U.S. patent application number 11/895131 was filed with the patent office on 2008-02-28 for heavy wall seamless steel pipe for line pipe and a manufacturing method thereof.
This patent application is currently assigned to SUMITOMO METAL INDUSTRIES, LTD.. Invention is credited to Yuji Arai, Nobuyuki Hisamune, Kunio Kondo.
Application Number | 20080047635 11/895131 |
Document ID | / |
Family ID | 37053160 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080047635 |
Kind Code |
A1 |
Kondo; Kunio ; et
al. |
February 28, 2008 |
Heavy wall seamless steel pipe for line pipe and a manufacturing
method thereof
Abstract
A heavy wall seamless steel pipe for line pipe with a high
strength and increased toughness, which has a chemical composition,
by mass %, that consists of C: 0.03 to 0.08%, Si: not more than
0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni:
0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to
0.008%, and 0.0002 to 0.005%, in total, of at least one selected
from Ca, Mg and REM, and the balance Fe and impurities, optionally
including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that
P and S among impurities are not more than 0.05% and not more than
0.005% respectively. It may contain 0.0003 to 0.01% of boron. A
manufacturing method thereof is characterized by cooling rate,
heating condition for piercing, and heat treating after pipe
making.
Inventors: |
Kondo; Kunio; (Sanda-Shi,
JP) ; Arai; Yuji; (Amagasaki-shi, JP) ;
Hisamune; Nobuyuki; (Kinokawa-Shi, JP) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 S. WACKER DRIVE, SUITE 6300
SEARS TOWER
CHICAGO
IL
60606
US
|
Assignee: |
SUMITOMO METAL INDUSTRIES,
LTD.
Osaka
JP
|
Family ID: |
37053160 |
Appl. No.: |
11/895131 |
Filed: |
August 23, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2006/304613 |
Mar 9, 2006 |
|
|
|
11895131 |
Aug 23, 2007 |
|
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|
Current U.S.
Class: |
148/541 ;
148/335; 148/593; 420/106; 420/109 |
Current CPC
Class: |
C21D 8/10 20130101; C22C
38/00 20130101; C22C 38/001 20130101; B21B 19/04 20130101; B21B
23/00 20130101; C22C 38/06 20130101; C22C 38/04 20130101; C22C
38/44 20130101; C22C 38/46 20130101; C22C 38/002 20130101; C22C
38/50 20130101; C22C 38/42 20130101; C22C 38/58 20130101; C22C
38/02 20130101 |
Class at
Publication: |
148/541 ;
148/593; 148/335; 420/106; 420/109 |
International
Class: |
C21D 8/10 20060101
C21D008/10; C21D 9/08 20060101 C21D009/08; C22C 38/50 20060101
C22C038/50 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 29, 2005 |
JP |
2005-095240 |
Claims
1. A heavy wall seamless steel pipe for line pipe with a high
strength and increased toughness, which has a chemical composition,
by mass %, that consists of C: 0.03 to 0.08%, Si: not more than
0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni:
0.02 to 1.0%, Mo: 0.02 to 1.2%, fi: 0.004 to 0.010%, N: 0.002 to
0.008%, and 0.0002 to 0.005%, in total, of at least one selected
from Ca, Mg and REM, and the balance Fe and impurities, optionally
including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that
P and S among impurities are not more than 0.05% and not more than
0.005% respectively.
2. A heavy wall seamless steel pipe for line pipe with a high
strength and increased toughness, which has a chemical composition,
by mass %, that consists of C: 0.03 to 0.08%, Si: not more than
0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni:
0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to
0.008%, B: 0.0003 to 0.01%, and 0.0002 to 0.005%, in total, of at
least one selected from Ca, Mg and REM, and the balance Fe and
impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or
Cu: 0 to 1.0%, and that P and S among impurities are not more than
0.05% and not more than 0.005% respectively.
3. A method of manufacturing a heavy wall seamless steel pipe for
line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (e): (a)
Forming a billet with a round cross section by continuous casting
of molten steel that has the chemical composition according to
claim 1 or 2. (b) Cooling the billet to the room temperature at not
less than 6.degree. C./min of an average cooling rate between 1400
and 1000.degree. C. (c) Heating the billet to the temperature
between 1150 and 1280.degree. C. at not more than 15.degree. C./min
of an average heating rate between 550 and 900.degree. C., then
piercing and rolling, to make a seamless pipe. (d) Cooling forcedly
the seamless pipe to a temperature of not higher than 100.degree.
C. at not less than 8.degree. C./min of an average cooling rate
between 800 and 500.degree. C., immediately after pipe making, or
after isothermal treating at the temperature between 850 and
1000.degree. C. immediately in succession with pipe making, or
after heating to the temperature between 850 and 1000.degree. C.
after once cooling in succession with pipe making. (e) Tempering
the seamless pipe at the temperature between 500 and 690.degree.
C.
4. A method of manufacturing a heavy wall seamless steel pipe for
line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (0): (a)
Forming a bloom or slab with a square cross section by continuous
casting of molten steel that has the chemical composition according
to claim 1 or 2. (b) Cooling the bloom or slab to the room
temperature at not less than 8.degree. C./min of an average cooling
rate between 1400 and 1000.degree. C. (c) Heating the bloom or slab
to the temperature between 1150 and 1280.degree. C. at not more
than 15.degree. C./min of an average heating rate between 550 and
900.degree. C., and then cooling to the room temperature, to form a
billet with a round cross section by forging and/or rolling. (d)
Heating the billet to the temperature between 1150 and 1280.degree.
C., then piercing and rolling, to make a seamless pipe. (e) Cooling
forcedly the seamless pipe to a temperature of not higher than
100.degree. C. at not less than 8.degree. C./min of an average
cooling rate between 800 and 500.degree. C., immediately after pipe
making, or after isothermal treating at the temperature between 850
and 1000.degree. C. immediately in succession with pipe making, or
after heating to the temperature between 850 and 1000.degree. C.
after once cooling in succession with pipe making. (f) Tempering
the seamless pipe at the temperature between 500 and 690.degree.
C.
5. A method of manufacturing a heavy wall seamless steel pipe for
line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (e): (a)
Forming a billet with a round cross section by continuous casting
of a molten steel that has the chemical composition according to
claim 1 or 2. (b) Cooling the billet to the room temperature at not
less than 6.degree. C./min of an average cooling rate between 1400
and 1000.degree. C. (c) Isothermal treating the billet during not
less than 15 minutes at the temperature between 550 and
1000.degree. C., and heating to the temperature between 1150 and
1280.degree. C., then piercing and rolling, to make a seamless
pipe. (d) Cooling forcedly the seamless pipe to a temperature of
not higher than 100.degree. C. at not less than 8.degree. C./min of
an average heating rate between 800 and 500.degree. C., immediately
after pipe making, or after isothermal treating at the temperature
between 850 and 1000.degree. C. immediately in succession with pipe
making, or after heating to the temperature between 850 and
1000.degree. C. after once cooling in succession with pipe making.
(e) Tempering the seamless pipe at the temperature between 500 and
690.degree. C.
6. A method of manufacturing a heavy wall seamless steel pipe for
line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (0): (a)
Forming a bloom or slab with a square cross section by continuous
casting of a molten steel that has the chemical composition
according to claim 1 or 2. (b) Cooling the bloom or slab to the
room temperature at not less than 8.degree. C./min of an average
cooling rate between 1400 and 1000.degree. C. (c) Isothermal
treating the bloom or slab during not less than 15 minutes at the
temperature between 550 and 1000.degree. C., and heating to the
temperature between 1150 and 1280.degree. C., then forging and/or
rolling, to form a billet with a round cross section, and then
cooling to the room temperature. (d) Heating the billet to the
temperature of 1150 to 1280.degree. C., then piercing and rolling,
to make a seamless pipe. (e) Cooling forcedly the seamless pipe to
a temperature of not higher than 100.degree. C. at not less than
8.degree. C./min of an average cooling rate between 800 and
500.degree. C., immediately after pipe making, or after isothermal
treating at the temperature between 850 and 1000.degree. C.
immediately in succession with pipe making, or after heating to the
temperature between 850 and 1000.degree. C. after once cooling in
succession with pipe making. (f) Tempering the seamless pipe at the
temperature between 500 and 690.degree. C.
Description
TECHNICAL FIELD
[0001] The present invention relates to a heavy wall seamless steel
pipe for line pipe excellent in strength, toughness and
weldability, and a manufacturing method thereof. The heavy wall
seamless steel pipe means a seamless steel pipe having a wall
thickness of 25 mm or more. The seamless steel pipe of the present
invention is a high-strength, high-toughness heavy wall seamless
steel pipe for line pipe having a strength of not less than X70
regulated in API (American Petroleum Institute) Standard, that is,
a strength of X70 (yield strength of 482 MPa or more), X80 (yield
strength of 551 MPa or more), X90 (yield strength of 620 MPa or
more), X100 (yield strength of 689 MPa or more), and X120 (yield
strength of 827 MPa or more), which is particularly suitably used
for submarine flow lines.
BACKGROUND ART
[0002] In recent years, petroleum and gas resources of oil fields
located on land or in shallow sea areas are running dry, and
development of offshore oil fields in deep sea areas has been
increasingly activated. In deep-sea oil fields, there is a need for
the transportation of crude oil or gas from a pit of an oil well or
gas well set on the sea bottom to a platform on the ocean by the
use of a flow line or riser.
[0003] An inner part of a pipe constituting the flow line laid in
the deep sea suffers a high internal fluid pressure in addition to
a deep stratum pressure, and is also subjected to repeated strains
by ocean waves and influenced by the sea water pressure of the deep
sea at the time of shutdown. Therefore, a heavy wall steel pipe
with high strength and toughness is desired as the pipe used for
this purpose.
[0004] Such a seamless steel pipe with high strength and toughness
has been manufactured by piercing a billet heated to high
temperature by a piercing mill, shaping into a pipe shape of
product by rolling and drawing, and then performing heat treatment.
In recent years, however, simplification of the manufacturing
process by applying an in-line heat treatment has been examined
from the viewpoint of energy and process saving. Particularly,
paying attention to effective use of the heat possessed by the
material after hot-worked, a process for performing quenching
without once cooling beforehand to room temperature has been
introduced. According to this method, substantial energy saving and
increased efficiency of the manufacturing process can be attained,
enabling significant reduction in manufacturing cost.
[0005] A steel pipe manufactured in the in-line heat treatment
process of performing quenching directly after finish rolling has
not been subjected to transformation and reverse transformation
since, unlike in the past, it is not reheated after once cooling to
room temperature and rolling. Therefore, the grains are apt to be
coarsened, and it is not easy to ensure the toughness and corrosion
resistance. Some techniques have been proposed in order to make
fine grains of a finish-rolled steel pipe and ensure the toughness
or corrosion resistance even if the grains are not so fine.
[0006] For example, the following Patent Document 1 (Japanese
Patent Unexamined Publication 2001-240913) discloses a technique
for making fine grains by adjusting the leading time to let it into
the reheating furnace after finish rolling. The following Patent
Document 2 (Japanese Patent Unexamined Publication 2000-104117)
discloses a technique for adjusting the chemical composition,
particularly, the contents of Ti and S to provide a satisfactory
performance even with a relatively large grain size.
[0007] However, the technique disclosed in Patent Document 1 cannot
respond to manufacture of a heavy wall steel pipe with high
strength for offshore oil fields in depth, which has been
increasingly demanded in recent years. For example, the heavy wall
steel pipe requires a high finish rolling temperature, and it takes
an excessive time to ensure an intended reheating furnace
temperature, and seriously reduces the production efficiency. The
method described in Patent Document 2 is also hardly applicable to
heavy wall materials. Since the cooling rate in the in-line heat
treatment is reduced in the case of heavy wall materials, the
toughness is deteriorated even if steel of the composition
disclosed in Patent Document 2 is applied.
[0008] [Patent Document 1] Japanese Patent Unexamined Publication
2001-240913.
[0009] [Patent Document 2] Japanese Patent Unexamined Publication
2000-104117.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0010] In order to solve the above-mentioned problems, it is the
objective of the present invention to provide a seamless steel pipe
for line pipe having high strength and stable toughness,
particularly, in a steel pipe with a heavy wall and also a
manufacturing method thereof.
Means for Solving the Problems
1. Fundamental Examination and Findings
[0011] Factors governing the toughness of a heavy wall seamless
steel pipe were analyzed first. As a result, the following
information was found.
[0012] (1) Cooling conditions in and after solidification of molten
steel greatly influence the toughness. Since a lower cooling rate
causes reduction in toughness, the cooling must be accomplished at
not less than a specific cooling rate.
[0013] (2) A blooming process for heating an ingot to a
high-temperature range for hot working does not have a good
influence on the toughness.
[0014] (3) The above-mentioned reduction in toughness is caused by
the precipitation form of Ti carbonitride due to the cooling rates
in and after solidification. In order to prevent this reduction in
toughness, it is important to finely precipitate the Ti
carbonitride.
[0015] (4) Precipitation strengthening deteriorates the balance
between strength and toughness in the case of in-line heat
treatment materials. Although it is disadvantageous for obtaining a
high strength, it is desirable to utilize the transformation
strengthening and the solid-solution strengthening, without
utilizing the precipitation strengthening, in order to obtain a
high toughness.
[0016] (5) It is necessary to prevent from the generation of a
retained austenite and a low-temperature transformed martensite in
order to obtain a homogenous metal microstructure.
[0017] (6) Regarding the chemical composition, it is desirable to
reduce the contents of Si, P and S, and to control the contents of
Nb and V so as not to exceed the specific upper limits, and also to
include a proper quantity of Ti, in addition to a proper quantity
of at least one elements selected from Ca, Mg and REM. Accordingly,
the toughness of heavy wall materials will be significantly
improved.
[0018] (7) The findings described in (1) to (6) above were obtained
on the assumption of the in-line heat treatment. However, if it is
applied to a steel pipe subjected to an off-line heat treatment, an
increased toughness can be obtained. Therefore, the above-mentioned
findings can also be used to manufacture a high-strength material
by an off-line heat treatment.
2. Basic Test and Result
[0019] Since an in-line heat treatment does not have a fine-grain
making process by "transformation-reverse transformation" unlike an
off-line heat treatment, a fine-graining itself at the end of
rolling is required to ensure the toughness.
[0020] It is generally said that although the as-solidified grains
are coarse, the grains become fine by reheating in order to perform
blooming. Therefore, optimization of the blooming process in the
in-line heat treatment materials was examined under laboratory
experiments. As a result, it was found that, when the blooming is
not executed, the grains tend to be fine in the in-line
heat-treatment material, improving the toughness. Namely, it was
found that the conventional general knowledge is not always
correct.
[0021] In order to understand this unexpected result, a simulation
test was further executed under laboratory experiments. In the
process including the blooming process, a cast ingot was heated to
1250.degree. C. and hot worked to form a block, which was then
further heated to 1250.degree. C. to perform hot rolling and water
cooling, whereby the piercing process and the in-line heat
treatment process were simulated.
[0022] In the process without including the blooming process, a
block of the same size as the block formed by the above hot working
was cut from the cast ingot by machining, and this block was heated
to 1250.degree. C. to perform hot rolling and water cooling,
whereby the piercing process and the in-line heat treatment process
were simulated.
[0023] As the results of the two simulation tests, the grain size
not subjected to blooming was overwhelmingly fine and the toughness
was improved.
[0024] However, the similar trial to the two simulation tests
executed under actual equipments, not under laboratory experiments,
resulted in the fact that the grains not subjected to blooming were
not finer than expected.
[0025] Therefore, the inventors examined why the grain size not
subjected to blooming differed extremely from the one subjected to
blooming about the two simulation tests under laboratory
experiments.
[0026] As a result, they found that most of added Ti precipitated
as Ti carbonitride in the blooming simulation process under
laboratory experiments, and that the number of precipitated grains
reduced with the grain growing of the Ti carbonitride during
heating and hot working in the blooming simulation process. The
reduce of the number of precipitated grains deteriorated the
capability of pinning the grain growth in the parent phase, which
resulted in not suppressing the coarse-graining during the
subsequent heating of the block for piercing simulation.
[0027] To the contrary, they found that, in the simulation test
without the blooming process under laboratory experiments, the Ti
carbonitride finely precipitated during the heating in the piercing
process because no carbonitride precipitation generated within the
ingot, and the Ti carbonitride pinned the grain growing in the
parent phase, wherein remarkably fine grains were made.
[0028] The reason why the grains not subjected to blooming under
the simulation tests under actual equipment were not finer than
expected was found that the Ti carbonitride was already
precipitated during the casting because the cooling rate during
casting was not high enough to dissolve Ti in the solid state.
[0029] The Ti carbonitride that precipitated during casting is apt
to coarsen with a reduced number of precipitated grains, since the
precipitation is caused at a high temperature. Therefore, the
capability of pinning the grains of the parent phase is reduced. On
the other hand, if a sufficient quantity of dissolved Ti is ensured
during the casting with minimized precipitation of Ti carbonitride,
the Ti carbonitride is finely precipitated with an increased number
of precipitated grains during the heating of the billet in the
subsequent pipe making process because the precipitation is
occurred at a low temperature. If the number of precipitated grains
is large, the effect of pinning the crystal grains of the parent
phase is increased to suppress the coarse-graining of the parent
phase. Accordingly, it is extremely important to properly control
the cooling rate during casting.
[0030] If the cooling rate after solidification is low, the Ti
carbonitride precipitates in a high temperature range during
cooling. However, this precipitate in an austenite range with
relatively low dislocation causes few nucleation sites, which leads
to a coarsely dispersed state. Once coarsely precipitated, the Ti
carbonitride cannot be finely dispersed since it is hardly
dissolved in a solid phase.
[0031] If the cooling rate after solidification is set to a rate
causing no precipitation of the Ti carbonitride, the cast ingot has
no Ti carbonitride, but Ti in a dissolved state. The Ti
carbonitride precipitates at a relatively low temperature during
the subsequent heating for hot working. Since, during heating, the
Ti carbonitride precipitates at a low temperature in a bainite
structure with high dislocation, the Ti carbonitride precipitates
as being finely dispersed with many nucleation sites. It was also
found that an excessively high heating rate makes fine
precipitation difficult because of the precipitation in a high
temperature range.
[0032] It is also effective for the sufficiently fine precipitation
of Ti carbonitride to execute isothermal treatment in a proper
temperature range during heating. The Ti carbonitride once finely
precipitated is hardly coarsened, and even if blooming is executed,
the effect of suppressing the coarse-graining can be exhibited.
However, since a slight coarse-graining of Ti carbonitride is
caused in the blooming, the dissolved Ti in solidification should
preferably be present more than that in the case of executing
without blooming.
[0033] Since precipitation strengthening by V or Nb makes easier to
obtain high strength, the precipitation strengthening has been
frequently applied to steel products, which require weldability in
addition to high strength. However, it is better not to use the
precipitation strengthening as much as possible, since it causes
serious deterioration of toughness in a heavy wall in-line heat
treatment material. Particularly, Nb seriously deteriorates the
toughness of the in-line heat treatment material. Therefore, if Nb
is included, it is necessary to strictly set an upper limit. With
respect to V, it is also necessary to perform an alloy designing in
order to ensure the strength based on the transformation
strengthening and solid-solution strengthening by restricting the
upper limit of the V content, although it is not as strict as in
Nb.
[0034] Further, in the case of the heavy wall material, it is
difficult to obtain a homogenous metal structure in quenching
treatment during the first stage of the heat treatment, and the
toughness tends to deteriorate. Since the cooling rate is reduced
in the heavy wall material, it is difficult to obtain a
homogenously transformed structure. Namely, although it is
successively transformed to martensite or bainite during cooling, C
is condensed to non-transformed austenite if the diffusion of C is
possible to some degree with a low cooling rate, and this part is
changed to martensite or bainite with a high C content or to
retained austenite with a high C content after the final
transformation. Accordingly, it is desirable to perform forced
cooling to a temperature as low as possible, at a cooling rate as
large as possible.
[0035] However, there are limits to increase the cooling rate in
the case of heavy wall steel pipes. Therefore, examinations were
made to develop a technique capable of forming a homogenous
structure at a cooling rate that is attainable even in the heavy
wall materials. Consequently, the inventors found that minimizing
the content of the Carbon element to be condensed and also
suppressing the content of Si can lead to reducing the condensation
of C in the second phase.
[0036] Based on the above findings, basic ideas of the alloy design
and the manufacturing process were clarified as follows to complete
the present invention. In the following description, "%" represents
"% by mass", unless otherwise specified.
[0037] The C content is limited to not more than 0.08%. The upper
limit of the Si content is set to not more than 0.25%, preferably
to not more than 0.15%, and more preferably to not more than 0.10%.
Ti content needs to be controlled in a narrow range of 0.004 to
0.010% suitable to precipitate as fine Ti carbonitride, without
precipitation in the solidification, during the subsequent billet
heating. Further, an addition of Nb is not performed in the case of
an in-line heat treatment since it causes strength dispersion in
addition to deterioration of the toughness, and the upper limit as
an impurity is preferably set to not more than 0.005%. Since V also
deteriorates the toughness, it is not added, or should be
controlled to not more than 0.08% if included.
[0038] Other elements are adjusted from the viewpoint of the
balance between high strength and satisfactory toughness. For P and
S that adversely affect the toughness, the allowable upper limit
values are set, respectively. Mn, Cr. Ni, Mo and Cu should be
selectively adjusted according to the intended strength,
considering the toughness and weldability. Al is added for
deoxidation. It is also effective to selectively add at least one
of Ca, Mg and REM to ensure the casting characteristic or improve
the toughness. Further, the content of N needs to be controlled in
a narrow range in order to precipitate stable Ti carbonitride.
[0039] For the manufacturing process, it is important to obtain a
solidified ingot in which dissolved Ti is ensured while suppressing
the precipitation of the Ti carbonitride. The present inventors
found that the Ti carbonitride is not precipitated immediately
after solidification if the contents of C, Ti, and N are set to the
above ranges. However, since a coarse-grained Ti carbonitride is
precipitated if the subsequent cooling rate is low, the cooling
after solidification needs to be performed at a specified rate or
more.
[0040] Regarding the casting method, a continuous casting to a
billet with a circular cross section (hereinafter, refers to "a
round billet") is ideal, but a process of continuously casting to a
square mold or casting thereto as an ingot, and then blooming to
the round billet can be adapted. In this case, it is important to
further strictly control the cooling rate after casting to ensure a
sufficient quantity of dissolved Ti while suppressing precipitation
of a coarse-grained TiN.
[0041] The round billet is reheated to a hot workable temperature
and piercing, drawing and shaping rolling are performed thereto. If
the dissolved Ti is sufficiently present, the Ti carbonitride is
precipitated during reheating. Since the precipitation temperature
is relatively low, remarkably fine Ti carbonitride is precipitated,
compared with the precipitation during cooling after
solidification. Since the number of grains of the finely
precipitated Ti carbonitride is large, grain migration during
heating or holding the billet can be suppressed to prevent the
coarse-graining. A quick heating causes no fine precipitation at
low temperature so that the effect of preventing the
coarse-graining cannot be obtained. Therefore, a gentle heating or
a holding in a middle stage is required to promote a precipitation
of fine-grained Ti carbonitride.
[0042] To obtain a homogenous structure is necessary for ensuring
the toughness in the heat treatment after pipe making. It is
important therefore to use steel with an adjusted chemical
composition and to sufficiently cool it at a forced cooling end
temperature set as low as possible. These ideas result in improving
the toughness because of preventing it from generating a
transformation strengthened structure with partially concentrated C
or retained austenite.
[0043] The present invention according to the above-mentioned basic
ideas involves the following seamless steel pipes for line pipe (1)
and (2) and the following methods of manufacturing a seamless steel
pipe for line pipe (3) to (6).
[0044] (1) A heavy wall seamless steel pipe for line pipe with high
strength and increased toughness, which has a chemical composition,
by mass %, that consists of C: 0.03 to 0.08%, Si: not more than
0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni:
0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to
0.008%, and 0.0002 to 0.005%, in total, of at least one selected
from Ca, Mg and REM, and the balance Fe and impurities, optionally
including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that
P and S among impurities are not more than 0.05% and not more than
0.005% respectively.
[0045] (2) A heavy wall seamless steel pipe for line pipe with high
strength and increased toughness, which has 0.0003 to 0.01% of
boron in addition to the chemical composition above.
[0046] (3) A method of manufacturing a heavy wall seamless steel
pipe for line pipe with high strength and increased toughness
characterized by comprising the following steps (a) to (e):
[0047] (a) Forming a billet with a round cross section by
continuous casting of molten steel that has the chemical
composition according to (1) or (2) above.
[0048] (b) Cooling the billet to the room temperature at not less
than 6.degree. C./min of an average cooling rate between 1400 and
1000.degree. C.
[0049] (c) Heating the billet to the temperature between 1150 and
1280.degree. C. at not more than 15.degree. C./min of an average
heating rate between 550 and 900.degree. C., then piercing and
rolling, to make a seamless pipe.
[0050] (d) Cooling forcedly the seamless pipe to a temperature of
not higher than 100.degree. C. at not less than 8.degree. C./min of
an average cooling rate between 800 and 500.degree. C., immediately
after pipe making, or after isothermal treating at the temperature
between 850 and 1000.degree. C. immediately in succession with pipe
making, or after heating to the temperature between 850 and
1000.degree. C. after once cooling in succession with pipe
making.
[0051] (e) Tempering the seamless pipe at the temperature between
500 and 690.degree. C.
[0052] (4) A method of manufacturing a heavy wall seamless steel
pipe for line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (f):
[0053] (a) Forming a bloom or slab with a square cross section by
continuous casting of molten steel that has the chemical
composition according to (1) or (2) above.
[0054] (b) Cooling the bloom or slab to the room temperature at not
less than 8.degree. C./min of an average cooling rate between 1400
and 1000.degree. C.
[0055] (c) Heating the bloom or slab to a temperature between 1150
and 1280.degree. C. at not more than 15.degree. C./min of an
average heating rate between 550 and 900.degree. C., and then
cooling to the room temperature, to form a billet with a round
cross section by forging and/or rolling.
[0056] (d) Heating the billet to the temperature between 1150 and
1280.degree. C., then piercing and rolling, to make a seamless
pipe.
[0057] (e) Cooling forcedly the seamless pipe to a temperature of
not higher than 100.degree. C. at not less than 8.degree. C./min of
an average cooling rate between 800 and 500.degree. C., immediately
after pipe making, or after isothermal treating at the temperature
between 850 and 1000.degree. C. immediately in succession with pipe
making, or after heating to the temperature between 850 and
1000.degree. C. after once cooling in succession with pipe
making.
[0058] (f) Tempering the seamless pipe at the temperature between
500 and 690.degree. C.
[0059] (5) A method of manufacturing a heavy wall seamless steel
pipe for line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (e):
[0060] (a) Forming a billet with a round cross section by
continuous casting of a molten steel that has the chemical
composition according to (1) or (2) above.
[0061] (b) Cooling the billet to the room temperature at not less
than 6.degree. C./min of an average cooling rate between 1400 and
1000.degree. C.
[0062] (c) Isothermal treating the billet during not less than 15
minutes at the temperature between 550 and 1000.degree. C., and
heating to the temperature between 1150 and 1280.degree. C., then
piercing and rolling, to make a seamless pipe.
[0063] (d) Cooling forcedly the seamless pipe to a temperature of
not higher than 100.degree. C. at not less than 8.degree. C./min of
an average heating rate between 800 and 500.degree. C., immediately
after pipe making, or after isothermal treating at the temperature
between 850 and 1000.degree. C. immediately in succession with pipe
making, or after heating to the temperature between 850 and
1000.degree. C. after once cooling in succession with pipe
making.
[0064] (e) Tempering the seamless pipe at the temperature between
500 and 690.degree. C.
[0065] (6) A method of manufacturing a heavy wall seamless steel
pipe for line pipe with a high strength and increased toughness
characterized by comprising the following steps (a) to (f):
[0066] (a) Forming a bloom or slab with a square cross section by
continuous casting of a molten steel that has the chemical
composition according to (1) or (2) above.
[0067] (b) Cooling the bloom or slab to the room temperature at not
less than 8.degree. C./min of an average cooling rate between 1400
and 1000.degree. C.
[0068] (c) Isothermal treating the bloom or slab during not less
than 15 minutes at the temperature between 550 and 1000.degree. C.,
and heating to the temperature between 1150 and 1280.degree. C.,
then forging and/or rolling, to form a billet with a round cross
section, and then cooling to the room temperature.
[0069] (d) Heating the billet to the temperature of 1150 to
1280.degree. C., then piercing and rolling, to make a seamless
pipe.
[0070] (e) Cooling forcedly the seamless pipe to a temperature of
not higher than 100.degree. C. at not less than 8.degree. C./min of
an average cooling rate between 800 and 500.degree. C., immediately
after pipe making, or after isothermal treating at the temperature
between 850 and 1000.degree. C. immediately in succession with pipe
making, or after heating to the temperature between 850 and
1000.degree. C. after once cooling in succession with pipe
making.
[0071] (f) Tempering the seamless pipe at the temperature between
500 and 690.degree. C.
BEST MODE FOR CARRYING OUT THE INVENTION
1. Chemical Composition of Steel Pipe of the Present Invention
[0072] The reason for limiting the chemical compositions of the
steel pipe of the present invention will be described as follows.
As described above, "%" showing the content (or concentration) of a
chemical composition means "% by mass".
[0073] C: 0.03 to 0.08%
[0074] C is an important element for ensuring the strength of
steel. A content of not less than 0.03% is needed in order to
improve the hardenability and strength in a heavy wall material.
Since a content exceeding 0.08% causes deterioration of toughness,
the C content is set to 0.03 to 0.08%.
[0075] Si: 0.25% or less
[0076] Si has an effect as a deoxidizer in steel production, but it
is better to add as little as possible. Because it seriously
deteriorates the toughness, particularly, of a heavy wall material.
If the content of Si exceeds 0.25%, the toughness of the heavy wall
material is remarkably deteriorated. Therefore, the content is set
to 0.25% or less if it is added as the deoxidizer. A content of
0.15% or less enables further improvement in toughness. The content
is most desirably controlled to less than 0.10%. Although it is
difficult to extremely reduce Si as impurity from the point of the
steel making process, extremely satisfactory toughness can be
obtained if the content is limited to less than 0.05%.
[0077] Mn: 0.3 to 2.5%
[0078] Mn needs to be included in a relatively large quantity since
it enhances the hardenability and therefore strengthens the center
even in a heavy wall material and also enhances the toughness. A
content of less than 0.3% cannot provide these effects, and a
content exceeding 2.5% causes deterioration of the HIC resisting
characteristic, therefore, the Mn content is set to 0.3 to
2.5%.
[0079] Al: 0.001 to 0.10%
[0080] Al is added as a deoxidizer in steel making. In order to
obtain this effect, it needs to be added so as to have a content of
not less than 0.001%. On the other hand, if the content of Al
exceeds 0.10%, the inclusions are clustered, thereby deteriorating
the toughness, and surface defects are frequently generated during
the bevel face working of pipe ends. Therefore, the content of Al
is set to 0.001 to 0.10%. From the point of preventing the surface
defects, it is desirable to provide an upper limit, and the upper
limit is preferably set to 0.03% and more preferably to 0.02%.
Since a high deoxidation effect by the addition of Si cannot be
expected in the steel pipe of the present invention, the lower
limit of Al content is preferably set to 0.010% for sufficient
deoxidation.
[0081] Cr: 0.02 to 1.0%
[0082] Cr is an element that improves the hardenability and
strength of steel in a heavy wall material. The effect becomes
remarkable when 0.02% or more of Cr is included. However, since an
excessive content thereof causes some deterioration of toughness,
the content is limited to 1.0% or less.
[0083] Ni: 0.02 to 1.0%
[0084] Ni is an element that improves the hardenability and
strength of steel in a heavy wall material. The effect becomes
remarkable when 0.02% or more of Ni is included. However, since Ni
is an expensive element, and the effect is saturated if it is
excessively included, the upper limit thereof is set to 1.0%.
[0085] Mo: 0.02 to 1.2%
[0086] Mo is an element that improves the strength of steel by
transformation strengthening and solid-solution strengthening. The
effect becomes remarkable when 0.02% or more of Mo is included.
However, since an excessive addition thereof causes deterioration
of the toughness, the upper limit is set to 1.2%.
[0087] Ti: 0.004 to 0.010%
[0088] The content of Ti needs to be controlled in a narrow range
of 0.004% to 0.010% suitable to precipitate as fine Ti
carbonitride, without precipitation in the solidification, during
the subsequent billet heating. If the content is less than 0.004%,
a sufficient number of precipitated grains of the Ti carbonitride
cannot be ensured, and if it exceeds 0.010%, the Ti carbonitride is
coarsely precipitated in the cooling after solidification.
Therefore, a proper content of Ti is 0.004 to 0.010%.
[0089] N: 0.002 to 0.008%
[0090] N needs to be included in a content of 0.002% or more to
ensure finely dispersed Ti carbonitride. Since a content exceeding
0.008% results in precipitation of coarse-grained Ti carbonitride
in the solidification, the content needs to be controlled in a
narrow range of 0.002 to 0.008%.
[0091] V: 0 to 0.08%
[0092] V is an element whose content is to be determined depending
on the balance between strength and toughness. If sufficient
strength can be ensured by other alloy elements, increased
toughness can be obtained without the addition thereof. When it is
added as a strength improving element, the content is preferably
set to 0.02% or more. Since the toughness is seriously deteriorated
if the content exceeds 0.08%, the upper limit of the content is set
to 0.08% if added.
[0093] Nb: 0 to 0.05%
[0094] Nb is remarkably effective for suppressing the
coarse-graining during heating for quenching in the case of an
off-line heat treatment. In order to obtain this effect, Nb is
desirably included in a content of 0.005% or more. However, if the
content of Nb exceeds 0.05%, a coarse-grained carbonitride is
precipitated to deteriorate the toughness. Therefore, the upper
limit is set to 0.05%.
[0095] In the case of an in-line heat treatment, basically, it is
better not to add Nb since the Nb carbonitride is inhomogeneously
precipitated, which increases the strength dispersion as well as
deterioration of the toughness. When the content exceeds 0.005%,
the strength dispersion is remarkable and problematic in
manufacturing. Therefore, when applying the in-line heat treatment,
the allowable upper limit should be set to 0.005%.
[0096] Cu: 0 to 1.0%
[0097] Cu does not need to be added. However, it can be added, if
improvement in the HIC resisting characteristic (hydrogen-induced
cracking resisting characteristic) is intended, since it has an
effect of improving the HIC resisting characteristic. The minimum
content that improves the HIC characteristic is 0.02%. Since the
effect is saturated even with a content that exceeds 1.0%, the
content may be set to 0.02 to 1.0% if added.
[0098] Ca, Mg, and REM: 00002 to 0.005%, in total of at least one
selected therefrom.
[0099] These elements are added for the purpose of improving the
toughness and the corrosion resistance by shape controlling of
inclusions and for the purpose of suppressing nozzle clogging in
casting to improve the casting characteristic. In order to obtain
such effects, a content of 0.0002% or more, in total of at least
one selected therefrom is needed. If the content exceeds 0.005%, in
total of at least one selected therefrom, not only the effects are
saturated, but also the inclusions are easily clustered, thereby
somewhat deteriorating the toughness and the HIC resisting
characteristic. Therefore, when one of these elements are added,
each content is set to 0.0002 to 0.005%, and when two or more
selected therefrom are added, the total content is set to 0.0002 to
0.005%. REM means 17 elements that include lanthanoide elements, Y
and Sc.
[0100] B: 0.0003 to 0.01%
[0101] B does not need to be added. However, since addition thereof
leads to improvement in hardenability even if it is a trace, the
addition is effective when further high strength is needed. In
order to obtain this effect, a content of 0.0003% or more is
desirable. However, since an excessive addition thereof causes
deterioration of the toughness, the content of B is set to 0.01% or
less if added.
[0102] The steel pipe for line pipe of the present invention
contains the above chemical composition and the balance Fe and
impurities. The upper limits of content of P and C among the
impurities should be controlled as follows.
[0103] P: Not more than 0.05%
[0104] P is an impurity element that deteriorates the toughness,
and the content is preferably as little as possible. Since a
content exceeding 0.05% causes remarkable deterioration of the
toughness, the allowable upper limit is set to 0.05%. The content
of P is preferably 0.02% or less and, further preferably, 0.01% or
less.
[0105] S: Not more than 0.005%
[0106] S is also an impurity element that deteriorates the
toughness, and the content is preferably as little as possible.
Since a content exceeding 0.005% causes remarkable deterioration of
the toughness, the allowable upper limit is set to 0.005%. The
content of S is preferably 0.003% or less and, further preferably,
0.001% or less.
2. Manufacturing Method
[0107] Suitable manufacturing conditions of the manufacturing
method of the present invention will be described as follows.
(1) Casting and Cooling after Solidification
[0108] Steel is refined in a basic oxygen furnace or the like so as
to have the above composition followed by casting and
solidification to obtain a bloom. At this time, it is important to
obtain a solidified ingot in which precipitation of the Ti
carbonitride is suppressed. If the contents of C, Ti and N are
restricted as described above, basically, the Ti carbonitride is
not precipitated during solidification. However, if the subsequent
cooling rate is low, a coarse-grained Ti carbonitride precipitates,
therefore, the cooling must be performed at not less than a
specified cooling rate.
[0109] A continuous casting to a round billet shape is ideal for
the manufacturing process, however, a process for continuously
casting to a square mold or casting thereto as an ingot and then
blooming to the round billet can be performed. In this case, it is
important to further strictly control the cooling rate after
solidification to suppress the precipitation of a coarse-grained
TiN.
[0110] An average cooling rate in a temperature range of 1400 to
1000.degree. C., where the Ti carbonitride is apt to be generated
after solidification, is required to be not less than 6.degree.
C./min in the case of casting to the round billet and to be not
less than 8.degree. C./min in the case of executing the blooming.
The average cooling rate is more preferably set to be not less than
8.degree. C./min in the case of casting to the round billet and to
be not less than 10.degree. C./min in the case of executing the
blooming. In each case, no upper limit is provided since the larger
average cooling rate is more desirable.
[0111] The cooling rate of the bloom varies depending on portions
of the bloom. In the case of continuous casting to the circular
mold, the cooling rate is controlled at a place distant from a
center by the distance of 1/2 of the radius. In the case of
continuously casting to a square mold, the cooling rate is
controlled at a middle position between the center of gravity and
the surface on a line passing the center of gravity of the square
in parallel to the long sides thereof. The temperature can be
measured by attaching a thermocouple, or instead, by numerical
simulation corrected with the temperature history of the
surface.
(2) Working of Billet or Ingot
[0112] The round billet is reheated to a hot workable temperature,
and piercing, drawing and shaping rolling are performed thereto.
The bloom or slab cast into a square cross section is reheated and
then made into a round billet by forging or/and rolling, and
piercing, drawing and shaping rolling are then performed
thereto.
[0113] A reheating temperature is required to be 1150.degree. C. or
higher since hot deformation resistance is increased at a lower
temperature than 1150.degree. C., which increases flaws. The upper
limit thereof is set to 1280.degree. C., since a temperature
exceeding 1280.degree. C. leads to an excessive increase in the
heating fuel raw unit, and a reduction in yield by increased scale
loss, which uneconomically shortened the life of the heating
furnace, and the like. Since a lower heating temperature makes
finer grains and increases the toughness, a preferable heating
temperature is 1200.degree. C. or lower.
[0114] When the dissolved Ti is sufficiently present, the Ti
carbonitride is precipitated during reheating, however, the
precipitation occurs at a relatively low temperature, unlike the
precipitation during the cooling after solidification. Therefore,
the precipitated Ti carbonitride is much more fine-grained than the
one during the cooling after solidification. An increased number of
fine-grained Ti carbonitride are formed, and this suppresses the
grain migration during heating the billet to prevent the
coarse-graining. However, since rapid heating disables minute
precipitation at a low temperature, the effect of preventing
coarse-graining cannot be obtained. It is effective for the
promotion of the minute precipitation at a low temperature during
the reheating that the average heating rate should be 15.degree.
C./min or less at a temperature between 550 and 900.degree. C. or
that the isothermal treatment should be executed for 15 minutes or
more at a temperature between 550 and 1000.degree. C.
[0115] The piercing, drawing and shaping rolling can be executed in
the manufacturing conditions for a general seamless steel pipe.
3. Heat Treatment after Pipe Making
[0116] In the heat treatment after pipe making, to obtain a
homogeneous structure is needed in order to ensure toughness. The
quenching treatment is based on the in-line heat treatment of
executing quenching, without once cooling to room temperature, in
succession with hot rolling. However, if the reheating and the
quenching are performed after cooling, finer grains are made,
improving the toughness. When the quenching is executed in
succession with isothermal treatment in an isothermal furnace after
the end of hot work, a steel pipe with minimized strength
dispersion can be obtained.
[0117] High strength and high toughness can be obtained more easily
in a heavy wall material if the cooling rate in the quenching is
set higher. As the cooling rate gets closer to a theoretically
limited cooling rate, higher strength and higher toughness can be
obtained. The necessary average cooling rate is 8.degree. C./sec or
more at a temperature between 800 and 500.degree. C., more
preferably, 10.degree. C./sec or more, and most preferably not less
than 15.degree. C./sec.
[0118] The cooling end temperature is also important for ensuring
excellent toughness, in addition to the cooling rate. It is
important to use a steel with an adjusted chemical composition and
forcedly cool it to an end temperature of not higher than
100.degree. C. The forced cooling is continuously performed,
preferably to 80.degree. C. or lower, more preferably to 50.degree.
C. or lower, and most preferably to 30.degree. C. or lower.
According to this, the generation of transformation strengthened
structure in which C is partially concentrated or retained
austenite can be prevented, therefore, the toughness is
significantly improved.
[0119] After the quenching, tempering is performed at a temperature
between 500 and 700.degree. C. The tempering is performed in order
to adjust the strength and improving the toughness. The holding
time at the tempering temperature may be properly determined
according the wall thickness or the like of the steel pipe, and is
generally set to about 10 to 120 minutes.
EXAMPLE
[0120] Steels that have the chemical compositions shown in Table 1
were molten in a converter. Two methods for manufacturing a round
billet were adopted: one is a method for casting to a continuous
mold with a round cross section, and the other is a manufacturing
method for casting to a square mold and then blooming. The
manufacturing conditions in the casting to the round continuous
casting mold are shown in Tables 2 and 3. The solidification
process is represented as "RCC". The process for casting to the
square mold is represented as "BLCC", and the manufacturing
conditions thereof are shown in Tables 4 and 5.
[0121] Round billets were heated in the pipe making heating
conditions shown in Tables 2 to 5, and hollow pipes were produced
by use of a feed roll piercing machine. The hollow pipes were
finish-rolled by use of a mandrel mill and a sizer, whereby steel
pipes having wall thickness of 30 mm to 50 mm were obtained.
Thereafter, these pipes were cooled in the quenching conditions
described in Tables 2 to 5. Namely, after pipe making, any one of
the following three processes were adopted: the first one is
cooling immediately; the second one is charging immediately to a
reheating furnace for isothermal treatment and then quenching; and
the last one is cooling once to room temperature and reheating and
then cooling again. Thereafter, tempering was executed in the
conditions described in Tables 2 to 5 to obtain the finished
products.
[0122] A JIS (Japan Industrial Standard) No. 12 tensile test piece
for tensile test was prepared from each of the resulting steel
pipes to measure tensile strength (TS) and yield strength (YS). The
tensile test was carried out according to JIS Z 2241. As an impact
test piece, a V notch test piece of 10 mm.times.10 mm, 2 mm was
prepared from the longitudinal direction of the heavy wall center
according to No. 4 test piece of JIS Z 2202, and subjected to the
test.
[0123] In Test No. 1 of Table 2, two examples of branch numbers of
1 and 2 are described. Steel A, the invention, is used in 1-1 and
1-2, and the manufacturing condition of 1-1 is within the range
restricted by the present invention, where increased toughness is
obtained. On the other hand, the manufacturing condition of 1-2 is
deviated from the manufacturing process defined by the present
invention with an excessively high heating rate for pipe making,
where increased toughness cannot be obtained. Each of Test Nos. 2
to 24 also has branch numbers 1 and 2, and the same steel grade is
used in the same test number. The manufacturing condition of each
branch number 1 is within the range restricted by the present
invention, where increased toughness can be obtained. On the other
hand, the manufacturing condition of each branch number 2 is
deviated from the manufacturing process defined by the present
invention, where increased toughness cannot be obtained.
[0124] In Tables 4 and 5, the same steel grade is also used in one
test number, and each branch number 1 corresponds to the
manufacturing process within the range restricted by the present
invention, where increased toughness is obtained. On the other
hand, since each branch number 2 is deviated from the manufacturing
process defined by the present invention, where increased toughness
is not obtained.
[0125] Test Nos. 25 to 30 are examples of comparative steels that
are deviated from the alloy composition range restricted by the
present invention. Each of the steels is insufficient in toughness,
and has insufficient performances as a line pipe requiring
increased thickness and high toughness. TABLE-US-00001 TABLE 1
Chemical Composition [mass % Fe: balance] Steel C Si Mn P S Cr Ni
Mo Ti sol.Al N A 0.05 0.07 1.77 0.008 0.0012 0.50 0.12 0.16 0.006
0.025 0.0036 B 0.05 0.08 1.46 0.006 0.0013 0.41 0.06 0.25 0.008
0.015 0.0053 C 0.03 0.07 1.77 0.012 0.0009 0.25 0.12 0.13 0.010
0.027 0.0035 D 0.06 0.05 1.19 0.011 0.0009 0.47 0.16 0.24 0.007
0.027 0.0048 E 0.03 0.09 1.54 0.010 0.0013 0.48 0.14 0.10 0.006
0.019 0.0065 F 0.06 0.08 1.69 0.007 0.0011 0.27 0.17 0.14 0.008
0.029 0.0039 G 0.07 0.09 1.53 0.006 0.0010 0.36 0.26 0.16 0.007
0.023 0.0057 H 0.07 0.08 1.60 0.006 0.0005 0.37 0.12 0.08 0.010
0.023 0.0026 I 0.07 0.05 1.53 0.008 0.0014 0.07 0.16 0.22 0.006
0.023 0.0040 J 0.03 0.07 1.60 0.009 0.0008 0.33 0.27 0.23 0.007
0.024 0.0026 K 0.07 0.06 1.49 0.009 0.0009 0.39 0.10 0.07 0.009
0.020 0.0067 L 0.03 0.10 1.46 0.008 0.0011 0.44 0.14 0.16 0.007
0.027 0.0040 M 0.05 0.09 1.75 0.007 0.0006 0.35 0.19 0.17 0.010
0.017 0.0048 N 0.04 0.06 1.43 0.005 0.0010 0.35 0.15 0.29 0.009
0.016 0.0050 O 0.04 0.07 1.31 0.011 0.0009 0.44 0.24 0.31 0.010
0.015 0.0025 P 0.06 0.06 1.47 0.009 0.0005 0.31 0.05 0.18 0.008
0.024 0.0068 Q 0.03 0.06 1.35 0.006 0.0005 0.42 0.12 0.21 0.006
0.020 0.0055 R 0.03 0.05 1.55 0.011 0.0007 0.23 0.18 0.19 0.009
0.029 0.0063 S 0.06 0.07 1.44 0.011 0.0014 0.33 0.16 0.19 0.009
0.017 0.0027 T 0.05 0.08 1.69 0.011 0.0005 0.38 0.11 0.06 0.007
0.026 0.0045 U 0.04 0.08 1.42 0.009 0.0009 0.32 0.20 0.30 0.007
0.020 0.0020 V 0.07 0.09 1.30 0.006 0.0009 0.59 0.14 0.26 0.006
0.028 0.0041 W 0.04 0.07 1.72 0.012 0.0010 0.50 0.20 0.22 0.008
0.018 0.0039 X 0.03 0.08 1.51 0.006 0.0009 0.47 0.35 0.33 0.009
0.025 0.0038 AA 0.11 0.09 1.36 0.008 0.0012 0.49 0.11 0.17 0.008
0.029 0.0036 BB 0.06 0.34 1.47 0.009 0.0011 0.49 0.10 0.28 0.007
0.023 0.0033 CC 0.04 0.06 1.59 0.008 0.0070 0.52 0.15 0.25 0.006
0.027 0.0035 DD 0.05 0.08 1.65 0.009 0.0009 0.57 0.14 0.23 0.019
0.028 0.0032 EE 0.04 0.08 1.80 0.008 0.0010 0.46 0.10 0.21 0.007
0.023 0.0036 FF 0.04 0.08 1.75 0.009 0.0008 0.45 0.10 0.19 0.009
0.020 0.0042 Chemical Composition [mass % Fe: balance] Steel V Cu
Nb B Ca Mg REM A 0.04 0.00 -- -- 0.0007 -- -- The invention B 0.04
0.00 -- -- 0.0009 -- -- C 0.01 0.00 -- -- 0.0025 -- -- D 0.01 0.00
-- -- 0.0022 -- -- E 0.00 0.00 -- -- 0.0011 -- -- F 0.03 0.00 -- --
0.0010 -- -- G 0.00 0.25 -- -- 0.0024 -- -- H 0.00 0.00 -- 0.0021
0.0015 -- -- I 0.03 0.33 -- -- 0.0020 -- -- J 0.03 0.00 -- --
0.0008 -- -- K 0.00 0.31 -- -- 0.0014 -- -- L 0.03 0.32 -- --
0.0012 -- -- M 0.00 0.00 -- -- 0.0016 -- -- N 0.04 0.00 -- --
0.0027 0.0008 -- O 0.04 0.00 -- 0.0011 0.0025 -- 0.0015 P 0.03 0.00
-- -- 0.0019 0.0012 -- Q 0.01 0.25 -- -- 0.0013 0.0011 -- R 0.00
0.00 -- -- 0.0017 -- -- S 0.00 0.00 -- 0.0022 0.0007 -- -- T 0.04
0.23 -- -- 0.0020 -- -- U 0.04 0.00 -- 0.0011 0.0017 -- -- V 0.02
0.30 0.022 -- 0.0029 -- -- W 0.00 0.18 0.008 0.0015 0.0020 0.0008
0.0007 X 0.03 0.31 0.019 -- 0.0027 -- -- AA 0.04 0.00 -- 0.0014
0.0008 -- -- The comparative BB 0.03 0.00 -- 0.0013 0.0011 -- -- CC
0.04 0.00 -- 0.0013 0.0008 -- -- DD 0.02 0.00 -- 0.0016 0.0008 --
-- EE 0.11 0.00 -- 0.0015 0.0012 -- -- FF 0.04 0.00 -- 0.0015 -- --
--
[0126] TABLE-US-00002 TABLE 2 Quenching after pipe making Charging
Cooling to room Heating during pipe making to a reheating
temperature and charging Solidification Holding in a furnace before
to a reheating furnace Cooling Heating Heating middle stage
quenching before quenching Rate Temp. Rate Temp. Time Temp. Time
Temp. Time Test No. Steel Process (.degree. C./min) (.degree. C.)
(.degree. C.) (.degree. C.) (min) (.degree. C.) (min) (.degree. C.)
(min) 1-1 A RCC 10.6 1250 -- 600 90 950 10 -- -- 1-2 A RCC 15.1
1200 16.5 -- -- 950 10 -- -- 2-1 B RCC 11.7 1250 9.4 -- -- 900 20
-- -- 2-2 B RCC 4.5 1150 7.9 -- -- 900 20 -- -- 3-1 C RCC 10.2 1150
-- 800 90 950 10 -- -- 3-2 C RCC 12.1 1300 6.5 -- -- 950 10 -- --
4-1 D RCC 9.7 1150 5.6 -- -- 950 10 -- -- 4-2 D RCC 12.5 1100 16.4
400 90 950 10 -- -- 5-1 E RCC 14.1 1200 9.9 -- -- 950 10 -- -- 5-2
E RCC 10.7 1250 7.7 -- -- 950 10 -- -- 6-1 F RCC 8.2 1200 7.1 -- --
980 5 -- -- 6-2 F RCC 13 1150 -- 750 120 980 5 -- -- 7-1 G RCC 11.8
1200 9.0 -- -- 950 10 -- -- 7-2 G RCC 13.6 1310 6.3 -- -- 950 10 --
-- 8-1 H RCC 8.4 1250 6.2 -- -- 940 10 -- -- 8-2 H RCC 15 1200 7.4
-- -- 940 10 -- -- 9-1 I RCC 10.9 1250 -- 650 30 950 10 -- -- 9-2 I
RCC 4.5 1100 -- 700 120 950 10 -- -- 10-1 J RCC 14.4 1100 9.9 -- --
950 10 -- -- 10-2 J RCC 10.9 1100 6.7 -- -- 950 10 -- -- 11-1 K RCC
15.7 1250 8.1 -- -- 950 10 -- -- 11-2 K RCC 12.7 1200 15.7 -- --
950 10 -- -- 12-1 L RCC 11.9 1150 7.7 -- -- 950 10 -- -- 12-2 L RCC
12.4 1200 5.8 -- -- 1080 10 -- -- 13-1 M RCC 8.7 1250 6.9 -- -- 950
30 -- -- 13-2 M RCC 12.6 1100 -- 600 60 780 30 -- -- 14-1 N RCC
10.1 1250 8.7 -- -- -- -- -- -- 14-2 N RCC 13.6 1200 -- 850 60 --
-- -- -- Quenching after Toughness pipe making Fracture Temp.
Strength Appearance Thickness Cooling after Tempering Yield
Transition of Rate cooling Temp. Time Strength Temp. (FATT) pipe
wall Test No. (.degree. C./sec) (.degree. C.) (.degree. C.)
(.degree. C.) (MPa) (.degree. C.) (mm) Note (*) 1-1 8.4 37 590 20
772 -38 42 I 1-2 11.6 55 630 30 752 -7 42 C 2-1 11.7 56 600 30 663
-58 40 I 2-2 9.7 59 600 30 620 -19 40 C 3-1 15.6 26 590 20 586 -45
35 I 3-2 10.1 60 610 20 568 0 35 C 4-1 14.6 50 640 30 605 -51 35 I
4-2 10.9 57 560 20 582 -4 35 C 5-1 11.5 62 590 20 611 -33 45 I 5-2
5.5 47 590 30 567 -16 45 C 6-1 14.8 53 550 20 630 -60 35 I 6-2 8.3
200 600 10 655 -18 35 C 7-1 9.2 31 610 20 682 -58 30 I 7-2 19.4 51
640 10 690 -5 30 C 8-1 17.8 46 600 10 755 -39 32 I 8-2 4.1 48 620
30 709 2 32 C 9-1 18 30 570 10 617 -51 32 I 9-2 8.9 40 620 20 612
-15 32 C 10-1 19.1 53 640 20 647 -31 30 I 10-2 14.3 179 580 10 641
-22 30 C 11-1 9.2 57 640 10 654 -42 50 I 11-2 8.7 50 550 10 643 2
50 C 12-1 10.6 51 580 20 605 -35 45 I 12-2 9.5 55 610 10 624 -12 45
C 13-1 11.8 31 560 20 706 -42 40 I 13-2 13.2 41 610 20 703 -9 40 C
14-1 16 40 640 30 640 -54 35 I 14-2 4.3 26 560 10 591 -2 35 C (*)
Note: The symbol I shows the invention and the symbol C shows the
comparative.
[0127] TABLE-US-00003 TABLE 3 Quenching after pipe making Charging
Cooling to room Heating during pipe making to a reheating
temperature and charging Solidification Holding in a furnace before
to a reheating furnace Cooling Heating Heating middle stage
quenching before quenching Rate Temp. Rate Temp. Time Temp. Time
Temp. Time Test No. Steel Process (.degree. C./min) (.degree. C.)
(.degree. C.) (.degree. C.) (min) (.degree. C.) (min) (.degree. C.)
(min) 15-1 O RCC 15.1 1100 6.8 -- -- -- -- -- -- 15-2 O RCC 15.8
1150 5.3 -- -- -- -- -- -- 16-1 P RCC 13.2 1150 -- 800 90 -- -- --
-- 16-2 P RCC 14.6 1150 -- 700 60 -- -- -- -- 17-1 Q RCC 8.3 1150
8.7 -- -- -- -- -- -- 17-2 Q RCC 15 1150 -- 800 60 -- -- -- -- 18-1
R RCC 13.4 1100 -- 700 90 -- -- -- -- 18-2 R RCC 5.2 1200 9.3 -- --
-- -- -- -- 19-1 S RCC 13.9 1200 9.3 -- -- -- -- -- -- 19-2 S RCC
15.3 1310 5.1 -- -- -- -- -- -- 20-1 T RCC 10.9 1100 -- 850 90 --
-- -- -- 20-2 T RCC 13.4 1250 19.7 -- -- -- -- -- -- 21-1 U RCC
11.8 1200 7.9 -- -- -- -- -- -- 21-2 U RCC 14 1200 18.2 300 30 --
-- -- -- 22-1 V RCC 10.3 1250 -- 750 30 -- -- 920 15 22-2 V RCC
14.6 1250 9.1 -- -- -- -- 920 15 23-1 W RCC 11.4 1150 -- 600 120 --
-- 920 30 23-2 W RCC 11.9 1250 -- 800 90 -- -- 920 30 24-1 X RCC
13.6 1250 6.8 -- -- -- -- 920 20 24-2 X RCC 9.5 1250 5.6 -- -- --
-- 1100 20 25 AA RCC 11.8 1250 8.0 -- -- -- -- 920 15 26 BB RCC
11.7 1250 8.1 -- -- -- -- 920 15 27 CC RCC 10.6 1250 7.6 -- -- --
-- 920 15 28 DD RCC 12.3 1250 7.2 -- -- -- -- 920 15 29 EE RCC 11.2
1250 8.6 -- -- -- -- 920 15 30 FF RCC 11.6 1250 7.5 -- -- -- -- 920
15 Quenching after Toughness pipe making Fracture Temp. Strength
Appearance Thickness Cooling after Tempering Yield Transition of
Rate cooling Temp. Time Strength Temp. (FATT) pipe wall Test No.
(.degree. C./sec) (.degree. C.) (.degree. C.) (.degree. C.) (MPa)
(.degree. C.) (mm) Note (*) 15-1 14.5 50 580 30 745 -31 30 I 15-2
19.6 163 640 20 766 -2 30 C 16-1 9.3 36 640 10 586 -43 40 I 16-2
14.3 30 400 30 573 -9 40 C 17-1 13.3 60 630 20 577 -47 30 I 17-2
18.6 45 750 30 535 -10 30 C 18-1 16.3 53 630 10 530 -42 35 I 18-2
15.1 52 630 30 519 -24 35 C 19-1 13.4 37 570 20 720 -51 30 I 19-2
18.9 44 560 10 698 -3 30 C 20-1 8.4 45 640 10 679 -47 40 I 20-2
13.9 39 620 30 673 2 40 C 21-1 11.3 43 590 20 751 -35 35 I 21-2
17.1 40 550 10 746 -14 35 C 22-1 9.6 34 620 30 875 -36 43 I 22-2
11.7 210 580 20 872 8 43 C 23-1 12.4 45 560 20 888 -42 40 I 23-2
3.7 45 640 20 837 16 40 C 24-1 12.9 48 630 30 889 -38 40 I 24-2
12.1 28 610 30 900 -19 40 C 25 10.6 34 600 30 860 -12 40 C 26 10.1
31 600 30 840 -11 40 C 27 11.2 27 600 30 825 -12 40 C 28 9.8 28 600
30 914 -6 45 C 29 10.4 30 600 30 870 -15 45 C 30 10.3 31 600 30 886
-17 45 C (*) Note: The symbol I shows the invention and the symbol
C shows the comparative.
[0128] TABLE-US-00004 TABLE 4 Quenching after pipe making Cooling
to room temperature and Heating during blooming Pipe Charging to a
charging to a Solidification Holding in a making reheating furnace
reheating furnace Cooling Heating Heating middle stage Heating
before quenching before quenching Rate Temp. Rate Temp. Time Temp.
Temp. Time Temp. Time Test No. Steel Process (.degree. C./min)
(.degree. C.) (.degree. C.) (.degree. C.) (min) (.degree. C.)
(.degree. C.) (min) (.degree. C.) (min) 1-1 A BLCC 15.0 1150 8.3 --
-- 1250 950 10 -- -- 1-2 A BLCC 14.3 1200 6.1 -- -- 1310 950 10 --
-- 2-1 B BLCC 15.2 1250 7.8 -- -- 1200 950 10 -- -- 2-2 B BLCC 16.1
1100 7.8 -- -- 1250 950 10 -- -- 3-1 C BLCC 15.1 1150 8.0 -- --
1150 950 10 -- -- 3-2 C BLCC 16.0 1200 17.1 350 30 1150 950 10 --
-- 4-1 D BLCC 13.8 1200 6.7 -- -- 1100 950 10 -- -- 4-2 D BLCC 11.7
1310 6.0 -- -- 1200 950 10 -- -- 5-1 E BLCC 11.7 1200 4.8 -- --
1250 950 10 -- -- 5-2 E BLCC 14.4 1150 7.9 -- -- 1250 950 10 -- --
6-1 F BLCC 12.0 1200 5.6 -- -- 1200 950 10 -- -- 6-2 F BLCC 16.3
1150 7.8 -- -- 1150 760 10 -- -- 7-1 G BLCC 13.1 1150 5.8 -- --
1250 950 10 -- -- 7-2 G BLCC 13.6 1200 7.1 -- -- 1100 950 10 -- --
8-1 H BLCC 14.7 1200 6.4 -- -- 1250 950 10 -- -- 8-2 H BLCC 7.1
1150 5.9 -- -- 1150 950 10 -- -- 9-1 I BLCC 15.5 1250 6.7 -- --
1150 950 10 -- -- 9-2 I BLCC 11.3 1200 5.7 -- -- 1320 950 10 -- --
10-1 J BLCC 12.3 1250 6.3 -- -- 1250 950 10 -- -- 10-2 J BLCC 12.3
1200 16.8 -- -- 1150 950 10 -- -- 11-1 K BLCC 15.5 1250 7.7 -- --
1200 950 10 -- -- 11-2 K BLCC 16.3 1250 -- 750 120 1200 950 10 --
-- 12-1 L BLCC 15.8 1100 8.2 -- -- 1200 950 10 -- -- 12-2 L BLCC
15.1 1250 6.8 -- -- 1100 1090 10 -- -- 13-1 M BLCC 17.3 1250 8.1 --
-- 1150 950 10 -- -- 13-2 M BLCC 6.7 1200 5.2 -- -- 1250 950 10 --
-- 14-1 N BLCC 14.1 1100 7.3 -- -- 1200 -- -- -- -- 14-2 N BLCC
10.0 1150 5.8 -- -- 1250 -- -- -- -- Toughness Quenching after
Fracture pipe making Appearance Temp. Strength Transition Thickness
Cooling after Tempering Yield Temp. of Rate cooling Temp. Time
Strength (FATT) pipe wall Test No. (.degree. C./sec) (.degree. C.)
(.degree. C.) (.degree. C.) (MPa) (.degree. C.) (mm) Note (*) 1-1
17 44 580 20 779 -30 35 I 1-2 16.9 35 580 20 778 4 35 C 2-1 11.3 42
590 10 649 -54 45 I 2-2 4.9 60 570 20 607 -13 45 C 3-1 10.1 65 640
30 597 -33 30 I 3-2 19.4 38 580 10 583 -21 30 C 4-1 11 48 560 20
580 -31 35 I 4-2 17.2 38 610 10 614 -17 35 C 5-1 17.1 27 600 20 606
-40 32 I 5-2 4.4 26 560 10 555 -19 32 C 6-1 11.8 41 590 20 662 -27
40 I 6-2 11.4 36 630 10 631 -9 40 C 7-1 15.5 42 600 30 714 -28 35 I
7-2 11.2 179 560 20 712 -7 35 C 8-1 17.7 69 560 30 771 -27 33 I 8-2
13.1 45 590 30 780 -1 33 C 9-1 16.5 33 600 30 616 -48 35 I 9-2 12
66 560 10 632 4 35 C 10-1 17.4 56 620 30 651 -43 30 I 10-2 19.9 39
600 30 640 0 30 C 11-1 8.8 33 580 20 633 -50 50 I 11-2 8.2 192 550
30 666 -3 50 C 12-1 15.3 26 580 20 612 -46 30 I 12-2 18.5 37 640 20
616 -23 30 C 13-1 16.2 71 550 30 683 -51 35 I 13-2 10.1 66 550 10
680 -3 35 C 14-1 19.3 55 630 20 607 -46 30 I 14-2 19 41 380 20 625
-23 30 C (*) Note: The symbol I shows the invention and the symbol
C shows the comparative.
[0129] TABLE-US-00005 TABLE 5 Quenching after pipe making Cooling
to room temperature and Heating during blooming Pipe Charging to a
charging to a Solidification Holding in a making reheating furnace
reheating furnace Cooling Heating Heating middle stage Heating
before quenching before quenching Rate Temp. Rate Temp. Time Temp.
Temp. Time Temp. Time Test No. Steel Process (.degree. C./min)
(.degree. C.) (.degree. C.) (.degree. C.) (min) (.degree. C.)
(.degree. C.) (min) (.degree. C.) (min) 15-1 O BLCC 16.9 1250 7.6
-- -- 1150 -- -- -- -- 15-2 O BLCC 15.5 1150 19.1 250 30 1150 -- --
-- -- 16-1 P BLCC 14.1 1200 6.4 -- -- 1100 -- -- -- -- 16-2 P BLCC
17.5 1100 7.7 -- -- 1200 -- -- -- -- 17-1 Q BLCC 12.0 1200 6.2 --
-- 1250 -- -- -- -- 17-2 Q BLCC 17.3 1100 -- 800 60 1150 -- -- --
-- 18-1 R BLCC 14.8 1100 7.5 -- -- 1250 -- -- -- -- 18-2 R BLCC 6.9
1250 5.6 -- -- 1100 -- -- -- -- 19-1 S BLCC 11.8 1200 6.8 -- --
1200 -- -- -- -- 19-2 S BLCC 17.9 1330 -- 750 120 1250 -- -- -- --
20-1 T BLCC 14.3 1250 6.4 -- -- 1250 -- -- -- -- 20-2 T BLCC 16.5
1250 8.5 -- -- 1200 -- -- -- -- 21-1 U BLCC 13.5 1250 7.5 -- --
1250 -- -- -- -- 21-2 U BLCC 10.4 1150 20.8 -- -- 1150 -- -- -- --
22-1 V BLCC 17.0 1200 -- 600 60 1200 -- -- 910 30 22-2 V BLCC 15.5
1150 -- 700 60 1250 -- -- 1090 30 23-1 W BLCC 15.6 1150 8.4 -- --
1200 -- -- 920 20 23-2 W BLCC 15.3 1250 7.6 -- -- 1250 -- -- 920 20
24-1 X BLCC 10.2 1150 6.0 -- -- 1250 -- -- 930 15 24-2 X BLCC 17.8
1100 -- 800 90 1100 -- -- 930 15 25 AA BLCC 15.9 1230 8.8 -- --
1250 -- -- 920 15 26 BB BLCC 14.7 1230 8.6 -- -- 1250 -- -- 920 15
27 CC BLCC 13.7 1230 7.2 -- -- 1250 -- -- 920 15 28 DD BLCC 17.7
1230 8.5 -- -- 1250 -- -- 920 15 29 EE BLCC 15.3 1230 7.3 -- --
1250 -- -- 920 15 30 FF BLCC 14.1 1230 9.2 -- -- 1250 -- -- 920 15
Toughness Quenching after Fracture pipe making Appearance Temp.
Strength Transition Thickness Cooling after Tempering Yield Temp.
of Rate cooling Temp. Time Strength (FATT) pipe wall Test No.
(.degree. C./sec) (.degree. C.) (.degree. C.) (.degree. C.) (MPa)
(.degree. C.) (mm) Note (*) 15-1 8.2 50 590 20 746 -32 35 I 15-2
15.7 37 590 20 769 4 35 C 16-1 13.6 66 560 20 572 -49 33 I 16-2
17.9 166 630 20 596 3 33 C 17-1 9.5 39 580 10 585 -41 40 I 17-2
14.4 54 740 10 460 -21 40 C 18-1 19.9 30 630 20 538 -31 30 I 18-2
17.5 64 610 20 556 -11 30 C 19-1 16.4 68 640 10 739 -27 35 I 19-2
10.8 40 620 30 712 -4 35 C 20-1 9.1 53 620 20 646 -53 50 I 20-2 3.1
30 630 30 597 -17 50 C 21-1 19.9 53 640 10 711 -41 30 I 21-2 8.2 51
610 10 733 -11 30 C 22-1 14.3 29 610 30 878 -27 32 I 22-2 18.3 64
570 20 882 5 32 C 23-1 13 58 640 30 876 -22 40 I 23-2 8 203 640 30
887 16 40 C 24-1 9.3 71 560 20 900 -27 45 I 24-2 4.8 30 620 20 845
4 45 C 25 14.4 40 600 30 848 -15 35 C 26 16 38 600 30 846 -8 35 C
27 13.3 32 600 30 855 -16 40 C 28 13.3 32 600 30 884 -14 40 C 29
14.5 36 600 30 901 -9 35 C 30 15.9 33 600 30 849 -6 35 C (*) Note:
The symbol I shows the invention and the symbol C shows the
comparative.
INDUSTRIAL APPLICABILITY
[0130] According to the present invention, by restricting the
chemical composition of a seamless steel pipe and the manufacturing
method thereof, a seamless steel pipe, even a heavy wall steel
pipe, for line pipe excellent in toughness while having high
strength such as yield strength of X70 class (yield strength of not
less than 482 MPa), X80 class (yield strength of not less than 551
MPa), X90 class (yield strength of not less than 620 MPa), X100
class (yield strength of not less than 689 MPa), and X120 class
(yield strength of not less than 827 MPa) can be manufactured. The
seamless steel pipe of the present invention is a steel pipe that
can be laid in a severer circumstance of the deep sea,
particularly, for the use of a submarine flow line. The present
invention is thus greatly contributable to stable supply of
energies.
* * * * *