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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
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  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • 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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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • 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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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • 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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  • 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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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • 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

Definitions

• the present invention relates to a ferritic-austenitic duplex stainless steel material.
• Ferritic-austenitic duplex stainless steel materials have excellent corrosion resistance and high strength, and are therefore used as building and structural materials.
• ferritic-austenitic duplex stainless steel materials have lower ductility than general-purpose austenitic stainless steel materials such as SUS304, limiting their use in applications that require workability.
• general-purpose austenitic stainless steel materials such as SUS304
• lean ferritic-austenitic duplex stainless steel materials that reduce alloying elements. For this reason, development of lean ferritic-austenitic duplex stainless steel materials with excellent ductility is underway.
• Patent Document 1 proposes a ferritic-austenitic duplex stainless steel material that contains, in mass%, 0.05% or less C, 1% or less Si, 2-8% Mn, 0.1% or less P, 0.02% or less S, 15-23% Cr, 4% or less Mo, 3.0% or less Ni, 2% or less Cu, 0.05-0.3% N, the balance being Fe and unavoidable impurities, and in which the Cr equivalent and Ni equivalent satisfy a predetermined relationship. It is described that the ductility of this duplex stainless steel material can be improved by optimizing the Cr equivalent and Ni equivalent.
• Patent Document 2 proposes a ferritic-austenitic duplex stainless steel material containing, by mass%, 0.08% or less C, 0.7-1.1% Si, 2.4-3.5% Mn, 17.9-20.7% Cr, 0.05-1.15% Ni, 0.18-0.3% N, 0.4-2.8% Cu, with the balance being Fe and unavoidable impurities, and having a pitting potential predicted by a predetermined formula of 360-440 mV. It is described that the ductility of this duplex stainless steel material can be improved by optimizing the contents of alloy components such as Ni, Si, Mn, and Cu.
• an object of the present invention is to provide a ferritic-austenitic duplex stainless steel material which is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials.
• the present inventors have conducted extensive research into lean ferritic-austenitic duplex stainless steel materials in order to solve the above problems, and as a result have obtained the following findings (1) to (3).
• (1) By reducing the C and N contents, the austenite phase can be softened while ensuring the corrosion resistance of the duplex stainless steel material.
• (2) By controlling the Md of the duplex stainless steel material and the austenite phase within a predetermined range, the stability of the austenite phase can be increased, and high ductility can be achieved by the TRIP (transformation induced plasticity) effect.
• TRIP transformation induced plasticity
• the inventors discovered that the above problems can be solved by controlling the proportion and Md of the austenitic phase as well as the composition and Md of the ferritic-austenitic duplex stainless steel material, and
• the present invention has a composition, on a mass basis, of C: 0.001 to 0.050%, Si: 0.01 to 0.50%, Mn: 1.0 to 4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5 to 3.5%, Cr: 19.6 to 24.0%, Mo: 0.01 to 1.00%, Cu: 0.01 to 1.20%, N: 0.010 to 0.090%, C + N is less than 0.130%, and the balance is Fe and impurities;
• Md 551 - 462 (C + N) - 9.2Si - 8.1Mn - 29 (Ni + Cu) - 13.7Cr - 18.5Mo ...
• the metal structure has an austenite phase of 25 to 49 volume %;
• the present invention relates to a ferritic-austenitic duplex stainless steel material, in which the austenite phase has an Md value represented by the above formula (1) of 35.0 to 100.0°C.
• the present invention makes it possible to provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials.
• the ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention (hereinafter simply referred to as "duplex stainless steel material”) contains C: 0.001-0.050%, Si: 0.01-0.50%, Mn: 1.0-4.5%, P: 0.050% or less, S: 0.030% or less, Ni: 1.5-3.5%, Cr: 19.6-24.0%, Mo: 0.01-1.00%, Cu: 0.01-1.20%, N: 0.010-0.090%, with C+N being less than 0.130%, and the balance being Fe and impurities.
• the term “stainless steel material” refers to a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the shape of the material include a plate shape (including a strip shape), a rod shape, a tube shape, and the like. In addition, the material may be various shaped steels having a cross-sectional shape such as a T-shape or an I-shape.
• the term “ferritic-austenitic” refers to a metal structure that is mainly composed of two phases, ferritic and austenitic, at room temperature.
• the term “ferritic-austenitic” also includes metal structures that contain small amounts of phases other than ferritic and austenitic phases (e.g., martensite phase, etc.).
• impurities refer to components that are mixed in due to various factors in raw materials such as ores and scraps and manufacturing processes during industrial production of stainless steel materials, and are acceptable within a range that does not adversely affect the present invention.
• impurities include unavoidable impurities.
• An example of an impurity is O.
• the content of O is, for example, 0.0001 to 0.0070%.
• "xx% or less” means that the content is xx% or less, but includes an amount exceeding 0% (particularly, above the impurity level).
• the duplex stainless steel material according to the embodiment of the present invention may further contain one or more selected from Nb: 0.010 to 0.500%, Ti: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.05 to 0.50%, Co: 0.01 to 0.30%, B: 0.0002 to 0.0050%, Sn: 0.010 to 0.500%, Al: 0.010 to 0.050%, Mg: 0.0002 to 0.0100%, Ca: 0.0002 to 0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01 to 0.50%, and REM: 0.0002 to 0.0100%.
• Nb 0.010 to 0.500%
• Ti 0.01 to 0.50%
• V 0.01 to 0.50%
• W 0.05 to 0.50%
• Co 0.01 to 0.30%
• B 0.0002 to 0.0050%
• Sn 0.010 to 0.500%
• Al 0.010 to 0.050%
• Mg 0.0002 to 0.0100%
• C is an element that has a large effect on the stability of the austenite phase. If the C content is too high, the ductility (workability) may decrease, or the precipitation of Cr carbides may be promoted, causing intergranular corrosion. Therefore, the C content is set to 0.050% or less, preferably 0.045% or less, more preferably 0.040% or less, even more preferably 0.35% or less, and particularly preferably 0.030% or less. From the viewpoint of corrosion resistance, the C content is preferably low, but if the C content is reduced too much, it will lead to an increase in costs. Therefore, the C content is set to 0.001% or more, preferably 0.002% or more, and more preferably 0.005% or more.
• Si 0.01 to 0.50%> Si is added as a deoxidizing element and is also useful for improving oxidation resistance.
• the Si content is set to 0.50% or less, preferably less than 0.50%, more preferably 0.45% or less, and even more preferably 0.40% or less.
• the Si content is set to 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more.
• Mn is an element that plays an important role in concentrating in the austenite phase and stabilizing the austenite phase.
• the Mn content is set to 4.5% or less, preferably 4.0% or less, and more preferably 3.5% or less.
• the Mn content is set to 1.0% or more, preferably 1.1% or more, and more preferably 1.2% or more.
• P is an element contained in raw materials such as Cr. If the P content is high, the formability decreases, so the P content is set to 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less. On the other hand, a lower P content is preferable, but there is a limit to reducing the P content.
• the lower limit of the P content is generally 0.001%, preferably 0.002%, and more preferably 0.003%.
• S is an element contained in various raw materials. S combines with Mn to form inclusions, which can become the starting point of rust, so the lower the S content, the better the corrosion resistance. Therefore, the S content is set to 0.030% or less, preferably 0.025% or less, and more preferably 0.020% or less. On the other hand, there is a limit to reducing the S content.
• the lower limit of the S content is generally 0.0001%, preferably 0.0005%.
• Ni is an austenite generating element and is an important element for adjusting the stability of the austenite phase. Ni also has the effect of suppressing the precipitation of nitrides and improving corrosion resistance. In order to exert these effects, the Ni content is set to 1.5% or more, preferably 1.6% or more, more preferably 1.7% or more, and even more preferably 1.8% or more. On the other hand, if the Ni content is too high, not only will the raw material cost increase, but the proportion of the austenite phase will increase, and problems such as stress corrosion cracking may occur. Therefore, the Ni content is set to 3.5% or less, preferably 3.4% or less, and more preferably 3.0% or less.
• Cr is an element necessary for ensuring corrosion resistance.
• the Cr content is set to 19.6% or more, preferably 20.0% or more, and more preferably 20.4% or more.
• the Cr content is set to 24.0% or less, preferably 23.5% or less, and more preferably 23.0% or less.
• Mo is an element that improves corrosion resistance.
• the Mo content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more.
• the Mo content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.
• Cu like Mn and Ni, is an austenite-forming element and has the effect of suppressing the precipitation of nitrides and improving corrosion resistance.
• the Cu content is set to 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more.
• the Cu content is set to 1.20% or less, preferably 1.00% or less, and more preferably 0.80% or less.
• N is an element that has a large effect on the stability of the austenite phase. N is also an element that dissolves in solid solution to increase corrosion resistance. In order to achieve these effects, the N content is set to 0.010% or more, preferably 0.020% or more. On the other hand, if the N content is too high, the ductility decreases and the corrosion resistance also decreases due to the precipitation of Cr nitrides. Therefore, the N content is set to 0.090% or less, preferably 0.080% or less, and more preferably 0.075% or less.
• the total content of C and N is less than 0.130%, preferably less than 0.120%, and more preferably 0.110% or less.
• the lower limit of the total content of C and N is not particularly limited, but is preferably 0.010%, preferably 0.020%, and more preferably 0.030%.
• Nb forms nitrides (NbN) and carbides (NbC) and has the effect of improving workability.
• the Nb content is set to 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more.
• the Nb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.200% or less.
• Ti like Nb, also has the effect of forming nitrides (TiN) and carbides (TiC) and improving workability.
• the Ti content is set to 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more.
• the Ti content is set to 0.50% or less, preferably 0.30% or less, and more preferably 0.20% or less.
• V has the effect of forming nitrides and improving workability.
• the V content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more.
• the V content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.
• W 0.05 to 0.50%> W is an element effective in improving corrosion resistance.
• the W content is set to 0.05% or more, preferably 0.08% or more, and more preferably 0.10% or more.
• the W content is set to 0.50% or less, preferably 0.45% or less, and more preferably 0.40% or less.
• Co is an element effective in increasing high-temperature strength and improving hot workability.
• the Co content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more.
• the Co content is set to 0.30% or less, preferably 0.25% or less, and more preferably 0.20% or less.
• B is an element that segregates at grain boundaries to improve hot workability.
• the B content is set to 0.0002% or more, preferably 0.0010% or more, and more preferably 0.0015% or more.
• the B content is set to 0.0050% or less, preferably 0.0040% or less, and more preferably 0.0030% or less.
• Sn is an element that improves corrosion resistance.
• the Sn content is set to 0.010% or more, preferably 0.020% or more, and more preferably 0.030% or more.
• the Sn content is set to 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.
• Al is an element effective for desulfurization and deoxidization.
• the Al content is set to 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more.
• the Al content is set to 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less.
• Mg is an element that has the effect of not only deoxidizing but also refining the solidified structure.
• the Mg content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more.
• the Mg content is set to 0.0100% or less, preferably 0.0095% or less, and more preferably 0.0090% or less.
• Ca is an element effective for desulfurization and deoxidization.
• the Ca content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more.
• the Ca content is set to 0.0100% or less, preferably 0.0080% or less, and more preferably 0.0050% or less.
• Ta is an element that improves corrosion resistance by modifying inclusions.
• the Ta content is 0.050% or less, preferably 0.045% or less, and more preferably 0.040% or less.
• the lower limit of the Ta content is not particularly limited, but in order to exert the effect of Ta, it is preferably 0.001%, more preferably 0.003%.
• Ga is an element that improves corrosion resistance and suppresses hydrogen embrittlement. However, if the Ga content is too high, the workability decreases. Therefore, the Ga content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. On the other hand, the lower limit of the Ga content is not particularly limited, but in order to exert the effect of Ga, it is preferably 0.001%, more preferably 0.003%.
• Zr is an element that has a similar effect to Nb and Ti and improves oxidation resistance.
• the Zr content is set to 0.01% or more, preferably 0.03% or more, and more preferably 0.05% or more.
• the Zr content is set to 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.
• REM 0.0002 to 0.0100%> REM (rare earth) is an element effective in improving hot workability.
• the REM content is set to 0.0002% or more, preferably 0.0005% or more, and more preferably 0.0010% or more.
• the REM content is set to 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.
• REM is a collective term for Sc, Y, and 15 elements (lanthanoids) from La to Lu. These elements can be used alone or in combination of two or more kinds as REM.
• the value of Md represented by the following formula (1) is 50.0 to 150.0°C, preferably 55.0 to 140.0°C, more preferably 60.0 to 130.0°C, and even more preferably 70.0 to 120.0°C.
• Md 551 - 462 (C + N) - 9.2Si - 8.1Mn - 29 (Ni + Cu) - 13.7Cr - 18.5Mo ...
• the element symbols represent the contents (mass%) of each element.
• Md is an index representing the stability of the austenite phase. The larger the value of Md (higher temperature), the more unstable the austenite phase is.
• Md is less than 50.0°C
• the stability of the austenite phase is too high, making it difficult to transform the austenite phase into the processing-induced martensite phase, and the desired strength and ductility cannot be obtained.
• the value of Md exceeds 150.0°C, the amount of processing-induced martensite phase transformed from the austenite phase increases, resulting in excessively high strength and failure to obtain the desired ductility.
• the duplex stainless steel material according to the embodiment of the present invention has a metal structure in which the austenite phase is 25 to 49 volume %, preferably 25 to 47 volume %, more preferably 25 to 40 volume %, even more preferably 26 to 38 volume %, and particularly preferably 28 to 37 volume %. If the austenite phase is less than 25 volume %, the proportion of the ferrite phase is high, and the desired ductility is not obtained. On the other hand, if the austenite phase is more than 49 volume %, the strength becomes excessively high, and the desired ductility is not obtained. In addition, by controlling the austenite phase to 40 volume % or less, the average grain size of the ferrite phase can be easily controlled to be large.
• the ratio of the austenite phase in the duplex stainless steel material can be determined by EBSD (electron backscatter diffraction). Specifically, EBSD measurement is performed using a mirror-polished sample of a thickness direction cross section of the duplex stainless steel material parallel to the rolling direction. For the data obtained by this EBSD measurement, a phase ratio map is created using analysis software, the ferrite phase and the austenite phase are separated, and the ratio of the austenite phase is determined.
• EBSD electron backscatter diffraction
• the austenite phase has a value of Md represented by the above formula (1) of 35.0 to 100.0 ° C, preferably 40.0 to 95.0 ° C, more preferably 50.0 to 90.0 ° C, even more preferably 52.0 to 85.0 ° C, and particularly preferably 53.0 to 80.0 ° C. If the value of Md of the austenite phase is less than 35.0 ° C, it is difficult to transform the austenite phase into a processing-induced martensite phase, and it is difficult to obtain the desired strength and ductility.
• the content of each element in the austenite phase used to calculate the Md of the austenite phase can be measured by EPMA (electron probe microanalyzer). Specifically, a sample in which a cross section in the thickness direction of a duplex stainless steel material parallel to the rolling direction is mirror-polished is used, and a qualitative analysis is performed by EPMA.
• C and N have the characteristic of being concentrated in the austenite phase
• a qualitative mapping of C or N is performed on the entire cross section to identify the austenite phase. Then, C, N, Si, Mn, Cr, Ni, Cu, and Mo are quantitatively analyzed at approximately the center of the austenite phase so that the electron beam does not hit the ferrite phase. The quantitative analysis is performed at three or more points, and the average value is taken as the content of each element.
• the average grain size of the ferrite phase is preferably 7.0 ⁇ m or more, more preferably 7.1 ⁇ m or more, and even more preferably 7.2 ⁇ m or more. If the average grain size of the ferrite phase is less than 7.0 ⁇ m, it is difficult to obtain the desired ductility.
• the upper limit of the average grain size of the ferrite phase is not particularly limited, but is typically 20.0 ⁇ m, preferably 18.0 ⁇ m, and more preferably 15.0 ⁇ m.
• the average grain size of the ferrite phase in the duplex stainless steel material can be determined by EBSD measurement.
• the EBSD measurement is performed using a mirror-polished sample of a thickness direction cross section of the duplex stainless steel material parallel to the rolling direction. From the data obtained by this EBSD measurement, the area of the crystal grains of the ferrite phase (BCC) can be determined by the area fraction method.
• the value of DF represented by the following formula (2) is preferably 50.0 to 80.0, more preferably 54.0 to 80.0, even more preferably 60.0 to 80.0, particularly preferably 63.0 to 78.0, and most preferably 65.0 to 75.0.
• DF 7.2 (Cr + 0.88Mo + 0.78Si) - 8.9 (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ...
• the element symbols represent the contents (mass%) of each element.
• DF is an index representing the amount of ferrite phase. Therefore, 100-DF is the amount of austenite phase.
• DF is an index determined based on the content of elements, and does not coincide with the amount of austenite phase actually measured. If the value of DF is less than 50.0, the strength becomes excessively high, and it is difficult to obtain the desired ductility. On the other hand, if the value of DF exceeds 80.0, the proportion of ferrite phase becomes high, and it is difficult to obtain the desired ductility.
• the duplex stainless steel material according to the embodiment of the present invention preferably has a tensile strength of 800 MPa or less, more preferably 790 MPa or less, and even more preferably 780 MPa or less. If the tensile strength is in this range, it can be said to be softer than conventional duplex stainless steel materials, and the desired ductility can be ensured.
• the lower limit of the tensile strength is not particularly limited, but is generally 500 MPa, preferably 550 MPa.
• the tensile strength of the duplex stainless steel material can be measured in accordance with JIS Z2241:2011.
• the duplex stainless steel material according to the embodiment of the present invention preferably has a uniform elongation of 30.0% or more, more preferably 31.0% or more, and even more preferably 32.0% or more. If the uniform elongation is within this range, it can be said that the duplex stainless steel material has superior ductility compared to conventional duplex stainless steel materials.
• the upper limit of the uniform elongation is not particularly limited, but is generally 50.0%, preferably 48.0%, and more preferably 45.0%.
• the uniform elongation of the duplex stainless steel material can be measured in accordance with JIS Z2241: 2011. The uniform elongation is determined as the permanent elongation at the maximum tensile load.
• the n-value ratio of the n-value in the 15-20% strain range to the n-value in the 20-25% strain range is preferably 0.80 or less, more preferably 0.79 or less. If the n-value ratio is in this range, it can be said that the ductility is excellent under general processing conditions.
• the lower limit of the n-value ratio is not particularly limited, but is generally 0.01, preferably 0.10.
• the strain rate affects the processing heat generation, and therefore changes the magnitude of the TRIP effect. For example, when the strain rate is high, the processing heat generation increases, so the TRIP effect decreases and the ductility decreases.
• the n value of the duplex stainless steel material can be measured in accordance with JIS Z2241:2011.
• the duplex stainless steel material according to the embodiment of the present invention preferably has a 0.2% yield strength of 480 MPa or less, more preferably 470 MPa or less. If the 0.2% yield strength is in this range, the duplex stainless steel material can be said to be soft.
• the lower limit of the 0.2% yield strength is not particularly limited, but is generally 300 MPa, preferably 350 MPa.
• the 0.2% yield strength of the duplex stainless steel material can be measured in accordance with JIS Z2241:2011.
• the duplex stainless steel material according to the embodiment of the present invention may be either a hot-rolled material or a cold-rolled material.
• the hot-rolled material or the cold-rolled material may be annealed or pickled.
• the thickness of the duplex stainless steel material according to the embodiment of the present invention is not particularly limited and may be adjusted appropriately depending on the application, but is generally 5.0 mm or less, preferably 4.0 mm or less, and more preferably 3.0 mm or less.
• the thickness refers to the circle-equivalent diameter of the cross section.
• the thickness refers to the thickness at any point on the cross section.
• duplex stainless steel material according to the embodiment of the present invention can be, for example, the following two forms A and B.
• the metal structure has an austenite phase of 25 to 49 volume %;
• the austenite phase has an Md value represented by the above formula (1) of 35.0 to 100.0°C.
• duplex stainless steel material described in [A1] further containing, by mass, one or more selected from Nb: 0.010-0.500%, Ti: 0.01-0.50%, V: 0.01-0.50%, W: 0.05-0.50%, Co: 0.01-0.30%, B: 0.0002-0.0050%, Sn: 0.010-0.500%, Al: 0.010-0.050%, Mg: 0.0002-0.0100%, Ca: 0.0002-0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01-0.50%, and REM: 0.0002-0.0100%.
• [A4] The following characteristics (a) and (b): The duplex stainless steel material according to any one of [A1] to [A3], satisfying at least one of: (a) a tensile strength of 800 MPa or less; and (b) a uniform elongation of 30.0% or more.
• (1) (wherein the element symbols represent the contents (mass%) of the respective elements) is 50.0 to 150.0° C.,
• the metal structure has an austenite phase of 25 to 40 volume %;
• the austenite phase has an Md value represented by the above formula (1) of 35.0 to 100.0° C.,
• a duplex stainless steel material having an average grain size of ferrite phase of 7.0 ⁇ m or more.
• duplex stainless steel material described in [B1] further containing, by mass, one or more selected from Nb: 0.010-0.500%, Ti: 0.01-0.50%, V: 0.01-0.50%, W: 0.05-0.50%, Co: 0.01-0.30%, B: 0.0002-0.0050%, Sn: 0.010-0.500%, Al: 0.010-0.050%, Mg: 0.0002-0.0100%, Ca: 0.0002-0.0100%, Ta: 0.050% or less, Ga: 0.050% or less, Zr: 0.01-0.50%, and REM: 0.0002-0.0100%.
• [B3] A duplex stainless steel material according to [B1] or [B2], in which the value of Md of the austenite phase as shown in formula (1) is 50.0 to 90.0°C.
• [B5] The duplex stainless steel material according to any one of [B1] to [B4], in which, when a tensile test is carried out at a strain rate of 3.3 x 10 -4 to 8.3 x 10 -3 s -1 , the n-value ratio of the n-value in the 15 to 20% strain region to the n-value in the 20 to 25% strain region is 0.80 or less.
• the method for producing a duplex stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method capable of producing a duplex stainless steel material having the above-mentioned characteristics.
• examples of the manufacturing method for the duplex stainless steel material according to the embodiment of the present invention (particularly, the duplex stainless steel materials of aspects A and B) will be described.
• the duplex stainless steel materials of the aspects A and B can be produced by vacuum melting the stainless steel having the above composition to produce a steel slab, hot rolling and annealing the slab, and then cold rolling and finish annealing the slab. In this production method, the key is to control the heat treatment conditions.
• the specific production methods of each aspect will be described below.
• the heat treatment (annealing) process it is necessary to suppress the precipitation of carbides and nitrides during heating and cooling, while maintaining the temperature at the target temperature to sufficiently dissolve the undissolved carbides and nitrides.
• the concentration of elements (especially carbon and nitrogen) that constitute the austenite phase changes depending on the ratio of the austenite phase, and the resulting Md varies, so it is also necessary to control the ratio of the austenite phase by adjusting the target temperature.
• the hot rolling conditions are not particularly limited, and may be carried out in accordance with a conventional method.
• Annealing after hot rolling is performed by raising the temperature at a rate of 20°C/sec or more, holding the reached temperature of 1050 to 1150°C for 10 seconds or more, and then cooling to 400°C or less at a cooling rate of 20°C/sec or more.
• the reason for performing annealing under such conditions is to sufficiently dissolve the carbides and nitrides precipitated during cooling after hot rolling, and to suppress the precipitation of carbides and nitrides during the cooling process after annealing.
• the reached temperature is lower than 1050°C, the solid solution of carbides and nitrides becomes insufficient, and the proportion of austenite phase becomes too high.
• the reached temperature is higher than 1150°C, although the carbides and nitrides are sufficiently dissolved, the proportion of austenite becomes too low. Furthermore, a certain amount of carbon and nitrogen are also dissolved in the ferrite phase, and there is a risk that precipitates will be formed during cooling in the ferrite phase, which has a small solid solubility limit, and corrosion resistance will be deteriorated.
• the conditions of the cold rolling are not particularly limited, but the rolling ratio is preferably 50 to 90%.
• the rolling ratio is set to 50% or more because the surface area of carbides and precipitates is enlarged by crushing or extending them, thereby accelerating solid solution during heat treatment.
• the rolling ratio is set to 90% or less in order to suppress edge breakage due to excessive rolling. From the viewpoint of stably obtaining this effect, the rolling ratio is more preferably 85% or less.
• intermediate annealing may be performed between each cold rolling.
• the conditions may be similar to those of the annealing after hot rolling.
• the conditions for finish annealing are a heating rate of 20°C/sec or faster, a temperature of 1040-1120°C is reached and held for 5 seconds or longer, followed by cooling to 850°C or lower at a cooling rate of 30°C/sec or faster, and then cooling to 400°C or lower at a cooling rate of 20°C/sec or faster. Finish annealing is performed under these conditions in order to suppress the precipitation of carbides and nitrides during heating, complete recrystallization, dissolve carbides and nitrides, control the proportion of austenite phase, suppress fluctuations in the proportion of austenite phase during cooling, and suppress the reprecipitation of carbides and nitrides.
• ⁇ Aspect B> In duplex stainless steel materials, in order to control the average grain size of the ferrite phase within a predetermined range, it is necessary to control the hot rolling conditions (temperature immediately after the final pass, cooling rate). In addition, in order to control the proportion of the austenite phase and Md within a predetermined range, it is necessary to control the heat treatment (annealing) conditions (heating rate, ultimate temperature, holding time, and cooling rate). All of these conditions affect the solid solution state of carbon and nitrogen. In addition, the ultimate temperature affects the thermodynamic fluctuation of the austenite amount. Furthermore, the control of the ultimate temperature and holding time is also aimed at sufficiently recrystallizing the entire structure.
• annealing heat treatment
• the ultimate temperature affects the thermodynamic fluctuation of the austenite amount.
• the ratio of the austenite phase and Md cannot be controlled within a predetermined range. Therefore, in the heat treatment (annealing) process, it is necessary to suppress the precipitation of carbides and nitrides during heating and cooling, while maintaining the temperature at the target temperature to sufficiently dissolve the undissolved carbides and nitrides.
• the concentration of elements (especially carbon and nitrogen) that constitute the austenite phase changes depending on the ratio of the austenite phase, and the resulting Md varies, so it is also necessary to control the ratio of the austenite phase by adjusting the target temperature.
• the temperature immediately after the final pass is set to 1030° C. or higher, and then the material is cooled at a cooling rate of 20° C./sec or higher to 800° C.
• the crystal grains of the ferrite phase can be coarsened and controlled to a predetermined range.
• the first reason is recrystallization during hot rolling or during subsequent annealing due to accumulation of hot rolling strain. The lower the hot rolling temperature, the more strain accumulates and induces recrystallization, making the crystal grains of the ferrite phase more likely to become fine, so it is necessary to raise the temperature to the temperature immediately after the final pass, where this is less likely to occur.
• the second reason is the suppression of growth of the crystal grains of the ferrite phase due to the generation of the austenite phase.
• the austenite phase precipitates in the crystal grain boundaries of the ferrite phase, the movement of the crystal grain boundaries of the ferrite phase becomes slower, and grain growth is suppressed.
• the austenite phase decreases as the temperature increases, with a peak at around 900°C, so the higher the temperature, the easier the crystal grains of the ferrite phase grow.
• it is effective to increase the heating temperature of the slab before hot rolling or to increase the rolling speed to shorten the heat dissipation time, but this increases fuel costs and manufacturing difficulties. Taking these circumstances into consideration, the lower limit of the final pass temperature in hot rolling is set to 1030°C.
• Annealing after hot rolling is performed at a heating rate of 20°C/sec or more, with the temperature of 1080 to 1150°C held for 10 seconds or more, and then cooled to 400°C or less at a cooling rate of 20°C/sec or more. Annealing is performed under these conditions in order to fully dissolve the carbides and nitrides that precipitated during cooling after hot rolling and to suppress the precipitation of carbides and nitrides during the cooling process after annealing.
• the proportion of austenite phase is relatively small, which reduces the inhibition of grain growth of the ferrite phase and makes it easier to coarsen the grains of the ferrite phase.
• the temperature is lower than 1080°C, the dissolution of carbides and nitrides is insufficient and the proportion of austenite phase becomes too high.
• the temperature is higher than 1150°C, the carbides and nitrides are fully dissolved, but the proportion of austenite becomes too low.
• a certain amount of carbon and nitrogen dissolve in the ferrite phase, and since the ferrite phase has a small solubility limit, precipitates may form during cooling, which may reduce corrosion resistance.
• the conditions of the cold rolling are not particularly limited, but the rolling ratio is preferably 50 to 80%.
• the rolling ratio is set to 50% or more because the surface area of precipitates such as carbides is expanded by crushing or extending them, thereby promoting solid solution during heat treatment.
• the rolling ratio is set to 80% or less in order to suppress edge breakage due to excessive rolling.
• it is also to suppress the structure of the finish annealed material from becoming too fine due to the accumulation of rolling strain.
• the rolling ratio is more preferably 75% or less.
• the conditions for finish annealing are a heating rate of 20°C/sec or more, a temperature of 1000-1150°C is reached and held for 5 seconds or more, followed by cooling to 850°C or less at a cooling rate of 30°C/sec or more, and then cooling to 400°C or less at a cooling rate of 20°C/sec or more.
• Finish annealing is performed under these conditions in order to suppress the precipitation of carbides and nitrides during heating, complete recrystallization, dissolve carbides and nitrides, control the proportion of austenite phase, suppress fluctuations in the proportion of austenite phase during cooling, and suppress the reprecipitation of carbides and nitrides.
• the ferrite phase crystal grains become coarse during finish annealing.
• the duplex stainless steel material according to the embodiment of the present invention is softer and more ductile than conventional ferritic-austenitic duplex stainless steel materials. This allows this duplex stainless steel material to suppress springback caused by excessively high strength during forming, and it also has good shape fixability. This duplex stainless steel material also has higher strength and better corrosion resistance than general-purpose austenitic stainless steel materials such as SUS304. This duplex stainless steel material can therefore be used in a variety of applications where these properties are required.
• Example of A cold-rolled annealed sheet was prepared as a duplex stainless steel material. Specifically, stainless steel having the composition of the steel type shown in Table 1 (the balance being Fe and impurities) was melted by vacuum melting to form a steel slab, and then hot-rolled and annealed according to a conventional method. The annealing was performed at a heating rate of 30 ° C / sec, and the reached temperature of 1100 ° C was held for 180 seconds, and then cooled to 400 ° C or less at a cooling rate of 25 ° C / sec.
• the hot-rolled sheet after annealing was cold-rolled at a rolling ratio of 80%, and then finish annealed to obtain a cold-rolled annealed sheet having a thickness of 1.0 mm.
• the finish annealing was performed at a heating rate of 30 ° C / sec, and the reached temperature of 1080 ° C was held for 30 seconds, and then cooled to 850 ° C or less at a cooling rate of 30 ° C / sec, and then cooled to 400 ° C or less at a cooling rate of 25 ° C / sec.
• Table 1 the values of Md and DF were calculated based on the content of each element.
• No. 1-M and 1-N are existing duplex stainless steel materials.
• the cold-rolled annealed sheets obtained above were evaluated as follows.
• ⁇ Proportion of austenite phase ( ⁇ phase) in duplex stainless steel material> After cutting out a test piece from the cold-rolled annealed sheet, the thickness direction cross section parallel to the rolling direction was mirror-polished and EBSD (electron backscatter diffraction) measurement was performed. EBSD measurement was performed using a scanning electron microscope and measurement software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.), and a 200 ⁇ m square area was measured at the center of the thickness direction of the test piece with a step size of 0.3 ⁇ m.
• phase ratio map was created using analysis software TSL OIM Analysis 7 (TSL Solutions Co., Ltd.) for the data obtained by the EBSD measurement, and the ferrite phase and the austenite phase were separated. Then, the proportion of the austenite phase in the entire observation area was determined.
• the measurement area was an area of about 2 ⁇ m square, and three or more points were measured in each test piece, and the average value was used as the result of the content of each element.
• the EPMA measurement was performed under the conditions of an acceleration voltage of 15 kV, a current of 0.2 ⁇ A, and a step size of 0.15 ⁇ m.
• the Md of the austenite phase was calculated based on the content of each element thus obtained.
• Example 1-7 the composition and Md of the cold-rolled annealed sheet (duplex stainless steel material) as well as the proportion of the austenite phase and Md were controlled within a predetermined range, and therefore both the tensile strength and uniform elongation showed good results.
• Comparative Example 1-1 the Ni content was too low and the N and C+N contents were too high, resulting in excessively high strength and insufficient ductility.
• Comparative Example 1-2 the Cr content was too high, and the proportion of the ferrite phase was high, so that the ductility was insufficient.
• Comparative Example 1-3 the Ni content was too high while the Cr content was too low, resulting in a high proportion of the austenite phase, which made the material susceptible to work hardening and resulted in excessively high strength.
• Comparative Example 1-4 the contents of Ni, Cr, and Sn were too small while the content of Al was too large, and the Md values of the cold-rolled annealed sheet and the austenite phase were too high, so the amount of processing-induced martensite phase was large, resulting in excessive strength and insufficient ductility.
• Comparative Example 1-5 the Si and Cr contents were too high, the Ti content was low, and the Md value of the cold-rolled annealed sheet was too low and the proportion of the ferrite phase was too high, so that the ductility was insufficient.
• Comparative Examples 1-6 and 1-7 were existing duplex stainless steel materials, and in particular had too high a content of N.
• the Md values of the cold-rolled annealed sheet and the austenite phase were too low, so that the strength was excessively high and the ductility was insufficient.
• Comparative Example 1-8 the Md of the austenite phase was too low, and therefore the ductility was insufficient.
• Example of Aspect B A cold-rolled annealed sheet was produced as a duplex stainless steel material.
• the cold-rolled annealed sheet was produced in the following order: hot rolling, annealing, cold rolling, and finish annealing. Specifically, first, stainless steel having the composition of the steel type shown in Table 3 (the balance being Fe and impurities) was melted by vacuum melting to produce a steel slab. Next, a hot rolling process was carried out on this steel slab to obtain a hot-rolled sheet having a thickness of 5 mm. In the hot rolling process, the temperature immediately after the final pass was set to the temperature shown in Table 4, and the sheet was cooled to 800 ° C. by water cooling (cooling rate of 20 ° C./sec or more).
• the temperature was increased at a rate of 25 ° C./sec, and the sheet was held at the reached temperature (annealing temperature) of 1100 ° C. for 30 seconds, and then cooled to 400 ° C. or less by water cooling (cooling rate of 20 ° C./sec or more).
• Cold rolling was performed at the rolling ratio shown in Table 4 to obtain a cold-rolled sheet having the thickness shown in Table 4.
• the temperature was increased at a rate of 30° C./sec, and the final temperature (annealing temperature) shown in Table 4 was held for 30 seconds, and then the steel was cooled to 400° C. or less by water cooling (cooling rate of 30° C./sec or more).
• the values of Md and DF were calculated based on the contents of each element.
• the cold-rolled annealed sheets obtained above were evaluated as follows.
• ⁇ Average particle size of ferrite phase ( ⁇ phase) in duplex stainless steel material> After cutting out a test piece from the cold-rolled annealed sheet, the cross section in the thickness direction parallel to the rolling direction was mirror-polished and subjected to EBSD (electron backscatter diffraction) measurement.
• the EBSD measurement was performed using a scanning electron microscope and measurement software TSL OIM Data Collection 7 (TSL Solutions Co., Ltd.), and a 200 ⁇ m square area was measured at the center of the thickness direction of the test piece with a step size of 0.3 ⁇ m.
• the area of the crystal grains of the ferrite phase (BCC) was obtained by the area fraction method for the data obtained by this EBSD measurement.
• a JIS No. 13B test piece was cut out from the cold-rolled annealed sheet so that the parallel portion was in the rolling direction, and a tensile test was performed using this test piece in accordance with JIS Z2241:2011.
• the tensile test was performed in an air atmosphere at room temperature (25°C) and at a tensile speed of 10 mm/min. In the tensile test, the elongation up to the maximum attained strength (tensile strength) was defined as the uniform elongation.
• the n-value was determined by measuring the relationship between stress ⁇ and strain ⁇ from 0.2% yield strength to the maximum load point, calculating the true stress and true strain from these measurements, and plotting them on a logarithmic scale with strain (ln ⁇ ) on the horizontal axis and stress (ln ⁇ ) on the vertical axis. The slope of the straight line obtained by plotting was the n-value.
• the strain rate was as shown in Table 3. In these evaluations, if the n-value ratio is 0.80 or less and the uniform elongation is 30.0% or more, it can be said that the ductility is excellent. Also, if the 0.2% yield strength is 480 MPa or less, it can be said that the material is soft.
• Example 2-1 to 2-7 the composition and Md of the cold-rolled annealed sheet (duplex stainless steel material) as well as the proportion and Md of the austenite phase were controlled within a predetermined range, so that the n-value ratio, 0.2% yield strength, and uniform elongation were all good.
• Comparative Example 2-1 the proportion of the austenite phase was too small, and therefore the ductility was insufficient.
• Comparative Example 2-2 since the Md of the austenite phase was too high, the average grain size of the ferrite phase was small, and softening was insufficient.
• Comparative Example 2-3 had a high amount of C+N due to the excessive N content.
• the present invention can provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials.
• the present invention can provide a ferritic-austenitic duplex stainless steel material that is softer and has higher ductility than conventional ferritic-austenitic duplex stainless steel materials by adopting the following configurations [1] to [10].
• (1) (wherein the element symbols represent the contents (mass%) of the respective elements) is 50.0 to 150.0° C.,
• the metal structure has an austenite phase of 25 to 49 volume %;
• the austenitic phase has an Md value represented by the above formula (1) of 35.0 to 100.0°C.
• [4] A ferritic-austenitic duplex stainless steel material according to any one of [1] to [3], in which the Md value of the austenite phase is 50.0 to 90.0°C.
• a ferritic-austenitic duplex stainless steel material according to any one of [1] to [4], in which the average particle size of the ferritic phase is 7.0 ⁇ m or more.
• a ferritic-austenitic duplex stainless steel material according to any one of [1] to [7], having a tensile strength of 800 MPa or less.
• a ferritic-austenitic duplex stainless steel material according to any one of [1] to [8], having a uniform elongation of 30.0% or more.
• a ferritic-austenitic duplex stainless steel material according to any one of [1] to [9], in which, when a tensile test is carried out at a strain rate of 3.3 x 10 -4 to 8.3 x 10 -3 s -1 , the n-value ratio of the n-value in the 15 to 20% strain region to the n-value in the 20 to 25% strain region is 0.80 or less.

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