The present invention relates to a seamless steel
pipe and a method for producing the same.
Background Art
[00021
Oil wells and gas wells on the land and in the
shallow sea are drying up in recent years. For that
reason, undersea oil wells and undersea gas wells
(hereinafter, undersea oil well and undersea gas well are
collectively referred to as "undersea oil wellu) of a
deep sea have been developed.
[00031
In the undersea oil well, a flow line and a riser
are arranged from a well mouth arranged on the seafloor
to an off-shore platform. A production fluid containing
crude oil or natural gas is carried from the undersea oil
well to the platform through the flow line or the riser.
The flow line means a line pipe laid along the
geographical features of the surface of the earth or the
undersea surface. The riser means a line pipe arranged
in such a way as to rise up in the direction of the
platform (in other words, in the upward direction) from
the undersea surface.
[0004]
The production fluid flowing in a steel pipe
constructing these line pipes (flow line and riser) has a
high pressure. Further, in the case where an operation
is stopped, a sea water pressure is applied to the flow
line and the riser from the outside. Still further, a
repeated stress is also applied to the flow line and the
riser by the wave and the sea current. For that reason,
a high strength steel pipe having a thick wall has been
required for the line pipe such as the flow line and the
riser.
[0005]
However, when a steel pipe has its wall thickness
and strength increased, the steel pipe has its toughness
reduced and hence easily causes brittle fracture. For
that reason, the high strength steel pipe having a thick
wall is required to have excellent toughness.
[00061
In this way, a steel pipe having a thick wall for an
undersea pipeline is required to have high strength and
high toughness. A welded steel pipe has a welded portion
(seam portion) along a lengthwise direction and hence the
welded portion is lower in toughness than a base material.
For that reason, a seamless steel pipe is more suitable
for the undersea pipeline than the welded steel pipe.
[00071
JP9-287028A (Patent Literature 1) proposes a method
for producing a seamless steel pipe. In Patent
Literature 1, a seamless steel pipe produced by hot
rolling is cooled to a temperature not more than an Ar3
transformation point at a cooling rate not less than
80°C/min and then is quenched and tempered. It is
described that the seamless steel pipe produced by the
producing method of Patent Literature 1 has crystal
grains fined and hence has high strength and high
toughness.
Disclosure of the Invention
[00081
However, in the case where, for example, a thickwalled
seamless steel pipe having a wall thickness as
large as not less than 50 mm is produced by the producing
method disclosed in Patent Literature 1, the seamless
steel pipe might be varied and reduced in toughness. In
order to increase strength, Nb is effective, but in the
case where a seamless steel pipe contains Nb, the
seamless steel pipe might be further reduced in toughness
in a near surface portion and the scatter of toughness
between a near surface portion and a central portion of a
wall thickness might become large.
[00091
An objective of the present invention is to provide
a seamless steel pipe having high strength and high
toughness even if having a thick wall.
[OOlO]
A seamless steel pipe according to the present
embodiment consists of: in mass%, C: 0.03 to 0.08%, Si:
not more than 0.25%, Mn: 0.3 to 2.0%, P: not more than
0.05%, S: not more than 0.005%, Al: 0.001 to 0.10%, Cr:
0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 0.8%, N:
0.002 to 0.008%, Ca: 0.0005 to 0.005%, and Nb: 0.01 to
0.1%, the balance being Fe and impurities, and has a wall
thickness not less than 50 mm. Further, in a cross
section perpendicular to an axial direction of the
seamless steel pipe, an average crystal grain size of
prior austenite grains in a near surface portion is less
than 80 p, the near surface portion being a 500 p x 500
p area centered at a position of a depth of 2 mm from a
surface, and a difference between the average crystal
grain size of the prior austenite grains in the near
surface portion and an average crystal grain size in a
center portion of a wall thickness in the section is less
than 50 p, the center portion being a 500 p X 500 p
area centered at a center position of the wall thickness
of the seamless steel pipe.
[00111
Further, a seamless steel pipe according to the
present embodiment may contain Ti: not more than 0.010%,
in place of a part of Fe. Still further, a seamless
steel pipe according to the present embodiment may
contain at least one selected from a group consisting of:
V: not more than 0.1%, and Cu: not more than 1.0%, in
place of a part of Fe.
[00121
A method for producing a seamless steel pipe
according to the present embodiment includes: a step of
heating a starting material consisting of: in mass%, C:
0.03 to 0.08%, Si: not more than 0.25%, M b : 0.3 to 2.0%,
P: not more than 0.05%, S: not more than 0.005%, Al:
0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo:
0.02 to 0.8%, N: 0.002 to 0.008%, Ca: 0.0005 to 0.005%,
and Nb: 0.01 to 0.1%, the balance being Fe and
impurities; a step of hot working the heated starting
material to thereby produce a material pipe; a step of
cooling the hot worked material pipe in an accelerated
manner; a step of reheating the material pipe cooled in
the accelerated manner and then of soaking the material
pipe at 990 to llOO°C; a step of quickly cooling the
soaked material pipe to thereby quench the material pipe;
and a step of tempering the quenched material pipe.
[00131
The seamless steel pipe according to the present
embodiment has excellent strength and excellent toughness
even if it has a thick wall.
Brief Description of Drawings
[00141
[Figure 11 Figure 1 is a graph showing a relationship
between an average crystal grain size and a reheating
temperature when steel having a chemical composition of
the present embodiment is heated to various temperatures
at 5OC/min after the steel is quenched.
[Figure 21 Figure 2 is a layout diagram showing one
example of a production equipment of a seamless steel
pipe according to the present embodiment.
[Figure 31 Figure 3 is a flow diagram showing one example
of a method for producing a seamless steel pipe according
to the present embodiment.
Best Mode for Carrying Out the Invention
[0015]
Hereinafter, an embodiment of the present invention
will be described.in detail with reference to the
drawings. Same parts or equivalent parts in the drawings
will be denoted by the same reference symbols and their
descriptions will be not repeatedly made. Hereinafter, a
percent ( % ) relating to an element means "a mass%".
[00161
The present inventors have completed a seamless
steel pipe according to the present embodiment on the
basis of the following findings.
[00171
In a seamless steel pipe having a thick wall not
less than 50 mm, Nb combines with carbon to form NbC,
thereby increasing the strength of the steel. Hence, in
order to produce a seamless steel pipe having a thick
wall and having high strength, especially, a strength of
not less than X 80 class (yield strength is not less than
551 MPa) specified in API (American Petroleum Institute)
standards, the seamless steel pipe preferably contains Nb.
[0018]
However, in the case where a seamless steel pipe
having a thick wall and containing Nb is produced, there
are cases where: a prior austenite grain (hereinafter
referred to as "prior y grain") after quenching and
tempering might be not fined; and a prior y grain in a
near surface portion, in particular, might become a
coarse grain. Here, the near surface portion means an
area of a depth of 1 to 3 mm from the surface of the
seamless steel pipe.
[00191
The reason why the prior y grain becomes the coarse
grain in the seamless steel pipe having a thick wall and
containing Nb (hereinafter referred to as "Nb thickwalled
pipe") is because NbC inhibits the growth of a
granular reversely transformed y grain when the y grain
is reheated. This point will be described in detail in
the following.
[00201
When a hot-worked Nb thick-walled pipe is quenched
and then reheated, the Nb thick-walled pipe which is
being reheated has acicular reversely transformed y
grains and granular reversely transformed y grains
produced in the near surface portion thereof. Here, the
reversely transformed y means y produced during heating.
Then, the crystal orientations of the acicular reversely
transformed y grains and the crystal orientations of the
granular reversely transformed y grains were measured by
an EBSD (Electron Back Scattering Diffraction) method.
As a result, it was found that the crystal orientations
of a plurality of acicular reversely transformed y grains
produced in the same prior y grain were equal to each
other. On the other hand, the crystal orientations of
the granular reversely transformed y grains were
different from those of the acicular reversely
transf ormed y grains.
[00211
From this result, the followings are presumed. The
plurality of acicular reversely transformed y grains have
the same crystal orientations and hence easily grow and
bond to each other. In this case, coarse y grains are
formed. On the other hand, the granular reversely
transformed y grains have crystal orientations different
from the acicular reversely transformed y grains and
hence are hard to bond to the acicular reversely
transformed y grains. For this reason, the more the
granular reversely transformed y grains are produced, the
more the fine y grains are formed.
[0022]
NbC develops a pinning effect in the reheating
process and inhibits the growth of the granular reversely
transformed y grains. For this reason, in the reheating
process, the rate of the acicular reversely transformed y
grains increases. As a result, the prior y grains easily
become coarse grains in the near surface portion of the
Nb thick-walled pipe after tempering.
[00231
When the prior y grains become coarse grains,
toughness becomes lower. Further, in the case of a
thick-walled pipe, variations in the grain size of the
prior y grain are caused in a center portion of a wall
thickness and in the near surface portion, which hence
causes also variations in toughness.
[0024]
Hence, in the Nb thick-walled pipe, by fining the
grain sizes of the prior y grains in the central portion
of the wall thickness and in the near surface portion to
thereby reduce the variations, the thick-walled pipe can
have excellent strength and excellent toughness.
Specifically, an average crystal grain size of the prior
y grains in the near surface portion is made less than 80
pm and a difference in the average crystal grain size of
the prior y grains between in the near surface portion
and in the central portion of the wall thickness is made
less than 50 p, which hence can make the thick-walled
pipe have excellent yield strength and excellent
toughness and can prevent variations in the toughness in
the near surface portion and in the central portion of
the wall thickness.
[0025]
In order to inhibit the prior y grains in the near
surface portion of the Nb thick-walled pipe from being
coarsened, a heating temperature when the Nb thick-walled
pipe is reheated is made 990°C to llOO°C. Figure 1 is a
graph showing a relationship between an average crystal
grain size and a reheating temperature when steel having
a chemical composition of the present embodiment, which
contains Nb, is heated to various temperatures after
quenching. Figure 1 was acquired by the following method
[0026]
A rectangular parallelepiped block was acquired from
steel whose chemical composition is within the chemical
composition described above by the same producing method
as Example 1 which will be described later. A small
cylindrical test specimen (diameter: 3 mm, height: 10 mm)
for a heat treatment was acquired from the block. The
test specimen was soaked at 1200°C for 5 minutes and then
was quickly cooled to a room temperature and subsequently
was heated to a temperature between 950°C and 1200°C. The
test specimen was soaked at each temperature for 5
minutes and then was again quickly cooled to the room
temperature. A heating rate was 5OC/min corresponding to
a heating rate when the thick-walled steel pipe is heated
in a heating furnace.
[00271
An average grain size (p)of the prior y grains
after the heat treatment was obtained based on a method
for measuring an average crystal grain size, which will
be described later. Figure 1 was acquired by the use of
the obtained average crystal grain sizes.
[0028]
Referring to Figure 1, as a reheating temperature
became higher from 950°C, the average crystal grain size
became smaller. In a general technical common knowledge,
when a heating temperature is increased, a crystal grain
grows to become coarse. Hence, in the Nb thick-walled
pipe was caused a phenomenon different from the
conventional technical common knowledge. In the case
where the reheating temperature was 990°C to llOO°C, the
average crystal grain size of the prior y grains was held
small, whereby the prior y grains were fined. On the
other hand, when the reheating temperature was higher
than llOO°C, the prior y grains were again coarsened.
[00291
The phenomenon shown in Figure 1 is specific to the
~b thick-walled pipe and the following reason can be
presumed. In the case where the reheating temperature is
less than 990°C, NbC inhibits the growth of the granular
reversely transformed y grains in the reheating process.
For this reason, the rate of the acicular reversely
transformed y grains is increased and hence the acicular
reversely transformed y grains bond to each other to
thereby coarsen the prior y grains after tempering.
[00301
On the other hand, in the case where the reheating
temperature is 990 to llOO°C, NbC is dissolved in the
reheating process. For that reason, the granular
reversely transformed y grains are produced and grown.
AS a result, the prior y grains after tempering become
fine. Further, a difference in the average crystal grain
size of the prior y grains between in the near surface
portion and in the central portion of the wall thickness
becomes small, which produces excellent toughness.
Further, the dissolved NbC is again precipitated finely
after tempering. For that reason, high strength can be
also acquired.
[0031]
When the reheating temperature is further increased
and is higher than llOO°C, the fined y grains are grown.
AS a result, the prior y grains after tempering are
coarsened.
[00321
A seamless steel pipe according to the present
embodiment was completed on the basis of the findings
described above. The details of the seamless steel pipe
according to the present embodiment will be described in
the following.
LOO331
[Chemical composition]
The chemical composition of the seamless steel pipe
according to the present embodiment contains the
following elements.
[00341
Carbon (C) increases the strength of steel. However,
when a C content is excessively high, the toughness of
the steel is decreased. Hence, the C content is 0.03 to
0.08%.
[0035]
Si: not more than 0.25%
Silicon (Si) not only is added for the purpose of
deoxidizing the steel but also contributes to increasing
strength and increasing resistance to softening when
tempering. However, when a Si content is excessively
high, the toughness of the steel is decreased. Hence,
the Si content is not more than 0.25%. A preferable
lower limit of the Si content is 0.05%.
[0036]
Mn: 0.3 to 2.0%
Manganese (Mn) increases the hardenability of the
steel. For that reason, Mn can increase the strength of
the central portion of the wall thickness. However, when
a Mn content is excessively high, the toughness of the
steel is decreased. Hence, the Mn content is 0.3 to 2.0%.
A preferable lower limit of the Mn content is more than
0.3%, more preferably 0.5%, and still more preferably
1.0%.
[0037]
P: not more than 0.05%
Phosphate (P) is an impurity. P decreases the
toughness of the steel. Hence, it is preferable that a P
content be lower. The P content is not more than 0.05%.
A preferable P content is not more than 0.028, and more
preferably not more than 0.01%.
[0038]
S: not more than 0.005%
Sulfur (S) is an impurity. S decreases the
toughness of the steel. Hence, it is preferable that a S
content be lower. The S content is not more than 0.005%.
A preferable S content is not more than 0.0038, and more
preferably not more than 0.001%.
[0039]
Al: 0.001 to 0.10%
An aluminum (Al) content in the present invention
means a content of acid-soluble A1 (so-called Sol. Al).
A1 deoxidizes the steel. However, when an A1 content is
excessively high, a cluster-like inclusion is formed to
thereby decrease the toughness of the steel. Hence, the
A1 content is 0.001 to 0.10%. A preferable lower limit
of the A1 content is higher than 0.001%, and more
preferably 0.01%. A preferable upper limit of the A1
content is less than 0.10%, and more preferably 0.07%.
[0040]
Cr: 0.02 to 1.0%
Chrome (Cr) increases the hardenability of the steel
and increases the strength of the steel. However, when a
Cr content is excessively high, the toughness of the
steel is decreased. Hence, the Cr content is 0.02 to
1.0%. A preferable lower limit of the Cr content is
higher than 0.02%, and more preferably 0.1%. A
preferable upper limit of the Cr content is less than
1.0%, and more preferably 0.8%.
[00411
Ni: 0.02 to 1.0%
Nickel (Ni) increases the hardenability of the steel
and increases the strength of the steel. However, when a
Ni content is excessively high, the effect of Ni is
saturated. Hence, the Ni content is 0.02 to 1.0%. A
preferable lower limit of the Ni content is higher than
0.02%, and more preferably 0.1%.
[00421
Mo: 0.02 to 0.8%
Molybdenum (Mo) increases the hardenability of the
steel and increases the strength of the steel. However,
when a Mo content is excessively high, the toughness of
the steel is decreased. Hence, the Mo content is 0.02 to
0.8%. A preferable lower limit of the Mo content is
higher than 0.02%, and more preferably 0.1%. A
preferable upper limit of the Mo content is less than
0.8%, and more preferably 0.5%.
Nitrogen (N) combines with Al, Ti or the like to
thereby form nitride. When a large amount of N is
contained, the nitride is coarsened to thereby produce an
adverse effect on the toughness. On the other hand, when
a N content is excessively small, the amount of the
nitride becomes too small, which makes it hard to produce
an effect of inhibiting austenite grains from being
coarsened at the time of hot rolling. Hence, the N
content is 0.002 to 0.008%. A preferable lower limit of
the N content is higher than 0.002%, and more preferably
0.004%. A preferable upper limit of the N content is
less than 0.008%, and more preferably 0.007%.
[0044]
Ca: 0.0005 to 0.005%
Calcium (Ca) deoxidizes the steel. Further, Ca
combines with S in the steel to form CaS. The production
of CaS inhibits the production of MnS. In other words,
Ca inhibits the production of the MnS to thereby enhance
the toughness and the hydrogen induced cracking (HIC)
resistance of the steel. However, when a Ca content is
excessively high, a cluster-like inclusion is formed and
hence the toughness and the HIC resistance are decreased.
[00451
Nb: 0.01 to 0.1%
Niobium (Nb) combines with C and N in the steel to
thereby form a fine Nb carbonitride, thereby increasing
the strength of the steel. Further, the fine Nb
carbonitride increases the strength of the steel by
dispersion strengthening. However, when a Nb content is
excessively high, the Nb carbonitride is coarsened to
thereby decrease the toughness of the steel. Hence, the
Nb content is 0.01 to 0.1%. A preferable upper limit of
the Nb content is less than 0.1%, and more preferably
less than 0.08%.
[00461
The balance of the seamless steel pipe according to
the present embodiment is iron (Fe) and impurities. The
impurities described here mean elements mixed from ores
and scraps used as the raw materials of the steel or from
the environment or the like in the producing process.
[0047]
The seamless steel pipe according to the present
embodiment may further contain Ti in place of a part of
Fe. Ti is not an essential element but a selective
element.
[00481
Ti: not more than 0.010%
Titanium (Ti) combines with N in the steel to form
TiN, thereby inhibiting a decrease in the toughness of
the steel caused by dissolved N. Further, fine T ~ Nis
dispersed and precipitated to thereby produce an effect
of inhibiting the austenite grains from being coarsened
at the time of a hot rolling process, thereby further
enhancing the toughness of the steel. When the steel
contains even a small amount of Ti, the effect can be
produced. However, when a Ti content is excessively high,
TiN is coarsened and coarse Tic is formed, which hence
decreases the toughness of the steel. In other words, in
order to finely disperse TiN, the Ti content is limited.
Hence, the Ti content is not more than 0.010%. A
preferable lower limit of the Ti content is 0.001%, and
more preferably 0.004%.
[0049]
The seamless steel pipe according to the present
embodiment may further contain at least one selected from
a group consisting of V and Cu in place of a part of Fe.
V and Cu are not essential elements but selective
elements. Both of these elements increase the strength
of the steel.
[0050]
V: not more than 0.1%
Vanadium (V) combines with C and N in the steel to
form a fine carbonitride, thereby increasing the strength
of the steel. When the steel contains even a small
amount of V, the effect described above can be produced.
However, when a V content is excessively high, a V
carbonitride is coarsened to thereby decrease the
toughness of the steel. Hence, the V content is not more
than 0.1%. A preferable lower limit of the V content is
0.01%. A preferable upper limit of the V content is less
than 0.1%, and more preferably 0.08%.
[0051]
Cu: not more than 1.0%
Copper (Cu) increases the hardenability of the steel
and increases the strength of the steel. However, when a
Cu content is excessively high, the toughness of the
steel is decreased. Hence, the Cu content is not more
than 1.0%. A preferable lower limit of the Cu content is
0.1%. A preferable upper limit of the Cu content is less
than 1.0%, and more preferably 0.6%.
[0052]
[Wall thickness]
The wall thickness of the seamless steel pipe
according to the present embodiment is not less than 50
mm. In other words, the seamless steel pipe according to
the present embodiment is a thick-walled steel pipe. An
upper limit of a wall thickness is preferably 80 mrn, and
more preferably 70 mm.
[00531
[prior austenite grain size (prior y grain)]
In the thick-walled steel pipe having the wall
thickness described above, generally, a crystal grain
size is easily varied in the near surface portion and in
the central portion of the wall thickness of the steel
pipe. However, in the seamless steel pipe according to
the present embodiment, the average crystal grain size of
the prior y grains in the near surface portion is less
than 80 p and a difference in the average size of the
prior y grains between in the near surface portion and in
the central portion of the wall thickness is less than 50
p. For that reason, the seamless steel pipe according
to the present embodiment has excellent toughness and can
inhibit variations in the toughness between in the near
surface portion and in the central portion of the wall
thickness.
[00541
Here, the average crystal grain size of the prior y
grains in the near surface portion can be obtained by the
following method. In a measuring area (500 p x 500 p)
centered at a position of a depth of 2 mrn from the
surface (outer surface or inner surface) of a cross
section (section perpendicular to an axial direction of
the seamless steel pipe) of the seamless steel pipe are
measured the prior y grain sizes (average crystal grain
size of the prior y grains). Prior y grain boundaries are
developed by the use of a picric acid saturated aqueous
solution. The measurement of the average crystal grain
size of the prior y grains in the measuring area is made
on the basis of a method shown by JIS GO551 (2005) and a
grain size number is converted into an average crystal
grain size and the average crystal grain size is
evaluated. This average crystal grain size is defined as
an average crystal grain size of the prior y grains of
the near surface portion.
[00551
Similarly, a measuring area (500 p X 500 w)
centered at a center position of the wall thickness in
the cross section described above is selected, and an
average crystal grain size of the prior y grains in the
measuring area is calculated by the same method described
above. This average crystal grain size is defined as an
average crystal grain size of the prior y grains of the
central portion of the wall thickness.
[00561
In the seamless steel pipe according to the present
embodiment, the prior y grains of the near surface
portion are inhibited from being coarsened. For that
reason, the seamless steel pipe can show excellent
toughness. Further, the seamless steel pipe can suppress
the hardness of the near surface portion and hence can
have also an excellent sour resistance. Still further, a
difference in the average crystal grain size of the prior
y grains between in the near surface portion and in the
central portion of the wall thickness is small. For that
reason, variations in the toughness can be reduced in the
near surface portion and in the central portion of the
wall thickness of the seamless steel pipe. In this
regard, the average crystal grain size is smaller in the
central portion of the wall thickness than in the near
surface portion.
[0057]
[Production method]
An example of a method for producing a seamless
steel pipe described above will be described. The
seamless steel pipe described above may be produced by
the other production method.
[0058]
[Equipment system]
Figure 2 is a block diagram showing an example of a
production line of a seamless steel pipe for a line pipe
according to the present embodiment. Referring to Figure
2, the production line includes a heating furnace 1, a
piercing machine 2, a drawing-rolling mill 3, a sizing
mill 4, a supplemental heating furnace 5, a water cooling
unit 6, a quenching unit 7, and a tempering unit 8. A
plurality of transfer rollers 10 are arranged between the
respective units. In Figure 2, the quenching unit 7 and
the tempering unit 8 are also included by the production
line. However, the quenching unit 7 and the tempering
unit 8 may be arranged separately from the production
line. In short, the quenching unit 7 and the tempering
unit 8 may be arranged off line.
[00591
[Production flow]
Figure 3 is a flow diagram showing a production
process of the seamless steel pipe according to the
present embodiment.
[0060]
Referring to Figure 3, in the method for producing a
seamless steel pipe according to the present embodiment,
first, a starting material is heated in the heating
furnace 1 (Sl). The starting material is, for example, a
round billet. The starting material may be produced by a
continuous casting machine such as a round CC. Further,
the starting material may be produced by forging or
billeting an ingot or a slab. In the present example,
assuming that the starting material is a round billet,
the description of the method will be continuously made.
The heated round billet is hot-worked, thereby being made
into a material pipe (S2 and S3). The produced material
pipe is heated to a specified temperature by the
supplemental heating furnace 5 as required (S4). Then,
the material pipe is water-cooled by the water cooling
unit 6 (accelerated cooling: S5). The water-cooled
material pipe is quenched by the quenching unit 7 (S6)
and then is tempered by the tempering unit 8 (S7). The
seamless steel pipe according to the present embodiment
is produced by the process described above. erei in after,
the respective steps of the process will be described in
detail.
[00611
[Heating step (Sl)]
~irstly, the round billet is heated by the heating
furnace 1. A preferable heating temperature is 1150 to
1280°C. When the heating temperature is not less than
1150°C, the starting material is small in a deformation
resistance in the hot working and hence is hard to cause
a flaw. When the heating temperature is not more than
1280"~a~ s cale loss can be reduced and also a fuel
consumption is reduced. A preferable upper limit of the
heating temperature is 1200°C. The heating furnace 1 is,
for example, a walking beam furnace or a rotary furnace
which is well-known.
[00621
[piercing step (S2)I
The round billet is extracted from the heating
furnace. Then, the heated round billet is piercingrolled
by the piercing machine 2. The piercing machine 2
has a pair of skew rolls and a plug. The plug is
arranged between the skew rolls. A preferable type of
the piercing machine 2 is a crossing type piercing
machine. his is because the crossing type piercing
machine can pierce the round billet at a high pipe
expanding rate.
[0063]
[Rolling step (S3)I
Next, the material pipe is rolled. Specifically,
the material pipe is drawn and rolled by the drawingrolling
mill 3. The drawing-rolling mill 3 includes a
plurality of roll stands arranged in series. The
drawing-rolling mill 3 is, for example, a mandrel mill.
Subsequently, the material pipe drawn and rolled is
sizing-rolled by the sizing mill 4 to produce a seamless
steel pipe. The sizing mill 4 includes a plurality of
roll stands arranged in series. The sizing mill 4 is,
for example, a sizer or a stretch reducer.
[0064]
A surface temperature of the material pipe rolled by
a roll stand arranged at a tail end of the plurality of
roll stands of the sizing mill 4 is defined as a
"finishing temperature". The finishing temperature is
measured, for example, by a temperature sensor arranged
on the exit side of the roll stand arranged at a tail end
of the sizing mill 4. A preferable finishing temperature
is 900°C to llOO°C. In order to acquire the preferable
finishing temperature, a soaking furnace may be arranged
between the drawing-rolling mill 3 and the sizing mill 4
and the material pipe drawn and rolled by the drawingrolling
mill 3 may be soaked.
[00651
[Reheating step (S4)]
A reheating step (S4) is performed as required. In
the case where the reheating step is not performed, in
~igure 2, the process proceeds from the step S3 to the
step S5. Further, in the case where the reheating step
is not performed, in Figure 3, the supplemental heating
furnace 5 is not arranged. The supplemental heating
furnace means a reheating furnace for holding a produced
seamless steel pipe at a quenching temperature without
cooling the seamless steel pipe.
roo661
The produced seamless steel pipe is inserted into
the supplemental heating furnace 5 and is heated. This
processing can make the distribution of the prior y
grains more uniform. A preferable heating temperature in
the supplemental heating furnace 5 is 900°C to llOO°C. A
preferable soaking time is not more than 30 minutes.
This is because when the soaking time is too long, the
prior y grains might be coarsened.
LOO671
[Accelerated cooling step (S5)l
The material pipe produced in the step S3 or the
material pipe reheated in the step 54 is cooled in an
accelerated manner. Specifically, the material pipe is
water-cooled by the water cooling unit 6. The
temperature (surface temperature) of the material pipe
just before water cooling is not less than an Ar3 point,
and preferably not less than 900°C. In the case where
the temperature of the material pipe which is not yet
cooled in the accelerated manner is less than the Ar3
point, the material pipe is reheated by the use of the
supplemental heating furnace 5, an induction heating unit,
or the like to thereby make the temperature of the
material pipe not less than the Ar3 point.
[00681
A preferable cooling rate of the material pipe when
the material pipe is cooled in the accelerated manner is
not less than 100°C/min and a preferable cooling stop
temperature is not more than the A,I point. A more
preferable water cooling stop temperature is not more
than 450°C.
[0069]
In the case where the material pipe is not cooled in
the accelerated manner but is naturally cooled after
rolling, a low cooling rate results in a microstructure
comprising mainly coarse and inhomogeneous ferrite
pearlite. In this case, the material pipe has little
nucleation sites of the reversely transformed y grains.
On the other hand, when the material pipe is cooled in
the accelerated manner as described above, the material
pipe has a parent phase structure martensitized or
bainitized and hence densified, thereby having many
nucleation sites of the reversely transformed y grains.
[00701
The water cooling unit 6 is constructed, for example,
in the following manner. The water cooling unit 6
includes a plurality of rotary rollers, a laminar water
flow section, and a jet water flow section. The
plurality of rotary rollers are arranged in two rows.
The material pipe is arranged between the plurality of
rotary rollers arranged in the two rows. At this time,
the rotary rollers arranged in the two rows are put into
contact with the lower portion of the outer surface of
the material pipe. When the rotary rollers are rotated,
the material pipe is rotated around an axis thereof. The
laminar water flow section is arranged over the rotary
rollers and pours water to the material pipe from above.
At this time, the water poured to the material pipe forms
a laminar water flow. The jet water flow section is
arranged near the end of the material pipe arranged on
the rotary rollers. The jet water flow section jets a
jet water flow to the inside of the material pipe from
the end of the material pipe. The outer surface and the
inner surface of the material pipe are cooled at the same
time by the laminar water flow section and the jet water
flow section. The construction of the water cooling unit
6 of this type is particularly suitable for cooling, in
the accelerated manner, the thick-walled seamless steel
pipe of the present embodiment, which has a wall
thickness not less than 50 mrn.
[00711
The water cooling unit 6 may be other unit other
than the rotary rollers, the laminar water flow section,
and the jet water flow section. The water cooling unit 6
may be, for example, a water tank. In this case, the
material pipe produced in the step S3 is dipped in the
water tank, thereby being cooled. The water cooling unit
6 may include only the laminar water flow section. In
short, the type of the water cooling unit 6 is not
limited to a specific type.
[0072]
[Quenching step (S6)I
The material pipe water-cooled by the water cooling
unit 6 is reheated and is quenched (S6). Firstly, the
seamless steel pipe is heated by the quenching unit 7
(reheating step). A reheating temperature at this time
is made to 990 to llOO°C. As described above, in the
case where the reheating temperature is less than 990°C,
NbC inhibits the growth of the granular reversely
transformed y grains, which are effective for fining the
prior y grains, by a pinning effect. For that reason, it
is hard for the prior y grains of a product to be fined
and the prior y grains in the near surface portion, in
particular, are coarsened. On the other hand, in the
case where the reheating temperature is higher than
llOO°C, the prior y grains of the product are coarsened
in both of the near surface portion and the central
portion of the wall thickness.
[00731
When the reheating temperature is 990 to 1100°~, NbC
is melted and Nb and C are dissolved. For that reason,
at the time of reheating, the granular reversely
transformed y grains are easy to grow, whereby the prior
y grains of the product are inhibited from being
coarsened. In this way, the average crystal grain size
of the prior y grains of the near surface portion, in
particular, is made less than 80 pn, which hence
increases the toughness. Further, a difference in the
average crystal grain size of the prior y grains between
in the near surface portion and in the central portion of
the wall thickness is made less than 50 pn, which hence
inhibits variations in the toughness of the steel.
[00741
The heated material pipe is quenched by cooling
(cooling step). For example, the heated material pipe is
quenched by water cooling. Preferably, the temperature
of the material pipe is cooled to a room temperature. In
this way, a parent phase structure is martensitized or
bainitized, thereby being densified.
[0075]
[~emperings tep (S7)]
The quenched material pipe is tempered. A tempering
temperature is not more than the AC1 point, preferably
550 to 70OoC. A preferable retention time (soaking time)
at the tempering temperature is 10 to 120 minutes. By a
tempering processing, the strength grade of the seamless
steel pipe can be made not less than an X 80 class based
on the API standards, in other words, the yield stress of
the seamless steel pipe can be made not less than 551 MPa.
LO0761
In this regard, in the tempering step, fine NbC is
again precipitated and hence the strength of the steel is
increased.
[00771
The seamless steel pipe produced in the production
process described above contains Nb and hence acquires
high strength. Further, even if the wall thickness is
not less than 50 mm, the average crystal grain size of
the prior y grains in the near surface portion is less
than 80 pn and the difference in the average size of the
prior y grains between in the near surface portion and in
the central portion of the wall thickness becomes less
than 50 pn. For that reason, the seamless steel pipe has
excellent toughness in both of the near surface portion
and the central portion of the wall thickness and has a
small variation in the toughness.
Example 1
LOO781
[Examination method]
[production of test material]
A plurality of steels having a chemical composition
show in Table 1 were melted in a vacuum melting furnace
and an ingot of 180 kg was produced for each steel number.
[0079]
[Table 11
I steel I Chemical com~osition( unit mass%, balance being Fe and im~urities) I
number
A
6
C
D
C
0.059
0.057
0.058
0.060
Si
0.14
0.15
0.15
0.15
Mn
1.45
1.46
1.46
1.60
P
0.012
0.010
0.010
0.01 1
S
0.001
0.001
0.001
0.001
Cu
0.36
0.26
0.26
-
Cr
0.30
0.40
0.40
0.44
N i
0.43
0.25
0.25
0.1 0
Mo
0.20
0.35
0.35
0.35
V
0.051
0.004
0.003
-
Ti
0.006
0.050
-
-
Nb
-
-
0.05
0.03
Ca
0.0019
0.0021
0.0021
0.0018
sol-A1
0.028
0.030
0.029
0.031
N
0.0040
0.0045
0.0046
0.0047
The produced ingots were inserted into the heating
furnace and were soaked at 1250°C for 1 hour. The ingots
extracted from the heating furnace were hot-forged to
produce rectangular parallelepiped blocks. The blocks
were inserted into the heating furnace and were soaked at
1250°C for 30 minutes. Hot rolling was performed to the
soaked blocks to produce thick plates each having a
thickness of 53 mm or 60 mm. All of the finishing
temperatures at the hot rolling for the respective blocks
were 1050°C. The produced thick plates were held at 950°C
for 5 minutes. Then, water quenching (accelerated
cooling) was performed to the thick plates.
[00811
After the water quenching, the test materials of
test numbers of 1 to 17 were reheated at reheating
temperatures (OC) shown in Table 2. At this time, all
test materials were heated at a temperature raising rate
of 5"C/min. A soaking time was 5 minutes for all test
materials. After soaking, water quenching was performed
to the test materials. Tempering at 650°C for 30 minutes
was performed to the quenched test materials. In this
way, the test materials were acquired.
[00821
[Table 21
N
Vickers
hardness
(HV)
22 1
Test
number
1
iar ~ ; ~ ~ c e ~ o & i o ; ~
Absorbed Average
energy crystal grain
size (ym)
Grain size
difference I Note
Steel
number
A 6 1 Comparative example (
2 1 Comparative example 1
Plate
thickness
(mm)
53
2 1 Comparative example I
Comparative example
Comparative example
Comparative example
Reheating
temperature
PC)
950
79 1 Comparative example I
74 1 Comparative example I
Central portion of wall thick
47 1 Present invention I
Yield
496
23 Present invention
82 Comparative example
40 Present invention
21 Present invention
Comparative example
Comparative example
Present invention
Comparative example
Absorbed
enera
(J)
27 1
Average crystal
grain size (pm)
31
[0083]
[Evaluation test]
[Measurement test of average crystal grain size of prior
y grains]
For each of the test materials (thick plates) of the
test numbers 1 to 17, an average crystal grain size (pn)
of the prior y grains of a near surface portion (an area
of 500 pm x 500 pn centered at a position of a depth of 2
mm from a surface (an upper surface or a lower surface)
in a section in a thickness direction of the thick plate)
and an average crystal grain size (pn) of the prior y
grains of a central portion of a wall thickness (an area
of 500 pn x 500 p centered at a center position in a
thickness direction in the section in the thickness
direction of the thick plate) were obtained on the basis
of the measurement and calculation methods described
above. Since the test materials of the prior y grain
size were the thick plates, the center position of the
plate thickness (in other words, in the case of the plate
having a thickness of 53 mm, a position of 26. 5 mm from
the surface, whereas in the case of the plate having a
thickness of 60 mm, a position of 30 mm from the surface)
was considered to be the center of the central portion of
the wall thickness and an average crystal grain size of
the prior y grains of the central portion of the wall
thickness was acquired. According to the measurement
method described above, the prior y grain boundaries were
developed by the use of the picric acid saturated aqueous
solution, and an average crystal grain size of the prior
y grains was obtained by converting a grain size number
into a grain size on the basis of a method shown by JIS
G0551.
LO0841
ensio ion test]
A 14A tension test specimen (D = 8.5 mm$) in
compliance with JIS 22201 (1998) was acquired from the
central portion of the plate thickness of each test
material. A tension test in compliance with JIS 22241
(1998) was performed at a normal temperature (25'C) in
the atmosphere by the use of the acquired test specimen,
whereby a yield strength (0.2% proof stress) was obtained
LO0851
[Toughness evaluation test]
A V-notch test specimen based on No. 4 test coupon
of JIS 22242 was acquired from the near surface portion
and the central portion of the wall thickness (center
portion of the plate thickness) of each test material of
the test numbers 1 to 17 in such a way to be parallel to
a cross sectional direction of the test material. Each
of the V-notch test specimens of the near surface portion
included the surface and had a cross section of 10 mm x
10 mm and had a V notch having a depth of 2 mm. Each of
the V-notch test specimens of the central portion of the
wall thickness had a center in the plate thickness
located at the center of a cross section (10 mm x 10 mm)
and had a V notch having a depth of 2 mm.
[00861
A Charpy impact test in compliance with JIS 22242
was performed by the use of the acquired V-notch test
specimens at -40°C, whereby an absorbed energy at -40°C
was obtained for each of the V-notch test specimens.
[0087]
[Hardness test of near surface portion]
In each of the test materials of the test numbers 1
to 17, a Vickers hardness of the near surface portion was
measured by the following method. Three arbitrary
measurement points were selected from a position of a
depth of 2 mm from the surface of each of the test
materials. In each of the measurement points, a Vickers
hardness test in compliance with JIS 22244 (2009) was
performed. At this time, a test force was 10 kg£. An
average of Vickers hardnesses obtained at the three
measurement points was defined as the hardness (W) of
the near surface portion of the test material.
[0088]
[Test result 1
Test results are shown in Table 2. Referring to
Table 2, all of the test materials of the test numbers 9,
10, 12, 13, and 16 had suitable chemical compositions.
For that reason, the yield strength was as high as not
less than 551 MPa. Further, the reheating temperature
was suitable and hence the average crystal grain size of
the prior y grains in the near surface portion was less
than 80 pn. For that reason, the Vickers hardness of the
near surface portion was as low as less than 245 HV.
[00891
Further, although the plate thickness is as large as
53 mm and 60 mm, a difference in the average crystal
grain size between in the near surface portion and in the
central portion of the wall thickness (hereinafter
referred to as "grain size difference") was less than 50
pm and the absorbed energies of both of the central
portion of the wall thickness and the near surface
portion were not less than 100 J. Still further, a
difference in the absorbed energy between in the central
portion of the wall thickness and in the near surface
portion was as small as less than 100 J and a variation
in the toughness was small.
[00901
On the other hand, in the chemical composition of
the test materials of a steel number A used for the test
numbers 1 to 3, Nb was not contained. For that reason,
the yield strengths of the test materials of the test
numbers 1 to 3 were lower than the yield strengths of the
test materials of the test numbers 9, 10, 12, 13, and 16.
[00911
In the chemical composition of the test materials of
a steel number B used for the test numbers 4 to 6, a Ti
content was high. For that reason, the absorbed energies
at -40°C in the central portion of the wall thickness and
in the near surface portion were low.
[00921
In the test materials of the test numbers 7, 8 and
11, the reheating temperature was low. For that reason,
the average crystal grain size of the prior y grains of
the near surface portion was as large as not less than 80
p . For that reason, the ~ickers hardness of the near
surface portion was high and the absorbed energy at -40°C
of the near surface portion was low. Further, the grain
size difference was as large as not less than 50 p, so
that a difference in the absorbed energy between in the
central portion of the wall thickness and in the near
surface portion was as large as not less than 100 J and a
variation in the toughness was large.
[00931
In the test material of the test number 15, the
reheating temperature was low. For that reason, a grain
size difference between in the near surface portion and
in the central portion of the wall thickness was as large
as not less than 50 pm. For that reason, a difference in
the absorbed energy at -40°C between in the near surface
portion and in the central portion of the wall thickness
was as large as not less than 100 J and a variation in
the toughness was large.
[00941
In the test materials of the test numbers 14 and 17,
the reheating temperature was high. For that reason,
average crystal grain sizes of the prior y grains of the
central portion of the wall thickness and the near
surface portion were as large as not less than 80 pm.
For that reason, the absorbed energies at -40°C of the
central portion of the wall thickness and the near
surface portion were low. Further, the Vickers hardness
of the near surface portion was high.
Example 2
[0095]
[Production of test material]
Steel having a chemical composition shown in Table 3
was melted in a converter and then a plurality of round
billets were produced by a continuous casting process.
[00961
[Table 31

[00971
The chemical composition shown in Table 3 was
appropriate. The round billets were heated to 1250°C by
the heating furnace. Subsequently, the round billets
were piercing-rolled by a piercing rolling mill to
produce material pipes. Subsequently, the material pipes
were drawn and rolled by a mandrel mill. Then, the
material pipes were sizing-rolled by a sizer to produce
material pipes having a wall thickness of 53 mm.
[0098]
Just after sizing-rolling, the material pipes were
not cooled to the room temperature but were soaked to
950°C by a supplemental heating furnace. Then, the
material pipes were quenched by water. The quenched
material pipes were reheated. A reheating temperature at
that time was 1050°C. After reheating, the material
pipes were again quenched by water. The quenched
material pipes were tempered at 600°C for 30 minutes to
produce seamless steel pipes.
[0099]
[Evaluation test]
As is the case with Example 1, each of the test
specimens was subjected to the measurement test of
average crystal grain size of the prior Y grains, the
tension test, the toughness evaluation test, and the
hardness test of the near surface portion. In this
regard, as for the near surface portion, in the near
surface portion on the inner surface side and in the near
surface portion on the outer surface side of the seamless
steel pipe were respectively obtained a Vickers hardness,
an absorbed energy and an average crystal grain size.
[OlOO]
[Test result I
A test result is shown in Table 4.
[0101]
[Table 4]
Referring to Table 4, the chemical composition of
the test number 18 was appropriate. For that reason, the
yield strength was as high as not less than 551 MPa.
Further, the reheating temperature was suitable and hence
an average crystal grain size of the prior y grains of
the near surface portions (on the inner surface side and
on the outer surface side) was less than 80 p. For that
reason, the Vickers hardness of the near surface portion
was as low as less than 245 HV. Still further, an
absorbed energy at -40°C was as high as not less than 100
J.
[01031
Further, although the wall thickness was as large as
53 mrn, a grain size difference between in the near
surface portion and in the central portion of the wall
thickness was less than 50 pxn, and a difference in the
absorbed energy between in the central portion of the
wall thickness and in the near surface portion was as
small as less than 100 J.
[0104]
Although the embodiments of the present invention
have been described above, the embodiments described
above are only examples for carrying out the present
invention. Hence, the present invention is not limited
to the embodiments described above but can be put intopractice by modifying the embodiments described above as
required within a scope not departing from the gist of
the present invention.
Industrial Applicability
[0105]
The seamless steel pipe according to the present
invention can be used, for example, as a line pipe and,
in particular, is suitable for an undersea line pipe
(flow line and riser).

We claim:
[Claim 1]
A seamless steel pipe consisting of: in mass%,
C: 0.03 to 0.088,
Si: not more than 0.25%,
Mn: 0.3 to 2.0%,
P: not more than 0.05%,
S: not more than 0.005%,
Al: 0.001 to 0.10%,
Cr: 0.02 to 1.0%,
Ni: 0.02 to 1.0%,
Mo: 0.02 to 0.8%,
N: 0.002 to 0.008%,
Ca: 0.0005 to 0.005%, and
Nb: 0.01 to 0.1%,
the balance being Fe and impurities, and
having a wall thickness not less than 50 mm,
wherein in a cross section perpendicular to an axial
direction of the seamless steel pipe, an average crystal
grain size of prior austenite grains in a near surface
portion is less than 80 pn, the near surface portion
being a 500 pn x 500 pn area centered at a position of a
depth of 2 mrn from a surface, and
wherein a difference between the average crystal
grain size of the prior austenite grains in the near
surface portion and an average crystal grain size of
prior austenite grains in a central portion of a wall
thickness in the cross section is less than 50 p, the
central portion being a 500 p X 500 p area centered at
a center position of the wall thickness of the seamless
steel pipe.
[Claim 2]
The seamless steel pipe according to claim 1,
further containing:
Ti: not more than 0.010%, in place of a part of Fe.
[Claim 3]
The seamless steel pipe according to claim 1 or
claim 2, further containing: at least one selected from a
group consisting of :
Cu: not more than 1.0%, and
V: not more than 0.1%, in place of a part of Fe.
[Claim 4]
A method for producing a seamless steel pipe, the
method comprising:
a step of heating a starting material consisting of:
in mass%, C: 0.03 to 0.08%, Si: not more than 0.25%, Mn:
0.3 to 2.0%, P: not more than 0.05%, S: not more than
0.0058, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to
1.0%, Mo: 0.02 to 0.8%, N: 0.002 to 0.008%, Ca: 0.0005 to
0.005%, and Nb: 0.01 to 0.1%, the balance being Fe and
impurities;
a step of piercing and rolling the heated starting
material to thereby produce a material pipe;
a step of cooling the rolled material pipe in an
accelerated manner;
a step of reheating the material pipe cooled in the
accelerated manner and then of soaking the material pipe at
990 to llOO°C;
a step of quickly cooling the soaked material pipe to
thereby quench the material pipe; and
a step of tempering the quenched material pipe.
[Claim 5]
The method for producing a seamless steel pipe according
to claim 4, wherein the starting material further contains:
Ti: not more than 0.010%, in place of a part of Fe.
[Claim 6]
The method for producing a seamless steel pipe according
to claim 4 or claim 5,
wherein the starting material further contains: at least
one selected from a group consisting of:
Cu: not more than 1.0%, and
V: not more than 0.1%, in place of a part of Fe.