Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/10—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
• • 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
  • C21D9/08—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for tubular bodies or pipes
• • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

• This disclosure relates to stainless steel materials.
• the stainless steel material used in such CO2 storage technology is required to have a yield strength of, for example, 80 ksi or more (552 MPa or more).
• a yield strength of 80 ksi or more 552 MPa or more.
• stainless steel materials having a yield strength of 80 ksi or more have been proposed in JP 2011-190521 A (Patent Document 1) and JP 2012-149317 A (Patent Document 2).
• This martensitic stainless steel material has a yield strength of 400 to 800 MPa.
• CO 2 carbon dioxide
• SOx is a general term for sulfur oxides such as SO 2.
• NOx is a general term for nitrogen oxides such as NO 2.
• SOx and NOx dissolve in water to form acidic compounds (sulfuric acid, sulfurous acid, nitric acid, nitrous acid, etc.), which cause general corrosion and cracks on the steel surface.
• supercritical CO2 containing SOx, NOx, and O2 forms an extremely severe corrosive environment.
• the corrosive environment formed by supercritical CO2 containing SOx, NOx, and O2 is referred to as a "supercritical corrosive environment.”
• steel materials used in a supercritical corrosive environment are required to have better general corrosion resistance and stress corrosion cracking resistance than conventional corrosive environments.
• stainless steel materials assumed to be applied to CO2 storage technology in such cryogenic environments are required to have high strength, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, as well as excellent low-temperature toughness in cryogenic environments of -70°C or lower.
• the stainless steel materials disclosed in Patent Documents 1 and 2 are not assumed to be used in such supercritical corrosion environments or cryogenic environments.
• the objective of this disclosure is to provide a stainless steel material that has high strength, excellent resistance to general corrosion and stress corrosion cracking in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.
• the stainless steel material according to the present disclosure is In mass percent, C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: more than 18.0 to 21.5%, Ni: more than 5.00 to 8.00%, Mo: more than 0.60 to 3.50%, Al: 0.005-0.050%, V: 0.01-0.30%, N: 0.0030-0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00% O: 0.020% or less, W: 0-2.00%, Nb: 0 to 0.150%, Co: 0 to 0.80%, B: 0 to 0.0050%, Ca: 0-0.0050%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, and The balance is composed of Fe and impurities.
• Fn1 defined by formula (1) is 150.0 or more
• Fn2 defined by formula (2) is 24.0 or more
• the microstructure has a volume fraction of 30.0 to 85.0% ferrite phase, 0.1 to 40.0% retained austenite phase, and the balance martensite phase;
• the yield strength is 552 to 758 MPa;
• the amount of precipitated V is 0.005 to 0.130 mass %.
• Fn1 576.5-2660.7 ⁇ C-7.8 ⁇ Cr-11.3 ⁇ Mo-20.9 ⁇ Ni-10.6 ⁇ Cu (1)
• Fn2 2 ⁇ Cr+2 ⁇ Mo+19 ⁇ V+28 ⁇ N-887 ⁇ C (2)
• the element symbols in formulas (1) and (2) are substituted with the contents of the corresponding elements in mass %.
• the stainless steel material disclosed herein has high strength, excellent resistance to general corrosion and stress corrosion cracking in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.
• the present inventors first investigated how to obtain a stainless steel material having a yield strength of 552 to 758 MPa , assuming application to CO2 storage technology. That is, the present inventors investigated and examined a method for obtaining a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment, and excellent low-temperature toughness in an extremely low-temperature environment in a stainless steel material assumed to be applied to CO2 storage technology. As a result, the present inventors obtained the following findings.
• the inventors of the present invention focused on the chemical composition and investigated how to obtain the desired stainless steel material.
• the following composition was obtained: C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: over 18.0% to 21.5%, Ni: over 5.00% to 8.00%, Mo: over 0.60% to 3.50%, Al: 0.005 to 0.050%, V: 0.01 to 0.30%, N: 0.0030 to 0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00%, O: 0.020% or less, W: 0 It was believed that a stainless steel material consisting of Cu: 0.000-2.00%, Nb: 0-0.150%, Co: 0-0.80%, B: 0-0.0050%, Ca: 0-0.0050%, Mg: 0-0.0100%, rare earth elements: 0-0.0100%, and the balance being Fe and impurities, could potentially achieve
• the inventors of the present invention focused on the microstructure and investigated how to obtain the desired stainless steel material.
• the stainless steel material having the above-mentioned chemical composition has a microstructure consisting of a ferrite phase, a retained austenite phase, and the remainder being martensite phase.
• the ferrite phase increases the corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) of the steel material in a supercritical corrosion environment.
• the retained austenite phase increases the low-temperature toughness in an extremely low-temperature environment.
• the strength of the stainless steel material decreases.
• the martensite phase increases the strength of the stainless steel material.
• the toughness decreases.
• the microstructure is one consisting of 30.0 to 85.0% by volume of ferrite phase, 0.1 to 40.0% by volume of retained austenite phase, and the remainder being martensite phase, it is possible to obtain a yield strength of 552 to 758 MPa, excellent general corrosion resistance and stress corrosion cracking resistance in supercritical corrosion environments, and excellent low-temperature toughness in cryogenic environments.
• Fn1 is an index of the volume fraction of the retained austenite phase in a stainless steel material having the above-mentioned chemical composition. If Fn1 is 150.0 or more, the volume fraction of the retained austenite phase in the microstructure can be stably kept at 40.0% or less. As a result, the yield strength can be made 552 MPa or more. Therefore, in the stainless steel material according to this embodiment, Fn1 is set to 150.0 or more, assuming that the stainless steel material has the above-mentioned chemical composition.
• Fn2 is an index of the corrosion resistance of a stainless steel material having the above-mentioned chemical composition in a supercritical corrosion environment. If Fn2 is 24.0 or more, the corrosion resistance of the steel material in a supercritical corrosion environment can be improved. Therefore, in the stainless steel material according to this embodiment, which has the above-mentioned chemical composition, Fn2 is set to 24.0 or more, assuming that Fn1 is 150.0 or more.
• the inventors speculate that if the amount of precipitated V is 0.005 to 0.130 mass%, the desired yield strength, excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment, and excellent low-temperature toughness in an extremely low-temperature environment will be obtained.
• a stainless steel material In mass percent, C: 0.030% or less, Si: 1.00% or less, Mn: 0.30% or less, P: 0.030% or less, S: 0.0050% or less, Cr: more than 18.0 to 21.5%, Ni: more than 5.00 to 8.00%, Mo: more than 0.60 to 3.50%, Al: 0.005-0.050%, V: 0.01-0.30%, N: 0.0030-0.1000%, Ti: 0.100% or less, Cu: 0.01 to less than 1.00% O: 0.020% or less, W: 0-2.00%, Nb: 0 to 0.150%, Co: 0 to 0.80%, B: 0 to 0.0050%, Ca: 0-0.0050%, Mg: 0 to 0.0100%, Rare earth elements: 0 to 0.0100%, and The balance is composed of Fe and impurities.
• the shape of the stainless steel material according to this embodiment is not particularly limited.
• the stainless steel material according to this embodiment may be a steel pipe, a round bar (solid material), or a steel plate.
• Round bar means a steel bar with a circular cross section perpendicular to the axial direction.
• the steel pipe may be a seamless steel pipe or a welded steel pipe.
• the stainless steel material according to this embodiment will be described in detail below.
• the stainless steel material will also be simply referred to as “steel material.”
• general corrosion resistance and stress corrosion cracking resistance will also be collectively referred to as “corrosion resistance.”
• C 0.030% or less Carbon (C) is inevitably contained. That is, the lower limit of the C content is more than 0%. C combines with Cr and Mo to form carbides. Therefore, if the C content is too high, even if the contents of other elements are within the range of this embodiment, the amount of solid solution of Cr and Mo decreases, and the corrosion resistance of the steel material in a supercritical corrosion environment decreases. Therefore, the C content is 0.030% or less.
• the preferred upper limit of the C content is 0.025%, more preferably 0.020%, and even more preferably 0.010%.
• the C content is preferably as low as possible. However, an extreme reduction in the C content significantly increases the manufacturing cost. Therefore, when considering industrial production, the preferred lower limit of the C content is 0.001%, and even more preferably 0.003%.
• Si Silicon (Si) is inevitably contained. That is, the lower limit of the Si content is more than 0%. Si deoxidizes the steel. On the other hand, if the Si content is too high, the hot workability of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Si content is 1.00% or less.
• the preferred upper limit of the Si content is 0.95%, and more preferably 0.90%.
• the preferred lower limit of the Si content to more effectively obtain the above effects is 0.10%, more preferably 0.15%, more preferably 0.20%, and even more preferably 0.25%.
• Mn 0.30% or less Manganese (Mn) is inevitably contained. That is, the lower limit of the Mn content is more than 0%. Mn deoxidizes and desulfurizes the steel. Mn further improves the hot workability of the steel. On the other hand, if the Mn content is too high, even if the contents of other elements are within the range of this embodiment, the volume fraction of the retained austenite phase may become too high, and the strength of the steel may decrease. Therefore, the Mn content is 0.30% or less.
• the preferred upper limit of the Mn content is 0.25%, more preferably 0.20%, and even more preferably 0.15%.
• the preferred lower limit of the Mn content for more effectively obtaining the above effects is 0.01%, more preferably 0.02%, and even more preferably 0.03%.
• P 0.030% or less Phosphorus (P) is inevitably contained. That is, the lower limit of the P content is more than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, the corrosion resistance of the steel material in a supercritical corrosion environment decreases even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.030% or less.
• the preferred upper limit of the P content is 0.025%, more preferably 0.020%, and even more preferably 0.015%.
• the P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, and even more preferably 0.003%.
• S 0.0050% or less Sulfur (S) is inevitably contained. That is, the lower limit of the S content is more than 0%. S segregates at grain boundaries. Therefore, if the S content is too high, the corrosion resistance of the steel material in a supercritical corrosion environment decreases even if the contents of other elements are within the range of this embodiment. Therefore, the S content is 0.0050% or less.
• the preferred upper limit of the S content is 0.0040%, more preferably 0.0030%.
• the S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0004%.
• O 0.020% or less
• Oxygen (O) is inevitably contained. That is, the lower limit of the O content is more than 0%. O forms oxides. Therefore, if the O content is too high, the corrosion resistance of the steel material in a supercritical corrosion environment decreases even if the contents of other elements are within the range of this embodiment. Therefore, the O content is 0.020% or less.
• the preferred upper limit of the O content is 0.018%, more preferably 0.016%, and even more preferably 0.014%.
• the O content is preferably as low as possible. However, an extreme reduction in the O content increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the O content is 0.001%, more preferably 0.002%, and even more preferably 0.003%.
• the remainder of the chemical composition of the stainless steel material according to this embodiment is made up of Fe and impurities.
• impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when the stainless steel material is industrially manufactured, and are acceptable to the extent that they do not adversely affect the stainless steel material according to this embodiment.
• the chemical composition of the stainless steel material according to this embodiment may further contain W instead of a portion of Fe.
• Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb is contained as an impurity. Nb may be further added and contained. When contained, Nb forms carbides to increase the strength of the steel material. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, the toughness of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the Nb content is 0 to 0.150%. The preferred upper limit of the Nb content is 0.145%, more preferably 0.140%, and even more preferably 0.135%. The lower limit of the Nb content may be more than 0%, may be 0.001%, or may be 0.003%.
• B 0-0.0050% Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B is contained as an impurity. B may be further added and contained. When contained, B segregates at grain boundaries to improve the hot workability of the steel material. If even a small amount of B is contained, the above effect can be obtained to a certain extent. On the other hand, if the B content is too high, boron nitride (BN) is formed even if the contents of other elements are within the range of this embodiment, and the toughness of the steel material decreases. Therefore, the B content is 0 to 0.0050%. The preferred upper limit of the B content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%. The lower limit of the B content may be more than 0%, may be 0.0001%, or may be 0.0003%.
• the chemical composition of the stainless steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, and rare earth elements. All of these elements are optional elements, and improve the hot workability of the steel material.
• the preferred lower limit of the Ca content is more than 0%, more preferably 0.0001%, more preferably 0.0005%, more preferably 0.0008%, and more preferably 0.0010%.
• the preferred upper limit of the Ca content is 0.0048%, more preferably 0.0045%, and more preferably 0.0040%.
• Mg 0-0.0100%
• Mg Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg fixes S in the steel as sulfides to render it harmless and improve the hot workability of the steel. If even a small amount of Mg is contained, the above effect can be obtained to a certain extent. However, if the Mg content is too high, even if the contents of other elements are within the range of this embodiment, the oxides in the steel will coarsen and the corrosion resistance of the steel in a supercritical corrosion environment will decrease. Therefore, the Mg content is 0 to 0.0100%.
• the preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%.
• the preferred upper limit of the Mg content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0080%.
• the stainless steel material according to this embodiment has the above-mentioned chemical composition, and Fn1 defined by the following formula (1) is 150.0 or more.
• Fn1 576.5-2660.7 ⁇ C-7.8 ⁇ Cr-11.3 ⁇ Mo-20.9 ⁇ Ni-10.6 ⁇ Cu (1)
• the element symbols in formula (1) are substituted with the contents of the corresponding elements in mass %.
• the microstructure "consisting of ferrite phase, retained austenite phase, and martensite phase” means that the amount of phases other than the ferrite phase, retained austenite phase, and martensite phase in the microstructure is negligibly small.
• the volume fraction of precipitates and inclusions is negligibly small compared to the volume fractions of the ferrite phase, retained austenite phase, and martensite phase.
• the microstructure of the stainless steel material according to this embodiment may contain minute amounts of precipitates, inclusions, etc., in addition to the ferrite phase, retained austenite phase, and martensite phase.
• the stainless steel material according to this embodiment has a microstructure with a volume fraction of 30.0 to 85.0% ferrite phase, 0.1 to 40.0% retained austenite phase, and the remainder martensite phase.
• the volume fraction of the martensite phase is not particularly limited, but is, for example, 5.0 to 50.0%.
• the preferred lower limit of the volume fraction of the martensite phase is 5.5%, more preferably 6.0%, and even more preferably 6.5%.
• the preferred upper limit of the volume fraction of the martensite phase is 49.0%, more preferably 48.5%, and even more preferably 48.0%.
• the volume fraction of each phase in the microstructure is determined by the following method. Specifically, the volume fraction (%) of the retained austenite phase and the volume fraction (%) of the ferrite phase in the microstructure of the steel material are determined by the following method. The volume fraction (%) of the martensite phase is determined by subtracting the determined volume fractions of the retained austenite phase and ferrite phase from 100%.
• the volume fraction of the retained austenite phase in the microstructure of the steel material is determined by X-ray diffraction.
• a test piece for measuring the volume fraction of the retained austenite phase is prepared from the steel material according to this embodiment.
• the steel material is a steel plate
• the test piece is taken from the center of the plate thickness.
• the steel material is a steel pipe
• the test piece is taken from the center of the wall thickness.
• the steel material is a round steel
• the test piece is taken from the R/2 position.
• the R/2 position of the round steel means the center position of the radius R in a cross section perpendicular to the axial direction of the round steel.
• the yield strength of the stainless steel material is determined by the following method. Specifically, a tensile test is performed according to ASTM E8/E8M (2022). A test piece is prepared from the steel material according to this embodiment. If the steel material is a steel plate, a tensile test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, a round bar test piece or a circular arc test piece is prepared as the tensile test piece from the center of the wall thickness. In this case, the longitudinal direction of the round bar test piece or the circular arc test piece is parallel to the axial direction of the steel pipe. If the steel material is a round bar, a tensile test piece is prepared from the R/2 position. In this case, the longitudinal direction of the tensile test piece is parallel to the axial direction of the round bar.
• the tensile test specimen is, for example, a round bar test specimen with a parallel part diameter of 8.9 mm and a gauge length of 35.6 mm. If a round bar test specimen cannot be made from a steel pipe, an arc-shaped test specimen is made. The size of the arc-shaped test specimen is, for example, the full thickness, a width of 25.4 mm, and a gauge length of 50.8 mm.
• a tensile test is carried out at room temperature (24 ⁇ 3°C) in accordance with ASTM E8/E8M (2022).
• the 0.2% offset yield strength (MPa) obtained from the tensile test is defined as the yield strength (MPa).
• the yield strength (MPa) is calculated by rounding the obtained value to the nearest tenth.
• the stainless steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, has the above-mentioned microstructure, has a yield strength of 552 to 758 MPa, and further, in the stainless steel material, the amount of precipitated V is 0.005 to 0.130 mass%.
• the V precipitates may include V nitrides, V carbonitrides, etc. in addition to V carbides.
• most of the V precipitates are V carbides. Therefore, by increasing the amount of precipitated V, the amount of C dissolved in the stainless steel material can be reduced. As a result, the general corrosion resistance and stress corrosion cracking resistance of the steel in a supercritical corrosion environment are improved.
• the preferred lower limit of the amount of precipitated V is 0.006 mass%, more preferably 0.007 mass%, and even more preferably 0.008 mass%.
• the preferred upper limit of the amount of precipitated V is 0.128 mass%, more preferably 0.126 mass%, and even more preferably 0.124 mass%.
• the amount of precipitated V in the stainless steel material is determined by the following method. Specifically, a cylindrical test piece with a diameter of 4 mm and a length of 50 mm is prepared from the stainless steel material according to this embodiment. If the steel material is a steel plate, the cylindrical test piece is prepared from the center of the plate thickness. In this case, the longitudinal direction of the cylindrical test piece is parallel to the rolling direction of the steel plate. If the steel material is a steel pipe, the cylindrical test piece is prepared from the center of the wall thickness. In this case, the longitudinal direction of the cylindrical test piece is parallel to the axial direction of the steel pipe. If the steel material is a round bar, the cylindrical test piece is prepared from the R/2 position. In this case, the longitudinal direction of the cylindrical test piece is parallel to the axial direction of the round bar.
• the cylindrical test piece is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) and subjected to preliminary electrolysis.
• the preliminary electrolysis is performed at a temperature of 15 to 30° C. and a current of 1000 mA, to a depth of about 100 ⁇ m from the surface of the steel material.
• the cylindrical test piece after preliminary electrolysis is immersed in an alcohol solution and subjected to ultrasonic cleaning to remove surface deposits.
• the mass M 0 (g) of the cylindrical test piece after ultrasonic cleaning is measured.
• the cylindrical test piece after the mass measurement is immersed in an electrolyte (10% acetylacetone-1% tetramethylammonium chloride-methanol solution) and constant current electrolysis is performed.
• the electrolyte used in the constant current electrolysis is a new electrolyte, not the electrolyte used in the preliminary electrolysis.
• the constant current electrolysis is performed at a temperature of 15 to 30°C and a current density of 20 mA/ cm2 .
• the mass M1 (g) of the cylindrical test piece after the constant current electrolysis is measured. Furthermore, the cylindrical test piece after the constant current electrolysis is further immersed in an alcohol solution and subjected to ultrasonic cleaning to remove surface deposits.
• the electrolyte used in the constant current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning are passed through a 0.2 ⁇ m filter to capture the residue.
• the captured residue is decomposed with an acid, and ICP (inductively coupled plasma) emission spectrometry is performed to quantify the amount of V in the residue.
• the amount of precipitated V (mass %) is determined by rounding off the obtained value to the fourth decimal place.
• the stainless steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, the above-mentioned microstructure, the yield strength is 552 to 758 MPa, and further, the amount of precipitated V in the stainless steel material is 0.005 to 0.130 mass%.
• the stainless steel material according to this embodiment has excellent corrosion resistance (general corrosion resistance and stress corrosion cracking resistance) even in a supercritical corrosion environment.
• the excellent general corrosion resistance and stress corrosion cracking resistance in a supercritical corrosion environment are evaluated by the following method.
• test pieces for corrosion testing are prepared from the stainless steel material according to this embodiment.
• the steel material is a steel plate
• the test piece is prepared from the center of the plate thickness.
• the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate.
• the steel material is a steel pipe
• the test piece is prepared from the center of the wall thickness.
• the longitudinal direction of the test piece is parallel to the axial direction of the steel pipe.
• the steel material is a round bar
• the test piece is prepared from the R/2 position. In this case, the longitudinal direction of the test piece is parallel to the axial direction of the round bar.
• the size of the test piece is, for example, 75 mm in length, 10 mm in width, and 2 mm in thickness.
• a stress equivalent to 100% of the actual yield stress is applied to the test specimen by four-point bending.
• the test specimen to which the stress has been applied is sealed in an autoclave together with the test jig.
• a 5.0 mass% sodium chloride aqueous solution is poured into the autoclave so that the test specimen is immersed.
• a mixed gas of SO 2 , O 2 , NO 2 , and CO 2 is pressurized and sealed in the autoclave to saturate the test solution and form a test bath.
• the total pressure of the mixed gas is 300 bar
• the SO 2 concentration in the mixed gas is 30 ppm
• the O 2 concentration in the mixed gas is 30 ppm
• the NO 2 concentration in the mixed gas is 30 ppm
• the remainder is CO 2.
• the mass, density, and surface area of the test piece before the stress load and the test piece after 96 hours are determined, and the corrosion rate (mm/year) of the test piece is determined.
• the corrosion rate is determined by rounding off the obtained value to the fourth decimal place.
• the surface of the test piece after 96 hours is observed with a loupe at a magnification of 10 times to confirm the presence or absence of cracks. If the occurrence of cracks is suspected based on the observation with the loupe, the portion where the occurrence of cracks is suspected is cut out in the longitudinal direction of the test piece, and the cross section is observed with an optical microscope at a magnification of 100 times to confirm the presence or absence of cracks.
• the stainless steel material according to this embodiment has the above-mentioned chemical composition, Fn1 is 150.0 or more, Fn2 is 24.0 or more, the above-mentioned microstructure, a yield strength of 552 to 758 MPa, and further, the amount of precipitated V in the stainless steel material is 0.005 to 0.130 mass%.
• the stainless steel material according to this embodiment has excellent low-temperature toughness in a cryogenic environment.
• the excellent low-temperature toughness in a cryogenic environment is evaluated by the following method.
• the absorbed energy at -70°C determined under the above conditions is 60 J or more, it is evaluated as having excellent low-temperature toughness in an extremely low-temperature environment.
• the absorbed energy at -70°C is also simply referred to as "absorbed energy.”
• the steel plate with test number 27 had too high a Cr content.
• the absorbed energy of this steel plate at -70°C in the Charpy impact test was less than 60 J, and it did not have excellent low-temperature toughness in an extremely low-temperature environment.
• the Ni content of the steel plate with test number 29 was too high. As a result, this steel plate had too much retained austenite phase and the yield strength was less than 552 MPa. In other words, the desired yield strength was not obtained.
• the steel plate with test number 30 had too high a Mo content.
• the absorbed energy of this steel plate at -70°C in the Charpy impact test was less than 60 J, and it did not have excellent low-temperature toughness in an extremely low-temperature environment.
• the Mo content of the steel plate of test number 31 was too low. As a result, the corrosion rate of this steel plate exceeded 0.100 mm/year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were confirmed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.
• the steel plate with test number 32 had too low a Cu content.
• the corrosion rate of this steel plate exceeded 0.100 mm/year in the corrosion test, and this steel plate did not have excellent general corrosion resistance in a supercritical corrosion environment.
• cracks were confirmed in this steel plate in the corrosion test, and this steel plate did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.
• the steel plates of test numbers 34 and 35 had the V carbide precipitation process maintained for too short a time. As a result, the amount of precipitated V in these steel plates was less than 0.005 mass%. As a result, the corrosion rate of these steel plates exceeded 0.100 mm/year in the corrosion test, and they did not have excellent general corrosion resistance in a supercritical corrosion environment. Furthermore, cracks were confirmed in these steel plates in the corrosion test, and they did not have excellent stress corrosion cracking resistance in a supercritical corrosion environment.
• the tempering temperature of the steel plates of test numbers 36 and 37 was too low. As a result, these steel plates had too little retained austenite phase. As a result, these steel plates had absorbed energy of less than 60 J at -70°C in the Charpy impact test, and did not have excellent low-temperature toughness in an extremely low-temperature environment.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Manufacturing & Machinery (AREA)
• Heat Treatment Of Steel (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=96386598

Family Applications (1)

Country Status (2)

Citations (6)

• 2024
  • 2024-12-05 JP JP2025517658A patent/JP7804246B2/ja active Active
  • 2024-12-05 WO PCT/JP2024/043009 patent/WO2025150315A1/ja active Pending

Patent Citations (6)

Also Published As

Similar Documents

Legal Events