Document Type] Specification
[Title of the Invention] SEAMLESS STEEL PIPE AND METHOD FOR
PRODUCING SAME
[Technical Field of the Invention]
[0001]
The present invention relates to a seamless steel pipe and a method for
producing the same and, more specifically, to a sean~lesst eel pipe suitable for a line
pipe and a nlethod for producing the same.
Priority is claimed on Japanese Patent Application No. 2012-188634, filed 011
August 29,2012, the content of which is incorporated herein by reference.
[Related Art]
[0002]
In recent years, oil wells and gas wells in a sour em~ironmentr,e presented by
the deep sea or cold districts, severer than the conventional enviro~ullenht ave been in
development. The offshore pipeline laid in such a severe sour environn~enits
required to have strength (pressure resistance) and toughness higher than co~~ventiotial
ones and is further required to have resistance to hydrogen induced cracking (HIC
resistance).
[0003]
For the offshore pipeline, which is required to have such properties, a
seanlless steel pipe is nlore suitable than a welded steel pipe. This is because the
ivelded steel pipe has a weld zone (seam portion) along the longitudinal direction.
The weld zone has a toughness lower than that of a base metal. Therefore, the
seanlless steel pipe is suitable for the offshore pipeline.
[0004]
When the thickness of the seanlless steel pipe is increased, high pressure
resistance can be obtained. However, the increase in thiclu~esse asily causes a brittle
fiactuse and decreases the toughness. In order to inlprove the strength and touglu~css
for the thick seamless steel pipe, it is necessary to increase the amount of alloying
elements such as carbon to inlprove the hardenability. Howevcl; in the case where thc
seamless steel pipes having inlproved hardenability are joined to each other by
circunlferential welding, the heat affected zone is likely to harden, and the touglu~ess
and HIC resistance of the circumferential weld zone are decreased.
[OOOS]
In Patent Documents I to 3, there are disclosed seamless steel pipes for line
pipe having improved strength and toughness and methods for producing the same.
[0006]
In the seamless steel pipe for line pipe disclosed in Patent Document 1, it is
described that a product of a Mn content and a Mo content is 0.8 to 2.6 and thus the
strength and the toughness are increased. Ful-thel; the seanlless steel pipe for line
pipe disclosed in Patent Docu~nent 1 contains at least one of Ca and rare earth metals
(REM), and thus the SSC resistance is i~lcreased.
[0007]
The sea~nlesss teel pipe for line pipe disclosed in Patent Document 2 has a
metallographic structure nlainly cotllposed of bainite and has cenlentite having a length
of 20 p111 or less. In Patent Document 2, it is described that even when the pipe is
fortned to be thick, high stre~~gtgho,o d touglu~essa, nd good corrosion resistance can
be obtained.
[OOOS]
In the seamless steel pipe disclosed in Patent Document 3, it is described that
the n~uiibero f oxide-based inclusions present in the steel and Itaving a diameter larger
than 300 [un is one or less per square centinleter and the number of oxide-based
inclusions having a diameter of 5 [un to 300 pni is 200 or less per square centimeter.
In Patent Document 3, it is described that whcn the nuniber of oxide-based inciusions
is limited as described above, the enibrittlement at the grain boundary is suppressed
and thus the toughness of the seaniless steel pipe can be increased.
[Prior Art Documet~t]
[Patent Document]
[0009]
[Patent Document I] PCT International Publication No. WO 20071023804
[Patent Document 21 PCT International Publication No. WO 20071023806
[Patent Docunicnt 31 Japanese Unexaniined Patent Application, First
Publication No. 2004-124158
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[OO lo]
However, when the strength of the seamless steel pipes disclosed in Patent
Documents 1 to 3 is X80 or no re according to the API standards, that is, the yield
strength of the seamless steel pipe is 550 MPa or more, the HIC resistance niay
decrease.
[OOll]
Further, when the seamless steel pipes disclosed in Patent Documents 1 to 3
are circumferentially welded on the spot, the hardness of a lieat affected zone (HAZ)
among the circumferential weld zones, pa~ticularlya, fusion line (bond portion), is
increased and t11~1st he HIC resistance is decreased in some cases.
[OO 121
An object of the present invention is to provide to a seatnless steel pipe
suitable for a line pipe having high strength and excellent HIC resistance, and having
excellent HIC resistance of HAZ even \&711e11b eing circun~ferentiallyw elded.
[Means for Solving the Problcm]
[0013]
(1) According to an aspect of the present invention, there is provided a
seanlless steel pipe including, as a chemical composition, by mass%, C: 0.02% to
0.10%, Si: 0.05% to 0.5%, Mn: 1.0% to 2.0%, Mo: 0.5% to 1.0%, Cr: 0.1% to 1.0%,
Al: 0.01% to 0.10%, P: 0.03% or less, S: 0.005% or less, Ca: 0.0005% to 0.005%, V:
0.010% to 0.040%, N: 0.002% to 0.007%, at least one selected fioni the group
consisting of Ti: 0.008% or less and Nb: 0.02% to 0.05%, and a balance consistillg of
Fe and impurities, in which a carbon equivalent Ceq defined by the following Fonnula
(a) is 0.50% to 0.58%, and specified carbides containing Mo at a ratio of 50 mass% or
more, V, and at least one selected from the group consisting of Ti and Nb, and having a
size defined by an average value of major axes of 20 nm or more are contained.
Ceq = C + M11I6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ... (a)
here, into each of the syrllbols of elements in Formula (a), the amount of a
unit mass% of a corresponding elenlent is substituted, and in the case where an
element corresponding to the synibol of the element is not contained, "0" is substituted
into the corresponding symbol of the elenlent.
[0014]
(2) The seamless steel pipe according to (1) may fin-ther include at lcast one
selected fro111 the group consisting of Cu: 1 .O% or less and Ni: 1 .O% or less in place of
sotile of Fe.
[0015]
(3) In the seamless steel pipe according to (1) or (2), the yield strength may
be 550 MPa or more and a Vickers hardness at a positiori on an inner side 1 mm away
from an inner surface may be 248 I-IVIO or less.
[0016]
(4) The seamless steel pipe according to any one of (1) to (3) may be
produced by a process including a quenching and a tempering at 660°C to 700°C.
[0017]
(5) According to another aspect of the present invention, there is provided a
method for producing a sea~lllesss teel pipe including heating a steel material including,
as a chemical composition, by mass%, C: 0.02% to 0.10%, Si: 0.05% to 0.5%, Mn:
1.0% to 2.0%, Mo: 0.5% to 1.0%, Cr: 0.1% to l.O%,AI: 0.01% to 0.10%, P: 0.03% or
less, S: 0.005% or less, Ca: 0.0005% to 0.005%, V: 0.010% to 0.040%, N: 0.002% to
0.007%, at least one selected from the group consisting of Ti: 0.008% or less and Nb:
0.02% to 0.05%, and a balance consisting of Fe and impurities atld having a carbon
equivalent Ceq defined by the following Formula (b) of 0.50% to 0.58%, producing a
raw pipe by piercing-rolling the heated steel material, producing a seamless steel pipe
by rolling the raw pipe, quenching the seamless steel pipe at a quenching temperature
of an Ac3p oint or higher, and tempering the sea~lllesss teel pipe after the quenching at a
tempering temperature of 660°C to 700°C.
Ceq = C + Mn16 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ... (b)
here, into each of the symbols of elements in the Formula (b), the aalilount
(mass%) of the corresponding eletnent is substituted, and in the case \vhere an element
corresponding to the symbol of the element is not contained, "0" is substituted into the
corresponding syn~bool f the element.
[0018]
(6) The method for producing a seamless steel pipe according to (5) may
further include acceleratedly cooling the sea~nlesss teel pipe at a cooling rate of
100 oC/~ninor higher until a temperature of the sea~nlesss tecl pipe reaches a
temperature of an A,I point or lo~verb etween the producing of the sean~lesst eel pipe
and the quenching of the sean~lesss teel pipe, and the acceleratedly-cooled seanllcss
steel pipe may be quenched in the qnenching of the sean~lesss teel pipe.
[00 191
(7) In the method for producing a sea~nlesss teel pipe according to (5) or (6),
the seamless steel pipe may further inclnde, as the cl~e~niccaoln lposition, at least one
selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in place of
some of Fe.
[Effects of the Invcntion]
[0020]
The above-described seamless steel pipe has high strength and excellent HIC
resistance and has excellent HIC resistance of IIAZ even when being circumferentially
welded.
[Brief Description of the Drawings]
[0021]
FIG. 1 is a block diagram of a production line of a seamless steel pipe
according to the present enlbodiment.
FIG. 2 is a flowchart sho\ving a production process for the seamless steel pipe
according to the present en~bodi~nent.
FIG. 3 is a schematic view showing the temperature of a steel material, a raw
pipe, and a seanlless steel pipe in each step s l ~ oi~n vFI~G~ 2.
FIG. 4 is a cross sectional view showing a groove shape of a seamless steel
pipe at the time whe11 a circumfere~ltiawl eld zone touglnless examination is carried out
in an exa~llplc.
FIG. 5 is a schematic view illustrating a Vickers hardness test piece sampled
from a circu~llferentiawl eld zone in an example.
FIG. 6 is a schematic view illustrating a square test piece satupled from a
circutnferet~tiawl eld zone in an example.
[Embodiment of the Invet~tion]
[0022]
I-Iereinaftel; an embodiment of the present invention will be described in
detail with reference to the acconlpanyi~lgd rawings. In the drawings, the satne
sy~l~boalrse applied to the same or equivalent portions, and the explatration thereof is
not repeated.
[0023]
The present inventors researched and examined the stretlgth and HIC
resistance of the seamless steel pipe. As a result, the present itlve~ltorso btained the
following findings.
[0024]
(A) When the strength of steel is increased, a C content may be increased.
However, wvl~en the C content is too high, the hardness of the steel becomes too high
and the HIC resistance is decreased. Particularly, when the seatuless steel pipe is
subjected to circumferetltial welding, the hardness of HAZ includi~lga fiision line is
increased and the HIC resistance of the HAZ is decreased. Accordingly, it is
preferable to limit the C content to 0.02% to 0.10%.
[0025]
(B) When the C content is low, high stretigtl~is not easily obtained. Here,
in the embodiment, a carbon equivalent Ceq expressed by the following Forn~ula(1 ) is
0.50% to 0.58%.
Ceq = C + M11/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ... (1)
Here, into each of the symbols of eletnents in Forlnula (I), the amount
(mass%) of each eletnent is substituted, and in the case ~vlierea n element
corl~espondingt o the syn~bool f the element is not contained, "0" is substituted into the
corresponding symbol of the element.
[0026]
In a case of a carbon equivalent Ceq of 0.50% to 0.58%, even when the C
content is within the above-described range, a yield strength of 550 MPa or tnore is
obtained. Further, even when circunifere~itialw elding is carried out, the hardness of
the HAZ is not increased excessively. Therefore, excellent NIC resistance of the
HAZ can be maintained.
[0027]
(C) In order to obtain high strength and excellent I-IIC resistance, it is
effective that a plurality of specified carbides is contained it1 the seamless steel pipe.
Here, the specified carbides referred to herein represent carbides containing Mo as a
main component, V, and at least one ofTi and Nb.
[0028]
It is preferable that the size of the specified carbide is 20 nm or more. When
the size of the specified carbide is too small, the hardness of the steel beconles too high
and the HIC resistance is decreased. It is important to set the size of the specified
carbide to 20 nm or Inore in order to increase the HIC resistance by controlling the
hardness of the steel to fall in an appropriate range. Further, when the size of the
specified carbide is 20 11111 or more, the hardness of the HAZ of the circu~nferentially
welded sca~nlesss teel pipes is not likely to increase excessively and also the HIC
resistance of the I-IAZ can be maintained.
[0029]
(D) In order to produce the above-described seamless steel pipe, it is
effective to quench and temper the seanlless steel pipe. In the tempering, it is
preferable that the tempering temperature is 660°C to 700°C. Accordingl~: the size of
the specified carbide becomes 20 ntn or nlore.
[0030]
The seanlless steel pipe according to gle enlbodinlent conlpleted based on the
above findings and the method for producing the same will be described.
[003 11
[Chemical Composition]
The seamless steel pipe according to this e~nbodi~nehnats the following
chemical conlposition.
[0032]
C: 0.02% to 0.10%
Carbon (C) increases the strength of the steel. When the C content is less
than 0.02%, the above-described effect cannot be obtained sufficiently. On the other
hand, when the C content is more than 0.10%, the toughness of the circunlferential
weld zone of the sean~lesss teel pipe is decreased. Therefore, the C content is 0.02%
to 0.10%. The lower linlit of the C content is preferably more than 0.02%, and nlore
preferably 0.04%. Tl~ucp per linlit of tile C content is preferably 0.08%.
[0033]
Si: 0.05% to 0.5%
Silicon (Si) deoxidizes tl~cst eel. Wl~ert he Si content is 0.05% or more, the
above-described effect can be obtained remarkably. However, when the Si content is
more than 0.5%, the touglu~esso f the steel is decreased. Accordingly, the upper linlit
of the Si content is 0.5%. The lower limit of tlle Si conte~lits preferably more than
0.05%, more preferably 0.08%, and still more preferably 0.10%. The upper litnit of
the Si content is preferably less than 0.5%, more preferably 0.25%, and still more
preferably 0.20%.
[0034]
Mn: 1 .O% to 2.0%
Manganese (Mn) improves the hardenability of the steel, and increases the
strength of the steel. When the Mn content is less than 1.0%, the above-described
effect is not easily effectively obtained and a yield strcngth of a grade of X80 or higher
is not easily obtained. On the other hand, when the Mn content is more than 2.0%,
Mn segregates in steel, and resultantly the toughness of a heat affected zone (HAZ)
fornled by circumferential welding and the toughness of the seamless steel pipe itself
(base metal) are decreased. Accordingly, the Mn content is 1.0% to 2.0%. The
lower limit of the Mn content is preferably nlore than 1.0%, more preferably 1.3%, and
still more preferably 1.4%. The upper limit of the Mn content is less than 2.0%, more
preferably 1.8%, and still more preferably I .6%.
100351
Mo: 0.5% to 1.0%
Molybdenu~n(M o) itnproves the hardenability of the steel and increases the
strength of the steel. Further, Mo conlbines with C and V in the steel to form fine
specified carbides containing at least one of Ti and Nb which will be described later.
As long as the size of the specified carbide is 20 11111 or more, a high strength can be
stably obtained. In addition, even when heat treatment is carried out after
circumferential welding, the specified carbides are not easily coarsened. Thus, even
wllen the size of the specified carbide is 20 nm or more, the strength of the steel can be
tnailltailtcd. The specified carbides will be described later. When the Mo content is
less than 0.5%, the above-described effect is not easily obtained. On the other hand,
when the Mo content is more than 1.0%, the weldability and the HAZ toughness of the
steel are decreased. Accordingly, the Mo content is 0.5% to 1.0%. The lower linlit
of the Mo colltetlt is preferably more than 0.5%, more preferably 0.6%, and still tllore
preferably 0.7%. The upper limit of the Mo content is preferably less than 1.0%,
more preferably 0.9%, and still lllore preferably 0.8%.
[0036]
Cr: 0.1% to 1.0%
Chromiutn (Cr) improves the hardenability of the steel and increases the
strength of the steel. Cr furthcr improves the temper softening resistance of the steel.
Howevel; when the Cr content is less than 0.1%, the above-described effect is not
easily effectively obtained. On the other hand, when the Cr content is more than
1.0%, the weldability and the HAZ toughness of the steel are decreased. Accordingly,
the Cr content is 0.1% to 1.0%. The lower lilllit of the Cr colltellt is preferably more
than 0.1% and more preferably 0.2%. The upper limit of the Cr content is preferably
less than 1 .O% and more preferably 0.8%.
[0037]
A1: 0.01% to 0.10%
Aluminum (Al) combines with N to form fine A1 nitrides and illcreases the
toughness of the steel. Howeve]; v\~llettlh e A1 content is less than 0.01%, the abovedescribed
effect calnlot be effectively obtained. On the other hand, when the A1
content is tilore than 0.10%, thc A1 nitrides are coarsened and the toughness of the steel
is decreased. Accordingly, the A1 content is 0.01% to 0.10%. The lower limit of the
A1 content is preferably Illore than 0.01% and more preferably 0.02%. The upper
litnit of the Al content is preferably less than 0.1%, more preferably 0.08%, and still
more preferably 0.06%. The A1 content in the specification represents tlie atuount of
acid-soluble A1 (what is called So1.AI).
[0038]
P: 0.03% or less
Phosphorous (P) is an impurity. P decreases the toughness of tlie steel.
Accordingly, the P content is preferably as low as possible. Therefore, the P content
is limited to 0.03% or less. The upper litnit of the P content is preferably less than
0.03%, nlore preferably 0.015%, and still Inore preferably 0.012%.
[0039]
S: 0.005% or less
Sulfur (S) is an inlpttrity. S conibities with Mn to form coarse MnS, and
decreases the toughness and HIC resistance of the steel. Accordingly, tlie S content is
preferably as low as possible. Therefore, the S content is limited to 0.005% or less.
The upper limit of the S content is preferably less than 0.005%, more preferably
0.003%, and still more preferably 0.002%.
[0040]
Ca: 0.0005% to 0.005%
Calcium (Ca) combines with S in the steel to form CaS. The fonnation of
CaS suppresses the production of MnS. Therefore, Ca increases the toughness and
HIC resistance of the steel. Hovever, wlien the Ca content is less than 0.0005%, the
above-described effect cannot be effectively obtained. On the other hand, when tlie
Ca content is nlore than 0.005%. the cleanliness of the steel is decreased and thc
tonghness and HIC resistance of the steel are decreased. Accordingly, the Ca content
is 0.0005% to 0.005%. The lower limit of the Ca content is preferably more than
0.0005%, more preferably 0.0008%, and still more preferably 0.001%. The upper
limit of the Ca content is preferably less than 0.005%, more preferably 0.003%, and
still more preferably 0.002%.
[0041]
V: 0.010% to 0.040%
Vanadium (V) combines with C in the steel to form V-carbides, and increases
the strength of the steel. Further, V is solid-solved in Mo carbides to form specified
carbides. When V is contained, the specified carbides are not easily coarsened.
When the V content is less than 0.010%, the above-described effect calnlot be
effectively obtained. On the other hand, when the V content is more than 0.040%, the
V-carbides are coarsened. Accordingl~t~h,e V content is 0.010% to 0.040%. The
lower limit of the V content is preferably tnore than 0.010%, and Inore preferably
0.02%. The upper limit of the V content is preferably less than 0.040%.
[0042]
N: 0.002% to 0.007%
Nitride O\J) combines with A1 to form fine A1 nitrides and increases the
toughness of the steel. In order to obtain the above-desciibed effect, the lower limit
of the N content is preferably 0.002%. However, when the N content is excessively
high, N solid-solved in the steel decreases the toughness of the steel. Further, when
the N content is excessively high, the carboaitrides are coarsened and the toughness of
the steel is decreased. Accordingly, the N content is 0.007% or less. The upper
limit of the N content is preferably less than 0.007%, more preferably 0.006%, and still
more preferably 0.005%.
[0043]
The chenlical cotnposition of the seanlless steel pipe according to this
embodiment fttrther contains at least one selected fio~nth e grouip consisting of Ti and
Nb. Both the components increase the touglmess of the steel and are solid-solved in
Mo carbides to form specified carbides.
[0044]
Ti: 0.008% or less
Titanium (Ti) combines with N in the steel to for111 TiN, and suppresses the
decrease in toughness of steel caused by N forming a solid solution. Further, fine TiN
that is dispersedly precipitated, increases the toughness of the steel. Furthermore, Ti
is solid-solved in Mo carbides to fort11 sl~ecifiedc arbides and suppresses coarsening of
the specified carbides. As long as even a small atnount of Ti is contained, the abovedescribed
effect can be obtained. When the Ti content is 0.001% or more, the abovedescribed
effect can be remarkably obtained. On the other hand, \vl~ent he Ti content
is more than 0.008%, TiN is coarsened and coarse Tic is formed, therefore, toughness
of the steel is decreased. That is, when Ti is contained, the Ti content needs to be
restricted in order to refine and disperse the nitrides and the specified carbides. The
tipper linlit of the Ti content is 0.008% or less. The upper limit of the Ti content is
preferably less than 0.008%, more preferably 0.005%, still more preferably 0.003%,
and still more preferably 0.002%.
[0045]
Nb: 0.02% to 0.05%
Niobium (Nb) conlbilles wit11 C andlor N in the steel to for111 fine Nb carbides,
Nb nitrides, or Nb carbonitrides, and increases the touglmess of the steel. Fu~Tller,
fine Nb is solid-solved in Mo carbides to for111 specified carbides, thereby suppressing
coarsening of the specified carbides. When the Nb content is less than 0.02%, the
above-described effect cannot be effectively obtained. Therefore, the lower limit of
the Nb content when being contained is 0.02%. On the other hand, when the Nb
content is more than 0.05%, the specified carbides are coarsened. Accordingly, the
Nb content is preferably 0.02% to 0.05%. The lower liniit of theNb content is
preferably more than 0.02%, and niore preferably 0.03%. The upper limit of the Nb
content is preferably less than 0.05%, and niore preferably 0.04%.
[0046]
The balance of the co~npositiono f the seanlless steel pipe according to the
embodiment includes Fe and inlpurities. Here, the itilpuritics referred to herein are
clcme~ltsth at mixedly enter from ore and scrap used as raw materials for steel, the
environnlent of the production process, and the like.
[0047]
The chetiiical co~llpositiono f the seamless steel pipe according to the
embodiment may fi~rtherin clude at least one selected from the group consisting of Cu
and Ni in place of some of Fe. Any of these elenlents increases the hardenability of
the steel arid improves the strength of the steel.
[0048]
Cu: 1 .O% or less
Copper (Cu) is an optional element. Cu illlproves the liardenability of the
steel and increases the strength of the steel. Any small atilount of Cu can provide the
above-described effects. When the Cu content is 0.05% or more, the above-described
effect is remarkably obtained. On the other hand, ~vhenth e Cu content is more than
1.0%, the weldability of the steel is decreased. Further~~lorwe,h en the Cu content is
- 15 -
too high, the intergranular strength of the steel at a high temperature is decreased and
the hot workability of the steel is decreased. Accordingly, tlie upper limit of the Cu
content is 1.0%. The lower linlit of the Cu content is preferably nlore than 0.05%,
Inore preferably 0.1%, and still more preferably 0.2%.
[0049]
Ni: 1 .O% or less
Nickel (Ni) is an optional element. Ni improves the hardenability of the
steel and increases the strength of the steel. Any small amount of Ni content can
provide the above-described effect. Whcn the Ni content is 0.05% or more, the
above-described effect is remarkably obtained. On the other hand, when the Ni
content is more than 1.0%, the SSC resistance is decreased. Accordingly, the upper
limit of the Ni content is 1.0%. Tile lower litilit of the Ni content is preferably more
than 0.05%, more preferably O.l%, and still more preferably 0.2%. The upper limit
of the Ni content is preferably less than 1.0%, more preferably 0.7%, and still more
preferably 0.5%.
[OOSO]
[Carbon Equivalent Ceq]
For the scamless steel pipe according to the embodiment, the carbon
equivalent Ceq defined by the following Fornlula (1) is 0.50% to 0.58%.
Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ... (1)
[005 11
Here, into each of the syti~bolso f elements in Formula (I), the amount
(mass%) of the corresponding element is substituted. In the chemical cornposition of
the seamless steel pipe according to the embodiment, in the case where an elenlent
corresponding to the symbol of tlie elenlent in Fornlula (1) is not contained in the
seamless steel pipe, "0" is substituted into the correspotiding synlbol of the element in
Formula (1). I-Iere, the "case wliere an element is not contained" referred to lierein
represents that the amount of the element is at the level of the impurities or lower.
[0052]
In the seamless steel pipe according to the embodiment, the C content is
limited. This is because that C remarkably decreases the toughness of the weld zone
formed by circ~unferentialw elding. Howevcl; when the C content is too low, the high
strength of steel cannot be obtained. In this embodiment, therefore, the lower limit of
the carbon equivalent Ceq defined by Fortnula (1) is 0.50%. In this case, even when
the C content is low, high strength can be obtained. More specifically, the strength
grade of the seamless steel pipe can be X8O or higher according to the APT standards,
that is, the yield stress of the seamless steel pipe can be 550 MPa or more. On the
other hand, wltett the carbon equivalent Ceq is too high, the hardenability of the steel
becomes too high and thus the hardness of the heat affected zone (HAZ) is excessively
increased. As a result, the toughness of the I3AZ is decreased and the HIC resistance
is also decreased. Accordingly, the upper limit of the carbon equivalent Ceq is 0.58%.
[0053]
[Specified carbide]
The seaniless steel pipe according to the e~nbodimenct ontains a plurality of
specified carbides having a size of 20 11111 or more. Here, the specified carbides
represent carbides containing Mo as a main conlponent, V, and at least one of Ti and
Nb. Tlie "Mo as a main con~ponent"r epresents that the Mo content in the carbide is
50 mass% or more wit11 respect to the mass of the entire carbide. In addition, the V
content is preferably 1 mass% to 50 mass% and the Ti content and the Nb content are
preferably 1 mass% to 30 inass% ~vitlrie spect to the tilass of the entire carbide.
[0054]
The size of the specified carbides can be measured by the follo\ving manner.
An extraction replica method is used to sample an extraction replica filtn from the
thick portion of the seatnless steel pipe. Specifically, an extraction replica filtn
(diameter of 3 mm) is sampled from a region including the center portion of an
arbitrary thick portion of the seamless steel pipe in the thickness direction, and an
extraction replica film (diameter of 3 mm) is sampled fiom a region including a portion
positioned on the inner side 1 mm away from the iinler surface in the thickness
direction. On each of the extraction replica films, four places (four fields of viexv) of
arbitrary regions of 10 pm2 are observed. That is, for one seamless steel pipe eight
regions are observed. A transmission electron microscope (TEM) is used to observe
the places at a lnagnification of 3,000 times.
[OOSS]
Fro111 a plurality of precipitates observed in each region, carbides and
carbonitrides are identified based on the electron beam diffraction pattern analysis.
Further, using an energy dispersive X-ray spectroscope (EDS), the chemical
cotnpositions of each of the identified carbides and carbonitrides are analyzed to
identify specified carbides. Ten specified carbides are selected from the plurality of
identified specified carbides. The major axis (nm) of each of the selected specified
carbides is nleasurcd. Here, the "major axis" represents the maximum of the straight
lines connecting two different points at the interface between the specified carbides
and the base metal. The nlajor axes of 80 specified carbides (10 carbides x 8 regions)
are measured by the above-described t~lethod. The average value of the measured
major axes is defined as the "size (~nno) f specified carbides".
[0056]
Tl~esp ecified carbides increase the strength of the steel. 13owever, when the
size of the specified carbide is too small, the hardness of the steel becomes too high
and the I-IIC resistance is decreased. When the size of the specified carbide is 20 111
or less, the l~ardtiesso f the steel is within an appropriate range wl~ileth e strength of the
steel is increased. Therefore, the HIC resistance is also increased. Specifically, the
yield strength of the seamless steel pipe is 550 MPa or more (X8O grade or higl~er).
In addition, the Vickers hardness at the position on the inner side 1 nlln away from the
inner surface of the seanlless steel pipe (hereinaftel; referred to as inner surface layer
hardness) is 195 HVlO to 248 HV10. Futthel; the toughness of the circutnferentially
welded sea~nlesss teel pipe is not easily decreased excessively or the hardness is not
easily excessively increased.
[0057]
The upper limit of the size of the specified carbide is not particularly limited.
The upper limit of the size of the specified carbide is, for exainple, 200 nm. The
upper litnit of the size is preferably 100 ~ man,d inore preferably 70 nm.
[0058]
[Production Method]
An example of a method for producing the seatnless steel pipe according to
this enlbodiment will be described. In the enlbodiment, a seamless steel pipe
produced by hot working is cooled (air-cooled or acceleratedly cooled). Tlien, the
cooled seamless steel pipe is quenched and tetnpered at a specific tempering
temperature. Hereinafter, the nlethod for producing the seatnless steel pipe according
to the embodinlent will be described in detail.
[0059]
[Production Line]
FIG. 1 is a block diagram showing an exatnple of a production line for the
seamless steel pipe according to the embodiment. Refening to FIG. I, the production
line includes a heating furnace 1, a piercing machine 2, an elongation rolling mill 3, a
sizing rnill4, a holding furnace 5, a water cooling apparatus 6, a quenching apparatus 7,
and a telnpering apparatus 8. Between these apparatuses, a plurality of transfer
rollers 10 is disposed. In FIG. 1, the quenching apparatus 7 and the tenlpcring
apparatus 8 are also included in the production line. However, the quenching
apparatus 7 and the tempering apparatus 8 may be disposed so as to be separate from
the production line. In other words, the quenching apparatus 7 and the tempering
apparatus 8 may be disposed off-line.
[0060]
[Production Flow]
FIG. 2 is a flowchart slowing a production process for the sea~nlesss teel pipe
according to the elnbodiment. FIG. 3 is a diagram showing a change of surface
temperature of work pieces (steel material, raw pipe, and seamless steel pipe) with
respect to time during production. Here, A1 in the drawing represents an& point
when the work pieces are heated, and represents an A,I point when the work pieces are
cooled. In addition, A3 in the drawing represents an Ac3 point when the work pieces
arc heated, and represents an A,, point when the work pieces are cooled.
In the embodiment, the A,I point, Ac3 point, A,, point, and Ar3 point are values
obtained by creating a CCT diagram of a test piece sampled fro111 steel having a
predetermined che~nicacl omposition in a fornlastor test and calculating the values
based on the obtained CCT diagram.
[006l]
Referring to FIGS. 1 to 3, in the production process, first, a steel material is
heated in the lleating furnace 1 (heating step: Sl). The steel material is, for exa~nple,
a round billet. The steel material may be produced by using a continuous casting
apparatus such as a round CC. Fu~thert,h e steel material also may be produced by
hot-working (forging or blootning) an ingot or a slab. In this example, the
explanation is continued assunling that the steel material is a round billet.
(00621
The heated round billet is hot-worked to form a seamless steel pipe (S2 and
S3). Specifically, the round billet is piercing-rolled by the piercing machine 2 to form
a raw pipe (piercing-rolling step: S2). FUI-thel; the raw pipe is mlled by the
elongation rolling tnill 3 and the sizing mill 4 to form a seamless steel pipe (elongation
rolling step and sizing step: S3). Then, the seamless steel pipe produced by hot
working is heated to a predetermined temperature by ihe holding furnace 5 as
necessary (reheating step: S4). Successively, the seamless steel pipe is cooled
(cooling step: S5). As the cooling method, the seamless steel pipe is cooled by water
cooling (accelerated cooling) using the water cooling apparatus 6 (accelerated cooling
step: S5 1) or the seamless steel pipe is cooled by air cooling (air cooling step: S52). '
[0063]
The cooled seamless steel pipe is quenched using the quenching apparatus 7
(quencl~ings tep: S6) and tempered at a specific te~nperingte mperature using the
teml~eringa pparatus 8 (tempering step: S7). Hereinafter, each of the steps will be
described in detail.
[0064]
[Heating Step (Sl)]
First, a round billet is heated in the heating filmace 1. The preferable heating
temperature is 1100°C to 1300°C. When the round billet is heated at a temperature in
this temperature range, carbonitrides it1 thc steel dissolve. In the case w11cre the
round billet is produced fi.0111 a slab or an ingot by hot working, the heating
telnperature of the slab and ingot may not necessarily be 1100°C to 1300°C. This is
because when the ingot or the slab is heated, carboaitrides in the steel dissolvc. The
heating furnace 1 is, for exatnple, a well-known walking bearn fi~rnacco r rotary
furnace.
[0065]
[Piercing-Rolling Step (S2)]
The round billet is taken out of the heating furnace 1 and then the heated
round billet is piercing-rolled by the piercing machine 2 to produce a raw pipe. The
piercing machine 2 is provided with a plurality of inclined rolls and a plug. The plug
is disposed between the inclined rolls. The preferable piercing machine 2 is a crosstype
piercer. When the cross-type piercer is used, piercing can be performed at a high
pipe expansion rate and thus the use of the cross-type piercer is preferable.
[0066]
[Elongation Rolling Step and Sizing Step (S3)]
Next, the raw pipe is rolled. Specifically, the raw pipe is elongated and
rolled by the elongation rolling mill 3. The elongation rolling mill 3 includes a
plurality of roll stands disposed in series. The elongation rolling mill 3 is, for
exatnple, a mandrel mill. Successively, the elongated and rolled raw pipe is drawn
and rolled by the sizing nlill4 to produce a seamless steel pipe. The sizing mill 4
includes a plurality of roll stands disposed in series. The sizing mill 4 is, for example,
a sizer or a stretch reducer. Further, the elongation rolling step and the sizing step are
collectively sintply referred to as a rolling step in sotne cases.
[0067]
[Reheating Step (S4)]
A reheating step (S4) is carried out as necessary. That is, the production
method according to the embodiment may not include the reheating step (S4).
Specifically, when water cooling is carried out by an accelerated cooling step (S5 l),
the reheating step (S4) is carried out in a case nrhere the temperature of the sea~llless
steel pipe is increased before the water cooling. In the case where the reheating step
is not carried out, in FIG. 2, the process proceeds from step S3 to step S5. In the case
where the reheating step is not required, in FIG. 1, the holding fi~rnace5 does not have
to be provided.
[0068]
When the accelerated cooling is carried out in following step at a finishing
tetnperature (the surface temperature olthe seamless steel pipe immediately after the
step of S3 ends) lower than Ar3, reheating is preferably carried out in the reheating step
(S4). In the reheating step (S4),t he sea~nlesss teel pipe is charged into the l~olding
furnace 5 and is heated. The preferable heating temperature in the holding filrnace 5
is 900°C to llOO°C. The preferable soaking time is 30 minutes or less. This is
because whetl the soaking time is too long, the carbonitrides composed of Ti, Nb, C,
and N (Ti, Nb) (C, N) may be precipitated and coarsened.
[0069]
I11 the reheating step (S4), an induction heating apparatus may be used in
place of the holding furnace 5.
[0070]
[Cooling Step (S5)]
The sea~illcsss teel pipe produced in step S3 or the seamless steel pipe
reheated in step S4 is cooled. For the cooling, any of an accelerated cooling step
(S51) and an air cooling step (S52) may be canied out. The accelerated cooling step
(S51) and the air cooling step (S52) are collectively refelxed to as a cooling step (S5).
[0071]
[Accelerated Coolit~gS tep (S51)l
When the toughness of the seamless steel pipe is increased, the sean~lesss teel
pipe is cooled not by the air cooling step (S52), but the accelerated cooling step (S5 1).
In the accelerated cooling step (S51), the seamless steel pipe is water-cooled
(acceleratedly cooled) by the water coolillg apparatus 6. The temperature (surface
temperature) of the seamless steel pipe before being water-cooled is Ar3 or higher, and
preferably 800°C or higher. The Ar3 point of the seamless steel pipe having the
chemical compositio~w~it hin the above-described range according to the embodiment
is 750°C or lower. When the tenlperatt~reo f the seamless steel pipe immediately
before being water-cooled is lower than Ar3, ferrite is produced and quenching is not
sufficient. Thus, the temperature is not preferable. When the temperature of the
seamless steel pipe before being acceleratedly cooled is lower than Ar3, the seamless
steel pipe is reheated in the reheating step (S4) and the temperature thereof is illcreased
to Ar3 or higher.
[0072]
The cooling rate in the accelerated cooling step is preferably 100 OCImin or
higher. When the cooli~lgra te is lower than a cooli~lgra te of 100 "Cltnin, since ferrite
is generated, the temperature is not preferable. In addition, the cooling stop
temperature is preferably A,[ or lower. At a cooling stop tenlperature of A,, or highel;
the amount of residual austenite is increased and thus the temperature is not preferable.
The A,, point of the sean~lesss teel pipe according to the embodiment having the
chemical co~npositionw ithin the above-described range is 550°C or Io\vel: The
preferable cooling stop temperature is 450°C or lower. Tlie ~iiicrostructureo f the base
metal (matrix) is transfor~ned into martensite or bainite by tlie accelerated cooling and
is refined. More specifically, a tllartelisite lath or a bainite lath is generated in the
martensite or bainite.
[0073]
The configuration of the water cooling apparatus 6 used for accelerated
cooling is, for example, as described belo\v. The water cooling apparatus 6 includes a
plurality of rotary rollers, a lanlinar water flow device, and a jet water flow device.
Tlie plurality of rotary rollers are disposed in two rows and the seamless steel pipe is
provided between the plurality of rotary rollers disposed in two rows. At this time,
each of the rotary rollers disposed in two rows conles into contact with the lower
po~tiono f the outer surface of the seaniless steel pipe. When the rotary rollers are
rotated, the sealnless steel pipe is rotated arout~dth e axis thereof. The lanlinar water
flow device is disposed above the rotary rollers, and pours water over the seamless
steel pipe from above. At this titne, the water poured over the seatuless steel pipe
forms a latninar water flow. The jet water flow device is disposed near the end of the
seamless steel pipe disposed on tlie rotary rollers. The jet water flow device injects
jet water flow toward tlie inside of the steel pipe from the end of the seatnless steel
pipe. The laminar water flow device and the jet water flow device are used to cool
the outer and inner surfaces of the seamless steel pipe at the satne time. Such a
configuration of the water cooling apparatus 6 is particularly suitable for accelerated
cooling of a thick seamless steel pipe having a thickness of 35 null or more.
[0074]
The water cooling apparatus 6 nlay be an apparatus other than tlie abovedescribed
rotary rollers, latninar water flow device, and jet water flo~vd evice. For
exatnple, the water cooling apparatus 6 nlay be a water tank. In this case, the
seanlless steel pipe is itnnlersed in the water tank and is acceleratedly cooled. Also,
the water cooling apparat~rs6 nlay includc the lanlinar water flow device only. That
is to say, the type of the water cooling apparatus 6 is not limited.
[0075]
After the water cooling is stopped at the water cooling stop tetnperature, air
cooling nlay be carried out until the surface temperature of the seamless steel pipe
reaches roo111 temperature. The seamless steel pipe may be cooled to root11
temperature by the water cooling apparatus 6.
[0076]
As described above, the accelerated cooling step (S51) is effective in a case
where higher toughness is obtained. However, when there is no need to obtain high
toughness, in place of the accelerated cooling step (S51), the air cooling step (S52)
which will be described below may be carried out.
[0077]
[Air Cooling Step (S52)I
In the production process of the seamless steel pipe according to the
embodinlent, in place of the accelerated cooling step (S51), the air cooling (S52) may
be carried out. In the air cooling step (S52), the searnless steel pipe produced in the
step S3 is air-cooled. Accordingly, \vhen the air cooling step (S52) is carried out, the
reheating step (S4) may not be carried out.
[0078]
In the air cooling step (S52), cooling is carried out until the surface
temperature of the seamless steel pipe reaches 400°C or lower. I11 the air cooling, the
seamless steel pipe may be cooled to room tenlperature.
[0079]
[Quenching Step (S6)]
The sea~itlesss teel pipe wllich has been subjected to the accelerated cooling
step (S51) or the air cooling step (S52) is quenched. Specifically, tlie seamless steel
pipe is heated by the quenching apparatus 7. By this heating, the metallographic
~nicrostructureo f the sean~lesst eel pipe is transformed into austenite. Then, the
heated scamless steel pipe is quenched by accelerated cooling. Thereby, the
metallographic microstructnre of the seamless steel pipe beco~nesa ~netallographic
structure which consists mainly of martensite or bainite.
[OOSO]
In the quenching step (S6), the seamless steel pipe is heated to a temperature
of tlie Ac3 point or higher by heating using the quenching apparatus 7. In addition,
soaking is preferably carried out for 5 minutes to 90 minutes at a temperature witl~in
the aforementioned range. The Ac3 point of tile seamless steel pipe according to tlie
embodiment having the chetnical co~npositionw ithin the above-described range is
800°C to 900°C.
[0081]
In the cooling step of the quenching step (S6), the seamless steel pipe heated
to the Aa point 01. higher is quenched by accelerated cooling. The quenching start
temperature is tlie Ac3 point or higher as described above. Further, tthe cooling rate
during the time when the temperature of the sea~nlesst eel pipe is 800°C to 500°C is
5 OCIsec (300 OCInlin) or higher. Accordingly, a unifor~nq uenching structure can be
obtained. The cooling stop temperature is tlie A,, point or lower. Wlien tthe cooling
stop temperature is higher than the A,I point, the amount of residual austenite is
increased and thus the temperature is not preferable. The preferable cooling stop
temperature is 450°C or lower. Also, the seamless steel pipe may be cooled to room
temperature by accelerated cooling.
[0082]
[Tempering Step (S7)]
The quenched steel pipe is tempered. The tempering temperature is 660°C
to 700°C. The retaining time is preferably 10 minutes to 120 minutes. By carrying
out tenlpering under such conditions, specified carbides having a size of 20 nm or
more can be finely dispersed in the seamless steel pipe. As a result, the strength
grade of the seamless steel pipe can be X80 or l~igl~aecrc ording to the API standards,
that is, the yield strength of the seamless steel pipe can be 550 MPa or more. Furthel;
since the size of the specified carbide is 20 nnl or more, good toughness and HIC
resistance can be obtained in tlie circumferetltiallp welded HAZ.
[0083]
By the above-described production processes, even for the seatnless steel pipe
having a thickness of 35 mm or more, excelle~lts trength, toughness, and HIC
resistance can be obtained. The above-described production nlethod is particularly
suitable for a seamless steel pipe having a thickness of 35 111111 or more and is also
applicable to a seamless steel pipe having a thickness of 40 mtn or more. The upper
litllit of tlie thickness is not particularly limited and is typically 60 mnt or less.
[Examples]
[0084]
A plurality of seamless steel pipes having various cllemical compositions were
produced, and the strength, toughness, inner surface layer hardness, and HIC resistance
of each of the seamless steel pipes were examined. Further, the sean~lesst eel pipes
were circumferentially welded and the touglnless, hardness, and IIIC resistance of the
circulnferelitial weld zone were examined.
[OOSS]
[Examination Mctl~od]
Aplurality of molten steels having the chemical conlpositions shown in Table
1 was produced by a 40t electric filmace. Ingots were produced from the moltell
steels. The ingots were hot-forged to produce round billets.
The syn~bo"l- " in Table 1 indicates that the content is equal to or less than the
mcasurelnent limit.

[0087]
Each of the produced round billets was heated to 1 100°C to 1300°C.
Successively, each of the rotuld billets was piercing-rolled by the piercer to form raw
pipes. Next, each of the raw pipes was elongated and rolled by the n~andreml ill.
Then, each of the raw pipes was drawn and rolled (sized) by the sizes to produce a
plurality of seamless steel pipes. The seatnless steel pipes each had a thickness of 40
111111.
[0088]
Tables 2-1 and 2-2 show production conditions of each production process
after sizing.
.paIIol-loy 2u!aq l a p Zu!~oo3 .IF Kq anqvladural
moo1 01 paloo3 s! Iaals ayl 1 - qs alvxpu! ~ 1areq dals 8u![oo:, pa$vlala33p~ w dals Su!lvaya~jos uum-[o:, ayl y3!q~U! aldmxa mj ,
(is)
daas Zu!~adma~ (15s)da as Su![oos paaeJa[aaav (9s)d als a'u!seaqax
[I-z alqv~1
[68001

[0091]
After sizing step, some of tlie seanlless steel pipes of test Nos. 1 to 18 were
heated in the holding filmace under the conditions of the heating temperature PC) and
soaking tirne (min) of the "reheating step (S4)" in Table 2-1. The blank indicates that
the reheating step (S4) is not carried out.
[0092]
Then, the seatiiless steel pipe which was subjected to the reheating step was
acceleratedly cooled by water cooling. Tl~est art temperature (OC) of the "accelerated
cooling step (S5 1)" in Table 2-1 indicates a tetnperature (surface temperature, "C) of
the seamless steel pipe after sizing or heating in the holding furnace and imtnediately
before the accelerated cooling. The accelerated cooling rate ("Clmin) at the time of
accelerated cooling 15as as shown in the accelerated cooling ratc ("Clnlin) of the
"accelerated cooling step (S51)" in Table 2-1. The cooling stop temperature of all of
the acceleratedly cooled seamless steel pipes was 450°C or lower as shown in Table 2-
1.
[0093]
Atllotig test Nos. 1 to 18, some test nunlbers with blanks in the start
temperature, the cooling rate, and the cooling stop tenlperature in the column
"accelerated cooling step (S51)" indicate that the seamless steel pipe was not
acceleratedly cooled but air-cooled to roonl temperature (25°C).
[0094]
After tlie accelerated cooling step or the air cooling step, each of the seaniless
steel pipes was heated and quenched. At this time, each of the seanlless steel pipes
was charged into the quenching apparatus 7 and heated to the quenching tenlperature
("C) in the colulnn of heating tenlperature in the "quenching step (S6)" in Table 2-1.
At tlte quencl~ingte ~nperatwee, ach of the seanlless steel pipes was soaked for the time
(mill) in the colunln of soaking time shown in tlte "quenching step (S6)". After the
soaki~~tghe, sea~nlesss teel pipes \yere acceleratedly cooled at the cooling rate ("Clnlili)
shown in the column of cooling rate of the "qoenching step (S6)" in Tablc 2-1. Then,
the accelerated cooling was stopped at the cooling stop temperature ("C) shown in
Table 2-1. After the accelerated cooling was stopped at the cooling stop temperature,
the seamless steel pipes wrerc air-cooled to room temperature.
[0095]
After the quenching step, each of the seamless steel pipes was tetnpered.
The tempering temperature was as shown in Table 2-1. The retaining time at the
tempering telnperature for each of the test in~rnbersw as 30 minutes.
[0096]
The seamless steel pipes produced by the above-described production
processes were subjected to the following evaluation tests.
[0097]
[Yield Strength and Tensile Strength Test]
The yield strength and the tensile strength of the seamless steel pipes of each
of test Nos. 1 to 18 were examined. Specifically, from each of the sea~nlcsss teel
pipes, a No. 12 test piece (width: 25 mm, gage length: 200 tnnt) specified in JIS Z
2201 was santpled along the longitudinal direction (L direction) of the seamless steel
pipe. The satnpled test piece was used to carry out the tensile test according to JIS Z
2241 in the atmospltere at room temperature (25°C) to obtain yield strength (YS) and
tensile strength (TS). The yield strength was obtained by the 0.5% total elongation
method. The obtained yield strength (MPa) and tensile strengths (MPa) are shown in
Table 2-2. The "YS" in Table 2-2 indicates the yield strength obtained by the test
piece of each test number, and the "TS" indicates the tensile stress.
[0098]
[Toughness Test]
The toughness of the sea~nlesst ecl pipes of each of test Nos. 1 to 18 was
examined. Specifically, from the central portion of the thickness of each of the
seatnlcss steel pipes, a V-notch test piece according to JIS Z 2242 was sampled
perpendicularly to the longitudinal direction of the seamless steel pipe (in the T
direction). The V-notch test piece was a square rod shape having a transverse cross
section of 10 mn x 10 nml. The depth of the V notch was 2 mm. T l ~Vs- notch
speci~ilew~la s used to carry out the Charpy inlpact test according to JIS Z 2242 at
various temperatures. Tllus, the fracture appearance transition temperature (50%
FATT) of each of the seamless steel pipes was obtained. Table 2-2 shows the 50%
FAT?' ("C) obtained frotn the test piece of each test number. The 50% FATT
represents a temperature at which the ductile fracture percent is 50% on the fracture
surface of the test piece.
[0099]
[Inner Surface Layer Hardness Test]
The Vickers hardness test was carried out according to JIS Z 2244 at three
arbitrary points on the inner side 1 rnnl away from the inner surface of the sca~nless
stecl pipe in thc thickness direction on the transverse cross section (cross section
perpendicular to the center axis) of each of the searnless steel pipes of test Nos. 1 to 18.
The test force F in the Vickers hardness test was 10 kgf (98.07 N). The obtained
average value of the \ralues of the three points was defined as the inner surface layer
hardness (HV10) of the seanlless steel pipe of the test number. The obtained inner
surface layer hardness is sho\vn in Table 2-2.
[OI 001
[Measurement Test of Size of Specified Carbide]
The size (nm) of the specified carbide was obtained by the above-described
method on the transverse cross section of eaeh of the seaniless steel pipes of test Nos. 1
to 18. When the specified carbides were specified, elements (Mo, V, Ti, and Nb)
contained in the specified carbides were also identified. Tlie size (nm) of the
specified carbide and the identified elements in the carbides are shown in Table 2-2.
[OlOI]
[HIC Resistance Test of Base Metal]
The I3IC resistance of the seamless steel pipes of test Nos. 1 to 18 was
examined. Specifically, fro111 each of the seamless steel pipes, a test piece including
tile i~ulesr urface of the sea~iilesss teel pipe, a test piece illcluding the thickness center,
and a test pieee including the outer surface were each sampled. That is, three test
pieces were sampled from eaeh of the seaniless steel pipes. The thickness of each test
piece was 30 mm, the width (in the circumferential direction) was 20 mm, and the
length was 100 ~nm. According to the National Association of Corrosion Engineers
(NACE) TM0284-2003, the HIC resistance of eaeh test pieee was evaluated. The test
bath in which the test pieces were immersed was an aqueous solution of 5% common
salt + 0.5% acetic acid at room temperature in which hydrogen sulfide gas of 1 atni
was saturated.
[O 1021
After 96 hours elapsed after immersion, eaeh test piece was cut into three
equal pieces in the longitudinal direction. The cross section at this titne was a cross
section of thickness x width (in the circumferential direction) of the test piece. Tlie
cut test piece was used to obtain a crack length ratio CLR (= crack length (ml~l)/\vidth
(mm) of test piece). The ~naximunvl alue in the CLR of the aforeme~ltionedth ree test
pieces sampled from each steel pipe was defined as the crack length ratio CLR of the
test piece. The obtained crack length ratio CLR is showv~l in Table 2-2.
[0 1031
Furthel; regarding the test piece which was subjected to tlle HIC resistance
test, an ultrasonic test (UT) was carried out on the surface of the test piece including
the inner surface of the seamless steel pipe, corresponding to the inner surface of the
seamless steel pipe, (20 mm x 100 mm) and it was checked wlvhether or not a blister
(swelling due to cracks near the surface) was present and the number of blisters
generated it1 the test piece was counted. The nu~nbero f blisters is shown in Table 2-2.
[0 1041
[Examiuation of Touglu~esso f Circu~nferentiaWl eld Zo~le]
A circu~nfere~ltiwale lding test was carried out on the sean~lesst eel pipes of
test Nos. 3, 5, 9, 12, 17, and 18. Specifically, each sean~lesss teel pipe of the
concerned test number was cut in the central poi-tion in the longitudinal direction.
The cut portion was subjected to edge preparation to take a longitudinally sectioned
shape sho\v~iln FIG. 4. Under the welding conditions shown in Table 3, the cut
portions of the two cut-off seamless steel pipes were circutllferentially welded to each
other.
[0 1051
[Table 31
Groove shape
Preheating
Welding heat input
Post-welding heat tleatment
(PWIIT)
As sliow~il~l F IG. 4
. Not - done
1 .SkJ/m~n
68O0CxS minutes
[0106]
From each of the circuniferentially welded seamless steel pipes, a Charpy Vnotch
test piece including a weld zone (including weld metal, heat affected zone, and
base metal) was sanipled in tlie longitudinal direction of the seaniless steel pipe (L
direction). Specifically, from each of the seamless steel pipes, three test pieces, in
which a V notch is disposed on tlie fusion line (FL) in which the toughness is easily
deteriorated in the heat affected zone (HAZ), were sampled (hereinafter, referred to as
a FL test piece), and fintliel; thee specimens, in which the V notcli is disposed in the
two-phase zone HAZ (hereinaftel; referred to as "V.HAZ test piece"), were sampled
(hereinaftel; referred to as a V.HA.7 test piece). Here, tlie two-phase zone HAZ is a
portion in which the base metal is heated to tlie two-phase zone in the I-IAZ by welding
heat (that is, a poltion heated at a temperature within a transfonnnation point range of
A,, to AC3) and also represents a portion having structures of ferrite and maltensite at
morn temperature.
[0 1071
The sampled specimens were used to carry out the Charpy test according to
JIS Z 2242 at a test tenlperatnre of -30°C to obtain absorbed energy. The lowest
value of thee absorbed cnergy values obtained from each test ntnnber was defined as
the absorbed energy in the FL test piece arid the V.HAZ test piece of each test number,
The absorbed energy obtained by the test is slowvn in Table 4.
[0108]
[Table 41
Fusio~l line hardness
[0 1091
[Circuniferential Weld Zone Hardness Test]
Frotii each of tlie circumferetltially welded sea~illesss teel pipes, as shown in
the region indicated by a broken line in FIG. 5, a micro tcst piece including a \veld
zone (thickness TH = 40 mnl, width WI = 20 tim, length 20 n~mw) as sampled. In
FIG. 5, OS refers to an outer suiface atid IS refers to an inner surface.
[OllO]
The cross section of the thickness TH and the width WI (hereinaftel; referred
to as an obset-vatioti surface) of the micro test piece was ~iiirror-polished. A nital
etching solution was used to exhibit a metallographic structure on the minor-polished
observation surface. Then, in a ratige fro111 at1 inner side 1 mtn away from the outer
surface OS to an inner side 1 mm away from the illtier surface IS along the fi~sionli ne
FL, at intervals of 1 nim, the Vickers hardness test was carried out according to JIS Z
2244. As a result, the liardncss of 38 poitits it1 each micro tcst piece was measured.
The test force F in the Vickers liardness test was 9.8 N. The largest value of the
obtained liardness values of 38 points was defined as the hardness (HV) of tlie fi~sion
line of the test piece.
[Olll]
[Test of HIC Resistance of Circumferential Weld Zone]
As shown in FIG. 6, from each of the circumfcrentially welded seanlless steel
pipes, a square test piece including an inner surface IS and a weld zone WL (thickness
TH = 30 mm, width WI = 20 mm, length = 100 mm) was sarnplzd. The square test
piece was immersed in the same test bath as in the above-described HIC resistance test
of the base metal for 96 hours. The square test piece was taken out from the test bath
and an ultrasonic test was carried out fiom a direction perpendicular to the fitsion line
FL to exanline where or not HIC was present. The test results are shown in Table 4.
" N in the Table 4 indicates that HIC was not observed. "F" indicates tliat HIC was
observed.
[0112]
[Exatninatioti Results]
Refelring to Tables 1, 2-1, and 2-2, for the seatniess steel pipes of test Nos. 1
and 3 to 13, the cl~emicacl omposition was within the range of the present invention,
and the carbon equivalent was 0.50% or more. Therefore, the yield strength of each
of the test numbers was 550 MPa or more, corresponding to the strength grade of X80
or higher according to the API standards. The 50% FATT of each of the test numbers
was -50°C or lowel; that is, the seamless steel pipes had excellent toughness. Further,
tlie size of the specified carbide of tlie test numbers was 20 11111 or more. Therefore,
tlie inner surface layer hardness was 248 HVlO or less. Thus, the crack length ratio
CLR was low and the number of blisters generated was small.
[0113]
When test Nos. 5 and 6 were cotnpared, the 50% FATI of test No. 5 which
was subjected to accelerated cooling wvas lower than tliat of test No. 6, and was
excellent. In the sanle manner, when test Nos. 10 and 11, and 12 and 13 were
coml~aredt,h e 50% FATT of test Nos. 10 and 12 was excellent co~nparedto test Nos.
11 and 13. That is, as long as tlie sea~nlesss teel pipes were niade of the same type of
steel, excellent toughness was obtained in a case where the accelerated cooling was
carried out.
[0114]
Further, referring to Table 4, the absorbed energy in the circumferential weld
zones of all of test Nos. 3, 5,9, and 12 exceeded 100 J. The hardness of the fusion
line of each of the test numbers was low. Therefore, even in the weld zone, excellent
HIC resistance was exhibited.
[0115]
On the other hand, for test No. 2, as shown in Table 2-1, tlie tempering
temperature was too lo\v. Therefore, the size of the specified casbide was less than 20
nm. ' h s , the inner surface layer hardness of test No. 2 was excessively increased
and exceeded 248 I-IVlO. In addition, the crack length ratio CLR was high and the
number of blisters was also large. That is, the HIC resistance was low.
[0116]
For test No. 14, the V content was too high. Therefore, the inner surface
layer hardness was excessively increased and exceeded 248 HV10. Tlie crack length
ratio CLR was high and the number of blisters was also large.
[0117]
For test No. 15, the Mo content was too low. Tlierefore, tlie yield strength
was less than 550 MPa.
[0118]
For test No. 16, the carbon equivalent Ceq was too low. Therefore, the yield
strength was less than 550 MPa.
[0119]
For test No. 17, the carboa equivalent Ceq was too high. Therefore, as
s11on~11 in Table 4, the hardness of the fusion line was excessively increased, the
absorbed energy was low, and HIC occurred in the HAZ of the \veld zone.
[O 1201
For test No. 18, the Mn content was too high. Therefore, as showa in Table
4, the hardness of the fusion line was excessively increased and I-IIC occurred in the
HAZ of the weld zone.
[0121]
The e~llbodimeto~ft the present invention has been described above.
~owvgvert,h e above-described embodiment is merely at1 illustration for carrying out
the present invention. Therefore, the present invention is not liulitcd to the abovedescribed
embodiment, and the present invention can be applied by appropriately
changing or modifying the above-described enlbodi~lle~wlti thout departing fiotn the
spirit and scope of the present invention.
[Industrial Applicability]
[O 1221
According to the present inve~ltioni, t is possible to provide the seamless steel
pipe having high strength and cxcellent HIC resistance and having excellent HIC
resistance of the HAZ even when being circumferentially \velded.
[Brief Description of the Reference Symbols]
[0123]
1 HEATING FURNACE
2 PIERCING MACHINE
3 ELONGATION ROLLING MILL
4 SIZING MILL
5 HOLDING FURNACE
6 WATER COOLING APPARATUS
7 QUENCHING APPARATUS
8 TEMPERING APPARATUS
FL FUSIONLINE
IS N E R SUWACE
OS OUTER SURFACE
[Doculnent Type] CLAIMS
1. A seamless steel pipe con~prisinga, s a cliemical conlposition, by mass%:
C: 0.02% to 0.10%;
Si: 0.05% to 0.5%;
Mn: 1.0% to 2.0%;
Mo: 0.5% to 1.0%;
Cr: 0.1% to 1.0%;
Al: 0.01% to 0.10%;
P: 0.03% or less;
S: 0.005% or less;
Ca: 0.0005% to 0.005%;
V: 0.010% to 0.040%;
N: 0.002% to 0.007%;
at least one selected from the group consisting of Ti: 0.008% or less and Nb:
0.02% to 0.05%; and
a balance colisisting of Fe and impurities,
wherein a carbon equivalent Ceq defined by the following Formula (1) is
0.50% to 0.58%, and
specified carbides containing Mo at a ratio of 50 mass% or more, V, and at
least one selected fro111 the group consisting of Ti and Nb, and having a size defined by
an average value of major axes of 20 1ui1 or more are contained.
Ceq = C + M1d6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ... (1)
liere, into each of the synibols of elements in tlie For~nula(I ), tlie anlount of a
unit mass% of a corresponding element is substituted, and in the case where an
ele~nenct orresponding to the symbol of the element is not contained, "0" is substituted
into tlie corresponding symbol of the elemcnt.
2. The seatnless steel pipe according to claim 1, further coniprising ,
at least one selected from tlie group consisting of Cu: 1 .O% or less and Ni:
1.0% or less in place of sotne of Fe.
3. The seamless steel pipe according to claim 1 or 2,
wherein the yield strength is 550 MPa or more, and a Vickers hardness at a
position on an inner side 1 mm away from an inner surface is 248 IIVIO or less.
4. The seamless steel pipe according to any one of clairils 1 to 3,
wherein the seamless steel pipe is produced by a proccss including a
quenching and a tempering at 660°C to 70OoC.
5. A tilethod for producing a seamless steel pipe comprising:
heating a steel material including, as a chenlical composition, by mass%,
C: 0.02% to 0.1 O%,
Si: 0.05% to 0.5%,
Mn: 1 .O% to 2.0%,
Mo: 0.5% to 1.0%,
Cr: 0.1% to 1.0%,
Al: 0.01% to 0.10%,
P: 0.03% or less,
S: 0.005% or less,
Ca: 0.0005% to 0.005%,
V: 0.010% to 0.040%,
N: 0.002% to 0.007%,
at least one selected fro111 tlie group consisting of Ti: 0.008% or less
and Nb: 0.02% to 0.05%, and
a balance consisting of Fe and inlpurities arid having a carbon
equivalent Ceq defined by the following Forniula (2) of 0.50% to 0.58%;
producing a raw pipe by piercing-rolling the heated steel material;
producing a sean~lesst eel pipe by rolling the raw pipe;
quenching the seamless steel pipe at a quenching temperature of an Acj point
or higher; and
tempering the seamless steel pipe after the quenching at a tempering
temperature of 660°C to 700°C.
Ceq = C + Mn16 + (Cr + Mo + V)/5 + (Ni + Cu)/15 ... (2)
here, into each of the synlbols of elements in tlie formula (2), the amount
(n~ass%)o f the corresponding element is substituted, and in the case where an element
corresponding to the symbol of the element is not contained, "0" is substituted into the
corresponding syn~bool f the element.
6. The method for producing a searnless steel pipe according to clainl 5, further
conlprising
acceleratedly cooling the sea~nlesst eel pipe at a cooling rate of 100 "C/min
or higher ~ ~ n tai tle nlperature of tlie seamless steel pipe reaches a temperature of an ArI
point or lower between the producing of the sean~lesss teel pipe and the quenching of
the seamless steel pipe,
~vllereinth e acceleratedly-cooled seamless steel pipe is quenched in the
q~~enchinogf the seamless steel pipe.
7. The method for producing a seanlless steel pipe according to clainl5 or 6,
wl~ereiath e seamless steel pipe further includes, as the chemical conlposition,
at least one selected from the group consisting of Cu: 1.0% or less and Ni: 1 .O% or less
in place of some of Fe.