WO2015133551A1 - オーステナイト系耐熱鋼 - Google Patents
オーステナイト系耐熱鋼 Download PDFInfo
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
Definitions
- the present invention relates to an austenitic heat resistant steel.
- the method of coarsening the crystal grains inhibits the formation of the Cr 2 O 3 protective film, so that the steam oxidation resistance may be lowered.
- the element addition amount it is necessary to increase the element addition amount. If the amount of element addition is increased, various basic characteristics other than creep strength may be adversely affected. Further, when the amount of element addition is increased, the raw material cost increases, and there is a possibility that economic efficiency is impaired. For this reason, a method of obtaining a solid solution strengthening action in a heat resistant material is not desirable as a method of obtaining a desired strength.
- the method of obtaining the effect of precipitation strengthening can strongly suppress the movement of dislocation accompanying deformation and can greatly improve the creep strength.
- many heat-resistant members are manufactured in the order of softening heat treatment, cold working, and final heat treatment.
- the final heat treatment is performed by heating to a high temperature followed by a rapid cooling treatment in the actual use environment or in the creep test. It is necessary to previously dissolve the element that precipitates therein.
- Patent Document 1 discloses an austenitic stainless steel containing one or two of Ti: 0.15 to 0.5 mass% and Nb: 0.3 to 1.5 mass%.
- the final softening temperature is set to over 1200 ° C. to 1350 ° C. and heated. After cooling at a cooling rate of 500 ° C./hr or more, 20 to 90% of cold working is added. Next, by heating to a temperature of 1070 to 1300 ° C. and 30 ° C. lower than the final softening temperature, and performing a final heat treatment of cooling at a cooling rate of air cooling or higher, the creep strength is high, and the fine grain structure has good corrosion resistance.
- a method for producing austenitic stainless steel is disclosed.
- Patent Document 1 a part of the element to be precipitated in an actual use environment or a creep test is precipitated in a small amount in the above-described final heat treatment stage, and crystallized due to the grain boundary pinning effect by the precipitate. This is to suppress grain coarsening. That is, the method disclosed in Patent Document 1 precipitates the difference in the amount of solid solution corresponding to this temperature difference by raising the softening heat treatment temperature before cold working to a certain level or more with respect to the final heat treatment. Thus, by devising two heat treatment temperatures, improvement in creep strength by high-temperature heat treatment and formation of a structure (fine crystal grain structure) containing a lot of fine crystal grains are achieved.
- the manufacturing equipment used in actual production has an upper limit temperature.
- the softening heat treatment temperature is raised to the equipment upper limit temperature, in order to provide a difference between the two heat treatment temperatures as in the method disclosed in Patent Document 1, it is necessary to set the final heat treatment temperature lower than the equipment upper limit temperature. .
- the decrease in the final heat treatment temperature decreases the amount of precipitation formed in the actual use environment or during the creep test, and as a result, the creep strength may not be sufficiently improved.
- the invention disclosed in Patent Document 1 obtains excellent steam oxidation resistance by making a fine grain structure, and obtains a pinning effect of grain boundaries by precipitating a small amount of precipitates. It has a creep strength.
- lowering the final heat treatment temperature to obtain a pinning effect means that the deposits that should be formed in the actual use environment or during the creep test are used in advance and sacrificed. Conceivable.
- the conventional technology cannot sufficiently utilize the precipitation strengthening that can be obtained from the steel material components.
- the creep strength is a limiting factor that determines the thickness of the member. Therefore, if the creep strength is improved, the thickness can be reduced and the cost can be reduced.
- austenitic heat-resistant steel has sufficient creep strength, and it can be said that it has not led to cost reduction.
- the final heat treatment temperature is lowered when the method disclosed in Patent Document 1 is applied. There must be. As described above, when the final heat treatment temperature is lowered, the solid solution amount of the precipitated element is lowered. Therefore, it is presumed that precipitation strengthening cannot be utilized to the maximum and the effect of improving the creep strength cannot be fully expressed.
- This invention is made
- the creep strength has been arranged focusing on the solid solution amount of the precipitated element depending on the temperature of the heat treatment. Therefore, generally, when the final heat treatment temperature is lowered, the amount of precipitated elements decreases, and the amount of fine precipitates newly precipitated in the actual use environment or during the creep test is reduced. Has been thought to be lower.
- the temperature difference between the softening heat treatment and the final heat treatment is set to 30 ° C. or more, and the precipitation of some precipitation elements is suppressed by the final heat treatment, thereby suppressing the coarsening of crystal grains. is doing.
- the precipitate that is precipitated by this operation is a precipitate that is supposed to be deposited in the actual use environment or during the creep test and contribute to the improvement of the creep strength. That is, the austenitic stainless steel manufactured by the method disclosed in Patent Document 1 cannot sufficiently improve the creep strength by the amount of the precipitated element to suppress the coarsening of crystal grains. The possibility is high.
- the present inventors have intensively studied whether or not the precipitate formed by this final heat treatment can directly affect the improvement of creep strength.
- the inventors of the present invention maintain specific amounts of precipitation elements added and solid solution amounts within a certain range, and within a certain range the precipitation particle size and precipitation amount contained in the steel. It was found that the precipitate obtained by performing the final heat treatment at a lower temperature than in the prior art can improve the creep strength. That is, the present inventors have found that precipitates formed by performing final heat treatment under specific heat treatment conditions contribute to improvement of creep strength as fine precipitates as they are. This finding exceeds the concept of the prior art that the creep strength is superior to conventional precipitates obtained by heat treatment at high temperatures. It was also found that the final heat treatment is performed under the specific heat treatment conditions described above (lower temperature than before), so that the fine grain structure can be maintained and the steam oxidation resistance can be maintained.
- the precipitate formed by the final heat treatment suppresses creep deformation more effectively than the precipitate formed during the creep test.
- precipitates formed during a creep test on an austenitic heat resistant steel are formed along dislocations introduced with deformation. Since dislocations are concentrated in the vicinity of the grain boundaries, the distribution of precipitates becomes nonuniform.
- the precipitate formed by the final heat treatment when manufacturing the austenitic heat resistant steel is uniformly formed in the grains.
- the precipitate formed by the final heat treatment can efficiently suppress the dislocation movement accompanying the creep deformation from the initial stage of deformation throughout the grain. For these reasons, it is presumed that good creep strength can be obtained when the final heat treatment is performed under specific heat treatment conditions as described above. This knowledge goes beyond the concept of the solid solution amount of the precipitation element depending on the temperature of the conventional heat treatment.
- the austenitic heat-resisting steel according to the present invention which is made on the basis of the above findings and solves the above-mentioned problems, is C: 0.05 to 0.16% by mass, Si: 0.1 to 1% by mass, Mn: 0.0.
- precipitation cumulative number particle size precipitates in the range of 100nm exceeds the 0nm density 0.1-2.0 units / [mu] m 2, the distribution of the precipitated particles size and the cumulative number density
- the precipitation particle diameter corresponding to the half value of the cumulative number density is 70 nm or less
- the average hardness is 160 Hv or less
- the crystal grain size number is 7.5 or more.
- the austenitic heat-resisting steel according to the present invention can have precipitates that can be obtained by performing the final heat treatment under a specific heat treatment condition with the steel material component in the above-described range.
- This precipitate is one in which the particle diameter and the amount of precipitation contained in the steel fall within a certain range, and contributes to the improvement of the creep strength as a fine precipitate as it is after the precipitation.
- this fine precipitate can improve the creep strength as compared with the conventional precipitate formed by final heat treatment at a high temperature.
- the final heat treatment is performed at specific heat treatment conditions, specifically at a lower temperature than before, the fine grain structure can be maintained and the steam oxidation resistance is excellent. can do.
- the austenitic heat-resisting steel according to the present invention further includes Zr: 0.3% by mass or less (not including 0% by mass), rare earth elements: 0.15% by mass or less (not including 0% by mass), and W : It is preferable to contain at least one of 3% by mass or less (not including 0% by mass).
- the high temperature strength can be improved by precipitation strengthening.
- the austenitic heat-resisting steel according to the present invention contains a rare earth element in the above-described range, the oxidation resistance of the stainless steel can be improved.
- the austenitic heat-resisting steel according to the present invention contains W in the above-described range, the high temperature strength can be improved by solid solution strengthening.
- the austenitic heat-resisting steel according to the present invention has the steel material component in the above-described range, and the precipitate particle diameter and the precipitation amount contained in the steel are within a certain range, so that it is excellent while maintaining a fine grain structure.
- the steel material components are C: 0.05 to 0.16 mass%, Si: 0.1 to 1 mass%, Mn: 0.1 to 2.5 mass%, P : 0.01 to 0.05 mass%, S: 0.005 mass% or less (excluding 0 mass%), Ni: 7 to 12 mass%, Cr: 16 to 20 mass%, Cu: 2 to 4 mass %, Mo: 0.1 to 0.8 mass%, Nb: 0.1 to 0.6 mass%, Ti: 0.1 to 0.6 mass%, B: 0.0005 to 0.005 mass%, N: 0.001 to 0.15 mass%, Mg: 0.005 mass% or less (not including 0 mass%) and Ca: 0.005 mass% or less (not including 0 mass%)
- the total of the Nb content and the Ti content is 0.3% by mass
- the austenitic heat-resistant steel according to the present embodiment further includes Zr: 0.3% by mass or less (excluding 0% by mass), rare earth element: 0.15% by mass or less (not including 0% by mass), and It is preferable to contain at least one of W: 3 mass% or less (excluding 0 mass%).
- the austenitic heat-resistant steel according to the present embodiment is a fire SUS321J2HTB steel (18 mass% Cr-10 mass% Ni-3 mass% Cu-Nb) using Ti as a precipitation element. , Ti steel).
- the austenitic heat-resisting steel according to the present embodiment comprising the above-described steel material components has a cumulative number density of precipitates in the range of the precipitate particle diameter exceeding 0 nm and 100 nm, 0.1 to 2.0 pieces / ⁇ m 2 ,
- the precipitated particle size corresponding to the half value of the cumulative number density is 70 nm or less
- the average hardness is 160 Hv or less
- the crystal grain size number is 7.5 or more.
- the precipitated particle diameter refers to a value calculated as the equivalent-circle diameter of the precipitated particles (precipitate).
- the means for solving the problem it is possible to obtain a precipitate in which the precipitate particle size and the precipitation amount contained in the steel are within a certain range by performing the final heat treatment under specific heat treatment conditions.
- the above average hardness and grain size number can also be controlled by controlling the heat treatment temperature. Specific heat treatment conditions and heat treatment temperatures will be described later.
- the austenitic heat-resistant steel according to the present embodiment is excellent in steam oxidation resistance.
- the austenitic heat-resistant steel according to the present embodiment is similar to the fire SUS321J2HTB steel using Ti as a precipitation element.
- the following steel material components have the following effects, and if they deviate from the predetermined contents, the following problems may occur.
- C has the effect
- 0.05 mass% or more is contained in order to acquire the effect
- the lower limit of the C content is preferably 0.08% by mass, and more preferably 0.09% by mass.
- the upper limit of the C content is preferably 0.15% by mass, and more preferably 0.13% by mass.
- Si has a deoxidizing action in the molten steel and effectively acts to improve oxidation resistance.
- Si is contained in an amount of 0.1% by mass or more in order to obtain a deoxidizing action in molten steel and an action for improving oxidation resistance.
- the steel material may be brittle, which is not preferable.
- the lower limit of the Si content is preferably 0.2% by mass, and more preferably 0.3% by mass.
- the upper limit of the Si content is preferably 0.7% by mass, and more preferably 0.5% by mass.
- Mn has a deoxidizing action in molten steel.
- 0.1 mass% or more of Mn is contained.
- the Mn content exceeds 2.5 mass%, it is not preferable because it promotes coarsening of carbide precipitation.
- the lower limit of the Mn content is preferably 0.2% by mass, and more preferably 0.3% by mass.
- the upper limit of the Mn content is preferably 2.0% by mass, and more preferably 1.8% by mass.
- P 0.01 to 0.05% by mass
- P has the effect of improving the high temperature strength.
- 0.01 mass% or more of P is contained in order to improve the high temperature strength.
- the lower limit of the P content is preferably 0.015% by mass, and more preferably 0.02% by mass.
- the upper limit of the P content is preferably 0.04% by mass, and more preferably 0.03% by mass.
- S 0.005 mass% or less (excluding 0 mass%)
- S is an inevitable impurity. If the S content becomes excessive and exceeds 0.005% by mass, the hot workability is deteriorated.
- the S content is set to 0.005 mass% or less so as not to deteriorate the hot workability. The smaller the S content, the better.
- the upper limit of the S content is preferably 0.002% by mass, and more preferably 0.001% by mass.
- Ni has the effect of stabilizing the austenite phase.
- 7% by mass or more of Ni is contained in order to stabilize the austenite phase.
- the lower limit of the Ni content is preferably 9% by mass, and more preferably 9.5% by mass.
- the upper limit of the Ni content is preferably 11.5% by mass, and more preferably 11% by mass.
- Cr 16 to 20% by mass
- Cr has the effect of improving the oxidation resistance and corrosion resistance of the steel material.
- Cr in order to improve the oxidation resistance and corrosion resistance of the steel material, Cr is contained in an amount of 16% by mass or more. However, if the Cr content exceeds 20% by mass, the steel material becomes brittle.
- the lower limit of the Cr content is preferably 17.5% by mass, and more preferably 18% by mass.
- the upper limit of the Cr content is preferably 19.5% by mass, and more preferably 19% by mass.
- Cu has the effect of forming precipitates in steel and improving high temperature strength.
- 2 mass% or more of Cu is contained.
- the lower limit of the Cu content is preferably 2.5% by mass, and more preferably 2.8% by mass.
- the upper limit of the Cu content is preferably 3.5% by mass, and more preferably 3.2% by mass.
- Mo 0.1 to 0.8% by mass
- Mo has the effect
- Mo is contained in an amount of 0.1% by mass or more. However, if the Mo content becomes excessive and exceeds 0.8% by mass, the steel material becomes brittle.
- the lower limit of the Mo content is preferably 0.2% by mass, and more preferably 0.3% by mass.
- the upper limit of the Mo content is preferably 0.6% by mass, and more preferably 0.5% by mass.
- Nb 0.1 to 0.6% by mass
- Ti 0.1 to 0.6% by mass
- Total of Nb content and Ti content is 0.3 mass% or more
- Nb and Ti can be precipitated as carbonitrides (carbides, nitrides or carbonitrides) to improve the high-temperature strength. Moreover, this precipitate suppresses the coarsening of crystal grains and promotes the diffusion of Cr. It can be said that it is a part of the most important element in the present invention because it exerts a secondary effect of improving corrosion resistance (water vapor oxidation resistance) by diffusion of Cr.
- Nb and Ti precipitates are formed to improve the high-temperature strength and to exert the effect of improving the steam oxidation resistance, so that Nb is 0.1% by mass or more and Ti is 0.1%. It is contained by mass% or more. By simultaneously containing Nb and Ti, the contribution to the improvement of the high temperature strength of the precipitate can be further increased. However, if these are not contained so that the total of the Nb content and the Ti content is 0.3% by mass or more, it is impossible to ensure the minimum necessary precipitation amount.
- the minimum of Nb content shall be 0.2 mass%.
- the lower limit of the Ti content is preferably 0.15% by mass.
- the lower limit of the total content of Nb and Ti is preferably 0.35% by mass.
- the upper limit of content of Nb and Ti is respectively 0.4 mass%, and it is more preferable to set it as 0.3 mass%.
- B has the effect of promoting the formation of M 23 C 6 type carbide (M is a carbide forming element) and improving the high temperature strength.
- M is a carbide forming element
- B in order to improve high temperature strength, 0.0005 mass% or more of B is contained.
- the lower limit of the B content is preferably 0.001% by mass, and more preferably 0.0015% by mass.
- the upper limit of the B content is preferably 0.004% by mass, and more preferably 0.003% by mass.
- N has the effect of improving the high-temperature strength by solid solution strengthening.
- N is added in an amount of 0.001% by mass or more in order to improve the high temperature strength.
- the lower limit of the N content is preferably 0.002% by mass, more preferably 0.003% by mass.
- the upper limit of the N content is preferably 0.08% by mass, and more preferably 0.04% by mass.
- Mg and Ca act as desulfurization / deoxidation elements and have an effect of improving the hot workability of the steel material.
- Ca and Mg may be contained in a range of 0.005% by mass or less.
- the upper limit of Ca and Mg is preferably 0.002% by mass.
- Zr 0.3% by mass or less (excluding 0% by mass)
- Zr is an optional component and has the effect of improving the high-temperature strength by precipitation strengthening. However, if the Zr content becomes excessive and exceeds 0.3% by mass, a coarse intermetallic compound is formed, resulting in a decrease in hot ductility. In addition, it is preferable that the upper limit of Zr content shall be 0.25 mass%. However, since inclusion of Zr increases the cost of the steel material, it may be included as necessary.
- Rare earth elements are optional components and have the effect of improving the oxidation resistance of stainless steel. That is, the generation of oxide scale can be suppressed by arbitrarily containing rare earth elements. However, if the content of the rare earth element becomes excessive and exceeds 0.15% by mass, a part of the grain boundary is melted in a high temperature environment and hot workability is hindered.
- the upper limit of the rare earth element content is preferably 0.1% by mass, and more preferably 0.05% by mass.
- the rare earth element is one or more elements selected from a total of 17 elements including Sc and Y and 15 lanthanoid elements represented by La, Ce, and Nd.
- the rare earth element content is a total content of one or more elements selected from 17 elements.
- W 3% by mass or less (excluding 0% by mass)
- W is an optional component and has the effect of improving the high temperature strength by solid solution strengthening. However, if the W content is excessive and exceeds 3% by mass, a coarse intermetallic compound is formed, resulting in a decrease in high temperature ductility.
- the upper limit of the W content is preferably 2.5% by mass, and more preferably 2.0% by mass.
- the steel material component described above exhibits the above-described action by being contained, but at the same time increases the cost. Therefore, what is necessary is just to set content according to a required reinforcement
- the balance is Fe and inevitable impurities
- the balance is Fe and other inevitable impurities.
- other inevitable impurities include Al, Sn, Zn, Pb, As, Bi, Sb, Te, Se, and In. Inevitable impurities are preferably reduced as much as possible.
- Al is 0.01% by mass or less
- Sn is 0.005% by mass or less
- Zn is 0.01% by mass or less
- Pb is 0.002%.
- Mass% or less As is 0.01 mass% or less
- Bi is 0.002 mass% or less
- Sb is 0.002 mass% or less
- Te 0.01 mass% or less
- Se is 0.002 mass% or less
- In It is recommended that the content be 0.002% by mass or less.
- the average hardness (Vickers hardness) is set to 160 Hv or less in order to secure the solid solution amount of the elements that are deposited in the actual use environment or the creep test after the above-described component range. If the average hardness exceeds 160 Hv, the solid solution amount of the element that precipitates in the actual use environment or during the creep test cannot be secured, so the creep strength decreases.
- the upper limit of the average hardness is preferably 140 Hv.
- the lower limit of the average hardness is preferably 100 Hv, more preferably 110 Hv.
- Vickers hardness can be measured based on JISZ2244: 2009, for example.
- the precipitate particle diameter corresponding to half the cumulative number density is kept as fine as 70 nm or less while forming a certain amount of precipitates of 100 nm or less, thus improving the creep strength.
- the lower limit of the cumulative number density mentioned above is preferably of a 0.3 / ⁇ m 2, and more preferably set to 0.4 / ⁇ m 2.
- the upper limit of the precipitated particle diameter corresponding to the half value of the cumulative number density is preferably 60 nm, and more preferably 50 nm.
- the lower limit of the precipitated particle diameter corresponding to the half value of the cumulative number density exceeds 0 nm. The method for measuring the precipitated particle diameter and the cumulative number density will be described later.
- the metal structure is in a sufficiently fine state and can be referred to as a fine crystal grain structure. Therefore, the steam oxidation resistance can be maintained.
- the final heat treatment may be performed under specific heat treatment conditions described later.
- the final heat treatment may be performed under the condition that the coarsening factor is 2000 ° C. ⁇ min or less. This “condition that the coarsening factor of the precipitate is 2000 ° C./min or less” is the specific heat treatment condition described above.
- the coarsening factor of the precipitate is an index representing the influence of heat on the coarsening of the precipitate, and is a value obtained by integrating over time a temperature of 900 ° C. or higher at which the growth of the precipitate proceeds with respect to the temperature history during the heat treatment. is there. Note that this coarsening factor must include not only the heat treatment holding time but also the heating time and cooling time of 900 ° C. or higher.
- the coarsening factor of a conventional austenitic heat resistant steel that contains Ti as a precipitation element and has sufficiently increased high-temperature strength, such as fire SUS321J2HTB steel is about 3000 to 7000 ° C./min.
- the coarsening factor is set to 2000 ° C. ⁇ min or less.
- the lower limit of the coarsening factor is preferably larger than 473 ° C./min, more preferably 500 ° C./min or more, and even more preferably 821 ° C./min or more.
- the maximum temperature reached and the holding time can be adjusted according to the constraints of the equipment.
- the softening heat treatment it is necessary to carry out the softening heat treatment at a temperature higher by 30 ° C. or more than the final heat treatment to dissolve the precipitated elements. That is, the temperature lower by 30 ° C. than the softening heat treatment is the upper limit temperature of the final heat treatment.
- the “cumulative number density of precipitates having a diameter exceeding 0 nm and in the range of 100 nm” can be understood from numerical values with a horizontal axis of 90 to 100 nm.
- precipitation particle diameter corresponding to half the cumulative number density of precipitates in the range where the precipitation particle diameter exceeds 0 nm and 100 nm in the example in the figure, the point of 50 to 60 nm and the point of 60 to 70 nm are It can be understood from the numerical value on the horizontal axis that intersects with the half value of the numerical value of 90 to 100 nm.
- the austenitic heat-resisting steel according to the present embodiment described above has the steel material component in the above-described range and the precipitated particle diameter and precipitation amount contained in the steel are within a certain range, the fine grain structure is While maintaining, it can have excellent creep strength. Therefore, conventionally, the grain size has been reduced while sacrificing the amount of precipitation formed in the actual use environment or during the creep test. However, in the austenitic heat-resisting steel according to the present embodiment, it has been conventionally sacrificed. Precipitation can also contribute to the improvement of creep strength. Therefore, the precipitation strengthening action can be maximized even when the upper limit temperature of the heat treatment exists due to equipment restrictions and the like.
- the austenitic heat-resistant steel using Ti as a precipitation element it is possible to provide a heat-resistant stainless steel having a further improved creep strength while having a fine grain structure. Since the austenitic heat-resistant steel according to the present embodiment can improve the creep strength, the thickness of the heat-resistant member can be made thinner than before, and the cost reduction as a heat-resistant component can be realized.
- the heat treatment temperature and time were changed in the range of 1040 to 1215 ° C. and 0.5 to 10 minutes, that is, the coarsening factor [° C./min] of the precipitate was changed, and Table 2 No. Steel materials shown in 1-31 were prepared.
- the grain size number and the creep rupture time were measured as follows. These measurement results are shown in Table 2 together with the coarsening factor.
- numerical values represented by underlines and italics indicate that the requirements of the present invention are not satisfied.
- Vickers hardness [Hv] The Vickers hardness is No. A Vickers hardness test was performed on each of the steel materials 1 to 31 in accordance with JIS Z 2244: 2009, and the hardness was measured. The load in the Vickers hardness test was measured at 10 kg. Those having a Vickers hardness of 160 Hv or less were evaluated as being excellent in average hardness, and those having a Vickers hardness exceeding 160 Hv were evaluated as being inferior in average hardness.
- the grain size number is No. With respect to each of the steel materials shown in 1-31, the structure was observed with a microscope in accordance with JIS G 0551: 2013, and the crystal grain size number was measured. Those having a crystal grain size number of 7.5 or more were accepted and those less than 7.5 were rejected.
- Creep rupture time [hours] The creep rupture time is no. Test pieces were prepared from the steel materials shown in 1 to 31 in accordance with JIS Z 2271: 2010, and were tested and measured. Those having a creep rupture time of 650 hours or more were evaluated as being excellent in creep strength, and those having a creep rupture time of less than 650 hours were evaluated as being inferior in creep strength.
- no. 4 and 7, no. 11 and 14, no. 16 and 18, no. 20 and 23, no. Nos. 25 and 28 are examples in which the latter number is lower than the former number in the heat treatment temperature.
- 4 and 7, no. 11 and 14, no. Nos. 25 and 28 are examples in which the temperature was lowered by 20 ° C.
- Nos. 16 and 18 are examples in which the temperature is lowered by 10 ° C. 20 and 23 are examples in which the temperature was lowered by 30 ° C.
- No. 9 The steel material shown in No. 9 is a comparative example in which the precipitation component could not be sufficiently dissolved because the coarsening factor of the precipitate was too low.
- the No. Although the steel material shown in No. 9 had a fine grain structure, it was confirmed that the Vickers hardness (average hardness) deviated from the definition of the present invention and the creep rupture time was reduced.
- the steel materials shown in 29 to 31 are comparative examples in which the chemical composition deviates from the definition of the present invention. Of these, No. Although the steel materials shown in 29 and 30 have coarse grains and include elements desirable for creep strength, the creep strength of all the steel materials is less than 650 hours, and an insufficient strength can be obtained as compared with the examples. It was. No. The steel material shown in No. 31 has a crystal grain size number of 7.5 and a good fine grain structure is obtained, but the creep strength is less than 650 hours, and only an insufficient strength is obtained as compared with the examples. There wasn't.
- the steel materials shown in 3, 6, 10, 13, 15, 22, and 27 have a good fine crystal grain structure with a crystal grain size number of 7.5 or more.
- the steel materials shown in 3, 6, 10, 13, 15, 22, and 27 are, in the distribution of the cumulative number density of precipitates and the number of precipitated particles in the range of the precipitate particle diameter exceeding 0 nm and 100 nm, The creep rupture time was inferior when compared with the examples (both were comparative examples) because at least one of the precipitate particle diameters corresponding to the half value of the cumulative number density was not satisfied.
- the steel material satisfying the provisions of the present invention (steel material according to the embodiment) has a fine grain structure compared with the steel material not satisfying the provisions of the present invention (steel material according to the comparative example). Was confirmed to be excellent.
- the austenitic heat-resistant steel of the present invention exhibits excellent creep strength even in a high temperature environment, it is useful for energy-related equipment such as boilers and reaction vessels. Excellent creep strength even in high temperature environments.
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Abstract
Description
また、固溶強化を作用させるためには元素添加量を多くすることが必要となる。元素添加量を多くすると、クリープ強度以外の様々な基本特性に悪影響を及ぼすおそれがある。
また、元素添加量を多くすると、原料費が増加し、経済性を損なう可能性もある。そのため、耐熱材料において固溶強化の作用を得る方法は、所望の強度を得る方法として望ましいとはいえない。
つまり、本発明者らは、特定の熱処理条件の最終熱処理を行って形成された析出物が、そのまま微細析出物としてクリープ強度の向上に寄与することを見出した。この知見は、高温で熱処理して得られる従来の析出物よりもクリープ強度が優れるという、従来技術の概念を超えるものである。
また、前記した特定の熱処理条件(従来よりも低温)で最終熱処理を行うので、微細結晶粒組織を保つことができ、耐水蒸気酸化性を維持することができることも見出した。
今回、本発明者らは、オーステナイト系耐熱鋼において、最終熱処理で形成された析出物が、クリープ試験中に形成される析出物よりも効果的にクリープ変形を抑制することを見出した。通常、オーステナイト系耐熱鋼に対するクリープ試験中に形成される析出物は、変形に伴って導入される転位に沿って形成される。転位は結晶粒界近傍に集中するので、析出物の分布も不均一になる。
これに対し、オーステナイト系耐熱鋼を製造する際の最終熱処理で形成される析出物は、粒内に均一に形成される。そのため、当該最終熱処理で形成される析出物は、クリープ変形に伴う転位運動を粒内全体で変形初期から効率よく抑制することができると考えられる。このような理由により、前記した如く特定の熱処理条件で最終熱処理を行うと、良好なクリープ強度を得ることができると推測される。この知見は、従来の熱処理の温度に依存する析出元素の固溶量の概念を超えたものである。
以下、本発明に係るオーステナイト系耐熱鋼を実施するための形態(実施形態)について詳細に説明する。
本実施形態に係るオーステナイト系耐熱鋼は、鋼材成分が、C:0.05~0.16質量%、Si:0.1~1質量%、Mn:0.1~2.5質量%、P:0.01~0.05質量%、S:0.005質量%以下(0質量%を含まない)、Ni:7~12質量%、Cr:16~20質量%、Cu:2~4質量%、Mo:0.1~0.8質量%、Nb:0.1~0.6質量%、Ti:0.1~0.6質量%、B:0.0005~0.005質量%、N:0.001~0.15質量%、Mg:0.005質量%以下(0質量%を含まない)およびCa:0.005質量%以下(0質量%を含まない)のうちの少なくとも一つを含有し、前記Nbの含有量と前記Tiの含有量の合計が0.3質量%以上、残部がFeおよび不可避不純物からなる。
なお、本実施形態に係るオーステナイト系耐熱鋼は、さらに、Zr:0.3質量%以下(0質量%を含まない)、希土類元素:0.15質量%以下(0質量%を含まない)およびW:3質量%以下(0質量%を含まない)のうちの少なくとも一つを含有しているのが好ましい。
前記した鋼材成分をみて分かるように、本実施形態に係るオーステナイト系耐熱鋼は、析出元素としてTiを使用している火SUS321J2HTB鋼(18質量%Cr-10質量%Ni-3質量%Cu-Nb,Ti鋼)に類するものである。
なお、前記したように、本実施形態に係るオーステナイト系耐熱鋼は、析出元素としてTiを使用している火SUS321J2HTB鋼に類する。火SUS321J2HTB鋼において下記鋼材成分はそれぞれ以下に記載する作用を奏し、所定の含有量を外れると以下に記載する不具合が生じる場合がある。
Cは、炭化物を形成して高温強度を向上させる作用がある。本実施形態では、高温強度を向上させる作用を得るため、Cを0.05質量%以上含有させている。しかしながら、C含有量が過剰になって0.16質量%を超えると、粗大な炭化物を形成し、高温強度を向上させることができない。
なお、C含有量の下限は0.08質量%とするのが好ましく、0.09質量%とするのがより好ましい。C含有量の上限は0.15質量%とするのが好ましく、0.13質量%とするのがより好ましい。
Siは、溶鋼中で脱酸作用を有するとともに、耐酸化性の向上に有効に作用する。本実施形態では、溶鋼中での脱酸作用と耐酸化性を向上させる作用とを得るため、Siを0.1質量%以上含有させている。しかしながら、Si含有量が過剰になって1質量%を超えると鋼材の脆化をもたらすことがあるため好ましくない。
なお、Si含有量の下限は0.2質量%とするのが好ましく、0.3質量%とするのがより好ましい。Si含有量の上限は0.7質量%とするのが好ましく、0.5質量%とするのがより好ましい。
Mnは、溶鋼中で脱酸作用を有する。本実施形態では、溶鋼中での脱酸作用を得るため、Mnを0.1質量%以上含有させている。しかしながら、Mn含有量が2.5質量%を超えると炭化物析出の粗大化を助長するため好ましくない。
なお、Mn含有量の下限は0.2質量%とするのが好ましく、0.3質量%とするのがより好ましい。Mn含有量の上限は2.0質量%とするのが好ましく、1.8質量%とするのがより好ましい。
Pは、高温強度を向上させる作用がある。本実施形態では、高温強度を向上させるため、Pを0.01質量%以上含有させている。しかしながら、P含有量が過剰になり、0.05質量%を超えると溶接性を損なうおそれがある。
なお、P含有量の下限は0.015質量%とするのが好ましく、0.02質量%とするのがより好ましい。P含有量の上限は0.04質量%とするのが好ましく、0.03質量%とするのがより好ましい。
Sは、不可避不純物である。S含有量が過剰となり、0.005質量%を超えると熱間加工性を劣化させる。本実施形態では、熱間加工性を劣化させないようにするため、S含有量を0.005質量%以下としている。S含有量は少ないほど好ましい。
なお、S含有量の上限は0.002質量%とするのが好ましく、0.001質量%とするのがより好ましい。
Niは、オーステナイト相を安定化させる作用がある。本実施形態では、オーステナイト相を安定化させるため、Niを7質量%以上含有させている。しかしながら、Ni含有量が12質量%を超えると鋼材のコスト増加をもたらすことになる。
なお、Ni含有量の下限は9質量%とするのが好ましく、9.5質量%とするのがより好ましい。Ni含有量の上限は11.5質量%とするのが好ましく、11質量%とするのがより好ましい。
Crは、鋼材の耐酸化性および耐食性を向上させる作用がある。本実施形態では、鋼材の耐酸化性および耐食性を向上させるため、Crを16質量%以上含有させている。しかしながら、Cr含有量が20質量%を超えると鋼材の脆化を招いてしまう。
なお、Cr含有量の下限は17.5質量%とするのが好ましく、18質量%とするのがより好ましい。Cr含有量の上限は19.5質量%とするのが好ましく、19質量%とするのがより好ましい。
Cuは、鋼中で析出物を形成し、高温強度を向上させる作用がある。本実施形態では、高温強度を向上させるため、Cuを2質量%以上含有させている。しかしながら、Cu含有量が過剰になり、4質量%を超えるとその効果は飽和する。
なお、Cu含有量の下限は2.5質量%とするのが好ましく、2.8質量%とするのがより好ましい。Cu含有量の上限は3.5質量%とするのが好ましく、3.2質量%とするのがより好ましい。
Moは、耐食性を向上させる作用がある。本実施形態では、耐食性を向上させるため、Moを0.1質量%以上含有させている。しかしながら、Mo含有量が過剰になり、0.8質量%を超えると鋼材の脆化を招いてしまう。
なお、Mo含有量の下限は0.2質量%とするのが好ましく、0.3質量%とするのがより好ましい。Mo含有量の上限は0.6質量%とするのが好ましく、0.5質量%とするのがより好ましい。
[Ti:0.1~0.6質量%]
[Nbの含有量とTiの含有量の合計が0.3質量%以上]
NbおよびTiは、炭窒化物(炭化物、窒化物または炭窒化物)として析出させることで、高温強度を改善することができる。また、この析出物が結晶粒の粗大化を抑制し、Crの拡散を促進する。Crの拡散によって副次的に耐食性(耐水蒸気酸化性)向上の作用を発揮するため、本発明において最も重要な元素の一部であるといえる。
本実施形態では、NbおよびTiの析出物を形成させ、高温強度を改善したり、耐水蒸気酸化性向上の作用を発揮させたりするため、Nbを0.1質量%以上、Tiを0.1質量%以上含有させている。NbとTiを同時に含有させることで析出物の高温強度の向上への寄与をより高めることができる。
ただし、これらはNbの含有量とTiの含有量の合計が0.3質量%以上となるように含有させなければ、最低限必要な析出量を確保することできない。
その一方で、Nb含有量が過剰となって0.6質量%を超えたり、Ti含有量が過剰となって0.6質量%を超えたりすると、いずれの場合も析出物が粗大化し、靭性の低下を招くことになる。
なお、NbおよびTiの含有量の上限はそれぞれ0.4質量%とするのが好ましく、0.3質量%とするのがより好ましい。
Bは、M23C6型炭化物(Mは炭化物形成元素)の形成を促進させ、高温強度を改善する作用がある。本実施形態では、高温強度を改善させるため、Bを0.0005質量%以上含有させている。しかしながら、B含有量が過剰になり、0.005質量%を超えると溶接性の低下を招いてしまう。
なお、B含有量の下限は0.001質量%とするのが好ましく、0.0015質量%とするのがより好ましい。B含有量の上限は0.004質量%とするのが好ましく、0.003質量%とするのがより好ましい。
Nは、固溶強化によって高温強度を向上させる作用がある。本実施形態では、高温強度を向上させるため、Nを0.001質量%以上含有させている。しかしながら、N含有量が過剰になり、0.15質量%を超えると粗大なTi窒化物やNb窒化物の形成を招いて靭性を悪化させるおそれがある。
なお、N含有量の下限は0.002質量%とするのが好ましく、0.003質量%とするのがより好ましい。N含有量の上限は0.08質量%とするのが好ましく、0.04質量%とするのがより好ましい。
MgおよびCaは、脱硫・脱酸元素として作用し、鋼材の熱間加工性を改善する作用がある。不可避不純物として含まれるSの含有量に応じて、CaおよびMgを0.005質量%以下の範囲で含有させるとよい。
なお、CaおよびMgの上限はいずれも0.002質量%とするのが好ましい。
Zrは、任意成分であり、析出強化によって高温強度を向上させる作用がある。しかしながら、Zr含有量が過剰となり、0.3質量%を超えると粗大な金属間化合物を形成して高温延性の低下を招いてしまう。
なお、Zr含有量の上限は0.25質量%とするのが好ましい。
但し、Zrを含有させると鋼材のコストが増加するため、必要に応じて含有させればよい。
希土類元素は、任意成分であり、ステンレス鋼の耐酸化性を向上させる作用がある。
つまり、希土類元素を任意に含有させることによって、酸化スケールの生成を抑制することができる。しかしながら、希土類元素の含有量が過剰となり、0.15質量%を超えると、高温環境で粒界の一部が溶融して熱間加工性を阻害するため好ましくない。
なお、希土類元素の含有量の上限は0.1質量%とするのが好ましく、0.05質量%とするのがより好ましい。
ここで、希土類元素は、ScおよびYと、La、Ce、Ndに代表されるランタノイド元素15種と、の合計17種の元素から選択された1種以上の元素である。また、希土類元素の含有量は、17種の元素から選択された1種以上の元素の合計含有量である。
Wは、任意成分であり、固溶強化によって高温強度を向上させる作用がある。しかしながら、W含有量が過剰となり、3質量%を超えると粗大な金属間化合物を形成して高温延性の低下を招いてしまう。
なお、W含有量の上限は2.5質量%とするのが好ましく、2.0質量%とするのがより好ましい。
残部は、Feおよびその他の不可避不純物である。その他の不可避不純物としては、例えば、Al、Sn、Zn、Pb、As、Bi、Sb、Te、Se、Inなどが挙げられる。
なお、不可避不純物は可能な限り少なくすることが望ましく、その目安として、Alは0.01質量%以下、Snは0.005質量%以下、Znは0.01質量%以下、Pbは0.002質量%以下、Asは0.01質量%以下、Biは0.002質量%以下、Sbは0.002質量%以下、Teは0.01質量%以下、Seは0.002質量以下、Inは0.002質量%以下とすることが推奨される。
前記した成分範囲としたうえで、実使用環境中やクリープ試験中に析出する元素の固溶量を確保するため、本実施形態では、平均硬さ(ビッカース硬さ)を160Hv以下としている。平均硬さが160Hvを超えると、実使用環境中やクリープ試験中に析出する元素の固溶量を確保することができないため、クリープ強度が低下する。平均硬さを160Hv以下とするには、前述の成分組成にもよるが、例えば1150℃以上の温度で熱処理を行い、水冷による冷却を行うことにより、容易に得ることができる。
なお、平均硬さの上限は140Hvとするのが好ましい。また、平均硬さの下限は100Hvとするのが好ましく、110Hvとするのがより好ましい。
なお、ビッカース硬さは、例えば、JIS Z 2244:2009に準拠して測定することができる。
[累積数密度と析出粒子径の分布において、前記累積数密度の半値に相当する析出粒子径が70nm以下]
析出粒子径が0nmを超え100nmの範囲にある析出物の累積数密度を0.1~2.0個/μm2とし、且つ、累積数密度と析出粒子径の分布において、前記累積数密度の半値に相当する析出粒子径が70nm以下とすることにより、クリープ強度を向上させることができる。
すなわち、最終熱処理で形成させる析出物について、100nm以下の析出物を一定量形成させながら、累積数密度の半値に相当する析出粒子径を70nm以下と微細なままにしているので、クリープ強度を向上させることができる。
前記した累積数密度の下限は0.3/μm2とするのが好ましく、0.4/μm2とするのがより好ましい。
また、前記した累積数密度の半値に相当する析出粒子径の上限は、60nmとするのが好ましく、50nmとするのがより好ましい。なお、前記累積数密度の半値に相当する析出粒子径の下限は0nmを超える。
当該析出粒子径および前記した累積数密度の測定方法については後記する。
結晶粒度番号が7.5以上であれば、金属組織が十分微細な状態であり、微細結晶粒組織ということができる。従って、耐水蒸気酸化性を維持することができる。
結晶粒度番号を7.5以上とするには、後記する特定の熱処理条件で最終熱処理を行えばよい。
鋼中に含まれる析出粒子径および析出量を一定範囲に収め、かつ、結晶粒度番号を7.5以上とするためには、前記鋼材成分、硬さ範囲を前提としたうえで、析出物の粗大化因子が2000℃・min以下となる条件で最終熱処理を行えばよい。なお、この「析出物の粗大化因子が2000℃・min以下となる条件」が、前記した特定の熱処理条件である。
粗大化因子が前記した条件を満たすか否かを判断するためには、析出物の数密度およびサイズ分布を定量化する必要がある。これは、鋼材断面にて析出物が分散した様子を捉えたミクロ画像を採取し、これを画像解析によって定量化することで得ることができる。ミクロ画像は、例えば、電解研磨された鋼材表面を走査型電子顕微鏡で撮影することで得ることができる。析出物が微細である場合には、走査型電子顕微鏡に替えて透過型電子顕微鏡を用いるとよい。なお、定量精度の観点から、析出物は少なくとも200個以上を定量し、0nmを超え100nmを10nm毎のヒストグラムで整理するとよい。
そのため、従来は実使用環境中やクリープ試験中に形成される析出量を犠牲にしながら結晶粒の微細化を図っていたが、本実施形態に係るオーステナイト系耐熱鋼では、従来、犠牲になっていた析出もクリープ強度の向上に寄与させることができる。従って、設備の制約等により、熱処理の上限温度が存在する場合であっても、析出強化作用を最大限に得ることができる。これにより、析出元素としてTiを使用するオーステナイト系耐熱鋼において、微細結晶粒組織としながらもクリープ強度をさらに高めた耐熱ステンレス鋼を提供することができる。本実施形態に係るオーステナイト系耐熱鋼は、クリープ強度を向上させることができるため、従来よりも耐熱部材の肉厚を薄くできることができ、耐熱部品としての低コスト化を実現することができる。
その後、1250℃で軟化熱処理を施し、冷間圧延によって厚さ13mmまで加工して母材とした。なお、表1に示すNo.A~Fの鋼材のうち、No.A~Eは、所謂火SUS321J2HTB鋼に類するものであり、本発明で規定する化学成分組成を満足する鋼材である。これに対し、No.Fは本発明で規定する化学成分組成を外れる鋼材である。
なお、表1中、下線および斜字体で表している数値は、本発明の要件を満たさないことを示している。
なお、表2中、下線および斜字体で表している数値は、本発明の要件を満たさないことを示している。
ビッカース硬さは、No.1~31に示すそれぞれの鋼材に対してJIS Z 2244:2009に準拠してビッカース硬さ試験を行い、その硬さを測定した。なお、ビッカース硬さ試験の荷重は10kgで測定した。ビッカース硬さが160Hv以下のものを平均硬さに優れると評価し、160Hvを超えるものを平均硬さに劣ると評価した。
(3)累積数密度と析出粒子径の分布において、前記累積数密度の半値に相当する析出粒子径[μm]
前記(2)の累積数密度および前記(3)の累積数密度の半値に相当する析出粒子径は、電解研磨された鋼材表面から走査型電子顕微鏡を用いて6000倍の画像を撮像し、最低200個以上の析出物を画像解析するとともに、前記した図1に示したようなグラフを作成して、累積数密度と析出粒子径の分布を算出した。
このとき、6000倍の倍率において20nmの物体を認識できるような画像が得られており、本実施例においてはそれ以上に微細な析出物は存在しないことを透過型電子顕微鏡で確認している。
結晶粒度番号は、No.1~31に示すそれぞれの鋼材に対し、JIS G 0551:2013に準拠して組織を顕微鏡観察し、結晶粒度番号を測定した。結晶粒度番号が7.5以上のものを合格とし、7.5未満のものを不合格とした。
クリープ破断時間は、No.1~31に示すそれぞれの鋼材から、JIS Z 2271:2010に準拠して試験片を作製し、試験を行って測定した。クリープ破断時間が650時間以上のものをクリープ強度に優れると評価し、650時間未満のものをクリープ強度に劣ると評価した。
なお、微細結晶粒組織であるこれらの例は、良好な耐水蒸気酸化特性を得ることができると推察される。
これらの内、No.16と18、No.20と23、No.25と28の結果から、高温で熱処理した前者の番号に比べて後者の番号の方が、クリープ破断時間が増加していることが分かった。かかる知見は、本発明で得られるクリープ強度の改善効果が“熱処理温度の高い方が、クリープ強度が高い”という析出元素の固溶量に着目した従来の知見とは異なる作用による可能性があることを示している。
これらのうち、No.29、30に示す鋼材は、結晶粒が粗大でクリープ強度に望ましい要素を含んでいるものの、何れの鋼材もクリープ強度は650時間を下回り、実施例と比較して不十分な強度しか得られなかった。
また、No.31に示す鋼材は、結晶粒度番号が7.5であり、良好な微細結晶粒組織が得られているものの、クリープ強度は650時間を下回り、実施例と比較して不十分な強度しか得られなかった。
本出願は、2014年3月5日出願の日本特許出願(特願2014-042889)に基づくものであり、その内容はここに参照として取り込まれる。
Claims (2)
- C :0.05~0.16質量%、
Si:0.1~1質量%、
Mn:0.1~2.5質量%、
P :0.01~0.05質量%、
S :0.005質量%以下(0質量%を含まない)、
Ni:7~12質量%、
Cr:16~20質量%、
Cu:2~4質量%、
Mo:0.1~0.8質量%、
Nb:0.1~0.6質量%、
Ti:0.1~0.6質量%、
B :0.0005~0.005質量%、
N :0.001~0.15質量%を含有し、かつ、
Mg:0.005質量%以下(0質量%を含まない)およびCa:0.005質量%以下(0質量%を含まない)のうちの少なくとも一つを含有し、
前記Nbの含有量と前記Tiの含有量の合計が0.3質量%以上、
残部がFeおよび不可避不純物からなり、
析出粒子径が0nmを超え100nmの範囲にある析出物の累積数密度が0.1~2.0個/μm2、
累積数密度と析出粒子径の分布において、前記累積数密度の半値に相当する析出粒子径が70nm以下、
平均硬さが160Hv以下、かつ、
結晶粒度番号が7.5以上である
ことを特徴とするオーステナイト系耐熱鋼。 - さらに、Zr:0.3質量%以下(0質量%を含まない)、希土類元素:0.15質量%以下(0質量%を含まない)およびW:3質量%以下(0質量%を含まない)のうちの少なくとも一つを含有していることを特徴とする請求項1に記載のオーステナイト系耐熱鋼。
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JP2017014575A (ja) * | 2015-07-01 | 2017-01-19 | 新日鐵住金株式会社 | オーステナイト系耐熱合金及び溶接構造物 |
JP6623719B2 (ja) * | 2015-11-25 | 2019-12-25 | 日本製鉄株式会社 | オーステナイト系ステンレス鋼 |
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CN109576580B (zh) * | 2018-11-29 | 2020-09-29 | 武汉华培动力科技有限公司 | 柴油机可变截面增压器喷嘴组件用耐热钢及冶炼方法 |
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US20220145174A1 (en) * | 2020-11-05 | 2022-05-12 | Seoul National University R&Db Foundation | Perovskite color converter and method of manufacturing the same |
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