Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
Title of Invention: DUPLEX STAINLESS STEEL SEAMLESS PIPE AND
METHOD FOR MANUFACTURING SAME
Technical Field
[0001]
The present invention relates to a duplex stainless steel
seamless pipe having excellent corrosion resistance and having
a small difference between its axial tensile yield strength
and compressive yield strength. The invention also relates
to a method for manufacturing such a duplex stainless steel
seamless pipe. Here, axial tensile yield strength and
compressive yield strength having a small difference means that
the ratio of axial compressive yield strength to axial tensile
yield strength falls within a range of 0.85 to 1.15.
Background Art
[0002]
Important considerations for seamless steel pipes used
for mining of oil wells and gas wells include corrosion
resistance that can withstand a highly corrosive environment
under high temperature and high pressure, and high strength
characteristics that can withstand the deadweight and the high
pressure when the pipes are joined and used deep underground.
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Of importance for corrosion resistance is the amounts by which
corrosion resistance improving elements such as Cr, Mo, W, and
N are added to steel. In this regard, for example, various
duplex stainless steels are available, including SUS329J3L
containing 22% Cr, SUS329J4L containing 25% Cr, and ISO S32750
and S32760 containing Cr with increased amounts of Mo.
[0003]
The most important strength characteristic is the axial
tensile yield strength, and a value of axial tensile yield
strength represents the specified strength of the product.
This is most important because the pipe needs to withstand the
tensile stress due to its own weight when joined and used deep
underground. With a sufficiently high axial tensile yield
strength against the tensile stress due to its weight, the pipe
undergoes less plastic deformation, and this prevents damage
to the passivation coating formed on pipe surface and is
important for maintaining the corrosion resistance.
[0004]
While the axial tensile yield strength is most important
with regard to the specified strength of the product, the axial
compressive yield strength is important for the pipe joint.
From the standpoint of preventing fire or allowing for repeated
insertion and removal, pipes used as oil country tubular goods
such as in oil wells and gas wells cannot be joined by welding,
and screws are used to fasten the joint, compressive stress
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is produced in the screw thread along the axial direction of
pipe in magnitudes that depend on the fastening force. This
makes axial compressive yield strength important to withstand
such compressive strength.
[0005]
A duplex stainless steel has two phases in its
microstructure: the ferrite phase, and the austenite phase
which, crystallographically, has low yield strength. Because
of this, a duplex stainless steel, in an as-processed form after
hot forming or heat treatment, cannot provide the strength
needed for use as oil country tubular goods. For this reason,
pipes to be used as oil country tubular goods are processed
to improve axial tensile yield strength by dislocation
strengthening using various cold rolling techniques. Cold
drawing and cold pilgering are two limited cold rolling
techniques intended for pipes to be used as oil country tubular
goods. In fact, NACE (The National Association of Corrosion
Engineers), which provides international standards for use of
oil country tubular goods, lists cold drawing and cold
pilgering as the only definitions of cold rolling. These cold
rolling techniques are both a longitudinal cold rolling process
that reduces the wall thickness and diameter of pipe, and
dislocation strengthening, which is induced by strain, acts
most effectively for the improvement of axial tensile yield
strength along the longitudinal axis of pipe. In the foregoing
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cold rolling techniques that longitudinally apply strain along
the pipe axis, a strong Bauschinger effect occurs along a pipe
axis direction, and the compressive yield strength along the
axial direction of pipe is known to show an about 20% decrease.
For this reason, it is common practice in designing strength
to take the Bauschinger effect into account, and reduce the
yield strength of the screw fastening portion where axial
compressive yield strength characteristics are needed.
However, this has become a limiting factor of the product
specifications.
[0006]
PTL 1 addresses this issue by proposing a duplex
stainless steel pipe that contains, in mass%, C: 0.008 to 0.03%,
Si: 0 to 1%, Mn: 0.1 to 2%, Cr: 20 to 35%, Ni: 3 to 10%, Mo:
0 to 4%, W: 0 to 6%, Cu: 0 to 3%, N: 0.15 to 0.35%, and the
balance being iron and impurities, and has a tensile yield
strength YSLT of 689.1 to 1000.5 MPa along an axial direction
of the duplex stainless steel pipe, and in which the tensile
yield strength, YSLT, a compressive yield strength, YSLc, along
the axial direction of the pipe, a tensile yield strength, YScT,
along a circumferential direction of the duplex stainless steel
pipe, and a compressive yield strength, YScc, along the
circumferential direction of the pipe satisfy predetermined
formulae.
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Citation List
Patent Literature
[0007]
PTL 1: Japanese Patent No. 5500324
Summary of Invention
Technical Problem
[0008]
However, PTL 1 does not give any consideration to
corrosion resistance.
[0009]
The present invention has been made under these
circumstances, and it is an object of the present invention
to provide a duplex stainless steel seamless pipe having
excellent corrosion resistance and having a small difference
between its axial tensile yield strength and compressive yield
strength. The invention is also intended to provide a method
for manufacturing such a duplex stainless steel seamless pipe.
Solution to Problem
[0010]
A duplex stainless steel contains increased
solid-solution amounts of Cr and Mo, and forms a highly
corrosion-resistant coating, in addition to reducing
localized progression of corrosion. In order to protect the
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material from various forms of corrosion, it is also of
importance to bring the fractions of ferrite phase and
austenite phase to an appropriate duplex state in the
microstructure. The primary corrosion-resistant elements, Cr
and Mo, are both ferrite phase-forming elements, and the phase
fractions cannot be brought to an appropriate duplex state
simply by increasing the contents of these elements. It is
accordingly required to add appropriate amounts of austenite
phase-forming elements. C, N, Mn, Ni, and Cu are examples of
austenite phase-forming elements. Increasing the C content
in steel impairs corrosion resistance, and the upper limit of
carbon content should be limited. In a duplex stainless steel,
the carbon content is typically 0.08% or less. Other austenite
phase-forming elements are inexpensive to add, and nitrogen,
which acts to improve corrosion resistance in the form of a
solid solution, is often used.
[0011]
A duplex stainless steel seamless pipe is used after a
solid-solution heat treatment performed at a high temperature
of at least 1,000 C following hot forming, in order to form
a solid solution of corrosion-resistant elements in steel, and
to bring the phase fractions to an appropriate duplex state.
This is followed by dislocation strengthening by cold rolling,
should strengthening be needed. The product, in an
as-processed form after the solid-solution heat treatment or
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cold rolling, shows high corrosion resistance performance with
the presence of a solid solution of the elements that
effectively provide corrosion resistance.
[0012]
A low-temperature heat treatment, such as that taught
in PTL 1, is effective when the yield strength at the screw
fastening portion needs to be reduced taking into account the
Bauschinger effect. However, in a low-temperature heat
treatment, the elements that dissolve into the steel in the
solid-solution heat treatment diffuse, and the elements
important for corrosion resistance performance are consumed
as these elements precipitate in the form of carbonitrides,
and lose their corrosion resistance effect. Here, a possible
adverse effect of nitrogen is of concern when this element is
intentionally added in large amounts to reduce cost and to
improve corrosion resistance, or when nitrogen is contained
in large amounts as a result of melting in the atmosphere or
binding to other metallic elements added.
Specifically,
nitrogen, because of its small atomic size, easily diffuses
even in a low-temperature heat treatment, and forms nitrides
by binding to surrounding corrosion-resistant elements, with
the result that the corrosion-resistant improving effect of
these elements is lost.
[0013]
For the issue of precipitation of carbonitrides in a
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low-temperature heat treatment, the present inventors thought
that the possible cause of the corrosion resistance drop due
to nitride formation is the nitrogen added in much larger
amounts than carbon, which is added only in trace amounts. The
present inventors tested this hypothesis from various
perspectives, and obtained the following information.
[0014]
First, the present inventors investigated a relationship
between N content and nitride content in a heat treatment. FIGS.
1 and 2 represent SUS329J3L (22% Cr stainless steel; FIG. 1)
and 5U5329J4L (25% Cr stainless steel; FIG. 2) with regard to
their N contents against the amounts of precipitated Cr and
Mo nitrides after a low-temperature heat treatment (590 C)
The results are based on thermal equilibrium calculations.
Without a heat treatment, there was no observable formation
of nitrides with corrosion-resistant elements, and these
elements all existed as a solid solution in the steel. In a
heat-treatment temperature range of 150 to 450 C, the amount
of nitride also increased with increasing N contents, as in
FIGS. 1 and 2. Most of the nitrides observed as precipitates
after the low-temperature heat treatment were of Cr and Mo,
two of important elements for corrosion resistance performance.
In both steels, the amount of nitride increased with increasing
contents, consuming increasing amounts of
corrosion-resistant elements in the form of precipitates.
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That is, after a solid-solution heat treatment, nitrogen is
present in the form of a solid solution, and improves the
corrosion resistance performance with other
corrosion-resistant elements in steel. However, in a
low-temperature heat treatment, the amount of nitrides
increase in proportion to increasing N contents, and the
concentrations of the corrosion-resistant elements decrease
as these elements become consumed by nitride formation. This
appears to be the possible cause of decrease of corrosion
resistance performance. When added in excess amounts,
nitrogen also appears to form nitrides with
corrosion-resistant elements other than Cr and Mo (for example,
W), and decreases the corrosion resistance.
[0015]
In PTL 1, the low-temperature heat treatment is an
essential condition, aside from cold drawing and cold rolling.
To describe more specifically, the technique of PTL 1 uses
ordinary cold drawing and cold pilgering, and fails to prevent
the generation of the Bauschinger effect itself along a pipe
axis direction. Instead, the anisotropy in the yield strength
after the generation of the Bauschinger effect is relieved by
heat treatment. However, the technique of PTL 1 performing
a heat treatment in addition to cold drawing and cold rolling
involves decrease of corrosion resistance due to decrease of
the corrosion-resistant elements in steel. That is, a
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possible explanation for the decreased corrosion resistance
performance of the duplex stainless steel seamless pipe of the
foregoing related art is that, despite the importance of solid
solution amounts of corrosion-resistant elements such as Cr,
Mo, W, and N in steel, these corrosion-resistant elements
precipitate in the form of nitrides in the heat treatment
performed to reduce the Bauschinger effect, and, as a result
of reduced solid solution amounts, the corrosion resistance
decreases.
[0016]
In order to elucidate the relationship between N content
and corrosion resistance performance, the present inventors
conducted evaluations of stress corrosion resistance
performance at various N contents. In the system of components
represented in FIG. 1, only the N content was adjusted to 0.050,
0.110, 0.149, 0.152, 0.185, and 0.252%, and the material was
melted and hot formed before being subjected to cold working
following a solid-solution heat treatment performed at 1,050 C.
After adjusting the yield strength to 865 to 931 MPa, a 4-point
bending corrosion test piece was prepared, and each test piece
was evaluated under two different conditions - without heat
treatment and with a heat treatment at 400 C - and the stress
corrosion resistance performance was compared.
[0017]
The stress applied in the 4-point bending test was 90%
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of the yield strength, fixed. A corrosive environment was
created by preparing an aqueous solution (a 20% NaCl + 0.5%
CH3000H + CH3000Na aqueous solution with added H2S gas; adjusted
to pH 3.5; test temperature 25 C), simulating a corrosive
environment of chloride and sulfide encountered in mining of
an oil well. In the test, the test piece was dipped in the
corrosive solution for 720 hours under applied stress, and the
N content was compared against the corrosion state after the
test. The test revealed that corrosion does not occur when
the test piece is not subjected to heat treatment, regardless
of the N content. However, with heat treatment, corrosion
involving fine pitting, and cracking occurred with a N content
of 0.152%, and serious propagation of cracks was observed at
higher N contents, though no corrosion occurred with N contents
up to 0.149%. Observation of fine, corroded areas revealed
that the corrosion was initiated by nitrides that precipitated
along the grain boundaries of the material microstructure, and
that the pitting corrosion was the result of consumption of
corrosion-resistant elements after the preferential nitride
formation by corrosion-resistant elements that existed near
the grain boundaries and diffused at faster rates in the heat
treatment, resulting in localized decrease of the amount of
a solid solution of corrosion-resistant elements. From the
test result, the maximum allowable N content was decided to
be less than 0.150%, taking into account variation.
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[0018]
The present invention was completed on the basis of these
findings, and the gist of the present invention is as follows.
[1] A duplex stainless steel seamless pipe of a
composition comprising, in mass%, C: 0.005 to 0.08%, Si: 0.01
to 1.0%, Mn: 0.01 to 10.0%, Cr: 20 to 35%, Ni: 1 to 15%, Mo:
0.5 to 6.0%, N: 0.005 to less than 0.150%, and the balance being
Fe and incidental impurities,
the duplex stainless steel seamless pipe having an axial
tensile yield strength of 689 MPa or more, and a ratio of 0.85
to 1.15 as a fraction of axial compressive yield strength to
axial tensile yield strength.
[2] The duplex stainless steel seamless pipe according
to item [1], which has a ratio of 0.85 or more as a fraction
of circumferential compressive yield strength to axial tensile
yield strength.
[3] The duplex stainless steel seamless pipe according
to item [1] or [2], which further comprises, in mass%, at least
one selected from W: 0.1 to 6.0%, and Cu: 0.1 to 4.0%.
[4] The duplex stainless steel seamless pipe according
to any one of items [1] to [3], which further comprises, in
mass%, at least one selected from Ti: 0.0001 to 0.51%, Al:
0.0001 to 0.29%, V: 0.0001 to 0.55%, and Nb: 0.0001 to 0.75%.
[5] The duplex stainless steel seamless pipe according
to any one of items [1] to [4], which further comprises, in
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mass%, at least one selected from B: 0.0001 to 0.010%, Zr:
0.0001 to 0.010%, Ca: 0.0001 to 0.010%, Ta: 0.0001 to 0.3%,
and REM: 0.0001 to 0.010%.
[6] A method for manufacturing the duplex stainless steel
seamless pipe of any one of items [1] to [5],
the method comprising stretching along a pipe axis
direction followed by a heat treatment at a heating temperature
of 150 to 600 C, excluding 460 to 480 C.
[7] A method for manufacturing the duplex stainless steel
seamless pipe of any one of items [1] to [5],
the method comprising stretching along a pipe axis
direction at a temperature of 150 to 600 C, excluding 460 to
480 C.
[8] The method according to item [7], wherein the
stretching is followed by a heat treatment at a heating
temperature of 150 to 600 C, excluding 460 to 480 C.
[9] A method for manufacturing the duplex stainless steel
seamless pipe of any one of items [1] to [5], the method
comprising circumferential bending and rebending.
[10] The method according to item [9], wherein the
circumferential bending and rebending is performed at a
temperature of 600 C or less, excluding 460 to 480 C.
[11] The method according to item [9] or [10], wherein
the bending and rebending is followed by a heat treatment at
a heating temperature of 150 to 600 C, excluding 460 to 480 C.
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Advantageous Effects of Invention
[0019]
The present invention can provide a duplex stainless
steel seamless pipe having high corrosion resistance
performance and having a small difference between its axial
tensile yield strength and circumferential compressive yield
strength. The duplex stainless steel seamless pipe of the
present invention thus enables a screw fastening portion to
be more freely designed while ensuring crushing strength, which
is often evaluated in terms of axial tensile yield strength.
Description of Embodiments
[0020]
FIG. 1 is a graph representing SUS329J3L (22% Cr
stainless steel) with regard to a relationship between N
content and the amount of Cr and Mo nitrides in a
low-temperature heat treatment.
FIG. 2 is a graph representing SUS329J4L (25% Cr
stainless steel) with regard to a relationship between N
content and the amount of Cr and Mo nitrides in a
low-temperature heat treatment.
FIG. 3 shows schematic views representing
circumferential bending and rebending of pipe.
Description of Embodiments
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[0021]
The present invention is described below.
[0022]
The reasons for limiting the composition of a steel pipe
of the present invention are described first. In the following,
"%" means "mass%", unless otherwise specifically stated.
[0023]
C: 0.005 to 0.08%
C is an austenite phase-forming element, and favorably
serves to produce appropriate phase fractions when contained
in appropriate amounts. However, when contained in excess
amounts, C impairs the corrosion resistance by forming carbides.
For this reason, the upper limit of C content is 0.08% or less.
The lower limit is not necessarily needed because decrease of
austenite phase due to reduced C contents can be compensated
by other austenite phase-forming elements. However, the C
content is 0.005% or more because excessively low C contents
increase the cost of decarburization in melting the material.
[0024]
Si: 0.01 to 1.0%
Si acts to deoxidize steel, and it is effective to add
this element to the molten steel in appropriate amounts.
However, any remaining silicon in steel due to excess silicon
content impairs workability and low-temperature toughness.
For this reason, the upper limit of Si content is 1.0% or less.
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The lower limit is 0.01% or more because excessively low Si
contents after deoxidation increase manufacturing costs.
From the viewpoint of reducing the undesirable effect of
remaining excess silicon in steel while producing sufficient
levels of deoxidation effect, the Si content is preferably 0.2%
or more, and is preferably 0.8% or less.
[0025]
Mn: 0.01 to 10.0%
Mn is a strong austenite phase-forming element, and is
available at lower costs than other austenite phase-forming
elements. Unlike C and N, Mn does not consume the
corrosion-resistant elements even in a low-temperature heat
treatment. It is therefore required to add Mn in an amount
of 0.01% or more, in order to bring the fraction of austenite
phase to an appropriate duplex state in a duplex stainless steel
seamless pipe of reduced C and N contents. On the other hand,
when contained in excess amounts, Mn decreases low-temperature
toughness. For this reason, the Mn content is 10.0% or less.
The Mn content is preferably less than 1.0%, in order not to
impair low-temperature toughness. As for the lower limit, the
Mn content is 0.01% or more because Mn is effective at canceling
the harmful effect of impurity element of sulfur that mixes
into the molten steel, and Mn has the effect to fix this element
by forming MnS with sulfur, which greatly impairs the corrosion
resistance and toughness of steel even when added in trace
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amounts. When there is a need to adequately take advantage
of Mn as an austenite phase-forming element to achieve cost
reduction while taking care not to impair low-temperature
toughness, the Mn content is preferably 2.0% or more, and is
preferably 8.0% or less.
[0026]
Cr: 20 to 35%
Cr is the most important element in terms of increasing
the strength of the pass ivation coating of steel, and improving
corrosion resistance performance. The duplex stainless steel
seamless pipe, which is used in severe corrosive environments,
needs to contain at least 20% Cr. Cr contributes more to the
improvement of corrosion resistance with increasing contents.
However, with a Cr content of more than 35%, precipitation of
embrittlement phase occurs in the process of solidification
from the melt. This causes cracking throughout the steel, and
makes the subsequent forming process difficult. For this
reason, the upper limit is 35% or less. From the viewpoint
of ensuring corrosion resistance and productivity, the Cr
content is preferably 21.5% or more, and is preferably 28.5%
or less.
[0027]
Ni: 1 to 15%
Ni is a strong austenite phase-forming element, and
improves the low-temperature toughness of steel. It is
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therefore desirable to make active use of nickel when the use
of manganese as an inexpensive austenite phase-forming element
is an issue in terms of low-temperature toughness. To this
end, the lower limit of Ni content is 1% or more. However,
Ni is the most expensive element among the austenite
phase-forming elements, and increasing the Ni content
increases manufacturing costs. It is accordingly not
desirable to add unnecessarily large amounts of nickel. For
this reason, the upper limit of Ni content is 15% or less. When
the low-temperature toughness is not of concern, it is
preferable to use nickel in combination with other elements
in an amount of 1 to 5%. On the other hand, when high
low-temperature toughness is needed, it is effective to
actively add nickel, preferably in an amount of 5% or more,
and in an amount of 13% or less.
[0028]
Mo: 0.5 to 6.0%
Mo increases the pitting corrosion resistance of steel
in proportion to its content. This element is therefore added
in amounts that depend on the corrosive environment. However,
when Mo is added in excess amounts, precipitation of
embrittlement phase occurs in the process of solidification
from the melt. This causes large numbers of cracks in the
solidification microstructure, and greatly impairs stability
in the subsequent forming. For this reason, the upper limit
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of Mo content is 6.0% or less. While Mo improves the pitting
corrosion resistance in proportion to its content, Mo needs
to be contained in an amount of 0.5% or more to maintain stable
corrosion resistance in a sulfide environment. From the
viewpoint of satisfying both the corrosion resistance and
production stability needed for the duplex stainless steel
seamless pipe, the Mo content is preferably 1.0% or more, and
is preferably 5.0% or less.
[0029]
N: 0.005 to Less Than 0.150%
N is a strong austenite phase-forming element, in
addition to being inexpensive. By itself, N is a corrosion
resistance improving element, and is actively used. However,
when the solid-solution heat treatment is followed by a
low-temperature heat treatment, excess addition of N leads to
nitride precipitation, and, by consuming the
corrosion-resistant elements, causes decrease of corrosion
resistance. For this reason, the upper limit of N content is
less than 0.150%. The lower limit is not particularly limited.
However, excessively low N contents complicate the melting
process, and lead to poor productivity. For this reason, the
lower limit of N content is 0.005% or more . Containing nitrogen
in amounts that are not an issue in terms of corrosion
resistance allows for cost reduction by allowing the other
austenite phase-forming elements Ni, Mn, and Cu to be contained
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in reduced amounts. To this end, the N content is preferably
0.08% or more, and is preferably 0.14% or less.
[0030]
The balance is Fe and incidental impurities. Examples
of the incidental impurities include P: 0.05% or less, S: 0.05%
or less, and 0: 0.01% or less. P, S, and 0 are incidental
impurities that unavoidably mix into material at the time of
smelting. When retained in excessively large amounts, these
impurity elements cause a range of problems, including decrease
of hot workability, and decrease of corrosion resistance and
low-temperature toughness. The contents of these elements
thus must be confined in the ranges of P: 0.05% or less, S:
0.05% or less, and 0: 0.01% or less.
[0031]
In addition to the foregoing components, the following
elements may be appropriately contained in the present
invention, as needed.
[0032]
At Least One Selected from W: 0.1 to 6.0%, and Cu: 0.1 to 4.0%
W: 0.1 to 6.0%
As is molybdenum, tungsten is an element that increases
the pitting corrosion resistance in proportion to its content.
However, when contained in excess amounts, tungsten impairs
the workability of hot working, and damages production
stability. For this reason, tungsten, when contained, is
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contained in an amount of at most 6.0%. Tungsten improves the
pitting corrosion resistance in proportion to its content, and
its content range does not particularly require the lower limit.
It is, however, preferable to add tungsten in an amount of 0.1%
or more, in order to stabilize the corrosion resistance
performance of the duplex stainless steel seamless pipe. From
the viewpoint of the corrosion resistance and production
stability needed for the duplex stainless steel seamless pipe,
the W content is more preferably 1.0% or more, and is more
preferably 5.0% or less.
[0033]
Cu: 0.1 to 4.0%
Cu is a strong austenite phase-forming element, and
improves the corrosion resistance of steel. It is therefore
desirable to make active use of Cu when sufficient corrosion
resistance cannot be provided by other austenite phase-forming
elements,
+ Mn and Ni. On the other hand, when contained in excessively
large amounts, Cu leads to decrease of hot workability, and
forming becomes difficult. For this reason, Cu, when
contained, is contained in an amount of 4.0% or less. The Cu
content does not particularly require the lower limit.
However, Cu can produce the corrosion resistance improving
effect when contained in an amount of 0.1% or more. From the
viewpoint of satisfying both corrosion resistance and hot
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workability, the Cu content is more preferably 1.0% or more,
and is more preferably 3.0% or less.
[0034]
The following elements may also be appropriately
contained in the present invention, as needed.
[0035]
At Least One Selected from Ti: 0.0001 to 0.51%, Al: 0.0001 to
0.29%, V: 0.0001 to 0.55%, and Nb: 0.0001 to 0.75%
When added in appropriate amounts, Ti, Al, V, and Nb bind
to the excess nitrogen, and reduce the amount of solid solution
nitrogen in steel, preventing nitrogen from binding to the
corrosion-resistant elements, and improving the corrosion
resistance. These elements may be added alone or in
combination, as may be appropriately selected. The contents
of these elements do not particularly require the lower limits.
However, when contained, these elements can produce a corrosion
resistance improving effect with contents of 0.0001% or more.
It should be noted, however, that, because excess addition of
these elements increases the alloy cost, the preferred upper
limits are Ti: 0.51% or less, Al: 0.29% or less, V: 0.55% or
less, and Nb: 0.75% or less. The more preferred upper limits
are Ti: 0.30% or less, Al: 0.20% or less, V: 0.30% or less,
and Nb: 0.30% or less.
[0036]
The following elements may also be appropriately
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contained in the present invention, as needed.
[0037]
At Least One Selected from B: 0.0001 to 0.010%, Zr: 0.0001 to
0.010%, Ca: 0.0001 to 0.010%, Ta: 0.0001 to 0.3%, and REM:
0.0001 to 0.010%
When added in trace amounts, B, Zr, Ca, and REM improve
bonding at grain boundaries. Trace amounts of these elements
alter the form of surface oxides, and improve formability by
improving the workability of hot working. As a rule, a duplex
stainless steel seamless pipe is not an easily workable
material, and often involves roll marks and shape defects that
depend on the extent and type of working. B, Zr, Ca, and REM
are effective against forming conditions involving such
problems. The contents of these elements do not particularly
require the lower limits. However, when contained, B, Zr, Ca,
and REM can produce the workability and formability improving
effect with contents of 0.0001% or more. When added in
excessively large amounts, B, Zr, Ca, and REM impair the hot
workability. Because B, Zr, Ca, and REM are rare elements,
these elements also increase the alloy cost when added in excess
amounts. For this reason, the upper limits of B, Zr, Ca, and
REM are 0.010% or less. When added in small amounts, Ta reduces
transformation into the embrittlement phase, and, at the same
time, improves the hot workability and corrosion resistance.
Ta is effective when the embrittlement phase persists for
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extended time periods in a stable temperature region in hot
working or in the subsequent cooling process. For this reason,
Ta, when contained, is contained in an amount of 0.0001% or
more. The upper limit of Ta content is 0.3% or less because
Ta increases the alloy cost when added in excessively large
amounts.
[0038]
The following describes the appropriate phase fractions
of ferrite and austenite phase in the product, a property
important for corrosion resistance.
[0039]
The two different phases of the duplex stainless steel
act differently on corrosion resistance, and produce high
corrosion resistance by being present together in the steel.
To this end, both the austenite phase and the ferrite phase
must be present in the duplex stainless steel, and the phase
fractions of these phases are also important for corrosion
resistance performance. For example, The Japan Institute of
Metals and Materials Newsletter, Technical Data, Vol. 17, No.
8 (1978) describes a relationship between the ferrite phase
fraction of a 21 to 23% Cr duplex stainless steel and time to
fracture of the material in a corrosive environment (Fig. 9,
662) . It can be read from this relationship that the corrosion
resistance is greatly impaired when the ferrite phase fraction
is 20% or less, or 80% or more. Based on evidence that the
24
CA 03108758 2021-02-04
fraction of ferrite phase has impact on corrosion resistance
performance as supported by literature including the foregoing
publication, ISO 15156-3 (NACE MR0175) specifies that a duplex
stainless steel should have a ferrite phase fraction of 35%
or more and 65% or less. The material used in the present
invention is a duplex stainless steel pipe intended for
applications requiring corrosion resistance performance, and
it is important for corrosion resistance to create an
appropriate duplex fraction state. As used herein,
"appropriate duplex fraction state" means that the fraction
of the ferrite phase in the microstructure of the duplex
stainless steel pipe is at least 20% or more and 80% or less.
When the product is to be used in an environment requiring even
higher corrosion resistance, it is preferable that the ferrite
phase be 35 to 65%, following ISO 15156-3.
[0040]
The following describes a method for manufacturing a
duplex stainless steel seamless pipe of the present invention.
[0041]
First, a steel material of the foregoing duplex stainless
steel composition is produced. The process for making the
duplex stainless steel may use a variety of melting processes,
and is not limited. For example, a vacuum melting furnace or
an atmospheric melting furnace may be used when making the steel
by electric melting of iron scrap or a mass of various elements.
CA 03108758 2021-02-04
As another example, a bottom-blown decarburization furnace
using an Ar-02 mixed gas, or a vacuum decarburization furnace
may be used when using hot metal from a blast furnace. The
molten material is solidified by static casting or continuous
casting, and formed into ingots or slabs before being formed
into a round billet by hot rolling or forging.
[0042]
The round billet is heated by using a heating furnace,
and formed into a steel pipe through various hot rolling
processes. The round billet is formed into a hollow pipe by
hot forming (piercing). Various hot forming techniques may
be used, including, for example, the Mannesmann process, and
the extrusion pipe-making process. It is also possible, as
needed, to use, for example, an elongator, an assel mill, a
mandrel mill, a plug mill, a sizer, or a stretch reducer as
a hot rolling process that reduces the wall thickness of the
hollow pipe, or sets the outer diameter of the hollow pipe.
[0043]
Desirably, the hot forming is followed by a
solid-solution heat treatment. In hot rolling, the duplex
stainless steel undergoes a gradual temperature decrease while
being hot rolled from the high-temperature state of heating.
The duplex stainless steel is also typically air cooled after
hot forming, and temperature control is not achievable because
of the temperature history that varies with size and variety
26
CA 03108758 2021-02-04
of products. This may lead to decrease of corrosion resistance
as a result of the corrosion-resistant elements being consumed
in the form of thermochemically stable precipitates that form
in various temperature regions in the course of temperature
decrease. There is also a possibility of phase transformation
into the embrittlement phase, which leads to serious impairment
of low-temperature toughness. The duplex stainless steel
needs to withstand a variety of corrosive environments, and
it is important to bring the fractions of austenite phase and
ferrite phase to an appropriate duplex state for use. However,
because the rate of cooling from the heating temperature is
not controllable, controlling the fractions of these two phases,
which vary in succession with the hold temperature, is
difficult to achieve. To address these issues, a
solid-solution heat treatment is often performed that involves
rapid cooling after the high-temperature heating to form a
solid solution of the precipitates in steel, and to initiate
reverse trans formation of embrittlement phase to
non-embrittlement phase, and thereby bring the phase fractions
to an appropriate duplex state. In this process, the
precipitates and embrittlement phase are dissolved into steel,
and the phase fractions are controlled to achieve an
appropriate duplex state. The solid-solution heat treatment
is typically performed at a high temperature of 1,000 C or more,
though the temperature that dissolves the precipitates, the
27
CA 03108758 2021-02-04
temperature that initiates reverse transformation of
embrittlement phase, and the temperature that brings the phase
fractions to an appropriate duplex state slightly vary with
the types of elements added. The heating is followed by
quenching to maintain the solid-solution state. This may be
achieved by compressed-air cooling, or by using various
coolants, such as mist, oil, and water.
[0044]
The raw seamless pipe after the solid-solution heat
treatment contains the low-yield-strength austenite phase,
and, in its as-processed form, cannot provide the strength
needed for mining of oil wells and gas wells. This requires
strengthening of the pipe by dislocation strengthening, using
various cold rolling techniques. The strength of the duplex
stainless steel seamless pipe after strengthening is graded
according to its axial tensile yield strength.
[0045]
In the present invention, the pipe is strengthened by
using (1) a method that axially stretches the pipe, or (2) a
method that involves circumferential bending and rebending of
pipe, as follows.
[0046]
(1) Axial Stretching of Pipe: Cold Drawing, Cold Pilgering
Cold drawing and cold pilgering are two standardized
methods of cold rolling of pipes intended for mining of oil
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wells and gas wells. Both of these techniques can achieve high
strength along a pipe axis direction, and can be used as
appropriate. These techniques bring changes mostly in rolling
reduction and the percentage of outer diameter change until
the strength of the required grade is achieved. Another thing
to note is that cold drawing and cold pilgering are a form of
rolling that reduces the outer diameter and wall thickness of
pipe to longitudinally stretch and greatly extend the pipe in
the same proportion along the pipe axis. Indeed, longitudinal
strengthening of pipe along the pipe axis is an easy process.
A problem, however, is that these processes produce a large
Bauschinger effect in a direction of compression along the pipe
axis, and reduces the axial compressive yield strength by as
large as about 20% relative to the axial tensile yield strength.
[0047]
To avoid this, in the present invention, a heat treatment
is performed in a temperature range of 150 to 600 C, excluding
460 to 480 C, after the pipe is stretched along the pipe axis.
Provided that the N content is less than 0.150%, this can reduce
decrease of axial compressive yield strength due to stretching
along the pipe axis, without causing a corrosion resistance
performance drop due to consumption of the corrosion-resistant
elements, even after the heat treatment.
[0048]
It is also effective to stretch the pipe along the pipe
29
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axis in a temperature range of 150 to 600 C, excluding 460 to
480 C. Provided that the N content is less than 0.150%, it is
also possible in this case to reduce decrease of axial
compressive yield strength due to stretching along the pipe
axis, without causing a corrosion resistance performance drop,
as in the heat treatment performed after stretching. This
should also produce a work load reducing effect against
softening of material. Decrease of axial compressive yield
strength due to stretching along the pipe axis can be reduced
without affecting the corrosion resistance, even when the
post-stretching heat treatment and stretching are performed
in combination at increased temperatures, provided that the
N content is less than 0.150%. In the present invention, the
heat treatment may follow stretching performed in a temperature
range of 150 to 600 C, excluding 460 to 480 C, and the heating
temperature of the heat treatment is preferably 150 to 600 C,
excluding 460 to 480 C.
[0049]
The upper limits of the stretching temperature and the
heating temperature of the heat treatment need to be
temperatures that do not cancel the dislocation strengthening
provided by the work, and the applied temperature should not
exceed 600 C. Work temperatures of 460 to 480 C should be
avoided because this temperature range coincides with the
embrittlement temperature of the ferrite phase, and possibly
CA 03108758 2021-02-04
cause cracking during the process, in addition to causing
deterioration of the product characteristics due to
embrittlement of pipe.
[0050]
A rapid yield strength drop occurs when the heating
temperature of the heat treatment and the stretching
temperature are below 150 C. In order to avoid this and to
sufficiently produce the work load reducing effect, these
processes are performed at a temperature of 150 C or more.
Preferably, the temperature is 350 to 450 C to avoid passing
the embrittlement phase during heating and cooling.
[0051]
(2) Circumferential Bending and Rebending of Pipe
Dislocation strengthening involving circumferential
bending and rebending of pipe can also be used for strengthening
of pipe, though this is not a standardized technique of cold
working of duplex stainless steel seamless pipes intended for
mining of oil wells and gas wells. This working technique is
described below, with reference to the accompanying drawing.
Unlike cold drawing and cold pilgering that produce a
longitudinal strain along a pipe axis direction, the foregoing
technique produces strain by bending and flattening of pipe
(first flattening), and rebending of pipe that restores full
roundness (second flattening), as shown in FIG. 3. In this
technique, the amount of strain is adjusted by repeating
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CA 03108758 2021-02-04
bending and rebending, or by varying the amount of bend. In
either case, the strain imparted is an additive shear strain
that does not involve a shape change before and after work.
The technique also involves hardly any strain along a pipe axis
direction, and high strength is achieved by dislocation
strengthening due to the strain imparted in the circumference
and wall thickness of the pipe. This makes it possible to
reduce the Bauschinger effect that generates along a pipe axis
direction. That is, unlike cold drawing and cold pilgering,
the technique does not involve decrease of axial compressive
strength, or causes only a small decrease of compressive
strength, if any. This makes it possible to more freely design
the screw fastening portion. The circumferential compressive
strength also improves when the pipe is worked to reduce its
outer circumference. In this way, a strong steel pipe can be
produced that can withstand the external pressure encountered
in mining of deep oil wells and gas wells. Circumferential
bending and rebending cannot produce a large change in outer
diameter and wall thickness to the same extent as cold drawing
and cold pilgering, but is particularly effective when there
is a need to reduce the strength anisotropy along a pipe axis
direction and along a circumferential compressional direction
against the axial stretch.
[0052]
FIG. 3, (a) and (b) show cross sectional views
32
CA 03108758 2021-02-04
illustrating a tool with two points of contact. FIG. 3, (c)
is a cross sectional view showing a tool with three points of
contact. Thick arrows in FIG. 3 indicate the direction of
exerted force flattening the steel pipe. As shown in FIG. 3,
for second flattening, the tool may be moved or shifted in such
a manner as to rotate the steel pipe and make contact with
portions of pipe that were not flattened by the first flattening
(portions flattened by the first flattening are indicated by
shadow).
[0053]
As illustrated in FIG. 3, the circumferential bending
and rebending that flattens the steel pipe, when intermittently
or continuously applied throughout the pipe circumference,
produces strain in the pipe, with bending strain occurring in
portions where the curvature becomes the largest, and rebending
strain occurring toward portions where the curvature is the
smallest. The strain needed to improve the strength of the
steel pipe (dislocation strengthening) accumulates after the
deformation due to bending and rebending. Unlike the working
that achieves reduced wall thickness and reduced outer diameter
by compression, a characteristic feature of the foregoing
method is that the pipe is deformed by being flattened, and,
because this is achieved without requiring large power, it is
possible to minimize the shape change before and after work.
[0054]
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A tool used to flatten the steel pipe, such as that shown
in FIG. 3, may have a form of a roll. In this case, two or
more rolls may be disposed around the circumference of a steel
pipe. Deformation and strain due to repeated bending and
rebending can be produced with ease by flattening the pipe and
rotating the pipe between the rolls. The rotational axis of
the roll may be tilted within 900 of the rotational axis of
the pipe. In this way, the steel pipe moves in a direction
of its rotational axis while being flattened, and can be
continuously worked with ease. When using such rolls for
continuous working, for example, the distance between the rolls
may be appropriately varied in such a manner as to change the
extent of flattening of a moving steel pipe. This makes it
easy to vary the curvature (extent of flattening) of the steel
pipe in the first and second runs of flattening. That is, by
varying the roll distance, the moving path of the neutral line
can be changed to uniformly produce strain in a wall thickness
direction. The same effect can be obtained when the extent
of flattening is varied by varying the roll diameter, instead
of roll distance. It is also possible to vary both roll
distance and roll diameter. With three or more rolls, the pipe
can be prevented from whirling around during work, and this
makes the procedure more stable, though the system becomes more
complex.
[0055]
34
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The circumferential bending and rebending of pipe may
be performed at ordinary temperature. With the
circumferential bending and rebending performed at ordinary
temperature, all the nitrogen can turn into a solid solution,
and this is preferable from the viewpoint of corrosion
resistance. However, when the N content is less than 0.150%,
it is effective to soften the material by increasing the work
temperature, when working is not easily achievable with a high
load put on cold working. The upper limit of the work
temperature needs to be a temperature that does not cancel the
dislocation strengthening provided by the work, and the applied
temperature should not exceed 600 C. Work temperatures of 460
to 480 C should be avoided because this temperature range
coincides with the embrittlement temperature of the ferrite
phase, and possibly cause cracking during the process, in
addition to causing deterioration of the product
characteristics due to embrittlement of pipe. The preferred
work temperature of circumferential bending and rebending of
pipe is therefore 600 C or less, excluding 460 to 480 C. The
lower limit of work temperature is preferably 150 C or more
because a work temperature of less than 150 C coincides with
the temperature region where rapid decrease of yield strength
takes place. More preferably, the upper limit of work
temperature is 450 C from a standpoint of saving energy and
avoiding passing the embrittlement phase during heating and
CA 03108758 2021-02-04
cooling. With an increased work temperature, the strength
anisotropy of the pipe after work can be reduced to some extent,
and increasing the work temperature is also effective when the
strength anisotropy is of concern.
[0056]
In the present invention, the foregoing method (1) or
(2) used for dislocation strengthening may be followed by a
further heat treatment. With a further heat treatment, the
strength anisotropy can improve while maintaining the
corrosion resistance. The heating temperature of the heat
treatment is preferably 150 C or more because a heating
temperature of less than 150 C coincides with a temperature
region where a rapid decrease of yield strength occurs. The
upper limit of the heating temperature needs to be a temperature
that does not cancel the dislocation strengthening provided
by the work, and the applied temperature should not exceed 600 C.
Heating temperatures of 460 to 480 C should be avoided because
this temperature range coincides with the embrittlement
temperature of the ferrite phase, and causes deterioration of
the product characteristics due to embrittlement of pipe. It
is accordingly preferable that the heat treatment, when
performed, be performed at 150 to 600 C, excluding 460 to 480 C.
More preferably, the heating temperature is 350 to 450 C from
a standpoint of saving energy and avoiding passing the
embrittlement phase during heating and cooling, in addition
36
CA 03108758 2021-02-04
to producing the anisotropy improving effect. The rate of
cooling after heating may be a rate achievable by air cooling
or water cooling.
[0057]
A duplex stainless steel seamless pipe of the present
invention can be produced by using the manufacturing method
described above. Grading of the strength of duplex stainless
steel seamless pipes intended for oil wells and gas wells is
based on tensile yield strength along the pipe axis, which
experiences the highest load. A duplex stainless steel
seamless pipe of the present invention has a tensile yield
strength of at least 689 MPa along a pipe axis direction.
Typically, a duplex stainless steel contains the soft austenite
phase in its microstructure, and a tensile yield strength of
689 MPa cannot be achieved along a pipe axis direction in an
as-processed form after the solid-solution heat treatment.
The axial tensile yield strength of the heat-treated duplex
stainless steel is thus adjusted by dislocation strengthening
achieved by the cold working described above (axial stretching
or circumferential bending and rebending of pipe). In terms
of cost, it is advantageous to have higher axial tensile yield
strengths because it allows for pipe design with a thinner wall
for mining of wells. However, when only the wall thickness
is reduced without varying the outer diameter of pipe, the pipe
becomes susceptible to crushing under the external pressure
37
CA 03108758 2021-02-04
exerted deep underground, and this makes the pipe useless. For
this reason, many pipes have an axial tensile yield strength
of at most 1033.5 MPa.
[0058]
In the present invention, the ratio of axial compressive
yield strength to axial tensile yield strength of pipe is 0.85
to 1.15 (axial compressive yield strength/axial tensile yield
strength). With the ratio falling in this range, the steel
pipe can withstand higher axial compressive stress when
fastening a screw or when the steel pipe is bent in a well.
This enables the steel pipe to have the reduced wall thickness
needed to withstand compressive stress. The improved
flexibility of design of pipe wall thickness, particularly,
the wider range of reducible wall thickness lowers the material
cost, which lowers the manufacturing cost and improves the
yield. With warm stretching or bending and rebending, the
ratio of axial compressive yield strength to axial tensile
yield strength of pipe can be brought to 0.85 to 1.15 while
maintaining the corrosion resistance, provided that the N
content is 0.005 to less than 0.150%. With warm bending and
rebending, or with a low-temperature heat treatment performed
after the foregoing processes, the ratio of axial compressive
yield strength to axial tensile yield strength of pipe can be
brought closer to 1, toward a smaller anisotropy.
[0059]
38
CA 03108758 2021-02-04
In the present invention, the ratio of circumferential
compressive yield strength to axial tensile yield strength of
pipe is preferably 0.85 or more (circumferential compressive
yield strength/axial tensile yield strength) . Given the same
wall thickness, the reachable depth of well mining depends on
the axial tensile yield strength of pipe. In order to prevent
crushing under the external pressure exerted deep underground,
the pipe should have strength with a ratio of circumferential
compressive yield strength to axial tensile yield strength of
0.85 or more. Having a higher circumferential compressive
yield strength than axial tensile yield strength is not
particularly a problem; however, the effect typically becomes
saturated when the ratio is about 1.50. When the strength ratio
is too high, other mechanical characteristics (e.g.,
low-temperature toughness) along a pipe circumferential
direction greatly decrease compared to that in a pipe axis
direction. The ratio is therefore more preferably 0.85 to
1.25.
[0060]
In the present invention, the aspect ratio of austenite
grains separated by a crystal orientation angle difference of
15 or more in a cross section across the wall thickness along
the pipe axis is preferably 9 or less. It is also preferable
that austenite grains with an aspect ratio of 9 or less have
an area fraction of 50% or more. A duplex stainless steel of
39
CA 03108758 2021-02-04
the present invention is adjusted to have an appropriate
ferrite phase fraction by heating in a solid-solution heat
treatment. Here, inside of the remaining austenite phase is
a microstructure having a plurality of crystal grains separated
by an orientation angle of 15 or more after the
recrystallization occurring during the hot working and heat
treatment. This makes the aspect ratio of austenite grains
smaller. In this state, the duplex stainless steel seamless
pipe does not have the axial tensile yield strength needed for
use as oil country tubular goods, and the ratio of axial
compressive yield strength to axial tensile yield strength is
close to 1. In order to produce the axial tensile yield
strength needed for oil country tubular goods applications,
the steel pipe is subjected to (1) axial stretching (cold
drawing, cold pilgering), and (2) circumferential bending and
rebending. In these processes, changes occur in the ratio of
axial compressive yield strength to axial tensile yield
strength, and in the aspect ratio of austenite grains. That
is, the aspect ratio of austenite grains, and the ratio of axial
compressive yield strength to axial tensile yield strength are
closely related to each other. Specifically, while (1) or (2)
improves the yield strength in a direction of stretch of
austenite grains before and after work in a cross section across
the wall thickness along the pipe axis, the yield strength
decreases in the opposite direction because of the Bauschinger
CA 03108758 2021-02-04
effect, with the result that the difference between compressive
yield strength and axial tensile yield strength increases.
This means that a steel pipe of small strength anisotropy along
the pipe axis can be obtained when austenite grains before and
after the process (1) or (2) have a small, controlled, aspect
ratio.
[0061
In the present invention, a stable steel pipe with a small
strength anisotropy can be obtained when the austenite phase
has an aspect ratio of 9 or less. A stable steel pipe with
a small strength anisotropy can also be obtained when austenite
grains having an aspect ratio of 9 or less have an area fraction
of 50% or more. An even more stable steel pipe with a small
strength anisotropy can be obtained when the aspect ratio is
or less. Smaller aspect ratios mean smaller strength
anisotropies, and, accordingly, the aspect ratio should be
brought closer to 1, with no lower limit. The aspect ratio
of austenite grains is determined, for example, as a ratio of
the longer side and shorter side of a rectangular frame
containing grains having a crystal orientation angle of 15
or more observed in the austenite phase in a crystal orientation
analysis of a cross section across the wall thickness along
the pipe axis. Here, austenite grains of small particle
diameters are prone to producing large measurement errors, and
the presence of such austenite grains of small particle
41
CA 03108758 2021-02-04
diameters may cause errors in the aspect ratio. It is
accordingly preferable that the austenite grain used for aspect
ratio measurement be at least 10 m in terms of a diameter of
a true circle of the same area constructed from the measured
grain.
[0062]
In order to stably obtain a microstructure of austenite
grains having a small aspect ratio in a cross section across
the wall thickness along the pipe axis, it is effective not
to stretch the pipe along the pipe axis, and not to reduce the
wall thickness in the process (1) or (2). The process (1),
in principle, involves stretching along the pipe axis, and
reduction of wall thickness. Accordingly, the aspect ratio
is larger after work than before work, and this tends to produce
strength anisotropy. It is therefore required to maintain a
small aspect ratio by reducing the extent of work (the wall
thickness reduction is kept at 40% or less, or the axial stretch
is kept at 50% or less to reduce stretch in microstructure),
and by decreasing the outer circumference of the pipe being
stretched to reduce the wall thickness (the outer circumference
is reduced at least 10% while stretching the pipe along the
pipe axis). It is also required to perform a low-temperature
heat treatment after work (softening due to recrystallization
or recovery does not occur with a heat-treatment temperature
of 560 C or less) so as to reduce the generated strength
42
CA 03108758 2021-02-04
anisotropy. The process (2) produces circumferential
deformation by bending and rebending, and, accordingly, the
aspect ratio basically remains unchanged. This makes the
process (2) highly effective at maintaining a small aspect
ratio and reducing strength anisotropy, though the process is
limited in terms of the amount of shape change that can be
attained by stretching or wall thickness reduction of pipe.
This process also does not require the post-work
low-temperature heat treatment needed in (1). Austenite
grains having an aspect ratio of 9 or less can have an area
fraction in a controlled range of 50% or more by controlling
the work temperature and the heating conditions of (1) within
the ranges of the present invention, or by using the process
(2).
[0063]
A heat treatment performed after the process (1) or (2)
does not change the aspect ratio. Preferably, the ferrite
phase should have a smaller aspect ratio for the same reasons
described for the austenite phase. However, the austenite
phase has a smaller yield strength, and its impact on the
Bauschinger effect after work is greater than ferrite phase.
[0064]
Examples
The present invention is further described below through
Examples.
43
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[0065]
The chemical components represented by A to L in Table
I were made into steel with a vacuum melting furnace, and the
steel was hot rolled into a round billet having a diameter 0
of 60 mm.
[0066]
44
[Table 1]
(mass%)
Steel type C Si Mn Cr Ni Mo W Cu
N Ti, Al, V, Nb B, Zr, Ca, Ta, REM Microstructure
Remarks
A 0 030 , 0 5 0.4 22.5 4.0 2 5
00 00 0 145 Ferrite + Austenite phase Present steel
A-2 0 060 03 0.3 228 4 1 15
00 0,0 0.085 Ferrite +Austenite phase Present steel
B 0 020 05 04 227 40 2.5
0.5 05 0 155 Ferrite +Austenite phase Comparative steel
0-2 0 020 03 0.3 25.1 6.8 30
00 00 0.131 Ferrite +Austenite phase Present steel
C-3 0.025 0,3 03 252 70 28
00 00 0 085 Ferrite +Austenite phase Present steel
C 0 016 08 07 247 7.7 30
09 0 0 0.054 Ferrite +Austenite phase =Present steel
D 0.016 08 0.7 25.2 71 31
13 12 0.146 Ferrite +Austenite phase Present steel
E 0 015 0 8 07 253 7,2 3.2
1 3 12 0 153 Ferrite +Austenite phase Comparative steel
F 0 015 0 7 , 07 253 7.0 32
1 8 1 2 0 135 Ti 0.1,Al 0.05, V 025, Nb 055 Ferrite +Austenite phase
Present steel
G 0.025 06 12 258 75 32 35 1 2
0 111 V 040, Nb 025 B 0005, Ca 0 005 Ferrite +Austenite phase Present
steel
H 0.028 05 05 275 85 5.5 1.9 05 0
082 ,Zr 0008, Ta 0 25, REM 0 008 Ferrite +Austenite phase Present steel
I 0 026 05 05 277 85 55 1 9 05
0 164 Zr 0008, REM 0 008 Ferrite +Austenite phase Comparative steel
J 0.075 0 5 05 335 146 4.2 46
35 0 130 V 0 1, NbØ05 B 0008, Ca 0008, Ta 0 1 Fernte +Austenite
phase Present steel
K 0.025 0.3 52 223 11 0.5 0.0
11 0 146 Ti 0 005 ---,REM 0 008 Ferrite +Austenite phase Present
steel
L 0 045 0.5 95 255 _ 25 24 0.0
0.5 0 092 Al 0 02 Zr 0 008 Ferrite +Austenite phase Present steel
0
CA 03108758 2021-02-04
After hot rolling, the round billet was recharged into
the heating furnace, and was held at a high temperature of
1,200 C or more. The material was then hot formed into a raw
seamless pipe having an outer diameter (I) of 70 mm, and an inner
diameter of 58 mm (wall thickness = 6 mm), using a Mannesmann
piercing roll mill. After hot forming, the raw pipes of
different compositions were each subjected to a solid-solution
heat treatment at a temperature that brings the fractions of
ferrite phase and austenite phase to an appropriate duplex
state. This was followed by strengthening. This was achieved
by drawing rolling, a type of axial stretching technique, and
bending and rebending, as shown in Table 2. After drawing
rolling or bending and rebending, a part of pipe was cut out,
and the microstructure was observed to confirm that the
fractions of ferrite phase and austenite phase had an
appropriate duplex state. The sample was then subjected to
an EBSD crystal orientation analysis that observed a cross
section across the wall thickness taken parallel to the pipe
axis, and austenite grains separated by a crystal orientation
angle of 15 were measured for aspect ratio. The measurement
was made over a 1.2 mm x 1.2 mm area, and the aspect ratio was
measured for austenite grains that had a grain size of 10 m
or more in terms of a diameter of an imaginary true circle.
[0067]
The drawing was performed under the conditions that
46
CA 03108758 2021-02-04
reduce the wall thickness by 10 to 30%, and the outer
circumference by 20%.
[0068]
For bending and rebending, a rolling mill was prepared
that had three cylindrical rolls disposed at a pitch of 1200
around the outer circumference of pipe (FIG. 3, (c) ) . The pipe
was processed by being rotated with the rolls rolling around
the outer circumference of pipe with a roll distance smaller
than the outer diameter of the pipe. In selected conditions,
the pipes were subjected to warm working at 150 to 550 C. In
selected conditions, the pipes after cold working and warm
working were subjected to a low-temperature heat treatment at
150 to 550 C.
[0069]
The steel pipes after the cold working, warm working,
and low-temperature heat treatment were measured for axial
tensile yield strength and compressive yield strength along
the length of pipe, and for circumferential compressive yield
strength. The steel pipes were also measured for axial tensile
yield strength, on which grading of steel pipes intended for
oil wells and gas wells is based. As an evaluation of strength
anisotropy, the steel pipes were measured for a ratio of axial
compressive yield strength to axial tensile yield strength,
and a ratio of circumferential compressive yield strength to
axial tensile yield strength.
47
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[0070]
The steel pipes were also subjected to a stress corrosion
test in a chloride-sulfide environment. The corrosive
environment was created by preparing an aqueous solution that
simulates a mining environment encountered by oil country
tubular goods (a 20% NaC1 + 0.5% CH3000H + CH3COONa aqueous
solution with added H2S gas under a pressure of 0.01 to 0.10
MPa; an adjusted pH of 3.0; test temperature ¨ 25 C). In order
to be able to longitudinally apply stress along the pipe axis,
a 4-point bending test piece with a wall thickness of 5mm was
cut out, and a stress 90% of the axial tensile yield strength
of pipe was applied before dipping the pipe in the corrosive
solution. For evaluation of corrosion, samples were evaluated
as acceptable when no crack was observed on the stressed surface
immediately after the sample dipped in the corrosive aqueous
solution for 720 hours under applied stress was taken out of
the solution. Samples were evaluated as unacceptable when a
crack was observed under the same conditions.
[0071]
The manufacturing conditions are presented in Table 2,
along with the evaluation results.
[0072]
48
[Table 2]
Processing Heat-treatment Axial tensile yield
Corrosion resistance
Runs Axial
compressive yield Circumferential compressive
Steel temp. temp strength Aspect
performance
No Process
strength/axial tensile yield yield strength/axial tensile
type Pass C C MPa ratio
Acceptable or
strength
yield strength unacceptable
1 A Drawing rolling OT - 862 68
0.83 1 08 Acceptable CE
2 A Drawing rolling OT 350 865 67 087
1 03 Acceptable PE
3 A Drawing rolling 450 - 86629_ 6.7
0.86 1.04 Acceptable PE
8 4 A Drawing rolling 550 280 67
089 1 03 Acceptable PE
A Bending and rebending OT - 865 4.5
0.96 092 Acceptable PE
6 A Bending and rebending OT 300 870 4.6
097 092 Acceptable PE
7 A Bending and rebending OT 550 86628_ 4.6
0.98 0.96 Acceptable PE
8 8 A Bending and rebending 250 250
4.6 0.96 092 Acceptable PE
9 A Bending and rebending 250 - 863 45 095
091 Acceptable PE
9-1 A Drawing rolling 620 200 672
6.7 0.92 0.99 Acceptable CE
A-2 Bending and rebending OT - 760 3.9
1.08 0.90 Acceptable PE
11 A-2 Bending and rebending 2 OT - 827 3.5
1.08 0.95 Acceptable PE
12 A-2 Bending and rebending OT 350 775 3.9
1.07 0.95 Acceptable PE
13 A-2 Drawing rolling OT - 759
8.6 0.81 1.04 Acceptable CE
14 A-2 Drawing rolling OT 350 764 8.7
0.86 1.03 Acceptable PE P
A-2 Drawing rolling 350 350 765
8.6 0.88 1.03 Acceptable PE
,..
16 B Drawing rolling OT - 868 9 2
0.79 1 05 Acceptable CE ,
17 B Drawing rolling OT 350 873 9.2
0.86 1 05 Unacceptable CE 00
-..,
.
18 B Bending and rebending OT 350 868 4.2
0.96 1.04 Unacceptable CE 0
19 C Drawing rolling OT 550 915 6.2
0.88 1.05 Acceptable PE " 20 C Bending and rebending
OT 915 1 6 11 091 Acceptable PE ,
,
21 C Bending and rebending OT 550 918 15
1 04 0.94 Acceptable PE 0
22 C-2 Bending and rebending OT 885 4 6
0 92 0.95 Acceptable PE '
.
2.
23 C-2 Bending and rebending . 0T 550 - 895 46
094 0.96 Acceptable PE
24 C-3 Bending and rebending 875 4 1 0
95 0.94 Acceptable PE
C-3 Bending and rebending OT 400 880 42
0 96 0.95 Acceptable PE
0 26 D Bending and rebending 925 46 098
0.92 Acceptable PE
27 E Bending and rebending OT 350 935 4.4
095 093 Unacceptable CE
28 E Bending and rebending 350 - 929 44
0.96 0.93 Unacceptable CE
29 F Bending and rebending OT 935 1 4
1 06 0.91 Acceptable PE
F Bending and rebending 350 350 931 1 3 1 04
0.96 Acceptable PE
31 G Bending and rebending -OT 935 47
0 91 091 Acceptable PE
32 G Bending and rebending OT 350 937 47
0 92 092 Acceptable PE
33 ,H Bending and rebending OT - 935
4.6 0.91 091 Acceptable PE
34 H Bending and rebending 350 - 933 46 0
93 0.93 Acceptable PE
I Bending and rebending 350 - 933 45 0 93
093 Unacceptable CE
36 I Bending and rebending 350 350 931 4.5
0.96 096 Unacceptable CE
37 J Bending and rebending OT - 935 47
111 091 Acceptable PE
38 J Bending and rebending OT 350 937 47
1 09 093 Acceptable PE
39 J Bending and rebending 350 350 931 47
1.06 0.96 Acceptable PE
K Bending and rebending OT - 935 3.1
091 091 Acceptable PE
41 K Bending and rebending OT 350 937 30
0.92 092 Acceptable PE
42 L Bending and rebending OT - 935 3.2
0.91 091 . Acceptable , FE
43 L Bending and rebending OT 350 937 3.3
092 093 Acceptable
OT Ordinary Temperature CE: Comparative Example, PE Present Example
49
CA 03108758 2021-02-04
As can be seen from the results shown in Table 2, the
corrosion resistance was desirable in all of the component
systems of the present examples, and the difference between
axial tensile yield strength and compressive yield strength
was small in the present examples.
