Technical Field
[0001] The present disclosure relates to an as-rolled electric resistance welded steel pipe
for a line pipe and a hot-rolled steel sheet.
Background Art
[0002] Conventionally, various measures have been considered for steel pipes for a line
pipe, which are used in the production of a pipeline, and hot-rolled steel sheets
used in the production of the steel pipes for a line pipe.
[0003] For example, as a high-strength hot-rolled steel sheet for a spiral line pipe, which
has excellent low-temperature toughness, Patent Document 1 discloses a hot-rolled
steel sheet including, in terms of % by mass, from 0.02 to 0.08% of C, from 0.05 to
0.5% of Si, from 1 to 2% of Mn, from 0.03 to 0.12% of Nb, from 0.005 to 0.05% of Ti,
and the balance being Fe and inevitable impurity elements, in which a pro-eutectoid
ferrite fraction is from 3% to 20% and the others are a low-temperature transformation
phase and pearlite of 1% or less in a microstructure at a depth of a half thickness
of a wall thickness from a steel sheet surface, a number average crystal grain diameter
of the whole of the microstructure is from 1 µm to 2.5 µm and an area average grain
diameter is from 3 µm to 9 µm, a standard deviation of the area average grain diameter
is from 0.8 µm to 2.3 µm, and a reflected X-ray intensity ratio {211}/{111} of a {211}
direction and a {111} direction with respect to a plane parallel to the steel sheet
surface at the depth of the half thickness of the wall thickness from the steel sheet
surface is 1.1 or more.
[0004] Patent Document 1 states that the hot-rolled steel sheet described therein can be
used in the production of an electric resistance welded steel pipe or a spiral steel
pipe.
SUMMARY OF INVENTION
Technical Problem
[0006] As welded steel pipes among steel pipes for a line pipe, UOE steel pipes produced
using heavy plates (for example, heavy plates having a wall thickness of 30 mm or
more), or electric resistance welded steel pipes or spiral steel pipes produced using
hot coils made of hot-rolled steel sheets are used.
[0007] For the steel pipes for a line pipe, low-temperature toughness evaluated by DWTT
(Drop Weight Tear Test) (hereinafter, also simply referred to as "low-temperature
toughness") may be required. Specifically, the lower a DWTT guarantee temperature,
which is the lowest temperature value at which a percent ductile fracture is 85% or
more, is, the more excellent the low-temperature toughness is.
[0008] Generally, the low-temperature toughness tends to be required for the steel pipes
for a line pipe, which have a thick wall thickness. This is because the wall thickness
of the steel pipes for a line pipe being thick is advantageous in the strength but
disadvantageous in the low-temperature toughness.
[0009] Accordingly, in the field of the UOE steel pipes having a relatively thick wall thickness,
conventionally, the low-temperature toughness has received attention.
[0010] In contrast, in the field of the electric resistance welded steel pipes having a
relatively thin wall thickness, the low-temperature toughness has received little
attention.
[0011] As the reason why the low-temperature toughness has received attention in the field
of the UOE steel pipes and the low-temperature toughness has received little attention
in the field of the electric resistance welded steel pipes, there is the following
reason in the production.
[0012] A heavy plate process for producing heavy plates as materials of the UOE steel pipes
has a relatively high degree of freedom with respect to production conditions. For
example, in the heavy plate process, low-temperature rolling is easily performed,
and, for cooling after the rolling, complex controlled cooling is easily performed.
Accordingly, in the field of the UOE steel pipes, in order to improve the low-temperature
toughness of the UOE steel pipes, in the heavy plate process, fine adjustment of a
metallographic microstructure by the low-temperature rolling, the complex controlled
cooling, and the like has been generally performed.
[0013] In contrast, a hot-rolling process for producing hot coils (specifically, hot-rolled
steel sheets in the form of hot coils) as materials of the electric resistance welded
steel pipes has a lower degree of freedom with respect to production conditions compared
to the heavy plate process due to limitations of equipment focusing on the productivity.
For example, in the hot-rolling process, a hot-rolled steel sheet after rolling is
cooled to a coiling temperature (CT) of, for example, about from 400 to 600°C, and
then coiled into a coil shape. In the hot-rolling process, low-temperature rolling
and complex controlled cooling after the rolling are more difficult to be performed
compared to the heavy plate process due to the limitations. Because of these circumstances,
in the field of the electric resistance welded steel pipes, the idea itself, of performing
fine adjustment of a metallographic microstructure in the hot-rolling process in order
to improve the low-temperature toughness of the electric resistance welded steel pipes,
was difficult to conceive of.
[0014] In many cases, even if the chemical compositions are the same, the heavy plates and
the UOE steel pipes as end products thereof, and the hot-rolled steel sheets and the
electric resistance welded steel pipes as end products thereof are totally different
in the metallographic microstructure and/or the strength. Because of these circumstances,
the problem which receives attention in the UOE steel pipes (i.e., low-temperature
toughness) does not necessarily also receive attention in the same manner in the electric
resistance welded steel pipes.
[0015] For example, in the heavy plate process, since a heavy plate after the stop of cooling
is air-cooled in a state of (not coiled) one heavy plate from the sides of both surfaces,
a cooling rate during the air-cooling is relatively fast. In contrast, in the hot-rolling
process, since a hot-rolled steel sheet after the stop of cooling is air-cooled in
the form of a hot coil, a cooling rate during the air-cooling is relatively slow.
In the hot-rolling process, since the cooling rate in the air-cooling in the form
of a hot coil is slow, the metallographic microstructure may be substantially tempered
during the air-cooling in the form of a hot coil.
[0016] As described above, conventionally, the low-temperature toughness has received attention
in the field of the UOE steel pipes for a line pipe, but the low-temperature toughness
has received little attention for the electric resistance welded steel pipes for a
line pipe.
[0017] However, recently, the low-temperature toughness is likely to be required for the
electric resistance welded steel pipes for a line pipe because of a circumstance in
which a laying environment of a pipeline becomes more severe, a circumstance in which
the production of electric resistance welded steel pipes having a thick wall thickness
becomes possible due to the progress of a production technology of electric resistance
welded steel pipes, and the like.
[0018] Patent Document 1 described above is one of the few documents focusing on the low-temperature
toughness of hot-rolled steel sheets which may be used in the production of electric
resistance welded steel pipes.
[0019] However, for the technology disclosed in Patent Document 1, the low-temperature toughness
may be required to be further improved.
[0020] The disclosure was made in view of the circumstances described above.
[0021] An object of the disclosure is to provide an as-rolled electric resistance welded
steel pipe for a line pipe, which has excellent low-temperature toughness evaluated
by DWTT, and a hot-rolled steel sheet suitable for the production of the as-rolled
electric resistance welded steel pipe for a line pipe.
Solution to Problem
[0022] Means of solving the problem described above includes the following aspects.
- <1> An as-rolled 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:
from 0.030 to 0.120% of C,
from 0.05 to 0.30% of Si,
from 0.50 to 2.00% of Mn,
from 0 to 0.030% of P,
from 0 to 0.0100% of S,
from 0.010 to 0.035% of Al,
from 0.0010 to 0.0080% of N,
from 0.010 to 0.080% of Nb,
from 0.005 to 0.030% of Ti,
from 0.001 to 0.20% of Ni,
from 0.10 to 0.20% of Mo,
from 0 to 0.010% of V,
from 0 to 0.0030% of O,
from 0 to 0.0050% of Ca,
from 0 to 0.30% of Cr,
from 0 to 0.30% of Cu,
from 0 to 0.0050% of Mg,
from 0 to 0.0100% of REM, and
the balance being Fe and impurities, wherein:
F1 defined by the following Formula (1) is from 0.300 to 0.350,
in a metallographic microstructure of a wall thickness direction central portion of
the base metal portion, a polygonal ferrite fraction is from 60 to 90%, an average
crystal grain diameter is 15 µm or less, and a coarse crystal grain ratio, which is
an areal ratio of crystal grains having a crystal grain diameter of 20 µm or more,
is 20% or less, and
a yield ratio in a pipe axis direction is from 80 to 95%.
(In Formula (1), each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents mass% of a corresponding
element.)
- <2> The as-rolled electric resistance welded steel pipe for a line pipe according
to <1>, wherein the chemical composition of the base metal portion contains, in terms
of % by mass, one or more selected from the group consisting of:
more than 0% but equal to or less than 0.010% of V,
more than 0% but equal to or less than 0.0030% of Ca,
more than 0% but equal to or less than 0.30% of Cr,
more than 0% but equal to or less than 0.30% of Cu,
more than 0% but equal to or less than 0.0050% of Mg, and
more than 0% but equal to or less than 0.0100% of REM.
- <3> The as-rolled electric resistance welded steel pipe for a line pipe according
to <1> or <2>, wherein a yield strength in the pipe axis direction is from 450 to
540 MPa, and a tensile strength in the pipe axis direction is from 510 to 625 MPa.
- <4> The as-rolled electric resistance welded steel pipe for a line pipe according
to any one of <1> to <3>, wherein a wall thickness is from 12 to 25 mm, and an outer
diameter is from 304.8 to 660.4 mm.
- <5> The as-rolled electric resistance welded steel pipe for a line pipe according
to any one of <1> to <4>, wherein the yield ratio in the pipe axis direction is from
80 to 93%.
- <6> A hot-rolled steel sheet used in the production of the as-rolled electric resistance
welded steel pipe for a line pipe according to any one of <1> to <5>,
wherein a chemical composition consists of, in terms of % by mass:
from 0.030 to 0.120% of C,
from 0.05 to 0.30% of Si,
from 0.50 to 2.00% of Mn,
from 0 to 0.030% of P,
from 0 to 0.0100% of S,
from 0.010 to 0.035% of Al,
from 0.0010 to 0.0080% of N,
from 0.010 to 0.080% of Nb,
from 0.005 to 0.030% of Ti,
from 0.001 to 0.20% of Ni,
from 0.10 to 0.20% of Mo,
from 0 to 0.010% of V,
from 0 to 0.0030% of O,
from 0 to 0.0050% of Ca,
from 0 to 0.30% of Cr,
from 0 to 0.30% of Cu,
from 0 to 0.0050% of Mg,
from 0 to 0.0100% of REM, and
the balance being Fe and impurities, wherein:
F1 defined by Formula (1) is from 0.300 to 0.350, and
in a metallographic microstructure of a wall thickness direction central portion,
a polygonal ferrite fraction is from 60 to 90%, an average crystal grain diameter
is 15 µm or less, and a coarse crystal grain ratio, which is an areal ratio of crystal
grains having a crystal grain diameter of 20 µm or more, is 20% or less.
- <7> The hot-rolled steel sheet according to <6>, wherein a yield strength in a rolling
direction is from 450 to 500 MPa, and a tensile strength in the rolling direction
is from 510 to 580 MPa.
Advantageous Effects of Invention
[0023] According to the disclosure, an as-rolled electric resistance welded steel pipe for
a line pipe, which has excellent low-temperature toughness evaluated by DWTT, and
a hot-rolled steel sheet suitable for the production of the as-rolled electric resistance
welded steel pipe for a line pipe are provided.
BRIEF DESCRIPTION OF DRAWINGS
[0024]
Fig. 1 is a KAM map used in measurement of a polygonal ferrite fraction in an example
of a metallographic microstructure of a base metal portion in the disclosure.
Fig. 2 is a 15° high angle grain boundary map used in measurement of an average crystal
grain diameter and a coarse crystal grain ratio in an example of the metallographic
microstructure of the base metal portion in the disclosure.
Fig. 3 is a scanning electron micrograph (SEM micrograph; a magnification of 500 times)
showing an example of the metallographic microstructure of the base metal portion
in the disclosure.
Fig. 4 is a schematic front view of a tensile test specimen in the disclosure.
Fig. 5 is a continuous cooling transformation diagram (CCT diagram) in the case of
producing a hot-rolled steel sheet according to an example of the disclosure.
Fig. 6 is a schematic front view of a DWTT test specimen in the disclosure.
DESCRIPTION OF EMBODIMENTS
[0025] A numerical range expressed by "from x to y" herein includes the values of x and
y in the range as the minimum and maximum values, respectively.
[0026] The content of a component (element) expressed by "%" herein means "% by mass".
[0027] The content of C (carbon) in a base metal portion may be herein occasionally expressed
as "C content". The content of another element in the base metal portion may be expressed
similarly.
[0028] 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.
[0029] Herein, an "as-rolled electric resistance welded steel pipe for a line pipe" may
be simply referred to as an "electric resistance welded steel pipe" or an "as-rolled
electric resistance welded steel pipe".
[0030] Herein, the as-rolled electric resistance welded steel pipe refers to an electric
resistance welded steel pipe which is not subjected to heat treatment other than seam
heat treatment after pipe-making.
[0031] Herein, the "pipe-making" refers to a process of making an open pipe by roll-forming
of a hot-rolled steel sheet and forming an electric resistance welded portion by electric
resistance welding of abutting portions of the obtained open pipe.
[0032] Herein, the "roll-forming" refers to forming of a hot-rolled steel sheet into an
open pipe shape by bending work.
[As-rolled Electric Resistance Welded Steel Pipe for Line Pipe]
[0033] An electric resistance welded steel pipe (i.e., an as-rolled electric resistance
welded steel pipe for a line pipe) of the disclosure includes 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: from 0.030 to 0.120% of C, from
0.05 to 0.30% of Si, from 0.50 to 2.00% of Mn, from 0 to 0.030% of P, from 0 to 0.0100%
of S, from 0.010 to 0.035% of Al, from 0.0010 to 0.0080% of N, from 0.010 to 0.080%
of Nb, from 0.005 to 0.030% of Ti, from 0.001 to 0.20% of Ni, from 0.10 to 0.20% of
Mo, from 0 to 0.010% of V, from 0 to 0.0030% of O, from 0 to 0.0050% of Ca, from 0
to 0.30% of Cr, from 0 to 0.30% of Cu, from 0 to 0.0050% of Mg, from 0 to 0.0100%
of REM, and the balance being Fe and impurities, wherein: F1 defined by the following
Formula (1) is from 0.300 to 0.350, in a metallographic microstructure of a wall thickness
direction central portion of the base metal portion, a polygonal ferrite fraction
is from 60 to 90%, an average crystal grain diameter is 15 µm or less, and a coarse
crystal grain ratio, which is an areal ratio of crystal grains having a crystal grain
diameter of 20 µm or more, is 20% or less, and a yield ratio in a pipe axis direction
is from 80 to 95%.
(In Formula (1), each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents % by mass of
a corresponding element.)
[0034] In the electric resistance welded steel pipe of the disclosure, the base metal portion
refers to a portion other than the electric resistance welded portion and a heat affected
zone in the electric resistance welded steel pipe.
[0035] 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).
[0036] The electric resistance welded steel pipe of the disclosure has excellent low-temperature
toughness (i.e., low-temperature toughness evaluated by DWTT).
[0037] Such an effect is achieved by the chemical composition of the base metal portion
described above (including F1 being from 0.300 to 0.350) and the metallographic microstructure
of the base metal portion described above (approximately speaking, the metallographic
microstructure in which crystal grains are refined).
[0038] The metallographic microstructure of the base metal portion is achieved by a chemical
composition and a production condition of a hot-rolled steel sheet as a material.
The chemical composition of the base metal portion and the metallographic microstructure
of the base metal portion, and a preferred production condition of the hot-rolled
steel sheet will be described later.
[0039] As described above, the electric resistance welded steel pipe of the disclosure has
excellent low-temperature toughness.
[0040] Thus, the electric resistance welded steel pipe of the disclosure is suitable as,
for example, one member for forming a submarine pipeline which undergoes cyclic straining
due to waves or one member for forming a line pipe for cold climates.
[0041] The electric resistance welded steel pipe of the disclosure has a yield ratio in
a pipe axis direction of from 80 to 95%
[0042] A yield ratio of the electric resistance welded steel pipe of 95% or less secures
a plastic deformation allowance required as a steel pipe for a line pipe. A yield
ratio of the electric resistance welded steel pipe of 95% or less more suppresses
buckling in the case of laying a pipeline formed using the electric resistance welded
steel pipe by a reeling method or the like.
[0043] A yield ratio of the electric resistance welded steel pipe of 80% or more has excellent
production suitability of the electric resistance welded steel pipe.
<Chemical Composition of Base Metal Portion>
[0044] The chemical composition of the base metal portion in the disclosure will be described.
[0045] Hereinafter, the chemical composition of the base metal portion in the disclosure
(including F1 being from 0.300 to 0.350) is referred to as the "chemical composition
in the disclosure".
C: from 0.030 to 0.120%
[0046] C enhances the strength of steel. In a case in which a C content is too low, the
effect cannot be obtained. Accordingly, the C content is 0.030% or more. The C content
is preferably 0.035% or more, and more preferably 0.045% or more.
[0047] In contrast, in a case in which the C content is too high, a carbide is generated,
and the low-temperature toughness and the ductility of steel are decreased. Accordingly,
the C content is 0.120% or less. The C content is preferably 0.110% or less.
[0048] The mere term "strength" herein means a tensile strength (hereinafter, also referred
to as "TS") and/or a yield strength (hereinafter, also referred to as "YS").
Si: from 0.05 to 0.30%
[0049] Si deoxidizes steel. In a case in which a Si content is too low, the effect cannot
be obtained. Accordingly, the Si content is 0.05% or more. The Si content is preferably
0.10% or more, and still more preferably 0.15% or more.
[0050] In contrast, in a case in which the Si content is too high, the low-temperature toughness
of steel is decreased. Accordingly, the Si content is 0.30% or less. The Si content
is preferably 0.25% or less, and more preferably 0.21% or less.
Mn: from 0.50 to 2.00%
[0051] Mn enhances the hardenability of steel and enhances the strength of steel. In a case
in which a Mn content is too low, the effect cannot be obtained. Accordingly, the
Mn content is 0.50% or more. The Mn content is preferably 0.80% or more, and more
preferably 1.00% or more.
[0052] In contrast, in a case in which the Mn content is too high, the strength of steel
becomes too high, and the low-temperature toughness of steel is decreased. Accordingly,
the Mn content is 2.00% or less. The Mn content is preferably 1.80% or less, and more
preferably 1.50% or less.
P: from 0 to 0.030%
[0053] P is an impurity. P decreases the low-temperature toughness of steel. Accordingly,
a P content is preferably small. Specifically, the P content is 0.030% or less. The
P content is preferably 0.020% or less, and more preferably 0.015% or less.
[0054] In contrast, the P content may be 0%. From the viewpoint of reducing a dephosphorization
cost, the P content may be more than 0%, may be 0.001% or more, and may be 0.005%
or more.
S: from 0 to 0.0100%
[0055] S is an impurity. S binds to Mn to form a Mn-based sulfide. Thus, in a case in which
a S content is too high, the low-temperature toughness and the sour resistance of
steel are decreased. Accordingly, the S content is 0.0100% or less. The S content
is preferably 0.0080% or less, and more preferably 0.0050% or less.
[0056] In contrast, the S content may be 0%. From the viewpoint of reducing a desulfurization
cost, the S content may be more than 0%, may be 0.0001% or more, may be 0.0010% or
more, and may be 0.0020% or more.
Al: from 0.010 to 0.035%
[0057] Al deoxidizes steel. In a case in which an Al content is too low, the effect cannot
be obtained. Accordingly, the Al content is 0.010% or more. The Al content is preferably
0.015% or more, and more preferably 0.020% or more.
[0058] In contrast, in a case in which the Al content is too high, an Al oxide is coarsened,
and the low-temperature toughness of steel is decreased. Accordingly, the Al content
is 0.050% or less. The Al content is preferably 0.040% or less, more preferably 0.035%
or less, and still more preferably 0.030% or less.
[0059] The Al content herein means the content of total Al in the steel.
N: from 0.0010 to 0.0080%
[0060] N forms a nitride to suppress coarsening of austenite grains in a heating step. In
this case, the austenite grains are refined in a rolling step, and crystal grains
after transformation become fine. Therefore, the low-temperature toughness of steel
is enhanced. N further enhances the strength of steel by solid-solution strengthening.
In a case in which a N content is too low, the effect cannot be obtained. Accordingly,
the N content is 0.0010% or more. The N content is preferably 0.0020% or more, and
more preferably 0.0025% or more.
[0061] In contrast, in a case in which the N content is too high, a carbonitride is coarsened,
and the low-temperature toughness of steel is decreased. Accordingly, the N content
is 0.0080% or less. The N content is preferably 0.0070% or less, more preferably 0.0060%
or less, and still more preferably 0.0050% or less.
Nb: from 0.010 to 0.080%
[0062] Nb binds to C and N in the steel to form a fine Nb carbonitride. The Nb carbonitride
suppresses coarsening of crystal grains, and the average crystal grain diameter becomes
small. Thus, the low-temperature toughness of steel is enhanced. Furthermore, the
fine Nb carbonitride enhances the strength of steel by dispersion strengthening. In
a case in which a Nb content is too low, the effect cannot be obtained. Accordingly,
the Nb content is 0.010% or more. The Nb content is preferably 0.015% or more.
[0063] In contrast, in a case in which the Nb content is too high, the Nb carbonitride is
coarsened, and the low-temperature toughness of steel is decreased. Accordingly, the
Nb content is 0.050% or less. The Nb content is preferably 0.040% or less, and more
preferably 0.030% or less.
Ti: from 0.005 to 0.030%
[0064] Ti binds to N in the steel to form a TiN and suppress a decrease in the low-temperature
toughness of steel due to a solid solution of N. Furthermore, the dispersion precipitation
of the fine TiN suppresses coarsening of crystal grains. As a result, the low-temperature
toughness of steel is enhanced. In a case in which a Ti content is too low, the effect
cannot be obtained. Accordingly, the Ti content is 0.005% or more. The Ti content
is preferably 0.007% or more, and more preferably 0.010% or more.
[0065] In contrast, in a case in which the Ti content is too high, the TiN is coarsened,
and a coarse TiC is formed. In this case, the low-temperature toughness of steel is
decreased. Accordingly, the Ti content is 0.030% or less. The Ti content is preferably
0.020% or less, and more preferably 0.017% or less.
Ni: from 0.001 to 0.20%
[0066] Ni enhances the hardenability of steel and enhances the strength of steel. In a case
in which a Ni content is too low, the effect cannot be obtained. Accordingly, the
Ni content is 0.001% or more. The Ni content is preferably 0.01% or more, more preferably
0.05% or more, and still more preferably 0.07% or more.
[0067] In contrast, in a case in which the Ni content is too high, the above-described effect
is saturated. Accordingly, the Ni content is 0.20% or less. The Ni content is preferably
0.15% or less.
Mo: from 0.10 to 0.20%
[0068] Mo enhances the hardenability of steel and enhances the strength of steel. Mo further
refines austenite grains and enhances the low-temperature toughness of steel. In a
case in which a Mo content is too low, the effect cannot be obtained. Accordingly,
the Mo content is 0.10% or more. The Mo content is preferably 0.15% or more.
[0069] In contrast, in a case in which the Mo content is too high, the field weldability
of steel is decreased. Accordingly, the Mo content is 0.20% or less. The Mo content
is preferably 0.19% or less, and more preferably 0.18% or less.
V: from 0 to 0.010%
[0070] V is an optional element. Accordingly, a V content may be 0%.
[0071] V binds to C and N in the steel in a coiling step to form a fine carbonitride and
enhance the strength of steel. The fine V carbonitride further suppresses coarsening
of crystal grains and enhances the low-temperature toughness of steel. From the viewpoint
of the effect, the V content may be more than 0%, may be 0.001% or more, and may be
0.002% or more.
[0072] In contrast, in a case in which the V content is more than 0.010%, the low-temperature
toughness is deteriorated by coarsening of the V carbonitride. Accordingly, the V
content is 0.010% or less.
O: from 0 to 0.0030%
[0073] O is an impurity. O forms an oxide and decreases the hydrogen induced cracking resistance
(hereinafter, also referred to as "HIC resistance") of steel. O further decreases
the low-temperature toughness of steel. Accordingly, an O content is 0.0030% or less.
The O content is preferably 0.0025% or less. The O content is preferably as low as
possible.
[0074] In contrast, the O content may be 0%. From the viewpoint of reducing a deoxidation
cost, the O content may be more than 0%, may be 0.0001% or more, may be 0.0010% or
more, may be 0.0015% or more, and may be 0.0020% or more.
Ca: from 0 to 0.0050%
[0075] Ca is an optional element. Accordingly, a Ca content may be 0%.
[0076] Ca controls the form of MnS and makes the form into a spherical shape, thereby improving
the low-temperature toughness of steel. From the viewpoint of such an effect, the
Ca content may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, may
be 0.0015% or more, and may be 0.0020% or more.
[0077] In contrast, in a case in which the Ca content is more than 0.0050%, a coarse oxide-based
inclusion is formed. Accordingly, the Ca content is 0.0050% or less. The Ca content
is preferably 0.0045% or less.
Cr: from 0 to 0.30%
[0078] Cr is an optional element. Accordingly, a Cr content may be 0%.
[0079] Cr is an element that improves the hardenability and enhances the strength of steel.
From the viewpoint of such an effect, the Cr content may be more than 0%, and may
be 0.01% or more.
[0080] In contrast, in a case in which the Cr content is more than 0.30%, the hardenability
becomes too high, and the low-temperature toughness of steel is decreased. Accordingly,
the Cr content is 0.30% or less. The Cr content is preferably 0.20% or less, more
preferably 0.10% or less, and still more preferably 0.05% or less.
Cu: from 0 to 0.30%
[0081] Cu is an optional element. Accordingly, a Cu content may be 0%.
[0082] Cu enhances the hardenability of steel and enhances the strength of steel. From the
viewpoint of such an effect, the Cu content may be more than 0%, may be 0.01% or more,
may be 0.05% or more, and may be 0.10% or more.
[0083] In contrast, in a case in which the Cu content is too high, the hardenability becomes
too high, and the low-temperature toughness of steel is decreased. Accordingly, the
Cu content is 0.30% or less. The Cu content is preferably 0.25% or less, and more
preferably 0.20% or less.
Mg: from 0 to 0.0050%
[0084] Mg is an optional element and may not be contained. In other words, a Mg content
may be 0%.
[0085] In a case in which Mg is contained, Mg functions as a deoxidizer and a desulfurizer.
Moreover, Mg forms a fine oxide and also contributes to improvement in the toughness
of an HAZ. From the viewpoint of the effect, the Mg content is preferably more than
0%, more preferably 0.0001% or more, and still more preferably 0.0010% or more.
[0086] In contrast, in a case in which the Mg content is too high, the oxide becomes easy
to be aggregated or coarsened, and therefore, the decrease in HIC resistance or the
decrease in the toughness of the base metal portion or the HAZ may be caused. Accordingly,
the Mg content is 0.0050% or less. The Mg content is preferably 0.0030% or less.
REM: from 0 to 0.0100%
[0087] REM is an optional element and may not be contained. In other words, an REM content
may be 0%.
[0088] "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.
[0089] In a case in which REM is contained, REM functions as a deoxidizer and a desulfurizer.
From the viewpoint of such an effect, the REM content is preferably more than 0%,
more preferably 0.0001% or more, and still more preferably 0.0010% or more.
[0090] In contrast, in a case in which REM is too high, a coarse oxide is generated, and
therefore, the decrease in the HIC resistance or the decrease in the toughness of
the base metal portion or the HAZ may be caused. Accordingly, the REM content is 0.0100%
or less. The REM content is preferably 0.0070% or less, and more preferably 0.0050%
or less.
[0091] The chemical composition of the base metal portion may contain one or more selected
from the group consisting of: more than 0% but equal to or less than 0.010% of V,
more than 0% but equal to or less than 0.0030% of Ca, more than 0% but equal to or
less than 0.30% of Cr, more than 0% but equal to or less than 0.30% of Cu, more than
0% but equal to or less than 0.0050% of Mg, and more than 0% but equal to or less
than 0.0100% of REM.
[0092] The more preferred content of each optional element has been described above.
Balance: Fe and Impurities
[0093] In the chemical composition of the base metal portion, the balance excluding each
element described above is Fe and impurities.
[0094] The impurities refer to components which are contained in a raw material (for example,
ore, scrap, and the like) or mixed into in a production step, and which are not intentionally
incorporated into a steel.
[0095] 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.
[0096] Examples of the impurities include B, Sb, Sn, W, Co, As, Pb, Bi, and H.
[0097] 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.
F1: from 0.300 to 0.350
[0098] In the chemical composition of the base metal portion, F1 defined by the following
Formula (1) is from 0.300 to 0.350.
(In Formula (1), each of C, Si, Mn, Ni, Cr, Mo, V, and Nb represents % by mass of
a corresponding element.)
[0099] Needless to say, in a case in which the chemical composition does not contain any
element corresponding to an element symbol in Formula (1), "0" is substituted into
the corresponding element symbol in Formula (1).
[0100] F1 is correlated to the metallographic microstructure of the base metal portion (in
particular, crystal grain diameter).
[0101] In a case in which F1 is less than 0.300, since polygonal ferrite grains (hereinafter,
also simply referred to as "ferrite grains") are coarsened, the average crystal grain
diameter may become large, and moreover, since the metallographic microstructure becomes
a mixed-grain microstructure, the coarse crystal grain ratio may become large. Therefore,
the low-temperature toughness may be deteriorated. In a case in which F1 is less than
0.300, since the hardenability is decreased, a sufficient strength may not be obtained.
Accordingly, F1 is 0.300 or more. F1 is preferably 0.305 or more.
[0102] In contrast, in a case in which F1 is more than 0.350, since the polygonal ferrite
fraction becomes too small, the average crystal grain diameter and/or the coarse crystal
grain ratio may become too large. Therefore, the low-temperature toughness may be
deteriorated. Accordingly, F1 is 0.350 or less. F1 is preferably 0.345 or less, and
more preferably 0.340 or less.
[0103] From the viewpoint of easily achieving F1 of from 0.300 to 0.350, in the chemical
composition of the base metal portion, F2 defined by the following Formula (2) is
preferably from 0.230 to 0.300, and more preferably from 0.230 to 0.290.
[0104] In a case in which F2 is 0.230 or more, F1 of 0.300 or more is more easily achieved.
[0105] In a case in which F2 is 0.300 or less, F1 of 0.350 or less is more easily achieved.
(In Formula (2), each of Si, Mn, Ni, Cr, Mo, V, and Nb represents mass% of a corresponding
element.)
[0106] Needless to say, in a case in which the chemical composition does not contain any
element corresponding to an element symbol in Formula (2), "0" is substituted into
the corresponding element symbol in Formula (2).
[0107] <Metallographic Microstructure of Wall thickness direction central portion of Base
Metal Portion>
[0108] The metallographic microstructure of the wall thickness direction central portion
of the base metal portion (hereinafter, also referred to as the "metallographic microstructure
of the base metal portion") will be described below.
[0109] In the metallographic microstructure of the wall thickness direction central portion
of the base metal portion, the polygonal ferrite fraction (hereinafter, also simply
referred to as "ferrite fraction") is from 60 to 90%, the average crystal grain diameter
is 15 µm or less, and the coarse crystal grain ratio, which is an areal ratio of crystal
grains having a crystal grain diameter of 20 µm or more, is 20% or less.
Ferrite Fraction: from 60 to 90%
[0110] In the metallographic microstructure of the wall thickness direction central portion
of the base metal portion, the ferrite fraction (i.e., polygonal ferrite fraction)
is from 60 to 90%. In other words, the metallographic microstructure of the wall thickness
direction central portion of the base metal portion is a metallographic microstructure
which is mainly composed of ferrite (i.e., polygonal ferrite).
[0111] In a case in which the ferrite fraction is less than 60%, the average crystal grain
diameter and/or the coarse crystal grain ratio becomes too large, and therefore, the
low-temperature toughness may be deteriorated. In a case in which the ferrite fraction
is 60% or more, the crystal grains are refined (specifically, the average crystal
grain diameter and the coarse crystal grain ratio are decreased), and therefore, the
low-temperature toughness is enhanced. Accordingly, the ferrite fraction is 60% or
more. The ferrite fraction is preferably 65% or more, and more preferably 70% or more.
[0112] In contrast, in the chemical composition containing C in the disclosure, a metallographic
microstructure having a ferrite fraction of 90% or less is easily formed. Accordingly,
the ferrite fraction in the metallographic microstructure of the wall thickness direction
central portion of the base metal portion is 90% or less. The ferrite fraction is
preferably 85% or less.
Average Crystal Grain Diameter: 15 µm or less
[0113] In the metallographic microstructure of the wall thickness direction central portion
of the base metal portion, the average crystal grain diameter is 15 µm or less.
[0114] In a case in which the average crystal grain diameter is more than 15 µm, the low-temperature
toughness is deteriorated. Accordingly, the average crystal grain diameter is 15 µm
or less, and preferably 12 µm or less.
[0115] From the viewpoint of the low-temperature toughness, the lower limit of the average
crystal grain diameter is not particularly restricted. From the viewpoint of the production
suitability of the steel, the average crystal grain diameter is preferably 3 µm or
more, more preferably 5 µm or more, and still more preferably 8 µm or more.
Coarse Crystal Grain Ratio: 20% or less
[0116] In the metallographic microstructure of the wall thickness direction central portion
of the base metal portion, the coarse crystal grain ratio is 20% or less.
[0117] As described above, the coarse crystal grain ratio herein means an areal ratio of
crystal grains having a crystal grain diameter of 20 µm or more.
[0118] In a case in which the coarse crystal grain diameter ratio is more than 20%, the
low-temperature toughness is deteriorated. Accordingly, the coarse crystal grain diameter
ratio is 20%. The coarse crystal grain diameter ratio is preferably 18% or less, and
still more preferably 15% or less.
[0119] From the viewpoint of the low-temperature toughness, the lower limit of the coarse
crystal grain diameter ratio is not particularly restricted. From the viewpoint of
the production suitability of the steel, the coarse crystal grain diameter ratio is
preferably 3% or more, more preferably 5% or more, and still more preferably 8% or
more.
[0120] The ferrite fraction (i.e., polygonal ferrite fraction) herein means an areal ratio
of ferrite (i.e., polygonal ferrite).
[0121] Confirmation of the metallographic microstructure of the wall thickness direction
central portion of the base metal portion herein is performed by confirming the metallographic
microstructure of the wall thickness direction central portion in an L cross-section
at a base metal 90° position of the electric resistance welded steel pipe.
[0122] The base metal 90° position refers to a position shifted from the electric resistance
welded portion by 90° in a pipe circumferential direction.
[0123] The L cross-section refers to a cross-section parallel to a pipe axis direction and
a wall thickness direction.
[0124] The ferrite fraction is measured by the following method.
[0125] A sample for observing the wall thickness direction central portion in the L cross-section
at the base metal 90° position is sampled from the electric resistance welded steel
pipe. An observation surface of the sampled sample is polished by colloidal silica
polish for from 30 to 60 minutes. The polished observation surface is analyzed using
EBSD-OIM (trademark) (Electron Back Scatter Diffraction Pattern-Orientation Image
Microscopy), and an areal ratio of polygonal ferrite in a visual field range of 200
µm (pipe axis direction) × 500 µm (wall thickness direction), centered at the wall
thickness direction central portion in the L cross-section at the base metal 90° position,
is determined as the ferrite fraction.
[0126] A visual field magnification (observation magnification) of EBSD-OIM is 400 times,
and a measurement step is 0.3 µm.
[0127] Specifically, the ferrite fraction is determined by KAM (Kernel Average Misorientation)
method equipped in EBSD-OIM.
[0128] Specifically, first, a visual field range is divided into regular hexagonal pixel
units, and one regular hexagonal pixel in the visual field range is selected as the
central pixel. In a total of 37 pixels composed of the selected central pixel, six
pixels located outside of the central pixel, 12 pixels further located outside of
the six pixels, and 18 pixels further located outside of the 12 pixels, misorientations
between the respective pixels are determined. The average value of the obtained misorientations
is determined as a KAM value of the central pixel. In the same manner, a KAM value
is determined for each pixel included in the visual field range. The calculating method
of these KAM values is a method which is sometimes referred to as "third approximation".
[0129] A KAM map indicating the KAM values of the respective pixels included in the visual
field range is produced based on the above result.
[0130] Based on the obtained KAM map, an areal fraction of pixels having a KAM value of
1° or less with respect to the total area of the visual field range is determined
as the ferrite fraction.
[0131] A microstructure of pixels having a KAM value of 1° or less is polygonal ferrite,
and a microstructure of pixels having a KAM value of more than 1° is at least one
of bainite or pearlite.
[0132] Fig. 1 is a KAM map used in measurement of the ferrite fraction in the electric resistance
welded steel pipe according to an example of the disclosure.
[0133] Although the KAM map is displayed by gray scale in Fig. 1, a KAM map is typically
displayed by color.
[0134] In Fig. 1 displayed by gray scale, black parts are polygonal ferrite. In this example,
an areal ratio of the black parts (polygonal ferrite) with respect to the whole of
Fig. 1 (the whole of the metallographic microstructure) is the polygonal ferrite fraction.
[0135] The average crystal grain diameter and the coarse crystal grain ratio herein are
measured as follows by EBSD-OIM method.
[0136] In the same manner as the above-described measurement of the ferrite fraction, a
sample for observing the wall thickness direction central portion in the L cross-section
at the base metal 90° position is sampled from the electric resistance welded steel
pipe, and an observation surface of the sampled sample is polished by colloidal silica
polish for from 30 to 60 minutes.
[0137] The polished observation surface is analyzed using EBSD-OIM, and an area average
grain diameter in a visual field range of 200 µm (pipe axis direction) × 500 µm (wall
thickness direction), the range is centered at the wall thickness direction central
portion in the L cross-section at the base metal 90° position, is determined as the
average crystal grain diameter.
[0138] An areal ratio of crystal grains having a crystal grain diameter of 20 µm or more
(i.e., coarse crystal grains) with respect to the whole of the visual field range
is determined as the coarse crystal grain ratio.
[0139] A visual field magnification (observation magnification) of EBSD-OIM is 400 times,
and a measurement step is 0.3 µm.
[0140] More specifically, in the measurement of the average crystal grain diameter, orientation
measurement for each measurement step of 0.3 µm is performed, and a 15° large inclination
grain boundary map in which a position where a misorientation between adjacent measurement
points is more than 15° is regarded as a crystal grain boundary is produced. Here,
15° is a threshold value of a high angle grain boundary and is generally recognized
as a crystal grain boundary.
[0141] Based on the produced 15° high angle grain boundary map, a region surrounded by the
crystal grain boundaries is regarded as a crystal grain, and a grain diameter and
an area of each crystal grain are respectively determined. The grain diameter of each
crystal grain is an equivalent circle diameter of each crystal grain.
[0142] Based on the grain diameter and the area of each crystal grain, an area average grain
diameter is determined as the average crystal grain diameter.
[0143] An areal ratio of crystal grains having a crystal grain diameter of 20 µm or more
(i.e., coarse crystal grains) with respect to the whole of the visual field range
is determined as the coarse crystal grain ratio.
[0144] Fig. 2 is a 15° high angle grain boundary map used in measurement of the average
crystal grain diameter and the coarse crystal grain ratio in the electric resistance
welded steel pipe according to an example of the disclosure.
[0145] Fig. 2 shows the metallographic microstructure at the same part as Fig. 1.
[0146] In Fig. 2, fine (i.e, small area) crystal grains are ferrite grains, and large area
crystal grains are bainite grains or pearlite grains.
[0147] In the electric resistance welded steel pipe of the disclosure, the balance in the
metallographic microstructure of the base metal portion (i.e., the balance other than
polygonal ferrite) is preferably composed of at least one of bainite or pearlite.
As a result, the low-temperature toughness is improved compared to a case in which
the balance contains, for example, martensite.
[0148] The concept of "bainite" herein includes bainitic ferrite, upper bainite, and lower
bainite. The concept of "bainite" herein further includes tempered bainite formed
during air-cooling after coiling the hot-rolled steel sheet (i.e., during air-cooling
in the form of a hot coil).
[0149] The concept of "pearlite" herein includes pseudo-pearlite.
[0150] The electric resistance welded steel pipe of the disclosure is an as-rolled electric
resistance welded steel pipe (i.e., an electric resistance welded steel pipe which
is not subjected to heat treatment other than seam heat treatment after pipe-making).
Thus, the balance easily becomes at least one of bainite or pearlite.
[0151] In an electric resistance welded steel pipe formed by being subjected to heat treatment
other than seam heat treatment after pipe-making unlike the electric resistance welded
steel pipe of the disclosure (as-rolled electric resistance welded steel pipe), martensite
may be formed as the metallographic microstructure of the base metal portion. The
electric resistance welded steel pipe in this case tends to have poor low-temperature
toughness.
[0152] Fig. 3 is a scanning electron micrograph (SEM micrograph; a magnification of 500
times) showing an example of the metallographic microstructure of the base metal portion
in the disclosure.
[0153] Specifically, the SEM micrograph shown in Fig. 3 was measured as follows.
[0154] A test specimen for observing the wall thickness direction central portion in the
L cross-section at the base metal 90° position was sampled from the electric resistance
welded steel pipe according to an example of the disclosure. The L cross-section in
the sampled test specimen was nital-etched, and a micrograph of the nital-etched metallographic
microstructure (hereinafter, also referred to as "metallographic micrograph") was
taken with a scanning electron microscope (SEM) at a magnification of 500 times.
[0155] According to Fig. 3, the metallographic microstructure according to this example
is revealed to be a metallographic microstructure which is mainly composed of ferrite
(i.e., polygonal ferrite).
[0156] Being an as-rolled electric resistance welded steel pipe can be confirmed by not
observing yield elongation in a case in which a pipe axis direction tensile test is
performed.
[0157] In an as-rolled electric resistance welded steel pipe, yield elongation is not observed
in a case in which a pipe axis direction tensile test is performed.
[0158] In contrast, in an electric resistance welded steel pipe which is subjected to heat
treatment other than seam heat treatment (for example, tempering) after pipe-making,
yield elongation is observed in a case in which a pipe axis direction tensile test
is performed.
<Yield Strength in Pipe Axis Direction (YS)>
[0159] The electric resistance welded steel pipe of the disclosure has preferably a yield
strength in a pipe axis direction (YS) of from 450 to 540 MPa.
[0160] A YS of 450 MPa or more easily satisfies the strength required as the electric resistance
welded steel pipe for a line pipe. The YS is preferably 460 MPa or more, and more
preferably 480 MPa or more.
[0161] In contrast, a YS of 540 MPa or less is advantageous in view of a bending deformation
property or the suppression of buckling in the case of laying a pipeline formed using
the electric resistance welded steel pipe for a line pipe. The YS is preferably 530
MPa or less, and more preferably 520 MPa or less.
<Tensile Strength in Pipe Axis Direction (TS)>
[0162] The electric resistance welded steel pipe of the disclosure has preferably a tensile
strength in a pipe axis direction (TS) of from 510 to 625 MPa.
[0163] A TS of 510 MPa or more easily satisfies the strength required as the electric resistance
welded steel pipe for a line pipe. The TS is preferably 530 MPa or more, more preferably
540 MPa or more, and still more preferably 545 MPa or more.
[0164] In contrast, a TS of 625 MPa or less is advantageous in view of a bending deformation
property or the suppression of buckling in the case of laying a pipeline formed using
the electric resistance welded steel pipe for a line pipe. The TS is preferably 620
MPa or less, more preferably 600 MPa or less, still more preferably 590 MPa or less,
and still more preferably 575 MPa or less.
[0165] The YS and the TS are measured by the following method.
[0166] A full thickness tensile test specimen is sampled from the base metal 90° position
of the electric resistance welded steel pipe. Specifically, the tensile test specimen
is sampled such that a longitudinal direction of the tensile test specimen is parallel
to the pipe axis direction of the electric resistance welded steel pipe and the shape
of a cross-section of the tensile test specimen (i.e., a cross-section parallel to
a width direction and a wall thickness direction of the tensile test specimen) is
an arcuate shape.
[0167] Fig. 4 is a schematic front view of the tensile test specimen used for a tensile
test. A unit of numerical values in Fig. 4 is mm.
[0168] As shown in Fig. 4, the length of a parallel part of the tensile test specimen is
set to be 50.8 mm, and the width of the parallel part is set to be 38.1 mm.
[0169] In the disclosure, the tensile test (i.e., pipe axis direction tensile test) is conducted
using the tensile test specimen in conformity with standard API, specification 5CT
at ordinary temperature.
[0170] The YS and the TS are determined based on the test result.
<Yield Ratio in Pipe Axis Direction (YR)>
[0171] As described above, the electric resistance welded steel pipe of the disclosure has
a yield ratio in a pipe axis direction (YR = (YS/TS) × 100) of from 80 to 95%.
[0172] From the viewpoint of more effectively suppressing buckling in the case of laying
a pipeline formed using the electric resistance welded steel pipe for a line pipe,
the YR is preferably 93% or less.
[0173] From the viewpoint of more improving the production suitability of the electric resistance
welded steel pipe, the YR is preferably 84% or more.
<Wall Thickness of Electric Resistance Welded Steel Pipe>
[0174] The wall thickness of the electric resistance welded steel pipe of the disclosure
is preferably from 12 to 25 mm.
[0175] A wall thickness of the electric resistance welded steel pipe of the disclosure of
12 mm or more improves the strength of the electric resistance welded steel pipe.
[0176] Generally, as the wall thickness becomes thicker, a brittle fracture becomes easy
to occur (i.e., the toughness is decreased). However, in the electric resistance welded
steel pipe of the disclosure, also in a case in which the wall thickness is 12 mm
or more, excellent low-temperature toughness is exhibited.
[0177] Accordingly, in a case in which the wall thickness of the electric resistance welded
steel pipe of the disclosure is 12 mm or more, both the strength and the low-temperature
toughness are satisfied at a higher level.
[0178] The wall thickness of the electric resistance welded steel pipe of the disclosure
is more preferably 14 mm or more, and still more preferably 16 mm or more.
[0179] In contrast, 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 roll-forming of a hot-rolled steel sheet as a material).
[0180] The wall thickness is preferably less than 25 mm, more preferably 22 mm or less,
and still more preferably 20 mm or less.
<Outer Diameter of Electric Resistance Welded Steel Pipe>
[0181] The outer diameter of the electric resistance welded steel pipe of the disclosure
is preferably from 304.8 to 660.4 mm (i.e., from 12 to 26 inches).
[0182] An outer diameter of 304.8 mm (i.e., 12 inches) or more has excellent transport efficiency
of a fluid (for example, natural gas). The outer diameter is preferably 355.6 mm (i.e.,
14 inches) or more, and more preferably 406.4 mm (i.e., 16 inches) or more.
[0183] In contrast, an outer diameter of 609.6 mm (i.e., 24 inches) or less has excellent
production suitability of the electric resistance welded steel pipe. The outer diameter
is more preferably 508 mm (i.e., 20 inches) or less.
[Hot-rolled Steel Sheet]
[0184] Next, a preferred hot-rolled steel sheet as a material of the electric resistance
welded steel pipe of the disclosure (hereinafter, also referred to as the "hot-rolled
steel sheet of the disclosure") will be described.
[0185] The hot-rolled steel sheet of the disclosure has a chemical composition which is
the above-described chemical composition in the disclosure, and, in a metallographic
microstructure of a wall thickness direction central portion, has a polygonal ferrite
fraction of from 60 to 90%, an average crystal grain diameter of 15 µm or less, and
a coarse crystal grain ratio, which is an areal ratio of crystal grains having a crystal
grain diameter of 20 µm or more, of 20% or less.
[0186] A preferred embodiment of the chemical composition in the hot-rolled steel sheet
of the disclosure is the same as a preferred embodiment of the above-described chemical
composition in the disclosure (i.e., the chemical composition in the base metal portion
of the electric resistance welded steel pipe of the disclosure).
[0187] A preferred embodiment of each of the polygonal ferrite fraction, the average crystal
grain diameter, and the coarse crystal grain ratio in the hot-rolled steel sheet of
the disclosure is the same as a preferred embodiment of each of the polygonal ferrite
fraction, the average crystal grain diameter, and the coarse crystal grain ratio in
the electric resistance welded steel pipe of the disclosure.
[0188] The form of the hot-rolled steel sheet of the disclosure is preferably the form of
a hot coil in which the sheet is coiled into a coil shape.
[0189] A preferred range of the wall thickness (i.e., sheet thickness) of the hot-rolled
steel sheet of the disclosure is the same as a preferred range of the wall thickness
of the electric resistance welded steel pipe of the disclosure.
[0190] Preferably, the hot-rolled steel sheet of the disclosure has a yield strength in
a rolling direction (YS) of from 450 to 500 MPa and a tensile strength in the rolling
direction (TS) of from 510 to 580 MPa.
[0191] The rolling direction in the hot-rolled steel sheet corresponds to a longitudinal
direction in the hot-rolled steel sheet uncoiled from the hot coil.
[0192] Measurement of the YS and the TS of the hot-rolled steel sheet is performed in the
same way as the measurement of the TS and the YS of the electric resistance welded
steel pipe.
[0193] The YS of the hot-rolled steel sheet is preferably from 465 to 495 MPa.
[0194] The TS of the hot-rolled steel sheet is preferably from 531 to 565 MPa.
[0195] The YR of the hot-rolled steel sheet is preferably from 82 to 92%.
[0196] In the case of producing the electric resistance welded steel pipe of the disclosure
using the hot-rolled steel sheet of the disclosure, the YS and the TS (in particular,
YS) increase by roll-forming the hot-rolled steel sheet of the disclosure.
[One Example of Production Method of Hot-rolled Steel Sheet]
[0197] Next, a production method A of the hot-rolled steel sheet, which is an example of
a preferred production method of the hot-rolled steel sheet of the disclosure, will
be described.
[0198] The production method A of the hot-rolled steel sheet includes:
a preparation step of preparing a slab having the chemical composition in the disclosure,
a hot-rolling step of heating the prepared slab to a temperature of from 1060 to 1200°C
and hot-rolling the heated slab, thereby obtaining a hot-rolled steel sheet,
a cooling step of strong-cooling the hot-rolled steel sheet subjected to the hot-rolling
at a cooling rate V1 of 5°C/s or more to a strong-cooling stop temperature T1 of from
580 to 680°C with a time from the end of the hot-rolling (specifically, the end of
finish rolling) to the start of the strong-cooling being set to 20 seconds or less,
and then gradual-cooling the hot-rolled steel sheet at a cooling rate V2 of from 2
to 4°C/s to a gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying
T1 > T2), and
a coiling step of coiling the gradual-cooled hot-rolled steel sheet at a coiling temperature
CT of from 500 to 600°C (satisfying T2 > CT), thereby obtaining a hot-rolled steel
sheet in the form of a hot coil.
[0199] In the production method A, the heating temperature of the slab means a surface temperature
of the slab.
[0200] In the production method A, the temperature of the hot-rolled steel sheet (FT, T1,
T2, CT) means a surface temperature of the hot-rolled steel sheet.
[0201] In the production method A, the cooling rate (VI, V2) means a cooling rate in the
wall thickness direction central portion. The cooling rate (VI, V2) is determined
by thermal conduction calculation.
[0202] The chemical composition of the hot-rolled steel sheet in the form of a hot coil
produced by the production method A can be considered to be the same as the chemical
composition of the slab which is a raw material. The reason is that each step in the
production method A does not affect the chemical composition of a steel.
[0203] According to the production method A, a metallographic microstructure mainly composed
of ferrite and a metallographic microstructure in which crystal grains are refined
can be formed.
[0204] Accordingly, according to the production method A, the hot-rolled steel sheet of
the disclosure can be produced, in which, in the metallographic microstructure of
the wall thickness direction central portion, a ferrite fraction is from 60 to 90%,
an average crystal grain diameter is 15 µm or less, and a coarse crystal grain ratio
is 20% or less.
[0205] The reason why a metallographic microstructure mainly composed of ferrite and a metallographic
microstructure in which crystal grains are refined can be formed by the production
method A can be presumed as follows.
[0206] In the production method A, the heating temperature in the hot-rolling step is made
to be 1200°C or less, so that coarsening of crystal grains (specifically, austenite
grains in a heated stage) is suppressed.
[0207] Furthermore, in the cooling step, the hot-rolled steel sheet formed in the hot-rolling
step is strong-cooled at the cooling rate V1 of 5°C/s or more to the strong-cooling
stop temperature T1 of from 580 to 680°C with a time from the end of the hot-rolling
(specifically, the end of finish rolling) to the start of the strong-cooling being
set to 20 seconds or less, so that numerous nucleation sites are generated in a non-recrystallization
structure of the hot-rolled steel sheet.
[0208] The strong-cooled hot-rolled steel sheet is gradual-cooled under the above condition,
and then coiled under the above condition, so that fine ferrite grains are generated
from each of the numerous nucleation sites generated in the strong-cooling, and a
metallographic microstructure mainly composed of polygonal ferrite is formed.
[0209] For the above reason, it is considered that, according to the production method A,
a metallographic microstructure mainly composed of ferrite and a metallographic microstructure
in which crystal grains (specifically, ferrite grains) are refined can be formed.
[0210] In contrast, in a case in which the metallographic microstructure is mainly composed
of bainite, although laths (elongated microstructure) are generated in crystal grains
directly inherited from prior austenite grains, orientations of these laths are aligned
in each block, and each block substantially becomes one crystal grain. Thus, the size
of the crystal grains in the metallographic microstructure mainly composed of bainite
depends on the size of the prior austenite grains. Thus, in a case in which the metallographic
microstructure is mainly composed of bainite, the crystal grains are easily coarsened.
[0211] Next, the reason why fine ferrite grains are generated by the production method A
will be described in more detail using a continuous cooling transformation diagram
(CCT diagram) of the hot-rolled steel sheet.
[0212] Fig. 5 is the continuous cooling transformation diagram (CCT diagram) of the hot-rolled
steel sheet in the production method A.
[0213] In Fig. 5, F indicates a ferrite region, P indicates a pearlite region, B indicates
a bainite region, Ar
3 indicates an Ar
3 transformation temperature, and Ms indicates a temperature at which martensite begins
to be generated.
[0214] As shown in Fig. 5, the ferrite region exists at a higher temperature position than
the pearlite region and the bainite region.
[0215] In this example, a finish rolling temperature (i.e., finish rolling finishing temperature)
is a temperature equal to or more than the Ar
3 transformation temperature.
[0216] The hot-rolled steel sheet after the finish rolling is cooled from a temperature
equal to or more than the Ar
3 transformation temperature.
[0217] A dashed line C1 in Fig. 5 is a cooling curve in a case in which the hot-rolled steel
sheet is cooled under a conventional cooling condition.
[0218] The conventional cooling condition passes through all of the ferrite region, the
pearlite region and the bainite region. Thus, the ferrite fraction in the metallographic
microstructure is decreased. For example, the metallographic microstructure mainly
composed of bainite is obtained.
[0219] In contrast to the conventional cooling condition, in the cooling step in the production
method A, the hot-rolled steel sheet is cooled along a cooling curve of a dashed line
C2.
[0220] Specifically, in the cooling step in the production method A, the hot-rolled steel
sheet is strong-cooled at the cooling rate V1 of 5°C/s or more to the strong-cooling
stop temperature T1 of from 580 to 680°C with the time from the end of the hot-rolling
(specifically, the end of finish rolling) to the start of the strong-cooling being
set to 20 seconds or less (S31 in Fig. 5). The strong-cooling stop temperature T1
is located in the vicinity of a ferrite nose. In a case in which the steel is rapidly
cooled by the strong-cooling, numerous strains are generated in the steel, and therefore,
numerous nucleation sites are generated in a non-recrystallization structure.
[0221] After the strong-cooling, the hot-rolled steel sheet is gradual-cooled to the gradual-cooling
stop temperature T2 of from 550 to 670°C (satisfying T1 > T2) (S32 in Fig. 5). By
setting the gradual-cooling stop temperature T2 to the above temperature, the temperature
of the steel is maintained in the ferrite region of Fig. 5. As a result, fine ferrite
grains are generated from each of the numerous nucleation sites generated in the strong-cooling.
[0222] Therefore, a metallographic microstructure mainly composed of fine ferrite grains
(specifically, a metallographic microstructure in which the ferrite fraction is high
and crystal grains are refined) is formed.
[0223] F1 defined by the above Formula (1) affects a position of an S curve of each phase
of ferrite, pearlite, and bainite in the CCT diagram.
[0224] As described above, in the chemical composition in the disclosure, F1 is from 0.300
to 0.350.
[0225] As a result, as shown in Fig. 5, the S curve of each phase is arranged at an appropriate
position in the CCT diagram. Thus, the hot-rolled steel sheet is cooled mainly through
the ferrite region as the cooling curve C2 in Fig. 5.
[0226] Therefore, the ferrite fraction in the microstructure is increased, and crystal grains
(i.e., ferrite grains) are refined.
[0227] In a case in which F1 is less than 0.300, the S curve of each phase is shifted too
much to the left side. In this case, in the cooling step, the temperature of the steel
enters the ferrite region before the nucleation sites are sufficiently generated.
Thus, ferrite grains are coarsened, and the average crystal grain diameter becomes
large. Furthermore, the metallographic microstructure is easy to become a mixed-grain
microstructure, and thus, the coarse crystal grain ratio becomes large.
[0228] In contrast, in a case in which F1 is more than 0.350, the S curve of each phase
is shifted too much to the right side. In this case, the cooling curve C2 becomes
difficult to pass through the ferrite region. Therefore, the amount of the microstructure
other than ferrite (pearlite, bainite, and the like) generated is increased, and the
ferrite fraction in the microstructure is decreased.
[0229] Each step of the production method A will be described below.
<Preparation Step>
[0230] The preparation step in the production method A is a step of preparing a slab having
the chemical composition in the disclosure.
[0231] The step of preparing a slab may be a step of producing a slab or a step of simply
preparing a slab produced in advance.
[0232] In the case of producing a slab, for example, molten steel having the chemical composition
described above is produced, and a slab is produced using the produced molten steel.
In this case, the slab may be produced by continuous casting, or the slab may be produced
by producing an ingot using molten steel and breaking down the ingot.
[0233] The chemical composition of the slab can be considered to be the same as the chemical
composition of the molten steel which is a raw material. The reason is that the step
of producing a slab does not affect the chemical composition of a steel.
<Hot-rolling Step>
[0234] The hot-rolling step in the production method A is a step of heating the slab to
a temperature of from 1060 to 1200°C and hot-rolling the heated slab, thereby obtaining
a hot-rolled steel sheet.
[0235] A temperature at which the slab is heated (hereinafter, also referred to as "heating
temperature") of 1200°C or less can refine austenite grains. The heating temperature
is preferably 1180°C or less.
[0236] A heating temperature of 1060°C or more can realize refining of crystal grains during
rolling. A heating temperature of 1060°C or more can realize precipitation strengthening
after rolling, and therefore, the strength of the hot-rolled steel sheet can also
be improved. From the viewpoint of the effect, the heating temperature is preferably
1100°C or more.
[0237] In the production method A, the heating temperature of the slab means a surface temperature
of the slab.
[0238] In the production method A, the temperature of the hot-rolled steel sheet (FT, T1,
T2, CT) means a surface temperature of the hot-rolled steel sheet.
[0239] In the production method A, the cooling rate (V1, V2) means a cooling rate in the
wall thickness direction central portion, which is determined by thermal conduction
calculation.
[0240] The hot-rolling is performed by carrying out rough rolling and finish rolling in
this order for the slab heated to the above heating temperature.
[0241] The rough rolling and the finish rolling are performed using a rough rolling mill
and a finish rolling mill, respectively. Both the rough rolling mill and the finish
rolling mill include multiple rolling stands in a row, and each of the rolling stands
includes a pair of rolls.
[0242] The following finish rolling temperature FT (i.e., finish rolling finishing temperature)
is a surface temperature of the hot-rolled steel sheet at the exit side of a final
stand of the finish rolling mill.
[0243] From the viewpoint of reducing the rolling resistance and improving the productivity,
the finish rolling temperature FT (°C) is preferably the Ar
3 transformation temperature or more. In a case in which the finish rolling temperature
(°C) is the Ar
3 transformation temperature or more, a phenomenon in which rolling is performed in
a two-phase region of ferrite and austenite is suppressed, and the formation of a
banded structure and the decrease in mechanical properties associated with the phenomenon
can be suppressed.
[0244] In the chemical composition in the disclosure, the Ar
3 transformation temperature can be 750 or more.
[0245] In the hot-rolling, the rolling reduction in an austenite non-recrystallization temperature
region is preferably from 60 to 80%. In this case, a non-recrystallization structure
is refined.
<Cooling Step>
[0246] The cooling step in the production method A is a step of strong-cooling the hot-rolled
steel sheet obtained in the hot-rolling step at a cooling rate V1 of 5°C/s or more
to a strong-cooling stop temperature T1 of from 580 to 680°C with a time from the
end of the hot-rolling (specifically, the end of finish rolling) to the start of the
strong-cooling being set to 20 seconds or less, and then gradual-cooling the hot-rolled
steel sheet to a gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying
T1 > T2).
[0247] The cooling step in the production method A is performed on a ROT (Run Out Table).
[0248] Hereinafter, the cooling step in the production method A may be referred to as a
"ROT cooling".
[0249] The surface temperature of the steel sheet before the strong-cooling is not particularly
limited, and is preferably the Ar
3 transformation temperature or more. In a case in which the surface temperature of
the steel sheet just before the strong-cooling is the Ar
3 transformation temperature or more, coarsening of crystal grains and a decrease in
the strength caused thereby can be suppressed.
[0250] The strong-cooling is started within 20 seconds (more preferably within 10 seconds)
from the end of the hot-rolling (specifically, the end of finish rolling).
[0251] The strong-cooling is performed at the cooling rate V1 of 5°C/s or more.
[0252] The cooling rate V1 is a cooling rate at the wall thickness direction central portion.
The cooling rate V1 is a value calculated with thermal conduction.
[0253] A cooling rate V1 of 5°C/s or more makes the degree of supercooling by the cooling
sufficient, and therefore, nucleation sites of ferrite are sufficiently obtained.
[0254] The cooling rate V1 is preferably 7°C/s or more, and more preferably 8°C/s or more.
[0255] The strong-cooling is performed to the strong-cooling stop temperature T1 of from
580 to 680°C.
[0256] A strong-cooling stop temperature T1 of 580°C or more can suppress a phenomenon in
which the temperature of the hot-rolled steel sheet passes through the ferrite region
and reaches the pearlite region and/or the bainite region in the CCT diagram, so that
a ferrite fraction of 60% or more is easily achieved. The strong-cooling stop temperature
T1 is preferably 600°C or more, and more preferably 610°C or more.
[0257] A strong-cooling stop temperature T1 of 680°C or less can suppress a phenomenon in
which Nb precipitation which strengthens pro-eutectoid ferrite is overaged, and therefore,
a decrease in the strength of the hot-rolled steel sheet can be suppressed. The strong-cooling
stop temperature T1 is preferably 670°C or less, and more preferably 655°C or less.
[0258] The strong-cooling is preferably performed by water-cooling.
[0259] The strong-cooling is performed using, for example, a water-cooling apparatus by
making a water flow density in the water-cooling apparatus higher than a usual condition.
[0260] The strong-cooling stop temperature T1 is, in other words, a gradual-cooling start
temperature.
[0261] In the cooling step, the strong-cooled hot-rolled steel sheet is gradual-cooled to
the gradual-cooling stop temperature T2 of from 550 to 670°C (satisfying T1 > T2).
[0262] The gradual-cooling is preferably performed at a cooling rate V2 of from 2 to 4°C/s.
[0263] In a case in which the cooling rate V2 is 2°C/s or more, since the gradual-cooling
stop temperature T2 and a coiling temperature CT can be made lower, coarsening of
crystal grains can be suppressed.
[0264] In a case in which the cooling rate V2 is 4°C/s or less, since a phenomenon in which
the temperature of the hot-rolled steel sheet passes through the ferrite region and
reaches the pearlite region and/or the bainite region in the CCT diagram can be suppressed,
a ferrite fraction of 60% or more is easily achieved.
[0265] The gradual-cooling is performed to the gradual-cooling stop temperature T2 of from
550 to 670°C (satisfying T1 > T2).
[0266] In a case in which the gradual-cooling stop temperature T2 is 550°C or more, since
a phenomenon in which the temperature of the hot-rolled steel sheet passes through
the ferrite region and reaches the pearlite region and/or the bainite region in the
CCT diagram can be suppressed, a ferrite fraction of 60% or more is easily achieved.
The gradual-cooling stop temperature T2 is preferably 580°C or more, and more preferably
590°C or more.
[0267] In a case in which the gradual-cooling stop temperature T2 is 670°C or less, coarsening
of crystal grains can be suppressed. The gradual-cooling stop temperature T2 is preferably
650°C or less, more preferably 635°C or less, and still more preferably 620°C or less.
[0268] The gradual-cooling is preferably performed by water-cooling.
[0269] The gradual-cooling is performed using, for example, a water-cooling apparatus by
making a water flow density in the water-cooling apparatus lower than the water flow
density in the strong-cooling.
<Coiling Step>
[0270] The coiling step in the production method A is a step of coiling the hot-rolled steel
sheet cooled in the cooling step at a coiling temperature CT of from 500 to 600°C
(satisfying T2 > CT), thereby obtaining a hot-rolled steel sheet in the form of a
hot coil.
[0271] A cooling rate in cooling from the gradual-cooling stop temperature T2 to the coiling
temperature CT is preferably from 0.1 to 1.5°C/s, more preferably from 0.3 to 1.5°C/s,
and still more preferably from 0.5 to 1.5°C/s.
[0272] The coiling temperature CT is from 500 to 600°C.
[0273] In a case in which the coiling temperature CT is 500°C or more, since a phenomenon
in which the temperature of the hot-rolled steel sheet passes through the ferrite
region and reaches the pearlite region and/or the bainite region in the CCT diagram
can be suppressed, a ferrite fraction of 60% or more is easily achieved. As a result,
an average crystal grain diameter of 15 µm or less and a coarse crystal grain ratio
of 20% or less are easily achieved. The coiling temperature CT is preferably 510°C
or more, and more preferably 520°C or more.
[0274] In a case in which the coiling temperature CT is 580°C or less, coarsening of ferrite
grains can be suppressed. As a result, an average crystal grain diameter of 15 µm
or less and a coarse crystal grain ratio of 20% or less are easily achieved. The coiling
temperature CT is preferably 590°C or less, and more preferably 580°C or less.
[One Example of Production Method of Electric Resistance Welded Steel Pipe]
[0275] Next, a production method X of the electric resistance welded steel pipe, which is
an example of a preferred production method of the electric resistance welded steel
pipe of the disclosure, will be described.
[0276] The production method X of the electric resistance welded steel pipe includes:
a step of preparing the above-described hot-rolled steel sheet of the disclosure (hereinafter,
also referred to as "hot-rolled steel sheet preparation step"), and
a step of making an open pipe by roll-forming of the hot-rolled steel sheet and forming
an electric resistance welded portion by electric resistance welding of abutting portions
of the obtained open pipe, thereby obtaining an electric resistance welded steel pipe
(hereinafter, also referred to as "pipe-making step").
[0277] The pipe-making step in the production method X does not affect the chemical composition,
the polygonal ferrite fraction, the average crystal grain diameter, and the coarse
crystal grain ratio. Accordingly, the electric resistance welded steel pipe of the
disclosure is produced by the production method X using the hot-rolled steel sheet
of the disclosure.
[0278] The hot-rolled steel sheet preparation step is preferably a step of preparing the
hot-rolled steel sheet of the disclosure in the form of a hot coil.
[0279] In this case, in the pipe-making step, the hot-rolled steel sheet of the disclosure
is uncoiled from the hot coil, and the uncoiled hot-rolled steel sheet of the disclosure
is roll-formed.
[0280] The hot-rolled steel sheet preparation step may be a step of producing the hot-rolled
steel sheet of the disclosure (preferably, the hot-rolled steel sheet of the disclosure
in the form of a hot coil) or a step of simply preparing the hot-rolled steel sheet
of the disclosure (preferably, the hot-rolled steel sheet of the disclosure in the
form of a hot coil) produced in advance.
[0281] In both cases, the hot-rolled steel sheet of the disclosure in the form of a hot
coil is preferably produced in accordance with the production method A described above.
[0282] Each operation in the pipe-making step is not particularly limited, and can be performed
in accordance with a known method.
[0283] The production method X of the electric resistance welded steel pipe may include
other steps, if necessary.
[0284] Examples of the other steps include a step of subjecting the electric resistance
welded portion of the electric resistance welded steel pipe to seam heat treatment
after the pipe-making step, and a step of adjusting the shape of the electric resistance
welded steel pipe by a sizing roll after the pipe-making step.
EXAMPLES
[0285] Examples of the disclosure will be described below. However, the disclosure is not
limited to the following Examples.
[Examples 1 to 13 and Comparative Examples 1 to 8]
<Production of Slab and Hot Coil>
[0286] Slabs were produced by continuous casting of molten steel having chemical compositions
of Steel A to Steel O set forth in Table 1.
[0287] REM in Steel J is specifically Ce.
[0288] Each of the slabs described above was heated in a heating furnace.
[0289] The heating temperature (°C) of the slab was set forth in Table 2. The slab after
the heating was rolled using a rough rolling mill, and was cooled to 920°C.
[0290] Then, finish rolling was performed by a finish rolling mill. The rolling reduction
in a non-recrystallization temperature region was from 60 to 80% in all Examples and
Comparative Examples. The finish rolling temperature was the Ar
3 or more (specifically, 750°C or more) in all Examples and Comparative Examples.
[0291] For the steel sheet after the finish rolling, the ROT cooling (i.e., cooling step)
was performed.
[0292] For the hot-rolled steel sheet obtained by the finish rolling, the ROT cooling was
performed by sequentially carrying out the strong-cooling and the gradual-cooling.
[0293] Time from the end of the finish rolling to the start of the strong-cooling was 10
seconds or less.
[0294] Both the strong-cooling and the gradual-cooling were performed using a water-cooling
apparatus. Both the cooling rate V1 in the strong-cooling and the cooling rate V2
in the gradual-cooling were adjusted by adjusting a water flow density in the water-cooling
apparatus.
[0295] The cooling rate V1 (°C/s) in the strong-cooling, the strong-cooling stop temperature
T1 (°C), and the gradual-cooling stop temperature T2 (°C) were set forth in Table
2.
[0296] The cooling rate V2 (°C/s) in the gradual-cooling was in the range of from 2 to 4°C/s
in all examples.
[0297] The hot-rolled steel sheet after the ROT cooling was cooled, and coiled at the coiling
temperature CT set forth in Table 2, thereby obtaining a hot coil (i.e., the hot-rolled
steel sheet in the form of a hot coil).
[0298] The cooling rate in cooling from the gradual-cooling stop temperature T2 (°C) to
the coiling temperature CT was estimated to be from 0.5 to 1.5°C/s in all Examples
and Comparative Examples.
<Production of Electric Resistance Welded Steel Pipe>
[0299] The hot-rolled steel sheet was uncoiled from the hot coil described above, the uncoiled
hot-rolled steel sheet was roll-formed to thereby make an open pipe, and abutting
portions of the obtained open pipe was subjected to electric resistance welding to
form an electric resistance welded portion, thereby obtaining an electric resistance
welded steel pipe (hereinafter, also referred to as "electric resistance welded steel
pipe before shape adjustment").
[0300] Then, the electric resistance welded portion of the electric resistance welded steel
pipe before shape adjustment was subjected to seam heat treatment, and the shape was
then adjusted by a sizing roll, thereby obtaining an electric resistance welded steel
pipe (i.e., as-rolled electric resistance welded steel pipe) having an outer diameter
of 406.4 mm and a wall thickness of 17 mm.
[0301] 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 molten steel which is a raw material.
<YS, TS, and YR of Hot-rolled Steel Sheet>
[0302] By uncoiling the hot-rolled steel sheet from the hot coil described above and performing
a tensile test in a rolling direction for the uncoiled hot-rolled steel sheet, the
YS in the rolling direction and the TS in the rolling direction were respectively
measured. Furthermore, the YR (%) in the rolling direction was calculated based on
the YS in the rolling direction and the TS in the rolling direction.
[0303] The results are set forth in Table 2.
[0304] A full thickness tensile test specimen used in the measurement of the YS and the
TS was sampled from a position where a distance from one end of the hot-rolled steel
sheet in a sheet width direction is 1/4 of the sheet width (i.e., a position corresponding
to the base metal 90° position in the electric resistance welded steel pipe).
<Measurement and Evaluation of Electric Resistance Welded Steel Pipe>
[0305] The following measurement and evaluation were performed for the electric resistance
welded steel pipe after the shape adjustment by a sizing roll.
[0306] The results are set forth in Table 2.
(YS, TS, and YR)
[0307] For the electric resistance welded steel pipe after the shape adjustment by a sizing
roll, by performing a tensile test in a pipe axis direction, the YS in the pipe axis
direction and the TS in the pipe axis direction were measured. The detailed measurement
method has been described above. Furthermore, the YR (%) in the rolling direction
was calculated based on the YS in the pipe axis direction and the TS in the pipe axis
direction.
[0308] In the tensile test in the pipe axis direction in the measurement of the YS and the
TS, yield elongation was not observed in all Examples and Comparative Examples. In
other words, the electric resistance welded steel pipes of all Examples and Comparative
Examples were confirmed to be as-rolled electric resistance welded steel pipes.
(Ferrite Fraction, Average Crystal Grain Diameter, and Coarse Crystal Grain Ratio)
[0309] For the electric resistance welded steel pipe after the shape adjustment by a sizing
roll, the ferrite fraction, the average crystal grain diameter, and the coarse crystal
grain ratio in the metallographic microstructure of the wall thickness direction central
portion of the base metal portion were respectively measured using EBSD-OIM by the
method described above.
[0310] As analysis software in EBSD-OIM, "TSL OIM Analysis 7" manufactured by TSL Solutions
Ltd. was used.
[0311] In the measurement of the ferrite fraction, the kind of the balance (i.e., microstructure
other than polygonal ferrite) in the metallographic microstructure of the wall thickness
direction central portion of the base metal portion was confirmed.
[0312] In Table 2, the expression "B, P" means at least one of bainite or pearlite.
(Average Crystal Grain Diameter, Coarse Crystal Grain Ratio)
[0313] The average crystal grain diameter and the coarse crystal grain ratio of the wall
thickness direction central portion of the base metal portion in the electric resistance
welded steel pipe after the shape adjustment by a sizing roll were measured by the
method described above.
(Evaluation of Low-temperature Toughness (Measurement of DWTT Guarantee Temperature))
[0314] By sampling an arcuate member from the electric resistance welded steel pipe after
the shape adjustment by a sizing roll and processing the sampled arcuate member into
a flat plate shape, a full thickness DWTT test specimen was obtained.
[0315] Fig. 6 is a schematic front view of the obtained DWTT test specimen.
[0316] A unit of numerical values in Fig. 6 is mm.
[0317] The longitudinal direction of the DWTT test specimen (a direction of a length of
300 mm) corresponds to a pipe circumferential direction of the electric resistance
welded steel pipe. The central portion of the DWTT test specimen in the longitudinal
direction corresponds to the base metal 90° position of the electric resistance welded
steel pipe.
[0318] As shown in Fig. 6, in the DWTT test specimen, a notch having a depth of 5 mm was
formed at the central portion in the longitudinal direction.
[0319] The DWTT test was performed using the DWTT test specimen in conformity with specification
ASTM E 436, and a DWTT guarantee temperature which is the lowest value of a temperature
at which a percent ductile fracture is 85% or more was determined.
[0320] The lower a DWTT guarantee temperature is, the more excellent the low-temperature
toughness is.
[0321] As set forth in Table 1 and Table 2, the electric resistance welded steel pipe of
each Example, which satisfies the chemical composition of the base metal portion in
the disclosure (including F1 being from 0.300 to 0.350), and, in the metallographic
microstructure of the wall thickness direction central portion of the base metal portion,
has a F fraction of from 60 to 90%, an average crystal grain diameter of 15 µm or
less, and a coarse crystal grain ratio of 20% or less, had a low DWTT guarantee temperature
and excellent low-temperature toughness.
[0322] The electric resistance welded steel pipe of each Example had a YR in the range of
from 80 to 95%, and was confirmed to secure a plastic deformation allowance required
as a steel pipe for a line pipe.
[0323] In contrast to each Example, in Comparative Examples 1 and 7 having F1 of less than
0.300, the average crystal grain diameter was too large, and the DWTT guarantee temperature
was too high (i.e., the low-temperature toughness was poor). The reason why the average
crystal grain diameter was too large is considered that ferrite grains were coarsened
because F1 was less than 0.300.
[0324] In Comparative Examples 2 and 8 having F1 of more than 0.350, the F fraction was
too low, and the DWTT guarantee temperature was too high (i.e., the low-temperature
toughness was poor).
[0325] In Comparative Examples 2 and 8, the average crystal grain diameter and the coarse
crystal grain ratio were too large. The reason thereof is considered that the F fraction
became too low because F1 was more than 0.350.
[0326] In Comparative Example 3, the average crystal grain diameter and the coarse crystal
grain ratio were too large, and the DWTT guarantee temperature was too high (i.e.,
the low-temperature toughness was poor).
[0327] The reason why the average crystal grain diameter and the coarse crystal grain ratio
were too large in Comparative Example 3 is considered that austenite grains were coarsened
in heating the slab because the heating temperature of the slab was too high.
[0328] In Comparative Example 4, the average crystal grain diameter and the coarse crystal
grain ratio were too large, and the DWTT guarantee temperature was too high (i.e.,
the low-temperature toughness was poor).
[0329] The reason why the average crystal grain diameter and the coarse crystal grain ratio
were too large in Comparative Example 4 is considered that the effect of refining
of crystal grains by rolling was insufficient because the heating temperature of the
slab was too low.
[0330] In Comparative Example 5, the F fraction was too low, and the DWTT guarantee temperature
was too high (i.e., the low-temperature toughness was poor).
[0331] The reason why the F fraction was too low in Comparative Example 5 is considered
that the strong-cooling stop temperature T1, the gradual-cooling stop temperature
T2, and the coiling temperature CT were too low.
[0332] In Comparative Example 6, the average crystal grain diameter and the coarse crystal
grain ratio were too large, and the DWTT guarantee temperature was too high (i.e.,
the low-temperature toughness was poor).
[0333] The reason why the average crystal grain diameter and the coarse crystal grain ratio
were too large in Comparative Example 6 is considered that the strong-cooling stop
temperature T1 and the coiling temperature CT became too high because the cooling
rate V1 in the strong-cooling was too low, and therefore, coarse ferrite grains were
generated.