EP4282990A1 - Tuyau en acier inoxydable duplex et son procédé fabrication - Google Patents

Tuyau en acier inoxydable duplex et son procédé fabrication Download PDF

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Publication number
EP4282990A1
EP4282990A1 EP22770955.7A EP22770955A EP4282990A1 EP 4282990 A1 EP4282990 A1 EP 4282990A1 EP 22770955 A EP22770955 A EP 22770955A EP 4282990 A1 EP4282990 A1 EP 4282990A1
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Prior art keywords
steel pipe
less
stainless steel
duplex stainless
oxide layer
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EP22770955.7A
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German (de)
English (en)
Inventor
Shunsuke Sasaki
Seigo Goto
Masao Yuga
Tatsuro Katsumura
Hideo Kijima
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JFE Steel Corp
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JFE Steel Corp
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Publication of EP4282990A1 publication Critical patent/EP4282990A1/fr
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C21D9/14—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes wear-resistant or pressure-resistant pipes
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C21D8/105—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • 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.
  • excellent axial tensile yield strength means a yield strength of 689 MPa or more.
  • 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.
  • steel needs to contain corrosion-resistance improving elements (e.g., Cr, Mo, W, and N) in adjusted amounts.
  • duplex stainless steels 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.
  • axial tensile yield strength 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.
  • 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.
  • NACE National Association of Corrosion Engineers
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • a highly lubricative chemical-treatment coating is formed to prevent galling during cold rolling.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 (25°C) with a crosshead speed of 1.0 mm/min.
  • abrasion resistance and indentation resistance are excellent when an indented portion created by a scratch test has an indentation height of 50 ⁇ m 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 60° (a point of contact with a steel pipe) in a triangular cross section perpendicular to the base of the circular cone).
  • the present inventors conducted intensive studies of a duplex stainless steel pipe.
  • 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.
  • a duplex stainless steel pipe can have high corrosion resistance performance by adjusting the chemical components and performing a solid solution heat treatment.
  • the austenitic phase decreases the yield strength of the duplex stainless steel pipe.
  • 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.
  • the solid solution heat treatment is followed by cold-rolling dislocation strengthening to provide the desired strength.
  • 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.
  • Cold rolling is also a process that exposes metal on steel pipe surfaces.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the present invention has been made on the basis of this finding, and the gist of the present invention is as follows.
  • 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 ⁇ m or more.
  • 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.
  • the Si content is 1.0% or less.
  • 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.
  • the Si content is preferably 0.01% or more.
  • the Si content is more preferably 0.2% or more.
  • 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.
  • the Mn content is more preferably 2.0% or more.
  • the Mn content is more preferably 8.0% or less.
  • 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.
  • 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.
  • 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%.
  • 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.
  • Mo increases the pitting corrosion resistance of steel in proportion to its content.
  • Mo needs to be uniformly present on surfaces of steel material exposed to a corrosive environment.
  • Mo content is 6.0% or less.
  • Mo increases the pitting corrosion resistance in proportion to its content.
  • the Mo content needs to be 0.5% or more to maintain stable corrosion resistance in a sulfide environment.
  • 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.
  • N is a strong austenitic phase-forming element, in addition to being inexpensive.
  • 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.
  • 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.
  • the balance in the composition above is Fe and incidental impurities.
  • W 6.0% or Less
  • Cu 4.0% or Less
  • V 1.0% or Less
  • Nb 1.0% or Less
  • W 6.0% or Less
  • W is an element that increases the pitting corrosion resistance in proportion to its content.
  • W when contained in excess amounts, W impairs the workability of hot working, and damages production stability.
  • 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.
  • the W content is more preferably 1.0% or more.
  • the W content is more preferably 5.0% 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the Al content is preferably 0.01% or more.
  • 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.
  • B, Zr, 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.
  • the Ta content is preferably 0.30% or less because an excessively high Ta content increases the alloying cost.
  • 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.
  • the Ta content is preferably 0.0001% or more when Ta is contained.
  • 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.
  • the fraction (volume fraction) of the ferritic phase in the microstructure of the duplex stainless steel pipe is preferably 20% to 80%.
  • the ferritic phase is preferably 35% to 65%, in compliance with ISO 15156-3. The remainder is preferably the austenitic phase.
  • 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.
  • 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.
  • the microstructure is required to contain both ferritic phase and austenitic phase.
  • 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.
  • 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.
  • 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.
  • a Vilella's solution a reagent prepared by mixing 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol
  • 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 ⁇ m ⁇ 800 ⁇ m measurement area (1,920 pixels ⁇ 2,560 pixels) after the captured image is transformed into a grayscale image with 256 intensities.
  • the minimum brightness between two peaks observed in a histogram plotting brightness (256 intensities) on the horizontal axis is set as the threshold.
  • 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.
  • Adjustment of axial tensile yield strength is important for the strength characteristics of a steel pipe.
  • 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.
  • axial compressive yield strength and axial tensile yield strength 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.
  • 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.
  • a load is applied to the test specimen placed between flat plates at room temperature (25°C), 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.
  • 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.
  • the test specimen is stretched to break at room temperature (25°C) 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.
  • the average aspect ratio of austenite grains separated by a crystal orientation angle difference of 15° or more in an axial wall-thickness plane section is preferably 9 or less.
  • austenite grains with an aspect ratio of 9 or less have an area fraction of 50% or more.
  • the average aspect ratio is preferably 9 or less for austenite grains having a grain size (diameter) of 10 ⁇ m or more by assuming that the grains are true circles (true circles created without changing the area).
  • a duplex stainless steel pipe of the present invention is adjusted to have appropriate fractions of two phases by a solid solution heat treatment.
  • the austenitic phase is a microstructure having a plurality of crystal grains separated by an orientation angle of 15° or more after recrystallization. This makes the aspect ratio of austenite grains smaller.
  • the duplex stainless steel pipe does not have the axial tensile yield strength required for oil well or geothermal well applications.
  • 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.
  • 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.
  • 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 ⁇ m 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.
  • 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 15° or more observed in the austenitic phase in a crystal orientation analysis of an axial wall-thickness plane section.
  • 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 ⁇ m or more in a 1.2 mm ⁇ 1.2 mm measurement area by assuming that the grains are true circles (true circles created without changing the area).
  • 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 ⁇ m or more by assuming that the grains are true circles.
  • 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.
  • Oxide layers (Surface Oxide Coatings) on Outer and Inner Surfaces of Steel Pipe Have Average Thickness of 1.0 ⁇ m 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 ⁇ m or less.
  • the oxide layer of interest in the present invention is a layer of primarily Cr, Fe, and O (oxygen) that is formed by heating at 600°C or more, and contains ferrioxides containing O and Cr.
  • the oxides that form the oxide layer are usually of a spinel form rich in Fe, O, Cr, and Si ((Fe,Cr,Si) 3 O 4 , (Fe,Cr) 3 O 4 , Fe 3 O 4 ).
  • 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 O (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 O diffused by heating.
  • the composition of oxide layer is not particularly limited, as discussed above.
  • 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.
  • 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 ⁇ m or more by satisfying the following formula (1). Tmax 2 ⁇ t / Cr 4 > 1,000
  • 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
  • [Cr] is the content of Cr (mass%) in the steel pipe.
  • 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 ⁇ m-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 ⁇ m.
  • 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 ⁇ m 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).
  • the oxide layer has an average thickness of 1.0 ⁇ m 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 ⁇ m or more, more preferably 5.0 ⁇ m 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 ⁇ m or less because the oxide layer may exfoliate when it is too thick.
  • 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 steel pipe is protected from abrasion, scratch defects, and indentations in areas covered by the oxide layer.
  • the oxide layer covers at least 50% of the total surface area of the steel pipe.
  • the coverage is 80% or more when larger outer surface areas need to be protected.
  • 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.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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.
  • the duplex stainless steel pipe is preferably a seamless steel pipe with no seams along the circumferential direction.
  • the following describes a method for manufacturing a duplex stainless steel pipe of the present invention.
  • 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.
  • 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.
  • a bottom-blown decarburization furnace using an Ar-O 2 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.
  • the steel pipe may be a UOE steel pipe using a steel sheet, or an electric resistance welded steel pipe produced by roll forming.
  • 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.
  • the highest heating temperature in the hot rolling is preferably 1,150°C 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.
  • 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.
  • 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.
  • 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.
  • 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 1,000°C or more), and quenches the heated steel to prevent reprecipitation.
  • the solid solution heat treatment is typically performed at a high temperature of 900°C 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.
  • the solid solution heat treatment temperature is preferably 900°C or more, even more preferably 1,000°C 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.
  • 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 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.
  • 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.
  • 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
  • [Cr] is the content (mass%) of Cr in the steel pipe.
  • Tmax is 900 to 1,150°C.
  • t is 600 to 3,600 s.
  • the oxide layers formed on the outer and inner surfaces of the steel pipe can have an average thickness of 1.0 ⁇ m or more.
  • 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.
  • 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.
  • dislocation strengthening is performed using various cold working techniques.
  • 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 .
  • 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 .
  • 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.
  • 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.
  • FIG. 2 shows cross-sectional views illustrating a tool with two points of contact.
  • FIG. 2 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.
  • 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 ) .
  • 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.
  • 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.
  • a tool used to flatten the steel pipe may have a form of a roll.
  • 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.
  • 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.
  • (Di/Do) ⁇ 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.
  • Di is the outside diameter after working of the steel pipe material (the steel pipe diameter after work)
  • Do is the outside diameter before working of the steel pipe material (the initial diameter of steel pipe)
  • the range of (Di/Do) ⁇ 100 is more preferably 80 to 95%.
  • (Li/Lo) ⁇ 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).
  • the rate of elongational change is preferably 105 to 115%.
  • a duplex stainless steel pipe of the present invention can be produced by using the manufacturing method 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.
  • steel materials of the compositions represented by steels A to O in Table 1 were smelted with a vacuum melting furnace, and each steel was hot rolled into a round billet having an outside diameter ⁇ of 80 mm.
  • 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.
  • steel O 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.
  • 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 (°C) 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.
  • 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 120° apart from one another.
  • the steel pipe was measured for (Di/Do) ⁇ 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%.
  • 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, 10 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.
  • a Vilella's solution a reagent prepared by mixing 2 g of picric acid, 10 ml of hydrochloric acid, and 100 ml of ethanol
  • SEM scanning electron microscope
  • 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 ⁇ m ⁇ 800 ⁇ m measurement area (1,920 pixels ⁇ 2,560 pixels) after the captured image was transformed into a grayscale image with 256 intensities.
  • 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.
  • 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.
  • 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.
  • Table 1 shows the observed duplex state of the microstructure in each steel pipe, along with the measured fractions of ferritic phase.
  • 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 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.
  • 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).
  • 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.
  • 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.
  • 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.
  • axial compressive yield strength and axial tensile yield strength 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.
  • 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.
  • 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.
  • 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.
  • the test specimen was stretched to break at room temperature (25°C) with a crosshead speed of 1.0 mm/min.
  • the tensile yield strength was calculated from a stress-strain curve obtained in the 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 60° (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 ⁇ m or less.
  • 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 ⁇ m or more in a 1.2 mm ⁇ 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 ⁇ m 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.
  • Thickness of oxide layer ( ⁇ m) Surface coverage by oxide layer (%) Axial strength characteristics Average aspect ratio of austenite grains Area fraction of austenite grains with aspect ratio of 9 or less (%) Indentation height ( ⁇ m) Remarks Inner surface Outer surface Inner surface Outer surface Tensile yield strength (MPa) Compressive/ tensile (yield strength) Inner surface Outer surface 1 0.0 0.0 0 0 875 0.82 10.2 20 75.2 76.2 Comparative example 2 0.0 0.0 0 0 880 0.84 12.5 10 74.2 74.0 Comparative example 3 1.4 1.6 100 100 882 1.04 4.8 80 38.0 35.0 Present example 4 0.8 0.8 85 75 881 1.03 4.8 80 55.0 52.0 Comparative example 5 0.0 0.0 0 0 883 1.04 4.7 80 63.6 64.2 Comparative example 6 28.0 27.0 100 80 887 1.02 3.3 90 9.9 10.0 Present example 7 0.0 0.0 0 0 877 0.82 11.1 20 85.2 86.0 Comparative example 8 21.0 20.0 100 100
  • 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).

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EP22770955.7A 2021-03-17 2022-02-09 Tuyau en acier inoxydable duplex et son procédé fabrication Pending EP4282990A1 (fr)

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JPS57194213A (en) 1981-05-25 1982-11-29 Nippon Kokan Kk <Nkk> Production of abrasion resistant steel pipe
JPS63290616A (ja) 1987-05-22 1988-11-28 Sumitomo Metal Ind Ltd 耐摩耗腐蝕性に優れたクラッド鋼管
JPS6415323A (en) 1987-07-08 1989-01-19 Kawasaki Steel Co Production of wear resistant steel pipe
JP3697853B2 (ja) * 1997-09-09 2005-09-21 Ｊｆｅスチール株式会社 耐食性に優れるステンレス熱延鋼板およびその製造方法
CN106011689B (zh) * 2015-03-30 2019-05-03 新日铁住金不锈钢株式会社 含臭氧水用双相不锈钢
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CA3028947A1 (fr) * 2016-06-29 2018-01-04 Nippon Steel & Sumitomo Metal Corporation Acier ferritique resistant a la chaleur et element de transfert thermique ferritique
BR112021003350B8 (pt) * 2018-08-31 2023-12-19 Jfe Steel Corp Tubo de aço inoxidável duplex sem costura e método para fabricar o mesmo

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