DESCRIPTION
SEAMLESS STEEL PIPE FOR LINE PIPE AND METHOD FOR PRODUCING THE
SAME
TECHNICAL FIELD
[OOOl]
The present invention relates to a seamless steel pipe for line pipe and a method
for producing the same and, more particularly, to a high-strength seamless steel pipe for
line pipe having a low surface hardness and a method for producing the same.
BACKGROUND ART
[0002]
In recent years, the mining conditions of oil wells and gas wells (hereinafter, oil
and gas wells are collectively referred to simply as "oil wells") of crude oil, natural gas,
and the like have become severer. Concerning the mining environments, as the mining
depth increases, the atmosphere contains C02, H2S, C1-, and the like, and the mined crude
oil and natural gas also contain large amounts of H2S. Therefore, the requirement on
perfomiance of a line pipe for transmitting the crude oil and natural gas has also become
more rigid, and a demand for steel pipes for line pipe having sulfide resistance has
increased.
[0003]
The standards of National Association of Corrosion Engineers (NACE) specify
the highest hardness of steel for steels used in H2S environments from the viewpoint of
sulfide stress cracking resistance (hereinafter, referred also to as "SSC resistance"), which
is 250 HVlO or lower for carbon steels. Also, to assure safety, steels may, in some cases,
be required to have the highest hardness of 230 HVlO or lower in consideration of the
hardening of weld heat affected zone. Therefore, for steels that are required to have
sulfide resistance, the improvement in technique for restraining hardness has become an
important problem. The "HV10" means a "hardness symbol" in the case where Vickers
hardness test is conducted with the test force being 98.07 N (10 kgf).
[0004]
In the case where a high-strength seamless steel pipe for line pipe is produced,
unlike the production process of UO steel pipes in which controlled rolling is performed,
it is a common practice to perform quenching followed by tempering, in order to assure
strength. For a low-alloy steel such as a steel for line pipe, by the ordinary quenching
tempering treatment, martensite is not formed, and a micro-structure consists mainly of
bainite. Because of great dependence on cooling rate, the micro-structure sometimes
differs between the surface and the interior of steel pipe. Therefore, as compared with
the interior in which the cooling rate is low, the surface, on which the cooling rate is high,
tends to have a high hardness. As the result, the highest hardness on the surface becomes
high with respect to the strength of steel. This tendency becomes remarkable as the
strength increases and the wall thickness increases because of the increase in amount of
added alloying elements.
[0005]
Patent Document 1 discloses a seamless steel pipe having a wall thickness of 30
mm or larger and a high strength of X65 grade or higher (yield strength: 448 MPa or
higher). Also, Patent Document 2 discloses a seamless steel pipe having a strength of
X70 grade or higher and excellent hydrogen-induced cracking resistance.
[0006]
To solve the problem of increase in the highest hardness, Patent Document 3
proposes a method for softening only the casehardened layer by means of local heating
such as induction heating due to high-frequency current. Also, Patent Document 4
proposes a method for performing the quenching tempering treatment from a two-phase
region of austenite and ferrite. Further, Patent Document 5 proposes a method in which
cooling is stopped by controlled cooling in mid-course and the outer near-surface portion
is subjected to heat recuperation by the heat in the high-temperature portion on the surface
side in a steel pipe, before cooling again the outer near-surface portion.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0007]
Patent Document 1 : US2007100898 13A
Patent Document 2: JP2004-143593A
Patent Document 3: JP63-143222A
Patent Document 4: JP2-282427A
Patent Document 5: JP2-243722A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0008]
Generally, a seamless steel pipe for line pipe is produced through a quenching
tempering process. Therefore, the hardness of steel pipe near-surface portion that is
inevitably cooled rapidly is higher than the hardness of the central portion of wall
thickness of steel pipe. In Patent Documents 1 and 2, however, the surface hardness is
not considered, and therefore there arises a following problem: it is difficult to stably
make the hardness of steel pipe near-surface portion 250 HVl 0 or lower.
[0009]
In the method disclosed in Patent Document 3, in addition to the cooling
equipment used during quenching, cooling equipment used after local heating is necessary,
and also the control of heat treatment temperature is complicated. Also, the method
disclosed in Patent Document 4 has a following problem: a good micro-structure cannot
necessarily be obtained from the viewpoint of the balance of strength, toughness, and
corrosion resistance. The method disclosed in Patent Document 5 has a following
problem: it is difficult to control the productivity and heat treatment temperature and to
assure the balance of strength, toughness, and the like.
[OO 1 01
The present invention has been made to solve the above-described problems, and
accordingly an objective thereof is to provide a high-strength and high-toughness
seamless steel pipe for line pipe, in which the surface hardness is kept low, and a method
for producing the seamless steel pipe.
MEANS FOR SOLVING THE PROBLEMS
[OO 1 11
The present inventors earnestly studied a method for increasing the strength and
toughness and for keeping the surface hardness low, and resultantly came to obtain the
following findings.
[OO 121
Steel pipes were subjected to quenching tempering treatment, and surface
hardness was measured at various places. Resultantly, it was found that the measured
values varied greatly. If the heat treatment conditions are fixed, the surface hardness of
steel pipe is determined by the chemical composition and the cooling rate. The chemical
composition on the steel pipe surface was analyzed, and resultantly the segregation of
components was not recognized. Therefore, it is thought that the variations in surface
hardness are attributable to the variations in local cooling rate.
[00 131
Accordingly, the cause for variations in cooling rate on the steel pipe surface was
further studied. The surface properties of steel pipe were observed in detail, and it was
found that the hardness is high in a location where the scale on the steel pipe surface is
detached, and the hardness is low in a location where the scale adheres firmly to the
surface. That is to say, the variations in cooling rate depend on whether the scale adheres
tightly to the surface or peels off. Therefore, it is thought that if the scale on the steel
pipe surface can be caused to adhere to the surface uniformly, the variations in hardness
are suppressed, and therefore the highest hardness can be restrained.
[00 141
The present inventors studied the method for improving the adhesion of scale,
and found out that by containing a predetermined amount of Ni or further Cu in the base
metal of steel pipe, metal particles consisting mainly of Ni or Cu are dispersed finely in
the scale, and thereby the adhesion of scale can be improved.
[00 1 51
The relationship between the adhesion of scale and the dispersing state of metal
particles was further examined. As the result, it was found that in order to improve the
adhesion of scale, a simple dispersion of metal particles consisting mainly of Ni or Cu in
scale is not enough, and it is important to cause the scale to grow sufficiently, and to cause
the metal particles to exist in a wide range from the boundary between base metal and
scale toward the scale side.
[00 161
Generally, if the thickness of scale increases, the scale adhesion deteriorates.
However, the scale in which metal particles consisting mainly of Ni or Cu are dispersed
exerted good adhesion even if the scale was thick. Also, a thick scale reduces the
cooling rate of the steel pipe surface portion due to the thermal insulation effect, so that
the increase in surface hardness can be suppressed.
[00 1 71
With the decrease in cooling rate in the near-surface portion, the cooling rate in
the central portion of wall thickness further decreases. This phenomenon establishes a
condition that the strength is less liable to increase. However, by containing Ni or
further Cu in the steel, the hardenability is guaranteed, so that high strength and toughness
can be maintained.
[OO 1 81
Furthermore, since the as-quenched hardness depends on the carbon content, by
keeping the C content low, the hardness can be decreased. Also, in order to restrain the
surface hardness, it is necessary to properly control the contents of Mo, V and Nb that
cause secondary hardening during tempering.
[0019]
The present invention has been accomplished on the basis of the above-described
findings, and involves the seamless steel pipe for line pipe and the method for producing
the same that are described below.
[0020]
(1) A seamless steel pipe for line pipe, wherein a chemical composition consists,
by mass percent, of
C: 0.03 to 0.10%,
Si: 0.50% or less,
Mn: 1.0 to 2.0%,
P: 0.050% or less,
S: 0.005% or less,
Cr: 0.05 to 1.0%,
Mo: 0.01 to 0.30%,
Al: 0.001 to 0.10%,
N: 0.01% or less,
Ni: 0.04 to 2.0%,
Ca: 0.0005 to 0.0050%,
Cu: 0 to 2.0%,
Ti: 0 to 0.05%,
Nb: 0 to 0.05%,
V: 0 to 0.10%,
the balance: Fe and impurities, and
the chemical composition satisfies the conditions of
Cu + Ni: 0.10% or more, and
Mo + V: 0.30% or less, wherein
in a scale formed on the surface of the steel pipe, metal particles consisting
mainly of Ni or Cu having an average circle-equivalent diameter of 0.1 to 5 pm exist, and
a distance from a boundary between the base metal of the steel pipe and the scale to a
region in which the metal particles do not exist is 20 pn or longer.
[002 11
(2) The seamless steel pipe for line pipe according to (I), wherein
the chemical composition contains, by mass percent,
one or more elements selected from
Cu: 0.01 to 2.0%,
Ti: 0.003 to 0.05%,
Nb: 0.01 to 0.05%, and
V: 0.02 to 0.10%.
[0022]
(3) The seamless steel pipe for line pipe according to (I), wherein
the chemical composition consists, by mass percent, of
C: 0.03 to 0.10%,
Si: 0.30% or less,
Mn: 1 .OO to 1 .80%,
P: 0.020% or less,
S: 0.003% or less,
Ti: 0.001 to 0.015%,
Al: 0.001 to 0.050%,
Ni: 0.04 to 0.30%,
Cu: 0 to 0.30%,
Cr: 0.05 to 0.40%,
Mo: 0.02 to 0.15%,
V: 0.02 to 0.09%,
Ca: 0.0005 to 0.0030%,
N: 0.008% or less,
the balance: Fe and impurities, and
the chemical composition satisfies the conditions of
Cu + Ni: 0.10 to 0.50%, and
Mo + V: 0.05 to 0.20%.
[0023]
(4) The seamless steel pipe for line pipe according to (3), wherein the highest
hardness at a position 1 mm deep from the surface is 230 HVlO or lower.
[0024]
(5) The seamless steel pipe for line pipe according to any one of (1) to (4),
wherein:
a metal micro-structure of the steel pipe contains bainite of 50% or more in area
fraction; and
a wall thickness of the steel pipe is 30 mm or larger.
100251
(6) The seamless steel pipe for line pipe according to any one of (1) to (5),
wherein at a position 10 pn distant from the boundary to the scale side, the number
density of the metal particles observed per unit area is 5 x lo3 pieces/mm2 or higher.
[0026]
(7) The seamless steel pipe for line pipe according to any one of (1) to (6),
wherein a difference between the average hardness at the position 1 rnrn deep from the
surface of the steel pipe and the average hardness in the central portion of the wall
thickness is 30 HVlO or smaller.
LO0271
(8) The seamless steel pipe for line pipe according to any one of (1) to (7),
wherein a difference between the maximum value and the minimum value of the hardness
at the position 1 rnrn deep from the surface of the steel pipe is 40 HVlO or smaller.
[0028]
(9) A method for producing the seamless steel pipe for line pipe, wherein the
steel pipe having the chemical composition described in any one of (1) to (3) is conveyed
into a furnace after the finish of hot rolling, being heated in an atmosphere with the
temperature of Ac3+50°C or higher and the water vapor concentration of 5% or higher,
and thereafter is subjected to quenching treatment in which accelerated cooling is
performed at a rate of 10°C/s or higher, subsequently being tempered at a temperature of
ACI-50°C or lower.
[0029]
(1 0) The method for producing the seamless steel pipe for line pipe according to
(9), wherein after the finish of hot rolling, the steel pipe is conveyed into a furnace before
the surface temperature of the steel pipe decreases to a temperature lower than the Ar3
transformation point.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0030]
According to the present invention, the steel pipe has a yield strength of 448 MPa
or higher, and also the highest hardness on the surface of steel pipe can be restrained to
250 HVlO or lower, preferably 230 HVlO or lower, so that a ligh-strength and hightoughness
seamless steel pipe excellent in SSC resistance can be obtained. Therefore,
the seamless steel pipe according to the present invention can be used suitably as a line
pipe for transmitting crude oil and natural gas containing large amounts of H2S.
BRIEF DESCRIPTION OF THE DRAWINGS
[003 11
[Figure 11 Figure la is a backscattered electron image near the boundary between base
metal and scale of the steel pipe of the present invention, and Figures lb and lc are
element mapping images obtained by EPMA.
[Figure 21 Figure 2 is diagrams showing the hardness distributions of the seamless steel
pipes in accordance with an inventive example and a comparative example.
MODE FOR CARRYING OUT THE INVENTION
100321
Components of the present invention is described below in detail.
[0033]
1. Scale
The seamless steel pipe for line pipe according to the present invention have
metal particles consisting mainly of Ni or Cu with an average circle-equivalent diameter
of 0.1 to 5 pm in the scale formed on the surface of steel pipe. Figure la is a
backscattered electron image near the boundary between base metal and scale of the steel
pipe according to the present invention, and Figures lb and lc are element mapping
images of a region that is the same as the region of Figure la, which are obtained by
EPMA (Electron Probe MicroAnalyser). Figures lb and lc represent the distributions
of Ni and Cu, respectively. In the present invention, the "metal particles consisting
mainly of Ni or Cu" include the "metal particles consisting mainly of Ni and Cu".
COO341
The backscattered electron image and element mapping images are those of the
steel pipe having been quenched; however, even if tempering is performed subsequently,
the properties of scale and the dispersion state of metal particles are scarcely changed.
The reason for this is as described below. Since the steel pipe is allowed to cool after
being tempered, the scale does not peel off. Also, since the tempering temperature is
lower than the quenching temperature, the difhsion rate of Ni and Cu is low, and the
growth or movement of metal particles is less liable to occur.
100351
It can be seen that, near the boundary between base metal and scale, metal
particles consisting mainly of Ni or Cu are dispersed as shown in Figure 1. However,
the metal particles do not exist in the whole of scale, and near the scale surface distant
from the boundary between base metal and scale, a region is present where the metal
particles do not exist.
[003 61
Even if the metal particles exist in the scale, if the distance from the boundary
between base metal and scale to the region in which the metal particles do not exist is
shorter than 20 pm, the adhesion of scale is insufficient. Therefore, in order to improve
the adhesion of scale and to suppress the variations in hardness and the highest hardness,
the metal particles have to be dispersed widely in the scale, and it is necessary that the
distance from the boundary between base metal and scale of the steel pipe to the region
in which the metal particles do not exist be 20 pm or longer.
[0037]
As the "distance from the boundary between base metal and scale to the region
in which the metal particles do not exist", in the region in which the backscattered electron
image (200 pm x 200 pm) was obtained, throughout the overall length of boundary, the
distance from the boundary to the region in which the metal particles do not exist is
measured, and the maximum value of the distance is used.
[0038]
Also, even if the metal particles are distributed in a wide range in the scale, if
the number of metal particles is small, the effect of improving the adhesion of scale is
sometimes insufficient. Therefore, at a position 10 pm distant from the boundary
between base metal and scale to the scale side, the number density of metal particles
consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1 to 5
pm observed per unit area is preferably 5 x lo3 pieces/mm2 or higher. Also, if the
number density of metal particles consisting mainly of Ni or Cu increases, that is, if the
sizes of metal particles become excessively small, the ductility of scale decreases.
Therefore, the number density of metal particles is preferably 5 x 1 O5 pieces/rnrn2 or lower.
[0039]
As the number density of metal particles at the "position 10 pn distant from the
boundary between base metal and scale to the scale side", around a position 1 Opm distant
from the boundary to the scale side, three regions each having a dimension of 20 pm in
the direction perpendicular to the boundary and a dimension of 60 pm in the direction
horizontal thereto are extracted randomly, and the mean value of measurement results of
number densities in those regions is used. Also, the number density of metal particles
is calculated as described below. The element mapping image of Ni or Cu obtained by
EPMA is subjected to binarization processing of black and white, the number of particles
in which Ni or Cu concentrates is counted, the particle numbers in three visual fields are
arithmetically averaged, and the averaged value is divided by the measurement area (1 200
pm2).
[0040]
2. Chemical composition
The seamless steel pipe for line pipe in accordance with the present invention
has the chemical composition consisting, by mass percent, of C: 0.03 to 0.10%, Si: 0.50%
or less, Mn: 1.0 to 2.0%, P: 0.050% or less, S: 0.005% or less, Cr: 0.05 to 1.0%, Mo: 0.01
to 0.30%, Al: 0.001 to 0.10%, N: 0.01% or less, Ni: 0.04 to 2.0%, Ca: 0.0005 to 0.0050%,
Cu: 0 to 2.0%, Ti: 0 to 0.05%, Nb: 0 to 0.05%, V: 0 to 0.10%, the balance: Fe and
impurities, and satisfying the conditions of Cu + Ni: 0.10% or more and Mo + V: 0.30%
or less.
[004 11
The term "impurities" means components that are mixed in on account of various
factors in the production process including raw materials such as ore and scrap when an
alloy is produced on an industrial basis, which components are allowed in the range of
not exerting an adverse influence on the present invention.
[0042]
The reason why the content of each element is restricted is as described below.
In the following explanation, the symbol "%" for the content means "mass%".
[0043]
C: 0.03 to 0.10%
C (carbon) is an element necessary for enhancing the hardenability and for
increasing the strength. If the C content is less than 0.03%, the required strength cannot
be assured. On the other hand, if the C content is more than 0.10%, the surface hardness
is increased, and the SSC resistance is deteriorated. Also, when welding is performed,
the hardening of weld heat affected zone and the deterioration in toughness occur.
Therefore, the C content must be set to 0.03 to 0.10%. The C content is preferably
0.04% or more, and preferably 0.08% or less.
[0044]
Si: 0.50% or less
Si (silicon) is an element having a deoxidizing function and contributing to the
increase in strength. However, if more than 0.50% of Si is contained, the precipitation
of cementite is suppressed, and an M-A constituent (Martensite-Austenite constituent) is
liable to precipitate. Therefore, the Si content is set to 0.50% or less. The Si content
is preferably 0.30% or less. For the seamless steel pipe of the present invention, the
lower limit of the Si content is not especially defined because too low a content of Si
presents no problem unless the deoxidation of steel is hindered.
[0045]
Mn: 1 .O to 2.0%
Mn (manganese) is an element for enhancing the hardenability without the
increase in tempering softening resistance, and also effective in assuring the strength. If
the Mn content is less than 1.0%, a high strength of 448 MPa or higher cannot be assured.
On the other hand, if more than 2.0% of Mn is contained, the segregation increases, and
also the hardenability becomes too high, so that the toughness is deteriorated in both of
the base metal and the weld heat affected zone. Therefore, the Mn content has to be set
to 1.0 to 2.0%. The Mn content is preferably 1.2% or more, and preferably 1.8% or less.
[0046]
P: 0.050% or less
P (phosphorus) is an element existing unavoidably in a steel as an impurity.
However, if the P content is more than 0.050%, P segregates at the grain boundaries, and
may deteriorate the toughness. Therefore, the P content is set to 0.050% or less. The
P content is preferably 0.020% or less.
[0047]
S: 0.005% or less
S (sulfur) is an element existing unavoidably in a steel as an impurity. However,
if the content of S is more than 0.005%, sulfide-based nonmetallic inclusions such as MnS
are formed, and thereby the hydrogen-induced cracking resistance may be deteriorated.
Therefore, the S content is set to 0.005% or less. The S content is preferably 0.003% or
less.
[0048]
Cr: 0.05 to 1.0%
Cr (chromium) is an element for enhancing the hardenability and the tempering
softening resistance and for increasing the strength, so that 0.05% or more of Cr has to be
contained. However, if more than 1 .O% of Cr is contained, the toughness is deteriorated.
Therefore, the Cr content is set to 0.05 to 1.0%. The Cr content is preferably 0.15% or
more, and preferably 0.60% or less.
[0049]
Mo: 0.01 to 0.30%
Mo (molybdenum) is an element for greatly enhancing the hardenability and the
tempering softening resistance and for increasing the strength, so that 0.01% or more of
Mo has to be contained. However, if more than 0.30% of Mo is contained, the tempering
softening resistance becomes excessive, and the surface hardness after tempering does
not decrease. Therefore, the content of Mo is set to 0.01 to 0.30%. The Mo content is
preferably 0.05% or more, and preferably 0.25% or less.
[0050]
Al: 0.001 to 0.10%
A1 (aluminum) is an element having a deoxidizing function. If the content of
A1 is low, deoxidation becomes insufficient, and thereby the steel properties are
deteriorated, so that 0.001% or more of A1 has to be contained. However, if more than
0.10% of A1 is contained, not only alumina-based nonmetallic inclusions such as A1203
are formed, but also the precipitation of cementite is suppressed, so that M-A constituent
is liable to precipitate. Therefore, the content of A1 is set to 0.001 to 0.10%. The A1
content is preferably 0.01% or more, and preferably 0.05% or less.
[005 11
N: 0.0 1 % or less
N (nitrogen) exists in a steel as an impurity. If the content of N is more than
0.01%, the steel properties are deteriorated. Therefore, the content of N is set to 0.01%
or less.
[0052]
Ni: 0.04 to 2.0%
Ni (nickel) is an element for improving the hardenability and toughness.
Further, in the present invention, by containing Ni either singly or together with Cu, the
metal particles consisting mainly of Ni or Cu are dispersed finely in the surface scale to
improve the adhesion of surface scale. Therefore, 0.04% or more of Ni has to be
contained. However, if more than 2.0% of Ni is contained, the SSC resistance of the
weld heat affected zone deteriorates. Therefore, the Ni content is set to 0.04 to 2.0%.
The Ni content is preferably 0.10% or more, and preferably 1.8% or less.. Ni is an
element effective also in preventing surface hot-shortness due to Cu during continuous
casting and during hot rolling. In the case where it is desired to achieve this effect, the
Ni content has to be made one third or more of the Cu content.
[0053]
Ca: 0.0005 to 0.0050%
Ca (calcium) is used for mode control of nonmetallic inclusions such as MnS
and A1203, and improves the toughness and hydrogen-induced cracking resistance.
Therefore, 0.0005% or more of da has to be contained. However, if more than 0.0050%
of Ca is contained, the Ca-based inclusions are liable to be clustered. Therefore, the Ca
content is set to 0.0005 to 0.0050%. The Ca content is preferably 0.0010% or more, and
preferably 0.0040% or less.
[0054]
Cu: 0 to 2.0%
Cu (copper) is an element for improving the toughness and hardenability.
Further, in the present invention, by containing Cu together with Ni, the metal particles
consisting mainly of Ni or Cu are dispersed finely in the surface scale, and the adhesion
of surface scale is improved. Therefore, Cu may be contained as necessary. However,
if more than 2.0% of Cu is contained, the SSC resistance of the weld heat affected zone
deteriorates. Therefore, the Cu content, if being contained, is set to 2.0% or less. The
Cu content is preferably 1.5% or less, further preferably 1.2% or less.
[0055]
The effect of improving the adhesion of scale can be achieved sufficiently even
in the case where Ni is contained singly, so that Cu needs not necessarily be contained
positively. However, since Ni is an expensive element, it is desirable to replace a part
thereof with Cu. Also, a steel usually contains Cu as an impurity element, so that it is
unfavorable in terms of economy to decrease the Cu content excessively. Therefore, the
Cu content is preferably 0.01% or more, further preferably 0.02% or more.
[0056]
Ti: 0 to 0.05%
Ti (titanium) is an element effective in preventing cracking of cast piece by
means of the fixation of N in steel, so that Ti may be contained as necessary. However,
if more than 0.05% of Ti is contained, the carbo-nitrides of Ti are coarsened, and the
toughness is deteriorated. Therefore, the Ti content, if being contained, is set to 0.05%
or less. The Ti content is preferably 0.01% or less. In the case where it is desired to
achieve the above-described effect, the Ti content is preferably 0.003% or more.
[0057]
Nb: 0 to 0.05%
Nb (niobium) is an element for greatly enhancing the hardenability and
tempering softening resistance and for increasing the strength, so that Nb may be
contained as necessary. However, if more than 0.05% of Nb is contained, the tempering
softening resistance becomes excessive, and the surface hardness after tempering does
not decrease. Therefore, the Nb content, if being contained, is set to 0.05% or less.
The Nb content is preferably 0.04% or less. In the case where it is desired to achieve
the above-described effects, the Nb content is preferably 0.01% or more, further
preferably 0.02% or more.
[0058]
V: 0 to 0.10%
V (vanadium) is an element for greatly enhancing the hardenability and
tempering softening resistance and for increasing the strength, so that V may be contained
as necessary. However, if more than 0.10% of V is contained, the tempering softening
resistance becomes excessive, and the surface hardness after tempering does not decrease.
Therefore, the V content, if being contained, is set to 0.10% or less. The V content is
preferably 0.07% or less. In the case where it is desired to achieve the above-described
effects, the V content is preferably 0.02% or more.
[0059]
Cu + Ni: 0.10% or more
As described above, Cu and Ni have an effect of improving the adhesion of scale
due to the dispersion in the surface scale as metal particles, and are important elements in
the present invention. Therefore, the total content of Cu and Ni has to be 0.10% or more.
Also, the total content of Cu and Ni has only to be 4.0% or less; however, if the total
content of Cu and Ni is excessive, the SSC resistance may be deteriorated, and the steel
pipe becomes uneconomical. Therefore, the total content of Cu and Ni is preferably
3.0% or less.
[0060]
Mo + V: 0.30% or less
As described above, Mo and V are elements for greatly enhancing the
hardenability and tempering softening resistance and for increasing the strength.
However, if Mo and V are contained excessively, the hardness does not decrease even
after tempering, since Mo and V remarkably increase the tempering softening resistance.
Therefore, the total content of Mo and V has to be restricted to 0.30% or less.
[006 11
In order to restrain the highest hardness on steel pipe surface to 230 HVlO or
lower more stably, the chemical composition of steel pipe preferably consists, by mass
percent, of C: 0.03 to 0.10%, Si: 0.30% or less, Mn: 1.00 to 1.80%, P: 0.020% or less, S:
0.003% or less, Ti: 0.001 to 0.015%, Al: 0.001 to 0.050%, Ni: 0.04 to 0.30%, Cu: 0 to
0.30%, Cr: 0.05 to 0.40%, Mo: 0.02 to 0.15%, V: 0.02 to 0.09%, Ca: 0.0005 to 0.0030%,
N: 0.008% or less, the balance: Fe and impurities, and satisfies the conditions of Cu + Ni:
0.10 to 0.50%, and Mo + V: 0.05 to 0.20%.
[0062]
3. Metal micro-structure
Since the seamless steel pipe for line pipe according to the present invention is
of a low-alloy steel, by the ordinary quenching tempering treatment, martensite is not
formed, and a micro-structure consists mainly of bainite. As described before, for the
micro-structure consisting mainly of bainite, the hardness greatly depends on the cooling
rate. Therefore, in a location where the scale on the steel pipe surface is detached, the
cooling rate is high and therefore the hardness is high, whereas in a location where the
scale adheres firmly to the steel pipe surface, the cooling rate is low and therefore the
hardness is low.
[0063]
In the present invention, since the scale can be caused to adhere evenly, the
highest hardness on the steel pipe surface can be restrained. That is to say, the effect of
the present invention is exerted remarkably for the steel pipe having the metal microstructure
consisting mainly of bainite. Therefore, the seamless steel pipe for line pipe
of the present invention preferably has a metal micro-structure containing bainite of 50%
or more in area fraction. The area fraction of bainite in the metal micro-structure is
preferably 70% or more, further preferably 85% or more. In the present invention,
bainite includes an M-A constituent.
[0064]
4. Wall thickness of steel pipe
As the wall thickness of steel pipe increases, a difference in cooling rate between
the surface and the interior of steel pipe occurs, and resultantly, the highest hardness on
the surface becomes high with respect to the strength of steel. In the present invention,
however, a thick scale can be attached closely to the steel pipe surface evenly. Therefore,
due to the thermal insulation effect brought about by the thick scale, the cooling rate of
the steel pipe surface portion is reduced, and the increase in surface hardness can be
suppressed. That is to say, the effect of the present invention is exerted remarkably for
the steel pipe having a large wall thickness. Therefore, the seamless steel pipe for line
pipe of the present invention preferably has a wall thickness of 30 rnrn or larger. The
wall thickness of steel pipe is more preferably 40 rnrn or larger, further preferably 45 mm
or larger.
[0065]
5. Surface hardness
For the seamless steel pipe for line pipe according to the present invention, the
difference between the average hardness at a position 1 mm deep from the surface and
the average hardness in the central portion of wall thickness is preferably 30 HVlO or
smaller, and the difference between the maximum value and the minimum value of the
hardness at the position 1 mm deep from the surface is preferably 40 HVlO or smaller.
[0066]
The hardness at the position 1 mm deep from the surface of steel pipe and in the
central portion of wall thickness thereof can be measured, for example, by the method
specified in API standards or the like. A test specimen is cut out of the cross section of
steel pipe, and Vickers hardness test is conducted in at least five locations at the position
1 mm deep from the inner and outer surfaces of steel pipe and in the central portion of
wall thickness thereof, respectively, whereby the mean value of hardness and the
difference between the maximum value and the minimum value are determined.
[0067]
As described above, in order to assure the SSC resistance while the strength of
steel pipe is enhanced, the surface hardness must be kept low. If the difference in
average hardness between at the position 1 mm deep from the surface of steel pipe and in
the central portion of wall thickness thereof is larger than 30 HV10, it is sometimes
difficult to assure both of high strength and SSC resistance. Therefore, the difference
between the average hardness at the position 1 mm deep from the surface of steel pipe
and the average hardness in the central portion of wall thickness is preferably 30 HVl 0.
[0068]
Also, even if the difference in average hardness between in a portion near the
outer layer of steel pipe and in the central portion of wall thickness thereof is small, if the
variations in hardness in the outer layer are large, a portion in which the SSC resistance
deteriorates locally is produced, and cracking may occur with this portion being a starting
point. Therefore, the difference between the maximum value and the minimum value
of the hardness at the position 1 mm deep fiom the surface is preferably 40 HVlO or
smaller.
[0069]
6. Production method
The production method for the seamless steel pipe for line pipe according to the
present invention is not subject to any special restriction. However, for example, by
using the method described below, a seamless steel pipe having a yield strength of 448
MPa or higher and having the highest hardness on the steel pipe surface of 250 HVlO or
lower can be produced.
[0070]

Concerning the melting and casting, the method carried out in the production
method for general seamless steel pipes can be used, and the casting may be ingot casting
or continuous casting.
[007 11

After the casting, hot working such as forging, piercing, or rolling is performed
to produce a seamless steel pipe. Concerning the conditions in hot working, general
conditions used in the production method for seamless steel pipes have only to be applied.
For example, a billet produced by continuous casting is heated to a temperature of 1200°C
or higher, and a hollow blank is obtained by using a skew roll piercing machine. This
hollow blank is finish rolled into a steel pipe by using a mandrel mill and a sizing mill.
Ln the case where the steel pipe is directly sent to a quenching fhmace without being
cooled to a temperature lower than the AT3 transformation point, the pipe-making
finishing temperature is preferably set at a temperature of 950°C or higher.
[0072]

After being allowed to cool after having been hot-worked, the steel pipe may be
reheated and quenched. However, in order to decrease the highest hardness, it is
preferable that the steel pipe be conveyed into the furnace without being allowed to cool
before the surface temperature of steel pipe becomes a temperature lower than the &
transformation point, be heated, and be quenched. The heating temperature during
quenching is not subject to any special restriction; however, the heating temperature is
preferably set at a temperature of Ac3+50°C or higher. Also, the heating time is not
subject to any special restriction; however, the soaking time is preferably set to 5 min or
longer.
COO731
In the present invention, in order to disperse the metal particles consisting mainly
of Ni or Cu in a wide range and to accelerate the growth rate of the scale having a high
adhesion, the atmosphere in the furnace is preferably made an oxidizing environment,
and specifically, the water vapor concentration is preferably made 5% or higher. In
order to accelerate the growth rate of scale stably, the water vapor concentration is further
preferably made 10% or higher. The upper limit of the water vapor concentration is not
subject to any special restriction. However, the upper limit thereof is preferably made
25% or lower because if the water vapor concentration is excessively high, the service
life of furnace wall is shortened.
[0074]
Concerning the cooling rate during quenching, if the cooling rate is lower than
10°C/s, a sufficient strength cannot be attained, so that accelerated cooling is preferably
performed at a cooling rate of 10°C/s or higher. Also, concerning the cooling method,
the cooling method is not subject to any special restriction as far as accelerated cooling
can be performed; however, water cooling is preferably performed.
10075)
cTempering>
After quenching, tempering is preferably performed. The tempering
temperature is not subject to any special restriction. However, the tempering
temperature is preferably set at a temperature of ACI -50°C or lower because if tempering
is performed at a temperature of higher than ACI-50°C, the strength is decreased
remarkably, and a yield strength of 448 MPa or higher cannot be assured in some cases.
[0076]
Hereunder, the present invention is explained more specifically with reference
to Examples; however, the present invention is not limited to these Examples.
EXAMPLE 1
LO0771
Steels having the chemical compositions given in Table 1 were melted in a
converter, and billets were obtained by continuous casting. Each of the billets was
formed into a pipe having an outside diameter of 323.9 rnrn, a wall thickness of 37.0 mm,
and a length of 12,000 mm by the Mannesmann-mandrel mill process. The hot working
was performed under the condition that, for all the billets, the starting temperature was
1200°C and the finishing temperature was 1050°C after the billet had been heated to
1250°C.
[0078]
[Table 11
The pipe having been hot-worked and having the temperature given in Table 2
was conveyed into a furnace, and was subjected to heating for quenching. The water
vapor concentration in the furnace at that time was as given in Table 2. Afier being held
at 950°C for 15 min, the pipe was subjected to accelerated cooling by water cooling, and
was subjected to quenching treatment. Thereafter, the pipe was subjected to tempering
treatment in which the pipe was held at the temperature given in Table 2 for 30 min.
[OOSO]
[Table 21
In a location near the boundary between base metal and scale of the obtained
steel pipe, a backscattered electron image and an element mapping image obtained by
EPMA were acquired, and on the basis of these images, the distribution of metal particles
consisting mainly of Ni or Cu was examined. The distance from the boundary between
base metal and scale to the region in which the metal particles do not exist and the number
density of metal particles observed per unit area at the position 10 pm distant from the
boundary to the scale side were measured. The results of these measurements are also
given in Table 2.
[0082]
Also, a test specimen was cut out of the seamless steel pipe. By using this test
specimen, the metal micro-structure was observed, and the yield strength and surface
hardness were measured. The observation of metal micro-structure was made by the
procedure described below. First, the metal micro-structure was made to appear with
nital etching reagent in the central portion of the wall thickness of steel pipe.
Subsequently, three optical micro-structure photographs of 500 pm square were taken in
the central portion of the wall thickness of steel pipe. On each of the micro-structure
photographs, straight lines were drawn in the longitudinal direction and the transverse
direction at a pitch of 25 pm, and the number of lattice points on the ferrite structure was
counted. Then, the number of lattice points on the ferrite structure was subtracted from
the number of all lattice points, and the ratio was determined in percentage. The
obtained ratio was made the area fraction of bainite on each micro-structure photograph.
The average bainite area fraction was determined by arithmetically averaging the bainite
area fractions obtained by each of the micro-structure photographs.
[0083]
The yield strength was measured by the procedure described below. From the
central portion of wall thickness of each steel pipe, a No.14A tension test specimen (a
round bar test specimen having a diameter of 8.5 mm) specified in JIS Z 2241 (201 1) was
sampled. By using the sampled test specimen, a tension test in conformity to JIS Z 2241
(2011) was conducted in the atmosphere at normal temperature (25"C), and the yield
strength (0.2% yield stress) was determined.
I
[0084]
Furthermore, on the cross section of the test specimen, in eight locations at
intervals of 1 mm from the surface, Vickers hardness test was conducted with the test
force of 98.07N (10 kgf). The area fraction of bainite, the highest hardness at a position
1 mm deep fiom the surface, the variation in hardness, and the difference in hardness
between surface and central portion of wall thickness are also given in Table 2. The
variation in hardness in Table 2 indicates the difference between the maximum value and
the minimum value of hardness.
[0085]
Figure 2 is diagrams showing the hardness measurement results in test Nos. I
and 6. The plot in the figure shows the mean value of measurement values at eight
locations, and the error bar shows the difference between the maximum value and the
minimum value of hardness. In Example 1, a highest hardness of 230 HVlO or lower
was judged to be a good result.
[0086]
As can be seen from Table 2 and Figure 2, in test No. 6, which was a comparative
example, metal particles consisting mainly of Ni or Cu did not exist in the scale, resulting
in the deteriorated adhesion of scale. And, the variation in hardness at a position 1 mm
deep from the surface was caused to be larger than 40 HVlO due to detachment of scale
and the occurrence of variations in cooling rate. Also, the difference between the
average hardness at the position 1 rnm deep from the surface and the average hardness in
the central portion of wall thickness was relatively large. Further, the highest hardness
was as high as 255 HV10, and the result was that the SSC resistance was poor.
[0087]
Likewise, in test No. 5, since the Ni content was outside of the defined range,
metal particles did not exist sufficiently in the scale, so that the result was that the
adhesion was bad.
[OOSS]
On the other hand, in test No. 1, which was an inventive example, 2.2 x lo4
pieceslrnrn2 of metal particles consisting mainly of Ni or Cu with an average circleequivalent
diameter of 1.1 pm existed, and the distance from the boundary between base
metal and scale to the region in which metal particles did not exist was 3 1 pm, so that the
adhesion of scale was good. Therefore, there is obtained good results that the cooling
rate on the surface was uniform, the difference between the average hardness at the
position 1 mm deep from the surface and the average hardness in the central portion of
wall thickness was as small as 10 HV 1 0, and the variation in hardness at the position 1
rnrn deep from the surface was 25 HV10. Also, it can be seen that the highest hardness
was as low as 218 HVlO, and the SSC resistance was excellent.
[0089]
Likewise, in test Nos. 2 to 4, the yield strength was 496 MPa or higher, and
therefore the steel pipes had excellent strengths. Also, the metal particles consisting
mainly of Ni or Cu existed to a position 20 pm or larger distant from the boundary
between base metal and scale, and the number density was 1.5 x lo4 pieces/mm2 or higher,
so that the adhesion of scale was good. Therefore, the highest hardness at the position
1 mm deep from the surface was as low as 229 HVlO or lower, and therefore the result
was that the SSC resistance was excellent.
EXAMPLE 2
[0090]
Steels having the chemical compositions given in Table 3 were melted in a
converter, and a plurality of billets were obtained by continuous casting. Each of the
billets was formed into a pipe having the wall thickness given in Table 4 and a length of
12,000 rnm by the Mannesmann-mandrel mill process. The hot working was performed
under the condition that, for all the billets, the starting temperature was 1200°C and the
finishing temperature was 1050°C after the billet had been heated to 1250°C. After the
hot working, the pipe was cooled by being allowed to cool.
[0091]
[Table 31
[0093]
After the pipe having been hot-worked had been allowed to cool, the pipe having
the temperature given in Table 4 was conveyed into a furnace, and was subjected to
heating for quenching. The water vapor concentration in the furnace at that time was as
given in Table 4. After being held at 950°C for 15 min, the pipe was subjected to
accelerated cooling by water cooling, and was subjected to quenching treatment.
Thereafter, the pipe was subjected to tempering treatment in which the pipe was held at
the temperature given in Table 4 for 30 min.
[0094]
On the obtained steel pipe, as in Example 1, the distance from the boundary
between base metal and scale to the region in which the metal particles do not exist and
the number density of metal particles observed per unit area at the position 10 pm distant
from the boundary to the scale side were measured.
[0095]
Also, a test specimen was cut out of the seamless steel pipe, and by the same
methods as those of Example 1, the metal micro-structure was observed, and the yield
strength and surface hardness were measured. Further, on the cross section of the test
specimen, in eight locations at intervals of 1 mm from the surface, Vickers hardness test
was conducted with the test force of 98.07 N (10 kgf), and the highest hardness at the
position 1 mrn deep from the surface was determined. These results are also given in
Table 4. In Example 2, a highest hardness of 250 HVlO or lower was judged to be a
good result.
[0096]
As can be seen from Table 4, in test Nos. 9 and 10, which were comparative
examples, metal particles consisting mainly of Ni or Cu did not exist in the scale, so that
a result that the adhesion of scale was bad was obtained. Due to detachment of scale
and the occurrence of variations in cooling rate, the highest hardness was as high as 255
HVlO or higher, and the result was that the SSC resistance was poor.
[0097]
On the other hand, in test Nos. 7 and 8, which were inventive examples, the yield
strength was 510 MPa or higher, and therefore the steel pipes had excellent strengths.
Also, the metal particles consisting mainly of Ni or Cu having an average circleequivalent
diameter of 1.1 pm or larger existed to a position 20 pm or larger distant from
the boundary between base metal and scale, and the number density was 1.9 x lo4
pieces/mm2 or higher, so that the adhesion of scale was good. Therefore, it can be seen
that the highest hardness at the position 1 mm deep from the surface was as low as 240
HVlO or lower, and therefore the SSC resistance was excellent.
EXAMPLE 3
[0098]
Next, in a laboratory, experiments were conducted by controlling the atmosphere
in a soaking pit. Steels having the chemical compositions given in Table 5 were melted
in a vacuum furnace, and a 180-kg ingot was produced for each of the steels. The
produced ingot was charged into a heating firnace, and was soaked at 1250°C for 1 h.
The ingot taken out of the heating furnace was hot-forged into a rectangular prismatic
block. The block was charged into the heating furnace, and was soaked at 1250°C for
30 min. The soaked block was hot-rolled, and a steel sheet with a thickness of 30 mm
was produced.
[0099]
[Table 51
The produced steel sheet having the temperature given in Table 6 was conveyed
into a furnace, and was subjected to heating for quenching. The water vapor
concentration in the furnace at that time is as given in Table 6. After being held at 950°C
for 15 min, the steel sheet was subjected to accelerated cooling by water cooling, whereby
being subjected to quenching treatment. Thereafter, the steel sheet was subjected to
tempering treatment in which the steel sheet was held at 650°C for 30 min.
[OlOl]
[Table 61
Subsequently, by the same methods as those in Examples 1 and 2, the distance
from the boundary between base metal and scale to the region in which the metal particles
do not exist and the number density of metal particles observed per unit area at the
position 10 pm distant from the boundary to the scale side were measured. Then, the
metal micro-structure was observed, and the yield strength and the hardness on the surface
and in the central portion of wall thickness was measured. In Example 3, a highest
hardness of 230 HV10 or lower was judged to be a good result.
[0 1031
The results are given in Table 6. In test No. 12, since the Ni content was outside
of the defined range of the present invention, even if the atmosphere in the soaking pit
was proper, fine metal particles consisting mainly of Ni or Cu did not exist. Therefore,
the adhesion of scale was low, and the variation in hardness at the position 1 rnm deep
from the surface was as large as 44 HV10. Also, the increase in hardness in the outer
layer cannot be suppressed, and the difference between the average hardness at the
position 1 rnm deep from the surface and the average hardness in the central portion of
wall thickness was as large as 54 HV10. Furthermore, the result was that the highest
hardness was as high as 253 HVlO and the SSC resistance was poor.
[0 1 041
Also, in test No. 13, although the chemical composition satisfies the definition
of the present invention, the atmosphere in the soaking pit was not proper. Therefore,
in most regions, the scale was detached. A small amount of scale adhering to the surface
was analyzed. As the result, the metal particles consisting mainly of Ni or Cu existed
merely to a position 13 pm distant fiom the boundary between base metal and scale,
which did not satisfy the definition of the present invention. Resultantly, since the
adhesion of scale was low, the increase in hardness in the outer layer could not be
suppressed, and the difference between the average hardness at the position 1 mrn deep
from the surface and the average hardness in the central portion of wall thickness was as
large as 43 HV10. Further, the result was that the highest hardness was as high as 254
HV10, and the SSC resistance was poor.
[0105]
On the other hand, in test No. 11, since the chemical composition and the
atmosphere in the soaking pit were proper, it was confirmed that fine metal particles
consisting mainly of Ni or Cu existed in a region ranging a distance of 63 pm from the
boundary between base metal and scale. Therefore, the adhesion of scale was high, and
the increase in hardness in the outer layer could be suppressed.
EXAMPLE 4
[0 1 061
As in Example 3, in a laboratory, experiments were conducted by controlling the
atmosphere in a soaking pit. Steels having the chemical compositions given in Table 7
were melted in a vacuum furnace, and a 180-kg ingot was produced for each of the steels.
The produced ingot was charged into a heating furnace, and was soaked at 1250°C for 1
h. The ingot taken out of the heating furnace was hot-forged into a rectangular prismatic
block. The block was charged into the heating furnace, and was soaked at 1250°C for
30 min. The soaked block was hot-rolled, and a steel sheet having the thickness given
in Table 8 was produced.
[0 1071
[Table 71
[0109]
After the produced steel sheet had been allowed to cool, the steel sheet having
the temperature given in Table 8 was conveyed into a furnace, and was subjected to
heating for quenching. The water vapor concentration in the furnace at that time was as
given in Table 8. After being held at the temperature given in Table 8 for 15 min, the
steel sheet was subjected to accelerated cooling by water cooling, whereby being
subjected to quenching treatment. Thereafter, the steel sheet was subjected to tempering
treatment in which the steel sheet was held at the temperature given in Table 8 for 30 min.
[OllO]
Subsequently, by the same methods as those in Examples 1 to 3, the distance
from the boundary between base metal and scale to the region in which the metal particles
do not exist and the number density of metal particles observed per unit area at the
position 10 pm distant from the boundary to the scale side were measured. Then, the
metal micro-structure was observed, and the yield strength and the surface hardness were
measured. In Example 4, a highest hardness of 250 HVlO or lower was judged to be a
good result.
[Olll]
The results are given in Table 8. In test Nos. 22 to 24, since the total content
of Mo and V falls outside of the defined range of the present invention, the result was that
the highest hardness was as high as 253 HV10, and the SSC resistance was poor. In
particular, in test No. 24, since the Ni content was also outside of the defined range of the
present invention, even if the atmosphere in the soaking pit was proper, fine metal
particles consisting mainly of Ni or Cu did not exist. Therefore, since the adhesion of
scale was low, the increase in hardness in the outer layer could not be suppressed.
[0112]
Also, in test Nos. 25 to 28, although the chemical composition satisfied the
definition of the present invention, the atmosphere in the soaking pit was not proper.
Therefore, the metal particles consisting mainly of Ni or Cu existed merely to a position
15 pm or shorter distant from the boundary between base metal and scale, which did not
satisfy the definition of the present invention. As the result, the adhesion of scale was
low, the highest hardness on the surface was as high as 251 HVlO or higher, and the SSC
resistance was poor.
[0113]
On the other hand, in test Nos. 14 to 2 1, since the chemical composition and the
atmosphere in the soaking pit were proper, it was confirmed that fine metal particles
consisting mainly of Ni or Cu existed to a region 20 pm or longer distant from the
boundary between base metal and scale. Therefore, the adhesion of scale was high, and
the increase in hardness in the outer layer could be suppressed.
INDUSTRIAL APPLICABILITY
[0114]
According to the present invention, the steel pipe has a yield strength of 448 MPa
or higher, and also the highest hardness on the surface of steel pipe can be restrained to
250 HVlO or lower, preferably 230 HVlO or lower, so that a high-strength and hightoughness
seamless steel pipe excellent in SSC resistance can be obtained. Therefore,
the seamless steel pipe in accordance with the present invention can be used suitably as a
line pipe for transmitting crude oil and natural gas containing large amounts of H2S.
We claim:
1. A seamless steel pipe for line pipe, wherein a chemical composition consists,
by mass percent, of
C: 0.03 to 0.10%,
Si: 0.50% or less,
Mn: 1 .O to 2.0%,
P: 0.050% or less,
S: 0.005% or less,
Cr: 0.05 to 1.0%,
Mo: 0.01 to 0.30%,
Al: 0.001 to 0.10%,
N: 0.01 % or less,
Ni: 0.04 to 2.0%,
Ca: 0.0005 to 0.0050%,
Cu: 0 to 2.0%,
Ti: 0 to 0.05%,
Nb: 0 to 0.05%,
V: 0 to 0.10%,
the balance: Fe and impurities, and
the chemical composition satisfies the conditions of
Cu + Ni: 0.10% or more, and
Mo + V: 0.30% or less, wherein
in a scale formed on the surface of the steel pipe, metal particles consisting
mainly ofNi or Cu having an average circle-equivalent diameter of 0.1 to 5 pm exist, and
a distance from a boundary between the base metal of the steel pipe and the scale to a
region in which the metal particles do not exist is 20 pm or longer.
2. The seamless steel pipe for line pipe according to claim 1, wherein
the chemical composition contains, by mass percent,
one or more elements selected from
Cu: 0.01 to 2.0%,
Ti: 0.003 to 0.05%,
Nb: 0.01 to 0.05%, and
V: 0.02 to 0.10%.
3. The seamless steel pipe for line pipe according to claim 1, wherein
the chemical composition consists, by mass percent, of
C: 0.03 to 0.10%,
Si: 0.30% or less,
Mn: 1 .OO to 1.80%,
P: 0.020% or less,
S: 0.003% or less,
Ti: 0.001 to 0.015%,
Al: 0.001 to 0.050%,
Ni: 0.04 to 0.30%,
Cu: 0 to 0.30%,
Cr: 0.05 to 0.40%,
Mo: 0.02 to 0.15%,
V: 0.02 to 0.09%,
Ca: 0.0005 to 0.0030%,
N: 0.008% or less,
the balance: Fe and impurities, and
the chemical composition satisfies the conditions of
Cu + Ni: 0.10 to 0.50%, and
Mo + V: 0.05 to 0.20%.
4. The seamless steel pipe for line pipe according to claim 3, wherein the
highest hardness at a position 1 mm deep fkom the surface is 230 HVlO or lower.
5. The seamless steel pipe for line pipe according to any one of claims 1 to 4,
wherein:
a metal micro-structure of the steel pipe contains bainite of 50% or more in area
fraction; and
a wall thickness of the steel pipe is 30 mm or larger.
6. The seamless steel pipe for line pipe according to any one of claims 1 to 5,
wherein at a position 10 pm distant from the boundary to the scale side, the number
density of the metal particles observed per unit area is 5 x lo3 pieces/mm2 or higher.
7. The seamless steel pipe for line pipe according to any one of claims 1 to 6,
wherein a difference between the average hardness at the position 1 mm deep from the
surface of the steel pipe and the average hardness in the central portion of the wall
thickness is 30 HVlO or smaller.
8. The seamless steel pipe for line pipe according to any one of claims 1 to 7,
wherein a difference between the maximum value and the minimum value of the hardness
at the position 1 rnm deep from the surface of the steel pipe is 40 HVlO or smaller.
9. A method for producing the seamless steel pipe for line pipe, wherein the
steel pipe having the chemical composition described in any one of claims 1 to 3 is
conveyed into a furnace after the finish of hot rolling, being heated in an atmosphere with
the temperature of Ac3+50°C or higher and the water vapor concentration of 5% or higher,
and thereafter is subjected to quenching treatment in which accelerated cooling is
performed at a rate of 10°C/s or higher, subsequently being tempered at a temperature of
Acl-50°C or lower.
10. The method for producing the seamless steel pipe for line pipe according
to claim 9, wherein after the finish of hot rolling, the steel pipe is conveyed into a furnace
before the surface temperature of the steel pipe decreases to a temperature lower than the
Ar3 transformation point.