WO2012153814A1 - 耐繰返し酸化特性に優れた耐熱オーステナイト系ステンレス鋼 - Google Patents
耐繰返し酸化特性に優れた耐熱オーステナイト系ステンレス鋼 Download PDFInfo
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- 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
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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
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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- 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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- 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
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- 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
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- 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
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- 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
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- 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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- 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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- 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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- 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
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
Definitions
- the present invention relates to a heat-resistant austenitic stainless steel suitably used as a heat transfer tube material such as a boiler, and more particularly to a heat-resistant austenitic stainless steel excellent in cyclic oxidation resistance.
- 25Cr-20Ni austenitic stainless steel (SUS310S) is known as a heat resistant material having excellent oxidation resistance in a broad sense including characteristics other than cyclic oxidation resistance.
- This stainless steel contains a large amount of expensive Ni. There is a problem that the cost is high. For this reason, it is important to use 18Cr-8Ni austenitic stainless steel (SUS304), which has a low Ni content and good high-temperature strength and corrosion resistance, as basic components for boiler heat transfer tube materials. It becomes.
- Patent Documents 1 and 2 have been proposed as technologies related to austenitic stainless steel using a Ti compound as a precipitation strengthening mechanism. ing.
- Patent Document 1 discloses that the oxidation resistance is improved by adding Al that contributes to the improvement of corrosion resistance and promoting the formation of a Cr 2 O 3 layer by surface polishing.
- the total amount of Al and Si is increased to 4% or more, and in addition, REM such as Ce, Y, La, etc. or Ca is added to be resistant to oxidation. Has been shown to be improved.
- the action of delaying the growth rate of the oxide formed on the steel pipe surface can be expected by the addition of Al and Si and the formation of the Cr 2 O 3 layer, the formation of the oxide itself is not completely prevented. Also, it cannot be expected to exhibit good cyclic oxidation resistance. Furthermore, the steel material to which Al is added has a problem that surface flaws are likely to occur during pipe making.
- Patent Document 2 discloses that Ce, La, and Hf are added in order to improve the oxidation resistance. However, similar to the above technique, it is expected that the cyclic oxidation resistance is low, and the repetition resistance is also reduced. It was not made in recognition of the improvement in oxidation characteristics.
- Patent Document 3 As a technique for improving the resistance to repeated oxidation, a technique such as Patent Document 3 has also been proposed. However, since this technique contains a large amount of Al and Si, there is a problem in that it causes embrittlement after a surface flaw on the steel pipe or a long-time heat treatment. Further, in this technique, it has been shown that adding REM such as La and Ce including Y exhibits an effect of improving the adhesion of the scale, but it does not have sufficient characteristics and is resistant to resistance. It was not made in recognition of the improvement of repeated oxidation characteristics.
- REM such as La and Ce including Y
- Patent Document 4 As a technique for improving the oxidation resistance of austenitic stainless steel for boilers, a technique such as Patent Document 4 has also been proposed.
- This technology is a component system of “Take SUS304J1HTB” that uses Nb and N for precipitation strengthening and solid solution strengthening.
- Ti is added in an amount of about 0.002 to 0.05% for the purpose of forming oxide inclusions, but a steel material using precipitation of Ti compounds such as fire SUS321J2HTB as a strengthening mechanism.
- this technique is not made by recognizing an improvement in the resistance to repeated oxidation, and is expected to have a low resistance to repeated oxidation.
- oxidation resistance is improved by adding REM and particle spray peening.
- the peening process has another problem that the cost is increased due to an increase in the manufacturing process, and is not made in recognition of the improvement of the resistance to repeated oxidation, and is expected to have a low resistance to repeated oxidation.
- the present invention has been made under such circumstances, and the object thereof is to have a chemical composition equivalent to that of 18Cr-8Ni austenitic stainless steel in which the contents of Ni and Cr are added, as well as the addition of Al and Si, and the surface.
- An object of the present invention is to provide a heat-resistant austenitic stainless steel having excellent resistance to repeated oxidation, which is less dependent on treatment and has less oxide peeling in a repeated oxidation environment and is less likely to cause thinning.
- the heat-resistant austenitic stainless steel of the present invention that has solved the above problems is C: 0.05-0.2% (meaning mass%, hereinafter the same for chemical composition), Si: 0.1-1%, Mn: 0.1 to 2.5%, Cu: 1 to 4%, Ni: 7 to 12%, Cr: 16 to 20%, Nb: 0.1 to 0.6%, Zr: 0.05 to 0 .4%, Ce: 0.005 to 0.1%, Ti: 0.1 to 0.6%, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S: It contains 0.005% or less (excluding 0%) and P: 0.05% or less (not including 0%), respectively, and the balance is made of iron and inevitable impurities.
- the heat-resistant austenitic stainless steel of the present invention further contains Mo: 3% or less (not including 0%) and / or W: 5% or less (not including 0%) as necessary. Yes, the inclusion of these components further improves the high temperature strength.
- the heat-resistant austenitic stainless steel of the present invention further contains Ca: 0.005% or less (not including 0%) and / or Mg: 0.005% or less (not including 0%) as necessary.
- Ca 0.005% or less (not including 0%)
- Mg 0.005% or less
- the chemical component composition as described above By adjusting the chemical component composition as described above, a heat-resistant austenitic stainless steel with improved resistance to repeated oxidation can be obtained. Further, the crystal grain size of the metal structure is 6 or more and less than 12 in terms of ASTM grain size number. As a result, higher resistance to repeated oxidation can be obtained and the characteristics can be exhibited stably.
- the heat-resistant austenitic stainless steel of the present invention is less susceptible to the progress of oxidation due to scale peeling and the accompanying thinning of the steel material even in a repetitive oxidation environment, so that it can be used as a heat transfer tube for coal-fired power generation. It is possible to improve the power generation efficiency by increasing the temperature of the tube, extending the life of the heat transfer tube compared to existing materials, and reducing the maintenance cost. Moreover, since there is little peeling of a scale, when it uses as a heat exchanger tube, scattering of the scale inside can be suppressed and damage to a turbine can also be reduced.
- the present inventors have studied from various angles in order to realize an austenitic stainless steel having improved resistance to repeated oxidation while maintaining necessary high-temperature strength. As a result, if a predetermined amount of Zr and Ce is contained in a stainless steel having a chemical composition equal to that of 18Cr-8Ni austenitic stainless steel, the remarkably excellent resistance to repeated oxidation. The present invention was completed by discovering that the characteristics can be exhibited.
- the heat resistant austenitic stainless steel of the present invention is characterized in that the contents of Ni and Cr contain a predetermined amount of Zr and Ce with respect to the chemical composition equivalent to that of 18Cr-8Ni austenitic stainless steel.
- the reasons for setting the ranges of the contents of Zr and Ce are as follows.
- Zr and Ce express the effect of suppressing the exfoliation of the oxide by these synergistic effects.
- it is necessary to contain Zr at 0.05% or more.
- the upper limit must be 0.4% or less.
- Ce in order to exhibit the effect, it is necessary to contain 0.005% or more.
- the Ce content exceeds 0.1% and becomes excessive, an economic cost increase is caused.
- the preferable lower limit of the Zr content is 0.10% or more (more preferably 0.15% or more), and the preferable upper limit is 0.3% or less (more preferably 0.25% or less).
- the preferable minimum of Ce content is 0.01% or more (more preferably 0.015% or more), and a preferable upper limit is 0.05% or less (more preferably 0.03% or less).
- pure Ce may be added as a raw material of Ce, but it is also possible to add the necessary pure Ce using a separately prepared Ce-containing mother alloy or Ce-containing misch metal. There is no problem even if La, Nd, Pr, etc. contained are contained in steel as impurities at a lower concentration than Ce, respectively, and melting work is performed by using a mother alloy or misch metal compared to pure Ce that is easily oxidized It is possible to simplify the handling of time.
- Patent Documents 1, 3, and 5 among the prior arts disclose that the adhesion of oxides is improved by adding REM containing Y, La, and Ce. , REM is assumed to be added alone, and no synergistic effect by adding Ce together with Zr is disclosed.
- Patent Document 2 discloses that Zr and Ce can be used in combination, but in this technique, none of them is an essential component, and it is added as necessary including non-addition.
- Zr is contained in an amount less than the range specified in the present invention in view of strengthening grain boundaries and improving creep ductility.
- the heat-resistant austenitic stainless steel of the present invention has a chemical composition equivalent to that of 18Cr-8Ni austenitic stainless steel in the contents of Ni and Cr, but the chemical composition of each element other than Zr and Ce ( C, Si, Mn, Cu, Ni, Cr, Nb, Ti, B, N, S, and P) need to be appropriately adjusted.
- the effects of these components and the reasons for setting the range are as follows.
- C is an element that has the effect of forming carbides in a high-temperature use environment and improving the high-temperature strength and creep strength necessary as a heat transfer tube.
- C is 0.05. % Or more must be contained.
- the preferable lower limit of the C content is 0.07% or more (more preferably 0.09% or more), and the preferable upper limit is 0.18% or less (more preferably 0.15% or less).
- Si 0.1 to 1%
- Si is an element having a deoxidizing action in molten steel. Even if it is contained in a very small amount, it effectively works to improve oxidation resistance. In order to exert these effects, the Si content needs to be 0.1% or more. However, if the Si content is excessive and exceeds 1%, the formation of the ⁇ phase is caused and the steel material becomes brittle ( ⁇ brittle).
- the preferable lower limit of the Si content is 0.2% or more (more preferably 0.3% or more), and the preferable upper limit is 0.9% or less (more preferably 0.8% or less).
- Mn 0.1 to 2.5%
- Mn is an element having a deoxidizing action in molten steel, and also has an action of stabilizing austenite.
- the Mn content needs to be 0.1% or more. However, if the Mn content is excessive and exceeds 2.5%, hot workability is impaired.
- the preferable lower limit of the Mn content is 0.2% or more (more preferably 0.3% or more), and the preferable upper limit is 2.0% or less (more preferably 1.8% or less).
- Cu 1 to 4%
- Cu is an element that forms consistent precipitates in the steel (precipitates whose atomic arrangement is continuous with the base metal) and significantly improves the high-temperature creep strength, and is one of the main strengthening mechanisms in stainless steel. It is. In order to exert this effect, the Cu content needs to be 1% or more. However, even if the Cu content is excessive and exceeds 4%, the effect is saturated.
- the preferable lower limit of the Cu content is 2.0% or more (more preferably 2.5% or more), and the preferable upper limit is 3.7% or less (more preferably 3.5% or less).
- Ni has an effect of stabilizing austenite, and it is necessary to contain 7% or more in order to maintain the austenite phase. However, if the Ni content becomes excessive and exceeds 12%, the cost will increase.
- the preferable lower limit of the Ni content is 7.5% or more (more preferably 8.0% or more), and the preferable upper limit is 11.5% or less (more preferably 11.0% or less).
- Cr 16-20%
- Cr is an essential element in order to develop corrosion resistance as stainless steel. In order to exert such effects, it is necessary to contain 16% or more of Cr. However, if the Cr content becomes excessive and exceeds 20%, the ferrite phase that causes a decrease in high-temperature strength increases.
- the preferable lower limit of the Cr content is 16.5% or more (more preferably 17.0% or more), and the preferable upper limit is 19.5% or less (more preferably 19.0% or less).
- Nb is an element effective for improving the high-temperature strength by precipitating carbonitride (carbide, nitride, or carbonitride), and this precipitate suppresses the coarsening of crystal grains and diffuses Cr. By promoting the above, a secondary effect of improving corrosion resistance is exhibited.
- Nb needs to be contained by 0.1% or more. However, if the Nb content exceeds 0.6% and becomes excessive, the precipitates become coarse and the toughness is reduced.
- a preferable lower limit of the Nb content is 0.12% or more (more preferably 0.15% or more), and a preferable upper limit is 0.5% or less (more preferably 0.3% or less).
- Ti 0.1 to 0.6%
- Ti exhibits the same effect as Nb, but by adding it in combination with Nb and Zr, the precipitates are further stabilized and effective in maintaining high-temperature strength for a long period of time.
- the Ti content needs to be 0.1% or more.
- the preferable lower limit of the Ti content is 0.12% or more (more preferably 0.15% or more), and the preferable upper limit is 0.5% or less (more preferably 0.3% or less).
- B has the effect of promoting the formation of M 23 C 6 type carbide (M is a carbide forming element), which is one of the main strengthening mechanisms, by forming a solid solution in steel.
- M is a carbide forming element
- the B content needs to be 0.0005% or more.
- a preferable lower limit of the B content is 0.001% or more (more preferably 0.0012% or more), and a preferable upper limit is 0.004% or less (more preferably 0.003% or less).
- N has the effect of improving high temperature strength by solid solution strengthening by dissolving in steel, and is effective in improving high temperature strength by forming nitrides with Cr and Nb under a long-term high temperature load. It is an element. In order to exhibit these effects effectively, the N content needs to be 0.001% or more. However, if the N content becomes excessive and exceeds 0.15%, the formation of coarse Ti nitrides and Nb nitrides is caused, and the toughness is deteriorated.
- the preferable lower limit of the N content is 0.002% or more (more preferably 0.003% or more), and the preferable upper limit is 0.10% or less (more preferably 0.08% or less, still more preferably 0.02%). The following).
- S 0.005% or less (excluding 0%)
- S is an unavoidable impurity, but when its content increases, hot workability deteriorates, so it is necessary to make it 0.005% or less. Further, S impairs the action of adding Ce by fixing Ce as a sulfide, so S is preferably suppressed to 0.002% or less (more preferably 0.001% or less).
- P 0.05% or less (excluding 0%)
- P is an inevitable impurity, but if its content increases, weldability is impaired, so it is necessary to make it 0.05% or less. Preferably it is good to suppress to 0.04% or less (more preferably 0.03% or less).
- the contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities, and in addition to La, Nd, Pr, etc. contained at a lower concentration than Ce when adding Ce raw material with misch metal Furthermore, mixing of elements brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. can be allowed. However, low melting point impurities such as Sn, Pb, Sb, As, and Zn derived from scrap raw materials reduce the strength of grain boundaries during hot working or when used in a high temperature environment. In order to improve the resistance to embrittlement cracking after use, it is desirable to keep the concentration low. Moreover, the steel material of this invention may contain Mo, W, Ca, Mg, etc. as needed, and the characteristic of steel materials is further improved according to the kind of element contained.
- Mo and W have the effect of improving the high temperature strength by solid solution strengthening, and the high temperature strength can be further increased by inclusion if necessary.
- Mo content is excessive, hot workability is hindered. More preferably, it is 2.5% or less (more preferably 2.0% or less).
- W content is excessive, a coarse intermetallic compound is formed and the high temperature ductility is lowered. More preferably, it is 4.5% or less (more preferably 4.0% or less).
- the preferable minimum for exhibiting the above effects effectively is 0.1% or more (more preferably 0.5% or more) in Mo, and 0.1% or more (more preferably 1) in W. 0.0% or more).
- the contents may be set according to the required amount of reinforcement and the allowable cost.
- Ca and Mg function as desulfurization / deoxidation elements, formation of Ce sulfide and Ce oxide can be suppressed, and Ce yield can be improved, and reduction in toughness due to inclusion formation can be suppressed.
- a preferable lower limit for effectively exhibiting such an effect is 0.0002% or more, and more preferably 0.0005% or more.
- the upper limit value is set to 0.005% or less because there are restrictions on work such as bumping of molten steel during melting work. More preferably, both are 0.002% or less.
- the heat-resistant austenitic stainless steel of the present invention can improve the resistance to repeated oxidation by containing a predetermined amount of Zr and Ce. However, in order to further improve the characteristics, the crystal grain size of the metal structure is controlled. It is effective. From such a viewpoint, it is preferable that the crystal grain size of the metal structure of the heat-resistant austenitic stainless steel is a microstructure having an ASTM (American Society for Testing and Materials) grain size number of 6 or more and less than 12.
- the grain size number (crystal grain size number) is determined by ASTM, and means a grain size number calculated by a counting method (Planimetric method).
- the crystal grain size of the metal structure is less than 6 in terms of ASTM grain size number, the effect of improving the repeated oxidation resistance by containing Zr and Ce can be obtained, but the improvement effect cannot be sufficiently enhanced.
- the particle size number is more preferably 7 or more, and still more preferably 9 or more.
- the upper limit of the crystal grain size is preferably less than 12. In consideration of production cost and productivity, it is more preferably 10 or less.
- the crystal grain size range as described above can be obtained by adjusting the amount of components contributing to pinning of grain boundaries and the conditions of hot and cold working and heat treatment such as drawing and extrusion during the pipe making process. . Although each optimum condition varies depending on these three factors, in order to make the crystal grain size fine, it is necessary to add a large amount of precipitated elements, to increase the degree of processing, and to lower the heat treatment temperature.
- Cold / hot working is aimed at adjusting the thickness and adjusting the grain structure by heat treatment after processing by introducing strain, and is usually carried out at a cross-section reduction rate of 30% or more.
- the heat treatment is intended to remove strain, and is generally performed in a temperature range of 1000 ° C. or higher and lower than 1300 ° C.
- the prescribed particle size range can be obtained by setting the heat treatment temperature to 1250 ° C. or less, preferably 1225 ° C. or less, particularly preferably 1150 ° C. or less. It is not limited to this condition depending on the balance between processing and heat treatment.
- Example 1 Various steel materials having the chemical composition shown in Table 1 below were melted, and a 20 kg ingot melted in a vacuum melting furnace (VIF) was hot forged into a dimension of width: 120 mm ⁇ thickness: 20 mm, at 1250 ° C. After heat treatment, it was processed to a thickness of 13 mm by cold rolling. Thereafter, heat treatment was again performed at 1150 ° C. for 5 minutes, and this was used as a base material. A 20 mm ⁇ 30 mm ⁇ 2 mm steel material was cut out from the base material by machining, and a test piece was prepared by smoothing and mirror-finishing the surface of the steel material by polishing using emery paper and buffing using diamond abrasive grains.
- VIF vacuum melting furnace
- test No. Nos. 1 to 10 are steel materials (invention steels) satisfying the requirements specified in the present invention.
- Nos. 11 to 16 are steel materials (comparative steels) that do not satisfy the requirements defined in the present invention.
- Reference numerals 14, 15, and 16 are “fire SUS304J1HTB equivalent steel”, “SUS304L equivalent steel”, and “SUS310S equivalent steel”, which are existing steels, respectively.
- Test No. 7 and 8 are steel materials to which Ce is added by misch metal, and include La, Pr, Nd and the like as impurities.
- Test No. 9 and 10 are steel materials to which Mg and Ca are added, respectively.
- fire SUS304J1HTB equivalent steel belongs to 18Cr-8Ni austenitic stainless steel and is a steel type that has been used as a boiler heat transfer tube (for example, “Materia” Vol. 46, No. 2). No. 2007, P99-101).
- SUS310S equivalent steel belongs to 25Cr-20Ni austenitic stainless steel and is more expensive because it contains more Ni than 18Cr-8Ni austenitic stainless steel, but essentially in terms of chemical composition. It is a steel type that has better corrosion resistance than 18Cr-8Ni austenitic stainless steel.
- the production and peeling of the scale does not occur because the steel of the present invention has a smoother scale surface.
- the steel according to the present invention exhibits the same properties as the existing steel SUS310S equivalent to 25Cr-20Ni (test No. 16), which has a high Ni content and is excellent in corrosion resistance, and is an 18Cr-8Ni austenitic stainless steel. It can be seen that, despite the low cost, the repeated oxidation resistance can be improved to the same level as the 25Cr-20Ni austenitic stainless steel.
- Example 2 Test No. shown in Tables 1 and 2 Inventive steels 1 to 6 and test no.
- the heat treatment temperature was changed in the temperature range of 1125 to 1275 ° C. after cold working with a cross-section reduction rate of 35%, and samples with crystal grain numbers of 4.5 to 10.0 were prepared for each steel material.
- the repeated oxidation test is a temperature cycle of 25 minutes for heating in the furnace and 5 minutes for cooling to the atmosphere. The sample is taken in and out of the atmospheric furnace at 1100 ° C., and the specimen mass after 40 cycles is compared with the specimen mass in the initial state The mass loss (thickness loss: mg ⁇ cm ⁇ 2 ) was determined.
- the amount of thinning was significantly improved in some steels with added Zr and Ce, and the amount of thinning after 20 cycles was an error depending on the particle size. Repeated. For the calculation of the crystal grain size, three visual fields were observed per steel type.
- the addition of Zr and Ce itself improves the resistance to repeated oxidation, and the chemical composition is within the range specified in the present invention. It can be seen that the finer the grain size, the better the characteristics.
- the heat-resistant austenitic stainless steel of the present invention is suitably used as a heat transfer tube material such as a boiler.
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Abstract
Description
Cは、高温の使用環境において炭化物を形成し、伝熱管として必要な高温強度、クリープ強度を向上させる作用を有する元素であり、強化機構となる炭化物の析出量を確保するためには0.05%以上含有させる必要がある。しかしながら、C含有量が過剰になって0.2%を超えると、固溶限を超えて粗大な炭化物となり、更なる強化が得られない。C含有量の好ましい下限は0.07%以上(より好ましくは0.09%以上)であり、好ましい上限は0.18%以下(より好ましくは0.15%以下)である。
Siは、溶鋼中で脱酸作用を有する元素である。また微量の含有であっても、耐酸化性の向上に有効に作用する。これらの効果を発揮させるためには、Si含有量は0.1%以上とする必要がある。しかしながら、Si含有量が過剰になって1%を超えると、σ相の形成を招き、鋼材の脆化(σ脆化)をもたらすことになる。Si含有量の好ましい下限は0.2%以上(より好ましくは0.3%以上)であり、好ましい上限は0.9%以下(より好ましくは0.8%以下)である。
MnはSiと同様に、溶鋼中で脱酸作用を有する元素であり、またオーステナイトを安定化させる作用がある。これらの効果を発揮させるためには、Mn含有量は0.1%以上とする必要がある。しかしながら、Mn含有量が過剰になって2.5%を超えると、熱間加工性を阻害することになる。Mn含有量の好ましい下限は0.2%以上(より好ましくは0.3%以上)であり、好ましい上限は2.0%以下(より好ましくは1.8%以下)である。
Cuは、鋼中に整合析出物(母材と原子配列が連続的であるような析出物)を形成し、高温クリープ強度を著しく向上させる元素であり、ステンレス鋼における主要な強化機構の一つである。この効果を発揮させるためには、Cu含有量は1%以上とする必要がある。しかしながら、Cu含有量が過剰になって4%を超えてもその効果は飽和する。Cu含有量の好ましい下限は2.0%以上(より好ましくは2.5%以上)であり、好ましい上限は3.7%以下(より好ましくは3.5%以下)である。
Niは、オーステナイトを安定化させる作用があり、オーステナイト相を維持するためには7%以上含有させる必要がある。しかしながら、Ni含有量が過剰になって12%を超えると、コストの増加をもたらすことになる。Ni含有量の好ましい下限は7.5%以上(より好ましくは8.0%以上)であり、好ましい上限は11.5%以下(より好ましくは11.0%以下)である。
Crは、ステンレス鋼としての耐食性を発現するために必須の元素である。こうした効果を発揮させるためには、Crは16%以上含有させる必要がある。しかしながら、Cr含有量が過剰になって20%を超えると、高温強度の低下を招くフェライト相が増加する。Cr含有量の好ましい下限は16.5%以上(より好ましくは17.0%以上)であり、好ましい上限は19.5%以下(より好ましくは19.0%以下)である。
Nbは、炭窒化物(炭化物、窒化物または炭窒化物)を析出させることで、高温強度の改善に有効な元素であり、またこの析出物が結晶粒の粗大化を抑制し、Crの拡散を促進することで、副次的に耐食性向上の作用を発揮する。必要な析出量を確保するためには、Nbは0.1%以上含有させる必要がある。しかしながら、Nb含有量が0.6%を超えて過剰になると、析出物が粗大化し靭性の低下を招くことになる。Nb含有量の好ましい下限は0.12%以上(より好ましくは0.15%以上)であり、好ましい上限は0.5%以下(より好ましくは0.3%以下)である。
TiもNbと同様な作用を発揮するものの、NbおよびZrと複合添加することで、析出物が更に安定化して長期間の高温強度の維持にも有効である。こうした効果を有効に発揮させるためには、Ti含有量は0.1%以上とする必要がある。しかしながら、Ti含有量が過剰になると、Nbの場合と同様に析出物が粗大化し靭性の低下を招くことになるので、0.6%以下とする必要がある。Ti含有量の好ましい下限は0.12%以上(より好ましくは0.15%以上)であり、好ましい上限は0.5%以下(より好ましくは0.3%以下)である。
Bは、鋼中に固溶することで、主要な強化機構の一つであるM23C6型炭化物(Mは炭化物形成元素)の形成を促進させる作用がある。こうした効果を有効に発揮させるためには、B含有量は0.0005%以上とする必要がある。しかしながら、B含有量が過剰になると熱間加工性や溶接性の低下を招くため、0.005%以下とする必要がある。B含有量の好ましい下限は0.001%以上(より好ましくは0.0012%以上)であり、好ましい上限は0.004%以下(より好ましくは0.003%以下)である。
Nは、鋼中に固溶することで固溶強化によって高温強度を向上させる作用があり、また長期間の高温荷重下において、CrやNbと窒化物を形成して高温強度の向上に有効な元素である。これらの効果を有効に発揮させるためには、N含有量は0.001%以上とする必要がある。しかしながら、N含有量が過剰になって0.15%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。N含有量の好ましい下限は0.002%以上(より好ましくは0.003%以上)であり、好ましい上限は0.10%以下(より好ましくは0.08%以下、更に好ましくは0.02%以下)である。
Sは、不可避不純物であるが、その含有量が増加すると熱間加工性を劣化させるため、0.005%以下とする必要がある。また、SはCeを硫化物として固定することでCeを添加することによる作用を損なうので、好ましくは0.002%以下(より好ましくは0.001%以下)に抑制するのが良い。
Pは、不可避不純物であるが、その含有量が増加すると溶接性を損なうため、0.05%以下とする必要がある。好ましくは0.04%以下(より好ましくは0.03%以下)に抑制するのが良い。
MoおよびWは、固溶強化によって高温強度を向上させる効果があり、必要によって含有させることで高温強度を更に上昇させることができる。しかしながら、Mo含有量が過剰になると熱間加工性を阻害するので、3%以下とすることが好ましい。より好ましくは、2.5%以下(更に好ましくは2.0%以下)である。また、W含有量が過剰にな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くため、5%以下とすることが好ましい。より好ましくは4.5%以下(更に好ましくは4.0%以下)である。尚、上記のような効果を有効に発揮させるための好ましい下限は、Moで0.1%以上(より好ましくは0.5%以上)であり、Wで0.1%以上(より好ましくは1.0%以上)である。但し、これらの元素は含有させることによって、上記のような作用を発揮するが、それと同時にコスト増を招くため、必要な強化量と許容されるコストに応じて含有量を設定すれば良い。
CaおよびMgは、脱硫・脱酸元素として働くため、Ce硫化物やCe酸化物の形成を抑制してCeの歩留り向上や、介在物形成による靭性低下の抑制が可能となる。こうした効果を有効に発揮させるための好ましい下限はいずれも0.0002%以上であり、より好ましくは0.0005%以上である。しかしながら、これらの含有量が過剰になると、溶解作業中に溶鋼の突沸が生じるなどの作業上の制約を受けるため、上限値をいずれも0.005%以下とした。より好ましくはいずれも0.002%以下である。
下記表1に示す化学成分組成からなる各種鋼材を溶解し、真空溶解炉(VIF)にて溶製した20kgインゴットを幅:120mm×厚さ:20mmの寸法に熱間鍛造加工し、1250℃で熱処理を施した後、冷間圧延によって厚さ:13mmまで加工した。その後、1150℃で5分の熱処理を再度実施して、これを母材とした。この母材から20mm×30mm×2mmの鋼材を機械加工によって切出し、エメリー紙を用いた研磨とダイヤモンド砥粒を用いたバフ研磨で、鋼材の表面を平滑・鏡面化して試験片を作製した。
表1、2に示した試験No.1~6の発明鋼と、試験No.14の比較鋼について、断面減少率35%の冷間加工後に熱処理温度を1125~1275℃の温度範囲で変化させ、各々の鋼材で結晶粒度番号が4.5~10.0の試料を作製した。繰返し酸化試験は炉内加熱25分、大気放冷5分の温度サイクルで、サンプルを1100℃の大気炉から出し入れし、40サイクル後の試験片質量を初期状態の試験片質量と比較することで質量減少量(減肉量:mg・cm-2)を求めた。
本出願は、2011年5月11日出願の日本特許出願(特願2011-106588)、2011年9月16日出願の日本特許出願(特願2011-203604)、2012年3月5日出願の日本特許出願(特願2012-048357)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 (3)
- C:0.05~0.2%(質量%の意味。以下、化学成分組成について同じ。)、Si:0.1~1%、Mn:0.1~2.5%、Cu:1~4%、Ni:7~12%、Cr:16~20%、Nb:0.1~0.6%、Zr:0.05~0.4%、Ce:0.005~0.1%、Ti:0.1~0.6%、B:0.0005~0.005%、N:0.001~0.15%、S:0.005%以下(0%を含まない)およびP:0.05%以下(0%を含まない)を夫々含有し、残部が鉄および不可避不純物からなることを特徴とする耐繰返し酸化特性に優れた耐熱オーステナイト系ステンレス鋼。
- 更に、下記元素の少なくとも1つを含有する請求項1に記載の耐熱オーステナイト系ステンレス鋼。
Mo:3%以下(0%を含まない)
W :5%以下(0%を含まない)
Ca:0.005%以下(0%を含まない)
Mg:0.005%以下(0%を含まない) - 金属組織の結晶粒度がASTM粒度番号で6以上、12未満である請求項1または2記載の耐熱オーステナイト系ステンレス鋼。
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EP12782655.0A EP2708611B1 (en) | 2011-05-11 | 2012-05-10 | Heat-resistant austenitic stainless steel having excellent cyclic oxidation resistance |
KR1020137029415A KR20130137705A (ko) | 2011-05-11 | 2012-05-10 | 내반복 산화 특성이 우수한 내열 오스테나이트계 스테인리스강 |
ES12782655.0T ES2590465T3 (es) | 2011-05-11 | 2012-05-10 | Acero inoxidable austenítico resistente al calor que tiene una excelente resistencia a la oxidación cíclica |
US14/115,570 US20140154128A1 (en) | 2011-05-11 | 2012-05-10 | Heat-resistant austenitic stainless steel having excellent cyclic oxidation resistance |
CN201280022304.7A CN103517998B (zh) | 2011-05-11 | 2012-05-10 | 抗循环氧化性能优异的耐热奥氏体系不锈钢 |
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JP2012048357A JP5143960B1 (ja) | 2011-05-11 | 2012-03-05 | 高温強度と耐繰返し酸化特性に優れた耐熱オーステナイト系ステンレス鋼 |
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JP5296186B2 (ja) * | 2011-12-27 | 2013-09-25 | 株式会社神戸製鋼所 | 耐スケール剥離性に優れた耐熱オーステナイト系ステンレス鋼およびステンレス鋼管 |
ES2734051T3 (es) | 2015-06-05 | 2019-12-04 | Nippon Steel Corp | Acero inoxidable austenítico |
CN106256920B (zh) * | 2015-06-17 | 2019-10-29 | 宝钢德盛不锈钢有限公司 | 一种具有良好抗氧化性能的含钛奥氏体不锈钢及其制造方法 |
EP3318651B1 (en) * | 2015-07-01 | 2019-11-13 | Nippon Steel Corporation | Austenitic heat-resistant alloy and welded joint |
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 |
BR112018069311A8 (pt) * | 2016-04-07 | 2021-10-13 | Nippon Steel & Sumitomo Metal Corp | Material de aço inoxidável austenítico |
KR101877786B1 (ko) * | 2016-12-21 | 2018-07-16 | 한국기계연구원 | 내산화성이 우수한 오스테나이트계 스테인리스강 및 그 제조 방법 |
KR20180111417A (ko) | 2017-03-31 | 2018-10-11 | 엘지전자 주식회사 | 연성 스테인리스 강관 |
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ES2590465T3 (es) | 2016-11-22 |
CN103517998B (zh) | 2016-08-17 |
JP2013076156A (ja) | 2013-04-25 |
KR20130137705A (ko) | 2013-12-17 |
CN103517998A (zh) | 2014-01-15 |
EP2708611A1 (en) | 2014-03-19 |
EP2708611A4 (en) | 2015-04-08 |
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US20140154128A1 (en) | 2014-06-05 |
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