U.S. patent application number 15/775409 was filed with the patent office on 2018-12-13 for seamless steel pipe and method of manufacturing the same.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Yuji ARAI, Hiroki KAMITANI, Keiichi KONDO, Taro OE, Yusuke SENDAI.
Application Number | 20180355451 15/775409 |
Document ID | / |
Family ID | 58666735 |
Filed Date | 2018-12-13 |
United States Patent
Application |
20180355451 |
Kind Code |
A1 |
KONDO; Keiichi ; et
al. |
December 13, 2018 |
SEAMLESS STEEL PIPE AND METHOD OF MANUFACTURING THE SAME
Abstract
A seamless steel pipe contains, (mass %), C: 0.02 to 0.15%; Si:
0.05 to 0.5%; Mn: 0.30 to 2.5%; Al: 0.01 to 0.10%; Ti: 0.001 to
0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02%
and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to
0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, where carbon
equivalent is not less than 0.430% and less than 0.500%, the
microstructure main phase from the surface to an in-the-wall
portion is tempered martensite or tempered bainite, prior austenite
grain size is lower than 6.0, a portion between 1 mm from the inner
surface and 1 mm from the outer surface has Vickers hardness of 250
Hv or lower, and yield strength is 555 MPa or higher.
Inventors: |
KONDO; Keiichi; (Chiyoda-ku,
Tokyo, JP) ; OE; Taro; (Chiyoda-ku, Tokyo, JP)
; ARAI; Yuji; (Chiyoda-ku, Tokyo, JP) ; SENDAI;
Yusuke; (Chiyoda-ku, Tokyo, JP) ; KAMITANI;
Hiroki; (Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
58666735 |
Appl. No.: |
15/775409 |
Filed: |
February 16, 2016 |
PCT Filed: |
February 16, 2016 |
PCT NO: |
PCT/JP2016/054381 |
371 Date: |
May 11, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/58 20130101; C21D 8/105 20130101; C22C 38/48 20130101; C22C
38/002 20130101; C22C 38/06 20130101; C22C 38/46 20130101; C21D
8/10 20130101; C22C 38/00 20130101; C21D 6/004 20130101; C21D 6/005
20130101; C21D 2211/002 20130101; C21D 9/08 20130101; C21D 2211/001
20130101; C22C 38/44 20130101; C22C 38/42 20130101; C21D 9/085
20130101; C22C 38/50 20130101; C21D 2211/005 20130101; C22C 38/02
20130101; C22C 38/04 20130101; C21D 6/008 20130101 |
International
Class: |
C21D 9/08 20060101
C21D009/08; C21D 8/10 20060101 C21D008/10; C21D 6/00 20060101
C21D006/00; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/46 20060101 C22C038/46; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Claims
1. A seamless steel pipe having a chemical composition of, in mass
%, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to
0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti:
0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less
than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%;
V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the
balance being Fe and impurities, where a carbon equivalent Ceq as
defined by equation (1) below is not less than 0.430% and less than
0.500%, a main phase of a microstructure from a surface layer to an
in-the-wall portion is tempered martensite or tempered bainite, a
size of prior austenite grains in the microstructure is lower than
6.0 in crystal grain size number according to ASTM E112-10, a
portion between a position at 1 mm from an inner surface and a
position at 1 mm from an outer surface has a Vickers hardness of
250 Hv or lower, and a yield strength is 555 MPa or higher,
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (1), where a symbol of each
element in equation (1) is substituted by a content of this element
in mass %.
2. The seamless steel pipe according to claim 1, wherein the
chemical composition contains, in mass %: Nb: 0.010 to 0.05%.
3. The seamless steel pipe according to claim 1, wherein a
difference between a Vickers hardness of a portion at 1 mm from the
inner surface and that of a portion in a middle in a wall
thickness, a difference between a Vickers hardness of a portion at
1 mm from the outer surface and that of a portion in the middle in
the wall thickness, and a difference between a Vickers hardness of
a portion at 1 mm from the inner surface and that of a portion at 1
mm from the outer surface are each 25 Hv or lower.
4. The seamless steel pipe according to claim 1, wherein: the
seamless steel pipe is produced by quenching and tempering, and a
Larson-Miller parameter PL as defined by equation (2) below is
18800 or higher: PL=(T+273).times.(20+log(t)) (2), in equation (2),
T is a tempering temperature and t is a holding time for that
temperature, T is in .degree. C., and t is in hours.
5. A method of manufacturing a seamless steel pipe, comprising:
preparing a raw material having a chemical composition of, in mass
%, C: 0.02 to 0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to
0.03%; S: up to 0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti:
0.001 to 0.010%; N: up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less
than 0.02% and less than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%;
V: 0.020 to 0.20%; Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the
balance being Fe and impurities; hot working the raw material to
produce a hollow shell; quenching the hollow shell by direct
quenching or in-line quenching; and tempering the quenched hollow
shell, no reheating-and-quenching is performed between the
quenching and tempering, a carbon equivalent Ceq as defined by
equation (3) below is not less than 0.430% and less than 0.500%, a
Larson-Miller parameter PL as defined by equation (4) below is not
less than 18800, Ceq=C+Mn/6+(Cr+Mo+V)/5.+-.(Ni+Cu)/15 (3), and
PL=(T+273).times.(20+log(t)) (4), a symbol of each element in
equation (3) is substituted by a content of this element in mass %,
and in equation (4), T is a tempering temperature, and t is a
holding period for this temperature, and T is in .degree. C., and t
is in hours.
6. The seamless steel pipe according to claim 2, wherein a
difference between a Vickers hardness of a portion at 1 mm from the
inner surface and that of a portion in a middle in a wall
thickness, a difference between a Vickers hardness of a portion at
1 mm from the outer surface and that of a portion in the middle in
the wall thickness, and a difference between a Vickers hardness of
a portion at 1 mm from the inner surface and that of a portion at 1
mm from the outer surface are each 25 Hv or lower.
7. The seamless steel pipe according to claim 2, wherein: the
seamless steel pipe is produced by quenching and tempering, and a
Larson-Miller parameter PL as defined by equation (2) below is
18800 or higher: PL=(T+273).times.(20+log(t)) (2), in equation (2),
T is a tempering temperature and t is a holding time for that
temperature, T is in .degree. C., and t is in hours.
8. The seamless steel pipe according to claim 3, wherein: the
seamless steel pipe is produced by quenching and tempering, and a
Larson-Miller parameter PL as defined by equation (2) below is
18800 or higher: PL=(T+273).times.(20+log(t)) (2), in equation (2),
T is a tempering temperature and t is a holding time for that
temperature, T is in .degree. C., and t is in hours.
9. The seamless steel pipe according to claim 6, wherein: the
seamless steel pipe is produced by quenching and tempering, and a
Larson-Miller parameter PL as defined by equation (2) below is
18800 or higher: PL=(T+273).times.(20+log(t)) (2), in equation (2),
T is a tempering temperature and t is a holding time for that
temperature, T is in .degree. C., and t is in hours.
Description
TECHNICAL FIELD
[0001] The present invention relates to a seamless steel pipe and a
method of manufacturing the same, and, more particularly, to a
seamless steel pipe suitable for line pipe and a method of
manufacturing the same.
BACKGROUND ART
[0002] Oil and gas resources from oil wells located on land and in
shallow seas are drying up and, to address this, increasing numbers
of offshore oil fields in deep seas are being developed. In an
offshore oil field, crude oil or gas must be transported from the
pithead of an oil well or gas well installed on the seabed to the
platform above the sea using a flow line or a riser. A flow line is
a line pipe laid along the topography of the surface of the earth
or the seabed. A riser is a line pipe disposed to rise from the
seabed toward the platform (i.e. upward).
[0003] The inner side of a steel pipe forming part of a flow line
laid in the deep sea is subject to a high interior fluid pressure
having a pressure from deep strata added thereto and, when the
operation is halted, is also affected by seawater pressures of the
deep sea. A steel pipe forming part of a riser is further affected
by repeated distortions by ocean waves. Accordingly, it is
desirable that steel pipes used for such applications have high
strength and high toughness. In addition, oil and gas wells are
being developed in sour environments, which are harsher than
conditions for conventional wells, such as deep seas and cold
regions. Offshore pipe lines laid in such harsh sour environments
are required to have a higher strength (i.e. pressure resistance)
and toughness than conventional ones, and are further required to
have hydrogen-induced cracking resistance (HIC resistance) and
sulfide stress corrosion cracking resistance (SSC resistance).
[0004] Patent Document 1 discloses a seamless steel pipe with a
large wall thickness for line pipe having high strength and good
toughness, containing C: 0.03 to 0.08%, Si: 0.15 or less, Mn: 0.3
to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%,
Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008% and one
or more of Ca, Mg and REM: 0.0002 to 0.005% in total, the balance
being Fe and impurities, where P in the impurities: 0.05% or less,
S: 0.005% or less, and the wall thickness is 30 to 50 mm.
[0005] Patent Document 2 discloses a high-strength seamless steel
pipe with a large wall thickness that is made by quenching and
tempering and having a yield strength higher than 450 MPa for line
pipe with good sour resistance where the Vickers hardness HV5
measurable at an outermost or innermost position of the pipe with
an applied load of 5 kgf (with a force in the test of 49 N) is 250
HV5 or lower.
[0006] Patent Document 3 discloses a seamless steel pipe for line
pipe containing, in mass %, C: 0.02 to 0.10%, Si: 0.5% or less, Mn:
0.5 to 2.0%, Al: 0.01 to 0.1%, Ca: 0.005% or less, and N: 0.007% or
less, and one or more selected from the group consisting of Ti:
0.008% or less, V: less than 0.06% and Nb: 0.05% or less, the
balance being Fe and impurities, where the total content of Ti, V
and Nb is smaller than 0.06%, the carbon equivalent Ceq defined by
the following equation is 0.38% or more, and the size of
carbonitride particles containing one or more of Ti, V, Nb and Al
is 200 nm or less.
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15
[0007] Patent Document 4 discloses a seamless steel pipe with a
chemical composition of, in mass %, C: 0.02 to 0.10%, Si: 0.05 to
0.5%, Mn: 1.0 to 2.0%, Mo: 0.5 to 1.0%, Cr: 0.1 to 1.0%, Al: 0.01
to 0.10%, P: 0.03% or less, S: 0.005% or less, Ca: 0.0005 to
0.005%, V: 0.010 to 0.040%, and N: 0.002 to 0.007% and one or more
selected from the group consisting of Ti: 0.001 to 0.008% and Nb:
0.02 to 0.05%, the balance being Fe and impurities, where the
carbon equivalent Ceq is 0.50 to 0.58%, the pipe containing a
specified carbide.
PRIOR ART DOCUMENT
Patent Document
[0008] [Patent Document 1] JP 2010-242222 A
[0009] [Patent Document 2] JP 2013.32584 A
[0010] [Patent Document 3] WO 2011/152240
[0011] [Patent Document 4] JP 5516831 B
DISCLOSURE OF THE INVENTION
[0012] Even when one or more of the above conventional techniques
are used, a seamless steel pipe having a strength of .times.80
grade or higher as defined by the American Petroleum Institute
(API) standards (i.e. a lower limit yield strength of 555 MPa or
higher) may not have good SSC resistance in a reliable manner.
[0013] To improve the strength and toughness of a seamless steel
pipe produced by quenching-and-tempering, the content of alloy
elements such as carbon may be increased to increase hardenability.
However, if the content of alloy elements such as carbon is
increased, the strength (i.e. hardness) of the surface of the steel
pipe increases. In a seamless steel pipe produced by
quenching-and-tempering, the surface layer is cooled at a high rate
during quenching and can easily be hardened, increasing the
hardness, while the in-the-wall portions have low hardness. This
tendency may remain after tempering. As such, in a seamless steel
pipe having a strength of .times.80 grade or higher, a surface
layer hardness may exceed 250 Hv, which is the higher limit
required in the sour resistance grade according to the API 5 L
standards.
[0014] Although the techniques of Patent Document 1 are effective
in achieving high strength and high toughness, they do not
sufficiently consider reducing the hardness of the surface layer or
thus improving SSC resistance. Patent Document 2 states that the
hardness of the surface layer of a steel pipe can be controlled to
be 250 HV5 or lower; however, it appears to require a special
manufacturing process. Patent Document 3 provides some
considerations about SSC resistance; however, after hot forming, it
is necessary to perform direct quenching or in-line quenching and
then reheating-and-quenching. Patent Document 4 provides some
considerations about the hardness of the surface layer of a steel
pipe and HIC resistance; however, a reheating-and-quenching step is
necessary and, after hot forming, direct quenching or in-line
quenching is used as necessary, which means manufacturing costs
that are not very reasonable.
[0015] An object of the present invention is to provide a seamless
steel pipe that can be manufactured by a relatively reasonable
manufacturing process and that provides a yield strength of 555 MPa
or higher and good SSC resistance in a reliable manner.
[0016] A seamless steel in an embodiment of the present invention
has a chemical composition of, in mass %, C: 0.02 to 0.15% Si: 0.05
to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to 0.006%; O: up
to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N: up to 0.007%;
Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less than 0.5%; Ni:
0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%; Ca: 0.0005 to
0.005%; and Nb: 0 to 0.05%, the balance being Fe and impurities,
where a carbon equivalent Ceq as defined by equation (1) below is
not less than 0.430% and less than 0.500%, a main phase of a
microstructure from a surface layer to an in-the-wall portion is
tempered martensite or tempered bainite, a size of prior austenite
grains in the microstructure is lower than 6.0 in crystal grain
size number according to ASTM E112-10, a portion between a position
at 1 mm from an inner surface and a position at 1 mm from an outer
surface has a Vickers hardness of 250 Hv or lower, and a yield
strength is 555 MPa or higher,
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (1),
[0017] where a symbol of each element in equation (1) is
substituted by a content of this element in mass %.
[0018] A method of manufacturing a seamless steel pipe in an
embodiment of the present invention includes: preparing a raw
material having a chemical composition of, in mass %, C: 0.02 to
0.15%; Si: 0.05 to 0.5%; Mn: 0.30 to 2.5%; P: up to 0.03%; S: up to
0.006%; O: up to 0.004%; Al: 0.01 to 0.10%; Ti: 0.001 to 0.010%; N:
up to 0.007%; Cr: 0.05 to 1.0%; Mo: not less than 0.02% and less
than 0.5%; Ni: 0.03 to 1.0%; Cu: 0.02 to 1.0%; V: 0.020 to 0.20%;
Ca: 0.0005 to 0.005%; and Nb: 0 to 0.05%, the balance being Fe and
impurities; hot working the raw material to produce a hollow shell;
quenching the hollow shell by direct quenching or in-line
quenching; and tempering the quenched hollow shell. No
reheating-and-quenching is performed between the quenching and
tempering. A carbon equivalent Ceq as defined by equation (3) below
is not less than 0.430% and less than 0.500%, a Larson-Miller
parameter PL as defined by equation (4) below is not less than
18800,
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (3), and
PL=(T+273).times.(20+log(t)) (4).
[0019] A symbol of each element in equation (3) is substituted by a
content of this element in mass %. In equation (4), T is a
tempering temperature, and t is a holding period for this
temperature. T is in .degree. C., and t is in hours.
[0020] The present invention provides a seamless steel pipe that
can be manufactured by a relatively reasonable manufacturing
process and that provides a yield strength of 555 MPa or higher and
good SSC resistance in a reliable manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a block diagram illustrating an example of a
manufacturing line.
[0022] FIG. 2 is a flow chart illustrating a process for
manufacturing the seamless steel pipe.
[0023] FIG. 3 shows changes in the surface temperature of a
workpiece during a manufacture versus time.
[0024] FIG. 4 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and yield strength YS for steel.
B.
[0025] FIG. 5 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and yield strength YS for steel
A.
[0026] FIG. 6 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and hardness at an outer
surface, an in-the-wall portion and an inner surface for steel
B.
[0027] FIG. 7 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and hardness at an outer
surface, an in-the-wall portion and an inner surface for steel
A.
[0028] FIG. 8 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and maximum difference in
hardness for steel B.
[0029] FIG. 9 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and maximum difference in
hardness for steel A.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0030] The present inventors did research to find a method of
providing a seamless steel pipe that ensures a yield strength of
555 MPa or higher and good SSC resistance in a reliable manner.
They found out that limiting the carbon equivalent of a steel to an
appropriate range and reducing the difference between the hardness
of the surface layer and the hardness of the in-the-wall portions
of the seamless steel pipe ensures a yield strength of 555 MPa or
higher and good SSC resistance in a reliable manner, where only
direct quenching or in-line quenching is performed after hot
forming and no reheating-and-quenching is performed.
[0031] During the quenching after rolling, the surface layer of a
seamless steel pipe is cooled at high rate and can easily be
hardened. As such, the surface layer of the seamless steel pipe
tends to be hard and may exceed the values of hardness specified by
the API 5 L standards or DNV-OS-F101 standards. On the other hand,
the portions located in the middle in the wall thickness of the
seamless steel pipe is cooled at a lower rate and cannot easily be
hardened such that non-quenched structures such as ferrite may be
included. Thus, there is typically a difference between the
hardness of the surface layer and that of the in-the-wall portions,
and this tendency may remain after tempering for certain tempering
conditions. Further, in a seamless steel pipe with high carbon
equivalent such as those used in high-strength steel with .times.80
grade or higher, the difference between the hardness of the surface
layer and that of the in-the-wall portions tends to be significant.
Such a high hardness of the surface layer may be a problem when
good sour resistance is to be achieved in a reliable manner.
[0032] If the carbon equivalent is too low, it is difficult to
ensure a certain strength of a seamless steel pipe. If the carbon
equivalent is too high, it is difficult to reduce the Vickers
hardness of the surface layer to 250 Hv or lower with a
manufacturing process in which reheating-and-quenching is
eliminated, direct quenching or in-line quenching being only one
step of the quenching. This is because, if the quenching after hot
forming is direct quenching or in-line quenching, the austenite
grains tend to be coarse compared with implementations where
reheating-and-quenching is performed, which increases overall
hardenability. In view of this, Ceq as defined by equation (1)
below is to be not less than 0.430% and less than 0.500%:
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 (1),
[0033] where the symbol of each element in equation (1) is
substituted by the content of this element in mass %.
[0034] To reduce the difference between the hardness of the surface
layer and that of the in-the-wall portions, it is effective to
limit the carbon equivalent and, in addition, the tempering
conditions appropriately. That is, if tempering is not sufficiently
done, the reduction in the hardness of the surface layer is
insufficient such that some portions may have a Vickers hardness
higher than 250 Hv. More specifically, the Larson-Miller parameter
PL as defined by equation (2) below is 18800 or higher.
PL=(T+273).times.(20+log(t)) (2).
[0035] In equation (2), T is a tempering temperature (in .degree.
C.) and t is a holding time (in hours) for that temperature.
[0036] The present invention was made based on the above findings.
A seamless steel pipe in one embodiment of the present invention
will now be described in detail with reference to the drawings. The
same or corresponding portions in the drawings are labeled with the
same characters and their description will not be repeated.
[0037] [Chemical Composition]
[0038] The seamless steel pipe in the present embodiment has the
chemical composition described below. In the following description,
"%" for the content of an element means mass %.
[0039] C: 0.02 to 0.15%
[0040] Carbon (C) increases the strength of the steel. If the C
content is lower than 0.02%, this effect cannot be sufficiently
achieved. If the C content is higher than 0.15%, the toughness of
the steel decreases. In view of this, the C content should be in
the range of 0.02 to 0.15%. The C content is preferably higher than
0.02%, and more preferably 0.04% or higher. The C content is
preferably 0.10% or lower, and more preferably 0.08% or lower.
[0041] Si: 0.05 to 0.5%
[0042] Silicon (Si) deoxidizes steel. This effect can be clearly
achieved if the Si content is 0.05% or higher. However, if the Si
content is higher than 0.5%, the toughness of the steel decreases.
In view of this, the Si content should be in the range of 0.05 to
0.5%. The Si content is preferably higher than 0.05%, and more
preferably 0.08% or higher, and still more preferably 0.10% or
higher. The Si content is preferably lower than 0.5%, and more
preferably 0.25% or lower, and still more preferably 0.20% or
lower.
[0043] Mn: 0.30 to 2.5%
[0044] Manganese (Mn) increases the hardenability of steel to
increase the strength of the steel. These effects cannot be
sufficiently achieved if the Mn content is lower than 0.30%. If the
Mn content is higher than 2.5%, Mn segregates in the steel,
decreasing the toughness of the steel. In view of this, the Mn
content should be in the range of 0.30 to 2.5%. The Mn content is
preferably higher than 0.30%, and more preferably 1.0% or higher,
and still more preferably 1.3% or higher. The Mn content is
preferably lower than 2.5%, and more preferably 2.0% or lower, and
still more preferably 1.8% or lower.
[0045] P: Up to 0.03%
[0046] Phosphorus (P) is an impurity. P decreases the toughness of
steel. Thus, lower P contents are preferable. In view of this, the
P content should be 0.03% or lower. The P content is preferably
lower than 0.03%, and more preferably 0.015% or lower, and still
more preferably 0.012% or lower.
[0047] S: Up to 0.006%
[0048] Sulphur (S) is an impurity. S bonds with Mn to form coarse
MnS particles and thus decreases the toughness and HIC resistance
of the steel. Thus, lower S contents are preferable. In view of
this, the S content should be 0.006% or lower. The S content is
preferably lower than 0.006%, and more preferably 0.003% or lower,
and still more preferably 0.002% or lower.
[0049] O: Up to 0.004%
[0050] Oxygen (O) is an impurity. O forms coarse oxide particles or
clusters of oxide particles, decreasing the toughness of the steel.
Thus, lower O contents are preferable. In view of this, the O
content should be 0.004% or lower. The O content is preferably
0.003% or lower, and more preferably 0.002% or lower.
[0051] Al: 0.01 to 0.10%
[0052] Aluminum (Al) bonds with N to form fine nitride particles,
increasing the toughness of the steel. This effect cannot be
sufficiently achieved if the Al content is lower than 0.01%. If the
Al content is higher than 0.10%, coarse Al nitride particles
result, decreasing the toughness of the steel. In view of this, the
Al content should be in the range of 0.01 to 0.10%. The Al content
is preferably higher than 0.01%, and more preferably 0.02% or
higher. The Al content is preferably lower than 0.10%, and more
preferably 0.08% or lower, and still more preferably 0.06% or
lower. As used herein, Al content means the content of acid-soluble
Al (i.e. so-called "sol. Al").
[0053] Ti: 0.001 to 0.010%
[0054] Titanium (Ti) bonds with N in a steel and forms TiN,
suppressing the reduction in the toughness of the steel due to
dissolved N. Further, the dispersed and precipitated fine TiN
particles increase the toughness of the steel. These effects cannot
be sufficiently achieved if the Ti content is lower than 0.001%. If
the Ti content is higher than 0.010%, coarse TiN particles result
or coarse TiC particles are produced, decreasing the toughness of
the steel. In view of this, the Ti content should be in the range
of 0.001 to 0.010%. The Ti content is preferably higher than 0.001%
and more preferably 0.002% or higher. The Ti content is preferably
lower than 0.010%, and more preferably 0.006% or lower, and still
more preferably 0.005% or lower.
[0055] N: Up to 0.007%
[0056] Nitrogen (N) bonds with Al and forms fine Al nitride
particles, increasing the toughness of the steel. However, if the N
content is higher than 0.007%, dissolved N decreases the toughness
of the steel. Further, if the N content is too high, coarse
carbonitride and/or nitride particles result, decreasing the
toughness of the steel. In view of this, the N content should be
0.007% or lower. The N content is preferably lower than 0.007%, and
more preferably 0.006% or lower, and still more preferably 0.005%
or lower. The N content is preferably 0.002% or higher.
[0057] Cr: 0.05 to 1.0%
[0058] Chromium (Cr) increases the hardenability of steel and
increases the strength of the steel. Cr further increases the
temper softening resistance of the steel. These effects cannot be
sufficiently achieved if the Cr content is lower than 0.05%. If the
Cr content is higher than 1.0%, the toughness of the steel
decreases. In view of this, the Cr content should be in the range
of 0.05 to 1.0%. The Cr content is preferably higher than 0.05%,
and more preferably 0.2% or higher. The Cr content is preferably
lower than 1.0%, and more preferably 0.8% or lower.
[0059] Mo: Not Less than 0.02% and Less than 0.5%
[0060] Molybdenum (Mo) improves the strength of steel by
transformation toughening and solute strengthening. This effect
cannot be sufficiently achieved if the Mo content is lower than
0.02%. If the Mo content is higher than 0.5%, the toughness of the
steel decreases. In view of this, the Mo content should be not
lower than 0.02% and lower than 0.5%. The Mo content is preferably
higher than 0.02%, and more preferably 0.05% or higher, and still
more preferably 0.1% or higher. The Mo content is preferably 0.4%
or lower, and more preferably 0.3% or lower.
[0061] Ni: 0.03 to 1.0%
[0062] Nickel (Ni) increases the hardenability of steel and
increases the strength of the steel. Further, Ni has the effect of
improving the adherence of scales formed on the surface of the
steel during the heating step for quenching, and also the effect of
reducing the increase in the hardness of the surface layer of the
steel since the scales reduce the cooling rate at the surface of
the steel during the cooling step for quenching. These effects
cannot be sufficiently achieved if the Ni content is lower than
0.03%. If the Ni content is higher than 1.0%, the SSC resistance
decreases. In view of this, the Ni content should be in the range
of 0.03 to 1.0%. The Ni content is preferably 0.05% or higher, and
more preferably 0.08% or higher, and still more preferably 0.10% or
higher. The Ni content is preferably lower than 1.0%, and more
preferably 0.7% or lower, and still more preferably 0.5% or
lower.
[0063] Cu: 0.02 to 1.0%
[0064] Copper (Cu) increases the hardenability of steel and
increases the strength of the steel. Further, Cu has the effect of
improving the adherence of scales formed on the surface of the
steel during the heating step for quenching, and also the effect of
reducing the increase in the hardness of the surface layer of the
steel since the scales reduce the cooling rate at the surface of
the steel during the cooling step for quenching. These effects
cannot be sufficiently achieved if the Cu content is lower than
0.02%. If the Cu content is higher than 1.0%, the weldability of
the steel decreases. Further, if the Cu content is too high, the
grain boundary strength of the steel at high temperatures
decreases, decreasing the hot workability of the steel. In view of
this, the Cu content should be in the range of 0.02 to 1.0%. The Cu
content is preferably 0.05% or higher, and more preferably 0.08% or
higher, and still more preferably 0.10% or higher. The Cu content
is preferably lower than 1.0%, and more preferably 0.7% or lower,
and still more preferably 0.5% or lower.
[0065] V: 0.020 to 0.20%
[0066] Vanadium (V) bonds with C in a steel and forms a V carbide
to increase the strength of the steel. Further, V is dissolved in
an Mo carbide to form a carbide. A carbide containing V is less
likely to form coarse particles. These effects cannot be
effectively achieved if the V content is lower than 0.020%. If the
V content is higher than 0.20%, coarse carbide particles result. In
view of this, the V content should be in the range of 0.020 to
0.20%. The V content is preferably higher than 0.020%, and more
preferably 0.04% or higher. The V content is preferably lower than
0.16%.
[0067] Ca: 0.0005 to 0.005%
[0068] Calcium (Ca) bonds with S in steel to form CaS. As CaS is
formed, the formation of MnS is suppressed. Thus, Ca increases the
toughness and HIC resistance of the steel. These effects cannot be
sufficiently achieved if the Ca content is lower than 0.0005%. If
the Ca content is higher than 0.005%, the cleanliness of the steel
decreases, decreasing the toughness and HIC resistance of the
steel. Thus, the Ca content should be in the range of 0.0005 to
0.005%. The Ca content is preferably higher than 0.0005%, and more
preferably 0.0008% or higher, and still more preferably 0.001% or
higher. The Ca content is preferably lower than 0.005%, and more
preferably 0.003% or lower, and still more preferably 0.002% or
lower.
[0069] The balance of the chemical composition of the seamless
steel pipe in the present embodiment is made of Fe and impurities.
Impurity in this context means an element originating from ore or
scraps used as a raw material of steel or an element that has
entered from the environment or the like during the manufacturing
process.
[0070] Further, the chemical composition of the seamless steel pipe
in the present embodiment may contain Nb in lieu of some of Fe.
[0071] Nb: 0 to 0.05%
[0072] Niobium (Nb) is an optional element. Nb bonds with C and/or
N in steel and forms fine Nb carbide and/or carbonitride particles
to increase the toughness of the steel. Further, Nb is dissolved in
an Mo carbide and forms a specified carbide, thereby preventing
coarse particles of a specified carbide from being produced. On the
other hand, if the Nb content is higher than 0.05%, coarse carbide
particles result. In view of this, the Nb content should be in the
range of 0 to 0.05%. The above effects can be clearly achieved if
the Nb content is 0.010% or higher. The Nb content is preferably
0.015% or higher, and more preferably 0.020% or higher. The Nb
content is preferably 0.040% or lower, and more preferably 0.035%
or lower.
[0073] [Carbon Equivalent Ceq]
[0074] In the seamless steel pipe in the present embodiment, a
carbon equivalent Ceq as defined by equation (1) is not less than
0.430% and less than 0.500%.
Ceq=C+Mn/6+(Cr+Mo+V)/5+F(Ni+Cu)/15 (1),
[0075] where the symbol of each element in equation (1) is
substituted by the content of this element in mass %.
[0076] If the carbon equivalent Ceq is lower than 0.430%, it is
difficult to ensure a certain strength of a seamless steel pipe. If
the carbon equivalent Ceq is 0.500 or higher, it is difficult to
reduce the Vickers hardness of the surface layer to 250 Hv or lower
with a manufacturing process in which the quenching after hot
forming is only one step of direct quenching or in-line
quenching.
[0077] [Microstructure]
[0078] In the microstructure of the seamless steel pipe in the
present embodiment, the main phase from the surface layer to the
in-the-wall portions is tempered martensite or tempered bainite.
The seamless steel pipe in the present embodiment contains no
recrystallized ferrite at least in a region deeper than a position
1 mm deep relative to the surface. Recrystallized ferrite extremely
reduces the hardness of a portion at 1 mm from the surface layer of
the seamless steel pipe.
[0079] The main phase being tempered martensite or tempered bainite
generally means a microstructure in which the volume fraction of
tempered martensite is 50% or higher, a microstructure in which the
volume fraction of tempered bainite is 50% or higher, or a
microstructure in which the sum of the volume fraction of tempered
martensite and the volume fraction of tempered bainite is 50% or
higher. In other words, the above phrase means a microstructure in
which the volume fraction of a structure that is neither tempered
martensite nor tempered bainite (for example, ferrite) is lower
than 50%.
[0080] [Crystal Grain Size Number]
[0081] In the microstructure of the seamless steel pipe of the
present embodiment, the size of the prior austenite grains is lower
than 6.0 in crystal grain size number, as defined in ASTM
E112-10.
[0082] The prior austenite grain size number may be measured in
accordance with ASTM E112-10 by cutting out a test specimen from
each steel pipe preferably before tempering and after quenching,
such that a cross section perpendicular to the length of the steel
pipe (i.e. pipe forming direction) forms the observed surface, and
imbedding the test specimen into a resin and then using the
Bechet-Beaujard method where it is corroded by a picric acid
saturated aqueous solution to let prior austenite grain boundaries
appear.
[0083] Alternatively, the ASTM grain size number of prior austenite
crystal grains of the tempered steel pipe may be determined by
using methods such as electron beam backward scattering diffraction
(EBSD) based on the orientation relationship of crystals. In such
cases, the metal microstructure of a steel pipe after tempering is
observed by EBSD in the following manner: A sample is obtained from
the middle in the wall thickness in a cross section of a tempered
seamless steel pipe (i.e. cross section perpendicular to the axial
direction of the seamless steel pipe); the obtained sample is used
to perform crystal orientation analysis by EBSD for an observed
area of 500.times.500 pmt, and lines are drawn where a prior
austenite grain boundary is defined as the boundary of grains in a
misorientation angle in the range of 15 to 51.degree. and, based on
the resulting drawing, the crystal grain size number is calculated
in accordance with ASTM E112-10.
[0084] Theoretically, the prior austenite grain size after
quenching and before tempering is the same as the prior austenite
grain size after tempering. The prior austenite grain size
determined by EBSD after tempering is substantially equal to the
value obtained by observing crystal grains that were caused to
appear by the Bechet-Beaujard method after quenching and before
tempering, with an error of about .+-.0.2 in grain size number.
Thus, "the size of the prior austenite grains is lower than 6.0 in
crystal grain size number, as defined in ASTM E112-10" as in the
present invention means that, if the crystal grain size after
quenching is not known, at least, a crystal grain size number
determined by EBSD after tempering being lower than 5.8 is in the
scope of the present invention. In the following description,
unless specifically stated, prior austenite grain size is a value
obtained by the Bechet-Beaujard method for a test specimen after
quenching and before tempering.
[0085] If the prior austenite grains are fine grains with a crystal
grain size number of 6.0 or higher, sufficient hardenability cannot
be achieved in a material with a low carbon equivalent Ceq, as in
the present embodiment. Thus, a predetermined strength may not be
obtained. Further, it is difficult to produce a microstructure with
such fine grains with a manufacturing process in which the
quenching after hot forming is only one step of direct quenching or
in-line quenching. The crystal grain size number of prior austenite
grains is preferably 5.5 or lower, and more preferably, 5.0 or
lower.
[0086] [Vickers Hardness and Yield Strength]
[0087] In the seamless steel pipe in the present embodiment, a
portion between a position at 1 mm from the inner surface and a
position at 1 mm from the outer surface has a Vickers hardness of
250 Hv or lower. More specifically, in the seamless steel pipe in
the present embodiment, the Vickers hardness measured in compliance
with JIS Z 2244 at any position between a position at 1 mm from the
inner surface and a position at 1 mm from the outer surface is 250
Hv or lower.
[0088] The seamless steel pipe of the present invention has smaller
variations in hardness along the wall thickness direction. More
specifically, the difference between the Vickers hardness of a
portion at 1 mm from the inner surface and that of a portion in the
middle in the wall thickness, the difference between the Vickers
hardness of a portion at 1 mm from the outer surface and that of a
portion in the middle in the wall thickness, and the difference
between the Vickers hardness of a portion at 1 mm from the inner
surface and that of a portion at 1 mm from the outer surface is 25
Hv or lower.
[0089] The seamless steel pipe in the present embodiment has a
yield strength of .times.80 grade or higher (i.e. 555 MPa or
higher) according to the API standards.
[0090] The seamless steel pipe in the present embodiment may be
suitably used as, although not limited thereto, a seamless steel
pipe with a wall thickness of 25 to 55 mm. More preferably, to
rationalize the use of alloys, the wall thickness of a seamless
steel pipe is in the range of 25 to 40 mm.
[0091] [Manufacturing Method]
[0092] An example of a method of manufacturing the seamless steel
pipe in the present embodiment will be described below. However,
the method of manufacturing the seamless steel pipe in the present
embodiment is not limited thereto.
[0093] [Manufacturing Line]
[0094] FIG. 1 is a block diagram illustrating an example of a
manufacturing line. Referring to FIG. 1, the manufacturing line
includes a heating furnace 1, a piercing machine 2, an elongation
rolling mill 3, a sizing rolling mill 4, a supplementary heating
furnace 5, a water-cooling apparatus 6, and a tempering apparatus
7. A plurality of transport rollers 10 are disposed between these
apparatuses.
[0095] [Manufacturing Flow]
[0096] FIG. 2 is a flow chart illustrating a process for
manufacturing the seamless steel pipe in the present embodiment.
FIG. 3 shows changes in the surface temperature of a workpiece
(i.e. a steel raw material, hollow shell or seamless steel pipe)
during a manufacture versus time. In the graph, A1 indicates the
Ac.sub.1 point when considering a workpiece being heated, and
indicates the Ar.sub.1 point when considering a workpiece being
cooled. Further, in the graph, A3 indicates the Ac.sub.3 point when
considering a workpiece being heated, and indicates the Ar.sub.3
point when considering a workpiece being cooled.
[0097] As shown in FIGS. 1 to 3, the manufacturing process involves
first heating a steel raw material using the heating furnace 1
(heating step: S1). The steel raw material may be a round billet,
for example. The steel raw material may be produced by a continuous
casting system such as round CC. The steel raw material may be
produced by hot working (e.g. forging or blooming) an ingot or
slab. A case with a steel raw material that is a round billet will
be described below.
[0098] The heated round billet is hot-worked to produce a seamless
steel pipe (S2 and S3). More specifically, the round billet is
piercing-rolled by the piercing machine 2 to produce a hollow shell
(piercing-rolling step: S2). Further, the hollow shell is rolled by
the elongation rolling mill 3 and sizing rolling mill 4 to produce
a seamless steel pipe (elongation rolling step and sizing rolling
step S3).
[0099] The seamless steel pipe produced by the hot working is
heated to a predetermined temperature by the supplementary heating
furnace 5 as necessary (supplementary heating step: S4). The
seamless steel pipe produced by the hot working or the heated
seamless steel pipe is quenched by the water-cooling apparatus 6
(quenching step: S5). In either case, the seamless steel pipe
produced by the hot working is quenched without being cooled to
lower than Ar.sub.3 temperature. The quenched seamless steel pipe
is tempered by the tempering apparatus 7 (tempering step S6).
[0100] That is, in the above manufacturing method, quenching is
performed promptly after the hot working is finished. More
specifically, after hot working, quenching is performed before the
temperature of the seamless steel pipe is left to cool to decrease
to around room temperature. A heat treatment where a seamless steel
pipe after hot working is rapidly cooled before the surface
temperature becomes lower than the Ar.sub.3 point will be
hereinafter referred to as "direct quenching", and a heat treatment
where a seamless steel pipe after hot working is supplementarily
heated at a temperature not lower than the Ac.sub.3 point and then
rapidly cooled will be hereinafter referred to as "in-line
quenching". The use of direct quenching or in-line quenching makes
the grains of the microstructure coarser than with a heat treatment
in which a pipe is cooled after its production and then rapidly
cooled (hereinafter referred to as reheating-and-quenching). More
specifically, the crystal grain size number after quenching is
smaller than 6.0. This improves the hardenability of a
microstructure compared with the reheating-and-quenching, and thus
ensures a high strength even when a steel material with a low
carbon equivalent Ceq is used.
[0101] The steps will be described in more detail below.
[0102] [Heating Step (S1)]
[0103] A round billet is heated in the heating furnace 1. The
heating temperature is preferably in the range of 1100 to
1300.degree. C. Heating the round billet to this temperature range
causes the carbonitride in the steel to dissolve. If a round billet
is to be produced from a slab or ingot by hot working, it is only
required that the slab or ingot be heated to a temperature of 1100
to 1300.degree. C., and the temperature to which the round billet
is heated by the heating furnace 1 does not have to be in the range
of 1100 to 1300.degree. C., because the carbonitride in the steel
dissolves when the ingot or slab is being heated. The heating
furnace 1 may be a walking-beam furnace or a rotary furnace, for
example.
[0104] [Piercing Step (S2)]
[0105] The round billet is removed from the heating furnace 1 and
the heated round billet is piercing-rolled by the piercing machine
2 to produce a hollow shell. The piercing machine 2 includes a
plurality of skewed rolls and a plug. The plug is disposed between
the skewed rolls. Preferably, the piercing machine 2 is a
cross-type piercer. A cross-type piercer is preferable because it
can do piercing at high pipe expansion rate.
[0106] [Elongation Rolling Step and Sizing Rolling Step (S3)]
[0107] Next, the hollow shell is rolled. More specifically, the
hollow shell is elongation-rolled by the elongation rolling mill 3.
The elongation rolling mill 3 includes a plurality of roll stands
disposed in series. The elongation rolling mill 3 may be a mandrel
mill, for example. Subsequently, the hollow shell that has been
subjected to elongation rolling is subjected to reduction rolling
by the sizing rolling mill 4 to produce a seamless steel pipe. The
sizing rolling mill 4 includes a plurality of roll stands disposed
in series. The sizing rolling mill 4 may be a sizer or stretch
reducer, for example. The elongation rolling step and sizing
rolling step together may be referred to simply as rolling
step.
[0108] [Supplementary Heating Step (S4)]
[0109] The supplementary heating step (S4) is performed as
necessary. That is, the manufacturing method in the present
embodiment need not include the supplementary heating step (S4).
More specifically, the supplementary heating step (S4) is performed
in such a way that the temperature of the seamless steel pipe is at
a predetermined level that is equal to or higher than the Acs point
directly before the water cooling of the quenching step (S5). If
the supplementary heating step (S4) is not performed, the method in
FIG. 2 proceeds from step S3 to step S5. If the supplementary
heating step (S4) is not performed, the supplementary heating
furnace 5 in FIG. 1 may not be provided.
[0110] If the finishing temperature of the rolling step (i.e.
surface temperature of the seamless steel pipe directly after the
rolling step is finished) is lower than 800.degree. C., it is
preferable to perform the supplementary heating step (S4). At the
supplementary heating step (S4), the seamless steel pipe is
inserted into the supplementary heating furnace 5 and heated. The
heating temperature in the supplementary heating furnace 5 is
preferably in the range of 900 to 1100.degree. C. The soaking time
is preferably 30 minutes or less. If the soaking time is too long,
carbonitrides made of Ti, Nb, C and N, i.e. (Ti, Nb) and (C, N),
may precipitate and form coarse particles. At the supplementary
heating step, the supplementary heating furnace 5 may be replaced
by an induction heating apparatus.
[0111] [Quenching Step (S5)]
[0112] The seamless steel pipe is water-cooled in the water-cooling
apparatus 6. The temperature (i.e. surface temperature) of the
seamless steel pipe directly before water cooling is equal to or
higher than the Ac.sub.3 point, and preferably equal to or higher
than 800.degree. C.
[0113] For water cooling, it is preferable that the cooling rate
for the temperature range of the seamless steel pipe from
800.degree. C. to 500.degree. C. is equal to or higher than
5.degree. C./sec (300.degree. C./min). This provides a uniform
quenched microstructure. The cooling is stopped at a temperature
that is equal to or lower than the Ar.sub.1 point. The temperature
at which cooling is stopped is preferably 450.degree. C. or lower,
and the cooling may be done down to room temperature. The quenching
step (S5) changes the structure of the matrix to a structure mainly
composed of martensite or bainite.
[0114] For example, the water-cooling aperture 6 used for the
quenching step (S5) may have the following construction: The
water-cooling apparatus 6 includes a plurality of rotating rollers,
laminar water flow device, and a jet water flow device. The
rotating rollers are disposed in two rows, and the seamless steel
pipe is positioned between the two rows of rotating rollers. At
this time, the rotating rollers in the two rows are in contact with
bottom portions of the outer surface of the seamless steel pipe.
When the rotating rollers rotate, the seamless steel pipe rotates
about its axis. The laminar water flow device is located above the
rotating rollers and pours water from above the seamless steel
pipe. At this time, the water poured toward the seamless steel pipe
forms a laminar water flow. The jet water flow device is located
near an end of the seamless steel pipe positioned on the rotating
rollers. The jet water flow device emits a jet water flow from the
end of the seamless steel pipe toward the interior of the steel
pipe. The laminar and jet water flow devices cool the outer and
inner surfaces of the seamless steel pipe at the same time. A
water-cooling device 6 with such a construction is suitable for
accelerated cooling for a seamless steel pipe with a large wall
thickness of 25 mm or larger.
[0115] The water-cooling device 6 may be a device other than the
one including rotating rollers, laminar water flow device and jet
water flow device discussed above. The water cooling device 6 may
be a water tank, for example. In such implementations, the seamless
steel pipe is immersed in the water tank and thus subjected to
accelerated cooling. Alternatively, the water-cooling device 6 may
include a laminar water flow device only. To sum up, the cooling
device 6 is not limited to a specific type.
[0116] [Tempering Step (S6)]
[0117] The quenched seamless steel pipe is subjected to tempering.
More specifically, the quenched seamless steel pipe is heated to a
predetermined tempering temperature that is lower than the Ac1
point, and is held at this temperature for a predetermined period
of time in such a way that the Larson-Millar parameter PL as
defined by equation (2) below is 18800 or higher:
PL=(T+273).times.(20+log(t)) (2).
[0118] In equation (2), T is a tempering temperature (.degree. C.),
t is a holding time (in hours) for that temperature. Log (t) is the
logarithm of t whose base is 10.
[0119] If PL is lower than 18800, the reduction in surface hardness
is insufficient and some portions may have a Vickers hardness
exceeding 250 Hv. PL is preferably 18900 or higher.
[0120] If PL is too high, the recrystallization of ferrite occurred
in a region of a depth of 1 mm or deeper from the surface, which
may cause an extreme reduction in strength, a reduction in the sour
resistance in the surface layer and production of blisters. PL is
preferably 20000 or lower, and more preferably 19500 or lower.
[0121] The lower limit of tempering temperature is preferably
600.degree. C., and more preferably 630.degree. C., and still more
preferably 650.degree. C. The upper limit of tempering temperature
is preferably 700.degree. C., and more preferably 680.degree. C.
The lower limit of holding time is preferably one hour, and more
preferably two hours, and still more preferably three hours. The
upper limit of holding time is preferably six hours, and more
preferably five hours, and still more preferably four hours.
[0122] The above manufacturing process provides a seamless steel
pipe with a wall thickness that is as large as 25 mm or more having
good strength, toughness and HIC resistance. The above
manufacturing method is particularly suitable for a seamless steel
pipe with a wall thickness of 25 mm or larger, and can even be used
for a seamless steel pipe with a wall thickness of 40 mm or larger.
The upper limit of wall thickness is not limited to a specific
value, but is typically 60 mm or lower.
[0123] The seamless steel pipe in one embodiment of the present
invention and the method of manufacturing the same have been
described. The present embodiment provides a seamless steel pipe
that can be manufactured by a relatively reasonable manufacturing
process and that provides a yield strength of 555 MPa or higher and
good SSC resistance in a reliable manner.
EXAMPLES
[0124] The present invention will be described using specific
examples. The present invention is not limited to these
examples.
[0125] A plurality of seamless steel pipes with various chemical
compositions were produced and their yield strength, tensile
strength, surface hardness and sour resistance were
investigated.
[0126] [Investigation Methods]
[0127] A plurality of steels having the chemical compositions shown
in Table 1 were melt and were subjected to continuous casting to
produce round billets for pipe forming. Steels A, C, D1, D2 and J
in Table 1 are steels in which the chemical composition or the
value of Ceq does not meet the requirements of the present
invention.
TABLE-US-00001 TABLE 1 Chemical Composition (in mass %, balance
being Fe and impurities) Steel C Si Mn P S Cu Cr Ni Mo Ti V Nb Al
Ca O N Ceq A 0.059 0.12 1.53 0.005 0.0007 0.20 0.28 0.23 0.10 0.003
0.05 -- 0.031 0.0008 0.0015 0.0036 0.429 B 0.061 0.11 1.51 0.008
0.0009 0.20 0.31 0.31 0.25 0.003 0.05 -- 0.032 0.0017 0.0018 0.0050
0.468 C 0.070 0.09 1.42 0.011 0.0005 0.41 0.31 0.39 0.35 0.005 0.05
-- 0.030 0.0013 0.0017 0.0048 0.502 D1 0.066 0.12 1.46 0.009 0.0010
0.02 0.23 0.08 0.09 0.010 0.05 -- 0.037 0.0017 0.0018 0.0039 0.390
D2 0.065 0.12 1.44 0.009 0.0010 0.08 0.26 0.09 0.06 0.007 0.05 --
0.041 0.0016 0.0016 0.0041 0.390 E 0.068 0.11 1.51 0.009 0.0019
0.37 0.28 0.49 0.25 0.004 0.05 -- 0.030 0.0012 0.0012 0.0032 0.493
F 0.061 0.11 1.51 0.010 0.0010 0.20 0.20 0.28 0.25 0.004 0.05 --
0.030 0.0014 0.0015 0.0043 0.445 G 0.060 0.12 1.52 0.009 0.0010
0.21 0.21 0.28 0.25 0.005 0.05 0.020 0.034 0.0012 0.0011 0.0050
0.448 I 0.062 0.12 1.52 0.005 0.0008 0.21 0.27 0.23 0.11 0.006 0.05
0.025 0.031 0.0008 0.0015 0.0036 0.431 J 0.061 0.11 1.42 0.011
0.0018 0.36 0.28 0.49 0.51 0.005 0.04 -- 0.030 0.0013 0.0013 0.0032
0.520 K 0.058 0.12 1.50 0.008 0.0010 0.20 0.31 0.32 0.26 0.003 0.05
-- 0.033 0.0016 0.0017 0.0055 0.467
[0128] The round billets produced were heated by the heating
furnace to a temperature in the range of 1100 to 1300.degree. C.
Subsequently, the round billets were piercing-rolled by the
piercing machine to produce hollow shells. Subsequently, the
mandrel mill was used to elongation-roll the hollow shells.
Subsequently, the sizer was used to reduction-roll (i.e.
sizing-roll) the hollow shells to produce seamless steel pipes
having the outer diameters and wall thicknesses shown in Tables 2
and 3.
TABLE-US-00002 TABLE 2 Pipe-Forming AsQ Mechanical Properties AsQ
Conditions Prior Hv10kgf (maximum Prior Wall .gamma. Tempering
Conditions among positions) .gamma. Outer Thick- grain Soaking
Holding Inner Dif- grain Ferrite Diameter ness size Time Time YS TS
Outer In Sur- fer- size Recrystal- No. Steel (mm) (mm) No.
(.degree. C.) (min) PL (MPa) (MPa) Surface Wall face ence No.
lization Remarks 1 A 273.1 25.0 4.3 660 204 19156 518 592 202 198
208 10 4.2 absent comparative ex. 2 A 273.1 25.0 4.3 660 219 19185
501 577 212 200 213 13 4.3 absent comparative ex. 3 A 273.1 25.0
4.5 665 234 19314 509 580 197 195 162 35 4.4 present comparative
ex. 4 A 273.1 25.0 4.5 650 204 18951 524 602 203 203 220 17 4.5
absent comparative ex. 5 A 273.1 25.0 4.3 650 219 18979 519 593 202
196 214 18 4.6 absent comparative ex. 6 A 273.1 25.0 4.3 650 234
19006 511 585 197 201 215 18 4.1 absent comparative ex. 7 A 273.1
25.0 4.3 650 249 19030 506 585 202 200 219 19 4.6 absent
comparative ex. 8 A 273.1 25.0 4.3 650 264 19054 514 588 200 201
219 19 4.5 absent comparative ex. 9 A 273.1 25.0 4.3 650 294 19097
497 573 199 194 198 5 4.4 absent comparative ex. 10 A 273.1 25.0
4.3 650 205 18953 544 619 218 218 220 2 4.3 absent comparative ex.
11 A 273.1 25.0 4.3 630 204 18540 543 622 213 212 248 36 4.3 absent
comparative ex. 12 A 273.1 25.0 4.3 630 219 18568 541 620 209 210
236 27 4.4 absent comparative ex. 13 A 273.1 25.0 4.3 630 234 18594
531 610 213 208 242 34 4.3 absent comparative ex. 14 A 273.1 25.0
4.5 630 249 18618 531 610 206 202 240 38 4.4 absent comparative ex.
15 A 273.1 25.0 4.5 630 264 18641 536 615 211 209 239 30 4.5 absent
comparative ax. 16 A 273.1 25.0 4.3 630 294 18683 526 602 210 203
238 35 4.3 absent comparative ex. 17 A 273.1 25.0 4.3 600 204 17924
531 622 210 213 257 47 4.4 absent comparative ax. 18 B 323.9 25.0
4.3 700 294 20132 503 582 209 193 170 39 4.5 present comparative
ex. 19 B 323.9 25.0 4.3 700 204 19977 575 641 192 193 209 17 4.3
absent inventive ex. 20 B 323.9 25.0 4.3 690 204 19772 578 646 203
206 211 8 4.4 absent inventive ex. 21 B 323.9 25.0 4.5 680 204
19566 579 646 212 220 222 10 4.5 absent inventive ex. 22 B 323.9
25.0 4.3 670 204 19361 597 659 236 220 239 19 4.2 absent inventive
ex. 23 B 323.9 25.0 4.3 665 149 19131 621 688 238 240 239 2 4.3
absent inventive ex. 24 B 323.9 25.0 4.3 660 204 19156 606 670 225
226 239 14 4.4 absent inventive ex. 25 B 323.9 25.0 4.3 660 219
19185 601 665 224 230 239 15 4.3 absent inventive ex. 26 B 323.9
25.0 4.3 665 234 19314 600 664 233 226 235 9 4.3 absent inventive
ex. 27 B 323.9 25.0 4.5 650 204 18951 631 697 248 244 249 5 4.5
absent inventive ex. 28 B 323.9 25.0 4.5 650 219 18979 620 683 235
235 248 13 4.7 absent inventive ex. 29 B 323.9 25.0 4.5 650 234
19006 620 681 235 235 248 13 4.5 absent inventive ex. 30 B 323.9
25.0 4.3 650 249 19030 617 683 242 226 248 22 4.3 absent inventive
ex.
TABLE-US-00003 TABLE 3 Pipe-Forming AsQ Mechanical Properties AsQ
Conditions Prior Hv10kgf (maximum Prior Wall .gamma. Tempering
Conditions among positions) .gamma. Outer Thick- grain Soaking
Holding Inner Dif- grain Ferrite Diameter ness size Time Time YS TS
Outer In Sur- fer- size Recrystal- No. Steel (mm) (mm) No.
(.degree. C.) (min) PL (MPa) (MPa) Surface Wall face ence No.
lization Remarks 31 B 323.9 25.0 4.5 650 264 19054 617 679 236 232
247 15 4.7 absent inventive ex. 32 B 323.9 25.0 4.5 650 294 19097
612 674 227 226 237 11 4.3 absent inventive ex. 33 B 323.9 25.0 4.3
650 315 19125 619 683 236 234 237 3 4.6 absent inventive ex. 34 B
323.9 25.0 4.3 630 204 18540 649 720 241 241 269 28 4.4 absent
comparative ex. 35 B 323.9 25.0 4.5 630 219 18568 644 715 251 242
268 26 4.3 absent comparative ex. 36 B 323.9 25.0 4.6 630 234 18594
654 725 256 240 266 26 4.7 absent comparative ex. 37 B 323.9 25.0
4.3 630 249 18618 627 698 229 244 268 39 4.3 absent comparative ex.
38 B 323.9 25.0 4.3 630 264 18641 624 693 228 240 266 38 4.3 absent
comparative ex. 39 B 323.9 25.0 4.6 630 294 18683 628 697 241 234
260 26 4.5 absent comparative ex. 40 B 323.9 25.0 4.3 600 204 17924
639 728 260 253 286 33 4.7 absent comparative ex. 41 B 323.9 25.0
4.5 655 105 18786 635 708 245 232 263 31 4.2 absent comparative ex.
42 B 323.9 25.0 4.5 650 105 18684 636 707 251 233 264 31 4.5 absent
comparative ex. 43 C 323.9 25.0 4.6 650 105 18684 636 714 248 244
282 38 4.6 absent comparative ex. 44 C 323.9 25.0 4.6 650 185 18911
573 657 232 228 269 41 4.5 absent comparative ex. 45 D1 323.9 25.4
4.5 656 117 18849 478 561 178 178 205 27 4.5 absent comparative ex.
46 D2 323.9 25.4 4.3 650 130 18770 468 582 179 191 207 28 4.2
absent comparative ex. 47 E 406.4 38.1 4.3 600 204 17924 618 701
267 228 222 45 4.1 absent comparative ex. 48 E 406.4 38.1 4.6 630
204 18540 603 673 255 220 227 35 4.5 absent comparative ex. 49 E
406.4 38.1 4.5 630 234 18594 609 679 254 217 219 37 4.4 absent
comparative ex. 50 E 406.4 38.1 4.3 630 264 18641 609 674 252 214
228 38 4.2 absent comparative ex. 51 E 406.4 38.1 4.3 630 294 18683
609 675 251 223 219 32 4.4 absent comparative ex. 52 E 406.4 38.1
4.3 650 204 18951 599 665 237 222 214 23 4.3 absent inventive ex.
53 E 406.4 38.1 4.5 650 234 19006 575 644 231 212 211 20 4.5 absent
inventive ex. 54 E 406.4 38.1 4.5 650 264 19054 575 641 230 209 208
22 4.6 absent inventive ex. 55 E 406.4 38.1 4.5 650 294 19097 565
641 226 204 208 22 4.4 absent inventive ex. 56 E 406.4 38.1 4.6 660
204 19156 565 635 228 205 206 23 4.6 absent inventive ex. 57 E
406.4 38.1 4.3 660 234 19211 559 638 220 196 200 22 4.4 absent
inventive ex. 58 F 323.9 25.0 4.5 650 200 18942 570 643 212 219 224
12 4.7 absent inventive ex. 59 G 323.9 25.0 5.4 650 200 18942 597
665 236 223 238 15 5.7 absent inventive ex. 60 I 273.1 25.0 5.5 650
200 18942 575 647 200 203 211 11 5.5 absent inventive ex. 61 J
323.9 25.0 4.7 665 149 19131 680 734 261 252 283 31 4.7 absent
comparative ex. 62* K 323.9 25.0 6.8 665 234 19314 543 625 236 208
228 28 6.9 absent comparative ex. *62: in-line quenching at
950.degree. C. + reheating at 950.degree. C. and quenching +
tempering
[0129] The seamless steel pipes that had been subjected to sizing
rolling were heated by the supplementary heating furnace to
950.degree. C., and quenching was then performed by the
water-cooling apparatus where the pipes were cooled to room
temperature at a cooling rate of 5.degree. C./sec or higher.
[0130] After the quenching, the seamless steel pipes were tempered
at the soaking temperatures and holding times shown in Tables 2 and
3. However, during the production of the steel of No. 62, after the
above quenching was performed, before tempering, quenching was
performed where the steel was reheated off-line to 950.degree. C.
and soaked for 20 minutes and then water-cooled.
[0131] The following evaluation tests were conducted on the
seamless steel pipes produced in the above production process.
[0132] [Yield Strength and Tensile Strength Tests]
[0133] The yield strength of the seamless steel pipe of each number
was investigated. More specifically, a No. 12 test specimen (with a
width of 25 mm and a gauge length of 50 mm) as specified in JIS Z
2241 was taken out so that the longitudinal direction of the
specimen for tensile testing was parallel to the longitudinal
direction of the steel pipe (i.e. L direction). The test specimen
that had been taken out was used to conduct a tensile test in
compliance with JIS Z 2241 in the atmosphere at room temperature
(25.degree. C.), and the yield strength (YS) and tensile strength
(TS) were determined. The yield strength was determined using a
0.5% total elongation method. The determined yield strength (in
MPa) and tensile strength (in MPa) are shown in Tables 2 and 3. The
columns labeled "YS" in Tables 2 and 3 have yield strength and the
columns labeled "TS" have tensile strengths determined for the test
specimens of the various test numbers.
[0134] [Surface Hardness Test]
[0135] Four test specimens were taken from the seamless steel pipe
of each number, the specimens being displaced from each other by
90.degree. along the pipe's circumference, and a Vickers hardness
test in compliance with JIS Z 2244 was conducted on arbitrary three
points on a transverse cross-section (i.e. cross-section
perpendicular to the center axis) of each test specimen, the points
being at 1 mm inwardly in the wall thickness direction from the
inner surface. The force in the Vickers hardness tests, F, was 10
kgf (i.e. 98.07 N). The maximum among the values for the 12 points
that had been obtained was used as the value of hardness "at 1 mm
from the inner surface".
[0136] Similarly, a Vickers hardness test was conducted on
arbitrary three points of each of the four test specimens of the
seamless steel pipe of each test number, the points being at 1 mm
inwardly in the wall thickness direction from the outer surface,
and the maximum among the values of the 12 points that had been
obtained was used as the value of hardness "at 1 mm from the outer
surface". Further, a Vickers hardness test was conducted on
arbitrary three points of each of the four test specimens of the
seamless steel pipe of each test number, the points being near the
middle in the wall thickness, and the maximum among the values of
the 12 points that had been obtained was used as the value of
hardness "in the wall".
[0137] For the seamless steel pipe of each test number, the value
of hardness "at 1 mm from the outer surface", the value of hardness
"at 1 mm from the inner surface" and the value of hardness "in the
wall" are shown in Tables 2 and 3, in the columns labeled "Outer
Surface", "In Wall" and "Inner Surface".
[0138] The largest value among the difference between the hardness
"at 1 mm from the outer surface" and the hardness "in the wall",
the difference between the hardness "at 1 mm from the inner
surface" and the hardness "in the wall", and the difference between
the hardness "at 1 mm from the outer surface" and the hardness "at
1 mm from the inner surface" (hereinafter referred to as "maximum
difference in hardness") is shown in the column labeled
"Difference" in Tables 2 and 3.
[0139] [Observation of Microstructure]
[0140] A sample was taken from the seamless steel pipe of each
number, the sample containing the inner surface, outer surface and
middle in the wall thickness, and the microstructure was observed.
More specifically, each sample was etched by a nital etching
solution to cause the microstructure to appear, which was observed
using optical microscopy.
[0141] The seamless steel pipe of each number had a microstructure
having a main phase of tempered martensite or tempered bainite.
However, in some seamless steel pipes, recrystallization of ferrite
had occurred in a region of a depth of 1 mm or deeper from the
surface. Whether recrystallization of ferrite occurred in a region
of a depth of 1 mm or deeper from the surface is shown in the
column labeled "Ferrite Recrystallization" in Tables 2 and 3.
[0142] The crystal grain size number of the prior austenite grains
of the microstructure was measured by the following method: First,
a test specimen was cut out from each steel pipe such that a cross
section perpendicular to the length of the steel pipe as quenched
(i.e. pipe forming direction) forms the observed surface, and was
imbedded into a resin; then the Bechet-Beaujard method was used
where it is corroded by a picric acid saturated aqueous solution to
let prior austenite grain boundaries appear, which were observed by
optical microscopy (with a magnification of 200 times), and the
prior austenite grain size number was measured in accordance with
ASTM E112-10. Such grain size numbers are shown in the column "AsQ
Prior .gamma. grain size No." in Tables 2 and 3.
[0143] Since the grain size number of prior austenite grains after
tempering cannot be measured using picric acid saturated aqueous
solution corrosion; in view of this, the number was measured with
the help of EBSD. EBSD was performed by cutting out a test specimen
such that a cross section perpendicular to the length of a tempered
steel pipe forms the observed surface, finishing the observed
surface by mirror polishing and electrolysis polishing, and an area
of 500.times.500 pmt in the middle in the thickness of the steel
pipe was observed. A detector for EBSD mounted on an FE-SEM
(DigiViewIV from EDAX) was used. Based on the obtained crystal
orientation data, analysis software (OIM Analysis ver. 6 from EDAX)
was used to draw lines along the boundaries between crystal grains
in misorientation angles of 15 to 51.degree., and the resulting
line drawing was used to measure the prior austenite grain size
number in accordance with ASTM E112-10. Such grain size numbers are
shown in the column "QT Prior .gamma. Grain Size No." in Tables 2
and 3.
[0144] [Results of Investigation]
[0145] As shown in Tables 1 to 3, the seamless steel pipes of Nos.
19 to 33 and 52 to 60 had a chemical composition falling in the
scope of the present invention and had a carbon equivalent Ceq not
lower than 0.430% and lower than 0.500%. In these seamless steel
pipes, recrystallization of ferrite did not occur in a region of a
depth of 1 mm or deeper from the surface, and a structure was
present having a main phase of tempered martensite or tempered
bainite from the surface layer to the in-the-wall portions, and the
crystal grain size number of the prior austenite grains was lower
than 6.0. Further, these seamless steel pipes had Vickers hardness
values "at 1 mm from the outer surface", "at 1 mm from the inner
surface" and "in the wall" that were not higher than 250 Hv and had
a yield strength of 555 MPa or higher. These seamless steel pipes
had a maximum difference in hardness of 25 Hv or lower.
[0146] The seamless steel pipes of Nos. 1 to 17 had a yield
strength lower than 555 MPa. This is presumably because the carbon
equivalent Ceq of steel A was too low.
[0147] In the seamless steel pipe of No. 18, recrystallization of
ferrite occurred in a region of a depth of 1 mm or deeper from the
surface. Consequently, the seamless steel pipe of No. 18 had a
yield strength lower than 555 MPa. This is presumably because the
Larson-Miller parameter PL of the seamless steel pipe of num No.
ber 18 was too high.
[0148] The seamless steel pipes of Nos. 34 to 42 and 47 to 51 had a
Vickers hardness value "at 1 mm from the outer surface", "at 1 mm
from the inner surface" or "in the wall" that was higher than 250
Hv. Further, these seamless steel pipes had a maximum difference in
hardness higher than 25 By. This is presumably because the
Larson-Miller parameters PL of the seamless steel pipes of Nos. 34
to 42 and 47 to 51 were too low.
[0149] The seamless steel pipes of Nos. 43 and 44 had a Vickers
hardness "at 1 mm from the inner surface" higher than 250 Hv. This
is presumably because the carbon equivalent Ceq of steel C was too
high.
[0150] The seamless steel pipes of Nos. 45 and 46 had yield
strengths lower than 555 MPa. This is presumably because the carbon
equivalents Ceq of steels D1 and D2 were too low.
[0151] In the seamless steel pipe of No. 61, the Vickers hardness
was higher than 250 Hv at all the measurement points. This is
presumably because the carbon equivalent Ceq of steel J was too
high.
[0152] The seamless steel pipe of No. 62 had a yield strength lower
than 555 MPa. This is presumably because both in-line quenching and
reheating-and-quenching were used, which produced too fine prior
austenite grains, reducing hardenability and thus leading to
insufficient strength.
[0153] FIG. 4 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and yield strength YS for steel
B. As shown in FIG. 4, the yield strength YS tended to decrease as
the Larson-Miller parameter PL increased. Steel B provided a yield
strength of 555 MPa or larger except for the seamless steel pipe of
No. 18, in which the recrystallization of ferrite progressed.
[0154] FIG. 5 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and yield strength YS for steel
A. Steel A did not provide a yield strength not lower than 555 MPa
even though tempering conditions were adjusted. This is presumably
because the carbon equivalent Ceq of steel A was too low.
[0155] FIG. 6 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and hardness at an outer
surface, an in-the-wall portion and an inner surface for steel B.
As shown in FIG. 6, the hardnesses at the outer surface,
in-the-wall portion and inner surface tended to decrease as the
Larson-Miller parameter PL increased. As shown in FIG. 6, when the
Larson-Miller parameter PL was 18800 or higher, the hardnesses at
the outer surface, in-the-wall portion and inner surface were 250
Hv or lower. On the other hand, when the Larson-Miller parameter PL
was lower than 18800, the hardness at the outer surface,
in-the-wall portion or inner surface was higher than 250 Hv.
[0156] FIG. 7 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and hardness at an outer
surface, an in-the-wall portion and an inner surface for steel A.
In steel A, similar to steel B, the hardnesses at the outer
surface, in-the-wall portion and inner surface tended to decrease
as the Larson-Miller parameter increased.
[0157] FIG. 8 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and maximum difference in
hardness for steel B. As shown in FIG. 8, when the Larson-Miller
parameter PL was 18800 or higher, the maximum difference in
hardness was not more than 25 Hv. The seamless steel pipe of No. 18
had a large maximum difference in hardness presumably because the
recrystallization of ferrite progressed in a region of a depth of 1
mm or deeper from the surface.
[0158] FIG. 9 is a scatter plot illustrating the relationship
between Larson-Miller parameter PL and maximum difference in
hardness for steel A. As shown in FIG. 9, the relationship between
Larson-Miller parameter PL and maximum difference in hardness in
steel A exhibited similar tendencies. The seamless steel pipe of
No. 3 had a large maximum difference in hardness presumably because
the recrystallization of ferrite progressed in a region of a depth
of 1 mm or deeper from the surface.
[0159] [Evaluation of Sour Resistance]
[0160] A sour resistance evaluation as described below (i.e. HIC
resistance test, four-point bending test) was conducted on the
seamless steel pipes of some of the numbers.
[0161] [HIC Resistance Test]
[0162] From each seamless steel pipe were taken out a test specimen
containing the inner surface, a test specimen containing the middle
in the wall thickness, and a test specimen containing the outer
specimen. Each test specimen had a thickness of 20 mm and a width
(along the circumference) of 20 mm, and a length of 100 mm. The MC
resistance of each test specimen was evaluated in accordance with
NACE (National Association of Corrosion Engineers) TM 0284-2011.
The testing bath in which the test specimens were immersed was a 5%
salt+0.5% acetic acid aqueous solution saturated with hydrogen
sulfide gas at 1 atm at a temperature of 24.degree. C.
[0163] After 96 hours of immersion, ultrasonic inspection (UT) was
conducted on the test specimens after being tested to determine the
location of the largest crack, and the specimen was cut at this
location. The cross-section at this time was a cross-section of
thickness.times.width of the test specimen, i.e. perpendicular to
the longitudinal direction of the steel pipe. The cut test specimen
was used to determine the crack-length ratio CLR (=crack length
(mm)/width of test specimen (mm)). The maximum value among the CLR
values of the test specimen taken from each steel pipe was used as
the crack-length ratio CLR for this test number.
[0164] Further, it was determined whether each test specimen after
being tested had a blister (i.e. a swollen part due to a crack near
the surface), and the number of blisters produced on the test
specimen was counted. The maximum among the numbers of blisters on
the test specimen taken from each steel pipe was used as the number
of blisters for this test number.
[0165] [Four-Point Bending Test]
[0166] A stress of 95% of the actual yield strength (i.e. yield
strength of the seamless steel pipe of each number) was applied to
a test specimen containing the middle in the wall thickness of this
seamless steel pipe using a four-point bending jig in accordance
with ASTM G39. The test specimens to which stresses were applied
were placed in a test bath. The test bath was a 5% salt+0.5% acetic
acid aqueous solution saturated with hydrogen sulfide gas at 1 atm
at a temperature of 24.degree. C. After 720 hours, it was visually
determined whether there was a crack in the test specimens. If a
plate material had no crack, it was determined that this material
had good SSC resistance.
[0167] [Evaluation Results]
[0168] The results of sour resistance evaluation were shown in
Table 4.
TABLE-US-00004 TABLE 4 Hv10kgf Four- (maximum for positions) HIC
Point Outer In Inner Resistance Number of Bending No. PL YS (MPa)
Surface Wall Surface Difference Test Blisters Test 10 18953 544 218
218 220 2 .largecircle. 0 .largecircle. 3 19314 509 197 195 162 35
.largecircle. 3 -- 18 20132 503 209 193 170 39 .largecircle. 2 --
23 19131 621 238 240 239 2 .largecircle. 0 .largecircle. 33 19125
619 236 234 237 3 .largecircle. 0 .largecircle. 37 18618 627 229
244 268 39 CLR 1% 0 -- 40 17924 639 260 253 286 33 CLR 2% 0 -- 43
18684 636 248 244 282 38 CLR 2% -- -- 44 18911 573 232 228 269 41
CLR 2% -- -- 52 18951 599 237 222 214 23 .largecircle. 0 -- 57
19211 559 220 198 200 22 .largecircle. 0 -- 58 18942 570 212 219
224 12 .largecircle. 0 -- 59 18942 597 236 223 238 15 .largecircle.
0 -- 60 18942 575 200 203 211 11 .largecircle. 0 -- In Table 4,
".largecircle." in the columns labeled "HIC Resistance Test" and
"Four-Point Bending Test" indicates that there was no crack in the
relevant test. " " in the columns labeled "HIC Resistance Test" and
"Four-Point Bending Test" indicates that the relevant test was not
conducted.
[0169] As shown in Table 4, in the seamless steel pipes with a
yield strength of 555 MPa or higher and Vickers hardness values "at
1 mm from the outer surface", "at 1 mm from the inner surface" and
"in the wall" not higher than 250 Hv, no crack occurs in both the
HIC resistance test and four-point bending test, and a good sour
resistance was provided in a reliable manner. On the other hand,
the seamless steel pipes with Vickers hardness values "at 1 mm from
the outer surface", "at 1 mm from the inner surface" or "in the
wall" higher than 250 Hv provided a poor sour resistance. These
results prove a relationship between Vickers hardness and sour
resistance.
[0170] Although embodiments of the present invention have been
described, these embodiments are merely examples that may be used
to carry out the present invention. Accordingly, the present
invention is not limited to the above embodiments and the above
embodiments can be modified as appropriate without departing from
the spirit of the invention.
* * * * *