WO2022196498A1 - 二相ステンレス鋼 - Google Patents
二相ステンレス鋼 Download PDFInfo
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- WO2022196498A1 WO2022196498A1 PCT/JP2022/010351 JP2022010351W WO2022196498A1 WO 2022196498 A1 WO2022196498 A1 WO 2022196498A1 JP 2022010351 W JP2022010351 W JP 2022010351W WO 2022196498 A1 WO2022196498 A1 WO 2022196498A1
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- stainless steel
- duplex stainless
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- 229910001039 duplex stainless steel Inorganic materials 0.000 title claims abstract description 32
- 239000000203 mixture Substances 0.000 claims abstract description 13
- 239000000126 substance Substances 0.000 claims abstract description 12
- 239000012535 impurity Substances 0.000 claims abstract description 5
- 229910000831 Steel Inorganic materials 0.000 claims description 31
- 239000010959 steel Substances 0.000 claims description 31
- 238000001816 cooling Methods 0.000 claims description 25
- 238000004519 manufacturing process Methods 0.000 claims description 14
- 229910045601 alloy Inorganic materials 0.000 claims description 12
- 239000000956 alloy Substances 0.000 claims description 12
- 238000005098 hot rolling Methods 0.000 claims description 11
- 238000005266 casting Methods 0.000 claims description 10
- 229910052751 metal Inorganic materials 0.000 claims description 10
- 239000002184 metal Substances 0.000 claims description 10
- 238000005096 rolling process Methods 0.000 claims description 10
- 238000000034 method Methods 0.000 claims description 8
- 238000009749 continuous casting Methods 0.000 claims description 6
- 230000008569 process Effects 0.000 claims description 5
- 239000012071 phase Substances 0.000 description 51
- 230000007797 corrosion Effects 0.000 description 35
- 238000005260 corrosion Methods 0.000 description 35
- 230000000694 effects Effects 0.000 description 32
- 229910000859 α-Fe Inorganic materials 0.000 description 24
- 229910001566 austenite Inorganic materials 0.000 description 17
- 239000000463 material Substances 0.000 description 16
- 238000001556 precipitation Methods 0.000 description 14
- 239000010949 copper Substances 0.000 description 11
- 229910052761 rare earth metal Inorganic materials 0.000 description 10
- 229910000765 intermetallic Inorganic materials 0.000 description 9
- 238000007711 solidification Methods 0.000 description 9
- 230000008023 solidification Effects 0.000 description 9
- 238000005336 cracking Methods 0.000 description 7
- 230000007423 decrease Effects 0.000 description 6
- 230000006872 improvement Effects 0.000 description 5
- 230000003287 optical effect Effects 0.000 description 5
- 239000002994 raw material Substances 0.000 description 5
- 239000002344 surface layer Substances 0.000 description 5
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 4
- 229910052804 chromium Inorganic materials 0.000 description 3
- 238000000866 electrolytic etching Methods 0.000 description 3
- 229910052750 molybdenum Inorganic materials 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 238000005097 cold rolling Methods 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000006477 desulfuration reaction Methods 0.000 description 2
- 230000023556 desulfurization Effects 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 229910052747 lanthanoid Inorganic materials 0.000 description 2
- 150000002602 lanthanoids Chemical class 0.000 description 2
- 229910052757 nitrogen Inorganic materials 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 239000000047 product Substances 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 229910052596 spinel Inorganic materials 0.000 description 2
- 239000011029 spinel Substances 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- 229910001220 stainless steel Inorganic materials 0.000 description 2
- 239000010935 stainless steel Substances 0.000 description 2
- 150000003568 thioethers Chemical class 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- 229910000885 Dual-phase steel Inorganic materials 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 238000004458 analytical method Methods 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 238000004138 cluster model Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 239000012467 final product Substances 0.000 description 1
- 238000009863 impact test Methods 0.000 description 1
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 150000002739 metals Chemical class 0.000 description 1
- 239000003595 mist Substances 0.000 description 1
- 238000004776 molecular orbital Methods 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 230000002265 prevention Effects 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 229910052706 scandium Inorganic materials 0.000 description 1
- 238000004904 shortening Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000007790 solid phase Substances 0.000 description 1
- 239000007921 spray Substances 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 230000035882 stress Effects 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
- 238000003466 welding Methods 0.000 description 1
- 229910052727 yttrium Inorganic materials 0.000 description 1
Classifications
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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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- 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
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
-
- 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
- C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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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
- C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
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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
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
- C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
- C22C38/002—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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- 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
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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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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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
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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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
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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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- 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
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
Definitions
- the present invention relates to duplex stainless steel.
- Duplex stainless steel is stainless steel that has both austenite and ferrite phases in the steel structure. Due to its excellent corrosion resistance and high strength, duplex stainless steel is being applied in various fields such as petrochemical equipment materials, pump materials, chemical tank materials, etc., taking advantage of its high corrosion resistance.
- Patent Document 1 discloses a Sn-containing duplex stainless steel, a duplex stainless steel slab, and a duplex stainless steel material that have good hot manufacturability and are inexpensive.
- PRE Platinum Resistance Equivalent: Cr + 3.3Mo + 16N
- a parameter that indicates the corrosion resistance, especially pitting resistance, of duplex stainless steel and the contents of Cr, Mo and N are generally adjusted to increase the value of PRE. component design is carried out.
- steel materials with a PRE of 40 or more have been required for the purpose of improving corrosion resistance.
- duplex stainless steel with increased Cr and Mo contents has the problem that intermetallic compounds such as the ⁇ phase, which reduce mechanical properties and corrosion resistance, tend to precipitate. If the ⁇ phase or the like precipitates, the material is significantly hardened, so that cracks are likely to occur and the hot workability is extremely deteriorated. In addition, even in the final product, the toughness around the intermetallic compound deteriorates, making it difficult to ensure desired performance.
- Patent Document 2 by suppressing the precipitation of intermetallic compounds such as ⁇ phase and ⁇ phase, which are embrittlement phases, during the production of highly corrosion-resistant duplex stainless steel, while maintaining high corrosion resistance, it is possible to achieve more excellent A method for continuously casting highly corrosion resistant duplex stainless steel having embrittlement resistance, castability and hot workability is disclosed.
- Patent Document 2 the content of Ni that contributes to stabilization of the austenite phase, improvement of toughness, and suppression of nitride precipitation is 7.0% or less, so these effects may not be sufficiently obtained. , there is room for improvement.
- An object of the present invention is to provide a duplex stainless steel that can suppress cracking due to a decrease in toughness even when the PRE value is high and the Ni content is high.
- the present invention has been made based on the above findings, and its gist is the following duplex stainless steel.
- a continuous casting step of continuously casting molten steel having the chemical composition described in (1) above In the continuous casting process, the cast slab is primarily cooled to a temperature range of 950 to 1050 ° C., then reheated to a maximum temperature of 1050 ° C. or higher, and then a residence time in the temperature range of 900 to 1000 ° C. Cool under conditions of 400 s or less, A method for producing duplex stainless steel.
- the cast slab is heated in a temperature range of 1150 to 1300°C for 1.5 hours or more, then hot rolled under conditions where the finish rolling temperature is 900 to 1110°C, and then 800 to 500°C. Cooling to a temperature range of 500 ° C. or less under conditions where the average cooling rate in the temperature range is 0.1 to 1.0 ° C./s, A method for producing a duplex stainless steel according to (2) above.
- C 0.10% or less
- C is an element that forms a solid solution in the austenite phase to increase the strength.
- the C content should be 0.10% or less, preferably 0.050% or less.
- the C content is more preferably 0.030% or less.
- it is not necessary to set a lower limit to the C content, it is preferably 0.010% or more, more preferably 0.015% or more, if the above effect is to be obtained.
- Si 3.0% or less Si is used as a deoxidizing element and may be added to improve oxidation resistance. However, if it is contained in a large amount, it causes hardening of the steel and deteriorates workability. Therefore, the Si content should be 3.0% or less, preferably 2.0% or less or 1.0% or less. Although it is not necessary to set a lower limit to the Si content, it is preferably 0.10% or more, more preferably 0.20% or more, in order to obtain the above effect.
- Mn 8.0% or less Mn has the effect of increasing the austenite phase, increasing the solid solubility of nitrogen, and suppressing bubble defects during production. However, a large amount of Mn reduces corrosion resistance. Therefore, the Mn content is 8.0% or less, preferably 3.0% or less or 1.0% or less. Although it is not necessary to set a lower limit to the Mn content, it is preferably 0.20% or more, more preferably 0.40% or more, if the above effects are to be obtained.
- P 0.040% or less
- P is an element that is unavoidably mixed in steel and is also contained in raw materials such as Cr, so it is difficult to reduce it. Reduces moldability.
- the P content is preferably 0.030% or less.
- S 0.020% or less S is an element that is inevitably mixed in steel, and may combine with Mn to form inclusions and become the origin of rust. Therefore, the S content should be 0.020% or less. Since the lower the S content, the better the corrosion resistance, the S content is preferably 0.010% or less, more preferably 0.0050% or less.
- Cr 20.0-38.0% Cr is an element necessary to ensure corrosion resistance.
- Cr is a ferrite stabilizing element, and in order to obtain a two-phase structure of austenite and ferrite, it is necessary to contain 20.0% or more of Cr in consideration of the phase ratio. However, if a large amount of Cr is contained, the corrosion resistance is rather lowered. Therefore, the Cr content should be 38.0% or less.
- the Cr content is preferably 22.0% or more or 24.0% or more, and preferably 33.0% or less, 28.0% or less or 27.0% or less.
- Ni 3.00-12.00%
- Ni is an austenite stabilizing element. Moreover, Ni has an effect of improving corrosion resistance. Therefore, the Ni content is set to 3.00% or more. However, if Ni is contained in a large amount, the cost of raw materials increases, and problems such as stress corrosion cracking may occur. Therefore, the Ni content should be 12.00% or less.
- the Ni content is preferably 5.00% or more, more preferably 7.50% or more, and preferably 10.00% or less.
- Mo 1.0-6.5%
- Mo content is an element that improves corrosion resistance. Therefore, Mo content is made 1.0% or more. However, if a large amount of Mo is contained, not only does the raw material cost increase, but corrosion resistance decreases. Therefore, the Mo content should be 6.5% or less.
- the Mo content is preferably 2.0% or more, more preferably 3.0% or more, preferably 5.5% or less, and more preferably 4.4% or less.
- Cu 3.0% or less
- Cu is an element that is very effective in improving sulfuric acid resistance. However, if a large amount of Cu is contained, the corrosion resistance is rather lowered. Therefore, the Cu content should be 3.0% or less.
- the Cu content is preferably 2.0% or less, more preferably 0.90% or less. Although it is not necessary to set a lower limit to the Cu content, the Cu content is preferably 0.10% or more, more preferably 0.20% or more, in order to obtain the above effect.
- N 0.200-0.700%
- N is an element that forms a solid solution in the austenite phase to increase strength and corrosion resistance, thereby contributing to alloy saving. Therefore, the N content is made 0.200% or more. However, a large amount of N degrades the corrosion resistance of the steel due to defects due to blowholes. Therefore, the N content should be 0.700% or less.
- the N content is preferably 0.240% or more and preferably 0.450% or less.
- Al 0-1.0%
- Al is an optional element and may not be contained. When contained, Al exerts desulfurization and deoxidation effects. However, if Al is contained in a large amount, hard spinel inclusions (MgO.Al 2 O 3 ) that cause nozzle clogging are precipitated, and manufacturing flaws increase and raw material costs increase. Therefore, the Al content is set to 1.0% or less.
- the Al content is preferably 0.50% or less or 0.10% or less. In order to obtain the above effects with certainty, the Al content is preferably 0.010% or more.
- Sn 0-1.0% Sn is an optional element and may not be contained. When included, Sn enhances the corrosion resistance of steel. However, Sn is an element that impairs the workability of steel. Therefore, the Sn content is set to 1.0% or less. The Sn content is preferably 0.50% or less or 0.10% or less. In order to obtain the above effects with certainty, the Sn content is preferably 0.002% or more.
- W 0-6.0% W is an optional element and may not be contained. When included, W enhances the SCC and pitting resistance of the steel. W is also less likely to form the ⁇ phase than Mo. Therefore, W may be contained instead of part of Mo. The above effect can be obtained to some extent if W is contained even in a small amount. However, if the W content is too high, the production costs will be high. Therefore, the W content should be 6.0% or less.
- the W content is preferably 3.0% or less, more preferably 1.0% or less. In order to reliably obtain the above effects, the W content is preferably 0.01% or more, more preferably 0.10% or more.
- Co 0-3.0% Co is an optional element and may not be contained. When included, Co increases the strength of steel. Co also stabilizes austenite. As long as even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content is too high, the corrosion resistance of the steel is lowered and the manufacturing cost is increased. Therefore, the Co content should be 3.0% or less. The Co content is preferably 2.0% or less or 1.0% or less. In order to reliably obtain the above effect, the Co content is preferably 0.01% or more, more preferably 0.05% or more.
- Nb 0-0.50%
- Nb is an optional element and may not be contained. When included, Nb increases the strength of steel. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, the corrosion resistance of the steel is reduced. Therefore, the Nb content should be 0.50% or less.
- the Nb content is preferably 0.30% or less, 0.10% or less or 0.050% or less. In order to reliably obtain the above effects, the Nb content is preferably 0.005% or more.
- Ti 0-1.5%
- Ti is an optional element and does not have to be contained. When included, Ti increases the strength of steel. If even a little Ti is contained, the above effect can be obtained to some extent. However, if the Ti content is too high, the corrosion resistance of the steel is reduced. Therefore, the Ti content should be 1.5% or less.
- the Ti content is preferably 0.50% or less, 0.10% or less or 0.050% or less. In order to obtain the above effects with certainty, the Ti content is preferably 0.005% or more.
- V 0-1.0%
- V is an optional element and may not be contained. When included, V increases the strength of the steel. If V is contained even in a small amount, the above effect can be obtained to some extent. However, if the V content is too high, the corrosion resistance of the steel is reduced. Therefore, the V content should be 1.0% or less.
- the V content is preferably 0.80% or less, 0.50% or less or 0.30% or less. In order to obtain the above effects with certainty, the V content is preferably 0.01% or more or 0.05% or more.
- Zr 0-0.50%
- Zr is an optional element and may not be contained. When contained, Zr contributes to the improvement of corrosion resistance. As long as even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content is too high, the effect saturates. Therefore, the Zr content should be 0.50% or less.
- the Zr content is preferably 0.40% or less or 0.30% or less. In order to reliably obtain the above effects, the Zr content is preferably 0.005% or more.
- Ta 0-0.100%
- Ta is an optional element and may not be contained. When contained, Ta improves corrosion resistance by modifying inclusions. However, when the Ta content is too high, the ductility at room temperature is lowered. Therefore, the Ta content should be 0.100% or less.
- the Ta content is preferably 0.050% or less. If the above effects are to be obtained reliably, the Ta content is preferably 0.005% or more.
- B 0-0.100%
- B is an optional element and may not be contained. When included, B enhances hot workability. If B is contained even in a small amount, the above effect can be obtained to some extent. However, when the B content is too high, the above effects are saturated. Therefore, the B content should be 0.100% or less.
- the B content is preferably 0.0100% or less, more preferably 0.0050% or less. In order to reliably obtain the above effect, the B content is preferably 0.0001% or more, more preferably 0.0003% or more.
- Ca 0-0.50% Ca is an optional element and may not be contained. When contained, Ca exerts the effect of desulfurization, deoxidation, and prevention of formation of spinel inclusions. However, if a large amount of Ca is contained, the corrosion resistance is lowered and the amount of spatter generated during welding is increased. Therefore, the Ca content should be 0.50% or less.
- the Ca content is preferably 0.050% or less, more preferably 0.010% or less, even more preferably 0.0040% or less. In order to reliably obtain the above effect, the Ca content is preferably 0.0010% or more, more preferably 0.0015% or more.
- Mg 0-0.50% Mg is an optional element and may not be contained. When included, Mg forms sulfides with S in the steel and reduces the segregation of S to grain boundaries. As a result, the corrosion resistance of the steel increases, contributing to the improvement of hot workability. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content is too high, it forms coarse oxides or sulfides and initiates pitting corrosion. As a result, the corrosion resistance of steel is reduced. Therefore, the Mg content should be 0.50% or less. The Mg content is preferably 0.050% or less, more preferably 0.010% or less, even more preferably 0.0040% or less. In order to reliably obtain the above effect, the Mg content is preferably 0.0005% or more.
- REM 0-0.10% REM is an optional element and may not be contained. When included, REM improves the hot workability of the steel. Therefore, it is desirable that REM is contained in a very small amount. However, since an excessive content lowers the corrosion resistance of steel, the REM content is made 0.10% or less.
- the REM content is preferably 0.050% or less, more preferably 0.010% or less. In order to obtain the above effects reliably, the REM content is preferably 0.0005% or more or 0.005% or more.
- REM refers to a total of 17 elements of Sc, Y and lanthanoids, and the REM content means the total content of these elements.
- lanthanoids are industrially added in the form of misch metals.
- the balance is Fe and impurities.
- impurities refers to components mixed in by various factors in raw materials such as ores, scraps, etc., and in the manufacturing process when steel is manufactured industrially. means something
- the content of each element is within the range described above, and the PRE value and the Creq/Nieq value calculated by the formula shown below are each predetermined. must be within the range of
- PRE 41.0 or more PRE is a general index indicating the corrosion resistance of stainless steel, and is calculated from the chemical composition of steel using the following formula (i).
- the value of PRE is preferably 60.0 or less.
- PRE Cr+3.3Mo+16N (i)
- the element symbol in the above formula is the content (% by mass) of each element contained in the steel.
- Creq/Nieq 2.360-2.530 Creq and Nieq are defined by the following formulas (ii) and (iii), respectively.
- the value of Creq/Nieq is 2.400 or greater.
- the value of Creq/Nieq is set to 2.530 or less.
- the metal structure is mainly composed of vermicular ferrite, in which austenite crystallizes during solidification.
- the interfacial consistency of the ferrite/austenite phases is low, and cracking tends to progress as if the phase boundary separates, which tends to reduce the toughness.
- F-mode solidification is performed in which ferrite is solidified in a single phase.
- austenite precipitates through solid-phase transformation, resulting in a metal structure mainly composed of acicular ferrite. If the metallographic structure is mainly composed of acicular ferrite, the interfacial consistency between ferrite and austenite is high, and a decrease in toughness can be suppressed.
- Md Value is one of the indices of the phase stability of a multicomponent system, and is a value that means the electron orbital energy in the d orbital of each component of the alloy. The higher the Md value, the more unstable the phase becomes, and the easier the formation of intermetallic compounds such as the ⁇ phase.
- the average Md value defined by the following formula (iv) is set to 0.9140 or less. 0.9090 or less is desirable from the viewpoint of further suppressing the precipitation of intermetallic compounds.
- Average Md value ⁇ i (Md) i (iv)
- the meanings of the symbols in the above formula (iv) are as follows. ⁇ i : atomic fraction (Md) of alloy component i i : Md value (eV) of alloy component i
- the average Md value In order to suppress the precipitation of intermetallic compounds, the lower the average Md value, the better, so there is no need to set a lower limit.
- the average Md value may be 0.8800 or more.
- the Md value of the alloy component i can be obtained by cluster calculation (a molecular orbital calculation method performed using a cluster model consisting of several to several tens of atoms) (M.Morinaga et al., J. Phys. Soc. Jpn., 53 (1984), p.653).
- cluster calculation a molecular orbital calculation method performed using a cluster model consisting of several to several tens of atoms
- For the average Md value of the alloy it is possible to organize the precipitation of intermetallic compounds by converting the composition of the grain boundary and the final solidification part obtained from the initial composition and the segregation ratio into the atomic fraction and calculating XI. is.
- the area ratio of the ⁇ phase contained in the metal structure is 2.0% or less.
- the high Ni content promotes the precipitation of the ⁇ phase.
- the area ratio of the ⁇ phase is set to 2.0% or less.
- the area ratio of the ⁇ phase is preferably 1.0% or less, more preferably 0.10% or less, and even more preferably 0.05% or less. Since the lower the area ratio of the ⁇ phase, the better, there is no need to set a lower limit.
- the metal structure contains acicular ferrite in an area ratio of 50% or less and the balance is austenite and unavoidable products.
- the area ratio of austenite is relatively high, so toughness can be improved.
- Inevitable products may include Cr 2 N and the like in addition to the ⁇ phase. Cr 2 N and the like are allowed as long as the total content is 2.0% or less.
- the area ratios of ferrite and austenite are measured using a ferrite scope according to JIS Z 3119:2017. Whether the ferrite mainly consists of vermicular ferrite or acicular ferrite can be determined by structural observation using an optical microscope at a magnification of 50 times.
- the ⁇ phase is revealed by KOH electrolytic etching. Then, microstructure images of 60 fields of view are acquired at 400x magnification using an optical microscope. Then, the obtained image is binarized to measure the ⁇ phase area ratio. Since the ⁇ phase is unevenly contained in the structure, samples are taken from five or more locations on the cast slab, and the average of the measured values for each sample is used as the ⁇ phase area ratio.
- the duplex stainless steel according to the present invention can be manufactured, for example, by continuously casting molten steel having the chemical composition described above. That is, the duplex stainless steel of the present invention may be a slab. It is important to control the casting conditions at this time.
- the present inventors first conducted the following study on casting conditions for suppressing the precipitation of the ⁇ phase.
- the slab is mainly cooled by two processes: primary cooling using a water-cooled copper mold and secondary cooling in which a cooling spray is sprayed from the surface of the slab.
- primary cooling using a water-cooled copper mold and secondary cooling in which a cooling spray is sprayed from the surface of the slab.
- secondary cooling in which a cooling spray is sprayed from the surface of the slab.
- the temperature history at the 5 mm position of the surface layer of the cast slab was investigated by heat transfer analysis when the amount of water in the secondary cooling was changed variously.
- the casting speed at this time was 1.1 m/min.
- the precipitation nose of the ⁇ phase is considered to be about 900 to 1000°C. Therefore, in the cooling process after casting, shortening the residence time within the temperature range of 900 to 1000° C. is effective in suppressing precipitation of the ⁇ phase.
- the surface layer of the cast piece is the precipitated nose of the ⁇ phase. It was found that the residence time can be minimized within the temperature range of 900 to 1000°C.
- the maximum temperature is raised to 1050° C. or higher by recuperation from the center of the slab, and then By standing to cool, it is possible to set the residence time in the temperature range of 900 to 1000° C. to 400 seconds or less and the area ratio of the ⁇ phase to 2.0% or less.
- the shorter the residence time the better, preferably 300 s or less.
- the duplex stainless steel of the present invention may be a plate-shaped or rod-shaped hot-rolled material.
- the method for producing a duplex stainless steel according to the present invention further comprises a hot rolling step of hot rolling the cast slab. Although there are no particular restrictions on the conditions in the hot rolling step, the following conditions are preferred, for example.
- the temperature and time during heating mean the average temperature in the furnace and the time in the furnace, respectively.
- the heated slab is subjected to rough rolling and finish rolling.
- the finish rolling temperature is preferably 900 to 1110°C.
- the cooling method at this time is not particularly limited, and air cooling may be used, for example.
- the cooling rate during cooling from 500°C or below to room temperature, and cooling may be performed by air cooling, mist water cooling, water cooling, or the like.
- the finish rolling temperature means the surface temperature of the hot-rolled material at the exit of the final stand of a rolling mill having a plurality of stands.
- the cooling rate after finishing rolling refers to the cooling rate on the surface of the hot-rolled material.
- the duplex stainless steel of the present invention may be a cold-rolled material obtained by subjecting the above hot-rolled material to cold rolling. Cold rolling may be performed using a normal method. Further, the above hot-rolled material or cold-rolled material may be annealed to obtain an annealed hot-rolled material or an annealed cold-rolled material. At this time, the annealing temperature is preferably 550 to 900° C., for example, from the viewpoint of suppressing precipitation of the ⁇ phase.
- a cylindrical slab with a diameter of 180 mm and having the chemical composition shown in Table 1 was manufactured under various manufacturing conditions.
- Table 2 shows the continuous casting conditions for each slab.
- the metallographic structure was specifically measured according to the following procedure. First, the area ratios of ferrite and austenite were measured using a ferrite scope according to JIS Z 3119:2017. Whether the ferrite mainly consisted of vermicular ferrite or acicular ferrite was determined by structural observation using an optical microscope at a magnification of 50 times.
- samples for microscopic observation were cut out from 5 locations so that the observation surface was at a depth of 5 mm from the surface of the cast slab, and then KOH electrolytic etching was performed to reveal the ⁇ phase.
- microstructure images of 60 fields of view were obtained at a magnification of 400 times using an optical microscope, and the obtained images were binarized to measure the ⁇ phase area ratio. Then, the average value of the measured values of the five samples was taken as the ⁇ phase area ratio.
- a V-notch test piece was prepared from a 5 mm position on the surface layer of each slab.
- the test piece had a size of 10 mm ⁇ 10 mm ⁇ 55 mm and was subjected to a Charpy impact test according to JIS Z 2242:2005.
- the impact properties were evaluated as good when the impact value at 100° C. was 30.0 J/cm 2 or more, and as poor when it was less than 30.0 J/cm 2 .
- a test piece with a diameter of 8 mm and a length of 110 mm was cut from the surface layer of each slab. After that, the temperature was raised from room temperature to 1250° C. over 30 seconds and held for 30 seconds. Subsequently, it was cooled to 1000°C at a cooling rate of 20°C/s and held for 30s. After that, a tensile test was performed to measure the tensile strength and reduction of area.
- the slabs of 2, 4, 6, 8, 10 and 14 to 16 were subjected to hot rolling to obtain hot-rolled rods (wire rods) having a diameter of 5.5 mm. Specifically, after heating the slab at 1200 ° C. for 2 hours, it is hot rolled under the condition that the finish rolling temperature is 1100 ° C., and then the average cooling rate within the temperature range of 800 to 500 ° C. is 0.5. It was air-cooled to 400°C under the condition of °C/s, and further water-cooled to room temperature.
- samples for microscopic observation were cut out from five locations of the obtained hot-rolled material so that cross sections perpendicular to the longitudinal direction and the radial direction were observation surfaces, and the ⁇ phase was revealed by KOH electrolytic etching. I put it out. After that, microstructure images of 60 fields of view were obtained at a magnification of 400 times using an optical microscope, and the obtained images were binarized to measure the ⁇ phase area ratio. Then, the average value of the measured values of the five samples was taken as the ⁇ phase area ratio.
- the reduction of area at 1000 ° C. is 60.0% or more, and has good hot workability. It was possible to suppress it to 0% or less.
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Abstract
Description
C:0.10%以下、
Si:3.0%以下、
Mn:8.0%以下、
P:0.040%以下、
S:0.020%以下、
Cr:20.0~38.0%、
Ni:3.00~12.00%、
Mo:1.0~6.5%、
Cu:3.0%以下、
N:0.200~0.700%、
Al:0~1.0%、
Sn:0~1.0%、
W:0~6.0%、
Co:0~3.0%、
Nb:0~0.50%、
Ti:0~1.5%、
V:0~1.0%、
Zr:0~0.50%、
Ta:0~0.100%、
B:0~0.100%、
Ca:0~0.50%、
Mg:0~0.50%、
REM:0~0.10%、
残部:Feおよび不純物であり、
下記(i)式で定義されるPREの値が41.0以上であり、
下記(ii)式で定義されるCreqと、下記(iii)式で定義されるNieqとの比Creq/Nieqの値が2.360~2.530であり、
下記(iv)式で定義される平均Md値が0.9140以下であり、
金属組織中に含まれるσ相の面積率が2.0%以下である、
二相ステンレス鋼。
PRE=Cr+3.3Mo+16N ・・・(i)
Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti ・・・(ii)
Nieq=Ni+0.31Mn+22C+14.2N+Cu ・・・(iii)
平均Md値=ΣΧi・(Md)i ・・・(iv)
ただし、上記(i)~(iii)式中の元素記号は各元素の含有量(質量%)であり、上記(iv)式中の記号の意味は以下のとおりである。
Χi:合金成分iの原子分率
(Md)i:合金成分iのMd値(eV)
前記連続鋳造工程では、鋳片を950~1050℃の温度範囲まで一次冷却した後に、最高到達温度が1050℃以上となるまで復熱させ、その後、900~1000℃の温度範囲での滞留時間が400s以下となる条件で冷却する、
二相ステンレス鋼の製造方法。
前記熱間圧延工程では、前記鋳片を1150~1300℃の温度範囲で1.5h以上加熱した後に、仕上圧延温度が900~1110℃となる条件で熱間圧延し、次いで、800~500℃の温度範囲内での平均冷却速度が0.1~1.0℃/sとなる条件で500℃以下の温度範囲まで冷却する、
上記(2)に記載の二相ステンレス鋼の製造方法。
各元素の限定理由は下記のとおりである。なお、以下の説明において含有量についての「%」は、「質量%」を意味する。
Cは、オーステナイト相に固溶して強度を高める元素である。しかし、多量に含有すると、炭化物の析出から耐食性が低下する。したがって、C含有量は0.10%以下、好ましくは0.050%以下とする。時効耐食性を考慮すると、C含有量は0.030%以下であるのがより好ましい。なお、C含有量に下限を設ける必要はないが、上記の効果を得たい場合は、0.010%以上であるのが好ましく、0.015%以上であるのがより好ましい。
Siは、脱酸元素として使われ、また耐酸化性向上のために添加される場合がある。しかし、多量に含有すると、鋼の硬質化をもたらし、加工性が劣化する。したがって、Si含有量は3.0%以下、好ましくは2.0%以下または1.0%以下とする。なお、Si含有量に下限を設ける必要はないが、上記の効果を得たい場合は、0.10%以上であるのが好ましく、0.20%以上であるのがより好ましい。
Mnは、オーステナイト相を増加させ、また窒素の固溶度を上げ、製造時の気泡欠陥などを抑制する効果を有する。しかし、Mnを多量に含有すると、耐食性を低下させる。したがって、Mn含有量は8.0%以下、好ましくは3.0%以下または1.0%以下とする。なお、Mn含有量に下限を設ける必要はないが、上記の効果を得たい場合は、0.20%以上であるのが好ましく、0.40%以上であるのがより好ましい。
Pは、鋼中に不可避的に混入する元素であり、またCrなどの原料にも含有されているため、低減することが困難であるが、Pを多量に含有すると成形性を低下させる。P含有量は少ないほど好ましく、0.040%以下とする。P含有量は0.030%以下であるのが好ましい。
Sは、鋼中に不可避的に混入する元素であり、Mnと結合して介在物を作り、発銹の基点となる場合がある。したがって、S含有量は0.020%以下とする。S含有量は低いほど耐食性が向上するので、0.010%以下であるのが好ましく、0.0050%以下であるのがより好ましい。
Crは、耐食性を確保するために必要な元素である。加えて、Crはフェライト安定化元素であり、オーステナイトおよびフェライトの二相組織を得るため、相比率を考慮して20.0%以上のCrを含有する必要がある。しかし、Crを多量に含有すると、かえって耐食性の低下を招く。したがって、Cr含有量は38.0%以下とする。Cr含有量は22.0%以上または24.0%以上であるのが好ましく、33.0%以下、28.0%以下または27.0%以下であるのが好ましい。
Niは、オーステナイト安定化元素である。また、Niは耐食性を向上させる効果を有する。そのため、Ni含有量を3.00%以上とする。しかし、Niを多量に含有すると、原料コストの増加をもたらし、また応力腐食割れなどの問題が生じる可能性がある。したがって、Ni含有量は12.00%以下とする。Ni含有量は5.00%以上であるのが好ましく、7.50%以上であるのがより好ましく、10.00%以下であるのが好ましい。
Moは、耐食性を向上させる元素である。そのため、Mo含有量を1.0%以上とする。しかし、Moを多量に含有すると、原料コストの増加をもたらすだけでなく、かえって耐食性の低下を招く。したがって、Mo含有量は6.5%以下とする。Mo含有量は2.0%以上であるのが好ましく、3.0%以上であるのがより好ましく、5.5%以下であるのが好ましく、4.4%以下であるのがより好ましい。
Cuは、耐硫酸性の向上に非常に有効な元素である。しかし、Cuを多量に含有すると、かえって耐食性の低下を招く。したがって、Cu含有量は3.0%以下とする。Cu含有量は2.0%以下であるのが好ましく、0.90%以下であるのがより好ましい。なお、Cu含有量に下限を設ける必要はないが、上記の効果を得たい場合は、Cu含有量は0.10%以上であるのが好ましく、0.20%以上であるのが好ましい。
Nは、オーステナイト相に固溶して強度および耐食性を高めて省合金化に寄与する元素である。そのため、N含有量は0.200%以上とする。しかし、Nを多量に含有すると、ブローホールの発生による欠陥等により鋼の耐食性を劣化させる。したがって、N含有量は、0.700%以下とする。N含有量は0.240%以上であるのが好ましく、0.450%以下であるのが好ましい。
Alは任意元素であり、含有されなくてもよい。含有される場合、Alは、脱硫、脱酸の効果を発揮する。しかし、Alを多量に含有すると、ノズル閉塞の原因となる硬質なスピネル系介在物(MgO・Al2O3)が析出するほか、製造疵の増加および原料コストの増加を招く。したがって、Al含有量は1.0%以下とする。Al含有量は0.50%以下または0.10%以下であるのが好ましい。上記の効果を確実に得たい場合は、Al含有量は0.010%以上であるのが好ましい。
Snは任意元素であり、含有されなくてもよい。含有される場合、Snは鋼の耐食性を高める。しかしながら、Snは鋼の加工性を阻害する元素である。そのため、Sn含有量は1.0%以下とする。Sn含有量は0.50%以下または0.10%以下であるのが好ましい。上記の効果を確実に得たい場合は、Sn含有量は0.002%以上であるのが好ましい。
Wは任意元素であり、含有されなくてもよい。含有される場合、Wは鋼の耐SCC性および耐孔食性を高める。Wはさらに、Moと比較してσ相を生成しにくい。そのため、Moの一部に代えてWを含有させてもよい。Wが少しでも含有されれば、上記効果はある程度得られる。しかしながら、W含有量が高すぎる場合、製造コストが高くなる。したがって、W含有量は6.0%以下とする。W含有量は3.0%以下であるのが好ましく、1.0%以下であるのがより好ましい。上記の効果を確実に得たい場合は、W含有量は0.01%以上であるのが好ましく、0.10%以上であるのがより好ましい。
Coは任意元素であり、含有されなくてもよい。含有される場合、Coは鋼の強度を高める。Coはさらに、オーステナイトを安定化させる。Coが少しでも含有されれば、上記効果はある程度得られる。しかしながら、Co含有量が高すぎる場合、鋼の耐食性が低下するほか、製造コストが高くなる。したがって、Co含有量は3.0%以下とする。Co含有量は2.0%以下または1.0%以下であるのが好ましい。上記の効果を確実に得たい場合は、Co含有量は0.01%以上であるのが好ましく、0.05%以上であるのがより好ましい。
Nbは任意元素であり、含有されなくてもよい。含有される場合、Nbは鋼の強度を高める。Nbが少しでも含有されれば、上記効果はある程度得られる。しかしながら、Nb含有量が高すぎる場合、鋼の耐食性が低下する。したがって、Nb含有量は0.50%以下とする。Nb含有量は0.30%以下、0.10%以下または0.050%以下であるのが好ましい。上記の効果を確実に得たい場合は、Nb含有量は0.005%以上であるのが好ましい。
Tiは任意元素であり、含有されなくてもよい。含有される場合、Tiは鋼の強度を高める。Tiが少しでも含有されれば、上記効果はある程度得られる。しかしながら、Ti含有量が高すぎる場合、鋼の耐食性が低下する。したがって、Ti含有量は1.5%以下とする。Ti含有量は0.50%以下、0.10%以下または0.050%以下であるのが好ましい。上記の効果を確実に得たい場合は、Ti含有量は0.005%以上であるのが好ましい。
Vは任意元素であり、含有されなくてもよい。含有される場合、Vは鋼の強度を高める。Vが少しでも含有されれば、上記効果はある程度得られる。しかしながら、V含有量が高すぎる場合、鋼の耐食性が低下する。したがって、V含有量は1.0%以下とする。V含有量は0.80%以下、0.50%以下または0.30%以下であるのが好ましい。上記の効果を確実に得たい場合は、V含有量は0.01%以上または0.05%以上であるのが好ましい。
Zrは任意元素であり、含有されなくてもよい。含有される場合、Zrは耐食性向上に寄与する。Zrが少しでも含有されれば、上記効果はある程度得られる。しかしながら、Zr含有量が高すぎる場合、効果が飽和する。したがって、Zr含有量は0.50%以下とする。Zr含有量は0.40%以下または0.30%以下であるのが好ましい。上記の効果を確実に得たい場合は、Zr含有量は0.005%以上であるのが好ましい。
Taは任意元素であり、含有されなくてもよい。含有される場合、Taは介在物の改質により耐食性を向上させる。しかしながら、Ta含有量が高すぎる場合、常温での延性の低下を招く。したがって、Ta含有量は0.100%以下とする。Ta含有量は0.050%以下であるのが好ましい。上記の効果を確実に得たい場合は、Ta含有量は0.005%以上であるのが好ましい。
Bは任意元素であり、含有されなくてもよい。含有される場合、Bは熱間加工性を高める。Bが少しでも含有されれば、上記効果はある程度得られる。しかしながら、B含有量が高すぎる場合、上記効果は飽和する。したがって、B含有量は0.100%以下とする。B含有量は0.0100%以下であるのが好ましく、0.0050%以下であるのがより好ましい。上記の効果を確実に得たい場合は、B含有量は0.0001%以上であるのが好ましく、0.0003%以上であるのがより好ましい。
Caは任意元素であり、含有されなくてもよい。含有される場合、Caは脱硫、脱酸のほか、スピネル系介在物生成防止の効果を発揮する。しかし、Caを多量に含有すると、耐食性が低下するほか、溶接時のスパッタ発生量の増大を引き起こす。したがって、Ca含有量は0.50%以下とする。Ca含有量は0.050%以下であるのが好ましく、0.010%以下であるのがより好ましく、0.0040%以下であるのがさら好ましい。上記の効果を確実に得たい場合は、Ca含有量は0.0010%以上であるのが好ましく、0.0015%以上であるのがより好ましい。
Mgは任意元素であり、含有されなくてもよい。含有される場合、Mgは鋼中のSと硫化物を形成し、Sの粒界への偏析を低減する。その結果、鋼の耐食性が高まり、熱間加工性の向上にも寄与する。Mgが少しでも含有されれば上記効果はある程度得られる。しかしながら、Mg含有量が高すぎる場合、粗大な酸化物または硫化物を形成し、孔食の起点となる。その結果、鋼の耐食性が低下する。したがって、Mg含有量は0.50%以下とする。Mg含有量は0.050%以下であるのが好ましく、0.010%以下であるのがより好ましく、0.0040%以下であるのがさら好ましい。上記の効果を確実に得たい場合は、Mg含有量は0.0005%以上であるのが好ましい。
REMは任意元素であり、含有されなくてもよい。含有される場合、REMは鋼の熱間加工性を改善する。そのため、REMは微量含有されることが望ましい。しかし、過剰な含有は鋼の耐食性を低下させるため、REM含有量は0.10%以下とする。REM含有量は0.050%以下であるのが好ましく、0.010%以下であるのがより好ましい。上記の効果を確実に得たい場合は、REM含有量は0.0005%以上または0.005%以上であるのが好ましい。
PREは、ステンレス鋼の耐食性を示す一般的な指標であり、鋼の化学組成から、下記(i)式で計算される。PREの値が41.0以上となるように合金設計をすることで、優れた耐食性を確保することが可能となる。PREの値に上限を設ける必要はないが、過剰に高いと合金コスト増加の問題を生じさせるおそれがある。そのため、PREの値は60.0以下であるのが好ましい。
PRE=Cr+3.3Mo+16N ・・・(i)
但し、上記式中の元素記号は、鋼中に含まれる各元素の含有率(質量%)である。
CreqおよびNieqは、それぞれ下記(ii)および(iii)式で定義される。Creq/Nieqの値を2.360以上に制御することで、Fモード凝固にすることができ、靱性を確保することが可能となる。Creq/Nieqの値は2.400以上であるのが好ましい。一方、Creq/Nieqが高すぎるとフェライト単相組織となり、二相鋼の特性を得られない。そのため、Creq/Nieqの値は2.530以下とする。
Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti ・・・(ii)
Nieq=Ni+0.31Mn+22C+14.2N+Cu ・・・(iii)
但し、上記式中の元素記号は、鋼中に含まれる各元素の含有率(質量%)である。
Md値とは、多元系の相安定性の指標の一つであり、合金の各成分のd軌道にある電子軌道エネルギーを意味する値である。そして、Md値が高いほど、相が不安定となり、σ相等の金属間化合物が形成しやすくなる。本発明においては、金属間化合物の析出を抑制する観点から、下記(iv)式で定義される平均Md値を0.9140以下とする。金属間化合物の析出をより抑制する観点から、0.9090以下が望ましい。
平均Md値=ΣΧi・(Md)i ・・・(iv)
ただし、上記(iv)式中の記号の意味は以下のとおりである。
Χi:合金成分iの原子分率
(Md)i:合金成分iのMd値(eV)
本発明に係る二相ステンレス鋼においては、金属組織中に含まれるσ相の面積率が2.0%以下である。上述のように、PREの値に加えて、Ni含有量が高い場合には、σ相の析出が促進される。特に、σ相の面積率が2.0%を超える場合には、靱性の劣化が顕著になる。そのため、σ相の面積率は2.0%以下とする。σ相の面積率は1.0%以下であるのが好ましく、0.10%以下であるのがより好ましく、0.05%以下であるのがさらに好ましい。σ相の面積率は低ければ低いほどよいため、下限を設ける必要はない。
本発明に係る二相ステンレス鋼は、例えば、上記の化学組成を有する溶鋼を連続鋳造することによって製造することができる。すなわち、本発明の二相ステンレス鋼は、鋳片であってもよい。この際の鋳造条件の制御が重要となる。本発明者らは、σ相の析出を抑制するための鋳造条件について、まず以下の検討を行った。
Claims (3)
- 化学組成が、質量%で、
C:0.10%以下、
Si:3.0%以下、
Mn:8.0%以下、
P:0.040%以下、
S:0.020%以下、
Cr:20.0~38.0%、
Ni:3.00~12.00%、
Mo:1.0~6.5%、
Cu:3.0%以下、
N:0.200~0.700%、
Al:0~1.0%、
Sn:0~1.0%、
W:0~6.0%、
Co:0~3.0%、
Nb:0~0.50%、
Ti:0~1.5%、
V:0~1.0%、
Zr:0~0.50%、
Ta:0~0.100%、
B:0~0.100%、
Ca:0~0.50%、
Mg:0~0.50%、
REM:0~0.10%、
残部:Feおよび不純物であり、
下記(i)式で定義されるPREの値が41.0以上であり、
下記(ii)式で定義されるCreqと、下記(iii)式で定義されるNieqとの比Creq/Nieqの値が2.360~2.530であり、
下記(iv)式で定義される平均Md値が0.9140以下であり、
金属組織中に含まれるσ相の面積率が2.0%以下である、
二相ステンレス鋼。
PRE=Cr+3.3Mo+16N ・・・(i)
Creq=Cr+1.37Mo+1.5Si+2Nb+3Ti ・・・(ii)
Nieq=Ni+0.31Mn+22C+14.2N+Cu ・・・(iii)
平均Md値=ΣΧi・(Md)i ・・・(iv)
ただし、上記(i)~(iii)式中の元素記号は各元素の含有量(質量%)であり、上記(iv)式中の記号の意味は以下のとおりである。
Χi:合金成分iの原子分率
(Md)i:合金成分iのMd値(eV) - 請求項1に記載の化学組成を有する溶鋼を連続鋳造する連続鋳造工程を備え、
前記連続鋳造工程では、鋳片を950~1050℃の温度範囲まで一次冷却した後に、最高到達温度が1050℃以上となるまで復熱させ、その後、900~1000℃の温度範囲での滞留時間が400s以下となる条件で冷却する、
二相ステンレス鋼の製造方法。 - 前記鋳片に対して熱間圧延を施す熱間圧延工程をさらに備え、
前記熱間圧延工程では、前記鋳片を1150~1300℃の温度範囲で1.5h以上加熱した後に、仕上圧延温度が900~1110℃となる条件で熱間圧延し、次いで、800~500℃の温度範囲内での平均冷却速度が0.1~1.0℃/sとなる条件で500℃以下の温度範囲まで冷却する、
請求項2に記載の二相ステンレス鋼の製造方法。
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