Classifications

• • 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
• • 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
• • 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
• • 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
• • 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
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
• • 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
• • 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
• • 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
• • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
  • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
  • C23C8/10—Oxidising
  • C23C8/16—Oxidising using oxygen-containing compounds, e.g. water, carbon dioxide
  • C23C8/18—Oxidising of ferrous surfaces

Definitions

• the present invention relates to an austenitic heat resistant alloy.
• Olefins such as ethylene (C 2 H 4 ) are produced by subjecting hydrocarbons (naphtha, natural gas, ethane, etc.) to heat decomposition.
• hydrocarbons naphtha, natural gas, ethane, etc.
• olefinic hydrocarbons ethylene, propylene, etc.
• a pipe that is installed in a reactor and made of a high Cr-high Ni alloy, typically 25Cr-25Ni alloys or 25Cr-38Ni alloys, or is made of a stainless steel, typically SUS304 or the like, and by adding heat from an outer surface of the pipe, so that a heat decomposition reaction of the hydrocarbons occurs on an inner surface of the pipe.
• Patent Document 1 proposes a Ni-based heat resistant alloy that is excellent in hot workability, weldability, and carburization resistance properties.
• a Ni-based alloy is difficult to produce because a ⁇ ′ phase, which is a brittle phase, precipitates at high temperature, narrowing a temperature range that allows hot working.
• Patent Document 2 proposes an austenitic heat resistant alloy that keeps a high creep strength and a high toughness even in a high-temperature environment.
• Patent Document 1 JP 2001-40443A
• Patent Document 2 WO 2017/119415
• the austenitic heat resistant alloy described in Patent Document 2 forms an alumina layer on its surface while being used at high temperature, which not only provides high corrosion resistances but also allows the austenitic heat resistant alloy to have a long-term high-temperature strength and an excellent toughness.
• Patent Document 2 has no sufficient investigation on the carburization resistance properties, leaving room for improvement.
• the present invention has an objective to provide an austenitic heat resistant alloy that keeps a high creep strength and excellent carburization resistance properties even in its use in a high temperature environment.
• the present invention is made to solve the problem described above, and the gist of the present invention is the following austenitic heat resistant alloy.
• An austenitic heat resistant alloy having a chemical composition consisting of, in mass percent:
• Nb 0.05 to 2.00%
• N 0.05% or less
• an austenitic heat resistant alloy that keeps a high creep strength and excellent carburization resistance properties even in its use in a high temperature environment can be obtained.
• the present inventors conducted investigations and studies about carburization resistance properties of an austenitic heat resistant alloy in a high-temperature environment at 1000° C. or more (hereinafter, referred to simply as “high temperature environment”), and obtained the following findings.
• Carburization resistance properties at high temperature can be kept by forming a continuous alumina layer on a surface of a base metal.
• the formation of the alumina layer is promoted by presence of Cr. This effect is called the third element effect (TEE) of Cr.
• TEE third element effect
• Cr is preferentially oxidized on the surface of the base metal, forming a chromia layer.
• Cr—Mn spinel layer the layer having the “Cr—Mn-based spinel structure” (in the following description, also referred to as “Cr—Mn spinel layer”) is produced excessively in the use, Cr in an outer layer of the base metal runs short. This restrains the TEE as a period of the use increases, which causes Al to undergo internal oxidation, forming discontinuous alumina layers on the surface. As a result, the alumina becomes unable to fulfill a function as the protective layer.
• C carbon forms carbides, increasing the creep strength. Specifically, C binds with alloying elements to form fine carbides in crystal grain boundaries and grains in the use in the high-temperature environment. The fine carbides increase deformation resistance, thereby increasing the creep strength. If a content of C is excessively low, this effect is not obtained. In contrast, if the content of C is excessively high, a large number of coarse eutectic carbides are formed in a solidification micro-structure of the heat resistant alloy after casting. The eutectic carbides remain coarse in the micro-structure even after solution treatment, thus decreasing a toughness of the heat resistant alloy.
• the content of C is to range from 0.03 to 0.25%.
• a lower limit of the content of C is preferably 0.04%, more preferably 0.05%.
• An upper limit of the content of C is preferably 0.23%, more preferably 0.20%.
• Si deoxidizes the heat resistant alloy.
• Si increases corrosion resistances (oxidation resistance and steam oxidation resistance) of the heat resistant alloy.
• Si is an element that is contained unavoidably, but in a case where the deoxidation can be performed sufficiently by other elements, a content of Si may be as low as possible. In contrast, if the content of Si is excessively high, the hot workability is decreased. Accordingly, the content of Si is to range from 0.01 to 2.0%.
• a lower limit of the content of Si is preferably 0.02%, more preferably 0.03%.
• An upper limit of the content of Si is preferably 1.0%, more preferably 0.3%.
• S Sulfur
• S is an impurity. S decreases the weldability and the hot workability of the heat resistant alloy. Accordingly, a content of S is to be 0.010% or less. The content of S is preferably as low as possible.
• Chromium (Cr) increases corrosion resistances (oxidation resistance, steam oxidation resistance, etc.) of the heat resistant alloy in the high temperature environment.
• Cr brings about the TEE, promoting the uniform formation of the alumina layer.
• the content of Cr is to range from 13.0 to 30.0%.
• a lower limit of the content of Cr is preferably 15.0%.
• An upper limit of the content of Cr is preferably 25.0%, and more preferably 20.0%.
• Ni binds with Al to form fine NiAl, increasing the creep strength.
• Ni has an effect of increasing the corrosion resistances of the heat resistant alloy as well as an effect of increasing the carburization resistance properties by decreasing a diffusion velocity of C in the steel. If a content of Ni is excessively low, these effects are not obtained. In contrast, if the content of Ni is excessively high, these effects level off, and furthermore, the hot workability is decreased. In addition, the excessively high content of Ni increases a raw-material cost. Accordingly, the content of Ni is to range from 25.0 to 45.0%. A lower limit of the content of Ni is preferably 30.0%. An upper limit of the content of Ni is preferably 40.0%, more preferably 35.0%.
• Aluminum (Al) forms the alumina layer, which is excellent in the carburization resistance properties, in the use in the high temperature environment.
• Al binds with Ni to form the fine NiAl, increasing the creep strength. If a content of Al is excessively low, these effects are not obtained. In contrast, if the content of Al is excessively high, a structural stability is decreased, and a strength is decreased. Accordingly, the content of Al is to range from 2.5 to 4.5%.
• a lower limit of the content of Al is preferably 2.8%, more preferably 3.0%.
• An upper limit of the content of Al is preferably 3.8%.
• the content of Al means a total amount of Al contained in the alloy.
• Niobium (Nb) forms intermetallic compounds (Laves phase and Ni3Nb phase) to be precipitation strengthening phases, so as to bring about precipitation strengthening in the crystal grain boundaries and the grains, increasing the creep strength of the heat resistant alloy.
• Nb is excessively high, the intermetallic compounds are produced excessively, decreasing the toughness and the hot workability of the alloy.
• the excessively high content of Nb additionally decreases a toughness after long-time aging. Accordingly, the content of Nb is to range from 0.05 to 2.00%.
• a lower limit of the content of Nb is preferably 0.50%, more preferably 0.80%.
• An upper limit of the content of Nb is preferably 1.20%, more preferably 1.00%.
• N Nitrogen
• the content of N is to be 0.05% or less.
• An upper limit of the content of N is preferably 0.01%.
• Zr Zirconium
• Zr brings about grain-boundary strengthening, increasing the creep strength. Therefore, Zr may be contained as necessary. However, if a content of Zr is excessively high, the weldability and the hot workability of the heat resistant alloy are decreased. Accordingly, the content of Zr is to be 0.10% or less. An upper limit of the content of Zr is preferably 0.06%. Note that the content of Zr is preferably 0.0005% or more, and more preferably 0.001% or more in a case where an intention is to obtain the above effect.
• Copper (Cu) promotes the formation of the alumina layer in proximity to the surface, increasing the corrosion resistances of the heat resistant alloy. Therefore, Cu may be contained as necessary. However, if a content of Cu is excessively high, the effect levels off, and furthermore, the high temperature ductility is decreased. Accordingly, the content of Cu is to be 5.0% or less. An upper limit of the content of Cu is preferably 4.8%, more preferably 4.5%. Note that the content of Cu is preferably 0.05% or more, and more preferably 0.10% or more in a case where an intention is to obtain the above effect.
• Ca immobilizes S in a form of its sulfide, increasing the hot workability. Therefore, Ca may be contained as necessary. However, if a content of Ca is excessively high, the toughness, the ductility, and a cleanliness are decreased. Accordingly, the content of Ca is to be 0.050% or less. An upper limit of the content of Ca is preferably 0.030%, more preferably 0.010%. Note that the content of Ca is preferably 0.0005% or more in a case where an intention is to obtain the above effect.
• the thickness of the alumina layer formed by the treatment is less than 0.5 ⁇ m, the layer is broken in a short time in a high temperature carburizing environment, failing to keep the corrosion resistances. In contrast, if the thickness of the layer is more than 15 ⁇ m, the layer cannot withstand its internal stress and is prone to form a crack. Note that whether the alumina layer is continuous is evaluated by observing a cross section of the layer under a scanning electron microscope (SEM).
• the thickness of the Cr—Mn spinel layer is more than 5 ⁇ m, a Cr depleted zone is produced in the outer layer of the base metal, due to which the TEE is restrained as a period of the use increases.
• the producing method in the present embodiment includes a preparation step, a hot forging step, a hot working step, a cold working step, and a solution heat treatment step described below.
• the producing method may further include a scale removing step after the solution heat treatment step. The steps will be each described below.
• a molten steel having the chemical composition described above is produced.
• the molten steel is subjected to a well-known degassing treatment as necessary.
• the molten steel is cast to be produced into a starting material.
• the starting material may be an ingot made by an ingot-making process, or may be a cast piece such as a slab, bloom, and billet made by a continuous casting process.
• Hot working is performed on the hot-forged cylindrical starting material to produce an alloy hollow shell.
• a through hole is formed at a center of the cylindrical starting material by machining.
• Hot extrusion is performed on the cylindrical starting material with the through hole formed to produce the alloy hollow shell.
• the alloy hollow shell may be produced by performing piercing-rolling on the cylindrical starting material.
• Cold working is performed on the hot-worked alloy hollow shell to produce an intermediate material.
• the cold working is, for example, cold drawing or the like.
• a micro-structure of the base metal becomes close-grained through recrystallization in heat treatment, which enables formation of a more close-grained alumina layer.
• Solution heat treatment is performed on the produced intermediate material.
• the carbides and the precipitates included in the intermediate material are dissolved.
• the solution heat treatment its heat treatment temperature is 1150 to 1280° C. If the heat treatment temperature is less than 1150° C., the carbides and the precipitates are not dissolved sufficiently, and as a result, the corrosion resistances deteriorate. In contrast, if the heat treatment temperature is excessively high, the crystal grain boundaries are melted. A duration of the solution heat treatment is 1 minute or more, in which the carbides and the precipitates are dissolved.
• the austenitic heat resistant alloy according to the present embodiment is produced.
• the above description is made about the method for producing an alloy pipe, a plate material, but a bar material, a wire rod, or the like may be produced by a similar producing method.
• Molten steels having chemical compositions shown in Table 1 were produced using a vacuum furnace.
• the molten steels were used to produce column-shaped ingots having an outer diameter of 120 mm.
• the hot forging at an area reduction ratio of 60% was performed on the ingots to produce rectangular-shaped starting materials.
• the hot rolling and the cold rolling were performed on the rectangular-shaped starting materials to produce plate-shaped intermediate materials having a thickness of 1.5 mm. In the cold rolling, its area reduction ratio was 50%.
• the intermediate materials were retained at 1200° C. for 10 minutes and then water-cooled to be produced into alloy plate materials.
• the once-treated material subjected to the carburizing treatment was cut into halves in a direction perpendicular to its rolling direction.
• One of the halves was embedded in resin, and its observation surface was polished, by which a test specimen for observation was fabricated. Then, a kind, a thickness, and a form of the formed layer were observed under a SEM.
• a surface of the other of the halves subjected to the carburizing treatment was subjected to manual dry polishing using #600 abrasive paper, by which scales and the like on the surface were removed.
• the other of the two alloy plate materials was subjected to a process including carburizing treatment in which the other alloy plate material was heated in the H 2 —CH 4 —CO 2 atmosphere, at 1100° C., for 96 hours, and after the carburizing treatment, heating the other alloy plate material at 900° C. for 20 hours in the atmosphere containing steam, and the process was repeated five times (five-time-treated material).
• Test Nos. 14 to 20 are comparative examples that did not satisfy the specification according to the present invention. Specifically, Test No. 14 had a high content of C, and Test No. 17 had a low content of Nb, and thus Test No. 14 and Test No. 17 resulted in poor creep strengths.

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• 2018
  • 2018-12-27 EP EP18897323.4A patent/EP3733913A1/en not_active Withdrawn
  • 2018-12-27 JP JP2019562475A patent/JPWO2019131954A1/ja not_active Withdrawn
  • 2018-12-27 WO PCT/JP2018/048342 patent/WO2019131954A1/ja unknown
  • 2018-12-27 CN CN201880084052.8A patent/CN111542639A/zh not_active Withdrawn
  • 2018-12-27 US US16/958,550 patent/US20210062314A1/en not_active Abandoned

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