U.S. patent application number 13/203102 was filed with the patent office on 2011-12-15 for hydrogen fatigue resistant ferritic steel and manufacturing method thereof.
This patent application is currently assigned to National Institute of Advanced Industrial Science and Technology. Invention is credited to Masao Hayakawa, Saburo Matsuoka, Nobuo Nagashima, Etsuo Takeuchi.
Application Number | 20110305595 13/203102 |
Document ID | / |
Family ID | 42665382 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110305595 |
Kind Code |
A1 |
Matsuoka; Saburo ; et
al. |
December 15, 2011 |
HYDROGEN FATIGUE RESISTANT FERRITIC STEEL AND MANUFACTURING METHOD
THEREOF
Abstract
A ferritic steel having tensile properties and fatigue
properties capable of withstanding use in a hydrogen environment
and a method of manufacture thereof are provided. By adding one or
more element selected from among vanadium (V), titanium (Ti) and
niobium (Nb) so that the steel includes, together with at least
ferrite grains in the structure, a carbide or carbides of one or
more element selected from among V, Ti and Nb, the reduction of
area and the fatigue crack propagation rate of the ferritic steel
in a hydrogen environment are improved. The advantages of the
invention were confirmed in cases where the ferrite grains are
small grains of 1 .mu.m or less in size, and in cases where the
ferrite grains are coarse grains from several micrometers to 20
.mu.m in size, and moreover in cases where the ferrite grains are
coarse grains from several micrometers to 60 .mu.m in size.
Inventors: |
Matsuoka; Saburo; (Fukuoka,
JP) ; Hayakawa; Masao; (Ibaraki, JP) ;
Takeuchi; Etsuo; (Ibaraki, JP) ; Nagashima;
Nobuo; (Ibaraki, JP) |
Assignee: |
National Institute of Advanced
Industrial Science and Technology
Tokyo
JP
|
Family ID: |
42665382 |
Appl. No.: |
13/203102 |
Filed: |
January 29, 2010 |
PCT Filed: |
January 29, 2010 |
PCT NO: |
PCT/JP2010/051245 |
371 Date: |
September 2, 2011 |
Current U.S.
Class: |
420/120 |
Current CPC
Class: |
C21D 2211/005 20130101;
C22C 38/12 20130101; C22C 38/02 20130101; Y02E 60/50 20130101; C21D
9/14 20130101; Y02P 70/50 20151101; C21D 8/005 20130101; H01M
8/04201 20130101; C22C 38/14 20130101; C22C 38/04 20130101; H01M
8/04208 20130101 |
Class at
Publication: |
420/120 |
International
Class: |
C22C 38/04 20060101
C22C038/04; C22C 38/14 20060101 C22C038/14; C22C 38/12 20060101
C22C038/12; C22C 38/02 20060101 C22C038/02; C22C 38/06 20060101
C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2009 |
JP |
2009-043183 |
Claims
1. A hydrogen fatigue-resistant ferritic steel, comprising, with
one or more element selected from among vanadium (V), titanium (Ti)
and niobium (Nb) being added, a carbide or carbides of one or more
element selected from among V, Ti and Nb in a structure composed
primarily of ferrite grains, wherein the ferritic steel exhibits
improvement in reduction of area and in a fatigue crack propagation
rate in a hydrogen atmosphere.
2. The hydrogen fatigue-resistant ferritic steel of claim 1,
wherein the structure is composed primarily of fine ferrite grains
having a diameter of 1 .mu.m or less.
3. The hydrogen fatigue-resistant ferritic steel of claim 1,
wherein the structure is composed primarily of coarse ferrite
grains having a diameter of from several micrometers to 20
.mu.m.
4. The hydrogen fatigue-resistant ferritic steel of claim 1,
wherein the structure is composed primarily of coarse ferrite
grains having a diameter of from several micrometers to 60.mu..
5. The hydrogen fatigue-resistant ferritic steel of any one of
claims 1 to 4, wherein the one or more element selected from among
V, Ti and Nb has been added in at least the amount required to fix
all carbon (C) in the structure as the carbide or carbides.
6. The hydrogen fatigue-resistant ferritic steel of any one of
claims 1 to 4, comprising carbides of all the elements V, Ti and
Nb, wherein the V, Ti and Nb have been added in a collective amount
which is substantially the same as the amount required to fix all
carbon (C) in the structure as the carbides.
7. The hydrogen fatigue-resistant ferritic steel of any one of
claims 1 to 4, wherein the one or more element selected from among
V, Ti and Nb has been added in an amount which is less than, but at
least one-fourth of, the amount required to fix all carbon (C) in
the structure as the carbide or carbides.
8. The hydrogen fatigue-resistant ferritic steel of any one of
claims 1 to 4, wherein, of the V, Ti and Nb, the only element that
has been used as an additive is V, and the V has been added in an
amount which is less than the amount required to fix all carbon (C)
in the structure as a carbide of V.
9. The hydrogen fatigue-resistant ferritic steel of any one of
claims 1 to 4, wherein, of the V, Ti and Nb, the only element that
has been used as an additives is V, and the V has been added in an
amount which is less than, but at least one-fourth of, the amount
required to fix all carbon (C) in the structure as a carbide of
V.
10. A method of manufacturing a hydrogen fatigue-resistant ferritic
steel, the method comprising a step of adding one or more element
selected from among vanadium (V), titanium (Ti) and niobium (Nb) so
as to include a carbide or carbides of one or more element selected
from among V, Ti and Nb in a structure composed primarily of
ferrite grains, and thereby improving the reduction of area and
fatigue crack propagation rate of ferritic steel in a hydrogen
environment.
11. The method of manufacturing a hydrogen fatigue-resistant
ferritic steel of claim 10, wherein the one or more element
selected from among V, Ti and Nb is added in at least the amount
required to fix all carbon (C) in the structure as the carbide or
carbides.
12. The method of manufacturing a hydrogen fatigue-resistant
ferritic steel of claim 10, wherein the ferritic steel includes
carbides of all the elements V, Ti and Nb, and V, Ti and Nb are
added in a collective amount which is substantially the same as the
amount required to fix all carbon (C) in the structure as the
carbides.
13. The method of manufacturing a hydrogen fatigue-resistant
ferritic steel of claim 10, wherein the one or more element
selected from among V, Ti and Nb is added in an amount which is
less than, but at least one-fourth of, the amount required to fix
all carbon (C) in the structure as the carbide or carbides.
14. The method of manufacturing a hydrogen fatigue-resistant
ferritic steel of claim 10, wherein, of the V, Ti and Nb, the only
element to be added is V, said V being added in an amount which is
less than the amount required to fix all carbon (C) in the
structure as a carbide of V.
15. The method of manufacturing a hydrogen fatigue-resistant
ferritic steel of claim 10, wherein, of the V, Ti and Nb, the only
element to be added is V, said V being added in an amount which is
less than, but at least one-fourth of, the amount required to fix
all carbon (C) in the structure as a carbide of V.
Description
TECHNICAL FIELD
[0001] The present invention relates to metal materials used in a
hydrogen environment. More particularly, the invention relates to
ferritic steels having excellent tensile properties and fatigue
properties in a hydrogen environment, and to a method of
manufacture thereof.
BACKGROUND ART
[0002] From a global environmental standpoint, large expectations
are being placed on having hydrogen energy systems such as fuel
cell vehicles and a hydrogen energy infrastructure of hydrogen
stations and the like become a reality. However, metal materials
exposed to hydrogen in a hydrogen atmosphere undergo declines in
tensile properties and fatigue properties due to hydrogen
embrittlement. In particular, given that fatigue failure is
associated with 80% of failure accidents, there is a need to
elucidate the mechanisms for the hydrogen-induced decline in
fatigue properties and to pay very close attention to the fatigue
design of hydrogen-related equipment. In light of the above, to
ensure the safety and reliability of hydrogen energy systems and
infrastructure, there exists a desire for high-performance metal
materials which do not experience hydrogen-induced declines in
tensile properties and fatigue properties.
[0003] For example, at present, only the austenitic stainless steel
SUS316L and the aluminum alloy 6061-T6 have been approved for use
as metal materials exposed to hydrogen in fuel cell vehicles, with
6061-T6 being used as the liner of hydrogen tank and SUS316L being
used in pipes and various types of valves and springs. Titanium
alloys are used in the hydrogen storage vessels disclosed in Patent
Documents 1 and 2. Moreover, austenitic stainless steel is almost
always assumed as the piping material to be used in the hydrogen
pipelines currently being proposed. [0004] Patent Document 1:
Japanese Patent Application Laid-open No. H10-38486 [0005] Patent
Document 2: Japanese Patent Application Laid-open No.
2007-298131
[0006] Today, the metal materials regarded as capable of
withstanding use in a hydrogen environment, including the
above-mentioned austenitic stainless steel, are all very expensive.
Were these to be used in the construction of a hydrogen
infrastructure, the costs calculated based on the amount of such
materials that would be needed in piping would be extremely high.
This has become an obstacle to the construction of a hydrogen
infrastructure. In addition, parts used in a hydrogen environment
end up being expensive, which is a major factor holding back the
popularization of fuel cell vehicles and the like. By contrast,
ferritic steels cost no more than one-tenth as much as austenitic
steel. However, when used in a hydrogen environment, their tensile
properties and fatigue properties are far inferior to those of
austenitic steels, making their use under conditions of exposure to
hydrogen difficult at present.
DISCLOSURE OF THE INVENTION
[0007] This invention was conceived in light of the above
circumstances. The object of the invention is to provide ferritic
steels having tensile properties and fatigue properties capable of
withstanding use in a hydrogen environment, and a method of
manufacturing the same.
[0008] The invention solves the above problems by the following
means.
[0009] By adding one or more element selected from among vanadium
(V), titanium (Ti) and niobium (Nb) so as to include, together with
at least ferrite grains in the structure, a carbide or carbides of
one or more element selected from among V, Ti and Nb, the reduction
of area and fatigue crack propagation rate of ferritic steel in a
hydrogen atmosphere are improved. The advantages of the invention
were confirmed in cases where the ferrite grains are small grains 1
.mu.m or less in size, in cases where the ferrite grains are coarse
grains from several micrometers to 20 .mu.m in size, and in cases
where the ferrite grains are coarse grains from several micrometers
to 60 .mu.m in size.
[0010] The one or more element selected from among V, Ti and Nb is
added in an amount which is preferably at least the amount required
to fix all carbon (C) in the structure as the carbide or carbides
thereof. That is, an amount sufficient to fix all the carbon in the
structure as vanadium carbide, titanium carbide, niobium carbide,
or two or more of these carbides. The amount C* of carbon which can
be fixed as the carbides VC, TiC and NbC having stoichiometric
compositions may be obtained from the following formula, wherein
V.sup.C, Ti.sup.C and Nb.sup.C represent the amounts of the
respective elements which bond with carbon.
C * = ( 12.01 50.94 ) V C + ( 12.01 47.86 ) Ti C + ( 12.01 92.90 )
Nb C = ( 1 4.24 ) V C + ( 1 3.99 ) Ti C + ( 1 7.74 ) Nb C ( 1 )
##EQU00001##
[0011] Here, the units of C*, V.sup.C, Ti.sup.C and Nb.sup.C are
mass %. The atomic weights of C, V, Ti and Nb were taken to be,
respectively, 12.01, 50.94, 47.86 and 92.90.
[0012] In order to fix all the carbon included in the structure,
the following must hold
C<C* (2)
(the units of C here are mass %).
[0013] Therefore, the amounts of addition for the respective
elements are
V=V.sup.C,Ti=Ti.sup.C,Nb=Nb.sup.C (3).
Here, the amounts of V, Ti and Nb are in units of mass %.
[0014] In cases where vanadium has been added, a similar
improvement in the reduction of area performance can be confirmed
even without reaching the amount required to fix all the carbon
(FIG. 14). The experimental examples shown below are examples in
which only one element selected from among V, Ti and Nb has been
added. However, it is of course acceptable for two selected from
among these elements, or for all three, to be added.
[0015] The invention has the effect of improving the tensile
properties and fatigue properties of ferritic steels in a hydrogen
environment, and enabling them to withstand use under circumstances
involving exposure to hydrogen. This makes it possible to markedly
reduce the expenses required for building a hydrogen
infrastructure. Moreover, the invention also makes it possible to
greatly reduce production costs for parts used in a hydrogen
environment, such as hydrogen tank liners, pipelines and various
types of valves and springs used in fuel cell vehicles, thus making
it possible to provide fuel cell vehicles at lower prices. In
addition, the invention makes it possible to hold down considerably
construction costs for hydrogen pipelines.
[0016] At present, SUS316L is used as the material in the pipelines
through which high-pressure hydrogen gas flows at hydrogen stations
for 701 Mpa. SUS316L lines manufactured in accordance with the
High-Pressure Gas Safety Act have the following dimensions: in 1/2
inch pipe, an outside diameter of 12.7 mm and an inside diameter of
3.1 mm; in 3/8 inch pipe, an outside diameter of 10 mm and an
inside diameter of 2.1 mm. In addition, hydrogen gas filling
nozzles have an inside diameter of 1.6 mm. Because of such a small
inside diameter, pressure loss sometimes occurs, causing the flow
rate during filling to become only a fraction of the initial design
value. It is possible to improve the fill rate to some degree by
using SUS316, but at a very high cost. By using the ferritic steel
of the invention as the pipe material, it is possible to construct
a hydrogen station at a much lower cost than at present.
[0017] In recent years, SGP and STPG370 carbon steel pipes have
been investigated as candidate materials for hydrogen pipelines.
However, from the standpoint of properties, the toughness decreases
due to the presence of pearlite in the carbon steel. Moreover, a
problem with pearlite is that it becomes a hydrogen trapping site.
Therefore, pearlite-free ferritic steel, owing to the improved
tensile properties and fatigue properties in a hydrogen
environment, is outstanding as a candidate material for hydrogen
pipelines.
BRIEF DESCRIPTION OF THE DIAGRAMS
[0018] FIG. 1 shows photographs of the structure of S45C, FIG. 1(a)
being a transverse section, and FIG. 1(b) being a longitudinal
section.
[0019] FIG. 2 is a photograph of the structure of the fine-grained
material Ti02-II (with 0.25 mass % Ti addition).
[0020] FIG. 3 is a photograph of the structure of the fine-grained
material V02-II (with 0.27 mass % Ti addition).
[0021] FIG. 4 is a photograph of the structure of the
coarse-grained material V02-I (with 0.2 mass % V addition) which
was annealed at 600.degree. C. for 1 hour.
[0022] FIG. 5 is a photograph of the structure of the
coarse-grained material V04-I (with 0.4 mass % V addition) which
was annealed at 600.degree. C. for 1 hour.
[0023] FIG. 6 is a photograph of the structure of the
coarse-grained material of Nb05-I (with 0.53 mass % Nb addition)
which was annealed at 600.degree. C. for 1 hour.
[0024] FIG. 7 is a photograph of the structure of the
coarse-grained material Ti03-I (with additions of 0.3 mass % Ti and
50 mass ppm boron (B)) which was annealed at 600.degree. C. for 1
hour.
[0025] FIG. 8 shows photographs of the structure of the
coarse-grained material V02-II (with 0.27 mass % V addition) which
was annealed at 700.degree. C. for 1 hour, FIG. 8(a) being a
transverse section and FIG. 8(b) being a longitudinal image.
[0026] FIG. 9 illustrates the relationship between the amount of
absorbed hydrogen content and immersion time in a test specimen in
the form of a round bar of 8 mm diameter, FIG. 9(a) showing the
relationship when the specimen is made of S45C, and FIG. 9(b)
showing the relationship when the specimen is made of the
comparison base steel (fine-grained material) or Ti02-II
(fine-grained material).
[0027] FIG. 10 shows test pieces used in fatigue life tests, FIG.
10(a) being a smooth specimen, and FIG. 10(b) being a notched
specimen.
[0028] FIG. 11 illustrates methods for reducing the stress
intensity factor range .DELTA.K in a fatigue crack propagation
test, FIG. 11(a) being a method for holding the stress ratio R
constant and decreasing both the maximum load (Pmax) and the
minimum load (Pmin), and FIG. 11(b) being a method for holding the
maximum load (Pmax) constant and increasing the minimum load (Pmin)
as the crack propagates.
[0029] FIG. 12 shows test pieces used in fatigue crack propagation
tests, FIG. 12(a) being a compact tension (CT) test piece, and FIG.
12(b) being a plate-type bending specimen.
[0030] FIG. 13 is a fatigue crack propagation test apparatus which
uses a plate-type bending specimen.
[0031] FIG. 14 is a graph showing the relationship between the
relative reduction of area .phi.H/.phi. and the amount of absorbed
hydrogen.
[0032] FIG. 15 is a graph showing the fatigue life properties of an
S45C smooth specimen.
[0033] FIG. 16 is a graph showing the fatigue life properties of an
S45C notched specimen.
[0034] FIG. 17 is a graph showing the fatigue life properties of
the comparison base steel (fine-grained material).
[0035] FIG. 18 is a graph showing the fatigue life properties of
Ti02-II (fine-grained material).
[0036] FIG. 19 is a graph showing the fatigue life properties of
V02-II (fine-grained material).
[0037] FIG. 20 is a graph showing the fatigue life properties of
Nb04-II (fine-grained material).
[0038] FIG. 21 is a graph showing the fatigue life properties of
V005 (fine-grained material).
[0039] FIG. 22 is a graph showing the fatigue life properties of
V007-Nb01-Ti007 (fine-grained material).
[0040] FIG. 23 is a graph showing the fatigue crack propagation
properties of S45C when the stress ratio R was set to 0.1.
[0041] FIG. 24 is a graph showing the fatigue crack propagation
properties of S45C when the stress ratio R was set to 0.5.
[0042] FIG. 25 is a graph showing the fatigue crack propagation
properties of S45C when the stress ratio R was varied.
[0043] FIG. 26 is a graph showing the fatigue crack propagation
properties at the various stress ratios in FIGS. 23 to 25.
[0044] FIG. 27 is a graph showing the fatigue crack propagation
properties of V02-I (fine-grained material).
[0045] FIG. 28 is a graph showing the fatigue crack propagation
properties of V02-II (coarse-grained material: annealed at
700.degree. C. for 1 hour).
[0046] FIG. 29 is a graph showing the fatigue crack propagation
properties of V005 (fine-grained material).
[0047] FIG. 30 is a graph showing the fatigue crack propagation
properties of V007-Nb01-Ti007 (fine-grained material).
[0048] FIG. 31 is a graph showing the relationship between the
fatigue relative crack propagation rate and the cycle speed for
hydrogen-charged material and uncharged material at R=0.5 and
.DELTA.K=10 MPam.sup.1/2.
[0049] FIG. 32 is a graph showing the relationship between the
relative fatigue life and the cycle speed for uncharged material
and hydrogen-charged material at .sigma.a=350 MPa.
BEST MODE FOR CARRYING OUT THE INVENTION
[0050] In the invention, it was discovered that when ferritic steel
to which a trace amount of at least one element selected from among
vanadium (V), titanium (Ti) and niobium (Nb) has been added is
hydrogen-charged then subjected to tensile testing and fatigue
testing, considerable improvements with regard to the effects of
hydrogen on the tensile properties and the fatigue properties are
achieved compared with conventional materials. Preferred modes for
carrying out the invention are described below in detail.
1. Test Materials
[0051] The chemical ingredients in the test materials are shown in
Tables 1 to 4. The balance in all of the materials was iron (Fe)
and inadvertent impurities. Table 1 shows the chemical ingredients
in the common carbon steel S45C for machine structural use which is
used here for the sake of comparison.
TABLE-US-00001 TABLE 1 Chemical ingredients (mass %) in carbon
steel S45C for machine structural use C Si Mn P S Cu Ni Cr S45C
0.47 0.18 0.63 0.014 0.003 0.11 0.1 0.08
[0052] Table 2 shows the chemical ingredients in the comparison
base steel.
TABLE-US-00002 TABLE 2 Chemical ingredients (mass %) in comparison
base steel C Si Mn Cu Al O O--I 0.022 0.28 1.28 0.001 0.0009
0.002
[0053] As shown in Table 3 (Series I) and Table 4 (Series II), the
materials of the invention are ferritic steels in which the base
steel is 0.05C-0.30Si-1.5Mn and to which a trace amount of at least
one element selected from among V, Ti and Nb has been added.
Chemical analysis of all the materials was carried out by
inductively coupled plasma emission spectroscopy. Here, the amounts
of V, Ti and Nb addition are determined based on above formulas (1)
to (3). When the carbon content is 0.05 mass %, the amounts of
these respective elements needed to fix the carbon are 0.212 mass %
of V, 0.199 mass % of Ti, and 0.387 mass % of Nb. These values
indicate the amount required to fix all the carbon when V, Ti or Nb
is added alone. As shown in Table 3, the amount of V addition in
V02-I is 0.2 mass %, the amount of V addition in V04-I is 0.4 mass
%, the amount of Ti addition in Ti03-I is 0.3 mass %, and the
amount of Nb addition in Nb05-I is 0.53 mass %. In V04-I, Ti03-I
and Nb05-I, the amount of these respective elements suffices as the
amount required to fix all the carbon. However, in V02-I, the
amount of vanadium is lower than the amount required to fix all the
carbon. As shown in Table 4, the amount of Ti addition in Ti02-II
is 0.25 mass %, the amount of V addition in V02-II is 0.27 mass %,
and the amount of Nb addition in Nb04-II is 0.45 mass %. In each of
these materials, the amount of these elements suffices as the
amount required to fix all the carbon. Thus, in the Series I
materials V02-I and V04-I shown in Table 3, the V additions are
respectively 0.2 mass % and 0.4 mass %; and in the Series II
material V02-II shown in Table 4, the V addition is 0.27 mass %.
Hence, materials were prepared which contained amounts of these
elements that ranged from less than to about twice as much as the
amount required to fix all the carbon; that is, an addition of
0.212 mass %, as determined based on formulas (1) to (3).
TABLE-US-00003 TABLE 3 Chemical ingredients (mass %) in Series I B
C Si Mn Ti V Nb (ppm) V02-I 0.05 0.30 1.5 0.2 V04-I 0.05 0.30 1.5
0.4 Nb05-I 0.05 0.30 1.5 0.53 Ti03-I 0.05 0.30 1.5 0.3 50
TABLE-US-00004 TABLE 4 Chemical ingredients (mass %) in Series II C
Si Mn P S Ti V Nb Al O Ti02-II 0.05 0.30 1.51 0.001 0.001 0.25
0.006 0.002 V02-II 0.05 0.31 1.51 0.002 0.001 0.27 0.005 0.003
Nb04-II 0.05 0.31 1.52 0.001 0.002 0.45 0.010 0.003
[0054] Table 5 shows the heat treatment conditions and the
thermomechanical treatment conditions for the test materials. As
shown in Table 5(a), S45C used in the experiments below was
obtained by annealing (heated at 845.degree. C. for 30 minutes,
then allowed to cool in air), followed by quenching (so-called
water quenching, which entails heating at 845.degree. C. for 30
minutes followed by cooling in water), then tempering (heated at
550.degree. C. for 60 minutes, then allowed to cool in air).
TABLE-US-00005 TABLE 5 (a) Heat treatment conditions for S45C
Annealing Quenching Tempering 30 minutes at 845.degree. C., 30
minutes at 845.degree. C., 60 minutes at 550.degree. C., cooled in
air cooled in water cooled in air (b) Thermomechanical treatment
conditions for fine-grained material Forging Rolling 60 minutes at
1170.degree. C., 560.degree. C., 95% rolling reduction, cooled in
air cooled in water (c) Thermomechanical treatment conditions for
coarse-grained material Forging Rolling Annealing 60 minutes at
560.degree. C., 95% rolling 60 minutes at 600.degree. C. or 60
1170.degree. C., reduction, minutes at 700.degree. C., cooled in
air cooled in water cooled in air
[0055] In addition, base material as a control and various Series I
and Series II ferritic steels subjected to the thermomechanical
treatment shown in Table 5(b) were also prepared. That is,
treatment entailed 60 minutes of forging at 1170.degree. C.,
followed in turn by cooling in air, rolling at 560.degree. C. and a
rolling reduction of 95%, and cooling in water to form a
fine-grained structure. Ferritic steels subjected to this treatment
are referred to herein as "fine-grained materials." In addition to
fine-grained materials, various Series I and Series II ferritic
steels subjected to the thermomechanical treatment shown in Table
5(c) were prepared. This treatment entailed carrying out the
thermomechanical treatment in Table 5(b) and additionally carrying
out 60 minutes of annealing at 600.degree. C. or 700.degree. C. so
as to obtain a grain size which is about the same as that in
conventional materials. Ferritic steels subjected to this treatment
are referred to herein as "coarse-grained materials."
[0056] The inventors also prepared two test materials having
reduced additions of V, Ti and Nb. The chemical ingredients of
those test materials are shown in Table 6. In both materials, the
balance was iron (Fe) and inadvertent impurities. In the material
V005 shown in Table 6, 0.05 mass % of V has been added. This is
substantially the amount required to fix all carbon (C). In the
material V007-Nb01-Ti007, 0.07 mass % of V, 0.13 mass % of Nb and
0.07 mass % of Ti have been added. That is, V has been added in
about one-third the amount required to fix all the carbon with V
alone, Nb has been added in about one-third the amount required to
fix all the carbon with Nb alone, and Ti has been added in about
one-third the amount required to fix all the carbon with Ti alone.
Collectively, these amounts of addition are approximately the same
as the amount required to fix all the carbon. Fine-grained
materials obtained by subjecting V005 and V007-Nb01-Ti007 to the
thermomechanical treatment in Table 5(b) were prepared.
TABLE-US-00006 TABLE 6 Chemical ingredients (mass %) in
fine-grained material having reduced additions C Si Mn Ti V Nb V005
0.05 0.30 1.5 0.05 V007--Nb01--Ti007 0.05 0.30 1.5 0.07 0.07
0.13
[0057] FIGS. 1 to 4 show photographs of the microstructures. FIG. 1
shows optical micrographs of S45C subjected to the heat treatment
shown in Table 5(a); the structure is tempered martensite. FIG.
1(a) is a transverse section, and FIG. 1(b) is a longitudinal
section. FIGS. 2 and 3 are photographs of the structure of
fine-grained materials, and were obtained by preparation of a
thin-film of the material and observation with a transmission
electron microscope (Hitachi, Ltd.). FIG. 2 is a photograph of the
structure of the fine-grained material Ti02-II (with 0.25 mass % of
Ti addition) shown in Table 4, and FIG. 3 is similarly a photograph
of the structure of the fine-grained material V02-II (with 0.27
mass % of V addition) shown in Table 4. It was possible to confirm
from these photographs that the structure as a whole is a fine
ferrite grain structure, and that the ferrite grains of which the
structure is primarily composed are fine grains having a size of 1
.mu.m or less.
[0058] FIGS. 4 to 7 are photographs of the structure of
coarse-grained materials obtained with one hour of annealing at
600.degree. C., and FIG. 8 shows photographs of the structure of a
coarse-grained material obtained with one hour of annealing at
700.degree. C. Both were examined under an optical microscope after
being corroded with an ordinary Nital solution (3 vol % nitric
acid+ethanol). FIGS. 4, 5, 6 and 7 are respectively photographs of
the structures of the materials V02-I (with 0.2 mass % V addition),
V04-I (with 0.4 mass % V addition), Nb05-I (with 0.53 mass % Nb
addition) and Ti03-I (with 0.3 mass % Ti and 50 mass ppm boron (B)
additions) shown in Table 3. From the photographs in FIGS. 4 to 7,
it was possible to confirm that the structure overall was a
relatively coarse ferrite grain structure, and that the ferrite
grains of which the structure is primarily composed are coarse
grains having a size of from several micrometers to 20 .mu.m. FIG.
8 shows photographs of the structure of the material V02-II (with
0.27 mass % V addition) shown in Table 4, FIG. 8(a) being a
transverse section, and FIG. 8(b) being a longitudinal section. It
was possible to confirm from the photographs in FIG. 8 that the
structure as a whole was a coarse ferrite grain structure, and that
the ferrite grains of which the structure is primarily composed are
coarse grains having a size of from several micrometers to 60
.mu.m.
2. Hydrogen Charging Method
[0059] An immersion charging method was used to hydrogen charge the
specimens. The hydrogen charging conditions used were in general
accordance with the proposed measures being studied for
standardization by the Iron and Steel Institute of Japan and the
Japan Society of Spring Engineers. That is, hydrogen charging was
carried out by immersion in an aqueous solution containing 20 mass
% of ammonium thiocyanate. The temperature of the aqueous solution
was held at 40.degree. C., and the charging time was 48 hours. FIG.
9 shows an example of the relationship between the amount of
absorbed hydrogen and the immersion time in round bars of 8 mm
diameter as the specimens. FIG. 9(a) shows the relationship when
the specimen is made of S45C, and FIG. 9(b) shows the relationship
when the specimen is made of a fine-grained comparison base steel
or of the fine-grained material Ti02-II. Because the amount of
absorbed hydrogen in FIG. 9 reaches saturation in about 15 hours,
the charging time in this invention was set to 48 hours.
3. Tensile Test Method
[0060] The tensile test was carried out using an autoclave having a
maximum capacity of 100 kN (Shimadzu Corporation) and based on JIS
B7721. The test rate was 0.5 mm/min. The specimens were No. JIS14A
bars having a diameter of 5 mm and a gauge length L of 25 mm.
4. Fatigue Test Methods
4-1. Fatigue Life Test
[0061] The fatigue life test was carried out using a hydraulic
servo-type tension-compression fatigue tester (Shimadzu
Corporation) having a maximum capacity of 50 kN, and under
sinusoidal uniaxial loading. The stress ratio R (minimum
stress/maximum stress: .sigma.min/.sigma.max) was -1. The test was
carried out only at a cycle speed of 30 Hz on the uncharged
material, and at three cycle speeds--0.2 Hz, 2 Hz and 30 Hz--on
hydrogen-charged materials. However, in the case of S45C and the
fine-grained comparison base steel, because the fatigue life had a
strong cycle speed dependence, tests at 0.02 Hz were also carried
out. The fatigue tests carried out were primarily low life-side
(high stress-side) tests in which hydrogen release from the
specimen was low. The specimens used in the fatigue life tests are
shown in FIG. 10. These are specimens with shapes that are commonly
used in fatigue life tests. Use was made of both the smooth
specimen shown in FIG. 10(a) and the notched specimen shown in FIG.
10(b). The smooth specimen was a round bar having parallel sections
with a diameter of 8 mm. In the notched specimen, an annular
V-shaped notch with a depth of 1.5 mm was introduced at the center
of a smooth specimen. The stress intensity factor Kt was 3.7.
Finishing treatment at the surface in the test region of the
specimen consisted of axial polishing with No. 600 sandpaper (JIS
R6252).
4-2. Fatigue Crack Propagation Test
[0062] To further clarify the effects of hydrogen on the fatigue
properties, a fatigue crack propagation test was performed on some
of the materials. The fatigue crack propagation test was carried
out using a hydraulic servo-type tension compression fatigue test
having a maximum capacity of 10 kN, and under sinusoidal loading at
a cycle speed of 30 Hz. The two methods shown in FIG. 11 were used
as the methods for reducing the stress intensity factor range
.DELTA.K. One was a method in which, as shown in FIG. 11(a), both
the maximum load (Pmax) and the minimum load (Pmin) are reduced
while keeping the stress ratio R constant (.DELTA.K reducing test
at constant R=0.1, or .DELTA.K reducing test at constant R=0.5).
This method is commonly used. The other is a method in which, as
shown in FIG. 11(b), the maximum load (Pmax) is kept constant and
the minimum load (Pmin) is raised as cracking proceeds (.DELTA.K
reducing test at constant PMax). The .DELTA.K reduction ratio was
set at d.DELTA.K/da=-2 Gpam.sup.1/2 in both tests.
[0063] In the .DELTA.K reducing tests at constant Pmax, when
.DELTA.K decreases as the crack progresses, the stress ratio R
gradually rises. Hence, by setting the initial test conditions to
R.gtoreq.0.5 and .DELTA.K.gtoreq.7 MPam.sup.1/2, complex crack
closing behavior can be avoided until the fatigue crack threshold
.DELTA.K.sub.th is reached. In this way, the influence of hydrogen
on fatigue crack propagation can be clearly understood. To clarify
the cycle speed dependency, 0.2 Hz and 2 Hz tests were also carried
out on Series I and Series II materials having trace element
additions. In these cases, the tests were carried out near
.DELTA.K=10 MPam.sup.1/2 and at R=0.5 and a constant load amplitude
.DELTA.P. The crack lengths were measured at intervals of 0.2 mm or
0.1 mm using both the alternating current potential method and the
compliance method (a method of measuring the crack length from the
output of a strain gauge attached to the back of the specimen). A 1
mm pre-crack was introduced at R=0.1 and .DELTA.K=15
MPam.sup.1/2.
[0064] The specimen had a shape commonly used in fatigue crack
propagation tests. Because the Series I and Series II stock was in
the form of 17 mm square bar, the plate-like bending specimens
having a width of 12 mm and a plate thickness of 10 mm shown in
FIG. 12(b) were used. In cases where plate-like bending specimens
were used, crack propagation tests were carried out using the test
apparatus shown in FIG. 13. On the other hand, because the S45C
stock was large, the compact tension (CT) specimens having a plate
width of 35 mm and a plate thickness of 6 mm shown in FIG. 12(a)
were used. In this case, the load was applied by pin connection.
Some plate-type bending specimen tests were also carried out on
S45C, from which there was confirmed to be no difference with the
results obtained using CT specimens.
4-3. Hydrogen Analysis Method
[0065] Following completion of the fatigue tests, samples were
immediately cut from the test specimens, and the amount of absorbed
hydrogen was measured with a gas chromatograph-type thermal
differential analyzer (TDA). The ramp-up rate was set at
100.degree. C./h up to an ultimate temperature of 600.degree. C.,
and the cumulative amount of hydrogen released up to 500.degree. C.
was treated as the amount of absorbed hydrogen.
5. Test Results
5-1. Tensile Properties
[0066] Table 7(a) shows the tensile test results for the S45C
uncharged material, and Table 7(b) shows the tensile test results
for the S45C hydrogen-charged material. Table 8(a) shows the test
results for Series I (fine-grained) uncharged materials, Table 8(b)
shows the test results for Series I (fine-grained) hydrogen-charged
materials, and Table 8(c) shows the test results for Series I
(fine-grained) hydrogen-charged materials which were 3%
pre-strained. Table 9(a) shows the test results for Series II
(fine-grained) uncharged materials, and Table 9(b) shows the test
results for Series II (fine-grained) hydrogen-charged materials. In
addition, Table 10(a) shows the test results for Series I
(coarse-grained: annealed at 600.degree. C. for 1 hour) uncharged
materials, and Table 10(b) shows the test results for Series I
(coarse-grained material: annealed at 600.degree. C. for 1 hour)
hydrogen-charged materials.
TABLE-US-00007 TABLE 7 (a) S45C Uncharged material 0.2% offset
yield Tensile Reduction Test strength strength Elongation of area
piece (N/mm.sup.2) (N/mm.sup.2) (%) (%) S45C 778 906 20 62
TABLE-US-00008 TABLE 8 (b) S45C Hydrogen-charged material 0.2%
offset yield Tensile Reduction Amount of Test strength strength
Elongation of area hydrogen piece (N/mm.sup.2) (N/mm.sup.2) (%) (%)
(mass ppm) S45C 808 921 18 50 1
TABLE-US-00009 TABLE 8 (a) Series I (fine-grained) uncharged
material 0.2% offset yield Tensile Reduction Test strength strength
Elongation of area piece (N/mm.sup.2) (N/mm.sup.2) (%) (%) V02-I
1003 1007 10 69 V04-I 1017 1060 18 74 Nb05-I 682 860 14 76 Ti03-I
638 786 18 74 (b) Series I (fine-grained) hydrogen-charged material
0.2% offset yield Tensile Reduction Amount of Test strength
strength Elongation of area hydrogen piece (N/mm.sup.2)
(N/mm.sup.2) (%) (%) (mass ppm) V02-I 982 991 13.5 74 1.4 V04-I
1046 1071 14.9 70 3.4 Nb05-I 692 869 12.4 58 4.4 Ti03-I 676 786
12.8 62 3.5 (c) Series I (fine-grained) hydrogen-charged material
that was 3% pre-strained 0.2% offset yield Tensile Reduction Amount
of Test strength strength Elongation of area hydrogen piece
(N/mm.sup.2) (N/mm.sup.2) (%) (%) (mass ppm) V02-I 964 966 12.6 75
1.2 V04-I 1078 1085 14.2 71 4.2 Nb05-I 832 888 9.7 56 4.2 Ti03-I
800 807 9.6 58 3.5
TABLE-US-00010 TABLE 9 (a) Series II (fine-grained) uncharged
material 0.2% offset yield Tensile Reduction Test strength strength
Elongation of area piece (N/mm.sup.2) (N/mm.sup.2) (%) (%) Ti02-II
671 790 20 79 V02-II 946 937 20 76 Nb04-II 772 883 18 74 (b) Series
II (fine-grained) hydrogen-charged material 0.2% offset yield
Tensile Reduction Amount of Test strength strength Elongation of
area hydrogen piece (N/mm.sup.2) (N/mm.sup.2) (%) (%) (mass ppm)
Ti02-II 683 789 18 69 5.37 V02-II 952 946 18 76 2.38 Nb04-II 778
885 17 65 4.31
TABLE-US-00011 TABLE 10 (a) Series I (coarse-grained for 1 hour at
600.degree. C.) uncharged material 0.2% offset yield Tensile
Reduction Test strength strength Elongation of area piece
(N/mm.sup.2) (N/mm.sup.2) (%) (%) V02-I 760 770 26 81 V04-I 808 861
22 77 Nb05-I 683 734 25 77 Ti03-I 455 546 33 78 (b) Series I
(coarse-grained for 1 hour at 600.degree. C.) hydrogen-charged
material 0.2% offset yield Tensile Reduction Amount of Test
strength strength Elongation of area hydrogen piece (N/mm.sup.2)
(N/mm.sup.2) (%) (%) (mass ppm) V02-I 731 746 28 78 1.2 V04-I 838
875 20 74 2.9 Nb05-I 635 737 19 69 2.4 Ti03-I 491 553 31 77 1.3
[0067] The amounts of hydrogen shown in these tables are the
amounts of absorbed hydrogen in the specimens, as measured
following tensile testing. Because the tensile test time is only
about 15 minutes long, hydrogen release during the test is low.
Here, in the test results shown in Table 7, even though hydrogen
charging was carried out on S45C, decreases in the 0.2% offset
yield strength and the tensile strength are not observed. On the
other hand, decreases in the elongation and reduction of area are
observed; in particular, the decrease in the reduction of area was
pronounced. The reduction of area is generally used to assess the
influence of hydrogen on the tensile properties. From the results
in Table 7, it was possible to confirm a decrease in the reduction
of area due to the influence of hydrogen in S45C having no
additions of V, Nb or Ti.
[0068] In the test results shown in Tables 8 to 10, on comparing
uncharged materials with hydrogen-charged materials, there were no
large differences in the 0.2% offset yield strength (stress at the
time of 0.2% plastic deformation) and the tensile strength in any
of the specimens. This was the same as for S45C. However, a
characteristic of Series I and Series II materials is that the
reduction of area in hydrogen-charged materials exhibits no
decrease or decreases only slightly if at all compared with that in
uncharged materials. That is, by adding any one of the elements V,
Nb or Ti to ferritic steel, the influence of hydrogen on the
tensile properties can be decreased.
[0069] FIG. 14 shows the relationship between the relative
reduction of area .phi.H/.phi. and the amount of absorbed hydrogen.
.phi.H is the reduction of area for a hydrogen-charged material,
and .phi. is the reduction of area for an uncharged material.
Results for the carbon steel STPG370, which is regarded as a
candidate material for gas pipelines, have also been included in
the graph for the sake of comparison. In S45C and STPG370, the
relative reduction of area decreases sharply as the amount of
absorbed hydrogen C.sub.H rises. However, in Series I (fine-grained
material, coarse-grained material) and Series II (fine-grained
material) ferritic steels with trace element (V, Nb, Ti) additions,
the decrease in the relative reduction of area is gradual and the
reduction of area performance is greatly improved. It was confirmed
from these results that the addition of a trace amount of V, Ti or
Nb has a desirable effect on recovery from a decline in the
reduction of area (ductility).
[0070] Here, on comparing two materials having 0.20 mass %
additions of V, namely V02-I (fine-grained material) and V02-I
(coarse-grained material), with V02-II (fine-grained material)
having a 0.27 mass % addition of V and V04-I (fine-grained
material) and V04-I (coarse-grained material) having 0.40 mass %
additions of V, it can be confirmed that there are no clear
differences in the relative reduction of area. Hence, it is not
necessarily the case that all the carbon must be fixed in order to
improve the reduction of area and make ferritic steel capable of
withstanding use in a hydrogen atmosphere. While adding sufficient
additive to fix all the carbon is of course acceptable, because V,
Ti and Nb are all expensive, when taking cost into account, it is
desirable to minimize the amounts in which these are used.
5-2. Fatigue Properties
[0071] FIG. 15 shows the relationship between the S--N properties
of a smooth specimen of S45C, i.e., the stress amplitude .sigma.a
(stress amplitude .DELTA..sigma.=(maximum stress .sigma.max-minimum
stress .sigma.min).times.1/2), and the failure life. In the
diagram, the residual amount of hydrogen (mass ppm) measured in a
sample cut from the fatigue failure specimen is also shown. Because
the testing time extends to 66 hours at the lowest cycle speed 0.02
Hz, the residual amount of hydrogen in this case was only 0.42 mass
ppm. However, in high-speed tests at 0.2 Hz and 2 Hz, the residual
amounts of hydrogen were respectively 0.71 mass ppm and 1.03 mass
ppm, indicating that much hydrogen remained. From FIG. 15,
regardless of the cycle speed, the failure life of the
hydrogen-charged material agreed substantially with the results for
the uncharged material, and so no influence by hydrogen was
observable.
[0072] FIG. 16 shows the S--N properties of S45C notched specimens.
In this case as well, with the exception of the lowest cycle speed
0.02 Hz, much hydrogen remained. When the fatigue lives of
hydrogen-charged materials at test rates of 0.2 Hz, 2 Hz and 30 Hz
were compared with the results for the respective uncharged
materials, the fatigue life of the hydrogen-charged material became
progressively shorter at lower cycle speeds. In this diagram, the
fatigue life at a cycle speed of 30 Hz was from about 15,000 to
about 35,000 cycles, the fatigue life at 2 Hz was about 6,000
cycles, and the fatigue life at 0.2 Hz was about 2,000 cycles. The
fatigue lives do not shorten from 0.2 Hz to 0.02 Hz, indicating
that saturation occurs once the fatigue life decreases to about 6
to 13% of the fatigue life at 30 Hz.
[0073] Summarizing the results in FIGS. 15 and 16, in ordinary
materials such as S45C, the S--N properties (fatigue crack
generation) obtained with a smooth specimen do not incur a hydrogen
influence, but the S--N properties obtained with a notched specimen
(fatigue crack propagation) do incur a hydrogen influence. Hence,
the S--N properties obtained with a notched specimen were examined
in fine-grained materials. As shown in FIG. 17, in the comparison
base steel, the fatigue life at a cycle speed of 30 Hz was about
30,000 to 100,000 cycles, the fatigue life at 2 Hz was about 12,000
cycles, and the fatigue life at 0.2 Hz was about 6,000 cycles. The
fatigue life did not shorten from 0.2 Hz to 0.02 Hz, indicating
that saturation occurs once the fatigue life decreases to about 6
to 20% of the life at 30 Hz.
[0074] As shown in FIG. 18, in the Series II material Ti02-II
(fine-grained material) with Ti addition, the fatigue life at a
cycle speed of 30 Hz is from about 80,000 to about 300,000 cycles,
the fatigue life at 2 Hz is about 50,000 cycles, and the fatigue
life at 0.2 Hz is about 45,000 cycles. Here, the fatigue life
decreases to about 15 to 56% of the fatigue life at 30 Hz, although
the degree of decrease is more gradual than in the above-described
S45C and comparison base steel. That is, Ti02-II has a fatigue life
(the ratio of the life at 0.2 Hz to the life at 30 Hz) which is
from 2.5 to 4.3 times that of S45C, and from 2.5 to 2.8 times that
of the comparison base steel.
[0075] As shown in FIG. 19, in the Series II material V02-II
(fine-grained material) with V addition, the fatigue life at a
cycle speed of 30 Hz is about 30,000 to 90,000 cycles, the fatigue
life at 2 Hz is about 10,000 cycles, and the fatigue life at 0.2 Hz
is about 8,000 cycles. Here, the fatigue life decreases to about 9
to 27% of the fatigue life at 30 Hz. That is, V02-II has a fatigue
life (the ratio of the life at 0.2 Hz to the life at 30 Hz) which
is from 1.5 to 2.1 times that of S45C, and about 1.5 times that of
the comparison base steel. As shown in FIG. 20, in the Series II
material Nb04-II (fine-grained material) with Nb addition, the
fatigue life at a cycle speed of 30 Hz is about 90,000 to 350,000
cycles, the fatigue life at 2 Hz is about 40,000 cycles, and the
fatigue life at 0.2 Hz is about 50,000 cycles. Here, the fatigue
life decreases to about 12 to 56% of the life at 30 Hz. That is,
Nb02-II has a fatigue life (the ratio of the life at 0.2 Hz to the
life at 30 Hz) which is from 2 to 4.3 times that of S45C, and about
2 to 2.8 times that of the comparison base steel. These results
indicate that, in fine-grained materials with trace additions of
Ti, V or Nb, the hydrogen-induced fatigue crack propagation
properties improve.
[0076] As shown in FIG. 21, with regard to V005 (fine-grained
material), the fatigue life at a cycle speed of 30 Hz is about
20,000 cycles, the fatigue life at 2 Hz is about 8,000 cycles, and
the fatigue life at 0.2 Hz is about 6,000 cycles. Here, the fatigue
life decreases to about 30% of the life at 30 Hz. That is, V0005
has a fatigue life (the ratio of the life at 0.2 Hz to the life at
30 Hz) which is from 2.3 to 5 times that of S45C, and from about
1.5 to about 5 times that of the comparison base steel. As
indicated by these results, even when V is added in about
one-fourth the amount required to fix all the carbon, the
hydrogen-induced fatigue crack propagation properties can be
improved.
[0077] As shown in FIG. 22, with regard to V007-Nb01-Ti007
(fine-grained material), the fatigue life at a cycle speed of 30 Hz
is about 30,000 cycles, the fatigue life at 2 Hz is about 15,000
cycles, and the fatigue life at 0.2 Hz is about 12,000 cycles.
Here, the fatigue life decreases to about 60% of the life at 30 Hz.
That is, this material has a fatigue life (the ratio of the life at
0.2 Hz to the life at 30 Hz) which is from 4.6 to 10 times that of
S45C, and from about 3 to about 10 times that of the comparison
base steel. As indicated by these results, when all three elements
V, Nb and Ti are added and the collective amount of these elements
is about the same as the amount required to fix all the carbon,
extremely good fatigue crack propagation properties were
obtained.
[0078] Based on the above results, the influence of hydrogen on the
fatigue crack propagation properties were carefully studied. FIGS.
23 to 28 show the fatigue crack propagation properties, i.e., the
relationship between the fatigue crack propagation rate da/dN
(mm/cycle) and the stress intensity factor range .DELTA.K
(MPam.sup.1/2). The fatigue crack propagation properties of S45C
are shown in FIGS. 23 to 26, those of the Series I material V02-I
(fine-grained material) are shown in FIG. 27, and those of the
Series II material V02-II (coarse-grained material: annealed one
hour at 700.degree. C.) are shown in FIG. 28. Also, the fatigue
crack propagation properties of V005 (fine-grained material) are
shown in FIG. 29, and those of V007-Nb01-Ti007 (fine-grained
material) are shown in FIG. 30. In each of these figures, the
amount of absorbed hydrogen (mass ppm) measured after testing is
shown below the fatigue crack propagation curves for the
hydrogen-charged materials. The testing time was set to 30 hours or
less in order to minimize hydrogen release during the test. Hence,
the influence of hydrogen release during testing on the fatigue
crack propagation properties was small.
[0079] FIG. 23 shows fatigue crack propagation curves for uncharged
material and hydrogen-charged material when the stress ratio R is
0.1. The difference between the two curves can be clearly
distinguished. In particular, when .DELTA.K=7.0 MPam.sup.1/2,
because the uncharged material has a da/dN of about
9.times.10.sup.-8 (mm/cycle) and the hydrogen-charged material has
a da/dN of about 3.times.10.sup.-6 (mm/cycle), the crack
propagation rate da/dN of the hydrogen-charged material can be seen
to represent an up to 30-fold acceleration over that in uncharged
material. Also, as shown in FIGS. 24, 25 and 26, even in cases
where differing stress ratios R were used, the fatigue crack
propagation curves for uncharged material and hydrogen-charged
material can be clearly distinguished; the crack propagation rate
da/dN of the hydrogen-charged materials was found to represent an
acceleration of at least 10-fold compared with uncharged
materials.
[0080] With regard to the Series I material V02-I (fine-grained
material) shown in FIG. 27, the fatigue crack propagation
properties of the uncharged material and the hydrogen-charged
material are very similar to each other, making it impossible to
distinguish between the fatigue crack propagation curves for both.
That is, even in cases where hydrogen charging was carried out,
substantially no acceleration of da/dN arose. Also, with regard as
well to the Series II material V02-II (coarse-grained material:
annealed one hour at 700.degree. C.) shown in FIG. 28, the fatigue
cracking propagation properties of the uncharged material and the
hydrogen-charged material were very similar, making it impossible
to distinguish between the fatigue crack propagation curves for
both. Hence, even when hydrogen charging was carried out,
substantially no acceleration of da/dN arose.
[0081] FIG. 29 shows the fatigue crack propagation properties of
V005 (fine-grained material) in which the amount of V addition has
been reduced. When the stress ratio R is 0.1, the fatigue crack
propagation curves for uncharged material and hydrogen-charged
material can be said to generally coincide. On the other hand, when
R=0.6 to 0.9, the two curves separate at and above .DELTA.K=5
MPam.sup.1/2, but below .DELTA.K=5 MPam.sup.1/2 the two curves
generally coincide and substantially no acceleration of fatigue
crack propagation due to hydrogen charging is observable.
Therefore, even with regard to the material V005, in which the
amount of V addition was set at one-fourth of the amount required
to fix all the carbon, it was possible to confirm effects due to V
addition. However, at R=0.6 and above and at .DELTA.K=5
MPam.sup.1/2 and above, such effects cease to be clear and so it
may be regarded as preferable to add one or more element selected
from among V, Nb and Ti in at least one-fourth the amount required
to fix all the carbon.
[0082] FIG. 30 shows the fatigue crack propagation properties of
V007-Nb01-Ti007 (fine-grained material). Regardless of the stress
ratio R, the fatigue crack propagation curves for the uncharged
material and the hydrogen-charged material are very similar, with
no separation being apparent. From these results, it is clearly
advantageous to add all of the elements V, Nb and Ti and to have
the collective amount of these be substantially the same as the
amount required to fix all the carbon.
[0083] Based on the experimental results in FIGS. 23 to 28 and also
the results of additional experiments that were carried out, FIG.
31 shows the relationship between the fatigue relative crack
propagation rate for hydrogen-charged materials and uncharged
materials between the cycle speed f at R=0.5 and .DELTA.K=10
MPam.sup.1/2. The ordinate represents the fatigue relative crack
propagation rate (da/dN).sub.H/(da/dN). Here, (da/dN).sub.H is the
fatigue crack propagation rate of the hydrogen-charged material,
and da/dN is the fatigue crack propagation rate of the uncharged
material. The abscissa represents the cycle speed f (Hz). The
fatigue relative crack propagation rate of S45C (indicated by the
symbol .omicron. in FIG. 31) is about 30 at a cycle speed f=0.2 Hz,
about 30 at f=2 Hz, and about 4 at f=30 Hz. With hydrogen charging,
the fatigue crack propagation rate is accelerated up to
30-fold.
[0084] In the case of the comparison base steel (fine-grained
material) (.DELTA. in FIG. 31), the fatigue relative crack
propagation rate is about 5 at the cycle speed f=0.2 Hz, about 4 at
f=2 Hz, and about 3 at f=30 Hz. However, with regard to the
fine-grained material V02-I V (.left brkt-top. in FIG. 31) and the
coarse-grained material V02-II (.diamond. in FIG. 31), each of
which has a trace amount of V addition, in both of these cases, the
fatigue relative crack propagation rate is about 2 at the cycle
speed f=0.2 Hz, about 2 at f=2 Hz, and about 1 at f=30 Hz,
demonstrating that the hydrogen-induced acceleration of fatigue
crack propagation rate can be greatly ameliorated due to a trace
amount of V addition. Moreover, from the results for the
fine-grained material V02-I (in FIG. 31), sufficient improvement
effects can be obtained, even with regard to the fatigue crack
propagation rate, by adding a smaller amount of V than the amount
V.sup.C required to fix the carbon. In other words, it is not
necessarily essential to fix all the carbon in order to improve the
fatigue crack propagation rate.
[0085] With regard to V005 (fine-grained material), as shown in
FIG. 31, the fatigue relative crack propagation rate is about 3.5
at a cycle speed f=0.2 Hz, is about 3 at f=2 Hz, and is about 2.8
at f=30 Hz. Hence, compared with the comparison base steel
(fine-grained material), it was possible to ameliorate
hydrogen-induced acceleration of the fatigue crack propagation
rate. However, the effects compared with the fine-grained material
V02-I (.left brkt-top. in FIG. 31) and the coarse-grained material
V02-II (.diamond. in FIG. 31) are limited and, as can be seen also
by comparison with other results presented in the same graph, when
the amount of V addition is reduced even further from that in V005,
advantageous effects cease to be observable relative to the
comparison base steel (fine-grained material). Based on these
results as well, it can be said that the lower limit in the amount
of addition for obtaining a fatigue crack propagation
acceleration-ameliorating effect is about one-fourth of the amount
required to fix all the carbon. With regard to the fine-grained
material V007-Nb01-Ti007, this exhibits properties comparable to or
better than those of the fine-grained material V02-I (.quadrature.
n FIG. 31) and the coarse-grained material V02-II (.diamond. in
FIG. 31), and so fatigue crack propagation rate-ameliorating
effects from adding all of elements V, Nb and Ti and setting the
collective amount of these elements to about the same amount as
that required to fix all the carbon were observed.
[0086] FIG. 32 is a graph showing the relationship between the
relative fatigue life Nf/(Nf).sub.H at .sigma.a=350 MPa and the
cycle speed f, based on the S--N curves for notched specimens
(FIGS. 16 to 20). Here, Nf is the number of cycles to failure of
the uncharged material, and (Nf).sub.H is the number of cycles to
failure of the hydrogen-charged material. When specimens having a
sharp notch (Kt=3.7) are used, the relative fatigue life of S45C is
about 10 at a cycle speed f=0.2 Hz, about 3.5 at f=2 Hz, and about
1.2 at f=30 Hz. The relative fatigue life of the comparison base
steel is about 5 for a cycle speed f=0.2 Hz, about 3 for f=2 Hz,
and about 1.1 for f=30 Hz.
[0087] In the case of the fine-grained material Ti02-II with a
trace amount of Ti addition, the relative fatigue life is about 1.2
at a cycle speed f=0.2 Hz, about 1.0 at f=2 Hz, and about 0.6 at
f=30 Hz. In the case of the fine-grained material V02-II with a
trace amount of V addition, the relative fatigue life is about 2.3
at a cycle speed f=0.2 Hz, about 1.9 at f=2 Hz, and about 0.9 at
f=30 Hz. Hence, the ratio Nf/(Nf).sub.H and the ratio
(da/dN).sub.H/(da)/(dN) shown in FIG. 31 substantially coincide. In
the case of the fine-grained material Nb04-II with a trace amount
of Nb addition, the relative fatigue life is about 1.3 at a cycle
speed f=0.2 Hz, about 1.9 at f=2 Hz, and about 0.9 at f=30 Hz. From
these results, not only in cases where a trace amount of V has been
added, but also in cases where a trace amount of Ti or Nb has been
added, there can be said to be an ameliorating (i.e., suppressing)
effect on the hydrogen-induced acceleration of fatigue crack
propagation.
[0088] With regard to the fine-grained material V005, as shown in
FIG. 32, compared with the comparison base steel (fine-grained
material), this has a slight relative fatigue life-improving
effect. Therefore, when trying to improve the relative fatigue
life, it may regarded as essential to set the amount of addition to
at least about one-fourth the amount required to fix all the
carbons. At the same time, the fine-grained material
V007-Nb01-Ti007 exhibits properties comparable to or better than
those of the fine-grained material V02-II (.diamond. in FIG. 32),
from which it was possible to confirm that adding all the elements
V, Nb and Ti and setting the collective amount of addition thereof
to about the same amount as that required to fix all the carbon has
a relative fatigue life-improving effect.
INDUSTRIAL APPLICABILITY
[0089] The invention provides ferritic steels which are capable of
being used under a hydrogen atmosphere. The ferritic steels of the
invention can be employed as structural materials in hydrogen
energy systems such as fuel cell vehicles, and in hydrogen energy
infrastructure such as hydrogen stations.
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