WO2023153184A1 - Austenitic stainless steel and method for producing austenitic stainless steel - Google Patents

Austenitic stainless steel and method for producing austenitic stainless steel Download PDF

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WO2023153184A1
WO2023153184A1 PCT/JP2023/001835 JP2023001835W WO2023153184A1 WO 2023153184 A1 WO2023153184 A1 WO 2023153184A1 JP 2023001835 W JP2023001835 W JP 2023001835W WO 2023153184 A1 WO2023153184 A1 WO 2023153184A1
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stainless steel
austenitic stainless
mass
phase
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French (fr)
Japanese (ja)
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太一朗 溝口
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日鉄ステンレス株式会社
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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
    • C21D9/46—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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to austenitic stainless steel and a method for producing austenitic stainless steel.
  • Metastable austenitic stainless steel represented by SUS301 is known as an austenitic stainless steel used for applications requiring corrosion resistance and strength. Such austenitic stainless steels are used, for example, as materials for spring products such as cylinder head gaskets for automobile engines or structural members such as frame materials for automotive batteries.
  • Patent Document 1 discloses a method for producing a spring material having a martensite phase in which precipitates composed of a Cu-rich phase are dispersed. A method of subjecting a spring steel plate to aging treatment has been proposed.
  • Cu-rich phase precipitation is effective in increasing the strength of stainless steel. Therefore, according to the method described in Patent Document 1, the steel plate for spring is subjected to aging treatment to precipitate a Cu-rich phase, thereby reducing the processing load in the manufacturing process of the steel plate for spring, and the spring, which is the final product. It is possible to increase the strength of the material. However, since an aging treatment process is required, there is a problem in the productivity of the spring material.
  • An object of one aspect of the present invention is to realize an austenitic stainless steel with high productivity while simultaneously reducing the processing load during manufacturing and increasing the strength of the final product.
  • N since the precipitation temperature of the Cu-rich phase and the precipitation temperature of Cr carbide are relatively close, it is preferable to use N without excessively increasing the amount of C in order to reduce the deterioration of corrosion resistance due to precipitation of Cr carbide. I focused on that. Reducing the amount of C in the austenitic stainless steel is also preferable for achieving the desired reduction in processing load.
  • the austenitic stainless steel according to one aspect of the present invention has C: 0.005% or more and 0.03% or less and Si: 0.1% or more and 2.0% by mass.
  • Mn 0.3% or more and 2.5% or less
  • P 0.04% or less
  • S 0.015% or less
  • Ni 3.0% or more and less than 6.0%
  • Cr 16.0% 18.5% or less
  • Cu 1.5% or more and 4.0% or less
  • N 0.08% or more and 0.25% or less
  • the balance being Fe and unavoidable impurities, 20% by volume or more and a Cu-rich phase with a number density of 1.0 ⁇ 10 3 ⁇ m ⁇ 3 or more and a major axis of 30 nm or less
  • the balance consists of a deformation-induced martensite phase and an unavoidable formation phase, and the following ( 1)
  • a method for producing an austenitic stainless steel comprises: C: 0.005% or more and 0.03% or less; Si: 0.1% or more 0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16 0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities.
  • a method for producing an austenitic stainless steel having an Md30 value of 0.0 or more and 80.0 or less represented by the formula comprising a finish annealing step of performing finish annealing at a temperature of 750°C or more and 980°C or less. , When the maximum temperature reached in the final annealing step is 850 ° C. or higher, the time to heat at 850 ° C. or higher is within 30 seconds, and in the final annealing step, the average temperature from 700 ° C. to 500 ° C.
  • an austenitic stainless steel with high productivity while achieving both reduction in processing load during production and high strength of the final product.
  • FIG. 2 shows an EBSD grain boundary map and a TEM image of an austenitic stainless steel according to one embodiment
  • FIG. 2 is a diagram showing the relationship between 0.2% proof stress (YS18%) and reference strength (HV60%) of austenitic stainless steel according to an example and a comparative example;
  • Austenitic stainless steel according to one embodiment of the present invention is stainless steel containing 20% by volume or more of an austenitic phase.
  • austenitic stainless steel refers to austenitic stainless steel according to one embodiment of the present invention unless otherwise specified.
  • Austenitic stainless steels may be, for example, steel sheets or strips.
  • Austenitic stainless steel contains a deformation-induced martensite phase, which is part of the austenite phase transformed by the deformation-induced transformation plasticity (TRIP) phenomenon.
  • the proportion of the deformation-induced martensite phase is preferably 5% by volume or more, more preferably 10% by volume or more, and 15% by volume or more. is more preferable, and 20% by volume or more is more preferable.
  • the austenitic stainless steel preferably has a deformation-induced martensitic phase ratio of less than 80% by volume, more preferably 75% by volume or less. As long as the austenitic stainless steel contains at least 20% by volume of the austenite phase, the proportion of the deformation-induced martensite phase may decrease as the proportion of the deformation-induced martensite phase increases.
  • Austenitic stainless steels also contain Cu-rich phases.
  • the Cu-rich phase is a phase containing 60 atom % or more of Cu (copper), such as an ⁇ -Cu phase.
  • Austenitic stainless steel contains at least a Cu-rich phase with a number density of 1.0 ⁇ 10 3 ⁇ m ⁇ 3 or more and a major axis of 30 nm or less.
  • the major diameter means the diameter of the maximum length among the diameters of the Cu-rich phase precipitated in the form of particles.
  • the austenitic stainless steel may contain a Cu-rich phase with a major axis of more than 30 nm.
  • the Cu-rich phase may be dispersed in the austenite phase, may be dispersed in the deformation-induced martensite phase, or may be dispersed in the inevitable formation phase described later.
  • the Cu-rich phase may be determined by structural observation using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • a TEM sample including an arbitrary cross section of austenitic stainless steel is prepared, and a predetermined range of the cross section is observed using a TEM.
  • the number of rich phases can be measured. Further, the number density per volume can be calculated by calculating the volume based on the thickness of the TEM sample used for the number measurement and the area of the range where the number measurement was performed.
  • a measured thickness of the TEM sample may be used, or an estimate of the thickness based on the method by which the TEM sample was made may be used. Examples of the method for producing a TEM sample include, but are not limited to, electropolishing.
  • the strength of austenitic stainless steel increases as the precipitated Cu-rich phase becomes finer and more abundant.
  • the amount and size of the Cu-rich phase as described above are effective in increasing the strength of the austenitic stainless steel.
  • Austenitic stainless steel does not precipitate a Cu-rich phase during production such as cold rolling before final annealing, and keeps the strength low to reduce the working load. By precipitating a Cu-rich phase in the final annealing step, the strength of the manufactured austenitic stainless steel is increased. Manufacturing processes such as the finish annealing process will be described later.
  • the austenitic stainless steel may contain unavoidably formed phases other than the austenite phase, the strain-induced martensite phase and the Cu-rich phase.
  • Inevitably formed phases are not particularly limited, but include, for example, delta ferrite phases and phases containing carbides, nitrides and/or oxides.
  • Phases containing carbides, nitrides and/or oxides include, for example, phases containing carbides and/or nitrides of Cr, Ti and/or Nb, and oxidation of Si, Ti, Al, Mg and/or Ca. phase containing substances.
  • Austenitic stainless steel preferably has an average crystal grain size of 10.0 ⁇ m or less.
  • the strength of austenitic stainless steel increases as the crystal grains become finer. Further, in austenitic stainless steel, it is common that ductility decreases as strength increases. However, by refining grains, it is possible to achieve both strength improvement and ductility improvement in austenitic stainless steel.
  • the average grain size may be measured using the EBSD (Electron Back Scattering Diffraction) method.
  • EBSD Electro Back Scattering Diffraction
  • grain sizes in multiple fields of view may be calculated by the EBSD method, and the average value of the grain sizes calculated in the multiple fields of view may be used as the average grain size.
  • the average crystal grain size may be measured using a method other than the EBSD method.
  • a method other than the EBSD method for example, a method of exposing grain boundaries by nitric acid electrolytic treatment as shown in JIS G0551 and measuring by an intercept method or the like may be used.
  • Austenitic stainless steel is, in mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and less than 2.5%, P : 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more3. 8% or less and N: 0.08% or more and 0.25% or less.
  • the balance of the austenitic stainless steel may consist of Fe (iron) and unavoidable impurities. The significance of the content of each element contained in the austenitic stainless steel will be described below.
  • (C) C is an austenite forming element that facilitates the formation of an austenite phase, has a high solid-solution strengthening action, and is also an effective element for obtaining strength.
  • Austenitic stainless steel contains 0.005% by mass or more and 0.03% by mass or less of C. When the C content is 0.005% by mass or more, an austenitic stainless steel having a sufficient solid-solution strengthening effect and good strength can be obtained.
  • Si Si
  • the austenitic stainless steel contains 0.1% by mass or more and 2.0% by mass or less of Si, preferably 0.2% by mass or more and 1.0% by mass or less of Si.
  • the Si content is 0.1% by mass or more, deoxidizing action and solid solution strengthening action are effectively exhibited in the austenitic stainless steel. More preferably, the Si content is 0.2% by mass or more.
  • Si is a ferrite-forming element that facilitates the formation of a ferrite phase.
  • the ⁇ ferrite phase causes edge splitting or splitting in hot rolling. From the viewpoint of reducing the formation of the ⁇ ferrite phase, the Si content is 2.0% by mass or less, preferably 1.0% by mass or less.
  • Mn manganese
  • Mn manganese
  • the austenitic stainless steel contains 0.3 mass % or more and 2.5 mass % or less of Mn, preferably 0.5 mass % or more and 2.0 mass % or less of Mn. If the content of Mn is 0.3% by mass or more, it is easy to secure the amount of precipitation of the Cu-rich phase, and if the content of Mn is 0.5% by mass or more, it is more preferable. Moreover, excessive addition of Mn causes deterioration of the hot workability of the austenitic stainless steel. Therefore, the content of Mn is set to 2.5% by mass or less, preferably 2.0% by mass or less.
  • the austenitic stainless steel may contain P of 0.04% by mass or less. If the P content is 0.04% by mass or less, the adverse effect on material properties such as ductility can be reduced in the austenitic stainless steel.
  • the austenitic stainless steel may contain 0.015% by mass or less of S. If the S content is 0.015% by mass or less, the adverse effects on material properties such as ductility can be reduced in the austenitic stainless steel.
  • Ni Ni (nickel) is an austenite-generating element and an element effective for maintaining the austenite phase.
  • the austenitic stainless steel contains 3.0% by mass or more and less than 6.0% by mass of Ni, preferably 3.5% by mass or more and 5.5% by mass or less of Ni, and 4.0% by mass or more and 5 More preferably, it contains less than 0.0% by mass of Ni.
  • the Ni content is 3.0% by mass or more, the austenite phase is well formed and maintained. More preferably, the Ni content is 4.5% by mass or more.
  • Ni is an expensive element, and when added in excess, it stabilizes the austenite phase and reduces the amount of deformation-induced martensite phase produced. Therefore, the Ni content is less than 6.0% by mass, preferably 5.5% by mass or less, and more preferably less than 5.0% by mass.
  • Cr Cr (chromium) is an effective element for ensuring the corrosion resistance of austenitic stainless steel.
  • the austenitic stainless steel contains 16.0% by mass or more and 18.5% by mass or less of Cr, preferably 16.5% by mass or more and 18.0% by mass or less of Cr. If the Cr content is 16.0% by mass or more, good corrosion resistance of the austenitic stainless steel can be ensured. More preferably, the Cr content is 16.5% by mass or more.
  • the Cr content is 18.5% by mass or less, preferably 18.0% by mass or less.
  • Cu is an austenite-forming element and an element effective for maintaining the austenite phase. It is also effective in increasing the strength of austenitic stainless steel by precipitation of Cu-rich phases. Cu is an element that effectively acts also for crystal grain refinement. This is probably because the Cu-rich phase exhibits an inhibitory effect on grain growth. In addition, Cu reduces the work hardening of the austenite phase in a solid solution state, so that the rolling load in the manufacturing process of the austenitic stainless steel can be reduced.
  • the austenitic stainless steel contains 1.5% by mass or more and 4.0% by mass or less of Cu, preferably 2.0% by mass or more and 3.5% by mass or less of Cu, and more than 2.0% by mass. It is more preferable to contain 0.5% by mass or less of Cu.
  • the Cu content is 1.5% by mass or more, the austenite phase is well formed and maintained, and the Cu-rich phase is well precipitated. More preferably, the Cu content is 2.0% by mass or more, and even more preferably more than 2.0% by mass.
  • the Cu content is 4.0% by mass or less, preferably 3.5% by mass or less.
  • N nitrogen
  • nitrogen is an austenite forming element, and is an element having a solid-solution strengthening effect and an effect of improving corrosion resistance. Since the austenitic stainless steel has a C content of 0.03% by mass or less in order to ensure corrosion resistance of the weld zone, the N content is 0.08% by mass or more, and 0.10% by mass or more. is preferably 0.11% by mass or more, and even more preferably 0.12% by mass or more. Such an N content is effective in ensuring the strength and corrosion resistance required for austenitic stainless steel.
  • the N content is 0.25% by mass or less, preferably 0.20% by mass or less.
  • the austenitic stainless steel contains, by mass%, Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, and B: 0.0001% or more and 0.0001% or less. 01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% Below, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metal): 0.5% or less. It may further contain one or more selected from 2% or less.
  • the austenitic stainless steel preferably contains one or more selected from Mo of 1.0% by mass or less, W of 1.0% by mass or less, and V of 0.5% by mass or less.
  • B B (boron) is an element that improves hot workability, and is an element that is effective in reducing the occurrence of edge splitting and double cracking in hot rolling.
  • the austenitic stainless steel preferably contains 0.0001% by mass or more and 0.01% by mass or less of B. If the B content is 0.0001% by mass or more, it is effective in improving hot workability and reducing edge splitting and split cracking in hot rolling. However, excessive addition of B to an austenitic stainless steel containing Cr causes deterioration of corrosion resistance due to precipitation of Cr 2 B. Therefore, the content of B is preferably 0.01% by mass or less.
  • Co Co
  • Co Co
  • Co is an effective element for ensuring the corrosion resistance of austenitic stainless steel. It also contributes to reducing the coarsening of the Cu-rich phase and maintaining it fine. In order to obtain such an effect, it is preferable to contain 0.10% by mass or more of Co. However, Co is an expensive element, and from the viewpoint of cost reduction, the Co content is preferably 0.8% by mass or less.
  • Sn Sn (tin) is an effective element for ensuring the corrosion resistance of austenitic stainless steel.
  • the Sn content is preferably 0.1% by mass or less.
  • Al, Ca, Mg, Ti Al (aluminum), Ca (calcium), Mg (magnesium) and Ti (titanium) are all deoxidizing elements.
  • the austenitic stainless steel is selected from 0.3 wt% or less Al, 0.03 wt% or less Ca, 0.03 wt% or less Mg, and 0.5 wt% or less Ti as a deoxidizing agent. It is preferable to include one or more of
  • Nb Nb niobium
  • the austenitic stainless steel preferably contains 0.5% by mass or less of Nb.
  • the austenitic stainless steel contains up to 0.5% by weight Sb, up to 0.5% by weight Zr, up to 0.03% by weight Ta, up to 0.03% by weight Hf, and up to 0.2% by weight It preferably contains one or more selected from REM.
  • the austenitic stainless steel has an Md30 value of 0.0 or more and 80.0 or less, preferably 20.0 or more and 70.0 or less, as indicated by the following formula (1).
  • Md 30 551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
  • the content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
  • the value of Md30 is the temperature at which 50% of the structure of the austenitic stainless steel transforms into the martensitic phase when 30% tensile strain is applied to the austenitic stainless steel of the austenitic single phase. (°C). Therefore, the value of Md30 can be used as an indicator of the stability of the austenite phase. In addition, the value of Md30 can also be used as an index that influences the likelihood of the TRIP phenomenon occurring in austenitic stainless steel.
  • the value of Md30 of the austenitic stainless steel according to one embodiment of the present invention is preferably 0.0 or more and 80.0 or less.
  • the higher the value of Md 30 the easier the transformation from the austenite phase to the deformation-induced martensite phase occurs, and the application of mild cold-rolling strain can provide high strength and ensure excellent ductility. Further, even when the austenitic stainless steel is subjected to forming processing, the portion to which processing strain is imparted, such as a bent portion, tends to obtain higher strength due to the TRIP phenomenon.
  • Md30 which is an index of stability of the austenite phase
  • Md30 which is an index of stability of the austenite phase
  • the coefficient of Cu is set smaller than the coefficient of Ni in the component regression equation of Md 30 .
  • Many of the component regression equations of Md 30 based on conventional knowledge are based on the results of austenitic stainless steels that are not Ni-saving types.
  • the Ni-saving type composition of the present invention it has been found that the influence of Cu on the stabilization of the austenite phase is clearly smaller than the conventional knowledge.
  • a method for producing an austenitic stainless steel according to an embodiment of the present invention includes, in mass %, C: 0.005% to 0.03%, Si: 0.1% to 2.0%, Mn: 0 .3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% Below, Cu: 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, Md 30 represented by the above formula (1) is 0.0 or more and 80.0 or less. Also, the method for producing austenitic stainless steel includes a finish annealing step.
  • the method for manufacturing austenitic stainless steel may include a general manufacturing process for austenitic stainless steel for processes other than the final annealing process.
  • An example of a method for producing an austenitic stainless steel according to one embodiment of the present invention is shown below, but the present invention is not limited to this.
  • a slab is produced by continuously casting molten steel whose composition is adjusted. Then, the slab produced by continuous casting is heated to 1100° C. or higher and 1300° C. or lower, and then hot rolled to produce a hot rolled steel strip.
  • the precipitation rate of the Cu-rich phase from the less strained austenite phase after hot rolling is slow. Therefore, the finishing temperature and coiling temperature of the hot-rolled steel strip after hot rolling may be the same conditions as in the general method for producing austenitic stainless steel.
  • the coiling temperature of the hot-rolled steel strip after hot rolling is preferably 850° C. or lower, more preferably 650° C. or lower.
  • the hot-rolled steel strip that has been hot-rolled may be pickled.
  • the hot-rolled steel strip may be annealed before pickling, or may be pickled without annealing.
  • the annealing temperature is preferably in the range of 900° C. or higher and 1150° C. or lower. It is more preferable to operate at a temperature within the range, but is not limited to the above range. Then, the pickled hot-rolled steel strip is cold-rolled to a predetermined thickness to obtain a cold-rolled steel strip.
  • the cold rolling process should be performed at a rolling reduction and a rate such that the strain-induced martensite phase in the cold-rolled steel strip accounts for 20% by volume or more of the total. It is preferable to carry out by rolling temperature. By performing such a cold rolling process, a Cu-rich phase can be effectively precipitated in the steel strip in the subsequent finish annealing process.
  • the value of Md30 is adjusted to 0.0 or more and 80.0 or less.
  • An austenitic stainless steel having such a value of Md 30 precipitates a Cu-rich phase in the amount specified in one embodiment of the present invention, regardless of the amount of deformation-induced martensitic phase in the cold-rolled steel strip.
  • increasing the rolling reduction in the cold-rolling process, controlling the temperature in the cold-rolling process to be low, etc. as necessary are more effective for the precipitation of the Cu-rich phase.
  • the rolling reduction in the cold rolling step is preferably 40% or more, more preferably 50% or more, It is more preferably 60% or more.
  • the temperature in the cold rolling step is preferably 90° C. or lower, more preferably 60° C. or lower.
  • the cold-rolled steel strip is subjected to finish annealing.
  • the finish annealing step is carried out under conditions that promote the precipitation of the Cu-rich phase.
  • a Cu-rich phase is effective in increasing the strength of austenitic stainless steel. Therefore, the strength of the hot-rolled steel strip and the cold-rolled steel strip before precipitation of the Cu-rich phase is rather low, and the rolling load in the cold rolling process can be reduced. Then, the Cu-rich phase is precipitated in the final annealing step, so that the austenitic stainless steel after the final annealing has high strength.
  • the precipitation of the Cu-rich phase is also effective in refining the recrystallized grains of the austenite phase. Therefore, the precipitation of the Cu-rich phase can be used to control the average crystal grain size to 10.0 ⁇ m or less.
  • the method for producing austenitic stainless steel according to one embodiment of the present invention, it is possible to achieve both a reduction in the processing load during production and an increase in the strength of the final product at a high level.
  • the productivity of the austenitic stainless steel is also good because the additional step of the aging treatment is not required for the precipitation of the Cu-rich phase unlike the conventional method.
  • the finish annealing temperature in the finish annealing step is preferably 750°C or higher and 980°C or lower, and preferably 800°C or higher and 925°C or lower so that the Cu-rich phase is effectively precipitated in the austenitic stainless steel. If the final annealing temperature is less than 750°C, recrystallization of the structure will be insufficient. Moreover, when the temperature of the final annealing exceeds 980° C., the Cu-rich phase dissolves, so the amount of the Cu-rich phase remaining after the final annealing becomes insufficient.
  • the Cu-rich phase that precipitates from the deformation-induced martensite phase is particularly likely to dissolve in the austenite phase if it is held at a temperature of 850°C or higher for a long time in the final annealing. Therefore, when the maximum temperature reached in the final annealing step is 850°C or higher, it is preferable to shorten the heating time at 850°C or higher. Specifically, when the maximum temperature reached in the final annealing step is 850° C. or higher, the heating time at 850° C. or higher is set to 30 seconds or less, preferably 15 seconds or less.
  • the term "heating time at 850° C. or higher" refers to the total time of the plurality of heating times when the final annealing step is divided into multiple times for heating to 850° C. or higher.
  • Austenitic stainless steel has a C content of 0.03% by mass or less, so precipitation of Cr carbide during cooling is unlikely to occur. Therefore, the cooling rate after finish annealing may be the same as in a general stainless steel manufacturing method. From the viewpoint of productivity, it can be said that a faster cooling rate is preferable, but the average cooling rate from 700 ° C. to 500 ° C. may be a relatively slow speed of 1 ° C./sec or more, for example. Taking this into consideration, 5° C./second or more is preferable. Also, considering the flatness of the steel sheet, the cooling rate is preferably less than 75° C./second, more preferably 50° C./second or less.
  • intermediate annealing and intermediate rolling may be performed as necessary. Further, in order to further increase the strength of the steel strip after finish annealing, skin pass rolling may be performed as necessary.
  • the temperature of the intermediate annealing is preferably 980° C. or higher and 1150° C. or lower in order to avoid precipitation of the Cu-rich phase when priority is given to reducing the rolling load. In order to increase the strength by repeating the precipitation treatment, it is preferable that the temperature of the intermediate annealing is the same as that of the finish annealing. In addition, the temperature of the intermediate annealing is not limited to the above range.
  • An austenitic stainless steel according to an embodiment of the present invention has a relatively low strength in the manufacturing process to reduce the rolling load and achieves high strength after manufacturing.
  • Such properties of austenitic stainless steel can be expressed, for example, by the relationship between 0.2% proof stress (YS18%, MPa) and reference strength (HV60%).
  • 0.2% proof stress (YS18%) is an indicator of the strength of austenitic stainless steel.
  • the 0.2% yield strength (YS18%) indicates the 0.2% yield strength when the austenitic stainless steel is further subjected to temper rolling with an elongation of 18% after finish annealing.
  • the 0.2% yield strength can be evaluated using a method conforming to JIS Z2241.
  • the reference strength (HV60%) is an index that hypothetically indicates the strength of the austenitic stainless steel before precipitation of the Cu-rich phase in the final annealing process.
  • the reference strength (HV60%) although the chemical composition of the austenitic stainless steel is the same, the manufacturing method is partially changed from the manufacturing method according to the embodiment of the present invention, and after hot rolling, annealing at 1050 ° C. is performed. It shows the Vickers hardness when cold rolling is applied at a rolling reduction of 60%. That is, the reference strength (HV60%) does not indicate the strength of the austenitic stainless steel according to one embodiment of the present invention, but may be the strength of a steel strip produced for evaluation, for example. Vickers hardness can be measured based on the Vickers hardness test method conforming to JIS Z2244.
  • the austenitic stainless steel which achieves both a reduction in the processing load during manufacturing and a high strength of the final product, has a relationship between the 0.2% proof stress (YS18%) and the reference strength (HV60%). satisfies the following formula (2).
  • Austenitic stainless steel has very high strength and corrosion resistance. Therefore, austenitic stainless steel is used for spring products that require high strength and corrosion resistance, such as cylinder head gaskets, spiral springs, springs for electronic device parts, train vehicle members, automotive battery frame materials, structural materials, and metal packings. It is suitable as a material for In particular, austenitic stainless steel is excellent in corrosion resistance (weldability) even when welded. Therefore, the austenitic stainless steel according to one embodiment of the present invention is suitable even for applications in which a relatively large number of welded structures are used, such as train vehicle members or automotive battery frame materials manufactured for welding. Available.
  • the austenitic stainless steel according to aspect 1 of the present invention has, in mass %, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: Contains 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, 20% by volume or more of the austenitic phase, and a number density of 1 .0 ⁇ 10 3 ⁇ m ⁇ 3 or more and a Cu-rich phase with a major axis of 30 nm or less, and the balance consists of a deformation-induced martensite phase and an unavoidable formation phase, and the value of Md 30 shown by the following formula (1) is 0.0 or more and 80.0 or less: Md 30 shown by the following formula
  • the austenitic stainless steel according to aspect 2 of the present invention in aspect 1 above, contains Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, and B: 0.5% by mass. 0001% or more and 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb : 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less and REM ( rare earth metal): may further contain one or more selected from 0.2% or less.
  • the austenitic stainless steel according to aspect 3 of the present invention in aspect 1 or 2 above, may have an average crystal grain size of 10.0 ⁇ m or less.
  • the method for producing an austenitic stainless steel according to aspect 4 of the present invention comprises, by mass %, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.1% or more, and 0.03% or less; 3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less , Cu: 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, Md 30 represented by the following formula (1)
  • the heating time at 850° C. or higher is 30 seconds or less, and the average cooling rate from 700° C. to 500° C. after the finish annealing is 1° C./second or more in the final annealing step.
  • Md 30 551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
  • the content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
  • Austenitic stainless steels according to each of the examples and comparative examples of the present invention were produced by the following method.
  • Austenitic stainless steel having the chemical composition shown in Table 1 was melted, and the production method according to one example of the present invention (invention examples C1 to C8) or the production method according to the comparative example (comparative examples D1 and D2), From hot rolling to finish annealing, a cold-rolled annealed material was obtained.
  • the conditions of each manufacturing method are shown in Table 2 below.
  • the time for reaching 850°C or higher was adjusted as shown in Table 2.
  • the heating was adjusted so that the temperature began to decrease when the final annealing temperature reached 850 ° C., but for convenience, in Table 2, the time to reach 850 ° C. or higher is described as "1 second". are doing.
  • a TEM sample was produced by an electropolishing method from the cold-rolled annealed material produced under each condition. A plane parallel to the rolling direction of the cold-rolled and annealed material in the TEM sample was observed in three fields of view in a range of 400 nm ⁇ 400 nm. Cu-rich phases were determined from the contrast of the TEM image, and the number of Cu-rich phases was counted. The thickness of the TEM sample was assumed to be 150 nm, and the number density per unit volume was determined. When the Cu-rich phase coarsened, it became observed in a clear shape instead of contrast. Cu-rich phases with longer diameters greater than 30 nm were excluded from the measurements.
  • Crystal grain size The average grain size was evaluated using the EBSD method. A cross section parallel to the rolling direction and perpendicular to the rolling surface of the cold-rolled annealed material manufactured under each condition was mechanically polished and then electrolytically polished. After that, EBSD analysis was performed on a 40 ⁇ m ⁇ 40 ⁇ m range of the cross section with a step interval of 0.2 ⁇ m in a field of view with a magnification of 2000 ⁇ .
  • misorientation in the orientation relationship satisfying the ⁇ 3 correspondence grain boundary except for annealing twins with misorientation of 1° or less, boundaries with misorientation of 2° or more are regarded as grain boundaries, and the area of each grain is S ( ⁇ m 2 ), and the diameter of a circle having the same area as the crystal grain was defined as D ( ⁇ m), and the crystal grain size was calculated by the following formula (3). This was performed for 5 fields of view, and the average of the grain sizes obtained in the 5 fields of view was calculated as the average grain size.
  • Crystal grain size ⁇ (D ⁇ S) ⁇ /40 ⁇ 40 (3) (Amount of martensite phase)
  • the amount of martensite phase (% by volume) is as it is when the plate thickness is 1.5 mm or more, and when the plate thickness is less than 1.5 mm, the material after cold rolling or temper rolling is adjusted so that the total is 1.5 mm or more. Later materials were layered. These materials were measured with a ferrite scope (Fischer FMP30, electromagnetic induction method), and the value obtained by dividing the measured value by 0.7475 was taken as the amount of martensite phase.
  • the amount (% by volume) of the austenite phase was regarded as a value obtained by subtracting the amount of the martensite phase from 100% by volume of the entire matrix of the austenitic stainless steel.
  • the amounts of Cu-rich phases and unavoidably formed phases in austenitic stainless steel may be calculated as extraneous numbers because their ratios are small and accurate measurement is difficult.
  • a cold-rolled annealed material having a plate thickness of 1.5 mm was subjected to TIG tanning welding at an electrode diameter of 1.6 mm, a welding speed of 70 cm/min, and a welding current of 90 A under the conditions of an Ar gas seal.
  • a 10 mm ⁇ 10 mm area including the weld was used as an evaluation surface, and #600 polishing was applied to remove the influence of the film, and the corrosion resistance of the evaluation surface was evaluated using the electrochemical reactivation rate as an index.
  • the reactivation rate was measured according to JIS G0580. Specifically, in a 0.5 mol/L sulfuric acid and 0.01 mol/L potassium thiocyanate aqueous solution at a liquid temperature of 30°C, polarization was performed from the spontaneous potential to 0.3 V (vs SCE) at a sweep rate of 100 mV/min ( hereinafter referred to as the “outward route”). After reaching 0.3 V (vs SCE), the potential was swept in the direction opposite to the forward trip, and after reactivation of the hot-rolled material, the sweep was terminated at a potential at which the anode current became 0 again (hereinafter, "return trip"). .
  • the ratio (ir/ia) of the maximum current density ia on the outward trip and the maximum current density ir on the return trip was calculated as the reactivation rate.
  • Such an evaluation method is strict as a sensitization determination method for evaluating corrosion resistance, so even if the reactivation rate is, for example, about 1.5%, it is considered that there is no problem in the actual environment. be done.
  • the cold-rolled annealed material according to one example of the present invention may have fine crystal grains, it is difficult to evaluate corrosion resistance. It can be said that it has Therefore, the corrosion resistance of the weld zone was evaluated as "O" (good) when the reactivation rate was 1% or less, and as "x" (poor) when the reactivation rate exceeded 1%.
  • Table 3 shows the amount of precipitated Cu-rich phase and the grain size of the cold-rolled annealed material obtained under the conditions shown in Table 2 for the invention steel A2.
  • 0.2% yield strength (YS18%) under each condition and the amount of martensite phase after cold rolling (before finish annealing) and after temper rolling at which the elongation is 18% after finish annealing. are also shown in Table 3 below.
  • the cold-rolled annealed materials produced under the respective conditions of invention examples C1 to C8 have a Cu-rich phase precipitation amount within the specified range of the present invention, and have a fine average grain size of 10.0 ⁇ m or less. Indicated. On the other hand, precipitation of the Cu-rich phase was not observed in any of the cold-rolled annealed materials produced under the conditions of Comparative Examples D1 and D2.
  • the EBSD grain boundary map is shown on the left side of FIG. 1, and the TEM imaged image is shown on the right side of FIG.
  • the TEM imaged image is shown on the right side of FIG.
  • precipitation of a Cu-rich phase indicated as “Cu” in FIG. 1 was observed in the austenitic stainless steel according to one embodiment of the present invention.
  • the 0.2% yield strength (YS18%) is preferably 1094 MPa or more based on the above formula (2).
  • the 0.2% proof stress (YS18%) of the cold-rolled annealed materials of the invention steel A2 manufactured under the respective conditions of invention examples C1 to C8 was 1094 MPa or more.
  • the 0.2% proof stress (YS18%) of the cold-rolled annealed materials manufactured under the respective conditions of Comparative Examples D1 and D2 was lower than 1094 MPa.
  • Table 4 below shows the Cu-rich phase precipitation amount and grain size after finish annealing of the cold-rolled and annealed materials manufactured from the invention steels A1 to A15 or the comparative steels B1 to B5 under the manufacturing conditions shown in invention example C2. show.
  • the 0.2% proof stress (YS18%), reference strength (HV60%), and corrosion resistance of the weld under these conditions are also shown in Table 4 below.
  • the cold-rolled and annealed materials of the invention steels A1 to A15 had a precipitation amount of the Cu-rich phase within the specified range of the present invention, and exhibited a fine average crystal grain size of 10.0 ⁇ m or less.
  • the 0.2% proof stress (YS18%) showed good values satisfying the above formula (2).
  • the cold-rolled and annealed material of comparative steel B1 had poor corrosion resistance at the weld.
  • the cold-rolled annealed materials of comparative steels B2 to B5 do not satisfy the above formula (2) in terms of 0.2% proof stress (YS18%), and have a good balance between workability before finish annealing and high strength after finish annealing. Austenitic stainless steel was not obtained.
  • Fig. 2 shows a plot of the relationship between the 0.2% proof stress (YS18%) and the reference strength (HV60%) under each condition in Table 4.
  • an example of the present invention is indicated by a white circle, and a comparative example is indicated by a black arrowhead.
  • the higher the upper left plot the better the balance between the workability before the finish annealing and the high strength after the finish annealing.
  • the cold-rolled annealed material manufactured by the manufacturing method according to one embodiment of the present invention using the austenitic stainless steel having the composition according to one embodiment of the present invention is It was shown that both the reduction of the processing load and the increase in strength of the product are compatible. It was also shown that such a cold-rolled annealed material is excellent in corrosion resistance of welded parts and suitable for applications in which many weldings are performed.

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Abstract

The present invention achieves an austenitic stainless steel having high productivity while reducing processing load during production and increasing the strength of the final product. The austenitic stainless steel contains, in mass%, 0.005-0.03% of C, 0.1-2.0% of Si, 0.3-2.5% of Mn, 0.04% or less of P, 0.015% or less of S, 3.0-6.0% of Ni, 16.0-18.5% of Cr, 1.5-4.0% of Cu, and 0.08-0.25% of N, with the remainder comprising Fe and inevitable impurities, includes 20 vol% or more of an austenite phase, and a Cu-rich phase having a number density of 1.0×103·μm-3 or more and a major axis of 30 nm or less, with the remainder comprising deformation-induced martensite phases and inevitable formation phases, and has an Md30 value of 0.0-80.0.

Description

オーステナイト系ステンレス鋼およびオーステナイト系ステンレス鋼の製造方法Austenitic stainless steel and method for producing austenitic stainless steel
　本発明は、オーステナイト系ステンレス鋼およびオーステナイト系ステンレス鋼の製造方法に関する。 The present invention relates to austenitic stainless steel and a method for producing austenitic stainless steel.
　耐食性および強度が要求される用途に用いられるオーステナイト系ステンレス鋼として、ＳＵＳ３０１に代表される準安定オーステナイト系ステンレス鋼が知られている。このようなオーステナイト系ステンレス鋼は、例えば自動車におけるエンジンのシリンダヘッドガスケットのようなばね製品または車載電池フレーム材のような構造部材の素材として用いられる。 　Metastable austenitic stainless steel represented by SUS301 is known as an austenitic stainless steel used for applications requiring corrosion resistance and strength. Such austenitic stainless steels are used, for example, as materials for spring products such as cylinder head gaskets for automobile engines or structural members such as frame materials for automotive batteries.
　このようなステンレス鋼は一般的に、冷間圧延等の圧延率を大きくすることで高強度化するため、製造工程における圧延等の加工負荷が大きい傾向にある。このような負荷を低減すべく、例えば特許文献１には、Ｃｕリッチ相からなる析出物が分散したマルテンサイト相を有するバネ材の製造法として、Ｃｕリッチ相の析出がない複相組織を呈するバネ用鋼板に時効処理を施す方法が提案されている。 Such stainless steel generally increases its strength by increasing the rolling rate of cold rolling, etc., so there is a tendency for the processing load such as rolling in the manufacturing process to be large. In order to reduce such a load, for example, Patent Document 1 discloses a method for producing a spring material having a martensite phase in which precipitates composed of a Cu-rich phase are dispersed. A method of subjecting a spring steel plate to aging treatment has been proposed.
日本国特開２００８－１９５９７６号公報Japanese Patent Application Laid-Open No. 2008-195976
　Ｃｕリッチ相の析出は、ステンレス鋼の高強度化に有効である。そのため、特許文献１に記載の方法によれば、バネ用鋼板に時効処理を施してＣｕリッチ相を析出させることで、バネ用鋼板の製造工程における加工負荷を低減しながら、最終製品であるバネ材の高強度化を実現できる。しかしながら、時効処理工程を要することから、バネ材の生産性に課題がある。 　Cu-rich phase precipitation is effective in increasing the strength of stainless steel. Therefore, according to the method described in Patent Document 1, the steel plate for spring is subjected to aging treatment to precipitate a Cu-rich phase, thereby reducing the processing load in the manufacturing process of the steel plate for spring, and the spring, which is the final product. It is possible to increase the strength of the material. However, since an aging treatment process is required, there is a problem in the productivity of the spring material.
　本発明の一態様は、製造時の加工負荷の低減と最終製品の高強度化とを両立し、かつ、生産性の高いオーステナイト系ステンレス鋼を実現することを目的とする。 An object of one aspect of the present invention is to realize an austenitic stainless steel with high productivity while simultaneously reducing the processing load during manufacturing and increasing the strength of the final product.
　また、Ｃｕリッチ相の析出温度とＣｒ炭化物の析出温度とが比較的近いことから、Ｃｒ炭化物の析出による耐食性低下を低減する上では過度にＣ量を高めることなく、Ｎを活用することが好ましいことに着目した。オーステナイト系ステンレス鋼中のＣ量を低めにすることは、目的とする加工負荷の低減にも好ましい。 In addition, since the precipitation temperature of the Cu-rich phase and the precipitation temperature of Cr carbide are relatively close, it is preferable to use N without excessively increasing the amount of C in order to reduce the deterioration of corrosion resistance due to precipitation of Cr carbide. I focused on that. Reducing the amount of C in the austenitic stainless steel is also preferable for achieving the desired reduction in processing load.
　上記の課題を解決するために、本発明の一態様に係るオーステナイト系ステンレス鋼は、質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、２０体積％以上のオーステナイト相と、個数密度が１．０×１０３個・μｍ－３以上の、長径３０ｎｍ以下のＣｕリッチ相とを含み、残部が加工誘起マルテンサイト相および不可避的形成相からなり、下記（１）式で示すＭｄ３０の値が０．０以上８０．０以下である：
　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。 In order to solve the above problems, the austenitic stainless steel according to one aspect of the present invention has C: 0.005% or more and 0.03% or less and Si: 0.1% or more and 2.0% by mass. Below, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% 18.5% or less, Cu: 1.5% or more and 4.0% or less, N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, 20% by volume or more and a Cu-rich phase with a number density of 1.0×10 3 μm −3 or more and a major axis of 30 nm or less, and the balance consists of a deformation-induced martensite phase and an unavoidable formation phase, and the following ( 1) The value of Md 30 represented by the formula is 0.0 or more and 80.0 or less:
Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
　上記の課題を解決するために、本発明の一態様に係るオーステナイト系ステンレス鋼の製造方法は、質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、下記（１）式で示すＭｄ３０の値が０．０以上８０．０以下であるオーステナイト系ステンレス鋼の製造方法であって、７５０℃以上９８０℃以下の温度により仕上焼鈍を行う仕上焼鈍工程を含み、前記仕上焼鈍工程での最高到達温度が８５０℃以上である場合、８５０℃以上で加熱する時間を３０秒以内とし、前記仕上焼鈍工程において、前記仕上焼鈍後の７００℃から５００℃までの平均冷却速度を１℃／秒以上とする：
　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。 In order to solve the above problems, a method for producing an austenitic stainless steel according to one aspect of the present invention comprises: C: 0.005% or more and 0.03% or less; Si: 0.1% or more 0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16 0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities. (1) A method for producing an austenitic stainless steel having an Md30 value of 0.0 or more and 80.0 or less represented by the formula, comprising a finish annealing step of performing finish annealing at a temperature of 750°C or more and 980°C or less. , When the maximum temperature reached in the final annealing step is 850 ° C. or higher, the time to heat at 850 ° C. or higher is within 30 seconds, and in the final annealing step, the average temperature from 700 ° C. to 500 ° C. after the final annealing A cooling rate of 1°C/sec or more:
Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
　本発明の一態様によれば、製造時の加工負荷の低減と最終製品の高強度化とを両立し、かつ、生産性の高いオーステナイト系ステンレス鋼を実現できる。 According to one aspect of the present invention, it is possible to realize an austenitic stainless steel with high productivity while achieving both reduction in processing load during production and high strength of the final product.
一実施形態に係るオーステナイト系ステンレス鋼のＥＢＳＤ粒界マップおよびＴＥＭ撮像画像を示す図である。FIG. 2 shows an EBSD grain boundary map and a TEM image of an austenitic stainless steel according to one embodiment; 一実施例および比較例に係るオーステナイト系ステンレス鋼の、０．２％耐力（ＹＳ１８％）と参考強度（ＨＶ６０％）との関係を示す図である。FIG. 2 is a diagram showing the relationship between 0.2% proof stress (YS18%) and reference strength (HV60%) of austenitic stainless steel according to an example and a comparative example;
　以下、本発明の一実施形態に係るオーステナイト系ステンレス鋼について詳細に説明する。以下の記載は発明の趣旨をよりよく理解させるものであり、特に指定のない限り、本発明を限定するものではない。 The austenitic stainless steel according to one embodiment of the present invention will be described in detail below. The following description provides a better understanding of the spirit of the invention and is not intended to limit the invention unless otherwise specified.
　〔組織構成〕
　本発明の一実施形態に係るオーステナイト系ステンレス鋼は、２０体積％以上のオーステナイト相を含むステンレス鋼である。本明細書では以下、「オーステナイト系ステンレス鋼」とは、特記しない限り本発明の一実施形態に係るオーステナイト系ステンレス鋼を示す。オーステナイト系ステンレス鋼は、例えば鋼板または鋼帯であってよい。 [Organizational structure]
Austenitic stainless steel according to one embodiment of the present invention is stainless steel containing 20% by volume or more of an austenitic phase. Hereinafter, "austenitic stainless steel" refers to austenitic stainless steel according to one embodiment of the present invention unless otherwise specified. Austenitic stainless steels may be, for example, steel sheets or strips.
　オーステナイト系ステンレス鋼は、オーステナイト相の一部が加工誘起変態塑性（ＴＲＩＰ）現象により変態した加工誘起マルテンサイト相を含んでいる。オーステナイト系ステンレス鋼は、高強度化の観点から、加工誘起マルテンサイト相の割合が、５体積％以上であることが好ましく、１０体積％以上であることがより好ましく、１５体積％以上であることがより好ましく、２０体積％以上であることがより好ましい。また、オーステナイト系ステンレス鋼は、加工誘起マルテンサイト相の割合が、８０体積％未満であることが好ましく、７５体積％以下であることがより好ましい。オーステナイト系ステンレス鋼が含むオーステナイト相の割合は、２０体積％以上であれば、加工誘起マルテンサイト相の割合の増加に従って低下してよい。 Austenitic stainless steel contains a deformation-induced martensite phase, which is part of the austenite phase transformed by the deformation-induced transformation plasticity (TRIP) phenomenon. From the viewpoint of increasing the strength of the austenitic stainless steel, the proportion of the deformation-induced martensite phase is preferably 5% by volume or more, more preferably 10% by volume or more, and 15% by volume or more. is more preferable, and 20% by volume or more is more preferable. In addition, the austenitic stainless steel preferably has a deformation-induced martensitic phase ratio of less than 80% by volume, more preferably 75% by volume or less. As long as the austenitic stainless steel contains at least 20% by volume of the austenite phase, the proportion of the deformation-induced martensite phase may decrease as the proportion of the deformation-induced martensite phase increases.
　オーステナイト系ステンレス鋼はさらに、Ｃｕリッチ相を含んでいる。Ｃｕリッチ相とは、Ｃｕ（銅）を６０原子％以上含む相であり、例えばε－Ｃｕ相である。オーステナイト系ステンレス鋼は少なくとも、個数密度が１．０×１０３個・μｍ－３以上の、長径３０ｎｍ以下のＣｕリッチ相を含む。長径とは、粒子状に析出するＣｕリッチ相の直径のうち、最大の長さの径を意味する。なお、オーステナイト系ステンレス鋼は、長径３０ｎｍ超のＣｕリッチ相を含んでいてもよい。Ｃｕリッチ相は、オーステナイト相中に分散していてもよく、加工誘起マルテンサイト相中に分散していてもよく、後述する不可避的形成相中に分散していてもよい。 Austenitic stainless steels also contain Cu-rich phases. The Cu-rich phase is a phase containing 60 atom % or more of Cu (copper), such as an ε-Cu phase. Austenitic stainless steel contains at least a Cu-rich phase with a number density of 1.0×10 3 μm −3 or more and a major axis of 30 nm or less. The major diameter means the diameter of the maximum length among the diameters of the Cu-rich phase precipitated in the form of particles. Note that the austenitic stainless steel may contain a Cu-rich phase with a major axis of more than 30 nm. The Cu-rich phase may be dispersed in the austenite phase, may be dispersed in the deformation-induced martensite phase, or may be dispersed in the inevitable formation phase described later.
　Ｃｕリッチ相は、透過型電子顕微鏡（ＴＥＭ）を用いた組織観察によって判別してよい。例えば、オーステナイト系ステンレス鋼の任意の断面を含むＴＥＭサンプルを作製し、ＴＥＭを用いて当該断面の所定の範囲を観察することで、当該範囲内におけるＣｕリッチ相の断面の長径が３０ｎｍ以下のＣｕリッチ相の個数を計測できる。また、上記個数計測に用いたＴＥＭサンプルの厚さと、上記個数計測を行った範囲の面積とに基づいて体積を算出することで、体積あたりの個数密度を算出できる。ＴＥＭサンプルの厚さについては、例えば、ＴＥＭサンプルの厚さの実測値を用いてもよく、ＴＥＭサンプルを作製した方法に基づく厚さの推定値を用いてもよい。ＴＥＭサンプルの作製方法としては、例えば、電解研磨法が挙げられるが、これに限られない。 The Cu-rich phase may be determined by structural observation using a transmission electron microscope (TEM). For example, a TEM sample including an arbitrary cross section of austenitic stainless steel is prepared, and a predetermined range of the cross section is observed using a TEM. The number of rich phases can be measured. Further, the number density per volume can be calculated by calculating the volume based on the thickness of the TEM sample used for the number measurement and the area of the range where the number measurement was performed. For the thickness of the TEM sample, for example, a measured thickness of the TEM sample may be used, or an estimate of the thickness based on the method by which the TEM sample was made may be used. Examples of the method for producing a TEM sample include, but are not limited to, electropolishing.
　オーステナイト系ステンレス鋼は、析出したＣｕリッチ相が微細であるほど、また、多量に存在するほど高強度化される。上述のような量および大きさのＣｕリッチ相は、オーステナイト系ステンレス鋼の高強度化に有効である。オーステナイト系ステンレス鋼は、仕上焼鈍前の冷間圧延時等の製造途中では、Ｃｕリッチ相を析出させず強度を低く抑えて加工負荷を低減する。そして、仕上焼鈍工程においてＣｕリッチ相を析出させることで、製造後のオーステナイト系ステンレス鋼について高強度化を実現する。仕上焼鈍工程等の製造工程については、後述する。 The strength of austenitic stainless steel increases as the precipitated Cu-rich phase becomes finer and more abundant. The amount and size of the Cu-rich phase as described above are effective in increasing the strength of the austenitic stainless steel. Austenitic stainless steel does not precipitate a Cu-rich phase during production such as cold rolling before final annealing, and keeps the strength low to reduce the working load. By precipitating a Cu-rich phase in the final annealing step, the strength of the manufactured austenitic stainless steel is increased. Manufacturing processes such as the finish annealing process will be described later.
　また、オーステナイト系ステンレス鋼は、オーステナイト相、加工誘起マルテンサイト相およびＣｕリッチ相以外の、不可避的形成相を含んでもよい。不可避的形成相は特に限定されないが、例えば、δフェライト相と、炭化物、窒化物および／または酸化物を含む相と、が挙げられる。炭化物、窒化物および／または酸化物を含む相としては、例えば、Ｃｒ、Ｔｉおよび／またはＮｂの炭化物および／または窒化物を含む相、並びに、Ｓｉ、Ｔｉ、Ａｌ、Ｍｇおよび／またはＣａの酸化物を含む相が挙げられる。 In addition, the austenitic stainless steel may contain unavoidably formed phases other than the austenite phase, the strain-induced martensite phase and the Cu-rich phase. Inevitably formed phases are not particularly limited, but include, for example, delta ferrite phases and phases containing carbides, nitrides and/or oxides. Phases containing carbides, nitrides and/or oxides include, for example, phases containing carbides and/or nitrides of Cr, Ti and/or Nb, and oxidation of Si, Ti, Al, Mg and/or Ca. phase containing substances.
　オーステナイト系ステンレス鋼は、平均結晶粒径が１０．０μｍ以下であることが好ましい。オーステナイト系ステンレス鋼は、結晶粒が微細になるにつれて強度が向上する。また、オーステナイト系ステンレス鋼では、強度を向上させると延性が低下することが一般的である。しかしながら、結晶粒の微細化によれば、オーステナイト系ステンレス鋼において強度の向上と延性の改善とを両立できる。 Austenitic stainless steel preferably has an average crystal grain size of 10.0 μm or less. The strength of austenitic stainless steel increases as the crystal grains become finer. Further, in austenitic stainless steel, it is common that ductility decreases as strength increases. However, by refining grains, it is possible to achieve both strength improvement and ductility improvement in austenitic stainless steel.
　平均結晶粒径は、ＥＢＳＤ（Electron Back Scattering Diffraction）法を用いて測定してよい。例えば、オーステナイト系ステンレス鋼の任意の断面について、複数の視野の結晶粒径をＥＢＳＤ法によりそれぞれ算出し、当該複数の視野で算出した結晶粒径の平均値を、平均結晶粒径としてよい。また、平均結晶粒径は、ＥＢＳＤ法以外の方法を用いて測定してもよい。ＥＢＳＤ法以外の方法としては、例えば、ＪＩＳ　Ｇ０５５１に示されるような、硝酸電解処理によって粒界を現出させ、切片法等によって測定する方法であってよい。 The average grain size may be measured using the EBSD (Electron Back Scattering Diffraction) method. For example, for an arbitrary cross section of austenitic stainless steel, grain sizes in multiple fields of view may be calculated by the EBSD method, and the average value of the grain sizes calculated in the multiple fields of view may be used as the average grain size. Also, the average crystal grain size may be measured using a method other than the EBSD method. As a method other than the EBSD method, for example, a method of exposing grain boundaries by nitric acid electrolytic treatment as shown in JIS G0551 and measuring by an intercept method or the like may be used.
　〔成分組成〕
　オーステナイト系ステンレス鋼は、質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％未満、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上３．８％以下およびＮ：０．０８％以上０．２５％以下を含有する。オーステナイト系ステンレス鋼の残部は、Ｆｅ（鉄）および不可避的不純物からなるものであってよい。以下、オーステナイト系ステンレス鋼に含まれる各元素の含有量の意義について説明する。 [Component composition]
Austenitic stainless steel is, in mass%, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and less than 2.5%, P : 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more3. 8% or less and N: 0.08% or more and 0.25% or less. The balance of the austenitic stainless steel may consist of Fe (iron) and unavoidable impurities. The significance of the content of each element contained in the austenitic stainless steel will be described below.
　（Ｃ）
　Ｃ（炭素）は、オーステナイト相を生成しやすくするオーステナイト生成元素であり、高い固溶強化作用を有し、また強度を得るためにも有効な元素である。オーステナイト系ステンレス鋼は、０．００５質量％以上０．０３質量％以下のＣを含む。Ｃの含有量が０．００５質量％以上であれば、十分な固溶強化作用を発揮すると共に、良好な強度を有するオーステナイト系ステンレス鋼が得られる。 (C)
C (carbon) is an austenite forming element that facilitates the formation of an austenite phase, has a high solid-solution strengthening action, and is also an effective element for obtaining strength. Austenitic stainless steel contains 0.005% by mass or more and 0.03% by mass or less of C. When the C content is 0.005% by mass or more, an austenitic stainless steel having a sufficient solid-solution strengthening effect and good strength can be obtained.
　Ｃの過剰添加は、比較的低温での焼鈍によりＣｒ炭化物が析出する原因となり、オーステナイト系ステンレス鋼の、特に溶接部の耐食性の低下を招くことから、Ｃの含有量は０．０３質量％以下とする。Ｃの含有量が０．０３質量％以下であれば、溶接部においても良好な耐食性を有するオーステナイト系ステンレス鋼が得られる。 Excessive addition of C causes the precipitation of Cr carbides due to relatively low temperature annealing, which leads to a decrease in the corrosion resistance of the austenitic stainless steel, especially the weld zone, so the C content is 0.03% by mass or less. and If the C content is 0.03% by mass or less, an austenitic stainless steel having good corrosion resistance even at the weld zone can be obtained.
　（Ｓｉ）
　Ｓｉ（ケイ素）は、脱酸剤として有効であり、また固溶強化作用を有する元素である。オーステナイト系ステンレス鋼は、０．１質量％以上２．０質量％以下のＳｉを含み、０．２質量％以上１．０質量％以下のＳｉを含むことが好ましい。Ｓｉの含有量が０．１質量％以上であれば、オーステナイト系ステンレス鋼において脱酸作用および固溶強化作用が効果的に発揮される。Ｓｉの含有量が０．２質量％以上であればより好ましい。 (Si)
Si (silicon) is an element that is effective as a deoxidizing agent and has a solid-solution strengthening action. The austenitic stainless steel contains 0.1% by mass or more and 2.0% by mass or less of Si, preferably 0.2% by mass or more and 1.0% by mass or less of Si. When the Si content is 0.1% by mass or more, deoxidizing action and solid solution strengthening action are effectively exhibited in the austenitic stainless steel. More preferably, the Si content is 0.2% by mass or more.
　また、Ｓｉは、フェライト相を生成しやすくするフェライト生成元素である。δフェライト相は、熱間圧延において耳切れまたは二枚割れが発生する原因となる。δフェライト相の生成を低減する観点から、Ｓｉの含有量は２．０質量％以下とし、１．０質量％以下とすることが好ましい。 In addition, Si is a ferrite-forming element that facilitates the formation of a ferrite phase. The δ ferrite phase causes edge splitting or splitting in hot rolling. From the viewpoint of reducing the formation of the δ ferrite phase, the Si content is 2.0% by mass or less, preferably 1.0% by mass or less.
　（Ｍｎ）
　Ｍｎ（マンガン）は、オーステナイト生成元素であり、またオーステナイト相を維持するために有効な元素である。また、ＭｎはＣｕリッチ相の析出を促進する作用を有する元素である。オーステナイト系ステンレス鋼は、０．３質量％以上２．５質量％以下のＭｎを含み、０．５質量％以上２．０質量％以下のＭｎを含むことが好ましい。Ｍｎの含有量が０．３質量％以上であれば、Ｃｕリッチ相の析出量を確保しやすく、Ｍｎの含有量が０．５質量％以上であればより好ましい。また、Ｍｎの過剰添加はオーステナイト系ステンレス鋼の熱間加工性の低下を招いてしまう。このため、Ｍｎの含有量は２．５質量％以下とし、２．０質量％以下とすることが好ましい。 (Mn)
Mn (manganese) is an austenite-forming element and an element effective for maintaining the austenite phase. Also, Mn is an element that has the effect of promoting the precipitation of the Cu-rich phase. The austenitic stainless steel contains 0.3 mass % or more and 2.5 mass % or less of Mn, preferably 0.5 mass % or more and 2.0 mass % or less of Mn. If the content of Mn is 0.3% by mass or more, it is easy to secure the amount of precipitation of the Cu-rich phase, and if the content of Mn is 0.5% by mass or more, it is more preferable. Moreover, excessive addition of Mn causes deterioration of the hot workability of the austenitic stainless steel. Therefore, the content of Mn is set to 2.5% by mass or less, preferably 2.0% by mass or less.
　（Ｐ）
　Ｐ（リン）は、不可避的不純物として混入する元素であり、Ｐの含有量は少ないほど好ましい。製造性の観点から、オーステナイト系ステンレス鋼は、０．０４質量％以下のＰを含んでよい。Ｐの含有量が０．０４質量％以下であれば、オーステナイト系ステンレス鋼において、延性等の材料特性への悪影響を低減できる。 (P)
P (phosphorus) is an element mixed as an unavoidable impurity, and the smaller the content of P, the better. From the viewpoint of manufacturability, the austenitic stainless steel may contain P of 0.04% by mass or less. If the P content is 0.04% by mass or less, the adverse effect on material properties such as ductility can be reduced in the austenitic stainless steel.
　（Ｓ）
　Ｓ（硫黄）は、不可避的不純物として混入する元素であり、Ｓの含有量は少ないほど好ましい。製造性の観点から、オーステナイト系ステンレス鋼は、０．０１５質量％以下のＳを含んでよい。Ｓの含有量が０．０１５質量％以下であれば、オーステナイト系ステンレス鋼において、延性等の材料特性への悪影響を低減できる。 (S)
S (sulfur) is an element mixed as an unavoidable impurity, and the smaller the content of S, the better. From the viewpoint of manufacturability, the austenitic stainless steel may contain 0.015% by mass or less of S. If the S content is 0.015% by mass or less, the adverse effects on material properties such as ductility can be reduced in the austenitic stainless steel.
　（Ｎｉ）
　Ｎｉ（ニッケル）は、オーステナイト生成元素であり、またオーステナイト相を維持するために有効な元素である。オーステナイト系ステンレス鋼は、３．０質量％以上６．０質量％未満のＮｉを含み、３．５質量％以上５．５質量％以下のＮｉを含むことが好ましく、４．０質量％以上５．０質量％未満のＮｉを含むことがより好ましい。Ｎｉの含有量が３．０質量％以上であれば、オーステナイト相の生成および維持が良好となる。Ｎｉの含有量が４．５質量％以上であればより好ましい。 (Ni)
Ni (nickel) is an austenite-generating element and an element effective for maintaining the austenite phase. The austenitic stainless steel contains 3.0% by mass or more and less than 6.0% by mass of Ni, preferably 3.5% by mass or more and 5.5% by mass or less of Ni, and 4.0% by mass or more and 5 More preferably, it contains less than 0.0% by mass of Ni. When the Ni content is 3.0% by mass or more, the austenite phase is well formed and maintained. More preferably, the Ni content is 4.5% by mass or more.
　一方、Ｎｉは高価な元素であり、また、過剰に添加するとオーステナイト相の安定化により加工誘起マルテンサイト相の生成量を低減させる。そのため、Ｎｉの含有量は６．０質量％未満とし、５．５質量％以下とすることが好ましく、５．０質量％未満とすることがより好ましい。 On the other hand, Ni is an expensive element, and when added in excess, it stabilizes the austenite phase and reduces the amount of deformation-induced martensite phase produced. Therefore, the Ni content is less than 6.0% by mass, preferably 5.5% by mass or less, and more preferably less than 5.0% by mass.
　（Ｃｒ）
　Ｃｒ（クロム）は、オーステナイト系ステンレス鋼の耐食性を確保するために有効な元素である。オーステナイト系ステンレス鋼は、１６．０質量％以上１８．５質量％以下のＣｒを含み、１６．５質量％以上１８．０質量％以下のＣｒを含むことが好ましい。Ｃｒの含有量が１６．０質量％以上であれば、オーステナイト系ステンレス鋼の耐食性を良好に確保できる。Ｃｒの含有量が１６．５質量％以上であればより好ましい。 (Cr)
Cr (chromium) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. The austenitic stainless steel contains 16.0% by mass or more and 18.5% by mass or less of Cr, preferably 16.5% by mass or more and 18.0% by mass or less of Cr. If the Cr content is 16.0% by mass or more, good corrosion resistance of the austenitic stainless steel can be ensured. More preferably, the Cr content is 16.5% by mass or more.
　一方で、ＣｒはＳｉと同様にフェライト生成元素でもあるため、Ｃｒを過剰添加すると、δフェライト相が過剰に生成してしまう。そのため、Ｃｒの含有量は１８．５質量％以下とし、１８．０質量％以下とすることが好ましい。 On the other hand, since Cr is also a ferrite forming element like Si, excessive addition of Cr results in excessive formation of the δ ferrite phase. Therefore, the Cr content is 18.5% by mass or less, preferably 18.0% by mass or less.
　（Ｃｕ）
　Ｃｕは、オーステナイト生成元素であり、またオーステナイト相を維持するために有効な元素である。また、Ｃｕリッチ相の析出によるオーステナイト系ステンレス鋼の高強度化にも有効である。Ｃｕは、結晶粒微細化にも効果的に作用する元素である。これは、Ｃｕリッチ相が結晶粒成長の阻害効果を示すためと考えられる。また、Ｃｕは、固溶状態ではオーステナイト相の加工硬化を低減するため、オーステナイト系ステンレス鋼の製造工程における圧延負荷を低減できる。 (Cu)
Cu is an austenite-forming element and an element effective for maintaining the austenite phase. It is also effective in increasing the strength of austenitic stainless steel by precipitation of Cu-rich phases. Cu is an element that effectively acts also for crystal grain refinement. This is probably because the Cu-rich phase exhibits an inhibitory effect on grain growth. In addition, Cu reduces the work hardening of the austenite phase in a solid solution state, so that the rolling load in the manufacturing process of the austenitic stainless steel can be reduced.
　オーステナイト系ステンレス鋼は、１．５質量％以上４．０質量％以下のＣｕを含み、２．０質量％以上３．５質量％以下のＣｕを含むことが好ましく、２．０質量％超３．５質量％以下のＣｕを含むことがより好ましい。Ｃｕの含有量が１．５質量％以上であれば、オーステナイト相の生成および維持が良好になるとともに、Ｃｕリッチ相の析出が良好に得られる。Ｃｕの含有量が２．０質量％以上であればより好ましく、２．０質量％超であればさらに好ましい。 The austenitic stainless steel contains 1.5% by mass or more and 4.0% by mass or less of Cu, preferably 2.0% by mass or more and 3.5% by mass or less of Cu, and more than 2.0% by mass. It is more preferable to contain 0.5% by mass or less of Cu. When the Cu content is 1.5% by mass or more, the austenite phase is well formed and maintained, and the Cu-rich phase is well precipitated. More preferably, the Cu content is 2.0% by mass or more, and even more preferably more than 2.0% by mass.
　一方で、Ｃｕを過剰添加すると、スラブが凝固する過程において当該スラブの中心にＣｕＭｎ相が生成してしまい、スラブの熱間加工性が低下する。そのため、Ｃｕの含有量は４．０質量％以下とし、３．５質量％以下とすることが好ましい。 On the other hand, if Cu is excessively added, a CuMn phase is generated at the center of the slab during the solidification process of the slab, which reduces the hot workability of the slab. Therefore, the Cu content is 4.0% by mass or less, preferably 3.5% by mass or less.
　（Ｎ）
　Ｎ（窒素）は、オーステナイト生成元素であり、また固溶強化作用および耐食性向上作用を有する元素である。オーステナイト系ステンレス鋼は、溶接部の耐食性を確保するためにＣの含有量を０．０３質量％以下としていることから、Ｎの含有量を０．０８質量％以上とし、０．１０質量％以上であることが好ましく、０．１１質量％以上であることがより好ましく、０．１２質量％以上がさらに好ましい。このようなＮの含有量であれば、オーステナイト系ステンレス鋼に要求される強度および耐食性の確保に有効である。 (N)
N (nitrogen) is an austenite forming element, and is an element having a solid-solution strengthening effect and an effect of improving corrosion resistance. Since the austenitic stainless steel has a C content of 0.03% by mass or less in order to ensure corrosion resistance of the weld zone, the N content is 0.08% by mass or more, and 0.10% by mass or more. is preferably 0.11% by mass or more, and even more preferably 0.12% by mass or more. Such an N content is effective in ensuring the strength and corrosion resistance required for austenitic stainless steel.
　また、Ｎを過度に添加すると、オーステナイト系ステンレス鋼の圧延負荷が増加する。したがって、Ｎの含有量は０．２５質量％以下とし、０．２０質量％以下とすることが好ましい。 Also, excessive addition of N increases the rolling load of the austenitic stainless steel. Therefore, the N content is 0.25% by mass or less, preferably 0.20% by mass or less.
　（その他の元素）
　オーステナイト系ステンレス鋼は、上述の元素に加えて、質量％で、Ｍｏ：１．０％以下、Ｗ：１．０％以下、Ｖ：０．５％以下、Ｂ：０．０００１％以上０．０１％以下、Ｃｏ：０．８％以下、Ｓｎ：０．１％以下、Ｃａ：０．０３％以下、Ｍｇ：０．０３％以下、Ｔｉ：０．５％以下、Ｎｂ：０．５％以下、Ａｌ：０．３％以下、Ｓｂ：０．５％以下、Ｚｒ：０．５％以下、Ｔａ：０．０３％以下、Ｈｆ：０．０３％以下およびＲＥＭ（希土類金属）：０．２％以下から選択される１種以上をさらに含有してもよい。 (other elements)
In addition to the above elements, the austenitic stainless steel contains, by mass%, Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, and B: 0.0001% or more and 0.0001% or less. 01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% Below, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metal): 0.5% or less. It may further contain one or more selected from 2% or less.
　（Ｍｏ、Ｗ、Ｖ）
　Ｍｏ（モリブデン）、Ｗ（タングステン）およびＶ（バナジウム）は、耐食性の向上に有効な元素である。一方、Ｍｏ、ＷおよびＶはフェライト生成元素であり、高価な元素でもあることから、過剰な添加は好ましくない。したがって、オーステナイト系ステンレス鋼は、１．０質量％以下のＭｏ、１．０質量％以下のＷ、および０．５質量％以下のＶから選択される１種以上を含むことが好ましい。 (Mo, W, V)
Mo (molybdenum), W (tungsten) and V (vanadium) are elements effective in improving corrosion resistance. On the other hand, Mo, W and V are ferrite-forming elements and are also expensive elements, so excessive addition is not preferable. Therefore, the austenitic stainless steel preferably contains one or more selected from Mo of 1.0% by mass or less, W of 1.0% by mass or less, and V of 0.5% by mass or less.
　（Ｂ）
　Ｂ（ホウ素）は、熱間加工性を改善する元素であり、熱間圧延における耳切れおよび二枚割れの発生の低減に有効な元素である。オーステナイト系ステンレス鋼は、０．０００１質量％以上０．０１質量％以下のＢを含むことが好ましい。Ｂの含有量が０．０００１質量％以上であれば、熱間加工性の改善および熱間圧延における耳切れおよび二枚割れの発生の低減に有効である。ただし、Ｃｒが含まれるオーステナイト系ステンレス鋼へのＢの過剰添加は、Ｃｒ２Ｂの析出による耐食性の低下を招く。したがって、Ｂの含有量は０．０１質量％以下であることが好ましい。 (B)
B (boron) is an element that improves hot workability, and is an element that is effective in reducing the occurrence of edge splitting and double cracking in hot rolling. The austenitic stainless steel preferably contains 0.0001% by mass or more and 0.01% by mass or less of B. If the B content is 0.0001% by mass or more, it is effective in improving hot workability and reducing edge splitting and split cracking in hot rolling. However, excessive addition of B to an austenitic stainless steel containing Cr causes deterioration of corrosion resistance due to precipitation of Cr 2 B. Therefore, the content of B is preferably 0.01% by mass or less.
　（Ｃｏ）
　Ｃｏ（コバルト）は、オーステナイト系ステンレス鋼の耐食性を確保するために有効な元素である。また、Ｃｕリッチ相の粗大化を低減させて微細に維持することにも寄与する。このような効果を得るためには、Ｃｏを０．１０質量％以上含有させることが好ましい。ただし、Ｃｏは高価な元素であり、コスト低減の観点から、Ｃｏの含有量は０．８質量％以下とすることが好ましい。 (Co)
Co (cobalt) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. It also contributes to reducing the coarsening of the Cu-rich phase and maintaining it fine. In order to obtain such an effect, it is preferable to contain 0.10% by mass or more of Co. However, Co is an expensive element, and from the viewpoint of cost reduction, the Co content is preferably 0.8% by mass or less.
　（Ｓｎ）
　Ｓｎ（スズ）は、オーステナイト系ステンレス鋼の耐食性を確保するために有効な元素である。ただし、Ｓｎの過剰添加はオーステナイト系ステンレス鋼の熱間加工性の低下を招いてしまうことから、Ｓｎの含有量は０．１質量％以下とすることが好ましい。 (Sn)
Sn (tin) is an effective element for ensuring the corrosion resistance of austenitic stainless steel. However, since excessive addition of Sn causes deterioration of the hot workability of the austenitic stainless steel, the Sn content is preferably 0.1% by mass or less.
　（Ａｌ、Ｃａ、Ｍｇ、Ｔｉ）
　Ａｌ（アルミニウム）、Ｃａ（カルシウム）、Ｍｇ（マグネシウム）およびＴｉ（チタン）は、いずれも脱酸作用を有する元素である。オーステナイト系ステンレス鋼は、脱酸剤として、０．３質量％以下のＡｌ、０．０３質量％以下のＣａ、０．０３質量％以下のＭｇ、および０．５質量％以下のＴｉから選択される１種以上を含むことが好ましい。 (Al, Ca, Mg, Ti)
Al (aluminum), Ca (calcium), Mg (magnesium) and Ti (titanium) are all deoxidizing elements. The austenitic stainless steel is selected from 0.3 wt% or less Al, 0.03 wt% or less Ca, 0.03 wt% or less Mg, and 0.5 wt% or less Ti as a deoxidizing agent. It is preferable to include one or more of
　（Ｎｂ）
　Ｎｂ（ニオブ）は、オーステナイト系ステンレス鋼の鋭敏化の低減に有効な元素である。また、組織の微細化および均一化にも有効である。オーステナイト系ステンレス鋼は、０．５質量％以下のＮｂを含むことが好ましい。 (Nb)
Nb (niobium) is an element effective in reducing sensitization of austenitic stainless steel. It is also effective for making the structure fine and uniform. The austenitic stainless steel preferably contains 0.5% by mass or less of Nb.
　（Ｓｂ、Ｚｒ、Ｔａ、Ｈｆ、ＲＥＭ）
　Ｓｂ（アンチモン）、Ｚｒ（ジルコニウム）、Ｔａ（タンタル）、Ｈｆ（ハフニウム）およびＲＥＭ（希土類金属）はいずれも、熱間加工性を改善すると共に、耐酸化性にも有効な元素である。オーステナイト系ステンレス鋼は、０．５質量％以下のＳｂ、０．５質量％以下のＺｒ、０．０３質量％以下のＴａ、０．０３質量％以下のＨｆ、および０．２質量％以下のＲＥＭから選択される１種以上を含むことが好ましい。 (Sb, Zr, Ta, Hf, REM)
Sb (antimony), Zr (zirconium), Ta (tantalum), Hf (hafnium) and REM (rare earth metal) are all elements that improve hot workability and are also effective in oxidation resistance. The austenitic stainless steel contains up to 0.5% by weight Sb, up to 0.5% by weight Zr, up to 0.03% by weight Ta, up to 0.03% by weight Hf, and up to 0.2% by weight It preferably contains one or more selected from REM.
　〔Ｍｄ３０の値〕
　オーステナイト系ステンレス鋼は、下記式（１）により示すＭｄ３０の値が０．０以上８０．０以下であり、２０．０以上７０．０以下であることが好ましい。 [Value of Md 30 ]
The austenitic stainless steel has an Md30 value of 0.0 or more and 80.0 or less, preferably 20.0 or more and 70.0 or less, as indicated by the following formula (1).
　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。 Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
　オーステナイト系ステンレス鋼において、Ｍｄ３０の値は、オーステナイト相単相のオーステナイト系ステンレス鋼に対し３０％の引張り歪みを与えた時に、オーステナイト系ステンレス鋼の組織の５０％がマルテンサイト相に変態する温度（℃）を示す。このため、Ｍｄ３０の値は、オーステナイト相の安定度の指標として用いることができる。また、Ｍｄ３０の値は、オーステナイト系ステンレス鋼においてＴＲＩＰ現象の生じやすさに影響する指標としても用いることができる。 In austenitic stainless steel, the value of Md30 is the temperature at which 50% of the structure of the austenitic stainless steel transforms into the martensitic phase when 30% tensile strain is applied to the austenitic stainless steel of the austenitic single phase. (°C). Therefore, the value of Md30 can be used as an indicator of the stability of the austenite phase. In addition, the value of Md30 can also be used as an index that influences the likelihood of the TRIP phenomenon occurring in austenitic stainless steel.
　本発明の一実施形態に係るオーステナイト系ステンレス鋼のＭｄ３０の値は、０．０以上８０．０以下であることが好ましい。Ｍｄ３０の値は、その値が大きいほど、オーステナイト相から加工誘起マルテンサイト相への変態が起こりやすく、軽度の冷延ひずみの付与で高強度が得られると共に、優れた延性を確保できる。また、オーステナイト系ステンレス鋼に成形加工が施される場合にも、曲げ部等の加工歪みが付与された部分は、ＴＲＩＰ現象によりさらに高い強度を得られやすい。 The value of Md30 of the austenitic stainless steel according to one embodiment of the present invention is preferably 0.0 or more and 80.0 or less. The higher the value of Md 30 , the easier the transformation from the austenite phase to the deformation-induced martensite phase occurs, and the application of mild cold-rolling strain can provide high strength and ensure excellent ductility. Further, even when the austenitic stainless steel is subjected to forming processing, the portion to which processing strain is imparted, such as a bent portion, tends to obtain higher strength due to the TRIP phenomenon.
　また、オーステナイト系ステンレス鋼の製造過程において、仕上焼鈍によって結晶粒を微細化するためには、仕上焼鈍前の圧延材における加工誘起マルテンサイト相の存在が有効に作用する。このような効果は、Ｍｄ３０の値が０．０以上の場合に顕著に現れる。また、Ｍｄ３０の値が８０．０を超えると、ＴＲＩＰ現象が過剰に起こりやすくなり、オーステナイト系ステンレス鋼の特性が安定しにくい。 In the process of manufacturing austenitic stainless steel, the presence of deformation-induced martensite phase in the rolled material before final annealing works effectively to refine grains by final annealing. Such an effect appears remarkably when the value of Md30 is 0.0 or more. Moreover, when the value of Md 30 exceeds 80.0, the TRIP phenomenon tends to occur excessively, and the properties of the austenitic stainless steel are difficult to stabilize.
　したがって、オーステナイト相の安定度の指標であるＭｄ３０の値が０．０以上８０．０以下であれば、高強度でかつ良好な延性を備えるオーステナイト系ステンレス鋼を安定して製造できる。 Therefore, if the value of Md30 , which is an index of stability of the austenite phase, is 0.0 or more and 80.0 or less, austenitic stainless steel having high strength and good ductility can be stably produced.
　なお、従来知られているＭｄ３０の成分回帰式では、ＮｉおよびＣｕの係数に同じ値を用いることが一般的である。一方、本発明の一実施形態では、Ｍｄ３０の成分回帰式において、Ｎｉの係数よりもＣｕの係数を小さく設定している。従来の知見によるＭｄ３０の成分回帰式は、省Ｎｉ型ではないオーステナイト系ステンレス鋼での実績に基づくものが多い。これに対して、本発明のような省Ｎｉ型成分では、オーステナイト相の安定化に及ぼすＣｕの影響が、従来の知見に比べて明らかに小さいことが判明した。これは、本発明者らによる鋭意検討の結果得られた新規な知見であり、当該知見に基づいて、Ｍｄ３０の成分回帰式におけるＣｕの係数を設定している。これにより、Ｃｕの含有量の調整が容易となり、オーステナイト系ステンレス鋼の製造自由度が大きくなる。 Incidentally, in the conventionally known component regression equation of Md 30 , it is common to use the same value for the coefficients of Ni and Cu. On the other hand, in one embodiment of the present invention, the coefficient of Cu is set smaller than the coefficient of Ni in the component regression equation of Md 30 . Many of the component regression equations of Md 30 based on conventional knowledge are based on the results of austenitic stainless steels that are not Ni-saving types. On the other hand, in the Ni-saving type composition of the present invention, it has been found that the influence of Cu on the stabilization of the austenite phase is clearly smaller than the conventional knowledge. This is a novel finding obtained as a result of diligent studies by the present inventors, and based on this finding, the coefficient of Cu in the component regression equation for Md 30 is set. This facilitates adjustment of the Cu content and increases the degree of freedom in manufacturing the austenitic stainless steel.
　〔製造方法〕
　本発明の一実施形態に係るオーステナイト系ステンレス鋼の製造方法は、質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、上記（１）式で示すＭｄ３０の値が０．０以上８０．０以下であるオーステナイト系ステンレス鋼の製造方法である。また、オーステナイト系ステンレス鋼の製造方法は、仕上焼鈍工程を含む。 〔Production method〕
A method for producing an austenitic stainless steel according to an embodiment of the present invention includes, in mass %, C: 0.005% to 0.03%, Si: 0.1% to 2.0%, Mn: 0 .3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% Below, Cu: 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, Md 30 represented by the above formula (1) is 0.0 or more and 80.0 or less. Also, the method for producing austenitic stainless steel includes a finish annealing step.
　オーステナイト系ステンレス鋼の製造方法は、仕上焼鈍工程以外の工程については、一般的なオーステナイト系ステンレス鋼の製造工程を含んでよい。以下に、本発明の一実施形態に係るオーステナイト系ステンレス鋼の製造方法の一例を示すが、これに限られるものではない。 The method for manufacturing austenitic stainless steel may include a general manufacturing process for austenitic stainless steel for processes other than the final annealing process. An example of a method for producing an austenitic stainless steel according to one embodiment of the present invention is shown below, but the present invention is not limited to this.
　本発明の一実施形態に係るオーステナイト系ステンレス鋼の製造方法では、例えば、成分を調整した溶鋼を連続鋳造することによってスラブを製造する。そして、連続鋳造により製造したスラブを１１００℃以上１３００℃以下に加熱した後、熱間圧延を施して熱延鋼帯を製造する。熱間圧延後の、ひずみの少ないオーステナイト相からのＣｕリッチ相の析出速度は遅い。そのため、熱間圧延後における熱延鋼帯の仕上温度および巻取温度は一般的なオーステナイト系ステンレス鋼の製造方法と同様の条件でよい。仕上焼鈍までのＣｕリッチ相の析出を極力減らす観点からは、熱間圧延後における熱延鋼帯の巻取温度は８５０℃以下が好ましく、６５０℃以下がさらに好ましい。 In the method for producing austenitic stainless steel according to one embodiment of the present invention, for example, a slab is produced by continuously casting molten steel whose composition is adjusted. Then, the slab produced by continuous casting is heated to 1100° C. or higher and 1300° C. or lower, and then hot rolled to produce a hot rolled steel strip. The precipitation rate of the Cu-rich phase from the less strained austenite phase after hot rolling is slow. Therefore, the finishing temperature and coiling temperature of the hot-rolled steel strip after hot rolling may be the same conditions as in the general method for producing austenitic stainless steel. From the viewpoint of minimizing the precipitation of the Cu-rich phase until the final annealing, the coiling temperature of the hot-rolled steel strip after hot rolling is preferably 850° C. or lower, more preferably 650° C. or lower.
　熱間圧延を施した熱延鋼帯に酸洗を行ってもよい。なお、熱延鋼帯の酸洗前に焼鈍を施してもよく、焼鈍を施さずに酸洗を行ってもよい。熱延鋼帯の酸洗前に焼鈍を施す場合、焼鈍温度は９００℃以上１１５０℃以下の範囲の温度が好ましく、Ｃｕを完全に固溶状態とするためには、９８０℃以上１１５０℃以下の範囲の温度で行うことがより好ましいが、上述の範囲に限定されない。そして、酸洗後の熱延鋼帯に、所定の板厚になるまで冷間圧延を施して冷延鋼帯を得る。 The hot-rolled steel strip that has been hot-rolled may be pickled. The hot-rolled steel strip may be annealed before pickling, or may be pickled without annealing. When the hot-rolled steel strip is annealed before pickling, the annealing temperature is preferably in the range of 900° C. or higher and 1150° C. or lower. It is more preferable to operate at a temperature within the range, but is not limited to the above range. Then, the pickled hot-rolled steel strip is cold-rolled to a predetermined thickness to obtain a cold-rolled steel strip.
　オーステナイト系ステンレス鋼の製造方法では、冷間圧延工程後の仕上焼鈍工程において再結晶およびＣｕリッチ相の析出が同時に進行する。Ｃｕリッチ相は、加工誘起マルテンサイト相から特に析出しやすいことから、冷間圧延工程は、冷延鋼帯における加工誘起マルテンサイト相が全体の２０体積％以上の割合となるような圧延率および圧延温度により行うことが好ましい。このような冷間圧延工程を行うことで、その後の仕上焼鈍工程にて鋼帯にＣｕリッチ相を効果的に析出させることができる。 　In the method of manufacturing austenitic stainless steel, recrystallization and precipitation of the Cu-rich phase proceed simultaneously in the finish annealing process after the cold rolling process. Since the Cu-rich phase is particularly likely to precipitate from the strain-induced martensite phase, the cold rolling process should be performed at a rolling reduction and a rate such that the strain-induced martensite phase in the cold-rolled steel strip accounts for 20% by volume or more of the total. It is preferable to carry out by rolling temperature. By performing such a cold rolling process, a Cu-rich phase can be effectively precipitated in the steel strip in the subsequent finish annealing process.
　なお、オーステナイト系ステンレス鋼は、Ｍｄ３０の値を０．０以上８０．０以下に調整する。このようなＭｄ３０の値を有するオーステナイト系ステンレス鋼は、冷延鋼帯における加工誘起マルテンサイト相の量を問わず、本発明の一実施形態において規定する量のＣｕリッチ相が析出する。しかしながら、必要に応じて冷間圧延工程の圧延率を高める、冷間圧延工程の温度を低く制御する等は、Ｃｕリッチ相の析出にさらに有効である。 In the austenitic stainless steel, the value of Md30 is adjusted to 0.0 or more and 80.0 or less. An austenitic stainless steel having such a value of Md 30 precipitates a Cu-rich phase in the amount specified in one embodiment of the present invention, regardless of the amount of deformation-induced martensitic phase in the cold-rolled steel strip. However, increasing the rolling reduction in the cold-rolling process, controlling the temperature in the cold-rolling process to be low, etc. as necessary are more effective for the precipitation of the Cu-rich phase.
　冷延鋼帯において、加工誘起マルテンサイト相を２０体積％以上とする観点から、例えば、冷間圧延工程における圧延率は４０％以上であることが好ましく、５０％以上であることがより好ましく、６０％以上であることがさらに好ましい。また、冷間圧延工程における温度は、９０℃以下であることが好ましく、６０℃以下であることがより好ましい。 In the cold-rolled steel strip, from the viewpoint of making the deformation-induced martensite phase 20% by volume or more, for example, the rolling reduction in the cold rolling step is preferably 40% or more, more preferably 50% or more, It is more preferably 60% or more. Also, the temperature in the cold rolling step is preferably 90° C. or lower, more preferably 60° C. or lower.
　（仕上焼鈍工程）
　冷延鋼帯には、仕上焼鈍が施される。仕上焼鈍工程は、Ｃｕリッチ相の析出が進行する条件により実施する。Ｃｕリッチ相はオーステナイト系ステンレス鋼の高強度化に有効である。そのため、Ｃｕリッチ相を析出させる前の熱延鋼帯および冷延鋼帯では強度が低めとなっており、冷間圧延工程における圧延負荷を低減できる。そして、仕上焼鈍工程によりＣｕリッチ相が析出することで、仕上焼鈍後のオーステナイト系ステンレス鋼では高強度が得られる。 (Finish annealing process)
The cold-rolled steel strip is subjected to finish annealing. The finish annealing step is carried out under conditions that promote the precipitation of the Cu-rich phase. A Cu-rich phase is effective in increasing the strength of austenitic stainless steel. Therefore, the strength of the hot-rolled steel strip and the cold-rolled steel strip before precipitation of the Cu-rich phase is rather low, and the rolling load in the cold rolling process can be reduced. Then, the Cu-rich phase is precipitated in the final annealing step, so that the austenitic stainless steel after the final annealing has high strength.
　また、Ｃｕリッチ相の析出は、オーステナイト相の再結晶粒の微細化にも有効である。そのため、Ｃｕリッチ相の析出を利用して、平均結晶粒径を１０．０μｍ以下に制御できる。 In addition, the precipitation of the Cu-rich phase is also effective in refining the recrystallized grains of the austenite phase. Therefore, the precipitation of the Cu-rich phase can be used to control the average crystal grain size to 10.0 μm or less.
　このように、本発明の一実施形態に係るオーステナイト系ステンレス鋼の製造方法によれば、製造時の加工負荷の低減と最終製品の高強度化とを高い次元で両立できる。また、従来のように、Ｃｕリッチ相の析出に時効処理工程という追加の工程を要さないため、オーステナイト系ステンレス鋼の生産性も良好である。 Thus, according to the method for producing austenitic stainless steel according to one embodiment of the present invention, it is possible to achieve both a reduction in the processing load during production and an increase in the strength of the final product at a high level. In addition, the productivity of the austenitic stainless steel is also good because the additional step of the aging treatment is not required for the precipitation of the Cu-rich phase unlike the conventional method.
　仕上焼鈍工程における仕上焼鈍の温度は、オーステナイト系ステンレス鋼にＣｕリッチ相が効果的に析出するように、７５０℃以上９８０℃以下とし、８００℃以上９２５℃以下とすることが好ましい。仕上焼鈍の温度が７５０℃未満であれば、組織の再結晶が不十分となる。また、仕上焼鈍の温度が９８０℃を超える場合、Ｃｕリッチ相が溶解してしまうため、仕上焼鈍後に残存するＣｕリッチ相の量が不十分となる。 The finish annealing temperature in the finish annealing step is preferably 750°C or higher and 980°C or lower, and preferably 800°C or higher and 925°C or lower so that the Cu-rich phase is effectively precipitated in the austenitic stainless steel. If the final annealing temperature is less than 750°C, recrystallization of the structure will be insufficient. Moreover, when the temperature of the final annealing exceeds 980° C., the Cu-rich phase dissolves, so the amount of the Cu-rich phase remaining after the final annealing becomes insufficient.
　また、加工誘起マルテンサイト相から析出するＣｕリッチ相は特に、仕上焼鈍において８５０℃以上の温度で長時間保持されると、オーステナイト相に溶解しやすくなる。そのため、仕上焼鈍工程の最高到達温度が８５０℃以上である場合、８５０℃以上で加熱する時間を短くすることが好ましい。具体的には、仕上焼鈍工程の最高到達温度が８５０℃以上である場合、８５０℃以上で加熱する時間を３０秒以下とし、１５秒以下とすることが好ましい。「８５０℃以上で加熱する時間」とは、仕上焼鈍工程において８５０℃以上となる時間が複数回に分かれる場合、当該複数回の合計時間を示す。 In addition, the Cu-rich phase that precipitates from the deformation-induced martensite phase is particularly likely to dissolve in the austenite phase if it is held at a temperature of 850°C or higher for a long time in the final annealing. Therefore, when the maximum temperature reached in the final annealing step is 850°C or higher, it is preferable to shorten the heating time at 850°C or higher. Specifically, when the maximum temperature reached in the final annealing step is 850° C. or higher, the heating time at 850° C. or higher is set to 30 seconds or less, preferably 15 seconds or less. The term "heating time at 850° C. or higher" refers to the total time of the plurality of heating times when the final annealing step is divided into multiple times for heating to 850° C. or higher.
　オーステナイト系ステンレス鋼はＣ量が０．０３質量％以下であるため、冷却中のＣｒ炭化物の析出は起こりにくい。そのため、仕上焼鈍後の冷却速度は、一般的なステンレス鋼の製造方法と同様であってよい。生産性の観点からすると冷却速度は速い方が好ましいとは言えるが、例えば７００℃から５００℃までの平均冷却速度が１℃／秒以上の、比較的遅い速度であってもよく、生産性を考慮すれば５℃／秒以上が好ましい。また、鋼板の平坦度を考慮すると冷却速度は７５℃／秒未満が好ましく、５０℃／秒以下がより好ましい。 　Austenitic stainless steel has a C content of 0.03% by mass or less, so precipitation of Cr carbide during cooling is unlikely to occur. Therefore, the cooling rate after finish annealing may be the same as in a general stainless steel manufacturing method. From the viewpoint of productivity, it can be said that a faster cooling rate is preferable, but the average cooling rate from 700 ° C. to 500 ° C. may be a relatively slow speed of 1 ° C./sec or more, for example. Taking this into consideration, 5° C./second or more is preferable. Also, considering the flatness of the steel sheet, the cooling rate is preferably less than 75° C./second, more preferably 50° C./second or less.
　なお、冷間圧延工程において、必要に応じて中間焼鈍および中間圧延を行なってもよい。また、仕上焼鈍後の鋼帯についてさらに強度を高めるため、必要に応じて調質圧延を実施してもよい。中間焼鈍の温度は、圧延負荷低減を優先する場合はＣｕリッチ相の析出を避けるために９８０℃以上、１１５０℃以下が好ましい。析出処理を繰り返すことでの高強度化を狙う上では、中間焼鈍の温度は仕上焼鈍と同条件が好ましい。なお、中間焼鈍の温度は上述の範囲に限定されない。 In the cold rolling process, intermediate annealing and intermediate rolling may be performed as necessary. Further, in order to further increase the strength of the steel strip after finish annealing, skin pass rolling may be performed as necessary. The temperature of the intermediate annealing is preferably 980° C. or higher and 1150° C. or lower in order to avoid precipitation of the Cu-rich phase when priority is given to reducing the rolling load. In order to increase the strength by repeating the precipitation treatment, it is preferable that the temperature of the intermediate annealing is the same as that of the finish annealing. In addition, the temperature of the intermediate annealing is not limited to the above range.
　〔強度評価〕
　本発明の一実施形態に係るオーステナイト系ステンレス鋼は、製造工程における強度を低めにして圧延負荷を低減し、かつ、製造後における高強度化を実現するものである。オーステナイト系ステンレス鋼におけるこのような特性は、例えば、０．２％耐力（ＹＳ１８％、ＭＰａ）と参考強度（ＨＶ６０％）との関係により表すことができる。 [Strength evaluation]
An austenitic stainless steel according to an embodiment of the present invention has a relatively low strength in the manufacturing process to reduce the rolling load and achieves high strength after manufacturing. Such properties of austenitic stainless steel can be expressed, for example, by the relationship between 0.2% proof stress (YS18%, MPa) and reference strength (HV60%).
　０．２％耐力（ＹＳ１８％）は、オーステナイト系ステンレス鋼の強度の指標である。０．２％耐力（ＹＳ１８％）は、オーステナイト系ステンレス鋼の仕上焼鈍後に、伸びが１８％となる調質圧延をさらに施した場合の０．２％耐力を示すものである。０．２％耐力は、ＪＩＳ　Ｚ２２４１に準拠する方法を用いて評価することができる。 　0.2% proof stress (YS18%) is an indicator of the strength of austenitic stainless steel. The 0.2% yield strength (YS18%) indicates the 0.2% yield strength when the austenitic stainless steel is further subjected to temper rolling with an elongation of 18% after finish annealing. The 0.2% yield strength can be evaluated using a method conforming to JIS Z2241.
　参考強度（ＨＶ６０％）は、仕上焼鈍工程においてＣｕリッチ相を析出させる前のオーステナイト系ステンレス鋼の強度を、仮定的に示す指標である。参考強度（ＨＶ６０％）は、オーステナイト系ステンレス鋼の成分組成は同じだが、製造方法を本発明の一実施形態に係る製造方法から一部変更し、熱間圧延後に１０５０℃での焼鈍を施し、６０％の圧延率により冷間圧延を施した場合のビッカース硬さを示す。すなわち、参考強度（ＨＶ６０％）は、本発明の一実施形態に係るオーステナイト系ステンレス鋼の強度を示すものではなく、例えば評価用に作製した鋼帯の強度であってよい。ビッカース硬さは、ＪＩＳ　Ｚ２２４４準拠のビッカース硬さ試験方法に基づいて測定できる。 The reference strength (HV60%) is an index that hypothetically indicates the strength of the austenitic stainless steel before precipitation of the Cu-rich phase in the final annealing process. For the reference strength (HV60%), although the chemical composition of the austenitic stainless steel is the same, the manufacturing method is partially changed from the manufacturing method according to the embodiment of the present invention, and after hot rolling, annealing at 1050 ° C. is performed. It shows the Vickers hardness when cold rolling is applied at a rolling reduction of 60%. That is, the reference strength (HV60%) does not indicate the strength of the austenitic stainless steel according to one embodiment of the present invention, but may be the strength of a steel strip produced for evaluation, for example. Vickers hardness can be measured based on the Vickers hardness test method conforming to JIS Z2244.
　本発明者らは、製造時の加工負荷の低減と最終製品の高強度化とを両立するオーステナイト系ステンレス鋼は、０．２％耐力（ＹＳ１８％）と、参考強度（ＨＶ６０％）との関係が下記式（２）を満たすことを見出した。 The present inventors have found that the austenitic stainless steel, which achieves both a reduction in the processing load during manufacturing and a high strength of the final product, has a relationship between the 0.2% proof stress (YS18%) and the reference strength (HV60%). satisfies the following formula (2).
　ＹＳ１８％≧３．７５ＨＶ６０％－５７５　（２）
　本発明の一実施形態に係る製造方法によれば、上記式（２）を満たし、製造時の加工負荷の低減と最終製品の高強度化とが両立したオーステナイト系ステンレス鋼を製造できる。 YS18%≧3.75HV60%-575 (2)
According to the production method of one embodiment of the present invention, it is possible to produce an austenitic stainless steel that satisfies the above formula (2) and achieves both a reduction in processing load during production and a high strength of the final product.
　〔好適な用途〕
　オーステナイト系ステンレス鋼は、非常に高い強度および耐食性を有する。したがって、オーステナイト系ステンレス鋼は、例えば、シリンダヘッドガスケット、ぜんまいばね、電子機器部品用ばね、電車車両部材、車載電池フレーム材、構造材およびメタルパッキン等の、高い強度および耐食性が要求されるばね製品の素材として好適である。特に、オーステナイト系ステンレス鋼は、溶接された場合でも、溶接部の耐食性（溶接性）に優れる。そのため、電車車両部材または溶接利用のため製造する車載電池フレーム材のような、溶接構造が比較的多くなる用途であっても、本発明の一実施形態に係るオーステナイト系ステンレス鋼であれば好適に利用できる。 [Preferred uses]
Austenitic stainless steel has very high strength and corrosion resistance. Therefore, austenitic stainless steel is used for spring products that require high strength and corrosion resistance, such as cylinder head gaskets, spiral springs, springs for electronic device parts, train vehicle members, automotive battery frame materials, structural materials, and metal packings. It is suitable as a material for In particular, austenitic stainless steel is excellent in corrosion resistance (weldability) even when welded. Therefore, the austenitic stainless steel according to one embodiment of the present invention is suitable even for applications in which a relatively large number of welded structures are used, such as train vehicle members or automotive battery frame materials manufactured for welding. Available.
　〔まとめ〕
　本発明の態様１に係るオーステナイト系ステンレス鋼は、質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、２０体積％以上のオーステナイト相と、個数密度が１．０×１０３個・μｍ－３以上の、長径３０ｎｍ以下のＣｕリッチ相とを含み、残部が加工誘起マルテンサイト相および不可避的形成相からなり、下記（１）式で示すＭｄ３０の値が０．０以上８０．０以下である：
　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。 〔summary〕
The austenitic stainless steel according to aspect 1 of the present invention has, in mass %, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: Contains 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, 20% by volume or more of the austenitic phase, and a number density of 1 .0×10 3 μm −3 or more and a Cu-rich phase with a major axis of 30 nm or less, and the balance consists of a deformation-induced martensite phase and an unavoidable formation phase, and the value of Md 30 shown by the following formula (1) is 0.0 or more and 80.0 or less:
Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
　本発明の態様２に係るオーステナイト系ステンレス鋼は、上記態様１において、質量％で、Ｍｏ：１．０％以下、Ｗ：１．０％以下、Ｖ：０．５％以下、Ｂ：０．０００１％以上０．０１％以下、Ｃｏ：０．８％以下、Ｓｎ：０．１％以下、Ｃａ：０．０３％以下、Ｍｇ：０．０３％以下、Ｔｉ：０．５％以下、Ｎｂ：０．５％以下、Ａｌ：０．３％以下、Ｓｂ：０．５％以下、Ｚｒ：０．５％以下、Ｔａ：０．０３％以下、Ｈｆ：０．０３％以下おおよびＲＥＭ（希土類金属）：０．２％以下から選択される１種以上をさらに含有していてもよい。 The austenitic stainless steel according to aspect 2 of the present invention, in aspect 1 above, contains Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, and B: 0.5% by mass. 0001% or more and 0.01% or less, Co: 0.8% or less, Sn: 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb : 0.5% or less, Al: 0.3% or less, Sb: 0.5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less and REM ( rare earth metal): may further contain one or more selected from 0.2% or less.
　本発明の態様３に係るオーステナイト系ステンレス鋼は、上記態様１または２において、平均結晶粒径が１０．０μｍ以下であってもよい。 The austenitic stainless steel according to aspect 3 of the present invention, in aspect 1 or 2 above, may have an average crystal grain size of 10.0 μm or less.
　本発明の態様４に係るオーステナイト系ステンレス鋼の製造方法は、質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、下記（１）式で示すＭｄ３０の値が０．０以上８０．０以下であるオーステナイト系ステンレス鋼の製造方法であって、７５０℃以上９８０℃以下の温度により仕上焼鈍を行う仕上焼鈍工程を含み、前記仕上焼鈍工程での最高到達温度が８５０℃以上である場合、８５０℃以上で加熱する時間を３０秒以内とし、前記仕上焼鈍工程において、前記仕上焼鈍後の７００℃から５００℃までの平均冷却速度を１℃／秒以上とする：
　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。 The method for producing an austenitic stainless steel according to aspect 4 of the present invention comprises, by mass %, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.1% or more, and 0.03% or less; 3% or more and 2.5% or less, P: 0.04% or less, S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less , Cu: 1.5% or more and 4.0% or less and N: 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, Md 30 represented by the following formula (1) A method for producing austenitic stainless steel having a value of 0.0 or more and 80.0 or less, comprising a finish annealing step of performing finish annealing at a temperature of 750° C. or more and 980° C. or less, wherein the final annealing step achieves the maximum When the temperature is 850° C. or higher, the heating time at 850° C. or higher is 30 seconds or less, and the average cooling rate from 700° C. to 500° C. after the finish annealing is 1° C./second or more in the final annealing step. do:
Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
　本発明は上述した各実施形態に限定されるものではなく、請求項に示した範囲で種々の変更が可能であり、異なる実施形態にそれぞれ開示された技術的手段を適宜組み合わせて得られる実施形態についても本発明の技術的範囲に含まれる。 The present invention is not limited to the above-described embodiments, but can be modified in various ways within the scope of the claims, and can be obtained by appropriately combining technical means disclosed in different embodiments. is also included in the technical scope of the present invention.
　本発明の発明例および比較例に係るオーステナイト系ステンレス鋼について評価した結果について、以下に説明する。 The results of evaluating the austenitic stainless steels according to invention examples and comparative examples of the present invention are described below.
　〔評価の条件〕
　＜成分組成＞
　本発明の一実施例に係るオーステナイト系ステンレス鋼（発明鋼Ａ１～Ａ１５）および比較例に係るオーステナイト系ステンレス鋼（比較鋼Ｂ１～Ｂ５）の成分組成（質量％）およびＭｄ３０の値を、下記表１に示す。Ｍｄ３０の値は、上記式（１）により算出した。なお、下記表１において下線が付されている値は、本発明の規定範囲外であることを示す。下記表２についても同様である。 [Evaluation conditions]
<Component composition>
The chemical compositions (% by mass) and Md 30 values of the austenitic stainless steels according to the examples of the present invention (invention steels A1 to A15) and the austenitic stainless steels according to the comparative examples (comparative steels B1 to B5) are shown below. Table 1 shows. The value of Md 30 was calculated by the above formula (1). The underlined values in Table 1 below are out of the specified range of the present invention. The same applies to Table 2 below.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
　＜製造方法＞
　本発明の各実施例および比較例に係るオーステナイト系ステンレス鋼は、次に示す方法により製造した。表１に示す成分組成を有するオーステナイト系ステンレス鋼を溶製し、本発明の一実施例に係る製造方法（発明例Ｃ１～Ｃ８）または比較例に係る製造方法（比較例Ｄ１、Ｄ２）により、熱間圧延から仕上焼鈍までを行って、冷延焼鈍材を得た。各製造方法の条件について、下記表２に示す。 <Manufacturing method>
Austenitic stainless steels according to each of the examples and comparative examples of the present invention were produced by the following method. Austenitic stainless steel having the chemical composition shown in Table 1 was melted, and the production method according to one example of the present invention (invention examples C1 to C8) or the production method according to the comparative example (comparative examples D1 and D2), From hot rolling to finish annealing, a cold-rolled annealed material was obtained. The conditions of each manufacturing method are shown in Table 2 below.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
　仕上焼鈍工程では、仕上焼鈍温度が８５０℃以上となる場合には、８５０℃以上となる時間について、表２に示す通りに調整した。なお、発明例Ｃ３では、仕上焼鈍の温度が８５０℃に達した時点で温度が低下し始めるように加熱を調整したが、便宜上、表２では８５０℃以上となる時間を「１秒」として記載している。 In the finish annealing process, when the finish annealing temperature was 850°C or higher, the time for reaching 850°C or higher was adjusted as shown in Table 2. In invention example C3, the heating was adjusted so that the temperature began to decrease when the final annealing temperature reached 850 ° C., but for convenience, in Table 2, the time to reach 850 ° C. or higher is described as "1 second". are doing.
　＜評価方法＞
　本発明の各実施例および比較例に係るオーステナイト系ステンレス鋼の各種指標について、以下に示す通り評価を実施した。 <Evaluation method>
Various indices of the austenitic stainless steel according to each of the examples and comparative examples of the present invention were evaluated as follows.
　（Ｃｕリッチ相の個数密度）
　各条件により製造した冷延焼鈍材から、電解研磨法にてＴＥＭサンプルを作製した。ＴＥＭサンプルにおける、冷延焼鈍材の圧延方向に平行な面について、４００ｎｍ×４００ｎｍの範囲を３視野観察した。ＴＥＭ画像のコントラストからＣｕリッチ相を判別し、Ｃｕリッチ相の個数を計測した。ＴＥＭサンプルの厚さを１５０ｎｍとみなし、単位体積あたりの個数密度を求めた。Ｃｕリッチ相が粗大化すると、コントラストではなく明瞭な形状で観察されるようになった。長径が３０ｎｍを超えるＣｕリッチ相は、計測から除外した。 (Number density of Cu-rich phase)
A TEM sample was produced by an electropolishing method from the cold-rolled annealed material produced under each condition. A plane parallel to the rolling direction of the cold-rolled and annealed material in the TEM sample was observed in three fields of view in a range of 400 nm×400 nm. Cu-rich phases were determined from the contrast of the TEM image, and the number of Cu-rich phases was counted. The thickness of the TEM sample was assumed to be 150 nm, and the number density per unit volume was determined. When the Cu-rich phase coarsened, it became observed in a clear shape instead of contrast. Cu-rich phases with longer diameters greater than 30 nm were excluded from the measurements.
　（結晶粒径）
　平均結晶粒径は、ＥＢＳＤ法を用いて評価した。各条件により製造した冷延焼鈍材の、圧延方向に平行かつ圧延面に垂直な断面に対して機械研磨後に電解研磨を施した。その後、倍率２０００倍の視野で、当該断面における４０μｍ×４０μｍの範囲について、ステップ間隔０．２μｍでＥＢＳＤ分析を行った。Σ３対応粒界を満たす方位関係における方位差について、方位差１°以下の焼鈍双晶は除いて、方位差２°以上の境界を粒界とみなし、個々の結晶粒の面積をＳ（μｍ２）、当該結晶粒と同じ面積を有する円の直径をＤ（μｍ）とし下記式（３）により結晶粒径を算出した。これを５視野について行い、当該５視野で得られた結晶粒径の平均を、平均結晶粒径として算出した。 (Crystal grain size)
The average grain size was evaluated using the EBSD method. A cross section parallel to the rolling direction and perpendicular to the rolling surface of the cold-rolled annealed material manufactured under each condition was mechanically polished and then electrolytically polished. After that, EBSD analysis was performed on a 40 μm×40 μm range of the cross section with a step interval of 0.2 μm in a field of view with a magnification of 2000×. With regard to the misorientation in the orientation relationship satisfying the Σ3 correspondence grain boundary, except for annealing twins with misorientation of 1° or less, boundaries with misorientation of 2° or more are regarded as grain boundaries, and the area of each grain is S (μm 2 ), and the diameter of a circle having the same area as the crystal grain was defined as D (μm), and the crystal grain size was calculated by the following formula (3). This was performed for 5 fields of view, and the average of the grain sizes obtained in the 5 fields of view was calculated as the average grain size.
　結晶粒径＝｛Σ（Ｄ×Ｓ）｝／４０×４０　（３）
（マルテンサイト相の量）
　マルテンサイト相の量（体積％）は、板厚が１．５ｍｍ以上の場合はそのまま、１．５ｍｍ未満の場合は合計で１．５ｍｍ以上となるように冷間圧延後の材料または調質圧延後の材料を重ねた。これらの材料について、フェライトスコープ（Ｆｉｓｃｈｅｒ製ＦＭＰ３０、電磁誘導法）により計測し、測定値を０．７４７５で除した値をマルテンサイト相の量とした。オーステナイト相の量（体積％）は、オーステナイト系ステンレス鋼のマトリクス全体を１００体積％として、そこからマルテンサイト相の量を差し引いた値とみなした。オーステナイト系ステンレス鋼における、Ｃｕリッチ相および不可避的形成相の量は、割合が小さく正確な計測が困難であるため、外数として計算してよい。 Crystal grain size = {Σ(D × S)}/40 × 40 (3)
(Amount of martensite phase)
The amount of martensite phase (% by volume) is as it is when the plate thickness is 1.5 mm or more, and when the plate thickness is less than 1.5 mm, the material after cold rolling or temper rolling is adjusted so that the total is 1.5 mm or more. Later materials were layered. These materials were measured with a ferrite scope (Fischer FMP30, electromagnetic induction method), and the value obtained by dividing the measured value by 0.7475 was taken as the amount of martensite phase. The amount (% by volume) of the austenite phase was regarded as a value obtained by subtracting the amount of the martensite phase from 100% by volume of the entire matrix of the austenitic stainless steel. The amounts of Cu-rich phases and unavoidably formed phases in austenitic stainless steel may be calculated as extraneous numbers because their ratios are small and accurate measurement is difficult.
　（引張特性）
　引張特性の指標として、伸びが１８％となる調質圧延を施した場合の０．２％耐力（ＹＳ１８％）を評価した。０．２％耐力（ＹＳ１８％）は、ＪＩＳ１３号Ｂ試験片を作製し、ＪＩＳ　Ｚ２２４１に準じた引張試験により測定した。０．２％耐力（ＹＳ１８％）は、クロスヘッド速度３ｍｍ／ｍｉｎにより測定した。 (tensile properties)
As an index of tensile properties, the 0.2% proof stress (YS18%) when subjected to temper rolling with an elongation of 18% was evaluated. The 0.2% proof stress (YS18%) was measured by a tensile test according to JIS Z2241 by preparing a JIS No. 13B test piece. 0.2% proof stress (YS18%) was measured at a crosshead speed of 3 mm/min.
　（強度）
　本発明の一実施例および比較例の各条件において製造条件を一部変更し、熱延鋼帯に対する焼鈍を１０５０℃にて実施し、圧延率を６０％として冷間圧延を施した６０％圧延材のビッカース硬さを、参考強度（ＨＶ６０％）として測定した。ビッカース硬さは、６０％圧延材の表面についてビッカース硬さ試験機によってビッカース硬さ試験（ＪＩＳ　Ｚ２２４４）を行い測定した。ビッカース硬さ試験における測定時の荷重は１０ｋｇとした。 (Strength)
60% rolling in which the manufacturing conditions were partially changed from the conditions of the examples and comparative examples of the present invention, and the hot rolled steel strip was annealed at 1050 ° C. and cold rolled at a rolling reduction of 60%. The Vickers hardness of the material was measured as a reference strength (HV60%). The Vickers hardness was measured by performing a Vickers hardness test (JIS Z2244) with a Vickers hardness tester on the surface of the 60% rolled material. The load at the time of measurement in the Vickers hardness test was 10 kg.
　（溶接部の耐食性）
　板厚１．５ｍｍの冷延焼鈍材に対して、電極径１．６ｍｍ、溶接速度７０ｃｍ／分、溶接電流９０Ａにて、Ａｒガスシールを施した条件でＴＩＧなめ付け溶接を施した。溶接部を含む１０ｍｍ×１０ｍｍの範囲を評価面とし、皮膜影響を除去するために＃６００研磨を施した上で、電気化学的な再活性化率を指標として、評価面の耐食性を評価した。 (Corrosion resistance of weld zone)
A cold-rolled annealed material having a plate thickness of 1.5 mm was subjected to TIG tanning welding at an electrode diameter of 1.6 mm, a welding speed of 70 cm/min, and a welding current of 90 A under the conditions of an Ar gas seal. A 10 mm × 10 mm area including the weld was used as an evaluation surface, and #600 polishing was applied to remove the influence of the film, and the corrosion resistance of the evaluation surface was evaluated using the electrochemical reactivation rate as an index.
　再活性化率は、ＪＩＳ　Ｇ０５８０に準じて測定した。具体的には、液温３０℃の０．５ｍｏｌ／Ｌ硫酸、０．０１ｍｏｌ／Ｌチオシアン酸カリウム水溶液中で、自然電位から０．３Ｖ（ｖｓＳＣＥ）まで、掃引速度１００ｍＶ／ｍｉｎで分極させた（以下、「往路」）。０．３Ｖ（ｖｓＳＣＥ）に到達後、往路とは逆方向に電位を掃引し、熱延材の再活性化後に、再びアノード電流が０となる電位で掃引を終了した（以下、「復路」）。 The reactivation rate was measured according to JIS G0580. Specifically, in a 0.5 mol/L sulfuric acid and 0.01 mol/L potassium thiocyanate aqueous solution at a liquid temperature of 30°C, polarization was performed from the spontaneous potential to 0.3 V (vs SCE) at a sweep rate of 100 mV/min ( hereinafter referred to as the “outward route”). After reaching 0.3 V (vs SCE), the potential was swept in the direction opposite to the forward trip, and after reactivation of the hot-rolled material, the sweep was terminated at a potential at which the anode current became 0 again (hereinafter, "return trip"). .
　往路の最大電流密度ｉａと、復路の最大電流密度ｉｒとの比（ｉｒ／ｉａ）を、再活性化率として算出した。このような評価方法は、耐食性を評価するための方法である鋭敏化判定方法としては厳しいものであるため、再活性化率が例えば１．５％程度であっても実環境では問題ないと考えられる。しかしながら、本発明の一実施例に係る冷延焼鈍材は微細な結晶粒を有し得るため、耐食性の評価が困難であることを考慮し、再活性化率１％以下であれば、好ましい耐食性を有しているといえる。したがって、溶接部の耐食性について、再活性化率１％以下の場合を「Ｏ」（良好）、１％を超えた場合を「×」（不良）と評価した。 The ratio (ir/ia) of the maximum current density ia on the outward trip and the maximum current density ir on the return trip was calculated as the reactivation rate. Such an evaluation method is strict as a sensitization determination method for evaluating corrosion resistance, so even if the reactivation rate is, for example, about 1.5%, it is considered that there is no problem in the actual environment. be done. However, since the cold-rolled annealed material according to one example of the present invention may have fine crystal grains, it is difficult to evaluate corrosion resistance. It can be said that it has Therefore, the corrosion resistance of the weld zone was evaluated as "O" (good) when the reactivation rate was 1% or less, and as "x" (poor) when the reactivation rate exceeded 1%.
　〔結果〕
　発明鋼Ａ２について、表２に示す各条件により得られた冷延焼鈍材のＣｕリッチ相析出量および結晶粒径を下記表３に示す。また、各条件における０．２％耐力（ＹＳ１８％）、並びに、冷間圧延後（仕上焼鈍前）および仕上焼鈍後に伸びが１８％となる調質圧延を施した後のマルテンサイト相の量についても下記表３に示す。 〔result〕
Table 3 below shows the amount of precipitated Cu-rich phase and the grain size of the cold-rolled annealed material obtained under the conditions shown in Table 2 for the invention steel A2. In addition, regarding the 0.2% yield strength (YS18%) under each condition, and the amount of martensite phase after cold rolling (before finish annealing) and after temper rolling at which the elongation is 18% after finish annealing. are also shown in Table 3 below.
　なお、下記表３において下線は、Ｃｕリッチ相の析出量が本発明の規定範囲外であることを示す。 The underlines in Table 3 below indicate that the precipitation amount of the Cu-rich phase is outside the specified range of the present invention.
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
　発明鋼Ａ２について、発明例Ｃ１～Ｃ８の各条件により製造した冷延焼鈍材は、Ｃｕリッチ相の析出量が本発明の規定範囲内であり、１０．０μｍ以下の微細な平均結晶粒径を示した。一方、比較例Ｄ１、Ｄ２の各条件により製造した冷延焼鈍材は、いずれもＣｕリッチ相の析出が見られなかった。 For the invention steel A2, the cold-rolled annealed materials produced under the respective conditions of invention examples C1 to C8 have a Cu-rich phase precipitation amount within the specified range of the present invention, and have a fine average grain size of 10.0 μm or less. Indicated. On the other hand, precipitation of the Cu-rich phase was not observed in any of the cold-rolled annealed materials produced under the conditions of Comparative Examples D1 and D2.
　発明鋼Ａ２について、発明例Ｃ２の条件により製造した冷延焼鈍材について、図１左側にＥＢＳＤ粒界マップを、図１右側にＴＥＭ撮像画像をそれぞれ示す。図１右側のＴＥＭ撮像画像に示すように、本発明の一実施形態に係るオーステナイト系ステンレス鋼において、Ｃｕリッチ相（図１において「Ｃｕ」として図示）の析出が観察された。 Regarding the invention steel A2, the EBSD grain boundary map is shown on the left side of FIG. 1, and the TEM imaged image is shown on the right side of FIG. As shown in the TEM image on the right side of FIG. 1, precipitation of a Cu-rich phase (indicated as “Cu” in FIG. 1) was observed in the austenitic stainless steel according to one embodiment of the present invention.
　また、発明鋼Ａ２の参考強度（ＨＶ６０％）は４４５であったことから、０．２％耐力（ＹＳ１８％）は上記式（２）に基づいて、１０９４ＭＰａ以上であることが好ましい。発明例Ｃ１～Ｃ８の各条件により製造した発明鋼Ａ２の冷延焼鈍材は、いずれも０．２％耐力（ＹＳ１８％）が１０９４ＭＰａ以上であった。一方、比較例Ｄ１、Ｄ２の各条件により製造した冷延焼鈍材は、いずれも０．２％耐力（ＹＳ１８％）が１０９４ＭＰａよりも低かった。このように、比較例Ｄ１、Ｄ２の各条件ではＣｕリッチ相が析出しないため、仕上焼鈍前の加工性と、仕上焼鈍後の高強度とのバランスが良好なオーステナイト系ステンレス鋼を得ることは困難であることが示された。 Also, since the reference strength (HV60%) of Inventive Steel A2 was 445, the 0.2% yield strength (YS18%) is preferably 1094 MPa or more based on the above formula (2). The 0.2% proof stress (YS18%) of the cold-rolled annealed materials of the invention steel A2 manufactured under the respective conditions of invention examples C1 to C8 was 1094 MPa or more. On the other hand, the 0.2% proof stress (YS18%) of the cold-rolled annealed materials manufactured under the respective conditions of Comparative Examples D1 and D2 was lower than 1094 MPa. Thus, under the conditions of Comparative Examples D1 and D2, Cu-rich phases do not precipitate, so it is difficult to obtain an austenitic stainless steel with a good balance between workability before final annealing and high strength after final annealing. was shown to be
　次に、発明鋼Ａ１～Ａ１５または比較鋼Ｂ１～Ｂ５から、発明例Ｃ２に示す製造条件により製造した冷延焼鈍材の、仕上焼鈍後のＣｕリッチ相析出量および結晶粒径を下記表４に示す。また、これらの各条件における０．２％耐力（ＹＳ１８％）、参考強度（ＨＶ６０％）および溶接部の耐食性についても、下記表４に示す。 Next, Table 4 below shows the Cu-rich phase precipitation amount and grain size after finish annealing of the cold-rolled and annealed materials manufactured from the invention steels A1 to A15 or the comparative steels B1 to B5 under the manufacturing conditions shown in invention example C2. show. The 0.2% proof stress (YS18%), reference strength (HV60%), and corrosion resistance of the weld under these conditions are also shown in Table 4 below.
　なお、下記表４において下線が付されている部分は、Ｃｕリッチ相の析出量が本発明の規定範囲外、０．２％耐力（ＹＳ１８％）が上記式（２）を満たさない値であること、または溶接部の耐食性が不良であることを示す。 The underlined portion in Table 4 below indicates that the precipitation amount of the Cu-rich phase is outside the specified range of the present invention, and the 0.2% proof stress (YS18%) is a value that does not satisfy the above formula (2). or that the corrosion resistance of the weld is poor.
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
　発明鋼Ａ１～Ａ１５の冷延焼鈍材は、Ｃｕリッチ相の析出量が本発明の規定範囲内であり、１０．０μｍ以下の微細な平均結晶粒径を示した。また、０．２％耐力（ＹＳ１８％）について、いずれも上記式（２）を満たす良好な値を示した。 The cold-rolled and annealed materials of the invention steels A1 to A15 had a precipitation amount of the Cu-rich phase within the specified range of the present invention, and exhibited a fine average crystal grain size of 10.0 μm or less. In addition, the 0.2% proof stress (YS18%) showed good values satisfying the above formula (2).
　一方、比較鋼Ｂ１の冷延焼鈍材は、溶接部の耐食性が不良であった。比較鋼Ｂ２～Ｂ５の冷延焼鈍材は、０．２％耐力（ＹＳ１８％）が上記式（２）を満たさず、仕上焼鈍前の加工性と、仕上焼鈍後の高強度とのバランスが良好なオーステナイト系ステンレス鋼が得られなかった。 On the other hand, the cold-rolled and annealed material of comparative steel B1 had poor corrosion resistance at the weld. The cold-rolled annealed materials of comparative steels B2 to B5 do not satisfy the above formula (2) in terms of 0.2% proof stress (YS18%), and have a good balance between workability before finish annealing and high strength after finish annealing. Austenitic stainless steel was not obtained.
　図２に、表４の各条件における、０．２％耐力（ＹＳ１８％）と参考強度（ＨＶ６０％）との関係をプロットした図を示す。図２では、本発明の一実施例を白抜きの丸により、比較例を黒塗りの矢頭によりそれぞれ示す。図２に示すグラフにおいて、左上にプロットされるほど、仕上焼鈍前の加工性と、仕上焼鈍後の高強度とのバランスが良好である。 Fig. 2 shows a plot of the relationship between the 0.2% proof stress (YS18%) and the reference strength (HV60%) under each condition in Table 4. In FIG. 2, an example of the present invention is indicated by a white circle, and a comparative example is indicated by a black arrowhead. In the graph shown in FIG. 2, the higher the upper left plot, the better the balance between the workability before the finish annealing and the high strength after the finish annealing.
　なお、上述の結果は何れも、仕上焼鈍後の冷却速度が２５℃／秒の条件により得られた冷延焼鈍材の結果である。ここで、発明鋼Ａ１、Ａ２、Ａ５を用い、表２に示す発明例Ｃ２の条件において、仕上焼鈍後の、７００℃から５００℃までの冷却速度を０．３～１００℃／秒の範囲内で変更して、冷延焼鈍材を製造した。得られた冷延焼鈍材のＣｕリッチ相の析出量を、下記表５に示す。 All of the above results are for cold-rolled annealed materials obtained under the condition that the cooling rate after final annealing is 25°C/sec. Here, using invention steels A1, A2, and A5, under the conditions of invention example C2 shown in Table 2, the cooling rate from 700 ° C. to 500 ° C. after finish annealing was within the range of 0.3 to 100 ° C./sec. A cold-rolled annealed material was produced by changing the Table 5 below shows the precipitation amount of the Cu-rich phase in the obtained cold-rolled annealed material.
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
　冷却速度が５℃／秒以上では、冷延焼鈍材におけるＣｕ析出量に変化はなかった。冷却速度が十分に速いと、冷却中のＣｕリッチ相の粗大化と、それに伴うＣｕリッチ相の消失は起こらないといえる。冷却速度が２℃／秒になると僅かながらＣｕ析出量が減少した。これは冷却中にＣｕリッチ相の粗大化と、それに伴うＣｕリッチ相の消失が起こったためと考えられる。これは、一般にオストワルト成長と呼ばれる現象である。冷却速度が１℃／秒未満の場合は冷却中のＣｕリッチ相の粗大化と、それに伴うＣｕリッチ相の消失がより進み、析出量が１．０×１０３個・μｍ－３未満となった。 At a cooling rate of 5° C./second or more, there was no change in the amount of Cu precipitation in the cold-rolled annealed material. If the cooling rate is sufficiently high, it can be said that coarsening of the Cu-rich phase during cooling and accompanying disappearance of the Cu-rich phase do not occur. When the cooling rate was 2° C./second, the amount of Cu precipitation decreased slightly. It is considered that this is because the Cu-rich phase coarsened during cooling and disappeared along with the Cu-rich phase. This is a phenomenon commonly called Ostwald growth. When the cooling rate is less than 1° C./sec, the Cu-rich phase coarsens during cooling and disappears accordingly, and the precipitation amount becomes less than 1.0×10 3 pieces·μm −3 . Ta.
　表４および図２に示すように、本発明の一実施形態に係る組成を有するオーステナイト系ステンレス鋼を用いて、本発明の一実施形態に係る製造方法により製造した冷延焼鈍材は、製造時の加工負荷の低減および製品の高強度化を両立することが示された。また、このような冷延焼鈍材は、溶接部の耐食性も優れており、溶接が多く施される用途にも好適であることが示された。 As shown in Table 4 and FIG. 2, the cold-rolled annealed material manufactured by the manufacturing method according to one embodiment of the present invention using the austenitic stainless steel having the composition according to one embodiment of the present invention is It was shown that both the reduction of the processing load and the increase in strength of the product are compatible. It was also shown that such a cold-rolled annealed material is excellent in corrosion resistance of welded parts and suitable for applications in which many weldings are performed.

Claims (4)

  1. 　質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、
    　２０体積％以上のオーステナイト相と、個数密度が１．０×１０３個・μｍ－３以上の、長径３０ｎｍ以下のＣｕリッチ相とを含み、残部が加工誘起マルテンサイト相および不可避的形成相からなり、
    　下記（１）式で示すＭｄ３０の値が０．０以上８０．０以下である、オーステナイト系ステンレス鋼：
    　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
    　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。     % by mass, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less , S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: containing 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities,
    20% by volume or more of an austenite phase, and a Cu-rich phase with a number density of 1.0×10 3 μm −3 or more and a major axis of 30 nm or less, and the balance being a deformation-induced martensite phase and an unavoidably formed phase. become,
    Austenitic stainless steel having an Md 30 value of 0.0 or more and 80.0 or less, represented by the following formula (1):
    Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
    The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
  2. 　質量％で、Ｍｏ：１．０％以下、Ｗ：１．０％以下、Ｖ：０．５％以下、Ｂ：０．０００１％以上０．０１％以下、Ｃｏ：０．８％以下、Ｓｎ：０．１％以下、Ｃａ：０．０３％以下、Ｍｇ：０．０３％以下、Ｔｉ：０．５％以下、Ｎｂ：０．５％以下、Ａｌ：０．３％以下、Ｓｂ：０．５％以下、Ｚｒ：０．５％以下、Ｔａ：０．０３％以下、Ｈｆ：０．０３％以下およびＲＥＭ（希土類金属）：０．２％以下から選択される１種以上をさらに含有する、請求項１に記載のオーステナイト系ステンレス鋼。 % by mass, Mo: 1.0% or less, W: 1.0% or less, V: 0.5% or less, B: 0.0001% or more and 0.01% or less, Co: 0.8% or less, Sn : 0.1% or less, Ca: 0.03% or less, Mg: 0.03% or less, Ti: 0.5% or less, Nb: 0.5% or less, Al: 0.3% or less, Sb: 0 .5% or less, Zr: 0.5% or less, Ta: 0.03% or less, Hf: 0.03% or less, and REM (rare earth metal): 0.2% or less The austenitic stainless steel according to claim 1, wherein
  3. 　平均結晶粒径が１０．０μｍ以下である、請求項１または２に記載のオーステナイト系ステンレス鋼。 The austenitic stainless steel according to claim 1 or 2, having an average grain size of 10.0 µm or less.
  4. 　質量％で、Ｃ：０．００５％以上０．０３％以下、Ｓｉ：０．１％以上２．０％以下、Ｍｎ：０．３％以上２．５％以下、Ｐ：０．０４％以下、Ｓ：０．０１５％以下、Ｎｉ：３．０％以上６．０％未満、Ｃｒ：１６．０％以上１８．５％以下、Ｃｕ：１．５％以上４．０％以下およびＮ：０．０８％以上０．２５％以下を含有し、残部はＦｅおよび不可避的不純物からなり、下記（１）式で示すＭｄ３０の値が０．０以上８０．０以下であるオーステナイト系ステンレス鋼の製造方法であって、
    　７５０℃以上９８０℃以下の温度により仕上焼鈍を行う仕上焼鈍工程を含み、
    　前記仕上焼鈍工程での最高到達温度が８５０℃以上である場合、８５０℃以上で加熱する時間を３０秒以内とし、
    　前記仕上焼鈍工程において、前記仕上焼鈍後の７００℃から５００℃までの平均冷却速度を１℃／秒以上とする、オーステナイト系ステンレス鋼の製造方法：
    　Ｍｄ３０＝５５１－４６２（Ｃ＋Ｎ）－９．２Ｓｉ－８．１Ｍｎ－２９Ｎｉ－１０．６Ｃｕ－１３．７Ｃｒ－１８．５Ｍｏ　（１）
    　上記（１）式の元素記号の箇所には、上記オーステナイト系ステンレス鋼が含有している各元素の含有量（質量％）が代入され、無添加の元素については０が代入される。     % by mass, C: 0.005% or more and 0.03% or less, Si: 0.1% or more and 2.0% or less, Mn: 0.3% or more and 2.5% or less, P: 0.04% or less , S: 0.015% or less, Ni: 3.0% or more and less than 6.0%, Cr: 16.0% or more and 18.5% or less, Cu: 1.5% or more and 4.0% or less, and N: Austenitic stainless steel containing 0.08% or more and 0.25% or less, the balance being Fe and unavoidable impurities, and having a value of Md 30 represented by the following formula (1) of 0.0 or more and 80.0 or less A manufacturing method of
    Including a finish annealing step of performing finish annealing at a temperature of 750 ° C or higher and 980 ° C or lower,
    When the maximum temperature reached in the final annealing step is 850° C. or higher, the heating time at 850° C. or higher is set to 30 seconds or less,
    A method for producing austenitic stainless steel, wherein in the finish annealing step, the average cooling rate from 700° C. to 500° C. after the finish annealing is 1° C./second or more:
    Md 30 =551-462(C+N)-9.2Si-8.1Mn-29Ni-10.6Cu-13.7Cr-18.5Mo (1)
    The content (% by mass) of each element contained in the austenitic stainless steel is substituted for the symbol of the element in the formula (1), and 0 is substituted for the non-additive element.
PCT/JP2023/001835 2022-02-10 2023-01-23 Austenitic stainless steel and method for producing austenitic stainless steel WO2023153184A1 (en)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH04191352A (en) * 1990-11-26 1992-07-09 Nisshin Steel Co Ltd Gasket material for internal combustion engine excellent in settling resistance
WO2016047734A1 (en) * 2014-09-25 2016-03-31 新日鐵住金株式会社 Austenitic stainless steel sheet and method for producing same
JP2017206725A (en) * 2016-05-17 2017-11-24 Ｊｆｅスチール株式会社 Ferritic stainless steel and manufacturing method therefor
JP2022064692A (en) * 2020-10-14 2022-04-26 日鉄ステンレス株式会社 Austenitic stainless steel and method for producing austenitic stainless steel

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH04191352A (en) * 1990-11-26 1992-07-09 Nisshin Steel Co Ltd Gasket material for internal combustion engine excellent in settling resistance
WO2016047734A1 (en) * 2014-09-25 2016-03-31 新日鐵住金株式会社 Austenitic stainless steel sheet and method for producing same
JP2017206725A (en) * 2016-05-17 2017-11-24 Ｊｆｅスチール株式会社 Ferritic stainless steel and manufacturing method therefor
JP2022064692A (en) * 2020-10-14 2022-04-26 日鉄ステンレス株式会社 Austenitic stainless steel and method for producing austenitic stainless steel

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