WO2024166947A1 - Austenitic stainless steel material - Google Patents

Austenitic stainless steel material Download PDF

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Publication number
WO2024166947A1
WO2024166947A1 PCT/JP2024/004116 JP2024004116W WO2024166947A1 WO 2024166947 A1 WO2024166947 A1 WO 2024166947A1 JP 2024004116 W JP2024004116 W JP 2024004116W WO 2024166947 A1 WO2024166947 A1 WO 2024166947A1
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Prior art keywords
steel material
less
stainless steel
steel
austenitic stainless
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PCT/JP2024/004116
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French (fr)
Japanese (ja)
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佳幸 藤村
純一 濱田
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日鉄ステンレス株式会社
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Publication of WO2024166947A1 publication Critical patent/WO2024166947A1/en

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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00Modifying the physical properties of iron or steel by deformation
    • C21D7/02Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/04Modifying the physical properties of iron or steel by deformation by cold working of the surface
    • C21D7/06Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/60Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23GCLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08Iron or steel

Definitions

  • the present invention relates to austenitic stainless steel materials and methods for manufacturing them.
  • ammonia is attracting attention as a fuel to replace carbon fuels.
  • the combustion reaction of ammonia is 4NH 3 + 3O 2 ⁇ 2N 2 + 6H 2 O.
  • ammonia burns, water and nitrogen are produced, and it is expected to be a circulable fuel with a small environmental load.
  • the combustion temperature of ammonia is 1750°C in adiabatic flame temperature, which is lower than 2120°C for hydrogen, 1970°C for methane, and approximately 2000°C for gasoline, and the combustion temperature in actual engines and gas turbines is also lower than these existing fuels.
  • the exhaust gas temperature is about 500 to 700°C, which is lower than that of existing fuels.
  • This temperature range of 500 to 700°C is a temperature range at which steel materials used in exhaust pipes, etc. are easily oxidized, and is a temperature range at which so-called red scale is easily generated.
  • Patent Document 1 proposes an austenitic stainless steel that has good corrosion resistance even in high sulfur (S) and high chlorine (Cl) containing environments such as those used in boiler superheater tubes, waste incinerators, and ammonia synthesis plants.
  • S sulfur
  • Cl chlorine
  • Patent Document 2 proposes an austenitic stainless steel that has good corrosion resistance in an ammonia atmosphere without the addition of chromic acid, for use in ammonia-water absorption heat exchangers and the like.
  • ammonia which has a lower combustion temperature than existing fuels, is added and burned, so the combustion exhaust gas temperature is lower than that of existing fuels, at around 500 to 700°C.
  • ammonia combustion gas contains a large amount of nitrogen and water vapor. The inclusion of water vapor makes it easy for steam oxidation and red scale to occur.
  • the combustion gas temperature is about 500 to 700°C, which is also a temperature range in which red scale is likely to occur. For this reason, oxidation resistance (red scale resistance) is required for steel materials used in ammonia combustion gas systems.
  • Patent Document 1 The stainless steel in Patent Document 1 is described as being suitable for use in ammonia synthesis equipment, but it is not intended for use with ammonia combustion gas (combustion exhaust gas), and no consideration is given to measures against grain boundary cracking caused by nitriding or red scale (oxidizing properties) at 500 to 700°C.
  • Patent Document 2 The stainless steel in Patent Document 2 is intended for use in ammonia-water absorption heat exchangers, i.e., for contact with ammonia gas or ammonia solution, and does not take into consideration measures against grain boundary cracking due to nitriding or red scale (oxidative) at 500 to 700°C.
  • the present invention aims to provide an austenitic stainless steel that has red scale resistance (oxidation resistance) and intergranular cracking resistance (nitridation resistance) against gases containing large amounts of nitrogen and water (water vapor) at temperatures of about 500 to 700°C, such as ammonia combustion exhaust gas.
  • the present invention was developed based on these findings, and its gist is as follows:
  • a method for producing an austenitic stainless steel material according to any one of the items [1] to [5], comprising the steps of heating and holding a steel material having the components according to the item [1] at 900 to 1100°C after final cold rolling, cooling to a temperature of 50°C or less, immersing the steel material in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 70°C for 60 to 90 seconds to pickle the steel material, and then polishing, shot blasting or shot peening the surface of the steel material.
  • [8] The method for producing an austenitic stainless steel material according to [7], wherein the holding time in the heating and holding at 900 to 1100 ° C is 30 seconds or more and 5 minutes or less, and the residence time in the temperature range from 800 ° C to 300 ° C in the cooling is 30 seconds or more and 120 seconds or less.
  • [11] The part according to [10] above, which is an ammonia burning appliance part.
  • the austenitic stainless steel according to the present invention makes it possible to obtain stainless steel with good oxidation and nitridation resistance, even when it comes into contact with gases containing large amounts of nitrogen and water (steam) at temperatures of about 500-700°C, such as ammonia combustion exhaust gas. Furthermore, austenitic stainless steel has better high-temperature strength and high-temperature corrosion resistance than ferritic stainless steel. Therefore, in environments that require corrosion resistance and strength at high temperatures, such as ammonia combustion exhaust gas at temperatures of 500-700°C, which are not so high and contain large amounts of nitrogen and steam, the austenitic stainless steel according to the present invention is an extremely effective material.
  • % indicates mass % in the steel.
  • hardness indicates hardness measured by Vickers hardness.
  • ⁇ Steel composition> C 0-0.150% C is an element that reduces formability (r value), so the less the better, with the upper limit set at 0.150%. From the viewpoint of formability, 0.140% or less, 0.120% or less, or 0.100% or less is preferred. There is no particular limit to the lower limit, but an excessive reduction leads to an increase in refining costs, so 0.001% or more is preferred, and 0.002% or more is even more preferred.
  • Si 0.05-4.50%
  • Silicon is an element that is effective in suppressing oxidation, particularly steam oxidation, and is also effective in suppressing nitridation. Furthermore, from the viewpoint of generating an SiO2 internal oxide layer just below the steel surface, the silicon content is set to 0.05% or more.
  • the lower limit of Si is preferably 0.10%, 0.20%, 0.30%, 0.50%, 0.80%, 1.00%, 1.25%, 1.50%, 1.
  • the Si content is increased, the Si oxide layer is formed.
  • the area ratio of the Si (or SiO2 internal oxide layer) increases, deteriorating workability and weldability, so the upper limit is set to 4.50%.
  • the upper limit of Si is preferably 4.30%, 4.10%, or Or it may be set to 4.00%.
  • Mn 0.05-3.00% Mn, like Si, is an element effective in improving oxidation resistance, so it is preferable to include 0.05% or more.
  • the lower limit of Mn is preferably 0.07%, 0.10%, 0.13%, or The content of Mn is preferably 0.15%.
  • the content of Mn is preferably 3.00% or less.
  • the upper limit of Mn is preferably 2.80%, 2.60%, or 2.00%. .50%, or 2.40%.
  • P 0.050% or less P is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and it is better to keep it at 0.050% or less, and preferably at 0.040% or less.
  • an excessive decrease in P content increases the load during refining or requires the use of expensive raw materials, so in reality it may contain 0.001% or more.
  • S 0.0050% or less S is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and the upper limit should be 0.0050% or less, preferably 0.0030% or less.
  • an excessive reduction in S content increases the load during refining or requires the use of expensive raw materials, so in reality, 0.0001% or more may be contained.
  • Ni 8.00-21.00%
  • Ni is an element that stabilizes the austenite phase and has the effect of improving corrosion resistance to various acids and further low-temperature toughness. Therefore, it is advisable to include 8.00% or more of Ni, preferably 9.00% or more, 10.00% or more, or On the other hand, since it is an expensive element, even if it is contained in a large amount, the effect is not worth the increase in alloy cost, so it is preferable to contain it in a content of 21.00% or less, and preferably 20.00% or less. % or less, or 18.00% or less.
  • Cr:15.00 ⁇ 30.00% Cr is an important element that imparts corrosion resistance to stainless steel. It is advisable to include 15.00% or more of Cr, and preferably 15.50% or more, 16.00% or more, 17.00% or more, 18.00% or more, 19.00% or more, or 20.00% or more of Cr. It is preferable that the content of Mn is 30.00% or less, or 29.00% or less, or 28.00% or more. On the other hand, since a large content of Mn leads to a decrease in workability, it is preferable that the content of Mn is 30.00% or less, and more preferably 29.00% or less, or 28.00% or less. % or less, 27.00% or less, or 26.00% or less.
  • N 0-0.350%
  • N reduces workability and reduces corrosion resistance by combining with Cr, so it is preferable that the content is small. , 0.350% or less, and preferably 0.300% or less, 0.280% or less, 0.260% or less, 0.240% or less, 0.220% or less, or 0.200% or less.
  • Ni may preferably be contained in an amount of 0.001% or more, 0.005% or more, or 0.010% or more.
  • Nb 0-1.00% Nb has the effect of improving formability and corrosion resistance.
  • the Nb content is preferably 0.80% or less, or 0.70% or less.
  • the Nb content is preferably 0.80% or less, or 0.70% or less.
  • Mo 0-3.00%
  • Mo is not only an element that promotes nitriding, but also forms a brittle sigma phase with high Cr content, which leads to embrittlement and a decrease in corrosion resistance.
  • the Mo content is preferably 3.00% or less, more preferably 2.50% or less, or even 2.20% or less. There is no particular lower limit for the Mo content, but in order to reliably obtain the effect of corrosion resistance, it is preferably 0. It is preferable that the content be 0.1% or more.
  • Cu 0-3.50%
  • the addition of Cu has the effect of further enhancing the high corrosion resistance of stainless steel.
  • the content should be 3.50% or less, and preferably 3.
  • the Cu content is preferably 20% or less, 3.00% or less, or 2.80% or less.
  • Al 0.002-0.800%
  • Al is an element that combines with N to form AlN and promotes nitriding. Furthermore, excessive addition reduces workability, so the Al content should be 0.800% or less, preferably 0.750% or less. , 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.200% or less. Therefore, the Al content should be set to 0.002% or more, preferably 0.004% or more, 0.006% or more, or 0.008% or more.
  • Ti 0-0.600% Ti ensures corrosion resistance by stabilizing C and N.
  • Ti is an element that promotes nitriding, and if added in excess, TiN is significantly generated, causing nozzle blockage during manufacturing and surface defects in the product.
  • the Ti content is not particularly limited, but it is preferable to set the Ti content to 0.500% or less, 0.400% or less, or 0.300% or less in order to reliably obtain the effect. Therefore, it is preferable to contain 0.001% or more.
  • B 0-0.0100%
  • the B content is preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less.
  • Ca 0-0.0150% If the Ca content is large, the concentration in the oxide that promotes the formation of TiN increases, and this ability is lost, so the Ca content should be 0.0150% or less, preferably 0.0120% or less, and 0.0090% or less.
  • the lower limit is not particularly limited, but Ca is the main component of slag, and some inclusion in the slag is unavoidable. In addition, it is difficult to completely remove Ca. However, an excessive decrease in the content increases the load during refining, so in practical operation, the content may be 0.0001% or more, or 0.0002% or more.
  • Sn 0-1.00%
  • the addition of Sn has the effect of further enhancing the high corrosion resistance of stainless steel.
  • excessive addition leads to a decrease in workability, so it is advisable to keep the content at 1.00% or less, preferably 0.70% or less, 0.05% or less.
  • the Sn content is not particularly limited, but in order to reliably obtain the effect, it is preferable to contain 0.01% or more, or 0.02% or more.
  • the alloy may contain, in mass %, Hf: 0-0.60%, Zr: 0-0.60%, Sb: 0-0.60%, Co: 0-1.50%, W: 0-2.00%, Ta: 0-1.00%, Ga: 0-0.50%, Mg: 0-0.0050%, and REM: 0-0.200%.
  • the addition of these elements has the effect of increasing the corrosion resistance of stainless steel.
  • these elements are expensive, the effect of adding them in excess is not commensurate with the increased cost, so an upper limit has been set. There is no particular lower limit for the content of these elements, but to ensure the effect of their inclusion, it is preferable to contain 0.0001% or more of Mg and 0.001% or more of each of the elements other than Mg.
  • impurities refer to components that are mixed in during the industrial production of steel due to various factors in the manufacturing process, including raw materials such as ores and scraps, and are acceptable within the scope of not adversely affecting the present invention.
  • ⁇ Nitriding tendency index> In optimizing the steel components from the viewpoint of suppressing nitridation of steel, the relationship of the content of elements that affect nitridation was considered. Cr, Mo, Ti, Al, and Cu are known as elements that promote nitridation, but a certain amount of them may be contained in order to ensure the functions such as corrosion resistance of stainless steel. Furthermore, the inventors have found that Si, which is an important contained element in the steel according to the present invention, not only has an effect of suppressing red scale caused by steam oxidation, but also has an effect of suppressing nitridation, although the reason is unclear. In addition, as will be described later, it is also effective to form a Si oxide film (SiO 2 film) on the steel surface.
  • SiO 2 film Si oxide film
  • Nitriding tendency index 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ⁇ 15.0 ....
  • Equation 1 In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
  • the nitriding tendency index is, in short, an index of the ease of nitriding, and a smaller value is preferable. Therefore, the upper limit of the nitriding tendency index is preferably 14.5, 14.0, 13.5, 13.0, 12.5, 12.0, 11.5, 11.0, or 10.0.
  • ⁇ Surface hardening difference in hardness between surface hardness and center hardness is 20Hv or more> Since forming a strong Cr-Si oxide film (Cr-Si oxide film) on the surface layer of steel material is effective for nitridation resistance and oxidation resistance, development was carried out from the perspective of forming a strong Cr-Si oxide film. As a result, it was found that when the surface layer of the steel material is hardened, a strong Cr-Si oxide film can be formed even by heating it to a temperature of about 500 to 700°C. This is believed to be due to the following reasons.
  • dislocations in order to harden the surface layer of the steel, dislocations (strain) are introduced into the surface layer, and it is believed that these dislocations act as diffusion paths for Cr and Si inside the steel. Furthermore, when steel is heated to 500°C or higher, thermal energy and high-density strain act as the driving force, accelerating the diffusion of Cr and Si from inside the steel toward the surface, which is believed to combine with oxygen from the external environment to form a strong Cr-Si oxide film on the surface of the steel.
  • this strong Cr-Si oxide film on the surface prevents oxygen and nitrogen from the external environment from penetrating and diffusing into the steel material, and prevents excess consumption of Cr and Si in the steel material, which is thought to prevent the generation of Cr- and Fe-based oxides in the steel material, prevent a decrease in Cr concentration, and maintain corrosion resistance.
  • the strong Cr-Si oxide film on the surface layer also prevents nitrogen from penetrating and diffusing, which is thought to prevent nitriding of the surface layer of the steel material and prevent grain boundary cracking due to nitriding.
  • the hardness of the steel surface should be 20 Hv or more higher than the hardness of the inside of the steel (refers to the inside of the steel other than the surface hardened layer).
  • the hardness of the inside of the steel may be represented by the hardness of the central part of the steel.
  • the central part of the steel refers to the vicinity of the center in the plate thickness direction when the steel is a steel plate, refers to the vicinity of the central axis of the steel when the steel is a rod-shaped steel such as a bar or wire rod, and refers to the central part in the wall thickness direction of the steel pipe when the steel is a steel pipe.
  • a cross section perpendicular to the surface of the steel refers to a region from 3/8 to 5/8 of the thickness (thickness refers to the thickness in the case of a plate-shaped steel and the diameter in the case of a rod-shaped steel) in the thickness direction (the thickness direction is the direction perpendicular to the steel surface and toward the center of the steel (the central direction).) in the cross section perpendicular to the surface of the steel (hereinafter simply referred to as "steel cross section”). That is, when the Vickers hardness of the steel surface is Hvs and the Vickers hardness of the steel center is Hvc, the following formula 2 is satisfied.
  • Hvs-Hvc ⁇ 20Hv ....(Equation 2)
  • the hardness difference between the hardness (Hvs) of the steel surface and the hardness (Hvc) of the central part of the steel is preferably 22Hv or more, 24Hv or more, 26Hv or more, 28Hv or more, 30Hv or more, 32Hv or more, 34Hv or more, or 35Hv or more.
  • ⁇ Hardened surface layer> In order to introduce dislocations (strain) into the steel surface, a surface hardened layer having a thickness in the depth direction exists in the surface layer of the steel (part directly below the surface). That is, the surface hardened layer is a region having a Vickers hardness that is 20 Hv or more higher than the Vickers hardness of the central part of the steel.
  • the depth direction refers to the direction perpendicular to the steel surface and toward the central part of the steel in a cross section perpendicular to the surface of the steel (hereinafter simply referred to as the "cross section of the steel").
  • the thickness of the surface hardened layer is not particularly limited, but may be at least 0.5 ⁇ m, and is preferably 0.7 ⁇ m or more, 0.9 ⁇ m or more, or 1.0 ⁇ m or more.
  • the upper limit of the thickness of the surface hardened layer is not particularly limited.
  • the thickness of the surface hardened layer may be determined according to the thickness of the Cr-Si oxide film formed on the steel surface. On the other hand, since it is difficult to form a thick surface hardened layer, it is sufficient for practical purposes that the surface hardened layer is a region having a thickness of 10.0 ⁇ m or less in the depth direction from the steel surface.
  • the thickness is 15.0 ⁇ m or less, 20.0 ⁇ m or less, 25.0 ⁇ m or less, 30.0 ⁇ m or less, 35.0 ⁇ m or less, 40.0 ⁇ m or less, 45.0 ⁇ m or less, or 50.0 ⁇ m or less.
  • a region having a thickness of 10 ⁇ m or less in the depth direction from the steel surface refers to a region from the steel surface to 10 ⁇ m in the depth direction.
  • the Vickers hardness is measured with an indentation load of 200 g at five points, and the average value of the measured values is regarded as the Vickers hardness of that portion.
  • the hardness of the steel surface can be measured by measuring five points at any desired location on the surface of the steel (for example, an arbitrarily selected location of 5 mm square) and taking the arithmetic average value as the surface hardness.
  • the Vickers hardness of the center of the steel and the hardened surface layer is measured on the cross section of the steel.
  • the Vickers hardness of the center of the steel should be measured at five points near the center of the cross section of the steel (for example, a 5 mm square area including the center of the plate thickness for steel plates and steel pipes, or a 5 mm square area including the central axis for steel bars and wire rods), and the arithmetic average value should be taken as the hardness of the center of the steel.
  • the Vickers hardness of the surface hardened layer is measured on the cross section of the steel material from the surface of the steel material in the depth direction of the steel material at intervals of 0.5 ⁇ m, 1 ⁇ m, 2 ⁇ m, and thereafter at 1 ⁇ m intervals, and the area (surface hardened layer) having a hardness of 20 Hv or more can be identified by comparing it with the hardness of the central part of the steel material.
  • the inventors have developed a method for preventing the intrusion of nitrogen from the outside of the steel material. As a result, they have found that the presence of precipitates in the surface layer of the stainless steel material limits the movement of nitrogen and prevents the nitriding of the surface layer of the stainless steel material.
  • the surface layer of the stainless steel material (base material) refers to the range from the surface of the steel material to 20 ⁇ m in the depth direction in the cross section of the steel material.
  • the surface layer precipitate density is preferably 8/1000 ⁇ m2 or more, 10/1000 ⁇ m2 or more, 12/1000 ⁇ m2 or more, 15/1000 ⁇ m2 or more, 18/1000 ⁇ m2 or more, or 20/1000 ⁇ m2 or more.
  • the density of precipitates in the surface layer is preferably 200 precipitates/ 1000 ⁇ m2 or less, 150 precipitates/1000 ⁇ m2 or less, 100 precipitates/ 1000 ⁇ m2 or less, 70 precipitates/ 1000 ⁇ m2 or less, or 50 precipitates/ 1000 ⁇ m2 or less.
  • the particle size of the precipitates to be measured there is no particular upper limit on the particle size of the precipitates to be measured, but if the precipitate particle size is too large, it will affect the strength and corrosion resistance of the steel, so it is preferable to prevent large precipitates from forming. From this perspective, it is preferable to keep the particle size of the precipitates to 2.0 ⁇ m or less, so the particle size of the precipitates to be measured may also preferably be 2.0 ⁇ m or less.
  • the type of precipitate is not particularly limited, but may be, for example, one or more of Nb(C,N), Ti(C,N), W(C,N), B(C,N), V(C,N), and ⁇ -Cu.
  • M(C,N) represents a carbonitride of element M (a composite compound of either or both carbide and nitride).
  • the grain boundary precipitate density is preferably 5/ ⁇ m or more, 7/ ⁇ m or more, 10/ ⁇ m or more, 15/ ⁇ m or more, or 20/ ⁇ m or more.
  • the grain boundary precipitate density is preferably 100 particles/ ⁇ m or less, 70 particles/ ⁇ m or less, 50 particles/ ⁇ m or less, 40 particles/ ⁇ m or less, or 30 particles/ ⁇ m or less.
  • particle size of the precipitates there is no upper limit to the particle size of the precipitates to be measured, but it may be preferably 2.0 ⁇ m or less.
  • the type of precipitates is not particularly limited, as with the surface precipitates, but may be, for example, one or more of Nb(C,N), Ti(C,N), W(C,N), B(C,N), V(C,N), and ⁇ -Cu.
  • ⁇ Method of measuring surface layer precipitate density and grain boundary precipitate density> A method for observing precipitates in the surface layer will be described.
  • a depth range from the surface to 20 ⁇ m is observed. For example, it is good to observe 20 ⁇ m from the surface in a 50 ⁇ m depth direction parallel to the surface, and the observation field area at this time is 1000 ⁇ m2.
  • the observation field of the sample is analyzed by image analysis of the obtained observation image.
  • the observation image obtained by setting the conditions of acceleration voltage: 15 kV, current: 1 ⁇ 10 ⁇ 7 A can be image analyzed to observe the above precipitates.
  • the observed precipitates can be mapped by software attached to the EPMA.
  • the particle size of the mapped precipitates is measured, and precipitates with a particle size of 0.1 to 2 ⁇ m or more are extracted.
  • the surface layer precipitate density can be calculated by dividing the number of extracted precipitates by the observation field area.
  • the particle size of the precipitate is determined by taking the maximum width of the precipitate observed as the long axis and the maximum width perpendicular to the long axis as the short axis, and then taking the average of the long axis and the short axis ((long axis + short axis)/2).
  • grain boundaries can be identified from the obtained observation images, precipitates present on the grain boundaries can be extracted, and the grain boundary precipitate density (number/ ⁇ m), which is the linear density of the precipitates, can be calculated from the grain boundary length and the number of precipitates extracted on the grain boundaries through image processing.
  • ⁇ Intergranular crack length> By forming the steel surface layer as described above, the nitridation of the steel surface is suppressed, and as a result, the grain boundary cracking due to the intrusion of nitrogen (N) is suppressed.
  • the grain boundary cracking can be measured by observing the crystal grain boundaries, and three arbitrary 50 ⁇ m square ranges of the surface layer part (at least the part including the nitrided part) of the steel cross section are selected, and the total grain boundary cracking length in those areas is preferably 15 ⁇ m or less. If the total grain boundary cracking length in the three observation surfaces is 15 ⁇ m or less, the embrittlement of the steel surface can be suppressed, and the steel strength in the temperature range of 500 to 700 ° C. can be ensured. The shorter the total grain boundary cracking length, the more preferable it is, and it is more preferable that it is 14 ⁇ m or less, 13 ⁇ m or less, 12 ⁇ m or less, 11 ⁇ m or less, or 10 ⁇ m or
  • the intergranular crack length in the surface layer of steel can be measured as follows.
  • the cross section of the sample steel is observed under an optical microscope with a square field of view of 50 ⁇ m on each side, and the intergranular crack length is measured.
  • the observation field is measured.
  • the intergranular crack area can be marked on the measurement image, and its length can be measured using image processing.
  • the nitriding depth is preferably as shallow as possible from the viewpoint of suppressing nitriding cracks on the surface.
  • the steel material according to the present invention has a composition adjusted to suppress nitriding and has a silicon oxide film on the surface, so that the nitriding depth is shallow on average.
  • the nitriding depth varies slightly depending on the nitrogen (N) content of the contacting gas, but it has been confirmed that surface embrittlement is suppressed if the nitriding depth is generally 100 ⁇ m or less.
  • the nitriding depth is preferably 90 ⁇ m or less, 80 ⁇ m or less, 70 ⁇ m or less, 60 ⁇ m or less, 50 ⁇ m or less, 40 ⁇ m or less, 30 ⁇ m or less, 20 ⁇ m or less, 10 ⁇ m or less, 5 ⁇ m or less, 4 ⁇ m or less, 3 ⁇ m or less, or 2 ⁇ m or less.
  • the range in which the nitrogen concentration is 2 mass% or more is defined as the nitriding layer.
  • the form of the steel material is not particularly limited, and may be, for example, a steel plate, a steel bar, a wire rod, a steel pipe, a steel section, etc.
  • the steel material may also be a part obtained by processing a steel plate, etc.
  • the processing method is not particularly limited, and may be, for example, a press process, a wire drawing process, a cutting process, etc.
  • the manufacturing method described below is one embodiment for obtaining the steel material according to the present invention, and the manufacturing method is not limited to this. As long as the steel material according to the present invention can be obtained, the manufacturing method is not limited.
  • the method for manufacturing steel material according to the present invention after manufacturing steel material by conventional methods, it is subjected to acid washing, and then the surface is polished, shot blasted, or shot peened to introduce strain, thereby obtaining a steel material according to the present invention in which the surface hardness is higher than that of the center.
  • the steel material is a steel plate and polishing is used as the method for introducing strain into the surface.
  • Steel sheets before final annealing can be manufactured using standard manufacturing methods. For example, they can be manufactured using the process of steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.
  • a suitable method is to melt steel containing the components adjusted to the composition described above in a converter or electric furnace, followed by secondary refining.
  • the molten steel thus adjusted to the desired composition is made into slabs using a known casting method (e.g., continuous casting).
  • the slabs are heated to a desired temperature and hot rolled to a desired thickness.
  • the steel may be cold rolled (cold rolling) if necessary. Cold rolling may also be performed using conventional methods.
  • the slab thickness and hot-rolled sheet thickness may be set as appropriate. After coiling the hot-rolled sheet, it may be immersed in a water-cooled pool.
  • the mechanical descaling method such as shot blasting, bending, or brushing, may be selected as appropriate.
  • the pickling solution after hot rolling either, so existing conditions such as sulfuric acid, nitric hydrofluoric acid, etc. may be used.
  • the coil surface may be ground after this.
  • the hot-rolled steel sheet, hot-rolled annealed steel sheet, and cold-rolled steel sheet thus obtained are then subjected to final annealing to recrystallize.
  • the annealing atmosphere is not particularly limited, and may be air. Annealing is preferably performed in the temperature range of 900 to 1100°C.
  • the annealing temperature is too high, the precipitation will not proceed in the solid solution state, so it is preferably 1100°C or less, or 1050°C or less.
  • the annealing temperature is too low, the number of precipitates will be small, so it is preferably 900°C or more, 930°C or more, or 950°C or more.
  • the mechanism behind this has not been clarified, it is presumed that if the annealing temperature is low, precipitation nuclei will not be generated.
  • the holding time is no particular restrictions on the holding time, but if it is too short, there is a risk of insufficient precipitation time, so it is preferably 30 seconds or more or 60 seconds or more.
  • the holding time is too long, there is a risk of the precipitates becoming coarse and affecting toughness and corrosion resistance, so it is preferably 5 minutes or less, 3 minutes or less, or 2 minutes or less.
  • the steel sheet After annealing, the steel sheet is cooled. There are no particular limitations on the cooling conditions, but the residence time of the steel sheet in the temperature range of 800°C to 300°C is preferably 30 seconds or more and 120 seconds or less in order to grow the precipitates to a specified grain size.
  • the residence time is more preferably 40 seconds or more, 50 seconds or more, or 60 seconds or more, and is preferably 110 seconds or less, 100 seconds or less, or 90 seconds or less.
  • the steel sheet After the final annealing, the steel sheet is cooled to below 50°C and then pickled to etch away the upper Cr oxide and Fe oxide layers.
  • the pickling solution used for this purpose is one containing 3.0% or less hydrofluoric acid (HF) and 6-15% nitric acid, and is adjusted to a temperature of 50-70°C and an immersion time of 60-90 seconds. This makes it possible to obtain a steel sheet surface from which the upper layers of Fe oxide, Cr oxide, and Si oxide generated during the manufacturing process have been removed.
  • the surface of the steel sheet after pickling is polished to give dislocations (distortion) to the surface and harden it.
  • the thickness of the surface hardened layer can be adjusted by the polishing conditions (polishing time, pressing strength, etc.).
  • the polishing conditions are not particularly limited, but dry polishing or wet polishing can be applied. For example, in the case of dry polishing, it is recommended to polish the steel sheet surface using a #220 abrasive, and then polish the steel sheet surface using a #400 to #600 abrasive. For example, it is recommended to polish the steel sheet surface using a #220 abrasive, followed by a #400 and/or #600 abrasive.
  • the surface roughness of the steel sheet can be reduced and defects can be eliminated by polishing in order from coarse to fine abrasives.
  • the combination of abrasives is not particularly limited.
  • the polishing amount is also not particularly limited, but it is preferable to use about 10 ⁇ m to 50 ⁇ m in actual production. In reality, since the conditions differ depending on the steel material, it is sufficient to polish in advance and check the surface hardness and surface defects before deciding appropriately.
  • the steel material according to the present invention can be used to obtain the same effects when applied to parts that require resistance to nitridation and oxidation.
  • the holding time for the final annealing was 30 seconds, and the subsequent cooling was performed with a residence time between 800°C and 300°C of 90 seconds.
  • the holding time for the final annealing was 30 seconds, and the subsequent cooling was performed with a residence time between 800°C and 300°C of 30 seconds.
  • polishing was performed using a dry polishing method, in which abrasive-coated abrasive paper was wrapped around a resin holder, and the holder was pressed against the surface of the test piece, and the holder was moved back and forth over a length of 300 mm at a constant speed (10 m/min) while the polishing pressure was set as shown in Table 2.
  • a #220 abrasive was used for the first pass, and a #400 abrasive was used for the second pass.
  • test pieces One of the obtained test pieces was cut to obtain a cross section perpendicular to the surface, and the surface Vickers hardness and central Vickers hardness were measured with a load of 50 g.
  • measurements were made from the surface in the depth direction at 0.5 ⁇ m, 1.0 ⁇ m, 2.0 ⁇ m, and then at 1.0 ⁇ m intervals to identify the depth of the surface hardened layer.
  • the observation field was set at 50 ⁇ m in the surface direction and 20 ⁇ m deep from the surface, and the precipitation status within the observation field was image-analyzed using FE-EPMA.
  • the precipitate density in the surface layer (surface layer precipitate density) and the number of precipitates present at the grain boundaries between crystal grains were divided by the total grain boundary length in the observation field to determine the number of precipitates per 1 ⁇ m of grain boundary (grain boundary precipitate density).
  • test pieces were nitrided and oxidized to simulate ammonia combustion gas.
  • 10 vol% ammonia, 10 vol% water vapor, and the balance nitrogen (N) were introduced into the annealing furnace as atmospheric gas, and the remaining test pieces were placed in the furnace and heated to a temperature of 600°C and held there for 50 hours, after which they were cooled and removed, and the intergranular crack length and nitriding depth were measured.
  • the intergranular crack length was measured by cutting the test piece after nitriding and oxidation treatment so that the cross section in the thickness direction could be observed, and observing the cross section of the test piece using an optical microscope. The observation was performed with a field of view of 50 ⁇ m x 50 ⁇ m just below the steel plate surface, and three randomly selected points on the sample cross section were observed to measure the intergranular crack length. If the total intergranular crack length on the three observation surfaces was 15 ⁇ m or less, the nitriding resistance was good.
  • the thickness of the nitrided layer was determined by cutting the test piece after nitriding and oxidation treatment, and measuring the nitrogen concentration distribution from the surface to the thickness direction by EPMA analysis of the cross section, and the area showing a nitrogen concentration of 2 mass% or more was defined as the nitrided layer.
  • red scale oxidation resistance
  • This invention can be used in all industries, including the automotive and general machinery industries.

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Abstract

The present invention addresses the problem of suppressing intergranular cracking (having nitriding resistance) and suppressing the occurrence of red scale (having oxidation resistance) even in a gas atmosphere containing nitrogen and water vapor such as an ammonia combustion gas in a medium to high temperature range from 500 to 700°C. The purpose of the present invention is to provide an austenitic stainless steel which solves such problems. An austenitic stainless steel material according to the present invention has a specific component composition with which the nitriding tendency index is 15 or less, a dislocation (strain) is introduced thereinto by polishing the steel material surface so as to harden the steel material surface, and a Cr oxide layer and an Si oxide layer, which are strong in an atmosphere at 500°C or higher, are formed in the surface layer by setting the difference between the surface hardness and the hardness of a central part of the steel material to 20 Hv or more. Consequently, this austenitic stainless steel material has achieved excellent oxidation resistance and excellent nitriding resistance. Nitriding tendency index: 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si

Description

オーステナイト系ステンレス鋼材Austenitic Stainless Steel
 本発明は、オーステナイト系ステンレス鋼材とその製造方法に関する。 The present invention relates to austenitic stainless steel materials and methods for manufacturing them.
 地球温暖化が国際的な環境問題となっており、カーボンゼロやカーボンニュートラルなどの脱炭素化社会の実現への技術開発が活発に行われている。そのような流れの中、炭素燃料に代わる燃料としてアンモニアが注目されている。アンモニアの燃焼反応式は4NH+3O→2N+6HOであり、アンモニアが燃焼すると水と窒素が生成され、環境負荷が小さく循環可能な燃料として期待されている。アンモニアの燃焼温度は、断熱火炎温度で1750℃であり、水素の2120℃、メタンの1970℃、ガソリンの約2000℃と比較しても低く、実際のエンジンやガスタービン中での燃焼温度も、これら既存の燃料と比べて低くなる。そのため、アンモニアを燃料として使用した場合、その排ガス温度も既存燃料の場合より低温である500~700℃程度になる。この500~700℃に温度域は排気管などに用いられる鋼材が酸化し易い温度であり、いわゆる赤スケールが発生し易い温度域である。 Global warming has become an international environmental issue, and technological development is being actively carried out to realize a decarbonized society, such as carbon zero and carbon neutral. In this trend, ammonia is attracting attention as a fuel to replace carbon fuels. The combustion reaction of ammonia is 4NH 3 + 3O 2 → 2N 2 + 6H 2 O. When ammonia burns, water and nitrogen are produced, and it is expected to be a circulable fuel with a small environmental load. The combustion temperature of ammonia is 1750°C in adiabatic flame temperature, which is lower than 2120°C for hydrogen, 1970°C for methane, and approximately 2000°C for gasoline, and the combustion temperature in actual engines and gas turbines is also lower than these existing fuels. Therefore, when ammonia is used as fuel, the exhaust gas temperature is about 500 to 700°C, which is lower than that of existing fuels. This temperature range of 500 to 700°C is a temperature range at which steel materials used in exhaust pipes, etc. are easily oxidized, and is a temperature range at which so-called red scale is easily generated.
 特許文献1には、ボイラ過熱器管、ごみ焼却炉、アンモニア合成装置などの高硫黄(S)、高塩素(Cl)含有環境下で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提案されている。 Patent Document 1 proposes an austenitic stainless steel that has good corrosion resistance even in high sulfur (S) and high chlorine (Cl) containing environments such as those used in boiler superheater tubes, waste incinerators, and ammonia synthesis plants.
 特許文献2には、アンモニア-水系吸収熱交換器などにおいて、クロム酸を添加せずともアンモニア雰囲気化で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提案されている。 Patent Document 2 proposes an austenitic stainless steel that has good corrosion resistance in an ammonia atmosphere without the addition of chromic acid, for use in ammonia-water absorption heat exchangers and the like.
特開平3-126842号公報Japanese Patent Application Publication No. 3-126842 特開平10-280100号公報Japanese Patent Application Publication No. 10-280100
 アンモニアの燃料としての活用は、単独燃焼だけでなく他の燃料(重油、軽油、水素等)との混焼での開発も行われている。しかし、混焼とはいえ、既存燃料よりも燃焼温度の低いアンモニアを加えて燃焼させるため、燃焼排ガス温度は既存燃料より低く500~700℃程度となる。さらにアンモニアの燃焼ガス中には多量の窒素と水蒸気が含有されている。
 水蒸気の含有により水蒸気酸化や赤スケールが発生し易くなる。さらに、燃焼ガス温度が500~700℃程度と赤スケールが発生し易い温度域でもある。このため、アンモニア燃焼ガス系などに用いられる鋼材には耐酸化性(耐赤スケール性)が要求される。
Development is being carried out not only for the use of ammonia as a fuel, but also for its co-combustion with other fuels (heavy oil, light oil, hydrogen, etc.). However, even in the case of co-combustion, ammonia, which has a lower combustion temperature than existing fuels, is added and burned, so the combustion exhaust gas temperature is lower than that of existing fuels, at around 500 to 700°C. Furthermore, ammonia combustion gas contains a large amount of nitrogen and water vapor.
The inclusion of water vapor makes it easy for steam oxidation and red scale to occur. Furthermore, the combustion gas temperature is about 500 to 700°C, which is also a temperature range in which red scale is likely to occur. For this reason, oxidation resistance (red scale resistance) is required for steel materials used in ammonia combustion gas systems.
 さらに、アンモニア燃焼排ガス中に含まれる多量の窒素により、鋼材表層に窒素が浸入(窒化)し粒界割れに起因する脆化が顕在化してくる。そのため、アンモニア燃焼ガス系用鋼材には耐窒化性(耐粒界割れ性)も要求される。 Furthermore, the large amount of nitrogen contained in ammonia combustion exhaust gas causes nitrogen to penetrate (nitridation) into the surface layer of the steel, resulting in embrittlement caused by grain boundary cracking. For this reason, steel for ammonia combustion gas systems is also required to be nitridation-resistant (grain boundary cracking-resistant).
 特許文献1のステンレス鋼は、アンモニア合成装置への適用性は記載されているが、これはアンモニアの燃焼ガス(燃焼排ガス)を対象としておらず、窒化による粒界割れや、500~700℃での赤スケール(酸化性)についての対策は考慮されていない。 The stainless steel in Patent Document 1 is described as being suitable for use in ammonia synthesis equipment, but it is not intended for use with ammonia combustion gas (combustion exhaust gas), and no consideration is given to measures against grain boundary cracking caused by nitriding or red scale (oxidizing properties) at 500 to 700°C.
 特許文献2のステンレス鋼は、アンモニア-水系吸収熱交換器への適用、即ちアンモニアガスやアンモニア溶液と接触することを前提としており、窒化による粒界割れや、500~700℃での赤スケール(酸化性)についての対策は考慮されていない。 The stainless steel in Patent Document 2 is intended for use in ammonia-water absorption heat exchangers, i.e., for contact with ammonia gas or ammonia solution, and does not take into consideration measures against grain boundary cracking due to nitriding or red scale (oxidative) at 500 to 700°C.
 本発明は、オーステナイト系ステンレス鋼であって、アンモニア燃焼排ガスのような500~700℃程度で多量の窒素と水(水蒸気)を含有したガスに対して、耐赤スケール性(耐酸化性)と耐粒界割れ性(耐窒化性)を有することを課題とし、そのような鋼材(オーステナイト系ステンレス鋼)を提供することを目的とする。 The present invention aims to provide an austenitic stainless steel that has red scale resistance (oxidation resistance) and intergranular cracking resistance (nitridation resistance) against gases containing large amounts of nitrogen and water (water vapor) at temperatures of about 500 to 700°C, such as ammonia combustion exhaust gas.
 上記課題を達成するため、本発明者らは鋭意検討し、以下の知見を得た。 To achieve the above objective, the inventors conducted extensive research and obtained the following findings.
(a)鋼材表層に強固な耐酸化性および耐窒化性を有する皮膜を形成することを想起して開発を進めた。その結果、鋼材表面に歪を導入し表面硬化することにより、強固なCr酸化物とSi酸化物の皮膜(以下両者を合わせてCr-Si酸化物皮膜と呼ぶ。)を形成できることを見出した。表面硬化深さは鋼材表面から10~50μm程度でも十分に強固なCr-Si酸化物皮膜を形成できることが分かった。 (a) Development was carried out with the idea of forming a film with strong oxidation and nitridation resistance on the surface of steel material. As a result, it was discovered that by introducing strain into the steel surface and hardening the surface, it is possible to form a strong film of Cr oxide and Si oxide (hereinafter, both will be referred to as the Cr-Si oxide film). It was found that a sufficiently strong Cr-Si oxide film can be formed even with a surface hardening depth of about 10 to 50 μm from the steel surface.
(b)窒化を抑制する観点から、窒化を助長する元素であるCr、Mo、Ti、Al、Cuの含有量を最適化するとよいことを考えた。さらにSiは、水蒸気酸化による赤スケールに対し抑制効果があるだけでなく、理由は解明されていないものの窒化を抑制する効果もあることも見出した。窒化を助長する元素Cr、Mo、Ti、Al、Cuの含有量とともに、Siの効果を考慮することで鋼材の窒化傾向を示す窒化傾向指数を導出し、この窒化傾向指数が15.0以下にするとよいことを見出した。
窒化傾向指数=0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si≦15.0
・・・・・(式1)
(b) From the viewpoint of suppressing nitridation, it was considered that it would be good to optimize the contents of Cr, Mo, Ti, Al, and Cu, which are elements that promote nitridation. Furthermore, it was found that Si not only has an effect of suppressing red scale caused by steam oxidation, but also has an effect of suppressing nitridation, although the reason for this is not yet clear. By considering the effect of Si along with the contents of the elements Cr, Mo, Ti, Al, and Cu that promote nitridation, a nitridation tendency index that indicates the nitridation tendency of steel material was derived, and it was found that it would be good to set this nitridation tendency index to 15.0 or less.
Nitriding tendency index = 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≦ 15.0
.... (Equation 1)
(c)鋼材表層硬化する方法は特に限定されないものの、鋼材表面に研磨、ショットブラスト、またはショットピーニングなどで歪を導入すればよいことを見出した。強固なCr酸化物やSi酸化物の皮膜を形成する観点から、通常のステンレス鋼の製造プロセスで製造した後に、表面に生成した酸化物皮膜を一度除去し、活性化された表面に歪を導入して加熱酸化することで強固な酸化物皮膜を形成できることを見出した。しかも、この加熱酸化時の温度は500~700℃程度であり、アンモニアの燃焼ガス温度と重複する。そのため表面硬化したステンレス鋼をアンモニア燃焼機器や排気部品に適用し燃焼ガスに接触させることで、鋼材表面に強固な酸化物皮膜が形成されることを見出した。 (c) Although there is no particular limitation on the method for hardening the surface of the steel material, it has been found that it is sufficient to introduce strain into the steel material surface by polishing, shot blasting, shot peening, or the like. From the viewpoint of forming a strong Cr oxide or Si oxide film, it has been found that a strong oxide film can be formed by removing the oxide film formed on the surface after manufacturing using a normal stainless steel manufacturing process, introducing strain into the activated surface, and then heating and oxidizing it. Moreover, the temperature during this heating and oxidation is about 500 to 700°C, which overlaps with the temperature of ammonia combustion gas. Therefore, it has been found that a strong oxide film can be formed on the steel material surface by applying surface-hardened stainless steel to ammonia combustion equipment or exhaust parts and exposing them to combustion gas.
(d)さらに、ステンレス鋼材の表層部に析出物が存在することにより、窒素の移動を制限して、ステンレス鋼材表層の窒化が抑制されることを見出した。具体的には、ステンレス鋼材(母材)の表層部(鋼材表面から深さ方向に20μmまでの範囲)において、粒径0.1μm以上の析出物(Nb、Ti、W、B、Vの炭窒化物およびε-Cuの析出物)が7個/1000μm以上存在すると窒化が抑制されることを見出した。 (d) Furthermore, it was found that the presence of precipitates in the surface layer of the stainless steel material limits the movement of nitrogen and suppresses nitriding of the surface layer of the stainless steel material. Specifically, it was found that nitriding is suppressed when precipitates having a particle size of 0.1 μm or more (carbonitrides of Nb, Ti, W, B, V and precipitates of ε-Cu) are present at a rate of 7 particles/1000 μm2 or more in the surface layer of the stainless steel material (base material) (within the range of 20 μm in the depth direction from the steel material surface).
(e)特に、窒素の移動経路になり易い粒界(結晶粒間の粒界。以下、単に粒界と呼ぶ。)に前記した析出物が存在するとより窒素の移動が制限され窒化が抑制されることを見出した。具体的には、表層部において粒界に粒径0.1μm以上の前記析出物が0.10個/μm以上存在すると効果的に窒化が抑制されることを見出した。 (e) In particular, it was found that when the above-mentioned precipitates are present at grain boundaries (grain boundaries between crystal grains; hereafter simply referred to as grain boundaries) that are likely to be paths for nitrogen migration, the migration of nitrogen is further restricted and nitridation is suppressed. Specifically, it was found that when the above-mentioned precipitates having a particle size of 0.1 μm or more are present at grain boundaries in the surface layer at a rate of 0.10 particles/μm or more, nitridation is effectively suppressed.
 本発明は、これら知見を基になしたものであり、その要旨は以下のとおりである。 The present invention was developed based on these findings, and its gist is as follows:
[1]
 質量%で、
C :0~0.150%、
Si:0.05~4.50%、
Mn:0.05~3.00%、
P :0.050%以下、
S :0.0050%以下、
Ni:8.00~21.00%、
Cr:15.00~30.00%、
N :0~0.350%、
Nb:0~1.00%
Mo:0~3.00%
Cu:0~3.50%
Al:0.002~0.800%、
Ti:0~0.600%
V :0~1.00%、
B :0~0.0100%、
Ca:0~0.0150%
Sn:0~1.00%、
Hf:0~0.60%、
Zr:0~0.60%、
Sb:0~0.60%、
Co:0~1.50%、
W :0~2.00%、
Ta:0~1.00%、
Ga:0~0.50%、
Mg:0~0.0050%、および
REM:0~0.200%を含み、
残部Feおよび不純物からなるオーステナイト系ステンレス鋼材であり、
以下の式1を満足し、前記鋼材表面のビッカース硬度(Hvs)が、前記鋼材中央部のビッカース硬度(Hvc)より20Hv以上高い(即ち、Hvs-Hvc≧20Hvである)ことを特徴とするオーステナイト系ステンレス鋼材。
0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si≦15.0
・・・・・(式1)
ただし、式1中の元素記号は、当該元素の含有量(質量%)を示し、含有しない場合は0を代入する。
[2]
 前記鋼材の表面に垂直な断面において、前記鋼材中央部のビッカース硬度より20Hv以上高いビッカース硬度を有する表層硬化層を有し、前記表層硬化層の厚さが前記鋼材の表面から深さ方向に0.5μm以上である、[1]に記載のオーステナイト系ステンレス鋼材。
[3]
 前記鋼材の表面に垂直な断面において、前記鋼材表面から深さ20μmの表層部に、粒径0.1μm以上の析出物が7個/1000μm以上存在する、[1]または[2]に記載のオーステナイト系ステンレス鋼材。
[4]
 前記鋼材の表面に垂直な断面において、前記鋼材表面から深さ20μmの表層部中の粒界に、粒径0.1μm以上の析出物が0.10個/μm以上存在する、[1]~[3]のいずれか1項に記載のオーステナイト系ステンレス鋼材。
[5]
 前記鋼材の表面に垂直な向断面で、50μm四方の範囲を1視野として、任意の3視野における粒界割れ長さの合計が15μm以下である、[1]~[4]のいずれか1項に記載のオーステナイト系ステンレス鋼材。
[6]
 前記[1]~[5]のいずれか1項に記載のアンモニア燃焼機器用オーステナイト系ステンレス鋼材。
[7]
 前記[1]~[5]のいずれか1項に記載のオーステナイト系ステンレス鋼材の製造方法であって、前記[1]に記載の成分を有する鋼材を、最終冷延後に、900~1100℃に加熱保持した後、50℃以下の温度まで冷却し、フッ酸2.0%以下、硝酸6~15%を含有し、温度50~70℃の酸洗液中に60~90秒間浸漬して酸洗し、その後前記鋼材表面を研磨、ショットブラストまたはショットピーニングする工程を有することを特徴とするオーステナイト系ステンレス鋼材の製造方法。
[8]
 前記900~1100℃に加熱保持における保持時間が30秒以上5分以下であり、前記冷却において800℃から300℃までの温度範囲での滞留時間が30秒以上120秒以下である[7]に記載のオーステナイト系ステンレス鋼材の製造方法。
[9]
 前記研磨が乾式研磨であって、#220に続いて#400または/および#600の順に研磨する、[7]または[8]に記載のオーステナイト系ステンレス鋼材の製造方法。
[10]
 前記[1]~[5]のいずれか1項に記載のオーステナイト系ステンレス鋼材を少なくとも一部に有する、部品。
[11]
 アンモニア燃焼機器用部品である前記[10]に記載の部品。
[1]
In mass percent,
C: 0 to 0.150%,
Si: 0.05-4.50%,
Mn: 0.05-3.00%,
P: 0.050% or less,
S: 0.0050% or less,
Ni: 8.00-21.00%,
Cr: 15.00-30.00%,
N: 0 to 0.350%,
Nb: 0-1.00%
Mo: 0-3.00%
Cu: 0-3.50%
Al: 0.002-0.800%,
Ti: 0-0.600%
V: 0 to 1.00%,
B: 0 to 0.0100%,
Ca: 0-0.0150%
Sn: 0-1.00%,
Hf: 0-0.60%,
Zr: 0 to 0.60%,
Sb: 0 to 0.60%,
Co: 0 to 1.50%,
W: 0-2.00%,
Ta: 0 to 1.00%,
Ga: 0-0.50%,
Mg: 0 to 0.0050% and REM: 0 to 0.200%;
The remainder is an austenitic stainless steel material consisting of Fe and impurities,
An austenitic stainless steel material characterized in that it satisfies the following formula 1, and the Vickers hardness (Hvs) of the surface of the steel material is 20 Hv or more higher than the Vickers hardness (Hvc) of the center part of the steel material (i.e., Hvs-Hvc≧20 Hv).
0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si≦15.0
.... (Equation 1)
In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
[2]
The austenitic stainless steel material according to [1], wherein in a cross section perpendicular to the surface of the steel material, the steel material has a surface hardened layer having a Vickers hardness that is 20 Hv or more higher than the Vickers hardness of a central part of the steel material, and the thickness of the surface hardened layer is 0.5 μm or more in a depth direction from the surface of the steel material.
[3]
The austenitic stainless steel material according to [1] or [2], wherein in a cross section perpendicular to the surface of the steel material, there are 7 or more precipitates per 1000 μm2 having a grain size of 0.1 μm or more in a surface layer portion at a depth of 20 μm from the surface of the steel material.
[4]
The austenitic stainless steel material according to any one of [1] to [3], wherein in a cross section perpendicular to a surface of the steel material, 0.10 precipitates/μm or more having a grain size of 0.1 μm or more are present at grain boundaries in a surface layer portion at a depth of 20 μm from the steel material surface.
[5]
The austenitic stainless steel material according to any one of [1] to [4], wherein the total length of intergranular cracks in any three visual fields, each of which is a 50 μm square area in a cross section perpendicular to the surface of the steel material, is 15 μm or less.
[6]
The austenitic stainless steel material for an ammonia burner according to any one of [1] to [5].
[7]
A method for producing an austenitic stainless steel material according to any one of the items [1] to [5], comprising the steps of heating and holding a steel material having the components according to the item [1] at 900 to 1100°C after final cold rolling, cooling to a temperature of 50°C or less, immersing the steel material in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 70°C for 60 to 90 seconds to pickle the steel material, and then polishing, shot blasting or shot peening the surface of the steel material.
[8]
The method for producing an austenitic stainless steel material according to [7], wherein the holding time in the heating and holding at 900 to 1100 ° C is 30 seconds or more and 5 minutes or less, and the residence time in the temperature range from 800 ° C to 300 ° C in the cooling is 30 seconds or more and 120 seconds or less.
[9]
The method for producing an austenitic stainless steel material according to [7] or [8], wherein the polishing is dry polishing, and polishing is performed in the order of #220, followed by #400 and/or #600.
[10]
A part having at least a part made of the austenitic stainless steel material according to any one of [1] to [5].
[11]
The part according to [10] above, which is an ammonia burning appliance part.
 本発明に係るオーステナイト系ステンレス鋼により、アンモニア燃焼排ガスのような温度500~700℃程度で窒素や水(水蒸気)を多量に含有するガスが接触しても、耐酸化性、耐窒化性の良好なステンレス鋼を得ることができる。さらにオーステナイト系ステンレス鋼は、フェライト系ステンレス鋼に比べ高温強度や高温耐食性も良好である。従って、高温での耐食性、強度を必要しつつアンモニア燃焼排ガスのように500~700℃というそれほど高温でなく窒素と水蒸気が多量に含まれる環境下では、本発明に係るオーステナイト系ステンレス鋼は非常に有効な材料である。 The austenitic stainless steel according to the present invention makes it possible to obtain stainless steel with good oxidation and nitridation resistance, even when it comes into contact with gases containing large amounts of nitrogen and water (steam) at temperatures of about 500-700°C, such as ammonia combustion exhaust gas. Furthermore, austenitic stainless steel has better high-temperature strength and high-temperature corrosion resistance than ferritic stainless steel. Therefore, in environments that require corrosion resistance and strength at high temperatures, such as ammonia combustion exhaust gas at temperatures of 500-700°C, which are not so high and contain large amounts of nitrogen and steam, the austenitic stainless steel according to the present invention is an extremely effective material.
 以下、本発明の一実施態様(以下、単に本発明という。)について説明する。特に断りのない限り、成分に関する「%」は鋼中の質量%を示す。特に下限を規定していない場合や下限が0%となっているものは、含有しない場合(0%)も含む。また、特に断りのない限り、硬度はビッカース硬さによる硬度を示す。 Below, one embodiment of the present invention (hereinafter simply referred to as the present invention) will be described. Unless otherwise specified, "%" for components indicates mass % in the steel. When no lower limit is specified or the lower limit is 0%, this also includes the case where no component is contained (0%). Furthermore, unless otherwise specified, hardness indicates hardness measured by Vickers hardness.
 <鋼成分について>
 C:0~0.150%
 Cは、成形性(r値)を低下させる元素であるため少ない方が好ましく、上限を0.150%とする。成形性の観点から0.140%以下、0.120%以下、または0.100%以下が好ましい。下限は特に限定しないが、過度な低減は精錬コストの上昇を招くため0.001%以上が好ましく、さらに好ましくは0.002%以上である。
<Steel composition>
C: 0-0.150%
C is an element that reduces formability (r value), so the less the better, with the upper limit set at 0.150%. From the viewpoint of formability, 0.140% or less, 0.120% or less, or 0.100% or less is preferred. There is no particular limit to the lower limit, but an excessive reduction leads to an increase in refining costs, so 0.001% or more is preferred, and 0.002% or more is even more preferred.
 Si:0.05~4.50%
 Siは酸化、特に水蒸気酸化の抑制に対し有効であるとともに、窒化の抑制にも有効な元素である。さらに、鋼材表面直下にSiO内部酸化層を生成する観点から0.05%以上含有する。Siの下限は、好ましくは0.10%、0.20%、0.30%、0.50%、0.80%、1.00%、1.25%、1.50%、1.70%、1.90%、2.00%、2.20%、2.40%、2.50%、または2.60%であるとよい。一方、Si含有量を多くするとSi酸化物層(もしくはSiO内部酸化層)の面積率が増加し加工性や溶接性を劣化させるため、4.50%を上限とする。Siの上限は、好ましくは4.30%、4.10%、または4.00%とするとよい。
Si: 0.05-4.50%
Silicon is an element that is effective in suppressing oxidation, particularly steam oxidation, and is also effective in suppressing nitridation. Furthermore, from the viewpoint of generating an SiO2 internal oxide layer just below the steel surface, the silicon content is set to 0.05% or more. The lower limit of Si is preferably 0.10%, 0.20%, 0.30%, 0.50%, 0.80%, 1.00%, 1.25%, 1.50%, 1. On the other hand, when the Si content is increased, the Si oxide layer is formed. The area ratio of the Si (or SiO2 internal oxide layer) increases, deteriorating workability and weldability, so the upper limit is set to 4.50%. The upper limit of Si is preferably 4.30%, 4.10%, or Or it may be set to 4.00%.
 Mn:0.05~3.00%
 Mnは、Siと同様で耐酸化性に有効な元素であることから0.05%以上含有するとよい。Mnの下限は、好ましくは0.07%、0.10%、0.13%、または0.15%であるとよい。一方、Mnを多く含有すると加工性を劣化させるので、3.00%以下含有するとよい。Mnの上限は、好ましくは2.80%、2.60%、2.50%、または2.40%であるとよい。
Mn: 0.05-3.00%
Mn, like Si, is an element effective in improving oxidation resistance, so it is preferable to include 0.05% or more. The lower limit of Mn is preferably 0.07%, 0.10%, 0.13%, or The content of Mn is preferably 0.15%. On the other hand, since a large content of Mn deteriorates workability, the content of Mn is preferably 3.00% or less. The upper limit of Mn is preferably 2.80%, 2.60%, or 2.00%. .50%, or 2.40%.
 P:0.050%以下
 Pは靱性や熱間加工性、耐食性を低下させる等、ステンレス鋼にとって有害であるため、少ないほど良く、0.050%以下にするとよく、好ましくは0.040%以下にするとよい。ただし、過剰な低下は精錬時の負荷を高くするか、または高価格の原料を用いる必要があるため、現実的には0.001%以上含有してもよい。
P: 0.050% or less P is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and it is better to keep it at 0.050% or less, and preferably at 0.040% or less. However, an excessive decrease in P content increases the load during refining or requires the use of expensive raw materials, so in reality it may contain 0.001% or more.
 S:0.0050%以下
 Sは靱性や熱間加工性、耐食性を低下させる等、ステンレス鋼にとって有害であるため、少ないほど良く、上限を0.0050%以下にするとよく、好ましくは0.0030%以下にするとよい。ただし、過剰な低下は精錬時の負荷が高くするか、または高価格の原料を用いる必要があるため、現実的には0.0001%以上含有してもよい。
S: 0.0050% or less S is harmful to stainless steels, as it reduces toughness, hot workability, and corrosion resistance, so the less the better, and the upper limit should be 0.0050% or less, preferably 0.0030% or less. However, an excessive reduction in S content increases the load during refining or requires the use of expensive raw materials, so in reality, 0.0001% or more may be contained.
 Ni:8.00~21.00%
 Niはオーステナイト相を安定化させる元素であり、各種酸に対する耐食性、さらに低温靭性を改善する作用がため8.00%以上含有するとよく、好ましくは9.00%以上、10.00%以上、または11.00%以上含有するとよい。一方、高価な元素であるため多量に含有しても合金コストの増大に見合う効果が得られないため、21.00%以下にするとよく、好ましくは20.00%以下、または18.00%以下にするとよい。
Ni: 8.00-21.00%
Ni is an element that stabilizes the austenite phase and has the effect of improving corrosion resistance to various acids and further low-temperature toughness. Therefore, it is advisable to include 8.00% or more of Ni, preferably 9.00% or more, 10.00% or more, or On the other hand, since it is an expensive element, even if it is contained in a large amount, the effect is not worth the increase in alloy cost, so it is preferable to contain it in a content of 21.00% or less, and preferably 20.00% or less. % or less, or 18.00% or less.
 Cr:15.00~30.00%
 Crはステンレス鋼に耐食性をもたらす重要な元素であり、15.00%以上含有するとよく、好ましくは15.50%以上、16.00%以上、17.00%以上、18.00%以上、19.00%以上、または20.00%以上にするとよい。その一方で多量の含有は加工性の低下を招くため、30.00%以下にするとよく、好ましくは29.00%以下、28.00%以下、27.00%以下、または26.00%以下にするとよい。
Cr:15.00~30.00%
Cr is an important element that imparts corrosion resistance to stainless steel. It is advisable to include 15.00% or more of Cr, and preferably 15.50% or more, 16.00% or more, 17.00% or more, 18.00% or more, 19.00% or more, or 20.00% or more of Cr. It is preferable that the content of Mn is 30.00% or less, or 29.00% or less, or 28.00% or more. On the other hand, since a large content of Mn leads to a decrease in workability, it is preferable that the content of Mn is 30.00% or less, and more preferably 29.00% or less, or 28.00% or less. % or less, 27.00% or less, or 26.00% or less.
 N:0~0.350%
 表面のNによる粒界割れを抑制する観点から、鋼材に元々含有するNは少ない方が好ましい。さらに、Nは加工性を低下させ、Crと結合して耐食性を低下させるため、少ない方が好ましく、0.350%以下にするとよく、好ましくは0.300%以下、0.280%以下、0.260%以下、0.240%以下、0.220%以下、または0.200%以下にするとよい。一方、過剰な低減は精錬工程上の負荷が大きいため、好ましくは0.001%以上、0.005%以上、または0.010%以上含有してもよい。
N: 0-0.350%
From the viewpoint of suppressing grain boundary cracking due to surface N, it is preferable that the original content of N in the steel material is small. Furthermore, N reduces workability and reduces corrosion resistance by combining with Cr, so it is preferable that the content is small. , 0.350% or less, and preferably 0.300% or less, 0.280% or less, 0.260% or less, 0.240% or less, 0.220% or less, or 0.200% or less. On the other hand, an excessive reduction in Ni imposes a large burden on the refining process, so Ni may preferably be contained in an amount of 0.001% or more, 0.005% or more, or 0.010% or more.
 Nb:0~1.00%
 Nbは成形性や耐食性を高める作用がある。一方、Nb含有量が多過ぎると再結晶しにくくなって組織が粗くなるため、1.00%以下にするよく、好ましくは0.90%以下、0.80%以下、または0.70%以下にするとよい。Nb含有量の下限は特に限定しないが、効果を確実に得るため好ましくは0.01%以上含有するとよい。
Nb: 0-1.00%
Nb has the effect of improving formability and corrosion resistance. On the other hand, if the Nb content is too high, recrystallization becomes difficult and the structure becomes coarse, so it is preferable to set the Nb content at 1.00% or less, preferably 0.90% or less, The Nb content is preferably 0.80% or less, or 0.70% or less. There is no particular lower limit for the Nb content, but in order to reliably obtain the effect, it is preferable to contain 0.01% or more.
 Mo:0~3.00%
 Moは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、窒化を助長する元素であるばかりか、高Crで脆いシグマ相を形成して脆化と耐食性の低下を招くため、3.00%以下にするとよく、好ましくは2.50%以下、または2.20%以下にするとよい。Mo含有量の下限は特に限定しないが、耐食性の効果を確実に得るため好ましくは0.01%以上含有するとよい。
Mo: 0-3.00%
The addition of Mo has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, Mo is not only an element that promotes nitriding, but also forms a brittle sigma phase with high Cr content, which leads to embrittlement and a decrease in corrosion resistance. The Mo content is preferably 3.00% or less, more preferably 2.50% or less, or even 2.20% or less. There is no particular lower limit for the Mo content, but in order to reliably obtain the effect of corrosion resistance, it is preferably 0. It is preferable that the content be 0.1% or more.
 Cu:0~3.50%
 Cuは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、過剰な添加は製造上のコストに見合う性能向上がなされないため、3.50%以下にするとよく、好ましくは3.20%以下、3.00%以下、または2.80%以下にするとよい。Cu含有量の下限は特に限定しないが、効果を確実に得るため好ましくは0.01%以上含有するとよい。
Cu: 0-3.50%
The addition of Cu has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, excessive addition does not improve performance to justify the manufacturing costs, so the content should be 3.50% or less, and preferably 3. The Cu content is preferably 20% or less, 3.00% or less, or 2.80% or less. There is no particular lower limit for the Cu content, but in order to reliably obtain the effect, it is preferable to contain 0.01% or more.
 Al:0.002~0.800%
 AlはNと結びつきAlNを生成し、窒化を助長する元素であり、さらに過剰な添加は加工性を低下させるため、Al含有量を0.800%以下にするとよく、好ましくは0.750%以下、0.700%以下、0.600%以下、0.500%以下、0.400%以下、0.300%以下、または0.200%以下にするとよい。一方、脱硫して耐食性を向上する効果があることから、Al含有量を0.002%以上にするとよく、好ましくは0.004%以上、0.006%以上、または0.008%以上にするとよい。
Al: 0.002-0.800%
Al is an element that combines with N to form AlN and promotes nitriding. Furthermore, excessive addition reduces workability, so the Al content should be 0.800% or less, preferably 0.750% or less. , 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.200% or less. Therefore, the Al content should be set to 0.002% or more, preferably 0.004% or more, 0.006% or more, or 0.008% or more.
 Ti:0~0.600%
 TiはCやNの安定化作用により耐食性を担保する。一方、Tiは窒化を助長する元素であり、過剰に添加するとTiNが著しく生成して製造時のノズル閉塞や製品の表面欠陥を招くため、0.600%以下にするとよく、好ましくは0.500%以下、0.400%以下、または0.300%以下にするとよい。Ti含有量の下限は特に限定しないが、効果を確実に得るため好ましくは0.001%以上含有するとよい。
Ti: 0-0.600%
Ti ensures corrosion resistance by stabilizing C and N. On the other hand, Ti is an element that promotes nitriding, and if added in excess, TiN is significantly generated, causing nozzle blockage during manufacturing and surface defects in the product. The Ti content is not particularly limited, but it is preferable to set the Ti content to 0.500% or less, 0.400% or less, or 0.300% or less in order to reliably obtain the effect. Therefore, it is preferable to contain 0.001% or more.
 V:0~1.00%
 Vは添加することでステンレス鋼の高い耐食性をさらに高める作用がある。一方、高濃度に含有すると靱性の低下を招くため、その上限を1.00%とするとよく、好ましくは0.90%以下、0.70%以下、または0.50%以下にするとよい。V含有量の下限は特に限定しないが、効果を確実に得るため好ましくは0.01%以上、または0.05%以上含有するとよい。
V: 0 to 1.00%
The addition of V has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, if it is contained in a high concentration, it will cause a decrease in toughness, so the upper limit should be set to 1.00%, and preferably 0.90% or less, 0.70% or less, or 0.50% or less. There is no particular lower limit for the V content, but in order to ensure the effect, it is preferable to contain 0.01% or more, or 0.05% or more.
 B:0~0.0100%
 Bは粒界の強度を高める元素であり、加工性の向上に寄与する。一方、過剰な添加は却って延びの低下による加工性低下を招くため、含有量を0.0100%以下にするとよく、好ましくは0.0090%以下、0.0070%以下、または0.0050%以下にするとよい。B含有量の下限は特に限定しないが、効果を確実に得るため好ましくは0.0001%以上、または0.0005%以上含有するとよい。
B: 0-0.0100%
B is an element that increases the strength of grain boundaries and contributes to improving workability. On the other hand, excessive addition of B leads to a decrease in elongation and hence a decrease in workability, so the content should be 0.0100% or less. The B content is preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. There is no particular lower limit for the B content, but in order to reliably obtain the effect, it is preferably 0.0001% or more, or It is preferable that the content be 0.0005% or more.
 Ca:0~0.0150%
 Caは多く含有すると、TiN生成を促進するための酸化物中の濃度が上昇し、その能力を失わせるため0.0150%以下含有するとよく、好ましくは0.0120%以下、0.0090%以下、0.0070%以下、または0.0050%以下にするとよい。下限は特に限定しないが、Caはスラグの主成分であり、多少の巻き込みは避けられない。また、完全に除去することは難しく、過剰な低下は精錬時の負荷が高くなるため、実操業としては0.0001%以上、または0.0002%以上含有してもよい。
Ca: 0-0.0150%
If the Ca content is large, the concentration in the oxide that promotes the formation of TiN increases, and this ability is lost, so the Ca content should be 0.0150% or less, preferably 0.0120% or less, and 0.0090% or less. The lower limit is not particularly limited, but Ca is the main component of slag, and some inclusion in the slag is unavoidable. In addition, it is difficult to completely remove Ca. However, an excessive decrease in the content increases the load during refining, so in practical operation, the content may be 0.0001% or more, or 0.0002% or more.
 Sn:0~1.00%
 Snは添加することでステンレス鋼の高い耐食性をさらに高める効果がある。一方で過剰な添加は加工性の低下につながるため、1.00%以下にするとよく、好ましくは0.70%以下、0.50%以下、または0.30%以下にするとよい。Sn含有量の下限は特に限定しないが、効果を確実に得るため好ましくは0.01%以上、または0.02%以上含有するとよい。
Sn: 0-1.00%
The addition of Sn has the effect of further enhancing the high corrosion resistance of stainless steel. On the other hand, excessive addition leads to a decrease in workability, so it is advisable to keep the content at 1.00% or less, preferably 0.70% or less, 0.05% or less. The Sn content is not particularly limited, but in order to reliably obtain the effect, it is preferable to contain 0.01% or more, or 0.02% or more.
 その他、さらに質量%で、Hf:0~0.60%、Zr:0~0.60%、Sb:0~0.60%、Co:0~1.50%、W:0~2.00%、Ta:0~1.00%、Ga:0~0.50%、Mg:0~0.0050%、REM:0~0.200%を含んでも良い。これら元素は添加することでステンレス鋼の耐食性を高める作用がある。一方で、高価な元素であるため、過剰に含有してもコストの増大に見合う効果が得られないため、上限を設けた。これら元素の含有量の下限は特に限定しないが、含有した効果を確実に得るため好ましくは、Mgは0.0001%以上、Mg以外の元素はそれぞれ0.001%以上含有するとよい。 In addition, the alloy may contain, in mass %, Hf: 0-0.60%, Zr: 0-0.60%, Sb: 0-0.60%, Co: 0-1.50%, W: 0-2.00%, Ta: 0-1.00%, Ga: 0-0.50%, Mg: 0-0.0050%, and REM: 0-0.200%. The addition of these elements has the effect of increasing the corrosion resistance of stainless steel. On the other hand, since these elements are expensive, the effect of adding them in excess is not commensurate with the increased cost, so an upper limit has been set. There is no particular lower limit for the content of these elements, but to ensure the effect of their inclusion, it is preferable to contain 0.0001% or more of Mg and 0.001% or more of each of the elements other than Mg.
 上記鋼成分の残部はFeおよび不純物である。ここで不純物とは、鋼を工業的に製造する際に、鉱石やスクラップ等のような原料をはじめとして、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。 The balance of the above steel components is Fe and impurities. Here, impurities refer to components that are mixed in during the industrial production of steel due to various factors in the manufacturing process, including raw materials such as ores and scraps, and are acceptable within the scope of not adversely affecting the present invention.
 <窒化傾向指数>
 鋼の窒化を抑制する観点から鋼成分を最適化するに当たり、窒化に影響する元素の含有量の関係を考えた。窒化を助長する元素としてCr、Mo、Ti、Al、Cuが知られているが、ステンレス鋼としての耐食性などの機能を確保する上では一定量含有してもよい。さらに、本発明に係る鋼で重要な含有元素であるSiには、水蒸気酸化による赤スケールに対し抑制効果があるだけでなく、理由は定かではないものの窒化を抑制する効果も有することを、本発明者らは見出した。また、後述するが、鋼表面にSi酸化物皮膜(SiO皮膜)を形成させることも有効である。そこで、これら窒化を助長する元素であるCr、Mo、Ti、Alと、窒化抑制に効果のあるSiとをバランスよく組み合わせることを想起し、オーステナイト系ステンレス鋼の場合は窒化傾向を示す指数として0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Siで評価することができることを見出した。窒化抑制の観点から、この窒化傾向指数が15.0以下にするとよいことを見出した。
 窒化傾向指数=0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si≦15.0
・・・・・(式1)
 ただし、式1中の元素記号は、当該元素の含有量(質量%)を示し、含有しない場合は0を代入する。
 窒化傾向指数は一言でいうと窒化し易さの指標であり、値が小さい方が好ましい。そのため、窒化傾向指数の上限値は、好ましくは14.5、14.0、13.5、13.0、12.5、12.0、11.5、11.0、または10.0であるとよい。
<Nitriding tendency index>
In optimizing the steel components from the viewpoint of suppressing nitridation of steel, the relationship of the content of elements that affect nitridation was considered. Cr, Mo, Ti, Al, and Cu are known as elements that promote nitridation, but a certain amount of them may be contained in order to ensure the functions such as corrosion resistance of stainless steel. Furthermore, the inventors have found that Si, which is an important contained element in the steel according to the present invention, not only has an effect of suppressing red scale caused by steam oxidation, but also has an effect of suppressing nitridation, although the reason is unclear. In addition, as will be described later, it is also effective to form a Si oxide film (SiO 2 film) on the steel surface. Therefore, it was envisioned to combine Cr, Mo, Ti, and Al, which are elements that promote nitridation, with Si, which is effective in suppressing nitridation, in a well-balanced manner, and it was found that in the case of austenitic stainless steel, an index indicating the nitridation tendency can be evaluated as 0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si. From the viewpoint of suppressing nitridation, it was found that it is good to make this nitridation tendency index 15.0 or less.
Nitriding tendency index = 0.5Cr + 10Al + 2Mo + 3Ti + 0.5Cu - 1.5Si ≦ 15.0
.... (Equation 1)
In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
The nitriding tendency index is, in short, an index of the ease of nitriding, and a smaller value is preferable. Therefore, the upper limit of the nitriding tendency index is preferably 14.5, 14.0, 13.5, 13.0, 12.5, 12.0, 11.5, 11.0, or 10.0.
<表面硬化:表面硬度と中央部硬度の硬度差が20Hv以上>
 鋼材の表層に強固なCr酸化物とSi酸化物の皮膜(Cr-Si酸化物皮膜)を形成させると耐窒化性、耐酸化性に有効であるため、強固なCr-Si酸化物皮膜を形成する観点から開発を進めた。その結果、鋼材表層を硬化すると、500~700℃程度の温度に加熱することでも強固なCr-Si酸化物皮膜を形成できることを見出した。これは、以下のような理由によるものと考えられる。
<Surface hardening: difference in hardness between surface hardness and center hardness is 20Hv or more>
Since forming a strong Cr-Si oxide film (Cr-Si oxide film) on the surface layer of steel material is effective for nitridation resistance and oxidation resistance, development was carried out from the perspective of forming a strong Cr-Si oxide film. As a result, it was found that when the surface layer of the steel material is hardened, a strong Cr-Si oxide film can be formed even by heating it to a temperature of about 500 to 700°C. This is believed to be due to the following reasons.
 即ち、鋼材表層を硬化するため、表層部に転位(歪)を導入することになるが、この転位が鋼材内部のCrやSiの拡散パスとなるものと考えられる。さらに、鋼材を500℃以上に加熱すると、熱エネルギや高密度の歪が駆動力となり、鋼材内部から表層に向かってCrやSiの拡散が加速し、外部環境の酸素を結合して、鋼材表層に強固なCr-Si酸化物皮膜を形成できるものと考えられる。 In other words, in order to harden the surface layer of the steel, dislocations (strain) are introduced into the surface layer, and it is believed that these dislocations act as diffusion paths for Cr and Si inside the steel. Furthermore, when steel is heated to 500°C or higher, thermal energy and high-density strain act as the driving force, accelerating the diffusion of Cr and Si from inside the steel toward the surface, which is believed to combine with oxygen from the external environment to form a strong Cr-Si oxide film on the surface of the steel.
 さらに、この表層の強固なCr-Si酸化物皮膜が形成されることで、外部環境の酸素や窒素が鋼材中に侵入拡散することを抑制することができ、鋼材中のCrやSiが余計に消費されることを抑制することができるため、鋼材中のCrやFe系酸化物が生成されることなく、Cr濃度の低下を抑制し、耐食性を維持することができるものと考えられる。また、同様に表層の強固なCr-Si酸化物皮膜により、窒素の侵入拡散も抑制されるため、鋼材表層部の窒化が抑制され、窒化による粒界割れを防止することができるものと考えられる。 Furthermore, the formation of this strong Cr-Si oxide film on the surface prevents oxygen and nitrogen from the external environment from penetrating and diffusing into the steel material, and prevents excess consumption of Cr and Si in the steel material, which is thought to prevent the generation of Cr- and Fe-based oxides in the steel material, prevent a decrease in Cr concentration, and maintain corrosion resistance. Similarly, the strong Cr-Si oxide film on the surface layer also prevents nitrogen from penetrating and diffusing, which is thought to prevent nitriding of the surface layer of the steel material and prevent grain boundary cracking due to nitriding.
 鋼材表面の硬度は、鋼材内部(表層硬化層以外の鋼材内部を指す。)の硬度より20Hv以上高いとよいことを確認した。ここで鋼材内部の硬度は、鋼材中央部の硬度で代表してもよい。鋼材中央部とは、鋼材が鋼板の場合は板厚方向の中央近傍を指し、鋼材が棒鋼や線材のような棒状の場合は鋼材の中心軸近傍を指し、鋼材が鋼管の場合は鋼管肉厚方向の中央部を指す。例えば、鋼材の表面に垂直な断面(以下、単に「鋼材断面」と呼ぶ。)において鋼材表面から厚さ方向(厚さ方向とは、鋼材表面に垂直で鋼材中心に向かう方向(中心方向)である。)に厚さ(厚さとは、板状の場合は厚さ、棒状の場合は直径を指す。)の3/8から5/8までの距離の領域を指す。即ち、鋼材表面のビッカース硬度をHvsとし、鋼材中央部のビッカース硬度をHvcとしたとき、以下の式2を満足することになる。
 Hvs-Hvc≧20Hv
・・・・・(式2)
 鋼材表面の硬度(Hvs)と鋼材中央部の硬度(Hvc)との硬度差は、好ましくは22Hv以上、24Hv以上、26Hv以上、28Hv以上、30Hv以上、32Hv以上、34Hv以上、または35Hv以上であるとよい。
It was confirmed that the hardness of the steel surface should be 20 Hv or more higher than the hardness of the inside of the steel (refers to the inside of the steel other than the surface hardened layer). Here, the hardness of the inside of the steel may be represented by the hardness of the central part of the steel. The central part of the steel refers to the vicinity of the center in the plate thickness direction when the steel is a steel plate, refers to the vicinity of the central axis of the steel when the steel is a rod-shaped steel such as a bar or wire rod, and refers to the central part in the wall thickness direction of the steel pipe when the steel is a steel pipe. For example, in a cross section perpendicular to the surface of the steel (hereinafter simply referred to as "steel cross section"), it refers to a region from 3/8 to 5/8 of the thickness (thickness refers to the thickness in the case of a plate-shaped steel and the diameter in the case of a rod-shaped steel) in the thickness direction (the thickness direction is the direction perpendicular to the steel surface and toward the center of the steel (the central direction).) in the cross section perpendicular to the surface of the steel (hereinafter simply referred to as "steel cross section"). That is, when the Vickers hardness of the steel surface is Hvs and the Vickers hardness of the steel center is Hvc, the following formula 2 is satisfied.
Hvs-Hvc≧20Hv
....(Equation 2)
The hardness difference between the hardness (Hvs) of the steel surface and the hardness (Hvc) of the central part of the steel is preferably 22Hv or more, 24Hv or more, 26Hv or more, 28Hv or more, 30Hv or more, 32Hv or more, 34Hv or more, or 35Hv or more.
<表層硬化層>
 鋼材表面に転位(歪)導入するため、鋼材表層部(表面直下の部分)には深さ方向に厚さをもった表層硬化層が存在する。即ち、表層硬化層は、鋼材中央部ビッカース硬度より20Hv以上高いビッカース硬度を有する領域である。ここで、深さ方向とは、鋼材の表面に垂直な断面(以下、単に「鋼材断面」と呼ぶ。)において鋼材表面に垂直で鋼材中央部方向を示す。
<Hardened surface layer>
In order to introduce dislocations (strain) into the steel surface, a surface hardened layer having a thickness in the depth direction exists in the surface layer of the steel (part directly below the surface). That is, the surface hardened layer is a region having a Vickers hardness that is 20 Hv or more higher than the Vickers hardness of the central part of the steel. Here, the depth direction refers to the direction perpendicular to the steel surface and toward the central part of the steel in a cross section perpendicular to the surface of the steel (hereinafter simply referred to as the "cross section of the steel").
 表層硬化層の厚さは特に限定されないが、少なくとも0.5μmであればよい。好ましくは0.7μ以上、0.9μm以上、または1.0μm以上であるとよい。
 表面硬化層の厚さの上限は特に限定されない。鋼材表面に形成するCr-Si酸化物皮膜の厚さに応じて表面硬化層の厚さを決定すればよい。一方、表面硬化層を厚く形成することも難しいことから、実用上鋼材表面から深さ方向に厚さ10.0μm以下の領域が表面硬化層であれば十分である。好ましくは、15.0μm以下、20.0μm以下、25.0μm以下、30.0μm以下、35.0μm以下、40.0μm以下、45.0μm以下、または50.0μm以下であるとよい。
 例えば、鋼材表面から深さ方向に厚さ10μm以下の領域とは、鋼材表面から深さ方向に10μmまでの領域を示す。
The thickness of the surface hardened layer is not particularly limited, but may be at least 0.5 μm, and is preferably 0.7 μm or more, 0.9 μm or more, or 1.0 μm or more.
The upper limit of the thickness of the surface hardened layer is not particularly limited. The thickness of the surface hardened layer may be determined according to the thickness of the Cr-Si oxide film formed on the steel surface. On the other hand, since it is difficult to form a thick surface hardened layer, it is sufficient for practical purposes that the surface hardened layer is a region having a thickness of 10.0 μm or less in the depth direction from the steel surface. Preferably, the thickness is 15.0 μm or less, 20.0 μm or less, 25.0 μm or less, 30.0 μm or less, 35.0 μm or less, 40.0 μm or less, 45.0 μm or less, or 50.0 μm or less.
For example, a region having a thickness of 10 μm or less in the depth direction from the steel surface refers to a region from the steel surface to 10 μm in the depth direction.
<ビッカース硬度の測定方法>
 ビッカース硬度の測定は押し込み荷重200g重で行い、5点のビッカース硬さを測定し、それらの平均値をもってその部位のビッカース硬さとする。
 鋼材表面の硬度測定は、鋼材の表面の任意の部位(例えば任意に選択した5mm四方の部位)において、5点測定しその算術平均値をもって表面硬度とすることができる。
<Method of measuring Vickers hardness>
The Vickers hardness is measured with an indentation load of 200 g at five points, and the average value of the measured values is regarded as the Vickers hardness of that portion.
The hardness of the steel surface can be measured by measuring five points at any desired location on the surface of the steel (for example, an arbitrarily selected location of 5 mm square) and taking the arithmetic average value as the surface hardness.
 鋼材中央部と表層硬化層のビッカース硬度の測定は、鋼材断面において測定する。鋼材中央部のビッカース硬度は、鋼材断面において中央部近傍の部位(例えば、鋼板や鋼管であれば板厚の中央を含む5mm四方の部位、棒鋼や線材であれば中心軸を含む5mm四方の部位)において5点測定しその算術平均値をもって、鋼材中央部の硬度とするとよい。 The Vickers hardness of the center of the steel and the hardened surface layer is measured on the cross section of the steel. The Vickers hardness of the center of the steel should be measured at five points near the center of the cross section of the steel (for example, a 5 mm square area including the center of the plate thickness for steel plates and steel pipes, or a 5 mm square area including the central axis for steel bars and wire rods), and the arithmetic average value should be taken as the hardness of the center of the steel.
 表層硬化層のビッカース硬度は、鋼材断面において、鋼材表面から鋼材深さ方向に0.5μm、1μm、2μmと以降1μmピッチでビッカース硬度を測定し、鋼材中央部硬度と比較して20Hv以上の硬度を有する領域(表面硬化層)を特定することができる。 The Vickers hardness of the surface hardened layer is measured on the cross section of the steel material from the surface of the steel material in the depth direction of the steel material at intervals of 0.5 μm, 1 μm, 2 μm, and thereafter at 1 μm intervals, and the area (surface hardened layer) having a hardness of 20 Hv or more can be identified by comparing it with the hardness of the central part of the steel material.
<表層部の析出物密度>
 さらに、鋼材外部からの窒素の侵入を抑制する観点から開発を進めた。その結果、ステンレス鋼材の表層部において析出物が存在することにより、窒素の移動を制限して、ステンレス鋼材表層の窒化が抑制されることを見出した。ここで、ステンレス鋼材(母材)の表層部とは、鋼材断面において、鋼材の表面から深さ方向に20μmまでの範囲を指す。
<Precipitate density in surface layer>
Furthermore, the inventors have developed a method for preventing the intrusion of nitrogen from the outside of the steel material. As a result, they have found that the presence of precipitates in the surface layer of the stainless steel material limits the movement of nitrogen and prevents the nitriding of the surface layer of the stainless steel material. Here, the surface layer of the stainless steel material (base material) refers to the range from the surface of the steel material to 20 μm in the depth direction in the cross section of the steel material.
 鋼材断面において、鋼材表層部に粒径0.1μm以上の析出物の個数密度(表層部析出物密度)が7個/1000μm以上存在すると窒化が抑制されることが分かった。表層部析出物密度は、好ましくは8個/1000μm以上、10個/1000μm以上、12個/1000μm以上、15個/1000μm以上、18個/1000μm以上、または20個/1000μm以上であるとよい。表層部析出物密度の上限は特に限定されないが、表層部析出物密度が高すぎると、窒化以外の特性(例えば靭性)に影響を及ぼすため好ましくは200個/1000μm以下、150個/1000μm以下、100個/1000μm以下、70個/1000μm以下、または50個/1000μm以下にするとよい。 It has been found that nitriding is suppressed when the number density of precipitates having a particle size of 0.1 μm or more (surface layer precipitate density) is 7/1000 μm2 or more in the steel surface layer cross section. The surface layer precipitate density is preferably 8/1000 μm2 or more, 10/1000 μm2 or more, 12/1000 μm2 or more, 15/1000 μm2 or more, 18/1000 μm2 or more, or 20/1000 μm2 or more. Although there is no particular upper limit to the density of precipitates in the surface layer, if the density of precipitates in the surface layer is too high, it will affect properties other than nitriding (e.g., toughness), so the density is preferably 200 precipitates/ 1000 μm2 or less, 150 precipitates/1000 μm2 or less, 100 precipitates/ 1000 μm2 or less, 70 precipitates/ 1000 μm2 or less, or 50 precipitates/ 1000 μm2 or less.
 測定対象とする析出物の粒径の上限は特に限定しないが、析出物粒径が大きすぎると鋼材の強度や耐食性にも影響がでるため、大きな析出物が生成しないようにすることが好ましい。この観点から、析出物の粒径は2.0μm以下に抑えることが好ましいので、測定対象析出物の粒径も好ましくは2.0μm以下にしてもよい。 There is no particular upper limit on the particle size of the precipitates to be measured, but if the precipitate particle size is too large, it will affect the strength and corrosion resistance of the steel, so it is preferable to prevent large precipitates from forming. From this perspective, it is preferable to keep the particle size of the precipitates to 2.0 μm or less, so the particle size of the precipitates to be measured may also preferably be 2.0 μm or less.
 析出物の種類は特に限定しないが、例えばNb(C、N)、Ti(C、N)、W(C、N)、B(C、N)、V(C、N)、およびε-Cuの1種以上があり得る。ここでM(C、N)は、元素Mの炭窒化物(炭化物または窒化物の一方または両方の複合化合物)を示す。 The type of precipitate is not particularly limited, but may be, for example, one or more of Nb(C,N), Ti(C,N), W(C,N), B(C,N), V(C,N), and ε-Cu. Here, M(C,N) represents a carbonitride of element M (a composite compound of either or both carbide and nitride).
<表層部中の粒界での析出物密度>
 析出物による窒素の移動制限について検討を進めたところ、窒素の移動経路になり易い粒界において析出物が存在すると、窒素の移動をブロックし、窒素の移動を効率的に制限できることが分かった。試験により窒化と表層部中の粒界に析出した析出物との関係を調査した結果、表層部中の粒界に、粒界の長さ当たり粒径0.1μm以上の析出物の個数(粒界析出物密度)が0.10個/μm以上存在すると効果的に窒化が抑制されることが分かった。粒界析出物密度は、好ましくは5個/μm以上、7個/μm以上、10個/μm以上、15個/μm以上、または20個/μm以上であるとよい。粒界析出物密度の上限は特に限定しないが、粒界析出物密度が高すぎると、窒化以外の特性(例えば靭性や耐食性)に影響を及ぼすため好ましくは100個/μm以下、70個/μm以下、50個/μm以下、40個/μm以下、または30個/μm以下であるとよい。
<Precipitate density at grain boundaries in the surface layer>
As a result of further investigation into the restriction of nitrogen migration by precipitates, it was found that the presence of precipitates at grain boundaries that are likely to be a migration path for nitrogen can block the migration of nitrogen and effectively restrict the migration of nitrogen. As a result of investigating the relationship between nitriding and precipitates precipitated at grain boundaries in the surface layer through tests, it was found that nitriding is effectively suppressed when the number of precipitates with a grain size of 0.1 μm or more per grain boundary length (grain boundary precipitate density) is 0.10/μm or more at the grain boundaries in the surface layer. The grain boundary precipitate density is preferably 5/μm or more, 7/μm or more, 10/μm or more, 15/μm or more, or 20/μm or more. There is no particular upper limit to the grain boundary precipitate density, but if the grain boundary precipitate density is too high, it will affect properties other than nitriding (e.g., toughness and corrosion resistance), so the density is preferably 100 particles/μm or less, 70 particles/μm or less, 50 particles/μm or less, 40 particles/μm or less, or 30 particles/μm or less.
 測定対象とする析出物の粒径の上限は、表層部析出物と同様に特に限定しないが、好ましくは2.0μm以下にしてもよい。 As with surface layer precipitates, there is no upper limit to the particle size of the precipitates to be measured, but it may be preferably 2.0 μm or less.
 析出物の種類は、表層部析出物と同様に特に限定しないが、例えばNb(C、N)、Ti(C、N)、W(C、N)、B(C、N)、V(C、N)、およびε-Cuの1種以上があり得る。 The type of precipitates is not particularly limited, as with the surface precipitates, but may be, for example, one or more of Nb(C,N), Ti(C,N), W(C,N), B(C,N), V(C,N), and ε-Cu.
<表層部析出物密度、粒界析出物密度の測定方法>
 表層部における析出物の観察方法について説明する。観察する鋼材試料の鋼材断面において、表面から20μmまでの深さ範囲を観察する。例えば、表面に平行方向に50μm深さ方向に表面から20μmを観察するとよく、この時の観察視野面積は1000μmになる。試料の観察視野をEF-EPMA(例えばJXA-8100、JXA―8350F)を用いて、得られた観測画像を画像解析する。例えば、加速電圧:15kV、電流:1×10-7Aの条件にすることにより得られた観察画像を画像解析し、上記析出物を観察することができる。観察された析出物はEPMAに付属するソフトによりマッピングすることができる。マッピングした析出物の粒径を測定し、粒径0.1~2μm以上の析出物を抽出する。抽出した析出物の個数を観察視野面積で除することにより表層部析出物密度を算出することができる。
<Method of measuring surface layer precipitate density and grain boundary precipitate density>
A method for observing precipitates in the surface layer will be described. In the steel cross section of the steel sample to be observed, a depth range from the surface to 20 μm is observed. For example, it is good to observe 20 μm from the surface in a 50 μm depth direction parallel to the surface, and the observation field area at this time is 1000 μm2. The observation field of the sample is analyzed by image analysis of the obtained observation image. For example, the observation image obtained by setting the conditions of acceleration voltage: 15 kV, current: 1×10 −7 A can be image analyzed to observe the above precipitates. The observed precipitates can be mapped by software attached to the EPMA. The particle size of the mapped precipitates is measured, and precipitates with a particle size of 0.1 to 2 μm or more are extracted. The surface layer precipitate density can be calculated by dividing the number of extracted precipitates by the observation field area.
 なお、析出物の粒径は、観察される析出物の最大幅を長径とし、長径に垂直方向での最大幅を短径として、長径と短径の平均値((長径+短径)/2)を粒径とする。 The particle size of the precipitate is determined by taking the maximum width of the precipitate observed as the long axis and the maximum width perpendicular to the long axis as the short axis, and then taking the average of the long axis and the short axis ((long axis + short axis)/2).
 同様に、得られた観察画像から粒界を特定し、粒界上に存在する析出物を抽出し、画像処理により粒界長さと粒界上の抽出した析出物個数から析出物の線密度である粒界析出物密度(個数/μm)を算出することができる。 Similarly, grain boundaries can be identified from the obtained observation images, precipitates present on the grain boundaries can be extracted, and the grain boundary precipitate density (number/μm), which is the linear density of the precipitates, can be calculated from the grain boundary length and the number of precipitates extracted on the grain boundaries through image processing.
 少なくとも3視野において測定し、得られた表層部析出物密度と粒界析出物密度を算術平均して、その鋼材の表層部析出物密度と粒界析出物密度とする。 Measure in at least three fields of view, and take the arithmetic average of the surface layer precipitate density and grain boundary precipitate density obtained to determine the surface layer precipitate density and grain boundary precipitate density of the steel.
 <粒界割れ長さ>
 以上説明した鋼材表層部にすることにより、鋼材表面の窒化が抑制された結果、窒素(N)の侵入による粒界割れが抑制される。粒界割れは、結晶粒界を観察して長さを測定することができ、鋼材断面の表層部(少なくとも窒化部分を含む部分)の任意の50μm四方の範囲を3箇所選定し、それらにおいて粒界割れ長さの合計が15μm以下であるとよい。3箇所の観察面での粒界割れ長さの合計が15μm以下であれば、鋼材表面の脆化を抑制することができ、500~700℃の温度域での鋼材強度を確保することができる。粒界割れ長さの合計は短いほど好ましく、14μm以下、13μm以下、12μm以下、11μm以下、または10μm以下であるとより好ましい。
<Intergranular crack length>
By forming the steel surface layer as described above, the nitridation of the steel surface is suppressed, and as a result, the grain boundary cracking due to the intrusion of nitrogen (N) is suppressed. The grain boundary cracking can be measured by observing the crystal grain boundaries, and three arbitrary 50 μm square ranges of the surface layer part (at least the part including the nitrided part) of the steel cross section are selected, and the total grain boundary cracking length in those areas is preferably 15 μm or less. If the total grain boundary cracking length in the three observation surfaces is 15 μm or less, the embrittlement of the steel surface can be suppressed, and the steel strength in the temperature range of 500 to 700 ° C. can be ensured. The shorter the total grain boundary cracking length, the more preferable it is, and it is more preferable that it is 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, or 10 μm or less.
 鋼材表層部の粒界割れ長さの測定は、次のように測定することができる。試料となる鋼材断面を光学顕微鏡下で観察視野を50μm四方の正方形として観察し、粒界割れ長さを測定する。この時、鋼材表面に近いほど窒素の影響を受けやすいので、鋼材表面の直下に相当する部分を観察視野にするとよい。測定に際し画像処理により測定することが好ましい。例えば、測定画像上に粒界割れ部分をマーキングし、画像処理によりその長さを測定することができる。 The intergranular crack length in the surface layer of steel can be measured as follows. The cross section of the sample steel is observed under an optical microscope with a square field of view of 50 μm on each side, and the intergranular crack length is measured. At this time, since the area closer to the steel surface is more susceptible to the effects of nitrogen, it is advisable to set the observation field to the area directly below the steel surface. It is preferable to carry out the measurement using image processing. For example, the intergranular crack area can be marked on the measurement image, and its length can be measured using image processing.
 <窒化深さ>
 窒化深さは、表面の窒化割れを抑制する観点から浅ければ浅いほどよい。本発明に係る鋼材は、窒化が抑制されるよう成分が調整されていることと、表面にSi酸化物皮膜を有していることから、窒化深さが平均すると浅くなる。特に、明らかに窒化傾向指数の値が小さい(負の値も含めて)ほど窒化深さも浅くなる傾向がある。窒化深さは接触するガスの窒素(N)含有量でも若干異なるが、概ね100μm以下であれば表面脆化を抑制されることを確認した。窒化深さは、好ましくは90μm以下、80μm以下、70μm以下、60μm以下、50μm以下、40μm以下、30μm以下、20μm以下、10μm以下、5μm以下、4μm以下、3μm以下、または2μm以下であるとよい。なお、窒素濃度が2質量%以上を示す範囲を窒化層とする。
 <鋼材の形態>
 鋼材の形態は特に限定せず、例えば鋼板、棒鋼、線材、鋼管、形鋼などがあり得る。また、鋼材は、鋼板などを加工した部品であってもよい。加工方法は特に限定せず、例えばプレス加工、伸線加工、切削加工などがあり得る。
<Nitriding depth>
The nitriding depth is preferably as shallow as possible from the viewpoint of suppressing nitriding cracks on the surface. The steel material according to the present invention has a composition adjusted to suppress nitriding and has a silicon oxide film on the surface, so that the nitriding depth is shallow on average. In particular, the smaller the nitriding tendency index value (including negative values), the shallower the nitriding depth tends to be. The nitriding depth varies slightly depending on the nitrogen (N) content of the contacting gas, but it has been confirmed that surface embrittlement is suppressed if the nitriding depth is generally 100 μm or less. The nitriding depth is preferably 90 μm or less, 80 μm or less, 70 μm or less, 60 μm or less, 50 μm or less, 40 μm or less, 30 μm or less, 20 μm or less, 10 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less. The range in which the nitrogen concentration is 2 mass% or more is defined as the nitriding layer.
<Steel material form>
The form of the steel material is not particularly limited, and may be, for example, a steel plate, a steel bar, a wire rod, a steel pipe, a steel section, etc. The steel material may also be a part obtained by processing a steel plate, etc. The processing method is not particularly limited, and may be, for example, a press process, a wire drawing process, a cutting process, etc.
 <製造方法>
 次に製造方法について説明する。以下に説明する製造方法は、本発明に係る鋼材を得るための一実施形態であり、この製造方法に限定されるものではない。本発明に係る鋼材が得られるのであれば、その製造方法は限定されない。
<Production Method>
Next, a manufacturing method will be described. The manufacturing method described below is one embodiment for obtaining the steel material according to the present invention, and the manufacturing method is not limited to this. As long as the steel material according to the present invention can be obtained, the manufacturing method is not limited.
 本発明に係る鋼材の製造方法の一実施形態は、常法により鋼材を製造したのち、酸洗浄を行い、その後表面を研磨やショットブラスト、またはショットピーニングなどにより歪を導入するものであり、これにより表面硬度が中央部より高い本発明に係る鋼材が得ることができる。以下、鋼材が鋼板であり、表面に歪を導入する方法として研磨であることを例として説明する。 In one embodiment of the method for manufacturing steel material according to the present invention, after manufacturing steel material by conventional methods, it is subjected to acid washing, and then the surface is polished, shot blasted, or shot peened to introduce strain, thereby obtaining a steel material according to the present invention in which the surface hardness is higher than that of the center. Below, an example will be described in which the steel material is a steel plate and polishing is used as the method for introducing strain into the surface.
 最終焼鈍前の鋼板は、常法の製造方法によって製造すればよい。例えば、製鋼-熱間圧延、製鋼-熱間圧延-焼鈍あるいは製鋼-熱間圧延-酸洗-冷延の工程で製造することができる。 Steel sheets before final annealing can be manufactured using standard manufacturing methods. For example, they can be manufactured using the process of steelmaking-hot rolling, steelmaking-hot rolling-annealing, or steelmaking-hot rolling-pickling-cold rolling.
 ただし、製鋼においては、前記説明した成分になるよう調整した成分を含有する鋼を転炉や電気炉にて溶製し、続いて2次精錬を行う方法が好適である。こうして所定の成分に調整した溶鋼は、公知の鋳造方法(例えば連続鋳造法)に従ってスラブにする。スラブは、所定の温度に加熱され、所定の板厚に熱間圧延される。熱延後、必要に応じて冷間圧延(冷延)してもよい。冷延も、常法にて行えばよい。 However, in steelmaking, a suitable method is to melt steel containing the components adjusted to the composition described above in a converter or electric furnace, followed by secondary refining. The molten steel thus adjusted to the desired composition is made into slabs using a known casting method (e.g., continuous casting). The slabs are heated to a desired temperature and hot rolled to a desired thickness. After hot rolling, the steel may be cold rolled (cold rolling) if necessary. Cold rolling may also be performed using conventional methods.
 製造工程における諸条件は適宜選択すれば良い。例えば、スラブ厚さ、熱間圧延板厚などは適宜設定すれば良い。熱延板の巻取後に水冷プールに浸漬しても構わない。熱延後あるいは熱延焼鈍後の酸洗工程については特に限定せず、ショットブラスト、ベンディング、ブラシ等のメカニカルデスケール方法については適宜選択すれば良い。熱延後の酸洗液についても特に限定しないので、例えば硫酸、硝弗酸等の既設条件で構わない。さらに、この後にコイル研削を表面に施しても構わない。 The various conditions in the manufacturing process may be selected as appropriate. For example, the slab thickness and hot-rolled sheet thickness may be set as appropriate. After coiling the hot-rolled sheet, it may be immersed in a water-cooled pool. There are no particular restrictions on the pickling process after hot rolling or after hot-rolling annealing, and the mechanical descaling method, such as shot blasting, bending, or brushing, may be selected as appropriate. There are no particular restrictions on the pickling solution after hot rolling, either, so existing conditions such as sulfuric acid, nitric hydrofluoric acid, etc. may be used. Furthermore, the coil surface may be ground after this.
 こうして得られた熱延鋼板、熱延焼鈍鋼板、冷延鋼板を再結晶させるため最終焼鈍する。焼鈍雰囲気は特に限定されず、大気雰囲気でもよい。温度900~1100℃の温度域にて焼鈍するとよい。 The hot-rolled steel sheet, hot-rolled annealed steel sheet, and cold-rolled steel sheet thus obtained are then subjected to final annealing to recrystallize. The annealing atmosphere is not particularly limited, and may be air. Annealing is preferably performed in the temperature range of 900 to 1100°C.
 表層部に析出物を析出する観点から、焼鈍温度が高過ぎても固溶状態のまま析出が進まないため、好ましくは1100℃以下、または1050℃以下にするとよい。一方、焼鈍温度が低すぎると析出物の個数が少なくなるため、好ましくは900℃以上、930℃以上、または950℃以上にするとよい。このメカニズムは解明されていないが、焼鈍温度が低いと析出物の析出核が生成しないものと推測される。保定時間は特に限定しないが、短過ぎると析出時間が不足するおそれがあるため好ましくは30秒以上または60秒以上にするとよい。一方、保定時間が長すぎても析出物が粗大化し靭性や耐食性に影響を与えるおそれがあるため、保定時間は好ましくは5分以下、3分以下、または2分以下であるとよい。 From the viewpoint of precipitating precipitates in the surface layer, if the annealing temperature is too high, the precipitation will not proceed in the solid solution state, so it is preferably 1100°C or less, or 1050°C or less. On the other hand, if the annealing temperature is too low, the number of precipitates will be small, so it is preferably 900°C or more, 930°C or more, or 950°C or more. Although the mechanism behind this has not been clarified, it is presumed that if the annealing temperature is low, precipitation nuclei will not be generated. There are no particular restrictions on the holding time, but if it is too short, there is a risk of insufficient precipitation time, so it is preferably 30 seconds or more or 60 seconds or more. On the other hand, if the holding time is too long, there is a risk of the precipitates becoming coarse and affecting toughness and corrosion resistance, so it is preferably 5 minutes or less, 3 minutes or less, or 2 minutes or less.
 焼鈍後、鋼板を冷却する。冷却条件は特に限定しないが、析出物を所定の粒径に成長させるために800℃から300℃までの温度範囲における鋼板の滞留時間を好ましくは30秒以上、120秒以下にするとよい。滞留時間は、さらに好ましくは40秒以上、50秒以上、または60秒以上であるとよく、110秒以下、100秒以下、または90秒以下であるとよい。 After annealing, the steel sheet is cooled. There are no particular limitations on the cooling conditions, but the residence time of the steel sheet in the temperature range of 800°C to 300°C is preferably 30 seconds or more and 120 seconds or less in order to grow the precipitates to a specified grain size. The residence time is more preferably 40 seconds or more, 50 seconds or more, or 60 seconds or more, and is preferably 110 seconds or less, 100 seconds or less, or 90 seconds or less.
 最終焼鈍後の鋼板を50℃以下まで冷却した後、酸洗して、上層のCr酸化物層とFe酸化物層をエッチング除去する。そのため酸洗液はフッ酸(HF)3.0%以下、硝酸6~15%を含む酸洗溶液で、温度50~70℃で浸漬時間60~90秒になるように調整するとよい。これにより、製造プロセスにて生成された上層のFe酸化物、Cr酸化物、Si酸化物が除去された鋼板表面を得ることができる。 After the final annealing, the steel sheet is cooled to below 50°C and then pickled to etch away the upper Cr oxide and Fe oxide layers. The pickling solution used for this purpose is one containing 3.0% or less hydrofluoric acid (HF) and 6-15% nitric acid, and is adjusted to a temperature of 50-70°C and an immersion time of 60-90 seconds. This makes it possible to obtain a steel sheet surface from which the upper layers of Fe oxide, Cr oxide, and Si oxide generated during the manufacturing process have been removed.
 酸洗後の鋼板を表面研磨して、表面に転位(歪)を付与して、表面を硬化させる。この時の研磨条件(研磨時間や押付け強度など)により、表層硬化層の厚さを調整することができる。研磨条件は特に限定しないが、乾式研磨や湿式研磨などを適用することができる。例えば乾式研磨の場合、#220の研磨剤を用いて鋼板表面を研磨し、その後#400~#600の研磨剤を用いて鋼板表面を研磨するとよい。例えば#220の研磨剤に続いて#400または/および#600の研磨剤を用いて鋼板の表面を研磨するとよい。粗い研磨剤から細かい研磨剤になるよう順に研磨することにより鋼板表面粗さの低減および疵を無くすことができるからである。研磨剤の組み合わせは特に限定されない。研磨量も特に限定されないが、実際の生産上10μm~50μm程度が好ましい。実際は、鋼材により条件が異なるため、事前に研磨して表面硬度や表面疵を確認しつつ適宜決定すればよい。 The surface of the steel sheet after pickling is polished to give dislocations (distortion) to the surface and harden it. The thickness of the surface hardened layer can be adjusted by the polishing conditions (polishing time, pressing strength, etc.). The polishing conditions are not particularly limited, but dry polishing or wet polishing can be applied. For example, in the case of dry polishing, it is recommended to polish the steel sheet surface using a #220 abrasive, and then polish the steel sheet surface using a #400 to #600 abrasive. For example, it is recommended to polish the steel sheet surface using a #220 abrasive, followed by a #400 and/or #600 abrasive. This is because the surface roughness of the steel sheet can be reduced and defects can be eliminated by polishing in order from coarse to fine abrasives. The combination of abrasives is not particularly limited. The polishing amount is also not particularly limited, but it is preferable to use about 10 μm to 50 μm in actual production. In reality, since the conditions differ depending on the steel material, it is sufficient to polish in advance and check the surface hardness and surface defects before deciding appropriately.
 湿式研磨の場合、研磨剤を混ぜた油を鋼板表面に塗布し、研磨布を回転させつつ、鋼板表面を研磨することができる。この時も、研磨剤を粗いものから細かいものになるように順に研磨するとよい。研磨後は、表面を水洗やアルカリ洗浄して、鋼板表面上に残存した研磨剤を除去すればよい。 In the case of wet polishing, oil mixed with an abrasive is applied to the surface of the steel plate, and the surface of the steel plate is polished while rotating the polishing cloth. In this case, too, it is recommended to polish the surface with abrasives in order from coarse to fine. After polishing, the surface can be washed with water or washed with an alkali to remove any abrasives remaining on the steel plate surface.
 表面に転位(歪)を付与する方法としてショットブラストやショットピーニングを採用する場合も、ショットブラストやショットピーニングの条件は特に限定されず、事前に試験して表面硬度や表面疵を確認しつつ適宜決定すればよい。 Even if shot blasting or shot peening is used as a method for imparting dislocations (distortion) to the surface, the conditions for shot blasting or shot peening are not particularly limited, and can be appropriately determined by checking the surface hardness and surface defects through prior testing.
<用途>
 本発明に係る鋼材は、窒素(N)含有量の多いガス環境において使用しても、良好な耐窒化性から鋼材表層の窒素浸入が少なく、粒界割れが抑制される。さらに、耐酸化性も備え、特に500~700℃程度の中高温域において、従来のステンレス鋼で問題となる赤スケール発生に対しても有効である。このことから、例えば窒素含有量が高く、ガス温度が500~700℃の中高温域であるアンモニアの燃焼機器用に用いることができる。特にアンモニア燃焼機器の排気部品などに使用することができる。
 もちろん、耐窒化性、耐酸化性の性質から、例えばアンモニアや尿素などに直接接する容器や配管部品などに使用しても、溶液中の窒素イオンやアンモニアや尿素が蒸発ガス中の窒素が鋼材表層へ侵入することが抑えられ、粒界割れが抑制される。
 その他、耐窒化性と耐酸化性を要求する部品に本発明に係る鋼材を適用すると、その効果を得ることができる。
<Applications>
Even when the steel material according to the present invention is used in a gas environment with a high nitrogen (N) content, the good nitriding resistance reduces nitrogen penetration into the surface layer of the steel material, suppressing grain boundary cracking. Furthermore, the steel material also has oxidation resistance, and is effective against the generation of red scale, which is a problem with conventional stainless steels, particularly in the medium-high temperature range of about 500 to 700°C. For this reason, the steel material according to the present invention can be used, for example, for ammonia combustion equipment with a high nitrogen content and a medium-high gas temperature range of 500 to 700°C. In particular, the steel material can be used for exhaust parts of ammonia combustion equipment.
Of course, due to its nitridation-resistance and oxidation-resistance, even if it is used for containers or piping parts that come into direct contact with ammonia or urea, the nitrogen ions in the solution and the nitrogen in the evaporated gas from ammonia or urea are prevented from penetrating into the surface layer of the steel material, thereby suppressing grain boundary cracking.
In addition, the steel material according to the present invention can be used to obtain the same effects when applied to parts that require resistance to nitridation and oxidation.
 以下、実施例によって本発明をより具体的に説明するが、本発明はこれらの実施例に限定されるものではない。 The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.
 表1に示す成分組成の鋼を溶製しスラブに鋳造し、スラブを熱間圧延して板厚4mmの熱延鋼板を得た。その後熱延版の焼鈍を900~1100℃の温度で行い、酸洗を行い冷間圧延して、板厚1.5mmの冷延鋼板を得た。得られた冷延鋼板を900~1100℃の温度で90秒保定して焼鈍(最終焼鈍)し、その後800℃~300℃の滞留時間が90秒となるよう冷却し、50℃以下になるまで冷却した。その後、50~70℃の酸洗液(2%フッ酸+10%硝酸+水)中に60~90秒浸漬し(最終酸洗)、水洗して試験材を得た。  Steel with the composition shown in Table 1 was melted and cast into a slab, which was then hot-rolled to obtain a hot-rolled steel sheet with a thickness of 4 mm. The hot-rolled sheet was then annealed at a temperature of 900-1100°C, pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.5 mm. The resulting cold-rolled steel sheet was annealed (final annealing) by holding it at a temperature of 900-1100°C for 90 seconds, then cooled so that the residence time at 800°C-300°C was 90 seconds, and then cooled to below 50°C. The steel was then immersed in a pickling solution (2% hydrofluoric acid + 10% nitric acid + water) at 50-70°C for 60-90 seconds (final pickling), and rinsed with water to obtain the test material.
 なお、試料3は最終焼鈍の保定時間を30秒とし、その後の冷却での800℃~300℃の滞留時間を90秒になるよう冷却した。試料8は、最終焼鈍の保定時間を30秒とし、その後の冷却での800℃~300℃の滞留時間も30秒になるよう冷却した。 For sample 3, the holding time for the final annealing was 30 seconds, and the subsequent cooling was performed with a residence time between 800°C and 300°C of 90 seconds. For sample 8, the holding time for the final annealing was 30 seconds, and the subsequent cooling was performed with a residence time between 800°C and 300°C of 30 seconds.
 得られた試験材から、20mm×25mmの試験片を4本切り出して、表面を研磨した。研磨は乾式研磨であり、研磨剤を塗布した研磨紙を樹脂製ホルダーに巻き付け、試験片表面にホルダーを押付け、速度一定(10m/分)で長さ300mmを往復動させ研磨圧力を表2に示すように設定して研磨した。研磨剤は、1回目に#220の研磨剤を、2回目に#400の研磨剤を用いた。 Four test pieces measuring 20 mm x 25 mm were cut from the obtained test material and the surfaces were polished. Polishing was performed using a dry polishing method, in which abrasive-coated abrasive paper was wrapped around a resin holder, and the holder was pressed against the surface of the test piece, and the holder was moved back and forth over a length of 300 mm at a constant speed (10 m/min) while the polishing pressure was set as shown in Table 2. A #220 abrasive was used for the first pass, and a #400 abrasive was used for the second pass.
 得られた試験片のうち1本について、表面に垂直な断面が得られるよう切断し、表面ビッカース硬度と中央部ビッカース硬度を荷重50g重で測定した。また、表面から深さ方向に0.5μm、1.0μm、2.0μmと以降1.0μmピッチで測定し、表層硬化層の深さを特定した。さらに、同じ鋼材断面で、表面方向に50μmで表面から深さ20μmを観察視野として、FE-EPMAにて観察視野内の析出物の析出状況を画像解析した。粒径0.1μm以上の析出物を対象として、表層部の析出物密度(表層部析出物密度)と、結晶粒間の粒界に存在する析出物個数を観察視野中の粒界長さの合計で除して粒界1μmあたりの析出物個数(粒界析出物密度)を求めた。 One of the obtained test pieces was cut to obtain a cross section perpendicular to the surface, and the surface Vickers hardness and central Vickers hardness were measured with a load of 50 g. In addition, measurements were made from the surface in the depth direction at 0.5 μm, 1.0 μm, 2.0 μm, and then at 1.0 μm intervals to identify the depth of the surface hardened layer. Furthermore, for the same steel cross section, the observation field was set at 50 μm in the surface direction and 20 μm deep from the surface, and the precipitation status within the observation field was image-analyzed using FE-EPMA. For precipitates with a grain size of 0.1 μm or more, the precipitate density in the surface layer (surface layer precipitate density) and the number of precipitates present at the grain boundaries between crystal grains were divided by the total grain boundary length in the observation field to determine the number of precipitates per 1 μm of grain boundary (grain boundary precipitate density).
 残り3本の試験片はアンモニア燃焼ガスを想定して窒化および酸化処理をした。 窒化・酸化処理は、焼鈍炉に雰囲気ガスにアンモニア10vol%、水蒸気10vol%、残部窒素(N)のガスを導入し、残りの試験片を炉内に載置し、温度600℃に加熱後50時間保持し、その後冷却して取り出し、粒界割れ長さと窒化深さを測定した。 The remaining three test pieces were nitrided and oxidized to simulate ammonia combustion gas. For the nitriding and oxidation treatment, 10 vol% ammonia, 10 vol% water vapor, and the balance nitrogen (N) were introduced into the annealing furnace as atmospheric gas, and the remaining test pieces were placed in the furnace and heated to a temperature of 600°C and held there for 50 hours, after which they were cooled and removed, and the intergranular crack length and nitriding depth were measured.
 粒界割れ長さは、窒化・酸化処理後の試験片を板厚方向の断面が観察できるように切断し、光学顕微鏡を用いて試験片の断面を観察した。観察は、鋼板表面直下の50μm×50μmの範囲を1視野とし、試料断面中のランダムに選択した3カ所を観察し、粒界割れ発生長さを測定した。3か所の観察面での粒界割れ長さの合計が15μm以下であれば耐窒化性は良好である。 The intergranular crack length was measured by cutting the test piece after nitriding and oxidation treatment so that the cross section in the thickness direction could be observed, and observing the cross section of the test piece using an optical microscope. The observation was performed with a field of view of 50 μm x 50 μm just below the steel plate surface, and three randomly selected points on the sample cross section were observed to measure the intergranular crack length. If the total intergranular crack length on the three observation surfaces was 15 μm or less, the nitriding resistance was good.
 窒化層厚さは、窒化・酸化処理後の試験片を切断し、断面をEPMA分析により表面から厚さ方向の窒素濃度分布を測定し、窒素濃度が2質量%以上を示す範囲を窒化層とした。 The thickness of the nitrided layer was determined by cutting the test piece after nitriding and oxidation treatment, and measuring the nitrogen concentration distribution from the surface to the thickness direction by EPMA analysis of the cross section, and the area showing a nitrogen concentration of 2 mass% or more was defined as the nitrided layer.
 赤スケール性(耐酸化性)は目視にて確認し、赤スケールが発見できなかったものを合格(〇)、赤スケールが少しでも発見されたものを不合格(×)とした。 The red scale (oxidation resistance) was checked visually, and samples with no detectable red scale were rated as passing (◯), while samples with even a small amount of red scale were rated as failing (×).
 これらの測定結果を表2に示す。表2のデータから、本発明に係る鋼板は粒界割れ長さが低減していることが分かる。 These measurement results are shown in Table 2. The data in Table 2 show that the intergranular crack length is reduced in the steel plate according to the present invention.
 本発明は、自動車工業、一般機械産業などあらゆる産業において利用することができる。 This invention can be used in all industries, including the automotive and general machinery industries.

Claims (11)

  1.  質量%で、
    C :0~0.150%、
    Si:0.05~4.50%、
    Mn:0.05~3.00%、
    P :0.050%以下、
    S :0.0050%以下、
    Ni:8.00~21.00%、
    Cr:15.00~30.00%、
    N :0~0.350%、
    Nb:0~1.00%
    Mo:0~3.00%
    Cu:0~3.50%
    Al:0.002~0.800%、
    Ti:0~0.600%
    V :0~1.00%、
    B :0~0.0100%、
    Ca:0~0.0150%
    Sn:0~1.00%、
    Hf:0~0.60%、
    Zr:0~0.60%、
    Sb:0~0.60%、
    Co:0~1.50%、
    W :0~2.00%、
    Ta:0~1.00%、
    Ga:0~0.50%、
    Mg:0~0.0050%、および
    REM:0~0.200%を含み、
    残部Feおよび不純物からなるオーステナイト系ステンレス鋼材であり、
    以下の式1を満足し、
    前記鋼材表面のビッカース硬度(Hvs)が、前記鋼材中央部のビッカース硬度(Hvc)より20Hv以上高いことを特徴とするオーステナイト系ステンレス鋼材。
    0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si≦15.0
    ・・・・・(式1)
    ただし、式1中の元素記号は、当該元素の含有量(質量%)を示し、含有しない場合は0を代入する。
    In mass percent,
    C: 0 to 0.150%,
    Si: 0.05-4.50%,
    Mn: 0.05-3.00%,
    P: 0.050% or less,
    S: 0.0050% or less,
    Ni: 8.00-21.00%,
    Cr: 15.00-30.00%,
    N: 0 to 0.350%,
    Nb: 0-1.00%
    Mo: 0-3.00%
    Cu: 0-3.50%
    Al: 0.002-0.800%,
    Ti: 0-0.600%
    V: 0 to 1.00%,
    B: 0 to 0.0100%,
    Ca: 0-0.0150%
    Sn: 0 to 1.00%,
    Hf: 0-0.60%,
    Zr: 0 to 0.60%,
    Sb: 0 to 0.60%,
    Co: 0 to 1.50%,
    W: 0-2.00%,
    Ta: 0 to 1.00%,
    Ga: 0 to 0.50%,
    Mg: 0 to 0.0050% and REM: 0 to 0.200%;
    The remainder is an austenitic stainless steel material consisting of Fe and impurities,
    The following formula 1 is satisfied:
    An austenitic stainless steel material, characterized in that the Vickers hardness (Hvs) of the surface of the steel material is 20 Hv or more higher than the Vickers hardness (Hvc) of the center of the steel material.
    0.5Cr+10Al+2Mo+3Ti+0.5Cu-1.5Si≦15.0
    .... (Equation 1)
    In the formula 1, the symbol of an element indicates the content (mass%) of the element, and 0 is substituted when the element is not contained.
  2.  前記鋼材の表面に垂直な断面において、前記鋼材中央部のビッカース硬度より20Hv以上高いビッカース硬度を有する表層硬化層を有し、前記表層硬化層の厚さが前記鋼材の表面から深さ方向に0.5μm以上である、請求項1に記載のオーステナイト系ステンレス鋼材。 The austenitic stainless steel material according to claim 1, which has a surface hardened layer having a Vickers hardness that is 20 Hv or more higher than the Vickers hardness of the center of the steel material in a cross section perpendicular to the surface of the steel material, and the thickness of the surface hardened layer is 0.5 μm or more in the depth direction from the surface of the steel material.
  3.  前記鋼材の表面に垂直な断面において、前記鋼材表面から深さ20μmの表層部に、粒径0.1μm以上の析出物が7個/1000μm2以上存在する、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材。 The austenitic stainless steel material according to claim 1 or 2, in which, in a cross section perpendicular to the surface of the steel material, there are 7 precipitates/1000 μm2 or more with a grain size of 0.1 μm or more in the surface layer at a depth of 20 μm from the surface of the steel material.
  4.  前記鋼材の表面に垂直な断面において、前記鋼材表面から深さ20μmの表層部中の粒界に、粒径0.1μm以上の析出物が0.10個/μm以上存在する、請求項1~3のいずれか1項に記載のオーステナイト系ステンレス鋼材。 The austenitic stainless steel material according to any one of claims 1 to 3, in which, in a cross section perpendicular to the surface of the steel material, there are 0.10 precipitates/μm or more with a grain size of 0.1 μm or more at the grain boundaries in the surface layer at a depth of 20 μm from the surface of the steel material.
  5.  前記鋼板の表面に垂直な断面で、50μm四方の範囲を1視野として、任意の3視野における粒界割れ長さの合計が15μm以下である、請求項1~4のいずれか1項に記載のオーステナイト系ステンレス鋼材。 An austenitic stainless steel material according to any one of claims 1 to 4, in which the total length of intergranular cracks in any three fields of view, each of which is a 50 μm square area in a cross section perpendicular to the surface of the steel plate, is 15 μm or less.
  6.  請求項1~5のいずれか1項に記載のアンモニア燃焼機器用オーステナイト系ステンレス鋼材。  The austenitic stainless steel material for ammonia combustion equipment according to any one of claims 1 to 5.
  7.  請求項1~6のいずれか1項に記載のオーステナイト系ステンレス鋼材の製造方法であって、請求項1に記載の成分を有する鋼材を、最終冷延後に、900~1100℃に加熱保持した後、50℃以下の温度まで冷却し、フッ酸2.0%以下、硝酸6~15%を含有し、温度50~70℃の酸洗液中に60~90秒間浸漬して酸洗し、その後前記鋼材表面を研磨、ショットブラストまたはショットピーニングする工程を有することを特徴とするオーステナイト系ステンレス鋼材の製造方法。 A method for producing an austenitic stainless steel material according to any one of claims 1 to 6, comprising the steps of heating and holding a steel material having the composition according to claim 1 at 900 to 1100°C after final cold rolling, cooling to a temperature of 50°C or less, immersing the steel material in a pickling solution containing 2.0% or less hydrofluoric acid and 6 to 15% nitric acid at a temperature of 50 to 70°C for 60 to 90 seconds to pickle the steel material, and then polishing, shot blasting or shot peening the surface of the steel material.
  8.  前記900~1100℃に加熱保持における保持時間が30秒以上90秒以下であり、前記冷却において800℃から300℃までの温度範囲での滞留時間が30秒以上120秒以下である請求項7に記載のオーステナイト系ステンレス鋼材の製造方法。 The method for manufacturing austenitic stainless steel material according to claim 7, wherein the holding time during heating to 900 to 1100°C is 30 seconds or more and 90 seconds or less, and the residence time during cooling in the temperature range from 800°C to 300°C is 30 seconds or more and 120 seconds or less.
  9.  前記研磨が乾式研磨であって、#220に続いて#400または/および#600の順に研磨する、請求項7または8に記載のオーステナイト系ステンレス鋼材の製造方法。 The method for manufacturing an austenitic stainless steel material according to claim 7 or 8, wherein the polishing is dry polishing, and polishing is performed in the order of #220, followed by #400 and/or #600.
  10.  請求項1~6のいずれか1項に記載のオーステナイト系ステンレス鋼材を少なくとも一部に有する、部品。 A part having at least a portion made of the austenitic stainless steel material described in any one of claims 1 to 6.
  11.  アンモニア燃焼機器用部品である請求項10に記載の部品。 The part according to claim 10, which is an ammonia burning device part.
PCT/JP2024/004116 2023-02-07 2024-02-07 Austenitic stainless steel material WO2024166947A1 (en)

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Citations (5)

* Cited by examiner, † Cited by third party
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JP2004043903A (en) * 2002-07-12 2004-02-12 Nisshin Steel Co Ltd Austenitic stainless steel superior in red scale resistance
JP2009068079A (en) * 2007-09-14 2009-04-02 Sumitomo Metal Ind Ltd Steel tube with excellent steam oxidation resistance
KR20110071508A (en) * 2009-12-21 2011-06-29 주식회사 포스코 Austenitic stainless steel for gas nitriding and gas nitriding method of the same
JP2022155181A (en) * 2021-03-30 2022-10-13 日鉄ステンレス株式会社 austenitic stainless steel
JP2023144254A (en) * 2022-03-28 2023-10-11 日本製鉄株式会社 Shape steel of austenitic stainless steel and manufacturing method thereof

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004043903A (en) * 2002-07-12 2004-02-12 Nisshin Steel Co Ltd Austenitic stainless steel superior in red scale resistance
JP2009068079A (en) * 2007-09-14 2009-04-02 Sumitomo Metal Ind Ltd Steel tube with excellent steam oxidation resistance
KR20110071508A (en) * 2009-12-21 2011-06-29 주식회사 포스코 Austenitic stainless steel for gas nitriding and gas nitriding method of the same
JP2022155181A (en) * 2021-03-30 2022-10-13 日鉄ステンレス株式会社 austenitic stainless steel
JP2023144254A (en) * 2022-03-28 2023-10-11 日本製鉄株式会社 Shape steel of austenitic stainless steel and manufacturing method thereof

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