WO2024111516A1 - Acier inoxydable austénitique - Google Patents

Acier inoxydable austénitique Download PDF

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WO2024111516A1
WO2024111516A1 PCT/JP2023/041434 JP2023041434W WO2024111516A1 WO 2024111516 A1 WO2024111516 A1 WO 2024111516A1 JP 2023041434 W JP2023041434 W JP 2023041434W WO 2024111516 A1 WO2024111516 A1 WO 2024111516A1
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content
less
mass
extraction residue
steel
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PCT/JP2023/041434
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Japanese (ja)
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工 西本
雅浩 ▲瀬▼戸
克樹 田中
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日本製鉄株式会社
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
    • C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

  • This disclosure relates to austenitic stainless steels.
  • Boilers installed in thermal power plants, chemical plants, and other plants are exposed to high temperatures. For this reason, heat transfer tubes used in boilers (hereinafter simply referred to as “boiler heat transfer tubes”) are required to have good high-temperature strength, specifically creep strength.
  • Patent Documents 1 and 2 disclose austenitic stainless steels with good creep strength.
  • SCC stress corrosion cracking
  • Patent Documents 1 and 2 do not consider SCC resistance and steam oxidation resistance. Therefore, the austenitic stainless steels disclosed in the above documents have room for further improvement in terms of SCC resistance and steam oxidation resistance.
  • the present disclosure aims to solve the above problems and provide an austenitic stainless steel that has good creep strength, SCC resistance, and steam oxidation resistance.
  • This disclosure has been made to solve the above problems, and is centered on the following austenitic stainless steel and steel pipes.
  • V ER V content (mass%) in the precipitate obtained by extraction residue analysis
  • TiER Ti content (mass%) in the precipitate obtained by extraction residue analysis
  • NbER Nb content (mass%) in the precipitate obtained by extraction residue analysis
  • NiER Ni content (mass%) in the precipitate obtained by extraction residue analysis
  • CrER Cr content (mass%) in the precipitate obtained by extraction residue analysis
  • Mo ER Mo content (mass%) in the precipitate obtained by extraction residue analysis
  • Hv 40 Vickers hardness measured at a position 40 ⁇ m from the surface in the thickness direction with a test load of 10 gf.
  • Hv t/2 Vickers hardness measured at a position 1/2t from the surface in the thickness direction with a test load of 10 gf, where t is the total thickness.
  • each symbol is defined as follows, and each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and when no element is contained, the symbol is set to zero.
  • V ER V content (mass%) in the precipitate obtained by extraction residue analysis
  • TiER Ti content (mass%) in the precipitate obtained by extraction residue analysis
  • NbER Nb content (mass%) in the precipitate obtained by extraction residue analysis
  • NiER Ni content (mass%) in the precipitate obtained by extraction residue analysis
  • CrER Cr content (mass%) in the precipitate obtained by extraction residue analysis
  • Mo ER Mo content (mass%) in the precipitate obtained by extraction residue analysis
  • Hv 40 Vickers hardness measured at a position 40 ⁇ m from the surface in the thickness direction with a test load of 10 gf.
  • Hv t/2 Vickers hardness measured at a position 1/2t from the surface in the thickness direction with a test load of 10 gf, where t is the total thickness.
  • An austenitic stainless steel according to any one of (1) to (4) above which is a steel pipe and the surface is the inner surface of the steel pipe.
  • an austenitic stainless steel having good creep strength, SCC resistance, and steam oxidation resistance.
  • FIG. 1 is a photograph of the structure of an Nb compound.
  • the inventors have investigated methods for improving the creep strength, SCC resistance, and steam oxidation resistance of austenitic stainless steels, and have obtained the following findings.
  • (a) C has the effect of increasing creep strength.
  • the austenitic stainless steel disclosed in Patent Document 1 contains 0.03% or more C.
  • C can be a factor in the occurrence of SCC.
  • the Cr in the steel combines with C to form Cr carbides.
  • a Cr-deficient layer is formed, and SCC occurs in a corrosive environment where chlorides and the like are present. Therefore, from the perspective of suppressing SCC, it is necessary to reduce the C content.
  • solute Nb inhibits excessive penetration of O into the steel in the usage environment, promoting the formation of Cr oxides, which, although their formation rate is slow, contribute to improving steam oxidation resistance. For this reason, it is effective to have Nb in a solute state in advance, not only from the standpoint of creep strength but also from the standpoint of steam oxidation resistance. Then, it is preferable to perform shot blasting or the like to form a processed layer near the surface.
  • C 0.002 to 0.020%
  • C (carbon) is an element necessary for ensuring high-temperature strength, particularly creep strength. Therefore, the C content is 0.002% or more.
  • the C content is preferably 0.003% or more, and more preferably 0.004% or more.
  • the C content is 0.020% or less.
  • the C content is preferably 0.015% or less, and more preferably 0.010% or less.
  • Si 0.10 to 0.60% Si (silicon) is an element having a deoxidizing effect. Therefore, the Si content is 0.10% or more.
  • the Si content is preferably 0.12% or more, and more preferably 0.14% or more. However, if Si is contained excessively, the workability decreases. Therefore, the Si content is 0.60% or less.
  • the Si content is preferably 0.50% or less, and more preferably 0.40% or less.
  • Mn 0.2 to 2.0% Mn (manganese) combines with the impurity S contained in the steel to form MnS, which has the effect of improving hot workability. For this reason, the Mn content is 0.2% or more.
  • the Mn content is preferably 0.4% or more, and more preferably 0.6% or more.
  • the Mn content is 2.0% or less.
  • the Mn content is preferably 1.5% or less, and more preferably 1.3% or less.
  • P phosphorus
  • P is an element contained in steel as an impurity, and reduces SCC resistance. P also reduces the hot workability and toughness of steel. For this reason, the P content is 0.035% or less.
  • the P content is preferably 0.030% or less, and more preferably 0.025% or less. It is preferable to reduce the P content as much as possible, but if the content is reduced excessively, the manufacturing cost increases. For this reason, the P content is preferably 0.010% or more.
  • S 0.010% or less
  • S sulfur
  • the S content is preferably 0.009% or less, and more preferably 0.008% or less. It is preferable to reduce the S content as much as possible, but if the content is reduced excessively, the manufacturing cost increases. For this reason, the S content is preferably 0.0001% or more.
  • Cu 2.50 to 4.50% Cu (copper) precipitates as a Cu phase in grains, and increases the creep strength and creep ductility of steel by precipitation strengthening. Therefore, the Cu content is 2.50% or more.
  • the Cu content is preferably 2.70% or more, and more preferably 2.90% or more. However, if Cu is contained in excess, hot workability and weldability are reduced. Therefore, the Cu content is 4.50% or less.
  • the Cu content is preferably 4.00% or less, and more preferably 3.50% or less.
  • Ni 9.00 to 16.00%
  • Ni nickel
  • Ni is an element that stabilizes the austenite structure and has the effect of improving SCC resistance and corrosion resistance. Ni also has the effect of improving creep strength. For this reason, the Ni content is 9.00% or more.
  • the Ni content is preferably 10.00% or more, and more preferably 10.50% or more. However, if Ni is contained in excess, the manufacturing cost increases. Moreover, the creep strength decreases. For this reason, the Ni content is 16.00% or less.
  • the Ni content is preferably 15.00% or less, and more preferably 14.00% or less.
  • Cr 15.00 to 20.00%
  • Cr chromium
  • Cr has the effect of improving creep strength. Therefore, the Cr content is 15.00% or more.
  • the Cr content is preferably 15.50% or more, and more preferably 16.00% or more.
  • the Cr content is set to 20.00% or less.
  • the Cr content is preferably 19.75% or less, and more preferably 19.50% or less.
  • Mo 0.20 to 1.50% Mo (molybdenum) has the effect of improving creep strength. Therefore, the Mo content is 0.20% or more.
  • the Mo content is preferably 0.35% or more, and more preferably 0.50% or more. However, if Mo is contained in excess, the stability of the austenitic structure decreases. Therefore, the Mo content is 1.50% or less.
  • the Mo content is preferably 1.25% or less, and more preferably 1.00% or less.
  • Nb 0.15 to 0.60%
  • Nb niobium
  • the Nb content is 0.15% or more.
  • the Nb content is preferably 0.20% or more, and more preferably 0.25% or more.
  • the Nb content is 0.60% or less.
  • the Nb content is preferably 0.55% or less, and more preferably 0.50% or less.
  • N 0.05 to 0.15%
  • nitrogen has the effect of improving strength by solid solution strengthening and precipitation strengthening by Nb carbonitride. Therefore, the N content is 0.05% or more.
  • the N content is preferably 0.06% or more, and more preferably 0.07% or more.
  • the N content is 0.15% or less.
  • the N content is preferably 0.13% or less, and more preferably 0.12% or less.
  • B 0.0010 to 0.0060%
  • B boron
  • the B content is preferably 0.0015% or more, and more preferably 0.0020% or more.
  • the B content is 0.0060% or less.
  • the B content is preferably 0.0050% or less, and more preferably 0.0045% or less.
  • the chemical composition of the austenitic stainless steel of this embodiment may further contain one or more elements selected from V and Ti (hereinafter, referred to as "Group A") within the ranges shown below. The reasons for limiting each element will be explained.
  • V 0.50% or less
  • V vanadium
  • V has the effect of reducing solute C and increasing SCC resistance.
  • V also has the effect of increasing creep strength. For this reason, V may be contained as necessary. However, if V is contained in excess, ⁇ ferrite is generated, and the creep strength, toughness, and weldability of the steel are reduced. For this reason, the V content is 0.50% or less.
  • the V content is preferably 0.40% or less, and more preferably 0.30% or less.
  • the V content may be 0% or more, but for example, in order to obtain the above effect, the V content is preferably 0.01% or more, and more preferably 0.02% or more.
  • Ti 0.500% or less
  • Ti titanium
  • Ti has the effect of reducing solute C and increasing SCC resistance, similar to V.
  • Ti also has the effect of increasing creep strength. For this reason, Ti may be contained as necessary. However, if Ti is contained in excess, creep strength decreases. For this reason, the Ti content is 0.500% or less.
  • the Ti content is preferably 0.400% or less, more preferably 0.100% or less, and even more preferably 0.050% or less.
  • the Ti content may be 0% or more, but for example, in order to obtain the above effect, the Ti content is preferably 0.001% or more, and more preferably 0.002% or more.
  • the chemical composition of the austenitic stainless steel of this embodiment may further contain one or more elements selected from Co, W, Ta, Sn, Al, Ca, Mg and REM (hereinafter referred to as "group B") within the ranges shown below. The reasons for limiting each element will be explained.
  • Co 1.00% or less
  • Co (cobalt) has the effect of stabilizing the austenite structure and increasing creep strength. Therefore, Co may be contained as necessary. However, if Co is contained in excess, the manufacturing cost increases. Therefore, the Co content is 1.00% or less.
  • the Co content is preferably 0.50% or less, and more preferably 0.30% or less.
  • the Co content may be 0% or more, but for example, in order to obtain the above effect, the Co content is preferably 0.02% or more.
  • W 1.00% or less W (tungsten) has the effect of increasing the creep strength of steel by dissolving in the parent phase. For this reason, W may be contained as necessary. However, if W is contained in excess, the stability of the austenite phase decreases, and creep strength and toughness decrease instead. Therefore, the W content is 1.00% or less.
  • the W content is preferably 0.50% or less, and more preferably 0.30% or less.
  • the W content may be 0% or more, but for example, in order to obtain the above effect, the W content is preferably 0.01% or more.
  • Ta 0.40% or less Ta (tungsten) combines with C to form carbonitrides and reduce the amount of solute C. As a result, Ta has the effect of increasing SCC resistance. Ta also has the effect of increasing creep strength. For this reason, Ta may be included as necessary. However, if Ta is excessively included, ⁇ ferrite is generated, and the creep strength, toughness, and weldability of the steel are reduced. Therefore, the Ta content is 0.40% or less. The Ta content is preferably 0.30% or less, and more preferably 0.10% or less. The Ta content may be 0% or more, but for example, in order to obtain the above effect, the Ta content is preferably 0.01% or more.
  • Sn 0.0300% or less Sn (tin) has the effect of improving corrosion resistance and high temperature characteristics. Therefore, Sn may be contained as necessary. However, if Sn is contained in excess, weldability and manufacturability decrease. Therefore, the Sn content is 0.0300% or less.
  • the Sn content is preferably 0.0200% or less, and more preferably 0.0100% or less.
  • the Sn content may be 0% or more, but for example, in order to obtain the above effect, the Sn content is preferably 0.0010% or more.
  • Al 0.035% or less
  • Al is an element having a deoxidizing effect, and has the effect of improving hot workability by fixing O as inclusions. Therefore, Al may be contained as necessary. However, if Al is contained excessively, inclusions are formed excessively, and the surface properties are deteriorated. In addition, hot workability is also deteriorated. Therefore, the Al content is 0.035% or less.
  • the Al content is preferably 0.030% or less, and more preferably 0.025% or less.
  • the Al content may be 0% or more, but for example, in order to obtain the above effect, the Al content is preferably 0.0005% or more.
  • Ca 0.0100% or less
  • Ca (calcium) has the effect of fixing S and O as inclusions and improving the hot workability and creep ductility of steel. For this reason, Ca may be contained as necessary. However, if Ca is contained in excess, the hot workability and creep ductility are reduced. Therefore, the Ca content is 0.0100% or less.
  • the Ca content is preferably 0.0050% or less, and more preferably 0.0030% or less.
  • the Ca content may be 0% or more, but for example, in order to obtain the above effect, the Ca content is preferably 0.0010% or more.
  • Mg 0.0100% or less
  • Mg magnetium
  • the Mg content is preferably 0.0050% or less, and more preferably 0.0030% or less.
  • the Mg content may be 0% or more, but for example, in order to obtain the above effect, the Mg content is preferably 0.0002% or more.
  • REM 0.0800% or less Like Ca and Mg, REM (rare earth elements) fix S and O as inclusions and have the effect of improving the hot workability and creep ductility of steel. For this reason, REM may be contained as necessary. However, if REM is contained in excess, hot workability and long-term creep ductility are reduced. For this reason, the REM content is 0.0800% or less.
  • the REM content is preferably 0.0600% or less, and more preferably 0.0400% or less.
  • the REM content may be 0% or more, but for example, in order to obtain the above effect, the REM content is preferably 0.0010% or more.
  • REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements.
  • REM is often added in the form of misch metal.
  • the optional elements may contain one or more elements selected from the group consisting of group A and group B, as necessary.
  • the balance is Fe and impurities.
  • impurities refer to components that are mixed in due to various factors in the raw materials, such as ores and scraps, and the manufacturing process when steel is industrially produced, and are acceptable within the range that does not adversely affect the properties of the austenitic stainless steel.
  • solute C combines with Cr to form Cr carbides in a high-temperature environment. As a result, SCC resistance is reduced. For this reason, it is desirable to reduce the amount of solute C before use as a boiler heat transfer tube. Specifically, it is preferable to fix C as a precipitate (compound) in advance using V, Ti, and Nb. Therefore, the amount of V, Ti, and Nb that combines with C and exists as a precipitate, that is, the value of the right side of formula (i), is controlled.
  • V ER V content (mass%) in the precipitate obtained by extraction residue analysis
  • TiER Ti content (mass%) in the precipitate obtained by extraction residue analysis
  • NbER Nb content (mass%) in the precipitate obtained by extraction residue analysis
  • the value on the right hand side of equation (i) is less than 0.010, the amount of solute C will increase, and in the operating environment, the solute C will combine with Cr to form Cr carbides. As a result, a Cr-deficient layer will form and SCC resistance will decrease. For this reason, the value on the right hand side of equation (i) is 0.010 or more.
  • the value on the right hand side of equation (i) is preferably 0.012 or more, more preferably 0.015 or more, and even more preferably 0.020 or more.
  • the value on the right hand side of equation (i) is preferably 0.120 or less. Also, from the viewpoint of the decrease in strength caused by excessive fixation of N, the value on the right hand side of equation (i) is preferably 0.100 or less.
  • the amount of Ni, Cr, Mo, and Nb dissolved in the matrix is controlled. This is because the amount of the above elements dissolved in the matrix can be improved by reducing the amount of C, which is effective for creep strength, while ensuring the amount of the above elements dissolved in the matrix.
  • the amount of the elements dissolved in the matrix can be calculated from the difference between the content (mass%) of each element and the content (mass%) of each element in the precipitate obtained by the extraction residue analysis.
  • the austenitic stainless steel of this embodiment must satisfy formula (ii).
  • each symbol is defined as follows, and each element symbol in the above formula represents the content (mass%) of each element contained in the steel, and when no element is contained, the symbol is set to zero.
  • NbER Nb content (mass%) in the precipitate obtained by extraction residue analysis
  • NiER Ni content (mass%) in the precipitate obtained by extraction residue analysis
  • CrER Cr content (mass%) in the precipitate obtained by extraction residue analysis
  • Mo ER Mo content (mass%) in the precipitate obtained by extraction residue analysis
  • the value in formula (ii) is 27.0 or more.
  • the value in formula (ii) is preferably 29.0 or more, and more preferably 31.0 or more.
  • the value in formula (ii) is 40.5 or more, there may be cases where, among the above elements, too much Nb is dissolved, and the amount of Nb that bonds with the dissolved C and forms precipitates cannot be secured sufficiently, making it impossible to satisfy formula (i). For this reason, the value in formula (ii) is less than 40.5.
  • the value in formula (ii) is preferably 39.0 or less, more preferably 37.0 or less, and even more preferably 35.0 or less.
  • a certain amount of Nb is dissolved in solid solution to improve steam oxidation resistance.
  • forming a processed layer by shot blasting or the like is effective in improving steam oxidation resistance, but when a composition is desired that has improved creep strength and SCC resistance, simply providing a processed layer does not sufficiently improve steam oxidation resistance.
  • Nb ER which is the amount of Nb in a precipitated state
  • the dissolved Nb promotes the formation of Cr oxide in the usage environment. Cr oxide grows slowly.
  • the dissolved Nb suppresses the excessive penetration of O into the steel, thereby suppressing the formation of other oxides. It is therefore believed that sufficient time can be secured for the growth of Cr oxide. Therefore, it is effective in improving steam oxidation resistance to provide a processed layer by shot blasting or the like after securing the amount of dissolved Nb.
  • Nb is precipitated as Nb carbonitrides containing Ti, while a certain amount of Nb is present as solid solution.
  • Figure 1 shows a microstructure photograph of Nb compounds extracted from precipitates obtained by extracting the austenitic stainless steel of this embodiment and observing them. Analysis of this microstructure photograph revealed that the above-mentioned Nb compounds were Nb carbonitrides containing Ti.
  • Nb may combine with, for example, N, Cr, etc. in the steel to form fine NbCr nitrides.
  • NbCr nitrides When these fine NbCr nitrides are formed, the Nb ER value increases and the amount of soluble Cr decreases. As a result, it becomes difficult for austenitic stainless steel to satisfy formula (ii). For this reason, Nb is controlled so that it becomes the above-mentioned Nb carbonitride, not NbCr nitride.
  • the austenitic stainless steel of this embodiment satisfies formula (iii). If the austenitic stainless steel does not satisfy formula (iii), i.e., if the Nb ER is 0.052 or more, even if a processed layer is formed thereafter, it is difficult to improve the steam oxidation resistance.
  • NbCr nitrides are formed, making it difficult to form the desired Nb carbonitrides.
  • Nb ER is preferably 0.050 or less, more preferably 0.045 or less, even more preferably 0.040 or less, and most preferably 0.035 or less.
  • the content (mass%) of each element in the precipitate obtained by the extraction residue analysis described in items 2 to 4 above can be measured by the following procedure. Specifically, about 0.4 g of the sample is electrolyzed at a current value of 20 mA/ cm2 using 10% acetylacetone-1% tetramethylammonium chloride/methanol. The electrolyzed sample solution is then filtered through a 0.2 ⁇ m filter, and the residue is decomposed with an acid. The amount (mass%) of the above elements analyzed as the electrolytic extraction residue is then calculated using an ICP emission spectrometer.
  • a processed layer is provided near the surface by shot blasting or the like. As described above, by making the Nb ER less than 0.052 and providing a processed layer, a recrystallized structure is formed during use at high temperatures. This recrystallized structure has a very fine grain size, so that Cr is likely to diffuse to grain boundaries.
  • the worked layer is a structure in which the surface vicinity is hardened, and therefore satisfies the following formula (iv). 0.5 ⁇ ( Hv40 ⁇ Hvt/2 )/Hvt /2 (iv)
  • each symbol is defined as follows.
  • Hv 40 Vickers hardness measured at a position 40 ⁇ m from the surface in the thickness direction with a test load of 10 gf.
  • Hv t/2 Vickers hardness measured at a position 1/2t from the surface in the thickness direction with a test load of 10 gf, where t is the total thickness.
  • the right-hand side of equation (iv) is called the hardness increase rate, and the larger this value is, the greater the processing that has been performed. If the right-hand side of equation (iv) is 0.5 or greater, recrystallization will occur in the processed layer formed within 40 ⁇ m of the surface in the thickness direction during use at high temperatures. As a result, even if the scale peels off during use at high temperatures, it will be possible to repair the peeled area. For this reason, the right-hand side of equation (iv) is 0.5 or greater.
  • the value of the right-hand side of equation (iv) is preferably 0.6 or more, and more preferably 0.7 or more.
  • the upper limit of the right-hand side of equation (iv) is not particularly limited. However, as the right-hand side of equation (iv) increases, the surface becomes significantly harder, which may cause problems in molding, welding, and the like. For this reason, the right-hand side of equation (iv) is preferably 2.0 or less.
  • the Vickers hardness is in the range of 100 to 245 Hv, for example.
  • Hv 40 and Hv t/2 may be measured by the following procedure.
  • a test piece of 15 mm square is cut out, embedded in resin, and cut and mirror-polished at a cross section.
  • the Vickers hardness of this test piece is measured at a position 40 ⁇ m from the surface in the thickness direction and a position t/2 from the surface in the thickness direction (when the total thickness is t) so that they are in the same line in the thickness direction, with a load of 10 gf.
  • This measurement is repeated at different positions of the test piece and at positions that are not affected by other measurements, so that the number of measurements at each position is five.
  • the average of the measurement results at five points at a position 40 ⁇ m in the thickness direction is taken as Hv 40.
  • the average of the measurement results at five points at a position t/2 in the thickness direction is taken as Hv t/2 .
  • measurements are taken at a position 40 ⁇ m from the surface in the thickness direction and at a position t/2 from the surface in the thickness direction so that they are in the same straight line in the thickness direction, and this is also done at other locations, with the above-mentioned measurements being taken a total of five times at each position.
  • the above-mentioned measurements are taken at a position 40 ⁇ m from the inner surface of the steel pipe in the thickness direction and at a position t/2 from the inner surface of the steel pipe in the thickness direction (where t is the total thickness).
  • the hardness of the inner surface of the steel pipe is measured because, when used as a heat transfer tube for a boiler, high-temperature steam passes through the inside of the tube, and steam oxidation resistance in particular is required on the inner surface of the tube.
  • the thickness of the austenitic stainless steel of this embodiment is preferably in the range of 2 to 100 mm, taking into consideration its application. If the austenitic stainless steel is in the form of a tube, the wall thickness is preferably in the range of 2 to 95 mm. If the austenitic stainless steel is in the form of a steel plate, the plate thickness is preferably in the range of 2 to 35 mm.
  • the austenitic stainless steel of this embodiment may be, for example, a steel plate or a steel pipe.
  • the resulting billet is then hot worked.
  • the hot working conditions there are no particular limitations on the hot working conditions, but in order to prevent harmful defects from occurring during pipe manufacturing, for example, the billet is heated to a temperature in the range of 900 to 1300°C before hot working.
  • the type of hot working There are no particular limitations on the type of hot working.
  • hot rolling can be used.
  • the billet can be shaped into a pipe by hot extrusion.
  • the steel is quenched under the following conditions: Time from completion of hot working to start of quenching: 5.0 minutes or less Steel temperature at start of quenching: 700°C or higher Cooling rate from completion of hot working to start of quenching: 15°C/min or higher
  • the time (in minutes) from the completion of hot working to the start of quenching is called the "standing time”.
  • this "standing time” refers to the time from the completion of hot working to the time the steel is transported to the water-cooling device and water-cooling begins. If the standing time exceeds 5.0 minutes, coarse precipitates will form.
  • NbCr nitrides will be more likely to form instead of the desired Nb carbonitrides.
  • the leaving time is set to 5.0 minutes or less.
  • the leaving time is preferably set to 4.5 minutes or less, more preferably to 4.0 minutes or less, and even more preferably to 3.5 minutes or less.
  • the temperature (°C) of the steel at the start of quenching is called the "quenching start temperature.” If the quenching start temperature is less than 700°C, coarse precipitates such as Cr carbides will form. In addition, NbCr nitrides are more likely to form rather than the desired Nb carbonitrides. Even if softening is performed, the precipitates will not be able to dissolve in the parent phase, and formula (ii) will not be satisfied. As a result, creep strength will decrease. For this reason, the quenching start temperature is set to 700°C or higher. The quenching start temperature is preferably 750°C or higher, more preferably 780°C or higher, even more preferably over 790°C, and extremely preferably 800°C or higher.
  • the cooling rate from the completion of hot working to the start of quenching should be 15°C/min or more, preferably 18°C/min or more, and more preferably 20°C/min or more.
  • the above cooling rate is the difference between the surface temperature of the steel immediately after the completion of hot working and the surface temperature of the steel immediately before the start of quenching divided by the standing time.
  • Softening treatment The variation in steel quality in the longitudinal direction and thickness direction caused by hot working is reduced by softening treatment. If softening treatment is not performed, the coarse precipitates generated by hot working are not sufficiently dissolved in the parent phase. Then, in the solution heat treatment described later, there are parts where the precipitation of Ti, V, and Nb is insufficient. As a result, the right-hand side value of equation (i) becomes less than the left-hand side value, and SCC resistance may decrease. Therefore, softening treatment is performed as a process after hot working to homogenize the steel quality.
  • the holding temperature in the softening treatment (hereinafter referred to as "softening treatment temperature T 1 ") may be equal to or higher than the recrystallization temperature and equal to or lower than the grain boundary melting temperature. In the softening treatment, it is preferable to hold the steel uniformly at 1040 to 1300°C.
  • the softening temperature T1 is set to 1040°C or higher.
  • the softening temperature T1 is preferably set to 1100°C or higher, and more preferably set to 1150°C or higher.
  • the softening temperature T1 exceeds 1300°C, the crystal grains tend to become coarse. Therefore, the softening temperature T1 is set to 1300°C or lower.
  • the softening temperature T1 is preferably set to 1290°C or lower, and more preferably set to 1280°C or lower.
  • the softening time is not particularly limited, but is preferably 1 to 10 minutes from the viewpoint of recrystallization in the thickness direction and manufacturing costs. If the softening time is less than 1 minute, SCC resistance may decrease. On the other hand, if the softening time exceeds 10 minutes, the crystal grains may become coarse.
  • first cooling After softening, the steel is cooled. This cooling is called the first cooling.
  • the steel In the first cooling, the steel is cooled by water cooling or at a cooling rate equal to or faster than water cooling. If water cooling or a cooling rate equal to or faster than water cooling is not performed, a non-uniform structure will form due to the generation and growth of precipitates. As a result, the non-uniformity will be carried over to the subsequent cold working and solution heat treatment, and the product quality may not be consistent. For this reason, in the first cooling, water cooling or a cooling rate equal to or faster than water cooling is performed. This cooling is usually performed to 700°C or lower. The cooling rate in water cooling is usually 2 to 8°C/s.
  • the cold working may be a drawing process to produce the desired steel pipe shape.
  • solution temperature T2 the soaking temperature in the solution heat treatment. If the solution temperature T2 is less than 1100°C, the solid solution of elements effective for improving creep strength is insufficient, the formula (ii) is not satisfied, and the creep strength is reduced. If the solution temperature T2 exceeds 1200°C, the elements to be precipitated are excessively dissolved, and the formula (i) and/or the formula (ii) may not be satisfied. In order to improve the SCC resistance and make the right-hand side value of the formula (i) 0.012 or more, the solution temperature T2 is preferably 1170°C or less.
  • the softening temperature T1 is adjusted to be higher than the solution temperature T2 . That is, it is controlled to satisfy the following formula (a). If the solution temperature T2 is higher than the softening temperature T1 , it becomes difficult to sufficiently improve the SCC resistance.
  • T1 Softening temperature (°C)
  • T2 Solution temperature (°C)
  • the difference between the softening temperature T1 and the solution temperature T2 is set to 150° C. or less. That is, the difference is controlled so as to satisfy the following formula (b).
  • T 1 ⁇ T 2 ⁇ 150 (b) In the above formula (b), each symbol is defined as follows.
  • T1 Softening temperature (°C)
  • T2 Solution temperature (°C)
  • the solution heat treatment time is preferably less than 10 minutes. If the solution heat treatment time is more than 10 minutes, the elements to be precipitated will be excessively dissolved, and formula (i) and/or formula (ii) may not be satisfied.
  • the lower limit of the solution heat treatment time is not particularly limited, but is usually 1 minute.
  • Second Cooling After the solution heat treatment, the steel is cooled. This cooling is called the second cooling.
  • the steel In the second cooling, the steel is cooled with water or at a cooling rate equal to or faster than the water cooling, just like in the first cooling. If cooling is not performed with water or at a cooling rate equal to or faster than the water cooling, the formation and growth of precipitates may reduce the creep strength and the product quality may not be consistent. It is usually preferable to cool the steel to 600°C or lower.
  • a processed layer is formed near the surface of the steel that has undergone the second cooling.
  • the method for forming the processed layer is not particularly limited.
  • various known spraying methods such as shot peening, shot blasting, shot processing, sand blasting, sand processing, air blasting, and water jet can be used.
  • the shape of the particles may be, for example, spherical, cut wire, grid, etc.
  • the particles may be sprayed using compressed air, centrifugal force from an impeller (impeller type), high-pressure water, ultrasound, etc.
  • the particles may also be mixed with a liquid and sprayed with compressed air, etc. (This is also called "liquid honing").
  • Other methods include polishing, ball milling, grinding, honing, and ultrasonic impact processing to provide a processed layer.
  • polishing ball milling, grinding, honing, and ultrasonic impact processing to provide a processed layer.
  • particle blasting which is easy to process uniformly over the entire surface.
  • the processed layer is formed on the pickled surface.
  • the processed layer is formed after pickling. This is because pickling improves the surface condition and makes it easier for distortion to occur on the surface.
  • solution heat treatment was performed under the conditions shown in Table 2, and water cooling was performed to obtain a steel pipe.
  • the obtained steel pipe was subjected to shot blasting on the inner surface of the steel pipe to form a processed layer.
  • conditions such as the injection pressure, injection amount, injection angle, and nozzle shape were controlled and adjusted.
  • the surface of the steel pipe during shot blasting was subjected to the same conditions as No. Except for No. 33, the surface was pickled.
  • the obtained steel pipes were subjected to the following procedures: measurement of electrolytic extraction residue, hardness (Hv 40 and Hv t/2 ), SCC test, creep test, and steam oxidation test.
  • the SCC resistance was evaluated by performing an SCC test in accordance with ASTM A262 Method E. Specifically, a test piece measuring 15 mm x 2 mm x 70 mm was taken from one end of the obtained austenitic stainless steel. After the test piece was subjected to a sensitization heat treatment at 700 ° C. for 30 minutes, about 70 g of copper cuttings were placed in a sulfuric acid/copper sulfate aqueous solution specified in the above standard, and the test piece was immersed in this boiling solution for 24 hours and subjected to a bending test. After the test, the test piece had no cracks, and even if the immersion time was set to 48 hours and a bending test was performed, no cracks were observed.
  • the SCC resistance was evaluated as being very good (A). If cracks occurred at an immersion time of 48 hours but no cracks occurred at an immersion time of 24 hours, the SCC resistance was evaluated as being good (B). If cracks occurred even at an immersion time of 24 hours, the SCC resistance was evaluated as being poor (D).
  • the scale thickness was measured at 500 times magnification in any 10 fields of view, and the average value of the thickness was calculated.
  • the scale thickness was 10 ⁇ m or less, the steam oxidation resistance was evaluated as very good (B), when it was more than 10 ⁇ m and 50 ⁇ m or less, the steam oxidation resistance was evaluated as slightly poor (C), and when it was 50 ⁇ m or more, the steam oxidation resistance was evaluated as poor (D).
  • B very good
  • C slightly poor
  • D the steam oxidation resistance was evaluated as poor
  • Test Nos. 1-20, 21-23, 24, 29, 31-33, 26, 37, and 38 showed no cracks in the SCC resistance test, whether the immersion time was 24 hours or 48 hours, and were rated "A.” These examples are considered to have better SCC resistance because the value on the right side of equation (i) was 0.012 or more. Also, Test Nos. 5 and 6 showed no cracks in the SCC resistance test when the immersion time was 24 hours, and were rated "B.” This is because, although the value on the right side of equation (i) was less than 0.012, it was 0.010 or more, so the precipitation of V, Ti, and Nb fixed C even after sensitization heat treatment, suppressing the formation of grain boundary Cr carbides and preventing cracks.
  • test numbers 25-28, 30, and 34-36 showed cracks in the SCC resistance test and were rated "D.” These examples did not satisfy formula (i), and therefore it is believed that the precipitation of V, Ti, and Nb was insufficient, and that after sensitization heat treatment, Cr carbides were formed at the grain boundaries, creating a Cr-deficient layer around them, which is why cracks occurred in the SCC resistance test. The reason for this is believed to be that the chemical composition, softening temperature, or solution temperature were outside the preferred range.
  • Test Nos. 1, 3, 8, 10, 12-17, 20-22, and 31 were rated "A" in the creep test. These examples are considered to have excellent creep strength because the side value in formula (ii) was equal to or greater than the more preferable lower limit.
  • Test Nos. 2, 4-7, 9, 11, 18, 23, 25, 29, 30, 32, and 33 were rated "B" in the creep test. Although these examples do not have the side value in formula (ii) equal to or greater than the more preferable lower limit, they satisfy formula (ii), so the elements that contribute to creep strength are sufficiently dissolved, and it is considered that they have obtained good creep strength that can withstand the present invention.
  • Test Nos. 19, 24, and 26-28 were rated “D” in the creep test.
  • the value in the middle of formula (ii) was less than 27.0, and it is believed that the solid solution of Ni, Cr, Mo, and Nb, which improve creep strength, was insufficient, and therefore sufficient creep strength was not obtained.
  • Nos. 24, 26, and 27 did not satisfy formula (iii), and therefore NbCr nitrides were formed, which is believed to have reduced creep strength.
  • Test Nos. 1 to 23, 25, and 28 to 30 were rated "B" in the steam oxidation test. These examples satisfied formulas (iii) and (iv), and are therefore considered to have good steam oxidation resistance.
  • Test Nos. 24, 26, 27, and 31 were rated "C” in the steam oxidation test. These examples satisfied formula (iv), but did not satisfy formula (iii), and are therefore considered to have failed to form a good processed layer, resulting in poor steam oxidation resistance.
  • Test Nos. 32 and 33 were rated "D" in the steam oxidation test. These examples were considered to have failed to form a sufficient processed layer, as the conditions for forming the processed layer were not within the preferred range.
  • V ER V content (mass%) in the precipitate obtained by extraction residue analysis
  • TiER Ti content (mass%) in the precipitate obtained by extraction residue analysis
  • NbER Nb content (mass%) in the precipitate obtained by extraction residue analysis
  • NiER Ni content (mass%) in the precipitate obtained by extraction residue analysis
  • CrER Cr content (mass%) in the precipitate obtained by extraction residue analysis
  • Mo ER Mo content (mass%) in the precipitate obtained by extraction residue analysis
  • Hv 40 Vickers hardness measured at a position 40 ⁇ m from the surface in the thickness direction with a test load of 10 gf.
  • Hv t/2 Vickers hardness measured at a position 1/2t from the surface in the thickness direction with a test load of 10 gf, where t is the total thickness.
  • the chemical composition is, in mass%, V: 0.01 to 0.50%, and Ti: 0.001 to 0.500%,
  • the austenitic stainless steel according to the above (1) comprising one or more selected from the following:
  • the chemical composition is, in mass%, Co: 0.02 to 1.00%, W: 0.01 to 1.00%, Ta: 0.01 to 0.40%, Sn: 0.0010 to 0.0300%, Al: 0.0005 to 0.035%, Ca: 0.0010 to 0.0100%, Mg: 0.0002 to 0.0100%, and REM: 0.0010 to 0.0800%,

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  • Crystallography & Structural Chemistry (AREA)
  • Physics & Mathematics (AREA)
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Abstract

Un acier inoxydable austénitique ayant une composition chimique prédéterminée et satisfaisant que [0,010 ≤ VER＋TiER＋NbER], [27,0 ≤ 1,13(Ni-NiER)＋(Cr-CrER) + 1,85(Mo-MoER) + 1,79(Nb-NbER) < 40,5], [NbER < 0,052], et [0,5 ≤ (Hv40 - Hvt/2) / Hvt/2].
PCT/JP2023/041434 2022-11-24 2023-11-17 Acier inoxydable austénitique WO2024111516A1 (fr)

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009068079A (ja) * 2007-09-14 2009-04-02 Sumitomo Metal Ind Ltd 耐水蒸気酸化性に優れた鋼管
WO2013001956A1 (fr) * 2011-06-28 2013-01-03 新日鐵住金株式会社 Tuyau en acier inoxydable austénitique
JP2021127517A (ja) * 2020-02-14 2021-09-02 日本製鉄株式会社 オーステナイト系ステンレス鋼材
WO2022255223A1 (fr) * 2021-05-31 2022-12-08 日本製鉄株式会社 Acier inoxydable austénitique et tuyau en acier

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2009068079A (ja) * 2007-09-14 2009-04-02 Sumitomo Metal Ind Ltd 耐水蒸気酸化性に優れた鋼管
WO2013001956A1 (fr) * 2011-06-28 2013-01-03 新日鐵住金株式会社 Tuyau en acier inoxydable austénitique
JP2021127517A (ja) * 2020-02-14 2021-09-02 日本製鉄株式会社 オーステナイト系ステンレス鋼材
WO2022255223A1 (fr) * 2021-05-31 2022-12-08 日本製鉄株式会社 Acier inoxydable austénitique et tuyau en acier

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