WO2018135554A1 - フェライト・オーステナイト系二相ステンレス鋼板 - Google Patents
フェライト・オーステナイト系二相ステンレス鋼板 Download PDFInfo
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- WO2018135554A1 WO2018135554A1 PCT/JP2018/001293 JP2018001293W WO2018135554A1 WO 2018135554 A1 WO2018135554 A1 WO 2018135554A1 JP 2018001293 W JP2018001293 W JP 2018001293W WO 2018135554 A1 WO2018135554 A1 WO 2018135554A1
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- 229910001039 duplex stainless steel Inorganic materials 0.000 title claims abstract description 34
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 26
- 229910052726 zirconium Inorganic materials 0.000 claims abstract description 20
- 239000000203 mixture Substances 0.000 claims abstract description 11
- 239000012535 impurity Substances 0.000 claims abstract description 8
- -1 B: 0.01% or less Substances 0.000 claims description 2
- 238000003466 welding Methods 0.000 abstract description 31
- 229910052748 manganese Inorganic materials 0.000 abstract description 7
- 229910052759 nickel Inorganic materials 0.000 abstract description 7
- 229910052802 copper Inorganic materials 0.000 abstract description 6
- 229910052750 molybdenum Inorganic materials 0.000 abstract description 6
- 229910052804 chromium Inorganic materials 0.000 abstract description 4
- 230000014509 gene expression Effects 0.000 abstract 2
- 229910052742 iron Inorganic materials 0.000 abstract 1
- 229910000831 Steel Inorganic materials 0.000 description 45
- 239000010959 steel Substances 0.000 description 45
- 230000007797 corrosion Effects 0.000 description 30
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- 230000000694 effects Effects 0.000 description 16
- 229910001220 stainless steel Inorganic materials 0.000 description 12
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- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 2
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- 230000002411 adverse Effects 0.000 description 2
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- 206010070834 Sensitisation Diseases 0.000 description 1
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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
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
- C21C5/005—Manufacture of stainless steel
-
- 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
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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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
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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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
- C22C38/06—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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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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
- C22C38/54—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
-
- 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 a ferritic / austenitic duplex stainless steel sheet having excellent weldability and strength.
- Ferritic / austenitic duplex stainless steel (hereinafter also referred to as duplex stainless steel) is a steel grade that has a dual phase structure of ferrite ( ⁇ ) and austenite ( ⁇ ) at room temperature and has high strength (high proof stress). And has excellent characteristics such as stress corrosion cracking resistance.
- Duplex stainless steel is a type of steel that has attracted attention in recent years from the viewpoint of saving rare elements because it has a lower Ni content than ⁇ -based stainless steel.
- JIS G 4304 and JIS G 4305 include general-purpose duplex stainless steels. Three types, one super duplex stainless steel, and two lean (resource-saving, low Ni content) duplex steels are defined.
- SUS821L1 (representative component: 22 mass% Cr-2 mass% Ni-0.5 mass% Mo-1 mass% Cu-0.18 mass% N) which is a resource-saving duplex stainless steel is SUS329J3L (representative component). : 22% by mass Cr-5% by mass Ni-3% by mass Mo-0.16% by mass N) and the like. Since SUS821L1 has low Ni and Mo contents, its corrosion resistance is inferior to other duplex stainless steels, and is the same as SUS304 (typical component: 18 mass% Cr-8 mass% Ni), which is a general-purpose ⁇ -based stainless steel. Degree.
- SUS821L1 is excellent in price stability because relatively inexpensive elements such as N, Mn, and Cu are used as the ⁇ -phase generating element instead of expensive Ni.
- the proof stress is higher than that of SUS304, it can be applied to structural members to which SUS304 could not be applied because of its low proof strength.
- duplex stainless steels having components similar to SUS821L1 are described in, for example, Patent Documents 1 to 3. All of the steels described in these documents are characterized in that the Ni content is reduced, and the N content, Mn content, and Cu content are increased.
- the solidified structure of the molten metal part is an ⁇ single phase, and the ⁇ phase is generated during the cooling process and returns to the ⁇ ⁇ ⁇ duplex structure.
- the heat-affected zone (HAZ zone) near the molten metal zone is once heated to the ⁇ single-phase temperature region, and then returns to the ⁇ ⁇ ⁇ two-phase structure in the cooling process.
- the process of changing from an ⁇ single-phase structure to an ⁇ / ⁇ two-phase structure in the cooling process after welding is followed, but the ⁇ phase is not sufficiently generated during cooling because the cooling rate is fast, and before welding.
- the ⁇ phase fraction may decrease.
- the ⁇ -phase fraction decreases and the N concentration in the ⁇ -phase increases, the ⁇ -phase has a smaller solid solution amount of N than the ⁇ -phase, so that the corrosion resistance of the grain boundary decreases due to the precipitation of Cr 2 N, N exceeding the melting limit may vaporize to generate bubbles, and defects (hereinafter referred to as blowholes) that are trapped in the weld bead during solidification may occur.
- an object of the present invention is to provide a ferritic / austenitic duplex stainless steel sheet which does not generate blowholes during welding and has excellent strength.
- “no blowholes are generated during welding” means that two cutting surfaces of 4.0 mm-thick steel plates are butted against each other and TIG welded, and the entire cross-section of the molten metal part and the HAZ part is observed. It means that there is no blow hole with a diameter of 3 ⁇ m or more.
- the groove is I type, and welding conditions are current: 220 A, voltage: 15 V, welding speed: 200 mm / min, welding wire: none, shield gas: Ar, gas flow rate: 15 l / min on both sides.
- excellent strength means that the 0.2% proof stress measured according to JIS Z 2241 is 480 MPa or more.
- the present inventors diligently studied to achieve the above object, and by appropriately controlling the balance of Zr and N, it is possible to increase the strength of the tissue without excessively increasing the amount of N in the ⁇ phase. I found out that Thereby, intensity
- the present invention has been made based on such findings, and the gist thereof is as follows.
- C 0.10% or less, Si: 1.0% or less, Mn: 2.0 to 7.0%, P: 0.07% or less, S: 0.030% or less, Cr: 18.0 to 24.0%, Ni: 0.1 to 3.0%, Mo: 0.01 to 1.0%, Cu: 0.1 to 3.0%, Al: 0.003 to 0.10%, Zr: 0.01 to 0.50%, N: 0.15 to 0.30%, satisfying the following formulas (1) and (2), the balance being Fe and inevitable impurities
- a ferritic / austenitic duplex stainless steel sheet having a composition comprising: N-Zr / 6.5 ⁇ 0.15% (1) N-Zr / 6.5 ⁇ 0.23% (2) However, in the formulas (1) and (2), N and Zr represent the content (% by mass) of each element.
- any one of B 0.01% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less in mass%.
- FIG. 1 is a graph for explaining that the contents of Zr and N affect the properties of steel.
- the balance control of the Zr content and the N content which is the point of the present invention, will be described.
- the N content is increased for the purpose of increasing the strength of the duplex stainless steel, there is a problem that blowhole defects are likely to occur during welding. Therefore, the present inventors diligently searched for a new strengthening technique that does not depend on an excessive increase in the amount of solute N. As a result, it was found that the yield strength is increased by containing an appropriate amount of Zr. By precipitating ZrN in the steel, it is estimated that the crystal grains are refined and the proof stress is improved.
- the present inventors made various steels with varying contents of Zr and N, and investigated the strength and the occurrence of blowholes during welding. .
- steels containing the components of Steel Nos. 1 to 6 and Steel Nos. 16 to 22 in Table 1 described later in the item of Example are melted, and 4.0 mm by the method described later in the item of Example.
- a thick hot-rolled annealed plate was produced.
- These hot-rolled annealed plates were similarly subjected to a tensile test and a welding test by the method described later in the item of Example, and the strength and the occurrence of blowholes were investigated.
- FIG. 1 is a graph for explaining that the contents of Zr and N affect the properties of steel.
- FIG. 1 the result of having evaluated the characteristic of steel in the following two items is shown.
- Strength Passed with 480 MPa ⁇ proof strength (0.2% yield strength)]
- Presence / absence of blowholes during welding [Pass without generating blowholes with a diameter of 3 ⁇ m or more]
- ⁇ the result of having evaluated the characteristic of steel in the following two items is shown.
- N and Zr represent the content (% by mass) of each element.
- the left side of the formulas (1) and (2): “N—Zr / 6.5” indicates that all of the contained Zr is precipitated as ZrN, and all the N not involved in the precipitation of ZrN is dissolved in the steel. Assuming that, the amount of solute N in the steel is shown. That is, these formulas indicate that the solid solution N amount needs to be controlled in the range of 0.15 to 0.23% in order to pass all the evaluations.
- the solid solution N content in the ⁇ phase is decreased and the strength is greatly reduced. Even if it contains, the target intensity
- N is also a ⁇ -phase generating element
- the ⁇ -phase fraction may be insufficient.
- the solid solution N amount becomes excessive and blow holes may occur during welding.
- the target strength cannot be satisfied within the range of the N content in which blowholes are not generated.
- the present inventors have studied the optimum balance of Zr and N content, and the lower limit of the solid solution N content is specified as 0.15%, and the upper limit is defined as 0.23%, resulting in the present invention. .
- the contents of Zr and N within the range of the present invention, it is possible to increase the strength by precipitation of ZrN while maintaining an appropriate amount of solute N, and the desired characteristics can be satisfied.
- the present invention makes it possible to secure the target strength and ⁇ phase fraction while utilizing the increase in strength due to the inclusion of Zr, and further, the amount of Zr and the amount of N are such that the amount of N does not generate blowholes. This is achieved by controlling the balance.
- the ferritic / austenitic duplex stainless steel sheet of the present invention based on the above technical idea is, in mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 2.0 to 7.0%, P: 0.07% or less, S: 0.030% or less, Cr: 18.0 to 24.0%, Ni: 0.1 to 3.0%, Mo: 0.01 to 1.0%, Cu : 0.1 to 3.0%, Al: 0.003 to 0.10%, Zr: 0.01 to 0.50%, N: 0.15 to 0.30%, the following (1)
- the composition satisfies the formulas (2) and the balance is composed of Fe and unavoidable impurities, no blowholes are generated during welding, and it has excellent strength.
- N-Zr / 6.5 ⁇ 0.23% However, in the formulas (1) and (2), N and Zr represent the content (% by mass) of each element.
- C 0.10% or less, Si: 1.0% or less, Mn: 2.0 to 7.0%, P: 0.07% or less, S: 0.030% by mass%
- Cr 18.0 to 24.0%
- Ni 0.1 to 3.0%
- Mo 0.1 to 1.0%
- Cu 0.1 to 3.0%
- Al 0.0. 003 to 0.10%
- Zr 0.01 to 0.50%
- N 0.15 to 0.30%
- N-Zr / 6.5 ⁇ 0.23% represent the content (% by mass) of each element.
- % which is a unit of content of component elements shown below, means “mass%”.
- the ferrite phase is also referred to as an ⁇ phase
- the austenite phase is also referred to as a ⁇ phase.
- C 0.10% or less
- C is an element that increases the ⁇ phase fraction. In order to acquire the said effect, it is preferable to contain C 0.003% or more.
- the C content is 0.10% or less.
- the C content is preferably less than 0.050%, more preferably less than 0.030%, and even more preferably less than 0.020%.
- Si 1.0% or less
- Si is an element contained as a deoxidizer and preferably contains 0.01% or more of Si.
- the Si content exceeds 1.0%, the steel material strength is increased and the cold workability is lowered.
- the Si content is 1.0% or less.
- Si content becomes like this.
- it is 0.70% or less, More preferably, it is 0.50% or less, More preferably, it is 0.35% or less.
- Mn 2.0 to 7.0% Mn increases the solid solution amount of N in the ⁇ phase, and is effective in preventing sensitization at the ⁇ phase grain boundary and suppressing the generation of blowholes during welding. In order to acquire the said effect, it is necessary to contain 2.0% or more of Mn. On the other hand, when the Mn content exceeds 7.0%, hot workability and corrosion resistance are deteriorated. Therefore, the Mn content is set to 2.0 to 7.0%.
- the Mn content is preferably 5.00% or less, more preferably 4.00% or less, and even more preferably 3.50% or less.
- P 0.07% or less
- P is an element that lowers corrosion resistance and hot workability. If the P content exceeds 0.07%, the adverse effect becomes significant, so 0.07% or less.
- the P content is preferably 0.05% or less, and more preferably 0.040% or less.
- S 0.030% or less S is an element that lowers corrosion resistance and hot workability. If the S content exceeds 0.030%, the adverse effect becomes significant, so 0.030% or less. S content becomes like this. Preferably it is 0.010% or less, More preferably, it is 0.005% or less.
- Cr 18.0 to 24.0% Cr is the most important component for imparting corrosion resistance to stainless steel. If the Cr content is less than 18.0%, sufficient corrosion resistance cannot be obtained. On the other hand, Cr is an ⁇ -phase generating element, and when the Cr content exceeds 24.0%, it is difficult to obtain a sufficient amount of ⁇ -phase fraction. Therefore, the Cr content is 18.0 to 24.0%.
- the Cr content is preferably 19.0% or more, and more preferably 20.5% or more. Moreover, Cr content becomes like this. Preferably it is 23.0% or less, More preferably, it is 22.0% or less.
- Ni 0.1-3.0%
- Ni is a ⁇ -phase generating element and has an effect of improving crevice corrosion resistance. Further, when Ni is added to the duplex stainless steel, the corrosion resistance of the ferrite phase is improved and the pitting potential is increased. In order to obtain these effects, it is necessary to contain 0.1% or more of Ni. On the other hand, if the Ni content exceeds 3.0%, the amount of Ni in the ⁇ phase increases, the ductility of the ⁇ phase decreases, and the moldability is reduced. Moreover, since Ni is an expensive element with a large price fluctuation, an increase in the content detracts from price stability and departs from the spirit of the present invention. Therefore, the Ni content is 0.1 to 3.0%. The Ni content is preferably 0.50% or more, and more preferably 1.50% or more. Further, the Ni content is preferably 2.50% or less.
- Mo 0.01 to 1.0% Mo has the effect of improving the corrosion resistance. In order to acquire this effect, it is necessary to contain Mo 0.01% or more. On the other hand, if the Mo content exceeds 1.0%, the high-temperature strength is increased and the hot workability is lowered. Further, since Mo is an expensive element with a large price fluctuation, an increase in the Mo content detracts from price stability and departs from the spirit of the present invention. Therefore, the Mo content is set to 0.01 to 1.0%. Mo content becomes like this. Preferably it is 0.1% or more, More preferably, it is 0.20% or more. Moreover, Mo content becomes like this. Preferably it is 0.60% or less, More preferably, it is 0.40% or less.
- Cu 0.1 to 3.0%
- Cu is a ⁇ phase generating element and has an effect of increasing the ⁇ phase fraction. In order to acquire this effect, it is necessary to contain Cu 0.1% or more.
- the Cu content is set to 0.1 to 3.0%.
- Cu content becomes like this. Preferably it is 0.20% or more, More preferably, it is 0.30% or more, More preferably, it is 0.50% or more. Moreover, Cu content becomes like this. Preferably it is 1.50% or less, More preferably, it is 1.20% or less.
- Al 0.003 to 0.10%
- Al is a deoxidizing agent, and its effect can be obtained with a content of 0.003% or more. However, if the Al content exceeds 0.10%, a nitride is formed, which causes surface defects. Therefore, the Al content is set to 0.003 to 0.10%.
- the Al content is preferably 0.005% or more, and more preferably 0.010% or more. Moreover, Al content becomes like this. Preferably it is 0.050% or less, More preferably, it is 0.030% or less.
- Zr 0.01 to 0.50%
- Zr is an important element that increases the strength of steel. The effect is obtained when the Zr content is 0.01% or more. On the other hand, even if it contains Zr exceeding 0.50%, not only the effect is saturated, but surface defects may occur due to Zr inclusions. Moreover, since the alloy cost increases, it is not preferable. Therefore, the Zr content is set to 0.01 to 0.50%.
- the Zr content is preferably 0.03% or more, more preferably 0.05% or more. Moreover, Zr content becomes like this. Preferably it is 0.20% or less, More preferably, it is 0.10% or less.
- N 0.15-0.30%
- N is a ⁇ -phase-forming element and is an important element that enhances corrosion resistance and strength. This effect is obtained when the N content is 0.15% or more.
- the N content is preferably 0.170% or more.
- N content becomes like this. Preferably it is 0.250% or less, More preferably, it is 0.200% or less.
- N-Zr / 6.5 is made 0.15% or more. Preferably it is 0.16% or more, more preferably 0.17% or more.
- N-Zr / 6.5 is 0.23% or less. Preferably it is 0.21% or less, More preferably, it is 0.20% or less.
- the balance other than the above components is Fe and inevitable impurities.
- O (oxygen) is preferably controlled to 0.05% or less from the viewpoint of preventing surface flaws due to inclusions.
- the stainless steel of the present invention may contain the following components as necessary in addition to the essential components described above.
- B 0.01% or less
- Ca 0.01% or less
- Mg 0.01% or less
- REM 0.1% or less
- any one or more of B, Ca, Mg are heat It is a component that improves the workability and can be contained as appropriate.
- the content of each of B, Ca, and Mg is preferably 0.0003% or more.
- B, Ca, and Mg exceeds 0.01%, the corrosion resistance decreases. Therefore, when B, Ca, and Mg are contained, the respective contents may be limited to 0.01% or less. preferable.
- B, Ca, and Mg are more preferably 0.005% or less.
- REM can be appropriately contained as a component for improving hot workability.
- the REM content is preferably 0.002% or more.
- the REM content is preferably limited to 0.1% or less. More preferably, the REM content is 0.05% or less.
- REM means Sc, Y and lanthanoid elements (elements having atomic numbers 57 to 71 such as La, Ce, Pr, Nd, and Sm).
- ⁇ phase fraction in the structure of the ferrite-austenitic duplex stainless steel sheet of the present invention is preferably 30% or more in order to obtain good strength. Further, the ⁇ phase fraction is preferably 70% or less in order to obtain good corrosion resistance.
- the production method is not particularly limited.
- a steel having the above component composition is melted in a converter or an electric furnace, refined by VOD (Vacuum Oxygen Decarburization), AOD (Argon Oxygen Decarburization), and the like.
- VOD Vauum Oxygen Decarburization
- AOD Aral Oxygen Decarburization
- the hot-rolled steel sheet obtained by this method is preferably descaled by pickling or polishing after continuous annealing at 900 to 1200 ° C. as necessary.
- pickling for example, sulfuric acid or a mixed solution of nitric acid and hydrofluoric acid can be used.
- the scale may be removed by shot blasting before pickling.
- the hot-rolled steel sheet may be annealed and cold-rolled to produce a cold-rolled steel sheet.
- the cold-rolled steel sheet obtained by this method is preferably descaled by pickling or polishing after continuous annealing at a temperature of 900 to 1200 ° C., if necessary. If necessary, bright annealing may be performed at a temperature of 900 to 1200 ° C.
- the steel having the chemical composition shown in Table 1 melted in a 50 kg small vacuum melting furnace was heated to 1250 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a thickness of 4.0 mm. Subsequently, it annealed in air
- ⁇ phase fraction A test piece having a length of 15 mm and a width of 10 mm was taken from a hot-rolled annealed plate, embedded in a resin so that a cross section parallel to the rolling direction was an observation surface, and the cross section was mirror-polished. Then, after coloring with Murakami's reagent (aqueous solution in which 100 g of potassium ferricyanide, 100 g of potassium hydroxide and 100 cm 3 of pure water were mixed), observation with an optical microscope was performed. In the coloring by Murakami reagent, only the ⁇ phase is colored gray (the surface is etched and light is diffusely reflected. Therefore, it becomes darker than the ⁇ phase and appears to be colored gray).
- Murakami's reagent aqueous solution in which 100 g of potassium ferricyanide, 100 g of potassium hydroxide and 100 cm 3 of pure water were mixed
- the ⁇ phase is not colored and remains white (the surface is not etched and remains a mirror-polished surface and is bright).
- the ⁇ phase fraction was calculated by image analysis. Observation was carried out at a magnification of 200 times for five visual fields, and the average value of the area ratio was defined as the ⁇ phase fraction.
- the proof stress of the 4.0 mm thick hot-rolled annealed plate of SUS304 was about 240 MPa.
- an alternative member of duplex stainless steel in which only the plate thickness is reduced from an existing product is applied to an existing SUS304 member.
- the target thickness was reduced to 50% of the conventional SUS304, and the target proof stress of the duplex stainless steel was set to a proof strength increased by 100% of SUS304.
- the target yield strength of the duplex stainless steel was set to 480 MPa or more with respect to the yield strength of 240 MPa of SUS304.
- Corrosion resistance was evaluated by pitting potential.
- a 20 mm square test piece was cut out from the hot-rolled annealed plate, sealed with a resin leaving a surface of 11 ⁇ 11 mm, and then immersed in 10% by weight nitric acid for passivation treatment, and further a surface of 10 ⁇ 10 mm. The part of was polished.
- the sample was left for 10 minutes after being immersed in a 3.5 mass% NaCl solution at 30 ° C., and a potential scan was started to measure the pitting corrosion potential.
- the measurement result of the pitting corrosion potential is less than 270 (mVvs SCE) x 270 (mVvs SCE) or more and less than 320 (mVvs SCE) ⁇ (pass), 320 (mVvs SCE) or more ⁇ (pass: excellent)
- ⁇ or ⁇ it was evaluated as having excellent corrosion resistance applicable to structural members such as sluices that require particularly corrosion resistance.
- Tables 1 and 2 show the results of various evaluations.
- steels within the scope of the present invention were all evaluated to be acceptable, no blowholes were generated during welding, and the strength was excellent. Furthermore, it was found that these steels had excellent corrosion resistance because the evaluation of corrosion resistance was good or bad.
- steel no. 1 to 8, 11, 12, 14, and 15 were evaluated as excellent in corrosion resistance, and particularly excellent in corrosion resistance.
- Steel No. having a Ni content of less than 0.50%. 9 Steel No. 1 with Ni content of less than 0.50% and Mn content of more than 5.00%. 10 and steel No. 1 with a Cr content of less than 19.0%. 13, Steel No. with Mo content of less than 0.1%. Steel Nos. 28 to 34 and Mn content exceeding 5.00%.
- No. 35 was evaluated as ⁇ for the corrosion resistance.
- steel outside the scope of the present invention was rejected in at least one evaluation and did not satisfy the desired characteristics.
- Steel No. No. 16 could not obtain the desired strength because the Zr content was less than the lower limit of the range of the present invention.
- Steel No. Since 20 and 22 do not satisfy Formula (2) blowholes occurred during welding.
- a ferrite-austenite duplex stainless steel having both excellent strength and weldability can be obtained, which is very useful in industry.
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Abstract
Description
[1]質量%で、C:0.10%以下、Si:1.0%以下、Mn:2.0~7.0%、P:0.07%以下、S:0.030%以下、Cr:18.0~24.0%、Ni:0.1~3.0%、Mo:0.01~1.0%、Cu:0.1~3.0%、Al:0.003~0.10%、Zr:0.01~0.50%、N:0.15~0.30%を含有し、下記(1)式および(2)式を満たし、残部がFeおよび不可避的不純物からなる成分組成を有する、フェライト・オーステナイト系二相ステンレス鋼板。
N-Zr/6.5≧0.15%・・・(1)
N-Zr/6.5≦0.23%・・・(2)
ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。
[2]質量%で、C:0.10%以下、Si:1.0%以下、Mn:2.0~7.0%、P:0.07%以下、S:0.030%以下、Cr:18.0~24.0%、Ni:0.1~3.0%、Mo:0.1~1.0%、Cu:0.1~3.0%、Al:0.003~0.10%、Zr:0.01~0.50%、N:0.15~0.30%を含有し、下記(1)式および(2)式を満たし、残部がFeおよび不可避的不純物からなる成分組成を有する、フェライト・オーステナイト系二相ステンレス鋼板。
N-Zr/6.5≧0.15%・・・(1)
N-Zr/6.5≦0.23%・・・(2)
ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。
[3]前記成分組成に加えてさらに、質量%で、B:0.01%以下、Ca:0.01%以下、Mg:0.01%以下、REM:0.1%以下のいずれか1種または2種以上を含有する、前記[1]または[2]に記載のフェライト・オーステナイト系二相ステンレス鋼板。
(1)強度[480MPa≦耐力(0.2%耐力)で合格]
(2)溶接時のブローホール有無[直径3μm以上のブローホール発生無しで合格]
これら2項目のうち、両方が合格評価である鋼を○、いずれか1項目でも不合格のものを×と表し図1に示した。これらの結果から、Zr:0.01~0.50%、N:0.15~0.30%を含有し、さらにN-Zr/6.5≧0.15%・・・(1)、N-Zr/6.5≦0.23%・・・(2)の関係を満たす鋼板であれば、いずれの評価も合格となることがわかる。ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。
N-Zr/6.5≧0.15%・・・(1)
N-Zr/6.5≦0.23%・・・(2)
ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。
N-Zr/6.5≧0.15%・・・(1)
N-Zr/6.5≦0.23%・・・(2)
ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。
Cは、γ相分率を高める元素である。上記効果を得るためには、Cを0.003%以上含有することが好ましい。一方で、C含有量が、0.10%を超えると、Cを固溶させるための熱処理温度が著しく高くなり、生産性が低下する。そのため、C含有量は0.10%以下とする。C含有量は、好ましくは0.050%未満であり、より好ましくは0.030%未満であり、さらに好ましくは0.020%未満である。
Siは、脱酸剤として含有される元素であり、Siを0.01%以上含有することが好ましい。一方で、Si含有量が1.0%を超えると、鋼材強度が高くなって冷間加工性を低下させる。また、Siはα相生成元素であるため、Si含有量が1.0%を超えると所望のγ相分率を得ることが困難となる場合がある。そのため、Si含有量は1.0%以下とする。Si含有量は、好ましくは0.70%以下であり、より好ましくは0.50%以下であり、さらに好ましくは0.35%以下である。
Mnは、α相中のNの固溶量を高め、α相粒界における鋭敏化の防止や、溶接時のブローホールの発生の抑制に効果がある。上記効果を得るためには、Mnを2.0%以上含有する必要がある。一方で、Mn含有量が7.0%を超えると、熱間加工性および耐食性が低下する。そのため、Mn含有量は2.0~7.0%とする。Mn含有量は、好ましくは5.00%以下であり、より好ましくは4.00%以下であり、さらに好ましくは3.50%以下である。
Pは、耐食性や熱間加工性を低下させる元素であり、P含有量が0.07%を超えると悪影響が顕著となるので0.07%以下とする。P含有量は、好ましくは0.05%以下であり、より好ましくは0.040%以下である。
Sは、耐食性や熱間加工性を低下させる元素であり、S含有量が0.030%を超えると悪影響が顕著となるので0.030%以下とする。S含有量は、好ましくは0.010%以下であり、より好ましくは0.005%以下である。
Crは、ステンレス鋼に耐食性を付与する最も重要な成分であり、Cr含有量が18.0%未満では、十分な耐食性が得られない。一方、Crはα相生成元素であり、Cr含有量が24.0%を超えると十分な量のγ相分率を得ることが困難となる。そのため、Cr含有量は18.0~24.0%とする。Cr含有量は、好ましくは19.0%以上であり、より好ましくは20.5%以上である。また、Cr含有量は、好ましくは23.0%以下であり、より好ましくは22.0%以下である。
Niは、γ相生成元素であり、耐隙間腐食性を向上させる効果を有する。さらに、二相ステンレス鋼にNiを添加すると、フェライト相の耐食性が向上して孔食電位が高まる。これらの効果を得るためにはNiを0.1%以上含有する必要がある。一方で、Ni含有量が3.0%を超えるとα相中のNi量が増加してα相の延性が低下し、成形性の低下を招く。また、Niは高価かつ価格変動の激しい元素であるため、含有量が増えると価格安定性を損ない本発明の趣旨をはずれる。そのため、Ni含有量は0.1~3.0%とする。Ni含有量は、好ましくは0.50%以上であり、より好ましくは1.50%以上である。また、Ni含有量は、好ましくは2.50%以下である。
Moは、耐食性を向上させる効果を有する。この効果を得るために、Moを0.01%以上含有する必要がある。一方で、Mo含有量が1.0%を超えると、高温強度が上昇して熱間加工性の低下を招く。また、Moは高価かつ価格変動の激しい元素であるため、Mo含有量が増えると価格安定性を損ない本発明の趣旨をはずれる。そのため、Mo含有量は0.01~1.0%とする。Mo含有量は、好ましくは0.1%以上であり、より好ましくは0.20%以上である。また、Mo含有量は、好ましくは0.60%以下であり、より好ましくは0.40%以下である。
Cuは、γ相生成元素であり、γ相分率を高める効果がある。この効果を得るために、Cuを0.1%以上含有する必要がある。一方で、Cu含有量が3.0%を超えると、高温強度が上昇して熱間加工性の低下を招く。そのため、Cu含有量は0.1~3.0%とする。Cu含有量は、好ましくは0.20%以上であり、より好ましくは0.30%以上であり、さらに好ましくは0.50%以上である。また、Cu含有量は、好ましくは1.50%以下であり、より好ましくは1.20%以下である。
Alは、脱酸剤であり、0.003%以上の含有でその効果が得られる。ただし、Al含有量が0.10%を超えると、窒化物を形成して表面疵の原因となる。そのため、Al含有量は0.003~0.10%とする。Al含有量は、好ましくは0.005%以上であり、より好ましくは0.010%以上である。また、Al含有量は、好ましくは0.050%以下であり、より好ましくは0.030%以下である。
Zrは、鋼の強度を高める重要な元素である。その効果は0.01%以上のZrの含有で得られる。一方、0.50%を超えてZrを含有しても効果が飽和するばかりか、Zr介在物により表面疵が生じる場合がある。また、合金コストが増加するため好ましくない。そのため、Zr含有量は0.01~0.50%とする。Zr含有量は、好ましくは0.03%以上であり、より好ましくは0.05%以上である。また、Zr含有量は、好ましくは0.20%以下であり、より好ましくは0.10%以下である。
Nは、γ相生成元素であり耐食性や強度も高める重要な元素である。この効果は0.15%以上のNの含有で得られる。一方で、N含有量が0.30%を超えると、Nは鋳造時や溶接時にブローホール発生の要因となる。そのため、N含有量は0.15~0.30%とする。N含有量は、好ましくは0.170%以上である。また、N含有量は、好ましくは0.250%以下であり、より好ましくは0.200%以下である。
N-Zr/6.5が0.15%未満の場合、γ相中の固溶N量が減少して強度が大幅に低下するため、Zr含有による高強度化の効果を持ってしても目的の強度が得られない。また、Nはγ相生成元素でもあるため、固溶Nが減少するとγ相分率が不足する場合もある。したがって、N-Zr/6.5は0.15%以上とする。好ましくは0.16%以上、より好ましくは0.17%以上である。
N-Zr/6.5が0.23%を超えると、固溶N量が過剰となり溶接時にブローホールが発生する場合がある。したがって、N-Zr/6.5は0.23%以下とする。好ましくは0.21%以下、より好ましくは0.20%以下である。
B、Ca、Mgは、熱間加工性を向上させる成分であり、適宜含有することができる。その効果を得るためには、B、Ca、Mgのそれぞれの含有量は0.0003%以上であることが好ましい。一方で、B、Ca、Mgのそれぞれが0.01%を超えると耐食性が低下するため、B、Ca、Mgを含有する場合は、それぞれの含有量は0.01%以下に制限することが好ましい。B、Ca、Mgは、より好ましくは、それぞれ0.005%以下である。同様に、REMは、熱間加工性を向上させる成分として適宜含有することができ、REMを含有する場合は、REM含有量は0.002%以上であることが好ましい。一方で、REM含有量が0.1%を超えると耐食性が低下するため、REM含有量は0.1%以下に制限することが好ましい。より好ましくは、REM含有量は0.05%以下である。なお、REMとは、Sc、Yおよびランタノイド系元素(La、Ce、Pr、Nd、Smなど原子番号57~71までの元素)を意味する。
熱延焼鈍板から長さ15mm幅10mmの試験片を採取し、圧延方向に平行な断面が観察面となるよう樹脂に埋め込んで断面を鏡面研磨した。その後、村上試薬(フェリシアン化カリウム100g・水酸化カリウム水溶液100g、純水100cm3を混合した水溶液)による着色処理を施してから、光学顕微鏡による観察を行った。村上試薬による着色では、α相のみが灰色に着色され(表面がエッチングされて光を乱反射するようになる。そのため、γ相の部分と比較して暗くなり、灰色に着色されたよう見える。)、γ相は着色されずに白色のままとなる(表面はエッチングされず鏡面研磨面のままで、明るい。)。この反応を利用してγ相とα相を区別した後、画像解析によりγ相分率を算出した。観察は5視野について倍率200倍で実施し、その面積率の平均値をγ相分率とした。
熱延焼鈍板から、圧延方向に平行な方向が試験片の長手となるようにJIS 13B号引張試験片を採取し、引張試験をJIS Z 2241に準拠して行い0.2%耐力を測定した。0.2%耐力が480MPa以上であれば合格(○)、480MPa未満であれば不合格(×)と評価した。なお、今回は二相ステンレス鋼を、既にSUS304が用いられている用途に対し薄肉軽量化目的で適用することを前提に、目標強度を設定した。本発明者らの測定によると、SUS304の4.0mm厚熱延焼鈍板の耐力はおよそ240MPaであった。ここで、既存のSUS304部材に対し、既存品から板厚のみ薄肉化した二相ステンレス鋼の代替部材を適用する場合を考える。例えば、これらの部材にかかる引張荷重が同一であれば、同一荷重条件で降伏しないためには、少なくとも板厚を薄肉化させた割合と同量だけ、二相ステンレス鋼の耐力を上昇させる必要があると考えられる。今回は、板厚を従来のSUS304の50%に減少させることを目標とし、二相ステンレス鋼の目標耐力をSUS304の100%上昇した耐力とした。具体的には、SUS304の耐力240MPaに対し、二相ステンレス鋼の目標耐力を480MPa以上と設定した。
熱延焼鈍板から35mm幅×150mm長さの試験片を切出し、長手方向の1方の端面を5mm切削して30mm幅×150mm長さとした。この試験片を2本作製し、切削面同士を突合せTIG溶接した。開先はI型とし、溶接条件は、電流:220A、電圧:15V、溶接速度:200mm/min、溶接ワイヤー:無し、シールドガス:Ar、ガス流量:表裏とも15l/minとした。こうして得られた溶接部から溶接長さ15mm間隔で断面観察用の試験片を10個採取し、光学顕微鏡(倍率200倍)にてブローホールの有無を判定した。溶融金属部およびHAZ部の断面全面を観察して、直径3μm以上のブローホールが存在しなければ合格(○)、直径3μm以上のブローホールが存在した場合は不合格(×)と評価した。
耐食性は孔食電位によって評価した。まず、熱延焼鈍板より20mm角の試験片を切り出し、表面11×11mmを残して樹脂でシールした後、10質量%濃度の硝酸に浸漬して不動態化処理を行い、さらに表面10×10mmの部分を研磨した。次に、JIS G0577に準拠して、30℃の3.5質量%NaCl溶液中に浸漬後10分放置し、電位走査を開始して孔食電位を測定した。孔食電位の測定結果は、270(mVvs SCE)未満を×、270(mVvs SCE)以上320(mVvs SCE)未満を○(合格)、320(mVvs SCE)以上を◎(合格:優れている)とし、○あるいは◎であれば水門などの特に耐食性が要求される構造部材に対しても適用可能な優れた耐食性を有するものと評価した。
具体的には、まず、鋼No.16は、Zr含有量が本発明の範囲の下限値未満であるため、所望の強度を得られなかった。
鋼No.17、18は、式(1)を満たさないため、所望の強度を得られなかった。
鋼No.19、21は、Zr含有量が本発明の範囲の下限値未満であると共に、式(2)を満たさないため、溶接時にブローホールが発生した。
鋼No.20、22は、式(2)を満たさないため、溶接時にブローホールが発生した。
鋼No.23は、Cr含有量が本発明の範囲の上限値超えであるため、γ相分率が低下し所望の強度を得られず、溶接時にブローホールが発生した。
鋼No.24は、N含有量が本発明の範囲の下限値未満であると共に、式(1)を満たさないため、所望の強度を得られなかった。
鋼No.25は、N含有量が本発明の範囲の上限値超えであると共に、式(2)を満たさないため、溶接時にブローホールが発生した。
鋼No.26は、Mn含有量が本発明の上限値超えであるため、熱延割れが発生し、評価をすることができなかった。
鋼No.27は、Mn含有量が本発明の範囲の下限値未満であるため、α相のN固溶量が低下し溶接時にブローホールが発生した。また、γ相分率が低下したため、所望の強度を得られなかった。
Claims (3)
- 質量%で、
C:0.10%以下、
Si:1.0%以下、
Mn:2.0~7.0%、
P:0.07%以下、
S:0.030%以下、
Cr:18.0~24.0%、
Ni:0.1~3.0%、
Mo:0.01~1.0%、
Cu:0.1~3.0%、
Al:0.003~0.10%、
Zr:0.01~0.50%、
N:0.15~0.30%
を含有し、下記(1)式および(2)式を満たし、残部がFeおよび不可避的不純物からなる成分組成を有する、フェライト・オーステナイト系二相ステンレス鋼板。
N-Zr/6.5≧0.15% ・・・(1)
N-Zr/6.5≦0.23% ・・・(2)
ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。 - 質量%で、
C:0.10%以下、
Si:1.0%以下、
Mn:2.0~7.0%、
P:0.07%以下、
S:0.030%以下、
Cr:18.0~24.0%、
Ni:0.1~3.0%、
Mo:0.1~1.0%、
Cu:0.1~3.0%、
Al:0.003~0.10%、
Zr:0.01~0.50%、
N:0.15~0.30%
を含有し、下記(1)式および(2)式を満たし、残部がFeおよび不可避的不純物からなる成分組成を有する、フェライト・オーステナイト系二相ステンレス鋼板。
N-Zr/6.5≧0.15% ・・・(1)
N-Zr/6.5≦0.23% ・・・(2)
ただし、(1)式、(2)式中、N、Zrは各元素の含有量(質量%)を表す。 - 前記成分組成に加えてさらに、質量%で、
B:0.01%以下、
Ca:0.01%以下、
Mg:0.01%以下、
REM:0.1%以下
のいずれか1種または2種以上を含有する、請求項1または2に記載のフェライト・オーステナイト系二相ステンレス鋼板。
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