EP4606918A1 - Duplexedelstahlmaterial - Google Patents

Duplexedelstahlmaterial

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Publication number
EP4606918A1
EP4606918A1 EP23879798.9A EP23879798A EP4606918A1 EP 4606918 A1 EP4606918 A1 EP 4606918A1 EP 23879798 A EP23879798 A EP 23879798A EP 4606918 A1 EP4606918 A1 EP 4606918A1
Authority
EP
European Patent Office
Prior art keywords
steel material
content
duplex stainless
stainless steel
present
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP23879798.9A
Other languages
English (en)
French (fr)
Inventor
Seiya OKADA
Kazuya NAKANE
Yuji Arai
Keiichi Kondo
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4606918A1 publication Critical patent/EP4606918A1/de
Pending legal-status Critical Current

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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
    • 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
    • C21D6/007—Heat treatment of ferrous alloys containing Co
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0268—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment between cold rolling steps
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/005—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/18—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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • 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

  • Oil wells and gas wells may be in a corrosive environment containing a corrosive gas.
  • corrosive gas means carbon dioxide gas and/or hydrogen sulfide gas. That is, steel materials for use in oil wells are required to have excellent corrosion resistance in a corrosive environment.
  • duplex stainless steel materials that achieve both high strength and excellent corrosion resistance.
  • Patent Literature 1 Japanese Patent Application Publication No. 2014-043616 (Patent Literature 1) and International Application Publication No. WO2021/246118 (Patent Literature 2) each proposes a duplex stainless steel material that has high strength and excellent corrosion resistance.
  • the duplex stainless steel material disclosed in Patent Literature 2 consists of, in mass%, C: 0.002 to 0.03%, Si: 0.05 to 1.0%, Mn: 0.10 to 1.5%, P: 0.040% or less, S: 0.0005 to 0.02%, Cr: 20.0 to 28.0%, Ni: 4.0 to 10.0%, Mo: 2.0 to 5.0%, Al: 0.001 to 0.05%, and N: 0.06 to 0.35%, with the balance being Fe and impurities.
  • this duplex stainless steel has a microstructure containing, by volume ratio, austenite phase: 20 to 70% and ferrite phase: 30 to 80%, and in the duplex stainless steel, a yield strength is 448 MPa or more, a number density of oxide-based inclusions with an average grain size of 1 ⁇ m or more is 15 pieces/mm 2 or less, and among the oxide-based inclusions, the proportion of oxide-based inclusions containing Al is 50% by mass or less. It is described in Patent Literature 2 that this duplex stainless steel has high strength, high toughness, and excellent corrosion resistance.
  • a duplex stainless steel material having high strength and excellent corrosion resistance can be obtained.
  • a duplex stainless steel material that achieves both high strength and excellent corrosion resistance may also be obtained by a technique other than the techniques disclosed in the aforementioned Patent Literatures 1 and 2.
  • An objective of the present disclosure is to provide a duplex stainless steel material that achieves both high strength and excellent corrosion resistance.
  • a duplex stainless steel material according to the present disclosure consists of, in mass%,
  • the duplex stainless steel material according to the present disclosure achieves both high strength and excellent corrosion resistance.
  • the present inventors attempted to obtain a duplex stainless steel material having, specifically, a yield strength of 758 MPa or more as high strength. Therefore, first, the present inventors conducted studies from the viewpoint of the chemical composition with regard to a duplex stainless steel material in which both a high yield strength of 758 MPa or more and excellent corrosion resistance are achieved.
  • a duplex stainless steel material consists of, in mass%, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.5 to 7.0%, P: 0.040% or less, S: 0.0200% or less, Al: 0.100% or less, Ni: 4.0 to 9.0%, Cr: 20.0 to 30.0%, Mo: 0.5 to 2.0%, Cu: 1.5 to 3.0%, N: 0.15 to 0.30%, V: 0.01 to 0.50%, Co: 0.05 to 1.00%, Sn: 0.001 to 0.050%, Nb: 0 to 0.300%, Ta: 0 to 0.100%, Ti: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, W: 0 to 0.200%, Sb: 0 to 0.100%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, B: 0 to 0.020%, and rare earth metal: 0 to 0.200%
  • the microstructure of a duplex stainless steel material having the chemical composition described above is composed of ferrite and austenite.
  • the present inventors have found that in a duplex stainless steel material having the chemical composition described above, if the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, the strength and corrosion resistance are stably increased. That is, in a duplex stainless steel material according to the present embodiment, the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite.
  • the phrase "composed of ferrite and austenite" means that the amount of any phase other than ferrite and austenite is negligibly small.
  • the present inventors conducted detailed studies regarding a technique for increasing corrosion resistance while maintaining the yield strength with respect to a duplex stainless steel material having the chemical composition and microstructure described above and having a yield strength of 758 MPa or more. Specifically, the present inventors focused their attention on dislocations in the duplex stainless steel material. When the dislocation density in a duplex stainless steel material is increased, the yield strength of the steel material increases. That is, in the duplex stainless steel material according to the present embodiment in which the yield strength is increased to 758 MPa or more, there is a possibility that the dislocation density will be increased to a certain level or more.
  • the dislocation density in a duplex stainless steel material having the chemical composition and microstructure described above is increased to a certain level or more as a result of increasing the yield strength to 758 MPa or more.
  • the dislocation density in the duplex stainless steel material is increased by work hardening or the like, dislocations are introduced locally in some cases, and thus the dislocation density is liable to increase locally.
  • a ratio between the dislocation density ⁇ ( ⁇ ) in the ferrite and the dislocation density p(y) in the austenite is controlled within a certain range, localization of the dislocation density in the duplex stainless steel material is lessened.
  • the present inventors surmise that it is likely that, as a result, while maintaining the yield strength, an increase in the local dislocation density is lessened and the corrosion resistance of the duplex stainless steel material increases.
  • the duplex stainless steel material has a yield strength of 758 MPa or more and excellent corrosion resistance because of a mechanism that is different from the mechanism described above.
  • the gist of the duplex stainless steel material according to the present embodiment which has been completed based on the above findings, is as follows.
  • the shape of the duplex stainless steel material according to the present embodiment is not particularly limited.
  • the duplex stainless steel material according to the present embodiment may be a steel pipe, may be a round steel bar (solid material), or may be a steel plate.
  • round steel bar refers to a steel bar in which a cross section perpendicular to the axial direction is a circular shape.
  • the steel pipe may be a seamless steel pipe or may be a welded steel pipe.
  • duplex stainless steel material according to the present embodiment is described in detail. Note that, in the following description, the duplex stainless steel material is also referred to as simply "steel material”.
  • the chemical composition of the duplex stainless steel material according to the present embodiment contains the following elements.
  • the symbol "%" relating to an element means “mass percent” unless otherwise noted.
  • Silicon (Si) deoxidizes the steel. If the content of Si is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Si is too high, toughness and hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Si is to be 0.20 to 1.00%. A preferable lower limit of the content of Si is 0.25%, and more preferably is 0.30%. A preferable upper limit of the content of Si is 0.95%, and more preferably is 0.90%.
  • Manganese (Mn) deoxidizes the steel and desulfurizes the steel. Mn also improves hot workability of the steel material. If the content of Mn is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, Mn segregates to grain boundaries together with impurities such as P and S. Therefore, if the content of Mn is too high, corrosion resistance of the steel material in a high temperature environment will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Mn is to be 0.5 to 7.0%. A preferable lower limit of the content of Mn is 0.6%, more preferably is 0.8%, and further preferably is 1.0%. A preferable upper limit of the content of Mn is 6.5%, and more preferably is 6.2%.
  • Phosphorus (P) is unavoidably contained. That is, the lower limit of the content of P is more than 0%. P segregates to grain boundaries. Therefore, if the content of P is too high, the corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of P is to be 0.040% or less.
  • a preferable upper limit of the content of P is 0.035%, and more preferably is 0.030%.
  • the content of P is preferably as low as possible. However, extremely reducing the content of P will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of P is 0.001%, and more preferably is 0.003%.
  • S Sulfur
  • the lower limit of the content of S is more than 0%. S segregates to grain boundaries. Therefore, if the content of S is too high, toughness and hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of S is to be 0.0200% or less.
  • a preferable upper limit of the content of S is 0.0180%, and more preferably is 0.0160%.
  • the content of S is preferably as low as possible. However, extremely reducing the content of S will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of S is 0.0001%, more preferably is 0.0005%, further preferably is 0.0010%, and further preferably is 0.0015%.
  • Nickel (Ni) stabilizes the austenitic microstructure of the steel material. That is, Ni is an element necessary for obtaining a stable duplex microstructure of ferrite and austenite. Ni also increases corrosion resistance of the steel material. If the content of Ni is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Ni is too high, even if the contents of other elements are within the range of the present embodiment, the volume ratio of austenite will be too high and the yield strength of the steel material will decrease. Therefore, the content of Ni is to be 4.0 to 9.0%. A preferable lower limit of the content of Ni is 4.1%, more preferably is 4.3%, and further preferably is 4.5%. A preferable upper limit of the content of Ni is 8.8%, more preferably is 8.5%, and further preferably is 8.0%.
  • a preferable lower limit of the content of Cr is 20.5%, more preferably is 21.0%, and further preferably is 21.5%.
  • a preferable upper limit of the content of Cr is 29.5%, more preferably is 29.0%, and further preferably is 28.5%.
  • Molybdenum (Mo) increases corrosion resistance of the steel material. Mo also dissolves in the steel and increases the yield strength of the steel material. In addition, Mo forms fine carbides in the steel and thereby increases the yield strength of the steel material. If the content of Mo is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Mo is too high, hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Mo is to be 0.5 to 2.0%. A preferable lower limit of the content of Mo is 0.6%, more preferably is 0.7%, and further preferably is 0.8%. A preferable upper limit of the content of Mo is 1.9%, more preferably is 1.7%, and further preferably is 1.5%.
  • N Nitrogen
  • N stabilizes the austenitic microstructure of the steel material. That is, N is an element necessary for obtaining a stable duplex microstructure of ferrite and austenite. N also increases the corrosion resistance of the steel material. If the content of N is too low, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of N is too high, toughness and hot workability of the steel material will decrease even if the contents of other elements are within the range of the present embodiment. Therefore, the content of N is to be 0.15 to 0.30%. A preferable lower limit of the content of N is 0.16%, more preferably is 0.18%, and further preferably is 0.20%. A preferable upper limit of the content of N is 0.29%, and more preferably is 0.27%.
  • V 0.01 to 0.50%
  • Tin (Sn) increases the corrosion resistance of the steel material. If the content of Sn is too low, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment. On the other hand, if the content of Sn is too high, even if the contents of other elements are within the range of the present embodiment, liquation cracking will occur at grain boundaries, which will cause hot workability of the steel material to decrease. Therefore, the content of Sn is to be 0.001 to 0.050%. A preferable lower limit of the content of Sn is 0.002%, more preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Sn is 0.045%, and more preferably is 0.040%.
  • the balance of the chemical composition of the duplex stainless steel material according to the present embodiment is Fe and impurities.
  • impurities in the chemical composition refers to substances which are mixed in from ore and scrap as the raw material or from the production environment or the like when industrially producing the duplex stainless steel material, and which are permitted within a range that does not adversely affect the duplex stainless steel material according to the present embodiment.
  • the chemical composition of the duplex stainless steel material described above may further contain one or more elements selected from the group consisting of Nb, Ta, Ti, Zr, Hf, and W in lieu of a part of Fe.
  • Each of these elements is an optional element, and each of these elements increases strength of the steel material.
  • Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%. When contained, Nb forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Nb is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Nb is to be 0 to 0.300%.
  • Tantalum (Ta) is an optional element, and does not have to be contained. That is, the content of Ta may be 0%. When contained, Ta forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Ta is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ta is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Ta is to be 0 to 0.100%.
  • a preferable lower limit of the content of Ta is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of Ta is 0.080%, and more preferably is 0.070%.
  • Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%. When contained, Ti forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ti is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Ti is to be 0 to 0.100%. A preferable lower limit of the content of Ti is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%. A preferable upper limit of the content of Ti is 0.080%, and more preferably is 0.070%.
  • Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%. When contained, Zr forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Zr is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Zr is to be 0 to 0.100%.
  • a preferable lower limit of the content of Zr is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of Zr is 0.080%, more preferably is 0.070%, further preferably is 0.060%, further preferably is 0.050%, and further preferably is 0.045%.
  • Hafnium (Hf) is an optional element, and does not have to be contained. That is, the content of Hf may be 0%. When contained, Hf forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of Hf is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Hf is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of Hf is to be 0 to 0.100%.
  • a preferable lower limit of the content of Hf is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of Hf is 0.080%, and more preferably is 0.070%.
  • Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%. When contained, W forms carbo-nitrides and thereby increases strength of the steel material. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of W is too high, even if the contents of other elements are within the range of the present embodiment, strength of the steel material will be too high and toughness of the steel material will decrease. Therefore, the content of W is to be 0 to 0.200%.
  • a preferable lower limit of the content of W is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of W is 0.180%, and more preferably is 0.150%.
  • the chemical composition of the duplex stainless steel material described above may further contain Sb in lieu of a part of Fe.
  • Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%. When contained, Sb increases corrosion resistance of the steel material. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Sb is too high, even if the contents of other elements are within the range of the present embodiment, high-temperature ductility of the steel material will decrease, and hot workability of the steel material will decrease. Therefore, the content of Sb is to be 0 to 0.100%. A preferable lower limit of the content of Sb is more than 0%, more preferably is 0.001%, further preferably is 0.002%, and further preferably is 0.003%. A preferable upper limit of the content of Sb is 0.080%, and more preferably is 0.070%.
  • the chemical composition of the duplex stainless steel material described above may further contain one or more elements selected from the group consisting of Ca, Mg, B, and rare earth metal in lieu of a part of Fe. Each of these elements is an optional element, and increases hot workability of the steel material.
  • Ca Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%. When contained, Ca fixes S in the steel material as a sulfide to make it harmless, and thereby increases hot workability of the steel material. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Ca is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the toughness of the steel material will decrease. Therefore, the content of Ca is to be 0 to 0.020%.
  • a preferable lower limit of the content of Ca is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of Ca is 0.018%, more preferably is 0.015%.
  • Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%. When contained, Mg fixes S in the steel material as a sulfide to make it harmless, and thereby increases hot workability of the steel material. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Mg is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and the toughness of the steel material will decrease. Therefore, the content of Mg is to be 0 to 0.020%.
  • a preferable lower limit of the content of Mg is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of Mg is 0.018%, and more preferably is 0.015%.
  • Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%. When contained, B suppresses segregation of S in the steel material to grain boundaries, and thereby increases hot workability of the steel material. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of B is too high, even if the contents of other elements are within the range of the present embodiment, boron nitride (BN) will be formed and will cause the toughness of the steel material to decrease. Therefore, the content of B is to be 0 to 0.020%.
  • a preferable lower limit of the content of B is more than 0%, more preferably is 0.001%, further preferably is 0.002%, further preferably is 0.003%, and further preferably is 0.005%.
  • a preferable upper limit of the content of B is 0.018%, and more preferably is 0.015%.
  • Rare earth metal 0 to 0.200%
  • Rare earth metal is an optional element, and does not have to be contained. That is, the content of REM may be 0%. When contained, REM fixes S in the steel material as a sulfide to make it harmless, and thereby increases hot workability of the steel material. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of REM is too high, even if the contents of other elements are within the range of the present embodiment, oxides in the steel material will coarsen and toughness of the steel material will decrease. Therefore, the content of REM is to be 0 to 0.200%.
  • a preferable lower limit of the content of REM is more than 0%, more preferably is 0.001%, further preferably is 0.005%, further preferably is 0.010%, and further preferably is 0.020%.
  • a preferable upper limit of the content of REM is 0.180%, and more preferably is 0.160%.
  • REM means one or more elements selected from the group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids.
  • scandium Sc
  • Y yttrium
  • Li lutetium
  • content of REM refers to the total content of these elements.
  • the yield strength of the duplex stainless steel material according to the present embodiment is 758 MPa or more.
  • the duplex stainless steel material according to the present embodiment has the chemical composition described above and has a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and in the duplex stainless steel material, a dislocation density ratio ⁇ ( ⁇ )/ ⁇ ( ⁇ ) to be described later is more than 0.3 to less than 4.0.
  • the duplex stainless steel material according to the present embodiment has excellent corrosion resistance even though the yield strength is 758 MPa or more.
  • a preferable lower limit of the yield strength of the duplex stainless steel material according to the present embodiment is 760 MPa, and more preferably is 765 MPa.
  • the upper limit of the yield strength of the duplex stainless steel material according to the present embodiment is not particularly limited, for example the upper limit is 1000 MPa.
  • the yield strength of the duplex stainless steel material according to the present embodiment can be determined by the following method. Specifically, a tensile test is carried out by a method in accordance with ASTM E8/E8M (2022). A test specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a tensile test specimen is prepared from a center portion of the thickness. In this case, the longitudinal direction of the tensile test specimen is to be made parallel to the rolling elongation direction of the steel plate. If the steel material is a steel pipe, an arc-shaped test specimen having a thickness which is the same as the wall thickness of the steel pipe and having a width of 25.4 mm and a gage length of 50.8 mm is prepared.
  • the longitudinal direction of the arc-shaped test specimen is to be made parallel to the pipe axis direction of the steel pipe.
  • a tensile test specimen is prepared from an R/2 position.
  • the longitudinal direction of the tensile test specimen is to be made parallel to the axial direction of the round steel bar.
  • R/2 position of a round steel bar means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar.
  • a tensile test is carried out at normal temperature (25°C) in air using the test specimen.
  • the 0.2% offset proof stress obtained by the tensile test is defined as the yield strength (MPa).
  • the decimals of the obtained numerical value is rounded off, and the resultant value is adopted as the yield strength (MPa).
  • the duplex stainless steel material according to the present embodiment has the chemical composition described above and has a microstructure composed of, in volume ratio, ferrite in an amount of 35 to 65%, with the balance being austenite, and in the duplex stainless steel material, a dislocation density ratio ⁇ ( ⁇ )/ ⁇ ( ⁇ ) to be described later is more than 0.3 to less than 4.0.
  • the duplex stainless steel material according to the present embodiment has excellent corrosion resistance even though the yield strength is 758 MPa or more.
  • the phrase that the microstructure is "composed of ferrite and austenite" means that the amount of any phase other than ferrite and austenite in the microstructure is negligibly small.
  • the volume ratios of precipitates and inclusions are negligibly small as compared with the volume ratios of ferrite and austenite. That is, the microstructure of the duplex stainless steel material according to the present embodiment may contain minute amounts of precipitates, inclusions and the like, in addition to ferrite and austenite.
  • the volume ratio of ferrite is 35 to 65%. If the volume ratio of ferrite is too low, in some cases the yield strength and/or corrosion resistance of the steel material may decrease. On the other hand, if the volume ratio of ferrite is too high, in some cases the toughness or hot workability of the steel material may decrease. Therefore, in the microstructure of the duplex stainless steel material according to the present embodiment, the volume ratio of ferrite is 35 to 65%. A preferable lower limit of the volume ratio of ferrite is 36%, and more preferably is 37%. A preferable upper limit of the volume ratio of ferrite is 64%, and more preferably is 63%.
  • the pipe circumferential direction of a steel pipe means the direction that is perpendicular to the pipe axis direction and the pipe diameter direction. If the steel material is a round steel bar, a test specimen having an observation surface with dimensions of 5 mm in the axial direction and 5 mm in the circumferential direction is prepared from an R/2 position.
  • the term "circumferential direction" of a round steel bar means the direction that is perpendicular to the axial direction and the radial direction. Note that, the size of the test specimen is not particularly limited as long as the aforementioned observation surface is obtained.
  • Fn1 is more than 0.3 to less than 4.0.
  • a preferable lower limit of Fn1 is 0.4, and more preferably is 0.5.
  • a preferable upper limit of Fn1 is 3.9, and more preferably is 3.8.
  • the dislocation density ratio Fn1 in the present embodiment can be determined by the following method.
  • a thin film sample for dislocation density measurement is prepared from the duplex stainless steel material according to the present embodiment. Specifically, a test specimen is cut out from the duplex stainless steel material. Further, electropolishing using a twin-jet method is performed to prepare a thin film sample from the test specimen that was cut out. Note that, if the steel material is a steel plate, a thin film sample having an observation surface perpendicular to the rolling elongation direction is prepared from a test specimen that was cut out from a center portion of the thickness.
  • a thin film sample having an observation surface perpendicular to the pipe axis direction is prepared from a test specimen that was cut out from a center portion of the wall thickness. If the steel material is a round steel bar, a thin film sample having an observation surface perpendicular to the axial direction is prepared from a test specimen that was cut out from an R/2 position. Further, the size of the test specimen and the size of the thin film sample are not particularly limited as long as an observation visual field to be described later is obtained.
  • Ferrite and austenite are each identified in the observation surface of the obtained thin film sample.
  • the ferrite and the austenite in the observation surface can be identified by identification of the crystal structure by electron diffraction.
  • the specified visual fields are subjected to microstructure observation using a transmission electron microscope (hereinafter, also referred to as "TEM").
  • TEM transmission electron microscope
  • the area of each observation visual field is not particularly limited, and it suffices that the area is an area obtained at a magnification at which dislocations can be easily observed.
  • the area of the observation visual field is, for example, within the range of 100 nm ⁇ 100 nm to 800 nm ⁇ 800 nm.
  • the volume (m 3 ) of each observation visual field is determined based on the area of the observation visual field and the thickness of the observation visual field.
  • the thickness of the observation region is determined based on the total integrated intensity of an electron energy loss spectrum (EELS) and the integrated intensity of a zero-loss spectrum with respect to the thin film sample.
  • EELS electron energy loss spectrum
  • the microstructure observation for each observation visual field is conducted using an accelerating voltage of 300 kV and diffraction conditions set to conditions suitable for observing dislocations.
  • the thin film sample in order to obtain diffraction conditions suitable for observing dislocations, the thin film sample is tilted and the observation region of the thin film sample is subjected to bright field observation.
  • dislocations instead of bright field observation, dislocations may be observed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). In observation by HAADF-STEM, dislocations can be observed more easily in comparison to bright field observation.
  • HAADF-STEM high-angle annular dark field scanning transmission electron microscopy
  • the dislocation density p(y) in the austenite (m -2 ) is determined based on the total (m) of the obtained lengths of the dislocations in austenite in five visual fields, and the total volume (m 3 ) of austenite in the five visual fields.
  • the dislocation density ⁇ ( ⁇ ) in the ferrite (m -2 ) and the dislocation density p(y) in the austenite (m -2 ) are not particularly limited as long as the yield strength is 758 MPa or more and Fn1 satisfies a condition of being within the range of more than 0.3 to less than 4.0.
  • the dislocation density ⁇ ( ⁇ ) in the ferrite (m -2 ) is, for example, 1.0 ⁇ 10 14 to 8.0 ⁇ 10 15 (m -2 ).
  • the dislocation density p(y) in the austenite (m -2 ) is, for example, 1.0 ⁇ 10 14 to 8.0 ⁇ 10 15 (m -2 ). If the dislocation density ⁇ ( ⁇ ) in the ferrite (m -2 ) is 1.0 ⁇ 10 14 to 8.0 ⁇ 10 15 (m -2 ) and the dislocation density p(y) in the austenite (m -2 ) is 1.0 ⁇ 10 14 to 8.0 ⁇ 10 15 (m -2 ), on the condition that the other requirements of the present embodiment are satisfied, a duplex stainless steel material that has a consistent yield strength of 758 MPa or more and also has excellent corrosion resistance can be obtained.
  • the yield strength of the duplex stainless steel material according to the present embodiment is 758 MPa or more.
  • Fn1 dislocation density ratio
  • the duplex stainless steel material according to the present embodiment has excellent corrosion resistance even though the yield strength thereof is 758 MPa or more.
  • whether or not the duplex stainless steel material has excellent corrosion resistance is evaluated as follows.
  • a test specimen for a four-point bending test is prepared from the duplex stainless steel material according to the present embodiment.
  • the test specimen has a thickness of 2 mm, a width of 10 mm, and a length of 75 mm.
  • the steel material is a steel plate
  • the test specimen is prepared from a center portion of the thickness.
  • the longitudinal direction of the test specimen is to be made parallel to the rolling elongation direction of the steel plate.
  • the steel material is a steel pipe
  • the test specimen is prepared from a center portion of the wall thickness.
  • the longitudinal direction of the test specimen is to be made parallel to the pipe axis direction of the steel pipe.
  • the steel material is a round steel bar
  • the test specimen is prepared from the R/2 position. In this case, the longitudinal direction of the test specimen is to be made parallel to the axial direction of the round steel bar.
  • the shape of the duplex stainless steel material according to the present embodiment is not particularly limited.
  • the duplex stainless steel material according to the present embodiment is a seamless steel pipe.
  • the duplex stainless steel material according to the present embodiment is a seamless steel pipe, even if the wall thickness is 5 mm or more, the duplex stainless steel material has a yield strength of 758 MPa or more and excellent corrosion resistance.
  • a method for producing the duplex stainless steel material according to the present embodiment which is composed as described above will now be described.
  • a method for producing the duplex stainless steel material according to the present embodiment is not limited to the production method described hereunder.
  • One example of a method for producing the duplex stainless steel material according to the present embodiment includes a starting material preparation process, a hot working process, a first cold working process, a solution treatment process, and a second cold working process.
  • each production process is described in detail.
  • a starting material having the chemical composition described above is prepared.
  • the starting material may be prepared by producing the starting material, or may be prepared by purchasing the starting material from a third party. That is, the method for preparing the starting material is not particularly limited.
  • the intermediate steel material is a hollow shell (seamless steel pipe)
  • the Ugine-Sejournet process or the Ehrhardt push bench process that is, hot extrusion
  • the intermediate steel material may be subjected to piercing-rolling (that is, hot rolling) according to the Mannesmann process.
  • hot working may be performed only one time or may be performed multiple times.
  • the aforementioned hot extrusion may be performed.
  • elongation rolling may be performed. That is, in the hot working process, hot working is performed by a well-known method to produce an intermediate steel material having the desired shape.
  • the first cold working process cold working is performed on the intermediate steel material subjected to the aforementioned hot working process.
  • the cold working may be cold rolling or may be cold drawing. That is, in the first cold working process, it suffices to perform well-known cold working under well-known conditions.
  • the temperature of the intermediate steel material during cold working may be within the range of room temperature to less than 150°C.
  • a solution treatment is performed on the intermediate steel material subjected to the aforementioned first cold working process.
  • a method for performing the solution treatment is not particularly limited, and it suffices to perform a well-known method.
  • the intermediate steel material is loaded into a heat treatment furnace, and after being held at a desired temperature, is rapidly cooled.
  • the temperature at which the solution treatment is performed means the temperature (°C) of the heat treatment furnace for performing the solution treatment.
  • the time for holding at the solution treatment temperature means the time (mins) for which the intermediate steel material is held at the heat treatment temperature.
  • the heat treatment temperature in the solution treatment process of the present embodiment is set within the range of 950 to 1150°C. If the heat treatment temperature is too low, in some cases the ferrite volume ratio in the duplex stainless steel material after the solution treatment will be less than 35%, and the strength and/or corrosion resistance of the produced duplex stainless steel material will decrease. On the other hand, if the heat treatment temperature is too high, in some cases the volume ratio of ferrite in the duplex stainless steel material after the solution treatment will be more than 65%, and the corrosion resistance of the steel material will, on the contrary, decrease.
  • the solution treatment temperature is set within the range of 950 to 1150°C.
  • a more preferable lower limit of the solution treatment temperature is 960°C, and further preferably is 970°C.
  • a more preferable upper limit of the solution treatment temperature is 1140°C, and further preferably is 1120°C.
  • the solution treatment time is not particularly limited, and it suffices that the solution treatment time is in accordance with a well-known condition.
  • the solution treatment time is, for example, 5 to 180 minutes.
  • the rapid cooling method is, for example, water cooling.
  • cold working is performed on the intermediate steel material subjected to the aforementioned solution treatment process.
  • the cold working may be cold rolling or may be cold drawing. That is, in the second cold working process, similarly to the first cold working process, it suffices to perform well-known cold working under well-known conditions.
  • the temperature of the intermediate steel material during cold working may be within the range of room temperature to less than 150°C.
  • an area reduction ratio Rd2 (%) of the intermediate steel material is defined as follows.
  • Rd2 (%) ⁇ 1 - (cross-sectional area perpendicular to working direction of intermediate steel material after second cold working process/cross-sectional area perpendicular to working direction of intermediate steel material before second cold working process) ⁇ ⁇ 100
  • the area reduction ratio Rd1 (%) in the first cold working process influences the variation in the sizes of the grains after the solution treatment. If the variation in the sizes of the grains after the solution treatment is small, dislocations are more likely to be uniformly distributed between ferrite and austenite by the cold working in the second cold working process. In such case, the dislocation density ratio Fn1 is more likely to be small.
  • Rd1 is defined relative to Rd2. That is, by increasing Rd1 to a certain level or more in accordance with Rd2, it is possible to regulate in advance the variation in the size of the grains of the intermediate steel material in the second cold working process. In other words, the occurrence of a local increase in the dislocation density p(y) in the austenite in the second cold working process can be suppressed. As a result, the dislocation density ratio Fn1 can be reduced.
  • FnA (Ni+20N+10Sn+4Co+0.5Mn+0.5Cu)/(Cr+3Mo+2Si).
  • FnA is an index that indicates the degree to which the variation in the size of the grains is regulated in the microstructure of a duplex stainless steel material having the chemical composition described above. The larger FnA is, the more likely it is for a variation in the grain size to be large. Therefore, even in a case where FnA is large, if Rd1 is increased in accordance with Rd2, the effect of regulating the variation in the size of the grains will increase.
  • the production method according to the present embodiment may also include production processes other than the production processes described above.
  • the duplex stainless steel material according to the present embodiment may be subjected to an aging heat treatment.
  • the term "aging heat treatment” means a treatment in which the produced duplex stainless steel material is held at a desired temperature.
  • the aging heat treatment is not particularly limited, and it suffices to perform the aging heat treatment using a well-known method.
  • the duplex stainless steel material according to the present embodiment may be subjected to a pickling treatment.
  • the pickling treatment is not particularly limited, and it suffices to perform the pickling treatment using a well-known method.
  • the duplex stainless steel material which was subjected to the second cold working process may also be subjected to other well-known post treatments.
  • the duplex stainless steel material according to the present embodiment can be produced by the processes described above. Note that, the method for producing the duplex stainless steel material that is described above is one example, and the duplex stainless steel material according to the present embodiment may be produced by other methods. Hereunder, the present invention is described in further detail by way of examples.
  • Molten steels having the chemical compositions shown in Table 1-1 and Table 1-2 were melted using a 50 kg vacuum furnace, and ingots were produced by an ingot-making process.
  • the symbol "-" in Table 1-2 means that the content of the corresponding element was at an impurity level.
  • the symbol "-" means that the content of Nb, the content of Ta, the content of Ti, the content of Zr, the content of Hf, the content of W, the content of Sb, the content of Ca, the content of Mg, the content of B, and the content of REM of steel A were each 0% when rounded off to the third decimal place.
  • the ingot of each steel type was subjected to hot working to produce a hollow shell (seamless steel pipe).
  • the hollow shell of each test number on which the hot working had been performed was subjected to a first cold working with an area reduction ratio Rd1 (%) that is described in Table 2.
  • the hollow shell of each test number was subjected to a solution treatment at a heat treatment temperature (°C) for a holding time (mins) which are each described in Table 2.
  • the hollow shell of each test number on which the solution treatment had been performed was subjected to a second cold working with an area reduction ratio Rd2 (%) that is described in Table 2.
  • the ratio of the area reduction ratio Rd1 (%) in the first cold working to the area reduction ratio Rd2 (%) in the second cold working in each test number is shown in the column "Rd1/Rd2" in Table 2. Note that, cold drawing was performed for each of the first cold working and the second cold working.
  • a seamless steel pipe of each test number was obtained by the above process.
  • the obtained seamless steel pipe of each test number was subjected to a tensile test, a microstructure observation test, a dislocation density ratio measurement test, and a corrosion resistance test.
  • the seamless steel pipe of each test number was subjected to a tensile test in accordance with ASTM E8/E8M (2022), and the yield strength was determined.
  • an arc-shaped test specimen for a tensile test was prepared from the seamless steel pipe of each test number.
  • the thickness of the arc-shaped test specimen was made the same as the wall thickness of the steel pipe, and the width thereof was set to 25.4 mm and the gage length was set to 50.8 mm.
  • the arc-shaped test specimen of each test number was used to carry out a tensile test at normal temperature (25°C) in air, and the 0.2% offset proof stress (MPa) was determined.
  • the determined 0.2% offset proof stress was defined as the yield strength (MPa).
  • the obtained yield strength of each test number is shown in the column "YS (MPa)" in Table 3.
  • the seamless steel pipe of each test number was subjected to microstructure observation, and the volume ratio of ferrite was determined.
  • a test specimen for microstructure observation having an observation surface with dimensions of 5 mm in the pipe axis direction ⁇ 5 mm in the pipe circumferential direction was prepared from a central portion of the wall thickness of the seamless steel pipe of each test number.
  • the observation surface of the test specimen of each test number was polished to obtain a mirror surface, and was then electrolytically etched in a 7% potassium hydroxide etching reagent.
  • the observation surface on which the microstructure had been revealed by the electrolytic etching was observed in 10 visual fields using an optical microscope.
  • the area of each visual field was 1.00 mm 2 (magnification of ⁇ 100).
  • a thin film sample was prepared from the seamless steel pipe of each test number by the method described above.
  • the thin film sample of each test number was used to determine the dislocation density ⁇ ( ⁇ ) in the ferrite (m -2 ) and the dislocation density p(y) in the austenite (m -2 ) by the method described above. Note that, in the present embodiment, the dislocations were observed by bright field observation.
  • the dislocation density ⁇ ( ⁇ ) in the ferrite was 1.0 ⁇ 10 14 to 8.0 ⁇ 10 15 (m -2 )
  • the dislocation density p(y) in the austenite was 1.0 ⁇ 10 14 to 8.0 ⁇ 10 15 (m -2 ).
  • the determined dislocation density ratio Fn1 is shown in the column "Dislocation Density Ratio p(y)/p( ⁇ )" in Table 3.
  • test bath was degassed, a gaseous mixture of H 2 S gas at 0.1 bar and CO 2 gas at 10 bar was charged under pressurization into the autoclave, and the test bath was stirred to cause the gaseous mixture to saturate. After sealing the autoclave, the test bath was stirred at 90°C for 720 hours.
  • Test specimens in which cracking was not confirmed after 720 hours elapsed were determined as "having excellent corrosion resistance” ("EX” (Excellent) in Table 3). On the other hand, test specimens in which cracking was confirmed after 720 hours elapsed were determined as "not having excellent corrosion resistance” ("NA” (Not Acceptable) in Table 3).
  • NA not having excellent corrosion resistance
  • the chemical composition was appropriate.
  • the production method used to produce these seamless steel pipes was the preferable production method described herein.
  • the yield strength was 758 MPa or more
  • the volume ratio of ferrite was 35 to 65%
  • the dislocation density ratio Fn1 satisfied a condition of being within the range of more than 0.3 to less than 4.0.
  • the corrosion resistance test it was determined that these seamless steel pipes had excellent corrosion resistance. That is, in the seamless steel pipes of Test Nos. 1 to 19, a high yield strength of 758 MPa or more and excellent corrosion resistance were both achieved.

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  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
EP23879798.9A 2022-10-18 2023-10-17 Duplexedelstahlmaterial Pending EP4606918A1 (de)

Applications Claiming Priority (2)

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JP2022166669 2022-10-18
PCT/JP2023/037585 WO2024085155A1 (ja) 2022-10-18 2023-10-17 二相ステンレス鋼材

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EP4606918A1 true EP4606918A1 (de) 2025-08-27

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EP23879798.9A Pending EP4606918A1 (de) 2022-10-18 2023-10-17 Duplexedelstahlmaterial

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US (1) US20260078458A1 (de)
EP (1) EP4606918A1 (de)
JP (1) JP7498420B1 (de)
CN (1) CN120051587A (de)
MX (1) MX2025004181A (de)
WO (1) WO2024085155A1 (de)

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Publication number Priority date Publication date Assignee Title
WO2025164106A1 (ja) * 2024-01-31 2025-08-07 日本製鉄株式会社 二相ステンレス鋼材
JP7695602B1 (ja) * 2024-01-31 2025-06-19 日本製鉄株式会社 二相ステンレス鋼材

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JP5842769B2 (ja) 2012-08-27 2016-01-13 新日鐵住金株式会社 二相ステンレス鋼およびその製造方法
JP6946737B2 (ja) * 2017-05-18 2021-10-06 日本製鉄株式会社 二相ステンレス鋼材及びその製造方法
WO2020203938A1 (ja) * 2019-03-29 2020-10-08 日鉄ステンレス株式会社 クラッド鋼板およびその製造方法
JP7518342B2 (ja) * 2020-04-10 2024-07-18 日本製鉄株式会社 二相ステンレス鋼材
JP7004118B1 (ja) 2020-06-02 2022-02-04 Ｊｆｅスチール株式会社 二相ステンレス鋼および二相ステンレス継目無鋼管

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JP7498420B1 (ja) 2024-06-12
US20260078458A1 (en) 2026-03-19
JPWO2024085155A1 (de) 2024-04-25
CN120051587A (zh) 2025-05-27
WO2024085155A1 (ja) 2024-04-25
MX2025004181A (es) 2025-05-02

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