TECHNICAL FIELD
[0001] The present invention relates to a seamless steel pipe. More particularly, it relates
to a seamless steel pipe for a line pipe that is used in sour environments containing
hydrogen sulfide (H
2S), which is a corrosive gas.
BACKGROUND ART
[0002] Crude oil and natural gas contain hydrogen sulfide and moisture. Such a wet hydrogen
sulfide environment is called a sour environment. Line pipes are used as pipelines
for transporting crude oil or natural gas produced from oil wells or gas wells. Therefore,
the line pipes are used in sour environments. A line pipe used in sour environments
has a problem of hydrogen embrittlement attributable to hydrogen absorbed into a steel
because of corrosion in environments containing hydrogen sulfide.
[0003] The hydrogen embrittlement includes sulfide stress cracking occurring on a steel
product under static external stresses and hydrogen induced cracking (hereinafter,
referred to as HIC) occurring in the interior of a steel product without external
stresses. The line pipe often has a problem of HIC. Therefore, a steel pipe for a
line pipe is especially required to have HIC resistance.
[0004] A steel pipe for a line pipe includes a welded steel pipe and a seamless steel pipe.
The welded steel pipe has a seam part (weld zone) extending in the axial direction
or in a spiral form. The steel plate used for the welded steel pipe has a center segregated
portion, which is produced at the time of continuous casting, in the center of plate
thickness, and the center segregated portion has high HIC susceptibility. Therefore,
as a steel pipe for a line pipe especially required to have HIC resistance, the seamless
steel pipe is preferably used.
[0005] In is well known that, in general, HIC occurs easily with the increase in steel strength.
International Application Publication No.
WO2005/075694 (Patent Document 1) proposes a seamless steel pipe having a high strength and excellent
HIC resistance.
[0006] Specifically, the steel product for a line pipe disclosed in Patent Document 1 has
a composition consisting, in mass%, of C: 0.03 to 0.15%, Si: 0.05 to 1.0%, Mn: 0.5
to 1.8%, P: 0.015% or less, S: 0.04% or less, O: 0.01% or less, N: 0.007% or less,
sol.Al: 0.01 to 0.1%, Ti: 0.024% or less, and Ca: 0.0003 to 0.02%, the balance being
Fe and impurities. Further, for the above-described steel product for a line pipe,
the size of TiN in the steel product is 30 µm or less. Patent Document 1 describes
that since TiN is fine, excellent HIC resistance can be attained.
PRIOR ART DOCUMENTS
PATENT DOCUMENTS
[0008] In the case where a high-strength seamless steel pipe is produced, usually, the strength
of the seamless steel pipe is enhanced by quenching and tempering treatment after
the hot-working. On the other hand, there is also a demand for low-strength seamless
steel pipe for a line pipe that does not need high strength and has yield strength
of less than 450 MPa. For such a low-strength seamless steel pipe, usually, the quenching
and tempering treatment is not performed and omitted.
[0009] Conventionally, as described above, it has been thought that if the strength is
low, HIC is less liable to occur. However, as the result of investigation conducted
by the present inventors, it was found, as a new finding, that in the case where not
only the strength is high but also the strength is low, a large number of blisters,
which are one type of HIC, and fine internal cracks may be generated.
[0010] The blister is a swell that is generated in the vicinity of the surface of a steel
product and extends in the axial direction of the steel product. In the HIC resistance
test (for example, NACE TM0284) specified by NACE, even in the high-strength seamless
steel pipe exhibiting excellent HIC resistance, the occurrence of blister may be observed.
However, the case where HIC (blister) is merely a crack in the vicinity of surface
does not lead to leakage or the like of a fluid being transmitted. Therefore, for
the conventional high-strength seamless steel pipe, the blister has not especially
posed a problem.
[0011] However, for a low-strength seamless steel pipe, and when a tensile stress is applied
thereto, a plurality of blisters and fine internal cracks in the steel may connect
with each other in the wall thickness direction of the seamless steel pipe, and stress
oriented hydrogen induced cracking (SOHIC) may occur.
[0012] Therefore, for the low-strength seamless steel pipe that is not subjected to quenching
and tempering treatment, it is desirable that the occurrence of blisters and fine
internal cracks be suppressed. For the low-strength steel product, since the cause
of the occurrence of fine internal cracks is the same as the cause of the occurrence
of blisters, it is only necessary to pay attention to the blisters and to suppress
the occurrence thereof.
SUMMARY OF INVENTION
[0013] An objective of the present invention is to provide a seamless steel pipe which is
not subjected to quenching and tempering treatment and in which, in the case where
the steel pipe is used in line pipe, which line pipe is used in sour environments,
the occurrence of blisters and fine internal cracks can be suppressed.
[0014] The seamless steel pipe according to an embodiment of the present invention is used
for a line pipe used in sour environments. This seamless steel pipe has a chemical
composition consisting, in mass%, of C: 0.08 to 0.24%, Si: 0.10 to 0.50%, Mn: 0.3
to 2.5%, P: 0.02% or less, S: 0.006% or less, Nb: 0.02 to 0.12%, Al: 0.005 to 0.100%,
Ca: 0.0003 to 0.0050%, N: 0.0100% or less, O: 0.0050% or less, Ti: 0 to 0.1%, V: 0
to 0.03%, Cr: 0 to 0.6%, Mo: 0 to 0.3%, Ni: 0 to 0.4%, Cu: 0 to 0.3%, and B: 0 to
0.005%, the balance being Fe and impurities, has a structure consisting of ferrite
and pearlite, and also has a yield strength of 350 to less than 450 MPa.
[0015] For the seamless steel pipe of this embodiment, quenching and tempering treatment
is not performed, and even if the strength is low, the occurrence of blisters and
fine internal cracks can be suppressed.
BRIEF DESCRIPTION OF DRAWINGS
[0016]
FIG. 1 is a graph showing the relationship between the yield strength of a seamless
steel pipe and the number of occurring blisters (per 20 cm2).
FIG. 2 is a photographic image of two surfaces (corresponding to the external surface
and internal surface of seamless steel pipe) of a coupon test specimen after the blister
number measuring test of Inventive Example of the present invention (steel A4, wall
thickness: 20 mm) in Examples.
FIG. 3 is a photographic image of two surfaces (corresponding to the external surface
and internal surface of seamless steel pipe) of a coupon test specimen after the blister
number measuring test of Comparative Example (steel B3, wall thickness: 20 mm) in
Examples.
DESCRIPTION OF EMBODIMENT
[0017] In the following, an embodiment of the present invention is explained in detail
with reference to the drawings.
[0018] The present inventors examined and studied the occurrence of blisters in a low-strength
seamless steel pipe that is not subjected to quenching and tempering treatment, and
obtained the findings described below.
[0019] A blister occurs through the mechanism described below. Hydrogen accumulates around
inclusions in the steel, and forms the starting point of hydrogen swell (blister).
If the steel product yields on account of the rise in hydrogen pressure at the starting
point, a crack is produced. If the crack is produced, the dislocation and hydrogen
further accumulate at the edges of crack, and the crack propagates. Thereby, a blister
is formed.
[0020] For the low-strength seamless steel pipe not subjected to quenching and tempering
treatment, the ratio of ferrite having a low yield strength is high. Therefore, it
is thought that the ferrite yields and thereby a blister is generated. For this reason,
in order to suppress the occurrence of blisters, it is effective to enhance the strength
of steel by strengthening the ferrite itself, or by increasing the ratio of pearlite
in the steel, or by the like means.
[0021] FIG. 1 is a graph showing the relationship between the yield strength of a seamless
steel pipe and the number of occurring blisters (per 20 cm
2). FIG. 1 was obtained by the method described below. Seamless steel pipes having
various chemical compositions were produced. At this time, each of the seamless steel
pipes subjected to hot working was allowed to cool or cooled at a cooling rate of
less than 5°C/s, and the quenching and tempering treatment was not performed.
[0022] For each of the produced seamless steel pipes, the yield strength was determined
in the later-described yield strength test. Further, the number of blisters (per 20
cm
2) occurring in each of the seamless steel pipes was determined in the later-described
blister number measuring test. Thereby, FIG. 1 was created.
[0023] Referring to FIG. 1, for the seamless steel pipes, until the yield strength increased
to 350 MPa, the number of blisters decreased remarkably with the increase in yield
strength. On the other hand, in the case where the yield strength was 350 MPa or more,
even if the yield strength increased, the number of blisters did not change so much.
[0024] In effect, the curve of FIG. 1 has an inflection point in the vicinity of the yield
strength of 350 MPa. Therefore, if the yield strength is 350 MPa or more, the number
of blisters can be kept small.
[0025] If the content of C is increased, the ratio of pearlite in the steel increases, and
thereby the yield strength of steel is enhanced. However, if the C content increases,
the weldability decreases. The seamless steel pipe for a line pipe is circumferentially
welded at the site at which the line pipe is laid. If the C content increases, the
toughness of the circumferentially welded joint part decreases, and also sulfide stress
cracking (SSC) is liable to occur. Therefore, it is difficult to excessively increase
the C content.
[0026] Also, the strength of seamless steel pipe can be enhanced by performing the quenching
and tempering treatment. However, the quenching and tempering treatment of a low-strength
seamless steel pipe leads to an increase in production cost.
[0027] Also, a welded steel pipe such as a UOE steel tube is subjected to cold working such
as pipe making and pipe expanding. Since the strength of welded steel pipe is enhanced
by cold working, the number of occurring blisters may possibly be reduced. However,
as described above, the seamless steel pipe is suitable as a line pipe used in hostile
sour environments. Therefore, it is difficult to raise the strength by means of cold
working or the like, and considering the production cost as well, the cold working
is unfavorable.
[0028] Accordingly, in this embodiment, the C content is increased, and further the Nb content
is increased. Specifically, the C content is set to 0.08 to 0.24%, and the Nb content
is set to 0.02 to 0.12%. In this case, even for the seamless steel pipe not subjected
to quenching and tempering treatment (for which the quenching and tempering treatment
is omitted), the strength thereof can be enhanced, and the occurrence of blisters
can be suppressed.
[0029] Preferably, the numerical value of Nb content (mass%) is made not less than the F1
value defined by Formula (1).
where, t is the wall thickness (unit: mm) of the seamless steel pipe.
[0030] The wall thickness of the seamless steel pipe for a line pipe used in sour environments
is, for example, 10 to 50 mm. If the wall thickness increases, the cooling condition
of the seamless steel pipe after hot-working also changes. The cooling rate decreases,
and the strength of steel tends to be degraded. If the Nb content is not less than
the F1 value of Formula (1), the strength of steel is 350 MPa or more, and the occurrence
of blisters can be suppressed.
[0031] The seamless steel pipe of this embodiment completed based on the above-described
findings is as described below.
[0032] The seamless steel pipe according to this embodiment is used for a line pipe used
in sour environments. This seamless steel pipe has a chemical composition consisting,
in mass%, of C: 0.08 to 0.24%, Si: 0.10 to 0.50%, Mn: 0.3 to 2.5%, P: 0.02% or less,
S: 0.006% or less, Nb: 0.02 to 0.12%, Al: 0.005 to 0.100%, Ca: 0.0003 to 0.0050%,
N: 0.0100% or less, O: 0.0050% or less, Ti: 0 to 0.1%, V: 0 to 0.03%, Cr: 0 to 0.6%,
Mo: 0 to 0.3%, Ni: 0 to 0.3%, Cu: 0 to 0.3%, and B: 0 to 0.005%, the balance being
Fe and impurities, has a structure consisting of ferrite and pearlite, and also has
a yield strength of 350 to less than 450 MPa.
[0033] Preferably, the numerical value of Nb content (mass%) is made not less than the F1
value defined by Formula (1).
where, t is the wall thickness (unit: mm) of the seamless steel pipe.
[0034] In the following, the seamless steel pipe of this embodiment is described in detail.
[Chemical composition]
[0035] The seamless steel pipe according to this embodiment has the chemical composition
described below.
C: 0.08 to 0.24%
[0036] Carbon (C) enhances the hardenability, and enhances the strength of steel. In the
case where heat treatment such as quenching and tempering is not performed after pipe
making as in the case of the seamless steel pipe of this embodiment, if the C content
is too low, the strength of steel decreases excessively. If the C content is too low,
furthermore, excellent HIC resistance is less liable to be attained. If the C content
is 0.08% or more, high-strength pearlite dispersedly precipitates in the steel. Therefore,
the yield of ferrite is restrained. For this reason, excellent HIC resistance is attained,
and the occurrence of blisters is suppressed. On the other hand, the seamless steel
pipe of this embodiment is circumferentially welded at the site as a line pipe. Therefore,
if the C content is too high, the heat affected zone (HAZ) of circumferential welding
hardens, and the SSC resistance decreases. Therefore, the C content is 0.08 to 0.24%.
The lower limit of the C content is preferably more than 0.08%, further preferably
0.10%. The upper limit of the C content is preferably less than 0.24%, further preferably
0.15%.
Si: 0.10 to 0.50%
[0037] Silicon (Si) deoxidizes a steel. If the Si content is too low, this effect cannot
be achieved. On the other hand, if the Si content is too high, the toughness of the
welding heat affected zone decreases. If the Si content is too high, furthermore,
the precipitation of ferrite, which is a softening phase, is promoted. Therefore,
the HIC resistance decreases, and blisters are liable to occur. For these reasons,
the Si content is 0.10 to 0.50%. The lower limit of the Si content is preferably more
than 0.10%, further preferably 0.15%, and still further preferably 0.20%. The upper
limit of the Si content is preferably less than 0.50%, further preferably 0.35%, and
still further preferably 0.30%.
Mn: 0.3 to 2.5%
[0038] Manganese (Mn) enhances the hardenability of steel, and enhances the strength of
steel. Furthermore, Mn enhances the toughness of steel. If the Mn content is too low,
these effects cannot be achieved. On the other hand, if the Mn content is too high,
HIC is liable to occur due to the hardening of steel caused by Mn segregation and
due to the formation of MnS. Therefore, the Mn content is 0.3 to 2.5%. The lower limit
of the Mn content is preferably more than 0.3%, further preferably 0.5%, and still
further preferably 0.8%. The upper limit of the Mn content is preferably less than
2.5%, further preferably 2.0%, and still further preferably 1.8%.
P: 0.02% or less
[0039] Phosphorus (P) is an impurity. Phosphorus decreases the toughness of steel. Therefore,
the P content is 0.02% or less. The P content is preferably less than 0.02%, further
preferably 0.01% or less. The P content is preferably as low as possible.
S: 0.006% or less
[0040] Sulfur (S) is an impurity. Sulfur forms MnS. The MnS serves as the starting point
of a blister. Therefore, the S content is preferably as low as possible. However,
the decreasing of the S content incurs high costs. For the seamless steel pipe of
this embodiment, in order to reduce the production cost, the S content should be 0.006%
or less. For the seamless steel pipe of this embodiment, even if 0.005% or more of
S is contained, excellent HIC resistance is exhibited and the occurrence of blisters
is suppressed if the C content and the Nb content are proper. However, the S content
is preferably as low as possible. The S content is preferably 0.003% or less.
Nb: 0.02 to 0.12%
[0041] Niobium (Nb) dissolves ferrite and enhances the strength of steel. Furthermore, Nb
combines with C and N to form carbo-nitrides, and performs grain refinement of steel
due to pinning hardening. By the grain refinement, the HIC resistance of steel is
enhanced. Furthermore, the grain refinement enhances the toughness of steel. In the
case where the seamless steel pipe is made from a steel material containing the above-described
range of C and the above-described range of Mn, and not containing Nb, and thereafter
heat treatment is not performed (that is, in the case where an as-rolled material,
for which quenching and tempering treatment is omitted, is produced), the yield strength
of the produced seamless steel pipe is about 250 MPa. However, if the above-described
range of Nb is contained, the yield strength of the seamless steel pipe rises to 350
MPa or more. Therefore, the occurrence of blisters is suppressed. If the Nb content
is too low, the above-described effects are not achieved. On the other hand, if the
Nb content is too high, coarse Nb carbo-nitrides are formed. A coarse Nb carbo-nitride
serves as the starting point of blister, and further the HIC resistance also decreases.
Therefore, the Nb content is 0.02 to 0.12%.
[0042] As described above, the wall thickness of the seamless steel pipe for a line pipe
used in sour environments is 10 to 50 mm. As the wall thickness of the seamless steel
pipe increases, the cooling rate of the seamless steel pipe becomes low, and the ferrite
grains become coarse, so that the yield strength of steel degrades. Therefore, the
lower limit of the Nb content is preferably not less than the F1 value (%) defined
by the following Formula (1).
where, t is the wall thickness (unit: mm) of the seamless steel pipe.
[0043] In the case where the seamless steel pipe satisfies Formula (1), not only in the
base metal but also in the welding heat affected zone formed by circumferential welding
between the seamless steel pipes, a sufficient yield strength can be assured, and
the occurrence of blisters is suppressed. The welding heat affected zone includes
a hardened zone in which the cooling rate after heating is high and which is hardened,
and a softened zone in which the cooling rate is low and which is softened by undergoing
thermal effects repeatedly. In the case where Formula (1) is satisfied, in the softened
zone, a sufficient yield strength is assured.
[0044] The lower limit of the Nb content is preferably more than 0.02%, further preferably
0.03%, and still further preferably 0.04%. The upper limit of the Nb content is preferably
less than 0.12%, further preferably 0.10%, and still further preferably 0.08%.
Al: 0.005 to 0.100%
[0045] Aluminum (Al) deoxidizes a steel. If the Al content is too low, this effect cannot
be achieved. On the other hand, if the Al content is too high, coarse cluster-form
alumina inclusion particles are formed when the circumferential welding is performed,
and thereby the toughness in the welding heat affected zone (HAZ) is decreased. Therefore,
the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably
more than 0.005%, further preferably 0.010%, and still further preferably 0.020%.
The upper limit of the Al content is preferably less than 0.100%, further preferably
0.060%, and still further preferably 0.040%. In this description, the Al content means
the content of acid-soluble Al (sol.Al).
Ca: 0.0003 to 0.0050%
[0046] Calcium (Ca) suppresses the clogging of a tundish nozzle when casting is performed.
Furthermore, Ca suppresses the formation of MnS, which serves as the starting point
of HIC, a blister, and a fine internal crack. Therefore, Ca suppresses the occurrence
of blisters and fine internal cracks. If the Ca content is too low, these effects
are insufficient. On the other hand, if the Ca content is too high, inclusions form
a cluster, and the toughness and HIC resistance of steel are decreased. Therefore,
the Ca content is 0.0003 to 0.0050%. The lower limit of the Ca content is preferably
more than 0.0003%, further preferably 0.0010%, and still further preferably 0.0015%.
The upper limit of the Ca content is preferably less than 0.0050%, further preferably
0.0040%, and still further preferably 0.0030%.
N: 0.0100% or less
[0047] Nitrogen (N) is an impurity. Nitrogen forms coarse nitrides, and decreases the toughness
and SSC resistance of steel. Therefore, the N content is preferably as low as possible.
For this reason, the N content is 0.0100% or less. The N content is preferably 0.0080%
or less, further preferably 0.0060% or less.
O: 0.0050% or less
[0048] Oxygen (O) is an impurity. Oxygen forms coarse oxides or a cluster of oxides, and
decreases the toughness and HIC resistance of steel. Therefore, the O content is preferably
as low as possible. For this reason, the O content is 0.0050% or less. The O content
is preferably 0.0040% or less, further preferably 0.0030% or less.
[0049] The balance of chemical composition of the seamless steel pipe of this embodiment
is Fe and impurities. The impurities referred to in this description mean elements
that are mixed from ore and scrap used as steel raw materials or from the environment
in the production process or the like.
[Concerning optional elements]
[0050] Furthermore, the seamless steel pipe of this embodiment may contain one or more types
of elements selected from a group consisting of Ti, V, Cr, Mo, Ni, Cu, and B. Any
of these elements enhances the strength of steel.
Ti: 0 to 0.1%
[0051] Titanium (Ti) is an optional element. Like Nb, Ti combines with C and N to form carbo-nitrides,
and performs grain refinement of steel due to pinning hardening. On the other hand,
if the Ti content is too high, this effect is saturated. Therefore, the Ti content
is 0 to 0.1%. The lower limit of the Ti content is preferably 0.002%, further preferably
0.005%. The upper limit of the Ti content is preferably less than 0.1%, further preferably
0.05%.
V: 0 to 0.03%
[0052] Vanadium (V) is an optional element. Vanadium forms carbides to strengthen a steel.
On the other hand, if the V content is too high, coarse carbides are formed, and SSC
is liable to occur. Therefore, the V content is 0 to 0.03%. The lower limit of the
V content is preferably 0.01%, further preferably 0.015%. The upper limit of the V
content is preferably less than 0.03%, further preferably 0.025%.
Cr: 0 to 0.6%
Mo: 0 to 0.3%
Ni: 0 to 0.4%
Cu: 0 to 0.3%
[0053] All of chromium (Cr), molybdenum (Mo), nickel (Ni), and copper (Cu) are optional
elements. Any of these elements enhances the hardenability of steel to strengthen
the steel, and enhances the HIC resistance for a low-strength steel. On the other
hand, if the content of any of these elements is too high, a hardened structure may
be formed locally, or uneven corrosion may be caused on the surface of steel. Therefore,
the Cr content is 0 to 0.6%, the Mo content is 0 to 0.3%, the Ni content is 0 to 0.4%,
and the Cu content is 0 to 0.3%. The lower limit of the Cr content is preferably 0.01%,
further preferably 0.05%. The lower limit of the Mo content is preferably 0.01%, further
preferably 0.05%. The lower limit of the Ni content is preferably 0.01%, further preferably
0.05%. The lower limit of the Cu content is preferably 0.01%, further preferably 0.05%.
The upper limit of the Cr content is preferably less than 0.6%, further preferably
0.5%. The upper limit of the Mo content is preferably less than 0.3%, further preferably
0.25%. The upper limit of the Ni content is preferably less than 0.4%, further preferably
0.3%, and still further preferably 0.25%. The upper limit of the Cu content is preferably
less than 0.3%, further preferably 0.25%.
[0054] Preferably, the total content of Cr, Mo, Ni and Cu satisfies the following Formula
(2).
where, each of the element symbols in the formula is the content (mass%) of the corresponding
element.
[0055] If Cr, Mo, Ni and Cu satisfy Formula (2), even for a large-thickness seamless steel
pipe, the yield strength is less than 450 MPa.
B: 0 to 0.005%
[0056] Boron (B) is an optional element. Boron enhances the hardenability of steel for a
low-strength seamless steel pipe, and enhances the HIC resistance for a low-strength
steel. On the other hand, if the B content is too high, the SSC resistance of steel
decreases. Therefore, the B content is set to 0 to 0.005%. The lower limit of the
B content is preferably 0.0001% or more, further preferably 0.0003%. The upper limit
of the B content is preferably less than 0.005%, further preferably 0.003%.
[Structure and strength]
[0057] The seamless steel pipe of this embodiment is not subjected to quenching and tempering
treatment after pipe making. That is to say, the seamless steel pipe of this embodiment
is a so-called as-rolled material for which the quenching and tempering treatment
is omitted. As described later, the seamless steel pipe having been made is allowed
to cool or is cooled at a cooling rate of less than 2°C/s. Therefore, the structure
of the seamless steel pipe of this embodiment consists of ferrite and pearlite. Most
part of the structure is ferrite, and the remaining part thereof is pearlite. The
structure referred to in this description means a matrix structure not containing
inclusions and precipitates.
[0058] Even if being cooled at so low a cooling rate as described above, the seamless steel
pipe of this embodiment has a yield strength of 350 MPa or more. In this description,
the yield strength means a 0.2% yield stress. The preferable yield strength of the
seamless steel pipe is 400 MPa or more. For the seamless steel pipe of this embodiment,
the yield strength is less than 450 MPa.
[Manufacturing method]
[0059] There is now explained one example of the manufacturing method for the seamless steel
pipe for a line pipe used in sour environments according to the embodiment.
[0060] A steel having the above-described chemical composition is melted, and is refined
by the well-known method. Successively, the molten steel is cast into a continuously
cast material by the continuous casting process. The continuously cast material is,
for example, a slab, a bloom, or a billet. Also, the molten steel may be made an ingot
by the ingot-making process.
[0061] The slab or bloom of the continuously cast material or the ingot is hot-worked to
produce a billet. For example, a slab, bloom or an ingot is rolled into a billet using
a blloming mill.
[0062] Successively, the produced billet is hot-rolled to produce a seamless steel pipe.
Specifically, the billet is heated in a heating furnace. If the heated billet is hot-rolled
in the state in which coarse Nb inclusions remain therein, at the cooling time after
hot rolling, the strengthening due to Nb cannot be attained sufficiently. In this
embodiment, therefore, the billet is heated to a further high temperature as compared
with the time of production of the ordinary seamless steel pipe. Specifically, at
the heating time, the billet is heated to a temperature of 1250°C or more.
[0063] The billet extracted from the heating furnace is hot-worked to produce a seamless
steel pipe. Specifically, piercing-rolling based on the Mannesmann process is performed
to produce a hollow shell. The produced hollow shell is further subjected to elongation
rolling and sizing by using a mandrel mill, a reducer, a sizing mill, or the like
to produce a seamless steel pipe.
[0064] The produced seamless steel pipe is cooled. At this time, the cooling rate in a high-temperature
region of 500°C or more, in which Nb carbo-nitrides precipitate, is preferably higher.
Therefore, until the temperature of seamless steel pipe decreases to 500°C, the seamless
steel pipe is cooled at a cooling rate of 0.5 to 5°C/s, and subsequently, it is cooled
at a cooling rate of less than 2°C. The cooling at a cooling rate of less than 2°C/s
includes the allowing to cool.
[0065] The cooling rate can be controlled, for example, by regulating the spacing between
the adjacent seamless steel pipes at the time of allowing to cool. For example, until
the temperature of seamless steel pipe decreases to 500°C, the spacing between the
adjacent seamless steel pipes is made distance D1, and at a temperature of 500°C or
less, the spacing is regulated to distance D2, which is shorter than distance D1.
Thereby, a gentle two-stage cooling rate can be realized.
[0066] In the above-described production method, the seamless steel pipe after hot-working
is not subjected to quenching and tempering treatment.
[Number of blisters]
[0067] For the seamless steel pipe produced by the manufacturing method, the occurrence
of blisters can be suppressed. In particular, in the case where the Nb content (%)
is not less than the F1 value defined by Formula (1), the number of blisters in the
surface is less than 10 per 20 cm
2. The number of blisters can be determined by the blister number measuring test described
below.
[Blister number measuring test]
[0068] Based on NACE TM0284-2011 specified by NACE (National Association of Corrosion Engineers)
International, a HIC test using a wet hydrogen sulfide environment (sour environment)
is conducted. Specifically, a coupon test specimen measuring plate thickness x 20
mm wide x 100 mm long (length in the axial direction of seamless steel pipe) is sampled.
This coupon test specimen has a pair of surfaces corresponding to the external surface
and internal surface of the seamless steel pipe.
[0069] In conformity to NACE TM0284, there is prepared a 25°C test bath in which 100% H
2S gas is saturated in (5%NaCl + 0.5%CH
3COOH) aqueous solution under the atmospheric pressure. The coupon test specimen is
immersed in the test bath for 96 hours. After the 96-hour immersion, the surfaces
(two surfaces each measuring 20 mm wide x 100 mm long corresponding to the external
surface and internal surface of seamless steel pipe) of the coupon test specimen are
observed visually. Then, the total number of blisters occurring in the surfaces is
counted to determine the number of blisters (per 20 cm
2).
[0070] As described above, for the seamless steel pipe according to this embodiment, by
enhancing the yield strength to 350 MPa or more by means of C and Nb, the occurrence
of blisters can be suppressed. Therefore, the HIC resistance is excellent, and furthermore,
when a tensile stress is applied, SOHIC is less liable to occur.
EXAMPLES
[0071] Ingots of steels A1 to A15 and B1 to B6 given in Table 1 were produced.
[0072] [Table 1]
Table 1
Steel |
Chemical composition (unit: mass%, balance being Fe and impurities) |
F2 |
C |
Si |
Mn |
P |
S |
Nb |
Al |
Ca |
Ti |
V |
Cr |
Mo |
Ni |
Cu |
B |
N |
O |
|
A1 |
0.20 |
0.20 |
1.00 |
0.01 |
0.005 |
0.030 |
0.037 |
0.0017 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0020 |
- |
A2 |
0.20 |
0.20 |
0.99 |
0.01 |
0.005 |
0.069 |
0.034 |
0.0018 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0020 |
- |
A3 |
0.10 |
0.19 |
0.98 |
0.01 |
0.004 |
0.050 |
0.034 |
0.0025 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0020 |
- |
A4 |
0.10 |
0.19 |
0.98 |
0.01 |
0.004 |
0.098 |
0.032 |
0.0023 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0010 |
- |
A5 |
0.11 |
0.20 |
0.99 |
0.01 |
0.006 |
0.044 |
0.038 |
0.0016 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0020 |
- |
A6 |
0.13 |
0.28 |
1.19 |
0.01 |
0.005 |
0.045 |
0.034 |
0.0018 |
0.013 |
- |
- |
- |
- |
- |
0.001 |
0.0010 |
0.0030 |
- |
A7 |
0.12 |
0.30 |
1.23 |
0.01 |
0.006 |
0.045 |
0.035 |
0.0015 |
- |
0.02 |
- |
- |
- |
- |
- |
0.0020 |
0.0020 |
- |
A8 |
0.12 |
0.30 |
1.21 |
0.01 |
0.006 |
0.046 |
0.041 |
0.0020 |
- |
- |
0.23 |
0.18 |
- |
- |
- |
0.0020 |
0.0020 |
0.082 |
A9 |
0.12 |
0.29 |
1.20 |
0.01 |
0.006 |
0.044 |
0.033 |
0.0017 |
- |
- |
- |
- |
0.19 |
0.19 |
- |
0.0020 |
0.0030 |
0.025 |
A10 |
0.09 |
0.11 |
0.61 |
0.01 |
0.004 |
0.021 |
0.031 |
0.0023 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0030 |
- |
A11 |
0.10 |
0.13 |
0.60 |
0.01 |
0.004 |
0.033 |
0.032 |
0.0019 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0030 |
- |
A12 |
0.20 |
0.21 |
1.01 |
0.01 |
0.004 |
0.049 |
0.035 |
0.0020 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0030 |
- |
A13 |
0.11 |
0.19 |
1.00 |
0.01 |
0.005 |
0.050 |
0.032 |
0.0015 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0030 |
- |
A14 |
0.11 |
0.21 |
1.29 |
0.01 |
0.003 |
0.043 |
0.038 |
0.0016 |
0.019 |
- |
0.11 |
0.01 |
0.36 |
0.24 |
0.001 |
0.0040 |
0.0020 |
0.064 |
A15 |
0.12 |
0.18 |
1.33 |
0.01 |
0.004 |
0.045 |
0.034 |
0.0021 |
0.011 |
- |
0.24 |
0.02 |
0.34 |
0.25 |
0.001 |
0.0010 |
0.0020 |
0.091 |
B1 |
0.19 |
0.20 |
0.98 |
0.01 |
0.006 |
- |
0.033 |
0.0020 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0020 |
- |
B2 |
0.20 |
0.19 |
1.00 |
0.01 |
0.005 |
0.011 |
0.036 |
0.0019 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0030 |
- |
B3 |
0.10 |
0.19 |
1.00 |
0.01 |
0.005 |
- |
0.036 |
0.0025 |
- |
- |
- |
- |
- |
- |
- |
0.0010 |
0.0010 |
- |
B4 |
0.01 |
0.19 |
0.99 |
0.01 |
0.005 |
0.045 |
0.045 |
0.0013 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0030 |
- |
B5 |
0.05 |
0.18 |
1.01 |
0.01 |
0.005 |
0.046 |
0.039 |
0.0015 |
- |
- |
- |
- |
- |
- |
- |
0.0020 |
0.0020 |
- |
B6 |
0.11 |
0.21 |
1.31 |
0.01 |
0.005 |
0.046 |
0.035 |
0.0022 |
0.013 |
- |
0.42 |
0.02 |
0.34 |
0.23 |
0.001 |
0.0020 |
0.0030 |
0.126 |
[0073] The symbol "-" in Table 1 indicates that the content is substantially "0"% (impurity
level). The F2 value in Table 1 is defined as described below.
In effect, F2 is the left side of Formula (2).
[0074] Referring to Table 1, the chemical compositions of steels A1 to A15 were within the
range of chemical composition of the seamless steel pipe of this embodiment. On the
other hand, steels B1 and B3 did not contain Nb, and the Nb content of steel B2 was
less than the lower limit of the Nb content of the seamless steel pipe of this embodiment.
The C content of each of steels B4 and B5 was less than the lower limit of the C content
of the seamless steel pipe of this embodiment. The F2 value of steel B6 did not satisfy
Formula (2).
[0075] The ingots of the steels were hot-forged to produce a plurality of billets of the
steels. After each of the billets had been heated at the heating temperature given
in Table 2, the billet is piercing-rolled by using a piercing mill (piercer) to produce
a seamless steel pipe. At this time, for each steel, three kinds of seamless steel
pipes having wall thicknesses of 12.7 mm, 25.4 mm, and 38.1 mm were produced. Each
of the produced seamless steel pipes was cooled at the first cooling rate given in
Table 2 until the temperature of seamless steel pipe decreases to 500°C, and was cooled
at the second cooling rate subsequently.
[0076] [Table 2]
Table 2
Steel |
Heating temperature (°C) |
No content (%) |
Wall thickness t=12.7 mm |
Wall thickness t=25.4 mm |
Wall thickness t=38.1 mm |
F1 value |
1st cooling rate |
2nd cooling rate |
YS (MPa) |
Number of blisters |
F1 value |
1st cooling rate |
2nd cooling rate |
YS (MPa) |
Number of blisters |
F1 value |
1st cooling rate |
2nd cooling rate |
YS (MPa) |
Number of blisters |
A1 |
1260 |
0.030 |
0.018 |
3.0 |
1.0 |
411 |
1 |
0.030 |
1.5 |
0.5 |
382 |
0 |
0.043 |
1.0 |
Less than 0.5 |
349 |
10 |
A2 |
1260 |
0.069 |
0.018 |
3.0 |
1.0 |
442 |
0 |
0.030 |
1.5 |
0.5 |
419 |
1 |
0.043 |
1.0 |
Less than 0.5 |
389 |
0 |
A3 |
1280 |
0.050 |
0.018 |
3.5 |
1.5 |
424 |
1 |
0.030 |
2.0 |
1.0 |
402 |
0 |
0.043 |
1.5 |
0.5 |
383 |
0 |
A4 |
1280 |
0.098 |
0.018 |
3.5 |
1.5 |
438 |
0 |
0.030 |
2.0 |
1.0 |
426 |
0 |
0.043 |
1.5 |
0.5 |
402 |
0 |
A5 |
1250 |
0.044 |
0.018 |
2.0 |
1.0 |
418 |
0 |
0.030 |
1.0 |
0.5 |
399 |
1 |
0.043 |
1.0 |
Less than 0.5 |
381 |
0 |
A6 |
1250 |
0.045 |
0.018 |
2.0 |
1.0 |
433 |
0 |
0.030 |
1.0 |
0.5 |
415 |
0 |
0.043 |
1.0 |
Less than 0.5 |
398 |
0 |
A7 |
1250 |
0.045 |
0.018 |
2.0 |
1.0 |
421 |
2 |
0.030 |
1.0 |
0.5 |
410 |
3 |
0.043 |
1.0 |
Less than 0.5 |
388 |
1 |
A8 |
1250 |
0.046 |
0.018 |
2.0 |
1.0 |
435 |
1 |
0.030 |
1.0 |
0.5 |
412 |
2 |
0.043 |
1.0 |
Less than 0.5 |
398 |
1 |
A9 |
1250 |
0.044 |
0.018 |
2.0 |
1.0 |
446 |
0 |
0.030 |
1.0 |
0.5 |
423 |
0 |
0.043 |
1.0 |
Less than 0.5 |
405 |
0 |
A10 |
1250 |
0.021 |
0.018 |
2.0 |
1.0 |
372 |
0 |
0.030 |
1.0 |
0.5 |
348 |
10 |
0.043 |
1.0 |
Less than 0.5 |
329 |
18 |
A11 |
1250 |
0.033 |
0.018 |
2.0 |
1.0 |
392 |
0 |
0.030 |
1.0 |
0.5 |
368 |
0 |
0.043 |
1.0 |
Less than 0.5 |
344 |
11 |
A12 |
1160 |
0.049 |
0.018 |
3.0 |
1.0 |
331 |
32 |
0.030 |
1.5 |
0.5 |
318 |
41 |
0.043 |
1.0 |
Less than 0.5 |
299 |
39 |
A13 |
1260 |
0.05 |
0.018 |
Less than 0.5 |
Less than 0.5 |
349 |
12 |
0.030 |
Less than 0.5 |
Less than 0.5 |
340 |
15 |
0.043 |
Less than 0.5 |
Less than 0.5 |
328 |
19 |
A14 |
1250 |
0.043 |
0.018 |
2.0 |
1.0 |
423 |
0 |
0.030 |
1.0 |
0.5 |
408 |
1 |
0.043 |
1.0 |
Less than 0.5 |
391 |
1 |
A15 |
1250 |
0.045 |
0.018 |
2.0 |
1.0 |
448 |
0 |
0.030 |
1.0 |
0.5 |
438 |
0 |
0.043 |
1.0 |
Less than 0.5 |
424 |
0 |
B1 |
1260 |
- |
0.018 |
3.0 |
1.0 |
300 |
33 |
0.030 |
1.5 |
0.5 |
269 |
62 |
0.043 |
1.0 |
Less than 0.5 |
253 |
91 |
B2 |
1260 |
0.011 |
0.018 |
3.0 |
1.0 |
349 |
11 |
0.030 |
1.5 |
0.5 |
340 |
13 |
0.043 |
1.0 |
Less than 0.5 |
325 |
25 |
B3 |
1250 |
- |
0.018 |
3.5 |
1.5 |
286 |
24 |
0.030 |
2.0 |
1.0 |
260 |
78 |
0.043 |
1.5 |
0.5 |
238 |
112 |
B4 |
1250 |
0.045 |
0.018 |
2.0 |
1.0 |
321 |
38 |
0.030 |
1.0 |
0.5 |
298 |
42 |
0.043 |
1.0 |
Less than 0.5 |
278 |
58 |
B5 |
1250 |
0.046 |
0.018 |
2.0 |
1.0 |
348 |
10 |
0.030 |
1.0 |
0.5 |
324 |
28 |
0.043 |
1.0 |
Less than 0.5 |
311 |
32 |
B6 |
1250 |
0.046 |
0.018 |
2.0 |
1.0 |
466 |
0 |
0.030 |
1.0 |
0.5 |
459 |
0 |
0.043 |
1.0 |
Less than 0.5 |
452 |
0 |
[Micro-structure observing test]
[0077] Each of the seamless steel pipes having three kinds of wall thicknesses that had
been produced for each steel was subjected to a micro-structure observing test. In
the transverse cross section (the surface perpendicular to the axial direction of
seamless steel pipe) of each seamless steel pipe, the wall thickness central portion
was etched by using nital or the like. One optional visual field (visual field area:
40,000 µm
2) of the etched wall thickness central portion was observed. For the observation,
an optical microscope having a magnification of x500 was used.
[0078] As the result of the micro-structure observing test, each of all the seamless steel
pipes had a structure consisting of ferrite and pearlite.
[Yield strength test]
[0079] From each of the three kinds of seamless steel pipes of each steel, a round-bar tensile
test specimen having a parallel part measuring 6 mm in outside diameter and 40 mm
in length was sampled. The parallel part was parallel to the axial direction of the
seamless steel pipe. By using the sampled round-bar tensile test specimen, a tension
test was conducted at normal temperature (25°C) to determine the yield strength YS
(0.2% yield stress) (MPa).
[Blister number measuring test]
[0080] Each of the three kinds of seamless steel pipes of each steel was subjected to the
above-described blister number measuring test to determine the number of blisters.
[Test results]
[0081] Table 2 gives the test results. FIG. 2 is a photographic image of two surfaces (corresponding
to the external surface and internal surface of seamless steel pipe) of the coupon
test specimen after the blister number measuring test of steel A4 (wall thickness:
20 mm), and FIG. 3 is a photographic image of two surfaces of the coupon test specimen
after the blister number measuring test of steel B3 (wall thickness: 20 mm). In FIGS.
2 and 3, the upper surface corresponds to the external surface of seamless steel pipe,
and the lower surface corresponds to the internal surface of seamless steel pipe.
[0082] Referring to Table 2, the chemical compositions of steels A1 to A11, A14 and A15
were proper. Therefore, each of the 12.7-mm seamless steel pipes of these steels,
which had a wall thickness of 15 mm or less, had a yield strength YS of 350 to less
than 450 MPa. For this reason, as shown in FIG. 2, the occurrence of blisters in the
surface was suppressed, and the number of blisters was less than 10 per 20 cm
2.
[0083] Furthermore, for the 25.4-mm seamless steel pipes, the Nb content of each of steels
A1 to A9, A11, A14 and A15 was not less than the F1 value defined by Formula (1).
Therefore, even for the seamless steel pipes having a wall thickness of more than
15 mm, a yield strength of 350 to less than 450 MPa was attained, and the number of
blisters was less than 10 per 20 cm
2.
[0084] Furthermore, for the 38.1-mm seamless steel pipes, the Nb content of each of steels
A2 to A9, A14 and A15 was not less than the F1 value. Therefore, even for the seamless
steel pipes having a wall thickness of more than 35 mm, a yield strength of 350 to
less than 450 MPa was attained, and the number of blisters was less than 10 per 20
cm
2.
[0085] On the other hand, although the chemical compositions of steels A12 and A13 were
proper, steel A12 had a too low heating temperature and steel A13 had a too low first
cooling rate. Therefore, the yield strength YS was less than 350 MPa, and for the
seamless steel pipes having any wall thickness, the number of blisters was not less
than 10 per 20 cm
2.
[0086] On the other hand, the Nb content of each of steels B1 to B3 was too low. Therefore,
even for the seamless steel pipes each having a wall thickness of less than 20mm,
the yield strength was less than 350 MPa. As the result, many blisters occurred in
the surface as shown in FIG. 3, and the number of blisters was not less than 10 per
20 cm
2.
[0087] Also, the C content of each of steels B4 and B5 was too low. Therefore, even for
the seamless steel pipes each having a wall thickness of less than 20mm, the yield
strength was less than 350 MPa, and the number of blisters was not less than 10 per
20 cm
2.
[0088] The F2 value of steel B6 did not satisfy Formula (2). Therefore, the yield strength
of steel B6 was more than 450 MPa.
[0089] The above is an explanation of the embodiment of the present invention, and the embodiment
is merely an illustration for carrying out the present invention. Therefore, the present
invention is not limited to the embodiment, and the embodiment can be carried out
by being modified or changed as appropriate without departing from the spirit and
scope of the present invention.