DESCRIPTION
STEEL PIPE
Teclt~~icFaile ld
[OOOI] The present invention relates to a steel pipe.
Background Art
[0002] Repeated application of internal lxessure to steel pipes may com~nonlyre sult in
generation of fatigue fractures (hereinafter also referred to as "interual pressure fatigue
fractures") in the steel pipes. In order to inhibit such internal pressure fatigue fractures,
improvement in the internal pressure fatigue resistance properties of the steel pipes may be
demanded.
Hitilerto, in order to improve the internal pressure fatigue resistance properties of
steel pipes, various tecl~nologiesh ave been examined.
For example, as a method of producing a steel pipe for a cylinder tube having
excellent internal pressure fatigue resistance properties, a method of producing a steel pipe for
a cylinder tube is known in which, in the process of producing a steel pipe for a cylinder tube
by drawing a steel pipe, heat treatment is performed at from 300°C to 350°C after the drawing
(see, for example, Japanese Patent Application Laid-Open (JP-A) No. H4-183820).
In order to improve the fatigue resistance propel-ties of the steel pipes, technologies
to reduce residual stress of steel pipes are also known. For example, as a method of
producing a steel pipe for a cylinder having reduced residual stress and excellent fatigue
strength, a method of producing a steel pipe for a cylinder is known in w\lliich both ends of an
original steel pipe are chucked, and then the original steel pipe is moved in one direction
while rotating the original steel pipe, and the11 a roll is pushed on a worked portion of the
original steel pipe while heating the worked portion to work the origiual steel pipe so that an
outer diameter of the original steel pipe become constant (see, for example, Japanese Patent
Application Laid-Open (JP-A) No. 2003-103329).
SUMMARY OF INVENTION
Technical Problem
[0003] Nowevet; there have been demands to further improve the internal pressure fatigue
resistance properties of steel pipes.
In particulal; in a steel pipe, a fla\\r is often imparted from the outside of the steel
1
pipe rather than from the inside of the steel pipe. Therefore, in order to more effcctively
improve the internal pressure fatigue resistance prope~tiesi,t is important to inhibit internal
pressure fatigue frac!urcs originating fio!n flws i!?:;?artec! from tl:e c::tslde.
In order to inhibit internal pressure fatigue fractures origieating from flaws imparted
fiom the outside, it is effective to increase con~pressivere sidual stresses at the outer surface of
the steel pipe and in the vicinity of the outer surface.
[0004] Examples of methods of increasing compressive residual stress at the outer surface of
a steel pipe include methods of perforu~ings hot blasting working, burnishing working, and
the like with respect to the outer surface of a steel pipe.
I-Iowevel; these methods may be incapable of inhibiting a fatigue fracture originating
from a flaw imparted from the outside because it is only the compressive residual stress in a
very sl~allowp ortion including the outer surface that is increased by these methods. For
example, in a case in which an outer surface is removed by cutting or the like of a steel pipe,
and a flaw is imparted in a region in which the outer surface is removed, an internal pressure
fatigue fracture originating from the flaw is easily generated. In a case in which a flaw
havit~ga depth from the outer surface to the inside of the steel pipe is imparted to the steel
pipe, an internal pressure fatigue fiachlre originating from the flaw is also easily generated.
[0005] The invention was made under such circumstances with an object of providing a steel
pipe having excellent internal pressure fatigue resistance properties.
Solution to Problem
[0006] The inventors found that an increase in compressive residual stress not o11ly at the
outer surface of a steel pipe but also in the vicinity ofthe outer surface is effective for
improving the internal pressure fatigue resistance properties of the steel pipe, thereby
accotnplishing the invention.
Namely, specific means for solving these problen~sa re as follows.
[0007] A steel pipe, consisting of, in terms of mass%:
0.06% to 0.25% of C,
0.50% or less of Si,
1.00% to 1.80% of Mn,
0.030% or less of P,
0.020% or less of S,
0.08% or less ofAI,
0.008% or less of N,
0.080% or less ofNb, and
a remainder cousisting of Fe and unavoidable impurities,
2
it1 \\~Ilicl1a compressive residual stress at an outer surface measured by an X-ray
neth hod is 250 MPa or more, and
a compressive residual stress at a position at a de!~tlto f I mi11 fi.n!n tl~roi !!er s~~rface
measured by tlie X-ray method is 70% or more of tlie compressive residual stress at the outer
surface measured by the X-ray method.
<2> The steel pipe according to , further comprising, in terms of mass%, one or more
of:
0.080% or less of V,
0.030% or less of Ti,
0.50% or less of Cu,
0.50% or less of Ni,
0.50% or less of CI;
0.50% or less of Mo,
0.0040% or less of B,
0.005% or less of Ca, or
0.005% or less of REM.
<3> The steel pipe according to < I > or <2>, having a wall thickness of fiorn 7 mm to 17
inm, in which a ratio of the wall thickness to an outer diameter (wall thickness/outer diameter)
is fiom 0.07 to 0.12.
<4> The steel pipe according to any one of to <3>, in which, in a case in wliich a full
thickness specimen is subjected to a pipe axis direction tensile test, a ratio ofyield strength to
tensile strength is 80% or more, and yield elongation is exhibited.
<5> The steel pipe according to any one of to <4>, in which the steel pipe is an
electric resistance welded steel pipe.
Advantageous Effects of Invention
[0008] According to the invention, a steel pipe having excellent internal pressure fatigue
resistance properties can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0009] Fig. 1 is a graph illustrating a reiationship between outer surface residual stress of a
steel pipe and the number of cycles required to fiactore the steel pipe at an applied stress of
400 MPa.
DESCRIPTION OF EMBODIMENTS
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[OOI 01 flerein, compressive residual stress may be expressed as a negative (-) residual stress
while tensile residual stress !nay be cn!,resred as a positive (t) rzsidoa! st!ass. Na!?lel;:
"compressive residual stress is high" herein means that the residual stress is a negative value,
and that the absolute value of the residual stress is high. For exa~nple", compressive residual
stress of 250 Pa or more" is synonymous with "residual stress of -250 MPa or less".
"Residual stress is high" herein means that the absolute value of a residual stress is
high.
"Compressive residual stress" and "residual stress" herein refer to compressive
residual stress measured by an X-ray method, and residual stress measured by the X-ray
method, respectively, unless otherwise specified.
A nutnerical range expressed by "x toy" herein includes the values of x and y in the
range as the minimum and maximum values, respectively.
[0011] A steel pipe of the invention will be described in detail below.
The steel pipe ofthe invention contains in term of mass%: from 0.06% to 0.25% of
C, 0.50% or less of Si, from 1.00% to 1.80% of Mn, 0.030% or less of P, 0.020% or less of S,
0.08% or less ofAl, 0.008% or less of N, 0.080% or less of Nb, and a remainder consisting of
Fe and unavoidable impul:ities, in which a compressive residual stress at an outer surface
nleasured by an X-ray method is 250 MPa or more, and a compressive residual stress at a
position at a depth of I Inn1 from the outer surface measured by the X-ray method is 70% or
more of the compressive residual stress at the outer surface measured by the X-ray method.
[0012] Herein, the ratio (%) ofthe compressive residual stress at a position at a depth of 1
mm from the outer surface to the compressive residual stress at the outer surface is defined as
ratio C. Namely, the ratio C is defined by the following For~nula1 .
In the invention, the ratio C is 70% or more.
[0013] Formula 1: Ratio C (%) =(compressive residual stress at a position at a depth of 1
nnn from the outer surface /compressive residual stress at the outer surface) x 100
[0014] In the steel pipe of the invention, strength as a base for improving the internal
pressure fatigue resistance properties can be secured by having the above-described
composition as the composition of the steel pipe. The composition of the steel pipe of the
invention will be described belowv.
The compressive residual stress at the outer surface (outer peripheral surface) of the
steel pipe ofthe invention, and the ratio C of the steel pipe are set in the above-described
ranges, in addition to having the above-described composition as the composition of the steel
pipe. As a result, internal pressure fatigue fractures originating eon1 a flaw imparted fron~
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the outside are inhibited.
Rascd on the above, such internal pressure fatigue resistance properties are improved
hy the steel pipe of the invention.
[OO I S ] In a case in which the residual stress at the outer surface of a steel pipe is
conipressive residual stress, the residual stress at the ii~riers urface of the steel pipe generally
tends to he tensile residual stress in order to maintain a balance between the residual stresses
at the outer surface (outer peripheral surface) and the inner surface (inner peripheral surface).
Thus, the tensile residual stress at the inner surface tends to increase in conjunction with an
increase in the compressive residual stress at the outer surface.
I-lowevet; the residual stress (absolute value) at the inner surface of the steel pipe is
reduced compared with that at the outer surface, depending on the wall thickness of the steel
pipe. Such a tendency is more remarkable in the case of satisfying at least one of a wall
thickness of 7 nlln or more, or a ratio [wall tl~ickness/outedr iameter] of 0.07 or more.
Moreover, it is more difficult to impart a flaw to the inner surface ofthe steel pipe than to the
outer surface of the steel pipe.
For such reasons, an internal pressure fatigue fracture originating from a flaw
imparted in the inner surface is not significantly problematic even in a case in which the
compressive residual stress at the outer surface is increased as described above.
Therefore, in order to improve the internal pressure fatigue resistance properties of
the steel pipe, a countenlleasure against an internal pressure fatigue fracture originating from
a flaw imparted to the outer surface is more important than a countermeasure against an
internal pressure fatigue fracture originating from a flaw imparted to the inner surface.
[0016] In the invention, the compressive residual stress at the outer surface is 250 MPa or
more. As a result, internal pressure fatigue fractures originating from a flaw imparted from
the outside are inhibited.
The compressive residual stress at the outer surface is preferably 350 MPa or more,
and more preferably 400 MPa or more.
The upper limit o f the compressive residual stress at the outer surface is not
particularly restricted. From the viewpoint of further reducing the tensile residual stress at
the inner surface, the compressive residual stress at the outer surface is preferably 600 MPa or
less.
[0017] A c&npressive residual stress of 250 MPa according to an X-ray method is
equivalent to a compressive residual stress of 150 MPa according to a Crampton method, a
compressive residual stless of430 MPa according to the X-ray method is equivalent to a
compressive residual stress of 300 MPa according to the Crampton method, ar~d a tensile
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residual stress of 120 MPa according to the X-ray method is equivalent to a tensile residual
stress of 100 MPa according to the Crampton method.
1001 81 Measurement of the compressive residual stress by tlie X-ray method can be
performed according to a usual method. An example of the measurement method is
described in the Examples below.
[0019] In the invention, the ratio C is 70% or more as described above.
As a result, internal pressure fatigue fractures originating fsom a flaw imparted fiom
the outside are inhibited.
In particular, this kind of intestla1 pressure fatigue fracture can be inhibited even in a
case in which the outer surface is peeled for any reason, or in a case in which a flaw having a
depth fiom the outer surface to tlie inside is imparted.
[0020] The ratio C is preferably 80% or more, and more preferably 90% or more.
In theor): tlie upper limit of the ratio C is 100%. However, the ratio C may be more
than 100% in a case, for example, ill which a measurement position at the outer surface and a
measurement position located at a depth of 1 mm are different from each other in an axial or
circumferential direction.
[0021] The effect of improve~nenitn internal pressure fatigue resistance properties by the
invention can be evaluated, for example, based on the number of cycles required for ~acture
at an applied stress of 400 MPa in the circumferential direction of the steel pipe. It goes
without saying that a greater number of cycles required for fsacture means superior internal
pressure fatigue resistance properties.
Fig. 1 is a graph illustrating a relationsl~ipb ehveen outer surface residual stress of a
steel pipe and the number of cycles required to fsacture the steel pipe at an applied stress of
400 MPa.
In the measurement of the graph, first, nine steel pipes (nine electric resistance
welded steel pipes) having the compositio~o~f Steel No. 2, which is describkd below, and
having residual stresses at outer surfaces according to an X-ray method ("OUTER SURFACE
RESIDUAL STRESSES" in Fig. 1) that are the values indicated in Fig. I, were prepared.
The tesidual stresses at tlie outer surfaces were varied by changing the conditions of heat
treatment in the Exan~plesd escribed below. The residual stresses at the outer surfaces in the
X-ray method were measured by a method described in the Examples belowv.
Then, a stress of 400 MPa was repeatedly applied onto each of the nine steel pipes
described above at a frequency of 0.8 Hz in a circumferential direction, and the number of
cycles (times) of the stress until each steel pipe was fractured was detern~ined. The obtained
nu~nber(t imes) of the cycles was regarded as "TI-IE NUMBER OF CYCLES TO
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FRACTURE AT APPLIED STRESS OF 400 MPa [TIMES]" in Fig. 1. The number of
cycles was measured under the condition that the steel pipe temperature was an ordina~y
te!nperaD.!rct (ahout 20°C).
As illustrated in Fig. 1, a residual stress of -250 MPa or less (i.e., a compressive
residual stress of 250 MPa or more) at the outer surface was found to result in a remarkable
increase in the number (times) of cycles to fracture; that is, in remarkable improvement in the
internal pressure fatigue resistance properties of the steel pipe.
[0022] The wall thickness of the steel pipe of the invention is preferably from 7 mm to 17
mm.
A wall thickness of 7 mm or more results in a greater improvement in resistance to
internal pressure. Because residual stress at the inner surface can be reduced due to the
increased wall thickness, an internal pressure fatigue fracture originating from a flaw imparted
to the inner surface is further inhibited.
An upper limit of the wall thick~~eossf 17 mm is an upper limit set in consideration
of formability for bending a hot rolled steel sheet to fonn a steel pipe (in particulat;
formability in a case in which the steel pipe of the invention is an electric resistance welded
steel pipe).
[0023] In the steel pipe of the invention, the ratio of the wall thickness to the outer diameter
(wall thicknesslouter diameter) is preferably from 0.07 to 0.12.
A ratio (wall thicknesslouter diameter) of 0.07 or more results in a greater
improvement in resistance to internal pressure. Because the residual stress at the in~ler
surface can be reduced, an internal pressure fatigue fiacture originating from a flaw imparted
to the inner surface is further inhibited.
An upper limit of the ratio (wall thicknesslouter diameter) of 0.12 is an upper litnit
set in consideration of formability for bending a hot rolled steel sheet to forni a steel pipe (in
particular, formability in a case in which the steel pipe of the invention is an electric
resistance welded steel pipe).
[0024] The steel pipe ofthe invention pal-ticularlp preferably has a wall thickness of from 7
mm to 17 mm, and a ratio (\ifall thicknesslouter diameter) of from 0.07 to 0.12.
Herein, the outer diametel; the wall thickness, and the ratio (wall thicknesslouter
diameter) map be referred to as "outer diameter D", "wall thickness t", and "ratio [t/D]",
respectively.
[0025] In view of further itnproving the internal pressure fatigue resistance properties, the
steel pipe of the invention preferably has a ratio ofyield strength to tensile strength
(hereinafter also referred to as "yield ratio") of 80% or more, and yield elongation is exhibited,
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in a case in which a ft~ltlh ickness specinlet1 is sul?jected to a pipe axis direction tensile test.
The properties of a yield ratio of 80% or more, and of the presence of yield
elongation, ace !~?o!~e?tipeesc ?!!lar to a steel pips subjected to the heat treatn?e!?td ssccibed
below.
A yield ratio of 80% or more can result in a wider elastic region. Fractures are
inhibited even with internal pressure fatigue in a case in which yield elongation is exhibited.
In theory, the upper litnit ofthe yield ratio is 100%.
[0026] The kind of the steel pipe of the invention is not particularly restricted, and may be a
\velded steel pipe such as an electric resistance welded steel pipe, or may be a sealnless steel
pipe.
It is preferable that the steel pipe of the invention is an electric resistance welded
steel pipe from the viewpoint of dintensional accuracy, manufacturing costs, and the like.
[0027] The steel pipe of the invention (in particular, a steel pipe having a yield ratio of 80%
or more, and exhibiting yield elongation, in a case in which a full thickness specimen is
subjected to a pipe axis direction tensile test) can be produced, for example, by heating the
whole of a steel pipe that has not yet been subjected to heat treatment after pipe making
(hereinafter also referred to as a "steel pipe as-rolled") to a temperature of not more tt~ana n
Acl point, and by quenching the outer surface of the heated steel pipe. Herein, a process
from the start of the heating to the end (stop) of the cooling may be referred to as "heat
treatment".
It is thought that the quenching can cause the difference between the temperatures of
the outer surface and the inner surface, and the difference behveen the temperatures can cause
a large compressive residual stress at the outer surface. The effect due to the quenching can
be inore effectively exerted in the case of satisfying at least one of a wall thickness of 7 mm
or more, or a ratio (wall tl~ickness/outer diameter) of 0.07 or more.
The outer surface can be quenched, for example, by spraying a coolit~gs olvent from
the vicinity of the outer surface onto the outer surface by a spray nozzle or the like. In such
a case, the compressive residual stress at the outer surface and a ratio C can be adjusted by
adjusting the heat treatment temperature, the cooling starting temperature, the cooling rate, 01.
the like. For example, the compressive residual stress at the outer surface tends to increase
with an increase in the cooling rate.
The steel pipe after pipe making and before heating (steel pipe as- rolled) may also
be subjected to other processes such as cold drawing.
[0028] The use of the steel pipe of the invention is not particularly restricted. The steel
pipe of the invention can be used ill any application requiring excellent internal pressure
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fatigue resistance propetties.
Exatnples of the steel pipc ofthe invention include a Steel pipe for a cylinder, a steel
pipe for a vibration control datii!>eb a steel pipe for an eartllqr~ake-resistatd~at n~pe!; an?.
hydraulic piping.
In particular, improvement in the internal pressure fatigue resistance properties is of
great significance in a case in which the steel pipe ofthe invention is a steel pipc for cylinder
use.
The steel pipe for a cylinder is preferably a steel pipe for a cylinder which is applied
to the outer cover of a cylinder that extends and contracts due to oil pressure or the like.
Examples of such a cylinder include cylinders in the vicinity of driving systems, such as the
buckets, anns, and booms of hydraulic sl~ovels. .
[0029] The con~positiono f the steel pipe of the invention will be described below.
"0h I, expressing the content of an element in a steel pipe is "mass%" in the follo\~~ing.
Further, xir11en simply the term "content" is used for the respective elements, this
refers to the content in a steel pipe.
[0030] The steel pipe of the invention contains ftom 0.06% to 0.25% of C, 0.50% or less of
Si, from 1.00% to 1 .SO% of Mn, 0.030% or less of P, 0.020% or less of S, 0.08% or less ofAl,
0.008% or less of N, and 0.080% or less of Nb, atid a remainder consisting of Fe and
unavoidable impurities, as described above.
Each element and its content, and the unavoidable impurities, will be described
below.
[003 11
C (carbon) is an element that is effective for improving the strength of a steel pipe.
The content of C in the steel pipe ofthe invention is 0.06% or more. As a result, the
strength of the steel pipe can be secured as a base for inlpluving the internal pressure fatigue
resistance properties.
An excessively high content of C results in excessively high strength of the steel pipe,
and in deterioration in toughness. Tluus, the upper limit of the content of C is 0.25%.
[0032]
Si (silicon) is effective as a deoxidizer.
However, an excessively high content of Si results in deterioration of
low-temperature toughness, and furthe]; in deterioration of electric resistance xireldability in a
case in whicli the steel pipe of the invention is an electric resistance welded steel pipe. Thus,
the upper limit of the content of Si is 0.50%. The content of Si is preferably 0.40% or less,
and more preferably 0.30% or less.
9
The content of Si is preferably 0.0 I % or more in view of being able to more
effectively achieve an effect as a deoxidizer: The content of Si is preferably 0.05% or more,
and more preferably 0.10% or z~~oricn ,v iew of fi.!rt!?er increesing the strengt!? oft!^ s!eel pipe
by solid solution strengthening.
Si may not ouly be contained intentionally in steel, but Si may also be mixed as an
impurity into steel. Tl~elo wer limit of the content of Si is not pa~ticularlyr estricted because
a lower content of Si is preferable in a case in whicli Si is mixed as an impurity into steel.
[0033]
Mn (tnanganese) is an element that enhances the hardenability of steel, thereby
allowing the steel to have high strength.
The content of Mn (manganese) in the steel pipe of the invention is 1.00% or !nore in
view of securing liigl~s trength. The content of Mn is preferably I . I 0% or more, and Inore
preferably 1.20% or more.
However, an excessively high content of Mn promotes the generation of martensite,
and in deterioration in toughness. Thus, the upper limit of the content of Mn is 1.80%.
[0034]
P (phosphorus) is an impurity.
The upper limit of the content of P is 0.030% because tougllness is improved by
reducing the content of P. The content of P is preferably 0.018% or less.
The lower linlit of the content of P is not patticularly restricted because a lower
content of P is preferable. However, the content of P is ordinarily 0.001% or Inore fiom the
viewpoint of balancing properties and costs.
[0035]
S (sulfi~r)is an impurity.
The upper linlit of the content of S is 0.020% because reduction of the content of S
enables MnS elongated by hot rolling to be reduced, and toughness to be improved. The
content of S is preferably 0.008% or less, and more preferably 0.005% or less.
The lower limit of the content of S is not particularly restricted because a lo\ver
conteut of S is preferable. Howevel; the cot~tenot f S is ordinarily 0.0001% or more from the
viewpoint of balancing properties and costs.
[0036]
N (nitrogen) is an elctnent that unavoidably exists in steel.
Howvevet; an excessively high content ofN results ill an excessive increase in
inclusions such 'as AIN, whereby an adverse effect such as a surface flaw or deterioration in
toughness may be caused. Thus, the upper litnit of the content of N is 0.008%. The
content ofN is preferably 0.007% or less, and particularly preferably 0.006% or less.
The lower litnit of the content ofN is not particularly restricted, and the content of N
is preferably 0.002% or more in consideration of the cost and economical efficiency of N
removal (denitration).
[0038] iNb: 0.080% or less>
Nb (niobium) is an element which lowers the rectystallization temperature, and
\vhich inhibits rectystallization of auster~ited uring hot rolling, thereby contributing to
refinement of the structure.
Howevel; an excessively high content ofNb results in deterioration in toughness due
to coarse precipitates. Thus, the upper limit of the content ofNb is 0.080%. The content of
Nb is preferably 0.070% or less, and more preferably 0.050% or less.
The content of Nb is preferably 0.008% or more, more preferably 0.010% or more,
and particularly preferably 0.015% or more, in view of more reliably obtaining a structure
refinement effect.
[0039]
In the invention, an unavoidable impuri@ tneans a cotnponent contained in a source
material or a component mixed into steel in a production process, and is not a cotnponent that
is intentionally included in the steel.
Specific examples of ut~avoidable impurities include 0 (oxygen), Sb (antimony), Sn
(tin), \$I (tungsten), Co (cobalt), As (arsenic), Mg (magnesium), Pb (lead), Bi (bismuth), B
(boron), and H (hydrogen).
Among these, 0 is preferably controlled to have a content of 0.006% or less.
Ordinarily, for other elements, it is possible to allow inclusion of a content of 0.1% or
less of Sb, SII, W, Co, or As, a content of 0.005% or less of Mg, Pb, or Bi, and a content of
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0.0004% 01. less of B or I-I; hov.~evet; \vith respect to any other elements, no patticular control
is required insofar as the content is within an ordina~yra nge.
[0040] Tlie stee! pipe of t!ie Invention !nay fi.~t?hers e!c~ti~.~cIoy~ tai!?o ne or more of
0.080% or less of V, 0.030% or less of Ti, 0.50% or less of Co, 0.50% or less ofNi, 0.50% or
less of CI; 0.50% or less of Mo, 0.0040% or less of B, 0.005% or less of Ca, and 0.005% or
less of REM.
These elements may be mixed into the steel as unavoidab!e impurities in cases other
than which they are intentionally included in the steel. Therefore, the lower limits of the
contents of the elements are not particularly restricted.
These elements, and the preferred contents of the elements contained in the steel pipe
ofthe invention will be described below.
[0041]
V (vanadium) is an element that generates a carbide and a nitride, and improves the
strengtll of steel by precipitation strengthening.
However, an excessively high content of V causes the carbide and the nitride to be
coarsened, whereby toughness may be deteriorated. Thus, the content of V is preferably
0.080% or less, and more preferably 0.060% or less.
Tlle content ofV is preferably 0.01 0% or more in view of further improving tlie
strength of tlle steel pipe.
[0042] q i : 0.030% or less>
Ti (titanium) is an element that forms a refined nitride (TiN), and inhibits an austenite
grain from coarsening during the heating of a slab, thereby contributing to refinement of the
structure.
Howevel; an excessively high content of Ti results in coarsening of TiN, or in
precipitation hardening due to Tic, whereby tougllness may be deteriorated. Tlms, tlle
content of Ti is preferably 0.030% or less, more preferably 0.025% or less, and particularly
preferably 0.020% or less.
The content of Ti is preferably 0.008% or more, and more preferably 0.0 10% or more,
from the viewpoint of further improving tougllness by refining the sttmcture. b
[0043]
Cu (copper) is an element that enhances the hardenability of steel, thereby allowing
the steel to have high strength. Cu is also an element that contributes to solid solution
strengthening.
However, an excessively high content of Cu may result in deterioration of the surface
quality of tlle steel pipe. Thus, the content of Cu is preferably 0.50% or less, and more
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preferably 0.30% or less.
The content of Cu is preferably 0.05% or more.
!!I a case in ~!?ic!t!! ~es tec! pipe contains Co, it is preferab!~t! lat t!ie stee! pipe also
contains Ni fiom the viewpoint of pre~~entinthge surface quality from deteriorating.
[0044]
B (boroa) is at1 element of which even a minute content results in remarkable
enhancement of the hardenability of steel, thereby contributing to higher strength of the steel.
However, since the hardenability is not further improved, and moreover, precipitates
may be generated, thereby reducing toughness, in a case in tvhich the content of B is more
than 0.0040%, the upper limit of the content of B is preferably 0.0040%. Although B may
be mixed fiom raw material impurities, a B content of 0.0004% or more is preferable ia order
to sufficiently obtain the effect ofhardenability.
[0048]
Ca (calciun~)is an element that controls the fo1.m of sulfide-based inclusions,
ituproves low-temperature toughness, and further refines oxides in an electric resistance weld,
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thereby improving the toughness of the electric rcsistance weld.
However, an excessively high content of Ca results in an increased size of oxides or
sulfides, whereby toughness may be adversely affected. Thus, the content of Ca is
preferably 0.005% or less.
The content of Ca is preferably 0.001 % or more.
[0049]
"EM" herein means a rare earth element, and is the general term for the 17 elements
Sc (scandium), Y (yttrioa~),L a (lanthanum), Ce (cerium), Pr (praseodymium), Nd
(neodymium), Pm (promethium), Sm (samarium), Eu (europium); Gd (gadolinium), Tb
(terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and
Lu (lutetium).
"REM : 0.005% or less" nleans that at least one of the 17 elements is contained, and ,
the total content of the 17 elements is 0.005% or less.
REM is an element that controls the form of sulfide-based inclusions, improves
low-temperature toughness, and further refines oxides in an electric resistance weld, thereby
improving the toughness of the electric resistance weld.
However, an excessively high content of REM results in increased size of oxides or
sulfides, whereby toughr~essm ay be adversely affected. Thus, the content of REM is
preferably 0.005% or less.
The content of REM is preferably 0.001% or more.
EXAMPLES
[0050] The invention will be still more specifically described with Examples, provided that
the iuveution is not limited to the following Examples.
[0051] [Examples 1 to 16, and Comparative Examples 1 to 51
Electric resistance welded steel pipes (electric resistance welded steel pipes asrolled)
having the compositions set forth in "Steel No. I to No. 5" in the following Table 1,
and the outer diameters D, mlall thicknesses t, and ratios [tD] set forth in the following Table
2 were prepared.
In the coulpositions of the electric resistance welded steel pipes, components
(remainder) other than the elements set forth in Table 1 were Fe (iron) and unavoidable
impurities.
"REM" in Steel No. 5 is specifically La (lanthanum).
[0052] A heat tccatnlent apparatus sequentially including a heating furnace, a soakiug
14
furnace, and a rapid water-cooling apparatus, 1vas prepared.
The heat treatment apparatus is configured such that a steel pipe to be heat-treated is
!ransported a!ong !!le pipe *xis direction of !!IS s ! ~p!ip e, and the stee! pipe is sequentia!!y
passed tl~rougltih e heating furnace, the soaking fi~rnacea, tid tl~era pid water-cooling
apparatus.
The rapid water-cooling apparatus includes a spray nozzle for spraying cooling water
from the periphely of tlie outer surface (outer peripheral surface) of the steel pipe over tlie
entire outer surface.
In the heat treatment apparatus, a radiation thermometer A for measuring tl~eco oling
start temperature of the steel pipe is included behveen the soaking furnace and the rapid
water-cooling apparatus, and a radiation thermometer B for measuring the cooling stop
temperature oftlle steel pipe is included at a downstream side in the direction of transporting
the steel pipe viewed from the rapid water-cooling apparatus.
The soaking furnace includes a tlier~nocouplefo r measuring the temperature of the
atmosphere in the furnace.
[0053]
The electric resistance welded steel pipes were subjected to heat treatment (heating
and quenching) by sequentially passing the electric resistance welded steel pipes througl~t lle
heating furnace, soaking furnace, and rapid water-cooling apparatus of the heat treatment
apparatus. The heating temperatures, cooling start temperatures, cooling rates, and cooling
stop temperatures here are as set forth in the following Table 2. In the Examples, "heat
treatment" refers to a process from the start of heating to the stop of cooling.
The heating temperatures were measured by the thermocouple included in the
soaking furnace, the cooling start temperatures were measured by the radiation thermometer A
included bepeen the soaking furnace and the rapid water-cooling apparatus, and the cooling
stop temperatures were measured by tlie radiation thermometer B included at the dowvnstream
side in tlie direction of transporting the steel pipes viewed fro111 the rapid water-cooling
apparatus. The respective cooling rates were calculated based on the respective cooling start
temperatures, cooling stop temperatures, distances between the radiation tliennometer A and
the radiation thermometer B, and speeds of transporting the steel pipes.
[0054]
Tl~ree sidual stresses of the electric resistance welded steel pipes subjected to the heat
treat~nenwt ere measured by an X-ray tnethod.
The residual stresses \\'ere measured at a position at a depth of 1 111111 frolii the outer
surfaces, and at the outer surfaces.
15
Based on the measurement results, each ratio C (i.e., ratio (%) of compressive
residual stress at a position at a depth of I mm from the outer surface to compressive residual
strrss mcaso!.~?b. y X-ray !??ct!?oca! t t!?e outer sorface) !vas deter!??l!ied according to Formula
I .
The results are set forth in the following Table 2.
[0055] The conditio~o~fs t he measurement of such residual stress by the X-ray method are as
follows.
-Conditions of Measure~nent of Residual Stress by X-Ray Method-
In measurement of residual stress by an X-ray method, a shorter length of a sample
may result in a reduced residual stress. Thus, it is preferable to secure a length for each
sample used in the measuren~entth at is 1.5 times the outer diameter of the sample or more.
Therefore, samples (electric resistance welded steel pipes) each having a length of 400 mm
were prepared in the meas~trement,
The measurement of residual stress by the X-ray method was performed by a
gradient method using a microfocus X-ray stress tneasurement device. The position of the
measurement wiras set at a middle position in the lengthwise direction of each sample.
With regard to the residual stresses at the outer surfaces, the residual stresses at the
outer surfaces of the samples were nnleasured by the above-described n~etl~od.
With regard to the residual stress at a position at a depth of 1 mm from each outer
surface, a recess having a depth of 1 mm from the outer surface was formed by subjecting
each of the satnples to electrolytic polishing, atid the residual stress at the bottom of the recess
(i.e., the position at a depth of 1 mrn fiom the outer surface) was measured by the
above-described method.
[0056]
A full thickness specitnen was sampled from each electric resistance welded steel
pipe subjected to the heat treatment. The fullthickness specimen was subjected to a pipe
axis direction tensile test, whereby the yield strength YS and tensile strength TS ofthe fill1
thickness specimen were measured. In the test, the presence or absence of yield elongation
was further confirmed. A yield ratio YRwas further determined as the ratio (%) of the yield
strength YS to the tensile strength TS.
The above results are set forth in the following Table 2.
[0057] The pipe axis direction tensile test was conducted in confonnity with JIS 22241
(201 1). The tensile direction ofthe specimen was set as the pipe axis direction.
The shape of the fitll thickness specinleu was set as the shape of Specin~eN~oi . 12.

[Table 21
Electric resistance welded steel pipe Heat treatment conditions Residual stress
Pipe axis direction tensile test rzsults
[0060] As set fort11 in Table 2, tlie electric resistance welded steel pipes of Examples I to I6
each had a residual stress at the outer surface of -250 MPa or less (i.e., compressive residual
stress at the outer surface of 250 MPa or more), and a ratio C (ratio of compressive residual
stress at a position at depth of I tnni from the outer surface to compressive residual stress at
tlie outer surface) of 70% or more.
In the pipe axis direction tensile test, tlie electric resistance welded steel pipes of
Exatnples 1 to 16 eacli had a yield ratio YR of 80% or more and each exhibited yield
elongation.
Based on the above, the electric resistance welded steel pipes of Exa~nples1 to I6 are
found to have excellent internal pressure fatigue resistance properties.
[0061] Then, a comparative sample A obtained not by subjecting the electric resistance
welded steel pipe as- lulled in Example 5 to the lieat treatment described above, but by
subjecting the outer surface of the electric resistance welded steel pipe as- rolled to shot
blasting working (ejection pressure: 0.8 MPa, grindinglpolishing material: 1.0 mm diameter
steel ball, working temperature: room temperature, and coverage: 100%) was prepared.
Measurement of the residual stresses of the comparative sample A by the
above-described method revealed that the residual stress at the outer surface was -300 MPa,
and the residual stress at a depth of 1 nnn from the outer surface was +I00 MPa.
[0062] A comparison sample B obtained not by subjecting the electric resistance welded
steel pipe as- rolled in Example 5 to the heat treatment described above, but by subjecting the
outer surface of the electric resistance welded steel pipe to burnishing working was further
prepared.
Measurement of the residual stresses of the comparative sample B by the
above-described method revealed that the residual stress at the outer surface was -1 00 MPa,
and the residual stress at a depth of I mni fi.orn the outer surface was +I00 MPa.
[0063] All docun~entsp, atent 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.
I . A steel pipe, consisting of, in ter!ns of mass%:
0.06% to 0.25% of C,
0.50% or less of Si,
1.00% to 1 .SO% of Mn,
0.030% or less of P,
0.020% or less of S,
0.08% or less ofAI,
0.008% or less ofN,
0.080% or less of Nb, and
a remainder consisting of Fe and unavoidable i~npurities,
wherein a compressive residual stress at an outer surface meas~eedb y an X-ray
method is 250 MPa or more, and
a compressive residual stress at a position at a depth of I mm from the oater surface
rneasuted by the X-ray method is 70% or inore of the compressive residual stress at the outer
surface tneasured by the X-ray method.
2. The steel pipe according to clain~ 1, fill-ther comprising, in terms of mass%, olle 01
Inore of:
0.080% or less of V,
0.030% or less of Ti,
0.50% or less of Cu,
0.50% or less ofNi,
0.50% or less of CI;
0.50% or less of Mo,
0.0040% or less of B,
0.005% or less of Ca, or
0.005% or less of REM.
3. The steel pipe according to claim 1 or claim 2, liavi~~a gw all thickness of fium 7 mtn
to 17 tnm, \vilerein a ratio of the wall thickness to an oater diameter (wall thickl~ess/outer
diameter) is from 0.07 to 0.12.
4. The steel pipe according to any one of claim 1 to claim 3, wherein, in a case in which
20
a full thickness specimen is subjected to a pipe axis direction tellsile test, a ratio of yield
stt.ength to te~lsiles tl.engtl~i s SO% 01. mol-e, and yieltl elollgatio~li s exhibitetl.
5. The steel pipe according to ally one ofclaitl~I to claim 4, wherein the steel pipe is an
electric resistance welded steel pipe.