Technical Field
[0001] The present disclosure relates to an as-rolled electric resistance welded steel pipe
for a line pipe.
Background Art
[0002] Crude oil or natural gas produced in recent years includes wet hydrogen sulfide (H
2S). An environment including hydrogen sulfide is referred to as a sour environment.
[0003] A pipeline for transporting drilled crude oil or natural gas is exposed to such a
sour environment. Thus, for a steel pipe for a line pipe, which is used in the production
of a pipeline, resistance to a sour environment (sour resistance) is required.
[0004] For example, Japanese Patent Application Laid-Open (
JP-A) No. 2013-11005 (Patent Document 1) discloses, as a thick-walled high-strength hot-rolled steel sheet
for a line pipe, which has excellent sour resistance, a thick-walled high-strength
hot-rolled steel sheet for a line pipe, which has a composition including, in terms
of % by mass, from 0.01 to 0.07% of C, 0.40% or less of Si, from 0.5 to 1.4% of Mn,
0.015% or less of P, 0.003% or less of S, 0.1% or less of Al, from 0.01 to 0.15% of
Nb, 0.1% or less of V, 0.03% or less of Ti, and 0.008% or less of N, such that Nb,
V, and Ti satisfy Nb + V + Ti < 0.15, and further Cm satisfies 0.12 or less, and the
balance of the composition consists of Fe and inevitable impurities, and has a structure
including a bainite phase or a bainitic ferrite phase with an areal ratio of 95% or
more, and wherein, in the thick-walled high-strength hot-rolled steel sheet, a maximum
hardness in a sheet thickness direction is 220 HV or less, and a yield strength is
450 MPa or more. Here, Cm = C + Si/30 + (Mn + Cu)/30 + Ni/60 + Mo/7 + V/10.
SUMMARY OF INVENTION
Technical Problem
[0006] The concept of "sour resistance" includes resistance to hydrogen induced cracking
(hereinafter also referred to as "HIC") generated mainly in the central portion of
the wall thickness of the steel pipe (hereinafter also referred to as "HIC resistance")
and resistance to sulfide stress cracking (hereinafter also referred to as "SSC")
generated mainly from the inner peripheral surface of the steel pipe as the initiating
point (hereinafter also referred to as "SSC resistance").
[0007] Regarding this point, in Patent Document 1, only the HIC resistance is evaluated,
and the SSC resistance is not evaluated as the sour resistance. Thus, the high-strength
hot-rolled steel sheet for a welded steel pipe for a line pipe of Patent Document
1 may have low SSC resistance.
[0008] From the viewpoint of the improvement in transport efficiency and the improvement
in operation efficiency, for an electric resistance welded steel pipe for a line pipe,
a certain amount of high strength (for example, a yield strength in a pipe axis direction
of 415 MPa or more, and a tensile strength in the pipe axis direction of 461 MPa or
more) is required.
[0009] In contrast, from the viewpoint of a bending deformation property and the suppression
of buckling in the case of laying a pipeline formed using the electric resistance
welded steel pipe for a line pipe, for the electric resistance welded steel pipe for
a line pipe, not-too-high strength (for example, the yield strength in the pipe axis
direction of 550 MPa or less, and the tensile strength in the pipe axis direction
of 625 MPa or less) is also required.
[0010] Therefore, an object of the disclosure is to provide an as-rolled electric resistance
welded steel pipe for a line pipe, which has a yield strength in a pipe axis direction
of from 415 to 550 MPa, which has a tensile strength in the pipe axis direction of
from 461 to 625 MPa, and which has excellent SSC resistance.
Solution to Problem
[0011] 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 0.01 to 0.10% of C,
from 0.01 to 0.40% of Si,
from 0.50 to 2.00% of Mn,
from 0 to 0.030% of P,
from 0 to 0.0015% of S,
from 0.010 to 0.050% of Al,
from 0.0030 to 0.0080% of N,
from 0.010 to 0.050% of Nb,
from 0.005 to 0.020% of Ti,
from 0 to 0.20% of Ni,
from 0 to 0.20% of Mo,
from 0 to 0.0050% of Ca,
from 0 to 1.00% of Cr,
from 0 to 0.100% of V,
from 0 to 1.00% of Cu,
from 0 to 0.0050% of Mg,
from 0 to 0.0100% of REM, and
the balance being Fe and impurities, wherein:
in a metallographic microstructure of the base metal portion, an areal ratio of polygonal
ferrite is from 80% to 98%, the balance is composed of at least one of bainite or
pearlite,
a yield strength in a pipe axis direction is from 415 to 550 MPa, a tensile strength
in the pipe axis direction is from 461 to 625 MPa, and
a maximum Vickers hardness of an inner surface layer of the base metal portion is
248 HV or less and is smaller than a maximum Vickers hardness of an outer surface
layer of the base metal portion by 5 HV or more.
- <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 0.20% of Ni,
more than 0% but equal to or less than 0.20% of Mo,
more than 0% but equal to or less than 0.0050% of Ca,
more than 0% but equal to or less than 1.00% of Cr,
more than 0% but equal to or less than 0.10% of V,
more than 0% but equal to or less than 1.00% 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 the chemical composition of the base metal portion contains,
in terms of % by mass, one or more selected from the group consisting of:
from 0.001 to 0.20% of Ni, and
from 0.1 to 0.20% of Mo.
- <4> The as-rolled electric resistance welded steel pipe for a line pipe according
to any one of <1> to <3>, wherein the chemical composition of the base metal portion
contains, in terms of % by mass, from 0.0005 to 0.0050% of Ca.
- <5> The as-rolled electric resistance welded steel pipe for a line pipe according
to any one of <1> to <4>, wherein a wall thickness is from 10 to 25 mm, and an outer
diameter is from 114.3 mm to 660.4 mm.
Advantageous Effects of Invention
[0012] According to the disclosure, an as-rolled electric resistance welded steel pipe for
a line pipe, which has a yield strength in a pipe axis direction of from 415 to 550
MPa, which has a tensile strength in the pipe axis direction of from 461 to 625 MPa,
and which has excellent SSC resistance, is provided.
BRIEF DESCRIPTION OF DRAWINGS
[0013]
Fig. 1 is a scanning electron micrograph (a magnification of 500 times) showing an
example of a metallographic microstructure of a base metal portion in the disclosure.
Fig. 2 is a scanning electron micrograph (a magnification of 2,000 times) obtained
by enlarging a part of Fig. 1.
Fig. 3 is a schematic front view of a tensile test specimen used for a tensile test
in the disclosure.
Fig. 4 is a schematic perspective view showing an example of a pipe-making step for
producing an electric resistance welded steel pipe of the disclosure.
DESCRIPTION OF EMBODIMENTS
[0014] 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.
[0015] The content of a component (element) expressed by "%" herein means "% by mass".
[0016] 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.
[0017] 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.
[0018] 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".
[0019] 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.
[0020] 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.
[0021] Herein, the "roll-forming" refers to forming of a hot-rolled steel sheet into an
open pipe shape by bending work.
[0022] 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 0.01 to 0.10% of C, from 0.01
to 0.40% of Si, from 0.50 to 2.00% of Mn, from 0 to 0.030% of P, from 0 to 0.0015%
of S, from 0.010 to 0.050% of Al, from 0.0030 to 0.0080% of N, from 0.010 to 0.050%
of Nb, from 0.005 to 0.020% of Ti, from 0 to 0.20% of Ni, from 0 to 0.20% of Mo, from
0 to 0.0050% of Ca, from 0 to 1.00% of Cr, from 0 to 0.100% of V, from 0 to 1.00%
of Cu, from 0 to 0.0050% of Mg, from 0 to 0.0100% of REM, and the balance being Fe
and impurities, wherein: in a metallographic microstructure of the base metal portion,
an areal ratio of polygonal ferrite is from 80% to 98%, the balance is composed of
at least one of bainite or pearlite, a yield strength in a pipe axis direction (hereinafter
also referred to as "YS") is from 415 to 550 MPa, a tensile strength in the pipe axis
direction (hereinafter also referred to as "TS") is from 461 to 625 MPa, and a maximum
Vickers hardness of an inner surface layer of the base metal portion is 248 HV or
less and is smaller than a maximum Vickers hardness of an outer surface layer of the
base metal portion by 5 HV or more.
[0023] 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.
[0024] 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).
[0025] In the electric resistance welded steel pipe of the disclosure, the maximum Vickers
hardness of the inner surface layer of the base metal portion means a value measured
as follows.
[0026] First, as measurement points of the Vickers hardness, in a C cross-section of the
electric resistance welded steel pipe (i.e., a cross-section perpendicular to the
pipe axis direction), nine points at 1 mm pitch, which are arranged on the circumference
at a depth of 0.1 mm from an inner peripheral surface of the electric resistance welded
steel pipe and centered at a base metal 180° position (i.e., a position shifted from
the electric resistance welded portion by 180° in a pipe circumferential direction),
are selected. A specimen including the above-described selected nine measurement points
is sampled from the electric resistance welded steel pipe. In each of the nine measurement
points in the specimen, the Vickers hardness is measured in conformity with JIS Z2244
(2009) under the condition of a test force of 100 gf (= 0.98 N) with the pipe axis
direction as a test direction. The maximum value among the obtained nine measurement
results is regarded as the maximum Vickers hardness of the inner surface layer of
the base metal portion.
[0027] In other words, the maximum Vickers hardness of the inner surface layer of the base
metal portion is, approximately speaking, a maximum Vickers hardness in the vicinity
of the inner peripheral surface of the base metal portion.
[0028] In the electric resistance welded steel pipe of the disclosure, the maximum Vickers
hardness of the outer surface layer of the base metal portion means a value measured
in the same way as the maximum Vickers hardness of the inner surface of the base metal
portion described above except that the "inner peripheral surface" is read as the
"outer peripheral surface".
[0029] In other words, the maximum Vickers hardness of the outer surface layer of the base
metal portion is, approximately speaking, a maximum Vickers hardness in the vicinity
of the outer peripheral surface of the base metal portion.
[0030] The electric resistance welded steel pipe of the disclosure has a certain amount
of strength (i.e., YS and TS in the ranges described above) and has excellent SSC
resistance.
[0031] In contrast to the electric resistance welded steel pipe of the disclosure, in a
conventional electric resistance welded steel pipe for a line pipe (for example, the
electric resistance welded steel pipe for a line pipe described in the above-described
Patent Document 1), the HIC resistance as the sour resistance was considered, but
the SSC resistance as the sour resistance was not considered.
[0032] However, locations of generation of cracking are different in HIC (hydrogen induced
cracking) and SSC (sulfide stress cracking). Specifically, HIC is generated mainly
in the central portion of the wall thickness of the electric resistance welded steel
pipe, whereas SSC is generated mainly from the inner peripheral surface of the electric
resistance welded steel pipe as the initiating point. More specifically, in a state
where a fluid containing wet hydrogen sulfide (specifically, crude oil or natural
gas; hereinafter also referred to as "sour fluid") is in contact with the inner peripheral
surface of the electric resistance welded steel pipe for a line pipe, SSC is generated
from the inner peripheral surface as the initiating point.
[0033] Therefore, even if an electric resistance welded steel pipe has excellent HIC resistance,
the electric resistance welded steel pipe may have poor SSC resistance.
[0034] In the electric resistance welded steel pipe of the disclosure, the maximum Vickers
hardness of the inner surface layer of the base metal portion is 248 HV or less, and
the maximum Vickers hardness of the inner surface layer of the base metal portion
is smaller than the maximum Vickers hardness of the outer surface layer of the base
metal portion by 5 HV or more. As a result, under a state where a sour fluid is in
contact with the inner peripheral surface of the electric resistance welded steel
pipe, SSC which is cracking generated from the inner peripheral surface as the initiating
point is suppressed (i.e., the SSC resistance is improved).
[0035] Moreover, SSC tends to be easily generated as the strength of the electric resistance
welded steel pipe becomes higher.
[0036] Regarding this point, in the electric resistance welded steel pipe of the disclosure,
the YS is limited to 550 MPa or less, and the TS is limited to 625 MPa or less, respectively.
As a result, the SSC resistance is improved.
[0037] In contrast, in the electric resistance welded steel pipe of the disclosure, the
maximum Vickers hardness of the inner surface layer of the base metal portion is smaller
than the maximum Vickers hardness of the outer surface layer of the base metal portion
by 5 HV or more, so that the maximum Vickers hardness of the outer surface layer of
the base metal portion is relatively secured to a certain degree.
[0038] As a result, a certain amount of high strength (specifically, YS of 415 MPa or more,
and TS of 461 MPa or more) is secured as the entire electric resistance welded steel
pipe.
[0039] In contrast to the electric resistance welded steel pipe of the disclosure, in the
conventional electric resistance welded steel pipe, the maximum Vickers hardness of
the inner surface layer of the base metal portion was almost the same as the maximum
Vickers hardness of the outer surface layer of the base metal portion, and the condition
that "the maximum Vickers hardness of the inner surface layer of the base metal portion
is smaller than the maximum Vickers hardness of the outer surface layer of the base
metal portion by 5 HV or more" was not satisfied owing to the following circumstances.
[0040] The electric resistance welded steel pipe is produced by using a hot coil consisting
of a hot-rolled steel sheet as a raw material and subjecting the hot-rolled steel
sheet, uncoiled from the hot coil, to pipe-making (i.e., roll-forming and electric
resistance welding). One of two surfaces of the hot-rolled steel sheet uncoiled from
the hot coil (hereinafter also referred to as "first surface") becomes an outer surface
of the electric resistance welded steel pipe, and the other of the two surfaces (hereinafter
also referred to as "second surface") becomes an inner surface of the electric resistance
welded steel pipe. A production process of the hot coil includes respective stages
of hot-rolling, cooling, and coiling in this order. From the viewpoint of suppressing
warpage of the hot-rolled steel sheet after cooling or from the viewpoint of the productivity,
this cooling was conventionally performed by water-cooling the two surfaces of the
hot-rolled steel sheet obtained by hot-rolling at cooling rates which are almost the
same. Under such a circumstance, in the conventional electric resistance welded steel
pipe, the maximum Vickers hardness of the inner surface layer of the base metal portion
was almost the same as the maximum Vickers hardness of the outer surface layer of
the base metal portion (i.e., the condition that "the maximum Vickers hardness of
the inner surface layer of the base metal portion is smaller than the maximum Vickers
hardness of the outer surface layer of the base metal portion by 5 HV or more" was
not satisfied).
[0041] For the above-described conventional electric resistance welded steel pipe, the present
inventors succeeded in making the maximum Vickers hardness of the inner surface layer
of the base metal portion smaller than the maximum Vickers hardness of the outer surface
layer of the base metal portion by 5 HV or more by providing a difference between
the cooling rates for the two surfaces when the two surfaces of the hot-rolled steel
sheet obtained by hot-rolling are cooled (specifically, by making the cooling rate
of the second surface corresponding to the inner peripheral surface slower than the
cooling rate of the first surface corresponding to the outer peripheral surface).
Furthermore, the present inventors found that, virtually, the warpage of the hot-rolled
steel sheet after cooling is not matter too much because the cooled hot-rolled steel
sheet is subsequently coiled.
[0042] The electric resistance welded steel pipe of the disclosure was completed based on
the above-described knowledge of the present inventors.
[0043] Not only the maximum Vickers hardness of the inner surface layer of the base metal
portion but also the chemical composition of the base metal portion, the metallographic
microstructure of the base metal portion, and being the as-rolled electric resistance
welded steel pipe contribute to the SSC resistance.
[0044] Moreover, the chemical composition of the base metal portion, the metallographic
microstructure of the base metal portion, and being the as-rolled electric resistance
welded steel pipe also contribute to the achievement of the YS in the range described
above and the TS in the range described above.
[0045] The chemical composition of the base metal portion and the metallographic microstructure
of the base metal portion will be described below.
[Chemical Composition of Base Metal Portion]
[0046] The chemical composition of the base metal portion will be described below.
C: from 0.01 to 0.10%
[0047] 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 0.01% or more. The C content
is preferably 0.03% or more, and more preferably 0.04% or more.
[0048] In contrast, in a case in which the C content is too high, a carbide is generated,
and the toughness and ductility of steel are decreased. Accordingly, the C content
is 0.10% or less. The C content is preferably 0.09%, and still more preferably 0.08%
or less.
Si: from 0.01 to 0.40%
[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 0.01% or more. The Si content is preferably
0.02% or more, and still more preferably 0.10% or more.
[0050] In contrast, in a case in which the Si content is too high, the toughness of steel
is decreased. Accordingly, the Si content is 0.40% or less. The Si content is preferably
0.38% or less, and more preferably 0.35% 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 0.60% or more, and more
preferably 0.80% or more.
[0052] In contrast, in a case in which the Mn content is too high, the toughness and SSC
resistance of steel are 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 segregates in a grain boundary and embrittles the grain boundary.
Thus, P decreases the toughness and SSC resistance of steel. Accordingly, a P content
is preferably small. Specifically, the P content is 0.030% or less. The P content
is preferably 0.021% or less, more preferably 0.015% or less, and still more preferably
0.010% 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%, and may be 0.001% or more.
S: from 0 to 0.0015%
[0055] S is an impurity. S binds to Mn to form a Mn-based sulfide. The Mn-based sulfide
is diffluent. Thus, the toughness and SSC resistance of steel are decreased. Accordingly,
a S content is preferably as low as possible. Specifically, the S content is 0.0015%
or less. The S content is preferably 0.0010% or less, and more preferably 0.0008%
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, and may be 0.0003%
or more.
Al: from 0.010 to 0.050%
[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
0.012% or more, and more preferably 0.013% or more.
[0058] In contrast, in a case in which the Al content is too high, an Al nitride is coarsened,
and the 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 0.0030 to 0.0080%
[0060] N 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 0.0030% or more.
[0061] In contrast, in a case in which the N content is too high, a carbonitride is coarsened,
and the SSC resistance 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 0.0040% or less.
Nb: from 0.010 to 0.050%
[0062] Nb binds to C and N in the steel to form a fine Nb carbonitride. 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 0.020% or more, and more preferably 0.030%
or more.
[0063] In contrast, in a case in which the Nb content is too high, the Nb carbonitride is
coarsened, and the SSC resistance of steel is decreased. Furthermore, in a case in
which the Nb content is too high, the toughness of the electric resistance welded
portion is decreased. Accordingly, the Nb content is 0.050% or less. The Nb content
is preferably 0.045% or less, and more preferably 0.040% or less.
Ti: from 0.005 to 0.020%
[0064] Ti binds to N in the steel to form a Ti nitride and/or Ti carbonitride. The Ti nitride
and/or Ti carbonitride refines crystal grains of the steel. 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, a coarse Ti nitride and/or
Ti carbonitride is formed. Thus, the SSC resistance of steel is decreased. Accordingly,
the Ti content is 0.020% or less. The Ti content is preferably 0.018% or less, and
more preferably 0.016% or less.
Ni: from 0 to 0.20%
[0066] Ni is an optional element and may not be contained. In other words, a Ni content
may be 0%.
[0067] In a case in which Ni is contained, Ni enhances the strength of steel by solid-solution
strengthening. Ni further enhances the toughness of steel. From the viewpoint of the
effect, the Ni content is preferably more than 0%, more preferably 0.001% or more,
more preferably 0.005% or more, still more preferably 0.01% or more, and still more
preferably 0.05% or more.
[0068] In contrast, in a case in which the Ni content is too high, the weldability of steel
is decreased. Accordingly, the Ni content is 0.20% or less. The Ni content is preferably
0.18% or less, and still more preferably 0.15% or less.
Mo: from 0 to 0.20%
[0069] Mo is an optional element and may not be contained. In other words, a Mo content
may be 0%.
[0070] In a case in which Mo is contained, Mo enhances the hardenability of steel and enhances
the strength of steel. Furthermore, since micro segregation of Mo is difficult to
be generated, generation of HIC caused by center segregation is suppressed. From the
viewpoint of the effect, the Mo content is preferably more than 0%, more preferably
0.10% or more, and still more preferably 0.12% or more.
[0071] In contrast, since Mo is expensive, in a case in which Mo is excessively included,
the production cost increases. Accordingly, the Mo content is 0.20% or less. The Mo
content is preferably 0.18% or less, and more preferably 0.15% or less.
Ca: from 0% to 0.0050%
[0072] Ca is an optional element and may not be contained. In other words, a Ca content
may be 0%.
[0073] In a case in which Ca is contained, Ca makes the form of MnS that becomes a initiating
point of generation of SSC into a spherical shape and suppresses the generation of
SSC. Ca further forms CaS and suppresses generation of MnS. From the viewpoint of
the effect, the Ca content is preferably more than 0%, more preferably 0.0005% or
more, still more preferably 0.0010% or more, and still more preferably 0.0020% or
more.
[0074] In contrast, in a case in which the Ca content is too high, the effect is saturated,
and the production cost increases. Accordingly, the Ca content is 0.0050% or less.
The Ca content is preferably 0.0045% or less.
Cr: from 0 to 1.00%
[0075] Cr is an optional element and may not be contained. In other words, a Cr content
may be 0%.
[0076] In a case in which Cr is contained, Cr contributes to improvement in hardenability.
From the viewpoint of such an effect, the Cr content is preferably more than 0%, and
more preferably 0.01% or more.
[0077] In contrast, in a case in which the Cr content is too high, the toughness of the
electric resistance welded portion may be deteriorated by Cr-based inclusions generated
in the electric resistance welded portion. Accordingly, the Cr content is 1.00% or
less. The Cr content is preferably 0.50% or less, more preferably 0.30% or less, and
still more preferably 0.20% or less.
V: from 0 to 0.100%
[0078] V is an optional element and may not be contained. In other words, a V content may
be 0%.
[0079] In a case in which V is contained, V contributes to improvement in toughness. From
the viewpoint of such an effect, the V content is preferably more than 0%, more preferably
0.001% or more, and still more preferably 0.005% or more.
[0080] In contrast, in a case in which the V content is too high, the toughness may be deteriorated
by a V carbonitride. Accordingly, the V content is 0.100% or less. The V content is
preferably 0.070% or less, more preferably 0.050% or less, and still more preferably
0.030% or less.
Cu: from 0 to 1.00%
[0081] Cu is an optional element and may not be contained. In other words, a Cu content
may be 0%.
[0082] In a case in which Cu is contained, Cu contributes to improvement in the strength
of the base metal portion. From the viewpoint of such an effect, the Cu content is
preferably more than 0%, more preferably 0.01% or more, and still more preferably
0.05% or more.
[0083] In contrast, in a case in which the Cu content is too high, fine Cu particles are
generated, and the toughness may be significantly deteriorated. Accordingly, the Cu
content is 1.00% or less. The Cu content is preferably 0.70% or less, more preferably
0.50% or less, and still more preferably 0.30% 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 0.20% of Ni,
more than 0% but equal to or less than 0.20% of Mo, more than 0% but equal to or less
than 0.0050% of Ca, 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 1.00%
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 O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.
[0097] Among the elements described above, O is preferably controlled to have a content
of 0.006% or less.
[0098] 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.
[Metallographic Microstructure of Base Metal Portion]
[0099] In the electric resistance welded steel pipe of the disclosure, in the metallographic
microstructure of the base metal portion, an areal ratio of polygonal ferrite (hereinafter
also referred to as "ferrite fraction") is from 80 to 98%, and the balance is composed
of at least one of bainite or pearlite.
[0100] A YS of 550 MPa or less and a TS of 625 MPa or less can be achieved by allowing a
ferrite fraction to be 80% or more. The ferrite fraction is preferably 81% or more,
and more preferably 82% or more.
[0101] In contrast, a YS of 415 MPa or more and a TS of 461 MPa or more can be achieved
by allowing a ferrite fraction to be 98% or less. The ferrite fraction is preferably
97% or less, and more preferably 95% or less.
[0102] The balance in the metallographic microstructure of the base metal portion is composed
of at least one of bainite or pearlite. As a result, the SSC resistance is improved
compared to a case in which the balance contains, for example, martensite.
[0103] The concept of "bainite" herein includes bainitic ferrite, upper bainite, and lower
bainite.
[0104] The concept of "pearlite" herein includes pseudo-pearlite.
[0105] The above-described metallographic microstructure of the base metal portion relates
to the electric resistance welded steel pipe of the disclosure being an as-rolled
electric resistance welded steel pipe (i.e., not subjected to heat treatment other
than seam heat treatment after pipe-making).
[0106] 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 has poor SSC resistance.
[0107] The measurement of the ferrite fraction and the identification of the balance in
the metallographic microstructure of the base metal portion are performed as follows.
[0108] A metallographic microstructure of the central portion of the wall thickness in an
L cross-section at a base metal 180° position is nital-etched, and micrographs of
the nital-etched metallographic microstructure (hereinafter also referred to as "metallographic
micrographs") are observed with a scanning electron microscope (SEM) at a magnification
of 500 times. Metallographic micrographs corresponding to ten 500-times visual fields
(corresponding to actual cross-sectional area of 0.48 mm
2) are taken. The measurement of the ferrite fraction and the identification of the
balance 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.
[0109] Fig. 1 is a scanning electron micrograph (SEM micrograph; a magnification of 500
times) showing an example of a metallographic microstructure of a base metal portion
in the disclosure, and Fig. 2 is a SEM micrograph (a magnification of 2,000 times)
obtained by enlarging a region of Fig. 1.
[0110] The SEM micrograph (500 times) in Fig. 1 is one (one visual field) of SEM micrographs
used in the measurement of the ferrite fraction and the identification of the balance
in Test Number 22 described later.
[0111] As shown in Fig. 1 and Fig. 2, the metallographic microstructure according to this
example is a metallographic microstructure which is mainly composed of polygonal ferrite
and in which the balance is pearlite.
[0112] More specifically, because cementite is not precipitated in a grain boundary of polygonal
ferrite and lamellar cementite in pearlite in the balance is not divided, the metallographic
microstructure is revealed to be a metallographic microstructure which is not subjected
to heat treatment after pipe-making (i.e., a metallographic microstructure of an as-rolled
electric resistance welded steel pipe).
[0113] Being an as-rolled electric resistance welded steel pipe can also be confirmed by
not observing yield elongation in a case in which a pipe axis direction tensile test
is performed.
[0114] 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.
[0115] In contrast, in an electric resistance welded steel pipe which is subjected to heat
treatment after pipe-making, yield elongation is observed in a case in which a pipe
axis direction tensile test is performed.
[Maximum Vickers Hardness of Inner Surface Layer of Base Metal Portion, Maximum Vickers
Hardness of Outer Surface Layer of Base Metal Portion]
[0116] In the electric resistance welded steel pipe of the disclosure, a maximum Vickers
hardness of an inner surface layer of the base metal portion is 248 HV or less, and
the Vickers hardness of the inner surface layer of the base metal portion is smaller
than a maximum Vickers hardness of an outer surface layer of the base metal portion
by 5 HV or more.
[0117] Each of the maximum Vickers hardness of the inner surface layer of the base metal
portion and the maximum Vickers hardness of the outer surface layer of the base metal
portion has been described above.
[0118] A difference obtained by subtracting the Vickers hardness of the inner surface layer
of the base metal portion from the maximum Vickers hardness of the outer surface layer
of the base metal portion (i.e., the maximum Vickers hardness of the outer surface
layer of the base metal portion - the Vickers hardness of the inner surface layer
of the base metal portion) is hereinafter also referred to as an "outer-inner hardness
difference".
[0119] For example, "the Vickers hardness of the inner surface layer of the base metal portion
is smaller than the maximum Vickers hardness of the outer surface layer of the base
metal portion by 5 HV or more" is hereinafter also referred to as "the outer-inner
hardness difference is 5 HV or more".
[0120] In a case in which the maximum Vickers hardness of the inner surface layer of the
base metal portion exceeds 248 HV, the toughness of steel is decreased, and the SSC
resistance of the electric resistance welded steel pipe is decreased. Accordingly,
the maximum Vickers hardness of the inner surface layer is 248 HV or less. The maximum
Vickers hardness of the inner surface layer is preferably 245 HV or less, and more
preferably 220 HV or less.
[0121] The lower limit of the maximum Vickers hardness of the inner surface layer is not
particularly limited. From the viewpoint of more improving the strength of the electric
resistance welded steel pipe (i.e., YS and TS), the maximum Vickers hardness of the
inner surface layer is preferably 175 HV or more, more preferably 180 HV or more,
and still more preferably 185 HV or more.
[0122] In a case in which the outer-inner hardness difference is less than 5 HV, depending
on the value of the maximum Vickers hardness of the inner surface layer of the base
metal portion, at least one of the deterioration of the SSC resistance, the deficiency
of the YS, or the deficiency of the TS occurs. Accordingly, the outer-inner hardness
difference is 5 HV or more, and preferably 6 HV or more.
[0123] The upper limit of the outer-inner hardness difference is not particularly restricted.
From the viewpoint of the production suitability of the electric resistance welded
steel pipe, the outer-inner hardness difference is preferably 20 HV or less, more
preferably 15 HV or less, and still more preferably 10 HV or less.
[0124] The maximum Vickers hardness of the outer surface layer of the base metal portion
may satisfy the maximum Vickers hardness of the inner surface layer of the base metal
portion and the outer-inner hardness difference described above, and others are not
particularly restricted.
[0125] The maximum Vickers hardness of the outer surface layer of the base metal portion
is preferably from 180 MPa to 250 MPa, and more preferably from 210 MPa to 230 MPa.
[0126] As described above, in the electric resistance welded steel pipe of the disclosure,
the Vickers hardness of the inner surface layer of the base metal portion is smaller
than the maximum Vickers hardness of the outer surface layer of the base metal portion
by 5 HV or more.
[0127] In the electric resistance welded steel pipe of the disclosure, the maximum Vickers
hardness of the inner surface layer may be lower than the maximum Vickers hardness
of the outer surface layer by 5 HV or more in not only the base metal portion but
also the electric resistance welded portion.
[0128] For example, in the case of producing the electric resistance welded steel pipe by
production method A described later, the maximum Vickers hardness of the inner surface
layer may be lower than the maximum Vickers hardness of the outer surface layer by
5 HV or more also in the electric resistance welded portion.
[Yield Strength in Pipe Axis Direction (YS)]
[0129] The electric resistance welded steel pipe of the disclosure has a yield strength
in a pipe axis direction (YS) of from 415 to 550 MPa.
[0130] A YS of 415 MPa or more secures the strength as the electric resistance welded steel
pipe for a line pipe. The YS is preferably 430 MPa or more.
[0131] In contrast, a YS of 550 MPa or less (i.e., not-too-high YS) is advantageous in view
of the improvement in the SSC resistance or a bending deformation property and 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.
[Tensile Strength in Pipe Axis Direction (TS)]
[0132] The electric resistance welded steel pipe of the disclosure has a tensile strength
in a pipe axis direction (TS) of from 461 to 625 MPa.
[0133] A TS of 461 MPa or more secures the strength as the electric resistance welded steel
pipe for a line pipe. The TS is preferably 500 MPa or more, and more preferably 510
MPa or more.
[0134] In contrast, a TS of 625 MPa or less (i.e., not-too-high TS) is advantageous in view
of the improvement in the SSC resistance or a bending deformation property and 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.
[0135] The YS and the TS are measured by the following method.
[0136] 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 thickness direction of the tensile test specimen) is an arcuate
shape.
[0137] Fig. 3 is a schematic front view of a tensile test specimen used for a tensile test.
[0138] A unit of numerical values in Fig. 3 is mm.
[0139] As shown in Fig. 3, 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.
[0140] The tensile test is conducted using the tensile test specimen in conformity with
standard API, specification 5CT at ordinary temperature. The YS and the TS are determined
based on the test result.
[Yield Ratio in Pipe Axis Direction (YR)]
[0141] The electric resistance welded steel pipe of the disclosure has preferably a yield
ratio in a pipe axis direction (YR = (YS/TS) × 100) of 95% or less.
[0142] A YR of 95% or less is advantageous in view of the suppression of buckling in the
case of laying a pipeline formed using the electric resistance welded steel pipe for
a line pipe.
[Wall Thickness of Electric Resistance Welded Steel Pipe]
[0143] The wall thickness of the electric resistance welded steel pipe of the disclosure
is preferably from 10 to 25 mm.
[0144] The wall thickness is more preferably 12 mm or more.
[0145] 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 Diameter of Electric Resistance Welded Steel Pipe]
[0146] The outer diameter of the electric resistance welded steel pipe of the disclosure
is preferably from 114.3 to 660.4 mm (i.e., 4.5 to 26 inches).
[0147] The outer diameter is preferably 152.4 mm (i.e., 6 inches) or more, and more preferably
254 mm (i.e., 10 inches) or more.
[0148] The outer diameter is preferably 609.6 mm (i.e., 24 inches) or less, and more preferably
508 mm (i.e., 20 inches) or less.
[One Example of Production Method]
[0149] One example of a method of producing the electric resistance welded steel pipe of
the disclosure is the following production method A.
[0150] The production method A includes:
a preparation step of preparing a slab having the chemical composition described above,
a hot-rolling step of heating the prepared slab and hot-rolling the heated slab, thereby
obtaining a hot-rolled steel sheet,
a cooling step of cooling a first surface of the hot-rolled steel sheet at a cooling
rate V1 and cooling a second surface which is the opposite side of the first surface
of the hot-rolled steel sheet at a cooling rate V2 which is slower than the cooling
rate V1,
a coiling step of coiling the cooled hot-rolled steel sheet, 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 in a direction such that the first surface is
an outer peripheral surface and the second surface is an inner surface 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 electric resistance welded steel pipe.
[0151] According to the production method A, since the hot-rolled steel sheet in which the
hardness of the second surface is lower than the hardness of the first surface is
easily produced, the electric resistance welded steel pipe in which the hardness of
the inner peripheral surface is lower than the hardness of the outer peripheral surface
is easily produced, and therefore, the electric resistance welded steel pipe of the
disclosure having an outer-inner hardness difference of 5 HV or more is easily produced.
(Preparation Step)
[0152] In the production method A, the step of preparing a slab is a step of preparing a
slab having the chemical composition described above.
[0153] 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.
[0154] 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.
(Hot-rolling Step)
[0155] In the production method A, the hot-rolling step is a step of heating the prepared
slab described above and hot-rolling the heated slab, thereby obtaining a hot-rolled
steel sheet.
[0156] The heating temperature in heating the slab is preferably from 1,100 to 1,250°C.
[0157] In a case in which the heating temperature is 1,100°C or more, refining of crystal
grains during hot-rolling and precipitation strengthening after hot-rolling easily
proceed, and therefore, the strength of steel is easily improved.
[0158] In a case in which the heating temperature is 1,250°C or less, since coarsening of
austenite grains can be more suppressed, crystal grains are easily refined, and therefore,
the strength of steel is easily improved.
[0159] The heating of the slab is performed by, for example, a heating furnace.
[0160] In the hot-rolling step, a hot-rolled steel sheet is obtained by hot-rolling the
heated slab described above.
[0161] The hot-rolling is preferably performed under the condition that a finish rolling
finishing temperature (hereinafter also referred to as "finish rolling temperature")
is from 780 to 830°C.
[0162] The hot-rolling is generally performed using a rough rolling mill and a finish rolling
mill. Both the rough rolling mill and the finish rolling mill generally include multiple
rolling stands in a row, and each of the rolling stands includes a pair of rolls.
In this case, the finish rolling temperature (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.
[0163] In a case in which the finish rolling temperature is 780°C or more, since the rolling
resistance of the steel sheet can be reduced, the productivity is improved.
[0164] Moreover, in a case in which the finish rolling temperature is 780°C 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.
[0165] In contrast, in a case in which the finish rolling temperature is 830°C or less,
since a phenomenon in which the steel becomes too hard is suppressed, a phenomenon
in which the YS and/or TS of the electric resistance welded steel pipe to be obtained
becomes too high is suppressed.
[0166] In the hot-rolling, the rolling reduction in an austenite non-recrystallization temperature
region is preferably from 70 to 80%. In this case, a non-recrystallization structure
is refined.
(Cooling Step)
[0167] The cooling step is a step of cooling a first surface of the hot-rolled steel sheet
at a cooling rate V1 and cooling a second surface which is the opposite side of the
first surface of the hot-rolled steel sheet at a cooling rate V2 which is slower than
the cooling rate V1.
[0168] In the cooling step, the first surface may be an upper surface (a surface on the
opposite side with respect to the gravity direction, the same shall apply hereinafter)
and the second surface may be a lower surface (a surface oriented in the gravity direction,
the same shall apply hereinafter), or the first surface may be the lower surface and
the second surface may be the upper surface.
[0169] Both the cooling of the first surface and the cooling of the second surface preferably
include water-cooling.
[0170] In this case, the hot-rolled steel sheet may be water-cooled immediately after the
hot-rolling, or the hot-rolled steel sheet immediately after the hot-rolling may be
first air-cooled and then water-cooled.
[0171] The cooling rate V1 and the cooling rate V2 preferably satisfy the following Formula
(1). As a result, the hot-rolled steel sheet in which the hardness of the second surface
is lower than the hardness of the first surface is more easily produced, and therefore,
the electric resistance welded steel pipe of the disclosure having an outer-inner
hardness difference of 5 HV or more is more easily produced.
[0172] (In Formula (1), V1 represents the cooling rate V1 (°C/s), and V2 represents the
cooling rate V2 (°C/s).)
[0173] The cooling rate V1 is preferably from 5 to 25°C/s.
[0174] The cooling rate V2 is not particularly limited. From the viewpoint of more increasing
the strength of the electric resistance welded steel pipe (YS and TS), the cooling
rate V2 is preferably 0.5°C/s or more, and more preferably 0.8°C/s or more.
[0175] The cooling rate V1 and the cooling rate V2 can be adjusted by, for example, adjusting
a water flow density in a water-cooling apparatus for performing water-cooling. For
example, on the presupposition that the water flow density on the second surface side
is made smaller than the water flow density on the first surface side (i.e., V2 <
V1), in order to satisfy the above Formula (1), the water flow density on the second
surface side and the water flow density on the first surface side are respectively
independently adjusted.
(Coiling Step)
[0176] The coiling step is a 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.
[0177] The surface temperature of the hot-rolled steel sheet at the start of coiling (hereinafter
also referred to as "coiling temperature") is preferably 620°C or less, and more preferably
600°C or less.
[0178] In a case in which the coiling temperature is 620°C or less, since coarsening of
crystal grains can be more suppressed, the strength of steel can be more improved.
[0179] The lower limit of the coiling temperature is not particularly limited.
[0180] From the viewpoint of the productivity, the coiling temperature is preferably 500°C
or more, and more preferably 530°C or more.
(Pipe-making Step)
[0181] The pipe-making step is a step of uncoiling the hot-rolled steel sheet from the hot
coil, roll-forming the uncoiled hot-rolled steel sheet in a direction such that the
first surface is an outer peripheral surface and the second surface is an inner surface
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 electric resistance welded steel pipe.
[0182] The pipe-making step can be performed in accordance with a known method except the
roll-forming in the direction such that the first surface is an outer peripheral surface
and the second surface is an inner surface.
[0183] Fig. 4 is a schematic perspective view showing an example of a pipe-making step.
[0184] As shown in Fig. 4, a hot-rolled steel sheet uncoiled from a hot coil is roll-formed
using a forming roll (not shown in the drawing) in a direction such that a first surface
is an outer peripheral surface 1 and a second surface is an inner peripheral surface
2, thereby making an open pipe. Abutting portions 3 of the open pipe are subjected
to electric resistance welding using a power feed terminal 60 and a welding roll 70,
thereby obtaining an electric resistance welded steel pipe 200.
[0185] The production method A may include other steps, if necessary.
[0186] 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
[0187] Examples of the disclosure will be described below. However, the disclosure is not
limited to the following Examples.
[Test Numbers 1 to 26]
[0188] An electric resistance welded steel pipe of each Test Number was produced in accordance
with the production method A described above.
[0189] The details will be described below.
<Production of Slab and Hot Coil>
[0190] Slabs were produced by continuous casting of molten steel having chemical compositions
of Steel A to Steel O set forth in Table 1. REM in Steel L is specifically Ce.
[0191] Each of the slabs described above was heated in a heating furnace, the heated slab
was hot-rolled using multiple hot rolling mills to obtain a hot-rolled steel sheet,
the obtained hot-rolled steel sheet was air-cooled and then water-cooled, and the
water-cooled hot-rolled steel sheet was coiled, whereby a hot coil consisting of the
hot-rolled steel sheet was obtained.
[0192] The heating temperature in heating the slab, the finish rolling temperature in the
hot-rolling, the cooling rates in water-cooling the hot-rolled steel sheet (V1 and
V2), and the coiling temperature in coiling the water-cooled hot-rolled steel sheet
are respectively set forth in Table 2.
[0193] In the water-cooling of the hot-rolled steel sheet, the upper surface of the hot-rolled
steel sheet was set as a first surface, the cooling rate of the first surface was
set as V1, the lower surface of the hot-rolled steel sheet was set as a second surface,
and the cooling rate of the second surface was set as V2.
[0194] The water-cooling of the hot-rolled steel sheet was performed by spraying the upper
surface (i.e., first surface) and the lower surface (i.e., second surface) of the
hot-rolled steel sheet, respectively, with a water-cooling shower. In this case, the
water flow density of the water-cooling shower for the upper surface and the water
flow density of the water-cooling shower for the lower surface were respectively adjusted,
so that V1 and V2 were adjusted to be values set forth in Table 2.
[0195] A conventional standard condition of water-cooling is a condition of Test Number
12 (Comparative Example).
<Production of Electric Resistance Welded Steel Pipe>
[0196] The hot-rolled steel sheet was uncoiled from the hot coil described above, the uncoiled
hot-rolled steel sheet was roll-formed in a direction such that the first surface
is an outer peripheral surface and the second surface is an inner peripheral surface
of a pipe 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"). 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 15.9 mm.
[0197] Only in Test Number 16 (Comparative Example), the electric resistance welded steel
pipe after the seam heat treatment (i.e., as-rolled electric resistance welded steel
pipe) was further subjected to heat treatment at a heating temperature of 760°C for
30 minutes, and then water-cooled.
[0198] 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.
<Measurement and Evaluation>
[0199] The following measurement and evaluation were performed for the electric resistance
welded steel pipe after the shape adjustment by a sizing roll in each Test Number.
[0200] The results are set forth in Table 2.
(Measurement of Ferrite Fraction and Confirmation of Kind of Balance)
[0201] By the method described above, the ferrite fraction (hereinafter also referred to
as "F fraction") was measured, and the kind of the balance was confirmed.
[0202] In Table 2, "B" means bainite, "P" means pearlite, and "M" means martensite.
(Maximum Vickers Hardness)
[0203] The maximum Vickers hardness of the inner surface layer of the base metal portion
(HV) and the maximum Vickers hardness of the outer surface layer of the base metal
portion (HV) were respectively measured based on the measurement method described
above.
[0204] The outer-inner hardness difference was calculated based on the measurement result
by the following Formula.
[0205] Outer-inner Hardness Difference (HV) = Maximum Vickers Hardness of Outer Surface
Layer of Base Metal Portion (HV) - Maximum Vickers Hardness of Inner Surface Layer
of Base Metal Portion (HV)
(YS, TS)
[0206] The YS (MPa) and the TS (MPa) in the pipe axis direction of the electric resistance
welded steel pipe were respectively measured based on the measurement method described
above.
[0207] In the tensile test in the pipe axis direction in the measurement of the YS and the
TS, yield elongation was observed in Test Number 16 (Comparative Example), but yield
elongation was not observed in all the other Test Numbers.
(Evaluation of SSC Resistance)
[0208] A full thickness specimen having a size of 120 mm (pipe circumferential direction)
× 25 mm (pipe axis direction) was sampled from the base metal 180° position of the
electric resistance welded steel pipe.
[0209] In a state where a load corresponding to 90% of the YS is applied to the sampled
specimen in accordance with EFC (European Federation of Corrosion Publications) No.
16 Method B (four-point bend test), the specimen was immersed for 720 hours in the
following test bath. As the test bath, a liquid obtained by saturating hydrogen sulfide
gas in an aqueous solution including 5% by mass of sodium chloride and 0.4% by mass
of sodium acetate was used. The temperature of the test bath during the immersion
was ordinary temperature (23°C).
[0210] Whether the specimen was fractured or not was confirmed after a lapse of 720 hours
since the start of the immersion. As a result of the confirmation, a case in which
a fracture was not observed in the specimen was determined to be "A" (i.e., the SSC
resistance of the steel is high), and a case in which a fracture was observed in the
specimen was determined to be "B" (i.e., the SSC resistance of the steel is low).
[Table 1]
Steel |
Chemical Composition (Unit is % by Mass, Balance is Fe and Impurities) |
C |
Si |
Mn |
P |
S |
Al |
N |
Nb |
Ti |
Ni |
Mo |
Ca |
Cr |
V |
Cu |
Mg |
REM |
A |
0.06 |
0.19 |
1.14 |
0.009 |
0.0008 |
0.027 |
0.0035 |
0.034 |
0.014 |
- |
- |
- |
- |
- |
- |
- |
- |
B |
0.10 |
0.38 |
1.00 |
0.010 |
0.0005 |
0.030 |
0.0030 |
0.050 |
0.015 |
- |
- |
- |
- |
- |
- |
- |
- |
C |
0.04 |
0.30 |
1.20 |
0.009 |
0.0003 |
0.035 |
0.0040 |
0.025 |
0.010 |
- |
- |
- |
- |
- |
- |
- |
- |
D |
0.03 |
0.20 |
1.25 |
0.010 |
0.0008 |
0.030 |
0.0035 |
0.025 |
0.015 |
0.09 |
- |
- |
- |
- |
- |
- |
- |
E |
0.08 |
0.25 |
1.30 |
0.010 |
0.0006 |
0.035 |
0.0032 |
0.040 |
0.013 |
- |
0.18 |
- |
- |
- |
- |
- |
- |
F |
0.07 |
0.18 |
1.22 |
0.010 |
0.0004 |
0.032 |
0.0034 |
0.028 |
0.015 |
- |
- |
0.0038 |
- |
- |
- |
- |
- |
G |
0.06 |
0.19 |
1.25 |
0.010 |
0.0005 |
0.035 |
0.0035 |
0.030 |
0.015 |
0.08 |
- |
0.0035 |
- |
- |
- |
- |
- |
H |
0.04 |
0.14 |
1.25 |
0.010 |
0.0004 |
0.027 |
0.0038 |
0.020 |
0.012 |
- |
- |
- |
0.04 |
- |
- |
- |
- |
I |
0.05 |
0.17 |
1.24 |
0.015 |
0.0007 |
0.022 |
0.0034 |
0.034 |
0.017 |
- |
- |
- |
- |
0.020 |
- |
- |
- |
J |
0.09 |
0.18 |
1.30 |
0.020 |
0.0008 |
0.027 |
0.0032 |
0.030 |
0.014 |
- |
- |
- |
- |
- |
0.20 |
- |
- |
K |
0.07 |
0.19 |
1.10 |
0.011 |
0.0005 |
0.024 |
0.0034 |
0.032 |
0.015 |
- |
- |
- |
- |
- |
- |
0.0020 |
- |
L |
0.06 |
0.21 |
1.14 |
0.021 |
0.0003 |
0.027 |
0.0035 |
0.021 |
0.016 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
M |
0.05 |
0.19 |
1.15 |
0.010 |
0.0008 |
0.027 |
0.0035 |
0.036 |
0.014 |
0.10 |
0.18 |
0.0026 |
- |
- |
- |
- |
- |
N |
0.06 |
0.18 |
1.21 |
0.010 |
0.0007 |
0.025 |
0.0035 |
0.030 |
0.014 |
- |
- |
0.0032 |
- |
- |
- |
- |
- |
O |
0.07 |
0.14 |
1.31 |
0.010 |
0.0004 |
0.027 |
0.0038 |
0.028 |
0.014 |
- |
- |
0.0042 |
- |
- |
- |
- |
- |
[Table 2]
Test Number |
Steel |
Heating Temperature (°C) |
Finish Rolling Temperature (°C) |
Cooling Rate (°C/s) |
Coiling Temperature (°C) |
Heat Treatment after Pipe-making |
F Fraction (%) |
Kind of Balance |
Maximum Vickers Hardness (HV) |
Outer-inner Hardness Difference (HV) |
YS (MPa) |
TS (MPa) |
SSC Resistance |
Remarks |
Upper Surface (VI) |
Lower Surface (V2) |
Outer Surface Layer |
Inner Surface Layer |
1 |
A |
1200 |
790 |
10 |
5 |
550 |
Absence |
81 |
P, B |
224 |
210 |
14 |
525 |
580 |
A |
Example |
2 |
B |
1230 |
780 |
23 |
1 |
540 |
Absence |
82 |
P, B |
210 |
200 |
10 |
550 |
585 |
A |
Example |
3 |
C |
1150 |
820 |
24 |
15 |
560 |
Absence |
85 |
P, B |
230 |
220 |
10 |
530 |
570 |
A |
Example |
4 |
D |
1120 |
790 |
20 |
10 |
580 |
Absence |
90 |
P, B |
225 |
215 |
10 |
520 |
580 |
A |
Example |
5 |
E |
1170 |
795 |
8 |
3 |
570 |
Absence |
82 |
P, B |
212 |
206 |
6 |
540 |
585 |
A |
Example |
6 |
F |
1100 |
790 |
17 |
10 |
530 |
Absence |
88 |
P, B |
230 |
220 |
10 |
550 |
585 |
A |
Example |
7 |
G |
1120 |
780 |
21 |
12 |
585 |
Absence |
84 |
P, B |
223 |
213 |
10 |
545 |
605 |
A |
Example |
8 |
A |
1270 |
790 |
10 |
5 |
550 |
Absence |
83 |
P, B |
224 |
210 |
14 |
530 |
585 |
A |
Example |
9 |
A |
1200 |
840 |
18 |
10 |
540 |
Absence |
95 |
P, B |
254 |
240 |
14 |
560 |
630 |
A |
Comparative Example |
10 |
A |
1180 |
780 |
30 |
20 |
560 |
Absence |
95 |
P, B |
250 |
230 |
20 |
562 |
632 |
A |
Comparative Example |
11 |
A |
1170 |
790 |
2 |
1 |
550 |
Absence |
82 |
P, B |
180 |
175 |
5 |
530 |
590 |
A |
Example |
12 |
A |
1180 |
790 |
25 |
24 |
540 |
Absence |
76 |
P, B |
253 |
250 |
3 |
560 |
630 |
B |
Comparative Example |
13 |
A |
1150 |
785 |
15 |
8 |
620 |
Absence |
84 |
P, B |
226 |
210 |
16 |
530 |
580 |
A |
Example |
14 |
A |
1080 |
792 |
10 |
5 |
550 |
Absence |
87 |
P, B |
210 |
205 |
5 |
545 |
585 |
A |
Example |
15 |
A |
1100 |
775 |
17 |
10 |
530 |
Absence |
90 |
P. B |
253 |
240 |
13 |
552 |
628 |
A |
Comparative Example |
16 |
A |
1120 |
790 |
21 |
10 |
550 |
Presence (760°C) |
60 |
B, M |
276 |
256 |
20 |
560 |
630 |
B |
Comparative Example |
17 |
H |
1130 |
790 |
11 |
6.2 |
560 |
Absence |
87 |
P, B |
217 |
210 |
7 |
545 |
590 |
A |
Example |
18 |
I |
1140 |
780 |
13 |
8.1 |
570 |
Absence |
89 |
P, B |
230 |
220 |
10 |
545 |
580 |
A |
Example |
19 |
J |
1150 |
820 |
20 |
10.1 |
550 |
Absence |
91 |
P, B |
220 |
214 |
6 |
538 |
575 |
A |
Example |
20 |
K |
1160 |
810 |
21 |
12.1 |
560 |
Absence |
92 |
P, B |
222 |
214 |
8 |
540 |
580 |
A |
Example |
21 |
L |
1140 |
800 |
14 |
8.1 |
580 |
Absence |
93 |
P, B |
214 |
206 |
8 |
545 |
585 |
A |
Example |
22 |
M |
1130 |
810 |
15 |
8.6 |
590 |
Absence |
95 |
P, B |
217 |
207 |
10 |
535 |
580 |
A |
Example |
23 |
N |
1120 |
800 |
17 |
6.3 |
540 |
Absence |
94 |
P, B |
218 |
210 |
8 |
539 |
578 |
A |
Example |
24 |
O |
1140 |
810 |
19 |
5.4 |
560 |
Absence |
97 |
P, B |
217 |
208 |
9 |
540 |
580 |
A |
Example |
25 |
A |
1150 |
800 |
27 |
0.3 |
560 |
Absence |
93 |
P, B |
267 |
170 |
97 |
400 |
430 |
A |
Comparative Example |
26 |
B |
1150 |
810 |
12 |
11 |
550 |
Absence |
92 |
P, B |
233 |
230 |
3 |
406 |
438 |
A |
Comparative Example |
[0211] As set forth in Table 1 and Table 2, the electric resistance welded steel pipe of
each Example, which satisfies the chemical composition and the metallographic microstructure
of the base metal portion in the disclosure, satisfies the YS (i.e., from 415 to 550
MPa) and the TS (i.e., from 461 to 625 MPa) in the disclosure, has the maximum Vickers
hardness of the inner surface layer of the base metal portion of 248 HV or less, and
has the outer-inner hardness difference of 5 HV or more, had excellent SSC resistance.
[0212] In contrast, in Test Number 12 (Comparative Example), the SSC resistance was deteriorated.
The reason thereof is considered that the maximum Vickers hardness of the inner surface
layer exceeded the upper limit, both the TS and the YS exceeded the upper limit, and
the outer-inner hardness difference was less than 5 HV.
[0213] Moreover, also in Test Number 16 (Comparative Example), the SSC resistance was deteriorated.
The reason thereof is considered that martensite was contained in the metallographic
microstructure of the base metal portion because tempering was performed after pipe-making.
[0214] Test Numbers 9, 10, and 15 are all Comparative Examples in which the TS and the YS
exceeded the upper limit, and Test Numbers 25 and 26 are Comparative Examples in which
the TS and the YS were lower than the lower limit.
[0215] In Test Number 26 (Comparative Example), since the outer-inner hardness difference
was less than 5 HV although the maximum Vickers hardness of the inner surface layer
of the base metal portion was 248 HV or less, the SSC resistance was excellent, but
the TS and the YS were lower than the lower limit.
[0216] The entire disclosure of Japanese Patent Application No.
2016-068749 is incorporated herein by reference.
[0217] 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.