U.S. patent application number 14/370999 was filed with the patent office on 2014-11-27 for low alloy steel.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Hiroyuki Hirata, Kaori Kawano, Kenji Kobayashi, Kazuhiro Ogawa, Tomohiko Omura, Kota Tomatsu.
Application Number | 20140348695 14/370999 |
Document ID | / |
Family ID | 48781354 |
Filed Date | 2014-11-27 |
United States Patent
Application |
20140348695 |
Kind Code |
A1 |
Hirata; Hiroyuki ; et
al. |
November 27, 2014 |
LOW ALLOY STEEL
Abstract
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 O: 0.03% or less. In the alloy steel, a HAZ has excellent
resistance to embrittlement attributable to hydrogen such as stress
corrosion cracking in wet hydrogen sulfide environments.
Inventors: |
Hirata; Hiroyuki; (Tokyo,
JP) ; Kobayashi; Kenji; (Tokyo, JP) ; Omura;
Tomohiko; (Tokyo, JP) ; Kawano; Kaori; (Tokyo,
JP) ; Tomatsu; Kota; (Tokyo, JP) ; Ogawa;
Kazuhiro; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
48781354 |
Appl. No.: |
14/370999 |
Filed: |
December 17, 2012 |
PCT Filed: |
December 17, 2012 |
PCT NO: |
PCT/JP2012/082606 |
371 Date: |
July 8, 2014 |
Current U.S.
Class: |
420/84 ; 420/104;
420/119; 420/120; 420/91 |
Current CPC
Class: |
C22C 38/08 20130101;
C21D 9/00 20130101; C22C 38/04 20130101; C21D 9/08 20130101; C22C
38/24 20130101; E21B 17/01 20130101; C22C 38/06 20130101; C22C
38/001 20130101; C22C 38/12 20130101; C22C 38/002 20130101; C21D
9/14 20130101; C22C 38/32 20130101; C22C 38/42 20130101; C22C 38/44
20130101; C22C 38/38 20130101; C22C 38/14 20130101; C22C 38/54
20130101; E21B 17/00 20130101; C21D 9/50 20130101; C22C 38/48
20130101; C22C 38/46 20130101; C22C 38/02 20130101; C22C 38/58
20130101 |
Class at
Publication: |
420/84 ; 420/91;
420/104; 420/119; 420/120 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/38 20060101 C22C038/38; C22C 38/32 20060101
C22C038/32; C22C 38/24 20060101 C22C038/24; C22C 38/14 20060101
C22C038/14; C22C 38/12 20060101 C22C038/12; C22C 38/08 20060101
C22C038/08; C22C 38/06 20060101 C22C038/06; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; E21B 17/00 20060101 E21B017/00; E21B 17/01 20060101
E21B017/01; C22C 38/54 20060101 C22C038/54 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 12, 2012 |
JP |
2012-004103 |
Claims
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 O: 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: 1.5% or less in total in lieu of a part of
Fe.
3. The low alloy steel according to claim 1, 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 claim 1, 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 claim 1, wherein the content of
B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
6. The low alloy steel according to claim 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.
7. The low alloy steel according to claim 2, 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.
8. The low alloy steel according to claim 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.
9. The low alloy steel according to claim 6, 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.
10. The low alloy steel according to claim 2, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
11. The low alloy steel according to claim 3, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
12. The low alloy steel according to claim 4, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
13. The low alloy steel according to claim 6, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
14. The low alloy steel according to claim 7, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
15. The low alloy steel according to claim 8, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
16. The low alloy steel according to claim 9, wherein the content
of B satisfies Formula (1): 0.005.times.Hv/300+0.0023.ltoreq.B (1)
where, "Hv" in the formula means the maximum value of Vickers
hardness of HAZ, and "B" means the content of B (mass %).
Description
TECHNICAL FIELD
[0001] The present invention relates to a low alloy steel.
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.
[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: 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.
[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 adding
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.
LIST OF PRIOR ART DOCUMENT(S)
[0006] Patent Document 1: JP5-255746 A [0007] Patent Document 2:
JP6-336639A [0008] Patent Document 3: JP2002-60894A [0009] Patent
Document 4: JP2010-24504A [0010] Non-Patent Document 1: [0011]
Masanori Kowaka, Corrosion damage and anticorrosion engineering of
metal, Aug. 25, 1983, issued by Agne Corporation, p. 198
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0012] In the invention of Patent Document 4, Mo, which is an
expensive element, is essential.
[0013] An objective of the present invention is to provide a low
alloy steel in which a HAZ has excellent hydrogen embrittlement
resistance in wet hydrogen sulfide environments or the like without
requiring much cost.
Means for Solving the Problems
[0014] 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").
[0015] 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 as-quenched 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.
[0016] As a result of repeated earnest studies, it was found that,
in order to enhance a resistance of 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.
[0017] 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).
[0018] (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 O: 0.03% or less.
[0019] (2) The low alloy steel described in item (1), wherein the
low alloy steel contains, by mass percent, of one or more elements
selected from Cr, Mo, Ni and Cu: 1.5% or less in total in lieu of a
part of Fe.
[0020] (3) The low alloy steel described in item (1) or (2),
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.
[0021] (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.
[0022] (5) The low alloy steel described in any one of items (1) to
(4), wherein the content of B satisfies Formula (1):
0.005.times.Hv/300+0.0023.ltoreq.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
[0023] 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
[0024] 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 %".
C: 0.01 to 0.15%
[0025] 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%.
Si: 3% or less
[0026] 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.
Mn: 3% or less
[0027] 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%.
B: 0.005 to 0.050%
[0028] 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 0.050%. 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%.
[0029] 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:
0.005.times.Hv/300+0.0023.ltoreq.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 Z2244. Al: 0.08% or less
[0030] 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 Al (so-called
"sol.Al").
[0031] 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.
N: 0.01% or less
[0032] 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%.
P: 0.05% or less
[0033] 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%.
S: 0.03% or less
[0034] 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%.
O; 0.03% or less
[0035] O (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.
[0036] The low alloy steel in accordance with the present invention
may contain the elements described below in lieu of a part of
Fe.
One or more elements selected from Cr, Mo, Ni and Cu: 1.5% or less
in total
[0037] 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%.
One or more elements selected from Ti, V and Nb: 0.2% or less in
total
[0038] 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%.
Ca and/or Mg: 0.05% or less in total
[0039] 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%.
EXAMPLE(S)
[0040] 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.degree. 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.
<Tension Test>
[0041] 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.
<Vickers Test>
[0042] In conformity to JIS Z2244, 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.
<SCC Resistance Test>
[0043] 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 5% common salt+0.5%
acetic acid aqueous solution of normal temperature (24.degree. C.),
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 4.degree. C., 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.
[0044] These test results are given in Table 2.
TABLE-US-00001 TABLE 1 Chemical composition(mass % Balance being Fe
and impurities) No. C Si Mn P S B Al N O Cr Ni Mo Ti Nb V Others A1
0.10 0.25 1.99 0.013 0.001 0.0087 0.024 0.0063 0.001 -- -- -- -- --
-- -- A2 0.10 0.25 1.98 0.013 0.001 0.0140 0.024 0.0062 0.002 -- --
0.03 0.003 -- -- -- A3 0.09 0.24 1.98 0.013 0.001 0.0170 0.024
0.0061 0.002 -- -- -- -- -- -- -- A4 0.05 0.24 2.04 0.013 0.001
0.0095 0.020 0.0047 0.002 -- -- -- -- -- -- -- A5 0.03 0.23 2.00
0.014 0.001 0.0140 0.016 0.0045 0.002 0.02 0.02 0.03 -- 0.01 0.01
Cu: 0.02 A6 0.05 0.24 1.62 0.014 0.001 0.0130 0.019 0.0050 0.001 --
0.03 -- 0.05 -- -- Ca: 0.003 A7 0.10 0.25 1.81 0.015 0.001 0.0079
0.021 0.0050 0.001 0.20 -- -- -- -- 0.01 -- A8 0.01 0.25 1.81 0.015
0.001 0.0065 0.020 0.0046 0.003 -- 0.60 -- -- -- -- Mg: 0.002 B1
0.10 0.24 2.02 0.014 0.001 0.0001* 0.021 0.0044 0.001 -- -- -- --
-- -- -- B2 0.05 0.23 2.93 0.014 0.001 0.0001* 0.021 0.0052 0.001
-- -- -- -- -- -- -- B3 0.05 0.24 2.53 0.014 0.001 0.0035* 0.017
0.0056 0.001 -- 0.05 0.01 0.01 -- -- -- B4 0.10 0.26 1.80 0.014
0.001 0.0048* 0.022 0.0048 0.003 0.03 -- -- -- 0.02 0.01 -- B5 0.05
0.24 1.61 0.014 0.001 0.0531* 0.020 0.0051 0.002 -- 0.03 0.03 -- --
-- -- *indicates it does not satisfy the claimed range.
TABLE-US-00002 TABLE 2 The left side B content Hardness value of
Evaluation of SSC test No. (mass %) (Hv) formula (1) 24.degree. C.
4.degree. C. A1 0.0087 328 0.0078 No SSC No SSC A2 0.0140 330
0.0078 No SSC No SSC A3 0.0170 331 0.0078 No SSC No SSC A4 0.0095
299 0.0073 No SSC No SSC A5 0.0140 262 0.0067 No SSC No SSC A6
0.0130 273 0.0069 No SSC No SSC A7 0.0079 279 0.0070 No SSC No SSC
A8 0.0065 249 0.0065 No SSC No SSC B1 0.0001 330 0.0078 SSC SSC B2
0.0001 317 0.0076 SSC SSC B3 0.0035 318 0.0076 SSC SSC B4 0.0048
267 0.0068 No SSC SSC B5 0.0531 251 0.0065 SSC SSC
[0045] As shown in Table 2, because the content of B contained in
the steel was less than 0.005% 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.degree. 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.degree. C.
INDUSTRIAL APPLICABILITY
[0046] 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.
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