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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a stainless steel, more particularly to an austenitic stainless steel.
• austenitic stainless steels containing increased contents of Cr and increased contents of Ni, or Ni-based alloys containing increased contents of Cr have been used as heat resistant steels.
• These heat resistant steels are austenitic stainless steels or Ni-based alloys each containing about 20 to 30% by mass of Cr and about 20 to 70% by mass of Ni.
• Pipes of the facilities such as thermal power generation boilers and chemical plants are produced from steel material pipes.
• the steel material pipe is produced by melting and thereafter performing hot working on the above austenitic stainless steel or Ni-based alloy. Therefore, heat resistant steels are requested to have high hot workabilities.
• austenitic stainless steels typically have high deformation resistances and low ductilities at high temperature. For that reason, there is a demand for austenitic stainless steels having excellent hot workabilities.
• Patent Literature 1 Stainless steels having increased anti-carburizing properties and anti-coking properties are proposed in, for example, Japanese Patent Application Publication No. 2005-48284 (Patent Literature 1).
• a stainless steel disclosed in Patent Literature 1 is made of a base material including a chemical composition consisting of, in mass percent, C: 0.01 to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 20 to 55%, Ni: 10 to 70%, N: 0.001 to 0.25%, O (oxygen): 0.02% or less, with the balance being Fe and unavoidable impurities.
• This stainless steel includes a Cr depleted zone in its near-surface portion, a Cr concentration in the Cr depleted zone is 10% or more and less than a Cr concentration in the base material, and a thickness of the Cr depleted zone is within 20 ⁇ m.
• Patent Literature 1 states that the anti-carburizing properties and the anti-coking properties are increased by forming a protection film mainly made of Cr 2 O 3 coating film.
• the protection film mainly includes the Cr 2 O 3 coating film. Therefore, the stainless steel suffers from an insufficient function of preventing oxygen and carbon from entering from an external atmosphere, in particular, under a high temperature carburizing environment. As a result, internal oxidation and carburizing may occur in the material.
• Patent Literature 2 International Application Publication No. WO2010/113830 (Patent Literature 2), International Application Publication No. WO2004/067788 (Patent Literature 3), and Japanese Patent Application Publication No. 10-140296 (Patent Literature 4) disclose techniques relating to protection films that are alternatives to Cr 2 O 3 coating films.
• a protection film mainly containing Al 2 O 3 which is thermodynamically stable, is formed on a surface of heat resistant steel, as a protection film that is an alternative to the Cr 2 O 3 coating films.
• a cast product disclosed in Patent Literature 2 includes a casting made of a heat resistant alloy that consists of, in mass percent, C: 0.05 to 0.7%, Si: more than 0% to 2.5% or less, Mn: more than 0% to 3.0% or less, Cr: 15 to 50%, Ni: 18 to 70%, Al: 2 to 4 %, and rare earth metals: 0.005 to 0.4 %, as well as W: 0.5 to 10% and/or Mo: 0.1 to 5%, with the balance being Fe and unavoidable impurities.
• the casting includes a barrier layer formed on its surface that is to be brought into contact with a high-temperature atmosphere, the barrier layer is an Al 2 O 3 layer having a thickness of 0.5 ⁇ m or more, 80% by area or more of an outermost surface of the barrier layer is Al 2 O 3 , and Cr-based particles disperse in an interface between the Al 2 O 3 layer and the casting, the Cr-based particles having a Cr concentration higher than that of a base of the alloy.
• Patent Literature 2 states that with added Al, a protection film mainly including an Al 2 O 3 protection film is formed, and anti-carburizing properties are increased.
• a nickel-chromium casting alloy disclosed in Patent Literature 3 consists of, up to 0.8 % of Carbon, up to 1% of silicon, up to 0.2% of manganese, 15% to 40% of chromium, 0.5% to 13% of iron, 1.5% to 7% of aluminum, up to 2.5% of niobium, up to 1.5% of titanium, 0.01% to 0.4% of zirconium, up to 0.06% of nitrogen, up to 12% of cobalt, up to 5% of molybdenum, up to 6% of tungsten, and 0.019 % to 0.089% of yttrium, with the rest being nickel.
• Patent Literature 3 states that with added REM as well as Al, the nickel-chromium casting alloy including Al 2 O 3 , which serves as a protection film, with enhanced anti-peeling properties can be provided.
• An austenitic stainless steel disclosed in Patent Literature 4 consists of, in mass percent, C: 0.15% or less, Si: 0.9% or less, Mn: 0.2 to 2%, P: 0.04% or less, S: 0.005% or less, S(%) and O(%) at 0.015% or less in total, Cr: 12 to 30%, Ni: 10 to 35%, Al: 1.5 to 5.5%, B: 0.001 to 0.01%, N: 0.025% or less, Ca: 0 to 0.008%, Cu: 0 to 2%, one or more elements of Ti, Nb, Zr, V, and Hf at 0 to 2% in total, one or more elements of W, Mo, Co, and Re at 0 to 3% in total, and one or more elements of rare earth metals at 0 to 0.05% in total, with the balance being Fe and unavoidable impurities.
• Patent Literature 4 states that with added Al, a protection film mainly including an Al 2 O 3 protection film is formed, and an oxidation resistance is increased.
• the heat resistant alloy contains Cr at 50% at the maximum. Therefore, in a high temperature carburizing environment such as a hydrocarbon gas atmosphere, Cr may form its carbide on a steel surface. In this case, Al 2 O 3 , which serves as a protection film, is not formed uniformly. As a result, carburizing may occur.
• the casting item and the nickel-chromium casting alloy disclosed in Patent Literatures 2 and 3 each have a high content of C, which significantly decreases their hot workabilities.
• Patent Literature 3 a content of Ni is high, which significantly increases a raw-material cost.
• Patent Literature 4 anti-carburizing properties are not considered. As a result, its anti-carburizing properties may be low.
• An objective of the present invention is to provide an austenitic stainless steel that has excellent anti-carburizing properties even in a high temperature carburizing environment such as a hydrocarbon gas atmosphere, and provides an excellent hot workability in its production.
• An austenitic stainless steel includes a chemical composition consisting of, in mass percent, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to less than 4.5%, Nb: 0.01 to 3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, Ti: 0 to less than 0.2%, Mo: 0 to 0.5%, W: 0 to 0.5%, Cu: 0 to 0.5%, V: 0 to 0.2%, and B: 0 to 0.01%, with the balance being Fe and impurities, and satisfying Formula (1). 0.40 ⁇ C Cr ′ / C Al ′ / C Cr / C Al ⁇ 0.80
• a Cr concentration (mass percent) in an outer layer of the austenitic stainless steel is substituted for Ccr' in Formula (1).
• An Al concentration (mass percent) in the outer layer of the austenitic stainless steel is substituted for C Al '.
• a Cr concentration (mass percent) in an other-than-outer-layer region of the austenitic stainless steel is substituted for C Cr .
• An Al concentration (mass percent) in the other-than-outer-layer region of the austenitic stainless steel is substituted for C Al .
• the austenitic stainless steel according to the present embodiment has excellent anti-carburizing properties even in a high temperature carburizing environment such as a hydrocarbon gas atmosphere, and provides an excellent hot workability in its production.
• the present inventors conducted investigations and studies about anti-carburizing properties of the austenitic stainless steel in a high temperature carburizing environment and a hot workability in its production, and obtained the following findings.
• the high temperature carburizing environment refers to an environment in a hydrocarbon gas atmosphere at 1000°C or more.
• An austenitic stainless steel according to the present embodiment that is made based on the above findings includes a chemical composition consisting of, in mass percent, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to less than 4.5%, Nb: 0.01 to 3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, Ti: 0 to less than 0.2%, Mo: 0 to 0.5%, W: 0 to 0.5%, Cu: 0 to 0.5%, V: 0 to 0.2%, and B: 0 to 0.01%, with the balance being Fe and impurities, and satisfying Formula (1).
• a Cr concentration (mass percent) in an outer layer of the austenitic stainless steel is substituted for Ccr' in Formula (1).
• An Al concentration (mass percent) in the outer layer of the austenitic stainless steel is substituted for C Al '.
• a Cr concentration (mass percent) in an other-than-outer-layer region of the austenitic stainless steel is substituted for C Cr .
• An Al concentration (mass percent) in the other-than-outer-layer region of the austenitic stainless steel is substituted for C Al .
• the above chemical composition may contain one or two or more types selected from the group consisting of Ti: 0.005 to less than 0.2%, Mo: 0.01 to 0.5%, W: 0.01 to 0.5%, Cu: 0.005 to 0.5%, V: 0.005 to 0.2%, and B: 0.0001 to 0.01.
• a chemical composition of the austenitic stainless steel according to the present embodiment contains the following elements.
• Carbon (C) binds mainly with Cr to form a Cr carbide in the steel, increasing a creep strength in use in the high temperature carburizing environment.
• An excessively low content of C results in failure to provide this effect.
• an excessively high content of C causes a large number of coarse eutectic carbides to be formed in a solidification micro structure after the steel is cast, resulting in a decrease in a toughness of the steel. Consequently, a content of C is 0.03 to less than 0.25%.
• a lower limit of the content of C is preferably 0.05%, more preferably 0.08%.
• An upper limit of the content of C is preferably 0.23%, more preferably 0.20%.
• Silicon (Si) deoxidizes steel. If the deoxidation can be sufficiently performed using another element, a content of Si may be reduced as much as possible. In contrast, an excessively high content of Si results in a decrease in the hot workability. Consequently, the content of Si is 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%.
• Mn Manganese
• MnS Manganese
• a lower limit of the content of Mn is preferably 0.1%, more preferably 0.2%.
• An upper limit of the content of Mn is preferably 1.2%.
• Phosphorus (P) is an impurity. P decreases the weldability and the hot workability of the steel. Consequently, a content of P is 0.04% or less. An upper limit of the content of P is preferably 0.03%. The content of P is preferably as low as possible. A lower limit of the content of P is, for example, 0.0005%.
• S Sulfur
• S is an impurity. S decreases the weldability and the hot workability of the steel. Consequently, a content of S is 0.01% or less. An upper limit of the content of S is preferably 0.008%. The content of S is preferably as low as possible. A lower limit of the content of S is, for example, 0.001%.
• Chromium (Cr) exhibits the above TEE effect to promote the formation of the Al 2 O 3 coating film in the heat treatment process and under the high temperature carburizing environment.
• Cr binds with C in the steel to form a Cr carbide, increasing the creep strength.
• An excessively low content of Cr results in failure to provide these effects.
• an excessively high content of Cr causes Cr to bind with C derived from atmospheric gas (hydrocarbon gas) under the high temperature carburizing environment and form a Cr carbide on the steel surface.
• the formation of the Cr carbide on the steel surface causes local depletion of Cr in the steel surface. This lessens the TEE effect, resulting in failure to form the uniform Al 2 O 3 coating film.
• a content of Cr is 10 to less than 22%.
• a lower limit of the content of Cr is preferably 11%, more preferably 12%.
• An upper limit of the content of Cr is preferably 21%, more preferably 20%.
• the Cr carbide is divided into a Cr carbide formed in the steel and a Cr carbide formed on the steel surface.
• the Cr carbide in the steel is allowed to form, and the Cr carbide on the steel surface is inhibited.
• Nickel (Ni) stabilizes an austenite, increasing the creep strength. In addition, Ni increases the anti-carburizing properties of the steel. An excessively low content of Ni results in failure to provide these effects. In contrast, an excessively high content of Ni results not only in saturation of these effects but also in an increase in raw-material costs. Consequently, a content of Ni is more than 30.0% to 40.0%.
• a lower limit of the content of Ni is preferably 31.0%, more preferably 32.0%.
• An upper limit of the content of Ni is preferably 39.0%, more preferably 38.0%.
• Aluminum (Al) forms the Al 2 O 3 coating film on the steel surface in the heat treatment process and under the high temperature carburizing environment, increasing the anti-carburizing properties of the steel.
• the Al 2 O 3 coating film is thermodynamically stable as compared with Cr 2 O 3 coating films conventionally used.
• An excessively low content of Al results in failure to provide these effects.
• an excessively high content of Al leads to a decrease in structural stability, resulting in a significant decrease in the creep strength. Consequently, a content of Al is more than 2.5% to less than 4.5%.
• a lower limit of the content of Al is preferably 2.55%, more preferably 2.6%.
• An upper limit of the content of Al is preferably 4.2%, more preferably 4.0%.
• the content of Al means a total amount of Al contained in the steel material.
• Niobium (Nb) forms intermetallic compounds to be precipitation strengthening phases (Laves phase and Ni 3 Nb phase) to cause precipitation strengthening in crystal grain boundaries and in grains, increasing the creep strength of the steel.
• an excessively high content of Nb causes the intermetallic compounds to be produced excessively, resulting in a decrease in the toughness of the steel.
• an excessively high content of Nb also results in a decrease in the toughness after long-time aging. Consequently, a content of Nb is 0.01 to 3.5%.
• a lower limit of the content of Nb is preferably 0.05%, more preferably 0.1%.
• An upper limit of the content of Nb is preferably less than 3.2%, more preferably 3.0%.
• N Nitrogen
• an excessively high content of N causes coarse nitride and/or carbo-nitride, which remains undissolved even after heat treatment, to be produced.
• the coarse nitride and/or carbo-nitride decreases the toughness of the steel. Consequently, a content of N is 0.03% or less.
• An upper limit of the content of N is preferably 0.01%.
• a lower limit of the content of N is, for example, 0.0005%.
• a content of Ca is 0.0005 to 0.05%.
• a lower limit of the content of Ca is preferably 0.0006%, more preferably 0.0008%.
• An upper limit of the content of Ca is preferably 0.01%, more preferably 0.008%.
• Mg Magnesium (Mg) immobilizes S in a form of its sulfide, increasing the hot workability of the steel.
• an excessively high content of Mg results in a decrease in the toughness and the ductility.
• the hot workability decreases.
• an excessively high content of Mg results in a decrease in cleanliness. Consequently, a content of Mg is 0.0005 to 0.05%.
• a lower limit of the content of Mg is preferably 0.0006%, more preferably 0.0008%.
• An upper limit of the content of Mg is preferably 0.01%, more preferably 0.008%.
• the balance of the chemical composition of the austenitic stainless steel according to the present embodiment is Fe and impurities.
• the impurities mean elements that are mixed from ores and scraps used as raw material, a producing environment, or the like when the austenitic stainless steel is produced in an industrial manner, and are allowed to be mixed within ranges in which the impurities have no adverse effect on the present invention.
• the above chemical composition of the austenitic stainless steel may further contain Ti in lieu of a part of Fe.
• Titanium (Ti) is an optional element and need not be contained. If contained, Ti forms intermetallic compounds to be precipitation strengthening phases (Laves phase and Ni 3 Ti phase) to cause precipitation strengthening, increasing the creep strength. In contrast, an excessively high content of Ti causes the intermetallic compounds to be produced excessively, resulting in a decrease in high-temperature ductility and the hot workability. In addition, an excessively high content of Ti results in a decrease in the toughness after long-time aging. Consequently, a content of Ti is 0 to less than 0.2%. A lower limit of the content of Ti is preferably 0.005%, more preferably 0.01%. An upper limit of the content of Ti is preferably 0.15%, more preferably 0.1%.
• the above chemical composition of the austenitic stainless steel may further contain, in lieu of a part of Fe, one or two elements selected from the group consisting of Mo and W. All of these elements are optional elements and increase the creep strength of the steel.
• Molybdenum (Mo) is an optional element and need not be contained. If contained, Mo is dissolved in the austenite, a parent phase. The dissolved Mo causes solid-solution strengthening, increasing the creep strength. In contrast, an excessively high content of Mo results in a decrease in the hot workability. Consequently, a content of Mo is 0 to 0.5%.
• a lower limit of the content of Mo is preferably 0.01%, more preferably 0.05%.
• An upper limit of the content of Mo is preferably 0.4%, more preferably 0.3%.
• Tungsten (W) is an optional element and need not be contained. If contained, W is dissolved in the austenite, the parent phase. The dissolved W causes solid-solution strengthening, increasing the creep strength. In contrast, an excessively high content of W results in a decrease in the hot workability. Consequently, a content of W is 0 to 0.5%.
• a lower limit of the content of W is preferably 0.01%, more preferably 0.05%.
• An upper limit of the content of W is preferably 0.4%, more preferably 0.3%.
• the above chemical composition of the austenitic stainless steel may further contain Cu in lieu of a part of Fe.
• Copper (Cu) is an optional element and need not be contained. If contained, Cu stabilizes the austenite. In addition, Cu causes precipitation strengthening, increasing a strength of the steel. In contrast, an excessively high content of Cu results in a decrease in the ductility and the hot workability of the steel. Consequently, a content of Cu is 0 to 0.5%.
• a lower limit of the content of Cu is preferably 0.005%, more preferably 0.01%.
• An upper limit of the content of Cu is preferably 0.3%, more preferably 0.1%.
• the above chemical composition of the austenitic stainless steel may further contain V in lieu of a part of Fe.
• Vanadium (V) is an optional element and need not be contained. If contained, V forms intermetallic compounds, as with Ti, increasing the creep strength of the steel. In contrast, an excessively high content of V makes a volume ratio of the intermetallic compounds in the steel excessively high, resulting in a decrease in the hot workability. Consequently, a content of V is 0 to 0.2%.
• a lower limit of the content of V is preferably 0.005%, more preferably 0.01%.
• An upper limit of the content of V is preferably 0.15%, more preferably 0.1%.
• the above chemical composition of the austenitic stainless steel may further contain B in lieu of a part of Fe.
• B Boron
• B is an optional element and need not be contained. If contained, B segregates in grain boundaries, promoting precipitation of intermetallic compounds in the grain boundaries. This increases the creep strength of the steel. In contrast, an excessively high content of B results in decreases in the weldability and the hot workability of the steel. Consequently, the content of B is 0 to 0.01%.
• a lower limit of the content of B is preferably 0.0001%, more preferably 0.0005%.
• An upper limit of the content of B is preferably 0.008%, more preferably 0.006%.
• the austenitic stainless steel according to the present embodiment further satisfies Formula (1). 0.40 ⁇ C Cr ′ / C Al ′ / C Cr / C Al ⁇ 0.80
• a Cr concentration (mass percent) in an outer layer of the austenitic stainless steel is substituted for Ccr' in Formula (1).
• An Al concentration (mass percent) in the outer layer of the austenitic stainless steel is substituted for C Al '.
• a Cr concentration (mass percent) in an other-than-outer-layer region of the austenitic stainless steel is substituted for C Cr .
• An Al concentration (mass percent) in the other-than-outer-layer region of the austenitic stainless steel is substituted for C Al .
• the outer layer of the austenitic stainless steel means a range of 2 ⁇ m depth from the surface of the austenitic stainless steel.
• the 2 ⁇ m depth from the surface means 2 ⁇ m depth from a surface of the base metal.
• the 2 ⁇ m depth from the surface of the base metal means 2 ⁇ m depth from the surface of the base metal after the Al 2 O 3 coating film is removed by descaling treatment.
• the Cr concentration (mass percent) in the range of 2 ⁇ m depth from the surface of the austenitic stainless steel is substituted for C Cr ' in Formula (1).
• the Al concentration (mass percent) in the range of 2 ⁇ m depth from the surface of the austenitic stainless steel is substituted for C Al ' in Formula (1).
• the Cr concentration of the other-than-outer-layer region means an average Cr concentration (mass percent) in a region of the base material other than the outer layer.
• the Al concentration of the other-than-outer-layer region means an average Al concentration (mass percent) in the region of the base material other than the outer layer.
• the ratio of the Cr concentration in the outer layer to the Al concentration in the outer layer is made moderately lower than the ratio of the Cr concentration of the base material to the Al concentration of the base material.
• the formation of the Al 2 O 3 coating film is promoted as described above.
• the anti-carburizing properties are increased in the high temperature carburizing environment.
• F1 (C Cr '/C Al ')/(C Cr /C Al ).
• F1 is an index of Cr behavior.
• the ratio of the Cr concentration of the outer layer to the Al concentration of the outer layer is excessively higher than the ratio of the Cr concentration of the base material to the Al concentration of the base material. That is, Ccr', the Cr concentration of the outer layer, is excessively high.
• a Cr carbide is formed on the steel surface, physically inhibiting the formation of the uniform Al 2 O 3 coating film.
• F1 is 0.40 to 0.80.
• a lower limit of F1 is preferably 0.42, more preferably 0.44.
• An upper limit of F1 is preferably 0.79, more preferably 0.78.
• the Cr concentration C Cr ' in the outer layer and the Al concentration C Al ' in the outer layer described above are determined by the following method.
• the austenitic stainless steel is cut perpendicularly to its surface. In the range of 2 ⁇ m depth from the surface of the cut austenitic stainless steel (when the austenitic stainless steel includes the Al 2 O 3 coating film on its surface, it is the surface of the base metal after the Al 2 O 3 coating film is removed by the descaling treatment), any five points (measurement points) are selected.
• the Cr concentrations and the Al concentrations at the measurement points are measured by EDX (Energy Dispersive X-ray Spectroscopy). Values determined by averaging the measured values are defined as C Cr ' (%) and C Al ' (%).
• the austenitic stainless steel includes the Al 2 O 3 coating film on its surface
• the Cr concentration C Cr ' in the outer layer and the Al concentration C Al ' in the outer layer are measured after the descaling treatment is performed.
• Conditions for descaling the austenitic stainless steels conform to JIS Z 2290(2004).
• Analysis of the Cr concentration C Cr in the other-than-outer-layer region and the Al concentration C Al in the other-than-outer-layer region described above can be conducted by a well-known component analysis method. Specifically, they are determined by the following method.
• the austenitic stainless steel is cut perpendicularly to its longitudinal direction (in a case of a steel pipe, it is its axis direction), and a measurement surface is prepared. A wall-thickness center portion of the measurement surface is pierced with a drill. By the piercing, machined chips are produced, and the machined chips are collected. The machined chips are collected at four spots of the same measurement surface.
• the machined chips are collected at four spots provided at 45° pitches.
• the collected machined chips are subjected to ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) to conduct an elemental analysis of its chemical composition.
• ICP-OES Inductively Coupled Plasma Optical Emission Spectrometry
• a procedure of the analysis according to the ICP-OES conforms to JIS G 1258(2007). Averages of the measured values for the four spots are defined as the Cr concentration C Cr in the other-than-outer-layer region (%) and the Al concentration C Al in the other-than-outer-layer region (%).
• the austenitic stainless steel according to the present embodiment includes the Al 2 O 3 coating film on its surface after the heat treatment process to be described later. Therefore, the austenitic stainless steel of the present embodiment may include the Al 2 O 3 coating film on its surface.
• the Al 2 O 3 coating film can be removed by a well-known method such as pickling treatment and shotpeening performed after the heat treatment process. Therefore, in the austenitic stainless steel of the present embodiment, the Al 2 O 3 coating film may be removed from its surface.
• the austenitic stainless steel according to the present embodiment preferably has a grain size of 30 to 80 ⁇ m.
• the grain size is 30 ⁇ m or more, the creep strength of the steel further increases.
• the grain size is 80 ⁇ m or less, grain boundary diffusion of Al is promoted, which further promotes the formation of the Al 2 O 3 coating film.
• the grain size is determined by the microscopic test method for a grain size specified in JIS G 0551(2013).
• a shape of the austenitic stainless steel according to the present embodiment is not limited to a particular shape.
• the austenitic stainless steel is, for example, a steel pipe.
• An austenitic stainless steel pipe is used as a reaction tube for a chemical plant.
• the austenitic stainless steel may be a plate material, a bar material, a wire rod, or the like.
• 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 produce a starting material.
• the starting material may be an ingot made by an ingot-making process, or a cast piece such as a slab, bloom, and billet made by a continuous casting process.
• a tube-shaped casting may be produced by a centrifugal casting process.
• Hot forging may be performed on the produced starting material to produce a cylindrical starting material.
• an interior structure of the molten steel produced in the preparation process can be modified from a solidification micro structure to a regulated-grain-sized structure, which is formed by homogeneous grains.
• a temperature of the hot forging is, for example, 900 to 1200°C.
• Hot working is performed on the starting material produced through the preparation process or the starting material produced by the hot forging (cylindrical starting material) to produce a steel material pipe.
• a through hole is formed at a center of the cylindrical starting material by machining.
• the cylindrical starting material with the through hole formed is subjected to hot extrusion to produce the steel material pipe.
• a machining temperature of the hot extrusion is, for example, 900 to 1200°C.
• the steel material pipe may be produced by performing piercing-rolling (the Mannesmann process etc.) on the cylindrical starting material.
• Cold working is performed on the steel material pipe subjected to the hot working to produce an intermediate material.
• the cold working is, for example, cold drawing or the like.
• giving strain to the steel surface allows elements such as Al and Cr to move to the steel surface easily.
• the TEE effect is provided sufficiently. It is thereby possible to obtain an austenitic stainless steel in which Cr is moderately depleted in an outer layer of the steel and that satisfies Formula (1).
• This effect cannot be provided when a working ratio of the cold working is excessively low.
• An upper limit of the working ratio of the cold working is not particularly specified, but cold working with an excessively high working ratio is practically difficult to perform. Consequently, the working ratio of the cold working is 10 to 90%.
• Heat treatment is performed on the produced intermediate material in an air atmosphere.
• the uniform Al 2 O 3 coating film is formed on the steel surface.
• Cr in the outer layer of the steel is moderately depleted by the TEE effect.
• a temperature of the heat treatment is 900 to less than 1100°C, and a duration of the heat treatment is 3.0 to 30.0 minutes.
• the temperature of the heat treatment is less than 900°C, or the duration of the heat treatment is less than 3.0 minutes, the TEE effect cannot be provided sufficiently.
• the Cr concentration C Cr ' in the outer layer of the steel becomes excessively high, failing to satisfy Formula (1). Accordingly, a Cr carbide is formed on the steel surface under the high temperature carburizing environment, and the uniform Al 2 O 3 coating film is not formed sufficiently. As a result, the anti-carburizing properties are decreased. Consequently, the temperature of the heat treatment is 900°C or more, and the duration of the heat treatment is 3.0 minutes or more.
• a grain size becomes 30 ⁇ m or more.
• the Al concentration C Al ' in the outer layer of the steel becomes excessively low, failing to satisfy Formula (1). Accordingly, the uniform Al 2 O 3 coating film is not formed sufficiently under the high temperature carburizing environment. As a result, the anti-carburizing properties are decreased. Consequently, the temperature of the heat treatment is less than 1100°C, and the duration of the heat treatment is 30.0 minutes or less. In addition, when the temperature of the heat treatment is less than 1100°C, and the duration of the heat treatment is 30.0 minutes or less, a grain size becomes 80 ⁇ m or less.
• the temperature of the heat treatment is 900 to less than 1100°C, and the duration of the heat treatment is 3.0 to 30.0 minutes, the TEE effect is provided sufficiently and appropriately, and the steel having a chemical composition satisfying Formula (1) is obtained. As a result, the anti-carburizing properties under the high temperature carburizing environment are increased.
• pickling treatment may be performed on the intermediate material subjected to the heat treatment.
• pickling for example, a mixed acid solution of nitric acid and hydrochloric acid is used.
• a duration of the pickling is, for example, 30 minutes to 60 minutes.
• shot peening may be performed on the steel surface.
• a starting material and a shape of shot media, and treatment conditions are not specified, but the starting material and the shape, and the treatment conditions are set to be sufficient for peeling the scales on the steel surface and giving the strain to the steel surface.
• the scales refer to, for example, Al 2 O 3 .
• the austenitic stainless steel according to the present embodiment is produced.
• the above description is made about the method for producing a steel pipe.
• a plate material, bar material, wire rod, or the like may be produced by a similar producing method (preparation process, hot forging process, hot working process, cold working process, heat treatment process). It is particularly preferable to apply the austenitic stainless steel according to the present embodiment to steel pipes.
• the austenitic stainless steel according to the present embodiment is preferably an austenitic stainless steel pipe.
• a test specimen for microscopic observation was fabricated.
• a surface corresponding to the above cross section (referred to as an observation surface) was used, and the microscopic test method specified in ASTM E 112 was performed, and the grain size was measured.
• the observation surface was subjected to mechanical polishing, and thereafter etched using etching reagent, and crystal grain boundaries in the observation surface were exposed. An average grain size of ten visual fields on the etched surface was determined. The area of each visual field was about 0.75 mm 2 .
• the steel plates of the respective test numbers were subjected to the descaling treatment under conditions conforming to JIS Z 2290(2004).
• Each of the steel plates subjected to the descaling treatment was cut perpendicularly to its rolling direction, and a sample including a surface of the steel plate was taken.
• Each of the samples was embedded in resin, and its observation surface including a cross section of the vicinity to the surface of the steel plate was polished. On the polished observation surface, the above method was used to determine Ccr', the Cr concentration and C Al ', the Al concentration in the outer layer (range of 2 ⁇ m depth from the surface of the steel plate).
• the above method was used to determine the Cr concentration C Cr in the other-than-outer-layer region and the Al concentration C Al in the other-than-outer-layer region.
• the steel plates of the respective test numbers were retained in H 2 -CH 4 -CO 2 atmosphere at 1100°C ⁇ 96 hours. After the carburizing, scales and the like were removed from surfaces of the steel plates by performing manual dry polishing on the surfaces using #600 abrasive paper. From the surfaces of the steel plates, machined chips for analysis of four layers were taken at 0.5 mm pitches. On the taken machined chips for analysis, the C concentrations were measured by a high frequency combustion infrared absorption method. Values obtained by subtracting the C concentration originally contained in the steel from results of the measurement were determined as C concentration increase quantities. An average of C concentration increase quantities of the four layers was determined as an entering C quantity.
• a column-shaped tensile test specimen having a diameter of 10 mm and a length of 130 mm was cut out.
• Each tensile test specimen was subjected to a tensile test at a tensile speed (strain rate) of 10/s, and its hot workability was evaluated.
• strain rate tensile speed
• the austenitic stainless steel according to the present invention is available even in the high temperature carburizing environment such as a hydrocarbon gas atmosphere, in which there are concerns about carburizing and coking.
• the austenitic stainless steel is suitable for application to steel for reaction tube in chemical industry plants such as ethylene producing plants, and the like.

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• 2017
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