Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02861740 2014-06-26
DESCRIPTION
TITLE OF THE INVENTION
LOW ALLOY STEEL
TECHNICAL FIELD
[Noll
The present invention relates to a low alloy steel. More particularly, it
relates to a low alloy steel in which a weld heat affected zone that has been
subjected to postweld heat treatment has excellent resistance to embrittlement
attributable to hydrogen, such as stress corrosion cracking in wet hydrogen
sulfide environments.
BACKGROUND ART
[0002]
In the development of submarine oilfields, a steel pipe called a riser,
flowline, or trunkline is used for transmission of crude oil or natural gas
between an oil well or gas well located at the bottom of the sea and a
platform
on the sea or between the platform and a refinery station on the land. On the
other hand, with the worldwide exhaustion of fossil fuels, oil fields
containing
much hydrogen sulfide having corrosiveness have been developed actively. A
steel pipe for transmitting crude oil or natural gas exploited from oil fields
containing such a corrosive gas is sometimes broken by embrittlement
attributable to hydrogen formed from a corrosion reaction called hydrogen
induced cracking (hereinafter, referred to as "HIC") and sulfide stress
cracking
(hereinafter, referred to as "SSC"). Many steels developed from the viewpoint
of improving the HIC resistance and SSC resistance have traditionally been
proposed.
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[0003]
For example, Patent Document 1 (JP5-255746A) proposes a steel
provided with excellent HIC resistance by defining the heat history and heat
treatment conditions at the production time without substantially containing
Ni, Cu and Ca. Also, Patent Document 2 (JP6-336639A) proposes a steel
provided with HIC resistance and SSC resistance by essentially adding Cr, Ni
and Cu. Further, Patent Document 3 (JP2002-60894A) proposes a steel in
which the HIC resistance and SSC resistance are enhanced by defining the
specific ranges of amounts of C, Ti, N, V and 0.
[0004]
When a structure is assembled by using any of these steels, for example,
when a steel pipe consisting of any of these steels is laid, welding work is
generally performed. Unfortunately, for example, as described in non-Patent
Document 1, it is widely known that the SSC susceptibility is increased by the
increase in hardness. When a steel undergoes heating due to welding, a
hardened portion is produced in a so-called weld heat affected zone
(hereinafter, referred to as a "HAZ"). As a result, however much the HIC
resistance and SSC resistance of the steel itself are enhanced, practically
sufficient performance of a welded structure cannot be achieved in many cases.
[0005]
Therefore, in recent years, as described in Patent Document 4 (JP2010-
24504A), there has also been proposed a high-strength steel in which, by
reducing the amounts of C and Mn and by containing 0.5% or more of Mo, the
hardening of weld heat affected zone is restrained, and both of HIC resistance
and SSC resistance of base metal and HAZ are achieved.
[0006]
As a method for reducing the hardness of weld heat affected zone,
postweld heat treatment (hereinafter, referred to as "PWHT") is widely used
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for a Cr-Mo steel or martensitic stainless steel used for pressure vessels and
the like in large amounts. For example, Patent Document 5 (JP2007-
321228A) proposes a low alloy steel containing 0.5% or more of Cr assuming
that PWHT of one hour per one-inch wall thickness.
LIST OF PRIOR ART DOCUMENT(S)
[0007]
[Patent Document 1] JP5-255746A
[Patent Document 21 JP6-336639A
[Patent Document 31 JP2002-60894A
[Patent Document 4] JP2010-24504A
[Patent Document 51 JP2007-321228A
[0008]
[Non-Patent Document 1] Masanori Kowaka, Corrosion damage and
anticorrosion engineering of metal, August 25, 1983, issued by Agne
Corporation, p.198
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0009]
According to the invention described in Patent Document 4, it is
described that the hardening of weld heat affected zone is restrained, and
both
of HIC resistance and SSC resistance of base metal and HAZ can be achieved.
However, since Mo is an expensive element, there has been desired a method
for improving hydrogen embrittlement resistance of HAZ without requiring
much cost.
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[0010]
As described in Patent Document 5, PWHT brings about a certain effect.
However, since importance is attached to efficiency in laying line pipes so
that
welding work is performed, for example, on a ship on the sea, it is generally
desirable that PWHT be eliminated, or, even if being performed, the PWHT be
performed for a very short period of time.
[0011]
An objective of the present invention is to provide a low alloy steel in
which a HAZ subjected to PWHT, especially short-time PWHT, has excellent
hydrogen embrittlement resistance in wet hydrogen sulfide environments or
the like.
MEANS FOR SOLVING THE PROBLEMS
[0012]
To enhance the hydrogen embrittlement resistance of HAZ of a steel
subjected to PWHT, the present inventors first examined hydrogen
embrittlement of as-welded HAZ to clarify necessary conditions. As a result,
it is considered that the hydrogen embrittlement of HAZ is produced by the
mechanism described below.
[0013]
In the case where a steel is exposed to a corrosive environment
containing hydrogen sulfide, hydrogen intrudes into the steel on account of
corrosion reaction. This hydrogen can move freely in the crystal lattice of
the
steel. This hydrogen is so-called diffusible hydrogen. The
intruding
diffusible hydrogen accumulates in a dislocation or a vacancy, which is one
kind of defects in the crystal lattice, and further, at the lattice strain of
interface between a carbide such as cementite and a matrix to embrittle the
steel. In particular, the HAZ is heated to a high temperature by the heat
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history of welding, being cooled rapidly, and becomes an as-quenched
martensite or bainite structure. Therefore, in the HAZ, the dislocations and
vacancies in which hydrogen is trapped exist densely as compared with a
thermally refined base metal, and cementite also disperses. For this reason,
it is considered that the HAZ is highly susceptible to hydrogen embrittlement
as compared with the base metal.
[0014]
In the case where PWHT is performed, the density of dislocations or
vacancies is reduced, and softening advances, and on the other hand,
cementite precipitates. Therefore, especially in the case where sufficient
softening does not occur due to short-time PWHT, it is considered that the
reduction effect of hydrogen embrittlement susceptibility is not high because
of
a trade-off with the precipitation of cementite.
[0015]
Accordingly, in order to enhance the hydrogen embrittlement resistance
of HAZ to which PWHT has been applied, the present inventors attempted to
optimize the alloying elements. As a result, it was found that, in order to
enhance the hydrogen embrittlement susceptibility of HAZ to which PWHT
has been applied, it is effective to contain one or more kinds of any of Ti, V
and
Nb. The reason for this is considered to be as follows.
[0016]
Each of the elements of Ti, V and Nb has a high affinity for carbon as
compared with iron, and therefore forms fine MX-type carbides in the process
of PWHT. The MX-type carbides have a high consistency with a parent phase
as compared with cementite, so that the lattice strain of interface with the
matrix is small, and the amount of occlusion of diffusible hydrogen in the
carbides is large. Therefore, it is considered that, when hydrogen intrudes
due to corrosion reaction, the accumulation site of diffusible hydrogen is
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dispersed, whereby remarkable hydrogen accumulation and formation of
embrittlement starting point due to this accumulation are restrained, and
embrittlement is resultantly alleviated.
[0017]
It was verified that a proper amount of Ti, V and Nb must be contained
as the amount of C increases, that is, as the hardenability of HAZ at the
cooling time of welding is higher, as the density of dislocations or vacancies
is
higher, and as the driving force of precipitation of cementite at the
application
time of PWHT is higher. Specifically, it was verified that one or more kinds
selected from Ti, V and Nb must be contained in the range satisfying Formula
(1):
0.1 x [C(%)] [Ti(%)] + [V(%)] + 0.5 x [Nb(%)] 0.2 ... (1)
where, the symbol of element in the formula represents the content (mass%) of
each element.
[0018]
The present invention has been made based on the above-described
findings, and the gist thereof is low alloy steels described in the following
items [1] to [61.
[0019]
[1] A low alloy steel subjected to post weld heat treatment, containing,
by mass percent, of
C: 0.01 to 0.15%,
Si: 3% or less,
Mn: 3% or less, and
Al: 0.08% or less,
one or more kinds of elements selected from Ti, V and Nb: the range satisfying
Formula (1), and
the balance being Fe and impurities,
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wherein in the impurities,
N: 0.01% or less,
P: 0.05% or less,
S: 0.03% or less, and
0: 0.03% or less:
0.1 x [C(%)] [Ti(%)] + [v(%)] + 0.5 x [Nb(%)] 5 0.2 ... (1)
where, the symbol of element in the formula represents the content (mass%) of
each element.
[0020]
[2] The low alloy steel described in item [1], wherein the low alloy steel
contains, by mass percent, Cr and/or Mo: 1.5% or less in total in lieu of a
part
of Fe.
[0021]
[3] The low alloy steel described in item [1] or [2], wherein the low alloy
steel contains, by mass percent, Ni and/or Cu: 0.8% or less in total in lieu
of a
part of Fe.
[0022]
[4] The low alloy steel described in any one of items [1] to [3], wherein
the low alloy steel contains, by mass percent, Ca and/or Mg: 0.05% or less in
total in lieu of a part of Fe.
[0023]
[5] The low alloy steel described in any one of items [1] to [4], wherein
the low alloy steel contains, by mass percent, B: the range satisfying Formula
(2) in lieu of a part of Fe:
[B(%)] <0.1 x [C(%)] ... (2)
where, the symbol of element in the formula represents the content (mass%) of
each element.
[0024]
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[6] The low alloy steel described in any one of items [1] to [5], wherein
post weld heat treatment is performed under the condition satisfying Formula
(3):
8000 T x {20 + log(t/3600)} 5_ 15000 ... (3)
where, T is treatment temperature ( C) of postweld heat treatment, and t is
treatment time (sec) of postweld heat treatment.
ADVANTAGEOUS EFFECT(S) OF THE INVENTION
[0025]
According to the present invention, there can be provided a low alloy
steel in which a HAZ subjected to PWHT, especially short-time PWHT, has
excellent hydrogen embrittlement resistance in wet hydrogen sulfide
environments or the like.
MODE FOR CARRYING OUT THE INVENTION
[0026]
Hereunder, the range of chemical composition of the low alloy steel in
accordance with the present invention and the reason for restricting the
chemical composition are explained. In the following explanation, "%"
representing the content of each element means "mass%".
[0027]
C: 0.01 to 0.15%
C (carbon) is an element effective in enhancing the hardenability of steel
and increasing the strength thereof. In order to achieve these effects, 0.01%
or more of C must be contained. However, if the content of C exceeds 0.15%,
when PWHT is performed, a large amount of cementite is precipitated, and the
hydrogen embrittlement susceptibility of HAZ is enhanced. Therefore, the C
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content is set to 0.01 to 0.15%. The lower limit of the C content is
preferably
0.03%. The C content is preferably 0.12% or less.
[0028]
Si: 3% or less
Si (silicon) is an element effective for deoxidation, but brings about a
decrease in toughness if being contained excessively. Therefore, the Si
content is set to 3% or less. The Si content is preferably 2% or less. The
lower limit of the Si content is not particularly defined; however, even if
the Si
content is decreased, the deoxidizing effect decreases, the cleanliness of
steel is
deteriorated, and an excessive decrease in the Si content leads to an increase
in production cost. Therefore, the Si content is preferably 0.01% or more.
[0029]
Mn: 3% or less
Like Si, Mn (manganese) is an element effective for deoxidation, and
also is an element contributing to the enhancement of hardenability of steel
and to the increase in strength thereof. However, if Mn is contained
excessively, remarkable hardening of HAZ is caused, and the hydrogen
embrittlement susceptibility is enhanced. Therefore, the Mn content is set to
3% or less. The lower limit of the Mn content is not particularly defined;
however, in order to achieve the strength increasing effect of Mn, 0.2% or
more
of Mn is preferably contained. The lower limit thereof is further preferably
0.4%, and the preferable upper limit thereof is 2.8%.
[0030]
Al: 0.08% or less
Al (aluminum) is an element effective for deoxidation, but if being
contained excessively, the effect is saturated, and also the toughness is
decreased. Therefore, the Al content is set to 0.08% or less. The Al content
is preferably 0.06% or less. The lower limit of the Al content is not
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particularly defined; however, an excessive decrease in the Al content does
not
sufficiently achieve the deoxidizing effect, deteriorates the cleanliness of
steel,
and also increases the production cost. Therefore, 0.001% or more of Al is
preferably contained. The Al content in the present invention means the
content of acid soluble Al (so-called "sol.AI").
[0031]
One or more kinds selected from Ti (titanium), V (vanadium) and Nb
(niobium): in the range satisfying Formula (1):
0.1 x [C(%)] [Ti(%)] + [V(%)] + 0.5 x [Nb(%)1 0.2 ... (1)
where, the symbol of element in the formula represents the content (mass%) of
each element.
These elements form fine MX-type carbides in the process of PWHT, and
enhance the hydrogen embrittlement resistance. In order to achieve this
effect, "[Ti(%)] + [V(%)] + 0.5 x [Nb(%)1" must be 0.1 x [C(%)] or more.
However, if the content of these elements is excessively high, the carbides
are
coarsened, and rather the hydrogen embrittlement susceptibility is enhanced
and the toughness is decreased. Therefore, "[Ti(%)] + [V(%)] + 0.5 x [Nb(%)1"
must be 0.2% or less. The upper limit thereof is preferably 0.18%, further
preferably 0.15%.
[0032]
The low alloy steel in accordance with the present invention contains the
above-described elements, and the balance consists of Fe and impurities. The
"impurities" mean components that are mixed on account of various factors
including raw materials such as ore or scrap when a steel material is produced
on an industrial scale. Of the impurities, concerning the elements described
below, the content thereof must be restricted stringently.
[0033]
N: 0.01% or less
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N (nitrogen) exists in the steel as an impurity. Nitrogen produces
embrittlement when fine carbo-nitrides are formed, and decreases the
toughness even when being dissolved. Therefore, the N content must be
restricted to 0.01% or less. The N content is preferably 0.008% or less. The
lower limit of the N content is not particularly defined; however, an
excessive
decrease in the N content leads to a remarkable increase in production cost.
Therefore, the lower limit of the N content is preferably 0.0001%.
[0034]
13; 0.05% or less
P (phosphorus) exists in the steel as an impurity. Phosphorus
segregates at grain boundaries in HAZ, and decreases the toughness.
Therefore, the P content is restricted to 0.05% or less. The lower limit of
the
P content is not particularly defined; however, an excessive decrease in the P
content leads to a remarkable increase in production cost. Therefore, the
lower limit of the P content is preferably 0.001%.
[0035]
S: 0.03% or less
Like P, S (sulfur) exists in the steel as an impurity. Sulfur forms
sulfides in a steel material, and since the interface with a matrix acts as an
accumulation site of hydrogen, S enhances the hydrogen embrittlement
susceptibility, and also decreases the HAZ toughness. Therefore, the S
content is restricted to 0.03% or less, more severely than P. The lower limit
of
the S content is not particularly defined; however, an excessive decrease in
the
S content leads to a remarkable increase in production cost. Therefore, the
lower limit of the S content is preferably 0.0001%.
[0036]
0; 0.03% or less
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=
0 (oxygen) exists in the steel as an impurity. If much 0 is contained,
large amounts of oxides are formed, and the workability and ductility are
deteriorated. Therefore, the 0 content must be set to 0.03% or less. The 0
content is preferably 0.025% or less. The lower limit of the 0 content need
not particularly be defined; however, an excessive decrease in the 0 content
leads to a remarkable increase in production cost. Therefore, the 0 content is
preferably 0.0005% or more.
[0037]
The low alloy steel in accordance with the present invention may contain
the elements described below in lieu of a part of Fe.
[0038]
Cr and/or Mo: 1.5% or less in total
At least one of Cr (chromium) and Mo (molybdenum) may be contained
because these elements enhance the hardenability and contribute to the
improvement in strength. However, if the contents thereof are excessively
high, these elements precipitate as carbides to hinder the carbides of Ti and
the like and to enhance the hydrogen embrittlement susceptibility. Therefore,
if Cr and/or Mo are contained, the contents thereof are set to 1.5% or less in
total. The lower limit of the contents of Cr and/or Mo is preferably 0.02%,
further preferably 0.05%. The upper limit thereof is preferably 1.2%.
[0039]
Ni and/or Cu: 0.8% or less in total
At least one of Ni (nickel) and Cu (copper) may be contained because
these elements enhance the hardenability and contribute to the improvement
in strength. However, even if these elements are contained excessively, not
only the effects are saturated, but also the cost is increased. Therefore, if
Ni
and/or Cu are contained, the contents thereof are set to 0.8% or less in
total.
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The lower limit of the contents of Ni and/or Cu, if added, is preferably
0.02%,
further preferably 0.05%. The upper limit thereof is preferably 0.7%.
[0040]
Ca and/or Mg: 0.05% or less in total
At least one of Ca (calcium) and Mg (magnesium) may be contained
because these elements improve the hot workability of steel. However, if the
contents thereof are excessively high, these elements combine with oxygen to
remarkably decrease the cleanliness, so that the hot workability may rather be
deteriorated. Therefore, if at least one kind of these elements is contained,
the contents thereof are set to 0.05% or less in total. The lower limit of the
contents of Ca and/or Mg is preferably 0.0005%, further preferably 0.001%.
The upper limit thereof is preferably 0.03%.
[0041]
B: in the range satisfying Formula (2)
[B(%)] <0.1 x [C(%)] ... (2)
where, the symbol of element in the formula represents the content (mass%) of
each element.
B (boron) may be contained because it segregates at the grain
boundaries, so that it restrains the precipitation of ferrite from the grain
boundaries, thereby enhancing the hardenability indirectly, and contributes to
the improvement in strength. However, if B is contained excessively, in the
process of PWHT, B precipitates as borides or is replaced with C and dissolves
in cementite, further increasing the lattice strain with a matrix, and
therefore
may decrease the hydrogen embrittlement resistance. Therefore, if B is
contained, the content of B is preferably in the range satisfying Formula (2).
The lower limit of the B content is preferably 0.0001%, further preferably
0.0005%.
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[0042]
The conditions of PWHT performed for the low alloy steel in accordance
with the present invention are not subject to any special restriction.
However,
when PWHT is performed under the condition satisfying Formula (3), the low
alloy steel in accordance with the present invention achieves excellent
effects:
8000 T x {20 + log(t/3600)} 15000 ... (3)
where, T is treatment temperature ( C) of postweld heat treatment, and t is
treatment time (sec) of postweld heat treatment.
If "T x {20 + log(t/3600)}" is less than 8000, there is a possibility that the
hydrogen embrittlement resistance of HAZ of steel material consisting of the
low alloy steel in accordance with the present invention cannot be enhanced.
On the other hand, if "T x {20 + log(t/3600)}" exceeds 15000, the coarsening
of
fine MX-type carbides consisting of Ti or the like advances, so that
sufficient
hydrogen embrittlement resistance cannot be obtained, and also the strength
of steel including the weld zone is decreased remarkably. Therefore, the
PWHT performed for the low alloy steel in accordance with the present
invention is preferably carried out under the condition satisfying Formula
(3).
[0043]
In particular, the PWHT is preferably carried out in the temperature
range of 500 to 750 C for 30 to 600 seconds. The reason for this is that fine
MX-type carbides are formed stably by short-time PWHT, whereby the
hydrogen embrittlement resistance is enhanced, and also an extreme increase
in cost caused by long-time PWHT in actual work is restrained. In particular,
the PWHT time is preferably set to 300 seconds or shorter.
[0044]
The low alloy steel of the present invention preferably has a yield
strength (YS) of 552 MPa or higher. The reason for this is that, for a low
alloy
steel having a high strength, by PWHT, the strength of steel including the
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weld zone is decreased remarkably, and the merit of improvement in hydrogen
embrittlement resistance brought about by short-time PWHT can be further
obtained.
EXAMPLE(S)
[0045]
To confirm the effects of the present invention, the experiments
described below were conducted. A test material was prepared by machining
a 12 mm-thick low alloy steel plate having the chemical composition given in
Table 1 into a 12 mm square and a 100 mm length. This test material was
subjected to HAZ-simulated thermal cycle in which the test material was
heated to a temperature of 1350 C, at which the hardening of HAZ was
remarkable, for 3 seconds by high-frequency induction heating, and thereafter
was rapidly cooled. By using this test material, the tests described below
were conducted.
[0046]
<Tension test>
In conformity to JIS Z2241, a round-bar tensile test specimen having a
parallel part diameter of 6 mm and a parallel part length of 10 mm was
sampled from the obtained test material, and a tension test was conducted at
normal temperature.
[0047]
<SCC resistance test>
A test specimen having a thickness of 2 mm, a width of 10 mm, and a
length of 75 mm was sampled from the obtained test material, and the SCC
resistance was evaluated by a four-point bending test in conformity to EFC16
specified by the European Federation of Corrosion. In the test, after a stress
corresponding to 50% of 0.2% yield stress, which was derived from the tension
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4
test, had been applied to the sampled test specimen by four-point bending, the
test specimen was immersed in a 5% common salt + 0.5% acetic acid aqueous
solution of normal temperature (24 C), in which 1 atm hydrogen sulfide gas is
saturated, for 336 hours, whereby the presence of occurrence of SSC was
examined. Test No. in which SSC did not occur was made acceptable, and
test No. in which SSC occurred was made unacceptable.
[0048]
These test results are given in Table 2.
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cs)
TABLE 1
Steel Chemical Composition (mass % balance being Fe
and impurities) Evaluation
No.
0 0
C Si Mn P S B Al N 0 Cr Ni Mo
Ti Nb V Other Formula (1) Formula (2)
Al 0.10 0.25 2.40 0.013 0.001 - 0.021
0.0046 0.003 - - - 0.029 - - 0.010 0.0290 satisfaction
A2 0.05 0.24 2.41 0.014 0.001 - 0.020 0.0051 0.003 -
- - - 0.011 - 0.005 0.0055 satisfaction
A3 0.11 0.25 2.00 0.012 0.001 - 0.020 0.0046 0.001 -
- - - 0.020 Ca:0.002 0.011 0.0200
satisfaction
A4 0.05 0.28 1.98 0.010 0.001 - 0.027 0.0045
0.002 0.31 - 0.75 - 0.012 0.009 - 0.005 0.0150 satisfaction
AS 0.12 0.25 1.29
0.013 0.001 0.0002 0.029 0.0046 0.001 0.50 0.25 0.49 - 0.028 -
Cu:0.02 0.012 0.0140 satisfaction satisfaction
A6 0.12 0.24 1.83 0.014
0.001 0.0110 0.019 0.0044 0.003 - - - 0.010 - 0.018 0.012
0.0280 satisfaction satisfaction
A7 0.09 0.26 1.96
0.014 0.001 0.0085 0.024 0.0060 0.001 0.02 - - 0.009 0.010 - -
0.009 0.0140 satisfaction satisfaction
A8 0.01 0.22 2.03 0.014 0.001 - 0.025 0.0046
0.002 - 0.02 - - - 0.180 Mg:0.001 0.001 0.1800 satisfaction
0
B1 0.10 0.22 2.05 0.014 0.001 - 0.020 0.0042
0.001 - - - -* -* 0.007* - 0.010 0.0070 dissatisfaction
1-+ B2 0.06 0.24 2.90 0.013 0.001 - 0.022 0.0050
0.001 - - - -* 0.009* -* 0.006 0.0045 dissatisfaction
0
83 0.10 0.25 1.78 0.015 0.001 - 0.020 0.0046
0.003 0.02 - - -* 0.060* 0.190* - 0.010 0.2200 dissatisfaction
84 0.10 0.27 1.82 0.014
0.001 0.0170* 0.023 0.0045 0.003 - - - - 0.020 0.010 - 0.010
0.0200 satisfaction dissatisfaction
85 0.05 0.26 2.50 0.014 0.001 - 0.020
0.0051 0.002 0.78* - 0.77* 0.008 - - 0.005 0.0080 satisfaction
* indicates it is not satisfy the claimed range.
0 indicates the calculated value of "0.1 x [C(%)]".
0 indicates the calculated value of "[Ti(%)] + [V(%)] + 0.5 x [Nb( /0)]".
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[0050]
TABLE 2
Condition of PWHT
Property
Evaluation
Steel
No. of of
No. Temperature Time Formula (3) SSC
Test
T ( C) t (sec)
X1 Al 500 30 8,960 o No SSC
X2 Al 600 30 10,752 0 No SSC
X3 Al 750 30 13,441 0 No SSC
X4 Al 500 600 9,611 0 No SSC
X5 Al 600 300 11,352 0 No SSC
X6 A2 600 30 10,752 0 No SSC
X7 A3 600 30 10,752 0 No SSC
X8 A4 600 30 10,752 0 No SSC
X9 A5 600 , 30 10,752 0 No SSC
X10 A6 600 30 10,752 0 No SSC
X11 A7 600 30 10,752 0 No SSC
X12 A8 600 30 10,752 0 No SSC
Y1 Al not performed - x SSC
Y2 A7 not performed - x SSC
Y3 Bl* 600 30 10,752 0 SSC
Y4 B2* 600 30 10,752 0 SSC
Y5 B3* 600 30 10,752 a SSC
Y6 B4* 600 30 10,752 o SSC
-
Y7 B5* 600 30 10,752 o SSC
* indicates it is not satisfy the claimed range.
0 indicates the calculated value of "T x { 20 + log(t/3600)".
[0051]
As shown in Table 2, in test Nos. X1 to X12, the occurrence of SSC was
not recognized in the four-point bending test. Contrarily, in test Nos. Y1 and
Y2, although the chemical components met the requirements of the present
invention, since PWHT was not performed, MX-type carbides did not
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precipitate, and SSC occurred. In test Nos. Y3 and Y4, since the addition
amounts of Ti, Nb and V, which were constituent elements of MX-type carbides
contained in the steel, were small, and the predetermined relationship with C
was not satisfied, sufficient amounts of MX-type carbides did not precipitate,
and SSC occurred. In test No. Y5, since the addition amounts of Ti, Nb and V
were inversely too large, MX-type carbides precipitate coarsely, and SSC
occurred. In test No. Y6, although B was added, the addition amount thereof
was excessive, so that SSC occurred. Further, in test No. Y7, since Cr and Mo
were contained excessively, the carbides thereof were precipitated by PWHT,
and MX-type carbides were not formed stably, so that SSC occurred.
INDUSTRIAL APPLICABILITY
[00521
According to the present invention, there can be provided a low alloy
steel in which a HAZ subjected to PWHT, especially short-time PWHT, has
excellent hydrogen embrittlement resistance in wet hydrogen sulfide
environments or the like. This low alloy steel is best suitable as a starting
material of a steel pipe for the transmission of crude oil or natural gas.
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