Technical field
[0001]
　The present invention relates to stainless steel, more particularly, it relates to an austenitic stainless steel.
BACKGROUND
[0002]
　Conventionally, in facilities such as thermal power boilers and chemical plants used in high temperature carburizing environment, as heat-resistant steel, austenitic stainless steel with increased Cr content and the Ni content, or, Ni group having an increased Cr content alloy is used. These heat-resistant steels, austenitic stainless steel or Ni-base alloy containing about 20 to 30 mass% Cr and 20 to 70 mass of about% Ni.
[0003]
　Equipment piping such as thermal power generation boilers and chemical plants are produced from steel containing tubes. Steel mother pipe, after the austenitic stainless steel or Ni-based alloy described above is melted, is manufactured is hot worked. Therefore, high hot workability is required to have heat-resistant steel. However, austenitic stainless steel is generally high deformation resistance at high temperatures, low ductility. Therefore, austenitic stainless steel is required to have an excellent hot workability.
[0004]
　Recently, the way, by the so-called shale revolution, inexpensive shale gas have been produced. In facilities such as chemical plants, when using the shale gas as a source gas, as compared with the conventional material such as naphtha, the carbon derived from the raw material gas (C), the metal tube used in equipment such as a chemical plant (e.g. reaction carburizing is likely to occur is a corrosion phenomenon of the tube). Therefore, the steel used in equipment such as chemical plants, obtained excellent carburization resistance.
[0005]
　Carburization resistance and coking resistance enhanced stainless steel, for example, has been proposed in JP 2005-48284 (Patent Document 1).
[0006]
　Stainless steel disclosed in Patent Document 1, by mass%, C: 0.01 ~ 0.6% , Si: 0.1 ~ 5%, Mn: 0.1 ~ 10%, P: 0.08% hereinafter, S: 0.05% or less, Cr: 20 ~ 55%, Ni: 10 ~ 70%, N: 0.001 ~ 0.25%, O ( oxygen): 0.02% or less, the balance being Fe and made from the base material having a chemical composition consisting of unavoidable impurities. The stainless steel has a Cr-depleted zone in the surface layer portion, the Cr concentration in the Cr-depleted zone, more than 10%, less than the Cr concentration of the base material, and the thickness of the Cr-depleted zone is within 20 [mu] m. In Patent Document 1, Cr 2 O 3 to form a protective film mainly composed of film, enhancing carburization resistance and coking resistance, it has been described as.
[0007]
　However, the stainless steel Patent Document 1, the subject of protective coating Cr 2 O 3 is film. Therefore, especially in a high-temperature carburizing environment, intrusion prevention of oxygen and carbon from the external atmosphere is not sufficient. As a result, there may occur an internal oxidation and carburization material.
[0008]
　Therefore, WO 2010/113830 (Patent Document 2), International Publication No. 2004/067788 (Patent Document 3), and JP-A-10-140296 (Patent Document 4), Cr 2 O 3 replaces the film It discloses a technique for the protection film. Specifically, in these documents, Cr 2 O 3 as a protective coating in place of the film, thermodynamically stable Al 2 O 3 to form a protective coating mainly composed of the surface of the heat-resisting steel.
[0009]
　Patent Document 2 disclosed cast product at mass%, C: 0.05 ~ 0.7% , Si: 0% Beyond 2.5%, Mn: more than 0% 3.0 % or less, Cr: 15 ~ 50%, Ni: 18 ~ 70%, Al: 2 ~ 4%, rare earth elements: 0.005 to 0.4%, and, W: 0.5 ~ 10% and / or Mo : contains 0.1 to 5% with the casting of the heat-resistant alloy and the balance Fe and unavoidable impurities. And a barrier layer is formed on the surface of the casting in contact with the high temperature atmosphere, the barrier layer has a thickness 0.5μm or more Al 2 O 3 a layer, more than 80 area% of the outermost surface of the barrier layer Al 2 O 3 is, Al 2 O 3 at the interface between the layers and the casting, high Cr based particles Cr concentration than the base alloy is characterized in that it dispersed. In Patent Document 2, the addition of Al, Al 2 O 3 to form a protective film mainly composed of film, enhancing carburization resistance, it has been described as.
[0010]
　Nickel disclosed in Patent Document 3 - chromium alloy cast, up to 0.8% carbon, silicon up to 1%, up to 0.2% manganese, 15% and 40% chromium, 0.5% to 13 % iron, 1.5% to 7% aluminum, niobium up to 2.5%, titanium up to 1.5% zirconium 0.01% to 0.4%, up to 0.06% nitrogen, cobalt up to 12%, up to 5% molybdenum, tungsten up to 6%, 0.019% ~ 0.089% of yttrium, remainder nickel. In Patent Document 3, the addition of REM in addition to Al, Al is a protective coating 2 O 3 Ni increased delamination resistance - chromium cast alloy is obtained, is described as.
[0011]
　Patent Document austenitic disclosed 4 stainless steel, in weight%, C: 0.15% or less, Si: 0.9% or less, Mn: 0.2 ~ 2%, P: 0.04% or less, S: 0.005% or less, and S (%) and O (%) 0.015% or less combined, Cr: 12 ~ 30%, Ni: 10 ~ 35%, Al: 1.5 ~ 5.5 %, B: 0.001 ~ 0.01% , N: 0.025% or less, Ca: 0 ~ 0.008%, Cu: 0 ~ 2%, Ti, Nb, Zr, of the V and Hf 1 0-2% or more species in total, W, Mo, 0 ~ 3 % in total of one or more of Co and Re, contain from 0 to 0.05% of one or more in total of rare earth elements , the balance consisting of Fe and unavoidable impurities. In Patent Document 4, the addition of Al, Al 2 O 3 to form a protective film mainly composed of film, enhances the oxidation resistance, has been described as.
CITATION
Patent Document
[0012]
Patent Document 1: JP 2005-48284 JP
Patent Document 2: WO 2010/113830
Patent Document 3: WO 2004/067788 Patent
Patent Document 4: JP-A 10-140296 JP
Summary of the Invention
Problems that the Invention is to Solve
[0013]
　However, in Patent Document 2, heat resistant alloy contains up to 50% Cr. Therefore, in the high-temperature carburizing environment such as a hydrocarbon gas atmosphere, there is a case where Cr in the steel surface to form a carbide. In this case, Al is a protective film 2 O 3 is not uniformly formed. Therefore, there is a case where carburization occurs.
[0014]
　Further, Patent Documents 2 and 3 to the disclosed casting and nickel - in chrome cast alloy, since a high C content, the hot workability is remarkably lowered.
[0015]
　Patent Document 3 further has high Ni content, the raw material cost increases significantly.
[0016]
　Patent Document 4, no consideration is given carburization resistance. For this reason, it may be low resistance to carburization.
[0017]
　An object of the present invention, even in a high temperature carburizing environment such as a hydrocarbon gas atmosphere, excellent have carburization resistance was further it is to provide an austenitic stainless steel having excellent hot workability during manufacturing .
Means for Solving the Problems
[0018]
　Austenitic stainless steel according to the present embodiment, by mass%, C: less than 0.03 ~ 0.25%, Si: 0.01 ~ 2.0%, Mn: 2.0% or less, P: 0.04 % or less, S: 0.01% or less, Cr: less than 10 ~ 22%, Ni: 30.0 % ultra-40.0%, Al: 2.5% ultra-than 4.5%, Nb: 0. 01 ~ 3.5%, N: 0.03 % or less, Ca: 0.0005 ~ 0.05%, Mg: 0.0005 ~ 0.05%, Ti: 0 ~ less than 0.2%, Mo: 0 ~ 0.5%, W: 0 ~ 0.5%, Cu: 0 ~ 0.5%, V: 0 ~ 0.2%, and, B: 0 contains ~ 0.01%, the balance being Fe and has a chemical composition consisting of impurities, it satisfies the equation (1).
　≦ 0.40 (C Cr '/ C Al ') / (C Cr / C Al ) ≦ 0.80 (1)
　where, C in the formula (1) Cr Cr in the surface layer of austenitic stainless steel to ' concentration (wt%) is substituted. C Al is the 'Al Also, C Cr in Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. C Al is the Al concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
Effect of the invention
[0019]
　Austenitic stainless steel according to the present embodiment, even in a high temperature carburizing environment such as a hydrocarbon gas atmosphere, has excellent carburization resistance, further has excellent hot workability at the time of manufacture.
DESCRIPTION OF THE INVENTION
[0020]
　The present inventors have carburization resistance of austenitic stainless steels at elevated temperatures carburization environment, and conduct research and investigated hot workability during production, and obtained the following findings. The high-temperature carburizing environment refers to 1000 ° C. or more environmental in hydrocarbon gas atmosphere.
[0021]
　It is contained the Cr in (A) austenitic stainless steel or Ni-based alloy, Cr is a protective film on the steel surface 2 O 3 is formed, increasing resistance to carburization is. However, as described above, Cr 2 O 3 are thermodynamically unstable. Therefore, in the present invention, Al in the steel surface 2 O 3 to form a film. Al 2 O 3 acts as a protective coating. Al 2 O 3 , in the high-temperature carburizing environment, Cr 2 O 3 is thermodynamically more stable than. That, Al 2 O 3 if coating, even 1000 ° C. or more environmental, it is possible to increase the carburization resistance of an austenitic stainless steel.
[0022]
　(B) Cr, when excess is contained in Al-containing austenitic stainless steel or Ni-base alloy, at a high temperature carburizing environment, binds to C from ambient gas. Cr conjugated with C forms a Cr carbide to the steel surface. Cr carbide, Al in the steel surface 2 O 3 physically inhibits uniform formation of the coating. As a result, resistance to carburization of the steel is lowered. Therefore, Cr content should be limited to a certain.
[0023]
　On the other hand, Cr is Al 2 O 3 to promote uniform formation of the coating. Since this effect, Cr of Third Element Effect (hereinafter referred to as TEE effect) that. Mechanism of TEE effect is as follows. In very early described later heat treatment step, the steel surface is first Cr is preferentially oxidized, Cr 2 O 3 is formed. Therefore, the oxygen partial pressure of the steel surface is reduced locally. Thus, Al is without internal oxidation, uniform Al near the surface 2 O 3 is formed as a film. Thereafter, Cr 2 O 3 oxygen was used as the Al 2 O 3 is incorporated into. Then, Al is in the heat treatment step end 2 O 3 protective coating of only is formed. Cr, like even under a high temperature carburizing environment, having TEE effect. That, Cr, even Al in a high-temperature carburizing environment 2 O 3 to promote uniform formation of the coating. Therefore, uniform Al 2 O 3 to form a coating, it is necessary to contain at least a certain Cr.
[0024]
　Therefore, to suppress the formation of Cr carbide in a high-temperature carburizing environment, Al 2 O 3 to promote the formation of the film, in the present invention, the Cr content is set to 10 to less than 22%.
[0025]
　In (C) austenitic stainless steels, the ratio of the Al concentration in the Cr concentration and the surface layer in the surface layer, it is effective to appropriately smaller than the ratio of the Cr concentration and the Al concentration than the surface layer other than the surface layer. That is, austenitic stainless steel satisfies Expression (1), increases the resistance to carburization in a high temperature carburizing environment.
　≦ 0.40 (C Cr '/ C Al ') / (C Cr / C Al ) ≦ 0.80 (1)
　where, C in the formula (1) Cr Cr in the surface layer of austenitic stainless steel to ' concentration (wt%) is substituted. C Al is the 'Al Also, C Cr in Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. C Al is the Al concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
[0026]
= 　F1 (C Cr '/ C Al ') / (C Cr / C Al is defined as). If F1 is 0.40 or more, in a high-temperature carburizing environment, TEE effect by Cr steel surface can be sufficiently obtained. In this case, Al 2 O 3 coating formation is promoted. If F1 is 0.80 or less, in a high temperature carburizing environment, the formation of Cr carbide in the steel surface is suppressed. Therefore, uniform Al 2 O 3 film is formed. As a result, increasing resistance to carburization is.
[0027]
　(D) if containing austenitic calcium chemical composition of 0.0005% or more of stainless steel (Ca) and 0.0005% or more magnesium (Mg), hot workability is improved. On the other hand, if the content of these elements is too high, decrease the toughness and ductility at high temperature austenitic stainless steel, hot workability is deteriorated. Therefore, Ca: 0.0005 to .05%, and, Mg: to contain 0.0005 to .05%.
[0028]
　Austenitic stainless steel according to the present embodiment has been completed based on the above findings, by mass%, C: less than 0.03 ~ 0.25%, Si: 0.01 ~ 2.0%, Mn: 2.0 % or less, P: 0.04% or less, S: 0.01% or less, Cr: less than 10 ~ 22%, Ni: 30.0 % ultra-40.0%, Al: 2.5% ultra-4. less than 5%, Nb: 0.01 ~ 3.5 %, N: 0.03% or less, Ca: 0.0005 ~ 0.05%, Mg: 0.0005 ~ 0.05%, Ti: 0 ~ 0 less than .2%, Mo: 0 ~ 0.5 %, W: 0 ~ 0.5%, Cu: 0 ~ 0.5%, V: 0 ~ 0.2%, and, B: 0 ~ 0.01 % containing, having a chemical composition the balance being Fe and impurities, satisfies the equation (1).
　≦ 0.40 (C Cr '/ C Al ') / (C Cr / C Al ) ≦ 0.80 (1)
　where, C in the formula (1) Cr Cr in the surface layer of austenitic stainless steel to ' concentration (wt%) is substituted. C Al is the 'Al Also, C Cr in Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. C Al is the Al concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
[0029]
　The chemical composition, Ti: less than 0.005 ~ 0.2%, Mo: 0.01 ~ 0.5%, W: 0.01 ~ 0.5%, Cu: 0.005 ~ 0.5%, V: 0.005 ~ 0.2%, and, B: 0.0001 ~ may contain one or more members selected from the group consisting of 0.01.
[0030]
　It described in detail below austenitic stainless steel of the present embodiment. "%" Related elements, unless otherwise specified, it means mass%.
[0031]
　[Chemical composition]
　The chemical composition of the austenitic stainless steel according to the present embodiment contains the following elements.
[0032]
　C: less than from 0.03 to 0.25%
　carbon (C) primarily bonded to Cr to Cr carbide is formed in the steel, increasing the creep strength during use at high temperatures carburization environment. If the C content is too low, the effect can not be obtained. On the other hand, if the C content is too high, coarse eutectic carbides in the solidified structure during subsequent casting of steel the many forms, reducing the toughness of the steel. Therefore, C content is less than 0.03 to 0.25 percent. The preferable lower limit of C content is 0.05%, more preferably 0.08%. The preferable upper limit of C content is 0.23%, and more preferably 0.20%.
[0033]
　Si: 0.01 ~ 2.0%
　silicon (Si) is deoxidized steel. If the deoxidation can be carried out sufficiently by other elements, the content of Si may be less as possible. On the other hand, if the Si content is too high, hot workability is deteriorated. Therefore, Si content is 0.01-2.0%. A preferable lower limit of Si content is 0.02%, more preferably from 0.03%. The preferable upper limit of the Si content is 1.0%.
[0034]
　Mn: 2.0% or less
　manganese (Mn) is inevitably contained. Mn forms MnS by combining with S contained in the steel, increasing the hot workability of the steel. However, if the Mn content is too high, the steel becomes too hard, hot workability and weldability is decreased. Therefore, Mn content is 2.0% or less. The preferable lower limit of the Mn content is 0.1%, more preferably 0.2%. The preferable upper limit of the Mn content is 1.2%.
[0035]
　P: 0.04% or less
　Phosphorus (P) is an impurity. P decreases the weldability and hot workability of the steel. Accordingly, P content is 0.04% or less. The preferable upper limit of the P content is 0.03%. P content is preferably as small as possible. The lower limit of the P content is, for example, 0.0005%.
[0036]
　S: 0.01% or less
　of sulfur (S) is an impurity. S decreases weldability and hot workability of the steel. Thus, S content is 0.01% or less. The preferable upper limit of the S content is 0.008%. S content is preferably as small as possible. The lower limit of the S content is, for example, is 0.001%.
[0037]
　Cr: less than 10 to 22%
　Chromium (Cr), the TEE effects described above, Al under and in high-temperature carburizing environment heat treatment step 2 O 3 to promote the formation of a film. Cr is further combined with C in steel to form a Cr carbide in the steel, increasing the creep strength. If the Cr content is too low, these effects can not be obtained. On the other hand, if the Cr content is too high, in a high-temperature carburizing environment, Cr is combined with C from ambient gas (hydrocarbon gas), to form a Cr carbide to the steel surface. When Cr carbide is formed on the steel surface Cr of the steel surface is locally deficient. Therefore TEE effect decreases, uniform Al 2 O 3 film is not formed. Further if the Cr content is too high, Cr carbide steel surface uniform Al 2 O 3 physically inhibit the formation of the coating. Therefore, Cr content is 10 to less than 22%. A preferable lower limit of the Cr content is 11%, more preferably 12%. The preferable upper limit of the Cr content is 21%, still more preferably 20%. In this specification, Cr carbide is divided into a Cr carbide is formed in the steel, the Cr carbide is formed on the steel surface. The austenitic stainless steel of the present embodiment, to form a Cr carbide in steel, Cr carbide steel surface inhibit.
[0038]
　Ni: 30.0% ultra-40.0%
　nickel (Ni), the austenite is stabilized, increasing the creep strength. Ni further, increase the carburization resistance of steel. If the Ni content is too low, these effects can not be obtained. On the other hand, if the Ni content is too high, not only these effects are saturated, the raw material cost becomes high. Therefore, Ni content is 30.0% ultra to 40.0%. A preferable lower limit of Ni content is 31.0%, still more preferably 32.0%. The preferable upper limit of the Ni content is 39.0%, still more preferably 38.0%.
[0039]
　Al: 2.5% ultra-4.5% less than
　aluminum (Al) is, Al in the steel surface under and in high-temperature carburizing environment heat treatment step 2 O 3 film-forming, enhancing the resistance to carburization of the steel. Especially in high-temperature carburizing environments that are assumed in the present invention, Cr is conventionally used 2 O 3 as compared to the film, Al 2 O 3 coating is thermodynamically stable. If the Al content is too low, these effects can not be obtained. On the other hand, if the Al content is too high, structural stability is lowered, creep strength decreases significantly. Therefore, Al content is less than 2.5% ultra to 4.5%. A preferable lower limit of the Al content is 2.55%, more preferably 2.6%. The preferable upper limit of Al content is 4.2%, more preferably from 4.0%. In austenitic stainless steel according to the present invention, Al content is meant the total amount of Al contained in the steel material.
[0040]
　Nb: 0.01 ~ 3.5%
　niobium (Nb) is an intermetallic compound serving as a precipitation strengthening phase (Laves phase and Ni 3 formed of Nb phase), and precipitation strengthening the grain boundaries and in crystal grains, increase the creep strength of the steel. On the other hand, if the Nb content is too high, the intermetallic compound is excessively generated, the toughness of the steel is lowered. Further if the Nb content is too high, also decreases long toughness after aging. Therefore, Nb content is 0.01 to 3.5%. The preferable lower limit of Nb content is 0.05%, more preferably 0.1%. The preferable upper limit of the Nb content is less than 3.2%, more preferably from 3.0%.
[0041]
　N: 0.03% or less
　Nitrogen (N) stabilizes the austenite is contained inevitably. On the other hand, if the N content is too high, coarse nitrides remain without forming a solid solution even after heat treatment and / or carbonitrides are formed. Coarse nitrides and / or carbonitrides decreases the toughness of the steel. Therefore, N content is 0.03% or less. The upper limit of the preferred N content is 0.01%. The lower limit of the N content is, for example, 0.0005%.
[0042]
　Ca: 0.0005 ~ 0.05%
　of calcium (Ca) fixes S as sulfides, improving the hot workability. On the other hand, if the Ca content is too high, toughness and ductility is decreased. Therefore, the hot workability is deteriorated. Further if the Ca content is too high, detergency is lowered. Therefore, Ca content is 0.0005 to 0.05%. The preferable lower limit of Ca is 0.0006%, more preferably 0.0008%. The preferable upper limit of the Ca content is 0.01%, more preferably 0.008%.
[0043]
　Mg: 0.0005 ~ 0.05%
　magnesium (Mg) fixes S as sulfides, enhancing the hot workability of steel. On the other hand, if the Mg content is too high, toughness and ductility is decreased. Therefore, the hot workability is deteriorated. Further if the Mg content is too high, detergency is lowered. Thus, Mg content is 0.0005 to 0.05%. A preferable lower limit of Mg is 0.0006%, more preferably 0.0008%. The preferable upper limit of the Mg content is 0.01%, more preferably 0.008%.
[0044]
　The remainder of the chemical composition of the austenitic stainless steel of the present embodiment is composed of Fe and impurities. Here, the impurity, when the industrial production of austenitic stainless steels, the ore as a raw material, there is to be mixed etc. Scrap or manufacturing environment, the allowable range of the present invention does not adversely affect It is the means something.
[0045]
　[For any element]
　The chemical composition of the above-mentioned austenitic stainless steels further, in place of part of Fe, and may contain Ti.
[0046]
　Ti: 0 ~ 0.2% less
　titanium (Ti) is optional element and may not be contained. If contained, Ti is an intermetallic compound serving as a precipitation strengthening phase (Laves phase and Ni 3 to form a Ti phase), increase the creep strength through precipitation strengthening. On the other hand, if the Ti content is too high, the intermetallic compound is excessively generated, high temperature ductility and hot workability is deteriorated. Further if the Ti content is too high, the toughness after aging long lowered. Therefore, Ti content is 0 to less than 0.2%. A preferable lower limit of the Ti content is 0.005%, more preferably 0.01%. The preferable upper limit of the Ti content is 0.15%, more preferably 0.1%.
[0047]
　Further the chemical composition of the austenitic stainless steels described above, instead of a part of Fe, may contain one or two kinds selected from the group consisting of Mo and W. All of these elements are optional elements, enhancing the creep strength of the steel.
[0048]
　Mo: 0 ~ 0.5%
　of molybdenum (Mo) is an optional element and may not be contained. If contained, Mo is a solid solution in the austenite the mother phase. Solid solution was Mo enhances the creep strength by solid-solution strengthening. On the other hand, if Mo content is too high, hot workability is deteriorated. Therefore, Mo content is 0 to 0.5%. A preferable lower limit of Mo content is 0.01%, more preferably 0.05%. The preferable upper limit of the Mo content is 0.4%, and more preferably 0.3%.
[0049]
　W: 0 ~ 0.5%
　tungsten (W) are optional elements may not be contained. If contained, W is a solid solution in the austenite the mother phase. Solid solution was W enhances the creep strength by solid-solution strengthening. On the other hand, if the W content is too high, hot workability is deteriorated. Therefore, W content is 0 to 0.5%. The preferable lower limit of the W content is 0.01%, more preferably 0.05%. The preferable upper limit of the W content is 0.4%, and more preferably 0.3%.
[0050]
　The chemical composition of the austenitic stainless steels described above further, in place of part of Fe, and may contain Cu.
[0051]
　Cu: 0 ~ 0.5%
　copper (Cu) is an optional element and may not be contained. If contained, Cu stabilizes austenite. Cu further enhance the strength of steel by precipitation strengthening. On the other hand, if the Cu content is too high, decrease the ductility and hot workability of the steel. Therefore, Cu content is 0 to 0.5%. The preferable lower limit of Cu content is 0.005%, more preferably 0.01%. The preferable upper limit of Cu content is 0.3%, more preferably 0.1%.
[0052]
　The chemical composition of the austenitic stainless steels described above further, in place of part of Fe, and may contain V.
[0053]
　V: 0 ~ 0.2%
　vanadium (V) are optional elements may not be contained. If contained, V is similar to the Ti to form an intermetallic compound, enhances the creep strength of the steel. On the other hand, if the V content is too high, the volume ratio of the intermetallic compound in the steel becomes excessively high, hot workability is deteriorated. Therefore, V content is 0-0.2%. The preferable lower limit of V content is 0.005%, more preferably 0.01%. The preferable upper limit of the V content is 0.15%, more preferably 0.1%.
[0054]
　Further the chemical composition of the austenitic stainless steels described above, instead of a part of Fe, and may contain B.
[0055]
　B: 0 ~ 0.01%
　boron (B) is optional element and may not be contained. If contained, B is segregated in the grain boundary, to promote the precipitation of intermetallic compounds in the grain boundaries. Thus, increasing the creep strength of the steel. On the other hand, if the B content is too high, decrease the weldability and hot workability of the steel. Therefore, B content is 0 to 0.01%. The preferable lower limit of the B content is 0.0001%, more preferably from 0.0005%. The preferable upper limit of B content is 0.008%, more preferably 0.006%.
[0056]
　[For formula (1)]
　austenitic stainless steel of the present embodiment further satisfies the formula (1).
　≦ 0.40 (C Cr '/ C Al ') / (C Cr / C Al ) ≦ 0.80 (1)
　where, C in the formula (1) Cr Cr in the surface layer of austenitic stainless steel to ' concentration (wt%) is substituted. C Al is the 'Al Also, C Cr in Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. C Al is the Al concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
[0057]
　In the present specification, the surface layer of austenitic stainless steel, means the range of from the surface of austenitic stainless steel to 2μm depth. The 2μm depth from the surface, means 2μm depth from the surface of the base material. Austenitic stainless steel Al on the surface 2 O 3 when comprising a film, and the 2μm depth from the surface of the base material, Al by descaling 2 O 3 of 2μm depth from the surface of the base material after removing the coating means. That, C in the formula (1) Cr is the 'Al on the surface (austenitic stainless steel surface of the austenitic stainless steel 2 O 3 when having the coating, Al by descaling 2 O 3 after removal of the film Cr concentration in the range of from the surface) to 2μm depth of the base metal (mass%) is substituted. C in the formula (1) Al is the 'Al the surface of austenitic stainless steel (austenitic stainless steel on the surface 2 O 3 when having the coating, Al by descaling 2 O 3 base after removing the coating Al concentration in a range from the surface) of the timber up to 2μm depth (mass%) is substituted. Further, the Cr concentration than the surface layer (wt%), means the average Cr concentration of the base material in the region other than the surface layer (wt%). The Al concentration than the surface layer (wt%) refers to the average Al concentration of the base material in the region other than the surface layer (wt%).
[0058]
　As shown in equation (1), in austenitic stainless steel of the present embodiment, the ratio between the surface layer of the Cr concentration and the surface of the Al concentration, moderately than the ratio of the Cr concentration and the Al concentration in the mother material of the base material small. In this case, as described above, Al 2 O 3 coating formation is promoted. As a result, in a high-temperature carburizing environment, increasing resistance to carburization is.
[0059]
= 　F1 (C Cr '/ C Al ') / (C Cr / C Al is defined as). F1 is an indication of Cr behavior.
[0060]
　If it exceeds F1 is 0.80, the ratio between the surface layer of the Cr concentration and the surface of the Al concentration is too larger than the ratio of the Cr concentration and the Al concentration in the mother material of the base material. That is, the surface of the Cr concentration C Cr 'is too high. In this case, Cr carbide is formed on the steel surface at a high temperature carburizing environment, uniform Al 2 O 3 formation of film is physically inhibited.
[0061]
　If F1 is less than 0.40, the ratio of the surface layer of the Cr concentration and the surface of the Al concentration is too small than the ratio of the Cr concentration and the Al concentration in the mother material of the base material. In other words, the surface layer of Cr concentration C Cr 'is too small. In this case, TEE effect of Cr can not be obtained in a high temperature carburizing environment. Therefore, uniform Al in the steel surface 2 O 3 film is not formed.
[0062]
　Thus, F1 is 0.40 to 0.80. A preferred lower limit of F1 is 0.42, more preferably 0.44. A preferred upper limit of F1 is 0.79, more preferably 0.78.
[0063]
　Cr concentration in the surface layer of the above-described C Cr 'and the surface layer of the Al concentration C Al ' is calculated by the following method. Cutting perpendicular to the surface of austenitic stainless steel. Cutting the surface of austenitic stainless steel (austenitic stainless steel Al on the surface 2 O 3 when having the coating, Al by descaling 2 O 3 surface of the base after removal of the coating) from up to 2μm depth in the range, it selects arbitrary five points (measurement points). The Cr concentration and the Al concentration of each measuring point is measured by EDX (energy dispersive X-ray spectroscopy). The value obtained for the measured values by averaging C Cr '(%) and C Al is defined as' (%).
[0064]
　Austenitic stainless steel Al on the surface 2 O 3 when having the coating, after the descaling process, the surface of the Cr concentration C Cr 'and the surface layer of the Al concentration C Al to measure'. Conditions for descaling austenitic stainless steel, which conforms to JIS Z 2290 (2004).
[0065]
　Cr concentration C other than the surface layer of the above Cr and except the surface layer Al concentration C Al analysis can be determined by a well-known component analysis. Specifically, determined by the following method. The austenitic stainless steels, to prepare the measurement surface and cut perpendicular to the (axial direction if the steel pipe) longitudinal direction. The thickness center portion of the measurement surface, to drilling using a drill. It generates chips by drilling, and collecting the chips. And collecting the chips from four of the same measurement surface. When austenitic stainless steels are steel pipes, collecting cuttings from four at 45 ° pitch. ICP-OES respect harvested chips (Inductively Coupled Plasma Optical Emission Spectrometry) was carried out to perform the elemental analysis of the chemical composition. Procedure for analysis by ICP-OES conforms to JIS G 1258 (2007). The average of four measurements, other than the surface layer Cr concentration C Cr (%) and a surface layer other than the Al concentration C Al and (%).
[0066]
　Austenitic stainless steel of the present embodiment, after the heat treatment step described later, Al to the surface 2 O 3 comprises a film. Accordingly, the austenitic stainless steel of the present embodiment, Al in the surface 2 O 3 may have a coating. However, Al 2 O 3 film can be removed by a known method, pickling or shot peening or the like after the heat treatment step. Accordingly, the austenitic stainless steel of the present embodiment, Al of the surface 2 O 3 in some cases in a state where the film has been removed.
[0067]
　[Grain size]
　Preferably, the grain size of the austenitic stainless steel of the present embodiment is 30 ~ 80 [mu] m. If the crystal grain size is 30μm or more, further increases creep strength of the steel. If the crystal grain size is 80μm or less, the grain boundary diffusion of Al is promoted, Al 2 O 3 coating formation is further promoted. Crystal grain size is determined by microscopic examination method for grain size prescribed in JIS G 0551 (2013).
[0068]
　The shape of austenitic resistant stainless steel according to the present embodiment is not particularly limited. Austenitic resistant stainless steel, for example, a steel pipe. Austenitic stainless steel is used as the reaction tube for chemical plants. Austenitic stainless steel, plate, bar, may be a wire or the like.
[0069]
　[Manufacturing Method]
　An example of a manufacturing method of austenitic resistant stainless steel of the present embodiment, a method for manufacturing a steel pipe.
[0070]
　[Preparation Step]
　to produce a molten steel having the chemical composition described above. Against the molten steel, carrying out the known degassing treatment if necessary. With molten steel, to produce a material by casting. Material may be a ingot by ingot-making method, a slab or bloom by continuous casting, may be a slab of billets or the like. Further, by centrifugal casting, may be prepared casting of tubular shape.
[0071]
　[Hot forging step]
　hot forging may be manufactured cylinder material was performed on manufactured material. By carrying out hot forging, the molten steel inside the tissue prepared in the preparation step can be changed into a homogeneous grain structure from the solidified structure. Temperature of hot forging for example, 900 ~ 1200 ° C..
[0072]
　[Hot working step]
　prepared in preparation step material, or by carrying out hot working against hot forged material (cylindrical material), to produce a steel-containing tubes. For example, to form a through hole in the cylindrical element center by machining. The hot extrusion was performed on a cylindrical material formed with a through-hole, to produce a steel-containing tubes. Working temperature of hot extrusion is, for example, 900 ~ 1200 ° C.. Piercing the cylindrical material (Mannesmann method) to be produced steel containing tube.
[0073]
　[Cold working step]
　The cold working was performed with respect to the steel element tube after hot working, to produce an intermediate material. Cold working example, a cold drawing or the like. If imparting strain to the steel surface in the cold working process, elements such as Al and Cr is easily moved to the steel surface. In this case, it obtained sufficiently TEE effect. This allows the steel surface layer of Cr is moderately depleted, obtain austenitic stainless steels satisfying the formula (1). If the processing rate of the cold working is too low, the effect can not be obtained. The upper limit of the working ratio of cold working is not particularly provided, the cold working processing rate is too high, practically implementations are difficult. Therefore, the processing rate of the cold working is 10 to 90%.
[0074]
　[Heat treatment step]
　with respect manufactured intermediate member, carrying out the heat treatment in an air atmosphere. The heat treatment in an air atmosphere, uniform Al in the steel surface 2 O 3 film is formed. In this case, Cr in the steel surface layer is moderately depleted by TEE effect. As a result, it is possible to obtain an austenitic stainless steel which satisfies the equation (1).
[0075]
　The heat treatment temperature is less than 900 ~ 1100 ° C., the heat treatment time is 3.0 to 30.0 minutes.
[0076]
　The heat treatment temperature is lower than 900 ° C., or if heat treatment time is less than 3.0 minutes, TEE effect is not sufficiently obtained. In this case, the steel surface of Cr concentration C Cr too high ', does not satisfy the equation (1). Therefore, in a high-temperature carburizing environment, Cr carbide is formed on the steel surface, uniform Al 2 O 3 film is not sufficiently formed. As a result, carburization resistance is lowered. Therefore, the heat treatment temperature is 900 ° C. or higher, and the heat treatment time is more than 3.0 minutes. Heat treatment temperature of 900 ° C. or higher, and further if the heat treatment time is 3.0 minutes or more, the crystal grains is equal to or greater than 30 [mu] m.
[0077]
　On the other hand, if the heat treatment temperature is 1100 ° C. or higher, Cr on the steel surface 2 O 3 scale mainly composed of are excessively formed. For this reason, the steel surface layer of Cr is too depleted. In this case, the steel surface of Cr concentration C Cr 'is too low, does not satisfy the equation (1). Therefore, reduces the TEE effect of Cr in a high-temperature carburizing environment, uniform Al 2 O 3 film is not sufficiently formed. As a result, carburization resistance is lowered. Further, if it exceeds the heat treatment time is 30.0 minutes, Al in the steel surface 2 O 3 scale mainly composed of are excessively formed. Therefore, the steel surface layer of Al is excessively deficient. In this case, the steel surface of the Al concentration C Al 'is too low, does not satisfy the equation (1). Therefore, uniform Al in a high-temperature carburizing environment 2 O 3 is not sufficiently formed film. As a result, carburization resistance is lowered. Therefore, the heat treatment temperature is lower than 1100 ° C., and the heat treatment time is less than 30.0 minutes. Less than the heat treatment temperature is 1100 ° C., and the heat treatment time is further equal to or less than 30.0 minutes, the crystal grains become 80μm or less.
[0078]
　Less than the heat treatment temperature is 900 ~ 1100 ° C., if the heat treatment time is 3.0 to 30.0 minutes, TEE effect sufficiently and suitably obtained, steel having a chemical composition satisfying the formula (1) is obtained. As a result, it increases the carburization resistance under high temperature carburizing environment.
[0079]
　The intermediate material after the heat treatment, the removal of scale formed on the surface may be subjected to a pickling treatment purposes. The pickling for example, a mixed acid solution of nitric acid and hydrochloric acid. Pickling time, for example, from 30 minutes to 60 minutes.
[0080]
　Moreover, the intermediate material after pickling, the purpose of distortion imparting to descaling and steel surfaces of the steel surface may be performed shots processed steel surface. Shot particles of the material in the shot processing, shape, and processing conditions are not specified, peel the scale of the steel surface, or a sufficient material, shape, and processing conditions to impart strain to the steel surface. Scale and, for example, Al 2 O 3 is. By a known method such as pickling and shot process, Al 2 O 3 film can be removed.
[0081]
　By the above manufacturing method, the austenitic stainless steel of the present embodiment is manufactured. In the above described method for manufacturing a steel pipe. However, the same manufacturing method (preparation step, hot forging step, hot working step, cold working step, heat treatment step), the plate material, rod material, may be manufactured wire or the like. Austenitic stainless steel of the present embodiment, it is particularly preferably applied to a steel pipe. Thus, preferably, an austenitic stainless steel of the present embodiment is an austenitic stainless steel tube.
Example
[0082]
　[Production Method]
　The molten steel having the chemical compositions shown in Table 1, were prepared using a vacuum melting furnace.
[0083]
[Table 1]

[0084]
　Using the above molten steel was produced with an outer diameter of 120mm cylindrical ingot (30kg). It was carried out hot forging and hot rolling with respect to the ingot. After hot rolling, by carrying out cold rolling under the conditions shown in Table 2, we were prepared intermediate material having a thickness of 15 mm. From the obtained intermediate material, a plate material of 8 mm × 20 mm × 30 mm by machining, were manufactured by two respective steels. At a temperature and time shown in Table 2 with respect to the plate material and heat-treated. After the heat treatment, the sheet was cooled with water to produce a steel sheet for testing.
[0085]
[Table 2]

[0086]
　[Austenite grain measured diameter]
　Test pieces were prepared for microscopic observation from a central portion in the rolling direction and the cross section perpendicular steel of each test number. Of the surface of the test piece, using surface (called observation plane) corresponding to the cross section, and carrying out the microscopic examination method specified in ASTM E 112, to measure the crystal grain size. Specifically, after mechanically polishing the observation surface, the etchant corrosion was used to appear the crystal grain boundaries of the viewing surface. In 10 field on the corroded surface, to obtain an average crystal grain size of each field. Area of each field of view, about 0.75 mm 2 is.
[0087]
　[Surface layer of Cr concentration C Cr 'and the surface layer of the Al concentration C Al ' Measurement
　against steel of each test number was performed descaling under conditions conforming to JIS Z 2290 (2004). The steel sheet after descaling cut perpendicular to the rolling direction were taken samples containing surface. Embedded sample to the resin and polished viewing surface including a cross-section near the surface. With respect to the observation surface after polishing, Cr concentration C of the surface layer using the method described above (ranging from the surface to 2μm depth) Cr 'and Al concentration C Al was determined'.
[0088]
　[Cr concentration C other than the surface layer Cr Al concentration C other than and the surface layer Al Measurement
　by the method described above, Cr concentration C other than the surface layer Cr and Al concentration C other than the surface layer Al was determined.
[0089]
　[Carburizing Test]
　The steel sheet of each test number, H 2 -CH 4 -CO 2 was held 1100 ° C. × 96 hours at ambient. The steel sheet surface after carburizing and dry hand polished with # 600 abrasive paper to remove the scale of the surface. Analysis chips of four layers at 0.5mm pitch from the steel sheet surface was collected. The obtained analytical cutting chips was measured C concentration in the high-frequency combustion-infrared absorption method. From the measurement results, by subtracting the C concentration is contained originally in the steel, and the C concentration increase. The average C concentration increase of four layers, was entering C amount.
[0090]
　[High-temperature tensile test]
　Preparative ingot, from thick central portion and a length in 10mm diameter were cut cylindrical tensile specimens of 130 mm. Each tensile test specimen, a tensile speed (strain rate) was carried out tensile test at 10 / s, to evaluate the hot workability. In the present invention, the diaphragm after the tensile test at 900 ° C., passed 60% (○), was judged as failure less than 60% (×).
[0091]
　[Test Results]
　Table 2 shows the test results.
[0092]
　Referring to Table 2, the chemical composition of Test No. 1 - test No. 12 is appropriate, since production conditions were also suitable, F1 satisfied the equation (1). As a result, intrusion C content is 0.4% or less showed excellent carburization resistance. Furthermore, the aperture value of the high-temperature tensile test is 60% or more showed excellent hot workability.
[0093]
　On the other hand, in Test No. 13, the working ratio of cold rolling was too low. Therefore, F1 is 0.35, did not satisfy the formula (1). As a result, intrusion C content is 0.51% carburization resistance was low.
[0094]
　In test number 14, the heat treatment temperature was too low. Therefore, F1 is 1.00, did not satisfy the formula (1). As a result, intrusion C content is 0.65%, carburization resistance was low. In Test No. 14 In addition, the crystal grain size was 21 [mu] m.
[0095]
　In test number 15, the heat treatment temperature was too high. Therefore, F1 is 0.39, did not satisfy the formula (1). As a result, intrusion C content is 0.58% carburization resistance was low. In Test No. 15 In addition, the crystal grain size was 131μm.
[0096]
　In Test No. 16, the heat treatment time was too short. Therefore, F1 is 1.06, did not satisfy the formula (1). As a result, intrusion C content is 0.69% carburization resistance was low. In Test No. 16 In addition, the crystal grain size was 22 .mu.m.
[0097]
　In Test No. 17, the heat treatment time is too long. Therefore, F1 is 0.95, did not satisfy the formula (1). As a result, intrusion C content is 0.54% carburization resistance was low. In Test No. 17 In addition, the crystal grain size was 95 .mu.m.
[0098]
　In Test No. 18, Cr content is too low. Therefore, TEE effect due to Cr is reduced. As a result, intrusion C content is 0.75%, carburization resistance was low.
[0099]
　In Test No. 19, Cr content is too high. Therefore, Al with Cr carbides 2 O 3 coating formation was inhibited. As a result, intrusion C content is 0.60% carburization resistance was low.
[0100]
　In Test No. 20, Al content was too low. Therefore, Al 2 O 3 film is not sufficiently formed. As a result, intrusion C content is 0.83% carburization resistance was low.
[0101]
　In Test No. 21, Ni content was too low. Therefore, intrusion C content is 0.52% carburization resistance was low.
[0102]
　In Test No. 22, Mg content was too low. Therefore, the aperture value is less than 60%, hot workability is low.
[0103]
　In Test No. 23, Mg content was too high. Therefore, the aperture value is less than 60%, hot workability is low.
[0104]
　It has been described an embodiment of the present invention. However, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by changing the above-described embodiments without departing from the scope and spirit thereof as appropriate.
Industrial Applicability
[0105]
　Austenitic stainless steels of the present invention can also be used in high-temperature carburizing environment, such as a hydrocarbon gas atmosphere, carburizing and coking is concerned. In particular, it is particularly suitable for use as the reaction tube for steel in chemical industrial plant such as ethylene production plant.

The scope of the claims
[Requested item 1]
　By
　mass%, C: less than ~ 0.25%
　0.03, Si: 0.01
　~ 2.0%, Mn: 2.0% or
　less, P: 0.04% or
　less, S: 0.01% or less ,
　Cr: less than ~ 22%
　10, Ni: 30.0%
　ultra-40.0% ~, Al: 2.5% ultra-less%
　4.5, Nb: 0.01 - 3.5%,
　N: 0 .03% or
　less,
　Ca: 0.0005 ~ 0.05%, Mg: 0.0005
　~ 0.05%, Ti: 0 ~ less than%
　0.2, Mo: 0 ~
　0.5%, W: 0 ~
　% 0.5, Cu: 0
　- 0.5% V: 0 ~ 0.2%
　and, B: 0 contains ~ 0.01%,
　having a chemical composition the balance being Fe and impurities,
　It satisfies the equation (1), austenitic stainless steel.
　≦ 0.40 (C Cr '/ C Al ') / (C Cr / C Al ) ≦ 0.80 (1)
　where, C in the formula (1) Cr Cr in the surface layer of austenitic stainless steel to ' concentration (wt%) is substituted. C Al is the 'Al Also, C Cr in Cr concentration than the surface layer of austenitic stainless steel (mass%) is substituted. C Al is the Al concentration than the surface layer of austenitic stainless steel (mass%) is substituted.
[Requested item 2]
　A austenitic stainless steel according to claim 1,
　wherein the chemical
　composition, Ti: less than ~ 0.2%
　0.005, Mo: 0.01
　~ 0.5%, W: 0.01 ~ 0.
　% 5,
　Cu: 0.005 ~ 0.5%, V: 0.005 ~ 0.2%,
　and, B: one or more members selected from the group consisting of 0.0001-0.01 containing, austenitic stainless steel.