AU2019329105A1 - Duplex Stainless Steel Seamless Pipe and Method for Manufacturing Same - Google Patents

Abstract

The purpose of the present invention is to provide: a duplex stainless steel seamless pipe which has a small difference between the tensile yield strength and the compressive yield strength in the pipe axial direction, while exhibiting excellent corrosion resistance; and a method for producing this duplex stainless steel seamless pipe. A duplex stainless steel seamless pipe which has a component composition containing, in mass%, from 0.005% to 0.08% of C, from 0.01% to 1.0% of Si, from 0.01% to 10.0% of Mn, from 20% to 35% of Cr, from 1% to 15% of Ni, from 0.5% to 6.0% of Mo and 0.005% or more but less than 0.150% of N, with the balance being made up of Fe and unavoidable impurities, and which is configured such that: the tensile yield strength in the pipe axial direction is 689 MPa or more; and the value of (compressive yield strength in pipe axial direction)/(tensile yield strength in pipe axial direction) is from 0.85 to 1.15.

Description

DESCRIPTION

Title of Invention: DUPLEX STAINLESS STEEL SEAMLESS PIPE AND

METHOD FOR MANUFACTURING SAME

Technical Field

[0001]

The presentinvention 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 strengthhavingasmalldifferencemeans 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.

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.

[00031

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 ofpreventing 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 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.

[00051

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, aduplex stainless steel, in an as-processed formafter

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 bothalongitudinalcoldrollingprocess

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 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.

[00061

PTL 1 addresses this issue by proposing a duplex

stainless steelpipe 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:

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 axialdirection of the pipe, a tensile yield strength, YSCT,

alongacircumferentialdirection ofthe duplex stainless steel

pipe, and a compressive yield strength, YScc, along the

circumferential direction of the pipe satisfy predetermined

formulae.

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 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 shouldbe 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 1,0000C 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 cold rolling, shows high corrosion resistance performance with the presence of a solid solution of the elements that effectively provide corrosion resistance.

[00121

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 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.

[00141

First, the presentinventors investigated arelationship

between Ncontent and nitride content in a heat treatment. FIGS.

1 and 2 represent SUS329J3L (22% Cr stainless steel; FIG. 1)

and SUS329J4L (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 ofimportant elements for corrosion resistance performance.

In both steels, the amount of nitride increased with increasing

N contents, consuming increasing amounts of

corrosion-resistant elements in the form of precipitates.

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 andMo (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

axisdirection. 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 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 Ncontents. In the systemofcomponents

representedin FIG. 1, only the Ncontent 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% of the yield strength, fixed. A corrosive environment was created by preparing an aqueous solution (a 20% NaCl + 0.5%

CH 3 COOH + CH3COONa aqueous solution with added H 2 S 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 Ncontents, thoughno corrosion occurred withNcontents

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 materialmicrostructure, 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.

[00181

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 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] Amethod formanufacturing the duplex stainless steel

seamless pipe of any one of items [1] to [5],

the method comprising stretching along a pipe axis

direction followedby aheat treatment at aheating temperature

of 150 to 6000C, excluding 460 to 480°C.

[7] Amethod formanufacturing 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 6000C, excluding 460 to

4800C.

[8] The method according to item [7], wherein the

stretching is followed by a heat treatment at a heating

temperature of 150 to 6000C, excluding 460 to 4800C.

[9] Amethod formanufacturing 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 6000C or less, excluding 460 to 4800C.

[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 6000C, excluding 460 to 4800C.

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

bemore freely designedwhile 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

[00211

The present invention is described below.

[0022]

The reasons for limiting the composition of a steel pipe

of the presentinvention 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, Cimpairs the corrosionresistance 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.

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 ofdeoxidation effect, the Sicontentis preferably 0.2%

or more, and is preferably 0.8% or less.

[00251

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, andMn 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 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.

[00261

Cr: 20 to 35%

Cr is the most important element in terms of increasing

the strength of the passivation 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 therefore desirable to make active use of nickel when the use ofmanganese 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 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 limitis not particularly limited.

However, excessively low N contents complicate the melting

process, and lead to poor productivity. For this reason, the

lower limit ofNcontentis 0.005% ormore. Containingnitrogen

in amounts that are not an issue in terms of corrosion

resistance allows for cost reduction by allowing the other

austenite phase-formingelements Ni, Mn, and Cu to be contained in reduced amounts. To this end, the N content is preferably

0.08% or more, and is preferably 0.14% or less.

[00301

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 ofproblems, 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 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.

[00331

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 providedby 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 workability, the Cu content is more preferably 1.0% or more, and is more preferably 3.0% or less.

[00341

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 canproduce acorrosion

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.

[00361

The following elements may also be appropriately contained in the present invention, as needed.

[00371

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 alsoincrease the alloy cost when addedinexcess

amounts. For this reason, the upper limits of B, Zr, Ca, and

REMare 0.010% or less. When addedin smallamounts, Tareduces

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 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.

[00391

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 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 steelmaterial 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 atmosphericmelting furnacemaybe usedwhenmaking the steel

by electricmelting ofiron scrap or amass ofvarious elements.

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 steelundergoes 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 ofproducts. 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 embrittlementphase, whichleads to seriousimpairment 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 ofthese twophases, 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 transformation 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 ahigh temperature of1,000°C or more, though the temperature that dissolves the precipitates, the 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 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 bringchangesmostlyin 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 effectin a direction ofcompression along the pipe

axis, and reduces the axial compressive yield strength by as

large as about 20% relative to the axialtensile yield strength.

[0047]

To avoid this, in the present invention, a heat treatment

is performed in a temperature range of 150 to 600C, excluding

460 to 480C, after the pipe is stretched along the pipe axis.

Provided that the Ncontent 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 axis in a temperature range of 150 to 6000C, excluding 460 to

4800C. 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 treatmentmay follow stretchingperformedin a temperature

range of 150 to 6000C, excluding 460 to 480C, 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 6000C. Work temperatures of 460 to 4800C 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.

[0050]

A rapid yield strength drop occurs when the heating

temperature of the heat treatment and the stretching

temperature are below 1500C. In order to avoid this and to

sufficiently produce the work load reducing effect, these

processes are performed at a temperature of 1500C or more.

Preferably, the temperature is 350 to 4500C 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 ofpipe 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 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.

[00521

FIG. 3, (a) and (b) show cross sectional views 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 maybe moved or shifted in such a manner as to rotate the steel pipe and make contact with portions ofpipe that were not flattenedby the first flattening

(portions flattened by the first flattening are indicated by

shadow).

[00531

As illustrated in FIG. 3, the circumferential bending

andrebending that flattens the steelpipe, whenintermittently

or continuously applied throughout the pipe circumference,

produces strain in the pipe, with bending strain occurring in

portions where the curvature becomes the largest, andrebending

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 reducedwallthickness and reducedouter 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]

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 90° 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, forexample, 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 systembecomes more

complex.

[0055]

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 strengtheningprovidedby the work, and the applied

temperature should not exceed 600C. Work temperatures of 460

to 4800C 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 4800C. The

lower limit of work temperature is preferably 1500C 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 4500C from a standpoint of saving energy and

avoiding passing the embrittlement phase during heating and 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.

[00561

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 1500C or more because a heating

temperature of less than 1500C 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 450C from

a standpoint of saving energy and avoiding passing the

embrittlement phase during heating and cooling, in addition 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 steelcontains 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 exerted deepunderground, and thismakes the pipe useless. For this reason, many pipes have an axial tensile yield strength of at most 1033.5 MPa.

[00581

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.

[00591

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

crushingunder the externalpressure exerted deepunderground,

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

saturatedwhen the ratiois about1.50. When the strengthratio

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.

[00601

In the present invention, the aspect ratio of austenite

grains separated by a crystal orientation angle difference of

° 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 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 amicrostructure having aplurality ofcrystal grains separated by an orientation angle of 150 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 andafterwork in a cross section across the wall thickness along the pipe axis, the yield strength decreases in the opposite direction because of the Bauschinger effect, with the result that the difference between compressive yield strength and axial tensile yield strength increases.

This means that a steelpipe 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.

[00611

In the presentinvention, a stable steelpipe 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 150

ormore observedin the austenite phase in acrystalorientation

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 diameters may cause errors in the aspect ratio. It is accordinglypreferable that the austenite grainused for aspect ratio measurement be at least 10 pm 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 thanbefore work, and this tends toproduce

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 axialstretch

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 5600C or less) so as to reduce the generated strength 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).

[00631

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.

[00651

The chemical components represented by A to L in Table

1 were made into steel with a vacuum melting furnace, and the

steel was hot rolled into a round billet having a diameter#

of 60 mm.

[0066]

E 0)0)c cL L , o L ci, o cL ci, L i, o cL ci, oL c

2 -92 -2 -92 -92 -92 -92 -92 -92 -92 -92 -92 -92

oo 0~ ~00~0~ ~ 0 0~0~ ~ 0 0~0~ c 0 u~u~u u ~ ~ ~u~u u u ~ ~uqu

Of c c u) -- Lij .... ...CDLo c CD C co qqqc q c6 0 9 - wD L6 oN oNoc C o

6 Dc (DC

C)C

L6 666

~~~~~~ Lfl~~~~~~~~C -o f =: f O~Lf -a : N ~C : 0 N - L~~ O C -

Lo LoL o q (Jm L~ clf qt C) o l

C) CO CNJ CNJ CNJ CNJ m - N - N -~

r-H r-L L o o L 0o r- r- r- r- - - o o

LO C - - NLo r o m L c < c U l o L o L O O r-r-c)cjL

C/)

cD D c cDLO o o oLo 45L

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 # 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 pm

or more in terms of a diameter of an imaginary true circle.

[0067]

The drawing was performed under the conditions that reduce the wall thickness by 10 to 30%, and the outer circumference by 20%.

[00681

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 5500C. In

selected conditions, the pipes after cold working and warm

working were subjected to a low-temperature heat treatment at

150 to 5500C.

[00691

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 steelpipes 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.

[00701

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% NaCl + 0.5% CH 3 COOH + CH3COONa aqueous

solution with added H 2 S 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 5 mm 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 whenno crack was observedon 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]

C cio 0 So o Q a E0E < 0 0 0 -) Q c-) c-) c-, c-, Q c-, Z., Q Z., Z., oi Q oi oi o Q o co, Q c ~ Ico o o o cc c, c-) c-) c-) c-) (. .5 ( . ci~ c , (. ,o i o i o o, c c w c , c- , c- , c- , c. , c. , c.) c, QQ Q QQ co c ro Q r QQ

C) C) 0CC CC CC CC CC CC CC -C- - -C- - - C) C) C) C) C) C) C101 C) C0 C) C) C) C) )C 0C) CDC D D

o I

o~ o

ci,.-

82 > i ce,

2E =s Eo

cE. c

c,

o O pl l lO 01 0 LO O OO M- q1 O110 0zlz-z-mP- -1 0LO LO O- P-P oE u to to to (o ( o to to to to to to m(m C- Is- s- Is- Lf- C- Co- Lf- I-- 0- Lf 0- Co Is- Co-N (N- N- s- Co Lf- C-o- s- o s- s-s s- I-- Is-C Co L- L- I- - Is- Is- C- (N e C o oo to oo co coc a o t - or-r-r-r-c oc m oo co co m m m m m m2m Om La Co a a aa C aa) a aa a a a LO LO -

o N-o - )-J-o c a cN cm cm - cNLa cC c cO oa uc- aa c -a m-o-a-oo co co) co) co 0 c)) c ) cc) c0) c

2 aEi, c~ 0o - 2 co

cm x LO LOl LO LO (N LO LO )L L L LO .E ) c'j c'j o cl LO m ~ ~ Cm l

u)'- ci CD CD C C C C CD C DC C C CD CD C D C D C C C C C C CCCCCCCCCCC CC

lol000 00000 0000000 0000 0 000, 00 000 C)C)bb I- - - - - - 02l ------------

u)J om mo o om m E

o __ 49

-03 - - - -c o o o- o io iF u1° 2 001)--,,22

0

C) 0- ci) JcN( (NJJo mN (o (NJ(NM lq O( . o0 ) cl l tu cP

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.

Claims (11)

1. [Claim 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. [Claim 2]

   The duplex stainless steel seamless pipe according to

   claim 1, which has a ratio of 0.85 or more as a fraction of

   circumferential compressive yield strength to axial tensile

   yield strength.
3. [Claim 3]

   The duplex stainless steel seamless pipe according to

   claim 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. [Claim 4]

   The duplex stainless steel seamless pipe according to

   any one of claims 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. [Claim 5]

   The duplex stainless steel seamless pipe according to

   any one of claims 1 to 4, which further comprises, in 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. [Claim 6]

   A method for manufacturing the duplex stainless steel

   seamless pipe of any one of claims 1 to 5,

   the method comprising stretching along a pipe axis

   direction followedby aheat treatment at aheating temperature

   of 150 to 600°C, excluding 460 to 480°C.
7. [Claim 7]

   A method for manufacturing the duplex stainless steel

   seamless pipe of any one of claims 1 to 5,

   the method comprising stretching along a pipe axis

   direction at a temperature of 150 to 600°C, excluding 460 to

   480 0 C.
8. [Claim 8]

   The method according to claim 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. [Claim 9]

   A method for manufacturing the duplex stainless steel

   seamless pipe of any one ofclaims 1 to 5, the method comprising

   circumferential bending and rebending.
10. [Claim 10]

    The method according to claim 9, wherein the

    circumferential bending and rebending is performed at a

    temperature of 6000C or less, excluding 460 to 480°C.
11. [Claim 11]

    The method according to claim 9 or 10, wherein the bending

    and rebending is followed by a heat treatment at a heating

    temperature of 150 to 6000C, excluding 460 to 480°C.