Technical Field
[0001] The present disclosure relates to an electric resistance welded steel pipe for a
line pipe.
Background Art
[0002] In recent years, a line pipe which is one of types of means of primarily transporting
crude oil or natural gas has increased in importance.
[0003] Electric resistance welded steel pipes used as line pipes (i.e., electric resistance
welded steel pipes for line pipes) may be required to have lower yield ratios in the
pipe axis directions of the electric resistance welded steel pipes.
[0004] For example, Patent Document 1 discloses a technology in which the yield ratio of
an obtained electric resistance welded steel pipe in a pipe axis direction is decreased
by repeatedly applying a strain to an uncoiled steel sheet which is a raw material,
for example, by bending-unbending processing, before pipe-making forming, thereby
inducing a Bauschinger effect.
[0005] Patent Document 2 proposes a technology in which the yield ratio of an electric resistance
welded steel pipe in a pipe axis direction is decreased by allowing the metallographic
microstructure of a hot-rolled steel sheet for producing an electric resistance welded
steel pipe to be a microstructure consisting of martensite having an areal ratio of
from 1 to 20% and ferrite.
[0006] Patent Document 3 proposes a method of producing an electric resistance welded steel
pipe using a slab in which the amount of Nb is from 0.003% to less than 0.02%, as
a method of producing an electric resistance welded steel pipe in which a rise in
yield ratio due to heating is suppressed, and a deformation property is improved,
and which has excellent strain aging resistance. In the paragraph 0019 of Patent Document
3, "In a conventional electric resistance welded steel pipe with a large amount of
Nb, a work strain introduced in pipe-making causes the precipitation of Nb carbide
to proceed, thereby increasing a yield strength and a tensile strength. Such precipitation
strengthening was found to particularly cause a yield strength to be greatly increased,
thereby resulting in an increase in yield ratio." is described.
SUMMARY OF INVENTION
Technical Problem
[0008] In the technology of Patent Document 1, however, a step of applying a strain to an
uncoiled steel sheet is needed, and therefore, the number of steps is increased, thereby
resulting in the possibility of increasing the cost of producing an electric resistance
welded steel pipe.
[0009] In the technology of Patent Document 2, further improvement in the toughness of the
base metal portion of an electric resistance welded steel pipe may be required.
[0010] In the technology of Patent Document 3, a decrease in YR of an electric resistance
welded steel pipe by a method other than a method of reducing the amount of Nb may
be required.
[0011] An object of the present disclosure is to provide an electric resistance welded steel
pipe for a line pipe, which has a certain amount of tensile strength and yield strength,
which has a decreased yield ratio, and a base metal portion and an electric resistance
welded portion thereof having excellent toughness.
Solution to Problem
[0012] Means for solving the problem described above includes the following aspects.
- <1> An electric resistance welded steel pipe for a line pipe, the steel pipe comprising
a base metal portion and an electric resistance welded portion,
wherein a chemical composition of the base metal portion consists of, in terms of
% by mass:
0.080 to 0.120% of C,
0.30 to 1.00% of Mn,
0.005 to 0.050% of Ti,
0.010 to 0.100% of Nb,
0.001 to 0.020% of N,
0.010 to 0.450% of Si,
0.001 to 0.100% of Al,
0 to 0.030% of P,
0 to 0.0100% of S,
0 to 0.50% of Mo,
0 to 1.00% of Cu,
0 to 1.00% of Ni,
0 to 1.00% of Cr,
0 to 0.100% of V,
0 to 0.0100% of Ca,
0 to 0.0100% of Mg,
0 to 0.0100% of REM, and
the balance being Fe and impurities, wherein:
CMeq, expressed by the following Formula (1), is from 0.170 to 0.300,
a ratio of % by mass of Mn to % by mass of Si is 2.0 or more,
LR, expressed by the following Formula (2), is 0.210 or more,
in a case in which a metallographic microstructure of the base metal portion is observed
using a scanning electron microscope at a magnification of 1,000 times, an areal ratio
of a first phase that is ferrite is from 60 to 98%, and a second phase, which is the
balance, comprises tempered bainite,
a yield strength in a pipe axis direction is from 390 to 562 MPa,
a tensile strength in the pipe axis direction is from 520 to 690 MPa,
a yield ratio in the pipe axis direction is 90% or less,
a Charpy absorbed energy in a circumferential direction of the pipe in the base metal
portion is 100 J or more at 0°C, and
a Charpy absorbed energy in the circumferential direction of the pipe in the electric
resistance welded portion is 80 J or more at 0°C;
wherein, in Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent
% by mass of respective elements.
- <2> The electric resistance welded steel pipe for a line pipe according to <1>,
wherein the chemical composition of the base metal portion comprises, in terms of
% by mass, one or more of:
more than 0% but equal to or less than 0.50% of Mo,
more than 0% but equal to or less than 1.00% of Cu,
more than 0% but equal to or less than 1.00% of Ni,
more than 0% but equal to or less than 1.00% of Cr,
more than 0% but equal to or less than 0.100% of V,
more than 0% but equal to or less than 0.0100% of Ca,
more than 0% but equal to or less than 0.0100% of Mg, or
more than 0% but equal to or less than 0.0100% of REM.
- <3> The electric resistance welded steel pipe for a line pipe according to <1> or
<2>, wherein an areal ratio of a precipitate having an equivalent circle diameter
of 100 nm or less is from 0.10 to 1.00% in a case in which the metallographic microstructure
of the base metal portion is observed using a transmission electron microscope at
a magnification of 100,000 times.
- <4> The electric resistance welded steel pipe for a line pipe according to any one
of <1> to <3>, wherein a content of Nb in the chemical composition of the base metal
portion is, in terms of % by mass, 0.020% or more.
- <5> The electric resistance welded steel pipe for a line pipe according to any one
of <1> to <4>, wherein the electric resistance welded steel pipe for a line pipe has
a wall thickness of from 10 to 25 mm and an outer diameter of from 114.3 to 609.6
mm.
Advantageous Effects of Invention
[0013] According to the present disclosure, an electric resistance welded steel pipe for
a line pipe, which has a certain amount of tensile strength and yield strength, which
has a decreased yield ratio, and a base metal portion and an electric resistance welded
portion thereof having excellent toughness can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0014] Fig. 1 is a scanning electron micrograph showing an example of the metallographic
microstructure of a base metal portion in the present disclosure.
DESCRIPTION OF EMBODIMENTS
[0015] A numerical range expressed by "x to y" herein includes the values of x and y in
the range as the minimum and maximum values, respectively.
[0016] The content of a component (element) expressed by "%" herein means "% by mass".
[0017] The content of C (carbon) may be herein occasionally expressed as "C content". The
content of another element may be expressed similarly.
[0018] The term "step" herein encompasses not only an independent step but also a step of
which the desired object is achieved even in a case in which the step is incapable
of being definitely distinguished from another step.
[0019] An electric resistance welded steel pipe for a line pipe of the present disclosure
(hereinafter also simply referred to as "electric resistance welded steel pipe") includes
a base metal portion and an electric resistance welded portion, wherein the chemical
composition of the base metal portion consists of, in terms of % by mass: 0.080 to
0.120% of C, 0.30 to 1.00% of Mn, 0.005 to 0.050% of Ti, 0.010 to 0.100% of Nb, 0.001
to 0.020% of N, 0.010 to 0.450% of Si, 0.001 to 0.100% of Al, 0 to 0.030% of P, 0
to 0.0100% of S, 0 to 0.50% of Mo, 0 to 1.00% of Cu, 0 to 1.00% of Ni, 0 to 1.00%
of Cr, 0 to 0.100% of V, 0 to 0.0100% of Ca, 0 to 0.0100% of Mg, 0 to 0.0100% of REM,
and the balance being Fe and impurities, wherein: CMeq, expressed by the following
Formula (1), is from 0.170 to 0.300, a ratio of % by mass of Mn to % by mass of Si
(hereinafter also referred to as "Mn/Si ratio") is 2.0 or more, LR, expressed by the
following Formula (2), is 0.210 or more, in a case in which the metallographic microstructure
of the base metal portion is observed using a scanning electron microscope at a magnification
of 1,000 times, an areal ratio of a first phase that is ferrite (hereinafter also
referred to as "ferrite fraction") is from 60 to 98%, and a second phase, which is
the balance, includes tempered bainite, a yield strength in a pipe axis direction
(hereinafter also referred to as "YS") is from 390 to 562 MPa, a tensile strength
in the pipe axis direction (hereinafter also referred to as "TS") is from 520 to 690
MPa, a yield ratio in the pipe axis direction (hereinafter also referred to as "YR")
is 90% or less, a Charpy absorbed energy in a circumferential direction of the pipe
in the base metal portion is 100 J or more at 0°C, and a Charpy absorbed energy in
the circumferential direction of the pipe in the electric resistance welded portion
is 80 J or more at 0°C;
wherein, Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent
% by mass of respective elements.
[0020] The electric resistance welded steel pipe of the present disclosure includes the
base metal portion and the electric resistance welded portion.
[0021] Commonly, an electric resistance welded steel pipe is produced by forming a hot-rolled
steel sheet into a pipe shape (hereinafter also referred to as "roll forming") to
thereby make an open pipe, subjecting abutting portions of the obtained open pipe
to electric resistance welding to form an electric resistance welded portion, and
then, if necessary, subjecting the electric resistance welded portion to seam heat
treatment.
[0022] In the electric resistance welded steel pipe of the present disclosure, the base
metal portion refers to a portion other than the electric resistance welded portion
and a heat affected zone.
[0023] The heat affected zone (hereinafter also referred to as "HAZ") refers to a portion
affected by heat caused by electric resistance welding (affected by heat caused by
the electric resistance welding and seam heat treatment in a case in which the seam
heat treatment is performed after the electric resistance welding).
[0024] Herein, the electric resistance welded portion may be simply referred to as "welded
portion".
[0025] The electric resistance welded steel pipe of the present disclosure has a certain
amount of YS and TS (i.e., YS and TS in the ranges described above), has YR decreased
to 90% or less, and has the excellent toughness of the base metal portion and the
electric resistance welded portion.
[0026] In the present disclosure, the excellent toughness means that a Charpy absorbed energy
(J) in the circumferential direction of the pipe at 0°C (hereinafter also referred
to as "vE") is high.
[0027] Specifically, the electric resistance welded steel pipe of the present disclosure
has a vE of 100 J or more in the base metal portion and a vE of 80 J or more in the
electric resistance welded portion.
[0028] The electric resistance welded steel pipe of the present disclosure has low YR, and
is therefore expected to exhibit the effect of being capable of suppressing the buckling
of the electric resistance welded steel pipe.
[0029] Examples of a case in which the suppression of the buckling of a steel pipe is demanded
include a case in which a steel pipe for a subsea pipeline is laid by reel-lay. In
the reel-lay, the steel pipe is produced on land in advance, and the produced steel
pipe is spooled on the spool of a barge. The spooled steel pipe is laid on a sea bottom
while being unspooled at sea. In the reel-lay, plastic bending is applied to the steel
pipe at the time of the spooling or unspooling of the steel pipe, and therefore, the
steel pipe may be buckled. The occurrence of the buckling of the steel pipe unavoidably
results in the stopping of a laying operation, and the damage caused by the stopping
is enormous.
[0030] The buckling of the steel pipe can be suppressed by reducing the YR of the steel
pipe.
[0031] Accordingly, the electric resistance welded steel pipe of the present disclosure
is expected to exhibit the effect of being capable of suppressing buckling at the
time of reel-lay, for example, in the case of being used as an electric resistance
welded steel pipe for a subsea pipeline.
[0032] Further, the electric resistance welded steel pipe of the present disclosure has
the excellent toughness of the base metal portion and the electric resistance welded
portion, and is therefore expected to exhibit the effect of having the excellent property
of arresting crack propagation at the time of burst in the case of being used as an
electric resistance welded steel pipe for a line pipe.
[0033] YS, TS, YR, the vE of the base metal portion, and the vE of the electric resistance
welded portion as described above are achieved by a combination of the chemical composition
(including CMeq, a Mn/Si ratio, and LR) and the metallographic microstructure in the
electric resistance welded steel pipe.
[Chemical Composition of Base Metal Portion]
[0034] With regard to the chemical composition of the base metal portion, each component
in the chemical composition will be first described below, and CMeq, a Mn/Si ratio,
and LR will be subsequently described.
C: 0.080 to 0.120%
[0035] C is an element necessary for forming a second phase.
[0036] A C amount of 0.080% or more facilitates achievement of a ferrite fraction of 98%
or less and achievement of an LR of 0.210 or more. As a result, achievement of a YR
of 90% or less is facilitated. Accordingly, the amount of C is 0.080% or more. The
amount of C is preferably 0.085% or more.
[0037] In contrast, a C amount of 0.120% or less facilitates achievement of a ferrite fraction
of 60% or more. As a result, achievement of a YR of 90% or less is facilitated. Accordingly,
the amount of C in the present disclosure is 0.120% or less. The amount of C is preferably
0.115% or less, and more preferably 0.110% or less.
Mn: 0.30 to 1.00%
[0038] Mn is an element that enhances the hardenability of steel. In addition, Mn is an
essential element for detoxification of S.
[0039] A Mn amount of less than 0.30% may result in embrittlement due to S and in the deterioration
of the toughness of the base metal and the electric resistance welded portion. Accordingly,
the amount of Mn is 0.30% or more. The amount of Mn is preferably 0.40% or more, and
more preferably 0.50% or more.
[0040] In contrast, a Mn amount of more than 1.00% may result in generation of coarse MnS
in the central portion of the sheet thickness, thereby degrading the toughness of
the base metal and the electric resistance welded portion. In addition, a Mn amount
of more than 1.00% may make it impossible to achieve an LR of 0.210 or more, thereby
consequently making it impossible to achieve a YR of 90% or less. Accordingly, the
amount of Mn is 1.00% or less. The amount of Mn is preferably 0.90% or less, and more
preferably 0.85% or less.
Ti: 0.005 to 0.050%
[0041] Ti is an element forming a carbonitride and contributing to crystal grain refining.
[0042] The amount of Ti is 0.005% or more from the viewpoint of securing the toughness of
the base metal and the electric resistance welded portion.
[0043] In contrast, a Ti amount of more than 0.050% may result in generation of coarse TiN,
thereby deteriorating the toughness of the base metal and the electric resistance
welded portion. Accordingly, the amount of Ti is 0.050% or less. The amount of Ti
is preferably 0.040% or less, still more preferably 0.030 or less, and particularly
preferably 0.025%.
Nb: 0.010 to 0.100%
[0044] Nb is an element contributing to improvement in toughness and a reduction in YR.
[0045] The amount of Nb is 0.010% or more for improvement in toughness due to rolling in
the region of unrecrystallization temperature.
[0046] The amount of Nb is preferably 0.020% or more.
[0047] In contrast, a Nb amount of more than 0.100% results in the deterioration of toughness
due to a coarse carbide. Therefore, the amount of Nb is 0.100% or less. The amount
of Nb is preferably 0.090% or less.
N: 0.001 to 0.020%
[0048] N is an element that forms a nitride, thereby suppressing the coarsening of crystal
grains and consequently improving the toughness of the base metal portion and the
electric resistance welded portion. From the viewpoint of such an effect, the amount
of N is 0.001% or more. The amount of N is preferably 0.003% or more.
[0049] In contrast, a N amount of more than 0.020% results in an increase in the amount
of generated alloy carbide, thereby deteriorating the toughness of the base metal
portion and the electric resistance welded portion. Accordingly, the amount of N is
0.020% or less. The amount of N is preferably 0.015% or less, more preferably 0.010%
or less, and particularly preferably 0.008% or less.
Si: 0.010 to 0.450%
[0050] Si is an element that functions as a deoxidizer for steel. More specifically, a Si
amount of 0.010% or more results in suppression of generation of a coarse oxide in
the base metal portion and the electric resistance welded portion, thereby resulting
in improvement in the toughness of the base metal portion and the electric resistance
welded portion. Accordingly, the amount of Si is 0.010% or more. The amount of Si
is preferably 0.015% or more, and more preferably 0.020% or more.
[0051] In contrast, a Si amount of more than 0.450% may result in generation of an inclusion
in the electric resistance welded portion, thereby decreasing a Charpy absorbed energy
and deteriorating toughness. Accordingly, the amount of Si is 0.450% or less. The
amount of Si is preferably 0.400% or less, more preferably 0.350% or less, and particularly
preferably 0.300% or less.
Al: 0.001 to 0.100%
[0052] Al is an element that functions as a deoxidizer, similar to Si. More specifically,
an Al amount of 0.001% or more results in suppression of generation of a coarse oxide
in the base metal portion and the electric resistance welded portion, thereby resulting
in improvement in the toughness of the base metal portion and the electric resistance
welded portion. Accordingly, the amount of Al is 0.001% or more. The amount of Al
is preferably 0.010% or more, and more preferably 0.015% or more.
[0053] In contrast, an Al amount of more than 0.100% may result in generation of an Al-based
oxide during electric resistance welding, thereby deteriorating the toughness of a
welded portion. Accordingly, the amount of Al is 0.100% or less. The amount of Al
is preferably 0.090% or less.
P: 0 to 0.030%
[0054] P is an impurity element. A P amount of more than 0.030% may result in segregation
in a grain boundary, thereby degrading toughness. Accordingly, the amount of P is
0.030% or less. The amount of P is preferably 0.025% or less, more preferably 0.020%
or less, still more preferably 0.015% or less, and more preferably 0.010% or less.
[0055] The amount of P may be 0%. From the viewpoint of reducing a dephosphorization cost,
the amount of P may be more than 0%, and may be 0.001% or more.
S: 0 to 0.0100%
[0056] S is an impurity element. A S amount of more than 0.0100% may result in degradation
in toughness. Accordingly, the amount of S is 0.0100% or less. The amount of S is
preferably 0.0070 or less, more preferably 0.0050% or less, and more preferably 0.0030%
or less.
[0057] The amount of S may be 0%. From the viewpoint of reducing a desulfurization cost,
the amount of S may be more than 0%, may be 0.0001% or more, and may be 0.0003% or
more.
Mo: 0 to 0.50%
[0058] Mo is an optional element. Accordingly, the amount of Mo may be 0%.
[0059] Mo is an element improving the hardenability of a steel and contributing to the high
strength of the steel. From the viewpoint of such an effect, the amount of Mo may
be more than 0%, may be 0.01% or more, and may be 0.03% or more.
[0060] In contrast, a Mo amount of more than 0.50% may result in generation of a Mo carbonitride,
thereby deteriorating toughness. Accordingly, the amount of Mo is 0.50% or less. The
amount of Mo is preferably 0.40% or less, more preferably 0.30% or less, still more
preferably 0.20% or less, and particularly preferably 0.10% or less.
Cu: 0 to 1.00%
[0061] Cu is an optional element. Accordingly, the amount of Cu may be 0%.
[0062] Cu is an element that is effective for improving the strength of a base metal. From
the viewpoint of such an effect, the amount of Cu may be more than 0%, may be 0.01%
or more, and may be 0.03% or more.
[0063] In contrast, a Cu amount of more than 1.00% may result in generation of fine Cu grains,
thereby considerably deteriorating toughness. Accordingly, the amount of Cu is 1.00%
or less. The amount of Cu is preferably 0.80% or less, more preferably 0.70% or less,
still more preferably 0.60% or less, and particularly preferably 0.50% or less.
Ni: 0 to 1.00%
[0064] Ni is an optional element. Accordingly, the amount of Ni may be 0%.
[0065] Ni is an element that contributes to improvement in strength and toughness. From
the viewpoint of such an effect, the amount of Ni may be more than 0%, may be 0.01%
or more, and may be 0.05% or more.
[0066] In contrast, a Ni amount of more than 1.00% may result in excessively high strength.
Accordingly, the amount of Ni is 1.00% or less. The amount of Ni is preferably 0.80%
or less, more preferably 0.70% or less, and still more preferably 0.60% or less.
Cr: 0 to 1.00%
[0067] Cr is an optional element. Accordingly, the amount of Cr may be 0%.
[0068] Cr is an element that improves hardenability. From the viewpoint of such an effect,
the amount of Cr may be more than 0%, may be 0.01% or more, and may be 0.05% or more.
[0069] In contrast, a Cr amount of more than 1.00% may result in the deterioration of the
toughness of the welded portion due to Cr-based inclusions generated in the electric
resistance welded portion. Accordingly, the amount of Cr is 1.00% or less. The amount
of Cr is preferably 0.80% or less, more preferably 0.70% or less, still more preferably
0.50% or less, and particularly preferably 0.30% or less.
V: 0 to 0.100%
[0070] V is an optional element. Accordingly, the amount of V may be 0%.
[0071] V is an element that contributes to improvement in toughness and a reduction in YR.
From the viewpoint of such an effect, the amount of V may be more than 0%, may be
0.005% or more, and may be 0.010% or more.
[0072] In contrast, a V amount of more than 0.100% may result in the deterioration of toughness
due to a V carbonitride. Accordingly, the amount of V is 0.100% or less. The amount
of V is preferably 0.080% or less, more preferably 0.070% or less, still more preferably
0.050% or less, and particularly preferably 0.030% or less.
Ca: 0 to 0.0100%
[0073] Ca is an optional element. Accordingly, the amount of Ca may be 0%.
[0074] Ca is an element controlling a shape of a sulfide-based inclusion and improving low-temperature
toughness. From the viewpoint of such an effect, the amount of Ca may be more than
0%, may be 0.0001% or more, may be 0.0010% or more, may be 0.0030% or more, and may
be 0.0050% or more.
[0075] In contrast, a Ca amount of more than 0.0100% may result in generation of a large-sized
cluster or large-sized inclusion including CaO-CaS, thereby adversely affecting toughness.
Accordingly, the amount of Ca is 0.0100% or less. The amount of Ca is preferably 0.0090%
or less, more preferably 0.0080% or less, and particularly preferably 0.0060% or less.
Mg: 0 to 0.0100%
[0076] Mg is an optional element. Accordingly, the amount of Mg may be 0%.
[0077] Mg is an element that is effective as a deoxidizer and a desulfurization agent and
that particularly forms a fine oxide, thereby contributing to improvement in the toughness
of an HAZ (heat affected zone). From the viewpoint of such an effect, the amount of
Mg may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, may be 0.0020%
or more, may be 0.0030% or more, and may be 0.0050% or more.
[0078] In contrast, a Mg amount of more than 0.0100% is prone to cause an oxide to be aggregated
or coarsened, thereby resulting in the deterioration of HIC resistance (hydrogen-induced
cracking resistance) or the deterioration of the toughness of the base metal or the
HAZ. Accordingly, the amount of Mg is 0.0100% or less. The amount of Mg is preferably
0.0060% or less.
REM: 0 to 0.0100%
[0079] REM is an optional element. Accordingly, the amount of REM may be 0%.
[0080] "REM" refers to a rare earth element, i.e., at least one element selected from the
group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,
and Lu.
[0081] REM is an element effective as a deoxidizer or a desulfurization agent. From the
viewpoint of such an effect, the amount of REM may be more than 0%, may be 0.0001%
or more, and may be 0.0010% or more.
[0082] In contrast, an REM amount of more than 0.0100% may result in generation of a coarse
oxide, thereby resulting in the deterioration of HIC resistance or in the deterioration
of the toughness of a base metal or HAZ. Accordingly, the amount of REM is 0.0100%
or less. The amount of REM is preferably 0.0070% or less, and more preferably 0.0050%
or less.
[0083] From the viewpoint of obtaining the effects offered by the optional elements described
above, the chemical composition of the base metal portion may contain one or more
of: more than 0% but equal to or less than 0.50% of Mo, more than 0% but equal to
or less than 1.00% of Cu, more than 0% but equal to or less than 1.00% of Ni, more
than 0% but equal to or less than 1.00% of Cr, more than 0% but equal to or less than
0.100% of V, more than 0% but equal to or less than 0.0100% of Ca, more than 0% but
equal to or less than 0.0100% of Mg, and more than 0% but equal to or less than 0.0100%
of REM.
[0084] The more preferred amount of each optional element has been described above.
Balance: Fe and Impurities
[0085] In the chemical composition of the base metal portion, the balance excluding each
element described above is Fe and impurities.
[0086] The impurities refer to components which are contained in a raw material or mixed
into in a production step, and which are not intentionally incorporated into a steel.
[0087] Examples of the impurities include any elements other than the elements described
above. Elements as the impurities may be only one kind, or may be two or more kinds.
[0088] Examples of the impurities include O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.
[0089] Among the elements described above, O is preferably controlled to have a content
of 0.006% or less.
[0090] For the other elements, typically, Sb, Sn, W, Co, or As may be included in a content
of 0.1% or less, Pb or Bi may be included in a content of 0.005% or less, B may be
included in a content of 0.0003% or less, H may be included in a content of 0.0004%
or less, and the contents of the other elements need not particularly be controlled
as long as being in a usual range.
CMeq: 0.170 to 0.300
[0091] In the chemical composition of the base metal portion, CMeq expressed by the following
Formula (1) is from 0.170 to 0.300.
[0092] [in Formula (1), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent % by mass of the respective
elements, respectively].
CMeq has a positive correlation with a yield strength.
[0093] CMeq is 0.170 or more from the viewpoint of facilitating achievement of a yield strength
of 390 MPa or more. CMeq is preferably 0.180 or more, more preferably 0.200 or more,
and still more preferably 0.230 or more.
[0094] In contrast, CMeq is 0.300 or less from the viewpoint of facilitating achievement
of a yield strength of 562 MPa or less. CMeq is preferably 0.290 or less, and more
preferably 0.275 or less.
LR: 0.210 or more
[0095] In the chemical composition of the base metal portion, LR expressed by the following
Formula (2) is 0.210 or more.
[0096] In the electric resistance welded steel pipe of the present disclosure, an LR of
0.210 or more may result in achievement of a YR of 90% or less.
[0097] An LR of less than 0.210 may result in a YR of more than 90%. The reason thereof
can be considered to be because the amount of precipitate in a steel is decreased,
and work hardenability is deteriorated (i.e., TS is decreased).
[0098] [in Formula (2), C, Nb, and Mn represent % by mass of the respective elements, respectively].
The technological meaning of Formula (2) is as follows.
[0099] The reason why the amounts of C and Nb are arranged in the numerator in Formula (2)
can be considered to be that C and Nb form precipitates, thereby improving the work
hardenability of a steel (i.e., increasing TS) and consequently decreasing the YR
of the steel.
[0100] The reason why the amount of C is multiplied by "2.1" can be considered to be because,
regarding the effect of improving work hardenability due to the formation of a precipitate
described above, the effect of the inclusion of C is about 2.1 times the effect of
the inclusion of Nb.
[0101] The reason why the amount of Mn is arranged in the denominator in Formula (2) is
because, although the inclusion of Mn enables a steel to be transformed at relatively
low temperature, the inclusion of Mn causes the work hardenability in itself of the
steel to be deteriorated (i.e., causes TS to be decreased), thereby increasing the
YR of the steel.
As described above, LR has a positive correlation with the amounts of Nb and C, and
has a negative correlation with the amount of Mn.
[0102] In the electric resistance welded steel pipe of the present disclosure, even in a
case in which the amount of Nb is relatively large, for example, more than the amount
of Nb in Patent Document 3 (International Publication No.
WO 2012/133558) (from 0.003% to less than 0.02%), LR may be allowed to be 0.210 or more depending
on the amounts of C and Mn by allowing LR to satisfy 0.210 or more. In this case,
a YR of 90% or less can be achieved.
[0103] In the electric resistance welded steel pipe of the present disclosure, a YR of 90%
or less can also be achieved by allowing LR to be 0.210 or more and allowing conditions
other than LR to be satisfied, in a case in which the amount of Nb is less than 0.02%.
[0104] From the viewpoint of further facilitating achievement of a YR of 90% or less, LR
is preferably 0.230 or more, and more preferably 0.270 or more.
[0105] The upper limit of LR is not particularly restricted. From the viewpoint of the production
suitability of the electric resistance welded steel pipe, LR is preferably 0.500 or
less, more preferably 0.450 or less, and particularly preferably 0.400 or less.
Mn/Si Ratio: 2.0 or more
[0106] In the chemical composition of the base metal portion, a Mn/Si ratio (i.e., a Mn/Si
ratio which is a ratio of % by mass of Mn to % by mass of Si) is 2.0 or more.
[0107] In the electric resistance welded steel pipe of the present disclosure, a Mn/Si ratio
of 2.0 or more results in improvement in the toughness of the electric resistance
welded portion, thereby allowing vE in the electric resistance welded portion (i.e.,
a Charpy absorbed energy in the circumferential direction of the pipe at 0°C) to be
80 J or more.
[0108] In a case in which the Mn/Si ratio is less than 2.0, vE may be less than 80 J. The
reason thereof can be considered to be because in a case in which the Mn/Si ratio
is less than 2.0, a MnSi-based inclusion initiates brittle fracture in the electric
resistance welded portion, whereby toughness is deteriorated.
[0109] The Mn/Si ratio is preferably 2.1 or more from the viewpoint of further improving
the toughness of the electric resistance welded portion.
[0110] The upper limit of the Mn/Si ratio is not particularly restricted. From the viewpoint
of further improving the toughness of the electric resistance welded portion and the
toughness of the base metal portion, the Mn/Si ratio is preferably 50 or less, more
preferably 40 or less, and particularly preferably 30 or less.
[Metallographic Microstructure of Base Metal Portion]
[0111] In the electric resistance welded steel pipe of the present disclosure, the metallographic
microstructure of the base metal portion has a ferrite fraction (i.e., an areal ratio
of a first phase that is ferrite) of from 60 to 98% and includes a second phase, which
is a balance, including at least either tempered bainite or pearlite in a case in
which the metallographic microstructure is observed using a scanning electron microscope
at a magnification of 1,000 times.
[0112] In the electric resistance welded steel pipe of the present disclosure, a YR of 90%
or less can be achieved by allowing a ferrite fraction to be 60% or more. The ferrite
fraction is preferably 65% or more, and more preferably 70% or more.
[0113] In the electric resistance welded steel pipe of the present disclosure, a ferrite
fraction of 98% or less as described above enables a TS of 520 MPa or more to be achieved.
The ferrite fraction is preferably 95% or less, and more preferably 92% or less.
[0114] In the electric resistance welded steel pipe of the present disclosure, the second
phase which is the balance includes tempered bainite.
[0115] The inclusion of tempered bainite in the second phase means that the electric resistance
welded steel pipe of the present disclosure is an electric resistance welded steel
pipe tempered after pipe-making (i.e., after electric resistance welding (after seam
heat treatment in the case of performing the seam heat treatment after the electric
resistance welding)).
[0116] The electric resistance welded steel pipe of the present disclosure is an electric
resistance welded steel pipe tempered after pipe-making, whereby a YR of 90% or less
can be achieved. The reason thereof can be considered to be because YR is decreased
by the tempering after the pipe-making. The reason why YR is decreased by the tempering
after the pipe-making can be considered to be because YS is decreased by decreasing
a dislocation density, and cementites are precipitated on a dislocation, thereby increasing
work hardening (i.e., increasing TS).
[0117] Herein, tempered bainite is distinguished from bainite which is not tempered bainite,
in view of including granular cementites in the structure of the tempered bainite.
[0118] The concept of "bainite" herein includes bainitic ferrite, granular bainite, upper
bainite, and lower bainite.
[0119] The second phase may include tempered bainite, may be a phase consisting of tempered
bainite, or may include a structure other than tempered bainite.
[0120] Examples of the structure other than tempered bainite include pearlite.
[0121] The concept of "pearlite" herein also includes pseudo-pearlite.
[0122] The measurement of the ferrite fraction and the identification of the second phase
in the metallographic microstructure of the base metal portion are performed by nital-etching
a metallographic microstructure at the 1/4 position of a wall thickness in an L cross-section
at a base metal 90° position, and observing micrographs of the nital-etched metallographic
microstructure (hereinafter also referred to as "metallographic micrographs") with
a scanning electron microscope (SEM) at a magnification of 1,000 times. Metallographic
micrographs corresponding to ten 1,000-times visual fields (corresponding to an actual
cross-sectional area of 0.12 mm
2) are taken. The measurement of the ferrite fraction and the identification of the
second phase are performed by performing image processing of the metallographic micrographs
that were taken. The image processing is performed using, for example, a small-sized
general-purpose image analysis apparatus LUZEX AP manufactured by NIRECO CORPORATION.
[0123] Herein, " base metal 90° position" refers to a position deviating at 90° in the circumferential
direction of the pipe from an electric resistance welded portion, "L cross-section"
refers to a cross section parallel to a pipe axis direction and a wall thickness direction,
and "1/4 position of wall thickness" refers to a position to which a distance from
the outer surface of the electric resistance welded steel pipe is 1/4 of a wall thickness.
[0124] Herein, the pipe axis direction may be referred to as "L-direction".
[0125] Fig. 1 is a scanning electron micrograph (SEM micrograph; a magnification of 1,000
times) showing an example of the metallographic microstructure of a base metal portion
in the present disclosure.
[0126] The SEM micrograph in Fig. 1 is one (one visual field) of SEM micrographs used in
the measurement of a ferrite fraction and the identification of a second phase in
Example 17 described later.
[0127] As shown in Fig. 1, a first phase that is ferrite and a second phase including tempered
bainite can be confirmed. In particular, the presence of white points (cementites)
reveals that the second phase includes tempered bainite.
[0128] The metallographic microstructure of the base metal portion preferably has an areal
ratio (hereinafter also referred to as "specific precipitate areal ratio") of precipitates
having an equivalent circle diameter of 100 nm or less (hereinafter also referred
to as "specific precipitates") of from 0.10 to 1.00% in a case in which the metallographic
microstructure is observed using a transmission electron microscope at a magnification
of 100,000 times.
[0129] The specific precipitate areal ratio of 0.10% or more further facilitates achievement
of a YR of 90% or less. The reason thereof can be considered to be because the specific
precipitates (i.e., precipitates having an equivalent circle diameter of 100 nm or
less) contribute to improvement in work hardening characteristic (i.e., an increase
in TS), thereby resulting in a decrease in YR.
[0130] In contrast, the specific precipitate areal ratio of 1.00% or less results in suppression
of brittle fracture (i.e., excellent toughness of the base metal portion). The specific
precipitate areal ratio is preferably 0.80% or less, and more preferably 0.70% or
less.
[0131] The specific precipitate areal ratio of from 0.10 to 1.00% can be achieved by performing
tempering at a temperature of from 400°C to an Ac1 point after pipe-making.
[0132] In the present disclosure, the precipitate areal ratio (i.e., the areal ratio of
precipitates having an equivalent circle diameter of 100 nm or less) is measured by
observing a metallographic microstructure at a position of 1/4 of a wall thickness
in an L cross-section at a base metal 90° position with a transmission electron microscope
(TEM) at a magnification of 100,000 times.
[0133] More specifically, at first, on the basis of a sample taken from the position of
1/4 of the wall thickness in the L cross-section at the base metal 90° position, a
replica for TEM observation is produced by SPEED method using an electrolytic solution
including 10% by volume of acetylacetone, 1% by volume of tetramethylammonium chloride,
and 89% by volume of methyl alcohol. Then, by observing the obtained replica for TEM
observation with TEM at a magnification of 100,000 times, TEM images with a field
size of 1 µm square, corresponding to ten visual fields, are obtained. The areal ratio
of precipitates having an equivalent circle diameter of 100 nm or less with respect
to the total area of the obtained TEM image is calculated, and the obtained result
is regarded as the specific precipitate areal ratio (%).
[0134] The condition of etching in the SPEED method is set at a condition in which a charge
of 10 coulombs is applied at a voltage of -200 mV with respect to a surface area of
about 80 square millimeters with the use of a saturated calomel electrode as a reference
electrode.
[0135] The specific precipitates (i.e., precipitates having an equivalent circle diameter
of 100 nm or less) can be specifically considered to be at least one selected from
the group consisting of carbides of metals other than Fe, nitrides of metals other
than Fe, and carbonitrides of metals other than Fe.
[0136] Conceivable examples of the metals other than Fe include Ti and Nb. In a case in
which the chemical composition contains at least one of V, Mo, or Cr, conceivable
examples of the metals other than Fe include at least one of V, Mo, or Cr.
[Yield Strength in Pipe Axis Direction (YS)]
[0137] The electric resistance welded steel pipe of the present disclosure has a yield strength
in a pipe axis direction (YS) of from 390 to 562 MPa.
[0138] YS in the pipe axis direction is preferably 430 MPa or more, more preferably 450
MPa or more, more preferably 470 MPa or more, and particularly preferably 500 MPa
or more.
[0139] YS in the pipe axis direction is preferably 550 MPa or less, more preferably 540
MPa or less, and particularly preferably 530 MPa or less.
[0140] A YS in the pipe axis direction of 562 MPa or less can be achieved by performing
tempering after pipe-making. The reason thereof can be considered to be because the
tempering after pipe-making results in a decrease in pipe-making strain, and a dislocation
density.
[Tensile Strength in Pipe Axis Direction (TS)]
[0141] The electric resistance welded steel pipe of the present disclosure has a tensile
strength in a pipe axis direction (TS) of from 520 to 690 MPa.
[0142] TS in the pipe axis direction is preferably 550 MPa or more, and more preferably
580 MPa or more.
[0143] TS in the pipe axis direction is preferably 680 MPa or less, more preferably 660
MPa or less, and particularly preferably 650 MPa or less.
[Yield Ratio in Pipe Axis Direction]
[0144] The electric resistance welded steel pipe of the present disclosure has a yield ratio
in a pipe axis direction (YR = (YS/TS) × 100) of 90% or less.
[0145] As a result, the buckling of the electric resistance welded steel pipe in laying
or the like is suppressed.
[0146] A YR in the pipe axis direction of 90% or less can be achieved by performing tempering
after pipe-making. The reason thereof can be considered to be because YS is decreased
by decreasing a dislocation density, and because work hardening is increased (i.e.,
TS is increased) by finely precipitating cementites on a dislocation.
[Wall Thickness of Electric Resistance Welded Steel Pipe]
[0147] The wall thickness of the electric resistance welded steel pipe of the present disclosure
is preferably from 10 to 25 mm.
[0148] A wall thickness of 10 mm or more is advantageous in view of facilitating a decrease
in YR by using a strain caused by forming a hot-rolled steel sheet into a pipe shape.
The wall thickness is more preferably 12 mm or more.
[0149] A wall thickness of 25 mm or less is advantageous in view of the production suitability
of the electric resistance welded steel pipe (specifically, formability in formation
of a hot-rolled steel sheet into a pipe shape). The wall thickness is more preferably
20 mm or less.
[Outer Shape of Electric Resistance Welded Steel Pipe]
[0150] The outer diameter of the electric resistance welded steel pipe of the present disclosure
is preferably from 114.3 to 609.6 mm (i.e., from 4.5 to 24 inches).
[0151] An outer diameter of 114.3 mm or more is more preferred as the electric resistance
welded steel pipe for a line pipe. The outer diameter is preferably 139.7 mm (i.e.,
5.5 inches) or more, and more preferably 177.8 mm (i.e., 7 inches) or more.
[0152] An outer diameter of 609.6 mm or less is advantageous in view of facilitating a decrease
in YR by using a strain caused by forming a hot-rolled steel sheet into a pipe shape.
The outer diameter is preferably 406.4 mm (i.e., 16 inches) or less, and more preferably
304.8 mm (i.e., 12 inches) or less.
[One Example of Production Method]
[0153] One example of a method of producing an electric resistance welded steel pipe of
the present disclosure is the following production method A.
[0154] The production method A includes:
a step of producing an as-rolled electric resistance welded steel pipe by using a
hot-rolled steel sheet having the chemical composition described above, and
a tempering step of obtaining an electric resistance welded steel pipe by tempering
the as-rolled electric resistance welded steel pipe.
According to the production method A, the inclusion of the tempering step facilitates
the production of an electric resistance welded steel pipe having a YR of 90% or less
by the reasons described above.
[0155] A tempering temperature (i.e., a retention temperature in the tempering) is preferably
from 400°C to an Ac1 point.
[0156] A tempering temperature of 400°C or more further facilitates precipitation of cementite
and a specific precipitate (precipitate having an equivalent circle diameter of 100
nm or less), and therefore further facilitates achievement of a YR of 90% or less.
The tempering temperature is more preferably 420°C or more.
[0157] A tempering temperature of an Ac1 point or less enables a precipitate to be further
inhibited from being coarse. Although tempering temperature depends on the Ac1 point
of a steel, it is also preferably 720°C or less, also preferably 710°C or less, and
also preferably 700°C or less.
[0158] The Ac1 point means a temperature at which transformation to austenite is started
in the case of increasing the temperature of a steel.
[0159] The Ac1 point is calculated by the following Formula:
[0160] [where C, Si, Mn, Ni, Cu, Cr, Mo, V, Ti, Nb, and Al represent % by mass of the respective
elements, respectively. Ni, Cu, Cr, Mo, and V are optional elements. Among the optional
elements, an element that is not contained in a slab is set at 0% by mass, and the
Ac1 point is calculated.
[0161] A tempering time (i.e., a retention time at the tempering temperature) in the tempering
step is preferably 5 minutes or more in view of facilitating a more decrease in YR.
[0162] In the production method A, the as-rolled electric resistance welded steel pipe refers
to an electric resistance welded steel pipe which is produced by roll-forming (i.e.,
forming into a pipe shape) a hot-rolled steel sheet, and which is not subjected to
heat treatment other than seam heat treatment after the roll-forming.
[0163] A preferred aspect of the step of producing the as-rolled electric resistance welded
steel pipe in the production method A will be described later.
[0164] The production method A preferably includes a sizer step of adjusting the shape of
the as-rolled electric resistance welded steel pipe by a sizer under a condition in
which the change in ovality before and after adjustment (hereinafter also referred
to as "change in ovality (%) by sizer step") is 1.0% or more, between the step of
producing an as-rolled electric resistance welded steel pipe and the tempering step.
[0165] In a case in which the production method A includes the sizer step, the electric
resistance welded steel pipe having the specific precipitate areal ratio of from 0.10
to 1.00% described above can be more easily produced.
[0166] The reason thereof can be considered to be because a dislocation of which the amount
is equal to or more than a certain amount is introduced into the as-rolled electric
resistance welded steel pipe by the sizer step under the condition in which the change
in ovality by sizer step is 1.0% or more, and the as-rolled electric resistance welded
steel pipe is then tempered at a temperature of from 400°C to an Ac1 point, thereby
facilitating precipitation of fine specific precipitates on the dislocation.
[0167] The ovality of the as-rolled electric resistance welded steel pipe is determined
as described below.
[0168] First, four measurement values are obtained by measuring the outer diameter of the
as-rolled electric resistance welded steel pipe in the circumferential direction of
the pipe with a 45° pitch. Each of the maximum value, minimum value, and average value
of the four measurement values is determined. The ovality of the as-rolled electric
resistance welded steel pipe is determined by the following Formula on the basis of
the maximum value, the minimum value, and the average value.
[0169] The change in ovality (%) by sizer step is determined by the following Formula on
the basis of the ovality of the as-rolled electric resistance welded steel pipe before
the adjustment of the shape by the sizer and the ovality of the as-rolled electric
resistance welded steel pipe after the adjustment of the shape by the sizer.
[0170] The step of producing an as-rolled electric resistance welded steel pipe in the production
method A preferably includes:
a hot-rolling step of heating a slab having the chemical composition described above
and hot-rolling the heated slab, thereby obtaining a hot-rolled steel sheet,
a cooling step of cooling the hot-rolled steel sheet obtained in the hot-rolling step,
a coiling step of coiling the hot-rolled steel sheet cooled in the cooling step, thereby
obtaining a hot coil consisting of the hot-rolled steel sheet, and
a pipe-making step of uncoiling the hot-rolled steel sheet from the hot coil, roll-forming
the uncoiled hot-rolled steel sheet to thereby make an open pipe, and subjecting abutting
portions of the obtained open pipe to electric resistance welding to form an electric
resistance welded portion, thereby obtaining an as-rolled electric resistance welded
steel pipe.
[0171] In the pipe-making step, the electric resistance welded portion may be subjected
to seam heat treatment after the electric resistance welding, if necessary.
[0172] In the hot-rolling step, the slab having the chemical composition described above
is preferably heated to a temperature of from 1150°C to 1350°C.
[0173] In a case in which the temperature to which the slab is heated is 1150°C or more,
the toughness of the base metal portion of the electric resistance welded steel pipe
can be further improved. The reason thereof can be considered to be because generation
of an insoluble Nb carbide can be suppressed in a case in which the temperature to
which the slab is heated is 1150°C or more.
[0174] In a case in which the temperature to which the slab is heated is 1350°C or less,
the toughness of the base metal portion of the electric resistance welded steel pipe
can be further improved. The reason thereof can be considered to be because coarsening
of a metallographic microstructure can be suppressed in a case in which the temperature
to which the slab is heated is 1350°C or less.
[0175] In the hot rolling step, the slab heated, for example, to a temperature of 1150°C
to 1350°C is preferably hot-rolled at a temperature that is equal to or more than
Ar3 point + 100°C. As a result, the hardenability of the hot-rolled steel sheet can
be improved.
[0176] The Ar3 point is determined from the chemical composition of the base metal portion
by the following Formula:
[0177] [where C, Mn, Ni, Cu, Cr, and Mo represent % by mass of the respective elements,
respectively. Ni, Cu, Cr, and Mo are optional elements. Among the optional elements,
an element that is not contained in the slab is set at 0% by mass, and the Ar3 point
is calculated.
[0178] In the cooling step, the hot-rolled steel sheet obtained in the hot-rolling step
is preferably cooled at a cooling start temperature set at the Ar3 point or more.
As a result, the strength and toughness of the base metal portion can be further improved.
The reason thereof can be considered to be because generation of coarse polygonal
ferrite is suppressed by setting the cooling start temperature at the Ar3 point or
more.
[0179] In the cooling step, the hot-rolled steel sheet obtained in the hot-rolling step
is preferably cooled at a cooling rate of from 5°C/s to 80°C/s.
[0180] In a case in which the cooling rate is 5°C/s or more, the degradation of the toughness
of the base metal portion is further suppressed. The reason thereof can be considered
to be because generation of coarse ferrite is suppressed by setting the cooling rate
in the cooling step at 5°C/s or more.
[0181] In a case in which the cooling rate is 80°C/s or less, the degradation of the toughness
of the base metal portion is suppressed. The reason thereof can be considered to be
because an excessive second phase fraction (i.e., a ferrite fraction of less than
60%) is suppressed by setting the cooling rate in the cooling step at 80°C/s or less.
[0182] In the coiling step, the hot-rolled steel sheet cooled in the cooling step is preferably
coiled at a coiling temperature of from 450 to 650°C.
[0183] A coiling temperature of 450°C or more results in suppression of the degradation
of the toughness of the base metal portion. The reason thereof can be considered to
be because a coiling temperature of 450°C or more results in suppression of generation
of martensite.
[0184] A coiling temperature of 650°C or less may result in suppression of an increase in
YR. The reason thereof can be considered to be because a coiling temperature of 650°C
or less results in suppression of excessive generation of a Nb carbonitride, thereby
resulting in suppression of an increase in YS.
EXAMPLES
[0185] Examples of the present disclosure will be described below. However, the present
disclosure is not limited to the following Examples.
[Examples 1 to 17, and Comparative Examples 1 to 26]
<Production of Hot Coil>
[0186] Each of slabs having chemical compositions set forth in Table 1 and Table 2 was heated
to a temperature of 1250°C, the heated slab was hot-rolled to obtain a hot-rolled
steel sheet, the obtained hot-rolled steel sheet was cooled at a cooling start temperature
set at 820°C and a cooling rate of 50°C/s, and the cooled hot-rolled steel sheet was
coiled at a coiling temperature of 550°C, whereby a hot coil consisting of the hot-rolled
steel sheet was obtained.
[0187] In each Example and each Comparative Example, the balance excluding the elements
set forth in Table 1 and Table 2 is Fe and impurities.
[0188] In Tables 1 and 2, REM in Example 11 is Ce, REM in Example 16 is Nd, and REM in Example
17 is La.
[0189] In Tables 1 to 3, the underlined numerical values show numerical values that fall
outside the scope of the present disclosure.
<Production of As-Rolled Electric Resistance Welded Steel Pipe>
[0190] A hot-rolled steel sheet was uncoiled from the hot coil, the uncoiled hot-rolled
steel sheet was roll-formed to thereby make an open pipe, abutting portions of the
obtained open pipe was subjected to electric resistance welding to form an electric
resistance welded portion (hereinafter also referred to as "welded portion"), and
the welded portion was then subjected to seam heat treatment, thereby obtaining an
as-rolled electric resistance welded steel pipe.
<Production of Electric Resistance Welded Steel Pipe (Sizer and Tempering)>
[0191] The shape of the as-rolled electric resistance welded steel pipe was adjusted by
a sizer under conditions achieving each of changes in ovality (%) by sizer step set
forth in Table 3.
[0192] The as-rolled electric resistance welded steel pipe of which the shape had been adjusted
was tempered at each tempering temperature and for each tempering time set forth in
in Table 3, thereby obtaining an electric resistance welded steel pipe.
[0193] The outer diameter of the obtained electric resistance welded steel pipe was 219
mm, and the wall thickness of this electric resistance welded steel pipe was 15.9
mm.
[0194] The above production step does not affect the chemical composition of a steel. Accordingly,
the chemical composition of the base metal portion of the obtained electric resistance
welded steel pipe can be considered to be the same as the chemical composition of
the slab which is a raw material.
<Measurement>
[0195] The following measurement was performed for the obtained electric resistance welded
steel pipe.
[0196] The results are set forth in Table 3.
(Measurement of Ferrite Fraction and Confirmation of Structure of Second Phase)
[0197] By the method described above, the ferrite fraction ("F fraction" in Table 3) was
measured, and the kind of a second phase was confirmed.
[0198] In Table 3, TB means tempered bainite, P means pearlite, and MA means martensite
island.
(Measurement of YS, TS, and YR)
[0199] A specimen for a tensile test was sampled in a direction where the test direction
(tensile direction) in a tensile test corresponds to the pipe axis direction (hereinafter
also referred to as "L-direction") of the electric resistance welded steel pipe from
the base metal 90° position of the electric resistance welded steel pipe. The shape
of the specimen was allowed to be a flat plate shape conforming to an American Petroleum
Institute standard API 5L (hereinafter simply referred to as "API 5L").
[0200] A tensile test in which a test direction was the L-direction of the electric resistance
welded steel pipe was conducted using the sampled specimen in conformity with API
5L at room temperature, and TS in the L-direction of the electric resistance welded
steel pipe and YS in the L-direction of the electric resistance welded steel pipe
were measured.
[0201] YR (%) in the L-direction of the electric resistance welded steel pipe was determined
based on a calculation formula "(YS/TS) × 100".
(Measurement of vE (J) (Charpy Absorbed Energy at 0°C) of Base Metal Portion)
[0202] A full-size specimen with a V-notch (a specimen for a Charpy impact test) was sampled
from the base metal 90°C position of the electric resistance welded steel pipe. The
full-size specimen with a V-notch was sampled so that a test direction was the circumferential
direction of the pipe (C-direction). The sampled full-size specimen with a V-notch
was subjected to a Charpy impact test in conformity with API 5L under a temperature
condition of 0°C to measure vE (J).
[0203] The above measurement was performed five times per one electric resistance welded
steel pipe, and the average value of five measurement values was regarded as vE (J)
of the base metal portion of the electric resistance welded steel pipe.
(Measurement of vE (J) (Charpy Absorbed Energy at 0°C) of Welded Portion)
[0204] The same operation as the measurement of vE (J) of the base metal portion was performed
except that a position from which a full-size specimen with a V-notch was sampled
was changed to the welded portion of the electric resistance welded steel pipe.
(Measurement of Specific Precipitate Areal Ratio)
[0205] The specific precipitate areal ratio (i.e., the areal ratio of precipitates having
an equivalent circle diameter of 100 nm or less, simply referred to as "precipitate
areal ratio (%)"in Table 3) was measured by the method described above.
[Table 1]
|
Component (% by mass) |
C |
Mn |
Ti |
Nb |
N |
Si |
Al |
P |
S |
Example 1 |
0.097 |
0.73 |
0.010 |
0.032 |
0.010 |
0.318 |
0.041 |
0.019 |
0.0019 |
Example 2 |
0.092 |
0.64 |
0.016 |
0.021 |
0.005 |
0.200 |
0.019 |
0.008 |
0.0044 |
Example 3 |
0.088 |
0.64 |
0.007 |
0.047 |
0.003 |
0.026 |
0.071 |
0.018 |
0.0050 |
Example 4 |
0.080 |
0.50 |
0.006 |
0.030 |
0.009 |
0.100 |
0.048 |
0.013 |
0.0026 |
Example 5 |
0.098 |
0.66 |
0.007 |
0.036 |
0.004 |
0.179 |
0.084 |
0.017 |
0.0020 |
Example 6 |
0.103 |
0.61 |
0.017 |
0.026 |
0.011 |
0.243 |
0.099 |
0.018 |
0.0029 |
Example 7 |
0.085 |
0.62 |
0.014 |
0.043 |
0.008 |
0.300 |
0.090 |
0.018 |
0.0006 |
Example 8 |
0.084 |
0.72 |
0.008 |
0.040 |
0.009 |
0.230 |
0.020 |
0.005 |
0.0030 |
Example 9 |
0.080 |
0.60 |
0.009 |
0.025 |
0.003 |
0.100 |
0.031 |
0.012 |
0.0036 |
Example 10 |
0.081 |
0.66 |
0.012 |
0.038 |
0.005 |
0.289 |
0.019 |
0.006 |
0.0024 |
Example 11 |
0.095 |
0.77 |
0.016 |
0.032 |
0.010 |
0.027 |
0.061 |
0.015 |
0.0009 |
Example 12 |
0.082 |
0.61 |
0.019 |
0.060 |
0.010 |
0.121 |
0.088 |
0.010 |
0.0008 |
Example 13 |
0.083 |
0.82 |
0.045 |
0.020 |
0.007 |
0.200 |
0.010 |
0.003 |
0.0020 |
Example 14 |
0.083 |
0.51 |
0.025 |
0.015 |
0.005 |
0.200 |
0.015 |
0.001 |
0.0019 |
Example 15 |
0.082 |
0.78 |
0.025 |
0.015 |
0.003 |
0.230 |
0.020 |
0.001 |
0.0020 |
Example 16 |
0.084 |
0.61 |
0.045 |
0.020 |
0.005 |
0.243 |
0.010 |
0.015 |
0.0008 |
Example 17 |
0.082 |
0.61 |
0.017 |
0.020 |
0.010 |
0.200 |
0.020 |
0.013 |
0.0009 |
Comparative Example 1 |
0.150 |
0.73 |
0.006 |
0.028 |
0.004 |
0.046 |
0.059 |
0.002 |
0.0032 |
Comparative Example 2 |
0.090 |
0.90 |
0.023 |
0.015 |
0.011 |
0.480 |
0.051 |
0.007 |
0.0029 |
Comparative Example 3 |
0.106 |
0.66 |
0.008 |
0.042 |
0.002 |
0.005 |
0.029 |
0.019 |
0.0030 |
Comparative Example 4 |
0.109 |
0.20 |
0.013 |
0.031 |
0.002 |
0.069 |
0.088 |
0.014 |
0.0038 |
Comparative Example 5 |
0.080 |
1.02 |
0.024 |
0.049 |
0.011 |
0.091 |
0.015 |
0.002 |
0.0017 |
Comparative Example 6 |
0.117 |
0.82 |
0.001 |
0.027 |
0.009 |
0.015 |
0.056 |
0.008 |
0.0049 |
Comparative Example 7 |
0.086 |
0.69 |
0.060 |
0.049 |
0.012 |
0.300 |
0.052 |
0.019 |
0.0047 |
Comparative Example 8 |
0.090 |
0.63 |
0.016 |
0.008 |
0.010 |
0.225 |
0.020 |
0.006 |
0.0035 |
Comparative Example 9 |
0.080 |
0.60 |
0.017 |
0.102 |
0.004 |
0.081 |
0.071 |
0.010 |
0.0011 |
Comparative Example 10 |
0.081 |
0.78 |
0.010 |
0.080 |
0.005 |
0.241 |
0.0008 |
0.011 |
0.0015 |
Comparative Example 11 |
0.093 |
0.65 |
0.014 |
0.051 |
0.011 |
0.093 |
0.110 |
0.018 |
0.0020 |
Comparative Example 12 |
0.100 |
0.90 |
0.021 |
0.080 |
0.010 |
0.100 |
0.015 |
0.002 |
0.0005 |
Comparative Example 13 |
0.080 |
0.30 |
0.021 |
0.010 |
0.008 |
0.100 |
0.015 |
0.002 |
0.0005 |
Comparative Example 14 |
0.080 |
0.90 |
0.014 |
0.010 |
0.010 |
0.300 |
0.006 |
0.010 |
0.0026 |
Comparative Example 15 |
0.113 |
0.64 |
0.017 |
0.056 |
0.008 |
0.184 |
0.054 |
0.000 |
0.0012 |
Comparative Example 16 |
0.089 |
0.62 |
0.014 |
0.068 |
0.002 |
0.097 |
0.074 |
0.012 |
0.0038 |
Comparative Example 17 |
0.087 |
0.73 |
0.021 |
0.050 |
0.012 |
0.046 |
0.016 |
0.006 |
0.0048 |
Comparative Example 18 |
0.116 |
0.61 |
0.012 |
0.063 |
0.010 |
0.173 |
0.004 |
0.001 |
0.0035 |
Comparative Example 19 |
0.087 |
0.65 |
0.008 |
0.044 |
0.0006 |
0.068 |
0.089 |
0.003 |
0.0003 |
Comparative Example 20 |
0.099 |
0.78 |
0.009 |
0.046 |
0.030 |
0.054 |
0.060 |
0.016 |
0.0015 |
Comparative Example 21 |
0.099 |
0.77 |
0.014 |
0.042 |
0.005 |
0.300 |
0.055 |
0.018 |
0.0049 |
Comparative Example 22 |
0.102 |
0.76 |
0.008 |
0.026 |
0.007 |
0.081 |
0.015 |
0.019 |
0.0004 |
Comparative Example 23 |
0.081 |
0.60 |
0.009 |
0.091 |
0.006 |
0.296 |
0.089 |
0.004 |
0.0014 |
Comparative Example 24 |
0.090 |
0.60 |
0.023 |
0.015 |
0.005 |
0.450 |
0.051 |
0.007 |
0.0029 |
Comparative Example 25 |
0.086 |
0.95 |
0.021 |
0.015 |
0.010 |
0.100 |
0.074 |
0.018 |
0.0035 |
Comparative Example 26 |
0.081 |
0.95 |
0.021 |
0.021 |
0.005 |
0.081 |
0.060 |
0.000 |
0.0003 |
[Table 2]
|
Component (% by mass) (continued from Table 1) |
CMeq |
Mn/Si |
LR |
Ac1 (°C) |
Mo |
Cu |
Ni |
Cr |
V |
Ca |
Mg |
REM |
Example 1 |
|
|
|
|
|
|
|
|
0.251 |
2.3 |
0.325 |
746 |
Example 2 |
|
|
|
|
|
|
|
|
0.219 |
3.2 |
0.336 |
746 |
Example 3 |
|
|
|
|
|
|
|
|
0.242 |
25.1 |
0.363 |
740 |
Example 4 |
|
|
|
|
|
|
|
|
0.193 |
5.0 |
0.396 |
743 |
Example 5 |
|
0.35 |
|
|
|
|
|
|
0.267 |
3.7 |
0.370 |
730 |
Example 6 |
|
|
0.44 |
|
|
|
|
|
0.261 |
2.5 |
0.396 |
724 |
Example 7 |
|
|
|
0.20 |
|
|
|
|
0.272 |
2.1 |
0.354 |
746 |
Example 8 |
|
|
|
0.08 |
|
0.0032 |
|
|
0.260 |
3.1 |
0.301 |
752 |
Example 9 |
0.05 |
|
|
|
|
|
|
|
0.222 |
6.0 |
0.322 |
745 |
Example 10 |
|
|
|
|
0.015 |
|
|
|
0.245 |
2.3 |
0.312 |
751 |
Example 11 |
|
|
|
|
|
|
|
0.0018 |
0.255 |
28.9 |
0.300 |
737 |
Example 12 |
|
0.06 |
0.06 |
|
|
|
|
|
0.251 |
5.0 |
0.382 |
740 |
Example 13 |
|
|
|
|
|
|
|
|
0.240 |
4.1 |
0.237 |
745 |
Example 14 |
0.20 |
|
|
|
|
|
|
|
0.250 |
2.6 |
0.371 |
752 |
Example 15 |
|
|
|
|
|
|
0.0060 |
|
0.227 |
3.4 |
0.240 |
744 |
Example 16 |
|
|
|
|
|
0.0025 |
|
0.0015 |
0.205 |
2.5 |
0.323 |
748 |
Example 17 |
|
|
|
|
|
0.0028 |
|
0.0020 |
0.204 |
3.1 |
0.314 |
746 |
Comparative Example 1 |
|
|
|
|
|
|
|
|
0.299 |
15.7 |
0.469 |
736 |
Comparative Example 2 |
|
|
|
|
|
|
|
|
0.255 |
2.1 |
0.227 |
741 |
Comparative Example 3 |
|
|
|
|
|
|
|
|
0.257 |
131.3 |
0.402 |
745 |
Comparative Example 4 |
|
|
|
|
|
|
|
|
0.173 |
2.9 |
1.296 |
739 |
Comparative Example 5 |
|
|
|
|
|
|
|
|
0.299 |
13.2 |
0.213 |
747 |
Comparative Example 6 |
|
|
|
|
|
|
|
|
0.281 |
52.9 |
0.334 |
735 |
Comparative Example 7 |
|
|
|
|
|
|
|
|
0.249 |
2.3 |
0.335 |
748 |
Comparative Example 8 |
|
|
|
|
|
|
|
|
0.203 |
2.8 |
0.312 |
744 |
Comparative Example 9 |
|
|
|
|
|
|
|
|
0.282 |
7.5 |
0.450 |
755 |
Comparative Example 10 |
|
|
|
|
|
|
|
|
0.290 |
3.2 |
0.322 |
762 |
Comparative Example 11 |
|
|
|
|
|
|
|
|
0.252 |
7.0 |
0.379 |
736 |
Comparative Example 12 |
|
|
|
|
|
|
|
|
0.330 |
9.0 |
0.322 |
755 |
Comparative Example 13 |
|
|
|
|
|
|
|
|
0.140 |
3.0 |
0.593 |
747 |
Comparative Example 14 |
|
|
|
|
|
|
|
|
0.240 |
3.0 |
0.198 |
745 |
Comparative Example 15 |
|
|
|
|
|
|
|
|
0.275 |
3.5 |
0.461 |
747 |
Comparative Example 16 |
|
|
|
|
|
|
|
|
0.261 |
6.4 |
0.408 |
746 |
Comparative Example 17 |
|
|
|
|
|
|
|
|
0.259 |
15.8 |
0.320 |
750 |
Comparative Example 18 |
|
|
|
|
|
|
|
|
0.279 |
3.5 |
0.503 |
758 |
Comparative Example 19 |
|
|
|
|
|
|
|
|
0.238 |
9.4 |
0.349 |
737 |
Comparative Example 20 |
|
|
|
|
|
|
|
|
0.275 |
14.5 |
0.324 |
741 |
Comparative Example 21 |
|
|
|
|
|
|
|
|
0.270 |
2.6 |
0.323 |
745 |
Comparative Example 22 |
|
|
|
|
|
|
|
|
0.255 |
9.4 |
0.318 |
744 |
Comparative Example 23 |
|
|
|
|
|
|
|
|
0.273 |
2.0 |
0.432 |
753 |
Comparative Example 24 |
|
|
|
|
|
|
|
|
0.205 |
1.3 |
0.340 |
744 |
Comparative Example 25 |
|
|
|
|
|
|
|
|
0.259 |
2.0 |
0.206 |
730 |
Comparative Example 26 |
|
|
|
|
|
|
|
|
0.260 |
2.0 |
0.201 |
734 |
[Table 3]
|
Change in ovality (%) by sizer step |
Tempering temperature (°C) |
Tempering time (min) |
Electric resistance welded steel pipe |
F fraction (%) |
Kind of second phase |
YS (MPa) |
TS (MPa) |
YR (%) |
vE (J) |
Precipitate areal ratio (%) |
Base metal portion |
Welded portion |
Example 1 |
1.8 |
620 |
16 |
72 |
TB+P |
536 |
657 |
82 |
181 |
146 |
0.38 |
Example 2 |
2.6 |
610 |
22 |
79 |
TB+P |
490 |
586 |
84 |
247 |
212 |
0.35 |
Example 3 |
2.5 |
570 |
25 |
81 |
TB |
527 |
631 |
83 |
262 |
228 |
0.31 |
Example 4 |
4.0 |
540 |
10 |
92 |
TB |
438 |
525 |
83 |
272 |
245 |
0.22 |
Example 5 |
1.2 |
680 |
7 |
68 |
TB+P |
541 |
633 |
85 |
200 |
178 |
0.49 |
Example 6 |
4.6 |
650 |
10 |
67 |
TB+P |
546 |
636 |
86 |
199 |
179 |
0.43 |
Example 7 |
4.5 |
420 |
26 |
69 |
TB+P |
560 |
663 |
84 |
260 |
229 |
0.10 |
Example 8 |
2.5 |
590 |
5 |
81 |
TB |
520 |
635 |
82 |
251 |
209 |
0.41 |
Example 9 |
4.4 |
680 |
19 |
85 |
TB |
534 |
623 |
86 |
308 |
262 |
0.57 |
Example 10 |
2.2 |
620 |
22 |
82 |
TB |
516 |
633 |
82 |
272 |
228 |
0.42 |
Example 11 |
2.5 |
640 |
24 |
70 |
TB+P |
544 |
676 |
81 |
188 |
150 |
0.42 |
Example 12 |
2.1 |
590 |
19 |
82 |
TB |
532 |
653 |
82 |
259 |
218 |
0.39 |
Example 13 |
4.0 |
680 |
10 |
78 |
TB+P |
515 |
600 |
86 |
300 |
268 |
0.12 |
Example 14 |
3.6 |
700 |
10 |
68 |
TB+P |
550 |
640 |
86 |
315 |
285 |
0.42 |
Example 15 |
3.6 |
650 |
10 |
65 |
TB+P |
520 |
590 |
88 |
325 |
298 |
0.35 |
Example 16 |
2.5 |
600 |
10 |
67 |
TB+P |
480 |
560 |
86 |
358 |
318 |
0.42 |
Example 17 |
3.2 |
650 |
10 |
63 |
TB+P |
490 |
560 |
88 |
380 |
369 |
0.37 |
Comparative Example 1 |
4.5 |
710 |
8 |
59 |
TB+P |
590 |
616 |
96 |
207 |
166 |
0.52 |
Comparative Example 2 |
1.6 |
520 |
10 |
65 |
TB+P |
579 |
679 |
85 |
262 |
10 |
0.19 |
Comparative Example 3 |
2.1 |
600 |
8 |
74 |
TB+P |
539 |
627 |
86 |
8 |
18 |
0.36 |
Comparative Example 4 |
1.2 |
480 |
29 |
95 |
TB+P |
424 |
530 |
80 |
5 |
12 |
0.13 |
Comparative Example 5 |
3.5 |
580 |
29 |
50 |
TB+P |
673 |
759 |
89 |
15 |
18 |
0.35 |
Comparative Example 6 |
1.2 |
610 |
26 |
60 |
TB+P |
575 |
705 |
82 |
10 |
15 |
0.33 |
Comparative Example 7 |
1.5 |
710 |
9 |
79 |
TB+P |
575 |
697 |
83 |
5 |
8 |
0.63 |
Comparative Example 8 |
1.7 |
720 |
21 |
81 |
TB |
474 |
588 |
81 |
15 |
171 |
0.53 |
Comparative Example 9 |
2.2 |
590 |
10 |
86 |
TB |
598 |
688 |
87 |
21 |
24 |
0.47 |
Comparative Example 10 |
2.7 |
530 |
5 |
74 |
TB+P |
560 |
660 |
85 |
15 |
21 |
0.34 |
Comparative Example 11 |
4.1 |
410 |
20 |
79 |
TB+P |
533 |
655 |
81 |
250 |
8 |
0.15 |
Comparative Example 12 |
3.1 |
690 |
27 |
61 |
TB+P |
668 |
829 |
81 |
218 |
194 |
0.62 |
Comparative Example 13 |
2.5 |
640 |
30 |
100 |
- |
406 |
495 |
82 |
350 |
312 |
0.39 |
Comparative Example 14 |
2.3 |
480 |
6 |
68 |
TB+P |
518 |
569 |
91 |
250 |
220 |
0.15 |
Comparative Example 15 |
4.6 |
350 |
27 |
72 |
TB+P |
578 |
635 |
91 |
113 |
99 |
0.01 |
Comparative Example 16 |
4.6 |
750 |
7 |
82 |
TB+MA |
450 |
594 |
76 |
10 |
15 |
0.02 |
Comparative Example 17 |
0.5 |
450 |
28 |
76 |
TB+P |
575 |
632 |
91 |
251 |
216 |
0.05 |
Comparative Example 18 |
4.5 |
620 |
1 |
73 |
TB+P |
581 |
638 |
91 |
106 |
85 |
0.05 |
Comparative Example 19 |
2.3 |
480 |
10 |
81 |
TB |
511 |
627 |
81 |
15 |
15 |
0.14 |
Comparative Example 20 |
2.5 |
530 |
15 |
69 |
TB+P |
570 |
666 |
86 |
5 |
8 |
0.24 |
Comparative Example 21 |
0.9 |
690 |
15 |
69 |
TB+P |
568 |
624 |
91 |
215 |
190 |
0.05 |
Comparative Example 22 |
0.8 |
710 |
15 |
69 |
TB+P |
536 |
585 |
92 |
143 |
110 |
0.02 |
Comparative Example 23 |
0.7 |
510 |
15 |
85 |
TB |
564 |
618 |
91 |
287 |
250 |
0.06 |
Comparative Example 24 |
2.5 |
510 |
10 |
83 |
TB |
495 |
592 |
84 |
279 |
10 |
0.17 |
Comparative Example 25 |
0.8 |
690 |
15 |
69 |
TB+P |
520 |
570 |
91 |
340 |
310 |
0.02 |
Comparative Example 26 |
4.6 |
350 |
10 |
69 |
TB+P |
515 |
568 |
91 |
330 |
300 |
0.42 |
[0206] As set forth in Table 1 to Table 3, the electric resistance welded steel pipe of
each Example satisfied TS, YS, YR, vE (base metal portion), and vE (welded portion)
in the present disclosure. In other words, it was shown that the electric resistance
welded steel pipe of each Example had a certain amount of tensile strength and yield
strength, which had a decreased yield ratio, and had the excellent toughness of a
base metal portion and a welded portion.
[0207] Comparative Example 1 in which the amount of C was more than the upper limit resulted
in a decrease in F fraction and an increase in YR.
[0208] Comparative Example 2 in which the amount of Si was more than the upper limit resulted
in the deterioration of the toughness of a welded portion.
[0209] Comparative Example 3 in which the amount of Si was less than the lower limit resulted
in the deterioration of the toughness of a base metal portion and a welded portion.
The reason thereof can be considered to be because deoxidization became insufficient,
thereby generating a coarse oxide.
[0210] Comparative Example 4 in which the amount of Mn was less than the lower limit resulted
in the deterioration of the toughness of a base metal portion and a welded portion.
The reason thereof can be considered to be because embrittlement due to S occurred.
[0211] Comparative Example 5 in which the amount of Mn was more than the upper limit resulted
in the deterioration of the toughness of a base metal portion and a welded portion.
The reason thereof can be considered to be because cracking due to MnS was facilitated.
[0212] Comparative Example 6 in which Ti was less than the lower limit resulted in the deterioration
of the toughness of a base metal portion and a welded portion. The reason thereof
can be considered to be because a crystal grain became coarse.
[0213] Comparative Example 7 in which Ti was more than the upper limit resulted in the deterioration
of the toughness of a base metal portion and a welded portion. The reason thereof
can be considered to be because coarse TiN was generated.
[0214] Comparative Example 8 in which Nb was less than the lower limit resulted in the deterioration
of the toughness of a base metal portion. The reason thereof can be considered to
be because rolling in the region of unrecrystallization temperature became insufficient.
[0215] Comparative Example 9 in which Nb was more than the upper limit resulted in the deterioration
of the toughness of a base metal portion and a welded portion. The reason thereof
can be considered to be because a coarse Nb carbonitride was generated.
[0216] Comparative Example 10 in which Al was less than the lower limit resulted in the
deterioration of the toughness of a base metal portion and a welded portion. With
regard to the reason thereof, the reason thereof can be considered to be because deoxidization
became insufficient.
[0217] Comparative Example 11 in which Al was more than the upper limit resulted in the
deterioration of the toughness of a welded portion. The reason thereof can be considered
to be because a large amount of Al-based inclusion was generated.
[0218] In Comparative Example 12 in which CMeq was more than the upper limit, YS and TS
were more than the upper limits.
[0219] In Comparative Example 13 in which CMeq was less than the lower limit, a F fraction
was more than the upper limit, and TS was less than the lower limit.
[0220] Comparative Example 14 in which LR was less than 0.210 resulted in a yield ratio
of more than 90%.
[0221] In Comparative Example 15, YS and YR were more than the upper limits. The reason
thereof can be considered to be because a tempering temperature was too low, thereby
resulting in the insufficient effect of reducing a pipe-making strain by tempering
(i.e., the effect of reducing a dislocation density by tempering) and in insufficient
precipitation on a dislocation.
[0222] In Comparative Example 16, the toughness of a base metal portion and a welded portion
was deteriorated (i.e., both the base metal portion and the welded portion showed
vE less than the lower limit). The reason thereof can be considered to be because
a tempering temperature was too high, transformation to austenite therefore occurred
in a partial region, the concentration of C was increased in the partial region, and
as a result, martensite island (MA) was generated by subsequent cooling.
[0223] In Comparative Example 17, YR was more than 90%. The reason thereof can be considered
to be because a change in ovality by sizer step was small, and therefore, the introduction
of a dislocation and the precipitation on the dislocation were insufficient.
[0224] In Comparative Example 18, YR was more than 90%. The reason thereof can be considered
to be because a tempering time was short, and therefore, precipitation on a dislocation
was insufficient.
[0225] Comparative Example 19 in which the amount of N was less than the lower limit resulted
in the deterioration of the toughness of a base metal portion and a welded portion.
The reason thereof can be considered to be because crystal grains became coarse.
[0226] Comparative Example 20 in which the amount of N was more than the upper limit resulted
in the deterioration of the toughness of a base metal portion and a welded portion.
The reason thereof can be considered to be because a carbide became coarse.
[0227] In Comparative Examples 21 to 23, YR was more than 90%. The reason thereof can be
considered to be because a change in ovality by sizer step was small, and therefore,
the introduction of a dislocation and the precipitation on the dislocation were insufficient.
[0228] In Comparative Example 24 in which a Mn/Si ratio was 2.0 or less, the toughness of
a welded portion was deteriorated.
[0229] In Comparative Examples 25 and 26 in which LR was less than 0.210, a yield ratio
was more than 90%.
[0230] The entire disclosure of Japanese Patent Application No.
2016-056858 is incorporated herein by reference.
[0231] All documents, patent applications, and technical standards described in this specification
are herein incorporated by reference to the same extent as if each individual document,
patent application, or technical standard was specifically and individually indicated
to be incorporated by reference.