AU2022240057A1 - Duplex stainless steel pipe and method for manufacturing same - Google Patents

Abstract

Provided are a high-strength duplex stainless steel pipe having an excellent wear resistance or dent resistance on the inner and outer surfaces of a steel pipe, and a method for manufacturing the same.　The present invention has a component composition containing, by mass, 0.005 to 0.150% of C, 1.0% or less of Si, 10.0% or less of Mn, 11.5 to 35.0% of Cr, 0.5 to 15.0% of Ni, 0.5 to 6.0% of Mo, and less than 0.400% of N, with the remainder made up by Fe and unavoidable impurities, and has a steel structure having a ferrite phase and an austenite phase. The pipe axial direction tensile yield strength is 689 MPa or above, and the present invention has, on the steel pipe outer surface and the steel pipe inner surface, an oxide layer having a thickness of 1.0 μm or above.

Description

DESCRIPTION

Title of Invention: DUPLEX STAINLESS STEEL PIPE AND METHOD

FOR MANUFACTURING SAME

Technical Field

[0001]

The present invention relates to a stainless steel

pipe having excellent axial tensile yield strength with

excellent abrasion resistance and indentation resistance,

and to a method for manufacturing such a stainless steel

pipe. Here, "excellent axial tensile yield strength" means

a yield strength of 689 MPa or more.

Background Art

[0002]

Steel pipes used for extraction of oil and gas from

oil wells and gas wells (hereinafter, also referred to

simply as "steel pipes for oil wells") or steels pipes for

geothermal wells are required to have corrosion resistance

performance that can withstand use in highly corrosive high

temperature and high-pressure environments, and high

strength characteristics that can withstand the tensile

stress due to the weight of pipes joined to extend deep into

the ground, and the thermal stress and high pressure associated with high temperature. In order to have excellent corrosion resistance performance, steel needs to contain corrosion-resistance improving elements (e.g., Cr,

Mo, W, and N) in adjusted amounts. In this connection,

various duplex stainless steels are available, including,

for example, SUS329J3L containing 22 mass% of Cr, SUS329J4L

containing 25 mass% of Cr, and ISO S32750 and S32760

containing increased amounts of Mo.

[00031

In order to provide high strength characteristics, it

is important to adjust the axial tensile yield strength,

and a value of axial tensile yield strength represents the

specified strength of the product. This is important

because the pipe needs to withstand the tensile stress due

to its own weight when joined to extend deep into the ground.

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 passive film that is important for keeping the pipe

surface corrosion resistant.

[0004]

In this respect, duplex stainless steels such as above

have a duplex microstructure with a ferritic phase

coexisting with an austenitic phase which is

crystallographically low in yield strength. Because of this, hot forming and a heat treatment alone are not enough to provide the tensile strength needed for oil well or geothermal well applications.

The axial tensile yield strength of a duplex stainless

steel pipe to be used for oil well or geothermal well

applications is therefore provided by dislocation

strengthening using various types of cold rolling. Cold

drawing and cold pilgering are two cold rolling techniques

available for pipes to be used for oil well or geothermal

well applications, as defined by NACE (The National

Association of Corrosion Engineers), which provides

international standards for use of oil well pipes. These

cold rolling techniques both represent a longitudinal

rolling process that reduces the wall thickness and the

diameter of a pipe. A steel pipe to be subjected to these

cold rolling processes needs to be cleaned with an acid, or

a lubricant coating needs to be formed by chemical treatment

before cold rolling, in order to reduce defects in the

product, or to protect the tools. When a lubricant coating

is formed, the steel pipe needs to be cleaned with an acid

after cold rolling.

[00051

Steel pipes intended for oil well or geothermal well

applications are used outdoors, often in places that are

not leveled. During extraction or passing of oil or hot water through the steel pipe, the steel pipe often collides with hard objects such as stones. Scraping or collision between steel pipes is also common when inserting a steel pipe into another steel pipe, or when transporting steel pipes. When joining steel pipes, clamping with a fastening tool exerts a high contact pressure on steel pipe surface.

Such collisions with hard objects, colliding and scraping

of steel pipes, and contact pressure of a fastening tool

cause scratch defects and indentations on inner and outer

surfaces of a steel pipe.

These scratch defects and indentations become

initiation points of corrosion. When excessively large,

indentations also affect product dimensions. For example,

the wall thickness decreases in proportion to the depth of

a scratch defect or an indentation, causing a decrease of

axial tensile strength, which is an important characteristic

of a steel pipe.

As discussed above, duplex stainless steel pipes to

be used for oil well or geothermal well applications require

not only high strength and high corrosion resistance but

the ability to reduce scratch defects and indentations on

inner and outer surfaces of a steel pipe. That is, the

inner and outer surfaces of steel pipes to be used for these

applications need to have excellent abrasion resistance and

indentation resistance.

[00061

In this regard, a duplex stainless steel pipe is

produced through dislocation strengthening by cold rolling,

in order to provide a high axial tensile yield strength, as

described above. Before cold rolling, steel pipe surfaces

are cleaned with an acid to remove the surface oxide layer,

in order to reduce damage such as that experienced by a

rolling tool during cold rolling. Alternatively, a highly

lubricative chemical-treatment coating is formed to prevent

galling during cold rolling. In this case, the surface

oxide layer is removed with the chemical-treatment coating

after cold rolling. Cold rolling increases the surface area

of a steel pipe by reducing the wall thickness and

stretching the pipe along its axis. Accordingly, a steel

pipe after cold rolling does not have the surface oxide

layer, and, because of an increased surface area, the steel

pipe has a bare metal surface with a metallic sheen.

However, with its metal surface exposed, the steel

pipe is more susceptible to scratch defects and indentations

such as above. That is, a conventional duplex stainless

steel pipe produced by cold rolling has a bare metal surface

to provide high strength, and is susceptible to scratch

defects and indentations.

[00071

Various techniques are available concerning steel pipes. For example, PTL 1 and PTL 2 disclose steel pipes having improved hardness and abrasion resistance of inner surfaces. PTL 3 discloses a clad steel pipe in which a material that is high in hardness and abrasion resistance is joined to a base material.

Citation List

Patent Literature

[00081

PTL 1: Japanese Unexamined Patent Application

Publication No. 57-194213

PTL 2: Japanese Unexamined Patent Application

Publication No. 1-15323

PTL 3: Japanese Unexamined Patent Application

Publication No. 63-290616

Summary of Invention

Technical Problem

[00091

However, the techniques described in PTL 1 to PTL 3

lack consideration with regard to improvement of all of

strength characteristics, abrasion resistance, and

indentation resistance, which are required for oil well or

geothermal well applications described above, and further

improvements are needed.

[0010]

The present invention has been made under these

circumstances, and it is an object of the present invention

to provide a duplex stainless steel pipe that is high in

strength and has excellent abrasion resistance and

indentation resistance of inner and outer surfaces of the

steel pipe. The invention is also intended to provide a

method for manufacturing such a stainless steel pipe.

[0011]

In the present invention, "high strength" means an

axial tensile yield strength of 689 MPa or more as measured

by a JIS Z2241 tensile test when a round-bar tensile test

specimen taken parallel to the pipe axis at a middle portion

of the wall thickness and having a diameter of 5.0 mm at a

parallel portion is stretched to break at room temperature

(250C) with a crosshead speed of 1.0 mm/min.

In the present invention, abrasion resistance and

indentation resistance are excellent when an indented

portion created by a scratch test has an indentation height

of 50 pm or less as measured in a middle portion of the

length of the indented portion relative to an unindented

raised portion after a pipe is scratched by sweeping a pipe

surface over a distance of 30 mm at 3 mm/s along the pipe

axis under a 59 N load of an indenter having a cemented

carbide tip (a circular cone indenter having a tip angle of

600 (a point of contact with a steel pipe) in a triangular

cross section perpendicular to the base of the circular

cone) .

Solution to Problem

[0012]

In order to achieve the foregoing objects, the present

inventors conducted intensive studies of a duplex stainless

steel pipe.

To increase the corrosion resistance of a duplex

stainless steel pipe, corrosion-resistant elements Cr and

Mo must be added, and a corrosion resistance-reducing

element C must be reduced. Addition of Cr and Mo and

reduction of C increase the ferritic phase in the

microstructure of the product. However, when the ferritic

phase increases excessively, the duplex microstructure

fails to provide excellent corrosion resistance performance,

and low-temperature toughness decreases. In order to

protect a duplex stainless steel pipe from various forms of

corrosion, it is therefore important that elements such as

Ni, N, and Mn, which increase the austenitic phase, are

added in a well-balanced manner to produce an appropriate

duplex ferritic and austenitic phase in the microstructure

of the product.

[0013]

In order to produce an appropriate duplex state in a

duplex stainless steel pipe, a solid solution heat treatment

is required, in addition to the appropriate addition of

chemical components that form the ferritic phase and

austenitic phase.

Stabilization of corrosion resistance performance is

possible when a solid solution heat treatment produces

appropriate fractions of the two phases, and when

precipitates and an embrittlement phase that are formed

during cooling and hot forming after solidification and are

harmful to corrosion resistance are dissolved in steel and

the corrosion-resistant elements are dispersed evenly in

the steel.

[0014]

A duplex stainless steel pipe can have high corrosion

resistance performance by adjusting the chemical components

and performing a solid solution heat treatment. However,

the austenitic phase decreases the yield strength of the

duplex stainless steel pipe. Accordingly, an axial tensile

yield strength of 689 MPa or more required for steel pipes

to be used for oil well or geothermal well applications

cannot be obtained by simply adjusting the chemical

components and performing a solid solution heat treatment.

For this reason, in manufacture of a duplex stainless steel

pipe, the solid solution heat treatment is followed by cold rolling dislocation strengthening to provide the desired strength.

[00151

Cold drawing or cold pilgering is a conventional

method of cold rolling for increasing steel pipe strength.

These rolling methods involve reduction of wall thickness

or axial stretching of a steel pipe.

The solid solution heat treatment discussed above must

be performed before these cold rolling processes. This is

because the dislocation provided by cold rolling is

annihilated, and the effect of cold rolling to improve yield

strength cannot be obtained when a steel pipe is subjected

to high temperature such as in a solid solution heat

treatment after cold rolling. The solid solution heat

treatment performed before cold rolling forms oxide layers

on inner and outer surfaces of a steel pipe.

The oxide layers on inner and outer surfaces of a

steel pipe before cold rolling are removed with an acid

before a commonly performed cold drawing or pilger rolling

because of a possibility of damaging tools used for cold

rolling. An alternative way of protecting tools is to form

a lubricative lubricant coating on a steel pipe surface by

a chemical treatment, and remove the coating with the oxide

layer by cleaning after cold rolling. Removal of oxide

layers before or after cold rolling results in bare metal surfaces inside and outside of the steel pipe.

[00161

Cold rolling is also a process that exposes metal on

steel pipe surfaces. Specifically, cold drawing and cold

pilgering are rolling methods that involve reduction of wall

thickness and stretching of a steel pipe, and, accordingly,

the metallic portion, which is the base material, increases

its surface area. Unlike the base material, the oxide layer

lacks ductility, and cannot follow the deformation. This

results in even more exposure of metal on newly-formed

surfaces of the steel pipe after cold rolling.

[0017]

For the reasons discussed above, a current duplex

stainless steel product inevitably has bare metal surfaces

if it were to have high corrosion resistance and high axial

tensile yield strength. When such a steel pipe is used in

oil well or geothermal well applications, defects or

indentations occur when the steel pipe scrapes or collides

with hard objects or with other steel pipe, or when contact

pressure is exerted upon by a joining tool. Such

degradation of the product surface leads to damage or

corrosion in the steel pipe, and the resulting decrease of

dimensional accuracy causes a decrease of axial compressive

yield strength and circumferential tensile yield strength.

[0018]

By focusing on these points, the present inventors

conducted investigation of a technique to produce a steel

pipe without removing surface oxide layers. The

investigation led to the finding that excellent abrasion

resistance and indentation resistance can be achieved while

ensuring high strength and high corrosion resistance when a

solid solution heat treatment is performed under specific

conditions, and when cold circumferential bending and

reverse bending is performed without removing the oxide

layers formed.

[0019]

The present invention has been made on the basis of

this finding, and the gist of the present invention is as

follows.

[1] A duplex stainless steel pipe having a composition

that contains, in mass%, C: 0.005 to 0.150%, Si: 1.0% or

less, Mn: 10.0% or less, Cr: 11.5 to 35.0%, Ni: 0.5 to 15.0%,

Mo: 0.5 to 6.0%, N: less than 0.400%, and the balance being

Fe and incidental impurities, and having a microstructure

with a ferritic phase and an austenitic phase,

the duplex stainless steel pipe having an axial

tensile yield strength of 689 MPa or more, and having an

outer surface and an inner surface each having an oxide

layer having an average thickness of 1.0 pm or more.

[2] The duplex stainless steel pipe according to [1], wherein the oxide layer covers at least 50% of the outer surface and at least 50% of the inner surface of the steel pipe in terms of an area percentage.

[3] The duplex stainless steel pipe according to [1]

or [2], which has an axial compressive yield strength-to

axial tensile yield strength ratio of 0.85 to 1.15.

[4] The duplex stainless steel pipe according to any

one of [1] to [3], wherein the composition further contains,

in mass%, one or two or more selected from W: 6.0% or less,

Cu: 4.0% or less, V: 1.0% or less, and Nb: 1.0% or less.

[5] The duplex stainless steel pipe according to any

one of [1] to [4], wherein the composition further contains,

in mass%, one or two selected from Ti: 0.30% or less and

Al: 0.30% or less.

[6] The duplex stainless steel pipe according to any

one of [1] to [5], wherein the composition further contains,

in mass%, one or two or more selected from B: 0.010% or

less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or

less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.010%

or less.

[7] A method for manufacturing a duplex stainless

steel pipe of any one of [1] to [6],

the method including:

hot rolling a steel pipe material into a shape of a

steel pipe; subjecting the steel pipe material after the hot rolling to a solid solution heat treatment that satisfies the formula (1) below; and performing cold circumferential bending and reverse bending without removing an oxide layer formed on the steel pipe material after the solid solution heat treatment,

Tmax 2 x t/[Cr] 4 > 1,000 (1),

wherein Tmax is a highest heating temperature(°C) of the

solid solution heat treatment, t is a retention time (s) at

the highest heating temperature of the solid solution heat

treatment, and [Cr] is the content of Cr (mass%) in the

steel pipe.

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

heating temperature in the hot rolling is 1,1500C or more.

[9] The method according to [7] or [8], wherein the

cold bending and reverse bending reduces a diameter of the

steel pipe material to (Di/Do) x 100 = 99% or less, where

Di is an outside diameter of the steel pipe material after

work, and Do is an outside diameter of the steel pipe

material before work.

[10] The method according to any one of [7] to [9],

wherein (Li/Lo) x 100 (%) is 125% or less after the cold

bending and reverse bending, where Li is an axial length of

the steel pipe material after work, and Lo is an axial

length of the steel pipe material before work.

Advantageous Effects of Invention

[0020]

With the present invention, high abrasion resistance

and high indentation resistance can be achieved while

ensuring excellent axial tensile yield strength. Scratch

defects and indentations caused by collision and scraping

can therefore be stably reduced even in oil well or

geothermal well applications where temperature and pressure

are high and the environment is highly corrosive.

Brief Description of Drawings

[0021]

FIG. 1 is a graph explaining the thickness of an oxide

scale layer and its effect to reduce surface defect.

FIG. 2 shows schematic views representing

circumferential bending and reverse bending.

Description of Embodiments

[0022]

The present invention is described below.

[0023]

A duplex stainless steel pipe of the present invention

has a composition that contains, in mass%, C: 0.005 to

0.150%, Si: 1.0% or less, Mn: 10.0% or less, Cr: 11.5 to

35.0%, Ni: 0.5 to 15.0%, Mo: 0.5 to 6.0%, N: less than

0.400%, and the balance being Fe and incidental impurities,

and has a microstructure with a ferritic phase and an

austenitic phase. The duplex stainless steel pipe of the

present invention has an axial tensile yield strength of

689 MPa or more, and has an outer surface and an inner

surface each having an oxide layer having an average

thickness of 1.0 pm or more.

[0024]

The reasons for limiting the composition of the duplex

stainless steel pipe of the present invention are described

first. It is to be noted that "%" used in conjunction with

the content of each component means "mass%".

[0025]

C: 0.005 to 0.150%

C deteriorates corrosion resistance. Increasing the

C content causes a transformation of austenitic phase into

martensitic phase, and makes cold rolling and cold working

difficult. The C content is therefore 0.150% or less to

obtain appropriate corrosion resistance performance and an

appropriate duplex structure. The C content is 0.005% or

more because the decarburization cost of smelting increases

when the C content is too low. The C content is preferably

0.080% or less.

[0026]

Si: 1.0% or Less

Remaining Si in steel due to excess Si content has a

possibility of impairing workability and low-temperature

toughness. For this reason, the Si content is 1.0% or less.

Preferably, the Si content is 0.8% or less. Si acts to

deoxidize steel, and it is effective to add this element to

the molten steel in appropriate amounts. To this end, the

Si content is preferably 0.01% or more. In view of providing

a sufficient deoxidizing effect while reducing the side

effects of remaining excess Si in steel, the Si content is

more preferably 0.2% or more.

[0027]

Mn: 10.0% or Less

An excessively high Mn content decreases low

temperature toughness. For this reason, the Mn content is

10.0% or less. The Mn content is preferably less than 1.0%

when low-temperature toughness needs to be increased. Mn

is a strong austenitic phase-forming element, and is

available at lower costs than other austenitic phase-forming

elements. Mn is also effective at neutralizing the impurity

element S that mixes into the molten steel, and Mn has the

effect to fix S by forming MnS with S, which greatly impairs

the corrosion resistance and toughness of steel even when

added in trace amounts. From this viewpoint, the Mn content

is preferably 0.01% or more. When there is a need to take advantage of Mn as an austenitic phase-forming element to achieve cost reduction while providing low-temperature toughness, the Mn content is more preferably 2.0% or more.

The Mn content is more preferably 8.0% or less.

[0028]

Cr: 11.5 to 35.0%

Cr is an element that increases the strength of the

passive film of steel, and improves corrosion resistance.

Cr is also an element that is needed to stabilize the

ferritic phase and obtain an appropriate duplex structure.

In the present invention, the Cr content needs to be 11.5%

or more to obtain a duplex structure and high corrosion

resistance. Cr is an underlying element that stabilizes

the passive film, and the passive film becomes stronger as

the Cr content increases. Accordingly, increasing the Cr

content contributes to improving the corrosion resistance.

However, with a Cr content of more than 35.0%, 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 of Cr

content is 35.0%. Taken together, the Cr content is 11.5

to 35.0% in the present invention. From the viewpoint of

ensuring corrosion resistance and manufacturability at the

same time, the Cr content is preferably 20% or more. The

Cr content is preferably 28% or less.

[00291

Ni: 0.5 to 15.0%

Ni is an expensive element compared to other

austenitic phase-forming elements, and an increased Ni

content leads to increased manufacturing costs. For this

reason, the Ni content is 15.0% or less. Ni is a strong

austenitic phase-forming element, and improves the low

temperature toughness of steel. It is therefore desirable

to make active use of Ni when the use of Mn as an inexpensive

austenitic phase-forming element is an issue for low

temperature toughness. To this end, the Ni content is 0.5%

or more. When low-temperature toughness is not of concern,

it is preferable to use Ni in combination with other

elements with the Ni content of 0.5 to 5.0%. On the other

hand, when high low-temperature toughness is needed, it is

effective to actively add Ni, preferably in an amount of

5.0% or more. The Ni content is preferably 13.0% or less.

[0030]

Mo: 0.5 to 6.0%

Mo increases the pitting corrosion resistance of steel

in proportion to its content. To this end, Mo needs to be

uniformly present on surfaces of steel material exposed to

a 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 solid microstructure, and greatly impairs stability in subsequent forming. For this reason, the Mo content is 6.0% or less. Mo increases the pitting corrosion resistance in proportion to its content.

However, the Mo content needs to be 0.5% or more to maintain

stable corrosion resistance in a sulfide environment. For

these reasons, the Mo content is 0.5 to 6.0% in the present

invention. From the viewpoint of satisfying both the

corrosion resistance and production stability needed for

the duplex stainless steel pipe, the Mo content is

preferably 1.0% or more. The Mo content is preferably 5.0%

or less.

[0031]

N: Less than 0.400%

While N itself is inexpensive, excessive addition of

N requires specialty equipment and time, and increases the

manufacturing cost. For this reason, the N content is less

than 0.400%. N is a strong austenitic phase-forming element,

in addition to being inexpensive. In the form of a solid

solution in steel, N is an element that is useful for

improving corrosion resistance performance and strength.

There is no particular need to set limits for N content, as

long as the product can have an appropriate duplex fraction

with N and other austenitic phase-forming elements. However, an overly low N content necessitates a high degree of vacuum for smelting and refining, and restricts the types of raw materials that can be used. For this reason, the N content is preferably 0.010% or more.

[0032]

The balance in the composition above is Fe and

incidental impurities.

[0033]

Additionally, the following elements may be

appropriately contained in the present invention, as needed.

[0034]

One or Two or More Selected from W: 6.0% or Less, Cu: 4.0%

or Less, V: 1.0% or Less, and Nb: 1.0% or Less

W: 6.0% or Less

As is Mo, W is an element that increases the pitting

corrosion resistance in proportion to its content. However,

when contained in excess amounts, W impairs the workability

of hot working, and damages production stability. For this

reason, W, when contained, is contained in an amount of 6.0%

or less. W improves pitting corrosion resistance in

proportion to its content, and the W content does not

particularly require a lower limit. It is, however,

preferable to add W in an amount of 0.1% or more, in order

to stabilize the corrosion resistance performance of the

duplex stainless steel pipe. From the viewpoint of the corrosion resistance and production stability needed for the duplex stainless steel pipe, the W content is more preferably 1.0% or more. The W content is more preferably

5.0% or less.

[00351

Cu: 4.0% or Less

Cu is a strong austenitic 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 austenitic 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

a lower limit. However, the corrosion resistance improving

effect can be obtained when the Cu content is 0.1% or more.

From the viewpoint of satisfying both improvement of

corrosion resistance and hot workability, the Cu content is

more preferably 1.0% or more. The Cu content is more

preferably 3.0% or less.

[00361

V: 1.0% or Less

Excess addition of V impairs low-temperature toughness,

and the V content is preferably 1.0% or less when V is contained. Because V is also effective for improving strength, this element can be contained when higher strength is required. The strength improving effect can be obtained with a V content of 0.01% or more. For this reason, the V content is preferably 0.01% or more when V is contained.

Because V is an expensive element, the V content is

preferably 0.40% or less from the view point of its strength

improving effect and cost. The V content is more preferably

0.10% or less, even more preferably 0.06% or less. The V

content is more preferably 0.05% or more.

[0037]

Nb: 1.0% or Less

Excess addition of Nb impairs low-temperature

toughness, and the Nb content is preferably 1.0% or less.

Because Nb is also effective for improving strength, this

element can be contained when higher strength is required.

The strength improving effect can be obtained with a Nb

content of 0.01% or more. For this reason, the Nb content

is preferably 0.01% or more when Nb is contained. As is V,

Nb is an expensive element, and the Nb content is preferably

0.40% or less from the view point of its strength improving

effect and cost. The Nb content is more preferably 0.10%

or less, even more preferably 0.06% or less. The Nb content

is more preferably 0.05% or more.

[0038]

The following elements may also be appropriately

contained in the present invention, as needed.

[00391

One or Two Selected from Ti: 0.30% or Less and Al: 0.30% or

Less

Ti: 0.30% or Less

The Ti content is preferably 0.30% or less because

increasing the Ti content decreases the low-temperature

toughness of steel pipe. Ti is capable of refining the

solidified microstructure and fixing the excess C and N,

and may be appropriately contained when control of

microstructure or adjustments of chemical components are

needed. When containing Ti, these effects can be obtained

with a Ti content of 0.0001% or more. The Ti content is

more preferably 0.001% or more. The Ti content is more

preferably 0.10% or less.

[0040]

Al: 0.30% or Less

Al impairs toughness when this element remains in

large amounts in steel pipe. For this reason, the Al content

is preferably 0.30% or less when Al is contained. The Al

content is more preferably 0.10% or less, even more

preferably 0.02% or less.

Al is also effective as a deoxidizing agent in

refining. To obtain this effect, the Al content is preferably 0.01% or more.

[0041]

The following elements may also be appropriately

contained in the present invention, as needed.

[0042]

One or Two or More Selected from B: 0.010% or less, Zr:

0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb:

0.30% or less, Sn: 0.30% or less, and REM: 0.010% or less

B, Zr, Ca, and REM impair hot workability when

contained in excessively large amounts, and, because these

are rare elements, B, Zr, Ca, and REM raise the alloying

cost when the content is excessively high. For this reason,

the content is preferably 0.010% or less for each of B, Zr,

Ca, and REM. The content is more preferably 0.0015% or less

for each of Ca and REM.

When contained in trace amounts, B, Zr, Ca, and REM

improve bonding at grain boundaries, and improve hot

workability and formability by altering the form of surface

oxide. A duplex stainless steel pipe is typically a

difficult-to-process material, and is susceptible to roll

marks and shape defects attributed to amounts and form of

work. B, Zr, Ca, and REM are effective when the forming

conditions involve this issue. The lower limit is not

particularly required for the content of each element.

However, when these elements are contained, the workability and formability improving effect can be obtained when the content of each element is 0.0001% or more.

When Ta is contained, the Ta content is preferably

0.30% or less because an excessively high Ta content

increases the alloying cost. When added in small amounts,

Ta reduces the transformation into the embrittlement phase,

and improves hot workability and corrosion resistance at

the same time. Ta is effective when the embrittlement phase

persists for a long time period in a stable temperature

region in hot working or in subsequent cooling. For these

reasons, the Ta content is preferably 0.0001% or more when

Ta is contained.

Formability decreases when the content of Sb and Sn

is overly high. For this reason, when Sb and Sn are

contained, the content is preferably 0.30% or less for each

of these elements. Sb and Sn improve corrosion resistance

when contained in small amounts. For this reason, the

content is preferably 0.0003% or more for each of Sb and Sn

when Sb and Sn are contained.

[0043]

Duplex Ferritic and Austenitic Phase

The following describes the ferritic phase and

austenitic phase, which affect corrosion resistance. The

ferritic phase and austenitic phase of the duplex stainless

steel pipe act differently on corrosion resistance, and provide high corrosion resistance by being present in a duplex state in steel. That is, the duplex stainless steel must have both austenitic phase and ferritic phase. Because the present invention provides a duplex stainless steel pipe used in applications requiring corrosion resistance, it is preferable that the fractions of the two phases is controlled from the viewpoint of corrosion resistance. In the present invention, the fraction (volume fraction) of the ferritic phase in the microstructure of the duplex stainless steel pipe is preferably 20% to 80%. For use in environments requiring even higher corrosion resistance, the ferritic phase is preferably 35% to 65%, in compliance with ISO 15156-3. The remainder is preferably the austenitic phase.

[0044]

A microstructure containing a martensitic phase or an

embrittlement phase cannot be used because hot workability

and cold workability decrease, and the stainless steel

cannot be formed into the shape of the product. When the

microstructure is not a duplex structure but is a single

phase structure of ferritic or austenitic phase, it is not

possible to obtain corrosion resistance performance, and

cold working fails to produce a high axial tensile strength.

In the present invention, the microstructure is required to

contain both ferritic phase and austenitic phase.

Specifically, the microstructure of the present

invention is a microstructure with a ferritic phase and an

austenitic phase, preferably a microstructure consisting of

a ferritic phase and an austenitic phase.

[0045]

For observation of the microstructure, a test specimen

for microstructure observation is taken to observe an axial

plane section. The volume fractions of ferritic phase and

austenitic phase can be determined by observing the surface

with a scanning electron microscope. Specifically, the test

specimen for microstructure observation is etched with a

Vilella's solution (a reagent prepared by mixing 2 g of

picric acid, 10 ml of hydrochloric acid, and 100 ml of

ethanol), and a microstructure image is captured with a

scanning electron microscope (SEM; 1,000 times). From the

micrograph of microstructure, the average area percentage

is calculated for the ferritic phase and the austenitic

phase to determine the volume fraction (volume%) of each

phase, using an image analyzer.

In a captured image, the ferritic phase, which is less

likely to be etched, appears white in color after

binarization, whereas the easier to be etched austenitic

phase appears black in the binarized image. The image is

binarized for a 600 pm x 800 pm measurement area (1,920

pixels x 2,560 pixels) after the captured image is transformed into a grayscale image with 256 intensities.

For binarization, the minimum brightness between two peaks

observed in a histogram plotting brightness (256

intensities) on the horizontal axis is set as the threshold.

[0046]

Axial Tensile Yield Strength: 689 MPa or More

For extraction of oil from oil wells or extraction of

hot water, steel pipes are joined to extend down from the

ground, and experience a high axial tensile stress. This

makes the adjustment of axial tensile yield strength

important from among different types of strengths. An

ordinary duplex stainless steel pipe cannot achieve an axial

tensile yield strength of 689 MPa or more after a solid

solution heat treatment performed to provide excellent

corrosion resistance performance. The yield strength is

therefore increased by cold-rolling dislocation

strengthening. The axial tensile yield strength is

preferably 757.9 MPa or more because material can be saved

by reducing the pipe thickness needed for strength

improvement. The axial tensile yield strength is more

preferably 861.25 MPa or more. There is no upper limit;

however, the axial tensile yield strength is preferably

1033.5 MPa or less because the effect to reduce the wall

thickness of steel pipe becomes lost when the axial tensile

yield strength exceeds 1033.5 MPa.

[00471

Axial Compressive Yield Strength/Axial Tensile Yield

Strength: 0.85 to 1.15

Adjustment of axial tensile yield strength is

important for the strength characteristics of a steel pipe.

However, a steel pipe also undergoes axial bending

deformation or experiences axial compressive stress during

fastened with threads or the like, though the extent of such

deformation or stress is small. It is therefore preferable

that the ratio of axial compressive yield strength to axial

tensile yield strength is 0.85 to 1.15, more preferably 0.90

or more. The ratio is more preferably 1.10 or less. When

the ratio of axial compressive yield strength to axial

tensile yield strength is 0.90 to 1.10, the steel pipe can

withstand an even higher compressive yield stress when

joined with threads.

[0048]

For the measurement of axial compressive yield

strength and axial tensile yield strength, a round-bar

tensile test specimen and a cylindrical compression test

specimen, each measuring 5.0 mm in outside diameter, are

taken from a middle portion of the wall thickness at an end

of a pipe prepared for pressure test. These are compressed

or stretched at a rate of 1.0 mm/min, and a stress-strain

curve is calculated in a tensile or compression test at room temperature. The axial tensile yield strength and axial compressive yield strength are then calculated from the stress-strain curve.

[00491

Specifically, a cylinder compression test is performed

for the measurement of axial compressive yield strength. A

cylindrical test specimen to be compressed is taken from a

middle portion of the wall thickness, parallel to the pipe

axis. The cylindrical test specimen cut out from a middle

portion of the pipe wall thickness has dimensions with an

outside diameter d of 5.0 mm, and a height h of 8.0 mm. In

the compression test, a load is applied to the test specimen

placed between flat plates at room temperature (250C), and

the compressive yield strength is calculated from a stress

strain curve obtained as a result of compression. The

stress-strain curve is obtained by compressing the test

specimen 30% at a crosshead speed of 1.0 mm/min, using a

compression testing machine.

[0050]

For the measurement of axial tensile yield strength,

a round-bar tensile test specimen, measuring 5.0 mm in

diameter at a parallel portion, is taken from a middle

portion of the pipe wall thickness, parallel to the pipe

axis in accordance with JIS Z2241. In a tensile test, the

test specimen is stretched to break at room temperature

(250C) with a crosshead speed of 1.0 mm/min. The tensile

yield strength is then calculated from a stress-strain curve

obtained as a result of the tensile test.

[0051]

In order to ensure that the ratio of axial compressive

yield strength to axial tensile yield strength stably falls

in the 0.85 to 1.15 range, it is preferable that the average

aspect ratio of austenite grains separated by a crystal

orientation angle difference of 150 or more in an axial

wall-thickness plane section 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.

Specifically, the average aspect ratio is preferably

9 or less for austenite grains having a grain size

(diameter) of 10 pm or more by assuming that the grains are

true circles (true circles created without changing the

area).

It is also preferable that austenite grains having an

aspect ratio of 9 or less have an area fraction of 50% or

more in austenite grains having a grain diameter of 10 pm

or more. That is, it is preferable to satisfy ((2)/(1)) x

100 (%) = 50% or more, where (1) represents the total area

of austenite grains having a grain diameter of 10 pm or more,

and (2) represents the area of austenite grains having a grain diameter of 10 pm or more and an aspect ratio of 9 or less.

[0052]

A duplex stainless steel pipe of the present invention

is adjusted to have appropriate fractions of two phases by

a solid solution heat treatment.

Here, the austenitic phase is a microstructure having

a plurality of crystal grains separated by an orientation

angle of 150 or more after recrystallization. This makes

the aspect ratio of austenite grains smaller. In this state,

the duplex stainless steel pipe does not have the axial

tensile yield strength required for oil well or geothermal

well applications. However, the ratio of axial compressive

yield strength to axial tensile yield strength is close to

an ideal value of 1. The duplex stainless steel pipe is

then subjected to cold working to provide the axial tensile

yield strength required for oil well or geothermal well

applications. However, a notable characteristic of metals,

including duplex stainless steels, is that the yield

strength of the direction opposite the direction stretched

by cold working decreases because of the Bauschinger effect.

That is, the relationship between axial compressive strength

and axial yield strength tends to become unstable when the

aspect ratio increases as a result of stretching of

microstructure by cold rolling.

[00531

For these reasons, in the present invention, a duplex

stainless steel pipe having an axial compressive yield

strength-to-axial tensile yield strength ratio of 0.85 to

1.15 can easily be obtained when austenite grains having a

grain diameter of 10 pm or more have an average 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. A duplex stainless steel pipe with a desirable

relationship between axial compressive yield strength and

axial tensile yield strength can be obtained even more

stably when the average aspect ratio is 5 or less. Smaller

aspect ratios mean smaller strength anisotropies, and,

accordingly, the aspect ratio should be brought closer to

1, with no lower limit.

[0054]

The aspect ratio of austenite grains is determined,

for example, as a ratio of the longer side and shorter side

of a rectangular enclosure containing grains having a

crystal orientation angle of 150 or more observed in the

austenitic phase in a crystal orientation analysis of an

axial wall-thickness plane section. Specifically, for the

measurement of aspect ratio, the aspect ratio of austenite

grains separated by a crystal orientation angle of 15° is measured by an EBSD crystal orientation analysis of an axial plane section of the steel pipe at a middle portion of the wall thickness. The aspect ratio is measured for austenite grains having a grain size (diameter) of 10 pm or more in a

1.2 mm x 1.2 mm measurement area by assuming that the grains

are true circles (true circles created without changing the

area).

[00551

Here, austenite grains of small grain diameters are

prone to producing large measurement errors, and the

presence of such austenite grains of small grain diameters

may cause errors in the aspect ratio. It is therefore

preferable that the aspect ratio is measured for austenite

grains having a grain diameter of 10 pm or more by assuming

that the grains are true circles.

[00561

The aspect ratio of ferritic phase is not particularly

limited. This is because the austenitic phase has a lower

yield strength, and, unlike the aspect ratio of austenite

grains that easily affects the Bauschinger effect after work,

the aspect ratio of ferrite grains has no effect on the

Bauschinger effect.

[0057]

Oxide layers (Surface Oxide Coatings) on Outer and Inner

Surfaces of Steel Pipe Have Average Thickness of 1.0 pm or

More

The surface of stainless steel has a passive film that

improves corrosion resistance. The passive film is

different from the surface oxide layer of interest in the

present invention. The passive film is a thin film with a

thickness of 0.01 pm or less. In contrast, the oxide layer

of interest in the present invention is a layer of primarily

Cr, Fe, and 0 (oxygen) that is formed by heating at 6000C

or more, and contains ferrioxides containing 0 and Cr.

In the case of a duplex stainless steel, the oxides

that form the oxide layer are usually of a spinel form rich

in Fe, 0, Cr, and Si ((Fe,Cr,Si)30 4 , (Fe,Cr)30 4 , Fe304).

A Si-rich oxide layer may occur in regions closer to

the base material different from the oxide layer. The outer

surface of oxide layer has a low Cr content, and hematite

may be present that is composed of Fe and 0 (OH). Regardless

of its composition, the oxide layer is harder than the base

material, and produces the desired effect, provided that

the oxide layer is an oxide with a composition containing 0

diffused by heating. In the present invention, it is

preferable to form a spinel-type oxide layer, which has good

adhesion to the base material, in a thickness of 1.0 pm or

more (average thickness).

[00581

In the present invention, the composition of oxide layer is not particularly limited, as discussed above.

However, the thickness of oxide layer needs to be adjusted.

The present inventors have elucidated the effect of the

chemical components in steel and the heat treatment

conditions (highest heating temperature and the retention

time at the highest heating temperature) on the thickness

of oxide layer, and how the thickness of oxide layer,

surface abrasion resistance, and indentation resistance are

related to one another, as follows.

[00591

First, the present inventors prepared sets of five

duplex stainless steel pipes containing 22.0 to 28.0 mass%

of Cr, and investigated the thickness of the oxide layer on

steel pipe surface by performing a solid solution heat

treatment with varying highest heating temperature and

varying retention time at the highest heating temperature.

It was confirmed after this and other investigations that

the oxide layer can stably have a thickness (average

thickness) of 1.0 pm or more by satisfying the following

formula (1).

Tmax 2 x t/[Cr] 4 > 1,000 (1)

In the formula (1), Tmax is the highest heating

temperature (°C) of a solid solution heat treatment, t is

the retention time (s) at the highest heating temperature

in the solid solution heat treatment, and [Cr] is the content of Cr (mass%) in the steel pipe.

[00601

Solid solution heat treatments were performed under

different conditions satisfying values calculated from

formula (1), and steel pipes (steel pipe materials) having

1.0 to 45.0 pm-thick surface oxide layers were obtained. A

steel pipe selected from each set of steel pipes sharing

the same components was cleaned with an acid or polished to

remove and reduce the thickness of the surface oxide layer

to less than 1.0 pm. At this stage, the steel pipe materials

had an axial tensile yield strength of 689 MPa or less.

The steel pipe materials with the oxide layers, and

the steel pipe materials with oxide layers less than 1.0 pm

thick after cleaning with an acid or polishing were all

subjected to cold bending and reverse bending that reduced

the outside diameter 10% and stretched the pipe 8% along

the axis, in order to increase the axial tensile yield

strength of steel pipe from 861 MPa to 931 MPa. The oxide

layer thickness measured after cold bending and reverse

bending was no different from that before the cold working.

The high-strength steel pipes so obtained were

subjected to a scratch test, in which the steel pipe surface

was scratched over a distance of 30 mm along the pipe axis

with an indenter (a stylus with a cemented carbide tip)

under a 59 N load. The oxide layer on steel pipe surface was then evaluated with regard to abrasion resistance and indentation resistance by measuring the oxide layer thickness and the height difference after the scratch test

(the height of the indented portion in the scratched surface

relative to the raised portion occurring after making the

indentation).

[00611

The results are shown in FIG. 1. Steel pipes having

oxide layers with a thickness of 1.0 pm or more had

significantly reduced height differences in the surfaces,

and showed improvement of abrasion resistance and

indentation resistance characteristics. In contrast, in

steel pipes that had the oxide layers removed by cleaning

with an acid, large height differences were observed after

the scratch test, and the dimensional accuracy was poor

because of defects and irregularities. The indented

portions created by scratching are portions where stress

concentrates, and it was confirmed that such indentations

have a possibility of adversely affecting corrosion

resistance performance against such as stress corrosion

cracking.

[0062]

From these results, it was found that excellent

abrasion resistance and indentation resistance can be

achieved when the oxide layer has an average thickness of

1.0 pm or more. It can also be seen from FIG. 1 that the

height difference decreases as the oxide layer becomes

thicker. It is therefore preferable that the average

thickness of the oxide layer is 3.0 pm or more, more

preferably 5.0 pm or more, provided that the conditions for

the temperature and retention time of the solid solution

heat treatment that provides the oxide layer are satisfied.

There is no upper limit for the thickness of oxide layer.

However, the preferred thickness of oxide layer is 200.0 pm

or less because the oxide layer may exfoliate when it is

too thick.

[00631

In the present invention, the oxide layer is a region

in a cross section of a sliced steel pipe where the oxygen

concentration is at least two times higher than in the base

metal when measured from the inner and outer sides of the

pipe by energy dispersive x-ray analysis after polishing

the cross-sectional surface to a mirror finish. The oxide

layer thickness (average thickness) is the average of

measured values from arbitrarily chosen 5 points (preferably,

equally spaced apart along the circumferential direction)

(oxide layer thickness = a value obtained by dividing a

total of thicknesses from 5 points by 5).

It is preferable not to perform pickling before the

solid solution heat treatment and cold working (so that the oxide layer from hot rolling remains on steel pipe surface) because it allows the oxide layer to have a thickness that effectively provides abrasion resistance and indentation resistance.

[0064]

Coverage of Oxide Layer on Outer and Inner Surfaces of Steel

Pipe is 50% or More in Terms of Area Percentage in Each

Surface

The steel pipe is protected from abrasion, scratch

defects, and indentations in areas covered by the oxide

layer. Preferably, the oxide layer covers at least 50% of

the total surface area of the steel pipe. Preferably, the

coverage is 80% or more when larger outer surface areas need

to be protected. Preferably, the oxide layer covers at

least 90% of the inner surface because the inner surface is

more susceptible to collision damage caused by hard objects

traveling inside the steel pipe.

[0065]

The coverage of a steel pipe surface by oxide layer

is a percentage determined from the pipe surface area of a

region with no oxide layer (uncoated area) divided by the

total surface area of pipe calculated from the outside

diameter, wall thickness, and length of the pipe. The

surface area of a region with no oxide layer is easily

measurable because these regions show a metallic sheen after abrasive polishing or pickling.

Specifically, an enclosure (a rectangle) that is

parallel to circumferential and axial directions is drawn

so as to include a region that, upon visual inspection,

appears to have been polished or pickled. The uncoated area

can then be calculated from the circumferential length (the

longer side of the rectangle) and the axial length (the

shorter side of the rectangle). Here, the area is

calculated as the product of the circumferential length (the

longer side of the rectangle) and the axial length (the

shorter side of the rectangle), and the sum of these areas

from the same steel pipe is determined.

In order to find the total surface area of a steel

pipe (the total surface area is the surface area excluding

the end portions where the pipe is cut), the outer

circumferential length and inner circumferential length of

the steel pipe are determined from its outside diameter and

wall thickness, and the outer circumferential length and

inner circumferential length are separately multiplied by

the axial length, and the products of these multiplications

are added to determine the total surface area. Here, the

outside diameter, wall thickness, and length are average

values. The coverage of a steel pipe surface by oxide layer

can then be determined as a percentage (%) by dividing the

uncoated area by the total surface area of the steel pipe.

[00661

In view of uniformity of properties along the

circumferential direction, the duplex stainless steel pipe

is preferably a seamless steel pipe with no seams along the

circumferential direction.

[0067]

The following describes a method for manufacturing a

duplex stainless steel pipe of the present invention.

[00681

First, a steel material of the foregoing duplex

stainless steel composition is produced. The process for

smelting 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. 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 hot rolled into a

sheet- or round billet-shaped material.

[00691

In the case of a welded steel pipe produced by forming

a sheet-shaped steel material into a cylindrical shape and welding the end portions, the steel pipe may be a UOE steel pipe using a steel sheet, or an electric resistance welded steel pipe produced by roll forming. In the case of a seamless steel pipe using a round billet, a round billet is heated with a heating furnace, and formed into a steel pipe through hot pierce rolling and subsequent wall thickness reduction sizing. The process used to form a round billet into a hollow pipe by hot forming (piercing) may be, for example, the Mannesmann process or extrusion pipe-making process. For wall thickness reduction and outside diameter sizing, it is possible to use, for example, an elongator, an assel mill, a mandrel mill, a plug mill, a sizer, or a stretch reducer.

[0070]

The highest heating temperature in the hot rolling is

preferably 1,1500C or more.

A thicker oxide layer can be obtained when the oxide

layer after the solid solution heat treatment and cold

working is not removed by, for example, pickling or surface

polishing, and when the highest heating temperature of hot

rolling is 1,150°C or more, as described below.

[0071]

Solid Solution Heat Treatment

A solid solution heat treatment is performed because

after the steel is hot-formed into a steel pipe, various carbonitrides and intermetallic compounds are formed in steel upon air cooling. Specifically, a duplex stainless steel in hot rolling undergoes a gradual temperature decrease while being hot rolled from the high-temperature state of heating. The steel pipe is typically air cooled after hot forming, and temperature control is not achievable because the temperature history varies with the size and type of product. 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 a phase transformation into the embrittlement phase, which leads to serious decrease of low-temperature toughness. A duplex stainless steel needs to withstand a variety of corrosive environments, and it is important that the austenitic phase and ferritic phase are in an appropriate duplex state in use. However, because the rate of cooling from the heating temperature is not controllable, it is difficult to control the fractions of the two phases consecutively varying with retention temperature.

To address these issues, a solid solution heat

treatment is often performed that involves rapid cooling

after hot forming, so as to form a solid solution of

precipitates in steel, and to initiate a reverse transformation of embrittlement phase to non-embrittlement phase, and bring the phase fractions to an appropriate duplex state.

The solid solution heat treatment is a process that

heat-decomposes the carbonitrides and embrittlement phase

without decomposing the duplex ferritic and austenitic phase

(for example, by heating at a heating temperature of 1,0000C

or more), and quenches the heated steel to prevent

reprecipitation.

This process dissolves the precipitates and

embrittlement phase into steel, and controls the phase

fractions to achieve an appropriate duplex state. The solid

solution heat treatment is typically performed at a high

temperature of 9000C or more, though the temperature that

dissolves the precipitates, the temperature that initiates

a 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. In the present invention, the solid

solution heat treatment temperature is preferably 9000C or

more, even more preferably 1,000C or more. The solid

solution heat treatment temperature is preferably 1,150°C

or less.

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. In the present invention, the surface oxide layer important for abrasion resistance and indentation resistance can occur after the hot rolling and after the solid solution heat treatment, and the oxide layer is not removed before or after cold working.

The oxide layer after the solid solution heat

treatment is not removed by pickling, and the way this is

achieved is not particularly limited, as long as the steel

pipe produced has an oxide layer having an average thickness

of 1.0 pm or more. For example, the oxide layer may be

removed over the smallest possible area by, for example,

polishing surfaces in areas affected by defects or galling,

instead of removing the oxide layer throughout the pipe.

Alternatively, the oxide layer in areas affected by defects

or galling may be removed by, for example, polishing the

surface before the solid solution heat treatment in which

growth of an oxide layer (oxide coating) takes place,

without removing the oxide layer by pickling after the solid

solution heat treatment.

[00721

Tmax 2 x t/[Cr] 4 > 1,000 (1)

In the formula (1), Tmax is the highest heating

temperature (°C) of the solid solution heat treatment, t is

the retention time (s) at the highest heating temperature of the solid solution heat treatment, and [Cr] is the content (mass%) of Cr in the steel pipe.

Preferably, Tmax is 900 to 1,1500C. Preferably, t is

600 to 3,600 s.

The solid solution heat treatment is performed so as

to satisfy the formula (1), as noted above. In this way,

the oxide layers formed on the outer and inner surfaces of

the steel pipe can have an average thickness of 1.0 pm or

more. In order to provide a thicker oxide layer, the left

hand side of the formula (1) is preferably more than 2,000,

more preferably 2,500 or more, even more preferably 3,000

or more. The left-hand side of the formula (1) is preferably

8,000 or less, even more preferably 6,000 or less because

the oxide layer may fall off in the furnace when there is

excessive growth of oxide layer.

[0073]

Cold Circumferential Bending and Reverse Bending

(hereinafter, also referred to as "bending and reverse

bending")

A steel pipe material after the solid solution heat

treatment contains the low-yield-strength austenitic phase,

and, with its as-processed form, the axial tensile yield

strength required for oil well or gas well applications and

for extraction of hot water cannot be obtained. To increase

strength, dislocation strengthening is performed using various cold working techniques.

[00741

In the present invention, the yield strength of pipe

is increased by circumferential bending and reverse bending.

This enables formation of the surface oxide layer required

for abrasion resistance and indentation resistance while

stably improving axial tensile yield strength, as described

below.

The cold working technique of the present invention

is a novel method that makes use of dislocation

strengthening by circumferential bending and reverse

bending. This technique is described below, with reference

to FIG. 2. Unlike cold drawing and cold pilgering that

improve the tensile yield strength of a steel pipe by

rolling that reduces the wall thickness and stretches the

pipe along the axis, the foregoing technique produces strain

by a bending process by flattening of a pipe (first

flattening), and a reverse bending process that restores

the full roundness (second flattening), as shown in FIG. 2.

In this technique, the amount of strain is adjusted by

repeating bending and reverse bending, or by varying the

amount of bend, without greatly changing the initial shape

of the steel pipe. That is, in contrast to the conventional

cold rolling method that uses the axial elongation strain,

the cold working method of the present invention that hardens the steel and increases steel pipe strength takes advantage of circumferential bending strain, and does not impart a large change in the shape of steel pipe after bending and reverse bending. That is, unlike cold drawing and cold pilgering that involve a newly-formed surface that occurs as a result of stretching that reduces the wall thickness, the method of the present invention, in principle, does not usually form such a new surface, and the steel pipe can have high yield strength while maintaining the surface oxide layers. The method of the present invention also differs from cold drawing and cold pilgering in that the method does not involve deformation occurring as a result of wall thickness reduction or stretching but involves bending that uses shear deformation. Bending is a form of deformation that requires a smaller force to provide the same deformation, and causes less damage to tools used for cold bending and reverse bending. Bending also does not require cleaning of the oxide layer with an acid before cold bending and reverse bending. There is also no need for a chemical-treatment coating process for lubrication because the extent of sliding against the tool is small. Another characteristic is that a tool does not need to be disposed on the inner side of the steel pipe. This makes it easier to maintain the oxide layer provided by the solid solution heat treatment.

[00751

In FIG. 2, (a) and (b) show cross-sectional views

illustrating a tool with two points of contact. In FIG. 2,

(c) is a cross-sectional view showing a tool with three

points of contact. Thick arrows in FIG. 2 indicate the

direction of an exerted force flattening the steel pipe.

As shown in FIG. 2, 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 shown in FIG.

2).

[0076]

As illustrated in FIG. 2, the circumferential bending

and reverse bending 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 reverse bending 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 reverse bending. Unlike the

working that achieves reduced wall thickness and reduced

outside 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.

[0077]

A tool used to flatten the steel pipe, such as that

shown in FIG. 2, 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 reverse bending 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°

with respect to 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.

[0078]

In the cold bending and reverse bending of the present

invention, it is preferable that (Di/Do) x 100 is 99% or

less, where Di is the outside diameter after working of the

steel pipe material (the steel pipe diameter after work),

and Do is the outside diameter before working of the steel

pipe material (the initial diameter of steel pipe),

regardless of the form of working. In this way, a

circumferential increase in the areas of inner and outer

surfaces can be reduced, and, accordingly, there is less

exposure of a newly-formed surface after deformation,

enabling the whole steel pipe to be stably coated with the

oxide layer that provides excellent abrasion resistance and

indentation resistance. In view of stably providing

strength characteristics and oxide layers for the steel pipe,

the range of (Di/Do) x 100 is more preferably 80 to 95%.

In the cold bending and reverse bending of the present

invention, it is preferable that (Li/Lo) x 100 (%) is 125%

or less, where Li is the axial length of the steel pipe

material after work, and Lo is the axial length of the steel pipe material before work (a rate of elongational change).

In this way, an axial increase in the areas of inner

and outer surfaces can be reduced, and, accordingly, there

is less exposure of a newly-formed surface after deformation,

enabling the whole steel pipe to be stably coated with the

oxide layer that provides excellent abrasion resistance and

indentation resistance. In view of stably providing

strength characteristics and oxide layers for the steel pipe,

the rate of elongational change is preferably 105 to 115%.

[0079]

A duplex stainless steel pipe of the present invention

can be produced by using the manufacturing method described

above.

[0080]

As described above, the present invention employs the

cold bending and reverse bending method that enables the

oxide layers to be maintained, and the duplex stainless

steel produced can have high yield strength characteristics,

and excellent abrasion resistance and indentation

resistance provided by the oxide layers. This makes it

possible to reduce defects and indentations that occur in a

steel pipe used in oil well or gas well applications or in

extraction of hot water (geothermal well applications), and

provide a duplex stainless steel pipe having excellent

corrosion resistance and dimensional accuracy.

Examples

[00811

The present invention is described below through

Examples.

[0082]

Steel materials of the compositions represented by

steels A to 0 in Table 1 were smelted with a vacuum melting

furnace, and each steel was hot rolled into a round billet

having an outside diameter 0 of 80 mm. In steels L, M, and

N, the microstructure did not have an appropriate duplex

state because the elements added to these steels were

outside of the ranges of the present invention. In steel 0

in which Cr and Mo were added beyond the range of the present

invention, cracking occurred in the process of

solidification from the melt or during hot rolling.

[0083]

A seamless steel pipe was formed by hot rolling, and

subjected to a solid solution heat treatment.

The solid solution heat treatment was performed at the

highest heating temperatures Tmax (0C) and with the

retention times t (s) at highest heating temperatures shown

in Table 2.

The axial tensile yield strength of steel pipe was

increased by dislocation strengthening using various types of cold rolling and cold working. Strength was increased by cold circumferential bending and reverse bending, which represents the cold working method of the present invention.

For comparison, draw rolling and pilger rolling were also

performed. Before cold drawing and cold pilgering, the

surface oxide layer was removed by cleaning with an acid.

For pickling, a mixture of nitric acid and hydrofluoric acid

was used, and the oxide layers on inner and outer surfaces

of steel pipe were removed by immersing the steel pipe in a

bath. The steel pipe was immersed until the oxide layers

were no longer observable by visual inspection.

Circumferential bending and reverse bending was

performed with two oppositely disposed mill rolls, and with

three mill rolls circumferentially disposed 1200 apart from

one another. The steel pipe was measured for (Di/Do) x 100

(%), where Di is the outside diameter of the steel pipe

material after work (the outside diameter of pipe after cold

working), and Do is the outside diameter of the steel pipe

material before work (the initial outside diameter of the

base pipe). The steel pipe was also measured for Lo, which

is the axial length of the steel pipe material before work

(initial axial length), and Li, which is the axial length

after work (the axial length after cold working). In table

2, these are presented as Di/Do and Li/Lo. In draw rolling

and pilger rolling, the steel pipe was stretched by rolling to reduce the wall thickness by 15 to 60%.

[00841

The microstructure was observed in the following

fashion. First, a test specimen for microstructure

observation was taken to observe an axial plane section.

The volume fractions of ferritic phase and austenitic phase

were determined by observing the surface with a scanning

electron microscope. Specifically, the test specimen for

microstructure observation was etched with a Vilella's

solution (a reagent prepared by mixing 2 g of picric acid,

ml of hydrochloric acid, and 100 ml of ethanol), and a

microstructure image was captured with a scanning electron

microscope (SEM; 1,000 times). From the micrograph of

microstructure, the average area percentage was calculated

for the ferritic phase and the austenitic phase to determine

the volume fraction (volume%) of each phase, using an image

analyzer.

[0085]

In a captured image, the ferritic phase, which is less

likely to be etched, appears white in color after

binarization, whereas the easier to be etched austenitic

phase appears black in the binarized image. The image was

binarized for a 600 pm x 800 pm measurement area (1,920

pixels x 2,560 pixels) after the captured image was

transformed into a grayscale image with 256 intensities.

For binarization, the minimum brightness between two peaks

observed in a histogram plotting brightness (256

intensities) on the horizontal axis was set as the threshold.

The martensitic phase is easy to be etched, and appears gray

in a captured image before binarization. Unlike the

austenitic phase that also appears gray, the martensitic

phase can be recognized by the shades of gray due to the

substructure including blocks and laths. The martensitic

phase was therefore determined by measuring the area of

regions where such substructures were observable in the gray

portions of the captured image. When present, the

embrittlement phase occurs at its grain boundary with the

ferritic phase, and appears black after being etched.

Accordingly, the embrittlement phase was determined by

measuring the area of black portions.

[00861

Table 1 shows the observed duplex state of the

microstructure in each steel pipe, along with the measured

fractions of ferritic phase.

[00871

The oxide layer is a region in a cross section of a

sliced steel pipe where the oxygen concentration was at

least two times higher than in the base metal when measured

from the inner and outer sides of pipe by energy dispersive

x-ray analysis after polishing the cross-sectional surface to a mirror finish. The oxide layer thickness (average thickness) is the average of measured values from arbitrarily chosen 5 points (equally spaced apart along the circumferential direction) (oxide layer thickness = a value obtained by dividing a total of thicknesses from 5 points by 5). Table 2 shows the thickness of the oxide layer of each steel pipe.

[00881

The coverage of a steel pipe surface by oxide layer

is a percentage determined from the pipe surface area of a

region with no oxide layer (uncoated area) divided by the

total surface area of pipe calculated from the outside

diameter, wall thickness, and length of the pipe. The

surface area of a region with no oxide layer is easily

measurable because these regions show a metallic sheen after

abrasive polishing or pickling.

Specifically, an enclosure (a rectangle) that is

parallel to circumferential and axial directions was drawn

so as to include a region that, upon visual inspection,

appeared to have been polished or pickled. The uncoated

area was then calculated from the circumferential length

(the longer side of the rectangle) and the axial length (the

shorter side of the rectangle). Here, the area was

calculated as the product of the circumferential length (the

longer side of the rectangle) and the axial length (the shorter side of the rectangle), and the sum of these areas from the same steel pipe was determined.

In order to find the total surface area of a steel

pipe (the total surface area is the surface area excluding

the end portions where the pipe is cut), the outer

circumferential length and inner circumferential length of

the steel pipe were determined from its outside diameter

and wall thickness. The outer circumferential length and

inner circumferential length were separately multiplied by

the axial length, and the products of these multiplications

were added to determine the total surface area. Here, the

outside diameter, wall thickness, and length are average

values. The coverage of a steel pipe surface by oxide layer

was then determined as a percentage (%) by dividing the

uncoated area by the total surface area of the steel pipe.

Table 2 shows the coverage of pipe surface by oxide

layer for each steel pipe.

[00891

For the measurement of axial compressive yield

strength and axial tensile yield strength, a round-bar

tensile test specimen and a cylindrical compression test

specimen, each measuring 5.0 mm in outside diameter, were

taken from a middle portion of the wall thickness at an end

of a pipe prepared for pressure test. These were compressed

or stretched at a rate of 1.0 mm/min and a stress-strain curve was calculated in a tensile or compression test at room temperature. The axial tensile yield strength and axial compressive yield strength were then calculated from the stress-strain curve.

[00901

Specifically, a cylinder compression test was

performed for the measurement of axial compressive yield

strength. A cylindrical test specimen to be compressed was

taken from a middle portion of the wall thickness, parallel

to the pipe axis. The cylindrical test specimen cut out

from a middle portion of the pipe wall thickness had

dimensions with an outside diameter d of 5.0 mm, and a

height h of 8.0 mm. In the compression test, a load was

applied to the test specimen placed between flat plates at

room temperature (25°C), and the compressive yield strength

was calculated from a stress-strain curve obtained as a

result of compression. The stress-strain curve was obtained

by compressing the test specimen 30% at a crosshead speed

of 1.0 mm/min, using a compression testing machine.

[0091]

For the measurement of axial tensile yield strength,

a round-bar tensile test specimen having a diameter of 5.0

mm at a parallel portion was taken parallel to the pipe axis

at a middle portion of the wall thickness, according to JIS

Z2241. In a tensile test, the test specimen was stretched to break at room temperature (250C) with a crosshead speed of 1.0 mm/min. The tensile yield strength was calculated from a stress-strain curve obtained in the test.

In a scratch test, the pipe was scratched by sweeping

a pipe surface over a distance of 30 mm at 3 mm/s along the

pipe axis under a 59 N load of an indenter provided as a

stylus having a cemented carbide tip (a circular cone

indenter having a tip angle of 600 (a point of contact with

a steel pipe) in a triangular cross section taken

perpendicular to the base of the circular cone). The height

of the indented portion relative to the raised portion was

then measured at a lengthwise middle portion of the indented

portion scratched in the metallic base material portion (the

maximum height of the indented portion along the wall

thickness relative to the raised portion formed by

scratching). Steel pipes were determined as having

excellent abrasion resistance and indentation resistance

and having passed the test when the indentation height was

50 pm or less.

[00921

The aspect ratio of austenite grains separated by a

crystal orientation angle of 15° was measured by an EBSD

crystal orientation analysis of an axial plane section of

the steel pipe at a middle portion of the wall thickness.

The aspect ratio was measured for austenite grains having a grain diameter of 10 pm or more in a 1.2 mm x 1.2 mm measurement area by assuming that the grains are true circles having the same area. The area fraction of austenite grains having an aspect ratio of 9 or less was also calculated. The area fraction was measured for austenite grains having a grain diameter of 10 pm or more by calculating the percentage of the total area of austenite grains with an aspect ratio of 9 or less with respect to the area of all austenite grains.

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[00951

As can be seen from the results presented in Table 2,

the present examples all had a high axial tensile yield

strength of 689 MPa or more, and formation of oxide layer

was confirmed. The scratch test showed that the abrasion

resistance and indentation resistance were excellent in the

present examples. In contrast, it was not possible to

obtain high yield strength and oxide layers in steel pipes

produced by cold drawing and cold pilgering representing

conventional cold rolling methods. Accordingly, the scratch

test showed inferior results, suggesting that the steel

pipes will have inferior abrasion resistance and indentation

resistance when used in oil well applications or in

geothermal well applications (collection of hot water).

Claims (10)

1. [Claim 1]

   A duplex stainless steel pipe having a composition

   that comprises, in mass%, C: 0.005 to 0.150%, Si: 1.0% or

   less, Mn: 10.0% or less, Cr: 11.5 to 35.0%, Ni: 0.5 to 15.0%,

   Mo: 0.5 to 6.0%, N: less than 0.400%, and the balance being

   Fe and incidental impurities, and having a microstructure

   with a ferritic phase and an austenitic phase,

   the duplex stainless steel pipe having an axial

   tensile yield strength of 689 MPa or more, and having an

   outer surface and an inner surface each having an oxide

   layer having an average thickness of 1.0 pm or more.
2. [Claim 2]

   The duplex stainless steel pipe according to claim 1,

   wherein the oxide layer covers at least 50% of the outer

   surface and at least 50% of the inner surface of the steel

   pipe in terms of an area percentage.
3. [Claim 3]

   The duplex stainless steel pipe according to claim 1

   or 2, which has an axial compressive yield strength-to-axial

   tensile yield strength ratio of 0.85 to 1.15.
4. [Claim 4]

   The duplex stainless steel pipe according to any one

   of claims 1 to 3, wherein the composition further comprises,

   in mass%, one or two or more selected from W: 6.0% or less,

   Cu: 4.0% or less, V: 1.0% or less, and Nb: 1.0% or less.
5. [Claim 5]

   The duplex stainless steel pipe according to any one

   of claims 1 to 4, wherein the composition further comprises,

   in mass%, one or two selected from Ti: 0.30% or less and

   Al: 0.30% or less.
6. [Claim 6]

   The duplex stainless steel pipe according to any one

   of claims 1 to 5, wherein the composition further comprises,

   in mass%, one or two or more selected from B: 0.010% or

   less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or

   less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.010%

   or less.
7. [Claim 7]

   A method for manufacturing a duplex stainless steel

   pipe of any one of claims 1 to 6,

   the method comprising:

   hot rolling a steel pipe material into a shape of a steel pipe; subjecting the steel pipe material after the hot rolling to a solid solution heat treatment that satisfies the formula (1) below; and performing cold circumferential bending and reverse bending without removing an oxide layer formed on the steel pipe material after the solid solution heat treatment,

   Tmax 2 x t/[Cr] 4 > 1,000 (1),

   wherein Tmax is a highest heating temperature(°C) of the

   solid solution heat treatment, t is a retention time (s) at

   the highest heating temperature of the solid solution heat

   treatment, and [Cr] is the content of Cr (mass%) in the

   steel pipe.
8. [Claim 8]

   The method according to claim 7, wherein the highest

   heating temperature in the hot rolling is 1,1500C or more.
9. [Claim 9]

   The method according to claim 7 or 8, wherein the cold

   bending and reverse bending reduces a diameter of the steel

   pipe material to (Di/Do) x 100 = 99% or less, where Di is

   an outside diameter of the steel pipe material after work,

   and Do is an outside diameter of the steel pipe material

   before work.
10. [Claim 10]

    The method according to any one of claims 7 to 9,

    wherein (Li/Lo) x 100 (%) is 125% or less after the cold

    bending and reverse bending, where Li is an axial length of

    the steel pipe material after work, and Lo is an axial

    length of the steel pipe material before work.