Note: Descriptions are shown in the official language in which they were submitted.
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SEAMLESS STEEL PIPE AND METHOD OF MANUFACTURING THE
SAME
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 %
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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
[00081 [Patent Document 1] JP 2010-242222 A
[Patent Document 21 JP 2013-32584 A
[Patent Document 3] WO 2011/152240
[Patent Document 4] JP 5516831 B
DISCLOSURE OF THE INVENTION
[0009] Even when one or more of the above conventional techniques are
used, a seamless steel pipe having a strength of X80 grade or higher as
defined by the American Petroleum Institute (API) standards (i.e. a lower
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limit yield strength of 555 MPa or higher) may not have good SSC resistance
in a reliable manner.
[0010] 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 X80 grade
or higher, a surface layer hardness may exceed 250 Hv, which is the higer
limit required in the sour resistance grade according to the API 5L
standards.
[0011] 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.
[0012] 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.
[0013] 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 %; 13: up to 0.03 %; S: up to 0.006 %; 0: 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
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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 %.
[0014] 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 %; 0: 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) x(20+log(t)) ... (4).
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 C, and t is in hours.
[0015] 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.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0016] [FIG. 1] FIG. 1 is a block diagram illustrating an example of a
manufacturing line.
[FIG. 2] FIG. 2 is a flow chart illustrating a process for
manufacturing the seamless steel pipe.
[FIG. 31 FIG. 3 shows changes in the surface temperature of a
workpiece during a manufacture versus time.
[FIG. 41 FIG. 4 is a scatter plot illustrating the relationship between
Larson-Miller parameter PL and yield strength YS for steel B.
[FIG. 5] FIG. 5 is a scatter plot illustrating the relationship between
Larson-Miller parameter PL and yield strength YS for steel A.
[FIG. 6] 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.
[FIG. 7] 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.
[FIG. 81 FIG. 8 is a scatter plot illustrating the relationship between
Larson-Miller parameter PL and maximum difference in hardness for steel
B.
[FIG. 91 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
[0017] 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.
[0018] 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
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surface layer of the seamless steel pipe tends to be hard and may exceed the
values of hardness specified by the API 5L standards or DNV-0S-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 X80 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.
[0019] 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),
where the symbol of each element in equation (1) is substituted by
the content of this element in mass %.
[0020] 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) x(20+log(t)) ... (2).
In equation (2), T is a tempering temperature (in C) and t is a
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holding time (in hours) for that temperature.
[0021] 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.
[0022] [Chemical Composition]
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 %.
[0023] C: 0.02 to 0.15 %
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.
[0024] Si: 0.05 to 0.5 %
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.
[0025] Mn: 0.30 to 2.5 %
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.
[0026] P: up to 0.03 %
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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.
[0027] S: up to 0.006 %
Sulphur (5) 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.
[0028] 0: up to 0.004 %
Oxygen (0) is an impurity. 0 forms coarse oxide particles or
clusters of oxide particles, decreasing the toughness of the steel. Thus,
lower 0 contents are preferable. In view of this, the 0 content should be
0.004 % or lower. The 0 content is preferably 0.003 % or lower, and more
preferably 0.002 % or lower.
[0029] Al: 0.01 to 0.10 %
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").
[0030] Ti: 0.001 to 0.010 %
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
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0.010 %, and more preferably 0.006 % or lower, and still more preferably
0.005 % or lower.
[0031] N: up to 0.007 %
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.
[0032] Cr: 0.05 to 1.0 %
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.
[0033] Mo: not less than 0.02 % and less than 0.5 %
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.
[0034] Ni: 0.03 to 1.0 %
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
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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.
[0035] Cu: 0.02 to 1.0 %
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.
[0036] V: 0.020 to 0.20 %
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 %.
[0037] Ca: 0.0005 to 0.005 %
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
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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.
[0038] 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.
[0039] Further, the chemical composition of the seamless steel pipe in the
present embodiment may contain Nb in lieu of some of Fe.
[0040] Nb: 0 to 0.05 %
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.
[0041] [Carbon Equivalent Ceq]
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-E(Ni+Cu)/15 ... (1),
where the symbol of each element in equation (1) is substituted by
the content of this element in mass %.
[0042] 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
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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.
[0043] [Microstructure]
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.
[0044] 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 %.
[0045] [Crystal Grain Size Number]
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.
[0046] 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.
[0047] 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
12
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
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 x 500 pm2,
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 510 and,
based on the resulting drawing, the crystal grain size number is calculated in
accordance with ASTM E112-10.
[0048] 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.
[0049] 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.
[0050] [Vickers Hardness and Yield Strength]
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
13
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
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.
[0051] 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.
[0052] The seamless steel pipe in the present embodiment has a yield
strength of X80 grade or higher (i.e. 555 MPa or higher) according to the API
standards.
[0053] 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.
[0054] [Manufacturing Method]
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.
[0055] [Manufacturing Line]
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.
[0056] [Manufacturing Flow]
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, Al
14
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
indicates the Aci point when considering a workpiece being heated, and
indicates the An point when considering a workpiece being cooled. Further,
in the graph, A3 indicates the Ac3 point when considering a workpiece being
heated, and indicates the Ar3 point when considering a workpiece being
cooled.
[0057] As shown in FIGS. 1 to 3, the manufacturing process involves first
heating a steel raw material using the heating furnace 1 (heating step: Si).
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.
[0058] 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).
[0059] 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 Ar3 temperature. The quenched seamless steel pipe is
tempered by the tempering apparatus 7 (tempering step S6).
[0060] 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 Ar3 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 Ac3 point and then rapidly cooled will be
hereinafter referred to as "in-line quenching". The use of direct quenching
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
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
6Ø
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.
[0061] The steps will be described in more detail below.
[0062] [Heating Step (Si)]
A round billet is heated in the heating furnace 1. The heating
temperature is preferably in the range of 1100 to 1300 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 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 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.
[00631 [Piercing Step (S2)1
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.
[0064] [Elongation Rolling Step and Sizing Rolling Step (S3)]
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
16
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
simply as rolling step.
[0065] [Supplementary Heating Step (S4)]
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 Ac3 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.
[0066] 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 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 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.
[0067] [Quenching Step (S5)]
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 AC3
point, and preferably equal to or higher than 800 C.
[0068] For water cooling, it is preferable that the cooling rate for the
temperature range of the seamless steel pipe from 800 C to 500 C is equal
to or higher than 5 C/sec (300 C/min). This provides a uniform quenched
microstructure. The cooling is stopped at a temperature that is equal to or
lower than the An point. The temperature at which cooling is stopped is
preferably 450 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.
17
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
[0069] 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.
[0070] 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.
[0071] [Tempering Step (S6)1
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 Ad l 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) x(20+log(t)) ... (2).
In equation (2), T is a tempering temperature ( C), t is a holding time
(in hours) for that temperature. Log (t) is the logarithm oft whose base is
10.
18
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
[00721 If Pb 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.
[0073] If PL is too high, the recrystallization of ferrite occurred in a
region of
a depth of lmm 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.
[0074] The lower limit of tempering temperature is preferably 600 C, and
more preferably 630 C, and still more preferably 650 C. The upper limit of
tempering temperature is preferably 700 C, and more preferably 680 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.
[0075] 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.
[0076] 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
[0077] The present invention will be described using specific examples.
The present invention is not limited to these examples.
[0078] 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.
[0079] [Investigation Methods]
A plurality of steels having the chemical compositions shown in Table
19
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
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.
cTABLE1
o F
o
00
00
1--, Chemical Composition an mass %, balance being
Fe and impurities)
Steel Ceci 2
C Si Mn P S Cu Cr Ni Ma Ti V Nb Al Ca 0 N
_
H . - .
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 V
(D 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 o-
O C 0.070
0.09 1.42 0.011_ 0.0005_ 0.41 , 0.31 , 0.39 0.35 0.005 0.05 -
0.0300.0013 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 0.010 005 ,
- _ 0.037_0.0017 0.0018 0.0039 am
_ D2 _ 0.065 0.12 , 144 , 0009 0.0010_ 0.08 0.26 0.09 006 0.007 0.05 , -
0.041 , 0.0016 0.0016 0.0041 0390
E 0.068 0.11 , 1.51 0.009 _ 0.0019, 0.37 , 0.28 0.40 0.25 0.004
0.05 , - 0.030 , 0.0012 0.0012 0.0032 0.493
rp- 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
e-t- _ .
_
m 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,
0.062 0.12 1.52 0.005 0.0008 ' 0.21 0.27 0.21 0.11 0.006 0.05 0.025 0.031
0.0008 0.0015 0.0036 0.431
o
.. _
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
CD
P
,
LND CD
w
Iv
,=1
a.
Iv
o
CD
1--µ
a)
DD
1
C-1-
o
CD
1
1--µ
cr
'c
CD
CD
Z
P
c-t-
ti)
1-..
0 CA
Crq
g
,
.
g:i ,P,,
i-t
AD
CD
IND ''Ci
11:5, CC:2D
CA 03013287 2018-07-31
NSSMC Ref. FP151209
Our Ref. 102-199P1
temperature in the range of 1100 to 1300 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.
22
TABLE2
75
0
Pipe-
CO
Forming Tempering Mechanical
t\
Properties
Conditions AsQ Conditions
_
AsQ
Ferrite
No. Thick-
wag Prior Soaking HoIcing YS TS
Prior H
N Steel Outer HvlOkgf Recrystah-
Remarks AD
7 grain
y grain C7
Diameter
ness size No. Time __ Time PL
(maximum among positions) size N. lization
(IT'
Outer In Inner
(mm) (mm) ('C) (min)(AVa) (MPa)
Difference I\D
Surface Wall
Surface
. ,
_
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 õ A 273.1 25.0 4.5 650 204 18951, 524 602
203 203 220 17 4.5 absent comparative ex..
, 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._
-
P
A 2711 _ 25.0 4.3 650 205 _18953 544 619 218
218 220 2 4.3 absent comparative ex. 0
11 A 273.1 , 25,0 4.3 , 630 204 , 18540
543 622 213 212 248 36 4.3 absent
comparative ex. ,..
0
12 _ A 2711 25.0 , 4.3 630 _ 219
18568 541 , 620 , 209 210 236 27 4.4 absent
comparative ex. 1-
,..
t\D , 13 A 273.1 25.0 4.3 630 234 _18594 531
, 610 213 208 242 34 4.3 absent
comparative ex. n,
c..0
...]
14 , A _ 273.1 25.0 , 4.5 _ 630 249 18618 531 610 206
202 240 38 4.4 absent comparative ex. n,
A , 273.1 25.0 4.5 , 630 , 264 18841 538 615 211
209 239 30 4.5 absent comparative ex. 0
1-
16 _ A 273.1 25.0 4.3 630 294 :18683 526
602 210 203 238 35 4.3 absent
comparative ex. 0,
-
i
17 , A 273.1 _ 25.0 4.3 600 204 17924
531 622 210 213 257 47 4.4 absent
comparative ex. 0
...]
I
18 B , 323.9 25.0 4.3 700 , 294
20132_503 582 , 209 193 170 39 4.5 present
comparative ex. ,..
19 5 323.9 _ 25.0 4.3 700 204 19977
575 641 192 193 209 17 4.3 absent
inventive ex. 1-
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 , 13 _ 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 13 323.9 25.0 4.3 660 204 _ 19156 606
670 225 226 _ 239 14 4.4 absent , inventive ex.
B 323.9 , 25.0 4.3 660 219 19185 601 665 224
230 239 15 4.3 absent inventive ex. Z
26 , B 323.9 25.0 4.3 665 234 19314 600 664
233 226 235 9 4.3 absent inventive ex.
27 ,_ 8 _ 323.9 25.0 4.5 , 650 204 18951 631 697 õ 248
244 249 5 4.5 absent inventive ex. Z ===
28 B 323.9 , 25.0 4.5 650 _ 219
_18979_ 620 683 235 , 235 248 13 4.7 absent inventive
ex. li (Th
29 B 323.9 25.0 , 4.5 650 , 234 ,19006, 620
681 235 235 248 13 4.5 absent inventive ex.
P:1 pd
B 323.9 25.0 4.3 650 249 19030 617 683 242 226
248 22 4.3 absent inventive ex. CD CD
t+, t-r,
0 .
I-L, Iji-:
t.0 ND
it 0
i--, CD
-c5 TABLE3
75
(1) CD
CD
'1
Pipe-00
Tempering Mechanical
.--, Forming
Donations Properties
CD Donations
MO As0
P 1-9
Ferrite
Wall c+-
Prior
Prior 1-3 No. Steel
CD (1) Outer
Thick- y grain TS y grain Soaking
Holding Hv10kgf Reorystal- Remarks P
P-, YS
Diameter Time Time PL.
(maximum among positions) lization Cr
rn
Cr' ct, ness size No.
size No. (LT'
Outer In Inner0..,
e--
(mm) (ram) (C) (min)
. (MPa) (MPa)
Surface Wall Surface Difference
_
CD (17' 31 _ B 323.9 25.0 _ 4.5 650 . 264 19054
617 679 236 232 247 15 4.7 absent inventive ex. ,
rn W 32 B 323.9 25.0 4.5 650 294 19097 612
674 227 226 237 11 4.3 absent inventive ex.
= rn
33 B 323.9 25.0 4.3 850315 ,19125 619 683
236 234 237 3 4.6 absent inventive ex.
e-l- 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.
ti 36 B 323.9
_ 25.0 4.6 630 234 ,18594 654 , 725 , 256 240
286 26 4.7 absent comparative ex.
a) ,..... 37 B _ 323.9 . 25.0 4.3 . 630
249 18618 627 , 698 , 229 , 244 268 39 4.3 absent
comparative ex.
O 'CI 38 B 323.9 25.0 _ 4.3 630 ,
264 .18841 624 693 228 , 240 266 38 4.3 absent comparative ex.
p cn 39 B _ 323.9 25.0 4.6 _ 630_ 294
18683 628 697 241 234 280 _ 26 4.5 absent
comparative ex. P
1-1 e-m' 40 B 323.9 25.0 4.3 600 204
17924 639 728 280 253 286 33 4.7
absent comparative ex. o
-'
L.
- 41 B 323.9 25.0 . 4.5 655 105 ,18786,
635 708 245 . 232 263 31 4.2 absent
comparative ex. o
1-
(E) = C* 42 . B 323.9 25.0 4.5 650 105
.18684 636 707 251 233 284 31 ._ 4.5
absent comparative ex. L.
43 , C 323.9 25.0 , 4.6 650 105 18684 636
714 248 244 282 38 4.6 absent comparative ex.
0,
_
...1
I., = ,_,, 44 C 323.9 25,0 4.6 650 185 .18911 573
657 232 228 269 41 4.5 absent comparative ex.
' 45 D1 cfq 323.9 - 25.4 4.5 656 117 ,
18849 478 561 178 178 205 27 4.5
absent comparative ex. o
1-
cr.
03
46 D2 323.9 25.4 4.3 650 130 18770 468 582
179 191 207 28 .2 absent comparative ex.
i
4
o
0 CD 47 E 406.4 38.1 4.3 600
204 .17924 618 701 267228 , 222 45 4.1 absent comparative ex.
.
...1
I
'1 48 ' E 406.4 38.1 4.6 630 204 .18540
603 673 255 , 220 227 35 4.5 absent comparative
ex. L.
1-
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.
5 E 406.4 38.1 4.3 630 , 294 18883, 609 ,
675 251 . 223 219 32 4.4 absent comparative ex.
1
_ -
o 0 52 E 406.4 38.1 4.3 : 650 , 204
_18951 , 599 , 665 237 , 222 214 23 4.3 absent inventive
ex.
e-i-
CD CD 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.
Z
c-t- 55 E 406.4 38.1 4.5 650 294 19097 565
641 226 204 208 22 4.4 absent inventive ex.
o 0 _ _
n 7, 56 E 406.4 38.1 4.6 660 204 19156 565
635 228 205 206 23 4.6 absent inventive ex.
Up
=_
_ CD 4
- 'E.; = 57 E 406.4 38.1 4.3 660 234
19211 559 628220 198 200 22 4.4 absent inventive ex
_ -
(--)
P - -58 F 323.9 25.0 4.5 - 650 _ 200 18942
570 643 212 219 224 12 4.7 absent inventive ex
_
0
59 0 323.9 25.0 5.4 , 650 _ 200 , 18942, 597 ,
665 236 223 238 15 5.7 absent inventive ex
pc1
60 _ 1 273.1 25.0 5.5 . 650 200 , 18942 ,
575 647 200 203 211 11 5.5 absent inventive ex
= 61 J 323.9 , 25.1) , 4.7 665 149 , 19131 680
734 261 252 283 31 4.7 absent comparative ex.
a) 1- .
1- = 62 K 323.9 25.0 6.8 665 234 19314 543
625 236 208 228 28 6.9 absent comparative
ex. t= ti
O cm
*62: in-line quenching at 950C + reheating at
950 C and quenching + tempering 1--, 0-1
1-. =
Cs:, t..D
ti o
CA 03013287 2018-07-31
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Our Ref. 102-199P1
was then performed by the water-cooling apparatus where the pipes were
cooled to room temperature at a cooling rate of 5 C/sec or higher.
[0085] 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 C and soaked for 20 minutes and then water-cooled.
[0086] The following evaluation tests were conducted on the seamless steel
pipes produced in the above production process.
[0087] [Yield Strength and Tensile Strength Tests]
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 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.
[0088] [Surface Hardness Test]
Four test specimens were taken from the seamless steel pipe of each
number, the specimens being displaced from each other by 90 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".
[0089] 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
CA 03013287 2018-07-31
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Our Ref. 102-199P1
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".
[0090] 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".
[0091] 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.
[0092] [Observation of Microstructure]
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.
[0093] 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 1mm or deeper from the surface. Whether
recrystallization of ferrite occurred in a region of a depth of lmm or deeper
from the surface is shown in the column labeled "Ferrite Recrystallization"
in Tables 2 and 3.
[0094] 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
26
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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 y grain size No." in Tables 2 and 3.
[0095] 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 x 500 pm2 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 , 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 Priory
Grain Size No." in Tables 2 and 3.
[0096] [Results of Investigation]
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 lmm 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 6Ø
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 Hy 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.
27
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[0097] 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.
[0098] In the seamless steel pipe of No. 18, recrystallization of ferrite
occurred in a region of a depth of lmm 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.
[0099] 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
Hv. 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.
[0100] The seamless steel pipes of Nos.43 and 44 had a Vickers hardness "at
lmm from the inner surface" higher than 250 Hv. This is presumably
because the carbon equivalent Ceq of steel C was too high.
[0101] 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.
[0102] 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.
[0103] 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.
[0104] FIG. 4 is a scatter plot illustrating the relationship between
Larson-Miller parameter PL and yield strength IS 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.
[0105] FIG. 5 is a scatter plot illustrating the relationship between
Larson-Miller parameter PL and yield strength IS for steel A. Steel A did
not provide a yield strength not lower than 555 MPa even though tempering
28
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conditions were adjusted. This is presumably because the carbon
equivalent Ceq of steel A was too low.
[0106] 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.
[0107] 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.
[0108] 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 lmm or deeper from the surface,.
[0109] 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 lmm or deeper from the surface.
[0110] [Evaluation of Sour Resistance]
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.
[0111] [HIC Resistance Test]
29
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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 HIC 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 C.
[0112] 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 x 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.
[0113] 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.
[0114] [Four-Point Bending Test]
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 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.
[0115] [Evaluation Results]
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The results of sour resistance evaluation were shown in Table 4.
[Table 4]
TAB LE4
Hv10kgf Four-
YS (maximum for positions) HIC
Number Point
No. PL Resistance
(MPa) Outer In InnerDfference est of
Bending
i T
Surface Wall Surface Blisters Test
18953 544 218 218 220 2 0 0 0
3 19314 509 197 195 162 35 0 3
18 20132 503 209 193 170 39 0 2
23 19131 621 238 240 239 2 0 0 0
33 19125 619 236 234 237 3 0 0 0
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 0 0 -
57 19211 559 220 198 200 22 0 0 -
58 18942 570 212 219 224 12 0 0 -
59 18942 597 236 223 238 15 0 0 -
60 _ 18942 575 200 203 211 11 0 0 -
[0116] In Table 4, "o" 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.
[0117] 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.
[0118] 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.
31
