-described effect becomes apparent when the area
ratio of a martensite microstructure to the entire microstructure
is 10% to 95% and the balance is basically composed of ferrite,
that is, when the steel microstructure is a dual-phase
microstructure primarily composed of ferrite and martensite. Thus,
in the present invention, the steel structure for hydrogen gas has
a steel microstructure including 10% to 95% of a martensite
microstructure on an area-ratio basis with the balance being
substantially a ferrite microstructure. The area ratio of
martensite is preferably 20% to 95% and is more preferably 25% to
95%. The area ratio of martensite is further preferably 30% to 70%.
Further preferably, the area ratio of martensite is 40% to 60%. The
fatigue crack propagation rate becomes the lowest when the area
ratios of the ferrite microstructure and the martensite
microstructure are substantially equal to each other. In other
words, the fatigue crack propagation rate becomes the lowest when
the proportion of the area ratio of the martensite microstructure
in the total area ratio of the ferrite microstructure and the
martensite microstructure, that is, namely, a martensite-area-ratio
proportion [Martensite-Area-Ratio Proportion: (Area Ratio of
Martensite Microstructure)/((Area Ratio of Ferrite
Microstructure)+(Area Ratio of Martensite Microstructure))], is 0.3
to 0.7. Thus, the martensite-area-ratio proportion is preferably
0.3 to 0.7 and is more preferably 0.4 to 0.6. Although the balance
other than the martensite microstructure is substantially ferrite,
microstructures other than martensite or ferrite (e.g., pearlite
and bainite) may be included in such a manner that the total area
ratio of the other microstructures is 2% or less. This is because
the advantageous effects of the present invention are not impaired
when the total area ratio of the other microstructures is 2% or
less. In other words, the other microstructures may be included in
such a manner that the total area ratio of martensite and ferrite
is 98% or more.
[0058] A preferable steel composition of the steel structure for
hydrogen gas according to the present invention (Invention 2),
which has the above-described steel microstructure including 10% to
95% of martensite on an area-ratio basis with the balance being
substantially ferrite, is described below. As described above, the
notation of "%" regarding compositions represents "% by mass"
unless otherwise specified.
C: 0.05% to 0.35%
[0059] Carbon (C) is added to a steel in order to ensure adequate
hardenability. However, this effect may become insufficient if the
C content is less than 0.05%. Accordingly, the C content is set to
0.05% or more and is preferably set to 0.08% or more. In
particular, in order to facilitate achieving the above-described
area ratio of martensite, the C content is preferably set to 0.10%
or more. However, if the C content exceeds 0.35%, the toughness of
a base metal and the toughness of a weld heat-affected zone may
become reduced and weldability may be significantly degraded.
Accordingly, the C content is set to 0.35% or less and is
preferably set to 0.27% or less. In particular, in order to
facilitate achieving the above-described area ratio of martensite,
the C content is preferably set to 0.25% or less. Thus, the C
content is limited to 0.05% to 0.35%.
Si: 0.05% to 0.50%
[0060] Silicon (Si) is added to a steel as an element that serves
as a deoxidizer in a steelmaking process and that ensures certain
hardenability. However, the effect may become insufficient if the
Si content is less than 0.05%. Accordingly, the Si content is set
to 0.05% or more and is preferably set to 0.08% or more. In
particular, in order to facilitate achieving the above-described
area ratio of martensite, the Si content is preferably set to 0.10%
or more. However, if the Si content exceeds 0.50%, embrittlement of
grain boundaries may occur, which leads to a reduction in
low-temperature toughness. Accordingly, the Si content is set to
0.50% or less and is preferably set to 0.45% or less. In
particular, in order to facilitate achieving the above-described
area ratio of martensite, the Si content is preferably set to 0.40%
or less. Thus, the Si content is limited to 0.05% to 0.50%.
Mn: 0.5% to 2.0%
[0061] Manganese (Mn) is added to a steel as an element that
ensures certain hardenability. However, this effect may become
insufficient if the Mn content is less than 0.5%. Accordingly, the
Mn content is set to 0.5% or more and is preferably set to 0.8% or
more. In particular, in order to facilitate achieving the
above-described area ratio of martensite, the Mn content is
preferably set to 1.0% or more. However, a Mn content exceeding
2.0% may reduce grain boundary strength, which leads to a reduction
in low-temperature toughness. Accordingly, the Mn content is set to
2.0% or less and is preferably set to 1.8% or less. In particular,
in order to facilitate achieving the above-described area ratio of
martensite, the Mn content is preferably set to 1.5% or less. Thus,
the Mn content is limited to 0.5% to 2.0%.
Al: 0.01% to 0.10%
[0062] Aluminium (Al) is added to a steel as a deoxidizer. Al also
forms a fine precipitate of an Al-based nitride, which causes
pinning of austenite grains to occur during heating and thereby
limits coarsening of the grains. However, these effects may become
insufficient if the Al content is less than 0.01%. Accordingly, the
Al content is set to 0.01% or more and is preferably set to 0.02%
or more. However, an Al content exceeding 0.10% may increase the
risk of formation of surface flaws in a steel plate. Accordingly,
the Al content is set to 0.10% or less and is preferably set to
0.08% or less. Thus, the Al content is limited to 0.01% to
0.10%.
N: 0.0005% to 0.008%
[0063] Nitrogen (N) is added to a steel because it reacts with Nb,
Ti, Al, or the like to form a nitride and then forms a fine
precipitate, which causes pinning of austenite grains to occur
during heating and thereby limits coarsening of the grains. This
leads to an increase in low-temperature toughness. However, if the
N content is less than 0.0005%, the sizes of microstructures may
fail to be reduced to a sufficient degree. Accordingly, the N
content is set to 0.0005% or more and is preferably set to 0.002%
or more. However, a N content exceeding 0.008% may increase the
amount of dissolved N, which reduces the toughness of a base metal
and the toughness of a weld heat-affected zone. Accordingly, the N
content is set to 0.008% or less and is preferably set to 0.006% or
less. Thus, the N content is limited to 0.0005% to 0.008%.
P: 0.05% or Less
[0064] Phosphorus (P), which is an impurity element, is likely to
segregate at grain boundaries. A P content exceeding 0.05% may
reduce the grain boundary strength of adjacent grains, which leads
to a reduction in low-temperature toughness. Thus, the P content is
limited to 0.05% or less and is preferably set to 0.03% or
less.
S: 0.01% or Less
[0065] Sulfur (S), which is an impurity element, is likely to
segregate at grain boundaries and is likely to form MnS, which is a
nonmetallic inclusion. A S content exceeding 0.01% may reduce the
grain boundary strength of adjacent grains and increase the amount
of inclusion, which leads to a reduction in low-temperature
toughness. Thus, the S content is limited to 0.01% or less and is
preferably set to 0.005% or less.
O: 0.01% or Less
[0066] Oxygen (O) reacts with Al or the like to form an oxide and
thereby affects ease of shaping materials. An O content exceeding
0.01% may increase the amount of inclusion and reduce the ease of
shaping. Thus, the O content is limited to 0.01% or less and is
preferably set to 0.006% or less.
3) Steel Microstructure Including Pearlite with Balance being
Substantially Ferrite (Invention 3)
[0067] One of steel microstructures of the steel structure for
hydrogen gas according to the present invention is a steel
microstructure including 10% to 95% of pearlite on an area-ratio
basis with the balance being substantially ferrite. This steel
microstructure of the steel structure for hydrogen gas according to
the present invention is a steel microstructure in which soft
ferrite and hard pearlite are dispersed. In the steel structure for
hydrogen gas according to the present invention, fatigue cracks
stagnate, divert, and/or split in the vicinity of the interface
between the dispersed soft ferrite and hard pearlite, which reduces
fatigue crack propagation rate and enables high hydrogen
embrittlement resistance to be achieved. The term "soft ferrite"
used herein refers to polygonal ferrite having a microstructure
having a hardness value of about 70 to 150 HV10, and the term "hard
pearlite" used herein refers to a microstructure having a hardness
value of about 150 to 300 HV10, in which ferrite and pearlite form
a lamellar structure or pearlite is dispersed in ferrite in
clusters.
[0068] The above-described effect becomes apparent when the area
ratio of a pearlite microstructure to the entire microstructure is
10% to 95% and the balance is basically composed of ferrite, that
is, when the steel microstructure is a dual-phase microstructure
primarily composed of ferrite and pearlite. Thus, in the present
invention, the steel structure for hydrogen gas has a steel
microstructure including 10% to 95% of a pearlite microstructure on
an area-ratio basis with the balance being substantially a ferrite
microstructure. The area ratio of pearlite is preferably 20% to 95%
and is more preferably 25% to 95%. The area ratio of pearlite is
further preferably 30% to 70%. Further preferably, the area ratio
of pearlite is 40% to 60%. The fatigue crack propagation rate
becomes the lowest when the area ratios of the ferrite
microstructure and the pearlite microstructure are substantially
equal to each other. In other words, the fatigue crack propagation
rate becomes the lowest when the proportion of the area ratio of
the pearlite microstructure in the total area ratio of the ferrite
microstructure and the pearlite microstructure, that is, namely, a
pearlite-area-ratio proportion [Pearlite-Area-Ratio Proportion:
(Area Ratio of Pearlite Microstructure)/((Area Ratio of Ferrite
Microstructure)+(Area Ratio of Pearlite Microstructure))], is 0.3
to 0.7. Thus, the pearlite-area-ratio proportion is preferably 0.3
to 0.7 and is more preferably 0.4 to 0.6. Although the balance
other than the pearlite microstructure is substantially ferrite,
microstructures other than pearlite or ferrite (e.g., bainite and
martensite) may be included in such a manner that the total area
ratio of the other microstructures is 2% or less. This is because
the advantageous effects of the present invention (Invention 3) are
not impaired when the total area ratio of the other microstructures
is 2% or less. In other words, the other microstructures may be
included in such a manner that the total area ratio of pearlite and
ferrite is 98% or more.
[0069] A preferable steel composition of the steel structure for
hydrogen gas according to the present invention, which has the
above-described steel microstructure including 10% to 95% of
pearlite on an area-ratio basis with the balance being
substantially ferrite, is described below. As described above, the
notation of "%" regarding compositions represents "% by mass"
unless otherwise specified.
C: 0.05% to 0.10%
[0070] Carbon (C) is added to a steel in order to ensure adequate
hardenability. However, this effect may become insufficient if the
C content is less than 0.05%. Accordingly, the C content is set to
0.05% or more and is preferably set to 0.06% or more. In
particular, in order to facilitate achieving the above-described
area ratio of pearlite, the C content is preferably set to 0.07% or
more. However, if the C content exceeds 0.10%, the toughness of a
base metal and the toughness of a weld heat-affected zone may
become reduced and weldability may be significantly degraded.
Accordingly, the C content is set to 0.10% or less and is
preferably set to 0.09% or less. In particular, in order to
facilitate achieving the above-described area ratio of pearlite,
the C content is preferably set to 0.08% or less. Thus, the C
content is limited to 0.05% to 0.10%.
Si: 0.05% to 0.50%
[0071] Silicon (Si) is added to a steel as an element that serves
as a deoxidizer in a steelmaking process and that ensures certain
hardenability. However, the effect may become insufficient if the
Si content is less than 0.05%. Accordingly, the Si content is set
to 0.05% or more and is preferably set to 0.08% or more. In
particular, in order to facilitate achieving the above-described
area ratio of pearlite, the Si content is preferably set to 0.10%
or more. However, if the Si content exceeds 0.50%, embrittlement of
grain boundaries may occur, which leads to a reduction in
low-temperature toughness. Accordingly, the Si content is set to
0.50% or less and is preferably set to 0.45% or less. In
particular, in order to facilitate achieving the above-described
area ratio of pearlite, the Si content is preferably set to 0.40%
or less. Thus, the Si content is limited to 0.05% to 0.50%.
Mn: 0.5% to 2.0%
[0072] Manganese (Mn) is added to a steel as an element that
ensures certain hardenability. However, this effect may become
insufficient if the Mn content is less than 0.5%. Accordingly, the
Mn content is set to 0.5% or more and is preferably set to 0.8% or
more. In particular, in order to facilitate achieving the
above-described area ratio of pearlite, the Mn content is
preferably set to 1.0% or more. However, a Mn content exceeding
2.0% may reduce grain boundary strength, which leads to a reduction
in low-temperature toughness. Accordingly, the Mn content is set to
2.0% or less and is preferably set to 1.8% or less. In particular,
in order to facilitate achieving the above-described area ratio of
pearlite, the Mn content is preferably set to 1.5% or less. Thus,
the Mn content is limited to 0.5% to 2.0%.
Al: 0.01% to 0.10%
[0073] Aluminium (Al) is added to a steel as a deoxidizer. Al also
forms a fine precipitate of an Al-based nitride, which causes
pinning of austenite grains to occur during heating and thereby
limits coarsening of the grains. However, these effects may become
insufficient if the Al content is less than 0.01%. Accordingly, the
Al content is set to 0.01% or more and is preferably set to 0.02%
or more. However, an Al content exceeding 0.10% may increase the
risk of formation of surface flaws in a steel plate. Accordingly,
the Al content is set to 0.10% or less and is preferably set to
0.08% or less. Thus, the Al content is limited to 0.01% to
0.10%.
N: 0.0005% to 0.008%
[0074] Nitrogen (N) is added to a steel because it reacts with Nb,
Ti, Al, or the like to form a nitride and then forms a fine
precipitate, which causes pinning of austenite grains to occur
during heating and thereby limits coarsening of the grains. This
leads to an increase in low-temperature toughness. However, if the
N content is less than 0.0005%, the sizes of microstructures may
fail to be reduced to a sufficient degree. Accordingly, the N
content is set to 0.0005% or more and is preferably set to 0.002%
or more. However, a N content exceeding 0.008% may increase the
amount of dissolved N, which reduces the toughness of a base metal
and the toughness of a weld heat-affected zone. Accordingly, the N
content is set to 0.008% or less and is preferably set to 0.006% or
less. Thus, the N content is limited to 0.0005% to 0.008%.
P: 0.05% or Less
[0075] Phosphorus (P), which is an impurity element, is likely to
segregate at grain boundaries. A P content exceeding 0.05% may
reduce the grain boundary strength of adjacent grains, which leads
to a reduction in low-temperature toughness. Thus, the P content is
limited to 0.05% or less and is preferably set to 0.03% or
less.
S: 0.01% or Less
[0076] Sulfur (S), which is an impurity element, is likely to
segregate at grain boundaries and is likely to form MnS, which is a
nonmetallic inclusion. A S content exceeding 0.01% may reduce the
grain boundary strength of adjacent grains and increase the amount
of inclusion, which leads to a reduction in low-temperature
toughness. Thus, the S content is limited to 0.01% or less and is
preferably set to 0.005% or less.
O: 0.01% or Less
[0077] Oxygen (O) reacts with Al or the like to form an oxide and
thereby affects workability of materials. An O content exceeding
0.01% may increase the amount of inclusion and reduce the
workability. Thus, the O content is limited to 0.01% or less and is
preferably set to 0.006% or less.
[0078] In the present invention, the balance of the above-described
steel composition is preferably Fe and inevitable impurities in any
of the above-described cases: 1) a microstructure includes bainite
and ferrite (Invention 1); 2) a microstructure includes martensite
and ferrite (Invention 2); and 3) a microstructure includes
pearlite and ferrite (Invention 3). Optionally, the components i)
and ii) below may be added to a steel alone or in combination
appropriately in accordance with desired properties.
[0079] i) One or more elements selected from Cu: 0.05% to 1.0%, Ni:
0.05% to 2.0%, Cr: 0.1% to 2.5%, Mo: 0.05% to 2.0%, Nb: 0.005% to
0.1%, V: 0.005% to 0.2%, Ti: 0.005% to 0.1%, W: 0.05% to 2.0%, and
B: 0.0005% to 0.005%.
[0080] ii) One or more elements selected from Nd: 0.005% to 1.0%,
Ca: 0.0005% to 0.005%, Mg: 0.0005% to 0.005%, and REM: 0.0005% to
0.005%.
Cu: 0.05% to 1.0%
[0081] Copper (Cu) enhances hardenability. This effect may, become
insufficient if the Cu content is less than 0.05%. However, a Cu
content exceeding 1.0% may increase the risk of cracking that may
occur during hot working when steel slabs are heated or welded.
Thus, when Cu is added to a steel, the Cu content is limited to
0.05% or more and 1.0% or less.
Ni: 0.05% to 2.0%
[0082] Nickel (Ni) enhances hardenability similarly to Cu and also
increases toughness. These effects may become insufficient if the
Ni content is less than 0.05%. However, a Ni content exceeding 2.0%
may result in poor economy. Thus, when Ni is added to a steel, the
Ni content is limited to 0.05% or more and 2.0% or less.
Cr: 0.1% to 2.5%
[0083] Chromium (Cr) is added to a steel as an element that ensures
certain hardenability. This effect may become insufficient if the
Cr content is less than 0.1%. However, a Cr content exceeding 2.5%
may deteriorate weldability. Thus, when Cr is added to a steel, the
Cr content is limited to 0.1% or more and 2.5% or less.
Mo: 0.05% to 2.0%
[0084] Molybdenum (Mo) enhances hardenability. This effect may
become insufficient if the Mo content is less than 0.05%. However,
a Mo content exceeding 2.0% may results in poor economy. Thus, when
Mo is added to a steel, the Mo content is limited to 0.05% or more
and 2.0% or less.
Nb: 0.005% to 0.1%
[0085] Niobium (Nb) enhances hardenability and forms a fine
precipitate of an Nb-based carbonitride, which causes pinning of
austenite grains to occur during heating and thereby limits
coarsening of the grains. These effects may become insufficient if
the Nb content is less than 0.005%. However, an Nb content
exceeding 0.1% may reduce the toughness of a weld heat-affected
zone. Thus, when Nb is added to a steel, the Nb content is limited
to 0.005% or more and 0.1% or less.
V: 0.005% to 0.2%
[0086] Vanadium (V) enhances hardenability and forms a fine
precipitate of a V-based carbide, which causes pinning of austenite
grains to occur during heating and thereby limits coarsening of the
grains. These effects may become insufficient if the V content is
less than 0.005%. However, a V content exceeding 0.2% may reduce
the toughness of a weld heat-affected zone. Thus, when V is added
to a steel, the V content is limited to 0.005% or more and 0.2% or
less.
Ti: 0.005% to 0.1%
[0087] Titanium (Ti) enhances hardenability and forms a fine
precipitate of a Ti-based carbonitride, which causes pinning of
austenite grains to occur during heating and thereby limits
coarsening of the grains. These effects may become insufficient if
the Ti content is less than 0.005%. However, a Ti content exceeding
0.1% may reduce the toughness of a weld heat-affected zone. Thus,
when Ti is added to a steel, the Ti content is limited to 0.005% or
more and 0.1% or less.
W: 0.05% to 2.0%
[0088] Tungsten (W) enhances hardenability. This effect may become
insufficient if the W content is less than 0.05%. However, a W
content exceeding 2.0% may deteriorate weldability. Thus, when W is
added to a steel, the W content is limited to 0.05% or more and
2.0% or less.
B: 0.0005% to 0.005%
[0089] Boron (B) is added to a steel as an element that ensures
certain hardenability. This effect may become insufficient if the B
content is less than 0.0005%. However, a B content exceeding 0.005%
may reduce toughness. Thus, when B is added to a steel, the B
content is limited to 0.0005% or more and 0.005% or less.
Nd: 0.005% to 1.0%
[0090] Neodymium (Nd) incorporates S as an inclusion, which reduces
the amount of S that segregates at grain boundaries and thereby
enhances low-temperature toughness and hydrogen embrittlement
resistance. This effect may become insufficient if the Nd content
is less than 0.005%. However, an Nd content exceeding 1.0% may
reduce the toughness of a weld heat-affected zone. Thus, when Nd is
added to a steel, the Nd content is limited to 0.005% or more and
1.0% or less.
Ca: 0.0005% to 0.005%
[0091] Calcium (Ca) forms CaS, which causes the form of a
sulfide-based inclusion to change from MnS, which is an inclusion
that is likely to be extended by rolling, into CaS, which is a
spherical inclusion that is less likely to be extended by rolling.
This effect may become insufficient if the Ca content is less than
0.0005%. However, a Ca content exceeding 0.005% may deteriorate
cleanliness, which results in degradation of material properties
such as toughness. Thus, when Ca is added to a steel, the Ca
content is limited to 0.0005% or more and 0.005% or less.
Mg: 0.0005% to 0.005%
[0092] Magnesium (Mg) may be used as a hot-metal desulphurization
agent. This effect may become insufficient if the Mg content is
less than 0.0005%. However, a Mg content exceeding 0.005% may
deteriorate cleanliness. Thus, when Mg is added to a steel, the Mg
content is limited to 0.0005% or more and 0.005% or less.
REM: 0.0005% to 0.005%
[0093] REM forms a sulfide in a steel in the form of REM(O,S) and
thereby reduces the amount of S dissolved at grain boundaries,
which enhances resistance to stress-relief cracking. This effect
may become insufficient if the REM content is less than 0.0005%.
However, a REM content exceeding 0.005% may cause a REM sulfide to
significantly accumulate at a sedimental zone, which leads to
degradation of material properties. Thus, when REM is added to a
steel, the REM content is limited to 0.0005% or more and 0.005% or
less. Note that REM is the abbreviation for rare earth metal.
[0094] The steel structure for hydrogen gas according to the
present invention has the above-described steel microstructure and
preferably has the above-described composition. There are no
particular limitations on a method for producing the steel
structure for hydrogen gas. A preferable method for producing the
steel structure for hydrogen gas according to the present invention
is described below taking a hydrogen line pipe and a hydrogen
storage tank as examples of the steel structure for hydrogen gas
according to the present invention. The steel structure for
hydrogen gas according to the present invention may be a steel
structure for hydrogen gas that is any of various steel materials
such as a thin sheet, a thick plate, a pipe, a shaped steel, and a
steel bar which have the above-described steel microstructure,
preferably have the above-described composition, and have high
resistance to fatigue crack propagation in high-pressure hydrogen
gas. Alternatively, the steel structure for hydrogen gas according
to the present invention may also be a steel structure for hydrogen
gas produced by forming any of the above-described steel materials
having high resistance to fatigue crack propagation in
high-pressure hydrogen gas into a predetermined shape.
[0095] The temperatures specified in the production conditions are
measured at the center of a steel material, that is, specifically,
the center of the steel material in the thickness direction for a
thin sheet, a thick plate, a pipe, and a profile and the center of
the steel material in the radial direction for a steel bar.
However, the portion at which the temperature is measured is not
limited to the exact center of the steel material because any
portion in the vicinity of the center of the steel material has the
substantially similar temperature history.
[0096] The hydrogen line pipe, which is the steel structure for
hydrogen gas according to the present invention, can be produced
by, for example, hot rolling a steel and subsequently performing
either accelerated cooling or direct quenching and tempering.
Steel Material
[0097] A steel material used for producing the hydrogen line pipe
according to the present invention is produced by casting molten
steel having any of the above-described compositions (Inventions 1
to 3). It is not necessary to particularly limit the casting
conditions. Various steel materials produced under different
casting conditions may be used. A method for producing a cast slab
from molten steel and a method for producing a steel slab by hot
rolling the cast slab are not particularly specified. Steel
materials produced by a converter steelmaking process, an electric
steelmaking process, or the like and steel slabs produced by
continuous casting, ingot casting, or the like can be used.
Production by Accelerated Cooling
[0098] The above-described steel materials is heated to the
Ac.sub.3 transformation temperature or more and hot-rolled to a
predetermined thickness. Subsequently, accelerated cooling from the
Ar.sub.3 transformation temperature or more to 600.degree. C. or
less at a cooling rate of 1.degree. C./sec. to 200.degree. C./sec.
is performed by water cooling or the like. If the heating
temperature is less than the Ac.sub.3 transformation temperature, a
portion of non-transformed austenite may remain, which results in
failure to form a desired steel microstructure after hot rolling
and accelerated cooling. Thus, the temperature to which heating is
performed before hot rolling is set to the Ac.sub.3 transformation
temperature or more. The heating temperature is more preferably set
to (Ac.sub.3+50).degree. C. or more. The heating temperature is
preferably set to 1250.degree. C. or less in order to prevent an
excessive increase in the diameters of initial austenite grains
from occurring and increase the production efficiency. If the
temperature at which cooling is started after hot rolling is less
than the Ar.sub.3 transformation temperature, transformation of a
portion of austenite may occur before cooling is started, which
results in failure to forming a desired steel microstructure after
accelerated cooling. Thus, cooling is started at the Ar.sub.3
transformation temperature or more after hot rolling. Cooling is
preferably started at (Ar.sub.3+50).degree. C. or more. The
temperature at which cooling is started is preferably set to
1000.degree. C. or less in consideration of hot rolling. The rate
at which cooling is performed from the Ar.sub.3 transformation
temperature or more is set to 1.degree. C./sec. or more and
200.degree. C./sec. or less in order to form a desired
microstructure. The cooling rate is an average cooling rate
measured at the center of the steel plate in the thickness
direction. The cooling rate is preferably set to 5.degree. C./sec.
or more and less than 20.degree. C./sec. in order to consistently
form a steel microstructure including 10% to 95% of bainite on an
area-ratio basis with the balance being substantially ferrite. The
cooling rate is preferably set to 20.degree. C./sec. or more and
200.degree. C./sec. or less in order to consistently form a steel
microstructure including 10% to 95% of martensite on an area-ratio
basis with the balance being substantially ferrite. The cooling
rate is preferably set to 1.degree. C./sec. or more and less than
5.degree. C./sec. in order to consistently form a steel
microstructure including 10% to 95% of pearlite on an area-ratio
basis with the balance being substantially ferrite. There are no
particular limitations on cooling means. For example, water cooling
may be performed. If cooling is stopped at a temperature exceeding
600.degree. C., desired transformation may fail to be completed,
which results in failure to form a desired steel microstructure.
Thus, accelerated cooling is performed until the temperature
reaches 600.degree. C. or less and is preferably performed until
the temperature reaches 550.degree. C. or less. The temperature at
which cooling is stopped is preferably set to 300.degree. C. or
more in consideration of transformation behavior.
Direct Quenching and Tempering
[0099] The above-described steel material is heated to the Ac.sub.3
transformation temperature or more and then hot-rolled.
Subsequently, quenching is performed from the Ar.sub.3
transformation temperature or more to 250.degree. C. or less at a
cooling rate of 1.degree. C./sec. to 200.degree. C./sec, and then
tempering is performed at the Ac.sub.1 transformation temperature
or less. If the heating temperature is less than the Ac.sub.3
transformation temperature, a portion of non-transformed austenite
may remain, which results in failure to form a desired steel
microstructure after hot rolling, quenching, and tempering. Thus,
the temperature to which heating is performed before hot rolling is
set to the Ac.sub.3 transformation temperature or more and is
preferably set to (Ac.sub.3+50).degree. C. or more. The heating
temperature is preferably set to 1250.degree. C. or less in order
to prevent an excessive increase in the diameters of initial
austenite grains from occurring and increase the production
efficiency. If the temperature at which quenching is started after
hot rolling is less than the Ar.sub.3 transformation temperature,
transformation of a portion of austenite may occur before quenching
is started, which results in failure to forming a desired steel
microstructure after quenching and tempering. Thus, quenching is
performed by starting cooling at the Ar.sub.3 transformation
temperature or more after hot rolling. Cooling is preferably
started at (Ar.sub.3+50).degree. C. or more. The temperature at
which quenching is started is preferably set to 1000.degree. C. or
less in consideration of hot rolling. The cooling rate at which
quenching is performed from the Ar.sub.3 transformation temperature
or more is set to 1.degree. C./sec. or more and 200.degree. C./sec.
or less in order to form a desired microstructure. The cooling rate
is an average cooling rate measured at the center of the steel
plate in the thickness direction. The cooling rate is preferably
set to 5.degree. C./sec. or more and less than 20.degree. C./sec.
in order to consistently form a steel microstructure including 10%
to 95% of bainite on an area-ratio basis with the balance being
substantially ferrite. The cooling rate is preferably set to
20.degree. C./sec. or more and 200.degree. C./sec. or less in order
to consistently form a steel microstructure including 10% to 95% of
martensite on an area-ratio basis with the balance being
substantially ferrite. The cooling rate is preferably set to
1.degree. C./sec. or more and less than 5.degree. C./sec. in order
to consistently form a steel microstructure including 10% to 95% of
pearlite on an area-ratio basis with the balance being
substantially ferrite. There are no particular limitations on
cooling means. For example, water cooling may be performed. If
quenching is stopped at a temperature exceeding 250.degree. C.,
desired transformation may fail to be completed, which results in
failure to form a desired steel microstructure after tempering.
Thus, quenching is performed until the temperature reaches
250.degree. C. or less and is preferably performed until the
temperature reaches 200.degree. C. or less. The temperature at
which quenching is stopped is preferably set to 100.degree. C. or
more in order to increase production efficiency. After quenching,
tempering is performed at the Ac.sub.1 transformation temperature
or less. If the tempering temperature exceeds the Ac.sub.1
transformation temperature, a portion of the microstructure may be
transformed into austenite, which results in failure to form a
desired steel microstructure after tempering. Tempering is
preferably performed at (Ac.sub.1--20).degree. C. or less. The
tempering temperature is preferably set to 300.degree. C. or more,
for example, in order to recover toughness and the like.
[0100] The hydrogen storage tank, which is the steel structure for
hydrogen gas according to the present invention, can be produced
by, for example, forming a steel material having a predetermined
composition into a predetermined shape, that is, the shape of a
desired hydrogen storage tank, and subsequently performing
reheating, quenching, and tempering.
Reheating, Quenching, and Tempering
[0101] A steel material having the above-described composition is
formed into a predetermined shape, and subsequently heating to the
Ac.sub.3 transformation temperature or more, quenching from the
Ar.sub.3 transformation temperature or more to 250.degree. C. or
less at a cooling rate of 0.5.degree. C./sec. to 100.degree.
C./sec, and tempering at the Ac.sub.1 transformation temperature or
less are performed. The steel material that is to be heated to the
Ac.sub.3 transformation temperature or more may have any
composition corresponding to the steel microstructure of a desired
hydrogen storage tank, and it is not necessary to particularly
specify the steel microstructure of the steel material. If the
temperature to which heating is performed after the steel material
is formed into a predetermined shape is less than the Ac.sub.3
transformation temperature, a portion of non-transformed austenite
may remain, which results in failure to form a desired steel
microstructure after hot rolling, quenching, and tempering. Thus,
the heating temperature is set to the Ac.sub.3 transformation
temperature or more and is preferably set to (Ac.sub.3+50).degree.
C. or more. The heating temperature is preferably set to
1250.degree. C. or less in order to prevent an excessive increase
in the diameters of initial austenite grains from occurring and
increase the production efficiency. If the temperature at which
quenching is started after heating is less than the Ar.sub.3
transformation temperature, transformation of a portion of
austenite may occur before quenching is started, which results in
failure to forming a desired steel microstructure after quenching
and tempering. Thus, quenching is performed by starting cooling at
the Ar.sub.3 transformation temperature or more after heating.
Cooling is preferably started at (Ar.sub.3+50).degree. C. or more.
The temperature at which quenching is started is preferably set to
1000.degree. C. or less in consideration of hot rolling. The
cooling rate at which quenching is performed from the Ar.sub.3
transformation temperature or more is set to 0.5.degree. C./sec. or
more and 100.degree. C./sec. or less in order to form a desired
microstructure and prevent quench cracking from occurring. The
cooling rate is an average cooling rate measured at the center of
the steel plate (i.e., wall of the storage tank) in the thickness
(i.e., wall thickness) direction. The cooling rate is preferably
set to 5.degree. C./sec. or more and less than 20.degree. C./sec.
in order to consistently form a steel microstructure including 10%
to 95% of bainite on an area-ratio basis with the balance being
substantially ferrite. The cooling rate is preferably set to
20.degree. C./sec. or more and 100.degree. C./sec. or less in order
to consistently form a steel microstructure including 10% to 95% of
martensite on an area-ratio basis with the balance being
substantially ferrite. The cooling rate is preferably set to
0.5.degree. C./sec. or more and less than 5.degree. C./sec. in
order to consistently form a steel microstructure including 10% to
95% of pearlite on an area-ratio basis with the balance being
substantially ferrite. There are no particular limitations on
cooling means. For example, oil cooling or water cooling may be
performed. If quenching, that is, cooling, is stopped at a
temperature exceeding 250.degree. C., desired transformation may
fail to be completed, which results in failure to form a desired
steel microstructure after tempering. Thus, quenching is performed
until the temperature reaches 250.degree. C. or less and is
preferably performed until the temperature reaches 200.degree. C.
or less. The temperature at which quenching is stopped is
preferably set to 100.degree. C. or less in order to increase
production efficiency. After quenching, tempering is performed at
the Ac.sub.1 transformation temperature or less. If the tempering
temperature exceeds the Ac.sub.1 transformation temperature, a
portion of the microstructure may be transformed into austenite,
which results in failure to form a desired steel microstructure
after tempering. The tempering temperature is preferably set to
(Ac.sub.1--20).degree. C. or less. The tempering temperature is
preferably set to 300.degree. C. or more, for example, in order to
recover toughness and the like.
[0102] Although a method for determining the Ac.sub.3
transformation temperature (.degree. C.), the Ar.sub.3
transformation temperature (.degree. C.), and the Ac.sub.1
transformation temperature (.degree. C.) is not particularly
specified in the present invention, for example, these
transformation temperatures can be calculated using the following
equations: Ac.sub.3=854-180C+44Si-14Mn-17.8Ni-1.7Cr;
Ar.sub.3=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo; and
Ac.sub.1=723-14Mn+22Si-14.4Ni+23.3Cr, where the symbols of elements
represent the contents (% by mass) of respective elements in a
steel material.
[0103] The hydrogen line pipe and hydrogen storage tank, which are
the steel structure for hydrogen gas, having a steel microstructure
including a predetermined amount of bainite with the balance being
substantially ferrite (Invention 1), a steel microstructure
including a predetermined amount of martensite with the balance
being substantially ferrite (Invention 2), or a steel
microstructure including a predetermined amount of pearlite with
the balance being substantially ferrite (Invention 3) can be
produced under the above-described conditions.
Example 1
Invention 1
[0104] An example in which the advantageous effects of the present
invention, that is, specifically, a steel structure for hydrogen
gas which has a steel microstructure including bainite with the
balance being substantially ferrite (Invention 1), were verified is
described below. In this example, a method for producing a steel
plate was studied in order to simulate the method for producing a
hydrogen line pipe or a method for producing a hydrogen storage
tank, and property evaluations of a steel plate were performed in
order to simulate the property evaluations of the hydrogen line
pipe or the hydrogen storage tank. Specifically, in the case where
the production method was accelerated cooling or direct quenching
and tempering, production of the hydrogen line pipe was simulated,
and, in the case where reheating, quenching, and tempering were
performed, production of the hydrogen storage tank was
simulated.
[0105] Steels BA to BH having the respective chemical compositions
shown in Table 1 (Tables 1-1 and 1-2) were each molten and cast
into a slab. Some of the slabs were heated to the respective
heating temperatures shown in Table 2 and then hot-rolled. The
hot-rolled steels were subjected to accelerated cooling (Steel
plate Nos. B1 and B4) or direct quenching and tempering (Steel
plate Nos. B2 and B5) by performing water cooling under the
respective conditions shown in Table 2 to prepare steel plates. The
other slabs were, after casting, temporarily formed into steel
plates, which were then quenched by water cooling or oil cooling
under the respective conditions shown in Table 2 to prepare steel
plates (Steel plate Nos. B3 and B6 to B15). That is, reheating,
quenching, and tempering were performed. The temperature of each
steel plate was measured using a thermocouple placed into the
center of the steel plate in the thickness direction. The cooling
rates shown in Table 2, at which water cooling or oil cooling was
performed, were 5.degree. C./sec. or more and less than 20.degree.
C./sec.
[0106] Table 2 summarizes the bainite area ratio, tensile strength,
and fatigue crack propagation rate (m/cycle) in 90-MPa
high-pressure hydrogen gas at a stress intensity factor range of 25
MPam.sup.1/2 of each steel plate. A material test and evaluation of
material properties were conducted in the following manner. In the
steel plates shown in Table 2, microstructures other than bainite
were principally ferrite, and the total area ratio of
microstructures other than bainite or ferrite was 2% or less. The
targeted fatigue crack propagation rate was 1.0.times.10.sup.-6
(m/cycle) or less, and it was considered that the steel plate had
high hydrogen embrittlement resistance when the targeted rate was
achieved.
(a) Microstructure of Steel Plate
[0107] A microstructure was caused to appear by performing 3% nital
etching. An optical microscope image of a cross section of each
steel plate which is parallel to the rolling direction was captured
at the 1/4-thickness position at an appropriate magnification of
200 to 400 times. Microstructures were visually distinguished, and
the area ratios of the microstructures were determined by an image
analysis.
(b) Tensile Properties
[0108] A tensile test conforming to JIS 22241 was conducted using
full-thickness tensile test specimens described in JIS Z2201
(1980), which were taken so that the longitudinal direction
(tensile direction) of each specimen was parallel to the rolling
direction, in order to make an evaluation.
(c) Fatigue Crack Propagation Test
[0109] A fatigue crack propagation characteristic was examined in
the following manner. Compact tension specimens (CT specimen)
conforming to ASTM E 647 were taken from the respective steel
plates so that the loading direction was parallel to the rolling
direction. The lengths of fatigue cracks formed in each specimen
were measured by a compliance method using a clip gage, and thereby
a fatigue crack propagation rate in 90-MPa high-pressure hydrogen
gas was determined. The test specimens were prepared by, when the
thickness of the steel plate was 10 mm or less, grinding the both
surfaces of the steel plate by 0.5 mm so that test specimens having
thicknesses of 2 mm, 5 mm, 8 mm, and 9 mm were prepared. When the
thickness of the steel plate was other than the above-described
thickness, that is, more than 10 mm, a test specimen having a
thickness of 10 mm was taken at the t/2 (t: plate thickness)
position. Both sides of each test specimen were subjected to mirror
polishing. A fatigue crack propagation rate (m/cycle) at a stress
intensity factor range .DELTA.K of 25 (MPam.sup.112), which is a
stable growth region in which Paris' law holds, was used as a
representative value for evaluation. The targeted fatigue crack
propagation rate was 1.0.times.10.sup.-6 (m/cycle) or less.
[0110] Steel plate Nos. B1 to B6, B8, B11, and B14 shown in Table
2, which satisfy all of the requirements for chemical composition
and production conditions according to the present invention, had a
dual-phase microstructure primarily composed of ferrite and
bainite, and the area ratio of bainite fell within the range of the
present invention. As summarized in Table 2, these steel plates had
a fatigue crack propagation rate of 1.0.times.10.sup.-6 (m/cycle)
or less, which confirms that these steel plates had high hydrogen
embrittlement resistance in high-pressure hydrogen gas.
[0111] On the other hand, in Steel plate No. B7, where the heating
temperature was lower than the lower limit of the range of the
present invention (i.e., Ac.sub.3), both the targeted bainite area
ratio and the targeted fatigue crack propagation rate were not
achieved. In Steel plate Nos. B9 and B12, where the cooling start
temperature (i.e., temperature at which water cooling or oil
cooling was started) was lower than the lower limit of the range of
the present invention (i.e., Ar.sub.3), that is, out of the range
of the present invention, both the targeted bainite area ratio and
the targeted fatigue crack propagation rate were not achieved. In
Steel plate Nos. B10 and B13, where the cooling stop temperature
(i.e., temperature at which water cooling or oil cooling was
stopped) was higher than the upper limit of the range of the
present invention (i.e., 250.degree. C.), that is, out of the range
of the present invention, both the targeted bainite area ratio and
the targeted fatigue crack propagation rate were not achieved. In
Steel plate No. B15, where the tempering temperature was higher
than the upper limit of the range of the present invention (i.e.,
Ac.sub.1), that is, out of the range of the present invention, both
the desired bainite area ratio and the desired fatigue crack
propagation rate were not achieved. However, Steel plate Nos. B7,
B9, B10, B12, B13, and B15, which are shown as comparative
examples, also had a dual-phase microstructure primarily composed
of ferrite and bainite.
[0112] As is clear from the above-described results, in Invention
examples, the fatigue crack propagation rate was
1.0.times.10.sup.-6 (m/cycle) or less and a good hydrogen
embrittlement characteristic was achieved. This confirms that a
steel structure for hydrogen gas such as a hydrogen storage tank or
a hydrogen line pipe which has high hydrogen embrittlement
resistance can be produced.
TABLE-US-00001 TABLE 1-1 Steel Composition (mass %) type C Si Mn Al
N P S O BA 0.05 0.17 0.67 0.031 0.0032 0.008 0.0032 0.0032 BB 0.07
0.25 1.01 0.035 0.0036 0.015 0.0041 0.0052 BC 0.05 0.19 0.72 0.027
0.0033 0.009 0.0015 0.0029 BD 0.06 0.31 0.88 0.031 0.0035 0.005
0.0011 0.0033 BE 0.08 0.21 0.59 0.034 0.0038 0.007 0.0024 0.0035 BE
0.12 0.31 0.72 0.063 0.0031 0.004 0.0013 0.0042 BG 0.15 0.42 1.12
0.030 0.0035 0.012 0.0008 0.0028 BH 0.19 0.45 1.71 0.063 0.0036
0.018 0.0023 0.0042
TABLE-US-00002 TABLE 1-2 Steel Composition (mass %) Ac.sub.3
Ar.sub.3 Ac.sub.1 type Cu Ni Cr Mo Nb V Ti B Nd W Ca Mg REM
(.degree. C.) (.degree. C.) (.degree. C.) BA -- -- -- -- -- -- --
-- -- -- -- -- -- 843 841 717 BB -- -- -- -- -- -- -- -- -- -- --
-- -- 838 808 714 BC -- -- 0.77 -- 0.018 -- 0.012 -- -- -- -- -- --
842 825 735 BD -- -- 0.52 0.12 -- 0.047 0.011 -- -- -- -- -- -- 844
804 730 BE -- -- 0.86 0.47 0.028 0.038 0.007 -- -- -- -- -- -- 839
788 739 BF -- -- 0.79 0.26 0.016 0.052 -- -- -- -- -- -- -- 835 783
738 BG 0.71 1.15 0.63 0.35 0.022 0.028 0.012 -- -- -- -- -- -- 808
659 715 BH -- -- 1.52 0.51 0.024 0.064 0.015 0.0008 0.011 0.08
0.0006 0.0005 0.0007 813 651 744 Note 1: Ac.sub.3 (.degree. C.) =
854 - 180C + 44Si - 14Mn - 17.8Ni - 1.7Cr, where the symbols of
elements represent the contents (mass %) of the respective
elements. Note 2: Ar.sub.3 (.degree. C.) = 910 - 310C - 80Mn - 20Cu
- 15Cr - 55Ni - 80Mo, where the symbols of elements represent the
contents (mass %) of the respective elements. Note 3: Ac.sub.1
(.degree. C.) = 723 - 14Mn + 22Si - 14.4Ni + 23.3Cr, where the
symbols of elements represent the contents (mass %) of the
respective elements.
TABLE-US-00003 TABLE 2 Water- Water- Oil-cooling Steel Heating
cooling start cooling stop start plate Steel Thickness temperature
temperature temperature temperature No. type (mm) Production method
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) B1 BA 3
Accelerated cooling 1100 900 550 -- B2 BB 6 Direct quenching and
1100 900 200 -- tempering B3 BB 6 Reheating, quenching, 920 850 200
-- and tempering B4 BC 9 Accelerated cooling 1100 850 500 -- B5 BD
10 Direct quenching and 1100 850 200 -- tempering B6 BE 12
Reheating, quenching, 920 850 200 -- and tempering B7 BE 12
Reheating, quenching, 750 850 200 -- and tempering B8 BF 25
Reheating, quenching, 920 850 200 -- and tempering B9 BF 25
Reheating, quenching, 920 650 200 -- and tempering B10 BF 25
Reheating, quenching, 920 850 400 -- and tempering B11 BG 25
Reheating, quenching, 920 -- -- 850 and tempering B12 BG 25
Reheating, quenching, 920 -- -- 600 and tempering B13 BG 25
Reheating, quenching, 920 -- -- 850 and tempering B14 BH 32
Reheating, quenching, 920 -- -- 850 and tempering B15 BH 32
Reheating, quenching, 920 -- -- 850 and tempering Oil-cooling
Bainite Fatigue crack Steel stop Tempering area Tensile propagation
plate temperature temperature ratio strength rate No. (.degree. C.)
(.degree. C.) (%) (MPa) (m/cycle) Remark B1 -- -- 14 338 0.02
.times. 10.sup.-6 Invention example B2 -- 650 23 354 0.03 .times.
10.sup.-6 Invention example B3 -- 650 17 341 0.02 .times. 10.sup.-6
Invention example B4 -- -- 15 363 0.03 .times. 10.sup.-6 Invention
example B5 -- 650 26 452 0.09 .times. 10.sup.-6 Invention example
B6 -- 650 23 666 0.17 .times. 10.sup.-6 Invention example B7 -- 650
8 589 1.15 .times. 10.sup.-6 Comparative example B8 -- 650 31 667
0.31 .times. 10.sup.-6 Invention example B9 -- 650 7 555 1.17
.times. 10.sup.-6 Comparative example B10 -- 650 9 576 1.16 .times.
10.sup.-6 Comparative example B11 150 650 51 703 0.33 .times.
10.sup.-6 Invention example B12 150 650 9 651 1.77 .times.
10.sup.-6 Comparative example B13 400 650 7 671 2.03 .times.
10.sup.-6 Comparative example B14 150 650 32 921 0.51 .times.
10.sup.-6 Invention example B15 150 800 6 812 2.23 .times.
10.sup.-6 Comparative example Note 1: The underlines indicate that
the values are out of the range of the present invention.
Example 2
Invention 2
[0113] An example in which the advantageous effects of the present
invention, that is, specifically, a steel structure for hydrogen
gas which has a steel microstructure including martensite with the
balance being substantially ferrite (Invention 2), were verified is
described below. In this example, as in Example 1, a method for
producing a steel plate was studied in order to simulate the method
for producing a hydrogen line pipe or a method for producing a
hydrogen storage tank, and property evaluations of a steel plate
were performed in order to simulate the property evaluations of the
hydrogen line pipe or the hydrogen storage tank. Specifically, in
the case where the production method was accelerated cooling or
direct quenching and tempering, production of the hydrogen line
pipe was simulated, and, in the case where reheating, quenching,
and tempering were performed, production of the hydrogen storage
tank was simulated.
[0114] Steels MA to MH having the respective chemical compositions
shown in Table 3 (Tables 3-1 and 3-2) were each molten and cast
into a slab. Some of the slabs were heated to the respective
heating temperatures shown in Table 4 and then hot-rolled. The
hot-rolled steels were subjected to accelerated cooling (Steel
plate Nos. M1 and M4) or direct quenching and tempering (Steel
plate Nos. M2 and M5) by performing water cooling under the
respective conditions shown in Table 4 to prepare steel plates. The
other slabs were, after casting, temporarily formed into steel
plates, which were then quenched by water cooling or oil cooling
under the respective conditions shown in Table 4 to prepare steel
plates (Steel plate Nos. M3 and M6 to M15). That is, reheating,
quenching, and tempering were performed. The temperature of each
steel plate was measured using a thermocouple placed into the
center of the steel plate in the thickness direction. The cooling
rates shown in Table 4, at which water cooling or oil cooling was
performed, were 20.degree. C./sec. or more and 200.degree. C./sec.
or less. In particular, in the case where reheating, quenching, and
tempering were performed, the cooling rate was set to 20.degree.
C./sec. or more and 100.degree. C./sec. or less both for water
cooling and oil cooling.
[0115] Table 4 summarizes the martensite area ratio, tensile
strength, and fatigue crack propagation rate (m/cycle) in 90-MPa
high-pressure hydrogen gas at a stress intensity factor range of 25
MPam.sup.1/2 of each steel plate. A material test and evaluation of
material properties were conducted as in Example 1 in the following
manner. In the steel plates shown in Table 4, microstructures other
than martensite were principally ferrite, and the total area ratio
of microstructures other than martensite or ferrite was 2% or less.
The targeted fatigue crack propagation rate was 1.0.times.10.sup.-6
(m/cycle) or less, and it was considered that the steel plate had
high hydrogen embrittlement resistance when the targeted rate was
achieved.
(a) Microstructure of Steel Plate
[0116] A microstructure was caused to appear by performing 3% nital
etching. An optical microscope image of a cross section of each
steel plate which is parallel to the rolling direction was captured
at the 1/4-thickness position at an appropriate magnification of
200 to 400 times. Microstructures were visually distinguished, and
the area ratios of the microstructures were determined by an image
analysis.
(b) Tensile Properties
[0117] A tensile test conforming to JIS 22241 was conducted using
full-thickness tensile test specimens described in JIS Z2201
(1980), which were taken so that the longitudinal direction
(tensile direction) of each specimen was parallel to the rolling
direction, in order to make an evaluation.
(c) Fatigue Crack Propagation Test
[0118] A fatigue crack propagation characteristic was examined in
the following manner. Compact tension specimens conforming to ASTM
E 647 were taken from the respective steel plates so that the
loading direction was parallel to the rolling direction. The
lengths of fatigue cracks formed in each specimen were measured by
a compliance method using a clip gage, and thereby a fatigue crack
propagation rate in 90-MPa high-pressure hydrogen gas was
determined. The test specimens were prepared by, when the thickness
of the steel plate was 10 mm or less, grinding the both surfaces of
the steel plate by 0.5 mm so that test specimens having thicknesses
of 2 mm, 5 mm, 8 mm, and 9 mm were prepared. When the thickness of
the steel plate was other than the above-described thickness, that
is, more than 10 mm, a test specimen having a thickness of 10 mm
was taken at the t/2 (t: plate thickness) position. Both sides of
each test specimen were subjected to mirror polishing. A fatigue
crack propagation rate (m/cycle) at a stress intensity factor range
.DELTA.K of 25 (MPam.sup.112), which is a stable growth region in
which Paris' law holds, was used as a representative value for
evaluation. The targeted fatigue crack propagation rate was
1.0.times.10.sup.-6 (m/cycle) or less.
[0119] Steel plate Nos. M1 to M6, M8, M11, and M14 shown in Table
4, which satisfy all of the requirements for chemical composition
and production conditions according to the present invention, had a
dual-phase microstructure primarily composed of ferrite and
martensite, and the area ratio of martensite fell within the range
of the present invention. As summarized in Table 4, these steel
plates had a fatigue crack propagation rate of 1.0.times.10.sup.-6
(m/cycle) or less, which confirms that these steel plates had high
hydrogen embrittlement resistance in high-pressure hydrogen
gas.
[0120] On the other hand, in Steel plate No. M7, where the heating
temperature was lower than the lower limit of the range of the
present invention (i.e., Ac.sub.3), both the targeted martensite
area ratio and the targeted fatigue crack propagation rate were not
achieved. In Steel plate Nos. M9 and M12, where the cooling start
temperature (i.e., temperature at which water cooling or oil
cooling was started) was lower than the lower limit of the range of
the present invention (i.e., Ar.sub.3), that is, out of the range
of the present invention, both the targeted martensite area ratio
and the targeted fatigue crack propagation rate were not achieved.
In Steel plate Nos. M10 and M13, where the cooling stop temperature
(i.e., temperature at which water cooling or oil cooling was
stopped) was higher than the upper limit of the range of the
present invention (i.e., 250.degree. C.), that is, out of the range
of the present invention, both the targeted martensite area ratio
and the targeted fatigue crack propagation rate were not achieved.
In Steel plate No. M15, where the tempering temperature was higher
than the upper limit of the range of the present invention (i.e.,
Ac.sub.1), that is, out of the range of the present invention, both
the desired martensite area ratio and the desired fatigue crack
propagation rate were not achieved. However, Steel plate Nos. M7,
M9, M10, M12, M13, and M15, which are shown as comparative
examples, also had a dual-phase microstructure primarily composed
of ferrite and martensite.
[0121] As is clear from the above-described results, in Invention
examples, the fatigue crack propagation rate was
1.0.times.10.sup.-6 (m/cycle) or less and a good hydrogen
embrittlement characteristic was achieved. This confirms that a
steel structure for hydrogen gas such as a hydrogen storage tank or
a hydrogen line pipe which has high hydrogen embrittlement
resistance can be produced.
TABLE-US-00004 TABLE 3-1 Steel Composition (mass %) type C Si Mn Al
N P S O MA 0.05 0.24 0.97 0.031 0.0034 0.021 0.0011 0.0038 MB 0.08
0.35 1.51 0.033 0.0041 0.017 0.0007 0.0041 MC 0.10 0.21 0.69 0.026
0.0035 0.010 0.0016 0.0032 MD 0.12 0.32 0.87 0.030 0.0036 0.006
0.0012 0.0031 ME 0.15 0.22 0.58 0.033 0.0039 0.008 0.0026 0.0032 MF
0.19 0.36 0.78 0.062 0.0030 0.006 0.0011 0.0040 MG 0.21 0.41 1.15
0.056 0.0033 0.012 0.0007 0.0026 MH 0.33 0.46 1.89 0.067 0.0035
0.017 0.0018 0.0044
TABLE-US-00005 TABLE 3-2 Steel Composition (mass %) Ac.sub.3
Ar.sub.3 Ac.sub.1 type Cu Ni Cr Mo Nb V Ti B Nd W Ca Mg REM
(.degree. C.) (.degree. C.) (.degree. C.) MA -- -- -- -- -- -- --
-- -- -- -- -- -- 842 817 715 MB -- -- -- -- -- -- -- -- -- -- --
-- -- 834 764 710 MC -- -- 0.75 -- 0.019 -- 0.014 0.0009 -- -- --
-- -- 834 813 735 MD -- -- 0.51 0.15 0.023 -- 0.012 0.0011 -- -- --
-- -- 833 784 730 ME -- -- 0.87 0.55 0.032 0.041 0.008 -- -- -- --
-- -- 827 760 740 MF -- -- 0.77 0.32 0.018 0.055 -- -- -- -- -- --
-- 823 752 738 MG 0.51 1.28 0.61 0.41 0.020 0.042 0.013 0.0012 --
-- -- -- -- 794 630 712 MH -- -- 2.12 0.68 0.023 0.082 0.016 0.0009
0.016 0.12 0.0008 0.0006 0.0005 785 570 756 Note 1: Ac.sub.3
(.degree. C.) = 854 - 180C + 44Si - 14Mn - 17.8Ni - 1.7Cr, where
the symbols of elements represent the contents (mass %) of the
respective elements. Note 2: Ar.sub.3 (.degree. C.) = 910 - 310C -
80Mn - 20Cu - 15Cr - 55Ni - 80Mo, where the symbols of elements
represent the contents (mass %) of the respective elements. Note 3:
Ac.sub.1 (.degree. C.) = 723 - 14Mn + 22Si - 14.4Ni + 23.3Cr, where
the symbols of elements represent the contents (mass %) of the
respective elements.
TABLE-US-00006 TABLE 4 Water-cooling Water- Oil-cooling Steel
Heating start cooling stop start plate Steel Thickness temperature
temperature temperature temperature No. type (mm) Production method
(.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.) M1 MA 3
Accelerated cooling 1100 900 550 -- M2 MB 6 Direct quenching and
1100 900 200 -- tempering M3 MB 6 Reheating, quenching, 920 850 200
-- and tempering M4 MC 9 Accelerated cooling 1100 850 500 -- M5 MD
10 Direct quenching and 1100 850 200 -- tempering M6 ME 12
Reheating, quenching, 920 850 200 -- and tempering M7 ME 12
Reheating, quenching, 750 850 200 -- and tempering M8 MF 25
Reheating, quenching, 920 850 200 -- and tempering M9 MF 25
Reheating, quenching, 920 650 200 -- and tempering M10 MF 25
Reheating, quenching, 920 850 400 -- and tempering M11 MG 25
Reheating, quenching, 920 -- -- 850 and tempering M12 MG 25
Reheating, quenching, 920 -- -- 600 and tempering M13 MG 25
Reheating, quenching, 920 -- -- 850 and tempering M14 MH 32
Reheating, quenching, 920 -- -- 850 and tempering M15 MH 32
Reheating, quenching, 920 -- -- 850 and tempering Oil-cooling
Fatigue crack Steel stop Tempering Martensite Tensile propagation
plate temperature temperature area ratio strength rate No.
(.degree. C.) (.degree. C.) (%) (MPa) (m/cycle) Remark M1 -- -- 15
417 0.06 .times. 10.sup.-6 Invention example M2 -- 650 21 442 0.08
.times. 10.sup.-6 Invention example M3 -- 650 17 431 0.07 .times.
10.sup.-6 Invention example M4 -- -- 12 451 0.09 .times. 10.sup.-6
Invention example M5 -- 650 23 546 0.12 .times. 10.sup.-6 Invention
example M6 -- 650 21 731 0.23 .times. 10.sup.-6 Invention example
M7 -- 650 6 661 1.20 .times. 10.sup.-6 Comparative example M8 --
650 27 758 0.46 .times. 10.sup.-6 Invention example M9 -- 650 5 651
1.31 .times. 10.sup.-6 Comparative example M10 -- 650 3 666 1.23
.times. 10.sup.-6 Comparative example M11 150 650 37 791 0.56
.times. 10.sup.-6 Invention example M12 150 650 7 732 1.92 .times.
10.sup.-6 Comparative example M13 400 650 5 751 2.22 .times.
10.sup.-6 Comparative example M14 150 650 21 998 0.68 .times.
10.sup.-6 Invention example M15 150 800 5 874 4.89 .times.
10.sup.-6 Comparative example Note 1: The underlines indicate that
the values are out of the range of the present invention.
Example 3
Invention 3
[0122] An example in which the advantageous effects of the present
invention, that is, specifically, a steel structure for hydrogen
gas which has a steel microstructure including pearlite with the
balance being substantially ferrite (Invention 3), were verified is
described below. In this example, as in Example 1, a method for
producing a steel plate was studied in order to simulate the method
for producing a hydrogen line pipe or a method for producing a
hydrogen storage tank, and property evaluations of a steel plate
were performed in order to simulate the property evaluations of the
hydrogen line pipe or the hydrogen storage tank. Specifically, in
the case where the production method was accelerated cooling or
direct quenching and tempering, production of the hydrogen line
pipe was simulated, and, in the case where reheating, quenching,
and tempering were performed, production of the hydrogen storage
tank was simulated.
[0123] Steels PA to PH having the respective chemical compositions
shown in Table 5 (Tables 5-1 and 5-2) were each molten and cast
into a slab. Some of the slabs were heated to the respective
heating temperatures shown in Table 6 and then hot-rolled. The
hot-rolled steels were subjected to accelerated cooling (Steel
plate Nos. P1 and P4) or direct quenching and tempering (Steel
plate Nos. P2 and P5) by performing water cooling under the
respective conditions shown in Table 6 to prepare steel plates. The
other slabs were, after casting, temporarily formed into steel
plates, which were then quenched by water cooling or oil cooling
under the respective conditions shown in Table 6 to prepare steel
plates (Steel plate Nos. P3 and P6 to P15). That is, reheating,
quenching, and tempering were performed. The temperature of each
steel plate was measured using a thermocouple placed into the
center of the steel plate in the thickness direction. The cooling
rates shown in Table 6, at which water cooling or oil cooling was
performed, were set to 1.degree. C./sec. or more and less than
5.degree. C./sec. in the case where accelerated cooling was
performed and in the case where direct quenching and tempering were
performed. In the case where reheating, quenching, and tempering
were performed, the cooling rate was set to 0.5.degree. C./sec. or
more and less than 5.degree. C./sec.
[0124] Table 6 summarizes the pearlite area ratio, tensile
strength, and fatigue crack propagation rate (m/cycle) in 90-MPa
high-pressure hydrogen gas at a stress intensity factor range of 25
MPam.sup.1/2 of each steel plate. A material test and evaluation of
material properties were conducted as in Example 1 in the following
manner. In the steel plates shown in Table 6, microstructures other
than pearlite were principally ferrite, and the total area ratio of
microstructures other than pearlite or ferrite was 2% or less. The
targeted fatigue crack propagation rate was 1.0.times.10.sup.-6
(m/cycle) or less, and it was considered that the steel plate had
high hydrogen embrittlement resistance when the targeted rate was
achieved.
(a) Microstructure of Steel Plate
[0125] A microstructure was caused to appear by performing 3% nital
etching. An optical microscope image of a cross section of each
steel plate which is parallel to the rolling direction was captured
at the 1/4-thickness position at an appropriate magnification of
200 to 400 times. Microstructures were visually distinguished, and
the area ratios of the microstructures were determined by an image
analysis.
(b) Tensile Properties
[0126] A tensile test conforming to JIS 22241 was conducted using
full-thickness tensile test specimens described in JIS Z2201
(1980), which were taken so that the longitudinal direction
(tensile direction) of each specimen was parallel to the rolling
direction, in order to make an evaluation.
(c) Fatigue Crack Propagation Test
[0127] A fatigue crack propagation characteristic was examined in
the following manner. Compact tension specimens conforming to ASTM
E 647 were taken from the respective steel plates so that the
loading direction was parallel to the rolling direction. The
lengths of fatigue cracks formed in each specimen were measured by
a compliance method using a clip gage, and thereby a fatigue crack
propagation rate in 90-MPa high-pressure hydrogen gas was
determined. The test specimens were prepared by, when the thickness
of the steel plate was 10 mm or less, grinding the both surfaces of
the steel plate by 0.5 mm so that test specimens having thicknesses
of 2 mm, 5 mm, 8 mm, and 9 mm were prepared. When the thickness of
the steel plate was other than the above-described thickness, that
is, more than 10 mm, a test specimen having a thickness of 10 mm
was taken at the t/2 (t: plate thickness) position. Both sides of
each test specimen were subjected to mirror polishing. A fatigue
crack propagation rate (m/cycle) at a stress intensity factor range
.DELTA.K of 25 (MPam.sup.1/2), which is a stable growth region in
which Paris' law holds, was used as a representative value for
evaluation. The targeted fatigue crack propagation rate was
1.0.times.10.sup.-6 (m/cycle) or less.
[0128] Steel plate Nos. P1 to P6, P8, P11, and P14 shown in Table
6, which satisfy all of the requirements for chemical composition
and production conditions according to the present invention, had a
dual-phase microstructure primarily composed of ferrite and
pearlite, and the area ratio of pearlite fell within the range of
the present invention. As summarized in Table 6, these steel plates
had a fatigue crack propagation rate of 1.0.times.10.sup.-6
(m/cycle) or less, which confirms that these steel plates had high
hydrogen embrittlement resistance in high-pressure hydrogen
gas.
[0129] On the other hand, in Steel plate No. P7, where the heating
temperature was lower than the lower limit of the range of the
present invention (i.e., Ac.sub.3), both the targeted pearlite area
ratio and the targeted fatigue crack propagation rate were not
achieved. In Steel plate Nos. P9 and P12, where the cooling start
temperature (i.e., temperature at which water cooling or oil
cooling was started) was lower than the lower limit of the range of
the present invention (i.e., Ar.sub.3), that is, out of the range
of the present invention, both the targeted pearlite area ratio and
the targeted fatigue crack propagation rate were not achieved. In
Steel plate Nos. P10 and P13, where the cooling stop temperature
(i.e., temperature at which water cooling or oil cooling was
stopped) was higher than the upper limit of the range of the
present invention (i.e., 250.degree. C.), that is, out of the range
of the present invention, both the targeted pearlite area ratio and
the targeted fatigue crack propagation rate were not achieved. In
Steel plate No. P15, where the tempering temperature was higher
than the upper limit of the range of the present invention (i.e.,
Ac.sub.1), that is, out of the range of the present invention, both
the desired pearlite area ratio and the desired fatigue crack
propagation rate were not achieved. However, Steel plate Nos. P7,
P9, P10, P12, P13, and P15, which are shown as comparative
examples, also had a dual-phase microstructure primarily composed
of ferrite and pearlite.
[0130] As is clear from the above-described results, in Invention
examples, the fatigue crack propagation rate was
1.0.times.10.sup.-6 (m/cycle) or less and a good hydrogen
embrittlement characteristic was achieved. This confirms that a
steel structure for hydrogen gas such as a hydrogen storage tank or
a hydrogen line pipe which has high hydrogen embrittlement
resistance can be produced.
TABLE-US-00007 TABLE 5-1 Steel Composition (mass %) type C Si Mn Al
N P S O PA 0.05 0.15 0.89 0.028 0.0035 0.016 0.0021 0.0032 PB 0.06
0.23 1.55 0.034 0.0041 0.027 0.0042 0.0041 PC 0.05 0.18 0.71 0.031
0.0032 0.008 0.0014 0.0028 PD 0.06 0.32 0.92 0.029 0.0034 0.006
0.0009 0.0032 PE 0.06 0.22 0.62 0.033 0.0037 0.008 0.0022 0.0034 PF
0.07 0.32 0.77 0.059 0.0029 0.005 0.0012 0.0041 PG 0.08 0.44 1.23
0.028 0.0033 0.011 0.0009 0.0029 PH 0.10 0.46 1.52 0.027 0.0035
0.017 0.0021 0.0041
TABLE-US-00008 TABLE 5-2 Steel Composition (mass %) Ac.sub.3
Ar.sub.3 Ac.sub.1 type Cu Ni Cr Mo Nb V Ti B Nd W Ca Mg REM
(.degree. C.) (.degree. C.) (.degree. C.) PA -- -- -- -- -- -- --
-- -- -- -- -- -- 839 823 714 PB -- -- -- -- -- -- -- -- -- -- --
-- -- 832 767 706 PC -- -- 0.81 -- -- -- 0.006 -- -- -- -- -- --
842 826 736 PD -- -- 0.51 0.11 -- -- -- -- -- -- -- -- -- 844 801
729 PE -- -- 0.79 0.06 -- -- 0.006 -- -- -- -- -- -- 843 825 738 PF
-- -- 0.77 0.12 0.016 0.025 -- -- -- -- -- -- -- 843 806 737 PG
0.53 0.79 0.65 0.07 0.019 0.026 0.015 -- -- -- -- -- -- 827 717 719
PH -- -- 1.25 0.23 0.023 0.032 0.016 0.0006 0.005 0.06 0.0007
0.0006 0.0006 833 720 741 Note 1: Ac.sub.3 (.degree. C.) = 854 -
180C + 44Si - 14Mn - 17.8Ni - 1.7Cr, where the symbols of elements
represent the contents (mass %) of the respective elements. Note 2:
Ar.sub.3 (.degree. C.) = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni -
80Mo, where the symbols of elements represent the contents (mass %)
of the respective elements. Note 3: Ac.sub.1 (.degree. C.) = 723 -
14Mn + 22Si - 14.4Ni + 23.3Cr, where the symbols of elements
represent the contents (mass %) of the respective elements.
TABLE-US-00009 TABLE 6 Water- Water- Oil-cooling Heating cooling
start cooling stop start Steel Thickness temperature temperature
temperature temperature No. type (mm) Production method (.degree.
C.) (.degree. C.) (.degree. C.) (.degree. C.) P1 PA 3 Accelerated
cooling 1100 900 550 -- P2 PB 6 Direct quenching and 1100 900 200
-- tempering P3 PB 6 Reheating, quenching, 920 850 200 -- and
tempering P4 PC 9 Accelerated cooling 1100 850 500 -- P5 PD 10
Direct quenching and 1100 850 200 -- tempering P6 PE 12 Reheating,
quenching, 920 850 200 -- and tempering P7 PE 12 Reheating,
quenching, 750 850 200 -- and tempering P8 PF 25 Reheating,
quenching, 920 850 200 -- and tempering P9 PF 25 Reheating,
quenching, 920 650 200 -- and tempering P10 PF 25 Reheating,
quenching, 920 850 400 -- and tempering P11 PG 25 Reheating,
quenching, 920 -- -- 850 and tempering P12 PG 25 Reheating,
quenching, 920 -- -- 600 and tempering P13 PG 25 Reheating,
quenching, 920 -- -- 850 and tempering P14 PH 32 Reheating,
quenching, 920 -- -- 850 and tempering P15 PH 32 Reheating,
quenching, 920 -- -- 850 and tempering Oil-cooling Pearlite Fatigue
crack stop Tempering area Tensile propagation temperature
temperature ratio strength rate No. (.degree. C.) (.degree. C.) (%)
(MPa) (m/cycle) Remark P1 -- -- 12 289 0.008 .times. 10.sup.-6
Invention example P2 -- 650 15 305 0.01 .times. 10.sup.-6 Invention
example P3 -- 650 14 303 0.01 .times. 10.sup.-6 Invention example
P4 -- -- 15 313 0.01 .times. 10.sup.-6 Invention example P5 -- 650
14 402 0.05 .times. 10.sup.-6 Invention example P6 -- 650 13 616
0.12 .times. 10.sup.-6 Invention example P7 -- 650 8 539 1.07
.times. 10.sup.-6 Comparative example P8 -- 650 16 617 0.17 .times.
10.sup.-6 Invention example P9 -- 650 6 505 1.09 .times. 10.sup.-6
Comparative example P10 -- 650 7 526 1.11 .times. 10.sup.-6
Comparative example P11 150 650 21 653 0.26 .times. 10.sup.-6
Invention example P12 150 650 8 601 1.42 .times. 10.sup.-6
Comparative example P13 400 650 6 621 1.87 .times. 10.sup.-6
Comparative example P14 150 650 12 871 0.46 .times. 10.sup.-6
Invention example P15 150 800 5 762 2.02 .times. 10.sup.-6
Comparative example Note 1: The underlines indicate that the values
are out of the range of the present invention.
* * * * *