DESCRIPTION
TITLE OF THE INVENTION
LOW ALLOY STEEL
TECHNICAL FIELD
roo011
The present invention relates to a low alloy steel.
BACKGROUND ART
[00021
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 suEde 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 fiom the viewpoint
of improving the HIC resistance and SSC resistance have traditionally been
proposed.
[00031
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.
[00041
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: Heat Affected Zone"). 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.
[00051
Therefore, in recent years, as described in Patent Document 4 (JP2010-
2460410, there has also been proposed a high-strength steel in which, by
reducing the amounts of C and Mn and by adding 0.6% 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.
LIST OF PRIOR ART DOCUMENT(S)
[0006l
Patent Document 1: JP5-255746A
Patent Document 2: JP6-336639A
Patent Document 3: JP2002-60894A
Patent Document 4: JP2010-24504A
[00071
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
[00081
In the invention of Patent Document 4, Mo, which is an expensive
element, is essential.
[00091
An objective of the present invention is to provide a low alloy steel in
which a HA2 has excellent hydrogen embrittlement resistance in wet hydrogen
sulfide environments or the like without requiring much cost.
MEANS FOR SOLVING THE PROBLEMS
loo 101
The present inventors conducted examinations and studies to optimize
the chemical composition capable of enhancing the hydrogen embrittlement
resistance of a weld heat affected zone (HAZ: Heat Affected Zone, hereinafter,
referred to as a "HAZ").
lo01 11
It is considered that the reason why the HAZ is highly susceptible to
hydrogen embrittlement is as follows. 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.
This hydrogen accumulates in a dislocation or a vacancy, which is one kind of
defects in the crystal lattice to embrittle the steel. The HAZ is an asquenched
structure being heated to a high temperature by the heat history of
welding, and cooled rapidly. Therefore, in the HAZ, the dislocations and
vacancies in which hydrogen is trapped exist densely as compared with a
thermally refined base metal. As a result, it is considered that the HAZ is
highly susceptible to hydrogen embrittlement as compared with the base metal.
[00121
As a result of repeated earnest studies, it was found that, in order to
enhance the hydrogen embrittlement susceptibility of HAZ, it was very
effective to positively contain B, specifically, to contain 0.005 to 0.050% of B.
The reason for this is considered to be as follows. Because having a small
atom radius like hydrogen, B exists in a crystal lattice, and can move in the
lattice. In addition, B has a tendency to segregate in a lattice defect and to
exist stably. Therefore, for the steel containing much B, it is considered that
hydrogen can be prevented from accumulating in the dislocation or vacancy
introduced into the HAZ, and embrittlement can be suppressed.
100 131
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 (5).
[00141
(1) A low alloy steel, containing, by mass percent, C: 0.01 to 0.15%, Si:
3% or less, Mn: 3% or less, B: 0.005 to 0.050%, and Al: 0.08% or less, and the
balance being Fe and impurities, 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.
[OO15 1
(2) The low alloy steel described in item (I), wherein the low alloy steel
contains, by mass percent, of one or more elements selected from Cr, Mo, Ni
and Cu: 1.6% or less in total in lieu of a part of Fe.
[00161
(3) The low alloy steel described in item (1) or (21, wherein the low alloy
steel contains, by mass percent, one or more elements selected from Ti, V and
Nb: 0.2% or less in total, of in lieu of a part of Fe.
[00171
(4) The low alloy steel described in any one of items (1) to (3), wherein
the low alloy steel contains, by mass percent, of Ca and/or Mg: 0.05% or less in
total in lieu of a part of Fe.
[oo 181
(5) The low alloy steel described in any one of items (1) to (4), wherein
the content of B satisfies Formula (1):
0.006xHv/300 + 0.0023 B ... (1)
where, "Hv" in the formula means the maximum value of Vickers hardness of
HAZ, and "B" means the content of B (mass%).
MEANS FOR SOLVING THE PROBLEMS
[0019]
According to the present invention, there can be provided a low alloy
steel in which a HAZ has excellent resistance to embrittlement attributable to
hydrogen such as stress corrosion cracking in wet hydrogen sulfide
environments. This low alloy steel is best suitable as a starting material of a
steel pipe for the transmission of crude oil or natural gas.
MODE FOR CARRYING OUT THE INVENTION
[00201
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%".
[00211
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%,
the hardness in the quenched state increases too much, and the HAZ is
hardened, so that the hydrogen embrittlement susceptibility of HAZ is
enhanced. Therefore, the C content is set to 0.01 to 0.15%. The lower limit
of the C content is preferably 0.02%, further preferably 0.03%. The C content
is preferably 0.12% or less, further preferably less than 0.10%.
100221
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.
[00231
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%.
100241
B: 0.005 to 0.050%
B (boron) is an element that constitutes the findings, which are the basis
of the present invention. As described before, B occupies the accumulation
site of hydrogen, such as the dislocation or vacancy in the HAZ. Therefore, B
is an element effective in enhancing the hydrogen embrittlement resistance.
Furthermore, when a steel material is produced, B segregates at grain
boundaries, thereby enhancing the hardenability indirectly, and contributes to
the improvement in strength. In order to achieve these effects, 0.005% or
more of B must be contained. On the other hand, if B is contained excessively,
borides precipitate in large amounts in the HAZ, the interface between a
matrix and borides acts as the accumulation site of hydrogen, and inversely
embrittlement is produced. Therefore, the B content is set to 0.005 to 0.060%.
The lower limit of the B content is preferably 0.006%, further preferably
0.008%. The upper limit thereof is preferably 0.045%, further preferably
0.040%.
100251
In the case where the hardness of HAZ increases, the dislocation density
increases, so that it is preferable that, in order to attain sufficient hydrogen
embrittlement resistance, the lower limit of the B content be controlled
according to the highest hardness of HAZ. That is to say, in order to attain
sufficient hydrogen embrittlement resistance, the B content is preferably in
the range satisfying Formula (1) in the relationship with the maximum value
of Vickers hardness of HAZ:
0.006xHv/300 + 0.0023 5 B ... (1)
where, "Hv" in the formula means the maximum value of Vickers hardness of
HAZ, and "B" means the content of B (mass%). The maximum value of
Vickers hardness of HAZ is a value that is determined by a Vickers test in
which the test force is 98.07N in conformity to JIS 22244.
Lo0261
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
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 A1 (so-called "so1.Al").
100271
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.
[ooasl
N: 0.01% or less
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%.
[00291
P: 0.06% 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%.
[00301
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%.
[003 11
0: 0.03% or less
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.
100321
The low alloy steel in accordance with the present invention may contain
the elements described below in lieu of a part of Fe.
100331
One or more elements selected from Cr, Mo, Ni and Cu: 1.6% or less in total
One or more elements selected from Cr (chromium), Mo (molybdenum),
Ni (nickel) and Cu (copper) may be contained because these elements enhance
the hardenability and contribute to the improvement in strength. However, if
the contents thereof are excessively high, the HAZ is hardened remarkably,
and therefore the hydrogen embrittlement susceptibility may be enhanced.
Therefore, if one or more elements of these elements are contained, the
contents thereof are set to 1.5% or less in total. The lower limit of the
contents of these elements is preferably 0.02%, further preferably 0.05%. The
upper limit thereof is preferably 1.2%.
LO0341
One or more elements selected from Ti, V and Nb: 0.2% or less in total
One or more elements selected from Ti (titanium), V (vanadium) and Nb
(niobium) may be contained because these elements are elements that form
fine carbo-nitrides and contribute to the improvement in strength, and also
stably supplement diffusible hydrogen, and bring about a considerable effect of
reducing the hydrogen embrittlement susceptibility. However, if the contents
thereof are excessively high, the formation of carbo-nitrides becomes excessive,
and therefore the toughness may be decreased. Therefore, if one or more
elements of these elements are contained, the contents thereof are set to 0.2%
or less in total. The lower limit of the contents of these elements is preferably
0.001%, further preferably 0.003%. The upper limit thereof is preferably
0.15%.
[00351
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 andlor Mg is preferably 0.0005%, further preferably 0.001%.
The upper limit thereof is preferably 0.03%.
EXAMPLE(S)
100361
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.
[00371

In conformity to JIS 22244, the cross section of the obtained test
material was caused to appear, and a Vickers test in which the test force was
98.07N was conducted to measure the Vickers hardness.
[00391

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
test, had been applied to the sampled test specimen by four-point bending, the
test specimen was immersed in a 6% common salt + 0.5% acetic acid aqueous
solution of normal temperature (240C), in which 1 atm hydrogen sulfide gas is
saturated, for 336 hours, whereby the presence of occurrence of SSC was
examined. In addition, the same test was also conducted in a 5% common salt
+ 0.5% acetic acid aqueous solution of 4OC, which temperature is more
stringent as an SSC environment. Test No. in which SSC did not occur was
made acceptable, and test No. in which SSC occurred was made unacceptable.
[00401
These test results are given in Table 2.
[0041]
Table 1
Chemicsl cornpcrsitlon(rnass% Balance being Fe I and itnpdes) No. .
C l S i l M n l P I S 1 B 1 Al 1 N 1 0 I C r I N i I M o I TI I N b I V (Others
* indicates H does not satlsfy the clained ranga
[00421
Table 2
LO0431
As shown in Table 2, because the content of B contained in the steel was
less than 0.006% in test Nos. B1 to B3, and because the content of B contained
in the steel exceeded 0.050% in test No. B5, large amounts of borides were
precipitated in the HAZ, and since the precipitated borides became the
starting point of embrittlement, SSC occurred in the four-point bending test at
normal temperature. Also, in test No. B4, although the B content was as low
as 0.0048%, and SSC did not occur at normal temperature, under the more
stringent condition of 4"C, SSC occurred. Contrarily, in test Nos. A1 to A8 in
which the requirements of the present invention were met, the occurrence of
SSC was not recognized in the four-point bending test under both of the test
conditions of normal temperature and 4°C.
No.
A1
A2
A3
A4
A5
, A6
A7
A8
61
82
63
B4
B5
The left side
value of
formula (1)
0.0078
0.0078
0.0078
0.0073
0.0067
0.0069
0.0070
0.0065
0.0078
0.0076
0.0076
0.0068
0.0065
B content
(mass%)
0.0087
0.0140
0.0170
0.0095
0.0140
0.01 30
0.0079
0.0065
0.0001
0.0001
0.0035
0.0048
0.0531
Hardness
( Hv )
328
330
331
299
262
273
279
249
330
31 7
31 8
267
25 1
Evaluation of SSC test
24°C
No SSC
No SSC
No SSC
No SSC
No SSC
No SSC
No SSC
No SSC
SSC
SSC
SSC
No SSC
SSC
4°C
No SSC
No SSC
No SSC
No SSC
No SSC
No SSC
No SSC
No SSC
SSC
SSC
SSC
SSC
SSC
INDUSTRIAL APPLICABILITY
[00443
According to the present invention, there can be provided a low alloy
steel in which a HAZ has excellent resistance to embrittlement attributable to
hydrogen such as stress corrosion cracking in wet hydrogen sulfide
environments. This low alloy steel is best suitable as a starting material of a
steel pipe for the transmission of crude oil or natural gas.
We claim:
1. A low alloy steel, containing, by mass percent,
C: 0.01 to 0.15%, Si: 3% or less,
Mn: 3% or less,
B: 0.005 to 0.050%, and
Al: 0.08% or less, and
the balance being Fe and impurities,
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.
2. The low alloy steel according to claim 1, wherein the low alloy steel
contains, by mass percent, of one or more elements selected from Cr,
Mo, Ni and Cu: 1.6% or less in total in lieu of a part of Fe.
3. The low alloy steel according to claim 1 or 2, wherein the low alloy
steel contains, by mass percent, of one or more elements selected
from Ti, V and Nb: 0.2% or less in total in lieu of a part of Fe.
4. The low alloy steel according to any one of claims 1 to 3, wherein the
low alloy steel contains, by mass percent, of Ca and/or Mg: 0.05% or
less in total in lieu of a part of Fe.
5. The low alloy steel according to any one of claims 1 to 4, wherein the
content of B satisfies Formula (1):
0.005xHv/300 + 0.0023 5 B ... (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass%).
Dated this 15th day of July, 2014.
Nippob Steel & Sumitomo Metal