EP4484598A1 - Duplexedelstahlmaterial - Google Patents

Duplexedelstahlmaterial Download PDF

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Publication number
EP4484598A1
EP4484598A1 EP23759818.0A EP23759818A EP4484598A1 EP 4484598 A1 EP4484598 A1 EP 4484598A1 EP 23759818 A EP23759818 A EP 23759818A EP 4484598 A1 EP4484598 A1 EP 4484598A1
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Prior art keywords
steel material
austenite
content
duplex stainless
stainless steel
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EP23759818.0A
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English (en)
French (fr)
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EP4484598A4 (de
Inventor
Seiya OKADA
Yuji Arai
Toshinobu Nishibata
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Nippon Steel Corp
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Nippon Steel Corp
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Priority claimed from PCT/JP2023/005219 external-priority patent/WO2023162817A1/ja
Publication of EP4484598A1 publication Critical patent/EP4484598A1/de
Publication of EP4484598A4 publication Critical patent/EP4484598A4/de
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    • 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
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    • 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
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
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    • 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
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • 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
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    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
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    • 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
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    • 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
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    • 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
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    • 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
    • C22C38/60—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
    • 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
    • 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
    • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • 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
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length

Definitions

  • Patent Literature 1 Japanese Patent Application Publication No. 10-60597 (Patent Literature 1), International Application Publication No. WO2012/111536 (Patent Literature 2), and Japanese Patent Application Publication No. 2016-3377 (Patent Literature 3) each propose a technique for increasing the strength and low-temperature toughness of a duplex stainless steel material.
  • Patent Literature 2 discloses a duplex stainless steel material that has a chemical composition consisting of, by mass%, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 8.00% or less, P: 0.040% or less, S: 0.0100% or less, Cu: more than 2.00 to 4.00% or less, Ni: 4.00 to 8.00%, Cr: 20.0 to 30.0%, Mo: 0.50 to less than 2.00%, N: 0.100 to 0.350%, and Al: 0.040% or less, with the balance being Fe and impurities, and that has a microstructure in which a ferrite ratio is 30 to 70%, and the hardness of ferrite is 300 Hv 10gf or more. It is described in Patent Literature 2 that this duplex stainless steel material has high strength and high toughness.
  • Patent Literature 3 discloses a duplex stainless steel material that is a duplex stainless steel tube which has a chemical composition consisting of, by mass%, C: 0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0%, P: 0.040% or less, S: 0.010% or less, Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to 4.0%, N: 0.1 to 0.35%, O: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, and B: 0.0005 to 0.02%, with the balance being Fe and impurities, and has a microstructure composed of a duplex microstructure of a ferrite phase and an austenite phase in which there is no precipitation of sigma phase, and in which a proportion occupied by the ferrite phase in the steel microstructure is 50% or less in area fraction, and the number of oxides having a particle size of 30 ⁇ m or more present in a
  • Patent Literatures 1 to 3 disclose duplex stainless steel materials that have high strength and excellent low-temperature toughness.
  • a duplex stainless steel material that has high strength and excellent low-temperature toughness may also be obtained by a technique other than the techniques disclosed in the aforementioned Patent Literatures 1 to 3.
  • An objective of the present disclosure is to provide a duplex stainless steel material that has high strength and excellent low-temperature toughness.
  • a duplex stainless steel material according to the present disclosure consists of, by mass%,
  • the duplex stainless steel material according to the present disclosure has high strength and excellent low-temperature toughness.
  • the present inventors conducted studies from the viewpoint of the chemical composition with respect to a duplex stainless steel material that achieves both a yield strength of 80 ksi or more and excellent low-temperature toughness. As a result, the present inventors considered that if a duplex stainless steel material has a chemical composition consisting of, by mass%, C: 0.030% or less, Si: 0.20 to 1.00%, Mn: 0.50 to 7.00%, P: 0.040% or less, S: 0.0200% or less, Al: 0.100% or less, Ni: 4.20 to 9.00%, Cr: 20.00 to 30.00%, Mo: 0.50 to 2.00%, Cu: 0.50 to 3.00%, N: 0.150 to 0.350%, V: 0.01 to 1.50%, Nb: 0 to 0.100%, 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%, Co: 0 to 0.500%, Sn
  • the microstructure of a duplex stainless steel material having the chemical composition described above is composed of ferrite and austenite.
  • the microstructure of a duplex stainless steel material having the chemical composition described above is composed of, in volume ratio, ferrite in an amount of 35 to 55% 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.
  • ferrite is harder than austenite. Therefore, especially in a low-temperature environment, minute cracks generated in the duplex stainless steel material tend to easily propagate through the ferrite.
  • the present inventors conducted studies regarding causing fine austenite to disperse in the ferrite so as to increase the low-temperature toughness of the ferrite. If austenite is finely dispersed in ferrite, there is a possibility that the low-temperature toughness of the ferrite can be selectively increased. In such case, there is a possibility that the low-temperature toughness of a duplex stainless steel material having the chemical composition described above can be increased.
  • the present inventors considered causing ferrite having a volume ratio of 35 to 55%, coarse austenite, and fine austenite to be intermixed in the microstructure of a duplex stainless steel material.
  • ferrite having a volume ratio of 35 to 55%, coarse austenite, and fine austenite to be intermixed in the microstructure of a duplex stainless steel material.
  • the yield strength can be maintained by the coarse austenite.
  • the present inventors classified austenite in the microstructure of a duplex stainless steel material into coarse austenite grains with a minor axis of 20 ⁇ m or more, and fine austenite grains which is the remaining austenite other than the coarse austenite grains. More specifically, in the austenite of the microstructure of a duplex stainless steel material, the present inventors defined an austenite grain with a minor axis of 20 ⁇ m or more as “primary austenite”, and defined the balance of the austenite as "secondary austenite”. This point will be described more specifically using the drawings.
  • FIG. 1 is a schematic diagram illustrating the appearance of microstructure during microstructure observation at a cross section at a central portion of the wall thickness of a duplex stainless steel seamless pipe that is one example of the duplex stainless steel material according to the present embodiment, the cross section being perpendicular to the pipe axis direction of the duplex stainless steel seamless pipe.
  • the vertical direction of an observation visual field region 10 in FIG. 1 corresponds to the pipe radial direction of the duplex stainless steel seamless pipe.
  • the horizontal direction of the observation visual field region 10 in FIG. 1 corresponds to the pipe circumferential direction of the duplex stainless steel seamless pipe. That is, the observation visual field region 10 in FIG. 1 corresponds to a plane perpendicular to the pipe axis direction.
  • the observation visual field region 10 in FIG. 1 has a length of 200 ⁇ m in the vertical direction and a length of 200 ⁇ m in the horizontal direction.
  • regions shown in black represent ferrite 20, and regions shown in white represent austenite 30.
  • austenite 30 an austenite grain with a minor axis of 20 ⁇ m or more is defined as primary austenite 31, and an austenite grain with a minor axis of less than 20 ⁇ m is defined as secondary austenite 32.
  • the ferrite 20, the primary austenite 31, and the secondary austenite 32 can be identified by a method described later.
  • FIG. 2 is a view illustrating the relation between a volume ratio (%) of secondary austenite and a lowest temperature (°C) at which an absorbed energy is 30 J/cm 2 or more in duplex stainless steel materials which satisfy the chemical composition described above among examples that are described later.
  • FIG. 2 was created using a volume ratio of secondary austenite (%) and a lowest temperature (°C), that is an index of low-temperature toughness, at which an absorbed energy was 30 J/cm 2 or more in, among examples described later, duplex stainless steel materials which satisfied the chemical composition described above and which had a microstructure including ferrite in an amount of 35 to 55% in volume ratio, and primary austenite in an amount of 40 to 55% in volume ratio.
  • a white circle (O) in FIG. 2 means a steel material whose yield strength was 552 MPa or more.
  • a black circle ( ⁇ ) in FIG. 2 means a steel material whose yield strength was less than 552 MPa.
  • FIG. 2 it can be confirmed that in a duplex stainless steel material having the chemical composition and microstructure described above, when the volume ratio of secondary austenite is 5% or more, the lowest temperature at which an absorbed energy is 30 J/cm 2 or more is -20°C or less, and thus excellent low-temperature toughness is exhibited.
  • FIG. 2 it can be confirmed that in a duplex stainless steel material having the chemical composition and microstructure described above, when the volume ratio of secondary austenite is more than 20%, although excellent low-temperature toughness is exhibited, a yield strength of 552 MPa or more is not obtained. That is, referring to FIG.
  • the duplex stainless steel material according to the present embodiment has the chemical composition described above, and is composed of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5 to 20%.
  • the duplex stainless steel material according to the present embodiment has a yield strength of 80 ksi (552 MPa) or more and has excellent low-temperature toughness.
  • 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 (a solid material), or may be a steel plate.
  • round steel bar means 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.
  • 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%” unless otherwise noted.
  • Carbon (C) is unavoidably contained. That is, the lower limit of the content of C is more than 0%. C forms Cr carbides at grain boundaries and increases corrosion susceptibility at the grain boundaries. Therefore, if the content of C is too high, 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 C is to be 0.030% or less. A preferable upper limit of the content of C is 0.028%, and more preferably is 0.025%. The content of C is preferably as low as possible. However, extremely reducing the content of C will significantly increase the production cost. Therefore, when industrial manufacturing is taken into consideration, a preferable lower limit of the content of C is 0.001%, more preferably is 0.003%, and further preferably is 0.005%.
  • 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, the low-temperature toughness 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 0.80%, more preferably is 0.70%, and further preferably is 0.60%.
  • Manganese (Mn) deoxidizes the steel, and desulfurizes the steel. Mn also increases 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 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 0.50 to 7.00%. A preferable lower limit of the content of Mn is 0.75%, and more preferably is 1.00%. A preferable upper limit of the content of Mn is 6.50%, and more preferably is 6.20%.
  • 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 low-temperature toughness and 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, the low-temperature toughness and 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 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 0.0005%, and more preferably is 0.0010%.
  • Aluminum (Al) is unavoidably contained. That is, the lower limit of the content of Al is more than 0%. Al deoxidizes the steel. On the other hand, if the content of Al is too high, coarse oxide-based inclusions will form and the low-temperature toughness 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 Al is to be 0.100% or less.
  • a preferable lower limit of the content of Al is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.
  • a preferable upper limit of the content of Al is 0.090%, and more preferably is 0.085%. Note that, as used in the present description, the term "content of Al" means the content of "acidsoluble Al," that is, the content of sol. Al.
  • Nickel (Ni) stabilizes the austenitic microstructure of the steel material. That is, Ni is an element necessary for obtaining a stable duplex microstructure composed of ferrite and austenite. Ni also enhances 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 4.20 to 9.00%.
  • a preferable lower limit of the content of Ni is 4.25%, more preferably is 4.30%, further preferably is 4.35%, further preferably is 4.40%, and further preferably is 4.50%.
  • a preferable upper limit of the content of Ni is 8.75%, more preferably is 8.50%, further preferably is 8.25%, further preferably is 8.00%, and further preferably is 7.75%.
  • Chromium (Cr) forms a passive film as an oxide on the surface of the steel material and thereby enhances corrosion resistance of the steel material. Cr also increases the volume ratio of the ferritic microstructure of the steel material. By obtaining a sufficient ferritic microstructure, corrosion resistance of the steel material is stabilized. If the content of Cr 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 Cr 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 Cr is to be 20.00 to 30.00%.
  • a preferable lower limit of the content of Cr is 20.50%, more preferably is 21.00%, further preferably is 21.50%, and further preferably is 22.00%.
  • a preferable upper limit of the content of Cr is 29.50%, more preferably is 29.00%, and further preferably is 28.00%.
  • Molybdenum (Mo) enhances 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 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 0.50 to 2.00%. A preferable lower limit of the content of Mo is 0.55%, more preferably is 0.60%, and further preferably is 0.70%. A preferable upper limit of the content of Mo is less than 2.00%, more preferably is 1.85%, and further preferably is 1.50%.
  • Copper (Cu) increases the yield strength of the steel material. If the content of Cu 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 Cu is too high, the 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 Cu is to be 0.50 to 3.00%.
  • a preferable lower limit of the content of Cu is 0.60%, more preferably is 0.80%, further preferably is 0.90%, further preferably is 1.00%, and further preferably is 1.50%.
  • a preferable upper limit of the content of Cu is 2.90%, more preferably is 2.75%, and further preferably is 2.50%.
  • N Nitrogen
  • N stabilizes the austenitic microstructure of the steel material. That is, N is an element necessary for obtaining a stable duplex microstructure composed of ferrite and austenite. N also enhances 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, the low-temperature 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 0.150 to 0.350%. A preferable lower limit of the content of N is 0.170%, more preferably is 0.180%, and further preferably is 0.190%. A preferable upper limit of the content of N is 0.340%, and more preferably is 0.330%.
  • 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 according to the present embodiment 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 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 the low-temperature toughness of the steel material will decrease. Therefore, the content of Nb is to be 0 to 0.100%.
  • a preferable lower limit of the content of Nb 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 Nb is 0.080%, and more preferably is 0.070%.
  • 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 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 the low-temperature toughness of the steel material will decrease. Therefore, the content of Ta is to be 0 to 0.100%.
  • 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 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 the low-temperature 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 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 the low-temperature 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%, and more preferably is 0.070%.
  • 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 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 the low-temperature 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 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 the low-temperature 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 according to the present embodiment may further contain one or more elements selected from the group consisting of Co, Sn, and Sb in lieu of a part of Fe.
  • Each of these elements is an optional element, and each of these elements enhances corrosion resistance of the steel material.
  • Co Co is an optional element, and does not have to be contained. That is, the content of Co may be 0%. When contained, Co forms a coating on the surface of the steel material, and thereby enhances corrosion resistance of the steel material. Co also increases hardenability of the steel material and stabilizes strength of the steel material. If even a small amount of Co is contained, the aforementioned advantageous effects will be obtained to a certain extent. However, if the content of Co is too high, the production cost will increase extremely, even if the contents of other elements are within the range of the present embodiment. Therefore, the content of Co is to be 0 to 0.500%.
  • a preferable lower limit of the content of Co is more than 0%, more preferably is 0.001%, further preferably is 0.010%, and further preferably is 0.020%.
  • a preferable upper limit of the content of Co is 0.480%, more preferably is 0.460%, and further preferably is 0.450%.
  • Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%. When contained, Sn enhances corrosion resistance of the steel material. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, 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 to 0.100%. A preferable lower limit of the content of Sn 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 Sn is 0.080%, and more preferably is 0.070%.
  • 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 enhances 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 according to the present embodiment 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 each of these elements 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 low-temperature 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%, and 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 low-temperature 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 low-temperature 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 the low-temperature 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 552 MPa or more.
  • the duplex stainless steel material according to the present embodiment has the chemical composition described above, and is composed of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5 to 20%.
  • the duplex stainless steel material according to the present embodiment has excellent low-temperature toughness even when the yield strength is 80 ksi (552 MPa) or more.
  • a preferable lower limit of the yield strength of the duplex stainless steel material according to the present embodiment is 560 MPa, more preferably is 570 MPa, and further preferably is 580 MPa.
  • the upper limit of the yield strength of the duplex stainless steel material according to the present embodiment is, for example, 724 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 performed by a method in accordance with ASTM E8/E8M (2021). A test specimen is prepared from the steel material according to the present embodiment. If the steel material is a steel plate, a round bar test specimen is prepared from a center portion of the thickness. 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.
  • a round bar test specimen is prepared from an R/2 position in a cross section perpendicular to the axial direction of the round steel bar.
  • R/2 position means the center position of a radius R in a cross section perpendicular to the axial direction of the round steel bar.
  • the size of the round bar test specimen is, for example, as follows: the parallel portion diameter is 6 mm and the gage length is 24 mm.
  • the longitudinal direction of the round bar test specimen and the arc-shaped test specimen is to be parallel to the rolling direction of the steel material.
  • a tensile test is carried out at normal temperature (25°C) in air using the test specimen, and the obtained 0.2% offset proof stress is defined as the yield strength (MPa).
  • the microstructure of the duplex stainless steel material according to the present embodiment is composed of ferrite and austenite.
  • the phrase "composed of ferrite and austenite” means that the amount of any phase other than ferrite and austenite 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 55%. If the volume ratio of ferrite is too low, the yield strength will decrease. On the other hand, if the volume ratio of ferrite is too high, the low-temperature toughness of the steel material will decrease. However, in the microstructure of a duplex stainless steel material which has the chemical composition described above and which is produced by a preferable production method to be described later, the volume ratio of ferrite is 35 to 55%. Therefore, in the microstructure of the duplex stainless steel material according to the present embodiment, the volume ratio of ferrite is 35 to 55%.
  • the volume ratio of primary austenite is 40 to 55% and the volume ratio of secondary austenite is 5 to 20%.
  • an austenite grain with a minor axis of 20 ⁇ m or more is defined as primary austenite, and the balance of the austenite is defined as secondary austenite.
  • the fine secondary austenite disperses in ferrite and increases the low-temperature toughness of the ferrite. As a result, the low-temperature toughness of the duplex stainless steel material increases. If the volume ratio of the secondary austenite is too low, the aforementioned advantageous effect will not be sufficiently obtained. On the other hand, if the volume ratio of the secondary austenite is too high, the yield strength of the duplex stainless steel material will decrease. As a result, a duplex stainless steel material having a yield strength of 80 ksi or more will not be obtained. Therefore, in the present embodiment, the volume ratio of secondary austenite is to be 5 to 20%.
  • a preferable lower limit of the volume ratio of secondary austenite is 6%, more preferably is 7%, and further preferably is 10%.
  • a preferable upper limit of the volume ratio of secondary austenite is 19%, more preferably is 18%, and further preferably is 15%.
  • the volume ratio of the primary austenite is to be 40 to 55%.
  • a preferable lower limit of the volume ratio of primary austenite is 41%, more preferably is 42%, and further preferably is 45%.
  • a preferable upper limit of the volume ratio of primary austenite is 54%, more preferably is 53%, and further preferably is 50%.
  • the duplex stainless steel material according to the present embodiment has the chemical composition described above, and is composed of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5 to 20%.
  • the duplex stainless steel material according to the present embodiment has a yield strength of 80 ksi or more and has excellent low-temperature toughness. Note that, in the present embodiment, as described later, microstructure observation is performed at a cross section perpendicular to the rolling direction of the duplex stainless steel material.
  • the microstructure is composed of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5 to 20%.
  • the volume ratios of ferrite, primary austenite, and secondary austenite of the duplex stainless steel material according to the present embodiment can be determined by the following method.
  • a test specimen for microstructure observation is prepared from the duplex stainless steel material according to the present embodiment. If the steel material is a steel plate, a test specimen having an observation surface with dimensions of 5 mm in the width direction and 5 mm in the thickness direction is prepared from a center portion of the thickness. If the steel material is a steel pipe, a test specimen having an observation surface with dimensions of 5 mm in the pipe radial direction and 5 mm in the pipe circumferential direction is prepared from a central portion of the wall thickness.
  • a test specimen having an observation surface (5 mm ⁇ 5 mm) perpendicular to the axial direction of the round steel bar is prepared from an R/2 position in a cross section perpendicular to the axial direction of the round steel bar.
  • the size of the test specimen is not particularly limited as long as the aforementioned observation surface can be obtained.
  • the observation surface of the prepared test specimen is mirror-polished.
  • the mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the microstructure.
  • the observation surface where the microstructure has been revealed is observed in 10 visual fields using an optical microscope.
  • the area of each visual field is, for example, 40000 ⁇ m 2 (200 ⁇ m ⁇ 200 ⁇ m).
  • ferrite and austenite are identified based on contrast.
  • the area fraction of the identified ferrite is determined.
  • the method for determining the area fraction of ferrite is not limited, and it suffices to use a well-known method.
  • the area fraction of ferrite can be determined using image analysis software.
  • the austenite grain with a minor axis of 20 ⁇ m or more is identified among the austenite of each visual field identified based on contrast.
  • the minor axis of austenite is defined as follows. First, any austenite is identified in each visual field. Note that, in the present description, the phrase "any austenite is identified” means one austenite grain whose outer circumference is surrounded by ferrite is identified. A line segment with the longest length among line segments linking an arbitrary two points of the outer circumference of the relevant austenite is defined as the major axis of the relevant austenite. A rectangle that circumscribes the relevant austenite is then drawn in a manner so that the major axis of the relevant austenite is the long side. At this time, the short side of the drawn rectangle is defined as the minor axis of the relevant austenite.
  • the area fraction of the identified austenite with a minor axis of 20 ⁇ m or more is determined.
  • the method for determining the area fraction of the primary austenite is not limited, and it suffices to use a well-known method.
  • the area fraction can be determined using image analysis software.
  • the area fraction (%) of secondary austenite can be determined using the area fraction (%) of ferrite and the area fraction (%) of primary austenite as determined by the above methods, and the following Formula (A).
  • (Area fraction (%) of secondary austenite) 100 - ⁇ (area fraction (%) of ferrite) + (area fraction (%) of primary austenite) ⁇
  • the arithmetic average value of the area fractions (%) of ferrite obtained in the 10 visual fields by the above method is defined as the volume ratio (%) of ferrite.
  • the arithmetic average value of the area fractions (%) of primary austenite obtained in the 10 visual fields by the above method is defined as the volume ratio (%) of primary austenite.
  • the arithmetic average value of the area fractions (%) of secondary austenite obtained in the 10 visual fields by the above method is defined as the volume ratio (%) of secondary austenite.
  • the microstructure includes precipitates or inclusions or the like in addition to ferrite, primary austenite, and secondary austenite.
  • the volume ratios of precipitates or inclusions or the like are negligibly small as compared with the volume ratios of ferrite, primary austenite, and secondary austenite. Therefore, in the present description, when calculating the volume ratios of ferrite, primary austenite, and secondary austenite by the above method, the volume ratios of precipitates, inclusions and the like are ignored.
  • the duplex stainless steel material according to the present embodiment has the chemical composition described above, and is composed of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5 to 20%.
  • the duplex stainless steel material according to the present embodiment has excellent low-temperature toughness even when the yield strength thereof is 80 ksi (552 MPa) or more.
  • excellent low-temperature toughness is defined as follows.
  • the low-temperature toughness of the duplex stainless steel material according to the present embodiment is evaluated by a Charpy impact test in accordance with ASTM E23 (2016).
  • a V-notch test specimen in accordance with ASTM E23 (2016) is prepared from the steel material according to the present embodiment. Specifically, if the steel material is a steel plate, a V-notch test specimen having a notched surface parallel to the thickness direction and the rolling direction is to be prepared from a center portion of the thickness. If the steel material is a steel pipe, a V-notch test specimen having a notched surface parallel to the wall thickness direction and the pipe axis direction is to be prepared from a central portion of the wall thickness.
  • a V-notch test specimen having a notched surface parallel to the radial direction of the cross section and to the rolling direction is to be prepared from an R/2 position in a cross section perpendicular to the axial direction. Note that, the longitudinal direction of the V-notch test specimen is to be parallel to the rolling direction of the steel material.
  • the prepared V-notch test specimen is subjected to a Charpy impact test in accordance with ASTM E23 (2016).
  • the Charpy impact test is performed under eight conditions set in increments of 10°C in the range of 0 to -70°C, and the absorbed energy (J) at each temperature is determined.
  • the determined absorbed energy (J) is divided by the cross-sectional area (cm 2 ) of the V-notch test specimen to determine the absorbed energy (J/cm 2 ) per unit area at each temperature.
  • cross-sectional area of the V-notch test specimen means the area of a cross section perpendicular to the longitudinal direction of the V-notch test specimen at a position at the bottom of the V-notch.
  • the absorbed energy (J/cm 2 ) per unit area can be determined by dividing the determined absorbed energy (J) by the cross-sectional area 0.8 cm 2 (width of 0.8 cm ⁇ thickness of 1.0 cm) of the V-notch test specimen.
  • the lowest temperature (°C) at which an absorbed energy is 30 J/cm 2 or more is determined.
  • the lowest temperature is -30°C.
  • the duplex stainless steel material has excellent low-temperature toughness. Note that, in the present description the absorbed energy per unit area is also referred to simply as "absorbed energy”.
  • 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 steel plate, may be a round steel bar, or may be a wire rod.
  • the duplex stainless steel material according to the present embodiment is a seamless steel pipe. In a case where 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 552 MPa or more and excellent low-temperature toughness.
  • 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 of the present embodiment includes a starting material preparation process, a hot working process, a secondary austenite precipitation treatment process, and a solution treatment process. Hereunder, 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 starting material is produced by the following method.
  • a molten steel having the chemical composition described above is produced.
  • a cast piece (a slab, a bloom, or a billet) is produced by a continuous casting process using the molten steel.
  • An ingot may also be produced by an ingot-making process using the molten steel.
  • a slab, a bloom, or an ingot may be subjected to blooming to produce a billet.
  • the starting material is produced by the above process.
  • the starting material prepared in the aforementioned preparation process is subjected to hot working to produce an intermediate steel material.
  • intermediate steel material refers to a plate-shaped steel material in a case where the end product is a steel plate, refers to a hollow shell in a case where the end product is a steel pipe, refers to a steel material having a circular cross-sectional shape in a case where the end product is a round steel bar, and refers to a wireshaped steel material in a case where the end product is a wire rod.
  • the hot working may be hot forging, may be hot extrusion, or may be hot rolling.
  • the hot working method is not particularly limited, and it suffices to use a well-known method.
  • the intermediate steel material is a hollow shell (seamless steel pipe)
  • the Ugine-Sejoumet 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.
  • tube drawing 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 intermediate steel material may be produced as follows. If the steel material is a round steel bar, first, the starting material is heated in a heating furnace. Although not particularly limited, the heating temperature is, for example, 1100 to 1300°C. After being extracted from the heating furnace, the starting material is subjected to hot working to produce an intermediate steel material in which a cross section perpendicular to the axial direction is a circular shape.
  • the hot working is, for example, blooming performed using a blooming mill or hot rolling performed using a continuous mill. In a continuous mill, a horizontal stand having a pair of grooved rolls arranged one on the other in the vertical direction, and a vertical stand having a pair of grooved rolls arranged side by side in the horizontal direction are alternately arranged.
  • the starting material is heated in a heating furnace.
  • the heating temperature is, for example, 1100 to 1300°C.
  • the starting material is subjected to hot rolling using a blooming mill and a continuous mill to produce an intermediate steel material in the shape of a steel plate.
  • the intermediate steel material produced by the aforementioned hot working process is subjected to a heat treatment to cause secondary austenite to precipitate in the intermediate steel material.
  • the intermediate steel material is heated, and is held for three minutes or more within the range of 900 to 960°C.
  • the heating rate from 400 to 800°C when heating the intermediate steel material is set to 0.35°C/sec or more.
  • the heating rate from 400 to 800°C when heating the intermediate steel material is set to 0.35°C/sec or more.
  • the upper limit of the heating rate from 400 to 800°C when heating the intermediate steel material is, for example, 0.60°C/sec.
  • a method for heating the intermediate steel material is not particularly limited, and a well-known method can be used.
  • the intermediate steel material may be heated using a holding furnace or a high-frequency heating furnace.
  • the intermediate steel material heated at the aforementioned heating rate is held for three minutes or more at a temperature within the range of 900 to 960°C.
  • the temperature at which the intermediate steel material is held (holding temperature) is too high, secondary austenite will not precipitate sufficiently. In such case, a sufficient volume ratio of secondary austenite will not be obtained in the produced duplex stainless steel material. As a result, sufficient low-temperature toughness will not be obtained in the produced duplex stainless steel material.
  • the holding temperature is too low, too large an amount of secondary austenite will precipitate. In such case, the volume ratio of primary austenite in the produced duplex stainless steel material will decrease. As a result, in some cases the yield strength of the steel material will be less than 552 MPa.
  • the holding temperature is set within the range of 900 to 960°C.
  • a more preferable lower limit of the holding temperature is 905°C, and further preferably is 910°C.
  • a more preferable upper limit of the holding temperature is 955°C, and further preferably is 950°C.
  • the holding time is set to three minutes or more.
  • a more preferable lower limit of the holding time is four minutes, and further preferably is five minutes.
  • the upper limit of the holding time is not particularly limited. However, even if the holding time is very long, the effect thereof will be saturated. Therefore, in the secondary austenite precipitation treatment process according to the present embodiment, the upper limit of the holding time is preferably set to 20 minutes. A more preferable upper limit of the holding time is 19 minutes, and further preferably is 18 minutes.
  • a solution treatment is performed on the intermediate steel material subjected to the aforementioned secondary austenite precipitation treatment 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 term "solution treatment temperature” means the temperature (°C) of the heat treatment furnace for performing the solution treatment.
  • solution treatment time means the time for which the intermediate steel material is held at the solution treatment temperature.
  • the solution treatment temperature in the solution treatment process of the present embodiment is set within the range of 980 to 1 1 10°C. If the solution treatment temperature is too low, precipitates (for example, ⁇ phase that is an intermetallic compound or the like) may remain in the intermediate steel material after the solution treatment. In such case, the corrosion resistance of the produced duplex stainless steel material will decrease. Furthermore, if the solution treatment temperature is too low, too much secondary austenite will precipitate. In such case, in the produced duplex stainless steel material, in some cases the yield strength of the steel material may be less than 552 MPa. On the other hand, if the solution treatment temperature is too high, in some cases the precipitated secondary austenite will dissolve and the volume ratio of secondary austenite in the produced duplex stainless steel material will decrease. In such case, the low-temperature toughness of the steel material will decrease.
  • 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.
  • the duplex stainless steel material on which the solution treatment was performed may be subjected to a pickling treatment.
  • the pickling treatment is not particularly limited and it suffices that the pickling treatment is performed by a well-known method.
  • the duplex stainless steel material on which the solution treatment was performed may be subjected to cold rolling. Even in a case where cold rolling is performed, as long as the aforementioned requirement regarding the volume ratio of ferrite, the volume ratio of primary austenite, and the volume ratio of secondary austenite is satisfied, a yield strength of 80 ksi (552 MPa) or more and excellent low-temperature toughness can both be achieved.
  • the duplex stainless steel material according to the present embodiment can be produced by performing the processes described above. Note that the method for producing the duplex stainless steel material described above is one example, and the duplex stainless steel material may also be produced by the other methods. Hereunder, the present invention is described in more detail by way of examples.
  • Molten steels having the chemical compositions shown in Table 1A and Table 1B were melted using a 50 kg vacuum furnace, and ingots were produced by an ingot-making process.
  • the symbol "-" in Table 1B means that the content of the corresponding element was at an impurity level.
  • it 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 Co, the content of Sn, the content of Sb, the content of Ca, the content of Mg, the content of B, and the content of REM of Test No. 1 were each 0% when rounded off to third decimal places.
  • the ingot of each steel type was heated to the heating temperature (°C) for hot working described in Table 2, and thereafter subjected to hot rolling to produce a hollow shell (seamless steel pipe) having an outer diameter of 177.8 mm and a wall thickness of 12.65 mm.
  • the hollow shell of each test number on which the hot working had been performed was subjected to a secondary austenite precipitation treatment in which the hollow shell was heated at a heating rate (°C/sec) from 400 to 800°C described in Table 2, and held for only a holding time (mins) at a holding temperature (°C) which are described in Table 2.
  • a solution treatment was performed at a solution treatment temperature (°C) for a solution treatment time (mins) which are described in Table 2.
  • 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, and a Charpy impact test. Note that, cracking was confirmed in the obtained seamless steel pipes of Test Nos. 34 and 35. Therefore, these seamless steel pipes were not subjected to evaluation tests.
  • the seamless steel pipe of each test number excluding Test Nos. 34 and 35 was subjected to a tensile test in accordance with ASTM E8/E8M (2021). Specifically, an arc-shaped test specimen for a tensile test was prepared from a central portion of the wall thickness of 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, the width was made 25.4 mm, and the gage length was made 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 arithmetic average value of the area fractions of ferrite in the 10 visual fields was defined as the ferrite volume ratio (%).
  • the arithmetic average value of the area fractions of primary austenite in the 10 visual fields was defined as the primary austenite volume ratio (%).
  • the arithmetic average value of the area fractions of secondary austenite in the 10 visual fields was defined as the secondary austenite volume ratio (%).
  • the determined ferrite volume ratio (%), primary austenite volume ratio (%), and secondary austenite volume ratio (%) of each test number are shown in Table 3.
  • the seamless steel pipe of each test number excluding Test Nos. 34 and 35 was subjected to a Charpy impact test in accordance with ASTM E23 (2016), and the low-temperature toughness was evaluated.
  • a V-notch test specimen for the Charpy impact test was prepared from the seamless steel pipe of each test number in accordance with ASTM E23 (2016). Specifically, a V-notch test specimen having a notched surface parallel to the wall thickness direction and the pipe axis direction was prepared from a central portion of the wall thickness of the seamless steel pipe of each test number. Note that, the longitudinal direction of the V-notch test specimen was parallel to the rolling direction of the steel material.
  • the V-notch test specimen was a full-size V-notch test specimen (having a width of 10 mm, a thickness of 10 mm, and a length of 55 mm), and the V-notch depth was made 2 mm.
  • the term "width" of the V-notch test specimen means the distance between the surface in which the V-notch was formed and the surface on the opposite side thereto in the V-notch test specimen.
  • each test number three of the prepared V-notch test specimens were cooled under each of the eight conditions, i.e. a temperature of 0°C, -10°C, -20°C, - 30°C, -40°C, -50°C, -60°C, and -70°C, respectively.
  • Each of the cooled test specimens was subjected to the Charpy impact test in accordance with ASTM E23 (2016), and the absorbed energy (J) was determined. Note that, the arithmetic average value for the three test specimens at each temperature was defined as the absorbed energy (J).
  • the determined absorbed energy (J) was divided by the cross-sectional area (cm 2 ) of the V-notch test specimen to determine the absorbed energy per unit area (J/cm 2 ) at each temperature. Note that, the cross-sectional area of the V-notch test specimen (cm 2 ) was taken as 0.8 cm 2 (width of 0.8 cm ⁇ thickness of 1.0 cm) as defined by the aforementioned method.
  • the lowest temperature (°C) at which the absorbed energy per unit area is 30 J/cm 2 or more was determined.
  • the determined lowest temperature of each test number is shown in Table 3.
  • the chemical composition was appropriate.
  • the production method was also the preferable production method described in the present description.
  • the yield strength was 552 MPa or more.
  • the volume ratio of ferrite was 35 to 55%
  • the volume ratio of primary austenite was 40 to 55%
  • the volume ratio of secondary austenite was 5 to 20%.
  • the lowest temperature at which the absorbed energy per unit area was 30 J/cm 2 or more was - 20°C or less. That is, the seamless steel pipes of Test Nos. 1 to 23 had a yield strength of 80 ksi or more and excellent low-temperature toughness.
  • the seamless steel pipes of Test Nos. 30 and 31 were not subjected to the secondary austenite precipitation treatment process. As a result, the volume ratio of secondary austenite was less than 5%. Consequently, the lowest temperature at which the absorbed energy per unit area was 30 J/cm 2 or more was more than -20°C. That is, these seamless steel pipes did not have excellent low-temperature toughness.
  • the solution treatment temperature in the solution treatment process was too low.
  • the volume ratio of primary austenite was less than 40%, and in addition, the volume ratio of secondary austenite was more than 20%. Consequently, the yield strength was less than 552 MPa. That is, these seamless steel pipes did not have a yield strength of 80 ksi or more.
  • the content of Ni was too low.
  • the volume ratio of ferrite was more than 55%
  • the volume ratio of primary austenite was less than 40%
  • the volume ratio of secondary austenite was less than 5%. Consequently, the lowest temperature at which the absorbed energy per unit area was 30 J/cm 2 or more was more than -20°C. That is, this seamless steel pipe did not have excellent low-temperature toughness.

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